Nanoscale Actuators Research Articles

Flexoelectricity in amorphous hafnium oxide (HfO2)

Flexoelectricity, inherent in all materials, offers a promising alternative to piezoelectricity for nanoscale actuation and sensing. However, its widespread application faces significant challenges: differentiating flexoelectric effects from those of piezoelectricity and other phenomena, verifying its universality across all material structures and thicknesses, and establishing a comprehensive database of flexoelectric coefficients across different materials. This work introduces a groundbreaking methodology that accurately isolates flexoelectricity from piezoelectric, electrostrictive, and electrostatic effects, with a detection threshold extending below 1 fC/m. The robustness of this method is demonstrated through its application to amorphous hafnium oxide, successfully measuring a flexoelectric coefficient of 105 ± 10 pC/m. This measurement signifies the first measurement of flexoelectricity in hafnia, as well as in any amorphous material. In addition, the study compiles a list of published flexoelectric coefficients, revealing an important insight. The relationship between the flexoelectric coefficient and the material’s relative permittivity is better approximated by a quadratic proportionality. This challenges the traditional linear assumption proposed in Kogan’s work and opens new avenues for future research in flexoelectric materials.

Controlled thermally-driven mass transport in carbon nanotubes using carbon hoops

Controlling mass transportation using intrinsic mechanisms is a challenging topic in nanotechnology. Herein, we employ molecular dynamics simulations to investigate the mass transport inside carbon nanotubes (CNT) with temperature gradients, specifically the effects of adding a static carbon hoop to the outside of a CNT on the transport of a nanomotor inside the CNT. We reveal that the underlying mechanism is the uneven potential energy created by the hoops, i.e., the hoop outside the CNT forms potential energy barriers or wells that affect mass transport inside the CNT. This fundamental control of directional mass transportation may lead to promising routes for nanoscale actuation and energy conversion.

Low-Field Actuating Magnetic Elastomer Membranes Characterized using Fibre-Optic Interferometry.

Smart robotic devices remotely powered by magnetic field have emerged as versatile tools for wide biomedical applications. Soft magnetic elastomer (ME) composite membranes with high flexibility and responsiveness are frequently incorporated to enable local actuation for wireless sensing or cargo delivery. However, the fabrication of thin ME membranes with good control in geometry and uniformity remains challenging, as well as the optimization of their actuating performances under low fields (milli-Tesla). In this work, the development of ME membranes comprising of low-cost magnetic powder and highly soft elastomer through a simple template-assisted doctor blading approach, is reported. The fabricated ME membranes are controllable in size (up to centimetre-scale), thickness (tens of microns) and high particle loading (up to 70 wt.%). Conflicting trade-off effects of particle concentration upon magnetic responsiveness and mechanical stiffness are investigated and found to be balanced off as it exceeds 60 wt.%. A highly sensitive fibre-optic interferometric sensing system and a customized fibre-ferrule-membrane probe are first proposed to enable dynamic actuation and real-time displacement characterization. Free-standing ME membranes are magnetically excited under low field down to 2 mT, and optically monitored with nanometer accuracy. The fast and consistent responses of ME membranes showcase their promising biomedical applications in nanoscale actuation andsensing.

Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII 4 FeII 2 } Compound.

Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)FeIII (CN)3 ]4 [FeII (Ppmp)]2 ⋅2 CH3 OH (1, Tp*=hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Ppmp=2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine), which has a rhombic core structure abbreviated as {FeIII 2 FeII 2 }. Magnetic susceptibility measurements and single-crystal X-ray diffraction analyses revealed that 1 underwent thermally-induced spin transition with the thermal hysteresis. The FeII site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the FeII centers actuated anisotropic deformation of the rhombic {FeIII 2 FeII 2 } core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.

Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII4FeII2} Compound

AbstractColossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)FeIII(CN)3]4[FeII(Ppmp)]2⋅2 CH3OH (1, Tp*=hydrotris(3,5‐dimethyl‐pyrazol‐1‐yl)borate and Ppmp=2‐[3‐(2′‐pyridyl)pyrazol‐1‐ylmethyl]pyridine), which has a rhombic core structure abbreviated as {FeIII2FeII2}. Magnetic susceptibility measurements and single‐crystal X‐ray diffraction analyses revealed that 1 underwent thermally‐induced spin transition with the thermal hysteresis. The FeII site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the FeII centers actuated anisotropic deformation of the rhombic {FeIII2FeII2} core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.

Evanescent field trapping and propulsion of Janus particles along optical nanofibers

Small composite objects, known as Janus particles, drive sustained scientific interest primarily targeted at biomedical applications, where such objects act as micro- or nanoscale actuators, carriers, or imaging agents. A major practical challenge is to develop effective methods for the manipulation of Janus particles. The available long-range methods mostly rely on chemical reactions or thermal gradients, therefore having limited precision and strong dependency on the content and properties of the carrier fluid. To tackle these limitations, we propose the manipulation of Janus particles (here, silica microspheres half-coated with gold) by optical forces in the evanescent field of an optical nanofiber. We find that Janus particles exhibit strong transverse localization on the nanofiber and much faster propulsion compared to all-dielectric particles of the same size. These results establish the effectiveness of near-field geometries for optical manipulation of composite particles, where new waveguide-based or plasmonic solutions could be envisaged.

Design and Experimental Investigation of Three Degrees of Freedom Nanoscale Microsuspension Actuators

Nanoscale actuators are one of the key technologies in high-precision servo systems. For optical storage systems, the performance of the actuator determines the maximum storage capacity and read/write speed. This article introduces a three-degrees of freedom nanoscale microsuspension actuator. Compared with an actuator from a traditional optical storage system, the proposed actuator can hold dual objective lenses with large apertures, the load capacity of which is increased by 3.14 times. With dual objective lenses operating at the same time, the actuator can achieve a large range of layer jumps. Thanks to the four-sided magnetic circuit and the overall structure, the actuator has an operating bandwidth greater than 10 000 Hz for dynamic performance, can compensate systematic errors in nanoscale, and provides a radial tilt servo direction. We have derived the actuator a unified electromechanical dynamic model, an equivalent stiffness model, an equivalent damping model, and a magnetic circuit analysis model in detail. Compared with other analysis models of microsuspension actuator, the proposed models provide a theoretical guidance for the determination of each key parameter, and can effectively improve design efficiency. The theoretical models and design method of the actuator are also applicable to microactuators design and active vibration isolation design of other high-precision servo systems. The simulation and experimental results verify the good dynamic performance of the actuator and show the effectiveness of the principal analysis and design method.

Interface dewetting as a source of void formation and aggregation in phase change nanoscale actuators

This paper reports a phenomenon occurring between phase change material (PCM) germanium telluride (GeTe) and a thin encapsulation layer of alumina when the PCM undergoes the phase transformation, consistent with dewetting of the PCM from the surrounding alumina. Massive structural change, including formation of large voids, which take up to 21.9% of the initial GeTe volume after 10 000 phase change cycles is observed. Electrical and mechanical characterization of the structure confirms this interpretation. A rapid thermal annealing test of blanket films on alumina that demonstrates dewetting further validates this conjecture. The dewetting and associated gross material displacement can lead to an extraordinary actuation corresponding to a one-time 44 nm height change for a 178 nm GeTe thick layer. However, control of this phenomenon is required to build reliable actuators that do not suffer from rupture of the encapsulation layer.

Small-Scale Dynamic Behavior of Embedded Beams Under Moving Force in Complex Environments

Forced and free dynamic responses of nanobeams resting on a three-parameter foundation subjected to a moving force in magneto-hygro-thermal conditions are investigated based on the nonlocal strain gradient theory. Also, a detailed parametric study is conducted to identify the impact of slender ratio, axial and distributed tangential loads on the vibration characteristics and dynamic phenomena of the small-scale system. Comparative studies with existing results in the literature are performed to ensure the accuracy of the presented model and solution approach. Analytical and numerical methods are implemented to detect the effects of foundation coefficients, environmental conditions, geometrical and scale parameters on the dynamic amplification factor, critical moving force velocity, cancellation, and maximum free response mechanisms of the system. The outcomes revealed that the critical moving force velocity is enhanced by ascending the slender ratio, elastic, and shear moduli of the foundation. It is found that when the damping factor is considered for the foundation, it is feasible to prevent the occurrence of the cancellation phenomenon. Also, it is concluded that by fine-tuning scale parameters and environmental conditions, unwanted vibration of the system can be suppressed. The attained results of this study can be helpful in the vibration control of nanoscale actuators, switches, and resonators.

Design of Mechanical and Electrical Properties for Multidirectional Control of Microtubules.

Microtubule (MT)-motor systems show promise as nanoscale actuator platforms for performing molecular manipulations in nanobiotechnology and micro total analysis systems. These systems have been demonstrated to exert a variety of functions, including the concentration, transportation, and detection of molecular cargos. Although gliding direction control of MTs is necessary for these applications, most direction control methods are currently conducted using micro/nanofabricated guiding structures and/or flow, magnetic, and electric field forces. These control methods force all MTs to exhibit identical gliding behaviors and destinations. In this chapter, we describe an active multidirectional control method for MT without guiding tracks. The bottom-up molecular design allowed MTs to be guided in designated directions under an electric field in a microfluidic device. By designing the stiffness and surface charge density of MTs, three types of MT (Stiff-MT, Soft-MT, and Charged soft-MT) with different mechanical and electrical properties are prepared. The gliding directions within an electric field are predicted according to the measured stiffness and electrophoretic mobility. Finally, the Stiff-MTs are separated from Soft-MTs and Charged soft-MTs with a microfluidic sorter.

Fast Light-Driven Motion of Polydopamine Nanomembranes.

The actuation of micro- and nanostructures controlled by external stimuli remains one of the exciting challenges in nanotechnology due to the wealth of fundamental questions and potential applications in energy harvesting, robotics, sensing, biomedicine, and tunable metamaterials. Photoactuation utilizes the conversion of light into motion through reversible chemical and physical processes and enables remote and spatiotemporal control of the actuation. Here, we report a fast light-to-motion conversion in few-nanometer thick bare polydopamine (PDA) membranes stimulated by visible light. Light-induced heating of PDA leads to desorption of water molecules and contraction of membranes in less than 140 μs. Switching off the light leads to a spontaneous expansion in less than 20 ms due to heat dissipation and water adsorption. Our findings demonstrate that pristine PDA membranes are multiresponsive materials that can be harnessed as robust building blocks for soft, micro-, and nanoscale actuators stimulated by light, temperature, and moisture level.

Sense–Analyze–Respond–Actuate (SARA) Paradigm: Proof of Concept System Spanning Nanoscale and Macroscale Actuation for Detection of Escherichia coli in Aqueous Media

Foodborne pathogens are a major concern for public health. We demonstrate for the first time a partially automated sensing system for rapid (~17 min), label-free impedimetric detection of Escherichia coli spp. in food samples (vegetable broth) and hydroponic media (aeroponic lettuce system) based on temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) nanobrushes. This proof of concept (PoC) for the Sense-Analyze-Respond-Actuate (SARA) paradigm uses a biomimetic nanostructure that is analyzed and actuated with a smartphone. The bio-inspired soft material and sensing mechanism is inspired by binary symbiotic systems found in nature, where low concentrations of bacteria are captured from complex matrices by brush actuation driven by concentration gradients at the tissue surface. To mimic this natural actuation system, carbon-metal nanohybrid sensors were fabricated as the transducer layer, and coated with PNIPAAm nanobrushes. The most effective coating and actuation protocol for E. coli detection at various temperatures above/below the critical solution temperature of PNIPAAm was determined using a series of electrochemical experiments. After analyzing nanobrush actuation in stagnant media, we developed a flow through system using a series of pumps that are triggered by electrochemical events at the surface of the biosensor. SARA PoC may be viewed as a cyber-physical system that actuates nanomaterials using smartphone-based electroanalytical testing of samples. This study demonstrates thermal actuation of polymer nanobrushes to detect (sense) bacteria using a cyber-physical systems (CPS) approach. This PoC may catalyze the development of smart sensors capable of actuation at the nanoscale (stimulus-response polymer) and macroscale (non-microfluidic pumping).

Electromechanical Analysis of Flexoelectric Nanosensors Based on Nonlocal Elasticity Theory.

Flexoelectric materials have played an increasingly vital role in nanoscale sensors, actuators, and energy harvesters due to their scaling effects. In this paper, the nonlocal effects on flexoelectric nanosensors are considered in order to investigate the coupling responses of beam structures. This nonlocal elasticity theory involves the nonlocal stress, which captures the effects of nonlocal and long-range interactions, as well as the strain gradient stress. Based on the electric Gibbs free energy, the governing equations and related boundary conditions are deduced via the generalized variational principle for flexoelectric nanobeams subjected to several typical external loads. The closed-form expressions of the deflection and induced electric potential (voltage) values of flexoelectric sensors are obtained. The numerical results show that the nonlocal effects have a considerable influence on the induced electric potential of flexoelectric sensors subjected to general transverse forces. Moreover, the induced electric potential values of flexoelectric sensors calculated by the nonlocal model may be smaller or larger than those calculated by the classical model, depending on the category of applied loads. The present research indicates that nonlocal effects should be considered in order to understand or design basic nano-electromechanical components subjected to various external loads.

Axial strain effect on translational motion property of carbon nanotube thermal actuator

ABSTRACT The possibility of strain engineering of linear motion property of the double walled carbon nanotube thermal actuator is explored in this paper. Systematical study of axial strain effect on linear motion property of the carbon nanotube thermal actuator is conducted using molecular dynamics, and real-time controlling method for double walled carbon nanotube thermal actuator by applying axial strains is proposed as well. It is found that the axial strain has great influences on the linear motion behaviour of the thermal actuator, implying that the strain effect has potential to control the property of nano actuator. The strain effect and its mechanism are revealed by investigating the variation of thermal driving force and intertube distance as the axial strain is applied. The intertube interaction affected by axial strain is analyzed using the intertube stiffness constant drawn from the analytical model. The valuable conclusions for strain engineering of nanoscale actuator are obtained. This work suggests the great potential of strain effect to be real-time control method to nanoelectromechanical system and provides new ideas for the application of carbon nanotube thermal actuator.

Applications of Casimir forces: Nanoscale actuation and adhesion

Here, we discuss possible applications of the Casimir forces in micro- and nanosystems. The main part of this paper is devoted to actuation with quantum fluctuations and to the relative contribution of van der Waals and Casimir interactions to adhesion. Switching between the amorphous and crystalline states of phase change materials could generate force contrast sufficient for actuation, though for practical applications, the influence of protective capping layers and volume compression have to be better understood. Resilience against the pull-in instability is also a critical point defined by the material choice, dissipation in the system, and roughness of the surfaces. The adhesion induced by the Casimir forces is omnipresent, and it can play a pivotal role in unwanted stiction demanding deeper understanding. The open problems are the distance upon contact and the relative area of the real contact since both of them control the adhesion. An experiment designed to answer these questions is briefly discussed.

Measuring the five elastic constants of a nematic liquid crystal elastomer

ABSTRACT The determination of the mechanical properties of materials is important for the creation of physical models and describing mechanisms for converting diverse stimuli into mechanical response. It is critical for the design of nanoscale transducers, sensors, and actuators such as motors, pumps, artificial muscles and medical microrobots. This paper reports the measurement of the five independent elastic constants of a transversely isotropic liquid crystal elastomer. We express the elastic constants , , , , and in terms of strains and stresses, then measure these and determine the moduli for three different nematic elastomer samples.

Impact of Transduction Scaling Laws on Nanoelectromechanical Systems.

We study the electromechanical transduction in nanoelectromechanical actuators and show that the differences in scaling laws for electrical and mechanical effects lead to an overall nontrivial miniaturization behavior. In particular, the previously neglected fringing fields considerably increase electrical forces and improve the stability of nanoscale actuators. This shows that electrostatics does not pose any limitations to the miniaturization of electromechanical systems; in fact, in several respects, nanosystems outperform their microscale counterparts. As a specific example, we consider in-plane actuation of ultrathin slabs and show that devices consisting of a few layers of graphene are feasible, implying that electromechanical resonators operating beyond 40GHz are possible with currently available technology.

Biocompatible Magnetic Micro- and Nanodevices: Fabrication of FePt Nanopropellers and Cell Transfection.

The application of nanoparticles for drug or gene delivery promises benefits in the form of single-cell-specific therapeutic and diagnostic capabilities. Many methods of cell transfection rely on unspecific means to increase the transport of genetic material into cells. Targeted transport is in principle possible with magnetically propelled micromotors, which allow responsive nanoscale actuation and delivery. However, many commonly used magnetic materials (e.g., Ni and Co) are not biocompatible, possess weak magnetic remanence (Fe3 O4 ), or cannot be implemented in nanofabrication schemes (NdFeB). Here, it is demonstrated that co-depositing iron (Fe) and platinum (Pt) followed by one single annealing step, without the need for solution processing, yields ferromagnetic FePt nanomotors that are noncytotoxic, biocompatible, and possess a remanence and magnetization that rival those of permanent NdFeB micromagnets. Active cell targeting and magnetic transfection of lung carcinoma cells are demonstrated using gradient-free rotating millitesla fields to drive the FePt nanopropellers. The carcinoma cells express enhanced green fluorescent protein after internalization and cell viability is unaffected by the presence of the FePt nanopropellers. The results establish FePt, prepared in the L10 phase, as a promising magnetic material for biomedical applications with superior magnetic performance, especially for micro- and nanodevices.

Hydrogenated graphene oxide (H-G-SiO2) Janus structure: experimental and computational study of strong piezo-electricity response

We have investigated the piezoelectric response of the hydrogenated graphene oxide (H-G-SiO2) stacks both experimentally and theoretically. The piezoresponse force microscopy method and density-functional theory (DFT) calculations were used to study the piezoresponse effect of this structure from both experimental and computational point of views. A mono-layer graphene, made by chemical vapour deposition method, is deposited on Si/SiO2 substrate and its surface is then functionalized with hydrogen atoms. The vertical piezoresponse, observed by piezoresponse force microscopy, is measured to be about 2146 pC N−1, that is comparable to the reported state of the art piezoelectric materials such as relaxor-based ferroelectric single crystals. In order to carry out the DFT modelling, a H-Graphene-O Janus structure has been adopted, where graphene is modified by oxygen atoms adsorbed on one side while hydrogen atoms are placed on the other side. Through modelling by DFT calculations, it is revealed that, by applying out-of-plane compressive uniaxial strain, the structure preforms different piezoelectric behaviours, up to three orders of magnitude alteration by the applied strain. The demonstrated approach for enhancing the piezo-response of graphene paves the way for realizing graphene-based nanoscale sensors, actuators and transducers.

Nanoscale silicon-based actuators with extremely large actuation strain and extremely low driving voltage

The coupling between lithiation reaction of silicon and the considerable volume change has been widely recognized as an adverse effect which hinders the practical application of silicon-based lithium-ion batteries. Here we theoretically demonstrate a novel class of nanoscale electrochemically-driven silicon actuators, in virtue of the “unfavorable” gigantic volume expansion engendered by lithiation. Two representative design prototypes are reported, namely, a nano-sized flat-film silicon actuator and a nanowire silicon actuator. Our thermodynamic analysis establishes the operation condition of the actuators by identifying the electrochemical driving force and mechanical resistance due to lithiation-induced stress. We show that the nano-actuator exhibits an extremely low driving voltage about 1 V and an extremely high strain of actuation up to 300%, which goes far beyond the features of most common actuator materials. Given a mechanical load, the flat-film silicon actuator features a constant actuation strain and the nanowire actuator can provide tunable actuation strain. The results from the study offer quantitative guidance to the design of the novel silicon-based nano-actuators.