Browsing by Author "Kordas, Krisztian"
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Item Carbon nanotube micropillars trigger guided growth of complex human neural stem cells networks(Springer, 2019) Lorite, Gabriela S.; Ylä-Outinen, Laura; Janssen, Lauriane; Pitkänen, Olli; Joki, Tiina; Koivisto, Janne T.; Kellomäki, Minna; Vajtai, Robert; Narkilahti, Susanna; Kordas, KrisztianNew strategies for spatially controlled growth of human neurons may provide viable solutions to treat and recover peripheral or spinal cord injuries. While topography cues are known to promote attachment and direct proliferation of many cell types, guided outgrowth of human neurites has been found difficult to achieve so far. Here, three-dimensional (3D) micropatterned carbon nanotube (CNT) templates are used to effectively direct human neurite stem cell growth. By exploiting the mechanical flexibility, electrically conductivity and texture of the 3D CNT micropillars, a perfect environment is created to achieve specific guidance of human neurites, which may lead to enhanced therapeutic effects within the injured spinal cord or peripheral nerves. It is found that the 3D CNT micropillars grant excellent anchoring for adjacent neurites to form seamless neuronal networks that can be grown to any arbitrary shape and size. Apart from clear practical relevance in regenerative medicine, these results using the CNT based templates on Si chips also can pave the road for new types of microelectrode arrays to study cell network electrophysiology.Item Flexible planar supercapacitors by straightforward filtration and laser processing steps(IOP, 2020) Pitkänen, Olli; Eraslan, Toprak; Sebők, Dániel; Szenti, Imre; Kukovecz, Ákos; Vajtai, Robert; Kordas, KrisztianThere is ever increasing demand for flexible energy storage devices due to the development of wearable electronics and other small electronic devices. The electrode flexibility is best provided by a special set of nanomaterials, but the required methodology typically consists of multiple steps and are designed just for the specific materials. Here, a facile and scalable method of making flexible and mechanically robust planar supercapacitors with interdigital electrode structure made of commercial carbon nanomaterials and silver nanowires is presented. The capacitor structure is achieved with vacuum filtration through a micropatterned contact mask and finished with simple laser processing steps. A maximum specific capacitance of 4 F cm−3 was measured with cyclic voltammetry at scan rate of 5 mV s−1. The reliability and charge transfer properties of devices were further investigated with galvanostatic charge-discharge measurements and electrochemical impedance spectroscopy, respectively. Furthermore, mechanical bending tests confirmed the devices have excellent mechanical integrity, and the deformations have no adverse effects on the electrochemical charge-discharge behavior and stability.Item High photoresponse of individual WS2 nanowire-nanoflake hybrid materials(AIP Publishing LLC, 2018) Asres, Georgies Alene; Järvinen, Topias; Lorite, Gabriela S.; Mohl, Melinda; Pitkänen, Olli; Dombovari, Aron; Tóth, Geza; Spetz, Anita Lloyd; Vajtai, Robert; Ajayan, Pulickel M.; Lei, Sidong; Talapatra, Saikat; Kordas, Krisztianvan der Waals solids have been recognized as highly photosensitive materials that compete conventional Si and compound semiconductor based devices. While 2-dimensional nanosheets of single and multiple layers and 1-dimensional nanowires of molybdenum and tungsten chalcogenides have been studied, their nanostructured derivatives with complex morphologies are not explored yet. Here, we report on the electrical and photosensitive properties of WS2 nanowire-nanoflake hybrid materials we developed lately. We probe individual hybrid nanostructured particles along the structure using focused ion beam deposited Pt contacts. Further, we use conductive atomic force microscopy to analyze electrical behavior across the nanostructure in the transverse direction. The electrical measurements are complemented by in situ laser beam illumination to explore the photoresponse of the nanohybrids in the visible optical spectrum. Photodetectors with responsivity up to ∼0.4 AW−1 are demonstrated outperforming graphene as well as most of the other transition metal dichalcogenide based devices.Item Self-assembled large scale metal alloy grid patterns as flexible transparent conductive layers(Nature Publishing Group, 2015) Mohl, Melinda; Dombovari, Aron; Vajtai, Robert; Ajayan, Pulickel M.; Kordas, KrisztianThe development of scalable synthesis techniques for optically transparent, electrically conductive coatings is in great demand due to the constantly increasing market price and limited resources of indium for indium tin oxide (ITO) materials currently applied in most of the optoelectronic devices. This work pioneers the scalable synthesis of transparent conductive films (TCFs) by exploiting the coffee-ring effect deposition coupled with reactive inkjet printing and subsequent chemical copper plating. Here we report two different promising alternatives to replace ITO, palladium-copper (PdCu) grid patterns and silver-copper (AgCu) fish scale like structures printed on flexible poly(ethylene terephthalate) (PET) substrates, achieving sheet resistance values as low as 8.1 and 4.9 Ω/sq, with corresponding optical transmittance of 79% and 65% at 500 nm, respectively. Both films show excellent adhesion and also preserve their structural integrity and good contact with the substrate for severe bending showing less than 4% decrease of conductivity even after 105 cycles. Transparent conductive films for capacitive touch screens and pixels of microscopic resistive electrodes are demonstrated.