Near-infrared (NIR)-light-emitting semiconductor nanocrystals (NCs) are regarded as promising biological markers for in vivo imaging due to their chemical stability and tunable emission properties depending on their size. To date, several high-quality NIR-emitting binary semiconductor NCs, such as CdTe, PbS and PbSe, have been developed, but they often contained highly toxic heavy metals such as Cd or Pb, limiting the range of their usage. Recently, we have successfully prepared highly luminescent and low toxic Ag-III-VI₂-based multinary semiconductor NCs of ZnS-AgInS₂ solid solution by using colloidal synthetic method, where these NCs have tunable photoluminescence (PL) (maximum quantum yield of 80 %) in visible light wavelength region.^[1,2]

   Since an Ag-III-VI₂ semiconductor of AgInTe₂ was reported to have a bulk band gap of approximately 1.0 eV, size-controlled AgInTe₂ NCs show promise as NIR-light-emitting markers. However, with the exception of the recent preparation of spherical, approximately 11-nm AgInTe₂ NCs,^[3] no attempts have been made to fabricate highly photoluminescent AgInTe₂ NCs with sizes less than several tens of nanometers. Here, we report rod-shaped AgInTe₂ NCs with controlled widths. The obtained NCs exhibit intense PL and the peak wavelength in the NIR region is tunable by changing the rod width.

   AgInTe₂ NCs were synthesized by a thermal reaction of silver and indium acetates and trioctylphosphine telluride in 1-dodecanethiol at 180-300°C for 10 min under N₂ atmosphere. Thus-obtained NCs were isolated by the addition of ethanol and then dissolved in octane to measure their optical properties.

   In XRD patterns, the NCs prepared at higher than 280°C exhibited three broad peaks corresponding to tetragonal AgInTe₂. On the other hand, different diffraction patterns were observed for particles prepared at less than 250°C, which was not consistent with any patterns reported for AgInTe₂ or other conceivable crystal phases such as AgIn₈Te₅, Ag₂Te, and In₂Te₃. We simulated the diffraction patterns of hexagonal crystal structure for bulk AgInTe₂ which was in good agreement with XRD pattern of NCs obtained at 180°C. To the best of our knowledge, this is the first report of the formation of hexagonal AgInTe₂ including bulk materials. TEM observations revealed that rod-shaped NCs were formed regardless of reaction temperature. The average width of the NCs increased monotonically from 5.5 to 8.3 nm with increasing reaction temperature from 180°C to 300°C, accompanied by an increase in the average length from 11.4 to 13.5 nm.

   The solutions containing AgInTe₂ NCs were deep-black in color due to strong light absorption in the visible and NIR wavelength regions. The NCs exhibited near-band-edge PL peak located near the corresponding absorption onset, and the peak wavelength was red-shifted with increasing reaction temperature as shown in Figure 1. The PL QY was enhanced with a decrease in the width of the rod-shaped AgInTe₂ NCs, in which the maximum PL QY of 18% was obtained for the hexagonal phase AgInTe₂ NCs prepared at 180°C. Although this value was approximately half of the PL QYs observed for conventional NIR-emissive semiconductor nanocrystals of PbS or PbSe, it was comparable to that of indocyanine green, which has practical uses as a dye for biological imaging. Hence, the relatively intense NIR emission of the AgInTe₂ NCs demonstrates their potential use as an NIR marker for biological imaging.

Reference

[1] T. Torimoto, et al., J. Am. Chem. Soc., 2007, 129, 12388.

[2] T. Kameyama, T. Torimoto, et al., J. Phys. Chem. C, 2015, 119, 24740.

[3] M. Langevin et al., Nanoscale Res. Lett., 2015, 10, 255.