19-Electron Intermediates and Cage-Effects in the Photochemical Disproportionation of [CpW(CO)₃]₂ with Lewis Bases

The role of 19-electron intermediates in the photochemical disproportionation of [CpW(CO)3]2 (Cp = C5H5) with Lewis bases (PR3; R = OMe, Bu, Ph) is investigated on the ultrafast time scale using femtosecond VIS-pump, IR-probe spectroscopy. Formation of a 19-electron (19e) species CpW(CO)3PR3• by coordination of PR3 with photogenerated 17-electron (17e) radicals CpW(CO)3• is directly observed, and equilibrium is established between the 17e radicals and the 19e intermediates favoring 19e intermediates in the order:  Bu > OMe ≫ Ph. Steric effects dominate the 17e/19e equilibrium when the cone-angle of the Lewis base exceeds a certain limiting value (between 132° and 145°), but below this value electronic properties of the Lewis base control the 17e/19e dynamics. Disproportionation occurs in less than 200 picoseconds by electron transfer between a solvent caged 17e radical and 19e, highly reducing species. The rate and extent of ultrafast disproportionation depends on both the identity and concentration of the Lewis base. In low concentrations of PR3 (typically 1−2 M or less) or with Lewis bases whose equilibrium heavily favors 17e radicals (e.g., PPh3), disproportionation is rate-limited by breakdown of the solvent cage. Density functional theory calculations on vibrational frequencies and charge distributions of the various complexes support the experimental results

Identifying Crystallization- and Incorporation-Limited Regimes during Vapor–Liquid–Solid Growth of Si Nanowires

The vapor–liquid–solid (VLS) mechanism is widely used for the synthesis of semiconductor nanowires (NWs), yet several aspects of the mechanism are not fully understood. Here, we present comprehensive experimental measurements on the growth rate of Au-catalyzed Si NWs over a range of temperatures (365–480 °C), diameters (30–200 nm), and pressures (0.1–1.6 Torr SiH₄). We develop a kinetic model of VLS growth that includes (1) Si incorporation into the liquid Au–Si catalyst, (2) Si evaporation from the catalyst surface, and (3) Si crystallization at the catalyst–NW interface. This simple model quantitatively explains growth rate data collected over more than 65 distinct synthetic conditions. Surprisingly, upon increasing the temperature and/or pressure, the analysis reveals an abrupt transition from a diameter-independent growth rate that is limited by incorporation to a diameter-dependent growth rate that is limited by crystallization. The identification of two distinct growth regimes provides insight into the synthetic conditions needed for specific NW-based technologies, and our kinetic model provides a straightforward framework for understanding VLS growth with a range of metal catalysts and semiconductor materials

Epitaxially Grown Silicon Nanowires with a Gold Molecular Adhesion Layer for Core/Shell Structures with Compact Mie and Plasmon Resonances

Noble-metal plasmonic nanostructures have attracted much attention because they can support deep-subwavelength optical resonances, yet their performance tends to be limited by high Ohmic absorption losses. In comparison, high-index dielectric materials can support low-loss optical resonances but do not tend to yield the same subwavelength optical confinement. Here, we combine these two approaches and examine the dielectric-plasmonic resonances in dielectric/metal core/shell nanowires. Si nanowires were grown epitaxially from (111) substrates, and direct deposition of Au on these structures by physical vapor deposition yielded nonconformal Au islands. However, by introduction of a molecular adhesion layer prior to deposition, cylindrical Si/Au core/shell nanostructures with conformal metal shells were successfully fabricated. Examining these structures as optical cavities using both optical simulations and experimental extinction measurements, we found that the structures support Mie resonances with quality factors enhanced up to ∼30 times compared with pure dielectric structures and plasmon resonances with optical confinement enhanced up to ∼5 times compared with pure metallic structures. Interestingly, extinction spectra of both Mie and plasmon resonances yield Fano line shapes, whose manifestation can be attributed to the combination of high quality factor resonances, Mie-plasmon coupling, and phase delay of the background optical field. This work demonstrates a bottom-up synthetic method for the production of freestanding, cylindrically symmetric semiconductor/metal core/shell nanowires that enables the efficient trapping of light on deep-subwavelength length scales for varied applications in photonics and optoelectronics

19-Electron Intermediates in the Ligand Substitution of CpW(CO)₃^• with a Lewis Base

The photochemical reactions of [CpW(CO)3]2 with the Lewis base P(OMe)3 are examined on the nanosecond and microsecond time scales using step-scan FTIR spectroscopy. Photolysis at 532 nm produces the 17-electron (17e) radicals CpW(CO)3•, which are in equilibrium with the 19-electron (19e) radicals CpW(CO)3P(OMe)3• on the nanosecond time scale. The reactions of the 19e radical are directly observed for the first time; the major reaction pathway is spontaneous loss of a carbonyl to form the 17e species CpW(CO)2P(OMe)3•, with a barrier of 7.6 ± 0.3 kcal/mol for this process. The minor reaction pathway (3 (85 mM) is disproportionation to form the products CpW(CO)3P(OMe)3+ and CpW(CO)3-. On the microsecond time scale, the 17e radicals CpW(CO)2P(OMe)3• dimerize to form the ligand substitution product [CpW(CO)2P(OMe)3]2. These results indicate that the 19e species is a stable intermediate rather than transition state in the ligand substitution reaction, and this type of reactivity is likely to be typical of 17e organometallic radicals which undergo associative substitution mechanisms

The Role of Odd-Electron Intermediates and In-Cage Electron Transfer in Ultrafast Photochemical Disproportionation Reactions in Lewis Bases

Femtosecond visible pump−IR probe studies of Cp2W2(CO)6 in P(OMe)3 and CH2Cl2 have allowed direct observation of a 19-electron intermediate and of disproportionation into CpW(CO)3- and CpW(CO)3P(OMe)3+ on the ultrafast time scale. A new disproportionation mechanism involving in-cage electron transfer between a 19-electron intermediate and a 17-electron radical has been proposed

Encoding Abrupt and Uniform Dopant Profiles in Vapor–Liquid–Solid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst

Semiconductor nanowires (NWs) are often synthesized by the vapor–liquid–solid (VLS) mechanism, a process in which a liquid dropletsupplied with precursors in the vapor phasecatalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this “reservoir effect” is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (≤250 nm/min) and low reactor pressures (≤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures

Influence of Geometry on Quasi-Ballistic Behavior in Silicon Nanowire Geometric Diodes

Diodes are a basic component of electrical circuits to control the flow of charge, and geometric diodes (GDs) are a special class that can operate using ballistic or quasi-ballistic transport in conjunction with geometric asymmetry to direct charge carriers preferentially in one direction, enabling an electron ratcheting effect. Nanomaterials present a unique platform for the development of GDs, and silicon nanowire (NW)-based GDscylindrically symmetric but translationally asymmetric three-dimensional nanostructureshave recently been demonstrated functioning at room temperature. These devices can theoretically achieve a near zero-bias turn-on voltage and rectify up to THz frequencies. Here, we synthesize silicon NW GDs and fabricate single-NW devices from which significant changes in diode performance are observed from relatively minor changes in geometry. To elucidate the interplay between geometry and ballistic behavior, we develop a Monte Carlo simulation that describes the quasi-ballistic behavior of electrons within a three-dimensional NW GD. We examine the effects of doping level, temperature, and geometry on charge carrier transport, revealing the relationships between charge carrier mean free path (MFP), specular reflection at surfaces, and geometry on GD performance. As expected, geometry strongly influences performance by directing or blocking charge carrier passage through the nanostructure. Interestingly, we find that the blocking effect is at least as important as the directing effect. Moreover, within certain geometric limits, the diode behavior is less sensitive to the MFP than might be initially expected because of the short relevant length scales and importance of the blocking effect. The results provide guidelines for the future design of NW GDs and enable the prediction and interpretation of trends in experimental results. An improved understanding of quasi-ballistic transport is crucial to guiding future experiments toward realizing THz rectification for applications in high-speed data transfer and long-wavelength energy harvesting

Waveguide Scattering Microscopy for Dark-Field Imaging and Spectroscopy of Photonic Nanostructures

Dark-field microscopy (DFM) is widely used to optically image and spectroscopically analyze nanoscale objects. In a typical DFM configuration, a sample is illuminated at oblique angles and an objective lens collects light scattered by the sample at a range of lower angles. Here, we develop waveguide scattering microscopy (WSM) as an alternative technique to image and analyze photonic nanostructures. WSM uses an incoherent white-light source coupled to a dielectric slab waveguide to generate an evanescent field that illuminates objects located within several hundred nanometers of the waveguide surface. Using standard microscope slides or coverslips as the waveguide, we demonstrate high-contrast dark-field imaging of nanophotonic and plasmonic structures such as Si nanowires, Au nanorods, and Ag nanoholes. Scattering spectra collected in the WSM configuration show excellent signal-to-noise with minimal background signal compared to conventional DFM. In addition, the polarization of the incident field is controlled by the direction of the propagating wave, providing a straightforward route to excite specific optical modes in anisotropic nanostructures by selecting the appropriate input wavevector. Considering the facile integration of WSM with standard microscopy equipment, we anticipate it will become a versatile tool for characterizing photonic nanostructures

Monolayer-like Exciton Recombination Dynamics of Multilayer MoSe₂ Observed by Pump–Probe Microscopy

Transition metal dichalcogenides (TMDCs) have garnered considerable interest over the past decade as a class of semiconducting layered materials. Most studies on the carrier dynamics in these materials have focused on the monolayer due to its direct bandgap, strong photoluminescence, and strongly bound excitons. However, a comparative understanding of the carrier dynamics in multilayer (e.g., >10 layers) flakes is still absent. Recent computational studies have suggested that excitons in bulk TMDCs are confined to individual layers, leading to room-temperature stable exciton populations. Using this new context, we explore the carrier dynamics in MoSe2 flakes that are between ∼16 and ∼125 layers thick. We assign the kinetics to exciton–exciton annihilation (EEA) and Shockley–Read–Hall recombination of free carriers. Interestingly, the average observed EEA rate constant (0.003 cm2/s) is nearly independent of flake thickness and 2 orders of magnitude smaller than that of an unencapsulated monolayer (0.33 cm2/s) but very similar to values observed in encapsulated monolayers. Thus, we posit that strong intralayer interactions minimize the effect of layer thickness on recombination dynamics, causing the multilayer to behave like the monolayer and exhibit an apparent EEA rate intrinsic to MoSe2