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A dual-photoelectrodedevice, consisting of a photoanode and photocathode with complementaryenergy bandgaps, has long been perceived as an ideal scheme for achievinghigh efficiency, unassisted solar-driven water splitting. Previouslyreported 2-photon tandem devices, however, generally exhibit an extremelylow efficiency (<0.1%), which has been largely limited by the incompatibilitybetween the two photoelectrode materials. Here we show that the useof metal-nitride nanowire photoelectrodes, together with the schemeof parallel illumination by splitting the solar spectrum spatiallyand spectrally, can break the efficiency bottleneck of conventional2-photon tandem devices. We have first investigated a dual-photoelectrodedevice consisting of a GaN nanowire photoanode and an InGaN nanowirephotocathode, which exhibited an open circuit potential of 1.3 V andnearly 20-fold enhancement in the power conversion efficiency undervisible light illumination (400–600 nm), compared to the individualphotoelectrodes in 1 mol/L HBr. We have further demonstrated a dual-photoelectrodedevice consisting of parallel-connected metal-nitride nanowire photoanodesand a Si/InGaN nanowire photocathode, which can perform unassisted,direct solar-to-hydrogen conversion. A power conversion efficiencyof 2% was measured under AM1.5G 1 sun illumination. [ABSTRACT FROM AUTHOR]

The guest cation preintercalation strategy has been widely adopted to improve the performance of zinc-vanadium batteries. However, existing studies always ignore the deintercalation of guest cations. This work focuses on the severe and universal deintercalation phenomenon and confirms the unaltered capacity after deintercalation, indicating that the capacity improvement mechanism cannot be attributed to the role of guest cations. Therefore, after excluding all of the previously researched factors for capacity improvement, the decisive factor is identified as the morphology (surface area). Based on the electrochemically active surface area (ECSA), a quantitative relationship with intrinsic capacity is established for the first time. This guides us to enhance battery capacity via enhancing ECSA through liquid-phase ultrasonic crushing to achieve the highest capacity of cation-preintercalated V 2 O 5 · n H 2 O (333.7 mAh g -1 at 10 A g -1 ). We believe that the enhanced ECSA is a plausible explanation for the improved performance of hydrated vanadium oxides.

Chronic obstructive pulmonary disease (COPD), the third leading cause of death worldwide, is caused by chronic exposure to toxic particles and gases, such as cigarette smoke. Free radicals, which are produced during a stress response to toxic particles, play a crucial role in disease progression. Measuring these radicals is difficult since the complex mixture of chemicals within cigarette smoke interferes with radical detection. We used a new quantum sensing technique called relaxometry to measure free radicals with nanoscale resolution on cells from COPD patients and healthy controls exposed to cigarette smoke extract (CSE) or control medium. Epithelial cells from COPD patients display a higher free radical load than those from healthy donors and are more vulnerable to CSE. We show that epithelial cells of COPD patients are more susceptible to the damaging effects of cigarette smoke, leading to increased release of free radicals.

Increasing near-field radiative heat transfer between two bodies separated by a vacuum gap is crucial for enhancing the power density in radiative energy transport and conversion devices. However, the largest radiative heat transfer coefficient between two realistic materials at room temperature is limited to around 2000 W/(m 2 ·K) for a gap of 100 nm. Here, analogous to conventional plate-fin heat exchangers based on convection, we introduce the concept of a nanophotonic heat exchanger, which enhances near-field radiative heat transfer using two bodies with interpenetrating gratings. Our calculations, based on rigorous fluctuational electrodynamics, show that the radiative heat transfer coefficient between the bodies separated by a 100 nm gap can significantly exceed 2000 W/(m 2 ·K) by increasing the aspect ratios of the gratings. We develop a semianalytical heat transfer model that agrees well with the rigorous calculations for design optimization. Our work opens new opportunities for enhancing near-field radiative heat transfer between any materials.

Tuning the interfacial Schottky barrier with van der Waals (vdW) contacts is an important solution for two-dimensional (2D) electronics. Here we report that the interlayer dipoles of 2D vdW superlattices (vdWSLs) can be used to engineer vdW contacts to 2D semiconductors. A bipolar WSe 2 with Ba 6 Ta 11 S 28 (BTS) vdW contact was employed to exhibit this strategy. Strong interlayer dipoles can be formed due to charge transfer between the Ba 3 TaS 5 and TaS 2 layers. Mechanical exfoliation breaks the superlattice and produces two distinguished surfaces with TaS 2 and Ba 3 TaS 5 terminations. The surfaces thus have opposite surface dipoles and consequently different work functions. Therefore, all the devices fall into two categories in accordance with the rectifying direction, which were verified by electrical measurements and scanning photocurrent microscopy. The growing vdWSL family along with the addition surface dipoles enables prospective vdW contact designs and have practical application in nanoelectronics and nano optoelectronics.

Three-dimensional (3D) hanging drop cell culture is widely used in organoid culture because of its lack of selection pressure and rapid cell aggregation. However, current hanging drop technology has limitations, such as a dependence on complex microfluidic transport channels or specific capillary force templates for drop formation, which leads to unchangeable drop features. These methods also hinder live imaging because of space and complexity constraints. Here, we have developed a hanging drop construction method and created a flexible 3D hanging drop construction platform composed of a manipulation module and an adhesion module. Their harmonious operation allows for the easy construction of hanging drops of varying sizes, types, and patterns. Our platform produces a cell hanging drop chip with small sizes and clear fields of view, thereby making it compatible with live imaging. This platform has great potential for personalized medicine, cancer and drug discovery, tissue engineering, and stem cell research.

Bismuth and bismuth-based compounds have been extensively studied as anodes as prospective candidates for rechargeable magnesium batteries (rMBs). However, the unsatisfactory magnesium-storage capability caused by the typical alloying reaction mechanism severely restricts the practical option for anodes in rMBs. Herein, polyaniline intercalated Bi 2 O 2 CO 3 nanosheets are prepared by an effective interlayer engineering strategy to fine-tune the layer structure of Bi 2 O 2 CO 3 , achieving enhanced magnesium-storage capacity, rate performance, as well as long cycle life. Excitedly, a stepwise insertion-conversion-alloying reaction is aroused to stabilize the performance, which is elucidated by in / ex situ investigations. Moreover, first-principles calculations confirm that the coupling of Bi 2 O 2 CO 3 and polyaniline not only increases the conductivity induced by the strong density of states and the interior self-built-in electric field but also significantly reduces the energy barrier of Mg shuttles. Our findings shed light on exploring new electrode materials with an appropriate working mechanism toward high-performance rechargeable batteries.

Single-atom nanozymes (SAzymes) open new possibilities for the development of artificial enzymes that have catalytic activity comparable to that of natural peroxidase (POD). So far, most efforts have focused on the structural modulation of the Fe-N 4 moiety to mimic the metalloprotein heme center. However, non-heme-iron POD with much higher activity, for example, HppE, has not been mimicked successfully due to its structural complexity. Herein, carbon dots (CDs)-supported SAzymes with twisted, nonplanar Fe-O 3 N 2 active sites, highly similar to the non-heme iron center of HppE, was synthesized by exploiting disordered and subnanoscale domains in CDs. The Fe-CDs exhibit an excellent POD activity of 750 units/mg, surpassing the values of conventional SAzymes with planar Fe-N 4 . We further fabricated an activatable Fe-CDs-based therapeutic agent with near-infrared enhanced POD activity, a photothermal effect, and tumor-targeting ability. Our results represent a big step in the design of high-performance SAzymes and provide guidance for future applications for synergistic tumor therapy.

Membrane morphology and its dynamic adaptation regulate many cellular functions, which are often mediated by membrane proteins. Advances in DNA nanotechnology have enabled the realization of various protein-inspired structures and functions with precise control at the nanometer level, suggesting a viable tool to artificially engineer membrane morphology. In this work, we demonstrate a DNA origami cross (DOC) structure that can be anchored onto giant unilamellar vesicles (GUVs) and subsequently polymerized into micrometer-scale reconfigurable one-dimensional (1D) chains or two-dimensional (2D) lattices. Such DNA origami-based networks can be switched between left-handed (LH) and right-handed (RH) conformations by DNA fuels and exhibit potent efficacy in remodeling the membrane curvatures of GUVs. This work sheds light on designing hierarchically assembled dynamic DNA systems for the programmable modulation of synthetic cells for useful applications.

Here, we report a novel, feasible, and cost-effective method for the preparation of one-dimensional TiO 2 nanowire arrays using a super-aligned carbon nanotube film as a template. Pure-anatase-phase TiO 2 nanowires were scalably prepared in a suspended manner, and a high-performance ultraviolet (UV) photodetector was realized on a flexible substrate. The large surface area and one-dimensional nanostructure of the TiO 2 nanowire array led to a high detectivity (1.35 × 10 16 Jones) and an ultrahigh photo gain (2.6 × 10 4 ), respectively. A high photoresponsivity of 7.7 × 10 3 A/W was achieved under 7 μW/cm 2 UV (λ = 365 nm) illumination at a 10 V bias voltage, which is much higher than those of commercial UV photodetectors. Additionally, by taking advantage of its anisotropic geometry, we found the TiO 2 nanowire array showed polarized photodetection. The concept of using nanomaterial systems shows the potential for realization of nanostructured photodetectors for practical applications.

Low-dimensional materials are bringing significant innovations to in situ TEM characterization. Here a new graphene microheater chip for TEM was developed by stacking graphene on a suspended SiN x membrane as the Joule heating element. It could be heated up to 800 °C within 26.31 ms with a low power consumption of 0.025 mW/1000 μm 2 . The bulging was only ∼50 nm at 650 °C, which is 2 orders of magnitude smaller than those of conventional MEMS heaters at similar temperatures. The performances benefit from the employment of graphene, since its monolayer structure greatly reduces the heat capacity, and the vdW contact significantly reduces the interfacial interaction. The TEM observation on the Sn melting process verifies its great potential in resolving thermodynamic processes. Moreover, more multifunctional in situ chips could be developed by integrating other stimuli to such chips. This work opens a new frontier for both graphene and in situ characterization techniques.

The broad application of silicon-based materials is limited by large volume fluctuation, high preparation costs, and complicated preparation processes. Here, we synthesized SiO x C y microspheres on 3D copper foams by a simple chemical vapor deposition method using a low-cost silane coupling agent (KH560) as precursors. The SiO x C y microspheres are available with a large mass loading (>3 mg/cm 2 ) on collectors and can be directly used as the electrode without any binders or extra conductive agents. As a result, the as-prepared SiO x C y shows a high reversible capacity of ∼1240 mAh g -1 and can be cycled more than 1900 times without decay. Ex situ characterizations show that the volume change of the microspheres is only 55% and the spherical morphology as well as the 3D structure remain intact after cycles. Full-cell electrochemical tests paired with LiFePO 4 as cathodes show 87% capacity retention after 500 cycles, better than most reported results, thus showing the commercial potential of the material.

Investigation of neural growth and connection is crucial in the field of neural tissue engineering. Here, using a femtosecond laser direct writing (fs-DLW) technique, we propose a directionally aligned porous microtube array as a culture system for accelerating the growth of neurons and directing the connection of neurites. These microtubes exhibited an unprecedented guidance effect toward the outgrowth of primary embryonic rat hippocampal neurons, with a wrap resembling the myelin sheaths of neurons. The speed of neurite growth inside these microtubes was significantly faster than that outside these microtubes. We also achieved selective/directing connection of neural networks inside the magnetic microtubes via precise microtube delivery to a gap between two neural clusters. This work not only proposes a powerful microtube platform for accelerated growth of neurons but also offers a new idea for constructing biological neural circuits by arranging the size, location, and pattern of microtubes.

Functional microdevices based on responsive hydrogel show great promise in targeted delivery and biomedical analysis. Among state-of-the-art techniques for manufacturing hydrogel-based microarchitectures, femtosecond laser two-photon polymerization distinguishes itself by high designability and precision, but the point-by-point writing scheme requires mechanical apparatuses to support focus scanning. In this work, by predesigning holograms combined with lens phase modulation, multiple femtosecond laser spots are holographically generated and shifted for prototyping of three-dimensional shape-morphing structures without any moving equipment in the construction process. The microcage array is rapidly fabricated for high-performance target capturing enabled by switching environmental pH. Moreover, the built scaffolds can serve as arrayed analytical platforms for observing cell behaviors in normal or changeable living spaces or revealing the anticancer effects of loaded drugs. The proposed approach opens a new path for facile and flexible manufacturing of hydrogel-based functional microstructures with great versatility in micro-object manipulation.

The Landsberg limit represents the ultimate efficiency limit of solar energy harvesting. Reaching this limit requires the use of nonreciprocal elements. The existing device configurations for attaining the Landsberg limit, however, are very complicated. Here, we introduce the concept of a nonreciprocal multijunction solar cell and show that such a cell can reach the Landsberg limit in the idealized situation where an infinite number of layers are used. We also show that such a nonreciprocal multijunction cell outperforms a standard reciprocal multijunction cell for a finite number of layers. Our work significantly simplifies the device configuration required to reach the ultimate limit of solar energy conversion and points to a pathway toward using nonreciprocity to improve solar energy harvesting.

As essential units in an artificial neural network (ANN), artificial synapses have to adapt to various environments. In particular, the development of synaptic transistors that can work above 125 °C is desirable. However, it is challenging due to the failure of materials or mechanisms at high temperatures. Here, we report a synaptic transistor working at hundreds of degrees Celsius. It employs monolayer MoS 2 as the channel and Na + -diffused SiO 2 as the ionic gate medium. A large on/off ratio of 10 6 can be achieved at 350 °C, 5 orders of magnitude higher than that of a normal MoS 2 transistor in the same range of gate voltage. The short-term plasticity has a synaptic transistor function as an excellent low-pass dynamic filter. Long-term potentiation/depression and spike-timing-dependent plasticity are demonstrated at 150 °C. An ANN can be simulated, with the recognition accuracy reaching 90%. Our work provides promising strategies for high-temperature neuromorphic applications.

Low-dimensional semiconductors have shown great potential in switches for their atomically thin geometries and unique properties. It is significant to achieve new tunneling transistors by the efficient stacking methodology with low-dimensional building blocks. Here, we report a one-dimensional (1D)-two-dimensional (2D) mixed-dimensional van der Waals (vdW) heterostructure, which was efficiently fabricated by stacking an individual semiconducting carbon nanotube (CNT) and 2D MoS 2 . The CNT-MoS 2 heterostructure shows specific reconfigurable electrical transport behaviors and can be set as a nn junction, pn diode, and band-to-band tunneling (BTBT) transistor by gate voltage. The transport properties, especially BTBT, could be attributed to the electron transfer from MoS 2 to CNT through the ideal vdW interface and the 1D nature of the CNT. The progress suggests a new solution for tunneling transistors by making 1D-2D heterostructures from the rich library of low-dimensional nanomaterials. Furthermore, the reconfigurable functions and nanoscaled junction show that it is prospective to apply CNT-MoS 2 heterostructures in future nanoelectronics and nano-optoelectronics.

Daytime radiative cooling presents an exciting new strategy for combating global warming, because it can passively cool buildings by reflecting sunlight and utilizing the infrared atmospheric window to eject heat into outer space. Recent progress with novel material designs showed promising subambient cooling performance under direct sunlight. However, large-scale implementation of radiative cooling technologies is still limited by the high-cost and complex fabrication. Here, we develop a nanoporous polymer matrix composite (PMC) to enable rapid production and cost reduction using commercially available polymer processing techniques, such as molding, extrusion, and 3D printing. With a high solar reflectance of 96.2% and infrared emissivity > 90%, the nanoporous PMC achieved a subambient temperature drop of 6.1 °C and cooling power of 85 W/m 2 under direct sunlight, which are comparable to the state-of-the-art. This work offers great promise to make radiative cooling technologies more viable for saving energy and reducing emissions in building cooling applications.

We combine Raman scattering spectroscopy and lattice dynamics calculations to reveal the fundamental excitations of the intercalated metal monolayers in the Fe x TaS 2 ( x = 1/4, 1/3) family of materials. Both in- and out-of-plane modes are identified, each of which has trends that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. We test these trends against the response of similar systems, including Cr-intercalated NbS 2 and RbFe(SO 4 ) 2 , and demonstrate that the metal monolayer excitations are both coherent and tunable. We discuss the consequences of intercalated metal monolayer excitations for material properties and developing applications.

Objects around us constantly emit and absorb thermal radiation. The emission and absorption processes are governed by two fundamental radiative properties: emissivity and absorptivity. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation. This restriction limits the degree of freedom to control thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff's law typically utilize magneto-optical effects with an external magnetic field. However, these approaches require either a strong magnetic field (∼3T) or narrow-band resonances under a moderate magnetic field (∼0.3T), because the nonreciprocity in conventional magneto-optical effects is weak in the thermal wavelength range. Here, we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters that nearly completely violate Kirchhoff's law over broad angular and frequency ranges without requiring any external magnetic field.

The conventional notion for achieving high efficiency in thermophotovoltaics (TPVs) is to use a monochromatic emission at a photon energy corresponding to the band gap of the cell. Here, we prove theoretically that such a notion is only accurate under idealized conditions and further show that, when nonradiative recombination is taken into account, efficiency improvement can be achieved by broadening the emission spectrum, due to an enhancement in the open-circuit voltage. Broadening the emission spectrum also improves the electrical power density, by increasing the short-circuit current. Hence, broadening the emission spectrum can simultaneously improve the efficiency and power density of practical TPV systems. To illustrate these findings, we focus on surface polariton-mediated near-field TPVs. We propose a versatile design strategy for broadening the emission spectrum via stacking of multiple plasmonic thin film layers. As an example, we consider a realistic ITO/InAs TPV and predict a conversion efficiency of 50% simultaneously with a power density of nearly 80 W/cm 2 at a 1300 K emitter temperature. The performance of our proposed system far exceeds previous works in similar systems using a single plasmonic layer emitter.

In extreme environments, such as at ultrahigh or ultralow temperatures, the amount of tape used should be minimal so as to reduce system contamination and unwanted residues. However, tapes made from conventional materials typically lose their adhesiveness or leave residues difficult to remove under such conditions. Thus, the development of more versatile, lightweight, and easily removable tapes for applications in such extreme environments has received considerable attention. Here, we report that horizontally superaligned carbon nanotube (SACNT) tapes can be used to provide perfect van der Waals (vdW) interface contacts over a wide range of temperatures (from -196 to 1000 °C), yielding outstanding adhesiveness with specific adhesion strengths up to ∼1.1 N/μg. With a surface density of only 0.5-5 μg/cm 2 , hundreds of times lighter than the vertically aligned CNT adhesives, the SACNT tapes can be cost-effectively provided in hundreds of meters. They have multipurpose adhesive abilities for versatile materials and are also easily separated from samples even after exposure to extreme temperature regimes. First-principles calculations confirm the mechanism of vdW adhesion and reveal that ultraflat and nanometer-thick SACNT tapes may yield far greater adhesive abilities. These SACNT tapes show great potential for use in mechanical bonding, electrical bonding, and thermal dissipation in electronic devices.

A superaligned carbon nanotube (SACNT) film can act as an ideal gate electrode in vacuum electronics due to its low secondary electron emission, high electron transparency, ultrasmall thickness, highly uniform electric field, high melting point, and high mechanical strength. We used a SACNT film as the gate electrode in a thermionic emission electron tube and field emission display prototype. The SACNT film gate in a thermionic emission electron tube shows a larger amplification factor. A triode tube with the SACNT film gate is used in an audio amplification circuit. The SACNT film gate electrode in field emission devices shows better field uniformity. The field emission display prototype is demonstrated to dynamically display Chinese characters.

Low-grade waste heat contains an enormous amount of exergy that can be recovered for renewable-energy generation. Current solid-state techniques for recovering low-grade waste heat, such as thermoelectric generators and thermophotovoltaics, however, are limited by low conversion efficiencies or power densities. In this work, we propose a solid-state near-field thermophotonic system. The system consists of a light-emitting diode (LED) on the hot side and a photovoltaic (PV) cell on the cold side. Part of the generated power by the PV cell is used to positively bias the LED. When operating in the near-field regime, the system can have power density and conversion efficiency significantly exceeding the performance of current solid-state approaches for low-grade waste-heat recovery. For example, when the gap spacing is 10 nm and the hot side and cold side are, respectively, 600 and 300 K, we show that the generated electric power density and thermal-to-electrical conversion efficiency can reach 9.6 W/cm 2 and 9.8%, respectively, significantly outperforming the current record-setting thermoelectric generators. We identify the alignment of the band gaps of the LED and the PV cell, the appropriate choice of thickness of the LED and PV cell to mitigate the effect of non-radiative recombination, and the use of highly reflective back mirrors as key factors that affect the performance of the system. Our work points to the significant potential of photonic systems for the recovery of low-grade waste heat.

Carbon nanotubes (CNTs) and other related CNT-based materials with a high thermal conductivity can be used as promising heat dissipation materials. Meanwhile, the miniaturization and high functionality of portable electronics, such as laptops and mobile phones, are achieved at the cost of overheating the high power-density components. The heat removal for hot spots occurring in a relatively narrow space requires simple and effective cooling methods. Here, an auxiliary passive cooling approach by the aid of a flat plate (aluminum-magnesium alloy) is investigated to accommodate heat dissipation in a narrow space. The cooling efficiency can be raised to 43.5%. The cooling performance of several CNT-based samples is compared under such circumstances. Heat dissipation analyses show that, when there is a nearby plate for cooling assistance, the heat radiation is weakened and natural convection is largely improved. Thus, improving heat radiation by increasing emissivity without reducing natural convection can effectively enhance the cooling performance. Moreover, the decoration of an auxiliary cooling plate with sprayed CNTs can further improve the cooling performance of the entire setup.

Organic two-dimensional (2D) crystals are fundamentally important for development of future devices. Despite that more than a half of man-made products contain polymers, 2D crystals consisting of long linear chains have yet to be explored. Here we report on the fabrication of 2D polyaniline (PANI) crystals via rational electrochemical polymerization followed by liquid-phase exfoliation. The 2D PANI is molecularly thin (∼0.8 nm) and composed of PANI chains with a number-average molecular weight of ∼31 000. The chains are parallel to each other with the benzene rings standing almost vertically to the surface, implying a face-to-face arrangement of the neighboring chains held together by abundant π-π interactions augmented with hydrogen bonds. The 2D PANI can be readily transferred to various solid surfaces and exhibit interesting electrical and optical properties, suggesting that they would be potentially useful in photoelectronic devices and other applications.

Despite many efforts to fabricate high-aspect-ratio atomic force microscopy (HAR-AFM) probes for high-fidelity, high-resolution topographical imaging of three-dimensional (3D) nanostructured surfaces, current HAR probes still suffer from unsatisfactory performance, low wear-resistivity, and extravagant prices. The primary objective of this work is to demonstrate a novel design of a high-resolution (HR) HAR AFM probe, which is fabricated through a reliable, cost-efficient benchtop process to precisely implant a single ultrasharp metallic nanowire on a standard AFM cantilever probe. The force-displacement curve indicated that the HAR-HR probe is robust against buckling and bending up to 150 nN. The probes were tested on polymer trenches, showing a much better image fidelity when compared with standard silicon tips. The lateral resolution, when scanning a rough metal thin film and single-walled carbon nanotubes (SW-CNTs), was found to be better than 8 nm. Finally, stable imaging quality in tapping mode was demonstrated for at least 15 continuous scans indicating high resistance to wear. These results demonstrate a reliable benchtop fabrication technique toward metallic HAR-HR AFM probes with performance parallel or exceeding that of commercial HAR probes, yet at a fraction of their cost.

Light polarization is extensively applied in optical detection, industry processing and telecommunication. Although aligned carbon nanotube naturally suppresses the transmittance of light polarized parallel to its axial direction, there is little application regarding the photodetection of carbon nanotube based on this anisotropic interaction with linearly polarized light. Here, we report a photodetection device realized by aligned carbon nanotube. Because of the different absorption behavior of polarized light with respect to polarization angles, such device delivers an explicit response to specific light wavelength regardless of its intensity. Furthermore, combining both experimental and mathematical analysis, we found that the light absorption of different wavelength causes characteristic thermoelectric voltage generation, which makes aligned carbon nanotube promising in optical detection. This work can also be utilized directly in developing new types of photoswitch that features a broad spectrum application from near-ultraviolet to intermediate infrared with easy integration into practical electric devices, for instance, a "wavelength lock".

We present a theoretical framework, based on plasmonic circuit models, for generating a multiresonant field intensity enhancement spectrum at a single "hot spot" in a plasmonic device. We introduce a circuit model, consisting of an array of coupled LC resonators, that directs current asymmetrically in the array, and we show that this circuit can funnel energy efficiently from each resonance to a single element. We implement the circuit model in a plasmonic nanostructure consisting of a series of metal bars of differing length, with nearest neighbor metal bars strongly coupled electromagnetically through air gaps. The resulting nanostructure resonantly traps different wavelengths of incident light in separate gap regions, yet it funnels the energy of different resonances to a common location, which is consistent with our circuit model. Our work is important for a number of applications of plasmonic nanoantennas in spectroscopy, such as in single-molecule fluorescence spectroscopy or Raman spectroscopy.

Carbon nanotube (CNT) is believed to be the most promising material for next generation IC industries with the prerequisite of chirality specific growth. For various approaches to controlling the chiral indices of CNTs, the key is to deepen the understanding of the catalytic growth mechanism in chemical vapor deposition (CVD). Here we show our discovery that the as-grown CNTs are all negatively charged after Fe-catalyzed CVD process. The extra electrons come from the charge generation and transfer during the growth of CNTs, which indicates that an electrochemical process happens in the surface reaction step. We then designed an in situ measurement equipment, verifying that the CVD growth of CNTs can be regarded as a primary battery system. Furthermore, we found that the variation of the Fermi level in Fe catalysts have a significant impact on the chirality of CNTs when different external electric fields are applied. These findings not only provide a new perspective on the growth of CNTs but also open up new possibilities for controlling the growth of CNTs by electrochemical methods.

Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending naturally into the surrounding environment, has been a great challenge in the past decades. Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temperature controlling that has to dissipate a large amount of excessive heat. However, practical devices with an active modulation of emissivity have yet to be well explored. In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. It is believed that this device will find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

The aprotic Li-O2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today's batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the O2 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/decomposition of Li2O2, i.e., the desired Li-O2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O2 cathode in aprotic Li-O2 batteries. The catalyzed Li-O2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. However, a combined analysis using differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy, revealed that these catalyzed Li-O2 cells (particularly those based on Pd-CNT cathodes) did not work according to the desired Li-O2 electrochemistry. Instead the presence of noble metal catalysts impaired the cells' reversibility, as evidenced by the decreased O2 recovery efficiency (the ratio of the amount of O2 evolved during recharge/that consumed in the preceding discharge) coupled with increased CO2 evolution during charging. The results reported here provide new insights into the O2 electrochemistry in the aprotic Li-O2 batteries containing noble metal catalysts and exemplified the importance of the quantitative assays for the Li-O2 reactions in the course of pursuing truly rechargeable Li-O2 batteries.

A dual-photoelectrode device, consisting of a photoanode and photocathode with complementary energy bandgaps, has long been perceived as an ideal scheme for achieving high efficiency, unassisted solar-driven water splitting. Previously reported 2-photon tandem devices, however, generally exhibit an extremely low efficiency (<0.1%), which has been largely limited by the incompatibility between the two photoelectrode materials. Here we show that the use of metal-nitride nanowire photoelectrodes, together with the scheme of parallel illumination by splitting the solar spectrum spatially and spectrally, can break the efficiency bottleneck of conventional 2-photon tandem devices. We have first investigated a dual-photoelectrode device consisting of a GaN nanowire photoanode and an InGaN nanowire photocathode, which exhibited an open circuit potential of 1.3 V and nearly 20-fold enhancement in the power conversion efficiency under visible light illumination (400-600 nm), compared to the individual photoelectrodes in 1 mol/L HBr. We have further demonstrated a dual-photoelectrode device consisting of parallel-connected metal-nitride nanowire photoanodes and a Si/InGaN nanowire photocathode, which can perform unassisted, direct solar-to-hydrogen conversion. A power conversion efficiency of 2% was measured under AM1.5G 1 sun illumination.

Semiconductor nanowires supporting leaky mode resonances have been used to increase light absorption in optoelectronic applications from solar cell to photodetector and sensor. The light conventionally illuminates these devices with a wide range of different incident angles from half space. Currently, most of the investigated nanowires have centrosymmetric geometry cross section, such as circle, hexagon, and rectangle. Here we show that the absorption capability of these symmetrical nanowires has an upper limit under the half-space illumination. Based on the temporal coupled-mode equation, we develop a reciprocity theory for leaky mode resonances in order to connect the angle-dependent absorption cross section and the radiation pattern. We show that in order to exceed such a half-space limit the radiation pattern should be noncentrosymmetric and dominate in the direction reciprocal to the illumination. As an example, we design a metal trough structure to achieve the desired radiation pattern for an embedded nanowire. In comparison to a single nanowire case the trough structure indeed overcomes the half-space limit and leads to 39% and 64% absorption enhancement in TM and TE polarizations, respectively. Also the trough structure enables the enhancement over a broad wavelength range.

Polarizers provide convenience in generating polarized light, meanwhile their adoption raises problems of extra weight, cost, and energy loss. Aiming to realize polarizer-free polarized light sources, herein, we present a plasmonic approach to achieve direct generation of linearly polarized optical waves at the nanometer scale. Periodic slot nanoantenna arrays are fabricated, which are driven by the transition dipole moments of luminescent semiconductor quantum dots. By harnessing interactions between quantum dots and scattered fields from the nanoantennas, spontaneous emission with a high degree of linear polarization is achieved from such hybrid antenna system with polarization perpendicular to antenna slot. We also demonstrate that the polarization is engineerable in aspects of both spectrum and magnitude by tailoring plasmonic resonance of the antenna arrays. Our findings will establish a basis for the development of innovative polarized light-emitting devices, which are useful in optical displays, spectroscopic techniques, optical telecommunications, and so forth.

H2 generation under sunlight offers great potential for a sustainable fuel production system. To achieve high efficiency solar-to-hydrogen conversion, multijunction photoelectrodes have been commonly employed to absorb a large portion of the solar spectrum and to provide energetic charge carriers for water splitting. However, the design and performance of such tandem devices has been fundamentally limited by the current matching between various absorbing layers. Here, by exploiting the lateral carrier extraction scheme of one-dimensional nanowire structures, we have demonstrated that a dual absorber photocathode, consisting of p-InGaN/tunnel junction/n-GaN nanowire arrays and a Si solar cell wafer, can operate efficiently without the strict current matching requirement. The monolithically integrated photocathode exhibits an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V versus normal hydrogen electrode and nearly unity Faradaic efficiency for H2 generation. Such an adaptive multijunction architecture can surpass the design and performance restrictions of conventional tandem photoelectrodes.

Decoupling the growth and the application of nanomaterials by transfer is an important issue in nanotechnology. Here, we developed an efficient transfer technique for carbon nanotube (CNT) arrays by using ice as a binder to temporarily bond the CNT array and the target substrate. Ice makes it an ultraclean transfer because the evaporation of ice ensures that no contaminants are introduced. The transferred superaligned carbon nanotube (SACNT) arrays not only keep their original appearance and initial alignment but also inherit their spinnability, which is the most desirable feature. The transfer-then-spin strategy can be employed to fabricate patterned CNT arrays, which can act as 3-dimensional electrodes in CNT thermoacoustic chips. Besides, the flip-chipped CNTs are promising field electron emitters. Furthermore, the ice-assisted transfer technique provides a cost-effective solution for mass production of SACNTs, giving CNT technologies a competitive edge, and this method may inspire new ways to transfer other nanomaterials.

Near-field heat transfer recently attracted growing interest but was demonstrated experimentally only in macroscopic systems. However, several projected applications would be relevant mostly in integrated nanostructures. Here we demonstrate a platform for near-field heat transfer on-chip and show that it can be the dominant thermal transport mechanism between integrated nanostructures, overcoming background substrate conduction and the far-field limit (by factors 8 and 7, respectively). Our approach could enable the development of active thermal control devices such as thermal rectifiers and transistors.

A binder-free nano sulfur-carbon nanotube composite material featured by clusters of sulfur nanocrystals anchored across the superaligned carbon nanotube (SACNT) matrix is fabricated via a facile solution-based method. The conductive SACNT matrix not only avoids self-aggregation and ensures dispersive distribution of the sulfur nanocrystals but also offers three-dimensional continuous electron pathway, provides sufficient porosity in the matrix to benefit electrolyte infiltration, confines the sulfur/polysulfides, and accommodates the volume variations of sulfur during cycling. The nanosized sulfur particles shorten lithium ion diffusion path, and the confinement of sulfur particles in the SACNT network guarantees the stability of structure and electrochemical performance of the composite. The nano S-SACNT composite cathode delivers an initial discharge capacity of 1071 mAh g(-1), a peak capacity of 1088 mAh g(-1), and capacity retention of 85% after 100 cycles with high Coulombic efficiency (∼100%) at 1 C. Moreover, at high current rates the nano S-SACNT composite displays impressive capacities of 1006 mAh g(-1) at 2 C, 960 mAh g(-1) at 5 C, and 879 mAh g(-1) at 10 C.

We propose theoretically and demonstrate experimentally a dislocated double-layer metal grating structure, which operates as a unidirectional coupler capable of launching surface plasmon polaritons in a desired direction under normal illumination. The structure consists of a slanted dielectric grating sandwiched between two gold gratings. The upper gold grating has a nonzero lateral relative displacement with respect to the lower one. Numerical simulations show that a grating structure with 7 periods can convert 49% of normally incident light into surface plasmons with a contrast ratio of 78 between the powers of the surface plasmons launched in two opposite directions. We explain the unidirectional coupling phenomenon by the dislocation-induced interference of the diffracted waves from the upper and lower gold gratings. Furthermore, we developed a simple and cost-effective technique to fabricate the structure via tilted two-beam interference lithography and subsequent shadow deposition of gold. The experimental results demonstrate a coupling efficiency of 36% and a contrast ratio of 43. The relatively simple periodic nature of our structure lends itself to large-scale low-cost fabrication and simple theoretical analysis. Also, unlike the previous unidirectional couplers based on aperiodic structures, the design parameters of our unidirectional coupler can be determined analytically. Therefore, this structure can be an important component for surface-plasmon-based nanophotonic circuits by providing an efficient interface between free-space and surface plasmon waves.

Here we present a simple yet powerful approach for the imaging of nanostructures under an optical microscope with the help of vapor condensation on their surfaces. Supersaturated water vapor will first form a nanometer-sized water droplet on the condensation nuclei on the surface of nanostructures, and then the water droplet will grow bigger and scatter more light to make the outline of the nanostructure be visible under dark-field optical microscope. This vapor-condensation-assisted (VCA) optical microscopy is applicable to a variety of nanostructures from ultralong carbon nanotubes to functional groups, generating images with contrast coming from the difference in density of the condensation sites, and does not induce any impurities to the specimens. Moreover, this low-cost and efficient technique can be conveniently integrated with other facilities, such as Raman spectroscope and so forth, which will pave the way for widespread applications.

A fundamental limitation of transparent conducting electrode design is thought to be the trade-off between photonic and electronic performances. The photonic transmission property of a transparent conducting electrode, however, is not intrinsic but depends critically on the electromagnetic environment where the electrode is located. We develop the concept of optical impedance transformation, and use this concept to design nanophotonic structures that provide broadband and omnidirectional reduction of optical loss in an ultrathin transparent conducting electrode, without compromising its electrical performance.

We present a detailed balance analysis of current density-voltage modeling of a single-nanowire solar cell. Our analysis takes into account intrinsic material nonidealities in order to determine the theoretical efficiency limit of the single-nanowire solar cell. The analysis only requires the nanowire's absorption cross-section over all angles, which can be readily calculated analytically. We show that the behavior of both the current and voltage is due to coherent effects that arise from resonances of the nanowire. In addition, we elucidate the physics of open-circuit voltage enhancement over bulk cells in nanowires, by showing that the enhancement is related to the removal of resonances in the immediate spectral vicinity above the bandgap.

We present a pure photonic approach to overcome the Shockley-Queisser limit. A single material can show different effective bandgap, set by its absorption spectrum, which depends on its photonic structure. In a tandem cell configuration constructed from a single material, one can achieve two different effective bandgaps, thereby exceeding the Shockley-Queisser limit.

We demonstrate numerically that sharp 90° bends and T-splitters can be designed in plasmonic coaxial waveguides at deep-subwavelength scale to operate without reflection and radiation over a broad range of wavelengths, including the telecommunication wavelength of 1.55 μm. We explain the principles of the operation using a transmission line model of the waveguide in the quasi-static limit. The compact bends and T-splitters open up a new avenue for the design of densely integrated optical circuits with minimal crosstalk.

Aligned carbon nanotube (CNT) films drawn from CNT arrays have shown the potential as thermoacoustic loudspeakers. CNT thermoacoustic chips with robust structures are proposed to promote the applications. The silicon-based chips can play sound and fascinating rhythms by feeding alternating currents and audio signal to the suspending CNT thin yarn arrays across grooves in them. In additional to the thin yarns, experiments further revealed more essential elements of the chips, the groove depth and the interdigital electrodes. The sound pressure depends on the depth of the grooves, and the thermal wavelength can be introduced to define the influence-free depth. The interdigital fingers can effectively reduce the driving voltage, making the chips safe and easy to use. The chips were successfully assembled into earphones and have been working stably for about one year. The thermoacoustic chips can find many applications in consumer electronics and possibly improve the audiovisual experience.

We report on the first demonstration of stable photoelectrochemical water splitting and hydrogen generation on a double-band photoanode in acidic solution (hydrogen bromide), which is achieved by InGaN/GaN core/shell nanowire arrays grown on Si substrate using catalyst-free molecular beam epitaxy. The nanowires are doped n-type using Si to reduce the surface depletion region and increase current conduction. Relatively high incident-photon-to-current-conversion efficiency (up to ~27%) is measured under ultraviolet and visible light irradiation. Under simulated sunlight illumination, steady evolution of molecular hydrogen is further demonstrated.

Silicon has been driving the great success of semiconductor industry, and emerging forms of silicon have generated new opportunities in electronics, biotechnology, and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processability. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 μm. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 μm thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper, and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processability on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.

If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m(2) at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.