Nav: Home

Beating the heat a challenge at the nanoscale

July 28, 2016

Rice University scientists who analyze the properties of materials as small as a single molecule have encountered a challenge that appears at very low temperatures.

In trying to measure the plasmonic properties of gold nanowires, the Rice lab of condensed matter physicist Douglas Natelson determined that at room temperature, the wire heated up a bit when illuminated by a laser; but confoundingly, at ultracold temperatures and under the same light, its temperature rose by far more.

This is an issue for scientists like Natelson whose experiments require ultracold materials to stay that way. Laser heating, while it may seem minimal, presents a thermal barrier to simultaneous inelastic electron tunneling spectroscopy and surface-enhanced optical spectroscopy, which measure a material's electrical and optical properties.

Their report on the phenomenon appears in the American Chemical Society journal ACS Nano.

"Over the years we've made nice progress doing electronic and optical measurements simultaneously on nanoscale junctions that contain one or a few molecules," Natelson said. "We could learn a lot more if we could extend those measurements to quite low temperatures; the features in the electronic conduction would sharpen up a lot."

But such optical measurements require lasers, which combine with the properties of the metal electrodes to focus optical energy down to scales below the diffraction limit of light. "The laser for the optical measurements tends to heat the system," he said. "This isn't too bad at moderately low temperatures, but as we show in the paper, direct optical heating can get much more severe when the sample, without the light on, is cooled down to a few kelvins."

In plasmonic materials, lasers excite the oscillating quasi-particles that ripple like waves in a pool when excited. Plasmonic materials are used to sense biological conditions and molecular interactions; they also are used as photodetectors and have been employed in cancer therapies to heat and destroy tumors.

For their experiments, Natelson and his colleagues placed bowtie-shaped gold nanowires on silicon, silicon oxide, sapphire or quartz surfaces with a 1-nanometer adhesive layer of titanium between. They fabricated and tested 90 such devices. At their narrowest, the wires were less than 100 nanometers wide, and the geometry was tuned to be appropriate for plasmonic excitation with near-infrared light at 785 nanometers.

The researchers took measurements for various laser strengths and surface temperatures. For the nanowire on silicon or silicon oxide, they found that as they decreased the temperature of the silicon from 60 kelvins (-351 degrees Fahrenheit) to 5 kelvins (-450 F), it became less able to dissipate heat from the nanowire. With no change in the strength of the laser, the temperature of the wire increased to 100 kelvins (-279 F).

Replacing the silicon with sapphire provided some relief, with a threefold decrease in the laser-driven temperature increase, they reported. This was a startling result as the thermal conductivity of sapphire is a thousand times higher than that of silicon oxide, said Pavlo Zolotavin, a Rice postdoctoral researcher and lead author of the paper. A comprehensive numerical model of the structure revealed thermal boundary resistance as a major source of the detrimental temperature increase, especially for the crystalline substrates.

"The big issue is in getting vibrational heat out of the metal and into the insulating substrate," he said. "It turns out that this thermal boundary resistance gets much worse at low temperatures. The consequence is that the local temperature can get jacked up a lot with a somewhat complicated dependence, which we can actually model well, on the incident light intensity."

Solving the problem is important to Natelson and his team, as they specialize in measuring the electrical and magnetic properties of single molecules by placing them in gaps cut into bowtie nanowires. If heat expands the nanowires, the gaps close and the experiments are ruined. Heating can also "smear out" features in the data, he said.

"What this all means is that we need to be clever about how we try to do simultaneous electronic and optical measurements, and that we need to think hard about what the temperature distribution looks like and how the heat really flows in these systems," Natelson said.
-end-
Co-authors are Rice postdoctoral researcher Alessandro Alabastri and Peter Nordlander, a Rice professor of physics and astronomy, of electrical and computer engineering and of materials science and nanoengineering. Natelson is a professor and chair of the Department of Physics and Astronomy and a professor of electrical and computer engineering and of materials science and nanoengineering.

The Robert A. Welch Foundation and the Air Force Office of Scientific Research supported the research.

Read the abstract at http://pubs.acs.org/doi/abs/10.1021/acsnano.6b02911.

This news release can be found online at http://news.rice.edu/2016/07/28/beating-the-heat-a-challenge-at-the-nanoscale/

Follow Rice News and Media Relations via Twitter @RiceUNews.

Related materials:

Natelson Group: http://natelson.web.rice.edu/group.html

Nanoscale Views (Natelson blog): http://nanoscale.blogspot.com

Wiess School of Natural Sciences: http://natsci.rice.edu

Nordlander Nanophotonics Group: http://nordlander.rice.edu/members/nordlander

Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,910 undergraduates and 2,809 graduate students, Rice's undergraduate student-to-faculty ratio is 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for best quality of life and for lots of race/class interaction by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger's Personal Finance. To read "What they're saying about Rice," go to http://tinyurl.com/RiceUniversityoverview.

Rice University

Related Nanowires Articles:

Nanowires, the future of electronics
The current demand for small-sized electronic devices is calling for fresh approaches in their design.
Improving silver nanowires for FTCEs with flash light interactions
A Korean research team led by Professor Keon Jae Lee of the Materials Science and Engineering Department at KAIST and Dr.
UC researchers use gold coating to control luminescence of nanowires
In electronics, the race for smaller is huge. Physicists at the University of Cincinnati are working to harness the power of nanowires, microscopic wires that have the potential to improve solar cells or revolutionize fiber optics.
Obtaining of silicon nanowires becomes eco-friendly
Scientists from the Faculty of Physics, the Lomonosov Moscow State University have devised a technique of silicon nanowires synthesis.
Nanowires as sensors in new type of atomic force microscope
A new type of atomic force microscope (AFM) uses nanowires as tiny sensors.
More Nanowires News and Nanowires Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#534 Bacteria are Coming for Your OJ
What makes breakfast, breakfast? Well, according to every movie and TV show we've ever seen, a big glass of orange juice is basically required. But our morning grapefruit might be in danger. Why? Citrus greening, a bacteria carried by a bug, has infected 90% of the citrus groves in Florida. It's coming for your OJ. We'll talk with University of Maryland plant virologist Anne Simon about ways to stop the citrus killer, and with science writer and journalist Maryn McKenna about why throwing antibiotics at the problem is probably not the solution. Related links: A Review of the Citrus Greening...