Electronic, optoelectronic and low frequency noise measurements in transition metal dichalcogenide (TMDC) field effect transistors. Fabrication and characterization of TMDC based heterostructures aimed at optical and ferroelectric applications.
Electronic and Structural properties of Interface engineered atomically thin 2-D crystals:
Transition Metal Di-Chalcogenides (TMDCs) have gained enormous attention due to their rich electronic and structural properties i.e. direct Eg in 1L, thickness dependent Eg, spin valley coupling etc. Although TMDCs are normally semiconductors, It has been theoretically predicted that they can have a variety of crystalline phases which are normally unstable. A novel technique to stabilize one such ferroelectric phase in MoS2 has been discovered. Our spectroscopic and electronic evidences confirm that 1L MoS2 can be converted into a ferroelectric. The ferroelectric negative capacitance is being subsequently used to break the theoretical lower limit of subthreshold slope in FET.
My research focuses on probing magnetism along grain boundaries in 2D materials. Grain boundaries have long been considered a bane in the large scale fabrication of 2D materials such as graphene and transition metal dichalcogenides (TMDCs) by chemical vapour deposition (CVD) leading to polycrystallinity and poor structural and electrical performance. However, recent theoretical studies have predicted the manifestation of magnetism along such grain boundaries due to the nature of defects that constitute them. It will be interesting to probe and quantify the magnetic field strength arising out of these structural defects and suggest potential applications for their use in spintronics, spin filters or even in the hunt for the elusiveMajorana fermions.
Strain engineering in TMDCs (WSe2) through Van der Waals epitaxy: Relative stabilization of the 1T’ phase (which has a higher electrical conductivity) against the 1H phase has been observed in MoS2 via VdW epitaxy. We try investigating a similar phenomenon in WSe2 by investigating the optical and electronic properties of the heterostructure and relating the signatures to the strain developed in the system.
I am currently looking into the electrical transport properties as well as the optical response of ultra-thin semiconducting Tellurium (Te) nanowires (NWs). The preliminary transport results show that the transport in Te NWs is thermally activated in high temperature (>170K) and in the low temperature (<170K), Mott Variable Range Hopping (VRH) takes place. On the other hand, Te NW serves as an excellent candidate for near-infra red detector, as it is a narrow band-gap semiconductor (~0.6 eV). This allows us to investigate this interesting and highly popular field full of new novel physics of light-matter interaction as well as basic applications.
My research work focuses on optoelectronic properties associated to defects in two-dimensional materials. Defects have been known to influence the mechanical and electronic properties in two-dimensional materials and are considered to be undesirable. However, recent studies have exploited in-gap defect states for emission of single photons. Employing techniques such as high resolution confocal microscopy and resonant excitation it will be interesting to reveal the morphological and electronic properties of these emitters.
My research interests have been the development and study of novel low dimensional materials (2D van der Waal heterostructures) through building atomically-thin-Lego from graphene, molybdenum disulfide (MoS2) and other similar van der Waals materials for highly sensitive photodetection and quantum sensing applications e.g. number resolved single photon detection etc.
Our experimental investigations have confirmed that the electrical property of one van der Waals material (graphene, bilayer-graphene) can be combined with the optical property of the other van der Waals material (molybdenum disulphide (MoS2)) by physically stacking these together and forming heterostructures (Nature Nanotechnology 8, 826-830 (2013)). Irrespective of the nanoscale thickness (< 10 nm), such combination allows achieving extremely large photoresponsivity which is not observed in conventional photo-sensitive materials/devices. Our recent results with bilayer-graphene-MoS2 heterostructures show that such devices can be operated as number-resolved single photon detectors (Advanced Materials, In press (October-2017)).
In collaboration with Paria et al., we have also demonstrated that nanostructure of dimers can be used to enhance light matter interaction in a monolayer graphene leading high photoresponsivity (Advanced Materials 27, 1751-1758 (2015)).
I study conductance properties of pristine graphenenano-ribbons, which are created by a novel mechanical nano-exfoliation technique using an STM tip. Graphene created this way, has pristine edges as it has not been exposed to any chemicals or the atmosphere. It has been possible to study the change in the conductance of a graphene nano-ribbon caused by atomic level structuralchanges.
I am an experimental physicist. My research focusses on electronic transport in mesoscopic devices. These are micron-scale structures defined on a semiconductor platform which, in my case, are GaAs/AlGaAs heterostructure or bilayer graphene. In my pursuit, I have become well versed with fabrication, measurements, data acquisition and analysis. I design and fabricate most of these devices myself, starting from base materials to several processes such as electron beam lithography, etching recipes, and metallization, all the way to device packaging. One of my distinguished experience in fabrication is in the assembly of heterostructures of 2D crystals, such as graphene, boron nitride and transition metal dichalcogenides.
My experiments on these devices are mainly in two low temperature set-ups that I operate, dilution refrigerator and He3 cryostat which can cool my devices down to 20 mK and 265 mK respectively. The experiments involve sensitive measurements of electrical signals in the nano-amperes and micro-volts range, which require careful optimizations with measuring instruments. My thesis work has been mainly to study the low carrier density regime in these devices. A significant aspect of my investigations have been to study the intrinsic electrical noise in the system (that’s why the sensitivity of measurements are important) and extract information about the nature of electronic transport.
Dissertation – Impact of disorder and topology in two dimensional systems at low carrier densities
A two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure offers an avenue to build a variety of mesoscopic devices, much of which is achieved by the capability to construct complex potential landscapes using surface gates. Trapping charge carriers in low densities on them gives the prospects for a broad range of phenomena to emerge. This was the central idea behind my studies in these systems.
Linear magnetoresistance in two-dimensional electron systems
Linear magnetoresistance in strongly inhmogeneous regime
Bilayer graphene (BLG), made of two layers of graphene sheets in Bernal stacking, is fundamentally different from an individual graphene layer. The most striking property is that a tunable band gap can be induced in BLG by applying an electric field perpendicular to its plane. As a result, BLG can be electrically driven from a metallic state, with high carrier mobility, to a strongly insulating state, thus being suitable for a diverse range of novel device applications. However, experiments revealed that a significant density of states exist inside the band gap, which leads to new transport mechanisms. My goal has been to understand its true nature by examining noise in the form of flicker noise and mesoscopic conductance fluctuations when it is strongly gapped.
Percolative and edge transport in gapped bilayer graphene
Random potential fluctuations enable percolative paths in gapped bilayer graphene
Observable by both standard electrical and flicker noise measurements
Possible evidence for edge transport
Publications are in preparation.
Mesoscopic conductance statistics in gapped bilayer graphene
Mesoscopic conductance fluctuations in gapped bilayer graphene
Anomalous increase in conductance variance near metal-insulator transition
Exploration of the scaling theory of localization in strongly gapped regime
Observation of log-normal spectrum of conductance in strong disorder
Plasmonic coupling for optoelectronics of 2D hetrostructures:
Recent advances in growth and synthesis 2D transition metal dichacogindes (TMDCs) have opened new avenues for their implementation into electronic as well as optoelectronic devices. However, weak absorption in ultrathin semiconducting 2D TMDCs is a critical issue for photovoltaic application. For enhancing light matter interaction we are coupling 2D hetrostructures with plasmonic nanostructures.
Ph. D Physics (2008-2014)
University of Pune, India
Topological Insulators :The in-built topological protection against direct backscattering and absence of localization makes two-dimensional (2D) surface states of bismuth chalcogenide-based strong topological insulators (TIs) a promising platform for electronic and spintronic applications. Our study of noise in these samples not only allow us to evaluate the quality of signal but also find out the mechanisms responsible for dynamic scattering for the surface states. Our experiments reveal that the noise induced in these surface states does not stem from any external factors like substrate-sample interface or sample adsorbates, rather it involves the generation-recombination processes involving different impurity bands in the bulk of the TI .
Magnetically Doped TI : Interplay of topologically protected states and broken time-reversal symmetry makes Ferro-magnetically doped topological insulators (FMTI) interesting systems for both fundamental science and technological applications. The interplay of ferromagnetic defects and magnetism makes FMTI a very interesting field of study but although average transport measurements and spectroscopic studies very efficiently captures the average characteristics, microscopically intuitive study of defect dynamics (1/f noise) has been lacking. Our experiments in magnetically doped TI films has shown that localized states induced by magnetic (Cr) impurities gives rise to exponentially enhanced resistance fluctuations at both low temperature and density.
The thesis endeavored to theoretically understand electronic properties of finite trapezoidal shaped graphene sheets, and understand zero energy edge states. The motivation for this thesis was experimental work at Low Temperature Nano-electronics Laboratory under Prof. Arindam Ghosh (Amogh et al Nature Nanotechnology 2017). This work systematically tries to understand graphene, a two dimensional material, and it ‘s confinement in spacial dimensions. We started with analytical study of bulk graphene with various hoppings (scalar and Kane Mele type) in a tight-binding formulation and its bandstructure. Then we confine graphene in one-dimension to form semi-infinite graphene nano-ribbons and numerically determine its energy spectra and wave-functions. Then graphene is confined in both spacial dimensions and starting from simplest case of finite rectangular sheet, we move on to the two different ways in which graphene can be torn. Here, numerical studies were done to determine the density of states and local density of states.Finally we study various kinds of edge and bulk disorders and how they result in localization of wave-functions along the edges using Inverse Participation Ratio and Spectral Statistics.
Bhupendra Kumar Sharma, currently engaged in various research activities such as low temperature solution process synthesis of semiconductor and dielectric metal oxides, 2D materials (Graphene and MOS2) and its hybrid with inorganic materials for electronic applications. He consider to develop the materials engineering, its compatibility, and new architecture for flexible, printable and various electronic applications.
I mainly work on opto-electronics of 2 dimensional materials; Graphene, MoS2 and their hetro-structures. I am also interested in automation of techniques for low temperature nano scale measurements. I am currently working in single photon detection on Gr/MoS2 hetro-structure.
Graphene and other 2D materials like transition metal dichalcogenides (TMDCs) have attracted a lot of attention in optoelectronics and sensing technology due to their extraordinary electronic and optical properties. The optoelectronic devices can be integrated with the plasmonic nanostructures to tailor the bandwidth and photoresponse. Our aim is to understand how the localization of electric field due to the plasmonic nanostructures affects the photoresponse of graphene heterostructures. I use computational electromagnetic techniques like BEM and FEM to simulate the localized surface plasmon resonance (LSPR) for a structure of random shape and size.