Quantum Dots: Applications in Modern. Technology

Transcription

1 Quantum Dots 1 Quantum Dots: Applications in Modern Technology K. Li and R. Lan Optical Engineering Dr. K. Daneshvar July 13, 2007

2 Quantum Dots 2 Abstract: As technology moves forward, the need for semiconductors becomes more and more apparent. Conventional semiconductors, however, fall short of our needs and expectations. They are too large, too slow, and too inefficient. The discovery of quantum dots presents the possibility of faster, smaller, and more reliable semiconductors. Quantum dots, sometimes called pseudoatoms, mimic the structure of atoms. They can be tuned and adjusted for many applications, including some in optics, medicine, and quantum computation. This paper will introduce the concepts behind quantum dots, explain the methods of creation, and explore applications in modern technology.

3 Quantum Dots 3 Quantum dots are minuscule semiconductor nanostructures that limit electron movement in all three spatial directions, confining them to a tiny area around the dot. These dots can range in size from 2 nanometers to 10 nanometers in diameter, with the entire dot encompassing 100 to 100,000 individual atoms. (evidenttech) Because quantum dots are semiconductor materials, their conductivity changes in relation to external stimuli. The conductivity of a material depends primarily on the bandgap, which scientists define as the space between the conduction band and the valence band of an atom where electrons cannot propagate. The bandgap determines how much energy is required to elevate an electron from the valence band into the conduction band. In conductors, the bandgap is small or does not exist and current freely passes through the material. In semiconductors and insulators, a large bandgap usually exists and external energy is required in order to energize an electron into the conduction band beyond the bandgap, which then allows the material to conduct a current. When this happens, the electron in the conduction band and the hole in the valence band it leaves behind are bound together and collectively called an exciton. Because quantum dots are semiconductors, the concepts of energy levels and bandgap energy apply. In bulk semiconductors, however, energy levels are considered to be continuous because almost no energy difference exists between each individual level. (evidenttech) Quantum dots on the other hand have distinct energy levels, much like individual atoms. For this reason, quantum dots are sometimes called artificial atoms or pseudo-atoms. The existence of discrete energy levels around quantum dots can be explained by the exciton Bohr radius, which is the average distance between an electron and the hole it leaves behind when it enters the conduction band. Different materials have different exciton Bohr radii. As a semiconductor crystal becomes smaller than its exciton Bohr radius, its energy levels will become discrete. By definition, a quantum dot must be smaller than the exciton Bohr radius of

4 Quantum Dots 4 the material with which it is made out of. The existence of discrete energy levels around a quantum dot, called quantum confinement, has important repercussions on the absorptive and emissive behavior of the semiconductor material. (evidenttech) One special property of quantum dots is that they emit light at very specific wavelengths depending on several factors including shape, the material makeup, and most importantly the size of the dot. Altering the size of a quantum dot will change the distances between energy levels, changing the bandgap and thus the energy required for an electron to cross it. As dots become larger, the wavelength of light they emit becomes longer, causing the color to move towards the red end of the visible light spectrum. As dots become smaller, the wavelength becomes shorter and the coloration shifts towards the blue end of the spectrum. This is because as a quantum dot grows larger, its energy levels move closer together. (Nanoscienceworks) Thus, a larger dot requires less energy to create an exciton and will release less energy when the electron returns to the valance band, corresponding to a longer wavelength of light. Thus, scientists can control the wavelength and energy of the light emitted by a quantum dot by tuning its size. Studies show that quantum dots have a high quantum yield, which means that they produce many excitons for each high-energy photon that they absorb. (Weiss, 2006) Theoretically, a quantum dot could yield as many as seven excitons for each photon, raising the possibility that they could be used in highly efficient photovoltaic cells and high yield lasers. Scientists can produce quantum dots through several methods including molecular beam epitaxy, electron beam lithography, and colloidal synthesis. When fabricating quantum dots, scientists must ensure that individual dots do not come in contact with each other. If quantum dots begin to cluster, they will reform into bulk material. Thus, quantum dots must form in a host medium that keeps the dots separated. Molecular beam epitaxy is a method that deposits layers

5 Quantum Dots 5 of crystals on a wafer. In this process, pure elements are heated in a vacuum chamber until they begin to evaporate, forming beams of evaporated atoms. They are called beams because the high vacuum prevents them from interacting with other particles until they reach the wafer. The beam of vapor then condenses and combines on a wafer, slowly depositing layers of individual crystals on the surface. A computer controls the thickness of each crystal layer. When the lattice structures of the crystals and substrate do not match, unique structures may form, including structures that confine the movement of electrons, namely quantum dots. In some systems the chamber must be chilled to a temperature of 196 degrees Celsius. This is accomplished via liquid nitrogen pumps. Unfortunately, vacuum levels must be significantly higher to deposit crystals under these conditions. In other systems, wafers are loaded onto rotating platters that are heated to several hundred degrees Celsius. The quantum dots formed through MBE lend themselves to quantum cryptography and quantum computation. However, the cost of this process is high and the positioning of the dots is random and cannot be controlled. Another process that can create individual quantum dots is electron beam lithography. This process uses a beam of electrons to etch a pattern onto a semiconductor chip, similar to the process used in photolithography, and then deposits conducting material on top. Usually, the pattern is an array of holes, where quantum dots will be formed. By applying voltage, the electrons can be confined, resulting in a quantum dot. This process allows for control over the positioning of individual quantum dots. However, the machinery required for this process is quite expensive and the process itself is very time consuming. MBE and EBL are both costly and high maintenance processes. In order to mass-produce quantum dots, scientists use a process called colloidal synthesis. The reason this process for this is its low cost and low toxicity. It can also occur at standard temperature and pressure, which means that no vacuum or extreme temperatures are

6 Quantum Dots 6 required for this method of fabrication. (Nanoscienceworks) Pure elements, usually from groups two and four of the periodic table, are dispersed in solution and come together to form quantum dots. The solution ensures that the dots do not clump together and form bulk material. This process is also favored because it allows scientists to control the size of the quantum dots. Their size is proportional to how long the quantum dots remain in the solution. Controlling the size allows scientists to engineer the properties of the dots because the size of the dot affects the amount of energy required to cross the bandgap. Quantum dots have many applications in optics such as in semiconductor lasers or light emitting diodes (LEDs). Semiconductor lasers are key components in technological products requiring optical scanning such as compact disk players and laser printers. Lasers function by amplifying light inside an optical cavity before allowing the light to exit the laser in a concentrated beam. The light bounces around the optical cavity, passing through a gain medium that has optical amplifying properties. To amplify the light, a pumping mechanism supplies energy to the gain medium in order to achieve the stimulated emission required to form a beam of coherent light. Coherent light is a beam where each particle has the same wavelength and is aligned in the same way, making the light very powerful. Prior to the 1970s, lasers only used bulk materials in the gain medium, but with the discovery of nanostructures, scientists began considering lasers that take advantage of quantum properties. Beginning in the 1980s, researchers predicted that lasers using quantum dots as their principle gain medium would operate more efficiently than conventional semiconductor lasers. (Loyola, 1999) Today's quantum dot lasers are capable of operating at high speeds and efficiencies. The use of quantum dots in the gain medium lowers the threshold current required to activate the laser, reducing the amount of power consumed by operating the laser. Quantum dot lasers also have high

7 Quantum Dots 7 adjustability because scientists can manipulate the wavelength that the laser emits by changing the size of the quantum dots used in the active layer. In 2004, researchers in Japan developed a quantum dot laser that operates at 10 gigabits per second (Fujitsu 2004). This laser can operate without fluctuations in performance at temperatures from 20 degrees Celsius to 70 degrees Celsius. Previous semiconductor lasers, called strained quantum-well lasers, experienced sharp drops in output at elevated temperatures, requiring input of a coolant. Thus, the development of quantum dot lasers reduced the impact that temperature had on the performance of the laser. A drawback to the quantum dot laser, however, is that all the dots used in the gain medium must be of high quality and uniform size or else the medium will act like bulk material, causing the laser to lose efficiency. Because quantum dots are able to generate light efficiently at specific wavelengths, they may very well be the basis for next generation light bulbs. Special LEDs coated with quantum dots have been found to be more efficient and emit up to 60% more light than conventional LEDs. (Sandia Corporation quantum dots as a new approach to solid-state lighting) In order to produce white light, conventional semiconductor LEDs must contain a carefully prepared mixture of red, blue, and green emitting materials. Such mixtures are costly to make and cannot compete with conventional fluorescent lighting. Conventional fluorescent lights emit UV radiation that has wavelength of 400 nanometers that is absorbed by a coating of phosphor, which then emits visible white light. However, the phosphors cannot efficiently absorb radiation at that wavelength. Quantum dot nanophosphors in LEDs work in a way similar to that of traditional phosphor coatings, but unlike the conventional phosphor layer, scientists can engineer quantum dots to efficiently absorb any wavelength of light depending on their size. Utilizing chemical changes that change the size of the dots, it is possible for a single quantum dot

8 Quantum Dots 8 to emit multiple colors. The dots used in LEDs will be engineered to absorb the 400 nanometer wavelength light and emit light in the visible spectrum. Scientists can also increase the intensity of light emitted by increasing the concentration of quantum dots in the coating around the LED. One drawback is that when the quantum dots are made to coat the LED, they tend to clump together, losing their light emitting properties. In order to prevent this, they must be bonded to the "backbone" of the encapsulating polymer. One of the more exciting applications of quantum dots is the fabrication of super efficient solar cells. The world's oil resources are running out and scientists have been searching for efficient alternative energy sources. Solar cells are relatively easy to maintain and are environmentally friendly. However, conventional solar cells are expensive and can only use 30% of the sunlight that reaches the cells. This is due to the fact that conventional solar cells can only absorb certain wavelengths of the suns's energy, with the rest lost as heat. By using quantum dots, it is possible to raise the theoretical yield of a solar cell by almost 30%. Quantum dots can have their bandgaps altered, allowing researchers to tune their absorptive properties to optimum efficiency. It has been established that quantum dots with larger bandgaps will produce greater output voltage toward electricity generation, and that dots with smaller bandgaps will produce less voltage but more current towards electricity generation. (EvidentTech - Quantum Dot Solar Cells) The bandgap that produces the best solar energy conversion is what researchers are striving for. In addition to having tunable bandgaps, quantum dots are also more flexible in form. Quantum dots produced by colloidal synthesis are suspended in solution, making it easier to mold them into any form. Quantum dots also have the advantage of being more stable and longer-lasting than standard photovoltaic cells. They can be made with protective shells and do

9 Quantum Dots 9 not need to be replaced as often as conventional solar cells. Super efficient solar cells using quantum dots may be the solution to the world's energy crisis. Another application of quantum dots is the quantum computer. Conventional computers process information in terms of binary bits. The information is transferred via transistors, which can be "on" or "off", representing a 1 or a 0. A quantum computer would utilize quantum bits, sometimes called "qubits". Quantum theory states that it is possible for objects like atoms and electrons to exist in two states at the same time. (Chang, 2001) Quantum bits would be able to process all on and off combinations simultaneously. Quantum dots can act like the transistors of a normal computer, each dot defining a single qubit. Each dot can be a 1 or a 0, based on its electrons. Electrons have a "spin" of either up or down, which corresponds to a 1 or a 0. So far, researchers have been able to identify the spin of two qubits linked together by analyzing the flow of electricity through the dots. (Chang, 2001) One obstacle that still needs to be overcome is scaling such devices up to workable computers. Continuing research will also aim to control the spins of each dot, not just to detect them. Another important application of quantum dots is in biomedical imaging. Conventional techniques use organic dyes and MRI scans to locate tumors when diagnosing and removing diseased tissue. There are several disadvantages to these techniques. The light emitted by organic dyes is not very bright and the dyes degrade quickly. Surgeons often must stop in the middle of an operation, get another MRI scan done, and then proceed with the operation. The properties of quantum dots can solve these problems. It is possible to inject a person with specially prepared quantum dots that attach themselves to specific types of cells, for instance diseased cells. This is sometimes accomplished by having macrophages deliver the dots to the diseased area. Once the dots attach themselves to the cells, doctors can shine infrared light on the person and the dots in

10 Quantum Dots 10 the affected area will emit visible light. Due to their high quantum yield, the dots will emit a bright light. Their lifetimes are also orders of magnitude longer than conventional dyes. (Weiss, 2005) With quantum dot dyes, doctors will not have to suspend operations to get more MRI scans. Quantum dot dyes can also be used to study biological processes in healthy cells. Minor drawbacks include the irregular blinking of quantum dots and the toxicity of the elements used to make quantum dot dyes. Quantum dots have the potential to revolutionize many fields in modern science and technology. As nanostructures, they have optical properties that bulk materials cannot replicate. Although research is still being conducted in methods of fabrication and applications, quantum dots have already demonstrated substantial success and efficiency.

11 Quantum Dots 11 References Chang, A (2001).Kondo effect in an artificial quantum dot molecule. Science. 293 Electron beam lithography. (2005). Retrieved Jul. 6, 2007, from Wikipedia: the Free Encyclopedia Web site: Evident Technologies, (2005). Quantum dots explained. Retrieved Jul. 6, 2007, from Evident Technologies Web site: (2004, September 10th). Fujitsu, University of Tokyo Develop World's First 10Gbps Quantum Dot Laser Featuring Breakthrough Temperature-Independent Output. Retrieved July 10, 2007, from Fujitsu Ltd. Web site: Molecular beam epitaxy. (2005). Retrieved Jul. 6, 2007, from Wikipedia: the Free Encyclopedia Web site: Quantum Dot. Retrieved July 9, 2007, from NanoScienceWorks Web site: Quantum Dot. (2004). Retrieved Jul. 6, 2007, from Wikipedia - the free encyclopedia Web site: (1999, September). Quantum Dot Lasers. Retrieved July 10, 2007, from Chapter 5: Quantum Dot Lasers Web site: (2003, July 14th). Sandia researchers use quantum dots as a new approach to solid-state lighting. Retrieved July 11, 2007, from Sandia National Laboratories Web site: Weiss, P. (2006, June 3). Quantum Dot Leap: Tapping tiny crystals inexplicable light harvesting

12 talent. Science News. Vol. 169, No. 22. Quantum Dots 12