3C-SiC,Si Quantum Dots And Si-based All-color Tunble Solid Films

Silicon based semiconductor nanostructures have a variety of special properties which make them useful in the area of optoelectronic, and biomedical applications. In recent years, Si-based semiconductor nanomaterials have attracted enough attention, especially going with the development of nanoscience and microelectronic technique. People have been trying various methods to fabricate different morphologic Si based nanostructures, expecting to achieve effective and stable light emission. However, the complexity of Si based nanostructures makes it difficult to obtain broad range light emitting. In this thesis, we study3C-SiC and Si nanocrystals with tunable light emis-sion in violet to blue-green range, which are fabricated via a chemical etching meth-od with ultrasonic treatment. Moreover, we fabricate solid light emitting films with3C-SiC and Si nanocrystals. In addition, we are the first comers who fabricate a sili-con nanostructures film, with tunable light emission in visible range. The obtained main results are described as follows:1. We have produced glycerol-bonded3C-SiC nanocrystal (NC) films, which when excited by photons of different wavelengths, produce strong and tunable violet to blue-green (360-540nm) emission as a result of the quantum confinement effects rendered by the3C-SiC NCs. The emission is so intense that the emission spots are visible to the naked eyes. The light emission is very stable and even after storing in air for more than six months, and no intensity degradation can be observed. X-ray photo-electron spectroscopy and absorption fine structure measurements indicate that the Si-terminated NC surfaces are completely bonded to glycerol molecules. Calculations of geometry optimization and electron structures based on the density functional the-ory for3C-SiC NCs with attached glycerol molecules show that these molecules are bonded on the NCs causing strong surface structural change, while the isolated levels in the conduction band of the bare3C-SiC NCs are replaced with quasi-continuous bands that provide continuous tunability of the emitted light by changing the frequen-cies of exciting laser. As an application, we demonstrate the potential of using3C-SiC NCs to fabricate full-color emitting solid films by incorporating porous silicon.2. By chemical etching of Si powder in a mixture of nitric acid and hydrofluoric acid, followed with ultrasonic vibration, a large quantity of Si nanocrystals is fabri-cated in water. The diameter of Si nanocrystals varies from1to4nm, with a maximal probability diameter of2nm. Chemical etching results in the formation of intercon-nected nanocrystal network on the surface of Si grains. Consiquent ultrasonic treat-ment separates Si nanocrystals from the network into water. Excited by the Xe lamp, the suspension of Si nanocrystals performances intensive, stable light emitting, wich the PL peak varies from400to520nm, corresponding to the increasing excitation wavelength from320to420nm, respectively.According to the results of a series of measurements, it implies that the tunable PL emissions originates from the band-to-band recombination in the quantum confined Si nanocrystals.3. We obtain the PL and PLE spectra of Si quantum dots solution with different pH values of1,4,7and11. It is found that whatever the pH value is, the photolumi-nescence of Si quantum dots solution is intensive, which the PL peak redshifts when the excitation wavelength increases. All above demonstrates that the tunable photo-luminescence of Si quantum dots mainly originates from quantum confined effect. However, the pattern of PL peak is slightly different from each other, of which the pH value is different:in an acidic environment, the intensity of light emitting decreases that the Raman peak of water comes out; when the solution is alkalescent, although the PL is intensive enough, a fixed shoulder appears at around370nm, with a great change of PLE spectra; the PL in neutral solution is bright as well as responsible to quantum confined effect. According to the results as above, we infer that high acidity will make a large quantity of ionized hydrogen atoms absorbed to the surface of Si quantum dots, which will result in a decreacing intensity of light emitting, while sur-face/defect states of Si quantum dots appear in an environment full of hydroxyl. In a neutral solution, the light emitting of Si quantum dots is intensive and no peaks rela-tive to surface/defect states can be observed. Therefore, it means that although the tunable PL of Si quantum dots mainly comes from quantum confined effect, the sur-face/defect states are still able to influence the light emitting of Si quantum dots. Con-siderting that the Si quantum dots are made from chemical etching, it is unrealistic to expect the Si dots are ideally smooth without hanging bonds or any defects. The chemical properties of Si atoms on the surface of dots are still so active that can be easily bonded to hydrogen atom and hydroxyl group, which helps the generation of surface/defect states. Thus the PL characters of Si quantum dots are influenced. Hence, the intensive light emitting of Si quantum dots comes from both quantum confined effect and surface passivation.4. By passivating the surface of Si quantum dots with glycerin, we fabricate a Si quantum dots/glycerin solid film, which emits photons tunably in the violet to blue-green range. We also fabricate a tunably photoluminescent Si composite nano film, by glycerol passivating the Si quantum dots and subsequently embed the pas-sivated quantum dots into the nano pores of porous silicon. The Si composite nano film shows stable, tunable photoluminescence in all visible range. In add, the quantum efficiency value of this Si composite film mostly varies during20%-30%, corre-sponding to different excitation wavelength, which is far more intensive than usual. This is the first time that we obseved full colored tunable photoluminescence in com-posite nanostructure composed of Si completely. During the study we found that the tunable photoluminescence in such a broad range originates from both quantum con-fined effect and glycerol passivation of Si quantum dots.