Author: Viliam Vretenár

Polarization dependent photoluminescence and optical anisotropy in CuPtB-ordered dilute GaAs1-xBi xalloys

Posted on by Viliam Vretenár

PAULAUSKAS, Tadas – ACEECHAVIČIUS, Bronislovas – KARPUS, Vytautas – JOČIONIS, Lukas – TUMĖNAS, Saulius – DEVENSON, Jan – PAČ̌EBUTAS, Vaidas – STANIONYTE, Sandra – STRAZDIENE, Viktorija – GEIAUTIS, Andrejus – ČAPLOVIČOVÁ, Mária – VRETENÁR, Viliam – WALLS, Michael G. – KROTKUS, Arunas

In Journal of Applied Physics. Vol. 128, iss. 19 (2020)

https://doi.org/10.1063/5.0030091

The GaAs1–xBix semiconductor alloy allows one to achieve large bandgap reduction and enhanced spin–orbit splitting energy at dilute Bi quantities. The bismide is currently being developed for near- to mid-infrared lasers, multi-junction solar cells, and photodetectors. In this structure–property relationship study of GaAsBi alloys, we report polarization dependent photoluminescence that reaches a polarization ratio up to 2.4 at room temperature. Polarization dependence is also presented using transmittance spectra, birefringence, and linear dichroism. The optical anisotropy observations agree with the predictions of point symmetry reduction in the CuPtB-type ordered GaAsBi phase. The structural ordering is investigated experimentally from the atomic scale in molecular-beam epitaxy (MBE) grown samples on exact and miscut (001) GaAs substrates, as well as on (001) Ge. The latter sample is composed of anti-phase domains in which the ordering axes are rotated by 90° angles. Since the conditions stabilizing the CuPtB ordered phase fall within the typical MBE growth regime of dilute bismides, the optical anisotropy in GaAsBi alloys is expected to be ubiquitous. These findings are important for the future development of GaAsBi-based optoelectronics and also provide new means to analyze structurally complex bismide alloys.

Atomic-Resolution EDX, HAADF, and EELS Study of GaAs1-xBix Alloys

Posted on by Viliam Vretenár

PAULAUSKAS, Tadas – PACEBUTAS, Vaidas – BUTKUTE, Renata – ČECHAVIČIUS, Bronislovas – NAUJOKAITIS, Arnas – KAMARAUSKAS, Mindaugas – SKAPAS, Martynas – DEVENSON, Jan – ČAPLOVIČOVÁ, Mária – VRETENÁR, Viliam – LIU, Xiaoyan – KOCIAK, Mathieu – KROTKUS, Arunas

In Nanoscale Research Letters. Vol. 15, iss. 1 (2020)

https://doi.org/10.1186/s11671-020-03349-2

Abstract

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs1−xBix alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors.

A comparative life cycle assessment of graphene and activated carbon in a supercapacitor application

Posted on by Viliam Vretenár

COSSUTTA, Matteo – VRETENÁR, Viliam – CENTENO, Teresa A. – KOTRUSZ, Peter – MCKECHNIE, Jon – PICKERING, Stephen J.

In Journal of Cleaner Production. Vol. 242, iss. 1 (2020)

https://doi.org/10.1016/j.jclepro.2019.118468

Abstract

Graphene shows substantial promise in improving the technical performance of a range of applications. For its development and before its potential mass adoption, it is critical to understand the associated cradle-to-grave life cycle environmental impacts. Previous studies on graphene environmental performance do not include end of life and the potential environmental credits generated by graphene reuse. This study undertakes a cradle-to-grave approach to evaluating graphene applications, considering a case study of supercapacitors manufactured with graphene and activated carbon active materials. The analysis includes active materials commercial-size production, supercapacitors production, supercapacitors use phase in an automotive application, and their end of life in which both devices are recycled. With current material performance and energy mixes, the graphene-based supercapacitor would increase impacts in all environmental categories analysed, ranging from 27% higher human toxicity to 213% greater ozone depletion and showing a 48% increase in GHG emissions. This unfavourable result arises due to the tested graphene material exhibiting inferior specific capacitance to the activated carbon comparator, as well as a more energy-intensive production process. Prospective analysis considers hypothetical performance where both active materials reach their theoretical specific capacitance, and the decarbonisation of electricity generation. The environmental impacts of both the activated carbon and graphene supercapacitors are reduced in these prospective scenarios, with the graphene based supercapacitor becoming the least impacting solution due to its lower active material requirements. The graphene-based device shows 36% lower GHG emission and overall shows lower impacts ranging from 14% less mineral, fossil and renewable resource depletion to 43% less photochemical ozone formation. These results support graphene as a valid candidate material for substituting activated carbon in supercapacitors provided graphene technical performance is improved, production optimised, and recycling developed.

Biomass waste-carbon/reduced graphene oxide composite electrodes for enhanced supercapacitors

Posted on by Viliam Vretenár

GUARDIA, Laura – SUÁREZ, Loreto – QUEREJETA, Nausika – VRETENÁR, Viliam – KOTRUSZ, Peter – SKÁKALOVÁ, Viera – CENTENO, Teresa A.

In Electrochimica Acta. Vol. 298, iss. 3 (2019)

Abstract

We present a simple and effective alternative which optimizes electrodes based on low-cost carbons for high-performance supercapacitors. The combination with reduced graphene oxide (rGO) greatly improves the operation of microporous carbons easily produced by one-pot activation of grape seeds. The use of composite electrodes with rGO lowers the supercapacitor resistance and enables a much higher rate capability.

The mixture of rGO flakes and particles of a highly porous carbon obtained by KOH activation allows retaining the high capacitance of 260 F g−1 of the standard electrodes at 1 mA cm−2 in aqueous H2SO4 whereas the value at 200 mA cm−2 is increased by around 2.4 times. Consequently, at high current density, the capacitor assembled with these composites stores eight times more energy and the power density is multiplied by four.

The synergy between rGO and an ultramicroporous carbon produced by CO2-activation results extremely profitable, the cell assembled with composite electrodes reaching three times more energy and power at 200 mA cm−2 than the best performance of the standard counterpart.

More importantly, the higher density of the composite electrodes leads to a capacitance of around 200 F cm−3 which translates into a remarkable improvement in the supercapacitor operation normalized to volume.

Chemical Oxidation of Graphite: Evolution of the Structure and Properties

Posted on by Viliam Vretenár

SKÁKALOVÁ, Viera – KOTRUSZ, Peter – JERGEL, Matej – SUSI, Toma – MITTELBERGER, Andreas – VRETENÁR, Viliam – ŠIFFALOVIČ, Peter – KOTAKOSKI, J. – MEYER, Jannik C. – HULMAN, Martin

In Journal of Physical Chemistry C. Vol. 122, iss. 1 (2018)

https://doi.org/10.1021/acs.jpcc.7b10912

Abstract

Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graphite and the electrical resistivity increased by seven orders of magnitude. After 4 days of chemical processing, the expanded structure of graphite oxide became unstable and spontaneously exfoliated; more than 2 weeks resulted in a significant decrease in the oxygen content accompanied by reaggregation of the GO sheets. These correlated measurements allow us to offer a comprehensive view into the complex oxidation process.