Preparation And Properties Of TiO₂ Nanodots On A Substrate

The important metal oxide,titanium oxide(TiO₂) has been intensively studied because of its widespread applications in solar energy conversion,gas sensors, catalysis and remediation of environmental pollution.TiO₂ nanostructures demonstrate remarkably high performance levels for these applications because of their large surface area compared to bulk TiO₂;these have attracted increasing attention over the last few decades.Among all the nanostructures,those prepared on substrate are more desirable for applications of photovoltaic cells,catalysis, separations,sensors and optical devices.TiO₂ nanodots on a substrate are considered to be a superior form in utilizing zero-dimensional effects as well as avoiding agglomeration,which is common in wet-chemical synthesized nanoparticles.In this paper,we make detailed studies of preparation,optical and electronic properties of TiO₂ nanodots on a substrate.We have developed a new method,i.e.,microscopic mass-point addition,to prepare high-density TiO₂ nanodots on a substrate.Firstly,anodic aluminum oxide membrane touch tightly with a substrate set on a certain temperature.After that, certain amount of sol is dropped on,spreads on and infiltrates into the membrane completely;upon gelling completes,the membrane and the substrate are detached. TiO₂ nanodots can be formed on substrate after heat-treatment.The control of the nanodot size and density can be realized through changing of the substrate temperature.When the substrate temperature fall from 80℃to 30℃,the nanodot size reduces from 11.5 nm to 5 nm and the dot density reaches～10¹² cm^-2.The optical absorbance of the TiO₂ nanodots was characterized by UV-vis spectroscopy.The absorption edge exhibits a blueshift with decreasing nanodot size.A decrease in size increases the bandgap of the TiO₂ nanodots,and blueshifts of 0.56 eV and 0.33 eV for bandgaps with indirect and direct characteristics are evident when the nanodot size reduces from 93 nm to 48 nm.The room-temperature photoluminescence (PL) spectra for all samples exhibit striking and unique peaks in the UV region ascribed to near band edge luminescence.The PL energy increase from 3.38 eV to 3.58 eV and a blueshift of 0.2 eV is observed when the nanodot size is reduced from 93 nm to 48 nm.A possible interpretation was proposed to explain the present phenomena:(â…°) the TiO₂ nanodots have a direct bandgap,and create the intense UV luminescence;(â…±) a decrease in size causes a variation of the oscillator strength of the allowed direct transitions(X_1a→X_1b:3.45 eV,X_2b→X_1b;3.59 eV,Γ_5'a→Γ_1b:4.05 eV) and increases the bandgap,which contributes to the blueshift of the absorption edge and UV luminescence.Low-field electron emission properties have been found from the TiO₂ nanodots on substrate.The electric fields required to reach current densities of 10μA cm^-2 and 1 mA cm^-2 could be as low as 1 Vμm^-1 and 3.7 Vμm^-1,the lowest values ever found in TiO₂ materials.The fine field emission properties can be attributed to(1) phase-separation-induced self-assembled nanodots have good contact with substrate as a electrode;(2) high density defects as well as the unique geometric shape of TiO₂ nanodots;(3) decrease of screening effect due to density control in large degree.We have investigated I-V,dielectric constant and dielectric loss of TiO₂ nanodots on a substrate.The resistance of TiO₂ nanodots increases as decreasing nanodot size. This is ascribed to the wider bandgap in smaller nanodots,which prevent the electron transitions into the conduction band.The dielectric constant increases with decreasing of nanodot size,this is because more amorphous region exists in smaller nanodots. The dielectric loss of the TiO₂ nanodots is very small,which is in the range of～0.15—0.2.Monodisperse Eu-doped TiO₂ nanodots with spherical shape were also synthesized on substrate by utilizing the phase-separation-induced self-assembly. Under UV light excitation,the Eu-doped TiO₂ nanodots display strong red light luminescence.The average luminescence intensity of the main peak(⁵D₀→⁷F₂) of Eu-doped TiO₂ nanodots can be more than 10 times strong than the film sample.