2D materials for energy storage: batteries, super capacitor, solar cells, thermoelectric

Resume : Application of 2D materials is at the core of continuous down-scaling and performance improvements of various technologies ranging from nanoelectronic, energy storage to thermoelectric and sensing devices. Design, performance and reliability of these devices strongly depend on thermal properties of 2D materials. In this respect, atomistic simulations of thermal properties represent an invaluable tool that can be used to support the R&D of these advanced technologies. Thermal transport simulations of large structures, including multilayer heterostructures combining different 2D materials, can be computationally very demanding or even unmanageable if performed using ab initio (DFT) molecular dynamics (MD), whereas computationally cheaper conventional Force Fields for 2D materials are very scarce and take a long time to be developed. Particularly, they can be cumbersome to generate for 2D material heterostructures. In this talk, we will show that this can be overcome by using machine learning (ML) based force fields which are 10x-1000x more efficient compared to DFT based simulations. We will present how accurate ML force fields, Moment Tensor Potentials (MTPs) [1] with the reverse non-equilibrium MD (RNEMD) method in the QuantumATK software package for atomistic simulations [2,3] developed by Synopsys are used to simulate thermal transport and thermal conductivity of large-scale (~ 100 nm long, ~100000 atoms) 2D material structures. We will also introduce the automated machine learning force field MTP training and simulation workflow in QuantumATK and show how MTPs for 2D materials are trained on DFT data, such as energies, forces and stresses, of reference structures. Finally, we will demonstrate a good agreement of results obtained with MTPs and DFT and experiments. References 1) A. V. Shapeev. ‘Moment tensor potentials: a class of systematically improvable interatomic potentials’ Multiscale Modeling & Simulation 14, 1153 (2016). 2) S. Smidstrup et al. ‘QuantumATK: An integrated platform of electronic and atomic-scale modelling tools’ Journal of Physics: Condensed Matter 32, 015901 (2019). 3) https://www.synopsys.com/silicon/quantumatk.html

Resume : The purpose of the present work was to study the impact of polycrystalline substrate thickness on solar cells performances. The thickness of a cell is a pertinent parameter during the manufacture of a solar cell since the penetration depth directly impacts the efficiency of solar cells. This work is geared towards the reduction of the substrate thickness to increase efficiency of PV cells using low cost material. Indeed, it has been shown that the production of solar cells with buried metal contacts improves carrier collection by: (i) increasing the collection surface, (ii) reducing the contact resistance and (iii) passivation of grain boundaries; this resulted in a significant decrease in leakage current and improved spectral response. It has been demonstrated that all these factors may lead to improved energy efficiency of low-grade metal made PV cell from 7% to nearly 12 % for a cell treated with buried contacts SP.

Resume : After obtaining Ti3C2 MXene structures terminated with O, S, Se, F, Cl, and Br, we calculate the energy barrier for Li-ion diffusion on the surface of each MXene, being the first to report on the Li-ion diffusivity in Cl and Br terminated Ti3C2. We find that the Ti3C2Cl2 MXene has the lowest diffusion barrier, substituting the Ti3C2S2 reported in the literature so far. In addition, a study on the adsorption energies indicates that the top binding position is the most stable adsorption position for the Li-ion. Furthermore, it is shown that the adsorption energy depends on the electronegativity of the termination atoms, as well as the distance between the terminations, the Li, and the surface Ti-atoms. Finally, we show that the bond valence sum method provides an indication of the transition state of the Li-ion and can serve as a comparison tool for the diffusion barriers of different structures

Resume : Recently, 2D materials MXene gained significant attention in various research fields due to their unique properties like chemical diversity, hydrophilicity, 2D morphology, metallic conductivity, etc. In spite of the promising properties of MXenes, these materials are not gaining momentum in various applications because of their poor chemical or oxidation stability. A few reported literatures stated that organic solvents with low temperature storage could prevent the oxidation of MXene flakes due to the presence of less number of dissolved oxygen molecules in the solvents. Till now, mostly investigated material regarding oxidation stability is Ti3C2Tx MXene only due to its high capacitance and excellent high metallic conductivity. In this regard, Ti2CTx MXene also delivers comparable electrical and electrochemical properties like the previous one, but it is getting less attention due to lack of knowledge about their suitable storage medium for prolonged period without oxidation/ degradation. To address this issue, this paper reports the best suitable medium for preventing or delaying the degradation or oxidation of Ti2CTx MXene. Herein, Ti2CTx MXene was dispersed in different solvents for duration of 3 months at ambient and freezing conditions and characterized thereafter using XRD, Raman spectroscopy, SEM, TEM and XPS. The results suggest that the best suitable mediums to store Ti2CTx MXene for 3 months are IPA or DMSO at -20°C or in water at -80°C.

Resume : The development of new photocatalytic systems such as a photocatalytic TiO2-based system associated with graphitic carbon nitride (g-C3N4) for effectively obtaining of hydrogen through photocatalytic water splitting is one of the most promising areas of renewable energy. For efficient light utilization, the fabrication of g-C3N4/TiO2 composite material has attracted much attention. However, both pristine g-C3N4 and g-C3N4/TiO2 composite exhibits photoactivity in blue region of visible spectrum only. It is found that the doping of carbon nitride by oxygen significantly improves its photocatalytic properties. Therefore, to improve the photocatalytic activity of semiconductor photocatalyst, the coupling O-doped g-C3N4 (O-g-C3N4) with anatase TiO2 is a good strategy. New composite material O-g-C3N4/TiO2 was synthesized by gas phase method under the special reactionary conditions of the pyrolysis of melamine. Deposition of O-g-C3N4 (~5% O) on the anatase nanopowder particles is confirmed by XRD, IR, XPS, SEM, EDX methods. It is found by UV-Vis-DRS method, that O-g-C3N4/TiO2 photosensitivity is observed in the significant part of the visible light region and the band gaps of product is determined to be less than 2.38 eV. Constructing heterojunction structures of TiO2 and O-g-C3N4 may be used as a cost-effective way to avoid the drawbacks of each component and realize a synergic effect for boosting the photocatalytic activity of material for energy applications.

Resume : Combining two-dimensional (2D) materials with organic materials can be very attractive for applications that require flexibility and where size and weight are important parameters to be considered, such as in wearable, portable and mobile applications. Organic materials usually exhibit excellent optical absorption efficiency and photo- and temperature-induced conformational changes, while 2D materials often show relatively high carrier mobility, superior mechanical flexibility, and tunable electronic and optical properties. Combining both systems can stabilize the organic materials and lead to heterostructures with both high carrier mobility and high optical absorption efficiency, which is promising for solar energy conversion. In this work we investigate, by means of density-functional-theory calculations, heterostructures composed of organic molecules (for example, pentacene, tetracene and azulene) and transition metal dichalcogenides (TMD) for application in photovoltaic devices. We examine the interaction between the molecules and monolayer TMDs as well as the band alignment of the heterostructures, considering effects of the molecular coverage and dielectric screening. Acknowledgement: This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 406901005.

Resume : Rapid development of photovoltaic power industry has pushed an active search for new materials for photovoltaics and photodetection, where heterostructures with an efficient charge-carrier separation are of particular interest. Emerging van der Waals heterostructures (vdWHs) based on two-dimensional (2D) transition metal dichalcogenides (TMDs) constitute a promising platform for novel electronic and optical devices, such as ultra-thin film solar cells or photodetectors [1], [2].. Single layers of TMDs, such as MoS2, WS2, MoSe2 and WSe2 are direct bandgap semiconductors, which can effectively absorb and emit light at visible and near infrared wavelengths (with photon energies between 1 and 2 eV) [3]. Moreover, 2D TMDs with various bandgaps can be used as building blocks in vertical vdWHs forming novel vdW systems with extraordinary broad absorption across a wide range of the solar spectrum. Pioneer results on vdWHs were demonstrated using physical crystal stacking. However, vertical stacking of 2D TMDs layers in a 3D architecture is not favorable for large-scale applications and direct growth methods must be further investigated. Here, we present a two-step growth process: (1) oxide bilayer films obtained by pulsed laser deposition (PLD) and (2) sulfurized in a tube furnace at high-temperature in a sulfur-rich environment for TMDs heterostructure synthesis. We show that native oxygen vacancies in the PLD-grown precursors serve as niches for sulfur atoms and facilitate MoS2 and WS2 lateral crystal growth. First, we explore how the two-step synthesis process of oxides with variable concentration of oxygen vacancies can be applied for vdWHs growth. Second, these results serve as a potential step to facilitate integration of dissimilar 2D TMDs layers. Lastly, we investigate the use of PLD for the direct growth of quasi-continuous thin films of MoSe2 and WSe2 with two-step grown MoS2 and/or WS2 layers to merge sulfides and selenides in a vdWHs. Thus, our study suggests a way towards fabrication of vdWHs without using individual layer transfer. Potential results of this study are formation of high quality crystals with a good photoluminescence (PL) and strong Raman signals from individual layers. Overlaid, MoS2 and WS2, or MoSe2 and MoS2 form a heterostructure, and display contributions to the Raman spectra from both layers, but significantly quenched PL signal. This may be a promising sign of an effective charge carrier separation within the heterostructure created. The synthesis of heterostructures was examined by XPS, Raman spectroscopy, PL, AFM, SEM, optical microscopy, and TEM. [1] M. K. S. Bin Rafiq et al., Sci. Rep., vol. 10, no. 1, pp. 1–11, 2020. [2] M. Zhong et al., 2D Mater., vol. 5, no. 3, 2018. [3] S. J. Liang, B. Cheng, X. Cui, and F. Miao,Adv. Mater., vol. 32, no. 27, pp. 1–27, 2020

Resume : Time Dependent Exfoliation of Bulk MoS2 for Electrochemical Studies-Ranging from Supercapacitors to Na-Ion Batteries Surbhi Priya1*, Debabrata Mandal2, Trilok Singh2 and Amreesh Chandra1,2# 1School of Energy Science & Engineering, 2School of Nano-Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India Email: *surbhipriya2010@gmail.com, #achandra@phy.iitkgp.ac.in ABSTRACT: Transition-metal dichalcogenides (TMDs) such as MX2 (M = Mo, W, Nb, Ta, etc. and X = S, Se, etc.) are becoming useful for energy storage applications. Among these, molybdenum disulfide (MoS2), has emerged as a promising candidatedue to its exceptional electrical, physicochemical, and mechanical properties. Bulk MoS2 has a layered crystal structure consisting of stacked S-Mo-S monolayers separated by approximately 0.62 nm interlayer spacing, with strong covalent Mo-S bonds and weak Van der Waals forces between adjacent monolayers. Due to their high specific surface area and flexibility, mono or layered MoS2 nanosheets have superior electrochemical properties than bulk MoS2. In this work we present a simple single-step ultrasonic exfoliation technique for producing layered MoS2 nanosheets from bulk MoS2 powder. The two major concerns of chemical exfoliation i.e. usage of highly flammable chemicals and yield production have been eliminated in this process. Using LiBr and toluene for exfoliation, almost 70% yield was achieved. Various physiochemical characterizations were performed for both non-ultrasonicated and ultrasonicated MoS2. In case of ultrasonicated sample, studies were performed on the basis of time, i.e. the sample was collected after every hour of exfoliation for up to 3 hours. The peak broadening of the XRD profiles confirmed successful exfoliation, suggesting formation of nanosized layered structure from the bulk MoS2. The electrochemical investigations were carried out in 3-electrode configuration. The specific capacitance for pristine MoS2, 1, 2 and 3 h exfoliated samples were calculated to be 96, 110, 140 and 200 F g-1, respectively, at 1 A g-1 current density. An enhancement of about 110% from bulk to 3 h exfoliated sample was observed. Further, the effect of electrolyte on the electrochemical activity of MoS2 was analysed. In acidic electrolyte (1 M H2SO4) much higher specific capacitance (200 F g-1) was obtained compared to the alkaline (90 F g-1 in 1 M KOH) and neutral ones (50 F g-1 in 1 M Na2SO4). Moreover, the Na and Li-ion battery performance of MoS2 as anode material was investigated. The results exhibit high rate capability and excellent cycling stability, thus establishing the advantages of MoS2 as next generation high-end energy storage material.

Resume : The exceptional semiconducting properties of two-dimensional transition metal dichalcogenides (2D-TMDs) makes them suitable for a range of optoelectronic applications including photodetectors, solar cells, and photoelectrodes (ex. Solar H2 production). Not only does this class of materials provide a range of tunable optoelectronic properties, but their fabrication as large-area thin films paves the way towards ultrathin, flexible solar energy conversion devices. A key challenge in commercializing these TMD-based devices, however, is the scalable production.[1] Our group has addressed several aspects of this issue (including exfoliation, thin film fabrication, and defect mitigation) all while maintaining solution-processable methods suitable for large- device fabrication. In this presentation we describe our recent work advancing the processes at each step of the thin-film formation process. We present a novel powder-based electrochemical intercalation technique yielding 2D nanosheet dispersions with superior optoelectronic properties compared to conventional solution processing methods (ex. ultrasonication) and demonstrate high photon-to-chemical energy conversion efficiencies in MoS2. Next, we show the importance of process parameters in the formation of nanosheet thin films using a liquid-liquid interfacial self-assembly (LLISA) technique to increase nanoflake-substrate contact and maximize charge extraction.[2] In particular we highlight our results employing WSe2 for solar water reduction.[2] We further advance this method towards continuous roll-to-roll deposition of TMD-based thin films demonstrating reproducible printing of 100mm wide TMD flake films on plastic substrates at 10 mm s-1 with nanoflake loadings of 35 mg m-2.[3] Importantly we show that these large-scale films of semiconducting TMDs are robust and capable of solar-to-chemical energy conversion comparable to that of their batch-scale counterparts. Finally, we address the effects of processing conditions on TMD nanoflake quality and resulting photoelectrochemical device performance. While choosing the correct exfoliation methods can assist in avoiding unnecessary nanoflake damage, we discuss defect mitigation strategies to overcome inescapable imperfections and drastically improve the solar energy conversion ability of TMD nanoflake thin films.[4] In particular, we achieve state-of-the-are performance for solution-processed WSe2 nanoflake films for solar H2 production with 1 sun photocurrent densities over 4 mA cm-2.[4] References: [1] Yu, X.; Sivula, K. ACS Energy Lett. 2016, 1 (1), 315–322. [2] Yu, X.; Prévot, M. S.; Guijarro, N.; Sivula, K. Nat. Commun. 2015, 6, 7596. [3] Wells, R. A.; Johnson, H.; Lhermitte, C. R.; Kinge, S.; Sivula, K. ACS Appl. Nano Mater. 2019, 2 (12), 7705–7712. [4] Yu, X.; Guijarro, N.; Johnson, M.; Sivula, K. Nano Lett. 2018, 18 (1), 215–222.

Resume : Epitaxy on two-dimensional Van der Waals crystals has attracted attention during the last years due to the possibility of synthetizing releasable semiconductor membranes on reusable substrates, and as a new pathway for the integration of 2D and 3D materials in multilayer stacks for next generation functional devices. In particular, the so-called remote epitaxy [1] on graphene-covered substrates is attractive for the fabrication of III-V epilayers for photovoltaic applications, as it may allow substrate reuse after growth, and therefore drastically reduce the cost of the currently most efficient solar cells, overcoming the limitations of mature transfer-recycle techniques. To synthetize transferrable layers in this way, large-area graphene grown on foreign substrates needs to be transferred to the target substrate with a high surface coverage, thickness uniformity, cleanliness, and no degradation of the underlying substrate surface. Despite the numerous research efforts carried out in the last years to improve transfer processes, the introduction of organic and metallic residues, wrinkles, and mechanical damage with the commonly used wet-transfer methods still hinders its use for many potential applications. In this contribution, we develop a dry transfer method of epitaxial graphene grown on Ge(110) by chemical vapor deposition to obtain substrates suitable for remote epitaxy. We avoid organic residues by using mechanical exfoliation with a strained nickel layer [2], where the accumulated strain energy is optimized controlling the deposited thickness. A high transfer yield and a clean 2D-3D interface are enabled by vacuum transfer and aircushion pressing of the graphene/metal stack on the target substrate. These improvements enabled the reproducible large-scale (~1 cm²) transfer to Si/SiO2 and GaAs without degradation of the graphene as shown by the Raman defect-related bands, negligible introduction of unintentional doping, a transfer yield exceeding 99 % and an optical transmission close to the theoretical value expected for pristine single-layer graphene. The surface chemistry was analyzed by X-ray photoelectron spectroscopy, that revealed a lower level of interface oxidation and C-O contamination in dry-transferred graphene as compared to wet-transferred graphene, while Ni oxides were found to be the main impurity in the former. Here, we will present our improved polymer-free transfer method and an extensive structural, chemical, and morphological characterization of transferred samples. We will discuss the challenges arising in each step of the process, from the graphene synthesis to the selective etching of the carrier layer. Finally, we will present our preliminary results on the molecular beam epitaxy growth and lift-off of GaAs on graphene-covered substrates, focusing on the effect of the graphene transfer method used, the introduced impurities and the interface quality. References: 1. Y. Kim et al., Nature, vol. 544, no. 7650, pp. 340–343, Apr. 2017. 2. J. Kim et al., Science, vol. 342, Issue 6160, pp. 833-836, 2013.

Resume : Solid state supercapacitors are intrinsically capable of sustaining substantially higher voltages, faster charging/discharging rates, higher temperatures, and much greater lifetimes than electrochemical double layer capacitors. However, the relatively smaller capacitance of solid-state supercapacitors using nonlinear dielectrics impedes their adoption as an auxiliary energy storage medium in energy technologies. Nonetheless, advances in the properties of dielectrics offer a promising route to enhance the energy storage density of solid-state supercapacitors. An emergent class of antiferroelectrics originating from reversible field-induced phase transitions in the HfO2 and ZrO2 material system are particularly extraordinary due to the facile fabrication of 3-dimensional capacitor structures, nanoscale size, lead-free chemistry, and the adjustable properties of the phase transition with composition. Whereas linear dielectrics store energy through the reversible displacement of the electronic cloud and ionic moments with an applied electric field, reversible field-induced phase transitions amplify electric charge storage through the conversion of the internal crystalline structure of the dielectric. A major leap forward in the supercapacitor performance of antiferroelectric ZrO2 capacitors is achieved by using a built-in electric field to simultaneously enhance both energy storage density and efficiency. The enhancement of the energy storage in 8 nm thick ZrO2-based thin films is achieved by exploiting the workfunction difference of RuOx and TiN electrodes. By replacing the top TiN electrode with RuOx in a capacitor where ZrO2 is grown by atomic layer deposition on a bottom TiN electrode, the energy storage density of ZrO2 was increased from 59.1 to 63 J cm-3 with a simultaneous increase in efficiency from 67.7 % to 84.1 % at room temperature. The workfunction engineered ZrO2 capacitors display superior cycling and maximum voltage performance compared to conventional symmetric antiferroelectric supercapacitors with TiN electrodes. A maximum energy storage density of 94 J cm-3 with 75 % efficiency up to 1E7 cycles at room temperature is demonstrated with workfunction engineered ZrO2 supercapacitors, which represents a 50% increase in the energy storage density for the HfO2 and ZrO2 material system. The reliability and performance of the ZrO2 supercapacitors are assessed up to 125 °C. The improvement in energy storage density and concurrent increase in efficiency by workfunction engineering is not only major advance for HfO2 and ZrO2-based thin films, but is an extremely promising method to enhance the energy storage for all nanoscale antiferroelectric supercapacitors.

Resume : The recent discovery of magnetism within the family of exfoliable van der Waals (vdW) compounds has attracted considerable interest for both fundamental research and technological applications.[1] However, current vdW ferromagnetic semiconductors are limited by their extreme sensitivity to air, poor charge transport properties and weak super-exchange interactions leading to very low magnetic ordering temperatures. Among the family of 2D magnetic semiconductors, CrSBr has recently gained attention because of its relatively high bulk interlayer ferromagnetism and intralayer antiferromagnetic order with Néel Temperature, TN ~ 132 K.[2] Furthermore, theoretical studies show that monolayers of CrSBr might have even higher ferromagnetic ordering temperature whereas semiconducting transport properties, large negative magnetoresistance, stability under ambient conditions, and gate-tunable magnetic ordering have been experimentally observed.[3] Here we report the angular and temperature dependent electron spin resonance (ESR) properties of bulk CrSBr.[4] In-plane and out-of-plane angular dependent ESR studies show room temperature magnetic anisotropy with the easy and hard axis of the magnetization along the crystallographic b and c axis, respectively. Interestingly, the ESR linewidth reveals a non trivial angular dependence for rotations of the CrSBr crystal in the c-b plane suggesting the coexistence of a 2-fold and a 4-fold symmetries whereas only a 2-fold symmetry is observed for rotations in the a-b plane. Finally, ESR temperature dependent studies performed by cooling the CrSBr crystal with the external magnetic field, B parallel to b and c crystallographic directions, show a shift of the resonance line to lower and higher magnetic fields respectively, thus unambiguously demonstrating the in plane ferromagnetic order and the antiferromagnetic order in between the layers. A minimum of the linewidth in the temperature dependence provides information on the magnetic order temperature[5] with transitions occurring in the ranges 190 - 220 K and 240 - 270 K for the FM and AF order, respectively. In conclusion, our ESR studies on CrSBr crystals show interesting magnetic resonance properties for spintronic applications: the existence of a magnetic anisotropy at room temperature, the onset of ferromagnetic and antiferromagnetic resonances at high temperatures and a complex magnetization dynamics as inferred from the analysis of the ESR linewidth. References: [1] N. P. Wilson et a. Nature Materials 20, 1657–1662 (2021) [2] E. J. Telford Adv. Mater. 32, 2003240 (2020) [3] Y. Guo, et al. Nanoscale 10,18036. (2018) [4] F. Moro et al. in preparation [5] M. Farle Rep. Prog. Phys. 61, 755–826 (1998).

Resume : Organometallic-Metal halide perovskites (OMHP) have received great attention in the recent years due to their potential use as materials in optoelectronics and ferroelectrics. Perovskites are materials having general formula ABX3 where A is a monovalent cation like Cs, formamidinum (FA), and methyl ammonium (MA), B is a bivalent cation like lead (Pb) or tin (Sn) and X is halogen (X = Cl, Br and I). By replacing traditional A site cations with organometallic cations, the chemical stability and optoelectronic properties can be enhanced. Till date researchers have explored nitrogen (ammonium) based organometallic metal halide perovskites. Phosphorus atom resides close to the nitrogen atom and both have a similar covalent character, while their differences in atomic radius, mass, and electron structure may open interesting variables within either chemical or physical properties. Recently Han-Yue Zhang et. al. reported phosphanium based 2D perovskite system which showed enhanced ferroelctric properties. The material showed enhanced dielectric coefficient in the ferroelectric perovskites. Here we report a new two-dimensional phosphonium cation based lead halide perovskite 2D material which has been synthesized by ligand-assisted solvent evaporation method at 260 degree centigrade for 1.5 hr using a capillary self-assembly set up. These two-dimensional materials form layered sheet like structures in micrometre regime. With increasing annealing time from 1.5 hrs to 2 hrs., the micro-sheets are found to be converted into coin shaped structures having enhanced surface smoothness. The XRD of both the structures are well matching with each other indicating no phase change during annealing process. The synthesized material is air stable and is showing an absorption edge nearly 600 nm corresponding to a band gap of approximately 2 eV which would be an excellent candidate for both ferroelectric and optoelectronic applications.

Resume : Transition metal oxides (TMO) lead to an innovative direction for the development of materials for electrochemical energy storage due to their excellent stability. Zinc oxide (ZnO), a primary TMO, represents a green choice due to its abundance and biocompatibility. Surface modification and structural design represent the most traveled ways to improve the conductivity of zinc oxides-based electrodes. Indeed, nanostructures with many different shapes have been produced by both sophisticated and costly techniques as well as by means of cheap methods [1]. Here we focus on a cost-effective mass production of nanostars by means of Chemical Bath Deposition (CBD) in aqueous solution. Nanostars appear as 2D self-assembled bundles of crystalline ZnO nanostrips (sized 1oo up to 1000 nm), with clear hexagonal symmetry on the assembly plane (building 6-point stars). These novel nanostructures are deeply characterized by X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Photoluminescence spectroscopy (PL) and electrochemical measurements (e.g CV, GCD), in order to evidence their structural, morphological, optical and electrical properties. The different preparative parameters, such as concentrations, thermal annealing and reaction time (growth kinetic) were deeply investigated. Specifically, with the kinetic nanostars with different arm lengths (range 80 nm up to 12 µm) have been obtained. We then tested and optimized the stars with different dimension as capacitors for Energy Storage applications. [1] Najib, S., Bakan, F., Abdullayeva, N., Bahariqushchi, R., Kasap, S., Franzò, G., Sankir, M., Sankir, N.D., Mirabella, S. and Erdem, E., 2020. Nanoscale, 2020, DOI: 10.1039/D0NR03921G

Resume : Magnesium-ion batteries (MIBs) are considered as promising alternatives to lithium-ion technologies due to the di-valent character of the Mg2+ cations, the high theoretical volumetric capacity of magnesium (3833 mA h cm¬-3 for the Mg-anode), as well as its low toxicity, natural abundance and safety (dendrite-free).1 However, the development of MIBs is hindered by the slow solid-state diffusion of Mg2+ ions into the cathode lattice, which leads to relatively low practical capacities, irreversible capacity losses and poor cyclic stability. Phosphate-based compounds are considered as suitable candidate electrode materials for multi-valent ion batteries due to their open three-dimensional frameworks, structural stability, and thermochemical safety under oxidising conditions.2,3 Therefore, in this work, the NASICON-based Mg0.5Ti2(PO4)2 (MTP) compound was synthesised by the solid-state method and investigated as a potential electrode active material for MIBs. However, in order to improve the slow kinetics of the Mg2+ insertion-extraction process via reducing the total diffusion distances of the Mg2+ ions during charge and discharge, nano-sized MTP-powders were prepared via ball milling the macro-sized MTP powder at different rotation speeds. All powders were subjected to comprehensive physico-chemical characterisation techniques (XRD, SEM and IR) to determine their phase purities as well as the changes in particle morphologies after milling. Furthermore, the electrochemical behaviour of MTP with Mg2+ ions was investigated using post-mortem analysis (XRD, IR, SEM and XPS), aiming to establish the nature of the charge storage reaction mechanism and quantify the influence of particle size on electrochemical performance. A second challenge in the development of MIBs is the formation of cationic insulating layers on the surface of the Mg anode when electrolytes prepared by mixing ionic salts (i.e. Mg(ClO4)2) and aprotic organic solvents are used. This leads to the passivation of the Mg-anode and poor performance of the overall cell. In order to address this problem, activated carbon (AC) was used as the counter electrode when using Mg(ClO4)2 in acetonitrile as the electrolyte. The electrochemical characterisation data showed an exponential drop in the discharge and charge capacities of MTP upon cycling which varied only slightly with particle size. This suggests that other factors such as the intrinsic electronic and ionic conductivities of MTP may play more critical roles in the electrochemical performance of the material. References 1. H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, D. Aurbach, Energy Environ. Sci., 2013, 6, 2265–2279; 2. P. Canepa, G. Sai Gautam, D. C. Hannah, R. Malik, M. Liu, K. G. Gallagher, K. A. Persson, G. Ceder, Chem. Rev., 2017, 117, 4287–4341; 3. B. Kang, G. Ceder, Nature, 2009, 458, 190.

Resume : Printable dielectric inks with desired dielectric properties are in high demand to enable high-performance printed electronics[1]. Two-dimensional (2D) materials based printed electronics have been demonstrated to outperform standard printed electronics, promising high-mobility devices[2] for printed integrated circuits[3]. Hexanol boron nitride (h-BN) is a highly suitable dielectric material for electronics based on 2D materials, because of its low atomic roughness, coupled with a wide band gap of 5.6 eV and a high in-plane dielectric constant of 6.8[4]. 2D h-BN nanosheets with single- and few-layer flakes (SFLF) can be obtained by liquid phase exfoliation (LPE)[5] from bulk h-BN powders. This method is a cost-effective technique with a prospect for mass production[6]. Here we report an optimized dielectric inks based on h-BN and BiOCl. Inks are produced by probe sonication and purified by centrifugation. Raman spectroscopy and Atomic Force Microscopy of the ink indicated the successful exfoliation of few-layer thick h-BN flakes. The lateral size and the thickness of the h-BN flakes in the inks both follow a log-normal distribution, which were peaked at 352 nm and 6.6 nm, respectively. A typical thickness of a LPE BiOCl flake is 5 nm. A metal-insulator-metal parallel plate capacitor configuration is designed to investigate the dielectric properties. Results showed that, compared to traditional polymer-based dielectric materials, capacitor performance using our dielectric inks is improved, revealing a dielectric constant of up to 9.68 based on h-BN, and 23.91 based on BiOCl. Increasing h-BN weight ratio against the polymers or adding high-κ dielectric additives could futher improve the dielectric constant and lower the leakage current density in electroncis. The work is beneficial to ultrathin printable electronics development with cost-effective dielectric inks, enabling wearable devices. References 1. F. Torrisi and T. Carey, Nano Today, 2018, 23, 73-96. 2. T. Carey, S. Cacovich, G. Divitini, J. Ren, A. Mansouri, J. M. Kim, C. Wang, C. Ducati, R. Sordan and F. Torrisi, Nature communications, 2017, 8, 1-11. 3. T. Carey, A. Arbab, L. Anzi, H. Bristow, F. Hui, S. Bohm, G. Wyatt‐Moon, A. Flewitt, A. Wadsworth and N. Gasparini, Advanced Electronic Materials, 2021, 2100112. 4. Y. Y. Illarionov, T. Knobloch, M. Jech, M. Lanza, D. Akinwande, M. I. Vexler, T. Mueller, M. C. Lemme, G. Fiori and F. Schwierz, Nature Communications, 2020, 11, 1-15. 5. V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N. Coleman, Science, 2013, 340. 6. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De and R. J. Smith, Science, 2011, 331, 568-571.

Resume : The development of clean light harvesting platforms and technologies is crucial in view of the urgent energy and environmental global challenges. The use of ultra-thin 2D-TMD semiconductor layers such as MoS2 show great promise in photonics, but they suffer from poor photon absorption and the fabrication is typically constrained to micrometer-scale devices. New approaches enabling large area photon harvesting applications in such ultra-thin devices are thus intensively searched. In this work we propose a novel method based on Laser Intereference Lithography combined with physical deposition to fabricate large area MoS2 nanorippled layers extending over cm2 areas [1]. We then demonstrate enhanced photochemical reactivity and photobleaching of highly polluting methylene blue molecules promoted by this ultra-thin platform. We investigate this effect by tailoring the spectral overlap between the molecule absorption band and the guided mode anomalies supported by the nanotextured 2D-TMD film. These results are promising in view of engineering and optimization of light harvesting platforms for waste water treatment, dye molecules sensing devices and a broad range of other light harvesting applications. [1] ACS Appl. Mater. Interfaces 2021, 13, 13508−13516

Resume : Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi5S9 with an ultralow κ value of 0.6–0.4 W m–1 K–1 in the temperature range 300–700 K. In contrast to prior studies, we show that this synthetic sulfide does not exhibit the ordered gladite mineral structure but instead forms a copper-deficient disordered aikinite structure with partial Pb replacement by Bi, according to the chemical formula Cu1/3□2/3Pb1/3Bi5/3S3. By combining experiments and lattice dynamics calculations, we elucidated that the ultralow κ value of this compound is due to very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints at substantial anharmonic effects that contribute to enhance phonon scattering. Importantly, we show that this aikinite-type sulfide, despite being a poor semiconductor, is a potential matrix for designing novel, efficient n-type thermoelectric compounds with ultralow κ values. A drastic improvement in the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The Cu1–x□xPb1–xBi1+xS3 series provides a new, interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.

Resume : Exploring the influence of dimension of electrode materials on electrochemical behavior is of great significance to the development of rechargeable batteries. Herein, three titanium niobium oxide (TiNb2O7) nanostructures with different dimensions (1D, 2D, and 3D) were successfully designed based on a facile solvothermal method. The dimension effect on the performance evolution for lithium-ion batteries (LIBs) were studied in detail. Comprehensive characterizations proved that 2D TiNb2O7 (TNO-2D) shows a superior electronic conductivity and a large Li+-ion diffusion coefficient, resulting in great electrochemical properties. Operando and ex-situ X-ray diffraction measurements reveal the robust structure stability and high reversibility of TNO-2D. Moreover, after coating with carbon, TNO-2D achieves excellent rate capability (205 mAh g-1 at 50 C) and superior long-term cycling stability (87% after 1000 cycles at 5 C). This work provides insights into the rational design of electrode materials for accelerated charge transport and hence enhanced fast-charging capability, pushing forward the development of high-power rechargeable batteries for the future energy storage.

Resume : Electrospun 1-D supercapacitor electrode materials which are based on FeOx/carbon nanofibers are typically synthesized using indirect carbothermal reduction (IDCR), which entails pre-oxidation of the polymer and Fe-salt nanofiber in air followed carbonization under inert atmosphere. This synthesis method has become the gold-standard because it stabilizes the polymer and Fe-salt, produces a high yield, and flexible binder-free electrode materials. However, it often generates aggregated FeOx nanoparticles with a large size, low crystallinity, and low transport kinetics which result in limited FeOx-pseudocapacitive reactions. Therefore, the much-coveted high theoretical capacitance of FeOx (i.e., 2299 F/g for Fe3O4 and 3625 F/g for Fe2O3) has so far not been achieved. In this study, we investigate how direct-carbothermal reduction (DCR) coupled with zinc doping (i.e., DCR-ZD) can mitigate the aforementioned limitations and improve energy storage performance. By DCR we mean the absence of a pre-oxidation step. Our results show that under DCR, early precipitation of Zn-salt in the form of ZnO is highly favored over FeOx, which exerts a physical confinement effect on FeOx and limit its migration and aggregation. This leads to the formation of uniformly dispersed ultra-small FeOx nanoparticle with high crystallinity. The micropores and mesopores created by the subsequent sublimation of ZnO(s) in the form of ZnO0(g) expose large areas of FeOx to high concentrations of polymer-derived CO(g) which catalyzes its carbothermal reduction into Fe3C nanoparticles. An added benefit of the porous structure is better electrolyte penetration to the Fe-sites. The dominant Fe3-C phase generates multiple pseudocapacitive redox reactions (i.e., Fe0/Fe2+, Fe2+/Fe3+, and Fe0/Fe3+) compared to the single one (i.e., Fe2+/Fe3+) by FeOx. Consequently, the specific capacitance of DCR-ZD-derived 1-D Fe3C/carbon is ~5X greater than that of IDCR-derived 1-D FeOx/carbon, at 1 Ag-1. More importantly, the DCR-ZD-derived 1-D Fe3C/carbon achieves ~25% of the theoretical capacitance of FeOx compared to just ~19.6% reported in literature. Therefore, DCR-ZD may prove to be a game-changer in the quest to synthesis high-performing iron-based supercapacitors for practical applications.

Resume : Two-dimensional transition metal dichalcogenides (TMDCs) are considered encouraging materials for photovoltaic applications, with differing electronic and optical properties from their bulk counterparts. Pentacene is an organic compound with high exciton mobility, and complimentary electronic properties to TMDCs. Here are investigated systems of adsorbed pentacene on to monolayers of Group-VI dichalcogenides; MoS2, MoSe2, WS2 and WSe2, for photovoltaic applications. Using ab initio methods within density functional theory, optimized atomic positions were calculated and energetically favourable adsorption sites of pentacene were determined. These sites were further investigated with a varying concentration of adsorbed pentacene, with the aim of investigating how molecule-molecule interactions affect the interaction between molecule and substrate. The electronic properties of the favourable systems were then probed and the charge balance of the systems analysed. *This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)– 406901005

Resume : Recently, two-dimensional (2D) transition metal dichalcogenides (TMDCs) open a great innovation potential for direct integration with silicon (Si) technology, due to their outstanding chemical, physical, electronic and optical properties. Among them, platinum diselenide (PtSe2) is new addition to the 2D family. Due to its layer dependent transition between semiconductor and semimetal and interesting optical/electronic properties it is of significant interest for applications in electronics, spintronics, sensors, catalysis, etc [1, 2]. Here, we report the synthesis details of PtSe2 layers by thermally assisted selenization of pre-deposited Pt films using Chemical Vapour Deposition (CVD) method. The Pt layers were pre-deposited as thin films using Magnetron Sputtering technique on Silicone, quartz, and glass substrates. The existence of 2D PtSe2 was confirmed using X-ray photoelectron spectroscopy (XPS) and Raman analysis. UV-VIS-NIR spectroscopy and ellipsometry spectroscopy were used to investigate the optical properties and to determine the thickness of synthesized 2D PtSe2 layers. In addition, the measured current-voltage characteristics indicates Ohmic behaviour as well as intensity dependent photoconductivity. All the studied properties reveal great potential of PtSe2 to design varieties of heterostructures for future nanoelectronic and optoelectronic structures and devices. Acknowledgments This research was funded by Bulgarian Science Fund under the project numbers DFNI КП-06-ДO 02/2 and DFNI КП-06-ДO 02/3 in the frames of M-ERA program project “Functional 2D materials and heterostructures for hybrid spintronic-memristive devices“ N. M and V. M. acknowledge the financial support by the European Regional Development Fund within the Operational Programme ‘Science and Education for Smart Growth 2014–2020’ under the Project CoE ‘National Center of Mechatronics and Clean Technologies’ BG05M2OP001-1.001-0008-C01. References: [1] Chanyoung Yim, Kangho Lee,Niall McEvoy, Maria O’Brien,Sarah Riazimehr, Nina C. Berner, Conor P. Cullen, Jani Kotakoski, Jannik C. Meyer, Max C. Lemme, and Georg S. Duesberg “High-Performance Hybrid Electronic Devices from Layered PtSe2 Films Grown at Low Temperature” ACS Nano, 10, 9550−9558 (2016)] [2] Li-Syuan Lu, Guan-Hao Chen, Hui-Yu Cheng, Chih-Piao Chuu, Kuan-Cheng Lu, Chia-Hao Chen, Ming-Yen Lu, Tzu-Hung Chuang, Der-Hsin Wei, Wei-Chen Chueh, Wen-Bin Jian, Ming-Yang Li, Yu-Ming Chang, Lain-Jong Li, and Wen-Hao

Resume : Semiconducting transition metal dichalcogenides (TMDs) are of great interest as absorbers for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and intrinsic self-passivated surfaces. Tungsten diselenide (WSe2), a semiconducting transition metal dichalcogenide, further shows great potential as active material in optoelectronic devices due to its ambipolarity and direct bandgap in its single-layer form. Recently [1,2], WSe2 have been further exploited to realize electrically tunable PN junctions, demonstrating its potential for digital electronics and solar cell applications. The present study is focused on WSe2 continuous layers synthesized on large area by using thermally assisted conversion (TAC) method of pre-deposited by magnetron sputtering tungsten (W) layers with defined thickness. The obtained WSe2 films on different substrates were characterized by AFM (Atomic force microscopy), Raman spectroscopy and XPS (X-ray photoelectron spectroscopy). Films thickness dependent optical properties were studied toward photovoltaic and photodetector applications. References: [1]Junli Du, et al. “Gate-Controlled Polarity-Reversible Photodiodes with Ambipolar 2D Semiconductors” Advanced Functional Materials, 31 (8) 2007559 (2021) [2]Ricardo Frisenda, et al. “Atomically thin p–n junctions based on two-dimensional materials” Chemical Society Reviews, 47, 3339 (2018) Acknowledgements: This research was funded by Bulgarian Science Fund under the project numbers DFNI КП-06-ДO 02/2 and DFNI КП-06-ДO 02/3 in the frames of M-ERA program project “Functional 2D materials and heterostructures for hybrid spintronic-memristive devices“ V. M. and D. D. acknowledge the financial support by the European Regional Development Fund within the Operational Programme ‘Science and Education for Smart Growth 2014–2020’ under the Project CoE ‘National Center of Mechatronics and Clean Technologies’ BG05M2OP001-1.001-0008-C01. Research equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian Ministry of Education and Science under contract D01-284/17.12.2019 was used for AFM measurements.

Resume : One of the greatest challenges we face as a society is fighting climate change without negatively affecting the quality of life of the population. In this line, it is essential to improve the efficiency in the production and storage of renewable energy, as well as the autonomy of devices and machinery powered by electricity. At this point, it is essential to improve Li-ion batteries (LIBs), which are a key technology for clean, efficient transport and sustainable development, and therefore play a fundamental role in the transition towards a future zero-emissions society. Within the framework of the "StressLIC" Consortium granted by M-Era.Net (EU), we have set up a facility of Pulsed Laser Deposition (PLD) — Multibeam Optical Stress Sensor (MOSS) to deposit and characterize in situ Li-ion components to be used in all-thin film solid state Li-ion battery (SSLB). We present here a comprehensive study on the coexistence of phases in LiCoO2 (LCO) thin films deposited by PLD. LCO, which was the first cathode (CAT) commercialized by Sony in 1991 and later replaced due to its poor thermal stability, has regained prominence in the development of "zero strain" composite CATs for SSLBs, since LCO is the only CAT with a negative chemical coefficient of expansion, and thus a basic mix component to prevent CAT breathing during charge-discharge cycles. Polycrystalline LCO thin films are deposited on Si(100), without and with diffusion barrier of thermal SiO2, in order to study the chemical phases that arise as a result of the Li diffusion towards Si(100). The films are analyzed in terms of: (i) composition by Raman spectroscopy (RS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD); (ii) microstructure by scanning electron (SEM) and atomic force microscopies (AFM); and (iii) Li stoichiometry by impedance spectroscopy (IS). RS and IS reveal LCO grows stoichiometry in the Li rich regions, while within Li-poor regions, Co oxides are formed. RS and RBS interpret the Co oxides as Co3O4 and CoO2. SEM shows that films exhibit a columnar compact microstructure. The results point to a heterogeneously nucleation of LCO grains whose coarsening is ruled by short-range Li diffusion. The LCO deposition on SiO2/Si gives rise to higher LCO/Co-oxide ratios and thicker films as expected. However, the fact that Co oxides are still formed on SiO2/Si from the evaporation of stoichiometry target indicates that there are other sources of Li loss in addition to diffusion towards Si bulk, probably, losses in the evaporation plume and/or Li reevaporation under PLD conditions. From the compilation of all these results, we propose a thermodynamic model of LCO growth by physical vapor deposition.

Resume : Electrochemical capacitors (ECs) are important high-power energy storage devices that gained significant attention for the potential applications in electric vehicles and portable or complex electronics. Different types of carbon-based materials, e.g., activated carbon or carbon fibers are mainly used as electrode materials thanks to relatively high capacitance as well as long cyclability and tunable porous texture. In comparison to other electrode materials such as metal polyoxometalates, conducting polymers, or transition metal dichalcogenides (TMDs), they possess relatively small capacitance mostly due to the lack of redox reactions. Among different materials, TMDs are highly promising due to the large variety of compounds, two-dimensional character, and presence of exposed active sites. TMDs can be represented by the MX2 formula, where M is a metal atom (Mo, W, V, Re, etc.) and X is a chalcogen atom (S, Se, or Te). TMDs consist of three atomic layers, where metal atoms are sandwiched between the chalcogen atoms and are connected with strong covalent bonds. The most commonly studied electrode material in ECs, among TMDs family, is molybdenum disulfide (MoS2). It possess two-dimensional layered structure, developed specific surface area and crystallography dependent conductivity. Carbons are often employed as structural supports for the deposition of TMD layered structures. Finally, they positively affect the composite conductivity as well as overall electrochemical performance. Modification of carbon surface with benzoquinone-hydroquinone compounds is already known to enhance specific capacitance due to exploring fast reversible redox reactions. However, the impact of the surface modification of carbon before hydrothermal synthesis of TMD/carbon composites on their electrochemical performance was never evaluated before. In this study, several MoS2 composites were prepared with different carbon materials such as carbon nanotubes or modified soot. Prior the reaction, surface of the carbon materials were modified with benzoquinone/hydroquinone species. Various structural and textural properties were evaluated by nitrogen sorption/desorption at 77K, X-Ray diffraction, Raman spectroscopy and scanning electron microscopy. Two- and three-electrode symmetric and asymmetric cells were employed for testing prepared materials. The electrochemical performance of the assembled cells was studied by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. Cells were tested with sulphate based electrolytes. Acknowledgments: The authors would like to acknowledge the National Science Centre, Poland, for the financial support in the framework of the project 2018/31/B/ST4/01852. M.T. would like to thank the National Science Centre for the financial support within the project Preludium 20 2021/41/N/ST5/01364.

Resume : Semiconducting transition metal dichalcogenides (TMD) have shown potential for a wide range of electronic and optical applications. TMD’s can be fabricated using laser ablation, chemical vapour deposition and intercalation reactions. However after fabrication, fast and easy identification of the produced nanolayers poses a significant challenge for scalability of the process. Several approaches exist for the identification of TMDs including optical microscopy, Raman spectroscopy and atomic force microscopy; the former of which offers the easiest and fastest method. Optical identification of TMD nanolayers and flakes requires a measurable optical contrast between the drop-casted or as-grown nanolayers and the substrate material. This quantity is simply dependent upon the refractive indices and the thicknesses of the material. Thus, optical contrast can be altered by changing the substrates. In this work, we have used a simple finite element modelling method to investigate the optical contrast of various TMDs including molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and molybdenum ditelluride (MoTe2) on various substrates. In particular, this work examines the optical contrast between TMDs and thin (~200 nm) diamond films that are either intrinsic or boron doped. We demonstrate differences in reflectance by using these different substrates.

Resume : Graphene is at the top of considerable interest due to the giant electron and hole mobility, flexibility, optical transparency, chemical inertness. One of the main limitations of stopping graphene's broader application in semiconductor device technology is a complex graphene transfer procedure. In this case, graphene is synthesized on the catalytic Cu or Ni foils. Afterward, follows the long process of the graphene transfer onto the targeted semiconductor or dielectric substrates. During that process, graphene can be contaminated by different adsorbents. Wrinkles or ripples can be formed on graphene. Thus, control of the graphene layer or graphene-semiconductor contact properties is complicated. Recently there was shown that direct synthesis of graphene on semiconducting or dielectric substrates is possible. However, the development of this technology is the very beginning. In the present research, graphene layers were directly synthesized by microwave plasma-enhanced chemical vapor deposition, ICP plasma beam source, and reactive magnetron sputtering on the flat and textured semiconducting monocrystalline Si(100) substrates. The films' structure was investigated by multiwavelength Raman scattering spectroscopy, atomic force microscopy, Kelvin probe microscopy, scanning electron microscopy. The composition was investigated by X-ray photoelectron spectroscopy. Graphene/Si(100) Schottky diodes were fabricated. Effects of the deposition conditions on the structure of the graphene layers were studied. The influence of the methane and hydrogen gas flow ratio, pressure, plasma power, and deposition time was considered. Nitrogen, fluorine, and boron-doped graphene were synthesized. There were revealed that both vertical graphene flakes and planar graphene layers could be synthesized by setting appropriate deposition conditions. Current-voltage characteristics, and photovoltaic and photoelectric properties of the graphene/Si diodes, were investigated. The relation between photovoltaic properties, graphene structure, and doping was found.