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Black phosphorous, the new wonder material for electronic, optoelectronic and thermoelectric devices.

Black phosphorous, named for its distinctive color, is a natural semiconductor with an energy bandgap, and highly promising material for electronic, optoelectronic and thermoelectric devices. Also called phosphorene as It belongs to class of two-dimensional crystals including graphene, boron nitride and molybdenum disulphide.

A band gap is an energy band in which no electron states can exist and is essential for creating the “on/off” flow of electrons that are needed in digital logic electronics. Black phosphorus’s bandgap range with wavelengths of 0.6 to 4.0 micrometers means it can absorb and emit light covering the visible to infrared range. Such a band structure allows its electrical conductance to be switched “on and off “and its application in field-effect transistosr (FET) and for the generation of photons for LEDs and lasers.

It is a layered material in which individual atomic layers are stacked together by van der Waals interactions, much like bulk graphite. Inside a single layer, each phosphorus atom is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure. The three bonds take up all three valence electrons of phosphorus, so, unlike graphene, monolayer black phosphorus (termed ‘phosphorene’) is a semiconductor with a predicted direct bandgap of 2 eV For few-layer phosphorene, interlayer interactions reduce the bandgap for each layer added, and eventually reach 0.3 eV for bulk black phosphorus.

 

Energy Efficient Nanoelectronics

Last year, researchers built a field-effect transistor out of black phosphorus and showed that it performed remarkably well. This research suggested that black phosphorous could have a bright future in nanoelectronic devices.

Researchers from Fudan University, Shanghai, China found, “Excellent transistor performances were achieved at room temperature. In particular, important metrics of our devices such as drain current modulation and mobility are either better or comparable to FETs based on other layered materials”

According to Thomas Szkopek, a McGill University electrical and computer engineering associate professor and senior author of the study, transistors are more efficient when they are made thin, with electrons moving within just two dimensions. Electrons in black phosphorus behave this way, suggesting that the material can help engineers address one of the biggest challenges in electronics: energy-efficient transistors.

Szkopek explained that what’s surprising is that electrons in phosphorus behaved two-dimensionally despite being several atoms thick. This finding has significance because it could simplify the manufacturing process for the material.

Black phosphorus (BP) 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. 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.
Few-layer BP is also a p-type semiconductor, by combining with few layer MoS2 sheet, it can be developed as solar cell and p-n diode.

Black phosphorus unleashes power of supercapacitors

Researchers in China have built powerful and durable all-solid-state supercapacitors using black phosphorus nanoflakes.

Photonics

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.

“Molybdenum disulfide (MoS2) another 2D material being studied, owns indirect band-gap at multilayer format, whereas BP has the direct transition for all thickness, which shows much more absorption and should be more suitable for optoelectronic applications particularly at the long wavelength range where strong motivation on optical communications and military purposes provoke,” according to researchers from China.

This large degree of tunability makes black phosphorus a unique material that can be used for a wide range of applications–from chemical sensing to optical communication.

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.

The University of Minnesota team demonstrated that the performance of the black phosphorus photodetectors even rivals that of comparable devices made of germanium–considered the gold standard in on-chip photodetection. Germanium, however, is difficult to grow on silicon optical circuits, while black phosphorus and other two-dimensional materials can be grown separately and transferred onto any material, making them much more versatile

Additionally, black phosphorus is a so-called “direct-band” semiconductor, meaning it has the potential to efficiently convert electrical signals back into light. Combined with its high performance photodetection abilities, black phosphorus could also be used to generate light in an optical circuit, making it a one-stop solution for on-chip optical communication.

“Because these materials are two-dimensional, it makes perfect sense to place them on chips with flat optical integrated circuits to allow maximal interaction with light and optimally utilize their novel properties,” said Professor Li, who led the research team.

“It is really exciting to think of a single material that can be used to send and receive data optically and is not limited to a specific substrate or wavelength,” said Nathan Youngblood, the lead author of the study. “This could have huge potential for high-speed communication between CPU cores which is a bottleneck in computing industry right now.”

The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for IR and mid-IR optoelectronics.

In the future, it is anticipated that BP-based IR or mid-IR devices such as passive Q-switcher, mode-locker, optical switcher or light modulator might emerge, encouraged by the broadband and strong nonlinear optical response in BP.

 

Thermoelectrics or Keeping cool with Black Phosphorous

Layered, crystalline black phosphorus could also lead to a new microchip design that lets heat flow away and keeps electrons moving. Designers can now take advantage of the orientation-dependent thermal properties of phosphorus to keep microelectronic devices cool and improve their efficiency

 

It has been theorized that in contrast to graphene, black phosphorous has opposite anisotropy in thermal and electrical conductivities—for example, heat flows more easily along a direction in which electricity flows with more difficultly. A team of researchers at the U.S. Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has experimentally confirmed strong in-plane anisotropy in thermal conductivity of single crystal black phosphorus nanoribbons, they conduct heat two times more in the zig-zag direction compared to another direction.

 

“Imagine the lattice of black phosphorous as a two-dimensional network of balls connected with springs, in which the network is softer along one direction of the plane than another,” says Junqiao Wu, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Div. and the Univ. of California (UC) Berkeley’s Dept. of Materials Science and Engineering. “Our study shows that in a similar manner heat flow in the black phosphorous nanoribbons can be very different along different directions in the plane.

“This in-plane anisotropy is not readily found in other 2-D crystals derived from layered materials,” Philip Feng, 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.

Such anisotropy would be a boost for designing energy-efficient transistors and thermoelectric devices, but experimental confirmation proved challenging because of sample preparation and measurement requirements.

“This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation. For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance,” said Wu.

Efficient Terahertz detection in black-phosphorus nano-transistors

Terahertz (THz) frequency range, has wide exceptional application possibilities in high data rate wireless communications, security, night-vision, biomedical or video-imaging and gas sensing, detection technologies.

Photodetection of light, i.e. conversion of photons into a stable electrical signal, at THz frequencies can be accomplished by several different mechanisms like photo-thermoelectric, photovoltaic, galvanic, bolometric, plasma-wave rectification or via a combination of them.

Researchers have engineered the first room-temperature terahertz (THz)-frequency nanodetector exploiting a 10 nm thick flake of exfoliated crystalline black phosphorus as an active channel of a field-effect transistor. By engineering and embedding planar THz antennas for efficient light harvesting, the first technological demonstration of a phosphorus-based active THz device is described.

The inherent electrical and thermal in-plane anisotropy of BP is exploited to selectively control the detection dynamics in the BP channel, at room-temperature (RT) and with state-of-the art detection efficiency.

 

Production Methods

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. That is a time-consuming task that severely limits potential applications.

Damien Hanlon at Trinity College Dublin in Ireland, and his colleagues have perfected a way of making large quantities of black phosphorus nanosheets with dimensions that they can control. And they have used this newfound ability to test black phosphorus in a number of new applications, such as a gas sensor, an optical switch, and even to reinforce composite materials to make them stronger.

The method of  Hanlon and co is to place the black phosphorus lump in a liquid solvent and then bombard it with acoustic waves that shake the material apart. The result is that the bulk mass separates into a large number of nanosheets that the team filters for size using a centrifuge. That leaves high-quality nanosheets consisting of only a few layers. “Liquid phase exfoliation is a powerful technique to produce nanosheets in very large quantities,” they say.

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.”

Hanlon and co has predicted that certain solvents should form a solvation shell around the sheet, which prevents oxygen or other oxidative species from reaching the phosphorus. The team use N-cyclohexyl-2-pyrrolidone or CHP as a solvent and because of this, the nanosheets are surprisingly long-lived.
But Hanlon and co say the newfound availability of black phosphorus nanosheets has allowed them to test a number of other ideas as well.

For example, they added the nanosheets to a film of polyvinyl chloride, thereby doubling its strength and increasing its tensile toughness sixfold. So it’s not just carbon allotropes that can increase strength!

They also determined the nonlinear optical response of the nanosheets to a pulsed laser by measuring the amount of light that is transmitted. It turns out that the amount of light black phosphorus absorbs decreases as the intensity rises, a property known as saturable absorption. What’s more, black phosphorus is better at this even than graphene.

Finally, they measured the current through the nanosheets while exposing them to ammonia. They found that the material’s resistance increased when it came into contact with ammonia, probably because ammonia donates electrons that neutralize holes in the black phosphorus sheets.
That immediately makes black phosphorous a decent ammonia detector. Hanlon and co say the material could detect ammonia at levels of around 80 parts per billion.

All this could mark an interesting step change in research associated with black phosphorus. Many people will have seen the excitement associated with the remarkable properties of graphene. If black phosphorous is half as remarkable, there should be an interesting future for material scientists.

 

Fast growing potential

All of the previous two-dimensional materials including grapheme and transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2), have serious tradeoffs, but black phosphorus provides the “best of both worlds” with a tunable band gap and high-speed capability.

“Black phosphorus is an extremely versatile material,” said Professor Steven Koester, “It makes great transistors and photodetectors, and has the potential for light emission and other novel devices, making it an ideal platform for a new type of adaptable electronics technology.”

 

The article sources also include:

 

http://spectrum.ieee.org/semiconductors/materials/black-phosphorus-has-renaissance-in-two-dimensions?

http://phys.org/news/2016-06-cool-black-semiconductor.html

http://www.nature.com/articles/srep20474

About Rajesh Uppal

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