Power/Performance Bits: Sept. 16

Rice University physicists find a 2D form of phosphorus makes a promising semiconductor and pays no heed to defects; a research team including Stanford engineers discovers that the benefits of slow draining and charging may have been overestimated.

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Phosphorus: a promising semiconductor
According to researchers at Rice University, defects damage the ideal properties of many 2D materials, like carbon-based graphene, but phosphorus just shrugs, making it a promising candidate for nano-electronic applications that require stable properties.

The team analyzed the properties of elemental bonds between semiconducting phosphorus atoms in 2D sheets, created through exfoliation from black phosphorus.

The researchers compared their findings to 2D metal dichalcogenides like molybdenum disulfide, which have also been considered for electronics because of their inherent semiconducting properties. In pristine dichalcogenides, atoms of the two elements alternate in lockstep but wherever two atoms of the same element bond, they create a point defect. This can be thought of as a temporary disturbance in the force that could slow electrons down.

The researchers reminded that semiconductors are the basic element of modern electronics that direct and control how electrons move through a circuit. When a disturbance deepens a band gap, the semiconductor is less stable; when chaos reigns in the form of multiple point defects or grain boundaries — where sheets of a 2-D material merge at angles, forcing like atoms to bond – the materials become far less useful.

The team’s calculations show phosphorus has no such problem. Even when point defects or grain boundaries exist, the material’s semiconducting properties are stable. Like perfect graphene – but unlike imperfect graphene — it performs as expected, they said.

This demonstrates good properties for application in solar cells. 2D phosphorus could potentially be used to harvest sunlight, as its band gap matches well with the solar spectrum, and unlike conventional absorbers, the presence of defects would not deteriorate the material’s performance.

The researchers also show it may be possible to tune the electronic properties of 2D phosphorus by doping it with foreign atoms, which should be of value to electronics manufacturers.

A point defect appears in a two-dimensional material when atoms don’t line up quite right, as in the puckered pair of a heptagon and a pentagon seen at top. In many materials, this disruption of regular six-atom rings (as seen at bottom) would change the material’s electronic properties. But Rice University theorists have determined that 2-D phosphorus would not be affected by such defects. (Source: Rice University)

A point defect appears in a two-dimensional material when atoms don’t line up quite right, as in the puckered pair of a heptagon and a pentagon seen at top. In many materials, this disruption of regular six-atom rings (as seen at bottom) would change the material’s electronic properties. But Rice University theorists have determined that 2-D phosphorus would not be affected by such defects. (Source: Rice University)

Interestingly, 2D phosphorus has more in common with 3D silicon, the most common element in semiconducting electronics like computer chips. As in 2D phosphorus, grain boundaries in silicon don’t cause band-gap changes but point defects in silicon can change its properties, unlike point defects in phosphorus. This suggests 2D phosphorus could also be a candidate for high-performance electronics.

Supercharging batteries is not so bad
A comprehensive look at how tiny particles in a lithium ion battery electrode behave shows that rapid-charging the battery and using it to do high-power, rapidly draining work may not be as damaging as researchers had thought – and that the benefits of slow draining and charging may have been overestimated.

The results challenge the prevailing view that “supercharging” batteries is always harder on battery electrodes than charging at slower rates, according to researchers from Stanford Engineering and the Stanford Institute for Materials & Energy Sciences (SIMES) at the Department of Energy’s SLAC National Accelerator Laboratory.
They also suggest that scientists may be able to modify electrodes or change the way batteries are charged to promote more uniform charging and discharging and extend battery life.

For more on this research and to view a video, click here.