Sonochemically Synthesized 3D Assemblies of Close-Packed In₂S₃Quantum Dots: Structure, Size Dependent Optical and Electrical Properties

Abstract

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 α-In 2 S 3 nanocrystals close packed in thin film form. Our observation of metastable cubic structure at room temperature (instead of the thermodynamically most stable tetragonal β 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.

Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

“…In the case of bulk cadmium selenide, the thermodynamically most stable modification at ambient temperature and pressure is the hexagonal (wurtzite) polymorph of this compound. The situation, however, that we encounter in the present case, is similar to those that we have reported before in the case of other compound semiconductor systems [59][60][61]. Namely, as a consequence of the ultrasmall dimensions of nanocrystals constituting the 3D array in the form of compact thin film or bulk precipitate, it is the cubic form that is actually being observed under ambient conditions.…”

“…We have further proceeded with careful analysis of the semiconductor absorption function of the form (28) to determine the band gap energy of the films deposited under various conditions, as well as of the thermally annealed films. We did this by a combined interpolation-extrapolation procedure, described in details in our previous publications [59][60][61]73]. It is necessary to implement such algorithm as the parabolic approximation for the dispersion relation is, strictly speaking, valid only in the neighborhood of the borders of the first Brillouin zone.…”

In2S3 Quantum Dots: Preparation, Properties and Optoelectronic Application

“…4f. It is evident that the V bi and W d are the same as similar QDs devices (such as the graphene quantum dots) [30], but the N is larger by an order of magnitude at zero bias. This explains the excellent performances of the device as compared to other QDs device [31].…”

Facile solvothermal synthesis of 3D flowerlike β-In2S3 microspheres and their photocatalytic activity performance

“…17,18 At atmospheric pressure, In 2 S 3 is found to crystallize into three different structural forms, defective cubic structure (a-In 2 S 3 ), defective spinel structure (b-In 2 S 3 ), and layered hexagonal structure (g-In 2 S 3 ). [19][20][21][22][23][24][25][26][27][28][29][30] b-In 2 S 3 is the stable state at room temperature with a tetragonal structure or cubic form, which is an n-type semiconductor with a suitable band gap of 1.9-2.3 eV corresponding to visible light region. 31 It is well-known that the size and morphology of nanomaterials have an important inuence on their properties, such as the movement of electrons and holes and the transportation related to phonons.…”