Vibration Analysis of Piezoelectric Microcantilever Sensors

The main objective of this dissertation is to comprehensively analyze vibration characteristics of microcantilever-based sensors with application to ultra small mass detection and low dimensional materials characterization. The first part of this work focuses on theoretical developments and experimental verification of piezoelectric microcantilevers, commercially named Active Probes, which are extensively used in most today\u27s advanced Atomic Force Microscopy (AFM) systems. Due to special geometry and configuration of Active Probes, especially multiple jump discontinuities in their cross-section, a general and comprehensive framework is introduced for forced vibration and modal analysis of discontinuous flexible beams. More specifically, a general formulation is obtained for the characteristics matrix using both boundary and continuity conditions. The formulation is then reduced to the special case of Active Probes with intentional geometrical discontinuities. Results obtained from experiment are compared with the commonly used uniform beam model as well as the proposed discontinuous beam model. It is demonstrated that a significant enhancement on sensing accuracy of Active Probes can be achieved using the proposed discontinuous beam model compared to a uniform model when a multiple-mode operation is desired. In the second part of this dissertation, a comprehensive dynamic model is proposed for vector Piezoforce Microscopy (PFM) system under applied electrical loading. In general, PFM is considered as a suspended microcantilever beam with a tip mass in contact with a piezoelectric material. The material properties are expressed in two forms; Kelvin-Voigt model for viscoelstic representation of the material and piezoelectric force acting on the tip as a result of response of material to applied electric field. Since the application of bias voltage to the tip results in the surface displacement in both normal and in-plane directions, the microcantilever is considered to vibrate in all three directions with coupled transversal/longitudinal and lateral/torsional motions. In this respect, it is demonstrated that the PFM system can be governed by a set of partial differential equations along with non-homogeneous and coupled boundary conditions. Using the method of assumed modes, the governing ordinary differential equations of the system and its state-space representation are derived under applied external voltage. The formulation is then reduced to vertical PFM, in which low dimensional viscoelestic and piezoelectric properties of periodically poled lithium niobate (PPLN) material can be detected. For this purpose, the experimental and theoretical frequency responses along with a minimization strategy for the percentage of modeling error are utilized to obtain optimal spring constant of PPLN. Finally, the step input responses of experiment and theory are used to estimate the piezoelectric and damping coefficients of PPLN. Overall in this dissertation, a precise dynamic model is developed for piezoelectric microcantilever for ultra small mass detection purpose. This model can also be utilized in AFM systems to replace laser-based detection mechanism with other alternative transductions. Moreover, a comprehensive model is proposed for PFM system to simultaneously detect low dimensional viscoelastic and piezoelectric properties of materials. This model can also be utilized for data storage purpose in ferroelectric materials

Direct Growth of High Mobility and Lowâ Noise Lateral MoS2â Graphene Heterostructure Electronics

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138199/1/smll201604301_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138199/2/smll201604301.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138199/3/smll201604301-sup-0001-S1.pd

Power Dissipation of WSe2 Field-Effect Transistors Probed by Low- Frequency Raman Thermometry

The ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry results in high power densities during device operation, which could cause a significant temperature rise within 2D channels. One challenge inRaman thermometry of 2D materials is that the commonly used high-frequency modes do not precisely represent the temperature rise in some 2D materials because of peak broadening and intensity weakening at elevated temperatures. In this work, we show that a low-frequency E2g 2 shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in backgated tungsten diselenide (WSe2) field-effect transistors, whereas the high-frequency peaks (E2g 1 and A1g) fail to provide reliable thermal information. Our calculations indicate that the broadening of high-frequency Raman-active modes is primarily driven by anharmonic decay into pairs of longitudinal acoustic phonons, resulting in a weak coupling with out-of-plane flexural acoustic phonons that are responsible for the heat transfer to the substrate. We found that the TBCat the interface of WSe2 and Si/SiO2 substrate is ∼16 MW/m2 K, depends on the number of WSe2 layers, and peaks for 3−4 layer stacks. Furthermore, the TBC to the substrate is the highest from the layers closest to it, with each additional layer adding thermal resistance. We conclude that the location where heat dissipated in a multilayer stack is as important to device reliability as the total TBC

Effects Of Gold Nanoparticles And Lithium Hexafluorophosphate On The Electrical Conductivity Of Pmma

An increase in electrical conductivity of a polymeric system can be realized by adding conductive fillers and/or dissolving a salt in a suitable solvent or polymer through formation of ionic conduction. An appropriate solvent that can form complexes with alkali metal cations is critical to providing electrical conductivity enhancements to a wide variety of polymers. In this study, we investigated the effects on electrical conductivity of lithium hexafluorophosphate (LiF6P) through the use of butyl glycidyl ether (BGE) as the solvent for dissolving the alkali metal compound LiF6P. Additionally we examined the effects of gold nanoparticles (AuNPs) alone and with the LiF6P/BGE for possible synergistic effects on electrical conductivity. Thin films of poly (methyl methacrylate) (PMMA) blended with LiF6P salt and AuNPs separately and together, were prepared. The electrical conductivity measurements were carried out on these films as a function of the salt and/or AuNP contents. PMMA with only 0.75 wt.% LiF6P decreased the resistivity by 3 orders of magnitude compared to PMMA, which showed the optimum conductivity value for this system. Formation of BGE-LiF6P complexes were studied by FTIR spectra. XRD studies confirmed the formation of complexes in thin film specimens. It was also found that the conductivity of PMMA with AuNPs is dependent on the size of the AuNPs. © 2007 Elsevier B.V. All rights reserved

Phase Separation and Ion Diffusion in Ionic Liquid, Organic Solvent, and Lithium Salt Electrolyte Mixtures

The highly desirable characteristics of ternary mixtures of ionic liquids, organic solvents, and metal salts make them a promising candidate for use in various electrothermal energy storage and conversion systems. In this study, using large-scale classical molecular dynamics simulations, we looked into 10 different ternary electrolyte mixtures using combinations of [EMIM]+, [BMIM]+, and [OMIM]+ cations with [NO3]−, [BF4]−, [PF6]−, [ClO4]−, [TFO]−, and [NTf2]− anions, tetraglyme, and Li salt to study the effect of ionic liquid composition on the phase behavior of ternary electrolyte mixtures. We uncovered that in these electrolytes, phase separation is mainly a function of pairwise binding energy of the constituents of the mixture. To corroborate this theory, several simulations are performed at various temperatures ranging from 260 to 500 K for each mixture, followed by calculating the binding energy of ionic liquid pairs using density functional theory. Our results verify that the transition temperature for the phase separation of each system is indeed a function of the pairwise binding energy of its ionic liquid pairs. It is also found that in some cases, the diffusion coefficient of the Li+ ions decreased even with the increase in the temperature, an effect that is attributed to the presence of condensed ionic domains in the electrolyte. This study provides a new insight for the design of multicomponent electrolyte mixtures for a wide range of energy applications

Preparation And Properties Of Natural Sand Particles Reinforced Epoxy Composites

An epoxy composite using Cancun natural hydrophobic sand particle as filler material was fabricated in this study. Three point bending tests demonstrated an enhancement of 7.5 and 8.7% in flexural strength and flexural modulus, respectively, of epoxy composite containing 1 wt-% sand particles without any chemical treatment involved, compared to the pristine epoxy. Scanning electron microscopy (SEM) studies revealed that the fracture toughness of the epoxy matrix was enhanced owing to the presence of sand particles in an epoxy/sand composite. Through dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA) methods, it was found that the storage modulus (E′), glass transition temperature (Tg) and dimensional stability of the sand particles/ epoxy composites were increased compared to the pristine epoxy. The friction behavior of epoxy/sand system reflected that the microstructure of epoxy composites was steady. These experimental results suggest that Cancun sand, as a freshly found natural micron porous material, may find promising applications in composite materials. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA

Interfacial Thermal Transport in Monolayer MoS2- and Graphene-Based Devices

In many device architectures based on 2D materials, a major part of the heat generated in hot-spots dissipates in the through-plane direction where the interfacial thermal resistances can significantly restrain the heat removalcapability of the device. Despite its importance, there is an enormous (1–2 orders of magnitude) disagreement in the literature on the interfacial thermal transport characteristics of MoS2 and other transition metal dichalcogenides (TMDs) (0.1–14 MW m−2 K−1). In this report, the thermal boundary conductance (TBC) across MoS2 and graphene monolayers with SiO2/Si and sapphire substrates is systematically investigated using acustom-made electrical thermometry platform followed by 3D finite element analyses. Through comparative experiments, the TBC at 295 K across MoS2 is found to be 20.3–33.5 MW m−2 K−1 on SiO2/Si, and 19–37.5 MW m−2 K−1 on c-sapphire, respectively, but far larger than the previous Raman-based measurements on TMDs with optical heating (0.1–2 MW m−2 K−1). This study also investigates the effects of processing quality and potential interface contaminants, substrate properties, and encapsulation on TBC across MoS2 and graphene monolayers. Our results reveal that the emergence of Rayleigh wave modes dramatically contributes to the interfacial conductance across encapsulated 2D monolayers. This finding opens up an additional pathway to improve heat dissipation in 2D-based devices through engineering of an encapsulating layer

Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor Deposition

Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and “puckering” mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles