2, Technion–Israel Institute of Technology, Haifa, , Israel
3, Technion–Israel Institute of Technology, Haifa, , Israel
Revolutionary as graphene, the inorganic two-dimensional (2D) nanostructures intrigue the scientific community, by offering large variability of chemical compositions, mechanical rigidity, possible engineering of electronic structure and tunability of energy band-gap. Thus, the inorganic 2D nanostructures are potentially useful for various opto-electronic applications. The current research focuses on the development of synthetic procedures for the growth and phase control of 2D semiconductor nanostructures, In2X3 (X=S) in a shape of nanoplatelets (NPLs). The materials were prepared using one step colloidal synthesis process. Metal-thiolate forms in the initial stage of reaction, which effectively acts as the precursors to decompose into γ-In2S3 nanocrystals. The size and shape of the γ-In2S3 NPLs can be easily controlled by varying the reaction growth temperature and the decomposition rate of Indium-thiolate. Additional heat supply lead to phase transition obtaining the defect spinel phase β-In2S3. The crystal structure and phase identities were confirmed by X-ray diffraction and HR-TEM analyses. The possible formation mechanism for γ-In2S3 NPLs with two anisotropic shapes is presented on the basis of the nanocrystals growth directions and the synthesis conditions. The HR-TEM images recorded at various synthesis stages indicated that the synthesized individual NPLs naturally restacked owing to mutual van der Waals (vdW) forces, and eventually led to moiré patterns with lattice fringes. Analysis of the FFT data supplied the number restacked hexagonal NPLs and the relative rotation angle between adjacent layers.
The absorption spectra and the Tauc analysis proposed the existence of a direct wide band-gap in both γ-In2S3 and β-In2S3 NPLs around ~ 3.5 eV -3.7 eV, in contradiction to a situation found in the analogous bulk materials. Furthermore, emission spectra were dominated by an intense exciton band slightly Stokes shifted from the absorption band-edge, with additional contribution from a defect radiative recombination at lower energies. The relative defects emission intensity change greatly during the reaction progress and especially in the course of phase transition due to defects migration and atomic rearrangement. The Raman spectra of the γ-In2S3 and β-In2S3 showed seven and sixteen resonance modes respectively, compatible with predicted transitions based on the corresponding crystal lattice symmetry. Overall, the preliminary results mentioned here revealed the phase transition process of γ-In2S3 to β-In2S3, and the formation of high quality and shape controlled 2D NPLs with unique optical properties emanated from the size confinement and morphology, like, direct band-gap, radiative exciton recombination and defects photoluminescence, with direct benefit for their for their integration in new and emerging solar cells, light sources and photodetector devices.