Scanning Tunneling Microscopy Features Research Articles

One-Dimensional Periodic Buckling at a Symmetry-Incompatible Heterointerface of the NaCl(001) Monolayer on Ir(111)

Monolayer and multilayer NaCl(001) films are grown on an Ir(111) substrate, and strain relaxation modes are investigated by scanning tunneling microscopy (STM) and density functional theory calculations. We report that monolayer NaCl(001) exhibits an apparent one-dimensional periodic buckling along the [010] direction, which separates one-dimensional domains of pristine lattices. An atomic model of the monolayer NaCl(001)-(2 × 6) lattice on an Ir(111)-(4 × 73) substrate is proposed, and related simulation reproduces all the STM features. The calculations suggest that the 4–6 symmetry incompatibility of the NaCl(001)/Ir(111) interface leads to the formation of domain boundaries with alternately arranged top-sited and bridge-sited adatom lines along [100], resulting in one-dimensional atomic buckling. We also observe that the atomic buckling is veiled as the NaCl(001) increases to bilayer and above, suggesting that this relaxation mode is finely tuned by the crystal thickness.

Seeking Out Heterogeneous Hydrogen Bonding in a Self-Assembled 2D Cocrystal of Croconic Acid and Benzimidazole on Au(111)

A two-dimensional (2D) hydrogen-bonded cocrystal was synthesized from croconic acid (CA) and benzimidazole (BI) on a gold surface under ultrahigh vacuum conditions. The network domains have a 1:1 CA/BI stoichiometry, can be synthesized over a range of temperatures, and contain one-dimensional chains of molecules connected by heterogeneous hydrogen bonds. Density functional theory (DFT) computations suggest that a tautomeric salt-like structure, with deprotonated CA and protonated BI, is the most stable model, which creates heterogeneous N–H···O contacts instead of N···H–O ones. The homogeneity of the network’s appearance in scanning tunneling microscopy (STM) and a habitual change in the STM features under certain tip conditions indicate that there is an equilibrium of tautomeric molecular states that may be influenced to some degree by STM stimuli. Overall, this study demonstrates how careful consideration of the precursor molecules can tune the architecture within a family of cocrystal networks and introduce desired bonding motifs that haven’t been achieved by solution-based synthesis for these species, such as the heterogeneous hydrogen bonds herein.

Imaging Individual Molecular-Like Orbitals of a Non-Planar Naphthalene Diimide on Pt(111): A Combined STM and DFT Study

Functionalized naphthalene diimides (NDIs) are attractive candidates as small molecule acceptors for various molecular electronic applications due to their reversible two-step one-electron reductions at relatively low potentials. Here, we utilize low-temperature scanning tunneling microscopy (STM) to study the spatial extent and the electronic structure of 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI) adsorbed on the Pt(111) surface. We succeeded to map in real space the electronic structure of this three-dimensional (3D) molecule with orbital resolution, and thus were able to image an in-plane π-nodal plane located at the benzyl side arms. Furthermore, on the basis of the comparison of voltage dependent STM images and ab initio density functional theory simulations, we are able to explain the STM features of BNTCDI in terms of a convolution between its 3D shape and electronic structure. Importantly, for this weakly coupled molecule on the Pt(111) substrate, the intramolecular N···H–C h...

Imaging of Formaldehyde Adsorption and Diffusion on TiO2(110)

Surface reactions of formaldehyde with reduced TiO2(110) surfaces have been studied using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen (Ob) vacancy (VO) defect sites. Bias-dependent STM images show that the STM features corresponding to both the Ti-bound CH2O and the VO-bound CH2O are positioned between the Ob row and the Ti row. While the VO-bound formaldehyde rotates at 95 K, the Ti-bound CH2O does not. The VO-bound CH2O starts to diffuse along the Ob row as –CH2– at ~170 K and starts to diffuse along the Ti row as an intact molecule at ~215 K. However, the stabilities and the configurations of the Ti-bound and VO-bound formaldehyde calculated using DFT are not in line with the experimental results. The discrepancy between the experiment and theory indicates the presence of a complex charge distribution related to the surface defects.

Imaging the buried MgO/Ag interface: Formation mechanism of the STM contrast

Scanning tunneling microscopy (STM) provides real-space electronic state information at the atomic scale that is most commonly used to study materials surfaces. An intriguing extension of the method is to attempt to study the electronic structure at an insulator/conductor interface by performing low-bias imaging above the surface of an ultrathin insulating layer on the conducting substrate. We use first-principles theory to examine the physical mechanisms giving rise to the formation of low-bias STM images in the MgO/Ag system. We show that the main features of the low-bias STM contrast are completely determined by the atoms on the surface of MgO which overcomes prior ambiguities in assigning observed STM features to atomic positions of the substrate or thin film in such an epitaxial thin film system. Hence, the low-bias contrast is formed by states at the Fermi level in the Ag that propagate evanescently through the lattice and atomic orbitals of the MgO on their way to the surface. We develop a number of analysis techniques based on an ab initio tight-binding representation that allows identification of the origin of the STM contrast in cases where previous approaches have proven ambiguous.

Adsorption of CO Molecules on Si(001) at Room Temperature

Initial adsorption of CO molecules on Si(001) is investigated by using room-temperature (RT) scanning tunneling microscopy (STM) and density functional theory calculations. Theoretical calculations show that only one adsorption configuration of terminal-bound CO (T-CO) is stable and that the bridge-bound CO is unstable. All the abundantly observed STM features due to CO adsorption can be identified as differently configured T-COs. The initial sticking probability of CO molecules on Si(001) at RT is estimated to be as small as ∼1 × 10 −4 monolayer/ Langmuir, which is significantly increased at high-temperature adsorption experiments implying a finite activation barrier for adsorption. Thermal annealing at 900 K for 5 min results in the dissociation of the adsorbed CO molecules with the probability of 60−70% instead of desorption, indicating both a strong chemisorption state and an activated dissociation process. The unique adsorption state with a large binding energy, a tiny sticking probability, and a finite adsorption barrier is in stark contrast with the previous low-temperature (below 100 K) observations of a weak binding, a high sticking probability, and a barrierless adsorption. We speculate that the low-temperature results might be a signature of a physisorption state in the condensed phase.

Oxygen Reduction at Cobalt Phthalocyanine Modified Au(111) Electrodes

Au(111) substrates were modified with cobalt phthalocyanine (CoPc) monolayers. Formation of the adsorbed layer was confirmed by in situ electrochemical scanning tunneling microscopy (STM) while its electrochemical activity was monitored by cyclic voltammetry (CV). The adsorbed molecules have demonstrated a noticeable electrocatalytic activity toward the reduction of oxygen. Significant changes in the STM features of the adsorbed molecules were observed when the measurements were performed in the presence of ca. 1 atm of oxygen at low overpotentials. The reported features were attributed to changes in the electronic structure of the molecule when it forms an intermediate during oxygen reduction. Geometries of two of the possible intermediates, HOOCoPc and O2CoPc, were optimized by density functional theoretical (DFT) method. Frontier orbitals of the hydroperoxo intermediate have electronic density similar to the STM features reported here, which could support that we are probing a reaction intermediate during the O2 reduction process.

Atomic Nanowires on thePt/Ge(001)Surface: Buried Pt-Ge Versus Top Pt-Pt Chains

Combining total-energy calculations, electronic-structure studies, and scanning tunneling microscopy (STM), we demonstrate that the observed one-dimensional nanowires are composed of Pt-induced Ge structures instead of Pt chains. Pt-Ge bonds are favored versus Pt-Pt ones. The novel tetramer-dimer-chain model explains STM features and the differential conductivity. The conduction path is related to the chain of alternating Pt-Ge atoms.

Multiple adsorption configurations ofNH3molecules on the Si(001) surface

Initial adsorption configurations of $\mathrm{N}{\mathrm{H}}_{3}$ molecules on the Si(001) surface are studied by using scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. In contrast to the previous consensus of single configuration, we demonstrate that there exist two distinct STM features. One is found to be the existing model of on-dimer configuration, that is characterized by the previously unreported ``U-shape'' empty-state feature. The other is identified to be $\mathrm{N}{\mathrm{H}}_{2}$ and H adsorbed at the same side of two adjacent dimers along the dimer row. The identification of all the adsorption configurations would be useful for the quantitative analysis of Si nitridation.

Oxygen vacancies on TiO 2(1 1 0) and their interaction with H 2O and O 2: A combined high-resolution STM and DFT study

From an interplay between high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) we discuss the origin of various point defects on reduced rutile TiO 2(1 1 0)-(1 × 1) surfaces. By means of adsorption and desorption experiments using water and oxygen as probe molecules we assign the different features observed in STM images to bridging oxygen vacancies, oxygen atoms on surface Ti atoms, and single as well as pairs of hydroxyls on bridging oxygen rows. These experimental results are discussed in comparison to previous STM reports where different assignments of the STM features were suggested. Based on DFT calculations we compare the interaction of water and oxygen with the reduced TiO 2(1 1 0) surface with the situation when these molecules encounter a perfect, stoichiometric TiO 2(1 1 0) surface. These DFT calculations strongly support the assignments of the features observed experimentally by STM. We report on how to produce clean, reduced TiO 2(1 1 0) surfaces and address criteria to ascertain cleanliness by STM.

Characterization of the surface states of Co(0001), Co(111), and ultrathin films of Co on Cu(111)

The surface electronic structure of Co(0001), Co(111), and $2\mathrm{Co}∕\mathrm{Cu}(111)$ is studied using a parametrized tight-binding Hamiltonian with $s$, $p$, and $d$ orbitals in the basis. We compare the calculated local density of states in the vacuum region near the surface with the differential conductance $dI∕dV$ obtained with scanning tunneling microscopy (STM). We make a detailed analysis of the band structure in order to settle the origin and nature of spectroscopic and STM features observed on Co surfaces as well as on Co thin films deposited on Cu(111), which have given rise to controversy in recent years.

Phosphine adsorption and dissociation on the Si(001) surface: Anab initiosurvey of structures

We report a comprehensive ab initio survey of possible dissociation intermediates of phosphine $({\mathrm{PH}}_{3})$ on the Si(001) surface. We assign three scanning tunneling microscopy (STM) features, commonly observed in room-temperature dosing experiments, to ${\mathrm{PH}}_{2}+\mathrm{H}$, $\mathrm{PH}+2\mathrm{H}$, and $\mathrm{P}+3\mathrm{H}$ species, respectively, on the basis of calculated energetics and STM simulation. These assignments and a time series of STM images which shows these three STM features converting into another, allow us to outline a mechanism for the complete dissociation of phosphine on the Si(001) surface. This mechanism closes an important gap in the understanding of the doping process of semiconductor devices.

Atomically resolved STM images of carbon nanotube defects produced byAr+irradiation

Multiwall carbon nanotubes (MWCNTs) dispersed on graphite on highly oriented pyrolytic graphite (HOPG) substrate were irradiated with ${\mathrm{Ar}}^{+}$ ions of $30\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The irradiated samples were investigated by scanning tunneling microscopy (STM) and spectroscopy (STS) in air. The irradiation-induced defects appear as hillocklike protrusions on the nanotube walls, similar to the hillocks observed earlier on ion irradiated HOPG. The results are in agreement with recent predictions, which attribute the STM features produced by ion irradiation to local modifications of the electronic structure. ``$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R$'' type superstructures are also observed near some of the defects. After annealing at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in nitrogen atmosphere, the irradiated MWCNTs were investigated again by STM. The effect of heat treatment on the irradiation-induced nanotube defects is also discussed.

CO2 + O on Ag(110): Stoichiometry of Carbonate Formation, Reactivity of Carbonate with CO, and Reconstruction-Stabilized Chemisorption of CO2

Surface carbonate species CO3(a) forms when exposing oxygen-covered Ag(110) surface to CO2. The stoichiometry of this reaction has been a subject of study for 20 years. Here, using a combined approach of TPRS (temperature programmed reaction spectroscopy), LEED (low energy electron diffraction), and STM (scanning tunneling microscopy), we establish the 1:1 stoichiometry, in contrast to conclusions reached by others. Further, we find that carbonate reacts readily with CO, resulting in a chemisorbed CO2 state. The reactivity of carbonate is as high as that of atomic oxygen under the same conditions. The chemisorbed CO2 desorbs at 485 K, distinct from physisorbed CO2 which desorbs at 130 K. The (1 × 2) reconstruction induced by carbonate formation appears essential to CO2 chemisorption. Direct CO2 exposure at 300 K on Ag(110)−(1 × 2) partially covered by chemisorbed CO2 leads to additional CO2 adsorption, whereas on clean Ag(110)−(1 × 1) no uptake is observed at this temperature. As a result of these reactions, background CO inherent in ultrahigh vacuum converts carbonate into chemisorbed CO2 within a few minutes in UHV or with large CO2 exposures (>30 L). STM images we have acquired of pure chemisorbed CO2 on Ag(110)−(1 × 2) bear strong resemblance to those previously assigned to carbonate by others; we reinterpret these STM features as due to chemisorbed CO2. Mobility and disorder of chemisorbed CO2 are also observed. At a sufficiently long time or large CO exposures chemisorbed CO2 is depleted from the surface, which reverts to clean Ag(110)−(1 × 1).

Self-Assembly of Tetraphenylporphyrin Monolayers on Gold Substrates

Two tetraphenylporphyrins, TPP-P-disulfide and TPP-P-thiol, were synthesized and used to form self-assembled monolayers (SAMs) on gold surfaces. The monolayers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet/visible absorption spectroscopy (UV/vis), and scanning tunneling microscopy (STM). XPS binding energy shifts revealed that the porphyrins were chemisorbed to the surface through a sulfur−gold bond. A red shift without a significant blue-shifted component of the Soret band in the absorption spectra demonstrated that the porphyrin molecules are aligned on the gold surface in a side-by-side orientation. Round STM features, approximately 2 nm in diameter, correspond closely to the diameter of tetraphenylporphyrin (1.8 nm). Taken together, these data indicated the formation of monolayers of uniformly spaced TPP-P-disulfide and TPP-P-thiol molecules, with the porphyrin ring oriented parallel to the gold surface. Furthermore, porphyrin monolayers were stable for at least a week a...

Kinetics of Carbon–NO Reaction Studied by Scanning Tunneling Microscopy on the Basal Plane of Graphite

The carbon–NO reaction was studied by scanning tunneling microscopy (STM) on the basal plane of graphite (HOPG) samples in a wide temperature range of 500–900°C. Monolayer etch pits, multilayer (deep) etch pits, and nascent etch pits were observed for the graphite–NO reaction in different reaction temperature regimes. The formation of deep pits and nascent pits became prevalent at higher temperatures, e.g., 850°C with 1% NO. No temperature “break” point in the Arrhenius plot of the turnover frequency (TOF) rates was observed. The activation energy for the graphite–NO reaction was 132 kJ/mol, which was relatively low compared to literature data. Careful analysis of basal plane STM features revealed that the often-reported (by others) temperature “break” point in the Arrhenius plot was caused by the contribution of NO basal plane c-attack in the higher temperature range, which forms nascent etch pits and deep pits. The formation of nascent and deep pits with higher activation energies (than that for monolayer etching) contributed to the higher apparent activation energy in the high temperature range. In global rate measurements for the NO-carbon reaction, it is not possible to distinguish between the monolayer etching process and basal plane c-attack. The TOF measured by the STM method were true intrinsic rates for edge site monolayer recession. For the NO-graphite reaction, the reaction is first order with respect to NO. The NO–graphite reaction is at least 15 times faster than the O2-graphite reaction. The NO-graphite and N2O–graphite reactions might have the same rate-limiting step in the low temperature range, but have different rate-limiting steps in the high temperature range.

Si(001)/B surface reconstruction.

We have used ab initio total-energy calculations to study the various possible structures resulting from surface reconstruction of Si(001) surface due to half a monolayer coverage of adsorbed B. The ranking of energies for the different structures considered can be attributed to the concentration of strains. The structures that we find to have the lowest energy are compared with the results of a recent scanning tunneling microscopy (STM) study. None of the structures, including those proposed by the authors of the STM work, accounts satisfactorily for the STM features reported. \textcopyright{} 1996 The American Physical Society.

Scanning tunneling microscopy and atomic force microscopy of perylene radical cation salt

Results of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) studies of perylene radical cation salt are presented. In the STM and AFM images obtained on the different surfaces of conductive monocrystal topographical features and details of atomic structure were distinguished. The correlation of STM and AFM features with crystallographic data is discussed.