20 Aug

Raman spectroscopy of monolayer and few-layer WTe2

Tungsten ditelluride (WTe2) is yet another member of the transition metal dichalcogenide family. Unlike other dichalcogenides, tungsten telluride is a semi-metal. In 2015, a Nature [1]  paper reported a non-saturating magnetoresistance in this compound which makes it an exciting systems for fundamental transport studies. As with other layered systems, Raman spectroscopy is a fast and non-destructive way to characterize this two-dimensional system. Don’t forget that we offer WTe2 is in our 2D crystal shop. In this article we examine WTe2 Raman spectrum.

The Raman spectrum of bulk WTeshows four main peaks in the region 100cm-1 – 300 cm-1, Figure 1. These are are approximately located at 118, 134, 164 and 212 cm-1. and are noted as the A13, A14, A17 and A19 respectively [2].

These peaks evolve with layer thickness:

  1. The A13  and A14 modes becomes completely extinct in monolayer WTe2
  2. The A14 mode modes completely extinct in bilayer WTe2
  3. The A17 mode does not shift with with layer thickness and finally
  4. The A19 mode blueshifts (shifts to higher wavenumber) for thinner layers.

In conclusion, the most reliable way to identify monolayer  WTe2  is the existence of only two Raman peaks in the region 100cm-1 – 300 cm-1.

Looking to buy a WTe2 single crystal? Get a quote at our webstore. We dispatch worldwide!

Figure 1. Raman spectrum of monolayer and few-layer Tungsten Ditelluride (WTe2).

Figure 1. Raman spectrum of monolayer and few-layer Tungsten Ditelluride (WTe2). The blue dash lines indicate the changes in the Raman spectrum with layer thickness.

References

[1] Large non-saturating magnetoresistance in WTe2. Ali et al Nature 2014

[2] Anomalous Lattice Dynamics of Mono-, Bi-, and Tri-layer WTe2. Kim et al arXiv 2015

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14 Aug

WS2 Raman spectroscopy

In this article we review the WS2 Raman spectrum. Its Raman spectrum is quite similar to it’s dichalcogenide neighbour material, molybdenum.

The Raman spectrum of bulk WS2 has two prominent peaks: an in-plane (E2g) mode located around 350cm-1 and an out-of-plane (A1g) mode which is located at 420cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the tungsten atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.

As WSbecomes single-layer these two modes evolve with thickness. The in-plane Raman mode upshifts, although very slowly (less than 1 cm-1), and the out-of-plane mode downshifts to 417cm-1. This means that monolayer WS2 can be reliably identified by the shift of the A1g mode.

Perhaps a more direct way of monitoring the transition to monolayer, in the case of WS2, is the observation of its photoluminescence. While bulk does not show any photoluminescence, as a result of its indirect band gap nature, monolayer shows strong and sharp photoluminesence which is a signature of a transition to monolayer state.

Don’t forget you can buy high quality tungsten disulphide crystals at our webstore.

References

[/fusion_builder_column][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][1] Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2 Zhao et al. Nanoscale, 2013

[2]  Identification of individual and few layers of WS2 using Raman Spectroscopy Berkdemir et al. Scientific Reports, 2013

[3] Electrical and optical characterization of atomically thin WS2 Georgiou et al. Dalton Transaction, 2014

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08 Aug

MoS2 Raman spectroscopy

In this blog post, we briefly describe the molybdenum disulfide’s, MoS2, Raman spectrum.

Like graphene, single-Raman spectroscopy of MoS2layer and few-layer molybdenum disulphide (moly) has distinctive signatures in its Raman spectrum. first, let’s discuss the Raman spectrum of bulk MoS2.

The Raman spectrum of bulk MoS2 has two prominent peaks: an in-plane (E2g) mode located around 383cm-1 and an out-of-plane (A1g) mode which is located at 407cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the Molybdenum atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.

As the layer thickness becomes single-layer these two modes evolve with thickness, a clear evolution in MoS2 Raman spectrum. The in-plane mode upshifts to 386cm-1 and the out-of-plane downshifts to 404cm-1The difference of these two modes (~18cm-1) can be used as a reliable identification for monolayer MoS2

Additionally, the photoluminescence of MoS2 changes with the number of layers. This means that while bulk MoS2 is an indirect semiconductor (1.3eV), monolayer MoS2 is a direct gap (~1.8eV). [4]

Don’t forget you can buy bulk MoS2 crystals at our store.

 

References
  1. Anomalous lattice vibrations of single- and few-layer MoS2Lee et al, ACS Nano, 2011.Thickness-dependent Raman spectroscopy of MoS2 flakes
  2. Phonons in single-layer MoS2 and WS2Molina-Sanchez et al, Physical Review B, 2011.Theoretical paper discussing evolution of phonon dispersions, focus on the out-of-plane and in-plane mode. It discusses in detail the mechanisms of shifting of the two peaks.
  3. From bulk to Monolayer MoS2: Evolution of Raman scatteringLi et al, Advanced
    Functional Materials, 2012
    .Multi-wavelength thickness-dependent Raman characterization of MoS2.
  4. Emerging photoluminescence in monolayer MoS2Splendiani et al, Nano letters 2010Layer-dependence of photoluminescence from atomically thin MoS2.

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