Search

Predmet istraživanja ove dizertacije su elektromagnetska svojstva grafena. U tu svrhu, korišten je numerički model električne vodljivosti grafena temeljen na Kubovoj jednadžbi koja uzima u obzir frekvenciju signala, kemijski potencijal, gubitke zbog raspršenja naboja, i temperaturu. Pokazano je da je vodljivost grafena moguće utvrditi analitički pomoću Kubove jednadžbe samo ako je frekvencija signala znatno niža od frekvencije raspršenja naboja. Usporedba analitički utvrđenog riješenja i numeričkih vrijednosti dobivenih rješavanjem Kubo jednadžbe pokazala je dobro slaganje. Analiza je pokazala da vodljivost grafena u radiofrekvencijskom području ima izraženu realnu komponentu vodljivosti dok je imaginarna komponenta vodljivosti manja od realne komponente za četiri reda veličine. Zbog ovoga svojstva se impedancija grafena u radiofrekvencijskom području može aproksimirati realnim otporom. Nadalje je pokazano da je u optičkom području frekvencija realna komponenta električne vodljivosti grafena veća od imaginarne komponente za jedan red veličine. Stoga se i u optičkom području frekvencija impedancija grafena može aproksimirati realnim otporom. U THz području impedancija grafena ima kompleksnu vrijednost u skladu s Lorentz-Drudeovim disperzijskim modelom. Najzanimljivije svojstvo grafena, s aspekta primjene, je efekt polja: U nekim slučajevima vanjsko električno polje može promijeniti realni i imaginarni dio kompleksne impedancije grafena. Opisano svojstvo koristi se za postizanje negativne permitivnosti u upravljivim metamaterijalima i metapovršinama. Eksperimentalno istraživanje utjecaja vanjskog električnog polja na električnu vodljivost grafena pokazalo je kako se vodljivost materijala može povećati nekoliko puta. U okviru ovog rada razvijen je reaktor za sintezu uzoraka jednoslojnog i višeslojnog grafena. Postupak sinteze je kemijska depozicija iz parne faze. Reaktor je izrađen upotrebom lako dobavljivih komponenti niske cijene što ga čini dostupnim većini istraživačkih grupa. Sintezirani grafen je visoke kvalitete što je potvrđeno karakterizacijom uz pomoć Ramanove spektroskopije i elektronske mikroskopije. Dizajnirana je i praktično realizirana upravljiva grafenska metapovršina koja je u osnovi mikrovalni apsorber baziran na Salisburyjevom zastoru. Punovalne numeričke simulacije i mjerenja u području frekvencija od 10 GHz pokazuju efikasnu apsorpciju mikrovalne energije iako njegova debljina iznosi samo jedan atom. Posebnost razvijenog apsorbera je mogućnost jednostavnog podešavanja njegovih parametara promjenom upravljačkog visokonaponskog signala. Focus of this thesis is the analysis of electromagnetic properties of graphene for use in electromagnetic metasurfaces at room temperature. Analysis of intrinsic graphene electric conductivity model based on Kubo formula in wide frequency range shows that real and imaginary part of complex electric conductivity change significantly with signal frequency and bias voltage. Below the THz, a real part of conductivity dominates over imaginary part by more than two orders of magnitude. Therefore, the lumped element model of the graphene can be approximated as a simple resistor with typical value of 1 k. In the visible part of electromagnetic spectrum, a real part of graphene conductivity is more than an order of magnitude higher than imaginary part. Thus, a lumped element model can again be simplified to a simple resistor of 10 k. In the infrared region, however, graphene has a complex impedance in accordance with Lorentz-Drude model and, therefore, complete RLC lumped model has to be used. In addition, an external bias voltage can be applied to graphene to vary both the real and imaginary part of its complex impedance. This phenomenon is called "field effect" and it is used to realize a negative permittivity in reconfigurable metamaterials and metasurfaces. The low-cost CVD reactor for graphene synthesis was designed and manufactured. Synthesized graphene samples were characterised by Raman spectroscopy and scanning electron microscopy. Measurement results revealed high quality of single-layer and multi-layer samples adequate for use in graphene-based metasurface devices. A simple contactless metod for measuring a surface resistance of single-layer and multi-layer graphene samples was designed and tested. This method is preferable over existing contact methods due to lack of variable contact resistance. In this method a graphene sample is supported by thin dielectric substrate and such composite material is placed within a rectangular waveguide. The scattering parameters were measured with calibrated network analyzer and the surface resistance was determinated using simple mathematical postprocessing. This method is similar to other published methods for measuring surface conductivity of thin conductive films although the mathematical postprocessing has a different form. Measurement results of various graphene samples were found to be in a good agreement with numerical simulation based on Kubo eqauation. The metasurface based on Salisbury screen was designed, constructed and measured. A conductive film needed for absorption was composed of single graphene layer whereas the section of WR-90 wavegude was used as a quater-wave transformer. Measurements and numerical simulations showed that the surface resistance of graphene should be optimised for the efficient absorption. If optimised, graphene-based absorber has a high-efficiency (>99% absorption) although the thickness of a conductive layer is only one-atom. Reconfigurable microwave absorber based on a single-layer graphene was designed, constructed and measured. It has a form of parallel-plate capacitor with the dimensions of WR-90 flange. A single-layer graphene of high-quality was used as first electrode (an active layer), a poor-quality graphene was used as a second electrode (bias layer) and the PET (polyethylene terephtalate) was used as a dielectric. The controll of absorption is performed by modifying active layer impedance with polarity and magnitude of applied high voltage at bias layer. To produce graphene-on-dielectric-based devices, a novel method for graphene transfer is developed using laminating pouches. This method, to the best of authors knowledge, has not been published in the literature so far. A transfer process using laminating pouches is faster, simpler and of lower cost than currently popular methods found in literature.

U ovom diplomskom radu dan je kratak pregled osnovnih stvari o metamaterijalima i metapovršinama. Izveden je matematički model za izračun osnovnih parametara koji opisuju neku metapovršinu. Pomoću tog modela napisan je programski kod koji pomoću raspršnih parametara određene metapovršine računa tražene parametre (primjerice površinsku impedanciju) te iscrtava pripadne grafove. Program je testiran na nekoliko poznatih struktura te je utvrđena njegova ispravnost. Stoga je isti program korišten za analizu pasivne jedinične ćelije kapacitivne metapovršine čiji je model prethodno izrađen pomoću CST-a. Potom se pasivnoj ćeliji dodaju nefosterovki elementi te se cijela struktura analizira korištenjem ADS-a. Provedeno istraživanje dalo je zadovoljavajuće rezultate te je uspješno ostvarena aktivna metapovršina. This thesis gives a short overview of the basic things about metamaterials and metasurfaces. The overview is followed by a detailed mathematical model for calculating the needed parameters that desrcibe a given metasurface. Then a code was written based on the mathematical model. The code calculates the needed parameters (such as surface impedance) using the scattering parameters of a metasurface. The code was tested on a few known structures and, after satisfying results, was used to analize a passive unit cell with a capacitive metasurface. This structure was then combined with non-Foster elements, and a full analysis was done using ADS. The total research gave satisfying results and it was concluded that it is indeed possible to create an active metasurface.