Variable-temperature Scanning Tunneling Microscopy Research Articles

Organic molecules acting as templates on metal surfaces.

The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.

Variable-temperature scanning tunneling microscopy investigation of CH3ONO adsorbed on Ag([formula omitted

Variable-temperature scanning tunneling microscopy (VT-STM) was used to image the CH3ONO–Ag(111) vacuum–solid interface held at 45, 105 and 160 K. With a few exceptions, atomic resolution is not achieved and only steps and terraces are resolved on the clean substrate. Images taken after dosing CH3ONO at 105 K where multilayers are not formed, reflect the presence of highly mobile two-dimensional (2D) islands. Streaks in these images indicate motion of CH3ONO with the STM tip as it is scanned. Island sizes range from 1×1 nm at low exposures to 10×10 nm at higher submonolayer exposures. The largest islands are generally located at the lower edges of steps. The images taken after lowering the temperature to 45 K show clear evidence for conversion of the 2D fluid-like islands into three-dimensional solids that form as ridges along steps on the Ag(111) surface. The ridges are ∼1 nm wide and 0.1 nm higher than the terrace connected to the upper step edge. Upon rewarming to 105 K, these ridges slowly form 2D islands on terraces. Upon heating to 160 K there is, as expected from temperature programmed desorption, no evidence for adsorbed CH3ONO.

Ultrahigh vacuum instrument that combines variable-temperature scanning tunneling microscopy with Fourier transform infrared reflection-absorption spectroscopy for studies of chemical reactions at surfaces

We describe the construction of an ultrahigh vacuum chamber that incorporates variable-temperature scanning tunneling microscopy (STM), Fourier transform infrared reflection-absorption spectroscopy (FT-IRAS), Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption, for studying structure and reactivity at surfaces. The chamber and manipulator design enables in situ sample preparation and analysis, and rapid access to several surface-analytical techniques by rotation only. This eliminates sample inconsistencies due to ex situ preparation or the necessity to run parallel experiments. Inclusion of FT-IRAS allows us to characterize surface species and identify adsorbates during studies using STM.

The oxidation mechanism of Pd(1 0 0)

The oxidation of Pd(1 0 0) was characterized using temperature programmed desorption (TPD), low-energy electron diffraction (LEED), and in situ variable-temperature scanning tunneling microscopy (STM). The results indicate that Pd(1 0 0) oxidation proceeds through four stages involving up to five surface phases. In the first stage, oxygen chemisorbs atop the Pd surface resulting in p(2×2) and c(2×2) overlayers that desorb at 800 and 700 K, respectively. As the overlayers saturated, island and peninsula growth was observed in STM movies. The islands are one Pd atom high and so the growth is attributed to Pd atoms ejected onto the terraces which then either nucleated islands or attached to preexisting steps. During this second stage, no change in the surface periodicity was observed. After growth stops, the surface reconstructs in the third stage. Above 475 K, STM images revealed the formation of a ( 5 × 5 ) R27° reconstruction. The images of the ( 5 × 5 ) R27° structure are twofold symmetric, consistent with the formation an epitaxial PdO(0 0 1)-like layer. In addition, a (5×5) reconstruction was observed with LEED prior to the ( 5 × 5 ) R27° reconstruction. As the oxygen coverage entered the regime where these reconstructions were observed, a new desorption peak appeared at 650 K. This temperature is higher than that expected for PdO dissociation but lower than that observed for chemisorption suggesting that the Pd–O bond strength in the reconstructed surfaces is intermediate between the bulk oxide and chemisorbed oxygen. In the fourth stage, the LEED patterns began to fade, the STM images showed three-dimensional clusters, and a low-temperature shoulder consistent with PdO decomposition developed in TPD traces. Thus the surface roughens as the bulk oxide forms. The mechanism outlined above is similar to that recently observed for Pd(1 1 1) oxidation.

Thermodynamics of Decanethiol Adsorption on Au(111): Extension to 0 °C

The coverage-dependent phase behavior of the thiolate formed from decanethiol, CH 3 (CH 2 ) 9 SH, on Au(111) was studied at 0 °C using variable-temperature scanning tunneling microscopy and compared to analogous results for temperatures between 25 and 65 °C. At 0 °C, the lowest density striped phase, β, converts to higher density striped phases, δ and Χ, at exposures that are significantly less than those required at room temperature. The upright saturation phase, o, is also obtained with a lower relative exposure. We discuss these results using an extrapolation of the schematic two-dimensional pressure (π) versus temperature phase diagram developed in previous work. The observed low-temperature phase behavior is rationalized on the basis of thermodynamic considerations. By use of a schematic plot of phase chemical potential versus lateral pressure, the range of exposures over which various phases are thermodynamically stable is assessed as a function of temperature between 0 and 65 °C.

Scanning tunneling microscope observations of Ge deposition on Si(111)-7×7 surfaces irradiated by Xe ions

The annealing behavior of Si(111)-7×7 surface defects created by Xe ion irradiation has been studied by a variable temperature scanning tunneling microscope (VT-STM). When Xe ions were irradiated on the Si surface at room temperature, traces with a diameter of a few nanometers could be observed on the surface. After 400 °C annealing, defects formed in the Si substrate diffused toward the surface, and new vacancies appeared on the surface. Upon annealing to 600 °C, the small vacancy clusters moved on the surface during annealing and were absorbed by large vacancy clusters. Thus, larger vacancy clusters were formed whose size corresponded to the number of defects created in the shallow surface region of the Si substrate. In addition, ion irradiation effects on nucleation and growth of Ge atoms deposited have been studied. For the growth of Ge islands on the Si surface without Xe ion irradiation, the density and the area of the islands increased with increase of deposition time. On the other hand, in the case of using Si substrates irradiated by Xe ions, followed by annealing, Ge islands were formed near the vacancies on the surface. It was found that these vacancies act as the nucleation sites for Ge deposition. Furthermore, to be compared with the case of the Si surface without Xe ion irradiation, the island density was higher and the area of the island was smaller.

Step Dynamics in 3D Crystal Shape Relaxation

Three-dimensional relaxation of small crystallites was imaged in real time using variable-temperature scanning tunneling microscopy. The micron-sized Pb crystallites, supported on Ru(0001), were equilibrated at 500--550 K, and the volume-preserving shape relaxation was induced by a rapid temperature decrease to 353--423 K. The (111) facet at the top of the crystallite grows by sequential peeling of single atomic layers, which shrink like circular islands. The rate of layer peeling slows dramatically as a new final state is reached.

Morphological instability of Cu vicinal surfaces during step-flow growth

The step-flow growth of Cu on vicinal Cu surfaces, Cu (1 1 17) and Cu (0 2 24), is investigated by variable-temperature scanning-tunneling microscopy. These vicinal surfaces have identical terrace widths but their step orientation differs by 45\ifmmode^\circ\else\textdegree\fi{}. Upon growth, the surfaces develop a step-meandering instability, resulting in an in-plane patterning of the surfaces with a temperature- and flux-dependent characteristic wavelength ${\ensuremath{\lambda}}_{u}.$ The instability-induced structural patterns depend on the step orientation and are the manifestation of the Bales-Zangwill instability in both cases. The selected characteristic wavelength is interpreted as the interplay of a destabilizing effect due to the presence of the Ehrlich-Schw\"obel barrier and a stabilizing mechanism presumably due to ``diffusion noise.'' As a result, ${\ensuremath{\lambda}}_{u}$ is proportional to the one-dimensional nucleation length ${l}_{n}$ along a straight step, involving the diffusion barrier along both the 〈110〉 and 〈100〉 step orientations on Cu(001).

Uniform-height island growth of Pb onSi(111)−Pb(3×3)at low temperatures

We have studied the growth of Pb on $\mathrm{Si}(111)\ensuremath{-}\mathrm{P}\mathrm{b}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ at low temperatures, $T<250\mathrm{K},$ and for coverages up to $\ensuremath{\theta}<20\mathrm{ML}.$ The study is carried out with the complementary techniques of high-resolution low-energy electron diffraction and variable-temperature scanning tunneling microscopy. Uniform-height Pb islands, of flat tops and steep edges, are observed on this phase [similar to the uniform island growth observed on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)].$ The origin of this unusual growth is quantum size effects (i.e., the dependence of the electron energy on the island thickness because of the quantization of the electron energy levels in the island). The preferred island height is five steps on the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ and seven steps on the $(7\ifmmode\times\else\texttimes\fi{}7)$ (for the same growth conditions T, \ensuremath{\theta}) because of the different electronic structure at the two interfaces. Since different types of phases [i.e., $(1\ifmmode\times\else\texttimes\fi{}1),\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3},$ which differ in the Pb stoichiometry] can form from the initial $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase, with increasing coverage, the growth on these phases can be also used to study island-height selection. Laterally larger uniform-height islands are observed on the $(1\ifmmode\times\else\texttimes\fi{}1)$ at lower temperatures, $T<170\mathrm{K};$ growth on the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ at higher temperatures, $T>195\mathrm{K},$ shows an open film with preference of the islands to grow to larger heights with uncovered regions of the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase between the islands, even after deposition of 20 ML of Pb.

Uniform island height selection in the low temperature growth of Pb/Si([formula omitted])-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7×7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage–temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

Scanning tunneling microscope study of dynamic phenomena on clean Si(111) surfaces: Si magic clusters and their role

On Si(111) surfaces, we observed a special type of Si magic clusters with a variable-temperature scanning tunneling microscope (STM). At temperatures above 400°C, these clusters migrate on Si(111)-(7×7) surfaces as a whole. They can hop within a half-cell of Si(111)-(7×7), but sometimes they hop away from their original halves, leaving the 7×7 structure intact. When this happens, the magic cluster usually reappears at a site a few hundred Å away. We characterize its structure and derive path-specific hopping parameters using Arrhenius analysis. In the long hops, interestingly, magic clusters exhibit a strong bias for moving in the direction of the heating current. Effects of the directed motion in electromigration and those in thermal migration are determined separately and quantitatively. We also observed fluctuations of step edges through detachment and attachment of magic clusters. The filling of two-dimensional (2-D) craters and the decay of 2-D islands are also found to occur preferentially at the cathode side. These observations provide important clues for understanding the atomic processes in epitaxial growth and in electromigration on Si(111) surfaces. Based on our observations, the phase transition of 7×7↔1×1 is also discussed.

Metal homoepitaxial growth at very low temperatures: Lattice-gas models with restricted downward funneling

We develop and analyze $1+1$- and $2+1$-dimensional (d) models for multilayer homoepitaxial growth of metal films at low temperatures $(T),$ where intralayer terrace diffusion is inoperative. This work is motivated by recent variable-temperature scanning tunneling microscopy studies of Ag/Ag(100) homoepitaxy down to 50 K. Adsorption sites are bridge sites in our $1+1d$ models, and fourfold hollow sites in our $2+1d$ models for fcc(100) or bcc(100) surfaces. For growth at 0 K, we introduce a ``restricted downward funneling'' model, wherein deposited atoms can be trapped on the sides of steep nanoprotrusions rather than always funneling down to lower adsorption sites. This leads to the formation of overhangs and internal defects (or voids), and associated ``rough'' growth. Upon increasing T, we propose that a series of interlayer diffusion processes become operative, with activation barriers below that for terrace diffusion. This leads to ``smooth'' growth of the film for higher T (but still within the regime where terrace diffusion is absent), similar to that observed in models incorporating ``complete downward funneling.''

Submonolayer homoepitaxial growth on Ag(1 1 0)

The submonolayer homoepitaxial growth on Ag(1 1 0) is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The evolution of island density is studied as a function of temperature at given flux (≈0.0025 ML s −1) and coverage (0.16 ML) to evaluate the energy barriers and the binding energies for the in-channel and cross-channel diffusion. The experimental results are compared with kinetic Monte Carlo simulation.

Low temperature phases of Pb/Si(1 1 1)

In this work the low temperature phases of Pb/Si(1 1 1) have been investigated by means of variable temperature scanning tunneling microscopy (STM). Our STM measurements, performed in the range of 55–215 K, show that two different phases, i.e. Pb/Si(1 1 1)-(3×3) and Pb/Si(1 1 1)- 3 2 −1 1 , coexist for samples with a nominal Pb coverage below 1 ML. The general morphology as well as the atomic structure of both surface reconstructions are discussed and compared to previous results.

A fast-scanning, low- and variable-temperature scanning tunneling microscope

The design and performance of a fast-scanning, low- and variable-temperature, scanning tunneling microscope (STM) incorporated in an ultrahigh vacuum system is described. The sample temperature can be varied from 25 to 350 K by cooling the sample using a continuous flow He cryostat and counter heating by a W filament. The sample temperature can be changed tens of degrees on a time scale of minutes, and scanning is possible within minutes after a temperature change. By means of a software implemented active drift compensation the drift rate can be as low as 1 nm/day. The STM is rigid, very compact, and of low weight, and is attached firmly to the sample holder using a bayonet-type socket. Atomic resolution on clean metal surfaces can be achieved in the entire temperature range. The performance of the instrument is further demonstrated by images of adsorbed hexa-tert-butyl-decacyclene molecules on Cu(110), by STM movies, i.e., sequential STM images with a time resolution down to 1 s/image (100×100 Å2 with 256×256 pixels), of the mobility of these molecules, and finally by constant current images of standing waves in the electronic local density of states on Cu(110).

Two-Dimensional Phase Diagram of Decanethiol on Au(111)

On the basis of variable-temperature ultrahigh vacuum scanning tunneling microscopy data, we propose a two-dimensional phase diagram of monolayer decanethiol on Au(111). Four triple-point temperatures were determined: T1 at ≈27 °C, T2 at ≈33 °C, T3 at ≈35 °C, and T4 at ≈56 °C. T1 defines the lowest temperature melting point, and T4 defines the temperature above which striped phases are metastable. These data provide a fundamental framework to understand and control mesoscale monolayer structure; moreover, they provide fundamental insight into the two-dimensional phase behavior of molecules with many degrees-of-freedom.

Sample mounting and transfer for coupling an ultrahigh vacuum variable temperature beetle scanning tunneling microscope with conventional surface probes

We present a new ultrahigh vacuum (UHV) chamber for surface analysis and microscopy at controlled, variable temperatures. The new instrument allows surface analysis with Auger electron spectroscopy, low energy electron diffraction, quadrupole mass spectrometer, argon ion sputtering gun, and a variable temperature scanning tunneling microscope (VT-STM). In this system, we introduce a novel procedure for transferring a sample off a conventional UHV manipulator and onto a scanning tunneling microscope in the conventional “beetle” geometry, without disconnecting the heating or thermocouple wires. The microscope, a modified version of the Besocke beetle microscope, is mounted on a 2.75 in. outer diameter UHV flange and is directly attached to the base of the chamber. The sample is attached to a tripod sample holder that is held by the main manipulator. Under UHV conditions the tripod sample holder can be removed from the main manipulator and placed onto the STM. The VT-STM has the capability of acquiring images between the temperature range of 180–500 K. The performance of the chamber is demonstrated here by producing an ordered array of island vacancy defects on a Pt(111) surface and obtaining STM images of these defects.

Effects of temperature and other experimental variables on single molecule vibrational spectroscopy with the scanning tunneling microscope

Inelastic electron tunneling spectroscopy (IETS) was performed on single molecules with a variable temperature scanning tunneling microscope. The peak intensity, width, position, and line shape of single molecule vibrational spectra were studied as a function of temperature, modulation bias, bias polarity, and tip position for the (C–H,C–D) stretching vibration of acetylene (C2H2,C2D2) on Cu(001). The temperature broadening of vibrational peaks was found to be a consequence of Fermi smearing as in macroscopic IETS. The modulation broadening of vibrational peaks assumed the expected form for IETS. Extrapolation of the peak width to zero temperature and modulation suggested an intrinsic width of ∼4 meV due primarily to instrumental broadening. The inelastic tunneling cross section at negative bias was reduced by a factor of 1.7 for the C–H stretch mode. Low energy modes of other molecules did not show such a reduction. There was no evidence of a tip-induced Stark shift in the peak positions. The spatial variation of the inelastic signal was measured to determine the junction stability necessary for the acquisition of single molecule vibrational spectra.

Inducing and Observing the Abstraction of a Single Hydrogen Atom in Bimolecular Reactions with a Scanning Tunneling Microscope

A variable-temperature scanning tunneling microscope (STM) was used to initiate and observe the abstraction of a single hydrogen atom in a simple bimolecular reaction. Dicarbon (CC) on the Cu(001) surface reacted separately with hydrogen sulfide (H2S) and water (H2O) to form CCH + SH and CCH + OH, respectively. At 9 K, hydrogen abstraction from H2O occurred spontaneously upon contact between CC and thermally diffusing H2O molecules. Hydrogen abstraction from H2S was effected at 9 K by inducing H2S diffusion via excitation with tunneling electrons. The thermal diffusion and reaction of H2S and CC were also observed at 45 K. By removing a single hydrogen atom from H2S and H2O using tunneling electrons, we established the identities of the sulfhydryl (SH) and hydroxyl (OH) reaction products. SH and OH did not react with CC under conditions that led to the reaction of the parent molecules. The bimolecular reactions H2O + O → 2OH and H2S + O → SH + OH did not occur thermally at 9 K but were induced by tunnelin...

Extrinsic structure changes by STM at 65 K on Si(001)

Using variable-temperature scanning tunneling microscopy to image buckled dimers in a $C(4\ifmmode\times\else\texttimes\fi{}2)$ phase on Si(001) at 65 K under conventional scanning conditions, we have observed symmetric dimers in the $P(2\ifmmode\times\else\texttimes\fi{}1)$ phase and the flip-flop motion of dimers at the boundary of two different phases. Careful investigation shows that higher tunneling currents produce larger areas of $P(2\ifmmode\times\else\texttimes\fi{}1)$ phase and higher flip-flop rates of individual dimers. We have measured and plotted the flip-flop rate as a function of current. The origin of the phase transition and the flip-flop dimer motion are discussed.