Search

Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO2 nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO2 nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C3v, which implies that the Ti exists in two local symmetries (D2d and C3v) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO2 nanocrystals.

The relationship between nanoscale friction and the surrounding environment has long been a critical issue in the field of nanotribology. Here, we utilized ambient pressure–atomic force microscopy ...

Bimetallic platinum–cobalt (Pt–Co) catalysts have highly enhanced performance for the oxygen reduction reaction (ORR), where this peculiar surface alloy structure contributes to efficient energy co...

We report a method that uses van der Waals interactions to transfer continuous, high-quality graphene films from Ge(110) to a different substrate held by hexagonal boron nitride carriers in a clean, dry environment. The transferred films are uniform and continuous with low defect density and few charge puddles. The transfer is effective because of the weak interfacial adhesion energy between graphene and Ge. Based on the minimum strain energy required for the isolation of film, the upper limit of the interfacial adhesion energy is estimated to be 23 meV per carbon atom, which makes graphene/Ge(110) the first as-grown graphene film that has a substrate adhesion energy lower than that of typical van der Waals interactions between layered materials. Our results suggest that graphene on Ge can serve as an ideal material platform to be integrated with other material systems by a clean assembly process.

Open in a separate window π-Conjugated molecule-bridged silicon quantum dot (Si QD) clusters were first synthesized by Sonogashira cross-coupling reaction between 4-ethynylstyryl and octyl co-capped Si QDs (4-Es/Oct Si QDs) and 2,5-dibromo-3-hexylthiophene. The formation of Si QD clusters was confirmed by field emission transmission electron microscopy. The electronic coupling between the QDs in the Si QD cluster is significantly enhanced as compared with that for 4-Es/Oct Si QDs, which is verified from the red shift in ultraviolet–visible absorption and photoluminescence spectra of the Si QD cluster with the possibility of exciton transport, the increased charging effect found in the core-level photoemission spectra, the shift to lower binding energy of the valence band photoemission spectrum, and more decisively, the increase in electrical conductance of the Si QD cluster thin film. To investigate the physical origin of the temperature dependence of the electrical conductance, we have merged the microscopic viewpoint, Marcus theory, on the electron transfer (W) between the adjacent QDs, with macroscopic concepts, such as the conductance (G), mobility (μ), and diffusion coefficient (D). The effective reorganizational energies of charge transfer between the neighboring Si QDs in 4-Es/Oct Si QD and Si QD cluster thin films are estimated to be 170 and 140 meV, respectively, while the ratio of the effective electronic coupling of the latter to that of the former is determined to be 7.3:1.

Ionic liquids and gels have attracted attention for a variety of energy storage applications, as well as for high-performance electrolytes for batteries and supercapacitors. Although the electronic structure of ionic electrolytes in these applications is of practical importance for device design and improved performance, the understanding of the electronic structure of ionic liquids and gels is still at an early stage. Here we report soft X-ray spectroscopic measurements of the surface electronic structure of a representative ammonia-based ionic gel (DEME-TFSI with PS-PMMA-PS copolymer). We observe that, near the outermost surface, the area of the anion peak (1s N– core level in TFSI) is relatively larger than that of the cation peak (N+ in DEME). This spontaneous ionic polarization of the electrolyte surface, which is absent for the pure ionic liquid without copolymer, can be directly tuned by the copolymer content in the ionic gel, and further results in a modulation in work function. These results shed...

Colloidal 4-ethynylstyryl and octyl cocapping silicon quantum dot (4-Es/Oct Si QD) and its spin-coated film were synthesized and fabricated at three different curing temperatures, 150, 250, and 350 °C under argon for 4 h. Thermal cross-linking of 4-ethynylstyryl-terminated 4-Es/Oct Si QD during the curing process was confirmed by differential scanning calorimetry for the 4-Es/Oct Si QD powder and by Fourier transform infrared spectroscopy for the Si QD thin films. The effect of thermal cross-linking of 4-ethynylstyryl capping groups on the electronic coupling between Si QDs in Si QD solids of thin film states was investigated by monitoring optical and electrical properties of the Si QD thin films at different curing temperatures. The valence-band spectra of 4-Es/Oct Si QD thin films obtain from high-resolution photoemission spectroscopy at 130 eV showed the shift to lower binding energy as curing temperature increases from 150 to 350 °C. The optical bandgap values estimated from the extinction-coefficient...

The capacities of graphene oxide (GO) and reduced graphene oxide (rGO) films grown on silicon substrate to cause the aniline to azobenzene oxidation reaction were compared by using Raman spectroscopy, high-resolution photoemission spectroscopy (HRPES), and work function measurements as well as scanning electron microscopy (SEM). The oxygen carriers’ existence on GO film, which includes a lot of oxygen carriers, can facilitate the aniline to azobenzene oxidation reaction with slightly partial conversion of aniline to nitrobenzene, as determined by the Raman shifts and core-level spectra resulting from exposure to aniline. The work function of the GO film was found to change dramatically in comparison with rGO film, indicating that aniline exposed to a GO film produced n-type doping characteristics by electron charge transfer from GO to aniline. These results indicate that the oxygen carriers on a GO film oxidize aniline to azobenzene and show that GO film prefers to act as a reaction reagent rather than rGO.

The modification of the electronic properties of a coverage-dependent graphene layer by nitrogen ions irradiation was investigated using core-level photoemission spectroscopy (CLPES). Here we describe preparation of monolayer and multilayer epitaxial graphene (EG) functionalized by nitrogen ions irradiation with 100 eV to minimize the damage of graphene layer as we annealed up to 1300 K to track the surface property changes using CLPES. As a result, on the monolayer EG, we found that pyridinic nitrogen mainly existed on the surface. On the multilayer EG, we observed the formation of graphitic nitrogen remaining as a major species confirmed using N 1s core-level spectra. Through a work function change (ΔΦ) measurement, both systems indicated p-type doping properties with 4.71 (monolayer EG) and 4.87 eV (multilayer EG) of work function values after N2 ion irradiation. Interestingly, we observed that monolayer EG maintained its p-type doping character, whereas multilayer EG changed the doping character from ...

High-resolution photoemission spectroscopy using synchrotron radiation was employed to understand the bonding state of N incorporated into a reduced TiO2(110)-1×1 surface. N-doping into TiO2(110) was accomplished both by dosing NH3 at elevated temperatures (800 K) and by using a NHx+ ion implantation method onto TiO2(110) at room temperature. Our results indicate that the N-doped TiO2(110) obtained after final annealing to 900 K contains N species incorporated into the subsurface region of TiO2substitutionally replacing the lattice O2–. Interestingly, we find that some of the N species are found to have a direct N–H bond (NH species). The valence band indicates that the band-gap state related to the Ti 3d state is enhanced with increasing subsurface N species, which explains an attractive interaction between N species and defects with a charge in the Ti 3d state, such as reduced Ti3+ species and oxygen vacancies.

We report the bonding structure of the selenophene molecules adsorbed on the Si(100)-2 × 1 surface at 300 K, and its evolution upon annealing investigated by adopting core-level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and ab initio calculations. The Si 2p, C 1s, Se 3d core-level spectra measured at two temperatures, 300 and 350 K, are consistently interpreted in terms of the two major structures suggested by theory, a twisted (T) 2,5-dihydroselenophene (T-DHS) and a T-deseleniumization where the selenium atom is dissociated from the selenophene ring. We find a significantly enhanced deseleniumization of selenophene molecules by mild thermal annealing indicating that these two equally abundant structures at 300 K become a single uniform phase of the T-deseleniumization structures at 350 K by overcoming a relatively low dissociation energy barrier between the two structures. In addition, we obtain an average tilt angle of a selenophene ring at 300 K from our NEXAFS sp...

A new kind of organic-inorganic hybrid polymer, poly(tetraphenyl)silole siloxane, was invented and synthesized for realization of its unique charge trap properties. The organic portions consisting of (tetraphenyl)silole rings were responsible for negative charge trapping, while the Si-O-Si inorganic linkages provided the intrachain energy barrier for controlling electron transport. The polysilole siloxane dielectric thin films were fabricated by spin-coating and curing of the polymers, followed by characterization with spectroscopic ellipsometry (SE), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and photoemission spectroscopy (PES). The abrupt increase in density and decrease in thickness of the thin film at a curing temperature of 100 °C was attributed to a thermodynamically preferred state in the nanoscopic arrangement of the polymer chains; this was due to cofacial π-π interactions in a skewed manner between peripheral phenyl groups of the (tetraphenyl)silole rings of the adjacent polymer chains. Using the NEXAFS spectrum to assess high electron affinity, the LUMO energy level of the dielectric thin film cured at 150 °C was positioned 1 eV above the Fermi energy level (E(F)). The electron trapping of the dielectric thin films was confirmed from the positive flat band shift (ΔV(FB)) in the capacitance-voltage (C-V) measurements performed within the metal-insulator-semiconductor (MIS) device structure, which strongly verified the polymer design concept. From the simple kinetics model of the electron transport, it was proposed that the flat band shift (ΔV(FB)) or trap density of the negative charges (|ρ|) was logarithmically proportional to the decay constant (β) for the electron-tunneling process. When a phenyl group of a silole ring in a polymer chain was inserted into the two available phenyl groups of another silole ring in another polymer chain, the electron transfer between the groups was enhanced, decreasing the trap density of the negative charges (|ρ|). For the thermodynamically preferred state generating the high refractive index, the distance between the two phenyl groups of the adjacent polymer chains was estimated to be in the range of 0.27-0.36 nm.

The adsorption configurations of S-proline on Ge(100) were studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations, and high-resolution photoemission spectroscopy (HRPES). We identified three adsorption structures of S-proline on Ge(100) through analysis of the STM images, DFT calculations, and HRPES results: (i) an “intrarow O−H dissociated and N dative bonded structure”, (ii) an “O−H dissociation structure”, and (iii) an “N dative bonded structure”. Moreover, because adsorption through the N atom of S-proline creates a new chiral center due to symmetry reduction produced by N dative bonding with the surface, the adsorption configurations of S-proline on Ge(100) have either (R,S) or (S,S) chirality. Through DFT calculations, we clearly demonstrated that the adsorption configurations have (R,S) chirality with a preference for reaction at the Re face. This work presents a novel method for generating stereoselective attachment using S-proline molecules adsorbed on a G...

Chemical doping of epitaxial graphene (EG) by organic free radicals (4-amino-2,2,6,6-tetramethyl-1-piperridinyloxy; 4-amino-TEMPO) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and high resolution photoemission spectroscopy (HRPES). STM images revealed that the empty density of states near the adsorption site significantly decreased as a result of radical doping. STS indicated that 4-amino-TEMPO radicals acted as n-type dopants on monolayer graphene. The radicals adsorbed onto EG through the nitroxide groups, leaving the amine group unreacted, which was confirmed by the binding energies of N 1s and O 1s core-level spectra. Furthermore, the measured work function changes verified that increased adsorption of the radicals on EG showed n-type doping characteristics.

The adsorption geometry of cis-2-butene-1,4-diol (BEDO, HOCH(2)CH=CHCH(2)OH) on Si(100)-2 x 1 was studied using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Bias-voltage-dependent STM images exhibited features characteristic of two distinct BEDO adsorption geometries. One feature was a bright protrusion located on the center of a single dimer, indicating an on-top configuration. The low bias-voltage STM image clearly showed dark features indicative of the formation of Si-H bonds on adjacent dimers in the same dimer row. The other feature was a bright protrusion bridged on end between two adjacent dimers in the same dimer row, indicating an end-bridge configuration. Accompanying this feature, two dark features attributed to Si-H bonds were observed on opposite positions to the bridged protrusion. From the XPS results, the Si 2p core level spectra revealed that the dimer atoms are involved in the formation of Si-O and Si-H bonds. On the other hand, carbon K-edge NEXAFS spectra showed that the C=C bond does not participate in the adsorption reaction and remains as an unreacted group. Collectively, the experimental results indicate that the adsorption of BEDO on Si(100)-2 x 1 occurs through the formation of two Si-O bonds via nucleophilic reaction between the two OH groups of BEDO and two Si-Si dimers. Importantly, the maintenance of the C=C bond means that the C=C functional group can be utilized as a new reaction site for further surface chemical reactions.

In the gas phase, electron beam irradiation of a furoxan molecule results in the production of two NO molecules and concomitant generation of a triple bond. In this study, we examined whether the selective cleavage of furoxan occurs on the surface of silicon wafers. A furoxan-substituted imine layer was prepared by the reaction of aminosilylated silicon wafers with 4-furoxancarbaldehyde. Formation of the imine layer was confirmed by UV-vis spectroscopy, contact angle goniometry, ellipsometry, and XPS. XPS spectroscopic monitoring of the electron beam (400 eV) induced reaction of the modified silicon wafers showed that two of the furoxan ring nitrogen atoms were lost. To determine if a carbon-carbon triple bond had been generated in the surface product of this reaction, FT-IR spectroscopy and NEXAFS (Near Edge X-ray Absorption Fine Structure) were performed. A weak absorption at 2203 cm -1 was observed in the FT-IR spectrum, reflecting the presence of a triple bond. The carbon K-edge NEXAFS spectrum contained a π*(C≡C) peak at 286.5 eV. Based on these results, we conclude that electron beam irradiation of the furoxan, incorporated on a silicon wafer surface, results in the release of nitrogen oxide and the formation of a triple bond containing product.

Chemically modified epitaxial graphene (EG) by azidotrimethylsilane (ATS) was investigated using high-resolution photoemission spectroscopy (HRPES). Through the spectral analysis, we clearly confirmed that EG is modified by thermally generated nitrene radicals and found that the bonding nature between the nitrene radicals and EG is covalent. As we observe bonding nature of N 1s peaks, we found that two distinct N peaks can be clearly distinguished in the spectra. Using a covalently bound stretched graphene (CSG) model, we elucidated that nitrene radicals adsorb on the graphene layer at two different adsorption sites. Moreover, we were able to control the band gap of EG using valence band spectra as we change the amount of the dosing of nitrene.

Graphene layers grown on 6H-SiC(0001) were irradiated with 1 MeV electron beam to functionalize its surface. A surface analysis using atomic force microscopy, the near edge X-ray adsorption fine structure spectra of C K-edge, and photoemission spectroscopy suggests that the electron beam irradiation in ambient condition can induce oxidation of the graphene layer.

The surface of a fused silica and oxidized silicon wafer (SiO2/Si(100)) was treated with (3-aminopropyl)triethoxysilane (1), (3-aminopropyl)diethoxymethylsilane (2), and (3-aminopropyl)ethoxydimethylsilane (3) for 72 h in solution. Thickness of the aminosilylated film out of 1 increases rapidly, and it turns out to be around 100 A in 72 h. Rather slow increase of the thickness is observed for 2, which produces the layers of 8 and 14 A in 10 and 72 h, respectively. The reagent 3 produces the monolayers of constant thickness 7−8 A during the whole span of the reaction. The aminosilane layer was allowed to react with 4-nitrobenzaldehyde to form an imine, and absorbance (Asurf) of the imine was recorded by UV−vis spectroscopy. The imine was hydrolyzed in a known volume of water to produce 4-nitrobenzaldehyde, and subsequently its absorbance was measured. Thus observed number of the aldehyde molecules is equivalent to the number of the imine molecules on the surface. With the known surface area of the substrat...

The adsorption and decomposition of AlCl3 on Ge(100) was studied using scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy (HRCLPES). Through the analysis of the STM image and Ge 3d and Cl 2p core-level spectra of AlCl3 on Ge(100), we found that an AlCl3 molecule reacts with two Ge atoms via a cycloaddition reaction, which forms Cl−Ge and AlCl2−Ge without breaking AlCl3. Additionally, by considering valence shell electron pair repulsion (VSEPR) arguments, the effect of molecular structure on the surface chemistry was explained. To our knowledge, the adsorption of Lewis acid molecules on a semiconductor surface has not been studied in detail. These are the first results for the adsorption structures of Lewis acid molecule on Ge(100).