Yildirim Handan, Kara Abdelkader, Rahman Talat S
Department of Physics, University of Central Florida, Orlando, FL 32816, USA.
J Phys Condens Matter. 2009 Feb 25;21(8):084220. doi: 10.1088/0953-8984/21/8/084220. Epub 2009 Jan 30.
We report results of a systematic study of structural, vibrational and thermodynamical properties of 34-atom bimetallic nanoparticles from the Ag(n)Cu(34-n) family using model interaction potentials as derived from the embedded atom method and invoking the harmonic approximation of lattice dynamics. Systematic trends in the bond length and dynamical properties can be explained largely from arguments based on local coordination and elemental environment. Thus an increase in the number of silver atoms in a given neighborhood introduces a monotonic increase in bond length, while an increase of the copper content does the reverse. Moreover, for the bond lengths of the lowest-coordinated (six and eight) copper atoms with their nearest neighbors (Cu atoms), we find that the nanoparticles divide into two groups with the average bond length either close to (∼2.58 Å) or smaller than (∼2.48 Å) that in bulk copper, accompanied by characteristic features in their vibrational density of states. For the entire set of nanoparticles, we find vibrational modes above the bulk bands of copper/silver. We trace a blue shift in the high-frequency end of the spectrum that occurs as the number of copper atoms increases in the nanoparticles, leading to shrinkage of the bond lengths from those in the bulk. The vibrational densities of states at the low-frequency end of the spectrum scale linearly with frequency as for single-element nanoparticles, with a more pronounced effect for these nanoalloys. The Debye temperature is found to be about one-third of that of the bulk for pure copper and silver nanoparticles, with a non-linear increase as copper atoms increase in the nanoalloy.
我们报告了一项系统性研究的结果，该研究使用从嵌入原子方法推导的模型相互作用势并调用晶格动力学的简谐近似，对Ag(n)Cu(34 - n)族的34原子双金属纳米颗粒的结构、振动和热力学性质进行了研究。键长和动力学性质的系统趋势在很大程度上可以从基于局部配位和元素环境的论据来解释。因此，在给定邻域内银原子数量的增加会导致键长单调增加，而铜含量的增加则产生相反的效果。此外，对于最低配位（六配位和八配位）的铜原子与其最近邻（铜原子）之间的键长，我们发现纳米颗粒分为两组，其平均键长要么接近（约2.58 Å）要么小于（约2.48 Å）体相铜中的键长，并伴随着其振动态密度的特征。对于整个纳米颗粒集合，我们发现了高于铜/银体带的振动模式。我们追踪到随着纳米颗粒中铜原子数量的增加，光谱高频端出现蓝移，导致键长从体相中的键长收缩。光谱低频端的振动态密度与频率呈线性关系，如同单元素纳米颗粒一样，对于这些纳米合金来说这种效应更为明显。发现纯铜和银纳米颗粒的德拜温度约为体相的三分之一，并且随着纳米合金中铜原子的增加呈非线性增加。
