Preparation Of 3D Anatase Titanium Dioxide Single Crystal Film Via Atmospheric Pressure Non-equilibrium Plasma And Its Luminescence Study

Titanium dioxide ?TiO₂? with possesses high chemical stability, avirulence and strong photo induced oxidation activity, has attracted much attention in recent years due to their potential applications in DSSCs, purification of air and water, photolysis of water to produce hydrogen and oxygen. As one of indirect wide band gap semiconductors with band gap 3.2 eV ?388 nm? for anatase and 3.0 eV ?414 nm? for rutile, TiO₂ has low luminescence efficiency. Although nanosize TiO₂ colloidal sols can emit PL at 390 nm by band-to-band transition or emit defects-induced PL ranged from 400 nm to 600 nm. There is any report for strong observable PL for TiO₂ until now.Recently, proposed by the stimulation studies of strong photoluminescence of porous silicon,much research attention has been paid to increase the luminescence efficiency of indirect semiconductors through band gap engineering or defect engineering. Titanium is one of the most abundant elements on the earth like silicon. By improving efficiency of TiO₂ through the method of band gap engineering or defect engineering, it will greatly widen the applications of TiO₂ and open a potential way for titanium-based luminescence and titanium-based optoelectronic integrated circuit.Plasma enhanced chemical vapor deposition ?PECVD? is a widely used technology for one step preparation of film materials in microelectronic industry and photoelectronic industry. Unlike wet chemical methods for preparing TiO₂ such as sol-gel, hydrothermal, thermal evaporation etc PECVD does not need shape-controlled agents, catalysts or mask and does not involve multilple long and tedious post heating, agents and catalysts removing process. Importantly, it can lower the film deposition temperature and in-situ modulates the band gap energy during the film deposition process through doping, ion bombardment or reactive etching. In this paper, a new non-equilibrium atmospheric plasma enhanced chemical vapor deposition ?NE-APCVD? of Ar/O₂/TiCl₄ has been developed to prepare luminescence TiO₂ film by taking the advantage of PECVD. We made big progresses in the aspects of simulation of TiO₂ APCVD process, one step and fast deposition of 3D anatase TiO₂ single crystal film with tunable oxygen vacanev concentration and observable strong PL. The simulation and diagnostics of Ar/O₂/TiCl₄ atmospheric pressure non-equilibrium plasma were done to study the physical-chemical process and the main reactions of TiO₂ deposition. By adjusting the key parameters for the physical-chemical deposition processes, we got 3D anatase TiO₂ single crystal film with very higt concentration of oxygen vacancies on the quartz substrate. This new 3D structure film is connected by anatase TiO₂ single crystal sheets and can emit very strong observable PL with excitation of a 325 nm He-Cd laser with pump power of 30 mW. The intensity of the PL comparable to that of commercial fluorescent lamp interior coatings.The research work in the paper can be categorized into four sections:Part I. Simulation and Diagnostic of Ar/O₂/TiCl₄ NE-APCVDIn order to understand the physical and chemical process of the Ar/O₂/TiCl₄ NE-APCVD, a global model was applied to simulate the discharge and reaction process. 197 main reactions and 26 main species were included in the simulation. According to the simulation results, main species are TiCl3, TiO₂Cl3, O, O₂*, Cl,Cl2, ClO. The discharge power, the total gas flow, oxy???and TiCl₄ play a key role on the deposition of TiO₂ film. Besides, the experimental diagnostics of this Ar/O₂/TiCl₄ concentration NE-APCVD was done through discharge current/voltage and emission spectrum measurement. Both the waveform of the applied voltage and current of thedischarge are sinusoidal and the phase of the current leads ahead of the voltage, indicating the capacitive coupling glow discharge of the plasma. The peak value of applied voltage and current increased as the discharge power increased. By study the Ar/O₂/TiCl₄ AP-NE Plasma op???emission spectroscopy, the election energy and ion temperature ?gas temperature? was calculated.The temperature of electron is about 2.08 eV and the average ion temperature of 900 K. Both the simulation and diagnostic results are for benefit of deposition of TiO₂ crystals.Part ?. Preparation of TiO₂ via Ar/O₂/TiCl₄ AP-NE PlasmaThe effect of discharge power, TiCl₄ and argon flow on the morphology, structure,crystallinity, chemical elements and defects of deposition of TiO₂ was studied in this paper to find the mechanism of Ar/O₂/TiCl₄ AP-NE Plasma deposition of TiO₂. It is found that the discharge power and argon flow play a key role on the morphologies and crystallization when the content of precursor TiCl₄ in the range of 0.2 sccm to 0.5 sccm. The deposited TiO₂ is mainly nanoparticle film when the discharge power is low and argon flow is large. However, the morphology abundant,crystallization controlled and rich of defects such as oxygen vacancies of 3D anatase TiO₂ single crystal film can be deposited in the quartz substrate in one step. This technology can afford a new method for fabrication of 3D micro/nano sized sheet-connected structures at low temperature and low energy. Besides, the effect of the concentration of main species and gas temperature on the deposition of 3D anatase TiO₂ single crystal film was studied. It was found that the 3D anatase TiO₂ single crystal film deposited from the glow discharge zone with high concentration of main species and high temperature has more compact structure, larger size and smaller {001} facets compared with that of the 3D anatase TiO₂ single crystal film deposited from the after glow discharge zone.Part III. Mechanism study of Ar/O₂/TiCl₄ AP-NE Plasma deposition of TiO₂From the point of view of generation, transport and self-assembly of reactive species, we studied the growth process of Ar/O₂/TiCl₄ AP-NE Plasma deposition of TiO₂. The effect of growth time, energy and temperature on the morphology, crystallinity and structure was studied. It was found that the multi-crystalline TiO₂ nanoparticles can be generated when the feed gas flow though the reaction zone with 2 cm length ?0.0054 second?. The TiO₂ particles can have very well crystallinity when the growth time was only about 1 min. When the growth time increased to 10 min, TiO₂ already has very clear crystal facets and very large size because of self-assembly and recrystallization. The growth of {001} facets was restricted due to its lower surface energy because of the high concentration high reactive of Cl. The growth of {101} facets dominated and the anatase single crystal TiO₂ with large percentage of {001} facets can be formed. The high concentration of oxygen vacancies can be in situ introduced due to the high temperature and desorption of ClO from the surface of TiO₂. The morphology and property of the TiO₂ deposited in the in glow discharge and after glow discharge are different due to the different of the concentration of reactive species and gas temperature. According to the above research results, a"self-confined growth" model was established.Part IV. Luminescence study of deposited TiO₂At last, the luminescence property of deposited TiO₂ films was studied in this paper. The PL spectra were measured at room temperature by using the 325 nm line of a He-Cd laser as the pump source with pump power of 30 mW. It was found that the deposited TiO₂ film exhibit strong observable visible light, which is strong enough to be viewed with naked eyes. The intensity and color of the PL of the TiO₂ prepared by different conditions are different. They can emit white and white-blue PL at the centers of 450 nm & 570 nm and 300 nm & 600 nm. The intensity of the PL is comparable to that of commercial fluorescent lamp interior coatings. Besides, the CL of the deposited TiO₂ film was also studied, the CL intensity of different facets of TiO₂ single crystal varied, indicating that concentration of different facets is different. The mechanism of luminescence was also studied. The PL included band-to-band emission, self-trapped excitations emission and defects emission, during which the oxygen vacancies are mainly responsible for the strong observable visible PL.