Effective optical bandgap

Effective optical bandgap of YH3, Yo.4Mgo.6H2.4 alloy and Y/Mg-H2.4 multilayers (Giebels et al., 2002b)... 

This effect is shown in Eigure 2.9 for CdSe films deposited from baths containing Cd complexed with NTA (nitrilotriacetate) and Na2SeS03 as a Se source. The nanocrystal size, measured by both XRD and TEM, varied from ca. 3 nm up to 20 nm with increase in temperature and/or change in mechanism from a cluster mechanism to an ion-by-ion deposition. The optical bandgap shifts from 1.8 eV (for bulk, zincblende CdSe) to ca. 2.4 eV for the smallest nanocrystals (ca. 3 nm). 

Near-IR solid-state lasers (e.g., Ti sapphire) with chirped pulse amplification produce laser light with high brightness and very short pulses around 800 nm [ 116]. 150 fs laser pulse experiments on PI, polycarbonates (PC), PET, and PMMA have shown an increase in the single pulse ablation threshold from 1 J/cm for PI to 2.6 J/cm for PMMA. This corresponds well with the optical bandgaps of these polymers and indicates a multiphoton process. Incubation effects were observed for all polymers, but are more pronounced for PMMA, PC, and PET than for PI and PTFE, which are more stable [117-120]. Clear signs of molt redeposition of material can be observed for all polymers, except PI, which is not surprising, as it decomposes and does not melt. 

Figure 8.7 Top Effect of density on the optical bandgap for aerogels prepared from naked CdSe nanopaiticles and CdSe-ZnS core-shell nanoparticles. Bottom CdSe-ZnS core-shell aerogel and xerogel monohths under normal (left) and UV (right) illumination. The native aerogel and xerogel are orange under normal hght and fluorescent green under UV stimulation.

For Eg O (nonmetal), the optical properties can generally be successfully modeled using the particle in a sphere model and its extensions." " Consideration of an electron and hole with negligible spatial correlation in a potential well, with neglect of polarization effects, leads to the following expression for the shift of the optical bandgap (l hlAe excited state) with the radius of the sphere ... 

The increase in the optical bandgap for decreasing particle size is caused by the confinement of the electron and hole wavefunctions of the generated exdton (electron-hole pair). Due to the lower effective mass of the electron, its wavefunction is more sensitive to the confinement than for the hole. This results in an unsymmetrical bandgap widening shown in Figure 13.4. 

Huang s group has systematically studied the structure-property relationships of fluorene-thiophene-based conjugated polymers 57-60 [95, 96], In contrast to poly thiophene homopolymers, the regiochemistry of substitution in bithiophene fragments in the studied copolymers shows little effect on the optical bandgap (59 and 60, respectively Eg = 2.49 and 2.58 eV [96] or 2.57 and 2.60 eV [97, 98]) or the emission maxima, but the head-to-head copolymer 60 was significantly more thermally stable. 

Quantum confinement A characteristic effect observed in semiconductors when the size of the particle is reduced beneath the Bohr radius of the electron-hole pair (exciton) generated by optical absorption. The effect manifests as an increase in optical bandgap with a decrease in particle size... 

The 3B and BOLS premises led to a systematic understanding of the effects of chemical bond formation and the physical relaxation on the performance of substance, which enables the process design for strengthening diamond-metal adhesion, functional materials for photo and electron emission, photonic crystal with tunable optical bandgap, etc. 

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