Formation of fe cluster superlattice in a metal-organic quantum-box network

Recent Progress of Imaging Chemical Bonds by Scanning Probe Microscopy: A Review

In the past decades, the invention of scanning probe microscopy (SPM) as the versatile surface-based characterization of organic molecules has triggered significant interest throughout multidisciplinary fields. In particular, the bond-resolved imaging acquired by SPM techniques has extended its fundamental function of not only unraveling the chemical structure but also allowing us to resolve the structure-property relationship. Here, we present a systematical review on the history of chemical bonds imaged by means of noncontact atomic force microscopy (nc-AFM) and bond-resolved scanning tunneling microscopy (BR-STM) techniques. We first summarize the advancement of real-space imaging of covalent bonds and the investigation of intermolecular noncovalent bonds. Beyond the bond imaging, we also highlight the applications of the bond-resolved SPM techniques such as on-surface synthesis, the determination of the reaction pathway, the identification of molecular configurations and unknown products, and the generation of artificial molecules created via tip manipulation. Lastly, we discuss the current status of SPM techniques and highlight several key technical challenges that must be solved in the coming years. In comparison to the existing reviews, this work invokes researchers from surface science, chemistry, condensed matter physics, and theoretical physics to uncover the bond-resolved SPM technique as an emerging tool in exploiting the molecule/surface system and their future applications.

Artificial kagome lattices of Shockley surface states patterned by halogen hydrogen-bonded organic frameworks

Artificial electronic kagome lattices may emerge from electronic potential landscapes using customized structures with exotic supersymmetries, benefiting from the confinement of Shockley surface-state electrons on coinage metals, which offers a flexible approach to realizing intriguing quantum phases of matter that are highly desired but scarce in available kagome materials. Here, we devise a general strategy to construct varieties of electronic kagome lattices by utilizing the on-surface synthesis of halogen hydrogen-bonded organic frameworks (XHOFs). As a proof of concept, we demonstrate three XHOFs on Ag(111) and Au(111) surfaces, which correspondingly deliver regular, breathing, and chiral breathing diatomic-kagome lattices with patterned potential landscapes, showing evident topological edge states at the interfaces. The combination of scanning tunnelling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding calculations, directly substantiates our method as a reliable and effective way to achieve electronic kagome lattices for engineering quantum states.

Two-Dimensional Artificial Ge Superlattice Confining in Electronic Kagome Lattice Potential Valleys

Constructing two-dimensional (2D) artificial superlattices based on single-atom and few-atom nanoclusters is of great interest for exploring exotic physics. Here we report the realization of two types of artificial germanium (Ge) superlattice self-confined by a 37 × 37 R25.3° superstructure of bismuth (Bi) induced electronic kagome lattice potential valleys. Scanning tunneling microscopy measurements demonstrate that Ge atoms prefer to be confined in the center of the Bi electronic kagome lattice, forming a single-atom superlattice at 120 K. In contrast, room temperature grown Ge atoms and clusters are confined in the sharing triangle corner and the center, respectively, of the kagome lattice potential valleys, forming an artificial honeycomb superlattice. First-principle calculations and Mulliken population analysis corroborate that our reported atomically thin Bi superstructure on Au(111) has a kagome surface potential valley with the center of the inner Bi hexagon and the space between the outer Bi hexagons being energetically favorable for trapping Ge atoms.

Chiral and Catalytic Effects of Site-Specific Molecular Adsorption

The changes of properties and preferential interactions based on subtle energetic differences are important characteristics of organic molecules, particularly for their functionalities in biological systems. Only slightly energetically favored interactions are important for the molecular adsorption and bonding to surfaces, which define their properties for further technological applications. Here, prochiral tetracenothiophene molecules are adsorbed on the Cu(111) surface. The chiral adsorption configurations are determined by Scanning Tunneling Microscopy studies and confirmed by first-principles calculations. Remarkably, the selection of the adsorption sites by chemically different moieties of the molecules is dictated by the arrangement of the atoms in the first and second surface layers. Furthermore, we have investigated the thermal effects on the direct desulfurization reaction that occurs under the catalytic activity of the Cu substrate. This reaction leads to a product that is covalently bound to the surface in chiral configurations.

Host guest chemistry and supramolecular doping in triphenylamine-based covalent frameworks on Au(111)

The post-synthetic modification of covalent organic frameworks (COFs) via host-guest chemistry is an important method to tailor their electronic properties for applications. Due to the limited structural control in the assembly of two-dimensional surface-supported COFs, supramolecular networks are traditionally used at present for host-guest experiments on surfaces, which lack structural and thermal stability, however. Here, we present a combined scanning tunneling microscopy and density functional theory study to understand the host-guest interaction in triphenylamine-based covalently-linked macrocycles and networks on Au(111). These triphenylamine-based structures feature carbonyl and hydrogen functionalized pores that create preferred adsorption sites for trimesic acid (TMA) and halogen atoms. The binding of the TMA through optimized hydrogen-bond interactions is corroborated by selective adsorption positions within the pores. Band structure calculations reveal that the strong intermolecular charge transfer through the TMA bonding reduces the band gap in the triphenylamine COFs, demonstrating the concept of supramolecular doping by host-guest interactions in surface-supported COFs. Halogen atoms selectively adsorb between two carbonyl groups at Au hollow sites. The mainly dispersive interaction of the halogens with the triphenylamine COF leads to a small downshift of the bands. Most of the halogens change their adsorption position selectively upon annealing near the desorption temperature. In conclusion, we demonstrate evidence for supramolecular doping via post-synthetic modification and to track chemical reactions in confined space.

Modulation on the Iron Centers by Selective Synthesis of Organic Ligands with Stereo-Specific Conformations

Advanced fabrication of surface metal-organic complexes with specific coordination configuration and metal centers will facilitate to exploit novel nanomaterials with attractive electronic/magnetic properties. The precise on-surface synthesis provides an appealing strategy for in situ construction of complex organic ligands from simple precursors autonomously. In this paper, distinct organic ligands with stereo-specific conformation are separately synthesized through the well-known dehalogenative coupling. More interestingly, the exo-bent ligands promote the mono-iron chelated complexes with the Fe center significantly decoupled from the surface and of high spin, while the endo-bent ligands lead to bi-iron chelated ones instead with ferromagnetic properties.

Metalated Graphyne-Based Networks as Two-Dimensional Materials: Crystallization, Topological Defects, Delocalized Electronic States, and Site-Specific Doping

Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report on the fabrication of large-area organometallic Ag-bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag-acetylide bonds' reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal-bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag-bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.

Artificial relativistic molecules

We fabricate artificial molecules composed of heavy atom lead on a van der Waals crystal. Pb atoms templated on a honeycomb charge-order superstructure of IrTe₂ form clusters ranging from dimers to heptamers including benzene-shaped ring hexamers. Tunneling spectroscopy and electronic structure calculations reveal the formation of unusual relativistic molecular orbitals within the clusters. The spin–orbit coupling is essential both in forming such Dirac electronic states and stabilizing the artificial molecules by reducing the adatom–substrate interaction. Lead atoms are found to be ideally suited for a maximized relativistic effect. This work initiates the use of novel two-dimensional orderings to guide the fabrication of artificial molecules of unprecedented properties.

Artificial molecules supported on templated surfaces attract enormous interest due to their tunable electronic properties. Here the authors use STM experiments and DFT calculations to show the formation of Pb artificial clusters on a IrTe₂ honeycomb template that are maximally stabilized by relativistic effects.

Electron Transmission through Coordinating Atoms Embedded in Metal-Organic Nanoporous Networks

On-surface metal-organic nanoporous networks generally refer to adatom coordinated molecular arrays, which are characterized by the presence of well-defined and regular nanopores. These periodic structures constructed using two types of components confine the surface electrons of the substrate within their nanocavities. However, the confining (or scattering) strength that individual building units exhibit is a priori unknown. Here, we study the modification of the substrate's surface electrons by the interaction with a Cu-coordinated TPyB metal-organic network formed on Cu(111) and disentangle the scattering potentials and confinement properties. By means of STM and angle-resolved photoemission spectroscopy we find almost unperturbed free-electron-like states stemming from the rather weak electron confinement that yields significant coupling between adjacent pores. Electron plane wave expansion simulations match the superlattice induced experimental electronic structure, which features replicating bands and energy renormalization effects. Notably, the electrostatic potential landscape obtained from our ab initio calculations suggests that the molecules are the dominant scattering entities while the coordination metal atoms sandwiched between them act as leaky channels. These metal atom transmission conduits facilitate and enhance the coupling among quantum dots, which are prone to be exploited to engineer the electronic structure of surface electron gases.

Surface state tunable energy and mass renormalization from homothetic quantum dot arrays

Quantum dot arrays in the form of molecular nanoporous networks are renowned for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the band bottom of the confined states can exhibit downward shifts accompanied by a lowering of the effective masses simultaneous to the appearance of tiny gaps at the Brillouin zone boundaries. We observed these effects by angle resolved photoemission for two self-assembled homothetic (scalable) Co-coordinated metal-organic networks. Complementary scanning tunneling spectroscopy measurements confirmed these findings. Electron plane wave expansion simulations and density functional theory calculations provide insight into the nature of this phenomenon, which we assign to metal-organic overlayer-substrate interactions in the form of adatom-substrate hybridization. To date, the absence of the experimental band structure resulting from single metal adatom coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer to be general for low interacting and well-defined adatom arrays.

Watching nanostructure growth: kinetically controlled diffusion and condensation of Xe in a surface metal organic network

Diffusion, nucleation and growth provide the fundamental access to control nanostructure growth. In this study, the temperature activated diffusion of Xe at and between different compartments of an on-surface metal organic coordination network on Cu(111) has been visualized in real space. Xe atoms adsorbed at lower energy sites become mobile with increased temperature and gradually populate energetically more favourable binding sites or remain in a delocalized 'fluid' form confined to diffusion along a topological subset of the on-surface network. These diffusion pathways can be studied individually under kinetic control via the chosen thermal energy kT of the sample and are determined by the network and sample architecture. The spatial distribution of Xe in its different modes of mobility and the time scales of the motion is revealed by Scanning Tunneling Microscopy (STM) at variable temperatures up to 40 K and subsequent cooling to 4 K. The system provides insight into the diffusion of a van der Waals gas on a complex structured surface and its nucleation and coarsening/growth into larger condensates at elevated temperature under thermodynamic conditions.

Engineering of entanglement and spin state transfer via quantum chains of atomic spins at large separations

Several recent experiments have shown that long-range exchange interactions can determine collective magnetic ground states of nanostructures in bulk and on surfaces. The ability to generate and control entanglement in a system with long-range interaction will be of great importance for future quantum technology. An important step forward to reach this goal is the creation of entangled states for spins of distant magnetic atoms. Herein, the generation of long-distance entanglement between remote spins at large separations in bulk and on surface is studied theoretically, based on a quantum spin Hamiltonian and time-dependent Schrödinger equation for experimentally realized conditions. We demonstrate that long-distance entanglement can be generated between remote spins by using an appropriate quantum spin chain (a quantum mediator), composed by sets of antiferromagnetically coupled spin dimers. Ground state properties and quantum spin dynamics of entangled atoms are studied. We demonstrate that one can increase or suppress entanglement by adding a single spin in the mediator. The obtained result is explained by monogamy property of entanglement distribution inside a quantum spin system. We present a novel approach for non-local sensing of remote magnetic adatoms via spin entanglement.

Two-dimensional delocalized states in organometallic bis-acetylide networks on Ag(111)

The electronic structure of surface-supported organometallic networks with Ag-bis-acetylide bonds that are intermediate products in the bottom-up synthesis of graphdiyne and graphdiyne-like networks were studied. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a frontier, unoccupied electronic state that is delocalized along the entire organometallic network and proves the covalent nature of the Ag-bis-acetylide bonds. Density-functional theory (DFT) calculations corroborate the spatial distribution of the observed delocalized state and attribute it to band mixing of carbon and silver atoms combined with n-doping of the metal surface. The metal-bis-acetylide bonds are typical metal-organic bonds with mixed character containing covalent and strong ionic contributions. Moreover, the organometallic networks exhibit a characteristic graphene-like band structure with linear band dispersion at each K point.

Adsorbate-Induced Modification of the Confining Barriers in a Quantum Box Array

Quantum devices depend on addressable elements, which can be modified separately and in their mutual interaction. Self-assembly at surfaces, for example, formation of a porous (metal-) organic network, provides an ideal way to manufacture arrays of identical quantum boxes, arising in this case from the confinement of the electronic (Shockley) surface state within the pores. We show that the electronic quantum box state as well as the interbox coupling can be modified locally to a varying extent by a selective choice of adsorbates, here C₆₀, interacting with the barrier. In view of the wealth of differently acting adsorbates, this approach allows for engineering quantum states in on-surface network architectures.

N-Heterocyclic carbenes on close-packed coinage metal surfaces: bis-carbene metal adatom bonding scheme of monolayer films on Au, Ag and Cu

By means of scanning tunnelling microscopy (STM), complementary density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) we investigate the binding and self-assembly of a saturated molecular layer of model N-heterocyclic carbene (NHC) on Cu(111), Ag(111) and Au(111) surfaces under ultra-high vacuum (UHV) conditions. XPS reveals that at room temperature, coverages up to a monolayer exist, with the molecules engaged in metal carbene bonds. On all three surfaces, we resolve similar arrangements, which can be interpreted only in terms of mononuclear M(NHC)2 (M = Cu, Ag, Au) complexes, reminiscent of the paired bonding of thiols to surface gold adatoms. Theoretical investigations for the case of Au unravel the charge distribution of a Au(111) surface covered by Au(NHC)2 and reveal that this is the energetically preferential adsorption configuration.

13-cis-Retinoic acid on coinage metals: hierarchical self-assembly and spin generation

Hierarchical self-assembly of 13-cis-retinoic acid on Au(111) and Ag(111) was investigated using low-temperature scanning tunnelling microscopy. On both surfaces, molecules form dimers by hydrogen bonds and the dimers arrange into ordered two-dimensional arrays through van der Waals forces. Three packing modes are observed on Au(111) and only one on Ag(111). We tentatively attribute the different patterns on the two surfaces to a stronger molecule-substrate interaction on Ag(111) and site-dependent molecular adsorption on different atomic lattices. In addition, 13-cis-ReA on Au(111) can be made to carry a localized spin.

Competition between Hydrogen Bonds and Coordination Bonds Steered by the Surface Molecular Coverage

In addition to the choices of metal atoms/molecular linkers and surfaces, several crucial parameters, including surface temperature, molecular stoichiometric ratio, electrical stimulation, concentration, and solvent effect for liquid/solid interfaces, have been demonstrated to play key roles in the formation of on-surface self-assembled supramolecular architectures. Moreover, self-assembled structural transformations frequently occur in response to a delicate control over those parameters, which, in most cases, involve either conversions from relatively weak interactions to stronger ones (e.g., hydrogen bonds to coordination bonds) or transformations between the comparable interactions (e.g., different coordination binding modes or hydrogen bonding configurations). However, intermolecular bond conversions from relatively strong coordination bonds to weak hydrogen bonds were rarely reported. Moreover, to our knowledge, a reversible conversion between hydrogen bonds and coordination bonds has not been demonstrated before. Herein, we have demonstrated a facile strategy for the regulation of stepwise intermolecular bond conversions from the metal-organic coordination bond (Cu-N) to the weak hydrogen bond (CH···N) by increasing the surface molecular coverage. From the DFT calculations we quantify that the loss in intermolecular interaction energy is compensated by the increased molecular adsorption energy at higher molecular coverage. Moreover, we achieved a reversible conversion from the weak hydrogen bond to the coordination bond by decreasing the surface molecular coverage.

Two-Orbital Kondo Screening in a Self-Assembled Metal-Organic Complex

Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl dicarbonitrile molecules exhibit strongly spatial anisotropic Kondo features with directionally dependent Kondo temperatures and line shapes, as evidenced by scanning tunneling spectroscopy. First-principles calculations find nearly full polarization for the half-filled Fe 3d_xz and 3d_yz orbitals, which therefore can give rise to Kondo screening with the experimentally observed directional dependence and distinct Kondo temperatures. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements confirm that the spin in both channels is effectively Kondo-screened. At ideal Fe coverage, these two-orbital Kondo impurities are arranged in a self-assembled honeycomb superlattice.

Epitaxy-Induced Assembly and Enantiomeric Switching of an On-Surface Formed Dinuclear Organocobalt Complex

We report on the surface-guided synthesis of a dinuclear organocobalt complex, its self-assembly into a complex nanoarchitecture, and a single-molecule level investigation of its switching behavior. Initially, an organic layer is prepared by depositing hexakis((trimethylsilyl)ethynyl)-benzene under ultrahigh-vacuum conditions onto Ag(111). After Co dosage at 200 K, low-temperature scanning tunneling microscopy (STM) reveals an epitaxy-mediated organization mechanism of molecules and on-surface formed organometallic complexes. The dinuclear complexes contain two bis(η²-alkynyl) π-tweezer motifs, each stabilizing a single Co atom and express two enantiomers due to a conformation twist. The chirality is transferred to the two-dimensional architecture, whereby its Co adatoms are located at the corners of a 3.4.6.4 rhombitrihexagonal tessellation due to the systematic arrangement and anchoring of the complexes. Extensive density functional theory simulations support our interpretation of an epitaxy-guided surface tessellation and its chiral character. Additionally, STM tip-assisted manipulation experiments on isolated dinuclear complexes reveal controlled and reversible switching between the enantiomeric states via inelastic electron processes. After activation by bias pulses, structurally modified complexes display a distinctive Kondo feature attributed to metastable Co configurations.

Magnetic Hysteresis in Er Trimers on Cu(111)

We report magnetic hysteresis in Er clusters on Cu(111) starting from the size of three atoms. Combining X-ray magnetic circular dichroism, scanning tunneling microscopy, and mean-field nucleation theory, we determine the size-dependent magnetic properties of the Er clusters. Er atoms and dimers are paramagnetic, and their easy magnetization axes are oriented in-plane. In contrast, trimers and bigger clusters exhibit magnetic hysteresis at 2.5 K with a relaxation time of 2 min at 0.1 T and out-of-plane easy axis. This appearance of magnetic stability for trimers coincides with their enhanced structural stability.

Two-Dimensional Ketone-Driven Metal-Organic Coordination on Cu(111)

Two-dimensional metal-organic nanostructures based on the binding of ketone groups and metal atoms were fabricated by depositing pyrene-4,5,9,10-tetraone (PTO) molecules on a Cu(111) surface. The strongly electronegative ketone moieties bind to either copper adatoms from the substrate or codeposited iron atoms. In the former case, scanning tunnelling microscopy images reveal the development of an extended metal-organic supramolecular structure. Each copper adatom coordinates to two ketone ligands of two neighbouring PTO molecules, forming chains that are linked together into large islands through secondary van der Waals interactions. Deposition of iron atoms leads to a transformation of this assembly resulting from the substitution of the metal centres. Density functional theory calculations reveal that the driving force for the metal substitution is primarily determined by the strength of the ketone-metal bond, which is higher for Fe than for Cu. This second class of nanostructures displays a structural dependence on the rate of iron deposition.

Confinement properties of 2D porous molecular networks on metal surfaces

Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article.

Intrinsic Two-Dimensional Organic Topological Insulators in Metal-Dicyanoanthracene Lattices

We predict theoretical existence of intrinsic two-dimensional organic topological insulator (OTI) states in Cu-dicyanoanthracene (DCA) lattice, a system that has also been grown experimentally on Cu substrate, based on first-principle density functional theory calculations. The pz-orbital Kagome bands having a Dirac point lying exactly at the Fermi level are found in the freestanding Cu-DCA lattice. The tight-binding model analysis, the calculated Chern numbers, and the semi-infinite Dirac edge states within the spin-orbit coupling gaps all confirm its intrinsic topological properties. The intrinsic TI states are found to originate from a proper number of electrons filling of the hybridized bands from Cu atomic and DCA molecular orbitals based on which similar lattices containing noble metal atoms (Au and Cu) and those molecules with two CN groups (DCA and cyanogens) are all predicted to be intrinsic OTIs.

Organics on oxidic metal surfaces: a first-principles DFT study of PMDA and ODA fragments on the pristine and mildly oxidized surfaces of Cu(111)

Using van der Waals corrected density-functional theory calculations, we study the fundamental physico-chemical properties of the molecular fragments of pyromellitic dianhydride oxydianiline (PMDA–ODA) on pristine and oxidized Cu(111) to investigate the effect of mild oxidation of the metal substrate on PMDA–ODA adsorption.

Two-Dimensional Superlattices of Bi Nanoclusters Formed on a Au(111) Surface Using Porous Supramolecular Templates

We used porous supramolecular structures as templates to make two-dimensional (2D) superlattices of Bi nanoclusters on a Au(111) surface. First, we applied on-surface self-assembly to prepare 2D porous supramolecular structures containing well-ordered nanopores. Then, we deposited Bi atoms on the surface. The Bi atoms were confined in the supramolecular pores and formed nanoclusters of a critical size that is defined by the pore size. These nanoclusters were arranged as a 2D superlattice dictated by the structure of the supramolecular templates. The nanocluster size and superlattice periodicity can be adjusted by appropriately designing the supramolecular structures. We further studied the formation mechanism of the nanoclusters. We found that Bi atoms could diffuse across the pore boundaries at room temperature and nucleated as clusters inside the pores. The clusters grew until they reached the critical size and became stable. We used kinetic Monte Carlo simulations to reproduce the experimental results and quantified the interpore diffusion barrier to be 0.65 eV.

Surface structure. Subatomic resolution force microscopy reveals internal structure and adsorption sites of small iron clusters

Clusters built from individual iron atoms adsorbed on surfaces (adatoms) were investigated by atomic force microscopy (AFM) with subatomic resolution. Single copper and iron adatoms appeared as toroidal structures and multiatom clusters as connected structures, showing each individual atom as a torus. For single adatoms, the toroidal shape of the AFM image depends on the bonding symmetry of the adatom to the underlying structure [twofold for copper on copper(110) and threefold for iron on copper(111)]. Density functional theory calculations support the experimental data. The findings correct our previous work, in which multiple minima in the AFM signal were interpreted as a reflection of the orientation of a single front atom, and suggest that dual and triple minima in the force signal are caused by dimer and trimer tips, respectively.

Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces

Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green's functions for finite geometries and electron plane wave expansion for periodic systems. The "particle in a box" picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

Visualization and thermodynamic encoding of single-molecule partition function projections

Ensemble averaging of molecular states is fundamental for the experimental determination of thermodynamic quantities. A special case occurs for single-molecule investigations under equilibrium conditions, for which free energy, entropy and enthalpy at finite temperatures are challenging to determine with ensemble averaging alone. Here we report a method to directly record time-averaged equilibrium probability distributions by confining an individual molecule to a nanoscopic pore of a two-dimensional metal-organic nanomesh, using temperature-controlled scanning tunnelling microscopy. We associate these distributions with partition function projections to assess real-space-projected thermodynamic quantities, aided by computational modelling. The presented molecular dynamics-based analysis is able to reproduce experimentally observed projected microstates with high accuracy. By an in silico customized energy landscape, we demonstrate that distinct probability distributions can be encrypted at different temperatures. Such modulation provides means to encode and decode information into position-temperature space.

Interplay of weak interactions in the atom-by-atom condensation of xenon within quantum boxes

Condensation processes are of key importance in nature and play a fundamental role in chemistry and physics. Owing to size effects at the nanoscale, it is conceptually desired to experimentally probe the dependence of condensate structure on the number of constituents one by one. Here we present an approach to study a condensation process atom-by-atom with the scanning tunnelling microscope, which provides a direct real-space access with atomic precision to the aggregates formed in atomically defined ‘quantum boxes’. Our analysis reveals the subtle interplay of competing directional and nondirectional interactions in the emergence of structure and provides unprecedented input for the structural comparison with quantum mechanical models. This approach focuses on—but is not limited to—the model case of xenon condensation and goes significantly beyond the well-established statistical size analysis of clusters in atomic or molecular beams by mass spectrometry.

Condensation in the regime of weakly interactions is of fundamental importance. Here, the authors study the condensation process one atom at a time, showing the forces driving the behaviour of xenon atoms as they condense into aggregate structures in nanoscale pores.

Controlled interaction of surface quantum-well electronic states

We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.

Cooperative assembly of magic number C60-Au complexes

We report the assembly of magic number (C60)m-(Au)n complexes on the Au(111) surface. These complexes have a unique structure consisting of a single atomic layer Au island wrapped by a self-selected number (seven, ten, or twelve) of C(60) molecules. The smallest structure consisting of 7 C60 molecules and 19 Au atoms, stable up to 400 K, has a preferred orientation on the surface. We propose a globalized metal-organic coordination mechanism for the stability of the (C(60))(m)-(Au)n complexes.