27 Jul

MoTe2 Raman spectrum

MoTe2 Raman spectrum

Yet another member of the two-dimensional family is molybdenum ditelluride. Like its other transition metal dichalcogenide siblings, MoTe2 is a semiconductor and when exfoliated to a monolayer shows a direct band gap of approximately 1.1 eV. In this article, we discuss the MoTe2 Raman spectrum.

The MoTe2 Raman spectrum shows a single prominent peak at approximately 233cm-1, corresponding to an in-plane E12d mode. The intensity of this mode increases with decreasing thickness.

The other two modes visible are one at 173cm-1, corresponding to an out-of-plane A1g and at 289cm-1, corresponding to an B12g mode.

Further information:

  1. Get a quote for MoTe2 crystals
  2. Recently published papers on MoTe2
02 Feb

MoS2 tunable band gap

A team of researchers from Purdue University and Global Foundries semiconductor foundry, led by Associate Professor Zhihong Chen write in Nano Letters regarding the band gap tenability of bilayer MoS2.

Why bilayer MoS2

It is well known that bilayer graphene has a tunable band gap, in the range of 0-300meV. However this band gap is a limitation for mid-infrared range applications which require larger gaps. On the contrary, TMDC demonstrate sizeable band gaps with many in the infrared to visible range. The paper, entitled “Electrically tunable bandgaps in bilayer MoS2” provides the experimental verification of continually tunable bandgap in bilayer MoS2 by means of a dual-gated transistor geometry.

Transport measurements

Samples were mechanically exfoliated MoS2 flakes and bilayers were identified with Raman and AFM measurements. A HfO2 top gated was defined by atomic layer deposition. The high-κ dielectric enables sufficient hole current injection to the MoS2 layer which shows ambipolar behaviour. The band gap can then be extracted by taking into consideration the applied electric field, the threshold voltages when the Fermi level enters the conduction or valence bands and the applied electrical bias. For monolayer MoS2, no changes are observed for the band gap, but for bilayer it was found that there is a reduction of 260meV per 1/Vnm displacement field.

Optical measurements

Such tunable bang gaps may also be optically verified from the photoluminescence spectrums. In order to allow optical access to the MoS2 flake graphene, instead of metal, was used as a top contact. In agreement with the transport measurements, A exciton peak at 1.83eV, changes its possible as the top gate is sweeped. Finally, density functional calculations complete the set by providing excellent agreement between transport, optical measurements and simulations.
Such bandgap tuning of TMDC can be immensely useful in electronic and optoelectronic applications, however, large area, controllable, low-temperature and self-limiting growth of bilayer MoS2 graphene remains to be addressed before MoS2 can realise its full potential.

  1. Read more: “Electrically tunable bandgaps in bilayer MoS2” Nano Letters Chu et Al.
  2. Recent papers on MoS2 http://2dresearch.com/tag/mos2/
  3. Get a quote for MoS2 single crystals.
26 Nov

Superacid makes brighter MoS2

Writing in Science, a group of scientists led by Berkeley professor Ali Javey report on significant improvement of quantum yield (QY) of monolayers of MoS2 when coated with the superacid bis(trifluoromethane)sulfonimide (TFSI).

More Light

Monolayers of transition metal dichalcogenides have been steadily attracting increasing attention in field-effect devices as well as in optical devices. This is due to their sizeable band gap which typically lie in the 1-2 eV range. However, they have been traditionally lacking in terms of their quantum yield, i.e. the number of photons emitted per photons absorbed, which is an important figure of merit for optoelectronic devices.  The paper described a technique which improved the quantum yield to near unity.

Soak and bake MoS2

The procedure is surprisingly simple: A 2g/L solution of the superacid TFSI is prepared in 1,2-dichloroethane (DCE), which is further diluted to 0.2g/L by either DCE or 1,2-dichlorobenzene (DCB). The samples, which are exfoliated flakes on oxidized silicon substrates, are then immersed in this solution for 10 minutes on a hotplate set for 100°C. The resulting samples show improvement of QY from 0.6% to >95%, and a photoluminescence (PL) improvement of 130 times.

Hydrogen peroxide and toluene

It should be noted that earlier in September, another similar technique was published in RSC Advances, which coated flakes with a layer of hydrogen peroxide and toluene. This technique improved the PL by 30 times.

Further reading:

(1) Near-unity photoluminescence quantum yield in MoS2 Amani et al. Science 2015

(2) Tuning photoluminescence of single-layer MoS2 using H2O2  Su et al. RSC Advances 2015

 

28 Oct

Review article on atomically thin MoS2 and its applications in sensing

Editorial 

Writing in Nanoscale scientists from the universities in Singapore and China review the two-dimensional material molybdenum disulfide. The material has been extensively studied in the past five years since, unlike graphene, has a sizeable bandgap which make it attractive for applications in electronics. Through their exhaustive 40-page article, the authors provide an extensive MoS2 review: properties (electronic/mechanical), production techniques (mechanical/ liquid exfoliation/CVD).

Applications in sensing

Finally, the authors of the MoS2 review paper, place a strong focus on sensing applications and they review electrochemical sensing, FET sensing, fluorescent sensing, gas sensing and several other types of biosensing of this atomically thin MoS2 semiconductor.

We offer MoS2 single crystals

Don’t forget that Manchester Nanomaterials offer single crystals of MoS2 at our webshop! We deliver worldwide with 1-3 day delivery.

Further reading: “Two dimensional atomically thin MoS2 nanosheets and their
sensing applications ” Huang et al. Nanoscale 2015

15 Oct

Reducing contact resistance in WS2

Editorial

Writing in Applied Materials & Interfaces  scientists from the Sejong University, Seoul report their technique in reducing contact resistance in tungsten disulfide (WS2) field-effect transistors with standard Cr/Au contacts. Although there are several techniques to do this (e.g. using scandium, or phase-engineering) this technique is noted for being particularly straightforward.

The technique relies on dipping the samples in 0.01M aqueous solutions of LiF (lithium fluoride) and baking the sample briefly at 80°C before drying under nitrogen. The technique reduces the contact resistance in WS2 by a least an order of magnitude and introduces n-doping to the sample.

Further reading:

1. “Highly Stable and Tunable Chemical Doping of Multilayer WS2 Field Effect Transistor: Reduction in Contact Resistance”  Applied Materials & Interfaces 2015.

2. Recent papers on WS2.

3. (Sponsored)  Buy WS2 crystals. USD$449 with free worldwide delivery.

10 Oct

Researchers find easy way to enhance photoluminescence in MoS2

Writing in RSC Advances, a team from  Hangzhou Dianzi University led by Weitao Su demonstrates a fast and easy way in order to improve the photoluminescence yield of monolayer molybdenum disulfide. The methodology begins with the typical micro mechanical exfoliation of atomically thin layers of MoS2, which are subsequently deposited on oxidized silicon substrate. Monolayer are identified through a combination of optical and Raman spectroscopy. The samples are process in order to increase their photoluminescence.

The technique involves coating the substrate and flakes with a layer of hydrogen peroxide  and toluene. This improves the photoluminescence in MoS2 by up to 30x times, a quite remarkable improvement.

Further reading “Tuning Photoluminescence of single-layer MoS2 using H2O2” Su et al, RSC Advances

29 Sep

Single layer WSe2 actually has indirect band gap

Writing in Nano Letters a team  led by Professor Chih-Kang Shih from the University of Texas at Austin studied the electronic structures of single layer transition metal dichalcogenides WSe2 and WS2, by scanning tunneling spectroscopy.

It is generally well understood within academia that monolayer semiconductor transition metals dichalcogenides, such as WS2, WSe2, MoS2 and others, have an indirect-to-direct band gap transition as they make the transition from bulk to single layer. This is typically manifested as a strong photoluminescence in monolayer flakes.  Although this is true for MoSe2, the team found that the band gap minimum for WS2 is in fact located at the Q-point of the Brillouin zone, instead of the K, although the two states are nearly degenerate.

Manchester Nanomaterials specialize in offering single crystals of transition metals dichalcogenides and more at our webshop!

Further reading: “Probing critical point energies of transition metal dichalcogenides:
surprising indirect gap of single layer WSe2

21 Sep

Superconductivity in graphene intercalated with Ca and hBN

In a paper published on arXiv teams from the University of Manchester and University College London, report superconductivity in graphene when intercalated with calcium.

In this work our hexagonal boron nitride crystals were also used, as a physical spacer for monolayer graphene flakes. The high purity of our crystals and large domain sizes were of imperative importance for this work.

Further reading Superconductivity in Ca-doped graphene

01 Sep

WSe2 Raman spectoscopy

Tungsten diselenide is another layered transition metal dichalcogenide. Like more of the other TMDCs, it is a semiconductor with a band gap in the infrared region. In this article we review the Raman of WSe2 monolayer.

Like MoS2 and WS2, the lattice structure of WSe2 is also trigonal prismatic, while the lattice constant is 3.28 Å. The band gap of WSe2 is around 1eV.

Don’t forget you can purchase WSe2 crystals in our webshop with free worldwide delivery.

The raman spectrum of bulk and monolayer tungsten diselenide.

The Raman spectrum of monolayer tungsten diselenide (left) and its strong photoluminescence near 750nm (right).

Further information

  1. Get a quote for a WSe2 single crystal
  2. Recent papers on WSe2
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