Ühedimensionaalsete nanostruktuuride tribomehaanilised omadused: lõplike elementide meetodi simulatsioonidega toetatud eksperimentaalmõõtmised

Väitekirja elektrooniline versioon ei sisalda publikatsiooneVäitekirja raames uuriti mitmeid olulisi küsimusi, mis käsitlevad 1D nanostruktuuride mehaanilisi ja triboloogilisi omadusi ja käitumist. Põhitegevused ja uudsed aspektid on esitatud allpool. 1DNS elemente manipuleeriti tasasel pinnal ja analüüsiti vastavaid triboloogilisi protsesse. Alljärgnevalt tuvastati: • Tasasel pinnal asetsev elastselt painutatud nanotraat on väga oluline nanotriboloogilistes mõõtmistes, kuna võimaldab nanotraadi profiilist lähtuvalt leida alusega seotud triboloogilised väärtused ilma välist jõusensorit kasutamata. Meetodi täpsuse parandamiseks töötati välja uudne analüütiline meetod, mis võtab arvesse staatilise hõõrde jaotuse tasasel pinnal asetseva elastselt painutatud nanotraadil. Erinevalt varasematest meetoditest pakub uus mudel realistlikuma jõuspektri ja arvesteb ääretingimusi. Meetodit rakendati edukalt staatilise hõõrdumise arvutamiseks ZnO nanotraatide korral, mis olid ränialusel AFM-i teravikuga manipuleerimisega suvalisse kujusse painutatud. • Uus FEM mudel töötati välja sellise konfiguratsiooni jaoks, kus osa nanotraadist toetub lamedale substraadile, samal ajal kui teine osa on vabalt üle serva. Üleulatuva vaba otsa painutatakse alusele fikseeritud osa nihkumiseni. Registreerides paindprofiili vahetult enne fikseeritud osa nihkumist, saame sisendi fikseeritud osa mõjutatava jõu arvutamiseks. Vanemate mudelite puhul võeti eelduseks staatilise hõõrdejõu ühtlast jaotust fikseeritud osale. Uue mudeli puhul näidati staatilise hõõrdumise ületamist väga lokaliseeritud protsessina, mis sarnaneb pragude tekkimisega. Näidati, et olemasolevad mudelid on staatilisest hõõrdumise rolli tunduvalt alahinnatud, samas kui uus mudel pakub reaalsusega paremat kooskõla. • Töötati välja dünaamiline FEM-mudel lamedal aluspinnal asetsevast mõlemast otsast sulanud Ag nanotraadi kirjeldamiseks. Näidati, et nanotraadis tekitatud mehaanilised pinged on tingitud asjaolust, et sulanud otsad moodustavad ümarad elemendid, mille tulemusena on võimalik ületada nanotraadi ja ränialuse vahelist adhesiooni. Selle tulemusena saavutatakse konfiguratsioon, kus ainult saadud nanoosakeste otsaelmemendid puutuvad kokku pinnaga, samas kui keskosa on pinna kohal. Selline “hantlisarnane” struktuur ja konfiguratsioon on tribologiliste mõõtmiste jaoks äärmiselt atraktiivne, kuna seda saab hõlpsasti manipuleerida väikese kontaktiala tõttu ja samal ajal säilivad kõik 1D geomeetria eelised. Lisaks uuriti alljärgnevid 1DNS mehaanilisi omadusi: • Karakteriseeriti paksude seintega torukujuliste 1DNS elastseid omadusi kasutades nii eksperimentaalseid kui ka teoreetili meetodeid. o SiO2 nanotorude elastset moodulit mõõdeti kolme erineva meetodi abil, kasutades konsooltala painutamist, nanoindetatsiooni ja kolme punkti paindekatseid. Tuvastati, et kolme punkti paindekatse on kõige täpsem meetod paksuseinaliste torukujuliste 1DNS elastusmooduli mõõtmiseksA number of important issues concerning mechanical and tribological properties and behavior of 1D nanostructures were studied within the framework of the thesis. Main activity and the novelty aspects are summarized below. First, tribological aspects of 1DNS manipulated on a flat substrate were considered. In particular: • Nanowire elastically bent of a flat substrate is highly attractive for nanotribological studies as profile of nanowire can be used for extracting frictional data without using external force sensors. In order to improve accuracy of the method, a novel analytical method was developed for the calculation of distributed static friction in elastically bent nanowire resting on a flat substrate. Unlike previously available methods, new model provides more realistic force spectrum and comply with boundary conditions. The method was successfully applied for calculation of distributed static friction in ZnO nanowires bent into arbitrary shapes in AFM manipulations on a Si substrate. • A novel FEM model was developed for configuration in which part of the nanowire is resting on a flat substrate while other part is suspended over the trench. Measurements consist in bending the free end until fixed part is displaced. The bending profile prior the displacement of fixed part is used for calculation of force acting on a fixed part. In older models static friction was considered to be uniformly distributed in adhered part. The new model considered overcoming of static friction as a highly localized process similar to crack formation. It was shown, that existing models severely underestimated static friction, while novel model provides more realistic results. • Dynamic FEM model of Ag nanowire that is being melted from both ends while resting on a flat substrate was created. It was shown that mechanical stresses, generated in nanowire due to the fact that molted ends form rounded bulbs, are able to overcome the adhesion between nanowire and silicon substrate. As a result, a configuration is achieved where only the end-bulbs of the obtained nanodumbell are in contact with the surface while intact midpart is suspended above the substrate. Such structure and configuration is highly attractive for tribological measurements as it can be easily manipulated due to the small contact area and at the same time it preserves all benefits of 1D geometry. Further, mechanical properties of 1DNS were considered: • Elastic properties of tubular 1DNS with thick walls were treated both experimentally and theoretically. o Elastic modulus of SiO2 nanotubes was measured by three different methods including cantilever beam bending, nanoindentation and three-point bending tests. Three-point bending tests were found to be the most appropriate method for measuring the Young’s modulus of thick-walled tubular 1DNS. o FEM model was created to investigate the behavior of tubular 1DNS in nanoindentation test. It was shown there are both compression and indentation present. Thus, neither of existing models where walls of nanotube are considered either as a thin membrane or rigid wall cannot be used for given system as they underestimate the Young modulus. • FEM model of composite core-shell nanowire consisting of elastic core and viscous shell was created to simulate the behavior of Ag/SiO2 core-shell nanowire in bending test under electron beam irradiation. By fitting the experimental result with FEM model it was found that even at moderate current and voltage e-beam is capable of inducing glass transition in amorphous oxide shell. Finally, two variations of three-point bending test of Au nanowires were compared: freely sliding ends and rigidly fixed ends. The effect of different boundary conditions on experimental results was determined and the adhesion forces acting between Au and substrate were estimated using the FEM modeling. In total, it was demonstrated that FEM is a powerful method for studying mechanical and tribological properties of nanoscale systems when used in combination with experimental result

Tribological and mechanical characterization of ZnO nanowires

http://tartu.ester.ee/record=b2653173~S1*es

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

Adhesion and mechanical properties of PDMS-based materials probed with AFM: A review

This work was supported by Russian Science Foundation project grant 18-19-00645 "Adhesion of polymer-based soft materials: from liquid to solid-.Polydimethylsiloxane (PDMS) is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological properties. PDMS has found extensive usage in various fields ranging from microfluidics and flexible electronics to cosmetics and food industry. In certain applications, like e.g. dry adhesives or dry transfer of 2D materials, adhesive properties of PDMS play crucial role. In this review we focus on probing the mechanical and adhesive properties of PDMS by means of atomic force microscopy (AFM). Main advantages and limitations of AFM-based measurements in comparison to macroscopic tests are discussed.Russian Science Foundation 18-19-00645; 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 CAMART

Investigating the mechanical properties of GeSn nanowires.

Germanium tin (GeSn) has been proposed as a promising material for electronic and optical applications due to the formation of a direct band-gap at a Sn content >7 at%. Furthermore, the ability to manipulate the properties of GeSn at the nanoscale will further permit the realisation of advanced mechanical devices. Here we report for the first time the mechanical properties of GeSn nanowires (7.1–9.7 at% Sn) and assess their suitability as nanoelectromechanical (NEM) switches. Electron microscopy analysis showed the nanowires to be single crystalline, with surfaces covered by a thin native amorphous oxide layer. Mechanical resonance and bending tests at different boundary conditions were used to obtain size-dependent Young's moduli and to relate the mechanical characteristics of the alloy nanowires to geometry and Sn incorporation. The mechanical properties of the GeSn nanowires make them highly promising for applications in next generation NEM devices

Mechanical properties of sol–gel derived SiO2 nanotubes

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

Analysis of static friction and elastic forces in a nanowire bent on a flat surface: A comparative study

International audienceZnO nanowires bent to a complex shape and held in place by static friction force from supporting flat surface are investigated experimentally and theoretically. The complex shapes are obtained by bending the nanowires inside a scanning electron microscope with a sharp tip attached to a nanopositioner. Several methods previously described in the literature are applied along with author's original method to calculate the distributed friction force and stored elastic energy in the nanowires from the bending profile. This comparative study evidences the importance of the usage of appropriate models for accurate analysis of the nanowires profile. It is demonstrated that incomplete models can lead up to an order of magnitude error in the calculated friction force for complex profiles. (C) 2013 Elsevier Ltd. All rights reserved

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