Free-standing Nanowire Arrays Research Articles

Robust and fast response solar-blind UV photodetectors based on the transferable 4H-SiC free-standing nanowire arrays

Wide band gap semiconductor 4 H -SiC has been considered as the most promising candidate for developing solar blind ultraviolet (UV) photodetectors (PDs). However, the absence of PDs constructed by the desired 4 H -SiC nanostructures greatly limits their performance and practical application. In this study, the free-standing 4 H -SiC bamboo-like nanowire arrays with an even diameter/height of 30 nm/35 µm prepared by the anodic oxidation of 4 H -SiC bulk single crystal in a top-down manner are used to develop the solar-blind PDs. Experiment results reveal that the developed PDs can have an excellent solar-blind detection performance such as high sensitivity to 275 and 365 nm UV light, a considerable light-dark ratio (∼44.3), remarkably low dark current, fast response (47.3 ms), excellent repeatability and stability. Additionally, the unencapsulated PDs can have an excellent high-temperature stability up to 300 ºC. It is expected that the 4 H -SiC nanowire arrays-based PDs can have more potential than their counterpart bulk material for the future optoelectronic device applications. • The MSM solar-blind UV photodetectors based on bamboo-like 4 H -SiC nanowire arrays have been successfully fabricated. • The developed photodetectors can have a large light-dark ratio, high photoresponsivity, and remarkably low dark current. • The resulting photodetectors can achieve the fast response and an excellent high-temperature stability up to 300 ºC.

Detailed balance analysis of vertical GaAs nanowire array solar cells: exceeding the Shockley Queisser limit.

We performed detailed balance analysis using rigorous coupled-wave analysis (RCWA) on vertical GaAs nanowire (NW) arrays. Both freestanding NW arrays as well as NW arrays on a perfect back reflector are assessed. Both types of vertical NW arrays demonstrate efficiencies that exceed the Shockley Queisser (SQ) or radiative efficiency limit when the NWs are sufficiently long. The use of a back reflector enhances the efficiency of NW solar cells by increasing solar absorption and suppressing emission from the backside of the solar cell. We study the light trapping and material reduction advantages of NWs. Furthermore, we compare simulations that evaluate detailed balance efficiency with ultimate efficiency and show that ultimate efficiency studies can determine near-optimal solar cells while vastly reducing the number of simulations that need to be performed. While open circuit voltages above the radiative limit can be achieved, tradeoffs with short circuit current must be carefully considered. We also compare our simulation results to other claims in the literature that NWs are capable of exceeding the SQ limit.

Augmentation of absorption channels induced by wave-chaos effects in free-standing nanowire arrays.

Plenty of issues on quantal features in chaotic systems have been raised since chaos was accepted as one of the intrinsic properties of nature. Through intensive studies, it was revealed that resonance spectra in chaotic systems exhibit complicated structures, which is deeply concerned with sophisticated resonance dynamics. Motivated by these phenomena, we investigate light absorption characteristics of chaotic nanowires in an array. According to our results, a chaotic cross-section of a nanowire induces a remarkable augmentation of absorption channels, that is, an increasing number of absorption modes leads to substantial light absorption enhancement, as the deformation of cross-section increases. We experimentally demonstrate the light absorption enhancement with free-standing Si-nanowire polydimethylsiloxane (PDMS) composites. Our results are applicable not only to transparent solar cells but also to complementary metal-oxide-semiconductor (CMOS) image sensors to maximize absorption efficiency.

Template-assisted electrodeposition of freestanding Sn, In and Sn-In nanowire arrays in an ionic liquid

In this paper we report on the template assisted electrochemical synthesis of Sn, In, and Sn-In nanowire arrays in the air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO). Polycarbonate membrane was employed as a template for the synthesis of nanowires. The electrochemical synthesis of nanowire arrays occurred in two steps; the electrodeposition inside the pores of the membrane and the chemical dissolution of the membrane. The results show that high quality, freestanding nanowire arrays of Sn, In and Sn-In, can be made in the employed ionic liquid electrolytes via a template-assisted electrodeposition regime. The reported synthesis process is quite facile and assures the formation of mechanically stable, freestanding nanowire arrays.

Double-Sided Brush-Shaped TiO2 Nanostructure Assemblies with Highly Ordered Nanowires for Dye-Sensitized Solar Cells

We describe a seeded hydrothermal process for the growth of unique double-sided brush-shaped (DSBS) TiO2 nanostructure assemblies consisting of highly ordered rutile nanowires vertically aligned around an annealed TiO2 nanoparticle layer. The annealed TiO2 nanoparticle layer seeds the nanowire growth and also supports the DSBS structure. The morphology of the DSBS TiO2 nanostructure depends on the hydrothermal reaction time. The diameter of the nanowires is about 6.6 nm, and with increasing reaction time from 1 to 8 h the nanowire length increases from 0.6 to 6.2 μm, whereas the thickness of the nanoparticle layer decreases from 4.3 to 2.8 μm. These free-standing nanowire arrays provide large internal surface area, which is essential for minimizing carrier recombination in high performance photovoltaic devices. Furthermore, the nanowire architecture can help increase the rate of charge transport as compared to particulate films because of lower concentration of grain boundaries. The power conversion efficiency of backside (DSBS TiO2/FTO photoanode) illuminated dye-sensitized solar cells fabricated using the DSBS TiO2 nanostructure assembly is found to be depended on the nanowire length. A cell fabricated using 15.2 μm thick nanostructures sensitized by N719 has a short-circuit current density of 12.18 mA cm(-2), 0.78 V open circuit potential, and a 0.59 filling factor, yielding a maximum power conversion efficiency of 5.61% under AM 1.5 illumination.

Nanotransfer Molding of Free-Standing Nanowire and Porous Nanomembranes Suspended on Microtrenches

Direct transfer printing of functional materials has been employed in the development of sensors, displays, and energy-harvesting devices. The transfer process can be applied advantageously to depositions onto nonplanar and flexible surfaces at low temperatures. In this work, we fabricated free-standing nanowire arrays and nanomembranes on micrometer-scale trenches by nanotransfer molding. We also investigated how deposition pattern types vary with trench dimensions as well as processing pressure and temperature. Finally, a free-standing polymer membrane fabricated by nanotransfer molding was employed as a novel mask in the preparation of three-dimensional nanodot arrays.

Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays

Flexible electronics such as wearable electronic clothing, paper-like electronic devices, and flexible biomedical diagnostic devices are expected to be commercialized in the near future. Flexible energy storage will be needed to power these devices. Supercapacitor devices based on freestanding nanowire arrays are promising high power sources for these flexible electronics. Electrodes for these supercapacitor devices consisting of heterogeneous coaxial nanowires of poly (3,4-ethylenedioxythiophene) (PEDOT)-shell and MnO(2)-core materials have been shown in a half cell system to have improved capacitance and rate capabilities when compared to their pure nanomaterials; however, their performance in a full cell system has not been fully investigated. Herein, these coaxial nanowires are tested in both a symmetric and an asymmetric (utilizing a PEDOT nanowire anode) full cell configuration in the aspect of charge storage, charge rate, and flexibility without using any carbon additives and polymer binders. It is found that the asymmetric cell outperforms the symmetric cell in terms of energy density, rate capability, and cycle ability. The asymmetric device's electrode materials display an energy density of 9.8 Wh/kg even at a high power density of 850 W kg(-1). This device is highly flexible and shows fast charging and discharging while still maintaining 86% of its energy density even under a highly flexed state. The total device is shown to have a total capacitance of 0.26 F at a maximum voltage of 1.7 V, which is capable of providing enough energy to power small portable devices.

Ambient Surfactantless Synthesis, Growth Mechanism, and Size-Dependent Electrocatalytic Behavior of High-Quality, Single Crystalline Palladium Nanowires

In this report, we utilize the U-tube double diffusion device as a reliable, environmentally friendly method for the size-controlled synthesis of high-quality, single crystalline Pd nanowires. The nanowires grown in 200 and 15 nm polycarbonate template pores maintain diameters of 270 ± 45 nm and 45 ± 9 nm, respectively, and could be isolated either as individual nanowires or as ordered free-standing arrays. The growth mechanism of these nanowires has been extensively explored, and we have carried out characterization of the isolated nanowires, free-standing nanowire arrays, and cross sections of the filled template in order to determine that a unique two-step growth process predominates within the template pores. Moreover, as-prepared submicrometer and nanosized wires were studied by comparison with ultrathin 2 nm Pd nanowires in order to elucidate the size-dependent trend in oxygen reduction reaction (ORR) electrocatalysis. Subsequently, the desired platinum monolayer overcoating was reliably deposited onto the surface of the Pd nanowires by Cu underpotential deposition (UPD) followed by galvanic displacement of the Cu adatoms. The specific and platinum mass activity of the core-shell catalysts was found to increase from 0.40 mA/cm(2) and 1.01 A/mg to 0.74 mA/cm(2) and 1.74 A/mg as the diameter was decreased from the submicrometer size regime to the ultrathin nanometer range.

Optical properties of heavily doped GaAs nanowires and electroluminescent nanowirestructures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vaporphase epitaxy. Electroluminescent structures were realized in both axial pn-junctions insingle GaAs nanowires and free-standing nanowire arrays with a pn-junctionformed between nanowires and substrate, respectively. The electroluminescenceemission peak from single nanowire pn-junctions at 10 K was registered at anenergy of around 1.32 eV and shifted to 1.4 eV with an increasing current. Theline is attributed to the recombination in the compensated region present in thenanowire due to the memory effect of the vapor–liquid–solid growth mechanism.Arrayed nanowire electroluminescent structures with a pn-junction formed betweennanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eVand two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed tothe recombination in the p-doped GaAs. The other two lines correspond to thetunneling-assisted photon emission and band-edge recombination in the abrupt junction,respectively. Electroluminescence spectra are compared with the micro-photoluminescencespectra taken along the single p-, n- and single nanowire pn-junctions to find the origin ofthe electroluminescence peaks, the distribution of doping species and the sharpness of thejunctions.

Freestanding nanowire arrays from soft-etch block copolymer templates.

Nanoporous poly(4-fluorostyrene) templates on gold-coated silicon/silicon oxide substrates were prepared by the electric field alignment of poly(4-fluorostyrene)-b-poly(d,l-lactide) block copolymer thin films followed by mild degradation of the polylactide phase using dilute aqueous base. Electrochemical deposition of nanowires was accomplished using a protocol for the preparation of copper oxide. Freestanding nanowires were observed after removal of the template by either simple dissolution of the poly(4-fluorostyrene) or by treatment with UV irradiation. The annealing time, the electric field strength used to align the block copolymer films, and the template removal method are shown to influence the freestanding nanowire arrays. The "soft-etch" method described is generally useful for the preparation of templates and nanostructures that are sensitive to more aggressive template removal processes.

Preparation of Free-Standing Nanowire Arrays on Conductive Substrates

By combining supercritical drying technique with AAO template-assisted electrodeposition, noncollapsed, vertically aligned, and free-standing nanowire arrays on a conductive Au film have been fabricated. We also demonstrate that these free-standing nanowire arrays can be feasibly used for fabricating nanowire-based electrically driven devices.

Electrochemical preparation and surface properties of gold nanowire arrays formed by the template technique

A new procedure for preparing free-standing nanowire arrays is described. This is based on a template method which entails electrochemical metal deposition into nanometer-wide parallel pores of porous anodic oxide films on aluminum. By varying the preparation conditions and by applying electrochemical posttreatment methods, gold nanowire arrays are prepared, whose morphological and dimensional properties and oxygen content in the nanowire surface can be varied and investigated by SEM and XPS-characterization. The voltammetric behaviour of such nanowire arrays shows a significant enhancement of the ratio between the total electrolytically exposed surface area and the diffusionally accessible surface area in comparison with macroscopic flat gold electrodes.