Department of Materials Science and Technology

III-V nanowires are comprehensively studied because of their suitability for optoelectronic quantum technology applications. However, their small dimensions and the spatial separation of carriers from the wire surface render electrical contacting difficult. Systematically studying ohmic contact formation by diffusion to n-doped GaAs nanowires, we provide a set of optimal annealing parameters for Pd/Ge/Au ohmic contacts. We reproducibly achieve low specific contact resistances of ∼ 2 × 10 −7 Ωcm 2 at room temperature becoming an order of magnitude higher at T 4.2 K. We provide a phenomenological model to describe contact resistances as a function of diffusion parameters. Implementing a transfer-matrix method, we numerically study the influence of the Schottky barrier on the contact resistance. Our results indicate that contact resistances can be predicted using various barrier shapes but further insights into structural properties would require a full microscopic understanding of the complex diffusion processes.

We present and analyze unique phenomena of enhanced transmission through systems of subwavelength dielectric cylinders embedded in an epsilon near zero host. Our analysis shows that these phenomena are due to Mie-resonance modes arisen in the dielectric cylinders. Subwavelength waveguides and lenses are proposed based on coupling of these modes between neighboring cylinders. Finally, the proposed phenomena and their possible applications are numerically demonstrated in the THz regime in a realistic polaritonic material of LiF rods in KCl.

Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Due to its exceptional electronic, optical and mechanical properties, it is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in visible and near-infrared spectra, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute. Solution-processed reduced graphene oxide combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This paper provides an overview of recent research progress in the application and consequent physical-chemical properties of solution-processed reduced graphene oxide-based films as transparent conductive electrodes (TCEs) in organic photovoltaic (OPV) cells. Reduced graphene oxide (rGO) can be effectively utilized as the TCE in flexible OPVs, where the brittle and expensive ITO is incompatible. The prospects and future research trend in graphene-based TCE are also discussed.

A novel top-down and universal optical technique for the effective decoration of two-dimensional (2D)
nanosheets (NS), graphene oxide (GO), boron nitride (BN) and tungsten disulfide (WS2), with noble
metallic nanoparticles (NPs) is reported. The NS–NP assemblies were formed under ambient conditions,
via a facile, rapid and solution compatible laser assisted process in the presence of a metallic precursor.
It is shown that a few seconds of irradiation is sufficient to decorate the NS lattice, while the NP density
can be readily controlled upon variation of the irradiation time. It is found that the laser induced
anchoring of Au NPs onto the NS basal plane and edges resulted in enhanced light harvesting that is
potentially useful for energy conversion and storage applications. To demonstrate the potential of the
approach for practical applications, the incorporation of WS2–Au NP assemblies into the photoactive
layer of ternary bulk heterojunction (BHJ) organic photovoltaic (OPV) cells is realized. The power
conversion efficiency (PCE) of the binary device consisting of a poly[N-90-heptadecanyl-2,7-carbazolealt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) : [6,6]-phenyl-C71-butyric-acid-methylester (PC71BM) blend as the donor–acceptor pair was 5.6%, while after the employment of WS2–Au NPs
the efficiency enhancement of the ternary device was approximately 13% reaching a total PCE of 6.3%.
The facile, rapid and room temperature nature of the photo-induced method proposed here provides
unique opportunities for the cost-effective synthesis of bulk amounts of NS–NP assemblies in solution
for many optoelectronic applications.

A novel top-down and universal optical technique for the effective decoration of two-dimensional (2D) nanosheets (NS), graphene oxide (GO), boron nitride (BN) and tungsten disulfide (WS2), with noble metallic nanoparticles (NPs) is reported.

Vanadium dioxide (VO 2) is a well-known thermochromic material that can potentially be used as a smart coating on glazing systems in order to regulate the internal temperature of buildings. Most growth techniques for VO 2 demand high temperatures (>250 • C), making it impossible to comply with flexible (polymeric) substrates. To overcome this problem, hydrothermally synthesized VO 2 particles may be dispersed in an appropriate matrix, leading to a thermochromic coating that can be applied on a substrate at a low temperature (<100 • C). In this work, we reported on the thermochromic properties of a VO 2 /Poly-Vinyl-Pyrrolidone (PVP) nanocomposite. More specifically, a fixed amount of VO 2 particles was dispersed in different PVP quantities forming hybrids of various VO 2 /PVP molar ratios which were deposited as films on fused silica glass substrates by utilizing the drop-casting method. The crystallite size was calculated and found to be 35 nm, almost independent of the PVP concentration. As far as the thermochromic characteristics are concerned, the molar ratio of the VO 2 /PVP nanocomposite producing VO 2 films with the optimum thermochromic properties was 0.8. These films exhibited integral solar transmittance modulation (overall wavelengths) ∆Tr sol = 0.35%-1.7%, infrared (IR) switching at 2000 nm ∆Tr IR = 10%, visible transmittance at 550 nm Tr Vis = 38%, critical transition temperature T C = 66.8 • C, and width of transmittance hysteresis loop ∆T C = 6.8 • C. Moreover, the critical transition temperature was observed to slightly shift depending on the VO 2 /PVP molar ratio.

Vanadium dioxide (VO2) is a well-known thermochromic material that can potentially be used as a smart coating on glazing systems in order to regulate the internal temperature of buildings. Most growth techniques for VO2 demand high temperatures (&gt;250 °C), making it impossible to comply with flexible (polymeric) substrates. To overcome this problem, hydrothermally synthesized VO2 particles may be dispersed in an appropriate matrix, leading to a thermochromic coating that can be applied on a substrate at a low temperature (&lt;100 °C). In this work, we reported on the thermochromic properties of a VO2/Poly-Vinyl-Pyrrolidone (PVP) nanocomposite. More specifically, a fixed amount of VO2 particles was dispersed in different PVP quantities forming hybrids of various VO2/PVP molar ratios which were deposited as films on fused silica glass substrates by utilizing the drop-casting method. The crystallite size was calculated and found to be 35 nm, almost independent of the PVP concentratio...

The presented work explores the structural properties, gasification reactivity, and syngas production of Greek lignite fuel (LG) and ex-situ produced chars during CO2 gasification. Three different slow pyrolysis protocols were employed for char production involving torrefaction at 300 °C (LG300), mild-carbonization at 500 °C (LG500), and carbonization at 800 °C (LG800). Physicochemical characterization studies, including proximate and ultimate analysis, X-ray Diffraction (XRD), and Raman spectroscopy, revealed that the thermal treatment under inert atmospheres leads to chars with increased fixed carbon content and less ordered surface structures. The CO2 gasification reactivity of pristine LG and as-produced chars was examined by thermogravimetric (TG) analysis and in batch mode gasification tests under both isothermal and non-isothermal conditions. The key parameters affecting the devolatilization and gasification steps in the overall process toward CO-rich gas mixtures were thorou...

Monoclinic vanadium dioxide VO2(M) is a well-known thermochromic material, with its critical transition temperature (68 °C) being the closest to room temperature (RT). Among the variety of grown methods, hydrothermal synthesis is a simple and cost-effective technique to grow thermochromic VO2 in the form of powder. In the present work, VO2 nanoparticles were prepared by hydrothermal synthesis in mild conditions, followed by a thermal annealing process at 700 °C under nitrogen flow for two hours. Vanadium pentoxide (V2O5) was used as the vanadium precursor, while two different reducing agents, namely oxalic and succinic acid, were employed for the reduction of V2O5 to VO2. Additionally, urea as well as thiourea were used as additives, in order to investigate their effects on the thermochromic performance of VO2. As a result, the VO2 (M) phase was obtained after annealing the crystalline powder, grown hydrothermally using oxalic acid and thiourea as a reducing agent and additive, resp...

In the present work, the individual or synergistic effect of Ni-based catalysts (Ni/CeO2, Ni/Al2O3) and an eutectic carbonate salt mixture (MS) on the CO2 gasification performance of olive kernels was investigated. It was found that the Ni/CeO2 catalyst presented a relatively superior instant gasification reaction rate (Rco) compared to Ni/Al2O3, in line with the significant redox capability of CeO2. On the other hand, the use of the binary eutectic carbonate salt mixture (MS) lowered the onset and maximum CO2 gasification temperatures, resulting in a notably higher carbon conversion efficiency (81%) compared to the individual Ni-based catalysts and non-catalytic gasification tests (60%). Interestingly, a synergetic catalyst-carbonate salt mixture effect was revealed in the low and intermediate CO2 gasification temperature regimes, boosting the instant gasification reaction rate (Rco). In fact, in the temperature range of 300 to 550 °C, the maximum Rco value for both MS-Ni/Al2O3 and...

The presented work explores the structural properties, gasification reactivity, and syngas production of Greek lignite fuel (LG) and ex-situ produced chars during CO 2 gasification. Three different slow pyrolysis protocols were employed for char production involving torrefaction at 300 • C (LG300), mild-carbonization at 500 • C (LG500), and carbonization at 800 • C (LG800). Physicochemical characterization studies, including proximate and ultimate analysis, X-ray Diffraction (XRD), and Raman spectroscopy, revealed that the thermal treatment under inert atmospheres leads to chars with increased fixed carbon content and less ordered surface structures. The CO 2 gasification reactivity of pristine LG and as-produced chars was examined by thermogravimetric (TG) analysis and in batch mode gasification tests under both isothermal and non-isothermal conditions. The key parameters affecting the devolatilization and gasification steps in the overall process toward CO-rich gas mixtures were thoroughly explored. The gasification performance of the examined fuels in terms of carbon conversion, instant CO production rate, and syngas generation revealed an opposite reactivity order during each stage. TG analysis demonstrated that raw lignite (LG) was more reactive during the thermal devolatilization phase at low and intermediate temperatures (da/dt max,devol. = 0.022 min -1 ). By contrast, LG800 exhibited superior gasification reactivity at high temperatures (da/dt max,gas. = 0.1 min -1 ). The latter is additionally corroborated by the enhanced CO formation of LG800 samples under both non-isothermal (5.2 mmol) and isothermal (28 mmol) conditions, compared to 4.1 mmol and 13.8 mmol over the LG sample, respectively. The pronounced CO 2 gasification performance of LG800 was attributed to its higher fixed carbon content and disordered surface structure compared to LG, LG300, and LG500 samples.