The engineering of complex architectures from functional molecules on surfaces provides new pathways to control matter at the nanoscale. In this article, we present a combined study addressing the self-assembly of the amino acid L-methionine on Ag(111). Scanning tunneling microscopy data reveal spontaneous ordering in extended molecular chains oriented along high-symmetry substrate directions. At intermediate coverages, regular biomolecular gratings evolve whose periodicity can be tuned at the nanometer scale by varying the methionine surface concentration. Their characteristics and stability were confirmed by helium atomic scattering. X-ray photoemission spectroscopy and high-resolution scanning tunneling microscopy data reveal that the L-methionine chaining is mediated by zwitterionic coupling, accounting for both lateral links and molecular dimerization. This methionine molecular recognition scheme is reminiscent of sheet structures in amino acid crystals and was corroborated by molecular mechanics calculations. Our findings suggest that zwitterionic assembly of amino acids represents a general construction motif to achieve biomolecular nanoarchitectures on surfaces.nanochemistry ͉ scanning tunneling microscopy ͉ supramolecular engineering ͉ surface chemistry ͉ x-ray photoemission spectroscopy T he controlled self-assembly of functional molecular species on well defined surfaces is a promising approach toward the design of nanoscale architectures (1). By using this methodology, regular low-dimensional systems such as supramolecular clusters, chains, or nanoporous arrays can be fabricated (2-6). A wide variety of molecular species as well as substrate materials proved to be useful (7), exploiting noncovalent directional interactions including dipole-dipole coupling (2, 3), hydrogen bridges (4,5,(8)(9)(10)(11), and metal-ligand interactions (6,(12)(13)(14)(15). With the exception of multiple H-bonded networks or coordination networks incorporating metal centers, it remains challenging to realize robust systems, and there is a need to develop protocols exploiting stronger intermolecular bonds to realize purely organic low-dimensional architectures. Small biological molecules such as amino acids or DNA base molecules represent an important class of building blocks that are of interest for molecular architectonic on surfaces because they inherently qualify for molecular recognition and self-assembly (16)(17)(18)(19)(20). The interaction between biomolecules and solid surfaces is decisive for the development of bioanalytical devices or biocompatible materials (21-23) as well as for a fundamental understanding of protein-surface bonding (24). Moreover, in three dimensions the amino acids provide assets to engineer distinct network structures based on zwitterionic coupling schemes (25-27), which may be categorized as subclass of ionic self-assembly (28), and thus are promising units to create robust nanoarchitectures. However, to date, the advantages of zwitterionic supramolecular synthons have not been exploited in ...

We present a combined multimethod experimental and theoretical study of the geometric and electronic properties of Co-tetraphenyl- porphyrin (Co-TPP) molecules adsorbed on a Ag(111) surface. Scanning tunneling microscopy (STM) topographs reveal that Co-TPP forms highly regular arrays with a square unit cell. Hereby, the Co-TPP molecules do not occupy a unique adsorption site on the Ag(111) atomic lattice. The central Co atom of the Co-TPP is found to reside predominantly above fcc and hcp hollow sites of the substrate, as determined from the photoelectron diffraction patterns. A strong adsorption-induced deformation of Co-TPP involving a saddle-shaped macrocycle is evidenced by high-resolution STM images and quantified by near-edge x-ray absorption fine-structure measurements. By scanning tunneling spectroscopy we resolved discrete molecular electronic states and mapped the pertaining spatial charge-density distribution. Specifically, we discuss the interaction of orbitals originating from the Co-metal center with the porphyrin macrocycle and show that the varying adsorption sites induce a modulation in the Co-TPP lowest unoccupied molecular orbital. These findings are corroborated by density-functional-theory calculations

We report on a high-resolution X-ray photoemission spectroscopy study on molecular-thick layers of L-cysteine deposited under ultrahigh vacuum conditions on Au(110). The analysis of core level shifts allowed us to distinguish unambiguously the states of the first-layer molecules from those of molecules belonging to the second layer. The first-layer molecules strongly interact with the metal through their sulfur headgroup. The multipeaked structure of the N 1s, O 1s, and C 1s core levels is interpreted in terms of different molecular moieties. The neutral acidic fraction (HSCH2CH(NH2)COOH) is abundant at low coverage likely associated with isolated molecules or dimers. The zwitterionic phase (HSCH2CH(NH3+)COO-) is largely dominant as the coverage approaches the monolayer limit and is related to the formation of ordered self-assembled molecular structures indicated by electron diffraction patterns. The occurrence of a small amount of cationic molecules (HSCH2CH(NH3+)COOH) is also discussed. The second-layer molecules mainly display zwitterionic character and are weakly adsorbed. Mild annealing up to 100 degrees C leads to the desorption of the second-layer molecules leaving electronic states of the first layer unaltered.