New Graphene __ Black Phosphorus

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.

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.

FOLLOW ME  :  @iam_arunmathew

Graphene Keeping It Cool In Electronics

By Dexter Johnson

Cooling fans and other system-level solutions are reaching their limits as circuit densities continue to grow. It’s no wonder then that graphene’s remarkable heat conductivity has led to a lot of research into using it to forthermal management in electronics.

Now an international team of researchers, organized by a team at the University of Michigan, has found that layered graphene can be an important tool for thermal management because of its ability to release heat efficiently.

In research published in the journal Nature Communications, the scientists demonstrated that the electrostatic interactions between electrically charged particles—known as Coulomb interactions—in  different layers of multi-layered graphene offers a key mechanism for dispersing heat. This occurs despite the fact that all electronic states are strongly confined within individual 2D layers.

“We believe that this cooling mechanism is not limited to multilayer graphene samples but is likely to be important in many other new, layered nanomaterials under active development by the scientific community,” said Theodore Norris, who led the research, in a press release.

This mechanism came as a bit of surprise to the researchers. They did not expect the heat building up in the electrons of the graphene to travel well through the layers because previous observations had shown that the graphene layers interact too strongly for this to occur. This stood in contrast to 3D pieces of silicon that are capable of conducting heat in any direction.

Momchil Mihnev, a doctoral student at the University of Michigan and first author on the paper, explained in the press release that while the electrons in the different layers can’t mechanically come in contact with each other, they do manage to interact with each other through their electrical charges.

When the negative charges repel each other, the electrons take on an effective size that extends between the layers. When the electrons come in contact with each other in this way, the hotter electrons transfer heat to the colder ones. This transfer of heat eventually channels down through the graphene towards the layer that is closest to the silicon carbide substrate the researchers used in these experiments. Once it gets to the final layer of graphene, the heat transfers into the silicon carbide.

The researchers have developed a detailed theory on why and how this mechanism works, and it could provide an important tool in keeping electronics cool well into the future.