Nanometals: Identifying the Onset of Metallic Relaxation Dynamics in Monolayer-Protected Gold Clusters Using Femtosecond Spectroscopy Chongyue Yi, †,§ Hongjun Zheng, †,§ Laura M. Tvedte, ‡ Christopher J. Ackerson, ‡ and Kenneth L. Knappenberger, Jr.* ,† † Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States ‡ Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States ABSTRACT: Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au 96 ( mMBA) 42 , Au 102 ( p MBA) 44 , Au 115 ( p MBA) 49 , Au 117 ( mMBA) 50 , Au 144 (pMBA) 60 , Au 250 (pMBA) 98 , and Au 459 (pMBA) 170 (pMBA = para- mercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au 144 (pMBA) 60 (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron-phonon coupling constant of (1.63 ± 0.25) × 10 16 W m -3 K -1 . Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum- confined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatial separation of photoexcited electrons and holes. Evidence for the latter was obtained by substituting mMBA for pMBA as a passivating ligand, which resulted in a 4-fold increase in the relaxation rate constant. ■ INTRODUCTION Photonic nanomaterials offer great potential for light-driven applications including photocatalysis, solar-to-electric energy conversion, medical therapeutics, sensing, and optical image contrast. 1 These unique opportunities arise because materials confined to nanoscale dimensions often display strikingly different chemical, physical, and optical properties than their bulk counterparts. For metal nanoparticles (≥∼ 2 nm), size- and structure-dependent properties, such as the localized surface plasmon resonance (LSPR), can be used to amplify light-matter interactions, resulting in increased optical signals and nanoparticle-mediated photocatalysis. 2-11 In contrast to metallic nanoparticles, quantum-confined metal nanoclusters (≤∼2 nm) exhibit structured absorption spectra and micro- second near-infrared photoluminescence. 12-18 These optical properties emerge for clusters because the manifold of electronic states near the Fermi level is nondegenerate, with energy gaps exceeding kT. 19 As a result of the large energy mismatch between typical phonon frequencies and the electronic energy gaps of quantum-confined metals, the electronic relaxation dynamics of small nanoclusters differ from the electron-phonon scattering models that accurately describe bulk and nanoparticle metallic systems. 20 For Au 25 (SC 8 H 9 ) 18 and Au 38 (SC 12 H 25 ) 24 , electronic relaxation is determined by the strength of the coupling between specific electronic states and the vibrational modes of protecting ligands. 21 Despite significant advances in describing the electronic structure and steady-state extinction spectra of ultrasmall metal nanoclusters and larger metallic nanoparticles, the inherent structural heterogeneity of this class of nanoma- terials has made it difficult to determine the size at which the transition from nonmetallic to metallic electronic behavior occurs. Monolayer-protected nanoclusters (MPCs) are a class of nanomaterials that can be synthesized and isolated with atomic precision, making them useful models for determining the size- and structure-dependent properties of metal particles in the 1- 2 nm domain. 22-25 Here, we provide the first comprehensive report on electronic relaxation dynamics for a range of gold MPCs, Au 96 (mMBA) 42 , Au 102 (pMBA) 44 , Au 115 (pMBA) 49 , Au 117 ( mMBA) 50 , Au 144 ( p MBA) 60 , Au 250 ( pMBA) 98 , and Au 459 (pMBA) 170 , where pMBA and mMBA are para- and meta-mercaptobenzoic acid, respectively. The sizes of the inorganic diameters of these MPCs range from 1.5 to 2.4 nm, where the transition from nonmetallic to metallic behavior is expected to occur. 26-32 Received: December 4, 2014 Revised: February 14, 2015 Published: February 24, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 6307 DOI: 10.1021/jp512112z J. Phys. Chem. C 2015, 119, 6307-6313