What happens when you squeeze a cluster of atoms into tiny space? Visualize hundreds of atoms contained in a sphere with a diameter that spans just 2 to 10 nanometers. A quantum dot (QD) is so small that it physically constrains its constituent atoms in all directions, so much that it affects the spacing between quantum shells.
With the successful fabrication of these nanoparticles in the 1980s, scientists found that they could control the distance between quantum energy shells, just by changing the size of a QD. The ability to control quanta as discrete energy levels has led scientists to create ultra-efficient semiconductors, precise wavelength-emitting LEDs, and other upgraded everyday technologies. As its technology improves, QDs are becoming an increasingly competitive market, expected to reach $4.6 billion by 2021.
As the space occupied by each atom in a QD approaches the size of the bandgap between quanta, it causes quantum confinement. At this point, the size of the QD begins to affect the size of the bandgap, or difference in energy levels between quanta. Scientists may model the QD’s constraint on quanta as a potential well with sides that extend infinitely upward (to ignore the possibility that an electron may exit the system via quantum tunneling) and a base width that equals the diameter of the QD.
Energy levels are confined within the boundaries of this potential well, so that when it is narrowed, the energy states are forced further apart (right). Think of water contained in a plastic cup. If a person squeezes the cup, it increases the water level. Since potential energy is not lost in the potential well, the quanta rise and spread apart as the diameter is constrained, since they have nowhere else to go in a continually shrinking the system.
A semiconductor in a solar cell uses light from the sun to enable electron transfer from the outer shell of one anion to an adjacent ion with a vacancy, or hole, in its outer shell. (This hole-electron pair, or exciton pair, is achieved with doping.) The outer atomic shell that supplies electrons is called the valence band, while the electron-accepting shell is called the conductance band. The energy difference between the valence and conductance bands is called the bandgap. An external energy source must be at least as intense as the bandgap for electron exchange to occur between bands.
The bandgap is relatively small, or low-energy, in semiconductors. Therefore, they can act as conductors rather than insulators in the presence of an energy source larger than or equal to the bandgap. However, the bandgap in macroscale-semiconductor solar panels is still larger than low-energy sunlight, such as infrared. Since solar energy is nearly 50% infrared, it can be argued that these semiconductors could be made more efficient.
In a QD, the valence and conductance bands no longer refer to the electron transfer between the shells of different ions; instead, they refer to the ground quanta (n=0) and next excited quanta (n=1), respectively. The bandgap between quanta is short compared to the bandgap between ions in a regular semiconductor. Therefore, by using larger quantum dots with a short bandgap in a solar cell, electrons can be excited by the infrared. An example can be seen in the image above (right), courtesy of the University of Texas at Austin.
Though they cost thousands of dollars, televisions backlit by QD LEDs are already commercially available. The color screens have extremely clear hues since the photon-emission wavelength of a QD can be precisely controlled directly by changing its diameter. (Recall that as an electron falls from its excited state back to its ground state, it emits a photon with a wavelength that is inversely related to its energy level as determined by the bandgap.)
Furthermore, dots of different bandgaps can be excited by the same light source and then release light of different wavelengths for a broad range of precise color. For example, a 2-nm QD will luminesce short-wavelength, high-energy light, which appears as blue. Meanwhile, a 7-nm QD will luminesce long-wavelength, low-energy light that shows up as red. QDs of different sizes can be combined in a matrix to produce white light.
QDs are claimed to last longer than regular LEDs, and are extremely energy-efficient. In addition, different color-emitting QDs may be manufactured at the same time without the need for toxic materials such as cadmium.
QDs are being developed to replace fluorescent dyes in in-vivo medical-imaging applications. Photo-luminescing atoms in quantum dots can be fabricated to release more than one photon during fluorescence, making QDs detectable deep in the tissue. In addition, they produce a real-time response to transient stimuli such as action potentials or chemical gradients. Medical QDs should be coated with a biomaterial to ensure safe interfacing with cells. The image above shows breast cancer cells stained for HER-2 protein with quantum dots.
A recent breakthrough announced by the Navy Research Lab enables QD interfacing with individual neurons in the brain. The image above shows the quantum dot’s photo-luminosity when influenced by an action potential across a neuron. The current across the dot induces the electron to jump to its excited state. As it falls back down, it releases two photons, causing it to luminesce for imaging.
The graph on the right shows the QD emission in real time with the action potential; the emission intensity corresponds with the intensity of the action potential. By seeding neurons with QDs, doctors will be able to track the action potentials across the brain as they happen, enabling them to view the exact neural pathways of action potential in the brain as a person performs everyday tasks, concentrates, falls down… anything.
Quantum dots are investigated for many uses, not limited to those covered in this gallery. Despite their apparent applicability, more cost-effective methods of mass production must be developed before QDs can replace current technologies. The image above shows QD research at the Johns Hopkins Engineering in Oncology Center.
This gallery explores the growing use of quantum dots in solar panels, TV screens and displays, and medical imaging.
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