where l B = e 2 /(4π ϵ r ϵ 0 k B T ) is the Bjerrum length of the medium. In essence, the Bjerrum length is a length scale through which the electrostatic energy is larger than the thermal energy. The Bjerrum length is the key length scale in the charged systems. Here, e is the elementary charge, ϵ 0 the vacuum permittivity, ϵ r the relative permittivity of the medium, k B the Boltzmann constant and T the absolute temperature. By simultaneously solving the above couple equations, we can obtain the following results,