Spin-Polarized Photoluminescence in Au25 (SC8 H9 )18 Monolayer-Protected Clusters.

Here, the observation of spin-polarized emission for the Au25 (SC8 H9 )18 monolayer-protected cluster (MPC) is reported. Variable-temperature variable-field magnetic circular photoluminescence (VTV H ⇀ -MCPL) measurements are combined with VT-PL spectroscopy to provide state-resolved characterization of the transient electronic structure and spin-polarized electron-hole recombination dynamics of Au25 (SC8 H9 )18 . Through analysis of VTV H ⇀ -MCPL measurements, a low energy (1.64 eV) emission peak is assigned to intraband relaxation between core-metal-localized superatom-D to -P orbitals. Two higher energy interband components (1.78 eV, 1.94 eV) are assigned to relaxation from superatom-D orbitals to states localized to the inorganic semirings. For both intraband superatom-based or interband relaxation mechanisms, the extent of spin-polarization, quantified as the degree of circular polarization (DOCP), is determined by state-specific electron-vibration coupling strengths and energy separations of bright and dark electronic fine-structure levels. At low temperatures (<60 K), metal-metal superatom-based intraband transitions dominate the global PL emission. At higher temperatures (>60 K), interband ligand-based emission is dominant. In the low-temperature PL regime, increased sample temperature results in larger global PL intensity. In the high-temperature regime, increased temperature quenches interband radiative recombination. The relative intensity for each PL mechanism is discussed in terms of state-specific electronic-vibrational coupling strengths and related to the total angular momentum, quantified by Landé g-factors.

State-Resolved Metal Nanoparticle Dynamics Viewed through the Combined Lenses of Ultrafast and Magneto-optical Spectroscopies.

Electronic carrier dynamics play pivotal roles in the functional properties of nanomaterials. For colloidal metals, the mechanisms and influences of these dynamics are structure dependent. The coherent carrier dynamics of collective plasmon modes for nanoparticles (approximately 2 nm and larger) determine optical amplification factors that are important to applied spectroscopy techniques. In the nanocluster domain (sub-2 nm), carrier coupling to vibrational modes affects photoluminescence yields. The performance of photocatalytic materials featuring both nanoparticles and nanoclusters also depends on the relaxation dynamics of nonequilibrium charge carriers. The challenges for developing comprehensive descriptions of carrier dynamics spanning both domains are multifold. Plasmon coherences are short-lived, persisting for only tens of femtoseconds. Nanoclusters exhibit discrete carrier dynamics that can persist for microseconds in some cases. On this time scale, many state-dependent processes, including vibrational relaxation, charge transfer, and spin conversion, affect carrier dynamics in ways that are nonscalable but, rather, structure specific. Hence, state-resolved spectroscopy methods are needed for understanding carrier dynamics in the nanocluster domain. Based on these considerations, a detailed understanding of structure-dependent carrier dynamics across length scales requires an appropriate combination of spectroscopic methods. Plasmon mode-specific dynamics can be obtained through ultrafast correlated light and electron microscopy (UCLEM), which pairs interferometric nonlinear optical (INLO) with electron imaging methods. INLO yields nanostructure spectral resonance responses, which capture the system's homogeneous line width and coherence dynamics. State-resolved nanocluster dynamics can be obtained by pairing ultrafast with magnetic-optical spectroscopy methods. In particular, variable-temperature variable-field (VTVH) spectroscopies allow quantification of transient, excited states, providing quantification of important parameters such as spin and orbital angular momenta as well as the energy gaps that separate electronic fine structure states. Ultrafast two-dimensional electronic spectroscopy (2DES) can be used to understand how these details influence state-to-state carrier dynamics. In combination, VTVH and 2DES methods can provide chemists with detailed information regarding the structure-dependent and state-specific flow of energy through metal nanoclusters. In this Account, we highlight recent advances toward understanding structure-dependent carrier dynamics for metals spanning the sub-nanometer to tens of nanometers length scale. We demonstrate the use of UCLEM methods for arresting interband scattering effects. For sub-nanometer thiol-protected nanoclusters, we discuss the effectiveness of VTVH for distinguishing state-specific radiative recombination originating from a gold core versus organometallic protecting layers. This state specificity is refined further using femtosecond 2DES and two-color methods to isolate so-called superatom state dynamics and vibrationally mediated spin-conversion and emission processes. Finally, we discuss prospects for merging VTVH and 2DES methods into a single platform.

Superatom spin-state dynamics of structurally precise metal monolayer-protected clusters (MPCs).

Electronic spin-state dynamics were studied for a series of Au25(SC8H9)18 q and Au24Pd(SC8H9)18 monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au25(SC8H9)18 -1 is a closed-shell superatomic cluster, but Au25(SC8H9)18 0 is an open-shell (7-electron) system; Au25(SC8H9)18 +1 and PdAu24(SC8H9)18 0 are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au25(SC8H9)18 0 in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired Pz electron to D states, whereas 1.97-eV was resonant with transitions between filled Px and Py subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au25(SC8H9)18, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu24(SC8H9)18 clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au25(SC8H9)18 +1 counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired Pz electron of the open-shell seven-electron Au25(SC8H9)18 MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems.

Variable-temperature variable-field magnetic circular photoluminescence (VTVH-MCPL) spectroscopy for electronic-structure determination in nanoscale chemical systems.

In this Letter, we describe variable-temperature variable-field magnetic circular photoluminescence (VTVH-MCPL) spectroscopy as a complementary technique to absorption-based magnetic circular dichroism. A paramagnetic model system, Au25(SC8H9)18, is chosen to demonstrate the information content that is obtained from VTVH-MCPL. Specifically, the methods and analyses for the determination of electronic Landé g-factors, zero-field energy splittings, and relative A-, B-, and C-term contributions to the MCPL response are detailed. The determination of these system properties from photoluminescence data suggests the feasibility of point-source-based super-resolution magneto-optical microscopy.

Chiral nanostructures studied using polarization-dependent NOLES imaging.

The Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging technique was used to generate optical images in which the position of a chiral object could be determined with nanometer precision. Asymmetric gold bowtie nanostructures were used as a model system with 2D chirality. The bowties functioned as a chiral nonlinear medium that converted the fundamental of a Ti:sapphire laser to its second harmonic frequency. The bowties consisted of two lithographically prepared equilateral triangles (base = 75 nm, height = 85 nm, thickness = 25 nm) separated by a 20 nm gap. Asymmetric bowties were formed by lateral displacement of one triangle by 10 nm, yielding C2 point group symmetry. The chirality of the bowtie nanostructures was confirmed via nonzero second-harmonic generation circular dichroism (SHG-CDR) ratios, which came from single-particle SHG measurements. The SHG-CDR ratios were validated using numerical finite difference time domain simulations that quantified the relative magnitudes of gap-localized electromagnetic fields at the harmonic frequency resulting from excitation by left and right circularly (LCP and RCP) and linearly polarized fundamental waves. The relative electric dipolar and magnetic dipolar contributions to the SHG responses were determined using single-particle continuous polarization variation (CPV) SHG measurements. The spatial localization precision obtainable for individual chiral nanostructures was determined by statistical analysis of the SHG image point spread function. Our results demonstrated that both the chiral image contrast, which resulted from LCP and RCP excitation, and the corresponding localization precision was dependent upon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13 nm and left-to-right image enhancements of 400% were obtained for bowties with the highest G/F ratios using 5 s frame exposure times. The polarization dependence and magnetic dipole amplification confirmed here demonstrate that the NOLES imaging technique is a powerful method for studying chiral specimens with high spatial precision.

Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters.

Near-infrared photoluminescence of a series of three gold monolayer protected clusters (MPCs) with volumes spanning 50-200 Å3 was studied by using variable-temperature photoluminescence (VT-PL) spectroscopy. The three MPCs, which included Au20(SC8H9)15-diglyme, Au25(SC8H9)18, and Au38(SC12H25)24, all exhibited temperature-dependent intensities that reflected a few-millielectronvolt energy gap that separated bright emissive and dark nonradiative electronic states. All clusters showed increased PL intensities upon raising the sample temperature from 4.5 K to a cluster-specific value, upon which increased sample temperature resulted in emission quenching. The increased PL in the low-temperature range is attributed to thermally activated carrier transfer from dark to bright states. The quenching at elevated temperatures is attributed to nonradiative vibrational relaxation through Au-Au stretching of the MPCs metal core. Importantly, the results show evidence of a common and size scalable metal-centered intraband PL mechanism that is general for ultrasmall metal nanoclusters, which are expected to show nonscalable optical properties.

Low-Temperature Magnetism in Nanoscale Gold Revealed through Variable-Temperature Magnetic Circular Dichroism Spectroscopy.

The low-temperature (0.35-4.2 K) steady-state electronic absorption of the monolayer-protected cluster (MPC) Au102( pMBA)44 was studied using magnetic circular dichroism (MCD) spectroscopy to investigate previously reported low-temperature (<50 K) magnetism in d10 nanogold systems. Variable-temperature variable-field analysis of resolvable MCD extinction components revealed two distinct magnetic anisotropic behaviors. A low-energy, diamagnetic component was correlated to excitation from states localized to the passivating ligands. A high-energy, paramagnetic component was attributed to excitation from the d-band of the Au core. The temperature dependence of the magnetic anisotropy for each component is discussed in terms of previously reported structural parameters of the atomically precise Au102( pMBA)44 MPC. It is concluded that temperature-sensitive structure-dependent Au d-d orbital interactions result in the promotion of 5d-band electrons to the 6sp-band via orbital rehybridization, inducing a 15× increase in the Landé g-factor over the temperature range spanning from 0.35 to 4.2 K.