Advances in Mathematical Physics

Volume 2018, Article ID 9268973, 24 pages

https://doi.org/10.1155/2018/9268973

## Nanobeams and AFM Subject to Piezoelectric and Surface Scale Effects

^{1}PGMAT, Departamento de Matemática, CCNE, Universidade Federal de Santa Maria, Av. Roraima 1000, 97105-900, Santa Maria, RS, Brazil^{2}PPGMAp-IME, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91509-900, Porto Alegre, RS, Brazil^{3}Centro de Engenharias, CENG, Universidade Federal de Pelotas, Almirante Barroso, 1734, Pelotas, RS, 96010-280, Brazil

Correspondence should be addressed to Julio Claeyssen; moc.oohay@nessyealcrcj

Received 22 July 2018; Accepted 16 September 2018; Published 18 October 2018

Academic Editor: Phuc Phung-Van

Copyright © 2018 Julio Claeyssen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Vibration dynamics of elastic beams that are used in nanotechnology, such as atomic force microscope modeling and carbon nanotubes, are considered in terms of a fundamental response within a matrix framework. The modeling equations with piezoelectric and surface scale effects are written as a matrix differential equation subject to tip-sample general boundary conditions and to compatibility conditions for the case of multispan beams. We considered a quadratic and a cubic eigenvalue problem related to the inclusion of smart materials and surface effects. Simulations were performed for a two stepped beam with a piezoelectric patch subject to pulse forcing terms. Results with Timoshenko models that include surface effects are presented for micro- and nanoscale. It was observed that the effects are significant just in nanoscale. We also simulate the frequency effects of a double-span beam in which one segment includes rotatory inertia and shear deformation and the other one neglects both phenomena. The proposed analytical methodology can be useful in the design of micro- and nanoresonator structures that involve deformable flexural models for detecting and imaging of physical and biochemical quantities.

#### 1. Introduction

Atomic force microscopy (AFM) is a scanning probe microscopy (SPM) technique to obtain images of surface topography at the atomic scale, in a noninvasive manner, from a wide variety of samples on a scale from angstroms to 100 microns [1]. A typical AFM consists of a microcantilever with a sharp tip, a sample positioning system, a detection system, and a control system. The associated length scales are sufficiently small to call the applicability of classical continuum models into question [2]. In this work, we seek to develop a vibration dynamics framework for beams that include smart materials and subject to surface effects. This framework is also considered in the case of a two-span beam in which the first segment is governed by the Timoshenko model and the second segment is an Euler-Bernoulli beam model [3].

Recently, new generations of active microcantilevers have included piezoelectric materials locally attached at the microbeam with the role of sensors and/or actuators linear and nonlinear [4–9], among others. This has led to the study of multispan beams for AFM. The inclusion of smart materials layers will modify material properties between neighboring segments, producing discontinuities and fulfilling compatibility conditions for the continuity of the displacement and rotation of the beam and for the equilibrium of bending moment and shear at the discontinuity points.

Surface effects often play a significant role in the physical properties of micro- and nanosized materials and structures. Since the atoms within a very thin layer near surfaces experience a different local environment form that is experienced by atoms in bulk, the physical properties and mechanical response of surfaces will be distinct from those of bulk materials. Contrary to macroscopic structures, surface effects can strongly influence the stress and deformation properties of nanodevices. This latter is due to the increasing ratio between surface/interface area and volume. For instance, for a bulk deformation energy ( elastic stiffness) and ( surface energy per area), we shall have with distance . In [10], it has been shown that with a modified Timoshenko beam model the dynamics of nanoscaled tubes with surface effects deviate considerably from results obtained with classic theories.

In some models, considering effective properties () of nanosized beams when compared with standard continuum mechanics (), the relative error scale is found as , where is a geometrical constant depending upon the nanobeam considered, is a surface elastic constant, is a bulk elastic modulus, and is a length defining the size of the structural element [11]. Here we shall consider free vibration of a nanoscale continuum beam models which incorporates surface energy [12–14]. The search for time exponential solutions leads to a quadratic and cubic eigenvalue problems associated with a second-order modal differential system and a singular third-order differential system, respectively.

Simulations have been performed for triple span beams with and without a patch in the first segment and with loads in the first and last segment with tip-sample interaction. These loads included a uniform pulse load and its second derivative associated with the moment due to a piezoelectric patch. It is observed how the inclusion of piezoelectric materials absorbs vibrations when compared with classical multispan beams. Also, the robustness of the impulse response method with varying parameters and its influence in the behavior of the responses are observed.

The size dependence effects in Timoshenko microbeams with surface effects (TMB) have been simulated for the nondimensional natural frequency and compared with those of the classical Timoshenko beam model (TB). It has been observed that for beam length on the order of nanometer to microns, the difference between natural frequencies is apparent, and by increasing the length of the microbeam, the results tend to Timoshenko classical theory; that is, the surface effects are significant only in nanoscale. This same behavior was observed in [14] for a microbeam simply supported.

This work involves a methodology that may be adequately adapted to study recent advances in micro-/nanosized structures that are intensively used to design advanced micro-/nanosensors and molecular transportation systems devices for various engineering and medical applications. These sensors due to their ultrahigh resonant frequencies are important in sensitive sensing, molecular transportation, molecular separation, high-frequency signal processing, and biological imaging. Some deposited processes in the formation of thin-film composite membranes can be similar to types of multispan vibrating beams. However, the complex configuration of materials used in real world devices requires the study of material properties stemming from their atomically thin layered structures. Chemical deposited processes in the manufacturing of thin films on semiconductors can involve low temperatures with silicon rich nitride. Conventional structural analysis methods assume ideal structures (free of irregularities) but material and/or geometrical variations in a structure may result in drastic changes in its dynamical behaviors. Diffusion- and reaction-controlled interfacial polymerization is an important and practical topic that is beyond our scope [15–19].

This paper is organized as follows. In Section 2, we consider the Euler-Bernoulli multispan beam model, their compatibility conditions, and the boundary conditions in a matrix form. In Section 3, the dynamic response of the matrix Euler-Bernoulli multispan beam model subject to tip-sample interactions and external forcing is given in terms of the distributed matrix impulse response. The case of a microcantilever with a piezoelectric layer is discussed in Section 4. In Section 5 are presented the results of a double-span beam, obtained by the expansion of the first segment of the triple piezoelectric beam. In Sections 6 and 7 is considered the eigenanalysis of Timoshenko beam models with surface effects and their comparison with the Timoshenko classical model, for micro- and nanoscale. In Section 8, we consider the multispan Timoshenko beam model. AFM-based nanoscale processing with continuum surrounding media such as that found in biology and nanomachining applications [20–22] suggests observing frequency effects that arise with an academic two span beam model in which one segment includes rotatory inertia and shear deformation and the other one neglects both effects.

#### 2. Flexural Vibrations Using the Euler-Bernoulli Multispan Beam Model

We consider a multispan microcantilever of length composed of segments in which the displacement in the segment is governed by the Euler-Bernoulli modelwhere is the mass per unit length, is the cross section area, is the mass density, is the flexural stiffness, and is the transverse dynamic load at the station, . Here and .

##### 2.1. Matrix Formulation

The stepped Euler-Bernoulli model (1) can be written as a second-order differential equationwith diagonal matrix coefficients

whereHere the displacement and the dynamic load and , at the multispan beam, are given in vector form as

##### 2.2. Boundary and Internal Conditions for a Multispan Cantilever Beam

For a cantilever beam, the boundary conditions can be written in a compact matrix formwhere

At the points , , located between two consecutive segments, the compatibility conditions for the displacement, slope, bending moment, and shear lead to the compatibility matrix relationshipwithbeing for

#### 3. Forced Vibrations

The dynamic response of a forced multispan Euler-Bernoulli model given in (2), subject to homogeneous boundary conditions, could be described in terms of the diagonal matrix impulse response or initial-value Green function satisfying the initial-value problemwhere is the identity matrix and , are as given in (10), by using the i-th diagonal component of .

To work on the frequency domain, we need to introduce transfer function as being the Laplace transform of with respect to time (For inputs we seek outputs where with defined in (17)). Thus

It turns out that acts as an integrating factor in Lagrange’s adjoint method for the forced equation (2). Multiplying (2) by and integrating by parts, we obtain the dynamic response

where and are given initial conditions.

The procedure mentioned above is also related to the Riemann function method for integrating partial differential equations. Dynamic responses have been considered in the field of control of distributed systems and in elastodynamics in connection with vibrations and cracking problems [23–26].

In practice, when computing the convolution integral, which corresponds to the forced response , with null initial values , we actually have , where is a free vibration introduced by the system, whose initial values are a priori unknown, and is a particular response. When can be determined by other means, then those initial values can be supplied as and [27]. Thus, the induced system free response, due to a permanent response , is given by

##### 3.1. Frequency Response

Harmonic and piecewise linear forcing are of interest in frequency analysis. When seeking a response of the same type, the transfer function is introduced. Given the harmonic inputwe have the harmonic output responsewhere

In particular, for a concentrated force at a point , of spatial amplitude , we have the permanent responseWith the initial values , , the induced free response is given bywhereFor a pulse amplitudewhere is the unit step function, the permanent response turns out

As before, by substituting the initial values in (14), the induced free response will now bewith given as in (20).

##### 3.2. Closed Form of the Transfer Function for a Multispan Cantilever Beam

The Green function of a multispan Euler-Bernoulli can be determined by using an appropriate solution basis in each segment. From (12), is a diagonal matrix, with entries that satisfy the differential equationwhereThe functions constitute a solution basis of the equation . Hereis the solution of the initial-value problemBy denotingin each segment we havefor In order to determine , we need to find the initial-value vectorsBy using the compatibility conditionsgiven similarly to (9), and by deriving (30) with respect to .

The above process will be carried out for the case of a two stepped cantilever beam with the intermediate discontinuity point .

The boundary conditions and the initial values of lead to a simplification of in the first and last segments. We have two null initial values at , and we can use the boundary conditions at as initial values, instead of those at . This is possible by changing the solution basis at the last segment in (30) to the one generated by .

Thus, for

By applying the compatibility conditions

we get the algebraic system for determining from (30).

*Remark*. The case of axially loaded stepped beams has been investigated in [28] with nonclassical conditions. There, the Green function was obtained by working with a solution basis that considers hyperbolic and trigonometric functions in the general solution. These functions depend upon simple roots of a complete quadratic polynomial due to the inclusion of a second derivative spatial term in (24). When the axial load is removed, the solution basis reduces to the one considered for particular cases of vibrating beams. We observe that through a limit process the use of , which involves a division by root parameters, can also handle the case of repeated roots, which is of interest in the static case or critical frequencies.

#### 4. A Cantilever Beam with a Piezoelectric Layer in AFM

Active microcantilever beams due to their structural flexibility and sensitivity to atomic and molecular forces have received increased attention in a variety of nanoscale sensing and measuring applications, including atomic force microscopy, thermal scanning microscopy, and biomass sensing.

In [9], a microcantilever model was proposed for studying atomic force microscope. It was formulated as a three stepped beam with a piezoelectric layer patch in the first segment and the other two segments were simple beams with different cross-sectional areas according to Figure 1(a).