Electronic Structure and Photochemistry of Molecular and Cluster Anions via Tandem Time-of-Flight Mass Spectroscopy and Photoelectron Imaging

Molecular and cluster anions have been investigated using a newly built tandem time-of-flight mass spectrometer combined with photoelectron imaging system. Solvation particularly hydration is shown not only to stabilize metastable anions such as CO₂⁻ in their ground state and impede autodetachment but also to alter the dynamics in the excited states. For instance, the 355 nm photoelectron image of mass-selected CO₂⁻(H₂O)(m) evolves from anisotropic to isotropic as m increases indicating excited state decay via electron autodetachment. Dissociation channels open at m=2 at 266 nm, resulting in O−(H₂O)m-k and CO₂⁻(H₂O)(m-k) products, the later becoming dominant as m increases. The photoelectron imaging of (CS₂)₂⁻ has revealed the coexistence of four electronic isomers: CS₂⁻•CS₂ [C(s)(₂A′)] and three covalent C₂S₄⁻ [C₂ᵥ(²B₁), D(2h)(²B(3g)), and D(2d)( ²A₁)] structures. Water-mediated intermolecular interactions have been shown to facilitate the formation of the global minimum C₂ᵥ(²B₁) structure rather than the less stable local minima C(s)(₂A′) and D(2d)(²A₁) structures that are favored in the dry source condition. In the (CS2)(n)⁻, n ≥ 3 and (CS₂)₂⁻ (H₂O)(m), m > 0 clusters, the population of the C₂ᵥ(²B₁) structure diminishes drastically due to more favorable solvent interactions with the CS2 − monomercore. Photoexcitation of the (CS₂)₂⁻ also results in the formation of CS₂⁻ and C₂S₂⁻ at 532 nm, and C₂S₂⁻, CS₂⁻, CS₃⁻, S₂⁻, and S⁻ at 355 and 266 nm. The relative yields of C₂S₂⁻ is significantly higher when (CS₂)₂⁻ is formed under wet source condition suggesting C₂ᵥ(²B₁) structure as the origin of C₂S₂⁻. An abrupt decrease in the relative yield of C₂S₂⁻ is observed upon adding CS₂ or H₂O to (CS₂)₂⁻. The CS₂⁻ based clusters are the likely origin of the S− photoproduct, while CS₃⁻ is formed through the secondary S⁻+CS₂ reaction. Novel anions (CS₂O₂⁻ and CS₃O⁻) are observed in the CS₂+O₂+e⁻ reaction. The photoelectron imaging and photodissociation results of these and other anionic products are presented. In addition, CS₂⁻•O₂ ion-neutral complex is formed depending on the conditions in the ion source. Despite the positive electron affinity of O₂, no clear signature of O₂⁻•CS₂ ion-neutral complex is seen in the photoelectron image. CO₃⁻ ion is also formed abundantly as a result of CS₂+CO₂+O₂+e⁻ reaction

Direct Near-Field Observation of Orientation-Dependent Optical Response of Gold Nanorods

The orientation-dependent optical response of short gold nanorods (length less than 100 nm) has been directly observed in the near-field, mapping the in-plane and out-of-plane vector components selectively using interferometric apertureless near-field scanning optical microscope. For the gold nanorods dispersed randomly on oxide-coated silicon wafer, the optical amplitude and phase contrast that are characteristic of the longitudinal and transverse mode dipolar plasmon resonances have been clearly resolved when the long axes of the nanorods are aligned parallel and perpendicular to the electric field of the laser, respectively. The near-field amplitude ratio of the longitudinal to the transverse plasmon mode is much smaller than the corresponding ratio of the scattering cross section, indicating the more efficient coupling of the longitudinal mode to the far-field than the transverse mode. This near-field amplitude ratio increases with the length-to-width aspect ratio of the nanorods, and electromagnetic simulation suggests a similar trend in the scattering cross section. In addition, by choosing the polarization of the laser light such that either the probe or the sample is preferentially excited, the near-field profiles of the dipolar surface plasmon modes induced by the incident light and by the field localized at the probing tip are identified. In accordance with the reciprocity relations of the tip–sample optical coupling, identical near-field optical amplitude and phase contrast have been obtained when the plasmon modes are excited by the incident field and by the field localized at the tip

Surface Ligand-Mediated Plasmon-Driven Photochemical Reactions

Contrary to the general expectation that surface ligands reduce the reactivity of surfaces by blocking the active sites, we present experimental evidence that surface ligands can in fact increase reactivity and induce important reaction pathways in plasmon-driven surface photochemistry. The remarkable effect of surface ligands is demonstrated by comparing the photochemistry of <i>p</i>-aminothiophenol (PATP) on resonant plasmonic gold nanorods (AuNRs) in the presence of citrate, hexadecyltrimethylammonium bromide (CTAB), and no surface ligands under visible light irradiation. The use of AuNRs with citrate and no surface ligand results in the usual azo-coupling reaction. In contrast, CTAB-coated AuNRs oxidize PATP primarily to <i>p</i>-nitrothiophenol (PNTP). Strong correlation has been observed between the N–O and Au–Br vibration band intensities, suggesting that CTAB influences the reaction pathway through the Br^– counterions that can minimize the electron–hole recombination rate by reacting with the hole and hence increasing the concentration of hot electrons that drive the oxidation reaction

Stripping and Transforming Alloyed Semiconductor Quantum Dots via Atomic Interdiffusion

We report the transformation of near-infrared CdSeTe/ZnS quantum dots (QDs) that are exposed to water. When the colloidal QDs with 840 nm emission wavelength and 75 nm spectral line width are self-assembled on water surface and transferred to an oxide-coated silicon wafer using a Langmuir–Blodgett (LB) procedure, two prominent relatively sharp photoluminescence (PL) bands are observed at ∼630 and ∼660 nm peak wavelengths with line width of ∼23 and ∼39 nm, respectively. On the other hand, the PL spectrum of the QDs as they are assembled on the water surface is essentially the same as that of the solution phase. Structural analysis of the LB films shows that the QDs are stripped off the stabilizing excess surfactant molecules by the preferential interaction at the water–air interface. After the film is transferred, the QDs are interfaced with each other and with the substrate directly, while covered with the stack of surfactant molecules from the top. Based on analysis of the chemical composition using X-ray photoelectron spectroscopy of the LB film, the transformation of the CdSeTe/ZnS nanocrystals is attributed to a diffusion of Te atoms from the core to the shell that can initiate inward diffusion of S atoms. This atomic interdiffusion minimizes lattice mismatch as the larger Te atoms are replaced by the smaller S atoms and can lead to formation of either CdSe/CdS or CdSeS nanocrystals that emit at 630 and 660 nm wavelengths, respectively

Plasmon-Enhanced Resonant Excitation and Demethylation of Methylene Blue

Using methylene blue (MB) as a model system, we demonstrate surface plasmon-enhanced resonant excitation that leads to N-demethylation reaction under visible light irradiation (λ = 633 nm) at low photon flux. The chemical changes are monitored by detecting the vibrational signatures of the reactant and product species in situ using surface-enhanced Raman scattering (SERS) spectroscopy. Drastic temporal evolution of SERS spectra has been observed upon continuous irradiation. While the SERS spectra acquired immediately after irradiation are the same as the Raman spectrum of MB solid powder, the spectra recorded a few seconds later are remarkably similar to that of thionine solid powder, indicating N-demethylation of MB. No demethylation reaction has been observed under resonant excitation (λ = 633 nm) of MB adsorbed on nonplasmonic surfaces. Similarly, excitations of plasmon resonances at 532 and 808 nm wavelengths that do not overlap with the MB electronic transition do not lead to transformation of MB to thionine. The reaction mechanism is discussed in terms of resonant excitation of MB and hot electron transfer to adsorbed species. Considering that both MB and thionine have large SERS signal due to the combination of resonance Raman and electromagnetic enhancement effects that provide high detection sensitivity, we suggest that this demethylation reaction serves as a convenient model system for future mechanistic studies

Near-Field Mapping of Optical Modes on All-Dielectric Silicon Nanodisks

We measure, for the first time to our knowledge, the near-field amplitudes and phases of localized optical modes of high-index all-dielectric nanoparticles using apertureless near-field optical microscopy. For individual silicon nanodisks, we observe a four-lobed mode pattern and the formation of deep-subwavelength hot-spots. Our numerical calculations of the optical near-fields of the nanodisks in combination with a multipole expansion of the scattered field based on vector spherical harmonics reveal that the observed modes are dominated by electric quadrupole contributions. The observed mode is of particular interest for the design of low-loss all-dielectric metasurfaces and nanoantennas for a broad range of applications, such as directional and complex-polarization controlled emission, light extraction from multipolar atomic transitions, and coherent multiple-emitter-nanocavity interactions

Hierarchical Self-Assembly of Carbon Dots into High-Aspect-Ratio Nanowires

We report a spontaneous and hierarchical self-assembly mechanism of carbon dots prepared from citric acid and urea into nanowire structures with large aspect ratios (>50). Scattering-type scanning near-field optical microscopy (s-SNOM) with broadly tunable mid-IR excitation was used to interrogate details of the self-assembly process by generating nanoscopic chemical maps of local wire morphology and composition. s-SNOM images capture the evolution of wire formation and the complex interplay between different chemical constituents directing assembly over the nano- to microscopic length scales. We propose that residual citrate promotes tautomerization of melamine surface functionalities to produce supramolecular shape synthons comprised of melamine-cyanurate adducts capable of forming long-range and highly directional hydrogen-bonding networks. This intrinsic, heterogeneity-driven self-assembly mechanism reflects synergistic combinations of high chemical specificity and long-range cooperativity that may be harnessed to reproducibly fabricate functional structures on arbitrary surfaces