Publications

DAMDID/RCDL 2016, 706, 248-262 (2017)
Metadata for Experiments in Nanoscience Foundries
Metadata is a key aspect of data management. This paper describes the work of NFFA-EUROPE project on the design of a metadata standard for nanoscience, with a focus on data lifecycle and the needs of data practitioners who manage data resulted from nanoscience experiments. The methodology and the resulting high-level metadata model are presented. The paper explains and illustrates the principles of metadata design for data-intensive research. This is value to data management practitioners in all branches of research and technology that imply a so-called “visitor science” model where multiple researchers apply for a share of a certain resource on large facilities (instruments).
Link to publication
Acta Biomaterialia 51, 21–52 (2017)
Controlling the morphology and outgrowth of nerve and neuroglial cells: The effect of surface topography
Unlike other tissue types, like epithelial tissue, which consist of cells with a much more homogeneous structure and function, the nervous tissue spans in a complex multilayer environment whose topographical features display a large spectrum of morphologies and size scales. Traditional cell cultures, which are based on two-dimensional cell-adhesive culture dishes or coverslips, are lacking topographical cues and mainly simulate the biochemical microenvironment of the cells. With the emergence of micro- and nano-fabrication techniques new types of cell culture platforms are developed, where the effect of various topographical cues on cellular morphology, proliferation and differentiation can be studied. Different approaches (regarding the material, fabrication technique, topographical characteristics, etc.) have been implemented. The present review paper aims at reviewing the existing body of literature on the use of artificial micro- and nano-topographical features to control neuronal and neuroglial cells’ morphology, outgrowth and neural network topology. The cell responses–from phenomenology to investigation of the underlying mechanisms- on the different topographies, including both deterministic and random ones, are summarized.
Link to publication
J. Vac. Sci. Technol. B 34, 06K702 (2016)
Comparative study of resists and lithographic tools using the Lumped Parameter Model
A comparison of the performance of high resolution lithographic tools is presented here. The authors use extreme ultraviolet interference lithography, electron beam lithography, and He ion beam lithography tools on two different resists that are processed under the same conditions. The dose-to-clear and the lithographic contrast are determined experimentally and are used to compare the relative efficiency of each tool. The results are compared to previous studies and interpreted in the light of each tool-specific secondary electron yield. In addition, the patterning performance is studied by exposing dense lines/spaces patterns, and the relation between critical dimension and exposure dose is discussed. Finally, the lumped parameter model is employed in order to quantitatively estimate the critical dimension of lines/spaces, using each tool specific aerial image. Our implementation is then validated by fitting the model to the experimental data from interference lithography exposures and extracting the resist contrast.
Link to publication
Nanoscale , 8 , 16197 (2016)
Spatial non-uniformity in exfoliated WS 2 single layers
Monolayers of transition metal dichalcogenides (TMDs) are atomically thin two-dimensional crystals with attractive optoelectronic properties, which are promising for emerging applications in nanophotonics. Here, we report on the extraordinary spatial non-uniformity of the photoluminescence (PL) and strain properties of exfoliated WS2 monolayers. Specifically, it is shown that the edges of such monolayers exhibit remarkably enhanced PL intensity compared to their respective central area. A comprehensive analysis of the recombination channels involved in the PL process demonstrates a spatial non-uniformity across the monolayer’s surface and reflects on the non-uniformity of the intrinsic electron density across the monolayer. Auger electron imaging and spectroscopy studies complemented with PL measurements in different environments indicate that oxygen chemisorption and physisorption are the two fundamental mechanisms responsible for the observed non-uniformity. At the same time Raman spectroscopy analysis shows remarkable strain variations among the different locations of an individual monolayer, however such variations cannot be strictly correlated with the non-uniform PL emission. Our results shed light on the role of the chemical bonding in the competition between exciton complexes in monolayer WS2, providing a method of engineering new nanophotonic functions using WS2 monolayers. It is therefore envisaged that our findings could find diverse applications towards the development of TMD-based optoelectronic devices.
Link to publication
our research
Optics Express Vol. 25, 14, pp. 15624-15634 (2017)
Zone plates as imaging analyzers for resonant inelastic x-ray scattering
We have implemented and successfully tested an off-axis transmission Fresnel zone plate as a novel type of analyzer optics for resonant inelastic x-ray scattering (RIXS). We achieved a spectral resolution of 64 meV at the nitrogen K-edge (E/dE = 6200), closely matching theoretical predictions. The fundamental advantage of transmission optics is the fact that it can provide stigmatic imaging properties. This opens up a variety of advanced RIXS configurations, such as efficient scanning RIXS, parallel detection for varying incident energy and time-resolved measurements.
Link to publication
Nanotechnology 28, 175301 (2017)
Thermal scanning probe lithography for the directed self-assembly of block copolymers
Thermal scanning probe lithography (t-SPL) is applied to the fabrication of chemical guiding patterns for directed self-assembly (DSA) of block copolymers (BCP). The two key steps of the overall process are the accurate patterning of a poly(phthalaldehyde) resist layer of only 3.5 nm thickness, and the subsequent oxygen-plasma functionalization of an underlying neutral poly(styrene-random-methyl methacrylate) brush layer. We demonstrate that this method allows one to obtain aligned line/space patterns of poly(styrene-block-methyl methacrylate) BCP of 18.5 and 11.7 nm half-pitch. Defect-free alignment has been demonstrated over areas of tens of square micrometres. The main advantages of t-SPL are the absence of proximity effects, which enables the realization of patterns with 10 nm resolution, and its compatibility with standard DSA methods. In the brush activation step by oxygen-plasma exposure, we observe swelling of the brush. This effect is discussed in terms of the chemical reactions occurring in the exposed areas. Our results show that t-SPL can be a suitable method for research activities in the field of DSA, in particular for low-pitch, high-χ BCP to achieve sub-10 nm line/space patterns.
Link to publication
Proceedings SPIE 10143, 1-10 (2017)
Extreme ultraviolet patterning of tin-oxo cages
We report on the extreme ultraviolet (EUV) patterning performance of tin-oxo cages: molecular building blocks that are known to turn insoluble upon EUV exposure, thus having the properties of a negative tone photoresist. In this work, we focus on contrast curves of the materials using open-frame EUV exposures and their patterning capabilities using EUV interference lithography. It is shown that baking steps, such as post-exposure baking (PEB) can significantly affect both the sensitivity and contrast in the open-frame experiments as well as the patterning experiments. In addition, we show that the exchange of the anions of the cage can make a difference in terms of their physical properties. Our results demonstrate the significance of process optimization while evaluating the resist performance of novel molecular materials.
Link to publication
Microelectron. Eng. 176, 75 (2017)
Fabrication of diamond diffraction gratings for experiments with intense hard x-rays
The demands on optical components to tolerate high radiation dose and manipulate hard x-ray beams that can fit the experiment requirements, are constantly increasing due to the advancements in the available x-ray sources. Here we have successfully fabricated the transmission type gratings using diamond, with structure sizes ranging from few tens of nanometres up to micrometres, and aspect ratio of up to 20. The efficiencies of the gratings were measured over a wide range of photon energies and their radiation tolerance was confirmed using the most intense x-ray source in the world. The fidelity of these grating structures was confirmed by the quality of the measured experimental results.
Link to publication
Microelectron. Eng. 177, 25 (2017)
Systematic efficiency study of line-doubled zone plates
Line-doubled Fresnel zone plates provide nanoscale, high aspect ratio structures required for efficient high resolution imaging in the multi-keV x-ray range. For the fabrication of such optics a high aspect ratio HSQ resist template is produced by electron-beam lithography and then covered with Ir by atomic layer deposition (ALD). The diffraction efficiency of a line-doubled zone plate depends on the width of the HSQ resist structures as well as on the thickness of the deposited Ir layer. It is very difficult to measure these dimensions by inspection in a scanning electron microscope (SEM) without performing laborious and destructive cross-sections by focus ion beams (FIB). On the other hand, a systematic measurement of the diffraction efficiencies using synchrotron radiation in order to optimize the fabrication parameters is not realistic, as access to synchrotron radiation is sparse. We present a fast and reliable method to study the diffraction efficiency using filtered radiation from an x-ray tube with a copper anode, providing an effective spectrum centered around 8 keV. A large number of Fresnel zone plates with varying dimensions of the resist structures and the ALD coating were measured in an iterative manner. Our results show an excellent match with model calculations. Moreover, this systematic study enables us to identify the optimum fabrication parameters, resulting in a significant increase in diffraction efficiency compared to Fresnel zone plates fabricated earlier without having feedback from a systematic efficiency measurement.
Link to publication
Appl. Surf. Sci. 385, 145 (2016)
Patterning of diamond like carbon films for sensor applications using silicon containing thermoplastic resist (SiPol) as a hard mask
Patterning of diamond-like carbon (DLC) and DLC:metal nanocomposites is of interest for an increasing number of applications. We demonstrate a nanoimprint lithography process based on silicon containing thermoplastic resist combined with plasma etching for straightforward patterning of such films. A variety of different structures with few hundred nanometer feature size and moderate aspect ratios were successfully realized. The quality of produced patterns was directly investigated by the means of optical and scanning electron microscopy (SEM). Such structures were further assessed by employing them in the development of gratings for guided mode resonance (GMR) effect. Optical characterization of such leaky waveguide was compared with numerical simulations based on rigorous coupled wave analysis method with good agreement. The use of such structures as refractive index variation sensors is demonstrated with sensitivity up to 319 nm/RIU, achieving an improvement close to 450% in sensitivity compared to previously reported similar sensors. This pronounced GMR signal fully validates the employed DLC material, the technology to pattern it and the possibility to develop DLC based gratings as corrosion and wear resistant refractometry sensors that are able to operate under harsh conditions providing great value and versatility.
Link to publication
Phys. Rev. B 94, 134306 (2016)
Electron-phonon scattering effects on electronic and optical properties of orthorhombic GeS
Group-VI monochalcogenides are attracting a great deal of attention due to their peculiar anisotropic properties. Very recently, it has been suggested that GeS could act as a promissory absorbing material with high input-output ratios, relevant features for designing prospective optoelectronic devices. In this work, we use the ab-initio many body perturbation theory to study the role of the electron-phonon coupling on orthorhombic GeS. We identify the vibrational modes that efficiently couple with the electronic states responsible for giving rise to the first and second excitonic state. We also study the finite-temperature optical absorption and show that even at T → 0K, the role of the electron-phonon interaction is crucial to properly describe the main experimental excitation peaks position and width. Our results suggest that the electron-phonon coupling is essential to properly describe the optical properties of the monochalcogenides family.
Link to publication
Phys. Rev. B 94, 245303 (2016)
First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra
In this work we put forward a first-principles approach and propose an accurate diagrammatic approximation to calculate the time-resolved (TR) and angle-resolved photoemission spectrum of systems with excitons. We also derive an alternative formula to the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the relaxed regime characterized by the presence of quasistationary excitons and vanishing polarization. The nonequilibrium self-energy diagrams are evaluated using excited Green's functions; since this is not standard, the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated in a first-principles manner. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR and angle-resolved photoemission.
Link to publication
Europhysics Lett., 116, 4, 43001 (2016)
Lamb shift of the Dirac cone of graphene
The fluctuations of the electromagnetic vacuum are one of the most powerful manifestations of the quantum structure of nature. Their effect on the Dirac electrons of graphene is known to induce some spectacular and purely quantistic phenomena, like the Casimir and the Aharanov-Bohm effects. In this work we demonstrate, by using a first-principles approach, that the Dirac cone of graphene is also affected by a sizeable Lamb shift. We show that the microscopic electronic currents flowing on the graphene plane are strongly coupled with the vacuum fluctuations causing a renormalisation of the electronic levels (as large as 4 meV). This shift is one order of magnitude larger than the value predicted for an isolated carbon atom, which imposes a reinterpretation of the Lamb shift as a collective effect.
Link to publication
Phys. Rev. B 93, 195205 (2016)
Non equilibrium optical properties in semiconductors from first principles: a combined theoretical and experimental study of bulk silicon
The calculation of the equilibrium optical properties of bulk silicon by using the Bethe–Salpeter equation solved in the Kohn–Sham basis represents a cornerstone in the development of an ab– initio approach to the optical and electronic properties of materials. Nevertheless calculations of the transient optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter–free calculation based on the non–equilibrium Bethe–Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter–free interpretation of concepts like bleaching, photo–induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.
Link to publication
Phys. Rev. B 93, 155435 (2016)
Temperature dependent excitonic effects in the optical properties of single-layer MoS2
Temperature influences the performance of two-dimensional materials in optoelectronic devices. Indeed, the optical characterization of these materials is usually realized at room temperature. Nevertheless most ab-initio studies are yet performed without including any temperature effect. As a consequence, important features are thus overlooked, such as the relative intensity of the excitonic peaks and their broadening, directly related to the temperature and to the non-radiative exciton relaxation time. We present ab-initio calculations of the optical response of single-layer MoS2, a prototype 2D material, as a function of temperature using density functional theory and manybody perturbation theory. We compute the electron-phonon interaction using the full spinorial wave functions, i.e., fully taking into account effects of spin-orbit interaction. We find that bound excitons (A and B peaks) and resonant excitons (C peak) exhibit different behavior with temperature, displaying different non-radiative linewidths. We conclude that the inhomogeneous broadening of the absorption spectra is mainly due to electron-phonon scattering mechanisms. Our calculations explain the shortcomings of previous (zero-temperature) theoretical spectra and match well with the experimental spectra acquired at room temperature. Moreover, we disentangle the contributions of acoustic and optical phonon modes to the quasi-particles and exciton linewidths. Our model also allows to identify which phonon modes couple to each exciton state, useful for the interpretation of resonant Raman scattering experiments.
Link to publication
ACS Nano 10, 1182 (2016)
Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2
Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2d) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics -- of pivotal importance for the understanding and optimization of device operation -- is still lacking. Here we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photo-excited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime.
Link to publication
Nano Lett. 16, 5095 (2016)
Anomalous temperature dependence of the band-gap in Black Phosphorus
Black Phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is, indeed, the BP temperature-induced band-gap opening: when temperature is increased the fundamental band-gap increases instead of decreasing. This anomalous thermal dependence has also been observed, recently, in its monolayer counterpart. In this work, based on \textit{ab-initio} calculations, we present an explanation for this long known, and yet not fully explained, effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are, in fact, highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band-gap opening and the frequency increase with increasing volume (tension effect).
Link to publication
Nature Communications 7, 11327 (2016)
Electron–vibration coupling induced renormalization in the photoemission spectrum of diamondoids
The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born–Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron–vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron–vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born–Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn–Teller effect.
Link to publication
Phys. Rev. B 93/15, (2016)
An unified theory of quantised electrons, phonons and photons out-of-equilibrium: a simplified ab-initio approach based on the Generalised Baym-Kadanoff ansatz
We present a full ab-inito description of the coupled out-of-equilibrium dynamics of photons, phonons, and electrons. In the present approach the quantised nature of the electromagnetic field as well as of the nuclear oscillations is fully taken into account. The result is a set of integro-differential equations, written on the Keldysh contour, for the Green's functions of electrons, phonons, and photons where the different kind of interactions are merged together. We then concentrate on the electronic dynamics in order to reduce the problem to a computationally feasible approach. By using the Generalised Baym-Kadanoff ansatz and the Completed Collision approximation we introduce a series of efficient but controllable approximations. In this way we reduce all equations to a set of decoupled equations for the density matrix that describe all kind of static and dynamical correlations. The final result is a coherent, general, and inclusive scheme to calculate several physical quantities: carrier dynamics, transient photo-absorption and light-emission. All of which include, at the same time, electron-electron, electron-phonon, and electron-photon interaction. We further discuss how all these observables can be easily calculated within the present scheme using a fully atomistic ab-initio approach.
Link to publication
Review of Scientific Instruments, 87/12, 123909 (2016)
A versatile UHV transport and measurement chamber for neutron reflectometry
We report on a versatile mini ultra high vacuum (UHV) chamber which is designed to be used on the MAgnetic Reflectometer with high Incident Angle (MARIA) of the J¨ulich Centre for Neutron Science at Heinz Maier-Leibnitz Zentrum in Garching, Germany. Samples are prepared in the adjacent thin film laboratory by molecular beam epitaxy and moved into the compact chamber for transfer without exposure to ambient air. The chamber is based on DN 40 CF flanges and equipped with sapphire view ports, a small getter pump and a wobble stick, which serves also as sample holder. Here, we present polarized neutron reflectivity measurements which have been performed on Co thin films at room temperature in UHV and in ambient air in a magnetic field of 200 mT and in the Q-range of 0.18 ˚A −1 . The results confirm that the Co film is not contaminated during the polarized neutron reflectivity measurement. Herewith it is demonstrated that the mini UHV transport chamber also works as measurement chamber which opens new possibilities for polarized neutron measurements under UHV conditions.
Link to publication
Phys. Rev. B 94, 035149 (2016)
Dielectrics in a time-dependent electric field: A real-time approach based on density-polarization functional theory
In the presence of a (time-dependent) macroscopic electric field the electron dynamics of dielectrics cannot be described by the time-dependent density only. We present a real-time formalism that has the density and the macroscopic polarization P as key quantities. We show that a simple local function of P already captures long-range correlation in linear and nonlinear optical response functions. Specifically, after detailing the numerical implementation, we examine the optical absorption, the second- and third-harmonic generation of bulk Si, GaAs, AlAs, and CdTe, at different levels of approximation. We highlight links with ultranonlocal exchangecorrelation functional approximations proposed within a linear response time-dependent density functional theory framework.
Link to publication
JINST, 12, P05024 (2017)
The hard X-ray Photon Single-Shot Spectrometer of SwissFEL—initial characterization
SwissFEL requires the monitoring of the photon spectral distribution at a repetition rate of 100 Hz for machine optimization and experiment online diagnostics. The Photon Single Shot Spectrometer has been designed for the photon energy range of 4 keV to 12 keV provided by the Aramis beamline. It is capable of measuring the spectrum in a non-destructive manner, with an energy resolution of Δ E/E = (2–5) × 10−5 over a bandwidth of 0.5% on a shot-to-shot basis. This article gives a detailed description about the technical challenges, structures, and considerations when building such a device, and to further enhance the performance of the spectrometer.
Link to publication