Prof. ZHOU Meng’s team from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), collaborating with Prof. WANG Quanming’s team from Tsinghua University, achieved near-unity room-temperature photoluminescence quantum yield (PLQY) (>99%) in the near-infrared (NIR) emission of metal nanoclusters in solution. The study was published in Science. 

Gold nanoclusters (Au NCs) as NIR-emissive materials hold potential in biomedical applications. However, the PLQY of Au NCs in NIR region is typically low, often less than 10%. To address this problem, the researchers synthesized Au₂₂(^tBuPhC≡C)₁₈(Au₂₂) and its copper-doped counterpart, Au₁₆Cu₆(^tBuPhC≡C)₁₈ (Au₁₆Cu₆), and studied their photophysical properties.  

Single-crystal x-ray diffraction analysis revealed that Au₂₂ and Au₁₆Cu₆ share similar structures. Au₂₂ showed an emission peak at 690 nm and Au₁₆Cu₆ at 720 nm. The absolute PLQY of Au₂₂ and Au₁₆Cu₆ in air were 9% and 95%, respectively. In deaerated solution, the PLQY of Au₁₆Cu₆ reached 100% measured by both absolute and relative methods. Time-correlated single-photon counting measured the photoluminescence lifetimes of Au₂₂ and Au₁₆Cu₆ to be 485 ns and 1.64 μs, respectively. 

Further investigation of NCs’ excited-state dynamics through transient-absorption spectroscopy revealed that both NCs' luminescent states originated from the triplet state (T₁), with distinct dynamic processes observed in femtosecond transient-absorption spectroscopy. Under 380 nm excitation, Au₂₂ showed a slow rise of 148 ps, while Au₁₆Cu₆ showed a rapid relaxation of 0.5 ps.  

Triplet sensitization experiments confirmed that these processes are attributed to ultrafast intersystem crossing (ISC) from singlet state (S₁) to T₁. Due to copper doping, Au₁₆Cu₆ has smaller ΔE_st, significantly accelerating its ISC rate. As a result, Au₁₆Cu₆ ultimately showcases PLQY close to 100%. 

The approach to achieve near-unity PLQY could enable the development of highly emissive metal cluster materials. This work demonstrates that near-unity PLQY can be attained with an alloy of gold-copper nanoclusters even in solution at room temperature, which enables applications ranging from biological imaging to luminescent device.