Thermal Properties of the Binary-Filler Composites with Few-Layer Graphene and Copper Nanoparticles

The thermal properties of an epoxy-based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the "synergistic" filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size-dissimilar fillers, has a well-defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of ~15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches ~40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, ~40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. The electrical percolation is observed at low graphene loading, less than 7 wt.%, owing to the large aspect ratio of graphene. At all concentrations of the fillers, below and above the electrical percolation threshold, the thermal transport is dominated by phonons. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.Comment: 25 pages, 4 figure

Low-Frequency Noise Spectroscopy of Charge-Density-Wave Phase Transitions in Vertical Quasi-2D Devices

We report results regarding the electron transport in vertical quasi-2D layered 1T-TaS2 charge-density-wave devices. The low-frequency noise spectroscopy was used as a tool to study changes in the cross-plane electrical characteristics of the quasi-2D material below room temperature. The noise spectral density revealed strong peaks - changing by more than an order-of-magnitude - at the temperatures closely matching the electrical resistance steps. Some of the noise peaks appeared below the temperature of the commensurate to nearly-commensurate charge-density-wave transition, possibly indicating the presence of the debated "hidden" phase transitions. These results confirm the potential of the noise spectroscopy for investigations of electron transport and phase transitions in novel materials.Comment: 16 pages; 5 figure

Fabrication and Testing of Room-Temperature Charge-Density-Wave Devices

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

Low-frequency noise spectroscopy of charge-density-wave phase transitions in vertical quasi-2D 1T-TaS2 devices

We report results regarding the electron transport in vertical quasi-two-dimensional (2D) layered 1T-TaS 2 charge-density-wave (CDW) devices. The low-frequency noise spectroscopy was used as a tool to study changes in the cross-plane electrical characteristics of the quasi-2D material below room temperature. The noise spectral density revealed strong peaks - changing by more than an order-of-magnitude - at the temperatures closely matching the electrical resistance steps. Some of the noise peaks appeared below the temperature of the commensurate to nearly-commensurate CDW transition, possibly indicating the presence of the debated hidden phase transitions. These results confirm the potential of the noise spectroscopy for investigations of electron transport and phase transitions in novel materials