K. M. F. Shahil

Crystal symmetry breaking in few-quintuple Bi2Te3 films: Applications in nanometrology of topological insulators

The authors report results of micro-Raman spectroscopy investigation of mechanically exfoliated single-crystal bismuth telluride films with thickness ranging from a few-nanometers to bulk limit. It is found that the optical phonon mode A1u, which is not-Raman active in bulk Bi2Te3 crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g modes to the in-plane Eg mode grow with decreasing film thickness. The evolution of Raman signatures with the film thickness can be used for identification of Bi2Te3 crystals with the thickness of few-quintuple layers important for topological insulator and thermoelectric applications.

Micro-Raman spectroscopy of mechanically exfoliated few-quintuple layers of Bi2Te3, Bi2Se3, and Sb2Te3 materials

Bismuth telluride (Bi2Te3) and related compounds have recently attracted strong interest, owing to the discovery of the topological insulator properties in many members of this family of materials. The few-quintuple films of these materials are particularly interesting from the physics point of view. We report results of the micro-Raman spectroscopy study of the “graphene-like” exfoliated few-quintuple layers of Bi2Te3, Bi2Se3, and Sb2Te3. It is found that crystal symmetry breaking in few-quintuple films results in appearance of A1u-symmetry Raman peaks, which are not active in the bulk crystals. The scattering spectra measured under the 633-nm wavelength excitation reveals a number of resonant features, which could be used for analysis of the electronic and phonon processes in these materials. In order to elucidate the influence of substrates on the few-quintuple-thick topological insulators, we examined the Raman spectra of these films placed on mica, sapphire, and hafnium-oxide substrates. The obtained results help to understand the physical mechanisms of Raman scattering in the few-quintuple-thick films and can be used for nanometrology of topological insulator films on various substrates.

Graphene-based thermal interface materials

Thermal management in electronic circuits is becoming an important integral part of design considerations. Increasing power densities and speed of advanced computer chips motivate the search for more efficient thermal interface materials. Here we report preliminary results of experimental and theoretical investigations of the epoxy composites, which use the liquid-phase exfoliated graphene and few-layer graphene as filler materials. Thermal properties of the obtained graphene-epoxy composites were measured using the “laser flash” technique. It was found that the thermal conductivity enhancement factor exceeded ∼ 1000% at 5% of the volume loading fraction. This enhancement is larger than anything that has been achieved with other filler materials. Our physics-based modeling analysis suggests that graphene can outperform other carbon allotropes and derivatives as the thermal filler material.

Graphene fillers for ultra-efficient thermal interface materials

Summary form only given. Continuous scaling of Si CMOS devices and circuits, increased speed and integration densities resulted in problems with thermal management of nanoscale device and computer chips. Further progress in information, communication and energy storage technologies requires more efficient heat removal methods and stimulates the search for thermal interface material (TIMs) with enhanced thermal conductivity. The commonly used TIMs are filled with the particles such as silver or silica. The conventional TIMs require high volume fractions of the filler (~70%) to achieve thermal conductivity of ~1-5 W/mK. Recently, some of us discovered that graphene has extremely high intrinsic thermal conductivity, which exceeds that of carbon nanotubes. To use this property for thermal management of nanoscale electronic devices, we utilized the inexpensive liquid-phase exfoliated graphene and multi-layer graphene (MLG) as filler materials in TIMs. The thermal properties of the obtained graphene-epoxy composites were measured using the “laser flash” technique. It was found that the thermal conductivity enhancement factor exceeded a factor of 23 at 10% of the graphene volume loading fraction. This enhancement is larger than anything that has been achieved using other fillers. We have also tested graphene flakes in the electrically-conductive hybrid graphene-metal particle TIMs. The thermal conductivity of resulting composites was increased by a factor of ~5 in a temperature range from 300 K to 400 K at a small graphene loading fraction of 5-vol.-%. The unusually strong enhancement of thermal properties was attributed to the high thermal conductivity of graphene, strong graphene coupling to matrix materials and the large range of the length-scale - from nanometers to micrometers - of the graphene and silver particle fillers. Graphene-based TIMs have a number of other advantages related to their viscosity and adhesion, which meet the industry requirements. Our results suggest that graphene can become excellent filler materials in the next generation of TIMs for the electronic, optoelectronic and photovoltaic solar cell applications.

Low-frequency 1/f noise in bismuth selenide Topological Insulators

Topological insulators is a newly discovered class of materials with the Dirac cone type dispersion at the surface and conventional band in the volume of the material. We present results of the study of the low-frequency excess noise in thin films made of Bi<inf>2</inf>Se<inf>3</inf> topological insulator material. The films were prepared through mechanically cleavage from the bulk crystal via the “graphene-like” exfoliation procedure. We verified the quality and crystallinity of Bi<inf>2</inf>Se<inf>3</inf> samples with the micro-Raman spectroscopy. Our fabricated devices have linear current voltage characteristics in the low bias region with the current fluctuation noise spectral density SI proportional to 1/f for frequency f less than 10 kHz. The noise spectral density SI showed the square law dependence on the source-drain current and changed from about ∼10⁻²² to 10⁻¹⁸ A²/Hz as current changes form ∼10⁻⁷ to 10⁻⁵ A. Our results can be used for understanding electron transport and trap dynamics, and for reducing low-frequency noise in topological insulator devices.