“Intercalating with specific molecules of our choice allows us to turn 2D nanostructures into functional nanomaterials whose properties can be tuned,” team leader Yi Cui told nanotechweb.org. “In this work, we focused our attention on tuning the optical and plasmonic properties of metal dichalcogenides, such as bismuth selenide (Bi2Se3) nanoplates and related compounds.”

The compounds studied are layered materials in which one layer is made up of five atomic planes (Se-Bi-Se-Bi-Se) bonded strongly together. These layers, each around 1 nm thick, are loosely stacked to form individual crystals. The Stanford researchers looked at how photons and collective oscillations of electrons (or plasmons) travel within the material.

These 2D structures are ideal for studying intercalation chemistry, because we can insert a variety of different molecules, atoms and ions into the gaps between the crystal layers, explains Cui. This allows us to control the optical and plasmonic properties in a controllable way thanks to the number and type of intercalates we employed.

Other ways of tuning the nanolayer properties

The scientists tuned the properties of the nanolayers in two other ways too. First, they controlled the thickness of the plates, something that was relatively easy to do since the crystals are formed by stacking 1 nm thick layers. Second, they changed the atomic composition of the nanoplates, going from 100% Bi2Se3 to Bi2(SexTe1–x)3, where Te is telluride, and then Bi2Te3. The material properties progressively change as Se atoms are gradually replaced with Te ones.

Although photons and plasmons do not travel very long distances in these dichalcogenides, there are many applications where long propagation of plasmon waves is not required – for example, in optical antennas, perfect absorbers and ultracompact modulators, says co-team leader Mark Brongersma. “Intriguingly, we might even be able to intercalate a gain medium between the crystal layers that can counteract the photon and plasmon losses. Once this issue has been solved, we believe that such 2D nanostructures might be ideal as new types of optical components in a variety of electronics devices.”

Cui adds that one possible application may be in transparent electrodes, where intercalation could be used to increase how optically transparent the dichalcogenides are while increasing their electrical conductivity.

The research is published in Nano Lett. DOI: 10.1021/nl402937g

Further reading

Defects help enhance photoluminescence in 2D semiconductors (Sep 2013)
TMDC "wallpaper" produces electricity (May 2013)
Surface charge transfer n-dopes semiconducting thin films (May 2013)