Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Core-Shell Nanowires

We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C₃N₅ is a low-band-gap (E_g = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C₃N₅ nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C₃N₅ heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm² V^-1 s^-1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm² V^-1 s^-1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J-V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C₃N₅ in CdS-MHP. C₃N₅, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core-shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.

Disassembly of microtubules by intense terahertz pulses

The biological effects of terahertz (THz) radiation have been observed across multiple levels of biological organization, however the sub-cellular mechanisms underlying the phenotypic changes remain to be elucidated. Filamentous protein complexes such as microtubules are essential cytoskeletal structures that regulate diverse biological functions, and these may be an important target for THz interactions underlying THz-induced effects observed at the cellular or tissue level. Here, we show disassembly of microtubules within minutes of exposure to extended trains of intense, picosecond-duration THz pulses. Further, the rate of disassembly depends on THz intensity and spectral content. As inhibition of microtubule dynamics is a mechanism of clinically-utilized anti-cancer agents, disruption of microtubule networks may indicate a potential therapeutic mechanism of intense THz pulses.

Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double-Helix SnIP Nanowires

Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, the understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, time-resolved terahertz (THz) spectroscopy is used to probe the transient photoconductivity of SnIP nanowire films and measure the carrier mobility. With insight into the highly anisotropic electronic structure from quantum chemical calculations, an electron mobility as high as 280 cm²   V^-1 s^-1 along the double-helix axis and a hole mobility of 238 cm²   V^-1 s^-1 perpendicular to the double-helix axis are detected. Additionally, infrared-active (IR-active) THz vibrational modes are measured, which shows excellent agreement with first-principles calculations, and an ultrafast photoexcitation-induced charge redistribution is observed that reduces the amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, it is shown that the carrier lifetime and mobility are limited by a trap density greater than 10¹⁸  cm^-3 . The results provide insight into the optical excitation and relaxation pathways of SnIP and demonstrate a remarkably high carrier mobility for such a soft and flexible material, suggesting that it could be ideally suited for flexible electronics applications.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Dynamical Control over Terahertz Electromagnetic Interference Shielding with 2D Ti3C2Ty MXene by Ultrafast Optical Pulses

High electrical conductivity and strong absorption of electromagnetic radiation in the terahertz (THz) frequency range by metallic 2D MXene Ti₃C₂T_y make it a promising material for electromagnetic interference shielding, THz detectors, and transparent conducting electrodes. Here, we demonstrate that ultrafast optical pulses with wavelengths straddling the visible range (400 and 800 nm) induce transient broad-band THz transparency in the MXene that persists for nanoseconds. We demonstrate that optically induced transient THz transparency is independent of temperature from 95 to 290 K. This discovery opens new possibilities for development of switchable electromagnetic interference shielding materials and devices that can be rendered partially transparent on demand for transmitting THz signals, or for designing new THz devices such as sensitive optically gated detectors.

Understanding the Origin of Phosphorescence in Bismoles: A Synthetic and Computational Study

A series of bismuth heterocycles, termed bismoles, were synthesized via the efficient metallacycle transfer (Bi/Zr exchange) involving readily accessible zirconacycles. The luminescence properties of three structurally distinct bismoles were explored in detail via time-integrated and time-resolved photoluminescence spectroscopy using ultrafast laser excitation. Moreover, time-dependent density functional theory computations were used to interpret the nature of fluorescence versus phosphorescence in these bismuth-containing heterocycles and to guide the future preparation of luminescent materials containing heavy inorganic elements. Specifically, orbital character at bismuth within excited states is an important factor for achieving enhanced spin-orbit coupling and to promote phosphorescence. The low aromaticity of the bismole rings was demonstrated by formation of a CuCl π-complex, and the nature of the alkene-CuCl interaction was probed by real-space bonding indicators derived from Atoms-In-Molecules, the Electron Localizability Indicator, and the Non-Covalent Interaction index; such tools are of great value in interpreting nonstandard bonding environments within inorganic compounds.

Synthesis and Surface Functionalization of Hydride-Terminated Ge Nanocrystals Obtained from the Thermal Treatment of Ge(OH)2

The synthesis of germanium nanocrystals (GeNCs) with well-defined surface chemistry is of considerable interest because of their potential applications in the optoelectronic, battery, and semiconductor industries. Modifying and tailoring GeNC surface chemistry provides an avenue by which reactivity, environmental compatibility (e.g., solubility, resistance to oxidation), and electronic properties may be tailored. Hydride-terminated GeNCs (H-GeNCs) are of particular interest because the reactivity of surface Ge-H bonds toward alkenes and alkynes via hydrogermylation affords the potential for convenient modification; however, these reactions and their scope have not been widely explored. This report describes a straightforward route for preparing a GeNC/GeO₂ composite via disproportionation of heretofore-unexplored Ge(II) oxide-based precursor from which the H-GeNCs were freed by subsequently chemical etching. The H-GeNCs were derivatized using a series of hydrogermylation approaches (i.e., thermally activated, radical-initiated, and borane-catalyzed). The presented findings indicate surface functionalization occurs under all conditions investigated; however the nature of surface species (i.e., monolayers vs multilayers) and surface coverage varies depending upon the conditions employed.

Generation of terahertz radiation by optical excitation of aligned carbon nanotubes

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis.

Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO₃

We use ultrafast phase-contrast imaging to directly observethe cone-like terahertz (THz) Cherenkov wave generated by optical rectification of femtosecond laser pulses focused into bulk lithium niobate (LiNbO₃) single crystals. The transverse imaging geometry allows the Cherenkov angle, THz wave velocity, and optical pump pulse group velocity to be measured. Furthermore, transition-like THz radiation generated by the femtosecond laser pulse at the air-crystal boundary is observed. The effect of optical pump pulse polarization on the generation of THz Cherenkov waves and transition-like radiation in LiNbO₃ is also investigated.

One-pot synthesis of functionalized germanium nanocrystals from a single source precursor

One-pot syntheses of surface functionalized germanium nanocrystals (GeNCs) based upon traditional hot injection and microwave-assisted heating of a Ge(II) dihydride single source precursor have been developed. The reported procedures offer in situ hydrogermylation-based covalent attachment of alkene/alkyne derived surface moieties that give access to hydrophobic or hydrophilic GeNCs.

Charge transfer state emission dynamics in blue-emitting functionalized silicon nanocrystals

Time-resolved photoluminescence spectroscopy reveals a charge transfer state dynamics responsible for the size-independent blue emission observed from dodecylamine and ammonia functionalized silicon nanocrystals.

Size vs surface: tuning the photoluminescence of freestanding silicon nanocrystals across the visible spectrum via surface groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Conductivity control of as-grown branched indium tin oxide nanowire networks

Branched indium tin oxide (ITO) nanowire networks are promising candidates for transparent conductive oxide applications, such as optoelectronic electrodes, due to their high porosity. However, these branched networks also present new challenges in assessing conductivity. Conventional four-point probe techniques cannot separate the effect of porosity on the long-range conductivity from the intrinsic material conductivity. Here we compare the average nanoscale conductivity within the film measured by terahertz time-domain spectroscopy (THz-TDS) to the film conductivity measured by four-point probe in a branched ITO nanowire network. Both techniques report conductivity increases with deposition flux rate from 0.5 to 3.0 nm s(-1), achieving a maximum of ~ 10 (Ω cm)(-1). Modeling the THz-TDS conductivity data using the Drude-Smith model allows us to distinguish between conductivity increases resulting from morphological changes and those resulting from the intrinsic properties of the ITO. In particular, the intrinsic material conductivity within the nanowires can be extracted, and is found to reach a maximum of ~ 3000 (Ω cm)(-1), comparable to bulk ITO. To determine the mechanism responsible for increasing conductivity with flux rate, we characterize dopant concentration and morphological changes (i.e., to branching behavior, nanowire diameter and nucleation layers). We propose that changes in the electron density, primarily due to changes in O-vacancy concentration at different flux rates, are responsible for the observed conductivity increase. This understanding will assist balancing structural and conductivity requirements in applications of transparent conductive oxide networks.

Intense THz pulses cause H2AX phosphorylation and activate DNA damage response in human skin tissue

Recent emergence and growing use of terahertz (THz) radiation for medical imaging and public security screening raise questions on reasonable levels of exposure and health consequences of this form of electromagnetic radiation. In particular, picosecond-duration THz pulses have shown promise for novel diagnostic imaging techniques. However, the effects of THz pulses on human cells and tissues thus far remain largely unknown. We report on the investigation of the biological effects of pulsed THz radiation on artificial human skin tissues. We observe that exposure to intense THz pulses for ten minutes leads to a significant induction of H2AX phosphorylation, indicating that THz pulse irradiation may cause DNA damage in exposed skin tissue. At the same time, we find a THz-pulse-induced increase in the levels of several proteins responsible for cell-cycle regulation and tumor suppression, suggesting that DNA damage repair mechanisms are quickly activated. Furthermore, we find that the cellular response to pulsed THz radiation is significantly different from that induced by exposure to UVA (400 nm).

Effective mass anisotropy of hot electrons in nonparabolic conduction bands of n-doped InGaAs films using ultrafast terahertz pump-probe techniques

The anisotropic effective mass of energetic electrons in an isotropic, nonparabolic conduction band is revealed using ultrafast THz-pump-THz-probe techniques in a n-doped InGaAs semiconductor thin film. A microscopic theory is applied to identify the origin of the observed anisotropy and to show that the self-consistent light-matter coupling contributes significantly to the THz response.

Terahertz pulse induced intervalley scattering in photoexcited GaAs

Nonlinear transient absorption bleaching of intense few-cycle terahertz (THz) pulses is observed in photoexcited GaAs using opticalpump--THz-probe techniques. A simple model of the electron transport dynamics shows that the observed nonlinear response is due to THz-electric- field-induced intervalley scattering over sub-picosecond time scales as well as an increase in the intravalley scattering rate attributed to carrier heating. Furthermore, the nonlinear nature of the THz pulse transmission at high peak fields leads to a measured terahertz conductivity in the photoexcited GaAs that deviates significantly from the Drude behavior observed at low THz fields, emphasizing the need to explore nonlinear THz pulse interactions with materials in the time domain.

Generation of 1.5 microJ single-cycle terahertz pulses by optical rectification from a large aperture ZnTe crystal

We demonstrate the generation muJ-level, single-cycle terahertz pulses by optical rectification from a large-aperture ZnTe single crystal wafer. Energies up to 1.5 muJ per pulse and a spectral range extending to 3 THz were obtained using a 100 Hz Ti:sapphire laser source and a 75-mmdiameter, 0.5-mm-thick, (110) ZnTe crystal, corresponding to an average power of 150 muW and an energy conversion efficiency of 3.1 x 10(-5). We also demonstrate real-time imaging of the focused terahertz beam using a pyroelectric infrared camera.

Two-photon absorption in two-dimensional conjugated quadrupolar chromophores

We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores.

Developing 1D nanostructure arrays for future nanophotonics

There is intense and growing interest in one-dimensional (1-D) nanostructures from the perspective of their synthesis and unique properties, especially with respect to their excellent optical response and an ability to form heterostructures. This review discusses alternative approaches to preparation and organization of such structures, and their potential properties. In particular, molecular-scale printing is highlighted as a method for creating organized pre-cursor structure for locating nanowires, as well as vapor–liquid–solid (VLS) templated growth using nano-channel alumina (NCA), and deposition of 1-D structures with glancing angle deposition (GLAD). As regards novel optical properties, we discuss as an example, finite size photonic crystal cavity structures formed from such nanostructure arrays possessing highQand small mode volume, and being ideal for developing future nanolasers.

Synthesis, structure, and nonlinear optical properties of diarylpolyynes

A series of alpha,omega-diarylpolyynes has been synthesized. In addition to the synthesis of three hexaynes (3a-c), a notably improved synthesis of 1,16-diphenylhexadecaoctayne (5) is described. The third-order nonlinear optical characteristics for these molecules have been studied and show a substantial increase in molecular hyperpolarizability (gamma) as a function of increasing length. The unusual solid-state structures of compounds 3a and 3b are reported.

Synthesis, Spectroscopic and Nonlinear Optical Properties of Multiple [60]Fullerene-Oligo(p-phenylene ethynylene) Hybrids

A series of multiple [60]fullerene terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds has been synthesized through a newly developed in situ ethynylation method. Structural and magnetic shielding properties of the highly unsaturated carbon-rich C(60) and OPE scaffolds were characterized by 1D and 2D NMR spectroscopic analyses. Electronic interactions between the [60]fullerenes and the OPE backbones were investigated by UV/Vis spectroscopic and cyclic voltammetry (CV) experiments. Our studies clearly show that although the multiple [60]fullerene groups are connected via pi-conjugated OPE frameworks, they present diminutive electronic interactions in the ground state, and the electronic behavior of the [60]fullerene cages are only affected by the OPE backbones through modest inductive effects. Interestingly, sizable third-order nonlinear optical (NLO) responses (gamma) and enhanced two-photon absorption (TPA) cross-sections (sigma((2))) were determined for the multifullerene-OPE hybrid 31 relative to its OPE precursor from differential optical Kerr effect (DOKE) experiments. Such enhanced NLO performance is presumably due to the occurrence of periconjugation and/or charge transfer effects in the excited state. In addition, comparatively strong excited-state absorption was observed and characterized for OPE pentamer 12. Thus, the use of such fullerene-derivatized conjugated oligomers aids the quest for molecules with large third-order NLO and TPA properties.

Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response

With the Fritsch-Buttenberg-Wiechell rearrangement as a primary synthetic route, a series of conjugated, triisopropylsilyl end-capped polyynes containing 2-10 acetylene units has been assembled. In a few steps, significant quantities of the polyynes are made available, which allow for a thorough analysis of their structural, physical, and optical properties. Molecules in the series have been characterized in detail using (13)C NMR spectroscopy, differential scanning calorimetry, mass spectrometry, and, for four derivatives including octayne 6, X-ray crystallography. UV-vis spectroscopy of the polyynes 1-7 shows a consistent lowering of the HOMO-LUMO gap (E(g)) as a function of the number of acetylene units (n), fitting a power-law relationship of E(g) approximately n(-)(0.379)(+/-)(0.002). The third-order nonlinear optical (NLO) properties of the polyyne series have been examined, and the nonresonant molecular second hyperpolarizabilities (gamma) increase as a function of length according to the power-law gamma approximately n(4.28)(+/-)(0.13). This result exhibits an exponent that is larger than theoretically predicted for polyynes and higher than is observed for polyenes and polyenynes. The combined linear and nonlinear optical results confirm recent theoretical studies that suggest polyynes as model 1-D conjugated systems. On the basis of UV-vis spectroscopic analysis, the effective conjugation length for this series of polyynes is estimated to be ca. n = 32, providing insight into characteristics of carbyne.

Synthesis, Structure, and Nonlinear Optical Properties of Cross-Conjugated Perphenylatediso-Polydiacetylenes

Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray crystallographic analysis demonstrate a non-planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red-shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25, a trend that has saturated by the stage of nonamer 22. Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n. The molecular third-order nonlinearities, gamma, for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n. Molecular modeling and spectroscopic studies suggest that iso-PDA oligomers (n>7) adopt a coiled, helical conformation in solution.

The surprising nonlinear optical properties of conjugated polyyne oligomers

Polyynes represent a unique class of conjugated organic compounds. The third-order nonlinear optical response of polyynes has been extensively modeled theoretically, and it is generally believed that the increase in molecular second hyperpolarizability (gamma) as a function of length for polyynes should be lower than that for polyenes. Experimental evidence to test this prediction, however, has been absent. We have synthesized conjugated polyynes that contain up to 20 consecutive sp-hybridized carbons, and we have determined their nonresonant gamma-values as a function of the number of acetylene repeat units (n). These gamma-values demonstrate a power-law behavior versus n(gamma approximately n(4.28+/-0.13)), with an exponent that is both larger than theoretically predicted for polyynes and substantially higher than that observed for polyenes or polyenynes. Furthermore, no saturation of the linear or nonlinear optical properties is observed.

Picosecond transient photoconductivity in functionalized pentacene molecular crystals probed by terahertz pulse spectroscopy

We have measured transient photoconductivity in functionalized pentacene molecular crystals using ultrafast optical pump-terahertz probe techniques. The single crystal samples were excited using 800 nm, 100 fs pulses, and the change in transmission of time-delayed, subpicosecond terahertz pulses was used to probe the photoconducting state over a temperature range from 10 to 300 K. A subpicosecond rise in photoconductivity is observed, suggesting that mobile carriers are a primary photoexcitation. At times longer than 4 ps, a power-law decay is observed consistent with dispersive transport.