Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities


Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Article: published in ACS Nano by César González, IFIMAC researcher and member of the Department of Theoretical Condensed Matter Physics.

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron−hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2 -a natural one and one prepared at high pressure and high temperature- and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials. [Full article]




Fabrication of Gold Nano-Crystal Arrays for Molecular Electronics: High Frequency Molecular Rectifiers and π-π Inter Molecular Interaction Energy


Fabrication of Gold Nano-Crystal Arrays for Molecular Electronics

Title: Fabrication of Gold Nano-Crystal Arrays for Molecular Electronics: High Frequency Molecular Rectifiers and π-π Inter Molecular Interaction Energy.
When: Monday, September 18, (2017), 12:00.
Place: Departamento de Física Teórica de la Materia Condensada, Facultad Ciencias, Module 5, Seminar Room (5th Floor).
Speaker: Dr. Jorge Trasobares, National University of Singapore, Singapore.

E-beam lithography was used for versatile fabrication of sub-15 nm single-crystal gold nanoarrays at wafer-scale by the so-called dot on the fly (DOTF) technique [1]. Here DOTF and other methods are compared evidencing the limiting factors for the writing speed. Wafer-scale fabrication of such arrays with 50 nm pitch allowed XPS analysis of a ferrocenylalkyl thiol self- assembled monolayer coated gold nanoarray. We exploit these arrays as a suitable test bed for Molecular Electronics (ME) [2] and propose two studies on high frequency molecular rectifiers [3] and inter molecular Interactions [4].

In a first study, we demonstrate molecular diodes operating up to 17.8 GHz. Direct current and radio frequency (RF) properties were simultaneously measured with the tip of an interferometric scanning microwave microscope and S11 parameters show a diode rectification ratio of 12 dB. In a second investigation, we explore the π-π intermolecular interactions. This factor is one of the most important to optimize the transport and optical properties of organic transistors, light-emitting diodes or (bio-) molecular devices. Electrochemical measurements indicate two different phases localized on top and facets of the nanocrystals with clear intermolecular interactions and electrical current statistics on ~3000 molecular junctions confirm the theoretical prediction [5] of asymmetrical histograms due to cooperative effects.

References

  1. J. Trasobares, F. Vaurette, M. François, H. Romijn, J-L. Codron, D. Vuillaume, D. Théron and N. Clément. High speed e-beam lithography for gold nanoarray fabrication and use in nanotechnology. Beilstein J. Nanotechnol. 5, 1918–1925 (2014).
  2. N. Clement, G. Patriarche, K, Smaali, F. Vaurette, K. Nishiguchi, D. Troadec, A. Fujiwara, D. Vuillaume. Large array of sub-10-nm single-grain Au nanodots for use in nanotechnology. Small, 7, 2607 (2011).
  3. J. Trasobares, D. Vuillaume, D. Théron, N. Clement, A 17 GHz Molecular Rectifier, Nat.Commun. 7, 12850 (2016).
  4. J. Trasobares, J. Rech, T. Jonckeere, T. Martin, O. Aleveque, E. Levillain, V. Diez-Cabanes, Y. Olivier, J. Cornil, J.P. Nys, R. Sivakumarasamy, K. Smaali, Ph. Leclère, A. Fujiwara, D. Théron, D. Vuillaume, N. Clément. Estimation of π-π Electronic Couplings from Current Measurements. Nano Letters, 17, 3215-3224 (2017).
  5. M.G. Reuter, M.C. Hersam, T. Seideman, M.A. Ratner. Signatures of cooperative effects and transport mechanisms in conductance histograms. Nano Letters, 12, 2243-2248 (2012).



XXIV International Summer School Nicolás Cabrera - 2017


XXIV International Summer School Nicolás Cabrera – 2017

When: September 4-8, 2017

Where: Residencia La Cristalera, Miraflores de la Sierra, Madrid, Spain

Contacts:

Inés Ortiz – e-mail: school@nicolascabrera.es
Manuela Moreno – e-mail: manuela.moreno@uam.es

Organizers:

Eduardo Lee, IFIMAC, UAM.
Elsa Prada, IFIMAC, UAM.
Alfredo Levy Yeyati, IFIMAC, UAM

School Scopes and Goals

Topological materials constitute an exciting and very active research area in condensed matter physics. It studies new states of matter whose bulk properties are similar to those of ‘ordinary’ materials but that, at the same time, display edge or boundary states with very exotic properties. Since the discovery of topological insulators, roughly a decade ago, the field has rapidly expanded with the identification of other topological materials, such as topological superconductors and Weyl semimetals. This Summer School will gather leading international experts to provide an introduction to the basic concepts underlying topology in condensed matter systems, followed by a discussion of recent developments, with a focus on quantum transport and hybrid devices. The goal is to cover not only theoretical aspects, but to also address the experimental progress, including the detection and manipulation of states associated with these materials.

School Topics

Invited Speakers

Organized with the collaboration of

 

 

 

Further information:

Nicolás Cabrera Institute – INC website

XXIV International Summer School Nicolás Cabrera in the newspapers – Atlasagencia




Postdoctoral Position at UAM-IFIMAC in Electron Transport Through Proteins and Peptides – Closed


Postdoctoral Position at UAM-IFIMAC in Electron Transport Through Proteins and Peptides

A postdoctoral position is available at UAM (Universidad Autónoma de Madrid) and funded by the Condensed Matter Physics Center – IFIMAC. The candidate will work under the supervision of Prof. Juan Carlos Cuevas and Dr. Linda A. Zotti on the theory of electron transport through proteins and peptides. Funding is available for 18 months.
The successful applicant will carry out theoretical simulations on the electron transport through proteins and peptides by means of Density Functional Theory (DFT) calculations, Non-Equilibrium Green’s Function Techniques and tight-binding models. It will be based on a close cooperation with experimental partners.
Applicants are invited to send a cover letter, a curriculum vitae and contact details of 2 referees who may be contacted to Dr. Linda A. Zotti (linda.zotti(at)uam.es)
Applications will be accepted until the position is filled, but those received before the 20th of July 2017 will be guaranteed full consideration.

Requirements:

For further information please visit IFIMAC’s website.




Enhancing Radiative Heat Transfer With Silicon Metasurfaces


Silicon metasurfaces made of 2D periodic arrays of square holes.

Silicon metasurfaces made of 2D periodic arrays of square holes.

Article: published in Physical Review Letters by Víctor Fernández Hurtado, Francisco J. García Vidal and Juan Carlos Cuevas, IFIMAC researchers and members of the Department of Theoretical Condensed Matter Physics.

Thermal radiation is a universal physical phenomenon of great importance for different disciplines of science and engineering. In recent years, there has been a renewed interest in this topic due to the discovery that radiative heat transfer between two bodies can be drastically enhanced if they are brought sufficiently close to each other. This enhancement, which occurs when the separation is smaller than the thermal wavelength (10 microns at room temperature), is due to the contribution of evanescent waves that dominate the near-field regime. The fact that this near-field radiative heat transfer (NFRHT) between closely spaced bodies can overcome the far-field limit set by the Stefan-Boltzmann law for black bodies has now been verified in a variety of experiments exploring different materials, geometrical shapes, and gaps ranging from micrometers to a few nanometers.

In this context, the question on the fundamental limits of thermal emission is attracting a lot of attention. So far, the largest NFRHT enhancements have been reported for polar dielectrics (SiC, SiO2, SiN, etc), in which thermal radiation is dominated by surface phonon polaritons. Now, in a work published in Physical Review Letters, the IFIMAC researchers Víctor Fernández Hurtado, Francisco J. García Vidal and Juan Carlos Cuevas, together with Professor Shanhui Fan (Stanford University), have shown that metasurfaces of doped silicon can be used to boost NFRHT. In particular, they demonstrate that one can design silicon metasurfaces that not only exhibit a room-temperature NFRHT much larger than that of bulk Si or other proposed periodic structures, but they also outperform the best unstructured polar dielectric. The underlying physical mechanisms responsible for this striking behavior are the existence of broadband spoof surface-plasmon polaritons (SPPs) in doped silicion and the ability to tune via nanostructuration the dispersion relation of these SPPs that dominate NFRHT in this structure. This work illustrates the great potential of metasurfaces for the field of radiative heat transfer. [Full article]




Molecular and Biomolecular Electron Transfer Processes: From the Single Molecule to the Cellular Length Scales


Molecular and Biomolecular Electron Transfer Processes: From the Single Molecule to the Cellular Length Scales

Title: Molecular and Biomolecular Electron Transfer Processes: From the Single Molecule to the Cellular Length Scales.
When: Tuesday, May 23, (2017), 12:00.
Place: Departamento de Física Teórica de la Materia Condensada, Facultad Ciencias, Module 5, Seminar Room (5th Floor).
Speaker: Prof. Spiros S. Skourtis, Department of Physics, University of Cyprus, Nicosia Cyprus.

Molecular electron transfer processes are ubiquitous in biology and chemistry and are central to the molecular electronics and energy materials technologies. Biological electron transfer mechanisms are particularly rich, ranging from coherent tunneling to incoherent thermally-activated hopping. I will give a review of recent trends in the theory and simulation of biomolecular electron transfer rates, focusing on the roles of electronic coupling and energy level fluctuations. I will also discuss electron-transport pathway control over length scales that range from the small-molecule to the cellular levels.

More information on IFIMAC Website




Bottom‐up Nanoelectronics: Contacting Single Molecules and Nanoparticles


Bottom‐up Nanoelectronics: Contacting Single Molecules and Nanoparticles

Title: Bottom‐up Nanoelectronics: Contacting Single Molecules and Nanoparticles.
When: Monday, May 22, (2017), 12:00.
Place: Departamento de Física de la Materia Condensada, Facultad Ciencias, Module 3, Seminar Room (5th Floor).
Speaker: Herre S.J. van der Zant, Delft University of Technology, Universidad Autonoma de Madrid and IMDEA Nanociencia Madrid.

Our research focus is on bottom-up nanoelectronics and in particular the electronic characterization of single molecules and nanoparticles for device applications. For this purpose, we employ several methods to create electrodes, such as direct e-beam patterning, electromigration of Au wires, electroburning of multilayer graphene flakes, (gateable) mechanically-controllable break junctions (MCBJs) and a self-aligned fabrication technique for fabricating nano-spaced electrodes over large lengths. Typical experiments consist of measuring current-voltage characteristics as a function of various external stimuli such as electrode separation, gate voltage, temperature, and/or magnetic field. In this talk, I will discuss experiments on spincrossover nanoparticles and molecules, protein networks, biological nanowires and the use of superconducting electrodes as a new direction to study Shiba states in one-level quantum dot systems.

More information on IFIMAC Website




Quantized Thermal Transport in Single-Atom Junctions


Artist view of quantized heat transport in a gold single-atom contact.

Artist view of quantized heat transport in a gold single-atom contact.

Article: published in Science by Juan Carlos Cuevas, IFIMAC researcher and member of the Department of Theoretical Condensed Matter Physics.

What does determine the heat flow through a single atom? This is the ultimate question in the field of nanoscale energy transport and its answer is crucial to establish the fundamental laws that should describe the thermal transport in a variety of nanoelectronic devices. In the context of electrical circuits, the atomic scale was first reached with the advent of metallic atomic-size contacts and single-molecule junctions in the 1990s. These systems constitute the ultimate limit of miniaturization and have emerged as an ideal playground to investigate quantum effects related to charge and energy transport. Thus for instance, in recent years it has been shown that transport properties of metallic atomic-size contacts such as the electrical conductance, shot noise, thermopower, or Joule heating are completely dominated by quantum effects, even at room temperature. However, the experimental study of thermal conduction in these atomic-scale systems continues to be a formidable challenge and it has remained elusive to date in spite of its fundamental interest.

This basic open problem has now been resolved in a work published in Science by a collaboration between the groups of Pramod Reddy and Edgar Meyhofer (University of Michigan), Fabian Pauly and Peter Nielaba (University of Konstanz), and the IFIMAC researcher Juan Carlos Cuevas. In this work, the authors made use of custom-designed picowatt-resolution calorimetric scanning probes to measure simultaneously the electrical and thermal conductance of gold and platinum atomic contacts all the way down to the single-atom level. This study reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature in units of the universal thermal conductance quantum. It also shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts, irrespective of the metal. Furthermore, this work shows that all these observations can be quantitatively explained within the Landauer picture for quantum coherent thermal transport. In particular, this theory clarifies that the observations described above are due to the fact that electrons dominate the thermal conductance in these metallic nanowires, and in the gold case electrons proceed ballistically through the contacts via fully open conduction channels.

The experimental techniques developed in this work will enable systematic studies of thermal transport in atomic chains and molecular junctions, which is key to investigating numerous fundamental issues that have remained inaccessible despite great theoretical interest. [Full article]

Quantized Thermal Transport in Single-Atom Junctions




Radiative Heat Transfer in Ångström and Nanometer-sized Gaps


A gold-coated scanning thermal microscopy probe is brought into close proximity of a heated gold substrate.

A gold-coated scanning thermal microscopy probe is brought into close proximity of a heated gold substrate.

Article: published in Nature Communications by Víctor Fernández-Hurtado, Johannes Feist, Francisco J. García-Vidal and Juan Carlos Cuevas, Department of Theoretical Condensed Matter Physics and IFIMAC researchers.

Radiative heat transfer between closely placed objects is attracting a lot attention for several reasons. First, recent experiments have finally verified the long-standing prediction that radiative heat transfer can be greatly enhanced over the classical far-field limit set by the Stefan-Boltzmann law for blackbodies if the gap between two objects is smaller than the thermal wavelength, which is of the order of 10 µm at room temperature. This is possible due to the contribution of the near field in the form of evanescent waves (or photon tunneling). Second, this confirmation has triggered the hope that near-field radiative heat transfer could have an impact in different technologies that make use of thermal radiation such as thermophotovoltaics, thermal management, lithography, data storage, and thermal microscopy.

In spite of the progress made in recent years in the understanding of thermal radiation at the nanoscale, several recent experiments exploring the radiative thermal transport in nanometric gaps have seriously questioned this understanding. In particular, measurements on two gold-coated surfaces with gap sizes in the range of 0.2-10 nm have suggested an extraordinarily large near-field enhancement more than 3 orders of magnitude larger than the predictions of fluctuational electrodynamics, which is presently the standard theory used for the description of near-field thermal radiation.

A possible solution to this puzzle has now been proposed in a work published in Nature Communications by a collaboration between the groups of Pramod Reddy and Edgar Meyhofer (University of Michigan) and IFIMAC researchers Víctor Fernández-Hurtado, Johannes Feist, Francisco J. García-Vidal and Juan Carlos Cuevas. In this work, the authors explore the radiative heat transfer in Ångström- and nanometer-sized gaps between an Au-coated scanning thermal microscopy probe and a planar Au substrate in an ultrahigh vacuum environment. Using the apparent tunneling barrier height as a measure of cleanliness, it was found that upon systematically cleaning via
plasma-cleaning or locally pushing the tip into the substrate by a few nanometers, the observed radiative conductances decreased from unexpectedly large values to extremely small ones-below the detection limit of the probe-as expected from computational results obtained within the framework of fluctuational electrodynamics. These results suggest that the huge signal reported in recent experiments might be an artifact due to the presence of contaminants bridging the gap between the tip and the substrate, thus providing an additional path for heat transfer via conduction.  Moreover, this work shows that it is possible to avoid the confounding effects of surface contamination and systematically study thermal radiation in Ångström- and nanometer-sized gaps. [Full article]

References

  1. Study of Radiative Heat Transfer in Ångström and Nanometre Sized Gaps, L. Cui, W. Jeong, V. Fernández-Hurtado, J. Feist, F.J. García-Vidal, J.C. Cuevas, E. Meyhofer, and P. Reddy, Nature Communications 8, 14479, (2017). [URL]



Unexpected Phenomena In The Quantum Transport Through Carbon Nanotubes


Unexpected Phenomena In The Quantum Transport Through Carbon NanotubesTitle: Unexpected Phenomena In The Quantum Transport Through Carbon Nanotubes.
When: Tuesday, February 14, (2017), 12:30.
Place: Departamento de Física de la Materia Condensada, Facultad Ciencias, Module 3, Seminar Room (5th Floor).
Speaker: Christoph Strunk, Regensburg University, Germany.

Carbon nanotubes have reached a quality that allows for stringent tests of theory. Applying high magnetic fields we perform transport spectroscopy on the first excess electron above the band gap. The observed single particle spectra allow to quantitatively probe the fine structure corrections to the simple Dirac Hamiltonian. The results only super¬ficially agree with expectations based upon accepted models. In parti-cular, we find an unexpected orbital degeneracy of the ground state, and a mis¬match of the orbital magnetic moments extracted from low and high magnetic field regimes.

In addition, the line intensities strongly vary, if the magnetic field shifts the levels across the Dirac cone. This effect can be traced back to deviations from the standard ‘particle in a box’ boundary conditions that apply to the bipartite graphene lattice. The boundary conditions couple the longitudinal and radial parts of the electronic wave functions and drastically affect the transmission amplitude in magnetic field.

Finally, we trace the signatures of the trigonal warping of the Dirac cone at higher energies in the Fabry-Perot-like interference pattern at the highly transmissive hole side of the spectrum. These can be exploited to determine the tube’s chiral angle from transport measurements.

More information on IFIMAC Website