载能碳离子撞击石墨烯中Stone-Wales缺陷的动力学研究

采用分子动力学模拟方法,研究了载能碳离子撞击石墨烯中Stone-Wales缺陷的动力学过程,计算了Stone-Wales缺陷中初级碰撞原子的离位阈能和载能碳离子使其移位的入射阈能,并与完美石墨烯结构计算结果进行对比。通过分析初级碰撞原子与入射离子动能和势能随时间的变化关系,研究了碰撞过程中能量转移过程。研究结果表明,初级碰撞原子产生离位并最终脱离石墨烯体系的最小能量为25.0 eV;当初始动能为23.0 eV时,Stone-Wales缺陷中2个七元环共用的碳-碳键旋转90°形成了完美的石墨烯结构;使Stone-Wales缺陷中初级碰撞原子发生离位的载能碳离子最小入射能为41.0 eV。     

碳离子碰撞单壁碳纳米管的动力学

采用分子动力学方法研究了碳离子碰撞碳纳米管中顶位、键中心和六元环中心的动力学过程。通过分析低、中、高3种入射能分别对碰撞过程的影响,探索了典型缺陷形成的微观演化过程。研究结果表明,碰撞碳纳米管中不同空间位置,其碰撞结果差异较大,其中顶位碰撞阈能最低,约为20 eV;碰撞六元环中心时碳管会发生严重变形,损伤最为严重。通过分析入射离子动能,碳纳米管热动能、质心动能以及势能随时间的演化规律,阐述了碰撞过程中的能量转移机制。     

锯齿形单壁碳纳米管的穿透能研究

基于分子动力学方法,研究了载能碳离子碰撞锯齿形单壁碳纳米管过程中初级碰撞原子(PKA)的运动过程和能量变化过程.分析了手性指数为(2n+1,0)(n=2~9)的单壁碳纳米管中PKA的穿透能与载能碳离子入射能间的关系.结果表明,穿透能与入射能之间呈线性增长关系,线性变化的斜率与碳纳米管直径有关.通过分析PKA势能随模拟时间的变化规律,阐述了初级碰撞原子的穿透能随入射能的增加而增加的物理机制.     

碳离子碰撞单壁碳纳米管的动力学

采用分子动力学方法研究了碳离子碰撞碳纳米管中顶位、键中心和六元环中心的动力学过程。通过分析低、中、高3种入射能分别对碰撞过程的影响,探索了典型缺陷形成的微观演化过程。研究结果表明,碰撞碳纳米管中不同空间位置,其碰撞结果差异较大,其中顶位碰撞阈能最低,约为20 e V;碰撞六元环中心时碳管会发生严重变形,损伤最为严重。通过分析入射离子动能,碳纳米管热动能、质心动能以及势能随时间的演化规律,阐述了碰撞过程中的能量转移机制。     

石墨烯中的Stone-wales缺陷对铂原子催化解离氧分子的影响

采用密度泛函理论中的UB3LYP方法,研究了石墨烯中的Stone-wales缺陷对铂原子催化解离氧气分子的影响.通过计算发现,氧气分子在以Stone-wales缺陷石墨烯片为载体的铂上(Pt-SW)形成3种吸附结构,通过4条路径,最终生成两种产物.氧气分子最易通过[2+1]环加成作用,吸附在以Stone-wales缺陷石墨烯片为载体的Pt的表面上,吸附能(Eads)为-0.64eV.由于石墨烯片上的Stone-wales缺陷的存在,氧气分子在Pt-SW上解离的4条路径中最有利的解离路径中的决速步能垒都明显高于氧气在以完美石墨烯为载体的Pt(Pt/Graphene)上解离的能垒(1.51eV vs 1.35eV),相应吸收的热量也高于在Pt/Graphene上吸收的热量(0.79eVvs0.15eV).     

缺陷石墨烯吸附Au、Ag、Cu的第一性原理计算

采用基于密度泛函理论的投影缀加波方法研究了Au、Ag、Cu吸附在缺陷石墨烯单侧和双侧的体系,对吸附体系的吸附能、磁性、电荷转移和电子结构进行了计算和分析.缺陷石墨烯吸附Au、Ag、Cu体系的吸附能比本征石墨烯增加2 eV以上,说明三种金属原子更容易吸附在缺陷位置;吸附体系的电荷密度差分和电子结构的结果表明,Au、Ag、Cu与缺陷石墨烯之间均为化学吸附.计算吸附体系的磁性发现,单侧吸附时三种吸附体系均有磁性,磁矩大约为1μB;双侧吸附时,三种吸附体系磁矩大约为2μB.     

缺陷石墨烯吸附Au、Ag、Cu的第一性原理计算

采用基于密度泛函理论的投影缀加波方法研究了Au、Ag、Cu吸附在缺陷石墨烯单侧和双侧的体系,对吸附体系的吸附能、磁性、电荷转移和电子结构进行了计算和分析. 缺陷石墨烯吸附Au、Ag、Cu体系的吸附能比本征石墨烯增加2 eV以上,说明三种金属原子更容易吸附在缺陷位置;吸附体系的电荷密度差分和电子结构的结果表明,Au、Ag、Cu与缺陷石墨烯之间均为化学吸附. 计算吸附体系的磁性发现,单侧吸附时三种吸附体系均有磁性,磁矩大约为1μ_B;双侧吸附时,三种吸附体系磁矩大约为2μ_B.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.     

石墨烯化聚合物:一种兼具电子和离子传输通道的三维富碳高分子能源材料

近年来,用于电化学能源存储和转化的石墨烯材料,得到了研究者们越来越多的关注。但是,这些石墨烯材料不同于严格定义的单原子碳层结构,往往具有孔洞、杂原子和化学官能团等缺陷结构。由于制备方法的不同,缺陷结构各不相同,其电化学性能也表现各异。结构分析表明,这类材料是由类似石墨烯片段的单元与聚合物链共价连接而成,使其具有石墨烯和聚合物的双重特性,我们称之为石墨烯化聚合物。由小分子通过自下而上的方法制备的多孔聚合物,也可以通过进一步热交联等方法,使其形成包含石墨烯片段单元与聚合物链的化学结构。这些材料与石墨烯衍生材料一起组成了石墨烯化聚合物的整个谱系;这个谱系涵盖了由聚合物到石墨烯的过渡区。更重要的是,这类材料特殊的结构与性质,使其成为一种兼具电子和离子传输通道的三维富碳高分子材料,非常适合作为电极材料应用于电化学能源存储和转化,这为我们深入研究储能器件中电极材料的结构与性能的相关关系提供了很好的材料平台。     

内掺Sc原子的扩展三明治结构graphene-Sc-graphene的几何结构,电子性质和储氢性能研究

使用密度泛函理论中的广义梯度近似对内掺Sc原子的graphene-Sc-graphene扩展三明治结构的几何结构、电子结构和储氢性能进行计算研究. 计算发现: Sc原子位于单层石墨烯中六元环上方的结构具有较大的结合能, 但小于固体Sc的内聚能实验值(3.90 eV), 然而, 当单个Sc原子或者多个Sc原子在双层石墨烯中间与底层相距2 Å时, Sc原子与基底的结合能增加到5 eV以上, 远远大于固体Sc的内聚能实验值(3.90 eV), 因此相邻的Sc原子可以有效避免成簇. 由此可见, 三明治结构的形成明显增加了Sc原子与基底的结合强度, 该结构可以进一步储氢来满足18电子规则而更加稳定, 从而成为理想的新型储氢纳米材料. 扩展三明治结构graphene-Sc-graphene的(2×3)单元中每个Sc原子最多可以吸附2个H₂分子, 对H₂的平均吸附能分别为0.67 eV和0.54 eV, 介于物理吸附和化学吸附(0.1～0.8 eV)之间, 因此该体系可以实现常温常压下对H₂的可逆吸附. 由储氢机制分析可知: 扩展三明治结构graphene-Sc-graphene主要通过Dewar-Kubas作用进行储氢, 形成了π-δ-π型的电子结构.     

Collision Dynamics of an Energetic Carbon Ion Impinging on the Stone-Wales Defect in a Single-walled Carbon Nanotube

By employing atomistic simulations based on an empirical potential model and a self-consistent-charge density-functional tight-binding method, the collision dynamics process of an energetic carbon ion impinging on the Stone-Wales defect in a single-walled carbon nanotube was investigated. The outwardly and inwardly displacement threshold energies for the primary knock-on atom in the Stone-Wales defect were calculated to be 24.0 and 25.0 eV, respectively. The final defect configuration for each case was a 5-1DB-T(DB=dangling bond) defect formed in the front surface of the nanotube. Moreover, the minimum incident energy of the projectile prompting the primary knock-on atom displacement was predicted to be 71.0 eV, and the time evolutions of the kinetic and potential energies of the projectile and the primary knock-on atom were both plotted to analyze the energy transfer process.     

Surface-induced dissociation and chemical reactions of C2D 4 + on stainless steel, carbon (HOPG), and two different diamond surfaces

Surface-induced interactions of the projectile ion C₂D₄⁺ with room-temperature (hydrocarbon covered) stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces (O-terminated and H-terminated) were investigated over the range of incident energies from a few eV up to 50 eV. The relative abundance of the product ions in dependence on the incident energy of the projectile ion [collision-energy resolved mass spectra, (CERMS) curves] was determined. The product ion mass spectra contained ions resulting from direct dissociation of the projectile ions, from chemical reactions with the hydrocarbons on the surface, and (to a small extent) from sputtering of the surface material. Sputtering of the surface layer by low-energy Ar⁺ ions (5–400 eV) indicated the presence of hydrocarbons on all studied surfaces. The CERMS curves of the product ions were analyzed to obtain both CERMS curves for the products of direct surface-induced dissociation of the projectile ion and CERMS curves of products of surface reactions. From the former, the fraction of energy converted in the surface collision into the internal excitation of the projectile ion was estimated as 10% of the incident energy. The internal energy of the surface-excited projectile ions was very similar for all studied surfaces. The H-terminated room-temperature diamond surface differed from the other surfaces only in the fraction of product ions formed in H-atom transfer surface reactions (45% of all product ions formed versus 70% on the other surfaces).     

Theoretical Simulations of Irradiation-Induced Sputtering at Tungsten Surface

The irradiation-induced sputtering and the structural damage at tungsten surface are investigated by using molecular dynamics simulations at the level of quantum mechanics. Our simulations indicate that the sputtered atoms appear when the energy of incident primary knock-on atom (PKA) is more than 200 eV and the incident angle of the PKA is larger than 65°. Meanwhile, the irradiation-induced vacancies are less when the incident angle of PKA is in the range of 45°-65°. So, the optimum incident angles of PKA are suggested to reduce the irradiation-induced damage of the W surface. Furthermore, we find that the interstitials contained in the systems accelerate the sputtering whereas the intrinsic vacancies suppress the sputtering when the PKA is near the defects.     

Ni adsorption on Stone-Wales defect sites in single-wall carbon nanotubes

Ni adsorption on Stone-Wales defect sites in (10,0) zigzag and (5,5) armchair single-wall carbon nanotubes was studied using the density functional theory. The stable adsorption sites and their binding energies on different Stone-Wales defect types were analyzed and compared to those on perfect side walls. It was determined that the sites formed via fusions of 7-7 and 6-7 rings are the most exothermic in the cases of (10,0) and (5,5) defective tubes. In addition C-C bonds associated with Stone-Wales defects are more reactive than the case for a perfect hexagon, thus enhancing the stability of the Ni adsorption. Moreover, the Ni adsorption was found to show a noticeable relationship to the orientation of the Stone-Wales defects with respect to the tube axis. The nature of the Ni adsorption on Stone-Wales defects that have the similar orientation is identical, in spite of the different chiralities.     

On the Addition of Aryl Radicals to Graphene: The Importance of Nonbonded Interactions

Dispersion‐corrected density functional theory is utilized to study the addition of aryl radicals to perfect and defective graphene. Although the perfect sheet shows a low reactivity against aryl diazonium salts, the agglomeration of these groups and the addition onto defect sites improves the feasibility of the reaction by increasing binding energies per aryl group up to 27 kcal mol^?1. It is found that if a single phenyl radical interacts with graphene, the covalent and noncovalent additions have similar binding energies, but in the particular case of the nitrophenyl group, the adsorption is stronger than the chemisorption. The single vacancy shows the largest reactivity, increasing the binding energy per aryl group by about 80 kcal mol^?1. The zigzag edge ranks second, enhancing the reactivity 5.4 times with respect to the perfect sheet. The less reactive defect site is the Stone–Wales type, but even in this case the addition of an isolated aryl radical is exergonic. The arylation process is favored if the groups are attached nearby and on different sublattices. This is particularly true for the ortho and para positions. However, the enhancement of the binding energies decreases quickly if the distance between the two aryl radicals is increased, thereby making the addition on the perfect sheet difficult. A bandgap of 1–2 eV can be opened on functionalization of the graphene sheets with aryl radicals, but for certain configurations the sheet can maintain its semimetallic character even if there is one aryl radical per eight carbon atoms. At the highest level of functionalization achieved, that is, one aryl group per five carbon atoms, the bandgap is 1.9 eV. Regarding the effect of using aryl groups with different substituents, it is found that they all induce the same bandgap and thus the presence of NO₂, H, or Br is not relevant for the alteration of the electronic properties. Finally, it is observed that the presence of tetrafluoroborate can induce metallic character in graphene.     

New horizons in sputter depth profiling inorganics with giant gas cluster sources: Niobium oxide thin films

X‐ray photoelectron spectroscopy is used to study a wide variety of material systems as a function of depth (“depth profiling”). Historically, Ar⁺ has been the primary ion of choice, but even at low kinetic energies, Ar⁺ ion beams can damage materials by creating, for example, nonstoichiometric oxides. Here, we show that the depth profiles of inorganic oxides can be greatly improved using Ar giant gas cluster beams. For NbO_x thin films, we demonstrate that using Ar_x⁺ (x = 1000‐2500) gas cluster beams with kinetic energies per projectile atom from 5 to 20 eV, there is significantly less preferential oxygen sputtering than 500 eV Ar⁺ sputtering leading to improvements in the measured steady state O/Nb ratio. However, there is significant sputter‐induced sample roughness. Depending on the experimental conditions, the surface roughness is up to 20× that of the initial NbO_x surface. In general, higher kinetic energies per rojectile atom (E/n) lead to higher sputter yields (Y/n) and less sputter‐induced roughness and consequently better quality depth profiles. We demonstrate that the best‐quality depth profiles are obtained by increasing the sample temperature; the chemical damage and the crater rms roughness is reduced. The best experimental conditions for depth profiling were found to be using a 20 keV Ar₂₅₀₀⁺ primary ion beam at a sample temperature of 44°C. At this temperature, there is no, or very little, reduction of the niobium oxide layer and the crater rms roughness is close to that of the original surface.