Luminescence from 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) surface excited by tunneling electrons in scanning tunneling microscopy

The electronic excitations induced with tunneling electrons into adlayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111) have been investigated by in situ fluorescence spectroscopy in scanning tunneling microscopy (STM). A minute area of the surface is excited by an electron tunneling process in STM. Fluorescence spectra strongly depend on the coverage of PTCDA on Ag(111). The adsorption of the first PTCDA layer quenches the intrinsic surface plasmon originated from the clean Ag(111). When the second layer is formed, fluorescence spectra are dominated by the signals from PTCDA, which are interpreted as the radiative decay from the manifold of first singlet excited state (S(1)) of adsorbed PTCDA. The fluorescence of PTCDA is independent of the bias polarity. In addition, the fluorescence excitation spectrum agrees with that by optical excitation. Both results indicate that S(1) is directly excited by the inelastic impact scattering of electrons tunneling within the PTCDA adlayer.     

Dissociation mechanism of 2-propanol on a Si(111)-(7x7) surface studied by scanning tunneling microscopy

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7x7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si-OCH(CH3)2 and Si-H on an adatom and rest atom pair. Si-OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si-OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si-OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si-OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si-OCH(CH3)2 in the half unit cell.     

4-Mercaptopyridine on Au(111): a scanning tunneling microscopy and spectroscopy study

The adsorption of 4-mercaptopyridine (4MPy) molecules on reconstructed Au(111) is investigated by Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) at low temperature and under ultra-high vacuum (UHV) conditions. As made visible by STM, at low coverage (<10%) 4MPy adsorbs preferentially at elbow sites of the Herringbone reconstruction and at step edges of the Au(111). Increasing coverage (but still <30%) results in formation of molecular chains followed, at even higher coverage, by a 3-dimensional growth. Detailed analysis of z-V spectroscopy (ramping the tunneling bias V while keeping the tunneling current constant) provides information on the bias dependent apparent height of a single 4MPy/Au(111) as well as on the local density of states (LDOS) of single and chain 4MPy molecules in comparison to the bare Au(111) surface revealing a significant shift of the lowest unoccupied molecular orbital (LUMO) towards lower energy for molecules within chains. Additionally, the data provide no evidence that for these samples prepared in UHV the adsorption of 4MPy on Au(111) requires mediating Au adatoms. Also, clear indications are given that the adsorption does not induce a strong reduction of the Au DOS close to its Fermi energy. Finally, in context of the apparent STM height of 4MPy molecules, the behavior of the differential barrier height Φ(diff)(V) = (?(z)?(V)I/?(V)I)(2) on bare Au(111) and 4MPy/Au(111) is analyzed and the corresponding experimental values are applied to recover the LDOS of the molecule for unoccupied states according to a previously published numerical recipe [B. Koslowski, H. Pfeifer and P. Ziemann, Phys. Rev. B, 2009, 80, 165419 and M. Ziegler, N. Néel, A. Sperl, J. Kr?ger, and R. Berndt, Phys. Rev. B, 2009, 80, 125402]. In this way, one obtains a spectrum comprising a constant DOS of the Shockley-like surface state of Au(111) and a Lorentzian line attributed to the LUMO of 4MPy.     

Self-assembly of manganese phthalocyanine on Pb(111) surface: a scanning tunneling microscopy study

The self-assembled structure of submonolayer manganese phthalocyanine (MnPc) on Pb(111) surface is investigated by using low-temperature scanning tunneling microscopy (STM). A "holelike" superlattice, which is superimposed on the self-assembled nearly quadratic network, is observed. High resolution STM images reveal that there are two distinct azimuthal orientations of MnPc molecules. It is found that by taking the two different orientations the self-assembly can further be optimized energetically by maximizing intermolecular orbital overlapping. It is this intralayer energy minimization process that leads to the characteristic holelike superlattice.     

Donor/acceptor complex of triphenylene and trinitrotoluene on Au(111): a scanning tunneling microscopy study

The co-adsorption of trinitrotoluene (TNT), a typical π-electron acceptor, and triphenylene (TP), a typical π-electron donor, on a Au(111) surface was investigated by in situ Electrochemical Scanning Tunneling Microscopy (ECSTM). DFT calculations proved that parallelly stacked and well-overlapped TP and TNT molecules can form Donor-Acceptor dyads through intermolecular π-π charge transfer, which agree well with the experimental results in the present work.     

Cadmium underpotential deposition on Cu(111) in situ scanning tunneling microscopy

Atomically resolved in situ STM images are presented for an underpotentially deposited (upd) cadmium layer on a Cu(111) electrode from a 10(-4) M CdCl2/10(-2) M HCl solution. The observed moiré-like structure seen in the images is analysed by means of an algebraic model for this long-range superstructure. A structure model for the upd layer is developed which reflects all features of the observed moiré pattern. Furthermore the height modulation was simulated by a hard-sphere model for the Cd overlayer and shows remarkable agreement with the detailed tunneling current density distribution of the measured STM images. The existence of translational and rotational domains is demonstrated. The results are also compared and shown to be fully consistent with previous (ex situ) low-energy electron diffraction (LEED) observations of this system. The mechanism of Cd upd involves a dynamic site exchange between preadsorbed Cl- anions and adsorbing Cd2+ cations as previously concluded from ex situ X-ray photoelectron (XPS) and low-energy ion scattering (LEIS) measurements.     

Understanding the contrast mechanism in scanning tunneling microscopy (STM) images of an intermixed tetraphenylporphyrin layer on Ag(111)

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.     

Scanning tunneling microscopy investigation of the structure of methanethiolate on Ag(111)

Scanning tunneling microscopy (STM) has been used to investigate the structure of the ordered methanethiolate overlayer formed on Ag(111) by reaction at room temperature with dimethyl disulfide. High-resolution images show an ordered structure with three inequivalent atomic-scale protrusions within each ( radical7 x radical7)R19 degrees surface unit mesh which can be reconciled with methanethiolate species on a regular lateral submesh, similar to that proposed in the multilayer ( radical7 x radical7)R19 degrees -S sulfide phase previously reported. STM imaging during dynamic dosing also provides evidence for a significant change in the outermost layer Ag atom density, consistent with a reconstructed surface model. Possible models for this reconstruction are presented and discussed in the light of available information.     

Rectangular (3 x 2 square root of 3) superlattice of a dodecanethiol self-assembled monolayer on Au(111) observed by ultra-high-vacuum scanning tunneling microscopy

A rectangular (3 x 2 radical3) surface lattice for long-term-annealed dodecanethiol self-assembled monolayers on Au(111) is observed by ultra-high-vacuum scanning tunneling microscopy. The new lattice has the same density and a unit cell of the same size as the well-known c(4 x 2) reconstruction. In contrast, it does not show hexagonal symmetry but rather a sort of thiol pairing, leading to a shift in the binding position of every second molecule. The described structure is believed to be an intermediate phase close to desorption.     

Growth of nanocrystalline MoO3 on Au(111) studied by in situ scanning tunneling microscopy

The growth of nanocrystalline MoO3 islands on Au(111) using physical vapor deposition of Mo has been studied by scanning tunneling microscopy and low energy electron diffraction. The growth conditions affect the shape and distribution of the MoO3 nanostructures, providing a means of preparing materials with different percentages of edge sites that may have different chemical and physical properties than atoms in the interior of the nanostructures. MoO3 islands were prepared by physical vapor deposition of Mo and subsequent oxidation by NO2 exposure at temperatures between 450 K and 600 K. They exhibit a crystalline structure with a c(4 x 2) periodicity relative to unreconstructed Au(111). While the atomic-scale structure is identical to that of MoO3 islands prepared by chemical vapor deposition, we demonstrate that the distribution of MoO3 islands on the Au(111) surface reflects the distribution of Mo clusters prior to oxidation although the growth of MoO3 involves long-range mass transport via volatile MoO3 precursor species. The island morphology is kinetically controlled at 450 K, whereas an equilibrium shape is approached at higher preparation temperatures or after prolonged annealing at the elevated temperature. Mo deposition at or above 525 K leads to the formation of a Mo-Au surface alloy as indicated by the observation of embedded MoO3 islands after oxidation by NO2. Au vacancy islands, formed when Mo and Au dealloy to produce vacancies, are observed for these growth conditions.     

Self-assembly of L-tryptophan on Cu(111) studied by low-temperature scanning tunneling microscopy

The self-assembly of L-tryptophan on Cu(111) is investigated by an ultrahigh vacuum scanning tunneling microscope (STM) at 4.4 K. A series of novel supramolecular structures have been prepared with different annealing temperatures.     

Adlayers of Sb irreversibly adsorbed on Pt(111): an electrochemical scanning tunneling microscopy study

This work presents an electrochemical scanning tunneling microscopy study of Sb irreversibly adsorbed on Pt(111) at various potentials. At an open circuit potential (0.46 V vs a Ag/AgCl electrode), well-ordered structures of SbO+ were found: four (4 x 3)-3SbO+ structures and one (2 square root(3) x 2 square root(3))R30 degrees-3SbO+ structure. In addition, several unidentifiable transient structures of SbO+ were observed, and their relations to the well-ordered structures of (4 x 3) and (2 square root(3) x 2 square root(3))R30 degrees, regarding structural evolution, were proposed. At a reducing potential (0 V), the Pt(111) surface was covered with irreversibly adsorbed Sb which consisted of three different domains: protruded domain, domain of uniaxially incommensurate (square root(3) x square root(2))-Sb, and domain of bare (1 x 1) Pt(111). During oxidation of elemental Sb at 0.30 V, the Sb domains of the (square root(3) x square root(2)) structure were oxidized, while the protruded domains were not oxidized. After underpotential deposition of additional Sb onto the Pt(111) covered with irreversibly adsorbed Sb, the whole surface was filled with the Sb domains where each Sb atoms were separated by the square root(2a) distance (a = one Pt-Pt distance, 0.277 nm). The observed electrochemical inactivity below 0.3 V was discussed in terms of the protruded domain of a presumable incommensurate (square root(2) x square root(2)) structure.     

Self-assembly of l-tryptophan on Cu(111) studied by low-temperature scanning tunneling microscopy

The self-assembly of l-tryptophan on Cu(111) is investigated by an ultrahigh vacuum scanning tunneling microscope (STM) at 4.4 K. When deposited onto the substrate at around 120 K with a coverage of 0.1 monolayer, molecular trimers, tetramers, hexamers, and chains coexist on Cu(111). Then almost all molecules self-assemble into chiral hexamers after being annealed at room temperature. When increasing molecular coverage to the full layer, a new type of chain is observed on the surface. Based on the high-resolution STM images at sub-molecular level, we suggest that the l-tryptophan molecules are present in neutral, zwitterionic or anionic states in these structures.     

In-situ scanning tunneling microscopy of carbon monoxide adsorbed on Au(111) electrode

In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine the adsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecular resolution STM revealed the formation of several commensurate CO adlattices, but the (9 x radical 3) structure eventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHE in CO-free 0.1 M HClO(4), as indicated by a broad and irreversible anodic peak which appeared at this potential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated for CO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging allowed direct visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (radical 3 x 22) reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 x radical 3) structure observed in CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determined from coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with no change in the (9 x radical 3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugation pattern with two nearest pairs separated by 74 +/- 1 A, corresponding to a 14% increase from the ideal value of 65.6 A known for the ( radical 3 x 22) reconstruction. Molecular resolution STM further revealed that protrusions resulting from CO admolecules in the (9 x radical 3) structure exhibited distinctly different corrugation heights, suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominated in the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 x radical 7) and ( radical 43 x 2 radical 13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min. The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed from the Au(111) surface at more positive potentials.     

Self-assembled gold nanoparticles on functionalized gold (111) studied by scanning tunneling microscopy

Nanogold colloidal solutions are prepared by the reduction of HAuClO4 with sodium citrate and sodium borohydride.4-Aminothiophenol (ATP) self-assembled monolayers (SAMs) are formed on gold(111) surface,on which gold nanopartides are immobilized and a sub-monolayer of the particles appears.This sub-monolayer of gold nanopartides is characterized with scanning tunneling microscopy (STM),and a dual energy barrier tunneling model is proposed to explain the imageability of the gold nanopartides by STM.This model can also be used to construct multiple energy barrier structure on solid/ liquid interface and to evaluate the electron transport ability of some organic monolayers with the aid of electrochemical method.     

Unisized two-dimensional platinum clusters on silicon(111)-7x7 surface observed with scanning tunneling microscope

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.     

Square, hexagonal, and row phases of PTCDA and PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag-Si(111)square root(3) x square root(3)R30 degrees surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.     

In situ scanning tunneling microscopy investigation of sulfur oxidative underpotential deposition on Ag(100) and Ag(110)

Underpotential (UPD) deposition of sulfur from Na(2)S solution in 0.1 M NaOH was studied on Ag(100) and Ag(110) using in situ scanning tunneling microscopy (STM). The cyclic voltammogram on Ag(100) presents two broad peaks, whereas three partial overlapping peaks and a sharper one are observed on Ag(110). STM measurements carried out during the whole UPD process show that progressively more compact structures are formed as the applied potential is scanned toward more positive potentials. More precisely, p(2×2), c(2×6), and c(2×2) were found on Ag(100) at E = -1.25, -1.0, and -0.9 V, respectively. Less definite conclusions can be drawn for the structures of S overlayers on Ag(110). However, the experimental findings are consistent with an incomplete p(2×1) at potentials preceding the sharp peak, and with a c(2×2) structure at E = -0.9 V vs Ag/AgCl, KCl(sat). The coverage values calculated on the basis of the hypothesized structures have been compared with the values obtained from chronocoulometric measurements at the most positive potentials investigated. Thus, the experimental coverage θ = 0.5 coincides with the coverage calculated for the c(2×2) structure found on Ag(110) at E = -0.9 V by STM, whereas the experimental coverage θ = 0.42 suggests that a mixture of structures c(2×6) and c(2×2) is formed on Ag(100).     

Ge adsorption on Ag(111): A density-functional theory investigation

First-principles density-functional theory has been used to investigate the adsorptions of Ge on Ag(111) surfaces for a wide range of coverage. Preferred adsorption sites, adsorption energies, surface structures, and the electronic properties are studied. Our results show that adsorption on the surface in fcc- sites is energetically favorable. The adsorption energies decrease as increasing Ge atoms, while the work functions of Ag surface decrease. The contour maps of the difference charge show that there exists covalent bonding in lower coverage systems to some extent for Ge on Ag(111) surface, and the interaction of Ge and Ag atoms becomes weaker with the increase of adsorption degree. The calculated density of states indicates that the adsorption structures have metallic character, while the number of electron transition is small and the interaction is not strong between Ge and Ag atoms.