The manipulation of individual intrinsic point defects is crucial for boosting the thermoelectric performances of n-Bi2Te3-based thermoelectric films, but was not achieved in previous studies. In this work, we realize the independent manipulation of Te vacancies VTe and antisite defects of TeBi and BiTe in molecular beam epitaxially grown n-Bi2Te3 films, which is directly monitored by a scanning tunneling microscope. By virtue of introducing dominant TeBi antisites, the n-Bi2Te3 film can achieve the state-of-the-art thermoelectric power factor of 5.05 mW m–1 K–2, significantly superior to films containing VTe and BiTe as dominant defects. Angle-resolved photoemission spectroscopy and systematic transport studies have revealed two detrimental effects regarding VTe and BiTe, which have not been discovered before: (1) The presence of BiTe antisites leads to a reduction of the carrier effective mass in the conduction band; and (2) the intrinsic transformation of VTe to BiTe during the film growth results in a built-in electric field along the film thickness direction and thus is not beneficial for the carrier mobility. This research is instructive for further engineering defects and optimizing electronic transport properties of n-Bi2Te3 and other technologically important thermoelectric materials.

Interactions among various film growth parameters, such as the substrate temperature (Tsub), film thickness (d), and composition, play a crucial role in controlling the type and density of the intrinsic point defects. In turn, the point defects modulate and control electronic transport properties of Bi2Te3 films. We have grown n-type Bi2Te3 films with different d by molecular beam epitaxy at different Tsub. The formation of point defects was analyzed by a combined use of angle-resolved photoelectron spectroscopy (ARPES) and electronic transport measurements. Two important findings were made: (i) the negatively charged vacancies, VTe··, initially the dominant intrinsic defects, transform gradually during the growth process into positively charged anti-site defects, BiTe′, driven by thermal annealing from a continuously heated substrate; and (ii) from the film's surface into the inner strata of the film, the density of VTe·· decreases while the density of BiTe′ increases, leading to a gradient of vacancies and anti-site defects along the film growth direction. As a result, the electron density in Bi2Te3 films decreases monotonically with increasing d. Moreover, elevating Tsub leads to a more significant in situ annealing effect and an eventual onset of intrinsic excitations that deteriorates electronic transport properties. The thinnest Bi2Te3 film (16 nm) grown at Tsub = 245 °C has the highest electron concentration of 2.03 × 1020 cm−3 and also the maximum room temperature power factor of 1.6 mW m−1 K−2 of all grown epitaxial films. The new insights regarding the defect formation and transformation pave the way for further optimization of electronic transport properties of n-type Bi2Te3-based films.

High-temperature thermoelectric properties of the double-perovskite ruthenium oxide (Sr1−xLax)2ErRuO6 J. Appl. Phys. 112, 073714 (2012) Thermoelectric properties of n-type Mn3−xCrxSi4Al2 in air J. Appl. Phys. 112, 073713 (2012) Thermoelectric effect in a graphene sheet connected to ferromagnetic leads J. Appl. Phys. 112, 073712 (2012) Thermoelectric properties of p-type Bi0.5Sb1.5Te2.7Se0.3 fabricated by high pressure sintering methodThe effective thermoelectric behavior of layered heterogeneous medium is studied, with the distribution of temperature, electric potential, and heat flux solved rigorously from the governing equations, and the effective thermoelectric properties defined through an equivalency principle. It is discovered that the effective thermoelectric figure of merit of a composite medium can be higher than all of its constituents even in the absence of size and interface effects, in contrast to previous studies. This points toward a new route for high figure of merit thermoelectric materials.

Monolayer (ML) 1T′-MoTe 2 has attracted intensive interest as a fascinating quantum spin Hall (QSH) insulator. However, there are two critical aspects impeding its exploration and potential applications of QSH effects. One is its semimetallic feature with a negative band gap, leading to nontrivial edge channels annihilated by the bulk states. The other is its fabrication always accompanied by a mixed phase of 1T′ and 2H. Based on first-principles calculations, it is shown that the large work-function difference results in strong interlayer interactions and proximity effects in ML 1T′-MoTe 2 via interfacing a 3D topological insulator Bi 2 Te 3 , facilitating the realization of pure 1T′ phase and even the band gap opening. It is further verified that the epi-grown ML 1T′-MoTe 2 on Bi 2 Te 3 is nearly in single phase. Furthermore, the measurements of angle resolved photoemission spectroscopy and scanning tunneling spectroscopy confirm the obvious separated-tendency of conduction and valence bands as well as the strong metallic edge states in ML 1T′-MoTe 2 . The results also reveal the nontrivial band topology in ML 1T′-MoTe 2 is preserved in 1T′-MoTe 2 /Bi 2 Te 3 heterostructure. This work offers a promising candidate to realize QSH effects and provides guidance for controlling the nontrivial band gap opening by proximity effects in van der Waals engineering.