Fig 10 - uploaded by Thomas Edwin Vandervelde
Content may be subject to copyright.
Mechanism of electrochemical deposition One drawback of electro-deposition is its tendency towards non-conformal growth on non- planar surfaces. This is not an issue when growing nanowires through a template, which serves to constrain the growth. However, on rough or textured surfaces (such as a nanowire array), electrons gather in the asperities on the surface. More deposition occurs at these points, which only exacerbates the issue as this make the surface rougher. Additives into the plating bath can alleviate this issue by gathering at the points of highest current density and inhibiting deposition, but this introduces impurities into the deposited material. The power of additives in creating conformal deposits is evidenced by the advent of copper
Context in source publication
Context 1
... lithographic processes. However, the use of lithography does present two distinct advantages when dealing with electronic devices. For one, the nanowires can be easily integrated into other devices and structures that are fabricated by lithographic methods. For example, interconnects, transistors, and MEMS structures can be made together with nanowires. Secondly, for standard top-down fabrication methods, there exist many well- characterized means to make electrical connections with the outside world (wirebonding, solder bumping, etc). This is often an issue when working with bottom-up methodologies. Finally, templated growth is a popular and versatile nanowire production method. This process is used in this work to fabricate core-shell nanowires. In this method, nanowires are masked by a template such as an anodized alumina membrane or a track etched polycarbonate membrane. Both of these membranes have extremely high aspect ratio pores. Anodized aluminum oxide (AAO) membranes have pores that range from nanometers to microns in diameter and up to hundreds of microns in length. To make nanowires, the desired material is filled into the template. A variety of methods have been used to achieve this including atomic layer deposition, centrifugation, electrophoretic deposition, and, most commonly, electrochemical deposition (ECD) (Cao and Liu 2008). In electrochemical growth a conductive seed layer is deposited on one side of the template, and the other side is introduced to a plating solution for the desired material. The final step in fabrication is the removal of the template, which is accomplished by dissolving the membrane in an appropriate solution (halogenated solvents for polycarbonate membranes or strong bases for AAOs). The materials investigated here are copper (II) oxide and zinc oxide. These have bandgaps of 1.2 eV (Jiang, Herricks et al. 2002) and 3.3 eV (Ozgur, Alivov et al. 2005), respectively. There is a fair amount of copper (I) oxide present as well which has a band-gap of 2.0 eV (Rakhshani 1986), but this material should decrease with optimization of our fabrication process. These materials were chosen for their ease of use in electrodeposition and their relative abundance. They have the added benefit of being natural n and p type materials. Undoped zinc oxide has a residual n-type conductivity (Look, Hemsky et al. 1999; Ozgur, Alivov et al. 2005), and CuO is naturally p-type. (Jiang, Herricks et al. 2002) Currently an indium tin oxide (ITO) thin film is used for the top contact, but future work will include investigating more economical alternatives. The structure of the nanowire consists of co-axially stacked layers creating a core-shell arrangement as seen in Figure 7. The benefits of the structure have been discussed in section 2.2. The base and core of the nanowires consist of a copper core, which acts as the bottom contact, next is a shell of copper oxide around the copper core, and then a second shell of zinc oxide. Finally, a layer of indium tin oxide is deposited on the top. Due to the sputtering deposition, this top layer is not very conformal and covers mostly the top of the wires. This process can be replaced by an ALD deposition to achieve a conformal coating. The band diagram of the cell can be seen in Figure 8. This shows the band-gaps of the materials and the voltage drop between them. This voltage drop is what pushes the electrons and holes away from each other to prevent them from immediately recombining. The band-gap of ZnO is 3.3eV and as such can absorb only UV light out to 376nm. Cuprous oxide (Cu 2 O) can absorb light out to 620nm, and cupric oxide, CuO, can absorb radiation out to 1033nm. This encompasses the vast majority of the solar spectrum. The ZnO layer is very thin and has a wide band-gap. Therefore its purpose is to act as the n-type material to create the built in voltage of the diode, which serves to extract charge carriers. The copper oxides serve as the absorber and p-type material in the diode. The geometric constraints when fabricating nanowires (especially core-shell structures) make the use of many common top-down techniques impractical. The dimensions of nanowires place constraints on fabrication with which many conventional deposition methods cannot comply. For example, physical vapor deposition (PVD) processes are line of sight methods, and, as such, these methods never yield impressive step coverage in high aspect ratio (length to width ratio) features such as nanowires (Madou 2002). Chemical vapor deposition performs better and is routinely used to coat or fill features with aspect ratios of five or ten (Gordon, Hausmann et al. 2003). However, this is still orders of magnitude removed from the typical nanowire aspect ratios. The following section details fabrication methods well-suited to the creation and alteration of nanowire arrays with special attention to copper and zinc oxides and core-shell structures. Electrochemical deposition, shown in Figure 10, is the utilization of electrically driven redox reactions to solidify ions out of solution. Most often, metallic cations are reduced at a cathode, while oxidation at an anode of the same metal replaces the reduced ions. It is possible to plate alloys and compounds through careful manipulation of the plating bath. For example, many metal oxides, including copper and zinc oxides, can be deposited by careful control of the solution and electrical conditions (Golden, Shumsky et al. 1996) interconnects in the integrated circuit industry where high-aspect ratio vias are filled by ECD (Andricacos 1999). However, aspect ratios are even more pronounced for the extreme geometries observed in nanowire arrays. There are also mass transfer concerns as the diffusion length for ions in solution exceeds the inter-nanowire spacing. Therefore, it is difficult (or improbable) for ions to travel the length of the nanowire to its base without being reduced and deposited prematurely. While ECD is difficult to implement for conformal coatings on nanowire arrays, it remains the method of choice to make high- quality nanowires through templated growth. Electroless and immersion deposition are chemical methods to deposit metal films without applying an electric potential and have traditionally been used to conformally coat difficult geometries. Electroless plating (also known as autocatalytic deposition) utilizes reducing agents in solution to drive the reaction at the surface, which acts as a catalyst. Electroless deposition does not suffer from the current crowding effects of electrolytic deposition, but mass transfer effects can influence the deposition rates at different points in a nanowire geometry (Paunovic, Schlesinger et al. 1998). Immersion plating is the displacement reaction of a more noble metal replacing a more active metal on a surface. For example, gold ions in solution would reduce and plate onto an iron bar. The iron surface would oxidize and dissolve into solution to maintain charge conservation. The reaction is self-limiting with deposits being only a few monolayers thick. Unlike autocatalytic and electrolytic plating, immersion deposits are exceptionally uniform regardless of surface topology. As such, immersion deposits are an attractive option for creating the ‘shell’ in nanowire core-shell applications. The main drawback is the material limitations: the depositing material must be higher in the galvanic series than the substrate (Langdon 1988). The material constraints of immersion plating can be solved through a process known as contact plating. In this process, the substrate to be coated is put into electrical contact with a more electropositive (less noble) metal. Oxidation occurs at the electropositive metal, driven by the constituents of the bath. This oxidation yields electrons, which travel through the electrical contact and allow for reduction of ions out of solution. If the less noble metal is also the coating material, this reaction is self-limiting in that the reaction ceases when the work is coated. Thus, conformal coatings of less-noble metals may be electrochemically deposited on difficult geometries (Durney 1984). While not electrochemical in nature, atomic layer deposition (ALD) should be mentioned as the ultimate method of conformal coating. Unlike the previous processes, ALD occurs from reactants in gas phase, generally at low pressures. In ALD a gas precursor is introduced and allowed to form a monolayer on the surface of a sample before being pumped out. Then, a second gas is introduced, which reacts with the monolayer to yield the desired film. This process is repeated to build up a film atomically, monolayer by monolayer. Because the monolayers are exceptionally uniform, conformal coatings on difficult topologies are easily achieved. Aspect ratios of almost 50 are easily coated uniformly (Ritala and Leskela 2001). The main drawback to ALD is the extremely low rate of deposition, which typically reaches a maximum at a few Angstroms per minute. Finally, oxidation reactions are perhaps the most facile and effective way of creating exceptionally uniform and conformal layers on complex structures. Oxidation can be performed in either aqueous or gaseous environments. The latter case, generally referred to simply as thermal oxidation, has been well-characterized over a wide-range of temperatures (Cabrera and Mott 1949). Thin, uniform oxides can be formed on metals simply by applying heat in an oxygen atmosphere (Rusu, G ı rtan et al. 2007; Njeh, Wieder et al. 2002). However, the high temperatures needed in thermal oxidation can have some undesired affects with regards to annealing, coefficients of thermal expansion, and diffusion. For example, a copper-zinc structure cannot be converted into a copper oxide – zinc oxide heterojunction by thermal oxidation because the materials will diffuse into one another. This forms brass, not a diode. In the case of wet oxidation, the reaction ...
Similar publications
Moderate fluorescence from aluminium doped zinc oxide films is detected. It is shown that this defect-related fluorescence can be controlled for the films deposited on the gilded glass and attached to the semicylinder prism. Namely the changes in the spectral shape, intensity and polarization of fluorescence were detected for certain resonant angle...
Herein, an efficient and stable fluorescent probe for Al3+ is established. The fluorescent probe bases on the fluorescent "turn on" mode of zinc sulfide crystal composite zinc oxide quantum dots (ZnS/ZnO QDs). The ZnS/ZnO QDs are synthesized via two-step method using L-Cysteine (L-Cys) as a sulfur source and stabilizer. In the synthesis of ZnS/ZnO...
A polyurethane (PU) is a multifunctional polymer prepared by using more than two types of monomers. The unique properties of PU come from monomers, thus broadening the applicability of PU in many different sectors. The properties can be further improved by using many nanoparticles. Different metal oxides as nanoparticles are also widely used in PU...
Citations
... Different kinds of manufacturing methods are used to produce these metamaterials including casting, additive manufacturing, [17][18][19][20][21] and electroforming of metallic layers. [19,22] Electroforming of these materials is a common way to produce a new generation of open-cell metal foams usually manufactured with a single layer of metallic shells including nickel, [23] copper, [24] iron, [25] magnesium, [26] etc. ...
... Different kinds of manufacturing methods are used to produce these metamaterials including casting, additive manufacturing, [17][18][19][20][21] and electroforming of metallic layers. [19,22] Electroforming of these materials is a common way to produce a new generation of open-cell metal foams usually manufactured with a single layer of metallic shells including nickel, [23] copper, [24] iron, [25] magnesium, [26] etc. Moreover, different kinds of experimental and computational methods used to investigate the micromechanical, [27][28][29] thermal, [30][31][32][33][34] acoustic, [35][36][37][38] and energy absorption properties of copper and nickel metal foams [39][40][41][42][43][44][45] are studied. ...
Metal foams are one of the unique materials that need more attention in terms of mechanical and microstructural characterizations. Herein, different kinds of metal foams such as Ni, Cu, and a novel type of multilayered metal foam with Ni–Cu and Cu–Ni coating layer orders are produced and characterized in terms of mechanical response and microstructure of these novel materials during a uniaxial compression test. These multilayered metallic foams have a strong mechanical adhesion at the interface due to the nature of electroforming process and the effect of solid‐solution area at these interfaces. Besides, multilayered metal foams are superior to Ni and Cu pure metal foams in terms of mechanical response; applying a multilayered metallic coat with 60 and 69 μm diameter for Ni and Cu improves the yield point of the Ni and Cu single‐layer metallic foams by 3 and 7 times, respectively. Moreover, in terms of energy absorption density, the multilayered metallic shell improves the energy absorption density by 3 and 5.5 times compared to Ni and Cu metal foams, respectively. This study shows that applying multi layered coatings to metal foams with Ni as the first layer has superior characteristics compared to single‐layer metal foams.
... This method is costly and displays other limitations, such as non-conformal growth on non-planar surfaces. Other restrictions include allowed morphologies and nanomaterial dimensions [91,92]. ...
Millions of people worldwide are affected by diabetes, a chronic disease that continuously grows due to abnormal glucose concentration levels present in the blood. Monitoring blood glucose concentrations is therefore an essential diabetes indicator to aid in the management of the disease. Enzymatic electrochemical glucose sensors presently account for the bulk of glucose sensors on the market. However, their disadvantages are that they are expensive and dependent on environmental conditions, hence affecting their performance and sensitivity. To meet the increasing demand, non-enzymatic glucose sensors based on chemically modified electrodes for the direct electrocatalytic oxidation of glucose are a good alternative to the costly enzymatic-based sensors currently on the market, and the research thereof continues to grow. Nanotechnology-based biosensors have been explored for their electronic and mechanical properties, resulting in enhanced biological signaling through the direct oxidation of glucose. Copper oxide and copper sulfide exhibit attractive attributes for sensor applications, due to their non-toxic nature, abundance, and unique properties. Thus, in this review, copper oxide and copper sulfide-based materials are evaluated based on their chemical structure, morphology, and fast electron mobility as suitable electrode materials for non-enzymatic glucose sensors. The review highlights the present challenges of non-enzymatic glucose sensors that have limited their deployment into the market.
... This method is costly and displays other limitations, such as non-conformal growth on non-planar surfaces. Other restrictions include allowed morphologies and nanomaterial dimensions [91,92]. ...
Millions of people worldwide are affected by diabetes, a chronic disease that continuously grows due to abnormal glucose concentration levels present in the blood. Monitoring blood glucose concentrations is therefore an essential diabetes indicator to aid in the management of the disease. Enzymatic electrochemical glucose sensors presently account for the bulk of glucose sensors in the market. However, their disadvantages are that they are expensive and are dependent on environmental conditions, hence affecting their performance and sensitivity. To meet the increasing demand, non-enzymatic glucose sensors based on chemically modified electrodes for the direct electrocatalytic oxidation of glucose are a good alternative to the costly enzymatic-based sensors currently on the market, and the research thereof continues to grow. Nanotechnology-based biosensors have been explored for their electronic and mechanical properties, resulting in enhanced biological signalling through the direct oxidation of glucose. Copper oxide and copper sulfide exhibit attractive attributes for sensor application, due to their non-toxic nature, abundance, and their unique properties. Thus, in this review, copper oxide and copper sulfide-based materials, are evaluated based on their chemical structure, morphology, and fast electron mobility as suitable electrode materials for non-enzymatic glucose sensors. The review highlights the present challenges of non-enzymatic glucose sensors that have limited their deployment into the market.
... Electrodeposition is a powerful technique and a convenient tool to create nanostructures by controlling the growth rate via altering the deposition potential [31]. One such surfactant and template-free route to create hierarchical dendritic gold microstructures (HDGMs) with secondary and tertiary branches have been used in the past using constant potential electrolysis at −0.6 V in 0.1 M Na 2 SO 4 and 30 mM HAuCl 4 serving as the electrolyte against indium tin oxide as the working electrode and platinum wire and saturated calomel electrode used as counter and reference electrode, respectively [32]. ...
The fundamental essence of material design towards creating functional materials lies in bringing together the competing aspects of a large specific surface area and rapid transport pathways. The generation of structural hierarchy on distinct and well-defined length scales has successfully solved many problems in porous materials. Important applications of these hierarchical materials in the fields of catalysis and electrochemistry are briefly discussed. This review summarizes the recent advances in the strategies to create a hierarchical bicontinuous morphology in porous metals, focusing mainly on the hierarchical architectures in nanoporous gold. Starting from the traditional dealloying method and subsequently moving to other non-traditional top-down and bottom-up manufacturing processes including templating, 3D printing, and electrodeposition, this review will thoroughly examine the chemistry of creating hierarchical nanoporous gold and other coinage metals. Finally, we conclude with a discussion about the future opportunities for the advancement in the methodologies to create bimodal structures with enhanced sensitivity.
... The electrochemical deposition process unlike most deposition techniques has the benefit of an enhanced interfacial bonding involving the material to be coated and the substrate prior to the application of heat. Though, it possesses the downside of producing non-conformal growths on uneven surfaces while using this technique [144]. Decorative coatings, energy storages, and separation techniques are appropriate for niobium oxide thin films produced from the electrochemical deposition process as against other techniques used for film production. ...
Niobium oxides (NbO, NbO2, Nb2O5), being a versatile material has achieved tremendous popularity to be used in a number of applications because of its outstanding electrical, mechanical, chemical, and magnetic properties. NbxOy films possess a direct band gap within the ranges of 3.2–4.0 eV, with these films having utility in different applications which include; optical systems, stainless steel, ceramics, solar cells, electrochromic devices, capacitor dielectrics, catalysts, sensors, and architectural requirements. With the purpose of fulfilling the requirements of a vast variety of the named applications, thin films having comprehensive properties span described by film composition, morphology, structural properties, and thickness are needed. The theory, alongside the research status of the different fabrication techniques of NbxOy thin films are reported in this work. The impact of fabrication procedures on the thin film characteristics which include; film thickness, surface quality, optical properties, interface properties, film growth, and crystal phase is explored with emphases on the distinct deposition process applied, are also described and discussed.
... 4,5 Holistically, electroless plating offers several distinct advantages over electrodeposition: it can coat nonconductive and semiconducting substrates, does not require external electrical energy, and can produce uniform deposits on acute morphologies where electrodeposition suffers from conformal buildup. 6,7 Among metals, nickel (Ni) has been broadly applied to metal surfaces for wear resistance, hardness enhancement, and corrosion protection, resulting in the development of several reliable plating recipes on conductive materials. [8][9][10][11] In particular, electroless Ni plating on semiconductors, such as silicon (Si), has garnered recent interest due to its potential applications spanning from microelectronics and solar cells to chemical catalysis. ...
Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory of the mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides more conformal Ni coating on complex and high-aspect ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.
... Considering two photocatalysts with the same 0.1 wt% Cu nominal content, the hydrogen production rate obtained with the FSP-made one is almost double (6.9 vs. 3.8 mmol h −1 g cat −1 ), with a halved selectivity to CO (6.1% vs. 10.8%). This might be a consequence of the formation of small NPs of crystalline copper oxides during the FSP synthesis, acting as semiconductors and thus forming a heterojunction with TiO 2 , which improves the separation of the photoproduced charge couples [36]. In Cu/TiO 2 photocatalysts produced by the grafting technique the oxidized copper species on the TiO 2 surface are expected to be in amorphous form, as grafting is carried out at room temperature and, thus, their action mechanism, consisting of switching between the Cu 2+ and Cu + oxidation states, may be different [12]. ...
The effect of Cu or Cu-Pt nanoparticles in TiO2 photocatalysts prepared by flame spray pyrolysis in one step was investigated in hydrogen production from methanol photo-steam reforming. Two series of titanium dioxide photocatalysts were prepared, containing either (i) Cu nanoparticles (0.05–0.5 wt%) or (ii) both Cu (0 to 0.5 wt%) and Pt (0.5 wt%) nanoparticles. In addition, three photocatalysts obtained either by grafting copper and/or by depositing platinum by wet methods on flame-made TiO2 were also investigated. High hydrogen production rates were attained with copper-containing photocatalysts, though their photoactivity decreased with increasing Cu loading, whereas the photocatalysts containing both Cu and Pt nanoparticles exhibit a bell-shaped photoactivity trend with increasing copper content, the highest hydrogen production rate being attained with the photocatalyst containing 0.05 wt% Cu.
... The measurements show that the use of a high transmittance substrate with a textured SiO 2 film results in a clear increase in the short-circuit current (J sc ) from 5.48 to 5.86 mA cm −2 . The J sc enhancement due to reduced reflection from the top surface of glass leads to increased light trapping and transmittance into the underlying cell [20]. The open-circuit voltage (V oc ) and fill factor are almost the same for all samples, at about 0.6 V and 61%, respectively. ...
The effects of a textured SiO2 antireflective layer with regard to the improved performance of an organic photovoltaic device are explored. The surface root mean square roughness of the textured SiO2 layer increases when 3 nm Au nanoparticles are deposited on it after texture patterning by a reactive ion etching process. The optical transmittance of the textured SiO2 layer is increased, thereby enhancing the short-circuit current density. The power conversion efficiency of an organic solar cell with a textured SiO2 layer is improved from 2.02 ± 0.02% to 2.13 ± 0.03%.
