Maxim G. Rybin

Broadband Light-Induced Absorbance Change in Multilayer Graphene

We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pump-probe measurements in 1100-1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.

Second harmonic generation in multilayer graphene induced by direct electric current

Optical second harmonic generation (SHG) is studied from multilayer graphene films in the presence of DC electric current flowing in the sample plane. Graphene layers are manufactured by chemical vapour deposition (CVD) technique and deposited on an oxidised Si(001) substrate. SHG intensity from graphene layer is found to be negligible in the absence of the DC current, while it increases dramatically with the application of the electric current. The current-induced change of the SHG intensity rises linearly with the current amplitude and changes its sign under the reversal of the current direction to the opposite. The observed effect is explained in terms of the interference of second harmonic radiation reflected from the Si surface and that induced by the DC current in multilayer graphene.

Experimental study of nitrogen-doped graphene by spectroscopic and probe methods of surface analysis

Abstract. This work demonstrates an integrated approach for studying graphene films with various doping levels of nitrogen. The graphene films grown by a chemical vapor deposition technique were doped by treatment in ammonia radio-frequency plasma discharge. The graphene samples were investigated by x-ray photoelectron spectroscopy with a parallel registration of photoemission angular dependence. The depth-dependent changes in the valence band structure and the nitrogen peak position were recorded. The shift of valence band maximum relative to the initial value (0.13±0.04  eV) was observed using ultraviolet photoelectron spectroscopy. The dispersion and the shift of π-plasmon maximum were registered while the percentage of nitrogen atoms in two-dimensional graphene network increased.

Manifestation of plasmonic response in the detection of sub-terahertz radiation by graphene-based devices

We report on the sub-terahertz (THz) (129-450 GHz) photoresponse of devices based on single layer graphene and graphene nanoribbons with asymmetric source and drain (vanadium and gold) contacts. Vanadium forms a barrier at the graphene interface, while gold forms an Ohmic contact. We find that at low temperatures (77 K) the detector responsivity rises with the increasing frequency of the incident sub-THz radiation. We interpret this result as a manifestation of a plasmonic effect in the devices with the relatively long plasmonic wavelengths. Graphene nanoribbon devices display a similar pattern, albeit with a lower responsivity.

Electrical properties of gas sensors based on graphene and single-wall carbon nanotubes

Abstract. Here, we present investigation of the influence of different gases (carbon dioxide, ammonia, and iodine vapor) on the sensory properties of graphene and single-wall carbon nanotube films. The gas molecules are adsorbed by carbon films (graphene or nanotubes) and change the film’s electrical resistance. In the course of this work, the setup for studying the electrophysical properties of carbon nanomaterials has been designed and constructed in the lab. With this home-made equipment, we have demonstrated a high efficiency of graphene and nanotubes as adsorbents of different gases and a possibility to use these materials as gas sensors. We have also performed a chemical modification of graphene and carbon nanotubes by attaching the nanoparticles of calcium carbonate (CaCO3) to improve the sensitivity and selectivity of sensors.

Graphene-based lateral Schottky diodes for detecting terahertz radiation

Demand for efficient terahertz radiation detectors resulted in intensive study of the carbon nanostructures as possible solution for that problem. In this work we investigate the response to sub-terahertz radiation of graphene field effect transistors of two configurations. The devices of the first type are based on single layer CVD graphene with asymmetric source and drain (vanadium and gold) contacts and operate as lateral Schottky diodes (LSD). The devices of the second type are made in so-called Dyakonov-Shur configuration in which the radiation is coupled through a spiral antenna to source and top electrodes. We show that at 300 K the LSD detector exhibit the room-temperature responsivity from R = 15 V/W at f= 129 GHz to R = 3 V/W at f = 450 GHz. The DS detector responsivity is markedly lower (2 V/W) and practically frequency independent in the investigated range. We find that at low temperatures (77K) the graphene lateral Schottky diodes responsivity rises with the increasing frequency of the incident sub-THz radiation. We interpret this result as a manifestation of a plasmonic effect in the devices with the relatively long plasmonic wavelengths. The obtained data allows for determination of the most promising directions of development of the technology of nanocarbon structures for the detection of THz radiation.

Modification of graphene electronic properties via controllable gas-phase doping with copper chloride

Molecular doping is an efficient, non-destructive, and simple method for changing the electronic structure of materials. Here, we present a simple air ambient vapor deposition method for functionalization of pristine graphene with a strong electron acceptor: copper chloride. The doped graphene was characterized by Raman spectroscopy, UV-vis-NIR optical absorption spectroscopy, scanning electron microscopy, and electro-physical measurements performed using the 4-probe method. The effect of charge transfer from graphene to a dopant results in shifting the Fermi level in doped graphene. The change of the electronic structure of doped graphene was confirmed by the tangential Raman peak (G-peak) shift and by the appearance of the gap in the UV-vis-NIR spectrum after doping. Moreover, the charge transfer resulted in a substantial decrease in electrical sheet resistance depending on the doping level. At the highest concentration of copper chloride, a Fermi level shift into the valence band up to 0.64 eV and a decrease in the sheet resistance value by 2.36 times were observed (from 888 Ω/sq to 376 Ω/sq for a single graphene layer with 97% of transparency).Molecular doping is an efficient, non-destructive, and simple method for changing the electronic structure of materials. Here, we present a simple air ambient vapor deposition method for functionalization of pristine graphene with a strong electron acceptor: copper chloride. The doped graphene was characterized by Raman spectroscopy, UV-vis-NIR optical absorption spectroscopy, scanning electron microscopy, and electro-physical measurements performed using the 4-probe method. The effect of charge transfer from graphene to a dopant results in shifting the Fermi level in doped graphene. The change of the electronic structure of doped graphene was confirmed by the tangential Raman peak (G-peak) shift and by the appearance of the gap in the UV-vis-NIR spectrum after doping. Moreover, the charge transfer resulted in a substantial decrease in electrical sheet resistance depending on the doping level. At the highest concentration of copper chloride, a Fermi level shift into the valence band up to 0.64 eV and a de...

Multi-gigahertz repetition rate ultrafast waveguide lasers mode-locked with graphene saturable absorbers

We report the development of an approach to build compact waveguide lasers that operate in the stable fundamental mode-locking regime with multigigahertz repetition rates. The approach is based on the use of depressed cladding multi- or single-mode waveguides fabricated directly in the active laser crystal using the femtosecond laser inscription method and a graphene saturable absorber. Using this approach we achieve the stable self-starting mode-locking operation of a diode-pumped waveguide Nd:YAG laser that delivers picosecond pulses at a repetition rate of up to 11.5 GHz with an average power of 12 mW at a central wavelength of 1064 nm. The saturable absorbers are formed through the chemical vapor deposition of single-layer graphene on the output coupler mirror or directly on the end facet of the laser crystal. The stable self-starting mode-locking operation is achieved by controlling the group delay dispersion in the laser cavity with an intracavity interferometer. The method developed for the creation of compact ultrashort pulse laser generators with gigahertz repetition rates can be extended further and applied for the development of compact high-repetition rate lasers that operate at a wide range of IR wavelengths.

Nonlinear transmission of CO{sub 2} laser radiation by graphene

The nonlinear transmission of multilayer ({approx}12 layers) graphene at the wavelength {lambda} {approx} 10 {mu}m is measured for the first time. The absorption saturation intensity in graphene ({approx}330 kW cm{sup -2}) and the ablation threshold of its outer layers ({approx}1 MW cm{sup -2}, 0.11 J cm{sup -2}) under action of a CO{sub 2} laser pulse with a duration of 70 - 85 ns at {lambda} = 10.55 {mu}m are determined. The residual absorption of graphene at its partial saturation was 48 % of the initial value. This is significantly smaller than the value measured previously for samples with a close number of layers at {lambda} = 1.55 {mu}m (92.3 % - 93.8 %). It is shown that the ablation threshold of two graphene layers adjacent to the BaF{sub 2} substrate (after successive ablation of outer layers) exceeds 0.27 J cm{sup -2}. (nonlinear optical phenomena)