Formulating Zwitterionic, Responsive Polymers for Designing Smart Soils.

The design of new remediation strategies and materials for treating saline-alkaline soils is of fundamental and practical importantance for many applications. Conventional soil remediation strategies mainly focus on the development of fertilizers or additives for water, nutrient, and heavy metal managements in soils, but they often overlook a soil sensing function for early detection of salinization/alkalization levels toward optimal and timely soil remediation. Here, new smart soils, structurally consisting of the upper signal soil and the bottom hygroscopic bed and chemically including zwitterionic, thermo-responsive poly(NIPAM-co-VPES) and poly(NIPAM-co-SBAA) aerogels in each soil layer are formulated. Upon salinization, the resultant smart soils exhibit multiple superior capacities for reducing the soil salinity and alkalinity through ion exchange, controlling the water cycling, modulating the degradation of pyridine-base ligands into water-soluble, nitrogenous salts-rich ingredients for soil fertility, and real-time monitoring salinized soils via pH-induced allochroic color changes. Further studies of plant growth in smart soils with or without salinization treatments confirm a synergy effect of soil remediation and soil sensing on facilitating the growth of plants and increasing the saline-alkaline tolerance of plants. The esign concept of smart soils can be further expanded for soil remediation and assessment.

Accurate Determination of the Quantity and Spatial Distribution of Counterions around a Spherical Macroion.

The accurate distribution of countercations (Rb+ and Sr2+ ) around a rigid, spherical, 2.9-nm size polyoxometalate cluster, {Mo132 }42- , is determined by anomalous small-angle X-ray scattering. Both Rb+ and Sr2+ ions lead to shorter diffuse lengths for {Mo132 } than prediction. Most Rb+ ions are closely associated with {Mo132 } by staying near the skeleton of {Mo132 } or in the Stern layer, whereas more Sr2+ ions loosely associate with {Mo132 } in the diffuse layer. The stronger affinity of Rb+ ions towards {Mo132 } than that of Sr2+ ions explains the anomalous lower critical coagulation concentration of {Mo132 } with Rb+ compared to Sr2+ . The anomalous behavior of {Mo132 } can be attributed to majority of negative charges being located at the inner surface of its cavity. The longer anion-cation distance weakens the Coulomb interaction, making the enthalpy change owing to the breakage of hydration layers of cations more important in regulating the counterion-{Mo132 } interaction.

Mechanism of UVA Degradation of Synthetic Eumelanin.

Eumelanin is a ubiquitous natural pigment that has a broad absorption across ultraviolet (UV, 100-400 nm) and visible wavelengths (400-700 nm) and can protect against radiation. Synthetic eumelanin with properties similar to natural eumelanin has been made using dopamine or dihydroxyindole. Here, we use solid-state nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy to elucidate the chemical structure of synthetic eumelanins (made from dopamine and l-3,4-dihydroxyphenylalanine precursors) and investigate how their structures change after intensive UVA (315-400 nm) exposure. We first confirm that polydopamine has indole units. Upon UV exposure, the pyrrole ring in this indole unit remains intact, and a fraction of the six-membered benzyl ring is broken and the indole potentially converted to furo[3,4-b]pyrrole. This change in the chemical structure is accompanied by a release of carbon dioxide. In addition, the sepia (natural) eumelanin used for comparison is more stable than the synthetic eumelanin. Understanding the UVA degradation mechanism of eumelanin will help reveal the role of eumelanin in skin cancer and in the design of more efficient UV stabilizers.

In situ infrared study of photo-generated electrons and adsorbed species on nitrogen-doped TiO2 in dye-sensitized solar cells.

Charge transfer between adsorbed dyes and the TiO2 surface plays a key role in controlling the efficiency of dye-sensitized solar cells (DSSCs). The lack of understanding of charge transfer steps has hindered further development of DSSCs and many solar energy conversion devices/processes. In this study, we used in situ infrared spectroscopy to investigate electron transfer and photo-electric energy conversion processes at the interface, i.e., surface hydroxyls, adsorbed species, as well as the dynamics of photo-generated electrons in TiO2 and N-TiO2 in DSSCs. Nitrogen (N-) doping of TiO2 blocked linear OH, giving more hydrophobic surface characteristics than undoped TiO2. N-Doping further increased the electron-hole separation caused by solar light on the working electrode and the current density in the DSSC. In situ infrared (IR) studies revealed that N-doping facilitated the electron transfer from the N719 dye (di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(ii)) to the conduction band in TiO2, reducing the impedance in the DSSC. Probing N-TiO2 with adsorbed ethanol showed that shallow traps in N-TiO2 can be accessed by electrons from adsorbed ethanol. Electron transfer from the N719 dye is significantly faster than that from adsorbed ethanol which involves C-H bond breaking.

A Spontaneous Structural Transition of {U24 Pp12 } Clusters Triggered by Alkali Counterion Replacement in Dilute Solution.

A transition between two isomeric clusters involving the change of the main skeleton structure of a well-defined, rigid molecular cluster [(UO2 )24 (O2 )24 (P2 O7 )12 ]48- , {U24 Pp12 }, is achieved by simply introducing proper alkali cations into its dilute aqueous solution. While the unique structural transition can be triggered by introducing any of the Na+ /K+ /Rb+ /Cs+ alkali ions, the two isomers, Li/Na-{U24 Pp12 } and Na/K-{U24 Pp12 }, as typical macroions, can accurately choose among different alkali counter-cations based on their hydrated sizes, and the ion selectivity process clearly showed endothermic features. The preferred K+ and Rb+ ions have suitable sizes to be incorporated into the proper windows on {U24 Pp12 } nanocapsules, as supported by the transition points in both ITC studies and IR measurements.

Ultrathin compound semiconductor on insulator layers for high-performance nanoscale transistors.

Over the past several years, the inherent scaling limitations of silicon (Si) electron devices have fuelled the exploration of alternative semiconductors, with high carrier mobility, to further enhance device performance. In particular, compound semiconductors heterogeneously integrated on Si substrates have been actively studied: such devices combine the high mobility of III-V semiconductors and the well established, low-cost processing of Si technology. This integration, however, presents significant challenges. Conventionally, heteroepitaxial growth of complex multilayers on Si has been explored-but besides complexity, high defect densities and junction leakage currents present limitations in this approach. Motivated by this challenge, here we use an epitaxial transfer method for the integration of ultrathin layers of single-crystal InAs on Si/SiO(2) substrates. As a parallel with silicon-on-insulator (SOI) technology, we use 'XOI' to represent our compound semiconductor-on-insulator platform. Through experiments and simulation, the electrical properties of InAs XOI transistors are explored, elucidating the critical role of quantum confinement in the transport properties of ultrathin XOI layers. Importantly, a high-quality InAs/dielectric interface is obtained by the use of a novel thermally grown interfacial InAsO(x) layer (~1 nm thick). The fabricated field-effect transistors exhibit a peak transconductance of ~1.6 mS µm(-1) at a drain-source voltage of 0.5 V, with an on/off current ratio of greater than 10,000.

Highly uniform and stable n-type carbon nanotube transistors by using positively charged silicon nitride thin films.

Air-stable n-doping of carbon nanotubes is presented by utilizing SiN(x) thin films deposited by plasma-enhanced chemical vapor deposition. The fixed positive charges in SiN(x), arising from (+)Si ≡ N3 dangling bonds induce strong field-effect doping of underlying nanotubes. Specifically, an electron doping density of ∼ 10(20) cm(-3) is estimated from capacitance voltage measurements of the fixed charge within the SiN(x). This high doping concentration results in thinning of the Schottky barrier widths at the nanotube/metal contacts, thus allowing for efficient injection of electrons by tunnelling. As a proof-of-concept, n-type thin-film transistors using random networks of semiconductor-enriched nanotubes are presented with an electron mobility of ∼ 10 cm(2)/V s, which is comparable to the hole mobility of as-made p-type devices. The devices are highly stable without any noticeable change in the electrical properties upon exposure to ambient air for 30 days. Furthermore, the devices exhibit high uniformity over large areas, which is an important requirement for use in practical applications. The work presents a robust approach for physicochemical doping of carbon nanotubes by relying on field-effect rather than a charge transfer mechanism.

MoS2 P-type transistors and diodes enabled by high work function MoOx contacts.

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

In situ infrared study of the effect of amine density on the nature of adsorbed CO2 on amine-functionalized solid sorbents.

In situ Fourier transform infrared spectroscopy was used to determine the nature of adsorbed CO2 on class I (amine-impregnated) and class II (amine-grafted) sorbents with different amine densities. Adsorbed CO2 on amine sorbents exists in the form of carbamate-ammonium ion pairs, carbamate-ammonium zwitterions, and carbamic acid. The adsorbed CO2 on high-amine density sorbents showed that the formation of ammonium ions correlates with the suppression of CH stretching intensities. An HCl probing technique was used to resolve the characteristic infrared bands of ammonium ions, clarifying that the band observed around 1498 cm(-1) is a combination of the deformation vibration of ammonium ion (NH3(+)) at 1508 and 1469 cm(-1) and the deformation vibration of NH in carbamate (NHCOO(-)) at 1480 cm(-1). Carbamate and carbamic acid on sorbents with low amine density desorbed at a rate faster than those on sorbents with high amine density after switching the flow from CO2 to Ar at 55 °C. Evaluation of the desorption temperature profiles showed that the temperature required to achieve the maximal desorption of CO2 (Tmax. des) increases with amine density. The adsorbed CO2 on sorbents with high amine density is stabilized via hydrogen bonding interactions with adjacent amine sites. These sorbents require higher temperature to desorb CO2 than those with low amine density.

Ballistic InAs nanowire transistors.

Ballistic transport of electrons at room temperature in top-gated InAs nanowire (NW) transistors is experimentally observed and theoretically examined. From length dependent studies, the low-field mean free path is directly extracted as ~150 nm. The mean free path is found to be independent of temperature due to the dominant role of surface roughness scattering. The mean free path was also theoretically assessed by a method that combines Fermi's golden rule and a numerical Schrödinger-Poisson simulation to determine the surface scattering potential with the theoretical calculations being consistent with experiments. Near ballistic transport (~80% of the ballistic limit) is demonstrated experimentally for transistors with a channel length of ~60 nm, owing to the long mean free path of electrons in InAs NWs.

High-performance single layered WSe2 p-FETs with chemically doped contacts.

We report high performance p-type field-effect transistors based on single layered (thickness, ∼0.7 nm) WSe(2) as the active channel with chemically doped source/drain contacts and high-κ gate dielectrics. The top-gated monolayer transistors exhibit a high effective hole mobility of ∼250 cm(2)/(V s), perfect subthreshold swing of ∼60 mV/dec, and I(ON)/I(OFF) of >10(6) at room temperature. Special attention is given to lowering the contact resistance for hole injection by using high work function Pd contacts along with degenerate surface doping of the contacts by patterned NO(2) chemisorption on WSe(2). The results here present a promising material system and device architecture for p-type monolayer transistors with excellent characteristics.

Nanoscale InGaSb heterostructure membranes on Si substrates for high hole mobility transistors.

As of yet, III-V p-type field-effect transistors (p-FETs) on Si have not been reported, due partly to materials and processing challenges, presenting an important bottleneck in the development of complementary III-V electronics. Here, we report the first high-mobility III-V p-FET on Si, enabled by the epitaxial layer transfer of InGaSb heterostructures with nanoscale thicknesses. Importantly, the use of ultrathin (thickness, ~2.5 nm) InAs cladding layers results in drastic performance enhancements arising from (i) surface passivation of the InGaSb channel, (ii) mobility enhancement due to the confinement of holes in InGaSb, and (iii) low-resistance, dopant-free contacts due to the type III band alignment of the heterojunction. The fabricated p-FETs display a peak effective mobility of ~820 cm(2)/(V s) for holes with a subthreshold swing of ~130 mV/decade. The results present an important advance in the field of III-V electronics.

Non-thermal plasma-assisted rapid hydrogenolysis of polystyrene to high yield ethylene.

The evergrowing plastic production and the caused concerns of plastic waste accumulation have stimulated the need for waste plastic chemical recycling/valorization. Current methods suffer from harsh reaction conditions and long reaction time. Herein we demonstrate a non-thermal plasma-assisted method for rapid hydrogenolysis of polystyrene (PS) at ambient temperature and atmospheric pressure, generating high yield (>40 wt%) of C1-C3 hydrocarbons and ethylene being the dominant gas product (Selectivity of ethylene, SC2H4 > 70%) within ~10 min. The fast reaction kinetics is attributed to highly active hydrogen plasma, which can effectively break bonds in polymer and initiate hydrogenolysis under mild condition. Efficient hydrogenolysis of post-consumer PS materials using this method is also demonstrated, suggesting a promising approach for fast retrieval of small molecular hydrocarbon modules from plastic materials as well as a good capability to process waste plastics in complicated conditions.

Patterned p-doping of InAs nanowires by gas-phase surface diffusion of Zn.

Gas phase p-doping of InAs nanowires with Zn atoms is demonstrated as an effective route for enabling postgrowth dopant profiling of nanostructures. The versatility of the approach is demonstrated by the fabrication of high-performance gated diodes and p-MOSFETs. High Zn concentrations with electrically active content of approximately 1 x 10(19) cm(-3) are achieved which is essential for compensating the electron-rich surface layers of InAs to enable heavily p-doped structures. This work could have important practical implications for the fabrication of planar and nonplanar devices based on InAs and other III-V nanostructures which are not compatible with conventional ion implantation processes that often cause severe lattice damage with local stoichiometry imbalance.

Improved Polydopamine Deposition in Amine-Functionalized Silica Aerogels for Enhanced UV Absorption.

Silica aerogels are interesting porous materials with extremely low density and high surface area, making them advantageous for a number of aerospace and catalysis applications. Here, we report the preparation of polydopamine (PDA)-functionalized silica aerogels using an in situ coating method, wherein the dopamine monomer was allowed to diffuse through the underlying structure of the gels in the absence of any external base and polymerize on the surface of the gel. The use of a siloxane precursor with an amine functionality decorates the silica backbone, allowing for a superior PDA coating, as evident in the darker color of PDA-coated amine-functionalized silica gels than PDA-coated silica-only gels and the X-ray photoelectron spectroscopy results. Furthermore, by varying the coating time, a series of aerogels with increasing optical absorption are prepared. Analyses using Brunauer-Emmett-Teller, scanning electron microscopy, and pycnometry show that the in situ PDA coating does not affect the inherent properties of the silica aerogels as opposed to PDA coatings deposited using an external base. Aerogels coated for 12 h and 24 h offer a surface area of 614 ± 35 and 658 ± 15 m2/g along with a porosity of 92.6 ± 0.9 and 92.4 ± 0.7%, respectively, properties similar to the native silica aerogels. PDA-coated aerogels have the potential to serve as UV ray mitigating materials due to the tortuosity of the underlying structure and the unique chemical properties of the PDA coating.

Tracing the reaction steps involving oxygen and IR observable species in ethanol photocatalytic oxidation on TiO2.

The rate-determining step of ethanol photocatalytic oxidation was identified to be the adsorption of O(2) by an infrared (IR) spectroscopy coupled with mass spectrometry method. Dosing O(2) during reaction showed that adsorption of O(2) controls the accumulation of photogenerated electrons and the formation of acetate (CH(3)COO(-)(ad)), acyl species (CH(3)CO(ad)), acetaldehyde (CH(3)CHO(ad)), CO(2), and H(2)O. Accumulation of CH(3)COO(-)(ad) on the TiO(2) surface slowed down the conversion of ethanol to CO(2) and H(2)O. Removal of CH(3)COO(-)(ad) from the TiO(2) surface holds the key to accelerating the rate of ethanol photocatalytic oxidation. This study bridges the gap between results of nanosecond and millisecond transient absorption studies and those of minute scale photocatalytic oxidation studies.

Direct growth of single-crystalline III-V semiconductors on amorphous substrates.

The III-V compound semiconductors exhibit superb electronic and optoelectronic properties. Traditionally, closely lattice-matched epitaxial substrates have been required for the growth of high-quality single-crystal III-V thin films and patterned microstructures. To remove this materials constraint, here we introduce a growth mode that enables direct writing of single-crystalline III-V's on amorphous substrates, thus further expanding their utility for various applications. The process utilizes templated liquid-phase crystal growth that results in user-tunable, patterned micro and nanostructures of single-crystalline III-V's of up to tens of micrometres in lateral dimensions. InP is chosen as a model material system owing to its technological importance. The patterned InP single crystals are configured as high-performance transistors and photodetectors directly on amorphous SiO2 growth substrates, with performance matching state-of-the-art epitaxially grown devices. The work presents an important advance towards universal integration of III-V's on application-specific substrates by direct growth.

Enhanced Thermoelectric Properties of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) by Binary Secondary Dopants.

To simultaneously increase the electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate ( PEDOT: PSS) was a challenge for realizing efficient organic thermoelectrics. In this study, for the first time, we report both increased electrical conductivities and Seebeck coefficients, hence, enhanced thermoelectric properties of PEDOT: PSS thin films by doped with binary secondary dopants, dimethyl sulfoxide (DMSO) and poly(ethylene oxide) (PEO). Without modifying film morphology, the molar ratios of PEDOT to PSS are tuned by PEO, resulting in increased proportions of PEDOT in the bipolaron states. Our study provides a facile route to optimizing thermoelectric properties of PEDOT: PSS thin films.

Effects of magnetic nanoparticles and external magnetostatic field on the bulk heterojunction polymer solar cells.

The price of energy to separate tightly bound electron-hole pair (or charge-transfer state) and extract freely movable charges from low-mobility materials represents fundamental losses for many low-cost photovoltaic devices. In bulk heterojunction (BHJ) polymer solar cells (PSCs), approximately 50% of the total efficiency lost among all energy loss pathways is due to the photogenerated charge carrier recombination within PSCs and low charge carrier mobility of disordered organic materials. To address these issues, we introduce magnetic nanoparticles (MNPs) and orientate these MNPS within BHJ composite by an external magnetostatic field. Over 50% enhanced efficiency was observed from BHJ PSCs incorporated with MNPs and an external magnetostatic field alignment when compared to the control BHJ PSCs. The optimization of BHJ thin film morphology, suppression of charge carrier recombination, and enhancement in charge carrier collection result in a greatly increased short-circuit current density and fill factor, as a result, enhanced power conversion efficiency.

In situ pulse diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) mass spectrometry study of the water-gas shift reaction on nickel(II) oxide-zinc(II) oxide catalysts.

The water-gas shift (WGS) reaction has been studied by pulsing carbon monoxide (CO) into a steady-state water (H2O)-Ar flow over nickel(II) oxide-zinc oxide (NiO-ZnO) catalysts using in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) coupled with a mass spectrometer method using the pulse technique (in situ pulse DRIFTS-MS) for different flow rates (gas hourly space velocity [GHSV] of 24,000-72,000 h(-1)) and reaction temperatures (250-350 °C). The results obtained from the in situ pulse DRIFTS-MS revealed that there are two types of water adsorption bands on the surface of the catalyst: (i) molecular adsorption (infrared [IR] bands in the 2500-3600 cm(-1) range and at 1640 cm(-1)), and (ii) dissociative adsorption at 3700 cm(-1), where carboxyl bands are formed at 1461 and 1368 cm(-1) and the gas-phase CO is adsorbed at 2187 and 2111 cm(-1) on the surface of the catalyst. After using a GHSV = 24,000 h(-1) H2O/Ar flow, we probed the existence of two active intermediates via the formation of two hydrogen production peaks. The products of hydrogen gas (H2) and carbon dioxide (CO2) had two pathways: the redox process and the associative process via the intermediate of the carboxyl group. In situ pulse DRIFTS-MS proves to be an effective approach for studying the nature of adsorbed species on the catalyst surface and the nature of the reaction product.