Grafeenil põhinevate struktuuride kompleksne nanoskoopiline karakteriseerimine

Väitekirja elektrooniline versioon ei sisalda publikatsiooneAntud 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.https://www.ester.ee/record=b528236

Atomic layer deposition of high-k oxides on graphene

Atomic layer deposition of high-k oxides on graphene.Comment: Graphene - Synthesis, Characterization, Properties and Applications, Jian Ru Gong (Ed.), ISBN: 978-953-307-292-0, InTech, Available from: http://www.intechopen.com/articles/show/title/atomic-layer-deposition-of-high-k-oxides-on-graphen

Nanostructured Coating for Aluminum Alloys Used in Aerospace Applications

The authors would like to acknowledge the Estonian Ministry of Education and Research by granting the projects IUT2–24, TLTFY14054T, PSG448, PRG4, SLTFY16134T and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-00). The atomic oxygen testing was performed in the framework of the “Announcement of opportunity for atomic oxygen in the ESTEC Materials and Electrical Components Laboratory/ESA-TECQE-AO-013375),” through a collaboration with Picosun Oy. The authors also thank Dr. Elo Kibena-Põldsepp for the electrodeposition of Ag onto the anodized substrates.A thin industrial corrosion-protection nanostructured coating for the Al alloy AA2024-T3 is demonstrated. The coating is prepared in a two-step process utilizing hard anodizing as a pre-treatment, followed by sealing and coating by atomic layer deposition (ALD). In the first step, anodizing in sulfuric acid at a low temperature converts the alloy surface into a low-porosity anodic oxide. In the second step, the pores are sealed and coated by low-temperature ALD using different metal oxides. The resulting nanostructured ceramic coatings are thoroughly characterized by cross-sectioning using a focused ion beam, followed by scanning electron microscopy, transmission electron microscopy, X-ray microanalysis, and nanoindentation and are tested via linear sweep voltammetry, electrochemical impedance spectroscopy, salt spray, and energetic atomic oxygen flow. The best thin corrosion protection coating, made by anodizing at 20 V, 1 °C and sealing and coating with amorphous Al2O3/TiO2 nanolaminate, exhibits no signs of corrosion after a 1000 h ISO 9227 salt spray test and demonstrates a maximum surface hardness of 5.5 GPa. The same coating also suffers negligible damage in an atomic oxygen test, which is comparable to 1 year of exposure to space in low Earth orbit. © 2022 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.Estonian Ministry of Education and Research by granting the projects IUT2–24, TLTFY14054T, PSG448, PRG4, SLTFY16134T; ERDF TK141 (2014-2020.4.01.15-00); Institute of Solid State Physics, University of Latvia as the Center of Excellence acknowledges funding from the European Union’s Horizon 2020 Framework Programme H2020- WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Magnetic and Electrical Performance of Atomic Layer Deposited Iron Erbium Oxide Thin Films

Mixed films of a high-permittivity oxide, Er2O3, and a magnetic material, Fe2O3, were grown by atomic layer deposition on silicon and titanium nitride at 375 degrees C using erbium diketonate, ferrocene, and ozone as precursors. Crystalline phases of erbium and iron oxides were formed. Growth into three-dimensional trenched structures was demonstrated. A structure deposited using tens to hundreds subsequent cycles for both constituent metal oxide layers promoted both charge polarization and saturative magnetization compared to those in the more homogeneously mixed films.Peer reviewe

Magnetic properties and resistive switching in mixture films and nanolaminates consisting of iron and silicon oxides grown by atomic layer deposition

SiO2-Fe2O3 mixture films and nanolaminates were grown by atomic layer deposition from iron trichloride, hexakis(ethylamino)disilane, and ozone at 300 degrees C. Orthorhombic -Fe2O3 was identified in Fe2O3 reference films and in Fe2O3 layers grown to certain thicknesses between amorphous SiO2 layers. SiO2-Fe2O3 films could be magnetized in external fields, exhibiting saturation and hysteresis in nonlinear magnetization-field curves. Electrical resistive switching, markedly dependent on the ratio of the component oxides, was also observed in films with proper composition. For relatively conductive films, application of small signal measurements allowed one to record memory maps with notable squareness and defined distinction between high and low conductance states.Peer reviewe

Structure and behavior of ZrO2-graphene-ZrO2 stacks

Producción CientíficaZrO2-graphene-ZrO2 layered structures were built and their crystallinity was characterized before resistive switching measurements. Thin nanocrystalline ZrO2 dielectric films were grown by atomic layer deposition on chemical vapor deposited graphene. Graphene was transferred, prior to the growth of the ZrO2 overlayer, to the ZrO2 film pre-grown on titanium nitride. Nucleation and growth of the top ZrO2 layer was improved after growing an amorphous Al2O3 interface layer on graphene at lowered temperatures. Studies on resistive switching in such structures revealed that the exploitation of graphene interlayers could modify the operational voltage ranges and somewhat increase the ratio between high and low resistance states.Fondo Europeo de Desarrollo Regional (project TK134)Estonian Research Agency (grants PRG753 and PRG4)Ministerio de Economía, Industria y Competitividad (grant TEC2017-84321-C4-2-R

Effects of post oxidation of SiO2/Si interfaces in ultrahigh vacuum below 450 °C

Growing SiO2 layer by wet-chemical oxidation of Si surfaces before growth of another insulating film(s) is a used method to passivate Si interfaces in applications (e.g., solar cell, photodiode) at low temperatures (LT) below 450 °C. We report on potential of LT ultrahigh-vacuum (UHV) treatments combined with the wet-chemical oxidation, by investigating effects of LT-UHV oxidation after the wet-chemical growth of SiO2 and before growing Al2O3 film on top of SiO2/Si. This method modifies the SiO2/Si and is found to (i) decrease defect-level density, (ii) increase negative fixed charge density, and (iii) increase carrier lifetime for Al2O3/SiO2/p-Si, as compared to state-of-the-art SiO2/p-Si reference interfaces without LT-UHV. X-ray photoelectron spectroscopy shows that the LT-UHV treatment decreases amount of Si+3 oxidized atoms in chemically grown SiO2 and also amount of carbon contamination. In order to pave the way for further tests of LT-UHV in silicon technology, we present a design of simple UHV instrument. The above-described benefits are reproduced for 4-inch silicon wafers by means of the instrument, which is further utilized to make LT-UHV treatments for complementary SiO2/Si interfaces of the native oxide at silicon diode sidewalls to decrease the reverse bias leakage current of the diodes.​​​​​​​</ul

Atomic layer deposition of titanium oxide films on As-synthesized magnetic Ni particles : Magnetic and safety properties

Spherical nickel particles with size in the range of 100-400 nm were synthesized by non-aqueous liquid phase benzyl alcohol method. Being developed for magnetically guided biomedical applications, the particles were coated by conformal and antimicrobial thin titanium oxide films by atomic layer deposition. The particles retained their size and crystal structure after the deposition of oxide films. The sensitivity of the coated particles to external magnetic fields was increased compared to that of the uncoated powder. Preliminary toxicological investigations on microbial cells and small aquatic crustaceans revealed non-toxic nature of the synthesized particles.Peer reviewe