Recent advances in nanoscale characterization and device fabrications have opened up opportunities for layered semiconductors in nanoelectronics and optoelectronics. Due to strong confinement in monolayer thickness, physical properties of this materials are greatly influenced by parameters such as strain, defects, and doping at the nanoscale. Therefore, understanding the effect of this parameters on layered semiconductors is the prerequisite for any device application. In this doctoral thesis, impact of such parameters on the optical properties of layered semiconductors are studied in nanoscale. MoS2, the most famous transition metal dechalcogenide (TMDC) (n-type semiconductor), and p-type GaSe, a member of metal monochalcogenide (MMC) are investigated in this work. Finally, in outlook, a device made of p-type few layer GaSe and n-type 1L-MoS2 is discussed.

This dissertation is devoted to the development of novel devices for optoelectronic and photovoltaic applications using the promise of inkjet printing with two-dimensional (2D) materials. A systematic approach toward the characterization of the liquid exfoliated 2D inks comprising of graphene, molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and 2D perovskites is discussed at depth. In the first study, the biocompatibility of 2D materials -- graphene and MoS2 -- that were drop cast onto flexible PET and polyimide substrates using mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) cell lines, was explored. The polyimide samples for both STO and HEF showed high biocompatibility with a cell survival rate of up to ~ 98% and a confluence rate of 70-98%. An inkjet printed, biocompatible, heterostructure photodetector was constructed using inks of photo-active MoS2 and electrically conducting graphene, which facilitated charge collection of the photocarriers. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet printed 2D heterojunction devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. Strain-dependent measurements were also conducted with bending, that showed Iph ~ 1.16 µA with strain levels for curvature up to ~ 0.262 cm-1, indicating the feasibility of such devices for large format arrays printed on flexible substrates. Alongside the optoelectronic measurements, temperature-dependent (~ 80 K to 573 K) frequency shifts of the Raman-active E12g and A1g modes of multilayer MoS2 exhibited a red-shift with increasing temperature, where the temperature coefficients for the E12g and A1g modes were determined to be ~ - 0.016 cm-1/K and ~ - 0.014 cm-1/K, respectively. The phonon lifetime τ was determined to be in the picosecond range for the E12g and A1g modes, respectively, for the liquid exfoliated multilayer MoS2. Secondly, an all inkjet printed WSe2-graphene hetero-structure photodetector on flexible polyimide substrates is also studied, where the device performance was found to be superior compared to the MoS2-graphene photodetector. The printed photodetector was photo responsive to broadband incoming radiation in the visible regime, where the photo responsivity R ~ 0.7 A/W and conductivity σ ~ 2.3 × 10-1 S/m were achieved at room temperature. Thirdly, the synthesis of solution-processed 2D layered organo-halide (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 2, 3, and 4) perovskites is presented here, where inkjet printing was used to fabricate heterostructure flexible photodetector devices on polyimide substrates. The ON/OFF ratio was determined to be high, ~ 2.3 × 103 while the photoresponse time on the rising and falling edges was measured to be rise ~ 24 ms and fall ~ 65 ms, respectively. The strain-dependent measurements, conducted here for the first time for inkjet printed perovskite photodetectors, revealed the Ip decreased by only ~ 27% with bending (radius of curvature of ~ 0.262 cm-1). This work demonstrates the tremendous potential of the inkjet printed, composition tunable, organo-halide 2D perovskite heterostructures for high-performance photodetectors, where the techniques are readily translatable toward flexible solar cell platforms as well. Fourthly, metal contacts and carrier transport in 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 4) perovskites is a critical topic, where the use of silver (Ag) and graphene (Gr) inks as metallic contacts to 2D perovskites was investigated. The all inkjet printed Gr-perovskite and Ag-perovskite photodetectors were found to be photo-responsive to broadband incoming radiation where measurements were conducted from λ ~ 400 nm to 2300 nm. The photoresponsivity R and detectivity D were compared between the Gr-perovskite and Ag-perovskite photodetectors, which revealed the higher performance for the Ag-perovskite photodetector. The superior performance of the Ag-perovskite photodetector was also justified with the Schottky barrier analysis using the thermionic emission model through temperature-dependent transport measurements. Finally, this dissertation ends with the description of the first steps for using solution-processed, inkjet printed perovskites for solar cells. The preliminary investigations include the discussion of the chemical formulations for the carrier separation layers, dispersion route, and the variation of solar cell figures of merit with processing.

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No / Here we report a hierarchical composite structure composed of few-layers molybdenum disulfide nanosheets supported by vertical graphene on conductive carbon cloth (MDNS/VG/CC) for high-performance electrochemical hydrogen evolution reaction (HER). In the fabrication, 3D vertical graphene is first prepared on carbon cloth by a micro-wave plasma enhanced chemical vapor deposition (MPCVD) and then few-layers MoS2 nanosheets are in-situ synthesized on the surface of the vertical graphene through a simple hydrothermal reaction. This integrated catalyst exhibits an excellent HER electrocatalytic activity including an onset potential of 50 mV, an overpotential at 10 mA cm(-2) (eta(10)) of 78 mV, a Tafel slop of 53 mV dec(-1), and an excellent cycling stability in acid solution. The excellent catalytic performance can be ascribed to the abundant active edges provided by the vertical MoS2 nanosheets, as well as the effective electron transport route provided by the graphene arrays on the conductive substrate. Moreover, the vertical graphene offers robust anchor sites for MoS2 nanosheets and appropriate intervals for electrolyte infiltration. This not only benefits hydrogen convection and release but also avoids the damaging or restacking of catalyst in electrochemical processes. / This work was financially supported by the National Natural Science Foundation of China (Grant nos. 61176007, 51372213, and 51402343).

This thesis deals with the research work carried out on the properties and applications such as GaN nanoparticles, Graphene etc. Chapter 1 of the thesis gives introduction to nanomaterials and various aspects of the thesis. Chapter 2 of the thesis describes the synthesis of GaN nanocrystals and their use as white light sources and as room temperature gas sensors. It also discusses negative differential resistance above room temperature exhibited by GaN. Electroluminescence from GaN-polymer heterojunction forms the last section of this chapter. Chapter 3 demonstrates the role of defect concentration on the photodetecting properties of ZnO nanorods with different defects prepared at different temperatures. Chapter 4 presents remarkable infrared and ultraviolet photodetector properties of reduced graphene oxide and graphene nanoribbons. Chapter 5 presents the infrared detecting properties of graphene-like few-layer MoS2. The summary of the thesis is given at the end of the thesis.

Technology scaling beyond Moore’s law demands cutting-edge solutions of the gate length scaling in sub-10 nm regime for low power high speed operations. Recently SOI technology has received considerable attention, however manufacturable solutions in sub-10 nm technologies are not yet known for future nanoelectronics. Therefore, to continue scalinginsub-10 nm region, new one(1D) and two dimensional(2D) “nano-materials” and engineering are expected to keep its pace. However, significant challenges must be overcome for nano-material properties in carrier transport to be useful in future silicon nanotechnology. Thus, it is very important to understand and modulate their electronic band structure and transport properties for low power nanoelectronics applications. This thesis tries to provide solutions for some problems in this area. In recent times, one dimensional Silicon nanowire has emerged as a building block for the next generation nano-electronic devices as it can accommodate multiple gate transistor architecture with excellent electrostatic integrity. However as the experimental study of various energy band parameters at the nanoscale regime is extremely challenging, usually one relies on the atomic level simulations, the results of which are at par with the experimental observations. Two such parameters are the band gap and effective mass, which are of pioneer importance for the understanding of the current transport mechanism. Although there exists a large number of empirical relations of the band gap in relaxed Silicon nanowire, however there is a growing demand for the development of a physics based analytical model to standardize different energy band parameters which particularly demands its application in TCAD software for predicting different electrical characteristics of novel devices and its strained counterpart to increase the device characteristics significantly without changing the device architecture. In the first part of this work reports the analytical modeling of energy band gap and electron transport effective mass of relaxed and strained Silicon nanowires in various crystallographic directions for future nanoelectronics. The technology scaling of gate length in beyond Moore’s law devices also demands the SOI body thickness, TSi0 which is essentially very challenging task in nano-device engineering. To overcome this circumstance, two dimensional crystals in atomically thin layered materials have found great attention for future nanolectronics device applications. Graphene, one layer of Graphite, is such 2D materials which have found potentiality in high speed nanoelectronics applications due to its several unique electronic properties. However, the zero band gap in pure Graphene makes it limited in switching device or transistor applications. Thus, opening and tailoring a band gap has become a highly pursued topic in recent graphene research. The second part of this work reports atomistic simulation based real and complex band structure properties Graphene-Boron nitride heterobilayer and Boron Nitride embedded Graphene nanoribbons which can improve the grapheme and its nanoribbon band structure properties without changing their originality. This part also reports the direct band-to-band tunneling phenomena through the complex band structures and their applications in tunnel field effect transistors(TFETs) which has emerged as a strong candidate for next generation low-stand by power(LSTP) applications due to its sub-60mV/dec Sub threshold slope(SS). As the direct band-to-band tunneling(BTBT) is improbable in Silicon(either its bulk or nanowire form), it is difficult to achieve superior TFET characteristics(i.e., very low SS and high ON cur-rent) from the Silicon TFETs. Whereas, it is explored that much high ON current and very low subthreshold slope in hybrid Graphene based TFET characteristics open a new prospect in future TFETs. The investigations on ultrathin body materials also call for a need to explore new 2D materials with finite band gap and their various nanostructures for future nanoelectronic applications in order to replace conventional Silicon. In the third part of this report, we have investigated the electronic and dielectric properties of semiconducting layered Transition metal dichalcogenide materials (MX2)(M=Mo, W;X =S, Se, Te) which has recently emerged as a promising alternative to Si as channel materials for CMOS devices. Five layered MX2 materials(exceptWTe2)in their 2D sheet and 1D nanoribbon forms are considered to study the real and imaginary band structure of thoseMX2 materials by atomistic simulations. Studying the complex dispersion properties, it is shown that all the five MX2 support direct BTBT in their monolayer sheet forms and offer an average ON current and subthresholdslopeof150 A/mand4 mV/dec, respectively. However, onlytheMoTe2 support direct BTBT in its nanoribbon form, whereas the direct BTBT possibility in MoS2 and MoSe2 depends on the number of layers or applied uniaxial strain. WX2 nanoribbons are shown to be non-suitable for efficient TFET operation. Reasonably high tunneling current in these MX2 shows that these can take advantage over conventional Silicon in future tunnel field effect transistor applications.

Technology scaling beyond Moore’s law demands cutting-edge solutions of the gate length scaling in sub-10 nm regime for low power high speed operations. Recently SOI technology has received considerable attention, however manufacturable solutions in sub-10 nm technologies are not yet known for future nanoelectronics. Therefore, to continue scalinginsub-10 nm region, new one(1D) and two dimensional(2D) “nano-materials” and engineering are expected to keep its pace. However, significant challenges must be overcome for nano-material properties in carrier transport to be useful in future silicon nanotechnology. Thus, it is very important to understand and modulate their electronic band structure and transport properties for low power nanoelectronics applications. This thesis tries to provide solutions for some problems in this area. In recent times, one dimensional Silicon nanowire has emerged as a building block for the next generation nano-electronic devices as it can accommodate multiple gate transistor architecture with excellent electrostatic integrity. However as the experimental study of various energy band parameters at the nanoscale regime is extremely challenging, usually one relies on the atomic level simulations, the results of which are at par with the experimental observations. Two such parameters are the band gap and effective mass, which are of pioneer importance for the understanding of the current transport mechanism. Although there exists a large number of empirical relations of the band gap in relaxed Silicon nanowire, however there is a growing demand for the development of a physics based analytical model to standardize different energy band parameters which particularly demands its application in TCAD software for predicting different electrical characteristics of novel devices and its strained counterpart to increase the device characteristics significantly without changing the device architecture. In the first part of this work reports the analytical modeling of energy band gap and electron transport effective mass of relaxed and strained Silicon nanowires in various crystallographic directions for future nanoelectronics. The technology scaling of gate length in beyond Moore’s law devices also demands the SOI body thickness, TSi0 which is essentially very challenging task in nano-device engineering. To overcome this circumstance, two dimensional crystals in atomically thin layered materials have found great attention for future nanolectronics device applications. Graphene, one layer of Graphite, is such 2D materials which have found potentiality in high speed nanoelectronics applications due to its several unique electronic properties. However, the zero band gap in pure Graphene makes it limited in switching device or transistor applications. Thus, opening and tailoring a band gap has become a highly pursued topic in recent graphene research. The second part of this work reports atomistic simulation based real and complex band structure properties Graphene-Boron nitride heterobilayer and Boron Nitride embedded Graphene nanoribbons which can improve the grapheme and its nanoribbon band structure properties without changing their originality. This part also reports the direct band-to-band tunneling phenomena through the complex band structures and their applications in tunnel field effect transistors(TFETs) which has emerged as a strong candidate for next generation low-stand by power(LSTP) applications due to its sub-60mV/dec Sub threshold slope(SS). As the direct band-to-band tunneling(BTBT) is improbable in Silicon(either its bulk or nanowire form), it is difficult to achieve superior TFET characteristics(i.e., very low SS and high ON cur-rent) from the Silicon TFETs. Whereas, it is explored that much high ON current and very low subthreshold slope in hybrid Graphene based TFET characteristics open a new prospect in future TFETs. The investigations on ultrathin body materials also call for a need to explore new 2D materials with finite band gap and their various nanostructures for future nanoelectronic applications in order to replace conventional Silicon. In the third part of this report, we have investigated the electronic and dielectric properties of semiconducting layered Transition metal dichalcogenide materials (MX2)(M=Mo, W;X =S, Se, Te) which has recently emerged as a promising alternative to Si as channel materials for CMOS devices. Five layered MX2 materials(exceptWTe2)in their 2D sheet and 1D nanoribbon forms are considered to study the real and imaginary band structure of thoseMX2 materials by atomistic simulations. Studying the complex dispersion properties, it is shown that all the five MX2 support direct BTBT in their monolayer sheet forms and offer an average ON current and subthresholdslopeof150 A/mand4 mV/dec, respectively. However, onlytheMoTe2 support direct BTBT in its nanoribbon form, whereas the direct BTBT possibility in MoS2 and MoSe2 depends on the number of layers or applied uniaxial strain. WX2 nanoribbons are shown to be non-suitable for efficient TFET operation. Reasonably high tunneling current in these MX2 shows that these can take advantage over conventional Silicon in future tunnel field effect transistor applications.