Showing papers on "Isotropic etching published in 2021"

Ambient-Stable Two-Dimensional Titanium Carbide (MXene) Enabled by Iodine Etching.

Abstract: MXene (e.g., Ti3 C2 ) represents an important class of two-dimensional (2D) materials owing to its unique metallic conductivity and tunable surface chemistry. However, the mainstream synthetic methods rely on the chemical etching of MAX powders (e.g., Ti3 AlC2 ) using hazardous HF or alike, leading to MXene sheets with fluorine termination and poor ambient stability in colloidal dispersions. Here, we demonstrate a fluoride-free, iodine (I2 ) assisted etching route for preparing 2D MXene (Ti3 C2 Tx , T=O, OH) with oxygen-rich terminal groups and intact lattice structure. More than 71 % of sheets are thinner than 5 nm with an average size of 1.8 μm. They present excellent thin-film conductivity of 1250 S cm-1 and great ambient stability in water for at least 2 weeks. 2D MXene sheets with abundant oxygen surface groups are excellent electrode materials for supercapacitors, delivering a high gravimetric capacitance of 293 F g-1 at a scan rate of 1 mV s-1 , superior to those made from fluoride-based etchants (<290 F g-1 at 1 mV s-1 ). Our strategy provides a promising pathway for the facile and sustainable production of highly stable MXene materials.

Enhanced photocatalytic activities of silicon nanowires/graphene oxide nanocomposite: Effect of etching parameters.

Abstract: Homogeneous and vertically aligned silicon nanowires (SiNWs) were successfully fabricated using silver assisted chemical etching technique. The prepared samples were characterized using scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Photocatalytic degradation properties of graphene oxide (GO) modified SiNWs have been investigated. We found that the SiNWs morphology depends on etching time and etchant composition. The SiNWs length could be tuned from 1 to 42 µm, respectively when varying the etching time from 5 to 30 min. The etchant concentration was found to accelerate the etching process; doubling the concentrations increases the length of the SiNWs by a factor of two for fixed etching time. Changes in bundle morphology were also studied as function of etching parameters. The SiNWs diameter was found to be independent of etching time or etchant composition while the size of the SiNWs bundle increases with increasing etching time and etchant concentration. The addition of GO was found to improve significantly the photocatalytic activity of SiNWs. A strong correlation between etching parameters and photocatalysis efficiency has been observed, mainly for SiNWs prepared at optimum etching time and etchant concentrations of 10 min and 4:1:8. A degradation of 92% was obtained which further improved to 96% by addition of hydrogen peroxide. Only degradation efficiency of 16% and 31% has been observed for bare Si and GO/bare Si samples respectively. The obtained results demonstrate that the developed SiNWs/GO composite exhibits excellent photocatalytic performance and could be used as potential platform for the degradation of organic pollutants.

N-polar ScAlN and HEMTs grown by molecular beam epitaxy

Abstract: Molecular beam epitaxy of single-phase wurtzite N-polar ScxAl1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc0.18Al0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc0.11Al0.89N and Sc0.30Al0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc0.18Al0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm2/V s is measured for a 15-nm-thick Sc0.18Al0.82N barrier sample with a sheet electron concentration of 4.1 × 1013 cm−2, and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of ScxAl1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.

Achieving a sub-10 nm nanopore array in silicon by metal-assisted chemical etching and machine learning

Abstract: Solid-state nanopores with controllable pore size and morphology have huge application potential. However, it has been very challenging to process sub-10 nm silicon nanopore arrays with high efficiency and high quality at low cost. In this study, a method combining metal-assisted chemical etching and machine learning is proposed to fabricate sub-10 nm nanopore arrays on silicon wafers with various dopant types and concentrations. Through a SVM algorithm, the relationship between the nanopore structures and the fabrication conditions, including the etching solution, etching time, dopant type, and concentration, was modeled and experimentally verified. Based on this, a processing parameter window for generating regular nanopore arrays on silicon wafers with variable doping types and concentrations was obtained. The proposed machine-learning-assisted etching method will provide a feasible and economical way to process high-quality silicon nanopores, nanostructures, and devices.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Abstract: Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Anisotropic silicon nanowire arrays fabricated by colloidal lithography

Abstract: The combination of metal-assisted chemical etching (MACE) and colloidal lithography allows for the affordable, large-scale and high-throughput synthesis of silicon nanowire (SiNW) arrays. However, many geometric parameters of these arrays are coupled and cannot be addressed individually using colloidal lithography. Despite recent advancements towards higher flexibility, SiNWs fabricated via colloidal lithography and MACE usually have circular, isotropic cross-sections inherited from the spherical templates. Here we report a facile technique to synthesize anisotropic SiNWs with tunable cross-sections via colloidal lithography and MACE. Metal films with an elliptical nanohole array can form from shadows of colloidal particles during thermal evaporation of the metal at tilted angles. The aspect ratio of these anisotropic holes can be conveniently controlled via the deposition angle. Consecutive MACE using these patterned substrates with or without prior removal of the templating spheres results in arrays of anisotropic SiNWs with either elliptical or crescent-shaped cross-sections, respectively. As a consequence of the anisotropy, long SiNWs with elliptical cross-sections preferentially collapse along their short axis, leading to a controlled bundling process and the creation of anisotropic surface topographies. These results demonstrate that a rich library of SiNW shapes and mesostructures can be prepared using simple spherical colloidal particles as masks.

Fabrication of single-mode circular optofluidic waveguides in fused silica using femtosecond laser microfabrication

Abstract: We demonstrate fabrication of single-mode circular optofluidic waveguides in fused silica glass substrates using femtosecond (fs) laser assisted chemical etching based on slit beam shaping. The fabrication procedure consists of four steps: slit-assisted fs laser direct writing of fused silica, selective chemical etching, polydimethylsiloxane film bonding, and vacuum-assisted liquid filling. The combination of slit beam shaping and high numerical aperture objectives ensures very narrowly modified lines with nearly circular cross-sections during laser direct writing. Introduction of a string of extra-access ports allows production of uniform circular microchannels with the diameters of ~10 µm and the lengths at the centimeter scale due to the improvement of wet chemical etching process. By vacuum-assisted filling a mixture of liquid paraffin and decane into a microchannel structure, a single-mode circular optofluidic waveguide embedded in glass can be obtained.

High Yield Transfer of Clean Large-Area Epitaxial Oxide Thin Films

Abstract: In this work, we have developed a new method for manipulating and transferring up to 5 mm × 10 mm epitaxial oxide thin films. The method involves fixing a PET frame onto a PMMA attachment film, enabling transfer of epitaxial films lifted-off by wet chemical etching of a Sr3Al2O6 sacrificial layer. The crystallinity, surface morphology, continuity, and purity of the films are all preserved in the transfer process. We demonstrate the applicability of our method for three different film compositions and structures of thickness ~ 100 nm. Furthermore, we show that by using epitaxial nanocomposite films, lift-off yield is improved by ~ 50% compared to plain epitaxial films and we ascribe this effect to the higher fracture toughness of the composites. This work shows important steps towards large-scale perovskite thin-film-based electronic device applications.

Growing Nanostructured CuO on Copper Foil via Chemical Etching to Upgrade Metallic Lithium Anode.

Abstract: Metallic lithium is one of the most promising anode materials to build next generation electrochemical power sources such as Li-air, Li-sulfur, and solid-state lithium batteries. The implementation of rechargeable Li-based batteries is plagued by issues including dendrites, pulverization, and an unstable solid electrolyte interface (SEI). Herein, we report the use of nanostructured CuO in situ grown on commercial copper foil (CuO@Cu) via chemical etching as a Li-reservoir substrate to stabilize SEI formation and Li stripping/plating. The lithiophilic interconnected CuO layer enhances electrolyte wettability. Besides, a mechanically stable Li2O- and LiF-rich SEI is generated on CuO@Cu during initial discharge, which permits dense and uniform lithium deposition upon subsequent cycling. Compared with bare Cu, the CuO@Cu electrode exhibits superior performance in terms of Coulombic efficiency, discharge/charge overpotentials, and cyclability. By pairing with the Li-CuO@Cu anodes, full cells with LiFePO4 and LiNi1/3Mn1/3Co1/3O2 cathodes sustain 300 cycles with 98.8% capacity retention at 1 C and deliver a specific capacity of 80 mAh g-1 at 10 C, respectively. This work would shed light on the design of advanced current collectors with SEI modulation to upgrade lithium anodes.

Tuning the Magnetic Properties of Two-Dimensional MXenes by Chemical Etching

Abstract: Two-dimensional materials based on transition metal carbides have been intensively studied due to their unique properties including metallic conductivity, hydrophilicity and structural diversity and have shown a great potential in several applications, for example, energy storage, sensing and optoelectronics. While MXenes based on magnetic transition elements show interesting magnetic properties, not much is known about the magnetic properties of titanium-based MXenes. Here, we measured the magnetic properties of Ti3C2Tx MXenes synthesized by different chemical etching conditions such as etching temperature and time. Our magnetic measurements were performed in a superconducting quantum interference device (SQUID) vibrating sample. These data suggest that there is a paramagnetic-antiferromagnetic (PM-AFM) phase transition and the transition temperature depends on the synthesis procedure of MXenes. Our observation indicates that the magnetic properties of these MXenes can be tuned by the extent of chemical etching, which can be beneficial for the design of MXenes-based spintronic devices.

Effect of Etching Duration on the Morphological and Opto-Electrical Properties of Silicon Nanowires Obtained by Ag-Assisted Chemical Etching

Abstract: Silicon nanowires (SiNWs) were obtained on p-Si (100) substrate by Ag-assisted chemical etching method in two-step process. The influence of the etching duration on the morphological, optical and electrical properties of SiNWs samples was investigated. SEM images show clearly the presence of nanowires and the existence of porous silicon structure especially for low etching duration. Fourier Transformed Infrared spectroscopy (FTIR) measurements allow identifying several particles on the SiNWs surfaces such as oxygen and hydrogen elements. The optical properties of the SiNWs layer were investigated by Photoluminescence spectroscopy (PL). A strong broad PL band was observed at room temperature for an etching time of 30 s. The PL spectra exhibit multi-band profile in the red-green region. The luminescence of SiNWs is mainly attributed to Si-oxide interface states and oxygen deficient centers. Ag/SiNWs/Si Schottky diodes were analyzed by X-ray diffraction (XRD) and studied by current - voltage (I-V) measurements. Diode parameters such as ideality factor (n), series resistance (Rs), saturation current (Is) and energy barrier (φb) were determined by analyzing the I-V characteristics. Ag/SiNWs/Si diode for an etching duration of 30 s exhibits a rectifying behavior with a factor ideality of 2.8 and a low threshold voltage of about 0.44 V in forward bias. Space charge limited current (SCLC) model is the dominant transport mechanism through Ag/SiNWs/Si diode. The trap states, on the Ag/SiNWs interface, play in important role in the conduction phenomenon through these structures.

Wet etch, dry etch, and MacEtch of β-Ga2O3: A review of characteristics and mechanism

Abstract: β-Ga2O3, a promising ultra-wide bandgap material for future high-power electronics and deep-ultraviolet optoelectronics applications, has drawn tremendous attention in recent years due to its wide bandgap of ~ 4.8 eV, high breakdown electric field, and availability of substrates. However, the reported etch behavior of β-Ga2O3 and the quality of etched surfaces, as well as the associated interface characteristics, could limit the performance of β-Ga2O3 devices. In this article, the etchings of β-Ga2O3, including regular wet etching, photoelectrochemical etching (PEC), reactive ion etching (RIE) and metal-assisted chemical etching (MacEtch), are reviewed. A comparison of the etch rate, orientation dependence, aspect ratio, etching mechanism, and surface quality for each of these etching methods is presented and the step-by-step reactions in PEC and MacEtch are proposed to elucidate the etch mechanism. The challenges for these etching techniques for β-Ga2O3 are discussed.

Effect of bionic hydrophobic structures on the corrosion performance of Fe-based amorphous metallic coatings

Abstract: Fe-based amorphous metallic coatings (AMCs) were prepared by the activated combustion−high velocity air fuel (AC − HVAF) coating method. A micro/nanoscale hydrophobic surface based on the lotus effect was constructed on the AMCs by chemical etching and surface modification, and the controllable preparation of bionic hydrophobic structures was realized by optimizing the chemical etching time and etching concentration. The influence of surface treatment on the corrosion resistance of the coatings was analyzed by electrochemical testing, and the correlation between the hydrophobicity of the coating structure and its corrosion resistance was determined. Results showed that the surface of the hydrophobic AMCs is composed of micro/nanoscale hierarchical structures of synapses formed by unmelted particles and corrosion products. Several pores and pits were observed after chemical etching. As the chemical etching concentration and etching time increased, the solid−liquid contact fraction and surface energy of the hydrophobic AMCs first decreased and then increased. The water contact angle peaked at 142.52° when the coatings were chemically etched in 3 mol/L HCl solution for 60 min, thereby conforming to the Cassie−Baxter model. The passivation current density and pitting tendency of the hydrophobic AMCs in 3.5% NaCl solution were closely related to the chemical etching process. The prepared hydrophobic interface could form an effective air wall that isolates the coating surface from the corrosive medium and enhances its uniform corrosion resistance. However, chemical etching aggravated the pores and intensified the dissolution of inclusions, thereby increasing the pitting sensitivity of these concavities. The construction of robust and compact bionic hydrophobic interfaces is an effective approach to enhance the uniform corrosion resistance of AMCs.

Optimization of the fused deposition modeling-based fabrication process for polylactic acid microneedles

Abstract: A microneedle (MN) array is a novel biomedical device adopted in medical applications to pierce through the stratum corneum while targeting the viable epidermis and dermis layers of the skin. Owing to their micron-scale dimensions, MNs can minimize stimulations of the sensory nerve fibers in the dermis layer. For medical applications, such as wound healing, biosensing, and drug delivery, the structure of MNs significantly influences their mechanical properties. Among the various microfabrication methods for MNs, fused deposition modeling (FDM), a commercial 3D printing method, shows potential in terms of the biocompatibility of the printed material (polylactic acid (PLA)) and preprogrammable arbitrary shapes. Owing to the current limitations of FDM printer resolution, conventional micron-scale MN structures cannot be fabricated without a post-fabrication process. Hydrolysis in an alkaline solution is a feasible approach for reducing the size of PLA needles printed via FDM. Moreover, weak bonding between PLA layers during additive manufacturing triggers the detachment of PLA needles before etching to the expected sizes. Furthermore, various parameters for the fabrication of PLA MNs with FDM have yet to be sufficiently optimized. In this study, the thermal parameters of the FDM printing process, including the nozzle and printing stage temperatures, were investigated to bolster the interfacial bonding between PLA layers. Reinforced bonding was demonstrated to address the detachment challenges faced by PLA MNs during the chemical etching process. Furthermore, chemical etching parameters, including the etchant concentration, environmental temperature, and stirring speed of the etchant, were studied to determine the optimal etching ratio. To develop a universal methodology for the batch fabrication of biodegradable MNs, this study is expected to optimize the conditions of the FDM-based fabrication process. Additive manufacturing was employed to produce MNs with preprogrammed structures. Inclined MNs were successfully fabricated by FDM printing with chemical etching. This geometrical structure can be adopted to enhance adhesion to the skin layer. Our study provides a useful method for fabricating MN structures for various biomedical applications.

X-ray Computed Tomography Procedures to Quantitatively Characterize the Morphological Features of Triply Periodic Minimal Surface Structures

Abstract: Additively manufactured (AM) metallic sheet-based Triply Periodic Minimal Surface Structures (TPMSS) meet several requirements in both bio-medical and engineering fields: Tunable mechanical properties, low sensitivity to manufacturing defects, mechanical stability, and high energy absorption. However, they also present some challenges related to quality control, which can prevent their successful application. In fact, the optimization of the AM process is impossible without considering structural characteristics as manufacturing accuracy, internal defects, as well as surface topography and roughness. In this study, the quantitative non-destructive analysis of TPMSS manufactured from Ti-6Al-4V alloy by electron beam melting was performed by means of X-ray computed tomography (XCT). Several advanced image analysis workflows are presented to evaluate the effect of build orientation on wall thicknesses distribution, wall degradation, and surface roughness reduction due to the chemical etching of TPMSS. It is shown that the manufacturing accuracy differs for the structural elements printed parallel and orthogonal to the manufactured layers. Different strategies for chemical etching show different powder removal capabilities and both lead to the loss of material and hence the gradient of the wall thickness. This affects the mechanical performance under compression by reduction of the yield stress. The positive effect of the chemical etching is the reduction of the surface roughness, which can potentially improve the fatigue properties of the components. Finally, XCT was used to correlate the amount of retained powder with the pore size of the functionally graded TPMSS, which can further improve the manufacturing process.

Fabrication of Black Silicon via Metal-Assisted Chemical Etching—A Review

Abstract: The metal-assisted chemical etching (MACE) technique is commonly employed for texturing the wafer surfaces when fabricating black silicon (BSi) solar cells and is considered to be a potential technique to improve the efficiency of traditional Si-based solar cells. This article aims to review the MACE technique along with its mechanism for Ag-, Cu- and Ni-assisted etching. Primarily, several essential aspects of the fabrication of BSi are discussed, including chemical reaction, etching direction, mass transfer, and the overall etching process of the MACE method. Thereafter, three metal catalysts (Ag, Cu, and Ni) are critically analyzed to identify their roles in producing cost-effective and sustainable BSi solar cells with higher quality and efficiency. The conducted study revealed that Ag-etched BSi wafers are more suitable for the growth of higher quality and efficiency Si solar cells compared to Cu- and Ni-etched BSi wafers. However, both Cu and Ni seem to be more cost-effective and more appropriate for the mass production of BSi solar cells than Ag-etched wafers. Meanwhile, the Ni-assisted chemical etching process takes a longer time than Cu but the Ni-etched BSi solar cells possess enhanced light absorption capacity and lower activity in terms of the dissolution and oxidation process than Cu-etched BSi solar cells.

On the Enhanced Phosphorus Doping of Nanotextured Black Silicon

Abstract: The integration of nanotextured black silicon (B-Si) into solar cells is often complicated by its enhanced phosphorus doping effect, which is typically attributed to increased surface area. In this article, we show that B-Si's surface-to-volume ratio, or specific surface area (SSA), which is directly related to surface reactivity, is a better indicator of reduced sheet resistance. We investigate six B-Si conditions with varying dimensions based on two morphology types prepared using metal-catalyzed chemical etching and reactive-ion etching. We demonstrate that for a POCl3 diffusion, B-Si sheet resistance decreases with increasing SSA, regardless of surface area. 2-D dopant contrast imaging of different textures with similar surface areas also indicates that the extent of doping is enhanced with increasing SSA. 3-D diffusion simulations of nanocones show that both the extent of radial doping within a texture feature and the metallurgical junction depth in the underlying substrate increase with increasing SSA. We suggest SSA should be considered more readily when studying B-Si and its integration into solar cells.

Improved Surface-Enhanced-Raman Scattering Sensitivity Using Si Nanowires/Silver Nanostructures by a Single Step Metal-Assisted Chemical Etching.

Abstract: In this study, we developed highly sensitive substrates for Surface-Enhanced-Raman-Scattering (SERS) spectroscopy, consisting of silicon nanowires (SiNWs) decorated by silver nanostructures using single-step Metal Assisted Chemical Etching (MACE). One-step MACE was performed on p-type Si substrates by immersion in AgNO3/HF aqueous solutions resulting in the formation of SiNWs decorated by either silver aggregates or dendrites. Specifically, dendrites were formed during SiNWs’ growth in the etchant solution, whereas aggregates were grown after the removal of the dendrites from the SiNWs in HNO3 aqueous solution and subsequent re-immersion of the specimens in a AgNO3/HF aqueous solution by adjusting the growth time to achieve the desired density of silver nanostructures. The dendrites had much larger height than the aggregates. R6G was used as analyte to test the SERS activity of the substrates prepared by the two fabrication processes. The silver aggregates showed a considerably lower limit of detection (LOD) for SERS down to a R6G concentration of 10−13 M, and much better uniformity in terms of detection in comparison with the silver dendritic structures. Enhancement factors in the range 105–1010 were calculated, demonstrating very high SERS sensitivities for analytic applications.