Correlation of p-doping in CVD Graphene with Substrate Surface Charges

Correlations between the level of p-doping exhibited in large area chemical vapour deposition (CVD) graphene field effect transistor structures (gFETs) and residual charges created by a variety of surface treatments to the silicon dioxide (SiO(2)) substrates prior to CVD graphene transfer are measured. Beginning with graphene on untreated thermal oxidised silicon, a minimum conductivity (σ(min)) occurring at gate voltage V(g) = 15 V (Dirac Point) is measured. It was found that more aggressive treatments (O(2) plasma and UV Ozone treatments) further increase the gate voltage of the Dirac point up to 65 V, corresponding to a significant increase of the level of p-doping displayed in the graphene. An electrowetting model describing the measured relationship between the contact angle (θ) of a water droplet applied to the treated substrate/graphene surface and an effective gate voltage from a surface charge density is proposed to describe biasing of V(g) at σ(min) and was found to fit the measurements with multiplication of a correction factor, allowing effective non-destructive approximation of substrate added charge carrier density using contact angle measurements

Non-contact method for measurement of the microwave conductivity of graphene

We report a non-contact method for conductivity and sheet resistance measurements of graphene samples using a high Q microwave dielectric resonator perturbation technique, with the aim of fast and accurate measurement of microwave conductivity and sheet resistance of monolayer and few layers graphene samples. The dynamic range of the microwave conductivity measurements makes this technique sensitive to a wide variety of imperfections and impurities and can provide a rapid non-contacting characterisation method. Typically the graphene samples are supported on a low-loss dielectric substrate, such as quartz, sapphire or SiC. This substrate is suspended in the near-field region of a small high Q sapphire puck microwave resonator. The presence of the graphene perturbs both centre frequency and Q value of the microwave resonator. The measured data may be interpreted in terms of the real and imaginary components of the permittivity, and by calculation, the conductivity and sheet resistance of the graphene. The method has great sensitivity and dynamic range. Results are reported for graphene samples grown by three different methods: reduced graphene oxide (GO), chemical vapour deposition (CVD) and graphene grown epitaxially on SiC. The latter method produces much higher conductivity values than the others.Comment: 8 pages, 2 figures and 2 table

Growth of Epitaxial Oxide Thin Films on Graphene

The transfer process of graphene onto the surface of oxide substrates is well known. However, for many devices, we require high quality oxide thin films on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices

Self-supporting graphene films and their applications

The self-supporting monolayer material which is graphene has excited enormous interest over the ten years since its discovery due to its remarkable electrical, mechanical thermal and chemical properties. In this paper we describe our work to develop chemical vapour deposition methods to grow monolayer graphene on copper foil substrates and the subsequent transfer process. Raman microscopy, scanning electron microscopy and atomic force microscopy (AFM) are used to examine the quality of the transferred material. To demonstrate the process we describe transfer onto patterned SiO2/Si substrates which forms freely suspended graphene with focus on circular wells forming graphene drums. These show interesting mechanical properties which are being explored as nanomechanical resonators.UK NMS Programme, the EU EMRP (European Metrology Research Programme) projects MetNEMS and GraphOh

Microwave study of field-effect devices based on graphene/aluminum nitride/graphene structures

Metallic gate electrodes are often employed to control the conductivity of graphene based field effect devices. The lack of transparency of such electrodes in many optical applications is a key limiting factor. We demonstrate a working concept of a double layer graphene field effect device that utilizes a thin film of sputtered aluminum nitride as dielectric gate material. For this system, we show that the graphene resistance can be modified by a voltage between the two graphene layers. We study how a second gate voltage applied to the silicon back gate modifies the measured microwave transport data at around 8.7 GHz. As confirmed by numerical simulations based on the Boltzmann equation, this system resembles a parallel circuit of two graphene layers with different intrinsic doping levels. The obtained experimental results indicate that the graphene-aluminum nitride-graphene device concept presents a promising technology platform for terahertz- to- optical devices as well as radio-frequency acoustic devices where piezoelectricity in aluminum nitride can also be exploited

On-chip integrated graphene aptasensor with portable readout for fast and label-free COVID-19 detection in virus transport medium

Graphene field-effect transistor (GFET) biosensors exhibit high sensitivity due to a large surface-to-volume ratio and the high sensitivity of the Fermi level to the presence of charged biomolecules near the surface. For most reported GFET biosensors, bulky external reference electrodes are used which prevent their full-scale chip integration and contribute to higher costs per test. In this study, GFET arrays with on-chip integrated liquid electrodes were employed for COVID-19 detection and functionalized with either antibody or aptamer to selectively bind the spike proteins of SARS-CoV-2. In the case of the aptamer-functionalized GFET (aptasensor, Apt-GFET), the limit-of-detection (LOD) achieved was about 103 particles per mL for virus-like particles (VLPs) in clinical transport medium, outperforming the Ab-GFET biosensor counterpart. In addition, the aptasensor achieved a LOD of 160 aM for COVID-19 neutralizing antibodies in serum. The sensors were found to be highly selective, fast (sample-to-result within minutes), and stable (low device-to-device signal variation; relative standard deviations below 0.5%). A home-built portable readout electronic unit was employed for simultaneous real-time measurements of 12 GFETs per chip. Our successful demonstration of a portable GFET biosensing platform has high potential for infectious disease detection and other health-care applications

Microwave Whispering-Gallery Resonators for Nanolitre Liquid Sensing

Within this thesis, a novel approach for the investigation of aqueous solutions interacting with electromagnetic radiation at micro-to-millimetre wave frequencies was developed and employed with relevance to biological applications. Whispering-gallery modes in cylindrically shaped dielectric disks machined from low-loss single crystalline materials such as sapphire or quartz allow for very high quality factors, because the electromagnetic fields are strongly confined inside the dielectric disk such that radiation losses can be neglected. Based on this resonator type, three different liquid sensing approaches were developed, analysed and optimized with emphasis on the determination of the complex dielectric permittivity of liquids. As a first method, whispering-gallery sapphire resonators at 35-37 gigahertz were developed and optimized for the investigation of small droplets of biochemical liquids with volumes ranging from 100 picolitres up to about a few nanolitres. Employing a micrometer controlled microinjection system, droplets could be spotted at arbitrary positions on the surface of the sapphire disk. At the surface of the disk, an evanescent field emerges, such that the droplet induces a slight change of the resonance frequency and a reduction of the inverse quality factor depending on complex dielectric permittivity of the liquid under test. The optimum spotting position on the resonator surface corresponding to the maximum electric field was determined experimentally. Two types of resonator excitation schemes were investigated and optimized for the Ka frequency band (26.5 – 40 Gigahertz). At first, resonators were embedded in a semi-open metal shielding cavity and excited in a standing wave regime by coaxial loop antennas fixed at the walls of the cavity. Secondly, dielectric disks without shielding cavity were investigated, comprising excitation of whispering-gallery running waves by dielectric image guides. In contrast to standing wave loop excitation, the electric field strength of the running whispering gallery waves is nearly independent of the azimuthal angle, which is of great advantage for the positioning of very small samples. In addition, this scheme can be extended up to the terahertz frequency range. As a first demonstration, whispering gallery resonances were excited in a quartz disk resonator at 170 Gigahertz with quality factors of order of 30000. The results on droplet induced changes of resonant frequency and inverse quality factor were found to be in quantitative agreement with perturbation theory in conjunction with numerical field simulation of the unperturbed resonator. In particular it was found, that the effect of real and imaginary part of permittivity could be separated. Both measured quantities were found to be nearly linear dependent on the droplet volume, as expected by perturbation theory. Due to the very high quality factor, droplets down to 100 picoliters volume could be analysed by this technique. Droplets of distilled water were compared with aqueous solutions of ethanol, sodium chloride, glucose and albumin. For all solutes, the measured results were clearly different from water for concentrations above 5%. Within perturbation theory it was found, that the ratio of droplet induced change of inverse quality factor and droplet induced frequency shift,[...

Do on Non-Standard Mathematics Lessons

Розглядається питання про проведення нестандартних уроків математики з метою підвищення пізнавальної активності учнів. Акцентується увага саме на проведені уроків з різновіковим складом учнів.The question of a non-standard mathematics lessons to improve cognitive activity. Stress is just held on lessons from the different age composition of students

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Microwave whispering-gallery resonators for nanolitre liquid sensing Dissertation zur Erlangung des akademischen Doktorgrades in Physik vorgelegt der Fakultät für Physik der Technischen Universität Dortmund vo