Open questions on the transition between nanoscale and bulk properties of metals

Nanoscience has progressed tremendously in the exploration of new phenomena not seen in bulk materials, however, the transition between nanoscale and bulk properties is not yet fully understood. Here the authors identify and discuss remaining open questions that call for future efforts.

the core/ligand interface structure, and the exact surface composition; all these are critically important for understanding the electron transport, catalysis, and many other events 1 .
Motivated by the above fundamental issues, nanochemists further expanded their synthetic capabilities, and significant efforts over the past years have finally established the atomically precise nanochemistry, at least in the size regime of 1-3 nm, with hopes toward larger sizes in future efforts. The size-focusing synthetic methodology played a major role in creating a series of atomically precise NPs (often called nanoclusters for differentiating from conventional NPs) 1 . This methodology has been extended from gold to silver and alloy nanoclusters 1 .
At the bottom end of the nanoscale (Fig. 1, about 1-3 nm), quantum phenomena start to emerge, manifested in the transition from the electronic-band structure to discrete energy levels 4 that are akin to those in molecules (Fig. 1). Such a quantization fundamentally alters the material properties and leads to emergence of many new phenomena 1 , such as the energy gap (E g ), excitonic absorption of light, luminescence, unique catalytic activity, single-electron magnetism, and redox properties, to name a few. In addition to the electronic structure alteration, the atom packing structure also starts to exhibit significant changes. For instance, ultrasmall gold NPs are no longer exclusively fcc. Exotic structures of gold, such as body-centered cubic (bcc) and hexagonal close-packed (hcp) structures not present in bulk gold or larger NPs, have indeed been attained in ultrasmall sizes 1 . These exotic packing modes of gold atoms lead to extraordinary properties, such as the three-orders-of-magnitude variance in photoexcited electron lifetime 5 .
The success of atomically precise nanochemistry has also led to insight into a decades-long fundamental question, that is, at what size the electronic band structure of metals (i.e. metallic state) evolves to discrete energy levels (i.e., semiconducting or molecular-like) [6][7][8] . This question came up in the 1930s shortly after the establishment of quantum mechanics, and it has since stimulated tremendous work in the physics and chemistry fields, but a major challenge was the synthesis of atomically precise NPs. With the advent of atomically precise nanochemistry, a sharp transition from metallic-state Au 279 (SR) 84 (where, SR = thiolate ligands) to semiconducting Au 246 (SR) 80 has recently been mapped out ( Fig. 2) 8 , manifested in several aspects including the steady-state optical spectra 8 , femtosecond transient absorption 8 and phonon dynamics 9 , as well as cryogenic spectroscopic features 8 . Back in the 1960s, Kubo 10 raised a theoretical criterion for the metal-to-nonmetal transition, i.e., when E g = k B T (where, E g is the energy gap, k B the Boltzmann constant, and T the temperature). This criterion indicates a temperature dependence and a smooth evolution, but neither was found experimentally 8 . Thus, the experimental results came as a surprise, which calls for a revisit to the electronic structure modeling of large-sized nanoclusters (of order~10 2 metal atoms). It should be pointed out that Kubo's treatment did not include the electron-electron interactions 10 . Apparently, as the size grows, the density of electronic states increases (hence, smaller E g ), and the electron correlation (e.g., screening) also becomes stronger, which ultimately leads to the collapse of discrete states and hence the emergence of the collective electron-gas.
Open questions on the transition of electronic and optical properties of atomically precise metal NPs Despite the discovery of the Au 246 -to-Au 279 sharp transition, some open questions remain for future work. First of all, understanding the effect of shape on the transition is worth pursuing. Compared to the spherical cases of Au 246 and Au 279 , nanoclusters of non-spherical shapes (e.g., one-dimensional rods, or two-dimensional oblate NPs) are more difficult to obtain; thus, new synthetic strategies are to be developed. The electronic transition in nonspherical cases could be more complex 11 and remains to be investigated in future work. Second, it remains elusive to what extent the surface ligands affect the transition 1 . By changing thiolate ligands to other types, future work will reveal whether the surface ligands on nanoclusters possess any major effect on the transition. Third, to what extent does the detailed atom-packing structure (e.g., fcc, hcp, bcc and other types) influence the transition? The Au 246 has a decahedral structure 7 , whereas Au 279 is fcc 8 , thus, the potential effect of structure remains unclear. Last but not least, the phonon dynamics in transition-sized nanoclusters still remains elusive, which pertains to the electron-phonon coupling and the power dependence in probing the transition 6,9 . In particular, the scaling relationship of phonon frequency with the number of atoms in the nanocluster is not clear yet, and how this scaling evolves to the well-known phonon frequency~1/d law (where, d = diameter of NPs) in metallic/plasmonic NPs 3 . Future studies on the metal-tononmetal transition will promote fundamental understanding on the origin of metallic state and nascent plasmons 12 , and will also lead to the discovery of new properties of nanoclusters.
Open questions on the catalytic properties of atomically precise metal NPs Among the various applications of atomically precise metal NPs, catalysis constitutes a major topic 13 . Many open questions remain, such as the precise size effect at the atomic level, the true active-sites in catalytic processes, and the fundamental mechanistic steps 13 . Recent work has demonstrated the promise of atomically precise NPs in pursuing such aspects 14 . When comparing different nanocatalysts, often many factors come into play simultaneously, for example, the investigation on the size effect at the atomic-level involves different-sized NPs, but other than the size, the structures or the surface ligands would often become different as well, which complicates the analysis of the size effect. For future work, key efforts should focus on the creation of correlated nanocatalysts with only one factor changed while other factors are kept the same (e.g., a correlated pair of nanoclusters with the same core but different surfaces) 15 . Such correlational studies will be extremely important for studying the effects of size, structure, composition, ligand, interface, and other factors in a manner of one at a time, rather than multiple factors being entangled. In revealing the catalytic mechanisms, site-specific tailoring approaches hold great promise, for instance, a local surgery for replacing the surface motif 13 . Another approach is the single-atom-level tailoring in the core, that is, one atom at a time to observe the catalytic effect of heteroatom doping 14,15 . By combining experiment and theory, future work is expected to unravel the fundamental principles of synergy in atomically precise nanoalloys.

Outlook
Finally, while we focus on metal NPs in the above discussions, semiconductor and magnetic NPs also have some remaining fundamental issues. Atomic precision should be pursued in future research in order to solve the issues of ambiguous surface composition and interface structure of quantum dots and magnetic NPs. With atomically precise NPs, there will also be new opportunities in assembling NPs into coherent artificial solids, which will open up new opportunities in research.
Overall, as a new class of nanomaterials, atomically precise NPs are expected to impact the fundamental research of nanoscience, in particular, the structure-property relationships at the atomic level. Based upon the new knowledge acquired, atomically precise nanomaterials will find new opportunities in a wide range of applications. Looking into the future of nanoscience, atomically precise nanochemistry will open up many exciting opportunities.