Controlled Synthesis And Property Of Cu-and Bi-based Functional Semiconductor Nanomaterials

First of all, we develop a sol gel solution route to synthesize shape-and struc-ture-controlled copper selenide nanocrystals （NCs） including CuSe nanosheets,Cu_2-xSe nanoparticles and Cu_2-xSe nanorods. The morphological features and composi-tion of the resulting nanocrystals can be tuned upon changing the chemical activities ofprecursors. Cu_2-xSe nanoparticles possess a distinct PL emission spectrum centered atca.422nm and the photoluminescence quantum yield （PL QY） is up to6%. Besides,the electrical transport properties of as-prepared products (Cu_2-xSe nanoparticles, na-norods and CuSe nanoplates) were systematically investigated. It is worth noting thatthe Cu_2-xSe nanorods as electrical transport materials present excellent performance.The phenomenon can be rationalized by the fact that its anisotropic rod-like shape andsingle-crystalline nature provide a physically continuous crystal-body and a more effi-cient conducting pathway in Cu_2-xSe nanorods. The present strategy offers a guidancefor other nanomaterials such as Cu-and Ag-based chalcogenides. Meanwhile the ex-cellent performance of copper selenide NCs as electrical transport materials may opennew opportunities for future nanoelectronic device applications.In addition, we demonstrate the synthesis of monodispersed hollow Cu_2-xTe NCswith tunable interior volume via a facile one-step solution-processing strategy based onthe Kirkendall effect. The as-prepared hollow Cu_2-xTe NCs exhibited greatly enhancedgas sensitivity owing to the small grain size and large surface-to-volume ratio. The re-sponse and recovery time of the sensor based on the hollow Cu_2-xTe NCs for100ppmCO were estimated to be about21and100s, respectively at the operating temperatureof260°C. The hollow Cu_2-xTe nanostructures may serve as microreactors in catalyticapplications and it is highly probable to provide new functional b locks for the designof gas sensors. Meanwhile, we believe that this one-step approach could be extended to the preparation of other hollow metal chalcogenides （such as copper sulfides, copperselenides and silver tellurides） based on the Kirkendall effect. Furthermore, we carryout an explorative high-pressure study on the bulk Cu2Te and hollow Cu2-xTe NCs, re-vealing that the phase transition of bulk Cu2Te starts at3.60GPa, and generates anunknown phase of Cu2Te at8.93GPa. It is important that this new phase remains whenthe pressure is released to ambient conditions completely. However, hollow Cu2-xTeNCs have a higher phase transition point compared with bulk Cu2-xTe, owning to thelarge surface energy. When the pressure reaches11.80GPa, the same unknown phaseof Cu2-xTe is produced. Likewise, the new phase is retained even when the pressure isreleased completely. As a consequence, it provides another new technique to acquirenew materials and new structures in the future: That is high pressure.Moreover, we demonstrate that the various size-and shape-controlled Bi₂S₃hie-rarchical architectures （HAs） were success fully prepared via a facile, low-cost, andenvironmentally-benign while effective one-step solution-based strategy. The morpho-logical features of the resultant branched products can be tuned upon changing the pa-rameters in the reaction system. By variation of the synthetic conditions such as initialadded temperature, the ratio of precursors and the concentration of ligand, thebranched Bi₂S₃NCs from sheaf-like to chrysanthemum-, clover-, dandelion-like andother novel architectures were obtained. Crystal splitting behavior was believed to beresponsible for the formation of Bi₂S₃HAs. It was demonstrated further that theas-prepared typical Bi₂S₃microstructures possess a direct band gap of¹.9eV ascribedto the quantum confinement effect. The photoresponse properties based on thesheaf-like Bi₂S₃as a representative system were also investigated in an ambient envi-ronment. It is worth nothing that the photocurrent significantly increases by¹order ofmagnitude compared with the dark state associated with response and decay time of0.5and0.8s, indicating excellent sensitivity with respect to potential applications inphotoelectric switch and light-sensitive devices.Finally, we observe an intriguing room-temperature ferromagnetism （RTFM） be- havior in dopant-free caterpillar-like Bi₂Te₃HAs prepared through a facile solu-tion-based route. First-principles DFT calculations indicate that the intrinsic point de-fect of BiTe2gives rise to a ferromagnetic order at room temperature even in the ab-sence of magnetic impurities. The features of coupling both topological insulators andintrinsic RTFM into a single role provide a possibility to realize the breaking oftime-reversal symmetry （TRS）, thus opening up the Dirac point on the surface and rea-lizing the quantum anomalous Hall effect. Meanwhile, it is anticipated to harness novelMajorana particles for performing fault-tolerant quantum computation in such magnet-ic Bi₂Te₃HAs with intrinsic RTFM. Therefore, one can take advantage of both the to-pological effect and the electronic spin for the promising applications not only in fun-damental condensed matter and particle physics, but also in dissipationless spintronics.At last, we perform the elementary high-pressure study of Bi₂Te₃HAs, which provesthat Bi₂Te₃will experience a continuous phase transition from phase I to phase II,phase II to phase III, phase III to phase IV at pressure of9.22,10.96and15.03GPa,respectively. Compared with bulk Bi₂Te₃, the phase transition points are lower due tothe lattice expansion, and―volume collapse‖resulting from large bulk modulus. It isbelieved that this primary exploration will lay a foundation for the future high-pressurestudy.