Static friction between silicon nanowires and elastomeric substrates

Frictional behavior of one-dimensional materials: an experimental perspective

The frictional behavior of one-dimensional (1D) materials, including nanotubes, nanowires, and nanofibers, significantly influences the efficient fabrication, functionality, and reliability of innovative devices integrating 1D components. Such devices comprise piezoelectric and triboelectric nanogenerators, biosensing and implantable devices, along with biomimetic adhesives based on 1D arrays. This review compiles and critically assesses recent experimental techniques for exploring the frictional behavior of 1D materials. Specifically, it underscores various measurement methods and technologies employing atomic force microscopy, electron microscopy, and optical microscopy nanomanipulation. The emphasis is on their primary applications and challenges in measuring and characterizing the frictional behavior of 1D materials. Additionally, we discuss key accomplishments over the past two decades in comprehending the frictional behaviors of 1D materials, with a focus on factors such as materials combination, interface roughness, environmental humidity, and non-uniformity. Finally, we offer a brief perspective on ongoing challenges and future directions, encompassing the systematic investigation of the testing environment and conditions, as well as the modification of surface friction through surface alterations.

Highly Stretchable and Sensitive Single-Walled Carbon Nanotube-Based Sensor Decorated on a Polyether Ester Urethane Substrate by a Low Hydrothermal Process

We report a highly stretchable sensor with low-concentration (1.5 wt %) single-walled carbon nanotubes (SWCNTs) on flexible polyether ester urethane (PEEU) yarn, fabricated using a low hydrothermal process at 90 °C. Although SWCNTs restrict the PEEU polymer chain mobility, the resulting ductility of our nanocomposites reduces only by 16.5% on average, initially from 667.3% elongation at break to 557.2%. The resulting electrical resistivity of our nanocomposites can be controlled systematically by the number of hydrothermal cycles. A high gauge factor value of 4.84 is achieved at a tensile strain below 100%, and it increases up to 28.5 with applying a tensile strain above 450%. We find that the piezoresistivity of our nanocomposite is sensitive to temperature variations of 25-85 °C due to the hopping effect, which promotes more charge transport at elevated temperatures. Our nanocomposites offer both a high sensitivity and a large strain sensing range, which is achieved with a relatively simple fabrication technique and low concentration of SWCNTs.

Interfacial adhesion of ZnO nanowires on a Si substrate in air

It is imperative to understand the interfacial adhesive behaviour of nanowires (NW) integrated into a nanoelectromechanical system in order to design commercialisable nanogenerators as well as ultrasensitive sensors. Currently available interfacial adhesion characterisation techniques that utilise in situ electron microscopy subject nanoscale systems to a high-vacuum, electron-irradiated environment, potentially altering their interfacial interactions. Alternatively, force-sensing techniques conducted in air do not provide visual feedback of the interface, and therefore can only indirectly deduce adhesive properties. Here, we present an interface characterisation technique that enforces ZnO NWs to remain partially delaminated on a Si substrate, and permits optical observation of their deformed condition in air. NWs are draped over a wedge and are allowed to conform to their minimum energy state. We evaluate the strain energy stored in the suspended segment of each NW by determining their deflected shape from interferometry. We show that utilising a tailored Euler-Bernoulli beam model which accounts for the tapering and irregularity of a NW is crucial for accurately evaluating their interfacial adhesion energy. A nominal energy per unit interface area value of [capital Gamma, Greek, macron]_{F-B,irr,taper} = 51.1 ± 31.9 mJ m^-2 is obtained for the ZnO NW-Si substrate interface; a magnitude lower than that found using electron microscopy, and higher than the upper-bound of the theoretically predicted van der Waals interaction energy of γ_vdW = 7.2 mJ m^-2. This apparent discrepancy has significant implications for any nanotribological study conducted inside an electron microscope. The results also implicate electrostatic and capillary interactions as significant contributors towards a NW's adhesive behaviour during device operation.

Stretchable Transparent Conductors: from Micro/Macromechanics to Applications

Stretchable transparent conductors (STCs), generally consisting of conducting networks and stretchable transparent elastomers, can maintain stable conductivity and transparency even at large tensile strain, beyond the reach of rigid/flexible transparent conductors. They are essential components in stretchable/wearable electronics for using on irregular 3D conformable surfaces and have attracted tremendous attention in recent years. This review aims to provide systematical correlation of the conducting element-substrate interaction with the structural stability of conducting networks, as well as the properties and device applications of STCs. It starts with the micromechanics for stretching of conducting elements on substrates, including the mechanical mismatch, distribution/level of interfacial shear stress, and the deformation behavior of conducting elements on substrates. The macromechanics for stretching of conducting networks on substrates are then further illustrated from a more statistical point of view, namely sliding/preferred orientation of percolation networks, unfolding of buckled structures, and unit cell distortion/distributed rupture of nanomeshes. The structure-dependent properties as well as the state-of-the-art applications of STCs are summarized before ending with the conclusions and outlooks for STCs.

Stretchable and sensitive sensor based on carbon nanotubes/polymer composite with serpentine shapes via molding technique

With the rapid development of wearable devices in recent years, stretchable strain sensors based on electrically conductive composites have attracted a great deal of attention owing to their good stretchability and piezoresistivity. However, due to the intrinsic restriction of these types of composites, the conventional stretchable strain sensors cannot do well in all aspect of sensing performance. A stretchable strain sensor based on carbon nanotubes/poly(dimethylsiloxane) composite with the serpentine shape was devised and fabricated. The sensor was readily manufactured through a molding technique. Not only can this sensor distinguish tension strain from transverse or longitudinal direction, but also exhibits good linearity of response to tensile strain. In terms of sensitivity, hysteresis and response time, the stretchable strain sensor showed significant performance. The sensing performance of this proposed stretchable sensor has been demonstrated to be good in this work and it also shows a good prospect for utilization in multifunctional wearable devices.

Characterizing the surface forces between two individual nanowires using optical microscopy based nanomanipulation

The adhesion and friction between two Al₂O₃ nanowires (NWs) was characterized by the use of optical microscopy based nanomanipulation, with which peeling, shearing and sliding was performed. The elastically deformed shape of the NWs during peeling and shearing was used to calculate the adhesion and frictional forces; force sensing was not required. The obtained adhesion stress between two Al₂O₃ NWs varied from 0.14 to 0.25 MPa, lower than that observed for carbon nanotube junctions, and was attributed to van der Waals attraction. Stick-slip was observed during the shearing and sliding of two NWs, and was the consequence of discrete contact between surface asperities. The obtained static and kinetic frictional stresses varied from 0.7 to 1.3 MPa and 0.4 to 0.8 MPa, respectively; significantly greater than the obtained adhesion stress.

Nanotribological behavior of a single silver nanowire on graphite

The nanotribological characteristics of silver nanowires (Ag NWs) are of great importance for the reliability of their applications in flexible nanodevices involving mechanical interactions. The frictional behaviors of Ag NWs on graphite substrate were directly investigated by atomic force microscopy (AFM) nanomanipulation. The relatively short NWs demonstrate three forms of motion-rotation, translation and a combination of the two-whose frictional behaviors behave like rigid rods. The relatively long Ag NW shows characteristics of a flexible beam, whose friction increases with an increase in the bending angle of the NW. The friction between the NW and substrate increases linearly with an increase in the length of the NW. The long Ag NW displays extraordinary flexibility that can be folded to different shapes, and the friction of the folded NW becomes smaller due to the decreased bending deformation. The critical aspect ratio of the Ag NW on graphite substrate for two different frictional behaviors between the relatively long and short NWs is found to be 12-15. These findings can deepen the understanding of the frictional characteristics of Ag NWs and contribute to their quantitative interface design.

Effects of surface defects on the mechanical properties of ZnO nanowires

The elastic modulus of ZnO nanowires was measured using a resonance method based on laser Doppler effect and their fracture strains were determined via two-point bending with the aid of optical nanomanipulation. The elastic moduli of ZnO nanowires with diameters of 78 to 310 nm vary from 123 to 154 GPa, which are close to the bulk value of 140 GPa and independent of the diameters and surface defects. However, the fracture strains of the ZnO nanowires depend significantly on their diameters, increasing from 2.1% to 6.0% with the decrease in diameter from 316 to 114 nm. Post-mortem TEM analysis of the ends of the fractured nanowires revealed that fracture initiated at surface defects. The Weibull statistical analysis demonstrated that a greater defect depth led to a smaller fracture strain. The surface-defect dominated fracture should be an important consideration for the design and application of nanowire-based nanoelectromechanical systems.

The effect of surface texture on the kinetic friction of a nanowire on a substrate

The friction between Al2O3 nanowires and silicon substrates of different surface textures was characterised by use of optical manipulation. It was found that surface textures had significant effect on both the friction and the effective contact area between a nanowire and a substrate. A genetic algorithm was developed to determine the effective contact area between the nanowire and the textured substrate. The frictional force was found to be nearly proportional to the effective contact area, regardless of width, depth, spacing and orientation of the surface textures. Interlocking caused by textured grooves was not observed in this study.

An on-chip micromagnet frictionometer based on magnetically driven colloids for nano-bio interfaces

A novel method based on remotely controlled magnetic forces of bio-functionalized superparamagnetic colloids using micromagnet arrays was devised to measure frictional force at the sub-picoNewton (pN) scale for bio-nano-/micro-electromechanical system (bio-NEMS/MEMS) interfaces in liquid. The circumferential motion of the colloids with phase-locked angles around the periphery of the micromagnets under an in-plane rotating magnetic field was governed by a balance between tangential magnetic force and drag force, which consists of viscous and frictional forces. A model correlating the phase-locked angles of the steady colloid rotation was formulated and validated by measuring the angles under controlled magnetic forces. Hence, the frictional forces on the streptavidin/Teflon interface between the colloids and the micromagnet arrays were obtained using the magnetic forces at the phase-locked angles. The friction coefficient for the streptavidin/Teflon interface was estimated to be approximately 0.036 regardless of both vertical force in the range of a few hundred pN and velocity in the range of a few tenths of μm s(-1).

In-plane trapping and manipulation of ZnO nanowires by a hybrid plasmonic field

In general, when a semiconductor nanowire is trapped by conventional laser beam tweezers, it tends to be aligned with the trapping beam axis rather than confined in the horizontal plane, and this limits the application of these nanowires in many in-plane nanoscale optoelectronic devices. In this work, we achieve the in-plane trapping and manipulation of a single ZnO nanowire by a hybrid plasmonic tweezer system on a flat metal surface. The gap between the nanowire and the metallic substrate leads to an enhanced gradient force caused by deep subwavelength optical energy confinement. As a result, the nanowire can be securely trapped in-plane at the center of the excited surface plasmon polariton field, and can also be dynamically moved and rotated by varying the position and polarization direction of the incident laser beam, which cannot be performed using conventional optical tweezers. The theoretical results show that the focused plasmonic field induces a strong in-plane trapping force and a high rotational torque on the nanowire, while the focused optical field produces a vertical trapping force to produce the upright alignment of the nanowire; this is in good agreement with the experimental results. Finally, some typical ZnO nanowire structures are built based on this technique, which thus further confirms the potential of this method for precise manipulation of components during the production of nanoelectronic and nanophotonic devices.

Characterising the nanoscale kinetic friction using force-equilibrium and energy-conservation models with optical manipulation

SiC nanowires were manipulated under an optic microscope to investigate the nanoscale friction between nanowires and a flat substrate. The deflection of the nanowires was modeled as that of an Euler-Bernoulli beam subjected to a uniformly distributed load. A simple formula was developed to calculate the kinetic friction from the normalized deflections at the two ends of a nanowire. The frictional force per unit area determined ranges from 0.18-0.51 MPa. Both experimental and simulated results demonstrated that the proposed approach was reliable. The results were also compared with those estimated using an energy-conservation model, which produced a frictional force ranging from 0.21-0.62 MPa. The results obtained from the two different methods are in excellent agreement.

Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes-Ecoflex nanocomposites

Super-stretchable, skin-mountable, and ultra-soft strain sensors are presented by using carbon nanotube percolation network-silicone rubber nanocomposite thin films. The applicability of the strain sensors as epidermal electronic systems, in which mechanical compliance like human skin and high stretchability (ϵ > 100%) are required, has been explored. The sensitivity of the strain sensors can be tuned by the number density of the carbon nanotube percolation network. The strain sensors show excellent hysteresis performance at different strain levels and rates with high linearity and small drift. We found that the carbon nanotube-silicone rubber based strain sensors possess super-stretchability and high reliability for strains as large as 500%. The nanocomposite thin films exhibit high robustness and excellent resistance-strain dependency for over ~1380% mechanical strain. Finally, we performed skin motion detection by mounting the strain sensors on different parts of the body. The maximum induced strain by the bending of the finger, wrist, and elbow was measured to be ~ 42%, 45% and 63%, respectively.

Free-Standing Kinked Silicon Nanowires for Probing Inter- and Intracellular Force Dynamics

Silicon nanowires (SiNWs) have emerged as a new class of materials with important applications in biology and medicine with current efforts having focused primarily on using substrate bound SiNW devices. However, developing devices capable of free-standing inter- and intracellular operation is an important next step in designing new synthetic cellular materials and tools for biophysical characterization. To demonstrate this, here we show that label free SiNWs can be internalized in multiple cell lines, forming robust cytoskeletal interfaces, and when kinked can serve as free-standing inter- and intracellular force probes capable of continuous extended (>1 h) force monitoring. Our results show that intercellular interactions exhibit ratcheting like behavior with force peaks of ∼69.6 pN/SiNW, while intracellular force peaks of ∼116.9 pN/SiNW were recorded during smooth muscle contraction. To accomplish this, we have introduced a simple single-capture dark-field/phase contrast optical imaging modality, scatter enhanced phase contrast (SEPC), which enables the simultaneous visualization of both cellular components and inorganic nanostructures. This approach demonstrates that rationally designed devices capable of substrate-independent operation are achievable, providing a simple and scalable method for continuous inter- and intracellular force dynamics studies.

A simple criterion for determining the static friction force between nanowires and flat substrates using the most-bent-state method

A simple criterion was developed to assess the appropriateness of the currently available models that estimate the static friction force between nanowires and substrates using the 'most-bent-state' method. Our experimental testing of the static friction force between Al2O3 nanowires and Si substrate verified our theoretical analysis, as well as the establishment of the criterion. It was found that the models are valid only for the bent nanowires with the ratio of wire length over the minimum curvature radius [Formula: see text] no greater than 1. For the cases with [Formula: see text] greater than 1, the static friction force was overestimated as it neglected the effect of its tangential component.

Nanomaterial-enabled stretchable conductors: strategies, materials and devices

Stretchable electronics are attracting intensive attention due to their promising applications in many areas where electronic devices undergo large deformation and/or form intimate contact with curvilinear surfaces. On the other hand, a plethora of nanomaterials with outstanding properties have emerged over the past decades. The understanding of nanoscale phenomena, materials, and devices has progressed to a point where substantial strides in nanomaterial-enabled applications become realistic. This review summarizes recent advances in one such application, nanomaterial-enabled stretchable conductors (one of the most important components for stretchable electronics) and related stretchable devices (e.g., capacitive sensors, supercapacitors and electroactive polymer actuators), over the past five years. Focusing on bottom-up synthesized carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal nanomaterials (e.g., metal nanowires and nanoparticles), this review provides fundamental insights into the strategies for developing nanomaterial-enabled highly conductive and stretchable conductors. Finally, some of the challenges and important directions in the area of nanomaterial-enabled stretchable conductors and devices are discussed.

Assembly and densification of nanowire arrays via shrinkage

Chemically synthesized semiconductor nanowires (NWs) have demonstrated substantial promise for nanoelectronics, nanoenergy, and nanobiotechnology, but the lack of an effective and controllable assembly process has limited the wide adoption of NWs in these areas. Here we demonstrate a facile, robust, and controllable approach to assembling and densifying a parallel array of NWs using shrinkable shape memory polymers. Using thermal-induced shrinkage of polystyrene, we were able to successfully assemble and densify NW arrays up to close-packing and, furthermore, achieve tunable density (up to ∼300% amplification of density) by controlling the shrinkage process. We also demonstrate scalable assembly and densification of NWs on a 2.5 × 6 inch scale to explore the manufacturability of the shrink-induced assembly process. Finally, we demonstrate the successful transfer of the shrink-assembled NW arrays onto various 2-dimensional and 3-dimensional substrates without compromising the integrity of NW assembly and density.

An improved loop test for experimentally approaching the intrinsic strength of alumina nanoscale whiskers

An improved loop test, which consists of twisting a loop in a nanoscale whisker adhered to a transmission electron microscopy (TEM) grid or other flat substrate and pulling the ends until the loop breaks, was developed for experimentally approaching the intrinsic strength of the whisker material. Alumina whiskers with diameters in the 82-320 nm range, as test targets, exhibited an average fracture strength of 39.1 GPa with a maximum of 48.8 ± 1.8 GPa, which is close to the theoretical prediction of ∼46 GPa. In situ TEM fracture analysis demonstrated the brittle fracture of alumina whiskers with thicknesses down to 82 nm. Conventional SEM/TEM imaging induced fracture of the whisker loops that had experienced high strain and a possible mechanism were discussed.

Sliding and rolling frictional behavior of a single ZnO nanowire during manipulation with an AFM

The frictional behavior during manipulation of a single ZnO nanowire with a mass of about 18.7 ng placed horizontally on a Si wafer was examined using atomic force microscopy (AFM). The frictional force measured was in the range of 36.4 nN to 69.3 nN, which corresponded to extremely high friction coefficients of 242 and 462, respectively. However, when the adhesion force of the nanowire was considered, the friction coefficients were similar to the values typically encountered in macro-scale systems. During manipulation of the nanowire, both rolling and sliding motions were observed depending on the nanowire-Si frictional interaction. Unlike macro-scale systems, the difference between the frictional forces of rolling/sliding and pure sliding motions of the nanowire was not drastic.

Fabrication of functional nanowire devices on unconventional substrates using strain-release assembly

We report three representative nanowire (NW) devices for applications in stretchable electronics, strain sensing, and optical sensing. Fabrication of such devices is based on a recently developed strain-release assembly method. NWs are first aligned transversely on an elastomeric substrate using the strain-release assembly. Constant resistance is achieved in silicon (Si) NW devices stretched up to ~40% of axial strain, highlighting a new concept of transverse buckling. Combining the NW assembly with transfer printing extends suitable device substrates beyond elastomers to other unconventional materials (e.g., flexible and transparent materials). Following this combined process, flexible SiNW strain sensors are fabricated on plastics capable of sensing up to 1.6% bending strain and gauge factors >1000; flexible zinc oxide NW ultraviolet sensors are demonstrated with quick recovery (~2 s) and excellent repeatability on plastics. Our results show promise for the strain-release assembly as a simple and cost-effective process to fabricate NW devices on unconventional substrates.

Buckling of aligned carbon nanotubes as stretchable conductors: a new manufacturing strategy

A new manufacturing strategy for buckling of aligned carbon nanotubes is developed, which does not involve prestretching the substrate but relies on the interface interaction between the nanotubes and the substrate. More specifically, upon stretching the substrate the nanotubes slide on the substrate, but upon releasing the nanotubes buckle. Following this manufacturing strategy, stretchable conductors based on aligned carbon nanotubes are demonstrated.