Ion-Current Rectification in Nanofluidic Devices - Nanofluidics: Nanoscience and Nanotechnology - page 175

Homogeneous nanochannel
with asymmetric concentration

Conical nanopore Nanofluidic diode

(1a) Equilibrium state (2a) Equilibrium state (3a) Equilibrium state

[K] n,R = [Cl] n,R

[K] n,R = [Cl] n,R [K] B,R = [Cl] B,R

K +

[K] n,R = [Cl] n,R [K] B,R = [Cl] B,R

K +

K +

[K] B,R = [Cl] B,R

Cl -

Cl -

C H > f /2

Cl -

[K] B,L = [Cl] B,L [K] n,L = [Cl] n,L

[K] B,L = [Cl] B,L [K] n,L = [Cl] n,L

[K] B,L = [Cl] B,L [K] n,L = [Cl] n,L

C L Bath Nanochannel C H Bath

C L Bath Nanochannel C R Bath

C L Bath Nanochannel C R Bath

(1b) Forward bias (2b) Forward bias (3b) Forward bias

K +

K + Cl -

K +

Cl -

Accumulation

Accumulation Accumulation

Cl -

Depletion Depletion

Depletion

---- + + + +

-------- J K B,R

J K B,L J K B,L J K B,R

J K n,L J K n,R

J K B,L J K B,R

J K n,L J K n,R

J K n,R

J K n,L
J Cl n,L

J Cl n,L J Cl n,L

J Cl B,L J Cl n,R J Cl B,R J Cl B,L J Cl n,R J Cl B,R

J Cl B,L J Cl n,R J Cl B,R

--------

---- + + + +

(1c) Reverse bias (3c) Reverse bias

(2c) Reverse bias

K + K + Cl -

K +

Cl -

Cl -

Accumulation Accumulation Accumulation

Depletion Depletion

Depletion

---- + + + +
J K n,R

------- J K n,R

J K B,R J K B,R

J K B,L

J K n,L J K B,L J K n,R J K B,R

J K B,L

J K n,L

J K n,L
J Cl n,L

J Cl n,L

J Cl n,R

J Cl B,R J Cl n,R J Cl n,L

J Cl n,R

J Cl B,R

J Cl B,L J Cl B,L

J Cl B,R

J Cl B,L

--------

---- + + + +

Figure 8.11 Interpretation of ionic rectification in different types of nanofluidic devices based on the
analysis of symmetric ion currents building up right after the external electric fields are applied.
For the same type of ion species, if the ion current flows in more than it flows out at a location,
there will be an accumulation of ions when the system reaches steady state. On the contrary, if
there is more out than in, depletion of ions takes place at the location.

8.3.3 Comparison of Rectifying Effects in Nanofluidic Diodes and
Semiconductor Diodes

As noted previously, since ionic and electronic systems share many similarities, it
would be interesting to compare the physics behind current rectification in a nanofluidic
and a semiconductor diode. When an n-type and a p-type semiconductor are put together, a
positive and a negative space-charge region is established at the junction, and mobile
charges are depleted. The resultant built-in potential, due to the formation of space-charge,
impedes the diffusion of majority carriers from both types of semiconductor across the
junction. As discussed, in a nanochannel p-n diode a similar situation exists.
While both nanofluidic diodes and semiconductor diodes exhibit similar rectifying
I-V characteristics, they operate differently in some ways. For instance, the forward bias
current in semiconductor diodes is dominated by the diffusion of charge carriers with
enough energy to overcome the built-in potential in the space-charge region. Once across
the junction, these charge carriers become the minority carriers, which will recombine with
the majority carriers within approximately a diffusion length from the edge of depletion or