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1.
Nano Lett ; 2020 May 19.
Artigo em Inglês | MEDLINE | ID: mdl-32406693

RESUMO

Spin-orbit coupling in graphene can be enhanced by chemical functionalization, adatom decoration, or proximity with a van der Waals material. As it is expected that such enhancement gives rise to a sizable spin Hall effect, a spin-to-charge current conversion phenomenon of technological relevance, it has sparked wide research interest. However, it has only been measured in graphene/transition-metal dichalcogenide van der Waals heterostructures with limited scalability. Here, we experimentally demonstrate the spin Hall effect up to room temperature in graphene combined with a nonmagnetic insulator, an evaporated bismuth oxide layer. The measured spin Hall effect arises most likely from an extrinsic mechanism. With a large spin-to-charge conversion efficiency, scalability, and ease of integration to electronic devices, we show a promising material heterostructure suitable for spin-based device applications.

2.
Adv Mater ; : e1906530, 2020 Jan 24.
Artigo em Inglês | MEDLINE | ID: mdl-31977111

RESUMO

Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale-confined volume modes in thin flakes. On the other hand, surface-confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons-representing typical linear waveguide structures-is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices.

3.
Adv Mater ; 32(8): e1906908, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31944432

RESUMO

The coupling of diverse degrees of freedom opens the door to physical effects that go beyond each of them individually, making multifunctionality a much sought-after attribute for high-performance devices. Here, the multifunctional operation of a single-layer p-type organic device, displaying both spin transport and photovoltaic effect at the room temperature on a transparent substrate, is shown. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device with both bias and light. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin photodetector with zero power consumption in the standby mode. The room-temperature exploitation of the interplay among light, bias, and magnetic field in the single device with a p-type molecule opens a way toward the development of efficient high-performance spin photovoltaic cells.

4.
Nano Lett ; 19(12): 8758-8766, 2019 12 11.
Artigo em Inglês | MEDLINE | ID: mdl-31661967

RESUMO

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration, unusual configurations are expected in low-symmetry transition-metal dichalcogenide semimetals, which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe2 flakes, which are stacked in graphene lateral spin valves. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration.

5.
ACS Appl Mater Interfaces ; 11(13): 12717-12722, 2019 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-30859806

RESUMO

Flexible and wearable devices are among the upcoming trends in the opto-electronics market. Nevertheless, bendable devices should ensure the same efficiency and stability as their rigid analogs. It is well-known that the energy barriers between the metal Fermi energy and the molecular levels of organic semiconductors devoted to charge transport are key parameters in the performance of organic-based electronic devices. Therefore, it is paramount to understand how the energy barriers at metal/organic semiconductor interfaces change with bending. In this work, we experimentally measure the interface energy barriers between a metallic contact and small semiconducting molecules. The measurements are performed in operative conditions, while the samples are bent by a controlled applied mechanical strain. We determine that energy barriers are not sensitive to bending of the sample, but we observe that the hopping transport of the charges in flat molecules can be tuned by mechanical strain. The theoretical model developed for this work confirms our experimental observations.

6.
Nano Lett ; 19(2): 1074-1082, 2019 02 13.
Artigo em Inglês | MEDLINE | ID: mdl-30608710

RESUMO

Graphene is an excellent material for long-distance spin transport but allows little spin manipulation. Transition-metal dichalcogenides imprint their strong spin-orbit coupling into graphene via the proximity effect, and it has been predicted that efficient spin-to-charge conversion due to spin Hall and Rashba-Edelstein effects could be achieved. Here, by combining Hall probes with ferromagnetic electrodes, we unambiguously demonstrate experimentally the spin Hall effect in graphene induced by MoS2 proximity and for varying temperatures up to room temperature. The fact that spin transport and the spin Hall effect occur in different parts of the same material gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Additionally, for a single graphene/MoS2 heterostructure-based device, we evidence a superimposed spin-to-charge current conversion that can be indistinguishably associated with either the proximity-induced Rashba-Edelstein effect in graphene or the spin Hall effect in MoS2. By a comparison of our results to theoretical calculations, the latter scenario is found to be the most plausible one. Our findings pave the way toward the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications.

7.
Chem Sci ; 9(1): 199-208, 2018 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-29629088

RESUMO

A new family of chloroquinolinate lanthanoid complexes of the formula A+[Ln(5,7Cl2q)4]-, with Ln = Y3+, Tb3+ and Dy3+ and A+ = Na+, NEt4+ and K0.5(NEt4)0.5+, is studied, both in bulk and as thin films. Several members of the family are found to present single-molecule magnetic behavior in bulk. Interestingly, the sodium salts can be sublimed under high vacuum conditions retaining their molecular structures and magnetic properties. These thermally stable compounds have been deposited on different substrates (Al2O3, Au and NiFe). The magnetic properties of these molecular films show the appearance of cusps in the zero-field cooled curves when they are deposited on permalloy (NiFe). This indicates a magnetic blocking caused by the interaction between the single-ion magnet and the ferromagnet. X-ray absorption spectroscopy confirms the formation of hybrid states at the molecule/metal interface.

8.
Chemistry ; 24(34): 8524-8529, 2018 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-29570869

RESUMO

Here we report for the first time an efficient fullerene-based compound, FU7, able to act as hole-transporting material (HTM) and electron blocking contact. It has been applied on perovskite solar cells (PSCs), obtaining 0.81 times the efficiency of PSCs with the standard HTM, spiro-OMeTAD, with the additional advantage that this performance is reached without any additive introduced in the HTM layer. Moreover, as a proof of concept, we have described for the first time efficient PSCs in which both selective contacts are fullerene derivatives, to obtain unprecedented "fullerene sandwich" PSCs.

9.
Science ; 359(6378): 892-896, 2018 02 23.
Artigo em Inglês | MEDLINE | ID: mdl-29472478

RESUMO

Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons), which promise opportunities for controlling light in photonic and optoelectronic applications. We developed a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride that supports deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits.

10.
Light Sci Appl ; 7: 17172, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-30839544

RESUMO

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.

11.
Nat Commun ; 8(1): 2198, 2017 12 19.
Artigo em Inglês | MEDLINE | ID: mdl-29259177

RESUMO

The observation of large nonlocal resistances near the Dirac point in graphene has been related to a variety of intrinsic Hall effects, where the spin or valley degrees of freedom are controlled by symmetry breaking mechanisms. Engineering strong spin or valley Hall signals on scalable graphene devices could stimulate further practical developments of spin- and valleytronics. Here we report on scale-invariant nonlocal transport in large-scale chemical vapor deposition graphene under an applied external magnetic field. Contrary to previously reported Zeeman spin Hall effect, our results are explained by field-induced spin-filtered edge states whose sensitivity to grain boundaries manifests in the nonlocal resistance. This phenomenon, related to the emergence of the quantum Hall regime, persists up to the millimeter scale, showing that polycrystalline morphology can be imprinted in nonlocal transport. This suggests that topological Hall effects in large-scale graphene materials are highly sensitive to the underlying structural morphology, limiting practical realizations.

12.
Nat Commun ; 8(1): 661, 2017 09 22.
Artigo em Inglês | MEDLINE | ID: mdl-28939841

RESUMO

Electrical generation and detection of pure spin currents without the need of magnetic materials are key elements for the realization of full electrically controlled spintronic devices. In this framework, achieving a large spin-to-charge conversion signal is crucial, as considerable outputs are needed for plausible applications. Unfortunately, the values obtained so far have been rather low. Here we exploit the spin Hall effect by using Pt, a non-magnetic metal with strong spin-orbit coupling, to generate and detect pure spin currents in a few-layer graphene channel. Furthermore, the outstanding properties of graphene, with long-distance spin transport and higher electrical resistivity than metals, allow us to achieve in our graphene/Pt lateral heterostructures the largest spin-to-charge output voltage at room temperature reported so far in the literature. Our approach opens up exciting opportunities towards the implementation of spin-orbit-based logic circuits and all electrical control of spin information without magnetic field.Spintronic devices with full electrical control rely on electrical generation and detection of spin currents in the absence of magnetic materials. Here, the authors use Pt, a non-magnetic metal, to generate and detect pure spin currents in a few-layer graphene channel, achieving a remarkable spin-to-charge voltage signal at room temperature.

13.
Science ; 357(6352): 677-680, 2017 08 18.
Artigo em Inglês | MEDLINE | ID: mdl-28818941

RESUMO

We fabricated a C60 fullerene-based molecular spin-photovoltaic device that integrates a photovoltaic response with the spin transport across the molecular layer. The photovoltaic response can be modified under the application of a small magnetic field, with a magnetophotovoltage of up to 5% at room temperature. Device functionalities include a magnetic current inverter and the presence of diverging magnetocurrent at certain illumination levels that could be useful for sensing. Completely spin-polarized currents can be created by balancing the external partially spin-polarized injection with the photogenerated carriers.

14.
Nat Commun ; 8(1): 95, 2017 07 21.
Artigo em Inglês | MEDLINE | ID: mdl-28733660

RESUMO

Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range-too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.Here the authors adapt a STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in hexagonal boron nitride, revealing that the electron loss is dominated by hyperbolic phonon polaritons.

15.
Nanoscale ; 9(29): 10178-10185, 2017 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-28517016

RESUMO

Organic field-effect transistors (OFETs) are fundamental building blocks for plastic electronics such as organic photovoltaics or bendable displays with organic light emitting diodes, and radio-frequency identification (RFID) tags. A key part in the performance of OFET is the organic material constituting the channel. OFETs based on solution-processed polymers represent a new class of organic electronic devices. Recent developments in upscale solution-processed polymers have advanced towards high throughput, low-cost, and environmentally friendly materials for high-performance applications. Together with the integration of high performance materials, another enduring challenge in OFET development is the improvement and control of the injection of charge carriers. Graphene, a two-dimensional layer of covalently bonded carbon atoms, is steadily making progress into applications relying on van der Waals heterointerfaces with organic semiconductors. Here, we demonstrate the versatile operation of solution-processed organic transistors both in lateral and vertical geometries by exploiting the weak-screening effect and work function modulation properties of graphene electrodes. Our results demonstrate a general strategy for overcoming traditional noble metal electrodes and to integrate graphene with solution-processed Polyera ActiveInk™ N2200 polymer transistors for high-performance devices suitable for future plastic electronics.

16.
Phys Rev Lett ; 118(14): 147202, 2017 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-28430518

RESUMO

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature that increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to the Pt/YIG bilayer, which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of the NiO layer. We explain the negative SMR by the NiO "spin flop" coupled with YIG, which can be overridden at higher temperature by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

17.
Nat Mater ; 16(5): 507-515, 2017 04 25.
Artigo em Inglês | MEDLINE | ID: mdl-28439116

RESUMO

The miniaturization trend in the semiconductor industry has led to the understanding that interfacial properties are crucial for device behaviour. Spintronics has not been alien to this trend, and phenomena such as preferential spin tunnelling, the spin-to-charge conversion due to the Rashba-Edelstein effect and the spin-momentum locking at the surface of topological insulators have arisen mainly from emergent interfacial properties, rather than the bulk of the constituent materials. In this Perspective we explore inorganic/molecular interfaces by looking closely at both sides of the interface. We describe recent developments and discuss the interface as an ideal platform for creating new spin effects. Finally, we outline possible technologies that can be generated thanks to the unique active tunability of molecular spinterfaces.

18.
Org Lett ; 19(7): 1718-1721, 2017 04 07.
Artigo em Inglês | MEDLINE | ID: mdl-28291368

RESUMO

An approach for the synthesis of pyrene-fused acenes that allows the introduction of electron-withdrawing cyano groups in key positions that simultaneously (i) induce twists in the aromatic framework and (ii) stabilize the LUMO level is reported. This combination of steric and electronic features provide a twisted, stable, and n-type tetrabenzoheptacene as confirmed by a combination of theoretical calculations and optical, electrochemical, thermal, and electrical characterization.

19.
Adv Mater ; 29(19)2017 May.
Artigo em Inglês | MEDLINE | ID: mdl-28295714

RESUMO

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

20.
Nat Nanotechnol ; 12(1): 31-35, 2017 01.
Artigo em Inglês | MEDLINE | ID: mdl-27775727

RESUMO

Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

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