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.

Tunneling mechanism and contact mechanics of colloidal nanoparticle assemblies.

Nanoparticle assemblies with thiol-terminated alkyl chains are studied by conducting atomic force microscopy (c-AFM) regarding their use as strain gauges for touch-sensitive panels. Current-force spectroscopy is used as a characterization tool complementary to the macroscopic setup since it allows a bias to be applied to a limited number of junctions, overcoming the Coulomb blockade energy and focusing on the contact electromechanics and the transport mechanism across the ligand. First, transition voltage spectroscopy is applied with varying force to target the underlying tunneling mechanism by observing whether the transition between the ohmic and exponential current-voltage behavior is force-dependent. Secondly, current-force spectroscopy in the ohmic range below the transition voltage is performed. The current-force behavior of the AFM probe in contact with a nanoparticle multilayer is associated with the spread of force and current within the nanoparticle lattice and at the level of adjacent particles by detailed contact mechanics treatment. The result is twofold: concerning the architecture of sensors, this work is a sample case of contact electromechanics at scales ranging from the device scale down to the individual ligand molecule. Regarding transport across the molecule, the vacuum tunneling mechanism is favored over the conduction by coherent molecular states, which is a decision-making aid for the choice of ligand in applications.

Directed assembly of single colloidal gold nanowires by AFM nanoxerography.

Ultrathin gold nanowires (NWs) dispersed in hexane were prepared by chemical reduction of HAuCl4 in oleylamine, along with nanospheres (NSs), side products of the reaction. X-ray photoelectron spectroscopy and small-angle X-ray scattering evidenced a stabilization of these nano-objects by oleylammonium chloride surfactants. The directed assembly of these nano-objects on surfaces was performed by atomic force microscopy (AFM) nanoxerography in a few seconds. Selective assembly of gold NWs only occurred on positively charged patterns, while NSs assembled more specifically on the negatively charged ones. This sorting suggests that the strong electric field generated by the charge patterns induced a negative effective charge on the gold NWs and a weak positive effective charge on the NSs. Such difference could be explained by the ion organization at the colloid surface, monolayered in the case of NWs, and bilayered in the case of NSs. By adjusting the design of the positive patterns and the experimental conditions of development, single gold nanowires were successfully assembled by AFM nanoxerography on predefined sites of surfaces without damaging them, opening the way for future electrical and mechanical characterizations.

Surface-enhanced spectroscopy on plasmonic oligomers assembled by AFM nanoxerography.

Surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) from individual plasmonic oligomers are investigated by confocal Raman micro-spectroscopy and time-resolved fluorescence microscopy coupled to steady state micro-spectroscopy. The nanoparticle (NP) oligomers are made of either ligand protected Au or Au@SiO2 core-shell colloidal NPs, which were assembled into ordered arrays by atomic force microscopy (AFM) nanoxerography. A strong dependence of the SERS emission on the polarization of incident light relative to the specific geometry of the plasmonic oligomer was observed. The SEF studies, performed on a large collection of NP oligomers of various known configurations showed interesting fluorophore decay rate modification and red-shift of the emission spectra. The experimental results are analyzed theoretically by employing finite-difference time-domain (FDTD) simulations on equivalent realistic structures, within the local density of optical states (LDOS) framework. The presented results, together with the proven potential of the LDOS approach as a useful common tool for analyzing both SERS and SEF effects further the general understanding of plasmon-related phenomena in nanoparticle oligomers.

Generation of living cell arrays for atomic force microscopy studies.

Atomic force microscopy (AFM) is a useful tool for studying the morphology or the nanomechanical and adhesive properties of live microorganisms under physiological conditions. However, to perform AFM imaging, living cells must be immobilized firmly enough to withstand the lateral forces exerted by the scanning tip, but without denaturing them. This protocol describes how to immobilize living cells, ranging from spores of bacteria to yeast cells, into polydimethylsiloxane (PDMS) stamps, with no chemical or physical denaturation. This protocol generates arrays of living cells, allowing statistically relevant measurements to be obtained from AFM measurements, which can increase the relevance of results. The first step of the protocol is to generate a microstructured silicon master, from which many microstructured PDMS stamps can be replicated. Living cells are finally assembled into the microstructures of these PDMS stamps using a convective and capillary assembly. The complete procedure can be performed in 1 week, although the first step is done only once, and thus repeats can be completed within 1 d.

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.

Small angle X-ray scattering coupled with in situ electromechanical probing of nanoparticle-based resistive strain gauges.

A comprehensive study on the electromechanical behavior of nanoparticle-based resistive strain gauges in action through normal and grazing incidence small angle X-ray scattering (SAXS/GISAXS) investigations is presented. The strain gauges were fabricated from arrays of colloidal gold nanoparticle (NP) wires assembled on flexible polyethylene terephthalate and polyimide substrates by convective self-assembly. Microstructural changes (mean interparticle distance variations) within these NP wires under uniaxial stretching estimated by SAXS/GISAXS are correlated to their macroscopic electrical resistance variations. SAXS measurements suggest a linear longitudinal extension and transversal contraction of the NP wires with applied strain (0 to ∼ 13%). The slope of this longitudinal variation is less than unity, implying a partial strain transfer from the substrate to the NP wires. The simultaneously measured electrical resistance of the strain gauges shows an exponential variation within the elastic domain of the substrate deformation, consistent with electron tunnelling through the interparticle gaps. A slower variation observed within the plastic domain suggests the formation of new electronic conduction pathways. Implications of transversal contraction of the NP wires on the directional sensitivities of strain gauges are evaluated by simulating electronic conduction in models mimicking a realistic NP arrangement. A loss of directionality of the NP-based strain gauges due to transversal current flow within the NP wires is deduced.

Dynamics of Dielectrophoretic-Force-Directed Assembly of NaYF4 Colloidal Nanocrystals into Tunable Multilayered Micropatterns.

The dynamics of dielectrophoretic-force-directed assembly of polarizable colloidal upconverting ß-NaYF4 nanocrystals into tunable multilayers on charge micropatterns written by atomic force microscopy is investigated. Multilayered nanocrystal assembly by this nanoxerography process occurs in two phases. During the first phase typically lasting a few minutes, the nanocrystal assemblies grow up to a maximum thickness under the influence of strong dielectrophoretic forces exerted by the charge patterns. Subsequently, the nanocrystals start to diffuse back into the solvent, leaving a single layer attached to the charge patterns. A theoretical model based on the Fokker-Planck equation is formulated to describe this dynamics involving an interplay of diffusive and dielectrophoretic forces. Being in good agreement with the experimental results, this approach may be reliably extended to simulate the directed assembly of other types of polarizable colloids from liquid phase by nanoxerography.

3D assembly of upconverting NaYF4 nanocrystals by AFM nanoxerography: creation of anti-counterfeiting microtags.

Formation of 3D close-packed assemblies of upconverting NaYF4 colloidal nanocrystals (NCs) on surfaces, by Atomic Force Microscopy (AFM) nanoxerography is presented. The surface potential of the charge patterns, the NC concentration, the polarizability of the NCs and the polarity of the dispersing solvent are identified as the key parameters controlling the assembly of NaYF4 NCs into micropatterns of the desired 3D architecture. This insight allowed us to fabricate micrometer sized Quick Response (QR) codes encoded in terms of upconversion luminescence intensity or color. Topographically hidden messages could also be readily incorporated within these microtags. This work demonstrates that AFM nanoxerography has enormous potential for generating high-security anti-counterfeiting microtags.

Electron transport in gold colloidal nanoparticle-based strain gauges.

A systematic approach for understanding the electron transport mechanisms in resistive strain gauges based on assemblies of gold colloidal nanoparticles (NPs) protected by organic ligands is described. The strain gauges were fabricated from parallel micrometer wide wires made of 14 nm gold (Au) colloidal NPs on polyethylene terephthalate substrates, elaborated by convective self-assembly. Electron transport in such devices occurs by inter-particle electron tunneling through the tunnel barrier imposed by the organic ligands protecting the NPs. This tunnel barrier was varied by changing the nature of organic ligands coating the nanoparticles: citrate (CIT), phosphines (BSPP, TDSP) and thiols (MPA, MUDA). Electro-mechanical tests indicate that only the gold NPs protected by phosphine and thiol ligands yield high gauge sensitivity. Temperature-dependent resistance measurements are explained using the 'regular island array model' that extracts transport parameters, i.e., the tunneling decay constant ß and the Coulomb charging energy E(C). This reveals that the Au@CIT nanoparticle assemblies exhibit a behavior characteristic of a strong-coupling regime, whereas those of Au@BSPP, Au@TDSP, Au@MPA and Au@MUDA nanoparticles manifest a weak-coupling regime. A comparison of the parameters extracted from the two methods indicates that the most sensitive gauges in the weak-coupling regime feature the highest ß. Moreover, the E(C) values of these 14 nm NPs cannot be neglected in determining the ß values.

Microarrays of gold nanoparticle clusters fabricated by Stop&Go convective self-assembly for SERS-based sensor chips.

SERS substrates fabricated from chemically synthesized nanoparticles (NPs) offer a distinct advantage of localizing and enhancing the electromagnetic fields by facile tuning of NP size, shape and interparticle distances. In this report, two-dimensional arrays of micrometre-sized clusters of gold nanoparticles protected by (i) sodium citrate and (ii) tris(2,4-dimethyl-5-sulfonatophenyl)phosphine (TDSP) ligands were directly assembled from colloidal suspensions onto flat, non-patterned substrates by discontinuous ('Stop&Go') convective self-assembly. The micrometric spacing between the NP clusters makes it easy to address them individually by confocal Raman microscopy. The packing of the gold NPs within these clusters with interparticle spacings of the order of nanometres leads to an optical response dominated by coupled surface plasmon resonances, and favours a strong enhancement of electromagnetic fields useful for surface enhanced Raman scattering (SERS). These NP clusters make very uniform SERS substrates, with reproducible SERS responses from cluster to cluster. The potential of these NP clusters for optical biosensing is demonstrated by the SERS detection of a biologically relevant molecule, cytosine, adsorbed onto the NP clusters. The presented results are promising for designing an original class of nanoparticle-based SERS microarrays. The new paradigm of convective self-assembly could be exploited generally for the patterning of various other types of colloidal micro- and nano-objects, such as semiconducting NPs, magnetic NPs, bacteria or proteins.

High-sensitivity strain gauge based on a single wire of gold nanoparticles fabricated by stop-and-go convective self-assembly.

High-sensitivity strain gauges based on single wires of close-packed 14 nm colloidal gold nanoparticles are obtained by a novel variant of convective self-assembly (CSA). This CSA mode named stop-and-go CSA enables the fabrication of nanoparticle wires only a few micrometers wide, separated by distances that can be easily tuned over tens to hundreds of micrometers. Nanoparticle wires are obtained in a single step by direct deposition of nanoparticles from suspensions onto flexible polyethylene terephthalate films, without any lithographic prepatterning. When connected between two electrodes, such single nanoparticle wires function as miniature resistive strain gauges. The high sensitivity, repeatability, and robustness demonstrated by these single-wire strain gauges make them extremely promising for integration into micro-electromechanical systems or for high-resolution strain mapping.

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.

Tunable conductive nanoparticle wire arrays fabricated by convective self-assembly on nonpatterned substrates.

Ordered arrays of centimeter-long nanoparticle wires are fabricated by convective self-assembly from aqueous suspensions of 18 nm gold colloids, on flat SiO(2)/Si substrates without any prepatterning. The orientation of the wires can be switched from parallel to perpendicular to the substrate-liquid-air contact line by controlling the substrate temperature. While the wires parallel to the meniscus are obtained by a stick-slip process, a mechanism based on critical density-triggered particle pinning is proposed to explain the formation of wires perpendicular to the meniscus. The geometry of the wire arrays is tuned by simply controlling the meniscus translation speed. Wires are typically characterized by widths of a few micrometers (1.8-8.2 µm), thicknesses of mono- to multilayers (18-70 nm), and spacings of few tens of micrometers. The fabricated nanoparticle wires are conductive, exhibiting a metallic resistive behavior in ambient conditions. Resistivity values of 5 × 10(-6) and 5 × 10(-2) Ωm are obtained on multilayer and monolayer nanoparticle wires, respectively. Such conductive nanoparticle wire arrays, fabricated by a simple and low-cost bottom-up strategy, offer opportunities for developing nanoparticle-based functional devices.