Theorized in the 1970s by Russian solid-state physicist Alexey Ekimov before actually becoming a reality in the following decade, the man-made, semi-conducting, nanoscale crystals called quantum dots — also called artificial atoms — are extraordinary things: designed to move electrons, one of the great things about them is when UV light strikes them they emit different coloured light at room temperature, a property called photoluminescence. Another wonderful property they possess is called quantum resonance, which is when they are physically close enough to each other, their electronic states are coupled. Their ability to do this drastically boosts the chances of transferring electrons between them. Being able to do this can improve outcomes in optical applications, quantum computing (QC) and biological and chemical applications.
Considering its difficulty, news out of Japan is that a group of scientists at Osaka City University have discovered a way to manipulate the interaction between these quantum dots. Their discovery, in theory, could exponentially improve charge transports, which could usher in the era of more efficient solar cells, and who knows, other areas of quantum information science (QIS) too.
The paper, Controlling the dimension of the quantum resonance in CdTe quantum dot superlattices fabricated via layer-by-layer assembly, was published in the journal Nature Communications in late October of this year.
“Our simple method for fine-tuning quantum resonance is an important contribution to both optical materials and nanoscale material processing,” said DaeGwi Kim, one of the co-authors of the paper. Nanomaterials engineer Kim, who leads a team of scientists at Osaka City University, RIKEN Center for Emergent Matter Science and Kyoto University, added: “Combining different types of semiconductor quantum dots, or combining semiconductor quantum dots with other nanoparticles, will expand the possibilities of new material design.”
Difficulties have arisen, however, in the form of controlling the distance between quantum dots in 1D, 2D and 3D structures. Fabrication methods currently employ long ligands, which unfortunately hamper their interactions.
The team discovered they could identify and manipulate quantum resonance through the use of cadmium telluride quantum dots connected with short N-acetyl-L-cysteine ligands. ‘They controlled the distance between quantum dot layers by placing a spacer layer between them made of oppositely charged polyelectrolytes. Quantum resonance is detected between stacked dots when the spacer layer is thinner than two nanometers. The scientists also controlled the distance between quantum dots in a single layer, and thus quantum resonance, by changing the concentration of quantum dots used in the layering process’.
Kim and his team’s next task — applying their ‘layer-by-layer approach’ — is to examine the optical properties, with a special focus on photoluminescence, of quantum dot superlattices. They are also excited that their fabrication method can be applied with different sorts of water-soluble quantum dots and nanoparticles. On this, Kim said: “This is extremely important for realizing new optical-electronic devices made with quantum dot superlattices.”
This is exciting news for the computing industry too. Computers are getting smaller, faster: Moore’s Law in motion. And with it, physical limits will start to be witnessed. One way of possibly offsetting the problem would be to adopt light instead of electrons for sending information. Photonic-based architecture could utilize quantum dots just like binary computers use transistors these days. These optical computers could be better than current technology, though whether or not they become widely adopted will depend on how quantum computers develop.
“Our simple method for fine-tuning quantum resonance is an important contribution to both optical materials and nanoscale material processing.”
— DaeGwi Kim
It’s an exciting time for physicists and those involved in quantum information science to be alive, and with Kim and his excellent team working hard to raise the bar, maybe quantum dots will be the go-to modality for most of the world’s technology in the future.