The development of faster and more efficient electronic devices involves the understanding of exotic properties of matter at the nanoscale. One of the classes of materials that present characteristics of interest for the electronics industry are the so-called topological insulators.

Topological insulators are materials only a few atoms thick that behave as insulators in the inner atomic layers, but as conductors in the surface. The electrical conductivity of these superficial layers is remarkably resistant to the atomic disorder caused by the presence of impurities, which is not the case in other materials.

Another important characteristic in these materials is that the spin magnetic moment of the surface electrons is “topologically protected”, locked in the direction perpendicular to the propagation of the electric current. Electron spin is already a property exploited for the storage of data on magnetic hard disks on computers. The special property of topological insulators can also open the door to the use of electron spin for processing information, and in future devices such as transistors for quantum computing.

Among the topological insulators, $\rm Bi_2 Te_3$ and $\rm Bi_2 Se_3$ are especially promising for use in nanodevices due to the formation of bismuth bilayers ($\rm Bi_2$) which, in addition to being conductive, also exhibit the so-called quantum spin Hall effect. However, the location of these bismuth bilayers in the three-dimensional structure of the material during the production of the topological insulators is still an open problem.

For this reason, Pedro Henrique R. Gonçalves et al. [1] investigated the formation of different phases of $\rm Bi_x Se_y $ during the controlled heating (annealing) of a system composed of $\rm Bi_2 Se_3 $. The sublimation of part of the selenium during the heating process allows new molecules to form in the material. In addition, the coexistence of distinct phases with $ \rm Bi_2 Se_3 $ can lead to distinct electronic properties.

As the temperature increases, decrease of $\rm Bi_2 Se_3$ is promptly observed, with the formation of $\rm Bi_4 Se_5$ and $\rm Bi Se $ phases. This indicates the sublimation of selenium and the creation of $ \rm Bi_2 $ bilayers inside the crystal.

Among the analyses carried out by the researchers, X-ray diffraction measurements were carried out at the XRD2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS) to follow very small changes in the composition of the material during thermal treatments. The researchers observed evolution of additional phases leading to the formation of bismuth bilayers both on the surface of the system and interspersed with quintuple layers of $\rm Bi_2 Se_3 $.

The formation of new phases shows that a possible device based on $\rm Bi_2 Se_3 $ should be used in a specific range of electric current and voltage so that local heating does not alter its chemical composition and, consequently, impairs its operation.

On the other hand, the results show that with the understanding of how to lead the system to new configurations of interest, it is possible to perform thermal treatments on a material for the controlled production of different chemical phases suitable to the final device. However, according to the researchers, this diversity of configurations still needs to be mastered so that future devices based on these materials can be designed.

Source: [1] P. H. R. Gonçalves, Thais Chagas, V. B. Nascimento, D. D. dos Reis, Carolina Parra, M. S. C. Mazzoni, Ângelo Malachias, and Rogério Magalhães-Paniago. Formation of BixSey Phases Upon Annealing of the Topological Insulator Bi2Se3: Stabilization of In-Depth Bismuth Bilayers, The Journal of Physical Chemistry Letters 2018 9 (5), 954-960. DOI: 10.1021/acs.jpclett.7b03172