Supramolecular coordination self-assembly on solid surfaces provides an effective route to form two-dimensional (2D) metal-organic frameworks (MOFs). In such processes, surface-adsorbate interaction plays a key role in determining the MOFs\' structural and chemical properties. Here, we conduct a systematic study of Cu-HAT (HAT = 1,4,5,8,9,12-hexaazatriphenylene) 2D conjugated MOFs (c-MOFs) self-assembled on Cu(111), Au(111), Ag(111), and MoS2 substrates. Using scanning tunneling microscopy and density functional theory calculations, we found that the as-formed Cu3HAT2 c-MOFs on the four substrates exhibit distinctive structural features including lattice constant and molecular conformation. The structural variations can be attributed to the differentiated substrate effects on the 2D c-MOFs, including adsorption energy, lattice commensurability, and surface reactivity. Specifically, the framework grown on MoS2 is nearly identical to its free-standing counterpart. This suggests that the 2D van der Waals (vdW) materials are good candidate substrates for building intrinsic 2D MOFs, which hold promise for next-generation electronic devices.

The concept of a solid catalyst with an ionic liquid layer (SCILL) is a promising approach to improve the selectivity of noble metal catalysts in heterogeneous reactions. In order to understand the origins of this selectivity control, we investigated the growth and thermal stability of ultrathin 1-ethyl-3-methylimidazolium trifluormethanesulfonate [C2C1Im][OTf] films on Pt(111) by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) in time-resolved and temperature-programmed experiments. We combined these spectroscopy experiments with scanning tunneling microscopy (STM) to obtain detailed insights into the orientation and adsorption geometry of the ions in the first IL layer. Furthermore, we propose a mechanism for the thermal evolution of [C2C1Im][OTf] on Pt(111). We observe an intact IL layer on the surface at temperatures below 200 K. Adsorbed [C2C1Im][OTf] forms islands, which are evenly distributed over the surface. The [OTf]- anion adsorbs via the SO3 group, with the molecular axis perpendicular to the surface. Anions and cations are arranged next to each other, alternating on the Pt(111) surface. Upon heating to 250 K, we observe changes in geometry and structural distribution. Whereas at low temperature, the ions are arranged alternately for electrostatic reasons, this driving force is no longer decisive at 250 K. Here, a phase separation of two different species is discernible in STM. We propose that this effect is due to a surface reaction, which changes the charge of the adsorbates. We assume that the IL starts to decompose at around 250 K, and thus, pristine IL and decomposition products coexist on the surface. Also, IRAS and XPS show indication of IL decomposition. Further heating leads to increased IL decomposition. The reaction products associated with the anions are volatile and leave the surface. In contrast, the cation fragments remain on the surface up to temperatures above 420 K.

The functionalization of AuNPs with different biological elements was achieved to investigate their possibility in biomedical applications such as drug delivery, vaccine development, sensing, and imaging. Biofunctionalized AuNPs are pursued for applications such as drug delivery, vaccine development, sensing, and imaging. In this study, AuNPs with diameters of 20 nm were functionalized with lipoic acid, mannose, or the cRGD peptide. By using UV-vis spectroscopy, Fourier transform infrared spectroscopy, dynamic light scattering, transmission electron microscopy, and scanning tunneling microscopy techniques, we showed that AuNPs can be functionalized by these biomolecules in a reliable way to obtain conjugates to explore potential biomedical applications. In particular, we demonstrate that the STM technique can be employed to analyze biofunctionalized AuNPs, and the obtained information can be valuable in the design of biomedical applications.

In a solid catalyst with ionic liquid layer (SCILL), ionic liquid (IL) coatings are used to improve the selectivity of noble metal catalysts. To understand the origins of this selectivity control, we performed model studies by surface science methods in ultrahigh vacuum (UHV). We investigated the growth and thermal stability of ultrathin IL films by infrared reflection absorption spectroscopy (IRAS). We combined these experiments with scanning tunneling microscopy (STM) to obtain information on the orientation of the ions, the interactions with the surface, the intermolecular interactions, and the structure formation. Additionally, we performed DFT calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2 C1 Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2 C1 Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ∼450 K. [C2 C1 Im][OTf] preferentially adsorbs at the step edges and elbows of the herringbone reconstruction of Au(111). The anion adsorbs via the SO3 group with the molecular axis perpendicular to the surface. At low coverage, the [C2 C1 Im][OTf] crystallizes in a glass-like 2D phase with short-range order. At higher coverage, we observe a phase transition to a 6-membered ring structure with long-range order.

The Cu(111) surface is an important substrate for catalysis and the growth of 2D materials, but a comprehensive understanding of the preparation and formation of well-ordered and atomically clean Cu(111) surfaces is still lacking. In this work, the morphology and structure changes of the Cu(111) surface after treatment by ion bombardment and annealing with a temperature range of 300-720 °C are investigated systematically by using in situ low-temperature scanning tunneling microscopy. With the increase of annealing temperature, the surface morphology changes from corrugation to straight edge, the number of screw dislocations changes from none to numerous, and the surface atomic structure changes from disordered to ordered structures (with many reconstructions). In addition, the changing trend of step width and step height in different stages is different (first increased and then decreased). A perfect Cu(111) surface with a step height of one atom layer (0.21 nm) and a width of more than 150 nm was obtained. In addition, two interesting superstructures and a new surface phase with a large number of line defects were found. This work serves as a strong foundation for understanding the properties of Cu(111) surface, and it also provides important guidance for the effective pretreatment of Cu(111) substrates, which are widely used.

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

Environmental control of single-molecule junction evolution and conductance was demonstrated for expanded pyridinium molecules by scanning tunneling microscopy break junction method and interpreted by quantum transport calculations including solvent molecules explicitly. Fully extended and highly conducting molecular junctions prevail in water environment as opposed to short and less conducting junctions formed in non-solvating mesitylene. A theoretical approach correctly models single-molecule conductance values considering the experimental junction length. Most pronounced difference in the molecular junction formation and conductance was identified for a molecule with the highest stabilization energy on the gold substrate confirming the importance of molecule-electrode interactions. Presented concept of tuning conductance through molecule-electrode interactions in the solvent-driven junctions can be used in the development of new molecular electronic devices.

Using a simulation protocol that mimics ultrafast scanning tunneling microscopy (STM) experiments, we demonstrate how pump-probe ultrafast STM may be used to image electron migration in molecules. Two pulses are applied to a model system, and the time-integrated current through the tip is calculated versus the delay time and tip position to generate STM images. With suitable pump and probe parameters, the images can track charge migration with atomistic spatial and femtosecond temporal resolutions.

Surface-assisted Ullmann coupling was widely applied to construct various molecular nanostructures on surfaces due to its reliability and controllability. By using 4,4\'\'-dichloro-1,1\':3\',1\'\'-terphenyl (DCTP) as the precursor, covalently bonded zig-zag oligophenylene chains and hexagonal hyperbenzene rings, e. g., [18]-honeycombenes, were successfully fabricated on Ag(111) via dechlorinated Ullmann coupling reaction. Stepwise annealing was applied to investigate the reaction process in detail. Scanning tunneling microscopy and synchrotron X-ray photoemission spectroscopy were utilized to explore the thermal evolution of the DCTP molecules on Ag(111) under ultrahigh vacuum conditions, evidencing the existence of intact DCTP molecules, chemisorbed Cl atoms, covalently bonded DCTP dimers as well as organometallic C-Ag-C-containing intermediates. These results may help understand dechlorinated Ullmann coupling reaction of aryl chlorides on metal surfaces.

Small silver clusters Agn (primarily probably Ag4 clusters which aggregate to Agn (400