Noble metals induced magnetic properties of graphene

The adsorption energies, stable configurations, electronic structures, and magnetic properties of the graphene with noble metal (NM=Pt, Ag, and Au) atom adsorption were investigated using first-principles density-functional theory. It is found that the bridge site is the most stable adsorption site for the Pt adatom; the Ag adatom can be stabilized almost equally at the bridge or the top site, while the Au adatom prefers to be adsorbed at top site. The Pt-graphene interaction is stronger than the interaction of Ag-graphene and Au-graphene, since the Pt atom has an unsaturated electronic d-shell (d⁹s¹). While there is no net magnetic moment for the Pt adatom, the Ag and Au adatoms still exhibit magnetic character on the graphene. The magnetic moments of the NM-graphene systems may be quenched (e.g., Pt-graphene), reduced (e.g., Ag-graphene) or not changed (e.g., Au-graphene) as compared with the values before adsorption. Therefore, the magnetic character of the adatom-graphene system can be turned by adsorbing different NM atoms on the graphene.     

Geometric stability and adsorption property of hydroxyl group on graphene sheets

The stable geometrics and adsorption behaviors of hydroxyl (OH) groups on graphene sheets are investigated using the first-principles calculations. The single hydroxyl adatom has small adsorption energy and diffusion barrier on pristine graphene. The binding strength of the hydroxyl group increases with the coverage, and the aggregations of the hydroxyl groups reduce the structural bucking of graphene sheet. On the graphene with single vacancy (SV-graphene), the large trapping zones mean the adsorbed OH would be easily trapped at the vacancy site. The hydroxyl groups prefer to aggregate on graphene surfaces and form the water molecule, leaving the epoxy group on pristine graphene or oxygen dopant in SV-graphene, which is used to constitute the structural model of oxidized graphene. These results would provide us a useful reference to understand the atomic structure and adsorption property of functional groups on graphene sheets.     

First–principles study of potassium adsorption and diffusion on graphene

Potassium–ion batteries (KIBs) are a new–type of energy storage devices that have attracted increasing attention due to their low cost and the abundant resource of K in the Earth’s crust. Monolayer and multilayer graphene are promising electrode materials for KIBs. Herein, the adsorption and diffusion of potassium atoms on the surface of graphene were studied using the first–principles calculations including the van der Waals interaction. It was determined that K atoms can stably adsorb on the surface of graphene. The climbing image nudged elastic band method was employed to calculate the diffusion barriers of a single K atom and two K atoms on the surface of graphene. The results demonstrated that the diffusion barrier of a single K atom on graphene was low. The interaction between K atoms was considered and it facilitates the K atom diffusion to the second and third nearest–neighbour site of the K adatom, but prevents the K atom diffusion to the far nearest–neighbour site of the K adatom. Moreover, the difference in charge density demonstrates that there was a significant charge transfer from two K adatoms to its nearest–neighbour carbon atoms.     

To the theory of adsorption on epitaxial graphene: Model approach

A model of adsorption on epitaxial graphene has been constructed in two stages: first, the density of states of a graphene monolayer adsorbed on a solid substrate has been found and then an adsorbed atom has been placed on the epitaxial graphene thus formed. Metallic and semiconductor substrates have been considered. Charge transfer between the adatom and epitaxial graphene has been calculated. The roles of the substrate and graphene layer in the formation of the electronic state of adatoms have been estimated.     

Comparative study on graphene growth mechanism using Ni films,Ni/Mo sheets,and Pt substrates

We demonstrate a comparative study on graphene growth mechanism using various catalytic metal substrates such as Ni thin films, Ni-deposited Mo (Ni/Mo) sheets, and Pt sheets during chemical vapor deposition (CVD). Depending on the substrates, two kinds of graphene growth mechanisms that involve either precipitation or surface adsorption of carbon have been reported. We synthesized graphene, focusing especially on the initial growth stage during CVD, by varying synthesis parameters such as synthesis time, amount of feedstock, and cooling rate after synthesis. We concluded that precipitation-driven synthesis is dominant in the case of Ni substrates whereas adsorption-driven growth is dominant in the Ni/Mo system. In the case of the Pt substrate, which is generally believed to grow by carbon precipitation, graphene growth by adsorption was found to be dominant. We believe that our results will contribute to a clearer understanding of the graphene synthesis mechanism, and development of manufacturing routes for controllable synthesis of high-quality graphenes.     

非金属与金属原子共掺杂石墨烯体系的气敏特性研究

采用基于密度泛函理论的第一性原理方法研究了非金属N原子和金属原子(M=Mo,Al,Co,Fe,Au和Pt)共掺杂石墨烯体系(M-GN4)的电子结构和表面活性.研究发现:单个金属原子掺杂的GN4体系表现出不同的稳定性,相比掺杂的Au原子,其它的金属原子都具有很高的稳定性( 6. 0 e V).掺杂的金属原子失去电荷显正电性将有助于调控气体分子的吸附特性. Mo-GN4和Al-GN4衬底对吸附的O_2表现出较高的灵敏性,单个CO和O_2分子在Co-GN4和Fe-GN4衬底的吸附能差别较小.此外,吸附不同的气体分子能够有效地调控M-GN4体系的电子结构和磁性变化.     

Edge effects on the characteristics of uranium diffusion on graphene and graphene nanoribbons

The first principles density-functional theoretical calculations of U adatom adsorption and diffusion on a planar graphene and quasi-one-dimensional graphene nanoribbons(GNRs) are performed. An energetic preference is found for U adatom diffusing to the hollow sites of both graphene and GNRs surface. A number of U distinctive diffusion paths either perpendicular or parallel to the ribbon growth direction are examined. The edge effects are evidenced by the calculated energy barriers of U adatom diffusion on armchair and zigzag nanoribbons surfaces. The calculation results indicate that the diffusion of U adatom from the inner site toward the edge site is a feasible process, particularly in zigzagGNR. It is viable to control the initial morphology of nuclear carbon material to retard the diffusion and concentration of nuclides.     

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercala- tion, seven different metals have been successfully intercalated at the interface of graphene/Ru（O001） and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru（O001） and on Ir（l I 1）, have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位;悬挂键的存在极大的增强了Pt在graphene空位处的吸附;B替位掺杂有利于Pt原子在杂质附近的吸附.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位；悬挂键的存在极大的增强了Pt在graphene空位处的吸附；B替位掺杂有利于Pt原子在杂质附近的吸附.     

Formation of carbon composite structures on the Ge(110) surfaces

We present our first-principles calculation of the adsorption and diffusion of a carbon adatom on the H-terminated and clean Ge(110) surfaces, which are essential processes in the nucleation and growth of a monolayer graphene on Ge(110) by chemical vapor deposition. On the H-terminated surface, the C adatom spontaneously substitutes H atom(s) to form a monohydride structure (CH) or a dihydride structure (CH₂) and makes direct bonds with the substrate Ge atoms. The resulting diffusion barriers of the C adatom are 2.67 and 6.45 eV parallel to and perpendicular to the zigzag Ge chains of the surface, respectively. On the clean surface, the C adatom embeds into the zigzag Ge chain with nearly no barrier, kicking out a Ge atom out of the chain at the same time. The kicked-out Ge atom, instead of the C adatom, becomes a diffusion species with the barrier less than 0.63 eV. The formation of the C composite structures makes the C adatom difficult to diffuse both on the H-terminated and clean Ge(110) surfaces, which suggests that the nucleation and growth of the graphene islands from C seeds is much suppressed. We propose a growth mechanism of graphene monolayer going round the diffusion of the C adatoms on the Ge(110) surfaces.     

First principles computational investigation on the possibility of Pt-decorated SiC hexagonal sheet as a suitable material for oxygen reduction reaction

First principles calculations play a significant role in developing and optimizing new energy storage and conversion materials especially at the nanoscale. In this work, the structural, energetics and, electronic properties of adsorbed Pt atom onto two-dimensional graphene, hexagonal BN (h-BN) and SiC (h-SiC) sheets have been investigated at DFT–B3LYP level of theory using coronene molecule as a suitable model. Spin-polarization and model size effects on the Pt adsorption properties have also been evaluated. Various positions for establishing Pt atom on the selected substrates have been considered and full structural optimization was carried out for all selected systems. The adsorption energies, electronic structures and charge population analysis indicated that in all the studied structures there were strong interaction between two interacting entities. It was also found that the adsorption ability of h-SiC is much stronger than the other counterparts with adsorption energy of −3.828 eV.We have also examined the O₂ adsorption properties of Pt-decorated graphene, h-BN and h-SiC sheets for possible tunability of O₂ adsorption strength of systems under study. We found that h-SiC sheet possess a weakened O₂ adsorption energy among the selected substrates. In view of the strong stability of adsorbed Pt atom on h-SiC sheet and relatively weaker O₂ adsorption energy, one can expect that h-SiC might be a promising material for support assistant as well as increasing the catalytic activity of Pt atoms compared to graphene and h-BN substrates. This may attribute to preventing aggregating of Pt atoms due to the strong fastening nature of the h-SiC sheet and also by affording a balance in the O₂ adsorption strength that lead to enhanced catalyst turnover. Therefore, our first principles findings offer a unique opportunity for design and applications of SiC-based nanoscale supports in fuel cell technology.     

氮原子修饰点缺陷石墨烯结构和性质的理论研究

氮原子掺杂石墨烯对基于石墨烯的器件和催化研究具有重要的应用价值.本文采用基于密度泛函理论的计算方法,研究了氮原子修饰的C-Bridge(碳原子吸附在石墨烯碳碳键桥位)、C-Top(碳原子吸附在石墨烯一个碳原子上方)和C7557(碳原子对吸附在石墨烯碳环上方)三种不同点缺陷类型的石墨烯物理性质.讨论不同缺陷石墨烯结构在用氮原子进行修饰前后体系的稳定性、电子结构等;计算得到了缺陷处原子的分波态密度(PDOS)图,分析了原子间的相互作用;模拟出氮原子修饰后缺陷石墨烯恒流模式的STM图像,以便和实验上得出的图像进行对比.计算结果表明,对于所选取的三种不同缺陷,氮原子能够较稳定地吸附在缺陷表面.C-Bridge和C-Top缺陷结构本身具有磁矩,经氮原子修饰后结构磁矩消失.与之相反,C7557缺陷结构本身没有磁矩,经氮原子修饰后缺陷体系带有磁矩.另外,C-Bridge和CTop两种不同缺陷结构石墨烯经过氮原子修饰后,体系几何结构变得完全一样.     

Mechanism for the self-diffusion of Au and Ir adatoms on Pt(110) surface

A molecular dynamics computer technique was used to simulate the diffusion of a Au and and Ir adatom on the Pt(110) surface. Details of the exchange mechanism associated with cross-channel diffusion were observed. For the Au adatom at the low temperature, the expected channel diffusion occurred, whereas at the higher temperature a temporary exchange of the Pt wall atom by the Au adatom was seen. In the Ir case at the low temperature no exchange was observed, although there was a partial displacement of the wall atom by the adatom. At the higher temperature, the exchange mechanism was observed. The results are consistent with experimental observations and also indicate the importance of the stability of the channel wall atoms (via their thermal motion) on the occurrence of the exchange mechanism.     

Intercalation of germanium oxide beneath large-area and high-quality epitaxial graphene on Ir(111) substrate

Epitaxial growth on transition metal surfaces is an effective way to prepare large-area and high-quality graphene.However,the strong interaction between graphene and metal substrates suppresses the intrinsic excellent properties of graphene and the conductive metal substrates also hinder its applications in electronics.Here we demonstrate the decoupling of graphene from metal substrates by germanium oxide intercalation.Germanium is firstly intercalated into the interface between graphene and Ir(111) substrate.Then oxygen is subsequently intercalated,leading to the formation of a GeO_x layer,which is confirmed by x-ray photoelectron spectroscopy.Low-energy electron diffraction and scanning tunneling microscopy studies show intact carbon lattice of graphene after the GeO_x intercalation.Raman characterizations reveal that the intercalated layer effectively decouples graphene from the Ir substrate.The transport measurements demonstrate that the GeO_x layer can act as a tunneling barrier in the fabricated large-area high-quality vertical graphene/GeO_x/Ir heterostructure.     

First principles study of hafnium intercalation between graphene and Ir(111) substrate

The intercalation of heteroatoms between graphene and metal substrates is a promising method for integrating epitaxial graphene with functional materials. Various elements and their oxides have been successfully intercalated into graphene/metal interfaces to form graphene-based heterostructures, showing potential applications in electronic devices. Here we theoretically investigate the hafnium intercalation between graphene and Ir(111). It is found that the penetration barrier of Hf atom is significantly large due to its large atomic radius, which suggests that hafnium intercalation should be carried out with low deposition doses of Hf atoms and high annealing temperatures. Our results show the different intercalation behaviors of a large-size atom and provide guidance for the integration of graphene and hafnium oxide in device applications.     

金属衬底上高质量大面积石墨烯的插层及其机制

石墨烯作为一种新型二维材料,因其优异的性质,在科学和应用领域具有非常重要的意义.而其超高的载流子迁移率、室温量子霍尔效应等,使其在信息器件领域备受关注.如何获得高质量并且与当代硅基工艺兼容的石墨烯功能器件,是未来将石墨烯应用于电子学领域的关键.近年来,研究人员发展了一种在外延石墨烯和金属衬底之间实现硅插层的技术,将金属表面外延石墨烯高质量、大面积的特点与当代硅基工艺结合起来,实现了无需转移且无损地将高质量石墨烯置于半导体之上.通过系统的实验研究并结合理论计算,揭示了插层过程包含四个主要阶段:诱导产生缺陷、异质原子插层、石墨烯自我修复和异质原子扩散成膜,并证实了这一插层机制的普适性.拉曼和角分辨光电子能谱实验结果表明,插层后的石墨烯恢复了本征特性,接近自由状态.此外,还实现了多种单质元素的插层.不同种类的原子形成不同的插层结构,从而构成了多种石墨烯/插层异质结.这为调控石墨烯的性质提供了实验基础,也展现了该插层技术的普适性.     

Density functional theory study on the adsorption of alkali metal ions with pristine and defected graphene sheet

The graphene-based materials along with the adsorption of alkali metal ions are suitable for energy conversion and storage applications. Hence in the present work, we have investigated the structural and electronic properties of pristine and defected graphene sheet upon the adsorption of alkali metal ions (Li⁺, Na⁺, and K⁺) using density functional theory (DFT) calculations. The presence of vacancies or vacancy defects enhances the adsorption of alkali ions than the pristine sheet. From the obtained results, it is found that the adsorption energy of Li⁺ on the vacancies defected graphene sheet is higher (3.05?eV) than the pristine (2.41?eV) and Stone–Wales (2.50?eV) defected sheets. Moreover, the pore radius of the pristine and defected graphene sheets are less affected by metal ions adsorption. The increase in energy gap upon the adsorption of metal ions is found to be high in the vacancy defected graphene than that of other sheets. The metal ions adsorption in the defective vacancy sheets has high charge transfer from metal ions to the graphene sheet. The bonding characteristic between the metal ions and graphene sheet are analysed using QTAIM analysis. The influence of alkali ions on the electronic properties of the graphene sheet is examined from the Total Density of States (TDOS) and Partial Density of States (PDOS).     

Theoretical investigation of structures and energetics of sodium adatom and its dimer on graphene: DFT study

Extensive ab initio calculations have been performed to study the energetics of a sodium (Na) atom and its dimer adsorbed on graphene using the SIESTA package Soler et al. (2002) [1] which works within a DFT(density functional theory)–GGA (generalized gradient approximation) pseudopotential framework. The adsorption energy, geometry, charge transfer, ionization potential and density of states (DOS), partial density states (PDOS) of adatom/dimer-graphene system have been calculated. After considering various sites for adsorption of Na on graphene, the center of a hexagonal ring of carbon atoms is found to be the preferred site of adsorption while the Na₂ dimer prefers to rest parallel to the graphene sheet. We find insignificant energy differences among adsorption configurations involving different possible sites in parallel orientation, which implies high mobility of the dimer on the graphene sheet. We also notice only a slight distortion of the graphene sheet perpendicular to its plane upon adatom adsorption. However, some lateral displacements seen are more perceptible.     

Anchoring platinum on graphene using metallic adatoms: a first principles investigation

First principles calculations based on spin-polarized density functional theory were used to identify metallic adatoms that would strengthen the Pt(111)/graphene interface (with a low work of separation of 0.009 J m(-2)), when the adatom was placed between the Pt(111) and the graphene. It was shown that the strength of the Pt-adatom bond, which had a metallic character, increased with the amount of charge transferred from the adatom to the Pt. The carbon-adatom bond, on the other hand, had a mixed ionic and covalent character and was weaker than the Pt-adatom bond for each of the 25 elements considered. Consequently, the total Pt(111)/graphene interface strength and, hence, the anchoring effect of the adatom were controlled by the carbon-adatom bond strength. Metals with unfilled d orbitals increased the Pt/graphene interface strength to above 0.5 J m(-2). The carbon-adatom bond strength was proportional to the ratio between the charge transferred from the adatom to the graphene (ΔZ(C)) and the charge transferred to the Pt surface (ΔZ(Pt)); i.e., the ΔZ(C)/ΔZ(Pt) ratio defined the ability of an adatom to anchor Pt to graphene. For Ir, Os, Ru, Rh and Re, ΔZ(C)/ΔZ(Pt) > 1.0, making these elements the most effective adatoms for anchoring Pt to graphene.