Discovery of NIR-II nanocrystals Ag3AuSe2 with near-unity quantum yield

The development of second near-infrared (NIR-II) emitting nanocrystals (NCs) with unity photoluminescence quantum yield (PLQY) is paramount for deep-tissue bioimaging and high-efficiency optoelectronics; however, the PLQY decreases markedly as the emission shifts to longer wavelengths. Herein, we have discovered a novel Ag3AuSe2 (fischesserite) NC system that extends this PLQY-wavelength frontier into the long-wavelength regime, achieving a near-unity absolute PLQY of 98.9% at 1000 nm with a monoexponential decay time of 4.1 µs. The synthesis of the near-single-crystal Ag3AuSe2 NCs hinges on the employment of an Au(I)-octanethiolate coordination polymer, [Au(OTT)]n, as the Au precursor. This precursor enables the regulated release of active gold species and provides a mild redox environment, which are essential for improving crystal quality and suppressing interfacial defects otherwise induced by side reactions. The exceptional brightness of these NCs provides a new paradigm for ultra-high signal-to-noise ratio imaging, fundamentally reshaping the application landscape for NIR-II emitters.