The Role of Carbon Contamination in Suspended Gold Nanowires
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The Role of Carbon Contamination in Suspended Gold Nanowires 1
Sergio B. Legoas, 1Douglas S. Galvão, 2Varlei Rodrigues, and 2Daniel Ugarte 1 Instituto de Física, Universidade Estadual de Campinas, 13083-970 Campinas, SP, Brazil 2 Laboratório Nacional de Luz Síncrotron, 13084-971 Campinas, SP, Brazil ABSTRACT Metallic nanowires represent very interesting systems due to new phenomena such as quantum conductance and unexpected long interatomic distances attaining 0.3-0.5 nm. These large distances represent a challenge for physical interpretation. In this work we present experimental data from transmission electron microscopy and results from ab initio density functional calculations for suspended gold chains. We show that large distances as 0.5 nm can be easily explained by the presence of carbon atoms as contaminants, while distances ranging from 0.29 up to 0.36 nm might be explained as resulting of a mixture of clean stressed and contaminated linear chains.
INTRODUCTION Metallic nanosized systems such as suspended atomic chains and metallic contacts have been object of intense investigations in the last years in order to understand the mechanisms of formation and conductance properties [1-4]. Experimental techniques such as scanning tunneling microscopy [1,5,6], high-resolution transmission electron microscopy (HRTEM) [2,3,7-10], and mechanically controllable break junction [9,11] have been used to generate these structures. In a recent work, Ohnishi et al. [2] have revealed the existence of suspended linear atom chains (ATCs) in gold nanowires (NWs) presenting quantized conductance. Long interatomic distances (ranging 0.35 – 0.40 nm) were observed which contrast with the usual nearest-neighbor distance in gold bulk (0.28 nm), or in Au dimer (0.25 nm). Other experimental studies also reported anomalously long distances, frequently varying in the 0.29-0.36 nm range [2,3,9,12]. However extremely long distances (close to 0.5 nm [13]) were rarely observed. Most of the reported long distances in metal ATCs are based on gold. However, they have been also observed in silver and platinum nanowires [14-16], indicating that some common features might be responsible for these unexpected results. Many theoretical studies have been carried out using different techniques [17,18], such as molecular dynamics and density functional formalisms trying to explain these unusual distances but large interatomic distances in the range of 0.5 nm have not been obtained from these simulations. However, preliminary calculations [19] considering the possibility of the presence of structural contamination produced an excellent agreement with the experimental data. In this work we report both experimental and theoretical studies on the formation of suspended gold NWs, addressing the calculations to analyze the origin of the anomalously long distances experimentally observed. Based on experimental information, the idea of nanoatomic chains being contaminated by some impurities [10] during sample fabrication was explored. Due to experimen
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