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Ion Implantation

Ion implantation is a well-established technique for doping bulk semiconductors, including Si. It offers flexibility in the dopant choice; performed in high vacuum environment it is clean and compatible with most demanding electronic devices. It can also be used locally to make lateral heterostructures. In this collaborative project, funded through VolkswagenStiftung program “Integration of Molecular Components in Functional Macroscopic Systems” we are working on the adaptation of the ion implantation to monolayer materials. Our goal is to implant dilute shallow dopants to localise excitons for single photon emission but we also explore a possibility to form quantum dots by converting submicron areas of MoS2 into MoSexS(2-x).

In order to achieve this goal we collaborate with experts in low energy ion implantation at University of Goettingen who developed techniques to perform implantation with ion energies down to 10 eV and thus very suitable for interactions with 2D materials. We study the effect of changing implantation conditions such as ion energy and fluence, as well as monolayer surface preparation, aiming at maximizing the retention rate of implanted ions, while minimizing the formation of structural defects. In collaboration with the University of Limerick, Ireland, we study the resulting material using scanning transmission electron microscopy (TEM), which provides information about the incorporation of implanted ions in the host lattice, about structural defects and optical dielectric functions. In collaboration with ER-C we further develop TEM methods to measure electrostatic potentials of the defects [1, 2]. The collaboration with a theory group at PGI-1 in the Forschungzentrum provides us with a useful guidance in the choice of elements to implant. We use optical spectroscopy (microRaman, micro-reflectance and micro-photoluminescence mapping) to study the impact of implantation on the electronic structure of the material. Recently we have achieved a conversion of MoS2 into MoSexS(2-x) at the estimated level of x=10% according to Raman spectroscopy.

MoSe2 STEMSe implantation into a MoS2 monolayer investigated using high-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy, the latter allowing us to distinguish between S and Se atoms.

MoSe2 RamanRaman spectra of Se implanted MoS2 samples with different ion fluence. The line at 280cm-1 indicated Mo-Se bond. The spectral shape and ratio of intensities of the blue line gives estimated 10% of Se of the compound.

Reference

[1] F. Winkler, A. H. Tavabi, J. Barthel, M. Duchamp, E. Yucelen, S. Borghardt, B. E. Kardynal and R. E. Dunin-Borkowski, Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe2, Ultramicroscopy 178, 83 (2017).

[2] S. Borghardt, F. Winkler, Z. Zanolli, M. J. Verstraete, J. Barthel, A. H. Tavabi, R. E. Dunin-Borkowski and B. E. Kardynal, Quantitative Agreement between Electron-Optical Phase Images of WSe2 and Simulations Based on Electrostatic Potentials that Include Bonding Effects, Physical Review Letters 118 (8), 086101 (2017).


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