An Electron‐Rich Cyclic (Alkyl)(Amino)Carbene on Au(111), Ag(111), and Cu(111) Surfaces

The structural properties and binding motif of a strongly σ‐electron‐donating N‐heterocyclic carbene have been investigated on different transition‐metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and molecular islands with short‐range order could be found at high coverage. A combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations highlights how CAACs bind to the surface, which is of tremendous importance to gain an understanding of heterogeneous catalysts bearing CAACs as ligands.     

Potential dependence of the kinetics of thiol self-organization on Au(111)

The self-assembly of thiol molecules from ethanolic solution on Au(111) depends significantly on the electrode potential. Especially at cathodic potentials, chemisorption of thiol molecules and the development of the highly ordered structure are slowed down significantly. At potentials near the point of zero charge, first a disordered thiol film of already high thiol density is formed, and then domains of the highly ordered phase develop and grow together. At cathodic potentials, first a disordered film of very low density of predominant flat adsorbed thiol molecules is formed; the formation of ordered domains takes time three orders of magnitude longer than at potentials near the point of zero charge. Received: 27 May 1997 / Accepted: 8 September 1997     

Two-dimensional Molecular Phase Transition of Alkylated-TDPB on Au(111) and Cu(111) Surfaces

The derivatives of aromatic cores bearing alkyl chains with different lengths are of potential interest in on-surface chemistry, and thus have been widely investigated both at liquid-solid interfaces and in vacuum. Here, we report on the structural evaluation of self-assembled 1,3,5-tri(4-dodecylphenyl)benzene(TDPB) molecules with increased molecular coverages on both Au(111) and Cu(111) surfaces. As observed on Au(111), rhombic and herringbone structures emerge successively depending on surface coverage. In the case of Cu(111), the same process of phase conversion is also observed, but with two distinct structures. In comparison, the self-assembled structures on Au(111) surface are packed more densely than that on Cu(111) surface under the same preparation conditions. This may fundamentally result from the higher adsorption energy of TDPB molecules on Cu(111), restricting their adjustment to optimize a thermodynamically favorable molecular packing.     

Aryl Radical Geometry Determines Nanographene Formation on Au(111)

The Ullmann coupling has been used extensively as a synthetic tool for the formation of C?C bonds on surfaces. Thus far, most syntheses made use of aryl bromides or aryl iodides. We investigated the applicability of an aryl chloride in the bottom‐up assembly of graphene nanoribbons. Specifically, the reactions of 10,10′‐dichloro‐9,9′‐bianthryl (DCBA) on Au(111) were studied. Using atomic resolution non‐contact AFM, the structure of various coupling products and intermediates were resolved, allowing us to reveal the important role of the geometry of the intermediate aryl radicals in the formation mechanism. For the aryl chloride, cyclodehydrogenation occurs before dehalogenation and polymerization. Due to their geometry, the planar bisanthene radicals display a different coupling behavior compared to the staggered bianthryl radicals formed when aryl bromides are used. This results in oligo‐ and polybisanthenes with predominantly fluoranthene‐type connections.     

Self-assembly of alkanols on Au(111) surfaces

Self-assembled monolayers (SAMs) of alkanols (1-C(N)H(2N+1)OH) with varying carbon-chain lengths (N = 10-30) have been systematically studied by means of scanning tunneling microscopy (STM) at the interfaces between alkanol solutions (or liquids) and Au(111) surfaces. The carbon skeletons were found to lie flat on the surfaces. This orientation is consistent with SAMs of alkanols on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces, and also with alkanes on reconstructed Au(111) surfaces. This result differs from a prior report, which claimed that 1-decanol molecules (N = 10) stood on their ends with the OH polar groups facing the gold substrate. Compared to alkanes, the replacement of one terminal CH3 group with an OH group introduces new bonding features for alkanols owing to the feasibility of forming hydrogen bonds. While SAMs of long-chain alkanols (N > 18) resemble those of alkanes, in which the aliphatic chains make a greater contribution, hydrogen bonding plays a more important role in the formation of SAMs of short-chain alkanols. Thus, in addition to the titled lamellar structure, a herringbone-like structure, seldom seen in SAMs of alkanes, is dominant in alkanol SAMs for values of N < 18. The odd-even effect present in alkane SAMs is also present in alkanol SAMs. Thus, the odd N alkanols (alkanols with an odd number of carbon atoms) adopt perpendicular lamellar structures owing to the favorable interactions of the CH3 terminal groups, similar to the result observed for odd alkanes. In contrast to alkanes on Au(111) surfaces, for which no SAMs on an unreconstructed gold substrate were observed, alkanols are capable of forming SAMs on either the reconstructed or the unreconstructed gold surfaces. Structural models for the packing of alkanol molecules on Au(111) surfaces have been proposed, which successfully explain these experimental observations.     

Graphene Grown from Flat and Bowl Shaped Polycyclic Aromatic Hydrocarbons on Cu(111)

The growth of carbon layers, defective graphene, and graphene by deposition of polycyclic aromatic hydrocarbons (PAHs) on Cu(111) is studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Two different PAHs are used as starting materials: the buckybowl pentaindenocorannulene (PIC) which contains pentagonal rings and planar coronene (CR). For both precursors, with increasing sample temperature during deposition, porous carbon aggregates (350 °C), dense carbon layers (400–450 °C), disordered defective graphene (500 °C–550 °C), and extended graphene (≥600 °C) are obtained. No significant differences for defective graphene grown from PIC and CR are observed. C 1s X-ray photoelectron spectra of PIC and CR derived samples grown at 350–550 °C exhibit a characteristic C−Cu low binding energy component. Preparation at ≥600 °C eliminates this C−Cu species and only C−C bonded carbon remains.     

Au(111)表面Verdazyl自由基的构象转换

Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.     

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.     

窄带隙IV-VI族半导体PbTe(111)的表面氧化及氧的热脱附机理

利用X射线光电子能谱(XPS)、扫描隧道显微镜(STM)以及低能电子衍射(LEED),对PbTe(111)薄膜的表面氧化及氧的热脱附机理进行了研究.结果表明:PbTe(111)薄膜经500VAr+轰击加上250℃高温退火循环处理,可得到呈(1×1)周期性排列的清洁表面.将此清洁表面暴露于大气两天后,表面被氧化形成了PbO2、PbO和TeO2,氧化层的厚度大于2个单原子层(ML),与清洁PbTe(111)表面相比,被氧化的PbTe(111)表面的Te3d5/2与Pb4f7/2芯态谱峰的面积比明显减小,表明被氧化的PbTe(111)表面是富Pb的.在热脱附处理过程中,PbO2和TeO2的芯态峰消失,且O1s芯态峰的强度迅速减弱,表明加热处理不仅使PbO2和TeO2发生了分解,同时也使氧发生了脱附,但PbO即使在350℃退火仍吸附于PbTe(111)表面.     

Self-assembly of normal alkanes on the Au (111) surfaces

The self-assembled monolayers (SAMs) of normal alkanes (n-C(n)H(2n+2)) with different carbon chain lengths (n=14-38) in the interfaces between alkane solutions (or liquids), and the reconstructed Au (111) surfaces have been systematically studied by means of scanning tunneling microscopy (STM). In contrast to previous studies, which concluded that some n-alkanes (n=18-26) can not form well-ordered structures on Au (111) surfaces, we observed SAM formations for all these n-alkanes without any exceptions. We find that gold reconstruction plays a critical role in the SAM formation. The alkane monolayers adopt a lamellar structure in which the alkane molecules are packed side-by-side, to form commensurate structures with respect to the reconstructed Au (111) surfaces. The carbon skeletons are found to lie flat on the surfaces, which is consistent with the infrared spectroscopic studies. Interestingly, we find that two-dimensional chiral lamellar structures form for alkanes with an even carbon number due to the specific packing of alkane molecules in a tilted lamella. Furthermore, we find that the orientation of alkane molecules deviates from the exact [011] direction, because of the intermolecular interactions among the terminal methyl groups of neighboring lamellae; this results in differences of molecular orientation between mirror structures of adjacent zigzag alkane lamellae. Structural models have been proposed, that shed new light on monolayer formation.     

Dispersion and STM Characterization of Au-CdSe Nanohybrids on Au(111)

We report the dispersion and scanning tunneling microscopy (STM) characterization of iso-lated Au-CdSe nanohybrids on atomically flat Au(111) through surface modifications. The top terminal groups of spacer molecules self-assembled on the surface are found critical for locking the nanohybrids into a well-separated state. The STM results indicate that both thiol and carboxylic terminals are effective in this aspect by making strong interaction with the Au portions of the nanohybrids. An argon ion sputtering technique is also proposed to clean up organic contaminants on the surface for improved STM imaging of individual Au-CdSe nanohybrids. These observations help to enrich technical approaches to dispersing individual nanostructures on the surface and provide opportunities to explore their local electroluminescent and energy transfer properties at the nanoscale.     

Ni在ZrO2(111)薄膜表面的生长、电子结构及热稳定性

利用X射线光电子能谱、紫外光电子能谱和低能电子衍射研究了Ni纳米颗粒在ZrO₂(111)薄膜表面的生长模式、电子结构及热稳定性. ZrO₂(111)薄膜外延生长于Pt(111)单晶表面,厚度约为3 nm.结果表明,当Ni气相沉积到ZrO₂(111)薄膜表面上时,遵循Stranski-Krastanov生长模式,即先二维生长至0.5 ML(monolayer),然后呈三维岛状生长.随着覆盖度的减小, Ni 2p_3/2峰逐渐向高结合能位移.利用俄歇参数法分析发现,引起该峰向高结合能位移的主要原因来源于终态效应的贡献,但在低的Ni覆盖度时,也有部分初态效应的贡献,说明Ni在ZrO₂表面初始生长时,两者存在较强的界面相互作用, Ni向ZrO₂衬底传递电荷,形成带部分正电荷的Ni_δ+.两种不同覆盖度(0.05和0.5 ML)的Ni/ZrO₂(111)模型催化剂热稳定性研究表明,当温度升高时, Ni逐渐被氧化成Ni₂₊,并伴随着向ZrO₂衬底的扩散.本文从原子水平上认识了Ni与ZrO₂表面的相互作用和界面结构,为理解实际ZrO₂担载的Ni催化剂结构提供了重要的依据.     

Growth and Electronic Properties of Ag Nanoparticles on Reduced CeO2-x(111) Films

Ag nanoparticles grown on reduced CeO_2-x thin films have been studied by X-ray photoelec-tron spectroscopy and resonant photoelectron spectroscopy of the valence band to understand the effect of oxygen vacancies in the CeO_2-x thin films on the growth and interfacial elec-tronic properties of Ag. Ag grows as three-dimensional particles on the CeO_2-x(111) surface at 300 K. Compared to the fully oxidized ceria substrate surface, Ag favors the growth of smaller particles with a larger particle density on the reduced ceria substrate surface, which can be attributed to the nucleation of Ag on oxygen vacancies. The binding energy of Ag3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. The interfacial interaction between Ag and CeO_2-x(111) is weak. The resonant enhancement of the 4f level of Ce³⁺ species in RPES indicates a partial Ce⁴⁺→Ce³⁺ re-duction after Ag deposited on reduced ceria surface. The sintering temperature of Ag on CeO_1.85(111) surface during annealing is a little higher than that of Ag on CeO₂(111) surface, indicating that Ag nanoparticles are more stable on the reduced ceria surface.     

Controlling Metalation Reaction of Phthalocyanine with Cobalt at Single-Molecule Level on Au(111) Surface

Metalation reaction of metal-free phthalocyanine molecule with Co atom adsorbed on Au(111) surface has been studied in situ at single atom/molecule scale by low-temperature scanning tunneling microscopy (STM) experiment combined with simulations based on density function theory calculations. Through manipulations using STM tip, we showed a controlled manner to have a single metal-free phthalocyanine molecule react with a Co atom to form Co phthalocyanine molecule. In this reaction process, an intermediate state originating from $\pi$-d interaction between the metal-free phthalocyanine molecule and Co atom has been identified. Moreover, we also revealed that the redox reaction represented as bond breaking and bond forming relative to the Co and pyrrolic N atoms, not pyrrolic H atoms, is a key process for dehydrogenation and metalation reaction. Our DFT calculations provided theoretical supporting for the above conclusions, and further understanding of the related mechanisms.     

Coverage-dependent Orientations of Dy@C82 Molecules on Au(111) Surface

The adsorption and molecular orientation of Dy@C82 isomer I on Au(111) has been investi-gated using ultrahigh-vacuum scanning tunneling microscopy at 80 K. At low coverages, the Dy@C82 molecules tend to grow along the step edges of Au(111), forming small clusters and molecular chains. Adsorption of Dy@C82 on the edges is dominated by the fullerene-substrate interaction and presents various molecular orientations. At higher coverages, the Dy@C82 is found to form ordered islands consisting of small domains of equally oriented molecules. The Dy@C82 molecules in the islands prefer the adsorption configurations with the major C2 axis being approximately parallel to the surface of the substrate. Three preferable orientations of the Dy@C82 molecules are found in a two-dimensional hexagonal close packed overlayer. These observations are attributed to the interplay of the fullerene-substrate interaction and dipole-dipole interaction between the metallofullerenes.     

First-principles Study of Single Tin-phthalocyanine Molecule on Ag(111) Surface

Adsorption behavior and electronic structure of tin-phthalocyanine (SnPc) on Ag(111) sur-face with Sn-up and Sn-down conformations are investigated using first-principles calcula-tions. Two predicted adsorption configurations agree well with the experimentally deter-mined structures. SnPc molecule energetically prefers to adsorb on Ag(111) surface with Sn-down conformation. The energy required to move the central Sn atom through the frame of a phthalocyanine molecule, switching from the Sn-up to Sn-down conformation, is about 1.68 eV. The simulated scanning tunneling microscopy images reproduce the main features of experimental observations. Moreover, the experimentally proposed hole attachment mech-anism is verified based on the calculated density of states of SnPc on Ag(111) with three different adsorption configurations.     

On-Surface Metathesis of an Ionic Liquid on Ag(111)

We investigated the adsorption, surface enrichment, ion exchange, and on-surface metathesis of ultrathin mixed IL films on Ag(111). We stepwise deposited 0.5 ML of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) and 1.0 ML of the aprotic IL 1-methyl-3-octylimidazolium hexafluorophosphate ([C₈C₁Im][PF₆]) at around 90 K. Thereafter, the resulting layered frozen film was heated to 550 K, and the thermally induced phenomena were monitored in situ by angle-resolved X-ray photoelectron spectroscopy. Between 135 and 200 K, [TfO]⁻ anions at the Ag(111) surface are exchanged by [PF₆]⁻ anions and enriched together with [C₈C₁Im]⁺ cations at the IL/vacuum interface. Upon further heating, [dema][PF₆] and [OMIm][PF₆] desorb selectively at ∼235 and ∼380 K, respectively. Hereby, a wetting layer of pure [C₈C₁Im][TfO] is formed by on-surface metathesis at the IL/metal interface, which completely desorbs at ∼480 K. For comparison, ion enrichment at the vacuum/IL interface was also studied in macroscopic IL mixtures, where no influence of the solid support is expected.     

Tailoring the Self-assembly of Melamine on Au (111) via Doping with Cu Atoms

The doping effect of Cu on the self-assembly film of melamine on an Au(111) surface has been investigated with scanning tunneling microscopy (STM). The evaporated Cu adatoms occupy the positions underneath the amino groups and change the hydrogen bonding pattern between the melamine molecules. Accordingly, the self-assembly structure has changed stepwise from a well-defined honeycomb into a track-like and then a triangular structure depending on the amount of Cu adatoms. The interaction between Cu adatom and melamine is moderate thus the Cu adatoms can be released upon mild heating to around 100℃. These findings are different from previous observations of either the coordination assembly or the physically trapped metal adatoms.     

Structure and growth of oxides on polycrystalline nickel surfaces

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O₂) at 25 and 300 °C and pressures near 130 Pa, was studied using X‐ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni²⁺) and trivalent (Ni³⁺) species could be distinguished using Ni 2p spectra, while surface adsorbed O₂ and atomic oxygen (O) species could be differentiated from bulk oxide (O^2?) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES^?, and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 °C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 °C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 °C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses of O₂ at 25 and 300 °C. The thickness of oxide was not strongly localized on this scale. However, the QUASES^? analysis suggests that there is localized growth on a nanometric scale—the result of island formation. Copyright © 2007 John Wiley & Sons, Ltd.