Tag Archives: nanoscience

Small World

It’s a New Year, and I went ahead and tried something new! This is a video version of some of the cool nanoscale and quantum things I’ve written about before, created in collaboration with director/editor Kevin Handy. I hope you enjoy it; I had a blast making it.

Happy Ada Lovelace Day!

Today is Ada Lovelace Day, a day of blogging about women in science! (Not necessarily blogging by women in science, which is every day here.) The day is named for Ada Lovelace, who was an important figure in the nascent days of computer science, back in the 1800s when it was more of a theoretical math field concerned with the creation of calculation engines. The idea of Ada Lovelace Day is to write about a woman in science, technology, engineering, or math, which raises awareness of all the great women, both now and in the past, who have done amazing things in the STEM fields. There will be lots of stories about various role models over at the official site once the day is concluded, but the scientist I wanted to tell you all about here is Mildred Dresselhaus.

For the last twenty years or so, materials made from carbon have been getting exponentially more and more attention. Carbon is an essential building block in many of the chemicals that are important for life, but there are also huge differences between materials made from carbon depending on how the carbon is bonded. Diamonds and coal are both forms of carbon, but with wildly different crystal structure. So many of the hot carbon materials from recent years have come from new ways that the carbon atoms can be arranged. For example, carbon nanotubes are like rolled up sheets of carbon, and graphene is a sheet of carbon that’s only one atom thick. Both carbon nanotubes and graphene have very high mechanical strength, electrical and thermal conductivity, and low permeability for their size. And there are a lot of other ways carbon can be nanostructured, collectively referred to as allotropes of carbon. You can see some of them in the image below, such as (a) diamond, (b) graphite (multiple sheets of graphene),  and (h) a carbon nanotube.

But Dresselhaus was into carbon before it was cool, and has been a professor at MIT since the 60s studying the physics of carbon materials. Her work has focused on the thermal and electrical properties of nanomaterials, and the way in which energy dissipation is different in nanostructured carbon. Her early work focused on difficult experimental studies of the electronic band structure of carbon materials and the effects of nanoscale confinement. And she was able to theoretically predict the existence of carbon nanotubes, some of their electronic properties, and the properties of graphene, years before either of these materials were prepared and measured. Her scientific achievements are extremely impressive, and she has gotten a lot of honors accordingly.

And as you can imagine, things have changed a lot for women in science over the course of her career. When she began at MIT, less than 5% of students were female, and these days it’s more like 40%. But of course, it helps female students quite a bit to see female role models, like Dresselhaus. Which is the entire point of Ada Lovelace Day!

You can read an interview with Mildred Dresselhaus here, and more about her scientific achievement here.

Why The Nanoscale Matters

One of the most intriguing experiences in science is observing something  completely counterintuitive. And then having to figure out, okay, why is everything I thought I knew wrong in this case, and what experimental or theoretical studies can I do that will help explain this? That’s the appeal and the challenge of nanoscience: things at a small scale behave very differently from what we’d guess based on observations at a large scale. So how does weirdness inherent in the quantum world come into play?

A single atom has different properties than a bulk material that has trillions of atoms together. That’s because there are a discrete number of possible configurations for the atom to be in, which means discrete energy levels. This is called quantization, as opposed to the continuous smear of available configurations and energy levels that a bulk material has. But if we move away from the extreme cases, where does bulk behavior stop and quantized behavior kick in? Are two atoms still a quantum object? Three? It turns out that quantization of material properties actually persists for awhile, and collections of hundreds or thousands of atoms can still show quantization.

So how do we know where the transition point is between bulk and quantized properties? If we think about shrinking a material down, at some point the size of the material will become smaller than the size of the electronic wavefunction in that bulk material. Below that size, the electrons are “confined” and the states available to them start to change depending on the material size. So we can define a length that is a “quantum confinement limit” for each material. Below the limit, the collection of atoms is confined and has quantum properties, and above the limit it’s approximately the same as a bulk material.

Once we know that limit, there are several ways to make nanomaterials that have quantized properties. We can make nanostructures that are confined in one dimension, like nanosheets, but bulk-like in the other two. We can also make nanowires, which are confined in two dimensions but have one bulk-like dimension. Or, we can make “quantum dots” which are confined in all three dimensions. Quantum dots are like little islands of material, with discrete energy levels just like an atom would have. And one consequence of quantization is that the wavelength of light that quantum dots absorb and emit actually depends on size, as in the image below. Smaller quantum dots absorb and emit bluer light, because the size of the quantum dot increases the spacing between energy levels. There are lots of applications for this effect, such as solar cells whose absorption spectrum is tuned to that of the sun, or LEDs that emit tunably-colored light.

But at the nanoscale, the surface of the object is a lot more important. In something macroscale, like a brick,  less than 0.0001% of the atoms are on the surface of the object, but in a quantum dot 30% of the atoms may be surface atoms. That makes surfaces very important! And surface atoms can be the sites of electronic defects, or the sites of bonding by various chemical species that change the properties of the quantum dot. So surface chemistry becomes important, and the quantum dot or nanowire or nanosheet may be very sensitive to small changes in the environment. This can be an asset, though, for example to make gas sensors from chemically functionalized nanomaterials.

Another consideration is that if a material has some features that are nanoscale, those features may be as small as or even smaller than the wavelength of visible light. Practically speaking, this means it’s often easier to image nano-objects with electrons rather than photons. But again, there’s an upside, because you can tune the nanoscale features to interact with light in specific ways or even be hidden from interactions light. This is one of the most interesting things about metamaterials, which I’ll write more about soon!

Nanowires, Memristors, and the Brain

I haven’t gone into much detail about what I work on here, though from the topics I pick to write about, you could probably guess that it involves electronics, materials science, and interesting technological applications. But a science communication student at my workplace made a short video about my project, which features me and a couple of my coworkers explaining what’s cool about nanowires, memristors, and the brain, all in less than four minutes. Check it out!