On the transition from nonmetallic to metallic state in metal nanoclusters

Metal nanoclusters (NCs) in the 1-3 nm size regime have been intensively pursued in recent years. With the ever-improving synthetic methodologies (i.e. size-focusing and ligand exchangeinduced size/structure transformation (LEIST)), monodispersed NCs of molecular purity are effectively synthesized and some of them are crystallized successfully. The total structures (including the metal core and the surface ligands) of nanoclusters can be unambiguously characterized by X-ray crystallography. Compared with monometallic NCs, their alloy counterparts often exhibit enhanced properties such as the largely improved catalytic activity and selectivity, photoluminescence, and other functionalities. While much work has been done on the smaller sizes of alloy NCs (less than 100 metal atoms), larger-size NCs (that is, >100 metal atoms), including alloy NCs, are still rare owing to major difficulties in the synthesis and crystallization. The major difficulties in large-size alloy NCs hamper the pursuit of clear answers to some fundamental questions, such as the dopant site patterns and the effects on functionality, the transition from non-metallic to metallic state in alloy NCs. My thesis work focuses on the controlled synthesis and characterization of bimetallic Au_144-xAg_x(SR)₆₀ NCs and the size series of monometal NC of Au₁₀₀, Au₁₀₂, Au₁₃₀, Au₁₄₄, Au₂₇₉ and Au₃₃₃ that span the non-metallic to metallic state. Overall, atomically precise nanoclusters will serve as a well-defined platform for fundamental studies. 

Chapter 1 of the thesis provides a general introduction to gold nanoclusters and the grand picture of NCs that are known up to now. The major criteria and the challenges in nonmetallic to metallic transition are also introduced. 

Chapter 2 focuses on the nascent plasmons and metallic bonding in atomically precise NCs including Au₂₇₉ and Au₃₃₃ protected by thiolates. The metallic bond is arguably the most intriguing one among the three types of chemical bonds, and the resultant plasmon excitation (e.g. in gold nanoparticles) has garnered wide interest since the 1990s. Recent progress in nanochemistry has led to successes in obtaining atomically precise nanoclusters (NCs) of hundreds of atoms per core. In this work, thiolate-protected Au₂₇₉(SR)₈₄ and Au₃₃₃(SR)₇₉ NCs, both in the nascent metallic state are investigated by cryogenic optical spectroscopy down to 2.5 K. At room temperature, both NCs exhibit distinct plasmon resonances, albeit the NCs possess a diminishing gap (estimated 0.02–0.03 eV, comparable to thermal energy at room temperature (r.t.)). Interestingly, we observe no thermal effect on plasmons with the expected transition from the metallic state at r.t. to the insulating state at cryogenic temperatures (down to 2.5 K), indicating a nonthermal origin for electron-gas formation. The electronic screening-induced birth of metallic state/ bonding is further discussed. The obtained insights offer a deeper understanding of the nascent metallic state and covalent-to-metallic bonding evolution, as well as the plasmon birth from concerted excitonic transitions. 

Chapter 3 illustrates the Ag alloying influences on the nonmetallic to metallic transition. Alloying is an important strategy in tailoring the functionality of materials. The introduction of a heterometal leads to alloy nanoclusters that often outperform the homometal ones in terms of the physical and chemical properties. In this work, a series of four M₁₄₄(PET)₆₀ alloy NCs (where, M = Au/Ag) are synthesized and characterized. The silver doping into the homogold template (Au144) leads to more prominent optical absorption features in the steady-state spectrum in the visible range. Femtosecond transient absorption spectroscopy reveals the effect of Ag doping on the electronic relaxation dynamics compared to Au₁₄₄ and the pump fluence-independent dynamics. Electrochemical results reflect a narrowing of HOMO-LUMO gap (E_g) induced by Ag doping. A temperature dependence of the single-electron charging is also observed for the series of alloy NCs, in which the Eg values of the alloy NCs enlarge as the temperature decreases, which is characteristic of semiconducting behavior. 

Chapter 4 and 5 present four new NCs: the 58 e Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄; and the 80 e Au₁₃₀(Napt)₅₀ and Au₁₃₀₍PET)₅₀. They provide a platform for the almost atom-by-atom evolution of NCs in the large size region. The HOMO–LUMO energy gaps of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ are comparable to other 58 e counterparts, e.g. Au₁₀₂(p-MBA)₄₄ and Au₁₀₃S₂(S-Nap)₄₁, indicating the decisive role of the metal kernel in the ground state electronic structure. The reported excited-state lifetimes of all the three organic phase NCs, Au₁₀₀(Napt)₄₂, Au₁₀₂(IPBT)₄₄ and Au₁₀₃S₂(S-Nap)₄₁, are almost 10 times shorter than the lifetime for the aqueous phase NC Au₁₀₂(pMBA)₄₄ (>3 ns). According to the similarity in steady-state absorption and the molecular formula, both Au₁₃₀(Napt)₅₀ and Au₁₃₀(PET)₅₀ nanoclusters are considered as having the same decahedral kernel (Au₁₀₅) as that of Au₁₃₀(pMBT)₅₀ that was reported previously. The electronic relaxation dynamics was also analyzed, and by comparison with the reported Au₁₃₀(pMBT)₅₀ and water-phase Au₁₃₀(pMBA)₅₀, a 100/200 times longer lifetime was observed for the water-phase Au₁₃₀(pMBA)₅₀. The results are consistent with the results obtained from the comparison between water-soluble Au₁₀₂(pMBA)₄₄ cluster and organic phase 58 e Au_100/102/103 NCs, which further indicates the kernel structure plays a more decisive role in the stead-state absorption while the surface influences the photoexcited electron relaxation dynamics for large size gold nanoclusters.