Optical and structural characterization of Copper Indium Sulphide Quantum Dots

Abstract

Copper indium sulfide (CIS) quantum dots (QDs) have been synthesized according to a simple heat up method starting from In(OAc) 3 and three different copper precursors: CuI, CuBr, or CuCl. The obtained nanoparticles (NPS) were characterized for their optical properties, the CIS QDs prepared using different copper(I) salts show different behaviors depending on the nature of the chosen precursor salt. The absorption spectra show a shift of the absorption edge towards longer wavelengths with increasing reaction time. This is in accordance with the quantum confinement effect: bigger nanoparticles display a wider band‐gap. The absorption spectra have been analyzed according to the Tauc interpretation in order to obtain information about the dimension of the nanosized semiconductors. The band gap energy (E g ) decreases with increasing reaction time. From the absorption analyses we observed a difference in the behavior of the nanoparticles synthesized in the presence of different anions. According to the Hofmeister interpretation, the three halogen ions possess different interacting properties with hydrophobic molecules. The charge distribution of the electronic cloud is rather diffuse in heavier halogen ions, the polarizability decreases according to the series I - >Br - >Cl - . The latter ion, having a high surface charge, interact weakly with hydrophobic surfaces. We tried to give an interpretation of the different behavior observed in the presence of different anions in the reaction environment. We can observe that when CuI is used the QDs obtained after 5 min of reaction have a smaller band gap than those obtained with CuBr or CuCl. Prolonging the reaction time the growth of the NPs continues and the absorption can be extended up above 800 nm. When copper salts with smaller counterions are used, this growth seems somehow inhibited. The same trend is confirmed by the photoluminescence analyses. The emission peak shifts upon increasing the reaction time: although not being excitonic in nature, the emission in this kind of QDs is known to be size‐related. The actual emission mechanism is not fully understood, but defect states (mainly copper vacancies) and bound band states have been demonstrated to play a role. For this reason, as the diameter of the quantum dots increases, the emission experiences a bathochromic shift accordingly. We also noted that the emission intensity is dependent upon the copper source used in the synthesis. The sample synthesized from CuI has better optical properties than the ones of the samples synthesized from CuBr or CuCl. The structural characterization has been done by TEM & XPS: for the analyses the NPs were washed using chloroform:methanol (1:1) to precipitate the QDs out from hexanes. From TEM observations, the NPs appear to have a good crystallinity after 2 hours of reaction. The morphology is polygonal, with a crystalline habitus typical of a tetragonal structure, supporting the attribution of the phase to the chalcopyrite polymorph. It is hard to determine the actual size of the NPs in each sample due to the rather irregular shape, from the FFT analysis the sample made starting from CuI shows smaller NPs. The EDX analysis returns the expected elements in the samples, with Cu, In, and S being the most abundant ones. Beside them, iodine and bromine lines can be observed in the spectra of the samples synthesized starting from CuI and CuBr, while no chlorine is detectable in the last sample. These results are in accordance with those coming from XPS analyses. The QDs are copper deficient, which is a characteristic beneficial from the point of view of the optical properties, since it has been demonstrated that the copper vacancies acts as defect levels involved in the electronic transitions responsible for the emission in this type of QDs. Sulfur vacancies are also supposed to give rise to intraband levels related to the emission mechanism. Our samples have a sulfur content slightly below the expected value. We observed the presence of halogen ions in the samples. It is also to be noted that the anion fraction in the sample increases in the order I − >Br − >Cl − . This trend is in accordance with the EDX analyses performed during the TEM observations. One of the mechanism affecting the properties of the QDs is the fact that the anions can be incorporated in the lattice, thus perturbing the chemical surrounding of the ions. The incorporation of higher amount of impurities should be also beneficial in terms of the optical quality of the NPs, according to the defect‐related nature of the emission of these QDs. The identification of the process through which the anions influence the synthesis and the properties of the NPs remains challenging. It is undoubtful that the use of different precursors leads to QDs with diverse properties, and that the trend follows that of the polarizability (Hofmeister series) of the copper counterions.