Ali Oguz Er
Nanotechnology has become one of the most intensely studied areas of research in material science over past few decades and is a rapidly growing field with potential applications ranging from electronics to cosmetics. There has been a tremendous effort to utilize some of its extraordinary opportunities within daily life. In today's modern world, many important electronic applications, such as light emitting diodes (LEDs) and personal computers, rely on semiconductors because their electrical conductivity can be greatly altered via external stimuli, such as photons, ions, electrons, and voltage.
As new technologies begin to rely more on semi-conductors, their shortcomings are more and more apparent. Conventional semi-conductors are used very frequently in electrical circuits; however, they have been found to be too big and too slow. There is a need for smaller size, low-power consumption semiconductor lasers that operate at high speeds and even high temperatures. Since conventional semiconductors have limited ranges of tolerance for the frequency of the current they carry, the traditional semi-conductors often create problems for circuits, such as overheating and eventually reducing performance. As engineers search for a faster and more compatible alternative to conventional semiconductors, they have discovered quantum dots, which could tolerate different current frequencies through a much larger range than conventional ones.
Quantum dots, also known as nanocrystals, are a special class of materials known as semiconductors; they are, quite simply, crystals composed of semiconductors. These tiny nano-scale dots can confine electrons in a three-dimensional space. After being excited by light, the electrons re-emit this light on a precise wavelength as they return to lower energy levels. The dot's small size gives researchers incredibly unusual properties to investigate potential applications. Although the study of nanomaterials has increased dramatically since the 1970s, quantum dots have a long and rich history. Early civilizations actually used them in many applications, some of which date back to the late fourth century.
Smaller is beautiful
The uniqueness of quantum dots comes from their sizes, which ranges from 2 to 40 nanometers (a billionth of a meter) in diameter, which is about the width of few hundred atoms. These artificial atoms take semi-conductors to a whole new level and can allow devices to work at almost the speed of light. As the size of the structures in material science approaches the nanometer scale, the laws of quantum mechanics come into play. These truly amazing materials start to behave much different at such a small size. Because quantum dots' electron energy levels are discrete rather than continuous, scientists could easily alter the band-gap, the energy required to lift an electron from its non-conductive states, by adding or removing just a few atoms to the quantum dot. The band-gap of a quantum dot is what determines which frequencies it will respond to, so being able to change the band-gap is what gives scientists more control and more flexibility when dealing with its applications. Quantum dots of the same material, but with different sizes, can emit light of different colors. For example, smaller dots would emit blue light while larger ones emit red light; this is a result of the quantum confinement effect. The coloration is directly related to the energy levels of the quantum dot. It is the band-gap energy that determines the energy (and hence color) of the fluorescent light, which is inversely proportional to the square of the size of the quantum dot.
Applications of quantum dots
Quantum dots (QDs) are particularly significant for optical applications due to their theoretically high quantum yield. In addition to the nano-scale size that they exhibit, they enable superior transportation and optical properties; this could be beneficial in many fields. They are currently being used in many areas, such as life science research, pathology, homeland security devices, detecting radiation and anti-espionage purposes, illness detection, and biologic tagging – not to mention many electronics and optical devices.
One of the many interesting possible applications would be for anti-counterfeiting solutions. The worldwide counterfeit goods trade is believed to be worth about $1 trillion annually. The U.N. estimates counterfeit drug sales alone were over $300 billion in 2008, while the World Customs Organization believes that other counterfeit goods sold for more than $600 billion (1). These figures represent hundreds of thousands of jobs lost.
A secure solution to the issue could come from these handy, miraculous nanoparticles. They could protect goods from counterfeiting. Due to their nanoscale size, quantum dots could easily be blended with polymers, gels, or inks, and printed onto most surfaces. The complexity of their manufacturing process also makes them nearly impossible to replicate or fake. Since quantum dots are also highly stable, extremely bright, and absorptive, they offer advancements in solid state lighting, solar collection, and electronic display technology, too. They can be engineered to respond to a specific frequency of light and can then be suspended in ink; thus, they can be printed onto money or any other good. If we shine the light with the same frequency as the ink solution onto the good, it would reveal whether or not the money is real or counterfeit.
The luminescent properties of quantum dots also make them incredibly ideal for applications to determine the results of clinical diagnostic and DNA analyses. Researchers from Indiana University have shown how to identify tens of thousands of genes, all at once, by using the tiny semiconducting crystals of cadmium selenide encased in a zinc sulfide shell; this gives off a fluorescent ultraviolet light (2). QDs are also believed to transform cell biology. They may also be used as biological tags by attaching them to cancer cells or tumors in the body. This will give scientists the ability to determine whether the malignant cells have spread to other locations or not. Compared to conventional organic dye for biological tagging, QDs are extremely stable and can be used to track several molecules at the same time, even for a longer period of time since they can be activated by a single wavelength.
The quantum properties of these dots makes them one of the most promising candidates for quantum computers, which will be much faster and provide more memory space than conventional ones. The flow of electrons could be controlled by applying a small voltage dose. Today's computer works by using bits. However, quantum computers are not limited to two numbers, as they exploit the quantum mechanics through the use of qubits. This allows them to work on a million computations at once. Quantum dots embedded in a semiconductor material could allow two photons to interact within the dot and hole combination.
There are, of course, some restrictions on quantum dots. Silicon is the basic material of microelectronics. More than 90% of all semiconductor devices are based on silicon. However, its optoelectronics applications are limited. Numerous efforts have been made to increase the efficiency of silicon. Ease of fabrication, full compatibility, and the ability to control growth makes these efforts very attractive. A single quantum dot on a silicon wafer can function as a transistor for nanoelectronics. This will reduce the number of transistors per circuit function, and open up opportunities for innovative architecture. Another possible solution would be to use germanium quantum dots on silicon wafers. These emit light throughout the visible spectrum and can be used to overcome some of the restrictions silicon has.
Manufacturing quantum dots
Manufacturing quantum dots is not easy, requires sophisticated devices and a pristine environment, and relies on complex processes. There are a variety of methods for producing quantum dots. In general, advanced manufacturing techniques are employed to grow quantum dots, depending on need. For instance, while chemical methods or ion implantation are preferable to produce quantum dots, wires, and wells are used for nanocrystals and solutions, and state-of-the-art lithographic techniques are used to grow them in nanodevices.
The future is bright
The usefulness and applications of quantum dots continues to expand and there are many companies constantly engaged in the production of new quantum dots as additional properties are demanded by consumers. Unfortunately, the production of quantum dots is still expensive, anywhere from US$3,000 to $10,000 per gram. This is the main reason restricting their use to specific applications for now (3). Although quantum dots are still a technology in their infancy, the ability to mass-produce consistently high quality quantum dots will enable engineers and scientists to envisage their use in a wide range of applications. It seems likely that quantum dots could become widespread in the next few years in banal household devices such as light bulbs, televisions, and mobile phones, and also such fields of technology as PV cells, biology, and medicine (2). Therefore, we can easily say that the future looks very bright from the quantum dot perspective.
- Y. Masumoto, Semiconductor quantum dots, Springer Verlag (2002)