Sol-Gel Spin Coating Research Articles

Effects of Al content on Zn0.95Mg0.05O Thin Films Deposited by Sol–Gel Spin Coating

Al-doped Zn0.95Mg0.05O films were deposited on glass and silicon substrates by a sol–gel spin coating and the coated samples were dried at 350 to 450 °C and annealed at 550, 650, and 750 °C. The effects of Al content and annealing temperature on the structural and optical properties of Al-doped Zn0.95Mg0.05O thin films were studied. Al-doped Zn0.95Mg0.05O alloy films have a preferred orientation in the (002) direction. By increasing Al content, crystallinity as well as transmittance was decreased. An X-ray photoelectron spectroscopy (XPS) analysis of Al-doped Zn0.95Mg0.05O thin films was performed to determine the chemical state on the surface of the thin films. The resistivities of the thin films were found to increase with increasing Al content and decrease with increasing annealing temperature. For an Al (0.5 wt %)-doped Zn0.95Mg0.05O thin film with a low resistivity of 3.15×10-2 Ω cm dried at 450 °C and annealed at 750 °C, optical band gap increased from 3.08 to 3.32 eV.

High quality c-axis oriented thin ZnO film obtained at very low pre-heating temperature

Highly oriented ZnO thin film has been obtained at a very low pre-heating temperature, with the spin-coating sol–gel technique. The dependence of the c-axis orientation on the pre-heating temperature has been studied with experimental design and response surface techniques to optimize the deposition process with respect to c-axis orientation, and surface uniformity. The optimization variables selected for this study are: pre-heating temperature, spin-coating speed and number of coating layers. The films are probed with X-ray diffraction and electron microscopy.

Formation and thermal-induced changes of mesostructures in fluorinated organosilicate films

Mesoporous fluorinated organosilicate films were synthesized from tetramethoxysilane and perfluoroalkylsilanes (RO) 3Si-R′ under acidic conditions in the presence of cationic surfactant or triblock copolymer by sol–gel spin-coating. The mesoporous fluorinated organosilicate films made from perfluoroalkylsilanes (PFASs) with long perfluoroalkyl chains, which acted as a template, displayed a hexagonal mesostructure with very low concentration of surfactants, regardless of the kind of surfactants. Although most of the surfactants and organic moieties were decomposed above 550 °C, the mesostructure of mesoporous fluorinated organosilicate films was maintained up to 650 °C. After calcination at 550 °C, their composition was similar to that of mesoporous silica films, and the cylindrical mesostructure was changed to less ordered and broken mesostructure when PFASs with long perfluoroalkyl chains were used. However, the cylindrical mesostructure was maintained when PFASs with short perfluoroalkyl chains were used. Therefore, PFASs with long perfluoroalkyl chains acted as a structure directing agent, and the chain length of PFASs affected the formation of mesostructure and thermal-induced mesostructural change. Also, the increase of calcination temperature caused the change of the composition, mesostructure, and optical property in the mesoporous fluorinated organosilicate films.

Low-temperature crystallization of sol–gel-derived lead zirconate titanate thin films using 2.45 GHz microwaves

Lead zirconate titanate (PZT) thin films of thickness 420 nm were deposited on Pt/Ti/SiO 2/Si substrate using a spin coating sol–gel precursor solution, and then annealed using 2.45 GHz microwaves at a temperature of 450 °C for 30 min. The film has a high perovskite content and high crystallinity with a full width at half maximum of 0.35°. The surface roughness of the PZT thin film was 1.63 nm. Well-saturated ferroelectric properties were obtained with a remanent polarization of 46.86 μC/cm 2 and coercive field of 86.25 kV/cm. The film also exhibited excellent dielectric properties with a dielectric constant of 1140 and a dissipation factor of 0.03. These properties are superior to those obtained by conventional annealing at a temperature of 700 °C for 30 min.

FABRICATION AND ELECTRON MICROSCOPY CHARACTERIZATION OF METAL-GATED CARBON NANOTUBE EMITTER ARRAYS

A technique was investigated for the fabrication of triode-type carbon nanotube (CNT) field emitter arrays, where an integrated extraction gate was built between the nanotube cathode and the anode. The gate improves the control capability of emission currents. To fabricate the metal-gated CNT field emitter arrays, well ordered cells were generated by focused ion beam (FIB) milling of platinum ( Pt ) coated silicon ( Si ) substrate and then modified by chemical etching. Two types of catalyst elements iron ( Fe ) and nickel ( Ni ), were used for growing the CNTs inside the cells. The methods for depositing catalysts into the cells include spin coating sol–gel Fe , FIB induced decomposition of ferrocene and sputter coating pure Ni . CNT growth was carried out by a chemical vapor deposition (CVD) process. The results suggest that the CNTs grew from inside the cells where the catalysts were located. In comparison, the CNTs synthesized from the sol–gel Fe catalyst were straighter than those from ferrocene Fe and pure Ni . The density and orientation of the CNTs in each cell are directly related to the type and quantity of the catalysts and are also affected by the size of the cells.

Fabrication Process of Sol-Gel Spin Coating forSrBi2Ta2O9 Films Applied to FeRAM

We investigate SrBi2Ta2O9 (SBT) films preparedby the sol-gel spin method with different spin rates or differentanneal conditions for the first layer of SBT, as promisingferroelectric layer materials applied to ferroelectric random accessmemory (FeRAM). All the specimens in this experiment have similarSBT crystal orientations of (115), (020), (220), and (135). ThePt/SBT/Pt capacitor with coating of 3000 rpm spin rate has a perfectrectangle shape of hysteresis loops, remanent polarization of7.57 μC/cm2 and coercive voltage of 0.816 V at 5 V voltageamplitude. These characteristics are better than those with coating of3500 rpm spin rate, which is attributed to the influence for thicknessand grain size of the film from depressed spin rate. Slow-rate annealin the furnace for the first layer of SBT can improve the crystallizationprocesses and properties for SBT layers slightly, compared with rapidthermal annealing. The ion damage from etching for the top electrodecan influence leakage current characteristics of the Pt/SBT/Ptcapacitor at positive voltage bias.

Seed-layer induced growth of high-quality oriented ZnO films by a sol–gel process

A two-step process was developed to grow high-quality oriented ZnO films on Si (1 0 0) substrates, which included the pre-deposition of a very thin ZnO film (seed layer) by pulsed laser deposition and the subsequent sol-gel spin-coating of ZnO films. The effects of the seed layer on the orientation, crystallinity, and morphology of ZnO films were investigated. Results show that ZnO films on the seed layer exhibit a single (0 0 2) orientation, but that on the bare silicon substrate is multi-oriented. Compared with ZnO films on the bare silicon substrate, the crystallinity of ZnO films on the seed layer is enhanced obviously at high preheating temperatures above 500 °C. In addition, ZnO films on the seed layer have a denser internal structure than that on Si (1 0 0). These results indicate the seed layer may effectively induce the growth of high-quality oriented ZnO films by sol–gel.

Photo-induced super-hydrophilic property and photocatalysis on transparent Ti-containing mesoporous silica thin films

The transparent Ti-containing mesoporous silica thin films can be prepared on quartz plate using a spin-coating sol–gel method. The spectroscopic characterization has revealed that the Ti-containing mesoporous silica thin films contain isolated and tetrahedrally-coordinated titanium oxide moieties in the frameworks. Compared with the common mesoporous silica thin films, these Ti-containing mesoporous silica thin films have demonstrated a strong hydrophilic surface property even before UV-irradiation. After UV-light irradiation, the contact angle of water droplet on the Ti-containing mesoporous silica thin films became lower, indicating the appearance of the super-hydrophilic property. Under UV-light irradiation Ti-containing mesoporous silica thin films also exhibited highly selective activity for the photocatalytic oxidation of propylene. The isolated and tetrahedrally-coordinated titanium oxide moieties are responsible for these photo-induced surface reactions.

Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces

A new one-step method is reported for the deposition of hybrid mesoporous thin films on various electrode surfaces (gold, platinum, glassy carbon). Deposition was achieved by spin-coating sol–gel mixtures in the presence of a surfactant template to get mesostructured thin layers on the various conducting substrates. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and/or organosilane precursors on the electrode surface. Extraction of the surfactant from the ordered mesoporous films led to a large increase of mass transport rates into the materials and imparted high accessibility to the organic moieties in case of functionalized mesoporous overlayers. The electrochemical properties of the film-modified electrodes have been studied by cyclic voltammetry (CV), and also via the chemical accumulation of mercury ions prior to their stripping analysis by differential pulse voltammetry (i.e. for thiol-functionalized thin films). Some evidences to support the formation of self-assembled monolayers (SAMs) on electrodes, have been also discussed. The formation of well-adhering mesoporous thin films on solid electrode surfaces is expected to have a high impact on the development of new electrochemical sensors.

Effects of Annealing Condition on the Preparation of Indium-Tin Oxide (ITO) Thin Films via Sol-Gel Spin Coating Process

Indium tin oxide (ITO) thin films were deposited on glass substrates via sol-gel spin coating process from a mixed solution of Indium (Ⅲ) acetylacetonate and Tin (Ⅳ) iso-propoxide. Then, ITO thin films were fired at 500°C, and then annealed at 500°C for 30 min with the sequential annealing process; VacuumN2Ar/H2, N2Ar/H2 and Ar/H2 gas. The effects of the different annealing processes on the surface morphologies and sheet resistance of ITO thin films were investigated. Sheet resistance values of ITO thin films treated under VacuumN2Ar/H2, N2Ar/H2 and Ar/H2 annealing process were 1.25 kohm/sq., 3.18 kohm/sq. and 4.92 kohm/sq., respectively. Actually, the sequential atmosphere gases and non-oxidizing gas, which were used in annealing process influenced the microstructural features or surface morphologies of ITO thin films: grain size and surface roughness. Thus, it was presumed that the sequential annealing condition influenced the densification behavior in the microstructural evolution of ITO thin films.

Silver negative-ion implantation to sol–gel TiO2 film for improving photocatalytic property under fluorescent light

We have investigated improvement of photocatalytic properties of spin-coated sol–gel anatase-type TiO2 with a bandgap of 3.2eV by negative ion-implantation. Silver-negative ions were implanted to anatase-type sol–gel TiO2 films (about 500nm in thickness) on a glass at 65 and 30keV with various doses in 1016–1017ions/cm2. Subsequent annealing was performed at up to 600°C for 1h. After annealing at above 300°C, all the samples showed strong optical absorption of surface plasmon resonance of Ag nanoparticles at around the photon energy of 2.2eV. This means that Ag nanoparticles were formed in the TiO2 films. In the photocatalytic properties under fluorescent light, the Ag-implanted sample (30keV, 1×1016ions/cm2) after annealing at 400°C showed an improved photocatalytic efficiency by 1.58 times than a rutile single crystal TiO2 with bandgap of 3.0eV less than that of the anatase. Ag nanoparticles formed at shallower depth are considered to work well for catalysis.

ZrO 2 thin films with controllable morphology and thickness by spin-coated sol–gel method

A spin-coated sol–gel method for the fabrication of highly porous or smooth ultra-thin ZrO 2 films was developed. By spin-coating sol solutions, which contained ZrCl 4 in anhydrous isopropanol, on substrates, the morphology and thickness of thin films were easily controlled by changing the concentrations of ZrCl 4 in sol solutions. A highly porous film with large specific surface area (180 m 2/g) was obtained at 150 g ZrCl 4/l without the addition of templates. When the concentration of ZrCl 4 decreased to 17 g/l, the porous surface turned into a smooth thin film. A linear relationship between film thicknesses and concentrations of ZrCl 4 ranging from 17 to 3 g ZrCl 4/l was observed, and an ultra-thin film with the thickness of 1.8 nm was achieved when 3 g ZrCl 4/l was used. Results obtained in this study clearly show that spin coating of anhydrous sol solution is an efficient method for the fabrication of ZrO 2 films with controllable morphology and thickness.

Au–Pt alloy nanocrystals incorporated in silica films

Au, Pt and Au–Pt alloy nanocrystals have been prepared in thin SiO2 film matrices by sol–gel spin-coating, followed by heating at 450 °C in 10% H2–90% Ar. X-Ray diffraction patterns reveal that the Au and Au–Pt nanocrystals have a preferential (111) orientation. Upon increasing the Pt concentration, part of the Pt does not alloy with Au, but instead forms a shell around the Au–Pt alloy core. The alloy composition itself goes up to Au(50) : Pt(50), and the Pt shells are formed around the alloy above an alloy composition of Au(75) : Pt(25). The surface plasmon resonance (SPR) band at 544 nm of Au gradually disappears due to the formation of Au–Pt alloys and core–shell structures.

Sol–gel derived (Li, Mg): ZnO films with high c-axis orientation and electrical resistivity

ZnO films with high orientation and resistivity have a great potential in piezoelectrical devices. In this work, Li- and Mg-doped ZnO films on glass substrates were prepared by sol–gel spin-coating and two-step growth. The effects of the dopants and heating temperature on c-axis orientation and resistivity of the resulting films were investigated. For the ZnO films formed by two-step growth, the orientation of the first grown layer played a significant role in c-axis orientation of the film. The results showed that Mg doping into Li:ZnO films could improve c-axis orientation and increase the resistivity, the optimized (Li,Mg):ZnO film had a highly c-axis orientation, small grain size and resistivity of 11.18×10 7 Ω·cm.

Sol–gel photonic bandgap materials and structures

In this work, one-dimensional multilayer structures of periodically alternating low refractive index (SiO2) and high index (TiO2) materials have been deposited by sol–gel spin-coating, in the form of dielectric mirrors (distributed Bragg reflectors) and Fabry-Perot microcavities. Some of the microcavities were doped with europium. These structures have been characterized structurally (by X-ray diffraction, Raman spectroscopy and field emission-scanning electron microscopy) and optically (by ellipsometry and visible–near IR spectroscopy); their optical properties, namely the stop band of high reflectivity and, in the case of the microcavities, the cavity mode and the photoluminescence behavior of the Eu3+ ions inserted in the cavity layer, were assessed. In addition, three-dimensional-like structures have also been synthesized by a colloidal/sol–gel route, starting with the self-organization of a colloidal dispersion of polystyrene microspheres, by dip-coating or convective self-assembly, followed by infiltration of the interstices with titania and removal of the polymeric template. The structural and optical properties of the inverse opals prepared by this method have also been studied, by scanning electron microscopy and by visible reflection spectroscopy, in order to assess their photonic bandgap properties.

Surface Skin Development and Rupture During Sol-Gel Spin-Coating

Spin coating is one of the standard methods for depositing sol-gel, nanocomposite, or polymer coatings onto flat substrates (silicon wafers, glass plates for displays, sensor substrates, etc.). Our recent research has been focused at understanding a wide variety of defect formation mechanisms and looking for ways to prevent these defects. A key aspect of spin-coating is the solvent evaporation that happens at the top surface of the coating fluid during spinning--even while the fluid is moving rapidly, radially, outward on the substrate. As the fluid gets physically thinner, concentration gradients can develop within the fluid as a result of the evaporation. The enrichment of the precursors at the top surface can cause premature gelation there, in other words a “skin” may form. This skin layer is also more resistant to stretching than the underlying fluid and can retard radial fluid motions as well as further solvent evaporation. It is even possible for the stretching forces to be large enough that fracture or tearing of the skin occurs. These surface ruptures create wispy locations of significantly thinner coating or small randomly located linear rips, but they do not typically result in complete penetration through the coating. The occurrence of these defects depends to a large degree on the volatility of key solvents used in the sol-gel process, with less volatile solvents being less likely to result in surface ruptures.

Photochromism in spiropyran impregnated fluorinated mesoporous organosilicate films

Fluorinated mesoporous organosilicate films were synthesized by sol-gel spin-coating on a quartz substrate using (heptadecafluorodecyl)trimethoxysilane, tetramethylorthosilicate, and n-cetyltrimethylammonium chloride as the fluoro-precursor, silica source, and supramolecular template, respectively. The photochromism efficiency of photochromic dyes (spiropyran) increases in the order non-porous < hexagonal mesoporous < fluorinated mesoporous organosilicate films. The relatively high efficiency of the photochromism of spiropyran in fluorinated mesoporous organosilicate films suggests that such films are promising materials for applications as optical shutters and light modulators.

Study of silica–titania films doped with Er and Ag by RBS and ERDA

The photoluminescence (PL) efficiency of Er-doped silica–titania planar waveguides, prepared by sol–gel processing, may be enhanced through the incorporation of neutral Ag metal particles. However, the presence of residual Si–OH groups and the limited reproducibility of the doping process limit the optical activity of the Er 3+ ions. We have used Rutherford backscattering spectrometry (RBS) combined with elastic recoil detection analysis (ERDA) to study the incorporation behaviour of Ag and H atoms during the heat treatments used to densify the films. These were deposited by sol–gel spin-coating on single crystal Si or silica-buffered Si substrates (∼5 μm thick silica buffer layer). The results indicate a homogeneous distribution of Er in the as-deposited films. On the contrary, Ag displays a bimodal in-depth profile centred at the film surface and at the film/substrate interface. Annealing up to 700 °C leads to in-diffusion and concentration of all the Ag species at the latter interface. The H concentration maintains nearly the same values after annealing at 500 and 900 °C, with larger values for temperatures in between. During heat treatment the Er profile remains stable.

Influences of processing parameters on the properties of ZnO thin films prepared by sol-gel spin-coating

Zinc oxide films were prepared on Si(111) substrates by sol-gel spin-coating technique.Structure,photoluminescence,and surface morphology of ZnO thin films processed at various temperatures and times were investigated.Results showed that ZnO thin films showed a hexagonal wurtzite structure,and the extent of c-axis orientation was dependent on the processing temperature.All the thin films emitted strong band to band UV emission without defects related visible emissions at room temperature when the processing temperature was below 550℃.When the processing temperature exceeded 550℃,a strong emission band appeared in the visible region.Results also showed that the photoluminescence spectrum was dependent not only on the processing temperature but also on the processing time.Long time(3 hours) processing resulted in the aggregation of ZnO grains in the film and a lower band to band emission in the photoluminescence spectrum.

Characterization and mesostructure control of mesoporous fluorinated organosilicate films

Mesoporous fluorinated organosilicate films were synthesized from tetramethoxysilane and perfluoroalkylsilanes (RO)3Si-R′ [R = Me, Et; R′ = (CH2)2CF3, (CH2)2(CF2)5CF3, or (CH2)2(CF2)7CF3] under acidic conditions in the presence of cetyltrimethylammonium chloride (CTACl) by sol–gel spin-coating. Calcination at 350 °C allowed the removal of surfactants without destroying the other organic contents. X-Ray diffraction patterns of the fluorinated films with short perfluoroalkyl chains indicated that both hexagonal and cubic mesoporous fluorinated organosilicate films can be formed by varying the surfactant to silane mole ratio. On the other hand, the mesoporous fluorinated organosilicate films made from perfluoroalkylsilane with longer perfluoroalkyl chains, which acted as a surfactant, displayed a hexagonal mesostructure across a broad range of surfactant concentrations. Mesoporous fluorinated organosilicate films had higher hydrophobicity and lower refractive indices than mesoporous silica films.