arXiv subjects

Coskun Kocabas

Publications and source records attributed to Coskun Kocabas.

24 records · Page 2Linked to original sources

arXiv2018

Observation of gate-tunable coherent perfect absorption of terahertz radiation in graphene

We report experimental observation of electrically-tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in graphene. We develop a reflection-type tunable THz cavity formed by a large-area graphene layer, a metallic reflective electrode and an electrolytic medium in between. Ionic gating in the THz cavity allows us to tune the Fermi energy of graphene up to 1eV and to achieve critical coupling condition at 2.8 THz with absorption of 99%. With the enhanced THz absorption, we were able to measure the Fermi energy dependence of the transport scattering time of highly doped graphene. Furthermore, we demonstrate flexible active THz surfaces that yield large modulation in the THz reflectivity with low insertion losses. We anticipate that the gate-tunable CPA will lead efficient active THz optoelectronics applications.

physics.app-ph↗

arXiv2018

Weighting graphene with QCM to monitor interfacial mass changes

In this Letter, using quartz crystal microbalance (QCM), we experimentally determined the mass density of graphene grown by chemical vapor deposition method. We developed a transfer printing technique to integrate large area single-layer graphene on QCM. By monitoring the resonant frequency of an oscillating quartz crystal loaded with graphene, we were able to measure the mass density of graphene as ~118 ng/cm2, which is significantly larger than the ideal graphene (~76 ng/cm2) mainly due to the presence of wrinkles and organic/inorganic residues on graphene sheets. High sensitivity of quartz crystal resonator allowed us to determine the number of graphene layers in samples. (The technique is very sensitive and able to determine the number of graphene layers in a particular sample.) Besides, we extended our technique to probe interfacial mass variation during adsorption of biomolecules on graphene surface, and plasma-assisted oxidation of graphene. (Besides, we extended our technique to probe interfacial mass variation during adsorption of biomolecules on graphene surface, and plasma-assisted oxidation of graphene.)

cond-mat.mtrl-sci↗

arXiv2018

Controlling phase of microwaves with active graphene surfaces

In this letter, we report a method to control reflection phase of microwaves using electrically tunable graphene devices. The device consists of mutually gated large-area graphene layers placed at a quarter-wave distance from a metallic surface. This device structure yields electrically tunable resonance absorbance and step-like phase shift around the resonance frequency when the impedance of graphene matches with the free space impedance. Electrostatic control of charge density on graphene yields an ability to control both intensity (> 50 dB) and phase (~pi) of the reflected electromagnetic waves with voltage. Furthermore, using the asymmetry of the doping at opposite polarity of the bias voltages, we showed bidirectional phase control with the applied voltage. We anticipate that our results will pave a new directions to control interaction of electromagnetic waves with matter for long wavelengths and could open a new avenue for microwave devices.

physics.app-ph↗

arXiv2018

Graphene Enabled Optoelectronics on Paper

The realization of optoelectronic devices on paper has been an outstanding challenge due to the large surface roughness and incompatible nature of paper with optical materials. Here, we demonstrate a new class of optoelectronic devices on a piece of printing paper using graphene as an electrically reconfigurable optical medium. Our approach relies on electro-modulation of optical properties of multilayer graphene on paper via blocking the interband electronic transitions. The paper based devices yield high optical contrast in the visible spectrum with fast response speed. Pattering graphene into multiple pixels, folding paper into 3-dimensional shapes or printing coloured ink on paper substrates enable us to demonstrate novel optoelectronic devices which cannot be realized with wafer-based techniques.

cond-mat.mtrl-sci↗

arXiv2011

Synthesis of Graphene on Gold

Here we report chemical vapor deposition of graphene on gold surface at ambient pressure. We studied effects of the growth temperature, pressure and cooling process on the grown graphene layers. The Raman spectroscopy of the samples reveals the essential properties of the graphene grown on gold surface. In order to characterize the electrical properties of the grown graphene layers, we have transferred them on insulating substrates and fabricated field effect transistors. Owing to distinctive properties of gold, the ability to grow graphene layers on gold surface could open new applications of graphene in electrochemistry and spectroscopy.

cond-mat.mes-hall↗

arXiv2006

Gate Capacitance Coupling of Singled-walled Carbon Nanotube Thin-film Transistors

The electrostatic coupling between singled-walled carbon nanotube (SWNT) networks/arrays and planar gate electrodes in thin-film transistors (TFTs) is analyzed both in the quantum limit with an analytical model and in the classical limit with finite-element modeling. The computed capacitance depends on both the thickness of the gate dielectric and the average spacing between the tubes, with some dependence on the distribution of these spacings. Experiments on transistors that use sub-monolayer, random networks of SWNTs verify certain aspects of these calculations. The results are important for the development of networks or arrays of nanotubes as active layers in TFTs and other electronic devices.

cond-mat.mtrl-sci↗