Sonochemically Synthesized 3D Assemblies of Close-Packed In2S3 Quantum Dots: Structure, Size Dependent Optical and Electrical Properties

Template-free conventional chemical and sonochemical approaches to 3D assemblies of indium(III) sulfide quantum dots were developed that allow deposition of strongly quantized cubic alpha-In2S3 nanocrystals close packed in thin film form. Our observation of metastable cubic structure at room temperature (instead of the thermodynamically most stable tetragonal beta modification in the case of bulk material) was related to the very small crystal size. Because of heterogeneous sonochemical effects, the average crystal radius of the QD solids reduces from 2.5 to 2.0 nm upon sonification of the reaction system by continuous high-intensity ultrasound. Upon postdeposition annealing treatment, these values increase to 4.1 nm. Structural, optical and electrical properties of the synthesized QD solids were studied in details. The band gap energy value of 2.85 eV for the as-deposited QD solids in thin film form is strongly blue-shifted (by 0.85 eV) with respect to the value characteristic for a macrocrystalline specimen. In the case of as-deposited films by sonochemical approach, band gap value is 3.00 eV, indicating the possibility for further control of the optoelectronic properties of this material by sonochemical approach. Upon postdeposition thermal treatment at 150 and 200 C, band gap energy red shifts to 2.20 and 2.00 eV were observed. Analysis of the size-quantization effects in the synthesized QD solids deposited in thin film form enabled us to estimate that the Bohr's excitonic radius in the studied semiconductor lies in the range from 2.5 to 4.1 nm. The absence of clearly defined excitonic peaks in the absorption spectra of the studied QD assemblies was attributed to the size-distribution of the nanoparticles and to the interdot electronic coupling effects. Analysis of the charge carrier transport properties in the QD assemblies within the Kazmerski's model indicated that the intercrystalline barrier height decreases by 0.04 eV upon thermal treatment of the films. Conductivity activation energy was found to be 0.82 eV, while the thermal band gap energy, calculated from the thermoelectrical measurements in the region where intrinsic conductivity mechanism is activated, was 2.22 eV. AFM measurements have shown that QD assemblies constituting the sonochemically deposited films show stronger tendency toward coagulation than those synthesized by conventional chemical approach.