(Keynote) Band-Edge Emission from AgInS2/Ga2S3 Core/Shell Quantum Dots and Enhancement of Their Quantum Yield
Authors: Kuwabata Susumu , Uematsu Taro , Hoisang Watcharaporn , Torimoto Tsukasa 
Published: December 2020

Semiconductor nanoparticles, diameter of which is 10 nm or less possess fascinate optical properties due to quantum confinement, causing, for instance, an increase in the band gap energy with decrease in the particle size that is called the quantum size effect. In case of semiconductor nanoparticles that emit photoluminescence (PL), their particle size determines the PL color. In other words, multiply color PL agents are producible from one kind of semiconductor. Such the fascinate optical properties were found mainly in II-VI semiconductors and they have been extensively studied as quantum dots (QDs). In particular, CdSe nanoparticles that became the first commercial products as QDs and the narrow band-edge emission from that materials attracts intense attention because they are one of ideal light sources for fabricating brilliant color displays. In fact, liquid-crystal displays including QDs backlights were on sale but they were discontinued because CdSe and related QDs containing toxic elements faced a ban in many countries. It was, therefore, necessary to search for non-toxic alternatives possessing comparable optical properties.
In 2007, we have published a paper regarding the first luminescent I-III-VI ternary semiconductor QDs that are AgInS2 QDs. In addition, if ZnS and AgInS2 are alloyed, PL color is tunable by particle size as well as composition ratio of ZnS and AgInS2.[1] However, although the quantum yield (QY) of the obtained PL was as high as 80%,[2] the spectra was relatively broad with large width as shown by (1) in Fig 1, indicating that the PL was emitted from defect sites of the particles. Since it is quite natural to suppose that possibility of defect site generation in ternary semiconductor is much higher than that in binary semiconductor like CdSe, it was, then, thought to be difficult or impossible to eliminate the defect sites from the I-III-VI semiconductor QDs. Nevertheless, we made several attempts and significant achievements as presented in this paper were obtained.
Modification of QDs with a shell material having wide band-gap is typical way to eliminate defect sites. ZnS is a frequently utilized but this is not appropriate because above-mentioned alloying reaction occurs between ZnS and I-III-VI semiconductor. Similarly, other materials that are often used as shell were useless. Then, we explored other materials that have never been employed as a shell material for QDs. One of them was In2S3 because the phase diagram predicted that this semiconductor and AgInS2 did not melt in each other. Then, PL spectrum of the prepared AgInS2/In2S3 QDs showed not only broad PL but also narrow PL peak that was located at shorter wavelength than the former, as shown by (2) in Fig. 1. Furthermore, when Ga2S3 having larger bandgap energy than In2S3 was chosen as a shell material, as shown by (3) in Fig. 1, the broad PL was almost suppressed, whereas the narrow PL peak distinctly appeared. However, QY of the narrow PL was as low as 29%, implying that defect sites causing non-radiative recombination were still remained.
The first breakthrough to remove most of defect sites was found when tri-n-octylphosphine was added to the QDs solution, as shwon by (4) in Fig. 1, that exhibited 56.0% of QY. [3] In order to remove defect sites generated in the interior of the AgInS core, we are now changing the way to synthesize AgInS2 core. Change of S source from thiourea to dimethylthiourea was effective because the latter species had higher solubility in the solvent we used, enabling slow reaction with dropwise of the S source. The resulting AgInS2/Ga2S3 core/shell QDs gave only narrow PL with 73.2% of QY. [4]

References

[1] T. Torimoto, S. Kuwabata, et al., J. Am. Chem. Soc., 2007, 129, 12388-12389.
[2] T. Torimoto, S. Kuwabata, et al., Chem. Commun, 2010, 46, 2082-2084.
[3] T. Uematsu, S. Kuwabata, et al., NPG Asia Mater. 2018, 10, 713-726.
[4] W. Hoisang, T. Uematsu, S. Kuwabata, et al., Nanomaterials, 2019, 9, 1763.

Figure 1