Optical Properties of Atomically Precise Gold Nanoclusters: Transition from Excitons to Plasmons

Tatsuya Higaki and Rongchao Jin

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA This email address is being protected from spam bots, you need Javascript enabled to view it

The optical properties constitute a major topic of metal nanoparticle research. Gold nanoparticles (NPs) are particularly attractive owing to their elegant colors that are tunable with particle size and shape. While the size-dependent optical properties of regular gold NPs (e.g., 5-100 nm) have been extensively studied, ultrasmall nanoparticles remain largely under-explored due to major difficulties in the synthesis of high-quality particles. Significantly, recent progress toward atomically precise nanochemistry has opened exciting avenues for exploring new optical properties of ultrasmall particles at the unprecedented atomic level (i.e., how adding or removing one atom changes the optical properties). This chapter illustrates the optical absorption of some typical sizes of atomically precise

Atomically Precise Nanoclusters

Edited by Yan Zhu and Rongchao Jin

Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4877-08-4 (Hardcover), 978-1-003-11990-6 (eBook) www.jennystanford.com nanoparticles (often called nanoclusters in order to differentiate them from the conventional nanoparticles without atomic precision). The grand exciton-to-plasmon transition has been discovered to occur between Au₂₄6 and AU279.

Introduction

Since the advent of nanotechnology in 2000, gold NPs have captured significant attention due to their diverse applications. This surge in new applications has led to a major effort to understand the fundamental underlying properties, in particular the optical properties. Conventional gold NPs are well known to be metallic, that is, thousands to millions of free electrons (depending on the particle size) roaming freely inside the particle. When light is used to excite such NPs, a strong resonant absorption band is observed, with Ap_eak being at 520-570 nm depending on the size of spherical NPs (Fig. 6.1) [1]. This excitation mode is called surface plasmon resonance (SPR) and it is unique to metallic nanoparticles. The nature of SPR is due to a collective excitation of free electrons in the particle upon absorbing light.

With decreasing size, the SPR band blue shifts and also becomes weaker (Fig. 6.1). Indeed, below ~3 nm, SPR fades out and new optical absorption features emerge, for example, multiple bands (as opposed to single SPR). This is due to the quantum size effect [2]. Naturally, many fundamental questions arise, such as (1) What are the origins of those multibands? (2) Does the evolution from metallic to nonmetallic states occur smoothly or abruptly? (3) If abruptly, at what precise size (i.e., the number of gold atoms in the particle) does the transition occur? Kubo indeed predicted in the 1960s a smooth progression of the electronic structure of ultrasmall metal particles with size and the HOMO-LUMO (highest occupied and lowest unoccupied molecular orbitals) gap should follow the scaling law £_g ~ 1/n, where n is the number of gold atoms [3]. However, experimental synthesis of ultrasmall particles has long remained a major challenge due to difficulties in controlling the particle monodispersity. For a size-dispersed sample, it was apparently impossible to investigate the precise scaling relationship. Thanks to the recently developed atomically precise nanochemistry [1], it has now become possible to tackle the above-mentioned fundamental questions, albeit half a century has passed since Kubo's theoretical prediction.

Figure 6.1 (A) Reflectance spectrum of bulk gold, (B) extinction spectra of

gold colloids (3-100 nm diameters), and (C) surface plasmon excitation (e.g., dipole mode) in metal nanoparticles [1].

Optical Properties of Small-Sized Gold Nanoclusters

Au25(SR)18 Nanoclusters

Among the reported sizes of atomically precise nanoclusters, Au₂₅(SR)₁₈ (SR = thiolate) is perhaps the most studied one, due in part to its early discovery. The success in size-focusing synthesis of Au₂₅(SR)₁₈ in high yield and molecular purity [4] finally led to crystallization and structure determination [5].

The Au₂₅(SR)₁₈ structure (Fig. 6.2A) is based on a centered icosahedral Au₁₃ core, which is capped by an exterior shell composed of the remaining 12 Au atoms, and the entire cluster is encapsulated by 18 thiolate ligands. The 12 surface gold atoms and 18 ligands are assembled into six dimeric staples (-SR-Au-SR-Au-SR-), which protect the Au₁₃ core in D_2h symmetry.

The availability of the crystal structure of Au₂₅(SR)₁₈ permitted a structure-property correlation by performing DFT calculations; thus, the origins of multibands were revealed (Fig. 6.2B-D). Due to quantum confinement caused by the ultrasmall size (1 nm) of Au₂₅(SR)₁₈, the electronic structure of Au₂₅(SR)₁₈ is significantly quantized (Fig. 6.2C). This is in striking contrast to the continuous band structure of plasmonic Au nanoparticles or bulk gold. The first optical transition (at 680 nm, Fig. 6.2B) corresponds to a

LUMCk— HOMO transition, which is essentially an intraband (sp<—sp) transition (note that the term sp band is inherited from solid state theory for the convenience of discussing the nonmetallic to metallic state evolution; one should bear in mind that the sp band in clusters is significantly quantized). Interestingly, the HOMO set (nearly threefold degenerate, above the dense d-band, see Fig. 6.2B) has essentially s character; thus, transitions arising out of the other occupied HOMO-n orbitals (belonging to d-band) are interband transitions (sp<— d). The HOMO and LUMO orbitals are comprised almost exclusively of atomic orbital contributions from 13 Au atoms in the icosahedral Au₁₃ core, rather than 12 exterior Au atoms. Thus, the first 680 nm peak in the absorption spectrum can be viewed as a transition that is due entirely to the electronic and geometric structure of the Au₁₃ core.

Figure 6.2 (A) X-ray structure of Au₂₅(SR);f₈(counterion: tetraoctylammonium, TOA⁺). Color labels: magenta = Au, yellow = S, and gray = C; all hydrogen atoms are omitted for clarity. (B) Experimental optical absorption spectrum. (C) Kohn- Sham orbital level diagram for Au₂5(SR)₁₈. (D) Theoretically simulated optical absorption spectrum [5].

Single-Atom Effect on Optical Properties

We chose the Au₂4(TBBM)₂ocase (TBBM = SCH₂Ph-p-^tBu) to illustrate the single-atom effect [6]. Although its size is only one atom less than Au₂₅(PET)₁₈ (PET = SCH₂CH₂Ph), drastic effects were observed in terms of structure and optical properties. The Au₂₄ structure exhibits a prolate shape (Fig. 6.3), as opposed to the quasi-spherical shape of Au₂₅. The core of Au₂₄ is a Au₈ bi-tetrahedron, in which the two tetrahedra are anti-prismatically joined together through two triangular faces of the tetrahedra, giving rise to a prolate structure [6]. The Au₈ kernel is protected by four tetrameric staples [i.e., Au₄(SR)s],

Figure 6.3 Total structure of [Аи₂₄(5СН₂Р11-^гВи)₂₀]⁰. (Color labels: magenta = Au, yellow = S, and gray = C; all H atoms are omitted for clarity) [6].

The optical absorption spectrum of Au₂₄ exhibits a distinct peak at ~500 nm and less prominent ones at shorter wavelengths (Fig. 6.4A) [6]. The number of peaks is less than that of Au₂₅ due to lower symmetry of Au₂₄. DFT calculations reproduced the optical spectrum and thus explained the origins of the electronic transitions involved (Fig. 6.4B) [6]. The experimental peak at ~500 nm is mainly contributed by the HOMO-LUMO electronic transition, but also in less part by the HOMO-2 to LUMO transition. Figure 6.4C,D illustrates the schematic diagrams of HOMO and LUMO orbitals, which are quite different from the superatomic orbitals (IP⁶ and 1D°) in Au₂₅(SR)₁₈". The HOMO-LUMO transition (at 500 nm) of Au₂₄(SR)₂₀ is also significantly blue-shifted compared to that of Au₂₅(SR)₁₈ (at 680 nm). This is due to the smaller core (Au₈ vs Au₁₃). Generally, for small sizes of nanoclusters, the HOMO-LUMO gap energies do not scale with size in a monotonic trend, but rather a zigzag behavior was observed, which is largely caused by the diverse structures of small nanoclusters [7].

Figure 6.4 (A) Experimental UV-Vis absorption spectrum of Au₂4(SR)₂o, (B) theoretical absorption spectrum, (C) schematic diagrams of HOMO, and (D) LUMO of the cluster [6].