Nanostructuring via electrodeposition

The small research group led by Dr L. Philippe at the Laboratory of Mechanics of Materials and Nanostruc-tures started its scientific activities at the interface of physics and chemistry in April 2008. In our research, we use electrochemical means to create well-ordered nanostructured materials, compact layers and microdevic-es, featuring exquisitely defined geometry, controlled surface chemistry, and tunable physical/mechanical properties.
We are interested in materials and structures whose properties can be tuned or optimized by variations in size, geometry, crystallinity, composition or surface. This “make and measure” strategy is applied primarily to problems related to mechanical devices, biomedical materials, culture heritage surfaces, renewable energies and magnetism.

Electrodeposition of metallic micro and nanostructures

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Optical image of stainless steel obtained by elec-trodeposition
Most electroformed components are nanocrystalline and are used as mechanical elements (e.g. gear, tools, etc). UV- and electron beam lithography in combination with electrodeposition of nanocrystalline Nickel is today the only established technique to synthesize metallic MEMS and watch parts. Our research focuses on NiCo, NiP alloys and ternary compounds of Fe-Ni-Cr (stainless steel) for applications in the watch and MEMS industry. In addition, compact layer of gold are developed with specific enhanced mechanical properties with ceramic nanoparticles co-deposition.

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Micro-tensile test specimen produced by elec-trodeposition into a UV-lithographic mold. The match head il-lustrates the size of the sample; the tensile bar has a width of 0.15 mm.
Electrodeposition into UV-lithographic molds (the so-called LIGA process) allow for the production of 2D and 2.5D micromechanical components with very good dimensional tolerances as they are used for example in the watch industry. Electrodeposition experiments can be conducted from the lab scale beaker level to prototypes of industrial sample size. The microstructural features of the deposited components are characterized by the in-house microanalysis capabilities. The mechanical properties of the developed material are evaluated by micromechanical testing in order to determine the hardness, tensile strength, elastic modulus, ductility, fatigue and creep properties.

Electrodeposition of metallic nanostructures

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STEM images of Co nan-owires and nanotubes ob-tained by controlled electro-chemical deposition conditions
For producing metallic (Co, Ni, Co, Pt, Au, etc.) nanowires and nanotubes with various functionalities (e.g. fer-romagnetic, Tip Enhanced Raman Scattering, etc.) we use electrochemical deposition process inside high as-pect ratio membrane templates (Anodic Alumina Oxide or polycarbonate). We control the main growth pa-rameters during the deposition in order to tailor crystal structure and morphology of the metallic nanostruc-tures. By using electrodeposition we are also able to synthesize multilayer, alloy and composites Moreover, their structural, electrical and mechanical properties were studied in correlation with the synthesis conditions employed.

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SEM images of flail Au nanostructures obtained by controlled electrochemical deposition conditions

Free-template electrodeposition is used for the synthesis of metallic nano/microstructures on solid substrates in order to facilitate their practical applications as nanobuilding blocks in nanodevices. For tailoring the nanostructures and the growth mechanism, organic molecules can be added to the electrochemical bath. The use of these metallic nano/microstructures in many applications (SERS, gas sensing, wettability, etc) is studied.

Mechanical Properties of electrodeposited structures

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Tensile testing of electrode-posited Au nanowires: Mi-cromanipulation enables po-sitioning of individual nanowires, which are fixated on MEMS devices by local deposition inside the SEM.

Mechanical properties of electrodeposited microstructures and nanostructures are investigated though micro tensile, microcompression and nanoindentation tests. Microdevices, shapes and intrinsic structures are tai-lored during the electrodeposition process. Investigation of mechanical properties of electrodeposited nan-owire is fundamental for the complete correlation between synthesis parameters/nanostructure and physical properties (see above). Combination of experimental tests and theoretical modeling allows determining in-trinsic material parameters of nanowires.

Electrodeposition of metallic microstructures

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TEM micrograph of electro-deposited nickel

UV-LIGA process for patterning of thick photoresist masks, the development of new alloys for microdevices by means of electrodeposition, having specific and well-controllable properties is the aim of the project. Development of galvanic processes for new and enhanced alloys interesting for microdevices such as NiFe, NiCo and NiFeCr is investigated. We developed a novel plating propeller cell inducing a thin, uniform and non-periodic boundary layer across the entire wafer surface. The temporal uniformity of the diffusion layer thickness is advantageous in the deposition of anomalous alloy pairs because of the compositional dependence on current density.

Combined electrochemical and mechanical nanopatterning of semiconductor surfaces

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Distribution of maximum shear stress during nanoscratching

We investigate selective dissolution of n-InP(1 0 0) triggered by surface defects. Nanometer size grooves were produced by sensitizing an InP surface with a mechanical contact patterning technique, i.e. nanoscratching at low loading forces at the threshold between elastic and plastic contacts. Subsequently, site-selective electrochemical dissolution was triggered in acidic electrolytes. Electrochemical characterization shows that there is an onset voltage for selective dissolution at the scratch-induced defects and that it is possible to define a processing potential window where dissolution occurs only inside the nanoscratches. Scanning electron microscopy (SEM) micrographs reveal that for optimized forces, highly confined dissolution in well-defined channels of 125 nm width can be obtained. Transmission electron microscopy (TEM) cross sectional analysis of the scratches before and after electrochemical experiments shows that the dislocations generated are confined in a well-defined volume below the surface, and that exactly this dislocated volume is dissoluted. Based on contact mechanics, the lateral resolution and the morphology of the dissolved groove can be explained by the dissolution time and the tip radius and the morphology by the applied load.

Prediction and Optimization of Electrodeposition Parameters and/or Mechanical Properties of Deposits using FEM tools

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Surface mismatch and Distri-bution of Current Density in Bi-Layer electro deposition coating

From one side, we develop optimization of the deposition processes by current density modeling during plating process and depending on the bath composition and substrate shape.
Internal stresses of the deposits depending of the synthesis parameters are then tested in order to produce a FEM modeling of the stress distribution depending on shape and synthesis parameters of the deposits.
A theoretical FEM model for electrodeposition is developed. With this model, we predict and optimize the parameters for electrodeposition of nanostructured materials or alloys. This is made, in order to improve the composition of the electrodeposited materials, and therefore, their mechanical properties.
These optimized parameters are compared with experiments, and then used to improve the bath composition and current density applied during the electroplating process, allowing us to obtain the desired mechanical properties, composition and patterning of the deposited material.