Using Ultrahigh Vacuum Scanning Tunneling Microscopy Research Articles

VO Cluster-Stabilized H2O Adsorption on a TiO2 (110) Surface at Room Temperature.

We probe the adsorption of molecular H2O on a TiO2 (110)-(1 × 1) surface decorated with isolated VO clusters using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and temperature-programmed desorption (TPD). Our STM images show that preadsorbed VO clusters on the TiO2 (110)-(1 × 1) surface induce the adsorption of H2O molecules at room temperature (RT). The adsorbed H2O molecules form strings of beads of H2O dimers bound to the 5-fold coordinated Ti atom (5c-Ti) rows and are anchored by VO. This RT adsorption is completely reversible and is unique to the VO-decorated TiO2 surface. TPD spectra reveal two new desorption states for VO stabilized H2O at 395 and 445 K, which is in sharp contrast to the desorption of water due to recombination of hydroxyl groups at 490 K from clean TiO2(110)-(1 × 1) surfaces. Density functional theory (DFT) calculations show that the binding energy of molecular H2O to the VO clusters on the TiO2 (110)-(1 × 1) surface is higher than binding to the bare surface by 0.42 eV, and the resulting H2O-VO-TiO2 (110) complex provides the anchor point for adsorption of the string of beads of H2O dimers.

Directing On-Surface Reaction Pathways via Metal-Organic Cu-N Coordination.

We demonstrate an enhancement of cyclization against polymerization for the on-surface debrominative coupling reaction, by using the metal-organic Cu-N coordination template to direct the reaction pathways. Experiments performed by using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM), with the substitution of metal-coordination centers, metallic substrates and functional organic ligands, corroborate the template effect of the Cu-N coordination.

Chain-Length-Dependent Reactivity of Alkanethiolate Self-Assembled Monolayers with Atomic Hydrogen

Gas–surface interactions are some of the most important yet complex chemical processes to occur, as they intrinsically involve many-body phenomena across a wide spectrum of energies and length scales. To understand these complicated interfacial interactions, we often use model systems such as thiolate self-assembled monolayers (SAMs) to study phenomena like reactivity and passivation, as these systems afford fine control over the surface parameters governing the events in question. In this study, we examine the effect of chain length on the reactivity of alkanethiolate SAMs with atomic hydrogen by monitoring morphological surface evolution throughout the reaction. These spatiotemporal data were obtained using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with directed in situ atomic hydrogen dosing. For a series of alkanethiolate SAMs 8- to 11-carbons long, we find that small increases in chain length cause disproportionately large decreases in reactivity. These reaction trends led us to develop a kinetic model characterized by two rate constants: a slow rate for hydrogen reactivity with close-packed domains, which is chain-length dependent, and a fast rate for reactivity with low-density regions, which is the same for all samples examined. In addition to reaction rates, we also tracked chain-length-dependent changes in surface morphology, notably how the size and shape of the SAMs’ etch pits evolved following hydrogen exposure. Few differences were observed in the 10C and 11C samples, while there was a significant increase in the mean etch pit area of the 8C and 9C SAMs. Overall, this study provides important quantitative insights into how surface packing and dynamic disorder of organic thin films can influence their passivation capabilities.

Absence of a Band Gap at the Interface of a Metal and Highly Doped Monolayer MoS2.

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS2 using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by two-dimensional metal-induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the MoS2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ∼0.55 nm near midgap to as long as 2 nm near the band edges and are nearly identical for Au, Pd, and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.

Effect of low-temperature-grown GaSb layer on formation of high-density and small GaSb islands on Si(100) substrate

The effect of a low-temperature-grown gallium antimonide (LT-GaSb) layer on the formation of high-density GaSb islands on a Si(100) substrate was studied using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy. By using an LT-GaSb layer in the initial growth stage (new growth method), high-density and small GaSb islands were formed at 500 °C regardless of the type of the reconstructed surface. The density and size of the GaSb islands with the LT-GaSb layer were estimated to be ∼1.0 × 1011 cm−2 and ∼20 nm, respectively. The STM results showed that the LT-GaSb layer was composed of an Sb-rich amorphous GaSb layer. With a rapid increase in temperature under the Sb flux, the LT-GaSb layer immediately changed into GaSb nuclei with a high density (2.1 × 1011 cm−2) and small size (11 nm). It is suggested that these high-density and small GaSb islands grow from the GaSb nuclei, which act as crystal seeds.

Substrate Level Control of the Local Doping in Graphene

Graphene exfoliated onto muscovite mica is studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) techniques. Mica provides an interesting dielectric substrate interface to measure the properties of graphene due to the ultraflat nature of a cleaved mica surface and the surface electric dipoles it possesses. Flat regions of the mica surface show some surface modulation of the graphene topography (24 pm) due to topographic modulation of the mica surface and full conformation of the graphene to that surface. In addition to these ultraflat regions, plateaus of varying size having been found. A comparison of topographic images and STS measurements show that these plateaus are of two types: one with characteristics of water monolayer formation between the graphene and mica, and the other arising from potassium ions trapped at the interfacial region. Immediately above the water induced plateaus, graphene is insulated from charge doping, while p-type doping is observed in areas adjacent to these water nucleation points. However, above and in the neighborhood of interfacial potassium ions, only n-type doping is observed. Graphene regions above the potassium ions are more strongly n-doped than regions adjacent to these alkali atom plateaus. Furthermore, a direct correlation of these Fermi level shifts with topographic features is seen without the random charge carrier density modulation observed in other dielectric substrates. This suggests a possible route to nanoscopic control of the local electron and hole doping in graphene via specific substrate architecture.

Formation of Rectangular Packing and One-Dimensional Lines of C60 on 11-Phenoxyundecanethiol Self-Assembled Monolayers on Au(111)

The behavior of C(60) molecules deposited onto 11-phenoxyundecanethiol (phenoxy) self-assembled monolayers (SAMs) is studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and spectroscopy. We observe that after thermally annealing between 350 and 400 K in vacuum a combination of hexagonally close-packed islands, rectangularly packed islands, and isolated single lines of C(60) is observed when the C(60) is initially deposited on an unannealed phenoxy SAM. However, only rectangularly packed islands are found when they are deposited on a preannealed phenoxy SAM. We determine the rectangular packing to have a (2√3 × 4) rectangular unit cell with respect to the underlying Au(111) substrate. This type of C(60) structure has not been observed previously for multicomponent self-assemblies on a surface. We discuss the possible causes for the formation of this structure as well as the differences between starting on an unannealed SAM and an annealed one. This study demonstrates the capability of functionalized alkanethiol SAMs to control the growth and structure of C(60) islands during annealing depending on the structural changes of the SAM itself; by preannealing the SAM, the motion of the C(60) can be confined and unique structures resulting from interactions between the SAM molecules and C(60) can be produced.

Thermal-Processing-Induced Structural Dynamics of Thiol Self-Assembly in Solution

Using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), we studied the thermal-processing-induced structural changes that occur during formation of a 1-octanethiol (OT) self-assembled monolayer (SAM) on Au(111) near melting temperature in an OT solution. A favorable ordered phase of a c(4√3 × 2√3) structure was achieved at high temperature (e.g., 343 K) below the melting temperature of (e.g., 353 K) alkanethiol SAMs, while a favorably ordered phase of a (√3 × √3) structure was achieved at the melting temperature. The high resolution STM observation indicated the following: (1) the growth process of OT SAMs in an OT solution induced a change in the structural phase via diffusion of OT molecules on gold by thermal energy at a high temperature below the melting temperature; (2) application of the melting temperature resulted in partial desorption of OT molecules from the surface due to melting of OT SAMs, showing a striped phase and a disordered phase; and (3) after melting of the SAM, time-dependent rearrangement of OT molecules adsorbed on Au(111) occurred through relaxation of surface Au atoms, which caused thiolate–gold complexes to diffuse and reform a (√3 × √3) phase. Furthermore, the ultimate structural transition to a c(4√3 × 2√3) structure at 343 K revealed the translational transition of molecular adsorption sites induced by lateral movements of OT–Au complexes at a high temperature in a solution. Also, the absence of domain boundaries among the three mobile phases (i.e., disordered, striped, and ordered (√3 × √3)) at 353 K revealed that there is no transition of molecular adsorption sites after melting of the SAM.

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.

A study on conformational changes by electron charges in viologen single molecules by using STM

The topography of self-assembled viologen derivatives (VC 8SH, VC 10SH, HSC 8VC 8SH, and HSC 10VC 10SH) molecules on an octanethiol (C 8) self-assembled monolayer (SAM) modified gold surface was measured using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We demonstrate here a novel matrix SAM appropriate for isolation of the viologen molecules. The C 8 was used for a matrix SAM, in which the VC 8SH, VC 10SH, HSC 8VC 8SH, and HSC 10VC 10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights (VC 8SH: 1.53 nm, VC 10SH: 2.01 nm, HSC 8VC 8SH: 2.71 nm, and HSC 10VC 10SH: 3.3 nm) of the molecular protrusions using STM. Also, changes in the central axis of viologen molecules were observed as VC 8SH (0.5–0.73 nm), VC 10SH (0.4–0.74 nm), HSC 8VC 8SH (0.67–0.84 nm), and HSC 10VC 10SH (0.67–0.99 nm), respectively.

Dissociative Chemisorption of Methanol on Ge(100)

The adsorption of methanol (CH 3 OH) on Ge(100) surface has been studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and density functional theory (DFT) calculations. At low coverages, high-resolution experimental STM shows that methanol undergoes O-H bond dissociative adsorption on a single Ge-Ge dimer. As the methanol coverage increases to saturation, a series of dissociative adsorptions of methanol results in the formation of dimer row-based chainlike arrays. The DFT calculations show that, at room temperature, O-H bond dissociative adsorption is kinetically more favorable than C-O bond dissociative adsorption, although the final product of C-O bond dissociative adsorption is more stable geometry thermodynamically. From the results of our STM experiments and DFT calculations, we conclude that the adsorption structure of methanol on Ge(100) at room temperature has a H-Ge-Ge-OCH 3 geometry as a result of O-H bond dissociative adsorption on a single dimer.

Study on NDR property of nitro-Benzene SAMs by using UHV-STM

Abstract Recently, research on conducting molecules containing thiol functional groups, such as benzenethiol has been progressing. This conducting molecule is applicable to the study of NDR and switching properties of logic devices. The 4,4′-di(ethynylphenyl)-2′-nitro-1-(thioacetyl)benzene molecule contains a thiol functional group as in benzenethiol. Thus, we measured the property of NDR by using the self-assembly method in STM. Au substrate was exposed to a 1 mM solution of 1-dodecanethiol in ethanol for 24 h to form a monolayer. After thoroughly rinsing the sample, it was exposed to a 0.1 mM solution of 4,4′-di(ethynylphenyl)-2′-nitro-1-(thioacetyl)benzene in dimethylformamide (DMF) for 30 min and kept in the dark during immersion to avoid photo-oxidation. After assembly, we measured the electrical properties of self-assembly monolayers (SAMs) by using ultra-high-vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy (STS) [L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones ll, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705]. As a result, we confirmed the property of NDR in a positive region between negative regions. The energy gap ( E g ) obtained by using differential conductance (d I /d V – V ) was verified through UV/visible. This molecule is applicable to the fabrication of molecular junctions.

Study on electrical conduction of viologen derivatives using scanning tunneling microscopy

The electrical conduction of self-assembled monolayers (SAMs) made from viologen derivatives was measured using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with a focus on the molecular structural effect on the electrical conduction. For viologen derivative SAMs, resistances through the monolayers increased exponentially with increases in molecular length when the decay constants of transconductance β were ca. 0.35 to 0.48 nm − 1 . The estimated monolayer resistances of the viologen derivatives such as N-methyl- N′-(10-mercaptodecyl)-4,4′-bipyridinium (VC 10SH), N-methyl- N′-di(8-mercaptooctyl)-4,4′-bipyridinium (HSC 8VC 8SH), and N-methyl- N′-di(10-mercaptodecyl)-4,4′-bipyridinium (HSC 10VC 10SH) SAMs were 1.3 × 10 8 Ω, 3.6 × 10 9 Ω, and 3.8 × 10 9 Ω, respectively.

Determination of the NDR and Electron Transport Properties of Self-Assembled Nitro-Benzene Monolayers Using UHV-STM

We investigated the negative differential resistance (NDR) property of self-assembled 4,4-di(ethynylphenyl)-2'-nitro-l-(thioacetyl)benzene (nitro-benzene), which has been well known as a conducting molecule [1]. Self-assembly mono layers (SAMs) were prepared on Au (111), which had been thermally deposited onto pre-treated (H₂SO₄: H₂O₂=3:1) Si. The Au substrate was exposed to a 1 mM solution of l-dodecanethiol in ethanol for 24 hours to form a monolayer. After thorough rinsing of the sample, it was exposed to a 0.1 μM solution of nitro-benzene in dimethylformamide (DMF) for 30 min and kept in the dark during immersion to avoid photo-oxidation. Following the assembly, the samples were removed from the solutions, rinsed thoroughly with methanol, acetone, and CH₂Cl₂, and finally blown dry with N₂. Under these conditions, we measured the electrical properties of SAMs using ultra high vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy (STS) [2]. As a result, we confirmed the properties of NDR in between the positive and negative region.

Multiple Two-Dimensional Structures Formed at Monolayer and Submonolayer Coverages of p-Sexiphenyl on the Au(111) Surface

The two-dimensional structures formed by monolayers and submonolayers of p-sexiphenyl (p-6P) molecules evaporated onto the Au(111) surface are investigated using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Five different 2D structures corresponding to different surface coverages are discovered and their 2D structures solved. The trends in the molecular alignment with respect to the underlying gold lattice are discussed. An unusual structure that consists of paired rows of p-6P molecules was discovered. A surface structure with alternating domains of slightly differently packed p-6P molecules was also found. The boundary between these two domains contains systematic molecular vacancies.

Investigation of negative differential resistance properties of self-assembled dipyridinium using STM

Recently, research on conducting molecules containing thiol functional groups such as benzenethiol has been progressing [X. Xiao, B. Xu, N.J. Tao, Nano Lett. 4 (2004) 267]. This conducting molecule is applicable to the study of the negative differential resistance (NDR) and switching properties of logic device. The 4-{4[4-(4-{1-[4-(4-acetylsulfanyl-phenylethynyl)-phenyl]-2,6-diphenyl-pyridinium-4-yl}-phenyl)-2,6-diphenyl-pyridinium-1-yl]-phenylethynyl}-phenylthioacetate (dipyridinium) molecule contains thiol functional groups such as benzenethiol. Thus, we have studied an NDR property of a dipyridinium molecule using the self-assembly method in scanning tunneling microscopy (STM). The Au substrate was exposed to a 1 mM solution of 1-dodecanethiol in ethanol for 24 h to form a monolayer. After thorough rinsing of the sample, it was exposed to a 0.1 μM solution of dipyridinium in dimethylformamide (DMF) for 30 min. After the assembly, we measured the electrical properties of the self-assembly monolayers (SAMs) using ultra high vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy (STS). As a result, we confirmed the properties of NDR in a negative region at −1.67 V and a positive region at 1.78 V. The energy gap ( E g) was found to be 3.12 eV [C. Arena, B. Kleinsorge, J. Robertson, W.I. Milne, M.E. Welland, J. Appl. Phys. 85 (1999) 1609]. This molecule is applicable to the fabrication of molecular junctions.

Growth of copper phthalocyanine on hydrogen passivated vicinal silicon(111) surfaces

Using ultra-high vacuum scanning tunneling microscopy (UHV-STM), we show that copper-phthalocyanine (CuPc) grows in a well ordered manner on hydrogen passivated vicinal silicon surfaces. CuPc grows one-dimensionally parallel to the monatomic steps on the vicinal silicon surface. Surprisingly, elongated clusters of the CuPc parallel to the step directions are formed even on the middle of the terraces well away from the step edges. The one-dimensional growth mode continues even after the full monolayer coverage on the substrate which results in strongly oriented growth mode of a thin film of CuPc on the vicinal silicon surfaces.

Epitaxial relation and island growth of perylene-3.4.9.10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(1 1 1) substrate

Surface of an off-cut Si(1 1 1) substrate with an average step distance of 100 Å was terminated with monohydride. On this substrate, thin film crystals of organic semiconductor PTCDA were grown by molecular beam epitaxy (MBE). Surface of the thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H-Si(1 1 1)) were observed using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy (AFM). The PTCDA thin film crystals had an island shape of Volmer–Weber type. From the direction of the vicinal steps of the substrate [1 1 ̄ 0] , we determined the size and orientation of the PTCDA thin film crystals on the H-Si(1 1 1). Long-axis of the two-dimensional (2D) unit cell of the thin film crystals matches the vector (6, 2) of the H-Si(1 1 1) surface. We proposed two conceivable epitaxial relations: one is 6 2 2/3 11/3 and the other is point-on-line coincidence. 2D unit cells of the thin film crystals have widely stretched structure (2–10%), and the island growth of Volmer–Weber type is probably due to this large lattice misfit.

An atomic structural model of ( [formula omitted])- R30° reconstruction proposed for 3C–SiC(1 1 1) crystallized islands on Si(1 1 1) by C 60 precursor

The surface structures of 3C–SiC(1 1 1) crystallized islands formed by thermal chemical reaction between silicon substrate and adsorbed C 60 molecules at high temperatures are studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). The STM images show a strong bias voltage dependence, transforming between (2×2), (2 3 ×2 3 ) - R30° and Y-type patterns under different bias voltages. A surface atomic structural model containing quarter coverage of vacancies and three different sites of surface silicon atoms is proposed to interpret the experimental observations. The dangling bond electronic states contributed by these different site Si atoms on the outermost surface layer and by the carbon atoms surrounding the vacancies are suggested to be responsible for the different tunneling conditions under different bias voltages in STM images.

Scanning Tunneling Microscopy Study of Formate Species Synthesized from CO2 Hydrogenation and Prepared by Adsorption of Formic Acid over Cu(111)

The adsorption structure of formate species on Cu(111), prepared by two different methods, was studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), that is, the synthesis by the hydrogenation of CO2 at atmospheric pressure and the adsorption of formic acid on an oxygen-precovered Cu(111). Linear chains of formate molecules were imaged by the hydrogenation of CO2 by STM at low formate coverage with the distance to the nearest neighbor of the formate species estimated to be 5.0 ± 0.2 Å, twice that of the nearest Cu−Cu neighbor. The adsorption phase of formate thus grows linearly at the initial stage by an anisotropic attractive interaction between the formate species. The ordered structure of the formate species changed in the order of p(2 × 4), c(2 × 8), (7 × 7), p(2 × 3), (5 × 5), and c(2 × 4) with increasing formate coverage, indicating that various ordered structures appeared corresponding to the small change in the formate coverage. All the formate structures, except for (7 × 7) and (5 × 5), consisted of the same chain as observed at low coverage, with the distance between the formate chains decreasing as the formate coverage increased. A repulsive interaction thus acts between the formate chains, resulting in a lowering of the adsorption energy of the formate species as reported in the literature. On the other hand, (4 × 4) and (3 × 7/2) structures were observed upon exposing the Cu(111) surface to formic acid at 2 and 15 L, respectively, showing no chain structure of the formate species. At low formate coverage, no molecular image was observed, indicating that the formate species diffuses more rapidly than the scanning of the STM tip. Thus, the adsorption structure of formate on Cu(111) was found to be different depending on the preparation method. It is suggested that the difference in the adsorption structure strongly influences the rate constant of the previously reported formate decomposition.