Types Of Defect Sites Research Articles

Nanoscale insights into graphene oxide reduction by tip-enhanced Raman spectroscopy.

Graphene oxide (GO) has attracted substantial interest for its tunable properties and as a possible intermediate for the bulk manufacture of graphene. GO and its reduced derivatives display electronic and optical properties that depend strongly on their chemical structure, and with proper functionalization, GO can have a desirable bandgap for semiconductor applications. However, its chemical activity leads to a series of unclear chemical changes under ambient conditions, resulting in changes in color and solubility upon exposure to light. In this paper, we study the properties of fresh and spontaneously reduced GO under ambient conditions using tip-enhanced Raman spectroscopy (TERS) to map its nanometer scale chemical and structural heterogeneity. We observe different types of defect sites on reduced GO (rGO) by spatially mapping the D to G band peak ratio and D and G band spectral positions. The higher spatial resolution and out-of-plane polarization compared to conventional micro-Raman spectroscopy enables us to resolve unusual features, including D-band shifting on rGO. Based on statistical analysis of the spatial variations in modes and theoretical calculations for different functional groups, we conclude the reduction mechanism of GO is a self-photocatalytic reduction with the participation of water and visible light, in which the rate determining step is electron transport through the metal substrate and ion diffusion on the GO surface. These results demonstrate that TERS can reveal structural and chemical details elucidating reduction mechanisms, through the examination of samples at different time points.

New Insights into the Bulk and Surface Defect Structures of Ceria Nanocrystals from Neutron Scattering Study

Neutron diffraction and pair distribution function studies coupled with Raman spectroscopy have successfully unraveled the detailed oxygen defect structures of ceria nanocubes and nanorods. Two types of defect sites are revealed for the ceria nanocrystals: surface and bulk defects. It is proposed that the surface oxygen defects in both types of CeO2 nanocrystals are predominantly the partially reduced Ce3O5+x, with the bulk defect structures dominated by interstitial Frenkel-type oxygen vacancies. Ceria nanorods possess much higher concentration of surface oxygen defects relative to the nanocubes, albeit with only slightly higher concentration of bulk Frenkel-type oxygen vacancies. Upon annealing the nanorod sample at 600 °C under vacuum (∼10–4 to 10–5 mbar), a partially reduced ceria phase with long-range oxygen vacancy ordering (Ce3O5+x) has been observed experimentally for the first time. This intriguing observation that surface defect phases can take on ordered defect sublattices under certain conditions is of great value in understanding the temperature-dependent catalytic performance of ceria nanocrystals. Furthermore, a drastic decrease of the surface vacancies in the ceria nanocrystals is observed upon exposure to SO2, especially for the nanorods, a likely origin for the sulfur poisoning effect on ceria-based materials. This study suggests that tailoring surface morphology is a promising strategy to control defect properties of ceria nanomaterials. It also provides fundamental insights to stabilize surface oxygen defects in CeO2 nanocrystals to achieve high redox performance under corrosive environments such as under SO2/SOx exposure.

Defect structure and property consequence when small Li+ ions meet BaTiO3

In the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped $\mathrm{BaTi}{\mathrm{O}}_{3}$. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance ($^{7}\mathrm{Li}$-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the $\mathrm{BaTi}{\mathrm{O}}_{3}$ host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in $\mathrm{BaTi}{\mathrm{O}}_{3}$ could not be determined by NPD, $^{7}\mathrm{Li}$-NMR revealed one broad ``disordered'' and multiple ``ordered'' peaks. Local structure models with different defect types involving ${\mathrm{Li}}^{+}$ were modeled and the corresponding chemical shifts $(\ensuremath{\delta})$ were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with $\mathrm{T}{\mathrm{i}}^{4+}$ being replaced by four ${\mathrm{Li}}^{+}$ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion $\mathrm{T}{\mathrm{i}}^{3+}$ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of ${\mathrm{Li}}^{+}$. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped $\mathrm{BaTi}{\mathrm{O}}_{3}$. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides.

Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1 × 4) Surface

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.

The effect of the preparation procedure on the morphology, texture and photocatalytic properties of ZnO

This paper focuses on the modification of ZnO properties by changes in the preparation procedures, which determine the surface and photocatalytic properties. A diversity of morphologies, structural/textural and surface properties of ZnO was achieved by using three different synthetic procedures: the sol-gel method, the precipitation method with the use of oxalic acid, and a solvent evaporation procedure. The degree of crystallinity, particle size, porosity, band gap energy, and the presence and types of defect sites were measured and correlated to the preparation procedure and photocatalytic activity toward Rhodamine B (RhB) degradation. It was found that the preparation method and the use of a re-dispersion route strongly influence the ZnO morphology. In turn, the morphology determines the nature and density of surface defects. The highest photocatalytic activity towards RhB decomposition is exhibited by ZnO prepared by the precipitation method. The reaction rate is strongly affected by the crystallinity of the ZnO.

Water Adsorption on TiOx Thin Films Grown on Au(111)

High resolution photoelectron spectroscopy has been used to investigate water adsorption on four different TiOx ultrathin film structures, grown on Au(111) by chemical vapor deposition. Two of the structures are reduced TiOx single layer phases, forming a honeycomb (HC) and a pinwheel (PW) structure, respectively. The other two phases have TiO2 stoichiometry, one in the form of islands and one in the form of a TiO2(B)(001) extended layer. Partial water dissociation is observed for all phases but the HC phase, and the dissociation propensity and adsorbate thermal stability structure result from interplay between the atomic structure of the particular TiOx phase and defects formed in the preparation. The dissociation on the TiO2(B) film is mainly related to different types of defect sites. The TiO2 islands, interpreted as surface reconstructed rutile TiO2(100), generate the highest amount of hydroxyls with a behavior consistent with reconstruction into a mixed (100) and (110) termination. Water dissociation on the PW layer can be assigned to particular sites of the structure and it stands out by leading to oxidation of Ti species.

Variations on the use of ion beam luminescence

Luminescence excited during ion beam bombardment offers information on both stable and metastable defect and impurity sites, and it is a sensitive and powerful analytical tool for insulating materials and semiconductors. Most studies so far have focused on visible and UV emission spectra as the spectra reveal information on the formation and types of defect sites. The modifications are dependent on ionisation density, and synergistic effects with nuclear collision damage. There is thus the ability to track defect production and amorphisation of crystalline hosts. Further, ion beam excited luminescence is equally a useful probe for studying nanoparticle growth. Such data can be obtained at different implantation temperatures or combined with other thermal treatments. Pulsed ion beams have the potential to reveal variations in defect formation by lifetime resolution of the signals. However, there is far greater analytical potential which has not been widely developed. The current article will describe how to extend the effects of lifetime resolution; to detect phase transitions, both stable and transient; and the opportunities which arise from combining the ion beam induced luminescence signals with simultaneous excitation of the materials with photons, electrons or X-rays. The combination of two methods can differ from the sum of separate excitation irradiations and so offer details on metastable structures related to ion implantation. Site specific probes using rare earth doped materials during implantation to form laser waveguide structures can track relaxations of both the guide and boundary regions. The measurement of ion beam luminescence spectra and lifetime data over a wide temperature range have historically been under exploited. Parallel data from optical absorption taken during implantation can be similarly informative. Benefits from all such measurements will be discussed.

Influence of electron‐beam treatment of TiO2/Ti on properties of deposited Pt films

AbstractSynchrotron‐radiation‐based photoemission spectroscopy was used to examine the changes in the TiO2 surface structures as a result of high‐energy e‐beam exposure (under atmospheric pressure). The influence of the e‐beam pre‐treatment on the surface structure of post‐deposited Pt on TiO2 and the CO oxidation activity of the Pt films were also investigated. Different types of defect sites (oxygen vacancies, OH‐groups) on TiO2 created under different e‐beam exposure conditions are suggested to either enhance or reduce the metal–support interface reaction, which influences structure and the catalytic activity of post‐deposited Pt films. High‐energy e‐beam under atmospheric pressure can be used for the modification of metal–support interactions, which can improve activity of a catalyst. Copyright © 2010 John Wiley & Sons, Ltd.

Enhanced bone formation by transforming growth factor‐β1‐releasing collagen/chitosan microgranules

Collagen/chitosan composite microgranules were fabricated as bone substitutes for the purpose of obtaining high bone-forming efficacy. The microgranules have the flexibility to fill various types of defect sites with closer packing. The interconnected pores formed spaces between the microgranules, which allowed new bone ingrowth and vascularization. In addition, the transforming growth factor-beta 1 (TGF-beta1) was incorporated into the microgranules in order to improve bone-healing efficacy. The collagen/chitosan microgranules were fabricated by dropping a mixed solution into a NaOH/ethanol solution. TGF-beta1 was loaded into the collagen/chitosan microgranules by soaking the microgranules in a TGF-beta1 solution. Scanning electron microscopy (SEM) observations and experiments examining the release of TGF-beta1 from chitosan and the collagen/chitosan microgranules were performed. SEM was used to examine the cell morphologies on the microgranules and cell proliferation was evaluated using a dimethylthiazole tetrazolium bromide assay. The differentiated cell function was assessed by measuring the alkaline phosphatase (ALPase) activity as well as detecting an osteocalcin assay. The in vivo bone-regeneration experiments were performed using a rabbit calvarial defect model. TGF-beta1 was released from the collagen/chitosan microgranules at a therapeutic concentration for 4 weeks. SEM indicated that the seeded osteoblastic cells were firmly attached to the microgranules and proliferated in a multilayer manner. The proliferation of the osteoblasts on the TGF-beta1-loaded microgranules was the highest among the different types of microgranules tested. The ALPase activity and osteocalcin level of all the samples increased during the culture period, and the TGF-beta1-loaded microgranules had a significantly higher ALPase activity and osteocalcin content than the other microgranules. The TGF-beta1-loaded microgranules demonstrated a higher bone-regenerative capacity in the rabbit calvarial defects after 4 weeks than the TGF-beta1-unloaded microgranules.

Transforming Growth Factor (TGF)- 1 Releasing Tricalcium Phosphate/Chitosan Microgranules as Bone Substitutes

Tricalcium phosphate (TCP)/chitosan composite microgranules were developed as bone substitutes and tissue engineering scaffolds with the aim of obtaining a high bone forming efficacy. The microgranules have the ability to fill various types of defect sites with closer packing. In addition, the transforming growth factor-beta 1 (TGF-beta1) was added to the microgranules in order to improve bone-healing efficacy. TCP/chitosan microgranules were fabricated by dropping a TCP suspended chitosan solution into a NaOH/ethanol solution. TGF-beta1 was incorporated into the TCP/chitosan microgranules by soaking the microgranules into the TGF-beta1 solution. Scanning electron microscopy (SEM) observations as well as experiments examining the release of TGF-beta1 from chitosan and TCP/chitosan microgranules were performed. SEM was used to examine the cell morphologies on the microgranules, and the extent of cell proliferation was evaluated using a dimethyl-thiazol tetrazolium bromide (MTT) assay. The differentiated cell function was assessed by measuring the alkaline phosphatase activity as well as performing an osteocalcin assay. The size of the prepared microgranules was 350-500 microm and TCP powders were observed on the surface of the microgranules. TGF-beta1 was released from the TCP/chitosan microgranules at a therapeutic concentration for 4 weeks. The proliferation of osteoblasts on the TGF-beta1 loaded microgranules was the highest among the microgranules. SEM indicated that the seeded osteoblastic cells were firmly attached to the microgranules and proliferated in a multilayer fashion. The ALPase activity and osteocalcin content of all the samples increased during the culture period. These results suggest that the TCP/chitosan microgranules are potential bone substitutes with a drug releasing capacity and a osteoblastic cells culture scaffold.

Characterization of defects generated by di-and trivalent cations in the potassium-dihydrophosphate structure and their influence on growth kinetics and face morphology

Different types of defect sites generated by the impurities of divalent (M2+) and trivalent (M3+) metals in the structure of potassium dihydrophosphate KH2PO4 (KDP) were revealed by crystal-chemical analysis and computer simulation. These sites cause different deformations of the crystal matrix by generating different local strains, which enhance the inhibiting effect of impurity atoms adsorbed on the surface. This fact accounts for the different influence of di-and trivalent cations on the growth kinetics and face morphology of KDP crystals. The effect of the M3+ ions is associated primarily with their adsorption on the face surfaces, whereas the influence of the M2+ ions results from their insertion into the surface layer of the crystal.

Converting Eu^3+ between defect sites in BaFCl for persistent spectral hole burning

Persistent spectral hole burning (PSHB) is demonstrated in the (7)F(0)-->(5)D(0) optical transition of Eu(3+) doped into crystals of BaFCl. For Eu(3+) ions at two different lattice sites, persistent holes can be burned at temperatures below 77 K. The characteristics of the hole-burning process suggest that the observed PSHB effect is due to laser-excitation-induced site-to-site conversion. One type of Eu(3+) site is converted into another type of defect site. This process is not optically reversible, and holes can be erased only when the temperature increases to 150 K.

CNDO calculation of energies of Ga atom ejection from defect sites on the GaP(110) surface

We have carried out a semi-empirical quantum mechanical calculation of the energy needed to eject a Ga atom from several types of defect sites on the GaP(110) surface. It is shown that the energy needed to remove a Ga atom from defect sites increases according to the sequence: a P(Ga) antisite, a GaP divacancy, a P vacancy, an adatom, a kink site on the surface and a Ga vacancy. All of which the energy lies in the region below 3 eV, and hence ejection of a Ga atom from each of these defects can be induced at least energetically by localization of two electron-hole pairs on a defect site.

CNDO calculation of energies of Ga atom ejection from defect sites on the GaP(110) surface

We have carried out a semi-empirical quantum mechanical calculation of the energy needed to eject a Ga atom from several types of defect sites on the GaP(110) surface. It is shown that the energy needed to remove a Ga atom from defect sites increases according to the sequence: a P(Ga) antisite, a Ga-P divacancy, a P vacancy, an adatom, a kink site on the surface and a Ga vacancy. All of which the energy lies in the region below 3 eV, and hence ejection of a Ga atom from each of theses defects can be induced at least energetically by localization of two electron-hole pairs on a defect site.

Real-time observation of the nucleation and propagation of reaction fronts on surfaces using photoemission electron microscopy

The catalytic oxidation of CO on Pt{100} has been studied in situ using photoemission electron microscopy (PEEM). The difference in work function between CO-covered regions of the surface and O-covered regions provides image contrast in PEEM using a deuterium lamp. The CO and O2 partial pressures were adjusted to give a CO coverage greater than 0.5 monolayer at a crystal temperature below 500 K corresponding to the low rate branch of the oxidation reaction in which CO inhibits the adsorption of oxygen. The transition to the high rate branch (characterized by low CO coverage) was then initiated by decreasing the CO partial pressure. This allows oxygen adsorption at various types of defect sites, forming holes in the CO film. The holes subsequently enlarge as reaction fronts propagate away from the defects at velocities from 0.5 to over 40 μm s−1 depending on temperature, pressure and directions of steps on the surface. The return to the low rate branch, induced by increasing the CO pressure again, does not give rise to propagating reaction fronts but rather to random adsorption of CO throughout the surface.

Dielectric Properties of Plasma-Polymerized Hexamethyldisiloxane Films: 2 Dielectric Breakdown

The dielectric breakdown characteristics of plasmapolymerized hexamethyldisiloxane (PPHMDSO) films prepared under different fabrication conditions have been investigated. The principal fabrication variable, substrate temperature Ts was varied from 25 to 400°C. Dielectric breakdown strength Eb for given fabrication conditions is substantially increased if the sample is first subjected to self-healing breakdown treatment, which leads to clearing of defect sites. As Ts is raised, the number of selfhealing healing breakdowns decreases, and Eb increases. A preliminary correlation study suggests that spheroidal particles included in the film structure constitute a principal type of defect site. For the best material investigated, PPHMDSO deposited at Ts=400°C, Eb= 12.5 MV cm-1 after self-healing, close to the published value for thermally-grown SiO2.

Nuclear quadrupole optical hole-burning in stoichiometric EuVO4

This study of EuVO4 is the first application of hole-burning techniques to the investigation of defect sites in a rare-earth compound. Laser excitation of the Eu3+ absorption spectrum in the region of the 7F0-5D0 forbidden transition ( approximately=580 nm) reveals some 30 absorption lines. Each corresponds to a distinct crystallographic site in EuVO4, as there are no crystal-field splittings in a 0-0 transition. Hole-burning of several of these lines indicates a wide range of quadrupole splitting parameters P and eta , as would be expected for different crystallographic sites. More detailed study of two of these hole-burning spectra shows that, once the parameters P and eta are determined, Zeeman spectra of selected holes directly determine the orientation of the quadrupole tensor relative to the crystallographic axes. In such a simple crystal structure as EuVO4 it is not at all obvious that there could be as many as 30 different type of defect site, and further studies will be needed to explain this.

Matrix-Isolated Silane. Infrared Spectra

The infrared spectra of silane and its deuterated species isolated in solid neon, argon, krypton, xenon, nitrogen, carbon monoxide, and methane have been obtained. In addition, argon—krypton mixed-matrix studies and oxygen-doping experiments have been done. It is shown that silane occupies at least two types of defect sites. There is no evidence for free or nearly free rotation of silane in any of the matrices.