We started out playing around with metal nanowires, and measuring the open-circuit voltage (that is, hook up a volt meter across the device, which nominally doesn't allow current to flow) across those wires as a function of where we illuminated them with a near-IR laser. Because the metal absorbs some of the light, that laser spot acts like a local heat source (though figuring out the temperature profile requires some modeling of the heat transfer processes). As mentioned here, particles tend to diffuse from hot locations to cold locations; in an open circuit, a voltage builds up to balance out this tendency, because in the steady state no net current flows in an open circuit; and in a metal, the way electron motion and scattering depend on the energy of the electrons gives you the magnitude and sign of this process. If the metal is sufficiently nanoscale that boundary scattering matters, you end up with a thermoelectric response that depends on the metal geometry. The end result is shown in the left portion of the figure. If you illuminate the center of the metal wire, you measure no net voltage - you shouldn't, because the whole system is symmetric. The junction where the wire fans out to a bigger pad acts like a thermocouple because of that boundary scattering, and if you illuminate it you get a net thermoelectric voltage (sign depends on how you pick ground and which end you're illuminating). Bottom line: Illumination heats the electrons a bit (say a few Kelvin), and you get a thermoelectric voltage because of that, to offset the tendency of the electrons to diffuse due to the temperature gradient. In this system, the size of the effect is small - microvolts at our illumination conditions.
Now we can take that same nanowire, and break it to make a tunnel junction somewhere in there - a gap between the two electrodes where the electrons are able to "tunnel" across from one side to the other. When we illuminate the tunnel junction, we now see open-circuit photovoltages that are much larger, and very localized to the gap region. So, what is going on here? The physics is related, but not true thermoelectricity (which assumes that it always makes sense to define temperature everywhere). What we believe is happening is something that was discussed theoretically here, and was reported in molecule-containing junctions here. As I said when talking about hot electrons, when light gets absorbed, it is possible to kick electrons way up in energy. Usually that energy gets dissipated by being spread among other electrons very quickly. However, if hot electrons encounter the tunnel junction before they've lost most of that energy, they have a higher likelihood of getting across the tunnel junction, because quantum tunneling is energy-dependent. Producing more hot electrons on one side of the junction than the other will drive a tunneling current. We still have an open circuit, though, so some voltage has to build up so that the net current in the steady state adds up to zero. Bottom line: Illumination here can drive a "hot" electron tunneling current, and you get a photovoltage to offset that process. This isn't strictly a thermoelectric effect because the electrons aren't thermally distributed - it's the short-lived high energy tail that matters most.
It's fun to think about ways to try to better understand and maximize such effects, perhaps for applications in photodetection or other technologies....
7 comments:
Hi Doug,
Very interesting result! I looked for the paper on your website, but only saw the link to the publisher's page. Can you please post the pdf?
I have a few questions:
Does the enhanced photovoltage increase when the nanogap width increases? In other words, do you get a correlation between photovoltage and gap resistance?
I imagine there must be some gap limit for which the effect breaks down... probably 5nm or so, when tunneling current is completely suppressed.
What is the ultimate limit of the enhancement, i.e. what is the maximum photovoltage you can get for a given illumination and fixed wire width, if the nanogap space can be varied?
This may not be an easy measurement, since the gap width is hard to control, but I imagine that with a sufficient number of structures, you get a distribution of nanogap widths, and can still find out if there is an upper bound to the photovoltage.
Thanks!
Hi Anon, see https://arxiv.org/abs/1704.07909.
Answering your questions:
Does the enhanced photovoltage increase when the nanogap width increases? In other words, do you get a correlation between photovoltage and gap resistance?
There is a lot of device-to-device variation, which we discuss in the paper. There is a figure in the supporting information that shows the aggregate of a large number of devices. It's hard to say, and one issue is that there is reason to expect atomic-scale variability in the surfaces, and that can affect things like work function that go very sensitively into tunneling.
I imagine there must be some gap limit for which the effect breaks down... probably 5nm or so, when tunneling current is completely suppressed.
You are right. In the limit that the tunneling gap is basically an open circuit, we detect no photovoltage. (I am still interested in what would happen in, e.g., the field emission regime, but we'd have to work with different electrode material and conditions to get stable device surfaces under those bias conditions.)
What is the ultimate limit of the enhancement, i.e. what is the maximum photovoltage you can get for a given illumination and fixed wire width, if the nanogap space can be varied?
The biggest numbers we've seen have been with Au devices, where we find evidence (polarization dependence) that plasmons help in the generation of hot carriers (possibly just by enhancing absorption), and we max out at tens of mV. You would not want to use these particular devices as energy harvesters b/c of poor efficiency, but I have plenty of ideas.
Thank you for the link and for your answers.
From the point of view of engineering a detector, this technique does produce large photovoltages, but at the expense of higher resistance, which in a way makes sense since the incident light acts as a current source, so it's a current-limited phenomenon.
Anon, true. There are things one could do to try to enhance the effect, and as made so far these are almost certainly not the optimal device structures. Usefulness depends very much on the application. These are not single-photon detectors by any means; they also have different relevant noise processes than semiconductor-based detectors. Fabrication of these kinds of structures for some wavelength ranges could be simpler in some ways than focal plane array devices based on narrow-gap semiconductors, for example. Fun to think about.
It is very interesting content and explained nicely.
Doug,
Does it work if the substrate is at room temperature in ambient conditions?
Hi Dan - Room temperature, yes. All the continuous nanowire stuff (including some weird things we have in a submitted paper that aren't out yet) works great at room temperature. It's funny - at low T, the Seebeck coefficient for the metal is smaller (b/c that's how the Mott expression in metals works), but the amount of heating due to the laser is larger (b/c of worse thermal conductivity + thermal boundary resistance), so the (substrate) room T numbers and the (substrate) low T numbers for photothermoelectric voltages are rather similar. For the nanogaps, the issue at room temperature is stability of the junction, b/c the metal atoms (and any unintended adsorbates) like to move around, especially when strongly illuminated. That aside, the photovoltages are about the same (see Fig. 3f in the paper, or S10 in the supplemental).
As for ambient conditions, we haven't tried in part b/c we wanted to minimize the likelihood that adsorbates contributed to what we were seeing somehow.
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