A Study On The Photoelectric Behavior Of Functional Materials By Kelvin Probe Technique

Functional materials with photoelectric properties will play a crucial role in the development of advanced science and technology in future, such as in the fields of energy, information and biology. The performance of the photoelectric functional materials is essentially determined by the intrinsic structure of the materials, which is mainly reflected by the photo-induced electron transition and transfer behavior in microscopic view. Especially for developing the advanced materials with photo-to-electric converting function, it is of great significance to study the relationship between the photoelectric activity and the characteristics of the photo-induced charges and explore the mechanisms of influencing the behavior of the charges. These researches are also beneficial to the practical application of the functional materials. In recent years, the introduction of nanomaterials into various photoactive functional systems has been proved to promote their performance significantly. Therefore, it is essential to study the behaviors of the photo-induced charges at the interface and surface of the photoactive systems with nanostructure for exploring their novel operating mechanisms and constructing and perfecting the photoelectric systems with higher performance.The Kelvin probe method has been traditionally and widely applied in the measurement of surface work function. However, only a few examples can be observed domestically, which mainly focus on the corrosion of metals under atmosphere. There are relatively rare domestic reports on the photoelectric behavior of functional nanomaterials by the Kelvin probe method, which is in rapid progress internationally at present. Therefore, one of the aims of this thesis is to explore the application of macroscopic scanning Kelvin probe in the study of the photoelectric behaviors of functional nanomaterials as well as their surface work functions.In the first chapter of this thesis, the traditional characterizing techniques on the photo-induced charges and the applications correlated with the photo-induced charge are intensively reviewed. The state of the art of the research on one typical functional material, ZnO nanomaterial is described briefly. In the second chapter, the fundamental concept, instrumental principle and applications of Kelvin probe technique including Kelvin probe force microscope are introduced in detail. Then in the following four chapters, the main thesis work is included.We first prepare ZnO nanorods with different sizes. Especially, ZnO nanorod arrays on transparent and conductive ITO are fabricated via chemistry method. The Kelvin probe (SKP, purchased from KP technology, UK) is used to study the surface work function variation of these materials under light excitation on visible-ultraviolet spectrum. By examining the onset of the photovoltage response and its decay upon light off, the kinetics of the photovoltage evolution are analyzed. We further discuss the contributions of the surface and the interface between ITO and ZnO nanorods to the photovoltage response of the arrays. By comparison of the photovoltaic features of different sizes and orientations of the nanorods, we reveal the relations between the photovoltaic behaviors of the nanorods and the geometrical parameters: size and orientation.Subsequently, the effects of ethanol solvent and a liquid crystalline porphyrin, [5-(para-dodecyloxy)phenyl-10,15,20-tri-phenyl] porphyrin (DPTPP) on the photovoltage enhancement in the ZnO nanorods array were studied via comparison between different irradiation directions on the system. We demonstrate that the ethanol adsorption could induce the space charge region to expand towards the ZnO/ITO interface. In the absence of ethanol, the ZnO nanorods array with the DPTPP adsorption showed enhanced SPV with reduced attenuation rate of photogenerated charge carriers observed on Kelvin probe. We found the separation of photogenerated charges could be further improved when coating the surface with DPTPP and ethanol together. Furthermore, the SPV spectra patterns of the composite system with opposite light-incident directions reveal that the DPTPP molecules adsorbed just at the surface of ZnO nanorods adopt more monomeric alignment in contrast to the aggregative state in the DPTPP bulk.Next, the ZnO array with different thicknesses of NiO film covered is studied by Kelvin probe technique. We compare the surface work functions and the photovoltage spectra of ZnO nanorods film and ZnO nanorods films with increasing thickness of NiO film as well as pure NiO film on ITO substrate. The work function of as-prepared NiO film is 0.36 eV higher than the gold reference probe. There are potential applications in photocatalysis and sensors for the composite films.Lastly, the surface work functions of ZnO nanomaterials with different grain shapes and sizes are comparatively studied by Kelvin probe. The surface work function of commercial ZnO powder on two different substrates (Al and Pt) was measured by scanning Kelvin Probe. As for both of the two substrates, we observed the same phenomenon that the apparent surface work function gradually changed towards the coherent value with increased thickness of ZnO powder layer. We eventually obtained the value of 4.3 eV, which is consistent with that reported in literature. The difference on surface work function between bulk materials and nanoparticle powder is also discussed. The possible geometrical contributions to the surface work function are qualitatively analyzed. We found that the flower-like nanocrystal aggregates with stabs at surface exhibited the lowest surface work function. The best reproducibility of work function result was observed for the nanorods array grown on ITO substrate, which should be attributed to the good electric contact between the ZnO nanorods and the probe. These results demonstrate the validity and issues with the surface work function measurement of nanostructured semiconductors by a macroscopic Kelvin probe.