Quantum energy bandgap

In addition to blue emission, the wide energy bandgap of SiC of 3.0eV allows for inherently low dark currents and high quantum efficiencies for UV photodiode detectors made from 6H-SiC, even at high temperatures. These devices typically exhibit a quantum efficiency of 80-100%, and a peak response of-250-280 nm that can be maintained to at least 623 K. 

We now consider a spherical CdSe QD in vacuum with a radius of a. The energy bandgap of bulk CdSe material is 1.74 eV (room temperature) and the exciton Bohr radius is 4.9 nm, the quantum confinement energies for CB electrons is 4.95 and 2.5 eV for the VB holes, and the electron and hole effective masses are m = 0.14mo and Wv = 0.46mo, respectively, mo... 

As the nanotube diameter increases, more wave vectors become allowed for the circumferential direction, the nanotubes become more two-dimensional and the semiconducting band gap disappears, as is illustrated in Fig. 19 which shows the semiconducting band gap to be proportional to the reciprocal diameter l/dt. At a nanotube diameter of dt 3 nm (Fig. 19), the bandgap becomes comparable to thermal energies at room temperature, showing that small diameter nanotubes are needed to observe these quantum effects. Calculation of the electronic structure for two concentric nanotubes shows that pairs of concentric metal-semiconductor or semiconductor-metal nanotubes are stable [178]. 

A very crude model to calculate the increase in bandgap energy is the effective-mass particle-in-a-box approximation. Assuming parabolic bands and infinitely high barriers the lowest conduction band (CB) level of a quantum wire with a square cross-section of side length w is shifted by AEC compared to the value Ec of the bulk crystal [Lei, Ho3] ... 

Time-resolved photoluminescence was also used to show that the spatial separation of the electron and hole wavefunctions due to the piezoelectric fields in GalnN/GaN QWs leads to a dramatic reduction in oscillator strength, particularly for thick quantum wells [6]. Due to the reduced oscillator strength for the lowest energy state, the optical absorption spectrum of the quantum wells is expected to be dominated by highly excited states close to the strained bulk bandgap. 

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