Smaller, faster, more efficient? – Review by A5 (Volz)

In their review paper, Andreas Beyer and Kerstin Volz describe in detail their investigation of novel composite materials, which may eventually replace today’s silicon-based electronic devices. As the latter increasingly reach their performance limit, one option to overcome these largely physics-based limitations is to cover silicon with a different material layer.

Experimentally measured and simulated structure of the galliumphospide/silicon-interface at atomic resolution. Electron diffraction patterns allow to determine the interfacial charge distribution.

However, covering silicon with different material layers like, for example, well-suited III/V semiconductors (containing elements of the 3rd and 5th group of the periodic system) is challenging. In joining different materials with their individual physico-chemical properties their interface may be marked by defects. Here, for example, “erroneous” attachments may lead to unwished-for local charges – rendering the combined material as unsuitable for application in devices.

The group of Kerstin Volz closely studied galliumphosphide on silicon as a model system of III/V semiconductors on silicon. In their invited review the authors now describe the various electron-microscopy-based approaches employed in the study of the internal interface between the two materials and its defects. By means of transmission-electron-microscopy the researchers were able to show that the interface between the two materials is far from smooth; in fact, it more resembles a pyramidal structure affecting several atomic layers. In addition, it was also possible to “see” the erroneous atomic attachments which cause the unwished-for charge effect and link the phenomenon directly to changes in preparatory procedures.

The insights gained will be applied to perfecting preparation methods in order to reduce the number of defects and to fine-tuning the interface with a focus on raising the efficiency of existing devices and encouraging the development of novel applications.

Publication

A. Beyer and K. Volz,
Quantitative Electron Microscopy for III/V on Silicon Integration
Adv. Mater. Interfaces (2019) DOI: 10.1002/admi.201801951

Contact

Prof. Dr. Kerstin Volz
Philipps-Universität Marburg
SFB 1083 project A5
Tel.: 06421 28 22297
EMAIL

PI Seminar 2019 in Oberheimbach

As the first half of SFB 1083’s second funding period comes to a close it was time for its principal investigators to get together to discuss in depth their individual project’s progress and how interdisciplinary research and discussion across projects are developing.

The secluded setting in Oberheimbach provided the right framework for ample conversation in changing smaller and bigger groups and discussion of possible new research agendas for the third funding period of SFB 1083.

For details of the program please follow the link.

SFB 1083 in partnership with Chemikum Marburg supports Girls’ Day 2019

SFB 1083’s Ö-project closely cooperates with Chemikum Marburg e.V. by installing at its premises several new experiments and workshops providing hands-on insights into the SFB’s research objective and the study methods employed. In 2019, for a second time, the partnership supported Girls’ Day activities with dedicated offerings.

Girls’ Day 2019 with its 50 young (agegroup 10-16) female participants benefitted from these offerings, which included the application of acid-base-reactions, measuring thermal signatures of chemical reactions and electrical conductivity in various substances. The experiments provide insight into the central research aspect of SFB 1083: “What are the reactions taking place at the interface, that is the contact between two materials?”

“Girls’ Day allows us to showcase how researchers work. In interesting experiments and workshops we can entice enthusiasm for MINT-disciplines in female pupils”, state Prof. Stefanie Dehnen and Dr. Christof Wegscheid-Gerlach, director and co-director of Chemikum Marburg and principal investigators of SFB 1083’s Ö-project.

A special informatics-focused workshop, in cooperation with Michael Szabo (Fachdidaktik Informatik PUM/MLS), showcased how disciplines work together and how modern research needs detailed programming for optimal analytical results. The SFB’s Atomic Force Microscope (AFM), for example, needs a complex range of operational programming to realize its full potential. This was demonstrated using the Lego-model and it rapidly became obvious to the riveted audience how programming controls the instrument’s operation. 18 girls were then guided in developing their own little programs using the language scratch. At the end of the 3-hour workshop the young participants had all succeeded in letting their “dog run around the lake” and teasing out the impact of minor changes to their code.

See also a press release in German.

Contact

Dr. Christof Wegscheid-Gerlach
Chemikum Marburg
SFB 1083 project Ö
Tel.: 06421 28 25252
EMAIL

Organic monolayers can reduce contact resistances in organic electronics – Publication by A2 (Witte)

In a detailed study, Felix Widdascheck, Alrun Hauke and Gregor Witte from SFB-project A2 show how phthalocyanine monolayers can be used to control the work function of noble metal electrodes, both in single crystalline model systems and for real life polycrystalline electrodes.

The work function of bare metal surfaces (yellow) can be modified by a thin layer of phthalocyanines (blue) to reduce injection barriers in organic electronic devices. (Image: F. Widdascheck).

Work function tailoring by means of organic monolayers is one of several promising approaches to reducing the contact resistance at the interface between metal electrodes and organic semiconductors in organic electronics devices.

In their study Felix Widdascheck and coauthors used several polar and non-polar phthalocyanines to modify the work functions of noble metal electrodes. As a starting point, they performed a detailed STM and Kelvin probe analysis of the coverage-dependent work function changes of Au and Ag single crystal surfaces. The authors find that the work function changes strongly depend on both coverage and the type of phthalocyanine used as the contact primer. Their phenomenological description of the observed trends provides important groundwork for more detailed theoretical modeling of the processes taking place at the complex internal interface between metal, monolayer and organic semiconductor.

In a further step towards actual device applications, the authors then transferred their findings and the developed preparation protocols to polycrystalline electrodes, demonstrating that the same work function changes can be observed also on “real-life” electrodes. With the end user in mind, the team also tested the air stability of their contact primers, proving that a sacrificial phthalocyanine multilayer serves well to protect the highly ordered mono- and bilayer contact primers during air transfer and can be removed by thermal desorption afterwards.

Publication

F. Widdascheck, A.A. Hauke and G. Witte,
A Solvent-Free Solution: Vacuum-Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes
Adv. Funct. Mater. (2019) DOI: 10.1002/adfm.201808385

See also press release in German.

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083 project A2
Tel.: 06421 28 21384
EMAIL

New ways of controlling and analyzing organic reactions on silicon surfaces – Publications by A8 (Koert/Dürr) & B5 (Höfer/Mette) and A6 (Tonner)

In a joint effort, the groups of SFB-projects A8 and B5 used scanning tunneling microscopy for controlling the final products of a textbook-type reaction of organic molecules on silicon surfaces. A detailed understanding of the driving forces of these reactions are obtained by means of energy decomposition analysis as developed in SFB-project A6.

The isomers naphthalene (left) and azulene (right) bind very differently to a copper surface: While naphthalene forms a weak bond (physisorption), azulene engages in a strong chemical bond with substantial charge transfer (chemisorption).

Ether cleavage on silicon is the surface analogue of an SN2 reaction; SN2 reactions represent the textbook example for how to control solution-based chemical reactions by means of steric hinderance or the choice of solvent. In an STM study published in Angewandte Chemie, the team around Gerson Mette and Michael Dürr has now shown that tip-induced ether cleavage on Si(001) leads to additional final products which are not obtained by thermal activation. Moreover, different final products can be selectively addressed by different excitation channels, either direct excitation by electron transfer or multiple excitation of vibrational modes. As the two channels can be selectively addressed by the tunneling bias, a new way of reaction control was achieved.

In parallel, the advances in the theoretical description of these systems are illustrated in a review article by Lisa Pecher and Ralf Tonner. Within the framework of density functional theory, the chemists of A8 successfully applied energy decomposition analysis to extended systems in order to derive bonding concepts for molecules on surfaces. This allows to interpret experimental results and predict new reaction schemes.

Publications

G. Mette, A. Adamkiewicz, M. Reutzel, U. Koert, M. Dürr, and U. Höfer,
Controlling an SN2 reaction by electronic and vibrational excitation ‐ tip‐induced ether cleavage on Si(001)
Angew. Chemie Int. Ed. 58/11 (2019) 3417-3420 DOI: 10.1002/anie.201806777
L. Pecher and R. Tonner,
Deriving bonding concepts for molecules, surfaces, and solids with energy decomposition analysis for extended systems
WIREs Comput. Mol. Sci. (2018) (21pp) DOI:10.1002/wcms.1401

See also joint press release by the universities of Gießen and Marburg under the auspices of the Forschungscampus Mittelhessen ( in German).

Contact

Prof. Dr. Michael Dürr
Justus-Liebig-Universität Gießen
SFB 1083 project A8
Tel.: 0641 993490
EMAIL

Molecular topology critically controls metal-organic interfaces in electronic devices – Publication by A4 (Gottfried), A6 (Tonner), & A12 (Tautz/Bocquet/Kumpf)

In organic electronic devices, such as modern displays with organic light-emitting diodes (OLEDs), organic materials connect to metal electrodes. The resulting metal–organic interfaces, which are in the focus of SFB 1083, determine important performance parameters such as rates of charge-carrier injection. Precise control over the interface properties, especially the wave-function overlap and the energy-level alignment, is therefore critical for rational improvement of organic electronic devices. Here, the SFB 1083 projects A4, A6 (both Univ. Marburg) and A12 (at FZ Jülich), together with groups in Utrecht (NL), Warwick (UK) and Erlangen (DE), show that the properties of metal-organic interfaces depend strongly on the linking pattern of the atoms in the organic material.

In organic semiconductors, the carbon atoms are typically laid out in a honeycomb-like sheet of abutting six-sided rings. If the sheet contains no odd-numbered rings, it is described as an “alternant topology.” Researchers rarely consider nonalternant topologies, which occur when the structure contains, for example, five- or seven-sided rings. To elucidate the influence of the topology on the interaction with a metal surface, the authors compare the aromatic hydrocarbon naphthalene to its nonalternant isomer, azulene, and study their interactions with a copper surface.

Azulene-like nonalternant 5-7 structural element embedded in a graphene lattice (right), compared to the ideal graphene lattice left. The figure shows sections through the charge density for both systems, according to DFT calculations. The 5-7 element accumulates negative charge (red) at the 5-membered ring and positive charge (blue/white) at the 7-membered ring. Copyright by CC-BY 4.0.

Benedikt Klein and his co-workers find that azulene forms a much stronger and shorter bond to copper than naphthalene. Spectroscopic analysis of the electronic structure reveals that azulene forms a true chemical bond and receives negative charge from the surface, whereas naphthalene bonds only weakly and does not exchange charge. Theoretical analysis reveals that the influence of the topology on the electronic structure, especially the lowest unoccupied molecular orbital, is responsible for the different behavior. This comprehensive analysis of a surface chemical bond was only possible through a multi-technique approach, which involved a collaboration between six research groups from experiment and theory, including three from SFB 1083. Important contributions were made by the groups of Ingmar Swart (Utrecht, NL), Reinhardt Maurer (Warwick, UK), and Wolfgang Hieringer (Erlangen, DE).

Based on their findings, the authors propose that the incorporation of nonalternant structural elements can be used to control and optimize performance-related properties of functional metal–organic interfaces.

Publication

B. P. Klein, N. J. van der Heijden, S. R. Kachel, M. Franke, C. K. Krug, K. K. Greulich, L. Ruppenthal, P. Müller, P. Rosenow, S. Parhizkar, F. C. Bocquet, M. Schmid, W. Hieringer, R. J. Maurer, R. Tonner, C. Kumpf, I. Swart, and J. M. Gottfried, Molecular topology and surface chemical bond: alternant versus nonalternant aromatic systems as functional structural elements, Physical Review X 9/1(2019) 011030 (17pp) DOI:10.1103/PhysRevX.9.011030

Contact

Prof. Dr. Michael Gottfried
Philipps-Universität Marburg
SFB 1083 project A4
Tel.: 06421 28-22541
EMAIL

“Frauenförderpreis 2018” for Prof. Dr. Stefanie Dehnen (A9)

Professor Dehnen with Prof. Dr. Katharina Krause (president of Philipps-Universität Marburg), Prof. Dr. Carmen Bickle, and Dr. Nina Schumacher (women and equal opportunity officer). (Photo: Henrik Isenberg)

As part of a special event organized for the last day of November, Prof. Dr. Stefanie Dehnen, PI of SFB-project A9 and Professor of Inorganic Chemistry, and Prof. Dr. Carmen Birkle, Professor for American Studies, jointly received the “Frauenförderpreis” of the Philipps-Universität Marburg.

The “Frauenförderpreis” of Philipps-Universität Marburg is awarded every two years since 1998 and worth 2500 EUR. It recognizes Prof. Dr. Stefanie Dehnen and Prof. Dr. Carmen Birkle for their strong mentorship and in the case of Professor Dehnen in particular for her participation as a mentor for early-career female academic staff in the Hessen-wide project “SciMento” and her enduring engagement for a family-friendly research environment.

In balancing a professorship and family-life with four young children, Stefanie Dehnen is living proof that having both, a research career and a family is possible.

ASOMEA-IX organized by SFB 951 & SFB 1083

The meeting was jointly organized by SFBs 951 “Hybrid Inorganic/Organic Systems for Opto-Electronics (HIOS)” Berlin and SFB 1083 “Structure and Dynamics of Internal Interfaces”. It attracted more than 80 participants from Europe, Asia and America to the Black Forest. The meeting consisted of 30 talks and 48 posters.

The series of biannual ASOMEA-workshops began in 2001 as a meeting of Swedish and Japanese scientists working with spectroscopic techniques and theoretical modeling for a better understanding of organic electronic materials and related interfaces. In 2016 the scope of the workshop was widened to include the German community and the intention to focus on organic materials at advanced stages, in situ/operando techniques, and time-resolved spectroscopy to name just a few.

For more information visit https://asomea9.internal-interfaces.de/.

Outreach Research Publication on SFB 1083

Copyright Scientia.

In the October issue of Scientia an outreach research publication entitled “Collaborating to Study Interfaces in Miniaturised Materials” covers our collaborative research center.

Scientia is a bimonthly science communication publication. The goal is to connect science and society by presenting research in an understandable, informative and attractive way to the general public. The article outlines motivation, scientific goals and methods of SFB 1083 “Structure and Dynamics of Internal Interfaces”. It highlights several results of the first funding period.

The publication is also accessible as a “scipod”-audiobook.

Publication

SFB 1083 – Collaborating to Study Interfaces in Miniaturised Materials
Scientia (2018) DOI: 10.26320/SCIENTIA255

Lightwave-driven Dirac currents – Publication by B6 (Höfer/Wallauer) in Nature

Johannes Reimann, Jens Güdde and Ulrich Höfer together with a team led by Rupert Huber in Regensburg have taken band structure movies of electrical currents carried by Dirac electrons as they are driven by an intense THz wave.

Artistic view of the experiment (image by Brad Braxley parttowhole.com).

The investigated currents consist of spin-polarized electrons confined to the uppermost atomic layers of the topological insulator Bi2Te3. The electrons were observed to react in an in­ertia-free fashion to the driving field, whereas spin-momentum locking lifts scattering times abo­ve 1 ps. This scenario enables giant sur­fa­ce cur­rent densities and bal­­listic mean free paths of several 100 nm, exceeding values obtained in conventional materials by or­ders of magni­tu­de. Based on this discovery, it might be possible to realize new lightwave-driven electronics in the future, combining low power consumption and clock rates that exceed those of conventional semiconductor devices by a factor of 1000 and more.

Animation of photoemission snapshots of the topological surface state in Bi2Te3 showing the back and forth acceleration of Dirac electrons at optical clock rates by an intensive Thz electric field.

The work of the two groups and collaborators in Novosibirsk and Hiroshima not only merges two novel and promising concepts in physics – topology and lightwave electronics. It also combines the expertise of the Regensburg group to manipulate electrons in solids with intense single-cycle terahertz (THz) transients, with the capabilities of time and angle-resolved photoelectron spectroscopy (ARPES) developed in Marburg.  The experiment conducted in Regensburg represents the first an­gle-re­­sol­ved pho­to­emis­sion spec­tro­scopy with subcycle resolution. It allowed Reimann and coworkers to directly ob­ser­ve how the car­rier wave of a te­rahertz pulse ac­celerates Di­rac fermions in the band­ struc­tu­­re. The resulting strong re­distribution in mo­men­tum space was directly mapped out in an ultrafast movie (Figure on larger screens).

In future experiments, it will be explored whether the topological protection responsible for the long scattering times at the Bi2Te3/vacuum interface survives when the material is covered by a protective cap layer as this is a prerequisite for device application.

Publication

J. Reimann, S. Schlauderer, C. P. Schmid, F. Langer, S. Baierl, K. A. Kokh, O. E. Tereshchenko, A. Kimura, C. Lange, J. Güdde, U. Höfer, and R. Huber, Subcycle observation of lightwave-driven Dirac currents in a topological surface band, Nature (2018) DOI: 10.1038/s41586-018-0544-x

SpringerNature Content Sharing Initiative: view-only full-text access

See also:
Joint press release of the Universities of Regensburg and Marburg (in German).
Blog Post “Topology at Cyberspeed” by Ulrich Höfer.
Homepage of the Huber group in Regensburg.

Contact

Prof. Dr. Ulrich Höfer
Philipps-Universität Marburg
SFB 1083 project B6
Tel.: 06421 28-24215
EMAIL

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