Synthesis and structural characterization of Ag2ZnSnS4 crystals

Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by "green" synthesis modified by seed nanocrystals or extra cations in the solution

The method of affordable colloidal synthesis of nanocrystalline Cu₂ZnSnS₄ (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these "seed" NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Machine learning and density functional theory simulation of the electronic structural properties for novel quaternary semiconductors

In order to accelerate the application of quaternary optoelectronic materials in the field of luminescence, it is crucial to develop new quaternary semiconductor materials with excellent properties. However, faced with vast alternative quaternary semiconductors, traditional trial-and-error methods tend to be laborious and inefficient. Here, we combined machine learning (ML) with density functional theory (DFT) calculation to predict the bandgaps of 2180 quaternary semiconductors, most of which were undeveloped but environmentally friendly. The evaluation coefficient (R²) of the model using a random forest algorithm was up to 0.93 in ML. Four novel quaternary semiconductors with direct bandgaps: Ag₂InGaS₄, AgZn₂InS₄, Ag₂ZnSnS₄, and AgZn₂GaS₄, were selected from the ML model. Then their electronic structures and optical properties were further verified and studied by DFT calculations, which demonstrated that the four quaternary semiconductors had direct bandgaps, a small effective mass, and a large exciton binding energy and Stokes shift. Our calculation could significantly speed up the discovery of novel optoelectronic semiconductors and has a certain reference value for the study of luminescent materials and devices.

Stepwise Li Substitution Induced Structure Evolution and Improved Nonlinear Optical Performance for Diamond-like Sulfides

One of the key scientific issues for exploration of novel infrared nonlinear optical (NLO) crystals is to obtain ones with good hybrid NLO behaviors. Herein, we report four diamond-like Ag-based sulfides via stepwise Li substitution of Ag in Ag₂ZnSnS₄, including Ag₂ZnSnS₄ (1), (Li_1.22Ag_0.78)ZnSnS₄ (2), (Li_1.58Ag_0.42)ZnSnS₄ (3), and Li₂ZnSnS₄ (4). With the increase of Li content, the sulfide's noncentrosymmetric crystal structure changes from tetragonal I42m for 1, to orthorhombic Pmn2₁ for 2 and 3, and to monoclinic Pn for 4. Accordingly, their NLO responses are improved along with the increase of Li content, viz. from non-NLO-active for 1, to non-phase-matchable for 2 and 3, and to phase-matchable for 4. Their optical band gaps also increase regularly. The relationship between their chemical compositions, crystal structures, and NLO activities is investigated by means of chemical structural analysis and theoretical calculation. This work offers a new systematic case for designing promising NLO-active compounds via rarely adopted cation's stepwise partial substitution and understanding the chemical composition-structure-NLO property relationship.

Raman and X-ray Photoelectron Spectroscopic Study of Aqueous Thiol-Capped Ag-Zn-Sn-S Nanocrystals

The synthesis of (Cu,Ag)-Zn-Sn-S (CAZTS) and Ag-Zn-Sn-S (AZTS) nanocrystals (NCs) by means of "green" chemistry in aqueous solution and their detailed characterization by Raman spectroscopy and several complementary techniques are reported. Through a systematic variation of the nominal composition and quantification of the constituent elements in CAZTS and AZTS NCs by X-ray photoemission spectroscopy (XPS), we identified the vibrational Raman and IR fingerprints of both the main AZTS phase and secondary phases of Ag-Zn-S and Ag-Sn-S compounds. The formation of the secondary phases of Ag-S and Ag-Zn-S cannot be avoided entirely for this type of synthesis. The Ag-Zn-S phase, having its bandgap in near infrared range, is the reason for the non-monotonous dependence of the absorption edge of CAZTS NCs on the Ag content, with a trend to redshift even below the bandgaps of bulk AZTS and CZTS. The work function, electron affinity, and ionization potential of the AZTS NCs are derived using photoelectron spectroscopy measurements.