Characteristics of Quantum Dots

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Characteristics of Quantum Dots
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Introduction

• Quantum dots, also known as semiconductor
  nanocrystals, are nanoscale materials composed of
  a small number of atoms. The number of atoms in a
  quantum dot is usually between a few and a few
  hundred, and their size in all three dimensions
  are less than 100 nm. The movement of carriers in
  the three dimensions of quantum dots is limited
  by the size effect. Due to the quantum
  confinement effect, the energy levels of carriers
  in quantum dots are similar to those of atoms
  having a discontinuous energy level structure, so
  quantum dots are also called artificial atoms.
  Due to their special energy level structure,
  quantum dots exhibit unique physical properties.
  This paper mainly discusses some properties of
  quantum dots, including quantum confinement
  effect, quantum size effect, surface effect and
  luminescence property.

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1 Quantum confinement effect

• Generally, the smaller the volume, the greater
  the bandwidth, so the optical and electrical
  properties of the quantum dots are highly
  dependent on the size of the material. Generally,
  when the size of the quantum dot is equal to or
  smaller than the exciton Bohr radius of the
  corresponding bulk material, the movement of the
  carrier electron-hole pair is in a strongly
  restricted state. When the energy gap increases
  as the particle size becomes smaller, the
  semiconductor material is quantified. The energy
  after quantification of the semiconductor
  material is E(R)Egh²p²/2uR²-1.8/eR. In the
  formula, Eg is the bulk band gap, u is the mass
  of electrons and holes, e is the dielectric
  constant of the quantum dot material, R is the
  radius of the particle, and E(R) is the lowest
  excitation energy. The value obtained by
  subtracting Eg from E(R) is the amount of
  increase in kinetic energy.

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2 Quantum size effect

• It can be seen from the above formula that the
  quantum confinement energy and the coulomb
  interaction energy are proportional to 1/R2 and
  1/R, respectively, the former can increase the
  band gap energy (blue shift), and the latter can
  reduce the band gap energy (red shift). When R is
  small, the quantum confinement can be more
  sensitive to R. As R decreases, the quantum
  confinement energy increases more than the
  Coulomb interaction energy, resulting in a blue
  shift of the spectrum.

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3 Surface effect

• Surface effect means that the specific surface
  area of quantum dots increases with the decrease
  of particle size, resulting in insufficient
  coordination of surface atoms and increased
  number of unsaturated bonds and dangling bonds,
  thus the atoms on the surface of quantum dots are
  extremely unstable and easily bind to other
  atoms. This surface effect gives the quantum dots
  a large surface energy and high activity, which
  not only causes changes in the atomic structure
  of the quantum surface, but also causes changes
  in the surface electron energy spectrum. Surface
  defects lead to trapped electrons or electron
  holes, which in turn affect the luminescent
  properties of quantum dots, causing nonlinear
  optical effects.

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4 Luminescence property

• The principle of luminescence of quantum dots is
  similar to that of conventional semiconductor
  luminescence, that is, carriers in a material
  reach an excited state after receiving external
  energy, and release energy when carriers return
  to the ground state, and this energy is usually
  released in the form of light. Unlike
  conventional luminescent materials, the
  luminescent materials of quantum dots have the
  following characteristics.

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4.1 Adjustable emission spectrum

• Semiconductor quantum dots are mainly composed of
  elements in IIB-VIA, IIIA-VA or IVA-VIA group.
  The luminescence spectra of quantum dots of
  different sizes or materials are in different
  bands. For example, the luminescence spectra of
  ZnS quantum dots covers the ultraviolet region,
  and the luminescence spectra of CdSe quantum dots
  covers the visible region, while the luminescence
  spectra of PbSe quantum dots covers the infrared
  region. Even for the same quantum dot material,
  the luminescence spectrum is different if the
  size is different.

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4.2 Wide excitation spectrum and narrow emission
spectrum

• The range of the spectrum that triggers the
  quantum dot to reach the excited state is wide,
  and the quantum dot can be excited as long as the
  excitation light energy is higher than the
  threshold value. Regardless of the wavelength of
  the excitation light, as long as the material and
  size of the quantum dots are not changed, the
  emission spectrum of the quantum dots is fixed,
  and the emission spectrum range is narrow and
  symmetrical.

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4.3 Large stokes movement

• The peak of the emission spectrum of a quantum
  dot material is usually red-shifted relative to
  the peak of the absorption spectrum. The
  difference between the peak of the emission and
  absorption spectrum is called the Stokes shift.
  The Stokes shift of quantum dots is larger than
  conventional materials. Stokes shift is widely
  used in the detection of fluorescence spectral
  signals.