Subsequent Chemical Etching Research Articles

Unveiling the 3D Morphology of Epitaxial GaAs/AlGaAs Quantum Dots.

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Polishing and etching damages of ZnO single crystals studied using time-resolved photoluminescence spectroscopy

Controlled thinning of wide bandgap semiconductors by chemo-mechanical polishing (CMP) and/or reactive ion etching (RIE) has been one of the versatile methods for various optoelectronic applications. The influences of CMP and subsequent wet chemical etching, as well as independent RIE, on the room-temperature photoluminescence lifetime for the near-band-edge emission [τPLNBE(RT)] of O-polarity c-plane ZnO single crystals were examined by using time-resolved photoluminescence measurements. τPLNBE(RT) decreased from a nanosecond range to a few picoseconds (ps) by a conventional CMP, indicating a generation of high-concentration midgap recombination centers, such as nonradiative recombination centers and deep radiative recombination centers. τPLNBE(RT) was progressively regained up to 600 ps by a subsequent etching using HCl aqueous solution. However, the recovery saturated at the etching depth of about 200 nm and τPLNBE(RT) was not restored even after etching by 350 nm. The results indicate the introduction of certain structural deformations during the CMP. Because x-ray diffraction measurement revealed the presence of incoherent surface domains right after the CMP and the HCl etching gave rise to inhomogeneously etched canyons, nonradiative recombination centers, such as dislocations and vacancy clusters, are likely generated by mechanical shear stresses. τPLNBE(RT) also decreased by the RIE. However, the degradation was less significant than the case for the CMP, because RIE scarcely gives mechanical stresses. Interestingly, τPLNBE(RT) for the samples etched under higher plasma power was longer than the lower power cases. From the results of x-ray photoelectron spectroscopy measurements, unintentionally deposited oxide films containing Si are proposed to act as an attenuating layer for the introduction of nonradiative recombination centers.

Highly Rectifying Conical Nanopores in Amorphous SiO2 Membranes for Nanofluidic Osmotic Power Generation and Electroosmotic Pumps

Nanopore membranes are a versatile platform for a wide range of applications ranging from medical sensing to filtration and clean energy generation. To attain high-flux rectifying ionic flow, it is required to produce short channels exhibiting asymmetric surface charge distributions. This work reports on a system of track etched conical nanopores in amorphous SiO$_2$ membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 1 $\mu$m thick SiO$_2$ windows with 2.2 GeV $^{197}$Au ions and subsequent chemical etching. Structural characterisation is performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), ellipsometry, and surface profiling. Conductometric characterisation of the pore surface is performed using a membrane containing 16 pores, including an in-depth analysis of ionic transport characteristics. The pores have a tip radius of (5.7 $\pm$ 0.1) nm, a half-cone angle of (12.6 $\pm$ 0.1)$^{\circ}$, and a length of (710 $\pm$ 5) nm. The $pK_a$, $pK_b$, and $pI$ are determined to 7.6 $\pm$ 0.1, 1.5 $\pm$ 0.2, and 4.5 $\pm$ 0.1, respectively, enabling the fine-tuning of the surface charge density between +100 and -300 mC $m^{-2}$ and allowing to achieve an ionic current rectification ratio of up to 10. This highly versatile technology addresses challenges that contemporary nanopore systems face, and offers a platform to improve the performance of existing applications.

Microchannels fabrication in nanoporous silicate matrix by femtosecond direct laser writing and subsequent chemical etching

Microchannels fabrication in nanoporous silicate matrix by femtosecond direct laser writing and subsequent chemical etching

Femtosecond-Laser-Induced All-Silicon Dielectric Metasurfaces Assisted by Wet Chemical Etching

All-dielectric metasurfaces offer low material loss and strong field localization and are, therefore, well suited for ultrathin and compact optical devices for electomagnetic wave manipulation at the nanoscale. All-silicon dielectric metasurfaces, in particular, may additionally offer the desired compatibility with complementary metal-oxide semiconductor technology and, hence, are ideal candidates for large-scale monolithic integration on a photonic chip. However, in conventional silicon microfabrication approaches, the combination of mask photolithography with reactive ion etching usually involves expensive masks and multiple preprocessing stages leading to increased cost and fabrication times. In this work, a single-step lithographical approach is proposed for the realization of all-silicon dielectric resonant metasurfaces that involves femtosecond laser processing of silicon below ablation threshold in combination with subsequent wet chemical etching. The method exploits the different etching rate between laser-modified and untreated regions, enabling large-area fabrication of patterned silicon surfaces in a facile and cost-efficient manufacturing approach. It is presented how two-dimensional silicon micro/nanostructures with controllable features, such as nanocones, can be effectively generated and, as a proof of concept, an all-silicon dielectric metasurface device supporting antiferromagnetic order is experimentally demonstrated.

Impact of sintering temperature on phase composition, microstructure, and porosification behavior of LTCC substrates

Phase development and changes in crystalline composition of LTCC material during the sintering process were investigated using in-situ X-ray diffraction (XRD) measurements. CeramTape GC was chosen as the chemically simplest model system composed of alumina particles and glass for the investigations. The chemical characterization and microstructural analyses of the tapes sintered with some representative firing profiles were performed by techniques such as (scanning) transmission electron microscope, energy-dispersive X-ray spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and XRD. Moreover, the porosification behavior of LTCC substrates fired at different peak temperatures was studied. These investigations are important for the subsequent wet chemical etching, representing an approach which allows to reduce locally the permittivity of LTCC tapes. Treatment with a KOH solution shows non-selective etching behavior for all substrates. In addition, highly porous silica structures corresponding to Ca and Al depletion from the anorthite phase were observed in all samples after etching treatment.

Recessed-Channel WSe2 Field-Effect Transistor via Self-Terminated Doping and Layer-by-Layer Etching.

Effective channel control with low contact resistance can be accomplished through selective ion implantation in Si and III-V semiconductor technologies; however, this approach cannot be adopted for ultrathin van der Waals materials. Herein, we demonstrate a self-aligned fabrication process based on self-terminated p-doping and layer-by-layer chemical etching to achieve low contact resistance as well as a high on/off current ratio in ultrathin tungsten diselenide (WSe2) field-effect transistors (FETs). Damage-free layer-by-layer thinning of the WSe2 channel is repeated up to a thickness of approximately 1.4 nm, while maintaining the selectively p-doped source/drain regions. The device characteristics of the recessed-channel WSe2 FET are systematically monitored during this layer-by-layer recess-channel process. The WSe2 etching rate is estimated to be 2-3 layers per cycle of oxidation and subsequent chemical etching. The self-terminated tungsten oxide (WOX) layer grown through ultraviolet-ozone treatment induces robust p-doping in the neighboring (or underlying) WSe2 through the electron withdrawal mechanism, which remains in the source/drain regions after channel oxide removal. The adopted self-terminated and self-aligned recess-channel process for ultrathin WSe2 FETs enables the realization of a high on/off output current ratio (>108) and field-effect mobility (∼190 cm2/V·s), while maintaining low contact resistance (0.9-6.1 kΩ·μm) without a postannealing process. The proposed facile and reproducible doping and atomic-layer-etching method for the fabrication of a recessed-channel FET with an ultrathin body can be helpful for high-performance two-dimensional semiconductor devices and is applicable to post-Si complementary metal-oxide semiconductor devices.

Achieving highly efficient pH-universal hydrogen evolution by superhydrophilic amorphous/crystalline Rh(OH)3/NiTe coaxial nanorod array electrode

Design of high-performance pH-universal electrocatalysts is critical to practical large-scale hydrogen generation as a carbon-neutral fuel, yet challenging. Herein, we report an unique motif with crystalline nickel tellurium nanorods enclosed by amorphous rhodium hydroxide (a-Rh(OH)3/NiTe), formed through a hydrothermal synthesis and a subsequent chemical etching process, to address this challenge. The as-prepared a-Rh(OH)3/NiTe cathode enables a current density of 100 mA cm−2 with low overpotentials of 51, 109, and 64 mV for HER in alkaline, neutral and acidic media, respectively. As revealed by density functional theory (DFT) calculations, the electronic interactions between a-Rh(OH)3 and NiTe enhance the performance of Rh active sites. More importantly, the motif possesses superhydrophilicity and aerophobicity features, which not only facilitates the access to electrolytes but also ensures the fast release of hydrogen bubbles, endowing the electrocatalyst with advanced pH-universal HER activity. This work provides insights for the design of highly efficient electrocatalysts for hydrogen evolution at both molecular and mesoscopic levels.

Electron Transport Mechanisms in Polyethylene Terephthalate Membranes

This paper describes carrier transport mechanisms in polyethylene terephthalate (PET) films and porous PET-based membranes (PMs) obtained by irradiating pristine PET film with swift heavy ions, with subsequent chemical etching in an alkali (NaOH) solution. The obtained PMs had through nanochannels (pores) with an average diameter of 720-750 nm. We observed that in the temperature range 240-300 K, the current-voltage characteristics I(V) of the initial Cu|PET|Cu structure obeyed the improved Mott--Gurney law, which is based on the Mark--Helfrich model for a space-charge-limited current (SCLC) mechanism for electron transport. It was found for the first time that creation of nanochannels in PMs resulted in a significant increase in the electric current density (by about three orders of magnitude) while maintaining the SCLC mechanism. The enhanced current density is explained by the formation of a highly conductive layer along the inner surface of the walls of the nanochannel that are covered with carboxyl end groups, which are created by alkaline hydrolysis. According to the model, the surface states formed by these groups enable the drift of additional electrons injected from the copper electrodes under the action of the bias voltage. Keywords: polyethylene terephthalate, electron transport, space-charge-limited current mechanism, Mark--Helfrich injection model.

Fabrication of Size- and Shape-Controlled Platinum Cones by Ion-Track Etching and Electrodeposition Techniques for Electrocatalytic Applications

The micro/nanocone structures of noble metals play a critical role as heterogeneous electrocatalysts that provide excellent activity. We successfully fabricated platinum cones by electrodeposition using non-penetrated porous membranes as templates. This method involved the preparation of template membranes by the swift-heavy-ion irradiation of commercially available polycarbonate films and subsequent chemical etching in an aqueous NaOH solution. The surface diameter, depth, aspect ratio and cone angle of the resulting conical pores were controlled in the ranges of approximately 70–1500 nm, 0.7–11 μm, 4–12 and 5–13°, respectively, by varying the etching conditions, which finally produced size- and shape-controlled platinum cones with nanotips. In order to demonstrate the electrocatalytic activity, electrochemical measurements were performed for the ethanol oxidation reaction. The oxidation activity was found to be up to 3.2 times higher for the platinum cone arrays than for the platinum plate. Ion-track etching combined with electrodeposition has the potential to be an effective method for the fabrication of micro/nanocones with high electrocatalytic performance.

Facile Fabrication of Composite Ag/Fe3O4 and Porous Fe3O4 Nanorods Based on Oblique Angle Codeposition

Composite Ag/Fe3O4 and porous Fe3O4 nanorods with controlled shape, size, composition, and porosity were fabricated by oblique angle codeposition as well as subsequent annealing and chemical etching. The codeposition supplies a way to evaporate two materials simultaneously to form composites, which would lead to a greater variety of nanorods. The composite Ag/Fe3O4 and porous Fe3O4 nanorods showed ferromagnetism, tunable optical properties, and strong carrier capability, which are believed to benefit the applications of biosensing, bioimaging, catalysis, targeted drug delivery, and therapeutic agent.

Laser-assisted etching of borosilicate glass in potassium hydroxide

We present a method for the selective etching of borosilicate glass (SCHOTT Borofloat 33), in which we modify the glass with an ultrashort pulse laser and subsequent wet chemical etching. The BF33 glass is often used in microtechnology to produce sensors, actors, and fluidic chips as it can be bonded to silicon wafers by anodic bonding. The glass is irradiated and modified by circular polarized laser light with a wavelength of 1030 nm. By etching the glass with potassium hydroxide, the modified material can be removed. In this study, the selectivity was analyzed dependent on the laser parameters pulse repetition rate, pulse duration, writing speeds, and pulse energy. A selectivity up to 540 could be observed in this study. Finally, the manufacturing capabilities for three-dimensional free form shapes in BF33 are demonstrated and compared with fused silica.

Aluminium-assisted chemical etching for fabrication of black silicon

This paper investigates aluminium-assisted chemical etching (AACE) process to fabricate b-Si absorber for the first time. The AACE process combines deposition of a thin aluminium (Al) film with thickness of 12–30 nm on crystalline silicon (c-Si) wafers, followed by subsequent thermal annealing and wet chemical etching processes. The AACE process produces random nanopores on the b-Si surface. From the results, thicker Al thickness leads to lower depth of nanopores, lower surface coverage of nanopores on the b-Si and exhibits higher weighted average reflection (Ravg) within 300–1100 nm wavelength region. The b-Si fabricated by Al film of 12 nm thickness demonstrates the lowest Ravg (11.9%) and the highest broadband light absorption. The b-Si reveals nanopores with average depth of 200 nm and surface coverage of 45.1%. The findings from this work demonstrate the potential of the AACE process to produce b-Si with superior broadband light absorption, which is crucial for photovoltaic (PV) applications.

Diffraction Structures Formed by Two Crossed Superimposed Diffraction Gratings

The diffraction patterns generated by surface relief diffraction structures that consisted of two identical crossed, superimposed gratings formed in the As2S3 thin films by electron beam recording and subsequent chemical etching were studied. The angle between the gratings with grating periods equal to 2 μm was varied between 2° and 90°. Additional diffraction gratings formed by a set of the intersection nodes of the basic grating lines were considered to simulate the diffraction patterns. The circular symmetric diffraction structure composed of four crossed superimposed gratings with equal periods has been considered as a combination of pairs of crossed, superimposed gratings. A good agreement between the calculated and experimental diffraction patterns is observed.

Ion transmission spectroscopy of pores filled with Au nanoparticles

Ion track membranes (ITM) are of long-term interest due to the high potential of applications in science and industry. However, the use of the membranes requires a detailed knowledge of (i) their structural parameters, i.e., the density and distribution of the ITM pores, the pore size (radius) and spatial shape, or (ii) the pore filling with other materials (demanding the knowledge of filling efficiency, filler confinement, or filler density). In this work, we studied the nuclear membranes prepared with pores of certain density and size and filled with Au nanoparticles. The analysis was carried out by Ion Transmission Spectroscopy (ITS). ITS is a nondestructive technique to determinate the spatial structure of (sub)micron inhomogeneities (pores, protrusions, etc.) in thin foils from the energy loss of even quasi-monoenergetic alpha particles (e.g., from a thin 241Am source) transmitted through the (empty or filled) pores. The reconstruction of the shape of the pores or pore fillings is performed by simulation of the transmission spectra using the MC code. The nuclear membranes were prepared by the irradiation of a thin polyethylene terephthalate with 157 MeV Xe+26 ions (with the fluence 106 cm−2) and the subsequent chemical etching in 9 M NaOH water solution at 50 °C for 40 min. The pores were filled with the Au nanoparticles (NPs) of a sub-micrometer size using the Pulse Laser Deposition (PLD), and studied by ITS. From the transmission spectra the shape of the pores filled (partially or fully) with Au NPs could be reconstructed. The ITM with pores filled with NPs were also analyzed by scanning electron microscopy (SEM). The results showed good agreement with the MC simulation of the ITS data.

Binder-free CuO nanoneedle arrays based tube-type sensor for H2S gas sensing

CuO nanoneedle arrays were directly prepared on the commercial ceramic tubes by magnetron sputtering and subsequent wet chemical etching and annealing. This brand-new fabrication of the tube-type gas sensor retains the high specific surface area of nanoneedle and avoids using binders that are generally used for coating gas-sensitive materials on the surface of the ceramic tube. The as-fabricated binder-free CuO nanoneedle arrays sensor exhibits a decent response of 76.50 % to 10 ppm H2S at a low temperature of 150 °C. The response decreases to 20.27 % as the relative humidity increases from 40 % to 80 %. Moreover, the sensor shows good selectivity, reproducibility, and long-term stability to a low concentration of 10 ppm H2S. This superior sensing performance can be attributed to the abundant surface reactive adsorption sites provided by CuO nanoneedle arrays and the sensitization effect of p-p nano-homojunctions that arose from the interlaced CuO nanoneedles.

Effect of surface bond and quantum confinement effect on photoluminescence properties of SiC nanowires in different solvents

Two kinds of SiC nanowires exhibiting different structures were fabricated by carbothermal reduction method and subsequent wet chemical etching. Strong and tunable ultraviolet to green emission peaks emerged in the photoluminescence spectra of both kinds of SiC nanowires in water and ethanol suspensions with the increase of excitation wavelength from 260 nm to 480 nm. The strong and distinctive ultraviolet emission originates from the quantum confinement effect of the nanoplatelets with a thickness less than 3 nm and the effects of Si–OH and Si–OR (R: ethoxy group) on the SiC nanowire surface. The blue emission derived from the SiC nanowires in different solvents is due to the combined effect of the nanoplatelet thickness, which meets the theoretical prediction value (3–5.4 nm), and surface bonds. The weaker green emission can be attributed to the large SiC nanowires and surface bonds caused by Si–OR and Si–OH.

Three‐Dimensional PdCuRu Alloy Porous Nanosheets as Efficient Electrocatalysts for Hydrogen Evolution Reaction in Varied Electrolytes

AbstractNoble metal materials with favorable electronic and geometric structure are considered as outstanding electrocatalysts for hydrogen evolution reaction (HER). However, it is still a grand challenge to increase the specific surface area of the catalyst to expose more active sites from structural advantages and optimize the adsorption/desorption energy of reaction intermediates. Here, we report the design and fabrication of a three‐dimensional (3D) PdCuRu nanostructure that is assembled by porous nanosheets through a facile wet‐chemical and subsequent chemical etching method. Interestingly, the prepared 3D porous nanostructures not only provide a wealth of surface active centers for reactants, but also provide a lot of ways for mass transport and charge transfer. More importantly, the alloy structure effectively optimizes the electronic structures of Ru and Cu, leading them to be favorable for the adsorption/desorption of Hads during HER. As a result, the 3D PdCuRu porous nanosheets (PdCuRu PNSs) display excellent electrocatalytic HER performance in both acidic and alkaline media. It is worth mentioning that the 3D PdCuRu PNSs could also show excellent kinetic stability with negligible activity loss representing a new class a class of promising electrocatalysts for HER.

Clarification of the false liquation crack existed in 9% Cr/CrMoV dissimilar welded joint by comparative etching process and in situ laser scanning confocal microscope

Liquation cracks are generally regarded as thermal defects in various materials, e.g. in steels and nickel-based alloys. However, direct observation of microstructure with crack-like morphology that may be revealed by chemical etching or electrochemical etching methods, might lead to the misinterpretation of liquation cracks. To avoid the possible misinterpretation, comparative etching was employed to reveal the effect of various etching processes on the determination of liquation cracks. As a result, the formation process and mechanism of the false liquation crack in 9% Cr/CrMoV welded joint were clarified. The results obtained by optical microscope (OM) and scanning electron microscope (SEM) in the heat treatment zone of 9% Cr steel (9% Cr-HAZ) after chemical etching clearly demonstrate the existence of crack-like morphology, and the disappearance of element segregation zone after regrinding and re-polishing. Besides, laser scanning confocal microscope (LSCM) was applied to further exhibit the microstructure evolution experiencing high temperature to confirm the possibility of the liquation crack. The results showed that the element segregation and subsequent chemical etching process were the crucial factors to cause the formation of false liquation crack.

Influence of the Finite-Size Effect on the Cluster Ion Emission of Silicon Nanostructures

The finite-size effect increasing the emission of polyatomic cluster ions from silicon nanostructures as compared to a macroscopic sample has been studied. This study has been performed for the first time in periodic structures with 10- and 50-nm-wide combs fabricated by electron beam lithography with subsequent plasma chemical etching. It has been shown that the yield of polyatomic cluster ions from structures with the comb whose effective width is comparable with the projective range of a bombarding bismuth ion in silicon is much higher than the yield from macroscopic samples, e.g., by more than a factor of 5 for 28Si10− (m/z = 280). This effect has been explained by a partial limitation of dissipation channels for the energy released by the bombarding ion in the volume of the structure, which stimulates a more efficient development of nonlinear collision cascades in such a volume and, as a result, increases the yield of polyatomic cluster ions.