Fundamental Rules to Construct Highly Integrated Organic
Nanowires as Nanodevices
Denille Brito de Lima 1,
Jordan Del Nero 2,*
1
Pós-graduação em Engenharia Elétrica, Universidade Federal do Pará, 66075-900,
Belém, Pará, Brazil.
2
Departamento de Física, Universidade Federal do Pará, 66075-110, Belém, Pará, Brazil.
* Corresponding Author: jordan@ufpa.br
(Date: October 23, 2007)
11:49
Keywords:
Organic Nanowire;
Donor – π Bridge – Acceptor;
Electrode – Donor – π Bridge – Acceptor – Electrode;
Abstract
Recent advances in inorganic nanowires techniques have led to more efficient and
reliable electronic nanodevices as bipolar transistors, field effect transistors, junctions,
logic-gate structures and applications from barcodes to LEDs. Otherwise, a true organic
nanowire (ONW) device has not been realized. In this paper we describe few rules that
will be useful to construct ONWs as well as the influence of mettalic contacts. Based on
an equilibrium technique, we present electronic transport simulation for a well-known
one-dimensional polymer prototype attached with donor/acceptor groups. Our results are
consistent with: (i) asymmetric electronic transport (Charge Accumulation/DepletionVoltage) curve for the ONW with and without the presence of contacts; (ii) for donorpolyacetylene-acceptor we find out assymetric nonresonance tunneling type conduction
in the I-V curve; (iii) the aluminum atoms show to be a good candidate as mettalic contact
when bonded with Sulfur atoms provoking rectification for lower values when compared
with the system without Aluminum; (iv) For forward and reverse bias the resonance takes
place suggesting in operational limit working as non-symetric bi-directional molecular
field effect transistor (FET).
Introduction
Since the first work on nanowires made by Knoedler in 1990
1
concerning
inorganic GaAs/AlGaAs semiconductors several works have been done. One of them,
made by Russel et al. where was proposed the fabrication of ultrahigh-density arrays of
nanopores with cobalt nanowires (1.9 x 1011 wires/cm2) using the self-assembled
morphology of asymmetric copolymers 2.
Leclere et al. 3 made a theoretical/experimental investigation using self-assembly
technique of copolymers block. The main consequence of these preliminaries results is to
open the possibility of producing organic semiconducting nanostructures with a
controlled shape via chemical synthesis.
A small review up to 2002 was done by Bohr
4
showing advances and goals to
nanotechnology. At that time, organic nanowires looks like a prominent material and a
direction to be explored.
Lieber et al. have been investigating several aspects to utilize inorganic nanowire
device as: It was presented parallel and crossed arrays working individually or together
5
and the transport measurements shown pattern as a resistor 5. After a technique
improvement by doping silicon nanowire with Boron & Phosphorous and indium
phosphide nanowires, they got bipolar transistors
6
and field effect transistors 7,
respectively. Also, nanowire junctions have been proposed as logic-gate structures (OR,
AND, and NOR) 8, as superlattices with several GaAs and GaP layers suggesting
applications from barcodes to nano-structured LEDs 9, 10, and electrically driven nanowire
lasers
11
. It was increase the nanowire field effect transistors performance within a
transconductance and current values of 3.3 mS μm-1 and 2.1 mA μm-1, respectively 12. In
the same way, inorganic nanowires with highly efficient electronic conduction properties
have been done 13, 14, 15, 16, 17, 18. As example, dense array of oriented crystalline ZnO 13, 14
working as solar cells, dielectric nanowire with self-organized gold nanoparticle chain
encapsulated in
Phosphorus
16
15
, Germanium nanowire p-n junction by doping with Boron and
, electroluminescence characteristic from a single nanowire based on by
tunnel injection properties
17
, and Silicon nanowire sensors in molecular and cellular
arrays 18.
In the otherwise, the development of organic nanowires is not, at the moment, in
the same development level as inorganic materials. Koswatta et al. shown based on a
theoretical semiclassical Boltzmann transport and a nonequilibrium Green’s function the
possibility of Carbon Nanotubes be used as MOSFET-like device 19. Also, experimental
works on an hybrid nanowire made by metal/organic is addressed: A metal nanocrystal
and carbon nanotube was proposed as a nonvolatile memory 20, a high performance ZnO
nanowire FETs and organic gate insulator (results with Al contacts show current of up to
4 μA)
21
. Likewise, recently done by Lieber et al., the 3D properties by multifunctional
electronics based on the layer-by-layer assembly
22
organized
over
components
(carbon
nanotubes)
was proposed as well as they utilize
large
areas
with
controlled
density/orientation and the electronic transport curves show that large arrays of nanowire
FETs could be efficiently fabricated on the wafer scale 23.
Summarizing, in counterpart of high advances presented on inorganic nanowires it
is clear the necessity to investigate deeply the fundamental properties concerning the
utilization of organic structures as nanowires and the conduction pattern through
molecules.
In this paper we investigate the electronic transport rule for a family of organic
nanowire that could be used as nanodevice as well as to investigate toward a rule to
design metal-molecule-metal and improve the conductivity efficiency of these materials.
Using ab initio methodologies we determine the electron charge distribution along the
structures giving a pattern about the current vs. voltage of this device family.
In the next two sections we present the system investigated with the methodology
and the results. The last section will be presented our final remarks including ours
perspectives and conclusions.
System and Methodology
In order to present how majority carriers transport effect could lead to the
modulation of electronic structure, we have performed a simulation to describe the
evolution of ONWs using a well-known polymer as polyacetylene 24 attached with Sulfur
and Aluminum as mettalic contacts.
For the equilibrium calculation, it has been employed Density Functional Theory
(DFT) methodology belonging in Gaussian program
25
. Based on DFT and using the
B3LYB/6-31G functionals, which is a well-established methodology 26, 27 and gives good
results about the electronic structure in correlated systems. The geometric parameters of
the analyzed structures were fully optimized including external electrical field in a closed
shell model for the Roothaan-Hall matrix
28, 29
. The effect of the external electrical field
on the molecule was considered with this methodology, and taking into account the
correlation between electric field and charge accumulation along the molecule.
It has been done by quantum mechanics methodologies several works to describe
organic structures interacting with electrodes as: (a) To produce a good electrical contact
between a molecule and a substrate working as conductor, a bond is necessary. Sulfur and
Selenium atoms are good candidates to bound with Gold, Silver and Aluminum 30; (b) A
theoretical/experimental work done with gold as electrode decrease the conductivity
when compared with the model without terminals and Sulfur atoms increases the
connection with metallic contacts 31; (c) An elegant way to construct electrodes by breakjunction was proposed showing that noble metal as Gold and divalent-Zinc offer the
possibility to form contacts with a well-transmitted single transport channel32; (d) A
coupled DFT/localized Wannier functions methodology was employed to investigate
different types of electrodes connected with nitrobenzene and benzene. The contacts were
made by Gold giving good conductivity results when anchored to thiol-groups provoking
an increase in the conductance for spacer as polyacetylene
33
; (e) Also, for benzene and
borazine were investigated connected with Gold atoms. It was used terminal groups as
cyano and Selenium atoms presenting a better linkage for benzene-CN and borazine-Se
34
; (f) For a system as Au-benzenedithiol-Au by DFT and nonequilibrium Green function
showing significant conductance resonance when it presents double Gold atoms, similar
results found with increasing the molecule-lead separation
35
; (g) Single-rectangle gate
electrode shows worst results when compared with four-rectangles gate electrode because
the coupling between the source/drain decrease 36.
Following these leads we improve our technique to design our device and we
track three possible backbones to study a two terminal device. The representation of
contact, connection, donor, bridge and acceptor are represented by Aluminum atoms,
Sulfur atoms, N(CH3)2, olygoacetylene (6, 8, 10 and 12 double bonds) and NO2,
respectively. The systems investigated in this work are: (i) Donor-Bridge-Acceptor; (ii)
Sulfur atoms-Donor-Bridge-Acceptor-Sulfur atoms; (iii) Aluminum atoms-Sulfur atomsDonor-Bridge-Acceptor-Sulfur atoms- Aluminum atoms. Overall, the systems presented
here [Figure 1] have the same grounds and they are additional point-of-view of models
recently investigated 26, 27, 28, 29.
Results and Discussion
In previous works it has been described that a Sourcebet-π bridge-Drainbet 26, 27 (bet
means betaine donor/acceptor type), Source-σ bridge-Drain
28
, and a three-terminal
Source- π and σ bridge –Drain with a Gate as charge controller
29
are important to
investigate devices with applications in nanotechnology.
In Figure 2 we present the charge accumulation/depletion-Voltage for the
molecular system presented in Figure 1 with 6, 8, 10, and 12 double bonds (dbs) in the
bridge of the nanodevice. The results show that the electronic transport increases as a
function of external applied electric field under forward and reverse polarization. For
positive external electric field it presents one rectification for specific bias between 2.24
V and 3.61 V reaching them the saturation region. In the otherwise, for negative external
electric field it presents three rectifications with similar behavior (The quantitative values
could be found inside the Figure 2). For small bridge (6 and 8 dbs) the third rectification
are not presented because the system doesn’t support high voltages values. As in
Schottky diodes, the conduction is lead by the electrons as carriers by drift transportation
and, also, as typical Silicon Controlled Rectifiers this system presents an assymmetric bidirectional electronic transportation with one [three] rectification(s) for positive
[negative] bias.
Applying an external electric field in the molecule with Sulfur atoms attached in
the donor/acceptor groups we observe (Figure 3) that under forward bias occurs
rectification and the operational value have a dependence with the size of dbs presented
between 2.47 V (6 dbs) up to 1.11 V (12 dbs). For negative bias occurs two competitive
effects: (i) For 6 and 8 dbs present a saturation value before break the molecular structure
and don’t show rectification; (ii) For 10 and 12 dbs present a rectification equal to 1.23 V
for both systems leaded the drift transportation of carriers having bi-directional electronic
FET signature.
The molecular device bonded with Aluminum atoms under forward bias (Figure
4) show an important signature as: (i) When the voltage goes to the operational forward
switch, a strong shift to lower energies occurs for rectification between 0.96 V (6 dbs) to
0.53 V (12 dbs); (ii) Also for reverse switch occurs a shift to lower energies presenting
values for rectification between -0.97 V (6 dbs) to -0.66 V (12 dbs). In these molecular
structures don’t appear steps as before (Figures 2 and 3) and it could be understood
analyzing the contact effects because when the charge is removed from the metal region
increase the majority carrier transport (in absolute values) with the applied voltage. In the
same way, it happens for both bias and the red shift in the voltage will appear in all
systems with these characteristic. The presented values for the calculated bias could be
understood as a breakdown voltage for forward (0.96 V, 0.82 V, 0.53 V and 0.53V) and
reverse (-0.97 V, -0.82 V, -0.76 V and –0.66 V) operational region. As a two-terminal
device the possibility of a bi-directional rectification occurs as a usual Diode for
Alternating Current.
Figure 5 presents the electronic absorption spectra of the ONW presented in
Figure 1. A similar feature is presented in these spectra and it it possible to raise up a few
topics as: (a) the |H→L> transitions (from the highest occupied molecular orbital to the
lowest unoccupied molecular orbital) are presented as the main transition for all SourceBridge-Drain compounds; (b) also with the increase of db the |H-1→L> appears as the
main transition; (c) the inclusion of Sulfur atoms increase the contribution of |H-1→L>
transition provoking that the weight of coefficient of linear combination of atomic
orbitals goes from 0.36 to 0.63 (for 12 db) showing the importance of more transitions
participating of ONW conduction process; (d) including contacts the previous behavior
increase the possibility of several others transitions as the main one.
Conclusions and Remarks
In this paper were investigated the majority transport behavior in a push-pull
system with a π bridge of polyacetylene type as spacer. Also, it was included Aluminum
atoms as contact with intention to investigate the competition between the organic
structure anchored in mettalic atoms.
The simulation of carrier transport as a function of an external electric field
applied was performed and by our results general rules were point out: (i) the rectification
process happens for conjugated bonds with and without explicity contacts showing that
this procedure is a behavior of the molecular structure investigated; (ii) the charge
accumulation/depletion – Voltage curves obtained by ab initio simulations give us a
feature about the majority transport signature and as saturable backbone systems could be
understood as a ballistic resonant model; (iii) For forward and reverse bias the system
have the similar characteristic as usual FET devices with lower operational bias region.
Overall, several properties on ONWs must be investigated before a summary
conclusion but we can stress out a few points/questions that deserve futher investigations:
(a) Are feasible to construct an efficient ONW using the experimental state-of-art
techniques?; (b) Is it possible to utilize functionalized conjugated polymers with large
nonlinearity χ(2) or χ(3) to produce efficient materials without (or at least minimize) signal
lost?; (c) Does the electronic transport mechanism the necessity of majority or minority
carriers excitations as solitons, polarons or bipolarons? (d) Is it possible to create an
hybrid organic-inorganic NW?
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Figure 1. (Color Online) Pictogram of molecular structure investigated composed by
push-pull like system with conjugated bonds in the bridge: (i) Donor – π Bridge –
Acceptor; (ii) Sulfur – Donor – π Bridge – Acceptor – Sulfur; (iii) Metallic contact –
Sulfur – Donor – π Bridge – Acceptor – Sulfur – Metallic contact.
Figure 1. (i)
Figure 1. (ii)
Figure 1. (iii)
Figure 2. (Color Online) Charge accumulation-Voltage and Charge depletion-Voltage
for the Donor – π Bridge – Acceptor investigated applying an external electrical field
(Represented by Figure 1 (i)).
Figure 3. (Color Online) Charge accumulation-Voltage and Charge depletion-Voltage
for the HS – Donor – π Bridge – Acceptor – SH investigated applying an external
electrical field (Represented by Figure 1 (ii)).
Figure 4. (Color Online) Charge accumulation-Voltage and Charge depletion-Voltage
for the Metallic contact – S – Donor – π Bridge – Acceptor – S – Metallic contact
investigated applying an external electrical field (Represented by Figure 1 (iii)).
Figure 5. (Color Online) Theoretical absorption spectra of ONW for 6, 8, 10 and 12
double bonds with and without contacts.