Thermoelectric Effect And Devices Of Two-Dimensional Bismuth

Thermoelectric materials can not only convert waste heat to electricity directly with zero carbon emissions,but can also monitor temperatures in human bodies,chips,etc.with high sensitivity.Among most metals,bismuth is featured with high mobility and the lowest thermal conductivity if mercury is not in the comparison.It is predicted that the quantum confinement effect can further boost the thermoelectric properties of bismuth.Therefore,this study focuses on the thermoelectric properties of two-dimensional(2D)bismuth,and reveals the effect of preparation process,thickness,magnetic field and material composition on its structure,morphology,electron and phonon transport.The main results are as follows:(1)2D bismuth(?30 nm)prepared by molecular beam epitaxy(MBE)and e-beam evaporation have different crystal orientations and thermoelectric properties.For instance,when MBE-synthesized 2D bismuth is single crystal with high crystallinity and(001)preferred orientation,e-beam evaporation produces polycrystalline 2D bismuth.Consequently,the former has anisotropic Seebeck coefficient varied by 2-5times,while the latter shows isotropic thermoelectric properties with higher electrical conductivity of?10⁴-10⁵ S/m.Compared with thr bulk counterpart,the thermal conductivity of 2D bismuth is reduced by?20%due to the phonon interface scattering effect.The z T value of 2D bismuth at room temperature is obtained,which is?10^-3-10^-2 and comparable to graphene or black phosphorus.In addition to the synthesis methods,substrate can also influence the thermoelectric properties.For example,when the substrate materials are Si,Si O₂ and glass,the Seebeck coefficient and electrical conductivity vary by?15.3%and?180.4%,respectively.(2)After Si substrate is selected,2D bismuth are deposited with different thicknesses(20 nm-100 nm).The highest power factor is achieved when the thickness is 100 nm.The value is?5.47×10^-5 W m^-1 K^-2 and increased by nearly 7.5 times when compared with20 nm.In this study,thickness plays a more important role in Seebeck coefficient than substrate,since the variation induced by different thicknesses is?8.3 times higher than different substrates.It is worth noting that 2D bismuth exhibits exotic electrical transport that is the opposite signs of Seebeck coefficient and Hall coefficient,and magnetoresistance response,i.e.anti-weak localization(WAL)effect at low temperatures.(3)2D bismuth-based composites are synthesized with different elements such as Ag,Cu and Sn or different microstructures such as Ag-nanowires and Ag-micro-grids.It is found the power factor of the composite thin films is increased by 69.4%.Thermoelectric measurement device is fabricated by photolithography method on 2D bismuth,which can also be utilized in microelectronics field for heat dissipation and energy recycling.In summary,this study investigates the thermoelectric performance of 2D bismuth in different statuses,and provides a way to explore the multifunctional electronic applications of2D bismuth-based thin films,such as thermoelectric energy devices,flexible magnetic field sensors,thermospintronic devices,etc.