Razvan Pascu

Bio: Razvan Pascu is an academic researcher from Politehnica University of Bucharest. The author has contributed to research in topics: Schottky diode & Schottky barrier. The author has an hindex of 10, co-authored 41 publications receiving 256 citations. Previous affiliations of Razvan Pascu include University of Bucharest.

Engineering Graphene Quantum Dots for Enhanced Ultraviolet and Visible Light p-Si Nanowire-Based Photodetector

Abstract: In this work, a significant improvement of the classical silicon nanowire (SiNW)-based photodetector was achieved through the realization of core–shell structures using newly designed GQDPEIs via simple solution processing. The poly(ethyleneimine) (PEI)-assisted synthesis successfully tuned both optical and electrical properties of graphene quantum dots (GQDs) to fulfill the requirements for strong yellow photoluminescence emission along with large band gap formation and the introduction of electronic states inside the band gap. The fabrication of a GQDPEI-based device was followed by systematic structural and photoelectronic investigation. Thus, the GQDPEI/SiNW photodetector exhibited a large photocurrent to dark current ratio (Iph/Idark up to ∼0.9 × 102 under 4 V bias) and a remarkable improvement of the external quantum efficiency values that far exceed 100%. In this frame, GQDPEIs demonstrate the ability to arbitrate both charge-carrier photogeneration and transport inside a heterojunction, leading to...

High-performance solid state supercapacitors assembling graphene interconnected networks in porous silicon electrode by electrochemical methods using 2,6-dihydroxynaphthalen

Abstract: The challenge for conformal modification of the ultra-high internal surface of nanoporous silicon was tackled by electrochemical polymerisation of 2,6-dihydroxynaphthalene using cyclic voltammetry or potentiometry and, notably, after the thermal treatment (800 °C, N2, 4 h) an assembly of interconnected networks of graphene strongly adhering to nanoporous silicon matrix resulted. Herein we demonstrate the achievement of an easy scalable technology for solid state supercapacitors on silicon, with excellent electrochemical properties. Accordingly, our symmetric supercapacitors (SSC) showed remarkable performance characteristics, comparable to many of the best high-power and/or high-energy carbon-based supercapacitors, their figures of merit matching under battery-like supercapacitor behaviour. Furthermore, the devices displayed high specific capacity values along with enhanced capacity retention even at ultra-high rates for voltage sweep, 5 V/s, or discharge current density, 100 A/g, respectively. The cycling stability tests performed at relatively high discharge current density of 10 A/g indicated good capacity retention, with a superior performance demonstrated for the electrodes obtained under cyclic voltammetry approach, which may be ascribed on the one hand to a better coverage of the porous silicon substrate and, on the other hand, to an improved resilience of the hybrid electrode to pore clogging.

Characterization technique for inhomogeneous 4H-SiC Schottky contacts: A practical model for high temperature behavior

Abstract: In this paper, a characterization technique for 4H-SiC Schottky diodes with varying levels of metal-semiconductor contact inhomogeneity is proposed. A macro-model, suitable for high-temperature evaluation of SiC Schottky contacts, with discrete barrier height non-uniformity, is introduced in order to determine the temperature interval and bias domain where electrical behavior of the devices can be described by the thermionic emission theory (has a quasi-ideal performance). A minimal set of parameters, the effective barrier height and peff, the non-uniformity factor, is associated. Model-extracted parameters are discussed in comparison with literature-reported results based on existing inhomogeneity approaches, in terms of complexity and physical relevance. Special consideration was given to models based on a Gaussian distribution of barrier heights on the contact surface. The proposed methodology is validated by electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC), ...

Characterization of non-uniform Ni/4H-SiC Schottky diodes for improved responsivity in high-temperature sensing

Abstract: The development of high-temperature sensors (up to 450 °C), based on SiC Schottky diodes, which are able to work in harsh conditions, requires using high-barrier Schottky contacts, such as nickel silicide. It is well known that post metallization annealing, targeting silicide formation, leads to Schottky barrier height (SBH) non-uniformity on the contact surface. On the other hand, sensor performances are intimately tied to the value and stability of SBH. This paper discusses solutions for assessing temperature sensing performances of inhomogeneous SiC Schottky diodes, particularly targeting high-temperature, harsh industrial applications where conventional solutions are unreliable. Microphysical investigations and electrical measurements are carried out on Ni/4H-SiC Schottky diodes with varying levels of non-uniformity. Devices are characterized using SEM and EDX analyses, together with electrical parameter evaluation techniques. It is demonstrated that, despite highly inhomogeneous electrical behavior, SiC-Schottky diodes exhibit stable sensitivity (up to 2.33 mV/°C) with excellent linearity (R2 > 99.9%) over wide temperature (up to 450 °C) and bias ranges (at least 100 nA–10 µA).

Heteroatom-mediated performance of dye-sensitized solar cells based on T-shaped molecules

Abstract: Two novel “sister” organic dyes featuring a T-shape molecular pattern based on phenothiazine/phenoxazine and triphenylamine framework as electron donor core and cyanoacrylic acid as acceptor and anchoring group were designed, synthesized and thoroughly characterized. Both dyes were functionalized at the nitrogen atom of phenoxazine/phenothiazine with the triphenylamine unit containing two hexyloxy substituents to obtain a T-shape configuration. The photo-optical and electrochemical data corroborated with time-dependent density functional theory computations and photovoltaic features were discussed in correlation with the structural architecture of each dye. Their performances in dye-sensitized solar cells (DSSCs) were comparatively surveyed with a special concern on the influence of the heteroatom variation on the most important DSSCs characteristics. In spite of its more favorable optical performance, the cells realized with phenoxazine-based dye are less efficient than those fabricated with phenothiazine ones. The cause is the stronger electron interception by the electrolyte in the case of phenoxazine dye which consistently degrades the open-circuit voltage, corroborated with a better regeneration of the phenothiazine by the iodine/iodide redox couple. In fact, to the best of our knowledge, this is one of the first studies which highlights how a single heteroatom in a T-shape dye structure can affect the dye interaction with the electrolyte and its regeneration, consequently the photovoltaic performances and brings out new opportunities for developing novel dyes for efficient DSSCs.

Recent Advances on Graphene Quantum Dots: From Chemistry and Physics to Applications

Abstract: Graphene quantum dots (GQDs) that are flat 0D nanomaterials have attracted increasing interest because of their exceptional chemicophysical properties and novel applications in energy conversion and storage, electro/photo/chemical catalysis, flexible devices, sensing, display, imaging, and theranostics. The significant advances in the recent years are summarized with comparative and balanced discussion. The differences between GQDs and other nanomaterials, including their nanocarbon cousins, are emphasized, and the unique advantages of GQDs for specific applications are highlighted. The current challenges and outlook of this growing field are also discussed.

Graphene quantum dots from chemistry to applications

Abstract: Graphene quantum dots (GQDs) have been widely studied in recent years due to its unique structure-related properties, such as optical, electrical and optoelectrical properties. GQDs are considered new kind of quantum dots (QDs), as they are chemically and physically stable because of its intrinsic inert carbon property. Furthermore, GQDs are environmentally friendly due to its non-toxic and biologically inert properties, which have attracted worldwide interests from academic and industry. In this review, a number of GQDs preparation methods, such as hydrothermal method, microwave-assisted hydrothermal method, soft-template method, liquid exfoliation method, metal-catalyzed method and electron beam lithography method etc., are summarized. Their structural, morphological, chemical composition, optical, electrical and optoelectrical properties have been characterized and studied. A variety of elemental dopant, such as nitrogen, sulphur, chlorine, fluorine and potassium etc., have been doped into GQDs to diversify the functions of the material. The control of its size and shape has been realized by means of preparation parameters, such as synthesis temperature, growth time, source concentration and catalyst etc. As far as energy level engineering is concerned, the elemental doping has shown an introduction of energy level in GQDs which may tune the optical, electrical and optoelectrical properties of the GQDs. The applications of GQDs in biological imaging, optoelectrical detectors, solar cells, light emitting diodes, fluorescent agent, photocatalysis, and lithium ion battery are described. GQD composites, having optimized contents and properties, are also discussed to extend the applications of GQDs. Basic physical and chemical parameters of GQDs are summarized by tables in this review, which will provide readers useful information.

Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review.

Abstract: The pH-dependent surface charging of metal (hydr)oxides is reviewed on the occasion of the 50th anniversary of the publication by G.A. Parks: "Isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems" in Chemical Reviews. The point of zero charge (PZC) and isoelectric point (IEP) became standard parameters to characterize metal oxides in aqueous dispersions, and they define adsorption (surface excess) of ions, stability against coagulation, rheological properties of dispersions, etc. They are commonly used in many branches of science including mineral processing, soil science, materials science, geochemistry, environmental engineering, and corrosion science. Parks established standard procedures and experimental conditions which are required to obtain reliable and reproducible values of PZC and IEP. The field is very active, and the number of related papers exceeds 300 a year, and the standards established by Parks remain still valid. Relevant experimental techniques improved over the years, especially the measurements of electrophoretic mobility became easier and more reliable, are the numerical values of PZC and IEP compiled by Parks were confirmed by contemporary publications with a few exceptions. The present paper is an up-to-date compilation of the values of PZC and IEP of metal oxides. Unlike in former reviews by the same author, which were more comprehensive, only limited number of selected results are presented and discussed here. On top of the results obtained by means of classical methods (titration and electrokinetic methods), new methods and correlations found over the recent 50years are presented.

Synthesis and applications of graphene quantum dots: a review

Abstract: Abstract As a new class of fluorescent carbon materials, graphene quantum dots (GQDs) have attracted tremendous attention due to their outstanding properties and potential applications in biological, optoelectronic, and energy-related fields. Herein, top-down and bottom-up strategies for the fabrication of GQDs, mainly containing oxidative cleavage, the hydrothermal or solvothermal method, the ultrasonic-assisted or microwave-assisted process, electrochemical oxidation, controllable synthesis, and carbonization from small molecules or polymers, are discussed. Different methods are presented in order to study their characteristics and their influence on the final properties of the GQDs. The respective advantages and disadvantages of the methods are introduced. With regard to some important or novel methods, the mechanisms are proposed for reference. Moreover, recent exciting progresses on the applications of GQD, such as sensors, bio-imaging, drug carriers, and solar cells are highlighted. Finally, a brief outlook is given, pointing out the issues still to be settled for further development. We believe that new preparation methods and properties of GQDs will be found, and GQDs will play more important roles in novel devices and various applications.

Absorption and solubility measurement of CO2 in water-based ZnO and SiO2 nanofluids

Abstract: The experiments of the absorption of carbon dioxide in the water-based nanofluids of spherical silica (SiO2) and zinc oxide (ZnO) nanoparticles are carried out in an isothermal stirred high pressure cell using quasi-static method through gas pressure measurement. The SiO2 and ZnO nanoparticle compounds were purchased and synthesized, respectively; and characterized through X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) images. Moreover, the zeta potential is measured to show the stability of these nanoparticles in water. The experiments are performed at 2, 5 and 8 °C and a range 1 bar up to gas hydrate equilibrium pressure; and in 15, 25 and 40 °C at 1–36 bar. The CO2 solubility measurement in pure water and the 0.1 wt% of the two nanofluids show that the amount of the absorbed CO2 enhances in presence of nanoparticles so that the ZnO nanofluids are more effective than SiO2 nanofluids at the all experimental conditions. In addition, the solubility of CO2 is measured in the 0.05, 0.1, 0.5 and 1 wt% of ZnO nanofluid at 5 °C and pressure range of 1–22 bar so that the amount of absorption was enhanced by increasing the ZnO mass loading at the all measurements. The mechanism of CO2 absorption in the nanofluids are fully discussed and compared with those gas solubility results in pure water.