Wavelength-Tunable Band-Edge Photoluminescence of Nonstoichiometric Ag-In-S Nanoparticles via Ga3+ Doping.

The nonstoichiometry of I-III-VI semiconductor nanoparticles, especially the ratio of group I to group III elements, has been utilized to control their physicochemical properties. We report the solution-phase synthesis of nonstoichiometric Ag-In-S and Ag-In-Ga-S nanoparticles and results of the investigation of their photoluminescence (PL) properties in relation to their chemical compositions. While stoichiometric AgInS2 nanoparticles simply exhibited only a broad PL band originating from defect sites in the particles, a narrow band edge PL peak newly appeared with a decrease in the Ag fraction in the nonstoichiometric Ag-In-S nanoparticles. The relative PL intensity of this band edge emission with respect to the defect-site emission was optimal at a Ag/(Ag + In) value of ca. 0.4. The peak wavelength of the band edge emission was tunable from 610 to 500 nm by increased doping with Ga3+ into Ag-In-S nanoparticles due to an increase of the energy gap. Furthermore, surface coating of Ga3+-doped Ag-In-S nanoparticles, that is, Ag-In-Ga-S nanoparticles, with a GaS x shell drastically and selectively suppressed the broad defect-site PL peak and, at the same time, led to an increase in the PL quantum yield (QY) of the band edge emission peak. The optimal PL QY was 28% for Ag-In-Ga-S@GaS x core-shell particles, with green band-edge emission at 530 nm and a full width at half-maximum of 181 meV (41 nm). The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic-element-free I-III-VI-based nanoparticles in a wide area of applications.

Influence of Zn on the photoluminescence of colloidal (AgIn)xZn2(1-x)S2 nanocrystals.

We study the effect of Zn on the photophysical properties of a family of group I-III-VI nanocrystals (NCs), namely in solid solutions of (AgIn)xZn2(1-x)S2 (ZAIS). We focus on the comparison of the photoluminescence (PL) properties of ZAIS NCs of comparable sizes and different amounts of Zn. This approach helps us to decouple the effects of size and varying chemical composition of the NCs which both influence the PL properties. We show that in the presence of Zn new radiative centers are generated which improve the NC quality in terms of PL quantum yield. However, an amount of Zn beyond a particular limit places the radiative recombination centers close to each other, leading to undesired interactions among charge carriers and non-radiative transitions. Proximity between the energy levels of these radiative centers and the conduction band leads to non-radiative localized-delocalized transitions, as evidenced from temperature dependent absorption, PL and lifetime measurements.

Single-particle spectroscopy of I-III-VI semiconductor nanocrystals: spectral diffusion and suppression of blinking by two-color excitation.

Ternary I-III-VI semiconductor nanocrystals have been explored as non-toxic alternatives to II-VI semiconductors for optoelectronic and sensing applications, but large photoluminescence spectral width and moderate brightness restrict their practical use. Here, using single-particle photoluminescence spectroscopy on nanocrystals of (AgIn)xZn2(1-x)S2 we show that the photoluminescence band is inhomogeneously broadened and that size distribution is the dominant factor in the broadening. The residual homogeneous linewidth of individual nanocrystals reaches up to 75% of the ensemble spectral width. Single nanocrystals undergo spectral diffusion which also contributes to the inhomogeneous band. Excitation with two lasers with energies above and below the bandgap reveals coexistence of two emitting donor states within one particle. Spectral diffusion in such particles is due to temporal activation and deactivation of one such state. Filling of a trap state with a lower-energy laser enables optical modulation of photoluminescence intermittency (blinking) and leads to an almost two-fold increase in brightness.