Suchergebnisse

We study the spatial distribution of the density of states (DOS) at zero bias N(r) in the mixed state of single and multigap superconductors. We provide an analytic expression for N(r) based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor β-Bi2Pd, we find that N(r) is governed by a length scale ξH=φ0/2πH, which decreases in rising fields. The vortex core size C, defined via the slope of the order parameter at the vortex center, C (dΔ/dr|r→0)-1, differs from ξH by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the 2H-NbSe1.8S0.2 show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor 2H-NbSe2. We fit the tunneling conductance of 2H-NbSe1.8S0.2 to a two gap model and calculate the vortex core size C for each band. We find that C is field independent and has the same value for both bands. We also analyze the two-band superconductor 2H-NbS2 and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands ; The work was supported by the Spanish Ministry of Economy and Competitiveness (FIS2014-54498-R, MAT2014-56143-R, MDM-2014-0377, and MDM2015-0538, Network of Excellence in Molecular Nanoscience MAT2014-52919-REDC), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT- 2850), the Generalidad Valenciana through program Prometeo, and by EU (Cost MP-1201 and COST CA-15128). E.H. acknowledges support of COLCIENCIAS Programa Doctorados en el Exterior Convocatoria 568-2012 and S.M. of MECD: FPU14/04407. M.G. acknowledges the European Union Horizon 2020 Marie Curie Actions under the project SPIN2D (H2020/2014-659378). We acknowledge SEGAINVEX workshop of UAM and Banco Santander. The work of I.G. receives support from Axa Research Fund, FP7-PEOPLE-2013-CIG 618321 and the European Research Council (Grant No. 679080). Work of V.K. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The Ames Laboratory is operated for the U.S. DOE by Iowa State University under Contract No. DE-AC02-07CH11358

arXiv:1608.00311v1.-- et al. ; Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors.Wehere report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure.Weshow that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance. ; E.Y.A, G.L and A L-M acknowledge support from DOE-FG02-99ER45742. This work has received funding from the European Union Seventh Framework Programme under grant agreement 604391 Graphene Flagship. S.R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluna and the Severo Ochoa Program (MINECO SEV-2013-0295). G.A. and O.V.Y. acknowledge support by the Swiss National Science Foundation (grant No. PP00P2_133552) and NCCR Marvel. First-principles calculations have been performed at the Swiss National Supercomputing Centre (CSCS) under project s675. JTF and MB are supported by the Lundbeck Foundation (R95-A10510) and the Center for Nanostructured Graphene (CNG) sponsored by the Danish National Research Foundation (Project DNRF103). ; Peer Reviewed

We describe a three axis vector magnet system for cryogenic scanning probe microscopy measurements. We discuss the magnet support system and the power supply, consisting of a compact three way 100 A current source. We obtain tilted magnetic fields in all directions with maximum value of 5T along z-axis and of 1.2T for XY-plane magnetic fields. We describe a scanning tunneling microscopy-spectroscopy (STM-STS) set-up, operating in a dilution refrigerator, which includes a new high voltage ultralow noise piezodrive electronics and discuss the noise level due to vibrations. STM images and STS maps show atomic resolution and the tilted vortex lattice at 150 mK in the superconductor β-Bi2Pd. We observe a strongly elongated hexagonal lattice, which corresponds to the projection of the tilted hexagonal vortex lattice on the surface. We also discuss Magnetic Force Microscopy images in a variable temperature insert ; This work was supported by Convocatoria Doctorados en el Exterior 568-2012 COLCIENCIAS, the Spanish MINECO (FIS2011-23488, MAT2011-27470-C02-02, CSD2009-00013), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT-2850) and by Marie-Curie actions under the project FP7-PEOPLE-2013- CIG-618321. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. We also acknowledge Banco Santander, COST MP1201. J.A. and C.M. acknowledge the FPI (BES- 2012-058600) and Juan de la Cierva (JCI-2011-08815) programs, respectively

7 pags., 4 figs. ; Light sources on the scale of single molecules can be addressed and characterized at their proper sub-nanometer scale by scanning tunneling microscopy-induced luminescence (STML). Such a source can be driven by defined short charge pulses while the luminescence is detected with sub-nanosecond resolution. We introduce an approach to concurrently image the molecular emitter, which is based on an individual defect, with its local environment along with its luminescence dynamics at a resolution of a billion frames per second. The observed dynamics can be assigned to the single electron capture occurring in the low-nanosecond regime. While the emitter's location on the surface remains fixed, the scanning of the tip modifies the energy landscape for charge injection into the defect. The principle of measurement is extendable to fundamental processes beyond charge transfer, like exciton diffusion. ; A. Rosławska acknowledges support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 771850) and the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement 894434. P. Merino acknowledges support from the A.v. Humboldt Foundation, the ERC Synergy Program (grant ERC-2013-SYG-610256, Nanocosmos), Spanish MINECO (MAT2017-85089-C2-1-R), and the "Comunidad de Madrid" for its support to the FotoArt-CM Project S2018/NMT-4367 through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds.

This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Nano, copyright © American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsnano.0c01837 ; The on-surface synthesis of edge-functionalized graphene nanoribbons (GNRs) is challenged by the stability of the functional groups throughout the thermal reaction steps of the synthetic pathway. Edge fluorination is a particularly critical case in which the interaction with the catalytic substrate and intermediate products can induce the complete cleavage of the otherwise strong C–F bonds before the formation of the GNR. Here, we demonstrate how a rational design of the precursor can stabilize the functional group, enabling the synthesis of edge-fluorinated GNRs. The survival of the functionalization is demonstrated by tracking the structural and chemical transformations occurring at each reaction step with complementary X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements. In contrast to previous attempts, we find that the C–F bond survives the cyclodehydrogenation of the intermediate polymers, leaving a thermal window where GNRs withhold more than 80% of the fluorine atoms. We attribute this enhanced stability of the C–F bond to the particular structure of our precursor, which prevents the cleavage of the C–F bond by avoiding interaction with the residual hydrogen originated in the cyclodehydrogenation. This structural protection of the linking bond could be implemented in the synthesis of other sp2-functionalized GNRs ; C.M. was supported by the Agency for Management of University and Research grants (AGAUR) of the Catalan government through the FP7 framework program of the European Commission under Marie Curie COFUND action 600385 funded by the CERCA Program/Generalitat de Catalunya. We acknowledge support from the Spanish Ministry of Economy and Competitiveness, MINECO (under Contracts Nos. MAT2016-78293-C6 ...

Glaser‐like coupling of terminal alkynes by thermal activation is extensively used in on‐surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser‐like coupling between terminal alkyne carbons within a custom‐synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway ; We thank the European Union (Project SPRING, contract no. 863098), the ERC grant AMSEL (682144), the Spanish Agencia Estatal de Investigación (MAT2016-78293-C6-3-R and CTQ2016-78157-R), Xunta de Galicia (Centro de Investigación de Galicia accreditation 2019–2022, ED431G 2019/03) and the EuropeanRegional Development Fund-ERDF for financial support ; SI

The Engineering and Physical Sciences Research Council (EPSRC) is acknowledged for the funding of FG (EP/M029077/1) and RM (EP/506631/1). FG also acknowledges the European Union for funding the SURMOF project under which part of this work was undertaken (contract number NMP4-CT-2006-032109). ; The structure and surface chemistry of ultrathin metallic films of one metal on another are strongly influenced by factors such as lattice mismatch and the formation of near-surface alloys. New morphologies may result in modified chemical properties which in turn open up different routes for molecular adsorption, desorption and surface functionalization, with important consequences in several fields of application. The Cu/Au(111) system has received the attention of many studies, only a few however have been performed in ultra-high vacuum (UHV), using surface sensitive techniques. In this contribution, the room temperature deposition of copper onto the (22×√3)-Au(111) surface, from submonolayer to thick film, is investigated using scanning tunnelling microscopy (STM). The onset of copper adsorption is seen to occur preferentially at alternate herringbone elbows, with a preference for hcp sites. With increasing coverage, copper-rich islands exhibit a reconstructed surface reminiscent of the clean Au(111) herringbone reconstruction. Disordered, pseudo-ordered and ordered surface layers are observed upon annealing. Models for the initial adsorption/incorporation mechanism, formation of adlayers and evolution with increasing coverage and annealing are qualitatively discussed. Further, the reactivity of copper-doped Au(111) systems is considered towards the adsorption of organic molecules of interest in nanotechnology and their catalytic applications. ; Publisher PDF ; Peer reviewed

Scanning electron microscopy (SEM) is a powerful imaging technique able to obtain astonishing images of the micro- and the nano-world. Unfortunately, the technique has been limited to vacuum conditions for many years. In the last decades, the ability to introduce water vapor into the SEM chamber and still collect the electrons by the detector, combined with the temperature control of the sample, has enabled the study of ice at nanoscale. Astounding images of hexagonal ice crystals suddenly became real. Since these first images were produced, several studies have been focusing their interest on using SEM to study ice nucleation, morphology, thaw, etc. In this paper, we want to review the different investigations devoted to this goal that have been conducted in recent years in the literature and the kind of information, beyond images, that was obtained. We focus our attention on studies trying to clarify the mechanisms of ice nucleation and those devoted to the study of ice dynamics. We also discuss these findings to elucidate the present and future of SEM applied to this field. ; A.V. acknowledges financial support by the Spanish Government under Project No. PID2019-110907GB-I00 and the "Severo Ochoa" Program for Centers of Excellence in R&D (CEX2019-000917-S). ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

We report the on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups. The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor. Whereas many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings. Scanning tunneling spectroscopy and density functional theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon and introducing deep impurity levels associated with the nitrogen electron lone pairs. ; This work was supported by FP7 FET-ICT "Planar Atomic and Molecular Scale devices" (PAMS) project (funded by the European Commission under Contract No. 610446), by the Agencia Estatal de Investigacion (Cooperative Grant No. MAT2016-78293 and Grant FIS2015-62538-ERC), the Basque Government (Dep. de Educacion and UPV/EHU, Grant No. IT-756-13, and Dep. Industry, Grant PI_2015_1_42), the Xunta de Galicia (Centro singular de investigacion de Galicia accreditation 2016−2019, ED431G/09), and the European Regional Development Fund (ERDF). ; Peer Reviewed

Airborne fine and ultrafine particles (aerodynamic diameters less than 2.5 and 0.1 μm respectively) have in recent years been recognized as major concerns for public health, due to their ability to penetrate deep into the lungs or even into the blood stream. Recent studies have furthermore found that parameters such as morphology, size, number concentration, surface area, as well as the chemical composition of particles have a high influence on the observed toxicological effects. However current aerosol legislations focus only on aerosol mass, meaning that small particles are poorly regulated due to their small contribution to the total aerosol mass concentration. New or additional legislations and measurement techniques are therefore needed to deal with the complexity of aerosols in order to establish standard procedures for measuring and regulating aerosol exposure. Here we present the current development of an automated software-based analysis of aerosols using Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS). The automated analysis will be capable of providing both detailed physical and chemical single particle information not provided by the current standard methods. Physical parameters such as area, diameter, aspect ratio, aggregation state, and estimates of surface area will be obtained for each individual particle based on the acquired images, while automated EDS analysis will yield single particle elemental composition data, allowing size resolved chemical classification of each individual particle. The automated analysis will furthermore be able to systematically map large areas of a sample without user intervention, enabling a fast and repeatable measurement, while obtaining sufficient data for statistical analysis.

This thesis is focused on combining the atomically resolved microscopic picture with the ultrafast sub picosecond picture of laser induced phase transitions in 1T-TaS2. The combination of scanning tunneling microscopy (STM) and pump-probe measurements, gives insight not only into the microscopic ordering and topological nature of different laser induced states in 1T-TaS2, but also into the conditions at which they are formed and the timescales at which they appear. The layered transition metal dichalcogenide 1T-TaS2 has undergone a recent resurgence of interest with the discovery of a stable hidden state, which can be induced by an ultrashort laser or electrical pulse. Since the transition is fast, the material became a very promising candidate for future cryogenic memory elements. I performed multiple excitation intensity dependent switching experiments at different temperatures in order to construct a phase diagram of photoinduced states in 1T-TaS2. It turns out that optical switching to the hidden state is very robust, while on the other hand switching to the other observed states cannot be controllably reproduced using an 800 nm laser. The threshold fluence for switching to the hidden state appears to be temperature independent, suggesting that only the density of the photoinduced carriers is important for the transition. Other photoinduced states such as the amorphous state and states with different chiralities seem to appear more randomly. At high laser powers, I discovered a novel kind of laser induced transformation that is a single layer polytype transformation from 1T to 1H polytype, where local heating and fast quench play a crucial role in formation of the new state. The polytype transformed layer shows a phase transition between the stripe phase and a network of hexagonal vertices, as seen with STM, which show interesting topological nature. I also examined the domain walls between the domains of different chiralities in 1T-TaS2, which appear to have a non-trivial structure. In order to combine the ultrafast measurements with atomically resolved STM scans, I built a prototype THz gated STM. I built and characterized several THz single cycle pulse sources and used them to excite the STM tip. I observed up to 20 pA of rectified THz current, which is expected enough to observe changes in time resolved measurements. The device was not finalized during my research work, but I am nevertheless showing the interesting and helpful experimental observations in order to make the future optimization of the THz resolved STM easier. ; Doktorsko delo je posvečeno povezavi med atomsko ločljivimi mikrokopskimi slikami in ultrahitrimi pod pikosekundnimi meritvami lasersko vzbujenih faznih prehodov v 1T-TaS2. Kombinacija vrstične tunelske mikroskopije (STM) in vzbuditveno testne tehnike nam da vpogled ne le v mikroskopsko sliko urejanja in topološko naravo različnih lasersko vzbujenih stanj, temveč tudi v pogoje pod katerimi se ta stanja pojavijo in v časovno skalo na kateri se oblikujejo. 1T-TaS2 je plastoviti dihalkogenid prehodnih kovin, ki je vzbudil veliko zanimanje z odkritjem stabilnega skritega stanja, ki ga lahko vzbudimo z ultrahitrim laserskim ali električnim sunkom. Ker je prehod hiter, je 1T-TaS2 postal zelo dober kandidat za ultrahitre nizkotemperaturne spominske elemente prihodnosti. Izvedel sem več meritev, kjer sem spreminjal moč in fluenco laserskega snopa pri različnih temperaturah z namenom sestaviti fazni diagram fotoinduciranih stanj v 1T-TaS2. Izkaže se, da je optični preklop v skrito stanje zelo robusten in da je mejna fluenca za preklop enaka pri vseh temperaturah. Po drugi strani pa je preklop med ostalimi stanji težko ponovljiv pri uporabi laserja z valovno dolžino 800 nm. Tako se amorfno stanje in stanja z domenami različnih kiralnosti pri preklapljanju pojavljajo naključno. Pri uporabi visokih moči laserja sem opazil novo vrsto laserske transformacije v kristalu, in sicer gre za politipno transformacijo vrhnje plasti materiala iz 1T v 1H politip, kjer ključno vlogo igra lokalno segrevanje in hitro ohlajanje. Na transformirani površini lahko opazujemo fazni prehod med fazama s črtami in trikotno mrežo, ki kažeta zanimive topološke lastnosti. Podrobneje sem preučil tudi domenske stene med domenami različne kiralnosti, ki nimajo trivialne strukture. Z namenom združiti ultrahitre meritve z atomsko ločljivimi STM slikami sem sestavil teraherčno (THz) prožen STM. Sestavil in karakteriziral sem nekaj virov ultrahitrih teraherčnih sunkov in jih uporabil za vzbujanje konice tunelskega mikroskopa. Opazil sem do 20 pA usmerjenega THz toka, kar je dovolj za opazovanje sprememb v časovno ločljivih meritvah. Naprava v času mojega dela ni bila popolnoma dokončana. Izsledki so kljub temu predstavljeni kot pomoč pri nadaljnji izgradnji in optimizaciji naprave.

Open-shell graphene nanoribbons have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone effectively adds one pz electron to the π-electron network, producing an unpaired π-radical. A similar scenario occurs for regular ketone-functionalized chGNRs in which one ketone is missing. Two such radical states can interact via exchange coupling, and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding ribbon, that is, with or without ketone functionalization. Besides, we determine the parameters whereby this type of system with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide insight to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties. ; We acknowledge financial support from the MCIN/AEI/10.13039/501100011033 (Grants PID2019-107338RB-C62, PID2019-107338RB-C63, PID2020-115406GB-I00, PCI2019-111933-2 (FLAG-ERA grant LEGOCHIP), and Juan de la Cierva Grant FJC2019-041202-I), the European Union's Horizon 2020 research and innovation program (FET-OPEN project SPRING, Grant No. 863098 and Marie Skłodowska-Curie Actions Individual Fellowship No. 101022150), the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019-2022, Grant ED431G2019/03), the European Regional Development Fund, the Basque Government (IT-1255-19), the Basque Government (PIBA Grant PI_2020_1_0014), the Basque Departamento de Educación through the Ph.D. scholarship No. PRE_2020_2_0049 (S.S.), the Spanish Research Council (LINKC20002). ; Peer reviewed

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes ; Recent advances in graphene-nanoribbon-based research have demonstrated the controlled synthesis of chiral graphene nanoribbons (chGNRs) with atomic precision using strategies of on-surface chemistry. However, their electronic characterization, including typical figures of merit like band gap or frontier band's effective mass, has not yet been reported. We provide a detailed characterization of (3,1)-chGNRs on Au(111). The structure and epitaxy, as well as the electronic band structure of the ribbons, are analyzed by means of scanning tunneling microscopy and spectroscopy, angle-resolved photoemission, and density functional theory ; The project leading to this publication has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 635919), from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant nos. MAT2016-78293-C6, FIS2015-62538-ERC), from the Basque Government (grant nos. IT-621-13, PI-2015-1-42, PI-2016-1-0027), from the European Commission in FP7 FET-ICT "Planar Atomic and Molecular Scale Devices" (PAMS) project (contract no. 610446), from the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016−2019, ED431G/09), and from the European Regional Development Fund (ERDF)