We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition (CVD) based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO 2 /Si substrates. Films of size up to several mm's have been synthesized. Structural characterizations by atomic force microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional transmission electron microscopy (XTEM) and Raman spectroscopy confirm that such large scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ~50% and carrier mobilities reaching ~2000 cm 2 /Vs. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 µm from the observation of 2D weak localization in low temperature magneto-transport measurements. Our results show that despite the non-uniformity and surface roughness, such largescale, flexible thin films can have electronic properties promising for device applications.

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 10 9 Jones, 3 × 10 7 W −1 , 9.1 × 10 −12 WHz −1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies. Two-dimensional (2D) materials have chronically been the most widely studied materials, particularly after the successful scotch tape test to exfoliate graphene by Andre Geim and Kostya Novoselov in 2004 1. 2D materials possess excellent electrical and mechanical properties toward diverse electronic device applications. Graphene, being the prototype of 2D materials 2,3 , has been studied broadly for its exotic electrical, optical, and mechanical properties 3,4. Besides, the group-IV transition metal dichalcogenides (TMDs) having a bandgap of around 1 to 2 eV 5-7 have attracted increasing interest because of their promising electronic and optoelectronic device applications 3,8-21. Graphene possesses extremely high carrier mobility (>10 5 cm 2 V −1 s −1), but the absence of band gap limits its electronic and optoelectronic applications 22. Therefore, TMDs with the properties of graphene-like stature, bandgap tunability, weak van der Waals-like forces and stability have intrigued the interest of the scientific community. TMDs are the family of 2D materials having the chemical composition of MX 2 , where M stands for the transition metal elements (M = Mo, W, Ta, Ge…etc) and X for the chalcogen elements (X = Se, S and Te). Among TMDs, Ge-based materials are preferred for applications due to their abundance on earth and environmentally friendly nature 23. With Se, the p-type Germanium from a narrow bandgap semiconductor material as p-GeSe having exciting application in near-infrared (NIR) photodetectors and electron tunnelling devices. p-GeSe has an indirect bandgap of 1.08 eV in the bulk 24,25 , and a direct bandgap of ~1.7 eV in monolayers 24,26,27. Few layers of p-GeSe can be obtained from bulk by mechanical exfoliation method 28. Among the many applications, p-GeSe shows tremendous capability in the realm of photovoltaics, because of its excellent optical, material and electrical properties. Therefore, it is well known as substitution of phosphorene 29. Moreover, GeSe is considered as an amb...

Two-dimensional (2D) heterostructure with atomically sharp interface holds promise for future electronics and optoelectronics because of their multi-functionalities. Here we demonstrate gate-tunable rectifying behavior and self-powered photovoltaic characteristics of novel p-GeSe/n-MoSe2 van der waal heterojunction (vdW HJ). A substantial increase in rectification behavior was observed when the devices were subjected to gate bias. The highest rectification of ~ 1 × 104 was obtained at Vg = − 40 V. Remarkable rectification behavior of the p-n diode is solely attributed to the sharp interface between metal and GeSe/MoSe2. The device exhibits a high photoresponse towards NIR (850 nm). A high photoresponsivity of 465 mAW−1, an excellent EQE of 670%, a fast rise time of 180 ms, and a decay time of 360 ms were obtained. Furthermore, the diode exhibits detectivity (D) of 7.3 × 109 Jones, the normalized photocurrent to the dark current ratio (NPDR) of 1.9 × 1010 W−1, and the noise equivalent power (NEP) of 1.22 × 10–13 WHz−1/2. The strong light-matter interaction stipulates that the GeSe/MoSe2 diode may open new realms in multi-functional electronics and optoelectronics applications.

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Here, novel lateral PtSe2 p-n junctions are fabricated based on the PtSe2/BN/graphene (Gr) van der Waals heterostructures upon illumination of visible light by optical excitation of mid-gap point defects in...

Based on this, 2D transition metal chalcogenide (TMD) has gained much attention as an excellent alternative to graphene. TMDs have unique properties that are useful for logic devices, such as bandgap tunability (approximately 1.3-1.9 eV) without surface dangling-bond, controllable valley and spin polarization, high I on /I off current ratio up to 10 8 in field-effect transistors (FETs), outstanding carrier transport mobility for both holes and electrons, high stability, and most importantly the exhibition of both unipolar as well as ambipolar behavior. Various 2D materials have been explored over the few past years and among the 2D transition metal chalcogenides (TMDs), WS 2 , WSe 2 , and MoS 2 are suitable for semiconductor devices. [2] Particularly, the semiconducting TMDCs exhibit compelling photovoltaic characteristics because of their tunable bandgap and relatively high mobilities. [3] Until now, the researchers have particularly focused on the electrical and photovoltaic properties of the II-VI compounds because of their excellent quantum-size effects and stable electrical behavior. Conversely, the IV-VI p-type TMDs, particularly GeSe, did not receive much attention despite their potential applications in photovoltaics (PV), transparent thin-film transistors, memristors, and flexible electronics. [4] GeSe is a layered p-type material with a relatively narrow bandgap and is utilized in electron tunneling devices and photo detectors. The direct (monolayer) and indirect (bulk) bandgaps of GeSe are approximately 1.7 and 1.08 eV, respectively. [5] GeSe has unique optical and electronic properties, and its optical properties are dominated by excitonic effects. [6] In 2D materials, the saddle points in electronic structure give rise to the diverging density of states. This leads to some intriguing physical phenomena that help improve optical absorption. GeSe possesses saddle points in both the highest valence band and the lowest conduction band. [7] Moreover, similar to the IV-VI chalcogenides, GeSe has a direct bandgap, and the indirect and direct bandgaps lie close to each other, making it promising for solar photovoltaic applications. [8] Despite some theoretical research on GeSe, not much attention has been paid to the heterojunctions (HJs) of GeSe and 2D n-type

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