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| 2. |
- Dankert, André, 1986, et al.
(author)
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High Performance Molybdenum Disulfide Field Effect Transistors with Spin Tunnel Contacts
- 2014
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 8:1, s. 476-482
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Journal article (peer-reviewed)abstract
- Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nanoelectronic, optoelectronic, and spintronic applications. Here, we investigate the field-effect transistor behavior of MoS2 with ferromagnetic contacts to explore its potential for spintronics. In such devices, we elucidate that the presence of a large Schottky barrier resistance at the MoS2/ferromagnet interface is a major obstacle for the electrical spin injection and detection. We circumvent this problem by a reduction in the Schottky barrier height with the introduction of a thin TiO2 tunnel barrier between the ferromagnet and MoS 2. This results in an enhancement of the transistor on-state current by 2 orders of magnitude and an increment in the field-effect mobility by a factor of 6. Our magnetoresistance calculation reveals that such integration of ferromagnetic tunnel contacts opens up the possibilities for MoS 2-based spintronic devices.
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| 3. |
- Dankert, André, 1986, et al.
(author)
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Origin and evolution of surface spin current in topological insulators
- 2018
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 97:12
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Journal article (peer-reviewed)abstract
- The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezin g out the bulk conductivity in semiconducting Bi1.5Sb0.5Te1.7Se1.3 below 100K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2and100K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.
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| 4. |
- Dankert, André, 1986, et al.
(author)
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Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators
- 2015
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 15:12, s. 7976-7981
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Journal article (peer-reviewed)abstract
- Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking of massless Dirac Fermions in their surface states. This helical spin polarization in three-dimensional (3D) TIs has been observed using photoemission spectroscopy up to room temperatures. Recently, spin polarized surface currents in 3D TIs were detected electrically by potentiometric measurements using ferromagnetic detector contacts. However, these electric measurements are so far limited to cryogenic temperatures. Here we report the room temperature electrical detection of the spin polarization on the surface of Bi2Se3 by employing spin sensitive ferromagnetic tunnel contacts. The current-induced spin polarization on the Bi2Se3 surface is probed by measuring the magnetoresistance while switching the magnetization direction of the ferromagnetic detector. A spin resistance of up to 70 mΩ is measured at room temperature, which increases linearly with current bias, reverses sign with current direction, and decreases with higher TI thickness. The magnitude of the spin signal, its sign, and control experiments, using different measurement geometries and interface conditions, rule out other known physical effects. These findings provide further information about the electrical detection of current-induced spin polarizations in 3D TIs at ambient temperatures and could lead to innovative spin-based technologies.
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| 5. |
- Dankert, André, 1986, et al.
(author)
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Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide
- 2017
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 11:6, s. 6389-6395
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Journal article (peer-reviewed)abstract
- The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS2 (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5–2% has been observed, corresponding to spin polarization of 5–10% in the measured temperature range of 300–75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance.
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| 6. |
- Dankert, André, 1986, et al.
(author)
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Spin transport and precession in graphene measured by nonlocal and three-terminal methods
- 2014
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:19, s. 192403 -
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Journal article (peer-reviewed)abstract
- We investigate the spin transport and precession in graphene by using the Hanle effect in nonlocal and three-terminal measurement geometries. Identical spin lifetimes, spin diffusion lengths, and spin polarizations are observed in graphene devices for both techniques over a wide range of temperatures. The magnitude of the spin signals is well explained by spin transport models. These observations rules out any signal enhancements or additional scattering mechanisms at the interfaces for both geometries. This validates the applicability of both the measurement methods for graphene based spintronics devices and their reliable extractions of spin parameters.
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| 7. |
- Dankert, André, 1986, et al.
(author)
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Tunnel Magnetoresistance with Atomically Thin Two‐Dimensional Hexagonal Boron Nitride Barriers
- 2015
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In: Nano Research. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 8:4, s. 1357-1364
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Journal article (peer-reviewed)abstract
- The two-dimensional atomically thin insulator hexagonal boron nitride (h-BN) constitutes a new paradigm in tunnel based devices. A large band gap, along with its atomically flat nature without dangling bonds or interface trap states, makes it an ideal candidate for tunnel spin transport in spintronic devices. Here, we demonstrate the tunneling of spin-polarized electrons through large area monolayer h-BN prepared by chemical vapor deposition in magnetic tunnel junctions. In ferromagnet/h-BN/ferromagnet heterostructures fabricated on a chip scale, we show tunnel magnetoresistance at room temperature. Measurements at different bias voltages and on multiple devices with different ferromagnetic electrodes establish the spin polarized tunneling using h-BN barriers. These results open the way for integration of 2D monolayer insulating barriers in active spintronic devices and circuits operating at ambient temperature, and for further exploration of their properties and prospects.
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| 8. |
- Dash, Saroj Prasad, 1975, et al.
(author)
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Spintronics with Graphene and van der Waals Heterostructures
- 2017
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In: World Scientific. - : WORLD SCIENTIFIC. - 9789813149823 ; , s. 241-258
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Book chapter (other academic/artistic)abstract
- The outstanding spin transport properties of graphene make it an ideal candidate for chip scale spin communication devices. Motivated by this prospect, over the past decade, remarkable progress has been made in enhancing the spin transport parameters in graphene. Apart from simple graphene devices, van der Waals heterostructures of graphene have been fabricated by laminating other two-dimensional crystals with graphene. Such heterostructures of graphene with insulating hexagonal boron nitride (h-BN) as a substrate and gate dielectric or as spin tunnel barrier have been used to achieve efficient spin injection, large spin coherence time and diffusion length in graphene. In this chapter, we present two important advancements in the field of graphene spintronics: First, the recent achievement of long distance spin communication in large scale chemical vapor deposited graphene, and second, the demonstration of enhanced spin injection and spin filtering effects in ferromagnet/h-BN-graphene van der Waals heterostructures. We discuss how these results feature in the present state-of-the art and open new avenues for future developments.
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| 9. |
- Devid, E. J., et al.
(author)
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Spin Transition in Arrays of Gold Nanoparticles and Spin Crossover Molecules
- 2015
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 9:4, s. 4496-4507
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Journal article (peer-reviewed)abstract
- We investigate if the functionality of spin crossover molecules is preserved when they are assembled into an interfacial device structure. Specifically, we prepare and investigate gold nanoparticle arrays, into which room-temperature spin crossover molecules are introduced, more precisely, [Fe(AcS-BPP)(2)](ClO4)(2), where AcS-BPP = (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethioate (in short, Fe(S-BPP)(2)). We combine three complementary experiments to characterize the molecule-nanoparticle structure in detail. Temperature-dependent Raman measurements provide direct evidence for a (partial) spin transition in the Fe(S-BPP)(2)-based arrays. This transition is qualitatively confirmed by magnetization measurements. Finally, charge transport measurements on the Fe(S-BPP)(2)-gold nanoparticle devices reveal a minimum in device resistance versus temperature, R(T), curves around 260-290 K. This is in contrast to similar networks containing passive molecules only that show monotonically decreasing R(T) characteristics. Backed by density functional theory calculations on single molecular conductance values for both spin states, we propose to relate the resistance minimum in R(T) to a spin transition under the hypothesis that (1) the molecular resistance of the high spin state is larger than that of the low spin state and (2) transport in the array is governed by a percolation model.
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| 10. |
- Devid, E. J., et al.
(author)
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The influence of molecular mobility on the properties of networks of gold nanoparticles and organic ligands
- 2014
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In: Beilstein Journal of Nanotechnology. - : Beilstein Institut. - 2190-4286. ; 5:1, s. 1664-1674
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Journal article (peer-reviewed)abstract
- We prepare and investigate two-dimensional (2D) single-layer arrays and multilayered networks of gold nanoparticles derivatized with conjugated hetero-aromatic molecules, i.e., S-(4-{[2,6-bipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl) thiolate (herein S-BPP), as capping ligands. These structures are fabricated by a combination of self-assembly and microcontact printing techniques, and are characterized by electron microscopy, UV-visible spectroscopy and Raman spectroscopy. Selective binding of the S-BPP molecules to the gold nanoparticles through Au-S bonds is found, with no evidence for the formation of N-Au bonds between the pyridine or pyrazole groups of BPP and the gold surface. Subtle, but significant shifts with temperature of specific Raman S-BPP modes are also observed. We attribute these to dynamic changes in the orientation and/or increased mobility of the molecules on the gold nanoparticle facets. As for their conductance, the temperature-dependence for S-BPP networks differs significantly from standard alkanethiol-capped networks, especially above 220 K. Relating the latter two observations, we propose that dynamic changes in the molecular layers effectively lower the molecular tunnel barrier for BPP-based arrays at higher temperatures.
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