28 January 2020 Özgür Burak Aslan


Speaker: Özgür Burak Aslan
Terabix, CA

Tuning Excitons in 2D Materials for Physics and Optoelectronics

Date: January 28, 2020
Time: 14:30
Cookie & Tea: SCI 103, 14:15
Place: SCI 103
web: https://physics-seminars.ku.edu.tr/


Excitons dominate the optical spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs; 2D Materials) due to their high binding energy and oscillator strength [1]. It is thus invaluable to understand them for many-body physics and fundamental studies on TMDCs. The ability to tune them is, moreover, very valuable for optoelectronic applications. Here, we first employ dielectric screening to tune the excitons. Contrary to most studies, we decrease  the dielectric screening by making suspended monolayer (1L) WSe2 samples [2]. We see that the binding energy of the ground state  exciton ( ) increases from about 0.3 eV (on substrate) to above 0.4 eV (suspended). Despite the significant change in the binding energy, the peak position of  is weakly affected and in a static way. To achieve larger and dynamic tuning, we next employ mechanical strain. We achieve a redshift of 100 meV in the optical band gap, providing a way of engineering the optical spectra of 2D materials. Surprisingly, we observe significant decrease in the spectral linewidth of the excitonic absorption and emission, due to the suppression of the intervalley exciton-phonon scattering [3]. We lastly combine the two cases and apply strain to the suspended 1Ls. As the TMDCs are impermeable to air [4],  we apply air pressure to the suspended 1Ls and realize an easy way of reversible tuning of the optical spectra. Due to the spectrally narrow excitonic features, we observe that the reflection spectra of the 1Ls are strongly dependent on strain and the wavelength of interest. In this manner, we establish strain as a means of probing the 2D materials and we pave the way to the use of 2D materials as active tunable elements for flat optics. We then propose novel ideas using graphene, quantum dots and TMDCs using strain engineering.

[1]  Chernikov, A.; et al. Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2. Phys. Rev. Lett. 2014, 113, 076802.

[2]  Aslan, O. B.; et al. Suspended Excitons in 2D Materials. Unpublished.

[3]  Aslan, O. B.; Deng, M.; Heinz, T. F. Strain Tuning of Excitons in Monolayer WSe2. Phys. Rev. B 2018, 98, 115308.

[4]  Bunch, J. S.; et al. Impermeable atomic membranes from graphene sheets. Nano Lett. 2008, 8, 2458-62.

 Short Bio:

Ozgur Burak Aslan will be joining the department of Physics at Bogazici University as a faculty member in June 2020. Burak currently works as a Senior Optical System Engineer at Terabix (a telecommunications company). He previously worked as a Postdoctoral Researcher at Stanford University in the department of Materials Science and Engineering where he utilized strain engineering of 2D materials as active tunable elements for flat optics (2018 Jan – 2019 Jan). Prior to that, he was a Visiting Graduate Researcher (2015 May-2017 May) and later a Postdoctoral Researcher (2017 June – 2017 Dec) at Stanford in the department of Applied Physics where he realized that strain can be a means of probing 2D materials for fundamental physics as well as of tuning them for engineering.

Burak received his Ph.D. in Physics from Columbia University (2017) and B.S. degrees in Electrical Engineering and Physics from Bogazici University (2012). He is interested in the strain engineering of low dimensional nanomaterials including graphene, quantum dots and semiconducting transition metal dichalcogenides for physics and optoelectronics.