Seminario J.M. Molecular and Nano Electronics. Analysis, Design and Simulation

Подождите немного. Документ загружается.

34 Luis A. Agapito and Jorge M. Seminario

and basis set (LANL2DZ) as the junction. The DOS for Au used for the current–voltage

calculations is shown in Figure 1. For compatibility, we use a Si DOS calculated using

the combination B3PW91/LANL2DZ (Figure 4).

The calculation of the current–voltage characteristic for the junction with the nitroOPE

in its coplanar conformation is shown in Figure 16A. Only the contribution of -electrons

is taken into account for the I-V calculation.

5.4.2. (4, 4) CNT contact

We test several (4, 4) CNT–nitroOPE–Si junctions, which include the coplanar, per-

pendicular, and charge states. The quantum-mechanical calculation of the values for the

Fermi levels is performed using the B3PW91 method and the full-electron 6-31G(d)

basis set. These values are reported in Table 8.

The Green functions, gE, for the Si and CNT contacts are based on B3PW91/

6-31G(d) calculations of the DOS using the Crystal 03 software. The Si DOS is shown

in Figure 3, it is calculated using the 6-31G(d) basis set. We point out that the Si

DOS use for the Au–nitroOPE–Si junction is calculated using a different basis set, the

LANL2DZ basis. The DOS of the (4, 4) CNT is shown in Figure 5.

Since the molecule is connected to one half of the Si bulk, we consider that only half

of the total Si electronic states are available to leak into the molecule through the bottom

contact. Then, a DOS factor (a scaling factor) of 0.5 is applied to the silicon DOS,

mostly on intuitive geometrical grounds [36]. Interestingly, the atoms in a CNT are both

surface and bulk atoms at the same time; therefore, DOS per atom of the bulk material

corresponds to the DOS of the atom to which the nitroOPE molecule is adsorbed and

no factor needs to be applied to the CNT DOS. The current–voltage characteristic for

the coplanar (4, 4) CNT–nitroOPE–Si junction is reported in Figure 16B.

For both metal–nitroOPE–semiconductor junctions, we notice a flat region in the

I-V curve at low bias voltage (approximately from −1.4 to 1.4 V). This flat region of

approximately zero conductance has also been observed in experimental calculations of

semiconductor–molecule–metal (metallic STM tip) junctions [81–84] and in insulator–

molecule–metal junctions [85], Al–AlO

–molecule–Ti–Al. The flat region in the I-V

curve constitutes the most notorious difference with respect to the metal–nitroOPE–metal

junctions (Figures 9 and 10).

Table 8 Summary of the -HOMO and -LUMO energies for the different charge

states and conformations of the (4, 4) CNT–nitroOPE–Si junction

Conformation Charge (e) 

or 

HOMO

(eV) 

LUMO

(eV)

Coplanar 0 −483 −450

−1 −306 −306

−2 −158 −153

−3007 012

Perpendicular 0 −481 −448

−1 −318 −301

−2 −157 −152

−3 −011 019

Metal–molecule–semiconductor junctions 35

Despite the fact that gold is a metal with higher Dos than the (4, 4) CNT (∼10 times

higher at their respective Fermi levels), the current in the CNT–nitroOPE–Si junction

is similar to or higher than the current in the Au–nitroOPE–Si junction. This indicates

that a better chemical contact between the molecule and the metallic contact can make

up for the lower density of electrons of the (4, 4) CNT.

Any junction composed of a metallic of a metallic and a semiconducting contact is

a simple Schottky diode. The interface between the metal and the semiconductor gives

rise to a potential barrier, which was first explained by Schottky. The Schottky barrier

obstructs the transport of carriers in one direction of the junction, acting as an electrical

rectifier. The rectifying behavior is one of the most characteristic features associated with

macroscopic Schottky diodes; however, lack of rectifying behavior in Schottky diodes

of nanometer sizes has been experimentally observed [64–66]. Likewise, no rectifying

behavior is seen in our I-V calculations of molecular metal–molecule–semiconductor

junctions (Figure 16).

Three mechanisms for electron transport can take place in a junction: thermionic

emission, tunneling, and diffusion. Thermionic emission and tunneling are the most

–5

–2.5 0 2.5 5

–0.15

–0.05

0.05

0.15

0.25

0.35

Voltage (V)

Current (µA)

Au–nitroOPE–Si

CNT–nitroOPE–Si

Figure 16 (A) Current–voltage characteristic for the Au–nitroOPE–Si in linear scale. (B) I-V

for the (4, 4) CNT–nitroOPE–Si junction

36 Luis A. Agapito and Jorge M. Seminario

important mechanisms in Schottky diodes. The thermionic transport depends mostly on

the height of the Schottky barrier whereas the tunneling transport depends on both the

height and the thickness of the barrier. The thermionic contribution to the current is

given by the following equation:

I ∝ T





−1



(70)

where 

is the height of the barrier and T , temperature. The tunneling current, for a

triangular barrier, is given by:

I ∝   = e

√

2m



(71)

where  is the tunneling probability, m is the effective mass, L is the thickness of the

barrier, and h is the reduced Planck constant (h = h/2).

Smit et al. used a theoretical model based on the Poisson equation to track the

behavior of the Schottky barrier for diodes of arbitrary sizes, from macroscopic to

ultra-small dimensions. Following that top–down methodology, they demonstrated that

for diodes smaller than a characteristic length (associated with the doping level of

the semiconductor), the thickness of the potential barrier no longer depends on the

concentration of the dopant in the semiconductor but on the size and shape of the

diode [86, 87]. “Molecular Schottky diodes” exhibit thin potential barriers; therefore,

the tunneling contribution to conduction outweighs the thermionic contribution [64, 65].

Then, contrary to macroscopic diodes, the I-V of molecular diodes does not exhibit the

characteristic diode-like shape of Eq. (70).

Because of the small length of molecular junctions, the electron transport is coherent.

Our DFT-GF is built upon the Landauer formalism, which deals with coherent transport.

In coherence transport, electrons travel non-interactively from one contact to the other

in a single quantum-mechanical process whose probability can be calculated directly

from the fundamental equations. The transport in molecular junctions can be seen as a

probability for an electron to cross from one side of the molecule to the other, which

is a tunneling process. Equation (53), used in our formalism to calculate the current–

voltage characteristic, reflects the fact that tunneling is the main mechanism for electron

transport through molecular junctions.

It is interesting to see how Smit et al., by using a top-down approach, reached

the conclusion that electron transport in ultra-small Schottky diodes is predominantly

by tunneling. This conclusion arises naturally when using an atomistic (bottom-up)

approach, such as our DFT-GF interpretation of the Landauer formalism.

5.5. Changes in the conformation and charge states

This section very importantly discusses how to determine the current-voltage charac-

teristics of the junctions. We will analyze the gold and the (4, 4) CNT contacts as

well as the electrostatic potential distribution along the junction and their molecular

orbitals.

Metal–molecule–semiconductor junctions 37

5.5.1. Gold contact

We also analyze the perpendicular conformation of the Au–nitroOPE–Si junction. For

that, we rotate by 90



the atoms of the middle phenyl ring and keep all the other

simulation parameters the same as for the coplanar conformation.

Our result predicts a drastic difference in conductance between the coplanar and per-

pendicular conformations (Figure 17). Such change in conductance has been attributed

to the rupture of the -orbital network [88]. When the phenyl rings are coplanar, they

form a conducting path across the molecule; however, when the phenyl rings are per-

pendicular to each other, this conducting path is broken, decreasing tremendously the

conductance.

This huge change of conductance was observed by Donhouser [48] in STM electrical

measurement of several types of OPE molecules, including the nitroOPE, over time.

The charge trapped in the molecular junction has been thought to cause a strong

change in the conductance of molecular junctions [15, 85]; we investigate that by

calculating the I-V of Au–nitroOPE–Si junctions containing an extra number of electrons

(anion, dianion, and trianion). In the coplanar conformation, we observe a strong decay

in conductance when the extended molecule gets negatively charged (Figure 18A). The

anion and dianion states present similar low conductance, but the trianion is seen to have

the lowest conductance. At 3.3 V the neutral junction conduces 23.6 nA, which is 34.6,

75.3, and 23.6 times the current conduced by the anion, the dianion, and the trianion

states respectively. At 1.0 V the ratios are even better, I

neutral

anion

=885I

neutral

dianon

1234I

neutral

trianion

=2462, where I

neutral

=55 ×10

−13

High ratios of the currents (I

neutral

charged

) are encouraging for the design of a bistable

electronic device. Although the absolute current values are too small (∼10

−13

A) to

be measured by present equipment (we point out that those values correspond to the

conduction through a single molecule), in reality, thousands of molecules are expected

to be self-assembled on parallel, with the net current being also thousand times higher.

–5 –2.5 0 2.5 5

–0.1

0.1

0.2

0.3

Voltage (V)

Current (μA)

coplanar

perpendicular

–5 –2.5 0 2.5 5

–3

Figure 17 Current–voltage characteristic for two different geometrical conformations of the

–nitroOPE–Si. The planar conformation is referred to when the three rings are coplanar, and

the perpendicular conformation refers to the case when the ring containing the nitro group is

perpendicular to the other two. The inset shows a zoomed view of the I-V characteristic for the

molecule in its perpendicular conformation

38 Luis A. Agapito and Jorge M. Seminario

–0.15

–0.05

0.05

0.15

0.25

0.35

–5 –2.5 0 2.5 5

Voltage (V)

(A)

neutral

anion

dianion

trianion

Current (

–2

× 10

–3

–5 –2.5 0 2.5 5

Voltage (V)

(B)

neutral

anion

dianion

Current (

Figure 18 (A) Current–voltage characteristic for different charge states of the Au–nitroOPE–

Si junction in their coplanar conformation. Only the contribution of  electrons is shown. (B)

Current–voltage characteristic for the different charge states of the Au–nitroOPE–Si junction in

its perpendicular conformational state

In the perpendicular conformation, the junctions already have a much lower con-

ductance for the neutral state with respect to the coplanar configuration, as seen in

Figure 17. The addition of charge to the perpendicular conformation lowers even more

the conductance as reported in Figure 18B.

5.5.2. (4, 4) CNT contact

We calculate the I-V for the perpendicular conformation of the (4, 4) CNT–nitroOPE–Si

junction. Figure 19 shows a comparison of the conductance between the coplanar and

the perpendicular conformations. The calculation confirms that the strong change in

Metal–molecule–semiconductor junctions 39

–5 –2.5 0 2.5 5

–0.4

–0.3

–0.2

–0.1

0.1

0.2

Voltage (V)

Current (

coplanar

perpendicular

Figure 19 Comparison of the current–voltage characteristics of the coplanar and perpendicular

configurations for the (4, 4) CNT–nitroOPE–Si junction

conductance between the two conformational states is still present if we use the metallic

CNT instead of gold.

For instance, at 3.3 V the molecule in its coplanar conformation conducts 53 nA,

which is 496 times the current that we find for the perpendicular conformation.

We calculate the I-V characteristic for several charge states (charges −1, −2, and −3)

for the coplanar and perpendicular configurations of the (4, 4) CNT–nitroOPE–Si junc-

tion, as summarized in Figure 20. The geometries of the charge states are kept fixed to

the geometry of the neutral molecule, only the wavefunctions are optimized. The opti-

mization of all the conformational and charge states is performed using the combination

of B3PW91 level of theory and the 6-31G(d) basis set.

Our I-V calculations for the negatively charged states of the coplanar (4, 4) CNT–

nitroOPE–Si junction show a reduction in conductance with respect to the neutral

(Figure 20A). The anion, dianion, and trianion present similar values of conduc-

tance, which is clearly distinctive from the neutral. For instance, at 3.3 V, the neutral

molecule conducts 52.9 nA, the anion 17.4 nA, the dianion 5.9 nA, and the trianion

10 nA. The reduction in conductance is not as drastic as when using Au for the top

contact.

For instance, at 3.3 V the molecule in its coplanar conformation conducts 53 nA,

which is 496 times the current that we find for the perpendicular conformation.

We calculate the I-V characteristic for several charge states (charges −1, −2, and

−3) for the coplanar and perpendicular configurations of the (4, 4) CNT–nitroOPE–Si

junction, as summarized in Figure 20. The geometries of the charge states are kept

fixed to the geometry of the neutral molecule, only the wavefunctions are optimized.

The optimization of all the conformational and charge states is performed using the

combination of B3PW91 level of theory and the 6-31G(d) basis set.

Our I-V calculations for the negatively charged states of the coplanar (4, 4) CNT–

nitroOPE–Si junction show a reduction in conductance with respect to the neutral

(Figure 20A. The anion, dianion, and trianion present similar values of conductance,

which is clearly distinctive from the neutral. For instance, at 3.3 V, the neutral molecule

40 Luis A. Agapito and Jorge M. Seminario

–5 –2.5 0 2.5 5

–0.4

–0.2

0.2

Voltage (V)

neutral

anion

dianion

trianion

(A)

–5 –2.5 0 2.5 5

–0.01

–0.005

0.005

0.01

0.015

Voltage (V)

Current (µA) Current (µA)

neutral

anion

dianion

trianion

(B)

Figure 20 Current–voltage characteristic for different charge states of (4, 4) CNT–nitroOPE–Si

(A) in its coplanar conformation. (B) in its perpendicular conformation. For all cases, only the

contribution of  electrons is shown

conducts 52.9 nA, the anion 17.4 nA, the dianion 5.9 nA, and the trianion 10 nA. The

reduction in conductance is not as drastic as when using Au for the top contact.

5.5.3. ESP distribution along the junction

We compare the ESP distribution between the neutral and anion states of the coplanar

nitroOPE junction. The ESP distributions for the neutral states of the Au–nitroOPE–Si

and the CNT–nitroOPE–Si are shown in Figure 21A and Figure 21C, respectively. The

value for the ESP in the junction ranges from positive (blue) to negative (red) values;

however, for the anion states (Figure 21, B and D) mostly regions of negative values

are found within the junction. Because of electrostatic repulsion, negatively charged

particles are scattered from regions of negative ESP (red). Therefore, the negatively

charged junctions (negative ions) behave as nearly closed channels for electron transport,

Metal–molecule–semiconductor junctions 41

(A) (B) (C) (D)

Figure 21 Distribution of the ESP for (A) the neutral and (B) anion states of the coplanar

Au–nitroOPE–Si junction. Distribution of the ESP for (C) the neutral and (D) anion charge states

for the coplanar (4, 4) CNT–nitroOPE–Si junction. The spatial region corresponds to a cylinder

of radius 4 Å centered on the main C−C axis. The scale ranges from −01V red to 0.1 V (blue)

which explains the noticeable reduction of current in the charged junctions as compared

to their neutral states.

5.5.4. Analysis of the molecular orbitals

Molecular orbitals (

) are the mathematical solutions to the one-electron Kohn–Sham

equation, given in Eq. (6). Despite controversies regarding their physical reality, MOs

have extensively been used as important qualitative indicators of the conductance of

molecular systems [15, 16, 89]. An MO that is delocalized throughout the molecular

junction represents a conducting channel; an MO that is localized only on specific

regions of the molecular junctions is not a good conducting channel. As a qualitative

rule, the more delocalized an MO is, the more conducting the channel it represents.

In the resonant tunneling picture, an electron from one contact jumps into an unoc-

cupied MO (HOMO, HOMO−1) then it jumps again from that MO to the other

contact, freeing the way for another electron to repeat the process. The transport of elec-

trons can also take place through the occupied MOs (LUMO, LUMO+1). In this

case, the electron occupying the MO jumps first into one contact, with a given tunneling

probability. Then, an electron from the other contact can jump into the available MO.

The tunneling probability depends not only on the shape of the MO, which indicates

how localized or delocalized the MO is, but also on the proximity of the energy of the

MO to the Fermi level of the molecular junction. As a qualitative rule, the MOs whose

energies are closest to the Fermi level of the molecule are more conducting than the

ones whose energies farther apart.

A simple electrostatic explanation suffices to account for the change in conductance

due to charge state, as discussed in section 5.5.3. However, we could not find a direct

electrostatic explanation for the change in conductance observed between the coplanar

42 Luis A. Agapito and Jorge M. Seminario

and the perpendicular. The explanation rests more in the quantum-mechanical nature of

the system. We use the MOs to explain the change in conductance between the coplanar

and the perpendicular conformational states. Since the Fermi level of the molecular

junction is taken as the energy of the HOMO, we expect most of the electron transport

to take place through the HOMO.

For the Au

–S–nitroOPE–S–Au

junction, the HOMO for the coplanar conforma-

tion is totally delocalized between the metallic contacts and the nitroOPE, as seen in

Table 9. This explains the high conductance of the coplanar configuration and the low

conductance of the perpendicular configuration.