Preparation of TCPP: Block copolymer composites and study of their memory behavior by tuning the loading ratio of TCPP in the polymer matrix

Article abstract of DOI:10.1039/c3tc30359d

In this paper, we report the synthesis, properties and electrical memory characteristics of block copolymer PStCH-b-P4VP and its TCPP composites. Both the optical absorption and photoluminescence indicate the strong interaction between the P4VP blocks and TCPP. The electrical switching behavior can be tuned through utilizing the loading ratio of TCPP in the PStCH-b-P4VP matrix. The memory device characteristics of the composites with 0%, 1% and 3% of TCPP in the PStCH-b-P4VP matrix are WORM, DRAM and DRAM, respectively. The OFF state for PStCH-b-P4VP and the 3% TCPP:PStCH-b-P4VP composite memory device can be fitted with the Poole-Frenkel (PF) emission and space-charge-limited-current (SCLC), respectively. Both of the ON-state currents are fitted with the Ohmic model. The 3% TCPP composite-based device exhibits a lower threshold voltage than the 1% TCPP composite-based device. The present study suggests that the electrical memory characteristics could be efficiently tuned by the PStCH-b-P4VP (donor)/TCPP (acceptor) ratio, which is good for potential application in polymer composite memory devices.

Full text of DOI:10.1039/c3tc30359d

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Preparation of TCPP: block copolymer composites and
study of their memory behavior by tuning the loading
ratio of TCPP in the polymer matrix†
Cite this: J. Mater. Chem. C, 2013, 1,
3947
a
a
a
ab
ab
Hua Liab
*
*
Juan Liu, Peiyang Gu, Feng Zhou, Qingfeng Xu,
and Lihua Wangab
Jianmei Lu,
In this paper, we report the synthesis, properties and electrical memory characteristics of block copolymer
PStCH-b-P4VP and its TCPP composites. Both the optical absorption and photoluminescence indicate the
strong interaction between the P4VP blocks and TCPP. The electrical switching behavior can be tuned
through utilizing the loading ratio of TCPP in the PStCH-b-P4VP matrix. The memory device
characteristics of the composites with 0%, 1% and 3% of TCPP in the PStCH-b-P4VP matrix are WORM,
DRAM and DRAM, respectively. The OFF state for PStCH-b-P4VP and the 3% TCPP:PStCH-b-P4VP
composite memory device can be fitted with the Poole–Frenkel (PF) emission and space-charge-limited-
current (SCLC), respectively. Both of the ON-state currents are fitted with the Ohmic model. The 3%
TCPP composite-based device exhibits a lower threshold voltage than the 1% TCPP composite-based
device. The present study suggests that the electrical memory characteristics could be efficiently tuned
by the PStCH-b-P4VP (donor)/TCPP (acceptor) ratio, which is good for potential application in polymer
composite memory devices.
Received 25th February 2013
Accepted 16th April 2013
DOI: 10.1039/c3tc30359d
www.rsc.org/MaterialsC
GO as charge storage materials.14 However, phase separation
and ion aggregation, arising from non-uniformly dispersed or
1 Introduction
Organic and polymer materials are promising candidates for
future molecular-scale memory applications due to their
advantages of simplicity in device structure, low-cost potential,
low-power operation and large capacity for data storage. Poly-
meric materials that exhibit memory switching characteristics
include conjugated polymers, side chain polymers, functional
polyimides (PIs) and polymer composites.1–4 The polymer
composites, using charge transfer5,6 or charge trapping-detrap-
ping,7,8 have been widely explored for memory device applica-
tion. The polymer composites usually contain carbon
nanotubes,9 fullerene,10 metal nanoparticles11 or graphene
oxide (GO)12 in the polymer matrix. For example, Yang et al.
tuned the electrical bistable memory effect in gold/polyaniline
nanober composite devices through controlling the size of the
gold nanoparticles.13 Chen et al. obtained a write-once-read-
many-times (WORM) memory device by using a supramolecular
hybrid route to hydrogen-bonded block copolymers (BCP) and
non-compatible components in a doped system, are unfavor-
able to the performance of a device. To reduce the inuence of
the problems, we try dissolving small conjugated organic
molecules, such as porphyrin-based derivatives, into the poly-
mer matrix. Studies have shown that some porphyrin-based
derivatives have been accepted as potential materials for
molecular information storage.15 The porphyrin-based deriva-
tives can serve as the charge-storage medium, and afford a
signicantly higher charge density.16
In addition, non-conjugated polymers containing pendant
p-conjugated moieties are of interest for their advantages of
excellent solubility, tunable morphology, and the possibility for
molecular design through chemical synthesis, which are good
for memory device fabrication.17–19 Moreover, the advantage of
functional block copolymers is obvious as they can self-organize
into an ordered nano-morphology,20 or be used to control the
aggregated size of polymer composites.21 The cooperative self-
assembly in polymer composites offers many possibilities to
develop novel electrical properties.22–24
aKey Laboratory of Organic Synthesis of Jiangsu Province, School of Chemistry,
Chemical Engineering and Materials Science, Soochow University (DuShuHu
Campus), 199 Renai Road, Suzhou, 215123, China
bKey Laboratory of Energy-saving and Environmental protection materials Test and
Technical service center of Jiangsu Province, Soochow University (DuShuHu
Campus), 199 Renai Road, Suzhou, 215123, China. E-mail: lujm@suda.edu.cn;
xuqingfeng@suda.edu.cn; Fax: +86-(0)512-6588-0367; Tel: +86-(0)512-6588-0368
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc30359d
In this paper, we synthesized a new AB-type block copolymer
containing pendant electron-donating 3-(4-(diethylamino)
phenyl)-1-(4-(4-vinylbenzyloxy)phenyl) prop-2-en-1-one (StCH)
and 4-vinylpyridine (4VP). The block copolymer exhibits stable
WORM memory behavior. Different amounts of 5,10,15,20-
tetra(4-carboxylphenyl) porphyrin (TCPP) were added into the
polymer system. 1,2-Dichloroethane was selected as a good
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 3947–3954 | 3947

View Article Online
Journal of Materials Chemistry C
Paper
solvent both for TCPP and PStCH-b-P4V. The hydrophilic pyridyl Hz, 1H), 5.77 (d, J ¼ 17.6 Hz, 1H), 5.27 (d, J ¼ 10.9 Hz, 1H), 5.12
groups on the side chains of PStCH-b-P4VP could interact with (s, 2H), 2.56 (s, 3H).
TCPP via hydrogen bonding and TCPP could participate in the
2.3.2 Synthesis of StCH. Compound 1 (2.52 g, 10 mmol), 4-
self-assembly of PStCH-b-P4VP. The morphology, physical diethylaminobenzaldehyde (1.77 g, 10 mmol), NaOH (0.6 g, 15
properties and memory device characteristics of self-assembled mmol) and ethanol (100 mL) were stirred at 60 ꢀC for 24 h, and
PStCH-b-P4VP and its composites with different amounts of then cooled to room temperature. The precipitate was ltered
TCPP were investigated in detail. The experimental results and recrystallized from ethanol. A yellow aky solid was
suggested that the memory characteristics could be efficiently obtained (yield: 82%).
tuned by the PStCH-b-P4VP (donor)/TCPP (acceptor) ratio.
1H NMR (300 MHz, DMSO-d6) d 8.10 (d, J ¼ 8.6 Hz, 2H), 7.73–
7.58 (m, 4H), 7.48 (dd, J ¼ 17.7, 7.9 Hz, 4H), 7.13 (d, J ¼ 8.6 Hz,
2H), 6.83–6.64 (m, 3H), 5.85 (d, J ¼ 17.8 Hz, 1H), 5.28 (d, J ¼ 11.0
Hz, 1H), 5.22 (s, 2H), 3.42 (q, 4H), 1.12 (t, J ¼ 6.8 Hz, 6H).
2.3.3 RAFT polymerization of PStCH. A typical polymeri-
zation is described as follows. StCH (2.47 g, 6 mmol), CPDN
(32.52 mg, 0.12 mmol) and AIBN (3.28 mg, 0.02 mmol) were
dissolved in 5 mL of cyclohexanone. The solution was degassed
by three freeze–pump–thaw cycles. The polymerization was
conducted at 100 ꢀC under stirring. Aer 24 h, the polymeri-
zation was stopped by transferring the reaction ask in liquid
nitrogen. The polymer solution was diluted by tetrahydrofuran
and precipitated in methanol. The polymer was dried under
vacuum.
2.3.4 Synthesis of PStCH-b-P4VP. 4VP (945 mg, 9 mol),
macromolecular chain transfer agent PStCH (240 mg, 0.03
mmol), and AIBN (1.64 mg, 0.01 mmol) were dissolved in 2 mL
of cyclohexanone. The solution was degassed by three freeze–
pump–thaw cycles. The polymerization was conducted at 100 ꢀC
under stirring. Aer 24 h, the polymerization was stopped by
transferring the reaction ask in liquid nitrogen. The polymer
solution was diluted by tetrahydrofuran and precipitated in
deionized water. The polymer was dried under vacuum.
2 Experimental section
2.1 Materials
4-Vinylbenzyl chloride (TCI, 90%) was obtained by column
chromatography (aluminum oxide, neutral). Cyclohexanone and
4-vinylpyridine (Sinopharm Chemical Reagent, 96%) were
distilled under vacuum. Azobisisobutyronitrile (AIBN) was
recrystallized from CH3OH. Potassium carbonate (K2CO3, 99%)
and sodium hydroxide (NaOH, 96%) were purchased from
Shanghai Sinopharm. 40-Hydroxyacetophenone (98%), 4-diethyl-
aminobenzaldehyde (98%) and 5,10,15,20-tetra(4-carboxyl-
phenyl) porphyrin (TCPP) (97%) were purchased from Aladdin.
All other reagents and solvents were analytically pure and were
used as received without further purication.
2.2 Synthesis of RAFT chain transfer agent CPDN
2-cyanoprop-2-yl-1-dithionaphthalate (CPDN) was synthesized
as described in the corresponding references.25
1H NMR (300 MHz, CDCl3), d (ppm): 8.16 (m, 1H), 7.88 (m,
2H), 7.53 (m, 2H), 7.46 (m, 2H), 1.98 (s, 6H).
2.3 Preparation of the monomer and polymers
The monomer was prepared in two steps and characterized by
1H NMR (Scheme 1).
2.4 Characterization
1H NMR spectra were measured on an INOVA 400 MHz NMR
spectrometer, using CDCl3 or DMSO-d6 as the solvent and tet-
ramethylsilane (TMS) as the internal standard at ambient
temperature. UV-vis spectra were recorded on a Perkin-Elmer l-
17 spectrometer using a 1 cm square quartz cell. The uores-
cence spectra were measured on an Edinburgh-920 uorescence
spectra photometer (Edinburgh Co. UK) with a slit of 5 nm.
Thermogravimetric analysis (TGA) was conducted on a TA
instrument Dynamic TGA 2950 at a heating rate of 20 ꢀC minꢁ1
under a N2 ow rate of 50 mL minꢁ1. Cyclic voltammetry was
performed at room temperature using an ITO working elec-
trode, 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
hexauorophosphate (n-Bu4NPF6) in acetonitrile (0.1 M). SEM
images were taken on a Hitachi S-4700 scanning electron
microscope. Molecular weights and the polydispersity values
were measured using Waters1515 GPC with DMF as the mobile
phase at a ow rate of 1 mL minꢁ1 and with a column
2.3.1 Synthesis of compound 1. 40-Hydroxyacetophenone
(8.16 g, 60 mmol), 4-vinylbenzyl chloride (9.15 g, 60 mmol), and
potassium carbonate (8.28 g, 60 mmol) were dissolved in 200
mL of acetone and reuxed for 24 h. The solution was cooled to
room temperature, and then poured into ice water. The
precipitate was ltered and recrystallized from ethanol. A white
aky solid was obtained (yield: 90%).
1H NMR (300 MHz, CDCl3) d 7.94 (d, J ¼ 8.8 Hz, 2H), 7.41 (q,
J ¼ 8.2 Hz, 4H), 7.00 (d, J ¼ 8.8 Hz, 2H), 6.73 (dd, J ¼ 17.6, 10.9
ꢀ
temperature of 30 C. TEM images were carried out with a FEI
TecnaiG220 microscope working at 200 kV. Differential scan-
ning calorimetry (DSC) analyses were performed on a Shimadzu
DSC-60A Instrument.
Scheme
1
The synthesis and chemical structure of PStCH-b-P4VP. (i) 40-
Hydroxyacetophenone, K2CO3; (ii) 4-diethylaminobenzaldehyde, NaOH; (iii) AIBN,
CPDN; (iv) AIBN, 4VP.
3948 | J. Mater. Chem. C, 2013, 1, 3947–3954
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
assigned to the characteristic peaks of the pyridyl group (spectra
b). The above results revealed the successful preparation of
PStCH-b-P4VP.
2.5 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. PStCH-b-P4VP
containing 0–3% TCPP (molar ratio of [TCPP]/[PStCH-b-P4VP
repeating unit]) in 1,2-dichloroethane were prepared and stirred
for 1 h to form homogenous aging solutions. The solutions were
ltrated through micro lters with a pinhole size of 0.22 mm,
aer 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. Finally, the top metal electrode was
obtained by a 100 nm thickness 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. Atomic force microscopy (AFM) measure-
ments were performed using a MFP-3DTM (Digital Instru-
ments/Asylum Research) AFM instrument.
The molecular weights (Mn) and polydispersity indices (PDI)
of PStCH and PStCH-b-P4VP were determined by GPC (Fig. S2†).
The molecular weights (Mn) of PStCH and PStCH-b-P4VP were
10 500 and 12 200 with corresponding PDIs of 1.10 and 1.19,
respectively. The number-average degrees of PStCH and P4VP
1
were 25 and 15.6, respectively. Hence, the H NMR spectrum
along with the GPC results indicates the block ratio of StCH to
4VP was 1 : 0.62.
The thermal stability, optical properties and electrochemical
properties of the PStCH-b-P4VP and TCPP:PStCH-b-P4VP
composites were investigated. All of them exhibited good
thermal stability. As shown in Table 1, the glass transition
temperatures (Tg) of PStCH-b-P4VP, the 1% TCPP:PStCH-b-P4VP
composite and 3% TCPP:PStCH-b-P4VP composite determined
by differential scanning calorimetry were 128 ꢀC, 125 ꢀC and
ꢀ
123 C, respectively. The thermal decomposition temperatures
(Td) of PStCH-b-P4VP, the 1% TCPP:PStCH-b-P4VP composite
and the 3% TCPP:PStCH-b-P4VP composite determined by
thermogravimetric analysis were 303 ꢀC, 287 ꢀC and 206 ꢀC,
respectively. All of them show suitable thermal stability for
electrical device applications. The Tg and Td decrease gradually
with the increasing TCPP content. The Tg and Td of the 3%
TCPP:PStCH-b-P4VP composite decrease signicantly, which
may be due to the decomposition of the carboxyl groups of TCPP.
3 Results and discussion
3.1 Characterization of PStCH-b-P4VP and its TCPP
composites
3.1.1 Preparation of TCPP:PStCH-b-P4VP composite. A
schematic of the PStCH-b-P4VP composite thin lms is pre-
sented in Scheme 2. The RAFT polymerization of PStCH was
carried out using CPDN as the initiator in cyclohexanone at
100 ꢀC with [StCH] : [CPDN] : [AIBN] ¼ 300 : 6 : 1, and the
preparation of the block polymer PStCH-b-P4VP was carried out
by using PStCH as the macroinitiator. The PStCH and PStCH-b-
P4VP were analyzed by 1H NMR (spectra a and b in Fig. S1†). In
the 1H NMR spectra of PStCH and PStCH-b-P4VP the signals of
the hydrogen atom from the vinyl group (5.2–6.0 ppm) dis-
appeared and a broad signal of methylene below 2.0 ppm
formed. The aromatic proton signals were observed around
6.0–8.17 ppm. The alkyl proton signals could be observed at
4.5–5.4 and 1.2–2.0 ppm. The proton peaks at 8.1–9.0 ppm were
3.2 Optical properties
The UV-vis absorption spectrum (Fig. S3†) of PStCH-b-P4VP in
DMF solution displays a p–p* transition of the aromatic rings
at 270 nm and an intramolecular charge transfer (ICT) transi-
tion at 417 nm.26 We also investigated the UV-vis absorption
spectra of the thin lms of PStCH-b-P4VP. Compared with the
absorption peaks in solution, both absorption peaks are slightly
bathochromically shied (from 270 to 273 nm and from 417 to
421 nm) and broadened. The red-shi and broadened absorp-
tion peaks from the solution to solid state might be associated
with the formation of molecular aggregation or orderly p–p*
stacking and/or increased polarizability of the lm, which is
benecial for the improvement of the charge carrier mobility of
the resulting nanolms.27
Fig. 1a shows the absorption spectra of TCPP:PStCH-b-P4VP
composite thin lms. As the TCPP content is enhanced from 0%
to 3%, the maximum absorption peak of the lms at 421 nm is
bathochromically shied to 426 nm (Fig. 1a) and a weak broad
band from 500 to 600 nm of TCPP appears with the increasing
amount of TCPP (Fig. 1a inset). The optical band gaps (Eg)
determined from the onset optical absorbance are summarized
in Table 1.
The interaction between the block polymer and TCPP was
further characterized by photoluminescence (PL) spectra of the
lms, as shown in Fig. 1b. The PL spectrum of PStCH-b-P4VP
exhibits a strong emission peak attributed to the chromophore
of chalcone conguration at 527 nm. The emission of PStCH-b-
P4VP in the range of 480–600 nm was quenched, while the
Scheme 2 Schematic illustration of the TCPP:PStCH-b-P4VP composite thin
films.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 3947–3954 | 3949

View Article Online
Journal of Materials Chemistry C
Paper
Table 1 Thermal, optical and electrochemical properties of the PStCH-b-P4VP and TCPP:PStCH-b-P4VP composite thin films
Film
lonset
Ec (V)
Eox
Sample
Tg (ꢀC)
Egb (eV)
HOMOd (eV)
LUMO (eV)
a
Td (ꢀC)
0% TCPP
1% TCPP
3% TCPP
128
125
123
303
287
206
484
495
501
2.56
2.50
2.47
0.98
1.01
1.09
ꢁ5.35
ꢁ5.38
ꢁ5.46
ꢁ2.79
ꢁ2.88
ꢁ2.99
Thermal decomposition temperature (5% weight-loss). b The data was calculated by the following equation: bandgap ¼ 1240/lonset of lms. c vs.
a
Ag/AgCl in MeCN. d The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (0.43 eV).
composites with 0%, 1% and 3% TCPP were determined to be
ꢁ5.35, ꢁ5.38 and ꢁ5.46 eV, respectively. The lowest unoccupied
molecular orbital (LUMO) energy levels were determined to be
ꢁ2.79, ꢁ2.88 and ꢁ2.99 eV, respectively. According to the results,
the half-peak potentials for oxidation (Eox) became higher with
increasing the TCPP content. Therefore, the HOMO and LUMO
energy levels simultaneously decrease, indicating a relatively
strong interaction between donor and acceptor moieties.
3.4 Morphology of the thin lms
The morphologies of the polymer PStCH-b-P4VP and
TCPP:PStCH-b-P4VP composites were studied by AFM (Fig. 2).
The PStCH-b-P4VP pristine lm possessed disordered aggre-
gated white spherical shapes with polar P4VP as the core and
non-polar PStCH as the corona, which were due to the solvent
selectivity on each block.29 When TCPP was added into the
system, the dispersion of PStCH-b-P4VP was obviously
improved and the surface root-mean-square roughnesses of the
thin lms were 0.588, 0.604 and 0.812 nm, respectively. This
suggests that the hydrogen bonding interaction between the
PStCH-b-P4VP and TCPP leads to the well dispersed PStCH-b-
P4VP and the good quality of the lms.
3.5 Memory device characteristics
The memory behavior of PStCH-b-P4VP and its TCPP compos-
ites was tested by the current–voltage (I–V) characteristics of the
Fig.
1
(a) Absorption and (b) fluorescence spectra of PStCH-b-P4VP and
TCPP:PStCH-b-P4VP composite thin films. lex ¼ 432 nm.
emission of TCPP in the range of 625–760 nm was enhanced
with the increasing amount of TCPP. This was probably due to
energy transfer between pyridyl groups in the P4VP block and
TCPP.28
3.3 Electrochemical properties
The electrochemical properties of PStCH-b-P4VP and the
TCPP:PStCH-b-P4VP composite thin lms were investigated by
using conducting cyclic voltammetry (CV) measurements.
Fig. S4† shows the cyclic voltammograms of the PStCH-b-P4VP
and TCPP:PStCH-b-P4VP composite lms and the related elec-
trochemical data is given in Table 1. The highest occupied
Fig. 2 (a–c) Tapping-mode (5 mm ꢂ 5 mm) AFM topography; (d–f) typical cross-
section profiles of AFM topographic images of PStCH-b-P4VP and TCPP:PStCH-b-
P4VP composite thin films spin-coated onto ITO substrates. The color bar of the
AFM images is displayed in the right of (c). The content of TCPP: (a and d) 0%; (b
molecular orbital (HOMO) energy levels of the polymer and e) 1% and (c and f) 3%.
3950 | J. Mater. Chem. C, 2013, 1, 3947–3954
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
ITO/polymer/Al device structures. Al was used as the anode and state at a threshold voltage of ꢁ3.6 V with an ON/OFF current
ITO was set as the cathode in all electrical measurements. All ratio of 104. The device remains in this high-conductivity state
the thicknesses of the active memory lms were adjusted to be during the subsequent negative scan from 0 to ꢁ6 V (the second
around 100 nm (Fig. S6†), as measured by scanning electron sweep). Subsequent application of the third scan performed
microscopy (SEM) through a cross-section of the device. The aer turning off power for about 3 min could be reprogrammed
current–voltage (I–V) characteristic of PStCH-b-P4VP is shown in from the OFF state to the ON state at ꢁ3.6 V again and the
Fig. 3a. In the rst sweep from 0 to ꢁ6 V, an abrupt increase in current was kept in the ON state. The OFF state could be further
current is observed at a switching threshold voltage of about ꢁ4 written to the ON state when the switching threshold voltage
V, indicating the device transition from a low-conductivity (OFF) was reapplied, indicating that the memory device was rewrit-
state to a high-conductivity (ON) state (the “writing” process). able. The short retention time of the ON state suggests that the
The device remains in this high-conductivity state during the memory device was volatile. This behavior is characteristic of a
subsequent negative scan from 0 to ꢁ6 V (the second sweep). dynamic random access memory (DRAM) device.
The distinct bielectrical states in the voltage range of about 0 to
Likewise, the memory effect of the 3% TCPP:PStCH-b-P4VP
ꢁ6 V, where the ON/OFF current ratio is up to 103, allowed the composite is shown in the current–voltage (I–V) characteristic of
voltage (e.g. ꢁ1 V) to read the “0” or “OFF” signal (before Fig. 3c. Similarly, the I–V characteristic denes the two electrical
writing) and “1” or “ON” signal (aer writing) of the memory conductance states of the 3% TCPP:PStCH-b-P4VP composite
device. However, the memory device also remained in the ON device and reveals the volatile nature of the memory effect. This
state when scanning from 0 to 6 V (sweep 3) and could not memory device switches from the OFF to ON state at about ꢁ1.8
return to the original OFF state. It is shown that the memory V, with an ON/OFF current ratio of about 102, when a voltage
device fabricated with PStCH-b-P4VP exhibited WORM memory sweep from 0 to ꢁ6 V is applied. The device remains in the ON
behavior.
state when the voltage sweep is repeated. Aer turning off the
The current–voltage (I–V) characteristic of the “ITO/1% power for about 3 min, the device drops back to the OFF state
TCPP:PStCH-b-P4VP composite/Al” is shown in Fig. 3b for when but can be switched ON again by applying the appropriate
a small amount of TCPP (1%) was incorporated into the polymer voltage ꢁ1.8 V (the third sweep). Thus, the behavior of this
matrix. As a negative bias voltage was applied, the current of the memory device also shows the characteristics of a DRAM device.
memory cell suddenly increased to the higher conductance
At a constant stress of ꢁ1 V, no obvious changes in the
current for the OFF and ON states of all the memory devices
were observed during the long-term testing of about 104 s in a
glove box, and also no signicant degradation was observed
aer 108 read cycles (Fig. 3d–f). The results suggest the excellent
device stability.
3.6 Mechanism of the memory device
The memory device characteristics of the composites with 0%,
1% and 3% of TCPP in the PStCH-b-P4VP matrix are WORM,
DRAM and DRAM, respectively. The devices with different
loading ratios of TCPP in the PStCH-b-P4VP matrix exhibit
different memory behavior, the obtained I–V characteristics for
the devices were further analyzed by tting appropriate
conductance models. As shown in Fig. 4a, the OFF state for the
ITO/PStCH-b-P4VP/Al device can be elucidated as Poole–Frenkel
(PF) emission30 in terms of the plots of log(I/V) versus (ꢁV)1/2
,
which were found to be linear in a range from ꢁ0.18 V to ꢁ3.97
V. On the other hand, the currents in the ON states are almost
linearly dependent on the applied voltage, just as in metallic
conduction, this indicates that the charge transport is domi-
nated by the Ohmic model in the ON states (Fig. 4b).31 There-
fore, 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 to the Ohmic model.
For PStCH-b-P4VP, the memory effects were dominated by
StCH units. In order to understand the charge transport
mechanism, the molecular HOMO, LUMO surfaces and elec-
trostatic potential (ESP) surfaces of StCH theoretical models
were carried out at the B3LYP/6-31G(d) level with the
Gaussian03 program package to understand the charge carriers'
Fig. 3 (a–c) Current–voltage (I–V) characteristics of the PStCH-b-P4VP and
TCPP:PStCH-b-P4VP composite memory devices based on the ITO–Al electrode.
(d–f) The effect of retention time on the ON and OFF states of the memory devices
under a constant stress of ꢁ1.0 V. The inset is the effect of read cycles on the ON
and OFF state under a read pulse voltage of ꢁ1.0 V. The contents of TCPP were: (a
and d) 0%; (b and e) 1% and (c and f) 3%.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 3947–3954 | 3951

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 4 Analysis of I–V characteristics of the (a) OFF-state with the Poole–Frenkel
model and (b) ON-state based on ITO/PStCH-b-P4VP/Al device; (c) OFF-state with
the Poole–Frenkel model and (d) ON-state based on ITO/1% TCPP:PStCH-b-P4VP/
Al device; (e) OFF-state with the SCLC model and (f) ON-state based on ITO/3%
TCPP:PStCH-b-P4VP/Al device.
Fig. 5 HOMO, LUMO and Electrostatic Potential (ESP) surfaces of the StCH
monomer units of PStCH-b-P4VP obtained by molecular simulation.
migration through the polymer nanolm. The molecular
surface with the continuous positive ESP (Fig. 5) in red areas
along with the side-chain of the block polymer indicates that
charge carriers can migrate through this open channel.
However, there are some negative ESP regions (blue), which
arise from the ether group and the carbonyl group. These
93imV2
Jf
(1)
8d3
negative regions can serve as “traps” to block the mobility of herein, m is the mobility of charge carriers and 3i is the dynamic
charge carriers and further hinder the formation of electronic permittivity of the insulator and d is the thickness of the lm.
transmission channels. Under a low applied voltage, the ITO/ Fig. 4e shows that the I–V curve could be tted rather well by
PStCH-b-P4VP/Al device is in a low conductivity state (OFF-state) this model. Close to the turn-on voltage the generated carriers
for the unlled traps. As the applied voltage increases, the traps ll some of the charge traps and the Al cathode also becomes an
are lled by charge carriers and the device can meet the electron-injecting contact (to the LUMO levels of TCPP), which
requirement of the ON-state when the transition voltage (ꢁ4.0 leads to double injection and thus enhanced carrier concen-
V) is reached. Upon undergoing the HOMO to LUMO excitation, tration and mobility; the current increases rapidly to switch the
the electron density obviously shied from the –N(Et)2 (donor) device to the high-conductivity state (ON state). In the ON state
to carbonyl group (acceptor) side, so a charge-separated state (>1.8V), the I–V curve again follows the Ohmic model (Fig. 4f).
occurred when the copolymer lm was activated by an electrical But the traps in the composite are not deep enough to maintain
eld. The trapped carriers may not be easily detrapped under the ON-state, turning off the power for a few minutes could turn
non-degrading reverse elds, making for a high conductivity the devices back to the OFF-state. This feature denes the
state that can be maintained for a long time. Accordingly, the volatile characteristics of the present memory device.
device shows typical nonvolatile WORM-type characteristics.
The work functions of the two electrodes were ꢁ4.3 and ꢁ4.8
The OFF-state current of devices based on ITO/1% eV for Al and ITO respectively. Calculated from the cyclic vol-
TCPP:PStCH-b-P4VP/Al was also tted with the PF emission tammetry (CV) and onset optical absorbance results, the energy
model in a range from ꢁ0.18 V to ꢁ3.97 V, whereas the ON-state levels HOMO and LUMO were ꢁ5.35 eV and ꢁ2.79 eV for PStCH-
current was tted with the Ohmic model, as shown in Fig. 4c b-P4VP and ꢁ5.83 eV and ꢁ3.37 eV for TCPP (Fig. S5†), respec-
and d. While, the OFF state for the ITO/3% TCPP:PStCH-b-P4VP/ tively. The corresponding energy diagram is shown in Fig. 6. The
Al memory device could be tted with the space charge-limited PStCH-b-P4VP and TCPP can be viewed as donors and acceptors
current (SCLC) model (Fig. 4e):31
in the thin lm respectively. In the ITO/PStCH-b-P4VP/Al device,
3952 | J. Mater. Chem. C, 2013, 1, 3947–3954
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
the pristine PStCH-b-P4VP is hole-transporting dominant due to detrapping. The system returns to the original OFF state when
the small energy barrier of ITO/HOMO (0.55 eV) compared to that the power is turned off.
of Al/LUMO (2.16 eV), which exhibits the p-type semiconductor
It has been reported that Al electrodes tend to penetrate into
characteristics. With a device conguration of ITO/TCPP:PStCH- the polymer composite lms during the vacuum deposition
b-P4VP the composite/Al, the low-lying LUMO of the TCPP can process, interacting with strongly coordinating heteroatoms,
serve as the electron trapping sites embedded in the polymer nitrogen or sulfur, in the conjugated polymers to form
matrix. When the negative voltage is applied, electrons are conductive metal laments.34–36 Under different electric elds,
injected into the composite lm, trapped by TCPP and then the formation and rupture process of the metal lament may
escape from the isolated TCPP trapping islands through a lead the polymer composite lms to have DRAM memory
hopping process if sufficient energy can be acquired. When the effects. In order to eliminate this phenomenon, an Au–Au
voltage swept above the threshold voltage, the electron-donating electrode was used instead of Al–ITO as the top-bottom elec-
pendent pyridine moiety of the PStCH-b-P4VP probably formed trode to measure the I–V curves on the same lms. The
charge transfer with the electron acceptor TCPP and the measured I–V characteristics are presented in Fig. S7.† Each
generated carriers consequently exhibited a sharp increase in device based on the Au–Au electrode showed similar memory
current density.
behaviors in contrast to the Al–ITO electrode. The Au–Au
The loading ratio of TCPP in the PStCH-b-P4VP matrix allows devices have smaller switch-on voltages than the Al–ITO
the device to manipulate the threshold voltage, since the sepa- devices. This may be due to the lower hole injection barrier
ration of the TCPP domain can be controlled through the TCPP between the work function of the Au electrode (ꢁ5.1 eV) and the
distribution in the PStCH-b-P4VP matrix, The 3% TCPP HOMO energy level of the polymer composites. The results
composite-based device exhibits a lower threshold voltage and indicate that the DRAM memory type is caused by the intrinsic
ON/OFF ratio than the 1% TCPP composite device, which may characteristics of the polymer composites themselves and not
be due to the decrease in distance between isolated TCPP decided by the device structure.
domains.32 The reduced distance can easily overcome the
percolation threshold of the trapping sites for the effective
charge carrier transport. The energy barrier for charge-transport
4 Conclusion
decreases with the increase in the TCPP content of the A new AB-type block copolymer PStCH-b-P4VP containing
composites, and more charges in the memory layer can be pendant chalcone moieties was successfully synthesized by
captured by TCPP molecules. Moreover, the blending approach RAFT polymerization. We have demonstrated the controllable
of PStCH-b-P4VP and TCPP could improve the dispersity of electrical switching behavior through utilizing the loading ratio
PStCH-b-P4VP, similar to those of the previously reported of TCPP in the PStCH-b-P4VP matrix. Both optical absorption
covalently linked polymer–GO conjugates.14 Charge transfer and photoluminescence indicated the formation of charge
interaction occurs electively between BCP (donor)–TCPP transfer complexation between the P4VP blocks and TCPP. The
(acceptor) and enhances the conductivity substantially, leading memory device characteristics of the composites with 0%, 1%
to a smaller ON/OFF ratio.33
and 3% of TCPP in the PStCH-b-P4VP matrix were WORM,
On the other hand, the ITO/TCPP:PStCH-b-P4VP composite/ DRAM and DRAM, respectively. The 3% TCPP composite-based
Al devices showed DRAM-type characteristics. In the presence of device exhibits a smaller threshold voltage and ON/OFF ratio
TCPP, the LUMO energy level and band gap of the polymer than the 1% TCPP composite-based device, which may be due to
composites decrease. The energy barrier for charge-transport the reduced distance between isolated TCPP domains and the
decreases with the increase of TCPP content in the polymer charge transfer interaction that occurred between BCP and
composites. The easier charge carrier mobility leads to electrons TCPP. The switching mechanism, based on the experimental
and tted I–V curves of PStCH-b-P4VP, was explained by the
Poole–Frenkel (PF) emission in the OFF state and the Ohmic
model in the ON state. While the switching mechanism of the
3% TCPP:PStCH-b-P4VP was explained by the space-charge-
limited-current (SCLC) in the OFF state and the Ohmic model in
the ON state. Overall, the memory devices fabricated with BCP
and TCPP have potential applications for high-density elec-
tronic memory devices.
Acknowledgements
The authors thank the Chinese Natural Science Foundation
(21071105 and 21176164), the Specialized Research Fund for
the Doctoral Program of Higher Education of China (Grant no.
20113201130003), and
a project funded by the Priority
Academic Program Development (PAPD) of the Jiangsu Higher
Education Institution.
Fig. 6 Energy level diagram for the ITO/TCPP:PStCH-b-P4VP/Al device.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 3947–3954 | 3953

View Article Online
Journal of Materials Chemistry C
Paper
20 M. R. Bockstaller, R. A. Mickiewicz and E. L. Thomas, Adv.
Mater., 2005, 17, 1331–1349.
Notes and references
1 S. Baek, D. Lee, J. Kim, S. H. Hong, O. Kim and M. Ree, Adv.
Funct. Mater., 2007, 17, 2637–2644.
2 X. D. Zhuang, Y. Chen, B. X. Li, D. G. Ma, B. Zhang and Y. Li,
Chem. Mater., 2010, 22, 4455–4461.
3 N. H. You, C. C. Chueh, C. L. Liu, M. Ueda and W. C. Chen,
Macromolecules, 2009, 42, 4456–4463.
21 J. L. Zhang, X. H. Yu, P. Yang, J. Peng, C. X. Luo, W. H. Huang
and Y. C. Han, Macromol. Rapid Commun., 2010, 31, 591–
608.
22 C. D. Rosa, F. Auriemma, R. D. Girolamo, G. P. Pepe,
T. Napolitano and R. Scaldaferri, Adv. Mater., 2010, 22,
5414–5419.
4 Y. Yang, J. Quyang, L. P. Ma, R. J. Tseng and C. W. Chu, Adv.
Funct. Mater., 2006, 16, 1001–1014.
5 R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner and Y. Yang, Nat.
Mater., 2004, 3, 918–922.
¨
23 T. Thurn-Albrecht, J. Schotter, G. A. Kastle, N. Emley,
T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C. T. Black,
M. T. Tuominen and T. P. Russell, Science, 2000, 290,
2126–2129.
6 J. C. Hsu, Y. Chen, T. Kakuchi and W. C. Chen,
Macromolecules, 2011, 44, 5168–5177.
7 H. T. Lin, Z. Pei and Y. J. Chan, IEEE Electron Device Lett.,
2007, 28, 569.
24 J. S. Lee, Y. M. Kim, J. H. Kwon, J. S. Sim, H. Shin, B.-H. Sohn
and Q. Jia, Adv. Mater., 2011, 23, 2064–2070.
25 J. Zhu, D. Zhou, X. L. Zhu and G. J. Chen, J. Polym. Sci., Part A:
Polym. Chem., 2004, 42, 2558–2565.
8 G. Liu, Q. D. Ling, E. Y. H. Teo, C. X. Zhu, D. S.-H. Chan,
K. G. Neoh and E. T. Kang, ACS Nano, 2009, 3, 1929–1937.
9 J. Borghetti, V. Derycke, S. Lenfant, P. Chenevier,
A. Filoramo, M. Goffman, D. Vuillaume and J. P. Bourgoin,
Adv. Mater., 2006, 18, 2535–2540.
¨
26 M. Surin, P. Sonar, A. C. Grimsdale, K. Mullen, S. De Feyter,
S. Habuchi, S. Sarzi, E. Braeken, A. V. Heyen, M. Van der
Auweraer, F. C. De Schryver, M. Cavallini, J. F. Moulin,
F. Biscarini, C. Femoni, L. Roberto and P. Leclere, J. Mater.
`
Chem., 2007, 17, 728–735.
10 J. Liu, Z. Yin, X. Cao, F. Zhao, A. Lin, L. Xie, Q. Fan, F. Boey,
H. Zhang and W. Huang, ACS Nano, 2010, 4, 3987–3992.
11 R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner and Y. Yang,
Nano Lett., 2005, 5, 1077–1080.
12 X. D. Zhuang, Y. Chen, G. Liu, P. P. Li, C. X. Zhu, E. T. Kang,
K. G. Neoh, B. Zhang, J. H. Zhu and Y. X. Li, Adv. Mater.,
2010, 22, 1731–1735.
13 C. O. Baker, B. Shedd, R. J. Tseng, A. A. Martinez-Morales,
C. S. Ozkan, M. Ozkan, Y. Yang and R. B. Kaner, ACS Nano,
2011, 5, 3469–3474.
14 A. D. Yu, C. L. Liu and W. C. Chen, Chem. Commun., 2012, 48,
383–385.
15 P. Thamyongkit, L. Yu, K. Padmaja, J. Y. Jiao, D. F. Bocian
and J. S. Lindsey, J. Org. Chem., 2006, 71, 1156–1171.
16 K. M. Roth, D. T. Gryko, C. Clausen, J. Z. Li, J. S. Lindsey,
W. G. Kuhr and D. F. Bocian, J. Phys. Chem. B, 2002, 106, 8641.
17 Q. D. Ling, S. L. Lim, Y. Song, C. X. Zhu, D. S. H. Chan,
E. T. Kang and K. G. Neoh, Langmuir, 2006, 23, 312–319.
18 C. M. Huang, Y. S. Liu, C. C. Chen, K. H. Wei and J. T. Sheu,
Appl. Phys. Lett., 2008, 93, 203301–203303.
27 R. Schmidt, J. H. Oh, Y. S. Sun, M. Deppisch, A.-M. Krause,
¨
K. Radacki, H. Braunschweig, M. Konemann, P. Erk, Z. Bao
¨
and F. Wurthner, J. Am. Chem. Soc., 2009, 131, 6215–6228.
28 J. C. Hsu, C. L. Liu, W. C. Chen, K. Sugiyama and A. Hirao,
Macromol. Rapid Commun., 2011, 32, 528–533.
29 A. F. M. Barton, Chem. Rev., 1975, 75, 731–753.
30 C. W. Chu, J. Ouyang, J. H. Tseng and Y. Yang, Adv. Mater.,
2005, 17, 1440–1443.
31 Q. D. Ling, Y. Song, S. L. Lim, E. Y. H. Teo, Y. P. Tan,
C. X. Zhu, D. S. H. Chan, D. L. Kwong, E. T. Kang and
K. G. Neoh, Angew. Chem., 2006, 118, 3013–3017.
32 J. C. Chen, C. L. Liu, Y. S. Sun, S. H. Tung and W. C. Chen,
So Matter, 2012, 8, 526–535.
33 G. Liu, X. Zhuang, Y. Chen, B. Zhang, J. Zhu, C. X. Zhu,
K. G. Neoh and E. T. Kang, Appl. Phys. Lett., 2009, 95,
253301.
34 S. Baek, D. Lee, J. Kim, S. H. Hong, O. Kim and M. Ree, Adv.
Funct. Mater., 2007, 17, 2637–2644.
35 S. Gao, C. Song, C. Chen, F. Zeng and F. Pan, J. Phys. Chem. C,
2012, 116, 17955–17959.
19 E. Y. H. Teo, Q. D. Ling, Y. Song, Y. P. Tan, W. Wang,
E. T. Kang, D. S. H. Chan and C. Zhu, Org. Electron., 2006,
7, 173–180.
36 H. Zhuang, X. P. Xu, Y. H. Liu, Q. H. Zhou, X. F. Xu, H. Li,
Q. F. Xu, N. J. Li, J. M. Lu and L. H. Wang, J. Phys. Chem.
C, 2012, 116, 25546–25551.
3954 | J. Mater. Chem. C, 2013, 1, 3947–3954
This journal is ª The Royal Society of Chemistry 2013