Nanostructured Black Silicon Research Articles

Investigations on black silicon nanostructures fabricated by reactive ion etching on highly curved surfaces

A large amount of valuable research on black silicon (b-Si) nanostructures has pushed its use further into application in recent years. However, the research results to be found are all related to plane substrates. For an investigation of b-Si structures on curved surfaces, they were fabricated on hemispherical silicon lenses using inductively coupled plasma reactive ion etching. It was possible to achieve different structure morphologies according to the selected etching parameters, analogous to their fabrication on Si wafers. Scanning electron microscopy inspections of the structure morphology were carried out to study the homogeneity across the lens surface as well as the orientation of the structures. Simulations of the bent electric field within the etching chamber and the resulting ion trajectories explain the underlying etching process that creates structures approximately perpendicular to the curved lens surface. Optical reflectance measurements confirm the findings on homogeneity and orientation.

Improvement in near-infrared absorbance attenuation by using nanometer black silicon composited with gold nanoparticles

In order to solve the problem of near-infrared (NIR) absorbance attenuation of silicon, a method of preparing gold nanoparticles (AuNPs) on the micro–nano-structured black silicon (B-Si) is proposed. In this study, the local surface plasmon resonance (LSPR) of AuNPs excited by a light field is used to achieve B-Si materials with broad spectrum and high absorption. The results show that nanometer B-Si composited with 25-nm AuNPs has an average absorption of 98.6% in the spectral range of 400–1100 nm and 97.8% in the spectral range of 1100–2500 nm. Compared with ordinary B-Si, the absorption spectrum is broadened from 400–1100 nm to 400–2500 nm, and the absorption is increased from 90.1 to 97.8% at 1100–2500 nm. It is possible to use the B-Si materials in the field of NIR-enhanced photoelectric detection and micro-optical night vision imaging due to the low cost, high compatibility, and reliability.

Stable and Reusable Lace-like Black Silicon Nanostructures Coated with Nanometer-Thick Gold Films for SERS-Based Sensing.

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 106. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 μm2). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10-5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10-9-10-4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare.

Wafer-scale nanostructured black silicon with morphology engineering via advanced Sn-assisted dry etching for sensing and solar cell applications.

Black-Si (b-Si) providing broadband light antireflection has become a versatile substrate for photodetectors, photo-electric catalysis, sensors, and photovoltaic devices. However, the conventional fabrication methods suffer from single morphology, low yield, or frangibility. In this work, we present a high-yield CMOS-compatible technique to produce 6-inch wafer-scale b-Si with diverse random nanostructures. b-Si is achieved by O2/SF6 plasma-based reactive ion etching (RIE) of the Si wafer which is coated with a GeSn layer. A stable grid of the SnOxFy layer, formed during the initial GeSn etching, acts as a self-assembled hard mask for the formation of subwavelength Si nanostructures. b-Si wafers with diverse surface morphologies, such as the nanopore, nanocone, nanohole, nanohillock, and nanowire were achieved. Furthermore, the responsivity of the b-Si metal-semiconductor-metal (MSM) photodetector in the near-infrared (NIR) wavelength range (1000-1200 nm) is 40-200% higher than that of a planar-Si MSM photodetector with the same level of dark current, which is beneficial for applications in photon detectors, solar cells, and photocatalysis. This work not only demonstrates a new non-lithography method to fabricate wafer-scale b-Si wafers, but may also provide a novel strategy to fabricate other nanostructured surface materials (e.g., Ge or III-V based compounds) with morphology engineering.

Comparison between One-Dimensional and Three-Dimensional Optical Modelling of Ordered Black Silicon Nanostructures Using Effective Medium Approach

The aim of this study is to determine whether the effective medium approach (EMA) is suitable to model black silicon structures. The present study focuses on comparing EMA paired with 1D FDTD simulations to full 3D FDTD simulations. Comparison was done for ordered cylindrical and hemispherical b-Si nanostructures. From the simulation results, the 1D simulation seems to underestimate the transmittance and overestimate the reflectance for these structures. This was attributed to the failure of the EMA to capture scattering and diffraction effects that were present in the nanostructures. The absorptance spectrum was comparable for both 1D and 3D simulations, hence it was concluded that the simplification may be suitable for simplifying problems where calculating the absorption of light is desired.

Two-dimensional metamaterials as meta-foams for optimized surface-enhanced solar steam generation

We report on a new class of metamaterials, which consists of meta-foams, optimized for high performance in enhanced solar steam generation. The design of these meta-foams involves precise dimensional control of a periodic two-dimensional micro-pore array. Proof-of-concept of meta-foams efficiency is demonstrated using two fabrication technologies. The first one involves microscale plasma etching onto a nanostructured black silicon substrate. As an alternative low-cost technique, we also use 3D printing of a graphene-containing polymer material. The experimental validation shows that the best evaporation rate reaches 1.34 kg/(h·m2) under 1 sun illumination, in open-air conditions under normal temperature and relative humidity conditions of 20 °C and 58%, an unmatched value so far under similar conditions, while the theoretical limit is estimated at 1.5 kg/(h·m2). This corresponding to a 89% conversion efficiency. Experimental data concur well with the predicted performance obtained by numerical simulations. The proposed model simultaneously accounts for both heat and mass transfer, including photothermal conversion and water phase change, to guide the optimization towards the maximization of the evaporation rate, an important figure of merit in many applications, including solar heat harvesting and solar water purification.

Light scattering from black silicon surfaces and its benefits for encapsulated solar cells

Black silicon (b-Si) has been widely investigated as a potential replacement for more traditional antireflective schemes for silicon solar cells, such as random pyramids, due to its reduced broadband reflectance and improved light-trapping properties. Wavelength and angle resolved scattering (WARS) reflectance measurements provide the means of analysing the amount of light scattered from a textured surface, which can be of interest when considering the amount of light trapped through total internal reflectance (TIR) at various interfaces in an encapsulated photovoltaic module. Here we present and analyse results from WARS measurements on b-Si surfaces fabricated using metal assisted chemical etching (MACE). Large angle scattering is observed for the entire spectrum, increasingly so for shorter incident wavelengths and increasing height of texture features. This is predicted to result in 35–40% of the reflected light being trapped by TIR at the glass-air interface and redirected back onto the sample, when the sample is encapsulated in standard PV module materials. This leads to a calculated additional boost of up to 0.45% in the photogenerated current of an encapsulated black silicon solar cell. This exceeds the calculated 0.21% boost due to TIR predicted for an encapsulated solar cell employing the industry-standard random pyramid texture with a thin film antireflective coating. • Black silicon nanostructures are fabricated for antireflection and light-trapping. • Nanostructures exhibit broadband reduced hemispherical reflectance below 3%. • Custom setup is used to measure wavelength and angular position of reflections. • Portion of reflected light trapped at various solar cell interfaces exceeds 40%. • Photocurrent relative gain of 0.45% is calculated for multiple nanostructure heights.

Optimized Deep Reactive-Ion Etching of Nanostructured Black Silicon for High-Contrast Optical Alignment Marks

High-contrast alignment marks are promising optical elements for the high-precision alignment of X-ray gratings in phase-contrast X-ray imaging systems. These marks contain micrometer-scale bands of optically absorptive and reflective areas that determine their relative degree of optical contrast. Nanostructured black silicon (n-BSi) enabling the broadband, quasi-omnidirectional absorption of light, is a promising material to define these absorptive areas. In this work, we have developed an optimized deep reactive-ion etching (DRIE) process to fabricate n-BSi for these alignment marks. The DRIE process was optimized by systematically changing the polymer deposition time, T, while keeping all other process conditions constant. As T varied from 0.6 to 2.5 s, three distinct etching regimes were observed: (1) smooth etching, (2) n-BSi, and (3) etching stop. During the n-BSi regime (T = 1.6 to 2.1 s), varying T altered the n-BSi morphology, resulting in a broad spectrum of nanostructures: nanopillars to nanopores. Our investigation of the process–structure–property relationships among the process etching, morphology evolution, and reflectance of n-BSi revealed that morphologies with an increased height, aspect ratio, and degree of tapering and a lower base spacing were most effective at suppressing reflection for λ = 500–800 nm. Our results demonstrated the lowest reflectance of the absorptive areas, Rabs = 0.01 (∼1%) and Rrel_abs = 0.03 (relative with respect to Si) at λ = 600 nm for T = 1.9 s (100 cycles), thus giving the highest contrast ratio, ϕ = 0.9. To increase the device throughput, we varied the cycle number from 5 to 100 times at T = 1.9 s to study its effect on growth, morphology, and reflectance of n-BSi. Our results demonstrate that Rabs, Rrel_abs, and ϕ plateaued at 50 cycles, cutting in half our total fabrication time, t. In addition, our alignment marks demonstrated rotational and translational precision alignment capabilities. Lastly, we present two design principles for T and the cycle number to fabricate the optimal n-BSi morphology for the targeted application using any DRIE tool.

Enhancing the light absorptance of stain-etched black silicon decorated by TiN nanoparticles

Due to the high energy, narrow distribution and breaking through the absorption limitation, plasmon-induced hot electrons have been widely applied to extend the photoresponse spectra of the semiconductor. In order to further enhance the resonance effect of local plasmon based on metallic nanostructures, we used hydrofluoric stain etching method to fabricate nanostructured black silicon (BSi) and deposited titanium nitride (TiN) nanoparticles on its surface by reactive magnetron sputtering. The results show that the BSi modified by plasmonic TiN nanoparticles has higher absorption in wavelength range from 1100 to 2500 nm compared to that of conventional acid etching of BSi. A PIN photoelectronic detector fabricated by the proposed BSi shows excellent device performance with responsivity of 0.45A/W at 1060 nm in near-infrared band.

A Magnetically Actuated Superhydrophobic Ratchet Surface for Droplet Manipulation.

A water droplet dispensed on a superhydrophobic ratchet surface is formed into an asymmetric shape, which creates a Laplace pressure gradient due to the contact angle difference between two sides. This work presents a magnetically actuated superhydrophobic ratchet surface composed of nanostructured black silicon strips on elastomer ridges. Uniformly magnetized NdFeB layers sputtered under the black silicon strips enable an external magnetic field to tilt the black silicon strips and form a superhydrophobic ratchet surface. Due to the dynamically controllable Laplace pressure gradient, a water droplet on the reported ratchet surface experiences different forces on two sides, which are explored in this work. Here, the detailed fabrication procedure and the related magnetomechanical model are provided. In addition, the resultant asymmetric spreading of a water droplet is studied. Finally, droplet impact characteristics are investigated in three different behaviors of deposition, rebound, and penetration depending on the impact speed. The findings in this work are exploitable for further droplet manipulation studies based on a dynamically controllable superhydrophobic ratchet surface.

Nanostructured pyramidal black silicon with ultra-low reflectance and high passivation

In this study, nanostructured pyramidal black silicon is prepared by metal assisted chemical etching method, in which the silver nitrate (AgNO3) is used as the metal catalyst. Effects of the concentration of AgNO3 on passivation and optical properties of the black silicon are investigated. The experimental results show that at the AgNO3 concentration of 0.03 M, the nanostructure length is about 300 nm, and the reflectance of the black silicon with a stack of silicon nitride (SiNx) and aluminum oxide (Al2O3) is 0.8%, which is comparable to that of the conventional black silicon with micrometer-long nanowires. In addition, an acceptably low surface recombination rate of 42 cm/s can be obtained. Plasma chemical vapor deposited SiNx is deposited well on the top of nanostructures of black silicon, but shows poor coverage at the bottom region. Spatial atomic layer deposited Al2O3 can conformally cover the nanostructures with high passivation quality. Simulation result indicates an improvement of 5.5% of conversion efficiency for the nanostructured pyramidal black silicon solar cell compared to industrial silicon solar cell. The short nanostructured pyramidal surface with low reflectance and high passivation is expected to be helpful for black silicon technology applied to photovoltaic applications.

A cost-effective and facile approach for realization of black silicon nanostructures on flexible substrate

In this paper, we report a cost effective and facile approach to produce uniform nanostructural black silicon over large-area by metal assist chemical etching followed by its transfer over pressure sensitive flexible substrate. Structural and optical properties of black silicon over flexible substrate were investigated. Field emission scanning electronic microscope (FESEM) reveals textured surface and dense morphology of silicon nanostructures; that appeared to be black. An intense and broadband UV and Visible photoluminescence spectra from these nanostructures was observed and suggested the optically active nature of black silicon. Broadening and asymmetric shifting of Raman line shape, corresponding to black silicon, confirmed quantum confinement of phonon. The crystal sizes of silicon (Si) nanostructures calculated using frequency shift in Raman spectra analytical model were 3.4 nm to 4.7 nm. Significant reduction of reflection below (1%) over the wide UV–Vis spectrum was recorded due to presence of textured surface as observed by FESEM. The present work aids our understanding of tuning the optical properties of black silicon and its’ realization on flexible substrate could be beneficial for flexible electronics including bio-sensing and optoelectronics devices. More interestingly low reflectance of black silicon could pave the way for realization of highly efficient flexible solar cells.

Black silicon with order-disordered structures for enhanced light trapping and photothermic conversion

Black silicon has become a versatile substrate for photoelectric and photothermic application due to its excellent light trapping ability. However, subsequent material engineering can conflict with its intrinsic anti-reflective behavior. In this paper, we present a two-step etching approach to create novel black silicon combining ordered micropores and disordered nanopores. The larger pores increase the optical path length and provide space to load functional materials, while the smaller ones facilitate coupling between the incident solar radiation and material. The proposed etching method increases the light-trapping ability of conventional nanostructured black silicon and achieves enhanced absorption in a broad spectrum. Besides, loading Au nanoparticles further improve the light absorbance in the near-infrared range of 1100–1700 nm. The intrinsic SiOx-rich layer on silicon surface produced during SF6/O2 plasma etching leads to photothermic conversion being the dominant dissipation route for the light energy, thereby yielding outstanding performance in photothermic and photo-thermal-electric demonstrations. This work establishes a robust relationship between three-dimensional structure and light-trapping property of black silicon and provides insights into substrate designs that facilitate the light conversion.

Simulations of Protein Adsorption on Nanostructured Surfaces

Recent technological advances have allowed the development of a new generation of nanostructured materials, such as those displaying both mechano-bactericidal activity and substrata that favor the growth of mammalian cells. Nanomaterials that come into contact with biological media such as blood first interact with proteins, hence understanding the process of adsorption of proteins onto these surfaces is highly important. The Random Sequential Adsorption (RSA) model for protein adsorption on flat surfaces was modified to account for nanostructured surfaces. Phenomena related to the nanofeature geometry have been revealed during the modelling process; e.g., convex geometries can lead to lower steric hindrance between particles, and hence higher degrees of surface coverage per unit area. These properties become more pronounced when a decrease in the size mismatch between the proteins and the surface nanostructures occurs. This model has been used to analyse the adsorption of human serum albumin (HSA) on a nano-structured black silicon (bSi) surface. This allowed the Blocking Function (the rate of adsorption) to be evaluated. The probability of the protein to adsorb as a function of the occupancy was also calculated.

Black Silicon: a New Manufacturing Method and Optical Properties

A new method for manufacturing nanostructured black silicon (b-Si) by growing cone-shaped (pointed) filamentous nanocrystals (nanowires, FNWs) on the surface of single-crystal Si-plates has been proposed. b-Si samples with reflectivity less than 0.1% have been obtained. It has been shown that the b-Si reflection coefficient depends on the sizes of the FNWs in the visible region of the spectrum: its minimum value (less than 0.1%) has been achieved at diameters at the base of the FNWs of 650–750 nm and a length of 1.5–2 μm.

Anti-Reflectance Optimization of Secondary Nanostructured Black Silicon Grown on Micro-Structured Arrays.

Owing to its extremely low light absorption, black silicon has been widely investigated and reported in recent years, and simultaneously applied to various disciplines. Black silicon is, in general, fabricated on flat surfaces based on the silicon substrate. However, with three normal fabrication methods—plasma dry etching, metal-assisted wet etching, and femtosecond laser pulse etching—black silicon cannot perform easily due to its lowest absorption and thus some studies remained in the laboratory stage. This paper puts forward a novel secondary nanostructured black silicon, which uses the dry-wet hybrid fabrication method to achieve secondary nanostructures. In consideration of the influence of the structure’s size, this paper fabricated different sizes of secondary nanostructured black silicon and compared their absorptions with each other. A total of 0.5% reflectance and 98% absorption efficiency of the pit sample were achieved with a diameter of 117.1 μm and a depth of 72.6 μm. In addition, the variation tendency of the absorption efficiency is not solely monotone increasing or monotone decreasing, but firstly increasing and then decreasing. By using a statistical image processing method, nanostructures with diameters between 20 and 30 nm are the majority and nanostructures with a diameter between 10 and 40 nm account for 81% of the diameters.

The study of the defect removal etching of black silicon for diamond wire sawn multi-crystalline silicon solar cells

We produced low-reflectivity nanostructured black silicon by metal-assisted chemical etching (MACE) technology on the 15.6 cm × 15.6 cm diamond wire sawn (DWS) multi-crystalline silicon (mc-Si) wafers. Subsequently, defect removal etching (DRE) processes were carried out for various times to form nanohills with different sizes on the wafer surface. The experimental results indicated that the DRE process could influence the size of the nanohills, reflectivity and the recombination velocities of the silicon wafers. It was found that for the black silicon wafer, with the increasing of DRE time, the reflectivity became larger and the passivation effect became better. The reflectivity and the passivation effect can be adjusted by suitable DRE processes. By this way, we successfully fabricated the DWS mc-Si solar cell (15.6 cm × 15.6 cm) with the highest conversion efficiency of 19.07%, showing an improvement in efficiency of 0.6% compared to a DWS mc-Si solar cell (15.6 cm × 15.6 cm) prepared by conventional acidic texturization.

Electroosmotic Flow in Microchannel with Black Silicon Nanostructures.

Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be suppressed by polymer coatings or introduction of nanostructures. In this work, we presented a novel technique that employs the Dry Etching, Electroplating and Molding (DEEMO) process along with reactive ion etching (RIE), to fabricate microchannel with black silicon nanostructures (prolate hemispheroid-like structures). The effect of black silicon nanostructures on EOF was examined experimentally by current monitoring method, and numerically by finite element simulations. The experimental results showed that the EOF velocity was reduced by 13 ± 7%, which is reasonably close to the simulation results that predict a reduction of approximately 8%. EOF reduction is caused by the distortion of local electric field at the nanostructured surface. Numerical simulations show that the EOF velocity decreases with increasing nanostructure height or decreasing diameter. This reveals the potential of tuning the etching process parameters to generate nanostructures for better EOF suppression. The outcome of this investigation enhances the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.

Conformal Antireflective Surface Formed onto 3-D Silicon Structure.

This letter reports on the fabrication, simulation and characterization of conformal antireflective black-silicon (BSi) nanowires on a 3D silicon structure. The BSi nanostructures were formed on various facets of a 3D Si structure including sharp tips and sidewalls using a metal-assisted chemical (MAC) etching process. The conformal BSi design was simulated using FDTD Lumerical software. The antireflection capability was indicated by the quantified reduction in normalized intensity after image processing of diffraction patterns. An optical iris of 1.00-mm circular aperture with conformal BSi nanowires was fabricated and characterized to demonstrate the anti-reflectivity capability at two visible wavelengths of 532 and 633 nm. The iris showed a significant reduction in glare around its Airy disc, up to 3× smaller than the same one but without the BSi nanostructures.

Черный кремний: новый метод изготовления и оптические свойства

AbstractA new method for manufacturing nanostructured black silicon ( b -Si) by growing cone-shaped (pointed) filamentous nanocrystals (nanowires, FNWs) on the surface of single-crystal Si-plates has been proposed. b -Si samples with reflectivity less than 0.1% have been obtained. It has been shown that the b -Si reflection coefficient depends on the sizes of the FNWs in the visible region of the spectrum: its minimum value (less than 0.1%) has been achieved at diameters at the base of the FNWs of 650–750 nm and a length of 1.5–2 μm.