Thermal Transport And Theroeletric Management Of Two Dimensional Bi₂O₂Se

Two-dimensional（2D）materials,such as graphene,have emerged as a promising platform for investigating heat transport and thermoelectric conversion in low-dimensional systems.These materials possess unique properties that offer appealing prospects for various applications.For example,the remarkable thermal conductivity of graphene has proved to be a great asset in dissipating heat in electronic devices.On the other hand,some 2D materials like Bi Cu OSe and Bi₂Te Se₂ exhibit a strong quantum confinement effect.And their weaker in-plane covalent bonds lead to a lower thermal conductivity,making them ideal for efficient thermoelectric conversion.These materials have significant applications in the areas of green clean energy and low-temperature refrigeration.In recent years,the 2D semiconductor Bi₂O₂Se has drawn extensive attention due to its exceptional properties,such as high electron mobility,moderate band gap,and excellent air stability.Its unique lattice structure contributes to an extremely low thermal conductivity,making it a promising material for thermal transport and thermoelectric conversion.However,previous studies has focused on the bulk counterpart and theoretical calculations.Therefore,there is a need for systematic investigation of key issues such as heat dissipation in Bi₂O₂Se-based electronic devices,heat transport,regulation of heat transport,and thermoelectric conversion of 2D Bi₂O₂Se.The main findings are organized into four parts:（1）Two-dimensional Bi₂O₂Se was synthesized by chemical vapor deposition,and its high quality was demonstrated by multiple characterization methods（optical microscopy,transmission electron microscopy,atomic force microscopy,scanning electron microscopy）.Meanwhile,the Bi₂O₂Se-based electronic/optoelectric devices were constructed.The high mobility(100–200 cm²V^-1s^-1)and high on/off ratio（～10⁴）were achieved in Bi₂O₂Se-based field-electric-transistor.And a high optoelectric response over a broadband was also realized.（2）The thermal conductivity of two-dimensional Bi₂O₂Se with varying thickness was investigated using Raman spectroscopy as a micro thermometer and heating source,through analysis of the Boltzmann heat transport equation.The results indicated a steady decline in thermal conductivity as the thickness decreased,and when it reached 8nm,the thermal conductivity was only～0.92±0.18 W/m K,a obvious reduction compared to most other 2D materials.The low thermal conductivity of Bi2O2Se is mainly attributed to three factors:low phonon group velocity due to strong anharmonic scattering,weak chemical bonds and strong acoustic phonon surface scattering.Additionally,the heat dissipation properties of 2D Bi2O2Se-based FET were also investigated.The good interfacial adhesion between the thin layer of Bi2O2Se and Si O₂/Si substrate resulted in a relatively large interfacial thermal conductivity(21MWm^-2K^-1).（3）The effect of oxygen defects on 2D Bi₂O₂Se heat transport was studied by thermal bridge method.The thermal conductivity of Bi₂O₂Se at room temperature can be reduced to0.68±0.05 W/m K by introducing oxygen vacancy through chemical vapor deposition.This phenomenon is attributed to full-band phonon scattering at high,medium,and low frequencies which cased by oxygen defects,strong anharmonicity,and nanometer-scale boundaries.In addition,due to the introduction of oxygen defects,the dominated scattering in Bi₂O_xSe transforms from polarized optical phonon scattering to acoustic phonon scattering at high temperatures（>200 K）,and its mobility is significantly improved,reaching 260～500 cm²V^-1S^-1 at 300K.（4）A thermoelectric device based on Bi₂O₂Se is designed.In comprehensive thermoelectric transport,the mobility of two-dimensional Bi₂O₂Se is governed by polar optical phonon scattering at high temperatures.And at low temperature,the mobility is governed by piezoelectric scattering and improved significantly.Importantly,the improvement of conductivity did not lead to the obvious drop of Seebeck coefficient,indicating that the management of scattering mechanism can achieve the decoupling of conductivity and Seebeck coefficient,which is completely different from the previous strategy of balancing conductivity and Seebeck coefficient by regulating carrier concentration.The modulation of mobility with thermoelectric power factor exhibits an almost linear correlation over two orders of magnitude.At the same time,the transition temperature of this scattering mechanism has a high degree of gate voltage tunability.By applying a certain amount of gate voltage,the transition temperature of polarized optical phonon scattering to piezoelectric scattering can be significantly improved,and finally the high thermal electric power factor(>400 W^-1m^-1K^-2)with a wide temperature range（80-200 K）can be achieved.