NW Density Research Articles

Optimized thermoelectric properties and geometry parameters of annular thin-film thermoelectric generators using n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thin films for energy harvesting

Thermoelectric generators can be used as energy harvesting for power supply in Internet of Thing sensors. Here, material properties and geometry parameters were investigated to improve the annular thin-film thermoelectric generator (ATTEG) output performances. The as-deposited and annealed n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 were deposited by radio frequency magnetron sputtering which resulted in higher Seebeck coefficient and electrical conductivity of annealed thin films. The temperature distributions and output performances in ATTEG were calculated using numerical simulation to identify the geometry parameters. The results showed that thinner substrate with higher output performances, larger legs number with higher output voltage, and a peak point in the maximum output power was observed for the effect of the angle ratio of p-type leg to n-type leg with optimal value of 1. We fabricated ATTEGs with different TE legs to identify the machining accuracy, and the ATTEG with 12 legs resulted in output voltage of 27.7 mV, the maximum output power of 169.0 nW and power density of 127.39 nW∕cm2 at a temperature difference of 23 K. Hollowing and nicking on substrate were proposed as further improvement, and resulted in increasement of 6.6% in output voltage and 13.6% in the maximum output power.

Physical, optical and electrical studies on hybrid Ag NPs/NiSi NWs electrode as a DNA template for biosensor

By means of Chemical Vapor Deposition (CVD), Nickel silicide nanowires (NiSi NWs) were grown on silica substrate followed by a solid-diffusion controlled growth mechanism at different substrate temperatures ranging from 400 to 520 °C. The NWs were then subsequently decorated with Ag nanoparticles (Ag NPs) via a simple thermal evaporation technique. The morphologies of the NiSi NWs and Ag NPs/NiSi NWs showed noticeable dependence on the growth substrate temperatures. NiSi NWs grown at 470 °C exhibited the highest NW density with a uniform decoration of spherical Ag NPs of ~13.7 ± 1.5 nm of diameter. The Ag NPs/NiSi NWs appeared to exhibit a better crystallinity compared to the as-grown NiSi NWs, which could be attributed to metal induced crystallization. Moreover, the decoration of Ag NPs on NiSi NWs showed a significant enhancement in Raman peak intensities mainly in the Ni2Si, NiSi, and NiSi2 phases. The Ag NPs/NiSi NWs demonstrated a well-defined surface plasmon resonance absorption bands centered at around 420 nm, which elucidate the effectiveness of the decoration of Ag NPs on NiSi NW surfaces with a thin amorphous dielectric barrier layer. Using current-voltage (I-V) measurement, Ag NPs/NiSi NWs and NiSi NWs shown to exhibit a surface type Schottky diode with rectifying behavior. The values of ideality factor for the NiSi NWs and Ag NPs/NiSi NWs Schottky diodes were calculated to be 6.992 and 2.559 respectively. Additionally, the values of series resistances were also calculated , where the series resistance obtained are ̃ 91.856 and ̃ 38.697 kΩ for NiSi NWs and Ag NPs/NiSi NWs DNA electrodes, respectively.

Core/shell copper nanowire networks for transparent thin film heaters

Copper nanowires (Cu NWs) appear as the strongest alternative to silver nanowires (Ag NWs) in transparent conductors. Cu NWs; however, are more prone to oxidation compared to Ag NWs even at room temperature. This problem becomes more severe when Cu NWs are used as transparent thin film heaters (TTFHs). In this work, we have utilized ALD deposited zinc oxide (ZnO) shell layers, and provide a comparison with typically used aluminum oxide (Al2O3) shell layers to improve the TTFH performance. While Cu NW network TTFHs barely withstood temperatures around 100 °C, critical thickness of ALD deposited Al2O3 and ZnO layers were determined to find out TTFH limits. Maximum stable and reproducible temperatures of 273 °C and 204 °C were obtained for Al2O3 and ZnO deposited Cu NW network TTFHs, respectively. An extensive parametric study on the NW density and oxide type in conjunction with the electrical conductivity and optical transmittance was conducted. A remarkably high heating rate of 14 °C s−1 was obtained from the fabricated core/shell networks with improved oxidation stability under ambient and high humidity conditions. Finally, these high performance core/shell Cu NW network TTFHs were utilized as efficient defrosters.

Morphology-controlled copper nanowire synthesis and magnetic field assisted self-assembly.

Morphology, dimensions, crystalline structure and compositions of nanomaterials are very critical in determining their unique characteristics. Here we report how the reducing agent concentration affects the surface morphology of copper nanowires and establish the optimized reaction conditions to synthesize high aspect ratio nanowires with a smooth surface. Also, reported is the magnetic field assisted technique to control the orientational and positional ordering of cupronickel nanowires (Cu/Ni NWs) on a large-scale area. A combination of magnetic field, surface derivatization and photolithographic techniques allowed self-assembly of Cu/Ni NWs into channels. The channel resistance as a function of the applied magnetic field during fabrication shows an anomalous decrease owing to the positional end-to-end alignment of NWs. Magnetic field and areal NW density dependence of NW sheet resistance in channels is presented and analyzed using scaling theoretical models.

THz Study on the Role of ZnO Crystallinity in ZnO/AgNW/ZnO Composite Electrodes

We present the conducting mechanism of silver nanowire (AgNW) networks embedded between zinc oxide (ZnO) bilayers studied by terahertz (THz) reflection spectroscopy. Percolatively connected AgNWs have been considered as one of the most promising alternatives for indium-tin-oxide transparent conductive electrodes (TCEs). Although AgNWs can be spin-coated on nearly any kind of substrate, they leave a large open area between randomly scattered NWs. In ZnO/AgNW/ZnO composite electrodes, the conductive ZnO underlayer has the advantage in charge transport through electrodes and another ZnO layer covering the AgNW/ZnO layer can suppress oxidation of NW surface and improve the thermal stability. The performance of AgNW-based TCEs is highly correlated to the connectivity of NWs at junctions, which can be realized by percolational transition near a critical NW density. Terahertz reflection spectroscopic measurement shows that the photoconductivity of ZnO/AgNW/ZnO composite electrodes is strongly influenced by the ZnO crystallinity. The percolational behavior of the confinement factor in THz spectroscopic analysis reveals that single-crystalline ZnO layers in ZnO/AgNW/ZnO increase the photoconductivity by forming wider effective charge collection area around AgNWs and filling the gap between AgNWs to improve the ohmic contact.

Optical properties of InxGa1−xN/GaN quantum-disks obtained by selective area sublimation

We study the cathodoluminescence (CL) and the photoluminescence (PL) properties of InxGa1−xN/GaN quantum disks (QDisks) included in nanowires obtained by selective area sublimation from an InxGa1−xN/GaN multiple quantum well using an in situ SixNy nanomasking performed in a molecular beam epitaxy reactor. The NW density can be adjusted as a function of the SixNy coverage. The mean NW diameter is found almost constant for NW density varying by two orders of magnitude. The light emission from the InxGa1−xN/GaN QDisks is blue-shifted compared to the quantum wells. The origin of this shift is discussed in terms of strain and lateral confinement effects in the QDisks. The CL and PL spectra reveal strongly reduced peak linewidths for experiments conducted on a few or single objects isolated by using sub-micrometer apertures in a metallic mask.

Temperature-controlled transparent-film heater based on silver nanowire–PMMA composite film

We fabricated a high-performance film heater based on a silver nanowire and polymethyl methacrylate (Ag NW–PMMA) composite film, which was synthesized with the assistance of mechanical lamination and an in situ transfer method. The films exhibit excellent conductivity, high figure of merit, and strong adhesion of percolation network to substrate. By controlling NW density, we prepared the films with a transmittance of 44.9–85.0% at 550 nm and a sheet resistance of 0.13–1.40 Ω sq−1. A stable temperature ranging from 130 °C–40 °C was generated at 3.0 V within 10–30 s, indicating that the resulting film heaters show a rapid thermal response, low driving voltage and stable temperature recoverability. Furthermore, we demonstrated the applications of the film heater in defrosting and a physical therapeutic instrument. A fast defrosting on the composite film with a transmittance of 88% was observed by applying a 9 V driving voltage for 20 s. Meanwhile, we developed a physical therapeutic instrument with two modes of thermotherapy and electronic-pulse massage by using the composite films as two electrodes, greatly decreasing the weight and power consumption compared to a traditional instrument. Therefore, Ag NW–PMMA film can be a promising candidate for diversified heating applications.

High-performance, bare silver nanowire network transparent heaters

Silver nanowire (Ag NW) networks are one of the most promising candidates for the replacement of indium tin oxide (ITO) thin films in many different applications. Recently, Ag-NW-based transparent heaters (THs) showed excellent heating performance. In order to overcome the instability issues of Ag NW networks, researchers have offered different hybrid structures. However, these approaches not only require extra processing, but also decrease the optical performance of Ag NW networks. So, it is important to investigate and determine the thermal performance limits of bare-Ag-NW-network-based THs. Herein, we report on the effect of NW density, contact geometry, applied bias, flexing and incremental bias application on the TH performance of Ag NW networks. Ag-NW-network-based THs with a sheet resistance and percentage transmittance of 4.3 Ω sq−1 and 83.3%, respectively, and a NW density of 1.6 NW μm−2 reached a maximum temperature of 275 °C under incremental bias application (5 V maximum). With this performance, our results provide a different perspective on bare-Ag-NW-network-based transparent heaters.

Three-Dimensional Arrays of Lead Halide Perovskite Nanowire Based Image Sensors

Lead halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention globally. In this work, we report for the first time a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) MAPbI3 NW arrays in nanoengineering templates. This unique CVD process utilizes Pb metal nanoclusters at the bottom of vertical nanochannels to initiate high quality MAPbI3 NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of 4×108/cm2~109/cm2 at a sizable scale of ~9 cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. It was found that the NW sensors can respond to dynamic optical input with reasonable speed, thus video capturing function of the NW image sensor was also successfully demonstrated. As the diameter of each NW can be as small as hundreds of nanometers and each NW can serve as one sensor pixel, this unique image sensor design can potentially lead to extremely high resolution approaching optical diffraction limit. In addition, fabrication and characterization of flexible image sensors have also been demonstrated in this work. The flexible optical sensor arrays can find broad applications for wearable electronics, electronic eyes, multifunctional robotic and artificial skins, etc. Lead halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention globally. In this work, we report for the first time a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) MAPbI3 NW arrays in nanoengineering templates. This unique CVD process utilizes Pb metal nanoclusters at the bottom of vertical nanochannels to initiate high quality MAPbI3 NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of 4×108/cm2~109/cm2 at a sizable scale of ~9 cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. It was found that the NW sensors can respond to dynamic optical input with reasonable speed, thus video capturing function of the NW image sensor was also successfully demonstrated. As the diameter of each NW can be as small as hundreds of nanometers and each NW can serve as one sensor pixel, this unique image sensor design can potentially lead to extremely high resolution approaching optical diffraction limit. In addition, fabrication and characterization of flexible image sensors have also been demonstrated in this work. The flexible optical sensor arrays can find broad applications for wearable electronics, electronic eyes, multifunctional robotic and artificial skins,etc.

The top-down fabrication of a 3D-integrated, fully CMOS-compatible FET biosensor based on vertically stacked SiNWs and FinFETs

A 3D vertically stacked silicon nanowire (SiNW) and Fin field effect transistor (FET) featuring a high density array (7 or 8×20 SiNWs,>4 Fins vertically stacked) of fully depleted, ultra-thin (SiNWs diameters ∼15–30nm, Fin width/height fw ∼30nm/fh ∼150nm), long (>2μm) and suspended channels has been successfully fabricated for the first time by a top-down, complementary metal oxide semiconductor (CMOS) compatible process on silicon on insulator (SOI) substrates for biosensing applications. SiNW and FinFETs continue to draw interest as biological sensors for their outstanding sensitivity due to their large surface to volume ratios (S/V), high selectivity towards a myriad of analytes through surface functionalization and the possibility for label free, direct monitoring of biological activities. In this paper we describe different strategies and limitations for the successful development and fabrication of a 3D Si nanostructure FET using conventional clean-room fabrication techniques and their integration into heterogeneous systems. The vertical stacking allows for higher utilization of the Si substrate, high output currents (1.3mA/μm, normalized to a NW diameter of 30nm at VSG=3V, and Vd=50mV, for a standard structure with 7×10NWs stacked) and high chances for biomolecule interaction as the number of conduction channels increases. Also, as the NWs/Fins are suspended, their entire surface area is exposed to the sensing analyte. The configuration of the 3D sensor furthermore offers excellent electrostatic control of the channels by the possibility of applying symmetric or asymmetric gate potentials to tune the sensitivity and optimize the power consumption. Our fabrication scheme is competitive in terms of scaling and NW density in comparison to bottom-up and other top-down approaches with the advantage of using a CMOS-compatible, high yield (>90%), reproducible and robust processes.

Growth of InAs quantum dots on Si-based GaAs nanowires by controlling the surface adatom diffusion

InAs/GaAs dots-on-nanowire (NW) hybrid heterostructures are grown on Si substrate by metal organic chemical vapor deposition. The formation of InAs quantum dots (QDs) is intimately associated with the NW density as well as the substrate surface properties, both of which strongly affect the surface adatom diffusion. InAs QDs are realized with a short deposition time by reducing the NW density. The QDs exhibit specific facets and pure zinc blende structure, residing on a wetting layer of several nanometers. Photoluminescence emission from the QDs is observed at room temperature, with a linewidth of 186meV. The results are promising for future integration of III–V NW hybrid devices, especially photovoltaic cells on Si.

Epitaxial growth of an antireflective, conductive, graded index ITO nanowire layer

Nanoporous and nanostructured films, assemblies and arrangements are important from an applied point of view in microelectronics, photonics and optical materials. The ability to minimize reflection, control light output and use contrast and variation of the refractive index to modify photonic characteristics can provide routes to enhanced photonic crystal devices, omnidirectional reflectors, antireflection coatings and broadband absorbing materials. This work shows how multiscale branching of defect-free ITO NWs grown as a layer with a graded refractive index improves antireflection properties and shifts the transparency window into the near-infrared (NIR). The measurements confirm the structural quality and growth mechanism of the NW layer without any heterogeneous seeding for NW growth. Optical reflectance measurements confirm broadband antireflection down to <5% between 1.3-1.6 um which is tunable with the NW density. The work also outlines how the suppression of the Burstein-Moss shifts using refractive index variation allows transparency in a conductive NW layer into NIR range.

Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth

Selective area growth of single crystalline Sn-doped In2O3 (ITO) nanowires synthesized via vapor–liquid–solid (VLS) method at 600°C was applied to improve the field emission behavior owing to the reduction of screen effect. The enhanced field emission performance reveals the reduction of turn-on fields from 9.3 to 6.6 V μm−1 with increase of field enhancement factors (β) from 1,621 to 1,857 after the selective area growth at 3 h. Moreover, we find that the screen effect also highly depends on the length of nanowires on the field emission performance. Consequently, the turn-on fields increase from 6.6 to 13.6 V μm−1 with decreasing β values from 1,857 to 699 after the 10-h growth. The detailed screen effect in terms of electrical potential and NW density are investigated in details. The findings provide an effective way of improving the field emission properties for nanodevice application.

Silicon Nanowires Obtained by Low Temperature Plasma-Based Chemical Vapor Deposition.

ABSTRACTSilicon Nanowires (Si-NWs) are obtained by vapor-liquid-solid growth using an inductively coupled chemical vapor deposition system which works at temperatures lower than 400 °C. Gold nanodots are used as metal catalyst. The selective growth of Si-NWs on the gold nanodots is obtained by controlling the contribution coming from the uncatalyzed growth on the bare Si substrate. In this way the final NW length can be controlled, and it is not influenced by the thickness of the uncatalyzed layer. The important parameter ruling the NW growth is found to be the plasma power which governs the dissociation of the Si precursor gas. Final NW lengths of 1 μm are obtained at temperatures of 380 °C with a thickness of uncatalyzed layer equal to zero. Also the NW density is addressed in this work and it is optimised by increasing the gold equivalent thickness. The NW density is increased from 2.9×108 to 1.3×1010 cm-2, when the gold equivalent thickness passes from 1.8 nm to 2.2 nm.

Nano-Structured ZnSe/CIS Heterojunction Solar Cells with ZnSe/ZnO Coaxial Nanowires

We have demonstrated superstrate type nano-structured solar cells with ZnSe/ZnO coaxial NWs embedded in the CuInSe2 (CIS) layer. Complete filling of the CIS film into the narrow spaces between the ZnO NWs was realized by growing the NWs sparsely. The ZnSe/CIS heterojunction was self-formed by converting a skin (∼50 nm) layer of ZnO after the selenization. The influences of the NW length and density on light trapping and on cell conversion efficiency have been investigated. A 30% improvement in Jsc and higher efficiency has been achieved by embedding NWs in the CIS layer. Conversion efficiency of 1.79% was obtained from a ZnSe/CIS heterojunction solar cell fabricated on sparse ZnO NWs with Voc = 599 mV, Jsc = 11.60 mA/cm2 and 25.71% fill factor.

Optical properties of laterally aligned Si nanowires for transparent electronics applications

We have investigated the optical properties of laterally aligned Si nanowire (SiNW) arrays in order to explore their potential applicability in transparent electronics. The SiNW array exhibited good optical transparency in the visible spectral range with a transmittance of ∼90% for a NW density of ∼20–25 per 10 μm. In addition, polarization-dependent measurements revealed a variation in transmittance in the range of 80%–95% depending on the angle between the polarization of incident light and the NW axis. Using the SiNWs, we demonstrated that transparent transistors exhibit good optical transparency (greater than 80%) and showed typical p-type SiNW transistor characteristics. Open image in new window

Recent Achievements in Nanostructured Photovoltaic Devices

This mini-review summarizes some key interesting applications and perspectives of nanostructured devices for future nanoelectronics, among them are photonic circuits, carbon nanostructures for chemisensors, unique Ag-Cu-nanocluster contacts for high-effective solar cells. Recent patents in the field are also discussed.

Reproduction reduces photosynthetic capacity in females of the subdioecious Honckenya peploides

As a consequence of the different reproductive functions performed by the sexes, sexually dimorphic/polymorphic plants may exhibit gender-related variations in the energy and resources allocated to reproduction, and in the physiological processes that underlie these differences. This study investigated whether the sexes of the subdioecious plant Honckenya peploides differ in ecophysiological traits related to photosynthetic capacity and whether possible differences depend on reproductive status and on the plant’s position (edge or centre) in the population. We registered in three sites in NW Spain, the sex and density of shoots of two segregated clumps of plants. These clumps represent an extreme case of sex-ratio variation across space, with separated single-sex clumps of plants. In two of these sites we measured photosynthetic efficiencies, chlorophyll content, and specific leaf areas. In females, reproduction reduced photochemical efficiency, chlorophyll content and increased the specific leaf area, which is a key leaf trait related to photosynthetic capacity. In males, no differences due to reproduction were detected. The position within the clump affected the specific leaf area of the shoots, with shoots growing at the edge having the lowest values, regardless of the sex. Finally, the effects of position in photosynthetic efficiency and chlorophyll content where highly variable among clumps. We conclude that the differential effects of reproduction on sexes may entail different costs that could be crucial in the outcome of interactions between them, contributing to their spatial segregation.

One-Dimensional Semiconductor Nanostructure Based Thin-Film Partial Composite Formed by Transfer Implantation for High-Performance Flexible and Printable Electronics at Low Temperature

Having high bending stability and effective gate coupling, the one-dimensional semiconductor nanostructures (ODSNs)-based thin-film partial composite was demonstrated, and its feasibility was confirmed through fabricating the Si NW thin-film partial composite on the poly(4-vinylphenol) (PVP) layer, obtaining uniform and high-performance flexible field-effect transistors (FETs). With the thin-film partial composite optimized by controlling the key steps consisting of the two-dimensional random dispersion on the hydrophilic substrate of ODSNs and the pressure-induced transfer implantation of them into the uncured thin dielectric polymer layer, the multinanowire (NW) FET devices were simply fabricated. As the NW density increases, the on-current of NW FETs increases linearly, implying that uniform NW distribution can be obtained with random directions over the entire region of the substrate despite the simplicity of the drop-casting method. The implantation of NWs by mechanical transfer printing onto the PVP layer enhanced the gate coupling and bending stability. As a result, the enhancements of the field-effect mobility and subthreshold swing and the stable device operation up to a 2.5 mm radius bending situation were achieved without an additional top passivation.