Narrow-Band Photoluminescence from Cadmium-Free I-III-VI Ternary Semiconductor Quantum Dots By Surface Modification
Authors: Uematsu Taro , Wajima Kazutaka , Hoisang Watcharaporn , Sharma Dharmendar Kumar , Hirata Shuzo , Yamamoto Takahisa , Kameyama Tatsuya , Vacha Martin , Torimoto Tsukasa , Kuwabata Susumu 
Published: December 2020

Semiconductor nanoparticles (quantum dots, QDs) are photoluminescent (PL) materials represented by cadmium selenide (CdSe), and they have recently been applied to the color conversion materials used in display devices due to prominent monochromaticity of their PL. Unfortunately, the use of cadmium compounds is no longer allowed for commercial products and a search for alternative materials is being continued. Silver indium sulfide (AgInS2) was one of the candidates of cadmium-free materials categorized as I-III-VI semiconductor, having similar structure to cadmium chalcogenide semiconductors. Our group was one of the first to propose this material and succeeded in generating PL more than ten years ago, but the broad PL spectrum of AgInS2-based QDs deriving from defect levels in the bandgap have remained as a largest problem.
Two years ago, we succeeded in obtaining a narrow shoulder peak at shorter wavelength side of the broad defect emission of the AgInS2 QDs while attempting various surface modification.1 The material that could passivate the AgInS2 QDs was indium sulfide (InSx), which is one of the III-VI semiconductors having two kinds of elements in common with the core. The still-remained broad defect emission was significantly decreased by changing the shell material from InSx to gallium sulfide (GaSx) that is same group of material having wider bandgap (E
g, bulk = 3.0 eV) than InSx (E
g, bulk = 2.4 eV) (Fig. 1). One of the important findings that we have done in a series of studies was that most of the defect emission of the I-III-VI semiconductor QDs was surface-derived, and not of lattice defects unlike many researchers have considered. In addition, STEM observation of the core/shell QDs revealed an interesting finding: the GaSx shell was amorphous while the core shows a good crystallinity. That is, the shell of the photoluminescent QDs should not necessarily be crystalline.
In fact, GaSx, a known defective material, have never been used as a shell material for QDs since a good passivating material have long believed to possess wide bandgap and be free of defect levels. For example, zinc sulfide (ZnS) that meets these criteria is frequently used for passivating CdSe QDs, but there has been no report to obtain the band-edge emission from AgInS2 QDs by passivating them with ZnS. However, it cannot be denied that the amorphous shell has its own fragility than conventional crystalline materials. In fact, the addition of tri-n-octylphosphine to the as-prepared AgInS2/GaSx core/shell QD solution doubled the PL quantum yield of the band-edge emission, and it has reached as high as 70%. This result indicated the poor quantum confinement of the GaSx shell although it is the only method to obtain band-edge emission.
The nature of AgInS2 core should also be important. Actually, cores of ~4 nm in diameter has tetragonal crystal structure and are prone to show higher band-edge emission when they are coated with GaSx shells than larger cores having orthorhombic crystal structure.2 We consider that such difference in the degree of passivation derives from the arrangement of surface atoms on a specific facet, leading to the difference in binding energy between AgInS2 and GaSx. We are attempting to reveal the criteria of good passivation as well as a search for third material to have band-edge emission from I-III-VI ternary semiconductor QDs.

Uematsu, T.; Wajima, K.; Sharma, D. K.; Hirata, S.; Yamamoto, T.; Kameyama, T.; Vacha, M.; Torimoto, T.; Kuwabata, S., Narrow band-edge photoluminescence from AgInS2 semiconductor nanoparticles by the formation of amorphous III–VI semiconductor shells. NPG Asia Mater.
2018, 10, 713-726.

Hoisang, W.; Uematsu, T.; Yamamoto, T.; Torimoto, T.; Kuwabata, S., Core Nanoparticle Engineering for Narrower and More Intense Band-Edge Emission from AgInS2/GaSx Core/Shell Quantum Dots. Nanomaterials
2019, 9, 1763.

Figure 1