Ram Sevak Singh

Laser Patterning of Epitaxial Graphene for Schottky Junction Photodetectors

Large-area patterning of epitaxial graphene for Schottky junction photodetectors has been demonstrated with a simple laser irradiation method. In this method, semimetal-semiconductor Schottky junctions are created in a controllable pattern between epitaxial graphene (EG) and laser-modified epitaxial graphene (LEG). The zero-biased EG-LEG-EG photodetector exhibits a nanosecond and wavelength-independent photoresponse in a broad-band spectrum from ultraviolet (200 nm) through visible to infrared light (1064 nm), distinctively different from conventional photon detectors. An efficient external photoresponsivity (or efficiency) of ∼0.1 A·W(-1) is achieved with a biased interdigitated EG-LEG-EG photodetector. The fabrication method presented here opens a viable route to carbon optoelectronics for a fast and highly efficient photoconductive detector.

A systematic study of the atmospheric pressure growth of large-area hexagonal crystalline boron nitride film

The growth of hexagonal boron nitride (h-BN) is of much interest owing to its outstanding properties and for scalable two dimensional (2D) electronics applications. Here, we report the controllable growth of h-BN on a copper substrate using the atmospheric pressure chemical vapor deposition (APCVD) method using ammonia borane as the precursor. The advantages of using APCVD include its ease of setup utilizing fewer resources, low cost and fast growth, all of which are essential for full film coverage and the mass production of 2D h-BN. In this study, we observed a substrate-position dependent evolution of h-BN domains at various stages of growth as the density and size of the domains increased downstream along the quartz tube. Other critical parameters such as growth temperature, deposition time, temperature and mass of precursor were also systemically investigated in order to understand the factors influencing the growth of the h-BN film. Importantly, with a slight increase in the growth temperature of 50 °C, we observe a significant (∼17-fold) increase in the average domain size, and its further expansion for a longer duration of growth. Likewise, our parametric study highlights the impact of other crucial parameters on domain size, coverage, and thickness of the h-BN film.

Tunable optical absorption and interactions in graphene via oxygen plasma

We report significant changes of optical conductivity in single layer graphene induced by mild oxygen plasma exposure, and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, the real part of the frequency-dependent conductivity is nearly completely suppressed below an exposure-dependent threshold in the near infrared range. The clear step-like suppression follows the Pauli blocking behaviour expected for doped monolayer graphene. The nearly zero residual conductivity at frequencies below 2Ef can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controlably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

Band gap effects of hexagonal boron nitride using oxygen plasma

Tuning of band gap of hexagonal boron nitride (h-BN) has been a challenging problem due to its inherent chemical stability and inertness. In this work, we report the changes in band gaps in a few layers of chemical vapor deposition processed as-grown h-BN using a simple oxygen plasma treatment. Optical absorption spectra show a trend of band gap narrowing monotonically from 6 eV of pristine h-BN to 4.31 eV when exposed to oxygen plasma for 12 s. The narrowing of band gap causes the reduction in electrical resistance by ∼100 fold. The x-ray photoelectron spectroscopy results of plasma treated hexagonal boron nitride surface show the predominant doping of oxygen for the nitrogen vacancy. Energy sub-band formations inside the band gap of h-BN, due to the incorporation of oxygen dopants, cause a red shift in absorption edge corresponding to the band gap narrowing.

Large room-temperature quantum linear magnetoresistance in multilayered epitaxial graphene: Evidence for two-dimensional magnetotransport

We report magnetoresistance (MR) properties from room temperature (300 K) to 2 K in multilayered epitaxial graphene (EG) prepared on C-face of SiC substrate. A large (∼50%) and linear MR is observed at 300 K, which is distinctively different from other carbon materials. This linear MR is attributed to the two-dimensional (2D) transport in the material as inferred from our angular dependence magnetotransport experiments. Furthermore, negative MR behaviour at a low field regime for temperatures ≤20 K is recognised as a weak localization in EG. This study underlines the potential of exploiting multilayered EG on C-face SiC for room temperature magneto-electronic device applications.

Photoresponse in epitaxial graphene with asymmetric metal contacts

We report photoresponse observations in epitaxial graphene (EG) devices with asymmetric metals (Au, Al) contacted in planar Au/EG/Al device format. The transient photocurrent measurements on the zero-bias device show photocurrent maxima at the Au/EG contact and minima at the EG/Al contact. This observed significant difference between the two types of junctions is responsible for the overall efficient device photoresponse. We have also found that the number of EG layers influences the photocurrent magnitude and response time regardless of incident photon energy or intensity. An external photoresponsivity (or efficiency) of ∼31.3 mA W−1 is achieved with a biased Au/EG/Al photodetector at excitation wavelength of 632.8 nm.

Effect of Oxygen Plasma on the Optical Properties of Monolayer Graphene

The significant alteration of absorption (A) of monolayer graphene under mild oxygen plasma exposure has been observed. The first important effect is the reduction of the excitonic resonance peak at 4.64 eV. Secondly, in the near infrared range, A is gradually suppressed below an exposure-dependent threshold in sense that A << A0. Quantity A0 (given by πα and α is the fine structure constant) denotes constant absorption and relates to universal optical conductivity σ0. The suppression of A0 can be thought as the weakening of electron-hole interaction as displayed by the reduction of the excitonic resonance peak at 4.64 eV. The weakening of this interaction is due to the disorder introduced by the oxygen plasma exposure.

Hydrogen adsorption on sulphur-doped SiC nanotubes

Hydrogen (H2) is an energy carrier and clean fuel that can be used for a broad range of applications that include fuel cell vehicles. Therefore, development of materials for hydrogen storage is demanded. Nanotubes, in this context, are appropriate materials. Recently, silicon carbide nanotube (SiCNTs) have been predicted as potential nanomaterials for hydrogen storage, and atomic doping into the nanotubes improves the H2 adsorption. Here, we report H2 adsorption properties of sulphur-doped (S-doped) SiCNTs using first-principles calculations based on density functional theory. The H2 adsorption properties are investigated by calculations of energy band structures, density of states (DOS), adsorption energy and Mulliken charge population analysis. Our findings show that, compared to the intrinsic SiCNT, S-doped SiCNT is more sensitive to H2 adsorption. H2 gas adsorption on S-doped C-sites of SiCNT brings about significant modulation of the electronic structure of the nanotube, which results in charge transfer from the nanotube to the gas, and dipole–dipole interactions cause chemisorptions of hydrogen. However, in the case of H2 gas adsorption on S-doped Si-sites of the nanotube, lesser charge transfer from the nanotube to the gas results in physisorptions of the gas. The efficient hydrogen sensing properties of S-doped SiCNTs, studied here, may have potential for its practical realization for hydrogen storage application.

Surface energy controlled growth of single crystalline two-dimensional hexagonal (h)-boron nitride

Two-dimensional (2D) nanomaterials, including graphene and boron nitride (BN), have been of intense interest in recent years due to their exceptional electronic, thermal, and mechanical properties. Tailoring these novel properties to their maximum potential requires precise control of the atomic layer growth process. In recent years, catalytic growth of 2-D nanomaterials using chemical vapor deposition (CVD) process has emerged as an attractive approach due to their low-cost, scalability, and ability to transfer the grown materials on various substrates. In this approach, the morphology and purity of the catalytic surface plays a critical role on the shape, size, and growth kintectics of the 2D nanomaterial. In this work, we present the results of our systematic studies of the role of catalytic surface morphology on the shape and domain size of CVD grown hexagonal (h)-BN films. The present work clearly demonstrates that the presence of surface roghness in the form of ridges leads to a preferential growth of small-domain triangular BN sheets. A 100-fold reduction in the surface roughness leads to increased domain BN triangles, eventually transitioning to large-domain hexagonal shaped BN sheets.