Two-dimensional transition metal dichalcogenides (TMDC) and MoS2 in particular are promising materials as sensitive layers for gas sensing due to room operation temperature, high sensitivity, low dimensions, vast methods of selectivity alteration, etc. MoS2 response to toxic gases exposure depends on applied electric field that expands capabilities of resistive detection techniques, therefore, requires in-depth study. We fabricated a back-gated MoS2 based field-effect transistor (MoS2-FET) with standard photolithography technique on Si/SiO2 substrate. AFM microscopy confirmed the single layer nature of MoS2 flakes by cross-section featuring a thickness of 0.7 nm. Raman spectroscopy revealed A1g and E12g modes position at 403.5 cm-1 and 382 cm-1 respectively. The mobility in the absence of passivation was about 10-1 cm2 V−1s−1. MoS2-FET exhibits room-temperature NH3 sensing with resistive response to 200 ppm exposure of about ~60%, signal-to-noise ratio about 8, and response/recovery time about 100 s.
In this work, we report a novel method of mask-less doping of graphene channel in field-effect transistor configuration by local inkjet printing of organic semiconducting molecules. Graphene-based transistor was fabricated via large-scale technology, allowing for upscaling electronic device fabrication and lowering the device cost. The altering of functionalization of graphene was performed through local inkjet printing of semiconducting molecules: N,N′-Dihexyl- 3,4,9,10-perylenedicarboximide (PDI-C6), 5,5′′′-Dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (HEX-4T-HEX) and polyalanine (PANI). We found the effect of UV treatment on fabrication of graphene/organic junctions because of change in graphene hydrophobic properties. We demonstrated the high resolution (about 50 μm) and accurate printing of organic ink on UV treated chemical vapor deposited (CVD) graphene. The PANI/graphene junction demonstrate more stable photoresponse characteristic for 470 nm diode illumination. The characteristics of PDI/graphene junction demonstrate the saturation for high diode power because of organic crystals degradation. The photoresponse of 1 mA/W was demonstrated for PANI/graphene junction at 0.3 V bias voltage. The developed method opens the way for local functionalization of on-chip array of graphene by inkjet printing of different semiconducting organic molecules for photonics and electronics application.
Carbon nanotubes (CNTs) are one of the most promising materials for advanced electronic applications, due to its extraordinary chemical and physical properties. Non-linear interactions between photons and carbon bonds provide the possibility to fabricate unique photonic devices. In this paper we present the new technological route of single walled CNTs (SWCNTs) modification using femtosecond (fs) laser pulses to produce junctions in nanotubes through multiphoton oxidation of the carbon lattice with nanoscale resolution. SWCNTs were deposited onto Si/SiO2 substrate using gas-phase process based on thermal decomposition of ferrocene in the presence of carbon monoxide. Source and drain 100/20 nm Au/Ti electrodes were fabricated by photolithography, the gate electrode was p++ Si substrate. Samples were irradiated via fs laser with different energy fluence. Fs laser pulses at low energies were used to perform photocurrent measurements. Not modified SWCNTs and structures modified upon fs laser demonstrate a huge difference for light induced charge generation. We observed significant changes in optical and electrical properties of SWCNTs after the modification. Varying the parameters of power and laser scanning speed we can change the level of local oxidation of SWCNT and photocurrent in produced photodetectors.
The development of planar functional junction provides continuous, single-atom thick, in-plane integrated circuits. The production of atomic contacts of different materials (hetero/homostructures) is still a challenging task for 2D materials technology. In this paper we describe a new method of formation of a photosensitive junction by femtosecond laser pulses patterning of graphene FET. The laser-induced oxidation of graphene goes under high intensity laser pulses, which provide nonlinear effects in graphene like multiphoton absorption and hot carrier generation. The process of laser induced local oxidation is studied on single-layer graphene FET produced by wet transfer of CVD grown graphene on copper foil onto a Si/SiO2 substrate. The 280 fs laser with 515 nm wavelength with various pulse energies is applied to modify of local electrical and optical properties of graphene. Thus, the developed process provides mask-less laser induced in-plane junction patterning in graphene. The scale of local heterojunction fabrication is about 1 μm. We observe that with an increasing of the laser fluence the number of defects increases according to two different mechanism for low and high fluences, respectively. The change of the charge carrier concentration causes the Dirac point shift in produced structures. We investigate the photoresponse in graphene junctions under fs pulsed laser irradiation with subthreshold energies. The response time is rather high while relaxation time is large because of charge traps at the graphene/SiO2 interface.
This paper describes the recent results in ultrafast (femtoseconds and picoseconds) pulsed laser patterning of graphene films (single layer graphene, graphene oxide (GO)). We investigated such effects of nonlinear optical interaction like selective laser ablation of graphene, laser reduction of graphene oxide and local functionalization (oxidation) of graphene based on multiphoton absorption for microelectrode patterning. The graphene oxide and reduction was demonstrated under femtosecond laser pulses as well as fine ablation for monolayer GO films under ps laser pulses. We demonstrated the patterned laser reduction over the GO film leads to minimum in resistance for laser fluence because of interplay of chemical and thermal effects in carbon lattice and photons. The micro-scale patterns in graphene on SiO2 substrates were fabricated using ultrashort 515 nm laser pulses. For both picosecond and femtosecond laser pulses two competitive processes, based on photo-thermal (ablation) and photochemical (oxidation/etching) effects, were observed. The laser-induced etching of graphene starts just below the threshold energy of graphene ablation. The mechanisms of ultrafast laser interaction with graphene are discussed. Patterned graphene was investigated by AFM, microRaman, SEM and sheet resistance measurements and other techniques. The mechanisms of ultrafast laser interaction with graphene are discussed. The comprehensive models of graphene oxidation/reduction are suggested.
Carbon nanotube (CNT) and SiO2 etching effects was studied and was found that using different techniques of focused ion beam (FIB) exposure and using two pass etching leads to a significant difference in the etching rate of CNTs relatively of SiO2 and directly individually oxide itself. The parameters annealing of the structures to remove the effects of the charge arising from the etching of CNT on SiO2 was determined and the effect of the charge on the effects of the deposition of organic molecules from solution was studied. Different behavior of deposition of polar and non-polar polymer materials on charged regions with width less than 100 nm was found. Obtained structures was investigated by SEM, AFM methods and for structures with polyaniline deposited CVC was measured and by comparison with literature and experimental data analysis of polyaniline structuring in nano-scale gap formed with FIB was carried out.
We have developed the ultra-short pulsed laser processing methods for patterning of graphene field effect transistors in topological and chemical way. We investigated in details the photoresponse in graphene FETs before and after laser-induced modification for laser influence below threshold energy. We observed two different mechanisms of the photoresponse under ultra-short laser pulses (280 fs). The photocurrent, observed for both pristine and laser processed graphene is raised because the laser induced charge is transferred from graphene to trapped levels in SiO2 surface resulting in electrostatic Dirac point shift. For laser oxidized areas we observed more pronounced photocurrent because of heterojunction formation in laser-processed area. While for electrostatic effect the relaxation time estimated as 50 seconds, the heterojunction relaxation was observed for less than 3 ms.
Nanocarbon materials are of great interest as field emission cathodes due to their low threshold voltage. In this work current-voltage characteristics of nanocarbon electrodes were studied. Low-threshold emission was found in planar samples where field enhancement is negligible (<10). Electron work function values, calculated by Fowler-Nordheim theory, are anomalous low (<1 eV) and come into collision with directly measured work function values in fabricated planar samples (4.1-4.4 eV). Non-applicability of Fowler-Nordheim theory for the nanocarbon materials was confirmed. The reasons of low-threshold emission in nanocarbon materials are discussed.
Carbon nanomaterials is among the most promising technologies for advanced electronic applications, due to their extraordinary chemical and physical properties. Nonetheless, after more than two decades of intensive research, the application of carbon-based nanostructures in real electronic and optoelectronic devices is still a big challenge due to lack of scalable integration in microelectronic manufacturing. Laser processing is an attractive tool for graphene device manufacturing, providing a large variety of processes through direct and indirect interaction of laser beams with graphene lattice: functionalization, oxidation, reduction, etching and ablation, growth, etc. with resolution down to the nanoscale. Focused laser radiation allows freeform processing, enabling fully mask-less fabrication of devices from graphene and carbon nanotube films. This concept is attractive to reduce costs, improve flexibility, and reduce alignment operations, by producing fully functional devices in single direct-write operations. In this paper, a picosecond laser with a wavelength of 515 nm and pulse width of 30 ps is used to pattern carbon nanostructures in two ways: ablation and chemical functionalization. The light absorption leads to thermal ablation of graphene and carbon nanotube film under the fluence 60-90 J/cm2 with scanning speed up to 2 m/s. Just under the ablation energy, the two-photon absorption leads to add functional groups to the carbon lattice which change the optical properties of graphene. This paper shows the results of controlled modification of geometrical configuration and the physical and chemical properties of carbon based nanostructures, by laser direct writing.
Influence of gas sorption and desorption on field emission current evolution from carbon nanotube cathodes was investigated. Two types of nanotube cathodes were made: nanotubes grown from gas phase on stainless steel and nanotubes deposited from solution on Si substrate. Exposure of cathode to air at atmospheric pressure leads to increase of starting current with rapid decrease. We associate this effect of reversible degradation with some kind of field desorption. Sorbed gases reduce the work function and thus increase emission current. Different gases demonstrate different effect on the current behavior. We observed desorption process in low field values ~104−105 V/cm.
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