Nanoelectromechanical Applications Research Articles

Clamping enables enhanced electromechanical responses in antiferroelectric thin films.

Thin-film materials with large electromechanical responses are fundamental enablers of next-generation micro-/nano-electromechanical applications. Conventional electromechanical materials (for example, ferroelectrics and relaxors), however, exhibit severely degraded responses when scaled down to submicrometre-thick films due to substrate constraints (clamping). This limitation is overcome, and substantial electromechanical responses in antiferroelectric thin films are achieved through an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition using operando electron microscopy, such that the in-plane clamping further enhances the out-of-plane expansion, as rationalized using first-principles calculations. In turn, a non-traditional thickness scaling is realized wherein an electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO3 film that is just 100 nm thick. The high performance and understanding of the mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems.

Design of High-Frequency Carbon Nanotube-Carbon Nanotorus Oscillators for Energy Harvesting: A Molecular Dynamics Study.

The objective of this study is to examine the feasibility of using carbon-based nanostructures as nano-oscillators for future nanoelectromechanical applications such as energy harvesting devices and vibration sensing. The proposed nano-oscillator is comprised of a carbon nanotube (CNT) oscillating through a fixed carbon nanotorus molecule. For the first time in the literature, molecular dynamics (MD) simulations in conjunction with the Tersoff-Brenner (TB) and 6-12 Lennard-Jones (LJ) potential functions are adopted to determine the molecular interactions of the introduced nanodevice. To simulate the oscillatory behavior, two different schemes, namely, rigid and flexible, are considered. A detailed parametric study is performed to investigate the effects of rigidity, flexibility, and size of nanostructures as well as initial velocity on the force distribution and time histories of displacement and velocity of the core. Numerical results reveal that unlike the rigid oscillators, the flexible oscillators damp out within a few cycles. It is shown that the escape velocity of the flexible scheme is ∼6 times greater than that of the rigid scheme. The operating frequency and the generated power of rigid and flexible schemes under different system parameters are also calculated and compared. It is demonstrated that with increasing the ratio of nanotube-to-nanotorus diameter, the operating frequencies of both schemes decrease, while the generated powers do not behave monotonically. For a determined system parameter, it is observed that the flexible scheme provides higher operating frequencies compared to the rigid one. Moreover, considering that the initial velocity of the system is identical to the escape velocity, the generated power of the flexible scheme is calculated to be ∼14 times greater than that of the rigid scheme.

Giant electrostriction-like response from defective non-ferroelectric epitaxial BaTiO3 integrated on Si (100)

Lead-free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04×10−14 m2/V2 at 1 kHz, and 3.87×10−15 m2/V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained upon cycling upto 6 million times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates well with the observed giant electrostriction-like response. These films show large coefficient of thermal expansion (2.36 × 10−5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films.

Unconventional Piezoelectricity of Two-Dimensional Materials Driven by the Hall Effect.

Piezoelectricity has been widely explored for nanoelectromechanical applications, yet its working modes are mainly limited in polar directions. Here we discover the intrinsic electro-mechanical response in crystal materials that is transverse to the conventional polarized direction, which is named unconventional piezoelectricity. A Hall-like mechanism is proposed to interpret unconventional piezoelectricity as charge polarization driven by a built-in electric field for systems with asymmetric Berry curvature distributions. Density functional theory simulations and statistical analyses justify such a mechanism and confirm that unconventional piezoelectricity is a general property for various two-dimensional materials with spin splitting or valley splitting. An empirical formula is derived to connect the conventional and unconventional piezoelectricity. The extended understanding of the piezoelectric tensor in quantum materials opens an opportunity for applications in multidirectional energy conversion, broadband operation, and multifunctional sensing.

Single-nanowire-morphed mechanical slingshot for directional shooting delivery of micro-payloads

Stretching elastomer bands to accumulate strain energy, for a sudden projectile launching, has been an old hunting skill that will continue to find new applications in miniaturized worlds. In this work, we explore the use of highly resilient and geometry-tailored ultrathin crystalline silicon nanowires (SiNWs) as elastic medium to fabricate the first, and the smallest, mechanical slingshot. These NW-morphed slingshots were first grown on a planar surface, with desired layout, and then mounted upon standing pillar frames, with a unique self-hooking structure that allows for a facile and reliable assembly, loading and shooting maneuver of microsphere payloads. Impressively, the elastic spring design can help to store 10 times more strain energy into the NW springs, compared with the straight ones under the same pulling force, which has been strong enough to overcome the sticky van der Waals (vdW) force at the touching interfaces that otherwise will hinder a reliable releasing onto soft surface with low-surface energy or adhesion force, and to achieve a directional shooting delivery of precise amount of tiny payload units onto delicate target with the least impact damage. This NW-morphing construction strategy also provides a generic protocol/platform to fast design, prototype, and deploy new nanoelectromechanical and biological applications at extremely low costs.

Two-Dimensional Ultrahigh Unconventional Piezoelectricity Driven by Charge Screening

In the past decade, piezoelectricity has been explored in a series of two-dimensional (2D) materials for nanoelectromechanical applications, while their piezoelectric coefficients are mostly much lower than those of prevalent piezoceramics. In this paper, we propose an unconventional approach of inducing 2D ultrahigh piezoelectricity dominated by charge screening instead of lattice distortion and show the first-principles evidence of such piezoelectricity in a series of 2D van der Waals bilayers, where the bandgap can be remarkably tuned via applying a moderate vertical pressure. Their polarizations can switch between the screened and unscreened state by a pressure-driven metal-insulator transition, which can be realized via tuning interlayer hybridization or inhomogeneous electrostatic potential by substrate layer to change the band splitting or tuning the relative energy shift between bands utilizing the vertical polarization of the substrate layer. Such 2D piezoelectric coefficients can be unprecedented and orders of magnitude higher than those of previously studied monolayer piezoelectrics, and their high efficiency of energy harvesting in nanogenerators can be expected.

Design and Validation of a Testing 4D Mechatronic System for Measurement and Integrated Control of Processes

Measurements are crucial for research in the fields of microelectromechanical (MEMS), nanoelectromechanical (NEMS) and industrial applications. In this work, the design approach for the development and construction of a testing 4D mechatronic system, and respective validation, including the detailed description of the used components and parts as well as the performed tests for respective validation in the working environment, are presented. Because this is testing equipment, the measurement feature is presented and validated, in detail, making this system available and reliable for the mentioned purposes of use. An important result in this work is the possibility of on-site control or the remote control of the 4D mechatronic system for measurement and integrated control of processes, with the aim of reducing the cost of obtaining the necessary measurements.

Flexoelectric-boosted piezoelectricity of BaTiO3@SrTiO3 core-shell nanostructure determined by multiscale simulations for flexible energy harvesters

Achieving lead-free piezoelectric nanoparticles with excellent electromechanical responses is in great demand for fabricating nanoscale electronic devices. Recently, the coupling effect of combining flexoelectricity and piezoelectricity has attracted attention as a promising approach to control electromechanical properties. However, the implication of the coupling for the nanoparticles in this regard has been challenging due to its difficulty in controlling and observing the internal strain gradient. In this study, we demonstrate the flexoelectric-boosted electromechanical properties of piezoelectric nanoparticles using an induced built-in strain gradient in heterogeneous core-shell nanostructure. The composition-graded core-shell structure of BaTiO3@SrTiO3 nanoparticles enables a significant increment of the effective piezoelectric charge coefficient via the chemical heterogeneities-induced lattice strain gradient. Through the combinations of ab-initio calculation and multiphysics simulations, the origin of the strain distribution over nanoparticles is theoretically interpreted with accompanying phase balance and diffusion criteria. In addition, our designed core-shell nanoparticles-based energy harvesting devices generate highly efficient and flexoelectric-boosted piezoelectric output signals. Individual core-shell nanoparticles and related elastomeric nanocomposites reported in this work represent state-of-the-art electromechanical properties compared to previously reported piezoelectric nanoparticles and composites. This study provides a new source of inspiration for achieving high-performance lead-free piezoelectric nanostructures, paving the way for developing nano-electromechanical applications.

Origin of the Repulsive Casimir Force in Giant Polarization-Interconversion Materials

Achieving strong repulsive Casimir forces through engineered coatings can pave the way for micro- and nano-electromechanical applications where adhesive forces currently cause reliability issues. Here, we exploit Lifshitz theory to identify the requirements for repulsive Casimir forces in gyrotropic media for two limiting cases (ultra-strong gyroelectric and non-gyroelectric). We show that the origin of repulsive force in media with strong gyrotropy such as Weyl semi-metals arises from the giant interconversion of polarization of vacuum fluctuations.

Robust Superlubricity of Gold–Graphite Heterointerfaces

AbstractNoncommensurate 2D interfaces hold great promise toward low friction and nanoelectromechanical applications. For identical constituents, the crystals interlock at specific rotational configurations leading to high barriers for slide. In contrast, nonidentical constituents comprising different lattice parameters should enable robust superlubricity for all rotational configurations. This is however not the case for gold–graphite interfaces, as both theory and experiments show scaling behavior of the sliding force as a function of the interface contact area. By simulating the sliding force for gold–graphite interfaces, this work shows that the origin for high force barriers at special angular configurations is a result of commensurability between the moiré structure and the contact geometry. Consequently, this paper suggests new geometries that can potentially overcome such commensurability effects to enable robust superlubricity.

Electronic and vibrational properties of van der Waals heterostructures of vertically stacked few-layer atomically thin MoS2 and BP

The state-of-the-art heterostructure-based devices often involve stacks of epilayers of few nanometer thick crystals. However, the ultimate limit would be a hitherto single-atomic-layer structure. Using material-by-design approach, the flexibility of two-dimensional (2D) materials could be explored to develop a multilayered artificial van der Waals (vdW) heterostructure where the synergistic coupling between the individual materials and the underlying interface creates new and novel functionalities. Herein, based on first-principles calculations, we report that vertically stacked few-layer vdW heterostructure of monolayer molybdenum disulfide (MoS2) and hexagonal boron phosphide (BP) is a strong electrically narrow direct bandgap material. More intriguingly, few-layer vdW heterostructure of monolayer MoS2 and BP exhibits superior mechanical properties. In striking contrast to heterostructures of MoS2/graphene, MoS2/WS2, and few-layer graphene, where the 2D moduli are lower than the sum of the 2D modulus of each nanosheet, the mechanical strength of few-layer hybrid vdW heterostructure of monolayer MoS2 and BP rather increased with increasing thickness. We attribute this difference to the unique interlayer and interface coupling, which affected the vibrational properties of the heterostructures including the emergence of shear and breathing phonon modes as well as the transformation of flexural phonon modes. Such superior mechanical properties could be explored for nanoelectromechanical device applications, e.g., as a nanoresonator. We demonstrate that the electronic structure could as well be tuned with both increasing numbers of monolayer stacks and defect-engineering promising for low-power applications.

Wrinkling C3N nano-grids in uniaxial tensile testing; a molecular dynamics study

Two-dimensional materials in recent years have found a special place in various sciences. Graphene, as the first two-dimensional material presented, has been able to reduce the size of electronic devices from what it was. Following successful successive graphene experiments, carbon-nitride materials such as C3N and C3N4 have been considered, which can have significant properties in nanoelectromechanical applications with regard to nitrogen doped in the graphene structure. In such applications, the first issue to be considered is the strength of the structure. For this purpose, in this paper, the mechanical properties and the strengths and weaknesses of the C3N nano-grids are evaluated using the molecular dynamics method. After verifying the used model, stress-strain diagrams of the structure are drawn in different states and the effects of circular and square holes in the C3N nano-grids are discussed. The stress distribution is depicted and it is seen that in certain dimensions, the type of strain energy release is different from others. The inductive shrinkage due to stretching is the most important reason for this, and it has finally been observed that the presence of holes of dimensions of 1.8 to 3 nm causes regular wrinkles in the structure, which can be remarkable for the construction of large area C3N nano-grids.

Molecular dynamics study of gold nano-clusters aggregation on a model defected graphene

Functionalized graphene sheets have attracted increasing attention due to their novel micro-/nano-electromechanical applications. In this paper, the aggregation of the gold nano-clusters on the defected graphene sheet is studied by using the molecular dynamics simulation method. It is shown that a model defected graphene with randomly distributed vacancies can affect the formation and aggregation of the Au nano-clusters on the graphene sheet. It is found that the Au nano-clusters agglomerate on the pristine parts of the surface rather than on the defected parts. In addition, the results show that increasing the temperature amplifies the above result and varies the Au nano-cluster sizes. Moreover, it is observed that the aggregation of Au clusters changes the surface roughness. The results presented here would help to design more efficient functionalized graphene-based electronic devices.

Phosphorene as a nanoelectromechanical material

Based on density functional simulations combined with the Landauer transport theory, the mechanical strain impacts on the chemical bonds of phosphorene and their effects on the electronic properties are studied. Moreover, the effect of the tensile strain along the zigzag direction on the charge transport properties of a two-terminal phosphorene device is evaluated. Enhancement of the intraplanar interactions, in particular between the next-nearest neighbors in strained phosphorene, is found to be essential in the band-structure evolution. The charge transport analyzing shows that phosphorene has a strong piezoconductance sensitivity, which makes this material highly desirable for high-pressure nanoelectromechanical applications. The piezoconductance gauge factor increases by strain from 46 in 5% tension to 220 in 12% tension, which is comparable to state-of-the-art silicon strain sensors. The transmission pathways monitor the current flowing in terms of the chemical bonds and hopping, however, the transport mostly arises from the charge transferring through the chemical bonds. The strong anisotropy in the transport properties along zigzag and armchair directions is observed.

Strain-engineering of Janus SiC monolayer functionalized with H and F atoms

Based on ab initio density functional theory calculations, the structural, electronic, mechanical, acoustic, thermodynamic, and piezoelectric properties of (F,H) Janus SiC monolayers are studied. The new set of derivatives shows buckled structures and different band gap values. Under strain, the buckling changes and the structures pass from semiconducting to metallic. The elastic limits and the metastable regions are determined. The Young's modulus and Poisson ratio reveal stronger behavior for the modified conformers with respect to graphene. The values of the Debye temperature make the new materials suitable for thermal application. Moreover, all the conformers show in-plane and out-of-plane piezoelectric responses comparable with known two-dimensional materials. If engineered, such piezoelectric Janus structures may be promising materials for various nanoelectromechanical applications.

Piezoresistive Strain Sensors Based on Carbon Nanotube Networks: Contemporary approaches related to electrical conductivity

Since they were discovered in 1991 [2], CARBON nanotubes (CNTs) have attracted enormous attention because of their remarkable properties [1], such as high electrical conductivity, ultrasmall diameter, and large aspect ratio. In addition, CNTs exhibit significant electromechanical properties [3] that could be useful in applications for piezoresistive-type sensors such as strain gauges, pressure sensors, chemical and biological sensors, microelectronic devices, and structural condition monitoring through strain sensing [4]. In addition to the application of a single CNT in various nanoelectromechanical applications, CNT researchers have gradually moved toward potential applications of CNTs as filler candidates in various types of multifunctional material systems, representing a new direction with broad applications [5], [6].

Characterization of SiO2/SiNx gate insulators for graphene based nanoelectromechanical systems

The structural and magnetotransport characterization of graphene nanodevices exfoliated onto Si/SiO2/SiNx heterostructures are presented. Improved visibility of the deposited flakes is achieved by optimal tuning of the dielectric film thicknesses. The conductance of single layer graphene Hall-bar nanostructures utilizing SiO2/SiNx gate dielectrics were characterized in the quantum Hall regime. Our results highlight that, while exhibiting better mechanical and chemical stability, the effect of non-stoichiometric SiNx on the charge carrier mobility of graphene is comparable to that of SiO2, demonstrating the merits of SiNx as an ideal material platform for graphene based nanoelectromechanical applications.

Dipolar polarization and piezoelectricity of a hexagonal boron nitride sheet decorated with hydrogen and fluorine

In contrast to graphene, a hexagonal boron nitride (h-BN) monolayer is piezoelectric because it is non-centrosymmetric. However, h-BN shows neither in-plane nor out-of-plane dipole moments due to its three-fold symmetry on the plane and the fact that it is completely flat. Here, we show that the controlled adsorption of hydrogen and/or fluorine atoms on both sides of a pristine h-BN sheet induces flatness distortion in a chair form and an out-of plane dipole moment. In contrast, a boat form has no out-of-plane dipole moment due to the alternating boron and nitrogen positions normal to the plane. Consequently, the chair form of surface-modified h-BN shows both in-plane and out-of-plane piezoelectric responses; while pristine h-BN and the boat form of decorated h-BN have only in-plane piezoelectric responses. These in-plane and out-of-plane piezoelectric responses of the modified h-BN are comparable to those in known three-dimensional piezoelectric materials. Such an engineered piezoelectric two-dimensional boron nitride monolayer can be a candidate material for various nano-electromechanical applications.

Size-dependent elastic properties of crystalline polymers via a molecular mechanics model

An analytical molecular mechanics model is developed to obtain the size-dependent elastic properties of crystalline polyethylene. An effective “stick-spiral” model is adopted in the polymer chain. Explicit equations are derived from the Lennard-Jones potential function for the van der Waals force between any two polymer chains. By using the derived formulas, the nine size-dependent elastic constants are investigated systematically. The present analytical results are in reasonable agreement with those from present united-atom molecular dynamics simulations. The established analytical model provides an efficient route for mechanical characterization of crystalline polymers and related materials toward nanoelectromechanical applications.

On the role of interband surface plasmons in carbon nanotubes

We review the properties of collective surface excitations—excitons and interband plasmons—in single-walled and double-walled carbon nanotubes. We show that an electrostatic field applied perpendicular to the nanotube axis can control the exciton-plasmon coupling in individual small-diameter (≲nm) singlewalled nanotubes, both in the linear excitation regime and in the non-linear excitation regime with the photoinduced biexcitonic states formation. For double-walled carbon nanotubes, we report a profound effect of interband surface plasmons on the inter-tube Casimir force at tube separations similar to their equilibrium distances. Strong overlapping plasmon resonances from both tubes warrant their stronger attraction. Nanotube chiralities possessing such collective excitation features will result in forming the most favorable innerouter tube combination in double-walled carbon nanotubes. These findings pave the way for the development of new generation of tunable optoelectronic and nano-electromechanical device applications with carbon nanotubes.