Water-dependent micromechanical and rheological properties of silica colloidal crystals studied by nanoindentation

Probing the Mechanical Properties of 2D Materials via Atomic-Force-Microscopy-Based Modulated Nanoindentation

As the field of low-dimensional materials (1D or 2D) grows and more complex and intriguing structures are continuing to be found, there is an emerging need for techniques to characterize the nanoscale mechanical properties of all kinds of 1D/2D materials, in particular in their most practical state: sitting on an underlying substrate. While traditional nanoindentation techniques cannot accurately determine the transverse Young's modulus at the necessary scale without large indentations depths and effects to and from the substrate, herein an atomic-force-microscopy-based modulated nanomechanical measurement technique with Angstrom-level resolution (MoNI/ÅI) is presented. This technique enables non-destructive measurements of the out-of-plane elasticity of ultra-thin materials with resolution sufficient to eliminate any contributions from the substrate. This method is used to elucidate the multi-layer stiffness dependence of graphene deposited via chemical vapor deposition and discover a peak transverse modulus in two-layer graphene. While MoNI/ÅI has been used toward great findings in the recent past, here all aspects of the implementation of the technique as well as the unique challenges in performing measurements at such small resolutions are encompassed.

Defects and plasticity in ultrastrong supercrystalline nanocomposites

Supercrystalline nanocomposites are nanoarchitected materials with a growing range of applications but unexplored in their structural behavior. They typically consist of organically functionalized inorganic nanoparticles arranged into periodic structures analogous to crystalline lattices, including superlattice imperfections induced by processing or mechanical loading. Although featuring a variety of promising functional properties, their lack of mechanical robustness and unknown deformation mechanisms hamper their implementation into devices. We show that supercrystalline materials react to indentation with the same deformation patterns encountered in single crystals. Supercrystals accommodate plastic deformation in the form of pile-ups, dislocations, and slip bands. These phenomena occur, at least partially, also after cross-linking of the organic ligands, which leads to a multifold strengthening of the nanocomposites. The classic shear theories of crystalline materials are found to describe well the behavior of supercrystalline nanocomposites, which result to feature an elastoplastic behavior, accompanied by compaction.

Effect of nanoparticle size on the mechanical properties of nanoparticle assemblies

Nanoparticle assemblies (NPAs) have attracted tremendous interests of various research communities. The particle-size-effect on mechanical properties of NPAs is systematically studied. With decreasing the particle size d from 300 nm to 10 nm, the SiO2 NPAs become drastically harder (∼39×), stiffer (∼15×), and tougher (>3.5×). The results are consistent with the data scattered in the literature for various nanoparticle (NP) systems, indicating a fundamentally universal d-effect for all NPAs. A model is developed to correlate the hardness and the NP junction (NPJ) strength f. Here, f is mainly due to van der Waals and capillary interactions, roughly a constant (140 nN) for d = 100-300 nm, and then f decreases with decreasing d from ∼100 nm. The deformation mechanism of NPAs (for indentation depth ≫d) is shear plasticity involving shear breaking of NPJs. The fundamental mechanism for the d-effect is that, with decreasing d, the NPJ's planar density increases much faster than the decrease of f. Moreover, three deformation mechanisms of NPAs, (1) nanoparticle dislodging, (2) shear-band formation, and (3) cracking are naturally d-dependent. These new findings can provide important insights into the fundamental understanding of the inter-NP interaction, the mechanical behavior of the NPAs, and the design of robust NP-based devices.

Å-Indentation for non-destructive elastic moduli measurements of supported ultra-hard ultra-thin films and nanostructures

During conventional nanoindentation measurements, the indentation depths are usually larger than 1-10 nm, which hinders the ability to study ultra-thin films (<10 nm) and supported atomically thin two-dimensional (2D) materials. Here, we discuss the development of modulated Å-indentation to achieve sub-Å indentations depths during force-indentation measurements while also imaging materials with nanoscale resolution. Modulated nanoindentation (MoNI) was originally invented to measure the radial elasticity of multi-walled nanotubes. Now, by using extremely small amplitude oscillations (<<1 Å) at high frequency, and stiff cantilevers, we show how modulated nano/Å-indentation (MoNI/ÅI) enables non-destructive measurements of the contact stiffness and indentation modulus of ultra-thin ultra-stiff films, including CVD diamond films (~1000 GPa stiffness), as well as the transverse modulus of 2D materials. Our analysis demonstrates that in presence of a standard laboratory noise floor, the signal to noise ratio of MoNI/ÅI implemented with a commercial atomic force microscope (AFM) is such that a dynamic range of 80 dB -- achievable with commercial Lock-in amplifiers -- is sufficient to observe superior indentation curves, having indentation depths as small as 0.3 Å, resolution in indentation <0.05 Å, and in normal load <0.5 nN. Being implemented on a standard AFM, this method has the potential for a broad applicability.

Disordered Nanoparticle Packings under Local Stress Exhibit Avalanche-Like, Environmentally Dependent Plastic Deformation

Nanoindentation experiments on disordered nanoparticle packings performed both in an atomic force microscope and in situ in a transmission electron microscope are used to investigate the mechanics of plastic deformation. Under an applied load, these highly porous films exhibit load drops, the magnitudes of which are consistent with an exponential population distribution. These load drops are attributed to local rearrangements of a small number of particles, which bear similarities to shear transformation zones and to the T1 process, both of which have been previously predicted for disordered packings. An increase in the relative humidity results in an increase in the number of observed load drops, indicating that the strength of the particle interactions has a significant effect on the modes of plastic deformation. These results suggest how disordered nanoparticle packings may be expected to behave in devices operating under varying environments.

The Template Determines Whether Chemically Identical Nanoparticle Scaffolds Show Elastic Recovery or Plastic Failure

Subtle variations in the preparation of ice-templated nanoparticle assemblies yield monoliths that are chemically identical but exhibit qualitatively different mechanical behavior. We ice template aqueous dispersions to prepare macroporous monoliths largely comprising silica nanoparticles held together by a crosslinked polymer mesh. When the polymer is crosslinked in the presence of ice crystals, we obtain an elastic sponge that is capable of recovery after imposition of large compressive strains (up to 80%). If, however, the ice is lyophilized before the polymer is crosslinked, we obtain a plastic monolith that fails even for modest strains (less than 10%). The elastic sponge and the plastic monolith are chemically identical; they have the same organic content, the same ratio of polymer to crosslinker, and the same average crosslink density. Atomic force microscopy (AFM) was used to probe the local mechanical properties of the crosslinked polymer mesh. These measurements indicate that plastic monoliths dissipate significantly more energy and have a larger spatial variation in local mechanical response relative to the elastic sponges. We believe that this behavior might correlate with a wider spatial distribution of crosslinks in plastic scaffolds relative to elastic scaffolds.

Colloidal crystals and water: Perspectives on liquid-solid nanoscale phenomena in wet particulate media

Solid colloidal ensembles inherently contain water adsorbed from the ambient moisture. This water, confined in the porous network formed by the building submicron spheres, greatly affects the ensemble properties. Inversely, one can benefit from such influence on collective features to explore the water behavior in such nanoconfinements. Recently, novel approaches have been developed to investigate in-depth where and how water is placed in the nanometric pores of self-assembled colloidal crystals. Here, we summarize these advances, along with new ones, that are linked to general interfacial water phenomena like adsorption, capillary forces, and flow. Water-dependent structural properties of the colloidal crystal give clues to the interplay between nanoconfined water and solid fine particles that determines the behavior of ensembles. We elaborate on how the knowledge gained on water in colloidal crystals provides new opportunities for multidisciplinary study of interfacial and nanoconfined liquids and their essential role in the physics of utmost important systems such as particulate media.

Exploration and exploitation of water in colloidal crystals

Water on solid surfaces is ubiquitously found in nature, in most cases due to mere adsorption from ambient moisture. Because porous structures have large surfaces, water may significantly affect their characteristics. This is particularly obvious in systems formed by separate particles, whose interactions are strongly influenced by small amounts of liquid. Water/solid phenomena, like adsorption, condensation, capillary forces, or interparticle cohesion, have typically been studied at relatively large scales down to the microscale, like in wet granular media. However, much less is known about how water is confined and acts at the nanoscale, for example, in the interstices of divided systems, something of utmost importance in many areas of materials science nowadays. With novel approaches, in-depth investigations as to where and how water is placed in the nanometer-sized pores of self-assembled colloidal crystals have been made, which are employed as a well-defined, versatile model system with useful optical properties. In this Progress Report, knowledge gained in the last few years about water distribution in such nanoconfinements is gathered, along with how it can be controlled and the consequences it brings about to extract new or enhance existing material functionalities. New methods developed and new capabilities of standard techniques are described, and the water interplay with the optical, chemical, and mechanical properties of the ensemble are discussed. Some lines for applicability are also highlighted and aspects to be addressed in the near future are critically summarized.

Energy dissipation of nanoconfined hydration layer: long-range hydration on the hydrophilic solid surface

The hydration water layer (HWL), a ubiquitous form of water on the hydrophilic surfaces, exhibits anomalous characteristics different from bulk water and plays an important role in interfacial interactions. Despite extensive studies on the mechanical properties of HWL, one still lacks holistic understanding of its energy dissipation, which is critical to characterization of viscoelastic materials as well as identification of nanoscale dissipation processes. Here we address energy dissipation of nanoconfined HWL between two atomically flat hydrophilic solid surfaces (area of ~120 nm²) by small amplitude-modulation, noncontact atomic force microscopy. Based on the viscoelastic hydration-force model, the average dissipation energy is ~1 eV at the tapping amplitude (~0.1 nm) of the tip. In particular, we determine the accurate HWL thickness of ~6 layers of water molecules, as similarly observed on biological surfaces. Such a long-range interaction of HWL should be considered in the nanoscale phenomena such as friction, collision and self-assembly.

Magnetic field tunable small-scale mechanical properties of nickel single crystals measured by nanoindentation technique

Nano- and micromagnetic materials have been extensively employed in micro-functional devices. However, measuring small-scale mechanical and magnetomechanical properties is challenging, which restricts the design of new products and the performance of smart devices. A new magnetomechanical nanoindentation technique is developed and tested on a nickel single crystal in the absence and presence of a saturated magnetic field. Small-scale parameters such as Young's modulus, indentation hardness, and plastic index are dependent on the applied magnetic field, which differ greatly from their macroscale counterparts. Possible mechanisms that induced 31% increase in modulus and 7% reduction in hardness (i.e., the flexomagnetic effect and the interaction between dislocations and magnetic field, respectively) are analyzed and discussed. Results could be useful in the microminiaturization of applications, such as tunable mechanical resonators and magnetic field sensors.

Synthesis and mechanical response of disordered colloidal micropillars

A method for synthesizing and uniaxially compressing free-standing colloidal micropillars is presented. The mechanical response of the micropillars is strongly dependent upon their initial defect population and water content.

Nanoscale Morphology of Water in Silica Colloidal Crystals

We show a simple method to visualize the morphology of water adsorbed within the pore network of colloidal crystals made of submicrometer silica spheres. Water is replicated into silica by modified silicon tetrachloride hydrolysation under standard ambient conditions, making it visible to standard electronic microscopy and thus allowing one to discern the original water distribution. Different distribution patterns are identified depending on the water content, surface condition, and spheres arrangement. The dimension and shape of wetting layers (covering the submicrometer spheres) and capillary bridges (joining them) are measurable at the nanoscale. We finally use these findings to demonstrate proof-of-principle of fabrication of isolated and freestanding silica nanorings by using hydrophobic polymeric templates and selective etching.