(aka Sherif Abdulkader Tawfik)
Dr. Sherif earned his BSc degree in Computer Science from the American University in Cairo, and received his Masters of Physics in 2011 from the same university. After an extensive experience in the IT sector, he moved to Sydney, Australia where he received his PhD in Physics from the University of Sydney in 2017. He joined Prof. Mike Ford’s group and the IBMD at UTS as Postdoctoral Research Fellow since 2016. He has a proven record of applying, as well as going beyond, state-of-the-art first principles methods in critical areas of research in the fields of van der Waals heterostructures, photonics, machine learning for physics and chemistry, upconversion nanoparticles, catalysis, superconductivity and molecular electronics.
Sherif is interested in the application of first principles atomistic simulations for the discovery of new materials with interesting physical and chemical properties. More specifically:
- The application of first principle methods, combined with machine learning, for the ARC project: the application of density-functional theory and machine learning for the rapid screening of hybrid van der Waals heterostructures
- The development of theoretical methods for the determination of the Huang-Rhys factor and the PL lineshape of point defects
- The utlization first principles approaches for the prediction of sperconductivity in novel materials, as well as the exporation of the suerpconducting mechanisms in high-temperature superconductors
- The pursuit of highly accurate van der Waals correlation functionals for the prediction of binding properties of 2D materials
- The investigation of the surface properties of upconversion nanoparticles
Batmunkh, M., Shrestha, A., Bat-Erdene, M., Nine, M.J., Shearer, C.J., Gibson, C.T., Slattery, A.D., Tawfik, S.A., Ford, M.J., Dai, S., Qiao, S. & Shapter, J.G. 2018, 'Electrocatalytic Activity of a 2D Phosphorene-Based Heteroelectrocatalyst for Photoelectrochemical Cells.', Angewandte Chemie (International ed. in English), vol. 57, no. 10, pp. 2644-2647.View/Download from: Publisher's site
Research into efficient synthesis, fundamental properties, and potential applications of phosphorene is currently the subject of intense investigation. Herein, solution-processed phosphorene or few-layer black phosphorus (FL-BP) sheets are prepared using a microwave exfoliation method and used in photoelectrochemical cells. Based on experimental and theoretical (DFT) studies, the FL-BP sheets are found to act as catalytically active sites and show excellent electrocatalytic activity for triiodide reduction in dye-sensitized solar cells. Importantly, the device fabricated based on the newly designed cobalt sulfide (CoSx ) decorated nitrogen and sulfur co-doped carbon nanotube heteroelectrocatalyst coated with FL-BP (FL-BP@N,S-doped CNTs-CoSx ) displayed an impressive photovoltaic efficiency of 8.31%, outperforming expensive platinum based cells. This work paves the way for using phosphorene-based electrocatalysts for next-generation energy-storage systems.
Gould, T., Johnson, E.R. & Tawfik, S.A. 2018, 'Are dispersion corrections accurate outside equilibrium? A case study on benzene', BEILSTEIN JOURNAL OF ORGANIC CHEMISTRY, vol. 14, pp. 1181-1191.View/Download from: Publisher's site
Tawfik, S.A., Gould, T., Stampfl, C. & Ford, M.J. 2018, 'Evaluation of van der Waals density functionals for layered materials', Physical Review Materials, vol. 2, no. 3.View/Download from: Publisher's site
Duong, H.T.T., Chen, Y., Tawfik, S.A., Wen, S., Parviz, M., Shimoni, O. & Ab, D.J. 2018, 'Systematic investigation of functional ligands for colloidal stable upconversion nanoparticles†', RSC Advances, vol. 8, no. 9, pp. 4842-4849.View/Download from: Publisher's site
© The Royal Society of Chemistry 2018. Despite intense efforts on surface functionalization to generate hydrophilic upconversion nanoparticles (UCNPs), long-term colloidal stability in physiological buffers remains a major concern. Here we quantitatively investigate the competitive adsorption of phosphate, carboxylic acid and sulphonic acid onto the surface of UCNPs and study their binding strength to identify the best conjugation strategy. To achieve this, we designed and synthesized three di-block copolymers composed of poly(ethylene glycol) methyl ether acrylate and a polymer block bearing phosphate, carboxylic or sulphonic acid anchoring groups prepared by an advanced polymerization technique, Reversible Addition Fragmentation Chain Transfer (RAFT). Analytical tools provide the evidence that phosphate ligands completely replaced all the oleic acid capping molecules on the surface of the UCNPs compared with incomplete ligand exchange by carboxylic and sulphonic acid groups. Meanwhile, simulated quantitative adsorption energy measurements confirmed that among the three functional groups, the calculated adsorption strength for phosphate anchoring ligands is higher which is in good agreement with experimental results regarding the best colloidal stability, especially in phosphate buffer solution. This finding suggests that polymers with multiple anchoring negatively charged phosphate moieties provide excellent colloidal stability for lanthanide ion-doped luminescent nanoparticles for various potential applications.
Ren, W., Wen, S., Tawfik, S.A., Su, Q.P., Lin, G., Ju, L.A., Ford, M.J., Ghodke, H., van Oijen, A.M. & Jin, D. 2018, 'Anisotropic functionalization of upconversion nanoparticles.', Chemical science, vol. 9, no. 18, pp. 4352-4358.View/Download from: UTS OPUS or Publisher's site
Despite significant advances toward accurate tuning of the size and shape of colloidal nanoparticles, the precise control of the surface chemistry thereof remains a grand challenge. It is desirable to conjugate functional bio-molecules onto the selected facets of nanoparticles owing to the versatile capabilities rendered by the molecules. We report here facet-selective conjugation of DNA molecules onto upconversion nanoparticles via ligand competition reaction. Different binding strengths of phosphodiester bonds and phosphate groups on DNA and the surfactant molecules allow one to create heterogeneous bio-chemistry surface for upconversion nanoparticles. The tailored surface properties lead to the formation of distinct self-assembly structures. Our findings provide insight into the interactions between biomolecules and nanoparticles, unveiling the potential of using nanoparticles as fundamental building blocks for creating self-assembled nano-architectures.
Zhou, J., Wen, S., Liao, J., Clarke, C., Tawfik, S.A., Ren, W., Mi, C., Wang, F. & Jin, D. 2018, 'Activation of the surface dark-layer to enhance upconversion in a thermal field', Nature Photonics, vol. 12, no. 3, pp. 154-158.View/Download from: Publisher's site
© 2018 The Author(s). Thermal quenching, in which light emission experiences a loss with increasing temperature, broadly limits luminescent efficiency at higher temperature in optical materials, such as lighting phosphors 1-3 and fluorescent probes 4-6 . Thermal quenching is commonly caused by the increased activity of phonons that leverages the non-radiative relaxation pathways. Here, we report a kind of heat-favourable phonons existing at the surface of lanthanide-doped upconversion nanomaterials to combat thermal quenching. It favours energy transfer from sensitizers to activators to pump up the intermediate excited-state upconversion process. We identify that the oxygen moiety chelating Yb 3+ ions, [YbO], is the key underpinning this enhancement. We demonstrate an approximately 2,000-fold enhancement in blue emission for 9.7 nm Yb 3+ -Tm 3+ co-doped nanoparticles at 453 K. This strategy not only provides a powerful solution to illuminate the dark layer of ultra-small upconversion nanoparticles, but also suggests a new pathway to build high-efficiency upconversion systems.
Fronzi, M., Tawfik, S.A., Stampfl, C. & Ford, M.J. 2018, 'Magnetic properties of stoichiometric and defective Co9S8.', Physical chemistry chemical physics : PCCP, vol. 20, no. 4, pp. 2356-2362.View/Download from: Publisher's site
In this paper, we present a detailed study of the stoichiometric and reduced Co9S8 pentlandite magnetic properties, based on density functional theory. We analyze both its geometry and electronic properties and show that only by the inclusion of the Hubbard term it is possible to correctly describe d-d splitting, which is necessary to accurately characterize the Co9S8 spin configuration and its antiferromagnetic nature. We also analyze the effect of sulfur vacancies and predict the formation of ferromagnetic clusters that give local ferromagnetic character to non-stoichiometric Co9S8, which may explain the contradictory experimental results reported in the literature.
Kratzer, P., Tawfik, S.A., Cui, C. & Stampfl, C. 2017, 'Detection of adsorbed transition-metal porphyrins by spin-dependent conductance of graphene nanoribbon', RSC Advances, vol. 7, pp. 29112-29121.View/Download from: UTS OPUS or Publisher's site
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2017, 'TDDFT Study of the Optical Excitation of Nucleic Acid Bases-C60 Complexes.', The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory, vol. 121, no. 47, pp. 9058-9063.View/Download from: UTS OPUS or Publisher's site
The potential of C60 as a nucleic acid base (NAB) optical sensor is theoretically explored. We investigate the adsorption of four NABs, namely, adenine, cytosine, guanine, and thymine, on C60 in the gas phase. For the optimal NAB@C60 adsorption configurations, obtained using a dispersion-corrected density functional, we calculate the vis-near-ultraviolet optical response using time-dependent density functional theory. While the isolated C60 and NAB molecules do not exhibit visible optical excitation, we find that C60/NAB conjugation gives rise to distinct spectral features in the visible range. These results suggest that C60 conjugation can be applied for photodetection of individual NABs.
Tawfik, S.A., Weston, L., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2017, 'Near-Perfect Spin Filtering and Negative Differential Resistance in an Fe(II)S Complex.', The journal of physical chemistry letters, vol. 8, no. 10, pp. 2189-2194.View/Download from: UTS OPUS or Publisher's site
Density functional theory and nonequilibrium Green's function calculations have been used to explore spin-resolved transport through the high-spin state of an iron(II)sulfur single molecular magnet. Our results show that this molecule exhibits near-perfect spin filtering, where the spin-filtering efficiency is above 99%, as well as significant negative differential resistance centered at a low bias voltage. The rise in the spin-up conductivity up to the bias voltage of 0.4 V is dominated by a conductive lowest unoccupied molecular orbital, and this is accompanied by a slight increase in the magnetic moment of the Fe atom. The subsequent drop in the spin-up conductivity is because the conductive channel moves to the highest occupied molecular orbital, which has a lower conductance contribution. This is accompanied by a drop in the magnetic moment of the Fe atom. These two exceptional properties, and the fact that the onset of negative differential resistance occurs at low bias voltage, suggests the potential of the molecule in nanoelectronic and nanospintronic applications.
Bat-Erdene, M., Batmunkh, M., Tawfik, S.A., Fronzi, M., Ford, M.J., Shearer, C.J., Yu, L.P., Dadkhah, M., Gascooke, J.R., Gibson, C.T. & Shapter, J.G. 2017, 'Efficiency Enhancement of Single-Walled Carbon Nanotube-Silicon Heterojunction Solar Cells Using Microwave-Exfoliated Few-Layer Black Phosphorus', Advanced Functional Materials, vol. 27, no. 48.View/Download from: UTS OPUS or Publisher's site
© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Carbon nanotube-silicon (CNT-Si)-based heterojunction solar cells (HJSCs) are a promising photovoltaic (PV) system. Herein, few-layer black phosphorus (FL-BP) sheets are produced in N-methyl-2-pyrrolidone (NMP) using microwave-assisted liquid-phase exfoliation and introduced into the CNTs-Si-based HJSCs for the first time. The NMP-based FL-BP sheets remain stable after mixing with aqueous CNT dispersion for device fabrication. Due to their unique 2D structure and p-type dominated conduction, the FL-BP/NMP incorporated CNT-Si devices show an impressive improvement in the power conversion efficiency from 7.52% (control CNT-Si cell) to 9.37%. Our density-functional theory calculation reveals that lowest unoccupied molecular orbital (LUMO) of FL-BP is higher in energy than that of single-walled CNT. Therefore, we observed a reduction in the orbitals localized on FL-BP upon highest occupied molecular orbital to LUMO transition, which corresponds to an improved charge transport. This study opens a new avenue in utilizing 2D phosphorene nanosheets for next-generation PVs.
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.
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2016, 'Communication: Electrical rectification of C59N: The role of anchoring and doping sites.', The Journal of chemical physics, vol. 144, no. 2, p. 021101.View/Download from: Publisher's site
Based on the nonequilibrium Green's function formalism and density-functional theory, we investigate the onset of electrical rectification in a single C59N molecule in conjunction with gold electrodes. Our calculations reveal that rectification is dependent upon the anchoring of the Au atom on C59N; when the Au electrode is singly bonded to a C atom (labeled here as A), the system does not exhibit rectification, whereas when the electrode is connected to the C-C bridge site between two hexagonal rings (labeled here as B), transmission asymmetry is observed, where the rectification ratio reaches up to 2.62 at ±1 V depending on the N doping site relative to the anchoring site. Our analysis of the transmission mechanism shows that N doping of the B configuration causes rectification because more transmission channels are available for transmission in the B configuration than in the A configuration.
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2016, 'Endohedral metallofullerenes, M@C60 (M = Ca, Na, Sr): selective adsorption and sensing of open-shell NOx gases.', Physical chemistry chemical physics : PCCP, vol. 18, no. 31, pp. 21315-21321.View/Download from: Publisher's site
Based on density-functional theory and non-equilibrium Green's function calculations, we demonstrate that endohedral metallofullerenes (EMFs) are reactive to open-shell gases, and therefore have the potential application as selective open-shell gas sensors. The adsorption of eight gas species (CO, H2O, H2S, NO2, NO, SO2, O2 and NH3) on three EMFs (M@C60, M = Ca, Na and Sr) shows that the adsorption energies of the EMFs towards NO2 and NO are significantly higher than the closed-shell species. Moreover, the high selectivity appears relatively insensitive to the inserted metal atoms. The calculated current-voltage characteristics of gold-M@C60-gold structures (M = Ca, Na) show that the adsorption of NO2 leads to significant change in conductivity, suggesting a potential application as an EMF gas resistive sensing device.
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2016, 'Enhanced oscillatory rectification and negative differential resistance in pentamantane diamondoid-cumulene systems.', Nanoscale, vol. 8, no. 6, pp. 3461-3466.View/Download from: Publisher's site
We propose a new functionality for diamondoids in nanoelectronics. Based on the nonequilibrium Green's function formalism and density functional theory, we reveal that when attached to gold electrodes, the pentamantane-cumulene molecular junction exhibits large and oscillatory rectification and negative differential resistance (NDR) - depending on the number of carbon atoms in cumulene (Cn). When n is odd rectification is greatly enhanced where the rectification ratio can reach 180 and a large negative differential resistance peak current of 3 A. This oscillatory behavior is well rationalised in terms of the occupancy of the carbon 2p states in Cn. Interestingly, different layers of C atoms in the pentamantane molecule have different contributions to transmission. The first and third layers of C atoms in pentamantane have a slight contribution to rectification, and the fifth and sixth layers have a stronger contribution to both rectification and NDR. Thus, our results suggest potential avenues for controlling their functions by chemically manipulating various parts of the diamondoid molecule, thus extending the applications of diamondoids in nanoscale integrated circuits.
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2016, 'Large spin-filtering effect in Ti-doped defective zigzag graphene nanoribbon.', Physical chemistry chemical physics : PCCP, vol. 18, no. 24, pp. 16224-16228.View/Download from: Publisher's site
Through first-principles calculations using the nonequilibrium Green's function formalism together with density functional theory, we study the conductance of double-vacancy zigzag graphene nanoribbons doped with four transition metal atoms Ti, V, Cr and Fe. We show that Ti doping induces large spin-filtering with an efficiency in excess of 90% for bias voltages below 0.5 V, while the other metal adatoms do not induce large spin filtering. This is despite the fact that the Ti dopant possesses small spin moment, while large moments reside on V, Cr and Fe dopants. Our analysis shows that the suppression of transmission in the spin-down channel in the Ti-doped graphene nanoribbon, thus the large spin filtering efficiency, is due to transmission anti-resonance arising from destructive quantum interference. These findings suggest that the decoration of graphene with titanium, and possibly other transition metals, can act as effective spin filters for nanospintronic applications.
Tawfik, S.A., El-Sheikh, S.M. & Salem, N.M. 2016, 'Erratum: Corrigendum: First principles calculation of field emission from nanostructures using time-dependent density functional theory: A simplified approach (Physica E: Low-Dimensional Systems and Nanostructures', Physica E: Low-Dimensional Systems and Nanostructures, vol. 83, p. 511.View/Download from: Publisher's site
© 2016 Elsevier B.V. Recently we have become aware that the description of the quantum wave functions in Sec. 2.1 is incorrect. In the published version of the paper, we have stated that the states are expanded in terms of plane waves. However, the correct description of the quantum states in the context of the real space implementation (using the Octopus code) is that states are represented by discrete points in a real space grid.
Tawfik, S.A., Cui, X.Y., Carter, D.J., Ringer, S.P. & Stampfl, C. 2015, 'Sensing sulfur-containing gases using titanium and tin decorated zigzag graphene nanoribbons from first-principles', PHYSICAL CHEMISTRY CHEMICAL PHYSICS, vol. 17, no. 10, pp. 6925-6932.View/Download from: UTS OPUS or Publisher's site
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2015, 'High On/Off Conductance Switching Ratio via H-Tautomerization in Quinone.', Journal of chemical theory and computation, vol. 11, no. 9, pp. 4154-4158.View/Download from: UTS OPUS or Publisher's site
Through first-principles electron transport simulations using the nonequilibrium Green's function formalism together with density functional theory, we show that, upon H-tautomerization, a simple derivative of quinone can act as a molecular switch with high ON/OFF ratio, up to 70 at low bias voltage. This switching behavior is explained by the quantum interference effect, where the positional change of hydrogen atoms causes the energies of the transmission channels to overlap. Our results suggest that this molecule could have potential applications as an effective switching device.
Tawfik, S.A., Cui, X.Y., Ringer, S.P. & Stampfl, C. 2015, 'Multiple CO<inf>2</inf> capture in stable metal-doped graphene: A theoretical trend study', RSC Advances, vol. 5, no. 63, pp. 50975-50982.View/Download from: UTS OPUS or Publisher's site
© The Royal Society of Chemistry 2015. Identifying stable systems with high CO < inf > 2 < /inf > adsorption capacity is an essential goal in CO < inf > 2 < /inf > capture and storage technologies. We have carried out a comprehensive first-principles study to explore the CO < inf > 2 < /inf > capture capacity of 16 representative metal-doped graphene systems where the metal dopants can be stabilized by single- and double-vacancies. The maximum number of adsorbed CO < inf > 2 < /inf > molecules was determined by a combination of adsorption energy and bond distance criteria. Generally, while the double-vacancy can bind metal dopants more strongly than the single-vacancy, single-vacancy graphene with metal dopants are better sorbents, with each Ca, Sc and Y dopant binding up to 5 CO < inf > 2 < /inf > molecules. CO < inf > 2 < /inf > capture involves significant charge transfer between the CO < inf > 2 < /inf > molecule and the dopant-vacancy complexes, where defective graphene acts as a charge reservoir for binding CO < inf > 2 < /inf > molecules. Some systems are predicted to involve the formation of a bent CO < inf > 2 < /inf > anion. Ca-doped single- and double-vacancy graphene systems, however, readily form oxides upon reaction with CO < inf > 2 < /inf > , thus they are less reusable for CO < inf > 2 < /inf > capture.
© Springer Science+Business Media New York 2013. We analytically study the interplay between group velocity dispersion and material dispersion due to femtosecond ultrafast pulse inside plasmonic slot waveguides with nonlinear dielectric core. The analytic investigation of the role of the core nonlinearity on pulse propagation has been investigated. Interestingly, our model shows that the focusing and defocusing effects of the material can be revered if the material is confined inside the core of a plas-monic slot. We confirm our analytical results with nonlinear finite difference time domain (FDTD) simulations.
Tawfik, S.A. 2012, 'Localized states in an ultracold atomic gas trapped in a bichromatic potential: The effect of a time-varying phase', Communications in Nonlinear Science and Numerical Simulation, vol. 17, no. 9, pp. 3552-3557.View/Download from: UTS OPUS or Publisher's site
Localization phenomena observed in an ultracold atomic gas trapped in a bichromatic optical lattice were found to be sensitive to the degree of lattice disorder, and are affected by the extent of interatomic interaction. In this work, I shall discuss the dependence of localization on the phase difference between the two superimposed optical waves. The step and sinusoidal phase functions in region III identified in the work by Larcher et al. , are studied. A step varying phase mildly distorts localization, whereas a sinusoidally varying phase will take the system to a unique form of localization, which we call logarithmic localization. © 2012 Elsevier B.V.
Tawfik, S.A., El-Sheikh, S.M. & Salem, N.M. 2011, 'First principles calculation of field emission from carbon nanotubes with nitrogen and boron doping', Physica E: Low-Dimensional Systems and Nanostructures, vol. 44, no. 1, pp. 111-114.View/Download from: UTS OPUS or Publisher's site
We investigate the field emission properties of nitrogenated and boronated carbon nanotubes using time-dependent density functional theory, where the wave function propagation is performed using the CrankNicholson algorithm. We extract the currentvoltage characteristics of the emitted electrons from nanotubes with different doping configurations. We found that boron doping alone either impedes, or slightly enhances, field emission. Nitrogen generally enhances the emission current, and the current is sensitive to the location of the nitrogen dopant in the nanotube. Doping with both nitrogen and boron will generally enhance emission, and the closer the nitrogen dopant is to the tip, the higher is the emitted current. The emitted charge cloud from nitrogen-doped carbon nanotubes, however, diffuse more than that from pristine ones, our simulations show the emergence of a branching structure from the charge cloud, which suggests that nitrogenated carbon nanotubes are less convenient for use in precision beam applications. © 2011 Elsevier B.V. All rights reserved.
Tawfik, S.A., El-Sheikh, S.M. & Salem, N.M. 2011, 'First principles calculation of field emission from nanostructures using time-dependent density functional theory: A simplified approach', Physica E: Low-Dimensional Systems and Nanostructures, vol. 43, no. 7, pp. 1360-1364.View/Download from: UTS OPUS or Publisher's site
We introduce a new simplified method for computing the electron field emission current in short carbon nanotubes and graphene sheets using ab-initio computation in slab-periodic simulation cells. The evolution of the wave functions using Time-Dependent Density Functional Theory is computed by utilizing the CrankNicholson propagator and using the Octopus code (Castro et al., 2006 ), where we skip the wave function relaxation step elaborated by Han et al. (2002)  , and apply a norm-conserving wave propagation method instead of the norm-nonconserving seventh-order Taylor Expansion method used by Araidai et al. (2004) . Our method is mainly geared towards reducing the time it takes to compute the wave function propagation and enhancing the calculation precision. We found that in pristine carbon nanotubes, the emitted charge tends to emerge mostly from electrons that are concentrated at the nanotube tip region. The charge beam concentrates into specific channel structures, showing the utility of carbon nanotubes in precision emission applications. © 2011 Elsevier B.V. All rights reserved.
Reimers, J.R., Tawfik, S.A. & Ford, M.J., 'Van der Waals forces control ferroelectric-antiferroelectric ordering in ABP2X6 laminar materials'.
We show how van der Waals (vdW) forces outcompete covalent and ionic forces
to control ferroelectric ordering in CuInP2S6 nanoflakes as well as in CuInP2S6
and CuBiP2Se6 crystals. While the self-assembly of these 2D layered materials
is clearly controlled by vdW effects, this result indicates that the internal
layer structure is also similarly controlled. Using up to 14 first-principles
computational methods, we predict that the bilayers of both materials should be
antiferroelectric. However, antiferroelectric nanoflakes and bulk materials are
shown to embody two fundamentally different types of inter-layer interactions,
with vdW forces strongly favouring one and strongly disfavouring the other
compared to ferroelectric ordering. Strong specific vdW interactions involving
the Cu atoms control this effect. Thickness-dependent significant cancellation
of these two large opposing vdW contributions results in a small net effect
that interacts with weak ionic contributions to control ferroelectric ordering.
Tawfik, S.A., Reimers, J.R., Stampfl, C. & Ford, M.J., 'van der Waals forces control the internal chemical structure of monolayers within ABP2X6 lamellar materials'.
Following the recent demonstration that van der Waals forces control the
ferroelectric ordering of layers within nanoflakes and bulk samples of
CuBiP2Se6 and CuInP2S6, it is demonstrated that they also control the internal
geometrical structure of isolated monolayers of these materials. This internal
structure involves large displacements of the copper atoms, either normal to
the layer plane or else within the plane, that change its ligation environment.
In both cases, the van der Waals dispersion force out-competes traditional
bonding effects to control structure. However, we find that the aspects of the
dispersion force giving rise to each effect are uncorrelated: long range
effects control inter-layer ferroelectric ordering whereas short-range effects
control internal layer structure. These conclusions are drawn considering
predicted properties of monolayers, bilayers, and bulk materials obtained using
14 density-functional-theory based methods. While the different methods used
often predict starkly different quantitative results, they concur as to the
basic nature of ABP2X6 materials. Of the methods used, only the PBE-D3 and
optPBEvdW methods were found to predict a wide range of observed properties
without serious disparity. Finding optimal computational methods remains a
significant challenge for which the unusual multi-scale nature of the van der
Waals interactions in ABP2X6 materials provides demanding criteria.
Tawfik, S.A., Stampfl, C. & Ford, M.J., 'Superconductivity in intercalated buckled two-dimensional materials: KGe$_2$'.
Germanene has emerged as a novel two-dimensional material with various
interesting properties and applications. Here we report the possibility of
superconductivity in a stable potassium intercalated germanene compound,
KGe$_2$, with a transition temperature $T_c \sim 11$ K, and an electron-phonon
coupling of 1.9. Applying a 5\% tensile strain, which reduces the buckling
height by 4.5\%, leads to the reduction of the electron-phonon coupling by 11\%
and a slight increase in $T_c \sim 12$ K. That is, strong electron-phonon
coupling results from the buckled structure of the germanene layers. Despite
being an intercalated van der Waals material similar to intercalated graphite
superconductors, it does not possess an occupied interlayer state.
Metal-insulator-metal plasmonic waveguides (plasmonic slot waveguides, PSW) are known to offer high propagation lengths and confinement factors and have recently been gaining increasing attention in the literature. We analytically study the interplay between group velocity dispersion and self-phase modulation on ultrafast surface plasmon-polariton (SPP) pulse-reshaping for plasmonic-slot waveguides with nonlinear dielectric core. The analytic investigation of the role of the core nonlinearity on pulse propagation has, to our knowledge, not been investigated in the literature. We correlate our analytical results with numerical FDTD simulations. © 2013 SPIE.