Why Size Matters: An Intro to Quantum Dots
Gems in STEM: Introduction to Quantum Dots
Does size matter? It’s an age-old question in all sorts of areas. Is it always “the bigger the better”? (If you think so, you might like Texas!) Or, can small prevail over tall (like a David v. Goliath type beat)? Honestly, the answer to this debate varies person-to-person.
We constantly make observations about our macroscale world, which we can luckily explore whenever we want. But in the past century, scientists have started taking things up a notch, or more accurately, down a notch. They started zooming in on the world to see where all the mysterious and interesting things happen on the micro and nanoscale.
This is the basis of nanotechnology, which is the study of how we can manipulate matter on an atomic and molecular scale. Unlike your side-view mirror, objects here are much smaller than they appear (in your mind).
That evil piece of paper that gave you a nasty paper cut? 100,000 nanometers thick. The ant you (hopefully accidentally) stepped on? A million nanometers long. Get this, the ratio of a meter to a nanometer (which is a billion) is approximately the ratio of a marble to the Earth. So yes, nano is (na)no joke.
Where did this not-so-miniscule idea even come from? Most scientists agree that the acclaimed physicist Richard Feynman first introduced the idea of nanotechnology in his lecture, “There’s Plenty of Room at the Bottom.” (Whose title, at first glance, seems like a pessimistic worldview.) In this speech, Feynman remarked on the possibilities of miniaturized machines and encoding large amounts of data in tiny spaces, which seemed almost unfathomable at the time–which is what makes nanotechnology so exciting.
Because the nanoscale is so incredibly small (around 1 to 100 nanometers), manipulating matter in this world isn’t too easy. It’d be super cool if we could boss a bunch of individual atoms around, from telling them when to “turn” on and off to store information or lighting them up with different colors (in what would be a fantastic glow show), or just directing them in some way. This isn’t quite currently possible, but don’t despair–we have the next best thing: quantum dots!
Quantum dots, often called artificial atoms, are tiny crystals that range in size from 2–10 nanometers. They’re so small that we can basically think of them as a concentrated single point, which is why they are often called zero-dimensional. Quantum dots are made from semiconductor material (like silicon)–they aren’t really a conductor or an insulator, but can be chemically treated to behave like either.
Quantum dots are proof that you don’t have to be big to be interesting, so it seems that size indeed doesn’t matter…or does it?
Quantum dots are able to absorb light, so when you shine a light on them, they release this absorbed energy as distinctive and precise colors that depend on the quantum dot’s size, shape, and material. Talk about shining in the spotlight!
Now, I have just a couple questions: what? Why? Where? When? How?
Never fear, dear reader, we will answer these together, but for now I’ll give you the mysterious answer that everyone says but never seems to actually explain: “because quantum!”
Specifically, quantum confinement is the culprit behind this optical correlation between the colors of QDs and their size + material.
Quantum Dots Are Exciting When Excited
To see this, let’s first understand why quantum dots are nicknamed artificial atoms! You might have already learned in your physics or chemistry class about how atoms can be excited with energy (like little kids) when an electron inside it jumps up to a higher energy level/electron shell (…unlike little kids). However, at higher levels they are generally unstable and thus eventually go down (or relax) to a lower level that is more stable (like little kids). In order to do this, they need to get rid of this extra energy, so the atom emits a photon of light with the same energy it absorbed!
This emitted light’s wavelength and frequency, and hence color, depends on the difference between the quantized (i.e., set) values of the energy levels it jumps between, based on what the atom is. You can sort of think of it as climbing rungs on a ladder!
Just like atoms, quantum dots can similarly be optically excited! QDs also have quantized energy levels, but even if they are made from the same material, they can emit different colors of light based on their size. Smaller quantum dots (~2 nm) produce shorter wavelengths and higher frequencies (bluer), and bigger quantum dots (~6 nm) produce longer wavelengths and lower frequencies (redder). QDs that are of a more intermediate size (~4 nm) emit green light–go photoluminescence (which is what we call this light emission process)!
It turns out that quantum dots have a broad absorption spectra, meaning that they can be excited across a pretty expansive range of light wavelengths. However, QDs actually have a fairly limited spectrum for the light they give off, which is why we get these super fine-tuned colors (like that vibrant rainbow we saw above). This means that we don’t see overlapping colors which gives way to some crazy applications in using these various QDs to track and label biomolecules in real-time (which we’ll talk more about in the next part about applications)! It’s especially useful because QDs photobleach really slowly, meaning that their color intensity lasts longer in comparison to other molecular markers, so we can track longer processes.
Let’s take it back now, y’all (and cha-cha real smooth) to what it means to be quantum.
Basically, QDs display quantum effects because of their incredibly small size, meaning that the electrons within them can only be at the discrete energy levels we just talked about. At this nanoscale, we’re restricting, aka confining, electrons’ motion and behavior in all three spatial directions — basically falling down a rabbit hole to the quantum wonderland (might as well change our names to Alice at this point and drink anything that says “drink me”)!
Band Gaps Aren’t Just for 1D, they’re for QDs too!
Because of this fascinating effect of quantum confinement, as nanocrystals decrease in size, their band gaps increase, which is what gives them this rainbow power.
To clarify, this band gap isn’t a gap a band takes after they inevitably lose a member. (I’m looking at you, One Direction — your “hiatus” has been 7 years long already with no end in sight, and it’s very upsetting.)
In actuality, the band gap is the minimum energy it takes to free electrons in a material so that they can move electricity through it. So, since they have stronger confinement, smaller QDs have larger band gaps, meaning that they need more energy to get excited (like adults). They then give off higher frequencies and shorter wavelengths and are thus bluer…also kind of like adults :(. As expected, larger QDs have smaller band gaps, so they need less energy and have lower frequencies and longer wavelength, which is why they are redder.
When a light is shone on a QD, an electron in it is excited to a higher energy level, which then drops back when it releases this energy as light emission, called photoluminescence. The color of the light depends on the energy difference, which is larger for smaller nanocrystals.
Since quantum dots’ band gaps are incredibly dependent on their size, QDs are thus tunable to our liking. Like we just talked about, by decreasing the size of the dots, you increase the band gap, and vice versa, meaning we can make different colored QDs just by changing their size (even if they’re made from the same materials).
Note that absorption and emission wavelengths are different because the absorption wavelength involves specific photon energies, while the emission wavelength represents these band energy gaps.
Moral of the story: when things get nano, things get quantum. And these quantum effects give us the superpowers to actually go in and tune the optical, electrical, magnetic properties of quantum dots to change the world as we know it — right at the nanoscale.
QDs Be Crazy
Quantum dots don’t just make for fun light shows, their unique optical properties and energy tunability mean that we can use them for all sorts of things! Especially situations where precise control of light is important, like photovoltaics, imaging, detecting, luminescent labels in biology, and so on! For the sake of time, we’ll look at just three application buckets here: solar cells, televisions, and medicine.
Solar Cell My Soul to QDs
Quantum dots are great at capturing light and converting it into electricity efficiently and, better yet, they require less space than the more standard materials! Talk about breakthrough technology — right now people are very excited about what QDs can do for solar cells.
In a traditional solar cell, photons of sunlight kick electrons out of a semiconductor into a circuit and make electric power at a low efficiency. Here come QDs to save the day! Quantum dots can generate more than one electron-hole pair (or exciton) per photon that knocks into them, which boosts their efficiency in converting sunlight to energy. This effect is called the multiple exciton generation (MEG), and is much preferred to the traditional solar cell’s single produced electron per incoming photon.
Beyond our friend MEG, quantum dots can be more easily manufactured with inexpensive materials that don’t need to be extensively purified (like silicon) and can be applied to cheap and flexible substrates (aka the underlying layer), like light-weight plastics.
Consider me solar sold on QDs!
Let’s Watch the QDs on TV
If channeling the literal sun wasn’t enough, quantum dots are also now being put into LCD TVs to make the pictures more colorful and vibrant (like my personality…and also kinda like LSD).
Their precise tunability gives us more realistic and precise colors, whereas a traditional LCD is made from tiny combinations of red, blue, and green crystals that are illuminated from behind a bright backlight (and don’t necessarily capture exact colors as we see them). Quantum dots don’t even need a backlight, they produce the light themselves, so they’re much more energy efficient! This makes a BIG difference for saving battery in small, portable devices like your cell phone (and helps make sure it doesn’t die at the exact moment you need it). QDs also let you watch cute dog and cat videos at a much higher-resolution.
From purer colors to long battery life to being cheap in price but not in quality, QDs are considered the future of displays in all sizes. That’s quantumazing.
Let There Be Light…to Detect
Because quantum dots can be mapped to a rainbow, scientists can use them to target and color code cells, helping them visualize and track molecular surfaces within cells.
Unlike most organic dyes (which generally have a limited color range and degrade quickly), quantum dots can be excited by various light sources since they have a broad absorption spectra, and their light emissions can also be easily distinguished since the emission spectra is narrower than traditional dyes. They’re basically incredibly bright, can virtually produce any color, and are photostable (meaning they could theoretically last forever). All in all, quantum dots are like nanoscopic light bulbs–a fantastic find for biological imaging, labeling, sensing, and more!
The color intensity of QDs also fades more slowly over time compared to other markers, meaning that they could track how molecules move inside a cell over a long period of time. We could also identify single-molecule binding events over time (which is usually difficult) because QDs have a blinking property that lets us identify individual quantum dots in a sample.
Let’s throw some other fun applications into the mix: QDs can be used as neuroscience sensors, signify when a therapy X-ray beam is correctly located, or for targeted drug delivery and cell labeling. They also have potential as radiation detectors for security, since quantum dots give off light near radiation. QDs are moreover being tested as sensors for chemical and biological warfare agents, which is insane. QDs definitely scooby-doo be crazy!
All of this sounds too good to be true–and it kind of is! Using quantum dots in these sorts of medical situations raises some safety issues. To be viable, Quantum dots need to be nontoxic and either dissipate or remain in a patient without harming them. However, a lot of the current materials used to make QDs don’t fit this criteria and could potentially cause health problems, but other element combinations can be more expensive.
Not only do we have to consider our own health, we have to take care of the environment’s health too. As we start using nanomaterials more, we must also address and carefully monitor their potential pollution and toxicity–let’s cross our t’s and dot our i’s for our Earth.
Alright, let’s finally climb out of this rabbit hole and scale back to our macro world. If there’s anything we learned on this journey in the quantum wonderland, size matters sometimes! (But not always. ❤)
Wait! Before you go, I have one last question for you: are you a semiconducting nanocrystal? Because you’re a QD & the light of my life. <3
Extra! Extra! Read All About It!
If you want to read about more interesting applications of quantum dots, look no further! Be like Dora and explore the links below.
Graphene Quantum Dots: https://en.wikipedia.org/wiki/Graphene_quantum_dot
SafeStamp has a mission of saving the 1 million people who die each year from counterfeit drugs by using QDs. With QDs they can build nanotech indicators that emit a blue light when purchases are counterfeit.
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As a reminder: this column, Gems in STEM, is a place to learn about various STEM topics that I find exciting, and that I hope will excite you too! It will always be written to be fairly accessible, so you don’t have to worry about not having background knowledge. However, it does occasionally get more advanced towards the end. Thanks for reading!