In recent years, there has been an increasing interest in thermal properties of materials. This arises mostly from the practical needs of heat removal and thermal management, which have now become critical issues for the continuing progress in electronic and optoelectronic industries. Another motivation for the study of thermal properties at nanoscale is from a fundamental science perspective. Thermal conductivity of different allotropes of carbon materials span a uniquely large range of values with the highest in graphene and carbon nanotube and the lowest in amorphous or disordered carbon. Here we describe the thermal properties of graphene and carbon-based materials and analyze the prospects of applications of carbon materials in thermal management.

Graphitic nanomaterials such as graphene, carbon nanotubes (CNT), and C60 fullerenes are promising materials for energy applications because of their extraordinary electrical and optical properties. However, graphitic materials are not readily dispersible in water. Strategies to fabricate all-carbon nanocomposites typically involve covalent linking or surface functionalization, which breaks the conjugated electronic networks or contaminates functional carbon surfaces. Here, we demonstrate a facile surfactant-free strategy to create such all-carbon composites. Fullerenes, unfunctionalized single walled carbon nanotubes, and graphene oxide sheets can be conveniently co-assembled in water, resulting in a stable colloidal dispersion amenable to thin film processing. The thin film composite can be made conductive by mild thermal heating. Photovoltaic devices fabricated using the all-carbon composite as the active layer demonstrated an on-off ratio of nearly 106, an open circuit voltage of 0.59V, and a power conversion efficiency of 0.21%. This photoconductive and photovoltaic response is unprecedented among all-carbon based materials. Therefore, this surfactant-free, aqueous based approach to making all-carbon composites is promising for applications in optoelectronic devices.

Processing graphene and graphene polymer nanocomposites in an aqueous medium has always been a big challenge due to the hydrophobic nature of graphene (or reduced graphene oxide) nanosheets. In this work, a waterborne latex of polyurethane has been used both as the matrix material for embedding the graphene nanosheets and as a unique stabilizer to help produce an up to 5 wt% graphene/PU nanocomposites. The graphene oxide/polyurethane latex aqueous suspension is reduced in-situ using hydrazine, without any trace of aggregation/agglomeration upon completion of the reduction process, which would otherwise have occurred severely were PU not present. A highly aligned nanostructure is produced when graphene content is increased beyond 2 wt%, resulting in a remarkable improvement in electrical and mechanical properties of the nanocomposite. The exceptionally low electrical percolation threshold of 0.078%, as well as 21-fold and 14 fold increases in tensile modulus and strength, respectively, have been attained thanks to the alignment of graphene nanosheets in the polymeric matrix.

Graphene oxide (GO) is a nonstoichiometric two-dimensional material obtained from the chemical oxidation and exfoliation of graphite, which has recently attracted intense research interest as a precursor for bulk production of graphene. GO has long been believed to be hydrophilic due to its dispersibility in water. Recent work in our group, however, has found that GO is actually a two-dimensional amphiphile; the edge of the sheet-like material is hydrophilic, while the basal plane of the material contains more hydrophobic graphitic nanodomains. To prove the concept, we demonstrate GO’s surface activity at an air-water interface, as well as its utility in dispersing insoluble aromatic materials such as toluene, graphite, and carbon nanotubes in water. As a colloidal surfactant which can be converted to a conducting material, GO presents unique possibilities for aqueous solution processing of organic electronic materials.

The layered oxide LiVO2 recently has received more attention due to its interesting structural and magnetic behaviors involving the two-dimensional magnetic frustration in these systems. We synthesized a series of F-doped LiVO2 samples, and reported the F-doping effect on the structure and transition temperature Tt . The samples LiVO2-x F x (x=0, 0.1, 0.2 and 0.3) were characterized by X-ray diffraction, scanning electron microscope (SEM), differential scanning calorimetry (DSC), magnetic susceptibility and specific heat measurement. The structural analysis shows that with increasing x, the ratio of lattice parameter c/a increasing, i.e. in the a-b plane the lattice is compressed while in the c-axis direction the lattice expands. The DSC measurements show that a first-order phase transition happens at around 500 K, and the thermal hysteresis around phase transition temperature Tt increases with increasing x. Substitution of O with F ions results in a change of two dimensional characteristics and the distortion of the VO6 block in structure, which significantly influence the magnetic ordering transition temperature Tt .

We investigated the electrical characteristics and digital data transmission performance few-layer graphene ribbons grown by chemical vapor deposition. Graphene ribbons having a mobility of 2,180 cm2V-1s-1 can sustain data rates up to 50 megabits per second at 1.5 μm length, thus the bandwidth is inversely proportional to resistance caused by defects in the graphene layers. Improving the graphene mobility to highest measured values (∼200,000 cm2V-1s-1) and using structures with multiple coplanar transmission lines in parallel could carry the bandwidth beyond the gigabits per second level.

We report results of the study of the low-frequency noise in thin films of bismuth selenide topological insulators, which were mechanically exfoliated from bulk crystals via “graphene-like” procedures. From the resistance dependence on the film thickness, it was established that the surface conduction contributions to electron transport were dominant. It was found that the current fluctuations have the noise spectral density S I ∞ 1/f (where f is the frequency) for the frequency range up to 10 kHz. The obtained noise data are important for transport experiments with topological insulators and for any proposed device applications of these materials.

We fabricated a number of top-gate graphene field-effect transistors on the ultrananocrystalline diamond (UNCD) – Si composite substrates. Raman spectroscopy, scanning electron microscopy and atomic force microscopy were used to verify the quality of UNCD and graphene device channels. The thermal measurements were carried out with the “hot disk” and “laser flash” methods. It was found that graphene on UNCD devices have increased breakdown current density by ∼50% compared to the reference devices fabricated on Si/SiO2. The relatively smooth surface of UNCD, as compared to other synthetic diamond films, allowed us to fabricate top gate graphene devices with the drift mobility of up to ∼ 2587 cm2V-1s-1.

Graphene reveals many extraordinary properties including extremely high room temperature carrier mobility and intrinsic thermal conductivity. Understanding how to controllably modify graphene’s properties is essential for its proposed applications. Here we report on a method for tuning the electrical properties of graphene via electron beam irradiation. It was observed that single-layer graphene is highly susceptible to the low-energy electron beams. We demonstrated that by controlling the irradiation dose one can change, by desired amount, the carrier mobility, shift the charge neutrality point, increase the resistance at the minimum conduction point, induce the “transport gap” and achieve current saturation in graphene. The change in graphene properties is due to defect formation on the graphene surface and in the graphene lattice. The changes are reversible by annealing until some critical irradiation dose is reached.

This paper addresses scaling issues in graphene nanoribbon transistors (GNRFETs) by using a two-dimensional (2-D) Poisson and drift-diffusion solver with finite element method (FEM). GNRFETs with the back gate control and the channel width down to less than 5nm have been reported to have I on =Ioff ratio up to 106. Our simulations show an agreement with the published experimental work and show a potential to reach unit current gain cut-off frequency, fT , up to more than 1THz with a satisfying Ion=Ioff ratio at the same time. This makes GNRFETs attractive for high speed logic.

The exceptional electrical, optical, and mechanical properties of graphene make it a promising material for many industrial applications such as solar cells, semiconductor devices, and thermal heat sinks. However, the greatest obstacle in the use of graphene in industry is high-throughput scaling of its production and characterization. Chemical-vapor deposition growth of graphene has allowed for industrial-scale graphene production. In this work we introduce complimentary high-throughput metrology technique for characterization of chemical-vapor deposition-grown graphene. This metrology technique provides quick identification of thickness and uniformity of entire large-area chemical-vapor deposition-grown graphene sheets on a glass substrate and allows for easy identification of folds and cracks in the graphene samples. This metrology technique utilizes fluorescence quenching microscopy, which is based on resonant energy transfer between a dye molecule and graphene, to increase allow graphene visualization on the glass substrate and increase the contrast between graphene layers.

Graphene is a promising candidate material for thermal management of high-power electronics owing to its high intrinsic thermal conductivity. Here we report preliminary results of the proof-of-concept demonstration of graphene lateral heat spreaders. Graphene flakes were transferred on top of GaN devices through the mechanical exfoliation method. The temperature rise in the GaN device channels was monitored in-situ using micro-Raman spectroscopy. The local temperature was measured from the shift in the Raman peak positions. By comparing Raman spectra of GaN devices with and without graphene heat spreader, we demonstrated that graphene lateral heat spreaders effectively reduced the local temperature by ~ 20oC for a given dissipated power density. Numerical simulation of heat dissipation in the considered device structures gave results consistent with the experimental data.

We used a “graphene-like” mechanical exfoliation to obtain atomically thin films of TiTe2. The building blocks of titanium ditelluride are atomic tri-layers separated by the van der Waals gaps. The exfoliation procedure allows one to obtain the few-atom-thick films with strong confinement of charge carriers and phonons. We have verified the crystallinity of the exfoliated films and fabricated the back-gated field-effect devices. The current – voltage characteristics of the TiTe2 devices revealed strong non-linearity, which suggests the charge-density wave effects. The obtained results are important for the proposed application of TiTe2 for the charge-density wave devices and thermoelectric energy conversion.

It was recently suggested theoretically that atomically thin films of Bi2Te3 topological insulators have strongly enhanced thermoelectric figure of merit. We used the “graphene-like” exfoliation process to obtain Bi2Te3 thin films. The films were stacked and subjected to thermal treatment to fabricate pseudo-superlattices of single crystal Bi2Te3 films. Thermal conductivity of these structures was measured by the “hot disk” and “laser flash” techniques. The room temperature in-plane and cross-plane thermal conductivity of the stacks decreased by a factor of ∼2.4 and 3.5 respectively as compared to that of bulk. The strong decrease of thermal conductivity with preserved electrical properties translates to ∼140-250% increase in the thermoelectric figure if merit. It is expected that the film thinning to few-quintuples, and tuning of the Fermi level can lead to the topological insulator surface transport regime with the theoretically predicted extraordinary thermoelectric efficiency.

We carried out a systematic experimental study of the low-frequency noise characteristics in a large number of single and bilayer graphene transistors. The prime purpose was to determine the dominant noise sources in these devices and the effect of aging on the current-voltage and noise characteristics. The analysis of the noise spectral density dependence on the surface area of the graphene channel indicates that the dominant contributions to the 1/f electronic noise come from the graphene channel region itself. Aging of graphene transistors due to exposure to ambient for over a month resulted in substantially increased noise, which was attributed to the decreasing mobility of graphene and increasing contact resistance. The noise spectral density in both single and bilayer graphene transistors shows a non-monotonic dependence on the gate bias. This observation confirms that the 1/f noise characteristics of graphene transistors are qualitatively different from those of conventional silicon metal-oxide-semiconductor field-effect transistors.

Graphene oxide sheets have recently gained immense interest as a building block for graphene based materials and devices. Rapid developments have been made in the chemistry and applications of GO. However, assembly, too, plays a critical role in the final properties of bulk graphene based materials as it determines the microstructures of the 2D sheets. There is thus a pressing need for controllable assembly strategies. Based on the recent identification of the pH dependent surfactant-like behavior of GO sheets, we are now able to control the tiling morphologies of such sheets to produce thin films with either wrinkled or overlapped types of microstructures. This allows for the deconvolution of the effects of these two basic morphological features in the electrical and optical properties of the resulting thin films, providing a well-defined example of the processing-microstructure-properties relationship for this unique soft material building block.

Ultracapacitors are promising candidate for alternative energy storage applications since they can store and deliver energy at relatively high rates. In this work, we integrated large area CVD graphene with multi-walled carbon nanotubes (MWNTs) to fabricate highly conductive, large surface-area composite thin films used as electrodes in ultracapacitors. Uniform, large area graphene layers were produced by CVD on copper foils and were chemically modified. Chemically shortened MWNTs, ranging in length of 200~500 nm, were deposited by dropping on graphene layers. Graphene/MWNT composite films with different thicknesses were obtained. The surface morphology was investigated by SEM. The results demonstrated relatively dense and homogeneous net nanostructure. The measurements of cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) are conducted to determine its performance of graphene/MWNT film structures.

Using fully reactive molecular dynamics methodologies we investigated the structural and dynamical aspects of the fluorination mechanism leading to fluorographene formation from graphene membranes. Fluorination tends to produce significant defective areas on the membranes with variation on the typical carbon-carbon distances, sometimes with the presence of large holes due to carbon losses. The results obtained in our simulations are in good agreement with the broad distribution of values for the lattice parameter experimentally observed. We have also investigated mixed atmospheres composed by H and F atoms. When H is present in small quantities an expressive reduction on the rate of incorporation of fluorine was observed. On the other hand when fluorine atoms are present in small quantities in a hydrogen atmosphere, they induce an increasing on the hydrogen incorporation and the formation of locally self-organized structure of adsorbed H and F atoms.

Using chemical vapor deposition technique, a novel 3D carbon nano-architecture called a pillared graphene nanostructure (PGN) is in situ synthesized. The fabricated novel carbon nanostructure consists of CNT pillars of variable length grown vertically from large-area graphene planes. The formation of CNTs and graphene occurs simultaneously in one CVD growth treatment. The detailed characterization of synthesized pillared graphene shows the cohesive structure and seamless contact between graphene and CNTs in the hybrid structure. The synthesized graphene-CNT hybrid has a tunable architecture and attractive material properties, as it is solely built from sp2 hybridized carbon atoms in form of graphene and CNT. Our methodology provides a pathway for fabricating novel 3D nanostructures which are envisioned for applications in hydrogen storage, nanoelectronics, and supercapacitors.