Vivek Goyal

Thermal properties of the hybrid graphene-metal nano-micro-composites: Applications in thermal interface materials

The authors report on synthesis and thermal properties of the electrically-conductive thermal interface materials with the hybrid graphene-metal particle fillers. The thermal conductivity of resulting composites was increased by ~500% 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 intrinsic 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. The obtained results are important for thermal management of advanced electronics and optoelectronics.

Mechanically-exfoliated stacks of thin films of Bi2Te3 topological insulators with enhanced thermoelectric performance

The authors report on “graphene-like” mechanical exfoliation of single-crystal Bi2Te3 films and thermoelectric characterization of the stacks of such films. Thermal conductivity of the resulting “pseudosuperlattices” was measured by the “hot disk” and “laser flash” techniques. The room temperature in-plane (cross-plane) thermal conductivity of the stacks decreases by a factor of ∼2.4 (3.5) as compared to bulk. The thermal conductivity reduction with preserved electrical properties leads to strong increase in the thermoelectric figure of merit. It is suggested that the film thinning to few-quintuples and tuning of the Fermi level can help in achieving the topological-insulator surface transport regime with an extraordinary thermoelectric efficiency.

Atomically-thin crystalline films and ribbons of bismuth telluride

The authors report on “graphene-like” exfoliation of the large-area crystalline films and ribbons of bismuth telluride with the thicknesses of a few atoms. It is demonstrated that Bi2Te3 crystal can be mechanically separated into its building blocks—Te–Bi–Te–Bi–Te atomic fivefolds—with the thickness of ∼1 nm and even further—to subunits with smaller thicknesses. The atomically-thin films can be structured into suspended crystalline ribbons providing quantum confinement in two dimensions. The quasi two-dimensional crystals of bismuth telluride revealed high electrical conductivity and low thermal conductivity. The proposed atomic-layer engineering of bismuth telluride opens up a principally new route for drastic enhancement of the thermoelectric figure of merit.

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-on-Diamond Devices with Increased Current-Carrying Capacity: Carbon sp2-on-sp3 Technology

Graphene demonstrated potential for practical applications owing to its excellent electronic and thermal properties. Typical graphene field-effect transistors and interconnects built on conventional SiO(2)/Si substrates reveal the breakdown current density on the order of 1 μA/nm(2) (i.e., 10(8) A/cm(2)), which is ~100× larger than the fundamental limit for the metals but still smaller than the maximum achieved in carbon nanotubes. We show that by replacing SiO(2) with synthetic diamond, one can substantially increase the current-carrying capacity of graphene to as high as ~18 μA/nm(2) even at ambient conditions. Our results indicate that graphenes current-induced breakdown is thermally activated. We also found that the current carrying capacity of graphene can be improved not only on the single-crystal diamond substrates but also on an inexpensive ultrananocrystalline diamond, which can be produced in a process compatible with a conventional Si technology. The latter was attributed to the decreased thermal resistance of the ultrananocrystalline diamond layer at elevated temperatures. The obtained results are important for graphenes applications in high-frequency transistors, interconnects, and transparent electrodes and can lead to the new planar sp(2)-on-sp(3) carbon-on-carbon technology.

Reduced thermal resistance of the silicon-synthetic diamond composite substrates at elevated temperatures

The authors report results of experimental investigation of thermal conductivity of synthetic diamond-silicon composite substrates. Although composite substrates are more thermally resistive than silicon at room temperature they outperform conventional wafers at elevated temperatures owing to different thermal conductivity dependence on temperature. The crossover point is reached near ∼360 K and can be made even lower by tuning the polycrystalline-grain size, film thickness, and interface quality. The reduction of thermal resistance of composite wafers at temperatures, typical for operation of electronic chips, may lead to better thermal management and new phonon-engineered methods for the electron mobility enhancement.

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.

Extraordinary thermal conductivity of graphene: Possibility of thermal management applications

We review the results of our investigation of the thermal conductivity of the suspended single-layer graphene. Using an original non-contact optical technique we discovered experimentally that the thermal conductivity of suspended graphene flakes with low coupling to the substrate is extremely high and can exceeds that of bulk graphite and carbon nanotubes. The high values of the thermal conductivity of graphene and their dependence on the graphene flake size were explained within the framework of Klemens theory. Superior thermal properties of graphene benefit all proposed graphene device applications and may lead to new high-heat flux thermal management solutions for advanced electronics.

Thermal conduction through diamond - silicon heterostructures

The interest to silicon - diamond structures was recently renewed motivated by industrys needs for composite substrates and better thermal management. In this work we investigated thermal conductivity and thermal boundary resistance (TBR) of ultrananocrystalline (UNCD) and microcrystalline diamond (MCD) films on silicon. The measurements were carried out using the transient plane source (TPS) technique. It was found that most of the silicon - synthetic heterostructures are rather resistive thermally with the TBR values of up to ∼ 10−6 m2K/W at room temperature. We established an importance of the trade-off between the structures characterized by the ultra-small diamond grain size with smooth silicon-diamond interface and those with larger grain size but rougher interface. It is shown that composite Si/Diamond wafers are promising at the elevated temperatures characteristic for operation of state-of-art electronic devices. The knowledge of TBR and heat conduction through silicon - diamond heterostructures is important for further development of composite substrates for electronic and optoelectronic industries.