Nanoscale Actuators Research Articles

A novel super-elastic carbon nanofiber with cup-stacked carbon nanocones and a screw dislocation

Carbon nanofibers (NFs) have been envisioned with broad promising applications, such as nanoscale actuators and energy storage medium. This work reports for the first-time super-elastic tensile characteristics of NFs constructed from a screw dislocation of carbon nanocones (NF–S). The NF–S exhibits three distinct elastic deformation stages under tensile, including an initial homogeneous deformation, delamination, and further stretch of covalent bonds. The delamination process endows the NF–S extraordinary tensile deformation capability, which is not accessible from its counterpart with a normal cup-stacked geometry. The failure of NF–S is governed by the inner edges of the nanocone due to the strain concentration, leading to a common failure force for NF–S with varying geometrical parameters. Strikingly, the delamination process is dominated by the inner radius and the apex angle of the nanocone. For a fixed apex angle, the yielding strain increases remarkably when the inner radius increases, which can exceed 1000%. It is also found that the screw dislocation allows the nanocones flattening and sliding during compression. This study provides a comprehensive understanding on the mechanical properties of NFs as constructed from carbon nanocones, which opens new avenues for novel applications, such as nanoscale actuators.

Paper-Based Mechanical Sensors Enabled by Folding and Stacking.

Electronics based on paper substrates can be foldable, inexpensive, and biodegradable, making such systems promising for low-cost sensors, smart packaging, and medical diagnostics. In this work, we saturate tissue paper with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) by using a simple and scalable process and construct pressure sensors that exhibit an enhanced response when the active material is folded or stacked. Nanoscale pressure actuation and current mapping reveals a sensing mechanism that takes advantage of the fibrous microstructure of the paper and relies on the formation and expansion of electrical contacts between fibers in adjacent paper layers as pressure is applied. The resulting paper-based pressure sensors respond to an impulse within 20 ms and are robust, showing only a 4.6% decrease in the operating current after 30 000 load/unload cycles. Pressure distribution mapping was achieved by using a sensor array with a stacked architecture, whereas folding was used to demonstrate multistate switching and to detect conformational change in a three-dimensional origami system. These strategies of folding and layering paper saturated with functional materials open up new avenues for building multifunctional paper electronics.

Measurement of the Casimir torque.

Intermolecular forces are pervasive in nature and give rise to various phenomena including surface wetting1, adhesive forces in biology2,3, and the Casimir effect4, which causes two charge-neutral, metal objects in vacuum to attract each other. These interactions are the result of quantum fluctuations of electromagnetic waves and the boundary conditions imposed by the interacting materials. When the materials are optically anisotropic, different polarizations of light experience different refractive indices and a torque is expected to occur that causes the materials to rotate to a position of minimum energy5,6. Although predicted more than four decades ago, the small magnitude of the Casimir torque has so far prevented direct measurements of it. Here we experimentally measure the Casimir torque between two optically anisotropic materials-a solid birefringent crystal (calcite, lithium niobite, rutile or yttrium vanadate) and a liquid crystal (5CB). We control the sign and strength of the torque, and its dependence on the rotation angle and the separation distance between the materials, through the choice of materials. The values that we measure agree with calculations, verifying the long-standing prediction that a mechanical torque induced by quantum fluctuations can exist between two separated objects. These results open the door to using the Casimir torque as a micro- or nanoscale actuation mechanism, which would be relevant for a range of technologies, including microelectromechanical systems and liquid crystals.

Quantitative Analysis of Thickness and pH Actuation of Weak Polyelectrolyte Brushes

Polymer brushes are widely used as surface coatings for various inert, functional, or responsive interfaces. If the polymer can alter its protonation state (a polyelectrolyte (PE)), the brush can switch between a collapsed and swollen state with pH, which enables applications such as nanoscale actuators. However, changes in brush height as the polymer alters its charge state are not straightforward to measure accurately. Here, we show how surface plasmon resonance can be used to determine the thickness of PE brushes both in their charged and neutral states. We use different methods to measure the heights of brushes consisting of poly(acrylic acid) and the polybasic poly(2-(diethylamino)ethyl methacrylate), both prepared by atom transfer radical polymerization. We find polymers in solution that can act as refractive index probes, which do not interact with the grafted polyelectrolytes, thus providing an “exclusion height” of the brush. Importantly, the angular reflection spectrum can be used to directly id...

Muscle-like Artificial Molecular Actuators for Nanoparticles

Summary Muscle tissue performs crucial contraction/extension motions that generate mechanical force and work by consuming chemical energy. Inspired by this naturally created biomolecular machine, artificial molecular muscles are designed and synthesized to undertake linear actuation functions. However, most of these muscle-like actuators are performed at large ensembles, while to realize the nanoscale actuation at the single- to few-molecule level remains challenging. Herein, we developed an artificial muscle-like molecular actuator that can reversibly control the proximity of the attached nano-objects, gold nanoparticles, within the single-molecule length level by its stimuli-responsive muscle-like linear contraction/extension motion. The molecular actuation motion is accompanied by an optical signal output resulting from the plasmonic resonance properties of gold nanoparticles. Meanwhile, the thermal noise of the muscle-like molecular actuator can be overcome by integrating the optical signal over a sufficiently long period.

Grain size and nanoscale effects on the nonlinear pull-in instability and vibrations of electrostatic actuators made of nanocrystalline material

Presented herein is the study of grain size, grain surface energy and small scale effects on the nonlinear pull-in instability and free vibration of electrostatic nanoscale actuators made of nanocrystalline silicon (Nc-Si). A Mori–Tanaka micromechanical model is utilized to calculate the effective material properties of Nc-Si considering material structure inhomogeneity, grain size and grain surface energy. The small-scale effect is also taken into account using Mindlin’s strain gradient theory. Governing equations are derived in the discretized weak form using the variational differential quadrature method based on the third-order shear defamation beam theory in conjunction with the von Kármán hypothesis. The electrostatic actuation is modeled considering the fringing field effects based upon the parallel plate approximation. Moreover, the Casimir force effect is considered. The pseudo arc-length continuation technique is used to obtain the applied voltage-deflection curve of Nc-Si actuators. Then, a time-dependent small disturbance around the deflected configuration is assumed to solve the free vibration problem. By performing a numerical study, the influences of various factors such as length scale parameter, volume fraction of the inclusion phase, density ratio, average inclusion radius and Casimir force on the pull-in instability and free vibration of Nc-Si actuators are investigated.

A Material Showing Colossal Positive and Negative Volumetric Thermal Expansion with Hysteretic Magnetic Transition

AbstractIt is an ongoing challenge to design and synthesize magnetic materials that undergo colossal thermal expansion and that possess potential applications as microscale or nanoscale actuators with magnetic functionality. A paramagnetic metallocyanate building block was used to construct a cyanide‐bridged Fe‐Co complex featuring both positive and negative colossal volumetric thermal‐expansion behavior. A detailed study revealed that metal‐to‐metal charge transfer between 180 and 240 K induced a volumetric thermal expansion coefficient of 1498 MK−1 accompanied with hysteretic spin transition. Rotation of the magnetic building blocks induced change of π⋅⋅⋅π interactions, resulting in a negative volume expansion coefficient of −489 MK−1, and another hysteretic magnetic transition between 300 and 350 K. This work presents a strategy for incorporating both colossal positive and negative volumetric thermal expansion with shape and magnetic memory effects in a material.

A Material Showing Colossal Positive and Negative Volumetric Thermal Expansion with Hysteretic Magnetic Transition.

It is an ongoing challenge to design and synthesize magnetic materials that undergo colossal thermal expansion and that possess potential applications as microscale or nanoscale actuators with magnetic functionality. A paramagnetic metallocyanate building block was used to construct a cyanide-bridged Fe-Co complex featuring both positive and negative colossal volumetric thermal-expansion behavior. A detailed study revealed that metal-to-metal charge transfer between 180 and 240 K induced a volumetric thermal expansion coefficient of 1498 MK-1 accompanied with hysteretic spin transition. Rotation of the magnetic building blocks induced change of π⋅⋅⋅π interactions, resulting in a negative volume expansion coefficient of -489 MK-1 , and another hysteretic magnetic transition between 300 and 350 K. This work presents a strategy for incorporating both colossal positive and negative volumetric thermal expansion with shape and magnetic memory effects in a material.

Large In-Plane and Vertical Piezoelectricity in Janus Transition Metal Dichalchogenides.

Piezoelectricity in 2D van der Waals materials has received considerable interest because of potential applications in nanoscale energy harvesting, sensors, and actuators. However, in all the systems studied to date, strain and electric polarization are confined to the basal plane, limiting the operation of piezoelectric devices. In this paper, based on ab initio calculations, we report a 2D materials system, namely, the recently synthesized Janus MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer and multilayer structures, with large out-of-plane piezoelectric polarization. For MXY monolayers, both strong in-plane and much weaker out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. For multilayer MXY, we obtain a very strong out-of-plane piezoelectric polarization when strained transverse to the basal plane, regardless of the stacking sequence. The out-of-plane piezoelectric coefficient d33 is found to be strongest in multilayer MoSTe (5.7-13.5 pm/V depending on the stacking sequence), which is larger than that of the commonly used 3D piezoelectric material AlN (d33 = 5.6 pm/V); d33 in other multilayer MXY structures are a bit smaller, but still comparable. Our study reveals the potential for utilizing piezoelectric 2D materials and their van der Waals multilayers in device applications.

Rise of the Nanorobots: Advances in Control, Molecular Detection, and Nanoscale Actuation Are Bringing Us Closer to a New Era of Technology Enhanced by Nanorobots

In 1988, a Scientific American article by A.K. Dewdney [1] on the work of nanotechnologist K. Eric Drexler spurred public interest in the nascent field of nanotechnology and its potential for advancing humanity into a new technological age. The article portrayed a world run by nanoscale machines that could operate in any environment (Figure 1), with uses ranging from fighting infections in the human body to building tomorrow's skyscrapers. Nearly 30 years later, these visions of the future are closer than ever. Advances in microscopic imaging, nanobiotechnology, and collective control are ushering in the age of nanoscale robotics.

Modeling and Identification of the Rate-Dependent Hysteresis of Piezoelectric Actuator Using a Modified Prandtl-Ishlinskii Model

Piezoelectric actuator (PEA) is an ideal microscale and nanoscale actuator because of its ultra-precision positioning resolution. However, the inherent hysteretic nonlinearity significantly degrades the PEA’s accuracy. The measured hysteresis of PEA exhibits strong rate-dependence and saturation phenomena, increasing the difficulty in the hysteresis modeling and identification. In this paper, a modified Prandtl-Ishlinskii (PI) hysteresis model is proposed. The weights of the backlash operators are updated according to the input rates so as to account for the rate-dependence property. Subsequently, the saturation property is realized by cascading a polynomial operator with only odd powers. In order to improve the efficiency of the parameter identification, a special control input consisting of a superimposition of multiple sinusoidal signals is utilized. Because the input rate of such a control input covers a wide range, all the parameters of the hysteresis model can be identified through only one set of experimental data, and no additional curve-fitting is required. The effectiveness of the hysteresis modeling and identification methodology is verified on a PEA-driven flexure mechanism. Experimental results show that the modeling accuracy is on the same order of the noise level of the overall system.

Modified Continuum Mechanics Modeling on Size-Dependent Properties of Piezoelectric Nanomaterials: A Review.

Piezoelectric nanomaterials (PNs) are attractive for applications including sensing, actuating, energy harvesting, among others in nano-electro-mechanical-systems (NEMS) because of their excellent electromechanical coupling, mechanical and physical properties. However, the properties of PNs do not coincide with their bulk counterparts and depend on the particular size. A large amount of efforts have been devoted to studying the size-dependent properties of PNs by using experimental characterization, atomistic simulation and continuum mechanics modeling with the consideration of the scale features of the nanomaterials. This paper reviews the recent progresses and achievements in the research on the continuum mechanics modeling of the size-dependent mechanical and physical properties of PNs. We start from the fundamentals of the modified continuum mechanics models for PNs, including the theories of surface piezoelectricity, flexoelectricity and non-local piezoelectricity, with the introduction of the modified piezoelectric beam and plate models particularly for nanostructured piezoelectric materials with certain configurations. Then, we give a review on the investigation of the size-dependent properties of PNs by using the modified continuum mechanics models, such as the electromechanical coupling, bending, vibration, buckling, wave propagation and dynamic characteristics. Finally, analytical modeling and analysis of nanoscale actuators and energy harvesters based on piezoelectric nanostructures are presented.

Two-dimensional multiferroics in monolayer group IV monochalcogenides

Low-dimensional multiferroic materials hold great promises in miniaturized device applications such as nanoscale transducers, actuators, sensors, photovoltaics, and nonvolatile memories. Here, using first-principles theory we predict that two-dimensional (2D) monolayer group IV monochalcogenides including GeS, GeSe, SnS, and SnSe are a class of 2D semiconducting multiferroics with giant strongly-coupled in-plane spontaneous ferroelectric polarization and spontaneous ferroelastic lattice strain that are thermodynamically stable at room temperature and beyond, and can be effectively modulated by elastic strain engineering. Their optical absorption spectra exhibit strong in-plane anisotropy with visible-spectrum excitonic gaps and sizable exciton binding energies, rendering the unique characteristics of low-dimensional semiconductors. More importantly, the predicted low domain wall energy and small migration barrier together with the coupled multiferroic order and anisotropic electronic structures suggest their great potentials for tunable multiferroic functional devices by manipulating external electrical, mechanical, and optical field to control the internal responses, and enable the development of four device concepts including 2D ferroelectric memory, 2D ferroelastic memory, and 2D ferroelastoelectric nonvolatile photonic memory as well as 2D ferroelectric excitonic photovoltaics.

Electro-mechano responsive properties of gelatin methacrylate (GelMA) hydrogel on conducting polymer electrodes quantified using atomic force microscopy.

Electrical stimulation of hydrogels has been performed to enable micro-actuation or controlled movement of ions and biomolecules such as in drug release applications. Hydrogels are also increasingly used as low modulus, biocompatible coatings on electrode devices and thus are exposed to the effects of electrical stimulation. As such, there is growing interest in the latter, especially on the dynamic and nanoscale physical properties of hydrogels. Here, we report on the electro-mechano properties of photocrosslinkable gelatin methacrylate (GelMA) hydrogel applied as coatings on conducting polymer polypyrrole-dodecylbenze sulfonate (PPy-DBSA) electrodes. In particular, Electrochemical-Atomic Force Microscopy (EC-AFM) was used to quantify the nanoscale actuation and dynamic changes in Young's modulus as the GelMA coating was electrically stimulated via the underlying PPy-DBSA electrode. Pulsed electrical stimulation was shown to induce dynamic expansion and contraction, or nanoscale actuation, of the GelMA hydrogel due to the reversible ingress of electrolyte ions and associated changes in osmotic pressure during oxidation and reduction of the PPy-DBSA film. In addition, dynamic changes in the Young's modulus of up to 50% were observed in the hydrogel and correlated with the actuation process and ion diffusion during oxidation and reduction of the underlying PPy-DBSA film. These dynamic properties were investigated for hydrogels with varying degrees of cross-linking, porosity and modulus, the latter ranging from ≈0.2-1 kPa. The study demonstrates an AFM-based approach to quantify the dynamic physical properties of hydrogels, which are shown to be modulated via electrical stimulation. This can enable a better understanding of the electro-mechano mechanisms that are important for the controlled release of drugs or controlling cell interactions at the hydrogel-cell interface.

Nanoscale mechanical actuation and near-field read-out of photonic crystal molecules

We employed the contact forces induced by a near-field tip to tune and probe the optical resonances of a mechanically compliant photonic crystal molecule. Here, the pressure induced by the near-field tip is exploited to control the spectral proprieties of the coupled cavities in an ultrawide spectral range, demonstrating a reversible mode shift of $37.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. Besides, by monitoring the coupling strength variation due to the vertical nanodeformation of the dielectric structure, distinct tip-sample interaction regimes have been unambiguously reconstructed with a nano-Newton sensitivity. These results demonstrate an optical method for mapping mechanical forces at the nanoscale with a lateral spatial resolution below 100 nm.

Investigation of the vdW Force-Induced Instability in Nano-scale Actuators Fabricated form Cylindrical Nanowires

The presence of van der Waals (vdW) force can lead to mechanical instability in freestanding nano-scale actuators. Most of the previous researches in this area have exclusively focused on modeling the instability in actuators with one actuating components. While, less attention has been paid to actuators consist of two actuating components. Herein, the effect of the vdW force on the instability of freestanding actuators with two parallel actuating components is investigated. Conventional configurations including cantilever and double-clamped geometries are investigated. A continuum mechanics theory in conjunction with Euler-beam model is applied to obtain governing equations of the systems. The nonlinear governing equations of the actuators are solved using two different approaches, i.e. the modified Adomian decomposition and the finite difference method. The maximum length of the nanowire and minimum initial gap which prevents the instability is computed.

Nanoscale optomechanical actuators for controlling mechanotransduction in living cells.

Herein we develop an approach for optically controlling receptor tension. This is achieved using optomechanical actuator nanoparticles that are controlled with non-invasive near-infrared light. Illumination leads to particle collapse, delivering piconewton forces to specific cell surface receptors with high spatial and temporal resolution. As a proof-of-concept, we applied optomechanical actuation to trigger integrin-based focal adhesion formation, cell protrusion and migration, as well as T cell receptor activation.

Biological Soft Robotics

In nature, nanometer-scale molecular motors are used to generate force within cells for diverse processes from transcription and transport to muscle contraction. This adaptability and scalability across wide temporal, spatial, and force regimes have spurred the development of biological soft robotic systems that seek to mimic and extend these capabilities. This review describes how molecular motors are hierarchically organized into larger-scale structures in order to provide a basic understanding of how these systems work in nature and the complexity and functionality we hope to replicate in biological soft robotics. These span the subcellular scale to macroscale, and this article focuses on the integration of biological components with synthetic materials, coupled with bioinspired robotic design. Key examples include nanoscale molecular motor-powered actuators, microscale bacteria-controlled devices, and macroscale muscle-powered robots that grasp, walk, and swim. Finally, the current challenges and future opportunities in the field are addressed.

Nanoscale directional motion towards regions of stiffness.

How to induce nanoscale directional motion via some intrinsic mechanisms pertaining to a nanosystem remains a challenge in nanotechnology. Here we show via molecular dynamics simulations that there exists a fundamental driving force for a nanoscale object to move from a region of lower stiffness toward one of higher stiffness on a substrate. Such nanoscale directional motion is induced by the difference in effective van der Waals potential energy due to the variation in stiffness of the substrate; i.e., all other conditions being equal, a nanoscale object on a stiffer substrate has lower van der Waals potential energy. This fundamental law of nanoscale directional motion could lead to promising routes for nanoscale actuation and energy conversion.

Observation of a giant two-dimensional band-piezoelectric effect on biaxial-strained graphene

Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and energy harvesters have received much attention. More recently, devices made of graphene are of particular interest because of graphene’s intriguing electronic and mechanical properties. Intrinsic graphene has long been considered devoid of the piezoelectric effect, although flexoelectricity has been exploited to demonstrate piezoelectricity in functionalized graphene and graphene nanoribbons. The perceived lack of this property has restricted graphene’s use in nanoelectromechanical systems (NEMS) for electromechanical coupling purposes. Here an unprecedented two-dimensional (2D) piezoelectric effect on a strained/unstrained graphene junction is reported. In stark contrast to the bulk piezoelectric effect that results from the occurrence of electric dipole moments in solids, the 2D piezoelectric effect arises from the charge transfer along a work function gradient introduced by the biaxial-strain-engineered band structure. The observed effect, termed the band-piezoelectric effect, exhibits an enormous magnitude due to the ultrathin structure of graphene. On the basis of the band-piezoelectric effect, a graphene nanogenerator and a pressure gauge were fabricated. The results not only provide a versatile NEMS platform for sensing, actuating and energy harvesting, but also pave the way for efficiently modulating graphene via strain engineering.