Quantum confinement effects of CdTe nanocrystals sequestered in TiO2 matrix: effect of oxygen incorporation

Thin films of TiO₂ with high volume fraction (40–55%) and crystallite size (6–40 nm) of CdTe nanoparticles had been prepared by rf magnetron sputtering from a composite TiO₂:CdTe target at room temperature and 373 K. A detailed optical properties of nanocrystalline CdTe:TiO₂ films as-deposited and after thermal treatment (300 °C) are studied. The absorbance of the TiO₂ films with CdTe nanocrystallite dispersions depends both on the nanocrystallite size and volume fraction. The blue-shifts of the optical absorption edge concurrent with the CdTe nanocystal size reduction for as-deposited and after thermal treatment of nanocrystalline CdTe:TiO₂ thin films with respect to the bulk semiconductor agrees quite well with the strong quantum confinement theory. A slight deviation in absorption edge values than the predicted values from the strong quantum confinement model can be attributed to change in interplanar distance due to oxygen incorporation and inhomogeneous size distribution of CdTe nanocrystallites in these films.     

Growth of nanocrystalline TiO2 films by pulsed-laser-induced liquid-deposition method and preliminary applications for dye-sensitized solar cells

A novel technique, the pulsed-laser-induced liquid-deposition (PLLD) method, has been employed to grow nanocrystalline TiO₂ films on fluorine-doped tin-oxide-coated (FTO) glass substrates at room temperature. The PLLD method was implemented by directing a pulsed laser into a liquid precursor and depositing the photosynthesized nanocrystalline TiO₂ on an FTO glass substrate immersed in the liquid precursor. The as-grown nanocrystalline TiO₂ films were found to have a rutile crystal structure and consist of a number of flower-like TiO₂ crystal units arrayed together on the FTO glass substrate. Each of the flower-like TiO₂ crystal units was composed of many nanostructured TiO₂ whiskers, and their building blocks were found to be bundles of TiO₂ nanorods with diameter of about 5 nm. The growth of these TiO₂ nanorods is highly anisotropic, with the preferential growth direction along [001]. As-grown nanocrystalline TiO₂ films were annealed at 450°C in air for 30 min for the applications of dye-sensitized solar cells, and the nanostructured characteristics with good porosity were preserved after annealing. A preliminary dye-sensitized solar cell was built based on the annealed nanocrystalline TiO₂ film. The results suggest that the PLLD method is a promising technique for growing nanocrystalline TiO₂ films for photovoltaic applications.     

Influence of nanocrystalline structure and composition on hardness of thin films based on TiO<Subscript>2</Subscript>

In this work, the influence of Tb-doping on structure, and especially hardness of nanocrystalline TiO₂ thin films, has been described. Thin films were formed by a high-energy reactive magnetron sputtering process in a pure oxygen atmosphere. Undoped TiO₂-matrix and TiO₂:Tb (2 at. % and 2.6 at. %) thin films, had rutile structure with crystallite sizes below 10 nm. The high-energy process produces nanocrystalline, homogenous films with a dense and close packed structure, that were confirmed by X-ray diffraction patterns and micrographs from a scanning electron microscope. Investigation of thin film hardness was performed with the aid of a nanoindentation technique. Results of measurements have shown that the hardness of all manufactured nanocrystalline films is above 10 GPa. In the case of undoped TiO₂ matrix, the highest hardness value was obtained (14.3 GPa), while doping with terbium results in hardness decreasing down to 12.7 GPa and 10.8 GPa for TiO₂:(2 at. % Tb) and TiO₂:(2.6 at. % Tb) thin films, respectively. Incorporation of terbium into TiO₂-matrix also allows modification of the elastic properties of the films.     

Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells

We have demonstrated an approach for the electrostatic assembly of CdTe quantum dots (QDs) with different charged ligands as sensitizers, achieving high coverage and good dispersion in TiO₂ porous films. The CdTe QD-sensitized TiO₂ porous films were subjected to thermal annealing in a high vacuum chamber to remove the ligand linker, resulting in the formation of direct heterojunctions between the bare CdTe QDs and TiO₂ for a favorable charge transfer. The as-received CdTe QD-sensitized TiO₂ porous films were employed as photoanodes for quantum dot-sensitized solar cells (QSSCs), and the photocurrent density reached as high as 4.69 mA/cm² under a standard illumination condition of simulated AM 1.5G (100 mW/cm²).     

Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures

Physical vapor deposition techniques such as sputtering and laser ablation – which are very commonly used in thin film technology – appear to hold much promise for the synthesis of nanocrystalline thin films as well as loosely aggregated nanoparticles. We present a systematic study of the process parameters that facilitate the growth of nanocrystalline metals and oxides. The systems studied include TiO₂, ZnO, γ-Al₂O₃, Cu₂O, Ag and Cu. The mean particle size and crystallographic orientation are influenced mainly by the sputtering power, the substrate temperature and the nature, pressure and flow rate of the sputtering gas. In general, nanocrystalline thin films were formed at or close to 300 K, while loosely adhering nanoparticles were deposited at lower temperatures. Received: 31 October 2000 / Accepted: 9 January 2001 / Published online: 26 April 2001     

Super-hydrophilic properties of TiO2-DLC nanocomposite films fabricated by the simple electrochemical process

Anatase TiO₂ nanoparticles incorporated DLC films were successfully deposited on single crystalline silicon substrates by the electrolysis of TiO₂-methanol solution under ambient atmospheric pressure and low temperature. Anatase TiO₂ nanoparticles were embedded into amorphous carbon matrix, forming the typical nanocrystalline/amorphous nanocomposite films, confirmed by transmission electron microscopy (TEM). TiO₂ incorporation effectively increased the sp³-hybridized carbon concentration in the composite film, and further regulated the microstructure and surface morphology. Furthermore, the static contact testing completely displayed, TiO₂ incorporation got the composite films super-hydrophilic, which fundamentally improved the wetting ability of DLC film.     

Photocatalytic decomposition of methylene blue and 4-chlorophenol on nanocrystalline TiO2 films under UV illumination: A ToF-SIMS study

The photocatalytic degradation of methylene blue and 4-chlorophenol on nanocrystalline TiO₂ (nc-TiO₂) under UV irradiation was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Nanocrystalline TiO₂ films were prepared from suspensions containing TiO₂-crystallites of different average sizes, the smallest one being 12 nm. The organic substances (either methylene blue or 4-chlorophenol) were applied to these films. The specimens were studied in the pristine state and upon UV exposure. The UV illuminations were carried out both under atmospheric conditions and in situ under ultrahigh vacuum in the ToF-SIMS instrument. Distinct mass signals from the parent molecules and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is significantly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signals are strongly diminished whereas fragmentation products are identified to be present at the TiO₂ surfaces. UV irradiations carried out under different vacuum conditions in the ToF instrument (ultrahigh vacuum, air or oxygen adsorption) indicate that varying ambient conditions may influence the photocatalytic reaction on the nanocrystalline TiO₂ films.     

Annealing effects on microstructural and optical properties of Nanostructured-TiO2 thin films prepared by sol-gel technique

In this paper we report on the effect of annealing on the microsctructural and optoelectronic properties of titanium dioxide (TiO₂) thin films prepared using sol-gel method onto silicon (Si) (100) and quartz substrates. The annealing temperatures range from 200 to 1000 °C. The Microstructural properties of annealed thin films were investigated by Thermal gravimetric analyses (TGA), X-ray diffraction (XRD) and Raman Spectroscopy. The surface morphology of the film was examined using Atomic Force Microscopy (AFM) method. The optical properties of TiO₂ thin films were characterized using UV-VIS and Spectroscopic ellipsometry. The results have shown that the TiO₂ thin films persist in the anatase phase even after annealing at 800 °C. The phase transformation from anatase to rutile occurred only when the films were annealed at 1000 °C. AFM studies revealed nanocrystalline structure where their shape and density depend strongly on the annealing temperatures. The elaborated nanostructured-TiO₂ thin films present a high transparency in the visible range. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO₂ thin films. Spectroscopic ellipsometry and UV-VIS shows that the band gap of TiO₂ thin films was found to decrease when the annealing temperature increases. The Anatase phase was find to show higher photocatalytic activity than the rutile one.     

Influence of annealing temperature on the photoluminescence property of ZnO thin film covered by TiO2 nanoparticles

In this work, ZnO thin films covered by TiO₂ nanoparticles (labeled as TiO₂-ZnO thin films) were prepared by electron beam evaporation. The influence of annealing temperature on the photoluminescence property of the samples was studied. The structures and surface morphologies of the samples were analyzed by X-ray diffraction (XRD) and atomic force microscope, respectively. The photoluminescence was used to investigate the fluorescent properties of the samples. The measurement results show that the ultraviolet emission of ZnO thin films is largely enhanced after they are covered by TiO₂ nanoparticles, while the green emission is suppressed. However, when the annealing temperature is relatively high (≥500 °C), the intensity of ultraviolet emission drops off and a violet emission peak along with a blue emission peak appears. This is probably connected with the atomic interdiffusion between TiO₂ nanoparticles and ZnO thin film. Therefore, selecting a suitable annealing temperature is a key factor for obtaining the most efficient ultraviolet emission from TiO₂-ZnO thin films.     

银/二氧化钛核壳纳米颗粒对碲化镉纳米晶的荧光增强研究

利用新合成的复合纳米结构银/二氧化钛核壳纳米颗粒,研究了金属银纳米颗粒对碲化镉纳米晶层荧光的增强情况.结果表明,这种新型复合金属纳米结构能极大地增强发光纳米晶层的荧光强度.银/二氧化钛核壳纳米颗粒是以水合肼、硝酸银和四异丙氧基钛为原材料,利用胶体化学法在水溶液中合成.透射电子显微镜图片表明这种新合成的银/二氧化钛纳米材料基本上呈球形,有较为明显的核壳结构,中间黑色的核是银纳米颗粒,外层颜色较浅部分是二氧化钛壳层.另外,包裹二氧化钛壳层后,银纳米颗粒的表面等离子吸收带从409 nm红移至430 nm,也证实了这种新型核壳纳米材料的形成.将此合成方法得到的银/二氧化钛纳米颗粒和碲化镉纳米晶用旋转涂覆方法进行直接组合后,得到了银纳米颗粒对碲化镉纳米晶荧光的明显增强,并对其增强的物理过程进行了讨论.这种能够增强荧光团发光的新型复合银纳米结构将在发光器件、荧光成像、生物探测等方面具有一定的应用价值.     

Magnetoelectric properties of Mn-substituted BiFeO3 thin films with a TiO2 barrier

Multiferroic thin films with the general formula TiO₂/BiFe_1−xMn_xO₃ (x=0.00, 0.05, 0.10 and 0.15) (TiO₂/BFMO) were synthesized on Au/Ti/SiO₂/Si substrates using a chemical solution deposition (CSD) method assisted with magnetron sputtering. X-ray diffraction analysis shows the thin films contained perovskite structures with random orientations. Compared with BFMO films, the leakage current density of the TiO₂/BFMO thin films was found to be lower by nearly two orders of magnitude, and the remnant polarizations were increased by nearly ten times. The enhanced ferroelectric properties may be attributed to the lower leakage current caused by the introduction of the TiO₂ layer. The J-E characteristics indicated that the main conduction mechanism for the TiO₂/BFMO thin film was trap-free Ohmic conduction over a wide range of electric fields (0-500 kV/cm). In addition, ferromagnetism was observed in the Mn doped BFO thin films at room temperature. The origin of ferromagnetism is related to the competition between distortion of structure and decrease of grain size and decreasing net magnetic moment in films due to Mn doping.     

Effects of thermally induced anatase-to-rutile phase transition in MOCVD-grown TiO2 films on structural and optical properties

Titanium dioxide (TiO₂) films with a thickness of 550 nm were deposited on quartz glass at 300 °C by metalorganic chemical vapor deposition. The effects of post-annealing between 600 °C and 1000 °C were investigated on the structural and optical properties of the films. X-ray diffraction patterns revealed that the anatase phase of as-grown TiO₂ films began to be transformed into rutile at the annealing temperature of 900 °C. The TiO₂ films were entirely changed to the rutile phase at 1000 °C. From scanning electron spectroscopy and atomic force microscopy images, it was confirmed that the microstructure of as-deposited films changed from narrow columnar grains into wide columnar ones. The surface composition of the TiO₂ films, which was analyzed by X-ray photoelectron spectroscopy data, was nearly constant although the films were annealed at different temperatures. When the annealing temperature increased, the transmittance of the films decreased, whereas the refractive index and the extinction coefficient calculated by the envelope method increased at high temperature. The values of optical band gap decreased from 3.5 eV to 3.25 eV at 900 °C. This abrupt decrease was consistent with the anatase-to-rutile phase transition. Received: 4 October 2000 / Accepted: 4 December 2000 / Published online: 23 May 2001     

Optical and electrical properties of nanocrystalline TiO<Subscript>2</Subscript>:Pd semiconducting oxides

Electrical and optical properties of TiO₂:Pd thin films deposited from Ti-Pd mosaic targets sputtered in reactive oxygen plasma have been studied. The properties were investigated for thin films with the Pd amount of 5.5 at. %, 8.4 at. % and 23 at. %. Based on resistivity measurements a drop from 10³ down to almost 10⁻³Ωcm has been recorded when the Pd amount was varied from 5.5 at. % to 23 at. %, respectively. Moreover, it was shown that doping with different amounts of Pd results in the possibility of obtaining both types of electrical conduction: n-type for the TiO₂ with 5.5 at. % and 8.4 at. % of Pd and p-type for the TiO₂ with 23 at. % of Pd thin films. From optical measurements it has been found that as the Pd amount was increased the transmission through the thin films was reduced and position of the fundamental absorption edge was shifted toward a longer wavelength range of up to 600 nm. The optical band gap was calculated for direct and indirect transitions from optical absorption spectra. Structural properties were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The XRD patterns displayed occurrence of the crystalline, TiO₂-rutile for lower Pd amounts (5.5 at. %, 8.4 at. %), while the TiO₂:Pd (23 at. %) thin films displayed XRD-amorphous behaviour. Images obtained from AFM displayed dense, nanocrystalline structure with homogenous distribution of crystallites. Additionally performed secondary ion mass spectroscopy investigation confirmed homogenous distribution of Pd in the whole thickness of the prepared thin films.     

Wavelet analysis of the surface morphologic of nanocrystalline TiO2 thin films

Surface morphologies of nanocrystalline TiO₂ thin films were studied by analyzing the surface profile of AFM images using wavelet transform method. Based on characterizing the fractal feature and computing the image details at different orientations and resolutions, the surface textures of nanocrystalline TiO₂ thin films before and after chemical treatment were examined. The results reveal that titanium isopropoxide treatment leads to an increase of surface roughness. The related mechanism of modification of the microstructure by chemical treatment associated with the improvement of the photocurrent response is discussed.     

Photoelectron spectroscopy of nanocrystalline anatase TiO2 films

Nanocrystalline TiO₂ (anatase) films were prepared using either colloidal suspensions or a sol-gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ∼4.6 eV was determined for the nanocrystalline anatase films.     

Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 °C

The article reports on correlations between the process parameters of reactive pulsed dc magnetron sputtering, physical properties and the photocatalytic activity (PCA) of TiO₂ films sputtered at substrate surface temperature T_surf ≤ 180 °C. Films were deposited using a dual magnetron system equipped with Ti (Ø50 mm) targets in Ar + O₂ atmosphere in oxide mode of sputtering. The TiO₂ films with highly photoactive anatase phase were prepared without a post-deposition thermal annealing. The decomposition rate of the acid orange 7 (AO7) solution during the photoactivation of the TiO₂ film with UV light was used for characterization of the film PCA. It was found that (i) the partial pressure of oxygen pO₂ and the total sputtering gas pressure p_T are the key deposition parameters influencing the TiO₂ film phase composition that directly affects its PCA, (ii) the structure of sputtered TiO₂ films varies along the growth direction from the film/substrate interface to the film surface, (iii) ∼500 nm thick anatase TiO₂ films with high PCA were prepared and (iv) the structure of sputtered TiO₂ films is not affected by the substrate surface temperature T_surf when T_surf < 180 °C. The interruption of the sputtering process and deposition in long (tens of minutes) pulses alternating with cooling pauses has no effect on the structure and the PCA of TiO₂ films and results in a decrease of maximum value of T_surf necessary for the creation of nanocrystalline nc-TiO₂ film. It was demonstrated that crystalline TiO₂ films with high PCA can be sputtered at T_surf ≤ 130 °C. Based on obtained results a phase zone model of TiO₂ films was developed.     

Characterization and antibacterial functions of Ag–TiO<Subscript>2</Subscript> and W–TiO<Subscript>2</Subscript> nanostructured thin films prepared by sol-gel/laser-induced technique

A novel sol-gel/laser-induced technique (SGLIT) has been developed to form nanocrystalline titanium dioxide (TiO₂) based thin films with an improved antibacterial performance. TiO₂ precursor films loaded with W⁺⁶ and Ag⁺² ions (W–TiO₂, Ag–TiO₂) were prepared separately by sol-gel method and spin-coated on microscopic glass slides. As-dried films were subjected to KrF excimer laser pulses at optimized parameters to generate mesoporous anatase and rutile phases at room temperature. The anatase phase was obtained after irradiation with 10 laser pulses only at 75–85 mJ/cm² fluence in W–TiO₂ films. However, higher number of laser pulses and higher W⁺⁶ content favored the formation of rutile. Whereas Ag–TiO₂ films exhibited anatase up to 200 laser pulses at the same fluence. The films were characterized by using XRD, FEG-SEM, TEM and UV-Vis spectrophotometer to investigate the crystallographic structure, phase transformation, surface morphology, film thickness and the optical properties. A crystallite size of approximately 20 nm was achieved from the anatase prepared by SGLIT. The films exhibited an enhanced antibacterial function against E-Coli cells under the UV excitation.     

Sol-gel synthesis, characterization and optical properties of mercury-doped TiO2 thin films deposited on ITO glass substrates

The Hg-doped and undoped nano-crystalline TiO₂ films on ITO glass substrates surface and polycrystalline powders were prepared by sol-gel dip coating technique. The crystal structure and surface morphology of TiO₂ were characterized by means of X-ray diffractometer (XRD), atomic force microscope (AFM), spectrophotometer, Fourier-transform infrared (FTIR), and spectroscopic ellipsometry (SE). The results indicated that the powder of TiO₂, doped with 5% Hg in room temperature was only composed of the anatase phase whereas in the undoped powder exhibits an amorphous phase were present. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 400 °C. The average crystallite size of the undoped TiO₂ films was about 8.17 nm and was increased with Hg-doping in the TiO₂ films. Moreover, the grains distributed more uniform and the surface roughness was greater in the Hg-doped TiO₂ films than in the undoped one. Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range (1.95-2.49) and the porosity is in the range (47-2.8). The coefficient of transmission varies from 60 to 90%. SE study was used to determine the annealing temperature effect on the optical properties in the wavelength range from 0.25 to 2 μm and the optical gap of the Hg-doped TiO₂ thin films.     

Photoconductivity studies on amorphous and crystalline TiO<Subscript>2</Subscript> films doped with gold nanoparticles

In this work, amorphous and crystalline TiO₂ films were synthesized by the sol–gel process at room temperature. The TiO₂ films were doped with gold nanoparticles. The films were spin-coated on glass wafers. The crystalline samples were annealed at 100°C for 30 minutes and sintered at 520°C for 2 h. All films were characterized using X-ray diffraction, transmission electronic microscopy and UV-Vis absorption spectroscopy. Two crystalline phases, anatase and rutile, were formed in the matrix TiO₂ and TiO₂/Au. An absorption peak was located at 570 nm (amorphous) and 645 nm (anatase). Photoconductivity studies were performed on these films. The experimental data were fitted with straight lines at darkness and under illumination at 515 nm and 645 nm. This indicates an ohmic behavior. Crystalline TiO₂/Au films are more photoconductive than the amorphous ones.     

Growth and characterization of nanostructured anatase phase TiO<Subscript>2</Subscript> thin films prepared by DC reactive unbalanced magnetron sputtering

Titanium dioxide thin films were deposited on three different unheated substrates by unbalanced magnetron sputtering. The effects of the sputtering current and deposition time on the crystallization of TiO₂ thin films were studied. The TiO₂ thin films were deposited at three sputtering current values of 0.50, 0.75, and 1.00 A with different deposition times of 25, 35, and 45 min, respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The film thickness was determined by field emission scanning electron microscopy (FE-SEM), and the optical property was evaluated with spectroscopic ellipsometry. The results show that polycrystalline anatase films were obtained at a low sputtering current value. The crystallinity of the anatase phase increases as the sputtering current increases. Furthermore, nanostructured anatase phase TiO₂ thin films were obtained for all deposition conditions. The grain size of TiO₂ thin films was in the range 10–30 nm. In addition, the grain size increases as the sputtering current and deposition time increase.