Fabrication and Characterization of Atomically thin Transition Metal Dichalcogenides (2D TMDs) and their heterostructure for the achievement of Novel Nano-devices

Session

PSA.1 - 1D & 2D Materials

Authors

Mr Samad Abdus (1)

Affiliations

1. University of Limerick

Keywords

2D Materials, TMDs, nanodevices, 

Abstract text

The interest in atomically thin 2D materials has grown remarkably since the discovery and isolation of graphene, the 2010 Nobel Prize 2D material, due to its structural flexibility and tuneable electrical properties[1][2][3]. Graphene has been one of the most well researched 2D materials, but its zero bandgap limits its applications in electronic devices[4]. This has fuelled research into layered Transition Metal Dichalcogenides (TMDs), which exhibit a direct bandgap when thinned down to monolayer[5]. Furthermore, these 2D sheets can be assembled into heterostructure for many novel properties[6][7][8].

 To observe and tailor these properties, the following are the aims of this study;

1.      Mechanical fabrication of the pristine TMD monolayers followed by characterization.

2.      To develop a pristine TMD heterostructure mechanically by vertically stacking tungsten dichalcogenides nanosheets (WS₂ and WSe₂) using the mechanical exfoliation method.

3.      Characterizing monolayers and heterostructures on different substrates using a combination of Raman and Photoluminescence (PL) spectroscopy.

4.      Investigate the sliding effect exhibited by exfoliated 2D sheets under ambient conditions.

 

 

WS2 and WSe2 monolayers and heterostructure were fabricated by the method previously reported [9]. In brief, flakes of both materials were obtained by mechanical exfoliation of high-quality monolayers on a PMMA coated Si/SiO₂ substrate and microscope glass slide for optical characterization (Olympus BX51 with 10x, 50x and 100x optical magnifications). Once the areas of interest were located, Raman and Photoluminescence (PL) spectroscopy were used for the spectroscopic characterization. The heterostructure sample was achieved by placing down two different monolayers and have them overlap to form a vertical heterostructure.

 

Raman spectroscopy was used for layer identification on both substrates. The samples were excited by 532nm green laser at ambient conditions, using Horiba LabRAM HR Evolution (LabSpec6 software) with 10x, 50x and 100x objectives. A 100x objective focuses the laser beam to a ~1 μm spot diameter in the ambient atmosphere at room temperature. Si Raman band at 520 cm⁻¹ was used as an internal frequency reference. For PL characterization, the samples were excited by a 405 nm laser diode (Thorlabs CPS405) through a 50x objective (ULWD MIRPlan NA=0.55). The same objective collected the PL signal and analyzed by a Spectrometer (ANDOR SR-193i-B1) with an 800 l/mm grating. PL spectra were acquired at ambient conditions.

 

The layer thickness of the samples was identified by observing the phonon shifts in the Raman spectra. WS₂ nanosheet layer thickness was identified by observing in-plane mode (E¹_2g), out-of-plane (A_1g) and acoustic phonon 2LA (M) mode on glass slide and PMMA coated Si/SiO₂ substrate. The heterostructure Raman spectra was a superposition of WS₂ and WSe₂ individual spectra. The shifts in A_1g and E¹_2g were attributed to strong interlayer vdW coupling between WSe₂ and WS₂ nanosheets.

 

Photoluminescence spectroscopy was used to identify the layer thickness and bandgap studies. The WS₂ spectra showed a sharp PL peak at 2.02eV which confirmed the nanosheet being monolayer and 2.02 eV was attributed to the bandgap. Interestingly, WS₂ PL spectra at room temperature showed a shoulder peak which was analyzed using Lorentzian fit and was attributed to ‘room temperature trion’ at 1.97eV.

 

SEM images indicated the non-uniformity of monolayers in a heterostructure, which is another evidence of slight shifts in Raman modes.  SEM degradation of nanosheets was observed at 5kV which started with monolayers. EDX was done to qualitatively confirm the elemental composition of these stacked sample flakes.

 

Preparing samples on glass substrates allowed sliding and rotation of 2D nanosheets in ambient conditions. This has the potential to improve the reproducibility and ease of heterostructure fabrication.  Using a micromanipulator TMDs can be moved and rotated on a glass slide substrate to align the target flakes in heterostructure fabrication. This phenomenon was investigated in SEM using microprobe but sliding did not occur. One idea was that a negative static charge on the substrate in the SEM might have an effect on the nanosheet binding with the substrate. To rule out any negative charging effects FIB was employed to charge the substrate positively, however, no sliding was observed in either case. It was surmised that the sliding phenomenon is most likely due to airborne water layers adsorbed between the substrate and the TMD flakes. The study of structure and nature of water confined between TMDs layer and substrate as well as between WS₂ and WSe₂ layers as a heterostructure is ongoing.  Also, a study of doping effects these confined water layers might have on individual monolayers and heterostructures is proposed as future works. 

References

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