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11.
Time-of-flight photoconductivity investigation of high charge carrier mobility in ▫$Ti_3C_2T_x$▫ MXenes thin-film
Jurij Urbančič, Erika Tomsic, Manisha Chhikara, Nadiia Pastukhova, Vadym Tkachuk, Alex Dixon, Andraž Mavrič, Payam Hashemi, Davood Sabaghi, Ali Shaygan Nia, Gvido Bratina, Egon Pavlica, 2023, original scientific article

Abstract: Charge transport through a randomly oriented multilayered network of two-dimensional (2D) Ti3C2Tx (where Tx is the surface termination and corresponds to O, OH and F) was studied using time-of-flight photoconductivity (TOFP) method, which is highly sensitive to the distribution of charge carrier velocities. We prepared samples comprising Ti3C2Tx with thickness of 12 nm or 6-monolayers. MXene flakes of size up to 16 μm were randomly deposited on the surface by spin-coating from water solution. Using TOFP, we have measured electron mobility that reached values up to 279 cm2/Vs and increase with electric-field in a Poole-Frenkel manner. These values are approximately 50 times higher than previously reported field-effect mobility. Interestingly, our zero-electric-field extrapolate approaches electron mobility measured using terahertz absorption method, which represents intra-flake transport. Our data suggest that macroscopic charge transport is governed by two distinct mechanisms. The high mobility values are characteristic for the intra-flake charge transport via the manifold of delocalized states. On the other hand, the observed Poole-Frenkel dependence of charge carrier mobility on the electric field is typical for the disordered materials and suggest the existence of an important contribution of inter-flake hopping to the overall charge transport.
Keywords: charge transport, multilayered network, flakes, time-of-flight photoconductivity, MXene exfoliation, high-mobility solution-cast thin-film, semiconducting MXene
Published in RUNG: 31.03.2023; Views: 1068; Downloads: 2
.pdf Full text (1,97 MB)

12.
Surface roughness and wetting properties of plastic pipes : Proof-of-concept experiment report
Andraž Mavrič, Mattia Fanetti, Simone Dal Zilio, 2022, final research report

Keywords: surface roughness, wetting
Published in RUNG: 21.03.2023; Views: 814; Downloads: 0
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13.
Voltage-dependent FTIR and 2D infrared spectroscopies within the electric double layer using a plasmonic and conductive electrode
Nan Yang, Matthew J. Ryan, Minjung Son, Andraž Mavrič, Martin Zanni, 2023, original scientific article

Abstract: Strong electric fields exist between the electric double layer and charged surfaces. These fields impact molecular structures and chemistry at interfaces. We have developed a transparent electrode with infrared plasmonic enhancement sufficient to measure FTIR and two-dimensional infrared spectra at submonolayer coverages on the surface to which a voltage can be applied. Our device consists of an infrared transparent substrate, a 10–20 nm layer of conductive indium tin oxide (ITO), an electrically resistive layer of 3–5 nm Al2O3, and a 3 nm layer of nonconductive plasmonic gold. The materials and thicknesses are set to maximize the surface number density of the monolayer molecules, electrical conductivity, and plasmonic enhancement while minimizing background signals and avoiding Fano line shape distortions. The design was optimized by iteratively characterizing the material roughness and thickness with atomic force microscopy and electron microscopy and by monitoring the plasmon resonance enhancement with spectroscopy. The design is robust to repeated fabrication. This new electrode is tested on nitrile functional groups using a monolayer of 4-mercaptobenzonitrile as well as on CO and CC stretching modes using 4-mercaptobenzoic acid methyl ester. A voltage-dependent Stark shift is observed on both monolayers. We also observe that the transition dipole strength of the CN mode scales linearly with the applied voltage, providing a second way of measuring the surface electric field strength. We anticipate that this cell will enable many new voltage-dependent infrared experiments under applied voltages.
Keywords: two-dimensional infrared spectroscopy, infrared transparent substrate, voltage-dependent infrared experiments
Published in RUNG: 24.02.2023; Views: 1087; Downloads: 0
.pdf Full text (6,07 MB)

14.
Non-covalent ligand-oxide interaction promotes oxygen evolution
Qianbao Wu, Junwu Liang, Mengjun Xiao, Chang Long, Lei Li, Zhenhua Zeng, Andraž Mavrič, Xia Zheng, Jing Zhu, Matjaž Valant, 2023, original scientific article

Abstract: Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak “non-bonding” interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO2 interaction that substantially elevates the population of Co4+ sites for improved water oxidation. We find that phenanthroline only coordinates with Co2+ forming soluble Co(phenanthroline)2(OH)2 complex in alkaline electrolytes, which can be deposited as amorphous CoOxHy film containing non-bonding phenanthroline upon oxidation of Co2+ to Co3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm−2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO2 through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.
Keywords: water oxidation, cobalt hydroxide, ligand-metal interactions
Published in RUNG: 23.02.2023; Views: 1297; Downloads: 15
.pdf Full text (1,77 MB)

15.
A comparative study of nanolaminate CrN/Mo2N and CrN/W2N as hard and corrosion resistant coatings
Marco Beltrami, Andraž Mavrič, Simone Dal Zilio, Mattia Fanetti, Gregor Kapun, Marco Lazzarino, Orfeo Sbaizero, Miha Čekada, 2013, original scientific article

Abstract: Nanolaminate coatings (NLC) consisting of alternated CrN coupled with either cubic tungsten nitride (β-W2N) or molybdenum nitride (γ-Mo2N) were deposited on cold worked tool steel substrates using reactive DC reactive magnetron sputtering for improved mechanical and corrosion resistance. The CrN/γ-Mo2N and CrN/β-W2N nanolaminate systems were found to perform better than the corresponding single-layer systems, with both mechanical and electrochemical properties improving by decreasing the individual layer thickness from 100 to 5 nm. The CrN/β-W2N NLC combined the high hardness value of W2N with the low corrosion current of CrN. The CrN/γ-Mo2N NLC showed synergistic improvements consisting of both higher hardness and lower corrosion currents with respect to the constituent materials alone. The dependence of mechanical and corrosion properties on the bilayer period is discussed in terms of the grain size, residual stresses and texture of the constituent materials and the nanostructured character of the multilayer architecture.
Keywords: Nanolayered coatings, Transition metal nitrides, Sputtering, Nanoindentation, Corrosion resistance, Mechanical properties
Published in RUNG: 12.01.2023; Views: 1088; Downloads: 0
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16.
Plasma-enhanced atomic layer deposition of amorphous Ga2O3 for solar-blind photodetection
Ze-Yu Fan, Min-Ji Yang, Bo-Yu Fan, Andraž Mavrič, Nadiia Pastukhova, Matjaž Valant, Bo-Lin Li, Kuang Feng, Dong-Liang Liu, Guang-Wei Deng, Qiang Zhou, Yan-Bo Li, 2022, original scientific article

Abstract: Wide-bandgap gallium oxide (Ga2O3) is one of the most promising semiconductor materials for solar-blind (200 nm–280 nm) photodetection. In its amorphous form, a-Ga2O3 maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature, thus is compatible with Si integrated circuits (ICs) technology. Herein, the a-Ga2O3 film is directly deposited on pre-fabricated Au interdigital electrodes by plasma enhanced atomic layer deposition (PE-ALD) at a growth temperature of 250 °C. The stoichiometric a-Ga2O3 thin film with a low defect density is achieved owing to the mild PE-ALD condition. As a result, the fabricated Au/a-Ga2O3/Au photodetector shows a fast time response, high responsivity, and excellent wavelength selectivity for solar-blind photodetection. Furthermore, an ultra-thin MgO layer is deposited by PE-ALD to passivate the Au/a-Ga2O3/Au interface, resulting in the responsivity of 788 A/W (under 254 nm at 10 V), a 250-nm-to-400-nm rejection ratio of 9.2×103, and the rise time and the decay time of 32 ms and 6 ms, respectively. These results demonstrate that the a-Ga2O3 film grown by PE-ALD is a promising candidate for high-performance solar-blind photodetection and potentially can be integrated with Si ICs for commercial production.
Keywords: Amorphous gallium oxide, Passivation layer, Plasma enhanced atomic layer deposition, Responsivity, Solar-blind photodetector
Published in RUNG: 25.10.2022; Views: 1052; Downloads: 0
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17.
Metal hydroxides for energy conversion and energy storage
Andraž Mavrič, invited lecture at foreign university

Abstract: Electrocatalysts, electrochromic devices, and pseudo-capacitors based on transition metal (oxy)hydroxides depend on the reversibility of the reduction-oxidation process of metal cations. Rapid switching between different redox states is often involved, particularly in electrocatalysis where redox metal sites act as active centers for electron transfer to the reactant. To ensure long-term durability, the reversibility of the redox metal sites should be robust. Nickel hydroxide is a model catalyst for the oxygen evolution reaction (OER) and the basic representative of the layered double hydroxides. It is frequently combined with other transition metals (e.g. Fe, Co, Mn), forming some of the most active OER electrocatalysts in alkaline media. [1] I will present the use of in-situ spectroscopy to track the reversibility of redox states of the Ni(OH)2 during its lifetime. During the operation at 200 mA cm-2 in 1 M KOH electrolyte, the catalytic activity of Ni(OH)2 gradually degrades until lastly, the catalyst breaks down. During the catalyst lifetime, the reduction-oxidation reversibility of the Ni2+/3+ redox couple is lost and the catalyst converts into an inactive phase. The reversibility of the redox couple is monitored by the in-situ UV/Vis spectroscopy. During the catalyst lifetime, the reversibility of the redox peak is lost. The activity collapse is attributed to the structural amorphization/disordering of the layered Ni(OH)2 catalyst, as confirmed by TEM investigations and in-situ Raman spectroscopy. [2] Similarly, the redox reversibility of metal sites is also important for long cycle life in supercapacitors, based on the pseudo-capacitance mechanism. Contrary to catalysts, for supercapacitors, the water oxidation needs to be suppressed to increase the working voltage range. I will discuss the mechanisms for the deactivation of transition metal hydroxides to serve as capacitors and approaches to increase power density. Finally, I will discuss the use of mixed metal hydroxides to serve as precursors for a copper oxide-based catalytic system for CO2 hydrogenation to methanol. Thermal decomposition of hydrotalcite-based hydroxide precursor is followed by in-situ x-ray diffraction. The conditions to prepare disordered oxide in contact with catalytical active Cu metal are identified and the catalytic performance of catalysts with crystalline and disordered oxide phases are compared. [1] A. Mavrič, C. Cui, (2021), Advances and Challenges in Industrial-Scale Water Oxidation on Layered Double Hydroxides, ACS Appl. Energy Mater., 4, 12032-12055. [2] A. Mavrič, M. Fanetti, Y. Lin, M. Valant, C. Cui, (2020), Spectroelectrochemical Tracking of Nickel Hydroxide Reveals Its Irreversible Redox States upon Operation at High Current Density, ACS Catal., 10, 9451-9457.
Keywords: electrochemistry, energy storage, CO2 hydrogenation, methnaol
Published in RUNG: 13.10.2022; Views: 1096; Downloads: 0
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18.
19.
Covalent polymer thin films for biosensor applications
Tina Skorjanc, Andraž Mavrič, Mads Nybo Sorensen, Gregor Mali, Changzhu Wu, Matjaž Valant, 2022, published scientific conference contribution abstract

Keywords: bacteria, sensing, electrochemical impedance spectroscopy, electrophoresis, electrode preparation, Menshutkin reaction
Published in RUNG: 26.09.2022; Views: 1107; Downloads: 0
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20.
Deposition of porphyrin thin films by electrophoresis
Andraž Mavrič, Tina Škorjanc, Mads Nybo Sørensen, Changzhu Wu, Matjaž Valant, 2022, published scientific conference contribution abstract

Abstract: Porphyrins are heterocyclic macrocycles consisting of interconnected pyrrole subunits acting as ligands for metal ions. Metallised metalloporphyrins naturally occur as cofactors in a series of enzymes, acting as active sites for biochemical transformations. Mimicking nature, a variety of functionalized porphyrins have been prepared for different catalytic purposes [1]. These organometallic complexes have isolated metal centers in tailored coordination environments to drive catalytic reactions in homogeneous solutions. The activity and selectivity of isolated metal ions acting as single-atom catalysts are defined by the coordination environment. Depending on the porphyrin structure, the solubility of these macrocycles and their processability in solutions can be altered. While such adjustments to the structure might ease the processability, the catalytic properties might also be altered. Because homogenous catalysis presents challenges with the separation and recyclability of the catalyst, it is common to fix metalloporphyrins into molecular organic frameworks or deposit them onto a substrate. Commonly used deposition techniques face several challenges. For instance, thermal evaporation can cause partial or complete degradation of some thermally-labile functional groups attached to the porphyrins. Similarly, spin coating commonly results in an uneven thickness and uneven morphology of the deposited films. To overcome these difficulties, we present an alternative method for the deposition of porphyrin thin films that is suitable for a wide range of functionalized porphyrins. The electrophoresis can force the molecules to deposit on a conductive substrate such as a metal foil or transparent conductive oxide by applying the electric field generated by a DC power supply. The film thickness can be precisely controlled by changing the voltage value, deposition time, or solution concentration using even a small amount of material [2, 3]. Six different functionalized porphyrin molecules have been successfully deposited onto the copper foil substrate by optimizing key parameters, including applied electric field, the duration of electrophoresis, the size of the copper electrodes, and solvent polarity. To demonstrate the generality of our approach, we have selected a broad range of porphyrins that incorporate the following functional groups: phenyl rings, carboxylic acids, pyridyl rings, methyl benzyl ethers, methyl benzoyl esters, and cobalt (II) metalized macrocycle. The impact of this study extends above catalysis to various applications of porphyrins thin films on conductive substrates such as optoelectronics and sensors.
Keywords: porphyrin, thin film, electrophoresis
Published in RUNG: 26.09.2022; Views: 1041; Downloads: 0
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