SCIENTIFIC HIGHLIGHTS

Fluoreszenz von Defektzentren in freischwebenden Diamanten

Forschern der AG Nanooptik vom Institut für Physik der Humboldt-Universität zu Berlin ist es gelungen, einzelne Diamantpartikel frei schweben zu lassen und das Fluoreszenzlicht der darin enthaltenen Defektzentren zu detektieren. Wie das Team um Oliver Benson, Mitglied von IRIS Adlershof, in der Fachzeitschrift Applied Physics Letters schreibt, wurden Cluster aus Diamant-Nanopartikeln in einer elektrodynamischen Quadrupolfalle, einer sogenannten Paul-Falle, stabilisiert. Die verwendeten Diamantcluster mit Größen zwischen einigen Mikro- und mehreren hundert Nanometern enthielten Stickstoff-Fehlstellenzentren (engl. nitrogen vacancy oder kurz NV center), bei denen ein Kohlenstoffatom des Diamantgitters durch ein Stickstoffatom ersetzt und ein angrenzender Gitterplatz leer ist. Unter den zahlreichen bekannten Diamantdefekten zeichnet sich dieser durch einige bemerkenswerte optische und elektronische Eigenschaften aus, die ihn zum perfekten Kandidaten für viele Anwendungen machen, ob als Einzelphotonen-Emitter in der Quantenoptik oder als Marker in biologischen Experimenten.
Um die Partikel im elektrischen Wechselfeld schweben lassen zu können, wurden sie per Elektrospray-Ionisation aufgeladen und in die Falle gesprüht. Die Forscher demonstrierten wie die Stabilisierung der Diamanten durch die Parameter der Falle kontrolliert werden kann. Die gemessenen Fluoreszenzspektren bewiesen, dass es sich bei den gefangenen Partikeln um Diamanten mit NV-Defekten handelte.
Aussagen über die Größe der gefangenen Partikel konnten nachträglich gemacht werden, indem die freischwebend charakterisierten Diamanten aus der Falle heraus auf den Endflächen von optischen Glasfasern abgelegt wurden. Der Vorteil dabei ist, dass die Partikel sehr leicht auf dem sauberen und abgegrenzten Bereich des Faserendes wiedergefunden werden können. Die anschließenden Messungen mit Licht- und Rasterkraftmikroskopen ergab, dass die bisher kleinsten gefangenen Diamantcluster mit detektierbarer Fluoreszenz eine Größe von ungefähr 500 nm haben. Das bedeutet, dass nicht ein einzelnes sondern zahlreiche NV-Zentren angeregt und detektiert wurden, was sich auch schon aus den Fluoreszenzspektren ableiten ließ.
Der Grund für die Begrenzung liegt in der Aufsammeleffizienz des verwendeten Mikroskopaufbaus. Die Elektroden der Falle schränken den optischen Zugang zum Fallenzentrum ein. Für den optischen Nachweis von einem einzigen NV-Defektzentrum in einem einzelnen, schwebenden Diamant-Nanokristall werden die Forscher in folgenden Experimenten andere Fallengeometrien ausprobieren.
 

 

Im Hinblick auf die Untersuchung von optomechanischen Wechselwirkungen in einzelnen nanometergroßen Diamantkristallen, ist die räumliche Isolation und die Beobachtung der Fluoreszenz der Diamantcluster aber ein erster wichtiger Schritt zum Erfolg.


Abbildung: a) Diamantpartikel in der linearen Paulfalle. 1: Fallenelektroden, 2: Endkappen-Elektroden, 3: Elektrodenhalterung, 4: gefangenes Diamantpartikel, 5: optische Faser. b) NV-Fluoreszenzspektren von einzelnen frei schwebenden Diamantclustern mit Größen von 900 nm (schwarze Kurve) und 500 nm (blaue Kurve). c) Optische Mikroskopaufnahme von einem auf dem Faserende abgelegten Diamantcluster. d) Rasterkraftmikroskopaufnahme von dem Partikel aus c)

Nitrogen vacancy fluorescence from a submicron diamond cluster levitated in a linear quadrupole ion trap
A. Kuhlicke, A. W. Schell, J. Zoll, and O. Benson
Applied Physics Letters 105, 073101, (2014)
DOI: 10.1063/1.4893575

Light-induced movements of single macromolecules on a modified graphite surface

A current challenge to be addressed in nanotechnology is the ability to precisely control the motion of single molecules on an atomically well-defined surface, with the aim of developing "machines" able to perform actual work at the smallest scale. A common strategy consists in creating compounds such that external energy―either chemical fuels or physical stimuli―is converted into movement through concerted conformational changes. Physical stimuli like electrons or photons are particularly beneficial, as they offer a non-invasive, clean way of addressing molecules with very high spatial precision. Several works have already been reported on the electro- or photo-induced motion of relatively small molecules, which were typically observed with very-high resolution microscopes, such as scanning tunneling or atomic force microscopes (STM and AFM). These examples include small molecules resembling wheels, pinions, or even cars.

       

Although the level of complexity already reached with small molecules is quite impressive, longer molecules, such as polymers, are more promising for obtaining mechanical movements over larger distances.

 

Now, writing in ACS Nano, David Bléger, Jürgen P. Rabe, and co-workers Chien-Li Lee, Tobias Liebig and Stefan Hecht, from IRIS Adlershof and the Departments of Physics and Chemistry of the Humboldt-Universität zu Berlin, have realized a system, in which single polymers can be reversibly contracted and stretched on a modified graphite surface by using two different types of light (UV and blue).



The macromolecules, visualized by AFM, were found to contract with large amplitudes, and sometimes move across the surface, resembling a "crawling" movement. The key to achieving these motions was first, to design polymers in which dramatic contractions could be reversibly induced by light, and second, to modify the surface for at the same time orienting the linear macromolecules, isolating them from each other, and decoupling them from the surface. The next steps will consist in finding ways to control the directionality of the movements, thus opening up new possibilities for the development of optomechanical nanosystems.

Light-Induced Contraction and Extension of Single Macromolecules on a Modified Graphite Surface
C.-L. Lee, T. Liebig, S. Hecht, D. Bléger, and J.P. Rabe
ACS Nano, Article ASAP
DOI: 10.1021/nn505325w

Neuer zweidimensionaler graphitischer Halbleiter entdeckt

Ein europäisches Team aus Chemikern und Physikern, darunter Dr. Nikolai Severin und Prof. Jürgen P. Rabe vom Institut für Physik der Humboldt-Universität und dem Joint Laboratory of Structural Research am IRIS Adlershof, haben einen neuen, dem Graphen verwandten, quasi zweidimensionalen Halbleiter entdeckt (s.a. Kommentar in ars technica). Das Material, ein Triazin-basiertes graphitisches Kohlenstoffnitrid (TGCN), wurde bereits 1996 theoretisch vorhergesagt. Jetzt konnte es erstmals vorgestellt werden. TGCN ist ein Mitglied der Graphen-Familie, von denen bisher nur fünf nichtmetallische Mitglieder bekannt waren: Graphen selbst, hexagonales Bornitrid, Bor-Kohlenstoff-Nitrid, Fluorgraphen und Graphenoxid. TGCN ist strukturell dem Graphit ähnlich, aber es ist halbleitend und daher hochinteressant für opto-elektronische Anwendungen.

 

Zu den Kooperationspartnern in diesem Projekt gehören Dr. Michael J. Bojdys und Professor Arne Thomas (TU Berlin), Professor Markus Antonietti (MPI für Kolloid- und Grenzflächenforschung) sowie fünf weitere Gruppen aus Großbritannien, Deutschland und Finnland. Im IRIS Adlershof spielen 2D Kristalle eine wichtige Rolle im Forschungsfeld "Hybridsysteme für Optik und Elektronik".

Triazine-Based Graphitic Carbon Nitride: a Two-Dimensional Semiconductor
G. Algara-Siller, N. Severin, S.Y. Chong, T. Björkman, R.G. Palgrave, A. Laybourn, M. Antonietti, Y.Z. Khimyak, A.V. Krasheninnikov, J.P. Rabe, U. Kaiser, A.I. Cooper, A. Thomas, and M.J. Bojdys
Angew. Chem. 126 (2014) 7580
DOI: 10.1002/ange.201402191
Angew. Chem. Int. Ed. 53 (2014) 7450
DOI: 10.1002/anie.201402191




 

Inner structure of adsorbed ionic microgel particles

Microgel particles of cross-linked poly(NIPAM-co-acrylic acid) with different acrylic acid contents are investigated in solution and in the adsorbed state. As a substrate, silicon with a poly(allylamine hydrochloride) (PAH) coating is used. The temperature dependence of the deswelling of the microgel particles was probed with atomic force microscopy (AFM). The inner structure of the adsorbed microgel particles was detected with grazing incidence small angle neutron scattering (GISANS). Small angle neutron scattering (SANS) on corresponding microgel suspensions was performed for comparison. Whereas the correlation length of the polymer network shows a divergence in the bulk samples, in the adsorbed microgel particles it remains unchanged over the entire temperature range.

 

In addition, GISANS indicates changes in the particles along the surface normal. This suggests that the presence of a solid surface suppresses the divergence of internal fluctuations in the adsorbed microgels close to the volume phase transition.

Inner Structure of Adsorbed Ionic Microgel Particles
S. Wellert, Y. Hertle, M. Richter, M. Medebach, D. Magerl, W. Wang, B. Demé, A. Radulescu, P. Müller-Buschbaum, T. Hellweg, and R. von Klitzing
Langmuir 30 (2014) 7168
DOI: 10.1021/la500390j




 

Energy-level alignment at metal/organic interfaces: Tying up the loose ends

Organic semiconductors have tremendous potential for complementing conventional, inorganic semiconductors as active materials in (opto-)electronic devices such as light-emitting diodes (OLEDs) for display and lighting applications or solar cells (OPVCs). Electrical connection of such devices to peripheral circuitry, however, is realized by metallic contacts and the resulting interfaces to the organic semiconductor have been found to play a central role: Energy barriers for injecting charge carriers into the organic (OLEDs) or energy losses upon their extraction (OPVCs) were found to detrimentally affect device performance. Attempts to minimize these energy barriers/losses are hampered by an incomplete understanding of their origin and the parameters that govern their magnitude.

Fig. 1: Schematic illustration of the electron transfer to acceptor states at a surface or interface of an n-doped semiconductor (middle and right). The development of a space-charge region induces band-bending, which brings the acceptor state closer to the Fermi level (right) In an undoped intrinsic semiconductor (left) no such electron transfer can take place resulting in an empty acceptor state in the band gap.

 

 

A multitude of sometimes conflicting views have been expressed in literature, each treating only a certain limiting case under particular assumptions for interface properties and dominant mechanism.

Now, a team including IRIS-member Norbert Koch implemented a detailed electrostatic model, which is capable of covering the full phenomenological range of interfacial energy-level alignment regimes within a single, consistent framework. Energy barriers/losses could be reproduced in both qualitative and quantitative agreement with a series of experiments. By continuously connecting the limiting cases described by previously proposed models, conflicting views in literature could be resolved. In particular, the team highlighted the key role played by the density of electronic states in the organic semiconductor: Its shape was found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt steps at the interface with the electrode to smoothly varying curves. Especially the latter – counter intuitively induced by introducing an ultrathin, electrically insulating interlayer between metal and organic – is beneficial for charge extraction in OPVCs, as illustrated below.

Organic semiconductor density of states controls the energy level alignment at electrode interfaces
M. Oehzelt, N. Koch, and G. Heimel
Nature Communications 5 (2014) 4174
DOI: 10.1038/ncomms5174

Scattering amplitudes in gauge theories – a new Lecture Notes in Physics Book by J. Henn and J. Plefka

At the fundamental level, the interactions of elementary particles are described by quantum gauge field theory. The quantitative implications of these interactions are captured by scattering amplitudes, traditionally computed using Feynman diagrams. In the past decade tremendous progress has been made in our understanding of and computational abilities with regard to scattering amplitudes in gauge theories, going beyond the traditional textbook approach. These advances build upon on-shell methods that focus on the analytic structure of the amplitudes, as well as on their recently discovered hidden symmetries. In fact, when expressed in suitable variables the amplitudes are much simpler than anticipated and hidden patterns emerge.

These modern methods are of increasing importance in phenomenological applications arising from the need for high-precision predictions for the experiments carried out at the Large Hadron Collider, as well as in foundational mathematical physics studies on the S-matrix in quantum field theory.

Bridging the gap between introductory courses on quantum field theory and state-of-the-art research, there has been a need for a focussed text book on the subject. Recently IRIS member Jan Plefka together with his former postdoc Dr. Johannes Henn, now at the Institute for Advanced Studies in Princeton (USA) have published the first monogrpahical text on this fundamental subject. The concise yet self-contained and course-tested lecture notes are well-suited for a one-semester graduate level course or as a self-study guide for anyone interested in fundamental aspects of quantum field theory and its applications.

The book contianes numerous exercises and solutions which help readers to embrace and apply the material presented in the main text.

Scattering Amplitudes in Gauge Theories
J. M. Henn and J. C. Plefka
ISBN: 978-3-642-54021-9 (Print) 978-3-642-54022-6 (Online)
(published 03-2014)


Band-bending in organic semiconductors

Band-bending in organic semiconductors, occurring at metal/alkali-halide cathodes in organic-electronic devices, is experimentally revealed and electrostatically modeled. Metal-to-organic charge transfer through the insulator, rather than doping of the organic by alkali-metal ions, is identified as the origin of the observed band-bending, which is in contrast to the localized interface dipole occurring without the insulating buffer layer.
 
Band-Bending in Organic Semiconductors: the Role of Alkali-Halide Interlayers 
This work was supported by the DGF (SFB 951).
H. Wang, P. Amsalem, G. Heimel, I. Salzmann, N. Koch, and M. Oehzelt

Adv. Mater. 26 (2014) 925
DOI: 10.1002/adma.201303467
 


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