Aleksei Emelianov

Photophysical and photochemical effects in ultrafast laser patterning of CVD graphene

The micro-scale patterns in graphene on Si/SiO2 substrate 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: 1.7 nJ per 280 fs pulse and 2.7 µJ per 30 ps pulse. Whilst etching is not sensitive to thermal properties of graphene and provides uniform patterns, the ablation, in contrast, is highly affected by defects in the graphene structure like wrinkles and bilayer islands. The mechanisms of ultrafast laser interaction with graphene are discussed.

Low-voltage field desorption in carbon nanotubes

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.

Molecular doping of single-walled carbon nanotube transistors: optoelectronic study

Single-walled carbon nanotubes (SWCNT) are a promising material for future optoelectronic applications, including flexible electrodes and field-effect transistors. Molecular doping of carbon nanotube surface can be an effective way to control the electronic structure and charge dynamics of these material systems. Herein, two organic semiconductors with different energy level alignment in respect to SWCNT are used to dope the channel of the SWCNT-based transistor. The effects of doping on the device performance are studied with a set of optoelectronic measurements. For the studied system, we observed an opposite change in photo-resistance, depending on the type (electron donor vs electron acceptor) of the dopants. We attribute this effect to interplay between two effects: (i) the change in the carrier concentration and (ii) the formation of trapping states at the SWCNT surface. We also observed a modest ~4 pA photocurrent generation in the doped systems, which indicates that the studied system could be used as a platform for multi-pulse optoelectronic experiments with photocurrent detection.

Photosensitive in-plane junction in graphene field effect transistor modified under femtoseconds laser irradiation

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.

Femtosecond laser-induced CNT heterojunction as visible light photodetector

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.

Photoresponse in graphene field effect transistor under ultra-short pulsed laser irradiation

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.