Metal-assisted Electroless Etching Research Articles

Elaboration of Silicon Nanostructures with Vapor-Phase Silver Assisted Chemical Etching: Correlation between Structural and Optical Properties.

Based on the widely used wet metal-assisted electroless etching, we develop in this work a novel vapor-phase silver-assisted chemical etching (VP-Ag-ACE) suitable for the elaboration of highly doped p-silicon (Si) nanostructures with strong, visible, and multi-peak photoluminescence (PL) emissions. The lateral and vertical etching rates (LER and VER) were discussed based on the etching mechanism of the VP-Ag-ACE. The antireflective suitability of the vapor-etched layer has been evaluated by a reflectivity measurement and exhibits reflectivity values lower than 3%. The PL emission at both room and low temperatures emissions were deeply discussed and correlated with the structural properties of the Si morphologies and their surface states based on the FTIR results.

Metal assisted chemical etching of silicon and solution synthesis of Cu-=SUB=-2-=/SUB=-O/Si radial nanowire array heterojunctions

Cu2O/Si radial nanowire (NWs) array heterojunctions were prepared by depositing Cu2O nanoparticles via chemical bath deposition on n-Si nanowire arrays that were fabricated by metal-assisted electroless etching. After 20 cycles of deposition, large numbers of Cu2O nanoparticles with form shells that wrap the upper segment of each Si nanowire. This method of etching offers exceptional simplicity, flexibility, environmental friendliness, and scalability for the fabrication of three-dimensional silicon nanostructures with considerable depths, because of replacement of harsh oxidants such as H2O2 and AgNO3. Keywords: Cu2O/Si NWs heterojunctions, Cu2O nanoparticles, metal-assisted electroless etching.

Kinetics of Tartrazine Photodegradation by Cu-Modified Silicon Nanowires

In the present work we investigated the photodegradation of tartrazine by silicon nanowires (SiNWs) without presence of H2O2. SiNWs were elaborated with a simple chemical method termed metal-assisted electroless etching. Also, SiNWs modified with copper was experimented as photocatalysts for the degradation of tartrazine and the effect of copper concentration was examined. The results show that silicon nanowires modified with high concentration of copper exhibit the best photocatalytic activity with a degradation rate follows the pseudo-first-order kinetics.

Strong light absorption capability directed by structured profile of vertical Si nanowires

Si nanowire arrays (SiNWAs) with random fractal geometry was fabricated using fast, mask-less, non-lithographic and facile approach by incorporating metal assisted electroless etching of n-type Si (111) substrates. The FESEM images demonstrate the formation of nano-porous surfaces that provide effective path for the incoming light to get trapped into the cavity of nanowires. The length of NWs increases from ∼1 to 10 μm with increase in the etching time having a diameter in the range of ∼25–82 nm. A transformation from zero to first order kinetics after a prolonged etching has been determined. The synthesized SiNWAs show high light trapping properties, including a maximum photon absorption across the entire visible and near IR range below the band gap of Si. The SiNWAs etched for 15 min exhibit extremely low specular and total reflectance of ∼0.2% and 4.5%, respectively over a broadband of wavelength. The reduction in the reflection loss is accompanied with the gradient of refractive index from air to Si substrate as well as due to the sub-wavelength structures, which manifests the light scattering effect. The COMSOL multiphysics simulation has been performed to study the high broadband light absorption capability in terms of the strong localized light field confinement by varying the length of the nanowire. Moreover, the SiNWs induces the dewetting ability at the solid/liquid interface and enhances the superhydrophobicity. Furthermore, a maximum length scale of 100–200 nm manifests a strong heterogeneity along the planar section of the surface of SiNWs. The study thus provides an insight on the light propagation into the random fractal geometries of Si nanowires. These outstanding properties should contribute to the structural optimization of various optoelectronic and photonic devices.

High efficiency conjugated polymer/Si hybrid solar cells with tetramethylammonium hydroxide treatment

Conjugated polymer/Si hybrid solar cells are fabricated based on a Si nanowire array (SiNW) substrate prepared by metal-assisted electroless etching.

Synthesis, optical properties and residual strain effect of GaN nanowires generated via metal-assisted photochemical electroless etching

Synthesis of GaN nanowiresviametal-assisted photochemical electroless etching.

Silicon Nanowire Photocathodes for Photoelectrochemical Hydrogen Production.

The performance of silicon for water oxidation and hydrogen production can be improved by exploiting the antireflective properties of nanostructured silicon substrates. In this work, silicon nanowires were fabricated by metal-assisted electroless etching of silicon. An enhanced photocurrent density of −17 mA/cm2 was observed for the silicon nanowires coated with an iron sulphur carbonyl catalyst when compared to bare silicon nanowires (−5 mA/cm2). A substantial amount of 315 µmol/h hydrogen gas was produced at low bias potentials for the silicon nanowires coated with an iron sulphur carbonyl catalyst.

Improved Efficiency of Silicon Nanoholes/Gold Nanoparticles/Organic Hybrid Solar Cells via Localized Surface Plasmon Resonance.

Silicon is the most widely used material for solar cells due to its abundance, non-toxicity, reliability, and mature fabrication process. In this paper, we fabricated silicon nanoholes (SiNHS)/gold nanoparticles (AuNPS)/organic hybrid solar cells and investigated their spectral and opto-electron conversion properties. SiNHS nanocomposite films were fabricated by metal-assisted electroless etching (EE) method. Then, we modified the surface of the nanocomposite films by exposing the samples in the air. After that, polymer poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) blended with AuNPS were spin-coated on the surface of the SiNHS nanocomposite films as a hole-transporting layer. The external quantum efficiency (EQE) values of the solar cells with AuNPS are higher than that of the samples without AuNPS in the spectral region of 600–1000 nm, which were essential to achieve high performance photovoltaic cells. The power conversion efficiency (PCE) of the solar cells incorporating AuNPS exhibited an enhancement of 27 %, compared with that of the solar cells without AuNPS. We thought that the improved efficiency were attributed to localized surface plasmon resonance (LSPR) triggered by gold nanoparticles in SiNHS nanocomposite films.

Extended short-wavelength spectral response of organic/(silver nanoparticles/Si nanoholes nanocomposite films) hybrid solar cells due to localized surface plasmon resonance

In this letter, we investigated spectral and opto-electronic conversion properties of the inorganic/organic hybrid cells by using silver nanoparticles (AgNPs)/Si nanoholes (SiNHs) nanocomposite films, which were fabricated by the modified metal-assisted electroless etching (EE) method. It was found that the optical absorption spectra of the films with AgNPs demonstrate a clear peak and show the enhancement of total absorption at the short wavelength. The results of current–voltage (I–V) measurements show that solar cells with AgNPs exhibit an increase of the power conversion efficiency by a factor of 2–3, in comparison with those of the samples without AgNPs. Moreover, higher external quantum efficiency (EQE) values in AgNPs-decorated solar cells were confirmed in the short-wavelength spectral region (400–700nm), which were essential to achieve high-performance photovoltaic cells. We thought these were mainly attributed to the localized surface plasmon resonance (LSPR) effects and increased light scattering of AgNPs.

Formation of Luminescent Silicon Nanowires and Porous Silicon by Metal-Assisted Electroless Etching

Metal-assisted etching of silicon in HF/H2O2 aqueous solutions has been used to fabricate luminescent silicon nanowires (SiNWs) and porous silicon. The impact of the gold catalyst layer thickness and the etching solution on the morphology of the synthesized nanostructures and the diameter of the obtained nanowires were systematically investigated. Scanning electron microscopy (SEM) analyses reveal that the morphology of the fabricated structures strongly depends on the composition of the solution and the thickness of the catalyst layer. It has been observed that SiNWs are formed in solutions with H2O2 ratios (ξ) below 10 %; increasing the H2O2 concentration above this critical value leads to mesoporous (10 % < ξ < 14 %) and macroporous (14 % < ξ < 17 %) structures. Photoluminescence measurements show that SiNWs emit light at about 430 nm. Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analyses were utilized to determine the origin of the emission in the silicon nanostructures. TEM imaging demonstrates that SiNWs are covered by a thin layer of porous silicon, which is assumed to be responsible for their light emission.

Fabrication of superhydrophobic and highly oleophobic silicon-based surfaces via electroless etching method

This study reports on a simple method for the preparation of superhydrophobic and highly oleophobic nanostructured silicon surfaces. The technique relies on metal-assisted electroless etching of silicon in sodium tetrafluoroborate (NaBF4) aqueous solution. Then, silver particles were deposited on the obtained surfaces, changing their overall physical morphology. Finally, the surfaces were coated by either C4F8, a fluoropolymer deposited by plasma, or by SiOx overlayers chemically modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) through silanization reaction. All these surfaces exhibit a superhydrophobic character (large apparent contact angle and low hysteresis with respect to water). In addition, they present high oleophobic properties, i.e. a high repellency to low surface energy liquids with various contact angle hysteresis, both depending on the morphology and type of coating.

Dependence of Performance of Si Nanowire Solar Cells on Geometry of the Nanowires

The dependence of performance of silicon nanowires (SiNWs) solar cells on the growth condition of the SiNWs has been described. Metal-assisted electroless etching (MAE) technique has been used to grow SiNWs array. Different concentration of aqueous solution containing AgNO3 and HF for Ag deposition is used. The diameter and density of SiNWs are found to be dependent on concentration of solution used for Ag deposition. The diameter and density of SiNWs have been used to calculate the filling ratio of the SINWs arrays. The filling ratio is increased with increase in AgNO3 concentration, whereas it is decreased with increase in HF concentration. The minimum reflectance value achieved is ~1% for SiNWs of length of ~1.2 μm in the wavelength range of 300–1000 nm. The performance and diode parameters strongly depend on the geometry of SiNWs. The maximum short circuit current density achieved is 35.6 mA/cm2. The conversion efficiency of solar cell is 9.73% for SiNWs with length, diameter, and wire density of ~1.2 μm, ~75 nm, and 90 μm−2, respectively.

Efficiency improvement of silicon nanostructure-based solar cells

Solar cells based on a high-efficiency silicon nanostructure (SNS) were developed using a two-step metal-assisted electroless etching (MAEE) technique, phosphorus silicate glass (PSG) doping and screen printing. This process was used to produce solar cells with a silver nitrate (AgNO3) etching solution in different concentrations. Compared to cells produced using the single MAEE technique, SNS-based solar cells produced with the two-step MAEE technique showed an increase in silicon surface coverage of ∼181.1% and a decrease in reflectivity of ∼144.3%. The performance of the SNS-based solar cells was found to be optimized (∼11.86%) in an SNS with a length of ∼300 nm, an aspect ratio of ∼5, surface coverage of ∼84.9% and a reflectivity of ∼6.1%. The ∼16.8% increase in power conversion efficiency (PCE) for the SNS-based solar cell indicates good potential for mass production.

Solution synthesis of Cu2O/Si radial nanowire array heterojunctions for broadband photodetectors

Cu2O/Si radial nanowire array heterojunctions were prepared by depositing Cu2O nanoparticles via surfactant-assisted chemical bath deposition on n-Si nanowire arrays that were fabricated by metal-assisted electroless etching. After 20 cycles of deposition, large numbers of Cu2O nanoparticles with diameters of around 70 nm form shells that wrap the upper segment of each Si nanowire. Using the rectifying effect of the Cu2O/Si pn junctions, high photodetection sensitivity under illumination at 405, 532 and 1064 nm is exhibited, with a maximal photoresponsivity of 13.05 mA W−1 at 532 nm. The photocurrents are superior to those of Cu2O/Si planar heterojunctions.

Rice-straw-like structure of silicon nanowire arrays for a hydrogen gas sensor

A rice-straw-like silicon nanowire (SiNW) array was developed for hydrogen gas sensing applications. The straight-aligned SiNW array sensor was first fabricated by the metal-assisted electroless etching (MAEE) technique. Rice-straw-like SiNW arrays were formed using a repeated MAEE technique. Hydrogen sensing characteristics were measured for gas concentrations from 20 to 1000 ppm at room temperature. The rice-straw-like SiNW-array-based hydrogen gas sensor performed with low noise and a high response (232.5%) for 1000 ppm hydrogen gas. It was found that the rice-straw-like SiNW-array hydrogen gas sensor had a much better response (approximately 2.5 times) than the straight-aligned SiNW-array sensor. The rice-straw-like SiNW-array structure effectively increased the surface area and the concentration of silicon oxide, which provided additional binding sites for gas molecules. Thus, the rice-straw-like SiNW-array-based hydrogen gas sensor possessed good sensing properties and has the potential for mass production of sensing devices.

Nanoporous GaN–Ag Composite Materials Prepared by Metal-Assisted Electroless Etching for Direct Laser Desorption-Ionization Mass Spectrometry

Three-dimensional nanoporous gallium nitride(PGaN) produced by metal-assisted electroless etching is chemically embedded with silver nanoparticles via electroless deposition, forming a metallized semiconductor membrane with large surface area and nanoscale metal features. A new application utilizing the unique chemical and morphological features of these composite nanostructures is described here, laser induced desorption-ionization(LDI) of biomolecules(e.g., cholesterol and nucleotides) for direct mass analysis, without use of additional organic matrix. Although PGaN itself is a poor matrix for direct LDI mass spectrometry, the combination of Ag and PGaN greatly improves ion signals relative to PGaN or Ag nanostructure surfaces alone. This behavior is attributed to the combination of strong UV absorption, enhanced surface area, and favorable thermal properties of PGaN. Importantly, Ag-PGaN is shown to facilitate the formation of Ag adduct ions in some cases, for example adenine, where adducts are not observed from either porous anodic aluminum membranes or surfaces presenting Ag nanoparticles in isolation. Nanopore-embedded Ag nanostructures serve a dual role: as cationization agents and to assist thermal desorption under UV laser irradiation. The results reported here suggest that the combination of Ag nanostructures embedded in PGaN has the capacity for high quality matrix-free LDI mass analysis.

Template-free fabrication of silicon micropillar/nanowire composite structure by one-step etching

A template-free fabrication method for silicon nanostructures, such as silicon micropillar (MP)/nanowire (NW) composite structure is presented. Utilizing an improved metal-assisted electroless etching (MAEE) of silicon in KMnO4/AgNO3/HF solution and silicon composite nanostructure of the long MPs erected in the short NWs arrays were generated on the silicon substrate. The morphology evolution of the MP/NW composite nanostructure and the role of self-growing K2SiF6 particles as the templates during the MAEE process were investigated in detail. Meanwhile, a fabrication mechanism based on the etching of silver nanoparticles (catalyzed) and the masking of K2SiF6 particles is proposed, which gives guidance for fabricating different silicon nanostructures, such as NW and MP arrays. This one-step method provides a simple and cost-effective way to fabricate silicon nanostructures.

Synthesis and Light Emission from ZnO-Coated Silicon Nanorods

We report the synthesis and thermal annealing of Si-core/ZnO-shell nanorods using a two-step process comprising the metal-assisted electroless etching of Si and the sputter deposition of ZnO. Transmission electron microscopy and X-ray diffraction analysis showed that the cores of the annealed core-shell nanorods were single crystal diamond cubic-type Si, whereas the shells of the annealed core-shell nanorods were single crystal wurtzite-type ZnO. The PL spectra of Si nanorods consisted of a broad red emission band and a weaker blue emission band. The major emission band of Si nanorods was shifted from 700 nm (in the red region) to 440 nm (in the violet region) by ZnO coating. The violet emission of the core-shell nanorods was enhanced in intensity considerably by annealing in an oxidizing atmosphere. The origin of the PL enhancement by annealing is also discussed.

Strategies for controlling Si nanowire formation during Au-assisted electroless etching

Dramatic changes in the morphologies of Si nanowires (NWs) ranging from vertical, to hooked, to ground-collapsed shapes are presented depending on the HF/H2O2 ratio in the etch solution during Au-assisted electroless etching. It is shown that the deviations from vertical NWs are not due to capillary action but correlated with the development of nanoporous structures, which is attributed to excess hole (h+) diffusion toward the NWs' sidewalls during the etching process. The role of hole diffusion is made more evident by observation of NWs with the neck formation when transitioning to different etching regimes during SiNW formation. Furthermore, the hooked and ground-collapsed SiNW can be suppressed at the cost of enhanced tapering by adding a strong electrolyte in solution, which is correlated to reduced etching rate resulting from electric screening of the charged Au surfaces. We provide an important finding that common salts such as NaCl, KCl, and CaCl2 can be useful retardants for the metal-assisted electroless etching. The new insights into structural transformation presented in this study afford greater control over the structure of SiNWs produced by metal-assisted electroless etching.

Porous Si layers—Preparation, characterization and morphology

Porous silicon layers (PSL) of nano- and micro-structures were prepared by metal-assisted electroless etching of silicon in HF–oxidizing agent aqueous solutions. The effect of oxidizing agent and HF content on the characteristics of the formed porous layers was investigated. A thin Pt film was electroless deposited on p-Si〈1 0 0〉 prior to immersion in the etching solution. The properties and morphology of the PSL formed by this method were investigated by electrochemical impedance spectroscopy (EIS) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) technique. The characteristics of the PSL were found to be affected by the constituents of the etching medium and also, the etching time. Potassium bromate (KBrO 3), potassium iodate (KIO 3), and potassium dichromate (K 2Cr 2O 7) have been used as oxidizing agents. Pt-assisted etching of p-Si for 1 h in an etching solution consisting of 22.0 M HF and 0.05 M of KBrO 3, results in the formation of nano- and micro-pores on the Si surface. The use of 0.05 M KIO 3 or K 2Cr 2O 7 as oxidizing agent has led to the formation of a deposit on the silicon surface. At relatively higher concentration [>0.05 M] of K 2Cr 2O 7 the surface deposit becomes clear and was found to consist of an insoluble passive solid-phase of K 2SiF 6 which increases the film impedance and blocks the porous structure formation. The use of higher concentration [>22 M] of HF in the etching electrolyte is accompanied by an increase in the dissolution rate of the insoluble K 2SiF 6 layer and a decrease in the PSL passivity. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model which accounts for the mechanism of the porous film formation at the Si/electrolyte interface.