Water splitting with silicon p-i-n superlattices suspended in solution.

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.

Electrostatic tip effects in scanning probe microscopy of nanostructures.

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.

Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy.

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 ∼1 × 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.

Interplay of Surface Recombination and Diode Geometry for the Performance of Axial p-i-n Nanowire Solar Cells.

Nanowires (NWs) with axial p-i-n junctions have been widely explored as microscopic diodes for optoelectronic and solar energy applications, and their performance is strongly influenced by charge recombination at the surface. We delineate how the photovoltaic performance of these diodes is dictated not only by the surface but also by the complex and seemingly counterintuitive interplay of diode geometry, that is, radius ( R) and intrinsic length ( Li), with the surface recombination velocity ( S). An analytical model to describe these relationships is developed and compared to finite-element simulations, which verify the accuracy and limitations of the model. The dependence of the dark saturation current ( I0), internal quantum efficiency (IQE), short-circuit current ( ISC), and open-circuit voltage ( VOC) on both geometric and recombination parameters demonstrates that no single set of parameters produces optimal performance; instead, various trade-offs in performance are observed. For instance, longer Li might be expected to produce higher ISC, yet at high values of S the ISC declines because of decreases in IQE. Moreover, longer Li produces a concurrent decline in VOC regardless of S due to increases in I0. We also find that ISC and VOC trends are radius independent, yet I0 is directly proportional to R, causing NWs with smaller R to display higher turn-on voltages. The analysis regarding the interplay of these parameters, verified by experimental measurements with various p-i-n geometries and surface treatments, provides clear guidance for the rational design of performance metrics for photodiode and photovoltaic devices.

Synthesis and structure-property evaluation of cellulose ω-carboxyesters for amorphous solid dispersions.

The use of amorphous solid dispersions (ASDs) is an effective and increasingly widely used approach for solubility enhancement of drugs and drug candidates with poor aqueous solubility. Successful molecular dispersion of drugs in polymer matrices requires new polymers that are designed to meet all ASD requirements, including drug release and prevention of drug recrystallization in storage or from solution. We describe herein design and synthesis of a new series of cellulose ω-carboxyalkanoates for ASDs, by reaction of cellulose with long-chain diacids that have been monoprotected as benzyl esters at one end, and monoactivated as acid chlorides at the other. Glass transition temperatures (Tg) of these cellulose ω-carboxyesters exceed ambient temperature by at least 50 °C, providing a sufficient ΔT to prevent drug mobility and crystallization. Cellulose acetate suberates and sebacates prepared in this way are extraordinary solution crystal growth inhibitors for the poorly soluble anti-HIV drug ritonavir. These new cellulose ω-carboxyesters have strong potential as ASD polymers for enhancement of drug solubility and bioavailability.