Controlled deposition of large-area and highly-ordered thin films: effect of dip-coating-induced morphological evolution on resistive memory performance

Article abstract of DOI:10.1039/c9tc00431a

Developing a simple, versatile and efficient technique that enables both large-scale production and nano-scale control is highly desirable but very challenging for achieving high-performance organic-based memory electronic devices. Herein, we employed a dip-coating method to fabricate reliable and cost-effective organic memory devices (OMDs). This technique enables us to deposit high-quality, homogeneous and large-area nanopatterns on the surfaces of thin films and realize uniform OMD performances with a record reproducibility up to 96%. To the best of our knowledge, this is the first report on dip-coated OMDs with the highest reproducibility observed to date, which demonstrates the promising versatility of the dip-coating technique to fabricate organic memory devices and its suitability to scale-up for high-throughput solution processing.

Full text of DOI:10.1039/c9tc00431a

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Controlled deposition of large-area and highly-ordered thin films:
effect of dip-coating-induced morphological evolution on resistive
memory performance†
Received 00th January 20xx,
Accepted 00th January 20xx
Yang Li,*,a,b,c Cheng Zhang,b Zhuang Li,b Peiyang Gu,c Zilong Wang,c Hua Li,*,b Jianmei Lu,*,b and
DOI: 10.1039/x0xx00000x
Qichun Zhang*,c
www.rsc.org/
Developing a simple, versatile and efficient technique that satisfies both large-scale production and nano-scale control is
highly desirable but very challenging for achieving high-performance organic-based memory electronics. Herein, we
employed a dip-coating method to fabricate reliable and cost-effective organic memory devices (OMDs). This technique
can allow us to deposit high-quality, homogeneous and large-area nanopatterns on the surfaces of thin films and realize
uniform OMD performances with a record reproducibility up to 96%. To the best of our knowledge, this is the first report
about dip-coated OMDs with the highest reproducibility observed to date, which demonstrates the promising versatility of
dip-coating technique to fabricate organic memories and its suitability to scale-up for high-throughput solution processing.
it is of high importance to develop a simple but efficient
solution-processed method for OMDs fabrication, which
combines both large-scale production and small-scale
Introduction
Organic memory devices (OMDs) have become a research
focus for a long time owing to their outstanding merits such as
low cost, light weight, high information-storage capacity and
possibility to mimic biological synapses.1-14 In particular,
attributing to the rapid development of solution-processing
techniques, a variety of solution-processable materials have
come to the forefront,15-19 which remarkably accelerate the
far-behind exploiting process of organic electronics. Recently,
most of the solution-processed OMDs are fabricated from
spin-coating or drop-casting methods,20-25 which have been
proved useful for small-scale film deposition. However, such
solution-coating processes usually suffer from a drawback that
restricts their range of application and makes scaling them up
difficult: Clearly, it is challenging to develop a general principle
for the preparation of uniform and continuous thin films over
large areas. Besides, these techniques often produce
inhomogeneous patches or films that contain lots of
disordered grains and grain boundaries,26,27 which could
severely impede their charge transport properties. Therefore,
deposition control.
Currently, some researchers have put forward the strategy
of thermal or solvent annealing as a post-processing step for
promoting the solution-coated film morphologies.28-31 For
instance, due to the controlled nucleation and crystallization of
film grains via solvent annealing, Loo et al. demonstrated that
the device mobility of soluble acene-based organic thin-film
transistors (OTFTs) was two orders of magnitude increased.32
Nevertheless, thermal or solvent annealing could hardly
guarantee uniformity over the entire film surface, which often
causes energy waste and complicates the device fabrication
processes.33 To address the aforementioned issues, herein, we
report a novel technique for OMDs fabrication that is precisely
controlled by a facile, straightforward and versatile dip-coating
method. Dip-coating can strategically induce the film growth
using
a suitable set of programmed parameters. This
technique can not only facilitate the self-assembly of well-
patterned films in nano-scale control,34,35 but also enable to
achieve large-area coverage,36,37 which benefits to integrate
devices into large scale. In addition, dip-coating does not
produce too much viscous stresses on organic molecules
comparing to current approaches (e.g., spin-coating),38
thereby effectively inhibiting the crack formation, shrinkage,
and irregular edge effects. Although dip-coating holds these
attractive advantages, it is very surprising that such method
has less been utilized to fabricate OMDs. To our best
knowledge, this study is the first demonstration of OMDs via
dip-coating with a record reproducibility, which provides a
simple and reliable solution-processing technique to realize a
a.Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy
Application, School of Mathematics and Physics, Suzhou University of Science and
Technology, Suzhou, Jiangsu 215009, P. R. China. E-mail: liyang@usts.edu.cn.
b.College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou 215123, P. R. China. E-mail: lihuaw@suda.edu.cn;
lujm@suda.edu.cn.
c. School of Materials Science and Engineering, Nanyang Technological University,
Singapore 639798, Singapore. E-mail: qczhang@ntu.edu.sg.
†
Electronic Supplementary Information (ESI) available: Experimental details,
thermal properties, AFM images of the spin-coated thin films, statistical
reproducibility and SET voltage distributions, the retention stabilities of dip-coated
OMDs, and X-ray crystallographic information file (CCDC 1870372). See
DOI: 10.1039/x0xx00000x
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Fig. 1 (a–c) Molecular geometries of (a) NIBe, (b) NIQu and (c) NIPy with the increasing molecular conjugation. (d) Schematic
presentation for the preparation of large-area, highly-ordered and uniform molecular films by an optimized dip-coating process.
controlled uniformity over large areas (> cm2) and batch-to- scalable for mass production of high-performance OMDs by a
batch OMD stability.
solution manufacturing process.
The analogues of 2-octyl-6-(5,6,7,8-tetraphenylnaphthalen-
2-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NIBe), 6-(9,14-
diphenylbenzo[f]tetraphen-11-yl)-2-octyl-1H-
Results and discussion
benzo[de]isoquinoline-1,3(2H)-dione
diphenyldibenzo[de,qr]tetracen-11-yl)-2-octyl-1H-
(
NIQu
)
and 6-(9,14-
Synthesis and Characterization of As-Prepared Molecules
NIBe, NIQu and NIPy were successfully synthesized by
benzo[de]isoquinoline-1,3(2H)-dione (NIPy) (Fig. 1a-c) were
employed here as prototypes for demonstrating the dip-
coated OMDs. The selection of these series of molecules relies
on the following considerations: (1) the intrinsic π-π
interaction between the aromatic planes and van der Waals
interaction between the alkyl chains can cooperate to tackle
the self-assembled nanostructures; (2) the increasing aromatic
plane of these molecules holds the promise to achieve
excellent charge mobility; and (3) the co-existence of phenyl
and alkyl chains guarantees the solution-processed ability. Fig.
1d presents a schematic diagram of the simple and efficient
fabrication process. The versatility of this method is
demonstrated by systematic atomic force microscopy (AFM)
inspection and more importantly, the ultimate OMDs
performance. In our study, devices fabricated from the dip-
coating technique showed far superior output performances
as compared to those based on the spin-coating method. The
stability and reproducibility of OMDs were nearly five-fold
increased (up to a record value of 96% obtained to date) owing
to the dip-coating-induced morphological evolution. This work
highlights a versatile technique and general guideline to
achieve nano-scale control over large areas, which would be
combining acene units and naphthalimide (NI) species
together through palladium-catalyzed Suzuki–Miyaura cross-
coupling reactions (Scheme S1
procedures are provided in the ESI
†
). The detailed synthetic
. These as-prepared three
†
molecules adopt the similar NI end-capped structure, and the
only difference is the increased conjugation at the acene part.
1
These three target products were confirmed by H NMR (Fig.
S1-S3 ), and elemental analysis. By
), 13C NMR (Fig. S4-S6
introducing alkyl chains, NIBe NIQu and NIPy show
†
†
,
satisfactory solubility in the most common organic solvents
such as chloroform, toluene, and dichlorobenzene. In addition,
these three compounds exhibit good thermal stability, with an
onset decomposition (5% weight-loss) temperature over 200
°C (Fig. S7†), respectively.
Moreover, the yellow plate-like crystals of NIPy were
obtained by slow evaporation of mixed solvents
(chloroform/isopropanol = 2:1) under ambient conditions,
which further demonstrate the structure of this series of
molecules. The single-crystal X-ray diffraction analysis reveals
that the as-prepared crystal of NIPy belongs to a triclinic
system with the following unit cell parameters: a = 9.7201(4), b
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Fig. 2 Single-crystal X-ray structure of NIPy: (a) Molecular structure (H atoms are omitted for clarity) shows that two adjacent
molecules form a dimer, where each acene part is marked with different colors. (b) The arrangement of the neighboring dimers
shows a dense overlap on the NI groups (highlight by blue color). (c) The crystal packing mode corresponds to a layer-by-layer
manner, separated by the acenes interdigitation. The mean central distance between the two π-overlapped NI planes is 3.53 Å.
(d) The side view of the crystal packing mode.
= 20.7651(7), c = 22.6267(8) Å, and the Z value is 2 (Table S1
Fig. 2a shows that one molecule is strongly affected by its to NIBe and NIQu
†
). the soluble propensity, leading to a lower solubility comparing
For the solution-processed organic
.
adjacent molecule, where two adjacent molecules form a electronics, the molecular growth in thin films is highly
“dimer” structure. The neighboring dimers self-assemble into a dependent on the molecular solubility.41,42 From this
layer-by-layer manner, which shows a dense overlap on the NI standpoint, we decided to optimize the dip-coating conditions
part (Fig. 2b), separated by the interdigitation of acenes. The for all three molecules through preferential systematic
mean central distance between the two nearest NI groups is investigation of NIPy
3.53 Å (Fig. 2c and 2d), which manifests that there are
.
considerable interactions among the π-planes of neighboring
NIPy molecules.39
Solvent Selection
The appropriate selection of solvents usually plays a significant
role in governing the molecular self-assembly behavior in thin
films.38,43 During the dip-coating process, molecular growth
Optimization of the Dip-Coating Process
To fabricate solution-processed devices, we firstly examined greatly relies on the synergistic effects of solute-exchange and
the solution-deposited behavior of these three compounds by solvent-evaporation rates. To determine the suitable dip-
spin-coating method. As shown in Fig. S8
†, all AFM images of coating solvent for prototype NIPy, we chose three typical
NIBe NIQu and NIPy based spin-coated films exhibit unevenly kinds of solvents (i.e., chloroform (CF), toluene (TL) and o-
,
aggregated domains, which are undesirable for high- dichlorobenzene (ODCB)) that possess apparently different
performance device fabrication. Thus, a convenient, simple but volatilities and investigated their corresponding film-forming
efficient film-forming technique for morphological evolution is properties. As shown in Fig. 3a, the film fabricated from CF
highly desirable. To this regard, the cost-effective dip-coating shows quite disordered and unevenly aggregated domains
method successfully arouses our attention. Dip-coating allows with root-mean-square roughness (RRMS) of 2.95 nm. Most of
precise nanostructural control over large areas, which offers these domains are larger than 1 um, which are undesirable for
the versatility for a large variety of organic materials.40 On the good device performance since they can largely impede charge
other hand, NIPy possesses the highest conjugated plane, injection. The film morphology from TL somewhat improves,
which could induce large molecular rigidity and thus decrease however, still suffers from the irregular surface structure (RRMS
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Fig. 3 AFM height images of NIPy dip-coated films from different solvents of (a) CF, (b) TL and (c) ODCB onto indium-tin-oxide
(ITO) substrates.
Fig. 4 AFM height images of NIPy dip-coated films from different concentrations of (a) 5 mg/mL, (b) 10 mg/mL and (c) 50 mg/mL
solutions in ODCB.
= 2.48 nm, Fig. 3b). When dip-coating from ODCB is employed, and film thickness at high concentration are larger than those
NIPy film becomes much more uniform with smaller roughness at low concentration. In order to determine the appropriate
of 0.80 nm (Fig. 3c), forming a high-quality and well-defined molecular concentration for processing, we prepared 5 mg/mL,
“granular” structure. The connectivity between neighboring 10 mg/mL and 50 mg/mL solutions of NIPy in ODCB, which
crystalline grains is developed, which enables the efficient represent low, moderate and high concentration, respectively.
charge carriers transport. Such morphological improvement The AFM image of film at low concentration (5 mg/mL) shows
can be attributed to the low solvent-evaporation rate of ODCB, a continuous surface, but the film thickness is insufficient and
which allows for sufficient solute exchange and arrangement. the morphology is relatively amorphous with some domains
Therefore, ODCB is a satisfactory solvent for dip-coated film (RRMS = 1.26 nm, Fig. 4a). At the moderate concentration of 10
fabrication.
mg/mL, the film exhibits a bark-like structure and the thickness
is enhanced. The uniformity of the film surface is guaranteed
over a large area with regular roughness of 0.86 nm (Fig. 4b).
However, when dip-coating from higher concentration of 50
mg/mL is conducted, the visible out-of-order crystalline islands
(larger than 1 μm) emerge and suppress the homogenous
surface (RRMS = 2.44 nm, Fig. 4c). Thus, the concentration of
10mg/mL in ODCB is preferred as suitable processing condition
for molecular dip-coating.
Solution Concentration Adjustment
The concentration of molecular solution is another parameter
that has a great impact on the film-forming behavior during
dip-coating process.27,44 The reliable concentration for dip-
coating is closely correlated to the molecular structure,
solubility and self-assembly property. Generally, the grain sizes
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Fig. 5 AFM height images of NIPy dip-coated films at different rates of (a) 10 μm/s, (b) 40 μm/s and (c) 160 μm/s.
Fig. 6 AFM height images of (a) NIBe, (b) NIQu and (c) NIPy dip-coated films under the optimized conditions.
increases to 40 μm/s, the aggregated crystalline domains
appear, and the film surface becomes coarse with uneven
grains (RRMS = 1.02 nm, Fig. 5b). Furthermore, when a higher
speed of 160 μm/s is utilized, the large-area arbitrarily shaped
crystalline islands form among the film surface, which could
severely impede the charge transport efficiency (RRMS = 1.88
nm, Fig. 5c). While pulling the substrate out of the solution at a
high speed, there is no enough time for the sufficient solute-
Pull-Out Speed Investigation
Generally, dip-coating is a powerful evaporation-controlled
process and allows to establish a fine control of the
evaporation rate through programming the pull-out speed of
substrate, which can largely influence the film
microstructures.36,45 The morphological evolution as a function
of withdrawal speed is related to the dynamical processes
occurring at the three-phase solution/substrate/air contact
line. By controlling the withdrawal speed, we are able to
obtain a high-quality and uniform thin film quantitatively. To
investigate the suitable pull-out speed, the pulling rate of
substrate from the solution reservoir was gradually changed in
three ranges: low, middle and high. Each substrate was dipped
into the 10 mg/mL NIPy solution in ODCB and pulled out at a
different rate of 10 μm/s (low), 40 μm/s (intermediate), and
160 μm/s (high), respectively. As shown in Fig. 5a, at low speed
of 10 μm/s, the film shows a homogeneous tiny-bark-like
structure with a regular RRMS of 0.86 nm. As the pulling speed
exchange
and
solvent-evaporation
at
the
air/solution/substrate contact line. Thus, the film growth is
mainly controlled by the subsequent evaporation of the
solution that adheres to the substrate at atmosphere,46 which
resembles the drop-casting process and suffers from the
discontinuous and uneven grains. Therefore, the withdrawal
speed of 10 μm/s is preferred, which warrants the adequate
solute concentration gradient and hence affords the driving
force for favorable film growth.
High-Quality Dip-Coated Thin Films of NIBe, NIQu and NIPy
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Fig. 7 (a-c) Optical absorption spectra of (a) NIBe, (b) NIQu and (c) NIPy in ODCB solutions and dip-coated films on the ITO
substrates. (d-f) Cyclic voltammogram curves of (d) NIBe, (e) NIQu and (f) NIPy at a scan rate of 50 mV s−1; the
ferrocene/ferrocenium redox couple is used as an internal standard (–4.80 eV, the onset potential is 0.42 eV).
According to the optimized dip-coating conditions, the highly- three molecules. In solutions, all three compounds exhibit
ordered films of NIBe
The substrates were smoothly dipped into the 10 mg/mL ODCB films, these series of molecules show two well-resolved
solutions of NIBe NIQu and NIPy, and pulled out at a constant absorption peaks located at ~340 and 450 nm, respectively.
, NIQu and NIPy were finally fabricated. absorption bands between 220 and 450 nm. In dip-coated thin
,
rate of 10 μm/s, respectively. The high-quality and uniform The high-energy absorption band at 340 nm arises from the
nanostructural patterns are formed over large areas, as proven molecular aromatic conjugation while the low-energy
by AFM images. In comparison with spin-coated films, the self- absorption band around 450 nm corresponds to the intra-
assembly behaviors of dip-coated thin films are significantly molecular charge transfer (ICT) process.47 Compared with
enhanced (Fig. 6a-c). All films of the three molecules present solution state, the onset absorption (λonset) peaks of NIBe
,
the appearance of “bark-shape” morphologies. As the NIQu and NIPy in films display obvious bathochromic shifts,
molecular conjugation increases, the grain sizes of the films which can be ascribed to the molecular self-assembly via dip-
become smaller and more homogenous throughout the coating. The λonset of NIBe, NIQu and NIPy are 553, 549 and
surface. The film roughness decreases from 3.06 nm of NIBe to 547 nm, thus, their corresponding optical band gaps (Egopt) are
1.75 nm of NIQu, and 0.86 nm of NIPy, which clearly indicates calculated to be 2.24, 2.26 and 2.27 eV (Egopt = 1240/λonset).
the increasing uniformity and flatness. More interestingly, for
NIPy, some nanofibers (about 100 nm in width) are developed electrochemical properties of NIBe
between the crystalline grains, which are conducive to the in Fig. 7d-f, all three compounds can exhibit oxidation and
crystalline connectivity and thus facilitates the efficient charge reduction waves. The onset oxidation potentials of NIBe NIQu
Cyclic voltammetry (CV) was utilized to examine the
,
NIQu and NIPy. As shown
,
transport.31 Whereas the spin-coated films are prone to and NIPy are 1.08, 1.34 and 1.33 V in the positive potential
irregular patterns, dip-coating permits a high degree of region, while the onset reduction potentials are –0.87, –0.94
morphological control, which offers particular advantages to and –0.90 V in the negative potential region, respectively.
implement large-area uniformity.
According to the equations EHOMO/LUMO = −e (4.80 − EFc
Eox/redonset) V, the highest occupied molecular orbital (HOMO)
energies of NIBe NIQu and NIPy are determined to be –5.46, –
5.72 and –5.71 eV, respectively, whilst the lowest unoccupied
molecular orbital (LUMO) energies are determined to be –
3.51, –3.44 and –3.48 eV. Therefore, the electrochemical band
+
,
Photophysical and Electronic Properties
The optical properties of NIBe, NIQu and NIPy in ODCB
solutions and dip-coated films were subsequently studied. Fig.
7a-c shows the normalized optical absorption spectra of these
gaps of NIBe
,
NIQu and NIPy are estimated to be 1.95, 2.28
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and 2.23 eV, which are similar to the aforementioned optical addition, the HCS can be programmed to theVioewriAgritnicalelOLnlCinSe
DOI: 10.1039/C9TC00431A
band gaps. Moreover, the hole injection energy barriers of through a reverse positive scan (sweep 3), which can be
ITO/HOMO are calculated to be 0.66, 0.92 and 0.91 eV, while regarded as a digital “erasing” process. The recovered LCS can
the electron injection energy barriers of aluminum (Al)/LUMO be stably read during the subsequent positive sweep (sweep 4)
are calculated to be 0.79, 0.86 and 0.82 eV. For NIBe, the hole and successfully reprogrammed to HCS (sweep 5) in the next
injection barrier is smaller than electron injection barrier, voltage scan. This “write−read−erase−read−rewrite” cycle
which indicates that hole injection dominates the charge corresponds to the binary FLASH memory behavior.14,20
conduction process.5 However, as the molecular conjugation
increases, the electron injection barrier becomes smaller than device,
Fig. 8c manifests the I−V characteristics of NIQu-based
which shows similar
a
hole injection barrier, which suggests the potential of NIQu “write−read−erase−read−rewrite” cycle. The I−V curve
and NIPy as ambipolar or n-type semiconducting materials.48 demonstrates that NIQu also exhibits FLASH-type binary
The extended aromatic conjugation benefits for the efficient memory performance. However, the electrical measurement
electron delocalization, which is conducive to the electron of NIPy-based device displays different resistive memory
transport.
behavior. When switching from LCS to HCS, the HCS can be
retained and could not be erased after removing the electric
field or under reverse voltage pulse (Fig. 8d, sweep 1-3). This
electrical switching phenomenon corresponds to WORM (i.e.,
write-once-read-many-times) type memory behavior. In
addition, the more repeatable I–V sweeps denote the reliable
memory performance of these devices (Fig. S9†). The LCS and
HCS are well-separated with the current ratio of ~103, which
benefits for running their OMDs accurately.
Moreover, we investigated the fifty independent units of
each kind of devices to verify their distinct memory types. As
shown in Fig. S10†, the statistic result reveals that for NIBe,
the proportion of FLASH-type memory remains around 74%.
While for NIQu, the FLASH-type ratio markedly decreases
(~50%) and several storage cells start to exhibit unrecoverable
NVM behavior. As it comes to NIPy, most of the device cells
Resistive Memory Device Performance
To inspect the electrical performances of NIBe, NIQu and NIPy,
we deposited Al metal onto their optimal dip-coated films and
fabricated the sandwiched OMDs as shown in Fig. 8a. The
current–voltage (I–V) measurement illustrates that the NIBe
-
based device shows a FLASH-type binary memory behavior (Fig.
8b). When applying the negative voltage from 0 to −5.0 V, an
abrupt increase in the current appears at −2.05 V (sweep 1),
indicating that the device switches from a low-conductance
state (LCS) to a high-conductance state (HCS). This LCS-to-HCS
transition can act as a “writing” process in digital storage. The
device remains its HCS during the next negative voltage pulse
(sweep 2) and even after the removal of the electric field,
which denotes the nonvolatile memory (NVM) nature. In
Fig. 8 (a) Scheme of the OMDs architecture (composed of ITO, organic dip-coated films and Al electrode), which is connected
with the I–V detector. (b-d) The detected I–V signals of (b) NIBe, (c) NIQu and (d) NIPy based memory devices; the numbers
represent the sweeping sequences whilst the insets denote the optimized large-area dip-coated films.
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Fig. 9 DFT calculated molecular conformation, ESP, HOMO and LUMO charge-density surfaces, electron affinities, and HOMO–
LUMO band gaps of NIBe NIQu and NIPy by the generalized gradient approximation (GGA) with unrestricted BLYP/DNP level.
,
(up to 96%) show non-erasable memory switching, which morphological improvement from NIBe to NIPy. The high-
transform from FLASH-type into WORM-type performance. quality and homogeneous films help reducing the series
Notably, different from the spin-coating-based devices, which contact resistances between the organic layer and top/bottom
show quite unstable and non-identifiable output performances, electrodes,49,50 and thus leading to reproducible behaviors
the dip-coating-based devices hold highly-raised batch-to- over large areas. Simultaneously, we have examined the
batch reproducibility with a record uppermost value of 96% stabilities of these dip-coated OMDs through retention time
reported so far, which strongly demonstrates the adaption of test under a constant stress of –1.0 V. No unfavourable
dip-coating to highly-reliable OMDs fabrication.
degradation of these devices in both LCS and HCS is detected
), which shows that these devices
It is noteworthy that along with the increasing molecular for at least 1×104 s (Fig. S12
†
conjugation, the molecular memory type changes obviously are endurable under external voltage stress and possess stable
from FLASH to WORM, which holds promise to realize different NVM nature.
functionalities in information storage. Besides, as the
molecular conjugation rises, the SET voltages (VSET, i.e., the
switching voltage of the memory device at which the device
Resistive Memory Mechanism Discussion
It has been reported that the metal electrode could have an
influence on the performance of memory devices (e.g., metal
switches from LCS to HCS) of these devices become more
narrowly-ranged (Fig. S11 ), which is in good accord with the
†
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filament conduction).1,20,51 To investigate this possibility, we initial state under reverse voltage bias. Therefore, the NIPy
altered the top Al electrode into inert gold (Au) and fabricated based device shows unrecoverable WORM-type memory
-
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memory devices with ITO/small-molecule/Au structure. The performance.
electrical characteristics of these memory devices were
measured under the same condition (Fig. S13†), which are
Conclusions
similar with those shown in Fig. 8. Together with the good
stability and the structure-dependent memory performance of
the devices, it can be concluded that the electrical transitions
are intrinsic to the organic small molecules.23
In summary, we have demonstrated dip-coating as a facile,
versatile and efficient technique to prepare high-performance
OMDs and gain precise control over molecular self-assembly.
Compared to the spin-coating method, dip-coating successfully
induces the morphological evolution, which produces high-
quality and homogeneous nanopatterns over large areas. As a
result, the corresponding solution-processed OMDs through
dip-coating method exhibited far superior performances and
stability. As far as we know, this is the first report about dip-
coated OMDs with uniform device reproducibility as high as
96%, which opens a new pathway for fabricating highly-
efficient organic memories in large-scale control. Further
studies on the versatility of dip-coating method for other
functional materials based OMD systems over both rigid and
flexible substrates, and the fine tuning of their self-assembly
behaviors are now in progress.
To get insight into the underlying memory mechanisms of
NIBe, NIQu and NIPy, density functional theory (DFT)
calculations were performed to further understand their
electrical properties. The molecular electrostatic potential
(ESP) isosurfaces (Fig. 9) show that the continuous positive ESP
(in yellow-green) occurs throughout the molecular surfaces,
forming an open channel, where the charge carriers can
transport smoothly. Yet, the negative ESP region aroused from
the electron-deficient NI acceptor (in blue) also exists, which
could act as “charge trap” to impede the charge migration.21,52
The electron affinities of NIBe, NIQu and NIPy are determined
to be quite similar (~3.0 eV), which attributes to the same
electron-withdrawing NI acceptor. Besides, the dihedral angles
between the NI unit and the acene specie are found to be
around 54°–55°, which might result in charge injection
barrier.53 Under a low voltage bias, charge carriers can hardly
get enough energy to overcome the injection barrier between
the acenes and NI acceptor. Thus, the device stays in the LCS.
When the bias increases, the charge carriers gradually acquire
sufficient energy and inject from the acenes to NI part to fill
the trap, leading to the LCS-to-HCS switching with “trap-free”
environment.
Experimental
Optimal Dip-Coated Films Preparation and OMDs Fabrication
The ITO-coated glass substrates (2×2 cm2) were cleaned
sequentially with distilled water, acetone, and ethanol by
ultrasonication. NIBe, NIQu and NIPy were dissolved in ODCB
(Aldrich 99%, anhydrous) to form 10 mg/mL solution,
respectively. Then the ITO substrates were vertically immersed
into the organic solutions and pulled out at a constant rate of
10 μm/s. Dip-coating was performed in an airtight box on a
shock-absorbing stage to eliminate the effects of air flow,
mechanical vibrations, and noise (temperature = 20−25 °C,
humidity = 35−60%). Afterwards, the top Al electrode with
thickness of about 100 nm was thermally deposited onto the
organic dip-coated films through a shadow mask with circular
patterns. The active area of the target memory device cell was
0.126 mm2 (a nummular Al electrode pattern with radius of 0.2
mm). Under ambient laboratory condition, current–voltage
measurements of the non-packaged OMDs were implemented
Moreover, the calculated HOMO and LUMO of NIBe, NIQu
and NIPy were also shown in Fig. 9. As noted, the HOMO and
LUMO surfaces were unevenly distributed. The electrons of
HOMOs are mainly-distributed over the acene part, while the
electrons of LUMOs are located at the NI acceptor side. The
calculated HOMO–LUMO band gaps were 2.16, 1.89 and 1.83
eV for NIBe, NIQu and NIPy, respectively. These electron
distributions indicate the NI-trap-induced intra-molecular
charge transfer (ICT) under electric field.23,54 However, it can
be noted that as the molecular conjugation increases from
NIBe to NIPy, the distance between the HOMOs and LUMOs
increases accordingly. For NIBe, the HOMOs and LUMOs are
partly overlapped at the NI side. Due to the overlap and short
distance between the HOMOs and LUMOs, the trapped
charges by NI acceptor can be easily de-trapped to the original
state under a reverse voltage bias,55 thus FLASH-type memory
behavior was observed. For NIQu, such HOMOs/LUMOs
distance relatively increases, which makes the charge
using
a Keilthley 4200-SCS semiconductor characteristic
system. During the device measurement, the top and bottom
electrodes of the OMDs were attached to the terminals of
Keilthley 4200-SCS instrument, respectively. Under the voltage
sweeping mode, voltage bias was applied to the device at a
normal speed without sweep delay. The sweep step of the
current–voltage characteristics was 0.05 V, and the compliance
recoverable process become difficult. As
a result, the
current was 0.1 A (Table S2†).
recovered FLASH-type ratio of NIQu-based device decreases.
When it comes to NIPy, the distance between the HOMOs and
LUMOs further extends to be larger than NIBe and NIQu. The
HOMOs and LUMOs are well-separated and exhibit few
overlap. Thus, the trapped charges could be facilitated by the
ICT process to form a stable charge-separated state.20,54 This
charge-separated state could not be easily recovered to the
Acknowledgements
Q.Z. acknowledges financial support from AcRF Tier 1 (RG
111/17, RG 2/17, RG 114/16, RG 8/16) and Tier 2 (MOE 2017-
T2-1-021), Singapore. Q.Z. also thanks the support from Open
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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Journal of Materials Chemistry C
Page 10 of 12
ARTICLE
Journal Name
24 G. Wen, Z. Ren, D. Sun, T. Zhang, L. Liu andViSe.w AYraticnle, OAndlinve.
Project of State Key Laboratory of Supramolecular Structure
and Materials (Grant number: sklssm201833), Jilin University,
China. J.L. acknowledges financial support from the NSF of
China (21336005). H.L. thanks financial support from the NSF
of China (21878199 and 21476152) and the National Excellent
Doctoral Dissertation funds (201455), China. This work is also
supported by Jiangsu Key Disciplines of the Thirteenth Five-
Year Plan (20168765) and Suzhou Key Laboratory for Low
Funct. Mater., 2014, 24, 3446.
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Journal of Materials Chemistry C
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Controlled deposition of large-area and highly-ordered thin films: effeDcOtI:o1f0.1039/C9TC00431A
dip-coating-induced morphological evolution on resistive memory performance
Yang Li,*,a,b,c Cheng Zhang,b Zhuang Li,b Peiyang Gu,c Zilong Wang,c Hua Li,*,b
Jianmei Lu,*,b and Qichun Zhang*,c
Table of Contents
A scalable dip-coating technique is reported to precisely control nano-patterned films
and resistive memory devices with a record reproducibility (96%).