Silicon Cones Research Articles

Ion Current Rectification and Long-Range Interference in Conical Silicon Micropores.

Fluidic devices exhibiting ion current rectification (ICR), or ionic diodes, are of broad interest for applications including desalination, energy harvesting, and sensing, among others. For such applications a large conductance is desirable, which can be achieved by simultaneously using thin membranes and wide pores. In this paper we demonstrate ICR in micrometer sized conical channels in a thin silicon membrane with pore diameters comparable to the membrane thickness but both much larger than the electrolyte screening length. We show that for these pores the entrance resistance is key not only to Ohmic conductance around 0 V but also for understanding ICR, both of which we measure experimentally and capture within a single analytic theoretical framework. The only fit parameter in this theory is the membrane surface potential, for which we find that it is voltage dependent and its value is excessively large compared to the literature. From this we infer that surface charge outside the pore strongly contributes to the observed Ohmic conductance and rectification by a different extent. We experimentally verify this hypothesis in a small array of pores and find that ICR vanishes due to pore-pore interactions mediated through the membrane surface, while Ohmic conductance around 0 V remains unaffected. We find that the pore-pore interaction for ICR is set by a long-ranged decay of the concentration which explains the surprising finding that the ICR vanishes for even a sparsely populated array with a pore-pore spacing as large as 7 μm.

Various multipole combinations for conical Si particles

In this work, we investigated the possibility of creating various multipole combinations in conical silicon nanoparticles. It was found that in conical silicon particles it seems possible to create key effects for nanophotonics, such as various kinds of Kerker effects (Generalized Kerker, Transverse Kerker), Hybrid anapole state, Bound states in the continuum. This greatly simplifies the manufacturing process of photonic devices due to the easier production of nanocones in practice. Also, conical particles allow an additional degree of freedom, which opens up new horizons for obtaining previously unknown effects.

Photovoltage Management Based on Enhanced Excitation Levels in Surface Arrays of Subwavelength Silicon Formations

Surface arrangements of subwavelength formations are extensively discussed in the context of photocurrent enhancement for photovoltaic (PV) applications. Recently, the potential contribution of such arrangements toward photovoltage augmentation is suggested. This study numerically demonstrates the potential for photovoltage management based on arrays composed of inverted silicon cones, referred to as light funnel (LF) arrays. The transition from an optimized nanopillar (NP) array into an LF array is examined. It is shown that a decrease in NP bottom diameter (Db) is accompanied by an increase in open‐circuit voltage (Voc). The highest photovoltage enhancement is recorded for the smallest considered Db = 50 nm with a Voc increase of 75 mV and reflects a 22% Voc enhancement compared with the NP Voc. It is shown that this Voc increase is due to 250% increase in the excitation level, and that the spatially resolved excitation level of the array‐nested LFs is more than two orders of magnitude higher than the highest spatially resolved excitation level in the array‐nested NPs. Finally, it is shown that the suggested photovoltage management entails almost a factor of 2 increase in the nominal power conversion efficiency upon the transition from an NP PV cell into LF PV cell.

Unprecedentedly Low CO2 Transport through Vertically Aligned, Conical Silicon Nanotube Membranes.

Nanotube membranes could show significantly enhanced permeance and selectivity for gas separations. Up until now, studies have primarily focused on applying carbon nanotubes to membranes to achieve ultrafast mass transport. Here, we report the first preparation of silicon nanotube (SiNT) membranes via a template-assisted method and investigate the gas transport behavior through these SiNT membranes using single- and mixed-gas permeation experiments. The SiNT membranes consist of conical cylinder-shaped nanotubes vertically aligned on a porous silicon wafer substrate. The diameter of the SiNT pore mouths are 10 and 30 nm, and the average inner diameter of the tube body is 80 nm. Interestingly, among the gases tested, we found an unprecedentedly low CO2 permeance through the SiNT membranes in single-gas permeation experiments, exceeding the theoretical Knudsen selectivity toward small gases/CO2 separation. This behavior was caused by the reduction of CO2 permeability through the blocking effect of CO2 adsorbed in the narrow pore channels of the SiNT cone regions, indicating that CO2 molecules have a high affinity to the native silicon oxide layer (∼2 nm) that is formed on the inner walls of SiNTs. SiNT membranes also exhibited enhanced gas permeance and water flux as compared to classic theoretical models and, as such, may prove useful as a new type of nanotube material for use in membrane applications.

Cellular Deformations Induced by Conical Silicon Nanowire Arrays Facilitate Gene Delivery.

Engineered cell-nanostructured interfaces generated by vertically aligned silicon nanowire (SiNW) arrays have become a promising platform for orchestrating cell behavior, function, and fate. However, the underlying mechanism in SiNW-mediated intracellular access and delivery is still poorly understood. This study demonstrates the development of a gene delivery platform based on conical SiNW arrays for mechanical cell transfection, assisted by centrifugal force, for both adherent and nonadherent cells in vitro. Cells form focal adhesions on SiNWs within 6 h, and maintain high viability and motility. Such a functional and dynamic cell-SiNW interface features conformational changes in the plasma membrane and in some cases the nucleus, promoting both direct penetration and endocytosis; this synergistically facilitates SiNW-mediated delivery of nucleic acids into immortalized cell lines, and into difficult-to-transfect primary immune T cells without pre-activation. Moreover, transfected cells retrieved from SiNWs retain the capacity to proliferate-crucial to future biomedical applications. The results indicate that SiNW-mediated intracellular delivery holds great promise for developing increasingly sophisticated investigative and therapeutic tools.

Optothermal response of a single silicon nanotip

The optical properties and thermal dynamics of conical single silicon nanotips are experimentally and theoretically investigated. The spectral and spatial dependencies of their optical extinction are quantitatively measured by spatial modulation spectroscopy (SMS). A nonuniform optical extinction along the tip axis and an enhanced near-infrared absorption, as compared to bulk crystalline silicon, are evidenced. This information is a key input for computing the thermal response of single silicon nanotips under ultrafast laser illumination, which is investigated by laser assisted atom probe tomography (La-APT) used as a highly sensitive temperature probe. A combination of these two experimental techniques and comparison with modeling also permits us to elucidate the impact of thermal effects on the laser assisted field evaporation process. Extension of this coupled approach opens up future perspectives for the quantitative study of the optical and thermal properties of a wide class of individual nano-objects, in particular elongated ones such as nanotubes, nanowires, and nanocones, which constitute promising nanosources for electron and/or ion emission.

Influence of deposition rate on the structural properties of plasma-enhanced CVD epitaxial silicon

Solar cells based on epitaxial silicon layers as the absorber attract increasing attention because of the potential cost reduction. In this work, we studied the influence of the deposition rate on the structural properties of epitaxial silicon layers produced by plasma-enhanced chemical vapor deposition (epi-PECVD) using silane as a precursor and hydrogen as a carrier gas. We found that the crystalline quality of epi-PECVD layers depends on their thickness and deposition rate. Moreover, increasing the deposition rate may lead to epitaxy breakdown. In that case, we observe the formation of embedded amorphous silicon cones in the epi-PECVD layer. To explain this phenomenon, we develop a model based on the coupling of hydrogen and built-in strain. By optimizing the deposition conditions to avoid epitaxy breakdown, including substrate temperatures and plasma potential, we have been able to synthesize epi-PECVD layers up to a deposition rate of 8.3 Å/s. In such case, we found that the incorporation of hydrogen in the hydrogenated crystalline silicon can reach 4 at. % at a substrate temperature of 350 °C.

Ordered Micro/Nanostructures with Geometric Gradient: From Integrated Wettability "Library" to Anisotropic Wetting Surface.

Geometric gradients within ordered micro/nanostructures exhibit unique wetting properties. Well-defined and ordered microsphere arrays with geometric gradient (OMAGG) are successfully fabricated through combining colloidal lithography and inclined reactive ion etching (RIE). During the inclined RIE, the graded etching rates in vertical direction of etcher chamber are the key to generating a geometric gradient. The OMAGG can be used as an effective mask for the preparation of micro/nanostructure arrays with geometric gradient by selective RIE. Through this strategy, a well-defined wettability "library" with graded silicon cone arrays is fabricated, and the possibility of screening one desired "book" from the designated wettability "library" is demonstrated. Meanwhile, the silicon cone arrays with geometric gradient (SCAGG) can be applied to control the wetting behavior of water after being modified by hydrophilic or hydrophobic chemical groups. Based on this result, a temperature-responsive wetting substrate is fabricated by modifying poly n-isopropyl acrylamide (PNIPAM) on the SCAGG. These wettability gradients have great potential in tissue engineering, microfluidic devices, and integrated sensors.

Gold Cone Metasurface MIC Sensor with Monolayer of Graphene and Multilayer of Graphite

This report makes a comparison between the spectrum features of plasmonic metamaterial metal-insulator-conductor (MIC) sensor with a monolayer of graphene and another MIC sensor with a multilayer of graphite as the back reflector. In both structures, the silicon substrate as an insulator layer was sandwiched between subwavelength periodic nanogold cones as the first layer and graphene and graphite as the third layer, respectively. Nanolayer of chromium nanorods was also considered in the structure of MIC sensors as an interface layer between silicon and nanogold cone metasurface. The performance of the sensor was evaluated under different incident polarized light angles and different thickness of the metasurface when the metasurface infiltrated with seawater and air. The transmission spectrum of monolayer graphene-based MIC sensor, respecting to s-polarized waves, reveals prominent feature to detect the air rather than seawater in invisible regime. Meanwhile, the reflection spectrum of graphite-based MIC sensor provides ∼0 % reflection under resonance condition regarding s- and p-polarized waves for detecting air in visible spectrum.

Gold nanoparticle-coated silicon cone array for surface-enhanced Raman spectroscopy

ABSTRACTWe demonstrate a silicon cone array substrate coated with gold nanoparticles and which was highly sensitive, homogeneous, and provided a large area for surface-enhanced Raman spectroscopy (SERS). A deep reactive ion-etching process was used to fabricate the high-density silicon cone array, and gold nanoparticles were formed on the silicon cone surface by magnetron sputtering. The substrate was tested with 10−6 M rhodamine 6 G solution. Enhancement of the substrate was about 60-fold greater than that of flat substrate. Moreover, SERS signals obtained from 24 random areas on the substrate showed good homogeneity with an average standard deviation of 3.9%.

Optical properties of antireflective flat or rough patterned topped silicon cones gratings

Achieving a broadband antireflection property from material surfaces is one of the highest priorities in photosensing applications. To demonstrate a low reflectance, we are concerned with the study of the optical response of flat-, patterned- and rough-top silicon cone gratings. Based on previous work exploring pyramid gratings, we first designed several flat top structures that theoretically demonstrate an antireflective character within the middle infrared region. In order to extend the antireflective properties up to the visible wavelengths, patterned- and rough-topped cones have been fabricated as well. Optical characterizations of several samples were carried out in the visible and mid infrared range, in order to determine the effect of structuration on the performances of the different cones gratings.

Size‐controllable fabrication of nano‐to‐microstructures on silicon surface using high‐density ion etching with pulsed bias

A ‘black silicon’ (BS) surface with low reflectance was fabricated by a standard pulsed deep reactive etching technology at room temperature. Aiming for a better understanding, a systematic experiment was conducted by varying the etching window size and bias power duty cycle. The samples were measured and analysed by a scanning electron microscope, the Bruker Optical Profiler and a UV-3400 spectrometer. It was observed that a broad scale range of the surface structures formed on the surface. With the duty cycle at 0.5, only about 100 nanometre scale replicable silicon cones formed on the surface, but as the duty cycle decreased to 0.25, the height of the silicon forest sharply increased to about 10 µm, leading to a low reflectance of 0.9% in the visible range for the surface. To clarify the reason for this trend, the bias effective voltage (BEV) was measured and it was confirmed that the BEV would decrease from 100 to 47 V with the duty cycle adjusted from 1 to 0.25. This suggested that this decrease in BEV leads to a reduction of ion energy and ion flux, and then modifies the fabricated structures. Besides, it was found that the broad etching window area only had a maximum promotion of 20% to the scale of the BS, indicating this method was almost free of loading effect.

Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF6 and a mixture of SF6 and O2 followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that Eto = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm2, and maximum current density J = 60.4 μA/cm2 at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.

Coupled optical and electrical modeling of solar cell based on conical pore silicon photonic crystals

We compare the efficiency of thin film photonic crystal solar cells consisting of conical pores and nanowires. Solving both Maxwell's equations and the semiconductor drift-diffusion in each geometry, we identify optimal junction and contact positions and study the influence of bulk and surface recombination losses on solar cell efficiency. We find that using only 1 μm of silicon, sculpted in the form of an inverted slanted conical pore photonic crystal film, and using standard contact recombination velocities, solar power conversion efficiency of 17.5% is obtained when the carrier diffusion length exceeds 10 μm. Reducing the contact recombination velocity to 100 cm s−1 yields efficiency up to 22.5%. Further efficiency improvements are possible (with 1 μm of silicon) in a tandem cell with amorphous silicon at the top.

Bimaterial microcantilevers with black silicon nanocone arrays

The performance of infrared (IR) sensing bimaterial cantilevers depends upon the thermal, mechanical and optical properties of the cantilever materials. This paper presents bimaterial cantilevers that have a layer of black silicon nanocone arrays, which has larger optical absorbance and mechanical compliance than single crystal silicon. The black silicon consists of nanometer-scale silicon cones of height 104–336nm, fabricated using a three-step O2–CHF3–Ar+Cl2 plasma process. The average cantilever absorbance was 0.16 over the 3–10μm wavelength region, measured using a Fourier transform infrared (FTIR) microspectrometer. The measured cantilever responsivity to incident IR light compares well to a model of cantilever behavior that relate the spectral absorbance, heat transfer, and thermal expansion. The model also provides further insights into the influence of the nanocone height on the absorbance and responsivity of the cantilever. Compared to a cantilever with smooth single crystal silicon, the cantilever with black silicon has about 2× increased responsivity. The nanocone array fabrication technique for silicon bimaterial cantilevers presented here could be applied to other IR sensors.

Light absorption in conical silicon particles

The problem of the absorption of light by a nanoscale dielectric cone is discussed. A simplified solution based on the analytical Mie theory of scattering and absorption by cylindrical objects is proposed and supported by the experimental observation of sharply localized holes in conical silicon tips after high-fluence irradiation. This study reveals that light couples with tapered objects dominantly at specific locations, where the local radius corresponds to one of the resonant radii of a cylindrical object, as predicted by Mie theory.

Study of black silicon obtained by cryogenic plasma etching: approach to achieve the hot spot of a thermoelectric energy harvester

In this paper, we study the enhanced absorption properties of micro/nano structured silicon surface under incident electromagnetic illumination and its capacity to convert light into heat. We simulate the optical reflectance of three-dimensional micro/nano silicon cones of different dimensions and under different electric field incident angles (źi). According to the favorable simulation results, we fabricate black silicon with conical microstructures that exhibits excellent anti-reflectivity behavior. Plasma etching under cryogenic temperatures is used for this purpose in an inductively coupled plasma-reactive ion etching reactor. The reflectance of the black silicon is measured to be approximately 1 % in the optical wavelength range, by using an integrating sphere coupled to a calibrated spectrometer. Furthermore, a device integrating a resistance temperature detector in a black silicon area is developed in order to investigate its efficiency as a photo-thermal converter.

Large Area Fabrication of Leaning Silicon Nanopillars for Surface Enhanced Raman Spectroscopy

Using a simple two step fabrication process substrates with a large and uniform Raman enhancement, based on flexible free standing nanopillars can be manufactured over large areas using readily available silicon processing equipment.

Cones fabricated by 3D nanoimprint lithography for highly sensitive surface enhancedRaman spectroscopy

We demonstrated a cost-effective and deterministic method of patterning 3D cone arraysover a large area by using nanoimprint lithography (NIL). Cones with tip radius ofless than 10 nm were successfully duplicated onto the UV-curable imprint resistmaterials from the silicon cone templates. Such cone structures were shown to bea versatile platform for developing reliable, highly sensitive surface enhancedRaman spectroscopy (SERS) substrates. In contrast to the silicon nanocones,the SERS substrates based on the Au coated cones made by the NIL offeredsignificant improvement of the SERS signal. A further improvement of the SERSsignal was observed when the polymer cones were imprinted onto a reflectivemetallic mirror surface. A sub-zeptomole detection sensitivity for a model molecule,trans-1,2-bis(4-pyridyl)-ethylene (BPE), on the Au coated NIL cone surfaces was achieved.

Photoinduced cleaning of water-soluble dyes on patterned superhydrophilic/superhydrophobic substrates

Herein we present a novel but simple method to fabricate photoinduced cleaning substrates with patterned superhydrophobic hierarchical silicon cone arrays (SCAs) and superhydrophilic TiO(2) nanorod clusters (TNCs). In our experiment, the photoinduced cleaning properties of the obtained substrate are investigated by repeatedly adsorbing and decomposing rhodamine B (RhB) molecules for at least six cycles. In addition, we demonstrate that the low-surface-energy coating on the superhydrophobic areas is stable, resulting in the high wettability contrast being well preserved during the renewal process. This straightforward method may open up new possibilities for the practical use of microchips with patterned superhydrophobic and superhydrophilic areas.