Applications In Molecular Spintronics Research Articles

Solvothermal Synthesis of Rare Earth Bisphthalocyanines.

Rare earth bisphthalocyanines (MPc2) are of particular interest because of their behavior as single-molecular magnets, which makes them suitable for applications in molecular spintronics, high-density data storage and quantum computation. Nevertheless, MPc2 are not commercially available, and the synthesis routes are mainly focused on obtaining substituted phthalocyanines. Two preparation routes depend on the precursor: synthesis from phthalonitrile (PN) and the metalation of free or dilithium phthalocyanine (H2Pc and Li2Pc). In both options, byproducts such as free-base phthalocyanine and in the first route additional PN oligomers are generated, which influence the MPc2 yield. There are three preparation methods for these routes: heating, microwave radiation and reflux. In this research, solvothermal synthesis was applied as a new approach to prepare yttrium, lanthanum, gadolinium and terbium unsubstituted bisphthalocyanines using Li2Pc and the rare earth(III) acetylacetonates. Purification by sublimation gave high product yields compared to those reported, namely 68% for YPc2, 43% for LaPc2, 63% for GdPc2 and 62% for TbPc2, without any detectable presence of H2Pc. Characterization by infrared, Raman, ultraviolet-visible and X-ray photoelectron spectroscopy as well as elemental analysis revealed the main featuresof the four bisphthalocyanines, indicating the success of the synthesis of the complexes.

Enhancing Spin-Transport Characteristics, Spin-Filtering Efficiency, and Negative Differential Resistance in Exchange-Coupled Dinuclear Co(II) Complexes for Molecular Spintronics Applications.

Single-molecule spintronics, where electron transport occurs via a paramagnetic molecule, has gained wide attention due to its potential applications in the area of memory devices to switches. While numerous organic and some inorganic complexes have been employed over the years, there are only a few attempts to employ exchange coupled dinuclear complexes at the interface, and the advantage of fabricating such a molecular spintronics device in the observation of switchable Kondo resonance was demonstrated recently in the dinuclear [Co2(L)(hfac)4] (1) complex (Wagner et al., Nat. Nanotechnol. 2013, 8, 575-579). In this work, employing an array of theoretical tools such as density functional theory (DFT), the ab initio CASSCF/NEVPT2 method, and DFT combined with nonequilibrium Green Function (NEGF) formalism, we studied in detail the role of magnetic coupling, ligand field, and magnetic anisotropy in the transport characteristics of complex 1. Particularly, our calculations not only reproduce the current-voltage (I-V) characteristics observed in experiments but also unequivocally establish that these arise from an exchange-coupled singlet state that arises due to antiferromagnetic coupling between two high-spin Co(II) centers. Further, the estimated spin Hamiltonian parameters such as J, g values, and D and E/D values are only marginally altered for the molecule at the interface. Further, the exchange-coupled state was found to have very similar transport responses, despite possessing significantly different geometries. Our transport calculations unveil a new feature of the negative differential resistance (NDR) effect on 1 at the bias voltage of 0.9 V, which agrees with the experimental I-V characteristics reported. The spin-filtering efficiency (SFE) computed for the spin-coupled states was found to be only marginal (∼25%); however, if the ligand field is fine-tuned to obtain a low-spin Co(II) center, a substantial SFE of 44% was noted. This spin-coupled state also yields a very strong NDR with a peak-to-valley ratio (PVR) of ∼56 - a record number that has not been witnessed so far in this class of compounds. Additionally, we have established further magnetostructural-transport correlations, providing valuable insights into how microscopic spin Hamiltonian parameters can be associated with SFE. Several design clues to improve the spin-transport characteristics, SFE and NDR in this class of molecule, are offered.

Spin Catalysis in Photochemical Reactions and Its Applications to Quantum Information Nanotechnology

Chemistry as a science about spin and electric charge of micro particles which provide driving forces of atomic interactions and molecular structure transformations fits pretty well to the modern Quantum Information Science (QIS) requirements. Today’s computers operate only electric current signals in the semiconductor networks but the electron-spin properties are not exploited in a large extend. Spintronics provides spin-polarized currents and manipulates magnetic spin interactions; it uses mostly solid state chemistry of heavy elements. But a rich organic chemistry of solvents and fin films offers a great potential for molecular electronics and quantum computing. Photo-excited organic complexes of the “chromophore–radical” type provide good promise for many technological applications in molecular spintronics and electronics, including QIS technology. The doublet state photo excitation of stable organic radical being delocalized onto the linked anthracene molecule within picoseconds and subsequently evolved into a quartet state for big radicals (a pure high spin state) of the mixed radical-triplet character presents a sensible spin-optical interface for qubit in quantum computing. This high-spin state is coherently addressable with EPR microwaves even at room temperature, with the optical read-out induced by intersystem crossing (ISC) to emissive triplet state. Such integration of radical luminescence and high-spin states EPR provides the organic materials involvement into emerging QIS technologies

Properties and applications of photoexcited chromophore-radical systems.

Photoexcited organic chromophore-radical systems hold great promise for a range of technological applications in molecular spintronics, including quantum information technology and artificial photosynthesis. However, further development of such systems will depend on the ability to control the magnetic properties of these materials, which requires a profound understanding of the underlying excited-state dynamics. In this Review, we discuss photogenerated triplet-doublet systems and their potential to be used for applications in molecular spintronics. We outline the theoretical description of the spin system in the different coupling regimes and the invoked excited-state mechanisms governing the generation and transfer of spin polarization. The main characterization techniques used to evaluate the optical and magnetic properties of chromophore-radical systems are discussed. We conclude by giving an overview of previously investigated covalently linked triplet-radical systems, and highlight the need for further systematic investigations to improve our understanding of the magnetic interactions in such systems.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state.

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

Molecular Compounds in Spintronic Devices: An Intricate Marriage of Chemistry and Physics.

Spintronics, a flourishing new field of microelectronics, uses the electron spin for reading and writing information in modern computers and other spintronic devices with a low power consumption and high reliability. In a quest to increase the productivity of such devices, the use of molecular materials as a spacer layer allowed them to perform equally well or even better than conventional all-inorganic heterostructures from metals, alloys, or inorganic semiconductors. In this review, we survey various classes of chemical compounds that have already been tested for this purpose─from organic compounds and coordination complexes to organic-inorganic hybrid materials─since the creation of the first molecule-based spintronic device in 2002. Although each class has its advantages, drawbacks, and applications in molecular spintronics, together they allowed nonchemists to gain insights into spin-related effects and to propose new concepts in the design and fabrication of highly efficient spintronic devices. Other molecular compounds that chemistry could offer in great numbers may soon emerge as suitable spacers or even electrodes in flexible magnetic field sensors, nonvolatile memories, and multifunctional spintronic devices.

A theoretical study of laser-induced ultrafast spin dynamics in trigonal monopyramidal iron and nickel complexes.

We present a first-principles study of the geometries, electronic structures, and laser-induced ultrafast spin dynamics in four trigonal monopyramidal complexes [tpat-BuFe]-, [tcmat-BuFe]-, [tpat-BuNi]-, and [tcmat-BuNi]- [tpa: tris-(pyrrolylmethyl)amine; tcma: tris(carbamoyl-methyl)amine; t-Bu: tert-butyl]. It is found that the low-lying level distribution of the four structures is similar, however, their spin and charge localization differs substantially. Detailed analysis demonstrates that the iron complexes have much more singly spin localized states located in the low energy region, while the nickel complexes have more charge-transfer (CT) states and more states with spin equally distributed between the Ni and the ligands. Affected by these features, more ultrafast spin-crossover (SCO) scenarios are achieved in the two iron complexes, and better CT dynamics is obtained in nickel complexes. In particular, for the CT scenarios combined with spin bifurcation, the charge is transferred from the tpa/tcma ligand to the Fe/Ni atoms, while spin-density transfer occurs in the opposite direction. Among the scenarios illustrated in the paper, the SCO processes turn out to be more complicated since they involve many more intermediate states and exhibit relatively low fidelity. In addition, the transferability of each scenario is analyzed from the absorption spectra of the initial and final states. All these results can provide significant insights into the electronic and magnetic natures of the four complexes, guide the experimental realization of the relevant scenarios, and thus promote their applications in molecular spintronics.

Understanding Charge Transport in Triarylmethyl-Based Spintronic Nanodevices

In this Article, we explore the potential of triarylmethyl (TAM) oligomers as devices for organic molecular spintronic applications. We use the Landauer formalism to compute global spin transport a...

Heterometallic Co-Dy SMMs grafted on iron oxide nanoparticles.

Heterometallic 3d-4f SMM [Co4Dy(OH)2(SALOH)5(chp)4(MeCN)(H2O)2] (1) has been deposited onto iron oxide nanoparticles (NPs) with an oleate self-assembled monolayer (SAM) as a surfactant. The obtained hybrid molecular-inorganic system 1-NP has been thoroughly characterized. The oleate SAM separates SMM 1 from the magnetic substrate to avoid the strong-coupling between the surface and molecule to ensure that 1 retains its magnetic properties in 1-NP. The magnetic properties of the hybrid system 1-NP have been characterized by element specific XMCD: the heterometallic SMM retains its magnetic properties on the surface of the iron oxide NPs while there is an enhancement of the magnetic properties of the NPs.

Real-Space Imaging of a Single-Molecule Monoradical Reaction.

Organic radicals consisting of light elements exhibit a low spin-orbit coupling and weak hyperfine interactions with a long spin coherence length, which are crucial for future applications in molecular spintronics. However, the synthesis and characterization of these organic radicals have been a formidable challenge due to their chemical instability arising from unpaired electrons. Here, we report a direct imaging of the surface chemical transformation of an organic monoradical synthesized via the monodehydrogenation of a chemically designed precursor. Bond-resolved scanning tunneling microscopy unambiguously resolves various products formed through a complex structural dissociation and rearrangement of organic monoradicals. Density functional theory calculations reveal detailed reaction pathways from the monoradical to different cyclized products. Our study provides unprecedented insights into complex surface reaction mechanisms of organic radical reactions at the single molecule level, which may guide the design of stable organic radicals for future quantum technology applications.

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene-Trityl Radical.

Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.

Multilevel Resistance Switching and Enhanced Spin Transition Temperature in Single- and Double-Molecule Spin Crossover Nanogap Devices

Spin crossover (SCO) molecules are promising bi-stable magnetic switches with applications in molecular spintronics. However, little is known about the switching effects of a single SCO molecule when it is confined between two metal electrodes. Here we examine the switching properties of a [Fe(III)(EtOSalPet )(NCS)] SCO molecule that is specifically tailored for surface deposition and binding to only one gold electrode in a nanogap device. Temperature dependent conductivity measurements on SCO molecule containing electromigrated gold break junctions show voltage independent telegraphic-like switching between two resistance states at temperature below 200 K. The transition temperature is very different from the transition temperature of 83 K that occurs in a bulk film of the same material. This indicates that the bulk, co-operative SCO phenomenon is no longer preserved for a single molecule and that the surface interaction drastically increases the temperature of the SCO phenomenon. Another key finding of this work is that some devices show switching between multiple resistance levels. We propose that in this case, two SCO molecules are present within the nanogap with both participating in the electronic transport and switching.

First-principles study of spin properties and laser-induced ultrafast spin dynamics in transition-metal oxide clusters TM3O30/+(TM=Fe,Co,andNi)

We present a first-principles study of spin properties of the triplet ${\mathit{TM}}_{3}{\mathrm{O}}_{3}^{0/+}\phantom{\rule{4pt}{0ex}}(\mathit{TM}=\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ clusters and their laser-induced ultrafast spin dynamics. The differences in geometries, infrared spectra, and level distributions of the three structures are discussed. It is found that the spin localization of the clusters is both structurally and magnetic-field dependent. Specifically, within the same magnetic field, the number of the spin-localized states decreases when the magnetic center changes from Fe via Co to Ni. Even for the same cluster, the spin-localized states may differ under different field directions, due to the fact that spin magnitudes and spin directions of the states change with magnetic field. For the spin dynamics, the results indicate that the system which possesses more spin-localized states normally exhibits rich spin functionalities. Among the various achieved laser-induced ultrafast spin-transfer scenarios, a spin-transfer cycle in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{3}$, a partial demagnetization process in ${\mathrm{Co}}_{3}{\mathrm{O}}_{3}^{+}$, and a spin bifurcation scenario in ${\mathrm{Ni}}_{3}{\mathrm{O}}_{3}$ are presented and analyzed, respectively. These results provide added information for the varieties of ultrafast optical control of magnetism in transition-metal oxide systems, and pave the way toward future related device design and molecular spintronics applications.

Triangulenes: From Precursor Design to On‐Surface Synthesis and Characterization

AbstractTriangulene and its higher homologues are a class of zigzag‐edged triangular graphene molecules (ZTGMs) with high‐spin ground states. These open‐shell molecules are predicted to host ferromagnetically coupled edge states with net spin values scaling with molecular size and are therefore considered promising candidates for future molecular spintronics applications. Unfortunately, the synthesis of unsubstituted [n]triangulenes and the direct observation of their edge states have been a long‐standing challenge due to a high reactivity towards oxygen. However, recent advances in precursor design enabled the on‐surface synthesis and characterization of unsubstituted [3]‐, [4]‐, and [5]triangulene. In this Minireview, we will highlight key aspects of this rapidly developing field, ranging from the principles of precursor design to synthetic strategies and characterization of a homologous series of triangulene molecules synthesized on‐surface. We will also discuss challenges and future directions.

Triangulenes: From Precursor Design to On-Surface Synthesis and Characterization.

Triangulene and its higher homologues are a class of zigzag-edged triangular graphene molecules (ZTGMs) with high-spin ground states. These open-shell molecules are predicted to host ferromagnetically coupled edge states with net spin values scaling with molecular size and are therefore considered promising candidates for future molecular spintronics applications. Unfortunately, the synthesis of unsubstituted [n]triangulenes and the direct observation of their edge states have been a long-standing challenge due to a high reactivity towards oxygen. However, recent advances in precursor design enabled the on-surface synthesis and characterization of unsubstituted [3]-, [4]-, and [5]triangulene. In this Minireview, we will highlight key aspects of this rapidly developing field, ranging from the principles of precursor design to synthetic strategies and characterization of a homologous series of triangulene molecules synthesized on-surface. We will also discuss challenges and future directions.

Coherent spin transport through a six-coordinate FeN6 spin-crossover complex with two different spin configurations

Due to the magnetic bistability, single-molecule spin-crossover (SCO) complexes have been considered to be the most promising building blocks for molecular spintronic devices. Here, we explore the SCO behavior and coherent spin transport properties of a six-coordinate FeN $_6$ complex with the low-spin (LS) and high-spin (HS) states by performing extensive first-principles calculations combined with non-equilibrium Green's function technique. Theoretical results show that the LS $\leftrightarrow$ HS spin transition via changing the metal-ligand bond lengths can be realized by external stimuli, such as under light radiation in experiments. According to the calculated zero-bias transmission coefficients and density of states as well as the $I$ - $V$ curves under small bias voltages of FeN $_6$ SCO complex with the LS and HS states sandwiched between two Au electrodes, we find that the examined molecular junction can act as a molecular switch, tuning from the OFF (LS) state to the ON (HS) state. Moreover, the spin-down electrons govern the current of the HS molecular junction, and this observed perfect spin-filtering effect is not sensitive to the detailed anchoring structure. These theoretical findings highlight this examined six-coordinate FeN $_6$ SCO complex for potential applications in molecular spintronics.

Accurate Magnetic Couplings in Chromium-Based Molecular Rings from Broken-Symmetry Calculations within Density Functional Theory.

We present a comprehensive analysis of magnetic coupling in a group of three popular chromium-based molecular rings, the homometallic Cr8 ring and the heterometallic Cr7Ni and Cr7Zn molecules. We show conclusively that the broken symmetry approach within density functional theory (DFT), based on suitable conventional or range-separated hybrid functionals, provides a quantitatively reliable tool to extract magnetic exchange coupling parameters in all rings considered, which opens a window for additional applications in molecular magnetism. We further show that a nonempirical model spin Hamiltonian, based on the parameters extracted from DFT, leads to excellent agreement with experimental susceptibility data and energy spectra. Moreover, based on an optimally tuned range-separated hybrid functional approach, we find that gas-phase gaps of the studied molecular rings are much larger than previously calculated and discuss the implications of the revised electronic structure to potential applications in molecular spintronics.

Endohedral Fullerene Fe@C28 Adsorbed on Au(111) Surface as a High-Efficiency Spin Filter: A Theoretical Study.

We present a theoretical study on the adsorption and spin transport properties of magnetic Fe@C28 using Ab initio calculations based on spin density functional theory and non-equilibrium Green’s function techniques. Fe@C28 tends to adsorb on the bridge sites in the manner of C–C bonds, and the spin-resolved transmission spectra of Fe@C28 molecular junctions exhibit robust transport spin polarization (TSP). Under small bias voltage, the transport properties of Fe@C28 are mainly determined by the spin-down channel and exhibit a large spin polarization. When compressing the right electrode, the TSP is decreased, but high spin filter efficiency (SFE) is still maintained. These theoretical results indicate that Fe@C28 with a large magnetic moment has potential applications in molecular spintronics.

Experimental and theoretical study of Tetrakis(dimethylamino)ethylene induced magnetism in otherwise nonmagnetic graphene derivatives

Abstract Magnetic nanocarbon offers unique opportunities for future molecular spintronic applications. In such nanocarbons, emergence of magnetization mainly comes from vacancy and adatom. Present work deals with comparison of magnetic interactions in Graphene-like nanocarbons (GNCs) and graphene by adding, tetrakis (dimethyl amino) ethylene (TDAE, (CH3)2N2C = CN2(CH3)2), an organic ferromagnet. Initially, synthesized nano-carbon compounds were functionalized by ∼10 at % of TDAE. Magnetic analysis, by SQUID, showed decrease in saturation level spin density and moments/nm2 in N-doped graphene like nanocarbons (N-GNCs), while enhancement in N-doped graphene (N-graphene). The FC-ZFC study showed significant irreversibility in magnetic anisotropy that obeys anti-ferromagnetic order up to ∼20 K in graphene. Moreover, it exhibits modifications in magnetic environment of molecular carbon due to alteration in centro-symmetry and skewness of central N2C CN2 molecule, as revealed by electron spectroscopy by chemical analysis. The tendency of donor nitrogen to transfer charge is disproportionate and results in acquiring different radical states by N-GNCs (−1) and N-graphene (−2). Computational studies have been carried out for pristine graphene nanoribbons (GNRs), both armchair and zigzag and GNR with monovacancy with and without passivation. Unpassivated zigzag GNR exhibits magnetic moments at the edges but the magnetic moment is quenched at the edge atoms on adsorption of the molecule.

Fullerene/layered antiferromagnetic reconstructed spinterface: Subsurface layer dominates molecular orbitals' spin-split and large induced magnetic moment.

The interfaces between organic molecules and magnetic metals have gained increasing interest for both fundamental reasons and applications. Among them, the C60/layered antiferromagnetic (AFM) interfaces have been studied only for C60 bonded to the outermost ferromagnetic layer [S. L. Kawahara et al., Nano Lett. 12, 4558 (2012) and D. Li et al., Phys. Rev. B 93, 085425 (2016)]. Here, via density functional theory calculations combined with evidence from the literature, we demonstrate that C60 adsorption can reconstruct the layered-AFM Cr(001) surface at elevated annealing temperatures so that C60 bonds to both the outermost and the subsurface Cr layers in opposite spin directions. Surface reconstruction drastically changes the adsorbed molecule spintronic properties: (1) the spin-split p-d hybridization involves multi-orbitals of C60 and top two layers of Cr with opposite spin-polarization, (2) the subsurface Cr atom dominates the C60 electronic properties, and (3) the reconstruction induces a large magnetic moment of 0.58 μB in C60 as a synergistic effect of the top two Cr layers. The induced magnetic moment in C60 can be explained by the magnetic direct-exchange mechanism, which can be generalized to other C60/magnetic metal systems. Understanding these complex hybridization behaviors is a crucial step for molecular spintronic applications.