Regulation Of The Electronic Band Structure And Electronic-Thermal Transport Properties Of MoTe₂ Based Compounds

The two-dimensional（2D）transition metal dichalcogenides（TMDs）,especially the MX₂（M=Mo and W;X=S,Se and Te）compounds,are attracting extensive attentions in the research fields of materials science and physics due to their unique structural,mechanical,optical,electrical,magnetic,and thermal properties.As one type of representative van der Waals layered materials,MX₂compounds are beneficial for constructing novel heterostructures or superlattices with intriguing physical and chemical properties.In addition,MX₂compounds provide an important platform in the frontier of fundamental researches in order to explore new structures,new physical effects and new physical mechanisms.On the one hand,the rapid development of strategic industries,such as 5G/6G communications,large-scale integrated circuits and wearable electronic products,have raised significant requirements for efficient thermoelectric materials and thermoelectric devices.They can realize the direct and reversible conversion of electrical energy and thermal energy,and are developing towards miniaturization,integration and flexibility for their emerging applications in the above fields.On the other hand,the electronic information field has put forward critical needs for exploring novel materials with the quantum spin Hall（QSH）effect and quantum anomalous Hall（QAH）effect as well as their performance optimization for developing quantum computation and next generation low dissipation electronic devices.The MoTe₂based compounds have the semiconductor 2H phase and topological 1T’（1T_d）phase,which endow MoTe₂based compounds the excellent candidates for developing low-dimensional high-performance thermoelectric materials and the regulation of topological electronic structure.Due to the current research deficiencies of MoTe₂based compounds in thermoelectricity,their significant advantages in the research of low-dimensional thermoelectric materials,and their unique properties in topological electronics,we choose MoTe₂based compounds as the research object in this work.And we have studied and elucidated the regulation of the electronic band structure and the new mechanisms for optimizing the electronic and thermal transport properties regarding the MoTe₂based compounds in the form of polycrystalline bulk,2D thin films,heterostructures and superlattices.The main research work and conclusions are summarized as follows:（1）Multi-valley band structure,impurity electronic states and electronic-thermal transport optimization mechanisms of the MoTe₂based compounds.A series of compacted 2H-and 1T’-MoTe₂based polycrystalline bulks with preferentially oriented microstructure have been successfully fabribated via solid-state reaction followed by plasma activated sintering（PAS）.The experimental results and the density functional theory（DFT）calculations have indicated that,the semiconductor 2H-MoTe₂based compounds possess the largest single valley effective mass_bm^*and a high band degeneracy NVas well as weak Mo-Te bonds,leading to much improved thermoelectric properties in comparision with other 2H-Mo X₂.The element Nb behaves as an effective p-type dopant for 2H-MoTe₂based compounds,which induces impurity electronic states near valence band maximum（VBM）,and leads to dramatically increased hole denstiy and carrier effective mass m^*as well as enhanced defect scattering of phonons.Furthermore,Nb doping in semimetal1T’-MoTe₂can suppress the bipolar electronic transport,resulting in simultaneous improvements of the electrical conductivitysand Seebeck coefficient S,and hence the improved power factor PF.Besides,Se alloying can remarkblely reduce the lattice thermal conductivityk_Lof the 2H-MoTe₂based compounds while the electrical properties are not deteriorated.Finally,the Mo_0.95Nb_0.05Se_1.2Te_0.8solid solution obtains the highest ZT_maxof 0.34 at 823 K in the direction perpendicular to the pressing direction（^P）,which is the best value among the 2H-Mo X₂based compounds.This work vertified that doping combined with solid solution alloying is an effective approach for optimizing the electronic band structure and the electronic-thermal transport properties of 2H-MoTe₂based bulk materials.（2）Regulation of the phase structure and the exploration of the electronic band structure and electronic transport properties of MoTe₂.The MoTe₂films were epitaxially grown on highly oriented pyrolytic graphite（HOPG）substrate using mocular beam expitaxy（MBE）technique.The in-situ characterization methods,such as reflection high-energy electron diffraction（RHEED）,X-ray photoelectron spectroscopy（XPS）,scanning tunneling microscopy（STM）,and angle-resolved photoemission spectroscopy（ARPES）,are comprehensively used to study the regulation of the phase structure and the characterastics of the electronic band structure of MoTe₂.The results indicated that,high growth temperature,annealing at high temperature and Se alloying are beneficial for obtaining 2H-MoTe₂,while the low growth temperature can promote the formation of the 1T’-MoTe₂.In monolayer（ML）2H-MoTe₂,there are abundant triangle network composed by mirror twin boundaries（MTBs）featured with one dimensional meatallic charge density wave（CDW）.While,the ML 1T’-MoTe₂shows the stripe-like surface morphology.Electronic structure study shows that,the ML 2H-MoTe₂is semiconductor with multi-valley and spin-spliting band structure.The ML 1T’-MoTe₂is semimetal with linear dispersion valence band and topological edge states.The electronic transport study shows that,the MBE-grown MoTe₂thick films with predominating 1T’phase have higher electrical conductivitysand power factor PF,and the maximum in-plane PF_maxof 0.31 m Wm^–1K^–2is obtained at 340 K.The study of phase structure and electronic band structure of the MoTe₂thin films has laid an important foundation for the subsequent explorations of functional units assembled superlattices with improved electronic-thermal transport properties and the regulation of topological electronic structure based on 1T’-MoTe₂.（3）The structure regulation,functional coupling,interfacial effect and electronic-thermal transport properties of functional units assembled superlattices based on 1T’-MoTe₂.The topological 1T’-MoTe₂phase with high conductivity and Bi₂Te₃or Sb₂Te₃with high thermoelectric properties are selected as the functional units.The MBE technique is effective for the successful fabrication of the n-type（1T’-MoTe₂）_x/（Bi₂Te₃）_yand the p-type（1T’-MoTe₂）_x/（Sb₂Te₃）_ysuperlattices with desirable stacking periods.This research demonstrated that,the interactions between functional units can promote the coupling and enhancement of functionality,while the functional units together with the spatially stacking order are effective for significantly improving the electronic-thermal transport performance of the assembled superlattices.The fabricated 1T’-MoTe₂based superlattices inherit the ferroelectric properties of the pristine 1T’-MoTe₂,with somehow intensified out-of-plane ferroelectric polarizations.The 1T’-MoTe₂and Bi₂Te₃units can form the metal-semiconductor heterostrucuture with ohmic contact,in which an obvious electron transfer from 1T’-MoTe₂layer to Bi₂Te₃layer is discovered.This leads to an over one magnitude increase of electron density for the n-type（1T’-MoTe₂）_x/（Bi₂Te₃）_ysuperlattices and hence the significantly improved power factor PF,as compared to the pristine Bi₂Te₃film.However,this effect is not valid in p-type（1T’-MoTe₂）_x/（Sb₂Te₃）_ysuperlattices.Meanwhile,the ferroelectric polarization,interfacial potential barrier and the grain boundary scattering at the interface can induce the carrier energy filtering effect,leading to obviously improved Seebeck coefficients of the superlattices.The（1T’-MoTe₂）₁/（Bi₂Te₃）₁₂superlattice with the period of x/y=1/12 possesses the maximum PF_maxof 2.72 m Wm^–1K^–2at room temperature and the average PF_aveof 2.38 m Wm^–1K^–2within 300–500 K,which are significantly larger than that of the pristine Bi₂Te₃film.Furthermore,the interfacial diffuse scattering of phonons and the ferroelectric instability induced soft transverse optical phonons modes lead to much reduced lattice thermal conductivityk_Lin comparison with the pristine Bi₂Te₃.Due to the synergistic optimization of electronic and thermal transport performances arising from the functional units combined with the spatially stacking order,the superlattice with the period of x/y=1/12 grown under the high Te flux condition obtains the maximum room temperature ZT_maxvalue along the in-plane and out-of-plane directions,which are 0.46 and 0.49,respectively.（4）Regulation of the topological electronic band structure of ML 1T’-MoTe₂and the proximity effect.The ML MoTe₂with nearly single phase of 1T’and the separating tendency of overlapping conduction and valence band tails are demonstrated in the formed 1T’-MoTe₂/Bi₂Te₃van der Waals heterostructure（vdw H）,which is fabricated by growing ML 1T’-MoTe₂on a three-dimensional topological insulator Bi₂Te₃（00l）substrate with strong spin-orbit coupling（SOC）.Based on the DFT calculations and the characterizations of ARPES and XPS,it is shown that the large work function defference between the two functional units results in strong interlayer charge transfer and interfacial protential,which is the main mechanism for the formation of the nearly single phase 1T’-MoTe₂.It is revealed that,the SOC proximity effect is the physical mechanism responsible for the apparent separation of the overlapping conduction band and valence band tails in ML 1T’-MoTe₂,which is due to the increased electron density in Mo-3d orbitals and the enhanced SOC originated from the interfacial potential.This study verifies that the vd WH engineering based on large work function differences is an effective approach for promoting the interlayer interactions and for introducing significant SOC proximity effects,which is beneficial for regulating the topological electronic band structure of various electronic materials.