Diamond brightens the performance of electronic devices

Two new studies performed at the U.S. Department of Energy’s Argonne National Laboratory have revealed a new pathway for materials scientists to use previously unexplored properties of nanocrystalline-diamond thin films. While the properties of diamond thin films are relatively well-understood, the new discovery could dramatically improve the performance of certain types of integrated circuits by reducing their "thermal budget."

For decades, engineers have sought to build more efficient electronic devices by reducing the size of their components. In the process of doing so, however, researchers have reached a "thermal bottleneck," said Argonne nanoscientist Anirudha Sumant.

In a thermal bottleneck, the excess heat generated in the device causes undesirable effects that affect its performance. "Unless we come-up with innovative ways to suck the heat off of our electronics, we are pretty much stuck with this bottleneck," Sumant explained.

The unusually attractive thermal properties of diamond thin films have led scientists to suggest using this material as a heat sink that could be integrated with a number of different semiconducting materials. However, the deposition temperatures for the diamond films typically exceed 800 degrees Celsius—roughly 1500 degrees Fahrenheit, which limits the feasibility of this approach.

"The name of the game is to produce diamond films at the lowest possible temperature. If I can grow the films at 400 degrees, it makes it possible for me to integrate this material with a whole range of other semiconductor materials," Sumant said.

By using a new technique that altered the deposition process of the diamond films, Sumant and his colleagues at Argonne’s Center for Nanoscale Materials were able to both reduce the temperature to close to 400 degrees Celsius and to tune the thermal properties of the diamond films by controlling their grain size. This permitted the eventual combination of the diamond with two other important materials: graphene and gallium nitride.

According to Sumant, diamond has much better heat conduction properties than silicon or silicon oxide, which were traditionally used for fabrication of graphene devices. As a result of better heat removal, graphene devices fabricated on diamond can sustain much higher current densities.

In the other study, Sumant used the same technology to combine diamond thin films with gallium nitride, which is used extensively in high-power light emitting devices (LED). After depositing a 300 nm-thick diamond film on a gallium nitride substrate, Sumant and his colleagues noticed a considerable improvement in the thermal performance. Because a difference within an integrated circuit of just a few degrees can cause a noticeable change in performance, he called this result "remarkable."

"The common link between these experiments is that we’re finding new ways of dissipating heat more effectively while using less energy, which is the key," Sumant said. "These processes are crucial for industry as they look for ways to overcome conventional limits on semiconducting circuits and pursue the next generation of electronics."

The results of the two studies were reported in Nano Letters and Advanced Functional Materials. Both of these studies were carried out in collaboration with Prof. Alexander Balandin at the University of California-Riverside and his graduate students Jie Yu, Guanxiong Liu and Dr. Vivek Goyal, a recent Ph.D. graduate.

Funding for the research conducted at the Center for Nanoscale Materials was provided by the Basic Energy Sciences program of the U.S. Department of Energy’s Office of Science.

More information: The papers can be found online at:
http://onlinelibra … 786/abstract
http://pubs.acs.or … 21/nl204545q

Provided by Argonne National Laboratory (news : web)

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Researchers show the way forward for improving organic and molecular electronic devices

"We have shown that when the molecules in organic thin films are aligned in particular directions, there is much better conductance," says Miquel Salmeron, a leading authority on nanoscale surface imaging who directs Berkeley Lab's Materials Sciences Division and who led this study. "Chemists already know how to fabricate organic thin films in a way that can achieve such an alignment, which means they should be able to use the information provided by our methodology to determine the molecular alignment and its role on charge transport across and along the molecules. This will help improve the performances of future organic electronic devices."

Salmeron and Shaul Aloni, also of the Materials Sciences Division, are the corresponding authors of a paper in the journal Nano Letters that describes this work. The paper is titled "Electron Microscopy Reveals Structure and Morphology of One Molecule Thin Organic Films." Other co-authors were Virginia Altoe, Florent Martin and Allard Katan.

Organic electronics, also known as plastic or polymer electronics, are devices that utilize carbon-based molecules as conductors rather than metals or semiconductors. They are prized for their low costs, light weight and rubbery flexibility. Organic electronics are also expected to play a big role in molecular computing, but to date their use has been hampered by low electrical conductance in comparison to metals and semiconductors.

"Chemists and engineers have been using their intuition and trial-and-error testing to make progress in the field but at some point you hit a wall unless you understand what is going on at the molecular level, for example, how electrons or holes flow through or across molecules, how the charge transport depends on the structure of the organic layers and the orientation of the molecules, and how the charge transport responds to mechanical forces and chemical inputs," Salmeron says. "With our experimental results, we have shown that we can now provide answers for these questions."

In this study, Salmeron and his colleagues used electron diffraction patterns to map the crystal structures of molecular films made from monolayers of short versions of commonly used polymers containing long chains of thiophene units. They focused specifically on pentathiophene butyric acid (5TBA) and two of its derivatives (D5TBA and DH5TBA) that were induced to self-assemble on various electron-transparent substrates.

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Electron diffraction patterns provide a wealth of information about the morphology, structure, and quality of monolayer organic thin films. Credit: Image from Berkeley Lab's Molecular Foundry

Pentathiophenes - molecules containing a ring of four carbon and one sulfur atoms - are members of a well-studied and promising family of organic semiconductors.

Obtaining structural crystallographic maps of monolayer organic films using electron beams posed a major challenge, as Aloni explains.

"These organic molecules are extremely sensitive to high energy electrons," he says. "When you shoot a beam of high energy electrons through the film it immediately affects the molecules. Within few seconds we no longer see the signature intermolecular alignment of the diffraction pattern. Despite this, when applied correctly, electron microscopy becomes essential tool that can provide unique information on organic samples."

Salmeron, Aloni and their colleagues overcame the challenge through the combination of a unique strategy they developed and a transmission electron microscope (TEM) at the Molecular Foundry's Imaging and Manipulation of Nanostructures Facility. Electron diffraction patterns were collected as a parallel electron beam was scanned over the film, then analyzed by computer to generate structural crystallographic maps.

"These maps contain uncompromised information of the size, symmetry and orientation of the unit cell, the orientation and structure of the domains, the degree of crystallinity, and any variations on the micrometer scale," says first author Altoe. "Such data are crucial to understanding the structure and electrical transport properties of the organic films, and allow us to track small changes driven by chemical modifications of the support films."

In their paper, the authors acknowledge that to gain structural information they had to sacrifice some resolution.

"The achievable resolution of the structural map is a compromise between sample radiation hardness, detector sensitivity and noise, and data acquisition rate," Salmeron says. "To keep the dose of high energy electrons at a level the monolayer film could support and still be able to collect valuable information about its structure, we had to spread the beam to a 90 nanometer diameter. However a fast and direct control of the beam position combined with the use of fast and ultrasensitive detectors should allow for the use of smaller beams with a higher electron flux, resulting in a better than 10 nanometer resolution."

While the combination of organic molecular films and substrates in this study conduct electrical current via electron holes (positively-charged energy spaces), Salmeron and his colleagues say their structural mapping can also be applied to materials whose conductance is electron-based.

"We expect our methodology to have widespread applications in materials research," Salmeron says.

Provided by Lawrence Berkeley National Laboratory (news : web)

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