Scanning tunneling microscopy of self-assembled phenylene ethynylene oligomers on Au(111) substrates

In this paper, we report the self-assembly, electrical characterization, and surface modification of dithiolated phenylene-ethynylene oligomer monolayers on a Au(111) surface. The self-assembly was accomplished by thiol bonding the molecules from solution to a Au(111) surface. We have confirmed the formation of self-assembled monolayers by scanning tunneling microscopy (STM) and optical ellipsometry, and have studied the kinetics of film growth. We suggest that self-assembled phenylene ethynylene oligomers on Au(111) surfaces grow as thiols rather than as thiolates. Using low-temperature STM, we collected local current-voltage spectra showing negative differential resistance at 6 K.     

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.     

Formation of supramolecular nanobelt arrays consisting of cobalt(II) "picket-fence" porphyrin on Au surfaces

Adlayers of cobalt(II) 5,10,15,20-tetrakis(alpha,alpha,alpha,alpha-2-pivalamidophenyl)porphyrin (CoTpivPP) were prepared by immersing either Au(111) or Au(100) substrate in a benzene solution containing CoTpivPP molecules, and they were investigated in 0.1 M HClO4 and 0.1 M H2SO4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The adlayer structure and electrochemical properties of CoTpivPP are compared to those of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP). Characteristic nanobelt arrays consisting of CoTpivPP molecules were produced on both Au(111) and Au(100) surfaces. The stability of the nanobelt arrays was controlled by manipulating the electrode potential. On the other hand, the formation of nanobelt arrays consisting of O2-adducted CoTpivPP molecules depended upon the crystallographic orientation of Au. The state of O2 trapped in the cavity of CoTpivPP was distinctly observed in STM images as a bright spot in the nanobelt array formed on reconstructed Au(100)-(hex) surface, but not on Au(111) surface. This result suggests that the arrangement of underlying Au atoms plays an important role in the formation of nanobelt arrays with the sixth ligand coordination.     

Interaction of carbon monoxide with Au(111) modified by ion bombardment: a surface spectroscopy study under elevated pressure

Gold based model systems exhibiting the structural versatility of nanoparticle ensembles and being accessible for surface spectroscopic investigations are expected to provide new information about the adsorption of carbon monoxide, a key process influencing the CO oxidation activity of this noble metal in nanoparticulate form. Accordingly, in the present work the interaction of CO is studied with an ion bombardment modified Au(111) surface by means of a combination of photoelectron spectroscopy (XPS and UPS), sum frequency generation vibrational spectroscopy (SFG), and scanning tunneling microscopy (STM). While no adsorption was found on intact Au(111), data collected on the ion bombarded surface at cryogenic temperatures indicated the presence of stable CO adsorbates below 190 K. A quantitative evaluation of the C 1s XPS spectra and the surface morphology explored by STM revealed that the step edge sites created by ion bombardment are responsible for CO adsorption. The identification of the CO binding sites was confirmed by density functional theory (DFT) calculations. Annealing experiments up to room temperature showed that at temperatures above 190 K unstable adsorbates are formed on the surface under dynamic exposure conditions that disappeared immediately when gaseous CO was removed from the system. Spectroscopic data as well as STM records revealed that prolonged CO exposure at higher pressures of up to 1 mbar around room temperature facilitates massive atomic movements on the roughened surface, leading to its strong reordering toward the structure of the intact Au(111) surface, accompanied by the loss of the CO binding capacity.     

High-resolution scanning tunneling microscopy images of molecular overlayers prepared by a new molecular beam deposition apparatus with spray-jet technique

We developed a new molecular beam deposition apparatus using a spray-jet technique for high-quality thin film preparation of nonsublimable molecules. The apparatus was used to deposit chloro[tri-tert-butyl-subphthalocyaninato]boron(III) (TBSubPc) molecules on an Au(111) surface for analysis by low-temperature scanning tunneling microscopy (STM). Highly resolved images, in which tert-butyl groups in a TBSubPc molecule were clearly identifiable, were obtained. The image quality and the resolution of these images compared favorably well to STM images taken on reference samples which were sublimed onto Au (111) from a heated crucible.     

Potential-induced phase transition of low-index Au single crystal surfaces in propylene carbonate solution

In situ scanning tunneling microscopy (STM) was employed to examine the surface structures of Au(111), Au(100), and Au(110) single crystals in propylene carbonate (PC) containing tetrabutylammonium perchlorate (TBAP). All three electrodes exhibited potential-induced phase transition between the reconstructed and unreconstructed (1 × 1) structures at negative and positive potentials, respectively. The potential-induced phase transition of the Au electrode surfaces is attributed to the interaction of the TBA cation and the perchlorate anion at the electrode surface, which is similar to that which takes place in aqueous solutions. In addition to static atomic structures, dynamic processes of both the reconstruction and the lifting of the reconstruction were investigated by means of in situ STM. The lifting of reconstructed Au(111)-(√3 × 22) on Au(111) to the (1 × 1) structure is completed within 1 min at a positive potential. The diffusion of Au atoms on the Au(100) plane in the PC solution proceeds more rapidly than that in the aqueous solution, suggesting that the PC solvent plays an important role in accelerating the diffusion of Au atoms.     

Time-dependent organization and wettability of decanethiol self-assembled monolayer on Au(111) investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.     

Adsorption and desorption processes of self-assembled monolayers studied by surface-sensitive microscopy and spectroscopy

Adsorption and desorption processes of self-assembled monolayers (SAMs) have been studied on an Au(111) surface by scanning tunnelling microscopy (STM), atomic force microscopy (AFM), X-ray photo-electron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At the initial growth stage, the ordered nucleation of SAM located at the herringbone turns of the Au(111) − (22 × √3) surface reconstruction and diffusion-controlled domain formation have been imaged by STM and AFM. Details of the oxidation process in UV desorption were also investigated by XPS. In addition, the dimerization reaction during desorption was confirmed by TDS for the first time in the alkanethiol SAM system.     

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.     

Substrate-induced pairing in 2,3,6,7,10,11-hexakis-undecalkoxy-triphenylene self-assembled monolayers on Au111

We present an STM study of self-assembled monolayers of 2,3,6,7,10,11-undecalkoxy-substituted triphenylene (T11) at the n-tetradecane/Au(111) interface under ambient conditions. T11 molecules self-organize as paired rows with molecules lying flat on the surface in an antiparallel position. Three alkyl chains of each T11 molecule align along the 110 direction of the underlying Au(111) substrate. The association of T11 in molecular pairs appears to result from a substrate-induced mechanism governed by the strong anisotropic interaction between T11 alkyl chains and Au(111).     

Toward Raman fingerprints of single dye molecules at atomically smooth Au(111)

The creation of a highly enhanced electromagnetic (EM) field underneath a scanning tunneling microscope (STM) tip enables Raman spectroscopic studies of organic submonolayer adsorbates at atomically smooth single crystalline surfaces. To study the sensitivity of this technique, tip-enhanced resonance Raman (TERR) spectra of the dye malachite green isothiocyanate on Au(111) in combination with the corresponding STM images of the probed surface region were analyzed. The detection limit for unambiguous identification of the dye and semiquantitative determination of the surface coverage reaches < or =0.7 pmol/cm(2), or approximately five molecules present in the enhanced-field region, which is confirmed by STM images. Because of well-defined adsorption sites at atomically smooth Au(111) surfaces, no variation in band positions or relative band intensities was observed at the single- or few-molecule detection level when employing TERR spectroscopy.     

The nature of alkanethiol self-assembled monolayer adsorption on sputtered gold substrates

A detailed study of the self-assembly and coverage by 1-nonanethiol of sputtered Au surfaces using molecular resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au surface composed predominantly of [111] oriented grains. The domains of the alkanethiol monolayer are observed with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. STM imaging shows that the (4 x 2) superlattice structure is observed as a (3 x 2) structure when imaged under noncontact AFM conditions. The 1-nonanethiol molecules reside in the threefold hollow sites of the Au[111] lattice and aligned along its [112] lattice vectors. The self-assembled monolayer (SAM) contains many nonuniformities such as pinholes, domain boundaries, and monatomic depressions which are present in the Au surface prior to SAM adsorption. The detailed observations demonstrate limitations to the application of 1-nonanethiol as a resist in atomic nanolithography experiments to feature sizes of approximately 20 nm.     

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.     

The interfaces of Au(111) and Au(100) in a hexaalkyl-substituted guanidinium ionic liquid: an electrochemical and in situ STM study

Five hexaalkylguanidinium-based ionic liquids have been synthesised, and based on their cyclic voltammograms the most suited one, N,N-dibutyl-N',N'-diethyl-N',N'-dimethylguanidinium bis(trifluoromethylsulfonyl)imide, has been chosen for electrochemical studies. The surface interaction of this room-temperature ionic liquid with single crystalline gold surfaces (Au(100) and Au(111)) has been investigated using cyclic voltammetry, impedance spectroscopy and in situ scanning tunnelling microscopy (STM). The interfacial capacitance was found to be very low; STM measurements revealed the hex-reconstruction and herringbone reconstruction for Au(100) and for Au(111), respectively, at negative potentials; that is, at these potentials no hints for ad-structures of the cation could be found.     

Effect of adlayer structure on the host-guest recognition between calcium and crown-ether-substituted phthalocyanine arrays on Au single-crystal surfaces.

Adlayers of 15-crown-5-ether-substituted cobalt(II) phthalocyanine (CoCRPc) were prepared by immersion of either Au(111) or Au(100) substrate into benzene-ethanol (9:1 v/v) mixed solutions containing CoCRPc. In situ STM imaging was carried out after transferring the CoCRPc-modified Au crystals into aqueous HClO(4) solution. The packing arrangement of the CoCRPc array on Au(111) was determined to be p(8 x 4 radical 3R - 30 degrees ), and the internal structure was clearly observed by high-resolution STM. Two adlayer structures of CoCRPc, (8 x 9) and (4 radical 5 x 4 radical 5)R26.7 degrees, were found on the Au(100)-(1 x 1) terrace. In the presence of 1 mM Ca(2+), two Ca(2+) ions were trapped in two diagonally located 15-crown-5-ether moieties of each CoCRPc molecule on Au(111), whereas encapsulation of Ca(2+) ions was not seen in the CoCRPc arrays on the Au(100)-(1 x 1) surface. The present study demonstrates that the relationship between crown moieties of CRPc and the underlying Au lattice is important in the trapping of Ca(2+) ions in crown rings.     

Investigation of the scanning tunneling microscopy image, the stacking pattern, and the bias-voltage-dependent structural instability of 1,10'-phenanthroline molecules adsorbed on Au(111) in terms of electronic structure calculations

A self-assembled monolayer of 1,10'-phenanthroline (phen) molecules on Au(111) was found to undergo a structural phase transition when the bias voltage is switched in scanning tunneling microscopy (STM) experiments (Phys. Rev. Lett. 1995, 75, 2376; Surf. Sci. 1997, 389, 19). The nature of two bright spots representing each phen molecule in the high-resolution STM images of phen molecules on Au(111) was identified by calculating the partial density plots for a monolayer of phen molecules adsorbed on Au(111) with tight-binding electronic structure calculations. The stacking pattern of chains of phen molecules on Au(111) was explained by studying the intermolecular interactions between phen molecules on the basis of first-principles electronic structure calculations for a phen dimer, (phen)(2). The structural instability of phen molecule arrangement caused by the bias-voltage switch was probed by estimating the adsorbate-surface interaction energy with the point-charge approximation for Au(111).     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Scanning tunneling microscopy studies of pulse deposition of dinuclear organometallic molecules on Au(111)

Ultrahigh-vacuum scanning tunneling microscopy (STM) was used to study trans-[Cl(dppe)2Ru(C Triple Bond C)6Ru(dppe)2Cl] [abbreviated as Ru2, diphenylphosphinoethane (dppe)] on Au(111). This large organometallic molecule was pulse deposited onto the Au(111) surface under ultrahigh-vacuum (UHV) conditions. UHV STM studies on the prepared sample were carried out at room temperature and 77 K in order to probe molecular adsorption and to characterize the surface produced by the pulse deposition process. Isolated Ru2 molecules were successfully imaged by STM at room temperature; however, STM images were degraded by mobile toluene solvent molecules that remain on the surface after the deposition. Cooling the sample to 77 K allows the solvent molecules to be observed directly using STM, and under these conditions, toluene forms organized striped domains with regular domain boundaries and a lattice characterized by 5.3 and 2.7 A intermolecular distances. When methylene chloride is used as the solvent, it forms analogous domains on the surface at 77 K. Mild annealing under vacuum causes most toluene molecules to desorb from the surface; however, this annealing process may lead to thermal degradation of Ru2 molecules. Although pulse deposition is an effective way to deposit molecules on surfaces, the presence of solvent on the surface after pulse deposition is unavoidable without thermal annealing, and this annealing may cause undesired chemical changes in the adsorbates under study. Preparation of samples using pulse deposition must take into account the characteristics of sample molecules, solvent, and surfaces.     

Thiol with an unusual adsorption-desorption behavior: 6-mercaptopurine on Au(111)

A multitechnique study of 6-mercaptopurine (6MP) adsorption on Au(111) is presented. The molecule adsorbs on Au(111), originating short-range ordered domains and irregular nanosized aggregates with a total surface coverage by chemisorbed species smaller than those found for alkanethiol SAMs, as derived from scanning tunneling microscopy (STM) and electrochemical results. X-ray photoelectron spectroscopy (XPS) results show the presence of a thiolate bond, whereas density functional theory (DFT) data indicate strong chemisorption via a S-Au bond and additional binding to the surface via a N-Au bond. From DFT data, the positive charge on the Au topmost surface atoms is markedly smaller than that found for Au atoms in alkanethiolate SAMs. The adsorption of 6MP originates Au atom removal from step edges but no vacancy island formation at (111) terraces. The small coverage of Au islands after 6MP desorption strongly suggests the presence of only a small population of Au adatom-thiolate complexes. We propose that the absence of the Au-S interface reconstruction results from the lack of significant repulsive forces acting at the Au surface atoms.