Tikalon Blog by Dev Gualtieri

Light-Emitting Diodes of Tungsten Diselenide

March 17, 2014

Layered materials have become the darling of our age of nanotechnology. It's easy to strip away flakes of just a few atomic layers from some materials. It all started with graphene , single-layer sheets of carbon atoms harvested from the graphite allotrope of carbon by the simple process of sticking a piece of cellophane tape on a piece of graphite and pulling it off. You can see a demonstration of this process in a YouTube video.[1]

Since the layers of graphite easily detach and slide against each other, graphite is used as a lubricant. The United States Geological Survey estimates that 1,350 metric tons of graphite were used as a lubricant in the United States in 2012.[2] The hexagonal polymorph of boron nitride, composed of elements flanking carbon in the periodic table, has a structure like graphite, and it's also a good lubricant.

Molybdenum disulfide (molybdenite, Mo S ₂), is another hexagonal layered material used as a lubricant (see figure). I wrote about molybdenum disulfide in a previous article (Molybdenum Disulfide Circuitry, December 9, 2011). The lubricity arises from the weak atomic forces binding the adjacent atomic planes of sulfur.

Crystal structure of molybdenum disulfide

A layer of molybdenum disulfide (molybdenite, MoS₂).[3]

(Via arXiv Preprint Server).

Tungsten diselenide (WSe₂), a compound formed from layers of tungsten and selenium , has an hexagonal crystal structure like that of molybdenum disulfide. As early as 1985, WSe₂ was shown to be an effective photovoltaic material with 14% efficiency under simulated solar radiation, albeit with a polyiodide liquid junction.[4] This presaged some recent work on making junction diodes of tungsten diselenide, and light emitting diodes as well.

Experiments on junction diodes of tungsten diselenide were reported in a trio of papers in a recent issue of Nature Nanotechnology.[5,8-9] One of these was authored by a group at the Massachusetts Institute of Technology.[8] Another at the Vienna University of Technology (Vienna, Austria).[9] The third was by a large international team of scientists from the University of Washington (Seattle, Washington), the Justus Liebig University (Giessen, Germany), the University of Tennessee (Knoxville, Tennessee), Oak Ridge National Laboratory (Oak Ridge, Tennessee), the National Institute for Materials Science (Tsukuba, Japan), and the University of Hong Kong (Hong Kong, China).[5] It appears that in this research, as in other examples, the research teams get larger as the materials get smaller.

The most interesting feature of the reported tungsten diselenide devices is the way that the junctions are formed. The n-type and p-type materials are formed by electrically biasing the thin WSe₂ sheets with metal electrodes. A thin boron nitride layer is used as an isolating dielectric between the biasing metal and WSe₂ film, to allow injection of electrons and holes to create the n-type and p-type regions.[5]

Photodetectors, photovoltaic cells, light-emitting diodes and transistors have all been demonstrated using this technique.[10] An MIT photodetector has shown a sensitivity of 210 mA/W, and photovoltaic power generation with a peak external quantum efficiency of 0.2%.[10] The quantum efficiency is small, but the material is so thin that very little light is absorbed.[8]

An MIT tungsten diselenide LED under a microscope (left), and in operation (right). (Microscope image, left, by Hugh Churchill and Felice Frankel and device in operation, right, by Britt Baugher and Hugh Churchill, via MIT.[10])

Such tungsten diselenide devices have shown electroluminescence with a thousand times smaller injection current and 10 times smaller linewidth than similar devices made from MoS₂.[5] Says Jason Ross, a materials science and engineering graduate student at the University of Washington, and an author of one of the papers,

"These are 10,000 times smaller than the thickness of a human hair, yet the light they emit can be seen by standard measurement equipment... This is a huge leap of miniaturization of technology, and because it's a semiconductor, you can do almost everything with it that is possible with existing, three-dimensional silicon technologies."[11]

One application of such devices would be as transmitters and receivers for high speed optical interconnections on integrated circuits.[11]

Tungsten diselenide photo- and electroluminescence.

Tungsten diselenide room temperature photoluminescence (blue) and electroluminescence (red), left, along with a photograph of the device with superimposed electroluminescence, right. The orange dashed lines outline the location of the tungsten diselenide layer. The scale bar is 2 μm. (Fig. 3 of ref. 7, via arXiv.[7])

Light-Emitting Diodes of Tungsten Diselenide

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