11 research outputs found
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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<sup>–</sup> 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