Selective formation of cumulative double bonds (C[double bond]C[double bond]N) in the attachment of multifunctional molecules on Si(111)-7 x 7

The cumulative double bond (C[double bond]C[double bond]N), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acrylonitrile to Si(111)-7 x 7. The covalent binding of acrylonitrile on Si(111)-7 x 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM) and DFT calculations. The observation of the characteristic vibrational modes and electronic structures of the C[double bond]C[double bond]N group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal C and N atoms of acrylonitrile and the neighboring adatom-rest atom pair, consistent with the prediction of DFT calculations. STM studies further show the preferential binding of acrylonitrile on the center adatom sites of faulted halves of Si(111)-7 x 7 unit cells.     

Detection of C-Si covalent bond in CH3 adsorbate formed by chemical reaction of CH3MgBr and H:Si(111)

High-resolution electron energy loss spectroscopy (HREELS) yielded evidence for the formation of single covalent bonds between Si(111) surface atoms and CH(3) groups from the reaction of CH(3)MgBr and hydrogen-terminated H:Si(111)(1 x 1). The vibration at 678 cm(-)(1), assigned to the C-Si bond, was isolated within the spectrum of CH(3) on deuterium-terminated D:Si(111)(1 x 1). The CH(3) groups were thermally stable at temperatures below 600 K. The C-Si bonds are essential for enhancing the usefulness of alkyl moieties, which will lead to a new prospective technology of nanoscale fabrication and biochemical application.     

Scanning tunneling microscopy and spectroscopy studies of 4-methyl- 4'-(n-mercaptoalkyl)biphenyls on Au(111)-(1x1).

4-methyl-4'-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n=3-6, BPn) monolayers assembled on Au(111)-(1x1) in 1,3,5,-trimethylbenzene (TMB) at various temperatures are studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images reveal that BP3 and BP5 form a (sqrt 3x2sqrt 3) repeating motif superimposed on a temperature-dependent Moire pattern. BP4 and BP6 adlayers are characterized by a coexisting (2sqrt 3x5sqrt 3) majority phase and a temperature-dependent (3xpsqrt 3) minority phase. Assembly at 60 degrees C or 90 degrees C leads to p=5. Compression of the adlayer was found at higher temperatures. Combined with high-resolution structure experiments, the electronic characteristics of BP3 and BP4 self-assembled monolayers (SAMs) were studied by monitoring current-distance (iT-Deltaz) and current-voltage (iT-Ebias) characteristics in TMB employing a gold STM tip|BPn|Au(111)-(1x1) configuration. The semilogarithmic (iT-Deltaz) plots yielded three linear regions in the range 10 pA相似文献   

Low-temperature STM images of methyl-terminated Si(111) surfaces

Low-temperature scanning tunneling microscopy (STM) has been used to image CH(3)-terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1 x 1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 +/- 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The C-H bonds in each methyl group were observed to be rotated by 7 +/- 3 degrees away from the center of an adjacent methyl group and toward an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms on neighboring methyl groups, and secondary interactions unique to the surface are also evident.     

Binding of glycine and L-cysteine on Si(111)-7 x 7

The adsorption of glycine and l-cysteine on Si(111)-7 x 7 was investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The observation of the characteristic vibrational modes and electronic structures of NH3+ and COO- groups for physisorbed glycine (l-cysteine) demonstrates the formation of zwitterionic species in multilayers. For chemisorbed molecules, the appearance of nu(Si-H), nu(Si-O), and nu(C=Omicron) and the absence of nu(O-H) clearly indicate that glycine and l-cysteine dissociate to produce monodentate carboxylate adducts on Si(111)-7 x 7. XPS results further verified the coexistence of two chemisorption states for each amino acid, corresponding to a Si-NH-CH2-COO-Si [Si-NHCH(CH2SH)COO-Si] species with new sigma-linkages of Si-N and Si-O, and a NH2-CH2-COO-Si [NH2CH(CH2SH)COO-Si] product through the cleavage of the O-H bond, respectively. Glycine/Si(111)-7 x 7 and l-cysteine/Si(111)-7 x 7 can be viewed as model systems for further modification of Si surfaces with biological molecules.     

Self-directed growth of contiguous perpendicular molecular lines on H-Si(100) surfaces

Future nanoscale integrated circuits will require the realization of interconnections using molecular-scale nanostructures; a practical fabrication scheme would need to be largely self-assembling and operate on a large number of like structures in parallel. The self-directed growth of organic molecules on hydrogen-terminated silicon(100) [H-Si(100)] offers a simple method of realizing one-dimensional molecular lines. In this work, we introduce the ability to change the growth direction and form more complex, contiguous shapes. Numerous styrene and trimethylene sulfide L shapes were grown on a H-Si(100)-3x1 surface in parallel with no intermediate surface lithography steps, and similar shapes were also grown using allyl mercaptan and benzaldehyde on H-Si(100)-2x1. Registered scanning tunneling microscopy (STM) images and high-resolution electron energy loss spectroscopy (HREELS) were used to investigate the growth process.     

Surface structure and interface dynamics of alkanethiol self-assembled monolayers on Au(111)

Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimensional c(4 x 2) domain structure. Our results will be very useful for a better understanding of the interface dynamics and stability of sulfur atoms in alkanethiol SAMs on Au(111) surfaces.     

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.     

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.     

Direct and indirect causes of Fermi level pinning at the SiO/GaAs interface

The correlation between atomic bonding sites and the electronic structure of SiO on GaAs(001)-c(2x8)/(2x4) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT). At low coverage, STM images reveal that SiO molecules bond Si end down; this is consistent with Si being undercoordinated and O being fully coordinated in molecular SiO. At approximately 5% ML (monolayer) coverage, multiple bonding geometries were observed. To confirm the site assignments from STM images, DFT calculations were used to estimate the total adsorption energies of the different bonding geometries as a function of SiO coverage. STS measurements indicated that SiO pins the Fermi level midgap at approximately 5% ML coverage. DFT calculations reveal that the direct causes of Fermi level pinning at the SiO GaAs(001)-(2x4) interface are a result of either local charge buildups or the generation of partially filled dangling bonds on Si atoms.     

Nitrate adsorption and reduction on Cu(100) in acidic solution

Nitrate adsorption and reduction on Cu(100) in acidic solution is studied by electrochemical methods, in situ electrochemical scanning tunneling microscopy (EC-STM), surface enhanced Raman spectroscopy (SERS), and density functional theory (DFT) calculations. Electrochemical results show that reduction of nitrate starts at -0.3 V vs Ag/AgCl and reaches maximum value at -0.58 V. Over the entire potential region interrogated adlayers composed of nitrate, nitrite, or other intermediates are observed by using in situ STM. From the open-circuit potential (OCP) to -0.22 V vs Ag|AgCl, the nitrate ion is dominant and forms a (2 x 2) adlattice on the Cu(100) surface while nitrate forms a dominantly c(2 x 2) structure from -0.25 to -0.36 V. The interconversion between the nitrate and nitrite adlattices is observed. DFT calculations indicate that both nitrate and nitrite are twofold coordinated to the Cu(100) surface.     

Long-range self-assembly of a polyunsaturated linear organosilane at the n-tetradecane/Au(111) interface studied by STM

We report on the formation of self-assembled monolayers of 13-(trimethylsilyl)-1-tridecene-6,12-diyne [C13H17-Si(CH3)3], an organosilane derivative with a linear polyunsaturated chain, on Au(111) substrates. Molecular resolution STM images recorded at the liquid-solid interface between gold and tetradecane reveal a long-range and densely packed hexagonal lattice with a ( radical3 x radical3)R30 degrees -like structure commensurate against gold adlattice.     

Scanning tunneling microscopy and spectroscopy of wet-chemically prepared chlorinated Si111 surfaces

Chlorine-terminated Si(111) surfaces prepared through the wet-chemical treatment of H-terminated Si(111) surfaces with PCl5 (in chlorobenzene) were investigated using ultrahigh vacuum scanning tunneling microscopy (UHV cryo-STM) and tunneling spectroscopy. STM images, collected at 77 K, revealed an unreconstructed 1 x 1 structure for the chlorination layer, consistent with what has been observed for the gas phase chlorination of H-terminated Si(111). However, the wet-chemical chlorination is shown to generate etch pits in the Si(111) surface, with an increase in etch pit density correlating with increasing PCl5 exposure temperatures. These etch pits were assumed to stabilize the edge structure through the partial removal of the <112> step edges. Tunneling spectroscopy revealed a nonzero density of states at zero bias. This is in contrast to the cases of H-, methyl-, or ethyl-terminated Si(111), in which similar measurements have revealed the presence of a large conductance gap.     

Fabrication of interconnected 1D molecular lines along and across the dimer rows on the Si(100)-(2 x 1)-H surface through the radical chain reaction

To realize nanoscale wiring in two dimensions (2D), the fabrication of interconnected one-dimensional (1D) molecular lines through the radical chain reaction of alkene molecules (CH2=CH-R) on the H-terminated Si(100)-(2 x 1) surface have been investigated using scanning tunneling microscopy (STM) at 300 K. By the judicious choice of R in the CH2=CH-R molecule and by creating a dangling bond (DB) at a desired point using the STM tip, the perpendicularly connected allyl mercaptan (ALM) and styrene lines have been fabricated on the Si(100)-(2 x 1)-H surface. The continuous growth of the styrene line at the end DB of a growing ALM line (or vice versa) does not occur, perhaps, due to steric hindrance or/and interaction between adsorbed molecules.     

Photoassisted adsorption of allylamine and 1-butene on H:Si(111) studied by surface vibrational spectroscopies

Ultraviolet photoassisted adsorption of terminally double-bonded molecules, allylamine (CH2=CH-CH2-NH2) and 1-butene (CH2=CH-CH2-CH3), on hydrogen-terminated silicon (111) surface was attempted to obtain adsorbates covalently terminating the surface Si atoms. The adsorption process was monitored by high-resolution electron energy loss spectroscopy, multiple internal infrared reflection-absorption spectroscopy, and Auger electron spectroscopy. Allylamine adsorbates emerged upon delivery of allylamine gas under ultraviolet irradiation. The N-H bonds in allylamine were evidenced to survive over the photoadsorption process by vibrational analysis and by the reaction with ketene. CH3- groups were detected at low coverage, indicating anchoring of the organic moieties by the secondary (sec-) type carbon atoms, which were taken over by the primary (n-) type with increasing coverage. C-D bonds were detected after deposition on deuterium-terminated Si(111) upon incorporation of Si-terminating H into the hydrocarbon part of adsorbates. In the case of 1-butene, not only the C=C end but also the CH3- end of a molecule might attach on Si, resulting in emergence of adsorbates composed of CH2 groups. The newly obtained adsorbates are prospective as a material applied for nanolithography, fine electrochemistry, and nano-biotechnology.     

STM/STS observation of polyoxoanions on HOPG surfaces: the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36-

A combination of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques have been performed on the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36- deposited on highly oriented pyrolytic graphite surfaces. Small, regular molecule clusters, as well as separated single molecules, were observed. The size of the molecules is in agreement with the data determined by X-ray crystallography. In STS measurements, we found a rather large contrast at the expected location of the Cu metal centers in our molecules, i.e., the location of the individual Cu ions in their organic matrix is directly addressable by STS.     

Self-assembled monolayers of cobalt(II)- (4-tert-butylphenyl)-porphyrins: the influence of the electronic dipole on scanning tunneling microscopy images

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70 degrees with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.     

A [4+2]-like cycloaddition of methyl methacrylate on Si(100)-2 x 1

The attachment of methyl methacrylate (MMA) on Si(100)-2x1 was investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. The HREELS spectra of chemisorbed MMA show the disappearance of characteristic vibrations of C=O (1725 cm(-1)) and C(sp(2))-H (3110, 1400, and 962 cm(-1)) coupled with the blue shift of the C=C stretching mode by 34 cm(-1) compared to those of physisorbed molecules. These results clearly demonstrate that both C=C and C=O in MMA directly participate in the interaction with the surface to form a SiCH(2)C(CH(3))=C(OCH(3))OSi species via a [4+2]-like cycloaddition. This binding configuration was further supported by XPS, UPS, and DFT studies.     

Adsorption of atomic hydrogen on ZnO(1010): STM study

The adsorption of atomic hydrogen on a single crystal ZnO(1010) surface has been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions at room temperature and at elevated temperatures. High resolution STM images indicate that a well-ordered (1x1) H adlayer is formed on the ZnO(1010) surface. The STM data strongly indicate that the hydrogen adsorbs on top of the oxygen atoms forming hydroxyl species. Scanning tunneling spectroscopy (STS) studies reveal a H atom induced metallization at room temperature. In contrast to the clean surface for the hydrogen-covered surface distinct defects structures consisting of missing O and Zn atoms could be identified.     

Controlled fabrication of 1D molecular lines across the dimer rows on the Si(100)-(2 x 1)-H surface through the radical chain reaction

Current interest in the fabrication of organic nanostructures on silicon surface is focused on the self-directed growth of 1D molecular lines with predefined position, structure, composition, and the length on the H-terminated Si(100)-(2 x 1) surface. To date, no studies have succeeded in growing the molecular line across the dimer rows on Si(100)-(2 x 1)-H, which is highly desirable. Using scanning tunneling microscopy (STM), we studied a new molecular system (allyl mercaptan, CH2=CH-CH2-SH) that undergoes chain reaction across the dimer row on the Si(100)-(2 x 1)-H surface at 300 K and produces covalently bonded 1D molecular lines. In combination with the previous findings of chain reaction along the rows, the present observations of self-directed growth of 1D molecular lines across the dimer rows on the Si(100)-(2 x 1)-H surface provide a means to connect any two points (through molecular lines) on a 2D surface.