Transition metal surface passivation induced graphene edge reconstruction

In vacuum, the bare zigzag (zz) edge of graphene is reconstructed into a line of pentagon-heptagon pairs, while the pristine armchair (ac) edge is retained. Our first-principle explorations of graphene edges on three metal surfaces [Cu(111), Co(111), and Ni(111)] indicate an opposite tendency, that is, the pristine zz edge is energetically favorable and the reconstructed ac edge with dangling C atoms is highly stable on Co(111) and Ni(111) surfaces. Insightful analysis shows that passivation of the graphene edge by metal surfaces is responsible for the dramatic differences. Beyond this, the unique edge configuration has a significant impact on the graphene CVD growth behavior.     

The Structure and Stability of Magic Carbon Clusters Observed in Graphene Chemical Vapor Deposition Growth on Ru(0001) and Rh(111) Surfaces

To improve the atomically controlled growth of graphene by chemical vapor deposition (CVD), understanding the evolution from various carbon species to a graphene nucleus on various catalyst surfaces is essential. Experimentally, an ultrastable carbon cluster on Ru(0001) and Rh(111) surfaces was observed, while its structure and formation process were still under debate. Using ab initio calculations and kinetic analyses, we disclose a specific type of carbon cluster, composed of a C₂₁ core and a few dangling C atoms, which is exceptional stable in a size range from 21 to 27 C atoms. The most stable one of them, an isomer of C₂₄ characterized by three dangling C atoms attached to the C₂₁ core (denoted as C₂₁‐3C), is the most promising candidate of the experimental observation. The ultrastability of C₂₁‐3C originates from both the stable core and the appropriate passivation of the dangling carbon atoms by the catalyst surface.     

金属衬底在石墨烯化学气相沉积生长中的作用

以过渡金属为催化衬底的化学气相沉积法(Chemical Vapor Deposition，CVD)已经可以制备与机械剥离样品相媲美的石墨烯，是实现石墨烯工业应用的关键技术之一。原子尺度理论研究能够帮助我们深刻理解石墨烯生长机理，为实验现象提供合理的解释，并有可能成为将来实验设计的理论指导。本文从理论计算的角度，总结了各种金属衬底在石墨烯CVD生长过程中的各种作用与相应的机理，包括在催化碳源裂解、降低石墨烯成核密度等，催化加快石墨烯快速生长，修复石墨烯生长过程中产生的缺陷，控制外延生长石墨烯的晶格取向，以及在降温过程中石墨烯褶皱与金属表面台阶束的形成过程等。在本文最后，我们对当前石墨烯生长领域中亟需解决的理论问题进行了深入探讨与展望。     

Graphene nucleation on transition metal surface: structure transformation and role of the metal step edge

The nucleation of graphene on a transition metal surface, either on a terrace or near a step edge, is systematically explored using density functional theory calculations and applying the two-dimensional (2D) crystal nucleation theory. Careful optimization of the supported carbon clusters, C(N) (with size N ranging from 1 to 24), on the Ni(111) surface indicates a ground state structure transformation from a one-dimensional C chain to a 2D sp(2) C network at N ≈ 10-12. Furthermore, the crucial parameters controlling graphene growth on the metal surface, nucleation barrier, nucleus size, and nucleation rate on a terrace or near a step edge are calculated. In agreement with numerous experimental observations, our analysis shows that graphene nucleation near a metal step edge is superior to that on a terrace. On the basis of our analysis, we propose the use of graphene seeds to synthesize high-quality graphene in large area.     

石墨烯纤维材料的化学气相沉积生长方法

石墨烯纤维材料是以石墨烯为主要结构基元沿某一特定方向组装而成或由石墨烯包覆纤维状基元形成的宏观一维材料。根据组成基元的不同可将石墨烯纤维材料分为石墨烯纤维和石墨烯包覆复合纤维。石墨烯纤维材料在一维方向上充分发挥了石墨烯高强度、高导电、高导热等特点,在智能纤维与织物、柔性储能器件、便携式电子器件等领域具有广阔的应用前景。随着化学气相沉积(Chemical Vapor Deposition,CVD)制备石墨烯薄膜技术的发展,CVD技术也逐渐应用于石墨烯纤维材料的制备。利用CVD法制备石墨烯纤维可避免传统纺丝工艺中繁琐的氧化石墨烯(Graphene Oxide,GO)还原过程。同时,通过CVD法直接将石墨烯沉积至纤维表面可以保证石墨烯与纤维基底之间强的粘附作用,提高复合纤维的稳定性,同时可实现对石墨烯质量的有效调控。本文综述了石墨烯纤维材料的CVD制备方法,石墨烯纤维材料优异的力学、电学、光学性质及其在智能传感、光电器件、柔性电极等领域的应用,并展望了CVD法制备石墨烯纤维材料未来的发展方向。     

Lattice mismatch induced nonlinear growth of graphene

As a two-dimensional material, graphene can be obtained via epitaxial growth on a suitable substrate. Recently, an interesting nonlinear behavior of graphene growth has been observed on some metal surfaces, but the underlying mechanism is still elusive. Taking the Ir(111) surface as an example, we perform a mechanistic study on graphene growth using a combined approach of first-principles calculations and kinetic Monte Carlo (kMC) simulations. Small carbon clusters on the terrace or at step sites are studied first. Then, we investigate how these small carbon species are attached to graphene edges. Generally, attachment of carbon atoms is thermodynamically favorable. However, due to substrate effect, there are also some edge sites where graphene growth must proceed via cluster attachment. The overall growth rate is determined by these cluster attachment processes, which have a much lower chance of happening compared to the monomer attachment. On the basis of such an inhomogeneous growth picture, kMC simulations are performed by separating different time scales, and the experimentally found quintic-like behavior is well reproduced. Different nonlinear growth behaviors are predicted for different graphene orientations, which is consistent with previous experiments. Inhomogeneity induced by lattice mismatch revealed in this study is expected to be a universal phenomenon and will play an important role in the growth of many other heteroepitaxial systems.     

Fast growth of graphene on SiO2/Si substrates by atmospheric pressure chemical vapor deposition with floating metal catalysts

Graphene on dielectric substrates is essential for its electronic applications. Graphene is typically synthesized on the surface of metal and then transferred to an appropriate substrate for fabricating device applications. This post growth transfer process is detrimental to the quality and performance of the as-grown graphene. Therefore, direct growth of graphene films on dielectric substrates without any transfer process is highly desirable. However, fast growth of graphene on dielectric substrates remains challenging. Here, we demonstrate a transfer-free chemical vapor deposition (CVD) method to directly grow graphene films on dielectric substrates at fast growth rate using Cu as floating catalyst. A large area (centimeter level) graphene can be grown within 15 min using this CVD method, which is increased by 500 times compared to other direct CVD growth on dielectric substrate in the literatures. This research presents a significant progress in transfer-free growth of graphene and graphene device applications.     

Oxygen-aided synthesis of polycrystalline graphene on silicon dioxide substrates

We report the metal-catalyst-free synthesis of high-quality polycrystalline graphene on dielectric substrates [silicon dioxide (SiO(2)) or quartz] using an oxygen-aided chemical vapor deposition (CVD) process. The growth was carried out using a CVD system at atmospheric pressure. After high-temperature activation of the growth substrates in air, high-quality polycrystalline graphene is subsequently grown on SiO(2) by utilizing the oxygen-based nucleation sites. The growth mechanism is analogous to that of growth for single-walled carbon nanotubes. Graphene-modified SiO(2) substrates can be directly used in transparent conducting films and field-effect devices. The carrier mobilities are about 531 cm(2) V(-1) s(-1) in air and 472 cm(2) V(-1) s(-1) in N(2), which are close to that of metal-catalyzed polycrystalline graphene. The method avoids the need for either a metal catalyst or a complicated and skilled postgrowth transfer process and is compatible with current silicon processing techniques.     

Dynamic factors in the reactions between the magic cluster Al(13)⁻ and HCl/HI

The icosahedral Al is a "magic" cluster with remarkable stability due to its high symmetry and closed valence shells. Its reactivity has provided a molecular model for understanding oxidation and dissolution processes in bulk metals. By first principles calculations, we demonstrated the importance of dynamic factors in the Al + HX reactions, with HX being either HCl or HI. There was a barrier to the dissociative adsorption of HX on the surface of an Al cluster, which involved charge transfer from Al. Furthermore, the H atom could be bonded to the cluster in multiple ways, similar to the top, bridge and hollow adsorption sites on Al(111) surface. With a large amount of energy (～40 kcal mol(-1)) deposited during the formation of Al(13)HX(-), the H atom could easily migrate among these sites, similar to the diffusion of hydrogen on metal surfaces. These factors were therefore important considerations in the formation and dissociation of Al(13)HX(-), and more generally in reactions involving other metal clusters.     

Adsorption of carbon on Pd clusters of nanometer size: a first-principles theoretical study

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.     

石墨烯晶圆的制备：从高品质到规模化

石墨烯晶圆是引领未来的战略材料,在集成电路、微机电系统和传感器等领域具有广阔的应用前景。实现石墨烯晶圆广泛应用的前提是高品质材料的规模化制备。可控性高、工艺兼容性强、成本低的化学气相沉积(chemical vapor deposition,CVD)法,是高品质石墨烯晶圆规模化制备的首选方法。本文将综述石墨烯晶圆的CVD制备进展：首先探讨石墨烯晶圆的制备需求,从实用牵引和应用场景出发,提出石墨烯晶圆的制备品质等级;随后重点介绍石墨烯的晶圆级制备方法和石墨烯晶圆材料的规模化制备技术;最后,对石墨烯晶圆可行的制备路线进行总结,并展望未来可能的发展方向。     

Structural Evolution of Boron Clusters on Ag(111) Surfaces – From Atomic Chains to Triangular Sheets with Hexagonal Holes

Unlike graphene and other 2D materials, borophene is 2D polymorphic with diverse hexagonal holes (HHs)-triangles ratios and the concentrations of HHs are highly substrate dependent. Here, we systematically explored the evolution of boron cluster on Ag(111) surface, B_N@Ag(111) (N=1∼36), to understand the nucleation of 2D boron sheet on metal surface. Our calculation showed that, with the size increasing, the structures of most stable B_N clusters undergo an evolution from compact triangular lattice, such as double-chains or triple-chains, to the ones with mixed triangular-hexagonal lattices. The first single-HH appears at N=12 and the first double-HH appears at N=27. The stability of large B_N clusters with mixed structures is derived from the charge transfer between triangular lattice and the HHs, as well as between the substrates and the B_N clusters. Our results provide a deep understanding on the formation of small boron clusters in the initial nucleation stage of borophene growth.     

小尺寸金石墨纳米颗粒的合成与表征

在表面增强拉曼光谱(SERS)的研究领域中,基于局域表面等离子体共振效应的等离子体SERS基底的制备成为过去几十年的研究热点。然而,通常开发的等离子体金属基底具有较差的稳定性和重现性。对于SERS而言,石墨烯类材料具有拉曼化学增强效应,除此之外,还具有分子富集、强的稳定性与荧光猝灭能力等优点,因此基于石墨金属复合纳米材料的SERS基底受到了研究人员的重视。我们利用化学气相沉积(CVD)法制备了小尺寸的金石墨核壳纳米颗粒(Au@G),其粒径约为17 nm。我们通过在Au NP上包覆介孔二氧化硅来控制Au@G的尺寸,同时还研究了包覆二氧化硅过程中,正硅酸乙酯(TEOS)的浓度对于石墨壳层形成的影响。结果表明当TEOS在一定浓度范围内,其浓度的降低有利于得到石墨化程度高的Au@G。进一步利用Au@G对结晶紫分子进行拉曼检测,也表明了Au@G具有较好的拉曼增强效果。这种小尺寸的Au@G在分子检测与细胞成像分析领域中具有广泛的应用潜力。     

(H2O)10 and (H2O)12 on a virtual metal surface: the growth of ice

(H2O)10 and (H2O)12 are used to investigate the growth of ice on metal surfaces with hexagonal symmetry. The model of the virtual metal surface was used to separate the electronic structure of the metal from that of the water cluster while maintaining the geometric constraints imposed by the metal surface on the water cluster. To complement the ab initio calculations on the water cluster, an additional multicenter analysis was done to analyze the hydrogen bonds within the clusters. These calculations suggested that the water bilayer structure adjacent to the virtual metal surface effectively shields the growing ice crystal from the metal surface.     

Superclean Growth of Graphene Using a Cold‐Wall Chemical Vapor Deposition Approach

Chemical vapor deposition (CVD) has become a promising approach for the industrial production of graphene films with appealing controllability and uniformity. However, in the conventional hot‐wall CVD system, CVD‐derived graphene films suffer from surface contamination originating from the gas‐phase reaction during the high‐temperature growth. Shown here is that the cold‐wall CVD system is capable of suppressing the gas‐phase reaction, and achieves the superclean growth of graphene films in a controllable manner. The as‐received superclean graphene film, exhibiting improved optical and electrical properties, was proven to be an ideal candidate material used as transparent electrodes and substrate for epitaxial growth. This study provides a new promising choice for industrial production of high‐quality graphene films, and the finding about the engineering of the gas‐phase reaction, which is usually overlooked, will be instructive for future research on CVD growth of graphene.     

Copper-phthalocyanine based metal-organic interfaces: the effect of fluorination, the substrate, and its symmetry

Metal-organic interfaces based on copper-phthalocyanine monolayers are studied in dependence of the metal substrate (Au versus Cu), of its symmetry [hexagonal (111) surfaces versus fourfold (100) surfaces], as well as of the donor or acceptor semiconducting character associated with the nonfluorinated or perfluorinated molecules, respectively. Comparison of the properties of these systematically varied metal-organic interfaces provides new insight into the effect of each of the previously mentioned parameters on the molecule-substrate interactions.     

Selective graphene formation on copper twin crystals

Selective graphene growth on copper twin crystals by chemical vapor deposition has been achieved. Graphene ribbons can be formed only on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces by tuning the growth conditions, especially by controlling the partial pressure of CH(4) in Ar/H(2) carrier gas. At a relatively low CH(4) pressure, graphene nucleation at steps on Cu (111) surfaces is suppressed, and graphene is preferentially nucleated and formed on twin crystal regions. Graphene ribbons as narrow as ～100 nm have been obtained in experiments. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which has been estimated by first principles calculations. Concentrations of reactants on a Cu surface have also been analyzed by solving a diffusion equation that qualitatively explains our experimental observations of the preferential graphene nucleation. Our findings may lead to self-organizing formation of graphene nanoribbons without reliance on top-down approaches in the future.     

气相反应对CVD生长石墨烯的影响

化学气相沉积法(CVD)制备的石墨烯薄膜具有质量高、均匀性好、层数可控且可放大等优点,近年来受到了学术界和工业界的广泛关注。在高温CVD生长过程中,除衬底表面的反应外,气相反应同样会影响石墨烯的生长行为和薄膜质量。本文将综述气相反应对CVD生长石墨烯的影响：首先对CVD体系内的气相传质过程和气相反应进行了详细讨论;随后系统介绍了基于气相调控提高石墨烯的结晶性、洁净度、畴区尺寸、层数和生长速度的相关策略及其机理;最后对气相反应影响CVD生长石墨烯的规律进行总结,并展望了未来可能的发展方向。     

CO2在金属表面活化的UBI-QEP方法研究

应用UBI-QEP方法估算了金属表面上形成的活化吸附态CO₂^-在Cu(111),Pd(111),Fe(111)和Ni(111)表面上的吸附热,计算了各种相关反应的活化能垒.结果表明,CO₂^-在4种过渡金属表面的相对稳定性的顺序为Fe>Ni>Cu>Pd;在Fe和Ni表面上CO₂^-较易生成,且容易进一步发生解离反应,在Fe表面会解离成C和O吸附原子,而在Ni表面上解离的最终产物为CO和O;在Cu表面上,CO₂^-虽较难形成,但其加氢反应的活化能比解离反应低,因此加氢反应是其进一步活化的有效模式;在Pd表面上,CO₂^-吸附态在能量上很不稳定,所以CO₂在Pd表面上不容易活化.     

超洁净石墨烯薄膜的制备方法

化学气相沉积(Chemical vapor deposition，CVD)法制备的石墨烯薄膜具有质量高、可控性好、可放大等优点，近年来受到了学术界和工业界的广泛关注。然而，近期研究结果表明，在高温CVD生长石墨烯的过程中，伴随着许多副反应，这些副反应会导致石墨烯薄膜表面沉积大量的无定形碳污染物，造成石墨烯薄膜的“本征污染”现象。同时，这些污染物的存在会导致转移后的石墨烯薄膜表面更脏，对石墨烯材料和器件的性能带来严重影响。这也是CVD石墨烯薄膜的性能一直无法媲美机械剥离石墨烯的重要原因之一。事实上，超洁净生长方法制备得到的超洁净石墨烯薄膜在诸多指标上都给出了目前文献报道的最好结果，代表着石墨烯薄膜材料制备技术的发展前沿。本文首先对CVD法制备石墨烯过程中表面污染物的形成机理进行分析，然后综述了超洁净石墨烯薄膜的制备方法，并列举了超洁净石墨烯薄膜的优异性质。最后，总结并展望了超洁净石墨烯未来可能的发展方向和规模化制备面临的机遇与挑战。