Scientists at the University of Washington have developed what they believe is the thinnest-possible semiconductor, a new class of nanoscale materials made in sheets only three atoms thick. Two of these single-layer semiconductor materials can be connected in an atomically seamless fashion known as a heterojunction. A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors, both of which have unequal band gaps. This result could be the basis for next-generation flexible and transparent computing, better light-emitting diodes (LEDs), and solar technologies.

A high-resolution scanning transmission electron microscopy (STEM) image shows the lattice structure of the heterojunctions in atomic precision. (University of Warwick)
The researchers discovered that two flat semiconductor materials can be connected edge-to-edge with crystalline perfection. They worked with two single-layer, or monolayer, materials – molybdenum diselenide and tungsten diselenide – that have very similar structures, which was key to creating the composite two-dimensional semiconductor.

Collaborators from the electron microscopy center at the University of Warwick in England found that all the atoms in both materials formed a single honeycomb lattice structure, without any distortions or discontinuities. This provides the strongest possible link between two single-layer materials, necessary for flexible devices. Within the same family of materials it is feasible that researchers could bond other pairs together in the same way.

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors. (University of Washington)
The researchers created the junctions in a small furnace at the UW. First, they inserted a powder mixture of the two materials into a chamber heated to 900 degrees Celsius (1,652°F). Hydrogen gas was then passed through the chamber and the evaporated atoms from one of the materials were carried toward a cooler region of the tube and deposited as single-layer crystals in the shape of triangles. After a while, evaporated atoms from the second material then attached to the edges of the triangle to create a seamless semiconducting heterojunction.

According to the researchers, the technique is scalable. With a larger furnace, it would be possible to mass-produce sheets of these semiconductor heterostructures. On a small scale, it takes about five minutes to grow the crystals, with up to two hours of heating and cooling time. It is quite possible that in the future it may be possible to combine two-dimensional materials using this technique to form all kinds of electronic structures such as in-plane quantum wells and quantum wires, superlattices, fully functioning transistors, and even complete electronic circuits.

This photoluminescence intensity map shows a typical piece of the lateral heterostructures. The junction region produces an enhanced light emission, indicating its application potential in optoelectronics. (University of Washington)
It has already been demonstrated that the junction interacts with light much more strongly than the rest of the monolayer, which is encouraging for optoelectric and photonic applications like solar cells.

For more information, contact Sanfeng Wu at This email address is being protected from spambots. You need JavaScript enabled to view it..

Lighting Technology Magazine

This article first appeared in the November, 2014 issue of Lighting Technology Magazine.

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