Sol-gel Deposition Process Research Articles

Distinct Magnetic Properties of CoFe₂O₄ by Tetrahedral Mg<SUB>2+</SUB> Substitution

Magnetic hysteresis curves of cobalt ferrites with a fraction of cobalt being replaced by magnesium (MgSUBx/SUBCoSUB1-x/SUBFe₂O₄) were investigated by vibrating sample magnetometry (VSM). The specimens with x ≤ 0.2 were prepared as thin films on silicon substrates by using a sol-gel deposition process. The VSM result revealed that the saturation magnetizations (MSUBS/SUB) of the Mg-substituted specimens were higher than that of CoFe₂O₄. The result of Raman spectral analysis suggested that the Mg-substituted specimens had higher tetrahedral CoSUP2+/SUP density in the spinel lattice compared to CoFe₂O₄. The increase in MS for MgSUBx/SUBCoSUB1-x/SUBFe₂O₄ is ascribed to two reasons, the increase in tetrahedral CoSUP2+/SUP density and significant MgSUP2+/SUP occupation in the tetrahedral sites.

Structural and Magnetic Properties of Thin-film ScFeO₃

Structural and magnetic properties of ScFeO₃ compound were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and vibrating sample magnetometry (VSM) in comparison with those of Fe₂O₃ and Sc₂O₃. The specimens were prepared as thin films under identical sol-gel deposition process. The XRD data indicated that the ScFeO₃ specimen have cubic bixbyite structure, identical to that of the Sc₂O₃ specimen but different from that of the Fe₂O₃ specimen (rhombohedral). The estimated lattice constant of ScFeO₃ is smaller than that of Sc₂O₃ by 2.9 %. According to the VSM data, the ScFeO₃ specimen showed a magnetic hysteresis curve that is alike in shape to that of the Fe₂O₃ specimen with the saturation magnetization of the former (2.6 emu/㎤) being smaller by 30 % than that of the latter (3.7 emu/㎤). The observed small magnetization in ScFeO₃ implies an antiferromagnetic spin alignment of neighboring FeSUP3+/SUP ions with canted spin magnetic moment.

A nanotubular TiO2/SiOx/Si composite derived from cellulosic cotton as an anode material for lithium-ion batteries with enhanced electrochemical performance

A TiO2/SiOx/Si composite derived from cellulosic cotton as the structural template was biomimetically synthesized. Modified sol–gel deposition processes were performed to coat each cellulose nanofiber with a SiO2 thin gel layer, followed by layer-by-layer self-assembly processes to form an external TiO2 coating with controllable thickness. The resulted matter was calcined in air to obtain the TiO2/SiO2 composite, which was successively put in N2 atmosphere and treated with magnesiothermic reduction reaction, resulting in the final TiO2/SiOx/Si composite. The composite possessed a typical hierarchical three-dimensional (3D) nanotubular network microstructure precisely inherited from the cellulosic cotton template, and each nanotube had a core-shell nanostructure with Si-based matters as the core and TiO2 coating as the shell. Compared with the bare nanotubular SiOx/Si material, the TiO2/SiOx/Si composite showed a highly enhanced electrochemical performance as an anode material for lithium-ion batteries (LIBs), delivering an initial discharge capacity of 2370 mAh g−1 at a current density of 0.2 A g−1 (an initial Coulombic efficiency of 56.4%), a reversible capacity of 682 mAh g−1 after 100 discharge/charge cycles, and a discharge capacity of 241 mAh g−1 at a high current density of 2.0 A g−1. The excellent specific capacity, cycling stability and rate capability are ascribed to the synergistic effect of the unique 3D porous nanotubular network structure and the uniform TiO2 coating layer, which accommodates the volume variation of Si-based components during cycling, offers sufficient ion transport pathways and improves the electron transfer efficiency. This work provides a facile biomass-based strategy for fabrication of metal oxide/Si composites with well-defined microstructures that exhibit significant potential as anode materials for LIBs.

Optimization of the production process of BZT–BCT sol–gel thin films obtained from a highly stable and green precursor solution

ABSTRACT Barium Zirconate Titanate–Barium Calcium Titanate (BZT–BCT) is an environmentally friendly piezoelectric-ferroelectric ceramic that is a promising alternative to lead zirconate titanate (PZT), whose toxicity is well known. Producing thin films by sol–gel deposition with properties comparable to their bulk counterparts is quite a difficult task, because of the substrate constraints which typically generate residual stresses during the production process, leading to the onset of cracks. Moreover, typically employed solutions contain toxic and harmful substances or are unstable over time. Recently, we produced a highly stable BZT–BCT precursor solution based on nontoxic and non-carcinogenic solvents. In this work, we optimized the BZT–BCT sol–gel deposition process by varying the spin coating velocity, which influences the thickness of each deposited layer, and the pyrolysis temperature, which impacts on the formation of cracks. Moreover, we explored the possibility to add a third heat treatment step to the deposition process with the aim of releasing the residual stresses, further improving the film properties. As a result, 250 nm thick films were obtained with optimized morphology and limited presence of cracks, thus enhancing the ferroelectric properties (Pr = 33.26 μC cm−2, Ec = 220.6 kV cm−1, ε = 1817).

Spectroscopic investigation on tetrahedral Co2+ in thin-film CoFe2O4

CoFe2O4 has been attracting attention for its ferrimagnetism applicable to spin transfer, resonance imaging, drug delivery etc. CoFe2O4 thin films were synthesized on Si(100) substrates by using a sol–gel deposition process. The CoFe2O4 specimen produced by post-annealing in air at 800 °C showed flat surface and polycrystalline grains with no secondary phase. The specimen exhibited magnetic hysteresis curve with magnetization up to 415 emu/cm3 and coercivity of 1.7 kOe. Such a large magnetization implies migration of a number of Co2+ ions from octahedral to tetrahedral sites of the spinel lattice. The distribution of Co2+ ions among tetrahedral and octahedral sites of CoFe2O4 was estimated by curve-fitting analysis on the Raman scattering spectrum of the specimen. The result suggests 30% of the Co2+ ions residing in the tetrahedral sites. The coexistence of Co2+ ions in both tetrahedral and octahedral sites of CoFe2O4 was also detectable by using Co 2p X-ray photoelectron spectroscopy.

Sol-gel synthesis of p-type zinc oxide using potassium permanganate

Polycrystalline, p-type ZnO were synthesized by KMnO4 doping using the sol-gel deposition process. Depending on the amount of doping, the addition of KMnO4 into ZnO resulted in either fibrous or porous film morphology. Optically, the films were transparent with transmittance ranging from 82∼88% depending on doping concentration. The p-type doping were confirmed using Hall effect measurements and the formation of rectifying pn junction with n-type ZnO.

Piezoelectric and Dielectric Characterization of MWCNT-Based Nanocomposite Flexible Films

PZT-epoxy-multiwalled carbon nanotube (MWCNT) flexible thick film actuators were fabricated using a sol-gel and spin coat and deposition process. Films were characterized in terms of their piezoelectric and dielectric properties as a function of MWCNT volume fraction and polarization process. Correlations between surface treatment of the MWCNTs and composite performance were made. The surface morphology and filler distribution were observed with the aid of SEM and TEM images. The volume fraction of PZT was held constant at 30%, and the volume fraction of MWCNTs varied from 1% to 10%. Two forms of dielectric polarization were compared. Corona discharge polarization induced enhanced piezoelectric and dielectric properties by a factor of 10 in comparison to the parallel-plate contact method (piezoelectric strain coefficient and dielectric constant were 0.59 pC/N and 61.81, respectively, for the parallel-plate contact method and 9.22 pC/N and 103.59 for the corona polarization method, respectively). The percolation threshold range was observed to occur at a MWCNT volume fraction range between 5% and 6%.

Aluminum Indium Antimonide (AlxIn1-xSb) Ternary Nanowires Synthesis and Characterization

As a direct bandgap semiconductor, indium antimonide (InSb) has the narrowest band gap (0.17 eV at 300 K) and the highest bulk electron mobility (77000 cm2V-1s-1) among all of III-V semiconductors. Due to the unique physical properties, it has considerable potential for applications in high-speed, low-power electronics and infrared optoelectronics.[1]-[3] Adding other elements is a possible method to adjust its bandgap, but it is very difficult to achieve doping during the electrodeposition and sol-gel deposition processes. In this study, the bandgap of InSb nanowires (NWs) is tuned by substitute of indium with varying the aluminum concentration. The AlxIn1-xSb nanowires were synthesized by using a vacuum hydraulic pressure injection process with anodic aluminum oxide (AAO) template assisted. Unlike vapor deposition processes or chemosynthesis, when a stoichiometric bulk material was produced, the following casting would not change the composite ratio of alloys until the injection process ended. This process has five main steps: first, fabrication of a nanotubular template by anodization process. Second, modification of the tube diameter of the oxide template by a chemical etching process. Third, preparation of an AlxIn1-xSb bulk with a desire composition. Binary and ternary alloys were prepared from powders of aluminum, indium and antimony by melting the elements with small sealed quartz tube in tube furnace under an argon atmosphere. Fourth, the entire chamber was vacuumed and heated up to melt the AlxIn1-xSb bulk, then load 50 kgf / cm2 for 5 min to press melting filling a liquid phase of AlxIn1-xSb into the AAO template to form AlxIn1-xSb nanowires. Finally, the entire chamber was cooled down under air atmosphere. Fifth, the AAO were removed by 1.8 wt.% chromic acid (H2CrO4) + 6 vol.% phosphoric acid (H3PO4) solution at 60 °C for 40 minute, and subsequently, the residual H2CrO4, H3PO4 solution were cleaned with de-ionized water three times then stored in alcohol solution for further processing and characterization. Moreover, the diameter of AlxIn1-xSb NW can be tuned by using the different AAO pore size. The length of the nanowire depends on the fluid properties of the material and the pressure of the hydraulic machine. These as-prepared nanowires were characterized by field-emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD), and transmission electron microscope (TEM) about the morphology, composition, crystal structure and defect. SEM image shows the diameter of nanowire has a good agreement with AAO pore size 100 nm and the length of nanowire is about 10~30 µm. SEM-EDX demonstrates the AlxIn1-xSb bulk has precise stoichiometric composition. However, due to the nanowires were placed on the Si Substrate, characteristic X-ray spectra of aluminum and silicon are too close cause the influence of peak overlapping. On the other hand, the TEM substrate (copper grid with carbon film) is proper to utilize. Therefore, the composition of nanowire was investigated by TEM-EDX and STEM mapping. In the TEM analysis, both of the high resolution image and diffraction pattern can prove the presence of lattice strain, and it reflects that the aluminum successfully is substituted for indium. Furthermore, Scanning Transmission Electron Microscope (STEM) mapping shows good uniformity of the whole ternary nanowire. In the future, Fourier transform infrared spectroscopy (FTIR) will be used to determine the band gap AlxIn1-xSb NWs and fabrication of device by using focused-ion-beam to carry out electrical measurement. Reference [1] Cheng-Hsiang Kuo, Jyh-Ming Wu, Su-Jien Lin, “Room temperature-synthesized vertically aligned InSb nanowires: electrical transport and field emission characteristics”, Nanoscale Research Letters, 2013, 8, 69. [2] Youwen Yang, Liang Li, Xiaohu Huang, Guanghai Li, Lide Zhang, “Fabrication and optical property of single-crystalline InSb nanowire arrays”, Journal of Materials Science , 2007, 42, 2753-2757. [3] D. L. Rode, “Electron Transport in InSb, InAs, and InP”, Phys Rev B, 1971, 3, 3287-3299.

Direct formation of p-type ZnO by using potassium hydroxide

In this study, p-type ZnO:K was successfully produced by potassium hydroxide using a sol-gel deposition process. The deposited ZnO:K samples were examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD), Hall-effect measurements, photoluminescence spectroscopy (PL) and Ultraviolet–visible ((UV–vis)) spectroscopy. It was found that the amount K-doping strongly affects the structure and optoelectronic properties of ZnO, with p-type conduction obtained in films doped between 2–5 at%.

Novel ZnO decorated SnO2 nanosheet for dye sensitized solar cell applications

Zinc oxide decrorated tin dioxide nanosheets have been synthesized by the sol-gel deposition process. Scanning electron microscope imaging conforms the formation these nanosheets within post annealing temperature at 500°C. The decoration of ZnO onto SnO2 was confirmed by high resolution imaging. The highest efficiency of the dye-sensitized solar cells utilizing the proposed material is 0.98%

Light harvesting, self-assembled SnO nanoflakes for dye sensitized solar cell applications

In this study, SnO2 nanoflakes were prepared by a sol-gel deposition process. The SnO2 nanoflakes were around 15μm in length and 200nm in width with a thickness of around 50nm. The SnO2 nanoflakes were bound with SnO2 nanoparticles with an average diameter of 10nm. As photoanodes, the dye-sensitized solar cell (DSSCs) based on the SnO2 nanoflakes show an improved power conversion efficiency of 0.62%, improved by 4 folds compared to pure SnO2 thin film (0.14%). The efficiency improvement is due to the vertically-aligned, crystalline sheet-like structure, which provides superior enhanced light scattering and electron transport.

ブリッジ構造の共振子と磁歪膜をもつMEMS磁気センサ

This paper reports on fabrication and characterization of resonant-type mechanical MEMS magnetic sensors using magnetostrictive FePd sputtered film. The sensor has a thin Si resonator bridge with a PZT piezoelectric thin film element for excitation. The resonator is pulled by other bridges with the FePd film when magnetic flux is applied. The sensor was fabricated from an SOI wafer by sol-gel PZT deposition, FePd sputtering, and Si etching processes. The completed sensor showed a resonant peak with a Q factor of 229 at 26.39 kHz. When magnetic flux was applied along the micro-bridges, resonant frequency drastically decreased due to relaxation of initial deflection of the resonant bridge. Magnetic flux can be detected by resonant frequency change with a sensing resolution of 0.2 Gauss per 0.1 Hz. When excitation frequency was fixed at slightly below the resonant frequency, amplitude of the bridge vibration slightly decreased with increasing magnetic flux in low magnetic flux region. High sensitive sensing of magnetic flux was difficult by the amplitude change. It was found that the magnetic sensor showed large thermal drift.

A close correlation between nucleation sites, growth and final properties of ZnO nanorod arrays: Sol-gel assisted chemical bath deposition process

Different ZnO seed layers are synthesized by changing sol-gel parameters, including precursor concentration, type of solvent, and type of additive in order to systematically investigate the importance of seed layer properties on the nucleation, growth, and final properties of zinc oxide (ZnO) nanorod (NR) arrays. The X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) show the importance of the seed layer on ZnO NRs properties. Results verify that the relative intensity (RI factor) of (002) polar planes in the XRD patterns of ZnO nanoparticles (NPs) together with the aspect ratio, density, and alignment of ZnO NRs control the structural characteristics of those arrays. For instance, the RI factor of ZnO NPs and NRs follow the same trend when changing precursor concentration in sol preparation step. However, the importance of other parameters, including aspect ratio, density, and alignment of NRs is confirmed by changing solvent and additive. In addition, FESEM images show that the density of ZnO NRs is proportional to NPs density and inversely proportional to the size of ZnO NPs in the seed layers. Besides, the significant role of wrinkled inter-layer on NRs properties, together with the formation mechanism of that inter-layer are profoundly investigated.

Synthesis and optical characterization of ZnO with varying nanogranular morphologies

Thin nano-granular ZnO layers have prepared using a sol-gel synthesis and spin-coating deposition process with varying elaboration parameters. The process has a low deposition rate allowing the formation of very thin layers (60 nm) which can present a large range of morphologies and correlated optical properties. An investigation of the correlation between granular morphology, dielectric function and luminescence properties is proposed. It is shown that, for such low thickness layers, simple variations of annealing temperature, sol concentration or Al-doping can greatly change the grain size, excitonic absorption and bandgap properties, or again optical density. Using these synthesis parameters, a large range of optical properties can be accessed.

Enhanced photocatalytic activity of sol–gel derived ZnO via the co-doping process

Thin films of intrinsic, N-doped and N/Al co-doped ZnO were prepared by a sol–gel deposition process. The effect of doping on structure, optoelectronics and photocatalytic properties of ZnO were studied. It was found that the doping process reduces the commonly observed wrinkled effect. XRD pattern measurements confirmed that all the films preferentially grow in the c-axis orientation, with degradation in crystallinity in doped films. The dopant created defect can be also observed in the PL spectra and UV–vis measurements of the NZO and NAZO thin film, with reduced band gap in doped films. Photocatalytic measurements confirm that the N and N/Al dopant process enhances photocatalytic degradation of MB by narrowing the band gap.

Sol-gel deposition of iridium oxide for biomedical micro-devices.

Flexible iridium oxide (IrOx)-based micro-electrodes were fabricated on flexible polyimide substrates using a sol-gel deposition process for utilization as integrated pseudo-reference electrodes for bio-electrochemical sensing applications. The fabrication method yields reliable miniature on-probe IrOx electrodes with long lifetime, high stability and repeatability. Such sensors can be used for long-term measurements. Various dimensions of sol-gel iridium oxide electrodes including 1 mm × 1 mm, 500 μm × 500 μm, and 100 μm × 100 μm were fabricated. Sensor longevity and pH dependence were investigated by immersing the electrodes in hydrochloric acid, fetal bovine serum (FBS), and sodium hydroxide solutions for 30 days. Less pH dependent responses, compared to IrOx electrodes fabricated by electrochemical deposition processes, were measured at 58.8 ± 0.4 mV/pH, 53.8 ± 1.3 mV/pH and 48 ± 0.6 mV/pH, respectively. The on-probe IrOx pseudo-reference electrodes were utilized for dopamine sensing. The baseline responses of the sensors were higher than the one using an external Ag/AgCl reference electrode. Using IrOx reference electrodes integrated on the same probe with working electrodes eliminated the use of cytotoxic Ag/AgCl reference electrode without loss in sensitivity. This enables employing such sensors in long-term recording of concentrations of neurotransmitters in central nervous systems of animals and humans.

Intrinsic Stress Control of Sol-Gel Derived PZT Films for Buckled Diaphragm Structures of Highly Sensitive Ultrasonic Microsensors

Intrinsic stress of sol-gel derived lead-zirconate-titanate (PZT) films has been investigated from the viewpoint of diaphragm buck- ling behavior for highly sensitive structures of piezoelectric ultrasonic microsensors. Since upward-buckled diaphragms of the sensors yield higher sensitivity than flat or downward ones, a fabrication process which enables the diaphragms to buckle sponta- neously upward was developed owning to intrinsic tensile stress of the PZT films. To control the intrinsic stress as adequate for the upward buckling, calcination temperature in the sol-gel deposition process has been modified in the range from 300∘C to 400∘C. The stress has decreased with increasing the temperature in the range and the 400∘C-calcined PZT films have shown a suitable stress for the buckling deflection and probability of upward buckling for the sensor diaphragms.

Adhesion Enabling Technology for Reliable Metallization and Patterning of Glass Interposers

In this study a thin-film adhesive metal oxide layer is described that forms chemical bonds with glass, and mechanically anchors to copper to create strong, reliable adhesion between the glass substrate and the deposited metal. A modified sol-gel deposition process followed by sintering was used to create the thin, transparent film that then enables electroless, and electrolytic copper plating directly onto glass. With this new approach, a copper film thickness of over 50μm can be applied without delamination from the smooth glass. Adhesion at 15μm Cu thickness as measured by 90° peel strength tests can achieve values of ≥5 N/cm. This is compared in the study to mechanical roughening and a control with no adhesion promoter applied. Thermal stress tests using lead free reflow conditions and Highly Accelerated Stress Testing (HAST) were carried out without significant adhesion loss. Through hole adhesion was demonstrated inside the Through Glass Vias (TGV's) without issues of blistering or delamination from the walls. The study also looks at filling of the TGV's with electrolytic copper. Photolithography of fine-lines as small as 5/5 μm was feasible, with very small undercut observed. Electro migration and high frequency tests were also conducted to test material compatibility for RDL patterning, showing no loss in surface insulation or signal integrity.

Sol–gel production of wrinkled fluorine doped zinc oxide through hydrofluride acid

Wrinkled ZnO:F (FZO) thin films have been derived through sol–gel deposition process, using a mixture of zinc acetate aluminum chloride, hydrofluoric acid and isopropanol. It is found that the deposited FZO thin films are highly transparent with sheet resistivity of around 0.07Ωcm. Scanning electron microscopy images and XRD measurements indicated that these FZO thin films are wrinkled with a preferential growth in the c-axis orientation. Electrical measurements revealed that all of the deposited films are n-type. The best figure of merit can be achieved by using the precursors with 6at% of F. Results of this study show the F incorporation is one of the effective factors to optimize the FZO thin films. As a demonstration crystalline silicon-based solar cells were fabricated using FZO derived from this study.

PMMA–SiO2 hybrid films as gate dielectric for ZnO based thin-film transistors

In this paper we report a low temperature sol–gel deposition process of PMMA–SiO2 hybrid films, with variable dielectric properties depending on the composition of the precursor solution, for applications to gate dielectric layers in field-effect thin film transistors (FE-TFT). The hybrid layers were processed by a modified sol–gel route using as precursors Tetraethyl orthosilicate (TEOS) and Methyl methacrylate (MMA), and 3-(Trimethoxysilyl)propyl methacrylate (TMSPM) as the coupling agent. Three types of hybrid films were processed with molar ratios of the precursors in the initial solution 1.0: 0.25, 0.50, 0.75: 1.0 for TEOS: TMSPM: MMA, respectively. The hybrid films were deposited by spin coating of the hybrid precursor solutions onto p-type Si (100) substrates and heat-treated at 90 °C for 24 h. The chemical bonding in the hybrid films was analyzed by Fourier Transform Infrared Spectroscopy to confirm their hybrid nature. The refractive index of the hybrid films as a function of the TMSPM coupling agent concentration, were determined from a simultaneous analysis of optical reflectance and spectroscopic ellipsometry experimental data. The PMMA–SiO2 hybrid films were studied as dielectric films using metal-insulator-metal structures. Capacitance–Voltage (C–V) and current–voltage (I–V) electrical methods were used to extract the dielectric properties of the different hybrid layers. The three types of hybrid films were tested as gate dielectric layers in thin film transistors with structure ZnO/PMMA–SiO2/p-Si with a common bottom gate and patterned Al source/drain contacts, with different channel lengths. We analyzed the output electrical responses of the ZnO-based TFTs to determine their performance parameters as a function of channel length and hybrid gate dielectric layer.