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Emissive Nanomaterials and Liquid Crystals
Published in Klaus D. Sattler, 21st Century Nanoscience – A Handbook, 2020
Marianne E. Prévôt, Julie P. Vanegas, Elda Hegmann, Torsten Hegmann, Julia Pérez-Prieto, Yann Molard
Luminescence stems from the Latin “lumen” meaning “light”. The German physicist Eilhardt Wiedemann in an attempt to design “cold” light, i.e. the energy involved does not derive from the temperature, introduced the expression in 1888 – interestingly enough at the same year liquid crystals were discovered. A luminophore refers to material characterized by the ability to emit visible light [15]. Luminophores can be either organic, inorganic, or hybrid organic/inorganic. A prefix is added to specify the nature of energy in the luminescence process; photoluminescence implies photon absorption, electroluminescence results from the application of an electric field. For example, hexanuclear molybdenum complexes involve d-d transitions, which according to the transition rules are normally prohibited. However, there is a probability that they take place. The simplified Perrin–Jablonski diagram represents the electronic states of a photoluminescent molecule and the transitions between these states. The vertical axis is in units of energy, the horizontal axis differentiates the states according to their spin multiplicity (Figure 6.11). Generally, photoluminescence phenomena obey to Stokes’ law; accordingly the energy of light emission (i.e. wavelength) is typically lower (the wavelength longer) than its excitation counterpart.
Analytical Use of Easily Accessible Optoelectronic Devices
Published in Krzysztof Iniewski, Smart Sensors for Industrial Applications, 2017
Luminescence is an emission of light resulting from electronically excited molecules via chemical reaction, light, or other stimuli including electricity, temperature, sound, and pressure. The molecule showing the luminescence is termed as a luminophore. In this chapter, we will focus on the sensing based on photoluminescence, i.e., luminescence induced by light. Furthermore, photoluminescence can be divided into fluorescence and phosphorescence depending on the electron spin states involved during the photon emission process.
Colorimetric Approaches Focused on Oxygen Quantification
Published in Khosla Ajit, Kim Dongsoo, Iniewski Krzysztof, Optical Imaging Devices, 2017
Luminescence is an emission of light resulting from electronically excited molecules via chemical reaction, light, or other stimuli, including electricity, temperature, sound, and pressure. The molecule showing the luminescence is called a luminophore. In this chapter, we focus on the sensing based on photoluminescence, i.e., luminescence induced by light. Furthermore, photoluminescence can be divided into fluorescence and phosphorescence depending on the electron spin states involved during the photon emission process.
Polymer dispersed liquid crystal device with integrated luminescent solar concentrator
Published in Liquid Crystals, 2018
Fahad Mateen, Heemuk Oh, Wansu Jung, Sae Youn Lee, Hirotsugu Kikuchi, Sung-Kyu Hong
The importance of energy-saving technologies is recently increasing in order to mitigate the effects of global warming and to overcome the energy crisis caused by the increasing prices of fossil fuels. A huge energy-saving potential lies in building sector, as the buildings are responsible for one-fifth of the world’s total energy consumption [1]. In line with the worldwide efforts, employment of ‘smart windows’ is of prime importance as they have a capability to control the throughput of thermal and solar radiations depending on the dynamic needs; consequently, helps to reduce artificial lighting-, cooling- and heating-related energy consumptions. An additional attractive feature of such windows includes a provision of indoor comfort along with improved decoration effects [2]. Generally, a smart window is regarded as a window that changes its transparency in response to the external signals. The technologies available for smart windows are commonly characterised by the materials that are used. At present, three different materials (chromic materials, polymer dispersed liquid crystals (PDLC) and suspended particles) have been reported for the fabrication of smart window [3]. Among these candidates, PDLC device owns the advantage of fast switching speed, better transparency and high durability and thus provides substantial benefits to the end users [4]. A working mechanism of a typical PDLC device is based on the reorientation of liquid crystal (LC) molecules between two conductive electrodes by applying an electric field, resulting in a transmittance modulation [3,5,6]. Moreover, provision of self-powering feature to PDLC window is an important consideration for achieving nearly zero energy consumption in buildings. This could be done by combing PDLC window with photovoltaic-active elements. Luminescent Solar Concentrator (LSC), on the other hand, is currently being considered a potential complement to integrate photovoltaic devices into built environment [7–10]. A standard LSC device comprises of transparent material (glass or polymer) embedded in or coated with a luminophore. Light is absorbed by the luminophore and is re-emitted at a longer wavelength. A portion of the emitted light is trapped by total internal reflection (TIR) and concentrated at the device edges wherein a solar cell can be attached for conversion of light into electricity [11–13]. Important aspect of LSC device is its capability to harvesting direct, diffused and ground-reflected light; therefore, the measurable amount of energy can be generated even in non-ideal illumination conditions [14,15]. Previously, fewer reports have been presented regarding the combination of LSC-based PDLC windows [16,17]. However, their focus was to explore the light harvesting through embedding a luminophore in the same PDLC device. Usually, in such configurations, absorption of luminophore decrease drastically owing to their homeotropic alignment on applying the voltage as the dipole moment of dye and the electric field vector of the incoming light are close to orthogonal to each other [16,18].