We present the investigations of hydroxylated TiO2 (110)−1 × 1 surfaces using scanning tunneling microscopy (STM). It is observed that in topographic images the protrusions of hydroxyls (OH) on TiO2 (110) surfaces are dependent on the tip−sample distance in STM measurements at room temperature. In comparison with those of the fivefold coordinated Ti atoms and the oxygen vacancies, the relative apparent height of OHs becomes smaller and smaller with decreasing tip−sample distance by changing the imaging conditions. The OHs even become invisible in much shorter tip−sample distances. Reversibly, the OHs are almost completely recovered when the imaging conditions are restored. It suggests that the OHs may be identified by varying the imaging conditions in STM measurement.

We report our investigation on the intrinsic chemical activity of the anatase TiO2(001)-(1 × 4) reconstructed surface, using epitaxially grown anatase TiO2(001) thin films and using methanol molecules as a probe, characterized by combining scanning tunneling microscopy and temperature-programmed desorption. Our results provide direct evidence that the perfect (1 × 4) lattice sites of the surface are intrinsically quite inert for the reaction of methanol. We obtain that the activation energy for desorption of molecular methanol is about 0.55–0.64 eV, which is in good agreement with our first-principle calculations based on the structural model with 5-fold coordinated Ti atoms at the ridges of (1 × 4) reconstruction. We find that two types of defect sites, that is, reduced Ti pairs and partially oxidized Ti pairs, are responsible for the chemical activity of the surface, evidenced by the desorption of water due to the dehydrogenation of methanol at the defect sites. The methoxy left at the reduced Ti-pair sites further produced CH3 radical, and the methoxy near the partially oxidized Ti-pair sites produced formaldehyde and methanol through disproportionation reaction. The determination of these intrinsic properties can be important to understanding the conflicting results from this surface in the literature and thus to reveal the actual reaction mechanisms.

•Atomically flat Cr-N codoped rutile TiO2(110) single crystal thin films were homoepitaxially grown by PLD method, using (Cr2O3)0.03•(TiN)0.94 mixture target.•The Cr-N codoped TiO2(110) thin films were systematically studied by STM/STS and XPS/UPS, in combination with the first-principle calculations.•The band gap in the Cr-N codoped TiO2(110) thin films was reduced to 1.9 eV.•Two types of structures (Cr-N and Cr-2N) and their electronic states in the TiO2(110) surface were identified.•The delocalized states introduced by the excess substitutional N atoms by the Cr-2N structure could be mainly responsible for the bandgap reduction.We report our investigation on the electronic properties of Cr-N codoped rutile TiO2(110) single crystal thin films, homoepitaxially grown by pulsed-laser-deposition method, and characterized using scanning tunneling microscopy and spectroscopy (STM/STS), X-ray/ultraviolet photoemission spectroscopy (XPS/UPS), in combination with first-principles calculations. Our results show that the bandgap reduction of the TiO2(110) surface is mainly contributed by the delocalized states whose position is at 2.0 eV below the Fermi level, introduced by the substitutional codoped Cr-2N pair, which is evidenced by the accordance of the results between the STS spectra and the calculated DOS. The codoped Cr-N pair contributes the gap state at about 0.8 eV below the Fermi level, in consistent with the theoretical calculations. While, the monodoped Cr contributes the states either close to the valence band maximum or the conduction band minimum, which should not contribute to the bandgap reduction too much. Our experimental results joint with theoretical calculations provide an atomic view of the bandgap reduction of the rutile TiO2(110) surface, which indicates that the excess substitutional N atoms should be important to efficiently narrow the bandgap by introducing the Cr-2N pairs.Download high-res image (160KB)Download full-size image

We report our investigation of the reversible reaction of methanol and the migration of hydrogen adatom (Had) on TiO2(110)-(1 × 1) surface at various temperatures and methanol coverages using scanning tunneling microscopy joint with density functional theory (DFT) calculations. At a relatively low coverage measured at room temperature, the methanol species adsorbed at the oxygen vacancy (OV) sites are immobile and appear only as a dissociative form, and the observed Had migration events are very few. However, when the OV sites are fully filled by methanol in the methanol-overdosed sample, the methanol species at the OV sites keep immobile but frequently switch between molecular and dissociative forms, accompanied by dramatically enhanced Had migration. Meanwhile, an established equilibrium shows a concentration ratio of 1:3 between the molecular and dissociative methanol. At 235 K, we directly visualized and confirmed that the reversible reactions of methanol and the enhanced Had migration are mediated by the diffusive methanol adsorbed at the 5-fold coordinated Ti sites. Our DFT calculations well elucidate the experimental results using the modeled configurations by considering the exchange processes of H atoms, reaching a clear atomistic picture for the dynamic equilibrium of the reversible reactions and the Had migration.

We report our investigation of adsorption and self-assembly of a nonplanar molecule 2,3,5,6-tetra(2′-pyridyl)pyrazine (TPPZ) on a Au(111) surface using ultrahigh vacuum low-temperature scanning tunneling microscopy joint with density functional theory (DFT) calculations. We find that the nonplanar TPPZ molecules exhibit various adsorption configurations depending on the coverage of molecules. The molecules mainly adsorb at step edges with a flat-lying configuration at low coverages and gather into chiral trimers almost equidistantly separated from each other in the fcc domains accompanied by diffusive molecules in the hcp domains of the herringbone reconstructed Au(111) surface at a coverage of about 0.2 monolayer (ML) and then form two dominant types of ordered domains, i.e., stripe-like (S-phase) and honeycomb-like (H-phase) superstructures, which may reflect the chiral separation characteristics at a coverage of about 1 ML. In the trimers and ordered domains, the adsorption configurations of molecules become declining or almost erect, i.e., an “edge-on” configuration, quite different from the flat-lying configuration at low coverages. After annealing to 380 K the S-phase transfers to the H-phase, and the H-phase may persist after annealing up to 410 K, which can be attributed to the existence of C–H···N hydrogen bonds between the TPPZ molecules with the same chirality. Our observations can be energetically interpreted by considering the interplay of molecule–substrate interaction and intermolecular interaction including van der Waals interaction and hydrogen bonds on the basis of the DFT calculations, where the hydrogen bonds should be a key factor for the formation of the stable ordered H-phase with chiral separation.

We systematically investigated the photocatalytic reaction of methanol on the TiO2 (110)-(1 × 1) surface under irradiation with ultraviolet (UV) light performed at various conditions, using scanning tunneling microscopy (STM) jointed with temperature-programmed desorption (TPD) techniques. Our STM and TPD results show that the photocatalytic reaction is indeed initiated from the molecular methanol at the 5-fold coordinated Ti sites, as commonly ascribed to the methanol oxidation by the photogenerated holes, reflecting the highly photoactive nature of methanol. The formaldehyde yield from the TPD results is much smaller by a factor of 2/3 than the amount of dissociated methanol from the STM results at 80 K. This observation can be assigned to the reverse reaction during the TPD measurement, and may explain the lower yield of formaldehyde using molecular methanol than using methoxy. From the fractal-like reaction kinetics of methanol, we can associate the coverage-dependence of the spectral dimensions with the change for the diffusion of holes across the surface from a one-dimensional to a two-dimensional behavior because of the increased scattering species at higher coverages. Our results here provide a clear picture for the photocatalytic reaction of molecular methanol and may rationalize the different observations performed at various conditions.

We investigate the dynamic processes of formaldehyde (HCHO) molecules on 5-fold-coordinated titanium (Ti5c) sites of rutile TiO2(110) surface using scanning tunneling microscopy (STM) together with density functional theory simulations. Our results show that the adsorbed HCHO molecules at Ti5c sites are present as two types of protrusions, either centered at Ti5c rows or centered at bridging oxygen (Ob) rows in the STM images, corresponding to the monodentate adsorption configuration through a O–Ti5c bond and to the bidentate adsorption configuration through both O–Ti5c and C–Ob bonds, respectively, which can be well supported by the simulated images. It is also observed that the monodentate adsorption tends to spontaneously switch to bidentate adsorption. Our results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti5c sites. We obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from monodentate adsorption to bidentate adsorption and reversely switching from bidentate adsorption to monodentate adsorption, respectively, from measurements of their dynamic processes. Our findings can well elucidate the missing signature of the energetically more favored bidentate configuration in some previous experiments and provide insightful understanding of formaldehyde on TiO2(110) surface.

We present an investigation of the structural and electronic properties of an ordered grain boundary (GB) formed by separated pentagon–heptagon pairs in single-layer graphene/SiO2 using scanning tunneling microscopy/spectroscopy (STM/STS), coupled with density functional theory (DFT) calculations. It is observed that the pentagon–heptagon pairs, i.e., (1,0) dislocations, form a periodic quasi-one-dimensional chain. The (1,0) dislocations are separated by 8 transverse rows of carbon rings, with a period of ∼2.1 nm. The protruded feature of each dislocation shown in the STM images reflects its out-of-plane buckling structure, which is supported by the DFT simulations. The STS spectra recorded along the small-angle GB show obvious differential-conductance peaks, the positions of which qualitatively accord with the van Hove singularities from the DFT calculations.

Scanning tunnelling microscopy (STM) has been a unique and powerful tool in the study of molecular systems among various microscopic and spectroscopic techniques. This benefits from the local probing ability for the atomically resolved structural and electronic characterization by the STM tip. Moreover, by using the STM tip one can modify a given structure and thus control the physical and chemical properties of molecules at a single-molecule level. The rapid developments in the past 30 years have extended the functions of STM far beyond characterization. It has shown the flexibility to combine STM with other techniques by making use of the advantages of the STM tip, demonstrating important applications in the growing nanotechnology. Here we review some recent progresses in our laboratory on single molecule chemistry by taking advantage of tip-assisted local approaches, such as the identification of specific orbitals or states of molecules on surfaces, tip-induced single-molecule manipulation, atomically resolved chemical reactions in photochemistry and tip-induced electroluminescence. We expect more joint techniques to emerge in the near future by using the unique advantages of STM tip, providing more powerful tools for the growing requirements of new materials design and the mechanism of chemical reactions at the molecular scale.

We investigate the growth of Cr–N codoped anatase TiO2(001) thin films, prepared with a pulsed-laser-deposition (PLD) method using a mixed Cr2O3 and TiN ceramic target (6 at.% Cr), and characterized using scanning tunneling microscopy (STM), X-ray and ultraviolet photoemission spectroscopy (XPS/UPS), and ultraviolet–visible (UV–Vis) absorption spectroscopy. We find that the doping concentration of N in the films can be finely tuned by the O2 pressure and the growth temperature. By optimizing the growth conditions, we obtain the anatase TiO2(001) films with relatively smooth (1 × 4) reconstructed surface at equally codoped contents of 6 at.% Cr and 6 at.% N. The roughness of the surface is about 0.9 nm in root mean square, and the typical size of the (1 × 4) terraces is about 20 nm. The XPS results indicate that Cr and N should be both substitutionally doped in the film. From the UPS spectrum for the codoped film, the valence band maximum is significantly lifted by about 1.3 eV, indicating a narrowing band gap of 1.9 eV. The optical absorption spectrum shows that the codoped film noticeably absorbs the light at less than 710 nm. Derived from the optical absorption spectrum, an estimated band gap value of 1.78 eV is obtained, which is consistent with the UPS result.

The water splitting reaction based on the promising TiO2 photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti5c) sites of the reduced TiO2 (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO2. This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OHbr, and the other either occurs as OHt at Ti5c sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OHt or oxygen adatoms exactly at the original Ti5c sites, without the trace of OHbr. Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.

The two-dimensional assemblies of truxenone, diindeno[1,2-a;1′,2′-c]fluorene-5,10,15-trione, on the Au(111) surface have been studied by scanning tunnelling microscopy in ultrahigh vacuum. It is found that the truxenone monolayer on Au(111) exhibits different two-dimensional supramolecular structures. The investigation using scanning tunnelling microscopy combined with the density functional theory calculations can be a helpful approach to understand the complicated supramolecular structures of truxenone self-assembly on Au(111).

It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)–phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co2+ ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S1 of the R3-Ag/Si surface and the localized unoccupied Co2+dz2 orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems.Keywords: cobalt(II)−phthalocyanine; density functional theory calculation; hybrid silicon-molecular electronics; negative differential resistance; scanning tunneling microscopy/spectroscopy; Si(111)-√3 × √3-Ag; surface-states

A knowledge of adsorption behaviors of oxygen on the model system of the reduced rutile TiO2(110)-1×1 surface is of great importance for an atomistic understanding of many chemical processes. We present a scanning tunneling microcopy (STM) study on the adsorption of molecular oxygen either at the bridge-bonded oxygen vacancies (BBOV) or at the hydroxyls (OH) on the TiO2(110)-1×1 surface. Using an in situ O2 dosing method, we are able to directly verify the exact adsorption sites and the dynamic behaviors of molecular O2. Our experiments provide direct evidence that an O2 molecule can intrinsically adsorb at both the BBOV and the OH sites. It has been identified that, at a low coverage of O2, the singly adsorbed molecular O2 at BBOV can be dissociated through an intermediate state as driven by the STM tip. However, singly adsorbed molecular O2 at OH can survive from such a tip-induced effect, which implies that the singly adsorbed O2 at OH is more stable than that at BBOV. It is interesting to observe that when the BBOVs are fully filled with excess O2 dosing, the adsorbed O2 molecules at BBOV tend to be nondissociative even under a higher bias voltage of 2.2 V. Such a nondissociative behavior is most likely attributed to the presence of two or more O2 molecules simultaneously adsorbed at a BBOV with a more stable configuration than singly adsorbed molecular O2 at a BBOV.

We report the investigation on the adsorption behaviors of CO on the rutile TiO2(110)-1 × 1 surface with preadsorbed O adatoms using scanning tunneling microscopy (STM) joint with density functional theory (DFT) calculations. The STM experimental results show that the diffusive CO molecules tend to adsorb at the site close to the O adatoms, forming CO−O and CO−O−CO complexes. These complexes are quite stable against the high bias voltages and UV illumination. DFT calculations give an activation energy barrier of 0.56 eV for CO oxidation through the CO−O complex to produce CO2. Our experimental and theoretical results both indicate that the dissociative O2, that is, the O adatoms on Ti4+, may not be directly responsible for the catalytic oxidation at low temperature.

We report our experimental study of the adsorption and diffusion of CO on clean TiO2(110)-1 × 1 at an atom-resolved scale with in situ scanning tunneling microscopy at 80 K, combined with density functional theory calculations. Our results reveal that the CO adsorption sites on the TiO2(110)-1 × 1 surface are preferentially located at next-nearest-neighbor five-coordinate Ti atoms close to a bridge-bonded oxygen vacancy (BBOV), in contrast to the idea that the BBOV itself acts as the adsorption site for CO. The equilibrium distribution of CO adsorption on the TiO2(110)-1 × 1 surface can be well-understood according to the calculated adsorption energies of different sites.

We present the growth of textured TiO2 thin films on muscovite mica using pulsed laser deposition. Atomic force microscopy, scanning electron microscopy, X-ray photoemission spectroscopy, and transmission electron microscopy were used to characterize the TiO2 films. Quasi-periodic wavy and comb-like buckles were observed. Below a critical thickness of about 25 nm, TiO2 films were relatively smooth, and buckles began to form when the nominal thickness of TiO2 films was larger than 25 nm. Co-existence of wavy and comb-like quasi-periodic buckles was observed when the nominal thickness of TiO2 films was larger than 100 nm. The film stress is compressive due to the competition of the tensile stress from the lattice mismatch and the growth stress. The buckle delamination occurred when the compressive stress overcome the adhesion strength of the TiO2 films on mica. A value of adhesion strength around 0.9 MPa for TiO2 on mica is obtained. The symmetric domains of buckles are assigned to the anisotropic lattice mismatch for TiO2 on mica.

We present structural studies of C60/Pd multilayer films. A new phase of a layered structure with a sixfold (or threefold) axis, which is similar to the metastable phase in Pd–Si alloys, has been observed by means of transmission electron microscope and X-ray diffraction in the C60/Pd multilayer films after annealed at 470 K 60 min. This new phase, which is believed to be one metastable phase of Pd–C, is a hexagonal structure with the lattice constant a = 0.438 nm. The reciprocal lattice points become continuous spikes in the direction c∗ of the reciprocal lattice, which indicates the Pd–C phase consists of layers.

Scanning tunnelling microscopy (STM) has been a unique and powerful tool in the study of molecular systems among various microscopic and spectroscopic techniques. This benefits from the local probing ability for the atomically resolved structural and electronic characterization by the STM tip. Moreover, by using the STM tip one can modify a given structure and thus control the physical and chemical properties of molecules at a single-molecule level. The rapid developments in the past 30 years have extended the functions of STM far beyond characterization. It has shown the flexibility to combine STM with other techniques by making use of the advantages of the STM tip, demonstrating important applications in the growing nanotechnology. Here we review some recent progresses in our laboratory on single molecule chemistry by taking advantage of tip-assisted local approaches, such as the identification of specific orbitals or states of molecules on surfaces, tip-induced single-molecule manipulation, atomically resolved chemical reactions in photochemistry and tip-induced electroluminescence. We expect more joint techniques to emerge in the near future by using the unique advantages of STM tip, providing more powerful tools for the growing requirements of new materials design and the mechanism of chemical reactions at the molecular scale.

The two-dimensional assemblies of truxenone, diindeno[1,2-a;1′,2′-c]fluorene-5,10,15-trione, on the Au(111) surface have been studied by scanning tunnelling microscopy in ultrahigh vacuum. It is found that the truxenone monolayer on Au(111) exhibits different two-dimensional supramolecular structures. The investigation using scanning tunnelling microscopy combined with the density functional theory calculations can be a helpful approach to understand the complicated supramolecular structures of truxenone self-assembly on Au(111).