The project goal to join experiences of two world leading group working in different scientific field: group of Prof. Kirill Bolotin from FUB who is expert in the optical and electric properties of 2D materials and group of Prof. Oleg Vyvenko at SPSU who is expert in physical and structural properties of defects in semiconductors as well as in creation of defects in one of 2D material, in graphene.

Two dimensional materials have been one of the most actively investigated topics in the field of condensed matter physics. The family of two-dimensional materials includes Dirac metal graphene, direct bandgap semiconductors such as molybdenum disulfide (MoS2) or tungsten selenide (WSe2) and a wide bandgap insulator hexagonal Boron Nitride. While properties of these materials in their pristine state are now mostly understood, much less is known regarding the defects in these materials. That lack of knowledge is related to the difficulties associated with controlled creation and imaging of such defects. At the same time, it is hypothesized that such defects dramatically change photophysical properties of 2D materials. Below, we briefly review two mechanisms of defects influencing the photophysics of 2D materials.

First, the presence of defects in monolayer MoS2 and WSe2 likely gives rise to a new excitonic state, a defect-bound exciton[1]. Very briefly, weak screening in monolayer gives raise to excitons, electron-hole pairs with binding energy of hundreds meV. Such excitons are visible event at room temperature. In presence of charged defects, the excitons bind to defects forming a defect-bound exciton, a three-body state reminiscent of a hydrogen ion. While this mechanism of exciton formation has been widely discussed, many questions still remain: What is the atomic mechanism of defect-bound exciton formation? What type of defects confine excitons? How stable are defect bound excitons?

Second, the presence of defects in hexagonal Boron Nitride causes formation of Single Quantum Emitters (SPEs)[2]. Such SPEs likely result from a meta stable long-lived mid-gap in hBN. Multiple researchers are now considering the use of SPEs towards application in quantum information technologies. At the same time, the exact origin of the SPEs remains unclear: What is the atomic structure of the defect that forms an SPE? What is the role played by local strain?

The goals of this project are 1) to controllably induce defects in 2D materials, 2) to image the defects with atomic resolution, and 3) to study the influence of the defects on photophysics of 2D materials. Specifically, we will study defect-bound excitons in MoS2 or WSe2 and SPEs in hBN. We will follow the following workplan:

1)At FUB, we will create high quality suspended membranes of monolayer MoS2 and hBN. Suspended membranes are needed as substrate interaction can significantly affect defect formation. (We note that working with high-quality suspended samples is a distinguishing feature of our approach, this has not been done before)
2)At SPSU defects in monolayers will be induced via either Focused Ion Beam lithography (FIB) or helium ion beam lithography (HIB). The defects will subsequently be studied via atomically resolved TEM. By adjusting the acceleration voltage, we intend to vary the defect type from single dislocation to nm-sized holes.
3)At SPSU low-temperature cathodoluminescence hyperspectral images of the samples with defects will be obtained
4)Materials with different irradiation doses will finally be studied in the Bolotin lab at FUB via room- and high- temperature photoluminescence spectroscopy and photon counting. Defect-bound excitons will be evident as a low-energy peak in the photoluminescence spectra of MoS2 seen at temperatures below 40K. SPEs will appear as sharp peaks in the PL spectra of hBN; their single-photon nature will be confirmed via Hanbury Brown-Twiss (HBT) measurement. In the end, we expect to correlate the type and the density of the defects to the spectral properties (binding energy, width, etc…) of the defect related peaks in the photoluminescence.


[1] Zhong Lin et al 2016 2D Mater. 3 022002
[2] Koperski et al, https://arxiv.org/pdf/1708.03612.pdf