Related Concept Videos
01:22Carrier Generation and Recombination
Carrier generation is the process by which electron-hole pairs (EHPs) are created within the semiconductor. In direct-bandgap semiconductors, such as gallium arsenide (GaAs), this occurs efficiently when energy absorption prompts valence electrons to leap into the conduction band, leaving behind holes.
This process is given by the generation rate G and is efficient due to the conservation of momentum between the valence band maximum and conduction band minimum.
Indirect generation involves an...
This process is given by the generation rate G and is efficient due to the conservation of momentum between the valence band maximum and conduction band minimum.
Indirect generation involves an...
02:53Conservative Site-specific Recombination and Phase Variation
Because the DNA segments are cut and reorganized in a direction-specific manner, site-specific recombination has emerged as an efficient genetic engineering technique. Flippase and Cyclization recombinases or Flp and Cre, respectively, are two members of the tyrosine recombinase family derived from bacteriophages, that are used to mediate site-specific DNA insertions, deletions, and targeted expression of proteins in mammalian cell lines.
The recognition sites for Cre recombinase called LoxP...
The recognition sites for Cre recombinase called LoxP...
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
01:30Crossing Over
Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I, duplicated...
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11:30
Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity
Published on: March 6, 2017
Recombination dynamics in CdTe/CdSe type-II quantum dots.
Chun Hsiung Wang1, Tzung Te Chen, Yang Fang Chen
1Department of Physics, National Taiwan University, Taipei 106, Taiwan.
Nanotechnology
|July 7, 2011
Summary
Recombination dynamics in type-II quantum dots (QDs) show long decay times due to spatially separated charges. Carrier recombination is influenced by temperature, QD size, and excitation power.
Area of Science:
- Materials Science
- Nanotechnology
- Quantum Physics
Background:
- Type-II core-shell quantum dots (QDs) exhibit unique electronic properties due to spatially separated charge carriers.
- Understanding recombination dynamics is crucial for optoelectronic applications.
Purpose of the Study:
- Investigate recombination dynamics in CdTe/CdSe core-shell type-II QDs.
- Determine the influence of temperature, QD size, and excitation power on carrier recombination.
Main Methods:
- Time-resolved photoluminescence (PL) spectroscopy.
- Analysis of PL decay time, radiative lifetime, and non-radiative lifetime.
- Temperature-dependent PL measurements.
Main Results:
- Observed very long PL decay times (hundreds of nanoseconds) at low temperatures, attributed to type-II heterostructure.
- Carrier recombination dominated by delocalized excitons below 150 K, transitioning to a mix of excitons and free carriers at higher temperatures.
- Established a third-power relationship between radiative lifetime and QD radius due to quantum confinement.
- Found radiative decay rate proportional to the square root of excitation power, linked to band bending effects.
Conclusions:
- Recombination dynamics in type-II QDs are complex and strongly dependent on external factors.
- Quantum confinement and band bending significantly influence carrier recombination pathways.
- Findings provide insights for designing efficient optoelectronic devices based on type-II QDs.