Molecular Resolution Scanning Tunneling Microscopy Research Articles

Highly Ordered Co-Assembly of Bisurea Functionalized Molecular Switches at the Solid-Liquid Interface.

Immobilization of stimulus-responsive systems on solid surfaces is beneficial for controlled signal transmission and adaptive behavior while allowing the characterization of the functional interface with high sensitivity and high spatial resolution. Positioning of the stimuli-responsive units with nanometer-scale precision across the adaptive surface remains one of the bottlenecks in the extraction of cooperative function. Nanoscale organization, cooperativity, and amplification remain key challenges in bridging the molecular and the macroscopic worlds. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) characterization of overcrowded alkene photoswitches merged in self-assembled networks physisorbed at the solid-liquid interface. A detailed anchoring strategy that ensures appropriate orientation of the switches with respect to the solid surface through the use of bis-urea groups is presented. We implement a co-assembly strategy that enables the merging of the photoswitches within physisorbed monolayers of structurally similar 'spacer' molecules. The self-assembly of the individual components and the co-assemblies was examined in detail using (sub)molecular resolution STM which confirms the robust immobilization and controlled orientation of the photoswitches within the spacer monolayers. The experimental STM data is supported by detailed molecular mechanics (MM) simulations. Different designs of the switches and the spacers were investigated which allowed us to formulate guidelines that enable the precise organization of the photoswitches in crystalline physisorbed self-assembled molecular networks.

Covalent Functionalization of Carbon Surfaces: Diaryliodonium versus Aryldiazonium Chemistry

Covalently modified carbon surfaces remain central to a number of applications. Surface modification is often achieved using aryldiazonium chemistry, which has become the gold standard for the functionalization of carbon-based surfaces. However, the higher reactivity and thus the lower stability of diazonium precursors at carbon interfaces have prompted a search for alternative chemistries. Diaryliodonium salts have emerged as stable alternatives to the conventional aryldiazonium precursors. In this contribution, we provide a detailed comparison between the two types of chemistries by carrying out electrochemical covalent grafting of graphite substrates using nitrophenyl groups generated from the corresponding aryldiazonium and diaryliodonium salts under identical experimental conditions. The electrochemical process was studied using cyclic voltammetry, whereas the covalently grafted substrates were characterized using a battery of surface analytical techniques spanning multiple length scales. Analysis of the modified substrates using Raman spectroscopy revealed that the efficiency of covalent grafting is higher for iodonium chemistry than that for aryldiazonium chemistry. This observation was further corroborated by detailed morphological characterization of the covalent films using atomic force microscopy and molecular-resolution scanning tunneling microscopy, which revealed higher surface coverage for iodonium modified substrates. The chemical composition of the films was probed using X-ray photoelectron spectroscopy. The detailed and systematic comparison presented here clearly demonstrates that diaryliodonium chemistry presents a robust and reliable alternative to the widely used aryldiazonium chemistry.

Surface Self-Assembly, Film Morphology, and Charge Transport Properties of Semiconducting Triazoloarenes.

Surface-assisted molecular self-assembly is a powerful strategy for forming molecular-scale architectures on surfaces. These molecular self-assemblies have potential applications in organic electronics, catalysis, photovoltaics, and many other technologies. Understanding the intermolecular interactions on a surface can help predict packing, stacking, and charge transport properties of films and allow for new molecular designs to be tailored for a required function. We have previously studied a molecular platform, tris( N-phenyltriazole) (TPT), that exhibits planar stacking through >20 molecular layers through donor-acceptor-type intermolecular π-π contacts between the electron-deficient tris(triazole) core and electron-rich peripheral phenyl units. Here, we investigate an expanded family of TPT-based molecules with variations made on the peripheral aryl groups to modulate the molecular electron distribution and examine the impact on molecular packing and charge transport properties. Molecular-resolution scanning tunneling microscopy was used to compare the molecular packing in the monolayer and to investigate the effects that the structural and electronic modifications have on the stacking in subsequent layers. Conductivity measurements were made using the four-point probe van der Pauw technique to demonstrate charge transport properties comparable to pentacene. Although molecular packing is clearly impacted by the chemical structure, we find that the charge transport efficiency is quite tolerant to small structural variations.

Adsorption and Electrochemical Polymerization of Pyrrole on Au(100) Electrodes as Examined by In Situ Scanning Tunneling Microscopy

Despite many efforts studying polypyrrole (PPy) with scanning probe techniques in the past several decades, its molecular structure and conformation have been elusive. This study presents the first atomic and molecular-resolution scanning tunneling microscopy (STM) images of pyrrole (Py) and PPy electrochemically produced on bare and iodine-modified Au(100) electrodes in pH 1 and 4 sulfate media. First, STM reveals a highly ordered Au(100)—(4√2 × 4√2) R45°—Py between 0.15 and 0.5 V (vs saturated calomel electrode) in 0.1 M H2SO4 containing 30 mM Py. By contrast, a disordered Py adlayer is seen in 0.1 M HClO4 medium, indicating the coadsorption of Py and an acid. This anion effect is manifested in voltammetric results obtained with Au(100), showing a pair of sharp peaks at 0.12 V in 0.1 M H2SO4 + 30 mM Py and a featureless profile seen with Au(111) or Au(100) in HClO4. Py and bisulfate (and perchloric acid) attract each other and are coadsorbed on Au(100) electrodes. The favorable adsorption of bisulfate a...

Effects of Benzenediols on the Oxidative Polymerization of Aniline on Au(111) – Electrochemistry and Scanning Tunneling Microscopy

This study used cyclic voltammetry (CV) and scanning tunneling microscopy (STM) to address the adsorption and oxidative polymerization of aniline on Au(111) electrode in sulfuric acids containing three isomers of benzenediol (BOH) - hydroquinone (HQ), catechol (CA), and resorcinol (RS). Molecular-resolution STM revealed highly ordered aniline adlattices of Au(111) – (√19×√21) at 0.7V (versus reversible hydrogen electrode) with all BOHs, compared with the (3×2√3) structure found without BOHs. Aniline molecules in the (√19×√21) structure were adsorbed in the zigzagged chains along the <121> direction of Au(111), which caused an anisotropic oxidative polymerization reaction that produced linear PAN molecules. BOHs slowed down polymerization of aniline on Au(111), and the effect increased in the order of CA<HQ<RS, due to their interactions with aniline molecules and their solubility in sulfuric acid. The corresponding reduction process of oxidized CA molecules at E>0.8V was eliminated by reacting with aniline molecules. Although BOHs all interacted with the PAN, they were not incorporated in the PAN films. PAN film supported by Au(111) resulted in 2 and 1.2 times higher currents of HQ and CA than those found with bare Au(111).

Self-Assembled Air-Stable Supramolecular Porous Networks on Graphene

Functionalization and modification of graphene at the nanometer scale is desirable for many applications. Supramolecular assembly offers an attractive approach in this regard, as many organic molecules form well-defined patterns on surfaces such as graphite via physisorption. Here we show that ordered porous supramolecular networks with different pore sizes can be readily fabricated on different graphene substrates via self-assembly of dehydrobenzo[12]annulene (DBA) derivatives at the interface between graphene and an organic liquid. Molecular resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigations reveal that the extended honeycomb networks are highly flexible and that they follow the topological features of the graphene surface without any discontinuity, irrespective of the step-edges present in the substrate underneath. We also demonstrate the stability of these networks under liquid as well as ambient air conditions. The robust yet flexible DBA network adsorbed on graphene surface is a unique platform for further functionalization and modification of graphene. Identical network formation irrespective of the substrate supporting the graphene layer and the level of surface roughness illustrates the versatility of these building blocks.

Long Range Chiral Imprinting of Cu(110) by Tartaric Acid

Restructuring of metals by chiral molecules represents an important route to inducing and controlling enantioselective surface chemistry. Tartaric acid adsorption on Cu(110) has served as a useful system for understanding many aspects of chiral molecule adsorption and ordering on a metal surface, and a number of chiral and achiral unit cells have been reported. Herein, we show that given the appropriate annealing treatment, singly deprotonated tartaric acid monolayers can restructure the Cu metal itself, and that the resulting structure is both highly ordered and chiral. Molecular resolution scanning tunneling microscopy reveals that singly deprotonated tartaric acid extracts Cu atoms from the Cu(110) surface layer and incorporates them into highly ordered, chiral adatom arrays capped by a continuous molecular layer. Further evidence for surface restructuring comes from images of atom-deep trenches formed in the Cu(110) surface during the process. These trenches also run in low symmetry directions and are...

Toward tunable doping in graphene FETs by molecular self-assembled monolayers

In this paper, we report the formation of self-assembled monolayers (SAMs) of oleylamine (OA) on highly oriented pyrolytic graphite (HOPG) and graphene surfaces and demonstrate the potential of using such organic SAMs to tailor the electronic properties of graphene. Molecular resolution Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) images reveal the detailed molecular ordering. The electrical measurements show that OA strongly interacts with graphene leading to n-doping effects in graphene devices. The doping levels are tunable by varying the OA deposition conditions. Importantly, neither hole nor electron mobilities are decreased by the OA modification. As a benefit from this noncovalent modification strategy, the pristine characteristics of the device are recoverable upon OA removal. From this study, one can envision the possibility to correlate the graphene-based device performance with the molecular structure and supramolecular ordering of the organic dopant.

Correction to Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionCorrection to Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling MicroscopyN. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam*, and R. P. Van Duyne*Cite this: Nano Lett. 2012, 12, 12, 6506Publication Date (Web):November 9, 2012Publication History Published online9 November 2012Published inissue 12 December 2012https://doi.org/10.1021/nl304082mCopyright © 2012 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views703Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (102 KB) Get e-Alerts Get e-Alerts

Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

In situ scanning tunneling microscopy of 5-(Dodecyloxy)-2-(5-(4-(pentyloxy)phenyl)-1H-pyrazol-3-yl)phenol adsorbed on Au(1 1 1) electrode

The devise and advance of scanning tunneling microscopy (STM) have yielded insights of molecular adsorption at well-ordered electrified interface. This study employed in situ STM in 0.1 M HClO 4 to examine the effect of potential on the spatial arrangements of 5-(Dodecyloxy)-2-(5-(4-(octyloxy)phenyl)-1H-pyrazol-3-yl)phenol ( 1) and 5-(Dodecyloxy)-2-(5-(4-(pentyloxy)phenyl)-1H-pyrazol-3-yl)phenol ( 2) on Au(1 1 1) electrode. High-quality molecular resolution STM imaging was used to study the anchoring configurations of these LC molecules on gold electrode. Only between 0 and 0.5 V (vs. reversible hydrogen electrode) did ordered molecular adlattices were found, including a chained phase formed between 0.5 and 0.25 V, and a lamellar phase formed between 0.05 and 0.25 V. The origin of this potential effect can be traced to the charge state of the gold electrode. Molecular resolution STM imaging of these ordered structures yielded intermolecular spacing and contour of admolecule, from which the adsorption configurations of 1 and 2 were deduced. They were adsorbed mostly parallel to the Au(1 1 1) substrate between 0.25 and 0.5 V, but their pyrazole moieties could turn upright as the potential was lowered to E < 0.25 V. The alkoxy side chains of adsorbed 1 and 2 were clearly imaged, which supports the notion that alkoxy interacted with Au(1 1 1) electrode more favorably at E < 0.25 V. This is the first report on the potential dependent adsorption of alkoxy moiety at metal electrode. Also, molecules comprising the lamellar phase were preferentially aligned with their long molecular axis normal to the lamellas, which resembles the molecular arrangement of the smectic A or C phase of liquid crystal.

Seeding Atomic Layer Deposition of High-k Dielectrics on Epitaxial Graphene with Organic Self-Assembled Monolayers

The development of high-performance graphene-based nanoelectronics requires the integration of ultrathin and pinhole-free high-k dielectric films with graphene at the wafer scale. Here, we demonstrate that self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) act as effective organic seeding layers for atomic layer deposition (ALD) of HfO(2) and Al(2)O(3) on epitaxial graphene on SiC(0001). The PTCDA is deposited via sublimation in ultrahigh vacuum and shown to be highly ordered with low defect density by molecular-resolution scanning tunneling microscopy. Whereas identical ALD conditions lead to incomplete and rough dielectric deposition on bare graphene, the chemical functionality provided by the PTCDA seeding layer yields highly uniform and conformal films. The morphology and chemistry of the dielectric films are characterized by atomic force microscopy, ellipsometry, cross-sectional scanning electron microscopy, and X-ray photoelectron spectroscopy, while high-resolution X-ray reflectivity measurements indicate that the underlying graphene remains intact following ALD. Using the PTCDA seeding layer, metal-oxide-graphene capacitors fabricated with a 3 nm Al(2)O(3) and 10 nm HfO(2) dielectric stack show high capacitance values of ∼700 nF/cm(2) and low leakage currents of ∼5 × 10(-9) A/cm(2) at 1 V applied bias. These results demonstrate the viability of sublimated organic self-assembled monolayers as seeding layers for high-k dielectric films in graphene-based nanoelectronics.

In Situ STM Imaging of Bis-3-sodiumsulfopropyl-disulfide Molecules Adsorbed on Copper Film Electrodeposited on Pt(111) Single Crystal Electrode

The adsorption of bis-3-sodiumsulfopropyldi-sulfide (SPS) on metal electrodes in chloride-containing media has been intensively studied to unveil its accelerating effect on Cu electrodeposition. Molecular resolution scanning tunneling microscopy (STM) imaging technique was used in this study to explore the adsorption and decomposition of SPS molecules concurring with the electrodeposition of copper on an ordered Pt(111) electrode in 0.1 M HClO(4) + 1 mM Cu(ClO(4))(2) + 1 mM KCl. Depending on the potential of Pt(111), SPS molecules could react, adsorb, and decompose at chloride-capped Cu films. A submonolayer of Cu adatoms classified as the underpotential deposition (UPD) layer at 0.4 V (vs Ag/AgCl) was completely displaced by SPS molecules, possibly occurring via RSSR (SPS) + Cl-Cu-Pt → RS(-)-Pt(+) + RS(-) (MPS) + Cu(2+) + Cl(-), where MPS is 3-mercaptopropanesulfonate. By contrast, at 0.2 V, where a full monolayer of Cu was presumed to be deposited, SPS molecules were adsorbed in local (4 × 4) structures at the lower ends of step ledges. Bulk Cu deposition driven by a small overpotential (η < 50 mV) proceeded slowly to yield an atomically smooth Cu deposit at the very beginning (<5 layers). On a bilayer Cu deposit, the chloride adlayer was still adsorbed to afford SPS admolecules arranged in a unique 1D striped phase. SPS molecules could decompose into MPS upon further Cu deposition, as a (2 × 2)-MPS structure was observed with prolonged in situ STM imaging. It was possible to visualize either SPS admolecules in the upper plane or chloride adlayer sitting underneath upon switching the imaging conditions. Overall, this study established a MPS molecular film adsorbed to the chloride adlayer sitting atop the Cu deposit.

Molecular Orientation of Individual Lu@C82Molecules Demonstrated by Scanning Tunneling Microscopy

We identified the orientation of individual Lu@C82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C82 molecule, suggesting that thermal rotation of Lu@C82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn−Sham molecular orbitals of Lu@C82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C82. Spatial mapping of the differential conductance on individual Lu@C82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO−LUMO gap of Lu@C82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the locally high conductivity around the Lu atom was attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C82 by applying a high electric field (about 1 × 107 V/cm) with a large tunneling current (1.5 nA).

Phenylacetylene One-Dimensional Nanostructures on the Si(100)-2 × 1:H Surface

Using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), many olefins have been shown to self-assemble on the hydrogen-passivated Si(100)-2 x 1 surface into one-dimensional nanostructures. This paper demonstrates that similar one-dimensional nanostructures can also be realized using alkynes. In particular, UHV STM, sum frequency generation (SFG), and density functional theory (DFT) are employed to study the growth mechanism and binding configuration of phenylacetylene (PA) one-dimensional nanostructures on the Si(100)-2 x 1:H surface. Molecular-resolution UHV STM images reveal the binding position and spacing of PA with respect to the underlying silicon dimer rows. Furthermore, UHV STM characterization of heteromolecular one-dimensional nanostructures of styrene and PA shows distinct electronic contrast between the two molecules, which is confirmed using simulated STM images derived from DFT and provides insight into the nature of PA binding to silicon. Additional evidence from SFG measurements corroborates the conclusion that the terminal carbon atoms of PA retain pi-conjugation following reaction to the Si(100)-2 x 1:H surface.

Molecular Resolution Imaging of an Antibiotic Peptide in a Lipid Matrix

In this work, we show molecular resolution scanning tunneling microscopy (STM) images of gramicidin, a model antibacterial peptide, inserted into a phospholipid matrix. The resolution of the images is superior to that obtained in previous attempts to image gramicidin in a lipid environment using atomic force microscopy (AFM). This breakthrough has allowed visualization of individual peptide molecules surrounded by lipid molecules. We have observed several important features: the peptide molecules do not aggregate, the peptide molecules adopt a single conformation corresponding to a specific ion channel form, and the lipid molecules adjacent to the peptide molecules are systematically longer than those in the lipid matrix. These results constitute a new approach to obtain structural characteristics of antibiotic peptides in lipid assemblies that is necessary for the understanding of their biological activity.

Self-Assembled Monolayer of Short Carboxyl-Terminated Molecules Investigated with ex Situ Scanning Tunneling Microscopy

Self-assembled monolayers (SAMs) of thiolated molecules have numerous potential applications and are ideal models for complex supramolecular systems. The structure of these two-dimensional crystals is not understood fully yet and deserves in-depth investigations. Here we show that a short carboxylic acid ω-terminated molecule (3-mercaptopropionic acid, MPA) forms highly ordered SAMs on Au(111) surfaces. Molecular resolution scanning tunneling microscopy (STM) images were taken at room temperature in air (ex situ). They showed a √3 × √3 unit cell that, when imaged at high set current, appears reconstructed as a c(4 × 2) superlattice. This structure is quantitatively the same as that of intermediate-length alkane thiols, but is different from the structure found previously for SAMs of the same molecule investigated with liquid-STM during the formation of the monolayer (in situ). This work highlights the possibility for a short ω-terminated molecule to form ordered monolayers over large areas with few defects.

Improvements in the Characterization of the Crystalline Structure of Acid-Terminated Alkanethiol Self-Assembled Monolayers on Au(111)

We report a study of acid-terminated self-assembled monolayers of alkanethiols of different length, 11-mercaptoundecanoic acid (11-MUA) and 16-mercaptohexadecanoic acid (16-MHDA), on Au(111). Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle techniques were used for characterization, and the results were compared with those obtained from n-alkanethiols of similar chain length, providing a detailed description of the two-dimensional crystalline structure. Molecular resolution STM images show that 11-MUA forms a dense-packed monolayer arranged in a (square root 3 x square root 3)R30 degrees structure with a c(2 x 4) superlattice, where the simple hexagonal phase, the c(2 x 4) superlattice, and nonordered areas coexist. 16-MHDA assembles in a uniform monolayer with similar morphology to that of 11-MUA, but molecular resolution could not be reached in STM due to both the hydrophilicity of the acid groups and the poor conductivity of the thick monolayer. Nevertheless, the monolayer thicknesses estimated by XPS and electrochemistry and the highly blocking character of the film observed by electrochemistry as well as the low water contact angle are consistent with 16-MHDA molecules forming a compact monolayer on the Au(111) substrate with fully extended alkyl chains and acid groups pointing away from the surface. The results obtained for 16-MHDA were reproducible under different preparation conditions such as the addition or omission of acetic acid to the ethanolic solution. Contrary to other reports, we demonstrate that ordered acid-terminated self-assembled monolayers are obtained with the same preparation conditions as those of the methyl-terminated ones, without any additional treatment.

Electrodeposition of Au Monolayer on Pt(111) Mediated by Self-Assembled Monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.

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.