Large-scale controlled coupling of single-photon emitters to high-index dielectric nanoantennas by AFM nanoxerography.

Improving the brightness of single-photon sources by means of optically resonant nanoantennas is a major stake for the development of efficient nanodevices for quantum communications. We demonstrate that nanoxerography by atomic force microscopy makes possible the fast, robust and repeatable positioning of model quantum nanoemitters (nitrogen-vacancy NV centers in nanodiamonds) on a large-scale in the gap of silicon nanoantennas with a dimer geometry. By tuning the parameters of the nanoxerography process, we can statistically control the number of deposited nanodiamonds, yielding configurations down to a unique single photon emitter coupled to these high index dielectric nanoantennas, with high selectivity and enhanced brightness induced by a near-field Purcell effect. Numerical simulations are in very good quantitative agreement with time-resolved photoluminescence experiments. A multipolar analysis reveals in particular all the aspects of the coupling between the dipolar single emitter and the Mie resonances hosted by these simple nanoantennas. This proof of principle opens a path to a genuine and large-scale spatial control of the coupling of punctual quantum nanoemitters to arrays of optimized optically resonant nanoantennas. It paves the way for future fundamental studies in quantum nano-optics and toward integrated photonics applications for quantum technologies.

On the in situ 3D electrostatic directed assembly of CdSe/CdZnS colloidal quantum nanoplatelets towards display applications.

HYPOTHESIS: Due to their unique quantum yield and photostability performances, quantum nanoplatelets are very promising building blocks for future generations of displays. The directed assembly of such colloidal nano-objects in the shape of micro-pixels is thus the next mandatory step to reach this goal. Selectively trapping them on electrostatically charged patterns by nanoxerography could be a versatile and appealing strategy but requires a full understanding of the assembly mechanisms in order to make the most of their integration. EXPERIMENTS: We propose an experimental platform based on a smart resealable microfluidic chip coupled to an inverted optical fluorescence microscope and a high-speed camera for in situ access of such assembly mechanisms, using CdSe/CdZnS quantum nanoplatelets as model nano-objects. The photoluminescence signal of the nanoplatelet patterns is thus recorded in real time during their assembly and data extracted after image processing. FINDINGS: The coupling of experimental results and numerical simulations evidences the main role of advection at the origin of this directed nanoparticle trapping. Deep understanding of the involved mechanisms and tuning of experimental parameters allow to make high resolution quantum nanoplatelet based micro-pixels with a fine control of their lateral and vertical dimensions.

Synthesis of hybrid colloidal nanoparticles for a generic approach to 3D electrostatic directed assembly: Application to anti-counterfeiting.

HYPOTHESIS: The capability of making 3D directed assembly of colloidal nanoparticles on surfaces, instead of 2D one, is of major interest to generate, tailor, and enhance their original functionalities. The nanoxerography technique, i.e. electrostatic trapping of nanoparticles on charged patterns, showed such 3D assembly potentialities but is presently restricted to polarizable nanoparticles with a diameter superior to 20 nm. Hence, it should be possible to exploit a generic approach based on hybrid systems using larger nanoparticles as cargos to anchor smaller ones. EXPERIMENTS: A synthesis of hybrid nanoparticles in a raspberry-like configuration was performed using 50 nm SiO2 nanoparticles and photoluminescent 3-5 nm InP@ZnS (visible emission) or PbS (infrared emission) nanoparticles. Complete topographical and photoluminescent characterizations were carried out on hybrid nanoparticle patterns assembled by nanoxerography and systematically compared to patterns obtained from single photoluminescent nanoparticles. FINDINGS: The synthesis approach is generic. Every hybrid nanoparticle system has led to 3D assemblies with improved photoluminescent signals compared to mono/bilayered assemblies. Straightforward applications for anti-counterfeiting are illustrated. The versatility of the proposed concept is expected to be applied to other nanoparticles to make the most of their magnetic, catalytic, optical etc. properties in a wide range of applications, sensors and devices.

Single-Step Binary Electrostatic Directed Assembly of Active Nanogels for Smart Concentration-Dependent Encryption.

Anionic and cationic (N-isopropylacrylamide derivatives) active colloidal hydrogel nanoparticles, i.e., nanogels, are electrostatically assembled on surfaces to form microscale patterns with complex geometries. While using mixed dispersions of these two kinds of nanogels, we demonstrate the capability of sorting the nanogels in one step to form binary nanogel patterns on a surface. These patterns appear independently or simultaneously depending on the relative proportion of each nanogel type in the mixture. Hence, the resulting nanogel patterns provide quantitative information regarding the dispersion composition and can be used to achieve smart concentration-dependent nanogel encryption. Moreover, atomic force microscopy characterization measurements performed in liquid prove that the assembled nanogels maintain their swelling/deswelling properties once attached to the surface. Consequently, this method paves the way for applying such active nanogel patterns to produce smart coatings and sensors.

Directed assembly of living Pseudomonas aeruginosa bacteria on PEI patterns generated by nanoxerography for statistical AFM bioexperiments.

Immobilization of living micro-organisms on predefined areas of substrates is a prerequisite for their characterizations by atomic force microscopy (AFM) in culture media. It remains challenging since micro-organisms should not be denatured but attached strongly enough to be scanned with an AFM tip, in a liquid phase. In this work, a novel approach is proposed to electrostatically assemble biological objects of interest on 2 nm thick polyethylenimine (PEI) patterns fabricated by nanoxerography. This nanoxerography process involves electrostatic trapping of PEI chains on negatively charged patterns written on electret thin films by AFM or electrical microcontact printing. The capability of this approach is demonstrated using a common biological system, Pseudomonas aeruginosa bacteria. These negatively charged bacteria are selectively assembled on large scale arrays of PEI patterns. In contrast to other PEI continuous films commonly used for cell anchoring, these ultrathin PEI patterns strongly attached on the surface do not cause any denaturation of the assembled Pseudomonas aeruginosa bacteria. AFM characterizations of large populations of individual living bacteria in culture media can thus be easily performed through this approach, providing the opportunity to perform representative statistical data analysis. Interestingly, this process may be extended to any negatively charged micro-organism in solution.

Electro-actuated hydrogel walkers with dual responsive legs.

Stimuli responsive polyelectrolyte hydrogels may be useful for soft robotics because of their ability to transform chemical energy into mechanical motion without the use of external mechanical input. Composed of soft and biocompatible materials, gel robots can easily bend and fold, interface and manipulate biological components and transport cargo in aqueous solutions. Electrical fields in aqueous solutions offer repeatable and controllable stimuli, which induce actuation by the re-distribution of ions in the system. Electrical fields applied to polyelectrolyte-doped gels submerged in ionic solution distribute the mobile ions asymmetrically to create osmotic pressure differences that swell and deform the gels. The sign of the fixed charges on the polyelectrolyte network determines the direction of bending, which we harness to control the motion of the gel legs in opposing directions as a response to electrical fields. We present and analyze a walking gel actuator comprised of cationic and anionic gel legs made of copolymer networks of acrylamide (AAm)/sodium acrylate (NaAc) and acrylamide/quaternized dimethylaminoethyl methacrylate (DMAEMA Q), respectively. The anionic and cationic legs were attached by electric field-promoted polyion complexation. We characterize the electro-actuated response of the sodium acrylate hydrogel as a function of charge density and external salt concentration. We demonstrate that "osmotically passive" fixed charges play an important role in controlling the bending magnitude of the gel networks. The gel walkers achieve unidirectional motion on flat elastomer substrates and exemplify a simple way to move and manipulate soft matter devices and robots in aqueous solutions.

Reversible patterning and actuation of hydrogels by electrically assisted ionoprinting.

The ability to pattern, structure, re-shape and actuate hydrogels is important for biomimetics, soft robotics, cell scaffolding and biomaterials. Here we introduce an 'ionoprinting' technique with the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions via their directed injection and complexation, assisted by electric fields. The ionic binding changes the local mechanical properties of the gel to induce relief patterns and, in some cases, evokes localized stress large enough to cause rapid folding. These ionoprinted patterns are stable for months, yet the ionoprinting process is fully reversible by immersing the gel in a chelator. The mechanically patterned hydrogels exhibit programmable temporal and spatial shape transitions, and serve as a basis for a new class of soft actuators that can gently manipulate objects both in air and in liquid solutions.

Self-healing stretchable wires for reconfigurable circuit wiring and 3D microfluidics.

This article describes the fabrication of self-healing stretchable wires formed by embedding liquid metal wires in microchannels composed of self-healing polymer. These stretchable wires can be completely severed with scissors and rapidly self-heal both mechanically and electrically at ambient conditions. By cutting the channels strategically, the pieces can be re-assembled in a different order to form complex microfluidic networks in 2D or 3D space.

Coulomb force directed single and binary assembly of nanoparticles from aqueous dispersions by AFM nanoxerography.

We present a simple protocol to obtain versatile assemblies of nanoparticles from aqueous dispersions onto charge patterns written by atomic force microscopy, on a 100 nm thin film of polymethylmethacrylate spin-coated on silicon wafers. This protocol of nanoxerography uses a two-stage development involving incubation of the desired aqueous colloidal dispersion on charge patterns and subsequent immersion in an adequate water-soluble alcohol. The whole process takes only a few minutes. Numerical simulations of the evolution of the electric field generated by charge patterns in various solvents are done to resolve the mechanism by which nanoparticle assembly occurs. The generic nature of this protocol is demonstrated by constructing various assemblies of charged organic/inorganic/metallic (latex, silica, gold) nanoparticles of different sizes (3 to 100 nm) and surface functionalities from aqueous dispersions onto charge patterns of complex geometries. We also demonstrate that it is possible to construct a binary assembly of nanoparticles on a pattern made of positive and negative charges generated in a single charge writing step, by sequential developments in two aqueous dispersions of oppositely charged particles. This protocol literally extends the spectra of eligible colloids that can be assembled by nanoxerography and paves the way for building complex assemblies of nanoparticles on predefined areas of surfaces, which could be useful for the elaboration of nanoparticle-based functional devices.