Harnessing the energy of sunlight can be as simple as tuning the optical and electronic properties of metal oxides at the atomic level by making an artificial crystal or super-lattice 'sandwich,' says a Binghamton University researcher in a new study published in the journal Physical Review B. "Metal oxides are cheap, abundant and 'green,'" said Louis Piper, assistant professor of physics at Binghamton University. "And as the study proved, quite versatile（通用的） . With the right touch, metal oxides can be tailored to meet all sorts of needs, which is good news for technological applications, specifically in energy generation and flat screen displays."

Here's how it works: semiconductors are an important class of materials in between metals and insulators. They are defined by the size of their band gap, which represents the energy required to excite an electron from the occupied shell to an unoccupied shell where it can conduct electricity. Visible light covers a range of 1 (infrared) to 3 (ultraviolet) electron volts. For transparent conductors, a large band gap is required, whereas for artificial photosynthesis（光合作用） , a band gap corresponding to green light is needed. Metal oxides provide a means of tailoring the band gap.

But whilst metal oxides are very good at electron conduction, they are very poor "hole" conductors. Holes refer to absence of electrons, and can conduct positive charge. To maximize their technologically potential, especially for artificial photosynthesis and invisible electronics, hole conducting metal oxides are required.

Knowing this, Piper has begun studying layered metal oxides systems, which can be combined to selectively 'dope' (replace a small number of one type of atom in the material), or 'tune' (control the size of the band gap). Recent work revealed that a super-lattice of two hole-conducting copper oxides could cover the entire solar spectrum. The goal is to improve the performance whilst using environmentally benign and cheap metal alternatives.

For instance, indium（铟） oxide is one of the most widely used oxides used in the production of coatings for flat screen displays and solar cells. It can conduct electrons really well and is transparent. But it is also rare and very expensive. Piper's current research is aimed towards using much cheaper tin（锡） oxide layers to get electron and hole conduction with optical transparency.