Article pubs.acs.org/JPCC
Optical Properties and Structural Relationships of the Silver Nanoclusters Ag32(SG)19 and Ag15(SG)11 Sung Hei Yau,† Brian A. Ashenfelter,‡ Anil Desireddy,‡ Adam P. Ashwell,§ Oleg Varnavski,† George C. Schatz,§ Terry P. Bigioni,‡ and Theodore Goodson, III*,† †
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States Department of Chemistry, University of Toledo, Toledo, Ohio 43606, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡
S Supporting Information *
ABSTRACT: The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag32 and Ag15 (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag32, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag32(SG)19 and Ag15(SG)11 have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metal−ligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of KEET ≥ 9.7 × 109 s−1. A “dark state” competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag15(SG)11, with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag32 recorded a possible metal core state at 530 nm, a metal−ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag32 are 100 nm blue-shifted in comparison to Au25. The transient spectrum for Ag15 shows a weak ground state bleach at ∼480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metal− ligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.
I. INTRODUCTION Metal nanoclusters are stable “closed-shell” small metal systems with unique properties that are fundamentally different from well-known colloidal metal nanoparticles.1−12 Recent developments of condensed (solution) phase metal clusters such as monolayer-protected clusters (MPCs) or metal nanobeacons have accelerated the field of metal cluster research.4,5,13−23 The increased availability of solution-phase stable metal clusters allows for detailed photophysical investigations of these systems.2,11,12,24−29 Some of the nanoclusters can be crystallized and studied, which led to detailed calculations of their electronic structure.9,14,21,23,30−32 Metal nanoclusters provide © 2016 American Chemical Society
exciting new ideas in fundamental nanomaterial research such as quantum size effects. New applications for metal nanoclusters are also being developed such as catalysis, medical imaging, electron transfers, and optical limiting.2,29,33−39 Research on metal nanoclusters has primarily been centered on Au clusters due to their high stability, but the recent developments of stable Ag metal nanoclusters have garnered significant attention.20,23,40−47 Received: October 16, 2016 Revised: December 11, 2016 Published: December 12, 2016 1349
DOI: 10.1021/acs.jpcc.6b10434 J. Phys. Chem. C 2017, 121, 1349−1361
Article
The Journal of Physical Chemistry C
product was cleaned by precipitation with methanol before being separated using polyacrylamide gel electrophoresis (PAGE).22 The reaction product was separated into a series of bands, each of which contained a different cluster size. Clusters in the sixth and second bands (as indexed from the bottom) were extracted from the gel and then filtered through a 0.22 μm syringe filter and concentrated with a 3 kDa cutoff filter before being dried.22 The samples were stored in powder form in a refrigerator and redissolved in water before measurements were made. Their optical densities were adjusted according to the needs of each experiment.37 II.B. Steady State Absorption and Emission. Steady state absorption and emission measurements were performed at room temperature, with a 10 mm thick quartz sample cell. The Ag nanoclusters were kept in powder form under refrigeration at −20 °C. The samples were dissolved in high purity water before the experiments to protect against degradation. Optical absorption measurements were carried out using the Agilent 8432 UV−vis absorption spectrometer. Emission was measured using a Fluoromax-2 fluorimeter. The QYs of the molecules were measured using crystal violet as the standard. The QY is calculated based on a series of solutions at different concentrations. The slopes of the concentration vs fluorescence of the standard and the sample were used to calculate the QY.20,29 To ensure sample aggregation or degradation did not affect the samples, UV−vis spectra were compared before and after each experiment. II.C. Femtosecond Transient Absorption Measurements. Transient absorption was used to investigate the excited state dynamics of the Ag nanoclusters; the system has been described previously.2,24,26,35,37 Briefly, the output of a Spectra Physics Spitfire amplified beam was split with a beam splitter to generate pump and probe beam pulses (85% and 15%). The pump beam was produced by an optical parametric amplifier (OPA-800c). The pump beam used in the present investigation (400−450 nm) was obtained from the fourth harmonic of the signal beam and was focused onto the sample cuvette. The probe beam was delayed with a computer controlled motion controller and focused into a 2 mm sapphire plate to generate a white light continuum. The white light was then overlapped with the pump beam in a 2 mm quartz cuvette containing the sample. The change in absorbance of the signal was collected by a CCD detector (Ocean optics). Data acquisition was controlled by the software from Ultrafast Systems Inc. The typical power of the probe beam was
