Preparation of homopolymers from new azobenzene organic molecules with different terminal groups and study of their nonvolatile memory effects

Article abstract of DOI:10.1016/j.polymer.2013.04.043

Two new azo organic molecules Azo-OCH₃ and Azo-Br were synthesized by using electron-donating moiety methoxyphenyl and electron-accepting moiety bromophenyl as a terminal group respectively. Two monomers MAzo-OCH₃ and MAzo-Br based on them were also synthesized and corresponding homopolymers PAzo-OCH₃ and PAzo-Br were prepared by free radical polymerization. Azo-OCH₃ and Azo-Br were fabricated as films by vacuum evaporation while PAzo-OCH₃ and PAzo-Br were fabricated as films by simple spin-coating and all of them were then prepared as sandwich memory devices ITO/Azo-OCH₃/Al, ITO/Azo-Br/Al, ITO/PAzo-OCH₃/Al and ITO/PAzo-Br/Al respectively. According to the measurements, all devices exhibited stable binary WORM-type (write once and read many times) memory effects. However, ITO/Azo-OCH₃/Al and ITO/Azo-Br/Al exhibited different turn-on threshold voltages of about -2, -3.6 V respectively. It illustrated that to organic molecule anchoring electron-donor as a terminal group shows lower turn-on threshold voltage, which was related to low-power consumption. Moreover, the ITO/polymer/Al devices have successfully preserved the memory performance of devices based on corresponding organic molecules. Therefore, we successfully achieved the advantages of low-cost and low-power consumption by designing molecular structures and easy fabrication by polymerization.

Full text of DOI:10.1016/j.polymer.2013.04.043

Polymer 54 (2013) 3324e3333
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Preparation of homopolymers from new azobenzene organic
molecules with different terminal groups and study of their
nonvolatile memory effects
,
,
,
,
,
,b
Fei-Long Ye a b, Pei-Yang Gu a b, Feng Zhou a b, Hai-Feng Liu a b, Xiao-Ping Xu a b, Hua Li a
,
Qing-Feng Xu a , Jian-Mei Lu
,
b,
a,b,
*
*
a Department of Polymer, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren’ai Road, Suzhou 215123, China
b Key Laboratory of Adsorption Technology in Petroleum and Chemical Industry for Wastewater Treatments, Soochow University, 199 Ren’ai Road,
Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new azo organic molecules Azo-OCH3 and Azo-Br were synthesized by using electron-donating
moiety methoxyphenyl and electron-accepting moiety bromophenyl as a terminal group respectively.
Two monomers MAzo-OCH3 and MAzo-Br based on them were also synthesized and corresponding ho-
mopolymers PAzo-OCH3 and PAzo-Br were prepared by free radical polymerization. Azo-OCH3 and Azo-Br
were fabricated as films by vacuum evaporation while PAzo-OCH3 and PAzo-Br were fabricated as films by
simple spin-coating and all of them were then prepared as sandwich memory devices ITO/Azo-OCH3/Al,
ITO/Azo-Br/Al, ITO/PAzo-OCH3/Al and ITO/PAzo-Br/Al respectively. According to the measurements, all
devices exhibited stable binary WORM-type (write once and read many times) memory effects. However,
ITO/Azo-OCH3/Al and ITO/Azo-Br/Al exhibited different turn-on threshold voltages of about ꢀ2, ꢀ3.6 V
respectively. It illustrated that to organic molecule anchoring electron-donor as a terminal group shows
lower turn-on threshold voltage, which was related to low-power consumption. Moreover, the ITO/poly-
mer/Al devices have successfully preserved the memory performance of devices based on corresponding
organic molecules. Therefore, we successfully achieved the advantages of low-cost and low-power con-
sumption by designing molecular structures and easy fabrication by polymerization.
Received 5 January 2013
Received in revised form
12 April 2013
Accepted 17 April 2013
Available online 25 April 2013
Keywords:
Azo organic molecules
Homopolymers
Terminal group
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction
In comparison with organic molecule, polymer can be fabri-
cated on the substrate by using a simple spin-coating technique
In recent years, a great deal of effort has been devoted to
developing fast, nonvolatile, and inexpensive techniques for data
storage [1]. Among various memory devices, nonvolatile memory
devices have been attracting considerable attention due to the low
fabrication cost and 3D stacking to achieve low-cost but high-
density memory [2e8]. Electroactive organic and polymeric ma-
terials are alternatives to traditional inorganic semiconductors that
have to face the problem of scaling down in feature size [9e15].
Organic materials have the advantages of facile tailoring through
organic synthesis and very low power consumption [16]. Thus,
organic electrical memory devices appear highly attractive and
have great potential application in data storage.
or surface-initiated ATRP method [17,18]. The advantages of easy
fabrication, high mechanical behavior and good scalability of
polymers and polymer composites have attracted considerable
attention in preparation of polymer-based devices and polymer
composites-based devices [19e22], such as poly(9-(2-(4-
vinyl(benzyloxy)ethyl)-9H-carbazole)) brushes and poly(N-
vinylcarbazole)-graphene composites [18,23]. Among the re-
ported polymers, flexible polymers pendent with functional
groups have the obvious advantages for their rich diversities of
structures, various polymerization methods. For example, the
Kang group reported two poly(N-vinylcarbazole) copolymers
with pendent azobenzene chromophores PVK-AZO-NO2 and PVK-
AZO-2CN. The terminal electron acceptor moieties of the two
copolymers are strong electron-withdrawing groups, eNO2 and e
CN respectively. Both of them exhibit write-once-read-many-
times (WORM) type memory effects owing to the deep electron
traps caused by the strong electron acceptors [24]. In addition, it’s
worthy of mention that quite a few organic molecules and
* Corresponding authors. Department of Polymer, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, 199 Ren’ai Road, Suzhou
215123, China. Tel./fax: þ86 512 6588 0367.
E-mail addresses: xuqingfeng@suda.edu.cn (Q.-F. Xu), lujm@suda.edu.cn
(J.-M. Lu).
0032-3861/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.043

F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
3325
polymers have been considered as active materials for memory
devices [25,26]. However, few reports have focused on the dif-
ferences and connections between organic molecules-based
memory devices and corresponding polymers-based memory
devices.
nitrite and other solvents were used as received without any further
purification.
2.2. Preparation of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br
In this paper, we synthesized two new azo organic molecules
Azo-OCH3 and Azo-Br firstly. Azo-OCH3 and Azo-Br have longer
conjugation length than most of other reported organic molecules
by introducing azobenzene and acrylonitrile groups. The long
conjugation length of Azo-OCH3 and Azo-Br was designed for the
stable memory property. The WORM memory effects can be pre-
dicted due to the strong electron-withdrawing ability of cyano
group. Meanwhile, we introduced donating and accepting elec-
tronic moieties to the end of azo organic molecules respectively for
studying the relationship between electrical memory behavior and
terminal electron moieties. In addition, the corresponding homo-
polymers (PAzo-OCH3 and PAzo-Br) were prepared for easier device
fabrication. Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br all exhibit
excellent WORM memory behavior. In polymer-based devices, the
memory effects were well-preserved. Interestingly, both Azo-OCH3
and PAzo-OCH3 based devices have an advantage of possessing low
turn-on threshold voltage due to the effective electron-donor ter-
minal group.
Synthesis scheme and molecular structures of Azo-OCH3, Azo-
Br, PAzo-OCH3 and PAzo-Br are shown in Scheme 1.
2.2.1. Synthesis of 4-((4-(ethyl(2-hydroxyethyl)amino)phenyl)
diazenyl)benzaldehyde (compound 1)
A solution of sodium nitrite (4.35 g, 0.063 mol) in water (24 mL)
was added dropwise to the mixture of 4-aminobenzaldehyde
polymer (6.03 g, 0.06 mol), water (32 mL) and concentrated hy-
drochloric (20 mL) at 0e5 ꢁC. The mixture was stirred at 0e5 ꢁC
for 30 min. The mixture of N-ethyl-N-(2-hydroxyethyl)aniline
(10.89 g, 0.066 mol), concentrated hydrochloric (10 mL) and water
(30 mL) was added slowly to the diazonium salt solution at 0e5 ꢁC.
After 1 h, sodium acetate anhydrous (18 g, 0.22 mol) was added to
the resultant mixture. The resultant mixture was stirred at 0e5 ꢁC
for 24 h. The solution was filtered, and the obtained crude product
was recrystallized from ethanol to give the brown powder com-
pound 1 (yield 68%). 1H NMR (300 MHz, DMSO-d6)
d (ppm): 10.05
(s, 1H), 8.04 (d, J ¼ 8.4 Hz, 2H), 7.91 (d, J ¼ 8.3 Hz, 2H), 7.81 (d,
J ¼ 9.1 Hz, 2H), 6.86 (d, J ¼ 9.2 Hz, 2H), 4.85 (t, J ¼ 5.2 Hz, 1H),
3.67e3.42 (m, 6H), 1.15 (t, J ¼ 6.9 Hz, 3H). Anal. calcd. for
C17H19N3O2: C, 68.61; H, 6.39; N, 14.12. Found: C, 68.52; H, 6.52;
N, 13.98.
2. Experimental section
2.1. Materials
4-Aminobenzaldehyde Polymer, 3,4-dimethoxyphenylacetonitrile,
4-bromobenzyl cyanide, sodium hydride, AIBN were all purchased
from Shanghai Chemical Reagent Co. Ltd. as analytical reagents and
used as received. N-Ethyl-N-(2-hydroxyethyl)aniline was purchased
from Tokyo Kasei Kogyo Co. Ltd. and used as received. Methacryloyl
chloride was obtained from Haimen Best Fine Chemical Industry Co.
Ltd. and used after distillation. Sodium acetate anhydrous, sodium
2.2.2. Synthesis of 2-(3,4-dimethoxyphenyl)-3-(4-(4-(ethyl(2-
hydroxyethyl)amino)phenyl diazen-yl)phenyl)acrylonitrile (Azo-
OCH3)
A
mixture of compound 1 (7.65 g, 0.0258 mol), 3,4-
dimethoxyphenylacetonitrile (4.64 g, 0.0262 mol) and sodium hy-
dride (312 mg, 0.013 mol) and anhydrous ethanol (100 mL) was
refluxed at 84 ꢁC for 12 h under N2 protection. The reaction mixture
Scheme 1. Synthesis scheme and molecular structures of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br.

3326
F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
was cooled down and poured into 300 mL deionized water. The
crude product was filtered off and recrystallized from ethanol to
give the red powder Azo-OCH3 (yield 82%). 1H NMR (300 MHz,
polymers were purified by Soxhlet extractor with ethanol to
remove of excess monomers and get the azo polymers PAzo-OCH3
(Fig. S1, Supplementary Material) and PAzo-Br (Fig. S2, Supple-
mentary Material) (yield 78%).
DMSO-d6)
d
(ppm): 8.11e8.00 (m, 3H), 7.88 (d, J ¼ 8.6 Hz, 2H), 7.80
(d, J ¼ 9.0 Hz, 2H), 7.38 (s, 1H), 7.30 (d, J ¼ 8.4 Hz, 1H), 7.10 (d,
J ¼ 8.6 Hz, 1H), 6.86 (d, J ¼ 9.2 Hz, 2H), 4.85 (s, 1H), 3.87 (s, 3H), 3.82
(s, 3H), 3.61 (s, 2H), 3.55e3.46 (m, 4H), 1.15 (t, J ¼ 6.9 Hz, 3H). Anal.
calcd. for C27H28N4O3: C, 70.97; H, 6.13; N, 12.27. Found: C, 70.89; H,
6.15; N, 12.32.
2.3. Characterization
1H NMR spectra were obtained on an Inova 300 MHz FT-NMR
spectrometer at ambient temperature. Molecular weights (Mn)
and polydispersity (Mw/Mn) were measured by gel permeation
chromatography (GPC) utilizing a Waters 515 pump and a differ-
ential refractometer. DMF was used as a mobile phase at a flow rate
of 1.0 mL/min. Thermogravimetric analysis (TGA) was conducted on
a TA Instruments Dynamic TGA 2950 at a heating rate of 10 ꢁC/min
and under an N2 flow rate of 50 mL/min. Ultravioletevisible (UVe
vis) absorption spectra were recorded on a PerkineElmer Lambda
spectrophotometer. Electrochemical properties of azo organic
molecules (Azo-OCH3 and Azo-Br) were investigated by using
conducting cyclic voltammetry measurements in DMF solution
(10ꢀ3 mol/L). A 0.1 M solution of tetrabutylammonium hexa-
fluorophosphate in anhydrous DMF was used as an electrolyte.
Cyclic voltammetry was performed at room temperature using a
platinum wire electrode as the working electrode, a platinum gauze
electrode as a counter electrode and an Ag/AgCl electrode as a
reference electrode at a sweep rate of 1 mV sꢀ1. The electrochemical
properties of azo polymers (PAzo-OCH3 and PAzo-Br) were inves-
tigated by using conducting cyclic voltammetry measurements in
acetonitrile solution. A 0.1 M solution of tetrabutylammonium
hexafluorophosphate in anhydrous acetonitrile was used as an
electrolyte. Cyclic voltammetry was performed at room tempera-
ture using an ITO electrode as the working electrode, a platinum
gauze electrode as a counter electrode and an Ag/AgCl electrode as
a reference electrode at a sweep rate of 1 mV sꢀ1 (CHI 604C elec-
trochemical analyzer). Atomic force microscopy (AFM) measure-
ments were obtained with a NanoScope 3D Controller atomic force
microscope (Digital Instruments) operated in the tapping mode at
room temperature.
2.2.3. Synthesis of 2-(4-bromophenyl)-3-(4-(4-(ethyl(2-
hydroxyethyl)amino)phenyl diazenyl) phenyl)acrylonitrile (Azo-Br)
A mixture of compound 1 (7.65 g, 0.0258 mol), 4-bromobenzyl
cyanide (5.08 g, 0.0262 mol) and sodium hydride (312 mg,
0.013 mol) and anhydrous ethanol (100 mL) was refluxed at 84 ꢁC
for 12 h under N2 protection. The reaction mixture was cooled
down, then evaporated the excess solvent. The crude product was
purified by column chromatography (petroleum ether:ethyl acetate
1:1, v/v) to give the red powder Azo-Br (yield 64%). 1H NMR
(300 MHz, DMSO-d6)
d
(ppm): 8.16 (s, 1H), 8.10 (d, J ¼ 8.6 Hz, 2H),
7.89 (d, J ¼ 8.5 Hz, 2H), 7.85e7.68 (m, 6H), 6.86 (d, J ¼ 9.2 Hz, 2H),
4.84 (s, 1H), 3.67e3.41 (m, 6H), 1.15 (t, J ¼ 6.9 Hz, 3H). Anal. calcd.
for C25H23BrN4O: C, 63.11; H, 4.84; N, 11.78. Found: C, 63.09; H,
5.00; N, 11.43.
2.2.4. Synthesis of 2-((4-(4-(2-cyano-2-(3,4-dimethoxyphenyl)
vinyl)phenyl diazenyl)phenyl)ethyl-amino)ethyl methacrylate
(MAzo-OCH3)
Azo-OCH3 (0.0025 mol) and Et3N (2.0 g, 0.0075 mol) were added
into freshly distilled tetrahydrofuran (THF; 40 mL) and stirred
vigorously at 0 ꢁC under nitrogen atmosphere. The acryloyl chloride
(0.005 mol) solution in THF (20 mL) was added dropwise to the
mixture. After 1 h, the ice-water bath was removed and the reaction
was allowed to continue for 24 h at room temperature. The solution
was filtered and poured into a large amount of water. The precip-
itated product was washed by column chromatography (petroleum
ether:ethyl acetate 1:1, v/v) to give the red powder MAzo-OCH3
(yield 72%). 1H NMR (300 MHz, DMSO-d6)
d (ppm): 8.13e8.00 (m,
3H), 7.90 (d, J ¼ 8.4 Hz, 2H), 7.81 (d, J ¼ 9.0 Hz, 2H), 7.39 (s, 1H), 7.31
(d, J ¼ 8.4 Hz, 1H), 7.10 (d, J ¼ 8.5 Hz, 1H), 6.93 (d, J ¼ 9.1 Hz, 2H),
6.02 (s, 1H), 5.69 (s, 1H), 4.32 (s, 2H), 3.88 (s, 3H), 3.82 (s, 3H), 3.77
(s, 2H), 3.54 (d, J ¼ 6.9 Hz, 2H), 1.86 (s, 3H), 1.16 (t, J ¼ 6.8 Hz, 3H).
Anal. calcd. for C31H32N4O4: C, 70.91; H, 6.10; N, 10.67. Found: C,
70.85; H, 6.15; N, 10.56.
2.4. Fabrication of the organic molecule memory device
The indium tin oxide (ITO) glass was precleaned with water,
acetone, and isopropanol each for 15 min. The organic thin films
were vacuum evaporated in a vacuum deposition apparatus, take
about 20 mg of organic compounds on the evaporator furnace and
drawn about the degree of vacuum to 3 ꢂ 10ꢀ4 Pa. The organic
compounds in the evaporator furnace were slowly sublimated to
the ITO glass surface. The organic molecule films were annealed in a
vacuum chamber at 10ꢀ5 torr and 80 ꢁC for 12 h. The organic
molecule film thickness was around 100 nm (Fig. S3, Supplemen-
tary Material).
2.2.5. Synthesis of 2-((4-(4-(2-cyano-2-(4-bromophenyl)vinyl)
phenyl diazenyl)phenyl)ethylami-no)ethyl methacrylate (MAzo-Br)
The synthesis method of MAzo-Br is similar with MAzo-OCH3.
(Yield 68%). 1H NMR (300 MHz, DMSO-d6)
d (ppm): 8.17 (s, 1H), 8.11
(d, J ¼ 8.5 Hz, 2H), 7.90 (d, J ¼ 8.5 Hz, 2H), 7.86e7.68 (m, 6H), 6.93 (d,
J ¼ 9.1 Hz, 2H), 6.02 (s, 1H), 5.68 (s, 1H), 4.32 (s, 2H), 3.77 (s, 2H),
3.54 (d, J ¼ 6.8 Hz, 2H), 1.86 (s, 3H), 1.16 (t, J ¼ 6.8 Hz, 3H). Anal.
calcd. for C29H27BrN4O2: C, 64.03; H, 4.97; N, 10.30. Found: C, 63.94;
H, 5.05; N, 10.24.
2.5. Fabrication of the polymer memory device
The azo polymer solution was prepared in 1,2-dichloroethane
(10 mg/mL) and filtered through microfilters with a pinhole size of
2.2.6. Synthesis of poly 2-((4-(4-(2-cyano-2-(3,4-dimetho-
xyphenyl)vinyl)phenyl diazenyl)phenyl) ethylamino)ethyl
methacrylate (PAzo-OCH3) and poly 2-((4-(4-(2-cyano-2-(4-
bromophenyl) vinyl)phenyl diazenyl)phenyl)ethylamino)ethyl
methacrylate (PAzo-Br)
Monomer (1 mmol) was polymerized via free-radical polymer-
ization in the presence of AIBN (0.1 mmol) initiator. The polymer-
ization was carried out at 60 ꢁC in cyclohexanone (5 mL) and under
a nitrogen atmosphere for 48 h. The crude polymers were precip-
itated in 200 mL of methanol under vigorous stirring. The crude
0.22 mm. Then, the filtered solution was spin-coated onto the pre-
cleaned ITO glass at a speed rate of 2000 rpm for 40 s and the
solvent was removed in a vacuum chamber at 10ꢀ5 torr and 60 ꢁC
for 12 h. The polymer film thickness was around 100 nm (Fig. S3,
Supplementary Material). Al top electrodes were thermally evap-
orated and deposited onto the polymer surface at about 10ꢀ7 torr
through a shadow mask. The currentevoltage (IeV) characteristics
were performed using an HP 4145B semiconductor parametric
analyzer. All electrical measurements of the devices were charac-
terized under ambient conditions.

F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
3327
3. Results and discussion
3.4. Electrochemical properties
3.1. Polymer characterization
The electrochemical properties of azo organic molecules (Azo-
OCH3 and Azo-Br) and azo polymers (PAzo-OCH3 and PAzo-Br)
were investigated by using conducting cyclic voltammetry mea-
surements, as shown in Fig. 2 and summarized in Table 1.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels can be calcu-
lated from the UV visible absorption spectra and the cyclic vol-
tammetry (CV) results by the following equations [30]:
Synthesis scheme and molecular structures of PAzo-OCH3 and
PAzo-Br are shown in Scheme 1. GPC showed that the number-
average molecular weights of PAzo-OCH3 and PAzo-Br were about
8030 with a polydispersity index (PDI) of 1.59 and 8120 with PDI of
1.64 respectively.
3.2. Thermal stabilities
EHOMO ¼ ꢀ½Eox þ 4:8 ꢀ EFocꢃ
ELUMO ¼ EHOMO þ Eg
The thermal properties of the azo organic molecules and the azo
polymers were evaluated by TGA under nitrogen atmosphere
(Fig. S4, Supplementary Material). The 5% weight-lost temperature
(Td5%) of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br was found to
be 274 ꢁC, 302 ꢁC, 295 ꢁC and 292 ꢁC respectively. The good thermal
stability of the azo organic molecules and the azo polymers is
essential for their applications as active materials in electronic
devices [27]. Glass transition temperatures of azo polymers were
evaluated by DSC under nitrogen atmosphere. The glass transition
temperatures (Tg) of PAzo-OCH3 and PAzo-Br were found to be
103 ꢁC and 108 ꢁC respectively.
where Eox is the onset oxidation potential, EFoc is the external
standard potential of the ferrocene/ferrocenium ion couple and Eg
is the band gap determined from the UVevisible absorption. The
optical band gaps of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br
estimated from the absorption edges are 2.25, 2.21, 2.19 and
2.18 eV, respectively. The external ferrocene/ferrocenium (Fc/Fcþ)
redox standard potential (EFoc1) was measured to be 0.55 eV in
DMF. The onset oxidation (Eox) estimated from the cyclic voltam-
metry (CV) curve for the thin films of Azo-OCH3 and Azo-Br is 1.03
and 1.20 eV respectively. The EFoc2 is 0.43 eV from the CV mea-
surement without any polymer film coated on ITO glass. The onset
oxidation (Eox) estimated from the cyclic voltammetry (CV) curve
for thin films of PAzo-OCH3 and PAzo-Br is 0.92 and 1.18 eV
respectively. The highest occupied molecular orbital (HOMO) en-
ergy levels of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br are
determined to be ꢀ5.28, ꢀ5.45, ꢀ5.29 and ꢀ5.55 eV, respectively.
The lowest unoccupied molecular orbital (LUMO) energy levels of
Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br are determined to
be ꢀ3.03, ꢀ3.24, ꢀ3.10 and ꢀ3.37 eV, respectively. Optoelectronic
properties and calculated energy levels of all compounds are
summarized in Table 1.
The energy barrier to hole injection from the ITO electrode to
the HOMO of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br was
estimated to be 0.48, 0.65, 0.49 and 0.75 eV, respectively. The en-
ergy barrier to electron injection from the Al electrode to the LUMO
of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br was estimated to be
1.25, 1.04, 1.18 and 0.91 eV, respectively.
It’s clearly that hole injection from ITO into the HOMO is easier
than electron injection from Al into the LUMO. Hence, Azo-OCH3,
Azo-Br, PAzo-OCH3 and PAzo-Br are all p-type materials and the
conduction process in these devices is dominated by hole injection
as well [31]. The results indicate that ITO/Azo-OCH3/Al device and
3.3. Optical properties
The absorption spectra of Azo-OCH3, Azo-Br, PAzo-OCH3 and
PAzo-Br in THF solution and thin film state are shown in Fig. 1 and
summarized in Table 1 respectively. All of them exhibit two major
absorption peaks according to Fig. 1. The absorption peak of azo
organic molecules at the shorter wavelength (around 350 nm) is
attributed to the
while the peak at the longer wavelength (around 480 nm) is due to
the coupling between the ne * and * transitions of the azo-
pep* electronic transition of the aromatic ring,
p
pep
benzene chromophore and acrylonitrile chromophore [24,28].
Absorption peaks are red-shifted when electron withdrawing
ability of the terminal substituent increased, which might be
attributed to the increased electron transition ability and the
increased electron delocalization. Compared to those of azo organic
molecules, the absorption bands of azo polymers are evidently
blue-shifted (459 nm for PAzo-OCH3 and 463 nm for PAzo-Br),
which is ascribed to the formation of flexible main chain. In addi-
tion, compared with the absorption spectra in solution, both the
pe
p
*
transition and the CT transition absorption peaks are
bathochromic-shifted and broadened in film state, ascribed to the
formation of molecular aggregation [29].
Fig. 1. (a) UVevis absorption spectra of Azo-OCH3, Azo-Br in dilute THF solution and solid thin film on ITO substrate; (b) UVevis absorption spectra of PAzo-OCH3, PAzo-Br in dilute
THF solution and solid thin film on ITO substrate.

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F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
Table 1
Optoelectronic properties and calculated energy levels of all compounds.
Compound
UVevis, lmax (nm)
Eg (eV)
E (V)
HOMO (eV)
Calc
LUMO (eV)
Calc
Solna
Film
Calc
Expb
Eox (onset)
Expc
Exp
Azo-OCH3
Azo-Br
PAzo-OCH3
PAzo-Br
474
477
459
463
484
487
461
467
2.92
2.86
2.91
2.86
2.25
2.21
2.19
2.18
1.03
1.20
0.92
1.18
ꢀ5.21
ꢀ5.33
ꢀ5.22
ꢀ5.34
ꢀ5.28
ꢀ5.45
ꢀ5.29
ꢀ5.55
ꢀ2.29
ꢀ2.47
ꢀ2.31
ꢀ2.48
ꢀ3.03
ꢀ3.24
ꢀ3.10
ꢀ3.37
a
In THF dilute solution.
b
c
Estimated from the absorption edge of the film.
The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV).
ITO/PAzo-OCH3/Al device have lower hole-injection barrier than
ITO/Azo-Br/Al device and ITO/PAzo-Br/Al device.
turn-on voltage of memory devices by adopting the electron
donating terminal moieties.
The memory behavior of PAzo-OCH3 and PAzo-Br was tested by
the IeV characteristics of ITO/polymer/Al devices. Fig. 4 shows
typical IeV characteristics of the memory devices fabricated with
azo polymers (PAzo-OCH3 and PAzo-Br). The ITO/polymer/Al de-
vices were measured under the same conditions with ITO/organic
molecule/Al devices. ITO/PAzo-OCH3/Al device has turn-on
threshold voltage of about ꢀ2.4 V and ON/OFF ratio of about
103e104 while ITO/PAzo-Br/Al device has turn-on threshold voltage
of about ꢀ4 V and ON/OFF ratio of about 105.
3.5. Electrical switching behavior of ITO/organic molecule/Al
memory devices and ITO/polymer/Al memory devices
The memory behavior of Azo-OCH3 and Azo-Br was tested by
the IeV characteristics of ITO/organic molecule/Al devices. Organic
molecule memory devices store data based on the high- and low-
conductivity response to the external applied voltages. Fig. 3
shows typical IeV characteristics of the memory devices fabri-
cated with azo organic molecules (Azo-OCH3 and Azo-Br).
The IeV curves suggested that the ITO/polymer/Al devices could
provide similar turn-on voltage and ON/OFF ratio with corre-
sponding ITO/organic molecule/Al devices. Hence, the ITO/poly-
mer/Al devices maintained the data storage performance of the
ITO/organic molecule/Al devices. Since organic molecule devices
require more elaborate and expensive processing such as vacuum
evaporation and deposition, it needs high fabrication cost to
fabricate organic molecules into memory devices. Compared to
organic molecule-based devices (ITO/Azo-OCH3/Al and ITO/Azo-Br/
Al), polymer-based devices (ITO/PAzo-OCH3/Al and ITO/PAzo-Br/Al)
not only have the advantages of easy fabrication, high mechanical
flexibility and good scalability, but also their properties can easily
be tailored through chemical synthesis. Thus, from the perspective
of device fabrication to consider, polymer-based devices (ITO/PAzo-
OCH3/Al and ITO/PAzo-Br/Al) are more suitable as nonvolatile
devices.
For ITO/Azo-OCH3/Al, the first sweep was from 0 to ꢀ6 V. In the
sweep from 0 to ꢀ2 V, the device was in the low-conductivity (OFF)
state. When the sweep voltage switching threshold voltage of ꢀ2 V
was reached, the current density increased abruptly from 10ꢀ6 to
10ꢀ2 A, indicating the device transition from the initial OFF state to
the high-conductivity (ON) state. This electrical transition can serve
as the “writing” process in a memory device. The device remained
in the ON state during the subsequent negative sweep (0 to ꢀ6 V)
and positive sweep (0e6 V), and it did not return to the OFF state
even after turning off the power or upon applying a reverse sweep.
The nonvolatile of the ON state suggests that ITO/Azo-OCH3/Al
device exhibits WORM type memory. ITO/Azo-Br/Al device was
measured under the same conditions and a similar phenomenon
was observed when voltage was added. More interestingly, the
turn-on threshold voltages of two devices are different about ꢀ2 V
for Azo-OCH3, ꢀ3.6 V for Azo-Br and ON/OFF ratio is about 103e104
(Azo-OCH3), 105 (Azo-Br) respectively. The results may indicate that
ITO/Azo-OCH3/Al device could provide a lower turn-on voltage,
which means a low-energy cost write process.
For considering the effect of the film thickness on the device
performance, we studied the data storage performance of the ITO/
polymer (70 nm)/Al devices (Fig. S5, Supplementary Material). The
results suggested that ITO/PAzo-OCH3 (70 nm)/Al, ITO/PAzo-Br
(70 nm)/Al exhibited turn-on threshold voltages of
Because the conduction process in these devices is dominated
by hole injection, ITO/Azo-OCH3/Al device has lower hole-injection
barrier, thus holes can very efficiently be injected into the HOMO
orbital of Azo-OCH3. Different HOMO energy levels affect the turn-
on threshold, while the higher HOMO level of Azo-OCH3 resulted
into a lower turn-on voltage [32]. Therefore, we can decrease the
about ꢀ2.3, ꢀ3.5
V respectively. Compared to ITO/polymer
(100 nm)/Al, the turn-on threshold voltages of ITO/polymer
(70 nm)/Al slightly decreased (Fig. S6, Supplementary Material).
Thus, the memory effect was almost kept unchanged when the
thickness of film was decreased from 100 nm to 70 nm.
Fig. 2. (a) Cyclic voltammetry (CV) curves of Azo-OCH3 and Azo-Br in DMF solution (10ꢀ3 mol/L) with 0.1 M n-Bu4NPF6 as the supporting electrolyte; (b) cyclic voltammetry (CV)
curves of PAzo-OCH3 and PAzo-Br film on ITO substrate in acetonitrile solution with 0.1 M n-Bu4NPF6 as the supporting electrolyte.

F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
3329
[33,34]. The small surface roughness in the AFM images of the
polymer films suggests the good quality films [32,35]. Compared to
the vacuum evaporated thin films of Azo-OCH3 and Azo-Br, the
spin-coated thin films of PAzo-OCH3 and PAzo-Br are more favor-
able in fabricating as memory devices.
3.7. Proposed storage mechanism
The HOMO, LUMO and molecular ESP surfaces of Azo-OCH3,
Azo-Br, PAzo-OCH3 and PAzo-Br are shown in Table 2. The HOMO
surface represents the localization of electron density on the elec-
tron donor in the ground state and the LUMO surface represents the
localization of electron density on the electron acceptor in the
excited state [36]. Through analysis of the electron density change
from the HOMO to LUMO surfaces, we can judge how the charge-
separated state will exist after the electric field is applied [37].
The results of theoretical calculation indicate that the HOMO of
Azo-OCH3 and PAzo-OCH3 is located on the ethylamino and
methoxyphenyl electron donor moieties, whereas the LUMO of
them is located on the azobenzene, cyano electron acceptor moi-
eties; the HOMO of Azo-Br and PAzo-Br is located on the ethyl-
amino electron donor moieties, whereas the LUMO of Azo-Br and
PAzo-Br is located on the azobenzene, cyano and bromophenyl
electron acceptor moieties. When electron density concentrated on
the HOMO orbital, the memory devices are in the OFF state. Under
excitations with sufficient energy, electrons in the ground state can
transit to the various excited states and a charge-transfer (CT)
interaction can occur between the electron donor moieties and the
electron acceptor moieties. At this stage, the majority of the charge
traps (azobenzene and cyano electron acceptor moieties) are filled,
and a “trap-free” environment with higher charge-carrier mobility
is formed [38], then a stable CT state is formed. The memory devices
are therefore transited from the OFF state to the ON state. This
indicates that the electron transfer subsequent to the excitation of
the electron donor moieties leads to the intra or intermolecular CT
state [24,38e40]. The theoretical high dipole moment of Azo-OCH3
(9.04 D), Azo-Br (11.33 D), PAzo-OCH3 (8.24 D) and PAzo-Br (12.11
D) probably leads to a more stable CT complex [37,38,41]. Mean-
while, from the ESP surfaces of Azo-OCH3, Azo-Br, PAzo-OCH3 and
PAzo-Br, it can be seen that there are some negative regions. These
negative regions can serve as “traps”. However, the traps in them
are deep and the filled traps cannot be detrapped easily due to the
strong electron-withdrawing abilities of the azobenzene and cyano
electron acceptor moieties. Thus, once a high-conductivity state is
formed, the memory devices will not return to the low-
conductivity state [42]. Thus the ITO/organic molecule/Al devices
and the ITO/polymer/Al devices all exhibit WORM type memory.
The results were similar with the reported devices of PVK-AZO-
NO2 and PVK-AZO-2CN [24,38], in which electron-withdrawing
groups lead to a stable charge-transfer (CT) complex and result in
a nonvolatile memory performance. Compared to PVK-AZO-NO2
and PVK-AZO-2CN, we introduced the different terminal group
methoxyphenyl electron donor and bromophenyl electron acceptor
to the end of azo organic molecules and azo polymers respectively.
Since the contribution of the terminal electron groups with the
charge-transfer (CT) process is opposite, Azo-OCH3 and Azo-Br
exhibit different memory performance in some aspects, such as
switching voltage and ON/OFF ratio. PAzo-OCH3 and PAzo-Br also
preserve the differences.
Fig. 3. Currentevoltage (IeV) characteristics of the azo organic molecule memory
devices.
The stabilities of all these devices under a constant stress of ꢀ1 V
are shown in Figs. 5 and 6. Under a constant stress of ꢀ1 V, no
obvious degradation in current is observed for both ON and OFF
states for at least 100 min during the readout test. These devices are
stable for at least 108 continuous read pulses of ꢀ1 V, as shown in
the inset of Figs. 5 and 6. These results indicate that these polymer
memory devices all exhibit good stability.
3.6. Morphology of films
The thin film surface image and roughness were examined by
atomic force microscopy at room temperature in a tapping mode, as
shown in Fig. 7. The film surface height images of Azo-OCH3, Azo-Br,
PAzo-OCH3 and PAzo-Br were examined by using atomic force
microscopy (AFM) after 12 h of thermal annealing at 110 ꢁC, the
scan size of the images is 1 ꢂ 1
m
m2.
The surface morphology of the vacuum evaporated thin films of
Azo-OCH3 and Azo-Br according to AFM scanning showed surface
with root-mean-square roughness of 1.01 and 0.79 nm, respec-
tively; the surface morphology of the spin-coated thin films of
PAzo-OCH3 and PAzo-Br according to AFM scanning showed sur-
face with root-mean-square roughness of 0.35 and 0.30 nm,
respectively. The results suggest that the azo polymer films could
provide smoother surface than the azo organic molecule films
3.8. Charge transport mechanism
Further information about the charge transport mechanism of
ITO/Azo-OCH3/Al device, ITO/Azo-Br/Al device, ITO/PAzo-OCH3/Al
device and ITO/PAzo-Br/Al device can be obtained from the IeV
Fig. 4. Currentevoltage (IeV) characteristics of the azo polymer memory devices.

3330
F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
Fig. 5. Retention times on the ON and OFF states of (a) ITO/Azo-OCH3/Al device and (b) ITO/Azo-Br/Al device under a constant stress of ꢀ1 V; effect of read pulse of ꢀ1 V on the ON
and OFF states of (c) ITO/Azo-OCH3/Al device and (d) ITO/Azo-Br/Al device.
curves in OFF and ON states according to various theoretical
models. Figs. 8a and 9a show that the IeV characteristics for ITO/
Azo-OCH3/Al device and ITO/PAzo-OCH3/Al device of the low
conductivity state can be fitted with the form J f 9εimV2/8d3,
where m is the mobility of charge carriers and εi is the dynamic
permittivity of the insulator and d is the thickness of film. Thus
the low conductivity state of ITO/Azo-OCH3/Al device and ITO/
PAzo-OCH3/Al device can be fitted with the space charge limited
Fig. 6. Retention times on the ON and OFF states of (a) ITO/PAzo-OCH3/Al device and (b) ITO/PAzo-Br/Al device under a constant stress of ꢀ1 V; effect of read pulse of ꢀ1 V on the
ON and OFF states of (c) ITO/PAzo-OCH3/Al device and (d) ITO/PAzo-Br/Al device.

F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
3331
Fig. 7. AFM height images of (a) Azo-OCH3 (b) Azo-Br (c) PAzo-OCH3 (d) PAzo-Br, the scan size of the images is 1 ꢂ 1
m
m2.
Table 2
HOMO, LUMO and ESP surfaces of Azo-OCH3, Azo-Br, PAzo-OCH3 and PAzo-Br.
Molecular structure HOMO
LUMO
ESP

3332
F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
Fig. 8. Experimental and fitted data of IeV curves for ITO/Azo-OCH3/Al device and ITO/Azo-Br/Al device in the OFF and ON states. Panels a and b are the OFF state with the SCLC
model and the ON state with the ohmic current model for ITO/Azo-OCH3/Al device. Panels c and d are the OFF state with the Poole Frenkel model (ꢀ0.49 to ꢀ3.6 V) and the ON state
with the ohmic current model for ITO/Azo-Br/Al device.
current (SCLC) model [43]. Figs. 8c and 9c show that the low
conductivity state of ITO/Azo-Br/Al device and ITO/PAzo-Br/Al
device can be elucidated as PooleeFrenkel (PF) emission in
terms of the plots of log(I/V) versus V1/2 found to be linear [44].
The PF emission is probably attributed to charge transport of
organic and polymeric materials filled with charge traps [34]. The
electron-accepting terminal moieties bromophenyl probably
facilitate the process of the carrier captured and released due to
the stronger electron withdraw ability of Azo-Br and PAzo-Br
[34,45]. Thus the low conductivity state of ITO/Azo-Br/Al device
Fig. 9. Experimental and fitted data of IeV curves for ITO/PAzo-OCH3/Al device and ITO/PAzo-Br/Al device in the OFF and ON states. Panels a and b are the OFF state with the SCLC
model and the ON state with the ohmic current model for ITO/PAzo-OCH3/Al device. Panels c and d are the OFF state with the Poole Frenkel model (ꢀ0.49 to ꢀ4 V) and the ON state
with the ohmic current model for ITO/PAzo-Br/Al device.

F.-L. Ye et al. / Polymer 54 (2013) 3324e3333
3333
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4. Conclusions
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In this paper, we have successfully synthesized two new azo
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the ITO/polymer/Al devices all exhibit WORM memory behavior.
Both the ON and OFF states of the devices investigated are stable
under a constant read voltage stress of ꢀ1 V and even can endure
108 read cycles under a pulse read voltage. The OFF state of ITO/
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fitted with PooleeFrenkel (PF) emission. The ON state of all mem-
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PAzo-OCH3 and PAzo-Br are all p-type materials. Both Azo-OCH3
and PAzo-OCH3 based devices have an advantage of possessing low
turn-on threshold voltage due to the lower hole-injection barrier,
which means low-power consumption. Polymers have successfully
retained the storage performance of organic molecules. Moreover,
polymer-based memory devices fabricated by spin coating polymer
materials have the advantages of easy fabrication, high mechanical
flexibility, good stability and many other advantages. Thus, from the
perspective of device fabrication to consider, we have successfully
achieved the advantages of easy fabrication, low-cost and low-
power consumption without destroying the storage performance.
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Acknowledgments
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The authors graciously thank the Chinese Natural Science Foun-
dation (21071105 and 21176164), Chinese-Singapore Joint Project
(2012DFG41900), the Specialized Research Fund for the Doctoral
Program of Higher Education of China (grant no. 20113201130003),
and the project funded by the Priority Academic Program Develop-
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Appendix A. Supplementary data
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