Defect engineering of 2D heterostructures with monolayer precision
Tailoring of heterostructure properties post-growth demands a modification technique with monolayer precision in order to address exclusively a specific layer. State-of-the-art techniques for material modification based on electron or laser illumination with this precision are still missing. Highly charged ions (HCIs)  are a new tool to manipulate and modify 2D materials and their heterostructures on the nanoscale at the topmost atomic surface layer due to their high charge state and subsequent neutralization at surfaces - a property unique to this class of ions. Recently, the Wilhelm group [2-4] and others  showed that HCIs feature an efficient tool to produce pores and defects in freestanding TMDs and TMDs on substrates.
© Richard Wilhelm
With this PhD project we will
- find the optimal HCI parameters to tune defect size distributions in freestanding TMDs, TMDs on substrate and in vdW structures.
- use the defect-engineered samples for Temperature Programmable Desorption (TPD) experiments together with the Parkinson group to evaluate the catalytic properties of ion-induced defects.
- explore the option to grow metal/metal-oxide clusters at defect sites together with the Eder group.
The kinetic energy and charge state dependence of the defect size distribution in TMDs is currently unknown, but is a key aspect of HCI defect-engineering. With the outcome of the project, we will be able to tune the defect size distribution in TMDs by HCI and determine the catalytic properties of the defect sites.
We will use freestanding MoS2, MoS2 on SiO2, and freestanding MoS2/graphene heterostructures in the beginning. At a later stage we will use also WSe2, WS2, and Janus-type TMDs (e.g., WSeS). Samples will be made available from our strong collaborations with A. Turchanin (U Jena; TMDs on substrate), M. Schleberger (U Duisburg-Essen; freestanding and Janus-TMDs), J. Kotakoski (U Wien, freestanding TMDs and heterostructures). We will perform ion irradiation with charge states from 10-40 and kinetic energies from 10- 400keV at the Wilhelm lab. Selected freestanding samples will be transferred to U Wien (Kotakoski group) with a UHV suitcase (10-9 mbar). Selected samples on SiO2 substrate will be transported under ambient conditions to the Parkinson group for atomically-resolved non-contact AFM (ncAFM) and TPD to check for catalytic activity of defective TMDs. Selected samples on SiO2 will be brought to the Eder group to deposit metal/metal-oxide clusters at defects. For very small defects we want to explore the extreme limit of single-atom deposition. These samples will then be used for TPD - and if promising ncAFM – together with the Parkinson group.
The PhD project combines our expertise on HCI--surface interaction with cutting edge microscopy imaging and catalysis studies (Parkinson) as well as state-of-the-art nanostructural analysis (Lendl) and ab-initio theoretical tools (Libisch). Defects and nm-sized pores produced by ion impact will serve as active adsorption sites for gaseous molecules (Filipovic) and metal/metal-oxide clusters (Eder). The modified 2D systems will be analyzed by PAR-XPS (Foelske) and IR (Lendl). Defective TMDs will be further characterized using AFM-IR (Lendl group). We will explore the option of gaseous molecule deposition (Filipov group). Theoretical descriptions of HCI-induced material modification will be done by the Libisch group.
Richard Wilhelm is an experimental physicist working in the field of surface and ion physics. His research focusses on the charge exchange dynamics of ions when they enter solid material. This charge exchange can trigger a plethora of different phenomena (transient surface charge up, emission of photons and electrons, etc.) and among them is the deposition of vast amounts of energy (10s of keV) in a shallow region at the material’s surface leading to layer-by-layer erosion on the nanoscale. Using different ion scattering techniques, secondary particle coincidence detection as well as atomically-resolved microscopy allows a comprehensive approach towards the underlying mechanism of surface erosion induced by the ion charge exchange. Exploiting it for nano-lithographic applications with 2D materials is a main driving force for this PhD project.
- Wilhelm, R. A. The charge exchange of slow highly charged ions at surfaces unraveled with freestanding 2D materials. Surface Science Reports 77, 100577 (2022), DOI: 10.1016/j.surfrep.2022.100577.
- Schwestka, J. et al. Atomic-Scale Carving of Nanopores into a van der Waals heterostructure with slow, highly charged ions. ACS Nano 14, 10536-10543 (2020), DOI: 10.1021/acsnano.0c04476.
- Niggas, A. et al. Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS2 Probed by the Emission of Electrons. Physical Review Letters 129, 086802 (2022), DOI: 10.1103/PhysRevLett.129.086802.
- Grossek, A. S., Niggas, A., Wilhelm, R. A., Aumayr, F., Lemell, C. Model for Nanopore Formation in Two-Dimensional Materials by Impact of Highly Charged Ions. Nano Lett. 22, 9679-9684 (2022), DOI: 10.1021/acs.nanolett.2c03894.
- Kozubek, R. et al. Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions. Journal of Physical Chemistry Letters 10, 904-910 (2019), DOI: 10.1021/acs.jpclett.8b03666.