Cadmium-free quantum dots (QDs) consisting of silver–indium–gallium–sulfide (AIGS) quaternary semiconductors were successfully synthesized using a metal–dithiocarbamate complex with sufficiently high reactivity to produce metal sulfides. The introduction of a gallium diethyldithiocarbamate precursor decreased the reaction temperature to produce active intermediates, which were subsequently converted into AIGS QDs at 150 °C with silver and indium acetates. Because of the low reaction temperature, AIGS QDs with a tetragonal crystal phase were produced selectively, which favorably generated band-edge emission whose full width at half-maximum is smaller than 40 nm after they were coated with gallium sulfide (GaS y ) shells. The compositional indium/gallium ratio was varied by changing the mixing ratio of the precursors used for the synthesis of the AIGS core, and the band-edge photoluminescence (PL) generated from the AIGS/GaS y core/shell QDs was blue-shifted with an increase in the gallium content in the core. Consequently, a pure green emission centered at 518 nm was obtained with a PL quantum yield as high as 68%.

Highly luminescent silver indium sulfide (AgInS 2 ) nanoparticles were synthesized by dropwise injection of a sulfur precursor solution into a cationic metal precursor solution. The two-step reaction including the formation of silver sulfide (Ag 2 S) nanoparticles as an intermediate and their conversion to AgInS 2 nanoparticles, occurred during the dropwise injection. The crystal structure of the AgInS 2 nanoparticles differed according to the temperature of the metal precursor solution. Specifically, the tetragonal crystal phase was obtained at 140 • C, and the orthorhombic crystal phase was obtained at 180 • C. Furthermore, when the AgInS 2 nanoparticles were coated with a gallium sulfide (GaS x ) shell, the nanoparticles with both crystal phases emitted a spectrally narrow luminescence, which originated from the band-edge transition of AgInS 2 . Tetragonal AgInS 2 exhibited narrower band-edge emission (full width at half maximum, FWHM = 32.2 nm) and higher photoluminescence (PL) quantum yield (QY) (49.2%) than those of the orthorhombic AgInS 2 nanoparticles (FWHM = 37.8 nm, QY = 33.3%). Additional surface passivation by alkylphosphine resulted in higher PL QY (72.3%) with a narrow spectral shape.in the visible region as well as large optical absorption coefficients, which are characteristic for direct semiconductors [9][10][11][12][13]. Among these QDs, silver indium sulfide (AgInS 2 ) nanoparticles (NPs) with a band gap energy of 1.87 eV have attracted increasing attention [14,15]. A universal challenge during the synthesis of AgInS 2 NPs is to balance the reactivity of two cationic precursors against one anionic precursor [10,16]. In the past, the thermal decomposition of a single molecular precursor was used to avoid the reactivity problem, which resulted in a high (~50%) PL QY for AgInS 2 -ZnS NPs [17][18][19]. The limitations of this method are the necessity of designing a molecular precursor for each composition and the difficulty of controlling the particle size and shape due to the complexity of the decomposition process [20]. Alternatively, the reactions of the cation mixture with sulfur-containing species at higher temperatures are common. A typical example uses silver nitrate (AgNO 3 ), indium acetate (In(OAc) 3 ), and 1-dodecanethiol (DDT) dissolved in 1-octadecene (ODE), which were heated until DDT reacted with metal cations to generate AgInS 2 NPs [21]. Although the "heating up" approach achieved better PL QY, it was still difficult to regulate the particle size because the growth process mainly occurred by Ostwald ripening [22,23]. At the same time, the method of particle size control by the rapid injection of precursors into a hot solvent, which has been commonly used for binary semiconductor QDs, was also adopted for the AgInS 2 NPs synthesis and achieved good size distribution with a PL QY as high as 59% [24][25][26]. Although the control over particle size and composition was achieved, none of these attempts generated a narrow band-edge emission corresponding to common II-VI semiconductor QD...

Ternary and quaternary semiconductor quantum dots (QDs) are candidates for cadmium-free alternatives. Among these, semiconductors containing elements from groups 11, 13, and 16 (i.e., I–III–VI2) are attracting increasing attention since...

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Liquid microextraction employing solidification of the floating organic droplet, with vortexing and heating to optimize extraction efficiency, was developed for the determination of seven insecticides in fruit juice, vegetables, and agricultural runoff water. The extracts were analyzed by gas chromatography with both flame ionization and mass spectrometry detection for the determination of chlorpyrifos, prothiofos, profenofos, ethion, λ-cyhalothrin, permethrin, and cypermethrin, respectively. Using 20 μL of 1-undecanol in 10 mL of aqueous solution containing 1% w/v sodium chloride provided preconcentration factor of 500. The enrichment factor of the analytes was in the range of 355 to 509 with extraction recovery >71%. The linearity ranges were 4-200 μg/kg for gas chromatography with flame ionization detection and 1-100 μg/kg for gas chromatography with mass spectrometry, with limits of detection ranging from 0.04 to 1.2 μg/kg, which are lower than the international maximum residue limits for vegetables and fruit juice. Intra-day and inter-day precisions are less than 5.4 and 7.0% relative standard deviation, respectively. The method was successfully applied to the determination of the seven insecticides in samples of vegetables, fruit juice and agricultural runoff, with recoveries ranging from 61.7 to 120.8%. The extraction method is simple, efficient and environmentally friendly.

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

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