Nanopillars Research Articles

Enhancement of optical and mechanical properties of Si nanopillars by ALD TiO2 coating

The mechanical and optical properties of Si and TiO2–Si nanopillars (NPl) were investigated.

Electrical, Luminescent and Structural Properties of Nanopillar GaN/InGaN Multi-Quantum-Well Structures Prepared by Dry Etching

GaN/InGaN multiple quantum well (MQW) structures with undoped n-GaN cap imitating true light emitting diodes were grown on sapphire and converted to deep nanopillar (NP) structures by dry etching beyond the MQW region. Structural measurements, electrical measurements on Schottky diodes, microcathodoluminescence (MCL) spectra measurements indicate a strong relaxation of strain in NP MQWs manifested in the prominent increase of the bowing radius of the structures and in the blueshift of the MQW peak in MCL spectra. Various treatments of the as-etched NP MQWs (annealing at 700°C, etching in KOH, and soaking in (NH4)2S) progressively decreased the leakage current of Schottky diodes and increased the MQW MCL peak intensity due to the suppression of the dry-etching damage of the nanopillar sidewalls.

Influence of wetting state on optical reflectance spectra of Si nanopillar arrays

Finite-difference time-domain (FDTD) simulations showed that the reflectance spectra of crystalline Si nanopillar (NP) arrays with diameters of 40, 70, 100, and 130 nm differed depending on wetting state. The observed reflectance dips of the 40-nm-diameter NP array were in good agreement with those estimated from destructive interference conditions at the top and bottom of the NPs: the NP arrays were treated as a homogeneous medium with an effective permittivity according to the effective medium approximation model. In contrast, the dip positions of the FDTD-simulated spectra for 70-, 100-, and 130-nm-diameter NP arrays deviated from the results of interference calculations, particularly for short wavelengths. This suggested that Mie resonances in individual NPs significantly increased the absorption cross-section at the resonant wavelengths, which was sensitive to the refractive index of the surrounding medium (i.e., the wetting state). Optical reflectance measurements provide an easy and efficient means of inspecting the wetting behavior of non-flat surfaces.

Surface photovoltage characterizations of Si nanopillar arrays for verifying field-effect passivation using a SiNx layer

The surface photovoltage (SPV) characteristics of periodic nanopillar (NP) arrays formed on Si wafers were investigated. The NP arrays exhibited broadband omnidirectional antireflection effects with Mie resonance. Kelvin probe force microscopy (KPFM) revealed that the positive fixed charges in SiNx layers induced band bending at the Si surface and increased surface photovoltage (SPV) at the NP top surface. Estimated SPV values, determined by the amount of surface band bending, were similar in NPs and planar counterparts. This finding suggests that field effect passivation by the dielectric layer coating could help improve photovoltaic performance of nanostructure-based Si solar cells and that KPFM may be a useful tool for the investigation of surface electrical properties of Si nanostructures.

Dark photovoltaic effects of Ti-rooted TiO2 nanopillars as the anode in CdSe-sensitised solar cell

TiO2 nanopillars (NPs) rooted in Ti were cultivated hydrothermally with the strongest organic base tetramethylammonium hydroxide (TMAOH) in aqueous solution in Ti substrate. TiCl4 was used to coat TiO2 NPs with a nanoscale compact thin TiO2 layer against charge recombination with oxidised species in electrolyte. A consecutive layer of CdSe nanoparticles was induced by successive ionic layer adsorption and reaction (SILAR) mode for sensitisation by light harvesting. J–V curves surprisingly indicate that untreated TiO2 NPs photoanode rooted in Ti produces considerable currents in the dark (thermal photovoltaic power generation) even higher compared to that under intense UV illumination, signifying TiCl4–CdSe treatment does not profit. Electrochemical impedance spectroscopy fitting parameters further reveal that the parameters associated with injection and recombination issues have been improved, as a comprehensive electrochemical result of TiCl4–CdSe treatment. Thus, the current-limiting problem probably arises from the remarkable continuous absorption bands in a wide spectrum range from visible to far-infrared, which is confirmed by UV–vis absorption spectra. However, the reason for the substantial photocurrent-limiting is that the intense irradiating UV light causes the innumerable electrons distributed in the inter-band-gap states to transit to much higher levels above the CB edge of TiO2 with excessive energy, then they collide with TiO2 crystall lattices to release thermal energy eventually to raise the transport resistance.

Direct spontaneous growth and interfacial structural properties of inclined GaN nanopillars on r-plane sapphire

The spontaneous growth of GaN nanopillars (NPs) by direct plasma-assisted molecular beam epitaxy on nitridated r-plane sapphire substrates has been studied. The emanation of metal-polarity NPs from inside an a-plane nonpolar GaN film was found to depend on both the substrate nitridation and the growth conditions. The density of NPs increased with increasing the duration of the nitridation process and the power applied on the radio-frequency plasma source, as well as the III/V flux ratio, while variation of the first two parameters enhanced the roughness of the substrate's surface. Transmission electron microscopy (TEM) techniques were employed to reveal the structural characteristics of the NPs and their nucleation mechanism from steps on the sapphire surface and/or interfacial semipolar GaN nanocrystals. Lattice strain measurements showed a possible Al enrichment of the first 5–6 monolayers of the NPs. By combining cross-sectional and plan-view TEM observations, the three-dimensional model of the NPs was constructed. The orientation relationship and interfacial accommodation between the NPs and the nonpolar a-plane GaN film were also elucidated. The NPs exhibited strong and narrow excitonic emission, suggesting an excellent structural quality.

Uniformly thick tri-color filters capitalizing on an etalon with a nanostructured cavity.

Highly efficient tri-color filters having uniform thickness are proposed based on an etalon tapping into a nanostructured cavity, where a hexagonal lattice of nanopillars (NPs) with a high refractive index is embedded in a base material with a low index. The nanostructured cavity is presumed to behave as a homogeneous medium, which provides a wide range for the effective refractive index in accordance with both the volume fraction of the NPs and the index contrast between the NPs and the base. Hence, for the etalon-based filters, the resonance wavelength can be efficiently tuned by simply altering the effective index rather than the thickness of the cavity, so as to span the entire visible regime including red, green, and blue (RGB) colors. In particular, a hexagonal lattice of NPs was introduced to extend the available range of effective index due to its highly flexible volume fraction. The NP-base index contrast has been pertinently maximized to achieve effective indices leading to RGB colors, to the extent that the nanostructured cavity can be safely modeled as a homogeneous medium. The proposed RGB color filters were finally designed to have an identical thickness of 240 nm by setting the diameters of the NPs at 95, 70, and 40 nm, to achieve a periodicity of 100 nm, considering that TiO2 and Al2O3 can be practically selected as the material candidate for the NPs and the base, respectively. A high transmittance of ∼78% and a suitable 1 dB bandwidth of ∼51 nm were obtained for the tri-color filters, which were further confirmed to exhibit polarization-independent transfer characteristics.

Surface modifications induced by high fluxes of low energy helium ions.

Several metal surfaces, such as titanium, aluminum and copper, were exposed to high fluxes (in the range of 1023 m−2s−1) of low energy (<100 eV) Helium (He) ions. The surfaces were analyzed by scanning electron microscopy and to get a better understanding on morphology changes both top view and cross sectional images were taken. Different surface modifications, such as voids and nano pillars, are observed on these metals. The differences and similarities in the development of surface morphologies are discussed in terms of the material properties and compared with the results of similar experimental studies. The results show that He ions induced void growth and physical sputtering play a significant role in surface modification using high fluxes of low energy He ions.

Human mesenchymal stem cell behavior on femtosecond laser-textured Ti-6Al-4V surfaces.

The aim of the present work was to investigate ultrafast laser surface texturing as a surface treatment of Ti-6Al-4V alloy dental and orthopedic implants to improve osteoblastic commitment of human mesenchymal stem cells (hMSCs). Surface texturing was carried out by direct writing with an Yb:KYW chirped-pulse regenerative amplification laser system with a central wavelength of 1030 nm and a pulse duration of 500 fs. The surface topography and chemical composition were investigated by scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. Three types of surface textures with potential interest to improve implant osseointegration can be produced by this method: laser-induced periodic surface structures (LIPSSs); nanopillars (NPs); and microcolumns covered with LIPSSs, forming a bimodal roughness distribution. The potential of the laser treatment in improving hMSC differentiation was assessed by in vitro study of hMSCs spreading, adhesion, elongation and differentiation using epifluorescence microscopy at different times after cell seeding, after specific stainings and immunostainings. Cell area and focal adhesion area were lower on the laser-textured surfaces than on a polished reference surface. Obviously, the laser-textured surfaces have an impact on cell shape. Osteoblastic commitment was observed independently of the surface topography after 2 weeks of cell seeding. When the cells were cultured (after 4 weeks of seeding) in osteogenic medium, LIPSS- and NP- textured surfaces enhanced matrix mineralization and bone-like nodule formation as compared with polished and microcolumn-textured surfaces. The present work shows that surface nanotextures consisting of LIPSSs and NPs can, potentially, improve hMSC differentiation into an osteoblastic lineage.

The origin of strain avalanches in sub-micron plasticity of fcc metals

Numerous experimental observations have recently demonstrated two aspects of sub-micron plasticity in nano pillars: (1) the statistical nature of the yield strength, and (2) the existence of dynamic temporal oscillations in the hardening regime. The present work is focused on the second aspect, where we study strain avalanches in nickel nano pillars via a series of 3-D dislocation dynamics (DD) simulations. The computer simulations reveal the existence of a key mechanism that explains the origin of oscillations in post-yield plastic flow in fcc metals. Small dipolar-loops easily form through several cross-slip mechanisms, and then act as internal glide dislocation sources. When dislocations exit pillar boundaries, they leave many dipolar-loops behind. At this stage, the pillar is starved from dislocations and an increase in stress is necessary to keep up with the applied strain rate (hardening stage). At some critical stress level, dipolar-loops become effective Frank–Read sources of new glide dislocations that collectively move in a burst mode, producing strain avalanches (softening stage). The time delay between these two states, which is determined by the average dislocation velocity and pillar diameter, leads to plastic strain oscillations and intermittent dislocation avalanches, consistent with experimental observations.

Spin-transfer torque switching observed in nanopillar superconducting-magnetic hybrid Josephson junctions

Spin-transfer torque switching observed in nanopillar superconducting-magnetic hybrid Josephson junctions

Plasmonic nanopillar array embedded microfluidic chips: an in situ SERS monitoring platform

Ag/Si nanopillar (NP) array embedded microfluidic chips as an in situ SERS monitoring platform.

Intrinsic and extrinsic parameters for controlling the growth of organic single-crystalline nanopillars in photovoltaics.

The most efficient architecture for achieving high donor/acceptor interfacial area in organic photovoltaics (OPVs) would employ arrays of vertically interdigitated p- and n- type semiconductor nanopillars (NPs). Such morphology could have an advantage in bulk heterojunction systems; however, precise control of the dimension morphology in a crystalline, interpenetrating architecture has not yet been realized. Here we present a simple, yet facile, crystallization technique for the growth of vertically oriented NPs utilizing a modified thermal evaporation technique that hinges on a fast deposition rate, short substrate-source distance, and ballistic mass transport. A broad range of organic semiconductor materials is beneficial from the technique to generate NP geometries. Moreover, this technique can also be generalized to various substrates, namely, graphene, PEDOT-PSS, ZnO, CuI, MoO3, and MoS2. The advantage of the NP architecture over the conventional thin film counterpart is demonstrated with an increase of power conversion efficiency of 32% in photovoltaics. This technique will advance the knowledge of organic semiconductor crystallization and create opportunities for the fabrication and processing of NPs for applications that include solar cells, charge storage devices, sensors, and vertical transistors.

Periodic Nanopillar N-I-P Amorphous Si Photovoltaic Cells Using Carbon Nanotube Scaffolds

© 2014 IEEE. Arrays of periodic one-dimensional nanomaterials offer tunable optical properties in terms of light-matter interaction which are attractive for designing efficient optoelectronic devices. This paper presents a fabrication of bottom-up grown nanopillar (NP) array solar cells based on n-i-p thin-film amorphous silicon using scaffolds of vertically aligned carbon nanotube (CNT) array. The effects of varying the CNT spacing over the range from 800 to 2000 nm on optical and electrical properties of the solar cells were investigated. The NP solar cell with CNT spacing of 800 nm exhibited 'moth-eye' broadband antireflection behavior, showing an average reflectance value lower than 10%. The enhanced optical absorption translated to significant enhancements in photocurrent and quantum efficiency compared to a conventional planar solar cell under low light condition. The open-circuit voltage (Voc) of the NP solar cell was found systematically correlated with the CNT spacing and the illumination condition. The results presented here is of importance for developing high efficiency one-dimensional nanostructured solar cells.

Geometry Controlled Growth of Single Crystalline CdS Nanopillar Arrays and the Investigation of Their Electrical Properties

Recently, one dimensional semiconductor nanomaterials have been extensively explored as building blocks for nanoscale electronic devices such as field-effect transistors (FETs), optoelectronic devices such as solar cells, light emitting diodes, and lasers. Apart from their advantageous small dimensions for device miniaturization, semiconductor nanowires (NWs) and nanopillars (NPLs) have shown intriguing properties that promise high device performance. Notably, many of these intriguing properties have shown strong geometry dependence, in terms of length, diameter, pitch, cross-section shape etc. Past studies have shown that well organized nanostructure arrays exhibit unique photon management ability, which further resulted in the potency to demonstrate light absorption exceeding conventional limits. Therefore, the capability to program and engineer the shape and morphology of NWs and NPLs is of paramount importance and in urgent need. However, conventional bottom-up growth of NWs in free space using a catalytic process lacks precise control over NW length and diameter. Template-assisted electrochemical deposition can produce NWs/NPLs with designed geometry. Nevertheless, the typically low crystalline quality of its products places them into disadvantageous situation for high performance electronic and opto-electronic devices. Herein, we utilize a template-assisted, Vapor-Liquid-Solid (VLS) growth process for the fabrication of regular and single-crystalline NW/NPL arrays with tunable shapes. Self-organized and perfectly ordered Anodized Alumina Membranes (AAMs) have been fabricated by properly controlling the anodization conditions in combination with the utilization of nanoimprint technique. With these nanoengineered AAM as templates, a variety of nanostructures, including nanopillar arrays, nanotower arrays, and nanocone arrays, have been successfully fabricated. It is worth noting that due to the capability of programmable structural design and fabrication of the AAM templates, the semiconductor arrays can be well defined as desired. ZnO nanostructures were also prepared with the same method. This is to demonstrate that the process is applied for the growth of CdS NPLs, but can be easily extended to other materials. All in all, the process reported here presented a generic platform toward the controlled growth of nanostructures with tunable shape and geometry. In addition, we have investigated the electrical properties of the CdS NPLs from AAM assisted growth. Vertical NPL array devices and horizontal individual NPL field-effect transistors have been fabricated and characterized. The intrinsic CdS NPLs showed poor conductivity and weak gate dependent performance, which is not desirable for device applications. Thus, Indium was chosen as dopant to tailor the electrical and optical properties of CdS NPLs. Specifically, in-situ Indium doping was conducted with 80 mg of Indium metal pellet. The measurements showed that the location of the Indium doping source significantly affected carrier concentration, conductivity and field-effect mobility of the prepared CdS NPLs. Particularly, it was found that conductivity could be improved by 4 orders of magnitude and field-effect mobility could be enhanced up to 50 cm2/Vs via proper doping control. These results enable further applications of CdS nanopillars for nano-optoelectronic applications such as photodetection and photovoltaics in the future.

Enhanced carrier extraction of a-Si/c-Si solar cells by nanopillar-induced optical modulation

We demonstrate improved short-wavelength internal quantum efficiency (IQE) of a-Si/c-Si heterojunction (HJ) solar cells with a surface nanopillar (NP) array via simulation. The gain in IQE is attributed to the light-field modulation caused by the cavity resonance inside the NPs, in which the light energy is effectively localized within the c-Si bulk rather than the a-Si layer. The average IQE in the short-wavelength range (330–450 nm) is enhanced from 43.94% to 62.88% by the optimal NP array, with a maximum IQE of 80.98% at λ = 400 nm. The resulting current gain is over 38.25% compared to a planar HJ cell in this wavelength range, showing a well suppressed recombination-induced current loss. This light-management scheme may also find applications in other types of cells.

Top-down fabrication of large-area GaN micro- and nanopillars

Large-area gallium nitride (GaN) micro- and nanopillar (NP) arrays were fabricated by plasma etching of lithographically patterned GaN thin-film grown on Si substrate. Deep-ultraviolet lithography, inductively coupled plasma (ICP) etching, and subsequent chemical treatments were effectively utilized to fabricate GaN pillars with diameters ranging from 250 nm to 10 μm. The impact of various plasma etching process parameters and chemical etchants on the morphology, strain, and surface defects of these NPs were studied using scanning-electron microscopy, photoluminescence (PL), and Raman spectroscopy. It was found that the shape of the NPs can be controlled by the substrate temperature during the plasma etch and by using different gas chemistries. Room-temperature PL and Raman spectroscopy measurements revealed significant strain relaxation in 250 nm diameter pillars as compared to 10 μm diameter pillars. PL measurement also indicated that the surface damage from the plasma etch can be removed by etching in KOH-ethylene glycol solution. Post-ICP selective wet chemical etch enabled us to fabricate functional structures such as micro- and nanodisks of GaN, which potentially could be utilized in nitride-based resonators and lasers.

Electrochemically synthesized broadband antireflective and hydrophobic GaOOH nanopillars for III-V InGaP/GaAs/Ge triple-junction solar cell applications.

We report the efficiency enhancement of III-V InGaP/GaAs/ Ge triple-junction (TJ) solar cells using a novel structure, i.e., vertically-oriented gallium oxide hydroxide (GaOOH) nanopillars (NPs), as an antireflection coating. The optical reflectance properties of rhombus-shaped GaOOH NPs, which were synthesized by a simple, low-cost, and large-scalable electrochemical deposition method, were investigated, together with a theoretical analysis using the rigorous coupled-wave analysis method. For the GaOOH NPs, the solar weighted reflectance of ~8.5% was obtained over a wide wavelength range of 300-1800 nm and their surfaces exhibited a high water contact angle of ~130° (i.e., hydrophobicity). To simply demonstrate the feasibility of device applications, the GaOOH NPs were incorporated into a test-grown InGaP/GaAs/Ge TJ solar cell structure. For the InGaP/GaAs/Ge TJ solar cell with broadband antireflective GaOOH NPs, the conversion efficiency (η) of ~16.47% was obtained, indicating an increased efficiency by 3.47% compared to the bare solar cell (i.e., η~13%).

Microscopic model for fracture of crystalline Si nanopillars during lithiation

Silicon (Si) nanostructures are attractive candidates for electrodes for Li-ion batteries because they provide both large specific charging capacity and less constraint on the volume changes that occur during Li charging. Recent experiments show that crystalline Si anodes expand highly anisotropically through the motion of a sharp phase boundary between the crystalline Si core and the lithiated amorphous Si shell. Here, we present a microscopic model to describe the size-dependent fracture of crystalline Si nanopillars (NPs) during lithiation. We derive a traction–separation law based on the plastic growth of voids, which, in turn, is used in a cohesive zone-finite element model. The model allows for both the initiation of cracking and crack growth. The initial size and spacing of the nanovoids, assumed to be responsible for the fracture, together with the computed facture toughness, are chosen to conform to recent experiments which showed the critical diameter of Si NPs to be 300–400 nm. The anisotropy of the expansion is taken into account and that leads naturally to the observed anisotropy of fracture. The computed work of fracture shows good agreement with recent experimental results and it may be possible to use it to describe the failure for other loading and geometries.

Effects of nanopillar array diameter and spacing on cancer cell capture and cell behaviors.

While substrates with nanopillars (NPs) have emerged as promising platforms for isolation of circulating tumor cells (CTCs), the influence of diameter and spacing of NPs on CTC capture is still unclear. In this paper, CTC-capture yield and cell behaviors have been investigated by using antibody functionalized NPs of various diameters (120-1100 nm) and spacings (35-800 nm). The results show a linear relationship between the cell capture yield and effective contact area of NP substrates where a NP array of small diameter and reasonable spacing is preferred; however, spacing that is too small or too large adversely impairs the capture efficiency and specificity, respectively. In addition, the formation of pseudopodia between captured cells and the substrate is found to be dependent not only on cell adhesion status but also on elution strength and shear direction. These findings provide essential guidance in designing NP substrates for more efficient capture of CTCs and manipulation of cytomorphology in future.