New way to characterise thickness of layered 2D materials

Context:

  • The isolation of single sheets of carbon, known as graphene, was awarded the Nobel Prize in 2010. This discovery has ushered in an era of two-dimensional materials. It offers immense possibilities in building electronics using similarly thin films.
  • A family of materials that have been investigated in this area are the transition metal dichalcogenides (TMDs), examples of which are molybdenum sulphide, molybdenum selenide, tungsten sulphide, tungsten telluride and so on.

Monolayer characteristics

  • It is necessary to know the characteristics of monolayers accurately if they are to be used in manufacturing or even designing devices. These include the thickness of the monolayers and the separation between pairs of monolayers.
  • A study by Dhirendra Vaidya and others from Indian Institute of Technology Bombay shows that it is incorrect to use the value of the actual physical thickness of the monolayer. The researchers compare earlier electrostatic calculations with an atomistic (density functional theory) approach that takes account of the ionic potentials.
  • While the qualitative features match, they find that the former is inaccurate as compared with the latter when it comes to the quantitative features.
  • The authors propose that this discrepancy is due to the tails in the ionic potentials, which extend into regions outside the edge of the ions themselves.
  • The genesis of the error is the protrusion of the chalcogen atomic potential into the space between the chalcogen-terminated layers.

Interlayer separation

  • This error becomes significant when the thickness of the device is small. “The disagreement is significant when the interlayer separation is comparable to the Van der Waals gap, or the natural separation between the bilayers,” 
  • If neglected, in the device analysis using the pure electrostatic model it can lead to a large error in the critical device parameters such as threshold voltage.
  • This finding would be important in designing futuristic devices such as the tunnel-field-effect transistor (TFET) which stand to challenge the CMOS switch – the standard device used in electronics today. It is expected that TFET could serve the demands of high energy efficiency of Internet-of-things applications.
  • The CMOS switch works by controlling the height of a thermal barrier and a potential of about 60 millivolt is needed to swing the current by an order of magnitude. On the other hand, the TFET works by controlling the thickness of a tunneling barrier, needing much less that 60 mV to effect the same swing.

Source:TH