Structural transformation of Ge dimers on Ge（001） surfaces induced by bias voltage

Scanning tunnelling microscopy is utilized to investigate the local bias voltage tunnelling dependent transformation between （2×1） and c（4×2） structures on Ge（001） surfaces, which is reversibly observed at room temperature and a critical bias voltage of -0.80 V. Similar transformation is also found on an epitaxial Ce islands but at a slightly different critical bias voltage of -1.00V. It is found that the interaction between the topmost atoms on the STM tip and the atoms of the dimers, and the pinning effect induced by Sb atoms, the nacancies or the epitaxial clusters, can drive the structural transformation at the critical bias voltage.     

Scanning tunneling microscopy study of molecular growth structures of Gd@C82 on Cu（111）

The coverage and temperature-dependent nucleation behaviors of the Gd@C82 metallofullerenes on Cu(111) have been studied by low-temperature scanning tunneling microscopy (LT-STM) in detail. Upon molecular deposition at low temperature, Gd@C82 molecules preferentially decorate the steps and nucleate into single layer islands with increasing coverage. Further annealing treatment leads some of the Gd@C82 molecules to assemble into bright and dim patches, which are correlated to the adsorption induced substrate reconstruction. Upon sufficient thermal activation, Gd@C82 molecules sink into the Cu(111) surface one-copper-layer-deep, forming hexagonal close-packed molecular islands with intra-molecular details observed as striped patterns. By considering the commensurability between the Gd@C82 nearest-neighbor distance and the lattice of the underlying Cu(111), we clearly identified two kinds of in-plane molecular arrangements as (19(1/2)×19(1/2))R23.4°and (19(1/2)×19(1/2))R36.6°with respect to Cu(111). Within the assembled Gd@C82 molecular, island molecules with dim-bright contrast are spatially distributed, which may be modulated by the preexisted species on Cu(111).     

Realization of semiconducting Cu2Se by direct selenization of Cu(111)

Bulk group IB transition-metal chalcogenides have been widely explored due to their applications in thermoelectrics. However, a layered two-dimensional form of these materials has been rarely reported. Here, we realize semiconducting Cu₂Se by direct selenization of Cu(111). Scanning tunneling microcopy measurements combined with first-principles calculations allow us to determine the structural and electronic properties of the obtained structure. X-ray photoelectron spectroscopy data reveal chemical composition of the sample, which is Cu₂Se. The observed moiré pattern indicates a lattice mismatch between Cu₂Se and the underlying Cu(111)-$\sqrt{3}$×$\sqrt{3}$ surface. Differential conductivity obtained by scanning tunneling spectroscopy demonstrates that the synthesized Cu₂Se exhibits a band gap of 0.78 eV. Furthermore, the calculated density of states and band structure demonstrate that the isolated Cu₂Se is a semiconductor with an indirect band gap of ～ 0.8 eV, which agrees quite well with the experimental results. Our study provides a simple pathway varying toward the synthesis of novel layered 2D transition chalcogenides materials.     

Image potential states mediated STM imaging of cobalt phthalocyanine on NaCl/Cu(100)

The adsorption and electronic properties of isolated cobalt phthalocyanine(Co Pc) molecule on an ultrathin layer of NaCl have been investigated. High-resolution STM images give a detailed picture of the lowest unoccupied molecular orbital(LUMO) of an isolated CoPc. It is shown that the Na Cl ultrathin layer efficiently decouples the interaction of the molecules from the underneath metal substrate, which makes it an ideal substrate for studying the properties of single molecules. Moreover, strong dependence of the appearance of the molecules on the sample bias in the region of relatively high bias( 3.1 V) is ascribed to the image potential states(IPSs) of NaCl/Cu(100), which may provide us with a possible method to fabricate quantum storage devices.     

Selective formation of ultrathin PbSe on Ag(111)

Two-dimensional (2D) semiconductors, such as lead selenide (PbSe), locate at the key position of next-generation devices. However, the ultrathin PbSe is still rarely reported experimentally, particularly on metal substrates. Here, we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111). The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag₅Se₂ and Pb, as further evidenced by the theoretical calculation. It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films. The detailed analysis demonstrates the bilayer structure of synthesized PbSe, which could be deemed to achieve the 2D limit. The differential conductance spectrum reveals a metallic feature of the PbSe film, indicating a certain interaction between PbSe and Ag(111). Moreover, the moiré pattern originated from the lattice mismatch between PbSe and Ag(111) is observed, and this moiré system provides the opportunity for studying physics under periodical modulation and for device applications. Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.     

Controllable growth of low-dimensional nanostructures on well-defined surfaces

The controllable growth of nanostructures with desired geometric order and well-defined shapes has stimulated great interest due to its applicability in the development of microelectronic devices. Self-assembly is an efficient and versatile way to guide the atoms or molecules into low-dimensional nanostructures as a consequence of balancing complex interplay between adsorbate-adsorbate and adsorbate-substrate interfacial interactions. The tailoring of low-dimensional nanostruc- tures by control of inter-adsorbate and adsorbate-substrate interfacial interactions is reviewed. Such inherent interactions greatly influence not only the size and shape of the growing nanostructures, but also their chemical identity. The degree of interaction between adsorbates can be controlled via preparation procedures, opening up the study of the influence of this phenomenon with respect to reactivity and catalytic behavior. The formation of well-defined molecular layers can be controlled not only by repulsive molecule-molecule interaction but also by symmetry matching or charge transfer be- tween adsorbed molecules and the substrate. It has become obvious that inter-adsorbate and adsorbate-substrate interfacial interactions can be tuned to fabricate diverse surface nanostructures from semiconductor, metallic, and molecular materials.     

Signatures of strong interlayer coupling in γ-InSe revealed by local differential conductivity

Interlayer coupling in layered semiconductors can significantly affect their optoelectronic properties. However, understanding the mechanisms behind the interlayer coupling at the atomic level is not straightforward. Here, we study modulations of the electronic structure induced by the interlayer coupling in the γ-phase of indium selenide (γ-InSe) using scanning probe techniques. We observe a strong dependence of the energy gap on the sample thickness and a small effective mass along the stacking direction, which are attributed to strong interlayer coupling. In addition, the moiré patterns observed in γ-InSe display a small band-gap variation and nearly constant local differential conductivity along the patterns. This suggests that modulation of the electronic structure induced by the moiré potential is smeared out, indicating the presence of a significant interlayer coupling. Our theoretical calculations confirm that the interlayer coupling in γ-InSe is not only of the van der Waals origin, but also exhibits some degree of hybridization between the layers. Strong interlayer coupling might play an important role in the performance of γ-InSe-based devices.     

气相沉积技术在原子制造领域的发展与应用

随着未来信息器件朝着更小尺寸、更低功耗和更高性能方向的发展,构建器件的材料尺寸将进一步缩小.传统的"自上而下"技术在信息器件发展到纳米量级时遇到瓶颈,而气相沉积技术由于其能在原子尺度构筑纳米结构引起极大关注,被认为是最有潜力突破现有制造极限进而在原子尺度构造、搭建物质形态的"自下而上"方法.本文重点讨论适用于低维材料的原子尺度制造的分子束外延技术和原子层沉积/刻蚀技术.简要介绍相关技术中蕴含的科学原理及其在纳米信息器件加工和制造领域的应用,并探讨如何在原子尺度实现对低维功能材料厚度和微观形貌的精密控制.     

类石墨烯锗烯研究进展

近年来,伴随石墨烯研究的深入开展,考虑到兼容半导体工业,构筑类石墨烯锗烯并探究其奇特电学性质已成为凝聚态物理领域的研究前沿.本文首先简要介绍了锗烯这一全新二维体系的理论研究进展,包括锗烯的几何结构、电子结构及其调控以及它们之间的关系.理论研究表明,因最近邻原子间距大,锗烯比硅烯更难构筑,实验上构筑锗烯颇具挑战性.针对这一问题,介绍了实验上制备锗烯的一些进展,重点介绍了金属表面外延制备锗烯,并对本征锗烯的制备及其在未来纳电子学器件的潜在应用做出了展望.     

STM study of In nanostructures formation on Ge（001） surface at different coverages and temperatures

Different In/Ge（001） nanostructures have been obtained by annealing the samples at 320℃ with different coverages of In. Annealing a sample with a critical coverage of 2.1 monolayer of In, different In/Ge（001） nanostructures can be obtained at different temperatures. It is found that thermal annealing treatments first make In atoms form elongated Ge{103}-faceted In-clusters, which will grow wider and longer with increasing temperature, and finally cover the surface completely.