Xiangfan Xu

Roll-to-roll production of 30-inch graphene films for transparent electrodes

The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.1 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), 2 Department of Chemistry, 3 Department of Mechanical Engineering, 4 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea. 5 NanoCore & Department of Physics, National University of Singapore, Singapore 117576 & 117542, 6 Digital & IT Solution Division, Samsung Techwin, Seongnam 462-807, Korea, 7 Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565 & Faculty of Science and Engineering, Meijo University, Nagoya 468-8502, Japan.

Interface Engineering of Layer‐by‐Layer Stacked Graphene Anodes for High‐Performance Organic Solar Cells

The major efforts in solar energy research are currently directed at developing cost-effective systems for energy conversion and storage. [ 1–3 ] The high cost of materials and preparation methods that are required for the fabrication of inorganic solar cells prevent their widespread deployment. Seeking a low-cost alternative in the form of solution-processable or roll-to-roll printable organic solar cells features prominently in the energy research roadmap. The conventional anode of choice for organic solar cells has been indium tin oxide (ITO), which consumes as much as 30% of the fabrication cost in solar cells. High quality ITO is expensive due to the dwindling supplies of indium. ITO also suffers from drawbacks like brittleness, sensitivity to acids and bases during processing, and reactive interface formation with copper indium sulfi de during high-temperature sintering. Graphene fi lms have been proposed as the new generation of multifunctional, transparent, and conducting electrodes. The attractiveness of graphene arises from their low cost, transparency, high electrical conductivity, chemical robustness, and fl exibility, as opposed to the rising cost and brittleness of ITO. [ 4–6 ]

Electrochemical delamination of CVD-grown graphene film: toward the recyclable use of copper catalyst.

The separation of chemical vapor deposited (CVD) graphene from the metallic catalyst it is grown on, followed by a subsequent transfer to a dielectric substrate, is currently the adopted method for device fabrication. Most transfer techniques use a chemical etching method to dissolve the metal catalysts, thus imposing high material cost in large-scale fabrication. Here, we demonstrate a highly efficient, nondestructive electrochemical route for the delamination of CVD graphene film from metal surfaces. The electrochemically delaminated graphene films are continuous over 95% of the surface and exhibit increasingly better electronic quality after several growth cycles on the reused copper catalyst, due to the suppression of quasi-periodical nanoripples induced by copper step edges. The electrochemical delamination process affords the advantages of high efficiency, low-cost recyclability, and minimal use of etching chemicals.

Length-dependent thermal conductivity in suspended single-layer graphene

Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and non-equilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-Layer MoS2

Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S ∼ T(1/3), suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

An innovative way of etching MoS2: Characterization and mechanistic investigation

We report a systematic study of the etching of MoS2 crystals by using XeF2 as a gaseous reactant. By controlling the etching process, monolayer MoS2 with uniform morphology can be obtained. The Raman and photoluminescence spectra of the resulting material were similar to those of exfoliated MoS2. Utilizing this strategy, different patterns such as a Hall bar structure and a hexagonal array can be realized. Furthermore, the etching mechanism was studied by introducing graphene as an etching mask. We believe our technique opens an easy and controllable way of etching MoS2, which can be used to fabricate complex nanostructures, such as nanoribbons, quantum dots, and transistor structures. This etching process using XeF2 can also be extended to other interesting two-dimensional crystals.Graphical abstract

Toward High Throughput Interconvertible Graphane-to-Graphene Growth and Patterning

We report a new route to prepare high quality, monolayer graphene by the dehydrogenation of graphane-like film grown by plasma-enhanced chemical vapor deposition. Large-area monolayer graphane-like film is first produced by remote-discharged radio frequency plasma beam deposition at 650 °C on Cu/Ti-coated SiO(2)-Si. The advantages of the plasma deposition include very short deposition time (<5 min) and a lower growth temperature of 650 °C compared to the current thermal chemical vapor deposition approach (1000 °C). Near edge X-ray adsorption, Raman spectroscopy, and transmission electron microscopy as well as scanning tunneling microscopy have been applied to characterize the graphane-to-graphene transition for the as-deposited films. The fingerprint quantum hall effect of monolayer graphene can be obtained on the fully dehydrogenated graphane-like film; four fully quantized half-integer plateaus are observed. The interconvertibility between graphane-like and graphene here opens up a possible route for the fabrication of regions with varying conductivity in a single deposition system using maskless, laser writing.

Thermal transport in nanostructures

This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fouriers law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.

Anomalous heat conduction and anomalous diffusion in low dimensional nanoscale systems

Abstract Heat conduction is an important energy transport process in nature. Phonon is the major energy carrier for heat in semiconductors and dielectric materials. In analogy to Ohm’s law of electrical conduction, Fourier’s law is the fundamental law of heat conduction in solids. Although Fourier’s law has received great success in describing macroscopic heat conduction in the past two hundred years, its validity in low dimensional systems is still an open question. Here we give a brief review of the recent developments in experimental, theoretical and numerical studies of heat conduction in low dimensional systems, including lattice models and low dimensional nanostructures such as nanowires, nanotubes and graphene. We will demonstrate that phonons transport in low dimensional systems superdiffusively, which leads to a size dependent thermal conductivity. In other words, Fourier’s law is not applicable in low dimensional structures.

A new route to graphene layers by selective laser ablation

Selectively creating regions of spatially varying thickness may enable the utilization of the electronic properties of N-layer (N=1 or more) graphene and other similar layered materials (e.g., topological insulators or layered superconductors) for novel devices and functionalities on a single chip. The ablation threshold energy density increases dramatically for decreasing layer numbers of graphene originating from the dimensional crossover of the specific heat. For the 2D regime of graphite (up to N≈7) the dominant flexural mode specific heat (due to its N-1 dependence) gives rise to a strong layer number-dependence on the pulsed laser ablation threshold energy density, while for 3D regime (N>>7) the ablation threshold saturates due to dominant acoustic mode specific heat. As a result, several energy density windows exist between the minimum energy densities that are required for ablating single, bi, or more layers of graphene, allowing layer number selectivity.