Comparison of graphene films grown on 6H-SiC and 4H-SiC substrates

Abstract

Atomic force microscopy, Kelvin-probe microscopy and Raman spectroscopy have been used to examine graphene films grown by thermal decomposition of the Si face of semi-insulating substrates of 6H-SiC and 4H-SiC polytypes in the atmosphere of argon. It was demonstrated that the quality of graphene grown on substrates of various polytypes at identical technological growth regimes is about the same. A conclusion was made that the differences in crystal structure between 6H-SiC and 4H-SiC does not lead to significant dissimilarities in the mechanism of sublimation of silicon carbide components from the surface of a crystal and in that of graphene crystallization.     

Transformation of the buffer layer grown on 4H-SiC to single-layer graphene by ex situ hydrogen intercalation

Abstract

The buffer layer samples grown on the Si-face of the 4H- and 6H-SiC substrates were heated in a hydrogen flow at different temperatures (600–900?°C) and heating times (40–60?min) in order to obtain quasi-freestanding monolayer graphene. Their structural properties were characterized before and after heating by using the methods of Raman spectroscopy, atomic force microscopy, Kelvin probe force microscopy, and low-energy electron diffraction. The dependence of the degree of coverage of the sample with quasi-freestanding graphene and the number of defects in the resulting films on the heating temperature was studied. As a result of optimization of the technological parameters, it is shown that the highest quality of the resulting quasi-freestanding graphene can be achieved by using the following parameters: heating time of 40?minutes and temperature of 800?°C.     

铂/石墨烯复合材料的合成及性能表征

以天然鳞状石墨为原料,采用化学氧化法合成氧化石墨,再经低温热解膨胀得到膨胀石墨;采用微波加热乙二醇法同时还原膨胀石墨和PtClO2-6离子得到铂/石墨烯(Pt/Gr)复合材料.分析了反应前溶液的pH值、微波加热时间以及乙二醇中水含量对Pt/Gr结构及催化性能的影响.通过循环伏安法对Pt/Gr的电化学性能进行了表征.采用透射电镜和扫描电镜观察了Gr和Pt/Gr的表面结构.研究结果表明,在优化的实验条件下可以得到高负载量高分散性的Pt/Gr纳米复合材料.实验得到的40％(质量分数)Pt/Gr的Pt粒子粒径分布在3.0～3.3nm范围内,对氢电极和氧电极反应有高催化活性,可作为质子交换膜燃料电池的电催化剂.     

Direct measurement of the Raman enhancement factor of rhodamine 6G on graphene under resonant excitation

Graphene substrates have recently been found to generate Raman enhancement. Systematic studies using different Raman probes have been implemented, but one of the most commonly used Raman probes, rhodamine 6G （R6G）, has yielded controversial results for the enhancement effect on graphene. Indeed, the Raman enhancement factor of R6G induced by graphene has never been measured directly under resonant excitation because of the presence of intense fluorescence backgrounds. In this study, a polarization-difference technique is used to suppress the fluorescence background by subtracting two spectra collected using different excitation laser polarizations. As a result, enhancement factors are obtained ranging between 1.7 and 5.6 for the four Raman modes of R6G at 611, 1,183, 1,361, and 1,647 cm-~ under resonant excitation by a 514.5 nm laser. By comparing these results with the results obtained under non-resonant excitation （632.8 nm） and pre-resonant excitation （593 nm）, the enhancement can be attributed to static chemical enhancement （CHEM） and tuning of the molecular resonance. Density functional theory simulations reveal that the orbital energies and densities for R6G are modified bv ~raphene dots.     

Effect of temperature on thermal expansion and anharmonicity in Cu2ZnSnS4 thin films grown by co-sputtering and sulfurization

Copper zinc tin sulfide (CZTS, made from the earth abundant and non-toxic materials) is a quaternary semiconducting compound which has received increasing interest for solar cells applications. In this study, CZTS thin film has been grown by co-sputtering Cu, Zn and Sn metal targets and sulfurizing it in H₂S. XRD, SEM, EDS, XRF and optical studies show that these films are suitable for solar cell applications. The temperature-dependent Raman spectroscopic study on CZTS thin ?lm was carried out in the temperature range of 80–450 K. A decrease in the intensity of “A” mode Raman peaks, shift of Raman peaks towards lower frequency and increase in the line width have been observed with increase in temperature. Raman “A” mode shifts from 337 cm⁻¹ at 80 K to 329 cm⁻¹ at 450 K. The peak at 288 cm⁻¹ disappears when the measurement was taken at 450 K. Based on experimental results and fittings of anharmonic equation; it has been shown that these effects are due to thermal expansion and interaction of the phonon with other phonons which arise at higher temperature. These phonons interact resulting in damping of the “A” mode phonon intensity. It was concluded that in order to truly analyze multicomponent compounds, Raman studies should be carried out at low temperature.     

Co~(2+)掺杂石墨烯/LiFePO_4锂离子电池复合正极材料的制备与表征

使用溶胶凝胶原位碳热还原制备了Co2+掺杂石墨烯/LiFePO4锂离子电池复合正极材料(石墨烯/LiCo0.03Fe0.97PO4),以期获得比容量高、充放电速率快和循环性能优良的锂离子电池正极材料。结构和形貌表征结果显示:石墨烯/LiCo0.03Fe0.97PO4复合材料具有三维导电网络结构,颗粒在石墨烯片层间生长均匀,粒径在200nm左右。电化学测试结果显示:石墨烯/LiCo0.03Fe0.97PO4复合材料具有高的可逆比容量和优异的循环倍率性能。2.0~4.0V充放电下0.1C时的首次放电比容量为159mA·h·g-1,在10.0C下首次放电比容量也有74mA·h·g-1;0.5C下循环100次,比容量保持率为99.7%。石墨烯/LiCo0.03Fe0.97PO4复合材料电化学性能提高的原因主要为Co2+掺杂和石墨烯包覆的协同作用。     

Infrared thermography for void mapping of a graphene/epoxy composite and its full‐field thermal simulation

Important challenges are faced during the manufacturing of graphene nanoplatelet (GNP)/polymer composites, associated with material quality and how to eliminate or reduce fabrication‐induced defects in the effort to improve performance. In the present work, infrared thermography (IRT) is used to measure void content and map void distribution, formed during fabrication of GNP/epoxy nanocomposites. Taking into consideration the size of each pixel (~100 μm), this method enables the non‐destructive detection of flaws with a size of approximately 200 μm. Their effect on thermal conductivity of the nanocomposite is studied by a 3D multiscale finite element analysis. Generic and full‐field comparisons demonstrate a good agreement between measurements and numerical predictions, validating assumptions and simplifications made in the proposed model.     

Study on the mechanism of rapid solid-phase recrystallization of hydrogenated amorphous silicon film by rapid thermal processing

High-quality hydrogenated amorphous silicon films (a-Si:H) were deposited on quartz glass substrates by radio-frequency plasma-enhanced chemical vapor deposition method. The films were then annealed at 800 °C for 3 min by rapid thermal processing (RTP). As confirmed by X-ray diffractometry and Raman spectrometry, hydrogenated microcrystalline silicon films were obtained after the annealing procedure. The mechanism of the rapid solid-phase recrystallization of a-Si:H film by RTP was theoretically mainly attributed to the interaction between short-wavelength photons and ground-state precursor radicals (silicon, SiH₂ and SiH₃).     

Investigation on the broadband electromagnetic wave absorption properties and mechanism of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>-nanosheets/reduced-graphene-oxide composite

A cobaltosic-oxide-nanosheets/reduced-graphene-oxide composite (CoNSs@RGO) was successfully prepared as a light-weight broadband electromagnetic wave absorber.The effects of the sample thickness and amount of composite added to paraffin samples on the absorption properties were thoroughly investigated.Due to the nanosheet-like structure of Co3O4,the surface-to-volume ratio of the wave absorption material was very high,resulting in a large enhancement in the absorption properties.The maximum refection loss of the CoNSs@RGO composite was-45.15 dB for a thickness of 3.6 mm,and the best absorption bandwidth with a reflection loss below-10 dB was 7.14 GHz with a thickness of 2.9 mm.In addition,the peaks of microwave absorption shifted towards the low frequency region with increasing thickness of the absorbing coatings.The mechanism of electromagnetic wave absorption was attributed to impedance matching of CoNSs@RGO as well as the dielectric relaxation and polarization of RGO.Compared to previously reported absorbing materials,CoNSs@RGO showed better performance as a lightweight and highly efficient absorbing material for application in the high frequency band.     

超声振动对石墨烯微片/聚丙烯复合材料导电导热性能的影响机制

在挤出过程加入超声振动作用,研究超声振动对高石墨烯微片(GNP)含量的聚丙烯基(GNP/PP)复合材料微观形态、结晶、导电性和导热性的影响。结果表明:由于超声振动提供强烈的冲击波与微射流,挤出过程中加入超声振动可有效地减薄GNP片层厚度,减少GNP团聚,增强GNP在PP中的分散均匀性,有利于构建导电导热网络,从而提高GNP/PP纳米复合材料的导电导热性能。相较于无超声振动,加入100 W超声振动后,GNP含量越高,GNP/PP电导率和热导率提升幅度越大,在GNP含量为15wt%时,电导率升幅为85%,热导率升幅为9.7%。而在GNP含量同为12wt%时,随着超声振动功率的增加,电导率和热导率呈现先增大后减少的规律。当超声功率为200 W时,电导率升幅为214%,热导率升幅为17.2%。而超声功率达到300 W时,较高功率的超声振动使部分石墨烯微片的片径减少,导致片层间更难以搭建完整的导电导热网络,使GNP/PP性能均略有下降。     

SiO2/环氧(E44)热膨胀性渐变的梯度功能材料(FGM)设计中的数学模型极其热应力分析

以基体树脂环氧(E44)为连续相，SiO2颗粒为分散相，设计了一种新型的热膨胀性渐变的梯度功能材料(FGM)，通过实验测定结合理论分析，确定了FGM材料组成与弹性模量、热膨胀系数及导热系数之间的关系式。应用经典层合板理论和热弹性力学理论分析了SiO2／环氧E44叠合层在稳态温度场内温度分布和热应力分布。结果表明，在稳态温度场内，FGM板的热应力分布依赖于体系的组成分布，当组成分布指数k=1时，最大热应力具有最小值。在该组成分布下，FGM板的热膨胀系数等各项性质都呈线性渐变。     

二维编织C/SiC复合材料的热膨胀系数预测

根据二维编织 C/ SiC复合材料的细观结构及其制备工艺特点 , 提出了一种预测该材料面内热膨胀系数的单胞模型。模型充分考虑了编织结构复合材料中的纤维束弯曲和 CVI工艺制备陶瓷基复合材料产生的孔洞对热膨胀系数的影响。利用单胞模型预测了二维编织 C/ SiC的结构参数、 纤维体积含量、 孔洞含量对复合材料热膨胀系数的影响规律 , 结果表明 : 随着纤维束扭结处产生间隙与纱线宽度比值的增大 , 热膨胀系数增大 ; 当其它参数不变时 , 随着纤维体积含量的增大 , 热膨胀系数反而下降; 随着孔洞含量的增加 , 热膨胀系数也出现了下降的趋势。利用 DIL402C热膨胀仪测试了二维编织 C/ SiC复合材料纵向热膨胀系数 , 试验结果与模型预测结果吻合较好。     

环保型电子封装用Sip/Al复合材料性能研究

为研究电子封装用Sip/Al复合材料热物理及力学性能,利用挤压铸造方法制备了体积分数为65%的Sip/LD11(Al-12%Si)可回收环保型复合材料.采用TEM观察、膨胀仪、激光导热综合测试仪以及万能电子拉伸试验机等设备及技术对复合材料进行研究.结果表明:Sip/LD11复合材料组织致密,材料中未观察到孔洞和缺陷;20～50℃时复合材料的热膨胀系数为8.1×10-6/℃,并随着温度的升高而增加,退火处理能够降低复合材料的热膨胀系数;Sip/LD11复合材料铸造状态和热处理状态的热导率分别为132.9、160.1 W/m·℃,复合材料的弯曲强度和比模量较高;可以采用化学镀方法在复合材料表面涂覆Ni层,镀层结合强度良好,是一种优异的电子封装用复合材料.     

Effects of GeO2 on the thermal stability and optical properties of Er/Yb-codoped oxyfluoride tellurite glasses

The aim of this article was to study the influence of GeO₂ on the thermal stability and optical properties of Er³⁺/Yb³⁺ codoped (70 − x)TeO₂–xGeO₂–PbF₂–BaF₂ (TGEYx) glasses prepared by using a melting method. The properties of Er³⁺/Yb³⁺ codoped glasses were investigated by using differential scanning calorimetry, upconversion luminescence, Raman and optical absorption spectra. The results indicated that TGEY35 glass with the germanate–tellurite mixed network showed the best thermal stability and poor crystallization tendency. With increasing the GeO₂ content, the maximum phonon energy of oxyfluoride tellurite glass network increased, while the phonon density decreased. The upconversion emission intensities enhanced obviously based on the decreasing phonon density of glass hosts, while the increasing red emission (657 nm) with the increase of GeO₂ concentration was attributed to the relative larger maximum phonon energy which matching the energy gap between the pertinent ⁴S_3/2 and ⁴F_9/2 levels.     

Synthesis and characterization of the open-framework cobalt borophosphate: (C4N2H12)Co[B2P3O12(OH)]

A high yield hydrothermal synthesis of the open-framework cobalt borophosphate (C₄N₂H₁₂)Co[B₂P₃O₁₂(OH)], has been developed. The compound was characterized by single crystal X-ray diffraction methods, thermogravimetric analysis, vibrational (IR and Raman) spectroscopy and magnetic measurements. In the structure Co(II)O₆ octahedra, BO₄ and PO₄ tetrahedra form nine-member rings which in turn are linked to form CoBPO layers parallel to the bc plane. The layers are joined together by another set of PO₄ tetrahedra and the (piperazinium)²⁺ cations occupy the channels running along [1 0 0]. The structure is compared with that of (C₂N₂H₁₀)Co[B₂P₃O₁₂(OH)].     

The effect of Si3N4 on the thermal expansion behavior of MoSi2

The effect of Si₃N₄ particulates on the thermal expansion coefficient (CTE) of MoSi₂ was investigated. It was observed that as the volume percent of Si₃N₄ increases, the CTE of the MoSi₂-Si₃N₄ composites decreases. In the temperature range 1000–1500 °C, typical of that required for glass melting, about 30–35 vol% Si₃N₄ particulates are needed in the MoSi₂-Si₃N₄ composites such that the CTE of the composite matches the CTE of Mo.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

纳米Fe3O4-还原氧化石墨烯复合修饰玻碳电极的制备及电化学检测多巴胺

通过电化学还原法制备纳米Fe₃O₄-还原氧化石墨烯复合修饰玻碳（Fe₃O₄-rGO/GCE）电极,用于多巴胺（DA）的检测。采用SEM、TEM和循环伏安对纳米Fe₃O₄-rGO复合材料进行表征。在pH为7.0的磷酸盐缓冲液（PBS）中,采用循环伏安法研究了DA在纳米Fe₃O₄-rGO/GC上的电化学行为。实验结果表明,较裸GC电极和rGO修饰（rGO/GC）电极,由于纳米Fe₃O₄与rGO的协同作用,纳米Fe₃O₄-rGO/GC显著增大了Fe₃O₄-rGO/GC复合材料电极电化学活性面积和氧化峰电流强度i_pa。DA的浓度在6.0×10^-8~2.0×10^-6 mol/L和2.0×10^-6~8.0×10^-5 mol/L范围内,与氧化峰电流强度i_pa呈良好的线性关系,检出限达4.0×10^-9 mol/L（信噪比S/N=3）。抗坏血酸和尿酸共存物几乎不干扰DA的测定,选择性高。Fe₃O₄-rGO/GC修饰电极用于盐酸DA注射液中的DA含量测定,获得结果较好,回收率为97.1%~103.9%。     

A unified model for the cohesive enthalpy, critical temperature, surface tension and volume thermal expansion coefficient of liquid metals of bcc, fcc and hcp crystals

First the cohesive enthalpy of pure liquid metals is modeled, based on experimental critical temperatures of alkali metals. The cohesive enthalpies are scaled to the melting points of pure metals. The temperature coefficient of cohesive enthalpy is the heat capacity of the liquid metal. The surface tension and its temperature coefficient for pure liquid metals are modeled through the excess surface enthalpy, excess surface entropy and molar surface area supposing that the outer two surface layers of liquid metals are similar to the {1 1 1} plane of fcc crystals. The volumetric thermal expansion coefficient of liquid metals is scaled to the ratio of the heat capacity and cohesion enthalpy. From known values of melting point, heat capacity and molar volume the following calculated properties of liquid metals are tabulated: (i) cohesive enthalpy at melting point, (ii) cohesive energy of the solid metal at 0 K, (iii) critical temperature, (iv) surface tension at melting point, (v) volume thermal expansion coefficient, and (vi) temperature coefficient of surface tension. The present models are valid only for liquid metals of bcc, fcc or hcp crystals as only their structure and nature of bonding are similar enough to be treated together.     

Effect of TiCl4 concentration on the structure of its aqueous solutions and on the properties and photoactivity of derived nanocrystalline titania

The influence of the concentration of aqueous TiCl₄ solution on the phase formation, morphology and particle size of the titanium dioxide hydrolysis product was investigated by XRD and TEM. Significant features, observed in the Raman spectra of the TiCl₄ solutions with a concentration >3 M, demonstrated that the TiCl₄ had hydrolysed. As the formal concentration of TiCl₄ decreased from 4.98 to 1 M, the Raman spectra changed qualitatively. Despite the changes in the Raman spectra of these precursor solutions, the TiO₂ product was mainly rutile in all cases. However, at low TiCl₄ concentrations small amounts of anatase were also observed. Electron microscopy suggested that the anatase particles were significantly smaller than the rutile and also indicated increasing aggregation of the product from the more dilute TiCl₄ solutions. The optical properties and photoactivities of the TiO₂ powders prepared at different concentrations were also investigated. The powder synthesized from 5 M TiCl₄ showed the highest UV extinction. The photoactivity of the product, determined by the photocatalytic oxidation of propan-2-ol (isopropanol) to propanone (acetone), was not significantly modified by changes in the concentration of the starting TiCl₄. The possibility that the relatively low area of most rutiles contributes to the reported photocatalytic activity of rutile being lower than that of anatase is discussed.