Electronic Transitions in Highly Symmetric Au₁₃₀ Nanoclusters by Spectroelectrochemistry and Ultrafast Spectroscopy

Abstract

Rich and discrete energy states in gold nanoclusters enable different combinations of electronic transitions and correspondingly electrochemical and optical properties for a variety of applications. The impacts on those electronic transitions by the emergence and magnitude/ alignment of a band gap and by the contributions from different atomic/molecular orbitals require further study. Au nanoclusters with 130 core Au atoms are of interest in this report because they are at the transition size regime where a small yet well-defined band gap can be resolved along with continuous quantized frontier core orbitals. Here, electrochemical analysis is combined with UV−vis−near infrared optical measurements to unveil previously unresolved electronic transitions. Finite changes in the steady-state optical absorption spectrum are captured by spectroelectrochemistry when the Au nanoclusters are charged to different states via electrolysis. Multiple previously unresolved peaks and valleys as well as isosbestic "points/regions" are observed in the differential spectrum. The detailed spectral features are explained by the respective electronic transitions to those affected energy states. Key features are also well correlated with ultrafast absorption analysis which provides additional insights, such as the lifetime of the corresponding transitions. The experimentally measured energy states and transitions could serve as references for future theoretical study to learn the respective contributions from different atomic orbitals and, importantly, to explore routes to enhance or suppress certain transition so as to modulate the corresponding electrochemical and optical properties for better applications.

Au130−xAgx Nanoclusters with Non‐Metallicity: A Drum of Silver‐Rich Sites Enclosed in a Marks‐Decahedral Cage of Gold‐Rich Sites

“…The slower 15.9 ps process is assigned to the relaxation from the lowest excited‐state to the ground state. Previous ultrafast spectroscopic studies revealed the long components of thiolate‐protected NCs with about 100 metal atoms to be 420 ps for Au 103 S 2 (SR) 41 , 180 ps for Au 130 (SR) 50 , 30 ps for Au 133 (SR) 52 , and 325 ps for Ag 146 Br 2 (SR) 80 , which are all significantly longer than 15.9 ps observed in Au 130− x Ag x . Considering the observed charge transfer in XAFS analysis, the shorter excited‐state lifetime of Au 130− x Ag x could originate from its bimetallic nature.…”

“…Thef aster 1.5 ps process is assigned to internal conversion since the excitation (480 nm, 2.58 eV) is far above the band gap (< 1eV, exact E g not determined). Thes lower 15.9 ps process is assigned to the relaxation from the lowest excited-state to the ground state.P revious ultrafast spectroscopic studies revealed the long components of thiolateprotected NCs with about 100 metal atoms to be 420 ps for Au 103 S 2 (SR) 41 , [16] 180 ps for Au 130 (SR) 50 , [25] 30 ps for Au 133 -(SR) 52 , [26] and 325 ps for Ag 146 Br 2 (SR) 80 , [27] which are all significantly longer than 15.9 ps observed in Au 130Àx Ag x . Considering the observed charge transfer in XAFS analysis, the shorter excited-state lifetime of Au 130Àx Ag x could originate from its bimetallic nature.Further spectroscopic studies on bimetallic NCs are expected to reveal the origin of this faster relaxation than homometal NCs,s uch as the effect of structure,l igands,o rA u/Ag ratios.S uch efforts will be essential in exploring the transition from non-metallic to metallic state in alloy NCs.…”

The Critical Number of Gold Atoms for a Metallic State Nanocluster: Resolving a Decades-Long Question

“…Both differential pulse voltammetry (DPV) and square wave voltammograms (SWV) have been used to investigate the electronic structures of Au NCs. ,− By subtracting the charging energy (potential spacing between successive oxidation or QDL peaks) from the electrochemical gap, one can estimate the HOMO–LUMO gap of a Au NC. In 2017, Kwak et al investigated the electrochemical properties of six Au NCs (Au 25 , Au 38 , Au 67 , Au 102 , Au 144 , and Au 333 ) using the SWV method .…”

“…For those very large metallic Au NPs (>3 nm), there is no peaks in SWV or DPV, and thus E g → 0. For those sizes in the transition regime with QDL features (∼150–300 gold atoms), one will observe almost evenly spaced peaks, and thus, it is difficult to determine if they are in the metallic state or not, and additional characterization methods are needed.…”

Controlling magnetism of Au133(TBBT)52 nanoclusters at single electron level and implication for nonmetal to metal transition

“…The similarity between the absorption spectra of [Au 133 (TBBT) 52 ] 0 and [Au 133 (TBBT) 52 ] + may be ascribed to the much less perturbation of one electron loss in Au 133 (1 out 81e) than Au 25 (1 out of 8e) as well as possibly less structural distortion in Au 133 than Au 25 when switching the charge state. 46,56,57 We also performed electrochemical oxidation of [Au 133 (-TBBT) 52 ] 0 by applying positive voltages and monitored the optical spectra in situ, 68,69 but no change in optical spectra was found, even aer applying a 1 V bias for 90 min (Fig. S1 †).…”