Versatile, rapid and robust nano-positioning of single-photon emitters by AFM-nanoxerography.

Atomic force microscopy (AFM) nanoxerography was successfully used to direct the assembly of colloidal nanodiamonds (NDs) containing nitrogen-vacancy (NV) centres on electrostatically patterned surfaces. This study reveals that the number of deposited NDs can be controlled by tuning the surface potentials of positively charged dots on a negatively charged background written by AFM in a thin PMMA electret film, yielding assemblies down to a unique single-photon emitter with very good selectivity. The mechanisms of the ND directed assembly are attested by numerical simulations. This robust deterministic nano-positioning of quantum emitters thus offers great opportunities for ultimate applications in nanophotonics for quantum technologies.

High-throughput fabrication of anti-counterfeiting colloid-based photoluminescent microtags using electrical nanoimprint lithography.

This work demonstrates the excellent capability of the recently developed electrical nanoimprint lithography (e-NIL) technique for quick, high-throughput production of well-defined colloid assemblies on surfaces. This is shown by fabricating micron-sized photoluminescent quick response (QR) codes based on the electrostatic directed trapping (so called nanoxerography process) of 28 nm colloidal lanthanide-doped upconverting NaYF4 nanocrystals. Influencing experimental parameters have been optimized and the contribution of triboelectrification in e-NIL was evidenced. Under the chosen conditions, more than 300 000 nanocrystal-based QR codes were fabricated on a 4 inch silicon wafer, in less than 15 min. These microtags were then transferred to transparent flexible films, to be easily integrated onto desired products. Invisible to the naked eye, they can be decoded and authenticated using an optical microscopy image of their specific photoluminescence mapping. Beyond this very promising application for product tracking and the anti-counterfeiting strategies, e-NIL nanoxerography, potentially applicable to any types of charged and/or polarizable colloids and pattern geometries opens up tremendous opportunities for industrial scale production of various other kinds of colloid-based devices and sensors.

Electrical nano-imprint lithography.

We present a novel technique called electrical nano-imprint lithography (e-NIL) for topographic and electrostatic patterning of thermoplastic electret films at the nanometer scale. This versatile parallel process consists of simultaneously transferring micro- or nano-patterns from a conductive mold into a thermoplastic electret film and injecting positive or negative electrical charges into the bottom of the imprinted patterns. As proof of concept, we used this e-NIL process to fabricate arrays of 5 µm and 300 nm wide topographic charged patterns into polymethylmethacrylate (PMMA) thin films coated on silicon wafers. We demonstrated that these patterned PMMA films, exhibiting thousands of topographically confined and electrostatically active sites, can be used for high-throughput directed assembly of colloidal nanoparticles.

Quantification of the electrostatic forces involved in the directed assembly of colloidal nanoparticles by AFM nanoxerography.

Directed assembly of 10 nm dodecanethiol stabilized silver nanoparticles in hexane and 14 nm citrate stabilized gold nanoparticles in ethanol was performed by AFM nanoxerography onto charge patterns of both polarities written into poly(methylmethacrylate) thin films. The quasi-neutral silver nanoparticles were grafted on both positive and negative charge patterns while the negatively charged gold nanoparticles were selectively deposited on positive charge patterns only. Numerical simulations were conducted to quantify the magnitude, direction and spatial range of the electrophoretic and dielectrophoretic forces exerted by the charge patterns on these two types of nanoparticles in suspension taken as models. The simulations indicate that the directed assembly of silver nanoparticles on both charge patterns is due to the predominant dielectrophoretic forces, while the selective assembly of gold nanoparticles only on positive charge patterns is due to the predominant electrophoretic forces. The study also suggests that the minimum surface potential of charge patterns required for obtaining effective nanoparticle assembly depends strongly on the charge and polarizability of the nanoparticles and also on the nature of the dispersing solvent. Attractive electrostatic forces of about 2 × 10( - 2) pN in magnitude just above the charged surface appear to be sufficient to trap silver nanoparticles in hexane onto charge patterns and the value is about 2 pN for gold nanoparticles in ethanol, under the present experimental conditions. The numerical simulations used in this work to quantify the electrostatic forces operating in the directed assembly of nanoparticles from suspensions onto charge patterns can easily be extended to any kind of colloid and serve as an effective tool for a better comprehension and prediction of liquid-phase nanoxerography processes.

Numerical simulations for a quantitative analysis of AFM electrostatic nanopatterning on PMMA by Kelvin force microscopy.

Electrostatic nanopatterning of electret thin films by atomic force microscopy (AFM) has emerged as an alternative efficient tool for the directed assembly of nano-objects on surfaces. High-resolution charge imaging of such charge patterns can be performed by AFM-based Kelvin force microscopy (KFM). Nevertheless, quantitative analysis of KFM surface potential mappings is not trivial because of side-capacitance effects induced by the tip cone and the cantilever of the scanning probe. In this paper, we developed numerical simulations of KFM measurements taking into account these artifacts, so as to estimate the actual surface charge density of square charge patterns (nominal sizes ranging from 100 nm to 10 microm) written by AFM into polymethylmethacrylate (PMMA) thin films. This work revealed that, under our conditions, such charge patterns exhibit a surface charge density between 1.5 x 10(-3) and 3.8 x 10(-3) C m(-2), depending on the assumed depth of injected charges. These results are crucial to quantify the actual electric field generated by such charge patterns and thus the electrostatic forces responsible for the directed assembly of nano-objects onto these electrostatic traps.

Combining convective/capillary deposition and AFM oxidation lithography for close-packed directed assembly of colloids.

We combine convective/capillary deposition and oxidation lithography by atomic force microscopy to direct the close-packed assembly of colloids on SiOx patterns fabricated on silicon substrates previously functionalized with a hydrophobic monolayer of octadecyltrimethoxysilane. The efficiency of this original generic method, which is well adapted to integrate colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. A three-step mechanism involving convective flow and capillary forces appears to be responsible for these close-packed assemblies of colloids onto SiOx patterns.