Si Nanopillars Research Articles

Characteristics of Carrier Collection of Nanopillar-Patterned Silicon Solar Cells.

Nanopillar-patterned Si solar cells were investigated. Ag nanoparticles were coated on a polished Si substrate as an etching mask. Reactive ion etching caused Si nanopillars to replicate in a reverse fashion on the Ag nanoparticles over a large area. The nanopillar structures efficiently reduced the light reflection on the surface and effectively drove the incident light into a Si absorber. This induced a significant enhancement of the photogenerated-current with an improved solar cell efficiency of 16.07%. The Si nanopillar-patterned solar cells showed improved carrier collection for long wavelengths; however, the surface-defect induced recombination degraded the quantum efficiency at short wavelengths. We suggest that the reduction of recombination loss should be considered for efficient nanostructure solar cells.

High aspect ratio PS-b-PMMA block copolymer masks for lithographic applications.

The control of the self-assembly (SA) process and nanostructure orientation in diblock copolymer (DBC) thick films is a crucial technological issue. Perpendicular orientation of the nanostructures in symmetric and asymmetric poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films obtained by means of simple thermal treatments was demonstrated to occur in well-defined thickness windows featuring modest maximum values, thus resulting in low aspect ratio (h/d < 2) of the final lithographic mask. In this manuscript, the thickness window corresponding to the perpendicular orientation of the cylindrical structures in asymmetric DBC is investigated at high temperatures (190 °C ≤ T ≤ 310 °C) using a rapid thermal processing machine. A systematic study of the annealing conditions (temperature and time) of asymmetric PS-b-PMMA (Mn = 67.1, polydispersity index = 1.09) films, with thicknesses ranging from 10 to 400 nm, allowed ordered patterns, with a maximum value of orientational correlation length of 350 nm, to be obtained for film thicknesses up to 200 nm. The complete propagation of the cylindrical structures through the whole film thickness in a high aspect ratio PS template (h/d ≈ 7) is probed by lift-off process. Si nanopillars are obtained having the same lateral ordering and characteristic dimensions of the DBC lithographic mask as further confirmed by grazing-incidence small-angle X-ray scattering experiments.

Highlights from recent advances in nanomedicine.

Highlights from recent advances in nanomedicine.

Surface Coating Mediated Swelling and Fracture in Lithiated Core-Shell Silicon Nanostructures

Despite tremendous effort in designing better Si electrodes based on nanostructured materials, composite and architectures as well as on novel binders and electrolyte additives, sustaining long cycle life at deep lithiation levels is still troublesome[1]. Recent studies have shown that surface coatings can positively impact the cycling stability of the silicon anodes by improving the current collection efficiency, by enabling the formation of a stable solid electrolyte interphase (SEI - and related to that, faster surface kinetics) and by acting as a potential mechanical clamping layer[2]. Despite intensive research, little knowledge is available about the electrochemical and especially mechanical stability of such layers provided that they have to sustain the 300% volume expansion of Si upon full lithiation. A key comprehension feature for the surface coated Si structures is the mechanical stability of such layers: will they fracture or not upon full lithiation? If cracks in the coating layer appear and propagate, much of the designed advantages are simply lost.Here, we will detail on the effect of metallic Ni coating on the lithiation behavior of crystalline Si nanostructures. To have a better understanding of the Ni shell influence upon Si lithiation we have fabricated composite structures with different morphological parameters: Ni shell thickness of 40nm, 80nm and 120nm; Si core diameter of 170nm, 330nm and 480nm. The used Si nanopillars have a perfect circular symmetry and the Ni layer forms a conformal coating of uniform thickness along radial and axial directions (Fig. 1). The composite structure is considered isotropic at equilibrium.The lithiation behavior is shown in Figure 1. Consistent with previous reports, pristine pillars display anisotropic swelling with preferential elongation along <110> planes[3]. For coated pillars, different swelling behavior and related to that, fracture regimes have been identified (Fig. 1). At small coating thickness, preferential fracture sites are attributed to high stress localization and plastic strain accumulation in the Ni coating resulting from anisotropic swelling of Si. For thick coatings the mechanical confinement induced by the Ni layer levels off the swelling anisotropy and, correspondingly, the plastic strain and stress concentrations, resulting in randomly oriented cracks that originate at the shell surface and propagate radially along the nanowire axis.The observed anomalous lithiation and fracture of the Si@Ni structures are explained within a combined thermodynamical - mechanical framework. The models we develop are extended to other type of coatings or nanostructures leading to correct understand of the capacity fading and ultimately, to a better design of high capacity anodes[4].[1] McDowell M. T. et al. Adv Mater 2013, 25, 4966.[2] Choi N.-S. et al. J Mater Chem 2011, 21, 9825.[3] Lee S. W. et al. Proc Natl Acad Sci 2012, 109, 4080; Liu X. H. et al. Nano Lett 2011, 11, 3312.[4] Vlad A. et al. Proc Natl Acad Sci 2012, 109, 15168; Sandu G et al. submitted.

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.

Fabrication and characterization of SiGe coaxial quantum wells on ordered Si nanopillars

Controlled SiGe coaxial quantum wells (CQWs) on periodic Si(001) nanopillars in a large area are explored systematically. The periodic SiGe CQW nanopillars are fabricated by a combination of nanosphere lithography, metal assisted chemical etching and epitaxial growth. The period, the radius, the height, the composition and the thickness of the SiGe alloy layer can all be intentionally modified. Considerably enhanced photoluminescence (PL) from the SiGe CQW nanopillars is observed, which is composed of four peaks. Such PL features are explained by the coupling between the spontaneous emissions of the SiGe CQW and the Mie resonant modes of the nanopillars, which can be further improved by optimizing the structural parameters of the SiGe CQW and the nanopillars. Our results demonstrate a feasible route to obtaining controlled SiGe CQW nanopillars, which have potential applications in optoelectronic devices.

Fabrication of N-doped TiO2 coatings on nanoporous Si nanopillar arrays through biomimetic layer by layer mineralization.

Si/N-doped TiO2 core/shell nanopillar arrays with a nanoporous structure are fabricated through a simple protein-mediated TiO2 deposition process. The Si nanopillar arrays are used as templates and alternatively immersed in aqueous solutions of catalytic molecules (protamine, PA) and the titania precursor (titanium(iv) bis(ammonium lactato)dihydroxide, Ti-BALDH) for the layer by layer mineralization of a PA/TiO2 coating. After a subsequent calcination, a N-doped TiO2 layer is formed, and its thickness could be controlled by varying the cycles of deposition. Moreover, the nanoporous structure of the Si nanopillars strongly affects the formation of the TiO2 layer. The obtained Si/TiO2 nanocomposites show significantly improved solar absorption compared with commercially purchased TiO2 nanoparticles.

300 mm Wafer-level, ultra-dense arrays of Au-capped nanopillars with sub-10 nm gaps as reliable SERS substrates.

The 193 nm deep UV immersion lithography is leveraged to fabricate highly dense and uniform arrays of Au-capped Si nanopillars on a 300 mm wafer level, and the substrates are applied in surface enhanced Raman spectroscopy for reliable molecule detection. Due to the sub-10 nm gap sizes and ultra-high array density with the lattice constant less than 100 nm, our nanopillar based substrates outperform the current commercial products in terms of the signal intensity, reproducibility and fabrication scale.

Droplet spreading on a two-dimensional wicking surface

The dynamics of droplet spreading on two-dimensional wicking surfaces were studied using square arrays of Si nanopillars. It was observed that the wicking film always precedes the droplet edge during the spreading process causing the droplet to effectively spread on a Cassie-Baxter surface composed of solid and liquid phases. Unlike the continual spreading of the wicking film, however, the droplet will eventually reach a shape where further spreading becomes energetically unfavorable. In addition, we found that the displacement-time relationship for droplet spreading follows a power law that is different from that of the wicking film. A quantitative model was put forth to derive this displacement-time relationship and predict the contact angle at which the droplet will stop spreading. The predictions of our model were validated with experimental data and results published in the literature.

Mie resonance-enhanced light absorption in periodic silicon nanopillar arrays

Mie-resonances in vertical, small aspect-ratio and subwavelength silicon nanopillars are investigated using visible bright-field µ-reflection measurements and Raman scattering. Pillar-to-pillar interactions were examined by comparing randomly to periodically arranged arrays with systematic variations in nanopillar diameter and array pitch. First- and second-order Mie resonances are observed in reflectance spectra as pronounced dips with minimum reflectances of several percent, suggesting an alternative approach to fabricating a perfect absorber. The resonant wavelengths shift approximately linearly with nanopillar diameter, which enables a simple empirical description of the resonance condition. In addition, resonances are also significantly affected by array density, with an overall oscillating blue shift as the pitch is reduced. Finite-element method and finite-difference time-domain simulations agree closely with experimental results and provide valuable insight into the nature of the dielectric resonance modes, including a surprisingly small influence of the substrate on resonance wavelength. To probe local fields within the Si nanopillars, µ-Raman scattering measurements were also conducted that confirm enhanced optical fields in the pillars when excited on-resonance.

Mid-infrared active graphene nanoribbon plasmonic waveguide devices

Doped graphene emerges as a strong contender for active plasmonic material in the mid-infrared wavelengths due to the versatile external-control of its permittivity-function and also its highly-compressed graphene surface plasmon (GSP) wavelength. In this paper, we design active plasmonic waveguide devices based on electrical-modulation of doped graphene nanoribbons (GNRs) on a voltage-gated inhomogeneous dielectric layer. We first develop figure-of-merit (FoM) formulae to characterize the performance of passive and active graphene nanoribbon waveguides. Based on the FoMs, we choose optimal GNRs to build a plasmonic shutter, which consists of a GNR placed on top of an inhomogeneous SiO$_2$ substrate supported by a Si nanopillar. Simulation studies show that for a simple 50nm-long plasmonic shutter, the modulation contrast can exceed 30dB. The plasmonic shutter is further extended to build a 4-port active power splitter and an 8-port active network, both based on GNR cross-junction waveguides. For the active power splitter, the GSP power transmission at each waveguide arm can be independently controlled by an applied gate-voltage with high modulation contrast and nearly-equal power-splitting proportions. From the construct of the 8-port active network, we see that it is possible to scale up the GNR cross-junction waveguides into large and complex active waveguide networks, showing great potential in an exciting new area of mid-infrared graphene plasmonic integrated nanocircuits.

In Situ Atomic Force Microscopy of Lithiation and Delithiation of Silicon Nanostructures for Lithium Ion Batteries

Using electron beam lithography, amorphous Si (a-Si) nanopillars were fabricated with a height of 100 nm and diameters of 100, 200, 300, 500, and 1000 nm. The nanopillars were electrochemically cycled in a 1 M lithium trifluoromethanesulfonate in propylene carbonate electrolyte. In situ atomic force microscopy (AFM) was used to qualitatively and quantitatively examine the morphology evolution of the nanopillars including volume and height changes versus voltage in real-time. In the first cycle, an obvious hysteresis of volume change versus voltage during lithiation and delithiation was measured. The pillars did not crack in the first cycle, but a permanent volume expansion was observed. During subsequent cycles the a-Si roughened and deformed from the initial geometry, and eventually pillars with diameters >200 nm fractured. Furthermore, a degradation of mechanical properties is suggested as the 100 and 200 nm pillars were mechanically eroded by the small contact forces under the AFM probe. Ex situ scanning electron microscopy (SEM) images, combined with analysis of the damage caused by in situ AFM imaging, demonstrate that during cycling, the silicon became porous and structurally unstable compared to as-fabricated pillars. This research highlights that even nanoscale a-Si suffers irreversible mechanical damage during cycling in organic electrolytes.

SiGe Quantum Dots Over Si Pillars for Visible to Near-Infrared Broadband Photodetection

We demonstrate a successful selective growth of Si0.3Ge0.7 quantum dots (QDs) over array of p+-Si nanopillars using a low-pressure chemical vapor deposition technique, and hereafter realized high-performance QD broadband photodiodes for visible to near-infrared photodetection based on heterostructures of indium tin oxide/Si0.3Ge0.7 QD/Si pillar. Thanks to effective hole confinement and thus a built-in electric field within the SiGe QD, high ratios of photocurrent to dark current of ~2200, 100, and 30, respectively, were measured on our SiGe QDs-based photodiodes under illumination of 9 mW/cm2 at wavelength of 500-800, 1300, and 1500 nm. The QD photodiode exhibits a very low dark current density of 3.2 × 10-8 A/cm2 and a tunable power-dependent linearity by applied voltage through the competition of electron drift and carrier recombination processes.

Fabrication and scanning tunneling microscopy characterization of suspended monolayer graphene on periodic Si nanopillars

We present the fabrication and scanning tunneling microscopy (STM) characterization of suspended monolayer graphene (SMG) on periodic Si nanostructure. Monolayer graphene (MG) was grown on Cu foils by chemical vapor deposition (CVD) and transferred onto a Si substrate with etched array of periodic nanopillars, obtaining partly suspended MG. Low-temperature STM characterization was performed on the suspension area of the MG with atomic resolution images obtained. The scanning tunneling spectroscopy of SMG shows a nonlinear behavior near the Fermi level (EF), which is attributed to the Dirac cone reshaped by electron-electron interaction.

Well Controlled Array of Si Nanopillar with Metal Core as Negative Electrode for Lithium-Ion Batteries

A promising Si negative electrode, that is a well controlled array of Si nanopillar, was fabricated by a semiconductor process. The electrochemical property of this electrode was examined by charge-discharge measurements, SEM and TEM observations. Although the result of charge-discharge measurements of this Si nanopillar electrode was comparable to those of a flat thin film Si electrode, pulverization of the Si film was successfully suppressed in the Si nanopillar electrode. In the Si thin film electrode, only the upper half of the Si film seemed to have reacted, and this caused cracking and pulverization. By contrast, in the Si nanopillar electrode, the entire Si film seemed to have reacted, and cracking and pulverization were suppressed. The experimental results demonstrated that the Si nanopillar electrode has a suitable structure for a Si negative electrode.

The Effect of the Deposition Conditions on the Electrodeposition of Si Nanopillars in TMHATFSI

Si nanopillars were prepared by electrodeposition from SiCl4 in an ionic liquid, specifically tri-methyl-n-hexyl ammonium bis-(trifluorosulfonyl) imide (TMHATFSI), and the effects of the various deposition conditions, such as bath temperature and deposition potential, were investigated. For the preparation of the patterned substrates, UV-nanoimprint lithography was employed. The thickness of resist was 90-100 nm, mold feature diameter was 150 nm, with a pitch of 450 nm. As a result, compact Si nanopillars were uniformly electrodeposited, and it was indicated that their morphology were affected mostly by the bath temperature.

Luminescent ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells

Abstract Ordered arrays of nanoporous Si nanopillars and Si nanopillars with nanoporous shells were fabricated using a combination of nanoimprint lithography and metal-assisted chemical etching. They show visible luminescence with an overlap of 2 red emission bands at room temperature. Nanoporous Si nanopillars are formed in highly doped p-Si after metal-assisted chemical etching, while Si nanopillars with nanoporous shells are formed in lightly doped p-Si. The cathodoluminescence intensity is clearly larger in the Si nanopillars with nanoporous shells because they exhibit more luminescence centers at the oxidized surfaces.

Pattern Transfer of 23-Nm-Diameter Block Copolymer Self-Assembled Nanodots Using CF&lt;sub&gt;4&lt;/sub&gt; Etching with Carbon Hard Mask (CHM) as Mask

The self-assembly of block copolymer (BCP) has demonstrated as promising alternative technology to overcome the limitation of conventional lithography owing to its ability in forming nanostructure with size 3-100 nm. In this study, we investigated a technique to transfer self-assembled nanodots of Poly(styrene-b-dimethyl siloxane) (PS-PDMS) BCP to Si. The pattern transfer of PS-PDMS nanodots with the pitch of 33 nm and the diameter of 23 nm using CF4 etching with Carbon Hard Mask (CHM) as Mask is demonstrated. Si nanopillar with height of 51 nm was fabricated. This result improves the potential use of PS-PDMS BCP self-assembly technique for fabrication nano-electronic devices, such as quantum dot solar cell and ultrahigh density of magnetic recording.

Ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells

The fabrication of ordered arrays of nanoporous Si nanopillars with and without nanoporous base and ordered arrays of Si nanopillars with nanoporous shells are presented. The fabrication route is using a combination of substrate conformal imprint lithography and metal-assisted chemical etching. The metal-assisted chemical etching is performed in solutions with different [HF]/[H2O2 + HF] ratios. Both pore formation and polishing (marked by the vertical etching of the nanopillars) are observed in highly doped and lightly doped Si during metal-assisted chemical etching. Pore formation is more active in the highly doped Si, while the transition from polishing to pore formation is more obvious in the lightly doped Si. The etching rate is clearly higher in the highly doped Si. Oxidation occurs on the sidewalls of the pillars by etching in solutions with small [HF]/[H2O2 + HF] ratios, leading to thinning, bending, and bonding of pillars.

Hierarchical structures of AlOOH nanoflakes nested on Si nanopillars with anti-reflectance and superhydrophobicity

A novel method to fabricate ultra-low reflective Si surfaces with nanoscale hierarchical structures is developed by the combination of AlOOH or boehmite nanoflakes nested on plasma-etched Si nanopillars. Using CF4 plasma etching, Si surfaces are nanostructured with pillar-like structures by selective etching with self-masking by fluorocarbon residues. AlOOH nanoflakes are formed by Al thin film coating with various thicknesses and subsequent immersion in boiling water, which induces the formation of nanoscale flakes through the hydrolysis reaction. AlOOH nanoflakes are formed on Si nanopillared surfaces for hierarchical structures, which are coated with a low-surface-energy material, resulting in a higher water wetting angle of over 150° and a very low contact angle hysteresis of less than 5°, and implying a self-cleaning surface. Reflectance reduced to 5.18% on average on hierarchical nanostructures in comparison to 9.63% on the Si nanopillar surfaces only. We found that Si nanopillars reduced reflection for wavelengths ranging from 200 to 1200 nm while AlOOH nanoflakes reduced reflection for wavelengths longer than 600 nm.