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

Charge Transport in Polycrystalline Graphene: Challenges and Opportunities

Graphene has attracted significant interest both for exploring fundamental science and for a wide range of technological applications. Chemical vapor deposition (CVD) is currently the only working approach to grow graphene at wafer scale, which is required for industrial applications. Unfortunately, CVD graphene is intrinsically polycrystalline, with pristine graphene grains stitched together by disordered grain boundaries, which can be either a blessing or a curse. On the one hand, grain boundaries are expected to degrade the electrical and mechanical properties of polycrystalline graphene, rendering the material undesirable for many applications. On the other hand, they exhibit an increased chemical reactivity, suggesting their potential application to sensing or as templates for synthesis of one-dimensional materials. Therefore, it is important to gain a deeper understanding of the structure and properties of graphene grain boundaries. Here, we review experimental progress on identification and electrical and chemical characterization of graphene grain boundaries. We use numerical simulations and transport measurements to demonstrate that electrical properties and chemical modification of graphene grain boundaries are strongly correlated. This not only provides guidelines for the improvement of graphene devices, but also opens a new research area of engineering graphene grain boundaries for highly sensitive electrobiochemical devices

Effect of Interface Structure on Mechanical Properties of Advanced Composite Materials

This paper deals with the effect of interface structures on the mechanical properties of fiber reinforced composite materials. First, the background of research, development and applications on hybrid composite materials is introduced. Second, metal/polymer composite bonded structures are discussed. Then, the rationale is given for nanostructuring the interface in composite materials and structures by introducing nanoscale features such as nanopores and nanofibers. The effects of modifying matrices and nano-architecturing interfaces on the mechanical properties of nanocomposite materials are examined. A nonlinear damage model for characterizing the deformation behavior of polymeric nanocomposites is presented and the application of this model to carbon nanotube-reinforced and reactive graphite nanotube-reinforced epoxy composite materials is shown

Ionic liquids in the electrochemical valorisation of CO2

The development of electrochemical processes for using captured CO2 in the production of valuable compounds appears as an attractive alternative to recycle CO2 and, at the same time, to store electricity from intermittent renewable sources. Among the different innovative attempts that are being investigated to improve these processes, the application of ionic liquids (ILs) has received growing attention in recent years. This paper presents a unified discussion of the significant work that involves the utilisation of ILs for the valorisation of CO2 by means of electrochemical routes. We discuss studies in which CO2 is used as one of the reactants to electrosynthesise value-added products, among which dimethyl carbonate has been the focus of particular attention in the literature. Approaches based on the electrochemical reduction of CO2 to convert it into products without the use of other carbon-based reactants are also reviewed, highlighting the remarkable improvements that the use of ILs has allowed in the CO2 electroreduction to CO. The review emphasises on different aspects related to process design, including the nature of IL anions and cations that have been used, the working conditions, the electrocatalytic materials, the electrode configurations, or the design of electrochemical cells, as well as discussing the most relevant observations, results and figures of merit that the participation of ILs has allowed to achieve in these processes. Several conclusions are finally proposed to highlight crucial challenges and recommendations for future research in this area.The financial support from the Spanish Ministry of Economy and Competitiveness Project CTQ2013-48280-C3-1-R is gratefully acknowledged. J. Albo particularly thanks Juan de la Cierva program (JCI-2012-12073)

Graphene Schottky diodes: an experimental review of the rectifying graphene/semiconductor heterojunction

In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.Comment: 85 pages. Review articl

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