A) Free-standing films of BN, MoS2 and WS2. On the right is a graphene free-standing film. The bottom row represents hybrids of graphene mixed with BN, MoS2 and WS2. In each case the mass ratio is 50:50. B) Stress strain curves for the films in A. C) Free standing hybrid WS2/SWNT films.
A new way of splitting layered materials to give atom thin “nanosheets” has been discovered. This has led to a range of novel two-dimensional nanomaterials with chemical and electronic properties that have the potential to enable new electronic and energy storage technologies. The new method is simple, fast, and inexpensive, and could be scaled up to work on an industrial scale. The work will open up over 150 similarly exotic layered materials – such as Boron Nitride, Molybdenum disulfide, and Bismuth telluride – that have the potential to be metallic, semiconducting or insulating, depending on their chemical composition and how their atoms are arranged. This new family of materials opens a whole range of new “super” materials.
Mechanical characterisation of free standing films and composites.
If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.
Of the many possible applications of these new nanosheets, perhaps the most important are as thermoelectric materials. These materials, when fabricated into devices, can generate electricity from waste heat. For example, in gas-fired power plants approximately 50% of energy produced is lost as waste heat while for coal and oil plants the figure is up to 70%. However, the development of efficient thermoelectric devices would allow some of this waste heat to be recycled cheaply and easily, something that has been beyond us, up until now.
“Our new method offers low-costs, a very high yield and a very large throughput: within a couple of hours, and with just 1 mg of material, billions and billions of one-atom-thick nanosheets can be made at the same time from a wide variety of exotic layered materials,” explained Dr Nicolosi, from the University of Oxford.
These new materials are also suited for use in next generation batteries – “supercapacitors” – which can deliver energy thousands of times faster than standard batteries, enabling new applications such as electric cars. Many of these new atomic layered materials are very strong and can be added to plastics to produce super-strong composites. These will be useful in a range of industries from simple structural plastics to aeronautics.
The collaborative* international research led by the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Ireland, and the University of Oxford has been published in this week’s Science
Thermoelectric properties of SWNT/WS2 composites studied in this work. A) DC
conductivity, B) Seebeck coefficient, C) power factor, D) thermal conductivity and E) figure of merit, zT. In panels A, B, C & E, the 0% data is taken from Ellmer et al(S30), while in panel D, the thermal conductivity of WS2 is taken from McLaren (~1.35 W/mK)
WS2 is not a promising thermoelectric material (ZT was only 0.002), it is used here as a model system. Within the TMDs, TiS2 has a much higher Seebeck coefficient. Exfoliation of this material and subsequent addition of nanotubes should result in much higher zTs. One of the most promising low temperature thermoelectric materials is Bi2Te3. We have shown that this material can be exfoliated using the same technology we describe here. Thus, preparation of Bi2Te3/SWNT films may result in extremely large zT values. Thus we believe that exfoliation of layered materials may lead to a breakthrough in the production of high zT materials.