Two-dimensional supramolecular honeycomb networks with cavities of an internal diameter of 2.95 nm were formed by the self-assembly of 4,4',4' '-benzene-1,3,5-triyl-tribenzoic acid (BTA) on a Ag(111) surface at room temperature. Annealing to higher temperatures resulted in two sequential phase transformations into closer-packed supramolecular arrangements. The phase transformations are associated with stepwise deprotonation of the carboxylic acid groups. The voids of the honeycomb network of BTA have a suitable size for the construction of hierarchical structures with guest molecules. Single molecules of the macrocyclic compound mt-33 were successfully confined inside 2D nanocavities of the honeycomb networks and released when the phase was transformed to the close-packed structure.

Hydrogen-bond formation between ionic adsorbates on an Ag(111) surface under ultrahigh vacuum was studied by scanning tunneling microscopy/spectroscopy (STM/STS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and molecular dynamics calculations. The adsorbate, 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA), self-assembles at low temperatures (250-300 K) into the known open honeycomb motif through neutral hydrogen bonds formed between carboxyl groups, whereas annealing at 420 K leads to a densely packed quartet structure consisting of flat-lying molecules with one deprotonated carboxyl group per molecule. The resulting charged carboxylate groups form intermolecular ionic hydrogen bonds with enhanced strength compared to the neutral hydrogen bonds; this represents an alternative supramolecular bonding motif in 2D supramolecular organization.

Controlling supramolecular self-assembly is a fundamental step towards molecular nanofabrication, which involves a formidable reverse engineering problem. It is known that a variety of structures are efficiently obtained by assembling appropriate organic molecules and transition metal atoms on well-defined substrates. Here we show that alkali atoms bring in new functionalities compared with transition metal atoms because of the interplay of local chemical bonding and long-range forces. using atomic-resolution microscopy and theoretical modelling, we investigate the assembly of alkali (Cs) and transition metals (mn) co-adsorbed with 7,7,8,8-tetracyanoquinodimethane (TCnQ) molecules, forming chiral superstructures on Ag(100). Whereas mn-TCnQ 4 domains are achiral, Cs-TCnQ 4 forms chiral islands. The specific behaviour is traced back to the different nature of the Cs-and mn-TCnQ bonding, opening a novel route for the chiral design of supramolecular architectures. moreover, alkali atoms provide a means to modify the adlayer electrostatic properties, which is important for the design of metal-organic interfaces.

Electron transport through metal-molecule contacts greatly affects the operation and performance of electronic devices based on organic semiconductors [1][2][3][4] and is at the heart of molecular electronics exploiting single-molecule junctions [5][6][7][8] . Much of our understanding of the charge injection and extraction processes in these systems relies on our knowledge of the potential barrier at the contact. Despite significant experimental and theoretical advances a clear rationale of the contact barrier at the single-molecule level is still missing. Here, we use scanning tunnelling microscopy to probe directly the nanocontact between a single molecule and a metal electrode in unprecedented detail. Our experiments show a significant variation on the submolecular scale. The local barrier modulation across an isolated 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid molecule bound to a copper(111) electrode exceeds 1 eV. The giant modulation reflects the interaction between specific molecular groups and the metal and illustrates the critical processes determining the interface potential. Guided by our results, we introduce a new scheme to locally manipulate the potential barrier of the molecular nanocontacts with atomic precision.The electronic structure at the interface between a bulk metal and an organic semiconductor thin film has been extensively studied [9][10][11] and is commonly described in the framework of a band alignment model at the interface. In the single-molecule case, however, this model faces its limits. Chemical bonding between an organic molecule and a metal surface can result in significant charge transfer and rearrangement, which depend critically on the local atomic geometry 5,12 . In this situation of a strongly hybridized electronic system, a good indicator of the physical and chemical processes determining the molecule-metal contact is the work function, which for metal substrates is defined as the energy difference between the vacuum level far above the surface and the Fermi level (see also Fig. 1a; ref. 13).The formation of induced dipoles at the interface owing to the bonding of molecules can substantially modify the work function, which thereby provides valuable information on the degree of charge reorganization at the interface [13][14][15] . Photoemission experiments, often used to determine the averaged, coverage-dependent work function of a surface 11,16,17 , have generated considerable progress in understanding the formation of the interface built-in potential. However, these experiments cannot provide any information at the molecular length scale. Lateral resolution can be achieved by photoemission of adsorbed Xe, scanning tunnelling or Kelvin probe measurements 13,[18][19][20] . revealed local modifications of the work function but could not probe a single-molecule contact with submolecular resolution. A quantity closely related to the work function is the local potential-energy barrier experienced by tunnelling electrons during scanning tunnelling microscopy (STM) measurement...

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