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Adsorption And Self-assembly Of Water Molecules On Bi(111)

Posted on:2017-05-03Degree:MasterType:Thesis
Country:ChinaCandidate:Y ZhangFull Text:PDF
GTID:2271330503983407Subject:Condensed matter physics
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Water is one of the most familiar and survival materials. In recent years, the research of interfacial water has become a hot research topic in the field of surface physics and nanomaterials. The research involves many fields. Especially after the invention of the STM, its high spatial resolution allows people come into the molecular and atomic age about water study. We can use STM to observe water molecules in the morphology of the solid surface, adsorption behavior, dynamic diffusion process and molecular orbital. And this makes us to have a deeper understanding of the structure of water molecules. It is helpful to preparing a variety of water nanoclusters. Although STM can provide high resolution images of molecular or atomic, it is not easy to water molecules. And the imaging process is easy to be disturbed by the STM tip. If STM tip adsorb a water molecule in the scanning process, it may open a new tunneling channel, and effect the tunneling current, thus affect the water molecule imaging. This study further reveals the imaging mechanism of STM, and deepens the understanding of the electronic structure of solid surface.We have investigated the adsorption and self-assembly of water molecules on the self-metallic Bi(111) surface. When the deposition rate is very low, individual water monomers and dimers have been observed on Bi(111) surface. High-resolution STM images indicate that the monomers are flat-lying and occupy the on-top site of Bi substrate. The dimers exhibit two protrusions which are aligned at the principal axes of Bi(111) lattices. When the deposition rate is high, large scale of monodispersed water hexamer have been found on the substrate. Each hexamer manifests as a triangle composing three bright protrusions aligned at the directions of Bi(111) principal axes. By further analysis, it is found that the three bright spots of hexamers arise from the three higher water molecules. One H atom is pointing to the vacuum while the other one takes part in the hydrogen-bond. The other three water molecules are absent due to their lower height. Water hexamer is a relatively stable type of adsorption. Under the premise of high beam current, increasing the coverage of the molecule, we can observe the hexamers connected to each other. Although they are very close, but the structure of these hexamer clusters has not been destroyed. We attribute the self-assembly of water on Bi(111) to a kinetics-limited growth process. The study provides a new idea for the preparation of water nanostructures.In this paper, the STM images’mechanism of the adsorption of water molecules and clusters on Bi (111) surface under different bias voltages by a modification tip is also studied. The Bi (111) thin film is used as the substrate, and the tungsten tip is used in the experiment, which can easily adsorb water molecules. The adsorb water molecules can function STM tip, thus open the new tunneling channel. Therefore, the water molecules’STM images change. It is found that in the experiment water molecules can make STM tip function at liquid nitrogen (77K) or helium (4.4K) temperature. Under different bias, water monomer, dimer, and hexamer’s STM images all change. Interestingly, when the bias is recovered to the original numerical value, the STM images change back to their original images. We make a conclusion that water molecules can modify STM tip and make STM tip functional. Thus the modified tip can image the molecular orbital selectively under different bias. Under the same high bias, there is a boundary of projections and depressions in the water hexamer clusters’STM image. Under low negative bias, all of them exhibit as depressions. Therefore, both of the water molecules functioning STM tip and the bias are the main factors to effect the STM images of the water molecules and their clusters on Bi (111) surface. Due to the draconian condition and the sensitivity of interference, we can not easily observe the water molecular orbital and water clusters’ orbital. So how to optimize STM tip, adjust the distance between tip and sample, and how to change the bias are the challenges we have encountered. It deepens the understanding of the mechanism of STM imaging and the understanding of the electronic structure of the solid surface.
Keywords/Search Tags:water, scanning tunneling microscopy, semi-metallic Bi(111) surface, hexamer, STM tip functionalization
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