Showing papers by "Michael S. Fuhrer published in 2016"
TL;DR: A comprehensive overview of 2D and layered transition metal oxides can be found in this paper, where the fundamentals and applications of planar TMOs are discussed and future prospects and pathways to new developments are presented.
378 citations
TL;DR: The recent progress on host 2D materials, various intercalation species, andintercalation methods, as well as tunable properties and potential applications enabled by intercalated materials are comprehensively reviewed.
Abstract: 2D materials have attracted tremendous attention due to their unique physical and chemical properties since the discovery of graphene. Despite these intrinsic properties, various modification methods have been applied to 2D materials that yield even more exciting results in terms of tunable properties and device performance. Among all modification methods, intercalation of 2D materials has emerged as a particularly powerful tool: it provides the highest possible doping level and is capable of (ir)reversibly changing the phase of the material. Intercalated 2D materials exhibit extraordinary electrical transport as well as optical, thermal, magnetic, and catalytic properties, which are advantageous for optoelectronics, superconductors, thermoelectronics, catalysis and energy storage applications. The recent progress on host 2D materials, various intercalation species, and intercalation methods, as well as tunable properties and potential applications enabled by intercalation, are comprehensively reviewed.
327 citations
TL;DR: In this paper, the authors investigated the strain relaxation of triangular WS2 deposited on polydimethylsiloxane substrate and found that the pattern of wrinkles was dependent on the relative angle between an edge of the triangular WS 2 crystal and tensile strain direction.
Abstract: Strain-dependent electrical and optical properties of atomically thin transition metal dichalcogenides may be useful in sensing applications However, the question of how strain relaxes in atomically thin materials remains not well understood Herein, the strain relaxation of triangular WS2 deposited on polydimethylsiloxane substrate is investigated The photoluminescence of trions (X–) and excitons (X0) undergoes linear redshifts of ≈20 meV when the substrate tensile strain increases from 0 to 016 However, when the substrate strain further increases from 016 to 032, the redshifts cease due to strain relaxation in WS2 The strain relaxation occurs through formation of wrinkles in the WS2 crystal The pattern of wrinkles is found to be dependent on the relative angle between an edge of the triangular WS2 crystal and tensile strain direction Finite element simulations of the strain distribution inside the WS2 crystals explain the experimental observations
100 citations
TL;DR: This work shows the possibility of reversibly tuning the photoluminescence of single and odd-numbered multilayered MoS2 using high frequency sound wave coupling, and reveals several key fundamentals governing acousto-optic properties of odd-layeredMoS2 that can be implemented in future optical and electronic systems.
Abstract: By exploiting the very recent discovery of the piezoelectricity in odd-numbered layers of two-dimensional molybdenum disulfide (MoS2), we show the possibility of reversibly tuning the photoluminescence of single and odd-numbered multilayered MoS2 using high frequency sound wave coupling. We observe a strong quenching in the photoluminescence associated with the dissociation and spatial separation of electrons–holes quasi-particles at low applied acoustic powers. At the same applied powers, we note a relative preference for ionization of trions into excitons. This work also constitutes the first visual presentation of the surface displacement in one-layered MoS2 using laser Doppler vibrometry. Such observations are associated with the acoustically generated electric field arising from the piezoelectric nature of MoS2 for odd-numbered layers. At larger applied powers, the thermal effect dominates the behavior of the two-dimensional flakes. Altogether, the work reveals several key fundamentals governing acou...
88 citations
TL;DR: In this paper, Li et al. showed that electron-hole compensation is not the mechanism for extremely large nonsaturating magnetoresistance in semimetal (100 nm) exfoliated lattice.
Abstract: The recently discovered large nonsaturating magnetoresistance in semimetal $\mathrm{WT}{\mathrm{e}}_{2}$ may result from near-perfect electron-hole compensation, however recent reports question whether the compensation is adequate to explain the observations. Experiments on significantly uncompensated $\mathrm{WT}{\mathrm{e}}_{2}$ are needed. We measure magnetoresistance ${\ensuremath{\rho}}_{xx}(H)$, Hall effect ${\ensuremath{\rho}}_{xy}(H)$, and an electrolyte gating effect in thin (100 nm) exfoliated $\mathrm{WT}{\mathrm{e}}_{2}$. We observe ${\ensuremath{\rho}}_{xy}(H)$ linear in $H$ at low $H$ consistent with near-perfect compensation, however ${\ensuremath{\rho}}_{xy}(H)$ becomes nonlinear and changes sign with increasing $H$, implying a breakdown of compensation. We break compensation more significantly by using an electrolytic gate for highly electron-doped $\mathrm{WT}{\mathrm{e}}_{2}$ with Li. In gated $\mathrm{WT}{\mathrm{e}}_{2}$ the nonsaturating ${\ensuremath{\rho}}_{xx}(H)$ persists to $H=14\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, even with significant deviation from perfect electron-hole compensation $(p/n=0.84)$ where the two-band model predicts a saturating ${\ensuremath{\rho}}_{xx}(H)$. Our results indicate electron-hole compensation is not the mechanism for extremely large magnetoresistance in $\mathrm{WT}{\mathrm{e}}_{2}$; alternative explanations are needed.
65 citations
TL;DR: Highly efficient broadband thermal radiation from reduced graphene oxide (RGO) paper mixed with single-walled carbon nanotubes (CNTs) is reported.
Abstract: Highly efficient broadband thermal radiation from reduced graphene oxide (RGO) paper mixed with single-walled carbon nanotubes (CNTs) is reported. These RGO-CNT paper ribbons routinely reach 3000 K before failure, with some samples exceeding 3300 K, higher than any other carbon nanomaterial. Excellent performance is achieved, with ≈90% radiation efficiency, 200 000 on/off cycles, and stable operation for more than 50 hours.
58 citations
TL;DR: Structural and electrical characterization of large-area epitaxial thin films of TDS Na3Bi on single crystal Al2O3[0001] substrates are reported, indicating that some aspects of the low field magnetotransport (μB < 1) in this TDS are yet to be explained.
Abstract: Topological Dirac semimetals (TDS) are three-dimensional analogues of graphene, with linear electronic dispersions in three dimensions. Nanoscale confinement of TDSs in thin films is a necessary step toward observing the conventional-to-topological quantum phase transition (QPT) with increasing film thickness, gated devices for electric-field control of topological states, and devices with surface-state-dominated transport phenomena. Thin films can also be interfaced with superconductors (realizing a host for Majorana Fermions) or ferromagnets (realizing Weyl Fermions or T-broken topological states). Here we report structural and electrical characterization of large-area epitaxial thin films of TDS Na3Bi on single crystal Al2O3[0001] substrates. Charge carrier mobilities exceeding 6,000 cm2/(V s) and carrier densities below 1 × 1018 cm–3 are comparable to the best single crystal values. Perpendicular magnetoresistance at low field shows the perfect weak antilocalization behavior expected for Dirac Fermion...
55 citations
Journal Article•
TL;DR: The positive piezoconductive effect observed in suspended bi- and multi-layer graphene can be understood as resulting from the strain-induced competition between interlayer coupling and intralayer transport, as confirmed by the numerical calculations based on the non-equilibrium Green's function method.
33 citations
TL;DR: Using 4D datasets in STEM from two specimens, monolayer MoS2 and bulk SrTiO3, this work demonstrates multiple STEM imaging modes on a quantitative absolute intensity scale, including phase reconstruction of the transmission function via differential phase contrast imaging.
28 citations
TL;DR: High-temperature transport and high-resolution photoelectron spectroscopy are performed on 20 nm thin film topological Dirac semimetal Na3Bi grown by molecular beam epitaxy to investigate the behavior of F4-TCNQ on the Na3 Bi surface.
Abstract: We perform low-temperature transport and high-resolution photoelectron spectroscopy on 20 nm thin film topological Dirac semimetal Na3Bi grown by molecular beam epitaxy. We demonstrate efficient electron depletion ∼1013 cm–2 of Na3Bi via vacuum deposition of molecular F4-TCNQ without degrading the sample mobility. For samples with low as-grown n-type doping (1 × 1012 cm–2), F4-TCNQ doping can achieve charge neutrality and even a net p-type doping. Photoelectron spectroscopy and density functional theory are utilized to investigate the behavior of F4-TCNQ on the Na3Bi surface.
27 citations
TL;DR: In this article, the phase of the transmission function can be related to magnetic and electric fields within the specimen and is sensitive to lighter elements, and the algorithm can be implemented using fast Fourier transforms, and this provides the possibility of reconstructing specimen transmission functions in real time.
Abstract: We introduce an algorithm for the reconstruction of the complex transmission function of a specimen using segmented detectors in scanning transmission electron microscopy geometry. The phase of the transmission function can be related to magnetic and electric fields within the specimen and is sensitive to lighter elements. The technique is demonstrated for simulated data and also using experimental datasets taken from a ${\mathrm{MoS}}_{2}$ monolayer and a ${\mathrm{SrTiO}}_{3}$ crystal. We present an extension to the algorithm to account for uncertainties in the illuminating probe. The algorithm can be implemented using fast Fourier transforms, and this provides the possibility of reconstructing specimen transmission functions in real time.
TL;DR: The conclusion is that intercalation does not occur during the process and that exfoliation is instead facilitated by the reduction of Mo centres leading to the exfolation of substoichiometric bilayers with distorted lattices.
Abstract: Substoichiometric molybdenum disulphide (MoSx) nanosheets are successfully synthesised following a novel reductive route using hydrazine salts. The resulting two dimensional crystals are found to be highly monodispersed in thickness, forming exclusively 1.9 ± 0.2 nm thick bilayers. The lateral dimensions of the nanosheets are governed by the precursor bulk particle's size. Exploring a range of hydrazine derivatives with various degrees of steric hindrance leads to the conclusion that intercalation does not occur during the process and that exfoliation is instead facilitated by the reduction of Mo centres leading to the exfoliation of substoichiometric bilayers with distorted lattices. The lattice distortion is found to be persistent across all samples with XPS analysis pointing towards a S to Mo ratio of 1.2. The resulting material features an electronic bandgap of 2.1 eV, which is wider than that of pristine monolayer MoS2 with relatively longer radiative decay time.
TL;DR: In this article, a solution-processable solid-state charge transport material composed of a blend of [Co(bpyPY4)](OTf)2 and Co(pyridin-2-yl)ethyl and trifluoromethanesulfonate anion is reported.
Abstract: Charge transport materials (CTMs) are traditionally inorganic semiconductors or metals. However, over the past few decades, new classes of solution-processable CTMs have evolved alongside new concepts for fabricating electronic devices at low cost and with exceptional properties. The vast majority of these novel materials are organic compounds and the use of transition metal complexes in electronic applications remains largely unexplored. Here, a solution-processable solid-state charge transport material composed of a blend of [Co(bpyPY4)](OTf)2 and Co(bpyPY4)](OTf)3 where bpyPY4 is the hexadentate ligand 6,6′-bis(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf− is the trifluoromethanesulfonate anion is reported. Surprisingly, these films exhibit a negative temperature coefficient of conductivity (dσ/dT) and non-Arrhenius behavior, with respectable solid-state conductivities of 3.0 S m−1 at room temperature and 7.4 S m−1 at 4.5 K. When employed as a CTM in a solid-state dye-sensitized solar cell, these largely amorphous, transparent films afford impressive solar energy conversion efficiencies of up to 5.7%. Organic–inorganic hybrid materials with negative temperature coefficients of conductivity generally feature extended flat π-systems with strong π–π interactions or high crystallinity. The lack of these features promotes [Co(bpyPY4)](OTf)2+ x films as a new class of CTMs with a unique charge transport mechanism that remains to be explored.
TL;DR: In this article, two research groups have, independently, fabricated and characterized a graphene-based device that is thought to be a promising platform for realizing an exciton condensate, but their measurements lay the groundwork for future searches.
Abstract: S uperfluids (fluids with zero viscosity) and superconductors (materials with zero resistance) have a common ingredient: bosons. These particles obey Bose-Einstein statistics, allowing a collection of them at low temperatures to collapse into a single quantummechanical state, or Bose-Einstein condensate. Bosons in superconductors consist of two paired electrons, but the pairing is weak and only occurs at low temperatures. In a quest to build devices that carry electricity with low dissipation at higher temperatures, researchers have therefore explored the possibility of engineering electrical condensates [1] out of strongly bound pairs of electrons and holes, or excitons. Now, two research groups have, independently, fabricated and characterized a graphene-based device that is thought to be a promising platform for realizing an exciton condensate [2, 3]. Neither group has yet found evidence for such a condensate—the ultimate goal of such experiments. But their measurements lay the groundwork for future searches.
TL;DR: In this paper, the authors report on successful tuning the electronic band structure of n-type Pr2−xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique.
Abstract: The occurrence of electrons and holes in n-type copper oxides has been achieved by chemical doping, pressure, and/or deoxygenation. However, the observed electronic properties are blurred by the concomitant effects such as change of lattice structure, disorder, etc. Here, we report on successful tuning the electronic band structure of n-type Pr2−xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique. Abnormal transport properties, such as multiple sign reversals of Hall resistivity in normal and mixed states, have been revealed within an electrostatic field in range of −2 V to + 2 V, as well as varying the temperature and magnetic field. In the mixed state, the intrinsic anomalous Hall conductivity invokes the contribution of both electron and hole-bands as well as the energy dependent density of states near the Fermi level. The two-band model can also describe the normal state transport properties well, whereas the carrier concentrations of electrons and holes are always enhanced or depressed simultaneously in electric fields. This is in contrast to the scenario of Fermi surface reconstruction by antiferromagnetism, where an anti-correlation is commonly expected.
TL;DR: A novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature reveals that near the charge neutral point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions.
Abstract: Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage-an effect that is unique to graphene.
TL;DR: In this article, high-resolution transmission electron microscopy analysis shows a morphologically degraded SiC/Si interface, while mechanical stress measurements indicate considerable relaxation of the interfacial stress.
Abstract: Epitaxial cubic silicon carbide on silicon is of high potential technological relevance for the integration of a wide range of applications and materials with silicon technologies, such as micro electro mechanical systems, wide-bandgap electronics, and graphene. The hetero-epitaxial system engenders mechanical stresses at least up to a GPa, pressures making it extremely challenging to maintain the integrity of the silicon carbide/silicon interface. In this work, we investigate the stability of said interface and we find that high temperature annealing leads to a loss of integrity. High–resolution transmission electron microscopy analysis shows a morphologically degraded SiC/Si interface, while mechanical stress measurements indicate considerable relaxation of the interfacial stress. From an electrical point of view, the diode behaviour of the initial p-Si/n-SiC junction is catastrophically lost due to considerable inter-diffusion of atoms and charges across the interface upon annealing. Temperature dependent transport measurements confirm a severe electrical shorting of the epitaxial silicon carbide to the underlying substrate, indicating vast predominance of the silicon carriers in lateral transport above 25 K. This finding has crucial consequences on the integration of epitaxial silicon carbide on silicon and its potential applications.
TL;DR: In this article, the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope were mapped to the Dirac point and shown to be significantly smaller than room temperature (nms = 4-6 meV = 40-70 K) and comparable to the highest quality graphene on h-BN.
Abstract: The close approach of the Fermi energy EF of a Dirac semimetal to the Dirac point ED uncovers new physics such as velocity renormalization,1,2,3 and the Dirac plasma 4,5 at |EF -ED| < kBT, where kBT is the thermal energy. In graphene, substrate disorder drives fluctuations in EF. Three-dimensional topological Dirac semimetals (TDS)6,7 obviate the substrate, and should show reduced EF fluctuations due to better metallic screening and higher dielectric constants. Here we map the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than room temperature ({\Delta}EF,rms = 4-6 meV = 40-70 K) and comparable to the highest quality graphene on h-BN;8 far smaller than graphene on SiO2,9,10 or the Dirac surface state of a topological insulator.11 Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provides a unique way to tune the surface density of states in a TDS thin-film material.
TL;DR: The intensity of the four-wave mixing signal is observed to grow monotonically as a function of the number of graphene layers, up to a maximum intensity corresponding to ∼32 layers, after which it decreases, well in agreement with theoretical predictions.
Abstract: It has been shown that graphene exhibits unique electronic, thermal, mechanical, and optical properties. In particular, due to its gapless band structure and linear dispersion relation around the Dirac points, graphene exhibits a strong nonlinear optical response, which has been theoretically predicted to depend on the number of graphene layers. In this Letter, we experimentally validate the theoretical predictions by probing multilayer graphene χ(3) nonlinearities. The intensity of the four-wave mixing signal is observed to grow monotonically as a function of the number of graphene layers, up to a maximum intensity corresponding to ∼32 layers, after which it decreases, well in agreement with theoretical predictions.
TL;DR: It was found that each few-layer graphene possesses a unique dispersionless optical index, which indicates that the optical conductivity does not simply scale with the number of layers, and that inter-layer electrodynamics are significant at visible energies.
Abstract: We have experimentally studied the dispersion of optical conductivity in few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a supercontinuum laser source measured the frequency dependence of the reflectance of exfoliated graphene flakes, including monolayer, bilayer and trilayer graphene, loaded on a Si/SiO2 Fabry-Perot resonator in the 545–700 nm range. The complex refractive index of few-layer graphene, n − ik, was extracted from the reflectance contrast to the bare substrate. It was found that each few-layer graphene possesses a unique dispersionless optical index. This feature indicates that the optical conductivity does not simply scale with the number of layers, and that inter-layer electrodynamics are significant at visible energies.
Patent•
14 Jun 2016TL;DR: In this paper, a hybrid graphene-metal design was proposed, in which the graphene acts as a gate-tunable inductor, and metal acts as capacitive reservoir for charge accumulation.
Abstract: A new approach to graphene-enabled plasmonic resonant structures in the THz is demonstrated in a hybrid graphene-metal design in which the graphene acts as a gate-tunable inductor, and metal acts as a capacitive reservoir for charge accumulation, A large resonant absorption in graphene can be achieved using the metal-graphene plasmonic scheme, and the peak can approach 100% in an optimized device, ideal for graphene-based THz detectors. Using high mobility graphene (μ > 50000 cm 2 V -1 S -1 ) will allow anomalously high resonant THz transmission (near 100%) through ultra-subwavelength graphene-filled metallic apertures at a resonance frequency that is gate tunable. This metal-graphene plasmonic scheme enables near perfect tunable THz filter or modulator.
TL;DR: High efficient, broadband lighting from printed hybrid nanocarbon structures with carbon nanotubes and reduced graphene oxides can find applications in a range of emerging applications where the shape and format, as well as being lightweight, are important.
Abstract: On page 4684, C. Dames, L. Hu and co-workers report highly efficient, broadband lighting from printed hybrid nanocarbon structures with carbon nanotubes and reduced graphene oxides. The fast response and excellent stability of the flexible lighting can find applications in a range of emerging applications where the shape and format, as well as being lightweight, are important.
Posted Content•
TL;DR: In this paper, the authors report on robust quantum emitters fabricated by thermal oxidation of tungsten disulphide multilayers and show robust, optically stable, linearly polarized luminescence at room temperature.
Abstract: Two dimensional systems offer a unique platform to study light matter interaction at the nanoscale. In this work we report on robust quantum emitters fabricated by thermal oxidation of tungsten disulphide multilayers. 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, our results provide important insights into understanding of defect formation and quantum emitter activation in 2D materials.
Posted Content•
TL;DR: In this paper, a photocurrent spectroscopy method was proposed to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature.
Abstract: Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. Our measurements reveal that near the charge-neutral-point the electron cooling is well described by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling mechanisms prevail. Furthermore, the method provides a unique frequency-domain measurement of the cooling rate that is not constrained by the width (temporal resolution) and the high intensity of the optical pulse. The photomixing spectroscopy method is well suited to measure the nonlinearity and response time of other photosensitive nanoscale materials and devices.
TL;DR: In this paper, the authors proposed a method to detect the presence of a single electron in an electron microscope at the Monash Centre for Electron Microscopy at Monash University in Australia.
Abstract: 1. School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia 2. Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia 3. Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia 4. Department of Civil Engineering, Monash University, Clayton 3800, Victoria, Australia 5. School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
01 Oct 2016
TL;DR: Graphene plasmonics have the potential to revolutionize terahertz technology as mentioned in this paper, the last great underdeveloped frequency band of electromagnetic waves, and have been shown to be suitable for use in large-area tunable graphene materials and devices.
Abstract: Graphene plasmonics has the potential to revolutionize terahertz technology — the last great underdeveloped frequency band of electromagnetic waves. We describe here recent research on large-area tunable graphene plasmonic materials and devices for use in terahertz detectors, modulators, and filters.
Journal Article•
TL;DR: In this article, unusual interactions between charges have been observed in two closely separated graphene bilayers, a promising system in which to create a condensate of electron-hole pairs.
Abstract: Unusual interactions between charges have been observed in two closely separated graphene bilayers, a promising system in which to create a condensate of electron-hole pairs.