In this study, nanoparticles (NPs) of various types and sizes are arranged to enhance both the omnidirectional light harvesting of solar cells and the light extraction of light emitting diodes (LEDs). A graded‐refractive‐index NP stack can minimize reflectance, not only over a broad range of wavelengths but also at different incident angles; the photocurrent of silicon‐based solar cells an also be significantly improved omnidirectionally. In addition, the optical gradient of an NP stack can also enhance the light‐extraction efficiency of LEDs, due to both the graded refractive index and the moderate surface roughness. Large particles having sizes on the same order of the wavelength of the incident light roughen the LED surfaces further and extract light from beyond the critical angle, as supported by three‐dimensional finite‐difference time‐domain simulations. Using this approach, the photoluminescence intensity can be increased by up to sevenfold. The developed technique: arranging sequences of different NPs in graded‐refractive‐index stacks, and considering their ability to scatter light due to their sizes and optical constants, may also significantly improve the performance of various optoelectronic devices.
Chirality control of helixes with the Δ (P) or Λ (M) form is interesting in various fields such as extended metal atom chains (EMACs), in which the metal backbones are helically wrapped by four ligands. Herein, we report two EMACs, Δ-[Ni5((-)camnpda)4] and Λ-[Ni5((+)camnpda)4], whose chiralities are controlled by chiral ligands with naphthyridine and camphorsulfonyl groups. There is a large energy difference (108 kcal mol(-1)) between the two helical structures with one chiral ligand. Furthermore, the electron communication between [Ni2](3+) units is more pronounced than in [Ni5(bna)4Cl2](2+) (bna=binaphthyridylamido). The results demonstrate control of small-scale helical structure and set the stage for future development of chiral controlled base and nanoelectronic devices.
Strain effects on optical properties of self-assembled InAs/GaAs quantum dots grown by epitaxy are investigated. Since a capping layer is added after the self-assembly process of the quantum dots, it might be reasonable to assume that the capping layer neither experiences nor affects the induced deformation of quantum dots during the self-assembly process. A new two-step model is proposed to analyse the three-dimensional induced strain fields of quantum dots. The model is based on the theory of linear elasticity and takes into account the sequence of the fabrication process of quantum dots. In the first step, the heterostructure system of quantum dots without the capping layer is considered. The mismatch of lattice constants between the wetting layer and the substrate is the driving source for the induced elastic strain. The strain field obtained in the first step is then treated as an initial strain for the whole heterostructure system, with the capping layer, in the second step. The strain from the two-step analysis is then incorporated into a steady-state effective-mass Schrödinger equation. The energy levels as well as the wavefunctions of both the electron and the hole are calculated. The numerical results show that the strain field from this new two-step model is significantly different from models where the sequence of the fabrication process is completely omitted. The calculated optical wavelength from this new model agrees well with previous experimental photoluminescence data from other studies. It seems reasonable to conclude that the proposed two-step strain analysis is crucial for future optical analysis and applications.
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