Silicon Nanocones Research Articles

Lithography-Free, Low-Cost Method for Improving Photodiode Performance by Etching Silicon Nanocones as Antireflection Layer

A three-step process has been demonstrated to improve the performance of photodiode by creating nanocone forest on the surface of photodiode as an antireflection layer. This high-throughput, low-cost process has been shown to decrease the reflectivity by 66.1%, enhance the quantum efficiency by 27%, and increase the responsivity by 25.7%. This low-cost manufacture process can be applied to increase the responsivity of silicon based photonic devices.

Structure Transition Mechanism of Single-Crystalline Silicon, g-C3N4, and Diamond Nanocone Arrays Synthesized by Plasma Sputtering Reaction Deposition

Single-crystalline silicon, g-C3N4, and diamond nanocone arrays were synthesized on nickel-covered silicon (100) substrates by a novel method of plasma sputtering reaction deposition. The experimental results show that the morphologies, structures, and composition of the as-grown nanocones strongly depend on the ratios of the inlet mixed gases. The silicon, g-C3N4, and diamond nanocone arrays could be grown at the CH4/(N2 + H2) ratios of about 1/20–1/10, 1/150–1/60, and 0, respectively. The analyses of the optical emission spectra from the discharged plasma indicate that the inlet methane controls the growth of the nanocones by suppressing the H+-sputtering effect to adjust the amounts of the silicon, carbon, and nitrogen atoms attaining the substrate, which determines the composition, structures, and crystallinity of the grown nanocones.

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures. [corrected

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/neff)3 < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

Three Dimensional Hybrids of Vertical Graphene-nanosheet Sandwiched by Ag-nanoparticles for Enhanced Surface Selectively Catalytic Reactions.

Three dimensional (3D) plasmonic nanostructure is perfect for the surface-enhanced Raman scattering (SERS) and also very suitable for surface catalytic reaction, but how to design and fabricate is still a robust task. Here, we show a 3D plasmonic nanohybrid of vertical graphene-nanosheet sandwiched by Ag-nanoparticles on the silicon nanocone array substrate for enhanced surface catalytic reaction. By SERS detection, we find that this hierarchical nanohybrid structure is highly efficient in the enhancement of catalytic reaction, even at a very low concentration of 10−11 M, which is far better than previous reports by four orders of magnitude. A strong electric field enhancement produced in the 3D framework nanohybrids of graphene nanosheet/Ag-nanoparticles is responsible for this great enhancement of catalytic reaction, due to larger electron collective oscillation in the composite system. Especially the oxygen adsorbed on the graphene and Ag nanoparticles can be excited to triplet excited states, and the electrons on the graphene and the nanoparticles can be effectively transferred to the oxygen, which plays very important role in molecular catalytic reactions. Our results demonstrate the contribution of graphene in plasmon-driven catalytic reactions, revealing a co-driven reaction process.This excellent SERS substrate can be used for future plasmon and graphene co-catalytic surface catalytic reactions, graphene-based surface plasmon sensors and so on.

A comparison of light-harvesting performance of silicon nanocones and nanowires for radial-junction solar cells.

Silicon nanorod based radial-junction solar cells are competitive alternatives to traditional planar silicon solar cells. In various silicon nanorods, nanocone is always considered to be better than nanowire in light-absorption. Nevertheless, we find that this notion isn’t absolutely correct. Silicon nanocone is indeed significantly superior over nanowire in light-concentration due to its continuous diameters, and thus resonant wavelengths excited. However, the concentrated light can’t be effectively absorbed and converted to photogenerated carriers, since its propagation path in silicon nanocone is shorter than that in nanowire. The results provide critical clues for the design of silicon nanorod based radial-junction solar cells.

Hybrid Si nanocones/PEDOT:PSS solar cell.

Periodic silicon nanocones (SiNCs) with different periodicities are fabricated by dry etching of a Si substrate patterned using monolayer polystyrene (PS) nanospheres as a mask. Hybrid Si/PEDOT:PSS solar cells based on the SiNCs are then fabricated and characterized in terms of their optical, electrical, and photovoltaic properties. The optical properties of the SiNCs are also investigated using theoretical simulation based on the finite element method. The SiNCs reveal excellent light trapping ability as compared to a planar Si substrate. It is found that the power conversion efficiency (PCE) of the hybrid cells decreases with increasing periodicity of the SiNCs. The highest PCE of 7.1% is achieved for the SiNC hybrid cell with a 400-nm periodicity, due to the strong light trapping near the peak of the solar spectrum and better current collection efficiency.PACS: 81.07.-b; 81.16.-c; 88.40.hj

Broadband optical absorption by tunable Mie resonances in silicon nanocone arrays.

Nanostructure arrays such as nanowire, nanopillar, and nanocone arrays have been proposed to be promising antireflection structures for photovoltaic applications due to their great light trapping ability. In this paper, the optical properties of Si nanopillar and nanocone arrays in visible and infrared region were studied by both theoretical calculations and experiments. The results show that the Mie resonance can be continuously tuned across a wide range of wavelength by varying the diameter of the nanopillars. However, Si nanopillar array with uniform diameter exhibits only discrete resonance mode, thus can't achieve a high broadband absorption. On the other hand, the Mie resonance wavelength in a Si nanocone array can vary continuously as the diameters of the cross sections increase from the apex to the base. Therefore Si nanocone arrays can strongly interact with the incident light in the broadband spectrum and the absorbance by Si nanocone arrays is higher than 95% over the wavelength from 300 to 2000 nm. In addition to the Mie resonance, the broadband optical absorption of Si nanocone arrays is also affected by Wood-Rayleigh anomaly effect and metal impurities introduced in the fabrication process.

Thick solid electrolyte interphases grown on silicon nanocone anodes during slow cycling and their negative effects on the performance of Li-ion batteries.

Thickness, homogeneity and coverage of the surface passivation layer on Si anodes for Li-ion batteries have decisive influences on their cyclic performance and coulombic efficiency, but related information is difficult to obtain, especially during cycling. In this work, a well-defined silicon nanocone (SNC) on silicon wafer sample has been fabricated as a model electrode in lithium ion batteries to investigate the growth of surface species on the SNC electrode during cycling using ex situ scanning electronic microscopy. It is observed that an extra 5 μm thick layer covers the top of the SNCs after 25 cycles at 0.1 C. This top layer has been proven to be a solid electrolyte interphase (SEI) layer by designing a solid lithium battery. It is noticed that the SEI layer is much thinner at a high rate of 1 C. The cyclic performance of the SNCs at 1 C looks much better than that of the same electrode at 0.1 C in the half cell. Our findings clearly demonstrate that the formation of the thick SEI on the naked nanostructured Si anode during low rate cycling is a serious problem for practical applications. An in depth understanding of this problem may provide valuable guidance in designing Si-based anode materials.

Silicon-based Organic/inorganic Hybrid Solar Cells

Organic-inorganic hybrid solar cells display potential to be high efficiency and low cost photovoltaics due to combined advantages of high stability, high mobility and well developed fabrication process from inorganic materials and the properties to adjust organic molecule structure, absorption spectrum and bandgap from solution processable organics. Het- erojunction photovoltaics formed by silicon and organics at low temperature has drawn great interests over past five years and the reported highest power conversion efficiency (PCE) has achieved up to 13.8%. The emerging of nanotechnology allows for silicon micro/nano structures including silicon nanowires, pyramids and nanocones with excellent light absorption properties which can greatly reduce the consumption of silicon materials as well as the purity dependence. The micro/nano structures also exhibit the advantages to offer larger junction area and more effective separation pathways for charge carriers. It is noticeable that silicon nanowire-based flexible hybrid solar cells with tens of micrometers silicon substrate thickness have achieved the PCE of above 12% adopting the most popular commercialized conjugated polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). With the rapid developments of new organic mate- rials and interface engineering methods, different kinds of organic-silicon hybrid solar cells has been reported and shown superior photovoltaic characteristics. The adopted organics include PEDOT:PSS, poly(3-hexylthiophene) (P3HT), 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-octylthiophene) (P3OT), fullerene derivative and so on. This paper reviews the device structures of silicon-based hybrid solar cells, working mecha- nism and related organic molecular. The hybrid heterojunction with different materials and fabrication processes has been discussed. The last section summarizes the method used to improve the performance of the hybrid solar cells and depicts the challenges and prospects of the silicon-based hybrid solar cells in the near future. Keywords silicon structures; organics; solution spin-coating process; optimization of interface; hybrid heterojunction solar cells

Fabrication of silver nanoplate hierarchical turreted ordered array and its application in trace analyses.

Silver nanoplate hierarchical turreted ordered arrays were fabricated through an electro-deposition method on ordered acuate silicon nanocone templates. Such arrays can be used as SERS substrates for trace analyses of streptomycin sulphate, and exhibit high activity and stability. This work is of importance in practical applications based on the SERS effect of noble metal micro/nano-structured arrays.

Hybrid silicon nanocone–polymer solar cells based on a transparent top electrode

We utilize fully covered thin layer of transparent MoO3/Ag/ZnS as the top electrode for Si nanocone/PEDOT:PSS hybrid solar cells. By adjusting the geometrical parameters systematically, the optimized PCE was realized as 5.12%.

The plasmonic enhancement in silicon nanocone hole solar cells with back located metal particles

A theoretical analysis of plasmonic enhancement in silicon nanocone hole (NCH) solar cells with back located periodic Ag particles is presented. The nanostructured Ag back particles can significantly enhance the absorption in the near-infrared spectrum. The ultimate efficiency of the optimized nanostructure can achieve 39%, which is 22% higher than that of NCH arrays without Ag particles. The angle-dependent ultimate efficiency is also investigated. The proposed nanostructure is expected to shed some light on the design and fabrication of high-efficiency solar cells.

Controlled growth of crystalline g-C3N4 nanocone arrays by plasma sputtering reaction deposition

Vertically and neatly arranged crystalline graphitic C3N4 (g-C3N4) nanocone arrays were firstly synthesized on nickel-covered silicon (100) substrates supported on a graphite frame. Synthesis was accomplished using an abnormal glow discharge plasma sputtering reaction deposition method with a feed gas mixture of CH4, N2 and H2. The experimental results show that the morphologies, structures, composition and photoluminescence of the grown nanostructures strongly depend on the CH4/(N2+H2) ratios (0–1/10). The hexagonal g-C3N4 nanocone arrays were well grown at a CH4/(N2+H2) ratio of around 1/150 as a result of the hydrogen ion sputtering of the graphite frame to generate a lot of carbon atoms, the hydrogen ion etching on the growing graphite structures, and the reaction between the active nitrogen atoms and the sputtered carbon atoms. The g-C3N4 nanocone arrays have the characteristic photoluminescence-peak of g-C3N4 and very nice wettability to the polymer absorber layers. At the higher or lower CH4/(N2+H2) ratios, the silicon or diamond nanocone arrays were grown respectively due to the ion-sputtering of the silicon substrate and the inhibition of CHn-radicals on the H+-sputtering of the graphite frame or the mismatch between the sputtered carbon atoms and the dissociated nitrogen atoms.

Morphology control of ordered Si nanowire arrays by nanosphere lithography and metal-assisted chemical etching

Metal-assisted chemical etching (MACE) using a nanosphere lithography (NSL) technique is regarded as a general fabrication method for silicon nanowire (SINW) arrays. However, morphology control of SiNWs using this method has not been reported. In this study, silicon nanowire (SINW) and silicon nanocone (SINC) arrays were fabricated by MACE using a NSL. Depending on the concentration of etchants in the etching solution, the morphology of the wires and etching rate were systemically changed. At high concentrations, the wires were etched cylindrically and at low concentrations, tapered (cone-like) wires were obtained. To quantify the degree of tapering, the volume ratio of the etched part was calculated from their morphology. The degree of tapering increased as the concentrations of etchants decreased. We suggest that the mechanism of the formation of nanocone is related to the degree of hole diffusion under the metal layer, which was supported by field emission scanning electron microscope (FE-SEM) images.

Hemispherical Arrays of Colloidal Crystals Fabricated by Transfer Printing

The self-assembly of colloidal spheres is an effective strategy for producing nanopatterns. To use colloidal crystals in lithographic applications, the key challenge is to fabricate a monolayer of colloidal crystals uniformly over a large area. A simple and effective method for fabricating a colloidal crystals monolayer from a 3D colloidal crystal is described. The top layer of a surface-etched 3D colloidal crystal is picked up on a PDMS stamp, and by simply heating the receiving substrate, the polymeric colloidal crystal can be easily transferred to many types of substrates, including curved or flexible materials, without utilizing a glue layer. Moreover, the colloidal spheres are deformed to hemispheres during the transfer process, which is a suitable form of a lithographic mask for both dry and wet etching processes. An array of silicon nanocones and gold dots is demonstrated by pattern transfer from an array of hemispherical polymeric particles. In addition, it is also shown that the transferred hemispherical array has good antireflective properties.

All-back-contact ultra-thin silicon nanocone solar cells with 13.7% power conversion efficiency

Thinner Si solar cells with higher efficiency can make a Si photovoltaic system a cost-effective energy solution, and nanostructuring has been suggested as a promising method to make thin Si an effective absorber. However, thin Si solar cells with nanostructures are not efficient because of severe Auger recombination and increased surface area, normally yielding <50% EQE with short-wavelength light. Here we demonstrate >80% EQEs at wavelengths from 400 to 800 nm in a sub-10-μm-thick Si solar cell, resulting in 13.7% power conversion efficiency. This significant improvement was achieved with an all-back-contact design preventing Auger recombination and with a nanocone structure having less surface area than any other nanostructures for solar cells. The device design principles presented here balance the photonic and electronic effects together and are an important step to realizing highly efficient, thin Si and other types of thin solar cells.

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.

Investigating pristine and carbon-decorated silicon nanocones: DFT studies

Electronic and structural properties of pristine and carbon-decorated (C-decorated) models of a representative silicon (Si) nanocone with 240° disclination angle have been investigated through density functional (DFT) theory calculations. Stabilized structures, dipole moments, energy gaps, binding energies, optimized bond lengths, and chemical shifts have been evaluated for both models. The results indicated that the molecular properties of C-decorated model could be meaningfully different from the pristine nanocone. The values of dipole moment and energy gap are increased in the C-decorated model and a better value of binding energy was obtained for this model. However, the values of Si–Si bond lengths are remained unchanged. Moreover, the values of chemical shifts revealed different chemical environments for both models of the Si nanocone. The C atoms could employ an electric field on the Si atoms, which made their chemical shifts go to lower or upper fields regarding to the pristine nanocone.

Enhanced field emission characteristics in metal-coated Si-nanocones

Metallic gold (Au) and platinum (Pt) thin films were deposited on silicon nanocones (Si-NCs) by sputtering to elucidate the effects of work function and conductivities on the field electron emission characteristics of surface-modified Si-NCs. The results showed that for Pt/Si-NCs and Au/Si-NCs, although the turn-on field defined at a corresponding current density of 10 μA cm(-2) only improved from 4.20 V μm(-1) for bare Si-NCs to 3.65 and 2.90 V μm(-1), respectively, the emission current density measured at 5.00 V μm(-1) was enhanced by orders of magnitude, reaching 1.82 mA cm(-2) for Au/Si-NCs. Compared to those obtained from various surface-modified Si-nanostructures, such as ZnO/Si-nanopillars and ferroelectrics/Si-nanotips, the current results represent an interesting alternative route for producing surface-modified Si-NCs that might be useful for optical and electronic applications.

Developing controllable anisotropic wet etching to achieve silicon nanorods, nanopencils and nanocones for efficient photon trapping

Controllable hierarchy of highly regular, single-crystalline nanorod, nanopencil and nanocone arrays with tunable geometry and etch anisotropy has been achieved over large areas (>1.5 cm × 1.5 cm) by using an [AgNO3 + HF + HNO3/H2O2] etching system. The etching mechanism has been elucidated to originate from the site-selective deposition of Ag nanoclusters. Different etch anisotropies and aspect ratios can be accomplished by modulating the relative concentration in the [AgNO3 + HF + HNO3/H2O2] etching system. Minimized optical reflectance is also demonstrated with the fabricated nano-arrays. Overall, this work highlights the technological potency of utilizing a simple wet-chemistry-only fabrication scheme, instead of reactive dry etching, to attain three-dimensional Si nanostructures with different geometrical morphologies for applications requiring large-scale, low-cost and efficient photon trapping (e.g. photovoltaics).