The preparation of graphene layers modified by Ga atoms and characterisation of their electrical properties

Tato diplomová práce se zabývá studiem charakterizace elektrických vlastností grafenových vrstev pokrytých Ga atomy. Substráty s elektrickými kontakty byly připraveny laserovou litografií a grafenová vrstva byla připravena pomocí metody chemické depozice z plynné fáze (CVD). Experimentálně byla zkoumán posuv Diracova bodu v závislosti na době depozice atomů galia na povrch grafenu. Byl také zkoumán vliv depozice atomárního vodíku na povrch grafenu. Výsledky měření a jejich zhodnocení jsou diskutovány v této práci.This master's thesis deals with the study of electric properties of graphene layers covered by Ga atoms in UHV conditions. The substrates were prepared by using laser litography and the graphene layer was prepared by using chemical vapor deposition (CVD). Dependence of Dirac point location on gallium atoms deposition time and influence of electrical properties of graphene on hydrogen atoms deposition time were studied. Experimental results and their evaluation are discussed.

Optimization of device for measurement field emission from surface CND

Tato bakalářská práce se zabývá studiem studené elektronové emise z povrchu nanokrystalického diamantu (NCD). Vzorky NCD byly připraveny pomocí metody chemické depozice z plynné fáze (CVD). Experimentálně byla zkoumána závislost emisního proudu a vlastností povrchu NCD na dopování vzorků různou koncentrací dusíku a metanu. Výsledky měření a jejich zhodnocení jsou diskutovány v této práci.This bachelor's thesis deals with the study of cold electron emission from the surface of nanocrystal diamond (NCD). The samples of NCD were prepared by using chemical vapor deposition (CVD). Dependences of emission current and properties of NCD surface on different concentration of nitrogen and methan were studied. Experimental results and their evaluation are discussed.

Density functional study of gallium clusters on graphene: electronic doping and diffusion

Motivated by experimental results on transport properties of graphene covered by gallium atoms, the density functional theory study of clustering of gallium atoms on graphene (up to a size of 8 atoms) is presented. The paper explains a rapid initial increase of graphene electron doping by individual Ga atoms with Ga coverage, which is continually reduced to zero, when bigger multiple-atom clusters have been formed. According to density functional theory calculations with and without the van der Waals correction, gallium atoms start to form a three-dimensional cluster from five and three atoms, respectively. The results also explain an easy diffusion of Ga atoms while forming clusters caused by a small diffusion barrier of 0.11 eV. Moreover, the calculations show this barrier can be additionally reduced by the application of an external electric field, which was simulated by the ionization of graphene. This effect offers a unique possibility to control the cluster size in experiments only by applying a gate-voltage to the graphene in a field-effect transistor geometry and thereby without growth temperature assistance.Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [17-21413S]; Ministry of Education, Youth and Sports from the Large Infrastructures for Research, Experimental Development and Innovations project 'IT4 Innovations National Super-computing Center' [LM2015070

Mechanism and Suppression of Physisorbed-Water-Caused Hysteresis in Graphene FET Sensors

Hysteresis is a problem in field-effect transistors (FETs) often caused by defects and charge traps inside a gate isolating (e.g., SiO2) layer. This work shows that graphene-based FETs also exhibit hysteresis due to water physisorbed on top of graphene determined by the relative humidity level, which naturally happens in biosensors and ambient operating sensors. The hysteresis effect is explained by trapping of electrons by physisorbed water, and it is shown that this hysteresis can be suppressed using short pulses of alternating gate voltages.Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [17-21413S]; H2020 Twinning Programme (Project SINNCE) [810626]; MEYS CR [LQ1601 CEITEC 2020, LM2015041

Correlative Raman imaging and scanning electron microscopy: The role of single Ga islands in surface-enhanced Raman spectroscopy of graphene

Surface-enhanced Raman spectroscopy (SERS) is a perspective nondestructive analytic technique enabling the detection of individual nanoobjects, even single molecules. In this paper, we have studied the morphology of Ga islands deposited on chemical vapor deposition graphene by ultrahigh vacuum evaporation and local optical response of this system by the correlative Raman imaging and scanning electron microscopy (RISE). Contrary to the previous papers, where only an integral Raman response from the whole ununiformed Ga nanoparticles (NPs) ensembles on graphene was investigated, the RISE technique has enabled us to detect graphene Raman peaks enhanced by single Ga islands and particularly to correlate the Raman signal with the shape and size of these single particles. In this way and by a support of numerical simulations, we have proved a plasmonic nature of the Raman signal enhancement related to localized surface plasmon resonances. It has been found that this enhancement is island-size-dependent and shows a maximum for medium-sized Ga islands. A reasonable agreement between the simulations of the plasmon enhancement of electric fields in the vicinity of Ga islands and the experimental intensities of corresponding Raman peaks proved the plasmonic origin of the observed effect known as SERS. © 2022 American Chemical Society.European Commission, EC: 71020004, 810626; Grantová Agentura České Republiky, GA ČRCzech Science FoundationGrant Agency of the Czech Republic [20-28573S]; European Commission (H2020-Twininning project)European Commission [810626.SINNCE, M-ERA NET HYSUCAP/TACR-TH71020004]; *BUT*.specific research [*FSI-S-20-648*5]; Ministry of Education, Youth and Sports of the Czech Republic (CzechNanoLab Research Infrastructure) [LM2018110

Mechanical strain and electric-field modulation of graphene transistors integrated on MEMS cantilevers

This work proposes a structure which allows characterization of graphene monolayers under combined electric field and mechanical strain modulation. Our approach is based on a cantilever integrated into a two-dimensional graphene-based Field effect transistor (FET). This allows us to change graphene properties either separately or together via two methods. The first way involves electric field induced by the gate. The second is induction of mechanical strain caused by external force pushing the cantilever up or down. We fabricated devices using silicon-on-insulator wafer with practically zero value of residual stress and a high-quality dielectric layer which allowed us to precisely characterize structures using both mentioned stimuli. We used the electric field/strain interplay to control resistivity and position of the charge neutrality point often described as the Dirac point of graphene. Furthermore, values of mechanical stress can be obtained during the preparation of thin films, which enables the cantilever to bend after the structure is released. Our device demonstrates a novel method of tuning the physical properties of graphene in silicon and/or complementary metal-oxide-semiconductor technology and is thus promising for tunable physical or chemical sensors. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Grantová Agentura České Republiky, GA ČR: GJ18-06498Y; Vysoké Učení Technické v Brně, BUT: FEKT-S-20-6206; Central European Institute of Technology, CEITEC: LM2015041Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GJ18-06498Y]; CEITEC Nano Research Infrastructure (MEYS CR, 2016 2019) CEITEC Brno University of Technology [LM2015041]; Brno University of Technology [FEKT-S-20-6206

Side charge propagation in simultaneous KPFM and transport measurement of humidity exposed graphene FET sensor

The surface diffusion (dissipation) of charge carriers enhanced by water molecules in solution-based biosensors and ambient operating gas sensors strongly influence their resistance response, sensitivity, and stability in time. Therefore, the information on the charge distribution at interfaces of conductive and insulating parts is essential for the operating sensors. This work presents the simultaneous measurement of the longitudinal macroscopic resistance response and local surface potential (SP) mapping by Kelvin probe force microscopy (KPFM) on a graphene Hall bar sensor. The results show the propagation of an electric charge from the main graphene channel onto the neighboring SiO2 surface. The charge propagation strongly increases with the relative humidity and can be controlled by a bottom-gate voltage used in most sensors based on a field effect transistor (FET) architecture. As proved by the longitudinal resistance measurements, the resulting side charge accumulation has a very small impact on the 2D resistivity of the graphene channel. It has been explained by an application of the Thomas-Fermi theory, proving an efficient screening of side accumulated charge potential caused by a redistribution of the charge inside the wide graphene channel. The combination of a transport resistance response and KPFM provides a deeper understanding of sensors/biosensors functionality and their design features than a simple resistance response usually observed. © 2023 Elsevier LtdMinisterstvo Školství, Mládeže a Tělovýchovy, MŠMT, (LM2023051, LQ1601 – CEITEC 2020); Grantová Agentura České Republiky, GA ČR, (23-07617S); European Regional Development Fund, ERDF, (CZ.1.05/1.1.00/02.0068

DFT study of water on graphene: Synergistic effect of multilayer p-doping

Recent experiments related to a study concerning the adsorption of water on graphene have demonstrated the p-doping of graphene, although most of the ab initio calculations predict nearly zero doping. To shed more light on this problem, we have carried out van der Waals density functional theory calculations of water on graphene for both individual water molecules and continuous water layers with coverage ranging from one to eight monolayers. Furthermore, we have paid attention to the influence of the water molecule orientation toward graphene on its doping properties. In this article, we present the results of the band structure and the Bader charge analysis, showing the p-doping of graphene can be synergistically enhanced by putting 4-8 layers of an ice-like water structure on graphene having the water molecules oriented with oxygen atoms toward graphene.AMISPEC, (TE01020233); INCHAR, (FW03010504); Technology Agency of the Czech Republic, TACR; Ministerstvo Školství, Mládeže a Tělovýchovy, MŠMT; Grantová Agentura České Republiky, GA ČR, (23-07617S); Vysoké Učení Technické v Brně, BUT, (FSI-S-23-8324)Grant Agency of the Czech Republic; Ministry of Education, Youth and Sports from the Large Infrastructures for Research; Technology Agency of the Czech Republic [TE01020233]; Specific Research grant of the Brno University of Technology [FSI-S-23-8324]; [23-07617S]Grant Agency of the Czech Republic; Ministry of Education, Youth and Sports from the Large Infrastructures for Research; Technology Agency of the Czech Republic [TE01020233]; Specific Research grant of the Brno University of Technology [FSI-S-23-8324]; [23-07617S

Kelvin probe force microscopy and calculation of charge transport in a graphene/silicon dioxide system at different relative humidity

The article shows how the dynamic mapping of surface potential (SP) measured by Kelvin probe force microscopy (KPFM) in combination with calculation by a diffusion-like equation and the theory based on the Brunauer-Emmett-Teller (BET) model of water condensation and electron hopping can provide the information concerning the resistivity of low conductive surfaces and their water coverage. This is enabled by a study of charge transport between isolated and grounded graphene sheets on a silicon dioxide surface at different relative humidity (RH) with regard to the use of graphene in ambient electronic circuits and especially in sensors. In the experimental part, the chemical vapor-deposited graphene is precisely patterned by the mechanical atomic force microscopy (AFM) lithography and the charge transport is studied through a surface potential evolution measured by KPFM. In the computational part, a quantitative model based on solving the diffusion-like equation for the charge transport is used to fit the experimental data and thus to find the SiO2 surface resistivity ranging from 107 to 1010 Ω and exponentially decreasing with the RH increase. Such a behavior is explained using the formation of water layers predicted by the BET adsorption theory and electron-hopping theory that for the SiO2 surface patterned by AFM predicts a high water coverage even at low RHs. © 2018 American Chemical Society.Grant Agency of the Czech Republic [17-21413S]; Technology Agency of the Czech Republic [TE01020233]; MEYS CR [LQ1601-CEITEC 2020]; CEITEC Nano Research Infrastructure (MEYS CR) [LM2015041