Targeted Additive Micromodulation of Grain Size in Nanocrystalline Copper Nanostructures by Electrohydrodynamic Redox 3D Printing.

The control of materials' microstructure is both a necessity and an opportunity for micro/nanometer-scale additive manufacturing technologies. On the one hand, optimization of purity and defect density of printed metals is a prerequisite for their application in microfabrication. On the other hand, the additive approach to materials deposition with highest spatial resolution offers unique opportunities for the fabrication of materials with complex, 3D graded composition or microstructure. As a first step toward both-optimization of properties and site-specific tuning of microstructure-an overview of the wide range of microstructure accessed in pure copper (up to >99.9 at.%) by electrohydrodynamic redox 3D printing is presented, and on-the-fly modulation of grain size in copper with smallest segments ≈400 nm in length is shown. Control of microstructure and materials properties by in situ adjustment of the printing voltage is demonstrated by variation of grain size by one order of magnitude and corresponding compression strength by a factor of two. Based on transmission electron microscopy and atom probe tomography, it is suggested that the small grain size is a direct consequence of intermittent solvent drying at the growth interface at low printing voltages, while larger grains are enabled by the permanent presence of solvent at higher potentials.

Metals by Micro-Scale Additive Manufacturing: Comparison of Microstructure and Mechanical Properties.

Many emerging applications in microscale engineering rely on the fabrication of 3D architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide flexible and facile access to these geometries. Yet, the synthesis of device-grade inorganic materials is still a key challenge toward the implementation of AM in microfabrication. Here, a comprehensive overview of the microstructural and mechanical properties of metals fabricated by most state-of-the-art AM methods that offer a spatial resolution ≤10 µm is presented. Standardized sets of samples are studied by cross-sectional electron microscopy, nanoindentation, and microcompression. It is shown that current microscale AM techniques synthesize metals with a wide range of microstructures and elastic and plastic properties, including materials of dense and crystalline microstructure with excellent mechanical properties that compare well to those of thin-film nanocrystalline materials. The large variation in materials' performance can be related to the individual microstructure, which in turn is coupled to the various physico-chemical principles exploited by the different printing methods. The study provides practical guidelines for users of small-scale additive methods and establishes a baseline for the future optimization of the properties of printed metallic objects-a significant step toward the potential establishment of AM techniques in microfabrication.

Polymer nanocomposite patterning by dip-pen nanolithography.

The ultimate aim of this study is to construct polymer nanocomposite patterns by dip-pen nanolithography (DPN). Recent investigations have revealed the effect of the amount of ink (Laplace pressure) on the mechanism of liquid ink writing. In this study it is shown that not only the amount of ink, but also physisorption and surface diffusion are relevant. After a few writing steps, physisorption and surface diffusion outweigh the influence of the amount of ink, allowing consistent patterning governed by dwell times and writing speeds. Polymer matrices can be utilized as a delivery medium to deposit functional particles. DPN patterning of polymer nanocomposites allows for local tuning of the functionality and mechanical strength of the written patterns in high resolution, with the benefit of pattern flexibility. Typically polymer matrices with volatile components are used as a delivery medium for nanoparticle deposition, with subsequent removal of loosely bound matrix material by heating or oxygen plasma. In our study, nanocomposite patterns were constructed, and the differences between polymer and nanocomposite patterning were investigated. Cross-sectional SEM and TEM analysis confirmed that nanoparticles can be deposited with the liquid-polymer ink and are evenly distributed in the polymer matrix.

Direct In- and Out-of-Plane Writing of Metals on Insulators by Electron-Beam-Enabled, Confined Electrodeposition with Submicrometer Feature Size.

Additive microfabrication processes based on localized electroplating enable the one-step deposition of micro-scale metal structures with outstanding performance, e.g., high electrical conductivity and mechanical strength. They are therefore evaluated as an exciting and enabling addition to the existing repertoire of microfabrication technologies. Yet, electrochemical processes are generally restricted to conductive or semiconductive substrates, precluding their application in the manufacturing of functional electric devices where direct deposition onto insulators is often required. Here, the direct, localized electrodeposition of copper on a variety of insulating substrates, namely Al2 O3 , glass and flexible polyethylene, is demonstrated, enabled by electron-beam-induced reduction in a highly confined liquid electrolyte reservoir. The nanometer-size of the electrolyte reservoir, fed by electrohydrodynamic ejection, enables a minimal feature size on the order of 200 nm. The fact that the transient reservoir is established and stabilized by electrohydrodynamic ejection rather than specialized liquid cells can offer greater flexibility toward deposition on arbitrary substrate geometries and materials. Installed in a low-vacuum scanning electron microscope, the setup further allows for operando, nanoscale observation and analysis of the manufacturing process.

Microparticle Impact Testing at High Precision, Higher Temperatures, and with Lithographically Patterned Projectiles.

In the first decade of high-velocity microparticle impact research, hardly any modification of the original experimental setup has been necessary. However, future avenues for the field require advancements of the experimental method to expand both the impact variables that can be quantitatively assessed and the materials and phenomena that can be studied. This work explores new design concepts for the launch pad (the assembly that launches microparticles upon laser ablation) that can address the root causes of many experimental challenges that may limit the technique in the future. Among the design changes contemplated, the substitution of a stiff glass launch layer for the standard elastomeric polymer layer offers a number of improvements. First, it facilitates a reduction of the gap between launch pad and target from hundreds to tens of micrometers and thus unlocks a reproducibility in targeting a specific impact location better than the diameter of the test particle itself (±1.75 µm for SiO2 particles 7.38 µm in diameter). Second, the inert glass surface enables experiments at higher temperatures than previously possible. Finally-as demonstrated by the launch of thin-film Au disks-a launch pad made of materials standard in microfabrication paves the way to facile microfabrication of advanced impactors.

Confined pulsed diffuse layer charging for nanoscale electrodeposition with an STM.

Regulating the state of the solid-liquid interface by means of electric fields is a powerful tool to control electrochemistry. In scanning probe systems, this can be confined closely to a scanning (nano)electrode by means of fast potential pulses, providing a way to probe the interface and control electrochemical reactions locally, as has been demonstrated in nanoscale electrochemical etching. For this purpose, it is important to know the spatial extent of the interaction between pulses applied to the tip, and the substrate. In this paper we use a framework of diffuse layer charging to describe the localization of electrical double layer charging in response to a potential pulse at the probe. Our findings are in good agreement with literature values obtained in electrochemical etching. We show that the pulse can be much more localized by limiting the diffusivity of the ions present in solution, by confined electrodeposition of cobalt in a dimethyl sulfoxide solution, using an electrochemical scanning tunnelling microscope. Finally, we demonstrate the deposition of cobalt nanostructures (<100 nm) using this method. The presented framework therefore provides a general route for predicting and controlling the time-dependent region of interaction between an electrochemical scanning probe and the surface.

Nanoscale electrochemical 3D deposition of cobalt with nanosecond voltage pulses in an STM.

To explore a minimal feature size of <100 nm with electrochemical additive manufacturing, we use a strategy originally applied to microscale electrochemical machining for the nanoscale deposition of Co on Au. The concept's essence is the localization of electrochemical reactions below a probe during polarization with ns-long voltage pulses. As shown, a confinement that exceeds that predicted by a simple model based on the time constant for one-dimensional double layer charging enables a feature size of <100 nm for 2D patterning. We further indirectly verify the potential for out-of-plane deposition by tracking growth curves of high-aspect-ratio deposits. Importantly, we report a lack of anodic stability of Au tips used for patterning. As an inert probe is the prerequisite for controlled structuring, we experimentally verify an increased resistance of Pt probes against degradation. Consequently, the developed setup and processes show a path towards reproducible direct 2D and 3D patterning of metals at the nanoscale.

Additive manufacturing of Zn with submicron resolution and its conversion into Zn/ZnO core-shell structures.

Electrohydrodynamic redox 3D printing (EHD-RP) is an additive manufacturing (AM) technique with submicron resolution and multi-metal capabilities, offering the possibility to switch chemistry during deposition "on-the-fly". Despite the potential for synthesizing a large range of metals by electrochemical small-scale AM techniques, to date, only Cu and Ag have been reproducibly deposited by EHD-RP. Here, we extend the materials palette available to EHD-RP by using aqueous solvents instead of organic solvents, as used previously. We demonstrate deposition of Cu and Zn from sacrificial anodes immersed in acidic aqueous solvents. Mass spectrometry indicates that the choice of the solvent is important to the deposition of pure Zn. Additionally, we show that the deposited Zn structures, 250 nm in width, can be partially converted into semiconducting ZnO structures by oxidation at 325 °C in air.

3D electrohydrodynamic printing and characterisation of highly conductive gold nanowalls.

3D printing research targets the creation of nanostructures beyond the limits of traditional micromachining. A proper characterisation of their functionalities is necessary to facilitate future implementation into applications. We fabricate, in an open atmosphere, high-aspect-ratio gold nanowalls by electrohydrodynamic rapid nanodripping, and comprehensively analyse their electronic performance by four-point probe measurements. We reveal the large-grained nanowall morphology by transmission electron microscopy and explain the measured low resistivities approaching those of bulk gold. This work is a significant advancement in contactless bottom-up 3D nanofabrication and characterisation and could also serve as a platform for fundamental studies of additively manufactured high-aspect-ratio out-of-plane metallic nanostructures.

Multi-metal electrohydrodynamic redox 3D printing at the submicron scale.

An extensive range of metals can be dissolved and re-deposited in liquid solvents using electrochemistry. We harness this concept for additive manufacturing, demonstrating the focused electrohydrodynamic ejection of metal ions dissolved from sacrificial anodes and their subsequent reduction to elemental metals on the substrate. This technique, termed electrohydrodynamic redox printing (EHD-RP), enables the direct, ink-free fabrication of polycrystalline multi-metal 3D structures without the need for post-print processing. On-the-fly switching and mixing of two metals printed from a single multichannel nozzle facilitates a chemical feature size of <400 nm with a spatial resolution of 250 nm at printing speeds of up to 10 voxels per second. As shown, the additive control of the chemical architecture of materials provided by EHD-RP unlocks the synthesis of 3D bi-metal structures with programmed local properties and opens new avenues for the direct fabrication of chemically architected materials and devices.

Additive Manufacturing of Metal Structures at the Micrometer Scale.

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro-stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser-assisted electrophoretic deposition, laser-induced forward transfer, local electroplating methods, laser-induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1-10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.

Template-Free 3D Microprinting of Metals Using a Force-Controlled Nanopipette for Layer-by-Layer Electrodeposition.

A novel 3D printing method for voxel-by-voxel metal printing is presented. Hollow atomic force microscopy (AFM) cantilevers are used to locally supply metal ions in an electrochemical cell, enabling a localized electroplating reaction. By exploiting the deflection feedback of these probes, electrochemical 3D metal printing is, for the first time, demonstrated in a layer-by-layer fashion, enabling the fabrication of arbitrary-shaped geometries.

Stretchable heterogeneous composites with extreme mechanical gradients.

Heterogeneous composite materials with variable local stiffness are widespread in nature, but are far less explored in engineering structural applications. The development of heterogeneous synthetic composites with locally tuned elastic properties would allow us to extend the lifetime of functional devices with mechanically incompatible interfaces, and to create new enabling materials for applications ranging from flexible electronics to regenerative medicine. Here we show that heterogeneous composites with local elastic moduli tunable over five orders of magnitude can be prepared through the site-specific reinforcement of an entangled elastomeric matrix at progressively larger length scales. Using such a hierarchical reinforcement approach, we designed and produced composites exhibiting regions with extreme soft-to-hard transitions, while still being reversibly stretchable up to 350%. The implementation of the proposed methodology in a mechanically challenging application is illustrated here with the development of locally stiff and globally stretchable substrates for flexible electronics.