Ultrahigh Vacuum Scanning Tunneling Research Articles

Initial Stages of Platinum Silicide Formation on Si(110) Studied by Scanning Tunneling Microscopy

Initial stages of formation of platinum (Pt) silicide have been studied by ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Stranski–Krastanov growth mode has been observed for the Pt silicide. Layer-by-layer growth occurs at submonolayer coverages resulting in four different surface structures. At coverages more than one monolayer (ML) Pt silicide islands start to nucleate. The morphology of the silicide is represented by separated islands or very long nanowires depending on preparation technique. The islands are supposed to grow endotaxially with a large top facet corresponding to PtSi(101) surface.

Carbon nanotubes on partially depassivatedn-dopedSi(100)−(2×1):Hsubstrates

We present a study on the mechanical configuration and the electronic properties of semiconducting carbon nanotubes supported by partially depassivated silicon substrates, as inferred from topographic and spectroscopic data acquired with a room-temperature ultrahigh vacuum scanning tunneling microscope and density-functional theory calculations. A mechanical distortion and doping for semiconducting carbon nanotubes on Si(100)-(2x1):H with hydrogen-depassivated stripes up to 100 Angstrom wide are ascertained from both experiment and theory. The results presented here point towards novel and local functionalities of nanotube-semiconductor interfaces.

Control and Characterization of Cyclopentene Unimolecular Dissociation on Si(100) with Scanning Tunneling Microscopy

Dissociation of individual cyclopentene molecules on the Si(100) surface is induced and investigated using cryogenic ultrahigh vacuum scanning tunneling microscopy (STM). Using a secondary feedback loop during elevated tunneling current and sample biasing conditions, the cyclopentene dissociation products are isolated and then characterized with atomic-scale spatial resolution. Using multibias STM and density functional theory, the cyclopentene dissociation products are shown to consist of a C(5)H(7) fragment and an individual H atom. The C(5)H(7) fragment contains a C=C double bond and is bound to the Si(100) surface via a single Si-C covalent bond, while the individual H atom can be induced to hop between two sites on a single silicon dimer with the STM tip. This study shows that the use of feedback control during STM-induced single molecule reactions allows transient reaction products to be captured and thus more thoroughly studied. While demonstrated here for cyclopentene on Si(100), this feedback-controlled approach can likely be applied to a wide array of chemical reactions on semiconductor surfaces.

超高真空電界放射型電子顕微鏡(UHV-FE-TEM)・走査トンネル顕微鏡(UHV-STM)複合システム

超高真空電界放射型電子顕微鏡(UHV-FE-TEM)・走査トンネル顕微鏡(UHV-STM)複合システム

Room-temperature molecular-resolution characterization of self-assembled organic monolayers on epitaxial graphene

Graphene, a two-dimensional sheet of carbon atoms, is a promising material for next-generation technology because of its advantageous electronic properties, such as extremely high carrier mobilities. However, chemical functionalization schemes are needed to integrate graphene with the diverse range of materials required for device applications. In this paper, we report self-assembled monolayers of the molecular semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) formed on epitaxial graphene grown on the SiC(0001) surface. The molecules possess long-range order with a herringbone arrangement, as shown by ultra-high vacuum scanning tunnelling microscopy at room temperature. The molecular ordering is unperturbed by defects in the epitaxial graphene or atomic steps in the underlying SiC surface. Scanning tunnelling spectra of the PTCDA monolayer show distinct features that are not observed on pristine graphene. The demonstration of robust, uniform organic functionalization of epitaxial graphene presents opportunities for graphene-based molecular electronics and sensors.

Atomic-Scale Templates Patterned by Ultrahigh Vacuum Scanning Tunneling Microscopy on Silicon

The ultrahigh vacuum (UHV) scanning tunneling microscope (STM) enables patterning and characterization of the physical, chemical, and electronic properties of nanostructures on surfaces with atomic precision. On hydrogen-passivated Si(100) surfaces, selective nanopatterning with the STM probe allows the creation of atomic-scale templates of dangling bonds surrounded by a robust hydrogen resist. Feedback-controlled lithography, which can remove a single hydrogen atom from the Si(100):H surface, demonstrates high-resolution nanopatterning. The resulting patterns can be used as templates for a variety of materials to form hybrid silicon nanostructures while maintaining a pristine background resist. The versatility of this UHV-STM nanolithography approach has led to its use on a variety of other substrates, including alternative hydrogen-passivated semiconductor surfaces, molecular resists, and native oxide resists. This review discusses the mechanisms of STM-induced hydrogen desorption, the postpatterning deposition of molecules and materials, and the implications for nanoscale device fabrication.

A low-temperature high resolution scanning tunneling microscope with a three-dimensional magnetic vector field operating in ultrahigh vacuum

We present a low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope setup with a combination of a superconducting solenoid coil and two split-pair magnets, providing a rotatable magnetic field up to 500 mT applicable in all spatial directions. An absolute field maximum of B=7 T(3 T) can be applied perpendicular (parallel) to the sample surface. The instrument is operated at a temperature of 4.8 K. Topographic and spectroscopic measurements on tungsten carbide and indium antimonide revealed a z-noise of 300 fm(pp), which barely changes in magnetic field. The microscope is equipped with a tip exchange mechanism and a lateral sample positioning stage, which allows exact positioning of the tip with an accuracy of 5 microm prior to the measurement. Additional contacts to the sample holder allow, e.g., the application of an additional gate voltage. The UHV part of the system contains versatile possibilities of in situ sample and tip preparation as well as low-energy electron diffraction and Auger analysis.

Translational transitions at domain boundaries in octanethiol monolayers on Au(1 1 1)

Systematic studies of domain boundaries of self-assembled monolayers of octanethiols on (1 1 1) oriented gold surfaces have been performed by ultrahigh vacuum scanning tunnelling microscopy (STM). The monolayers consist of domains that exhibit the c ( 4 × 2 ) superstructure of the hexagonal ( 3 × 3 ) R 30 ∘ structure of alkanethiols on gold. By high-resolution images domain boundaries are displayed with molecular resolution. These are used to deduce the exact relation between the adsorption sites of the molecules belonging to different domains on the gold substrate. A translational transition is shown here in detail and it is demonstrated that hexagonal close-packed (hcp), and face-centred cubic (fcc) triple hollow sites are occupied similarly by octanethiols.

STM images of a large organic molecule adsorbed on a bare metal substrate or on a thin insulating layer: Visualization of HOMO and LUMO

An isomer of the methylterrylene molecule was adsorbed both on Cu(111) and on a NaCl bilayer deposited on Cu(111) and imaged by ultra high vacuum scanning tunneling microscopy at low temperature (5 K). On the bare metal surface, the STM images do not reveal any intramolecular resolution and do not depend on the applied tunnel bias. On the contrary, the images acquired at specific bias voltages for the molecule on the salt layer show a striking similarity with the spatial distribution of the electronic probability density in the highest occupied molecular orbital (HOMO) and in the lowest unoccupied molecular orbital (LUMO) of free methylterrylene. They are well reproduced by elastic scattering quantum chemistry calculations. These data provide a direct view of the hyperconjugative interaction between the methyl group and the frontier orbitals of terrylene.

In situ characterization of initial growth of HfO2

The initial growth of HfO2 on Si (111) is monitored in situ by ultrahigh vacuum (UHV) scanning probe microscopy. UHV scanning tunneling microscopy and UHV atomic force microscopy reveal the topography of HfO2 films in the initial stage. The chemical composition is further confirmed by x-ray photoelectron spectroscopy. Scanning tunneling spectroscopy is utilized to inspect the evolution of the bandgap. When the film thickness is less than 0.6 nm, the bandgap of HfO2 is not completely formed. A continuous usable HfO2 film with thickness of about 1.2 nm is presented in this work.

Low-flux elucidation of initial growth of Ge clusters deposited onSi(111)−7×7observed by scanning tunneling microscopy

Deposition of Ge on $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ under very low Ge flux is examined using ultrahigh vacuum scanning tunneling microscopy; Ge atoms are deposited at $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ under a flux of $\ensuremath{\sim}0.005$ or 0.05 ML/min. Initially Ge atoms are substituted for Si atoms on corner adatom sites of faulted half unit cells. At a Ge coverage of 0.08 ML under the lower flux, hollow-centered hexagonal Ge clusters with six protrusions are formed preferentially on faulted half unit cells, which are uniform and separated from other clusters. At the higher flux various types of clusters grow, frequently neighboring with others. This indicates that the low flux is needed to elucidate the stable type of Ge clusters grown on $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$.

Nanofabrication of PTCDA molecular chains on rutileTiO2(011)-(2 × 1) surfaces

The adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on a rutileTiO2(011)-(2 × 1) surface is studied using ultra-high vacuum scanning tunneling microscopy. Theself-assembly process is dominated by the fine interplay between the lateral intermolecularinteractions and the binding to the substrate. By means of temperature-induced change inthe adsorption configuration and the activation of diffusion, the molecules are assembledinto one-dimensional chains oriented along the crystallographic direction.

Understanding the disorder of the DNA base cytosine on the Au(111) surface

Using ultrahigh vacuum scanning tunneling microscopy (STM) and ab initio density functional theory, we have investigated in detail structures formed by cytosine on the Au(111) surface in clean ultrahigh vacuum conditions. In spite of the fact that the ground state of this DNA base on the surface is shown to be an ordered arrangement of cytosine one-dimensional branches (filaments), this structure has never been observed in our STM experiments. Instead, disordered structures are observed, which can be explained by only a few elementary structural motifs: filaments, five- and sixfold rings, which randomly interconnect with each other forming bent chains, T junctions, and nanocages. The latter may have trapped smaller structures inside. The formation of such an unusual assembly is explained by simple kinetic arguments as a liquid-glass transition.

Chain-Length Effects on the Self-Assembly of Short 1-Bromoalkane and n-Alkane Monolayers on Graphite

The structural properties of self-assembled monolayers of short 1-bromoalkanes and n-alkanes on graphite were investigated by a combination of ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at 80 K and theoretical methods. STM images of 1-bromohexane reveal a lamellar packing structure in which the molecules form a 57° ± 3° lamella-molecular backbone angle and a head-to-head assembly of the bromine atoms (Muller, et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5315). STM images of 1-bromoheptane also show a head-to-head 60° ± 3° lamella-molecular backbone pattern; however, the molecules pack in a herringbone structure. The odd/even chain-length alternation in the monolayer morphologies of 1-bromoalkanes is similar to that observed for the self-assembly of short n-alkanes on graphite, suggesting that the bromine atom acts effectively as an extension of the carbon backbone. The analogy, however, is incomplete. Odd and even short n-alkanes (hexane, heptane, octane) display 60° herringbone and recta...

Self-assembly of rubrene on Cu(111)

We performed an ultra-high vacuum scanning tunneling microscopy (STM) investigation ofthe self-assembly of rubrene at room temperature on Cu(111), a metal surface withthreefold symmetry. Rubrene self-assembles into two different structures called row andtrimer. Both are different than the structures already observed on Cu(110) and Cu(100).Row and trimer structures have comparable molecular packing densities and are equallydistributed across the surface. In the row structure the molecules are oriented withtheir backbone along the same high symmetry directions of the surface: [], [] or []. The trimer structure is composed of units of three rubrene molecules, oriented along thehigh symmetry surface directions. These units are chiral, as revealed by height profilemeasurements by STM, and self-assemble in domains containing only one type ofenantiomer.

Orthogonal Self-Assembly of Interconnected One-Dimensional Inorganic and Organic Nanostructures on the Si(100) Surface

Orthogonal, interconnected inorganic and organic one-dimensional nanostructures have been fabricated by parallel self-assembly on the Si(100) surface and investigated using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, bismuth nanowires were self-assembled on the clean Si(100)-2 x 1 surface perpendicular to the Si dimer rows, followed by hydrogen passivation of the surrounding Si surface. Styrene molecular chains were then self-assembled on the H-passivated Si(100)-2 x 1 surface to intersect perpendicularly with the Bi nanowires. This general approach can likely be applied to the wide range of inorganic and organic species that spontaneously form one-dimensional nanostructures on the Si(100) surface.

Scanning Tunneling Microscopy Imaging and Spectroscopy of a Single Viologen Molecule

A low-temperature ultrahigh-vacuum scanning tunneling microscope (UHV-STM) was used to image viologen (N-methyl-N'-di (8-mercaptooctyl)-4,4'-bipyridinium; HSC8VC8SH) molecules and to perform local spectroscopic measurements on these molecules. Self-assembly of viologen molecules was conducted on Au (111), which had been thermally deposited onto freshly cleaved, heated mica. Here, we demonstrate a novel SAM matrix appropriate for the isolation of viologen molecules composed of octanethiol (C8) in which HSC8VC8SH was inserted at defects in the molecular lattice. The isolated single molecules of viologen inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. STM images at 298 K showed protrusions with a topographic height of about 2.71 nm (HSC8VC8SH) with viologen molecules that self-assembled on the substrate. The current-voltage (I-V) characteristics were measured while the electrical properties of the formed monolayer were scanned using scanning tunneling spectroscopy (STS). We found the high peak current-like rectification at +1.14 V (HSC8VC8SH). The rectification ratios, RR = J (at +2.5 V)/J (at -2.5 V), are in the range of 4.47.

On‐Surface Covalent Coupling in Ultrahigh Vacuum

The formation of 2D nanostructures through self-assembly on surfaces is a promising strategy for the fabrication of nanoscale devices by a bottom-up approach. Complex molecular structures held together by weak and reversible van der Waals interactions, hydrogen bonds, and metal complexation have been obtained under ultraclean conditions, namely in an ultrahigh vacuum (UHV). However, such structures are inherently fragile and the intermolecular interactions are weak, which precludes, for example, mechanical stability or intermolecular charge transport. Interconnection of the molecules in a controlled way directly on a surface through robust and irreversible covalent bonding offers a way to overcome these limitations. Such on-surface chemistry under ultraclean conditions potentially presents several advantages over solution synthesis: a) on-surface and UHV experiments allow a much broader range of reaction temperatures to be used: Sublimation cell or substrate temperatures can be easily controlled from 4 to 600 K without risk of air oxidation or solvent decomposition; b) the 2D confined geometry could favor some reactions or supramolecular aggregates that are not usually observed. These can arise as a result of entropic or kinetic effects or through interaction with the substrate; c) it could allow the preparation, from suitable small precursors, of extended 1D or 2D arrays of rigid oligomers or polymers that are impossible to synthesize in solution for solubility reasons; d) on-surface reactions can be followed by UHV scanning tunneling microscopy (STM). This powerful technique not only allows imaging at the submolecular level, but also very local spectroscopic measurements, tip-induced reactions, and molecular manipulation.

Interrupted-flux deposition: Ni on Ru(0001)

Nickel was deposited on Ru(0001) in a vacuum chamber at a temperature of 295 K. The depositions were performed using both a continuous and an interrupted flux of Ni, up to the same coverage of 0.25 monolayer. The growth of Ni on Ru(0001) was studied by ultra-high vacuum scanning tunneling microscopy. It is shown that changing the deposition or the pause duration leads to a change of the dominant growth carriers in the postnucleation period, affecting thus the morphology of the obtained Ni/Ru(0001) surfaces. For a pause duration of 10 s, the saturation cluster density obtained by interrupted-flux deposition was two times higher than one obtained with continuous deposition, and increases with the number of periods. With further increase of the pause duration, the saturation cluster density decreased due to the formation of larger clusters, and might even fall below the value obtained for continuous deposition.

Observation from Scanning Tunneling Microscopy of a Striped Phase for Octanethiol Adsorbed on Au(111) from Solution

A self-assembled monolayer of 1-octanethiol was prepared on a Au(111) surface via liquid-phase adsorption. An investigation of the surface using ultrahigh-vacuum scanning tunneling microscopy revealed a striped phase of the octanethiol molecules under the conditions examined. This phase resembles the well-known "pinstripe" structure of alkanethiols on Au(111), with a registry that is similar to that of the previously observed p x radical3 structures. We discuss the nature of this structure with respect to those that have been observed for other n-alkanethiols.