Local Electronic Properties of Corrugated Silicene Phases
Local Electronic Properties of Corrugated Silicene Phases
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DOI：
10.1002/adma.201202100
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发表时间：
2012-09-25
影响因子：
29.4
通讯作者：
Molle, Alessandro
中科院分区：
文献类型：
--
作者：
Chiappe, Daniele;Grazianetti, Carlo;Molle, Alessandro
探索原子级薄石墨材料新的物理化学性质一直是人们的兴趣所在，并有望对未来纳米电子器件的发展产生巨大影响。到目前为止，对石墨烯[1，2]的巨大考虑已经使其他二维（2D）对应物黯然失色，其可以克服石墨烯作为活性材料的固有限制。实际上，尽管石墨烯带隙开放[3-5]或在自组织器件结构中的集成方面的最新进展，[6，7]独立石墨烯的零带隙特性对于适合于基于石墨烯的逻辑器件的电场调制造成了严重的缺点。目前正在努力提供其他第IV族半导体的稳定的石墨烯状纳米晶格的理论和实验证据，所谓的硅烯和锗烯[8-11]，其可能受益于与Si技术的天然兼容性和固有的更高的自旋轨道耦合。与石墨烯不同，硅烯的自发形成预计不会在能量上有利，因为Si原子的sp 3杂化比sp 2杂化更稳定。然而，理论上已经预测了硅烯的石墨烯样形式[8，9，12]，并且在短短几年内，实验研究已经将这个迷人的假设提升为具体的证据。最近的实验[13，14]报道了硅烯在Ag（111）衬底上的外延生长，具有非平凡的表面排列和相关的电子特征，例如K点处的线性电子色散和估计的费米速度v F= 1.3× 10 6 ms − 1。[13]本文报道了用分子束外延（MBE）方法在Ag（111）衬底上生长Si（亚）单层的热力学相图。选择不反应的金属基底，如银，预期有利于超薄纯硅结构的生长;此外，作为（111）终止的Ag表面的特征的6重对称性，预期有利于蜂窝状Si ad层的形成。相对于先前的报告，[13，14]在此探索了更高的沉积通量方案，目的是研究增加缺陷部位（例如晶界）密度的可能性。从应用的角度来看，这方面可能是至关重要的精细定制的硅烯广告层的电子性能，如在石墨烯的情况下观察到的。[15-21]在这种情况下，石墨烯蜂窝晶格的对称性是决定石墨烯许多独特性质的关键因素，特别是其接近费米能级的电子结构。为了探索这些fundamental方面，开创性的原位表征的态密度（DOS）的Si层在这里解开的空间分辨隧道光谱（STS）的不同结构的硅烯相揭示屈曲依赖的局部效应。为了深入了解超薄硅层在Ag（111）表面上的形成，通过仔细调整最相关的参数，即控制Si吸附原子迁移率的衬底温度Ts和控制构成Si层的吸附原子密度的覆盖率θ，进行了几个生长实验。图1显示了在Ts = 250 ℃时覆盖度θ值（从左到右）增加的扫描隧道显微镜（STM）形貌的选择（图1 a，B，c）。结果表明，沉积θ = 0.45 ML的硅层（图1 a）涉及形成2D平坦域（黑色轮廓-表示为2D-fl），其不显示任何特征表面。
Exploring new physical-chemical properties of atomically thin graphitic materials is retaining a booming interest and it is expected to have a tremendous impact on the development of future nanoelectronic devices. So far, the enormous consideration for graphene [1, 2] has overshadowed other two dimensional (2D) counterparts which may overcome the intrinsic limitations of graphene as active material. Indeed, despite the recent advances either in graphene bandgap opening [3–5] or integration in ad hoc device structures,[6, 7] the zero-gap character of free standing graphene poses a severe drawback for an electric field modulation suitable to graphene based logic devices. A strong effort is currently made to provide theoretical and experimental evidence of stable graphene-like nanolattices of the other group-IV semiconductors, the so called silicene and germanene,[8–11] which might take benefit from being naturally compatible with the Si technology and from an intrinsically higher spin orbit coupling. Unlike graphene, the spontaneous formation of silicene is not expected to be energetically favorable because the sp 3 hybridization of Si atoms is more stable than the sp 2 one. However, a graphene-like form of silicene has been theoretically predicted [8, 9, 12] and, in just a few years, experimental investigations have moved up this fascinating hypothesis to concrete evidence. Recent experiments [13, 14] report on the epitaxial growth of silicene on Ag (111) substrates with a non-trivial surface arrangement and relevant electronic features such as a linear electronic dispersion at the K points and an estimated Fermi velocity v F= 1.3× 10 6ms− 1.[13] In this manuscript, molecular beam epitaxy (MBE) experiments are reported which shed a light into the thermodynamic phase diagram of Si (sub) monolayers grown on Ag (111). The choice of an unreactive metal substrate, such as silver, it is expected to favor the growth of ultra-thin pure silicon structures; furthermore, the 6-fold symmetry which is characteristic of the (111) terminated Ag surface, is expected to favor the formation of a honeycomb Si ad-layer. A higher deposition flux regime, with respect to previous reports,[13, 14] is here explored with the aim to investigate the possibility of increasing the density of defect sites (eg grain boundaries). From the applicative point of view, this aspect might be crucial for finely tailoring the electronic properties of the silicene ad-layer, as observed in the case of graphene.[15–21] In this context, the symmetry of the graphene honeycomb lattice is a key element for determining many of the graphene’s unique properties, in particular its electronic structure close to the Fermi level. In order to explore these foundamental aspects, a pioneering in situ characterization of the density of states (DOS) of the Si layers is here unraveled by means of spatially resolved tunneling spectroscopy (STS) for differently structured silicene phases therein revealing buckling dependent local effects. To gain insights into the formation of ultra-thin silicon layers on the Ag (111) surface, several growth experiments have been performed by carefully tuning the most relevant parameters, ie the substrate temperature T s which governs the mobility of the Si adatoms, and the coverage θ which dictates the density of adatoms constituting the Si layer. Figure 1 shows a selection of scanning tunneling microscopy (STM) topographies for increasing values of coverage θ (left-to-right) at T s≈ 250 C (Figure 1 a, b, c). It turns out that the deposition of a silicon layer with θ≈ 0.45 ML (Figure 1 a) involves the formation of 2D flat domains (black contour-denoted as 2D-fl) which do not exhibit any characteristic surface …