Browsing by Author "Kovalska, E."

Now showing 1 - 4 of 4

Open Access
Graphene as plasma-compatible blocking layer material for area-selective atomic layer deposition: a feasibility study for III-nitrides
(AVS Science and Technology Society, 2018) Deminskyi, P.; Haider A.; Kovalska, E.; Bıyıklı, Necmi
Plasma-assisted atomic layer deposition (PA-ALD) is a promising method for low-temperature growth of III-nitride materials. However, selective film deposition using PA-ALD is challenging mainly due to the plasma-incompatibility of conventional deactivation/blocking layers including self-assembled monolayers and polymers. The main motivation behind this work is to explore alternative plasma-resistant blocking layer materials. Toward this goal, single/multilayered graphene (SLG/MLG) sheets were investigated as potential growth-blocking layers for III-nitride grown via PA-ALD. Prior to PA-ALD growth experiments, partially graphene-covered Si(100) samples were exposed to N2/H2 and N2-only plasma cycles to evaluate the plasma resistance of SLG and MLG. While SLG degraded fairly quickly showing signs of completely etched areas and rough surface morphology, MLG surface displayed certain degree of plasma-resistance. Based on this result, III-nitride PA-ALD experiments were carried out on MLG-patterned Si(100) samples. Crystalline III-nitride film deposition was observed on both Si(100) and graphene surfaces, confirming the rather ineffective nucleation blocking property of graphene surface against PA-ALD process. However, as graphene layers feature relatively weak van der Waals bonds at the substrate/graphene interface as well as between the multilayer graphene interfaces, conventional lift-off process was sufficient to remove the deposited excessive nitride films. InN and AlN-coated samples were ultrasonicated, and blocked/unblocked surfaces were characterized using scanning electron microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometer. While ∼50 nm thick films were measured in the open Si(100) areas, graphene-coated sample portions exhibited limited material growth in the range of 5-15 nm. Although not completely, the MLG surface has considerably blocked the PA-ALD growth process resulting in a usable thickness difference, enabling growth selectivity with postgrowth etch process. An Ar-based physical dry etching recipe was utilized to completely etch the unwanted nitride films from graphene coated area, while about 30 and 40 nm thick InN and AlN films remained on the nonblocked parts of the samples, respectively. As a result, selective deposition of PA-ALD grown AlN and InN has been achieved via graphene-assisted lift-off technique along with subsequent dry-etch process, achieving a maximum growth selectivity of ∼40 nm. With further process recipe optimization and integrating with a suitable patterning technique, the demonstrated graphene-assisted lift-off technique might offer an alternative feasible pathway toward area-selective deposition of III-nitrides and other plasma-necessitating materials.
Open Access
Graphene-enabled optoelectronics on paper
(American Chemical Society, 2016-06) Polat, E. O.; Uzlu, H. B.; Balci, O.; Kakenov, N.; Kovalska, E.; Kocabas, C.
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 a fast response. Pattering graphene into multiple pixels, folding paper into three-dimensional shapes or printing colored ink on paper substrates enable us to demonstrate novel optoelectronic devices which cannot be realized with wafer-based techniques.
Open Access
NLL-Assisted Multilayer Graphene Patterning
(American Chemical Society, 2018) Kovalska, E.; Pavlov, I.; Deminskyi, P.; Baldycheva, A.; İlday, Fatih Ömer; Kocabas, C.
The range of applications of diverse graphene-based devices could be limited by insufficient surface reactivity, unsatisfied shaping, or null energy gap of graphene. Engineering the graphene structure by laser techniques can adjust the transport properties and the surface area of graphene, providing devices of different nature with a higher capacitance. Additionally, the created periodic potential and appearance of the active external/inner/edge surface centers determine the multifunctionality of the graphene surface and corresponding devices. Here, we report on the first implementation of nonlinear laser lithography (NLL) for multilayer graphene (MLG) structuring, which offers a low-cost, single-step, and high-speed nanofabrication process. The NLL relies on the employment of a high repetition rate femtosecond Yb fiber laser that provides generation of highly reproducible, robust, uniform, and periodic nanostructures over a large surface area (1 cm2/15 s). NLL allows one to obtain clearly predesigned patterned graphene structures without fabrication tolerances, which are caused by contacting mask contamination, polymer residuals, and direct laser exposure of the graphene layers. We represent regularly patterned MLG (p-MLG) obtained by the chemical vapor deposition method on an NLL-structured Ni foil. We also demonstrate tuning of chemical (wettability) and electro-optical (transmittance and sheet resistance) properties of p-MLG by laser power adjustments. In conclusion, we show the great promise of fabricated devices, namely, supercapacitors, and Li-ion batteries by using NLL-assisted graphene patterning. Our approach demonstrates a new avenue to pattern graphene for multifunctional device engineering in optics, photonics, and bioelectronics.
Open Access
Organic Electrolytes for Graphene-Based Supercapacitor: Liquid, Gel or Solid
(Elsevier Ltd, 2016) Kovalska, E.; Kocabas, C.
The electrolyte is an important and decisive factor in battery, capacitor and supercapacitor fabrication. Here we report how electrolyte's provenance and structure effect on the electro-optical properties of the graphene-based supercapacitor. The three organic electrolytes were synthesized: liquid electrolyte, which on the basis of propylene carbonate (PC), gel electrolyte - polyvinyl alcohol (PVA) and solid electrolyte - polyvinylidene fluoride (PVDF). As an application, we demonstrate an optical modulator using supercapacitor structure built by graphene electrodes and prepared electrolytes. All organic electrolyte-based supercapacitors potentiate optical modulation of graphene electrodes over a broad range of wavelengths, under ambient conditions. We reveal higher capacitance (78 μF/cm2) for a supercapacitor with gel electrolyte during various bias voltages. We represent the increasing of light transmission at 3 times using solid electrolyte, in comparison with liquid and gel electrolytes and illustrate the supercapacitor possibility with gel electrolyte to operate under negative voltage. Consequently, we suggest applying of solid electrolyte as a more appropriate electrolyte for fabrication of graphene-based supercapacitor. We anticipate that using of solid electrolyte allows us to get desired electro-optical properties, minimize the size of the device and vary it shape.