High-quality Monolayer Graphene Research Articles

(Invited) All-Chemical Vapor Deposited Graphene/Silicon Nitride TFTs

All-chemical vapor deposited silicon nitride / monolayer graphene TFTs have been fabricated. Polychromatic Raman spectroscopy shows high quality monolayer graphene channels with uniform coverage and significant interfacial doping at the source-drain contacts. Nominal mobilities of approximately 1900 cm2V-1s-1 have been measured opening up a potentially useful platform for analogue and RFR-based applications fabricated through all-chemical vapor deposition processes.

Optimizing performance parameters of graphene–silicon and thin transparent graphite–silicon heterojunction solar cells

We investigated heterojunctions of Si with large-area high-quality monolayer and multi-layer graphene, as well as thin transparent graphite. We show that by controlling the transmittance and sheet resistance of large-area graphitic electrodes, it is possible to obtain solar cells with power conversion efficiency (PCE) exceeding 3% without any doping requirements. Our calculations indicate that such junctions can form extremely robust interfaces with near-100% internal quantum efficiency. Under optimized doping conditions, power conversion efficiencies increase almost universally by a factor of 2.5. Optimized conditions for reproducibly obtaining cells with PCE>5% are presented, with the best PCE obtained ∼7.5% with short-circuit current density exceeding 24mA/cm2.

Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm

We report, for the first time to our knowledge, femtosecond pulse generation from a graphene mode-locked Cr:ZnSe laser at 2500 nm. To minimize the insertion losses at the lasing wavelength, high-quality monolayer graphene transferred on a CaF(2) substrate was used in the experiments. Once mode-locking was initiated, the laser generated a stable train of 226 fs pulses with a time-bandwidth product of 0.39. The mode-locked laser operated at a pulse repetition rate of 77 MHz and produced 80 mW output power with an incident pump power of 1.6 W. To our knowledge, this is the longest laser wavelength at which graphene-based passive mode-locking has been demonstrated to date.

High-quality monolayer graphene directly synthesized on insulating substrate by rapid-heating plasma CVD

High-quality monolayer graphene directly synthesized on insulating substrate by rapid-heating plasma CVD

Graphene mode-locked femtosecond Yb:KLuW laser

Large-area monolayer graphene, synthesized by chemical vapor deposition, was transferred to a 1-in. quartz substrate. The high-quality monolayer graphene has been subject to characterization of the nonlinear properties near 1 μm and was successfully applied as saturable absorber for passive mode-locking of a femtosecond Yb:KLuW laser. The diode-pumped mode-locked Yb:KLuW laser was tunable around 1.04 μm and delivered pulses as short as 160 fs. The maximum output power of 160 mW was demonstrated for 203 fs pulse duration. The mode-locked laser results are comparable to those demonstrated with the same laser gain medium using single-walled carbon nanotubes as saturable absorbers.

Synthesis of High Quality Monolayer Graphene at Reduced Temperature on Hydrogen-Enriched Evaporated Copper (111) Films

We report new findings on the chemical vapor deposition (CVD) of monolayer graphene with negligible defects (≥95% negligible defect-peak over 200 μm × 200 μm areas) on evaporated copper films. Compared to copper foils used in the CVD of graphene, several new unexpected results have been observed including high-quality monolayer synthesis at temperatures <900 °C, a new growth window using a hydrogen-free methane precursor for low-defects, and electron microscope evidence of commensurate growth of graphene grains on underlying copper grains. These thermal, chemical, and physical growth characteristics of graphene on copper films can be attributed to the distinct differences in the dominant crystal orientation of copper films (111) versus foils (100), and consequent dissimilar interplay with the precursor gas. This study suggests that reduced temperature, hydrogen-free synthesis of defect-negligible monolayer graphene is feasible, with the potential to shape and scale graphene grains by controlling the size and crystal orientation of the underlying copper grains.

High-quality, large-area monolayer graphene for efficient bulk laser mode-locking near 1.25 μm.

High-quality monolayer graphene as large as 1.2×1.2 cm2 was synthesized by chemical vapor deposition and used as a transmitting saturable absorber for efficient passive mode-locking of a femtosecond bulk solid-state laser. The monolayer graphene mode-locked Cr:forsterite laser was tunable around 1.25 μm and delivered sub-100 fs pulses with output powers up to 230 mW. The nonlinear optical characteristics of the monolayer graphene saturable absorber and the mode-locked operation were then compared with the case of the bilayer graphene saturable absorber.

Synthesis of high-quality monolayer and bilayer graphene on copper using chemical vapor deposition

The mechanisms determining the growth of high-quality monolayer and bilayer graphene on Cu using chemical vapor deposition (CVD) were investigated. It is shown that graphene growth on Cu is not only determined by the process parameters during growth, but also substantially influenced by the quality of Cu substrate and how the Cu substrate is pretreated. It is found that the micro-topography of the Cu surface strongly affects the uniformity of grown graphene while the purity of the Cu film determines the number of synthesized graphene layers at low pressure conditions. On the other hand, a minimum partial pressure of hydrocarbon is required for graphene to cover the Cu surface during graphene growth. The optimized bilayer graphene exhibits a maximum hole (electron) mobility of 5500 cm 2V –1s –1 (3900 cm 2V –1s –1). A new growth mode resulting in tetragonal shaped graphene domain, which is different from the known lobe structure (for monolayer) or hexagonal (for few layer) mode, is also discovered under our experimental conditions. Furthermore, high resolution transmission electron microscopy has revealed the non-ideal nature of CVD graphene structure for the first time, indicating an important cause of electron/hole mobility degradation that is typically observed in CVD graphene. This observation could be crucial for optimization of the CVD process to further improve the quality of graphene.

Extrinsic Corrugation‐Assisted Mechanical Exfoliation of Monolayer Graphene

Figure 1 . a) The optical microscopy (OM) image of ≈ 100 nm thick graphite fl akes on silicon substrate after thermal annealing at 350 ° C for 2 h. b) The OM image of the same position after the reverse exfoliation [∗] S. Pang , Dr. H. N. Tsao , Dr. Y. Hernandez , Dr. X. Feng , Prof. K. Mullen Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany) E-mail: feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de J. M. Englert , Prof. A. Hirsch Zentralinstitut fur Neue Materialien und Prozesstechnik Dr.-Mack Str. 81 D-90762 Furth (Germany) Since the reports of the fi rst isolation and observation of the exceptional electronic, mechanical, and chemical properties, single layer graphene has attracted intense interest from both academic and industrial communities. [ 1–3 ] While the mechanical exfoliation method led to many exciting discoveries, several promising approaches have been reported for the preparation of monolayer graphene including solution exfoliation of graphite, [ 4 , 5 ] epitaxial growth from SiC, [ 6 , 7 ] reduction of graphene oxide, [ 8–10 ] and chemical vapor deposition (CVD) on metal surfaces. [ 11–13 ] Up to now, the CVD growth method seems to be the most promising technique for the production of large-scale few layer graphene fi lms. [ 13 , 14 ] However, this surface-mediated process requires very high temperatures, and tedious additional steps involving etching and transfer, thus rendering the production of graphene-based electronic devices diffi cult. Ultra-large monolayer graphene [ 15–17 ] and patterned graphene structures [ 18–22 ] constitute two important aspects for graphene fabrication technology. A simple way to directly “print” a high-quality graphene monolayer on insulating substrates from a graphite stamp would be particularly appealing for electronic applications. Covalent immobilization [ 23 , 24 ] and electrostatic forces [ 18 , 25 ] have been devised for the modifi cation of mechanical exfoliation, with the aim to improve the yield of monolayer graphene or to achieve patterned graphene structures. Despite the success in deposition of graphene patterns over a large area, these procedures suffer from a low monolayer yield ( < 10%). [ 18 , 19 , 26 ] Here, we describe an extrinsic corrugationassisted mechanical exfoliation (ECAME) for synthesizing monolayer graphene on substrates. This strategy involves a simple thermal treatment of deposited graphite on a silicon wafer in association with a wafer processing tape peeling process. This work reveals that underlayer graphene sheets can corrugate following the rough SiO 2 surface when the thick graphite fl ake is thermally annealed. Such a surface-mediated extrinsic corrugation process thus serves as the key driving force for exfoliation leading to more than 60% high-quality monolayer graphene. This protocol can be further employed to fabricate graphene patterns on the surface, a technique that may be explored for graphene-based electronic device fabrication.

Structural Coherency of Graphene on Ir(111)

Low-pressure chemical vapor deposition allows one to grow high structural quality monolayer graphene on Ir(111). Using scanning tunneling microscopy, we show that graphene prepared this way exhibits remarkably large-scale continuity of its carbon rows over terraces and step edges. The graphene layer contains only a very low density of defects. These are zero-dimensional defects, edge dislocation cores consisting of heptagon-pentagon pairs of carbon atom rings, which we relate to small-angle in-plane tilt boundaries in the graphene. We quantitatively examined the bending of graphene across Ir step edges. The corresponding radius of curvature compares to typical radii of thin single-wall carbon nanotubes.