arXiv subjects

James Hone

Publications and source records attributed to James Hone.

At least 109 records · Page 6Linked to original sources

arXiv2017

Electrical detection of hyperbolic phonon-polaritons in heterostructures of graphene and boron nitride

Light properties in the mid-infrared can be controlled at a deep subwavelength scale using hyperbolic phonons-polaritons (HPPs) of hexagonal boron nitride (h-BN). While propagating as waveguided modes HPPs can concentrate the electric field in a chosen nano-volume. Such a behavior is at the heart of many applications including subdiffraction imaging and sensing. Here, we employ HPPs in heterostructures of h-BN and graphene as new nano-optoelectronic platform by uniting the benefits of efficient hot-carrier photoconversion in graphene and the hyperbolic nature of h-BN. We demonstrate electrical detection of HPPs by guiding them towards a graphene pn-junction. We shine a laser beam onto a gap in metal gates underneath the heterostructure, where the light is converted into HPPs. The HPPs then propagate as confined rays heating up the graphene leading to a strong photocurrent. This concept is exploited to boost the external responsivity of mid-infrared photodetectors, overcoming the limitation of graphene pn-junction detectors due to their small active area and weak absorption. Moreover this type of detector exhibits tunable frequency selectivity due to the HPPs, which combined with its high responsivity paves the way for efficient high-resolution mid-infrared imaging.

physics.app-ph↗

arXiv2017

Near-unity quantum yield from carbon nanotube excitons coupled to plasmonic nanocavities

Single-walled carbon nanotubes (SWCNTs) are promising absorbers and emitters to enable novel photonic and optoelectronic applications but are also known to severely suffer from low optical quantum yields. Here we demonstrate SWCNTs excitons coupled to plasmonic nanocavities reaching deeply into the Purcell regime with FP=234 (average FP=76), near unity quantum yields of 70% (average 41%), and a photon emission rate of 1.7 MHz into the first lens. The measured ultra-narrow exciton linewidth (18 micro eV) implies furthermore generation of indistinguishable single photons from a SWCNT. To demonstrate utility beyond quantum light sources we show that nanocavity-coupled SWCNTs perform as single-molecule thermometers detecting plasmonically induced heat (Delta T=150K) in a unique interplay of excitons, phonons, and plasmons at the nanoscale.

cond-mat.mes-hall↗

arXiv2017

Surface buckling of phosphorene materials: determination, origin and influence on electronic structure

The surface structure of phosphorene crystals materials is determined using surface sensitive dynamical micro-spot low energy electron diffraction (μLEED) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (\textit{i}) crystalline cleaved black phosphorus (BP) at 300 K and (\textit{ii}) exfoliated few-layer phosphorene (FLP) of about 10 nm thicknes, which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 Å and 0.30 Å, respectively, is measured, which is one order of magnitude larger than previously reported. Using first principle calculations, the presence of surface vacancies is attributed not only to the surface buckling in BP and FLP, but also the previously reported intrinsic hole doping of phosphorene materials.

cond-mat.mtrl-sci↗

arXiv2017

Tuning quantum non-local effects in graphene plasmonics

The response of an electron system to electromagnetic fields with sharp spatial variations is strongly dependent on quantum electronic properties, even in ambient conditions, but difficult to access experimentally. We use propagating graphene plasmons, together with an engineered dielectric-metallic environment, to probe the graphene electron liquid and unveil its detailed electronic response at short wavelengths.The near-field imaging experiments reveal a parameter-free match with the full theoretical quantum description of the massless Dirac electron gas, in which we identify three types of quantum effects as keys to understanding the experimental response of graphene to short-ranged terahertz electric fields. The first type is of single-particle nature and is related to shape deformations of the Fermi surface during a plasmon oscillations. The second and third types are a many-body effect controlled by the inertia and compressibility of the interacting electron liquid in graphene. We demonstrate how, in principle, our experimental approach can determine the full spatiotemporal response of an electron system.

cond-mat.mes-hall↗

arXiv2017

Interfacial Charge Transfer Circumventing Momentum Mismatch at 2D van der Waals Heterojunctions

Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technical of femtosecond pump-probe spectroscopy. Upon photo-excitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and, thus, momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiatively recombination, with rates varying by up to three-orders of magnitude from sample to sample, but with no correlation with inter-layer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.

cond-mat.mtrl-sci↗

arXiv2017

Coulomb engineering of the bandgap in 2D semiconductors

The ability to control the size of the electronic bandgap is an integral part of solid-state technology. Atomically-thin two-dimensional crystals offer a new approach for tuning the energies of the electronic states based on the interplay between the environmental sensitivity and unusual strength of the Coulomb interaction in these materials. By engineering the surrounding dielectric environment, we are able to tune the electronic bandgap in monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behavior to present an in-plane dielectric heterostructure with a spatially dependent bandgap, illustrating the feasibility of our approach for the creation of lateral junctions with nanoscale resolution. This successful demonstration of bandgap engineering based on the non-invasive modification of the Coulomb interaction should enable the design of a new class of atomically thin devices to advance the limits of size and functionality for solid-state technologies.

cond-mat.mes-hall↗

arXiv2017

Ultra-confined acoustic THz graphene plasmons revealed by photocurrent nanoscopy

Terahertz (THz) fields are widely applied for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated - well below the classical diffraction limit - through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, lp, compared to the photon wavelength, l0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene-insulator-metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. While infrared GPs have been visualised by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far - only GPs with nearly free-space wavelength have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. The on-chip GP detection simplifies GP imaging, as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (lp = l0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering. Acoustic GPs could thus strongly benefit the development of deep subwavelength-scale THz devices.

cond-mat.mes-hall↗

arXiv2016

Frictional magneto-Coulomb drag in graphene double-layer heterostructure

Coulomb interaction between two closely spaced parallel layers of electron system can generate the frictional drag effect by interlayer Coulomb scattering. Employing graphene double layers separated by few layer hexagonal boron nitride (hBN), we investigate density tunable magneto- and Hall-drag under strong magnetic fields. The observed large magneto-drag and Hall-drag signals can be related with Laudau level (LL) filling status of the drive and drag layers. We find that the sign and magnitude of the magneto- and Hall-drag resistivity tensor can be quantitatively correlated to the variation of magneto-resistivity tensors in the drive and drag layers, confirming a theoretical formula for magneto-drag in the quantum Hall regime. The observed weak temperature dependence and $\sim B^2$ dependence of the magneto-drag are qualitatively explained by Coulomb scattering phase-space argument.

cond-mat.mes-hall↗

arXiv2016

Nanobubble induced formation of quantum emitters in monolayer semiconductors

The recent discovery of exciton quantum emitters in transition metal dichalcogenides (TMDCs) has triggered renewed interest of localized excitons in low-dimensional systems. Open questions remain about the microscopic origin previously attributed to dopants and/or defects as well as strain potentials. Here we show that the quantum emitters can be deliberately induced by nanobubble formation in WSe2 and BN/WSe2 heterostructures. Correlations of atomic-force microscope and hyperspectral photoluminescence images reveal that the origin of quantum emitters and trion disorder is extrinsic and related to 10 nm tall nanobubbles and 70 nm tall wrinkles, respectively. We further demonstrate that hot stamping results in the absence of 0D quantum emitters and trion disorder. The demonstrated technique is useful for advances in nanolasers and deterministic formation of cavity-QED systems in monolayer materials.

cond-mat.mes-hall↗

arXiv2016

Graphene mechanical oscillators with tunable frequency

Oscillators, which produce continuous periodic signals from direct current power, are central to modern communications systems, with versatile applications such as timing references and frequency modulators. However, conventional oscillators typically consist of macroscopic mechanical resonators such as quartz crystals, which require excessive off-chip space. Here we report oscillators built on micron-size, atomically-thin graphene nanomechanical resonators, whose frequencies can be electrostatically tuned by as much as 14\%. The self-sustaining mechanical motion of the oscillators is generated and transduced at room temperature by simple electrical circuitry. The prototype graphene voltage controlled oscillators exhibit frequency stability and modulation bandwidth sufficient for modulation of radio-frequency carrier signals. As a demonstration, we employ a graphene oscillator as the active element for frequency modulated signal generation, and achieve efficient audio signal transmission.

cond-mat.mes-hall↗

arXiv2016

SU-8 clamped CVD graphene drum resonators

Graphene mechanical resonators are the ultimate two-dimensional nanoelectromechanical systems (NEMS) with applications in sensing and signal processing. While initial devices have shown promising results, an ideal graphene NEMS resonator should be strain engineered, clamped at the edge without trapping gas underneath, and electrically integratable. In this letter, we demonstrate fabrication and direct electrical measurement of circular SU-8 polymer-clamped chemical vapor deposition (CVD) graphene drum resonators. The clamping increases device yield and responsivity, while providing a cleaner resonance spectrum from eliminated edge modes. Furthermore, this resonator is highly strained, indicating its potential in strain engineering for performance enhancement.

cond-mat.mes-hall↗

arXiv2016

Modulation of mechanical resonance by chemical potential oscillation in graphene

The classical picture of the force on a capacitor assumes a large density of electronic states, such that the electrochemical potential of charges added to the capacitor is given by the external electrostatic potential and the capacitance is determined purely by geometry. Here we consider capacitively driven motion of a nano-mechanical resonator with a low density of states, in which these assumptions can break down. We find three leading-order corrections to the classical picture: the first of is a modulation in the static force due to variation in the internal chemical potential, the second and third are change in static force and dynamic spring constant due to the rate of change of chemical potential, expressed as the quantum (density of states) capacitance. As a demonstration, we study a capacitively driven graphene mechanical resonators, where the chemical potential is modulated independently of the gate voltage using an applied magnetic field to manipulate the energy of electrons residing in discrete Landau levels. In these devices, we observe large periodic frequency shifts consistent with the three corrections to the classical picture. In devices with extremely low strain and disorder, the first correction term dominates and the resonant frequency closely follows the chemical potential. The theoretical model fits the data with only one adjustable parameter representing disorder-broadening of the Landau levels. The underlying electromechanical coupling mechanism is not limited the particular choice of material, geometry, or mechanism for variation in chemical potential, and can thus be extended to other low-dimensional systems.

cond-mat.mes-hall↗

arXiv2016

Tunable Ultrafast Thermal Relaxation in Graphene Measured by Continuous-Wave Photomixing

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.

cond-mat.mes-hall↗

arXiv2016

High Electric Field Carrier Transport and Power Dissipation in Multilayer Black Phosphorus Field Effect Transistor with Dielectric Engineering

This study addresses high electric field transport in multilayer black phosphorus (BP) field effect transistors (FETs) with self-heating and thermal spreading by dielectric engineering. Interestingly, we found that multilayer BP device on a SiO2 substrate exhibited a maximum current density of 3.3 x 10E10 A/m2 at an electric field of 5.58 MV/m, several times higher than multilayer MoS2. Our breakdown thermometry analysis revealed that self-heating was impeded along BP-dielectric interface, resulting in a thermal plateau inside the channel and eventual Joule breakdown. Using a size-dependent electro-thermal transport model, we extracted an interfacial thermal conductance of 1-10 MW/m2 K for the BP-dielectric interfaces. By using hBN as a dielectric material for BP instead of thermally resistive SiO2 (about 1.4 W/m K), we observed a 3 fold increase in breakdown power density and a relatively higher electric field endurance together with efficient and homogenous thermal spreading because hBN had superior structural and thermal compatibility with BP. We further confirmed our results based on micro-Raman spectroscopy and atomic force microscopy, and observed that BP devices on hBN exhibited centrally localized hotspots with a breakdown temperature of 600K, while the BP device on SiO2 exhibited a hotspot in the vicinity of the electrode at 520K.

cond-mat.mtrl-sci↗

arXiv2016

Measurement of the optical dielectric function of transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2 and WSe2

We report a determination of the complex in-plane dielectric function of monolayers of four transition metal dichalcogenides: MoS2, MoSe2, WS2 and WSe2, for photon energies from 1.5 - 3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials.

cond-mat.mtrl-sci↗

arXiv2016

Influence of the Substrate Material on the Optical Properties of Tungsten Diselenide Monolayers

Monolayers of transition-metal dichalcogenides such as WSe2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe2. To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290K and 10K for exfoliated WSe2 on SiO2/Si, sapphire (Al2O3), hBN/Si3N4/Si, and MgF2, indicating substrate-dependent appearance and strength of exciton, trion, and biexciton modes. Additionally, one CVD-grown WSe2 monolayer on sapphire is included in this study for direct comparison with its exfoliated counterpart. Time-resolved micro-photoluminescence shows how radiative decay times strongly differ for different substrate materials. Our data indicates exciton-exciton annihilation as a shortening mechanism at room temperature, and subtle trends in the decay rates in correlation to the dielectric environment at cryogenic temperatures. On the measureable time scales, trends are also related to the extent of the respective 2D-excitonic modes' appearance. This result highlights the importance of further detailed characterization of exciton features in 2D materials, particularly with respect to the choice of substrate.

cond-mat.mes-hall↗

arXiv2016

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.

cond-mat.mes-hall↗

arXiv2016

The hot pick-up technique for batch assembly of van der Waals heterostructures

The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices we found semiclassical mean free paths up to 0.9 micrometer, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.

cond-mat.mes-hall↗