Lorentz Force-Actuated Bidirectional Nanoelectromechanical Switch with an Ultralow Operation Voltage.

The high operating voltage of conventional nanoelectromechanical switches, typically tens of volts, is much higher than the driving voltage of the complementary metal oxide semiconductor integrated circuit (∼1 V). Though the operating voltage can be reduced by adopting a narrow air gap, down to several nanometers, this leads to formidable manufacturing challenges and occasionally irreversible switch failures due to the surface adhesive force. Here, we demonstrate a new nanowire-morphed nanoelectromechanical (NW-NEM) switch structure with ultralow operation voltages. In contrast to conventional nanoelectromechanical switches actuated by unidirectional electrostatic attraction, the NW-NEM switch is bidirectionally driven by Lorentz force to allow the use of a large air gap for excellent electrical isolation, while achieving a record-low driving voltage of <0.2 V. Furthermore, the introduction of the Lorentz force allows the NW-NEM switch to effectively overcome the adhesion force to recover to the turn-off state.

Delay characteristics of quasi-nonvolatile memory devices operating in positive feedback mechanism.

This study examines the memory and read delay characteristics of quasi-nonvolatile memory (QNVM) devices operating in a positive feedback mechanism through technology computer-aided design simulation. The QNVM devices exhibit a rapid operation speed of 5 ns, a significant sensing margin of approximately 8.0µA, and a retention time of around 1 s without any external bias. These devices showcase an exceptionally brief read delay of 0.12 ns. The energy band diagrams during the memory operation are analyzed to clarify the factors influencing the read delay. The write and standby conditions modulate the potential barrier height during the standby operation, thereby affecting the read delay. Moreover, the shorter rising time causes the reduction of the read delay. This study demonstrates that the QNVM device has the potential to resolve energy consumption and speed issues in nonvolatile memory devices.

Silicon nanowire piezoresistor and its applications: a review.

Monocrystalline bulk silicon with doped impurities has been the widely preferred piezoresistive material for the last few decades to realize micro-electromechanical system (MEMS) sensors. However, there has been a growing interest among researchers in the recent past to explore other piezoresistive materials with varied advantages in order to realize ultra-miniature high-sensitivity sensors for area-constrained applications. Of the various alternative piezoresistive materials, silicon nanowires (SiNWs) are an attractive choice due to their benefits of nanometre range dimensions, giant piezoresistive coefficients, and compatibility with the integrated circuit fabrication processes. This review article elucidates the fundamentals of piezoresistance and its existence in various materials, including silicon. It comprehends the piezoresistance effect in SiNWs based on two different biasing techniques, viz., (i) ungated and (ii) gated SiNWs. In addition, it presents the application of piezoresistive SiNWs in MEMS-based pressure sensors, acceleration sensors, flow sensors, resonators, and strain gauges.

Modulating the density of silicon nanowire arrays for high-performance hydrovoltaic devices.

Hydrovoltaic devices (HDs) based on silicon nanowire (SiNW) arrays have received intensive attention due to their simple preparation, mature processing technology, and high output power. Investigating the impact of structure parameters of SiNWs on the performance of HDs can guide the optimization of the devices, but related research is still not sufficient. This work studies the effect of the SiNW density on the performance of HDs. SiNW arrays with different densities were prepared by controlling the react time of Si wafers in the seed solution (tseed) in metal-assisted chemical etching. Density of SiNW array gradually decreases with the increase oftseed. HDs were fabricated based on SiNW arrays with different densities. The research results indicate that the open-circuit voltage gradually decreases with increasingtseed, while the short-circuit current first increases and then decreases with increasingtseed. Overall, SiNW devices withtseedof 20 s and 60 s have the best output performance. The difference in output performance of HDs based on SiNWs with different densities is attributed to the difference in the gap sizes between SiNWs, specific surface area of SiNWs, and the number of SiNWs in parallel. This work gives the corresponding relationship between the preparation conditions of SiNWs, array density, and output performance of hydrovoltaic devices. Density parameters of SiNW arrays with optimized output performance and corresponding preparation conditions are revealed. The relevant results have important reference value for understanding the mechanism of HDs and designing structural parameters of SiNWs for high-performance hydrovoltaic devices.

Implantable pH Sensing System Using Vertically Stacked Silicon Nanowire Arrays and Body Channel Communication for Gastroesophageal Reflux Monitoring.

Silicon nanowires (SiNWs) are emerging as versatile components in the fabrication of sensors for implantable medical devices because of their exceptional electrical, optical, and mechanical properties. This paper presents a novel top-down fabrication method for vertically stacked SiNWs, eliminating the need for wet oxidation, wet etching, and nanolithography. The integration of these SiNWs into body channel communication (BCC) circuits was also explored. The fabricated SiNWs were confirmed to be capable of forming arrays with multiple layers and rows. The SiNW-based pH sensors demonstrated a robust response to pH changes, and when tested with BCC circuits, they showed that it was possible to quantize based on pH when transmitting data through the human body. This study successfully developed a novel method for SiNW fabrication and integration into BCC circuits, which could lead to improvements in the reliability and efficiency of implantable medical sensors. The findings demonstrate significant potential for bioelectronic applications and real-time biochemical monitoring.

Binder-free boron-doped Si nanowires toward the enhancement of lithium-ion capacitor.

Lithium-ion capacitors (LICs) are next-generation electrochemical storage devices that combine the benefits of both supercapacitors and lithium-ion batteries. Silicon materials have attracted attention for the development of high-performance LICs owing to their high theoretical capacity and low delithiation potential (∼0.5 V versus Li/Li+). However, sluggish ion diffusion has severely restricted the development of LICs. Herein, a binder-free anode of boron-doped silicon nanowires (B-doped SiNWs) on a copper substrate was reported as an anode for LICs. B-doping could significantly improve the conductivity of the SiNW anode, which could enhance electron/ion transfer in LICs. As expected, the B-doped SiNWs//Li half-cell delivered a higher initial discharge capacity of 454 mAh g-1with excellent cycle stability (capacity retention of 96% after 100 cycles). Furthermore, the near-lithium reaction plateau of Si endows the LICs with a high voltage window (1.5-4.2 V), and the as-fabricated B-doped SiNWs//AC LIC possesses the maximum energy density value of 155.8 Wh kg-1at a battery-inaccessible power density of 275 W kg-1. This study provides a new strategy for using Si-based composites to develop high-performance LIC.

Silicon nanowire FET biosensor and its application in acute myocardial infarction.

Over the last two decades, silicon nanowire field-effect transistors (SiNW-FETs) with prominent merits of high surface-to-volume ratio, excellent biocompatibility and mature fabrication with standard silicon technology, have been widely studied as ultrahigh sensitive biosensors for the detection of target biomolecules, such as proteins, nucleic acids, cells and viruses so on. Herein we present a comprehensive review of the fundamental aspects of SiNW-FET biosensors, involving the working principle and the device fabrication, surface functionalization, and system integration with fluid exchange and electrical detection. Futhermore, we emphatically discuss the electrical detection of cardiac-specific biomarkers related to acute myocardial infarction disease. SiNW-FET biosensors are being increasingly exploited as promising diagnostic devices, which provide high sensitivity, high integration density, high speed sampling, strong specificity, and real-time and label-free detection for simple and cheap clinical testing.

Array density effect on the optical and photoelectric properties of silicon nanowire arrays via Ag-assisted chemical etching.

Ag-assisted chemical etching (AgACE) is a low-cost method to produce silicon nanowires (SiNWs) for photoelectric applications. Structure parameters of SiNWs have great impact on their optical and photoelectric properties, which are worth studying for fabricating high-performance devices. However, array density of SiNWs via AgACE, as an important structural parameter, has not been sufficiently investigated. Here, array density effect on the optical and photoelectric properties of SiNWs is experimentally investigated. SiNW arrays with different densities (silicon occupation ratio of 7%-34.5%) were prepared through controlling the reaction time of silicon wafers in the seed solution (tseed). The SiNW array with atseedof 90 s shows optimum light absorption over 98% in the wavelength range of 300-1000 nm, though all the samples have light absorption over 95% due to the light trapping effect of nanowire array structure. In addition, the SiNW array with atseedof 90 s exhibits the best photoelectric property. SiNW arrays with shortertseedand higher density suffer more surface recombination, harming the photoelectric property. In SiNW arrays with longertseedthan 90 s and lower density, some SiNWs topple down and break, which has an adverse effect on transport and collection of carriers. These results indicate that the array density of SiNWs via AgACE has obvious effect on their photoelectric property. SiNW arrays via AgACE with atseedof 90 s are ideal for photoelectric devices. This work is potential to guide SiNW fabrication for photoelectric applications.

Silicon Nanowire Phototransistor Arrays for CMOS Image Sensor Applications.

This paper introduces a new design of silicon nanowire (Si NW) phototransistor (PT) arrays conceived explicitly for improved CMOS image sensor performance, and comprehensive numerical investigations clarify the characteristics of the proposed devices. Each unit within this array architecture features a top-layer vertical Si NW optimized for the maximal absorption of incoming light across the visible spectrum. This absorbed light generates carriers, efficiently injected into the emitter-base junction of an underlying npn bipolar junction transistor (BJT). This process induces proficient amplification of the output collector current. By meticulously adjusting the diameters of the NWs, the PTs are tailored to exhibit distinct absorption characteristics, thus delineating the visible spectrum's blue, green, and red regions. This specialization ensures enriched color fidelity, a sought-after trait in imaging devices. Notably, the synergetic combination of the Si NW and the BJT augments the electrical response under illumination, boasting a quantum efficiency exceeding 10. In addition, by refining parameters like the height of the NW and gradient doping depth, the proposed PTs deliver enhanced color purity and amplified output currents.

Application of p and n-Type Silicon Nanowires as Human Respiratory Sensing Device.

Accurate and fast breath monitoring is of great importance for various healthcare applications, for example, medical diagnoses, studying sleep apnea, and early detection of physiological disorders. Devices meant for such applications tend to be uncomfortable for the subject (patient) and pricey. Therefore, there is a need for a cost-effective, lightweight, small-dimensional, and non-invasive device whose presence does not interfere with the observed signals. This paper reports on the fabrication of a highly sensitive human respiratory sensor based on silicon nanowires (SiNWs) fabricated by a top-down method of metal-assisted chemical-etching (MACE). Besides other important factors, reducing the final cost of the sensor is of paramount importance. One of the factors that increases the final price of the sensors is using gold (Au) electrodes. Herein, we investigate the sensor's response using aluminum (Al) electrodes as a cost-effective alternative, considering the fact that the electrode's work function is crucial in electronic device design, impacting device electronic properties and electron transport efficiency at the electrode-semiconductor interface. Therefore a comparison is made between SiNWs breath sensors made from both p-type and n-type silicon to investigate the effect of the dopant and electrode type on the SiNWs respiratory sensing functionality. A distinct directional variation was observed in the sample's response with Au and Al electrodes. Finally, performing a qualitative study revealed that the electrical resistance across the SiNWs renders greater sensitivity to breath than to dry air pressure. No definitive research demonstrating the mechanism behind these effects exists, thus prompting our study to investigate the underlying process.

Pushing the Limits of Biosensing: Selective Calcium Ion Detection with High Sensitivity via High-k Gate Dielectric Engineered Si Nanowire Random Network Channel Dual-Gate Field-Effect Transistors.

Calcium ions (Ca2+) are abundantly present in the human body; they perform essential roles in various biological functions. In this study, we propose a highly sensitive and selective biosensor platform for Ca2+ detection, which comprises a dual-gate (DG) field-effect transistor (FET) with a high-k engineered gate dielectric, silicon nanowire (SiNW) random network channel, and Ca2+-selective extended gate. The SiNW channel device, which was fabricated via the template transfer method, exhibits superior Ca2+ sensing characteristics compared to conventional film channel devices. An exceptionally high Ca2+ sensitivity of 208.25 mV/dec was achieved through the self-amplification of capacitively coupled DG operation and an enhanced amplification ratio resulting from the high surface-to-volume ratio of the SiNW channel. The SiNW channel device demonstrated stable and reliable sensing characteristics, as evidenced by minimal hysteresis and drift effects, with the hysteresis voltage and drift rate measuring less than 6.53% of the Ca2+ sensitivity. Furthermore, the Ca2+-selective characteristics of the biosensor platform were elucidated through experiments with pH buffer, NaCl, and KCl solutions, wherein the sensitivities of the interfering ions were below 7.82% compared to the Ca2+ sensitivity. The proposed Ca2+-selective biosensor platform exhibits exceptional performance and holds great potential in various biosensing fields.

Optical Trapping and Positioning of Silicon Nanowires via Photonic Nozzling.

We have improved the maximum two-dimensional translation rate of optically tweezed silicon nanowires to 30 µm/s while lowering the power usage by an order of magnitude from the ∼100 mW range to 6 mW using a silicon film substrate at 532 nm laser wavelength. We then explain the mechanism for the enhanced tweezing using finite difference time domain simulation as "waveguide nozzling" of the incident radiation, directing the light underneath the nanowire where it is confined and forced to propagate opposite to the direction of nanowire motion. We then demonstrate the robust and deterministic placement of the nanowires on the Si film surface using a nanosecond laser at the same wavelength.

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.

Nanomaterial-Based Biosensors using Field-Effect Transistors: A Review.

Field-effect transistor biosensors (Bio-FET) have attracted great interest in recent years owing to their distinctive properties like high sensitivity, good selectivity, and easy integration into portable and wearable electronic devices. Bio-FET performance mainly relies on the constituent components such as the bio-recognition layer and the transducer, which ensures device stability, sensitivity, and lifetime. Nanomaterial-based Bio-FETs are excellent candidates for biosensing applications. This review discusses the basic concepts, function, and working principles of Bio-FETs, and focuses on the progress of recent research in Bio-FETs in the sensing of neurotransmitters, glucose, nucleic acids, proteins, viruses, and cancer biomarkers using nanomaterials. Finally, challenges in the development of Bio-FETs, as well as an outlook on the prospects of nano Bio-FET-based sensing in various fields, are discussed.

Photoelectrochemical Nitrate Reduction to Ammonia on Ordered Silicon Nanowire Array Photocathodes.

As a new path to "green" ammonia production, photoelectrochemical nitrate reduction reaction (PEC NO3 RR) is investigated for the first time. An Au-decorated ordered silicon nanowire (O_SiNW) array photocathode demonstrates 95.6 % of Faradaic efficiency (FE) to ammonia at 0.2 VRHE , which represents a more positive potential than the thermodynamic reduction potential of nitrate by utilizing photovoltage. The high FE is possible because both Si and Au surfaces are inactive for competing water reduction to hydrogen. The O_SiNW array structure is favorable to promote the PEC NO3 RR relative to planar Si or randomly-grown Si nanowire, by enabling the uniform distribution of small Au nanoparticles as an electrocatalyst and facilitating the mass transport during the reaction. The results demonstrate the feasibility of PEC nitrate conversion to ammonia and would motivate further studies and developments.

A Nanowire Nest Structure Comprising Copper Silicide and Silicon Nanowires for Lithium-Ion Battery Anodes with High Areal Loading.

High loading (>1.6 mg cm-2 ) of Si nanowires (NWs) is achieved by seeding the growth from a dense array of Cu15 Si4 NWs using tin seeds. A one-pot synthetic approach involves the direct growth of CuSi NWs on Cu foil that acts as a textured surface for Sn adhesion and Si NW nucleation. The high achievable Si NW loading is enabled by the high surface area of CuSi NWs and bolstered by secondary growth of Si NWs as branches from both Si and CuSi NW stems, forming a dense Si active layer, interconnected with an electrically conducting CuSi array (denoted Si/CuSi). When employed as Li-ion battery anodes, the Si/CuSi nest structure demonstrates impressive rate performance, reaching 4.1 mAh cm-2 at C/20, 3.1 mAh cm-2 at C/5, and 0.8 mAh cm-2 at 6C. Also, Si/CuSi shows remarkable long-term stability, delivering a stable areal capacity of 2.2 mAh cm-2 after 300 cycles. Overall, complete anode fabrication is achieved within a single reaction by employing an inexpensive Sn powder approach.

Inverting logic-in-memory cells comprising silicon nanowire feedback field-effect transistors.

In this paper, we propose inverting logic-in-memory (LIM) cells comprising silicon nanowire feedback field-effect transistors with steep switching and holding characteristics. The timing diagrams of the proposed inverting LIM cells under dynamic and static conditions are investigated via mixed-mode technology computer-aided design simulation to verify the performance. The inverting LIM cells have an operating speed of the order of nanoseconds, an ultra-high voltage gain, and a longer retention time than that of conventional dynamic random access memory. The disturbance characteristics of half-selected cells within an inverting LIM array confirm the appropriate functioning of the random access memory array.

Highly Sensitive and Selective Sodium Ion Sensor Based on Silicon Nanowire Dual Gate Field-Effect Transistor.

In this study, a highly sensitive and selective sodium ion sensor consisting of a dual-gate (DG) structured silicon nanowire (SiNW) field-effect transistor (FET) as the transducer and a sodium-selective membrane extended gate (EG) as the sensing unit was developed. The SiNW channel DG FET was fabricated through the dry etching of the silicon-on-insulator substrate by using electrospun polyvinylpyrrolidone nanofibers as a template for the SiNW pattern transfer. The selectivity and sensitivity of sodium to other ions were verified by constructing a sodium ion sensor, wherein the EG was electrically connected to the SiNW channel DG FET with a sodium-selective membrane. An extremely high sensitivity of 1464.66 mV/dec was obtained for a NaCl solution. The low sensitivities of the SiNW channel FET-based sodium ion sensor to CaCl2, KCl, and pH buffer solutions demonstrated its excellent selectivity. The reliability and stability of the sodium ion sensor were verified under non-ideal behaviors by analyzing the hysteresis and drift. Therefore, the SiNW channel DG FET-based sodium ion sensor, which comprises a sodium-selective membrane EG, can be applied to accurately detect sodium ions in the analyses of sweat or blood.

Process Variability in Top-Down Fabrication of Silicon Nanowire-Based Biosensor Arrays.

Silicon nanowire field-effect transistors (SiNW-FET) have been studied as ultra-high sensitive sensors for the detection of biomolecules, metal ions, gas molecules and as an interface for biological systems due to their remarkable electronic properties. "Bottom-up" or "top-down" approaches that are used for the fabrication of SiNW-FET sensors have their respective limitations in terms of technology development. The "bottom-up" approach allows the synthesis of silicon nanowires (SiNW) in the range from a few nm to hundreds of nm in diameter. However, it is technologically challenging to realize reproducible bottom-up devices on a large scale for clinical biosensing applications. The top-down approach involves state-of-the-art lithography and nanofabrication techniques to cast SiNW down to a few 10s of nanometers in diameter out of high-quality Silicon-on-Insulator (SOI) wafers in a controlled environment, enabling the large-scale fabrication of sensors for a myriad of applications. The possibility of their wafer-scale integration in standard semiconductor processes makes SiNW-FETs one of the most promising candidates for the next generation of biosensor platforms for applications in healthcare and medicine. Although advanced fabrication techniques are employed for fabricating SiNW, the sensor-to-sensor variation in the fabrication processes is one of the limiting factors for a large-scale production towards commercial applications. To provide a detailed overview of the technical aspects responsible for this sensor-to-sensor variation, we critically review and discuss the fundamental aspects that could lead to such a sensor-to-sensor variation, focusing on fabrication parameters and processes described in the state-of-the-art literature. Furthermore, we discuss the impact of functionalization aspects, surface modification, and system integration of the SiNW-FET biosensors on post-fabrication-induced sensor-to-sensor variations for biosensing experiments.

Combination of Aptamer Amplifier and Antigen-Binding Fragment Probe as a Novel Strategy to Improve Detection Limit of Silicon Nanowire Field-Effect Transistor Immunosensors.

Detecting proteins at low concentrations in high-ionic-strength conditions by silicon nanowire field-effect transistors (SiNWFETs) is severely hindered due to the weakened signal, primarily caused by screening effects. In this study, aptamer as a signal amplifier, which has already been reported by our group, is integrated into SiNWFET immunosensors employing antigen-binding fragments (Fab) as the receptors to improve its detection limit for the first time. The Fab-SiNWFET immunosensors were developed by immobilizing Fab onto Si surfaces modified with either 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde (GA) (Fab/APTES-SiNWFETs), or mixed self-assembled monolayers (mSAMs) of polyethylene glycol (PEG) and GA (Fab/PEG-SiNWFETs), to detect the rabbit IgG at different concentrations in a high-ionic-strength environment (150 mM Bis-Tris Propane) followed by incubation with R18, an aptamer which can specifically target rabbit IgG, for signal enhancement. Empirical results revealed that the signal produced by the sensors with Fab probes was greatly enhanced compared to the ones with whole antibody (Wab) after detecting similar concentrations of rabbit IgG. The Fab/PEG-SiNWFET immunosensors exhibited an especially improved limit of detection to determine the IgG level down to 1 pg/mL, which has not been achieved by the Wab/PEG-SiNWFET immunosensors.