Scientists detect thermal boundary that hinders ultracold experiments

Scientists detect thermal boundary that hinders ultracold experiments
Rice University scientists trying to measure the plasmonic properties of a gold nanowire (right) found the wire heated up a bit when illuminated by a laser at room temperature, but its temperature rose far more when illuminated in ultracold conditions. The effect called thermal boundary resistance (Rbd) blocks heat deposited in the gold (Q) from being dissipated by the substrate. Credit: Pavlo Zolotavin/Rice University

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 , 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 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, , 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 . 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.


Explore further

Engineer creates new technique for testing nanomaterials

More information: Pavlo Zolotavin et al. Plasmonic Heating in Au Nanowires at Low Temperatures: The Role of Thermal Boundary Resistance, ACS Nano (2016). DOI: 10.1021/acsnano.6b02911
Journal information: ACS Nano

Provided by Rice University
Citation: Scientists detect thermal boundary that hinders ultracold experiments (2016, July 28) retrieved 20 September 2019 from https://phys.org/news/2016-07-scientists-thermal-boundary-hinders-ultracold.html
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Jul 30, 2016
"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"

Is this a misprint? It shouldn't be startling that replacing a low conductivity material with a material 1000 times more conductive would increase dissipation via the substrate, in fact it's common sense.

What was the author trying to say?

Jul 30, 2016
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Jul 31, 2016
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Jul 31, 2016
An important discovery in measurement of cold nanosystems. Seems kinda boring at first glance, but it will direct quite a lot of research into finding substrates that dissipate the heat better.

Jul 31, 2016
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Jul 31, 2016
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Jul 31, 2016
t shouldn't be startling that replacing a low conductivity material with a material 1000 times more conductive would increase dissipation via the substrate, in fact it's common sense.
What's not "common sense" is that it should only be 3 times better.

it could be also consequence of the Casimir effect, because this effect is based on shielding of virtual photons between nearby surfaces.
The radiative transfer is going to be far smaller than the conductive transfer. We're talking about atoms bumping into atoms, not transfer of photons.

What we're looking at here is the ability of either very thin films (note the titanium layer) or material boundaries to block conduction, and this ability was heretofore unknown.

I'll also point out that it isn't just any surfaces close together that create the Casimir effect. They must be conductive. And, the cherry on top, the Casimir effect doesn't block conductive heat transfer.

Jul 31, 2016
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Jul 31, 2016
@epoxy, you haven't offered any evidence that the Casimir effect applies to conductive heat transfer.

And that's your problem: you never do.

Jul 31, 2016
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Jul 31, 2016
fluid
What "fluid?"

And sorry, but evidence drives theory. Without evidence it's just speculation- and as I pointed out elsewhere, not particularly good speculation at that.

Jul 31, 2016
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Jul 31, 2016
Do you have a better one?
Did you read the article? They have a better one. Maybe you should read it.

It says they did finite element modeling and found that it's a general effect due to increased importance of thermal boundary resistance at very low temperatures. You can read up on thermal boundary resistance here: https://en.wikipe...sistance

It's pretty well-known. They've been studying it since 1935. They use finite element models to examine it theoretically, and they're well-tested models.

What is responsible for the lack of thermal conductivity of narrow gaps according to you?
There aren't any "narrow gaps." The gold is deposited directly onto the surface of the substrate as a thin film, probably by sputtering. In this case to make sure it sticks, they first deposit a very thin film of titanium (1 or 2 nm) onto the substrate. As for what is responsible, see above.

Jul 31, 2016
Just so we're fully clear here, these guys did some experiments that showed increased thermal boundary resistance at very cold temperatures <- dingdingding experimental results

and when they went and modeled thermal boundary resistance at very cold temperatures they found out that their models predicted it <- dingdingding theoretical results using current models

which confirmed the models are indeed correct in the regime of very cold temperatures. Theory was confirmed by experiment. <- dingdingding real science occurred

Get it?

Jul 31, 2016
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Jul 31, 2016
This is just a description of the problem, not its explanation
The explanation is in the mathematics the finite element modeling program calculates. For a look at this: https://en.wikipe...dynamics

Since this is inherently a many-body problem, there is no analytical description of it. The description resides in the implications of many-body atomic interactions in solids.

This is typical of your intuitive approach; you think because there's no analytical description "we don't know anything about it." This is a huge flaw in your thinking.

Why the "pretty well-known effect" studied since 1935 suddenly got "startling"?
Because nobody before had either observed that it changed at very low temperatures or run the simulations that showed it did, and everyone would have assumed that there would be as big a difference in thermal conductivity of sapphire over glass at very low temperatures as there is at room temperature.

Jul 31, 2016
Don't know if anyone checked the "more info" link , but this appears to be an update (re-wording?) of a 2010 article on the same thing. Even the places (Rice U) and party named (Douglas Natelson) are the same...
Anyway, I would be interested in how MUCH the resistance increased....

Jul 31, 2016
Don't know if anyone checked the "more info" link , but this appears to be an update (re-wording?) of a 2010 article on the same thing. Even the places (Rice U) and party named (Douglas Natelson) are the same...
This appears to be a side-effect of low temperature that they discovered while testing their analytic technique detailed in the 2010 article.

Anyway, I would be interested in how MUCH the resistance increased....
About 300 times, relative to the comparative resistance between silica and sapphire at room temperature.

Jul 31, 2016
Worth noting I suppose that this increase in thermal resistance appears to be a surface effect rather than a bulk effect, and related to temperature.

Jul 31, 2016
Don't know if anyone checked the "more info" link , but this appears to be an update (re-wording?) of a 2010 article on the same thing. Even the places (Rice U) and party named (Douglas Natelson) are the same...
This appears to be a side-effect of low temperature that they discovered while testing their analytic technique detailed in the 2010 article.

Anyway, I would be interested in how MUCH the resistance increased....
About 300 times, relative to the comparative resistance between silica and sapphire at room temperature.

Fab! Thanks!

Jul 31, 2016
Worth noting I suppose that this increase in thermal resistance appears to be a surface effect rather than a bulk effect,

Not quite sure what you mean since I thought these were single atom thick layers for some reason...
and related to temperature.

Well, yeah...
but again, not quite sure the point being made...

Jul 31, 2016
No, they're not single atom thick. The titanium film is there only to make the gold stick to the crystal (silica or sapphire); the gold is not very thick, but the substrate is quite thick.

What's happening is that surface effects are preventing the transmission of the heat between the gold and the crystal. These surface effects become much more important at very low temperatures, in fact more important than the heat resistance in bulk. As a result they prevent the gold from losing its heat to the crystal at these low temperatures.

Aug 01, 2016
No, they're not single atom thick. The titanium film is there only to make the gold stick to the crystal (silica or sapphire); the gold is not very thick, but the substrate is quite thick.

What's happening is that surface effects are preventing the transmission of the heat between the gold and the crystal. These surface effects become much more important at very low temperatures, in fact more important than the heat resistance in bulk. As a result they prevent the gold from losing its heat to the crystal at these low temperatures.

Okay. Thanks...

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