小尺寸金属团簇熔化过程的分子动力学模拟

运用分子动力学方法模拟了小尺寸金属团簇的熔化过程, 原子之间的作用采用嵌入原子法(EAM)模型, 计算了均方根键长涨落δ随温度的变化, 以及升温过程中团簇热容的变化. 包含55、56个原子的面心立方(FCC)结构Au团簇的熔化过程是基本相同的. 而同样结构和数目Cu团簇的熔化过程却呈现出不同的趋势. Cu55、Cu56在模拟过程中都出现了FCC结构到二十面体结构的转变. 但由于表面多出了一个原子, Cu56的热容曲线比Cu55多了一个峰, 体系出现了预熔化现象. 这表明小尺寸团簇的固液转变的过程与团簇的原子类型、几何结构和原子数目密切相关.     

中介尺度Au纳米团簇熔化的分子动力学模拟

采用分子动力学模拟技术，研究了原子个数为16～8628的 Au纳米团簇的熔化过程.采用 Johnson的EAM (embedded atom method) 模型,模拟结果表明，金属纳米团簇存在一中介尺度区域.对Au纳米团簇而言，当原子个数N >456时，团簇的热力学性质与团簇尺寸呈线性关系，熔化首先从表面开始，逐步向中心区域推进，且满足Tmb－Tmc(N)＝aN(－1/3)的关系.另外，计算了中介区域的团簇的尺寸、熔化温度、表面能、熵、焓等热力学量以及均方根位移（RMSD）等动力学量，为研究纳米团簇提供定量数据.     

Pt-Au合金团簇热力学性质分子动力学模拟研究

采用分子动力学方法和原子嵌入法模型势模拟了Pt原子和Au原子合金纳米团簇的熔化过程,研究了这些金属原子纳米团簇熔点与团簇组分的关系,发现不同组分纳米团簇的熔点不是单调变化的,同时均出现了负热容现象.通过对各种团簇溶化前后结构的比较研究,分析了导致这种现象的原因.     

从头算分子动力学模拟Pd团簇负载UiO-66材料结构及稳定性

采用从头算分子动力学模拟与密度泛函理论相结合的方法,计算模拟了不同尺寸的Pdn(n=1~32)金属团簇在UiO-66孔道中的稳定构型,并对金属团簇与材料骨架之间的作用方式、结合能及骨架形变能等进行了讨论.采用Bader电荷分析方法对该体系的电荷转移情况进行了计算分析.结果表明,Pdn团簇稳定负载于UiO-66材料的四面体笼中,且均呈堆积型构型.当Pd原子个数为28时体系的热力学稳定性最好,这与金属团簇和有机配体的成键方式相关,是金属团簇内部结合能和骨架形变能综合作用的结果.     

[BxAl13-x]-(x=0～13)二元团簇的密度泛函理论研究

采用遗传算法（Genetic algorithm,GA）对B-Al二元团簇[B_xAl_13-x]^-（x=0～13）进行了全局结构搜索,并在密度泛函理论（Density Functional Theory,DFT）下对其进行了优化计算. 结果表明在二元团簇中,随着硼原子的数量逐渐增加,团簇的结构由二十面体（3D）逐渐转化为准平面（2D）且团簇结构由B与Al原子的比例所决定的. 当x=0～7时,Al原子占多数,团簇保持3D结构;反之团簇呈现2D构型;在x=7～8时,团簇的结构发生由3D向2D的转变. 能量决定着团簇的稳定性（例如相对能量）. 能量越小,其结构越稳定. 在计算相对能量E_rel,当x=1时,其相对能量最低,团簇结构最稳定. 为了进一步了解团簇的稳定性,计算了[B_xAl_13-x]^- 团簇的HOMO-LUMO能级差（E_H-L）和垂直电离能（Vertical Detachment Energies,VDE）,并且发现它们的值在整体上是随着B原子数量的增加而减小,表明其团簇的稳定性逐渐减弱. 在所有的团簇中,BAl₁₂^-的能级差最大,结构最稳定. 因此文章中对其进行了分子轨道分析,发现当一个B原子替代了Al₁₃^-团簇中的中心Al原子时,所得到的BAl₁₂^-的电子壳层结构的1s²和1p⁶几乎和Al₁₃^-团簇保持一致. 此外,对于x=13时的准平面全硼团簇B₁₃^-,文章中用适应性自然密度划分（Adaptive Natural Density Partitioning,AdNDP）对其化学成键进行了分析,结果显示B₁₃^-有8个π电子,具有π反芳香性.     

硅基稀土掺杂团簇MSi_7~q(M=Eu,Sm,Yb;q=0,-1)结构预测与光电子能谱研究

采用随机踢球模型结合密度泛函理论,在PBEPBE/RE/SDD/Si/6-311+G(d)水平下研究了中性和阴性的硅基稀土掺杂团簇MSi_7~q(M=Eu,Sm,Yb;q=0,-1)的几何结构、稳定性及电子和磁学性质.计算结果表明,阴性团簇的基态结构是在五角双锥的双锥侧面外法向方向加入一个Si原子而形成的3D结构,并且稀土原子M处于五角双锥的顶点;中性团簇的最低能结构是一个畸变的双帽八面体,并且M原子处于八面体的赤道面上.SmSi_7~-团簇在这3种稀土掺杂的团簇中具有最高的平均结合能和掺杂能,是这3种稀土掺杂团簇中最稳定的一种.Si_7团簇是非磁性团簇,但是当M原子(M=Eu,Sm,Yb)掺入其中时,由于镧系元素独特的原子磁性,使其变成了磁性团簇.此外,还模拟了各团簇前几种低能异构体的光电子能谱.     

液态金属Al凝固过程中的团簇结构与幻数特性

采用分子动力学方法,对含有100000个Al原子的液态金属系统在凝固过程中团簇结构的形成特性进行了模拟研究,并采用原子团类型指数法(CTIM)来描述各种类型的团簇结构组态.研究结果显示:在液态金属Al的凝固过程中,只有与1551键型相关的二十面体原子团(12 0 12 0)及其组合形成的各种团簇结构,对微结构的演变起着关键的、决定性的作用;由不同数目、不同类型基本原子团组成的各种层次的团簇结构,都在一定的原子数区段内呈现出峰值,即幻数点;系统的幻数序列为:13(13), 19(21), 25～28(27), 31～33(29～30) ,37、39,…(括号内为液态时对应的幻数值),与Harris等人的实验结果甚为相符.本模拟研究所用的团簇结构按层次区段来研究幻数序列的方法,可为实验结果提供更为合理的模型解释.     

InAs管状团簇及单壁InAs纳米管的结构、稳定性和电子性质

采用密度泛函理论研究了InnAsn (n≤90)管状团簇以及单壁InAs纳米管的几何结构、稳定性和电子性质. 小团簇InnAsn (n=1-3)基态结构和电子性质的计算结果与已有报道相一致. 当n≥4时优化得到了一族稳定的管状团簇, 其结构基元(In原子与As原子交替排列的四元环和六元环结构)满足共同的衍化通式. 团簇的平均结合能表明横截面为八个原子的管状团簇稳定性最好. 管状团簇前线轨道随尺寸的变化规律有效地解释了一维稳定管状团簇的生长原因, 同时也说明了实验上之所以能合成InAs纳米管的微观机理. 此外, 研究结果表明通过管状团簇的有效组装可得到宽带隙的InAs半导体单壁纳米管.     

嵌入纳米Fe颗粒的Fe液凝固过程的分子动力学模拟

采用Sutton-Chen 势函数及分子动力学(MD)方法对嵌入了Fe纳米团簇(半径从0.4-1.8 nm)的Fe液凝固过程进行了模拟. 模拟结果表明只有当嵌入的纳米团簇半径超过0.82 nm才能降低凝固时所需要的临界过冷度(ΔT^*), 起到诱导凝固的作用. 同时采用原子键型指数法(CTIM-2)对样本在凝固过程中的原子结构进行了标定, 通过观察微观结构演变发现当嵌入纳米团簇能够作为凝固核心时, 体系按照hcp-fcc 交叉形核的方式长大. 同时还发现嵌入纳米团簇对体系凝固过程晶核的生长方向及凝固的最终构型存在“结构遗传效应”.     

Cu_nNi(n=2-13)团簇基态结构和电子特性的密度泛函研究

基于第一性原理,利用密度泛函理论中广义梯度近似(GGA)对团簇Cun-1Ni和Cun(n=3-14)进行了结构优化和能量计算,结果表明,单质Cu团簇不是以密实结构而是以类平面结构生长,但Ni的掺杂使得Cu团簇结构以二十面体为基础生长并且增加了团簇的稳定性;团簇结合能的二阶差分计算表明Cu3Ni,Cu7Ni和Cu9Ni结构最为稳定;在团簇的最稳定结构中Ni原子趋于占据团簇的中心位置和更多的Cu原子形成化学键;位于表面的Cu原子成为Mülliken电荷的接受体而带负电性,这也可能是Ni掺杂Cu合金耐腐蚀性增强的原因之一;Ni的掺杂使原来没有磁性的铜团簇显示了磁性且总自旋磁矩表现明显的奇偶振荡,为1或2μB,与团簇的尺寸无关.     

Molecular-dynamics simulation of the structural stability,energetics, and melting of Cu n(n = 13–135) clusters

Cluster properties of copper have been investigated using the Molecular-Dynamics md technique. The structural stability and energetics of spherical Cu_n (n = 13–135) clusters have been investigated at temperatures T = 1 K and T = 300 K. It has been found that the average interaction energy per atom in the cluster decreases and reaches an asymptotic value as cluster size increases. The melting behaviour of clusters n = 13 and n = 55 have been investigated. It has been found that the melting temperature decreases as cluster size increases, and for clusters with multishell structures melting starts from the outermost shell. In the simulation an emprical potential energy function (PEF) proposed by Erkoç has been used, which contains two-body atomic interactions.     

Density functional study of small neutral and charged silver cluster hydrides

Small neutral, anionic, and cationic silver cluster hydrides AgnH and anionic HAgnH (n=1-7) have been studied using the PW91PW91 density functional method. It was found that the most stable structure of the AgnH complex (neutral or charged) does not always come from that of the lowest energy bare silver cluster plus an attached H atom. Among various possible adsorption sites, the bridge site is energetically preferred for the cationic and most cases of neutral Agn. For anionic Agn, the top site is preferred for smaller Agn within n相似文献   

Structure and electronic properties of PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters: theoretical investigations based on first principles calculations

A systematic theoretical study of the PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters have been investigated to explore the effect of impurity atoms on the structure and electronic properties of lead clusters. The calculations were carried out using the density functional theory with generalized gradient approximation for exchange-correlation potential. Extensive search based on large numbers of initial configurations has been carried out to locate the stable isomers of PbnM clusters. The results revealed that the location of the impurity atom depends on the nature of interaction between the impurity atom and the host cluster and the size of the impurity atom. Whereas, the impurity atoms smaller than Pb favor to occupy the endohedral position, the larger atoms form exohedral capping of the host cluster. The stability of these clusters has been analyzed based on the average binding energy, interaction energy of the impurity atoms, and the energy gap between the highest occupied and lowest unoccupied energy levels (HLG). Based on the energetics, it is found that p-p interaction dominates over the s-p interaction and smaller size atoms interact more strongly. The stability analysis of these clusters suggests that, while the substitution of Pb by C or Al enhances the stability of the Pbn clusters, Mg lowers the stability. Further investigations of the stability of PbnM clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of these clusters. The energy gap analysis reveals that, while the substitution of Mg atom widens the HLG, all other elements reduce the gap of the PbnM clusters.     

Insights into the early stages of metal nanoparticle formation via first-principle calculations: the roles of citrate and water

The early stages of silver nanoparticle formation in the presence of citrate and water have been investigated via first-principle theoretical calculations. Our study revealed that the charge density of the clusters is a key factor determining the selectivity among various growth pathways. An optimal charge density appears to control the selection between neutral and charged species in cluster growth; partially positively charged clusters are thermodynamically preferred and can serve as seeds for further growth. They interact favorably with both the solvent, leading to their solubility, and the citrate. The solvent (water) plays an important role in cluster growth both on the energetics of reactions including highly charged clusters and on the geometry of the resulting silver structures by preventing the formation of asymmetric ones (a structure directing action). Contrary to the common belief we found, from an energetic viewpoint, that growth of small clusters is not blocked by the citrate. Citrate, by acting as a reducing agent, opens up new channels for cluster growth involving highly charged species. By regulating the cluster charge, cluster-cluster associations may be promoted by the citrate, providing a new mechanistic interpretation for the effect of citrate concentration on nanoparticle size substantially different from the classic nucleation theory. From the citrate-silver and water-silver cluster interactions, linear free energy relationships have been retrieved that provide insights into metal nanoparticle growth mechanisms.     

Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R(2)O(3) (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO(2). Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.     

An improved clusterion-photoelectron coincidence technique for the investigation of the ionisation dynamics of clusters

The introduction of photoion-photoelectron coincidence techniques has made it possible to investigate photoionisation properties of heavy clusters, which are not accessible by conventional mass spectrometry. This technique has been further developed in combination with a zero-volt electron energy analyser and greatly improved in performance. The method has been applied to the investigation of different homogeneous and heterogeneous clusters. This type of cluster experiment requires both a very high resolution and a large dynamic range in order to identify also clusters present in low abundance. As an example, a series of coincidence mass spectra of Xe clusters has been recorded at different wavelengths. Below a photon energy of 11.1 eV, the range of observable clusters shifts to higher cluster sizes with decreasing energy. Appearance potentials and the binding energy of different cluster ions were obtained. Intensity fluctuations, already observed in spectra with electron bombardment ionisation (magic numbers), have also been detected in the coincidence spectra and become most pronounced near the ionisation threshold. This indicates that these fluctuations are caused by the size-dependent stability of the ionic and not the neutral cluster. Furthermore, the threshold size does not change linearly with cluster size. The binding energy per particle seems to change drastically aroundn=13 which indicates the existence of a shell structure in the cluster ion.     

Density functional study of the interaction of carbon monoxide with small neutral and charged silver clusters

CO adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n = 1-7) has been studied with use of the PW91PW91 density functional theory (DFT) method. The adsorption of CO on-top site, among various possible sites, is energetically preferred irrespective of the charge state of the silver cluster. The cationic silver clusters generally have a greater tendency to adsorb CO than the anionic and neutral silver ones, except for n = 3 and 4, and the binding energies reach a local minimum at n = 5. The binding energies on the neutral clusters, instead, reach a local maximum at n = 3, which is about 0.87 eV, probably large enough to be captured in the experiments. Binding of CO to the silver clusters is generally weaker than that to the copper and gold counterparts at the same size and charge state. This is due to the weaker orbital interaction between silver and CO, which is caused by the larger atomic radius of the silver atom. In contrast, Au atoms with a larger nuclear charge but a similar atomic radius to silver owing to the lanthanide contraction are able to have a stronger interaction with CO.     

Electron solvation in water-ammonia mixed clusters: Structure, energetics, and the nature of localization states of the excess electron

The structure and energetics of water-ammonia mixed clusters with an excess electron, [(H2O)n(NH3)m]- with m=1, n=2-6 and m=2, n=2, and also the corresponding neutral clusters are investigated in detail by means of ab initio quantum chemical calculations. The authors focus on the localization structure of the excess electron with respect to its surface versus interiorlike states, its binding to ammonia versus water molecules, the spatial and orientational arrangement of solvent molecules around the excess electron, the changes of the overall hydrogen-bonded structure of the clusters as compared to those of the neutral ones and associated dipole moment changes, vertical detachment energies of the anionic clusters, and also the vertical attachment energies of the neutral clusters. It is found that the hydrogen-bonded structure of the anionic clusters are very different from those of the neutral clusters unlike the case of water-ammonia dimer anion, and these changes in structural arrangements lead to drastically different dipole moments of the anionic and the neutral clusters. The spatial distribution of the singly occupied molecular orbital holding the excess electron shows only surface states for the smaller clusters. However, for n=5 and 6, both surface and interiorlike binding states are found to exist for the excess electron. For the surface states, the excess electron can be bound to the dangling hydrogens of either an ammonia or a water molecule with different degrees of stability and vertical detachment energies. The interiorlike states, wherever they exist, are found to have a higher vertical detachment energy than any of the surface states of the same cluster. Also, for interiorlike states, the ammonia molecule with its dangling hydrogens is always found to stay on top or on a far side of the charge density of the excess electron without participating in the hydrogen bond network of the cluster; the intermolecular hydrogen bonds are formed by the water molecules only which add to the overall stability of these anionic clusters.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.