Debye length

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In plasmas and electrolytes the Debye length (also called Debye radius), named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers (e.g. electrons) screen out electric fields. In other words, the Debye length is the distance over which significant charge separation can occur. A Debye sphere is a volume whose radius is the Debye length, in which there is a sphere of influence, and outside of which charges are screened. The notion of Debye length plays an important role in plasma physics, electrolytes and colloids (DLVO theory).

Contents

Physical origin

The Debye length arises naturally in the thermodynamic description of large systems of mobile charges. In a system of N different species of charges, the j-th species carries charge q_j and has concentration n_j(\mathbf{r}) at position \mathbf{r}. According to the so-called "primitive model", these charges are distributed in a continuous medium that is characterized only by its relative static permittivity, \varepsilon_r. This distribution of charges within this medium gives rise to an electric potential \Phi(\mathbf{r}) that satisfies Poisson's equation:

 \nabla^2 \Phi(\mathbf{r}) = -\frac{1}{\varepsilon_r \varepsilon_0} \, \sum_{j = 1}^N q_j \, n_j(\mathbf{r}),

where \varepsilon_0 is the electric constant.

The mobile charges not only establish \Phi(\mathbf{r}) but also move in response to the associated Coulomb force, - q_j \, \nabla \Phi(\mathbf{r}). If we further assume the system to be in thermodynamic equilibrium with a heat bath at absolute temperature T, then the concentrations of discrete charges, n_j(\mathbf{r}), may be considered to be thermodynamic (ensemble) averages and the associated electric potential to be a thermodynamic mean field. With these assumptions, the concentration of the j-th charge species is described by the Boltzmann distribution,

 n_j(\mathbf{r}) = n_j^0 \, \exp\left( - \frac{q_j \, \Phi(\mathbf{r})}{k_B T} \right),

where k_B is Boltzmann's constant and where n_j^0 is the mean concentration of charges of species j.

Identifying the instantaneous concentrations and potential in the Poisson equation with their mean-field counterparts in Boltzmann's distribution yields the Poisson-Boltzmann equation:

 \nabla^2 \Phi(\mathbf{r}) = -\frac{1}{\varepsilon_r \varepsilon_0} \, \sum_{j = 1}^N q_j n_j^0 \, \exp\left(- \frac{q_j \, \Phi(\mathbf{r})}{k_B T} \right).

Solutions to this nonlinear equation are known for some simple systems. Solutions for more general systems may be obtained in the high-temperature (weak coupling) limit, q_j \, \Phi(\mathbf{r}) \ll k_B T, by Taylor expanding the exponential:

 \exp\left(- \frac{q_j \, \Phi(\mathbf{r})}{k_B T} \right) \approx  1 - \frac{q_j \, \Phi(\mathbf{r})}{k_B T}.

This approximation yields the linearized Poisson-Boltzmann equation

 \nabla^2 \Phi(\mathbf{r}) = \left(\sum_{j = 1}^N \frac{n_j^0 \, q_j^2}{\varepsilon_r \varepsilon_0 \, k_B T} \right)\, \Phi(\mathbf{r}) - \frac{1}{\varepsilon_r \varepsilon_0} \, \sum_{j = 1}^N n_j^0 q_j

which also is known as the Debye-Hückel equation:[1][2][3][4][5] The second term on the right-hand side vanishes for systems that are electrically neutral. The term in parentheses has the units of an inverse length squared and by dimensional analysis leads to the definition of the characteristic length scale

 \lambda_D =  \left(\frac{\varepsilon_r \varepsilon_0 \, k_B T}{\sum_{j = 1}^N n_j^0 \, q_j^2}\right)^{1/2}

that commonly is referred to as the Debye-Hückel length. As the only characteristic length scale in the Debye-Hückel equation, \lambda_D sets the scale for variations in the potential and in the concentrations of charged species. All charged species contribute to the Debye-Hückel length in the same way, regardless of the sign of their charges.

The Debye-Hückel length may be expressed in terms of the Bjerrum length \lambda_B as

 \lambda_D = \left(4 \pi \, \lambda_B \, \sum_{j = 1}^N n_j^0 \, z_j^2\right)^{-1/2},

where z_j = q_j/e is the integer charge number that relates the charge on the j-th ionic species to the elementary charge e.

Typical values

In space plasmas where the electron density is relatively low, the Debye length may reach macroscopic values, such as in the magnetosphere, solar wind, interstellar medium and intergalactic medium (see table):

PlasmaDensity
ne(m-3)
Electron temperature
T(K)
Magnetic field
B(T)
Debye length
λD(m)
Solar core1032107--10−11
Tokamak10201081010−4
Gas discharge1016104--10−4
Ionosphere101210310−510−3
Magnetosphere10710710−8102
Solar wind10610510−910
Interstellar medium10510410−1010
Intergalactic medium1106--105
Source: Chapter 19: The Particle Kinetics of Plasma
http://www.pma.caltech.edu/Courses/ph136/yr2004/

Hannes Alfvén pointed out that: "In a low density plasma, localized space charge regions may build up large potential drops over distances of the order of some tens of the Debye lengths. Such regions have been called electric double layers. An electric double layer is the simplest space charge distribution that gives a potential drop in the layer and a vanishing electric field on each side of the layer. In the laboratory, double layers have been studied for half a century, but their importance in cosmic plasmas has not been generally recognized."

Debye length in a plasma

In a plasma, the background medium may be treated as the vacuum (\varepsilon_r = 1), and the Debye length is

 \lambda_D = \sqrt{\frac{\varepsilon_0 k_B/q_e^2}{n_e/T_e+\sum_{ij} j^2n_{ij}/T_i}}

where

λD is the Debye length,
ε0 is the permittivity of free space,
kB is the Boltzmann constant,
qe is the charge of an electron,
Te and Ti are the temperatures of the electrons and ions, respectively,
ne is the density of electrons,
nij is the density of atomic species i, with positive ionic charge jqe

The ion term is often dropped, giving

 \lambda_D = \sqrt{\frac{\varepsilon_0 k_B T_e}{n_e q_e^2}}

although this is only valid when the mobility of ions is negligible compared to the process's timescale.[6]

Debye length in an electrolyte

In an electrolyte or a colloidal suspension, the Debye length[7] is usually denoted with symbol κ−1

 \kappa^{-1} = \sqrt{\frac{\varepsilon_r \varepsilon_0 k_B T}{2 N_A e^2 I}}

where

I is the ionic strength of the electrolyte, and here the unit should be mole/m3,
ε0 is the permittivity of free space,
εr is the dielectric constant,
kB is the Boltzmann constant,
T is the absolute temperature in kelvins,
NA is the Avogadro number.
e is the elementary charge,

or, for a symmetric monovalent electrolyte,

 \kappa^{-1} = \sqrt{\frac{\varepsilon_r \varepsilon_0 R T}{2 F^2 C_0}}

where

R is the gas constant,
F is the Faraday constant,
C0 is the molar concentration of the electrolyte.

Alternatively,

 \kappa^{-1} = \frac{1}{\sqrt{8\pi \lambda_B N_A I}}

where

\lambda_B is the Bjerrum length of the medium.

For water at room temperature, λB ≈ 0.7 nm.

At room temperature (25 °C), one can consider in water the relation [8] :

 \kappa^{-1}(\mathrm{nm}) = \frac{0.304}{\sqrt{I(\mathrm{M})}}

where

κ−1 is expressed in nanometers (nm)
I is the ionic strength expressed in molar (M or mol/L)

Debye length in silicon

The Debye length has become increasingly significant in the modeling of solid state devices as improvements in lithographic technologies have enabled smaller geometries.[9][10][11]

The Debye length of silicon is given:

 \mathit{L}_D = \sqrt{\frac{\varepsilon_{\mathrm{Si}} k_B T}{q^2N_d}}

where

εSi is the dielectric constant of silicon,
kB is the Boltzmann's constant,
T is the absolute temperature in kelvins,
q is the elementary charge, and
Nd is the density of donors in a substrate.

When doping profiles exceed the Debye length, majority carriers no longer behave according to the distribution of the dopants. Instead, a measure of the profile of the doping gradients provides an “effective” profile that better matches the profile of the majority carrier density.

See also

Debye-Falkenhagen effect

References

  1. ^ Kirby BJ.. Micro- and Nanoscale Fluid Mechanics: Transport in Microfluidic Devices. http://www.kirbyresearch.com/textbook. 
  2. ^ Li D (2004). Electrokinetics in Microfluidics. 
  3. ^ PC Clemmow & JP Dougherty (1969). Electrodynamics of particles and plasmas. Redwood City CA: Addison-Wesley. pp. §7.6.7, p. 236 ff.. ISBN 0-201-47986-9. http://books.google.com/books?id=SBNNzUrTjecC&pg=PP1&dq=particles+plasmas+inauthor:Clemmow#PPA236,M1. 
  4. ^ RA Robinson &RH Stokes (2002). Electrolyte solutions. Mineola NY: Dover Publications. p. 76. ISBN 0-486-42225-9. http://books.google.com/books?id=6ZVkqm-J9GkC&pg=PR3#PPA76,M1. 
  5. ^ See DC Brydges & Ph A Martin Coulomb Systems at Low Density: A Review
  6. ^ I. H. Hutchchinson - Principles of plasma diagnostics; ISBN 0-521-38583-0
  7. ^ Russel, W.B., Saville, D.A. and Schowalter, W.R. Colloidal Dispersions, Cambridge University Press, 1989
  8. ^ Israelachvili, J., Intermolecular and Surface Forces, Academic Press Inc., 1985, ISBN 0-12-375181-0
  9. ^ Stern, Eric; Robin Wagner, Fred J. Sigworth, Ronald Breaker, Tarek M. Fahmy, Mark A. Reed (2007-11-01). "Importance of the Debye Screening Length on Nanowire Field Effect Transistor Sensors". Nano Letters 7 (11): 3405–3409. Bibcode 2007NanoL...7.3405S. doi:10.1021/nl071792z. 
  10. ^ Guo, Lingjie; Effendi Leobandung, Stephen Y. Chou (1997). "A room-temperature silicon single-electron metal–oxide–semiconductor memory with nanoscale floating-gate and ultranarrow channel". Applied Physics Letters 70 (7): 850. Bibcode 1997ApPhL..70..850G. doi:10.1063/1.118236. ISSN 0003-6951. http://link.aip.org/link/APPLAB/v70/i7/p850/s1&Agg=doi. Retrieved 2010-10-25. 
  11. ^ Tiwari, Sandip; Farhan Rana, Kevin Chan, Leathen Shi, Hussein Hanafi (1996). "Single charge and confinement effects in nano-crystal memories". Applied Physics Letters 69 (9): 1232. Bibcode 1996ApPhL..69.1232T. doi:10.1063/1.117421. ISSN 0003-6951. http://link.aip.org/link/APPLAB/v69/i9/p1232/s1&Agg=doi. Retrieved 2010-10-25. 

Further reading