Yufeng Hao

The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper

Oxygen Control of Graphene Growth The growth of graphene on copper surfaces through the decomposition of hydrocarbons such as methane can result in a wide variety of crystal domain sizes and morphologies. Hao et al. (p. 720, published online 24 October; see the cover) found that the presence of surface oxygen could limit the number of nucleation sites and allowed centimeter-scale domains to grow through a diffusion-limited mechanism. The electrical conductivity of the graphene was comparable to that of exfoliated graphene. Oxygen treatment of a copper surface promoted the faster growth of compact, centimeter-scale graphene domains. The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment–limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

Probing Layer Number and Stacking Order of Few‐Layer Graphene by Raman Spectroscopy

Graphene is a two-dimensional material defined as a planar honeycomb lattice of close-packed carbon atoms, where the electrons exhibit a linear dispersion near Dirac K points and behave as massless Dirac fermions. However, the valence and conduction bands in an AB stacked graphene bilayer split into two parabolic branches near the K point originating from the interaction of p electrons, and the electrons are hence described by massive Dirac fermions. Moreover, a graphene bilayer is a tunable-gap semiconductor under electric-field biasing. With a further increase in the number of layers along with AB stacking, the electronic structure reveals stepwise variations that eventually approach that of the three-dimensional counterpart. Considering the close relation between the electronic properties and layer number of few-layer graphene (FLG), the ability to accurately determine the layer number and correlating this with the electronic structure is a prerequisite in understanding the evolution of the electronic properties from twoto threedimensional graphitic materials. In addition to graphene layers with AB stacking, FLG with arbitrary stacking (Figure 1) is considered to possess distinct properties arising from its different crystalline structure and p electron interactions. Experimentally, it has been observed that the electroand magnetotransport properties for folded graphene sheets are different to thoseofAB stackedbilayers. Furthermore,FLG grown on SiC, Ni, and Ru also have non-AB stacking order. Therefore, elucidating the detailed character-

Enhancement of the Electrical Properties of Graphene Grown by Chemical Vapor Deposition via Controlling the Effects of Polymer Residue

Residual polymer (here, poly(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO2/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the -NH2 functional group in formamide that is absorbed in the polymer residue. This work provides a route to enhancing the electrical properties of CVD-grown graphene even when it has a thin polymer coating.

Two-dimensional antimonene single crystals grown by van der Waals epitaxy

Unlike the unstable black phosphorous, another two-dimensional group-VA material, antimonene, was recently predicted to exhibit good stability and remarkable physical properties. However, the synthesis of high-quality monolayer or few-layer antimonenes, sparsely reported, has greatly hindered the development of this new field. Here, we report the van der Waals epitaxy growth of few-layer antimonene monocrystalline polygons, their atomical microstructure and stability in ambient condition. The high-quality, few-layer antimonene monocrystalline polygons can be synthesized on various substrates, including flexible ones, via van der Waals epitaxy growth. Raman spectroscopy and transmission electron microscopy reveal that the obtained antimonene polygons have buckled rhombohedral atomic structure, consistent with the theoretically predicted most stable β-phase allotrope. The very high stability of antimonenes was observed after aging in air for 30 days. First-principle and molecular dynamics simulation results confirmed that compared with phosphorene, antimonene is less likely to be oxidized and possesses higher thermodynamic stability in oxygen atmosphere at room temperature. Moreover, antimonene polygons show high electrical conductivity up to 104 S m−1 and good optical transparency in the visible light range, promising in transparent conductive electrode applications.

Selective-area fluorination of graphene with fluoropolymer and laser irradiation

We have devised a method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser. This fluoropolymer produces active fluorine radicals under laser irradiation that react with graphene but only in the laser-irradiated region. The kinetics of C-F bond formation is dependent on both the laser power and fluoropolymer thickness, proving that fluorination occurs by the decomposition of the fluoropolymer. Fluorination leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained. Considering the simplicity of the fluorination process and that it allows patterning with a nontoxic fluoropolymer as a solid source, this method could find application to generate fluorinated graphene in graphene-based electronic devices such as for the electrical isolation of graphene.

Inductive Tuning of Fano-Resonant Metasurfaces Using Plasmonic Response of Graphene in the Mid-Infrared

Graphene is widely known for its anomalously strong broadband optical absorptivity of 2.3% that enables seeing its single-atom layer with the naked eye. However, in the mid-infrared part of the spectrum graphene represents a quintessential lossless zero-volume plasmonic material. We experimentally demonstrate that, when integrated with Fano-resonant plasmonic metasurfaces, single-layer graphene (SLG) can be used to tune their mid-infrared optical response. SLGs plasmonic response is shown to induce large blue shifts of the metasurfaces resonance without reducing its spectral sharpness. This effect is explained by a generalized perturbation theory of SLG-metamaterial interaction that accounts for two unique properties of the SLG that set it apart from all other plasmonic materials: its anisotropic response and zero volume. These results pave the way to using gated SLG as a platform for dynamical spectral tuning of infrared metamaterials and metasurfaces.

Metal Contacts on Physical Vapor Deposited Monolayer MoS2

The understanding of the metal and transition metal dichalcogenide (TMD) interface is critical for future electronic device technologies based on this new class of two-dimensional semiconductors. Here, we investigate the initial growth of nanometer-thick Pd, Au, and Ag films on monolayer MoS2. Distinct growth morphologies are identified by atomic force microscopy: Pd forms a uniform contact, Au clusters into nanostructures, and Ag forms randomly distributed islands on MoS2. The formation of these different interfaces is elucidated by large-scale spin-polarized density functional theory calculations. Using Raman spectroscopy, we find that the interface homogeneity shows characteristic Raman shifts in E2g(1) and A1g modes. Interestingly, we show that insertion of graphene between metal and MoS2 can effectively decouple MoS2 from the perturbations imparted by metal contacts (e.g., strain), while maintaining an effective electronic coupling between metal contact and MoS2, suggesting that graphene can act as a conductive buffer layer in TMD electronics.

Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic

Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.

P-type electrical, photoconductive, and anomalous ferromagnetic properties of Cu2O nanowires

Cu2O nanowires are synthesized by reduction of CuO nanowires with hydrogen gas. Strong green photoluminescence dominated by band-edge emission is observed. Field effect transistors fabricated from individual Cu2O nanowires present high on-off ratio (>106) and high mobility (>95 cm2/V s). Furthermore, the device demonstrates a fast photoelectric response to blue illumination in air at room temperature. In addition, anomalous ferromagnetism appears in Cu2O nanowires, which may originate from the defects in Cu2O nanowires. This work shows the application potentials of the Cu2O nanowires, especially in an electrical and photonic device.

Gold on graphene as a substrate for surface enhanced Raman scattering study

In this paper, we report our study on gold (Au) films with different thicknesses deposited on single layer graphene (SLG) as surface enhanced Raman scattering (SERS) substrates for the characterization of rhodamine (R6G) molecules. We find that an Au film with a thickness of ∼7 nm deposited on SLG is an ideal substrate for SERS, giving the strongest Raman signals for the molecules and the weakest photoluminescence (PL) background. While Au films effectively enhance both the Raman and PL signals of molecules, SLG effectively quenches the PL signals from the Au film and molecules. The former is due to the electromagnetic mechanism involved while the latter is due to the strong resonance energy transfer from Au to SLG. Hence, the combination of Au films and SLG can be widely used in the characterization of low concentration molecules with relatively weak Raman signals.