Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) has been implemented to investigate the structure and activity of iron(II) phthalocyanine (FePc)a model catalyst for the oxygen reduction ...reaction (ORR). Using EC-TERS, both reversible change and irreversible degradation to FePc have been observed during ORR. The reversible change in the Raman spectrum of FePc can be related to the FePc molecules that adapt a nonplanar geometry during catalysis. In contrast, the irreversible degradation of FePc is a consequence of FePc demetalation, leading to the subsequent formation of free base phthalocyanine. This observation affirms that FePc demetalation during ORR proceeds via a direct loss of Fe2+ and that carbon corrosion is not the operative mechanism. Importantly, the FePc demetalation process can be correlated with a loss of ORR activity suggesting that Fe-containing sites are essential for FePc to achieve high catalytic activity. This study establishes EC-TERS as a promising technique for the operando characterization of electrocatalytic reactions at the molecular scale.
Singlet fission (SF) in two or more electronically coupled organic chromophores converts a high-energy singlet exciton into two low-energy triplet excitons, which can be used to increase solar cell ...efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability and low triplet state energies. The results described here show that efficient SF occurs in polycrystalline thin films of 9,10-bis(phenylethynyl)anthracene (BPEA), a commercial dye that has singlet and triplet energies of 2.40 and 1.11 eV, respectively, in the solid state. BPEA crystallizes into two polymorphs with space groups C2/c and Pbcn, which undergo SF with k SFA = (109 ± 4 ps)−1 and k SFB = (490 ± 10 ps)−1, respectively. The high triplet energy and efficient SF evidenced from the 180 ± 20% triplet yield make BPEA a promising candidate for enhancing solar cell performance.
Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) is used to investigate adsorption of molecular oxygen (O2) on cobalt(II) phthalocyanine (CoPc) supported on Ag(111) single crystal ...surfaces, which is the initial step for the oxygen reduction reaction (ORR) using metal Pc catalysts. Two adsorption configurations are primarily observed, assigned as O2/CoPc/Ag(111) and O/CoPc/Ag(111) based on scanning tunneling microscopy (STM) imaging, TERS, isotopologue substitution, and density functional theory (DFT) calculations. Distinct vibrational features are observed for different adsorption configurations such as the 18O–18O stretching frequency at 1151 cm–1 for O2/CoPc/Ag(111), and Co–16O and Co–18O vibrational frequencies at 661 and 623 cm–1, respectively, for O/CoPc/Ag(111). DFT calculations show vibrational mode coupling of O–O and Co–O vibrations to the Pc ring, resulting in different symmetries of oxygen-related normal modes. This study establishes UHV-TERS as a chemically sensitive tool for probing catalytic systems at the molecular scale.
The use of a nonclassical light source for studying molecular electronic structure has been of great interest in many applications. Here we report a theoretical study of entangled two-photon ...absorption (ETPA) in organic chromophores, and we provide new insight into the quantitative relation between ETPA and the corresponding unentangled TPA based on the significantly different line widths associated with entangled and unentangled processes. A sum-over-states approach is used to obtain classical TPA and ETPA cross sections and to explore the contribution of each electronic state to the ETPA process. The transition moments and energies needed for this calculation were obtained from a second linear-response (SLR) TDDFT method
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, 204105, which enables the treatment of relatively large polythiophene dendrimers that serve as two-photon absorbers. In addition, the SLR calculations provide estimates of the excited state radiative line width, which we relate to the entangled two-photon density of states using a quantum electrodynamic analysis. This analysis shows that for the dendrimers being studied, the line width for ETPA is orders of magnitude narrower than for TPA, corresponding to highly entangled photons with a large Schmidt number. The calculated cross sections are in good agreement with the experimentally reported values. We also carried out a state-resolved analysis to unveil pathways for the ETPA process, and these demonstrate significant interference behavior. We emphasize that the use of entangled photons in TPA process plays a critical role in probing the detailed electronic structure of a molecule by probing light-matter interference nature in the quantum limit.
Surface-enhanced Raman spectroscopy (SERS) harnesses the confinement of light into metallic nanoscale hotspots to achieve highly sensitive label-free molecular detection that can be applied for a ...broad range of sensing applications. However, challenges related to irreversible analyte binding, substrate reproducibility, fouling, and degradation hinder its widespread adoption. Here we show how in-situ electrochemical regeneration can rapidly and precisely reform the nanogap hotspots to enable the continuous reuse of gold nanoparticle monolayers for SERS. Applying an oxidising potential of +1.5 V (vs Ag/AgCl) for 10 s strips a broad range of adsorbates from the nanogaps and forms a metastable oxide layer of few-monolayer thickness. Subsequent application of a reducing potential of -0.80 V for 5 s in the presence of a nanogap-stabilising molecular scaffold, cucurbit5uril, reproducibly regenerates the optimal plasmonic properties with SERS enhancement factors ≈10
. The regeneration of the nanogap hotspots allows these SERS substrates to be reused over multiple cycles, demonstrating ≈5% relative standard deviation over at least 30 cycles of analyte detection and regeneration. Such continuous and reliable SERS-based flow analysis accesses diverse applications from environmental monitoring to medical diagnostics.
Photoexcitation of molecular chromophore aggregates can form excimer states that play a significant role in photophysical processes such as charge and energy transfer as well as singlet fission. An ...excimer state is commonly defined as a superposition of Frenkel exciton and charge transfer states. In this work, we investigate the dynamics of excimer formation and decay in π-stacked 9,10-bis(phenylethynyl)anthracene (BPEA) covalent dimers appended to a xanthene spacer, where the electronic coupling between the two BPEA molecules is adjusted by changing their longitudinal molecular slip distances. Using exciton coupling calculations, we quantify the relative contributions of Frenkel excitons and charge transfer states and find that there is an upper and lower threshold of the charge transfer contribution for efficient excimer formation to occur. Knowing these thresholds can aid the design of molecular aggregates that optimize singlet fission.
Nitrogen-doped graphene has been increasingly utilized in a variety of energy-related applications, serving as a catalyst or support material for fuel cells, and as an anode material for lithium-ion ...batteries, among others. The thermal reduction of graphene oxide (GO) in nitrogenous sources to incorporate nitrogen, producing nitrogen-doped reduced graphene oxide (NRGO), is the most favored method. Controlling atomic configurations of nitrogen-doped sites is the key factor for tailoring the physico-chemical properties of NRGO, but major challenges remain in identifying detailed atomic arrangements at nitrogen binding sites on highly defective and chemically functionalized GO surfaces. In this paper, we present atomistic-scale modeling of the nitrogen doping process of GO with different types of vacancy defects. Molecular dynamics simulations using a reactive force field indicate that the edge carbon atoms on defect sites are the dominant initiation location for nitrogen doping. Further, first-principles calculations using density functional theory present energetically favorable chemical transition pathways for nitrogen doping. The significance of this work lies in providing important chemical insights for the effective control of the desired properties of NRGO by suggesting a detailed mechanism of the nitrogen doping process of GO.
Atomic layer deposition (ALD) of TiO2 was performed in tandem with in situ surface-enhanced Raman spectroscopy (SERS) to monitor changes in the transient surface species across multiple ALD cycles. A ...self-assembled monolayer of 3-mercaptopropionic acid was used as a capture agent to ensure that nucleation of the titanium precursor (titanium tetraisopropoxide TTIP) occurs. Comparisons between the Raman spectra of the neat precursor and the SER spectra of the first ALD cycle of TiO2 reveal typical ligand exchange chemistry taking place, with self-limiting behavior and intact isopropoxide ligands. However, subsequent cycles show drastically different chemistry, with no isopropoxide ligands remaining at any point during the second and third cycles. Continuous exposure of either TTIP or isopropyl alcohol after the first cycle shows unlimited chemical vapor deposition (CVD)-type growth. Comparisons with alternative precursors (aluminum isopropoxide, titanium tert-butoxide, and titanium propoxide) and DFT calculations reveal that, for the TTIP precursor, isolated TiO2 sites play a role in the dehydration of off-gassing isopropyl alcohol. The resulting propene then undergoes oligomerization into six-carbon olefins before polymerizing into indistinguishable carbon products that accumulate on the surface. The emergence of the dehydration chemistry is expected to be exclusively the result of these isolated TiO2 sites and, as such, is expected to occur on other surfaces where TiO2 ALD is feasible. This work showcases how seemingly innocuous ALD can evolve into a CVD process when the products can participate in various side reactions with newly made surface sites.
Abstract
Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we ...show how plasmonic nano-cavities activate anti-Stokes PL in WSe
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monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe
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and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.
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