Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO2-supported graphene devices

By combining atomic force microscopy and trans-port measurements, we systematically investigated effects of thermal annealing on surface morphologies and electrical properties of single-layer graphene devices fabricated by electron beam lithography on silicon oxide (SiO(2)) substrates. Thermal treatment above 300 °C in vacuum was required to effectively remove resist residues on graphene surfaces. However, annealing at high temperature was found to concomitantly bring graphene in close contact with SiO(2) substrates and induce increased coupling between them, which leads to heavy hole doping and severe degradation of mobilities in graphene devices. To address this problem, a wet-chemical approach employing chloroform was developed in our study, which was shown to enable both intrinsic surfaces and enhanced electrical properties of graphene devices. Upon the recovery of intrinsic surfaces of graphene, the adsorption and assisted fibrillation of amyloid β-peptide (Aβ1-42) on graphene were electrically measured in real time.     

镀膜浮法玻璃的风化性能

采用人工加速风化的方法研究了镀有SiO2薄膜的浮法玻璃的风化性能,并与相同风化条件下的浮法玻璃原片的表面状态进行了比对,从风化机理上进行了分析。红外反射光谱和扫描电镜分析结果表明,镀有SiO2薄膜的浮法玻璃和未镀膜玻璃随着风化温度和风化时间的增加,均呈现风化现象逐渐加重的现象。但前者风化程度明显低于后者,尤其当风化条件(70℃,75%RH,风化15d)恶劣时,镀膜玻璃表面只出现了小块斑点和红外光谱的略微变化;而未镀膜玻璃表面则出现大面积的侵蚀斑块,在红外谱图上不仅≡Si—O—Si≡特征峰发生位移,而且出现了C?O的特征峰。说明SiO2膜层能够很好地阻挡玻璃表面Na+与空气中H+、H3O+的交换,有效地抑制玻璃表面风化的发生。     

Mapping the density of scattering centers limiting the electron mean free path in graphene

Recently, giant carrier mobility μ (>10(5) cm(2) V(-1) s(-1)) and micrometer electron mean free path (l) have been measured in suspended graphene or in graphene encapsulated between inert and ultraflat BN layers. Much lower μ values (10000-20000 cm(2) V(-1) s(-1)) are typically reported in graphene on common substrates (SiO(2), SiC) used for device fabrication. The debate on the factors limiting graphene electron mean free path is still open with charged impurities (CI) and resonant scatterers (RS) indicated as the most probable candidates. As a matter of fact, the inhomogeneous distribution of such scattering sources in graphene is responsible of nanoscale lateral inhomogeneities in the electronic properties, which could affect the behavior of graphene nanodevices. Hence, high resolution two-dimensional (2D) mapping of their density is very important. Here, we used scanning capacitance microscopy/spectroscopy to obtain 2D maps of l in graphene on substrates with different dielectric permittivities, that is, SiO(2) (κ(SiO2) = 3.9), 4H-SiC (0001) (κ(SiC) = 9.7) and the very-high-κ perovskite strontium titanate, SrTiO(3) (001), briefly STO (κ(STO) = 330). After measuring l versus the gate bias V(g) on an array of points on graphene, maps of the CI density (N(CI)) have been determined by the neutrality point shift from V(g) = 0 V in each curve, whereas maps of the RS density (N(RS)) have been extracted by fitting the dependence of l on the carrier density (n). Laterally inhomogeneous densities of CI and RS have been found. The RS distribution exhibits an average value ～3 × 10(10) cm(-2) independently on the substrate. For the first time, a clear correlation between the minima in the l map and the maxima in the N(CI) map is obtained for graphene on SiO(2) and 4H-SiC, indicating that CI are the main source of the lateral inhomogeneity of l. On the contrary, the l and N(CI) maps are uncorrelated in graphene on STO, while a clear correlation is found between l and N(RS) maps. This demonstrates a very efficient dielectric screening of CI in graphene on STO and the role of RS as limiting factor for electron mean free path.     

Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices

By stacking various two-dimensional (2D) atomic crystals on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces. Here we employ cross sectional TEM to take a side view of several graphene-boron nitride heterostructures. We find that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean. Moreover, we observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene. This work proves the concept of heterostructures assembled with atomic layer precision and provides their first TEM images.     

Preparation of silica thin films by novel wet process and study of their optical properties

Silicon dioxide (SiO2) thin films have gained considerable attention because of their various industrial applications. For example, SiO2 thin films are used in superhydrophilic self-cleaning surface glass, UV protection films, anti-reflection coatings, and insulating materials. Recently, many processes such as vacuum evaporation, sputtering, chemical vapor deposition, and spin coating have been widely applied to prepare thin films of functionally graded materials. However, these processes suffer from several engineering problems. For example, a special apparatus is required for the deposition of films, and conventional wet processes are not suitable for coating the surfaces of substrates with a large surface area and complex morphology. In this study, we investigated the film morphology and optical properties of SiO2 films prepared by a novel technique, namely, liquid phase deposition (LPD). Images of the SiO2 films were obtained by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in order to study the surface morphology of these films: these images indicate that films deposited with different reaction times were uniform and dense and were composed of pure silica. Optical properties such as refractive index and transmittance were estimated by UV-vis spectroscopy and ellipsometry. SiO2 films with porous structures at the nanometer scale (100-250 nm) were successfully produced by LPD. The deposited film had excellent transmittance in the visible wavelength region.     

Low-temperature peculiarities of density of electronic states and electron transport characteristics in the disordered 2D graphene

We propose an approach that allows us to take into account the location of atomic defects in the graphene structure and describe the effect of electron scattering on certain configurations of foreign atoms in a graphene matrix on density of electronic states (DOS) and the electron transport characteristics in 2D graphene. The local disorder is shown to play a decisive role in formation of the low-temperature behavior of the DOS and electron transport characteristics in the disordered 2D graphene.     

Application of atomic force microscopy to the study of glass decay

Conventional methods such as scanning electron microscopy with energy dispersive X-ray spectrometry are commonly used to characterize corroded glasses. However, their use is often restricted when glass pieces come from historical artworks and may not be damaged. Atomic force microscopy can be an alternative method for characterizing such glasses since it is an essentially non-destructive technique which allows their topographic analysis with good vertical and lateral resolutions. In addition, samples do not require any previous manipulation. The application of atomic force microscopy to study glass decay is reported in this paper. The main goals of the research were to study the corroded texture of both historical glass pieces and model glasses weathered in the laboratory, and to determine and compare the chemical corrosion mechanisms which occurred in both cases. The resulting data suggest that atomic force microscopy can be a useful technique for characterizing decay mechanisms in historical glasses.     

Pressure-mediated doping in graphene

Exfoliated graphene and few layer graphene samples supported on SiO(2) have been studied by Raman spectroscopy at high pressure. For samples immersed on a alcohol mixture, an electron transfer of ?n/?P ～ 8 × 10(12) cm(-2) GPa(-1) is observed for monolayer and bilayer graphene, leading to giant doping values of n ～ 6 × 10(13) cm(-2) at the maximum pressure of 7 GPa. Three independent and consistent proofs of the doping process are obtained from (i) the evolution of the Raman G-band to 2D-band intensity ratio, (ii) the pressure coefficient of the G-band frequency, and (iii) the 2D band components splitting in the case of the bilayer sample. The charge transfer phenomena is absent for trilayer samples and for samples immersed in argon or nitrogen. We also show that a phase transition from a 2D biaxial strain response, resulting from the substrate drag upon volume reduction, to a 3D hydrostatic compression takes place when going from the bilayer to the trilayer sample. By model calculations we relate this transition to the unbinding of the graphene-SiO(2) system when increasing the number of graphene layers and as function of the surface roughness parameters. We propose that the formation of silanol groups on the SiO(2) substrate allows for a capacitance-induced substrate-mediated charge transfer.     

Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.     

Remote catalyzation for direct formation of graphene layers on oxides

Direct deposition of high-quality graphene layers on insulating substrates such as SiO(2) paves the way toward the development of graphene-based high-speed electronics. Here, we describe a novel growth technique that enables the direct deposition of graphene layers on SiO(2) with crystalline quality potentially comparable to graphene grown on Cu foils using chemical vapor deposition (CVD). Rather than using Cu foils as substrates, our approach uses them to provide subliming Cu atoms in the CVD process. The prime feature of the proposed technique is remote catalyzation using floating Cu and H atoms for the decomposition of hydrocarbons. This allows for the direct graphitization of carbon radicals on oxide surfaces, forming isolated low-defect graphene layers without the need for postgrowth etching or evaporation of the metal catalyst. The defect density of the resulting graphene layers can be significantly reduced by tuning growth parameters such as the gas ratios, Cu surface areas, and substrate-to-Cu distance. Under optimized conditions, graphene layers with nondiscernible Raman D peaks can be obtained when predeposited graphite flakes are used as seeds for extended growth.     

Crystallization and void formation in ZnO–B2O3–SiO2–MgO sintered solder glasses

The evolution of crystallization and porosity changes with firing temperature were studied in ZnO–B2O3–SiO2–MgO glasses. Those glasses presintered at 610 °C to a low porosity were crystallized in the temperature range of 690–870 °C. The glasses were crystallized by a surface crystallization mechanism. The porosity increased with the crystallization temperature. In the temperature range of 710–790 °C, several crystalline phases, such as 3ZnO–B2O3, willemite (2ZnO–SiO2), 5ZnO–2B2O3, and another form of zinc silicate (2ZnO–SiO2), produced at relatively low temperatures, were produced, while above 800 °C only the 2ZnO–SiO2 phase co-existed with a glass phase. Only an observed density difference between the glass and the crystallized glass cannot be attributed to the void formation during the crystallization reaction. Due to the crystallization the composition of the remaining glass around the crystalline phases is expected to change. The depletion of a certain component in the remaining glass, probably the SiO2 due to the production of the 2ZnO–SiO2 phase, might result in the increase in the vapour pressure of the remaining glass and lead to the observed increase in porosity. Below 800 °C, at which temperature the crystallization rate is fast and only a small amount of the glass phase remained, the porosity remained constant after the completion of the crystallization. Contrarily at 860 °C the porosity continuously increased with firing time. This revised version was published online in November 2006 with corrections to the Cover Date.     

硅烷偶联剂/二氧化硅界面层的表征研究

本文为研究γ-MPS/SiO₂界面层厚度与形态,使用自编程序,通过计算机数值模拟和图形分析后确定:当SiO₂膜厚为1250～1450?,激光源入射角为65～75°的最佳椭偏仪测定条件时,灵敏度可提高至1?,从而能在国产椭偏仪上对10?左右的界面超薄层膜具有足够的测试灵敏度。实验中应用的SiO₂/Si平片是一种新的适宜于作界面研究的玻纤模型。由此模型凭借椭偏仪和扫描电镜,可获得经γ-MPS水溶液处理再经溶剂淋洗后形成的γ-MPS/SiO₂界面层的重要信息:界面层结构是1～2个单分子层;形态为连续的膜层,偶尔可见被淋洗去的γ-MPS水解介聚物残留的边缘隆起的椭圆形凹坑。      

Two-dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks

Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics.     

Graphene Glass from Direct CVD Routes: Production and Applications

Recently, direct chemical vapor deposition (CVD) growth of graphene on various types of glasses has emerged as a promising route to produce graphene glass, with advantages such as tunable quality, excellent film uniformity and potential scalability. Crucial to the performance of this graphene‐coated glass is that the outstanding properties of graphene are fully retained for endowing glass with new surface characteristics, making direct‐CVD‐derived graphene glass versatile enough for developing various applications for daily life. Herein, recent advances in the synthesis of graphene glass, particularly via direct CVD approaches, are presented. Key applications of such graphene materials in transparent conductors, smart windows, simple heating devices, solar‐cell electrodes, cell culture medium, and water harvesters are also highlighted.     

Evaluation of optical glass composition by optimization methods

Optical glass comprises SiO(2) and various other oxides that create the basic glass structure network. The Huggins-Sun-Davis (HSD) model, later modified by several authors, explains the influence of glass composition on glass properties, such as refractive index and density. A new technique for calculating the composition of a given glass whose Buchdahl or Schott dispersion coefficients and density are known is described. The well-known damped-least-squares method implementing Lagrange multipliers for boundary constraints on the composition parameters is used to provide a powerful iteration scheme with a high rate of convergence. The method based on the modified HSD model has been tested on several commercial glasses and is found to converge to very realistic composition values. The method can be easily programmed and provides a good tool in graded-index profile computations and in forming new optical glasses.     

Upconversion luminescence of Er3+ in transparent SiO2—PbF2—ErF3 glass ceramics

Oxyfluoride glasses with the composition 50SiO2 · 50PbF2 · xErF3 (x=4 and 5) by molar ratio were developed. Transparent glass ceramics were obtained by heat-treating the 50SiO2 · 50PbF2 · xErF3 glasses at the first crystallization temperatures. X-ray diffraction analysis of the transparent glass ceramics revealed that fluorite type -PbF2:Er3+ solid solution regions of about 13.0 nm in diameter are precipitated in the glass matrix. The formation of this -PbF2:Er3+ solid solution was also supported by Eu3+ fluorescence spectra which were measured on specimens in which Eu substituted for Er. Under 800 nm laser excitation, the Er3+ upconversion luminescence of 50SiO2 · 50PbF2 · xErF3 glasses was barely detectable, but the 50SiO2 · 50PbF2 · xErF3 glass ceramics gave Er3+ upconversion luminescence at a very high efficiency. The reason for the highly efficient Er3+ upconversion luminescence in the 50SiO2 · 50PbF2 · xErF3 glass ceramics can be explained in terms of the very small multiphonon relaxation rates that are anticipated from consideration of the Eu3+ emission spectra.     

Graphene annealing: how clean can it be?

Surface contamination by polymer residues has long been a critical problem in probing graphene's intrinsic properties and in using graphene for unique applications in surface chemistry, biotechnology, and ultrahigh speed electronics. Poly(methyl methacrylate) (PMMA) is a macromolecule commonly used for graphene transfer and device processing, leaving a thin layer of residue to be empirically cleaned by annealing. Here we report on a systematic study of PMMA decomposition on graphene and of its impact on graphene's intrinsic properties using transmission electron microscopy (TEM) in combination with Raman spectroscopy. TEM images revealed that the physisorbed PMMA proceeds in two steps of weight loss in annealing and cannot be removed entirely at a graphene susceptible temperature before breaking. Raman analysis shows a remarkable blue-shift of the 2D mode after annealing, implying an anneal-induced band structure modulation in graphene with defects. Calculations using density functional theory show that local rehybridization of carbons from sp(2) to sp(3) on graphene defects may occur in the random scission of polymer chains and account for the blue-shift of the Raman 2D mode.     

A self-templated etching route to surface-rough silica nanoparticles for superhydrophobic coatings

A simple, mild, and effective self-templated etching strategy has been developed to directly convert SiO(2) nanospheres into surface-rough SiO(2) (SR-SiO(2)) nanoparticles (NPs) by reaction with NaBH(4). Small SiO(2) NPs on the surface of SR-SiO(2) NPs can be tailored by carefully regulating the reaction time. SiO(2) nanospheres with varied sizes were etched under varied reaction conditions. Subsequently, particulate coatings were constructed on slide glass using SR-SiO(2) NPs as building blocks through the Layer-by-Layer assembly. Slide glasses just coated with two cycles of SR-SiO(2) NPs followed by calcination and hydrophobic modification exhibited superhydrophobicity because of their dual-size surface roughness.     

Few layers of graphene as transparent electrode from botanical derivative camphor

Here, we report synthesis of large area graphene sheets by control pyrolysis of solid botanical derivative camphor (C₁₀H₁₆O) and fabrication of transparent electrodes. Raman study shows highly ordered graphene sheet with minimum defects. Second order Raman spectrum shows that graphene layers are more than single layer and can be controlled with amount of camphor pyrolyzed. Transmission electron microscopic images show presence of 4 layers for thinner and 13 layers for thicker graphene sheets. Transferred graphene sheets on glass substrates show very good transparency in wide range of wavelength (0.3-2 μm). Electrical measurements of the graphene sheets show thickness dependent sheet resistance. A sheet resistance of 203 Ω/sq is obtained at a transmittance of 63.5% of the graphene sheet. The technique to fabricate few layer of graphene as transparent electrode from camphor is both viable and scalable for potential large area optoelectronic applications.