Molecular Exchange Bias at Ferromagnet/Organic Interfaces

Department of Materials Science and EngineeringAn organic molecule, the carbon-based complex of several atoms, is an innovative and essential element to create nano-structural platforms, as a building block. Because of its variety and functionality via widely studied synthetic methods, molecules have played an important role in electronics as not only a transport channel by bulk-forms but also a tuning layer at the interface of heterostructures. The potential of molecular layers has also stood out in spintronics, owing to its mass-low composition producing long spin life time. Beyond this advantage, the on-surface configuration of molecules to a metal film displays unique phenomena as it can control the molecular spins and interfacial coupling between them, resulting in the emergence of molecular spinterface. With a great effort to unveil fundamental mechanism of the hybrid interface via various spectroscopies and theoretical simulations, future-oriented research of molecular spintronics to practical device application has received enormous attentions. Among them, exchange bias is an attractive phenomenon, because it is closely related to the fundamental concept of spinterface and is a critical factor in practical magnetic devices. Thus, introducing molecular spinterface to exchange bias will take advantage of its functionality for device applications and enrich the research of molecular spintronics. This thesis will show the comprehensive study of molecular exchange bias induced by newly developed molecular spinterfaces between paramagnetic metalloporphyrins and ferromagnetic layers. Magnetometry showed that various kinds of metalloporphyrins resulted in different degree of interfacial couplings and a wide range of exchange bias on the ferromagnetic layers. Varying the underlying ferromagnetic layers allowed to control the interfacial exchange interaction such as ferromagnetic and antiferromagnetic coupling, and the latter can even tune the degree of exchange bias depending on cooling magnetic field. Magnetotransport measurement provided an alternative assessment of the exchange bias through anisotropic and angle-dependent magnetoresistance which are essential ingredients to expand spintronics applications of the hybrid magnetic layers. The emergence of magnetic moment and interfacial coupling were calculated by theoretical approaches with demonstrations of incommensurately antiferromagnetic interlayers of the molecules and indirect exchange coupling between the molecule and metal layer. These fundamental studies of the hybrid interfacial coupling and its impact on the magnetic and magnetotransport characteristics open a new channel for controlling hybrid magnetic layers. The demonstrated tuning of magnetic exchange bias via the molecular approach will certainly extend versatile functionalities of organic spinterfaces.clos

Tuning of undoped ZnO thin film via plasma enhanced atomic layer deposition and its application for an inverted polymer solar cell

We studied the tuning of structural and optical properties of ZnO thin film and its correlation to the efficiency of inverted solar cell using plasma-enhanced atomic layer deposition (PEALD). The sequential injection of DEZn and O2 plasma was employed for the plasma-enhanced atomic layer deposition of ZnO thin film. As the growth temperature of ZnO film was increased from 100 ??C to 300??C, the crystallinity of ZnO film was improved from amorphous to highly ordered (002) direction ploy-crystal due to self crystallization. Increasing oxygen plasma time in PEALD process also introduces growing of hexagonal wurtzite phase of ZnO nanocrystal. Excess of oxygen plasma time induces enhanced deep level emission band (500 ??? 700 nm) in photoluminescence due to Zn vacancies and other defects. The evolution of structural and optical properties of PEALD ZnO films also involves in change of electrical conductivity by 3 orders of magnitude. The highly tunable PEALD ZnO thin films were employed as the electron conductive layers in inverted polymer solar cells. Our study indicates that both structural and optical properties rather than electrical conductivities of ZnO films play more important role for the effective charge collection in photovoltaic device operation. The ability to tune the materials properties of undoped ZnO films via PEALD should extend their functionality over the wide range of advanced electronic applications.open2

A scalable molecule-based magnetic thin film for spin-thermoelectric energy conversion

Spin thermoelectrics, an emerging thermoelectric technology, offers energy harvesting from waste heat with potential advantages of scalability and energy conversion efficiency, thanks to orthogonal paths for heat and charge flow. However, magnetic insulators previously used for spin thermoelectrics pose challenges for scale-up due to high temperature processing and difficulty in large-area deposition. Here, we introduce a molecule-based magnetic film for spin thermoelectric applications because it entails versatile synthetic routes in addition to weak spin-lattice interaction and low thermal conductivity. Thin films of Cr-II[Cr-III(CN)(6)], Prussian blue analogue, electrochemically deposited on Cr electrodes at room temperature show effective spin thermoelectricity. Moreover, the ferromagnetic resonance studies exhibit an extremely low Gilbert damping constant -(2.4 +/- 0.67) x10(-4), indicating low loss of heat-generated magnons. The demonstrated STE applications of a new class of magnet will pave the way for versatile recycling of ubiquitous waste heat

Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.ope

Local control of superconductivity in a NbSe2/CrSBr van der Waals heterostructure

Two-dimensional magnets and superconductors are emerging as tunable building-blocks for quantum computing and superconducting spintronic devices, and have been used to fabricate all two-dimensional versions of traditional devices, such as Josephson junctions. However, novel devices enabled by unique features of two-dimensional materials have not yet been demonstrated. Here, we present NbSe2/CrSBr van der Waals superconducting spin valves that exhibit infinite magnetoresistance and nonreciprocal charge transport. These responses arise from a unique metamagnetic transition in CrSBr, which controls the presence of localized stray fields suitably oriented to suppress the NbSe2 superconductivity in nanoscale regions and to break time reversal symmetry. Moreover, by integrating different CrSBr crystals in a lateral heterostructure, we demonstrate a superconductive spin valve characterized by multiple stable resistance states. Our results show how the unique physical properties of layered materials enable the realization of high-performance quantum devices based on novel working principles.This work was supported by the Spanish MCIN/AEI under Projects PID2020-117152RB-100, PID2020-114252GB-I00, PID2021-128004NB-C21, TED2021-130292B-C42, and PID2021-122511OB-I00. This work was also supported by the FLAG-ERA grant MULTISPIN, by the Spanish MCIN/AEI with grant number PCI2021-122038-2A. This work was supported by CEX2019-000919-M and CEX2020-001038-M /AEI /10.13039/501100011033 under the María de Maeztu Units of Excellence Program. Funding from the European Union’s Horizon 2020 Research and Innovation Program under Project SINFONIA, Grant 964396 as well as ERC AdG Mol-2D 788222 and ERC-2021-StG-101042680 2D-SMARTiES is acknowledged. The work was supported by Generalitat Valenciana (PROMETEO Program, PO FEDER Program IDIFEDER/2018/061 and CDEIGENT/2019/022). This study forms part of the Advanced Materials program and was supported by MCIN withfunding from European Union NextGenerationEU (PRTR-C17.I1), from the Bask country and from Generalitat Valenciana. M.G. acknowledges support from the “Ramón y Cajal” Program by the Spanish MCIN/AEI (grant no. RYC2021-031705-I). J.J. acknowledges the funding from the Ayuda FJC2020-042842-I financiada por MCIN/AEI/10.13039/501100011033 y por la Unión Europea NextGenerationEU/PRTR. Y.L. and F.S.B acknowledge funding from the Basque Government through the IKUR initiative on Quantum technologies, and grant IT-1591-22. The authors acknowledge E. Goiri for helpful discussions.With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2019-000919-M).With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2020-001038-M).Peer reviewe

Gate-dependent spin Hall induced nonlocal resistance and the symmetry of spin-orbit scattering in Au-clustered graphene

Engineering the electron dispersion of graphene to be spin-dependent is crucial for the realization of spin-based logic devices. Enhancing spin-orbit coupling in graphene can induce spin Hall effect, which can be adapted to generate or detect a spin current without a ferromagnet. Recently, both chemically and physically decorated graphenes have shown to exhibit large nonlocal resistance via the spin Hall and its inverse effects. However, these nonlocal transport results have raised critical debates due to the absence of field dependent Hanle curve in subsequent studies. Here, we introduce Au clusters on graphene to enhance spin-orbit coupling and employ a nonlocal geometry to study the spin Hall induced nonlocal resistance. Our results show that the nonlocal resistance highly depends on the applied gate voltage due to various current channels. However, the spin Hall induced nonlocal resistance becomes dominant at a particular carrier concentration, which is further confirmed through Hanle curves. The obtained spin Hall angle is as high as similar to 0.09 at 2 K. Temperature dependence of spin relaxation time is governed by the symmetry of spin-orbit coupling, which also depends on the gate voltage: asymmetric near the charge neutral point and symmetric at high carrier concentration. These results inspire an effective method for generating spin currents in graphene and provide important insights for the spin Hall effect as well as the symmetry of spin scattering in physically decorated graphene

Impedance spectroscopy analysis on the effects of TiO2 interfacial atomic layers in ZnO nanorod polymer solar cells: Effects of interfacial charge extraction on diffusion and recombination

The study of impedance spectroscopy (IS) provides essential information on the electrical processes at the interfaces in photovoltaic devices. In this work, the IS was performed to elucidate the role of TiO2 interfacial atomic layer in ZnO nanorod polymer solar cells. The studied devices have active layers of bulk heterojunction P3HT (poly-3-hexylthiophene):PCBM (Phenyl-C71-butyric acid methyl ester) and electron collecting ZnO nanorod array. The ultrathin TiO2 interfacial layer was deposited on ZnO nanorods by using plasma enhanced atomic layer deposition (PEALD) to facilitate charge extraction at the oxide/organic interface. The impedance measurements were performed upon applying a constant DC bias in order to study the effects of interfacial charge extraction under the different charge concentration. Addition of the TiO2 layer in our hybrid polymer solar cell increased recombination resistance and effective lifetime of charge carriers, especially at forward bias. The bias dependent effect of the TiO2 layer in impedance spectra indicates the concentration dependent charge extraction. These IS results provide important insight into the role of functional interfacial layers in polymer solar cellsclose0

Correlation between a Structural Change and a Thermoelectric Performance of a Glassy Carbon Thin Film Induced by Electron Beam Irradiation

Glassy carbon can be utilized in a variety of harsh environment due to exceptional thermal stability and chemically impermeability along with scalability and low electrical resistance. In this work, we studied effects of electron(e)-beam irradiation on thermoelectric properties of the glassy carbon film. E-beam irradiation triggered local crystallization and/or amorphization of glassy carbon thin films, which was determined by a Raman spectroscopy. The structural change by e-beam irradiation leads to the change in the doping level of the glassy carbon, which can be inferred from the change of a Seebeck coefficient and an electric conductivity. The optimal power factor we obtained for the irradiated glassy carbon film was ~200% higher than that of the non-irradiated sample.clos