In early October, scientists Moungi G. Bawendi, Louis E. Brus and Aleksey Yekimov were announced as the winners of the 2023 Nobel Prize in Chemistry "for the discovery and synthesis of quantum dots".
Quantum dots are atoms "glued together" in the lab, which can number between 10 and 10,000. For this reason, they are sometimes called "artificial atoms" or "artificial molecules". Interestingly, their properties change depending on their size; dots of different sizes can glow in different colours, from violet to bright red (or infrared or ultraviolet, which are not visible to the naked eye). Quantum dots have therefore added colour to nanotechnology, as the Royal Swedish Academy of Sciences has aptly pointed out.
It's also interesting to see the concrete, tangible benefits of this discovery already today - all you have to do is walk into any home appliances store and stop by the TV section. Or perhaps quantum dots are already "at work" in your home!
Millions upon millions of these physical particles, invisible to the naked eye, make the image on a TV screen far more lifelike and the colours more natural, warmer and brighter. It's called QLED (Quantum Light Emitting Diodes) technology, and it was the subject of much of the media coverage of this year's Nobel Prize in Chemistry.
But displays are far from the only area where quantum dots can play an important role. And Nobel Prize winners in chemistry are not the only ones in the world working on this technology. Chemists and physicists at the Center for Physical Sciences and Technology (FTMC) in Lithuania are developing it too.
(Dr. Agnė Sukovienė. Photo: FTMC)
Red signal when enemy detected
First, chemistry. How can quantum dots be useful in this field? They were created (or, more scientifically speaking, synthesised) by Dr. Agnė Sukovienė of the Department of Electrochemical Material Science at FTMC in 2019 as part of her PhD thesis.
The researcher used gold atoms, which she "glued" together in a solution to form quantum dots made up of 56 such atoms. To make the formation glow brighter and more protected, it also had a globule - a 'shell' made of denatured (lost its natural properties) bovine serum albumin.
By the way, there is a slight difference of opinion among scientists on the terms. Molecules with luminescent properties are commonly referred to as quantum dots by physicists, but in a chemistry group you're likely to hear the term "cluster". So, according to Agnė, she works with clusters of 56 gold atoms, which are also called quantum dots in other contexts. But technically they are the same.
(Production (synthesis) of gold clusters-quantum dots. Illustration: FTMC)
Although the cluster of 56 gold atoms is considered large at the quantum level, it is only about two nanometres in size. A nanometre is one millionth of a millimetre. And these tiny little things are expected to have enormous significance in cancer diagnostics.
FTMC chemist says that gold clusters of this size - quantum dots - glow red when excited by light at the visible spectrum's 470 nanometre wavelength. This is crucial: by targeting receptors (complementary compounds), such structures can bind exclusively to cancer cells, and the glow of the gold clusters allows scientists to pinpoint the locations of these cell aggregations.
"We first used the gold clusters to study malignant and non-malignant breast tumour cells. We were able to obtain traceability and reproducibility, and we saw that the clusters bind and glow more strongly to malignant cells," says A. Sukovienė.
(Production (synthesis) of gold clusters-quantum dots. Illustration: FTMC)
FTMC is conducting this research in collaboration with the National Cancer Institute of Lithuania and the technology is promising. However, as is often the case, many more experiments and refinements are needed before a real treatment can be delivered to the human body:
"All the studies so far have been done in vitro, and it is still problematic to assess cytology, i.e. how the cancer cell works. Our solution medium was quite alkaline, pH 12. As we lowered this to a neutral, blood pH (by washing, dialysis, removal of unnecessary substances from the solution), the intensity of the glow of the gold clusters decreased. They emitted red light, but the duration of the glow was shortened, so we had to carry out the tests very quickly.
So, part of the cytology is still unfinished, and the National Cancer Institute is already trying to address this issue."
Agnė Sukovienė and her team have also tried using smaller quantum dots - 32 gold atoms - that emit blue light to detect cancer. But the scientist found that this was not suitable for her experiments.
FTMC chemist continues to look for solutions to bring the day closer when the Nobel Prize authors' discovery will help treat people. Her PhD thesis topic combined two areas - gold clusters and magnetic nanoparticles - and further research is ongoing. What is expected from this "tandem"?
"Gold clusters glowing together would show us where a specific cancer site is in the body, and an external magnetic field could provide localised treatment. This is what we are aiming for, but so far we have either lost the magnetic properties or the photoluminescence. We don't have one 'optimal point' yet," says Sukovienė.
(Dr. Augustas Vaitkevičius. Photo: FTMC)
What is the link between the laboratory and the stained glass windows in the church?
While chemists "stick" quantum dots in a solution, physicists grow them in "solid form" in the lab. This is also happening at FTMC, where quantum dots of the chemical element bismuth, a semiconductor, are born. This process is called quantization.
"I liked the idea expressed by the Royal Swedish Academy of Sciences in a popular article that the ability to shrink particles opens up a whole new dimension in materials science. The properties of materials depend on the number of electrons in the atom from which they are formed. From this we have the Mendeleev table.
If we think of it as a piece of paper, quantization would "lift it up", giving it another dimension, from which the properties of materials change. There have been papers published on this, and we are now trying to do it with bismuth," says Dr. Augustas Vaitkevičius, a Scientist Trainee at the Department of Optoelectronics.
In simple terms, physical quantization occurs when a material is squeezed, reduced so much that its electrons are severely limited. These particles are "boxed in" (or rather, matrixed, forming multilayered atomic formations between sheets of semiconductors) - so that they are just where they need to be. And to do only the things that the scientist needs.
"In metals, the electron is completely unconstrained, whereas in semiconductors there are certain restrictions. And in very small structures, there are very specific energies and positions that are allowed for the electron," says Augustas, who adds that the semiconducting aluminium gallium arsenide sheets "imprison" the atoms so that they don't go anywhere.
(Quantum dots of the chemical element bismuth. Photo: FTMC)
It is between these sheets, inside the "sandwich", that atoms are trapped and quantum dots are formed. This still requires high heat (around 750 °C). And to help us imagine this, let's think about... stained glass windows in churches!
"Quantization is a physical phenomenon that happens whether you want it to or not. We've been doing it for thousands of years. Churches used to need coloured windows, so glass of a special composition would be heated until it was the desired colour. One of the people who won the Nobel Prize in Physics, Aleksey Yekimov, showed that such glass produces particles of the same size - quantum dots," explains Vaitkevičius.
FTMC physicists do something similar - they take a material, heat it up, and its atoms "click" to the desired size. Except, of course, it's much more complicated than creating stained glass.
Quantum channels and wells
The quantum "sandwich" is heated in FTMC clean rooms, specifically in a large molecular beam epitaxy (MBE) unit or in dedicated furnaces. According to Dr. Renata Butkutė, Senior Researcher at the Department of Optoelectronics, who works in these laboratories, this is flexible and convenient: all the samples are grown, put into a "sandwich" and "baked" practically in one place.
What's even more convenient is that you can change the thickness of the gap between the two quantum sheets. This will determine the size (and light spectrum) of the quantum dots inside. "What's more, we have these dots instantly in a semiconductor that can be added to another device," says Butkutė.
(Dr. Renata Butkutė at the molecular beam epitaxy unit where quantum dots are "born". Photo: FTMC)
Because quantum dots are supertiny, there are only a few scientific tools available to determine whether an observation is actually a quantum dot. One such tool, the transmission electron microscope, is mastered by Dr. Martynas Skapas, a Researcher in the Department of Characterisation of Materials Structure. Meanwhile, Augustas Vaitkevičius is using another piece of equipment that allows him to detect quantum dots from a very small area and observe their glow (microluminescence).
R. Butkutė says that she and her colleagues are also exploring a completely different way of forming quantum dots that has never been demonstrated before. This is the formation of bismuth quantum dots due to segregation. What does this mean?
"We are growing a semiconductor material called gallium arsenide bismide. For this to happen, a bismuth atom needs to be inserted into the gallium arsenide layer. This is very difficult to do because the bismuth atom is relatively large, it 'floats' on the surface, and that excess is a real nuisance. But if scientists can't deal with the side-effect, they come up with a way to turn it into a positive thing."
The physics team decided to embed bismuth elsewhere, between the aforementioned aluminium gallium arsenide quantum sheets. Once the structure is grown and then heated at high temperatures, the bismuth can no longer escape to the surface and begins to accumulate inside the channel, forming quantum dots.
"There is a term called dots-in-a-well (DWELL). We have managed to make bismuth not evaporate, but to aggregate into quantum dots and control their size.
Dr. Evelina Dudutienė and Dr. Vytautas Karpus, colleagues in the Department of Optoelectronics, have calculated that if bismuth is thicker than 60 nanometres, it is a semimetal, but if you make it thinner, it becomes a semiconductor. And only semiconductors have the ability to quantize, to change their properties like chameleons," says Butkutė.
(Quantum dots of the chemical element bismuth. Photo: FTMC)
Quantum dots for faster internet
We've had some clarification on the technical stuff. But where will it be applied in practice?
FTMC physicists highlight the field of telecommunications. The aim is to create stable quantum dots between 7 and 8 nanometres in size and to emit near-infrared light of 1.55 micrometres in length. According to A. Vaitkevičius, such a glow would be very useful in fiber-optic cables that provide the internet:
"We always want appliances to use less electricity. We want them to have the narrowest wavelengths of light, the fastest pulses, to turn on and off faster... There are many parameters for which new materials, devices, approaches are being sought. We are still a long way from that - we are just making a crystal that someone else will put in another object, which will then emit light. But the idea is this: we want a better internet."
E. Dudutienė, who studies bismuth quantum dots, was awarded this year's DigiTech Sector Association INFOBALT scholarship which is aimed at young scientists and to promote cooperation between science and business. The award was given to research that could be promising for the development of lasers in the field of telecommunications. Such lasers would be used in server rooms and would not need to be cooled as they would operate at room temperature. And the active core of the laser is expected to "work" with quantum dots emitting the 1.55 micrometre wavelength of light mentioned above.
(Dr. Evelina Dudutienė. Photo: FTMC)
Lasers of this kind are already showing improved performance, but it's still at the experimental stage and challenges abound. How to make quantum dots appear in very large numbers (e.g. a billion to a trillion per square centimetre)? To make them repeat themselves, to shine long enough at a time? This requires looking at the very foundations of physics, which makes it a very deep science. However, according to R. Butkutė, progress is slowly being made:
"The dissertation research of Augustas [Vaitkevičius], Evelina [Dudutienė] and Martynas [Skapas] leads to a common fundamental knowledge that will later yield results. This is very important."
According to physicist Renata, the most important area where quantum dots are expected to make a big difference is in medicine, which we mentioned at the very beginning: lasers that could make extremely precise tests for cancer markers, or the quantum dots themselves, which could show exactly where in the body cancer is hiding and starting to grow.
"It's very hard to say what a quantum dot breakthrough might look like, because it involves very deep fundamental stuff. But there could be all sorts of applications - TVs, tissue imaging, drug delivery (where the drug affects the specific tissue needed), better lasers, faster internet, etc.," adds A. Vaitkevičius.
Written by Simonas Bendžius