Σε αυτή τη Διπλωματική εργασία μελετάται η ταλαντωτική συμπεριφορά μιας κυκλικής πλάκας γραφενίου προκειμένου να διαπιστωθεί αν μπορεί να λειτουργήσει σαν νανομηχανική συσκευή ανίχνευσης μάζας. Για το λόγο αυτό δημιουργήθηκε και χρησιμοποιήθηκε ένα μοντέλο πεπερασμένων στοιχείων. Σε αυτό το μοντέλο οι αλληλεπιδράσεις μεταξύ των ατόμων και οι σχετικές κινήσεις αυτών προσομοιώθηκαν με κατάλληλα ελατήρια των οποίων η δυσκαμψία έχει υπολογιστεί χρησιμοποιώντας την ενέργεια δεσμού από τη θεωρία των μορίων. Ακόμη, τα άτομα του άνθρακα προσομοιώνονται με μάζες στη θέση των ατόμων άνθρακα. Το αναπτυχθέν μοντέλο αναπαριστά το δίσκο του γραφενίου σαν ορθότροπο υλικό. Προκείμενου να αποκτηθεί μια καλή εικόνα της συμπεριφοράς του κυκλικού γραφενίου γίνεται αρχικά διερεύνηση της ελεύθερης ταλάντωσης για διάφορες διαμέτρους αυτού. Επιπλέον, εξετάζεται η ταλαντωτική συμπεριφορά του γραφενίου με την προσκόλληση μάζας διαφόρων μεγεθών και σε διάφορες θέσεις πάνω στο γραφένιο κατά μήκος της ακτίνας του. Η επεξεργασία των μοντέλων έχει ως αποτέλεσμα τις φυσικές συχνότητες ταλάντωσης του γραφενίου και τις αντίστοιχες ιδιομορφές των φύλλων γραφενίου με ή χωρίς την προσκολλημένη μάζα παρέχοντας τα ταλαντωτικά τους χαρακτηριστικά. Επομένως, έχοντας δημιουργήσει μοντέλα για διάφορες διαμέτρους και μάζες μπορεί να εξεταστεί και να αποκτηθεί μια πλήρης εικόνα για την ενδεχόμενη ταλαντωτική συμπεριφορά του κυκλικού μονοστρωματικού δίσκου γραφενίου σαν αισθητήριο μάζας. / In this Diploma thesis is studied the vibrational behavior of a circular graphene sheet so as to investigate if this can operate as a nanomechanical system of mass sensing. For this purpose a finite elements’ model was created and being used. In this model the interatomic interactions and their relative positions are simulated by equivalent spring elements whose stiffness derived by corresponding potential energies provided by molecular theory and expressing the resistance in relative movements between carbon atoms under deformation. Moreover, the inertial effects of the system are simulated by using appropriate lumped masses in atomic positions. The model created represents the graphene plate as an orthotropic material. In order to obtain a full view of the behavior of circular graphene, firstly is been done investigation of its free vibration for different diameters of the plate. After that, is studied the Vibrational behavior with a mass lying on it. The solution gives the natural frequencies and corresponding mode shapes of vibration of individual graphene sheets and graphene-attached mass systems providing their vibrational characteristics. Subsequently, assuming the geometric characteristics of graphene as well as the weight and position of the attached mass as the global design parameters, a parametric study on mass sensing characteristics is presented in order to examine the potential behavior of a circular graphene monolayer as mass sensor.

Micromachined biosensors are used in chemical and biological applications. A biosensor which uses mass based transduction is called a mass sensor. Mass sensors are used to detect extremely small mass of biomolecules such as proteins, viruses or even parts of DNA in the range of femtograms (10-15 gm) to zeptograms (10−21 gm). Highly effective and reliable microcantilevers are used for detecting the mass of biomolecules using either static deflection or dynamic resonant peak shifts. The main objective of our work is to investigate the effect of elasticity of the attached mass on the shift in the resonant frequency and examine the validity of the rigid mass assumption used in the literature. The natural frequencies of a resonator are either found by solving the governing differential equation or approximately using Rayleigh-Ritz method. The mass of a body, attached to a resonator beam is determined using resonant frequency shift method. In our study, we derive an analytical expression for ‘δm’ based on the shift in frequency ‘δf’ that accounts for the elasticity of the added mass and the location of the mass on the beam. We study the simplest model to incorporate these effects where the added mass is itself modeled as a single degree of freedom spring-mass system. The entire system is represented as a 2-DOF lumped model of cantilever and the attached elastic mass. The natural frequencies are obtained using eigenvalue analysis. We study the mass estimation of Escherichia Coli (E. Coli), a food borne pathogen, using experimental results reported in the literature. We treat E.Coli as an elastic mass and model it as a single degree of freedom system to account for its elasticity. We use the elastic model as well as the rigid mass model to check the results available in the literature and point out the difference that results in mass estimation using the two models. To demonstrate the effect of elasticity on mass sensing using the resonant peak shift technique, we conduct mesoscale experiments. Since the fundamental principle does not depend on any phenomenon exclusively dependent on micro scales, the mesoscale experiments are justified. For this purpose, an experimental set-up with metallic cantilevers and flexible rubber strands as attached masses are used. We also use our experimental set-up to study the effect of positional inaccuracy of the added mass (rigid) in the computation of its mass from the shift in the resonance frequency. The results obtained show that elasticity of the added mass as well as its position on the resonator affect the computed mass but this effect is dependent on the relative stiffness and mass of the resonator and the added mass. We also observe the limitations of the experiments in carrying out studies over the desired range of parameters. We also create a computational model of the system and carry out simulations to explore a larger range of parameter values. In particular, we create an FEM model of our system in ANSYS, and carry out modal analysis for the cantilever beam resonator with and without the added mass, varying the relative stiffness and mass of the two components (the cantilever beam and the added mass). We compare the results of shift in the resonant frequency with those obtained from the rigid mass model. The results show the effect of elasticity clearly in certain ranges of relative stiffness and mass.

<p>To facilitate indoor air quality (IAQ) monitoring, the research described herein develops and implements methods for the electronic integration of two types of gas sensor, each functionalized with a polymer blend tailored for CO₂ detection. A highly sensitive and tunable electronic chemiresistive sensor interface was developed and experimentally validated. This device achieved analog-to-digital conversion (ADC) through a pulse width modulated (PWM) signal, temporary data storage with an efficient data buffering system, and noise reduction and signal amplification utilizing an instrumentation amplifier integrator circuit. These techniques can used beyond CO₂-specific applications to compensate for certain undesirable chemiresistive sensor characteristics, such as low response magnitude and signal noise. Additionally, resonant mass sensing circuitry was combined with an on-chip field programmable gate array (FPGA) implemented frequency counter. Hz-level resolution was achieved with an Alorium Snō FPGA board and a Verilog data acquisition and communication program. This device can monitor up to 16 sensor channels simultaneously and has a straightforward interface with a controllable output. Furthermore, the functionality of each integrated sensor was experimentally validated. With additional work, these integrated designs have the potential to be inexpensive, low-power, highly sensitive devices that are suitable for practical use in IAQ monitoring applications.</p>