The Stratakis Lab

The Stratakis Lab

Ultrafast Laser Micro and Nano Processing Laboratory

Laser engineering of spectroscopy on graphene and RELATED 2d materials

The research topic aims at the investigation and manipulation of the optoelectronic properties of graphene and other 2D materials.

Graphene and monolayers of transition metal dichalcogenides (TMDs) are atomically thin two-dimensional crystals with exotic optoelectronic properties, which are promising for emerging applications in optoelectronics and nanophotonics. We aim at the investigation and interpretation of the photoluminescence (PL), Raman, differential reflectivity (δR) and valley polarization properties of Chemical Vapor Deposition-grown (CVD) and exfoliated graphene & TMD monolayers, in temperatures ranging from 4K up to 340K. The excited state electron relaxation dynamics is investigated using time-resolved femtosecond pump-probe spectroscopy. The research activity is carried out with a collaboration of the Ultrafast Laser Micro- and Nano- Processing (ULMNP) group of the Institute of Electronic Structure and Laser (IESL) of the foundation for Research and Technology Hellas (FORTH) and the Laboratory of Applied Physics of the Physics Department of the Aristotle University of Thessaloniki, headed by Prof. Panos Patsalas. Auger electron imaging and spectroscopy, as well as X-ray photoelectron spectroscopy (XPS) studies complemented with the aforementioned spectroscopic measurements offer a thorough analysis of the optoelectronic properties of graphene and 2D monolayers. In addition, we introduce a simple photochemical doping technique to control the crystals’ electron density via pulsed UV laser irradiation in gas media. Furthermore, irradiation of 2D crystals with ultrashort laser pulses, provides an easy approach to form arbitrary patterns on the monolayer’s surface resulting in extraordinary phenomena. We envisage that our novel findings could find diverse applications in the development of graphene and TMDs-based optoelectronic devices.

Contact Person(s):
Dr. Emmanuel Stratakis

Laser engineering & spectroscopy of graphene and other 2D materials provide the principal knowledge towards the development of state-of-the-art optoelectronic devices.

Research Topics
·  Spectroscopy of graphene and other 2D materials

·  Photochemical doping of graphene and other 2D materials

·  Laser patterning of graphene and other 2D materials

Abstract
Graphene and monolayers of transition metal dichalcogenides (TMDs) are promising new materials for future 2D nanoelectronic systems. With their tunable direct gap in the visible range of the optical spectrum and high surface-to-volume ratio, these 2D semiconducting systems are ideal for field-effect transistors, photovoltaics, light-emitting diodes (LEDs), single-atom storage, molecule sensing, quantum-state metamaterials and electrocatalytic water splitting applications. Furthermore, the outstanding stretchability of 2D crystals is promising for strain engineering and related applications. In this research topic we aim at the investigation and manipulation of the optoelectronic properties of graphene and other 2D materials. For example, the boundaries of 2D TMDs crystals are reported to be non-atomically sharp and extremely susceptible to their environment which results into significant variations of the electronic properties across the TMDs’ monolayers’ surface. This affects not only the optical but also their transport properties. Moreover, we focus on the investigation of the temperature dependent spin-valley polarization properties of TMD monolayers in a range of temperatures from 4K up to room temperature. Intervalley scattering, electron-hole radiative recombinations, electron-phonon interactions and Auger effects are dominant mechanisms that negatively affect the temperature dependence of the valley polarization properties in 2D TMDs. Towards the development of 2D valleytronic devices, we target on the suppression of the undesirable aforementioned depolarization mechanisms in elevated temperatures. In addition, the development of a new photochemical doping technique that will allow fine tunability of the optoelectronic properties of TMDs monolayers is introduced. Intentional doping that allows fine control of the crystal’s carrier density can be achieved by exploiting the interaction of UV nanosecond pulses with TMD monolayers in different environments. Future investigations involve fundamental studies and laser assisted photochemical doping on a large variety of 2D crystals in several gas media.

Figure 1: (a) Typical optical microscopy image of exfoliated WS2 flakes. (b) Fluorescence image of WS2 monolayer. The stronger PL emission originates from the edges, creating a “donut” effect. (c) Intensity profile across the flake, following the line scan of Fig. 1b. (d) PL spectra comparison of two different spots on the flake (inset of Figure 1d). The PL emission from the central area is dominated by charged exciton (trion) recombination (X, at 1.97eV) while the emission from the edges is mainly due to the neutral excitons (X0 at 2.01eV).

Figure 2: Peak position of trion (black bullets) and neutral (red bullets) excitons in exfoliated WS2 monolayers as a function of the number of irradiation pulses in rich Cl2 environment. Significant shifts of both excitonic features indicate considerable changes in the electron density of the material.

Figure 3: Temperature-dependent PL spectra of an exfoliated WS2 monolayer under 3.8kW/cm2 excitation power density. The existence of biexcitons in mechanically exfoliated WS2 flakes from 78K up to room temperature is shown.

Figure 4: Temperature dependent circular polarization degree of WS2 on SiO2 substrate. A similar trend between neutral (X0) and charged (X) excitons is observed.

Collaborators
Prof. George Kioseoglou, Dept. of Materials Science and Technology, University of Crete

Prof. Panos Patsalas, Laboratory of Applied Physics, Dept. of Physics, Aristotle University of Thessaloniki

Publications

  • Room Temperature observation of biexcitons in exfoliated WS2 monolayers
  1. Paradisanos, S. Germanis, N.T. Pelekanos, C. Fotakis, E. Kymakis, G. Kioseoglou and E. Stratakis, Appl. Phys. Lett., 110 (2017), 193102.

 

  • Spatial Nonuniformity in Exfoliated WS2 single layers
    Paradisanos, N. Pliatsikas, P. Patsalas, C. Fotakis, E. Kymakis, G. Kioseoglou and E. Stratakis, Nanoscale, 8 (2016), 16197.
  • Intense Femtosecond Photoexcitation of bulk and monolayer MoS2
  1. Paradisanos, E. Kymakis, C. Fotakis, G. Kioseoglou, and E. Stratakis, Appl. Phys. Lett., 105 (2014), 041108.

 

Project Members

Dr. Emmanuel Stratakis

Prof. George Kioseoglou

Dr. Sotiris Psilodimitrakopoulos

Dr. Leonidas Mouchliadis

Mr. Ioannis Paradisanos
Ms. Ioanna Demeridou

Mr. Antonis Papadopoulos

Mr. Georgios Kourmoulakis

Mr. Dimitrios Labrinoudakis

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