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A Review of Carbon Dots – A Versatile Carbon Nanomaterial
Published in Swamini Chopra, Kavita Pande, Vincent Shantha Kumar, Jitendra A. Sharma, Novel Applications of Carbon Based Nano-Materials, 2023
Jayanta Sarmah Boruah, Ankita Deb, Jahnabi Gogoi, Kabyashree Phukan, Neelam Gogoi, Devasish Chowdhury
Phosphorescence: Phosphorescence is a spin forbidden process and refers to a radiative transition from the triplet state (T1) to the singlet ground state (S0). The phosphorescence lifetime is, therefore, longer than the fluorescence lifetime. CDs are reported to exhibit phosphorescence, monitored by steady-state PL spectroscopy and time-resolved PL spectroscopy. Deng et al. (2013) synthesized CDs from disodium salt of EDTA (ethylene diamine tetraacetic acid) in polyvinyl alcohol (PVA) by forming a CD-PVA composite. The composite obtained a phosphorescence peak at 500 nm, having a lifetime of 380 ms by exiting the sample with 325 nm. It has been assumed that the phosphorescence originated from the triplet excited states of aromatic carbonyls on the surface of the CDs, wherein the PVA molecules can effectively protect the triplet excited state from being quenched by rigidifying these groups with hydrogen bonding and oxygen. Apart from using in anti-counterfeiting applications, the room-temperature phosphorescent material can also be used in chemical and biological sensing and time-resolved imaging because of the water-soluble and biocompatible nature of the material (Anwar et al. 2019).
MOFs as SensorsMethods and Merits
Published in Ram K. Gupta, Tahir Rasheed, Tuan Anh Nguyen, Muhammad Bilal, Metal-Organic Frameworks-Based Hybrid Materials for Environmental Sensing and Monitoring, 2022
Selva Balasubramanian, Noel Nesakumar, Arockia Jayalatha Kulandaisamy, John Bosco Balaguru Rayappan
Luminescence in MOFs can originate from the building components: i) ligand-based luminescence (π-conjugation), ii) metal-centered luminescence, iii) charge transfer luminescence, and iv) guest-host framework interactions [5,42]. In detail, the extended π-conjugation system features the spin-allowed radiative transition (fluorescence) and the spin-forbidden radiative transition (phosphorescence). The phenomenon of effective quenching by d10 transition-metal ions with unpaired electrons [43] and lanthanide metal ions with exceptional 4f-4f transitions and coordination with high absorption struts (antenna molecules) are solely attributed to metal-centered emission. The charge-transfer process is referred to as charge transfer luminescence, and it is made up of ‘metal to ligand charge transfer (MLCT), the ligand to metal charge transfer (LMCT), metal to metal charge transfer (MMCT), and ligand to ligand charge transfer (LLCT)’ [5]. It is worth noting that none of these interactions are mutually exclusive; the emission pathway of one interaction can competitively coexist with that of another. For luminescent-based MOF (LMOF) sensors, changes in their spectroscopic features can be denoted as sensing signals, and they predominantly occur via two effects: quenching and guest interactions. Moreover, the intensity of both these effects was strongly influenced by the nature of the host-guest interactions [6].
Optical Transitions in Organic and Inorganic Semiconductors
Published in Juan Bisquert, The Physics of Solar Energy Conversion, 2020
The direct transition from the ground singlet state to a triplet state requires a spin flip that is forbidden on the basis of the conservation of the spin angular momentum alone. However, the transition in which an electron is moved between orbitals of differing symmetry becomes possible if assisted by the spin–orbit coupling, since the change in spin angular momentum compensates for the change in orbital angular momentum. The most effective way to increase the efficiency of these transitions is the presence of heavy atoms incorporated in the molecule that provoke a very large, metal-induced spin–orbit coupling. The excited triplet state is usually more stable than the excited singlet manifold due to larger stabilization by the exchange interaction between the electrons. Therefore, T1 is lower in energy than S1. The transition S1 → T1 is termed intersystem crossing (ISC) and depends on spin–orbit coupling and on the vibrational overlap between the singlet and triplet states, see Figure 19.23. From the relaxed state, a luminescent transition T1 → S0 is weakly allowed. This process occurs at lower energy and longer wavelengths than fluorescence, and with much longer radiative lifetime (microseconds to miliseconds), and is called phosphorescence. Since ISC is facilitated if the states have similar energy, the transition from S1 may preferentially occur to a higher-lying triplet state Tn that subsequently relaxes vibronically to T1.
A socio-technical system framework for risk-informed performance-based building regulation
Published in Building Research & Information, 2018
Brian J. Meacham, IJsbrand J. van Straalen
In studying the building regulatory systems in several countries, it has been recognized that the rule of law that is applied within a country can have a significant impact of how risk levels are set in regulation. By rule of law we refer to the civil law tradition and the common law tradition. A fundamental aspect of civil law is that the law defines what are unlawful or unjust acts, and for each of these, the penalty. As such, anything not expressly forbidden is allowed. By contrast, common law is based on judicial precedent and is applicable in the absence of relevant statutory provisions. As such, it could be said that what is not explicitly allowed is forbidden, unless it can be justified where necessary in court. See Appendix B in the supplemental data online for additional discussion.