Single Crystal Growth of Topological Insulator Bi2Se3: Structural Properties, Correlation Study and Applications

Topological insulators (TI), materials behaving as insulator in bulk but possessing gapless conducting surface states have emerged as pioneering materials due to the discovery of a new state of matter with time reversal symmetry protecting itself against perturbation and non-magnetic disorder. As such, the surfaces of these materials are fruitful environments for studying a variety of interesting phenomena including proximity induced superconductivity and Majorana fermions. In addition to their intellectual appeal, they also hold promise as candidates for use in fledgling technologies such as spintronics and quantum computing. Bi2Se3 a well-known TI exhibits semiconducting behavior with bulk band gap of approximately 300 meV. It also possesses an inverted band structure owing to the presence of strong spin-orbit coupling resulting from the large Bi mass. This TI can be turned to superconductor at very low temperature ~ 4K when intercalated with some particular transition metal atoms inside the van der Waals gap of its crystal structure. High purity single crystals of pure and Cu intercalated Bi2Se3 were grown through modified two step melt growth method which itself is a challenge. In addition to the superconductivity, the T2 variation of resistance with temperature at relatively high temperature indicate the presence of magnetic correlations in the sample. The nature of the magnetic correlation was found to be antiferromagnetic and was confirmed by the magnetoresistance measurements. As Bi2Se3 is a 3D TI, it has high band velocity and high carrier mobility at room  temperature due to topologically protected surface states which allows the electron to move dissipation less. This unique surface properties will be very fruitful to study the applications which involve surface electrons like humidity sensing and field electron emission. The humidity sensing properties of Bi2Se3 single crystals were investigated by a set-up which was designed in the lab. From the experimental results and corresponding theoretical analysis, it was found that the humidity detection abilities of the Bi2Se3 single crystals were improved by Cu intercalation. The chemi-resistive humidity sensing performance of the Cu0.25Bi2Se3 single crystal exhibits high sensitivity (~849%) with comparable response (24s) and recovery time (25s), negligible hysteresis (<1%) and excellent stability within 8-97% relative humidity (RH) range at room temperature. On the basis of the literature survey and scientific understanding, it is well established that nanomaterials are more suitable than single crystals to study the field electron emission property of a material. So nanoflakes of Bi2Se3 and Ni doped Bi2Se3 were synthesized by hydrothermal method to explore their electron emission dynamics. The results (a field enhancement factor (β) of 5.7 X 103 and a threshold field value of 2.5 V/μm for Ni doped Bi2Se3) suggested that Ni doping enhanced the field emission properties of TI Bi2Se3 nanomaterials and can be potentially used in X ray generators and flat panel displays.