Precision at the nanoscale: on the structure and property evolution of gold nanoclusters

Atomic precision in the nanoparticle systems is a prerequisite for precise understanding and controlling of the nanoworld. In the past ten years, the Jin group pioneered the work of syntheses, structure determinations, and property characterizations of thiolate-protected gold nanoclusters [denoted as Aun(SR)m], which have served as aparadigm system in atomically precise nanomaterials. My contributions to this field include the following major aspects: (i) identifying the important roles of surface ligandsin determining the sizes and structures of gold nanoclusters; (ii) establishing a precise nanoscale transformation methodology and significantly increasing the diversity of goldnanoclusters; (iii) pushing the limit of nanoparticle crystallization from the ultrasmall Au20(SR)16 to the hitherto largest Au133(SR)52; (iv) applying the concept of nanoscaleprecision to various fields and providing important insights into critical issues in nanoscience, quantum physics, surface science and cluster science with unprecedentedatomic resolution; and (v) unraveling the deep beauties in nanoparticle systems. The first chapter provides a general introduction to the major challenges in current nanoscience associated with the imperfection of nanomaterials, as well as broad impacts of nanoscale precision to different fields. Chapter 2 lays the foundation for the major synthetic work described in the entire thesis. A “ligand-exchange induced size/structure transformation” (abbreviated as LEIST) reaction is established. The LEIST process is aprecise nanoscale reaction, which not only permits the detailed study on the reaction pathway, but also leads to the rapid and rational expansion of the “size library” of Aun(SR)m nanoclusters. The total structure determination of Aun(SR)m nanoclusters by X-ray crystallography provides atomic insights into the packing structure of metal atoms (i.e. phases) and the binding structure of organic ligands (i.e. surfaces). Chapter 3 discusses the identification of the first face-centered cubic (FCC) phase in gold nanoclusters. The emergence of the FCC core in a Au36(SR)24 implies the important contribution of surface ligands to the total energy minimization of nanoparticles. Chapter 4 focuses on theevolution of the protecting motifs on the surface of gold nanoclusters. As the size of nanocluster decreases to ultrasmall Au20(SR)16, the ubiquitous surface-protecting staple motifs [Aux(SR)x+1] close up, evolving into a ring motif [i.e. Au8(SR)8], which is consistent with the macrocycle structures observed in Au-SR complexes. Therefore, the Au20((SR)16 represents a structural transition point from complexes to nanoclusters. The expansion of the structure library of Aun(SR)m nanoclusters also enables thedeciphering of some mysterious issues as well as the discovery of novel phenomena at the nanoscale. Chapter 5 tackles a fundamental question in cluster science — the origin of “magic sizes”. Based on the Kekule-like superstructure identified in the Au40(SR)24 and the DNA-like superstructure in Au52(SR)32, a “supermolecular” model is proposed to explain the stability in gold nanoclusters. Compared to the prevalent “superatom” model, the supermolecule model encompasses more magic sizes, just as the fact that an unlimited number of molecules can be assembled from a limited number of atoms (~90 naturalelements in the Periodic Table). Chapter 6 reports an intriguing phenomenon, i.e. the existence of periodicities in magic-sized nanoclusters. The first periodic series with aunified formula of Au8n+4(SR)4n+8 (n = 3, 4, 5, 6) is identified in gold nanoclusters. The periodicity in the formula is correlated with the uniform anisotropic layer-by-layer growth along the [100] direction of the FCC lattice, which leads to a series of size-varied “gold quantum boxes”. Their optical properties are further explained by the threedimensional“particle in a box” model. In Chapter 7, Aun(SR)m nanoclusters are applied to solve the structural enigma of self-assembled monolayers (SAMs). The translationalsymmetry exhibited in both kernel and facets of a tetragonal-shaped Au92(SR)44 allows the deduction of the total structures of bulk Au(100) SAMs as well as a series of larger gold nanoparticles. Finally, the hitherto largest crystal structure of gold nanocluster containing 133 gold atoms and 52 thiolate ligands is unraveled in Chapter 8. This giantstructure exhibits a kaleidoscope of structural patterns reminiscent of structures at different length scales, with the icosahedral kernel resembling the viral capsid, the helicalstripe pattern of staple motifs at the Au-S interface mimicking the helical structure in DNA, and the swirl pattern of carbon tails analogous to the galaxies. The quantum-sized, thiolate-protected gold nanoclusters with atomic precision is a highly interdisciplinary field, which embraces the ideas and sheds light onto many fundamental questions in quantum physics, surface science, cluster science, andnanoscience. This thesis work has illustrated the great promises of the nanoscale precision, which would eventually lead to the rational design and precise engineering ofcomplex architectures at the nanoscale.