Power/Performance Bits: July 14

Photo-doping semiconductors; cooling off with graphene; heating up with ceramics.

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Photo-doping semiconductors

Scientists at Michigan State University found that by shooting an ultrafast laser pulse into a semiconducting material, its properties would change as if it had been chemically doped, in a process known as photo-doping.

“The material we studied is an unconventional semiconductor made of alternating atomically thin layers of metals and insulators,” said Chong-Yu Ruan, an associate professor of physics and astronomy at MSU. “This combination allows many unusual properties, including highly resistive and also superconducting behaviors to emerge, especially when doped.”

The image on the right is an illustrated view of a material irradiated by the laser pulses. The one of the left is an image of the material showing subtle structural changes as a result of photo-doping. (Source: MSU Department of Physics and Astronomy)

The image on the right is an illustrated view of a material irradiated by the laser pulses. The one of the left is an image of the material showing subtle structural changes as a result of photo-doping. (Source: MSU Department of Physics and Astronomy)

An ultrafast electron-based imaging technique developed by the team allowed them to observe the changes in the materials. By varying the wavelengths and intensities of the laser pulses, the researchers were able to observe phases with different properties that are captured on the femtosecond timescale.

“The laser pulses act like dopants that temporarily weaken the glue that binds charges and ions together in the materials at a speed that is ultrafast and allow new electronic phases to spontaneously form to engineer new properties,” Ruan said. “Capturing these processes in the act allows us to understand the physical nature of transformations at the most fundamental level.”

Philip Duxbury, a team member and chairperson of the Department of Physics and Astronomy, said ultrafast photo-doping “has potential applications that could lead to the development of next-generation electronic materials and possibly optically controlled switching devices employing undoped semiconductor materials.”

Cooling off with graphene…

Researchers at Chalmers developed a method for efficiently cooling electronics using graphene-based film. The film has a thermal conductivity capacity four times that of copper. The graphene film is attachable to electronic components made of silicon, which favors the film’s performance compared to typical graphene characteristics shown in previous, similar experiments.

A couple of years ago, a research team at Chalmers showed that graphene can have a cooling effect on silicon-based electronics. That was the starting point for researchers conducting research on the cooling of silicon-based electronics using graphene.

“The methods that have been in place so far have presented the researchers with problems”, Johan Liu, professor at Chalmers, says. “It has become evident that those methods cannot be used to rid electronic devices off great amounts of heat, because they have consisted only of a few layers of thermal conductive atoms. When you try to add more layers of graphene, another problem arises, a problem with adhesiveness. After having increased the amount of layers, the graphene no longer will adhere to the surface, since the adhesion is held together only by weak van der Waals bonds. We have now solved this problem by managing to create strong covalent bonds between the graphene film and the surface, which is an electronic component made of silicon.”

Graphene-based film on an electronic component with high heat intensity. (Source: Johan Liu / Chalmers University of Technology)

Graphene-based film on an electronic component with high heat intensity. (Source: Johan Liu / Chalmers University of Technology)

“Increased thermal capacity could lead to several new applications for graphene,” said Liu. “One example is the integration of graphene-based film into microelectronic devices and systems, such as highly efficient LEDs, lasers and radio frequency components for cooling purposes. Graphene-based film could also pave the way for faster, smaller, more energy efficient, sustainable high power electronics.”

…And heating up with ceramics

Researchers at the University of Tokyo have discovered a new type of material that stores heat energy for a prolonged period. The new material, which they termed heat-storage ceramic, can be used as heat storage material for solar heat energy generation systems or efficient use of industrial heat waste, enabling recycling of heat energy.

The material, called stripe-type-lambda-trititanium-pentoxide, is composed of only titanium atoms and oxygen atoms, and can absorb and release a large amount of heat energy (230 kJ L-1). This heat energy stored is large at approximately 70% of the latent heat energy of water at its melting point. Applying a weak pressure of 60 MPa (mega Pascal) to stripe-type-lambda-trititanium-pentoxide induces a phase transition to beta-trititanium-pentoxide, releasing the stored heat energy.

A novel "heat-storage ceramic" demonstrated in stripe-type-lambda-trititanium-pentoxide. (a) The material stores heat energy of 230 kJ L-1 by heating and releases the energy by a weak pressure (60 MPa). In addition, this material stores heat energy by various approaches such as (b) electric current flow or (c) light-irradiation. (Source: Shin-ichi Ohkoshi/ University of Tokyo)

A novel “heat-storage ceramic” demonstrated in stripe-type-lambda-trititanium-pentoxide. (Source: Shin-ichi Ohkoshi/ University of Tokyo)

Besides direct application of heat, heat energy can be stored by passing an electric current through the material or irradiating it with light, enabling the repeated absorption and release of heat energy by a variety of methods.

The material also has possibilities for use for advanced electronic devices such as pressure-sensitive sheets, reusable heating pads, pressure-sensitive conductivity sensors, electric current driven type ReRAM, and optical memory.