Tuning adhesion forces between functionalized gold colloidal nanoparticles and silicon AFM tips: Role of ligands and capillary forces

Adhesion forces between functionalized gold colloidal nanoparticles (Au NPs) and scanning probe microscope silicon tips were experimentally investigated by atomic force microscopy (AFM) equipped with PeakForce QNM (Quantitative Nanoscale Mechanics) module. Au NPs were synthesized by a seed-mediated process and then functionalized with thiols containing different functional groups: amino, hydroxy, methoxy, carboxy, methyl, and thiol. Adhesion measurements showed strong differences between NPs and silicon tip depending on the nature of the tail functional group. The dependence of the adhesion on ligand density for different thiols with identical functional tail-group was also demonstrated. The calculated contribution of the van der Waals (vdW) forces between particles was in good agreement with experimentally measured adhesive values. In addition, the adhesion forces were evaluated between flat Au films functionalized with the same molecular components and silicon tips to exclude the effect of particle shape on the adhesion values. Although adhesion values on flat substrates were higher than on their nanoparticle counterparts, the dependance on functional groups remained the same.</p

The effect of heat treatment on the morphology and mobility of Au nanoparticles

This work was supported by The Centre National de la Recherche Scientifique (CNRS) of France and the French Embassy Program. The authors are also grateful for partial support by COST Action CA15216, the Estonian Science Foundation (grants PUT1689 and PUT1372), the Estonian Centre of Excellence in Zero Energy and Resource Efficient Smart Buildings and Districts, ZEBE, grant 2014-2020.4.01.15.0016 and Latvian Science Council grant lzp-2018/2-0083.In the present paper, we investigate the effect of heat treatment on the geometry and mobility of Au nanoparticles (NPs) on a Si substrate. Chemically synthesized Au NPs of diameter ranging from 5 to 27 nm were annealed at 200, 400, 600 and 800 °C for 1 h. A change in the geometry from faceted to more rounded shapes were observed with increasing annealing temperature. Kinetic Monte Carlo simulations indicate that the NPs become rounded due to the minimization of the surface area and the transition to lower energy surface types (111) and (100). The NPs were manipulated on a silica substrate with an atomic force microscope (AFM) in tapping mode. Initially, the NPs were immovable by AFM energy dissipation. However, annealed NPs became movable, and less energy was required to displace the NPs annealed at higher temperature. However, after annealing at 800 °C, the particles became immovable again. This effect was attributed to the diffusion of Au into the Si substrate and to the growth of the SiO2 layer.Centre National de la Recherche Scientifique; Latvian Council of Science lzp-2018/2-0083; Eesti Teadusfondi PUT1372,PUT1689,2014-2020.4.01.15.0016; European Cooperation in Science and Technology CA15216; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²https://www.beilstein-journals.org/bjnano/content/pdf/2190-4286-11-6.pd

Electro-Optics of TiO2 Nanowire Dispersions in PDMS Matrix

Low-cost electro-optical devices with fast, highly controllable and simple operation mechanisms, as well as long term stability are highly desirable for future generations of smart window technologies. There are different types of smart window technologies, for example thermochromics, photochromics, electrochromics, gaschromics, reflective metal hydrides, polymer-dispersed liquid crystals, gel-glass dispersed liquid crystal and dipole particle suspension (DPS) (also called suspended particle devices). From these solutions, the DPS devices allow great control of the transmittance of solar radiation and are also relatively simple. Here we report electro-optical properties of electrospun anatase TiO2 nanowire dispersions in polydimethylsiloxane (PDMS). It was found that changes in optical properties during TiO2 nanowire orientation in PDMS under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was found for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. Reported experimental findings could be important for smart window applications for the regulation of visible or even infrared transparency. In addition concentrations used in this study were 200 times lower than previously reported for titanium-based DPS devices, because the change in transmittance is attributed to a decrease of light scattering not absorption cross-section during the nanowire orientation along the direction of the electric field. The measurements of the influence of nanowire concentration and electric field strength on the nanowire orientation kinetics show that the device can be operated using electric fields as low as 0.1 V/μm and the magnitude of the change in transmittance can be adjusted by varying the TiO2 nanowire concentration in the PDMS matrix. A transmittance change of 23% was obtained with a DPS device using a suspension of 0.05 wt% electrospun anatase TiO2 in PDMS

TiO2 Nanowire Dispersions in Viscous Polymer Matrix: Electrophoretic Alignment and Optical Properties

The changes in optical properties during TiO2 nanowire orientation in polydimethylsiloxane (PDMS) matrix under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was demonstrated for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. These effects can be explained by the interplay between scattering and reflectance. Experimental findings reported could be important for smart window applications for the regulation of visible or even infrared transparency, thus reducing the energy consumption by air conditioning systems in buildings and automobiles in the future

Solvothermal Synthesis of One-Dimensional Transition Metal Doped ZnO Nanocrystals and Their Applications in Smart Window Devices

Oxide semiconductor nanowire (NW) suspension based devices have been attracted growing interest in smart window applications due to their great controllability of light transmittance, simplicity and long term stability. Recently, we demonstrated smart window device using the suspension of electrospun TiO2 or solvothermally synthesized ZnO NWs in viscous polydimethylsiloxane (PDMS) matrix. The operating principle of the oxide semiconductor NW and PDMS device is based on the alterable orientation, alignment or spatial distribution of the NWs in an electric field, by changing light scattering cross-section and thus reversibly increasing or decreasing transmittance [1]. One-dimensional (1D) nanostructures such as NWs exhibit a good response to electric field due to the highly anisotropic shape. Herein we report transition metal doped ZnO NW and PDMS based smart window devices. Doping ZnO NWs with other chosen metal ions, such as transition metals, may lead to the emergence of new targeted material properties. For instance, high quality ZnO nanocrystals doped with transition metal cations lead to enhanced optical absorption of light in visible range. The pristine ZnO NWs used in our previous work did not exhibit visible light absorption and transmittance of ZnO NWs and PDMS based smart window device was regulated by changing scattering cross-section. The visible light absorption could increase efficiency of smart window device, because during electrophoretic alignment of NWs towards direction of electric field will change not only scattering, but also visible light absorption cross-section. One-dimensional ZnO nanostructures have been synthesized by wet chemical techniques including microemulsion hydrothermal synthesis, surfactant-assisted hydrothermal orientation growth and alcohol solution refluxing. However, synthesis of high quality transition metal doped ZnO nanowires with small diameter and high aspect ratio is still a challenge. Most wet chemical methods fail to produce high aspect ratio doped ZnO NWs in large quantities. Here we are demonstrating large-scale, single-step, direct solvothermal method and have successfully prepared high aspect ratio single crystalline transition metal (Co, Cu, Fe, Ni, Mn) doped ZnO NWs. The solvothermal synthesis process presented here can be scaled up to macro scale production and the fact that NWs need no further modification increases the technological potential of oxide semiconductor nanowire suspension based devices electro-optical smart window devices

Counterintuitive Increase in Optical Scattering Efficiency during Negentropic Orientational Transition in Dilute ZnO Nanowire Suspensions

We demonstrate experimentally that the electrophoretic manipulation of a ZnO nanowire (NW) suspension in polydimethylsiloxane (PDMS) causes a remarkable change in optical scattering. Counterintuitively, as an electric field is applied to the suspension and a negentropic orientational transition from a chaotically oriented state to a partially ordered (aligned) state is induced, the geometrical cross-section of the particles decreases whereas the scattering efficiency increases significantly, indicating an increase in the scattering cross-section. The alignment of the longer axis of oblong ZnO nanoparticles in the direction of incident light unexpectedly resulted in up to a 40% decrease in transmittance in the middle of the visible spectral range in the case of 150 mm thick composite films with below 0.1 vol% NW concentration. A prepared prototype smart window device exhibited spontaneous restoration of transmittance, persistent electro-optical performance (0% change in contrast after more than 10 cycles), and temporal stability against nanoparticle sedimentation and agglomeration

Thermal Conductivity of PDMS Filled with Hollow Glass Microspheres

Polydimethylsiloxane (PDMS) is the most widely used silicon-based polymer due to its versatility and the range of attractive properties. Fabrication of PDMS involves liquid phase cross-linking to obtain hydrophobic and mechanically flexible material in the final solid form. This gives opportunity to add various fillers to affect the properties of resulting material. In the present work, we describe simple and reliable method of making a PDMS-based composite material with significantly improved thermal insulation properties by adding hollow glass microspheres (HGMs) to the mixture of liquid base and cross-linker (10:1 ratio) followed by degassing and heat-assisted crosslinking. We obtained 31% reduction of thermal conductivity for samples with HGMs content of 20% by mass. At the same time, sound insulation capacity slightly decreased as a result of lower density of PDMS-HGMs composite in comparison to pure PDMS. The wettability of the samples had no dependence on HGMs content

Manipulation of nanoparticles of different shapes inside a scanning electron microscope

In this work polyhedron-like gold and sphere-like silver nanoparticles (NPs) were manipulated on an oxidized Si substrate to study the dependence of the static friction and the contact area on the particle geometry. Measurements were performed inside a scanning electron microscope (SEM) that was equipped with a high-precision XYZ-nanomanipulator. To register the occurring forces a quartz tuning fork (QTF) with a glued sharp probe was used. Contact areas and static friction forces were calculated by using different models and compared with the experimentally measured force. The effect of NP morphology on the nanoscale friction is discussed

Mechanical properties of sol–gel derived SiO 2

The mechanical properties of thick-walled SiO2 nanotubes (NTs) prepared by a sol–gel method while using Ag nanowires (NWs) as templates were measured by using different methods. In situ scanning electron microscopy (SEM) cantilever beam bending tests were carried out by using a nanomanipulator equipped with a force sensor in order to investigate plasticity and flexural response of NTs. Nanoindentation and three point bending tests of NTs were performed by atomic force microscopy (AFM) under ambient conditions. Half-suspended and three-point bending tests were processed in the framework of linear elasticity theory. Finite element method simulations were used to extract Young’s modulus values from the nanoindentation data. Finally, the Young’s moduli of SiO2 NTs measured by different methods were compared and discussed