Arrays of horizontal carbon nanotubes of controlled chirality grown using designed catalysts.

The semiconductor industry is increasingly of the view that Moore's law-which predicts the biennial doubling of the number of transistors per microprocessor chip-is nearing its end. Consequently, the pursuit of alternative semiconducting materials for nanoelectronic devices, including single-walled carbon nanotubes (SWNTs), continues. Arrays of horizontal nanotubes are particularly appealing for technological applications because they optimize current output. However, the direct growth of horizontal SWNT arrays with controlled chirality, that would enable the arrays to be adapted for a wider range of applications and ensure the uniformity of the fabricated devices, has not yet been achieved. Here we show that horizontal SWNT arrays with predicted chirality can be grown from the surfaces of solid carbide catalysts by controlling the symmetries of the active catalyst surface. We obtained horizontally aligned metallic SWNT arrays with an average density of more than 20 tubes per micrometre in which 90 per cent of the tubes had chiral indices of (12, 6), and semiconducting SWNT arrays with an average density of more than 10 tubes per micrometre in which 80 per cent of the nanotubes had chiral indices of (8, 4). The nanotubes were grown using uniform size Mo2C and WC solid catalysts. Thermodynamically, the SWNT was selectively nucleated by matching its structural symmetry and diameter with those of the catalyst. We grew nanotubes with chiral indices of (2m, m) (where m is a positive integer), the yield of which could be increased by raising the concentration of carbon to maximize the kinetic growth rate in the chemical vapour deposition process. Compared to previously reported methods, such as cloning, seeding and specific-structure-matching growth, our strategy of controlling the thermodynamics and kinetics offers more degrees of freedom, enabling the chirality of as-grown SWNTs in an array to be tuned, and can also be used to predict the growth conditions required to achieve the desired chiralities.

Twist-angle-dependent interlayer exciton diffusion in WS2-WSe2 heterobilayers.

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures have emerged as a platform for designing exciton superlattices. However, our understanding of the motion of excitons in moiré potentials is still limited. Here we investigated interlayer exciton dynamics and transport in WS2-WSe2 heterobilayers in time, space and momentum domains using transient absorption microscopy combined with first-principles calculations. We found that the exciton motion is modulated by twist-angle-dependent moiré potentials around 100 meV and deviates from normal diffusion due to the interplay between the moiré potentials and strong exciton-exciton interactions. Our experimental results verified the theoretical prediction of energetically favourable K-Q interlayer excitons and showed exciton-population dynamics that are controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. These results form a basis to investigate exciton and spin transport in van der Waals heterostructures, with implications for the design of quantum communication devices.

Phenethylammonium Functionalization Enhances Near-Surface Carrier Diffusion in Hybrid Perovskites.

Understanding semiconductor surface properties and manipulating them chemically are critical for improving their performance in optoelectronic devices. Hybrid halide perovskites have emerged as an exciting class of highly efficient solar materials; however, their device performance could be limited by undesirable surface properties that impede carrier transport and induce recombination. Here we show that surface functionalization of methylammonium lead iodide (MAPbI3) perovskite with phenethylammonium iodide (PEAI), a commonly employed spacer cation in two-dimensional halide perovskites, can enhance carrier diffusion in the near-surface regions and reduce defect density by more than 1 order of magnitude. Using transient transmission and reflection microscopy, we selectively imaged the transport of the carriers near the (001) surface and in the bulk for single-crystal MAPbI3 microplates. The surface functionalization increases the diffusion coefficient of the carriers in the 40 nm subsurface region from ∼0.6 cm2 s-1 to ∼1.0 cm2 s-1, similar to the value for bulk carriers. These results suggest the PEA ligands are effective in reducing surface defect and phonon scattering and shed light on the mechanisms for enhancing photophysical properties and improving solar cell efficiency.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals. These measurements demonstrate the charge transfer time around 10 ps and long-lived charge-separated state over the nanosecond time scale in two-dimensional ligand-perovskite heterostructures. The efficient charge transfer processes coupled with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in two-dimensional perovskite solar cells.

Selective Growth of Subnanometer Diameter Single-Walled Carbon Nanotube Arrays in Hydrogen-Free CVD.

Small diameter single-walled carbon nanotube (SWNT) arrays with larger bandgap are more desirable as near-infrared optical absorbers for the fabrication of high performance photovoltaic and photodetector devices. We report herein a rational approach to selective growth of well-aligned subnanometer diameter (∼84% between 0.75 and 0.95 nm) SWNT arrays with a density of 0.3-0.5 tubes/µm on quartz surfaces using solid Mo2C catalysts for short-time growth by low carbon feeding in hydrogen-free CVD. These subnanometer diameter SWNTs have a narrow chirality distribution (the ratio of (8,4), (8,5) and (7,6) is higher than 73%). During nanotube growth, only small size Mo nanoparticles are carbonized into stable Mo2C for catalyzing the growth of SWNTs through low carbon feeding rate over short time in the hydrogen-free environment, whereas larger catalysts are inactive due to underfeeding. Meanwhile, solid Mo2C catalysts are effective in reducing the chirality distributions of the as-grown SWNTs. Additionally, combining an annealing process after loading catalyst on the sapphire substrates, the average density is increased to ∼15 tubes/µm while maintaining small diameter and narrow chirality distribution. Our results offer more choices for structurally controlled growth of aligned-SWNTs, with potential applications in nanoelectronics.

Optical Anisotropy of Black Phosphorus in the Visible Regime.

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

Lighting up the Raman signal of molecules in the vicinity of graphene related materials.

Surface enhanced Raman scattering (SERS) is a popular technique to detect the molecules with high selectivity and sensitivity. It has been developed for 40 years, and many reviews have been published to summarize the progress in SERS. Nevertheless, how to make the SERS signals repeatable and quantitative and how to have deeper understanding of the chemical enhancement mechanism are two big challenges. A strategy to target these issues is to develop a Raman enhancement substrate that is flat and nonmetal to replace the conventional rough and metal SERS substrate. At the same time, the newly developed substrate should have a strong interaction with the adsorbate molecules to guarantee strong chemical enhancement. The flatness of the surface allows better control of the molecular distribution and configuration, while the nonmetal surface avoids disturbance of the electromagnetic mechanism. Recently, graphene and other two-dimensional (2D) materials, which have an ideal flat surface and strong chemical interaction with plenty of organic molecules, were developed to be used as Raman enhancement substrates, which can light up the Raman signals of the molecules, and these substrates were demonstrated to be a promising for microspecies or trace species detection. This effect was named "graphene enhanced Raman scattering (GERS)". The GERS technique offers significant advantages for studying molecular vibrations due to the ultraflat and chemically inert 2D surfaces, which are newly available, especially in developing a quantitative and repeatable signal enhancement technique, complementary to SERS. Moreover, GERS is a chemical mechanism dominated effect, which offers a valuable model to study the details of the chemical mechanism. In this Account, we summarize the systematic studies exploring the character of GERS. In addition, as a practical technique, the combination of GERS with a metal substrate incorporates the advantages from both conventional SERS and GERS. The introduction of graphene to the Raman enhancement substrate extended SERS applications in a more controllable and quantitative way. Looking to the future, we expect the combination of the SERS concept with the GERS technology to lead to the solution of some important issues in chemical dynamics and in biological processes monitoring.

Tuning the redox activity of encapsulated metal clusters via the metallic and semiconducting character of carbon nanotubes.

We demonstrate that reactions confined within single-walled carbon nanotube (SWCNT) channels are modulated by the metallic and semiconducting character of the hosts. In situ Raman and X-ray absorption near-edge structure spectroscopies provide complementary information about the electronic state of carbon nanotubes and the encapsulated rhenium species, which reveal electronic interactions between encapsulated species and nanotubes. More electrons are transferred from metallic tubes (m-SWCNTs) to oxidic rhenium clusters, leading to a lower valence state rhenium oxide than that in semiconducting tubes (s-SWCNTs). Reduction in 3.5% (vol/vol) H2/Ar leads to weakened host-guest electronic interaction. The high valence state Re within s-SWCNTs is more readily reduced when raising the temperature, whereas only a sluggish change is observed for Re within m-SWCNTs. Only at 400 °C does Re reach a similar electronic state (mixture of Re(0) and Re(4+)) in both types of tubes. Subsequent oxidation in 1% O2/Ar does not show changes for Re in s-SWCNTs up to 200 °C. In comparison, m-SWCNTs facilitate the oxidation of reduced rhenium (160 °C). This can be exploited for rational design of active catalysts with stable species as a desired valence state can be obtained by selecting specific-type SWCNTs and a controlled thermal treatment. These results also provide a chemical approach to modulate reversibly the electronic structure of SWCNTs without damaging the sidewalls of SWCNTs.

Boosting exciton mobility approaching Mott-Ioffe-Regel limit in Ruddlesden-Popper perovskites by anchoring the organic cation.

Exciton transport in two-dimensional Ruddlesden-Popper perovskite plays a pivotal role for their optoelectronic performance. However, a clear photophysical picture of exciton transport is still lacking due to strong confinement effects and intricate exciton-phonon interactions in an organic-inorganic hybrid lattice. Herein, we present a systematical study on exciton transport in (BA)2(MA)n-1PbnI3n+1 Ruddlesden-Popper perovskites using time-resolved photoluminescence microscopy. We reveal that the free exciton mobilities in exfoliated thin flakes can be improved from around 8 cm2 V-1 s-1 to 280 cm2V-1s-1 by anchoring the soft butyl ammonium cation with a polymethyl methacrylate network at the surface. The mobility of the latter is close to the theoretical limit of Mott-Ioffe-Regel criterion. Combining optical measurements and theoretical studies, it is unveiled that the polymethyl methacrylate network significantly improve the lattice rigidity resulting in the decrease of deformation potential scattering and lattice fluctuation at the surface few layers. Our work elucidates the origin of high exciton mobility in Ruddlesden-Popper perovskites and opens up avenues to regulate exciton transport in two-dimensional materials.

Preparation and Physico-Chemical Characterization of OSA-Modified Starches from Different Botanical Origins and Study on the Properties of Pickering Emulsions Stabilized by These Starches.

Native starch (NS) from different botanical origins (native rice/tapioca/oat starch, NRS/NTS/NOS) were hydrophobically modified by octenyl succinic anhydride (OSA), and the octenyl succinic (OS) groups were successfully introduced in the starch molecules which obtained OS-starch (OSRS, OSTS and OSOS) with different levels of modification (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%) and degree of substitution (DS). The structural properties of the OS-starch, such as granule size, crystal, wettability and morphology were studied, and the OS-starch was used as particulate stabilizers to produce oil-in-water (O/W) Pickering emulsions. The emulsion index, droplet size distribution and microstructures of Pickering emulsions produced by different OS-starches were compared. OSA modification had almost no effect on the morphology or crystal structure types of three kinds of NS and OS-starch but markedly increased the contact angle and particle size distribution of OSRS, OSTS and OSOS. Esterification reaction of OSA and starch mainly occurred in amorphous regions of starch, and the OSA significantly improved the emulsifying capacity of OSRS, OSTS and OSOS granules and thus stabilized emulsions formed at higher levels (2.5% and 3.0%) of modification of OS-Starch exhibited better stability; the ability of OS-starch to stabilize Pickering emulsion was 3.0% OSRS > 3.0% OSOS > 3.0% OSTS, respectively. Observation and structural properties analysis of OS-starch granules and Pickering emulsion droplets showed that the number and thickness of the starch granules on the oil-water interface of the emulsion droplets increased with improvement of the OSA modification level, and an aggregation state was formed between the OS-starch granules, which was also enhanced with the OSA modification levels. These were all necessary for the Pickering emulsion stabilized by starch granules to remain in a steady state.

Reduced asynchronism between regenerative cysteine and fragments of deoxyosones promoting formation of sulfur-containing compounds through extra-added xylose and elevated temperature during thermal processing of 2­threityl-thiazolidine-4-carboxylic acid.

The flavor intensity of thermally processed 2­threityl-thiazolidine-4-carboxylic acid (TTCA) was significantly improved to 1.56 times of that generated from MRPs, but its flavor profile was not as desirable as that of fresh MRPs. The synergistic effect between the additional xylose (Xyl) and elevated temperature was proposed and confirmed via the quantitative analyses of regenerative cysteine (Cys) and fragments of deoxyosones (MGO/GO), which reduced the asynchronism between the formation of released Cys from degraded TTCA and retro-aldolisation products of the intermediate deoxyosones. This synergistic effect further enhanced the Strecker degradation of Cys as well as its thermal degradation and thereby promoted the formation of characteristic flavor substances including sulfur-containing compounds and pyrazines, and the total concentrations of TTCA reaction model reached 205.954 µg/L with additional Xyl at 140 °C. Model reaction systems were employed to verify this hypothesis and the proposed mechanism was further elucidated through isotope labeling technique.

Strong Dipolar Repulsion of One-Dimensional Interfacial Excitons in Monolayer Lateral Heterojunctions.

Repulsive and long-range exciton-exciton interactions are crucial for the exploration of one-dimensional (1D) correlated quantum phases in the solid state. However, the experimental realization of nanoscale confinement of a 1D dipolar exciton has thus far been limited. Here, we demonstrate atomically precise lateral heterojunctions based at transitional-metal dichalcogenides (TMDCs) as a platform for 1D dipolar excitons. The dynamics and transport of the interfacial charge transfer excitons in a type II WSe2-WS1.16Se0.84 lateral heterostructure were spatially and temporally imaged using ultrafast transient reflection microscopy. The expansion of the exciton cloud driven by dipolar repulsion was found to be strongly density dependent and highly anisotropic. The interaction strength between the 1D excitons was determined to be ∼3.9 × 10-14 eV cm-2, corresponding to a dipolar length of 310 nm, which is a factor of 2-3 larger than the interlayer excitons at two-dimensional van der Waals vertical interfaces. These results suggest 1D dipolar excitons with large static in-plane dipole moments in lateral TMDC heterojunctions as an exciting system for investigating quantum many-body physics.

Exciton annihilation in molecular aggregates suppressed through quantum interference.

Exciton-exciton annihilation (EEA), an important loss channel in optoelectronic devices and photosynthetic complexes, has conventionally been assumed to be an incoherent, diffusion-limited process. Here we challenge this assumption by experimentally demonstrating the ability to control EEA in molecular aggregates using the quantum phase relationships of excitons. We employed time-resolved photoluminescence microscopy to independently determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates featuring contrasting excitonic phase envelopes. Low-temperature EEA rates were found to differ by more than two orders of magnitude for the two compounds, despite comparable diffusion constants. Simulated rates based on a microscopic theory, in excellent agreement with experiments, rationalize this EEA behaviour based on quantum interference arising from the presence or absence of spatial phase oscillations of delocalized excitons. These results offer an approach for designing molecular materials using quantum interference where low annihilation can coexist with high exciton concentrations and mobilities.

Comparison of pyrazines formation in methionine/glucose and corresponding Amadori rearrangement product model.

The generation of pyrazines in a binary methionine/glucose (Met/Glc) mixture and corresponding methionine/glucose-derived Amadori rearrangement product (MG-ARP) was studied. Quantitative analyses of pyrazines and methional revealed that MG-ARP generated more methional compared to Met/Glc, whereas lower content and fewer species of pyrazines were observed in the MG-ARP model. Comparing the availability of α-dicarbonyl compounds generated from the Met/Glc model, methylglyoxal (MGO) was a considerably effective α-dicarbonyl compound for the formation of pyrazines during MG-ARP degradation, but glyoxal (GO) produced from MG-ARP did not effectively participate in the corresponding formation of pyrazines due to the asynchrony on the formation of GO and recovered Met. Diacetyl (DA) content was not high enough to form corresponding pyrazines in the MG-ARP model. The insufficient interaction of precursors and rapid drops in pH limited the formation of pyrazines during MG-ARP degradation. Increasing reaction temperature could reduce the negative inhibitory effect by promoting the content of precursors.

Mechanism of Pyrazine Formation Intervened by Oxidized Methionines during Thermal Degradation of the Methionine-Glucose Amadori Compound.

Methionine (Met) oxidation was observed during thermal degradation of methionine/glucose-derived Amadori rearrangement product (MG-ARP). The effects of oxidized methionine products, methionine sulfoxide (MetSO) and methionine sulfone (MetSO2), on pyrazine yields of the MG-ARP model were investigated. The pyrazine contents in the MG-ARP/Met and MG-ARP/MetSO models were found lower compared to those in the MG-ARP/MetSO2 model, and the inefficiency of pyrazine formation in the MG-ARP/Met model was proposed due to the fact that Met oxidation competitively inhibited the oxidation of dihydropyrazines for pyrazine formation in spite of relatively high methylglyoxal (MGO) content. The models of MGO mixed with Met, MetSO, or MetSO2 were established for further investigation of the mechanism for the involvement of Met oxidation in pyrazine formation. It was observed that the aldolization or carbonyl-amine reaction of MetSO with MGO was another important reason for the inhibition of pyrazine formation, except for the competitive inhibition of oxidative formation of MetSO on dihydropyrazine oxidation, and the adduct of MGO-MetSO was identified by MS/MS. These results also accounted for the phenomenon of low pyrazine yields but high yields of long-chain substituted pyrazines, which were converted from dihydropyrazines with the aldehyde involvement.

Soft-lock drawing of super-aligned carbon nanotube bundles for nanometre electrical contacts.

The assembly of single-walled carbon nanotubes (CNTs) into high-density horizontal arrays is strongly desired for practical applications, but challenges remain despite myriads of research efforts. Herein, we developed a non-destructive soft-lock drawing method to achieve ultraclean single-walled CNT arrays with a very high degree of alignment (angle standard deviation of ~0.03°). These arrays contained a large portion of nanometre-sized CNT bundles, yielding a high packing density (~400 µm-1) and high current carrying capacity (∼1.8 × 108 A cm-2). This alignment strategy can be generally extended to diverse substrates or sources of raw single-walled CNTs. Significantly, the assembled CNT bundles were used as nanometre electrical contacts of high-density monolayer molybdenum disulfide (MoS2) transistors, exhibiting high current density (~38 µA µm-1), low contact resistance (~1.6 kΩ µm), excellent device-to-device uniformity and highly reduced device areas (0.06 µm2 per device), demonstrating their potential for future electronic devices and advanced integration technologies.

Degradation of 2-Threityl-Thiazolidine-4-Carboxylic Acid and Corresponding Browning Accelerated by Trapping Reaction between Extra-Added Xylose and Released Cysteine during Maillard Reaction.

2-Threityl-thiazolidine-4-carboxylic acid (TTCA), a nonvolatile precursor of flavor and color, is considered to be more stable than its isomeric Amadori compound (ARP). The degradation behavior of TTCA favors higher temperatures and pH. In order to adjust and control the thermal degradation of TTCA to improve its food processing adaptability, a TTCA-Xyl thermal reaction model was constructed to explore the effect of extra-added Xyl on the thermal degradation behavior of TTCA. The results confirmed that the extra-added Xyl was involved in the degradation pathway of TTCA and accelerated its depletion, thus promoting the formation of characteristic downstream products of TTCA including some α-dicarbonyl compounds, and consequently accelerating the browning formation. The isotope-labeling technique was further applied to confirm that the added Xyl could trap the Cys released from the decomposition of ARP and formed additional TTCA, which could promote the movement of chemical equilibrium and gradually accelerate the degradation rate of TTCA as well as melanoidins formation. The higher pH value could even promote this phenomenon.

Effect of Methionine on the Thermal Degradation of N-(1-Deoxy-d-fructos-1-yl)-methionine Affecting Browning Formation.

The effect of additional dl-methionine (Met) on the thermal degradation of a methionine-glucose-derived Amadori rearrangement product (MG-ARP) was investigated under different reaction conditions. The resulting color formation and changes in the concentrations of MG-ARP, Met, and α-dicarbonyl compounds were investigated. Additional Met did not affect the degradation rate of MG-ARP but got involved in subsequent reactions and resulted in a decrease in the contents of C6-α-dicarbonyl compounds. During MG-ARP degradation, the formation of glyoxal (GO) and methylglyoxal (MGO) was facilitated by additional Met, through retro-aldolization reaction of C6-α-dicarbonyl compounds. This effect of Met addition was dependent on the reaction temperature, and the consistent conclusion could be made in a buffer system. The improvement of GO and MGO formation as color precursors caused by the additional Met contributed to the acceleration of browning formation.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites.

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm2 s-1, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Transformation between 2-Threityl-thiazolidine-4-carboxylic Acid and Xylose-Cysteine Amadori Rearrangement Product Regulated by pH Adjustment during High-Temperature Instantaneous Dehydration.

2-Threityl-thiazolidine-4-carboxylic acid (TTCA) was found to be the predominant product rather than the Amadori rearrangement product (ARP) during the formation of xylose-cysteine-derived (Xyl-Cys-derived) Maillard reaction intermediates (MRIs) through a thermal reaction coupled with vacuum dehydration. To regulate the existence forms of Xyl-Cys-derived MRIs, an effective method carried out by pH adjustment during high-temperature instantaneous dehydration through spray-drying was proposed in this research to promote the conversion from TTCA to ARP. The increased inlet air temperature of spray-drying could properly facilitate the shift of chemical equilibrium between the MRIs and promote the transformation from TTCA to ARP while raising the total yield of TA (TTCA + ARP). The conversion to ARP was increased to 20.83% at 190 °C of hot blast compared to the product without spray-drying (6.03%). The conversion from TTCA to ARP was further facilitated in the pH range of 7.5-9.5. When the pH of the aqueous reactants was adjusted to 9.5, the equilibrium conversion to ARP was improved to 47.23% after spray-drying, which accounted for 59.48% of the TA formation. Accordingly, MRIs with different TTCA/ARP proportions could be selectively obtained by pH adjustment of the stock solution during high-temperature instantaneous dehydration of spray-drying.