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Photoelectrochemical (PEC) water splitting to produce hydrogen fuel was first reported 50 years ago1, yet artificial photosynthesis has not become a widespread technology. Although planar Si solar cells have become a ubiquitous electrical energy source economically competitive with fossil fuels, analogous PEC devices have not been realized, and standard Si p-type/n-type (p-n) junctions cannot be used for water splitting because the bandgap precludes the generation of the needed photovoltage. An alternative paradigm, the particle suspension reactor (PSR), forgoes the rigid design in favour of individual PEC particles suspended in solution, a potentially low-cost option compared with planar systems2,3. Here we report Si-based PSRs by synthesizing high-photovoltage multijunction Si nanowires (SiNWs) that are co-functionalized to catalytically split water. By encoding a p-type-intrinsic-n-type (p-i-n) superlattice within single SiNWs, tunable photovoltages exceeding 10 V were observed under 1 sun illumination. Spatioselective photoelectrodeposition of oxygen and hydrogen evolution co-catalysts enabled water splitting at infrared wavelengths up to approximately 1,050 nm, with the efficiency and spectral dependence of hydrogen generation dictated by the photonic characteristics of the sub-wavelength-diameter SiNWs. Although initial energy conversion efficiencies are low, multijunction SiNWs bring the photonic advantages of a tunable, mesoscale geometry and the material advantages of Si-including the small bandgap and economies of scale-to the PSR design, providing a new approach for water-splitting reactors.
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Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.
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Stability and current-voltage hysteresis stand as major obstacles to the commercialization of metal halide perovskites. Both phenomena have been associated with ion migration, with anecdotal evidence that stable devices yield low hysteresis. However, the underlying mechanisms of the complex stability-hysteresis link remain elusive. Here we present a multiscale diffusion framework that describes vacancy-mediated halide diffusion in polycrystalline metal halide perovskites, differentiating fast grain boundary diffusivity from volume diffusivity that is two to four orders of magnitude slower. Our results reveal an inverse relationship between the activation energies of grain boundary and volume diffusions, such that stable metal halide perovskites exhibiting smaller volume diffusivities are associated with larger grain boundary diffusivities and reduced hysteresis. The elucidation of multiscale halide diffusion in metal halide perovskites reveals complex inner couplings between ion migration in the volume of grains versus grain boundaries, which in turn can predict the stability and hysteresis of metal halide perovskites, providing a clearer path to addressing the outstanding challenges of the field.
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Aryl diazonium electrografting is a powerful method for imparting molecular functionality onto various substrates by forming a stable carbon-surface covalent bond. While the high reactivity of the aryl radical intermediate makes this method fast and reliable, it can also lead to the formation of an insulating and disordered multilayer film. These thick films affect electrochemical performance, especially for semiconductor substrates used in photoelectrochemical applications. We studied the effects of film thickness and composition by electrografting in situ-generated aminobenzene diazonium salts onto both n-type and p-type silicon electrodes at fixed potentials. Next, we attached ferrocene to the amine-terminated films and probed their (photo)electrochemical behavior. Cyclic voltammetry measurements showed decreased electrochemical reversibility with increasing diazonium film thickness; this reversibility was restored when ferrocene was incorporated throughout the film with a layer-by-layer deposition process. Finally, we compared the behavior of dark p-type electrodes to n-type photoelectrodes and observed differences in the electrochemical reversibility that we attribute to the change in potential drop across the two interfaces.
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Graphene oxide (GO) has attracted substantial interest for its tunable properties and as a possible intermediate for the bulk manufacture of graphene. GO and its reduced derivatives display electronic and optical properties that depend strongly on their chemical structure, and with proper functionalization, GO can have a desirable bandgap for semiconductor applications. However, its chemical activity leads to a series of unclear chemical changes under ambient conditions, resulting in changes in color and solubility upon exposure to light. In this paper, we study the properties of fresh and spontaneously reduced GO under ambient conditions using tip-enhanced Raman spectroscopy (TERS) to map its nanometer scale chemical and structural heterogeneity. We observe different types of defect sites on reduced GO (rGO) by spatially mapping the D to G band peak ratio and D and G band spectral positions. The higher spatial resolution and out-of-plane polarization compared to conventional micro-Raman spectroscopy enables us to resolve unusual features, including D-band shifting on rGO. Based on statistical analysis of the spatial variations in modes and theoretical calculations for different functional groups, we conclude the reduction mechanism of GO is a self-photocatalytic reduction with the participation of water and visible light, in which the rate determining step is electron transport through the metal substrate and ion diffusion on the GO surface. These results demonstrate that TERS can reveal structural and chemical details elucidating reduction mechanisms, through the examination of samples at different time points.
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Per- and polyfluoroalkyl substances (PFASs), with their water- and heat-resistant properties, have been widely used in industrial and consumer products, including floor waxes. Adverse health effects are associated with PFAS exposures (e.g., increased risk of cancer and immunotoxicity); however, exposures resulting from the use of PFAS-containing products are poorly understood. This study examines PFAS emissions during professional floor stripping/waxing and their potential for occupational exposures. We measured PFASs in dust and airborne particulate matter (PM2.0, aerodynamic diameter ≤ 2.0 µm) before, during, and after floor stripping/waxing activities in three rooms in a university building. PM2.0 samples were analyzed for 34 targeted PFASs by ultra-high performance liquid chromatography coupled to electrospray ionization triple quadrupole mass spectrometer (UHPLC/ESI-MS/MS). In total, ten PFASs were detected in PM2.0 collected during floor waxing. Five were consistently higher during floor stripping/waxing compared to before (two with 95% confidence interval): perfluoro-2-methoxyacetic acid, perfluorobutanoic acid, perfluorohexanoic acid, perfluoroheptanoic acid, and perfluorooctane sulfonic acid. For these five, estimated exposures during floor stripping were 80.6, 320.5, 83.8, 29.6, and 157.7 pg m-3 per hour of floor stripping, respectively, one order of magnitude greater than typical residential indoor and two orders of magnitude greater than ambient outdoor concentrations. Estimated emission rates were 3.0, 9.6, 3.4, 1.5, and 6.5 ng h-1m-2, respectively (34.6% uncertainty). Inhalation occupational exposures were in the range of 9.42-23.2 pg per kg body weight per hour of floor stripping across the five PFASs.
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Water is essential to protein structure and stability, yet our understanding of how water shapes proteins is far from thorough. Our incomplete knowledge of protein-water interactions is due in part to a long-standing technological inability to assess experimentally how water removal impacts local protein structure. It is now possible to obtain residue-level information on dehydrated protein structures via liquid-observed vapor exchange (LOVE) NMR, a solution NMR technique that quantifies the extent of hydrogen-deuterium exchange between unprotected amide protons of a dehydrated protein and D2O vapor. Here, we apply LOVE NMR, Fourier transform infrared spectroscopy, and solution hydrogen-deuterium exchange to globular proteins GB1, CI2, and two variants thereof to link mutation-induced changes in the dehydrated protein structure to changes in solution structure and stability. We find that a mutation that destabilizes GB1 in solution does not affect its dehydrated structure, whereas a mutation that stabilizes CI2 in solution makes several regions of the protein more susceptible to dehydration-induced unfolding, suggesting that water is primarily responsible for the destabilization of the GB1 variant but plays a stabilizing role in the CI2 variant. Our results indicate that changes in dehydrated protein structure cannot be predicted from changes in solution stability alone and demonstrate the ability of LOVE NMR to uncover the variable role of water in protein stability. Further application of LOVE NMR to other proteins and their variants will improve the ability to predict and modulate protein structure and stability in both the hydrated and dehydrated states for applications in medicine and biotechnology.
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Proteínas de Bactérias/química , Peptídeos/química , Proteínas de Plantas/química , Água/química , Proteínas de Bactérias/genética , Hordeum/química , Mutação , Ressonância Magnética Nuclear Biomolecular/métodos , Peptídeos/genética , Proteínas de Plantas/genética , Estabilidade Proteica , Estrutura Secundária de Proteína , Staphylococcus/químicaRESUMO
Electrical scanning probe microscopies (SPM) use ultrasharp metallic tips to obtain nanometer spatial resolution and are a key tool for characterizing nanoscale semiconducting materials and systems. However, these tips are not passive probes; their high work functions can induce local band bending whose effects depend sensitively on the local geometry and material properties and thus are inherently difficult to quantify. We use sequential finite element simulations to first explore the magnitude and spatial distribution of charge reorganization due to tip-induced band bending (TIBB) for planar and nanostructured geometries. We demonstrate that tip-induced depletion and accumulation of carriers can be significantly modified in confined geometries such as nanowires compared to a bulk planar response. This charge reorganization is due to finite size effects that arise as the nanostructure size approaches the Debye length, with significant implications for a range of SPM techniques. We then use the reorganized charge distribution from our model to describe experimentally measured quantities, using in operando scanning microwave impedance microscopy measurements on axial p-i-n silicon nanowire devices as a specific example. By incorporating TIBB, we reveal that our experimentally observed enhancement (absence) of contrast at the p-i (i-n) junction is explained by the tip-induced accumulation (depletion) of carriers at the interface. Our results demonstrate that the inclusion of TIBB is critical for an accurate interpretation of electrical SPM measurements, and is especially important for weakly screening or low-doped materials, as well as the complex doping patterns and confined geometries commonly encountered in nanoscale systems.
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Tip-enhanced Raman spectroscopy (TERS) is a promising technique for structural studies of biological systems and biomolecules, owing to its ability to provide a chemical fingerprint with sub-diffraction-limit spatial resolution. This application of TERS has thus far been limited, due to difficulties in generating high field enhancements while maintaining biocompatibility. The high sensitivity achievable through TERS arises from the excitation of a localized surface plasmon resonance in a noble metal atomic force microscope (AFM) tip, which in combination with a metallic surface can produce huge enhancements in the local optical field. However, metals have poor biocompatibility, potentially introducing difficulties in characterizing native structure and conformation in biomolecules, whereas biocompatible surfaces have weak optical field enhancements. Herein, a novel, biocompatible, highly enhancing surface is designed and fabricated based on few-monolayer mica flakes, mechanically exfoliated on a metal surface. These surfaces allow the formation of coupled plasmon enhancements for TERS imaging, while maintaining the biocompatibility and atomic flatness of the mica surface for high resolution AFM. The capability of these substrates for TERS is confirmed numerically and experimentally. We demonstrate up to five orders of magnitude improvement in TERS signals over conventional mica surfaces, expanding the sensitivity of TERS to a wide range of non-resonant biomolecules with weak Raman cross-sections. The increase in sensitivity obtained through this approach also enables the collection of nanoscale spectra with short integration times, improving hyperspectral mapping for these applications. These mica/metal surfaces therefore have the potential to revolutionize spectromicroscopy of complex, heterogeneous biological systems such as DNA and protein complexes.
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Silicatos de Alumínio/química , Materiais Biocompatíveis/química , Ouro/química , Microscopia de Força Atômica/instrumentação , DNA/análise , Microscopia de Força Atômica/métodos , Análise Espectral Raman/métodos , Ressonância de Plasmônio de Superfície/métodosRESUMO
We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above â¼1 × 1019 cm-3. We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the nondestructive characterization of nanostructured material synthesis and local electronic properties without the need for contacts or inert atmosphere.
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Structure, dynamics, and coupling involving single-molecules determine function in catalytic, electronic or biological systems. While vibrational spectroscopy provides insight into molecular structure, rapid fluctuations blur the molecular trajectory even in single-molecule spectroscopy, analogous to spatial averaging in measuring large ensembles. To gain insight into intramolecular coupling, substrate coupling, and dynamic processes, we use tip-enhanced Raman spectroscopy (TERS) at variable and cryogenic temperatures, to slow and control the motion of a single molecule. We resolve intrinsic line widths of individual normal modes, allowing detailed and quantitative investigation of the vibrational modes. From temperature dependent line narrowing and splitting, we quantify ultrafast vibrational dephasing, intramolecular coupling, and conformational heterogeneity. Through statistical correlation analysis of fluctuations of individual modes, we observe rotational motion and spectral fluctuations of the molecule. This work demonstrates single-molecule vibrational spectroscopy beyond chemical identification, opening the possibility for a complete picture of molecular motion ranging from femtoseconds to minutes.
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Many phase transitions in correlated matter exhibit spatial inhomogeneities with expected yet unexplored effects on the associated ultrafast dynamics. Here we demonstrate the combination of ultrafast nondegenerate pump-probe spectroscopy with far from equilibrium excitation, and scattering scanning near-field optical microscopy (s-SNOM) for ultrafast nanoimaging. In a femtosecond near-field near-IR (NIR) pump and mid-IR (MIR) probe study, we investigate the photoinduced insulator-to-metal (IMT) transition in nominally homogeneous VO2 microcrystals. With pump fluences as high as 5 mJ/cm(2), we can reach three distinct excitation regimes. We observe a spatial heterogeneity on â¼50-100 nm length scales in the fluence-dependent IMT dynamics ranging from <100 fs to â¼1 ps. These results suggest a high sensitivity of the IMT with respect to small local variations in strain, doping, or defects that are difficult to discern microscopically. We provide a perspective with the distinct requirements and considerations of ultrafast spatiotemporal nanoimaging of phase transitions in quantum materials.
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We study the micro-Raman spectra of colloidal silicon nanocrystals as a function of size, excitation wavelength, and excitation intensity. We find that the longitudinal optical (LO) phonon spectrum is asymmetrically broadened toward the low energy side and exhibits a dip or antiresonance on the high-energy side, both characteristics of a Fano line shape. The broadening depends on both nanocrystal size and Raman excitation wavelength. We propose that the Fano line shape results from interference of the optical phonon response with a continuum of electronic states that become populated by intraband photoexcitation of carriers. The asymmetry exhibits progressive enhancement with decreasing particle size and with increasing excitation energy for a given particle size. We compare our observations with those reported for p- and n-doped bulk Si, where Fano interference has also been observed, but we find opposite wavelength dependence of the asymmetry for the bulk and nanocrystalline Si. Our results have important implications for potentially controlling carrier energy relaxation channels in strongly confined Si nanocrystals.
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With nanosecond radiative lifetimes, quenching dominates over enhancement for conventional fluorescence emitters near metal interfaces. We explore the fundamentally distinct behavior of photoluminescence (PL) with few-femtosecond radiative lifetimes of a coupled plasmonic emitter. Controlling the emitter-surface distance with subnanometer precision by combining atomic force and scanning tunneling distance control, we explore the unique behavior of plasmon dynamics at the transition from long-range classical resonant energy transfer to quantum coupling. Because of the ultrafast radiative plasmon emission, classical quenching is completely suppressed. Field-enhanced behavior dominates until the onset of quantum coupling dramatically reduces emission intensity and field enhancement, as verified in concomitant tip-enhanced Raman measurements. The entire distance behavior from tens of nanometers to subnanometers can be described using a phenomenological rate equation model and highlights the new degrees of freedom in radiation control enabled by an ultrafast radiative emitter near surfaces.
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We study the decrease in group velocity of broadband surface plasmon polariton propagation on a conical tip, using femtosecond time-domain interferometry. The group delay of (9±3) fs measured corresponds to a group velocity at the apex of less than 0.2c. The result agrees in general with the prediction from adiabatic plasmonic nanofocusing theory, yet is sensitive with respect to the exact taper geometry near the apex. This, together with the sub 25 fs(2) second-order dispersion observed, provides the fundamental basis for the use of plasmons for broadband slow-light applications.
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A sonochemical-based hydrosilylation method was employed to covalently attach a rhenium tricarbonyl phenanthroline complex to silicon(111). fac-Re(5-(p-Styrene)-phen)(CO)3Cl (5-(p-styrene)-phen = 5-(4-vinylphenyl)-1,10-phenanthroline) was reacted with hydrogen-terminated silicon(111) in an ultrasonic bath to generate a hybrid photoelectrode. Subsequent reaction with 1-hexene enabled functionalization of remaining atop Si sites. Attenuated total reflectance-Fourier transform infrared spectroscopy confirms attachment of the organometallic complex to silicon without degradation of the organometallic core, supporting hydrosilylation as a strategy for installing coordination complexes that retain their molecular integrity. Detection of Re(I) and nitrogen by X-ray photoelectron spectroscopy (XPS) further support immobilization of fac-Re(5-(p-styrene)-phen)(CO)3Cl. Cyclic voltammetry and electrochemical impedance spectroscopy under white light illumination indicate that fac-Re(5-(p-styrene)-phen)(CO)3Cl undergoes two electron reductions. Mott-Schottky analysis indicates that the flat band potential is 239 mV more positive for p-Si(111) co-functionalized with both fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene than when functionalized with 1-hexene alone. XPS, ultraviolet photoelectron spectroscopy, and Mott-Schottky analysis show that functionalization with fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene introduces a negative interfacial dipole, facilitating reductive photoelectrochemistry.
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The simultaneous nanometer spatial confinement and femtosecond temporal control of an optical excitation has been a long-standing challenge in optics. Previous approaches using surface plasmon polariton (SPP) resonant nanostructures or SPP waveguides have suffered from, for example, mode mismatch, or possible dependence on the phase of the driving laser field to achieve spatial localization. Here we take advantage of the intrinsic phase- and amplitude-independent nanofocusing ability of a conical noble metal tip with weak wavelength dependence over a broad bandwidth to achieve a 10 nm spatially and few-femtosecond temporally confined excitation. In combination with spectral pulse shaping and feedback on the second-harmonic response of the tip apex, we demonstrate deterministic arbitrary optical waveform control. In addition, the high efficiency of the nanofocusing tip provided by the continuous micro- to nanoscale mode transformation opens the door for spectroscopy of elementary optical excitations in matter on their natural length and time scales and enables applications from ultrafast nano-opto-electronics to single molecule quantum coherent control.
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Protein-based pharmaceuticals are increasingly important, but their inherent instability necessitates a "cold chain" requiring costly refrigeration during production, shipment, and storage. Drying can overcome this problem, but most proteins need the addition of stabilizers, and some cannot be successfully formulated. Thus, there is a need for new, more effective protective molecules. Cytosolically, abundant heat-soluble proteins from tardigrades are both fundamentally interesting and a promising source of inspiration; these disordered, monodisperse polymers form hydrogels whose structure may protect client proteins during drying. We used attenuated total reflectance Fourier transform infrared spectroscopy, differential scanning calorimetry, and small-amplitude oscillatory shear rheometry to characterize gelation. A 5% (wt/vol) gel has a strength comparable with human skin, and melts cooperatively and reversibly near body temperature with an enthalpy comparable with globular proteins. We suggest that the dilute protein forms α-helical coiled coils and increasing their concentration drives gelation via intermolecular ß-sheet formation.
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Dessecação , Tardígrados , Humanos , Animais , Espectroscopia de Infravermelho com Transformada de Fourier/métodos , Estrutura Secundária de Proteína , Varredura Diferencial de Calorimetria , ProteínasRESUMO
Real materials are disordered. This disorder influences the properties of these materials and the chemical processes that occur at their interfaces. Gaining a molecular-level understanding of the underlying physical manifestations caused by disordered materials is crucial to unraveling and ultimately controlling the efficiency and performance of these materials in a range of energy-related devices. This understanding necessitates measurement techniques through which disorder can be detected, quantified, and monitored. However, such quantitative measurements are notoriously difficult, as effects often average out in ensemble measurements. In this work, we describe how a combination of electrochemical and spatially resolved surface spectroscopy measurements illuminate a molecular-level picture of disorder in materials. Using amorphous carbon as an intrinsically disordered material, we covalently attached a monolayer of ferrocene. Interfacial electron transfer across the amorphous carbon-ferrocene interface is highly sensitive to disruptions of order. By systematically varying linker properties and surface loadings, the influence of lateral interactions between nonuniformly distributed ferrocene headgroups on ensemble electrochemical measurements is demonstrated. Electrochemical and imaging data collectively indicate that conformational flexibility of the ferrocene moieties provides a mechanism to elude repulsive and unbalanced lateral interactions, while rigid linkages provide direct information about the underlying disorder of the material. This study is the first of its kind to quantify and visualize molecular disorder and heterogeneity with an experimental model accessed through ensemble measurements.