Milos Toth is a Professor in the School of Mathematical and Physical Sciences. He received a PhD in 2001 from UTS where he worked on the luminescence properties and defect structure of wide bandgap semiconductors.
He subsequently worked as a postdoctoral researcher at Cavendish Laboratory, University of Cambridge, a research scientist at FEI Company laboratories in Boston, Massachusetts and Portland, Oregon (USA). His work in industry was focused on the research and development of charged particle beam nanofabrication and imaging techniques.
Milos returned to UTS in 2011 to take up a Professorial Chair, and to set up a research group focused on electron and ion beam nanofabrication techniques.
Can supervise: YES
My research is focused on charged particle interactions with solids, self assembly and reaction kinetics at surfaces.
Applied aspects of the work include the development of electron and ion beam nanofabrication techniques and their application in nanophotonics and optoelectronics.
Berhane, A.M., Jeong, K.Y., Bradac, C., Walsh, M., Englund, D., Toth, M. & Aharonovich, I. 2018, 'Photophysics of GaN single-photon emitters in the visible spectral range', Physical Review B, vol. 97, no. 16.View/Download from: Publisher's site
© 2018 American Physical Society. In this work, we present a detailed photophysical analysis of recently discovered, optically stable single-photon emitters (SPEs) in gallium nitride (GaN). Temperature-resolved photoluminescence measurements reveal that the emission lines at 4 K are three orders of magnitude broader than the transform-limited width expected from excited-state lifetime measurements. The broadening is ascribed to ultrafast spectral diffusion. The photophysical study on several emitters at room temperature (RT) reveals an average brightness of (427±215)kCounts/s. Finally, polarization measurements from 14 emitters are used to determine visibility as well as dipole orientation of defect systems within the GaN crystal. Our results underpin some of the fundamental properties of SPEs in GaN both at cryogenic and RT, and define the benchmark for future work in GaN-based single-photon technologies.
Bray, K., Kato, H., Previdi, R., Sandstrom, R., Ganesan, K., Ogura, M., Makino, T., Yamasaki, S., Magyar, A.P., Toth, M. & Aharonovich, I. 2018, 'Single crystal diamond membranes for nanoelectronics.', Nanoscale, vol. 10, no. 8, pp. 4028-4035.View/Download from: Publisher's site
Single crystal, nanoscale diamond membranes are highly sought after for a variety of applications including nanophotonics, nanoelectronics and quantum information science. However, so far, the availability of conductive diamond membranes has remained an unreachable goal. In this work we present a complete nanofabrication methodology for engineering high aspect ratio, electrically active single crystal diamond membranes. The membranes have large lateral directions, exceeding 500 500 m2 and are only several hundreds of nanometers thick. We further realize vertical single crystal p-n junctions made from the diamond membranes that exhibit onset voltages of 10 V and a current of several mA. Moreover, we deterministically introduce optically active color centers into the membranes, and demonstrate for the first time a single crystal nanoscale diamond LED. The robust and scalable approach to engineer the electrically active single crystal diamond membranes offers new pathways for advanced nanophotonic, nanoelectronic and optomechanical devices employing diamond.
Kianinia, M., Bradac, C., Sontheimer, B., Wang, F., Tran, T.T., Nguyen, M., Kim, S., Xu, Z.-.Q., Jin, D., Schell, A.W., Lobo, C.J., Aharonovich, I. & Toth, M. 2018, 'All-optical control and super-resolution imaging of quantum emitters in layered materials', Nature Communications, vol. 9, no. 1.View/Download from: UTS OPUS or Publisher's site
Kianinia, M., Bradac, C., Sontheimer, B., Wang, F., Tran, T.T., Nguyen, M., Kim, S., Xu, Z.-.Q., Jin, D., Schell, A.W., Lobo, C.J., Aharonovich, I. & Toth, M. 2018, 'All-optical control and super-resolution imaging of quantum emitters in layered materials.', Nature communications, vol. 9, no. 1, p. 874.View/Download from: UTS OPUS or Publisher's site
Layered van der Waals materials are emerging as compelling two-dimensional platforms for nanophotonics, polaritonics, valleytronics and spintronics, and have the potential to transform applications in sensing, imaging and quantum information processing. Among these, hexagonal boron nitride (hBN) is known to host ultra-bright, room-temperature quantum emitters, whose nature is yet to be fully understood. Here we present a set of measurements that give unique insight into the photophysical properties and level structure of hBN quantum emitters. Specifically, we report the existence of a class of hBN quantum emitters with a fast-decaying intermediate and a long-lived metastable state accessible from the first excited electronic state. Furthermore, by means of a two-laser repumping scheme, we show an enhanced photoluminescence and emission intensity, which can be utilized to realize a new modality of far-field super-resolution imaging. Our findings expand current understanding of quantum emitters in hBN and show new potential ways of harnessing their nonlinear optical properties in sub-diffraction nanoscopy.
We propose and design photonic crystal cavities (PCCs) in hexagonal boron nitride (hBN) for diverse photonic and quantum applications. Two dimensional (2D) hBN flakes contain quantum emitters which are ultra-bright and photostable at room temperature. To achieve optimal coupling of these emitters to optical resonators, fabrication of cavities from hBN is therefore required to maximize the overlap between cavity optical modes and the emitters. Here, we design 2D and 1D PCCs using anisotropic indices of hBN. The influence of underlying substrates and material absorption are investigated, and spontaneous emission rate enhancements are calculated. Our results are promising for future quantum photonic experiments with hBN.
Ngoc My Duong, H., Nguyen, M.A.P., Kianinia, M., Ohshima, T., Abe, H., Watanabe, K., Taniguchi, T., Edgar, J.H., Aharonovich, I. & Toth, M. 2018, 'Effects of High-Energy Electron Irradiation on Quantum Emitters in Hexagonal Boron Nitride.', ACS applied materials & interfaces.View/Download from: Publisher's site
Hexagonal boron nitride (hBN) mono and multilayers are promising hosts for room-temperature single photon emitters (SPEs). In this work we explore high-energy (MeV) electron irradiation as a means to generate stable SPEs in hBN. We investigate four types of exfoliated hBN flakes-namely, high-purity multilayers, isotopically pure hBN, carbon-rich hBN multilayers and monolayered material-and find that electron irradiation increases emitter concentrations dramatically in all samples. Furthermore, the engineered emitters are located throughout hBN flakes (not only at flake edges or grain boundaries) and do not require activation by high-temperature annealing of the host material after electron exposure. Our results provide important insights into controlled formation of hBN SPEs and may aid in identification of their crystallographic origin.
Nguyen, M., Kim, S., Tran, T.T., Xu, Z.-.Q., Kianinia, M., Toth, M. & Aharonovich, I. 2018, 'Nanoassembly of quantum emitters in hexagonal boron nitride and gold nanospheres.', Nanoscale, vol. 10, no. 5, pp. 2267-2274.View/Download from: Publisher's site
The assembly of quantum nanophotonic systems with plasmonic resonators is important for fundamental studies of single photon sources as well as for on-chip information processing. In this work, we demonstrate the controllable nanoassembly of gold nanospheres with ultra-bright narrow-band quantum emitters in 2D layered hexagonal boron nitride (hBN). We utilize an atomic force microscope (AFM) tip to precisely position gold nanospheres to close proximity to the quantum emitters and observe the resulting emission enhancement and fluorescence lifetime reduction. The extreme emitter photostability permits analysis at high excitation powers, and delineation of absorption and emission enhancement caused by the plasmonic resonators. A fluorescence enhancement of over 300% is achieved experimentally for quantum emitters in hBN, with a radiative quantum efficiency of up to 40% and a saturated count rate in excess of 5 106 counts per s. Our results are promising for the future employment of quantum emitters in hBN for integrated nanophotonic devices and plasmonic based nanosensors.
Tran, T.T., Kianinia, M., Nguyen, M., Kim, S., Xu, Z.Q., Kubanek, A., Toth, M. & Aharonovich, I. 2018, 'Resonant Excitation of Quantum Emitters in Hexagonal Boron Nitride', ACS Photonics, vol. 5, no. 2, pp. 295-300.View/Download from: Publisher's site
© 2017 American Chemical Society. Quantum emitters in layered hexagonal boron nitride (hBN) have recently attracted a great deal of attention as promising single photon sources. In this work, we demonstrate resonant excitation of a single defect center in hBN, one of the most important prerequisites for employment of optical sources in quantum information processing applications. We observe spectral line widths of an hBN emitter narrower than 1 GHz while the emitter experiences spectral diffusion. Temporal photoluminescence measurements reveal an average spectral diffusion time of around 100 ms. An on-resonance photon antibunching measurement is also realized. Our results shed light on the potential use of quantum emitters from hBN in nanophotonics and quantum information processing applications.
Xu, Z.-.Q., Elbadawi, C., Tran, T.T., Kianinia, M., Li, X., Liu, D., Hoffman, T.B., Nguyen, M., Kim, S., Edgar, J.H., Wu, X., Song, L., Ali, S., Ford, M., Toth, M. & Aharonovich, I. 2018, 'Single photon emission from plasma treated 2D hexagonal boron nitride.', Nanoscale, vol. 10, no. 17, pp. 7957-7965.View/Download from: Publisher's site
Artificial atomic systems in solids are becoming increasingly important building blocks in quantum information processing and scalable quantum nanophotonic networks. Amongst numerous candidates, 2D hexagonal boron nitride has recently emerged as a promising platform hosting single photon emitters. Here, we report a number of robust plasma and thermal annealing methods for fabrication of emitters in tape-exfoliated hexagonal boron nitride (hBN) crystals. A two-step process comprising Ar plasma etching and subsequent annealing in Ar is highly robust, and yields an eight-fold increase in the concentration of emitters in hBN. The initial plasma-etching step generates emitters that suffer from blinking and bleaching, whereas the two-step process yields emitters that are photostable at room temperature with emission wavelengths greater than 700 nm. Density functional theory modeling suggests that the emitters might be associated with defect complexes that contain oxygen. This is further confirmed by generating the emitters via annealing hBN in air. Our findings advance the present understanding of the structure of quantum emitters in hBN and enhance the nanofabrication toolkit needed to realize integrated quantum nanophotonic circuits.
Bishop, J., Fronzi, M., Elbadawi, C., Nikam, V., Pritchard, J., Fröch, J.E., Duong, N.M.H., Ford, M.J., Aharonovich, I., Lobo, C.J. & Toth, M. 2018, 'Deterministic Nanopatterning of Diamond Using Electron Beams.', ACS nano, vol. 12, no. 3, pp. 2873-2882.View/Download from: UTS OPUS or Publisher's site
Diamond is an ideal material for a broad range of current and emerging applications in tribology, quantum photonics, high-power electronics, and sensing. However, top-down processing is very challenging due to its extreme chemical and physical properties. Gas-mediated electron beam-induced etching (EBIE) has recently emerged as a minimally invasive, facile means to dry etch and pattern diamond at the nanoscale using oxidizing precursor gases such as O2 and H2O. Here we explain the roles of oxygen and hydrogen in the etch process and show that oxygen gives rise to rapid, isotropic etching, while the addition of hydrogen gives rise to anisotropic etching and the formation of topographic surface patterns. We identify the etch reaction pathways and show that the anisotropy is caused by preferential passivation of specific crystal planes. The anisotropy can be controlled by the partial pressure of hydrogen and by using a remote RF plasma source to radicalize the precursor gas. It can be used to manipulate the geometries of topographic surface patterns as well as nano- and microstructures fabricated by EBIE. Our findings constitute a comprehensive explanation of the anisotropic etch process and advance present understanding of electron-surface interactions.
Berhane, A.M., Jeong, K.-.Y., Bodrog, Z., Fiedler, S., Schröder, T., Triviño, N.V., Palacios, T., Gali, A., Toth, M., Englund, D. & Aharonovich, I. 2017, 'Bright Room-Temperature Single-Photon Emission from Defects in Gallium Nitride.', Advanced materials (Deerfield Beach, Fla.), vol. 29, no. 12.View/Download from: UTS OPUS or Publisher's site
Room-temperature quantum emitters in gallium nitride (GaN) are reported. The emitters originate from cubic inclusions in hexagonal lattice and exhibit narrowband luminescence in the red spectral range. The sources are found in different GaN substrates, and therefore are promising for scalable quantum technologies.
Tran, T.T., Wang, D., Xu, Z.-.Q., Yang, A., Toth, M., Odom, T.W. & Aharonovich, I. 2017, 'Deterministic Coupling of Quantum Emitters in 2D Materials to Plasmonic Nanocavity Arrays.', Nano Letters, vol. 17, no. 4, pp. 2634-2639.View/Download from: UTS OPUS or Publisher's site
Quantum emitters in two-dimensional materials are promising candidates for studies of light-matter interaction and next generation, integrated on-chip quantum nanophotonics. However, the realization of integrated nanophotonic systems requires the coupling of emitters to optical cavities and resonators. In this work, we demonstrate hybrid systems in which quantum emitters in 2D hexagonal boron nitride (hBN) are deterministically coupled to high-quality plasmonic nanocavity arrays. The plasmonic nanoparticle arrays offer a high-quality, low-loss cavity in the same spectral range as the quantum emitters in hBN. The coupled emitters exhibit enhanced emission rates and reduced fluorescence lifetimes, consistent with Purcell enhancement in the weak coupling regime. Our results provide the foundation for a versatile approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays and 2D materials.
Webb, J.R., Martin, A.A., Johnson, R.P., Joseph, M.B., Newton, M.E., Aharonovich, I., Toth, M. & Macpherson, J.V. 2017, 'Fabrication of a single sub-micron pore spanning a single crystal (100) diamond membrane and impact on particle translocation', Carbon, vol. 122, pp. 319-328.View/Download from: UTS OPUS or Publisher's site
© 2017 Elsevier Ltd The fabrication of sub-micron pores in single crystal diamond membranes, which span the entirety of the membrane, is described for the first time, and the translocation properties of polymeric particles through the pore investigated. The pores are produced using a combination of laser micromachining to form the membrane and electron beam induced etching to form the pore. Single crystal diamond as the membrane material, has the advantages of chemical stability and durability, does not hydrate and swell, has outstanding electrical properties that facilitate fast, low noise current-time measurements and is optically transparent for combined optical-conductance sensing. The resulting pores are characterized individually using both conductance measurements, employing a microcapillary electrochemical setup, and electron microscopy. Proof-of-concept experiments to sense charged polystyrene particles as they are electrophoretically driven through a single diamond pore are performed, and the impact of this new pore material on particle translocation is explored. These findings reveal the potential of diamond as a platform for pore-based sensing technologies and pave the way for the fabrication of single nanopores which span the entirety of a diamond membrane.
Zachreson, C., Wolff, C., Whitchurch, C.B. & Toth, M. 2017, 'Emergent pattern formation in an interstitial biofilm.', Physical Review E, vol. 95, no. 1-1, pp. 1-14.View/Download from: UTS OPUS or Publisher's site
Collective behavior of bacterial colonies plays critical roles in adaptability, survivability, biofilm expansion and infection. We employ an individual-based model of an interstitial biofilm to study emergent pattern formation based on the assumptions that rod-shaped bacteria furrow through a viscous environment and excrete extracellular polymeric substances which bias their rate of motion. Because the bacteria furrow through their environment, the substratum stiffness is a key control parameter behind the formation of distinct morphological patterns. By systematically varying this property (which we quantify with a stiffness coefficient ), we show that subtle changes in the substratum stiffness can give rise to a stable state characterized by a high degree of local order and long-range pattern formation. The ordered state exhibits characteristics typically associated with bacterial fitness advantages, even though it is induced by changes in environmental conditions rather than changes in biological parameters. Our findings are applicable to a broad range of biofilms and provide insights into the relationship between bacterial movement and their environment, and basic mechanisms behind self-organization of biophysical systems.
Zachreson, C., Yap, X., Gloag, E.S., Shimoni, R., Whitchurch, C.B. & Toth, M. 2017, 'Network patterns in exponentially growing two-dimensional biofilms.', Physical review. E, vol. 96, no. 4-1, p. 042401.View/Download from: UTS OPUS or Publisher's site
Anisotropic collective patterns occur frequently in the morphogenesis of two-dimensional biofilms. These patterns are often attributed to growth regulation mechanisms and differentiation based on gradients of diffusing nutrients and signaling molecules. Here, we employ a model of bacterial growth dynamics to show that even in the absence of growth regulation or differentiation, confinement by an enclosing medium such as agar can itself lead to stable pattern formation over time scales that are employed in experiments. The underlying mechanism relies on path formation through physical deformation of the enclosing environment.
Gloag, E.S., Elbadawi, C., Zachreson, C.J., Aharonovich, I., Toth, M., Charles, I.G., Turnbull, L. & Whitchurch, C.B. 2017, 'Micro-Patterned Surfaces That Exploit Stigmergy to Inhibit Biofilm Expansion.', Frontiers in Microbiology, vol. 7, pp. 1-10.View/Download from: UTS OPUS or Publisher's site
Twitching motility is a mode of surface translocation that is mediated by the extension and retraction of type IV pili and which, depending on the conditions, enables migration of individual cells or can manifest as a complex multicellular collective behavior that leads to biofilm expansion. When twitching motility occurs at the interface of an abiotic surface and solidified nutrient media, it can lead to the emergence of extensive self-organized patterns of interconnected trails that form as a consequence of the actively migrating bacteria forging a furrow network in the substratum beneath the expanding biofilm. These furrows appear to direct bacterial movements much in the same way that roads and footpaths coordinate motor vehicle and human pedestrian traffic. Self-organizing systems such as these can be accounted for by the concept of stigmergy which describes self-organization that emerges through indirect communication via persistent signals within the environment. Many bacterial communities are able to actively migrate across solid and semi-solid surfaces through complex multicellular collective behaviors such as twitching motility and flagella-mediated swarming motility. Here, we have examined the potential of exploiting the stigmergic behavior of furrow-mediated trail following as a means of controlling bacterial biofilm expansion along abiotic surfaces. We found that incorporation of a series of parallel micro-fabricated furrows significantly impeded active biofilm expansion by Pseudomonas aeruginosa and Proteus vulgaris. We observed that in both cases bacterial movements tended to be directed along the furrows. We also observed that narrow furrows were most effective at disrupting biofilm expansion as they impeded the ability of cells to self-organize into multicellular assemblies required for escape from the furrows and migration into new territory. Our results suggest that the implementation of micro-fabricated furrows that exploit stigmergy may be a ...
Walia, S., Balendhran, S., Ahmed, T., Singh, M., El-Badawi, C., Brennan, M.D., Weerathunge, P., Karim, M.N., Rahman, F., Rassell, A., Duckworth, J., Ramanathan, R., Collis, G.E., Lobo, C.J., Toth, M., Kotsakidis, J.C., Weber, B., Fuhrer, M., Dominguez-Vera, J.M., Spencer, M.J.S., Aharonovich, I., Sriram, S., Bhaskaran, M. & Bansal, V. 2017, 'Ambient Protection of Few-Layer Black Phosphorus via Sequestration of Reactive Oxygen Species.', Advanced Materials, vol. 29, no. 27, pp. 1-8.View/Download from: UTS OPUS or Publisher's site
Few-layer black phosphorous (BP) has emerged as a promising candidate for next-generation nanophotonic and nanoelectronic devices. However, rapid ambient degradation of mechanically exfoliated BP poses challenges in its practical deployment in scalable devices. To date, the strategies employed to protect BP have relied upon preventing its exposure to atmospheric conditions. Here, an approach that allows this sensitive material to remain stable without requiring its isolation from the ambient environment is reported. The method draws inspiration from the unique ability of biological systems to avoid photo-oxidative damage caused by reactive oxygen species. Since BP undergoes similar photo-oxidative degradation, imidazolium-based ionic liquids are employed as quenchers of these damaging species on the BP surface. This chemical sequestration strategy allows BP to remain stable for over 13 weeks, while retaining its key electronic characteristics. This study opens opportunities to practically implement BP and other environmentally sensitive 2D materials for electronic applications.
Tawfik, S.A., Ali, S., Fronzi, M., Kianinia, M., Tran, T.T., Stampfl, C., Aharonovich, I., Toth, M. & Ford, M.J. 2017, 'First-principles investigation of quantum emission from hBN defects.', Nanoscale, vol. 9, no. 36, pp. 13575-13582.View/Download from: UTS OPUS or Publisher's site
Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.
Kianinia, M., Regan, B., Tawfik, S.A., Tran, T.T., Ford, M.J., Aharonovich, I. & Toth, M. 2017, 'Robust Solid-State Quantum System Operating at 800 K', ACS Photonics, vol. 4, no. 4, pp. 768-773.View/Download from: UTS OPUS or Publisher's site
© 2017 American Chemical Society. Realization of quantum information and communications technologies requires robust, stable solid-state single-photon sources. However, most existing sources cease to function above cryogenic or room temperature due to thermal ionization or strong phonon coupling, which impedes their emissive and quantum properties. Here we present an efficient single-photon source based on a defect in a van der Waals crystal that is optically stable and oper ates at elevated temperatures of up to 800 K. The quantum nature of the source and the photon purity are maintained upon heating to 800 K and cooling back to room temperature. Our report of a robust high-temperature solid-state single photon source constitutes a significant step toward practical, integrated quantum technologies for real-world environments.
Tran, T.T., Choi, S., Scott, J.A., Xu, Z.Q., Zheng, C., Seniutinas, G., Bendavid, A., Fuhrer, M.S., Toth, M. & Aharonovich, I. 2017, 'Room-Temperature Single-Photon Emission from Oxidized Tungsten Disulfide Multilayers', Advanced Optical Materials, vol. 5, no. 5, pp. 1-5.View/Download from: UTS OPUS or Publisher's site
Robust quantum emitters fabricated by thermal oxidation of tungsten disulfide multilayers are reported. The emitters show robust, optically stable, linearly polarized luminescence at room temperature, can be modeled using a threelevel system, and exhibit moderate bunching. Overall, the results provide important insights into understanding of defect formation and quantum emitter activation in twodimensional materials.
Martin, A.A., Filevich, J., Straw, M., Randolph, S., Botman, A., Aharonovich, I. & Toth, M. 2017, 'Radiation-Induced Damage and Recovery of Ultra-Nanocrystalline Diamond: Toward Applications in Harsh Environments.', ACS Applied Materials and Interfaces, vol. 9, no. 45, pp. 39790-39794.View/Download from: UTS OPUS or Publisher's site
Ultra-nanocrystalline diamond (UNCD) is increasingly being used in the fabrication of devices and coatings due to its excellent tribological properties, corrosion resistance, and biocompatibility. Here, we study its response to irradiation with kiloelectronvolt electrons as a controlled model for extreme ionizing environments. Real time Raman spectroscopy reveals that the radiation-damage mechanism entails dehydrogenation of UNCD grain boundaries, and we show that the damage can be recovered by annealing at 883 K. Our results have significant practical implications for the implementation of UNCD in extreme environment applications, and indicate that the films can be used as radiation sensors.
© 2016 Macmillan Publishers Limited, part of Springer Nature. Single-photon emitters play an important role in many leading quantum technologies. There is still no 'ideal' on-demand single-photon emitter, but a plethora of promising material systems have been developed, and several have transitioned from proof-of-concept to engineering efforts with steadily improving performance. Here, we review recent progress in the race towards true single-photon emitters required for a range of quantum information processing applications. We focus on solid-state systems including quantum dots, defects in solids, two-dimensional hosts and carbon nanotubes, as these are well positioned to benefit from recent breakthroughs in nanofabrication and materials growth techniques. We consider the main challenges and key advantages of each platform, with a focus on scalable on-chip integration and fabrication of identical sources on photonic circuits.
Kianinia, M., Shimoni, O., Bendavid, A., Schell, A.W., Randolph, S.J., Toth, M., Aharonovich, I. & Lobo, C.J. 2016, 'Robust, directed assembly of fluorescent nanodiamonds.', Nanoscale, vol. 8, no. 42, pp. 18032-18037.View/Download from: UTS OPUS or Publisher's site
Arrays of fluorescent nanoparticles are highly sought after for applications in sensing, nanophotonics and quantum communications. Here we present a simple and robust method of assembling fluorescent nanodiamonds into macroscopic arrays. Remarkably, the yield of this directed assembly process is greater than 90% and the assembled patterns withstand ultra-sonication for more than three hours. The assembly process is based on covalent bonding of carboxyl to amine functional carbon seeds and is applicable to any material, and to non-planar surfaces. Our results pave the way to directed assembly of sensors and nanophotonics devices.
Kolíbal, M., Novák, L., Shanley, T., Toth, M. & Šikola, T. 2016, 'Silicon oxide nanowire growth mechanisms revealed by real-time electron microscopy', Nanoscale, vol. 8, no. 1, pp. 266-275.View/Download from: UTS OPUS or Publisher's site
© 2016 The Royal Society of Chemistry. Growth of one-dimensional materials is possible through numerous mechanisms that affect the nanowire structure and morphology. Here, we explain why a wide range of morphologies is observed when silicon oxide nanowires are grown on silicon substrates using liquid gallium catalyst droplets. We show that a gallium oxide overlayer is needed for nanowire nucleation at typical growth temperatures, and that it can decompose during growth and, hence, dramatically alter the nanowire morphology. Gallium oxide decomposition is attributed to etching caused by hydrogen that can be supplied by thermal dissociation of H2O (a common impurity). We show that H2O dissociation is catalyzed by silicon substrates at temperatures as low as 320 °C, identify the material supply pathways and processes that rate-limit nanowire growth under dry and wet atmospheres, and present a detailed growth model that explains contradictory results reported in prior studies. We also show that under wet atmospheres the Ga droplets can be mobile and promote nanowire growth as they traverse the silicon substrate.
Martin, A.A., Lin, T., Toth, M., Westphal, A.J., Vicenzi, E.P., Beeman, J. & Silver, E.H. 2016, 'Exposure and analysis of microparticles embedded in silica aerogel keystones using NF3-mediated electron beam–induced etching and energy-dispersive X-ray spectroscopy', Meteoritics and Planetary Science, vol. 51, no. 7, pp. 1223-1232.View/Download from: Publisher's site
© The Meteoritical Society, 2016. In 2006, NASA's Stardust spacecraft delivered to Earth dust particles collected from the coma of comet 81P/Wild 2, with the goal of furthering the understanding of solar system formation. Stardust cometary samples were collected in a low-density, nanoporous silica aerogel making their study technically challenging. This article demonstrates the identification, exposure, and elemental composition analysis of particles analogous to those collected by NASA's Stardust mission using in-situ SEM techniques. Backscattered electron imaging is shown by experimental observation and Monte Carlo simulation to be suitable for locating particles of a range of sizes relevant to Stardust (down to submicron diameters) embedded within silica aerogel. Selective removal of the silica aerogel encapsulating an embedded particle is performed by cryogenic NF3-mediated electron beam–induced etching. The porous, low-density nature of the aerogel results in an enhanced etch rate compared with solid material, making it an effective, nonmechanical method for the exposure of particles. After exposure, elemental composition of the particle was analyzed by energy-dispersive X-ray spectroscopy using a high spectral resolution microcalorimeter. Signals from fluorine contamination are shown to correspond to nonremoved silica aerogel and only in residual concentrations.
Tran, T.T., Bray, K., Ford, M.J., Toth, M. & Aharonovich, I. 2016, 'Quantum emission from hexagonal boron nitride monolayers.', Nature nanotechnology, vol. 11, no. 1, pp. 37-41.View/Download from: UTS OPUS or Publisher's site
Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Single-photon emission from two-dimensional materials has been reported, but only at cryogenic temperatures. Here, we demonstrate room-temperature, polarized and ultrabright single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing.
Choi, S., Tran, T.T., Elbadawi, C., Lobo, C., Wang, X., Juodkazis, S., Seniutinas, G., Toth, M. & Aharonovich, I. 2016, 'Engineering and Localization of Quantum Emitters in Large Hexagonal Boron Nitride Layers', ACS APPLIED MATERIALS & INTERFACES, vol. 8, no. 43, pp. 29642-29648.View/Download from: Publisher's site
Tran, T.T., Elbadawi, C., Totonjian, D., Lobo, C.J., Grosso, G., Moon, H., Englund, D.R., Ford, M.J., Aharonovich, I. & Toth, M. 2016, 'Robust Multicolor Single Photon Emission from Point Defects in Hexagonal Boron Nitride.', ACS nano, vol. 10, no. 8, pp. 7331-7338.View/Download from: UTS OPUS or Publisher's site
Hexagonal boron nitride (hBN) is an emerging two-dimensional material for quantum photonics owing to its large bandgap and hyperbolic properties. Here we report two approaches for engineering quantum emitters in hBN multilayers using either electron beam irradiation or annealing and characterize their photophysical properties. The defects exhibit a broad range of multicolor room-temperature single photon emissions across the visible and the near-infrared spectral ranges, narrow line widths of sub-10 nm at room temperature, and a short excited-state lifetime, and high brightness. We show that the emitters can be categorized into two general groups, but most likely possess similar crystallographic structure. Remarkably, the emitters are extremely robust and withstand aggressive annealing treatments in oxidizing and reducing environments. Our results constitute a step toward deterministic engineering of single emitters in 2D materials and hold great promise for the use of defects in boron nitride as sources for quantum information processing and nanophotonics.
Elbadawi, C., Tran, T.T., Kolíbal, M., Šikola, T., Scott, J., Cai, Q., Li, L.H., Taniguchi, T., Watanabe, K., Toth, M., Aharonovich, I. & Lobo, C. 2016, 'Electron beam directed etching of hexagonal boron nitride.', Nanoscale, vol. 8, no. 36, pp. 16182-16186.View/Download from: UTS OPUS or Publisher's site
Hexagonal boron nitride (hBN) is a wide bandgap van der Waals material with unique optical properties that make it attractive for two dimensional (2D) photonic and optoelectronic devices. However, broad deployment and exploitation of hBN is limited by alack of suitable material and device processing and nano prototyping techniques. Here we present a high resolution, single step electron beam technique for chemical dry etching of hBN. Etching is achieved using H2O as a precursor gas, at both room temperature and elevated hBN temperatures. The technique enables damage-free, nano scale, iterative patterning of supported and suspended 2D hBN, thus opening the door to facile fabrication of hBN-based 2D heterostructures and devices.
Scott, J.A., Totonjian, D., Martin, A.A., Tran, T.T., Fang, J., Toth, M., McDonagh, A.M., Aharonovich, I. & Lobo, C.J. 2016, 'Versatile method for template-free synthesis of single crystalline metal and metal alloy nanowires.', Nanoscale, vol. 8, no. 5, pp. 2804-2810.View/Download from: UTS OPUS or Publisher's site
Metal and metal alloy nanowires have applications ranging from spintronics to drug delivery, but high quality, high density single crystalline materials have been surprisingly difficult to fabricate. Here we report a versatile, template-free, self-assembly method for fabrication of single crystalline metal and metal alloy nanowires (Co, Ni, NiCo, CoFe, and NiFe) by reduction of metal nitride precursors formed in situ by reaction of metal salts with a nitrogen source. Thiol reduction of the metal nitrides to the metallic phase at 550-600 °C results in nanowire growth. In this process, sulfur acts as a uniaxial structure-directing agent, passivating the surface of the growing nanowires and preventing radial growth. The versatility of the method is demonstrated by achieving nanowire growth from gas-phase, solution-phase or a combination of gas- and solution-phase precursors. The fabrication method is suited to large-area CVD on a wide range of solid substrates.
Shanley, T.W., Bonnie, F., Scott, J. & Toth, M. 2016, 'Role of gas molecule complexity in environmental electron microscopy and photoelectron yield spectroscopy.', ACS applied materials & interfaces, vol. 8, pp. 27305-27310.View/Download from: UTS OPUS or Publisher's site
Environmental scanning electron microscopy (ESEM) and environmental photoelectron yield spectroscopy (EPYS) enable electron imaging and spectroscopy of surfaces and interfaces in low vacuum, gaseous environments. The techniques are both appealing and limited by the range of gases that can be used to amplify electrons emitted from a sample, and used to form images/spectra. However, to date, only H2O and NH3 gases have been identified as highly favorable electron amplification media. Here we demonstrate that ethanol vapor (CH3CH2OH) is superior to both of these, and attribute its performance to molecular complexity and valence orbital structure. Our findings improve present understanding of what constitutes a favorable electron amplification gas, and will help expand the applicability and usefulness of the ESEM and EPYS techniques.
Bray, K., Sandstrom, R., Elbadawi, C., Fischer, M., Schreck, M., Shimoni, O., Lobo, C., Toth, M. & Aharonovich, I. 2016, 'Localization of Narrowband Single Photon Emitters in Nanodiamonds.', ACS applied materials & interfaces, vol. 8, no. 11, pp. 7590-7594.View/Download from: UTS OPUS or Publisher's site
Diamond nanocrystals that host room temperature narrowband single photon emitters are highly sought after for applications in nanophotonics and bioimaging. However, current understanding of the origin of these emitters is extremely limited. In this work, we demonstrate that the narrowband emitters are point defects localized at extended morphological defects in individual nanodiamonds. In particular, we show that nanocrystals with defects such as twin boundaries and secondary nucleation sites exhibit narrowband emission that is absent from pristine individual nanocrystals grown under the same conditions. Critically, we prove that the narrowband emission lines vanish when extended defects are removed deterministically using highly localized electron beam induced etching. Our results enhance the current understanding of single photon emitters in diamond and are directly relevant to fabrication of novel quantum optics devices and sensors.
Tran, T.T., Zachreson, C., Berhane, A.M., Bray, K., Sandstrom, R.G., Li, L.H., Taniguchi, T., Watanabe, K., Aharonovich, I. & Toth, M. 2016, 'Quantum Emission from Defects in Single-Crystalline Hexagonal Boron Nitride', Physical Review Applied, vol. 5, no. 3.View/Download from: UTS OPUS or Publisher's site
© 2016 American Physical Society. Bulk hexagonal boron nitride (hBN) is a highly nonlinear natural hyperbolic material that attracts major attention in modern nanophotonics applications. However, studies of its optical properties in the visible part of the spectrum and quantum emitters hosted by bulk hBN have not been reported to date. In this work, we study the emission properties of hBN crystals in the red spectral range using sub-band-gap optical excitation. Quantum emission from defects is observed at room temperature and characterized in detail. Our results advance the use of hBN in quantum nanophotonics technologies and enhance our fundamental understanding of its optical properties.
Cullen, J., Bahm, A., Lobo, C.J., Ford, M.J. & Toth, M. 2015, 'Localized probing of gas molecule adsorption energies and desorption attempt frequencies', Journal of Physical Chemistry C, vol. 119, no. 28, pp. 15948-15953.View/Download from: UTS OPUS or Publisher's site
© 2015 American Chemical Society. Gas-mediated electron beam induced etching (EBIE) and deposition (EBID) can be used to measure activation energies that are interpreted as the adsorption energies of surface-adsorbed precursor molecules. However, the measured quantities often disagree with adsorption energies measured by conventional analysis techniques such as thermally programmed desorption and have anomalous dependencies on parameters such as the electron beam current used to perform EBID. Here, we use the theory of EBIE and EBID rate kinetics to explain this behavior and identify conditions under which the activation energies and the associated pre-exponential factors correspond to gas molecule adsorption energies and desorption attempt frequencies, respectively. Under these conditions, EBIE and EBID can be used as robust, nanoscale techniques for the analysis of adsorbates.
Cullen, J., Lobo, C.J., Ford, M.J. & Toth, M. 2015, 'Electron-Beam-Induced Deposition as a Technique for Analysis of Precursor Molecule Diffusion Barriers and Prefactors.', ACS applied materials & interfaces, vol. 7, no. 38, pp. 21408-21415.View/Download from: UTS OPUS or Publisher's site
Electron-beam-induced deposition (EBID) is a direct-write chemical vapor deposition technique in which an electron beam is used for precursor dissociation. Here we show that Arrhenius analysis of the deposition rates of nanostructures grown by EBID can be used to deduce the diffusion energies and corresponding preexponential factors of EBID precursor molecules. We explain the limitations of this approach, define growth conditions needed to minimize errors, and explain why the errors increase systematically as EBID parameters diverge from ideal growth conditions. Under suitable deposition conditions, EBID can be used as a localized technique for analysis of adsorption barriers and prefactors.
Martin, A.A., Bahm, A., Bishop, J., Aharonovich, I. & Toth, M. 2015, 'Dynamic Pattern Formation in Electron-Beam-Induced Etching.', Physical review letters, vol. 115, no. 25, p. 255501.View/Download from: UTS OPUS or Publisher's site
We report highly ordered topographic patterns that form on the surface of diamond, span multiple length scales, and have a symmetry controlled by the precursor gas species used in electron-beam-induced etching (EBIE). The pattern formation dynamics reveals an etch rate anisotropy and an electron energy transfer pathway that is overlooked by existing EBIE models. We, therefore, modify established theory such that it explains our results and remains universally applicable to EBIE. The patterns can be exploited in controlled wetting, optical structuring, and other emerging applications that require nano- and microscale surface texturing of a wide band-gap material.
© 2015 AIP Publishing LLC. Nanopatterning of graphene and diamond by low energy ( 30keV) electrons has previously been attributed to mechanisms that include atomic displacements caused by knock-on, electron beam heating, sputtering by ionized gas molecules, and chemical etching driven by a number of gases that include N2. Here, we show that a number of these mechanisms are insignificant, and the nanopatterning process can instead be explained by etching caused by electron induced dissociation of residual H2O molecules. Our results have significant practical implications for gas-mediated electron beam nanopatterning techniques and help elucidate the underlying mechanisms.
Martin, A.A., Randolph, S., Botman, A., Toth, M. & Aharonovich, I. 2015, 'Maskless milling of diamond by a focused oxygen ion beam', SCIENTIFIC REPORTS, vol. 5.View/Download from: UTS OPUS or Publisher's site
Toth, M., Lobo, C., Friedli, V., Szkudlarek, A. & Utke, I. 2015, 'Continuum models of focused electron beam induced processing', Beilstein Journal of Nanotechnology, vol. 6, no. 1, pp. 1518-1540.View/Download from: UTS OPUS or Publisher's site
© 2015 Toth et al. Focused electron beam induced processing (FEBIP) is a suite of direct-write, high resolution techniques that enable fabrication and editing of nanostructured materials inside scanning electron microscopes and other focused electron beam (FEB) systems. Here we detail continuum techniques that are used to model FEBIP, and release software that can be used to simulate a wide range of processes reported in the FEBIP literature. These include: (i) etching and deposition performed using prec ursors that interact with a surface through physisorption and activated chemisorption, (ii) gas mixtures used to perform simultaneous focused electron beam induced etching and deposition (FEBIE and FEBID), and (iii) etch processes that proceed through multiple reaction pathways and generate a number of reaction products at the substrate surface. We also review and release software for Monte Carlo modeling of the precursor gas flux which is needed as an input parameter for continuum FEBIP models.
Tran, T.T., Fang, J., Zhang, H., Rath, P., Bray, K., Sandstrom, R.G., Shimoni, O., Toth, M. & Aharonovich, I. 2015, 'Facile Self-Assembly of Quantum Plasmonic Circuit Components.', Advanced materials (Deerfield Beach, Fla.), vol. 27, no. 27, pp. 4048-4053.View/Download from: UTS OPUS or Publisher's site
A facile and cost-effective self-assembly route to engineering of vital quantum plasmonic circuit components is reported. By modifying the surface energy of silver nanowires, the position and density of attached nanodiamonds can be maneuvered leading to silver nanowire/nanodiamond(s) hybrid nanostructures. These structures exhibit strong plasmonic coupling effects and thus hold promise to serve as quantum plasmonic components.
Martin, A.A. & Toth, M. 2014, 'Cryogenic electron beam induced chemical etching.', ACS Applied Materials and Interfaces, vol. 6, no. 21, pp. 18457-18460.View/Download from: UTS OPUS or Publisher's site
Cryogenic cooling is used to enable efficient, gas-mediated electron beam induced etching (EBIE) in cases where the etch rate is negligible at room and elevated substrate temperatures. The process is demonstrated using nitrogen trifluoride (NF3) as the etch precursor, and Si, SiO2, SiC, and Si3N4 as the materials volatilized by an electron beam. Cryogenic cooling broadens the range of precursors that can be used for EBIE, and enables high-resolution, deterministic etching of materials that are volatilized spontaneously by conventional etch precursors as demonstrated here by NF3 and XeF2 EBIE of silicon.
Martin, A.A., Aharonovich, I. & Toth, M. 2014, 'Gas-Mediated Electron Beam Induced Etching - From Fundamental Physics to Device Fabrication', Microscopy and Microanalysis, vol. 20, no. S3, pp. 364-365.View/Download from: UTS OPUS or Publisher's site
Gas-mediated electron beam induced etching (EBIE) is a nanoscale, direct-write technique analogous to gas-assisted focused ion beam (FIB) milling. The main advantage of EBIE is the elimination of sputtering and ion implantation during processing as well as greater material selectivity . Here we discuss recent developments that expand the scope of EBIE applications in nanofabrication and defect generation analysis, and show advances in hardware that open the door for new studies in reaction kinetics using a scanning electron microscope (SEM).
Shanley, T.W., Martin, A.A., Aharonovich, I. & Toth, M. 2014, 'Localized chemical switching of the charge state of nitrogen-vacancy luminescence centers in diamond', APPLIED PHYSICS LETTERS, vol. 105, no. 6.View/Download from: UTS OPUS or Publisher's site
Toth, M. 2014, 'Advances in gas-mediated electron beam-induced etching and related material processing techniques', Applied Physics A: Materials Science and Processing, vol. 117, no. 4, pp. 1623-1629.View/Download from: UTS OPUS or Publisher's site
© 2014, Springer-Verlag Berlin Heidelberg. Electron beam-induced etching (EBIE) has traditionally been used for top-down, direct-write, chemical dry etching, and iterative editing of materials. The present article reviews recent advances in EBIE modeling and emerging applications, with an emphasis on use cases in which the approaches that have conventionally been used to realize EBIE are instead used for material analysis, surface functionalization, or bottom-up growth of nanostructured materials. Such applications are used to highlight the shortcomings of existing quantitative EBIE models and to identify physicochemical phenomena that must be accounted for in order to enable full exploitation and predictive modeling of EBIE and related electron beam fabrication techniques.
Toth, M., Zachreson, C. & Aharonovich, I. 2014, 'Role of recombination pathway competition in spatially resolved cathodoluminescence spectroscopy', Applied Physics Letters, vol. 105, no. 24.View/Download from: UTS OPUS or Publisher's site
© 2014 AIP Publishing LLC. Cathodoluminescence (CL) analysis enables characterization of optoelectronic materials and devices with high spatial resolution. However, data interpretation is complicated by the competitive nature of the CL generation process. Specifically, spatially resolved CL profiles are affected by both CL center distributions, and by the unknown distributions of recombination centers that do not generate peaks in measured CL spectra. Here, we use depth-resolved CL to show that the contribution of the latter can be deduced and removed from spatially resolved CL data. The utility of this technique is demonstrated using CL depth profiles of color centers in diamond.
Zachreson, C., Martin, A.A., Aharonovich, I. & Toth, M. 2014, 'Electron Beam Controlled Restructuring of Luminescence Centers in Polycrystalline Diamond', ACS APPLIED MATERIALS & INTERFACES, vol. 6, no. 13, pp. 10367-10372.View/Download from: UTS OPUS or Publisher's site
Direct-write three-dimensional nanolithography is demonstrated using cryogenic electron beam-induced deposition (EBID). Cryogenic cooling and an electron beam were used to condense and expose the precursor methylcyclopentadienyl(trimethyl) platinum (MeCp
Martin, A.A., Phillips, M. & Toth, M. 2013, 'Dynamic surface site activation: A rate limiting process in electron beam induced etching', ACS Applied Materials & Interfaces, vol. 5, no. 16, pp. 8002-8007.View/Download from: UTS OPUS or Publisher's site
We report a new mechanism that limits the rate of electron beam induced etching (EBIE). Typically, the etch rate is assumed to scale directly with the precursor adsorbate dissociation rate. Here, we show that this is a special case, and that the rate can instead be limited by the concentration of active sites at the surface. Novel etch kinetics are expected if surface sites are activated during EBIE, and observed experimentally using the electron sensitive material ultra nanocrystalline diamond (UNCD). In practice, etch kinetics are of interest because they affect resolution, throughput, proximity effects, and the topography of nanostructures and nanostructured devices fabricated by EBIE.
Randolph, S., Botman, A. & Toth, M. 2013, 'Capsule-free fluid delivery and beam-induced electrodeposition in a scanning electron microscope', RSC Advances, vol. 3, no. 43, pp. 20016-20023.View/Download from: UTS OPUS or Publisher's site
Gold coated borosilicate nanocapillaries are used to locally deliver aqueous, electrolytic CuSO4 solution into the low vacuum chamber of an environmental scanning electron microscope (ESEM). Capillary flow of the liquid is induced by bringing a nanocapil
Randolph, S., Botman, A. & Toth, M. 2013, 'Deposition of Highly Porous Nanocrystalline Platinum on Functionalized Substrates Through Fluorine-Induced Decomposition of Pt(PF3)(4) Adsorbates', Particle and Particle Systems Characterization, vol. 30, no. 8, pp. 672-677.View/Download from: UTS OPUS or Publisher's site
Nanocrystalline platinum is synthesized at room temperature by co-injecting Pt(PF3)4 and XeF2 vapors onto solid supports in vacuum. The Pt nucleation time scales with chemisorbed fluorine coverage, which is controlled by pre-dosing supports with XeF2, and by optional electron or ion beam irradiation under flowing XeF2. The latter is used to increase the chemisorbed fluorine coverage and localize the Pt growth process.
Elbadawi, C., Toth, M. & Lobo, C. 2013, 'Pure Platinum Nanostructures Grown by Electron Beam Induced Deposition', ACS Applied Materials & Interfaces, vol. 5, no. 19, pp. 9372-9376.View/Download from: UTS OPUS or Publisher's site
Platinum has numerous applications in catalysis, nanoelectronics, and sensing devices. Here we report a method for localized, mask-free deposition of high-purity platinum that employs a combination of room-temperature, direct-write electron beam induced deposition (EBID) using the precursor Pt(PF3)4, and low temperature (=400 °C) postgrowth annealing in H2O. The annealing treatment removes phosphorus contaminants through a thermally activated pathway involving dissociation of H2O and the subsequent formation of volatile phosphorus oxides and hydrides that desorb during annealing. The resulting Pt is indistinguishable from pure Pt films by wavelength dispersive X-ray spectroscopy (WDS).
Botman, A., Bahm, A., Randolph, S., Straw, M. & Toth, M. 2013, 'Spontaneous Growth of Gallium-Filled Microcapillaries on Ion-Bombarded GaN', PHYSICAL REVIEW LETTERS, vol. 111, no. 13.View/Download from: UTS OPUS or Publisher's site
Bishop, J., Toth, M., Phillips, M. & Lobo, C. 2012, 'Effects of oxygen on electron beam induced deposition of SiO2 using physisorbed and chemisorbed tetraethoxysilane', Applied Physics Letters, vol. 101, p. 211605.View/Download from: UTS OPUS or Publisher's site
Electron beam induced deposition (EBID) is limited by low throughput and purity of as-grown material. Co-injection of O2 with the growth precursor is known to increase both the purity and deposition rate of materials such as SiO2 at room temperature. Here, we show that O2 inhibits rather than enhances EBID from tetraethoxysilane (TEOS) precursor at elevated temperatures. This behavior is attributed to surface site competition between chemisorbates at elevated temperature, and TEOS decomposition by atomic oxygen produced through electron dissociation of physisorbed O2 at room temperature.
Bishop, J.D., Lobo, C., Martin, A.A., Ford, M., Phillips, M. & Toth, M. 2012, 'Role of activated chemisorption in gas-mediated electron beam induced deposition', Physical Review Letters, vol. 109, p. 146103.View/Download from: UTS OPUS or Publisher's site
Models of adsorbate dissociation by energetic electrons are generalized to account for activated sticking and chemisorption, and used to simulate the rate kinetics of electron beam induced chemical vapor deposition (EBID). The model predicts a novel temperature dependence caused by thermal transitions from physisorbed to chemisorbed states that govern adsorbate coverage and EBID rates at elevated temperatures. We verify these results by experiments that also show how EBID can be used to deposit high purity materials and characterize the rates and energy barriers that govern adsorption.
Lobo, C., Martin, A.A., Phillips, M. & Toth, M. 2012, 'Electron beam induced chemical dry etching and imaging in gaseous NH3 environments', Nanotechnology, vol. 23, pp. 375302-375302.View/Download from: UTS OPUS or Publisher's site
We report the use of ammonia (NH3) vapor as a new precursor for nanoscale electron beam induced etching (EBIE) of carbon, and an efficient imaging medium for environmental scanning electron microscopy (ESEM). Etching is demonstrated using amorphous carbonaceous nanowires grown by electron beam induced deposition (EBID). It is ascribed to carbon volatilization by hydrogen radicals generated by electron dissociation of NH3 adsorbates. The volatilization process is also effective at preventing the buildup of residual hydrocarbon impurities that often compromise EBIE, EBID and electron imaging. We also show that ammonia is a more efficient electron imaging medium than H2O, which up to now has been the most commonly used ESEM imaging gas.
Bresin, M., Thiel, B.L., Toth, M., Dunn, K.A. & Toth, M. 2011, 'Focused electron beam-induced deposition at cryogenic temperatures', Journal of Materials Research, vol. 26, no. 3, pp. 357-364.View/Download from: UTS OPUS or Publisher's site
Direct-write, cryogenic electron beam-induced deposition (EBID) was performed by condensing methylcyclopentadienyl-platinum-trimethyl precursor onto a substrate at 155 °C, exposing the condensate by a 15 keV electron beam, and desorbing unexposed precursor molecules by heating the substrate to room temperature. Dependencies of film thickness, microstructure, and surface morphology on electron beam flux and fluence, and Monte Carlo simulations of electron interactions with the condensate are used to construct a model of cryogenic EBID that is contrasted to existing models of conventional, room temperature EBID. It is shown that material grown from a cryogenic condensate exhibits one of three distinct surface morphologies: a nanoporous mesh with a high surface-to-volume ratio; a smooth, continuous film analogous to material typically grown by room temperature EBID; or a film with a high degree of surface roughness, analogous to that of the cryogenic condensate. The surface morphology can be controlled reproducibly by the electron fluence used for exposure.
Steven, R., Toth, M., Cullen, J.C., Chandler, O. & Lobo, C. 2011, 'Kinetics of gas mediated electron beam induced etching', Applied Physics Letters, vol. 99, no. 21, pp. 213103-213105.View/Download from: UTS OPUS or Publisher's site
Electron beam induced etching (EBIE) is a high resolution, direct write, chemical dry etch process in which surface-adsorbed precursor molecules are activated by an electron beam. We show that nanoscale EBIE is rate limited through at least two mechanisms ascribed to adsorbate depletion and the transport of gaseous precursor molecules into an etch pit during etching, respectively. The latter has, to date, not been accounted for in models of EBIE and is needed to reproduce etch kinetics which govern the time-evolution of etch pits, EBIE throughput, and spatial resolution.
Li, J., Toth, M., Dunn, K.A. & Thiel, B.L. 2010, 'Interfacial mixing and internal structure of Pt-containing nanocomposites grown by room temperature electron beam induced deposition', Journal of Applied Physics, vol. 107, no. 10.View/Download from: Publisher's site
Material grown by room temperature electron beam induced deposition (EBID) using (CH 3 ) 3 CH 3 C 5 H 4 Pt precursor consists of platinum nanocrystals embedded in an amorphous matrix. The crystallites are shown to intermix with the amorphous oxide on a Si substrate. The extent of intermixing scales with the electron energy density delivered to the material during growth. Dependencies on electron flux, fluence, and exposure time indicate that the intermixing process is athermal, electron-activated, and rate limited by mass transport inside the solid. Furthermore, the degree of deposit crystallinity is shown to scale with the electron flux and fluence used for EBID. We discuss mechanisms behind the observed changes in nanostructure and implications for the growth of functional materials by EBID. © 2010 American Institute of Physics.
Toth, M., Thiel, B.L. & Knowles, W. 2010, 'Gas Cascade Amplification in Ultra-High-Resolution Environmental Scanning Electron Microcopy', Microscopy And Microanalysis, vol. 16, no. 6, pp. 805-809.View/Download from: UTS OPUS or Publisher's site
We describe a feedback mechanism in the gas cascade amplification process used in magnetic immersion lens environmental scanning electron microcopy (ESEM). Feedback dominates gas gain under the conditions typically used for ultra-high-resolution ESEM and gives rise to novel dependencies of the imaging signal and noise on microscope operating parameters. It is ascribed tentatively to the generation of free electrons upon de-excitation of metastable species in the gas cascade. The results have implications for optimization of ESEM systems for applications such as critical dimension metrology and real-time imaging of nanostructure growth by gas mediated electron beam induced deposition.
Botman, A., Hagen, C.W., Li, J., Thiel, B.L., Dunn, K.A., Mulders, J.J., Randolph, S. & Toth, M. 2009, 'Electron postgrowth irradiation of platinum-containing nanostructures grown by electron-beam-induced deposition from Pt(PF3)4', Journal of Vacuum Science and Technology B, vol. 27, no. 6, pp. 2759-2763.View/Download from: UTS OPUS or Publisher's site
The material grown in a scanning electron microscope by electron beam-induced deposition (EBID) using Pt(PF3)4 precursor is shown to be electron beam sensitive. The effects of deposition time and postgrowth electron irradiation on the microstructure and resistivity of the deposits were assessed by transmission electron microscopy, selected area diffraction, and four-point probe resistivity measurements. The microstructure, notably the platinum nanocrystallite grain size, is shown to evolve with electron fluence in a controllable manner. The resistivity was observed to decrease as a result of postgrowth electron irradiation, with the lowest observed value of 215±15 µO?cm. The authors demonstrate that electron beam-induced changes in microstructure can be caused using electron fluences similar to those used during the course of EBID and suggest that the observed effects can be used to tailor the microstructure and functionality of deposits grown by EBID in situ without breaking vacuum.
Moody, S.J., Phillips, M.R. & Toth, M. 2009, 'Assessment of sem image quality using 1d power spectral density estimation', Microscopy and Microanalysis, vol. 15, no. SUPPL. 2, pp. 48-49.View/Download from: Publisher's site
Tileli, V., Knowles, W., Toth, M. & Thiel, B.L. 2009, 'Noise characteristics of the gas ionization cascade used in low vacuum scanning electron microscopy', Journal Of Applied Physics, vol. 106, no. 1, pp. 0-0.View/Download from: UTS OPUS or Publisher's site
The noise characteristics of gas cascade amplified electron signals in low vacuum scanning electron microscopy (LVSEM) are described and analyzed. We derive expressions for each component contributing to the total noise culminating in a predictive, quantitative model that can be used for optimization of LVSEM operating parameters. Signal and noise behavior is characterized experimentally and used to validate the model. Under most operating conditions, the noise is dominated by the excess noise generated in the gas amplification cascade. At high gains, the excess noise increases proportionally with gain such that the signal-to-noise ratio is constant. The effects of several instrument operating parameters, including working distance, gas pressure, beam current, and detector bias, are condensed and presented in the form of a master curve.
Toth, M., Lobo, C.J., Lysaght, M.J., Vladar, A.E. & Postek, M.T. 2009, 'Contamination-free imaging by electron induced carbon volatilization in environmental scanning electron microscopy', Journal Of Applied Physics, vol. 106, pp. 034306-034306.
Many ultraviolet, x-ray and charged particle beam techniques are inhibited by the growth of carbonaceous films caused by cross linking of hydrocarbon contaminant adsorbates. In electron microscopy, such films obscure surface features and reduce resolution. We demonstrate how resolution degradation can be alleviated using a H2O environment via gas mediated, electron beam induced carbon volatilization, a process that competes with film growth. Net behavior is a function of electron flux, which provides control over growth kinetics during imaging. Under optimized conditions, film growth can be eliminated, removing contamination as a factor limiting image information content and resolution.
Li, J., Toth, M., Tileli, V., Dunn, K.A., Lobo, C.J. & Thiel, B.L. 2008, 'Evolution of the nanostructure of deposits grown by electron beam induced deposition', Applied Physics Letters, vol. 93, pp. 023130-023130.
Environmental scanning electron microscopy (ESEM) was used to perform electron beam induced deposition (EBID) using a WF6 precursor. The deposits consist of WO3 nanocrystals embedded in an amorphous matrix. Oxide formation is attributed to residual oxidizers present in the ESEM chamber during EBID. Under conditions of fixed low electron flux, the WO3 grain size and the degree of deposit crystallinity increase with time. These changes correlate with the degree of electron energy deposition into the material during growth, indicating that electron beam induced modification of as-grown material is significant in controlling the nanostructure and functionality of materials fabricated by EBID. (C) 2008 American Institute of Physics.
Lobo, C.J., Toth, M., Wagner, R., Thiel, B.L. & Lysaght, M. 2008, 'High resolution radially symmetric nanostructures from simultaneous electron beam induced etching and deposition', Nanotechnology, vol. 19, pp. 025303-025303.View/Download from: UTS OPUS
Kucheyev, S.O., Toth, M., Baumann, T., Hamza, A., Ilavsky, K., Knowles, W., Saw, C., Thiel, B.L., Tileli, V., van Buuren, T., Wang, Y. & Willey, T. 2007, 'Structure of low-density nanoporous dielectrics revealed by low-vacuum electron microscopy and small-angle X-ray scattering', Langmuir, vol. 23, no. 2, pp. 353-356.View/Download from: UTS OPUS or Publisher's site
Aerogels (AGs) are ultralow-density nanoporous solids that have numerous potential applications. However, as most AGs are strong insulators with poor mechanical properties, direct studies of the complex nanoporous structure of AGs by methods such as atomic force and conventional scanning electron microscopy (SEM) have not proven feasible. Here, we use low-vacuum SEM to image directly the ligament and pore size and shape distributions of representative AGs over a wide range of length scales (similar to 10(0)-10(5) nm). The structural information obtained is used for unambiguous, real-space interpretation of small-angle X-ray scattering curves for these complex nanoporous systems. Low-vacuum SEM permits imaging of both cross-sections and skin layers of AG monoliths. Images of skin layers reveal the presence of microcracks, which alter the properties of cast monolithic AGs.
The distribution of deep traps in a bulk dielectric (Al2O3) is imaged by low vacuum scanning electron microscopy (LVSEM). The image contrast corresponds to spatial variations in radiation-induced, field-enhanced conductivity. A methodology is presented f
Toth, M., Lobo, C.J., Hartigan, G. & Knowles, W.R. 2007, 'Electron flux controlled switching between electron beam induced etching and deposition', Journal Of Applied Physics, vol. 101, pp. 054309-054309.
Electron beam induced deposition (EBID) and etching (EBIE) are promising methods for the fabrication of three-dimensional nanodevices, wiring of nanostructures, and repair of photolithographic masks. Here, we study simultaneous EBID and EBIE, and demonstrate an athermal electron flux controlled transition between material deposition and etching. The switching is observed when one of the processes has both a higher efficiency and a lower precursor partial pressure than the other. This is demonstrated in two technologically important systems: during XeF2-mediated etching of chrome on a photolithographic mask and during deposition and etching of carbonaceous films on a semiconductor surface. Simultaneous EBID and EBIE can be used to enhance the spatial localization of etch profiles. It plays a key role in reducing contamination buildup rates during low vacuum electron imaging and deposition of high purity nanostructures in the presence of oxygen-containing gases. (c) 2007 American Institute of Physics.
Toth, M., Lobo, C.J., Knowles, W.R., Phillips, M.R., Postek, M.T. & Vladar, A.E. 2007, 'Nanostructure fabrication by ultra-high-resolution environmental scanning electron microscopy', Nano Letters, vol. 7, pp. 525-530.View/Download from: UTS OPUS
Electron beam induced deposition (EBID) is a maskless nanofabrication technique capable of surpassing the resolution limits of resist-based lithography. However, EBID fabrication of functional nanostructures is limited by beam spread in bulk substrates, substrate charging, and delocalized film growth around deposits. Here, we overcome these problems by using environmental scanning electron microscopy (ESEM) to perform EBID and etching while eliminating charging artifacts at the nanoscale. Nanostructure morphology is tailored by slimming of deposits by ESEM imaging in the presence of a gaseous etch precursor and by pre-etching small features into a deposit (using a stationary or a scanned electron beam) prior to a final imaging process. The utility of this process is demonstrated by slimming of nanowires deposited by EBID, by the fabrication of gaps (between 4 and 7 nm wide) in the wires, and by the removal of thin films surrounding such nanowires. ESEM imaging provides a direct view of the slimming process, yielding process resolution that is limited by ESEM image resolution (similar to 1 nm) and surface roughening occurring during etching.
Toth, M., Uncovsky, M., Ralph Knowles, W. & Baker, F.S. 2007, 'Secondary electron imaging at gas pressures in excess of 1 kPa', Applied Physics Letters, vol. 91, no. 5.View/Download from: Publisher's site
Environmental scanning electron microscopy (ESEM) enables electron imaging of gas-mediated, direct-write nanolithography processes, liquids, and hydrated biomaterials. However, ESEM is limited by poor image quality at gas pressures in excess of 600 Pa. Here the authors achieve high quality secondary electron imaging at 2 kPa of H2 O by optimizing boundary conditions that govern beam scatter and the energy distribution of low energy electrons in the gas, dielectric breakdown of the gas, and detector collection efficiency. The presented high pressure imaging method will enable imaging of hydrated materials at close to room temperature, and gas-mediated surface modification processes occurring at high pressures. © 2007 American Institute of Physics.
Thiel, B.L., Toth, M., Schroemges, R., Scholtz, H., van Veen, G. & Knowles, W.R. 2006, 'Two-stage gas amplifier for ultrahigh resolution low vacuum scanning electron microscopy', Review of Scientific Instruments, vol. 77, p. 033705.
Toth, M., Knowles, W.R. & Thiel, B.L. 2006, 'Secondary electron imaging of nonconductors with nanometer resolution', Applied Physics Letters, vol. 88, no. 2, p. 023105.
Toth, M., Knowles, W.R. & Thiel, B.L. 2006, 'Secondary electron imaging of nonconductors with nanometer resolution', Applied Physics Letters, vol. 88, no. 2, pp. 1-3.View/Download from: Publisher's site
The resolution of secondary electron (SE) images in scanning electron microscopy (SEM) is limited by the SE diffusion length. However, most materials are poor electrical conductors and in practice, resolution and image information content are often limited by charging. We demonstrate how charging can be eliminated as the resolution-limiting factor using a gaseous SE detector for magnetic immersion electron lenses. Charging is stabilized by ions produced in a magnetic field-assisted gas ionization cascade. The charge control self-regulation process does not quench the SE imaging signal, thereby enabling high resolution image contrast mechanisms that are suppressed in high vacuum SEM. © 2006 American Institute of Physics.
Thiel, B.L. & Toth, M. 2005, 'Secondary electron contrast in low-vacuum/environmental scanning electron microscopy of dielectrics', Journal of Applied Physics, vol. 97, p. 051101.
Thiel, B.L., Toth, M. & Craven, J.P. 2004, 'Charging processes in low vacuum scanning electron microscopy', Microscopy & Microanalysis, vol. 10, no. 6, pp. 711-720.View/Download from: Publisher's site
Phillips, M., Telg, H., Kucheyev, S.O., Gelhausen, O. & Toth, M. 2003, 'Cathodoluminescence efficiency dependence on excitation density in n-type gallium nitride', Microscopy And Microanalysis, vol. 9, no. 2, pp. 144-151.View/Download from: UTS OPUS or Publisher's site
Cathodoluminescence (CL) spectra from silicon doped and undoped wurtzite n-type GaN have been measured in a SEM under a wide range of electron beam excitation conditions, which include accelerating voltage, beam current, magnification, beam diameter, and specimen temperature. The CL intensity dependence on excitation density was analyzed using a power-law model (ICL [is proportional to] Jm) for each of the observed CL bands in this material. The yellow luminescence band present in both silicon and undoped GaN exhibits a close to cube root (m = 0.33) dependence on electron beam excitation at both 77 K and 300 K. However, the blue (at 300 K) and donor-acceptor pair (at 77 K) emission peaks observed in undoped GaN follow power laws with exponents of m = 1 and m = 0.5, respectively. As expected from its excitonic character, the near band edge emission intensity depends linearly (m = 1) in silicon doped GaN and superlinearly (m = 1.2) in undoped GaN on the electron beam current. Results show that the intensities of the CL bands are highly dependent not only on the defect concentration but also on the electron-hole pair density and injection rate. Furthermore, the size of the focussed electron beam was found to have a considerable effect on the relative intensities of the CL emission peaks. Hence SEM parameters such as the objective lens aperture size, astigmatism, and the condenser lens setting must also be considered when assessing CL data based on intensity measurements from this material.
Kucheyev, S.O., Toth, M., Phillips, M., Williams, J.S., Jagadish, C. & Li, G. 2002, 'Chemical origin of the yellow luminescence in GaN', Journal of Applied Physics, vol. 91, no. 9, pp. 5867-5874.View/Download from: UTS OPUS or Publisher's site
The influence of ion-beam-produced lattice defects as well as H, B, C, N, O, and Si, introduced by ion implantation, on the luminescence properties of wurtzite GaN is studied by cathodoluminescence spectroscopy. Results indicate that intrinsic lattice defects produced by ion bombardment mainly act as nonradiative recombination centers and do not give rise to the yellow luminescence (YL) of GaN. Experimental data unequivocally shows that C is involved in the defect-impurity complex responsible for YL. In addition, C-related complexes appear to act as efficient nonradiative recombination centers. Implantation of H produces a broad luminescent peak which is slightly blueshifted with respect to the C-related YL band in the case of high excitation densities. The position of this H-related YL peak exhibits a blueshift with increasing excitation density. Based on this experimental data and results reported previously, the chemical origin of the YL band is discussed
Kucheyev, S.O., Toth, M., Phillips, M., Williams, J.S., Jagadish, C. & Li, G. 2002, 'X-ray spectometry investigation of electrical isolation in GaN', Journal of Applied Physics, vol. 91, no. 6, pp. 3940-3942.View/Download from: UTS OPUS or Publisher's site
Electrical isolation of n-type GaN epilayers bombarded with MeV light ions is studied by energy dispersive x-ray spectrometry (EDS). We show that the maximum bremsstrahlung x-ray energy (the Duane-Hunt limit) can be used to monitor the isolation process in GaN. This method allows the dose region above the threshold dose for isolation to be conveniently studied, whereas the application of conventional (low-voltage) electrical techniques in this dose range with large sheet resistances of the material (>~1011 ?/sq) is often impossible due to comparable parasitic resistances of the experimental setup. A correlation of EDS and resistance measurements of GaN strongly suggests that the magnitude of sample charging scales with the number of ion-beam-produced deep electron traps which are empty at equilibrium. The results presented demonstrate the utility of EDS as a powerful and simple technique to study electrical isolation in wide band-gap semiconductors.
Toth, M., Daniels, D.R., Thiel, B.L. & Donald, A.M. 2002, 'Quantification of electron-ion recombination in an electron-beam-irradiated gas capacitor', Journal of Physics D: Applied Physics, vol. 35, pp. 1796-1804.View/Download from: Publisher's site
Toth, M., Phillips, M., Craven, J.P., Thiel, B.L. & Donald, A.M. 2002, 'Electric fields produced by electron irradiation of insulators in a low vacuum environment', Journal of Applied Physics, vol. 91, no. 7, pp. 4492-4499.View/Download from: UTS OPUS or Publisher's site
We report on the properties of electric fields generated as a result of electron irradiation of dielectrics in a low vacuum scanning electron microscope. Individual field components produced by (i) ionized gas molecules located outside the sample surface and (ii) subsurface trapped charge were detected by measurements of changes in (i) primary electron landing energy and (ii) secondary electron (SE) emission current, respectively. The results provide experimental evidence for a recently proposed model of field-enhanced SE emission from electron irradiated insulators in a low vacuum environment [Toth et al., J. Appl. Phys. 91, 4479 (2002)]. Errors introduced into x-ray microanalysis by the electric fields generated by ionized gas molecules can be alleviated by minimizing the steady state ion concentration by the provision of efficient ion neutralization routes. It is demonstrated how this can be achieved using simple sample-electrode geometries.
Toth, M., Phillips, M., Thiel, B.L. & Donald, A.M. 2002, 'Electron imaging of dielectrics under simultaneous electron-ion irradiation', Journal of Applied Physics, vol. 91, no. 7, pp. 4479-4491.View/Download from: UTS OPUS or Publisher's site
We demonstrate that if charging caused by electron irradiation of an insulator is controlled by a defocused flux of soft-landing positive ions, secondary electron (SE) images can contain contrast due to lateral variations in (i) changes in the SE yield caused by subsurface trapped charge and (ii) the SE-ion recombination rate. Both contrast mechanisms can provide information on microscopic variations in dielectric properties. We present a model of SE contrast formation that accounts for localized charging and the effects of gas ions on the SE emission process, emitted electrons above the sample surface, and subsurface trapped charge. The model explains the ion flux dependence of charge-induced SE contrast, an increase in the sensitivity to surface contrast observed in SE images of charged dielectrics, and yields procedures for identification of contrast produced by localized sample charging.
Toth, M., Thiel, B.L. & Donald, A.M. 2002, 'On the role of electron-ion recombination in low vacuum scanning electron microscopy', Journal of Microscopy, vol. 205, pp. 86-95.View/Download from: Publisher's site
Gelhausen, O., Phillips, M. & Toth, M. 2001, 'Depth-Resolved Cathodoluminescence Microanalysis of Near-Edge Emission in Ill-Nitride Thin Films', Journal of Applied Physics, vol. 89, no. 6, pp. 3535-3537.View/Download from: UTS OPUS or Publisher's site
Kucheyev, S.O., Bradby, J.E., Williams, J.S., Swain, M.V., Toth, M., Phillips, M.R. & Jagadish, C. 2001, 'Mechanical properties of As-grown and ion-beam-modified GaN films', Materials Research Society Symposium-Proceedings, vol. 649, pp. Q5.5.1-Q5.5.6.
The deformation behavior of as-grown and ion-beam-modified wurtzite GaN films is studied by nanoindentation with a spherical indenter. Atomic force microscopy (AFM) and cathodoluminescence are used to characterize the deformation mode. No systematic dependence of the mechanical properties on the film thickness (at least for thicknesses from 1.8 to 4 m) as well as on doping type is observed. Results strongly suggest that (i) slips is the major contributor to the plastic deformation of crystalline GaN and (ii) slip nucleation (rather than a phase transformation) is responsible for "pop-in" events observed during loading. Indentation with an 4.2 m radius spherical indenter at maximum loads up to 900 mN does not produce any cracking visible by AFM in crystalline GaN. Instead, under such loads, indentation results in a pronounced elevation of the material around the impression. Implantation disorder dramatically changes the deformation behavior of GaN. In particular, implantation-produced defects in crystalline GaN suppress (i) "pop-in" events during loading, (ii) slip bands observed by AFM, and (iii) the plastic component of deformation. GaN amorphized by ion bombardment exhibits plastic flow even for very low loads. The values of hardness and elastic modulus of amorphous GaN are dramatically reduced compared to those of as-grown GaN. © 2001 Materials Research Society.
Kucheyev, S.O., Toth, M., Phillips, M., Williams, J.S., Jagadish, C. & Li, G. 2001, 'Cathodoluminescnece Depth Profiling of Ion-Implanted GaN', Applied Physics Letters, vol. 78, no. 1, pp. 34-36.View/Download from: UTS OPUS or Publisher's site
Kucheyev, S.O., Toth, M., Phillips, M., Williams, J.S., Jagadish, C. & Li, G. 2001, 'Effects of Excitation Density on Cathodoluminescence from GaN', Applied Physics Letters, vol. 79, no. 14, pp. 2154-2156.View/Download from: UTS OPUS or Publisher's site
Kucheyev, S.O., Bradby, J., Williams, J.S., Jagadish, C., Toth, M., Phillips, M. & Swain, M.V. 2000, 'Nanoindentation of epitaxial GaN Films', Applied Physics Letters, vol. 77, no. 21, pp. 3373-3375.View/Download from: Publisher's site
Toth, M., Kucheyev, S.O., Williams, J.S., Jagadish, C., Phillips, M. & Li, G. 2000, 'Imaging Charge Trap Distirbution in GaN Using Environmental Scanning Electron Microscopy', Applied Physics Letters, vol. 77, no. 9, pp. 1342-1344.View/Download from: Publisher's site
Fleischer, K., Toth, M., Phillips, M., Zou, J., Li, G. & Chua, S.J. 1999, 'Depth profiling of GaN by cathodoluminescence microanalysis', Applied Physics Letters, vol. 74, no. 8, pp. 1114-1116.View/Download from: Publisher's site
We present the results of a depth-resolved cathodoluminescence (CL) and transmission electron microscopy study of autodoped GaN grown on sapphire. Depth-resolved CL analysis can be used for depth profiling of the yellow luminescence (YL) center concentra
Phillips, M., Toth, M. & Drouin, D. 1999, 'Depletion Layer Imaging Using A Gaseous Secondary Electron Detector In An Environmental Scanning Electron Microscope', Applied Physics Letters, vol. 75, no. 1, pp. 76-78.View/Download from: UTS OPUS or Publisher's site
We present a method for imaging depletion layers using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. GSED images of a p-n junction were obtained from a Si P+PN power diode. Behavior of the junctio
Stevens Kalceff, M.A., Phillips, M.R., Toth, M., Moon, A.R., Jamieson, D.N., Orwa, J.O. & Prawer, S. 1999, 'Cathodoluminescence microanalysis of electron irradiation damage in wide band gap materials', Materials Research Society Symposium - Proceedings, vol. 540, pp. 43-48.
Cathodoluminescence(CL)microanalysis(spectroscopyandmicroscopy) inanelectronmicroscopeenablesbothpre- existingandirradiationinducedlocalvariationsinthebulkandsurfacedefectstructureof widebandgapmaterialstobecharacterizedwithhighspatial(lateralanddepth) resolutionandsensitivity. CLmicroanalyticaltechniquesallowtheinsitumonitoringofelectronirradiationinducedd amage,thepostirradiationassessmentofdamageinducedbyotherenergeticradiation, andtheinvestigationofirradiationinducedelectromigrationofmobilechargeddefectspec ies.ElectronirradiatedsilicondioxidepolymorphsandMeVH + ionimplantedTypeHadiamondhavebeeninvestigatedusingCLmicroanalyticaltechnique s.©1999MaterialsResearchSociety.
Toth, M. & Phillips, M. 1999, 'Detection of Cr impurities in GaN by room temperature cathodoluminescence spectroscopy', Applied Physics Letters, vol. 75, no. 25, pp. 3983-3985.View/Download from: Publisher's site
Trace levels of Cr impurities in epitaxial GaN grown on sapphire substrates were investigated using cathodoluminescence (CL) spectroscopy. CL emissions characteristic of Cr in an octahedral crystal field were observed from beta-Ga2O3 overlayers produced
Toth, M., Fleischer, K. & Phillips, M. 1999, 'Direct experimental evidence for the role of oxygen in the luminescent properties of GaN', Physical Review B, vol. 59, no. 3, pp. 1575-1578.View/Download from: Publisher's site
We present experimental evidence of electron-beam-induced diffusion of O and H in unintentionally doped n-type GaN grown on a sapphire substrate. Impurity diffusion was investigated using cathodoluminescence kinetics and imaging at 4 and 300 K and by wav
This work demonstrates the validity of approximating cathodoluminescence generation throughout the electron interaction volume by the total electron energy loss profile. The energy loss profiles in multilayer specimens were accurately calculated using th
Toth, M., Fleischer, K. & Phillips, M. 1998, 'Electron beam induced impurity electro-migration in unintentionally doped GaN', MRS Internet Journal of Nitride Semiconductor Research, vol. 537, pp. 1-6.
Electron beam induced electromigration of O-N(+) and H+ impurities in unintentionally n-doped GaN was investigated using cathodoluminescence (CL) kinetics profiling, CL imaging of regions pre-irradiated with a stationary electron beam, and wavelength dis
Xu, Z.-.Q., Elbadawi, C., Tran, T.T., Kianinia, M., Hoffman, T.B., Edgar, J.H., Toth, M. & Aharonovich, I., 'Atomic Engineering of Single Photon Sources in 2D Boron Nitride Zai-Quan'.
Artificial atomic systems in solids such as single photon emitters are
becoming increasingly important building blocks in quantum information
processing and scalable quantum nanophotonic networks. Here, we report on a
controllable way to engineer emitters in two-dimensional (2D) hexagonal boron
nitride (hBN) crystals using plasma processing. The method is robust, and
yields a 7-fold increase in the density of emitters in hBN, which is promising
for their deployment in practical devices. While as-fabricated emitters suffer
from blinking and bleaching, a subsequent annealing step yields photo-stable
emitters. The presented process is the first step towards controllable
placement of quantum emitters in hBN for integrated on-chip quantum
nanophotonics based on 2D materials.
Lobo, C. & Toth, M. 2012, 'Continuum modeling of electron beam induced processes' in Utke, I., Moshkalev, S.A. & Russell, P. (eds), Nanofabrication Using Focused Ion and Electron Beams: Principles and Applications, Oxford University Press, New York, USA, pp. 286-320.View/Download from: UTS OPUS
Gas-mediated focused electron beam induced etching (FEBIE) and deposition (FEBID), collectively referred to here as FEBIED, permit nanoscale modification of surface material via chemical reactions involving electron-dissociated precursor molecules. Electrons crossing the solid-vacuum interface usually possess a wide range of energies, are capable of breaking most bonds in typical precursor adsorbates and dissociation products, and can therefore generate a wide range of mobile, chemically active species. Adsorption, desorption, diffusion, and dissociation of these species all contribute to the development of nanostructures fabricated by FEBIED processes. Furthermore, these nanostructures are often electron-sensitive, and thus their structure evolves during deposition. The wide range of processes behind FEBIED yields very complex behavior that is yet to be modeled realistically fifty years after Christy first proposed a simple analytical model of deposition induced by a broad (defocused) electron beam . A complete description of FEBIED requires a realistic model of electron-gas and electron-solid interactions, the spatial and energy distributions of secondary and backscattered electrons, electron interactions with adsorbates, and the behavior of adsorbates and dissociation products at the solid-vacuum interface.
Berhane, A.M., Choi, S., Kato, H., Makino, T., Mizuochi, N., Yamasaki, S., Toth, M. & Aharonovich, I. 2016, 'Defect luminescence in diamond and GaN: Towards single photon emitting devices', 2016 Conference on Lasers and Electro-Optics, CLEO 2016.View/Download from: UTS OPUS
½ 2016 OSA. Narrow band single photon emitters in diamond and gallium nitride are investigated. The work provides foundation towards electrically driven sources at room temperature that are important prerequisites for solid state quantum technologies.
© 2016 OSA.We demonstrate first room temperature, and ultrabright single photon emission from a color center in two-dimensional multilayer hexagonal boron nitride. Density Functional Theory calculations indicate that vacancy-related centers are a likely source of the emission.
Elbadawi, C., Tran, T.T., Shimoni, O., Totonjian, D., Lobo, C.J., Grosso, G., Moon, H., Englund, D.R., Ford, M.J., Aharonovich, I. & Toth, M. 2016, 'Ultra-bright emission from hexagonal boron nitride defects as a new platform for bio-imaging and bio-labelling', Proceedings of SPIE - The International Society for Optical Engineering.View/Download from: UTS OPUS or Publisher's site
© 2016 SPIE. Bio-imaging requires robust ultra-bright probes without causing any toxicity to the cellular environment, maintain their stability and are chemically inert. In this work we present hexagonal boron nitride (hBN) nanoflakes which exhibit narrowband ultra-bright single photon emitters1. The emitters are optically stable at room temperature and under ambient environment. hBN has also been noted to be noncytotoxic and seen significant advances in functionalization with biomolecules2,3. We further demonstrate two methods of engineering this new range of extremely robust multicolour emitters across the visible and near infrared spectral ranges for large scale sensing and biolabeling applications.
Toth, M., Martin, A.A., Shanley, T.W. & Aharonovich, I. 2014, 'Direct-write electron beam fabrication of optically active diamond nanostructures', 2014 Conference on Optoelectronic and Microelectronic Materials and Devices, COMMAD 2014, Conference on Optoelectronic and Microelectronic Materials and Devices, IEEE, Perth, Western Australia, Australia, pp. 6-10.View/Download from: UTS OPUS or Publisher's site
© 2014 IEEE. Controlled fabrication of semiconductor nanostructures is a prerequisite step in the engineering of next generation photonic and optoelectronic devices. Here we describe two advances in electron beam processing of single crystal diamond: (i) chemical dry etching of optically active nanostructures, and (ii) chemical switching of the charge state of nitrogen-vacancy centers by surface fluorination. Etching and fluorination are realized by irradiating diamond by kiloelectronvolt electrons at room temperature in the presence of H 2 O and NF 3 vapor, respectively. The techniques do not generate defects that quench luminescence, thereby enabling the fabrication and editing of optically active nanostructures and diamond-based devices.
Lobo, C.J., Elbadawi, C. & Toth, M. 2014, 'Localized deposition of pure platinum nanostructures', 2014 Conference on Optoelectronic and Microelectronic Materials and Devices, COMMAD 2014, Conference on Optoelectronic and Microelectronic Materials and Devices, IEEE, Perth, Western Australia, Australia, pp. 15-16.View/Download from: UTS OPUS or Publisher's site
© 2014 IEEE. Localized deposition of pure platinum nanostructures was achieved using a combination of focused electron beam induced processing (FEBID) of an inorganic platinum precursor and low temperature annealing in water vapour. This technique enables fabrication of Pt nanostructures with high spatial resolution and purity, for applications in nanoelectronics, sensing devices and catalysis.
Lobo, C.J., Martin, A.A., Elbadawi, C., Bishop, J., Aharonovich, I. & Toth, M. 2014, 'Gas-mediated charged particle beam processing of nanostructured materials', Proceedings of SPIE - The International Society for Optical Engineering, The International Society for Optical Engineering Conference, SPIE-INT SOC OPTICAL ENGINEERING, San Francisco, CA.View/Download from: UTS OPUS or Publisher's site
Gas mediated processing under a charged particle (electron or ion) beam enables direct-write, high resolution surface functionalization, chemical dry etching and chemical vapor deposition of a wide range of materials including catalytic metals, optoelectronic grade semiconductors and oxides. Here we highlight three recent developments of particular interest to the optical materials and nanofabrication communities: fabrication of self-supporting, three dimensional, fluorescent diamond nanostructures, electron beam induced deposition (EBID) of high purity materials via activated chemisorption, and post-growth purification of nanocrystalline EBID-grown platinum suitable for catalysis applications. © 2014 SPIE.
Evertsen, J.J., Toth, M., Thiel, B.L. & Lifshin, E. 2006, 'Generation of spurious x-rays by focused ion beams in dual beam instruments', Microscopy and Microanalysis, pp. 1250-1251.View/Download from: Publisher's site
Toth, M., Knowles, W.R., Hartigan, G. & Lobo, C.J. 2006, 'Electron flux controlled switching between electron beam induced etching and deposition', Microscopy and Microanalysis, pp. 168-169.View/Download from: Publisher's site
Knowles, W.R., Toth, M., Thiel, B. & Schroemges, R. 2005, 'Ion control for optimized imaging with a novel gaseous detector', Microscopy and Microanalysis, pp. 28-29.View/Download from: Publisher's site
Toth, M., Thiel, B.L. & Knowles, W.R. 2005, 'Self-regulating charge control for ultra high resolution scanning electron microscopy', Proceedings of SPIE - The International Society for Optical Engineering, pp. 49-59.View/Download from: Publisher's site
We present a low vacuum tool for ultra high resolution scanning electron microscopy of insulators and floating conductors. Charging is stabilized by ionized gas molecules generated using an environmental secondary electron detector designed to operate within the magnetic field on an immersion objective lens. The charge stabilization mechanism yields consistent charge control that is transparent to the operator, and independent of the tasks performed during imaging. This is illustrated by series of artifact-free, high resolution images of an insulating test sample acquired as a function of magnification and scan speed, at a number of accelerating voltages. The low vacuum method is compared to the high vacuum technique of adjusting the electron beam landing energy so as to minimize charging artifacts (i.e., the "total yield" method). The low vacuum approach is less sensitive to changes in beam current density (determined by the beam current, magnification, scan speed and beam diameter) and yields higher ultimate image resolution. The resolution improvement results from effective suppression of both charge-induced defocusing of the electron beam and distortion of the scan pattern.
Knowles, W.R., Thiel, B.L., Toth, M., Schroemges, R.P.M., Scholtz, J.J., Van Veen, G., Elders, M. & Donald, A.M. 2004, 'Design of a two-stage gas amplification secondary electron detector for imaging insulating samples at the sub-1_nm scale', Microscopy and Microanalysis, pp. 1060-1061.View/Download from: Publisher's site
Stokes, D.J., Baker, F.S. & Toth, M. 2004, 'Raising the pressure: Realizing room temperature/high humidity applications in ESEM', Microscopy and Microanalysis, pp. 1074-1075.View/Download from: Publisher's site
Thiel, B.L., Toth, M. & Craven, J.P. 2002, 'Charging processes in low vacuum scanning electron microscopy', MICROSCOPY AND MICROANALYSIS, Topical Conference on Microbeam Characterization of Nonconductive Materials, CAMBRIDGE UNIV PRESS, McGill Univ, Dept Min, Met & Mat Engn, Montreal, CANADA, pp. 711-720.View/Download from: Publisher's site
Thiel, B.L., Toth, M., Schroemges, R.P.M., Scholtz, J.J., Van Veen, G. & Knowles, W.R. 2004, 'A two-stage gas amplification secondary electron detector for ultra-high resolution imaging', Microscopy and Microanalysis, pp. 1058-1059.View/Download from: Publisher's site
Thiel, B.L. & Toth, M. 2002, 'Considerations for secondary electron imaging of dielectric materials in low-vacuum and environmental SEM', Microscopy and Microanalysis, pp. 448-449.
Toth, M., Craven, J.P., Phillips, M., Thiel, B.L. & Donald, A.M. 2002, 'X-ray microanalysis of insulators in a variable pressure environment', Proceedings of Microscopy and Microanalysis 2002, Vol 11 (suppl 2), Microscopy & Microanalysis, Cambridge University Press, Quebec, Canada, pp. 1478-1479.View/Download from: UTS OPUS
Kucheyev, S.O., Bradby, J.E., Williams, J.S., Swain, M.V., Toth, M., Phillips, M.R. & Jagadish, C. 2001, 'Mechanical properties of As-grown and ion-beam-modified GaN films', Materials Research Society Symposium - Proceedings.
The deformation behavior of as-grown and ion-beam-modified wurtzite GaN films is studied by nanoindentation with a spherical indenter. Atomic force microscopy (AFM) and cathodoluminescence are used to characterize the deformation mode. No systematic dependence of the mechanical properties on the film thickness (at least for thicknesses from 1.8 to 4 m) as well as on doping type is observed. Results strongly suggest that (i) slips is the major contributor to the plastic deformation of crystalline GaN and (ii) slip nucleation (rather than a phase transformation) is responsible for "pop-in" events observed during loading. Indentation with an 4.2 m radius spherical indenter at maximum loads up to 900 mN does not produce any cracking visible by AFM in crystalline GaN. Instead, under such loads, indentation results in a pronounced elevation of the material around the impression. Implantation disorder dramatically changes the deformation behavior of GaN. In particular, implantation-produced defects in crystalline GaN suppress (i) "pop-in" events during loading, (ii) slip bands observed by AFM, and (iii) the plastic component of deformation. GaN amorphized by ion bombardment exhibits plastic flow even for very low loads. The values of hardness and elastic modulus of amorphous GaN are dramatically reduced compared to those of as-grown GaN.
Kucheyev, S.O., Toth, M., Phillips, M.R., Williams, J.S., Jagadish, C. & Li, G. 2000, 'Cathodoluminescence study of ion implanted GaN', Conference on Optoelectronic and Microelectronic Materials and Devices, Proceedings, COMMAD, pp. 190-193.View/Download from: Publisher's site
© 2000 IEEE. The effect of ion-beam-produced defects as well as H, C, and O, introduced by ion implantation, on the luminescence from wurtzite GaN is studied by cathodoluminescence (CL) spectroscopy. Results indicate that even relatively low dose keV light-ion bombardment results in a dramatic quenching of CL emission. Postimplantation annealing at temperatures up to 1050 °C generally causes a partial recovery of measured CL intensities. However, CL depth profiles indicate that, in most cases, such a recovery results from CL emission from virgin GaN, beyond the implanted layer due to a reduction in the extent of defect-related light absorption within the implanted layer. Results also show that H, C, and O, presumably in combination with point defects, give rise to yellow luminescence in GaN, while lattice defects alone (as well as the other species implanted such as B, N, and Si) do not give rise to yellow luminescence.
Kucheyev, S.O., Williams, J.S., Jagadish, C., Zou, J., Toth, M., Phillips, M.R. & Li, G. 2000, 'Ion implantation into GaN: Opportunities and problems', IEEE Semiconducting and Semi-Insulating Materials Conference, SIMC, pp. 47-50.View/Download from: Publisher's site
© 2000 IEEE. We summarize here our recent results on structural and optical characteristics of wurtzite GaN films bombarded under a wide range of implant conditions, as studied by Rutherford backscattering/channeling (RBS/C) spectrometry, transmission electron microscopy (TEM), atomic force microscopy (AFM), and cathodoluminescence (CL). Our results highlight current problems, associated with implantation-produced disorder, which may hinder a successful application of ion implantation in the fabrication of GaN-based devices.
Kucheyev, S.O., Williams, J.S., Jagadish, C., Zou, J., Toth, M., Phillips, M.R., Tan, H.H., Li, G. & Pearton, S.J. 2000, 'Surface disordering and nitrogen loss in GaN under ion bombardment', Materials Research Society Symposium-Proceedings, pp. T791-T796.
The damage build-up and amorphization behavior in wurtzite GaN films under a wide range of implant conditions are studied by Rutherford backscattering / channeling spectrometry, transmission electron microscopy, and cathodoluminescence spectroscopy. A strong surface peak of lattice disorder, in addition to the expected damage peak in the region of the maximum of nuclear energy loss, has been observed for all implant conditions of this study. Capping of GaN with SiO x and Si x N y layers prior to implantation does not eliminate surface disordering. This may suggest that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer.
Toth, M., Phillips, M., Kucheyev, S.O., Williams, J.S., Jagadish, C. & Li, G. 2000, 'Charge Contrast in SE Images Obtained Using the ESEM', 2nd Conference of the Internatioanl Union Microbeam Analysis Societies, Institute of Physics, Hawaii, pp. 275-276.
Electron beam induced electromigration of O N + and H + impurities in unintentionally n-doped GaN was investigated using cathodoluminescence (CL) kinetics profiling, CL imaging of regions pre-irradiated with a stationary electron beam, and wavelength dispersive x-ray spectrometry (WDS). The presented results (i) illustrate induced impurity diffusion in wide bandgap semiconductors, (ii) provide experimental evidence for the (V Ga -O N ) 2- model of yellow luminescence in GaN with low Si content, (iii) confirm the roles of O in frequently reported bound exciton and donor-acceptor pair emissions and (iv) suggest the involvement of O N + and hydrogenated gallium vacancies in a blue emission in autodoped GaN.
Recent advances in focused ion beam technology have enabled high-resolution,
direct-write nanofabrication using light ions. Studies with light ions to date
have, however, focused on milling of materials where sub-surface ion beam
damage does not inhibit device performance. Here we report on direct-write
milling of single crystal diamond using a focused beam of oxygen ions. Material
quality is assessed by Raman and luminescence analysis, and reveals that the
damage layer generated by oxygen ions can be removed by nonintrusive
post-processing methods such as localised electron beam induced chemical