Study On The Optical Properties Of The ZnO Low Dimensional Material And Carbon Nanotubes

Nanomaterial has attracted significant attention because of their unique physical and chemical properties as well as their prospects for practical applications. This thesis introduces the author's achievements during her doctor degree. The modern analysis techniques, such as scanning electron microscope (SEM), laser excitation, fluorescence, UV-visible absorption spectrum, Raman spectrum, X-ray diffraction (XRD), excitation spectrum, and optical limiting effects, were used to study the properties of the absorption and level structure, PL mechanisms, as well as nonlinear optical properties in the ZnO films, ZnO nanowires, and carbon nanotubes. The mechanism of optical limiting effects in miultiwalled carbon nanotubes (MWNTs) has been presented.The main contents of this thesis are as following:Part 1: The present situation of nanomaterial optical properties is introduced. Then the present situation of the optical properties of carbon nanotubes and ZnO low dimensional materials is presented. The questions investigating further the optical properties of carbon nanotubes and ZnO low dimensional materials are introduced.Part 2: The electronic structure and applications of carbon nanotubes are introduced. The modern analysis techniques, such as laser excitation, fluorescence, UV-visible absorption spectrum, Raman spectrum, SEM, and optical limiting effects, were used to study the properties of the photoluminescence (PL) mechanisms and nonlinear optical property in the carbon nanotubes. This part is detailedly described in chapter two and three. First of all, some achievements are theoretically obtained. When the interlayer interaction between the layers is not considered, the bandgap of MWNTs is 2 y o. However, when the interlayer interaction between the layers isconsidered, the bandgap of MWNTs is 2( /0 - /, / V2 ). It is shown that the bandgap ofMWNTs is the smaller when intershell coupling is considered. Secondly, some energy dispersion relations are split by the intershell coupling. Finally, it is theoretically shown that there are van Hove singularities (VHS) in MWNTs. The intensity of luminescence is not only related with transition probability, but also depends on density of states (DOS) corresponding to energy level. For a given transition probability, the higher DOS is, the greater the emission intensity. Therefore, the location of the emission peak is certainly related with VHS.Raman, absorption and photoluminescence spectrum of MWNTs are studies. There are many defects in MNWTs by analyzed Raman spectrum of MNWTs. The sample has absorption in the ultraviolet, visible and near-infrared region by investigated absorption spectra of solid state MWNTs. The UV absorptioncorresponds to electron transition from n orbit to 7t* orbit in the outermost tubesof MWNTs. These absorption features associate with VHS in the density of states. It is experimentally shown that there are VHS in MWNTs. The upconversion emission is found and different PL spectra are observed when MWNTs are excited with light of different wavelength by the research on PL spectra. The mechanism describing the underlying processes, which involve two-photon absorption followed by upconverted luminescence from VHS energy states. The excitation at 248 nm result in transition of the electron7t-7t * in MWNTs and the MWNTs electrons in the exited sates experience radiative transitions and form the PL spectrum peaking at 480nm. But the excitation at 514nm produces the PL spectrum peaking at 750 nm and 860 nm, respectively, mainly due to the some defects on the MWNTs surface. And the mechanism of optical limiting effects for MWNTs has been studied.Part 3: The developing history, essentiality, possible applications, and the present situation of optical property for ZnO material are introduced in chapter four. Chapter five, at first, the XRD spectrum for ZnO films synthesized by rf magnetron sputtering was measured. The diffraction peaks can be well indexed the samples withhight crystal quality. The Raman spectra of ZnO films synthesized by rf magnetron sputtering and sol-gel progress have been investigated. The Raman spectra of two samples are compared. And the cause appearing different Raman spectra has been explained. Secondly, the absorption spectrum of ZnO films synthesized on quartz substrate by rf magnetron sputtering was measured. The absorption spectrum exhibits a clear peak at 370 nm, which is freedom excitons absorption peak. And the bandwidth (Eg~3.3eV) is calculated. Then the PL spectrum of ZnO films synthesized on quartz substrate by rf magnetron sputtering was measured. The peak of near-ultraviolet emission located at 389 nm has been observed. Finally, PL spectra of ZnO films with soaking ethanol and ultrasonic treatment has been studied at first time. For the ZnO films treated by ultrasonic, not only the ultraviolet peak of ZnO films, but also the strong green peak at 508 nm have been observed. It is also found that the intensity of the green peak (508 nm) is stronger than that of the ultraviolet peak. The ultraviolet peak also appears red-shift. And the mechanism of appearance the phenomenon is interpreted and testified at first time. The foundation of experiment and theory are provided for further research on the emission mechanism of ZnO.Part 4: PL spectra of ZnO nanowires exited with 355 nm laser pulse at room temperature are measured. The narrow violet band (peaking at 382 nm) and a broad green band (peaking at 507 nm) are observed in the PL spectrum. The intensity of luminescence depending on the excitation power density and the saturation effect of luminescence are also observed. But the variation and saturation value are different for the two luminescence peaks. It is shown in experiment that the violet peak of ZnO nanowires appears red-shift as excitation power density increases. The mechanism, which includes luminescence peaks appearance, saturation value, luminescence intensity variation, and wavelength red-shift has been analyzed. The PL spectra of ZnO films and nanowires depending on excitation power density are compared. It is found that the variation of excitation power density only alters the intensity of the violet peak of ZnO films and does not change the location of the peak. However, the variation of excitation power density not only alters the intensity of the violet peak ofZnO nanowires, but also changes the location of the peak. It is found that the emission property of material depend on material form.