Fabrication Of Flexible Bismuth Telluride Films And Investigation Of Their Photoelectric Properties

Bismuth Telluride(Bi2Te3)is a semiconductor with a unique layered structure.Since 1960 s when Bi2Te3 and its alloys were found to possess some excellent thermoelectric properties,much attention has been paid to the study of these materials for thermoelectric applications.In recent years,there is renewed interest in studying Bi2Te3 as it was predicted that the material belongs to a group of three dimensional topological insulations in 2009 and shortly after that the prediction was confirmed in the experiments in the same year.Furthermore,it has been recognized that Bi2Te3 is a material suitable for fabricating wearable,flexible electronic and power devices due to its unique layered structure and the efficient energy conversion efficiency at room temperature.The objectives of the project described in this dissertation are to use a modified hot wall epitaxy(HWE)technique to grow flexible Bi2Te3 films on soft polyimide(PI)plastic substrates for fabricating flexible electronic devices,and to systematically investigate the thermoelectricity and photoelectricity of the flexible Bi2Te3 films.The major contents and results in this dissertation are summarized as follows:At first,we grew the high quality Bi2Te3 films on flexible PI substrates in a modified HWE system.The HWE technique is a simple vapor phase deposition method.Although the films grown by HWE technique do not have as high quality as the films grown by the molecular beam epitaxial method,the HWE system is still able to grow high quality films after the improvement.In the experiment,the crystal qualities of Bi2Te3 films on PI substrates were characterized using an X-ray diffractometer and scanning electron microscope.We made improvements to the growth conditions on the basis of the results of the measurements.We obtained Bi2Te3 films which exhibit better crystalline quality than the result from literature.After that,we fabricated the Bi2Te3 flexible photoelectronic devices with the as-grown flexible films,and the photoconductive response properties of these devices were characterized under different degrees of bending.The experimental results and SEM images revealed that the flexible device can maintain a stable respond even in larger degrees of bending conditions,which also confirmed that the Bi2Te3 films grown by modified HWE technique were suitable for fabrication the flexible electronic devices.Secondly,we studied the thermoelectricity property of the strained Bi2Te3 films in the flexible electronic devices.The Seebeck coefficients of the strained Bi2Te3 films under different temperature gradient directions were measured using a homemade experimental set up.The directional dependent Seebeck coefficients in x-y plane were found in c-oriented Bi2Te3 films which under the influence of the stain gradient in x-y plane.However,this result is inconsistent with the previous literature which reported that the Seebeck coefficient of Bi2Te3 has no relation with the direction of temperature gradient.After more experiment and analysis,we proved that the flexoelectric effect induced by the strain gradient between films and substrates should be responsible for the anomalous thermoelectric effect.Besides,the coupling effect of flexoelectric field and thermoelectric field on the measurements of open-circuit voltage was also been discussed.Finally,the bulk photovoltaic effect(BPVE)induced by the flexoelectric effect has been observed in the strained Bi2Te3 films was studied.Since the Bi2Te3 is an efficient thermoelectric material,a tiny temperature gradient can generate a significant thermoelectric voltage.In order to exclude the influence of temperature gradient on film surface induced by the thermal effect of light,the films were horizontally rotated by 180 degrees to reverse the direction of flexoelectric field and keep the direction of thermoelectric field unchanged at the same time.Then,the designed comparative experiments were carried to distinguish the BPV effect induced by the flexoelectricity and thermoelectric effect induced by the temperature gradient.The experimental results revealed that the strained Bi2Te3 film is a promising BPV material,and the subsequent experiments found that the value and direction of the voltage can be modulated by bending the films.