Raman spectroscopy and in situ Raman spectroelectrochemistry of bilayer ¹²C/¹³C graphene

Bilayer graphene was prepared by the subsequent deposition of a (13)C single-layer graphene and a (12)C single-layer graphene on top of a SiO(2)/Si substrate. The bilayer graphene thus prepared was studied using Raman spectroscopy and in situ Raman spectroelectrochemistry. The Raman frequencies of the (13)C graphene bands are significantly shifted with respect to those of (12)C graphene, which allows us to investigate the single layer components of bilayer graphene individually. It is shown that the bottom layer of the bilayer graphene is significantly doped from the substrate, while the top layer does not exhibit a signature of the doping from the environment. The electrochemical doping has the same effect on the charge carrier concentration at the top and the bottom layer despite the top layer being the only layer in contact with the electrolyte. This is here demonstrated by essentially the same frequency shifts of the G and G' bands as a function of the electrode potential for both the top and bottom layers. Nevertheless, analysis of the intensity of the Raman modes showed an anomalous bleaching of the Raman intensity of the G mode with increasing electrode potential, which was not observed previously in one-layer graphene.     

Li absorption and intercalation in single layer graphene and few layer graphene by first principles

We present an exhaustive first-principles investigation of Li absorption and intercalation in single layer graphene and few layer graphene, as compared to bulk graphite. For single layer graphene, the cluster expansion method is used to systemically search for the lowest energy ionic configuration as a function of absorbed Li content. It is predicted that there exists no Li arrangement that stabilizes Li absorption on the surface of single layer graphene unless that surface includes defects. From this result follows that defect-poor single layer graphene exhibits significantly inferior capacity compared to bulk graphite. For few layer graphene, we calibrate a semiempirical potential to include the effect of van der Waals interactions, which is essential to account for the contribution of empty (no Li) gallery to the total energy. We identify and analyze the Li intercalation mechanisms in few layer graphene and map out the sequence in stable phases as we move from single layer graphene, through few layer, to bulk graphite.     

石墨烯及其复合材料导热性能的研究现状

阐述了单层石墨烯、石墨烯带及石墨烯复合材料的导热性能。介绍了各种测试模型,综述了石墨烯的层数、纵横比、几何结构、边缘粗糙度、衬底耦合作用、温度等因素对其导热性能的影响。提出了石墨烯及其复合材料导热性能可深入研究的方面。     

Improvement of carrier mobility of top-gated SiC epitaxial graphene transistors using a PVA dielectric buffer layer

The effects of treatment with polyvinyl alcohol (PVA) and a dielectric film of HfO(2) on the properties of SiC based epitaxial graphene have been explored and analyzed. We have characterized the carrier mobility of graphene on Si-face and C-face SiC with a layer of HfO(2), with or without an initial PVA treatment on the device active layer. Epitaxial graphene grown on the C-face displays a higher mobility than a film grown on the silicon face. Also, the mobility in the presence of the PVA treatment with HfO(2) dielectric layer has been improved, compared with the mobility after deposition of only gate dielectric: ～20% in C-face graphene and ～90% in Si-face graphene. This is a major improvement over the degradation normally observed with dielectric/graphene systems.     

磁性石墨烯的制备初探

目的制备磁性石墨烯,实现其定向排列,为进一步制备性能良好的热界面材料提供定向导热相。方法采用聚合物包覆和层层组装技术,在石墨烯表面均匀包覆带负电荷的聚磺苯乙烯(PSS)聚电解质层,然后在静电力作用下包覆一层带正电荷的聚二烯丙基二甲基氯化铵(PDDA)聚电解质。最后,借助静电力使带负电荷的磁性Fe3O4纳米粒子在附着有PDDA层的石墨烯表面形成均匀的致密覆盖层,得到磁性石墨烯,并在外磁场作用下使磁性石墨烯进行定向排列。结果采用聚电解质层层组装技术可有效地对石墨烯进行磁性化处理;粉状石墨烯比片状石墨烯定向排列效果明显;以片状石墨烯为原材料的实验条件下,加入聚电解PSS、PDDA的量越多,磁性化效果更佳。结论通过聚电解质层层组装技术可有效地对石墨烯进行磁性化处理,并在磁场作用下实现定向排列。     

Vapour-phase graphene epitaxy at low temperatures

We report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO₂ and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.      

Synthesis of large area, homogeneous, single layer graphene films by annealing amorphous carbon on Co and Ni

The synthesis of large area, homogenous, single layer graphene on cobalt (Co) and nickel (Ni) is reported. The process involves vacuum annealing of sputtered amorphous carbon (a-C) deposited on Co/sapphire or Ni/sapphire substrates. The improved crystallinity of the metal film, assisted by the sapphire substrate, proves to be the key to the quality of as-grown graphene film. The crystallinity of the Co and Ni metal films was improved by sputtering the metal at elevated temperature as was verified by X-ray diffraction (XRD). After sputtering of a-C and annealing, large area, single layer graphene that occupies almost the entire area of the substrate was produced. With this method, 100 mm²-area single layer graphene can be synthesized and is limited only by the substrate and vacuum chamber size. The homogeneity of the graphene film is not dependent on the cooling rate, in contrast to syntheses using polycrystalline metal films and conventional chemical vapor deposition (CVD) growth. Our facile method of producing single layer graphene on Co and Ni metal films should lead to large scale graphene-based applications.     

Direct Synthesis of Few‐Layer Graphene on NaCl Crystals

Chemical vapor deposition is used to synthesize few‐layer graphene on micro crystalline sodium chloride (NaCl) powder. The water‐soluble nature of NaCl makes it convenient to produce free standing graphene layers via a facile and low‐cost approach. Unlike traditional metal‐catalyzed or oxygen‐aided growth, the micron‐size NaCl crystal planes play an important role in the nucleation and growth of few‐layer graphene. Moreover, the possibility of synthesizing cuboidal graphene is also demonstrated in the present approach for the first time. Raman spectroscopy, optical microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy are used to evaluate the quality and structure of the few‐layer graphene along with cuboidal graphene obtained in this process. The few‐layer graphene synthesized using the present method has an adsorption ability for anionic and cationic dye molecules in water. The present synthesis method may pave a facile way for manufacturing few‐layer graphene on a large scale.     

Graphene as an anti-permeation and protective layer for indium-free transparent electrodes

We show that graphene can be used as a protective layer for transparent electrodes made of materials which would otherwise deteriorate when exposed to the environment. In particular, we investigate aluminum-doped zinc oxides and ultrathin copper films capped with a one-atom graphene layer in damp heat (95% relative humidity and 95?°C) and high temperature (up to 180?°C) conditions. The results clearly indicate that a graphene layer can strongly reduce degradation of the electrodes' electrical, optical properties and surface morphology, thus preserving the functionality of the transparent electrodes. The proposed technique is particularly suitable for flexible optoelectronic devices thanks to the mechanical strength of graphene when subjected to bending.     

Intercalation of iridium atoms into a graphene layer on a metal

The intercalation of iridium atoms into a graphene (two-dimensional graphite) layer on a metal substrate (iridium (111) crystal face) has been studied. It is established that a thin film of iridium deposited at room temperature onto the graphene surface in a vacuum is completely destroyed on heating to 1000–1200 K and iridium atoms pass to an intercalated state between the graphene layer and the substrate. Vacuum deposition of iridium directly onto a heated sample of graphene/Ir(111) at 1000–1500 K leads to the accumulation of Ir atoms only in the intercalated state, while the outer surface of graphene remains free of the adsorbate.     

Adlayer‐Free Large‐Area Single Crystal Graphene Grown on a Cu(111) Foil

To date, thousands of publications have reported chemical vapor deposition growth of “single layer” graphene, but none of them has described truly single layer graphene over large area because a fraction of the area has adlayers. It is found that the amount of subsurface carbon (leading to additional nuclei) in Cu foils directly correlates with the extent of adlayer growth. Annealing in hydrogen gas atmosphere depletes the subsurface carbon in the Cu foil. Adlayer‐free single crystal and polycrystalline single layer graphene films are grown on Cu(111) and polycrystalline Cu foils containing no subsurface carbon, respectively. This single crystal graphene contains parallel, centimeter‐long ≈100 nm wide “folds,” separated by 20 to 50 µm, while folds (and wrinkles) are distributed quasi‐randomly in the polycrystalline graphene film. High‐performance field‐effect transistors are readily fabricated in the large regions between adjacent parallel folds in the adlayer‐free single crystal graphene film.     

Layer number and stacking sequence imaging of few-layer graphene by transmission electron microscopy

A method based on dark field transmission electron microscopy is developed to quantitively investigate the layer number and stacking order of multilayer graphene, demonstrated here on multilayer crystalline graphene synthesized by chemical vapor deposition. Our results show that the relative intensities of first- and second-order diffraction spots and contrast in corresponding dark field images are sufficient to identify the layer number and stacking order of graphene with layer number up to seven (7) or more with few-nanometer spatial resolution.     

Doped graphene electrodes for organic solar cells

In this work graphene sheets grown by chemical vapor deposition (CVD) with controlled numbers of layers were used as transparent electrodes in organic photovoltaic (OPV) devices. It was found that for devices with pristine graphene electrodes, the power conversion efficiency (PCE) is comparable to their counterparts with indium tin oxide (ITO) electrodes. Nevertheless, the chances for failure in OPVs with pristine graphene electrodes are higher than for those with ITO electrodes, due to the surface wetting challenge between the hole-transporting layer and the graphene electrodes. Various alternative routes were investigated and it was found that AuCl(3) doping on graphene can alter the graphene surface wetting properties such that a uniform coating of the hole-transporting layer can be achieved and device success rate can be increased. Furthermore, the doping both improves the conductivity and shifts the work function of the graphene electrode, resulting in improved overall PCE performance of the OPV devices. This work brings us one step further toward the future use of graphene transparent electrodes as a replacement for ITO.     

One‐Step Formation of a Single Atomic‐Layer Transistor by the Selective Fluorination of a Graphene Film

In this study, the scalable and one‐step fabrication of single atomic‐layer transistors is demonstrated by the selective fluorination of graphene using a low‐damage CF₄ plasma treatment, where the generated F‐radicals preferentially fluorinated the graphene at low temperature (<200 °C) while defect formation was suppressed by screening out the effect of ion damage. The chemical structure of the C–F bonds is well correlated with their optical and electrical properties in fluorinated graphene, as determined by X‐ray photoelectron spectroscopy, Raman spectroscopy, and optical and electrical characterizations. The electrical conductivity of the resultant fluorinated graphene (F‐graphene) was demonstrated to be in the range between 1.6 kΩ/sq and 1 MΩ/sq by adjusting the stoichiometric ratio of C/F in the range between 27.4 and 5.6, respectively. Moreover, a unique heterojunction structure of semi‐metal/semiconductor/insulator can be directly formed in a single layer of graphene using a one‐step fluorination process by introducing a Au thin‐film as a buffer layer. With this heterojunction structure, it would be possible to fabricate transistors in a single graphene film via a one‐step fluorination process, in which pristine graphene, partial F‐graphene, and highly F‐graphene serve as the source/drain contacts, the channel, and the channel isolation in a transistor, respectively. The demonstrated graphene transistor exhibits an on‐off ratio above 10, which is 3‐fold higher than that of devices made from pristine graphene. This efficient transistor fabrication method produces electrical heterojunctions of graphene over a large area and with selective patterning, providing the potential for the integration of electronics down to the single atomic‐layer scale.     

Improved Optical Sintering Efficiency at the Contacts of Silver Nanowires Encapsulated by a Graphene Layer

Graphene/silver nanowire (AgNWs) stacked electrodes, i.e., graphene/AgNWs, are fabricated on a glass substrate by air‐spray coating of AgNWs followed by subsequent encapsulation via a wet transfer of single‐layer graphene (SLG) and multilayer graphene (MLG, reference specimen) sheets. Here, graphene is introduced to improve the optical sintering efficiency of a xenon flash lamp by controlling optical transparency and light absorbing yield in stacked graphene/AgNW electrodes, facilitating the fusion at contacts of AgNWs. Intense pulsed light (IPL) sintering induced ultrafast (<20 ms) welding of AgNW junctions encapsulated by graphene, resulting in approximately a four‐fold reduction in the sheet resistance of IPL‐treated graphene/AgNWs compared to that of IPL‐treated AgNWs. The role of graphene in IPL‐treated graphene/AgNWs is further investigated as a passivation layer against thermal oxidation and sulfurization. This work demonstrates that optical sintering is an efficient way to provide fast welding of Ag wire‐to‐wire junctions in stacked electrodes of graphene/AgNWs, leading to enhanced conductivity as well as superior long‐term stability under oxygen and sulfur atmospheres.     

Centimeter-scale high-resolution metrology of entire CVD-grown graphene sheets

A high-throughput metrology method for measuring the thickness and uniformity of entire large-area chemical vapor deposition-grown graphene sheets on arbitrary substrates is demonstrated. This method utilizes the quenching of fluorescence by graphene via resonant energy transfer to increase the visibility of graphene on a glass substrate. Fluorescence quenching is visualized by spin-coating a solution of polymer mixed with fluorescent dye onto the graphene then viewing the sample under a fluorescence microscope. A large-area fluorescence montage image of the dyed graphene sample is collected and processed to identify the graphene and indicate the graphene layer thickness throughout the entire graphene sample. Using this metrology method, the effect of different transfer techniques on the quality of the graphene sheet is studied. It is shown that small-area characterization is insufficient to truly evaluate the effect of the transfer technique on the graphene sample. The results indicate that introducing a drop of acetone or liquid poly(methyl methacrylate) (PMMA) on top of the transfer PMMA layer before soaking the graphene sample in acetone improves the quality of the graphene dramatically over immediately soaking the graphene in acetone. This work introduces a new method for graphene quantification that can quickly and easily identify graphene layers in a large area on arbitrary substrates. This metrology technique is well suited for many industrial applications due to its repeatability and flexibility.     

Effects of Pb Intercalation on the Structural and Electronic Properties of Epitaxial Graphene on SiC

The effects of Pb intercalation on the structural and electronic properties of epitaxial single‐layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X‐ray photoelectron spectroscopy, and angle‐resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π‐bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of Pb? Si chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole‐doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene‐capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.     

Negative terahertz conductivity of graphene when pumping by optical plasmons

The diffusion pumping of graphene by optical plasmons, propagating in metal, and separated from the graphene by a semiconductor layer has been investigated theoretically. It is shown that pumping of graphene with optical plasmons provides maximum negative terahertz conductivity of graphene at a lower (approximately by 25%) pumping power compared to a diffusion pumping of graphene with optical radiation.     

A strong controllable absorber using graphene-metal nanostructure

In this paper, a strong absorption is obtained by a graphene-metal nanostructure at near-infrared wavelengths. The proposed absorber consists of a single graphene layer on a metal film array with semicircle and L-shaped grooves. There are two absorption peaks of 0.48 and 0.86 at 1488?nm and 1538?nm wavelengths, respectively, in the absorption spectrum of the structure without the graphene layer. The absorption is enhanced to 0.89 at 1498?nm wavelength and the full-width at half-maximum (FWHM) is increased as the graphene layer is used. Also, the absorption value of 0.99 can be attained as three graphene layers are utilized and the FWHM can be enhanced to 173?nm when four graphene layers are on the structure. Moreover, the effects of geometrical parameters of the structure and graphene chemical potential on the absorption spectrum are studied to tune the absorption value and wavelength. By tuning the geometrical parameters, a three-band absorber is proposed.     

A self-assembled Ag nanoparticle agglomeration process on graphene for enhanced light output in GaN-based LEDs

We introduce Ag nanoparticles fabricated by a self-assembled agglomeration process in order to enhance the electrical properties, adhesive strength, and reliability of the graphene spreading layer in inorganic-based optoelectronic devices. Here, we fabricated InGaN/GaN multi-quantum-well (MQW) blue LEDs having various current spreading layers: graphene only, graphene with Ag nanoparticles covering the surface, and graphene with Ag nanoparticles only in selectively patterned micro-circles. Although the Ag nanoparticles were found to act as an additional current path that increases the current spreading, optical properties such as transmittance also need to be considered when the Ag nanoparticles are combined with graphene. As a result, LEDs having a graphene spreading layer with Ag nanoparticles formed in selectively patterned micro-circles displayed more uniform and stable light emission and 1.7 times higher light output power than graphene only LEDs.