Electronics are based on crystals known as semiconductors, most famously silicon. To make semiconductors useful, engineers must tweak their crystalline lattice to start and stop the flow of electrons. Semiconductor engineers must know precisely how much energy it takes to move electrons in a crystal lattice. This energy measure is the band gap, which helps determine which material is best for which electronic task.

Now, an interdisciplinary team at Stanford has made a semiconductor crystal with a variable band gap. Among other potential uses, this variable semiconductor could lead to solar cells that absorb more energy from the sun by being sensitive to a broader spectrum of light.

The material itself is not new. Molybdenum disulfide, or MoS2, is a rocky crystal, like quartz, that is refined for use as a catalyst and a lubricant. But Stanford mechanical engineer Xiaolin Zheng and physicist Hari Manoharan have proved that MoS2 has some unique electronic properties that derive from how this crystal forms its lattice. Molybdenum disulfide is what scientists call a monolayer: a molybdenum atom links to two sulfurs in a triangular lattice that repeats sideways like a sheet of paper. Each MoS2 monolayer has semiconductor potential, which would triple the potential of each individual solar cell.

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