Synthesis and nonvolatile memristive switching effect of a donor-acceptor structured oligomer

Article abstract of DOI:10.1039/c4tc02285h

As promising nonlinear dynamic electronic devices with widespread potential applications in computer data storage and neuromorphic implementations, memristors have attracted tremendous attention in both materials science and condensed-matter physics. In this work, self-rectified memristive behavior was for the first time observed in a highly soluble metal-free all-organic material containing electron-rich fluorene and dithieno[3,2-b:2′,3′-d]pyrrole moieties in the oligomer mainchain as conjugated channels for charge carriers and electron-poor 9,9-bis(3,4-bis(3,4-dicyanophenoxy)phenyl side chains in the C-9 position of the fluorene units (hereafter denoted as "PFD-8CN"). Electrical conductance of PFD-8CN can be switched between two states with a rectification ratio of ～10, incrementally and repeatedly. Non-linear transmission characteristics of a biological synapse are also realized through consecutive multilevel conductance switchings. The novel electrical response of the device arises from electric field-induced charge transfer interactions in the donor-acceptor structured oligomer. The development of the organic memristor may offer new opportunities for the construction of molecular electronics that can greatly enhance the performance of modern computer systems.

Full text of DOI:10.1039/c4tc02285h

Journal of
Materials Chemistry C
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Synthesis and nonvolatile memristive switching
effect of a donor–acceptor structured oligomer
Cite this: J. Mater. Chem. C, 2015, 3,
Run-Wei Li, b Wenbin Zhang,b Luxin Wanga
664
a
b
ac
*
*
Cheng Wang,† Gang Liu,† Yu Chen,
and Bin Zhanga
As promising nonlinear dynamic electronic devices with widespread potential applications in computer data
storage and neuromorphic implementations, memristors have attracted tremendous attention in both
materials science and condensed-matter physics. In this work, self-rectified memristive behavior was for
the first time observed in a highly soluble metal-free all-organic material containing electron-rich
fluorene and dithieno[3,2-b:20,30-d]pyrrole moieties in the oligomer mainchain as conjugated channels
for charge carriers and electron-poor 9,9-bis(3,4-bis(3,4-dicyanophenoxy)phenyl side chains in the C-9
position of the fluorene units (hereafter denoted as “PFD-8CN”). Electrical conductance of PFD-8CN can
be switched between two states with a rectification ratio of ꢀ10, incrementally and repeatedly. Non-
linear transmission characteristics of
a biological synapse are also realized through consecutive
Received 10th October 2014
Accepted 9th November 2014
multilevel conductance switchings. The novel electrical response of the device arises from electric field-
induced charge transfer interactions in the donor–acceptor structured oligomer. The development of
the organic memristor may offer new opportunities for the construction of molecular electronics that
can greatly enhance the performance of modern computer systems.
DOI: 10.1039/c4tc02285h
www.rsc.org/MaterialsC
neurons, viz. short term and long term memory behavior, is
simply realized through spike-dependent incremental resis-
Introduction
tance (or conductance), switching characteristics in two-
terminal devices.7,8 According to the redenition of L. Chua in
2011,9 all two-terminal nonvolatile resistance switching
memory devices are memristors, regardless of the device
material or the physical operating mechanisms.
Designing novel device concepts and discovering new func-
tionalities through rational design and synthesis of novel
functional materials are always major tasks for chemists and
material scientists. Over the past few decades, great efforts have
been devoted to developing innovative technologies that exploit
new concepts and materials for the evolution of electronics.1–4
As the fourth passive circuit element beyond the fundamental
resistor, capacitor and inductor, the memristor was originally
envisioned by L. Chua5 in 1971 and considered capable of
mimicking the function of a mammalian synapse with the
ability to learn and memorize new information, and to allow
powerful and energy-efficient processing capability for complex
open-ended computing tasks. Aer having being proposed for
thirty seven years, the missing memristor was rst physically
implemented by physicists and electrical engineers in 2008 with
metal oxide crossbar arrays.6 Emulation of the learning and
memorizing ability of synapses and the connected neighboring
Organic and oligomeric functional materials carry inherent
compatibility with living tissues for the construction of bio-
logically implantable neuromorphic memristors. The wide
scope of organic chemistry and vast reservoir of functional
molecules also allow ne tuning of the structural, chemical and
electronic properties of the switching media. Nevertheless, less
attention has been paid to the development of high-perfor-
mance organic memristors. In pioneering works on organic
memristors, oligomeric and polymeric materials were used
either in metal-containing or multi-component redox
systems.10,11 The involvement of metal ions or foreign species,
however, may not always give rise to uniformly dispersed and
compatible components, and may lead to gradual degradation
of the functioning oligomers, therefore resulting in less favor-
able performance of the memory behaviors. To solve such
compatibility and stability issues, a solution-processable olig-
omer that can provide the necessary electronic properties
within a single molecule and still possess good chemical,
mechanical and morphological characteristics is desired for
memristor device applications. Beneting from the well-devel-
oped areas of organic electronics, in particular, the photovoltaic
aKey Laboratory for Advanced Materials, Institute of Applied Chemistry, East China
University of Science and Technology, 130 Meilong Road, Shanghai 200237, China.
E-mail: chentangyu@yahoo.com
bKey Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material
Technology and Engineering, Chinese Academy of Sciences, 1219 West Zhongguan
Road, Ningbo, Zhejiang 315201, China. E-mail: runweili@nimte.ac.cn
cThe State Key Laboratory of ASIC & System, Fudan University, 220 Handan Road,
Shanghai 200433, China
† These authors contribute equally to this work.
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and bistable switching devices,12 rational design and synthesis Moreover, this oligomer demonstrates learning and memo-
of memristive oligomers can be made possible, both theoreti- rizing capability with its conductance showing multilevel
cally and experimentally.
characteristics that are strongly dependent on the history of the
Basically, the molecular structure and memristive properties applied electrical elds, and bringing neuromorphic computing
of organic materials can be ne-tailored by functionalizing closer by imitating the performance of synapses. Experimental
them with electron donors (D) and acceptors (A)13 of different observation and molecular simulation indicate that the mem-
strengths, spacer moieties of different steric effects for the ristive switching is related to the formation of electrically more
electroactive pendant groups, or nanostructured electroactive conductive CT complexes of the cationic DTPc+ and anionic
materials. By utilizing coplanar fused aromatic molecules as CNcꢁ pairs.
building blocks, which can form a highly crystalline thin lm
microstructure domain leading to efficient charge transport,
one can easily achieve high-performing organic/oligomeric
Experimental
General
semiconducting materials. As an analogue of the carbazole
molecule, dithieno[3,2-b:20,30-d]pyrrole (DTP) and its derivatives
have emerged as popular building blocks for the construction of
oligomeric semiconducting materials due to their good struc-
tural planarity and strong electron-donating ability arising from
nitrogen atoms.14–16 In this work, we synthesized a novel DTP
and uorene-based D–A type conjugated oligomer, PFD-8CN, as
shown in Scheme 1. The utilization of coplanar donor molecule
of DTP, in conjunction with the hole-transporting uorene
moiety,16 can lead to enhanced intermolecular packing, a lower
energy bandgap and higher electrical conductance for fast
access of the programmed states. The presence of multiple
electron accepting cyano (–CN) moieties may tune the CT
interaction and the transport behavior of the oligomer succes-
sively. Therefore, a stable and incrementally switching of the
material conductance is observed in the tantalum/oligomer/
platinum structure with a high OFF state conductivity over 100 S
mꢁ1 and an interesting self-rectifying nature for over 700 cycles.
Weight-average (Mw) and number-average (Mn) molecular
weights of the oligomer were determined with a Waters 2690 gel
permeation chromatography (GPC), using monodispersed
polystyrene samples as the molecular weight standards and
tetrahydrofuran (THF) as the eluent. Fourier transform infrared
(FTIR) spectra were measured on a Nicolet Nagma-IR 550
spectrophotometer by dispersing the samples in KBr pellets. 1H
nuclear magnetic resonance (1H NMR) spectra were measured
on a 400 MHz Bruker Advance 500 NMR spectrometer with
CDCl3 as the solvents and tetramethylsilane (TMS) as the
internal standard. UV-visible (UV-vis) absorption spectra of the
oligomer solution and lm samples were obtained on a Shi-
madzu UV-2540 spectrophotometer. Steady-state uorescence
spectra were measured on a HORIBA-JOBIN-YVON Fluoromax-4
spectrouorophotometer. The sample for the uorescence
measurement was dissolved in dry solvents, ltered and trans-
ferred to a long quartz cell, where it was capped and degassed
Scheme 1 Synthesis of PFD-8CN.
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with ultrapure nitrogen for 10 min before measurement. Cyclic evaporation. The devices were characterized under ambient
voltammetric measurement was performed on a CHI 650D conditions, using a Keithley 4200 semiconductor characteriza-
electrochemical workstation (Chenhua, Shanghai, China) using tion system equipped with a Keithley 3402 pulse generator, in
a three-electrode cell under a nitrogen atmosphere. The olig- both the voltage sweeping and pulse mode. The sweeping step
omer lm coated on a Pt disk electrode (working electrode) was was 0.01 V. A compliance current (CC) presetting of 1 mA was
scanned anodically and cathodically in an electrolyte solution of used to avoid over-striking or permanent breakdown of the
tetrabutylammonium perchlorate (n-Bu4NClO4) in acetonitrile sample. An atomic force microscopy (Dimension V, Veeco)
(0.1 M), with Ag/AgCl and a platinum wire as the reference and equipped with a conducting cantilever coated with Pt/Ir was
counter electrode, respectively. The Pt working electrode was employed for the conductive atomic force microscopic (C-AFM)
polished with BAS polishing alumina suspension and rinsed measurements of the oligomer/Pt devices. During the
with acetone before use.
measurement, the tip was always grounded while a bias voltage
was applied on the Pt lm.
Synthesis and characterization
Molecular simulation
The synthesis of monomers and the oligomer were carried out
in an ultrapure nitrogen atmosphere following the procedure as
shown in Scheme 1. The monomers 1 and 2 were synthesized
according to the literature.17,18 The PFD-8CN oligomer was
synthesized through Stille coupling reaction. In general, 135 mg
of 1 (0.22 mmol) and 209 mg of 2 (0.20 mmol) were dissolved in
8 mL anhydrous toluene in a 25 mL Schlenk tube and degassed
with nitrogen for 15 min. Aer adding 10 mg of Pd(PPh3)4 into
the Schlenk tube, the reaction mixture was degassed for another
15 min and then heated to 120 ꢂC for 48 h. Upon being cooled to
room temperature, the crude product was collected by precipi-
tation in methanol (100 mL) with vigorously stirring. The as-
received solid was re-dissolved in CHCl3 and ran through a
siliceous earth column quickly to remove the metal catalyst
residue. Then the crude oligomer was subjected to Soxhlet
extraction with acetone, hexane, and CHCl3 sequentially to
remove the unreacted monomers and the trace catalyst. CHCl3
portion was concentrated and re-precipitated in methanol, and
dried under vacuum overnight to obtain the dark-red target
oligomer (PFD-8CN) with a yield of 65.1% (153 mg). Mn ¼ 4.52 ꢃ
103 (four repeating units per molecule), Mw/Mn ¼ 1.13. 1H-NMR
(CDCl3, 400 MHz): d/ppm ¼ 7–8 (m, 26H), 4.08 (m, 2H), 1.97
(m, 1H), 1.31 (m, 8H), 0.90 (m, 6H). The resultant oligomer does
not show a glass-transition temperature when heated up to 350 ꢂC.
Td (5%) ¼ 379 ꢂC (in nitrogen). All chemicals were purchased from
Aldrich and used without further purication. Organic solvents
were puried, dried and distilled under dry nitrogen.
Calculations of the optimized geometry and electronic struc-
ture, including the dipole moment, electrostatic potential (ESP)
surface, as well as HOMO and LUMO of the basic unit of the
PFD-8CN oligomer were carried out on a Compaq ES40 super-
computer using the Gaussian 09 program package and DFT
calculations at the B3LYP/6-31G(d) level.19 Based on the ground
state calculations using the above-mentioned method, the cor-
responding electronic properties of the basic unit of PFD-8CN
in the excited (or, ON) state were calculated using the congu-
ration interaction approach involving the single-electron exci-
tation method at the CIS/6-31G(d) level with the Gaussian 09
program package.20,21
Results and discussion
PFD-8CN was synthesized through the palladium-catalyzed
coupling reaction of monomers 1 and 2 in the presence of tet-
rakis(triphenylphosphine)palladium(0) as the catalyst. This
oligomer is soluble in many common organic solvents. By GPC
analysis against a linear polystyrene standard, PFD-8CN was
found to have Mn of 4.52 ꢃ 103 (four repeating units per
molecule), and a polydispersity of 1.13. The chemical structure
of PFD-8CN can be veried through 1H-NMR and Fourier
1
transform infrared (FTIR) spectroscopy. Its H-NMR spectrum
reveals proton signals of the alkyl side chain of the DTP donor
with the chemical shis of 4.08 (m, 2H), 1.97 (m, 1H), 1.31 (m,
8H), and 0.90 (m, 6H) ppm, as well as that of the benzene rings
at d/ppm ¼ ꢀ7–8 (m, 26H) for the conjugated D–A system. The
Fabrication and measurements
The electrical properties of PFD-8CN were evaluated in Ta/ moderate absorption peak at 2230 cmꢁ1 appeared in the IR
oligomer/Pt sandwich structures. The Pt/Ti/SiO2/Si substrates spectrum can be attributed to the stretching vibration mode of
were pre-cleaned with ethanol, acetone and isopropanol in an the cyano groups, while the absorption peaks at 1597, 1488 and
ultrasonic bath, each for 20 min in that order. Aer being 1407 cmꢁ1, as well as that at 1260 and 1100 cmꢁ1, are associated
ltered through a polytetrauoroethylene (PTFE) membrane with the skeleton and C–H bending vibrations of the aromatic
micro-lter with a pore size of 0.45 mm, a 50 mL cyclohexanone rings, respectively.
solution of PFD-8CN (1.0 wt% oligomer) was spin-coated onto
PFD-8CN demonstrates a distinctive asymmetric resistive
the substrates at a spinning speed of 400 rpm for 12 s and then switching behavior at room-temperature, as shown in the
2000 rpm for 40 s, followed by vacuum-drying at 80 ꢂC over- current–voltage (I–V) characteristics of Fig. 1a. Before the elec-
night. The thickness of the oligomer lm was about 100 nm as trical measurements, a 100 nm thin lm of PFD-8CN was spin-
measured by spectroscopic ellipsometer (model M2000DI, coated from the solution onto a grounded Pt/Ti/SiO2/Si
Woollam). Ta top electrodes with a diameter of 100 mm and substrate. A metal/insulator/metal sandwiched structure was
thickness of 50 nm were deposited on the lm surface at room formed and tested with an evaporated tantalum pad of 100 mm
temperature under reduced pressure (below 10ꢁ5 Pa) by E-beam in diameter as the top electrode (inset of Fig. 1a). The biased-
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Fig. 1 Memristive characteristics of PFD-8CN. (a) 3D current–voltage profile of PFD-8CN in a metal/oligomer/metal sandwich structure; (b)
distribution of sample conductance in both the OFF and ON states; (c) Weibull exponent (k) versus standard deviation to mean ratio (D/m) for the
ON and OFF state conductance of PFD-8CN and reported devices (green,26 pink,28 blue,29 magenta30); and (d) switching cyclability of the sample
under pulse operation mode. Inset of (a) is the schematic illustration of Ta/oligomer/Pt structure.
sweeping voltage was applied across the sandwich structure,
with a compliance current presetting of 1 mA employed to avoid
over striking and permanent breakdown of the switching
media. Initially present in the OFF state, PFD-8CN shows a
moderate conductivity of 100 S mꢁ1 ꢀ800 S mꢁ1, probably
arising from the ground state charge transfer interaction (see
later section for detail discussion). This conductivity value is
comparable with that of the chemically-doped conducting
polymers, and is favorable for the fast reading of the device
states. By sweeping the voltage in the direction of 0 V / ꢁ1 V
/ 0 V / 2 V / 0 V, PFD-8CN can be switched smoothly
between two resistance states with a stable ON/OFF ratio of ꢀ10
(read at ꢄ0.5 V). Differing from the bistable resistive switching
showing abrupt resistance or conductance jumps, the electrical
transition observed here demonstrates a smoother tuning of the
sample conductance during the voltage sweeping processes.
Together with a “pinch-off” feature in the I–V curve (Fig. 2a), this
resistive switching phenomenon is denitely the characteristic
of a memristor device.22 Interestingly, a self-rectifying effect
with the rectication ratio of ꢀ10 in both ON and OFF states,
which may arise from the Schottky barrier for charge carrier
injection at the Ta/PFD-8CN interface (see later section for
details), was also observed in the oligomer. Although the self-
rectifying effect of the present PFD-8CN device is relatively less
obvious, it still provides a simple but effective approach to
Fig. 2 (a) 2D current–voltage profile of the PFD-8CN memristive
device that demonstrates a “pinch-off” characteristic; and (b) retention
capability of the PFD-8CN memristive device in both the ON and OFF
states.
eliminate the incorporation of additional control elements to
solve the cross-talking problem in crossbar-structured devices,
or to emulate the single-direction information transmission in
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biological systems.23–25 Further improvement of the self-recti- to a narrower distribution of the variable.28,29 In comparison, the
fying effect may be further achieved by ne-tuning the elec- PFD-8CN devices exhibit k and D/m values comparable to that of
tronic structure of the electroactive materials. The atmospheric the reported devices with either oxide or organic materials as
moisture or oxygen may cause minor uctuation in the device the switching media. The standard deviation (D) to mean (m)
resistances, but are unlikely to be the governing factors that ratios of the OFF and ON state conductance are found to be
induce the observed resistive switching in PFD-8CN 0.283 and 0.364, respectively, which are lower than or compa-
devices.10,26,27
rable to that of the other reported devices.28,30–32 Moreover, the
To investigate the uniformity of the conductance-switching conductance of the oligomer can be well maintained under a
behavior, cyclic sweeping operations of the Ta/oligomer/Pt constant read voltage stress of 0.1 V for 104 s, or switched
device was performed. A narrow distribution of the device between the ON and OFF states by pulse operation for 6000
conductance at both the OFF and ON states in over 700 cycles, respectively (Fig. 1d and 2b), again suggesting the
switching cycles is demonstrated in Fig. 1b. Weibull distribu- excellent uniformity and stability in the switching behavior of
tion is employed to further evaluate the uniformity of the PFD-8CN.
resistive switching parameters by the following equation:
The other characteristics of a synaptic memristor is that its
synaptic weight, usually dened as the conductance of the two
terminal device, can be modulated and memorized using
consecutive spikes. As shown in Fig. 3a and b, the conductance
of the PFD-8CN device can be decreased continuously, with the
corresponding positive (0 V to 2 V) and negative (0 V to ꢁ1 V)
voltage sweeps executed consecutively. A better illustration of
the degressive change of the oligomer memristor's synaptic
weight, demonstrated as the sweeping voltage and responding
current versus time characteristics, as well as the device
conductance obtained at the end of each sweep, are plotted in
Fig. 3c. It is interesting that a single negative or positive sweep
can increase or decrease the device conductance (or synaptic
weight), respectively, while both the consecutive negative and
positive sweeps serve as synaptic spikes to inhibit the bio-
mimicking memristor devices, where similar phenomena
observed in biological synapse are dened as nonlinear trans-
mission characteristics. The reason for consecutive negatives
F(x) ¼ 1 ꢁ exp(ꢁ(x/x0)k)
(1)
where x is a random variable, x0 is the scale parameter of the
distribution of x, k is dened as the Weibull exponent and F(x) is
the cumulative probability of nding a randomly-distributed
variable (a switching parameter in the present work) below x
relative to a scaling constant x0. The Weibull exponent can be
correlated with the standard deviation (D) to mean (m) ratio (D/
m) using the following equation:
 Á
À
ÁÃ
1=2
G 1 þ 2=k ꢁ G2 1 þ 1=k
Gð1 þ 1=kÞ
D=m ¼
(2)
where G is the Gamma function. The plots of Weibull exponent
k as a function of the standard deviation to mean ratio D/m, for
the conductance of the PFD-8CN and other reported devices, are
shown in Fig. 1c, where a larger Weibull exponent corresponds
Fig. 3 Nonlinear transmission characteristics of the PFD-8CN memristor. (a and b) Current–voltage characteristics of the memristor at negative
and positive biased sweeps; and (c) current and voltage versus time profiles of the memristor plotted from the data in (a) and (b).
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acting as the inhibitory synaptic spikes is still not clear at the stabilized notably through solute–solvent interaction, lowering
moment. the energy gap between the ground and the CT states, resulting
The switching mechanism of the oligomer is proposed as in a red-shi of the optical absorption maximum consequently.
charge transfer enhanced conductance in the thin solid lms, Therefore, the observed bathochromic (or positive sol-
and is veried via both theoretical and experimental analysis. It vatochromic) phenomena in PFD-8CN also conrm the occur-
is noteworthy that PFD-8CN sample demonstrate charge trans- rence of charge transfer interaction in the ground state. When
ferred nature at ground state. In dilute solution, PFD-8CN the oligomer concentration is increased in the chloroform
shows a moderate optical absorption peak at 280 nm, which can solution, additional absorption shoulder associated with
be assigned to the p–p* transition of the conjugated backbone intermolecular (charge transfer) interaction between neigh-
(Fig. 4a). Intramolecular CT characteristics can be evidenced boring oligomer chains is observed at ꢀ500 nm (Fig. 4c). This
from the strong absorption in the wavelength range of 400 to absorption shoulder, together with the strong CT peak, merges
600 nm, ascribed to the coupling between the n–p* and p–p* into a single broader band when PFD-8CN is cast into thin lms,
transitions of the N-aryl rings and the pendant cyano moieties. suggesting that the intermolecular CT interaction has been
With an increase in the solvent polarity, the charge separated further intensied in the solid state, where the rigid-rod olig-
state (or, dipole moment) of the D–A macromolecule will get omer chains are stacked more closely to give an extended p–p
Fig. 4 UV-vis absorption (a) and fluorescence ((b), lex ¼ 440 nm) spectra of PFD-8CN in different organic solvents; (c) UV-vis absorption spectra
of PFD-8CN in solid thin film and in chloroform solution with different concentrations; (d) fluorescence spectra of the monomers 1, 2 and PFD-
8CN in chloroform, lex ¼ 440 nm; (e) UV-vis absorption spectra of the monomers 1, 2 and PFD-8CN in chloroform; and (f) UV-vis absorption and
fluorescence spectra of PFD-8CN in chloroform. Concentration: ꢀ1 mg Lꢁ1
.
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stacking between the donor and acceptor unities. However, the visible region (for instance, 480 nm in toluene, 517 nm in
such aggregates are not efficient due to the large amount of chloroform, 519 nm in benzonitrile and 521 nm in dime-
pendant groups tethered to the C-9 position of the uorene thylformamide). The nding that the uorescence intensity
unit, therefore hindering the coplanar association of the decreases with increasing solvent polarity again suggests the
aromatic backbone, and resulting in the unchanged wavelength occurrence of electron transfer process from the DTP donor to
of the CT absorption maximum.
the –CN acceptor moieties.
Given the donor–acceptor conjugation structure of the PFD-
By using electrochemical measurements, optical character-
8CN oligomer, intramolecular interaction is further explored ization and molecular simulation, the thermodynamics of the
through photoinduced event studies. Upon being exposed to an charge transfer process in PFD-8CN is furthered assessed. Cyclic
excitation light beam of the 440 nm wavelength, the DTP voltammetry (CV) measurements reveal oxidation of the DTP
monomer 2 exhibits an intensive emission peak at 512 nm, unit with doublet anodic responses at Eonset(OX1) ¼ 0.55 V and
which is similar to the reported values of the DTP moieties Eonset(OX2) ¼ 1.30 V, respectively.34 As the oligomer lm dis-
(Fig. 4d), while the monomer 1 with eight CN groups has a solved aer being electrochemically oxidized in electrolyte
much reduced emission at the longer wavelength of 527 nm. In solutions, the reversible reduction of the formed cationic radi-
comparison to monomer 2, the chloroform solution of the PFD- cals of the electron donating backbone was not detected.
8CN oligomer shows signicantly quenched dimeric emission Cathodic reduction of PFD-8CN was also observed at ꢁ1.79 V.
bands of the DTP entities at 517 and 550 nm (Fig. 4b), from Due to considerable ground-state CT interaction between the
which the energy of the singlet state is estimated to be 2.40 eV DTP donor and the –CN acceptor entities, the oligomer exhibits
with the following formula:
a low energy bandgap of ꢀ2.34 eV which well agrees with that
(ꢀ2.36 eV) derived from the optical absorption edge of Fig. 4a.
Thus, the energy levels of the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) of PFD-8CN are derived to be ꢁ4.97 eV and ꢁ2.63 eV,
respectively. With the working functions of ꢁ4.25 eV for Ta and
ꢁ5.65 eV for Pt,35 respectively, Schottky and Ohmic contacts are
formed at the TA/PFD-8CN and PFD-8CN/Pt interfaces
accordingly.
c
Esinglet ¼ h
(3)
lem
where h is the Planck constant (6.62 ꢃ 10ꢁ34 m2 kg sꢁ1), c is the
speed of light (3 ꢃ 108 m sꢁ1) and lem is the characteristic
emission wavelength of the oligomer (517 nm). The uores-
cence quantum yield of the PFD-8CN oligomer is only ꢀ4%,
which is much smaller than that of the other DTP-based
materials.33 The observed uorescence quenching in PFD-8CN,
as well as the poor matching between the emissive spectrum of
the DTP donor and the absorption spectrum of the –CN
acceptor (Fig. 4e), also indicate the occurrence of CT between
the electron donor and the acceptor at ground states. Similar to
the bathochromic effect, increasing the polarity of the solvents
will stabilize the charge separated state of the oligomer, leading
to a quenched, broader and red-shied emissive spectrum in
According to the Rehm–Weller equations:
ꢁDGCR ¼ Eonset(OX1) ꢁ Eonset(RED) + DGS
(4)
(5)
(6)
ꢁDGCS ¼ DE0–0 ꢁ (ꢁDGCR
DGS ¼ ꢁe2/4p330RCC
)
Fig. 5 Plausible electric field-induced electronic transition in PFD-8CN.
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where DE0–0 is the energy of the 0–0 transition between the
lowest excited state and ground state of the donor moiety, and
evaluated as 2.52 eV from the intersection of absorbance and
uorescence spectra of PFD-8CN at 493 nm (Fig. 4f), e is the
elementary charge (1.6 ꢃ 10ꢁ19 C), 30 is the vacuum permittivity
(8.85 ꢃ 10ꢁ12 F mꢁ1), 3R is static dielectric constant of the
solvent (37.5 for acetonitrile and 4.81 for chloroform, respec-
tively) used for the electrochemical measurements, RCC is the
distance between the positive and negative charges in the
charge-separated state of PFD-8CN and estimated to be 1.15 nm
(distance between the pyrrole nitrogen atom and the cyano
nitrogen atom) through Gaussian molecular simulation, the
coulombic potential energy between the cation and anion
radicals (DGS), as well as the driving forces (DGCR and DGCS
,
changes of free Gibbs energy) for the charge recombination and
separation processes, are derived as ꢁ0.26, ꢁ2.08 and ꢁ0.44 eV
(in chloroform solution), respectively. The negative DGCS indi-
cates that the occurrence of the charge-separation process is
exothermic and simultaneous to form the (DTPc+–CNcꢁ)n
radical ion pair, while the charge-separated state is metastable
with the absolute value of DGCS smaller than that of DGCR.
36 The
calculated driving forces are not completely accurate, as PFD-
8CN cannot be dissolved in acetonitrile, and the uorescence
spectra obtained in chloroform solution is used instead.
Nevertheless, such metastable charge-transferred state may be
further enhanced under the stimuli of external electric eld,
and may be responsible for the observed conductance switching
behavior of the PFD-8CN device.
Finally, the plausible electronic scenario of the CT process
and conductance switching of the PFD-8CN oligomer is
proposed with the assistance of molecular simulation and
shown in Fig. 5 and Table 1. To accelerate the simulation
process, as well as gaining a better understanding of the elec-
tronic transition process, a simplied model of the oligomer
with only one pendant phenyl-cyano acceptor moiety was
employed for molecular simulation. As shown, both the HOMO
and LUMO electrons are located on the DTP-uorene backbone.
The calculated HOMO energy level well agrees with that of the
experimental data deduced from the electrochemical measure-
ments, while the difference in the calculated and experimental
LUMO energy levels arises from the difference in effective
conjugation between the oligomer chain and the basic unit.
Under electric stimuli, electrons will accumulate sufficient
energy, get easily excited from the HOMO into the LUMO, and
then transit to the LUMO + 1 that is associated with the –CN
acceptor. Since HOMO ꢁ 1, HOMO and LUMO are all distrib-
uted along the conjugate backbone, the vacancies in the HOMO
and LUMO can be partially compensated by electrons from the
HOMO ꢁ 2 and HOMO ꢁ 1 through energy-decay transition.
Consequently, a charge transfer state of the PFD-8CN oligomer
has been established. The delocalization of electrons among the
HOMO ꢁ 1 to LUMO + 1 orbitals gives rise to half-lled energy
bands similar to that of the metallic conductors, and switches
the device from a low conductance state to a high conductance
state with increased amount of effective (free) charge carriers.
Upon being transited to the rst excited state, the separated
charges distributed on the DTP donor and the –CN acceptor
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Fig. 6 (a) AFM image of the PFD-8CN film with the scanning size of 2 ꢃ 2 mm2; (b) OFF state and (c) ON state C-AFM mapping of the PFD-8CN
film with the scanning size of 2 ꢃ 2 mm2.
lead to strong attraction between the charged Dc+–Acꢁ pairs, molecular electronics, and is useful for enhancing the pro-
therefore leading to a stabilized excited state with smaller cessing capability of modern computer systems with promising
dipole moment as compared to that of the ground state basic parallel capacities.
unit of the oligomer, and non-volatile nature of the device
memristive behavior. With the inclusion of multiple –CN
acceptors in the repeating unit of the oligomer, charge transfer
Acknowledgements
occurs stepwisely in PFD-8CN, which gives rise to the contin-
uous and consecutive modulation of the device conductance
that mimicks the nonlinear transmission characteristics of
biological synapses.
The authors are grateful for the nancial support of the National
Natural Science Foundation of China (51333002, 21074034,
51303194, 61328402), the Research Fund for the Doctoral
Program of Higher Education of China (20120074110004), the
State Key Laboratory of ASIC & System of Fudan University
(11KF007), the State Key Project of Fundamental Research of
China (973 Program, 2012CB933004), the Ningbo Science and
Technology Innovation Team (2011B82004), the Ningbo Natural
Science Foundation (2013A610031) and the Shanghai Leading
Talents program (to Prof. Dr Yu Chen).
As observed by the C-AFM mapping of the oligomer device,
the charge-transfer induced conductance switching occurs in
localized regions with the diameter of ꢀ20 nm (Fig. 6). Once
such highly conductive regions are formed, the electrical
current will ow through them preferentially. Consequently, the
other parts of the lm are no longer subjected to the applied
voltage, and will not undergo any changes to the physico-
chemical properties. Considering the much larger device total
area and the light beam spot size of either UV-visible absorption
or uorescence spectroscopy, nano-scaled optical diffraction or
absorption variation can hardly be detected by the current
technique. Complete elucidation of the underlying switching
mechanism requires further development in the characteriza-
tion techniques and instruments, and is of immense impor-
tance for the optimization of material structure and device
performance.
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