Magnetic Dipolar Interactions in Solid Gold Nanosphere Dimers

We report the first observation of a magnetic dipolar contribution to the nonlinear optical (NLO) response of colloidal metal nanostructures. Second-order NLO responses from several individual solid gold nanosphere (SGN) dimers, which we prepared by a bottom-up approach, were examined using polarization-resolved second harmonic generation (SHG) spectroscopy at the single-particle level. Unambiguous circular dichroism in the SH signal was observed for most of the dimeric colloids, indicating that the plasmon field located within the interparticle gap was chiral. Detailed analysis of the polarization line shapes of the SH intensities obtained by continuous polarization variation suggested that the effect resulted from strong magnetic-dipole contributions to the nanostructure’s optical properties

Optical Properties and Electronic Energy Relaxation of Metallic Au₁₄₄(SR)₆₀ Nanoclusters

Electronic energy relaxation of Au₁₄₄(SR)₆₀^q ligand-protected nanoclusters, where SR = SC₆H₁₃ and <i>q</i> = −1, 0, +1, and +2, was examined using femtosecond time-resolved transient absorption spectroscopy. The observed differential transient spectra contained three distinct components: (1) transient bleaches at 525 and 600 nm, (2) broad visible excited-state absorption (ESA), and (3) stimulated emission (SE) at 670 nm. The bleach recovery kinetics depended upon the excitation pulse energy and were thus attributed to electron–phonon coupling typical of metallic nanostructures. The prominent bleach at 525 nm was assigned to a core-localized plasmon resonance (CLPR). ESA decay kinetics were oxidation-state dependent and could be described using a metal-sphere charging model. The dynamics, emission energy, and intensity of the SE peak exhibited dielectric-dependent responses indicative of Superatom charge transfer states. On the basis of these data, the Au₁₄₄(SR)₆₀ system is the smallest-known nanocluster to exhibit quantifiable electron dynamics and optical properties characteristic of metals

Three-Dimensional Interfacial Structure Determination of Hollow Gold Nanosphere Aggregates

The boundary regions between hollow gold nanospheres (HGNs) comprising an extended aggregate were examined using 3-D electron tomography. The images obtained from these experiments allowed for precise determination of the 3-D arrangement of the HGNs within the aggregate and revealed structural heterogeneities that were not resolvable with traditional two-dimensional techniques. These features included particle necking, point contacts, lattice pinholes, and HGN cavities that were joined by pores. The theoretical influence of these interfacial substructures on nanoscale plasmon properties was assessed using finite difference time domain (FDTD) numerical simulations. These results demonstrated the prospective impact of 3-D imaging techniques on the development of complete-structure descriptions of nanoscale optical properties

Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of Au₂₅(SR)₁₈^– Monolayer-Protected Clusters

Electronic relaxation dynamics of Au₂₅(PET)₁₈^–1 and Au₂₅(PET*)₁₈^–1 monolayer-protected clusters (MPCs) were examined using femtosecond time-resolved transient absorption spectroscopy (fsTA). The use of two different excitation wavelengths (400 and 800 nm) allowed for quantification of state-resolved and ligand-dependent carrier dynamics for gold MPCs. Specifically, one-photon 400 nm (3.1 eV) and two-photon 800 nm (1.55 eV) interband excitations promoted electrons from the MPC ligand band into gold superatom d states. Following rapid internal conversion, carriers generated by interband excitation exhibited picosecond relaxation dynamics that depended upon both ligand structure and the dielectric of the dispersing medium. These solvent- and ligand-dependent effects were attributed to charge-transfer processes mediated by the manifold of ligand-based states. In contrast, one-photon intraband (gold sp–sp) excitation by 800 nm light resulted in solvent- and ligand-independent relaxation dynamics. The observed solvent independences of these data were attributed to internal relaxation via superatom p and d states localized to the MPC core. Effectively, these core-based transitions were screened from dielectric influences of the dispersing medium by the MPC gold–thiolate protecting units. Additionally, a low frequency (2.4 THz) modulation of TA signal amplitude was detected following intraband excitation. The 2.4 THz mode was consistent with Au–Au expansion in the MPC core. Based on these data, we conclude that intraband relaxation among the MPC Superatom states is mediated by low-frequency vibrations of the gold core. Structure-specific and state-resolved descriptions of MPC electron dynamics are necessary for integration of metal clusters as functional components in photonic materials

Plasmon Dephasing in Gold Nanorods Studied Using Single-Nanoparticle Interferometric Nonlinear Optical Microscopy

We report the polarization-dependent and time-resolved photoluminescence (PL) properties of gold nanorods (AuNRs). AuNRs corresponding to three different length-to-diameter aspect ratios (AR)1.86, 2.91, and 3.90were examined using single-nanorod spectroscopy and imaging; the nanorod volume was approximately constant over the three sample types. For each AuNR, an aspect ratio-independent transverse plasmon resonance (TSPR) was detected at 2.41 eV. Aspect-ratio-dependent longitudinal surface plasmon resonances (LSPRs) were observed at 2.08 ± 0.19 eV, 1.76 ± 0.12 eV, and 1.53 ± 0.15 eV for the 1.86-AR, 2.91-AR, and 3.90-AR samples, respectively. On the basis of both excitation and emission polarization-resolved two-photon photoluminescence (TPPL) measurements, AuNR PL emission proceeded by plasmon-mediated radiative electron–hole recombination. The resonant LSPR mode frequencies of the nanorods were determined from interferometrically detected TPPL signals. For these measurements, the interpulse time delays of a spectrally broad laser pulse (1.48–1.65 eV) were changed systematically with attosecond time resolution, and the TPPL signal amplitude was recorded. The 1.86-AR AuNR did not support a plasmon mode that was resonant within the laser bandwidth, whereas the 2.91-AR and 3.90-AR samples had LSPR frequencies that overlapped the high- and low-energy components of the excitation pulse. The LSPR frequencies were obtained by Fourier transformation of the time-domain TPPL data and compared to dark-field scattering spectra. The accuracy of the interferometric TPPL measurement for recovering plasmon resonance frequencies was confirmed by polarization-dependent measurements; alignment of the laser electric field parallel to the nanorod major axis was LSPR resonant, whereas projection of the laser pulse into an orthogonal plane was not. Finally, dephasing times (<i>T</i>₂) for resonant plasmon modes were extracted from analysis of interferometric TPPL and second harmonic generation data. These results showed that the dephasing time increased from 22 ± 4 to 31 ± 9 fs as the LSPR resonance energy decreased from 1.76 to 1.53 eV, as a result of less efficient plasmon dephasing due to interband scattering for lower energy resonances. These results demonstrate the capability of interferometric nonlinear optical imaging with single-nanostructure sensitivity for determining structure-specific dephasing times, which influence the efficiency of metal nanoparticle light-harvesting applications. Therefore, interferometric nonlinear optical (NLO) imaging is likely to make a significant impact on the rational design of photonic nanostructures

On the pH-Dependent Quenching of Quantum Dot Photoluminescence by Redox Active Dopamine

We investigated the charge transfer interactions between luminescent quantum dots (QDs) and redox active dopamine. For this, we used pH-insensitive ZnS-overcoated CdSe QDs rendered water-compatible using poly (ethylene glycol)-appended dihydrolipoic acid (DHLA-PEG), where a fraction of the ligands was amine-terminated to allow for controlled coupling of dopamine–isothiocyanate onto the nanocrystal. Using this sample configuration, we probed the effects of changing the density of dopamine and the buffer pH on the fluorescence properties of these conjugates. Using steady-state and time-resolved fluorescence, we measured a pronounced pH-dependent photoluminescence (PL) quenching for all QD-dopamine assemblies. Several parameters affect the PL loss. First, the quenching efficiency strongly depends on the number of dopamines per QD-conjugate. Second, the quenching efficiency is substantially increased in alkaline buffers. Third, this pH-dependent PL loss can be completely eliminated when oxygen-depleted buffers are used, indicating that oxygen plays a crucial role in the redox activity of dopamine. We attribute these findings to charge transfer interactions between QDs and mainly two forms of dopamine: the reduced catechol and oxidized quinone. As the pH of the dispersions is changed from acidic to basic, oxygen-catalyzed transformation progressively reduces the dopamine potential for oxidation and shifts the equilibrium toward increased concentration of quinones. Thus, in a conjugate, a QD can simultaneously interact with quinones (electron acceptors) and catechols (electron donors), producing pH-dependent PL quenching combined with shortening of the exciton lifetime. This also alters the recombination kinetics of the electron and hole of photoexcited QDs. Transient absorption measurements that probed intraband transitions supported those findings where a simultaneous pronounced change in the electron and hole relaxation rates was measured when the pH was changed from acidic to alkaline

Dynamic Diglyme-Mediated Self-Assembly of Gold Nanoclusters

We report the assembly of gold nanoclusters by the nonthiolate ligand diglyme into discrete and dynamic assemblies. To understand this surprising phenomenon, the assembly of Au₂₀(SC₂H₄Ph)₁₅-diglyme into Au₂₀(SC₂H₄Ph)₁₅-diglyme-Au₂₀(SC₂H₄Ph)₁₅ is explored in detail. The assembly is examined by high-angle annular dark field scanning transmission electron microscopy, size exclusion chromatography, mass spectrometry, IR spectroscopy, and calorimetry. We establish a dissociation constant for dimer to monomer conversion of 20.4 μM. Theoretical models validated by transient absorption spectroscopy predict a low-spin monomer and a high-spin dimer, with assembly enabled through weak diglyme oxygen–gold interactions. Close spatial coupling allows electron delocalization between the nanoparticle cores. The resulting assemblies thus possess optical and electronic properties that emerge as a result of assembly

Plasmon-Mediated Two-Photon Photoluminescence-Detected Circular Dichroism in Gold Nanosphere Assemblies

We report plasmon-mediated two-photon photoluminescence (TPPL)-detected circular dichroism (CD) from colloidal metal nanoparticle assemblies. Two classes of solid gold nanosphere (SGN) dimersheterodimers and homodimerswere examined using polarization-resolved TPPL, second harmonic generation (SHG), and one-photon photoluminescence (OPPL). Unambiguous CD was detected in both the TPPL and SHG signals, and the magnitudes of the CD responses in these measurements showed agreement for individual nanostructures. Heterodimers gave larger CD responses (average TPPL-CDR = 0.62 ± 0.33; average SHG-CDR = 0.51 ± 0.21) than homodimers (average TPPL-CDR = 0.19 ± 0.04; average SHG-CDR = 0.18 ± 0.06). OPPL-CD was not detected for either structure. Analysis of dimer emission properties suggested the CD responses were determined by properties of the one-photon-resonant mode excited by the laser. Average TPPL signals were (4.3 ± 0.6)× larger than those for SHG. Because signal amplitude is a primary determinant for spatial accuracies and precisions obtained from optical microscopy, CD contrast generated from plasmon-mediated TPPL, which we report for the first time, can extend the suite of super-resolution imaging techniques

Superatom State-Resolved Dynamics of the Au₂₅(SC₈H₉)₁₈^– Cluster from Two-Dimensional Electronic Spectroscopy

Superatom state-resolved dynamics of the Au₂₅(SC₈H₉)₁₈^– monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au₂₅(SC₈H₉)₁₈^– MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump–probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices

Nonlinear Chiro-Optical Amplification by Plasmonic Nanolens Arrays Formed via Directed Assembly of Gold Nanoparticles

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these <i>C</i>_∞v heterodimers (ratio of radii <i>R</i>₁/<i>R</i>₂ = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures