Asymmetric Etching Research Articles

Au@Ag Nanopencil with Au Tip and Au@Ag Rod: Multimodality Plasmonic Nanoprobe based on Asymmetric Etching for the Detection of SCN- and ClO.

In this paper, Au@Ag nanopencil is designed as a multimodality plasmonic nanoprobe based on asymmetric etching for the detection of SCN- and ClO- . Au@Ag nanopencil with Au tip and Au@Ag rod is prepared by asymmetric tailoring of uniformly grown silver-covered gold nanopyramids under the combined effect of partial galvanic replacement and redox reaction. By asymmetric etching in different systems, Au@Ag nanopencil exhibits diversified changes in the plasmonic absorption band: O2 •- facilitated by SCN- etches Au@Ag rod from the end to the tip, causing a blue shift of the localized surface plasmon resonance (LSPR) peak as the aspect ratio decreases; while the ClO- can retain Au@Ag shell and etch Ag within rod from the tip to the end, causing a redshift of the LSPR peak as the coupling resonance weakens. Based on peak shifts in different directions, a multimodality detection of SCN- and ClO- has been established. The results demonstrate the detection limits of SCN- and ClO- are 160 and 6.7nm, and the linear ranges are 1-600µm and 0.05-13µm, respectively. The finely designed Au@Ag nanopencil not only broadens the horizon of designing heterogeneous structures, but also enriches the strategy of constructing multimodality sensing platform.

Ultra-low concentrations of detection for fluoride and trivalent chromium ions by multiple biomimetic nanochannels in a PET membrane

The development of a detection method for fluoride and chromium (III) ions using a PET membrane with multiple biomimetic nanochannels is descibed. An asymmetric chemical etching technique was used to create conical nanochannels with a PET (polyethylene terephthalate) nuclear track membrane. SEM images showed that the pore diameter of the tip and base sides of the nanochannel were ∼40 nm and ∼180 nm, respectively. The compounds (4-aminophenyl) boronic acid (APBA) and 5-amino-2-nitrobenzoic acid (ANBA), with a specific recognition for F− and Cr3+ respectively, were modified by a conventional method with EDC and NHS. The current-voltage (I–V) curves of the modified PET membranes in response to the target ions at different concentrations were measured with a picoammeter. Excellent linear relationships between the current value of the I–V curve at −2 V and the concentration of Cr3+ and F− were established to obtain standard detection curves for Cr3+ and F−. From these relationships unknown concentrations of Cr3+ and F− can be quantitatively detected, and the results showed that the detection limits of Cr3+ and F− were as low as 4.10 nM and 0.46 nM, respectively. The performance of the novel method was compared to conventional ICP-MS analysis by the determination of Cr3+ in tap water and samples of the Jing Mi diversion canal, and good agreement was found between the two methods. The accuracy and reliability of the method has demonstrated its potential applicability to a wide range of other ionic contaminants.

Plasma etching of the trench pattern with high aspect ratio mask under ion tilting

In this study, we developed a method to qualify the plasma etching result in high-aspect-ratio trench with ion tilting using the natural sheath curvature at the wafer edge. Etching was performed using a patterned wafer with trenches composed of an amorphous carbon mask/SiO2/Si in a radio-frequency-biased inductively coupled plasma with an Ar/C4F6 mixture. It is revealed that even a slight ion tilt (1–2°) induces dramatic changes in etch characteristics, such as etch-stop, asymmetric and vertical etching, which strongly depend on the arrangement, location, and aspect ratio of the trench. These phenomena were analyzed using the relationship between the electric field angle (θE) at the sheath curvature and the critical angle (θc) for ions reaching the trench bottom. It was confirmed that: i) bottom etch stops when θE>θc, ii) asymmetric etching occurs when θE<θc, and iii) vertical etching occurs when θE≈0°. This study provides a new concept for quantifying the etching performance at the wafer edge, as well as the etching mechanism of high-aspect-ratio trench in the situation of ion tilting, which is a central issue in current and next-generation etch processes.

Resistive‐pulse Sensing of DNA with a Polymeric Nanopore Sensor and Characterization of DNA Translocation

AbstractThe ability to detect DNA strands and determine their characteristics in solution plays a critical role to design a sensor in terms of its biophysical properties and receives attention in whole‐ genome sequencing. In this study, we report a simple method to detect 100‐bp DNA based on resistive‐pulse sensing and its characteristics in terms of analyte concentration, potential, tip size. and electrolyte concentration. Track‐etched polyethyleneterephtalate (PET) nanopores were used and modified (w/EDC‐NHS coupling) to decrease the negative surface charge and promote DNA translocation. We tuned up the tip diameters of nanopores by using symmetric and asymmetric chemical etching. The tip diameters ranged from 21 nm to 42 nm. To show the electrophoretic nature of translocation, we investigated the concentration and potential dependence. The current‐pulse events were observed down to 600 mV (4±1 events/min) and 0.25 nM was the minimum concentration. Both potential and concentration dependence showed linear behavior which agreed to previous studies. The effect of tip size was also studied and its effect on translocation frequency was discussed. Finally, the change in current‐pulse amplitude and duration as a function of electrolyte concentration was studied and longer duration values (14.3±1.4 ms) were observed. Our findings provide a guide to translocate the charged molecules more efficiently and help to understand the translocation dynamics of short DNA molecules in resistive‐pulse sensing.

Graphene Oxide Membrane/Conical Nanoporous Polyimide Composites for Regulating Ion Transport

The ionic rectification effect, an important phenomenon existing in the solid-state nanopore system, has attracted much attention due to its ability to significantly regulate ion transport in the nanopores. However, current research mainly focuses on the single nanopore systems with low ionic currents, and thus further practical applications are limited. In this work, the conical nanoporous polyimide (CNPI) membrane was prepared by ion irradiation and asymmetric chemical etching to increase the ionic current, and the graphene oxide membrane (GOM) was spin-coated on it to improve the corresponding ionic flux and rectification effect after that. It was found that the ionic rectification ratio (0.05 M KCl at 3 V) increased from 17 (CNPI membrane) to 63 (GOM/CNPI composite) with the ionic current in microamperes. The enhanced rectification effect was ascribed to the increased geometry asymmetry and charge asymmetry, which further increased the difference of ionic conductance at opposite bias. Moreover, the corresponding COMSOL simulation revealed that the enhanced ion depletion and enrichment appeared in the nanochannel, especially at the tip side of conical nanopores after the GOM was spin-coated on the CNPI membrane. Finally, it was also confirmed that the GOM/CNPI composite can exhibit a good rectification effect and chemical durability in a wide acidic condition (pH 1 to 6), which extends the practical applications of the conical nanopore in a nanofluidic system.

Three-dimensional independent CoZnAl-LDH nanosheets via asymmetric etching of Zn/Al dual ions for high-performance supercapacitors

The design and synthesis of LDH materials with high conductivity and more exposed active sites are the current hotspots in supercapacitor energy storage. However, the practical specific capacity of LDHs is still far below their theoretical value, due to their intrinsic insulation nature, irreversible face-to-face stacking and limited exposed active surface area. Here, it is first proposed to utilize the feature of LDH morphological changes induced by Zn2+ doping, which achieves the transformation of 2D close-packed CoAl-LDH nanosheets into 3D independent loose-packed CoZnAl-LDH nanosheets. Furthermore, the surface electronic environment of Co is changed greatly and certain concentration of oxygen defects is generated of CoZnAl-LDH by partial dissolution of Zn/Al dual ions of LDH laminates in alkaline solution. These will improve the properties in both electrons conduction and ions diffusion of CoZnAl-LDH nanosheets, and thus improve the specific capacity, rate capacities and cycle stability of CoZnAl-LDH nanosheets. Compared with the non-etching CoZnAl-LDH (491 F g−1 at 1 A g−1, 76.9% capacitance retention after 4000 cycles), the obtained E-CoZnAl-LDH-8 h delivered a higher specific capacitance (946 F g−1 at 1 A g−1) and excellent long cycle life (92.3% capacitance retention after 4000 cycles). Moreover, the assembled E-CoZnAl-LDH-8 h//active carbon asymmetric supercapacitors device also showed a remarkable energy density (36.75 Wh kg−1 at 400 W kg−1) and long-term stability (72.7% capacitance retention after 8000 cycles). The “doping” and “dual ion etching” strategies proposed in this work provide theoretical guidance and experimental basis for the development of high-performance supercapacitors.

Switching the plasmon coupling of fractional hollow AuAg nanobox by asymmetrical etching of the inner Ag core

Herein, a sub-40 nm fractional hollow AuAg nanobox is asymmetrically filled with Ag atoms inside the nanobox until it is gradually transformed into a solid AuAg nanobox, and the discrete dipole approximation (DDA) method is applied to calculate the corresponding extinction spectra in this process. It is observed that the height of the inner Ag core could cause a non-monotonous shift of the surface plasmon resonance (SPR) band, which first has a linear blue shift and then a near-exponential red shift, indicating that the height of the inner Ag core could be presented as a key factor to tune the core-dependent non-monotonous SPR shift. A local electric field analysis reveals that the non-monotonous SPR shift may be attributed to the switching of plasmon coupling, then inducing the SPR properties of nanoshell and dimer, respectively. Furthermore, we find that the refractive index of both the dielectric cavity and ambient medium could tune the switching threshold of the Ag core height corresponding to the plasmon coupling. It is also revealed that the switching of plasmon coupling could significantly affect the local electric field intensity and its distribution. Consequently, we believe that the asymmetrical Ag core could increase the tunability of an SPR shift and change the local electric field distribution, which provides the potential for SPR-based biosensing and surface-enhanced Raman scattering (SERS) applications.

Optimization of photoresist development and DRIE processes to fabricate high aspect ratio Si structure in 5 nm scale

The fabrication of high aspect ratio (AR) Si structure with extremely small feature size is attracting more and more attention recently. In this work, a top-down scheme for fabricating nanostructures with 4.4 nm line width and 40:1 AR has been proposed and demonstrated. This technique utilizes electron beam lithography for nano-scale patterning and deep reactive ion etching (DRIE) for pattern transfer. In this method, a special development strategy is discussed to improve the perpendicularity of the photoresist mask, and a novel nano-scale barrel is utilized as the testing structure for the DRIE process optimization. To study the critical dimension (CD) variation in the DRIE process, the geometric morphology of the obtained vertical barrels is simulated by an ion transport model. It is found that the asymmetrical lateral etching encroachment decreased the CD loss by 50%. Without using a hard mask, this technique is fully compatible with Si-based microelectronic processes. Therefore, after further study of geometric properties of the obtained nano structure, this proposed microelectronic-compatible approach sheds light on manufacturing 3D nano devices towards 5 nm scale and beyond.

Transformation of Single Ag Nanowires into Super‐Large AgAu Elliptic Rings via a Water/Oil Interface‐Oriented Asymmetrical Etching

AbstractNanoring as one of fascinating geometries exhibits unique size‐dependent physical and chemical properties endowed by its circular feature. However, to the best of our knowledge, super‐large elliptical nanorings cannot be fabricated by the existing techniques. Herein, we successfully fabricated super‐large elliptical AgAu nanorings creatively by splitting single Ag nanowires along their longitudinal axes involving oil/water interfacial self‐assembly of Ag nanowires and the galvanic replacement between interfacial Ag nanowires and AuCl4− located in the water phase. The formation of AgAu nanoring is mainly attributed to the local nanomasking of Au atoms and asymmetrical etching on interfacial Ag nanowire. Additionally, as‐prepared super‐large elliptical AgAu nanorings with rough surfaces exhibit enhanced catalytic performance towards the reduction of p‐nitrophenol due to the combination of the compositional and structural effects. This strategy, without doubt, provides a convenient way for precisely carving nanoparticles, and has the potential for processing other metallic or non‐metallic nanoparticles.

Osmotic Effects in Track-Etched Nanopores.

Asymmetrically etched ion-track membranes attract great interest for both fundamental and technical reasons because of a large variety of applications. So far, conductometric measurements during track etching provide only limited information about the complicated asymmetric etching process. In this paper, monitoring of osmotic phenomena is used to elucidate the initial phase of nanopore formation. It is shown that strong alkaline solutions generate a considerable osmotic flow of water through newborn conical pores. The interplay between diffusion and convection in the pore channel results in a substantially nonlinear alkali concentration gradient and a rapid change in the pore geometry after breakthrough. Similar phenomena are observed in experiments with cylindrical track-etched pores of 15-30 nm in radius. A theoretical description of the diffusion-convection processes in the pores is provided.

Asymmetrical etching of Ag nanoparticles into symmetry-reduced bi-metallic nanocups at the single-nanoparticle level.

Asymmetrical etching of nanoparticles was achieved by employing interface-confined galvanic replacement reactions. Interfacial Ag nanocubes, nanowires and nanospheres were fabricated into symmetry-reduced cubic, trough-like and spherical nanocups, respectively. The formation of nanocups is attributed to in situ local nanomasking and selective etching of interfacial nanoparticles at the single-nanoparticle level.

Large Rectification Effect of Single Graphene Nanopore Supported by PET Membrane.

Graphene is an ideal candidate for the development of solid state nanopores due to its thickness at the atomic scale and its high chemical and mechanical stabilities. A facile method was adopted to prepare single graphene nanopore supported by PET membrane (G/PET nanopore) within the three steps assisted by the swift heavy ion irradiation and asymmetric etching technology. The inversion of the ion rectification effect was confirmed in G/PET nanopore while comparing with bare PET nanopore in KCl electrolyte solution. By modifying the wall charge state of PET conical nanopore with hydrochloric acid from negative to positive, the ion rectification effect of G/PET nanopore was found to be greatly enhanced and the large rectification ratio up to 190 was obtained during this work. Moreover, the high ionic flux and high ion separation efficiency was also observed in the G/PET nanopore system. By comparing the "on" and "off" state conductance of G/PET nanopore while immersed in the solution with pH value lower than the isoelectric point of the etched PET (IEP, pH = 3.8), the voltage dependence of the off conductance was established and it was confirmed that the large rectification effect was strongly dependent on the particularly low off conductance at higher applied voltage.

The rectification of mono- and bivalent ions in single conical nanopores

The polyethylene terephthalate (PET) films were irradiated with single 6.9MeV/u 58Ni19+ ions at the Lanzhou Interdisciplinary Heavy Ion Microbeam (LIHIM), and single conical nanopores were produced by asymmetric chemical etching of the latent ion tracks. Then, the current-voltage (I–V) characteristic was measured in LiCl, NaCl, KCl, MgCl2, and CaCl2 solution at different concentrations to study the transport properties of different cations in the single conical nanopores respectively. The measured I–V data showed that the conical nanopores have rectified transportation of these cations at the applied voltage of between +2V and −2V. The rectification coefficient γ of the mono- and bivalent ions was determined in their solution of 0.0001–1M measured at 1V, the result showed that the rectification coefficient is dependent on the valence of the ions and the electrolyte solution.

Nanoscrews: Asymmetrical Etching of Silver Nanowires.

World's smallest screws with helical threads are synthesized via mild etching of Ag nanowires. With detailed characterization, we show that this nanostructure arises not from the transformation of the initial lattice, but the result of a unique etching mode. Three-dimensional printed models are used to illustrate the evolution of etch pits, from which a possible mechanism is postulated.

Bivalent ion transport through graphene/PET nanopore

The PET suspended single graphene nanopore (G/PET) was produced by heavy ion irradiation and asymmetric chemical etching. The solutions of NiSO4, NiCl2, CuSO4 and CuCl2 with different concentration were adopted to study the transport properties of bivalent ion in single G/PET nanopore by measuring the I–V curves. The perfect “diode effect” and excellent rectification effect of G/PET nanopore were observed, and the huge rectification ratio up to 43.3 was obtained in NiSO4 solution. The great solution selectivity and ion current magnification effect of graphene/PET nanopore were also confirmed in our study.

Shedding light on the mechanism of asymmetric track etching: an interplay between latent track structure, etchant diffusion and osmotic flow.

The method of producing single track-etched conical nanopores has received considerable attention and found many applications in diverse fields such as biosensing, nanofluidics, information processing and others. The performance of an asymmetric nanopore is largely determined by its geometry, especially by the size and shape of its tip. In this paper we reconstruct the profiles of so-called conical pores fabricated by asymmetric chemical etching of ion tracks in polymer foil. Conductometric measurements during etching and field emission scanning electron microscopy examinations of the resulting pores were employed in order to determine the pore geometry. We demonstrate that the pore constriction geometry evolves through a variety of configurations with advancing time after breakthrough. While immediately after breakthrough the pore tips are trumpet-shaped, further etching is strongly affected by osmotic effects which eventually lead to bullet-shaped pore tips. We evidence that the osmotic flow appearing during asymmetric track etching has a determinative effect on pore formation. A convection-diffusion model is presented that semi-quantitatively explains the effect of osmotic processes under asymmetric track etching conditions. In addition, a phenomenon of reagent contaminant precipitation in nanopores is reported and discussed.

Nanopore detection of double stranded DNA using a track-etched polycarbonate membrane

We investigate the resistive-pulse sensing of 50-bp DNA using track-etched polycarbonate (PC) nanopores and show the translocation dynamics originating from the electrophoretic transport of DNAs. Conically shaped PC nanopore membranes have been prepared with asymmetric chemical etching technique. We show the potential and concentration dependence of DNA translocation through a PC nanopore. We find that the translocation of DNA scales linearly with both potential and concentration. Additionally, the threshold potential is determined to complete the translocation. Finally, by investigating the current-pulse amplitudes of nanopores with different tip sizes, we show that the nanopore size can be successfully used to distinguish the DNA molecules. These results suggest great promise for the sensing of short DNAs and understanding the dynamics of the translocation process using chemically-etched PC nanopores.

Asymmetrical formation of etching residues and their roles in inner-gate-recessed-channel-array-transistor

Different etching rates subjected to the poly-Si/W gate-stacks of state-of-the-art inner-gate-recessed-channel-array-transistor under HBr/O2 plasma environment result in the asymmetrical SiBrx deposition on the sidewalls of the gate-stacks, causing both the fluctuation of the critical dimension (CD) and the degradation of electrical properties as a consequence. If a HF cleaning process either cannot completely remove SiBrx or was postponed for a certain period of time after finishing the reactive ion etching of the gate-stacks, asymmetrically deposited SiBrx residue unexpectedly reacted with air to form the oxidized etching residue such as SiOxBry. The growth of SiOxBry film on the sidewalls can eventually affect the gate CD and the overlap capacitance, resulting in the degradation of both saturation current and propagation time (tPD). A simple model for the diffusion of the SiBrx by-product during the asymmetrical etching process and the growth of SiOxBry film as a function of time delay is suggested.

A gaas phononic crystal with shallow noncylindrical holes

A square lattice of shallow, noncylindrical holes in GaAs is shown to act as a phononic crystal (PnC) reflector. The holes are produced by wet-etching a GaAs substrate using a citric acid:H2O2 etching procedure and a photolithographed array pattern. Although nonuniform and asymmetric etch rates limit the depth and shape of the phononic crystal holes, the matrix acts as a PnC, as demonstrated by insertion loss measurements together with interferometric imaging of surface acoustic waves propagating on the GaAs surface. The measured vertical displacement induced by surface phonons compares favorably with finite-difference time-domain simulations of a PnC with rounded-square holes.

Conical nanopores fabricated via a pressure-biased chemical etch

Controlling the size and shape of nanopores in polymer membranes can significantly impact transport of molecular or ionic species through these membranes. Here we describe a facile method to controllably form conical nanopores in ion-tracked polycarbonate membranes. Commercial polycarbonate ion-tracked membranes were placed between a concentrated alkaline solution and an acidic solution. By varying the height of the acidic solution, the hydrostatic pressure was controlled, regulating the acid flux through the nanopores. The resulting asymmetric etching of the membrane produced conical pores with controllable aspect ratios. Scanning electron microscopy of both the pores and nickel nanostructures electrolessly templated in the pores confirms their conical shape. This safe, straightforward approach obviates the need to use large voltages, currents, and/or plasma etching equipment traditionally employed to create conical nanopores.