Ultrasmall Au nanoparticles (often called nanoclusters) are being intensively studied for both fundamentals and applications because of their unique non-metallic state and related properties. Recent progress in controlling nanoclusters with atomic precision provide new opportunities for investigating many fundamental questions, such as (1) the transition from non-metallic to metallic state, and (2) strategies to control the non-metallic properties. With decreasing size, the disappearance of plasmonic state occurs around ~2 nm in diameter for gold, and unique non-metallic properties emerge with intriguing geometric structures. The critical size of the transition is a central question, as it provides insights into the origin of plasmons as well as the formation of metallic bond, but this question has long remained elusive because of the extreme difficulty in preparing atomically precise NPs. In this presentation, I will discuss (1) at what size and how the transition occurs at atomic level, and (2) how to tailor the properties at atomic level in the ultrasmall size region of Au nanoclusters. To answer the first question, I will discuss the synthesis and optical properties of an atomically precise Au279(SR)84. Femtosecond spectroscopic analysis revealed that the Au279 shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au246(SR)80. Steady-state absorption spectra revealed that the nascent plasmon band of Au279 at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. To answer the second question, I will talk about a Au30(SR)18 with an exotic crystalline phase: hexagonal close-packed (hcp). The Au30 is compared with the body-centered cubic (bcc) Au38 and the face-centered cubic (fcc) Au36 in terms of optical and electronic properties. Further exploration of nanoclusters smaller than Au246 is expected by crystalline phase approach for applications in optoelectronic, catalysis, and biomedicine.

Sharp transition from metallic to non-metallic state in gold nanoclusters with atomically tailored optical properties

2019 Fall National Meeting

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