Composition-Controlled Synthesis of Near-IR-Light-Emitting Ag-Ιn-Ga-Se Nanocrystals for Biological Imaging
Authors: Kameyama Tatsuya , Yamauchi Hiroki , Mizumaki Toshiki , Yukawa Hiroshi , Baba Yoshinobu , Kuwabata Susumu , Torimoto Tsukasa 
Published: July 2021

Colloidal semiconductor nanocrystals have been intensively developed for the application to photovoltaics, light-emitting diodes, electroluminescent device, and photoluminescent markers for bio-imaging. Recently, low-toxic I-III-VI semiconductor-based multinary chalcogenide nanocrystals have attracted much attention as a promising alternative for Cd-based ones, because these nanocrystals are composed of less-toxic elements and exhibited intense photoluminescence (PL) in the visible and near-IR wavelength regions. In our previous paper, we reported that chemically synthesized Zn-Ag-In-S nanocrystals exhibited a broad PL peak (FWHM > 100 nm) originating from donor-acceptor pair radiative recombination, the wavelength of which was tunable from ca. 500 to 800 nm by changing the particle size and the chemical composition.[1] However, these multinary nanocrystals did not exhibit a band-edge emission, though narrowing PL peak has been earnestly desired for luminescent applications. In this study, we report the preparation of novel multinary Ag-In-Ga-Se nanocrystals via solution phase synthesis and control the peak wavelength of band-edge emission in the range from visible to near-IR lights. We also demonstrated the usefulness of thus-obtained Ag-In-Ga-Se nanocrystals for in vivo bio-imaging.[2]

Semiconductor nanocrystals composed of Ag-In-Ga-Se (AIGSe) solid solution were prepared by the thermal decomposition of corresponding metal salts and selenourea in a hot mixture solution of oleylamine/1-dodecanethiol under an N2 atmosphere. The molar ratios of precursors were varied with the synthesis of Ag0.67InyGa(1-y)Se2 (AIGSe) in mind. The individual XRD peaks of thus-obtained AIGSe nanocrystals were located between those of tetragonal AgInSe2 and AgGaSe2 crystal structures and shifted to a higher angle with a decrease in the y value, indicating that the Ga3+ fraction in the Ag(In,Ga)Se2 solid solution increased with a decrease in the y value in preparation. TEM measurements revealed that spherical nanocrystals were formed, the average size of which were varied in the range of 3~5 nm depending on the chemical composition. The PL spectra of as-prepared AIGSe nanocrystal exhibited both a narrow band-edge emission peak and a broad defect-site emission peak. By surface-coating AIGSe nanocrystals with GaSx (AIGSe@GaS
x
), a broad PL peak was remarkably diminished especially for y = 0.25-0.5, owing to the removal of the surface defect sites. With a decrease in the y value, that is, with an increase in the Ga/In ratio of nanocrystals, the peak wavelength of band-edge emission of AIGSe@GaS
x
nanocrystals was blue-shifted from 890 to 610 nm because of the increase in their energy gap. The PL quantum yield (QY) was enlarged by the GaS
x
shell coating, in which the maximum PL QY of 38% was obtained for the PL peak at 800 nm AIGSe@GaS
x
of y = 0.7. Thus-obtained nanocrystals were successfully used as near-IR PL probes for three-dimensional in vivo bioimaging, in which the wavelengths of excitation and detection lights could be selected in the first biological window. The imaging signals were clearly detected from AIGSe@GaS
x
nanocrystals injected into biological tissues by ca. 5 mm in depth.

Reference

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