Optoelectronic Properties of Graphene Based Van-der-Waals Hybrids
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Light matter interactions in atomically thin van der Waals materials have attracted significant attention in recent day [1–6]. Although the thickness does not exceed few nano-meters, such an atomically thin materials alone or in combination with other nanostructures show exciting and unexpected photodetection properties [7–16]. Fabrication of atomically sharp junctions can be achieved with 2D van der Waals heterostructures, which significantly enhances the scope to design new type of physical systems, where novel phenomena can be studied [15, 17, 18]. Heterostructures combine properties of dissimilar materials resulting improved device performances and hence, can be applied to multiple fields [19–21]. This thesis encompasses photo-response study of various atomically thin heterostructures made of graphene, bilayer-graphene (BLG) and MoS2. A graphene-on-MoS2 heterostructure, made of monolayer graphene and few atomic layers of MoS2, combine superior electronic transport properties of graphene with the optical properties of MoS2. Such hybrids exhibit enormous photo-responsivity, with values as high as ∼ 1010 A W−1 at ∼ 130 K and ∼ 108 A W−1 at room temperature, which make these the most photo-responsive material available till date. Presence of tunable persistent photo-response allows these to function as optoelectronic memory devices; where the persistent state shows near perfect charge retention within the experimental time scale of operation (∼ 12 hrs). Noise-free large gain (109 − 1010) mechanism is one of the salient features of graphene-MoS2 hybrids. Devices made from BLG-on-MoS2 hybrids further help in improving the photoresponsive gain in these devices, and a large photoresponsivity (∼ 109 A W−1) is maintained even when operating these devices at low channel bias (VDS < 50 mV), or at a low range of channel current (IDS < 10 nA). In an optimized operating condition, where circuit noise is lower than the signal from a single photoelectron, BLG-on-MoS2 devices function as a number resolved single photon detector. High specific detectivity and low noise equivalent power of these devices, allow investigation of photon noise present in an optical source. Along with the optoelectronic property study, various optical and electrical character-izations are adapted that explain the interface properties of graphene-MoS2 heterostructures. For example, Raman spectroscopy and photoluminicence study at the interface suggest strong interlayer coupling and efficient dissociation of excitons respectively, which play a key role in attaining large photoresponse. Interfacial barrier characteristics are also investigated in a vertical graphene-MoS2 geometry, which shows that the barrier height can be tuned by applying an electrostatic field. Various experimental techniques and instruments, such as heterostructure fabrication technique and setup, optical cryostat etc., were developed in house to accomplish experimental investigation, which are discussed in details. Results of photo-response study in van der Waals materials have opened up the possibility of designing a new class of photosensitive devices which can be utilized in various optoelectronic applications such as in biomedical sensing, astronomical sensing, optical communications, optical quantum information processing and in applications where low intensity photodetection and number resolved single photon detection attracts tremendous interest.
- Physics (PHY) 
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