Quantum Dots
Abstract
Quantum Dots is a new revolutionary technology from the field of nano technology. Quantum Dots are basically nano crystals made from semiconducting nano materials. Nano technology deals with matter at the atomic level or molecular level and the properties of matter changes drastically at this molecular level. Like that, quantum dots also exhibits unusual properties. These properties and the structure of quantum dots will be discussed along with its history and how it’s been synthesized over these years. Besides these, the applications of quantum dots in multi-disciplinary fields which includes electroluminescence (photonics), biological imaging, solar cells etc has also been reviewed.
1. Introduction
Quantum Dots as the name infers works on the basis on quantum theory where each quantum dot is like an atom which has a well defined energy level and can be individually controlled. They are manufactured in laboratories, hence also called artificial atoms. Their size ranges from about 2 to 10 nm scale. They are made from semiconductor materials such as silicon, cadmium selenide, cadmium sulfide, or indium arsenide etc.
Quantum dots with size ranging from about 6 nm and above emits light of higher wavelength (Red) and the ones with the size of about 2 nm emits light of smaller wavelength (blue). This is because as the size of the nano structure decreases the band gap increases and a larger band gap requires more energy for excitation from the valence band to the conduction band. Also, since energy is proportional to frequency, light of higher frequency and lower wavelength would be absorbed. This finely tunable speciality of quantum dots make it easier to meet specific applications in multi — disciplinary fields.
2. History
The first discovery of Quantum Dots was made by a Russian physicist Alexei Ekimov while at the Vavilov State Optical Institute in 1980. He produced the first Quantum Dots in a molten glass matrix. Later on, in 1982 an American chemist named Louis E. Brus observed the same phenomenon in colloidal solutions while at the AT&T Bell Laboratories. He was the first one to produce colloidal Quantum Dots. He observed the change in light emitted and absorbed by the Quantum Dots over a period of days as the crystal grew and inferred that the confinement of electrons is what providing the particle quantum properties. There was one more scientist called Alexander Efros who was the first to theorize the Quantum Dots. In 2006, the scientists Alexei Ekimov, Alexander Efros and Louis E. Brus shared the Optical Society of America’s R.W. Wood Prize for their groundbreaking work.
In 1993, Moungi Bawendi, one of the post-doctoral researcher of Louis Brus produced the first “high quality” Quantum Dots that have less than 5% size variation in the colloidal suspension. This enabled researchers to control the size of Quantum Dots and fine-tune the color of their fluorescence.
By the time, Philippe Guyot-Sionnest, a young professor at the University of Chicago, with his team synthesized the first Quantum Dots with the core surrounded by a shell in 1996.
3. Structure of Quantum Dots
Quantum Dots are generally spherical in shape and mainly consists of a core surrounded by a layer of outer shell. A nano particle (Quantum Dot) composing of approximately 100–10000 atoms, exhibits distinct narrow optical line spectra. A remarkable amount of research is focused at the optical properties of quantum dots such as LED’s, Display Screen, Solar cells and even in biological fields. The fabrication of Quantum Dots requires extremely controlled conditions and environment as its intrinsic properties depends on several factors such as size, shape, defect, impurities and crystallinity.
3.1 Core Structure
3.1.1 Phase Transition
Another structural quality of Quantum Dots is their solid-solid phase transition capabilities like bulk semi conductors. Phase transition in bulk materials can be performed by varying the composition, temperature and pressure. For example, by using a pressure of more than 3 Gpa the CdSe bulk semiconductor can be converted from low pressure wurzite to high pressure rock salt structures. Similarly, using high pressure x-ray diffraction (XRD) and optical absorption the wurzite to rock salt structural phase transition can be made in CdSe Quantum Dots.
3.1.2 Quantum Dots Doping
Doping is another important aspect that needs to be considered for Quantum Dots for several applications that includes opto electronics, magnetic, biological applications etc. Doping is the adding of impurities which acts as the activators that creates localized quantum states within the energy gaps. These dopants will be auto ionized due to quantum confinement property. Quantum Dots are doped with elements such as Phosphorous (P), Boron (B), Sodium (Na) etc according to different applications. The optical properties can be varied by varying the amount of dopants.
3.2 Surface Structure
The surface to volume ratio of a Quantum Dot is very large and as a result the surface of the dots also play a significant role in defining its optical properties. For example, 15 percent of atoms in a 5 nm CdS Quantum Dot is present in its surface. So the surface is vital in determining the efficiency, luminescence and other electronic properties of the dot.
3.2.1 Surface Passivation
Surface Passivation or capping is done to increase the stability of Quantum Dots. An ideally passivated Quantum Dot will have perfect internal quantum confinement i.e. there won’t be any atoms present at the surface of the dots. Surface passivation is usually carried out by depositing an organic or inorganic capping layer on the surface of Quantum Dots.
Organic capping of Quantum Dots is done by introducing organic compounds that adsorb on the surface of the dots. There is always a certain amount of atoms present in the surface of the dots when passivated by organic agents. This leads to instability of the dots. But recent researches have been able to develop DNA passivated stable Quantum Dots. For the inorganic passivation of Quantum Dots, inorganic layers that are usually a material with a larger band gap are used. The passivating shell can be grown either epitaxially or as non-epitaxial crystalline.
Epitaxial growth is done using layer by layer deposition of monocrystalline films. Here, a wider band gap shell is created around the Quantum dots core as a potential barrier to confine the exciton. This confinement of charge carriers into the core region results in increased efficiency and photostable luminescence. Except some, most of the Quantum Dots contain toxic ions due to the presence of tellurium, cadmium, selenium etc. This existence of toxicity is referred to as non-epitaxial growth. To reduce the toxicity, an oxide coating is given to the dots. Also, for the proper functionality of the dots a silica shell is grown on the outer layer of the dots. This silica shell prevents the leakage of toxic ions like cadmium ions etc.
4. Properties
4.1 Quantum Confinement
The most appealing property of Quantum Dots is quantum confinement. This property is only visible in particles of size in the range of 10 nm or below. Quantum Confinement is the spatial confinement of electron-hole pairs or excitons. The reduction in the number of atoms in a material results in the confinement of normally delocalized energy states. Electron-hole pairs become spatially confined when the diameter of a particle approaches the de Broglie wavelength of electrons in the conduction band. As a result the energy difference between energy bands increases with decrease in particle size. Due to these factors, semiconductors usually exhibit this phenomenon as they have a energy gap unlike metals in their electronic band structure. This distance between the electron and hole is called the exciton Bohr radius (rB). The exciton Bohr radius is a threshold value and the confinement effect becomes more predominant as the radius of Quantum Dots decreases. One of the major methods used for the study of these excitons are Effective Mass Approximation (EMA) model and Linear Combination of Atomic Orbital (LCAO) theory.
The EMA method was proposed by Alexander Efros and Alexei L. Efros in 1982 and was later modified by Brus. The method describes about the electronic structure of donors in bulk semiconductors and the range where quantum confinement starts to interfere with the electronic structure. The Bohr radius is being calculated in this method which helps to determine the size of the Quantum Dot. While the LCAO method provides a more detailed prediction of the confinement and also helps in defining the dependence of band gap on size of crystals. In LCAO method superposition of molecular orbitals takes place by combining two atomic orbitals of a single diatomic molecule. This combining of orbitals produces bonding and anti-bonding molecular orbitals. As the number of atoms increases, the discrete energy band becomes more continuous. The occupied bonding molecular orbitals (valence band) are called the Highest Occupied Molecular Orbital (HOMO) levels. The unoccupied anti-bonding orbitals (conduction band) are called the Lowest Unoccupied Molecular Orbital (LUMO) levels. The energy difference between the HOMO and the LUMO (band-gap) increases and the bands will split into discrete energy levels and thus reduces the mixing of atomic orbitals for small number of atoms. Thus, the small size of the Quantum Dots results in quantized electronic band structures.
4.2 Luminescence
When a Quantum Dot is excited using external energy there will be a transition from the ground state to the excited state and at this excited state the electrons possess a great amount of energy and may combine with holes to form an exciton i.e. electron-hole recombination occurs and the exciton will get relaxed to the ground state. The excess energy that forms due to recombination and relaxation can be either radiative or non-radiative.
Radiative relaxation results in spontaneous luminescence from the Quantum Dots. This may result in band-edge transition, defect or activator quantum states. The band-edge emission is the recombination of excited electron in the conduction band with a hole in the valence band. In defect quantum state, the radiative emissions come from localized impurities. Depending on the type of impurity i.e. excess electrons or less electrons, it can act as a donor or acceptor respectively. Activator quantum state occur when impurities are incorporated purposefully.
Unlike radiative recombination, non-radiative recombination produces phonons i.e. vibrations or sound waves are emitted as a result. Non-radiative relaxation can be categorized as internal conversion, external conversion and Auger recombination. In internal conversion an excited nucleus interacts with one of the orbital electrons in an atom and causes it to get emitted from the atom’s orbit. In external conversion, the excited molecules goes to the ground state by losing energy by colliding with other molecules. This collision results in heat generation. The Auger recombination occurs due to strong carrier to carrier interaction of molecules. Here, the excess energy is not released as photon or phonon but is transferred to another electron.
5. Synthesis
There are a number of methods for the synthesis of Quantum Dots. The most used methods for the synthesis of Quantum Dots are the top-down and the bottom-up approach.
5.1 Top — Down Approach
In this approach, bulk semiconductors are thinned to produce Quantum Dots. Electron beam lithography, reactive-ion etching and wet chemical etching are commonly used methods to achieve Quantum Dots of diameter approximately of about 30 nm. But there is a large possibility for the incorporation of impurities and occurrence of structural imperfections while employing these methods and is a major drawback.
In Electron-beam lithography (EBL), a focused beam of electrons is used to generate the desired shape for the Quantum Dots. The electron beam changes the solubility of the semiconductor and is selectively removed by immersing the exposed and unexposed areas of the semiconductor in a solvent.
Etching plays one of the most vital role in the manufacturing of Quantum Dots. In dry etching, a reactive gas is inserted into an etching chamber and a radio frequency voltage is applied to create a plasma which breaks down the gas molecules to more reactive fragments. These high kinetic energy ions strike the surface and form a volatile reaction product to etch a patterned sample. This etching process is called Reactive Ion Etching (RIE).
Wet chemical etching is done by using certain liquid chemicals to remove the unwanted material from the substrate. Most of the wet etching process are isotropic in nature. This method is simple and have a high selective rate but the chemicals used are highly expensive.
5.2 Bottom — Up Approach
It is a type of self assembly technique and is broadly divided into two. They are Wet Chemical and Vapour Phase methods.
5.2.1 Wet Chemical Method
Wet chemical method is a type of precipitation process with careful control of parameters for a single or a mixture of solutions. The precipitation process involves both nucleation and limited growth of nanoparticles. Nucleation is the process of formation of crystal structure from a solution by combining the atoms in the solution. By controlling certain factors like temperature, electrostatic double layer thickness, stabilizers or micelle formation, ratios of anions to cations etc, Quantum Dots of desired shape and size can be formed. Some major examples of the wet chemical methods are Sol-Gel process, Microemulsion Process and Hot-Solution Decomposition Process.
5.2.2 Vapour Phase Method
In vapour-phase methods layers of quantum dots are grown in an atom by atom process without any patterning instead by hetero-epitaxial growth of highly strained materials. In general, the layered materials grow as a uniform epitaxial layer, initially as a smooth layer followed by nucleation and growth of small islands on the substrate. Depending on the surface energies and lattice mismatch, one of these growth modes is observed. For example, In the case of substrates with a large lattice mismatch and appropriately small surface and interface energies, initial growth of the overlayer occurs through a layer by layer growth. However, when the film is thick consisting of few monolayers, to induce a large strain of energy the system lowers its total free energy by breaking the film into isolated islands or Quantum Dots. Production of Quantum Dots using vapour phase method is effective but there is a possibility for fluctuation of its size which often results in differing optoelectronic properties. Important examples of vapour phase method include Molecular beam epitaxy (MBE), Physical vapour deposition (PVD) and Chemical vapour deposition (CVD).
6. Applications
Due to immense flexibility there are wide variety of applications for Quantum Dots and some of the major research areas includes quantum dot displays, medical applications, Solar cells etc.
6.1 Quantum Dot Display
Quantum Dot display has been an intense field of research for the past decades due to its immense optical properties. A normal LCD is backlit by fluorescent lamps or conventional white LEDs that are color filtered to produce red, green, and blue pixels. Quantum dot displays use blue-emitting LEDs rather than white LEDs as the light sources. The emitted light is converted into pure green and red light by the corresponding color quantum dots placed in front of the blue LED. This white light as the backlight of an LCD panel allows for more accurate color production and high quality images at a cost lower than a RGB LED display. Quantum Dots has several advantages over OLED’s (Organic LED) also. Due to the high thermal stability of inorganic materials compared to organic materials, even at high brightness the displays will have a longer lifetime.
6.2 Quantum Dots for Solar Cell Fabrication
The maximum efficiency that can be obtained using a normal solar cell is nearly about 35%. One of leading factors for this less efficiency is that organic semi conductors that are used for the production of solar cells have high absorption coefficients than inorganic semi conductors. Therefore, a combination of organic and inorganic materials can improve the conversion efficiency of the solar cell. Compounds used for making hybrid solar cells include CdSe, TiO2, ZnO, PbS, PbSe etc. Other factors that makes the Quantum Dots more advantageous is that they are more stable and resistant to oxygen, moisture and UV radiation. Quantum Confinement enables size-tuned tunable band-gaps so that a multi-junction device is possible using the same Quantum Dot composition to absorb entire sunlight from UV to visible to IR spectrum. The cost of fabrication of hybrid solar cells is also relatively less.
6.3 Medical Applications
The major imaging technology advancements in the medical field includes magnetic resonance imaging (MRI), optical imaging, nuclear imaging etc. However, they differ in complexity, resolution, sensitivity and operational cost. But most of this imaging equipment’s use colored dyes for their optical bioimaging process. Using Quantum Dots instead of dyes have an enormous effect on the image results. Due to high stability of dots, photobleaching can be reduced. Absorbance and emissions can be tuned with size which enables the quantum dots for highly sensitive cellular imaging in high resolutions. The interference between the dots are less. The toxicity of inorganic dyes are more compared to Quantum Dots. The dots are more photostable under ultraviolet excitation than organic molecules and their fluorescence is more saturated, therefore, imaging of deeper tissues is possible. Another potential application is that quantum dots are being used as the inorganic fluorophore for intra-operative detection of tumors using fluorescence spectroscopy.
References
1. ResearchGate
2. NanoHub
3. Wikipedia
4. AZoNano
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