Zaiquan (Sean) Xu obtained his B.Sc. in Chemistry in 2006, and then he spent three years in the industry before he went back to Soochow University, China to study his M.Sc. After he completed his Master’s degree in 2012, he was awarded an International Postgraduate Research Scholarship (IPRS) and Australian Postgraduate Award (APA). He joined Department of Materials Science and Engineering, MONASH University for his Ph.D in late 2012. He joined University of Technology, Sydney (UTS) after submission of his thesis in 2016. He starts Chancellor's Research Fellow from 2018 in UTS. His research area includes solar cells, interface engineering, 2D materials, synthesis and opto-electronic applications.
Guest Editor in Photonics and Optoelectronics of Low-Dimensional Materials, 2017-2018
Guest Editor in Applied Sciences, 2016-2018
Can supervise: YES
Materials Science and Chemistry
Nelz, R, Radtke, M, Slablab, A, Xu, Z, Kianinia, M, Li, C, Bradac, C, Aharonovich, I & Neu, E 2020, 'Near‐Field Energy Transfer between a Luminescent 2D Material and Color Centers in Diamond', Advanced Quantum Technologies, vol. 3, no. 2, pp. 1900088-1900088.View/Download from: Publisher's site
Liu, B, Chen, C, Di, X, Liao, J, Wen, S, Su, QP, Shan, X, Xu, Z-Q, Ju, LA, Mi, C, Wang, F & Jin, D 2020, 'Upconversion nonlinear structured illumination microscopy.', Nano letters.View/Download from: Publisher's site
Video-rate super-resolution imaging through biological tissue can visualize and track biomolecule interplays and transportations inside cellular organisms. Structured illumination microscopy allows for wide-field super resolution observation of biological samples but is limited by the strong extinction of light by biological tissues, which restricts the imaging depth and degrades its imaging resolution. Here we report a photon upconversion scheme using lanthanide-doped nanoparticles for wide-field super-resolution imaging through the biological transparent window, featured by near-infrared and low-irradiance nonlinear structured illumination. We demonstrate that the 976 nm excitation and 800 nm up-converted emission can mitigate the aberration. We found that the nonlinear response of upconversion emissions from single nanoparticles can effectively generate the required high spatial frequency components in the Fourier domain. These strategies lead to a new modality in microscopy with a resolution below 131 nm, 1/7th of the excitation wavelength, and an imaging rate of 1 Hz.
Li, C, Xu, ZQ, Mendelson, N, Kianinia, M, Toth, M & Aharonovich, I 2019, 'Purification of single-photon emission from hBN using post-processing treatments', Nanophotonics, vol. 8, no. 11, pp. 2049-2055.View/Download from: Publisher's site
© 2019 Zai-Quan Xu, Igor Aharonovich et al., published by De Gruyter 2019. Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) are promising components for on-chip quantum information processing. Recently, large-area hBN films prepared by chemical vapor deposition (CVD) were found to host uniform, high densities of SPEs. However, the purity of these emitters has, to date, been low, hindering their applications in practical devices. In this work, we present two methods for post-growth processing of hBN, which significantly improve SPEs in hBN films that had been transferred from substrates used for CVD. The emitters exhibit high photon purities in excess of 90% and narrow linewidths of ~3 nm at room temperature. Our work lays a foundation for producing high-quality emitters in an ultra-compact two-dimensional material system and paves the way for deployment of hBN SPEs in scalable on-chip photonic and quantum devices.
Mendelson, N, Xu, Z-Q, Tran, TT, Kianinia, M, Scott, J, Bradac, C, Aharonovich, I & Toth, M 2019, 'Engineering and Tuning of Quantum Emitters in Few-Layer Hexagonal Boron Nitride.', ACS nano, vol. 13, no. 3, pp. 3132-3140.View/Download from: Publisher's site
Quantum technologies require robust and photostable single photon emitters (SPEs). Hexagonal boron nitride (hBN) has recently emerged as a promising candidate to host bright and optically stable SPEs operating at room temperature. However, the emission wavelength of the fluorescent defects in hBN has, to date, been shown to be uncontrolled, with a widespread of zero phonon line (ZPL) energies spanning a broad spectral range (hundreds of nanometers), which hinders the potential development of hBN-based devices and applications. Here we demonstrate chemical vapor deposition growth of large-area, few-layer hBN films that host large quantities of SPEs: ∼100-200 per 10 × 10 μm2. More than 85% of the emitters have a ZPL at (580 ± 10) nm, a distribution that is an order of magnitude narrower than reported previously. Furthermore, we demonstrate tuning of the ZPL wavelength using ionic liquid devices over a spectral range of up to 15 nm-the largest obtained to date from any solid-state SPE. The fabricated devices illustrate the potential of hBN for the development of hybrid quantum nanophotonic and optoelectronic devices based on two-dimensional materials.
Duong, NMH, Xu, Z-Q, Kianinia, M, Su, R, Liu, Z, Kim, S, Bradac, C, Li, L-J, Solntsev, A, Liu, J & Aharonovich, I 2018, 'Enhanced Emission from WSe2 Monolayers Coupled to Circular Bragg Gratings', ACS Photonics, vol. 5, pp. 3950-3955.View/Download from: Publisher's site
Two-dimensional transition-metal dichalcogenides (TMDC) are of great interest
for on-chip nanophotonics due to their unique optoelectronic properties. Here,
we propose and realize coupling of tungsten diselenide (WSe2) monolayers to
circular Bragg grating structures to achieve enhanced emission. The interaction
between WSe2 and the resonant mode of the structure results in Purcell-enhanced
emission, while the symmetric geometrical structure improves the directionality
of the out-coupling stream of emitted photons. Furthermore, this hybrid
structure produces a record high contrast of the spin valley readout (> 40%)
revealed by the polarization resolved photoluminescence (PL) measurements. Our
results are promising for on-chip integration of TMDC monolayers with optical
resonators for nanophotonic circuits.
Kianinia, M, Bradac, C, Sontheimer, B, Wang, F, Tran, TT, Nguyen, M, Kim, S, Xu, Z-Q, Jin, D, Schell, AW, Lobo, CJ, Aharonovich, I & Toth, M 2018, 'All-optical control and super-resolution imaging of quantum emitters in layered materials.', Nature communications, vol. 9, no. 1, pp. 874-874.View/Download from: 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.
Nguyen, M, Kim, S, Tran, TT, 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, TT, Kianinia, M, Nguyen, M, Kim, S, Xu, ZQ, 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.
Bao, X, Ou, Q, Xu, ZQ, Zhang, Y, Bao, Q & Zhang, H 2018, 'Band Structure Engineering in 2D Materials for Optoelectronic Applications', Advanced Materials Technologies.View/Download from: Publisher's site
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Recent advances in the development of 2D-layered materials have witnessed the use of these materials as intriguing building blocks for various optoelectronic applications. The versatility of 2D material systems makes them particularly attractive for photodetection with fast response and high sensitivity over a broad spectral coverage, ranging from ultraviolet, visible to infrared. However, due to the atomically thin nature and inherent electronic structure, light that is harvested by monolayer 2D materials is extremely low and the photodetector devices often operate as Schottky junctions, which significantly limit the efficiency for photocurrent generation. Here, recent progress on the exploration of 2D material–based heterostructures and the engineering of the band structures for energy-efficient optoelectronic applications is reviewed. First, the strategies to introduce a bandgap in graphene are reviewed and discussed. This is followed by a discussion on the engineering of electronic structures in 2D transition metal dichalcogenides by localized chemical doping, dual gating, liquid gating, thickness modulation, and constructing heterojunctions. It is concluded by a summary and perspective on the challenges and future directions.
Cheng, L-W, Ma, J, Cao, C-R, Xu, Z-Z, Lan, T, Yang, J-P, Chen, H-T, Yu, H-Y, Wu, S-D, Yao, S, Zeng, X-H & Xu, Z-Q 2018, 'Improved carrier injection and confinement in InGaN light-emitting diodes containing GaN/AlGaN/GaN triangular barriers', CHINESE PHYSICS B, vol. 27, no. 8.View/Download from: Publisher's site
Dai, S, Tymchenko, M, Xu, Z-Q, Tran, TT, Yang, Y, Ma, Q, Watanabe, K, Taniguchi, T, Jarillo-Herrero, P, Aharonovich, I, Basov, DN, Tao, TH & Alù, A 2018, 'Internal Nanostructure Diagnosis with Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride.', Nano letters, vol. 18, no. 8, pp. 5205-5210.View/Download from: Publisher's site
Imaging materials and inner structures with resolution below the diffraction limit has become of fundamental importance in recent years for a wide variety of applications. We report subdiffractive internal structure diagnosis of hexagonal boron nitride by exciting and imaging hyperbolic phonon polaritons. On the basis of their unique propagation properties, we are able to accurately locate defects in the crystal interior with nanometer resolution. The precise location, size, and geometry of the concealed defects are reconstructed by analyzing the polariton wavelength, reflection coefficient, and their dispersion. We have also studied the evolution of polariton reflection, transmission, and scattering as a function of defect size and photon frequency. The nondestructive high-precision polaritonic structure diagnosis technique introduced here can be also applied to other hyperbolic or waveguide systems and may be deployed in the next-generation biomedical imaging, sensing, and fine structure analysis.
Ou, Q, Zhang, Y, Wang, Z, Yuwono, JA, Wang, R, Dai, Z, Li, W, Zheng, C, Xu, Z-Q, Qi, X, Duhm, S, Medhekar, NV, Zhang, H & Bao, Q 2018, 'Strong Depletion in Hybrid Perovskite p-n Junctions Induced by Local Electronic Doping.', Advanced materials (Deerfield Beach, Fla.), vol. 30, no. 15.View/Download from: Publisher's site
A semiconductor p-n junction typically has a doping-induced carrier depletion region, where the doping level positively correlates with the built-in potential and negatively correlates with the depletion layer width. In conventional bulk and atomically thin junctions, this correlation challenges the synergy of the internal field and its spatial extent in carrier generation/transport. Organic-inorganic hybrid perovskites, a class of crystalline ionic semiconductors, are promising alternatives because of their direct badgap, long diffusion length, and large dielectric constant. Here, strong depletion in a lateral p-n junction induced by local electronic doping at the surface of individual CH3 NH3 PbI3 perovskite nanosheets is reported. Unlike conventional surface doping with a weak van der Waals adsorption, covalent bonding and hydrogen bonding between a MoO3 dopant and the perovskite are theoretically predicted and experimentally verified. The strong hybridization-induced electronic coupling leads to an enhanced built-in electric field. The large electric permittivity arising from the ionic polarizability further contributes to the formation of an unusually broad depletion region up to 10 µm in the junction. Under visible optical excitation without electrical bias, the lateral diode demonstrates unprecedented photovoltaic conversion with an external quantum efficiency of 3.93% and a photodetection responsivity of 1.42 A W-1 .
Elbadawi, C, Queralt, RT, Xu, Z-Q, Bishop, J, Ahmed, T, Kuriakose, S, Walia, S, Toth, M, Aharonovich, I & Lobo, CJ 2018, 'Encapsulation-Free Stabilization of Few-Layer Black Phosphorus.', ACS applied materials & interfaces, vol. 10, no. 29, pp. 24327-24331.View/Download from: Publisher's site
Under ambient conditions and in H2O and O2 environments, reactive oxygen species (ROS) cause immediate degradation of the mobility of few-layer black phosphorus (FLBP). Here, we show that FLBP degradation can be prevented by maintaining the temperature in the range ∼125-300 °C during ROS exposure. FLBP devices maintained at elevated temperature show no deterioration of electrical conductance, in contrast to the immediate degradation of pristine FLBP held at room temperature. Our results constitute the first demonstration of stable FLBP in the presence of ROS without requiring encapsulation or a protective coating. The stabilization method will enable applications based on the surface properties of intrinsic FLBP.
Xu, Z-Q, Elbadawi, C, Tran, TT, Kianinia, M, Li, X, Liu, D, Hoffman, TB, Nguyen, M, Kim, S, Edgar, JH, 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.
Carey, BJ, Ou, JZ, Clark, RM, Berean, KJ, Zavabeti, A, Chesman, ASR, Russo, SP, Lao, DWM, Xu, ZQ, Bao, Q, Kavehei, O, Gibson, BC, Dickey, MD, Kaner, RB, Daeneke, T & Kalantar-Zadeh, K 2017, 'Corrigendum: Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals', Nature communications, vol. 8, p. 15116.View/Download from: Publisher's site
Carey, BJ, Ou, JZ, Clark, RM, Berean, KJ, Zavabeti, A, Chesman, ASR, Russo, SP, Lau, DWM, Xu, ZQ, Bao, Q, Kevehei, O, Gibson, BC, Dickey, MD, Kaner, RB, Daeneke, T & Kalantar-Zadeh, K 2017, 'Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals', Nature Communications, vol. 8, pp. 1-9.View/Download from: Publisher's site
© The Author(s) 2017. A variety of deposition methods for two-dimensional crystals have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we introduce a technique for depositing and patterning of wafer-scale two-dimensional metal chalcogenide compounds by transforming the native interfacial metal oxide layer of low melting point metal precursors (group III and IV) in liquid form. In an oxygen-containing atmosphere, these metals establish an atomically thin oxide layer in a self-limiting reaction. The layer increases the wettability of the liquid metal placed on oxygen-terminated substrates, leaving the thin oxide layer behind. In the case of liquid gallium, the oxide skin attaches exclusively to a substrate and is then sulfurized via a relatively low temperature process. By controlling the surface chemistry of the substrate, we produce large area two-dimensional semiconducting GaS of unit cell thickness (∼1.5 nm). The presented deposition and patterning method offers great commercial potential for wafer-scale processes.
Li, Z, Zheng, J, Zhang, Y, Zheng, C, Woon, WY, Chuang, MC, Tsai, HC, Chen, CH, Davis, A, Xu, ZQ, Lin, J, Zhang, H & Bao, Q 2017, 'Synthesis of Ultrathin Composition Graded Doped Lateral WSe2/WS2 Heterostructures', ACS Applied Materials and Interfaces, vol. 9, no. 39, pp. 34204-34212.View/Download from: Publisher's site
© 2017 American Chemical Society. Lateral transition-metal dichalcogenide and their heterostructures have attracted substantial attention, but there lacks a simple approach to produce large-scaled optoelectronic devices with graded composition. In particular, the incorporation of substitution and doping into heterostructure formation is rarely reported. Here, we demonstrate growth of a composition graded doped lateral WSe 2 /WS 2 heterostructure by ambient pressure chemical vapor deposition in a single heat cycle. Through Raman and photoluminescence spectroscopy, we demonstrate that the monolayer heterostructure exhibits a clear interface between two domains and a graded composition distribution in each domain. The coexistence of two distinct doping modes, i.e., interstitial and substitutional doping, was verified experimentally. A distinct three-stage growth mechanism consisting of nucleation, epitaxial growth, and substitution was proposed. Electrical transport measurements reveal that this lateral heterostructure has representative characteristics of a photodiodes. The optoelectronic device based on the lateral WSe 2 /WS 2 heterostructure shows improved photodetection performance in terms of a reasonable responsivity and a large photoactive area.
Tran, TT, Choi, S, Scott, JA, Xu, ZQ, Zheng, C, Seniutinas, G, Bendavid, A, Fuhrer, MS, 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: 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 three‐level system, and exhibit moderate bunching. Overall, the results provide important insights into understanding of defect formation and quantum emitter activation in two‐dimensional materials.
Tran, TT, Wang, D, Xu, Z-Q, Yang, A, Toth, M, Odom, TW & 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: 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.
Tran, TT, Kianinia, M, Bray, K, Kim, S, Xu, Z-Q, Gentle, A, Sontheimer, B, Bradac, C & Aharonovich, I 2017, 'Nanodiamonds with photostable, sub-gigahertz linewidth quantum emitters', APL Photonics, vol. 2, no. 11, pp. 116103-116103.View/Download from: Publisher's site
Single-photon emitters with narrow linewidths are highly sought after for applications
in quantum information processing and quantum communications. In this
letter, we report on a bright, highly polarized near infrared single photon emitter
embedded in diamond nanocrystals with a narrow, sub-GHz optical linewidth
at 10 K. The observed zero-phonon line at ∼780 nm is optically stable under low
power excitation and blue shifts as the excitation power increases. Our results highlight
the prospect for using new near infrared color centers in nanodiamonds for
Jumabekov, AN, Della Gaspera, E, Xu, Z-Q, Chesman, ASR, van Embden, J, Bonke, SA, Bao, Q, Vak, D & Bach, U 2016, 'Back-contacted hybrid organic–inorganic perovskite solar cells', Journal of Materials Chemistry C, vol. 4, pp. 3125-3130.View/Download from: Publisher's site
A novel architecture for quasi-interdigitated electrodes (QIDEs) allows for the fabrication of back-contacted perovskite solar cells. The devices showed a stable power output of 3.2%. The design of the QIDEs avoids the defects that cause short-circuiting in conventional IDEs, while enhancing the collection area of the electrodes. Photoluminescence and photocurrent mapping is used to probe the charge generation and transport properties of the perovskite solar cells.
Lin, S, Liu, S, Yang, Z, Li, Y, Ng, TW, Xu, Z, Bao, Q, Hao, J, Lee, C-S, Surya, C & others 2016, 'Solution-Processable Ultrathin Black Phosphorus as an Effective Electron Transport Layer in Organic Photovoltaics', Advanced Functional Materials, vol. 26, pp. 864-871.View/Download from: Publisher's site
2D van der Waals crystals, possessing excellent electronic and physical properties, have been intriguing building blocks for organic optoelectronic devices. Most of the 2D materials are served as hole transport layers in organic devices. Here,it is reported that solution exfoliated few layers black phosphorus (BP) can be served as an effective electron transport layer (ETL) in organic photovoltaics (OPVs) for the first time. The power conversion efficiencies (PCEs) of the BP-incorporated OPVs can be improved to 8.18% in average with the relative enhancement of 11%. The incorporation of BP flakes with the optimum thickness of approximate to 10 nm can form cascaded band structure in OPVs, which can facilitate electron transport and enhance the PCEs of the devices. This study opens an avenue in using solution exfoliated BP as a highly efficient ETL for organic optoelectronics.
Liu, J, Xue, Y, Wang, Z, Xu, Z-Q, Zheng, C, Weber, B, Song, J, Wang, Y, Lu, Y, Zhang, Y & others 2016, 'Two-Dimensional CH3NH3PbI3 Perovskite: Synthesis and Optoelectronic Application', ACS nano, vol. 10, pp. 3536-3542.View/Download from: Publisher's site
Ponraj, JS, Xu, Z-Q, Dhanabalan, SC, Mu, H, Wang, Y, Yuan, J, Li, P, Thakur, S, Ashrafi, M, Mccoubrey, K, Zhang, Y, Li, S, Zhang, H & Bao, Q 2016, 'Photonics and optoelectronics of two-dimensional materials beyond graphene', NANOTECHNOLOGY, vol. 27, no. 46.View/Download from: Publisher's site
Wang, Y, Della Gaspera, E, Carey, BJ, Atkin, P, Berean, KJ, Clark, RM, Cole, IS, Xu, Z-Q, Zhang, Y, Bao, Q & others 2016, 'Enhanced quantum efficiency from a mosaic of two dimensional MoS 2 formed onto aminosilane functionalised substrates', Nanoscale, vol. 8, no. 24, pp. 12258-12266.View/Download from: Publisher's site
Wang, Z, Liu, J, Xu, Z-Q, Xue, Y, Jiang, L, Song, J, Huang, F, Wang, Y, Zhong, YL, Zhang, Y & others 2016, 'Wavelength-tunable waveguides based on polycrystalline organic–inorganic perovskite microwires', Nanoscale, vol. 8, no. 12, pp. 6258-6264.View/Download from: Publisher's site
Hybrid organic–inorganic perovskites have emerged as new photovoltaic materials with impressively high power conversion efficiency due to their high optical absorption coefficient and long charge carrier diffusion length. In addition to high photoluminescence quantum efficiency and chemical tunability, hybrid organic–inorganic perovskites also show intriguing potential for diverse photonic applications. In this work, we demonstrate that polycrystalline organic–inorganic perovskite microwires can function as active optical waveguides with small propagation loss. The successful production of high quality perovskite microwires with different halogen elements enables the guiding of light with different colours. Furthermore, it is interesting to find that out-coupled light intensity from the microwire can be effectively modulated by an external electric field, which behaves as an electro-optical modulator. This finding suggests the promising applications of perovskite microwires as effective building blocks in micro/nano scale photonic circuits.
Xu, ZQ, Zhang, Y, Wang, Z, Shen, Y, Huang, W, Xia, X, Yu, W, Xue, Y, Sun, L, Zheng, C, Lu, Y, Liao, L & Bao, Q 2016, 'Atomically thin lateral p-n junction photodetector with large effective detection area', 2D Materials, vol. 3, no. 4, pp. 1-9.View/Download from: Publisher's site
© 2016 IOP Publishing Ltd. The widely used photodetector design based on atomically thin transition metal dichalcogenides (TMDs) has a lateral metal-TMD-metal junction with a fairly small, line shape photoresponsive active area at the TMD-electrode interface. Here, we report a highly efficient photodetector with extremely large photoresponsive active area based on a lateral junction of monolayer-bilayer WSe 2 . Impressively, the separation of the electron-hole pairs (excitons) extends onto the whole 1L-2L WSe 2 junction surface. The responsivity of the WSe 2 junction photodetector is over 3200 times higher than that of a monolayer WSe 2 device and leads to a highest external quantum efficiency of 256% due to the efficient carrier extraction. Unlike the TMDp-n junctions modulated by dual gates or localized doping, which require complex fabrication procedures, our study establishes a simple, controllable, and scalable method to improve the photodetection performance by maximizing the active area for current generation.
Xue, Y, Zhang, Y, Liu, Y, Liu, H, Song, J, Sophia, J, Liu, J, Xu, Z, Xu, Q, Wang, Z & others 2016, 'Scalable production of a few-layer MoS2/WS2 vertical heterojunction array and its application for photodetectors', ACS nano, vol. 10, pp. 573-580.View/Download from: Publisher's site
Zhang, Q, Chang, Z, Xu, G, Wang, Z, Zhang, Y, Xu, Z-Q, Chen, S, Bao, Q, Liu, JZ, Mai, Y-W, Duan, W, Fuhrer, MS & Zheng, C 2016, 'Strain Relaxation of Monolayer WS2 on Plastic Substrate', ADVANCED FUNCTIONAL MATERIALS, vol. 26, no. 47, pp. 8707-8714.View/Download from: Publisher's site
Zhang, Y, Wang, Y, Xu, Z-Q, Liu, J, Song, J, Xue, Y, Wang, Z, Zheng, J, Jiang, L, Zheng, C, Huang, F, Sun, B, Cheng, Y-B & Bao, Q 2016, 'Reversible Structural Swell-Shrink and Recoverable Optical Properties in Hybrid Inorganic-Organic Perovskite', ACS NANO, vol. 10, no. 7, pp. 7031-7038.View/Download from: Publisher's site
Chen, C, Qiao, H, Lin, S, Man Luk, C, Liu, Y, Xu, Z, Song, J, Xue, Y, Li, D & Yuan, J 2015, 'Highly responsive MoS2 photodetectors enhanced by graphene quantum dots', Scientific Reports, vol. 5, pp. 1-9.View/Download from: Publisher's site
Molybdenum disulphide (MoS2), which is a typical semiconductor from the family of layered transition metal dichalcogenides (TMDs), is an attractive material for optoelectronic and photodetection applications because of its tunable bandgap and high quantum luminescence efficiency. Although a high photoresponsivity of 880–2000 AW−1 and photogain up to 5000 have been demonstrated in MoS2-based photodetectors, the light absorption and gain mechanisms are two fundamental issues preventing these materials from further improvement. In addition, it is still debated whether monolayer or multilayer MoS2 could deliver better performance. Here, we demonstrate a photoresponsivity of approximately 104 AW−1 and a photogain of approximately 107 electrons per photon in an n-n heterostructure photodetector that consists of a multilayer MoS2 thin film covered with a thin layer of graphene quantum dots (GQDs). The enhanced light-matter interaction results from effective charge transfer and the re-absorption of photons, leading to enhanced light absorption and the creation of electron-hole pairs. It is feasible to scale up the device and obtain a fast response, thus making it one step closer to practical applications.
Hunag, W, Gann, E, Xu, Z-Q, Thomsen, L, Cheng, Y-B & McNeill, C 2015, 'A facile approach to alleviate photochemical degradation in high efficiency polymer solar cells', Journal of Materials Chemistry A, vol. 3, no. 31, pp. 16313-16319.View/Download from: Publisher's site
In this paper, it is found that residual 1,8-diiodoctane (DIO) which is used as a solvent additive in the manufacture of high-performance polymer solar cells accelerates chemical degradation of the donor polymer PBDTTT-EFT under light and air exposure especially in its blend with PC71BM. Here we report an anti-solvent treatment which can significantly improve device stability while maintaining a device efficiency of ~ 9%. The morphological changes after anti-solvent treatment are also investigated by a combination of synchrotron based techniques.
Qiao, H, Yuan, J, Xu, Z, Chen, C, Lin, S, Wang, Y, Song, J, Liu, Y, Khan, Q, Hoh, HY & others 2015, 'Broadband Photodetectors Based on Graphene-Bi2Te3 Heterostructure', ACS Nano, vol. 9, no. 2, pp. 1186-1194.View/Download from: Publisher's site
Xu, Z-Q, Zhang, Y, Lin, S, Zheng, C, Zhong, YL, Xia, X, Li, Z, Sophia, PJ, Fuhrer, MS, Cheng, Y-B & others 2015, 'Synthesis and transfer of large-area monolayer WS2 crystals: moving toward the recyclable use of sapphire substrates', ACS nano, vol. 9, pp. 6178-6187.View/Download from: Publisher's site
Zheng, C, Xu, Z-Q, Zhang, Q, Edmonds, MT, Watanabe, K, Taniguchi, T, Bao, Q & Fuhrer, MS 2015, 'Profound effect of substrate hydroxylation and hydration on electronic and optical properties of monolayer MoS2', Nano Letters, pp. 3096-3102.View/Download from: Publisher's site
Atomic force microscopy, Kelvin probe force microscopy, and scanning photoluminescence spectroscopy image the progressive postgrowth hydroxylation and hydration of atomically flat Al2O3(0001) under monolayer MoS2, manifested in large work function shifts (100 mV) due to charge transfer (>1013 cm–2) from the substrate and changes in PL intensity, energy, and peak width. In contrast, trapped water between exfoliated graphene and Al2O3(0001) causes surface potential and doping changes one and two orders of magnitude smaller, respectively, and MoS2 grown on hydrophobic hexagonal boron nitride is unaffected by water exposure.
Si, KJ, Sikdar, D, Chen, Y, Eftekhari, F, Xu, ZQ, Tang, Y, Xiong, W, Guo, PZ, Zhang, S, Lu, Y, Bao, QL, Premaratne, M & Cheng, WL 2014, 'Giant Plasmene Nanosheets, Nanoribbons, and Origami', ACS Nano, vol. 8, no. 11, pp. 11086-11093.View/Download from: Publisher's site
We introduce Plasmene— in analogy to graphene—as free-standing, one-particle-thick, superlattice sheets of nanoparticles (“meta-atoms”) from the “plasmonic periodic table”, which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75 000. In conjunction with top–down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.
Shi, A-L, Li, Y-Q, Xu, Z-Q, Sun, F-Z, Li, J, Shi, X-B, Wei, H-X, Lee, S-T, Kera, S, Ueno, N & others 2013, 'Inverted polymer solar cells integrated with small molecular electron collection layer', Organic Electronics, vol. 14, pp. 1844-1851.View/Download from: Publisher's site
Sun, F-Z, Shi, A-L, Xu, Z-Q, Wei, H-X, Li, Y-Q, Lee, S-T & Tang, J-X 2013, 'Efficient inverted polymer solar cells with thermal-evaporated and solution-processed small molecular electron extraction layer', Applied Physics Letters, vol. 102, pp. 1-4.View/Download from: Publisher's site
Efficient inverted polymer solar cell is reported upon by integrating with a small molecular 1,3,5-
tri(phenyl-2-benzimi-dazolyl)-benzene (TPBi) electron extraction layer (EEL) at low processing
temperature with thermal-evaporation and solution-process, resulting in the power conversion
efficiencies of 3.70% and 3.47%, respectively. The potential of TPBi as an efficient EEL is
associated with its suitable electronic energy level for electron extraction and hole blocking
from the active layer to the indium tin oxide cathode.
Cheng, P-P, Ma, G-F, Li, J, Xiao, Y, Xu, Z-Q, Fan, G-Q, Li, Y-Q, Lee, S-T & Tang, J-X 2012, 'Plasmonic backscattering enhancement for inverted polymer solar cells', Journal of Materials Chemistry, vol. 22, pp. 22781-22787.View/Download from: Publisher's site
Fan, G-Q, Zhuo, Q-Q, Zhu, J-J, Xu, Z-Q, Cheng, P-P, Li, Y-Q, Sun, X-H, Lee, S-T & Tang, J-X 2012, 'Plasmonic-enhanced polymer solar cells incorporating solution-processable Au nanoparticle-adhered graphene oxide', Journal of Materials Chemistry, vol. 22, pp. 15614-15619.View/Download from: Publisher's site
Li, J, Bao, Q-Y, Wei, H-X, Xu, Z-Q, Yang, J-P, Li, Y-Q, Lee, S-T & Tang, J-X 2012, 'Role of transition metal oxides in the charge recombination layer used in tandem organic photovoltaic cells', Journal of Materials Chemistry, vol. 22, pp. 6285-6290.View/Download from: Publisher's site
The mechanism of charge recombination in transition metal oxide-based interconnectors for tandem organic photovoltaic cells is investigated, where the interconnector is composed of an abrupt heterointerface between a Mg-doped 4,7-diphenyl-1,10-phenanthroline (Mg:BPhen) layer and a MoO3 film. Based on the results of the interface energetics determined by ultraviolet photoelectron spectroscopy, as well as the corresponding device characteristics, it is revealed that the MoO3 layer pronouncedly modifies the energy level alignment of the interconnector, which is beneficial for the charge recombination process at the interface between MoO3 and the adjacent donor material for electrons and holes injected from stacked subcells. The incorporation of Mg:BPhen is essential for the conduction of the generated electrons from the bottom subcell into the conduction band of MoO3.
Xiao, Y, Yang, JP, Cheng, PP, Zhu, JJ, Xu, ZQ, Deng, YH, Lee, ST, Li, YQ & Tang, JX 2012, 'Surface plasmon-enhanced electroluminescence in organic light-emitting diodes incorporating Au nanoparticles', Applied Physics Letters, vol. 100, pp. 013308-013308.View/Download from: Publisher's site
Xu, Z-Q, Yang, J-P, Sun, F-Z, Lee, S-T, Li, Y-Q & Tang, J-X 2012, 'Efficient inverted polymer solar cells incorporating doped organic electron transporting layer', Organic Electronics, vol. 13, pp. 697-704.View/Download from: Publisher's site
Wei, HX, Li, J, Cai, Y, Xu, ZQ, Lee, ST, Li, YQ & Tang, JX 2011, 'Electronic structures of planar and mixed C 70/CuPc heterojunctions in organic photovoltaic devices', Organic Electronics, vol. 12, pp. 1422-1428.View/Download from: Publisher's site
Understanding the electronic structures of organic donor–acceptor heterojunction is of pronounced importance for the optimization of organic photovoltaic cells. Here, the detailed electronic structures of a planar fullerene (C70)/copper phthalocyanine (CuPc) bilayer and a mixed C70:CuPc bulk heterojunction (BHJ) have been studied via in situ photoemission spectroscopy. The results show that the energy level alignment by lining up separately observed energy levels of individual organic materials is not valid for these organic heterojunctions. The energy offset between the highest occupied molecular orbital of donor-like CuPc and the lowest unoccupied molecular orbital of acceptor-like C70, which is regarded as the origin of open-circuit voltage (VOC), is found to increase from 0.55 eV in the bilayer structure to 0.8 eV in the BHJ, which is possibly associated with the polarizability changes of C70 and CuPc molecules in the BHJs. This change is confirmed by the VOC variation in devices, where the VOC dramatically increased from 0.35 to 0.46 V by replacing the C70/CuPc bilayer with C70:CuPc BHJ. The thermal annealing effect on the mixed C70:CuPc BHJ reveals vertical phase separation, resulting in inhomogeneous concentration distribution in profile.
Xu, ZQ, Li, J, Yang, JP, Cheng, PP, Zhao, J, Lee, ST, Li, YQ & Tang, JX 2011, 'Enhanced performance in polymer photovoltaic cells with chloroform treated indium tin oxide anode modification', Applied Physics Letters, vol. 98, pp. 253303-253303.View/Download from: Publisher's site
Xu, Z-Q, Sun, F-Z, Li, J, Lee, S-T, Li, Y-Q & Tang, J-X 2011, 'Irradiation-induced molecular dipole reorientation in inverted polymer solar cell using small molecular electron collection layer', Applied Physics Letters, vol. 99, pp. 203301-203301.View/Download from: Publisher's site
Zhu, J-J, Xu, Z-Q, Fan, G-Q, Lee, S-T, Li, Y-Q & Tang, J-X 2011, 'Inverted polymer solar cells with atomic layer deposited CdS film as an electron collection layer', Organic Electronics, vol. 12, pp. 2151-2158.View/Download from: Publisher's site
An efficient inverted polymer solar cell (PSC) is reported by employing an atomic layer deposited (ALD) cadmium sulfide (CdS) film between the indium tin oxide (ITO) cathode and the photoactive layer as the electron collection layer (ECL), on which a active layer is composed of a blended poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction. As determined by photoelectron spectroscopy, the sulfur vacancy induces an n-type semiconducting property in the ALD-grown CdS films, and suitable energy level alignment at the ITO/CdS/PCBM interface is favorable to electron extraction through CdS to the ITO electrode. With the optimized CdS film thickness, the power conversion efficiency increases to 3.33%, with short-circuit current of 8.94 mA/cm2, open-circuit voltage of 0.61 V, and fill factor of 61.1% under AM1.5G 100 mW/cm2 irradiation.
Wei, HX, Li, J, Xu, ZQ, Cai, Y, Tang, JX & Li, YQ 2010, 'Thermal annealing-induced vertical phase separation of copper phthalocyanine: Fullerene bulk heterojunction in organic photovoltaic cells', Applied Physics Letters, vol. 97, pp. 083302-083302.
Yang, JP, Bao, QY, Xu, ZQ, Li, YQ, Tang, JX & Shen, S 2010, 'Light out-coupling enhancement of organic light-emitting devices with microlens array', Applied Physics Letters, vol. 97, pp. 223303-223303.
© 2017 Pan Stanford Publishing Pte. Ltd. All rights reserved. Polarization is a common property of electromagnetic waves. If the electric vectors are all along the same plane, the light is polarized; otherwise, the light is unpolarized. A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of unpolarized or mixed polarization into a beam with well-defined polarization, polarized light. All fiber in-line polarizer is one of the most important components in fiber-optic communication and sensor systems. The in-line devices are constructed by first polishing a short section of the lateral surface of the cladding to within the evanescent field around the fiber core, followed with cover with crystals [1,2], thin metal films [3-5], or graphene [6,7]. In a typical device, unpolarized 58or mix polarization light is coupled into and guided by the fiber from one end. The evanescent field of the light guided by the fiber interacts with the selective material covered on the polished parts along the fiber. The desired polarized light remains unaffected by the overlay material and is still guided by the fiber, while the light with unwanted polarization interacts with atop material and is no longer guided by the fiber. Only light with the desired polarization is coupled out in this manner.
Kianinia, M, Bradac, C, Sontheimer, B, Wang, F, Tran, TT, Nguyen, M, Kim, S, Xu, Z-Q, Jin, D, Schell, AW, Lobo, C, Aharanovich, I & Toth, M 2019, 'Enhanced Super-Resolution Imaging of Quantum Emitters in Hexagonal Boron Nitride', 2019 Compound Semiconductor Week (CSW), Compound Semiconductor Week, IEEE, Nara, Japan.View/Download from: 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. Amongst 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 which 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.
Nguyen, M, Kim, S, Tran, TT, Kianinia, M, Xu, Z, Wang, D, Yang, A, Aharonovich, I, Toth, M & Odom, T 2019, 'Nanophotonic integration of hexagonal boron nitride (Conference Presentation)', 2D Photonic Materials and Devices II, 2D Photonic Materials and Devices II, SPIE.View/Download from: Publisher's site
Mendelson, N, Nikolay, N, Xu, ZQ, Tran, TT, Sadzak, N, Bohm, F, Sontheimer, B, Benson, O, Toth, M & Aharonovich, I 2019, 'Tuning of Quantum Emitters in Hexagonal Boron Nitride', 2019 Conference on Lasers and Electro-Optics, CLEO 2019 - Proceedings.View/Download from: Publisher's site
© 2019 The Author(s) 2019 OSA. We demonstrate two different techniques to tune quantum emitters in hBN, achieving record tuning magnitudes for a solid state quantum emitter, as well as dynamic and reversible modulation of the emitters through both methods).
Mendelson, N, Nikolay, N, Xu, ZQ, Tran, TT, Sadzak, N, Böhm, F, Sontheimer, B, Benson, O, Toth, M & Aharonovich, I 2019, 'Tuning of quantum emitters in hexagonal boron nitride', Optics InfoBase Conference Papers.View/Download from: Publisher's site
© 2019 The Author(s). We demonstrate two different techniques to tune quantum emitters in hBN, achieving record tuning magnitudes for a solid state quantum emitter, as well as dynamic and reversible modulation of the emitters through both methods).
Opaluch, O, Nelz, R, Challier, M, Radtke, M, Slablab, A, Xu, Z, Aharonovich, I & Neu, E 2019, 'Toward coupling color centers in single crystal diamond to two-dimensional materials', 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019.View/Download from: Publisher's site
© 2019 IEEE. Individual nitrogen vacancy (NV) color centers in diamond are bright, photo-stable, atomic-sized dipole emitters . Consequently, they represent optimal candidates for novel scanning near field microscopy techniques . Here, NV centers form one member of a Forster Resonance Energy Transfer (FRET) pair. Due to their broadband emission (> 100 nm), NVs are versatile donors for FRET to systems absorbing in the near infrared spectral range. Highly-promising applications include, e.g., nanoscale imaging of fluorescent molecules or nanomaterials like graphene .
Toan, TT, Kianinia, M, Kim, S, Nguyen, M, Froch, J, Xu, Z-Q, Toth, M & Aharonovich, I 2018, 'Quantum Emitters in Flatland', 2018 INTERNATIONAL CONFERENCE ON OPTICAL MEMS AND NANOPHOTONICS (OMN), International Conference on Optical MEMS and Nanophotonics (OMN), IEEE, Lausanne, SWITZERLAND, pp. 1-2.
Lin, H, Xu, Z, Bao, Q & Jia, B 2016, 'Laser fabricated ultrathin flat lens in sub-nanometer thick monolayer transition metal dichalcogenides crystal', CLEO: Science and Innovations, Optical Society of America, pp. SF2E-4.
Xu, Z, Chen, C, Wu, SQY, Wang, B, Teng, J, Zhang, C & Bao, Q 2013, 'Graphene-polymer multilayer heterostructure for terahertz metamaterials', SPIE Micro+ Nano Materials, Devices, and Applications, SPIE Micro+Nano Materials, Devices, and Applications, SPIE, Melbourne, Victoria, Australia, pp. 89230C-89230C.View/Download from: Publisher's site
The optical response and plasmon coupling between graphene sheets for graphene/polymer multilayer heterostructures with controlled separation were systematically investigated. Anomalous transmission of light was experimentally observed in mid-infrared range. The position of the broad passband in the transmission spectra was observed to red-shift with the increase of the number of layers.