Kyeongjae Cho

Thermal conductivity of isotopically modified graphene

In addition to its exotic electronic properties graphene exhibits unusually high intrinsic thermal conductivity. The physics of phonons - the main heat carriers in graphene - was shown to be substantially different in two-dimensional (2D) crystals, such as graphene, than in three-dimensional (3D) graphite. Here, we report our experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of 13C were synthesized by chemical vapor deposition (CVD). The regions of different isotopic composition were parts of the same graphene sheet to ensure uniformity in material parameters. The thermal conductivity, K, of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal Raman technique, was higher than 4000 W/mK at the measured temperature Tm~320 K, and more than a factor of two higher than the value of K in a graphene sheets composed of a 50%-50% mixture of 12C and 13C. The experimental data agree well with our molecular dynamics (MD) simulations, corrected for the long-wavelength phonon contributions via the Klemens model. The experimental results are expected to stimulate further studies aimed at better understanding of thermal phenomena in 2D crystals.In addition to its exotic electronic properties graphene exhibits unusually high intrinsic thermal conductivity. The physics of phonons--the main heat carriers in graphene--has been shown to be substantially different in two-dimensional (2D) crystals, such as graphene, from in three-dimensional (3D) graphite. Here, we report our experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of 13C were synthesized by chemical vapour deposition (CVD). The regions of different isotopic compositions were parts of the same graphene sheet to ensure uniformity in material parameters. The thermal conductivity, K, of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal Raman technique, was higher than 4,000 W mK(-1) at the measured temperature T(m)~320 K, and more than a factor of two higher than the value of K in graphene sheets composed of a 50:50 mixture of 12C and 13C. The experimental data agree well with our molecular dynamics (MD) simulations, corrected for the long-wavelength phonon contributions by means of the Klemens model. The experimental results are expected to stimulate further studies aimed at a better understanding of thermal phenomena in 2D crystals.

Unusual infrared-absorption mechanism in thermally reduced graphene oxide

Infrared absorption of atomic and molecular vibrations in solids can be affected by electronic contributions through non-adiabatic interactions, such as the Fano effect. Typically, the infrared-absorption lineshapes are modified, or infrared-forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, we report here the observation of a giant-infrared-absorption band in reduced graphene oxide, arising from the coupling of electronic states to the asymmetric stretch mode of a yet-unreported structure, consisting of oxygen atoms aggregated at the edges of defects. Free electrons are induced by the displacement of the oxygen atoms, leading to a strong infrared absorption that is in phase with the phonon mode. This new phenomenon is only possible when all other oxygen-containing chemical species, including hydroxyl, carboxyl, epoxide and ketonic functional groups, are removed from the region adjacent to the edges, that is, clean graphene patches are present.

Near-unity photoluminescence quantum yield in MoS2

Brighter molybdenum layers The confined layers of molybdenum disulphide (MoS2) exhibit photoluminescence that is attractive for optolectronic applications. In practice, efficiencies are low, presumably because defects trap excitons before they can recombine and radiate light. Amani et al. show that treatment of monolayer MoS2 with a nonoxidizing organic superacid, bis(trifluoromethane) sulfonimide, increased luminescence efficiency in excess of 95%. The enhancement mechanism may be related to the shielding of defects, such as sulfur vacancies. Science, this issue p. 1065 Superacid treatment enhances the luminescence efficiency of monolayer molybdenum disulfide from 1% to >95%. Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude. The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8 ± 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

Band alignment of two-dimensional transition metal dichalcogenides: Application in tunnel field effect transistors

Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me = W, Mo; X = Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X = S, Se) as the p-type drain by density functional theory calculations.

The Role of Intercalated Water in Multilayered Graphene Oxide

A detailed in situ infrared spectroscopy analysis of single layer and multilayered graphene oxide (GO) thin films reveals that the normalized infrared absorption in the carbonyl region is substantially higher in multilayered GO upon mild annealing. These results highlight the fact that the reduction chemistry of multilayered GO is dramatically different from the single layer GO due to the presence of water molecules confined in the ∼1 nm spacing between sheets. IR spectroscopy, XPS analysis, and DFT calculations all confirm that the water molecules play a significant role interacting with basal plane etch holes through passivation, via evolution of CO(2) leading to the formation of ketone and ester carbonyl groups. Displacement of water from intersheet spacing with alcohol significantly changes the chemistry of carbonyl formation with temperature.

The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces

Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is shown to be unique for metal-2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.

Chemical control of nanotube electronics

The possibility of modifying the electronic properties of nanotubes using gas molecule adsorption is investigated using the first-principles total energy density functional calculations. Detailed analysis of the electronic structures and energetics is performed for the semiconducting (10,0) single-walled carbon nanotube interacting with several representative gas molecules (NO2, NH3, CO, O2, and H2O). The results elucidate the mechanisms of the adsorption-induced nanotube doping and illustrate an example of the simulation-based design characterization of nanoelectronic components.

Nanomechanics of carbon nanotubes and composites

Computer simulation and modeling results for the nanomechanics of carbon nanotubes and carbon nanotube-polyethylene composite materials are described and compared with experimental observations. Young’s modulus of individual single-wall nanotubes is found to be in the range of 1 TPa within the elastic limit. At room temperature and experimentally realizable strain rates, the tubes typically yield at about 5–10% axial strain; bending and torsional stiffness and different mechanisms of plastic yielding of individual single-wall nanotubes are discussed in detail. For nanotube-polyethylene composites, we find that thermal expansion and diffusion coefficients increase significantly, over their bulk polyethylene values, above glass transition temperature, and Young’s modulus of the composite is found to increase through van der Waals interaction. This review article cites 54 references. @DOI: 10.1115/1.1538625#

First-principles study of metal-graphene interfaces

Metal-graphene contact is a key interface in graphene-based device applications, and it is known that two types of interfaces are formed between metal and graphene. In this paper, we apply first-principles calculations to twelve metal-graphene interfaces and investigate the detailed interface atomic and electronic structures of physisorption and chemisorption interfaces. For physisorption interfaces (Ag, Al, Cu, Cd, Ir, Pt, and Au), Fermi level pinning and Pauli-exclusion-induced energy-level shifts are shown to be two primary factors determining graphene’s doping types and densities. For chemisorption interfaces (Ni, Co, Ru, Pd, and Ti), the combination of Pauli-exclusion-induced energy-level shifts and hybridized states’ repulsive interactions lead to a band gap opening with metallic gap states. For practical applications, we show that external electric field can be used to modulate graphene’s energy-levels and the corresponding control of doping or energy range of hybridization.

Conformal Al2O3 dielectric layer deposited by atomic layer deposition for graphene-based nanoelectronics

We present a facile route which combines the functionalization of a highly oriented pyrolytic graphite surface with an atomic layer deposition (ALD) process to allow for conformal Al2O3 layers. While the trimethylaluminum (TMA)∕H2O process caused selective deposition only along step edges, the TMA∕O3 process began to provide nucleation sites on the basal planes of the surface. O3 pretreatment, immediately followed by the ALD process with TMA∕O3 chemistry, formed Al2O3 layers without any preferential deposition at the step edges. This is attributed to functionalization of graphene by ozone treatment, imparting a hydrophilic character which is desirable for ALD deposition.