As transistors continue to decrease in size and packing densities increase, thermal management becomes a critical bottleneck for development of the next generation of compact and flexible electronics. The increase in computer usage and ever-growing dependence on cloud systems require better methods for dissipating heat away from electronic components. The important ingredients of thermal management are the thermal interface materials. The discovery of excellent heat conduction properties of graphene and few-layer graphene stimulated research on practical applications of graphene fillers in thermal interface materials. The initial studies of graphene fillers in thermal interface materials were focused almost exclusively on curing epoxy-based composites. However, many thermal management applications require specifically noncuring thermal paste type materials. This dissertation reports on the synthesis and thermal conductivity measurements of noncuring thermal paste based on mineral oil with the mixture of graphene and few-layer graphene flakes as the fillers. The relatively simple composition has been selected in order to systematically compare the performance and understand the mechanisms governing heat conduction. It was found that graphene thermal paste exhibits a distinctive thermal percolation threshold with the thermal conductivity revealing a sublinear dependence on the filler loading. This behavior contrasts with the thermal conductivity of curing graphene thermal interface materials, based on epoxy, where super-linear dependence on the filler loading is observed. The performance of graphene thermal paste was benchmarked against top-of-the-line commercial thermal pastes. The obtained results show that noncuring graphene thermal interface materials outperforms the best commercial pastes in terms of thermal conductivity, at substantially lower filler concentration. The results of this dissertation research shed light on the thermal percolation mechanism in noncuring polymeric matrices laden with quasi-two-dimensional fillers. Considering recent progress in graphene production via liquid phase exfoliation and oxide reduction, it is possible that the undertaken approach will open a pathway for large-scale industrial application of graphene in thermal management of electronics.

As the electronic industry aggressively moves towards nanometer designs thermal issues are becoming increasingly important for the high-end electronic chips. One of the approaches to mitigate the self-heating problems is the high-heat-flux hot-spot removal via incorporation into the chip designs of materials with the high thermal conductivity. Graphene is found to be one of the best known heat conductors, thus it can be used in nanoelectronic and optoelectronic devices as a heat spreader component. Graphene, the latest isolated allotrope of carbon made of individual atomic sheets bound in two dimensions, shows many remarkable properties. A non-contact method of measuring G peak position of the Raman spectrum as a function of both the temperature of the graphene sample and the power of the heat source was used to measure the thermal conductivity of graphene. The samples in the experiment had approximately rectangle geometry and the assumption about the plane heat wave was used for the data extraction. In this dissertation research we propose to develop a model and numerical procedure for the (i) accurate modeling-based data extraction for the thermal conductivity measurements; and (ii) simulate heat propagation in semiconductor device structures with graphene layers incorporated as heat spreaders. To achieve the goals of this dissertation research we simulated the heat transport in graphene using the finite element method (FEM) with the help of COMSOL software package, which solves numerically the partial differential equations. The modeling based data extraction was necessary to determine thermal conductivity of the graphene flakes of arbitrary shape. It also substantially improved the accuracy of the measurements. The simulation of heat propagation in device structures with graphene heat spreaders allows one to assess the feasibility of the graphene high-heat-flux thermal management. We focused on understanding how thermal transport is influenced by a surface geometry of the sample and geometries of the heat sources. The simulation results showed that the size, shape and heat source geometry impact heat propagation in different ways and have to be included in the experimental data extraction. The simulation procedure provided a necessary input for next experiments on heat conduction in graphene structures e.g., graphene multi-layers and graphene-heat sink structures and other device-level thermal management applications. It was found that the incorporation of graphene or few-layer graphene (FLG) layers with proper heat sinks can substantially lower the temperature of the localized hot spots. The developed model and obtained results are important for the design of graphene heat spreaders and interconnects and lead to a new method of heat removal from nanoelectronic and 3-D chips.

Power dissipation rates of modern semiconductor devices continually increases year after year. Despite this, safe operating temperatures of these devices remain moderately low, typically below 90 °C, after which point devices performance deviates from design and lifespan significantly diminishes. As a result, ever-increasing thermal dissipation capacity is required to achieve acceptable operating temperature. Polymer composites are often applied between two imperfect surfaces to replace air gaps and act as a better thermal bridge to facilitate heat flow between solids or as chip encapsulants to protect from environmental contamination. In each application, the overall thermal dissipation, and thus operating temperature, can be improved by using more thermally conductive polymers. The thermal conductivity of most polymers, certainly all of those in common general use in industry are quite low, below 0.3 W/mK. It is a common tactic to composite the polymers with other, highly thermally conductive materials to then raise the overall composite thermal conductivity. The extraordinary thermal conductivity of graphene has prompted significant study into composites composed of it for these applications. However, since graphene is also electrically conductive, composites filled to a sufficient level exhibit substantial electrical conductivity, which can be problematic in the electronics industry. The composite may make immediate contact or over the course of a realistic lifespan may make unavoidable, inadvertent contact with active circuit elements resulting from migration phenomena, which may threaten circuit stability depending on the composite’s electrical conductivity. This dissertation research developed a methodology for the independent control of thermal and electrical conductivity of graphene composites through hybridization by finely controlling filler levels of graphene and electrically insulating hexagonal boron nitride. Additionally, the lifespan performance of graphene polymer composites was studied in a custom-built accelerated aging instrument which revealed an unexpected enhancement of thermal conductivity in up to 500 power cycling treatments. The improved performance was attributed to an increased coupling between graphene and epoxy polymer resultant from increased cross-linking density, resulting in more firm contact and lower Kapitza resistance. The obtained results are important for future thermal management technologies that leverage the unique heat conduction properties of graphene.

Quasi-2D charge-density-wave van der Waals materials demonstrated potential for novel electronic device functionality, which can be achieved at room temperature. This dissertation reports on fabrication and testing of quasi-2D 1T-TaS2 charge-density-wave devices, focusing on the switching mechanisms of such devices. We tested the switching of 1T-TaS2 thin-film charge-density-wave devices, using nanosecond-duration electrical pulsing to construct their time-resolved current-voltage characteristics. The switching action was based upon the nearly-commensurate to incommensurate charge-density-wave phase transition in this material. For sufficiently short pulses, with rise times in the nanosecond range, self-heating of the devices is suppressed, and their current-voltage characteristics are weakly non-linear and free of hysteresis. This changes as the pulse duration is increased to ~200 ns, where the current develops pronounced hysteresis that evolves non-monotonically with the pulse duration. By combining the results of our experiments with a numerical analysis of transient heat diffusion in these devices, we established the thermal origins of their switching. In spite of this thermal character, our modeling suggests that suitable reduction of the size of these devices should allow their operation at GHz frequencies. We found that the charge-density-wave depinning process in 1T-TaS2 devices is not accompanied by an abrupt increase in electric current – in striking contrast to depinning in the “classical” charge-density-wave materials with quasi-1D crystal structure. It was demonstrated that the low-frequency noise spectroscopy and the differential current-voltage characteristics provide unambiguous metrics for the depinning threshold field in quasi-2D materials. It was also established that the depinning of the charge-density waves in quasi-2D materials is of the field-induced nature, and the threshold fields are substantially larger than those in quasi-1D van der Waals materials. The results obtained in this dissertation research are important for the proposed applications of the charge-density-wave devices in electronics.

Depend on a number of unique properties, Graphene is beneficial for microelectronic devices. To design logical gates using graphene without bandgap is a chanllenge. As a result, "conventional" design for construction is required to make this breakthrough. The theory of negative differential resistance is used in this design to build logical gate with gapless graphene. And the non-boolean computational architecture shows exciting result for the future of special construction. In this Section, we propose and discuss several special task grpahene-based logic circuits and illustrate their operation by numerical modeling.

This dissertation presents results of the experimental studies of graphene thermal properties and discusses possibilities of graphene applications for thermal management of high-power density electronic devices. The results reported here are divided into two parts. In part one, I describe fabrication and testing of graphene and few-layer graphene (FLG) heat spreaders for high-power AlGaN/GaN transistors. The mechanically exfoliated graphene-graphite layers have been transferred on top of AlGaN/GaN field-effect transistors and attached close to the heat generating regions near the metal contacts. The micro-Raman spectroscopy was used for in situ monitoring of the temperature of the hotspots. It was demonstrated that the temperature of the hot spots can be lowered by ~20 °C in transistors operating at ~13 W/mm power. The latter corresponds to an order-of-magnitude increase in the device lifetime. Simulations results supported our conclusions and indicated that graphene-graphite heat spreaders perform even better in AlGaN/GaN transistors on sapphire substrates. The proposed approach for heat removal at micrometer and nanometer scale represents a transformative change in thermal management of electronics.

In part two of this dissertation, I report the results of investigation of thermal conductivity of thin films made of a novel nanostructured graphene material, which consists of graphene nanoribbons encapsulated in single walled carbon nanotubes. The temperature dependent Raman spectrum of this material was measured in order to obtain the temperature coefficients of G+ peak and 2D peaks. Using the Raman optothermal technique, I determine the local temperature rise due to laser heating from the shifts in Raman peak positions. A finite element analysis method was conducted to simulated heat dissipation in the samples and to determine their effective thermal conductivity. The obtained results suggest that this hybrid graphene - carbon nanotube material can be used as fillers in thermal interface materials.

This thesis describes the development and experimental testing of the cryogenic Brillouin – Mandelstam light scattering spectrometer system. The advanced features of the spectrometer system include the specially designed rotating microscopy stage and imaging system for measuring the energy dispersion of phonons and magnons in samples with smaller dimensions than those acceptable for conventional spectrometers. In this thesis, I provide an overview of the Brillouin light scattering fundamentals, summarize the principles of operation of the tandem Fabry-Perot interferometer, and describe the steps in the hardware assembly of the Brillouin – Mandelstam spectrometer system. Details of the beam-path design, optical assemblies, and sample holders are also provided. In order to test the operation of the spectrometer, I acquired light scattering spectra from silicon, diamond, pure epoxy, and graphene-epoxy composites at various light incident angles and in the temperature range from 4 K to 300 K. The characteristic features of the measured spectra and experimental capabilities of the spectrometer are discussed in details. The developed and built cryogenic Brillouin – Mandelstam spectrometer system is expected to become an essential tool enabling cutting-edge research, advanced industrial material characterization, and graduate student training.

Nanostructure-based solar cells are attracting significant attention as possible candidates for drastic improvement in photovoltaic (PV) energy conversion efficiency. Although such solar cells are expected to be more expensive there is growing need for the efficient and light-weight solar cells in aero-space and related industries. In this dissertation I present results of the theoretical, computational and experimental investigation of novel designs for quantum dot superlattice (QDS) based PV elements and advanced materials for transparent solar cells. In the first part of the dissertation I describe possible implementation of the intermediate-band (IB) solar cells with QDS. The IB cells were predicted to have PV efficiency exceeding the Shockley-Queisser limit for a single junction cell. The parameters of QDS structure have to be carefully tuned to achieve the desired charge carrier dispersion required for the IB operation. The first-principles models were used to calculate the electrical properties and light absorption in QDS. This approach allowed me to determine the dimensions of QDS for inducing the mini-band which plays the role of the IB. Using the detailed balance theory it was determined that the upper-bound PV efficiency of such IB solar cells can be as high as ~51%. The required QDS dimensions on the basis of InAsN/GaAsSb are technologically challenging but feasible: ~2-6 nm. Using the developed simulation tools I proposed several possible designs of QDS solar cells including one, which combined the benefits of the IB concept and the advanced tandem cell design. The second part of the dissertation presents a study of graphene layers as transparent electrodes for the PV cells. The graphene layers were mechanically exfoliated from bulk graphite and characterized with micro-Raman spectroscopy. It was found that graphene electrodes have good electrical conductivity, which reveals unusual temperature dependence beneficial for the proposed application. The decrease in resistance with temperature was explained by the thermal generation of the electron-hole pairs in the conditions when the carrier mobility is limited by the defect scattering. The final part of the dissertation presents simulation results of electrical current transport in graphene ribbons, which can be used as transparent electrodes or interconnects.

Graphene demonstrates potential in practical applications due to its excellent electronic and thermal properties. Typical graphene field-effect transistors and interconnects built on conventional SiO2/Si substrates reveal a breakdown current density on the order of 1 uA/nm2 (i.e. 10^8 A/cm2) which is approximately 100 times larger than the fundamental limit for metals, but still smaller than the maximum achieved in carbon nanotubes. In this dissertation research I investigated different methods of fabricating graphene devices and interconnects on synthetic diamond and tested their current-voltage and thermal characteristics. Several types of these graphene-on-diamond devices have been compared to graphene-on-SiO2/Si devices as a baseline reference. It was established that by replacing SiO2 with synthetic diamond one can substantially increase the current carrying capacity of graphene to a level as high as ~18 uA/nm2 in ambient conditions. The obtained results indicate that graphene's current-induced breakdown is thermally activated. It was also found that the current carrying capacity of graphene can be improved not only on single-crystal diamond substrates, but also on inexpensive ultrananocrystalline diamond (UNCD). Additionally, UNCD can be produced in low-temperature processes compatible with conventional Si technology which is attributed to the decreased thermal resistance of ultrananocrystalline diamond layers at elevated temperatures. The obtained results are important for graphene's applications in interconnects and transistors and can lead to the new planar sp2-on-sp3 carbon-on-carbon technology.