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Vishal Panchal

Publications and source records attributed to Vishal Panchal.

14 recordsLinked to original sources
arXiv2018

Probing the nanoscale origin of strain and doping in graphene-hBN heterostructures

We use confocal Raman microscopy and modified vector analysis methods to investigate the nanoscale origin of strain and carrier concentration in exfoliated graphene-hexagonal boron nitride (hBN) heterostructures on silicon dioxide (SiO2). Two types of heterostructures are studied: graphene on SiO2 partially coved by hBN, and graphene fully encapsulated between two hBN flakes. We extend the vector analysis methods to produce spatial maps of the strain and doping variation across the heterostructures. This allows us to visualise and directly quantify the much-speculated effect of the environment on carrier concentration as well as strain in graphene. Moreover, we demonstrate that variations in strain and carrier concentration in graphene arise from nanoscale features of the heterostructures such as fractures, folds and bubbles trapped between layers. For bubbles in hBN-encapsulated graphene, hydrostatic strain is shown to be greatest at bubble centres, whereas the maximum of carrier concentration is localised at bubble edges. Raman spectroscopy is shown to be a non-invasive tool for probing strain and doping in graphene, which could prove useful for engineering of two-dimensional devices.

cond-mat.mtrl-sci↗
arXiv2018

Detection of ultra-low concentration NO2 in complex environment using epitaxial graphene sensors

We demonstrate proof-of-concept graphene sensors for environmental monitoring of ultra-low concentration NO2 in complex environments. Robust detection in a wide range of NO2 concentrations, 10-154 ppb, was achieved, highlighting the great potential for graphene-based NO2 sensors, with applications in environmental pollution monitoring, portable monitors, automotive and mobile sensors for a global real-time monitoring network. The measurements were performed in a complex environment, combining NO2/synthetic air/water vapour, traces of other contaminants and variable temperature in an attempt to fully replicate the environmental conditions of a working sensor. It is shown that the performance of the graphene-based sensor can be affected by co-adsorption of NO2 and water on the surface at low temperatures (<70 C). However, the sensitivity to NO2 increases significantly when the sensor operates at 150 C and the cross-selectivity to water, sulphur dioxide and carbon monoxide is minimized. Additionally, it is demonstrated that single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene.

physics.ins-det↗
arXiv2018

Wide-Band Nano-Imaging of Plasmon Dispersion and Hotspots in Quasi-Free-Standing Epitaxial Graphene

We report observation of graphene plasmon interference fringes across a wide spectral range using a scattering scanning near-field optical microscope (s-SNOM) that employs a widely tunable bank of quantum cascade lasers. We use plasmon interference to measure the dispersion curve of graphene plasmons over more than an order of magnitude of plasmon wavelength, from $λ_{sp}$ ~140 to ~1700 nm, and extract the electron Fermi energy of 298$\pm$4 meV for hydrogen-intercalated single layer epitaxial graphene on SiC. Furthermore, we demonstrate the appearance of wavelength tuneable graphene plasmon reflection "hotspots" at single-layer/bi-layer interfaces. This work demonstrates the capability of wide-band nano-imaging to precisely measure the electrical properties of graphene and spatially control plasmon reflection focusing.

cond-mat.mes-hall↗
arXiv2018

Unique excitonic effects in tungsten disulphide monolayers on two-layer graphene

Light emission in atomically thin heterostructures is known to depend on the type of materials, number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulphide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures. We bring evidence that the excitonic properties of WS2 can be effectively tuned by the number of supporting graphene layers. Integrating WS2 monolayers with two-layer graphene leads to a significant enhancement of the photoluminescence response, up to one order of magnitude higher compared to WS2 supported on one-layer graphene. Our findings highlight the importance of substrate engineering when constructing atomically thin layered heterostructures.

cond-mat.mtrl-sci↗
arXiv2018

Tuning epitaxial graphene sensitivity to water by hydrogen intercalation

The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.

cond-mat.mtrl-sci↗
arXiv2018

Water on graphene: Review of recent progress

The sensitivity of graphene to the surrounding environment is given by its π electrons, which are directly exposed to molecules in the ambient. The high sensitivity of graphene to the local environment has shown to be both advantageous but also problematic for graphene-based devices, such as transistors and sensors, where the graphene carrier concentration and mobility change due to ambient humidity variations. In this review, recent progress in understanding the effects of water on different types of graphene, grown epitaxially and quasi-free standing on SiC, by chemical vapour deposition on SiO2, as well as exfoliated flakes, are presented. It is demonstrated that water withdraws electrons from graphene, but the graphene-water interaction highly depends on the thickness, layer stacking, underlying substrate and substrate-induced doping. Moreover, we highlight the importance of clear and unambiguous description of the environmental conditions (i.e. relative humidity) whenever a routine characterisation for carrier concentration and mobility is reported (often presented as a simple figure-of-merit), as these electrical characteristics are highly dependent on the adsorbed molecules and the surrounding environment.

cond-mat.mtrl-sci↗
arXiv2018

Calibration of multi-layered probes with low/high magnetic moments

We present a comprehensive method for visualisation and quantification of the magnetic stray field of magnetic force microscopy (MFM) probes, applied to the particular case of custom-made multi-layered probes with controllable high/low magnetic moment states. The probes consist of two decoupled magnetic layers separated by a non-magnetic interlayer, which results in four stable magnetic states: +/-ferromagnetic (FM) and +/-antiferromagnetic (A-FM). Direct visualisation of the stray field surrounding the probe apex using electron holography convincingly demonstrates a striking difference in the spatial distribution and strength of the magnetic flux in FM and A-FM states. In situ MFM studies of reference samples are used to determine the probe switching fields and spatial resolution. Furthermore, quantitative values of the probe magnetic moments are obtained by determining their real space tip transfer function (RSTTF). We also map the local Hall voltage in graphene Hall nanosensors induced by the probes in different states. The measured transport properties of nanosensors and RSTTF outcomes are introduced as an input in a numerical model of Hall devices to verify the probe magnetic moments. The modelling results fully match the experimental measurements, outlining an all-inclusive method for the calibration of complex magnetic probes with a controllable low/high magnetic moment.

physics.app-ph↗
arXiv2018

Visualisation of edge effects in side-gated graphene nanodevices

Using local scanning electrical techniques we study edge effects in side-gated Hall nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ~60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene.

physics.app-ph↗
arXiv2018

Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical measurements

We directly correlate the local (20-nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions, where the starting state of the surface can be reproducibly defined, we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected by the atmospheric dopants. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate the state of the EG surface in ambient air. All doping effects can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.

cond-mat.mtrl-sci↗
arXiv2018

Confocal laser scanning microscopy: A tool for rapid optical characterization of 2D materials

Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characterization method for large-area analysis of graphene on different substrates, which can be applied in ambient air, does not require additional sample preparation, and is insusceptible to surface charging and surface contamination. CLSM leverages optical properties of graphene and provides greatly enhanced optical contrast and mapping of thickness down to a single layer. We demonstrate the effectiveness of CLSM by measuring mechanically exfoliated and chemical vapor deposition graphene on Si/SiO2, and epitaxial graphene on SiC. In the case of graphene on Si/SiO2, both CLSM intensity and height mapping is powerful for analysis of 1-5 layers of graphene. For epitaxial graphene on SiC substrates, the CLSM intensity allows us to distinguish features such as dense, parallel 150 nm wide ribbons of graphene (associated with the early stages of the growth process) and large regions covered by the interfacial layer and 1-3 layers of graphene. In both cases, CLSM data shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping, with a greatly reduced acquisition time. We demonstrate that CLSM is an indispensable tool for rapid analysis of mass-produced graphene and is equally relevant to other 2D materials.

physics.app-ph↗
arXiv2017

Rapid characterization of wafer-scale 2D material: Epitaxial graphene and graphene nanoribbons on SiC

We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, highly-efficient characterization method for large-area epitaxial graphene and graphene nanostructures on SiC substrates, which can be applied in ambient air without sample preparation and is insusceptible to surface charging or surface contamination. Based on the variation of reflected intensity from regions covered by interfacial layer, single layer, bilayer, or few layer graphene, and through the correlation to the results from Raman spectroscopy and SPM, CLSM images with a high resolution (around 150 nm) reveal that the intensity contrast has distinct feature for undergrown graphene (mixing of dense, parallel graphene nanoribbons and interfacial layer), continuous graphene, and overgrown graphene. Moreover, CLSM has a real acquisition time hundreds of times faster per unit area than the supplementary characterization methods. We believe that the confocal laser scanning microscope will be an indispensable tool for mass-produced epitaxial graphene or applicable 2D materials.

physics.app-ph↗
arXiv2017

Observation of Coulomb blockade in nanostructured epitaxial bilayer graphene on SiC

We study electron transport in nanostructures patterned in bilayer graphene patches grown epitaxially on SiC as a function of doping, magnetic field, and temperature. Away from charge neutrality transport is only weakly modulated by changes in carrier concentration induced by a local side-gate. At low n-type doping close to charge neutrality, electron transport resembles that in exfoliated graphene nanoribbons and is well described by tunnelling of single electrons through a network of Coulomb-blockaded islands. Under the influence of an external magnetic field, Coulomb blockade resonances fluctuate around an average energy and the gap shrinks as a function of magnetic field. At charge neutrality, however, conduction is less insensitive to external magnetic fields. In this regime we also observe a stronger suppression of the conductance below $T^*$, which we interpret as a sign of broken interlayer symmetry or strong fluctuations in the edge/potential disorder.

cond-mat.mes-hall↗
arXiv2013

Standardization of surface potential measurements of graphene domains

We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains with values ~4.55+/-0.02 eV and ~4.44+/-0.02eV for single- and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.

cond-mat.mes-hall↗