Enhancing the coplanarity of the donor moiety in a donor-acceptor molecule to improve the efficiency of switching phenomenon for flash memory devices

Article abstract of DOI:10.1016/j.dyepig.2013.09.002

Two conjugated small molecules were synthesized by Heck coupling reaction, wherein 1,8-naphthalimide acted as an electron acceptor, while either carbazole or triphenylamine, which contributed the different coplanarity, acted as electron donors, respectively. The devices based on both materials show non-volatile flash memory characteristics, and the effect of molecular coplanarity of the donor groups on the device performance was precisely studied. The results indicated that the reproducibility of the switching phenomenon for the memory device based on the carbazole containing naphthalimide derivative was much better than that based on the triphenylamino based naphthalimide due to the rigid carbazole moiety which improved the surface morphology as revealed by atomic force microscopy measurement. Therefore, the significance of the coplanarity of the donor moiety on improving reproducibility of switching phenomenon for memory device applications was revealed.

Full text of DOI:10.1016/j.dyepig.2013.09.002

Dyes and Pigments 100 (2014) 127e134
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Enhancing the coplanarity of the donor moiety in a donor-acceptor
molecule to improve the efficiency of switching phenomenon for flash
memory devices
*
*
Wusheng Ren, Hao Zhuang, Qing Bao, Shifeng Miao, Hua Li , Jianmei Lu , Lihua Wang
College of Chemistry, Chemical Engineering and Materials Science, China Petroleum and Chemical Industry Key Laboratory of Organic Wastewater
Adsorption Treatment & Resource, Soochow University, Suzhou, Jiangsu 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2013
Received in revised form
28 August 2013
Accepted 2 September 2013
Available online 12 September 2013
Two conjugated small molecules were synthesized by Heck coupling reaction, wherein 1,8-
naphthalimide acted as an electron acceptor, while either carbazole or triphenylamine, which contrib-
uted the different coplanarity, acted as electron donors, respectively. The devices based on both materials
show non-volatile flash memory characteristics, and the effect of molecular coplanarity of the donor
groups on the device performance was precisely studied. The results indicated that the reproducibility of
the switching phenomenon for the memory device based on the carbazole containing naphthalimide
derivative was much better than that based on the triphenylamino based naphthalimide due to the rigid
carbazole moiety which improved the surface morphology as revealed by atomic force microscopy
measurement. Therefore, the significance of the coplanarity of the donor moiety on improving repro-
ducibility of switching phenomenon for memory device applications was revealed.
Keywords:
Conjugated small molecules
Heck coupling
Coplanarity
Ó 2013 Elsevier Ltd. All rights reserved.
Non-volatile
Flash memory
Surface morphology
1. Introduction
precisely studied. A better understanding of the effect of terminal
moiety coplanarity on device performance is essential to help
Organic materials have been widely investigated for data stor-
age devices due to scalability, structural flexibility, low cost, and
three-dimensional stacking capability [1‒8]. A large number of
organic compounds, including conjugated polymers [9e15], oligo-
mers [16,17], blends of nanoparticles [18e20] and small organic
molecules [21e25], have been reported for electrical switching and
memory devices. Especially, organic donor-acceptor (DeA) mole-
cules, due to their ability to exhibit electrical bistability and
versatility in molecular design, have received considerable atten-
tion [26e29]. Currently, many groups use donor (D) e acceptor (A)
switching materials to meet the requirements of high-density data
storage. However, the research had been mainly focused on the
exploration of different DeA molecules with better electrical
properties for data storage. The coplanarity of the terminal donor
moiety which changes the surface morphology of the thin film
formed by vacuum evaporation affecting the reproducibility of the
switching phenomenon for the memory devices, had rarely been
design new DeA molecules for memory devices.
Herein, we designed and synthesized two novel DeA molecules
(NI-Cz and NI-TPA) with the same electron-accepting 1,8-
naphthalimide (NI) moiety which was an attractive deficient elec-
tron moiety and had been widely utilized in both small molecule and
polymer based memory devices [30,31]. Triphenylamine (TPA) de-
rivatives have good optical properties and hole-transporting ability,
buttheirmorphologywasamorphouswithroughersurfaceduetothe
twisted triphenylaminemoiety[32e35]. Thecarbazole(Cz)molecule,
also a well-known electron-donor and hole-transporting group, and
its substituted analogues have been widely used in the photovoltaic
devices and memory devices due to its unique electronic and
photochemical properties [36e39]. Compared to the triphenylamine
derivatives, the surface morphology of carbazole derivatives was
more coplanar due to the rigid carbazole moiety. The study showed
that the coplanarity of the terminal group played a key role for the
reproducibilityof switching phenomenon of the memory devices. For
the device based NI-Cz, the turn-on voltage was mainly
between ꢀ2.2 V and ꢀ2.6 V (with yield of 80%), and turn-off voltage
was mainly between 2.6 V and 3 V (with yield of 56%). For NI-TPA, the
turn-on voltage varied from ꢀ0.4 V to ꢀ2.0 V, and the turn-off voltage
was from 2.4 V to 4.0 V, and it showed no obvious distributed areas.
* Corresponding authors. Tel.: þ86 512 65880368; fax: þ86 512 65880367.
E-mail addresses: jianmeilu_group@hotmail.com (H. Li), lujm@suda.edu.cn
(J. Lu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.002

128
W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
This work takes an important step towards rational design of new
organic DeA molecules with suitable structures for better reproduc-
ibility of switching phenomenon for the memory devices.
4-Bromo-N-(2-ethylhexyl)-1,8-naphthalimide (1). A solution of 4-
bromo-1,8-naphtalic anhydride (5 g, 18.05 mmol) in ethanol
(100 mL) was added to a 250 mL three-neck round bottom equip-
ped with a reflux condenser and a magnetic stirrer. Then 2-
ethylhexan-1-amine (2.33 g, 18.05 mmol) was added drop-wise
and the reaction mixture was heated up to 110 ꢁC and stirred un-
der nitrogen overnight. The mixture was cooled to room temper-
ature, and it was filtered. The filtrate was purified by silica gel
2. Experimental
2.1. Materials
4-Bromo-1,8-naphthalic anhydride, 2-ethylhexan-1-amine,
palladium diacetate, tri-o-tolylphosphine, methyltriphenyl-
phosphonium bromide, potassium tert-butanolate, triphenylamine
and carbazole were all purchased from Energy Chemical as
analytical regents and used without further purification. N,N-
Dimethylformamide (DMF) was dried by distillation over CaH2.
Other reagents were used as received without any further
purification.
column chromatography using a mixture of petroleum and
dichloromethane (vol. ratio 5:1) as an eluent. After recystallization
from the eluent mixture of solvents, yellow crystals were obtained
with a yield of 95.01%. Mp ¼ 82e83 ꢁC. IR (in KBr), cmꢀ1: 3071,
2959, 2926, 2871, 2855, 1702, 1653, 1615, 1590, 1571, 1504,
1460,1435,1404,1344,1278,1231,1182,1150,1102,1083,1044,1024,
949, 897, 855, 782, 749, 730, 716, 664. 1H NMR (400 MHz, CDCl3)
d
8.65 (d, J ¼ 7.3 Hz, 1H), 8.56 (d, J ¼ 8.5 Hz, 1H), 8.41 (d, J ¼ 7.8 Hz,
1H), 8.04 (d, J ¼ 7.9 Hz, 1H), 7.85 (t, J ¼ 7.9 Hz, 1H), 4.18e4.04 (m,
2H), 2.01e1.85 (m, 1H), 1.43e1.27 (m, 8H), 0.93 (t, J ¼ 7.4 Hz, 3H),
0.87 (t, J ¼ 6.8 Hz, 3H). Anal. Calcd. for C20H22BrNO2: C, 61.86; H,
5.71; N, 3.61. Found: C, 61.82; H, 5.75; N, 3.57.
2.2. Instruments
All electrical measurements of the device were characterized
under ambient conditions without any encapsulation using an HP
4145B semiconductor parameter analyzer equipped with an HP
8110A pulse generator. NMR spectra were obtained from an Inova
400 FT-NMR spectrometer. High-resolution mass spectra (HRMS)
were determined on Micromass GCT-TOF mass spectrometer with
an ESI source. Elemental analysis was performed using a Carlo-Erba
EA-1110 instrument. UVeVis absorption spectra were measured at
room temperature by using a Shimadzu UV-3600 spectrophotom-
eter. 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 100 mV/s (CorrTest CS
Electrochemical Workstation analyzer). A 0.1 M solution of tetra-
butylammonium perchlorate (TBAP) in anhydrous DMF was used.
SEM images were taken on a Hitachi S-4700 scanning electron
microscope. Atomic force microscopy (AFM) measurements were
performed by using an MFP-3DTM (Digital Instruments/Asylum
Research) AFM instrument.
4-(9H-carbazol-9-yl)benzaldehyde (2). A mixture of 9H-Carba-
zole (5 g, 0.03 mol), and sodium hydroxide (1.2 g, 0.03 mol) in dry
dimethylformamide (DMF) was heated with stirring. Then 4-
fluorobenzaldehyde (3.72 g, 0.03 mol) was added, and the reac-
tion mixture was stirred at 130 ꢁC for 6 h. After cooling to room
temperature, the mixture was poured into distilled water and
extracted with ethyl acetate. The organic layer was dried with
anhydrous magnesium sulfate and concentrated by vacuum evap-
oration. The crude product was purified by column chromatog-
raphy using the mixture of petroleum and dichloromethane (vol.
ratio 3:1) as an eluent to get the desired compound as a yellow solid
(6.43 g) with a yield of 79.08%. Mp ¼ 156e157 ꢁC. IR (in KBr), cmꢀ1
:
2733,1910, 1702,1597, 1566, 1510, 1478, 1451, 1389, 1361, 1336,1316,
1300, 1226, 1208, 1181, 1161, 1123, 1103, 1023, 1000, 914, 829, 750,
720, 633, 619, 566. 1H NMR (400 MHz, CDCl3)
d 10.12 (s, 1H), 8.20e
8.10 (m, 4H), 7.80 (d, J ¼ 8.3 Hz, 2H), 7.51 (d, J ¼ 8.2 Hz, 2H), 7.44 (t,
J ¼ 7.6 Hz, 2H), 7.34 (t, J ¼ 7.4 Hz, 2H). Anal. Calcd. for C19H13NO: C,
84.11; H, 4.83; N, 5.16. Found: C, 84.07; H, 4.87; N, 5.15.
2.3. Synthesis of NI‒Cz and NI‒TPA
9-(4-Vinylphenyl)-9H-carbazole (3). A mixture of 2 (2.71 g,
10 mmol), potassium tert-butylate (1.68 g, 15 mmol), and
CH3PPh3Br (4.28 g, 12 mmol) in dry THF (100 mL) under nitrogen
was stirred for 24 h the mixture was poured into distilled water and
The two molecules were synthesized by Heck coupling reaction
in good yields as shown in Scheme 1.
Scheme 1. Synthetic routes and molecular structures of NI‒Cz and NI‒TPA.

W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
129
extracted with ethyl acetate. The organic layer was dried with
anhydrous magnesium sulfate and concentrated by vacuum evap-
oration. The crude product was purified by column chromatog-
raphy using the mixture of petroleum and dichloromethane (vol.
ratio 5:1) as an eluent to get the desired compound as a white solid
C
40H36N2O2: C, 83.30; H, 6.29; N, 4.86. Found: C, 83.25; H, 6.32; N,
4.84.
(E)-6-(4-(N,N-diphenyl)styryl)-2-(2-ethylhexyl)-1,8-
naphthalimide (NI‒TPA). NI-TPA was prepared by the same proce-
dure as compound NI-Cz using 1 (0.50 g, 1.29 mmol), 5 (0.42 g,
1.55 mmol), Pd(OAc)2 (5 mg, 0.016 mmol), P(o-tolyl)3 (16 mg,
0.071 mmol), DMF (10 mL), and triethylamine (3 mL). The crude
product was purified by column chromatography using the mixture
of petroleum and dichloromethane (vol. ratio 2:1) as an eluent to
get the desired compound as a red solid (0.39 g) with a yield of 52%
[31]. Mp ¼ 157e158 ꢁC. IR (in KBr), cmꢀ1: 3034, 2955, 2926, 2857,
1698, 1656, 1585, 1508, 1491, 1439, 1385, 1350, 1282, 1233, 1178,
1092, 1026, 964, 846, 831, 782, 751, 695, 617, 526. 1H NMR
(2.43 g) with a yield of 90.34%. Mp ¼ 117e118 ꢁC. IR (in KBr), cmꢀ1
:
1655,1628,1595,1513,1478, 1453,1361,1335, 1319,1229, 1186, 1112,
1014, 985, 912, 838, 749, 724, 654, 618, 568. 1H NMR (400 MHz,
CDCl3)
d
8.16 (d, J ¼ 7.7 Hz, 2H), 7.65 (d, J ¼ 8.3 Hz, 2H), 7.54 (d,
J ¼ 8.4 Hz, 2H), 7.47e7.40 (m, J ¼ 0.9 Hz, 4H), 7.30 (d, J ¼ 2.3 Hz, 2H),
6.84 (dd, J ¼ 17.6, 10.9 Hz, 1H), 5.87 (d, J ¼ 17.6 Hz, 1H), 5.38 (d,
J ¼ 10.9 Hz, 1H). Anal. Calcd. for C20H15N: C, 89.19; H, 5.61; N, 5.20.
Found: C, 89.16; H, 5.64; N, 5.20.
4-(Diphenylamino)benzaldehyde (4). Phosphorus oxychloride
(24 mL, 315 mmol) was added dropwise to DMF (30 mL) at 0 ꢁC, and
the mixture was stirred for 1 h at this temperature. Then triphe-
nylamine (10 g, 40 mmol) was added, and the reaction mixture was
stirred at 100 ꢁC for 6 h. When the reaction was finished the
mixture was cooled to room temperature, poured into ice water,
and neutralized to pH 7 with 5% NaOH aqueous solution. The so-
lution was extracted with ethyl acetate. The organic layer was dried
with anhydrous magnesium sulfate and concentrated by vacuum
evaporation. The crude product was purified by column chroma-
tography using the mixture of petroleum and dichloromethane
(vol. ratio 2:1) as an eluent to get the desired compound as a pale
yellow solid (5.76 g) with a yield of 52.74%. Mp ¼ 120e121 ꢁC. IR (in
KBr), cmꢀ1: 2741, 1688, 1584, 1503, 1488, 1450, 1427, 1330, 1304,
1288, 1269, 1219, 1185, 1155, 1111, 1074, 1028, 906, 824, 769, 757,
(400 MHz, CDCl3)
d
8.63 (d, J ¼ 7.2 Hz, 1H), 8.58 (d, J ¼ 8.0 Hz, 2H),
7.98 (d, J ¼ 7.8 Hz, 1H), 7.82 ‒ 7.70 (m, 2H), 7.51 (d, J ¼ 8.4 Hz, 2H),
7.34e7.27 (m, 5H), 7.15 (d, J ¼ 7.8 Hz, 4H), 7.10 (d, J ¼ 10.4 Hz, 4H),
4.21e4.05 (m, 2H), 1.96 (s, 1H), 1.40e1.27 (m, 8H), 0.94 (t, J ¼ 7.3 Hz,
3H), 0.88 (t, J ¼ 6.6 Hz, 3H). Anal. Calcd. for C40H38N2O2: C, 83.01; H,
6.62; N, 4.84. Found: C, 82.93; H, 6.71; N, 4.88.
2.4. Device fabrication and characterization
The electrical properties of the two D‒A molecules were eval-
uated in Al/molecule/ITO sandwich structures. The ITO glass sub-
strates were pre-cleaned in ultrasonic bath for 15 min each in
detergent, de-ionized water, acetone, and alcohol. The substituted
naphthalimide (25 mg) was deposited onto the pre-cleaned ITO
substrates by vacuum evaporation. The resultant film thickness was
around 70 nm which was measured by scanning electron micro-
scope (SEM) as shown in the Fig.1. Al top electrodes were deposited
onto the film surface via thermal evaporation at 10 ꢀ6 Torr through
a shadow mask.
696, 616, 535. 1H NMR (400 MHz, CDCl3)
d 9.81 (s, 1H), 7.68 (d,
J ¼ 8.6 Hz, 2H), 7.34 (t, J ¼ 7.7 Hz, 4H), 7.17 (d, J ¼ 6.9 Hz, 6H), 7.01 (d,
J ¼ 8.5 Hz, 2H). Anal. Calcd. for C19H15NO: C, 83.49; H, 5.53; N, 5.12.
Found: C, 83.47; H, 5.57; N, 5.12.
4-Vinyltriphenylamine (5). 5 was prepared by the same proce-
dure as compound 3 using 4 (2.73 g, 10 mmol), potassium tert-
butylate (1.68 g, 15 mmol), and CH3PPh3Br (4.28 g, 12 mmol). The
crude product was purified by column chromatography using the
mixture of petroleum and dichloromethane (vol. ratio 5:1) as an
eluent to get the desired compound as a white solid (2.23 g) with a
yield of 82.28%. Mp ¼ 90e91 ꢁC. IR (in KBr), cmꢀ1: 1625, 1590, 1506,
1486, 1411, 1328, 1283, 1267, 1175, 1074, 1026, 990, 890, 839, 758,
3. Results and discussion
3.1. Thermal gravimetric analysis of NI‒Cz, NI‒TPA
The thermal properties of NI‒Cz and NI‒TPA were investigated
by TGA as shown in Fig. 2. Both compounds exhibit good thermal
stability with thermal decomposition temperatures (5% weight loss
temperature) of 323.86 ꢁC (NI‒Cz) and 407.06 ꢁC (NI‒TPA),
respectively. It indicates that the materials can endure heat expo-
sure in the memory devices.
699, 649, 616, 581, 512, 490. 1H NMR (400 MHz, CDCl3)
d 7.28 (d,
J ¼ 8.6 Hz, 2H), 7.23 (d, J ¼ 8.0 Hz, 4H), 7.09 (d, J ¼ 7.8 Hz, 4H), 7.02
(d, J ¼ 8.0 Hz, 4H), 6.66 (dd, J ¼ 17.6, 10.9 Hz,1H), 5.63 (d, J ¼ 17.6 Hz,
1H), 5.15 (d, J ¼ 10.9 Hz, 1H). Anal. Calcd. for C20H17N: C, 88.52; H,
6.31; N, 5.16. Found: C, 88.50; H, 6.34; N, 5.16.
3.2. Optical properties and electrochemical properties
(E)-6-(4-(4aH-carbazol-9(9aH)-yl)styryl)-2-(2-ethylhexyl)-1,8-
naphthalimide (NI-Cz). A flask was charged with a mixture of 1
(0.50 g, 1.29 mmol), 3 (0.42 g, 1.55 mmol), Pd(OAc)2 (5 mg,
0.016 mmol), P(o-tolyl)3 (16 mg, 0.071 mmol), DMF (10 mL), and
triethylamine (3 mL). The flask was degassed and purged with N2.
The mixture was heated at 90 ꢁC for 24 h under N2. Then, it was
filtered, and the filtrate was poured into distilled water and
extracted with ethyl acetate. The organic layer was dried with
anhydrous magnesium sulfate and concentrated by vacuum evap-
oration. The crude product was purified by column chromatog-
raphy using the mixture of petroleum and dichloromethane (vol.
ratio 2:1) as an eluent to get the desired compound as a orange-
yellow solid (0.46 g) with a yield of 61.33%. Mp ¼ 210e211 ꢁC. IR
(in KBr), cmꢀ1: 3034, 2957, 2925, 1694, 1656, 1584, 1478, 1452, 1384,
1232, 1184, 1090, 964, 780, 752, 722. 1H NMR (400 MHz, CDCl3)
The UVevis absorption spectra of the two conjugated com-
pounds in dilute DCM solution and in the thin films are shown in
Fig. 3. In solution the high-energy absorption bands (294 nm,
307 nm and 342 nm) can be attributed to the
p‒p* transition of
d
8.70e8.59 (m, 3H), 8.17 (d, J ¼ 7.7 Hz, 2H), 8.07 (d, J ¼ 7.7 Hz, 1H),
8.00 (d, J ¼ 16.0 Hz, 1H), 7.89 (d, J ¼ 8.3 Hz, 2H), 7.85e7.79 (m, 1H),
7.67 (d, J ¼ 8.4 Hz, 2H), 7.53e7.40 (m, 5H), 7.32 (t, J ¼ 7.3 Hz, 2H),
4.15 (qd, J ¼ 12.8, 7.3 Hz, 2H), 1.96 (d, J ¼ 6.8 Hz, 1H), 1.43e1.27 (m,
8H), 0.95 (t, J ¼ 7.4 Hz, 3H), 0.89 (t, J ¼ 7.0 Hz, 3H). Anal. Calcd. for
Fig. 1. Schematic diagram of the memory device with thin film of NI‒Cz or NI‒TPA
sandwiched between an indium-tin-oxide (ITO) substrate and an aluminum top
electrode. The scale bar is 100 nm.

130
W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
Fig. 2. TGA curves of NI‒Cz (1), NI‒TPA (2) measured in nitrogen atmosphere at a heating rate of 10 ꢁC/min.
carbazole, triphenylamine and 1,8-naphthalimide moieties, and the
low-energy absorption bands (403 nm and 457 nm) arise from the
intramolecular charge transfer (ICT) [31]. Compared with the ab-
sorption spectra in solution state, the intramolecular charge
transfer (ICT) transition absorption peaks in the solid film state are
obviously red-shifted (NI‒Cz from 403 to 416 nm, and NI‒TPA from
457 to 467 nm) and significantly broadened. The red-shift and
broadened absorption peaks from solution to solid state might be
associated with the formation of molecular aggregation or orderly
stacking, so the absorption spectrum of the film of the NI‒Cz
molecule broadened more than NI‒TPA due to its coplanar carba-
p‒p
zole moiety that more readily participates in
p‒p stacking. In
addition, the optical bandgaps (Eogpt) of NI‒Cz and NI‒TPA films
determined from the onset of optical absorbance can be estimated
to be about 2.51 and 2.19 eV, respectively.
Fig. 4 shows the cyclic voltammograms of the two molecules
recorded in solution. Pertinent electrochemical data are summa-
rized in Table 1. Both molecules exhibited one irreversible oxidation
wave corresponding to the oxidation of the carbazole or triphe-
nylamine donor moieties with the oxidation peaks at 1.42 and
0.98 eV for NI‒Cz and NI‒TPA, respectively. The HOMO energy
levels can be calculated from the onset oxidation potential with
reference to ferrocene (4.8 eV) by the following equation HOMO
(eV) ¼ ‒[Eox (onset) vs Ag/AgCl þ 4.8 ‒ E
(ferrocene)], whereas
1/2
the LUMO energy levels can be determined from the difference
between the HOMO and the optical bandgap. The HOMO and LUMO
values were determined to be ꢀ5.66 (or ꢀ5.22) and ꢀ3.15
(or ꢀ2.54) eV for the NI‒Cz (or NI‒TPA) film.
The HOMO and LUMO energy levels of the two molecules and
the work function of Al top and ITO bottom electrodes were
considered here to understand the memory behavior of the mole-
cule based devices. For NI-Cz the energy barrier between ITO (‒
4.8 eV) and HOMO energy which was 0.86 eV which was lower than
that between Al (‒4.28 eV) and the LUMO energy level was 1.13 eV,
this indicated that hole injection from ITO into the HOMO of NI‒Cz
was much easier than the electron injection from Al into LUMO of
the compound. Thus, NI‒Cz was a p-type material and holes
controlled the conduction process. NI-TPA was also p-type material
Fig. 3. UVeVis absorption spectra of NI‒Cz and NI‒TPA in dilute DCM solution and
solid thin film on quartz glass substrate.
Fig. 4. Cyclic voltammograms of NI‒Cz (a), NI‒TPA (b) and Ferrocene (c) in anhydrous CH2Cl2 containing Bu4NClO4 (0.1 mol/L) with Ag/AgCl as reference electrode and Pt wire as
counter electrode. A scan rate 100 mV/s was used.

W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
131
Table 1
planar structure of the triphenylamine moiety. Compared to the NI-
Cz film with the coplanar carbazole moiety, the RMS roughness was
more than 30 nm which may cause metal penetration during the
evaporation process that affected the performance test. We also
observed similar morphology on other substrates with different
polarities, such as quartz and glass, indicating that it is the inter-
action between the molecules rather than the interaction between
the molecules and the substrate that drives the molecular self-
organization. Thus, altering the terminal group in D‒A molecules
can adjust the film morphology.
HOMOꢀLUMO energy levels, optical and electrochemical band-gap energies, and
ionization potentials of NI‒Cz and NI‒TPA.
Molecule
EHOMO (eV)a
ELUMO (eV)b
Eeglc (eV)c
Eogpt (eV)d
NI-Cz
NI-TPA
‒5.66 (ꢀ5.50)e
‒3.15 (ꢀ2.72)e
2.78
2.6
2.51
2.19
‒5.22 (ꢀ5.14)e
‒3.03 (ꢀ2.54)e
a
EHOMO ¼ ‒(Ep þ 4.8) (eV).
b
ELUMO ¼ ‒(En þ 4.8) (eV) (where En and Ep are onset oxidation potentials versus
the Fc/Fcþ).
c
Having HOMO and LUMO bands value, it is simple to calculate electrochemical
band gap Egelc ¼ ELUMO ‒ EHOMO (eV).
d
The optical band gap estimated from the onset wavelength of optical absorption
according to the formula Eg ¼ 1240/ledge, in which the ledge is the onset value of
3.4. Currentevoltage (IeV) characteristics of the memory devices
absorption spectrum in long-wave direction.
e
Corresponding to the model compounds NI-CZ, NI-TPA calculated at the B3LYP/
The electrical switching and memory effects of NI‒Cz and NI‒
TPA were illustrated by IeV characteristics of an electronic device
with the molecule film sandwiched between the ITO and Al elec-
trodes. Rather than encoding “0” and “1” as the amount of charge
stored in a cell in silicon devices, small molecular memory stored
date in an entirely different form which was based on the high- and
low-conductivity response to an applied voltage. Fig. 6 (a, c) shows
the typical IeV curves of the small molecular devices fabricated
with NI‒Cz and NI‒TPA. Taking NI‒Cz for example, as the initial
voltage swept from 0 to ꢀ2 V, current is very low and in the order of
10ꢀ7‒10ꢀ4 A. It reveals that the device is in a low-conductivity state
and assigned as the OFF-state or “0” signal in data storage. How-
ever, the continuous sweeping voltage induces an abrupt increase
in current around ꢀ2.44 V, which is defined as the switching
threshold voltage. After switching, the current reaches at almost 3
orders of magnitude higher than that in a low-conductivity state.
This increase in current indicates that the device is transformed
6-31G level.
deduced from UVevis absorption spectra and the CV
measurements.
3.3. Morphology of the thin films
Atomic force microscopy (AFM) was used to characterize the
surface morphology of the vacuum-deposited electroactive layers.
As shown in Fig. 5, the surface morphology of the NI‒Cz film
vacuum-deposited on the ITO glass substrate was uniform and
homogenenerous, and the surface root-mean-square (RMS)
roughness was only several nanometers which may be beneficial
for improving the device’s performance. However, the surface
morphology of the NI‒TPA film was agglomerative due to the non-
Fig. 5. Tapping-mode (20
respectively.
mm ꢂ 20
mm) AFM topography (a, c); typical cross-section profile of AFM topographic image (b, d) of NI‒Cz and NI‒TPA films on ITO substrates,

132
W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
from the low-conductivity state to a high-conductivity state and
assigned as ON-state or “1” signal in data storage. This electronic
transition from OFF-state to ON-state serves as the “writing” pro-
cess. The device remains in a high conductivity (ON-state) during
the subsequent second sweep from 0 to ꢀ4 V and does not relax to
the OFF-state even after the power is turned off for 20 min. As the
voltage sweeps negatively from 0 to 4 V, an abrupt decrease in
current is observed at a threshold voltage of about 2.88 V, corre-
sponding to the “erasing” process for the memory device. Similarly,
the device remains in a low conductivity (OFF-state) during the
subsequent fourth sweep from 0 to 4 V and even after the power is
turned off. The OFF-state can be further turned on to the ON-state
by reapplying the switching threshold voltages, as the fifth sweep
shown in Fig. 6a. Therefore, this memory device could be used as a
flash type memory. Compared to NI‒Cz, the device based on NI‒TPA
was also a flash type memory, but it showed a much lower turn on
threshold voltage of ꢀ0.85 V, which revealed by cyclic voltammetry
and UVevis absorption spectroscopy measurement.
from 50 cells (10 times for each cell) for the two devices based on
either NI‒Cz or NI‒TPA. For NI‒Cz, the turn-on voltage was mainly
between ꢀ2.2 V and ꢀ2.6 V (with yield of 80%), and turn-off voltage
was mainly between 2.6 V and 3 V (with yield of 56%). For NI-TPA,
the turn-on voltage varied from ꢀ0.4 V to ꢀ2.0 V, and the turn-off
voltage was from 2.4 V to 4.0 V, so it showed no obvious distributed
areas. Thus the device based on NI‒Cz has better stability and
repeatability compared to NI‒TPA due to the coplanar carbazole
moiety which suggested that the film with the smooth surface was
beneficial for improving the probability and stability of high-
density data-storage devices.
3.6. Proposed data-storage mechanism
To understand the mechanism for the observed flash-type
memory behavior of NI‒Cz and NI‒TPA system, quantum calcula-
tions were carried out at the B3LYP/6-31G(d) level with the
Gaussian 03 program package. The carbazole and triphenylamine
moieties are well-known as electron donors and 1,8-naphthalimide
was also known as electron acceptor. The charge transfer process
can occur when the conjugated molecule is activated by electrical
stimulation, transferring charges from either the carbazole or tri-
phenylamine to the 1,8-naphthalimide and forming a charge-
transfer (CT) state.
It is obvious that the 1,8-naphthalimide moiety contributed
more to the LUMO while carbazole/triphenylamine contributed
more on the HOMO as shown in Fig. 8. Under an applied field, a hole
would be easier to transfer from the Al electrode to NI‒Cz (or NI-
TPA) memory layer due to the lower hole injection barrier be-
tween ITO (‒4.8 eV) and HOMO energy which was 0.7 eV than that
between Al (‒ 4.28 eV) and the LUMO energy level was 1.56 eV.
With the increase of the bias up to the threshold voltage, more and
The stability of the memory effects was also evaluated under the
same atmosphere. Fig. 6 (b, d) showed the retention time tests
under a constant stress of 1 V for both the ON and OFF states of the
NI‒Cz and NI‒TPA device, respectively. The ON/OFF current ratios of
103w104 could be maintained and no obvious degradation in cur-
rent was observed for both the ON and OFF states after periods of at
least 104 s.
3.5. The probability and stability of the memory devices
Considering the fact that the bistability phenomenon in organic
devices is sometimes non-reproducible and unreliable, it is
important to investigate the cell-to-cell uniformity for small
molecule based devices. Fig. 7 shows the statistical data obtained
Fig. 6. Current density-voltage (IeV) characteristics and effect of the operation time (at 1 V) on the device current density in the OFF and ON states of the device.

W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
133
Fig. 7. The statistical data of reproducibility of the devices based NI‒Cz (a) or NI‒TPA(b).
more charges continue to be generated with high electric field and
4. Conclusions
the positive and negative charges are segregated, leading to the
formation of a stable CT complex. Then the device is switched from
the OFF state to the ON state. Even after the driving power is turned
off, the ON state of the memory device could still remain due to the
good stability of the CT complex formed. However, applying an
opposite voltage causes the CT complex decomposition and the
device returns to the original OFF state. Hence, the memory device
based on NI‒Cz or NI‒TPA exhibits flash memory behavior.
In this work, we have successfully synthesized two DeA mole-
cules and demonstrated the effects of the coplanarity of the donor
moiety on the reproducibility of switching phenomenon for the
memory device, which based on NI‒Cz was much better than that
based on NI‒TPA due to the rigid carbazole moiety in favor of
improving the surface morphology, which was revealed by atomic
force microscopy measurement. Both devices show flash memory
Fig. 8. Optimized geometries and electronic density counters of molecular orbital of NI‒Cz and NI‒TPA.

134
W. Ren et al. / Dyes and Pigments 100 (2014) 127e134
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Acknowledgments
This work was financially supported by the Chinese Natural
Science Foundation (21076134, 21176164 and 21206102), NSF of
Jiangsu Province (BK2010208), a project of the Department of Ed-
ucation of Jiangsu Province (12KJB430011), Suzhou Nano-project
(ZXG2012023) and Project supported by the Specialized Research
Fund for the Doctoral Program of Higher Education of China (Grant
No. 20113201130003 and 20123201120005). A Project Funded by
the Priority Academic Program Development of Jiangsu Higher
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