Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology

Abstract

Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor-polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

“…The nanoplatelets provide the highest color purity as a result of the possibility to control their thickness down to the atomic level,95 and were also confirmed to show moderate emission polarization 96…”

Consequence of shape elongation on emission asymmetry for colloidal CdSe/CdS nanoplatelets

“…The overgrowth of a shell (typically of a wider band gap such as CdS or ZnS on CdSe) on the core material has been proven as an efficient way for 1) increasing both the quantum yield and stability of the emitters, and 2) decreasing their emission blinking [21][22][23][24][25]. The luminescence dynamics have been studied, for example, as a function of the lateral dimensions of the nanoplatelets [26][27][28][29], and partial emission polarization has been found for elongated platelets [30,31]. However, the dipolar nature of these emitters has not yet been investigated.…”

Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios

“…QDQRs are applied as fluorescent reporters e.g., in bioanalysis and microscopy, 1,2 and optoelectronic components in photovoltaics, light-emitting diodes (LEDs), and liquid crystal displays (LCDs). [3][4][5][6] Such QDQRs can have different aspect ratios, i.e., different ratios of their length and thickness, and are commonly covered by a coordinatively bound shell of organic ligands controlling their stability and dispersibility. A typical example are CdSe/ZnS or CdSe/CdS QDQRs bearing non-polar surface ligands like hydrophobic amines, phosphonic acids, TOP/TOPO, or mixtures of these ligands.…”