Kinetics of hydrogen constrained graphene growth on Cu substrate

Chemical vapor deposition (CVD) has shown great promise for the large-scale production of high-quality graphene films for industrial applications. Atomic-scale theoretical studies can help experiments to deeply understand the graphene growth mechanism, and serve as theoretical guides for further experimental designs. Here, by using density functional theory calculations, ab-initio molecular dynamics simulations, and microkinetic analysis, we systematically investigated the kinetics of hydrogen constrained graphene growth on Cu substrate. The results reveal that the actual hydrogen-rich environment of CVD results in CH as the dominating carbon species and graphene H-terminated edges. CH participated island sp2 nucleation avoids chain cyclization process, thereby improving the nucleation and preventing the formation of non-hexameric ring defects. The graphene growth is not simply C-atomic activity, rather, involves three main processes: CH species attachment at the growth edge, leading to a localized sp3 hybridized carbon at the connecting site;excess H transfer from the sp3 carbon to the newly attached CH;and finally dehydrogenation to achieve the sp2 reconstruction of the newly grown edge. The threshold reaction barriers for the growth of graphene zigzag (ZZ) and armchair (AC) edges were calculated as 2.46 and 2.16 eV, respectively, thus the AC edge grows faster than the ZZ one. Our theory successfully explained why the circumference of a graphene island grown on Cu substrates is generally dominated by ZZ edges, which is a commonly observed phenomenon in experiments. In addition, the growth rate of graphene on Cu substrates is calculated and matches well with existing experimental observations.

Invisible vapor catalysis in graphene growth by chemical vapor deposition

Vapor catalysis was recently found to play a crucial role in superclean graphene growth via chemical vapor decomposition(CVD).However,knowledge of vapor-phase catalysis is scarce,and several fundamental issues,including vapor compositions and their impact on graphene growth,are ambiguous.Here,by combining density functional theory(DFT)calculations,an ideal gas model,and a designed experiment,we found that the vapor was mainly composed of Cui clusters with tens of atoms.The vapor pressure was estimated to be~10^(-12)-10^(-1)1 bar under normal low-pressure CVD system(LPCVD)conditions for graphene growth,and the exposed surface area of Cui clusters in the vapor was 22-269 times that of the Cu substrate surface,highlighting the importance of vapor catalysis.DFT calculations show Cu clusters,represented by Cu17,have strong capabilities for adsorption,dehydrogenation,and decomposition of hydrocarbons.They exhibit an adsorption lifetime and reaction flux six orders of magnitude higher than those on the Cu surface,thus providing a sufficient supply of active C atoms for rapid graphene growth and improving the surface cleanliness of the synthesized graphene.Further experimental validation showed that increasing the amount of Cu vapor improved the as-synthesized graphene growth rate and surface cleanliness.This study provides a comprehensive understanding of vapor catalysis and the fundamental basis of vapor control for superclean graphene rapid growth.

Adsorption of 1,3,5-Triphenylbenzene Molecules and Growth of Graphene Nanoflakes on Cu(100)Surface

Adsorption of 1,3,5-triphenylbenzene （TPB） molecules on Cu（100） surface is studied using ultraviolet photo- electron spectroscopy （UPS） and density functional theory （DFT） calculations. Researches on the bottom-up fabrication of graphene nanoflakes （GNFs） with TPB as a precursor on the Cu（100） surface are carried out based on UPS and DFT calculations. Three emission features d, e and f originating from the TPB molecules are located at 3.095, 7.326 and 9.349 eV below the Fermi level, respectively. With the increase of TPB coverage on the Cu（100） substrate, the work function decreases due to the formation of interfacial dipoles and charge （electron） rearrangement at the TPB/Cu（100） interface. Upon the formation of GNFs, five emission characteristic peaks of g, h, i, j and k originating from the GNFs are located at 1.100, 3.529, 6.984, 8.465 and 9.606eV below the Fermi level, respectively. Angle resolved ultraviolet photoelectron spectroscopy （ARUPS） and DFT calculations indicate that TPB molecules adopt a lying-down configuration with their molecular plane nearly parallel to the Cu（100） substrate at the monolayer stage. At the same time, the lying-down configuration for the GNFs on the Cu（100） surface is also unveiled by ARUPS and DFT calculations.

Theoretical investigations on the growth of graphene by oxygenassisted chemical vapor deposition

Recently,graphene has drawn considerable attention in the field of electronics,owing to its favorable conductivity and high carrier mobility.Crucial to the industrialization of graphene is its high-quality microfabrication via chemical vapor deposition.However,many problems remain in its preparation,such as the not fully understood cracking mechanism of the carbon source,the mechanism of its substrate oxidation,and insufficient defect repair theory.To help close this capability gap,this study leverages density functional theory to explore the role of O in graphene growth.The effects of Cu substrate oxidation on carbon source cracking,nucleation barriers,crystal nucleus growth,and defect repairs are discussed.OCu was found to reduce energy change during dehydrogenation,rendering the process easier.Moreover,the adsorbed O in graphene or its Cu substrate can promote defect repair and edge growth.

Complete coverage of reduced graphene oxide on silicon dioxide substrates

Reduced graphene oxide （RGO） has the advantage of an aqueous and industrial-scale production route. No other approaches can rival the RGO field effect transistor platform in terms of cost （〈US$1） and portability （millimetre scale）. However the large deviations in the electrical resistivity of this fabricated material prevent it from being used widely. After an ethanol chemical vapor deposition （CVD） post-treatment to graphene oxide with ethanol, carbon islets are deposited preferentially at the edges of existing flakes. With a 2-h treatment, the standard deviation in electrical resistance of the treated chips can be reduced by 99.95%. Thus this process could enable RGO to be used in practical electronic devices.

Oxygen-assisted direct growth of large-domain and high-quality graphene on glass targeting advanced optical filter applications

Growing high quality graphene films directly on glass by chemical vapor deposition(CVD)meets a growing demand for constructing high-performance electronic and optoelectronic devices.However,the graphene synthesized by prevailing methodologies is normally of polycrystalline nature with high nucleation density and limited domain size,which significantly handicaps its overall properties and device performances.Herein,we report an oxygen-assisted CVD strategy to allow the direct synthesis of 6-inch-scale graphene glass harvesting markedly increased graphene domain size(from 0.2 to 1.8μm).Significantly,as-produced graphene glass attains record high electrical conductivity(realizing a sheet resistance of 900Ω·sq^(-1)at a visible-light transmittance of 92%)amongst the state-of-the-art counterparts,readily serving as transparent electrodes for fabricating high-performance optical filter devices.This work might open a new avenue for the scalable production and application of emerging graphene glass materials with high quality and low cost.

Safe growth of graphene from non-flammable gas mixtures via chemical vapor deposition

Chemical vapor deposition has emerged as the most promising technique for the growth of graphene.However, most reports of this technique use either flammable or explosive gases, which bring safety concerns and extra costs to manage risk factors. In this article, we demonstrate that continuous monolayer graphene can be synthesized via chemical vapor deposition technique on Cu foils using industrially safe gas mixtures. Important factors, including the appropriate ratio of hydrogen flow and carbon precursor,pressure, and growth time are considered to obtain graphene films. Optical measurements and electrical transport measurements indicate graphene films are with comparable quality to other reports. Such continuous large area graphene can be synthesized under non-flammable and non-explosive conditions, which opens a safe and economical method for mass production of graphene. It is thereby beneficial for integration of graphene into semiconductor electronics.

An Effort Towards Full Graphene Photodetectors

Graphene as a truly 2-dimensional(2D)system is a promising candidate material for various optoelectronic applications.Implementing graphene as the main building material in ultra-broadband photodetectors has been the center of extensive research due to its unique absorption spectrum which covers most of the electro-magnetic spectra.However,one of the main challenges facing the wide application of pure graphene photodetectors has been the small optical absorption of monolayer graphene.Although novel designs were proposed to overcome this drawback,they often need complicated fabrication processes in order to integrate with the graphene photodetector.In this regard,fabrication of purely graphene photodetectors is a promising approach towards the manufacturing of simple,in expensive,and high photosensitive devices.The fabrication of full graphene photodetectors(FGPDs)is mainly based on obtaining an optimal technique for the growth of high quality graphene,modification of electronic and optical properties of the graphene,appropriate techniques for transfer of graphene from the grown substrate to the desire position,and a proper design for photodetection.Therefore,the available states of the art techniques for each step of device fabrication,along with their pros and cons,are reviewed and the possible approaches for optimization of FGPDs have been proposed.

Effects of substrate and transfer on CVD-grown graphene over sapphire-induced Cu films

We differentiated the effects of Cu films deposited on single crystalline a-,r-,and c-plane sapphire substrates upon graphene films synthesized with atmospheric pressure chemical vapor deposition(CVD).The data illustrate that the realization of high-crystalline Cu film is dependent not only on the crystallinity of underlying substrate,but also on the symmetric match of crystallographic geometry between metal film and substrate.We also systematically investigated the effects of PMMA removal on the Raman ID/IG and IG/I2D values of transferred graphene.The results reveal that different PMMA removal methods do not alter the ID/IG values;instead,the residue of PMMA increases the IG/I2D values and the thermal decomposition of PMMA leads to higher IG/I2D values than the removal of PMMA with acetone.The effects of PMMA removal on variations of the Raman spectra are also discussed.

Steric Hindrance Effect in High-Temperature Reactions

High-temperature reactions widely exist in nature.However,they are difficult to characterize either experimentally or computationally.The minimum energy path(MEP)model routinely used in computational modeling of chemical reactions is not justified to describe high-temperature reactions since high-energy structures are actively involved at high temperatures.In this study,we used methane(CH4)decomposition on Cu(111)surface as an example to compare systematically results obtained from the MEP model with those obtained from an explicit sampling of all relevant structures via ab initio molecular dynamics(AIMD)simulations at different temperatures.Interestingly,we found that,for reactions protected by strong steric hindrance effects,the MEP was still followed effectively even at a temperature close to the Cu melting point.In contrast,without such protection,the flexibility of the surface Cu atoms could lead to a significant reduction of the free-energy barrier at a high temperature.Accordingly,some earlier conclusions made about graphene growth mechanisms based on MEP calculations should be revisited.The physical insights provided by this study could deepen our understanding of high-temperature surface reactions.