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Graphene, a monolayer of carbon atoms, has gained prominence to augment existing chip-scale photonic and optoelectronic applications, especially for sensing in optical radiation, owing to its distinctive electrical properties and bandgap as well as its atomically thin profile. As a building block of photodetection, graphene has been co-integrated with mature silicon technology to realize the on-chip, high-performance photo-detecting platforms with broad spectral response from the deep-ultraviolet (UV) to the mid-infrared (MIR) regime. The recent state-of-the-art graphene-based photodetectors utilizing the combination of colloidal quantum dots (QDs) and graphene have been intensively studied, where QDs function as the absorber and the role of graphene is as a fast carrier recirculating channel. With such a configuration, an ultrahigh sensitivity can be achieved on account of the photogating mechanism; however, the response time is slow and limited to the millisecond-to-second range. To achieve balance between high sensitivity and fast response time, we have demonstrated a new photodetector that is based on graphene/two-dimensional heterostructures. The homogeneous thickness and the large contact of the heterostructure give rise to fast carrier transporting between the thin absorber layer and the graphene, leading to a fast response time. This thesis carefully investigates the optimization of fabrication as well as optoelectronic characterization of photodetectors based on graphene/semiconductor heterostructures field-effect transistors (GFETs). GFETs with different architectures were demonstrated and systematically studied under optical illumination ranging from deep-UV to MIR at varying optical powers. Noise behaviors have been studied under different device parameters such as device structure, area and gate-bias. Results show that the flicker noise of graphene-based devices can be explained by the McWhorter model in which the fluctuation of carrier numbers is the dominant process of noise in low frequencies; thus, it can be scaled down by reducing the number of introduced charged carriers with optimized fabrication. Besides, the impact of absorber on top of graphene and the bottom substrate has been comprehensively explored through various experimental techniques including current-voltage (IV), photo-response dynamics, and noise characterization measurements. With our configuration, the high sensitivity and fast response time of photodetectors can be obtained at the same time. In addition to this, the study of the bottom substrate with different doping levels suggests a concept of dual-photogating effect which is induced by the top absorbent material and the photoionization of the doped silicon substrate. In summary, this thesis showcases novel device architecture and fabrication procedures of GFETs photodetectors, optimizes device structure, quantifies the performance and evaluates the effect of various absorbent materials and substrate. It provides insight into the improvement of possible routes to achieve a broadband photo-detecting system with higher sensitivity, faster response time and lower noise level. / Doctor of Philosophy / The rapid expansion of networked devices and the development of the Internet of Things have given rise to an internet traffic and data explosion. Since conventional electrical interconnects are unable to rise to the occasion of the ever-growing demands of information technology and communication networking, next-generation alternative interconnects with higher performance and lower loss are attractive alternatives as the chip-scale optical interconnection. Among various optical interconnects, photodetectors play significant roles by converting optical input into electrical signal output. Sensing of light has a great impact in daily applications such as telecommunications, night vision, biomedical imaging and biochemical sensing. Graphene, belonging to the class of 2-dimensional materials, shows enormous potential as a building block of photodetection owing to its outstanding optical and electrical properties. One possible route to develop a sensitive and fast-operating on-chip photodetector is to integrate graphene into silicon photonics platforms since the latter has been widely studied and driven to maturity. In this thesis, graphene-based photodetectors with novel architectures have been fabricated, demonstrated and systematically investigated. Various measurements have been taken to quantify the performance of photodetectors in a wide detecting range from deep ultraviolet to mid-infrared.

Nanocomposites consist of graphene dispersed in matrices of soft materials are promising thermal management materials. A fundamental understanding of the thermal transport at graphene-soft material interfaces is essential for developing these nanocomposites. In this dissertation, thermal transport at graphene-octane interfaces was investigated using molecular dynamics simulations, and the results revealed several important characteristics of such thermal transport. The interfacial thermal conductance of graphene-octane interfaces were studied first. It was found that the interfacial thermal conductance exhibits a distinct duality: if heat enters graphene from one side of its basal plane and leaves it through the other side, the corresponding interfacial thermal conductance, Gacross, is large; if heat enters graphene from both sides of its basal plane and leaves it at a position far away on its basal plane, the corresponding interfacial thermal conductance, Gnon-across, is small. Gacross is ~30 times larger than Gnon-across for a single-layer graphene immersed in liquid octane. Additional analysis showed that this duality originates partially from the strong, positive correlations between the heat fluxes at the two surfaces of a graphene layer. The interfacial thermal conductance of the graphene-soft material interfaces in presence of defects in the graphene was then studied. The results showed that the heat transfer at the interfaces is enhanced by defects. Estimations based on effective medium theories showed that the effective thermal conductivity of the graphene-based composites could even be enhanced with defects in graphene when heat transfer at the graphene-soft material interface is the bottleneck for the thermal transport in these composites. To describe the interfacial thermal transport at graphene interfaces uniformly, a nonlocal constitutive model was proposed and validated to replace the classical Kapitza model. By characterizing the thermal transport properties of graphene interfaces using a pair of thermal conductance, the model affords a uniform description of the thermal transport at graphene interfaces for different thermal transport modes. Using this model, the data interpretation in time domain thermalreflectance (TDTR) measurements was investigated, and the results showed that the interfacial thermal conductance measured in typical TDTR tests is that of the across mode for thin-layered materials. / Ph. D.

The development of a new class of alternative plastics has been encouraged in the past few years due to the serious environmental issues, such as toxicity and carbon dioxide emissions. Hence, the introduction of renewable, biodegradable, and biocompatible materials is becoming critical as substituents of conventional synthetic plastics. To design a new system of novel TEMPO-oxidized cellulose nanofibrils (TOCNs)/graphene oxide (GO) composite, the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation was utilized to disintegrate never-dried wood nanofibrillated cellulose (NFC). GO was incorporated through high intensity homogenization and ultrasonication with varying degree of oxidation (0.5X, 1X, and 2X) of NFC and GO percent loadings: 0.4, 1.2, and 2.0wt %. As a result, despite the presence of carboxylate groups and graphene oxide (GO), X-ray diffraction (XRD) test showed the crystallinity of the bio-nanocomposite was not altered. Scanning electron microscopy (SEM) was used to characterize their morphologies. In addition, the thermal stability of TOCN/GO composite decreased upon oxidation level, and dynamic mechanical analysis (DMA) signified strong intermolecular interactions with the improvement in Young's storage modulus, and tensile strength. Fourier transform infrared spectroscopy (FTIR) was employed to see the hydrogen bonds between GO and cellulosic polymer matrix. The oxygen transmission rate (OTR) of TOCN/GO composite decreased. The water vapor permeability (WVP) was not significantly affected by the reinforcement with GO, but the moderate oxidation enhanced the barrier properties. Ultimately, the newly fabricated TOCN/GO composite can be utilized in a wide range of life science applications, such as food and medical industries. / Master of Science / In recent years, petroleum-based polyolefins have been contributing to severe environmental issues. With this in perspective, the development of a new class of alternative plastics has been encouraged. Hence, the introduction of renewable, biodegradable, and biocompatible materials is becoming critical as a substitute for non-degradable synthetic plastics. In this study, a new system of novel cellulose-based plastic composites was designed by incorporating carbon nanofillers at various percent loadings and different degree of surface modification of the plastics. These treatments are the economical way to achieve the targeted properties for industrial applications, exhibiting the obvious improvement in tensile strength due to the strong interaction between nanofillers and cellulose. In addition, water vapor and oxygen barrier properties play significant roles in food packaging since food decay is vulnerable to these two factors. The barrier performance was enhanced by hindering the permeation of oxygen gases, whereas the water vapor permeability was not significantly affected by the reinforcement with carbon nanofillers. Ultimately, the newly fabricated cellulose plastic can be utilized in various applications, especially, such as the pharmaceutical and biomedical areas, packaging for food and goods, and agriculture due to their high availability, sustainability, and biodegradability.