Tags on Sebastian Stäter

Publication: Excited-State Energy Landscape of P3HT-Nanofibers

Fri, 23 Jun 2023 00:00:00 +0000

In the field of organic optoelectronics, enhancing the diffusion of excitons has been a longstanding research focus [¹], due to its crucial role in the functionality of (e.g.) organic solar cells and OLEDs. However, directing these randomly diffusing excitons towards a specific target requires precise modification of the nanometer-scale assembly of organic materials. In a recent JACS publication with our colleagues from Bayreuth, we present a fresh approach to inscribe such energy-gradients into P3HT-nanofibers:

Stäter, S. et al. “Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers.” Journal of the American Chemical Society

For sample fabrication, we employ a needle-like nucleation seed that initiates the growth of P3HT-nanofibers in solution, resulting in a configuration resembling shish-kebab superstructures [²]. Our approach produces P3HT-nanofibers that exhibit remarkable length (several µm), parallel alignment, and close proximity.

We use hyperspectral imaging to collect 1680 PL and absorption spectra, comprehensively characterizing a 40x40µm area containing the superstructure. Analyzing the optical properties measured along nanofibers results in the energy landscape shown below:

Figure: Excited-state energy landscape along P3HT nanofibers within a NA/P3HT superstructure. Adopted without changes from Stäter et al., under CC BY-NC-ND 4.0 license.

Note that the location of the superstrucutre is shown schematically on the bottom of the graph: The P3HT-nanofibers (blue) extend around 5µm left and right from the needle-like nucleation seed (orange square, shown as cross-section). Plotted above this, we see how the edges of the exciton band are modified by the presence of the superstructure, leading to an energy-gradient along the P3HT-nanofibers.

We attribute the continuous variation of energy levels along these P3HT-nanofibers to defect-fractionation during nanofiber growth, as described by Oosterbaan et al. and Roehling et al [³, ⁴]. We hope that our approach, which involves employing a specialized nucleation seed to induce fiber growth in solution, will facilitate the design of systems that achieving directed long-range exciton transport.

Haedler, A. T.; Kreger, K.; Issac, A.; Wittmann, B.; Kivala, M.; Hammer, N.; Köhler, J.; Schmidt, H.-W.; Hildner, R. Long-Range Energy Transport in Single Supramolecular Nanofibres at Room Temperature. Nature 2015, 523 (7559), 196–199. https://doi.org/10.1038/nature14570. ↩︎
Brinkmann, M.; Chandezon, F.; Pansu, R. B.; Julien-Rabant, C. Epitaxial Growth of Highly Oriented Fibers of Semiconducting Polymers with a Shish-Kebab-Like Superstructure. Advanced Functional Materials 2009, 19 (17), 2759–2766. https://doi.org/10.1002/adfm.200900966. ↩︎
Oosterbaan, W. D.; Vrindts, V.; Berson, S.; Guillerez, S.; Douhéret, O.; Ruttens, B.; D’Haen, J.; Adriaensens, P.; Manca, J.; Lutsen, L.; Vanderzande, D. Efficient Formation, Isolation and Characterization of Poly(3-Alkylthiophene) Nanofibres: Probing Order as a Function of Side-Chain Length. J. Mater. Chem. 2009, 19 (30), 5424. https://doi.org/10.1039/b900670b. ↩︎
Roehling, J. D.; Arslan, I.; Moulé, A. J. Controlling Microstructure in Poly(3-Hexylthiophene) Nanofibers. J. Mater. Chem. 2012, 22 (6), 2498–2506. https://doi.org/10.1039/C2JM13633C. ↩︎

Spotlight in the Zernike Insitute's Advent Calendar

Sun, 25 Dec 2022 00:00:00 +0000

I want to thank the Zernike Insitute for publishing a short spotlight about me and my research as the 24th entry in this year’s Advent Calendar:

In the Zernike Institute Advent Calendar, we are presenting 24 short spotlights in December. In these specials, we highlight PhD students, postdocs, support staff, and technicians of our research groups and team - providing a glimpse in their typical day at work. In Episode 24 meet Sebastian Stäter, PhD student in the Optical Spectroscopy of Functional Nanosystems group of Prof. Richard Hildner.

“Plastic electronics” have the potential to revolutionize photovoltaics, illumination, and microelectronics, but their performance relies on the nanoscale structure of the underlying materials, e.g. on the semicrystalline order of a conjugated polymer film. As a PhD student in the group of Prof. Richard Hildner, I study this relationship between structure and function by employing optical spectroscopy.

In our lab, we build and employ versatile optical setups that operate with diffraction-limited spatial resolution, which allows me to examine the homogeneity of a thin film and compare specific spots within such films based on their absorption and fluorescence spectrum, lifetime, polarization, and more. Furthermore, our experiments work with single-molecule sensitivity, i.e., we measure only one molecules at a time. This allows me to study changes of spectra from individual molecules to solutions and thin films, helping me understand the effect of molecular interactions and aggregation.

Figure: A green laser beam is passing various optical elements in our home-built fluorescence experiment.

I believe these experimental techniques are an important contribution to optoelectronic research. I’m excited to be part of the Zernike Institute’s scientific community, where researchers from all over the world come together to exchange ideas and work on cutting-edge projects. If you need help with advanced optical measurements, our lab can get it done!

Publication: Highly Efficient Supramolecular Nucleating Agents for P3HT

Thu, 17 Mar 2022 00:00:00 +0000

New publication! 🎉 A longstanding collaboration with the group of Prof. Hans-Werner Schmidt in Bayreuth published exciting results in Macromolecules:

Wenzel, Felix A. et al. “Highly Efficient Supramolecular Nucleating Agents for Poly(3-hexylthiophene).” Macromolecules 2022, 55, 7, 2861–2871.

For everyday plastics, nucleating agents are a valuable ingredient to reduce the product’s processing time, or to clarify the material [¹]. As such, nucleating agents are routinely employed in the production of mundane things like lunch boxes and plastic bottles. However, for functional conjugated polymers (a material class that is used for optoelectronic devices such as OLEDs or organic solar cells), there’s still a lack of good nucleating agents, which limits our control over the microscale film morphology, which is an important factor for device performance [²].

In our publication, we present an extemely efficient nucleating agent for the famous polymer Poly(3-hexylthiophene) (in short: P3HT). The compounds’s chemical structure perfectly matches P3HT’s crystal structure (see below), so that local electronic push/pull interactions assist P3HT’s crystal growth.

Figure: The good structural match between nucleating agent and P3HT supports P3HT’s crystallization. Right: Polarization micrograph of trans-crystallized P3HT. Adopted without changes from Felix A. Wenzel et al., ACS Publications.

Using hyperspectral imaging, I compared the fluorescence response of thin P3HT films prepared with and without nucleating agent. Our setup’s spatial resolution of about 500nm (Abbe limit) allows us to uncover inhomogeneities in the local optical response of the films, such as impurities of various kinds. However, the lack of such local inhomogeneities indicates that the nucleating agent disperses well within the material, and should be readily applicable in solution-processing for organic optoelectronic devices.

“Designer” Nucleating Agents for Polypropylene, Markus Blomenhofer, Sandra Ganzleben, Doris Hanft, Hans-Werner Schmidt, Magnus Kristiansen, Paul Smith, Klaus Stoll, Dietmar Mäder, and Kurt Hoffmann. Macromolecules 2005 38 (9), 3688-3695 ↩︎
The Next Breakthrough for Organic Photovoltaics?, Nicholas E. Jackson, Brett M. Savoie, Tobin J. Marks, Lin X. Chen, and Mark A. Ratner. The Journal of Physical Chemistry Letters 2015 6 (1), 77-84 ↩︎

Publication: Spatially resolved absorption

Mon, 09 Aug 2021 00:00:00 +0000

In a recent publication, I employed spatially resolved absorption spectroscopy to visualize inhomogeneities in n-doped donor-acceptor copolymer films. The project was spearheaded by the groups of Ryan Chiechi and Jan Anton Koster, who investigated how the position of polar sidechains influences the material’s thermoelectric performance.

While I recommend reading the whole paper, let me take this occasion to show the beauty of spatially resolved spectroscopy. Our optical setup employs various high-quality optics to focus and collect light from spots as small as 500nm (the Abbe diffraciton limit). This allows us to map a certain absorption spectrum to the corresponding spot on a sample, and compare the spectra of various spots.

In the left column of the figure below (taken straight from the paper), I compare the absorption spectrum of four samples. As thick blue line, I show the average absorption spectrum found on a 20x20µm region of the sample. However, the absorption spectrum in this region actually varies, which is shown with the blue shadows in a)-d). You can see that the spectra in figure c) show the largest spatial variation.

Figure: (a−d) Average spatially resolved absorption spectra of doped copolymer films (thick blue line) and spectral variations (blue shaded area). (e−h) 20×20 μm maps of the absorption ratio A600/A850. Adopted without changes from the publication of Gang Ye et al., under CC BY-NC-ND 4.0 license

You may notice how the (normalized) spectra show the largest deviations at around 600nm. I therefore extracted the

Publication: Controlling n-Type Molecular Doping via Regiochemistry

Sat, 03 Jul 2021 00:00:00 +0000

Good news! 🎉 Our collaboration with the groups of Ryan Chiechi and Jan Anton Koster recently resulted in a publication in Macromolecules:

Ye, Gang, et al. “Controlling n-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor–Acceptor Copolymers.” Macromolecules 2021, 54, 8, 3886–3896.

In this paper, we investigate the morphology, doping-efficiency, and thermoelectric performance of n-doped donor-acceptor copolymer films with polar side chains. Our goal was to optimize these properties by varying the position (regiochemistry) of the side chains. To visualize inhomogeneities in such samples, I contributed spatially resolved absorption measurements.

My experiments are a powerful example of hyperspectral imaging: Our optical setup uses high-quality optics to focus light into spots as small as 500nm (the Abbe diffraciton limit). This allows us to detect absorption (and emission) spectra from small spots on the sample, and compare the spectra of different spots on the same sample.

Figure: Absorption spectra of doped copolymer films (left column), and 20x20µm maps of the absorption ratio A600/A850 (right column). Adopted without changes from Gang Ye et al., under CC BY-NC-ND 4.0 license.

In the figure above, note how the left column shows the average absorption spectrum of each sample as thick blue line, but also visualizes the degree of spectral variation as blue shaded area. This spot-to-spot variation can be translated into maps (right column) that reveal at which positions the spectra deviate most. In particular, we see that the third sample (PNDI2TEG-T2DO) exhibits structures with a larger spatial extent than other samples. This is consistent with our AFM measurements, and implies that the sidechain arrangement of this compound greatly impacts the packing (and thus doping) characteristics of the copolymer.

Photo: Home-built helium-neon laser

Sun, 14 Feb 2021 00:00:00 +0000

A photo from last year’s lab course: In “Build Your Own Laser”, students learned to align the components of a simple helium-neon laser, and characterize its properties.

In the photo above, you see the (noncoherently) glowing He-Ne-tube, enclosed in a transparent box for safety (high voltage!). It is sandwiched between a high-reflective mirror and a 98% reflective output coupler (both barely visible in this photo). Good alignment provided, the arrangement leads to a coherent narrow red laser beam propagating to the right/front. I blocked this beam with a piece of paper to make it visible, leading to the rightmost red spot on the photo.

Fascinating Microscopy Image

Sat, 16 Jan 2021 00:00:00 +0000

Let me show you something I enjoy: This fascinating fluorscence microscopy image shows a thin film of conjugated polymer mixed with a non-fluorescent matrix material. I obtained it when working towards single molecule experiments, where the amount of fluorescent material embedded in the matrix will be many orders of magnitudes smaller.

My Favorite Books in 2020

Sat, 12 Dec 2020 00:00:00 +0000

I found a lot of time to read in 2020 (not surprising, he?) and here are the five books I enjoyed most:

Hans Rosling: Factfulness

We’re irrational pessimists! Driven by emotion rather than data, we consistently perceive the state of the world as worse than it is, and underestimate how fast and consistently things improve. Subtitled “Ten Reasons We’re Wrong About the World - and Why Things Are Better Than You Think”, this book helped me get a better understanding of global health, population growth, economical perspectives, and the misconception of developing countries.

For a quick impression, I can also recommend his excellent TED talk “The Magic Washing Machine”, where he illustrates the impact of (technological) progress with a childhood memory.

My mother explained the magic with this washing machine the very, very first day. She said, ‘Now Hans, we have loaded the laundry. The machine will make the work. And now we can go to the library.’ (from the video)

More info on the book on goodreads / amazon

James Clear: Atomic Habits

Covering an admittedly stale topic (habit formation for personal growth), James Clear’s book stands out with concise and conclusive writing. Full of actionable advice on how to implement good habits and avoid bad habits, the book captivated me from the get-go with the author’s compelling praise of systems over goals.

We think we need to change our results, but the results are not the problem. What we really need to change are the systems that cause those results. (from the book)

More info on the book on goodreads / amazon / video review by Ali Abdaal

Sarah Frier: No Filter - The Inside Story of Instagram

When Facebook bought Instagram for the surreal price of $1 billion in 2012, the start-up had 13 employees (!) and was shaped by co-founder Kevin Systrom’s passion for curated and artistic experiences. After the acquisition, Instagram subsequently succumbed to Facebook’s business logic, e.g. by implementing ads, introducing the algorithm-driven feed, or copying Snapchat’s “Stories” feature (remember Snapchat?), finally evolving into a cornerstone of Facebook’s empire. Following Instagram’s history, Sarah Frier’s book gives an excellent insight into the various competing forces that shape social media (and broadly, technology) today.

Between that [Facebook’s acquisition of WhatsApp] and the Snapchat pursuit, there were suddenly no more doubts about whether Instagram was worth $1 billion to Facebook. Instead, Systrom was getting constant questions - from the media, from his peers in the industry, from everyone - about whether he’d sold too soon. (from the book)

More info on the book on goodreads / amazon / review by Financial Times

Jerry Seinfeld: Is This Anything?

I (re-) discovered Jerry Seinfeld through his excellent Netflix special “Jerry before Seinfeld”. This book collects his best jokes of five decades, framed by a few biographical notes, and it’s both interesting and genuinely funny. 👍

I saw a study that said, the number one fear of the average person is public speaking. Number two is death. Death is number two! How in the world is that? That means to most people, if you have to go to a funeral, you would rather be in the casket than doing the eulogy. (from the book)

More info on the book on goodreads / amazon

Claudia Gray - Master & Apprentice

There’s no denying: Star Wars books are a guilty pleasure of mine. This one follows the young Obi-Wan Kenobi (everone’s favorite Jedi, getting a Disney+ series!) and his master Qui-Gon Jinn. While the plot is predictable as ever, the dynamics between master and apprentice is what carries this book. I hope to get more content on Qui-Gon Jinn, realizing he may be the most quintessential Jedi in the whole saga.

Qui-Gon Jinn or Seneca, whose quote is it? 😉

Always remember: Your focus determines your reality.

More info on the book on goodreads / amazon

Happy reading! 👋
Sebastian

Image sources:

Stockholm Library from Susan Yin (unsplash)
book clipart by wikimedia commons

Hello World

Fri, 11 Dec 2020 00:00:00 +0000

Hi, thanks for stopping by. 👋

This is my first blog post. I’ll use this blog to share both research results and things of personal interest with you. In the long term, I hope to convert the blog content into a more curated site (meaning plenty categories and subcategories full of interesting stuff, e.g. the minimalistic patrickcollison.com).

For the moment however, this is just a blog. You can subscribe via RSS, or connect on LinkedIn or Mastodon, where I intend to crosspost interesting things.

Bye,
Sebastian