Quantum mechanics is term relates to the natural physics at small scales, and “quantum” means “quantization” which refers each physical amount to the smallest quantity of that physical property exists in nature (including energy, charge, mass, …etc), for instance; the quantization of charge states that the charge of any object is an integer number of the charge of electron (the smallest unit of charge). These physics governs the nature at scale < 100nm, where the materials shows different properties due to the quantization of charge. Significant improvements in various applications have emerged, as well as many novel applications based on miniaturization especially in electronic and optoelectronic devices by quantum physics, advanced fabrication and characterization methods, and the use of phenomena at nano-scale. Optical and electrical properties change as the size of nano particles changes due to surface-to-volume ratio. When the size of these particles becomes comparable with de Broglie wavelength of electron and with the magnificence of quantum physics, what is known as "electron confinement" is formed. Electron confinement, potential well or quantum dot is a small region in semiconductor where the electron confined in spatial dimensions, this confinement could be in one dimension, two dimension or three dimension, which refers to as quantum well, quantum wire and quantum dot, respectively. This region exhibit discrete sublevels of energy in the density of states (spacing of 10-100 meV) because it has a small number of atoms compared with macro-scale materials, electron exhibit atomic-like wave function in each energy level.
Also has a discrete tunable emission and absorption levels, which made a revolution in the field of LEDs. Quantum dot’s properties determined by its size, shape, impurities, defects and crystallization, different colors emits from different sizes of QD as shown in figure.1. Unlike materials which have bulk properties, QD’s properties which are intermediate between bulk and atomic properties, depend on the crystal structure in elemental or compound structure.
The number of electrons in QD can be tuned byPage 2charging energy which is similar to the ionization energy of atom (the sufficient energy to add or remove an electron). Table.1 shows a comparison between QD and atoms.Structure and properties of QD-LED:As mentioned before quantum dot with different sizes could enable multi-color imaging with need to one excitation wavelength, which make it good choice to use in LED. Also quantum dot based LEDs have a very large efficiency compared with other light sources, like, incandescent with about 5% of efficiency and fluorescent with about 25%.
The most amounts of losses in these kind of light sources is due to release heat, while solid state lighting, especially, quantum dot based LEDs reduce small amount of heat and need small energy to produce the same light amount.Generally, nano particles with diameter less than 30nm show difference in optical absorption, excitation energy, electron-hole pair recombination. Also have a transition like bulk semiconductors; these transitions have large impact on the optical properties. For instance high surface-to-volume ratio allows an increasing or decreasing the transfer ratio of photo-generated charge carriers due to high density of surface states. Surface states lies in band gap, therefore they can trap charge carriers (electron-hole), and then behave as reducing agents (electron), or oxidizing agent (hole), which will affect the overall conductivity and optical properties.There are two other properties affect by size; band-gap energy which increase when the diameter of QD becomes less than specific value related to the type of semiconductor called “confinement effect” which helps tuning energy band gap by size. In addition to observation of discrete which is define as well separated energy states due to small number of atoms in QD comparing with bulk.Quantum dot based LEDs have semiconductor based structure, often consist of anions (elements from sixth group in periodic table), like oxygen, sulfur, selenium and tellurium, with cations (elements from second group in periodic table), like zinc, cadmium and mercury.
These structures of QDs with (II-VI) compounds, like, CdSe, CdTe, ZnS, GaAs, InAs, InP, or any complex combination of these elements have either face-centered cubic structure, or hexagonal (wurtzite) crystal structure. These elements considered as direct semiconductors (have direct band-gap), so they have an excellent luminescence and often use as host for luminescent activator (dopants).Optical properties change with changing of the amount and the position of doping; keeping the size constant and engineer the band gap by doping and alloying will change the optoelectronic properties.
Dopants which is called activators creates local quantum states within band-gap, also progress an auto ionization without thermal activation due to quantum confinement. When the quantum confinement energy (band gap energy which is increasing by decreasing the size) exceeds columbic interaction (between carriers (electron & hole) with impurities) auto-ionization occurred. On the other hand alloying semiconductors provide different and nonlinear optoelectronic properties, mixed or intermediate optoelectronic properties when QD alloyed with multiple semiconductors, improve photoluminescence (PL) emission efficiency by minimizing bulk and surface defects, and get narrow full-width-half-maximum (FWHM) of PL. Table.2 shows the changes in band-gap of alloyed semiconductors with different alloying ratios.The basic idea of QLED is to make its band gap tunable, therefore make the color of its light emission tunable. The band gap tuning fundamentally depends on the density of states, which can be tuned by doping or alloying of the semiconductor material. Because QDs characterized by its high surface-to-volume ratio, what is important in its design is the density of states related to the surfacePage 4(surface density of states).
These states can impact the optical absorption, luminescent intensity, quantum efficiency, and spectrum and aging effects.Surface defects in QD work as a wall of the electron confinement dot, therefore it had to be covered “passivated” to improve optical stability of QD. The surface passivation or capping by organic or inorganic materials is important because it confines the carriers inside the core. Also passivation improves the optical properties and acts as a barrier or insulator for the conduction of charge.The most perfect surface passivation the most dangling bands saturation, the less surface states and the most internally confinement near band-edges states.
If the semiconductor compound with anion dangling bands at the surface passivated, the surface states’ bands located just above the valance band. Otherwise, if it is passivated with cation leave the dangling bands at the surface, the surface states become just below the conduction band edge.Table.2 changing in band-gap of alloyed semiconductors with different alloying ratios.
Page 5Synthesis: In general, the preparation of nano particles, including the quantum dots, follow one of two methods, either top-down where it starts with a bulk semiconductor and treated to reach the nano size, or bottom-up which starts from nothing and growing until reach the nano particles. The fabrication methods that classified under top-down approach are basically working through etching of the semiconductors to get quantum dot. The most common methods used to fabricate QDs with about 30nm in diameter are reactive-ion etching (RIE), focused ion beam (FIB), electron beam lithography and wet chemical etching.
The method used in fabrication depends on the type of quantum dot, where the electron beam lithography give the flexibility in design and used to fabricate any shape of quantum confinements; quantum dots with 3D confinement system, quantum wire or rings with 2D confinement system or quantum well which considered as 1D confinement system. Focused ion beam give very high precision where the shape, size and particle distance inside QD depend on the ion beam size. Bottom-up approach’ methods of fabrication listed into two categories; wet chemical and vapor-phase. Wet chemical includes sol-gel, competitive reaction chemistry, micromulsion, hot solution decomposition and electrochemistry methods. While vapor-phase methods include self-assembly, aggregation of gaseous monomers, liquid metal ion sources and sputtering. There are other synthesis techniques used in QD fabrication; like using acoustic waves (ultrasound waves, microwaves or sonic waves) through water to grow QD. Moreover inorganic solution used to crystallize QD by control pressure and temperature which known as hydrothermal synthesis or similar synthesis process.
Other applications of quantum dot: The quantum confinement or quantum dot is used dramatically nowadays; many researchers take advantage of its distinctive features by using them in a lot of domains and applications. Biologically, it is commonly used in medical imaging due to its continuous stable image and long fluorescent life-time, also the excitation wavelength of QD is far from emission which make it biocompatible. In addition, it is used in labeling and observation of detailed biological processes and monitoring cancerous cells. Nanodevices also had a good share of QD based researches; it is widely used in transistors and the most revolutionary application is single electron transistor (SET). Moreover researchers nowadays use quantum dot to enhance computer memories, telecommunication waveguides, carbon nanotubes quantum wires (CNTQ-wires), sensors and quantum computing.
Page 6The special optical properties of quantum dot make it preferable in optical applications; In addition to QLED, QD used in laser diodes, optical amplifiers, photo detectors and to manufacture inexpensive, high quality and industrial white-light. Furthermore, QD based solar cell of third generation was fabricated with about 60% of efficiency due to the wide broadband absorption of spectrum.Conclusion:To wrap up, quantum confinement is a quantum mechanical phenomenon, and like other phenomena discovered by quantum mechanics, it made a revolution in many fields; by improve or enhance the exits application or even by introduce new applications. Its special optical properties made it applicable in optical application, especially in LEDs, with low power consumption, high efficiency and wide range of optical absorption QLED start to be used in lighting systems, television, imaging and even printing. But it is still challenging technology due to the complexity and difficulty of its fabrication. However, the latest researches of QD fabrication techniques show an optimistic future of QD.
References:1. A literature review on quantum dots, Bal Krishan, Meenu Rani Garg2. Wikipedia, https://en.
wikipedia.org/wiki/Quantum_dot3. Quantum dots and their multimodal applications: a review, Debasis Bera*,Lie Qian, Teng-Kuan Tseng and Paul H.
Holloway*4. Quantum Dots and their potential impact on lighting and display applications, by Paul W.Brazis, Jr., PhD5. Review-Quantum dots and their Application in Lighting, Displays, and Biology, Talitha Frecker, Danielle Bailey, Xochitl Arzeta-Ferrer, James McBride, and Sandra J.Rosenthal6. Quantum dots: a bright future for photonic nanosensors, Sensor Review, Vol.30 Iss: 4pp.
279-2847. A Literature Review on Quantum Dots, Bal Krishan and Meenu Rani Garg8. Vanessa Wood & Vladimir Bulovic (2010) Colloidal quantum dot light-emmiting devices, Nano Review, 1:1, 5202, DOI: 10.3402/nano.v1i0.52029.
Quantum Dots-Converted Light-Emitting Diodes Packaging for Lighting and Display: Status and perspectives, Bin Xie, Run Hu, and Xiaobing Luo10. Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization, Xingliang Dai, Yunzhou Deng, Xiaogang Peng, and Yizheng Jin*11. Quantum Dots for Light Emitting Diodes, Khan Qasim*, Wei Lei, and Qing Li, Display Research Centre, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, P.