Influence of film thickness and surface orientation on melting behaviors of copper nanofilms

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ng Changa) and Fu-Rong Chang Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan (Received 4 August 2013; accepted 31 December 2013)

The effects of ﬁlm thickness and surface orientation on melting behaviors of copper nanoﬁlms were investigated by molecular dynamics simulations. A stepwise heating scheme was adopted to make sure that the nanoﬁlms reached thermal equilibrium before further temperature increase. Melting of the nanoﬁlms was monitored by examining the equilibrium potential energy, radial distribution function, and mean square displacement of the simulated nanoﬁlms. From the simulation, the melting was observed to occur at a speciﬁc temperature within 1 K error, unlike the progressive melting process reported in the literature. The melted temperature and the latent heat of fusion of the nanoﬁlms were found to increase with ﬁlm thickness and approach the bulk value. The nanoﬁlms with (111) surface have the highest melted temperature and the largest latent heat of fusion as compared to the ones with (001) and (011) surfaces, which could be explained by the lowest surface energy of (111) surface.

I. INTRODUCTION

Owing to great applications in novel electronic devices, such as thermal transducers1 and chip interconnects,2,3 copper (Cu) nanoﬁlms have attracted a lot of research interest in recent years. For example, Dingreville et al.4 developed a semianalytical method to compute elastic moduli of copper nanoﬁlms and to examine the effects of size and surface orientation on the elastic moduli. Gan and Chen5 studied the inﬂuence of size, temperature, and strain rate on thermomechanical properties (such as the strength and modulus) of copper nanoﬁlms. Chang and Ding6 examined the effects of grain size and ﬁlm thickness on elastic and plastic properties of copper nanoﬁlms. Recently, Jing et al.7 investigated deposition and annealing behaviors of Cu atoms onto a copper substrate. Besides mechanical properties, melting temperatures and the corresponding melting behaviors of copper nanoﬁlms are another critical issue for the development and design of electronic devices because they impose restrictions on the operation range of the devices. Concerning the melting temperatures, it has been reported that the melting temperatures of nanomaterials, such as nanoparticles, nanowires, and nanoﬁlms, are size dependent and signiﬁcantly deviate from their bulk values.8–10 Because only few research works on melting behaviors of copper nanoﬁlms are available in the literature, several related research works are illustrated for ease of a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.13 J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014

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discussion. Yang and Wu11 studied the melting point, the melting mechanism, and the corresponding melting behavior of the Nb(110) nanoﬁlm. They found that the melting point of the Nb(110) nanoﬁlm is about 2568 K, which is much lower than that of 2740 K for

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