The Light-induced Transverse Thermoelectric Effect In Layered Cobalt Oxide Thin Films

Light-induced transverse thermoelectric effect?LITT effect?is an unconventional thermoelectric phenomenon in which heat and electric energy flow through a material perpendicular to each other,and it becomes effective when the c axis of the film is tilted by an angle with respect to the samples surface normal.The origin of LITT effect is the anisotropy in Seebeck coefficient of the material,and it is a conversion process of photo-thermal-electric in nature.Since the LITT effect had been found in YBa2Cu3O7 thin films in 1990,almost all research about it has been focused on the influence of the geometric parameters of the sample on LITT effect.However,the optimization of LITT effect by adjusting electrical or thermal transport parameters in the material has never been reported,which also restricts its development in some extent.In this paper,Bi₂Sr₂Co₂O_y thin films have been epitaxially grown on the c-axis tilted LaAlO3?001?single crystal substrates by using a chemical solution deposition method.The effect of some modified approaches,such as doping,microstructure,oxygen content as well as adding an additional light absorption layer on film surface,on the LITT effect in Bi₂Sr₂Co₂O_y thin films have been systematically investigated.We hope which can optimize the electrical or thermal transport parameter in the material and improve the conversion efficiency of photo-thermal or thermal-electric in LITT effect.The main conclusions are as follows:1.The effect of some basic experimental parameters,including the film thickness and tilt angle as well as the laser incident direction,wavelength and energy,on LITT effect in Bi₂Sr₂Co₂O_y thin films have been investigated.When the film surface is irradiated by pulsed lasers with different wavelength and even a point-like thermal heater,obvious open-circuit voltage signals have been observed,and they show significant dependence on above mentioned parameters.We suggest that the origin of the observed signals is the anisotropic Seebeck coefficient of the material,which is also called the transverse thermoelectric effect.2.The effect of microstructure,oxygen content and Pb-doping of the film on LITT effect have been investigated.a)The samples with larger grain size or sufficient oxygen content show a better crystal quality and a stronger anisotropy,and have a voltage signalwith greater amplitude and faster response time.b)The LITT signals in Bi1.6Pb0.4Sr₂Co₂O_y film or Bi₂Sr₂Co₂O_y-Pb₂Sr₂Co₂O_y mutilayers structures exhibit an enhanced voltage sensitivity due to the increased ?S?the difference of Seebeck coefficient between ab-plan and the c-axis direction of the film?.3.Graphite,single-wall carbon nanotubes as well as nano-structured silver were coated on Bi₂Sr₂Co₂O_y thin film surface used as light absorption layer,respectively,and their influence on LITT effect has been investigated.a)Upon the illumination of a continuous laser,all light absorption layer can improve the voltage sensitivity in the film due to the increased photo-thermal conversion at the absorption layer.b)When the area of the light absorption layer is equal to that of laser spot,and the thickness is appropriate for just complete light absorption,the improvement degree for LITT signal is optimal.c)Among above-mentioned three materials,single-wall carbon nanotubes light absorption layer is the best for the improvement of LITT effect in Bi₂Sr₂Co₂O_y thin film.4.Temperature-dependent LITT effect of Bi₂Sr₂Co₂O_y thin film has been investigated.The voltage magnitude Vp increases with temperature at range of 30-150 K as well as20-250?.We believe that the decreased Vp can be explained by the compressed ?S of Bi₂Sr₂Co₂O_y thin film at temperature range of 150 to 30 K.While,at high temperature?20-250??,the physical mechanism for the improvement of Vp is complicated,and further investigation is necessary.In addition to LITT effect,the lateral photovoltaic?LPV?effect of untilted Bi₂Sr₂Co₂O_y thin film as well as temperature-dependent rectifying and photovoltaic characteristics of an oxygen-deficient Bi₂Sr₂Co₂O_y/Si heterojunction have also been investigated.These results suggest that the Bi₂Sr₂Co₂O_y thin films have great potential applications in many fields,such as thermoelectric devices,wide-band photodetectors,position-sensitive photodetector or photoelectronic devices.So the research about its structure and properties is of a great significance.