Research Highlights

Graphene — an ultrathin material consisting of a single layer of carbon atoms — has the potential to revolutionize electronic devices by making them smaller, faster and more efficient. One limitation of graphene, however, is that it can absorb only a small fraction of visible light. This prevents it from being used in photodetectors — optoelectronic components critical to the operation of solar cells and digital cameras.

Katsumi Tanigaki and colleagues from the AIMR at Tohoku University have now discovered a novel nanostructured material — gallium telluride (GaTe) flakes — that can detect light signals with a higher sensitivity and a faster electrical response than any existing two-dimensional (2D) layered material¹.

Although electrons can move through the hexagonally bonded framework of graphene at extraordinary speeds, graphene’s ‘indirect’ optical bandgap means that any interaction with light must proceed via a time- and energy-consuming detour involving intermediate states. ‘Direct’ bandgap materials, in contrast, can absorb large quantities of photons by immediately converting them into a photoelectric current, making such materials extremely photosensitive.

The researchers’ extensive search for a direct-bandgap material that is atomically thin led them to GaTe crystals. This material’s unique structure — 2D sheets of Te–Ga–Ga–Te atoms (see image) stacked into weakly bonded, multilayered complexes — provides it with intriguing optoelectronic properties. Furthermore, GaTe is lightweight and easy to synthesize.

But when the researchers began their investigation, they encountered a crucial problem: the bandgap of GaTe changes from being direct to indirect when its bulk crystal structure is thinned down to a monolayer. Through collaborating with theoreticians from the United Kingdom to better understand the optical properties of this system, the team came to realize that finite, ten-layer GaTe stacks could yield the improved conductivity of 2D systems while retaining a direct bandgap.

To achieve this goal, the researchers resorted to a low-tech but effective method: using sticky Scotch tape to strip off very thin flakes from bulk GaTe crystals. This technique — pioneered for isolating single-layer graphene — produced the desired multilayered GaTe flakes, which they then transferred to a silicon device. Experiments showed that the tiny flakes could respond to a wide range of light within milliseconds. Importantly, the nanomaterials produced a photocurrent of light-generated electrons several orders of magnitude higher than that generated by graphene.

“A direct bandgap is very important for achieving both a high sensitivity and a fast response time in a photodetector,” says Tanigaki. “The intrinsic direct bandgap available from GaTe nanoflakes makes them a promising candidate for future photodetectors beyond graphene.”