Raimonds Meija

Nanomechanics of individual aerographite tetrapods.

Carbon-based three-dimensional aerographite networks, built from interconnected hollow tubular tetrapods of multilayer graphene, are ultra-lightweight materials recently discovered and ideal for advanced multifunctional applications. In order to predict the bulk mechanical behaviour of networks it is very important to understand the mechanics of their individual building blocks. Here we characterize the mechanical response of single aerographite tetrapods via in situ scanning electron and atomic force microscopy measurements. To understand the acquired results, which show that the overall behaviour of the tetrapod is governed by the buckling of the central joint, a mechanical nonlinear model was developed, introducing the concept of the buckling hinge. Finite element method simulations elucidate the governing buckling phenomena. The results are then generalized for tetrapods of different size-scales and shapes. These basic findings will permit better understanding of the mechanical response of the related networks and the design of similar aerogels based on graphene and other two-dimensional materials.

An AC-assisted single-nanowire electromechanical switch

A unique two-source controlled nanoelectromechanical switch has been assembled from individual, single-clamped Ge nanowires. The switching behaviour was achieved by superimposing the control signals of specific frequencies to the electrostatic potential of the output terminals, eliminating the need for an additional gate electrode. Using an in situ manipulation technique inside a scanning electron microscope, we demonstrate that the pull-out force required to overcome adhesion at the contact can be significantly reduced by exciting mechanical resonant modes within the nanowire.

Determination of Young’s modulus of Sb2S3 nanowires by in situ resonance and bending methods

Summary In this study we address the mechanical properties of Sb2S3 nanowires and determine their Young’s modulus using in situ electric-field-induced mechanical resonance and static bending tests on individual Sb2S3 nanowires with cross-sectional areas ranging from 1.1·104 nm2 to 7.8·104 nm2. Mutually orthogonal resonances are observed and their origin explained by asymmetric cross section of nanowires. The results obtained from the two methods are consistent and show that nanowires exhibit Young’s moduli comparable to the value for macroscopic material. An increasing trend of measured values of Young’s modulus is observed for smaller thickness samples.

Sensing properties of assembled Bi2S3 nanowire arrays

Bismuth sulfide (Bi2S3) nanowires were grown in porous aluminium oxide template and a selective chemical etching was applied to transfer the nanowires to a solution. Well aligned nanowire arrays were assembled on pre-patterned silicon substrates employing dielectrophoresis. Electron beam lithography was used to connect aligned individual nanowires to the common macroelectrode. In order to evaluate the conductometric sensing performance of the Bi2S3 nanowires, current–voltage characteristics were measured at different relative humidity (RH) levels (5–80%) / argon medium. The response of the Bi2S3 nanowires depending of RH is found to be considerably different from those reported for other types of nanowire RH sensor devices.

Electric current induced modification of germanium nanowire NEM switch contact

We present an investigation of contact properties of a germanium (Ge) nanowire based nanoelectromechanical (NEM) switch in its ON state. The contact stiffness in the ON state was evaluated by detecting the nanowires resonance frequency. It was found that the resonance frequency increases when electric current flows through the nanowire/counter electrode contact area. The reason for modification in the contact area is referred to as electric-current-induced processes in the native oxide layer covering the nanowires. The presented resonance shift method is a simple way to indicate strengthening of the nanowire/counter electrode contact area without disassembling the contact.

Vapor–solid synthesis and enhanced thermoelectric properties of non-planar bismuth selenide nanoplates on graphene substrate

In this work, stoichiometric separate bismuth selenide (Bi2Se3) nanoplates and continuous Bi2Se3 coatings are synthesized on graphene substrate by catalyst-free vapor–solid deposition method. The orientation of synthesized nanoplates relative to the substrate surface varies from heteroepitaxial (planar) to oriented under different angles (non-planar). The non-planar growth of the nanoplates was achieved for the first time by short-term carrier inert gas flow in certain temperature interval of the synthesis process. The crystallographic growth directions of non-planar nanoplates were determined from HRTEM images as well as estimated from the slope angles of non-planar nanoplates. Bi2Se3 coatings consisting of combination of planar and non-planar nanoplates exhibit significantly enhanced in comparison with coating consisting of only planar coalescent Bi2Se3 nanoplates thermoelectric properties. Demonstrated graphene/Bi2Se3/graphene devices may find applications in thermoelectric and photo-detection sensors.

Review: Electrostatically actuated nanobeam-based nanoelectromechanical switches – materials solutions and operational conditions

This review summarizes relevant research in the field of electrostatically actuated nanobeam-based nanoelectromechanical (NEM) switches. The main switch architectures and structural elements are briefly described and compared. Investigation methods that allow for exploring coupled electromechanical interactions as well as studies of mechanically or electrically induced effects are covered. An examination of the complex nanocontact behaviour during various stages of the switching cycle is provided. The choice of the switching element and the electrode is addressed from the materials perspective, detailing the benefits and drawbacks for each. An overview of experimentally demonstrated NEM switching devices is provided, and together with their operational parameters, the reliability issues and impact of the operating environment are discussed. Finally, the most common NEM switch failure modes and the physical mechanisms behind them are reviewed and solutions proposed.

Young’s modulus and indirect morphological analysis of Bi2Se3 nanoribbons by resonance measurements

An electrostatically induced resonance behaviour of individual topological insulator Bi2Se3 nanoribbons grown by a catalyst free vapour-solid synthesis was studied in situ by scanning electron microscopy. It was demonstrated that the relation between the resonant frequencies of vibrations in orthogonal planes can be applied to distinguish the nanoribbons with rectangular cross-sections from the nanoribbons having step-like morphology (terraces). The average Youngs modulus of the Bi2Se3 nanoribbons with rectangular cross-sections was found to be 44 ± 4 GPa.

Relative Humidity Dependent Resistance Switching of Bi2S3 Nanowires

Electrical properties of Bi2S3 nanowires grown using a single source precursor in anodic aluminum oxide templates are sensitive to the relative humidity in an inert gas environment. Dynamic sensing dependency is obtained and shows presence of spontaneous resistance switching effect between low and high relative humidity states. Employing the thermionic field emission theory, heights of Schottky barriers are estimated from the current-voltage characteristics and in relation to the humidity response. The change of Schottky barrier height is explained by local changes in physically adsorbed water molecules on the surface of the nanowire.

Devices based on semiconductor nanowires

Recently, nanoelectromechanical systems (NEMS) have attracted much attention due to their unique properties and possible applications that differ greatly from those of microelectromechanical systems. NEMS operating frequencies may achieve giga- and terahertz levels and their power consumption and heat capacity is extremely low. Moreover, integration levels may reach 1012 devices per cm−2. In this review, we present techniques for integrating semiconductor materials in NEMS. In particular, we examine fabrication, structure, properties and potential applications of two main classes of NEMS, namely, resonators and switches.