Deposition Of Nanowires Research Articles

Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes.

We report a solution-phase approach to the synthesis of crystalline copper nanowires (Cu NWs) with an aspect ratio >1000 via a new catalytic mechanism comprising copper ions. The synthesis involves the reaction between copper(II) chloride and copper(II) acetylacetonate in a mixture of oleylamine and octadecene. Reaction parameters such as the molar ratio of precursors as well as the volume ratio of solvents offer the possibility to tune the morphology of the final product. A simple low-cost spray deposition method was used to fabricate Cu NW films on a glass substrate. Post-treatment under reducing gas (5% H2 + 95% N2) atmosphere resulted in Cu NW films with a low sheet resistance of 24.5 Ω/sq, a transmittance of T = 71% at 550 nm (including the glass substrate), and a high oxidation resistance. Moreover, the conducting Cu NW networks on a glass substrate can easily be transferred onto a polycarbonate substrate using a simple hot-press transfer method without compromising on the electrical performance. The resulting flexible transparent electrodes show excellent flexibility ( R/ Ro < 1.28) upon bending to curvatures of 1 mm radius.

Thermal chemical vapor deposition of tin oxide nanowires in a hydrogen reduction atmosphere

We fabricated tin oxide nanowires (NWs) on Au/Si3N4/Si(001) substrates using thermal chemical vapor deposition (CVD) in a hydrogen reduction atmosphere. Unlike typical thermal CVD conditions, a mixture of H2 and Ar gas was used as the carrier gas, and a catalyst powder such as graphite was not used. The hydrogen gas maintained the reducing atmosphere and easily vaporized the SnO2 powder without a catalyst, to supply vapors of Sn and O. This process enables vapor-liquid-solid (VLS) growth of SnO2 NWs at temperatures below 600 °C. The formation of a eutectic AuSn alloy that acts as a liquid seed during the VLS process contributes to lowering the growth temperature. X-ray diffraction and transmission electron microscopy analysis show that SnO2 NWs and SnO NWs coexist in the as-prepared sample. The oxygen-deficient atmosphere results in nucleation and growth of SnO NWs.

Site-Specific Growth and in Situ Integration of Different Nanowire Material Networks on a Single Chip: Toward a Nanowire-Based Electronic Nose for Gas Detection.

A new method for the site-selective synthesis of nanowires has been developed to enable material growth with defined morphology and, at the same time, different composition on the same chip surface. The chemical vapor deposition approach for the growth of these nanowire-based resistive devices using micromembranes can be easily modified and represents a simple, adjustable fabrication process for the direct integration of nanowire meshes in multifunctional devices. This proof-of-concept study includes the deposition of SnO2, WO3, and Ge nanowires on the same chip. The individual resistors exhibit adequate gas sensing responses toward changing gas concentrations of CO, NO2, and humidity diluted in synthetic air. The data have been processed by principal component analysis with cluster responses that can be easily separated, and thus, the devices described herein are in principle suitable for environmental monitoring.

Patterned electromagnetic alignment of magnetic nanowires

A combination of electromagnetic alignment and topological pattern assisted alignment to position magnetic nanowires, which is referred to as the Patterned Electromagnetic Alignment (PEA), is developed and examined. Electrodeposited, FeNiCo nanowires with different lengths were used as the test nanomaterial, and the microscale grooved surface was formed by UV nanoimprint lithography. The accuracy of the PEA with FeNiCo nanowires was evaluated by measuring the deviation angle from the direction of the magnetic field line for different magnetic field strengths and nanowire lengths, and a statistical alignment distribution was reported for different nanowire length groups. The results were compared with those of the electromagnetic alignment on flat surfaces and in grooved-patterned substrates without electromagnetic alignment. Overall, the deviation angle for the PEA was lower than that for the electromagnetic alignment when all other experimental conditions were identical, indicating that the alignment accuracy along the direction of the magnetic field lines was enhanced in the presence of surface micro grooves. This can be attributed to the fact that, upon attachment of nanowires to the substrate surface, the surface micro grooves in the PEA add additional deterministic characteristics to the otherwise stochastic nature of the nanowire deposition and solvent evaporation processes compared to the sole electromagnetic alignment.

Modification of the Electrolyte/Electrode Interface for the Template-free Electrochemical Synthesis of Metal Nanowires from Ionic Liquids.

In electrochemistry, the electrode/electrolyte interface (EEI) governs the charge/mass-transfer processes and controls the nucleation/growth phenomena. The EEI in ionic liquids (ILs) can be controlled by changing the cation/anion of the IL, salt concentration, electrode potential, and temperature. Here, we show that adding a dopant salt leads to the deposition of nanowires. To illustrate, zinc nanowires were electrodeposited from ZnCl2/1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate in the presence of GaCl3 as a dopant salt. The choice of Zn salt and its ratio to GaCl3 were found to be crucial for Zn nanowires formation. AFM studies revealed that the solvation structure of Au(111)/IL changes significantly in the presence of GaCl3 and ZnCl2. Chronoamperometry showed changes in the nucleation/growth process, consequently leading to the formation of nanowires. A similar approach was adopted to synthesize Sn nanowires. Thus, modification of the EEI by adding a dopant to ILs can be a viable method to obtain nanowires.

Simultaneously improving the mechanical strength and electromagnetic interference shielding of carbon/carbon composites by electrophoretic deposition of SiC nanowires

Electrophoretic deposition of SiC nanowires in C/C composites reinforces the matrix and creates additional conduction paths in the matrix.

Pulsed laser deposition of aluminum nitride nanowires

The possibility of AlN nanowires deposition on single-crystal silicon substrates by pulsed laser deposition in vacuum is shown in this work. Experimental samples were investigated by scanning electron microscopy and infrared Fourier spectroscopy. It is shown that the possible mechanism for the AlN nanowires formation is the "vapor-liquid-crystal" mechanism.

Eutectic-Bismuth Indium as a Growth Solvent for the Electrochemical Liquid-Liquid-Solid Deposition of Germanium Microwires and Coiled Nanowires

Crystalline germanium (Ge) micro- and nanowires have been grown from aqueous electrolytes through an electrochemical liquid-liquid-solid (ec-LLS) process using eutectic bismuth indium (e-BiIn) alloy as the liquid metal electrode. This alloy represents the first non-Hg or non-Ga containing liquid metal to be used for the growth of a group IV semiconductor crystal below the boiling point of water. The electrochemical stability of e-BiIn in aqueous electrolyte was assessed through cyclic voltammetry, which showed the metal alloy was destabilized either by anodic oxidation at potentials more positive than −1.0 V vs E(Ag/AgCl) or by cathodic reduction of Bi to BiH3 at potentials more negative than −1.4 V vs E(Ag/AgCl). Within this potential window, ec-LLS produced Ge micro- and nanowires that were evaluated with scanning electron microscopy, high resolution transmission electron microscopy, and selected area electron diffraction. The cumulative analyses showed pronounced crystallinity in the as-prepared Ge mic...

Effect of supersonic spraying impact velocity on opto-electric properties of transparent conducting flexible films consisting of silver nanowire, ITO, and polyimide multilayers

We demonstrate the use of supersonic spraying for the deposition of silver nanowires (AgNWs) on a flexible polyimide (PI) substrate for the formation of transparent and conducting films (TCF) as an alternative to nonflexible ITO (indium tin oxide). The self-fused intersections of the NWs resulted in films with a low sheet resistance (Rs = 31 Ω/sq) and fairly high transmittance (Tr = 92%) on a glass substrate. The effect of the impact speed of the supersonically sprayed AgNWs on the opto-electric properties of the flexible TCF was evaluated by varying the spray coating conditions. The fabricated films were characterized by X-ray diffraction analysis, atomic force microscopy, ultraviolet–visible spectroscopy, and scanning electron microscopy. Finally, cyclic bending tests were performed on the PI/AgNW films as well as PI/ZnO/indium tin oxide/AgNW films, and the changes in their electrical properties with bending were compared.

Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia.

The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH)4- complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

Solution-Processable transparent conducting electrodes via the self-assembly of silver nanowires for organic photovoltaic devices

Solution-processed transparent conducting electrodes (TCEs) were fabricated via the self-assembly deposition of silver nanowires (Ag NWs). Glass substrates modified with (3-aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTES) were coated with Ag NWs for various deposition times, leading to three different Ag NWs samples (APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH)). Controlling the deposition time produced Ag NWs monolayer thin films with different optical transmittance and sheet resistance. Post-annealing treatment improved their electrical conductivity. The Ag NWs films were successfully characterized using UV–Vis spectroscopy, field emission scanning electron microscopy, optical microscopy and four-point probe. Three Ag NWs films exhibited low sheet resistance of 4–19Ω/sq and high optical transmittance of 65–81% (at 550nm), which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device by using Ag NWs as the anode instead of ITO electrode, and optimized device with Ag NWs exhibited power conversion efficiency of 1.72%.

Influence of boric acid (H3BO3) concentration on the physical properties of electrochemical deposited nickel (Ni) nanowires

Authors have investigated the influence of the stabilizer (Boric Acid) concentration during the template-assisted electrochemical deposition of Nickel (Ni) nanowires in Anodic Alumina Oxide (AAO) templates. The synthesis was performed using Ni Sulfate Hexahydrate (NiSO4.6H2O) as metal salts and Boric Acid (H3BO3) as a stabilizer. The mixture of both solutions creates electrolyte and utilized for the electrochemical deposition of Ni nanowires. During the experiment, the boric acid concentration varied between 5 g/L, 37.5 g/L and 60 g/L with a deposition temperature of 80 °C (constant). After the electrochemical deposition process, AAO templates were cleaned with distilled water before dissolution in Sodium Hydroxide (NaOH) solution to obtain the freestanding Ni nanowires. Physical properties of the synthesized Ni nanowires were analyzed using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDX) and X-ray Diffraction (XRD). The physical properties of obtained Ni nanowires has eloborated by taking into account the effect of boric acid concentration on the surface morphology, growth length, elemental composition and crystal orientation crystal of the synthesized nickel nanowires. The finding exposes that the boric acid concentration does not influence all aspects in the physicals properties of the synthesized Ni nanowires. The boric acid concentration did not affect the surface texture and crystal orientation. However, shorter Ni nanowires obtained as the concentration of boric acid increased.

Electrochemical deposited nickel nanowires: influence of deposition bath temperature on the morphology and physical properties

This paper investigates the influence of the electrolytic bath temperature on the morphology and physical properties of nickel (Ni) nanowires electrochemically deposited into the anodic alumina oxide porous membrane (AAO). The synthesis was performed using nickel sulfate hexahydrate (NiSO4.6H2O) and boric acid (H3BO3) as an electrolytic bath for the electrochemical deposition of Ni nanowires. During the experiment, the electrolyte bath temperature varied from 40°C, 80°C, and 120°C. After the electrochemical deposition process, AAO templates cleaned with distilled water preceding to dissolution in sodium hydroxide (NaOH) solution to obtain free-standing Ni nanowires. Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDX) and X-ray Diffraction (XRD) analysis were employed to characterize the morphology and physical properties of the synthesized Ni nanowires. Finding reveals the electrodeposition bath temperature significantly influences the morphology and physical properties of the synthesized Ni nanowires. Rougher surface texture, larger crystal size, and longer Ni nanowires obtained as the deposition bath temperature increased. From the physical properties properties analysis, it can be concluded that deposition bath temperature influence the physical properties of Ni nanowires.

(Invited) Use of Supercritical Solvents for Electrodeposition of Thermoelectric Materials

Background: Thermoelectric materials (TEMs) are of interest for both cooling (Peltier effect) and voltage generation applications (Seebeck effect). Thermoelectric generators (TEGs) convert heat energy into electrical voltage via the Seebeck effect. In a typical TEG the thermoelectric elements, usually alternating p-type and n-type semiconductors, are arranged electrically in series and thermally in parallel. The power that can be generated depends on many factors: geometrical design of the module, thermal coupling to the heat exchangers, thermal and electrical conductivity of the materials supporting and separating the TEM elements and fundamentally the thermal and electrical conductivity of the TEM itself. The latter is summed up by the materials dimensionless “figure of merit”: zT = S2σT/κ. Where: S = Seebeck coefficient (thermoelectric sensitivity often given in μV/K); σ = electrical conductivity and κ = thermal conductivity (usually in W/mK). The current consensus is that, to be truly competitive for power generation, TEMs should have zT of 2-3[1]; although there are plenty of cases where this is less important. For instance where waste heat can be used on a large scale for power generation; here the conversion efficiency and the cost of the TEM is far more important. For certain niche applications requiring low to moderate power and low maintenance (e.g. for remote, standalone power supplies) cost is less important and there are a range of exotic materials with zT on the order of 1. One such material that has been widely studied is bismuth Telluride (Bi2Te3) which has a figure of merit typically between 0.7 and 0.8. This can be doped (often with antimony to produce p-type or selenium to produce n-type) to give zT >1.4[2, 3]. Modifying the carrier concentration to improve electrical conductivity (σ) is one way of improving zT but as can be seen from the equation above the figure of merit is also inversely proportional to the thermal conductivity (κ). Unfortunately σ and κ tend to vary likewise and good electrical conductivity usually goes hand in hand with good thermal conductivity. One strategy for reducing the thermal conductivity of a TEM, with minimal impact on the electrical conductivity, is nanostructuring[4, 5]. In in bidimensional or monodimensional structures (i.e. very thin films or nanowires) as the critical dimension is reduced it approaches the same scale as the mean free path of the phonons and their propagation is limited by surface scattering. Thus by tuning the size of the TE elements along with surface roughness and grain boundaries it should be possible to optimise both the thermal and electrical conductivities and produce superior TEGs. In this work: Previously we have demonstrated that, through judicious choice of electrolyte and design of precursor complex, supercritical solvents such as CO2 and difluoromethane (R32) may be used to electroplate a range of metal and semiconductor elements[6]. Furthermore supercritical fluid electrodeposition (SCFED) has distinct advantages over electroplating from conventional solvents: the broad potential window offered by R32 and CO2 allowing for electrodeposition of elements such as germanium; an ability to operate at high temperatures (since the reaction cells are all sealed); tunability of solvent properties (such as density and viscosity) and the absence of surface tension and very high rates of diffusion. The latter is a particular advantage when trying to electrodeposit into high aspect ratio nanopores and we have shown that filling of pores of 2-3 nm is viable by SCFED[7]. Here we demonstrate electrodeposition from binary solutions in supercritical R32 to produce bismuth telluride and antimony telluride films and nanowires of 13-55 nm diameter. We also demonstrate deposition of bismuth-tellurium and antimony-tellurium heterojunction nanowires. It is thought that Bi2Te3nanowires with a diameter <30 nm would offer a significant improvement in thermoelectric figure of merit[8, 9]. This work is conducted as part of the ADEPT project funded by EPSRC (EP/N035437/1).

Selective deposition of ZnO nanowires on silicon micro-pyramidal arrays

Growth of ZnO nanowires on various crystallographic facets of silicon was carried out in order to promote spatially-selective growth. Spontaneous growth of these structures at specific locations of a wafer is necessary for many applications, including photovoltaics and field-emission. Silicon micro-pyramidal arrays with a base of 80µm square and a plateau 30µm wide at the top were fabricated using the crystallographic wet chemical etching process. ZnO nanowires were grown onto these structures using the vapor-liquid-solid technique employing Au nanoparticles generated by annealing a sputter deposited layer. Several variations were made in the crystallographic etch process to generate different facet morphologies. Furthermore, the gold annealing step was carried out under different conditions, and its effect on the nature and density of ZnO nanowires was determined. It was observed that even for identical deposition conditions for the ZnO nanowires, modification of these two parameters strongly affected the selective deposition on various silicon facets. For relatively low Au annealing temperatures the entire surface of the micro-pyramidal structures was coated densely and uniformly with ZnO nanowires which were mostly aligned vertical to the facet surfaces. SEM as well as Cathodoluminescence (CL) imaging studies show that annealing of the Au film at higher temperature led to deposition of ZnO nanowires on the plateau at the top of the micro-pyramidal structures, while the side facets were completely bare. The nanowires deposited on the plateau on top the pyramids were as long as 40µm with a diameter of ~200nm. CL spectra obtained from the ZnO nanowires showed clear and sharp band edge luminescence along with emission at ~500nm due to surface states. It is envisioned that further reduction of the plateau dimensions will lead to single nanowire deposition on top of each pyramidal structure, which is appropriate for future field-emission devices.

Tailoring the Aqueous Synthesis and Deposition of Copper Nanowires for Transparent Electrodes and Heaters

AbstractDue to the high abundance of copper on the earth and its high intrinsic electrical conductivity, copper nanowires (CuNWs) represent a promising material for transparent electrodes. In this work, an environmentally friendly and scalable synthesis that requires a low process temperature is studied. The optimum temperature is found at 79 °C, which results in nanowires with the lowest diameters. The as‐synthesized solution is sprayed to transparent conducting films, which are in turn subjected to various post‐treatments such as thermal sintering or washing with propionic acid to enhance their electro‐optical performance. Following both the optimum protocol for the synthesis and post‐treatment, a sheet resistance of 10.3 Ω ◻−1 at a transparency of 83.4% is achieved. Moreover, the CuNW‐films are tested as transparent heaters and show a homogeneous heat distribution. For the electrical properties of the films, a temperature dependence of resistance that is lowered around 28% compared to the one for bulk copper is found.

Effect of electrolyte temperature on the cathodic deposition of Ge nanowires on in and Sn particles in aqueous solutions

The results of studying the growth of filamentous Ge structures in aqueous electrolytes at various temperatures using In and Sn nanoparticle arrays as nucleation sites are given. The temperature of Ge cathodic deposition process from aqueous solutions has a significant effect on the layer structure deposited onto the surface. In the presence of metal particles in the molten state, filamentous Ge structures grow due to the cathodic reduction of Ge-containing ions on the electrode surface, followed by dissolution and crystallization in the melt at the substrate interface. The results obtained show the crucial role of liquid metal particles during the electrochemical formation of germanium nanowires.

Study on controlled assembly various kinds of nanotubes and nanowires devices

ABSTRACTFloating potential dielectrophoresis (FPDEP) method is used to the assembly various kinds of nanotubes and nanowires devices. The method involves the optimization of the electrode geometry, applied voltage and frequency, load resistance, and type of nanotube sample used. In this work, we report a novel floated electrode structure to realize the controlled deposition of nanotubes or nanowires, only across those edges of a floating electrode, which are facing a counter electrode. Floating electrode dielectrophoresis allows preparation of devices without the unwanted nanotubes or nanowires thus preventing creation of parasitic pathways between microelectrodes. Electrostatic field simulations were also carried out to show the effect of floating electrode.

Fabrication of a transparent conducting electrode based on graphene/silver nanowires via layer-by-layer method for organic photovoltaic devices

A solution-processed transparent conducting electrode was fabricated via layer-by-layer (LBL) deposition of graphene oxide (GO) and silver nanowires (Ag NWs). First, graphite was oxidized with a modified Hummer’s method to obtain negatively-charged GO sheets, and Ag NWs were functionalized with cysteamine hydrochloride to acquire positively-charged silver nanowires. Oppositely-charged GO and Ag NWs were then sequentially coated on a 3-aminopropyltriethoxysilane modified glass substrate via LBL deposition, which provided highly controllable thin films in terms of optical transmittance and sheet resistance. Next, the reduction of GO sheets was performed to improve the electrical conductivity of the multilayer films. The resulting GO/Ag NWs multilayer was characterized by a UV–Vis spectrometer, field emission scanning electron microscope (FE-SEM), optical microscope (OM) and sheet resistance using a four-point probe method. The best result was achieved with a 2-bilayer film, resulting in a sheet resistance of 6.5Ω sq−1 with an optical transmittance of 78.2% at 550nm, which values are comparable to those of commercial ITO electrodes. The device based on a 2-bilayer hybrid film exhibited the highest device efficiency of 1.30% among the devices with different number of graphene/Ag NW LBL depositions.

Pulse, Electrodeposited Fe-Ni-Co Metal and Oxide Layered Nanowires

Introduction A challenge in depositing iron-group elements into nanoporous membranes from aqueous electrolytes is the avoidance of gas bubble generation that can impede deposition. One solution is to use pulse deposition. A number of studies have reported successful deposition of Ni,1, 2 Co,3 Fe,4 and Ni-Fe5 nanowires with a pulse potential or current. Also, Fe-Ni-Co nanowires using nanoporous alumina6 or polycarbonate membranes7 have been reported from our lab. Since the local pH is expected to increase within nanopores with gas evolving side reactions, an applied anodic potential step can be used to create a stable oxide layer in the same electrolyte that is used for deposition. In this paper we use this idea to (1) create a selective area for subsequent etching for the fabrication of novel nanotips and to (2) better understand the pulse response during deposition by examining the influence of oxide surfaces that may be generated during the off time in a pulse cycle. Experimental A rotation cylinder electrode (RCE) was used to characterize the deposition and etching behavior of Fe-Ni-Co alloy thin films. These results were then used to determine the conditions for depositing the nanowires. The nanowires were electrodeposited into a polycarbonate template. The first layer of Fe-Ni-Co was pulse deposited (-50 mA/cm2 for 2 s, 0 mA/cm2for 2 s). The surface of the nanowires was then treated with an applied potential. The potential was stepped to 0 V vs OCP for 10 min, or to an anodic potential for 10 s, before a second layer of Fe-Ni-Co was deposited under the same condition. After deposition, the nanowires were released from the membrane by dissolving the membrane in dichloromethane, followed by etching in a pH 5 citrate-acid solution. The resulting structure was characterized with Field Emission Scanning Electron Microscope (FESEM) and High-resolution transmission electron microscopy (HRTEM). Results and Discussion RCE results show that the current efficiency decreases as the rotation rate increases, while there is no change in composition. Thus, in the nanoporous template a decreasing current efficiency in the direction of growth can occur, as the proton boundary layer size changes, during Fe-Ni-Co nanowire deposition. The influence of the intermediate oxide layer upon exposure in the sodium citrate solution resulting in different morphological changes. If the intermediate anodic potential in the middle layer was at its open circuit potential, then upon etching the nanowires formed a ~ 10 nm nanogap. However, as the applied potential in this second step was adjusted to values more positive than +0.2 V vs SCE, nanotips were observed after etching. The pulse transients in the deposition of the metal in the third layer was helpful in evaluating the reduction of the oxide. When a cathodic current is applied the “on” potential reaches -0.8 V vs SCE, and this region is extended in time with larger overpotentials, which contributes to the increasing amount of iron oxide produced in the second step (Fig. 1). The reduction of this oxide layer creates gradient region in structure in between two pulsed deposited Fe-Ni-Co layers. Conclusion In this study, a novel approach for creating nanotips at the interfacial region of two Fe-Ni-Co layers was introduced via a combination of pulse electrodeposition and an intermediate potential reverse procedure. A sacrificial region was created in the middle of the nanowires as the result of a combination of oxide reduction and alloy deposition, which was the fundamental reason for the formation of nanotips after etching. Acknowledgement The authors acknowledge and thank Roche Diagnostics for support of this project. Reference 1. K. Nielsch, F. Müller, A.-P. Li and U. Gösele, Adv. Mater., 12, 582 (2000). 2. C. Jin, W. Liu, C. Jia, X. Xiang, W. Cai, L. Yao and X. Li, J. Cryst. Growth, 258, 337 (2003). 3. J. Zhang, G. A. Jones, T. H. Shen, S. E. Donnelly and G. Li, J. Appl. Phys., 101, 054310 (2007). 4. J. M. Baik, M. Schierhorn and M. Moskovits, J. Phys. Chem. C, 112, 2252 (2008). 5. M. A. Kashi, A. Ramazani, S. Doudafkan and A. Esmaeily, Appl. Phys. A, 102, 761 (2011). 6. H. Kim, S. A. Soper and E. J. Podlaha-Murphy, ECS Trans., 53, 9 (2013). 7. X. Geng and E. J. Podlaha, Nano Lett., 16, 7439 (2016). Figure 1