Desalegne Teweldebrhan

Superior Thermal Conductivity of Single-Layer Graphene

We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range approximately (4.84+/-0.44)x10(3) to (5.30+/-0.48)x10(3) W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.

Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits

The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si∕SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼3080–5150W∕mK and phonon mean free path of ∼775nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.

Modification of Graphene Properties due to Electron-Beam Irradiation

The authors report micro-Raman investigation of changes in the single and bilayer graphene crystal lattice induced by the low and medium energy electron-beam irradiation (5–20 keV). It was found that the radiation exposures result in the appearance of the strong disorder D band around 1345 cm−1, indicating damage to the lattice. The D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests graphene’s transformation to the nanocrystalline and then to amorphous form. The results have important implications for graphene characterization and device fabrication, which rely on the electron microscopy and focused ion beam processing.

High-temperature quenching of electrical resistance in graphene interconnects

The authors reported on the experimental investigation of the high-temperature electrical resistance of graphene. The test structures were fabricated by using the focused ion beam from the single and bilayer graphene produced by mechanical exfoliation. It was found that as temperature increases from 300to500K, the resistance of the single, and bilayer graphene interconnects drops down by 30% and 70%, respectively. The quenching and temperature dependence of the resistance were explained by the thermal generation of the electron-hole pairs and carrier scattering by acoustic phonons. The obtained results are important for the proposed graphene interconnect applications in integrated circuits.

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.

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.

Flicker Noise in Bilayer Graphene Transistors

We present results of the experimental investigation of the low-frequency noise in bilayer graphene transistors. The back-gated devices were fabricated using the electron beam lithography and evaporation. The charge neutrality point for the transistors was around +10 V. The noise spectra at frequencies f > 10-100 Hz were of the 1/f type with the spectral density on the order of S1 ~ 10-23-10-22 A2/Hz at the frequency of 1 kHz. The deviation from the 1/f spectrum at f < 10-100 Hz suggests that the noise is of the carrier-number fluctuation origin due to the carrier trapping by defects. The Hooge parameter was determined to be as low as ~ 10-4. The gate dependence of the normalized noise spectral density indicates that it is dominated by the contributions from the ungated parts of the device and can be reduced even further. The obtained results are important for graphene electronic and sensor applications.

Tuning of Graphene Properties via Controlled Exposure to Electron Beams

The controlled modification of graphene properties is essential for its proposed electronic applications. Here, we describe a possibility of tuning electrical properties of graphene via electron-beam (e-beam) irradiation. We show that by controlling the irradiation dose one can change the carrier mobility and increase the resistance at the minimum conduction point in the single layer graphene. The bilayer graphene is less susceptible to the e-beam irradiation. The modification of graphene properties via irradiation can be monitored and quantified by the changes in the disorder D peak in Raman spectrum of graphene. The obtained results may lead to a new method of defect engineering of graphene physical properties. They are also important implications for fabrication of graphene nanodevices, which involve scanning electron microscopy and e-beam lithography.

Thermal properties of the optically transparent pore-free nanostructured yttria-stabilized zirconia

The authors report results of investigation of thermal conductivity of nanocrystalline yttria-stabilized zirconia. The optically transparent pore-free bulk samples were prepared via the spark plasma sintering process to ensure homogeneity. Thermal conductivity K was measured by two different techniques. It was found that the pore-free nanostructured bulk zirconia is an excellent thermal insulator with the room-temperature K∼1.7–2.0 W/m K. It was also shown that the “phonon-hopping” model can accurately describe specifics of K dependence on temperature and the grain size. The obtained results are important for optimization of zirconia properties for specific applications in advanced electronics and coatings.