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Valence Bond Theory02:42

Valence Bond Theory

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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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Enhancement-mode MOSFETs are pivotal components in electronics, distinguished by their capacity to act as highly efficient switches. They are part of the larger family of metal-oxide Semiconductor Field-Effect Transistors (MOSFETs). They are available in two types: p-channel and n-channel, each tailored to specific polarity operations.
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MOSFET: Depletion Mode01:20

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Depletion-mode MOSFETs represent a unique subset of MOSFET technology, functioning fundamentally differently from their enhancement-mode counterparts. Unlike enhancement MOSFETs, which require a positive gate-source voltage (Vgs) to turn on, depletion-mode MOSFETs are inherently conductive and "normally on" devices.
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Carrier Generation and Recombination01:22

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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.
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Standing Waves in a Cavity01:28

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A household microwave and lasers are examples of standing electromagnetic waves in a cavity. When two conducting metal plates are placed parallel at the nodal planes, it creates a cavity where standing waves are formed. The cavity between the two planes is analogous to a stretched string held at the points x = 0 and x = L. Here, the distance 'L' between the two planes must be an integer multiple of half of the wavelength. The wavelengths that satisfy this condition are given by:
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Metal-Semiconductor Junctions01:24

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The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
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Schottky barriers arise when a metal with a work function (Φm) contacts a semiconductor with a different work function (Φs). Initially, electrons transfer until the Fermi levels of the metal and semiconductor align at equilibrium. For instance, if Φm > Φs, the semiconductor Fermi level is higher than the metal's before contact. The...
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Updated: Jan 10, 2026

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope
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Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

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Defect-engineered competition between exciton annihilation and trapping in MOCVD WS2.

Ruofei Zheng1, Leon Daniel2, Dedi Sutarma2

  • 1Department of Chemistry, University of Waterloo Waterloo Ontario N2L 3G1 Canada gsciaini@uwaterloo.ca.

Chemical Science
|November 24, 2025
PubMed
Summary

Sulfur vacancies in WS₂ monolayers cause defect trapping, but exciton-exciton annihilation dominates at high excitation. Understanding this competition is key for optoelectronic device performance.

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Nanotechnology

Background:

  • Exciton dynamics are crucial for the optoelectronic performance of 2D transition metal dichalcogenides (TMDCs).
  • Sulfur vacancies in MOCVD-grown WS₂ monolayers create in-gap states, leading to nonradiative recombination via defect trapping (DT).
  • At higher excitation levels, exciton-exciton annihilation (EEA) becomes a competing nonradiative pathway.

Purpose of the Study:

  • To quantitatively disentangle the competing exciton decay mechanisms (DT and EEA) in MOCVD-grown WS₂ monolayers.
  • To investigate the influence of defect concentration on exciton dynamics across various excitation regimes.
  • To establish a framework for defect engineering to tailor TMDC optoelectronic properties.

Main Methods:

  • Femtosecond broadband transient absorption spectroscopy.
  • Steady-state quantum efficiency measurements.
  • Rate-equation modeling incorporating DT and EEA.

Main Results:

  • Demonstrated partial occupation of defect states and their influence on photo-induced band renormalization.
  • Quantitatively extracted constants for DT (0.02 cm²/s) and EEA (0.1 cm²/s), revealing diffusion-limited behavior.
  • Identified a critical defect-to-exciton density ratio (≈3.5) for EEA activation and observed defect saturation suppressing DT at high exciton densities.

Conclusions:

  • Provided unprecedented quantitative insights into defect-modulated exciton decay in WS₂ monolayers.
  • Established a critical density ratio for the onset of exciton-exciton annihilation.
  • Highlighted the potential for controlled defect engineering to optimize TMDC optoelectronics.