Nanopillar Structures Research Articles

Highly Water-Repellent Nanostructure on Quartz Surface Based on Cassie-Baxter Model With Filling Factor

We fabricated a highly water-repellent quartz nanopillar (NP) structure to investigate the effect of varying the contact angle (CA) by using 10-nm-order gaps and 10-nm-diameter NPs. Gaps from 15 to 30 nm led to CAs of more than 100°, showing hydrophobicity, to a maximum of 105°. The mechanism of repelling water on quartz could be explained by the Cassie-Baxter model with a filling factor. A gap of more than 30 nm fills with water due to capillarity, but a gap of less than 30 nm causes water to be repelled by air. We were able to repeatedly fabricate a quartz NP structure with a controllable gap by using a combination of a bio-template and neutral-beam etching and found this structure to be highly water-repellent. The structure has high durability and optical transparency. As a result, we conclude that it can be used in sensors and lenses on various devices such as cameras and radars.

Feasibility study of nanopillar LED array for color-tunable lighting and beyond.

An LED chip containing monolithically integrated red, green, and blue channels was fabricated and characterized. Using local strain engineering in gallium nitride p-i-n nanopillar structures, each color channel emits a distinct color with emission wavelength determined entirely by the diameter of the nanopillar. The crosstalk between color channels is negligible. As a result, individually addressable color channels can be integrated on the same substrate which will be suitable for color-tunable lighting applications. Optical and electrical properties were measured and discussed. Fabrication challenges which degraded power efficiency of the shorter-wavelength channel were analyzed. Potential strategies for improvements were proposed.

Optical and Surface-Enhanced Raman Scattering Properties of Black and Metallic Silver Micro/Nano Structures Fabricated on Cicada Wings by Silver Mirror Reaction.

Silver was deposited on cicada (Cryptotympana facialis) wings to fabricate black and metallic silver micro/nano structures on the wings (black-Ag wings and metallic-Ag wings) by the silver mirror reaction. The transparent cicada wings with close-packed nanopillar array structures were used as the substrates based on natural materials. As the silver mirror reaction proceeded, the color of the wing gradually changed from transparent to brown, then to black, and finally to metallic silver. A thin Ag film coated nanopillar array structure is seen in the black-Ag wing. The transmittance and reflectance of the black-Ag wing were nearly 0% and an average of about 4% in the wavelength range of 200-800 nm, respectively. Densely stacked colloidal Ag particles with an average diameter of 155 nm were observed on the surface of the metallic-Ag wing. The surface enhanced Raman scattering (SERS) signal intensities of Rhodamine 6G (R6G) dripped and dried on the black-Ag wing, thin Ag film sputtered on glass slides, and metallic-Ag wing increased in that order. In particular, the SERS signal intensities of R6G on metallic-Ag wing were greater than two orders of magnitude than those on the black-Ag wing and thin Ag film.

Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centers in diamond

We investigate the influence of plasma treatments, especially a 0 V-bias, potentially low damage O2 plasma as well as a biased Ar/SF6/O2 plasma on shallow, negative nitrogen vacancy (NV−) centers. We ignite and sustain our 0 V-bias plasma using purely inductive coupling. To this end, we pre-treat surfaces of high purity chemical vapor deposited single-crystal diamond (SCD). Subsequently, we create ∼10 nm deep NV− centers via implantation and annealing. Onto the annealed SCD surface, we fabricate nanopillar structures that efficiently waveguide the photoluminescence (PL) of shallow NV−. Characterizing single NV− inside these nanopillars, we find that the Ar/SF6/O2 plasma treatment quenches NV− PL even considering that the annealing and cleaning steps following ion implantation remove any surface termination. In contrast, for our 0 V-bias as well as biased O2 plasma, we observe stable NV− PL and low background fluorescence from the photonic nanostructures.

Boosting Cell Performance of LiNi0.8 Co0.15 Al0.05 O2 via Surface Structure Design.

Although the high energy density and environmental benignancy of LiNi0.8 Co0.15 Al0.05 O2 (NCA) holds promise for use as cathode material in Li-ion batteries, present low rate capabilities, and fast capacity fade limit its broad commercial applications. Here, it is reported that surface modification of NCA cathode (R-3m) with 5 nm-thick nanopillar layers and Fm-3m structures significantly improves electrode structure, morphology, and electrochemical performance. The formation of nanopillar layers increases cycling and working voltage stability of NCA by shielding the host material from hydrofluoric acid and improves structural stability with the electrolyte. The modified NCA cathode exhibits an enhanced 89% capacity retention at a rate of 1 C over that of pristine NCA (75.2%) after 150 cycles and effectively suppresses working voltage fade (a drop of 0.025 V after 300 cycles) during repeated charge-discharge cycles. In addition, the diffusion barrier of Li ions in NCA crystals at 0.80 V is noticeably smaller than that of Li ions in pristine NCA (0.87 eV). These findings demonstrate that this unique surface structure design considerably enhances cycle and rate performance of NCA, which has potential applications in other Ni-rich layered cathode materials.

Ultra-compact structured light projector with all-dielectric metalenses for 3D sensing

The current structured light projector is generally large in size. In order to realize a compact and small-sized module, this paper proposes to substitute the conventional refractive lenses with the most popular meta-lenses to achieve compactness. This paper focuses on the analysis of the metalens performance requirements, design methods and simulation results. Varying diameters of these nano-pillar structures are used to adjust the phase of the electromagnetic wave to realize flat lens. Two types of all-dielectric meta-lenses are designed for using in ultra-compact structured light projector. Parameters of two meta-lenses at wavelength of 850nm are depicted as follows: the focal length f=500um, aperture size D=100um, numerical aperture NA=0.1, FWHM=0.323λ and the focal length is f=50um, aperture size is D=100um, numerical aperture is about NA=0.7, FWHM=0.598λ, respectively. At last, simulation results show that this solution can achieve an ultra-compact structured light projector, which is 77% smaller than the traditional technology.

Preparation of CdS nanorods on silicon nanopillars surface by hydrothermal method

Cadmium sulfide (CdS) nanorods are fabricated on Silicon (Si) nanopillars surface via hydrothermal reaction method for the first time. To optimize the growth condition, the thickness of CdS seed film, temperature and time of hydrothermal reaction are researched in this work. It is found that 30–60 nm seed layer at the 180 °C for 5 h is the optimal condition for CdS nanorods growth on Si nanopillars surface. The energy dispersive spectroscopy results show that Cd and S elements with a ratio of 1:1.02 is in agreement with the stoichiometric composition of CdS. The X-ray diffraction curves indicate that the CdS nanorods are well crystallized on Si nanopillars substrate. The CdS nanorods can reduce the reflection and increase the absorption of the incoming light obviously. And the CdS nanorods-Si nanopillars structure has obvious photosensitive effect, which has great potential in the field of photoelectric.

Meridian whispering gallery modes sensing in a sessile microdroplet on micro/nanostructured superhydrophobic chip surfaces

A liquid microdroplet could be a naturally simple, miniaturized and effective optical cavity by itself due to the intriguing optofluidic properties associated with its surface tension-induced spherical shape. It had been shown that optical whispering gallery modes (WGMs) can be present along the circular rim in the equatorial plane of a sessile microdroplet, and this phenomenon had been leveraged for biosensing demonstrations. However, optical coupling to such equatorial modes for their excitation and monitoring is mostly based on either tapered fiber coupling or free-space beam coupling, each of which demandingly requires precise alignment of the tapered fiber or the free-space beam adjacent to the equatorial surface of the resonator. In this paper, we show that WGMs could also be stimulated along the meridian plane of a liquid microdroplet resting on a properly designed nanostructured chip surface. The geometrical morphology and optical characteristics of a microdroplet cavity are critical to achieve a high-quality Q factor and therefore to realize high-resolution in situ and in vivo monitoring of trace analytes. The unavoidable deformation along the meridian rim of the sessile microdroplet can be controlled and regulated by tailoring the nanopillar structures and their associated hydrophobicity. The nanostructured superhydrophobic chip surface and its impact on the microdroplet morphology are modeled by Surface Evolver, which is subsequently validated by the Cassie–Wenzel theory of wetting. The influence of the microdroplet morphology on the optical characteristics of WGMs is further numerically studied using the finite-difference time-domain method and it is found that meridian WGMs with intrinsic quality factor Q exceeding 104 can exist. Importantly, such meridian WGMs can be efficiently excited by a waveguiding structure embedded in the planar chip, which could significantly reduce the overall system complexity by eliminating conventional mechanical coupling parts. Our simulation results also demonstrate that this optofluidic resonator can achieve a sensitivity as high as 530 nm/RIU. This on-chip coupling scheme could pave the way for developing lab-on-a-chip resonators for high-resolution sensing of trace analytes in various applications ranging from chemical detections, biological reaction processes to environmental protection.

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring.

Here, a single-step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self-organized nanopillar structures on fused silica (SiO2 ). The laser-induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar-like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser-processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s-p linearly polarized configurations. However, in the near-infrared spectrum, the laser-textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.

Stability Analysis of Spin-Torque Nano-oscillator in the Rotating Frame

Spin-torque nano-oscillators (STNOs) have become one of the emerging and novel microwave devices with the high performance and tunability of GHz range frequency. The nanopillar structure with an out-of-plane (OP) spin polarizer and an in-plane (IP) magnetized free layer (FL) has been considered as a good candidate for the STNOs. Using the local rotational coordinate transformation, a nonstationary process describing magnetization dynamics in the laboratory frame is therefore transformed into a stationary one in the rotating frame. In this way, the state phase diagram of this type of STNOs is well established as a function of an applied current and external field, which is also evidenced by the macrospin simulations. Also, we show that the frequency tunability of the STNOs through electrical current can be well elevated by applying a static magnetic field anti-parallel to the spin-polarizer vector.

Efficient Si-ZnO-ZnMgO heterojunction solar cell with alignment of grown hexagonal nanopillar

The bulk heterojunction (BHJ) grown with nanopillar has been predicted to be high performance solar cell due to having high photon and electron collection capability. Here, we report a reproducible high efficient Si-ZnO-ZnMgO BHJ solar cell having nanopillar structure (efficiency >20.9%). Due to its tendency to formation of single crystal orientation (002) in its structure and reduced contact and series resistance, long term stability is guaranteed as the device maintains its initial efficiency for 2400 h at 75% stability/reliability.

Convective dropwise condensation heat transfer in mini-channels with biphilic surface

For dropwise condensation (DWC) in confined channels, droplets slide on surface and may spoil the nano-pillars, thus the nano-pillars structure is not suitable. Here, we provide a robust biphilic surface with polymer film as hydrophobic wettability. Mesh screen is sintered on copper surface to form welding junction array. Dipping the sintered package in a Teflon solution followed by baking technique forms a hydrophobic coating film. Hydrophilic dots are exposed by separating mesh screen from copper substrate. Condensation experiment was performed in a mini-channel with dimensions of 30.0 × 3.13 × 0.97 mm3 and pure water–vapor as the working fluid. Mass flux covered a range of 10.5–63.8 kg/m2 s. It is shown that condensation heat transfer coefficients on biphilic surface were 5.54 times of those on polished copper surface, and 1.89 times of those on hydrophobic polymer layer surface, maximally. Pressure drops are almost identical in condensers with biphilic surface and hydrophobic surface. For DWC on biphilic surface, growing droplet on hydrophilic dot attracts and merges neighboring droplets to generate directional-droplets-coalescence, explained by the Gibbs energy analysis. Higher droplet departure frequency was observed on biphilic surface than hydrophobic surface. Compared with hydrophobic surface, droplet on biphilic surface has different contact angle, covers several hydrophilic dots and reduces droplet thermal resistance. The above effects explain the heat transfer enhancement mechanism when biphilic surface is present.

Thermal conductivity of carbon nanotubes grown by catalyst-free chemical vapor deposition in nanopores

A graphitic structure was synthesized by catalyst-free chemical vapor deposition on an anodized aluminum oxide (AAO) template using acetylene as the carbon source at a temperature of 620 °C. The AAO template was removed by chemical etching, which yielded a three-dimensional structure featuring planar layers seamlessly joined together by nanotube pillars via continuous carbon-carbon bonding. Raman spectroscopy and transmission electron microscopy measurements reveal that the deposited carbon is nanocrystalline graphite with a thickness of about 10 nm. Carbon nanotubes were isolated from the three-dimensional nano-pillar graphitic structure and measured with a thermal four-probe method to obtain the intrinsic thermal conductance. Discrete modulated heating and Fourier transform analysis were used to improve the signal to noise ratio of the thermal measurement of the low-conductance nanostructure. The measured thermal conductivity of the nanotube wall increased with increasing temperature and was 3.9 ± 0.3 Wm−1K−1 at room temperature. Both the temperature dependence and the magnitude are consistent with the nanocrystalline graphitic structure.

Near-infrared photodetection with plasmon-induced hot electrons using silicon nanopillar array structure

A plasmon-induced hot-electron photodetector based on silicon nanopillar array is developed. The nanostructure is fabricated by reactive ion etching with a monolayer of self-assembled polystyrene nanosphere in hexagonal close-packed lattice as the mask. Light absorption and hot-electron generation are mainly enhanced by the surface plasmon polaritons formed at the surface of the gold film on the nanopillar sidewalls. The photoresponse spans two telecom wavebands, viz. the range of 1250–1600 nm, and has a value of 2.5 mA W−1 at 1310 nm. The proposed silicon nanopillar-based hot-electron infrared detector has great potentials for device integration in silicon photonics relying on the economic large-area fabrication process.

Lateral nonlocal spin-transfer structures with separately tailored polarizer and free-layer interfaces

We investigate spin-transfer effects in mesoscopic lateral nonlocal structures. The structure consists of laterally separated spin polarizer and magnetic free layer bridged by a copper channel. The larger (300 nm × 80 nm) oxide interface on the polarizer reduces charge current density, and the small (80 nm × 80 nm) ohmic interface on the free layer draws a high-density spin current. Reversible and bistable switching of the 3 nm Ni81Fe19 free-layer, induced by a pure spin current, is observed between 5 K and 150 K. The interfacial critical charge current density required for switching is 1.5 × 107 A cm−2 at 5 K and ~6 × 106 A cm−2 between 100 K and 150 K. The ratio of spin current density to charge current density is estimated to be 0.19. The results demonstrate that lateral nonlocal spin-transfer structures can be potentially as efficient and robust as the nanopillar structures.

Absorption enhancement in a-Si thin-film solar cells based on silver nanopillar arrays

PurposeThe purpose of this paper is to investigate a light-trapping structure based on Ag nanograting for amorphous silicon (a-Si) thin-film solar cell. Silver nanopillar arrays on indium tin oxide layer of the a-Si thin-film solar cells were designed.Design/methodology/approachThe effects of the geometrical parameters such as nanopillar radius (R) and array period (P) were investigated by using the finite element simulation.FindingsThe optimization results show that the absorption of the solar cell with Ag nanopillar structure and anti-reflection film is enhanced up to 29.5 per cent under AM1.5 illumination in the 300- to 800-nm wavelength range compared with the reference cell. Furthermore, physical mechanisms of absorption enhancement at different wavelength range are discussed according to the electrical field amplitude distributions in the solar cells.Research limitations/implicationsThe research is still in progress. Further studies mainly focus on the performance of solar cells with different nanograting materials.Practical implicationsThis study provides a feasible method for light-trapping structure based on Ag nanograting for a-Si thin-film solar cell.Originality/valueThis study is promising for the design of a-Si thin-film solar cells with enhanced performance.

Room-temperature polarized spin-photon interface based on a semiconductor nanodisk-in-nanopillar structure driven by few defects

Owing to their superior optical properties, semiconductor nanopillars/nanowires in one-dimensional (1D) geometry are building blocks for nano-photonics. They also hold potential for efficient polarized spin-light conversion in future spin nano-photonics. Unfortunately, spin generation in 1D systems so far remains inefficient at room temperature. Here we propose an approach that can significantly enhance the radiative efficiency of the electrons with the desired spin while suppressing that with the unwanted spin, which simultaneously ensures strong spin and light polarization. We demonstrate high optical polarization of 20%, inferring high electron spin polarization up to 60% at room temperature in a 1D system based on a GaNAs nanodisk-in-GaAs nanopillar structure, facilitated by spin-dependent recombination via merely 2–3 defects in each nanodisk. Our approach points to a promising direction for realization of an interface for efficient spin-photon quantum information transfer at room temperature—a key element for future spin-photonic applications.

Tuning the morphology of electrosprayed BiVO4 from nanopillars to nanoferns via pH control for solar water splitting

Electrosprayed BiVO4 adopts a nanopillar structure formed by diffusion-limited aggregation, which maximizes the surface area of the nanopillars. However, increasing the interfacial area between an electrode and the electrolyte through nanostructuring enhances the overall interfacial activity for solar water splitting. For this purpose, the pH of the precursor solution used for electrospraying BiVO4 was altered by adding ammonium hydroxide, thereby inducing a drastic change in the morphology of BiVO4. The previously demonstrated nanopillar morphology of electrosprayed BiVO4 was transformed into a nanofern structure that increased the photocurrent density of BiVO4 from 0.82 to 1.23 mA·cm−2 at 1.2 V vs. Ag/AgCl. The produced films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrochemical impedance spectroscopy.

Dropwise condensation on superhydrophobic nanostructure surface, part II: Mathematical model

Inspired by our finding that nano-grass-surface (NGS) with smaller droplets has poorer condensation heat transfer performance than smooth-single-molecule-layer surface (SSML) and the long-term operation of NGS deteriorates heat transfer to approach limit values, the secrets of droplets interacting with nano-pillars structure are explored. A comprehensive dropwise condensation model is developed. The contact angles are treated to show correct trend with respect to Cassie, partial Wenzel and Wenzel morphologies. A mixed droplet detachment model is developed to consider the coalescence-induced-jumping, rolling and sliding modes simultaneously. The maximum drop radius is rmax = min(rmax,jump, rmax,roll, rmax,slide), where rmax,jump, rmax,roll and rmax,slide are maximum drop radii in jumping, rolling and sliding modes, respectively. The number of drop nucleation sites on NGS is f times of that on SSML, where f is the surface roughness. Our model predictions match the measured heat transfer data well. It is concluded that the dropwise condensation is the outcome of a series of positive and negative effects by introducing the nanostructure. The increased droplet population density and number of drop nucleation sites are the positive contributions, while the decreased single drop heat transfer rate and additional nano-porous thermal resistance are the negative contributions. The densely populated nano-pillars structure has the largest capability to enhance heat transfer. The Heterogeneous hydrophilicity/hydrophobicity surface limits droplet sweeping distance within neighboring hydrophilic dots, to avoid that stone rolls on the lawn to spoil the grasses, the heterogeneous surface is recommended to resist nanostructure failure for long-term operation.

Enhancing the light extraction of AlGaN-based ultraviolet light-emitting diodes in the nanoscale

AlGaN-based ultraviolet light-emitting diodes (UV LEDs) are promising next-generation UV sources for a wide variety of applications. The state-of-the-art AlGaN-based UV LEDs exhibit much lower output power and external quantum efficiency than highly commercialized GaN visible LEDs. One key issue for UV LEDs is the poor light-extraction efficiency. We have reviewed the recent progress in the light extraction approaches for AlGaN-based UV LEDs, including the highly reflective techniques, and the surface/interface modification for total internal reflection mitigating. Moreover, AlGaN-based UV LEDs in the nanoscale structures, such as nanopillar, nanorod, and nanowire structures, are also discussed.