Nucleation of ordered Ni island arrays on Au(111) by surface-lattice dislocations

Narrow Au(111) terraces decorated by self-organized Co nanowires: a low-temperature STM/STS investigation

Deposition of Co atoms on Au(111) surfaces leads to the formation of self-organized bilayer Co nanowires at the step edges between adjacent narrow Au(111) terraces. Scanning tunneling microscopy and spectroscopy at low temperatures is used to probe the influence of the finite dimensions on the local density of states for both the Co wires and the narrow Au terraces. Confinement of Au surface state electrons to narrow Au terraces causes a shift of the Au surface state onset energy to higher energies. For the Co nanowires discrete energy levels are observed. Moreover, standing wave patterns occur at the surface of both the Au and the Co. The patterns increase in complexity with increasing energy. All Co nanowires formed at the edges of narrow Au terraces reveal a characteristic maximum in the local density of states at a significantly different energy when compared to the Co islands that are formed on large Au terraces. The experimental results can be interpreted in terms of a simple particle-in-a-box model.

Formation of thick, large-area nanoparticle superlattices in lithographically defined geometries

Superlattices of colloidal nanocrystals hold the promise of new nanomaterials with tunable properties. The positioning and size of these structures are often poorly controlled after self-assembly from the solution phase, making studies of their properties difficult. We report the fabrication of approximately 100 layer thick, three-dimensional superlattices on a substrate with controlled lateral placement. This novel fabrication technique generates long-range order over the micrometer scale and controlled placement by employing lithographic patterning and microfluidic flow.

Site-specific chemistry directed by a bifunctional nanostructured surface

Using the scanning tunneling microscope (STM), we have created a bifunctional nanostructured surface which consists of parallel stripes of gold atoms on the Au(111) substrate. Each stripe has two parallel step-edges separated by a few nanometers in distance. The two step-edges have very different binding properties to molecules, and they are able to separate C(60) molecules into two types of adsorbed structures, giving rise to a controlled formation of two-dimensional closely spaced multiple molecular nanostructures.

Atomic-scale geometry and electronic structure of catalytically important pd/au alloys

Pd/Au bimetallic alloys catalyze many important reactions ranging from the synthesis of vinyl acetate and hydrogen peroxide to the oxidation of carbon monoxide and trimerization of acetylene. It is known that the atomic-scale geometry of these alloys can dramatically affect both their reactivity and selectivity. However, there is a distinct lack of experimental characterization and quantification of ligand and ensemble effects in this system. Low-temperature, ultrahigh vacuum scanning tunneling microscopy is used to investigate the atomic-scale geometry of Pd/Au111 near-surface alloys and to spectroscopically probe their local electronic structure. The results reveal that the herringbone reconstruction of Au111 provides entry sites for the incorporation of Pd atoms in the Au surface and that the degree of mixing is dictated by the surface temperature. At catalytically relevant temperatures the distribution of low coverages of Pd in the alloy is random, except for a lack of nearest neighbor pairs in both the surface and subsurface sites. Scanning tunneling spectroscopy is used to examine the electronic structure of the individual Pd atoms in surface and subsurface sites. This work reveals that in both surface and subsurface locations, Pd atoms display a very similar electronic structure to the surrounding Au atoms. However, individual surface and subsurface Pd atoms are depleted of charge in a very narrow region at the band edge of the Au surface state. dI/dV images of the phenomena demonstrate the spatial extent of this electronic perturbation.

Manipulated nucleation of Fe nanostructures on Au(111) with combined growth methods

The manipulated nucleation of Fe on hexagonal close-packed (hcp) and face centered cubic (fcc) areas of an Au(111) herringbone surface was demonstrated with the combined growth methods of Xe buffer layer, seed effect and low temperature deposition. Besides the herringbone kinks, bi-layer stacking at fcc gaps was observed. This preference resulted in various nucleation patterns, such as zigzag stripes and doublet islands, by properly choosing the growth condition. The idea of nucleation control through combined growth methods can be applied in other nanopatterned templates for the fabrication of self-assembled nanostructures.

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.

Critical stability of gold nanofingers on a zero-gradient stepped surface

Numerous methods and techniques have been developed in recent years for the fabrication of two-dimensional and one-dimensional nanostructures that exhibit quantum effects. In order to understand the size-dependent stability of such structures, we have developed a method where the tip of the scanning tunnelling microscope (STM) is used to extract atoms from pre-fabricated gold nanofingers. The rate of atom removal as a function of the finger width at a constant temperature provides a direct evaluation of the size-dependent stability of the fingers. Our study reveals that the stability of the nanofinger has a strong dependence on the width of the finger.

Modification of the Growth of Iron on Ag(111) by Predeposited Organic Monolayers

We study the modification of the growth behaviour of iron on Ag(111) by pread sorbed monolayers of 2H-phthalocyanine (2HPc) and 2H-tetraphenylporphyrin (2HTPP) by scanning tunneling microscopy, upon Fe deposition in ultra-high vacuum at room temperature. The growth of iron on clean Ag(111) follows the scheme expected from reports of similar systems: For low coverages (up to ~0.3 ML) an almost exclusive agglomeration of iron clusters at step edges occurs, which continue to grow in a dendritic fashion at higher coverages. Upon deposition onto Ag(111) precovered with monolayers of 2HTPP and 2HPc, at coverages up to 0.037 ML no growth of Fe islands but complete metalation, i.e., incorporation of the Fe atoms in the macrocyle of the preadsorbed molecules is found. For higher amounts of deposited Fe (0.037 < θ Fe < 0.30 ML), the formation of Fe islands is observed, which occurs differently for 2HPC and 2HTPP. While on 2HTPP two-dimensional islands are formed presumably underneath a FeTPP monolayer, for 2HPc the Fe growth is dominated by the formation of small 3D clusters within a FePc monolayer. At θ Fe = 2.0 ML, both systems exhibit small three dimensional Fe islands. These results are discussed considering different aspects like the molecular conformation of FeTPP and FePc on Ag(111) and the diffusional properties of Fe on the clean and modified substrate.

Molecular recognition and supramolecular self-assembly of a genetically engineered gold binding peptide on Au{111}

The understanding of biomineralization and realization of biology-inspired materials technologies depends on understanding the nature of the chemical and physical interactions between proteins and biominerals or synthetically made inorganic materials. Recently, combinatorial genetic techniques permit the isolation of peptides recognizing specific inorganic materials that are used as molecular building blocks for novel applications. Little is known about the molecular structure of these peptides and the specific recognition mechanisms onto their counterpart inorganic surfaces. Here, we report high-resolution atomic force microscopy (AFM), molecular simulation (MS), and geometrical docking studies that detail the formation of an ordered supramolecular self-assembly of a genetically engineered gold binding peptide, 3rGBP(1) ([MHGKTQATSGTIQS](3)), correlating with the symmetry of the Au{111} surface lattice. Using simulated annealing molecular dynamics (SA/MD) studies based on nuclear magnetic resonance (NMR), we confirmed the intrinsic disorder of 3rGBP(1) and identified putative Au docking sites where surface-exposed side chains align with both the <110> and <211> Miller indices of the Au lattice. Our results provide fundamental insight for an atomistic understanding of peptide/solid interfaces and the intrinsic disorder that is inherent in some of these peptide sequences. Analogous to the well-established atomically controlled thin-film heterostructure formation on semiconductor substrates, the basis of today's microelectronics, the fundamental observations of peptide-solid interactions here may well form the basis of peptide-based hybrid molecular technologies of the future.

Preparation of TiO2 nanocrystallites by oxidation of Ti-Au111 surface alloy

Ti-Au surface alloy oxidation is used to form nanocrystals of TiO(2) on Au(111). In situ scanning tunneling microscopy (STM) studies show that the approach yields arrays of 8-11 nm wide crystals with relatively narrow size dispersion and uniform crystallography. STM imaging shows that their crystallographic form is rutile with a triangular or hexagonal geometry. Scanning tunneling spectroscopy indicates that the crystals have a well-developed band gap, comparable to that in bulk TiO(2).

Fabrication of a well-ordered nanohole array stable at room temperature

We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm (2) and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature.

Solvent effects on the adsorption and self-organization of Mn12 on Au(111)

A sulfur-containing single molecule magnet, [Mn12O12(O2CC6H4SCH3)16(H2O)4], was assembled from solution on a Au(111) surface affording both submonolayer and monolayer coverages. The adsorbate morphology and the degree of coverage were inspected by scanning tunneling microscopy (STM), while X-ray photoelectron spectroscopy (XPS) allowed the determination of the chemical nature of the adsorbate on a qualitative and quantitative basis. The properties of the adsorbates were found to be strongly dependent on the solvent used to dissolve the magnetic complex. In particular, systems prepared from tetrahydrofuran solutions gave arrays of isolated and partially ordered clusters on the gold substrate, while samples prepared from dichloromethane exhibited a homogeneous monolayer coverage of the whole Au(111) surface. These findings are relevant to the optimization of magnetic addressing of single molecule magnets on surfaces.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Self-organized nanotemplating on misfit dislocation networks investigated by scanning tunneling microscopy

Self-ordering growth of nanoarrays on strained metallic interfaces is an attractive option for preparing highly ordered nanotemplates. The great potential of this natural templating approach is that symmetry, feature sizes, and density are predicted to depend on the interfacial stress in these strained layers, which can be adjusted by changing the substrate-thin film composition, temperature, and adlayer coverage. This bottom-up approach of growing nanostructured two-dimensional ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films and surfaces requires a detailed understanding of the nucleation and film-adsorbate interaction processes. Here we show how high resolution, large scale, variable temperature scanning tunneling microscopy imaging can improve our understanding of these self-assembly processes.

Deterministic positioning of three-dimensional structures on a substrate by film growth

A process to fabricate three-dimensional crystalline structures at controlled locations on a substrate during film growth and annealing is demonstrated. Low-energy electron microscopy reveals that silver is transported to regions on a tungsten surface with closely spaced atomic steps. By controlling the substrate topography using a focused ion beam to machine small holes, this general mechanism produces an array of cylinders as a silver film dewets the substrate.

Synthesis of TiO2 nanoparticles on the Au(111) surface

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.

Self-organized growth of nanopucks on Pb quantum islands

Electronic Moirè patterns found on lead (Pb) quantum islands can serve as a template to grow self-organized cluster (nanopucks) arrays of various materials. These patterns can be divided into fcc- and hcp-stacked areas, which exhibit different binding strengths to the deposited adatoms. For Ag adatoms, the binding energy can differ substantially and the confined nucleation thus occurs in the fcc sites. Both the size distribution and spatial arrangement of the Ag nanopucks are analyzed and found to be commensurate with the characteristics of the template island, which exhibits a bilayer oscillatory behavior.

Self-assembled periodical polycrystalline-ZnO/a-C nanolayers on Zn nanowire

Zn nanowires with an epitaxial thin surface layer of zinc oxide were dispersed onto amorphous carbon films and stored at room temperature. After 1500 h, a self-organized equal-spaced zinc oxide (approximately 2 nm)/carbon (approximately 2.5 nm) multilayer structure was found to form outside the Zn nanowire, taking the place of the original ZnO surface layer. We carried a systematic study to clarify the self-formation mechanism of the periodical multilayers outside the Zn nanowire and found out that such a configuration originated from a chemical reaction between Zn and CO2 and were formed via a gas phase diffusion-interfacial chemical reaction-phase separation process.

Self-assembly via adsorbate-driven dislocation reactions

Deposition of S onto a monolayer of Ag/Ru(0001) transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [Nature (London) 397, 238 (1999)]]. Time-resolved scanning tunneling microscopy reveals that this nanometer-scale restructuring occurs by a cooperative mechanism involving the sequential formation of triangular regions with fcc and hcp stacking. Using a 2D Frenkel-Kontorova model, we can simulate the creation of these triangular building blocks via basic dislocation motions and reactions.

Electrodeposition of Au-Cd alloy nanostructures on Au(111)

This report concerns an in-situ scanning tunneling microscopy study of the initial stages in the formation of a Au-Cd alloy on the Au(111) herringbone reconstruction. Although Au-Cd nanoclusters of alloy have been observed in sulfate electrolyte by this group, alloy "nanowires" were observed to form preferentially in the hcp regions between the sets of "soliton" walls of the reconstruction only in the presence of chloride. The nanowires were formed at -0.55 V versus 3 M Ag/AgCl, corresponding to Cd underpotential deposition (upd). Upd is electrodeposition at a potential prior to that needed to deposit the bulk element.

Linking surface stress to surface structure: measurement of atomic strain in a surface alloy using scanning tunneling microscopy

Annealed submonolayer CoAg/Ru(0001) films form an alloy with a structure that contains droplets of Ag surrounded by Co [G. E. Thayer, V. Ozolins, A. K. Schmid, N. C. Bartelt, M. Asta, J. J. Hoyt, S. Chiang, and R. Q. Hwang, Phys. Rev. Lett. 86, 660 (2001)]. To understand how surface stress contributes to the formation of this structure, we use scanning tunneling microscopy to extract atomic displacements at the boundaries between regions of Co and Ag. Comparing our measurements to Frenkel-Kontorova model calculations, we show how stress due to lattice mismatch contributes to the formation of the alloy droplet structure. In particular, we quantitatively evaluate how competing strain and chemical energy contributions determine surface structure.

Selective assembly on a surface of supramolecular aggregates with controlled size and shape

The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining molecular building blocks into desired functional structures, ideally with these structures supported on suitable substrates 1-4. Supramolecular aggregation occurs spontaneously and can lead to controlled structures if selective and directional non-covalent interactions are exploited. But such selective supramolecular assembly has yielded almost exclusively crystals or dissolved structures 5; the self-assembly of absorbed molecules into larger structures 6-8, in contrast, has not yet been directed by controlling selective intermolecular interactions. Here we report the formation of surface-supported supramolecular structures whose size and aggregation pattern are rationally controlled by tuning the non-covalent interactions between individual absorbed molecules. Using low-temperature scanning tunnelling microscopy, we show that substituted porphyrin molecules adsorbed on a gold surface form monomers, trimers, tetramers or extended wire-like structures. We find that each structure corresponds in a predictable fashion to the geometric and chemical nature of the porphyrin substituents that mediate the interactions between individual adsorbed molecules. Our findings suggest that careful placement of functional groups that are able to participate in directed non-covalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.

Direct observation of misfit dislocation glide on surfaces

Using scanning tunneling microscopy we have observed thermally induced dislocation glide in monolayer Cu films on Ru(0001) at room temperature. The motion is governed by a Peierls barrier that depends on the detailed structure of the dislocations, in particular upon whether the threading dislocations that terminate them are dissociated or not. Calculations based on the Frenkel-Kontorova model reproduce the threading dislocation structure and provide estimates of the Peierls barrier and dislocation stiffness which are consistent with experiment.

Atomic-scale structure of single-layer MoS2 nanoclusters

We have studied using scanning tunneling microscopy (STM) the atomic-scale realm of molybdenum disulfide ( MoS2) nanoclusters, which are of interest as a model system in hydrodesulfurization catalysis. The STM gives the first real space images of the shape and edge structure of single-layer MoS2 nanoparticles synthesized on Au(111), and establishes a new picture of the active edge sites of the nanoclusters. The results demonstrate a way to get detailed atomic-scale information on catalysts in general.

Self-organized patterning of an insulator-on-metal system by surface faceting and selective growth: NaCl/Cu(211)

We report experimental results on an insulator-on-metal system which is inherently unstable against lateral pattern formation on the nanometer scale. NaCl deposition on Cu(211) at substrate temperatures >300 K leads to faceting into (311) and (111) facets and selective NaCl growth on (311) facets only, thereby creating alternating stripes of bare Cu and NaCl-covered areas. The mesoscopic restructuring process is brought about by (1) the tendency to form (100)-terminated NaCl layers, (2) epitaxial matching between NaCl(100) and Cu(311), and (3) sufficient mobility of the Cu substrate surface.