Quasiparticle dynamics and optical properties of alkali metal doped few-layered transition metal dichalcogenides

• Applicant: Professor Dr. Alexander Grüneis
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 278161773

• Final Report Year: 2019

Final Report Abstract

In the first part of this project, the electronic properties of two-dimensional transition metal dichalcogenides (TMDCs) have been tuned by chemical functionalization and spectroscopically investigated. In particular, we have performed chemical doping of MoS2 by Li adsorption onto the surface of MoS2. For low doping, we have found a change of the luminescence intensity and helicity. That is, Li adsorbed to the surface of MoS2 causes quenching of the luminescence and a loss of the valley polarization. For large Li doping, we have performed angle-resolved photoemission spectroscopy (ARPES) and found degenerate doping of the material. In particular, we have found, using ARPES, that the Fermi level can be shifted by about 80 meV into the conduction band of MoS2. Further work on doping TMDCs by Fe was performed and we found that even sub-monolayers of Fe adsorbed to the surface of a TMDC can substantially shift the energy band structure and change the luminescence. We understand this effect from consideration of the dielectric environment which influences the electronic and optical properties. This effect is particularly important for 2D materials since all their atoms are on the surface. For example, the electric field lines of electron and hole (the constituents of an exciton) reach out to the environment (i.e., field lines pass through the environment and hence probe the environment’s dielectric function). As a consequence, the exciton binding energy, and hence the luminescence of a 2D material depend sensitively on the substrate and adsorbates. The dielectric function also affects the screening of electrons and the electron energy band structure. Thus, the energy band structure and the optical properties are dependent on adsorbate atoms. We have used this effect to control a transition from direct to indirect energy band gap semiconductor and energy of the optical emission. To that end we have carried out ARPES and luminescence measurements on in situ Fe doped MoS2. Regarding metallic TMDCs, we have investigated alkali metal doped ZrSe2 and VSe2 in their bulk form using ARPES and Raman spectroscopies. We have found evidence for a charge density wave in the latter by combining Raman, ARPES and tunneling spectroscopies. In the second part of the proposal, the growth of TMDC nanoribbons on stepped crystals has been started by molecular beam epitaxy. As a reference material, and in order to test the transfer methods from the Au crystal to SiO2/Si, we have performed growth of graphene nanoribbons (GNRs). We have developed a transfer method that allows us to transfer oriented ribbons onto arbitrary substrates. We expect that this method can be applicable also for the transfer of MoS2 ribbons. The corresponding publication has been chosen as a "Cover page" of the journal "Nano Letters". Surprisingly, we have found that luminescence in GNRs could be strongly enhanced by hydrogenation in ultra-high vacuum conditions which causes defect states inside the band gap. This unexpected result led us to carry out further investigations of GNRs grown on stepped Au-surfaces and after their transfer to insulating substrates.

Publications

• Making graphene nanoribbons photoluminescent. Nano letters 17 (7), 4029 (2017)
  BV Senkovskiy, M Pfeiffer, SK Alavi, A Bliesener, J Zhu, S Michel, AV Fedorov, R German, D Hertel, D Haberer, L Petaccia, FR Fischer, K Meerholz, PHM Van Loosdrecht, K Lindfors, A Grüneis
  
• Effect of lithium doping on the optical properties of monolayer MoS2. Appl. Phys. Lett., 112(12):121902, 2018
  N. Saigal, I. Wielert, D. Capeta, N. Vujiic, B.V. Senkovskiy, M. Hell, M. Kralj, A. Grüneis
  
• Narrow photoluminescence peak of epitaxial MoS2 on graphene/Ir(111). 2D Materials 6 011006 (2018)
  N. Ehlen, J. Hall, B. V. Senkovskiy, M. Hell, J. Li, A. Herman, D. Smirnov, A. Fedorov, V. Yu. Voroshnin, G. di Santo, L. Petaccia, T. Michely, A. Grüneis
  
• Resonance Raman spectrum of doped epitaxial graphene at the Lifshitz transition. Nano letters
  M.G. Hell, N. Ehlen, B. V. Senkovskiy, E. H. Hasdeo, A. Fedorov, D. Dombrowski, C.Busse, T. Michely, G.di Santo, L. Petaccia, R. Saito and A. Grüneis
  
• Synthesis and spectroscopic characterization of alkali-metal intercalated ZrSe2. Dalton Trans., 47:2986–2991, (2018)
  K. Nikonov, N. Ehlen, B. Senkovskiy, N. Saigal, A. Fedorov, A. Nefedov, C. Wöll, G. Di Santo, L. Petaccia, and A. Grüneis