Scanning Tunneling Microscopy Observations Research Articles

Influence of Multidirectional Interactions on Domain Size and Shape of 2-D Molecular Assemblies

The effect of molecule-molecule interactions on the size and shape of two-dimensional (2-D) ordering domains was investigated using scanning tunneling microscopy (STM) at the liquid/solid interface. Synthesized alkoxybenzene derivatives bearing amide group tend to form very large domains of 2-D orderings (>0.25 μm2), whereas the domain size of corresponding urea derivatives was significantly small in the same condition. The quantitative analysis of the concentration dependence of surface coverage using a nucleation-elongation model suggests that the replacement of amide functionality by urea increased the equilibrium constant of the nucleation process of 2-D self-assembly, which is a key factor for the domain size at the liquid/solid interface. Interestingly, the STM observation revealed that a slight difference of molecular structure influences the shape of 2-D ordering domains, and needle-shaped domains with large aspect ratio around 10 were observed by tuning molecule-molecule intramolecular interactions in 2-D self-assembly formed at a liquid/graphite interface.

Atomistic study of comblike structure on the MoO2/Mo(110) surface by scanning tunneling microscopy and density functional theory calculations

The origin of comblike step formation, which was previously observed in the initial oxidation stage of the Mo(110) surface, was investigated by comparing between scanning tunneling microscopy (STM) observations and ab initio density functional theory (DFT) calculations. The comblike steps were obtained by the formation and evaporation of cluster arrays during oxidation and thermal treatment. On the terraces surrounding the comblike steps formed by the evaporation of clusters, a characteristic “overstripe” pattern was found. On the basis of this pattern, it is assumed that a reliable unit cell with MoO2(010) placed on Mo(110) with in-plane rotation can be constructed and ab initio structural relaxations can be performed. The calculations included “small” model calculations for investigating stable interfacial oxygen sites and “large” model calculations for simulating STM images, and calculation results were compared with experimental results. The simulated STM images show good agreement with the experimental observations, indicating the relevance of the large model. From these analyses, it is pointed out that the local stresses on molybdenum in the oxide layer were important for the comblike step formation.

Temperature-Triggered Sulfur Vacancy Evolution in Monolayer MoS2 /Graphene Heterostructures.

The existence of defects in 2D semiconductors has been predicted to generate unique physical properties and markedly influence their electronic and optoelectronic properties. In this work, it is found that the monolayer MoS2 prepared by chemical vapor deposition is nearly defect-free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations. However, after thermal annealing process at ≈900 K, the existence of dominant single sulfur vacancies and relatively rare vacancy chains (2S, 3S, and 4S) is convinced in monolayer MoS2 as-grown on Au foils. Of particular significance is the revelation that the versatile vacancies can modulate the band structure of the monolayer MoS2 , leading to a decrease of the bandgap and an obvious n-doping effect. These results are confirmed by scanning tunneling spectroscopy data as well as first-principles theoretical simulations of the related morphologies and the electronic properties of the various defect types. Briefly, this work should pave a novel route for defect engineering and hence the electronic property modulation of three-atom-thin 2D layered semiconductors.

Effect of Fe-doping on CDW state in 1T-TaS2 investigated by STM/STS

1T-TaS2 has been known to undergo Mott transition from nearly commensurate charge density wave (CDW) state to commensurate CDW state at about 200 K. Recently, Fe doping was found to suppress the Mott transition and induce superconductivity. In this study, we report on the scanning tunneling microscopy (STM) and spectroscopy (STS) study on Fe doped 1T-TaS2 with different Fe concentration. STM observations reveal that the CDW superlattice becomes irregular as increasing Fe concentration. STS measurements uncover the spatial change in the local density of state (LDOS). We find two types of David-star: one has symmetric LDOS with respect to Fermi energy, and the other has asymmetric one. We identify that the asymmetric spectrum is due to an Fe dopant.

Development of simulation approach for two-dimensional chiral molecular self-assembly driven by hydrogen bond at the liquid/solid interface

Two-dimensional (2D) chiral self-assembly system of 5-(benzyloxy)-isophthalic acid derivative/(S)-(+)-2-octanol/highly oriented pyrolytic graphite was studied. A combined density functional theory/molecular mechanics/molecular dynamics (DFT/MM/MD) approach for system of 2D chiral molecular self-assembly driven by hydrogen bond at the liquid/solid interface was thus proposed. Structural models of the chiral assembly were built on the basis of scanning tunneling microscopy (STM) images and simplified for DFT geometry optimization. Merck Molecular Force Field (MMFF) was singled out as the suitable force field by comparing the optimized configurations of MM and DFT. MM and MD simulations for hexagonal unit model which better represented the 2D assemble network were then preformed with MMFF. The adhesion energy, evolution of self-assembly process and characteristic parameters of hydrogen bond were obtained and analyzed. According to the above simulation, the stabilities of the clockwise and counterclockwise enantiomorphous networks were evaluated. The calculational results were supported by STM observations and the feasibility of the simulation method was confirmed by two other systems in the presence of chiral co-absorbers (R)-(-)−2-octanol and achiral co-absorbers 1-octanol. This theoretical simulation method assesses the stability trend of 2D enantiomorphous assemblies with atomic scale and can be applied to the similar hydrogen bond driven 2D chirality of molecular self-assembly system.

Synthesis and Scanning Tunneling Microscopy Observation of 1,3,5-Triazine Bearing Azo-Carboxylate.

A new compound (2,4,6-tris-(3,5-di-methyl formate-4′-hydroxy azobenzene)-1,3,5-triazine,TDHAT) has been synthesized and the scanning tunneling microscopy (STM) is utilized to clarify the geometrical configuration on the highly oriented pyrolytic graphite (HOPG) surface. The star-shaped molecule self-assembles into uniform and regular triangular petal structure at liquid–solid interface, and high-resolution images has been obtained, which implied a high stability of this two dimensional configuration. The distance between the scattered bright spot and the center of the petal is measured to be about 1.2 nm (L₂), which corresponds to the actual size of each arm. Moreover, a comparison has been made between the TDHAT molecule and the TMA 4a molecule which has similar star-shaped structure with a long alkyl chain at the each end of the three arms, drawing a conclusion that both the structure of arms and the substituent groups would impact the nanoarchitecture. The results give us insight into a better comprehension of the self-assembly of the star-shaped molecule, which benefits the construction of functional nanostructures.

Hydrogen-induced atomic structure evolution of the oxygen-chemisorbed Cu(110) surface.

Using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) modeling, we determine the mechanism of the atomic structural evolution of the oxygenated Cu(110) surface induced by the reaction of adsorbed hydrogen with chemisorbed oxygen in the Cu(110)-c(6 × 2)-O structure. Our STM observations show that the reconstructed Cu(110)-c(6 × 2)-O surface undergoes a phase transition to the (2 × 1)-O reconstruction in the course of oxygen loss induced by the reaction with H2 gas. Using DFT modeling, we find that the surface phase transition is initiated via the adsorption of molecular hydrogen on the chemisorbed oxygen, which results in the formation of H2O molecules that desorb spontaneously from the surface. The loss of chemisorbed oxygen induces the c(6 × 2) → (2 × 1) transition that involves the diffusion of Cu-O-Cu chains along the ⟨1¯10⟩ direction.

One-atom-layer 4×4 compound in (Tl, Pb)/Si(111) system

An ordered 4×4-periodicity 2D compound has been found in the (Tl, Pb)/Si(111) system and its composition, structure and electronic properties have been characterized using low-energy electron diffraction, scanning tunneling microscopy observations and density-functional-theory calculations. The compound has been concluded to contain 9 Tl atoms and 12Pb atoms per 4×4 unit cell, i.e., 0.56ML of Tl and 0.75ML of Pb. Structural model was proposed for the 4×4-(Tl, Pb) compound where building blocks are a hexagonal array of 12Pb atoms, a triangular array of 6 Tl atoms and a Tl trimer. The proposed structure has a C3 symmetry and occurs in the two equivalent orientations. The electron band structure of the compound contains two metallic spin-split surface-state bands. Bearing in mind the advanced properties of the known 3×3 2D compound in the same (Tl, Pb)/Si(111) system (i.e., combination of giant Rashba effect and superconductivity), the found 4×4-(Tl, Pb) compound is believed to be a promising object for exploration of its superconductive properties.

Covalent Functionalization of GaP(110) Surfaces via a Staudinger-Type Reaction with Perfluorophenyl Azide

Despite the markedly low chemical reactivity of the nonpolar (110) surfaces of III–V semiconductors, the covalent functionalization of GaP(110) surfaces with perfluorophenyl azide (PFPA) molecules by a Staudinger-type reaction occurs only slightly above room temperature (325 K). Scanning tunneling microscopy observations, combined with density functional theory calculations, support the formation of stable, covalent perfluorophenyl nitride (PFPN) molecule–surface bonds, which can be described as Lewis acidic Ga-stabilized phosphine imides. π–π stacking between aromatic, electron-deficient PFPN units results in compact, commensurate 2D molecular assembly at the surface. PFPA deposition on GaP(110) at room temperature with no additional annealing leads to an intermediate phase consistent with an alternating 1D array of physisorbed and chemisorbed molecular units. This work provides a new route for covalently bonding molecular linkages to the (110) surfaces of III–V semiconductors.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

C70 self-assembly on In- and Tl-adsorbed Si(111)[formula omitted]-Au surfaces

Behavior of C70 fullerenes adsorbed onto the In- and Tl-modified Si(111)3×3-Au surfaces at room temperature (RT) and 112K has been studied using scanning tunneling microscopy observations and compared with the known results for the C60s on the same surfaces. During island growth at 112K, both C70 and C60 are characterized by the same critical island size i=1. The difference is in an island shape as C70s tend to form chain-like islands built of double or triple molecular rows along the [1¯10] Si(111) substrate directions. At RT, C70s have a lower mobility as compared to C60s. In contrast to the C60 magic islands, the C70 islands do not demonstrate any strong preference for specific shapes or sizes. Extended C70 arrays exhibit a stripe-like 3×1 reconstruction where a single row of bright fullerenes mediated by a double row of dim fullerenes. The reconstruction is due to the different orientations of C70s within the layer which does not relate to the structure of the underlying Au/Si(111) substrate. This is in contrast to the hexagonal reconstructions of C60s where the bright fullerenes occupy the specific adsorption site atop Au trimers. The difference in the behavior of C70 and C60 is plausibly affected by the non-spherical shape of C70 molecule and a greater intermolecular C70-C70 interaction.

Comparative STM analysis of C60 and C70 fullerene adsorption sites on pristine and Al-modified Si(111)7 × 7 surfaces

Using scanning tunneling microscopy observations, preferable adsorption sites of C60 and C70 fullerenes on pristine Si(111)7 × 7 and Al-modified Si(111)α–7 × 7-Al surfaces at 300 and 450 K were determined. The C60 and C70 molecules display similar, albeit not identical, adsorption behavior while the most essential variance is related with the hosting surfaces, pristine 7×7 or α–7 × 7-Al. Both C60 and C70 prefer to occupy positions above the Si rest atoms upon adsorption onto Si(111)7 × 7 at 300 K and change their preferable sites for those located at the Si edge adatom at 450 K, that is plausibly accompanied by releasing the Si adatom. Upon C60 and C70 adsorption onto Si(111)α–7 × 7-Al surface at 300 K, the fullerenes prefer to occupy the asymmetric sites at the dimer rows. At 450 K, the C70 fullerenes preserve this location, while about half of the C60 fullerenes change it for that at the corner Si adatom position. The present experimental data set serves as a useful basis for theoretical analysis.

(Invited) Physical Properties of Novel 2D Semimetal WTe2: From Microscopic Study to Devices

In this study, multiple measurements, including scanning tunneling microscopy (STM), electrical transport, corroborated by DFT calculations are performed on freshly cleaved surfaces of semimetallic tungsten ditelluride (WTe2) crystals. High sample quality with very low density of defects is confirmed by STM observation and in-plane anisotropic conductance (originated from quasi-one dimensional Fermi surfaces) can be conclude based on transport measurement and DFT calculation.We reveal that strong Rashba type spin-orbit effects due to the non-centrosymmetric structure splits the electron and hole bands crossing the Fermi level and lifts the spin degeneracy of these bands. Low temperature STM/S measurements show the umklapp-type interference pattern on WTe2 surface at 4.2 K, proven to be a spectroscopic feature dominated by spin-conserving processes. Furthermore, we investigate electronic properties of devices made from few-layer WTe2 down to a few nanometers. Our results reveal that few-layer WTe2 could be an ideal candidate for future 2D electronic, optoelectronic after solving the ambient sensitivity of device process.

Molecular simulations of C60 self-assembly on metal-adsorbed Si(111) surfaces

The authors have proposed a simulation procedure for the evaluation of energetics of C60 islands on crystalline surfaces that allows questions relating to shape, size, and orientation of the islands to be addressed. Simulation consists of placing a patch of close-packed C60 array of a given shape and size on a surface potential relief and finding energy minima by variation of island position and orientation. Upon appropriate adjustment of the surface potential relief, simulations reproduce well all the main results of the scanning tunneling microscopy observations. For C60 islands forming on In-adsorbed Si(111)3×3-Au and pristine Si(111)3×3-Ag surfaces, the optimal surface relief shows up as a periodic array of cosine-shaped peaks. The proposed approach provides a hint for understanding the driving mechanisms of C60 self-assembly, and, in principle, it can be applied to other adsorbate-substrate systems.

(Invited) Physical Properties of Novel 2D Semimetal WTe2: From Microscopic Study to Devices

An increasing number of alternative two-dimensional (2D) materials are being explored in the “post-graphene age”, such as transition metal dichalcogenides (TMDs), silicene, and germanene. In this study, multiple measurements, including scanning tunneling microscopy (STM), electrical transport and Raman spectroscopy are performed on freshly cleaved surfaces of semimetallic tungsten ditelluride (WTe2) crystals. High sample quality with very low density of defects is confirmed by STM observation and four Fermi pockets can be revealed by Shubnikov–de Haas (SdH) oscillations based on transport measurement. Furthermore, we investigate Raman spectroscopy of thin-layer WTe2. Our results reveal that few-layer WTe2 could be an ideal candidate for future 2D electronic, optoelectronic devices.

Vibrational spectroscopic evidence for (NO)3 formation on Cu(111).

The formation of (NO)3 on Cu(111) was recently reported based on scanning tunneling microscopy observations [A. Shiotari et al., J. Chem. Phys. 141, 134705 (2014)]. We herein report studies into this system using electron energy loss spectroscopy and verify the above findings through vibrational analysis. For the surface covered with mixed isotopes of N(16)O and N(18)O, we observed four peaks corresponding to N-O stretching vibrations, which were ascribed to the four isotopic combinations of the trimer. Dynamic coupling within the trimer was evaluated from model calculations of the coupled oscillators. Furthermore, we observed hindered rotation and translation modes in the dipole scattering regime, suggesting that the molecular axis is tilted from the surface normal. These results provide spectroscopic support for the formation of (NO)3 on Cu(111).

Nanoscale Structure, Dynamics, and Aging Behavior of Metallic Glass Thin Films.

Scanning tunnelling microscopy observations resolve the structure and dynamics of metallic glass Cu100−xHfx films and demonstrate scanning tunnelling microscopy control of aging at a metallic glass surface. Surface clusters exhibit heterogeneous hopping dynamics. Low Hf concentration films feature an aged surface of larger, slower clusters. Argon ion-sputtering destroys the aged configuration, yielding a surface in constant fluctuation. Scanning tunnelling microscopy can locally restore the relaxed state, allowing for nanoscale lithographic definition of aged sections.

Tailoring Kinetics on a Topological Insulator Surface by Defect-Induced Strain: Pb Mobility on Bi2Te3.

Heteroepitaxial structures based on Bi2Te3-type topological insulators (TIs) exhibit exotic quantum phenomena. For optimal characterization of these phenomena, it is desirable to control the interface structure during film growth on such TIs. In this process, adatom mobility is a key factor. We demonstrate that Pb mobility on the Bi2Te3(111) surface can be modified by the engineering local strain, ε, which is induced around the point-like defects intrinsically forming in the Bi2Te3(111) thin film grown on a Si(111)-7 × 7 substrate. Scanning tunneling microscopy observations of Pb adatom and cluster distributions and first-principles density functional theory (DFT) analyses of the adsorption energy and diffusion barrier Ed of Pb adatom on Bi2Te3(111) surface show a significant influence of ε. Surprisingly, Ed reveals a cusp-like dependence on ε due to a bifurcation in the position of the stable adsorption site at the critical tensile strain εc ≈ 0.8%. This constitutes a very different strain-dependence of diffusivity from all previous studies focusing on conventional metal or semiconductor surfaces. Kinetic Monte Carlo simulations of Pb deposition, diffusion, and irreversible aggregation incorporating the DFT results reveal adatom and cluster distributions compatible with our experimental observations.

Modulation of Coordinate Bonds in Hydrogen-Bonded Trimesic Acid Molecular Networks on Highly Ordered Pyrolytic Graphite Surface

The modulation effects of metal nitrites on the hydrogen-bonded supramolecular structure of trimesic acid on a graphite surface have been investigated at the liquid–solid interface by scanning tunneling microscopy (STM). The STM observations demonstrate that the self-assembled hydrogen-bonded molecular networks of trimesic acid (TMA) have been significantly transformed after the introduction of various metal nitrites. It is found that for the nitrite containing full orbital metals (AgNO3 and Zn(NO3)2), the pristine nitrite molecules are directly embedded into the pores within the TMA networks. In contrast, for the nitrite with vacant orbital metals (Cu(NO3)2, Mn(NO3)2 and Fe(NO3)3), the metal–organic coordination networks with various structures are formed due to the incorporations of the metal ions.

Point defects in epitaxial silicene on Ag(111) surfaces

Silicene, a counterpart of graphene, has achieved rapid development due to its exotic electronic properties and excellent compatibility with the mature silicon-based semiconductor technology. Its low room-temperature mobility of ∼100 cm2 V−1 s−1, however, inhibits device applications such as in field-effect transistors. Generally, defects and grain boundaries would act as scattering centers and thus reduce the carrier mobility. In this paper, the morphologies of various point defects in epitaxial silicene on Ag(111) surfaces have been systematically investigated using first-principles calculations combined with experimental scanning tunneling microscope (STM) observations. The STM signatures for various defects in epitaxial silicene on Ag(111) surface are identified. In particular, the formation energies of point defects in Ag(111)-supported silicene sheets show an interesting dependence on the superstructures, which, in turn, may have implications for controlling the defect density during the synthesis of silicene. Through estimating the concentrations of various point defects in different silicene superstructures, the mystery of the defective appearance of and silicene in experiments is revealed, and 4 × 4 silicene sheet is thought to be the most suitable structure for future device applications.