Surface Structures of CsSnI₃(001) Films on Au(111)

Although vacancies in perovskite CsSnI3 have been widely investigated to reveal the stumbling block effects on its optoelectronic properties, information concerning its surface structures and defects is very rare so far. Here, the surface structures of CsSnI3 films are studied by scanning tunneling microscopy, together with density functional theory calculations. Three coexisting phases, zigzag, double-chain, and tetramer structures, are found at the CsI terminated (001) surface of the CsSnI3 films grown on Au(111). The zigzag phase should be assigned to a perfect CsI-termination surface. The latter two phases show quasi-square structures belonging to 2×2 reconstructions, which are due to the orderly Cs vacancies on the surface. Our DFT calculations further indicate that the Cs vacancies on the surface of CsSnI3 perovskite not only play an important role in surface reconstruction but also have an additive effect on reducing the band gap

Tuning On-Surface Synthesis of Graphene Nanoribbons by Noncovalent Intermolecular Interactions

On-surface synthesis has been widely used for the precise fabrication of surface-supported covalently bonded nanostructures. Here, we report on tuning the on-surface synthesis of graphene nanoribbons by noncovalent intermolecular interactions on Au(111) surfaces. By introducing noncovalent intermolecular interactions with the companion molecules (dianhydride derivative), intramolecular cyclodehydrogenation of nonplanar precursor molecules (bianthryl derivative) are promoted at 200 °C, with the monomers interlinked by gold atoms instead of the formation of polyanthrylene. By adjusting the deposition sequence of precursor and companion molecules, conjugated graphene nanoribbons can be finally obtained at a temperature of 240 °C, much lower than the synthesis procedures without companion molecules. Density functional theory calculations indicate that intermolecular interactions result in a dramatic shrinkage of the torsional angle between the adjacent anthryl groups of the precursor molecule, aiding the cyclodehydrogenation process. Our work demonstrates an intermolecular strategy for controllable fabrication of covalently bonded nanostructures by on-surface synthesis

Surface-Mounted Molecular Rotors with Variable Functional Groups and Rotation Radii

A strategy for designing and activating surface-mounted molecular rotors with variable rotation radii and functional groups is proposed and demonstrated. The key point of the strategy is to separate the anchor and the rotating functional group from each other by using a connector of adjustable length. The three independent parts of the molecule are responsible for different functions to support the rotating movement of the molecule as a whole. In this way, one can easily change each part to obtain molecular rotors with different sizes, anchors, and functional rotating groups

Multilevel Supramolecular Architectures Self-Assembled on Metal Surfaces

We report the controllability of the complexity of surface-supported supramolecular assembly on metal surfaces. By introducing mismatch between the molecular packing and the surface atomic periodicity in the systems with comparable strength of intermolecular and molecule−substrate interactions, a homomolecular assembly exhibiting two-dimensional multilevel structures up to quaternary level was observed. In such a multiperiodicity modulated system, neither the intermolecular nor molecule−substrate interactions solely dominate the assembly, resulting in complicated multilevel structures. We further demonstrated that the multilevel assemblies can serve as templates for site-selective adsorption of guest molecules

Multilevel Supramolecular Architectures Self-Assembled on Metal Surfaces

We report the controllability of the complexity of surface-supported supramolecular assembly on metal surfaces. By introducing mismatch between the molecular packing and the surface atomic periodicity in the systems with comparable strength of intermolecular and molecule−substrate interactions, a homomolecular assembly exhibiting two-dimensional multilevel structures up to quaternary level was observed. In such a multiperiodicity modulated system, neither the intermolecular nor molecule−substrate interactions solely dominate the assembly, resulting in complicated multilevel structures. We further demonstrated that the multilevel assemblies can serve as templates for site-selective adsorption of guest molecules

Linear Alkane Polymerization on Au-Covered Ag(110) Surfaces

Efficiently utilizing alkanes as the precursors to realize various chemical reaction processes is challenging due to the inertness of alkane C–H bonds. We report here the selective C–H activation and C–C coupling reaction of linear alkanes on Au-covered Ag(110) surfaces. Based on density functional theory calculations, thin gold films with a thickness of several atomic layers deposited on Ag(110) resemble the bulk Au(110) surface for alkane C–H bond activation. By using scanning tunneling microscopy (STM) we have observed that, instead of linear C–C coupling on unalloyed Au(110), alkane molecules desorb from Ag(110) surfaces at elevated temperatures. The featured missing-row (1 × 2) reconstruction of Au(110) surfaces has been obtained by deposition of ∼20 ML (<3 nm) gold atoms on Ag(110). On such reconstructed surface, linear alkane polymerization through selective C–H activation and C–C coupling has been achieved at mild temperatures. Our work demonstrates the possibility to utilize thin gold films replacing bulk Au(110) substrates for selective alkyl C–H bond activation

On-Surface Reaction of 1,4-Dibromo-2,5-Diiodobenzene on Au(111) and Ag(100)

The on-surface reaction processes of 1,4-dibromo-2,5-diiodobenzene (C6H2Br2I2) on Au(111) and Ag(100) were systematically investigated under ultra-high vacuum conditions by using scanning tunneling microscopy. Deiodination underwent at RT on both surfaces with the formation of organometallic intermediates connected by C–Au–C and C–Ag–C linkages. In particular, partial deiodination on Au(111) resulted in self-assembled ordered structures of organometallic trans-trimers, which were converted into longer organometallic chains after complete deiodination at 100 °C. On Ag(100), complete deiodination gave rise to disordered molecular clusters at RT and organometallic intermediates were formed via the debromination process after annealing to 150 °C. Further annealing to higher temperatures resulted in covalent C–C bonded polyphenylene chains and finally disordered graphene nanostructures via cyclodehydrogenation. Our study provides fundamental comprehension of temperature-selective on-surface dehalogenation reactions of multi-halogen-substituted precursors for constructing designer covalently bonded networks

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl₂ Monolayer

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott–Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials

Effect of Metal Surfaces in On-Surface Glaser Coupling

The homocoupling of alkynes at metal surfaces, which was disclosed recently, is a promising reaction for efficient construction of conjugated nanostructures at metal surfaces. However, the role of the metal substrate as well as the mechanistic course of this process have not been investigated. The metal surface could act cooperatively (a) for two-dimensional confinement to properly orient the organic reactant and (b) also as an active mediator in the C–C bond-forming reaction. Herein we report covalent coupling of the dimers of 1,4-diethynylbenzene at various metal surfaces. The model reaction was investigated experimentally by STM and also by theoretical DFT calculations. Detailed statistical analysis and the theoretical results strongly support the involvement of the metal surface in the C–C bond-forming process. On the basis of these investigations, a model with two possible reaction pathways is suggested to describe the process: C–C coupling via direct CH activation and C–C coupling via alkynyl activation by π-complex formation