Patterns and instabilities in colloidal nanoparticle assemblies

Colloidal nanoparticles exhibit unusual individual and collective behaviour, often associated with interesting electrical, optical or electromagnetic properties. Thiol-passivated colloidal gold nanoparticles possess in addition a self-organising property, which, when the particles are deposited on a substrate, yields a plethora of fascinating patterns. The conditions of formation of these patterns are investigated, in order to understand the principles of - and gain control over - non-equilibrium self-organisation following drop evaporation. The work presented in this thesis relies mostly on experimental observations, although the results are supported by numerical simulations carried out in the group and based on modified versions of the model developed by Rabani et al. in 2003 [1]. A novel deposition method is introduced, which provides controllable conditions for the occurrence of a wide variety of patterns, including close-packed monolayers of nanoparticles. Pattern and surface characterisation is achieved by combined microscopy techniques - atomic force microscopy (AFM) and real-time contrast-enhanced optical microscopy. The influence on pattern formation of the nanoparticle-solvent-substrate interactions is studied by altering the physical properties of all three components (substrate, solvent and nanoparticles). The experimental set-up allows a meniscus-driven evaporation of the solvent of the nanoparticle solution and enables monitoring of drying front instabilities during the dewetting process. The effects of these instabilities on pattern formation are investigated and highlight a strong contribution of free excess ligands. We have focused on two specific types of patterns which emerge in these experiments : fingering structures and nanoparticle rings. The former are reminiscent of patterns that form in a number of other systems, a process usually called "viscous fingering". A thorough investigation reveals that the mechanism of formation of such patterns involves the combination of specific experimental conditions and at least two different dewetting processes, with different time and length scales. A "pseudo-3D" Monte Carlo model recreates such conditions and yields simulated results which are in good qualitative and quantitative agreement with experimental results. On the other hand, nanoparticle rings, although they are a recurrent type of pattern observed in nanoparticle assemblies [2, 3], form according to a mechanism which is not yet fully understood. We show however that wetting properties play a central role in ring formation and growth. As in the case of fingering structures, a narrow range of parameters has been determined, via an exhaustive experimental investigation, which favours the occurrence of nanoparticle rings. For all the nanoparticle assemblies studied in this thesis (close-packed monolayers, fingering structures and nanoparticle rings), the deduction of pattern formation mechanisms from experimental observation (and simulations) relies on the very high degree of reproducibility that it is possible to attain using the combination of a meniscus-driven evaporation, a very fine tuning of experimental conditions and nanoparticle-solvent-substrate interactions, and a systematic cross-characterisation by complementary imaging techniques

Patterns and instabilities in colloidal nanoparticle assemblies

Colloidal nanoparticles exhibit unusual individual and collective behaviour, often associated with interesting electrical, optical or electromagnetic properties. Thiol-passivated colloidal gold nanoparticles possess in addition a self-organising property, which, when the particles are deposited on a substrate, yields a plethora of fascinating patterns. The conditions of formation of these patterns are investigated, in order to understand the principles of - and gain control over - non-equilibrium self-organisation following drop evaporation. The work presented in this thesis relies mostly on experimental observations, although the results are supported by numerical simulations carried out in the group and based on modified versions of the model developed by Rabani et al. in 2003 [1]. A novel deposition method is introduced, which provides controllable conditions for the occurrence of a wide variety of patterns, including close-packed monolayers of nanoparticles. Pattern and surface characterisation is achieved by combined microscopy techniques - atomic force microscopy (AFM) and real-time contrast-enhanced optical microscopy. The influence on pattern formation of the nanoparticle-solvent-substrate interactions is studied by altering the physical properties of all three components (substrate, solvent and nanoparticles). The experimental set-up allows a meniscus-driven evaporation of the solvent of the nanoparticle solution and enables monitoring of drying front instabilities during the dewetting process. The effects of these instabilities on pattern formation are investigated and highlight a strong contribution of free excess ligands. We have focused on two specific types of patterns which emerge in these experiments : fingering structures and nanoparticle rings. The former are reminiscent of patterns that form in a number of other systems, a process usually called "viscous fingering". A thorough investigation reveals that the mechanism of formation of such patterns involves the combination of specific experimental conditions and at least two different dewetting processes, with different time and length scales. A "pseudo-3D" Monte Carlo model recreates such conditions and yields simulated results which are in good qualitative and quantitative agreement with experimental results. On the other hand, nanoparticle rings, although they are a recurrent type of pattern observed in nanoparticle assemblies [2, 3], form according to a mechanism which is not yet fully understood. We show however that wetting properties play a central role in ring formation and growth. As in the case of fingering structures, a narrow range of parameters has been determined, via an exhaustive experimental investigation, which favours the occurrence of nanoparticle rings. For all the nanoparticle assemblies studied in this thesis (close-packed monolayers, fingering structures and nanoparticle rings), the deduction of pattern formation mechanisms from experimental observation (and simulations) relies on the very high degree of reproducibility that it is possible to attain using the combination of a meniscus-driven evaporation, a very fine tuning of experimental conditions and nanoparticle-solvent-substrate interactions, and a systematic cross-characterisation by complementary imaging techniques

Self-Powered Conformable Deformation Sensor Exploiting the Collective Piezoelectric Effect of Self-Organised GaN Nanowires

International audienceWe present a novel integration-driven approach to the design of multi-scale multi-physics sensors and systems. We implement this method to model, design, fabricate and characterize a thin, conformable low-cost impact detection sensor based on assemblies of piezoelectric GaN nanowires. When suitably assembled, the latter demonstrate a macroscale additivity of their nanoscale intrinsic properties, which enables to appeal to classical fabrication techniques and exploitable electronic readouts at the system level. We also exploit multi-level simulations to provide useful insights of adapted application-driven integration solutions for these new forms of sensors. We demonstrate the potential of such application-targeted, fully-integrated and modular systems to accommodate to the stringent requirements of structural health monitoring (SHM)

Anisotropic Assembly of Colloidal Nanoparticles: Exploiting Substrate Crystallinity

We show that the crystal structure of a substrate can be exploited to drive the anisotropic assembly of colloidal nanoparticles. Pentanethiol-passivated Au particles of approximately 2 nm diameter deposited from toluene onto hydrogen-passivated Si(111) surfaces form linear assemblies (rods) with a narrow width distribution. The rod orientations mirror the substrate symmetry, with a high degree of alignment along principal crystallographic axes of the Si(111) surface. There is a strong preference for anisotropic growth with rod widths substantially more tightly distributed than lengths. Entropic trapping of nanoparticles provides a plausible explanation for the formation of the anisotropic assemblies we observe

Patterns and instabilities in colloidal nanoparticle assemblies

Colloidal nanoparticles exhibit unusual individual and collective behaviour, often associated with interesting electrical, optical or electromagnetic properties. Thiol-passivated colloidal gold nanoparticles possess in addition a self-organising property, which, when the particles are deposited on a substrate, yields a plethora of fascinating patterns. The conditions of formation of these patterns are investigated, in order to understand the principles of - and gain control over - non-equilibrium self-organisation following drop evaporation. The work presented in this thesis relies mostly on experimental observations, although the results are supported by numerical simulations carried out in the group and based on modified versions of the model developed by Rabani et al. in 2003 [1]. A novel deposition method is introduced, which provides controllable conditions for the occurrence of a wide variety of patterns, including close-packed monolayers of nanoparticles. Pattern and surface characterisation is achieved by combined microscopy techniques - atomic force microscopy (AFM) and real-time contrast-enhanced optical microscopy. The influence on pattern formation of the nanoparticle-solvent-substrate interactions is studied by altering the physical properties of all three components (substrate, solvent and nanoparticles). The experimental set-up allows a meniscus-driven evaporation of the solvent of the nanoparticle solution and enables monitoring of drying front instabilities during the dewetting process. The effects of these instabilities on pattern formation are investigated and highlight a strong contribution of free excess ligands. We have focused on two specific types of patterns which emerge in these experiments : fingering structures and nanoparticle rings. The former are reminiscent of patterns that form in a number of other systems, a process usually called "viscous fingering". A thorough investigation reveals that the mechanism of formation of such patterns involves the combination of specific experimental conditions and at least two different dewetting processes, with different time and length scales. A "pseudo-3D" Monte Carlo model recreates such conditions and yields simulated results which are in good qualitative and quantitative agreement with experimental results. On the other hand, nanoparticle rings, although they are a recurrent type of pattern observed in nanoparticle assemblies [2, 3], form according to a mechanism which is not yet fully understood. We show however that wetting properties play a central role in ring formation and growth. As in the case of fingering structures, a narrow range of parameters has been determined, via an exhaustive experimental investigation, which favours the occurrence of nanoparticle rings. For all the nanoparticle assemblies studied in this thesis (close-packed monolayers, fingering structures and nanoparticle rings), the deduction of pattern formation mechanisms from experimental observation (and simulations) relies on the very high degree of reproducibility that it is possible to attain using the combination of a meniscus-driven evaporation, a very fine tuning of experimental conditions and nanoparticle-solvent-substrate interactions, and a systematic cross-characterisation by complementary imaging techniques.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Pixel analysis of a force-sensing device based on individually contacted vertical piezoelectric nanowires

International audienceWe report on the static finite element (FEM) simulations of the representative pixel of a force-sensing device, with the aim of predicting the influence of technically tunable parameters on pixel response. This pixel was based on an individually contacted vertical nanowire. It was found that piezopotential collection efficiency was higher for thinner seed-layers, reaching up to 69 % for a 5 nm-thick layer. The degradation resulting from a gap between the NW and its contacts was quantified, lowering this value to 33 % for a 3 nm gap. The values chosen for technological parameters were based on experimental results and set to a range of plausible values for selective growth of ZnO nanowires on pre-patterned substrates. Our results provide important guidelines for the optimization of sensor pixel piezoelectric response, with resulting constraints on NW growth and substrate patterning

Flexible Capacitive Piezoelectric Sensor with Vertically Aligned Ultralong GaN Wires

International audienceWe report a simple and scalable fabrication process of flexible capacitive piezoelectric sensors using vertically aligned gallium nitride (GaN) wires as well as their physical principles of operation. The as-grown N-polar GaN wires obtained by self-catalyst metal–organic vapor phase epitaxy are embedded into a polydimethylsiloxane (PDMS) matrix and directly peeled off from the sapphire substrate before metallic electrode contacting. This geometry provides an efficient control of the wire orientation and an additive contribution of the individual piezoelectric signals. The device output voltage and efficiency are studied by finite element calculations for compression mechanical loading as a function of the wire geometrical growth parameters (length and density). We demonstrate that the voltage output level and sensitivity increases as a function of the wire length and that a conical shape is not mandatory for potential generation as it was the case for horizontally assembled devices. The optimal design to improve the overall device response is also optimized in terms of wire positioning inside PDMS, wire density, and total device thickness. Following the results of these calculations, we have fabricated experimental devices exhibiting outputs of several volts with a very good reliability under cyclic mechanical excitation

Towards a force-displacement sensor based on vertical ZnO piezoelectric nanowires

session B: Nanomaterials propertiesInternational audienc

Bending piezoelectric nanowires: application to force-displacement sensors based on individually interconnected vertical piezo-semi-conductive nanowires

session: posterInternational audienc

Static Finite Element Modeling for Sensor Design and Processing of an Individually Contacted Laterally Bent Piezoelectric Nanowire

International audienceWe report on the static finite element simulations of the representative elementary pixel of an arrayed force-sensing device. Our goal is to provide quantified guidelines for adjusting pixel geometry and tuning technological parameters in a view to on-chip device fabrication, according to the targeted device performance or usage. The force sensitive pixel consists of an individual piezoelectric nanowire (NW), selectively grown on a material stack representative of the targeted process, and with side electrodes. The model takes into account the full pixel environment but makes the assumption of purely insulating materials (no free charges). We found that the piezopotential collection factor was strongly dependent on the stack characteristics. A 69% collection factor was obtained when a 5-nm-thick growth-seeding ZnO layer was introduced directly below the NW. We also simulated micro-fabrication related defects, such as the loss of physical contact between the NW and the electrodes, and found that the collection factor dropped to 33% for a 3 nm gap before stabilizing. Our results provide important guidelines for the optimization of the overall sensor response and calibration, with resulting constraints on NW growth conditions and substrate patterning, as well as collected data dispersion sources for posttreatment purposes