Department of Physics, University of South Florida , Tampa, Florida 33620, United States.
Institut für Festkörperphysik, Leibniz Universität Hannover , Appelstraße 2, 30167 Hannover, Germany.
ACS Nano. 2017 May 23;11(5):5130-5139. doi: 10.1021/acsnano.7b02172. Epub 2017 May 1.
Twin grain boundaries in MoSe are metallic and undergo a metal to insulator Peierls transition at low temperature. Growth of MoSe by molecular beam epitaxy results in the spontaneous formation of a high density of these twin grain boundaries, likely as a mechanism to incorporate Se deficiency in the film. Using scanning tunneling microscopy, we study the grain boundary network that is formed in homoepitaxially grown MoSe and for MoSe grown heteroepitaxially on MoS and HOPG substrates. No statistically relevant variation of the grain boundary concentration has been found for the different substrates, indicating that the grain boundary formation is substrate independent and depends mainly on the growth conditions. Twin grain boundaries exhibit three crystallographically identical orientations, and thus they form an intersecting network. Different intersection geometries are identified that imply varying defect configurations. These intersection points act as preferential nucleation sites for vapor-deposited metal atoms, which we demonstrate on the example of selective gold cluster formation at grain boundary intersections. Scanning tunneling spectroscopy shows a band gap narrowing of MoSe in the immediate vicinity of the metallic grain boundary, which may be associated with lattice strain induced at the grain boundary. Tunneling noise spectra taken over the grain boundaries indicate random telegraphic noise, suggestive of pinning/depinning behavior of conductive channels in the metallic grain boundaries or their intersection points. Finally, indications for incommensurate and commensurate Peierls-driven charge density wave formation were observed in microprobe transport measurements at 205 and 227 K, respectively.
MoSe 中的孪晶界是金属性的，并在低温下经历金属到绝缘的 Peierls 转变。分子束外延生长 MoSe 会导致这些孪晶界的高密度自发形成，这可能是在薄膜中掺入 Se 缺陷的一种机制。使用扫描隧道显微镜，我们研究了在同质外延生长的 MoSe 中和在 MoS 和 HOPG 衬底上异质外延生长的 MoSe 中形成的晶界网络。对于不同的衬底，晶界浓度没有发现具有统计学意义的变化，这表明晶界的形成与衬底无关，主要取决于生长条件。孪晶界表现出三种晶体学上相同的取向，因此它们形成了一个相交的网络。确定了不同的交点几何形状，这意味着存在不同的缺陷配置。这些交点作为蒸气沉积金属原子的优先成核位点，我们在金原子选择性在晶界交点处形成的例子中证明了这一点。扫描隧道光谱表明，在金属晶界的紧邻区域，MoSe 的带隙变窄，这可能与晶界处的晶格应变有关。在晶界上采集的隧道噪声谱表明存在随机电报噪声，这表明金属晶界或其交点处的导电通道的钉扎/去钉扎行为。最后，在微探针输运测量中分别在 205 和 227 K 观察到了不相容和相容 Peierls 驱动的电荷密度波形成的迹象。
