Photoconversion Physics in Quantum-Wire Arrays with Double-Gyroid Topology.
Hugh Hillhouse is trying to squeeze more energy out of each photon. Albert Einstein did much to develop the concept of the photon, which, in broad terms, is defined as the tiniest packet of electromagnetic energy. Light is simply electromagnetic energy. When photons of light strike a solar cell, they’re absorbed, boosting the energy of the electrons in the atoms that make up the active part of the cell. Then those “excited” electrons are allowed to fall back down to their normal energy levels by first moving through a circuit, creating useable electricity, somewhat like a controlled jet of water coming out of the bottom of a dam though the electric power generator. Unfortunately, Hillhouse says, “in a conventional single-junction solar cell, about 30 percent of each photon’s energy gets lost as heat – even in the best case scenario.” Think of it as the energy lost when the rain falls from the sky – with a dam, we harness only the energy of the last few hundred feet of the raindrop’s journey to the bottom of the dam. When a photo cell isn’t operating perfectly, there are additional losses, called “recombination.” Think of that as water over the dam – useless for generating electricity. What Hillhouse wants to do with his Scialog grant is to explore the properties of a novel type of nanomaterial his group has developed – “nano,” the International System of Units prefix meaning 10−9; a nanometer is one-billionth of a meter – to capture some of the photon’s energy normally lost in the typical solar cell conversion process. The material he’s looking at is called a “double-gyroid nanowire array,” and it’s every bit as odd as its name. “It’s pretty easy to paint a picture of it,” Hillhouse says. “Imagine a wire segment that is straight, but then branched at a ‘y’ junction. Then imagine you follow, say, the left branch another unit of distance, and you branch again, also at a ‘y’ junction. But this one is precisely rotated relative to the one before it. Now, if you just repeat that along each branch with the correct rotations you can create an interconnected 3D structure that continues towards infinity every direction – it’s periodic. Imagine being a little nanoscopic creature crawling along a wire—no matter where you start, after only 10 y-junctions you can return to where you started! ” But when it comes to describing the oddity of this stuff, Hillhouse is only warming up. He adds: “We make the nanowire arrays from a template (think of a mold) that forms along what’s called the ‘gyroid surface;’ that’s a surface that has zero mean curvature.” Huh? “You’ve seen a saddle, right? If it’s convex left and right, then it would be concave forward and backward. When you picture a saddle, that’s kind of what your mind is seeing. And at that point right in the center of the saddle—mathematically, the “mean curvature” has a value of zero there.” At any rate, Hillhouse and his associates are the first people who’ve made semiconducting nanowires with that structure. And whether it turns out to be a big deal will depend upon what’s unique about the experiment Hillhouse will be conducting with his Scialog grant. To appreciate that uniqueness, though, you have to know a little something about quantum dots. The weird thing about them is that these incredibly tiny bits of semiconductor material behave differently than a big chunk of the same stuff. (“Semiconductor,” by the way, just refers to a material that has a limited ability to conduct electric current due to the absence of electronic energy levels.) For some reason – still hotly debated by scientists– a quantum dot can capture more energy in the form of excited electrons when it’s hit by a photon. That is, more electrons in this tiny bit of semiconductor become excited than in a much bigger chunk of material. The technical term for this phenomenon is “multi-exciton generation,” or MEG for short. But whatever you call it, it’s puzzling. So, where were we? Hillhouse has a new way of organizing matter, in this double-gyroid nanostructure. This nanostructure, with all of its twisty little “y” junctions, can be made out of semiconductor material, some of it really cheap, like lead sulfide. The thing about the “y” structure is that each one has a little stub at one end. Hillhouse and his associates will be checking to see how this “nanostub” – newly invented word there – behaves as a quantum dot. Another thing about quantum dots – an unfortunate thing, really—is that if you put a bunch of them close together, so that they can easily pass around electrons, that can suddenly cancel their tiny super powers, and they start behaving like one big, boring chunk of stuff again. Hillhouse is hoping that the nanostub in his double- gyroid material will be isolated enough to retain the excitability of a quantum dot, but since it’s wired into a grid of millions and millions of other nanostubs, the entire structure will power up like an enormous quantum dot. Of course, that’s a real long shot, precisely the kind of high-risk project Scialog funds. But if Hillhouse succeeds, the payoff could be huge. Did we mention that his double-gyroid structure is dirt-cheap to make? “It sounds like an intricate structure that would be nearly impossible to make, but here’s where nature helps us—you don’t need a machine,” he says. “We make them by what’s called self-assembly. You mix up a solution of the right stuff, all of it cheap and benign. We start with a super inexpensive surfactant (already used by industry), water, ethanol and hydrochloric acid – and you literally dip a substrate into that solution and you pull it out and it forms this thin film that has the double-gyroid structure.” Then, to make the semiconductor, Hillhouse removes one component of the film, the organic surfactant part. “And we fill those areas with semiconductor. And that can be done in different ways. We can do it by electrodeposition, solution deposition, drop casting—there are a whole bunch of ways to do that.” Part of what he’ll be exploring in this project is what might be the right way to make those semiconductor wire structures such that the film yields the physics he’s looking for – namely, getting more energy from a single photon. “People have actually observed this multi-exciton generation in silicon quantum dots,” he notes. “So it appears to be something general. Our philosophy is, let’s demonstrate our technique with one semiconductor and then we can put all sorts of time and effort into demonstrating it with other semiconductors.” Eventually, if things were to work out – and again, that’s a long shot—Hillhouse and his colleagues envision long sheets of inexpensive double-gyroid films going up on rooftops everywhere. Most people wouldn’t even suspect some exotic new arrangement of matter was involved. “To the naked eye it looks just like a normal coating,” he says. “And to even a reasonably high-resolution microscope it looks just like an ordinary coating. But when you really go in with the highest resolution microscopes that we have, or with X-rays or neutrons, you see that it’s composed of this perfectly ordered array of wires.” One would be hard pressed to come up with a better quote describing precisely where the frontier of solar energy conversion research is these days. It’s in the nanodetails.