Memory devices based on functionalized copolymers exhibiting a linear dependence of switch threshold voltage with the pendant nitro-azobenzene moiety content change

Article abstract of DOI:10.1039/c2jm33426g

A nonvolatile nanoscale memory device based on pendent copolymer exhibits write-once-read-many-times (WORM) characteristics with the highest ON/OFF current ratio up to 10⁴ and a long retention time. Moreover, it was observed that switch thresho

Full text of DOI:10.1039/c2jm33426g

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PAPER
Memory devices based on functionalized copolymers exhibiting a linear
dependence of switch threshold voltage with the pendant nitro-azobenzene
moiety content change
a
a
a
a
a
a
Naiyong Fan, Haifeng Liu, Qianhao Zhou, Hao Zhuang, Yang Li, Hua Li, Qingfeng Xu,a Najun Lia
*
ab
*
and Jianmei Lu
Received 28th May 2012, Accepted 13th August 2012
DOI: 10.1039/c2jm33426g
A nonvolatile nanoscale memory device based on pendent copolymer exhibits write-once-read-many-
times (WORM) characteristics with the highest ON/OFF current ratio up to 104 and a long retention
time. Moreover, it was observed that switch threshold voltages of the device varied almost linearly with
the functional moiety content in the copolymer. The cyclic voltammetry (CV) curves and UV-vis
spectra of the copolymer’s nanofilms were investigated and the obtained results associated with the
linear decreasing memory behavior. The mechanisms of the device exhibiting WORM characteristics
were elucidated from molecular simulation results showing that the electron density transition from the
HOMO to LUMO surfaces is permanently under electric field and would not revert to the original state
after the external voltage was removed.
acts as an electron donor and the nitro-azobenzene as an electron
Introduction
acceptor. The pull–push electron groups would undergo charge
transfer under the electric field,23,24 which is important for the
device to achieve OFF to ON state switching. A simple and
accessible free radical polymerization was adopted in this paper to
prepare PAzo-co-St (P(Azo)x(St)y) in which the monomer styrene
was introduced to enhance both the material’s solubility and
scalability. The memory devices based on copolymers with
different contents of functional moieties exhibited excellent write-
once-read-many times (WORM) performances with their ON/
OFFcurrentratioupto104 andtheswitchthresholdvoltages ofthe
device varied almost linearly with functional moiety content in the
copolymer. Thisisimportant informationfor the designofpendent
polymer structures to obtain good electronic performance.
In recent years, memory devices based on nanoscale thin films
have been widely investigated by scientists.1–4 Currently, there is
increasing interest in the use of organic and polymeric materials
as novel memory elements.5–9 Compared with organic small
molecule materials, polymeric materials show rapid, easy and
cost effective processing of the thin layers required in devices.10–19
The pendant polymers containing donor–acceptor structures in
the side-chains have been precisely studied and could achieve
different memory performance through adjusting the push–pull
ability of electron donors and acceptors.20–24 Whereas, there are
other important factors that may influence devices’ performance,
such as the film thickness, the content of functional moieties in
the copolymer and the energy barrier between the film and the
electrode’s surface. In this paper, we mainly investigate how the
varied functional moiety content in the copolymer would affect
the performance of the fabricated sandwiched devices.
Experimental section
Materials
Carbazole, containing a large conjugated planar structure, has
good hole transport capability for optoelectronic applications.14
The nitro-benzene has the advantages of a simple synthesis route
and low cost, and thus is frequently incorporated as an electron
acceptor in organic semiconductors.19 Herein, one azobenzene
derivative was prepared as shown in Scheme 1, where the carbazole
1-Chloro-6-hydroxyhexane, tetrahydrofuran (THF), azobisisobu-
tyronitrile (AIBN) (99%), carbazole (99%), cyclohexanone (99%),
potassium hydroxide (KOH) (99%) and other solvents were all
purchased from Shanghai Chemical Reagent Co. Ltd. Cyclohexa-
none and Styrene were purified by reduced pressure distillation.
AIBN was recrystallized from methanol before use. Other reagents
were used as received without any further purification.
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou, Jiangsu 215123, China. E-mail: lujm@suda.edu.cn;
Fax: +86 512 65880367; Tel: +86 512 65880368
bInstitute of Chemical Power Sources, Soochow University, Suzhou,
Jiangsu 215006, China
Preparation of P(Azo)x(St)y
Synthesis of CZ. Potassium hydroxide (5.0 g, 88.9 mmol) was
added into 120 mL N,N-dimethylacetamide (DMF). After
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Scheme 1 The synthesis route and molecular structure of the copolymers.
(CDCl3, d, ppm): 8.12 (d, 2H), 7.46–7.41 (m, 4H), 7.23–7.20 (m,
2H), 4.32 (t, 2H), 3.61–3.59 (t, 2H), 1.91–1.89 (m, 2H), 1.55 (m,
2H), 1.41 (m, 4H), 1.30 (s, 1H). Elemental analysis calculated for
C18H21NO (wt%): C, 80.86; H, 7.92; N, 5.24; found: C, 80.80; H,
7.87; N, 5.28.
Synthesis of CA. A mixed solution of 4-nitroaniline (1.38 g,
10 mmol), concentrated hydrochloric acid (4 g, 37%) and
deionized water (4 g) was stirred below 4 ꢀC in an ice bath. Then,
sodium nitrite (0.83 g, 12 mmol) dissolved in deionized water
(4 g) was slowly added dropwise to the above solution and
stirring continued for an hour at 0–4 ꢀC. After the solution had
become transparent, the mixture was filtered and the filtrate
(diazonium salt solution) was kept in the ice bath. Sodium
dodecyl sulfate (107 mg) was added into the filtrate. CZ (2.67 g,
10 mmol) was dissolved in methylene chloride (100 mL), and
then added dropwise to the above filtrate under intense stirring at
room temperature for 48 hours. Dichloromethane was removed
under reduced pressure, the mixture was filtered to get a red
precipitate and this was rinsed with plenty of water. The crude
product was purified through silica gel column with chloroform
and petroleum ether (volume ratio ¼ 5 : 1), yield 50%.1H-NMR
(d6-DMSO, d, ppm): 8.85 (s, 1H), 8.43 (d, 2H), 8.34 (d, 1H), 8.12
(d, 1H), 8.07 (d, 2H), 7.80 (d, 1H), 7.71–7.68 (t, 1H), 7.56–7.54 (t,
1H), 7.31 (t, 1H), 4.50–4.46 (t, 2H), 4.32–4.29 (t, 2H), 1.82–1.79
(m, 2H), 1.32–1.29 (m, 6H). Elemental analysis calculated for
C24H24N4O3 (wt%): C, 69.21; H, 5.81; N, 13.45; found: C, 69.25;
H, 5.87; N, 13.38.
Fig. 1 1H-NMR spectrum of P(Azo)1(St)1 in CDCl3.
Table 1 Characterization data of P(Azo)x(St)y
GPC result
P(Azo)x(St)y
Mn
Mw/Mn
Wt%a
TGA (ꢀC)
Tg (ꢀC)
PAzo
3682
3447
3899
4582
6344
1.71
1.40
1.73
1.49
1.62
100
80
67
36
308
304
301
298
293
129
126
123
119
117
P(Azo)1(St)1
P(Azo)1(St)5
P(Azo)1(St)10
P(Azo)1(St)20
26
a
The weight percentage of Azo moiety in the copolymers were measured
by UV-vis absorption standard curve.
Synthesis of Azo. A mixture of CA (1.04 g, 2.5 mmol), tetra-
hydrofuran (10 mL) and triethylamine (0.5 mL) was kept in the
ice bath. Then, methacryloyl chloride (0.36 g, 3.45 mmol) diluted
in 5 mL of THF was added dropwise to the mixture and the
reaction continued for 10 hours in the ice bath. The reaction
solution was concentrated and added into 50 mL chloroform and
then washed with 1% hydrochloric acid solution and 3% sodium
hydroxide solution. Finally, the solution was washed with
deionized water to neutral pH and chloroform was removed to
stirring for 10 min, carbazole (3.34 g, 20 mmol) was added to the
mixture and continued stirring for 45 min. Then, 1-chloro-6-
hydroxyhexane (2.85 g, 21 mmol) was added slowly to the
mixture and the reaction kept at 50 ꢀC for 24 hours. The resulting
mixture was poured into a large amount of water and the
precipitate was collected by filtration. Then, the final white
product CZ was dried and purified via silica gel column with
chloroform and petroleum ether (petroleum ether boiling point
range 60–90 ꢀC) (volume ratio ¼ 2 : 1), yield 69%. 1H-NMR
1
get red product, yield 99%. H-NMR (d6-DMSO, d, ppm): 8.85
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Instruments
1H-NMR spectra were obtained on an Inova 400 MHz FT-NMR
spectrometer (Fig. 1). The elemental analysis was performed by
Italian 1106 FT analyzer. UV-vis absorption spectra were
recorded by a PerkinElmer Lambda-17 spectrophotometer at
room temperature. Thermogravimetric analysis (TGA) was
conducted on a TA instrument Dynamic TGA 2950 at a heating
rate of 10 ꢀC minꢁ1 under a N2 flow rate of 50 mL minꢁ1. Cyclic
voltammetry was performed at room temperature using an ITO
working electrode, a reference electrode Ag/AgCl, and a counter
electrode (Pt wire) at a sweep rate of 20 mV sꢁ1 (CorrTest CS
Electrochemical Workstation analyzer) in a solution of tetra-
butylammonium hexafluorophosphate (n-Bu4NPF6) in acetoni-
trile (0.1 M). SEM images were taken on a Hitachi S-4700
scanning electron microscope. Molecular weights (Mn) and
polydispersity (Mw/Mn) was measured by gel permeation chro-
matography (GPC) utilizing a Waters 1515 pump and a differ-
ential refractometer, THF was used as a mobile phase at a flow
rate of 1.0 mL minꢁ1. Differential scanning calorimetry (DSC)
analyses were performed on a Shimadzu DSC-60A Instrument.
Device fabrication
The indium tin oxide (ITO) glass substrate was carefully pre-
cleaned sequentially with deionized water, acetone and alcohol
by ultrasonic bath for 20 minutes, respectively. The copolymer
solution was prepared in cyclohexanone (10 mg mLꢁ1) and
filtered through micro filters with a pinhole size of 0.22 mm, after
then the solutions were spin-coated onto ITO controlled at 2000
rpm and the solvent was removed in a vacuum chamber at 10ꢁ1
Pa and 50 ꢀC for 12 h. The thickness of the polymer nano-layer
was about 100 nm (Fig. 3b). Finally, the top metal electrode was
obtained by a 100 nm thick nanofilm of Al, which was thermally
evaporated at about 10ꢁ6 torr through a shadow mask. The
active area of the fabricated device was 0.126 mm2 (a nummular
point with a radius of 0.2 mm). All electrical measurements of the
device were characterized under ambient conditions, without any
encapsulation, using a HP 4145B semiconductor parameter
analyzer.
Fig. 2 UV-visible absorption spectra of P(Azo)x(St)y with different Azo
moiety content (a) in THF solution and (b) nanofilms spin-coated on ITO
glasses; (c) fluorescence intensity spectra of the nanofilms of P(Azo)x(St)y
withdifferentAzomoietycontentattheexcitationwavelengthlex ¼327nm.
Results and discussion
(s, 1H), 8.43–8.41 (d, 2H), 8.32 (d, 1H), 8.06–8.04 (m, 3H), 7.78
(d, 1H), 7.70 (d, 1H), 7.54 (t, 1H), 7.30 (t, 1H), 5.97 (s, 1H), 5.62
(s, 1H), 4.45 (t, 2H), 4.02 (t, 2H), 2.02 (s, 3H), 1.81 (m, 4H), 1.33–
1.31 (m, 4H). Elemental analysis calculated for C28H28N4O4 (wt
%): C, 69.41; H, 5.82; N, 11.56; found: C, 69.37; H, 5.86; N,
11.60.
The synthetic route and molecular structures of monomers and
copolymers are shown in Scheme 1. The weight percentages of
azobenzene moieties in the copolymers were determined by UV-
vis absorption standard curve. The Mn, PDIs, Azo moiety
content and the onset decomposition temperatures of the
copolymers for different monomer ratios are summarized in
Table 1. The Azo moiety content in the copolymers decreases
from 100% to 26% with the decreasing proportion of Azo
monomer. All the polymers exhibit good thermal sꢀtability, with
an onset decomposition temperature of about 300 C and Tg of
about 120 ꢀC, which promise good heat deterioration endurance
for the material in the memory devices.
Synthesis of P(Azo)x(St)y. The free radical copolymerization of
Azo with styrene was carried out in cyclohexanone solution, a
typical procedure is as follows: styrene (0.52 g, 5 mmol), Azo
(0.242 g, 0.5 mmol), 2,20-azobis(isobutyronitrile) (AIBN, 4.1 mg,
0.025 mmol) and 1 mL cyclohexanone were added to a dry glass
tube underꢀ N2 atmosphere and the reaction temperature was
kept at 90 C. After 20 hours, the mixture was cooled, diluted
with a little tetrahydrofuran and precipitated in a large amount
of methanol. The precipitated product was filtrated and further
purified by extraction with methanol in a Soxhlet apparatus to
remove small molecules to give the red product.
Fig. 2 shows the UV-vis spectra of P(Azo)x(St)y with different
Azo moiety content in THF solution and in solid nanofilms. In
THF, the peak maxima are almost the same for all copolymers
with different Azo content (Fig. 2a), this result indicates that the
polymer backbones are flexible and the arrangement of Azo
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Fig. 3 (a) A schematic diagram of the ITO/polymer/Al nano device; (b) a SEM image of a cross section of ITO/PAzo/Al nano device; (c) current–
voltage (I–V) characteristics of the memory device based on PAzo nanofilm; (d) the effect of retention time on the ON and OFF states of the memory
device under a constant stress of ꢁ1.0 V, and the inset is the effect of read cycles on the ON and OFF state under a read pulse voltage of ꢁ1.0 V.
Fig. 4 HOMO, LUMO and Electrostatic Potential (ESP) surfaces of the
Fig. 5 Experimental and fitted data of I–V curves for the ITO/PAzo/Al
Azo monomer units of P(Azo)x(St)y obtained by molecular simulation.
nano device in the OFF and ON states; (a) is the OFF state with the
Poole–Frenkel model (ꢁ0.19 to ꢁ2.30 V); (b) is the ON state with the
moieties in the side chains are irregular in the solution so that the
Ohmic current model.
steric influence of the side chain is negligible. For nanofilms of
P(Azo)x(St)y on ITO substrate, both the UV-vis spectra and the
fluorescence spectra show the largest absorbance peak or emit-
ting peak red shifted as the Azo moiety content in the copolymer
increased from 26% to 100% (Fig. 2b and c), which indicated that
the functional moieties in the copolymer side-chains stacked
more orderly and more closely as the functional moieties content
increasing.
P(Azo)x(St)y nanofilm, and ITO glass, respectively. Electrical
transitions in the ITO/copolymer/Al devices were investigated in
the measurements of the current regarding to an external applied
voltage. ITO was maintained as the ground electrode in all
electrical measurements. The current–voltage (I–V) characteris-
tics of the ITO/P(Azo)x(St)y/Al nano device in Fig. 3c showed
two distinct conductivity states (herein, Azo content in the
copolymer is 100%). In the first round scan from 0 to ꢁ4 V, the
current increased dramatically at a threshold voltage around
ꢁ2.31 V, indicating a transition from a low-conductivity state
(OFF state) to a high-conductivity state (ON state). The device
remained in this ON state during a subsequent scan from 0 to
Nano memory devices of an ITO/copolymer/Al sandwich
structure were fabricated using nanofilms of the copolymers
mentioned above. The device structure is shown schematically in
Fig. 3a. Fig. 3b is the scanning electronic microscopic (SEM)
image of one storage cell from the direction of a cross-section
view, from the top to bottom is aluminum electrode,
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devices are stable for up to 107 continuous read pulses of ꢁ1 V
without any resistance degradation. The high ON/OFF current
ratios of the devices promise a low misreading rate by precise
control over the OFF and ON states. The retention ability of the
OFF and ON states of a device was investigated under a constant
voltage stress of ꢁ1 V, recording the current passing through the
device at regular intervals. Fig. 3d shows that the device has both
stable OFF and ON state currents within a 300 min time frame,
with a high ON/OFF current ratio sustained. Thus, both states
are insensitive to read pulses and are stable under voltage stress.
Further information about the charge transport mechanism
can be obtained from I–V curves in the OFF and ON states
according to various theoretical models. As shown in Fig. 5a, the
OFF states for the ITO/P(Azo)x(St)y/Al nano device can be
elucidated as Poole–Frenkel (PF)25 emission in terms of the plots
of log(ꢁI/V) versus (ꢁV)1/2, which were found to be linear in a
range from ꢁ0.32 V to ꢁ2.31 V. In order to investigate the reason
why the log(ꢁI/V) and (ꢁV)1/2 didn’t have a strictly linear rela-
tionship in the whole OFF state, calculation of electrostatic
potentials (ESP) for the functional groups of Azo was further
carried out at the B3LYP/6-31G(d) level with the Gaussian03
program package to understand the charge carriers’ migration
through the polymer nanofilm (Fig. 4). We found that carbazole
acted as an electron donor and nitro-azobenzene groups as the
electron acceptor in the functional monomer. The molecular
surface with the continuous positive ESP (Fig. 4) in red areas
along with the side-chain of the copolymer indicates that charge
carriers can migrate through this open channel. However, there
are some negative ESP regions (blue), which arise from the nitro
groups, ester groups and the azo groups. These negative regions
can serve as ‘‘traps’’ to block the mobility of charge carriers.
Thus, the low current level in the OFF-state was due to the PF
emission models and the ‘‘traps’’ lying in the conjugated back-
bone of the side-chains. On the other hand, the currents in the
ON states are almost linearly dependent on the applied voltage,
just as in metallic conduction (Fig. 5b), this indicates that the
charge transport is dominated by the Ohmic model in the ON
states. Therefore, the change of the current conduction from the
OFF-state to the ON-state for both types of devices is attributed
to the PF emission model and ‘‘traps limited’’ model, and to the
Ohmic model.
Fig. 6 Current–voltage (I–V) curves of several nano devices based on
P(Azo)x(St)y nanofilms with different Azo moiety content.
Under a low bias, the ITO/P(Azo)x(St)y/Al nano device is in a
low conductivity state (OFF state) for the unfilled traps. As the
applied voltage increases, the traps are filled by charge carriers
and the device would be activated to the ON-state when the
transition voltage is reached. However, trapped charges are
stabilized by intramolecular charge transfer to form a charge
separated state, which can be illustrated by the molecular simu-
lation (Fig. 4). The HOMO surface represents that the electron
density is primarily located on the electron donor group
(carbazole); upon undergoing the HOMO to LUMO transition,
the electron density obviously underwent a transition from the
carbazole (electron donor) side to the nitro-azobenzene (electron
acceptor) side, so a charge-separated state occurred when the
copolymer film was activated by an electrical field. Therefore the
filled traps could hardly be detrapped and the device showed
WORM type characteristics.23
Fig. 7 (a) A UV-visible absorption spectrum of P(Azo)1(St)5 nanofilm;
(b) cyclic voltammetry (CV) curves of P(Azo)1(St)5 nanofilm on an ITO
substrate in acetonitrile with 0.1 M n-Bu4NPF6 as the supporting elec-
trolyte. The inset is the CV curves of the ferrocene standard, swept in the
same conditions as for P(Azo)1(St)5.
ꢁ4 V (sweep 2). The distinct bi-electrical states in the voltage
range of about 0 to ꢁ2.31 V, where the ON/OFF current ratio is
up to 104, allowed a voltage (e.g., ꢁ1.0 V) to read the ‘‘0’’ or
‘‘OFF’’ signal (before writing) and ‘‘1’’ or ‘‘ON’’ signal (after
writing) of the nano memory device. However, the nano device
also remained in the ON state when scanning from 0 to 4 V
(sweep 3) and could not return to the original OFF state. It is
shown that the nano device fabricated with PAzo exhibits
WORM memory behavior. The ON and OFF states of the
The relationship between the memory performance and the
functional moieties content in the copolymers was investigated
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Table 2 Characterization data of the P(Azo)x(St)y nanofilms
lEdge
(nm)
Eox(onset)
(eV)
a
DEp (eV)
P(Azo)x(St)y
Eg (eV)
HOMO (eV)
LUMO (eV)
DEnb (eV)
PAzo
576
563
554
543
537
2.15
2.20
2.24
2.28
2.31
0.92
0.85
0.79
0.65
0.57
ꢁ5.38
ꢁ5.31
ꢁ5.25
ꢁ5.11
ꢁ5.03
ꢁ3.23
ꢁ3.11
ꢁ3.01
ꢁ2.83
ꢁ2.72
0.58
0.51
0.45
0.31
0.23
1.07
1.19
1.29
1.47
1.58
P(Azo)1(St)1
P(Azo)1(St)5
P(Azo)1(St)10
P(Azo)1(St)20
a
The energy barrier. b The energy barrier between the work function of Al (ꢁ4.3 eV) top electrode and LUMO level of P(Azo)x(St)y nanofilms.
(LUMO) and UV-vis spectra of all copolymer nanofilms were
further investigated (Fig. 7, herein, azobenzene content in the
copolymer is 67%). The HOMO and LUMO energy levels can be
calculated from the onset oxidation potential and UV-vis spectra
of all copolymers, according to the following equations:
HOMO (eV) ¼ ꢁ[(Eox(onset) ꢁ EFoc) + 4.80]
LUMO (eV) ¼ HOMO + Eg
Eg ¼ hc/lEdge
where Eox is the onset oxidation potential, EFoc is the external
standard potential of the ferrocene/ferricenium ion couple, 4.80
is the reference energy level of ferrocene, Eg is the energy
bandgap of the copolymer nanofilms, h is the Planck constant
Fig. 8 (A) DEp (the energy barrier between the HOMO of copolymer
(6.63 ꢂ 10ꢁ34 m2 kg sꢁ1), c is the speed of light (3 ꢂ 108 m sꢁ1
)
nanofilms and the ITO work function) and (B) switch threshold voltages
of several devices based on P(Azo)x(St)y with different Azo moiety
content.
and lEdge is the optical absorbance band edge of the copolymer
nanofilms. Therefore, the lEdge, Eg, Eox(onset), DEp, DEn,
HOMO and LUMO energy levels of the P(Azo)x(St)y nanofilms
were determined and are summarized in Table 2.
It can be seen from Table 2 that the energy barrier between the
work function of the ITO bottom electrode (ꢁ4.8 eV) and the
HOMO level of P(Azo)x(St)y is smaller than that between the
LUMO level of P(Azo)x(St)y and the work function of the Al
(ꢁ4.3 eV) top electrode. The hole injection from ITO into the
HOMO level of P(Azo)x(St)y is more favorable than electron
injection from the Al top electrode into the LUMO level of
P(Azo)x(St)y. Thus, P(Azo)x(St)y is a p-type material and holes
dominate the conduction process. Meanwhile, the measured
HOMO of the corresponding copolymer nanofilm reduced
gradually as the azobenzene moiety content decreased in the
nanofilms, and the energy barrier between the HOMO of the
copolymer film and the ITO work function also reduced, which
illustrated the turn-on voltage showing a linear decline (as
showed in Fig. 8).
Fig. 9 Current–voltage (I–V) curves for the devices fabricated with
PAzo nano-films of various thicknesses (20–80 nm).
The device’s performance based on different film thickness,
such as 20, 40, 60, 80 nm, is also investigated (Fig. 9). The devices
fabricated with 80–40 nm thick films revealed WORM memory
characteristics, as observed for the 100 nm thick films device.
However, the OFF-state current level increases as the film
thickness decreases; the ON-state current level is almost the same
and is independent of the film thickness. The 20 nm thick film
never shows electrical switching behavior and always stays in the
ON state, which maybe arising from the top Al electrode pene-
trating through the whole film during the vacuum deposition
process. Meanwhile, the turn-ON voltage does not change with
(as shown in Fig. 6). The functional monomer Azo contents have
nothing to do with the device storage type, all of the fabricated
devices are WORM type. The switching threshold voltage varied
approximately linearly with the nitrobenzene moiety content in a
range from 100% to 26% in the inter-medium copolymer. In
order to figure out why the switching threshold voltage varied
approximately linearly with the decreasing azobenzene moieties
content, the experimental values of the highest occupied molec-
ular orbital (HOMO) and lowest unoccupied molecular orbital
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In summary, we have demonstrated that uniform P(Azo)x(St)y
nanofilms prepared by simple spin-coating can be successfully
utilized in a novel nonvolatile nano memory device. This device
exhibited WORM behavior with high ON/OFF ratios up to 104.
It was found that the switching threshold voltage varied almost
linearly with the functional moiety content, which was attributed
to the reduced energy barrier between the HOMO of copolymer
nanofilm and the ITO work function. The mechanism of the
WORM behavior was further elucidated from the molecular
simulations of electron density from the HOMO to LUMO
surfaces, and the obviously electron density shifting from donor
to acceptor side indicates that the charge separated state could
remain permanently and wouldn’t revert to its original state even
after applying a reverse voltage scan. Overall, the nano memory
devices fabricated with P(Azo)x(St)y copolymers have potential
applications in the future.
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Acknowledgements
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This work was supported by the Chinese Natural Science
Foundation (NSFC 21076134, 21176164), the Natural Science
Foundation of Jiangsu Province (BK2010208), and the Doctoral
Program of Higher Education of China (Grant no.
20113201130003), and a project funded by the Priority Academic
Program Development (PAPD) of Jiangsu Higher Education
Institutions.
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 19957–19963 | 19963