Sung Youb Kim

Near Room-temperature Synthesis of Transfer-free Graphene Films

Large-area graphene films are best synthesized via chemical vapour and/or solid deposition methods at elevated temperatures (~1,000 °C) on polycrystalline metal surfaces and later transferred onto other substrates for device applications. Here we report a new method for the synthesis of graphene films directly on SiO(2)/Si substrates, even plastics and glass at close to room temperature (25-160 °C). In contrast to other approaches, where graphene is deposited on top of a metal substrate, our method invokes diffusion of carbon through a diffusion couple made up of carbon-nickel/substrate to form graphene underneath the nickel film at the nickel-substrate interface. The resulting graphene layers exhibit tunable structural and optoelectronic properties by nickel grain boundary engineering and show micrometre-sized grains on SiO(2) surfaces and nanometre-sized grains on plastic and glass surfaces. The ability to synthesize graphene directly on non-conducting substrates at low temperatures opens up new possibilities for the fabrication of multiple nanoelectronic devices.

The importance of edge effects on the intrinsic loss mechanisms of graphene nanoresonators.

We utilize classical molecular dynamics simulations to investigate the intrinsic loss mechanisms of monolayer graphene nanoresonators undergoing flexural oscillations. We find that spurious edge modes of vibration, which arise not due to externally applied stresses but intrinsically due to the different vibrational properties of edge atoms, are the dominant intrinsic loss mechanism that reduces the quality (Q) factors. We additionally find that while hydrogen passivation of the free edges is ineffective in reducing the spurious edge modes, fixing the free edges is critical to removing the spurious edge-induced vibrational states. Our atomistic simulations also show that the Q factor degrades inversely proportional to temperature; furthermore, we also demonstrate that the intrinsic losses can be reduced significantly across a range of operating temperatures through the application of tensile mechanical strain.

Negative Poisson’s ratios in metal nanoplates

The Poissons ratio is a fundamental measure of the elastic-deformation behaviour of materials. Although negative Poissons ratios are theoretically possible, they were believed to be rare in nature. In particular, while some studies have focused on finding or producing materials with a negative Poissons ratio in bulk form, there has been no such study for nanoscale materials. Here we provide numerical and theoretical evidence that negative Poissons ratios are found in several nanoscale metal plates under finite strains. Furthermore, under the same conditions of crystal orientation and loading direction, materials with a positive Poissons ratio in bulk form can display a negative Poissons ratio when the materials thickness approaches the nanometer scale. We show that this behaviour originates from a unique surface effect that induces a finite compressive stress inside the nanoplates, and from a phase transformation that causes the Poissons ratio to depend strongly on the amount of stretch.

Multilayer friction and attachment effects on energy dissipation in graphene nanoresonators

We utilize classical molecular dynamics to study the effects of intrinsic, interlayer friction between graphene monolayers, as well as extrinsic attachment or clamping strength between graphene and a model silicon substrate on the energy dissipation (Q-factors) of oscillating graphene nanoresonators. Both interlayer friction and attachment effects are found to significantly degrade the graphene Q-factors, with an increase in energy dissipation with increasing temperature, while both effects are found to be strongly dependent on the strength of the van der Waals interactions, either between adjacent layers of graphene or between graphene and the underlying substrate.

Monolithic graphene oxide sheets with controllable composition

Graphene oxide potentially has multiple applications and is typically prepared by solution-based chemical means. To date, the synthesis of a monolithic form of graphene oxide that is crucial to the precision assembly of graphene-based devices has not been achieved. Here we report the physical approach to produce monolithic graphene oxide sheets on copper foil using solid carbon, with tunable oxygen-to-carbon composition. Experimental and theoretical studies show that the copper foil provides an effective pathway for carbon diffusion, trapping the oxygen species dissolved in copper and enabling the formation of monolithic graphene oxide sheets. Unlike chemically derived graphene oxide, the as-synthesized graphene oxide sheets are electrically active, and the oxygen-to-carbon composition can be tuned during the synthesis process. As a result, the resulting graphene oxide sheets exhibit tunable bandgap energy and electronic properties. Our solution-free, physical approach may provide a path to a new class of monolithic, two-dimensional chemically modified carbon sheets.

On the utility of vacancies and tensile strain-induced quality factor enhancement for mass sensing using graphene monolayers

We have utilized classical molecular dynamics to investigate the mass sensing potential of graphene monolayers, using gold as the model adsorbed atom. In doing so, we report two key findings. First, we find that while perfect graphene monolayers are effective mass sensors at very low (T < 10 K) temperatures, their mass sensing capability is lost at higher temperatures due to diffusion of the adsorbed atom at elevated temperatures. We demonstrate that even if the quality (Q) factors are significantly elevated through the application of tensile mechanical strain, the mass sensing resolution is still lost at elevated temperatures, which demonstrates that high Q-factors alone are insufficient to ensure the mass sensing capability of graphene. Second, we find that while the introduction of single vacancies into the graphene monolayer prevents the diffusion of the adsorbed atom, the mass sensing resolution is still lost at higher temperatures, again due to Q-factor degradation. We finally demonstrate that if the Q-factors of the graphene monolayers with single vacancies are kept acceptably high through the application of tensile strain, then the high Q-factors, in conjunction with the single atom vacancies to stop the diffusion of the adsorbed atom, enable graphene to maintain its mass sensing capability across a range of technologically relevant operating temperatures.

Auxetic nanomaterials: Recent progress and future development

Auxetic materials (materials with negative Poissons ratio) and nanomaterials have independently been, for many years, two of the most active research fields in material science. Recently, these formerly independent fields have begun to intersect in new and interesting ways due to the recent discovery of auxeticity in nanomaterials like graphene, metal nanoplates, black phosphorus, and others. Here, we review the research emerging at the intersection of auxeticity and nanomaterials. We first survey the atomistic mechanisms, both intrinsic and extrinsic, that have been found, primarily through atomistic simulations, to cause auxeticity in nanomaterials. We then outline the available experimental evidence for auxetic nanomaterials. In order to lay the groundwork for future work in this exciting area, we close by discussing several future prospects as well as the current challenges in this field.

Facile Synthesis of Few-Layer Graphene with a Controllable Thickness Using Rapid Thermal Annealing

Few-layer graphene films with a controllable thickness were grown on a nickel surface by rapid thermal annealing (RTA) under vacuum. The instability of nickel films in air facilitates the spontaneous formation of ultrathin (<2-3 nm) carbon- and oxygen-containing compounds on a nickel surface; thus, the high-temperature annealing of the nickel samples without the introduction of intentional carbon-containing precursors results in the formation of graphene films. From annealing temperature and ambient studies during RTA, it was found that the evaporation of oxygen atoms from the surface is the dominant factor affecting the formation of graphene films. The thickness of the graphene layers is strongly dependent on the RTA temperature and time, and the resulting films have a limited thickness (<2 nm), even for an extended RTA time. The transferred films have a low sheet resistance of ~0.9 ± 0.4 kΩ/sq, with ~94% ± 2% optical transparency, making them useful for applications as flexible transparent conductors.

Growth of Wrinkle-Free Graphene on Texture-Controlled Platinum Films and Thermal-Assisted Transfer of Large-Scale Patterned Graphene

Growth of large-scale patterned, wrinkle-free graphene and the gentle transfer technique without further damage are most important requirements for the practical use of graphene. Here we report the growth of wrinkle-free, strictly uniform monolayer graphene films by chemical vapor deposition on a platinum (Pt) substrate with texture-controlled giant grains and the thermal-assisted transfer of large-scale patterned graphene onto arbitrary substrates. The designed Pt surfaces with limited numbers of grain boundaries and improved surface perfectness as well as small thermal expansion coefficient difference to graphene provide a venue for uniform growth of monolayer graphene with wrinkle-free characteristic. The thermal-assisted transfer technique allows the complete transfer of large-scale patterned graphene films onto arbitrary substrates without any ripples, tears, or folds. The transferred graphene shows high crystalline quality with an average carrier mobility of ∼ 5500 cm(2) V(-1) s(-1) at room temperature. Furthermore, this transfer technique shows a high tolerance to variations in types and morphologies of underlying substrates.

On the effective plate thickness of monolayer graphene from flexural wave propagation

We utilize classical molecular dynamics to study flexural, or transverse wave propagation in monolayer graphene sheets and compare the resulting dispersion relationships to those expected from continuum thin plate theory. In doing so, we determine that regardless of the chirality for monolayer graphene, transverse waves exhibit a dispersion relationship that corresponds to the lowest order antisymmetric (A0) mode of wave propagation in a thin plate with plate thickness of h=0.104 nm. Finally, we find that the achievable wave speeds in monolayer graphene are found to exceed those reported previously for single walled carbon nanotubes, while the frequency of wave propagation in the graphene monolayer is found to reach the terahertz range, similar to that of carbon nanotubes.