Grazing incidence X-ray study of Ge-nanoparticle formation in (Ge:SiO2)/SiO2 multilayers

We present the study of formation of Ge-nanoparticles (Ge-NP) in germanosilicate (Ge:SiO₂) multilayer (ML) films under thermal treatment. In anticipation of controllable formation of Ge-NP, ML films were prepared by magnetron deposition at room temperature as 20 bi-layer stacks, each bi-layer comprised of a 7 nm thick layer of (Ge + SiO₂) (molar ratio: 60:40) succeeded by a 7 nm thick layer of pure SiO₂, and then annealed for 1 h, up to T_a = 900 °C. Formation and morphology of Ge-NP were analyzed by combining the information obtained from the grazing incidence small angle X-ray scattering and X-ray diffraction. It was found that precipitation of Ge-NP starts at T_a = 600 °C, while high degree of in-plane confinement and lateral ordering of rather uniform precipitated particles is achieved at T_a =  700-800 °C range. At still higher annealing temperature T_a > 800 °C, volume fraction of precipitated Ge-NP in SiO₂ matrix diminishes due to the out-diffusion of Ge atoms from the film, while Ge-NP are no more well confined to (Ge + SiO₂) layers.     

Growth of Ge nanoparticles on SiO2/Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films

Multilayer germanosilicate (Ge:SiO₂) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO₂ layer is separated by a pure SiO₂ layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO₂ layer closest to the interface through a pure SiO₂ layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.     

Quantum confinement effect in SiO2 films containing Ge microcrystallites

SiO₂ thin films embedded with Ge microcrystallites (Ge-SiO₂ films) were prepared by RF-magnetron co-sputtering method from a composite target of Ge and SiO₂. The average size of Ge crystallites can be modulated by the experiment parameters. The optical absorption and non-linear optical properties of Ge-SiO₂ films were measured. The blue shift of the optical absorption edge, the saturated absorption and two-photon absorption under the condition of resonant absorption have been observed, and are discussed according to the quantum confinement effect.     

Melting, crystallization and optical behaviors of poly (ethylene terephthalate)-silica/polystyrene nanocomposite films

Poly (ethylene terephthalate) (PET)-silica (SiO₂)/polystyrene (PS) nanocomposite films were prepared by melting PET with the core-shell SiO₂/PS nanoparticles. Differential scanning calorimetry (DSC) results showed that the crystallization temperature of PET-SiO₂/PS nanocomposite films with 2 wt.% PS-encapsulated SiO₂ nanoparticles reached 205.1 °C, 11.6 °C higher than that of PET. For crystallized PET-SiO₂/PS nanocomposite films, double melting peaks appeared in DSC curves similar to PET. Scanning electron microscopy revealed a netlike fibre morphology for the amorphous PET-SiO₂/PS nanocomposite films with 2 wt.% PS-encapsulated SiO₂ nanoparticles. The light transmittance of these amorphous PET-SiO₂/PS nanocomposite films reached 87.9%, compared to 84.2% for PET. With the increase of annealing temperature from 110 to 150 °C, the transmittance of PET-SiO₂/PS nanocomposite films decreased slowly from 69.9 to 46.9%, while their haziness increased slightly from 45.8 to 48.2%. All these phenomena are suggested to result from the strongly heterogeneous nucleation of PS-encapsulated SiO₂ nanoparticles in PET.     

Optical properties and hydrophilic behaviors of TaOxNy thin films with and without rapid thermal annealing

TaO_xN_y thin films were prepared using DC reactive sputtering of Ta target with the variation of (O₂ + N₂)/Ar ratio, followed by rapid thermal annealing (RTA) at 800 °C for 5 min. During RTA, the amorphous structure of the as-deposited films would transform to crystallized phases that might contain either TaON, or Ta₂O_5, or both. With the increase of (O₂ + N₂)/Ar ratio, Ta₂O₅ phase becomes more dominant, while TaON was formed in an opposite way. Elemental analysis also shows the same trend. Hydrophilicity and optical properties of these films were found to be dependent on phases formed after annealing. The optical band gap was measured and calculated to be in the range of 2.43 eV (510 nm) to 3.94 eV (315 nm). The films with low band gap values exhibited super hydrophilicity behavior under visible light irradiation, mainly due to their light absorption had been extended to visible light range. Clearly, it was due to the existence of TaON phase.     

Effect of swift heavy ion irradiation on bare and coated ZnS quantum dots

The present study compares structural and optical modifications of bare and silica (SiO₂) coated ZnS quantum dots under swift heavy ion (SHI) irradiation. Bare and silica coated ZnS quantum dots were prepared following an inexpensive chemical route using polyvinyl alcohol (PVA) as the dielectric host matrix. X-ray diffraction (XRD) and transmission electron microscopy (TEM) study of the samples show the formation of almost spherical ZnS quantum dots. The UV-Vis absorption spectra reveal blue shift relative to bulk material in absorption energy while photoluminescence (PL) spectra suggests that surface state and near band edge emissions are dominating in case of bare and coated samples, respectively. Swift heavy ion irradiation of the samples was carried out with 160 MeV Ni¹²⁺ ion beam with fluences 10¹² to 10¹³ ions/cm². Size enhancement of bare quantum dots after irradiation has been indicated in XRD and TEM analysis of the samples which has also been supported by optical absorption spectra. However similar investigations on irradiated coated quantum dots revealed little change in quantum dot size and emission. The present study thus shows that the coated ZnS quantum dots are stable upon SHI irradiation compared to the bare one.     

CdS quantum dot sensitized nanocrystalline Gd-doped TiO2 thin films for photoelectrochemical solar cells

CdS quantum dot sensitized Gd-doped TiO₂ nanocrystalline thin films have been prepared by chemical method. X-ray diffraction analysis reveals that TiO₂ and Gd-doped TiO₂ nanocrystalline thin films are of anatase phase. The absorption spectra revealed that the absorption edge of CdS quantum dot sensitized Gd-doped TiO₂ thin films shifted towards longer wavelength side (red shift) when compared to that of CdS quantum dot sensitized TiO₂ films. CdS quantum dots with a size of 5 nm have been deposited onto Gd-doped TiO₂ film surface by successive ionic layer adsorption and reaction method and the assembly of CdS quantum dot with Gd-doped TiO₂ has been used as photo-electrode in quantum dot sensitized solar cells. CdS quantum dot sensitized Gd-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.18 %, which is higher than that of CdS quantum dot sensitized TiO₂ (0.91 %).     

Origin of photoluminescence peaks in Ge–SiO2 thin films

Ge–SiO₂ thin films were prepared by the RF magnetron sputtering technique on p-Si substrates from a Ge–SiO₂ composite target. The as-deposited films were annealed in the temperature range of 300–1000 °C under nitrogen ambience. The structure of films was evaluated by X-ray diffraction, X-ray photoemission spectroscopy and Fourier transform infrared absorption spectroscopy. Results show that the content of Ge and its oxides in the films change with increasing annealing temperature (T_a), the photoluminescence (PL) characteristics are closely dependent on the contents of Ge and its oxides in SiO₂ matrix. The dependence observed strongly suggests that the PL peak at 394 nm is related to the existence of GeO and 580 nm to that of Ge nanocrystal (nc-Ge) in the films.     

SiO2–K2O–MgO vitreous films doped with erbium and silver nanoparticles for optical applications

The search of new glass compositions for films produced with high optical transparency throughout the visible spectrum from 0.5 μm to the near infrared region is important for optical integrated applications. In this work, we present the preparation of SiO₂–K₂O–MgO vitreous sol–gel films on SiO₂ substrates co-doped with Er³⁺ and Ag nanoparticles. The silver quantum dots were synthesized in continuous media (ex situ) by chemical reduction and isolation from media by anchoring chemical solution-compatible modifiers (aminosilanes) on their surfaces; then, they were successfully integrated into the glass matrix. We present the study of the preparation process and characterization of sol–gel matrices doped with Ag⁰ nanoparticles and Er³⁺, analyzing the role of synthesis parameters and optical properties, and comparing them with other wave-guide compositions, such as PLZT and SiO₂–B₂O₃.     

Enhanced effect of vacuum-deposited SiO2 overlayer on photo-induced hydrophilicity of TiO2 film

Photo-induced hydrophilicity of SiO₂/TiO₂ multilayer film prepared by using the vacuum deposition method was investigated by means of water contact angle measurement. Using black light irradiation of the films centering at a wavelength of 365 nm, an extreme photo-induced hydrophilicity was achieved when the TiO₂ film was covered by SiO₂ overlayer ranging from 10 to 20 nm in thickness. These multilayer films exhibited much more extreme hydrophilicity than the TiO₂ film without SiO₂ overlayer. The surface analyses revealed that the enhanced photo-induced hydrophilic surface of the multilayer films exhibited an improved photo-catalytic activity towards decomposition of organic substances on their surfaces. It was found that significant growth of the SiOH group occurred in the uppermost surface of the SiO₂ overlayer of the multilayer films through the depth profile measurement of TOF-SIMS. This result suggests that the photo-generated reactive species such as hole created in the TiO₂ film may transmit the SiO₂ layer to reach the surface. The enhanced photo-induced hydrophilicity of the films can be explained by a synergetic effect of the improved photo-catalytic activity of the multilayer film and the stable hydrophilicity of SiO₂ itself.     

A Novel Fabrication and Properties Investigation of Permalloy-SiO2 Granular Films with Induced Anisotropy

Permalloy-SiO₂ (PS) granular films with various metallic volume fraction (x_V) have been fabricated by non-continuum multilayer alternate sputtering with different power to generate appropriate controllable anisotropy and control the microstructure precisely. Transmission Electron Microscopy (TEM) investigation shows that the PS granular films consist of Permalloy crystalline nano-sized granules embedded in amorphous SiO₂ matrix. The films exhibit excellent soft magnetic properties and high resistivity because of the unique microstructure and composition variety, both of which have been controlled by adjusting the preparation conditions. Soft magnetic properties of the films are improved with increasing metallic x_V and sputtering power, while resistivity of the films goes down with increasing x_V and sputtering power. Furthermore, the controllable anisotropy generated in alternated sputtering process is applicable to integrated planar magnetic inductor which operates in the range of Radio Frequency (RF) for device performance enhancement.     

Characteristics of post-annealed SrTiO3 thin films prepared by mirror-confinement-type ECR plasma sputtering

SrTiO₃ films were synthesized on Pt/Ti/SiO₂/Si multilayer substrates by mirror-confinement-type ECR plasma sputtering without substrate heating. All films were found to be well crystallized at a substrate temperature below 450 K. A low temperature post-annealing of the films by electromagnetic-wave radiation drastically improved the crystallographic and electric properties of Pt/SrTiO₃/Pt/Ti/SiO₂/Si capacitors. The crystallinity of the films indicated little variation by post-annealing, but irradiation of electromagnetic wave was confirmed to be effective for decreasing the post-annealing time and temperature. The electric properties of films annealed without Pt upper electrodes were better than those with them, and the film dielectric constant reached a value of 260, which is nearly equal to thebulk one, at an annealing temperature of 573 K.     

Characteristics of post-annealed SrTiO3 thin films prepared by mirror-confinement-type ECR plasma sputtering

SrTiO₃ films were synthesized on Pt/Ti/SiO₂/Si multilayer substrates by mirror-confinement-type ECR plasma sputtering without substrate heating. All films were found to be well crystallized at a substrate temperature below 450 K. A low temperature post-annealing of the films by electromagnetic-wave radiation drastically improved the crystallographic and electric properties of Pt/SrTiO₃/Pt/Ti/SiO₂/Si capacitors. The crystallinity of the films indicated little variation by post-annealing, but irradiation of electromagnetic wave was confirmed to be effective for decreasing the post-annealing time and temperature. The electric properties of films annealed without Pt upper electrodes were better than those with them, and the film dielectric constant reached a value of 260, which is nearly equal to the bulk one, at an annealing temperature of 573 K.     

Dielectric engineering of Ge nanocrystal/SiO2 nanocomposite thin films with Ge ion implantation: Modeling and measurement

Ge nanocrystal (nc-Ge) embedded SiO₂ nanocomposite thin films have been synthesized with the ion implantation technique. The distribution profile of nc-Ge in the SiO₂ matrix can be tailored by varying the implantation energy and dose in the Ge ion implantation process; thus the effective dielectric constant of the nc-Ge/SiO₂ nanocomposite thin films can be engineered. The effective metal–oxide-semiconductor (MOS) capacitance of the nanocomposite thin films has been calculated using the sub-layer model and the Maxwell–Garnett effective medium approximation, taking the reduced dielectric constant corresponding to the nanometer size of nc-Ge into account. On the other hand, capacitance–voltage measurements on the MOS structures based on the nc-Ge/SiO₂ thin films have been conducted to extract the capacitance experimentally. The modeling and measurement results have shown good agreement, suggesting that the nanocomposite dielectric engineering can be easily realized through the energy- and dose-controlled Ge⁺ implantation technique.     

Growth and characterization of nc-Ge prepared by microwave annealing

Ge nanocrystals embedded in Silicon oxide matrix have been synthesized on Si substrate by co-sputtering of SiO₂ and Ge using RF magnetron sputtering technique. The as deposited films were subjected to microwave annealing at 800 and 900 °C. The structural characterization was performed by using X-ray diffraction (XRD) and Raman spectrometry. XRD measurements confirmed the formation of Ge nanocrystals. Raman scattering spectra showed a peak of Ge-Ge vibrational mode around 299 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Variation of the nanocrystal size with annealing temperature has been discussed. Advantages of microwave annealing are explained in detail.     

Effects of methane flow rate on the optical properties and chemical bonding of germanium carbon films deposited by reactive sputtering

Amorphous hydrogenated germanium carbon (a-Ge_1−xC_x:H) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (111) target in a CH₄ + H₂+Ar mixture and their composition, optical properties, chemical bonding were investigated as a function of gas flow rate ratio of CH₄/(Ar + H₂). The results showed that the deposition rate first increased and then decreased as gas flow rate ratio of CH₄/(Ar + H₂) was increased from 0.125 to 0.625. And the optical gap of the a-Ge_1−xC_x:H films increased from 1.1 to 1.58 eV accompanied with the increase in the carbon content and the decrease in the relative content of Ge–C bonds of the films as the CH₄ flow rate ratio was increased, while refractive index of the films decreased and the absorption edge shifted to high energy. Through the analysis of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, it was found that the formation of Ge–C bonds in the films was promoted by low CH₄ flow rate which is connected with relatively high H₂ concentration and Ge content. Especially in low CH₄ concentration, the formation of sp²-hybridised C–C bonds was suppressed considerably both due to the etching effect on weak bonds of H and the fact that chemical bonding for germanium can be only sp³ hybridization.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Ferromagnetism in One Layer of Self-organized InMnAs Quantum Dots

One layer of self-assembled InMnAs quantum dots with InGaAs barrier was grown on high-resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of ferromagnetic structure was confirmed in the InMnAs dilute magnetic quantum dots. The one layer of self-assembled InMnAs quantum dots was found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T _C =80 K. It is likely that the ferromagnetic exchange coupling of sample with T _C =80 K is hole-mediated resulting in Mn substituting Ge. PL emission spectra of InMnAs samples grown at temperature of 210°C and 285°C show that the interband transition peak centered at 1.31 eV comes from the InMnAs quantum dot.     

Ultrasensitive Surface‐Enhanced Raman Spectroscopy Detection Based on Amorphous Molybdenum Oxide Quantum Dots

Surface‐enhanced Raman spectroscopy (SERS) based on plasmonic semiconductive material has been proved to be an efficient tool to detect trace of substances, while the relatively weak plasmon resonance compared with noble metal materials restricts its practical application. Herein, for the first time a facile method to fabricate amorphous H_xMoO₃ quantum dots with tunable plasmon resonance is developed by a controlled oxidization route. The as‐prepared amorphous H_xMoO₃ quantum dots show tunable plasmon resonance in the region of visible and near‐infrared light. Moreover, the tunability induced by SC CO₂ is analyzed by a molecule kinetic theory combined with a molecular thermodynamic model. More importantly, the ultrahigh enhancement factor of amorphous H_xMoO₃ quantum dots detecting on methyl blue can be up to 9.5 × 10⁵ with expending the limit of detection to 10^?9 m . Such a remarkable porperty can also be found in this H_xMoO₃‐based sensor with Rh6G and RhB as probe molecules, suggesting that the amorphous H_xMoO₃ quantum dot is an efficient candidate for SERS on molecule detection in high precision.