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.