Oxidation of Au/Ag films by oxygen plasma: phase separation and generation of nanoporosity.

The oxidation of Au/Ag alloy thin films using radio-frequency oxygen plasma was studied in this work. It was demonstrated that there is a phase separation occurring between silver and gold. In addition, it was shown that the preferential oxidation of silver resulted in a solid-state diffusion of silver toward the surface where it oxidized and formed nanoporous microspheres. The gold phase remaining in the film exhibited nanoporosity due to the injected vacancies at the metal/silver oxide interface. Based on the scanning transmission electron microscopy analysis coupled with energy dispersive X-ray mapping a mechanism was proposed based on solid-state diffusion and the Kirkendall effect to explain the different steps occurring during the oxidation process.

Co-sputtering of gold and copper onto liquids: a route towards the production of porous gold nanoparticles.

Effective methods for the synthesis of high-purity nanoparticles (NPs) have been extensively studied for a few decades. Among others, cold plasma-based sputtering metals onto a liquid substrate appears to be a very promising technique for the synthesis of high-purity NPs. The process enables the production of very small NPs without using any toxic reagents and complex chemical synthesis routes, and enables the synthesis of alloy NPs which can be the first step towards the formation of porous NPs. In this paper, the synthesis of gold-copper alloy NPs has been performed by co-sputtering gold and copper targets over pentaerythritol ethoxylate. The resulting solutions contain a mixture of gold, copper oxide, and alloy NPs having a radius of few angstroms. The annealing of these NPs, inside the solution, has been performed in order to increase their size and further induce the dealloying of the Au-Cu NPs. The resulting NPs exhibit either a nanoporous structure or are self-organized in an agglomerate of small NPs.

Near interface ionic transport in oxygen vacancy stabilized cubic zirconium oxide thin films.

The cubic phase of pure zirconia (ZrO2) is stabilized in dense thin films through a controlled introduction of oxygen vacancies (O defects) by cold-plasma-based sputtering deposition. Here, we show that the cubic crystals present at the film/substrate interface near-region exhibit fast ionic transport, which is superior to what is obtained with similar yttrium-stabilized cubic zirconia thin films.

Dealloying of gold-copper alloy nanowires: From hillocks to ring-shaped nanopores.

We report on a novel fabrication approach of metal nanowires with complex surface. Taking advantage of nodular growth triggered by the presence of surface defects created intentionally on the substrate as well as the high tilt angle between the magnetron source axis and the normal to the substrate, metal nanowires containing hillocks emerging out of the surface can be created. The approach is demonstrated for several metals and alloys including gold, copper, silver, gold-copper and gold-silver. We demonstrate that applying an electrochemical dealloying process to the gold-copper alloy nanowire arrays allows for transforming the hillocks into ring-like shaped nanopores. The resulting porous gold nanowires exhibit a very high roughness and high specific surface making of them a promising candidate for the development of SERS-based sensors.

Direct nanopatterning of polymer/silver nanoblocks under low energy electron beam irradiation.

In this communication, we report on the growth, direct writing and nanopatterning of polymer/silver nanoblocks under low energy electron beam irradiation using a scanning electron microscope. The nanoblocks are produced by placing a droplet of an ethylene glycol solution containing silver nitrate and polyvinylpyrrolidone diluted in ethanol directly on a hot substrate heated up to 150 °C. Upon complete evaporation of the droplet, nanospheres, nano- and micro-triangles and nanoblocks made of silver-containing polymers, form over the substrate surface. Considering the nanoblocks as a model system, we demonstrate that such nanostructures are extremely sensitive to the e-beam extracted from the source of a scanning electron microscope operating at low acceleration voltages (between 5 and 7 kV). This sensitivity allows us to efficiently create various nanopatterns (e.g. arrays of holes, oblique slits and nanotrenches) in the material under e-beam irradiation. In addition to the possibility of writing, the nanoblocks revealed a self-healing ability allowing them to recover a relatively smooth surface after etching. Thanks to these properties, such nanomaterials can be used as a support for data writing and erasing on the nanoscale under low energy electron beam irradiation.

In situ conversion of nanostructures from solid to hollow in transmission electron microscopes using electron beam.

With the current development of electron beam sources, the use of transmission electron microscopes is no more limited to imaging or chemical analysis but has rather been extended to nanoengineering. This includes the e-beam induced growth, etching and structural transformation of nanomaterials. In this review we summarize recent progress on the e-beam induced morphological transformation of nanostructures from solid to hollow. We provide a detailed account of the processes reported so far in the literature with a special emphasis on the mechanistic understanding of the e-beam induced hollowing of nanomaterials. Through an important number of examples, we discuss how one can achieve a precise control of such hollowing processes by understanding the fundamental mechanisms occurring at the atomic scale during the irradiation of solid nanostructures. Finally, we conclude with remarks and our own view on the prospective future directions of this research field.

Planar Arrays of Nanoporous Gold Nanowires: When Electrochemical Dealloying Meets Nanopatterning.

Nanoporous materials are of great interest for various technological applications including sensors based on surface-enhanced Raman scattering, catalysis, and biotechnology. Currently, tremendous efforts are dedicated to the development of porous one-dimensional materials to improve the properties of such class of materials. The main drawback of the synthesis approaches reported so far includes (i) the short length of the porous nanowires, which cannot reach the macroscopic scale, and (ii) the poor organization of the nanostructures obtained by the end of the synthesis process. In this work, we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with a length up to a few centimeters. This two-step approach consists of the growth of gold/copper alloy nanowires by magnetron cosputtering on a nanograted silicon substrate, serving as a physical template, followed by a selective dissolution of copper by an electrochemical anodic process in diluted sulfuric acid. We demonstrate that the pore size of the nanowires can be tailored between 6 and 21 nm by tuning the dealloying voltage between 0.2 and 0.4 V and the dealloying time within the range of 150-600 s. We further show that the initial gold content (11 to 26 atom %) and the diameter of the gold/copper alloy nanowires (135 to 250 nm) are two important parameters that must carefully be selected to precisely control the porosity of the material.

Creating nanoporosity in silver nanocolumns by direct exposure to radio-frequency air plasma.

Nanoporous materials are of great importance for a broad range of applications including catalysis, optical sensors and water filtration. Although several approaches already exist for the creation of nanoporous materials, the race for the development of versatile methods, more suitable for the nanoelectronics industry, is still ongoing. In this communication we report for the first time on the possibility of generating nanoporosity in silver nanocolumns using a dry approach based on the oxidation of silver by direct exposure to a commercially available radio-frequency air plasma. The silver nanocolumns are created by glancing angle deposition using magnetron sputtering of a silver target in pure argon plasma. We show that upon exposure to the rf air plasma, the nanocolumns transform from solid silver into nanoporous silver oxide. We further show that by tuning the plasma pressure and the exposure duration, the oxidation process can be finely adjusted allowing for precisely controlling the morphology and the nanoporosity of the silver oxide nanocolumns. The generation of porosity within the silver nanocolumns is explained according to a cracking-induced oxidation mechanism based on two repeated events occurring alternately during the oxidation process: (i) oxidation of silver upon exposure to the air plasma and (ii) generation of nanocracks and blisters within the oxide layer due to the high internal stress generated within the material during oxidation.

The Kirkendall effect and nanoscience: hollow nanospheres and nanotubes.

Hollow nanostructures are ranked among the top materials for applications in various modern technological areas including energy storage devices, catalyst, optics and sensors. The last years have witnessed increasing interest in the Kirkendall effect as a versatile route to fabricate hollow nanostructures with different shapes, compositions and functionalities. Although the conversion chemistry of nanostructures from solid to hollow has reached a very advanced maturity, there is still much to be discovered and learned on this effect. Here, the recent progress on the use of the Kirkendall effect to synthesize hollow nanospheres and nanotubes is reviewed with a special emphasis on the fundamental mechanisms occurring during such a conversion process. The discussion includes the oxidation of metal nanostructures (i.e., nanospheres and nanowires), which is an important process involving the Kirkendall effect. For nanospheres, the symmetrical and the asymmetrical mechanisms are both reviewed and compared on the basis of recent reports in the literature. For nanotubes, in addition to a summary of the conversion processes, the unusual effects observed in some particular cases (e.g., formation of segmented or bamboo-like nanotubes) are summarized and discussed. Finally, we conclude with a summary, where the prospective future direction of this research field is discussed.

KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells.

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

Unusual dealloying effect in gold/copper alloy thin films: the role of defects and column boundaries in the formation of nanoporous gold.

Understanding the dealloying mechanisms of gold-based alloy thin films resulting in the formation of nanoporous gold with a sponge-like structure is essential for the future design and integration of this novel class of material in practical devices. Here we report on the synthesis of nanoporous gold thin films using a free-corrosion approach in nitric acid applied to cosputtered Au-Cu thin films. A relationship is established between the as-grown Au-Cu film characteristics (i.e., composition, morphology, and structure) and the porosity of the sponge-like gold thin films. We further demonstrate that the dealloying approach can be applied to nonhomogenous Au-Cu alloy thin films consisting of periodic and alternate Au-rich/Au-poor nanolayers. In such a case, however, the dealloying process is found to be altered and unusual etching stages arise. Thanks to defects and column boundaries playing the role of channels, the nitric acid is found to quickly penetrate within the films and then laterally (i.e., parallel to the film surface) attacks the nanolayers rather than perpendicularly. As a consequence to this anisotropic etching, the Au-poor layers are etched preferentially and transform into Au pillars holding the Au-rich layers and preventing them against collapsing. A further exposure to nitric acid results in the collapsing of the Au-rich layers accompanied by a transition from a multilayered to a sponge-like structure. A scenario, supported by experimental observations, is further proposed to provide a detailed explanation of the fundamental mechanisms occurring during the dealloying process of films with a multilayered structure.

Electron beam nanosculpting of Kirkendall oxide nanochannels.

The nanomanipulation of metal nanoparticles inside oxide nanotubes, synthesized by means of the Kirkendall effect, is demonstrated. In this strategy, a focused electron beam, extracted from a transmission electron microscope source, is used to site-selectively heat the oxide material in order to generate and steer a metal ion diffusion flux inside the nanochannels. The metal ion flux generated inside the tube is a consequence of the reduction of the oxide phase occurring upon exposure to the e-beam. We further show that the directional migration of the metal ions inside the nanotubes can be achieved by locally tuning the chemistry and the morphology of the channel at the nanoscale. This allows sculpting organized metal nanoparticles inside the nanotubes with various sizes, shapes, and periodicities. This nanomanipulation technique is very promising since it enables creating unique nanostructures that, at present, cannot be produced by an alternative classical synthesis route.

Highly ordered hollow oxide nanostructures: the Kirkendall effect at the nanoscale.

Highly ordered ultra-long oxide nanotubes are fabricated by a simple two-step strategy involving the growth of copper nanowires on nanopatterned template substrates by magnetron sputtering, followed by thermal annealing in air. The formation of such tubular nanostructures is explained according to the nanoscale Kirkendall effect. The concept of this new fabrication route is also extendable to create periodic zero-dimensional hollow nanostructures.

Highly ordered ultralong magnetic nanowires wrapped in stacked graphene layers.

We report on the synthesis and magnetic characterization of ultralong (1 cm) arrays of highly ordered coaxial nanowires with nickel cores and graphene stacking shells (also known as metal-filled carbon nanotubes). Carbon-containing nickel nanowires are first grown on a nanograted surface by magnetron sputtering. Then, a post-annealing treatment favors the metal-catalyzed crystallization of carbon into stacked graphene layers rolled around the nickel cores. The observed uniaxial magnetic anisotropy field oriented along the nanowire axis is an indication that the shape anisotropy dominates the dipolar coupling between the wires. We further show that the thermal treatment induces a decrease in the coercivity of the nanowire arrays. This reflects an enhancement of the quality of the nickel nanowires after annealing attributed to a decrease of the roughness of the nickel surface and to a reduction of the defect density. This new type of graphene-ferromagnetic-metal nanowire appears to be an interesting building block for spintronic applications.