IS BLACK PHOSPHORUS THE NEW GRAPHENE?

IS BLACK PHOSPHORUS

THE NEW GRAPHENE ?

Atoms-thin flakes of phosphorus have a crucial

property that graphene lacks

Chemists first synthesized

black phosphorus over a hundred

years ago. But it was only last year

when anybody really took interest in the

flaky black stuff. In a series of experiments

reported in the first half of 2014,

researchers were able to exfoliate black

phosphorus into very thin films of only

about 10 to 20 atoms thick. Now black

phosphorus has become the new darling

of two-dimensional materials research

and a new hope for a postsilicon world.

The excitement around black phosphorus,

which is also called phosphorene in

reference to its 2-D cousin graphene, stems

mainly from the fact that it has an inherent

bandgap, something that graphene lacks.

A bandgap, an energy band in which no

electron states can exist, is essential for

creating the on/off flow of electrons that

are needed in digital logic and for the generation

of photons for LEDs and lasers.

Black phosphorus doesn’t just have any

bandgap. Its bandgap can be fine-tuned

by adjusting the number of layers of the

material, explains Philip Feng, an assistant

professor of electrical engineering

and computer science at Case Western

Reserve University. His team has demonstrated

some of the first black phosphorous

mechanical and electronic devices

The bandgap can be dialed up from

0.3 to 2.0 electron volts. That’s a range

covering a regime otherwise unavailable

to all other recently discovered 2-D

materials. It bridges the bandgaps of

graphene (0 eV) and of transition-metal

dichalcogenides such as molybdenum

disulfide, which range from 1.0 to 2.5 eV.

By combining this bandgap tuning with

different choices of contact materials, scientists

at Sungkyunkwan University, in

South Korea, were recently able to build

both n-type transistors—those conducting

electrons—and ambipolar transistors,

which conduct both holes and electrons.

Such a mix brings the material closer to

mimicking the complementary logic used

in today’s silicon chips.

Scientists are also excited about black

phosphorus for photonics, “since optoelectronic

functions, including light

absorption, emission, and modulation, of

semiconductor materials depend on the

size of the bandgap,” says Mo Li, a photonics

expert at the University of Minnesota.

Black phosphorus’s bandgap range

means it can absorb and emit light with

wavelengths of 0.6 to 4.0 micrometers—

covering the visible to infrared. That spectrum

could be key to its use in sensors and

in optical communications.

Li’s group built a black phosphorous photodetector

that was able to convert 3 gigabits per second

of optical data to electronic signals.

Another cool property, Feng points

out, is that black phosphorus possesses

an intrinsic, strong in-plane anisotropy,

which means its properties are dependent

on the direction of the crystal. “This

in-plane anisotropy is not readily found

in other 2-D crystals derived from layered

materials,” he says. His team recently

demonstrated the first black phosphorous

high-frequency nanoelectromechanical

systems resonator. The resonator

took advantage of the material’s in-plane

anisotropy to generate new elastic behaviors

and frequency scaling abilities.

Unfortunately, black phosphorus is

hard to make and hard to keep. Currently,

it’s made by treating an amorphous form

of the element called red phosphorus

with high pressure (1 gigapascal) and high

temperature (1,000 °C). The resulting

millimeter-scale crystals are then exfoliated

into atoms-thick flakes for making

nanostructures and nanoscale devices.

More troubling is that “when exposed

in air, black phosphorous film degrades

within a few hours, due to reaction with

water vapor and oxygen in air,” explains

Li. “Luckily, many inert materials can

be used as passivation to preserve black

phosphorous devices for weeks or longer.”

If the manufacturing and preservation

problems can be solved, perhaps silicon

could finally fade to black.