Fast Photoresponse and Long Lifetime UV Photodetectors and Field Emitters Based on ZnO/Ultrananocrystalline Diamond Films

We have designed photodetectors and UV field emitters based on a combination of ZnO nanowires/nanorods (ZNRs) and bilayer diamond films in a metal–semiconductor–metal (MSM) structure. The ZNRs were fabricated on different diamond films and systematic investigations showed an ultra‐high photoconductive response from ZNRs prepared on ultrananocrystalline diamond (UNCD) operating at a lower voltage of 2 V. We found that the ZNRs/UNCD photodetector (PD) has improved field emission properties and a reduced turn‐on field of 2.9 V μm^?1 with the highest electron field emission (EFE) by simply illuminating the sample with ultraviolet (UV) light. The photoresponse (I_photo/I_dark) behavior of the ZNRs/UNCD PD exhibits a much higher photoresponse (912) than bare ZNRs (229), ZNRs/nanocrystalline diamond (NCD; 518), and ZNRs/microcrystalline diamond (MCD; 325) under illumination at λ=365 nm. A photodetector with UNCD films offers superior stability and a longer lifetime compared with carbon materials and bare ZNRs. The lifetime stability of the ZNRs/UNCD‐based device is about 410 min, which is markedly superior to devices that use bare ZNRs (92 min). The ZNRs/UNCD PD possesses excellent photoresponse properties with improved lifetime and stability; in addition, ZNRs/UNCD‐based UV emitters have great potential for applications such as cathodes in flat‐panel displays and microplasma display devices.     

Synthesis and properties of oxynitrides (La,Sr)Ti(O,N)3 thin films

We report on the synthesis, optical and electrical properties of thin films of La_xSr_(1−x)Ti(O,N)₃, x = 0, 0.25, 0.50, 0.75 and 1. The films were grown by a soft chemistry process from polymeric precursor solutions, which were deposited by spin coating on 100-oriented SrTiO₃ substrates. After drying, the organic matrix was burned at 400 °C. These steps were repeated six times to obtain a reasonable thickness of the films. Afterwards, the samples were heated in a tube furnace at 950 °C in flowing ammonia. The oxynitride films showed different colours from red-orange for LaTiO₂N to greenish-blue for SrTiO₃:N. The films were characterised by X-ray diffraction, scanning electron microscopy, atomic force microscopy, UV–vis spectroscopy, secondary ion mass spectrometry and electrical measurements. The results show that the films are polycrystalline and have the perovskite structure. Their thickness is about 440 nm and the average roughness value is 7.5 nm. UV–vis transmittance measurements showed a strong decrease in the band gap values for the oxynitrides compared to the respective oxides. The values change from approximately 3.2 eV for the oxides to an average value of 2.4 eV for the oxynitrides. The electrical measurements indicate a change in the electrical behaviour from insulator for LaTiO₂N to semiconductor for SrTiO₃:N.     

Properties of chemical vapour deposited nanocrystalline TiO2 thin films and their use in dye-sensitized solar cells

Nanocrystalline titanium dioxide (TiO₂) thin films have been prepared using titanium(IV) isopropoxide as a precursor onto the glass and fluorine doped tin oxide coated glass substrates by chemical vapour deposition technique at 400 °C substrate temperature. X-ray diffraction study confirms the polycrystalline nature of TiO₂ with anatase phase having tetragonal crystal structure. The films are 975 nm thick and transparent having transmittance grater than 80%. Atomic force microscopy (AFM) images reveal the nanocrystalline morphology with grain size of 200 nm. The film shows a sharp absorption edge near 350 nm. Photoelectrochemical study shows that TiO₂ thin film sensitized with Brown Orange dye is found to exhibit relatively maximum I_sc and V_oc among the studied dyes. The values of fill factor (FF) and efficiency (η) for the dye-sensitized solar cell (Brown Orange dye-sensitized TiO₂) are 0.54 and 0.17%, respectively. Such films would serve as better prospects for dye-sensitized solar cells.     

Chemical grafting of biphenyl self-assembled monolayers on ultrananocrystalline diamond

We have investigated the formation of self-assembled monolayers (SAMs) of 4'-nitro-1,1-biphenyl-4-diazonium tetrafluoroborate (NBD) onto ultrananocrystalline diamond (UNCD) thin films. In contrast to the common approach to modify diamond and diamond-like substrates by electrografting, the SAM was formed from the saturated solution of NBD in acetonitrile by pure chemical grafting. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and near edge X-ray absorption fine structure spectroscopy (NEXAFS) have been used to verify the direct covalent attachment of the 4'-nitro-1,1-biphenyl (NB) SAM on the diamond substrate via stable C-C bonds and to estimate the monolayer packing density. The results confirm the presence of a very stable, homogeneous and dense monolayer. Additionally, the terminal nitro group of the NB SAM can be readily converted into an amino group by X-ray irradiation as well as electrochemistry. This opens the possibility of in situ electrochemical modification as well as the creation of chemical patterns (chemical lithography) in the SAM on UNCD substrates and enables a variety of consecutive chemical functionalization for sensing and molecular electronics applications.     

Investigation of the photocatalytic degradation of organochlorine pesticides on a nano-TiO2 coated film

The photocatalytic degradation of organochlorine pesticides including -, β-, γ-, δ-hexachlorobenzene (BHC), dicofol and cypermethrin were carried out on a nano-TiO₂ coated films under UV irradiation in the air. The photocatalytic conditions, including the amount of TiO₂, irradiation time and the intensity of light were optimized. The pesticides were most effectively degraded under the condition of 2.24 mg/cm² on TiO₂ film and a 400 W UV irradiation of high-pressure mercury lamp with a wavelength of 365 nm. A typical organochlorine pesticide, 20 μg -BHC, was dipped onto the TiO₂ film surface and degraded completely within 20 min. In addition, the photocatalytic degradation pathways on the nano-TiO₂ coated film were discussed.     

Highly sensitive detection of transient absorption in dye-doped ultrathin polymer films by the TiO(2)/K(+) composite optical waveguide method upon pulsed laser excitation

A composite optical waveguide (OWG) composed of a 10–18 nm thick titanium dioxide (TiO₂) film sputtered on a conventional K⁺-doped optical waveguide was first applied to detect transient absorption of organic dyes in ultrathin polymer films upon excitation with ns laser. The thickness of the TiO₂ film considerably affected the relative sensitivity of the composite OWG. The composite OWG with 10 nm thick TiO₂ gave much stronger transient absorption for 30–415 nm thick polymer films containing organic dyes than that with 18 nm TiO₂. Transient absorption of phthalocyanine and spiropyran in 20–135 nm thick polymer films was detected 3–20 times more sensitively by the composite OWG with 10 nm TiO₂ than the conventional K⁺-doped OWG which showed a 150-fold sensitivity as compared with the usual normal incidence method. The relative sensitivity of the composite waveguide was also affected by the thickness and refractive index of polymers.     

Effect of various treatment and glazing (coating) techniques on the roughness and wettability of ceramic dental restorative surfaces

Surface treatment procedures such as grinding and polishing are needed to provide the ceramic dental restorative materials with proper fitting and occlusion. The treated surfaces are customarily glazed to improve the strength and smoothness. Though smoothness and wetting of the dental surfaces are important to minimize bacterial plaque retention, influence of the surface treatment and glazing procedures on the final surface roughness and its correlation to wettability are overlooked.

In this work, effect of various treatment (diamond fraising, stoning, sanding and aluminum oxide and rubber polishing) and glazing (auto and overglazing) techniques on the final roughness and the resulting wettability of dental ceramic surfaces were investigated using scanning electron microscopy (SEM) observations and atomic force microscopy (AFM) scans, 75 scans per sample. The surfaces were characterized and assigned an average roughness measure, R_a. The wettability of the same surfaces was evaluated using micro-contact angle measurements (25 micro-bubbles placed on a grid on each surface) to correlate the final surface roughness and wettability.

The results show that overglazing prevails over surface irregularities from different treatment procedures and provides homegeneously smooth surfaces with mean R_a < 10 nm. It also produces uniformly wetted surfaces with low contact angles around 20°. The autoglazed surfaces are less smooth (mean R_a around 50 nm) and displays sporadic topographic irregularities. They display larger and less uniform contact angles ranging between 35° and 50°. The results suggest that overglazing should be preferred after surface treatment to obtain a smooth and well-wetted dental ceramic surface.     

Nanocrystalline Diamond Thin Films Synthesis on Curved Surface

Thin films of curved surface nanocrystalline diamond (CS-NCD) are a category of important materials. However, the development of such materials is still a highly challenging task. Here we present a novel approach to synthesizing CS-NCD thin films deposited on non-spherical surfaces of molybdenum substrate using direct current plasma jet chemical vapor deposition. A special cooling system was designed and applied to ensure uniform substrate temperature. It is demonstrated from simulation and experimental results that this system is favorable for the production of thin films. The results show that the quality of CS-NCD thin films depends on the selection of optimal values of parameters including CH₄ concentration, substrate temperature, and chamber pressure. If the CH₄ concentration and/or the substrate temperature is too high or low, it results in non-diamond phase or micron-crystalline diamond thin films. Synthetic CS-NCD thin films using the proposed method have a smooth surface and uniform thickness. The average grain size and the mean surface roughness are approximately 30 and 4.3 nm respectively. Characteristics of CS-NCD thin film spectra comprised of the full width at half maximum with broad Raman peaks around 1,140 and 1,480 cm^?1, confirming the presence of the NCD phase.     

Does the nanometre scale topography of titanium influence protein adsorption and cell proliferation?

To investigate the influence of titanium films with nanometre scale topography on protein adsorption and cell growth, three different model titanium films were utilized in the present study. The chemical compositions, surface topographies and wettability were investigated by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and water contact angle measurement, respectively. The films share the same surface chemistry but exhibit different topographies on a nanometre scale. Thus, they act as model systems for biological studies regarding surface topography effects. The films were obtained by varying the deposition rate and the film thickness, respectively. These films displayed nanometre scale surface roughness (root mean square roughness, R_rms) from 2 to 21 nm over areas of 50 μm × 50 μm, with different grain sizes at their surfaces. Albumin and fibrinogen adsorption on these model titanium films were performed in this study. Bicinchoninic acid assay was employed to determine the amount of adsorbed protein on titanium film surfaces. No statistically significant differences, however, were observed for either albumin or fibrinogen adsorption between the different groups of titanium films. No statistically significant influence of surface roughness on osteoblast proliferation and cell viability was detected in the present study.     

Synthesis, characterization and gas permeation properties of a silica membrane prepared by high-pressure chemical vapor deposition

Cylindrical silica membranes with dead-end structure were prepared by an extended counter-diffusion chemical vapor deposition (CVD) method, in which a tetramethylorthosilicate (TMOS) silica source was fed from the outside of a cylindrical membrane support with γ-alumina interlayer (the membrane side), and oxygen gas was fed from the inside (the support side). Extended counter-diffusion CVD is a method of depositing silica films under highly pressurized conditions applied to the membrane side where TMOS is supplied. Two silica membranes were deposited for 10 h at 573 K under differential pressures of 0.1 MPa and 0.0 MPa applied between the cylindrical membranes. The hydrogen permeances for these silica membranes were unaffected (5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ at 573 K), although the methane and carbon dioxide permeances were greatly reduced for dense silica films prepared by high-pressure CVD (HPCVD). Therefore, the selectivity of hydrogen over methane and carbon dioxide was 24,000, and 1200, respectively. It is suggested from energy dispersive X-ray microanalysis (EDX) observations in scanning electron microscopy (SEM) and scanning probe microscopy (SPM) results that this high selectivity was due to the reduced number of defects and/or pinholes formed in the dense silica membranes by HPCVD.     

脉冲激光沉积制备SrSn1-xCoxO3外延薄膜的微结构表征及能带调控

钙钛矿结构SrSnO₃因其独特的介电和半导体性质而备受关注,通过掺杂可显著调控其电学、磁学性能,拓宽其应用范围。本研究在单晶SrTiO₃(001)衬底上通过脉冲激光方法外延生长了SrSn_1-xCo_xO₃ (x = 0, 0.16, 0.33, 0.5) (SSCO)薄膜,探究了Co含量对薄膜结晶性、微观结构、光学性能以及介电性能的影响。结果表明, SrSn_1-xCo_xO₃薄膜可在SrTiO₃(001)衬底上外延生长, Co掺杂不会导致薄膜结晶质量的劣化。薄膜表面形貌平整、致密,膜厚200 nm,表面粗糙度为0.44 nm。随薄膜中Co掺杂量增加,薄膜透过率从90%降至25%,光学带隙从4.24 eV降至2.44 eV。介电性能测试表明,掺杂薄膜在10⁶Hz时介电常数为70.1,比无掺杂SrSnO₃薄膜提高57%。室温时SSCO薄膜表面电阻率为172 MΩ,在1000℃范围内薄膜结构稳定。     

Surface functionalization of ultrananocrystalline diamond films by electrochemical reduction of aryldiazonium salts

The surface functionalization of ultrananocrystalline diamond (UNCD) thin films via the electrochemical reduction of aryl diazonium cations is described. The one-electron-transfer reaction leads to the formation of solution-based aryl radicals, which in turn react with the UNCD surface forming stable covalent C-C bonds. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ac impedance spectroscopy, and contact angle measurements have been employed to characterize the organic overlayer and estimate the surface coverage. The grafting of 3,5-dichlorophenyl groups renders the UNCD surface hydrophobic, whereas the attachment of 4-aminophenyl groups makes the surface relatively hydrophilic. The surface coverage, estimated from the electrochemical and XPS measurements, is as high as 70% of a compact monolayer. The aminophenyl terminated surface was obtained by electrochemical reduction of the tethered nitrophenyl groups. This two-step approach yields a UNCD surface with functional moieties available for the potential covalent coupling of a wide variety of biomolecules (e.g., DNA and proteins).     

Preparation of epitaxial La0.6Ca0.4Mn1−xFexO3 (x = 0, 0.2) thin films: Variation of the oxygen content

Perovskite thin films with a nominal composition of La_0.6Ca_0.4Mn_1−xFe_xO₃ (x = 0, 0.2) were deposited by pulsed reactive crossed beam laser ablation. The film properties, such as electrical conductivity and magnetoresistance are studied as a function of the oxygen content and substrate type. The oxygen content of the thin films was determined by Rutherford Backscattering and controlled by varying the background gas pressure, pressure of the gas pulse and by using alternatively O₂ and N₂O as the gas pulse.

LaAlO₃ and SrTiO₃ were used as substrates at deposition temperature of 650 °C. The grown films were analyzed by X-ray diffraction in order to optimize the growth conditions, i.e. to obtain epitaxial thin films. Thin films doped with 20% Fe were grown under the same experimental conditions as the undoped LCMO films and the effect of the doping on the structural and transport properties of the thin films has been investigated.

The temperature of the metal–insulator transition was measured as a function of the oxygen content and substrate type.     

Dye-sensitized solar cells made from BaTiO3-coated TiO2 nanoporous electrodes

We reported on the preparation of a thin BaTiO₃-coated layer (2.27 nm) on the surface of TiO₂ and its further application in the dye-sensitized solar cells (DSCs). The as-prepared BaTiO₃–TiO₂ films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The performances of the DSCs with and without BaTiO₃ coating were analyzed by cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS), and current–voltage measurements. It was found that the BaTiO₃–TiO₂ films with about 12 μm thickness increased the dye adsorption, resulting in increased J_sc. In the meantime, the BaTiO₃ modification on the TiO₂ surface is beneficial to the formation of an energy barrier against the electron transfer from TiO₂ to I₃⁻, providing the increase of V_oc due to the increased electron density in the TiO₂ that is caused by the increased electron lifetime.     

Interface geometry of Ag thin films supported at the MgO(1 0 0) surface: an ab-initio periodic study

In this Letter, the interface geometry of silver thin films of thickness T between 1 and 3 ML epitaxially deposited at the MgO surface has been accurately characterized employing DFT periodic calculations. The Ag–Ag out-of-plane interlayer spacing is considerably shorter than the bulk values because of: (i) the reduced dimension; (ii) the perfect epitaxy constraint; (iii) the interaction with the substrate. The interface distance between the silver monolayer and the MgO substrate, d(Ag–O) = 2.60 Å, is considerably longer than the estimates computed for the bilayer and the trilayer, d(Ag–O) = 2.47 and 2.48 Å, respectively. The difference between the values of the interface distance computed for the monolayer and the results obtained for thicker films, lies in the peculiar electronic properties of the silver monolayer.     

UV-vis-infrared optical and AFM study of spin-cast chitosan films

Optical properties of spin-cast chitosan films have been determined in the infrared, visible, and ultraviolet region of the spectrum using spectroscopic ellipsometry. Optical constants for the UV–vis–near IR spectra from 130 to 1700 nm were determined using Cauchy dispersion forms combined with Lorentzian oscillator models in the absorptive shorter wavelength regions. Infrared index of refraction and extinction coefficients from 750 to 4000 cm⁻¹ were determined using ellipsometric data fits to dispersion models based on harmonic oscillators. This modeling determined that optical anisotropy was present and measurable over all wavelength regions of ellipsometric data.

To obtain information on the micro- and nano-scale surface structure, tapping mode atomic force microscopy (AFM) imaging was employed to determine morphology and roughness information of dry spin-cast chitosan films.     

Size effect in the titanium/diamond‐like carbon bilayer films: effect of relative thickness on their structure and mechanical properties

Titanium/diamond‐like carbon (Ti/DLC) bilayer films with different relative thickness were fabricated by direct‐current and pulsed cathode arc plasma method. Microstructure, morphological characteristics, and mechanical properties of the films were investigated in dependence of the thickness of Ti and DLC layers by Raman spectroscopy, atomic force microscopy, Knoop sclerometer, and surface profilometer. Raman spectra of Ti/DLC bilayers show the microstructure evolution (the size and ordering degree of sp²‐hybridized carbon clusters) with varying the thicknesses of Ti interlayer and DLC layer. Nano‐scaled Ti interlayer of 12–20 nm thickness presents the largest size effect. The catalytic effect of the sublayer is most pronounced in the carbon layer of less than 106 nm. In these thickness ranges, the bilayer films possessed the highest micro‐hardness and reactivity between atoms at interface. Internal stress in the bilayer monotonically decreases, with the thickness of Ti interlayer increasing to 30 nm and then becomes stable with the thickness. These results are associated with the occurrence of atomic diffusion process at Ti/C interface, and they are of cardinal significance to optimize the structure and mechanical properties of carbon‐based multilayer films. Copyright © 2016 John Wiley & Sons, Ltd.     

Layered double hydroxides for ultrathin hybrid film preparation using layer-by-layer and spin coating methods

In our work, the synthesis and the structural properties of various layered double hydroxides (LDHs) were investigated. LDHs were prepared from metal salts with sodium hydroxide in aqueous medium at various molar ratios. Nitrate salts of, Mg²⁺, Ca²⁺, Co²⁺, Al³⁺ and acetate salt of Zn²⁺ were applied as precursors. The concentrations of LDH suspensions were in the range of 0.1–1 g/100 ml. The particle size and zeta-potential of LDH particles were measured by dynamic light scattering (DLS). The structure properties of LDHs were analyzed by several methods, namely XRD, UV-vis spectroscopy and AFM. We used the layer-by-layer dipping immersion technique and the spin coating method for the preparation of multilayer films from LDHs on glass surfaces. The cleaned glass surface has a negative surface charge, which is compensated by the positively charged LDH particles. Different negatively charged binders poly(sodium 4-styrenesulfonate), (PSS); poly(acryl-acid), (PAA) were utilized for the preparation of layer-by-layer films. Layer thickness was calculated from absorbance measurements by spectrophotometry and mass determination of the glass surface. Typical film thicknesses were in the range of 19–70 nm/layer and 6–40 μg/cm²/layer depending on the type of LDH and preparation technique. After n = 20 immersion cycles, maximal layer thickness was 1405 nm for 20 layers of 2:1 ZnAl–LDH/PSS film. The film preparation methods tested were compared and it was established that the films obtained by the layer-by-layer method are more stable but thicker than those prepared using the spin coating method.     

CoFe2-xRExO4(RE=Tb,Dy)纳米晶薄膜的化学合成及磁性

以溶胶-凝胶法制备了稀土铽或镝掺杂的钴尖晶石铁氧体纳米晶薄膜.考察了Tb或Dy的掺杂量及晶化条件对晶相的影响.结果表明,稀土离子的掺杂量x超过0.3时,样品很难形成尖晶石单相.原子力显微镜对纳米晶薄膜表面形貌的观测显示,溶胶-凝胶法制备的薄膜粒度可控制在20～50nm,且具有较高的表面平整度.对于厚度为200nm的薄膜,均方根粗糙度仅为4～5nm.磁特性研究发现,掺杂Tb或Dy的样品矫顽力明显提高.     

Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films

Na_xCoO₂ [x = 0.51, 0.54, and 0.59] thin films have been grown on SrTiO₃ (100)-oriented single crystals with a 5° vicinal cut towards [010] by pulsed laser deposition. We analysed the films by X-ray diffractometry, atomic force microscopy (AFM), and dc-transport measurements. X-ray diffraction patterns of the films show single phase and c-axis textured growth with the film plane closely aligned to the [001]-direction of 5° miscut SrTiO₃ (001) substrates. In addition to the structural analysis of these films we performed transport measurements along and perpendicular to the substrate tilt direction and determined the resistivity anisotropy as a function of temperature. The results enable the development of a strategy for the fabrication of Na_xCoO₂ based thermoelectric thin film devices.