Prof. Dr. Hubert Krenner
University of Augsburg
Lehrstuhl für Experimentalphysik I

Prof. Ludwig Bartels
University of California, Riverside
Chemistry Department

Molybdenum disulphide is the prototype two-dimensional transition metal dichalcogenide; it has recently attracted widespread attention since – in contrast to graphene – it is ideally suited for optoelectronic and spintronic applications due to its direct bandgap of ~1.9 eV and significant spin-orbit interaction. The main goal of this project is the investigation of the dynamics of photogenerated charge carriers in monolayer MoS₂ by means of a radio frequency surface acoustic wave (SAW) method. This project uniquely combines the expertise at Riverside in the preparation of single layer MoS₂ films and the expertise at Augsburg in the investigation and control of optoelectronic properties of functional nanosystems using SAWs. The latter will afford contactless measurement of transport properties, providing transformative insight into the native material properties.

Final report:

Two-dimensional materials have taken the scientific community by storm. In particular, single-layer transition metal dichalcogenides (TMDs) such as MoS₂ are considered of paramount importance because of their – in contrast to graphene – sizable (~1.5 – 2 eV) and direct electronic bandgap. Single-layer materials can be characterized by a wide range of spectroscopic techniques. Investigation of the electrical transport measurements conventionally involves lithographic fabrication of 2- or 4- contact devices on top of the material. While such techniques have provided important and exciting information about TMD materials, they are intrinsically hampered by contributions of Schottky barriers that form between the metal electrodes and the TMD layer. To overcome this limitation a contact-free approach would be highly desirable.

Our groups in Augsburg and Riverside combined their complementary expertise on TMD growth and spectroscopy (Bartels-Group) as well as advanced, contact-free surface acoustic wave (SAW) spectroscopy (Krenner-Group). In particular, we (i) succeeded in the direct growth of MoS₂ on technologically-relevant 128° rot Y X-propagating LiNbO₃ substrates, (ii) realized a prototypical hybrid acousto-electric and field effect device and (iii) utilized the latter for contact-free detection of photoconductivity of MoS₂ via the attenuation of the SAW.

Seed-funding provided by BaCaTeC enabled multiple mutual visits of students. So far, three students from UCR (Edwin Preciado [two times], Miguel Isarraraz & Michael Gomez) came to Bavaria and Sebastian Hammer spent a month in California conducting research in the Bartels lab.

The visits offered the students the opportunity to conduct joint experiments, which lead to high-profile publications in Nature Communications [1] and Applied Physics Letters [2], respectively. Our joint article in Nature Communications [1] reports on pioneering, original work that established SAW-spectroscopy as capable of addressing single layers of the most established 2D TMD material. The publication demonstrated unique advantages of this method including (i) its contact-free nature, (ii) its capability to probe electrical transport at high fidelity, and (iii) its high-sensitivity as a photoconductivity sensor.

Moving forward, we obtained preliminary results on novel hybrid MoS₂-SiO₂ photonic crystal nanocavities to amplify the light emission from the TMD layer. A manuscript of this work has just been submitted for publication [3]. We also anticipate, in case sufficient funds will be available, that UC Riverside student Michelle Wurch will spend 1-2 months this summer at the Krenner lab and plan for a return visit of Sebastian Hammer to UC Riverside later this year.

Publications of this project and crediting support by this BaCaTeC project:

[1] E. Preciado, F. J. R. Schülein, A. E. Nguyen, D. Barroso, M. Isarraraz, G. von Son, I-H. Lu, W. Michailow, B. Möller, V. Klee, J. Mann, A. Wixforth, L. Bartels, H. J. Krenner, “Scalable Fabrication of a Hybrid Field-Effect and Acousto-Electric Device by Direct Growth of Monolayer MoS₂/LiNbO₃.” Nature Communications 6, 8953 (2015)

Selected press releases covering this publication:
https://idw-online.de/de/news640338
https://idw-online.de/de/news640335
https://www.eurekalert.org/pub_releases/2015-10/uoc--npn102615.php
https://www.ecnmag.com/news/2015/10/nanoquakes-probe-new-2-dimensional-material
http://www.nano-initiative-munich.de/press/press-releases/meldung/n/nanoquakes-probe-new-2-dimensional-material/

[2] W. Michailow, F. J. R. Schülein, B. Möller, E. Preciado, A. E. Nguyen, G. v. Son, J. Mann, A. L. Hörner, A. Wixforth, L. Bartels, H. J. Krenner, “Combined electrical transport and capacitance spectroscopy of a MoS₂-LiNbO₃ field effect transistor.” Applied Physics Letters 110, 023505 (2017)

[3] S. Hammer, H.-M. Mangold, A. E. Nguyen, D. Martinez-Ta, S. Naghibi Alvillar, L. Bartels, H. J. Krenner, “Scalable fabrication of an array of hybrid MoS₂-SiO₂ photonic crystal nanocavities with quality factors exceeding 4000”, submitted (2017)