Doctoral College TU-D Unravelling advanced 2D materials
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Localized Intervalley Defect Excitons as Single-Photon Emitters in WSe2

L. Linhart, M. Paur, V. Smejkal, J. Burgdörfer, T. Mueller, and F. Libisch

Phys. Rev. Lett. 123, 146401 (2019)

 Single-photon emitters play a key role in present and emerging quantum technologies. Several recent measurements have established monolayer WSe2 as a promising candidate for a reliable single-photon source. The origin and underlying microscopic processes have remained, however, largely elusive. We present a multiscale tight-binding simulation for the optical spectra of WSe2 under nonuniform strain and in the presence of point defects employing the Bethe-Salpeter equation. Strain locally shifts excitonic energy levels into the band gap where they overlap with localized intragap defect states. The resulting hybridization allows for efficient filling and subsequent radiative decay of the defect states. We identify intervalley defect excitonic states as the likely candidate for antibunched single-photon emission. This proposed scenario is shown to account for a large variety of experimental observations including brightness, radiative transition rates, the variation of the excitonic energy with applied magnetic and electric fields as well as the variation of the polarization of the emitted photon with the magnetic field.

https://doi.org/10.1103/PhysRevLett.123.146401

Charge-Exchange-Driven Low-Energy Electron Splash Induced by Heavy Ion Impact on Condensed Matter

J. Schwestka, A. Niggas, S. Creutzburg, R. Kozubek, R. Heller, M. Schleberger, R. A. Wilhelm, and  F. Aumayr*

J. Phys. Chem. Lett.201910XXX4805-4811(2019)

See also this Press Release (in german) https://www.tuwien.at/tu-wien/aktuelles/news/news/langsame-elektronen-gegen-den-krebs/ !

 

 

Low-energy electrons (LEEs) are of great relevance for ion-induced radiation damage in cells and genes. We show that charge exchange of ions leads to LEE emission upon impact on condensed matter. By using a graphene monolayer as a simple model system for condensed organic matter and utilizing slow highly charged ions (HCIs) as projectiles, we highlight the importance of charge exchange alone for LEE emission. We find a large number of ejected electrons resulting from individual ion impacts (up to 80 electrons/ion for Xe40+). More than 90% of emitted electrons have energies well below 15 eV. This “splash” of low-energy electrons is interpreted as the consequence of ion deexcitation via an interatomic Coulombic decay (ICD) process.

doi.org/10.1021/acs.jpclett.9b01774

Ultrathin calcium fluoride insulators for two-dimensional field-effect transistors

Y. Illarionov, A. Banshchikov, D. Polyushkin, S. Wachter, T. Knobloch, M. Thesberg, L. Mennel, M. Paur, M. Stöger-Pollach, A. Steiger-Thirsfeld, M. Vexler, M. Waltl, N. Sokolov, T. Mueller, and T. Grasser

Nature Electronics 10.1038/s41928-019-0256-8 (2019)

See also this press release: https://www.tuwien.at/en/tu-wien/news/news-articles/news/ultrathin-transistors-for-faster-computer-chips/

 

Two-dimensional semiconductors could be used to fabricate ultimately scaled field-effect transistors and more-than-Moore nanoelectronic devices. However, these targets cannot be reached without appropriate gate insulators that are scalable to the nanometre range. Typically used oxides such as SiO2, Al2O3 and HfO2 are, however, amorphous when scaled, and 2D hexagonal boron nitride exhibits excessive gate leakage currents. Here, we show that epitaxial calcium fluoride (CaF2), which can form a quasi van der Waals interface with 2D semiconductors, can serve as an ultrathin gate insulator for 2D devices. We fabricate scalable bilayer MoS2 field-effect transistors with a crystalline CaF2 insulator of ~2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. Our devices exhibit low leakage currents and competitive device performance characteristics, including subthreshold swings down to 90 mV dec−1, on/off current ratios up to 107 and a small hysteresis.

www.nature.com/articles/s41928-019-0256-8

Pushing the detection of cation nonstoichiometry to the limit

M. Riva, G. Franceschi, Q. Lu, M. Schmid, B. Yildiz, and U. Diebold

Phys. Rev. Materials 3, 043802, (2019)

 

Nanoscale complex-oxide thin films prepared by well-established growth techniques, such as pulsed-laser deposition or molecular-beam epitaxy, often exhibit compositions that deviate from the ideal stoichiometry. Even small variations in composition can lead to substantial changes in the technologically relevant electronic, magnetic, and optical properties of these materials. To assess the reasons behind this variability, and ultimately to allow tuning the properties of oxide films with precise control of the deposition parameters, high-resolution detection of the nonstoichiometry introduced during growth is needed. The resolution of current techniques, such as x-ray diffraction, fluorescence, or spectroscopy, is limited to estimating composition differences in the percent level, which is often insufficient for electronic-device quality. We develop an unconventional approach based on scanning tunneling microscopy for enabling the determination of cation imbalance introduced in thin films with exceptionally small detection limit. We take advantage of the well-controlled surface reconstructions on SrTiO3(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown SrTiO3(110) films. We demonstrate that a <0.1% change in cation nonstoichiometry is detectable by our approach. Furthermore, we show that, for thin films that accommodate all the nonstoichiometry at the surface, this method has no fundamental detection limit.

https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.3.043802

Second harmonic generation in strained transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2, and WSe2

L. Mennel, M. Paur, and T. Mueller

APL Photonics 4, 034404 (2019)

 

Second-harmonic generation (SHG) is a powerful measurement technique to analyze the symmetry properties of crystals. Mechanical strain can reduce the symmetry of a crystal and even weak strain can have a considerable impact on the SHG intensity along different polarization directions. The impact of strain on the SHG can be modeled with a second-order nonlinear photoelastic tensor. In this work, we determined the photoelastic tensors at a fundamental wavelength of 800 nm for four different transition metal dichalcogenide (TMD) monolayers: MoS2, MoSe2, WS2, and WSe2. Strain is applied using a three-point bending scheme, and the polarization-resolved SHG pattern is measured in backscattering geometry. Furthermore, we connected the strain dependent SHG with the strain dependence of the A-exciton energy. With the second-order nonlinear photoelastic tensor, full strain information can be accurately extracted from polarization-resolved SHG measurements. Accordingly, uniaxial strain, induced by polydimethylsiloxan (PDMS) exfoliation and transfer, is measured. We find that TMD monolayers fabricated with PDMS are strained by ∼0.2%. With the experimentally determined nonlinear photoelastic tensors, it will be possible to optically probe arbitrary strain fields in TMD monolayers.

https://doi.org/10.1063/1.5051965

Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors

M. Paur, A. J. Molina-Mendoza, R. Bratschitsch, K. Watanabe, T. Taniguchi, and T. Mueller

Nature Communications 10, 1709 (2019)

Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe2 and WS2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in valleytronics or other novel optoelectronic devices. However, electrically-driven light emission from such exciton species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe2 and WS2 with graphene as electrode. By tailoring the pulse parameters we are able to tune the emission intensity of the different exciton species in both materials. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV.

www.nature.com/articles/s41467-019-09781-y

Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface

F. Kraushofer, Z. Jakub, M. Bichler, J. Hulva, P. Drmota, M. Weinold, M. Schmid, M. Setvin, U. Diebold, and G. S. Parkinson

J. Phys. Chem. C 122, 1657 (2018)

The α-Fe2O3(11̅02) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultrahigh vacuum (UHV) conditions; a stoichiometric (1 × 1) surface can be prepared by annealing at 450 °C in ≈10–6 mbar O2, and a reduced (2 × 1) reconstruction is formed by UHV annealing at 540 °C. The (1 × 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below EF. The images obtained from the (2 × 1) reconstruction cannot be reconciled with previously proposed models, and a new “alternating trench” structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favored in reducing conditions and that 4-fold-coordinated Fe2+ cations at the surface introduce gap states approximately 1 eV below EF. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV.

https://dx.doi.org/10.1021/acs.jpcc.7b10515

Neutralization Dynamics of Slow Highly Charged Ions in 2D Materials

R. Wilhelm, E. Gruber, J. Schwestka, R. Heller, S. Fascko, and F. Aumayr

Applied Sciences 8, 1050 (2018)

We review experimental and theoretical work on the interaction of slow highly charged ions with two-dimensional materials. Earlier work in the field is summarized and more recent studies on 1 nm thick amorphous carbon nanomembranes and freestanding single layer graphene by the authors are reviewed. To explain the findings, models for energy loss determination as well as qualitative model descriptions for the observed ultrafast neutralization dynamics are discussed. The results shown in this paper will be put into context with findings of nanostructure formation on two-dimensional materials, both freestanding and on substrate, as well as on surfaces of bulk insulators.

doi.org/10.3390/app8071050

 

Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide

Nature Communication 9, 3710 (2018)

Michele Riva, Markus Kubicek, Xianfeng Hao, Giada Franceschi, Stefan Gerhold, Michael Schmid, Herbert Hutter, Juergen Fleig, Cesare Franchini, Bilge Yildiz and Ulrike Diebold

Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.

http://dx.doi.org/10.1038/s41467-018-05685-5

 

Optical imaging of strain in two-dimensional crystals

Nature Communications 9, 516 (2018)
Lukas Mennel, Marco M. Furchi, Stefan Wachter, Matthias Paur, Dmitry K. Polyushkin and Thomas Mueller

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows extraction of the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.
https://doi.org/10.1038/s41467-018-02830-y

 

A versatile ion beam spectrometer for studies of ion interaction with 2D materials

Review of Scientific Instruments 89 (2018) 085101 (8 pages)
J. Schwestka, D. Melinc, R. Heller, A. Niggas, L. Leonhartsberger, H. Winter, S. Facsko, F. Aumayr, and R. A. Wilhelm

We present an ultrahigh vacuum setup for ion spectroscopy of freestanding two-dimensional solid targets. An ion beam of different ion species (e.g., Xe with charge states from 1 to 44 and Ar with charge states from 1 to 18) and kinetic energies ranging from a few 10 eV to 400 keV is produced in an electron beam ion source. Ions are detected after their transmission through the 2D target with a position sensitive microchannel plate detector allowing the determination of the ion’s exit charge state as well as the scattering angle with a resolution of approximately 0.04 . Furthermore, the spectrometer is mounted on a swiveling frame covering a scattering angle of ±8 with respect to the incoming beam direction. By utilizing a beam chopper, we measure the time-of-flight of the projectiles and determine the energy loss when passing a 2D target with an energy uncertainty of about 2%. Additional detectors are mounted close to the target to observe emitted secondary particles and are read-out in coincidence with the position and time information of the ion detector. A signal in these detectors can also be used as a start trigger for time-of-flight measurements, which then yield an energy resolution of 1% and an approximately 1000-fold larger duty cycle. First results on the interaction of slow Xe 30+ ions with a freestanding single layer of graphene obtained with the new setup are compared to recently published data where charge exchange and energy were measured by means of an electrostatic analyzer. Published by AIP Publishing.

https://doi.org/10.1063/1.5037798

Adsorption of CO on the Fe3O4 surface

J. Phys. Chem. B 122 (2018) 721 (9 pages)
Jan Hulva, Zdenek Jakub, Zbynek Novotny, Niclas Johansson, Jan Knudsen, Joachim Schnadt, Michael Schmid, Ulrike Diebold, and Gareth S. Parkinson

The interaction of CO with the Fe3O4(001)-(√2 × √2)R45° surface was studied using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), the latter both under ultrahigh vacuum (UHV) conditions and in CO pressures up to 1 mbar. In general, the CO–Fe3O4 interaction is found to be weak. The strongest adsorption occurs at surface defects, leading to small TPD peaks at 115, 130, and 190 K. Desorption from the regular surface occurs in two distinct regimes. For coverages up to two CO molecules per (√2 × √2)R45° unit cell, the desorption maximum shows a large shift with increasing coverage, from initially 105 to 70 K. For coverages between 2 and 4 molecules per (√2 × √2)R45° unit cell, a much sharper desorption feature emerges at ∼65 K. Thermodynamic analysis of the TPD data suggests a phase transition from a dilute 2D gas into an ordered overlayer with CO molecules bound to surface Fe3+ sites. XPS data acquired at 45 K in UHV are consistent with physisorption. Some carbon-containing species are observed in the near-ambient-pressure XPS experiments at room temperature but are attributed to contamination and/or reaction with CO with water from the residual gas. No evidence was found for surface reduction or carburization by CO molecules.
http://pubs.acs.org/doi/full/10.1021/acs.jpcb.7b06349

The role of radiative de-excitation in the neutralization process of highly charged ions interacting with a single layer of graphene

Nucl. Instr. Meth B 422 (2018) 63  (5 pages)
J. Schwestka, R.A. Wilhelm, E. Gruber, R. Heller, R. Kozubek, M. Schleberger, S.Facsko, and F. Aumayr

X-ray emission of slow (<1 a.u.) highly charged Argon and Xenon ions is measured for transmission through a freestanding single layer of graphene. To discriminate against X-ray emission originating from the graphene's support grid a coincidence technique is used. X-ray emission of 75 keV Ar17+ and Ar18+ ions with either one or two K-shell vacancies is recorded. Using a windowless Bruker XFlash detector allows us to measure additionally Ar KLL and KLM Auger electrons and determine the branching ratio of radiative vs. non-radiative decay of Ar K-shell holes. Furthermore, X-ray spectra for 100 keV Xe22+-Xe35+ ions are compared, showing a broad M-line peak for all cases, where M-shell vacancies are present. All these peaks are accompanied by emission lines at still higher energies indicating the presence of a hollow atom during X-ray decay. We report a linear shift of the main M-line peak to higher energies for increasing incident charge state, i.e. increasing number of M-shell holes.
https://doi.org/10.1016/j.nimb.2018.02.022

Accurate modeling of defects in graphene transport calculations    

Phys. Rev. B 97 , 035430 (2018)

L. Linhart, J. Burgdörfer, and F. Libisch   

Figure 2

We present an approach for embedding defect structures modeled by density functional theory into large-scale tight-binding simulations. We extract local tight-binding parameters for the vicinity of the defect site using Wannier functions. In the transition region between the bulk lattice and the defect the tight-binding parameters are continuously adjusted to approach the bulk limit far away from the defect. This embedding approach allows for an accurate high-level treatment of the defect orbitals using as many as ten nearest neighbors while keeping a small number of nearest neighbors in the bulk to render the overall computational cost reasonable. As an example of our approach, we consider an extended graphene lattice decorated with Stone-Wales defects, flower defects, double vacancies, or silicon substitutes. We predict distinct scattering patterns mirroring the defect symmetries and magnitude that should be experimentally accessible.

https://doi.org/10.1103/PhysRevB.97.035430

Interatomic Coulombic Decay - the mechanism for rapid de-excitation of hollow atoms

Physical Review Letters 119 (2017) 13401 (6 pages)
R.A. Wilhelm, E. Gruber, J. Schwestka, R. Kozubek, T.I. Madeira, J.P. Marques, J. Kobus, A.V. Krasheninnikov, M. Schleberger, and F. Aumayr


The impact of a highly charged ion onto a solid gives rise to charge exchange between the ion and target atoms, so that a slow ion gets neutralized in the vicinity of the surface. Using highly charged Ar and Xe ions and the surface-only material graphene as a target, we show that the neutralization and deexcitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple Interatomic Coulombic Decay (ICD) model can describe the observed ultrafast deexcitation. Other deexcitation mechanisms involving nonradiative decay and quasimolecular orbital formation during the impact are not important, as follows from the comparison of our experimental data with the results of first-principles calculations. Our method also enables the estimation of ICD rates directly.

dx.doi.org/10.1103/PhysRevLett.119.103401