Electrochemical Deposition of Conformal and Functional Layers on High Aspect Ratio Silicon Micro/Nanowires

Electrodeposited coatings for neural electrodes: A review

Neural electrodes play a pivotal role in ensuring safe stimulation and high-quality recording for various bioelectronics such as neuromodulation devices and brain-computer interfaces. With the miniaturization of electrodes and the increasing demand for multi-functionality, the incorporation of coating materials via electrodeposition to enhance electrodes performance emerges as a highly effective strategy. These coatings not only substantially improve the stimulation and recording performance of electrodes but also introduce additional functionalities. This review began by outlining the application scenarios and critical requirements of neural electrodes. It then delved into the deposition principles and key influencing factors. Furthermore, the advancements in the electrochemical performance and adhesion stability of these coatings were reviewed. Ultimately, the latest innovative works in the electrodeposited coating applications were highlighted, and future perspectives were summarized.

Self-Assembly Hybrid Manufacture of Nanoarrays for Metasurfaces

The development of metasurfaces necessitates the rapid fabrication of nanoarrays on diverse substrates at large scales, the preparation of patterned nanoarrays on both planar and curved surfaces, and even the creation of nanoarrays on prefabricated structures to form multiscale metastructures. However, conventional fabrication methods fall short of these rigorous requirements. In this work, a novel self-assembly hybrid manufacturing (SAHM) method is introduced for the rapid and scalable fabrication of shape-controllable nanoarrays on various rigid and flexible substrates. This method can be easily integrated with other fabrication techniques, such as lithography and screen printing, to produce patterned nanoarrays on both planar and non-developable surfaces. Utilizing the SAHM method, nanoarrays are fabricated on prefabricated micropillars to create multiscale pillar-nanoarray metastructures. Measurements indicate that these multiscale metastructures can manipulate electromagnetic waves across a range of wavelengths. Therefore, the SAHM method demonstrates the potential of multiscale structures as a new paradigm for the design and fabrication of metasurfaces.

The rise of electrically tunable metasurfaces

Metasurfaces, which offer a diverse range of functionalities in a remarkably compact size, have captured the interest of both scientific and industrial sectors. However, their inherent static nature limits their adaptability for their further applications. Reconfigurable metasurfaces have emerged as a solution to this challenge, expanding the potential for diverse applications. Among the series of tunable devices, electrically controllable devices have garnered particular attention owing to their seamless integration with existing electronic equipment. This review presents recent progress reported with respect to electrically tunable devices, providing an overview of their technological development trajectory and current state of the art. In particular, we analyze the major tuning strategies and discuss the applications in spatial light modulators, tunable optical waveguides, and adaptable emissivity regulators. Furthermore, the challenges and opportunities associated with their implementation are explored, thereby highlighting their potential to bridge the gap between electronics and photonics to enable the development of groundbreaking optical systems.

Automating CAR-T Transfection with Micro and Nano-Technologies

Cancer poses a significant health challenge, with traditional treatments like surgery, radiotherapy, and chemotherapy often lacking in cell specificity and long-term curative potential. Chimeric antigen receptor T cell (CAR-T) therapy,utilizing genetically engineered T cells to target cancer cells, is a promising alternative. However, its high cost limits widespread application. CAR-T manufacturing process encompasses three stages: cell isolation and activation, transfection, and expansion.While the first and last stages have straightforward, commercially available automation technologies, the transfection stage lags behind. Current automated transfection relies on viral vectors or bulk electroporation, which have drawbacks such as limited cargo capacity and significant cell disturbance. Conversely, micro and nano-tool methods offer higher throughput and cargo flexibility, yet their automation remains underexplored.In this perspective, the progress in micro and nano-engineering tools for CAR-T transfection followed by a discussion to automate them is described. It is anticipated that this work can inspire the community working on micro and nano transfection techniques to examine how their protocols can be automated to align with the growing interest in automating CAR-T manufacturing.

Grafting Block Copolymer Nanoparticles to a Surface via Aqueous Photoinduced Polymerization-induced Self-Assembly at Room Temperature

The creation of well-defined surface nanostructures is important for a diverse set of applications such as cell adhesion, superhydrophobic coating, and lithography. In this study, we describe a robust bottom-up method for surface functionalization that involves surface-initiated reversible deactivation radical polymerization (RDRP) and the grafting of block copolymer nanoparticles to material surfaces via aqueous photoinduced polymerization-induced self-assembly (photo-PISA) at room temperature. Using silica nanoparticles as a model substrate, colloidal mesoscale hybrid assemblies with various morphologies were successfully prepared. The morphologies can be easily tuned by changing the lengths of macromolecular chain transfer agents and parameters of the silica nanoparticles. The surface-initiated photo-PISA approach can also be employed for other large-scale substrates such as silicon wafer. Taking advantage of mild reaction conditions of this method (room temperature, aqueous medium, and visible light), enzymatic deoxygenation was introduced to develop oxygen-tolerant surface-initiated photo-PISA that can fabricate well-defined nanostructures on large-scale substrates under open-to-air conditions.

Optoelectronic Properties of a Cylindrical Core/Shell Nanowire: Effect of Quantum Confinement and Magnetic Field

This study investigates the effect of quantum size and an external magnetic field on the optoelectronic properties of a cylindrical AlxGa1-xAs/GaAs-based core/shell nanowire. We used the one-band effective mass model to describe the Hamiltonian of an interacting electron-donor impurity system and employed two numerical methods to calculate the ground state energies: the variational and finite element methods. With the finite confinement barrier at the interface between the core and the shell, the cylindrical symmetry of the system revealed proper transcendental equations, leading to the concept of the threshold core radius. Our results show that the optoelectronic properties of the structure strongly depend on core/shell sizes and the strength of the external magnetic field. We found that the maximum probability of finding the electron occurs in either the core or the shell region, depending on the value of the threshold core radius. This threshold radius separates two regions where physical behaviors undergo changes and the applied magnetic field acts as an additional confinement.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Recent Advances in Structuring and Patterning Silicon Nanowire Arrays for Engineering Light Absorption in Three Dimensions

Vertically aligned silicon nanowire (VA-SiNW) arrays can significantly enhance light absorption and reduce light reflection for efficient light trapping. VA-SiNW arrays thus have the potential to improve solar cell design by providing reduced front-face reflection while allowing the fabrication of thin, flexible, and efficient silicon-based solar cells by lowering the required amount of silicon. Because their interaction with light is highly dependent on the array geometry, the ability to control the array morphology, functionality, and dimension offers many opportunities. Herein, after a short discussion about the remarkable optical properties of SiNW arrays, we report on our recent progress in using chemical and electrochemical methods to structure and pattern SiNW arrays in three dimensions, providing substrates with spatially controlled optical properties. Our approach is based on metal-assisted chemical etching (MACE) and three-dimensional electrochemical axial lithography (3DEAL), which are both affordable and large-scale wet-chemical methods that can provide a spatial resolution all the way down to the sub-5 nm range.

Silicon Nanowires Length and Numbers Dependence on Sensitivity of the Field-Effect Transistor Sensor for Hepatitis B Virus Surface Antigen Detection

Silicon nanowire field effect transistor (NWFET) sensors have been demonstrated to have high sensitivity, are label free, and offer specific detection. This study explored the effect of nanowire dimensions on sensors’ sensitivity. We used sidewall spacer etching to fabricate polycrystalline silicon NWFET sensors. This method does not require expensive nanoscale exposure systems and reduces fabrication costs. We designed transistor sensors with nanowires of various lengths and numbers. Hepatitis B surface antigen (HBsAg) was used as the sensing target to explore the relationships of nanowire length and number with biomolecule detection. The experimental results revealed that the sensor with a 3 µm nanowire exhibited high sensitivity in detecting low concentrations of HBsAg. However, the sensor reached saturation when the biomolecule concentration exceeded 800 fg/mL. Sensors with 1.6 and 5 µm nanowires exhibited favorable linear sensing ranges at concentrations from 800 ag/mL to 800 pg/mL. The results regarding the number of nanowires revealed that the use of few nanowires in transistor sensors increases sensitivity. The results demonstrate the effects of nanowire dimensions on the silicon NWFET biosensors.

Polypyrrole-Silicon Nanowire Arrays for Controlled Intracellular Cargo Delivery

Intracellular cargo delivery is a critical and challenging step in controlling cell states. Silicon nanowire (NW) arrays have emerged as a powerful platform for accessing the intracellular space through a combination of their nanoscale dimensions and electrical properties. Here, we develop and characterize a conductive polypyrrole (PPy)-NW device for temporally controlled intracellular delivery. Fluorescent cargos, doped in electroresponsive PPy matrices at wire tips as well as entire NW arrays, are released with an applied reducing potential. Intracellular delivery into endothelial cells from PPy-Si substrates demonstrated comparable kinetics to solution-based delivery methods while requiring an order of magnitude less cargo loading. This hybrid polymer-semiconductor platform extends methods available for intracellular delivery and links electrical signaling from artificial systems with living molecular transduction.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Tutorial: using nanoneedles for intracellular delivery

Intracellular delivery of advanced therapeutics, including biologicals and supramolecular agents, is complex because of the natural biological barriers that have evolved to protect the cell. Efficient delivery of therapeutic nucleic acids, proteins, peptides and nanoparticles is crucial for clinical adoption of emerging technologies that can benefit disease treatment through gene and cell therapy. Nanoneedles are arrays of vertical high-aspect-ratio nanostructures that can precisely manipulate complex processes at the cell interface, enabling effective intracellular delivery. This emerging technology has already enabled the development of efficient and non-destructive routes for direct access to intracellular environments and delivery of cell-impermeant payloads. However, successful implementation of this technology requires knowledge of several scientific fields, making it complex to access and adopt by researchers who are not directly involved in developing nanoneedle platforms. This presents an obstacle to the widespread adoption of nanoneedle technologies for drug delivery. This tutorial aims to equip researchers with the knowledge required to develop a nanoinjection workflow. It discusses the selection of nanoneedle devices, approaches for cargo loading and strategies for interfacing to biological systems and summarises an array of bioassays that can be used to evaluate the efficacy of intracellular delivery.

Solar cell design using graphene-based hollow nano-pillars

In this paper, the full solar spectrum coverage with an absorption efficiency above 96% is attained by shell-shaped graphene-based hollow nano-pillars on top of the refractory metal substrate. The material choice guarantees the high thermal stability of the device along with its robustness against harsh environmental conditions. To design the structure, constitutive parameters of graphene material in the desired frequency range are investigated and its absorption capability is illustrated by calculating the attenuation constant of the electromagnetic wave. It is observed that broadband absorption is a consequence of wideband retrieved surface impedance matching with the free-space intrinsic impedance due to the tapered geometry. Moreover, the azimuthal and longitudinal cavity resonances with different orders are exhibited for a better understanding of the underlying wideband absorption mechanism. Importantly, the device can tolerate the oblique incidence in a wide span around 65°, regardless of the polarization. The proposed structure can be realized by large-area fabrication techniques.

Electrochemical Polymer Pen Lithography

The development of a massively parallel lithographic technique called electrochemical polymer pen lithography is reported. Pyramidal pen arrays, consisting of more than 10 000 hydrogel pens loaded with metal salts, are integrated into a three-electrode cell and used to locally reduce ions at each pen tip. This system enables high-throughput patterning of a variety of metallic inks (e.g., Ni²⁺ , Pt²⁺ , Ag⁺ ) on the nanometer to micrometer length scale. By incorporating a z-direction piezo actuator, the extension length and dwell time can be used to precisely define feature dimensions (210 to 10 µm in width, and up to 900 nm in height, thus far). Furthermore, by controlling the potential and precursor concentrations, more than one element can be simultaneously deposited, creating a new tool for the synthesis of alloy features, such as NiCo, which are relevant for catalysis. Importantly, this methodology enables fine control over feature size and composition in a single pattern, which may make it ultimately useful for rapid, high-throughput combinatorial screening of metallic features.

Boosting the performance of ZnO microrod metal-semiconductor-metal photodetectors via surface capping of thin amorphous Al2O3 shell layer

1D ZnO nanostructures have been widely explored due to their potential applications in ultraviolet (UV) region photodetectors because of their unique structural and optoelectronic properties. However, a large number of surface defect states leading to a noticeable dark current hinders their practical applications in UV photodetection. In this work, we have shown improved ZnO/Al₂O₃ core-shell microrod photodetectors, whose performance is significantly enhanced by defect passivation and the introduction of trap states by atomic layer deposition grown thin amorphous Al₂O₃ shell layer, as evidenced by steady-state and transient photoluminescence investigations. The photodetectors demonstrated suppressed dark current and increased photocurrent after capping the Al₂O₃ layer. Specifically, the ZnO/Al₂O₃ core-shell microrod photodetector exhibited a photoresponsivity as high as 0.019 A/(W cm^-2) with the dark current as low as ∼1 × 10^-11 A, and a high I _light/I _dark ratio of ∼10⁴ under relatively weak light illumination (∼10 μW cm^-2). The results presented in this work provide valuable pathways to boost the performance of 1D ZnO microrod-based photodetectors for future practical applications.

Spatioselective Deposition of Passivating and Electrocatalytic Layers on Silicon Nanowire Arrays

Metal-silicon nanowire array photoelectrodes provide a promising architecture for water-splitting because they can afford high catalyst loading and decouple charge separation from the light absorption process. To further improve and understand these hybrid nanowire photoelectrodes, control of the catalyst amount and location within the wire array is required. Such a level of control is currently synthetically challenging to achieve. Here, we report the synthesis of cm²-sized hybrid silicon nanowire arrays with electrocatalytically active Ni-Mo and Pt patches placed at defined vertical locations within the individual nanowires. Our method is based on a modified three-dimensional electrochemical axial lithography (3DEAL), which combines metal-assisted chemical etching (MACE) to produce Si nanowires with spatially defined SiO₂ protection layers to selectively cover and uncover specific areas within the nanowire arrays. This spatioselective SiO₂ passivation yields nanowire arrays with well-defined exposed Si surfaces, with feature sizes down to 100 nm in the axial direction. Subsequent electrodeposition directs the growth of the metal catalysts at the exposed silicon surfaces. As a proof of concept, we report photoelectrocatalytic activity of the deposited catalysts for the hydrogen evolution reaction on p-type Si nanowire photocathodes. This demonstrates the functionality of these hybrid metal/Si nanowire arrays patterned via 3DEAL, which paves the way for investigations of the influence of three-dimensional geometrical parameters on the conversion efficiency of nanostructured photoelectrodes interfaced with metal catalysts.

Sensing refractive index gradients along dielectric nanopillar metasurfaces

Metasurfaces exhibit unique optical properties that depend on the ratio of their refractive index and that of their surroundings. As such, they are effective for sensing global changes in refractive index based on the shifts of resonances in their reflectivity spectra. However, when used as a biosensor, the metasurface can be exposed to a spatial distribution of biomolecules that brings about gradients in refractive index along the plane of the metasurface. Such gradients produce complex global reflectivity spectrum but with distinct optical enhancements in localized areas along the metasurface. Here, we propose a unique sensing paradigm that images and maps out the optical enhancements that are correlated with the spatial distribution of the refractive index. Moreover, we designed a metasurface whose resonances can be tuned to detect a range of refractive indices. Our metasurface consists of silicon nanopillars with a cylindrical nanotrench at their centers and a metal plane at the base. To assess its feasibility, we performed numerical simulations to show that the design effectively produces the desired reflectivity spectrum with resonances in the near-infrared. Using an incident light tuned to one of its resonances, our simulations further show that the field enhancements are correlated with the spatial mapping of the gradients of refractive indices along the metasurface.

Vertical nanowire array-based biosensors: device design strategies and biomedical applications

Biosensors have been extensively studied in the areas of biology, electronics, chemistry, biotechnology, medicine, and various engineering fields. The interdisciplinarity creates an ideal platform for scientists to analyze biological species and chemical materials in a direct, efficient, and sensitive manner; this is expected to revolutionize the life sciences, basic medicine, and the healthcare industry. To carry out high-performance biosensing, nanoprobes - with specific nanoscale properties - have been proposed for ultrasensitive and in situ monitoring/detection of tracer biomolecules, cellular behavior, cellular microenvironments, and electrophysiological activity. Here, we review the development of vertical nanowire (VNW) array-based devices for the effective collection of biomedical information at the molecular level, extracellular level, and intracellular level. In particular, we summarize VNW-based technologies in the aspects of detecting biochemical information, cellular information, and bioelectrical information, all of which facilitate the understanding of fundamental biology and development of therapeutic techniques. Finally, we present a conclusion and prospects for the development of VNW platforms in practical biomedical applications, and we identify the challenges and opportunities for VNW-based biosensor systems in future biological research.

Broadband continuous/discrete spectrum optical absorber using graphene-wrapped fractal oligomers

In this paper, a second-order fractal oligomer constructed by graphene-coated cylindrical nano-rods is proposed as the unit cell of a wideband optical absorber. Nano-rods have resided on a dielectric substrate with a thick metallic mirror. The fractional bandwidth of the designed structure is 88.67% for the absorption above 90%. Broadband absorption originates from the cooperative excitation of localized surface plasmon resonances (LSPRs) of the bottom, top, and lateral surfaces of the rods, engineered by the geometrical parameters through the fractal concept. Designed full absorber has an acceptable performance concerning the incident angles up to around 35° and it is polarization insensitive. Moreover, broadband absorption can be altered to multi-band performance in the same spectrum with the desired number of frequency bands. This feature is obtained by manipulating the substrate thickness to excite multiple orders of Fabry-Perot cavity resonances. Our proposed structure has potential applications in various optical devices such as filters, sensors, and modulators.

Tailored multifunctional micellar brushes via crystallization-driven growth from a surface

The creation of nanostructures with precise chemistries on material surfaces is of importance in a wide variety of areas such as lithography, superhydrophobicity, and cell adhesion. We describe a platform for surface functionalization that involves the fabrication of cylindrical micellar brushes on a silicon wafer through seeded growth of crystallizable block copolymers at the termini of immobilized, surface-confined crystallite seeds. The density, length, and coronal chemistry of the micellar brushes can be precisely tuned, and post-growth decoration with nanoparticles enables applications in catalysis and antibacterial surface modification. The micellar brushes can also be grown on ultrathin two-dimensional materials such as graphene oxide nanosheets and further assembled into a membrane for the separation of oil-in-water emulsions and gold nanoparticles.

Imaging low-dimensional nanostructures by very low voltage scanning electron microscopy: ultra-shallow topography and depth-tunable material contrast

We demonstrate the implications of very low voltage operation (<1 kV) of a scanning electron microscope for imaging low-dimensional nanostructures where standard voltages (2-5 kV) involve a beam penetration depth comparable to the cross-section of the nanostructures. In this common situation, image sharpness, contrast quality and resolution are severely limited by emission of secondary electrons far from the primary beam incidence point. Oppositely, very low voltage operation allows reducing the beam-specimen interaction to an extremely narrow and shallow region around the incidence point, enabling high-resolution and ultra-shallow topographic contrast imaging by high-angle backscattered electrons detection on the one hand, and depth-tunable material contrast imaging by low-angle backscattered electrons detection on the other. We describe the performance of these imaging approaches on silicon nanowires obtained by the vapor-liquid-solid mechanism. Our experimental results, supported by Monte Carlo simulations of backscattered electrons emission from the nanowires, reveal the self-assembly of gold-silica core-shell nanostructures at the nanowire tips without any ad-hoc thermal oxidation step. This result demonstrates the capacity of very low voltage operation to provide optimum sharpness, contrast and resolution in low-dimensional nanostructures and to gather information about nanoscaled core-shell conformations otherwise impossible to obtain by standard scanning electron microscopy alone.

Electrodeposition of Large Area, Angle-Insensitive Multilayered Structural Colors

We demonstrate structural colors produced by a simple, inexpensive, and nontoxic electrodeposition process. Asymmetric metal-dielectric-metal (MDM) multilayered structures were achieved by sequential electrodeposition of smooth gold, thin cuprous oxide, and finally thin gold on conductive substrates, forming an effective optical cavity with angle-insensitive characteristics. Different colors of high brightness were achieved by simply tuning the thickness of the electrodeposited middle cavity layer. This process is compatible with highly nonplanar substrates of arbitrary shape, size, and roughness. This work is the first demonstration of solution-processed, electrodeposited, MDM film stacks that are uniform over large areas and highlights the clear advantages of this approach over traditional deposition or assembly methods for preparing colored films.

Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes

Gold electrocatalysts have been a research focus due to their ability to reduce CO₂ into CO, a feedstock for further conversion. Many methods have been employed to modulate CO₂ reduction (CDR) vs hydrogen evolution reaction (HER) selectivity on gold electrodes such as nano-/mesostructuring and crystal faceting control. Herein we show that gold surfaces with very different morphologies (planar, leaves, and wires) lead to similar bell-shaped CO faradaic efficiency as a function of applied potential. At low overpotential (E > -0.85 V vs standard hydrogen electrode (SHE)), HER is dominant via a potential quasi-independent rate that we attribute to a rate limiting process of surface dissociation of competent proton donors. As overpotential is increased, CO faradaic efficiency reaches a maximal value (near 90%) because CO production is controlled by an electron transfer rate that increases with potential, whereas HER remains almost potential independent. At high overpotential (E < -1.2 V vs SHE), CO faradaic efficiency decreases due to the concurrent rise of HER via bicarbonate direct reduction and leveling off of CDR as CO₂ replenishment at the catalyst surface is limited by mass transport and homogeneous coupled reactions. Importantly, the analysis shows that recent attempts to overcome mass transport limitations with gas diffusion electrodes confront low carbon mass balance owing to the prominence of homogeneous reactions coupled to CDR. The comprehensive kinetics analysis of the factors defining CDR vs HER on gold electrodes developed here provides an activation-driving force relationship over a large potential window and informs on the design of conditions to achieve desirable high current densities for CO₂ to CO conversion while maintaining high selectivity.

Strong correlation between optical properties and mechanism in deficiency of normalized self-assembly ZnO nanorods

Although, post annealing is an efficient way to annihilate/restructure deficiencies in self-assembly (SA) ZnO nanorods (ZNRs), the detailed investigation about the surface properties of annealed SA-ZNRs is a long standing issue and the major discrepancy is mainly due to single step annealing. We demonstrate the strategic two step annealing process to create reliable structural configuration in SA-ZNRs during the first round of annealing at 800 °C in vacuum (VA process), and create intrinsic defects in the second step of annealing in oxygen rich atmosphere (OA process) to correlate the formation of the defects related to green/orange-red emission. SA-ZNRs annealed in VA-OA processes reveal positive correlations between the oxygen flow rate and formation of oxygen interstitials (O_i) and zinc vacancies (V_Zn). The OA-VA processes exhibit the relation of residual O_i and additional V_o. According to VA-OA and OA-VA processes, we propose that the green emission in ZnO annealed in oxygen poor/rich condition is mainly due to the formation of V_o/V_Zn and annealing at oxygen rich condition creates O_i that lead to strong orange-red emission. Rather than O1s, we propose a reliable method by considering the peak shift of Zn2p in XPS to inspect the ZnO matrix, which has good interdependence with the characteristics of PL.

Inorganic semiconductor biointerfaces

Biological systems respond to and communicate through biophysical cues, such as electrical, thermal, mechanical and topographical signals. However, precise tools for introducing localized physical stimuli and/or for sensing biological responses to biophysical signals with high spatiotemporal resolution are limited. Inorganic semiconductors display many relevant electrical and optical properties, and they can be fabricated into a broad spectrum of electronic and photonic devices. Inorganic semiconductor devices enable the formation of functional interfaces with biological material, ranging from proteins to whole organs. In this Review, we discuss fundamental semiconductor physics and operation principles, with a focus on their behaviour in physiological conditions, and highlight the advantages of inorganic semiconductors for the establishment of biointerfaces. We examine semiconductor device design and synthesis and discuss typical signal transduction mechanisms at bioelectronic and biophotonic interfaces for electronic and optoelectronic sensing, optoelectronic and photothermal stimulation and photoluminescent in vivo imaging of cells and tissues. Finally, we evaluate cytotoxicity and highlight possible new material components and biological targets of inorganic semiconductor devices.

Three-Dimensional Electrochemical Axial Lithography on Si Micro- and Nanowire Arrays

A templated electrochemical technique for patterning macroscopic arrays of single-crystalline Si micro- and nanowires with feature dimensions down to 5 nm is reported. This technique, termed three-dimensional electrochemical axial lithography (3DEAL), allows the design and parallel fabrication of hybrid silicon nanowire arrays decorated with complex metal nano-ring architectures in a flexible and modular approach. While conventional templated approaches are based on the direct replication of a template, our method can be used to perform high-resolution lithography on pre-existing nanostructures. This is made possible by the synthesis of a porous template with tunable dimensions that guides the deposition of well-defined metallic shells around the Si wires. The synthesis of a variety of ring architectures composed of different metals (Au, Ag, Fe, and Ni) with controlled sequence, height, and position along the wire is demonstrated for both straight and kinked wires. We observe a strong enhancement of the Raman signal for arrays of Si nanowires decorated with multiple gold rings due to the plasmonic hot spots created in these tailored architectures. The uniformity of the fabrication method is evidenced by a homogeneous increase in the Raman signal throughout the macroscopic sample. This demonstrates the reliability of the method for engineering plasmonic fields in three dimensions within Si wire arrays.

Rapid, Wet Chemical Fabrication of Radial Junction Electroluminescent Wires

A wet chemical process involving two electrodeposition steps followed by a solution casting step, the "EESC" process, is described for the fabrication of electroluminescent, radial junction wires. EESC is demonstrated by assembling three well-studied nanocrystalline (or amorphous) materials: Au, CdSe, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The tri-layered device architecture produced by EESC minimizes the influence of an electrically resistive CdSe emitter layer by using a highly conductive gold nanowire that serves as both a current collector and a negative electrode. Hole injection, at a high barrier CdSe-PEDOT:PSS interface (ϕ_h ≈ 1.1 V), is facilitated by a contact area that is 1.9-4.7-fold larger than the complimentary gold-CdSe electron-injecting contact (ϕ_e ≈ 0.6 V), contributing to low-voltage thresholds (1.4-1.7 V) for electroluminescence (EL) emission. Au@CdSe@PEDOT:PSS wire EL emitters are 25 μm in length, amongst the longest so far demonstrated to our knowledge, but the EESC process is scalable to nanowires of any length, limited only by the length of the central gold nanowire that serves as a template for the fabrication process. Radial carrier transport within these multishell wires conforms to the back-to-back diode model.

Long-range surface plasmon polariton detection with a graphene photodetector

We present an integration of a single Ag nanowire (NW) with a graphene photodetector and demonstrate an efficient and compact detection of long-range surface plasmon polaritons (SPPs). Atomically thin graphene provides an ideal platform to detect the evanescent electric field of SPPs extremely bound at the interface of the Ag NW and glass substrate. Scanning photocurrent microscopy directly visualizes a polarization-dependent excitation and detects the SPPs. The SPP detection responsivity is readily controlled up to ∼17  mA/W by the drain-source voltage. We believe that the graphene SPP detector will be a promising building block for highly integrated photonic and optoelectronic circuits.

Interfacing Cells with Vertical Nanoscale Devices: Applications and Characterization

Measurements of the intracellular state of mammalian cells often require probes or molecules to breach the tightly regulated cell membrane. Mammalian cells have been shown to grow well on vertical nanoscale structures in vitro, going out of their way to reach and tightly wrap the structures. A great deal of research has taken advantage of this interaction to bring probes close to the interface or deliver molecules with increased efficiency or ease. In turn, techniques have been developed to characterize this interface. Here, we endeavor to survey this research with an emphasis on the interface as driven by cellular mechanisms.

Layer-by-Layer Assembly of Free-Standing Nanofilms by Controlled Rolling

A water surface not only provides a habitat to many living organisms but also opens up new possibilities to develop state-of-the-art technologies. Here, we show a technology for the layer-by-layer assembly of free-standing nanofilms by controlled rolling. The water surface is exploited as an ideal platform for rolling a nanofilm, allowing adhesion control and frictionless feeding. The nanofilm floating on the water surface is attached to a tube by van der Waals adhesion and is rolled up by the rotation of the tube. This method can assemble diverse film materials including metals, polymers, and two-dimensional materials, with an easy control of the number of layers. Furthermore, heterogeneous and spiral structures of the nanofilm are achieved. Various applications such as a stretchable tubular electrode, an electroactive polymer tube actuator, and a superelastic nanofilm tube are demonstrated. We believe this work can potentially lead to a breakthrough in the nanofilm assembly processes.

Influence of Polycation Composition on Electrochemical Film Formation

The effect of polyelectrolyte composition on the electrodeposition onto platinum is investigated using a counterion switching approach. Film formation of preformed polyelectrolytes is triggered by oxidation of hexacyanoferrates(II) (ferrocyanide), leading to polyelectrolyte complexes, which are physically crosslinked by hexacyanoferrate(III) (ferricyanide) ions due to preferential ferricyanide/polycation interactions. In this study, the electrodeposition of three different linear polyelectrolytes, namely quaternized poly[2-(dimethylamino)ethyl methacrylate] (i.e., poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride}; PMOTAC), quaternized poly[2-(dimethylamino)ethyl acrylate] (i.e., poly{[2-(acryloyloxy)ethyl]trimethylammonium chloride}; POTAC), quaternized poly[N-(3-dimethylaminopropyl)methacrylamide] (i.e., poly{[3-(methacrylamido)propyl]trimethylammonium chloride}; PMAPTAC) and different statistical copolymers of these polyelectrolytes with N-(3-aminopropyl)methacrylamide (APMA), are studied. Hydrodynamic voltammetry utilizing a rotating ring disk electrode (RRDE) shows the highest deposition efficiency DE for PMOTAC over PMAPTAC and over POTAC. Increasing incorporation of APMA weakens the preferred interaction of the quaternized units with the hexacyanoferrate(III) ions. At a sufficient APMA content, electrodeposition can thus be prevented. Additional electrochemical quartz crystal microbalance measurements reveal the formation of rigid polyelectrolyte films being highly crosslinked by the hexacyanoferrate(III) ions. Results indicate a different degree of water incorporation into these polyelectrolyte films. Hence, by adjusting the polycation composition, film properties can be tuned, while different chemistries can be incorporated into these electrodeposited thin hydrogel films.

Silicon nanowire-based multifunctional platform for chemo-photothermal synergistic cancer therapy

The new type of silicon nanowire-based pH/NIR/magnetism triple-responsive system shows high-efficacy synergistic photothermal-chemotherapy on cancer cells.

Subwavelength core/shell cylindrical nanostructures for novel plasmonic and metamaterial devices

In this review, we introduce novel plasmonic and metamaterial devices based on one-dimensional subwavelength nanostructures with cylindrical symmetry. Individual single devices with semiconductor/metal core/shell or dielectric/metal core/multi-shell structures experience strong light-matter interaction and yield unique optical properties with a variety of functions, e.g., invisibility cloaking, super-scattering/super-absorption, enhanced luminescence and nonlinear optical activities, and deep subwavelength-scale optical waveguiding. We describe the rational design of core/shell cylindrical nanostructures and the proper choice of appropriate constituent materials, which allow the efficient manipulation of electromagnetic waves and help to overcome the limitations of conventional homogeneous nanostructures. The recent developments of bottom-up synthesis combined with the top-down fabrication technologies for the practical applications and the experimental realizations of 1D subwavelength core/shell nanostructure devices are briefly discussed.