Complex characterization of graphene structures on nanometer level

Abstract

Antud töö raames valmistati ja karakteriseeriti puhtal ja funktsionaliseeritud kujul mõne- ja mitmekihilisel grafeenil põhinevaid struktuure, kasutades erinevaid spektroskoopia ja kõrglahutusmikroskoopia meetodeid. Selleks juurutati mõnekihilise ja mitmekihilise grafeeni süntees keemilise aurufaassadestamise meetodil nikkelkatalüsaatoril ning valmistatud grafeenikihtide omadusi võrreldi grafiidi mikromehhaanilise lõhestamise teel või keemilise aurufaasist sadestamise meetodil saadud ühekihilise grafeeni omadustega. Näidati grafeeni kasvu sõltuvust nikkelaluse paksusest ja kristalliitide orientatsioonist. Leiti, et nikli pinnal sünteesitud mitmekihilise grafeeni kiles esinevad monokihtide erinevate pakmetega alad. Seejuures, kui pöördenurk grafeenikihtide vahel oli suurem kui kriitiline nurk (~13 kraadi), siis ilmnesid mitmekihilise grafeeni ramanhajumise spektris monokihilise grafeeni spektrile iseloomulikud tunnused. Samuti näidati, et grafeeni kasvuga kaasnevad polükristalliliste nikkelaluste pinna morfoloogia muutused.

Edasi testiti Ni-alusel sünteesitud mitmekihilise grafeeni elektrokeemilisi omadusi ning uuriti selle funktsionaliseerimise võimalust ilma grafeeni metallaluselt eemaldamata. Uuringutest selgus, et nikkelkatalüsaatoril keemilise aurufaasist sadestamise meetodil kasvatatud mitmekihilist grafeeni saab kasutada elektrokeemiliselt passiivse alusmaterjalina elektrokatalüütiliselt aktiivsete materjalide uurimiseks. Kasutades arüüldiasooniumsoolade elektrokeemilist redutseerimist, saab sünteesitud mitmekihilise grafeeni pinda modifitseerida arüülrühmadega, mis võimaldab laiendada sünteesitud grafeeni kasutamist. Lisaks uuriti transistorstruktuuri paisudielektrikkile kasvatamise võimalust grafeeni pinnale aatomkihtsadestamise meetodi abil. Nendest uuringutest selgus, et dielektrikkilede sadestamine grafeenile, kasutades metallkloriid-vesi protsessi, põhjustab küll lateraalsete pingete tekkimist grafeenis, kuid ei genereeri struktuuridefekte selle võres. Siiski esineb grafeeni pinnal oksiidikile nukleatsiooni viivitus, mis teeb keeruliseks pidevate õhukeste oksiidikihtide kasvatamise. See on tingitud mitte ainult nukleatsioonitsentrite vähesusest, vaid ka nende tiheduse tugevalt ebaühtlasest jaotusest üle grafeeni pinna.

In this thesis, pristine and functionalized few-layer graphene (FLG)- and multilayer graphene (MLG)-based structures were prepared and characterized using various spectroscopy and high-resolution microscopy and methods. For this purpose, the FLG and MLG have been synthesized by chemical vapor deposition (CVD) on nickel catalyst, and the properties of the prepared graphene sheets have been compared to the single-layer graphene (SLG) obtained by CVD or micromechanical exfoliation of natural graphite. The dependence of the graphene growth on the thickness and crystallographic orientation of the nickel substrate has been shown. It has been found that MLG synthesized on polycrystalline nickel is characterized by a variety of stacking order between the graphene layers presented in the sheets. It should be pointed out that if the rotation angles in synthesized MLG are larger than the critical angle of ~13 degrees, the Raman spectrum even of MLG becomes similar to the spectrum of SLG. The pronounced morphological changes of the nickel substrate accompanied the graphene growth has been demonstrated as well. Further, the electrochemical properties of the synthesized MLG sheets on nickel have been studied, and the possibility of their subsequent functionalization using electroreduction of various diazonium salts has been examined. It has been found that the CVD-grown graphene on nickel can be used as electrochemically passive supporting material for exploring new electro-catalytically active materials. The surface of the CVD-grown graphene can be further modified with aryl groups using electroreduction of diazonium salts, which allows broadening the use of the synthesized MLG. In addition, the feasibility of the chloride-water atomic layer deposition (ALD) of gate dielectric on top of nonfunctionalized graphene has been explored. It has been found that the deposition of dielectric films with the chloride-water ALD processes leads to a compressive strain of graphene but does not generate structural defects in the lattice. Still, there is a delay in the nucleation of the oxide layer on graphene, caused by the fact that the nucleation sites are not only at a deficit on graphene, but their density varies considerably. This makes it much more challenging to achieve the growth of a continuous oxide layer on graphene.