Si Nanopillar Arrays Research Articles

Hyperspectral imaging for high-throughput, spatially resolved spectroscopic scatterometry of silicon nanopillar arrays.

Modern high-throughput nanopatterning techniques, such as nanoimprint lithography, make it possible to fabricate arrays of nanostructures (features with dimensions of 10's to 100's of nm) over large area substrates (cm2 to m2 scale) such as Si wafers, glass sheets, and flexible roll-to-roll webs. The ability to make such large-area nanostructure arrays (LNAs) has created an extensive design space, enabling a wide array of applications including optical devices, such as wire-grid polarizers, transparent conductors, color filters, and anti-reflection surfaces, and building blocks for electronic components, such as ultracapacitors, sensors, and memory storage architectures. However, existing metrology methods will have trouble scaling alongside fabrication methods. Scanning electron microscopy (SEM) and atomic force microscopy (AFM), for instance, have micron scale fields of view (FOV) that preclude comprehensive characterization of LNAs, which may be manufactured at m2 per minute rates. Scatterometry approaches have larger FOVs (typically 100's of µm to a few mm), but traditional scatterometry systems measure samples one point at a time, which also makes them too slow for large-scale LNA manufacturing. In this work, we demonstrate parallelization of the traditional spectroscopic scatterometry approach using hyperspectral imaging, increasing the throughput of the technique by a factor of 106-107. We demonstrate this approach by using hyperspectral imaging and inverse modeling of reflectance spectra to derive 3-dimensional geometric data for Si nanopillar array structures over both mm and cm-scale with µm-scale spatial resolution. This work suggests that geometric measurements for a variety of LNAs can be performed with the potential for high speed over large areas which may be critical for future LNA manufacturing.

Decreasing of the thermal conductivity of Si nanopillar/SiGe composite films investigated by using a piezoelectric photothermal spectroscopy

To investigate the decrease of thermal conductivity () of nanoscale Si materials, we conducted the piezoelectric photothermal (PPT) method for the highly periodic Si nanopillar arrays embedded in Si0.7Ge0.3. The PPT is an electrode free method that can measure a heat propagation in the parallel to the nanopillars direction. A distinctive dip was observed in the frequency-dependent PPT signal intensity. By focusing the dip frequency, was estimated from the comparison with the model analysis based on the one-dimensional multilayer thermal diffusion equation. The estimated was 0.19 ± 0.07 W m–1 K, in the parallel to the nanopillars direction. Since the considerable decrease of was confirmed from the non-radiative recombination point of view, we found the present non-destructive PPT method is very useful to estimate in the nanostructured devices for the thermoelectric application.

Adhesion and bactericidal properties of nanostructured surfaces dependent on bacterial motility.

Different nanostructured surfaces have bactericidal properties that arise from the interaction between the bacteria and the nanostructured surface. In this study, we focused on the relationship between bacterial motility and bactericidal properties. The motility of Escherichia coli (E. coli) was tuned by genetic engineering, and four types of E. coli (wild type (WT), lacking flagella, and flagellated with deficient motility or deficient chemotaxis) were used to evaluate the adhesion and bactericidal properties of nanostructured surfaces. Cicada (Cryptotympana facialis) wings and Si nano-pillar array substrates were used as natural and artificial nanostructured surfaces, respectively. Differences in motility and chemotaxis strongly influenced the adhesion behavior and to some extent, the damage to the cell membrane. These results suggest that the bactericidal properties of nanostructured surfaces depend on bacterial motility.

Broadband anti-reflection in Si substrate via Ag nanospheres on Si nanopillar arrays

A structure of Ag nanospheres on Si nanopillar arrays which sit on a Si substrate is presented to obtain a broadband low reflection. The simulated results show that the average reflection can reach 2.66% in the wavelength range of 400–1100 nm due to the forward scattering effects of Ag nanospheres and the antireflective effects of Si nanopillars, much lower than that of Ag nanosphere arrays on Si substrates and that of Si nanopillar arrays on Si substrates. The diameter of Ag nanospheres, the period of the arrays, the diameter and height of Si nanopillars have important influences on the reflection. This work provide a practicable method for the optimization of anti-reflection in different device applications.

Variance Reduction during the Fabrication of Sub-20 nm Si Cylindrical Nanopillars for Vertical Gate-All-Around Metal-Oxide-Semiconductor Field-Effect Transistors.

The variance of sub-20 nm devices is a critical issue for large-scale integrated circuits. In this work, uniform sub-20 nm Si nanopillar (NP) arrays with a reduced diameter variance (to ±0.5 nm) and a cylindrical shape, which can be used for vertical gate-all-around metal-oxide-semiconductor field-effect transistors, were fabricated. For the fabrication process, an array of tapered Si NPs with a diameter of approximately 62.7 nm and a diameter variance of ±2.0 nm was initially fabricated by an argon fluoride lithography followed by dry etching. Then, the NPs were oxidized in a self-limiting region. After the oxide removal, a similar oxidation process was used again for the NPs. It is determined that by controlling oxidation in the self-limiting region, the diameter variance can be reduced in the height direction of Si NPs (as well as shape control) and between NPs, simultaneously with a controllable diameter decrease. This approach decreases the variance in size caused by conventional nanoprocessing and helps overcome the position-dependent variance for 300 mm φ wafers, which is caused by current semiconductor processing.

Wafer-scale few-layer graphene growth on Cu/Ni films for gas sensing applications

Pristine, few-layer graphene (FLG)/Si nanopillar assemblies are introduced as gas sensitive chemiresistors showing unprecedented sensitivity towards NO2 when operated at room temperature (25 °C) and in humid air. To achieve this, we first developed wafer-scale (∼50 cm2) FLG growth using sub-micrometer thick films of thermally evaporated Cu/Ni on a SiO2/Si substrate. The Ni film was deposited and annealed to induce the formation of a Cu-rich binary alloy. This alloy formation limited the inter-diffusion of Cu and SiO2, a phenomenon known to take place during the CVD growth of graphene on Cu/SiO2/Si. The as-grown high structural quality FLG was transferred, using a conventional wet chemical method, to lithographically patterned arrays of Si nanopillars (non-flat substrate). Testing of the FLG/Si assembly revealed a NO2 sensitivity that outperforms what is reported in the literature for pristine graphene. Overall, our growth and device fabrication work-flow demonstrate a way to design graphene-based gas sensing systems without incurring inconvenient processing steps such as metal foil etching, surface functionalization or particle loading.

(Invited) Electrodeposition As a Versatile Method to Prepare Materials for the Storage and Conversion of Sustainable Energy

Sustainable energy carriers such as sun light, wind or biomass will play a crucial role in our future. After converting the energy, one should be able to store it and transport it to places where it is needed most. Therefore, the research on materials for storage systems, such as batteries, supercapacitors and fuel cells is an important aspect. Electrodeposition is a powerful technique for the preparation of such materials. As industry strives to produce smaller portable devices, one needs also smaller and lighter batteries with a high storage capacity per gram of active material. The commercial graphite anodes are limited to ca. 370 mAh/g, but silicon and C-Si composites have a theoretical storage capacity beyond 1000 mAh/g. Thus, nanostructured Si, such as nanoporous Si layers, ordered arrays of Si (porous) nanopillars produced by using a combination of nanoimprint lithography and metal-assisted chemical etching or silicon thin films deposited from ionic liquids could be used as high-capacity anodes in Li-ions batteries. Another interesting semiconductor material besides Si is Cu2O. Photo-cathodes from Cu2O can be used to produce hydrogen as a chemical fuel by utilizing direct solar energy and its narrow bandgap of 2.0-2.2 eV that can lead to the theoretical value of solar-to-hydrogen efficiency of 18 %. One can fabricate Cu/Cu2O porous electrodes using the hydrogen bubbles as a dynamic and effective template by electrochemical deposition. These foam electrodes can then be used as photocathodes for water electrolysis under visible light irradiation. This contribution will discuss recent results from the authors’ labs on developing new anode materials for Li-ion batteries (Si based), new electrodes for electrochemical water splitting (Cu2O based) and for electrocatalysis. Further, approaches using light induced plating for electrical front side contacts of solar cells will be presented.

Polarization-Independent, Narrowband, Near-IR Spectral Filters via Guided Mode Resonances in Ultrathin a-Si Nanopillar Arrays

We report the optical properties obtained through experiments, simulation, and theory of ultrathin (<0.1λ), amorphous Si nanopillar arrays embedded in a thin film of SiO_2 designed for narrowband filtering for multi- and hyperspectral imaging in the near-infrared. The fabricated nanopillar arrays are square-packed with subwavelength periodicity, heights of ∼100 nm, and a radius-to-spacing ratio, r/a, of ∼0.2. Specular reflection measurements at normal incidence demonstrate that these arrays behave as narrow stopband filters in the near-infrared (λ = 1300–1700 nm) and attain ∼90% reflectivity in band and a full width at half-maximum as low as 20 nm. Using a combination of full-wave simulations and theory, we demonstrate that these narrowband filtering properties arise from efficient grating coupling of light into guided modes of the array because the nanopillar arrays serve as photonic crystal slabs. This phenomenon is known as a guided mode resonance. We discover that the spectral location of these resonances is passively tunable by modifying array geometry and is most sensitive to nanopillar spacing. Theoretical photonic crystal slab band diagrams accurately predict the spectral locations of the observed resonance and provide physical insights into and support the guided mode resonance formulation. This work demonstrates that these ultrathin all-dielectric nanopillar arrays have advantages over existing hyperspectral filter designs because they are polarization independent, do not suffer from material absorption loss, and have significant implications for minimizing imaging device size.

Adhesion and Bactericidal Properties of a Wettability-Controlled Artificial Nanostructure

The adhesion behavior and bactericidal properties of the nanostructured surface of Si nanopillar array, which mimicked a cicada wing surface, were evaluated using Escherichia coli. Wettability of the nanostructured surface was controlled by using self-assembled monolayer (SAM). The adhesion behavior was strongly dependent on the wettability of the surface and whether it was a nanostructured or a flat surface. The number of adhered cells on the nanostructured surface was higher than that on the flat Au surface. In addition, the cell membrane was more strongly damaged at a higher contact angle than at a lower contact angle on the nanostructured surface. Time-lapse imaging was used to analyze the changes in fluorescence intensity caused by the effusion of an intercellular fluid, including fluorescent protein (mCherry), at the single cell level on the cicada wing surface and the artificial nanostructured surface. We found that there were three stages of changes in the fluorescence intensity gradient.

Hybrid structures of Fe3O4 and Ag nanoparticles on Si nanopillar arrays substrate for SERS applications

Surface-enhanced Raman scattering (SERS)-based sensing is the most promising approach to detect trace amounts of chemical and biological molecules. Three-dimensional (3D) nanostructures have been considered as one of the most promising SERS substrates due to ordered arrangement of high-density hot spots and large surface area for adsorbing analyte. This paper designed a hybrid SERS substrate containing structures of Fe3O4 and Ag nanoparticles on Si nanopillar arrays, which combines the advantages of highly ordered periodic 3D Si nanopillar arrays and Ag coated Fe3O4 core-satellite microspheres, leading to intense light metal and light molecule interactions. The detection limit of the substrate for malachite green (MG) is as low as 10−8 M, meeting the requirements for trace detection of analytes. This work provides a new insight for fabricating SERS substrates with high sensitivity.

(Invited) Silicon Nanopillars Coated with Earth-Abundant Electrocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) water splitting for hydrogen production promises to solve the impending energy and environmental crisis. The key to increase the efficiency of PEC hydrogen generation is developing high-performance catalysts and photocathodes. 3D p-type silicon (p-Si) nanopillar (NP) arrays are promising architectures due to the high light harvesting and the large interfacial areas. We demonstrate its enhanced PEC performance with a photocurrent density of -37.5 mA/cm2 at 0 V (vs. RHE) under simulated 100 mW/cm2 (1 Sun) with an AM 1.5 G filter, which is the highest value reported for p-type Si photocathodes. The synergic effects of the excellent light harvesting of Si NP array core and the good optical transparency, as well as excellent electrocatalytic activity of NiCoSex shell boost the production and utilization of photogenerated electrons. The Faradaic efficiency of H2 and O2 on p-Si/NiCoSex was approximately 100%, which confirmed that the photocurrent during PEC reaction was attributed to hydrogen generation. The completely enclosed core-shell structure isolated the Si NP from air and aqueous electrolyte and minimized the oxidation of silicon, leading to good stability. The design of p-Si/NiCoSex core/shell NP arrays offers a new strategy for preparing highly efficient photoelectrochemical solar energy conversion devices.

Space confined electroless deposition of silver nanoparticles for highly-uniform SERS detection

We demonstrate an efficient method to improve the reproducibility of SERS detection by controlling the distribution of silver nanoparticles (AgNPs) using Si nanopillar arrays as space confined templates for generating AgNPs with electroless deposition. In the detection of 4-MPy, the relative standard deviation of SERS intensity is decreased to ca. 5.27% from 23.2% by using the AgNPs selectively deposited on Si nanopillar array instead of that randomly deposited on blank Si. Moreover, the SERS-active substrate has good performance in detecting milk contaminated by melamine. This method could be used in real detection.

Tailored periodic Si nanopillar based architectures as highly sensitive universal SERS biosensing platform

We report a skeleton key platform for surface enhanced Raman spectroscopy (SERS) based biosensor, utilizing ordered arrays of Si nanopillars (SiNPLs) with plasmonic silver nanoparticles (AgNPs). The optimized SiNPLs based SERS (SiNPLs-SERS) sensor exhibited high enhancement factor (EF) of 2.4×108 for thiophenol with sensitivity down to 10−13M of R6G molecules. The ordered array of SiNPLs stabilizes the distribution of AgNPs along with the light trapping properties, which resulted in high EF and excellent reproducibility. The uniformity in the arrangement of AgNPs makes a single SiNPLs-SERS substrate to work for all types of biomolecules such as positively and negatively charged proteins, hydrophobic proteins, cells and dyes, etc. The experiments conducted on differently charged proteins, amyloid beta (the protein responsible for alzheimers), E. coli cells, healthy and malaria infected RBCs provide a proof of concept for employing universal SiNPLs-SERS substrate for trace biomolecule detection. The FDTD simulations substantiate the superior performance of the sensor achieved by the tremendous increase in the hotspot distribution compared to the bare Si sensor.

Droplet-Confined Electroless Deposition of Silver Nanoparticles on Ordered Superhydrophobic Structures for High Uniform SERS Measurements.

Surface-enhanced Raman scattering spectroscopy (SERS) is a nondestructive testing technique. To increase reproducibility of the SERS measurement is the key issue for improving the performance of SERS. In this article, we demonstrate an efficient method to improve the reproducibility, using confined silver nanoparticles (AgNPs) as a substrate. The AgNPs are formed uniformly on the tops of the prepared nanopillars by droplet-confined electroless deposition on the hydrophobic Si nanopillar arrays. The AgNPs present an excellent reproducibility in Raman measurement; the relative standard deviation is down to 3.40%. There exists a great linear correlation between the concentration of Rhodamine 6G (R6G) and the Raman intensity in the log-log plot; R2 is 0.998, indicating that this SERS substrate can be applied for the quantitative SERS analysis. Meanwhile, the minimum detection concentration is down to 10-11 M on the hydrophobic substrate, with R6G as a probe molecule.

Refractive index sensing in the visible/NIR spectrum using silicon nanopillar arrays.

Si nanopillar (NP) arrays are investigated as refractive index sensors in the visible/NIR wavelength range, suitable for Si photodetector responsivity. The NP arrays are fabricated by nanoimprint lithography and dry etching, and coated with thin dielectric layers. The reflectivity peaks obtained by finite-difference time-domain (FDTD) simulations show a linear shift with coating layer thickness. At 730 nm wavelength, sensitivities of ~0.3 and ~0.9 nm/nm of SiO2 and Si3N4, respectively, are obtained; and the optical thicknesses of the deposited surface coatings are determined by comparing the experimental and simulated data. The results show that NP arrays can be used for sensing surface bio-layers. The proposed method could be useful to determine the optical thickness of surface coatings, conformal and non-conformal, in NP-based optical devices.

Antibacterial Property of Si Nanopillar Array Fabricated Using Metal Assisted Etching; Mimic a Cicada Wing

We focused on a cicada wing which has superhydrophobic surface, antireflection to the light, unique tribology, and antibacterial property. From the SEM view, circular truncated cones in nanoscale were widely spread with regularly. It was just like nanopillar array. After the signature pattern was analyzed, we mimicked its structure composed of Si. Si nanostructure was fabricated mainly by wet process to decrease the cost but its dimension was controllable. We reported antibacterial property of the structure. Fabricated Si nanopillar array showed antibacterial property to E. Coli.

Mie Resonance-Modulated Spatial Distributions of Photogenerated Carriers in Poly(3-hexylthiophene-2,5-diyl)/Silicon Nanopillars.

Organic/silicon hybrid solar cells have great potential as low-cost, high-efficiency photovoltaic devices. The superior light trapping capability, mediated by the optical resonances, of the organic/silicon hybrid nanostructure-based cells enhances their optical performance. In this work, we fabricated Si nanopillar (NP) arrays coated with organic semiconductor, poly(3-hexylthiophene-2,5-diyl), layers. Experimental and calculated optical properties of the samples showed that Mie-resonance strongly concentrated incoming light in the NPs. Spatial mapping of surface photovoltage, i.e., changes in the surface potential under illumination, using Kelvin probe force microscopy enabled us to visualize the local behavior of the photogenerated carriers in our samples. Under red light, surface photovoltage was much larger (63 meV) on the top surface of a NP than on a planar sample (13 meV), which demonstrated that the confined light in the NPs produced numerous carriers within the NPs. Since the silicon NPs provide pathways for efficient carrier transportation, high collection probability of the photogenerated carriers near the NPs can be expected. This suggests that the optical resonance in organic/silicon hybrid nanostructures benefits not only broad-band light trapping but also efficient carrier collection.

Large-area, size-tunable Si nanopillar arrays with enhanced antireflective and plasmonic properties

In this paper, a novel method using the modified Langmuir–Blodgett and float-transfer techniques was introduced to construct the perfect PS monolayer nanosphere template with large area up to cm2. Based on such templates, the diameter, length, packing density, and the shape of Si nanopillar arrays (Si NPAs) could be precisely controlled and tuned through the modified nanosphere lithography combined with a metal-assisted chemical etching (NSL-MACE) method. Manipulation of the etching time can effectively avoid permanent deformation/clumping to generate size-tunable Si NPAs. The optical properties of the Si NPAs can be controlled by the Si NPA morphologies resulting from the different reactive ion etching (RIE) time and chemical etching time. The enhanced antireflective property and electromagnetic field effect of Au/Si NPAs were proved by the results. The new modified NSL-MACE technique with the capability of scale-up fabrication of Si NPAs would be helpful for potential applications in optoelectronic devices.

Experimental demonstration of the optical lattice resonance in arrays of Si nanoresonators

Optical resonances of crystalline Si nanopillar arrays on a Si substrate are studied using optical reflectivity and Raman spectroscopy. When the nanopillars are arranged in a two-dimensional lattice, a collective resonance is observed in the reflection spectra which is absent for randomly distributed nanopillars. The resonance is due to coherent oscillations in nanopillars, can be tuned spectrally by the nanopillar diameter and lattice period, and strongly suppresses reflection from the Si surface. Raman scattering demonstrates that the reduced reflectivity is accompanied by increased electromagnetic field confined in Si, thus suggesting potential application of the lattice resonance in surface enhanced spectroscopy and thin film solar cells.

Ultra-large suspended graphene as a highly elastic membrane for capacitive pressure sensors.

In this work, we fabricate ultra-large suspended graphene membranes, where stacks of a few layers of graphene could be suspended over a circular hole with a diameter of up to 1.5 mm, with a diameter to thickness aspect ratio of 3 × 10(5), which is the record for free-standing graphene membranes. The process is based on large crystalline graphene (∼55 μm) obtained using a chemical vapor deposition (CVD) method, followed by a gradual solvent replacement technique. Combining a hydrogen bubbling transfer approach with thermal annealing to reduce polymer residue results in an extremely clean surface, where the ultra-large suspended graphene retains the intrinsic features of graphene, including phonon response and an enhanced carrier mobility (200% higher than that of graphene on a substrate). The highly elastic mechanical properties of the graphene membrane are demonstrated, and the Q-factor under 2 MHz stimulation is measured to be 200-300. A graphene-based capacitive pressure sensor is fabricated, where it shows a linear response and a high sensitivity of 15.15 aF Pa(-1), which is 770% higher than that of frequently used silicon-based membranes. The reported approach is universal, which could be employed to fabricate other suspended 2D materials with macro-scale sizes on versatile support substrates, such as arrays of Si nano-pillars and deep trenches.