Optics in Our Time by Mohammad D. Al-Amri Mohamed El-Gomati & M. Suhail Zubairy

Author:Mohammad D. Al-Amri, Mohamed El-Gomati & M. Suhail Zubairy
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

This model results in three effects. First the energy levels become discrete according to: E n  = (h 2 n 2)/(8 m c R 2) where n is an integer designating energy levels, h is Planck’s constant, m c is the effective mass of the electron and hole system, and R is the radius of the quantum dot (particle). In this simplified model, the bandgap of the material widens to E g = E g0 + (h 2)/(8 m c R 2). Second, this result shows that as the particle decreases in size, the bandgap energy increases, as shown in Fig. 10.14 (left). More energy is then needed to excite the dot, and concurrently, more energy is released when the crystal returns to its ground state, resulting in a color shift towards blue in the emitted light. Third, in addition to the shift towards the blue the emission becomes stronger due to the discrete nature of the levels, making the nanoparticles much brighter than bulk [25]. As a result of this phenomenon, quantum dots can emit any color of light from the same material simply by changing the dot size. Additionally, with control over the size of the nanocrystals, quantum dots can be tuned during manufacturing to emit any color of light, as shown in Fig. 10.14 (right).

Fig. 10.14(Left) The conduction and valence bands with decreasing size of a semiconductor nanoparticle (quantum dot), showing splitting of energy levels due to the quantum confinement effect. The bandgap increases with decrease in size of the nanocrystal (Images from (http://​www.​sigmaaldrich.​com/​materials-science/​nanomaterials/​quantum-dots.​html)). (Right) Vials of quantum dot colloids of increasing average size from left to right emitting light with color from blue to red respectively (Image from http://​nanocluster.​mit.​edu/​research.​php [24])

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