Narrow band-edge photoluminescence from AgInS2 semiconductor nanoparticles by the formation of amorphous III–VI semiconductor shells

Nanoparticles of I–III–VI semiconductors are promising candidates for novel non-toxic fluorescent materials. However, removal of defect levels responsible for their broad-band emission has not been successful to date. The present study demonstrates, for the first time, the coating of core AgInS2 nanoparticles—one of the I–III–VI group semiconductors with a bandgap in the visible region—with III–VI group semiconductors. The AgInS2/InSx and AgInS2/GaSx (x = 0.8–1.5) core/shell structures generate intense narrow-band photoluminescence originating from a band-edge transition at a wavelength shorter than that of the original defect emission. Microscopic analyses reveal that the GaSx shell has an amorphous nature, which is unexpected for typical shell materials such as crystalline lattice-matching ZnS. Single-particle spectroscopy shows that the average linewidth of the band-edge photoluminescence is as small as 80.0 meV (or 24 nm), which is comparable with that of industry-standard II–VI semiconductor quantum dots. In terms of photoluminescence quantum yield, a value of 56% with nearly single-band emission has been achieved as a result of several modifications to the reaction conditions and post-treatment to the core/shell nanoparticles. This work indicates the increasing potential of AgInS2 nanoparticles for use as practical cadmium-free quantum dots. A method for controlling the optical properties of tiny semiconductor particles and making them useful for medical, optical, and display application has been demonstrated by scientists in Japan. Crystals of semiconducting material with nanometer-scale dimensions absorb and emit light across a very narrow range of wavelengths. This makes them potentially useful for bioimaging and other optical applications. Nanoparticles of silver indium sulfide (AgInS2) is one such non-toxic fluorescent material, but their intrinsic spectral responses are broadened. Taro Uematsu, Osaka University, and colleagues show that much narrower optical emission can be achieved by coating AgInS2 nanoparticles with gallium sulfide or indium sulfide. The sharpness of the the resulting spectal response was comparable to that of commercially available cadmium selenide or cadmium telluride nanoparticles, but had the advantage of avoiding the use of toxic cadmium. Silver indium sulfide (AgInS2) semiconductor nanoparticles are cadmium-free quantum dots emitting in visible to near infrared regions. The present study demonstrates the narrowing of their broad defect emission by coating AgInS2 core nanoparticles with amorphous indium sulfide or gallium sulfide shells. The new emission from the core/shell nanoparticles originating from the band-edge transition is substantially narrower (FWHM of 28.6 nm) than the defect emission of core nanoparticles (FWHM of 220 nm). The photoluminescence quantum yield is increased to 56% after giving several modifications to the synthetic procedures so that we can see the vibrant yellow emission under room light.