Exploring graphene formation on the C-terminated face of SiC by structural,chemical and electrical methods

The properties of epitaxial graphene on the C-face of SiC are investigated using comprehensive structural, chemical and electrical analyses. By matching similar nanoscale features on the surface potential and Raman spectroscopy maps, individual domains have been assigned to graphene patches of 1–5 monolayers thick, as well as bare SiC substrate. Furthermore, these studies revealed that the growth proceeds in an island-like fashion, consistent with the Volmer-Weber growth mode, illustrating also the presence of nucleation sites for graphene domain growth. Raman spectroscopy data shows evidence of large area crystallites (up to 620 nm) and high quality graphene on the C-face of SiC. A comprehensive chemical analysis of the sample has been provided by X-ray photoelectron spectroscopy investigations, further supporting surface potential mapping observations on the thickness of graphene layers. It is shown that for the growth conditions used in this study, 5 monolayer thick graphene does not form a continuous layer, so such thickness is not sufficient to completely cover the substrate.     

High sensitivity surface enhanced Raman spectroscopy of R6G on in situ fabricated Au nanoparticle/graphene plasmonic substrates

Plasmonic gold nanoparticles (AuNP) with controllable dimensions have been fabricated in situ on graphene at moderately elevated temperature for high sensitivity surface enhanced Raman spectroscopy (SERS) of Rhodamine 6G (R6G) dye molecules. Significantly enhanced Raman signature of R6G dyes were observed on AuNP/graphene substrates as compared to the case without graphene with an improvement factor of 400%, which is remarkably greater than previous results obtained in ex situ fabricated SERS substrate. Simulation of localized electromagnetic field around AuNPs with and without the underneath graphene layer reveals an enhanced local electromagnetic field due to the plasmonic effect of AuNPs, while additional Ohmic loss occurs when graphene is present. The enhanced local electromagnetic field by plasmonic AuNPs is unlikely the dominant factor contributing to the observed high SERS sensitivity on R6G/AuNP/graphene substrate. Instead, the p-doped graphene, which is supported by the large positive Dirac point shift away from “zero” observed in AuNP/graphene field effect transistors, promotes SERS signals through enhanced molecule adsorption and non-resonance molecular–substrate chemical interaction.     

The origin of sub-bands in the Raman D-band of graphene

In Raman spectroscopy investigations of defective suspended graphene, splitting in the D band is observed. Four double resonance Raman scattering processes: the outer and inner scattering processes, as well as the scattering processes with electrons first scattered by phonons (“phonon-first”) or by defects (“defect-first”), are found to be responsible for these features of the D band. The D sub-bands associated with the outer and inner processes merge with increasing defect concentration. However a Stokes/anti-Stokes Raman study indicates that the splitting of the D band due to the separate “phonon-first” and “defect-first” processes is valid for suspended graphene. For graphene samples on a SiO₂/Si substrate, the sub-bands of D band merge due to the increased Raman broadening parameter resulting from the substrate doping. Moreover, the merging of the sub-bands shows excitation energy dependence, which can be understood by considering the energy dependent lifetime and/or scattering rate of photo-excited carriers in the Raman scattering process.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I_2D/I_G ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I_2D/I_G ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

Tuning doping and strain in graphene by microwave-induced annealing

We propose microwave-induced annealing as a rapid, simple, and effective method of controlling surface doping and strain in graphene. Raman spectroscopy was used to confirm that heavy and uniform p-type (1.2 × 10¹³ cm⁻²) doping can be achieved within only 5 min without unintended defects by placing graphene onto a substrate with a sufficiently high dielectric constant and exposing graphene and its substrate to microwave irradiation. Further, we showed that ripples are formed in suspended graphene when it is exposed to microwave irradiation. Silicon has a sufficiently high dielectric constant (11.9) and graphene is commonly deposited on silicon-based substrates, so our proposed microwave-induced annealing technique can be used for the rapid manipulation of the properties of graphene at low cost.     

Controllable growth of single-layer graphene on a Pd(1 1 1) substrate

Using a surface segregation technique, single-layer graphene can be grown on a carbon-doped Pd(1 1 1) substrate. The growth was monitored and visualized using Auger electron spectroscopy, X-ray photoelectron spectroscopy, Raman microscopy, atomic force microscopy and scanning tunneling microscopy. Appropriate adjustment of annealing parameters enables controllable growth of single-layer graphene islands and homogeneous, wafer-scale, single-layer graphene. The chemical state of the C 1s peak from X-ray photoelectron spectroscopy indicates there is almost no charge transfer between graphene and the Pd(1 1 1) substrate, suggesting weak graphene–substrate interaction. These findings show surface segregation to be an effective method for synthesizing large-scale graphene for fundamental research as well as potential applications.     

Multilayer graphene grown by precipitation upon cooling of nickel on diamond

Multilayer graphene is grown by precipitation upon cooling of a thin nickel film deposited by e-beam evaporation on single crystal diamond (0 0 1) oriented substrates. Nickel acts as a strong catalyst inducing the dissolution of carbon from diamond into the metal. Carbon segregation produces multilayers of graphene on the top surface. Characterization by Raman spectroscopy reveals that these thin layers display relatively narrow Raman phonon peaks that are typically associated with graphene. Atomic force microscope measurements reveal a multigrain structure that reproduces small domains in the nickel film. The multilayer graphene is transferred onto a optical microscope glass slide for further analysis. The thickness of the layers estimated from optical transmission measurements is 12 nm. The catalytic reaction found for nickel on diamond is not observed when glassy carbon is used as substrate. This method provides a venue for the fabrication of large area graphene films.     

Electronic structure study of ordering and interfacial interaction in graphene/Cu composites

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.     

Buffer layer free graphene on SiC(0 0 0 1) via interface oxidation in water vapor

Intercalation of various elements has become a popular technique to decouple the buffer layer of epitaxial graphene on SiC(0 0 0 1) from the substrate. Among many other elements, oxygen can be used to passivate the SiC interface, causing the buffer layer to transform into graphene. Here, we study a gentle oxidation of the interface by annealing buffer layer and monolayer graphene samples in water vapor. X-ray photoelectron spectroscopy demonstrates the decoupling of the buffer layer from the SiC substrate. Raman spectroscopy is utilized to investigate a possible introduction of defects. Angle-resolved photoemission spectroscopy shows that the electronic structure of the water vapor treated samples. Low-energy electron microscopy (LEEM) measurements demonstrate that the decoupling takes place without changes in the surface morphology. The LEEM reflectivity spectra are discussed in terms of two different interpretations.     

The influence of thermal annealing to remove polymeric residue on the electronic doping and morphological characteristics of graphene

The impact of polymer removal by forming gas and vacuum annealing on the doping, strain, and morphology of chemical vapor deposited (CVD) and mechanically exfoliated (ME) graphene is investigated using Raman spectroscopy and atomic force microscopy (AFM). The behavior of graphene exposed and unexposed to polymer is compared. It is found that the well-known doping effect after forming gas annealing is induced in CVD–ME graphene by polymeric residue/hydrogen-functionalization. Further, forming gas annealing of ME graphene is shown to induce strain via pinning of the graphene layer to the substrate. It is found that vacuum annealing removes most polymeric residue, with minor doping and strain effects. Finally, a study of AFM step height and roughness measurements provides a comprehensive understanding of those annealing-based processes which create morphological changes and directly influence doping and strain in the graphene layer, such as removal of polymer, removal of the interfacial graphene–substrate water layer, environmental doping effects and deformation of the graphene layer.     

绝缘衬底上化学气相沉积法生长石墨烯材料

利用化学气相沉积法,在Si衬底、蓝宝石衬底和SiC衬底上生长石墨烯材料,研究石墨烯的表面形貌、缺陷、晶体质量和电学特性。原子力显微镜、光学显微镜和拉曼光谱测试表明,Si₃N₄覆盖层可以有效抑制3C-SiC缓冲层的形成;低温生长有利于保持材料表面的平整度,高温生长有利于提高材料的晶体质量。5.08 cm蓝宝石衬底上石墨烯材料,室温下非接触Hall测试迁移超过1000 cm²·V^-1·s^-1,方块电阻不均匀性为2.6%。相对于Si衬底和蓝宝石衬底,SiC衬底上生长石墨烯材料的表面形态学更好,缺陷更低,晶体质量和电学特性更好,迁移率最高为4900 cm²·V^-1·s^-1。     

Layer-by-layer synthesis of large-area graphene films by thermal cracker enhanced gas source molecular beam epitaxy

A thermal cracker enhanced gas source molecular beam epitaxy system was used to synthesize large-area graphene. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200 °C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800 °C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage of single layer and bilayer graphene films was grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method.     

Direct dry transfer of chemical vapor deposition graphene to polymeric substrates

We demonstrate the direct dry transfer of large area chemical vapor deposition graphene to several polymers (low density polyethylene, high density polyethylene, polystyrene, polylactide acid and poly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat and pressure, and the later mechanical peeling of the original graphene substrate. Simulations of the graphene–polymer interactions, rheological tests and graphene transfer at various experimental conditions show that controlling the graphene–polymer interface is the key to controlling graphene transfer. Raman spectroscopy and optical microscopy were used to identify and quantify graphene transferred to the polymer substrates. The results showed that the amount of graphene transferred to the polymer can be achieved by fine tuning the transfer conditions. As a result of the direct dry transfer technique, the graphene–polymer adhesion being stronger than graphene to Si/SiO₂ wafer.     

光电化学刻蚀方法去除SiC衬底外延石墨烯缓冲层及其表征

高温条件下裂解碳化硅（SiC）单晶,在直径5 cm的4H-SiC（0001）面制备出单层石墨烯。利用光电化学刻蚀方法,使KOH刻蚀液与SiC发生反应,降低石墨烯与衬底之间的相互作用力,去掉原位生长过程中SiC衬底与石墨烯之间存在的缓冲层,获得准自由的双层石墨烯。首先通过对比不同的电流密度和光照强度,总结出电流密度为6 mA·cm^-2、紫外灯与样品间距为3 cm时,石墨烯缓冲层的去除效率以及石墨烯质量皆为最佳。采用此优化后工艺处理的样品,拉曼光谱表明原位生长的缓冲层与衬底脱离,表现出准自由石墨烯的特性。X射线光电子能谱（XPS）C1s谱图中代表上层石墨烯与衬底Si悬键结合的S1、S2特征峰消失,即石墨烯缓冲层消失。通过分析刻蚀过程中的电化学曲线,提出了刻蚀过程的化学反应过程中的动态特性。     

石墨烯改性熔喷聚丙烯非织造材料及其吸附性能

以甲基丙烯酸丁酯(BMA)为单体,过氧化苯甲酰(BPO)为引发剂,对GO进行功能化,将功能化氧化石墨烯(GO)接枝到熔喷聚丙烯非织造材料(MBPP)表面,然后将氧化石墨烯还原,制得石墨烯改性熔喷聚丙烯非织造材料(RGO-MBPP)。通过FTIR、Raman和SEM表征了RGO-MBPP的结构,并考察了RGO-MBPP的饱和吸油率、保油率和重复使用率。结果表明:石墨烯附着在MBPP纤维表面;MBPP的饱和吸油率为29.65g/g,当氧化石墨烯的质量浓度为1.0 g/L、BMA用量为3 m L时,得到RGO-MBPP饱和吸油率可达34.66 g/g。RGO-MBPP使用5次的饱和吸油率分别为34.39、34.10、34.73、35.29、31.72 g/g,表明RGO-MBPP重复使用4次后仍具有良好的吸附性能。     

Direct growth of few layer graphene on hexagonal boron nitride by chemical vapor deposition

Few layer graphene was grown on hexagonal boron nitride single crystal flakes by chemical vapor deposition without using metal catalysts. High quality and thickness controllability of the graphene layers are confirmed by Raman spectroscopy and transmission electron microscopy. Chemical vapor deposition of graphene on this perfect-lattice-matching dielectric substrate offers many advantages including cost effectiveness, easy scalability and compatibility with standard intergraded circuit processes and promises an advance to graphene’s applications in microelectronics and optoelectronics.     

The role of SiC as a diffusion barrier in the formation of graphene on Si(111)

In this paper, we investigate the role of SiC as a diffusion barrier for Si in the formation of graphene on Si(111) via direct deposition of solid-state carbon atoms in ultra-high vacuum. Therefore, various thicknesses of the SiC layer preformed on the Si substrates were produced in order to evaluate its influence on the quality of graphene formation at different substrate temperatures from 900 °C to 1100 °C. At a given temperature of 1100 °C, we found that a thicker SiC layer can suppress silicon-out diffusion from the substrate and improve the structural quality of the graphene layer. The samples were analyzed by low energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and scanning tunneling microscopy.     

Molecular beam growth of micrometer-size graphene on mica

We demonstrate molecular beam growth of graphene on biotite mica substrates at temperatures below 1000 °C. As indicated by optical and atomic force microscopy, evaporation of carbon from a high purity solid-state source onto biotite surface results in the formation of single-, bi-, and multilayer graphene with size in the micrometer regime. It is shown that the graphene grown directly on mica surface is of very high crystalline quality with the defect density below the threshold detectable by Raman spectroscopy. The interaction between graphene and the mica substrate is studied by comparison of the Raman spectroscopy and atomic force microscopy data with the corresponding results obtained for graphene flakes mechanically exfoliated onto biotite substrates. Experimental insights are combined with density functional theory calculations to propose a model for the initial stage of the van der Waals growth of graphene on mica surfaces. This work provides hints on how the direct growth of high quality graphene on insulators can be realized in general without exceeding the thermal budget limitations of Si technologies.     

Multilayered graphene films prepared at moderate temperatures using energetic physical vapour deposition

Carbon films were energetically deposited onto copper and nickel foil using a filtered cathodic vacuum arc deposition system. Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and UV–visible spectroscopy showed that graphene films of uniform thickness with up to 10 layers can be deposited onto copper foil at moderate temperatures of 750 °C. The resulting films, which can be prepared at high deposition rates, were comparable to graphene films grown at 1050 °C using chemical vapour deposition (CVD). This difference in growth temperature is attributed to dynamic annealing which occurs as the film grows from the energetic carbon flux. In the case of nickel substrates, it was found that graphene films can also be prepared at moderate substrate temperatures. However much higher carbon doses were required, indicating that the growth mode differs between substrates as observed in CVD grown graphene. The films deposited onto nickel were also highly non uniform in thickness, indicating that the grain structure of the nickel substrate influenced the growth of graphene layers.     

Synthesis of graphene nano-sheets using eco-friendly chemicals and microwave radiation

A novel approach to synthesize graphene nano-sheets by combining chemical treatment and microwave radiation is demonstrated using eco-friendly chemicals. Microwave radiation was used to produce strong expansion of the graphite worm in the thickness direction. The graphene nano-sheets obtained in this study were characterized by morphology images, nanoprofilometry, Raman and X-ray photoelectron spectroscopy. Besides being eco-friendly, the proposed method has also the attractive advantage of being rapidly able to produce a lot of graphene nano-sheets.