A triphenylamine-containing donor-acceptor molecule for stable, reversible, ultrahigh density data storage

Article abstract of DOI:10.1021/ja074226e

A triphenylamine-containing donor-acceptor molecule was designed and synthesized. Its thin film showed excellent bistable electronic switching behavior with a high ON/OFF current ratio. With this molecule, stable, reliable, and reversible ultrahigh densit

Full text of DOI:10.1021/ja074226e

Published on Web 08/29/2007
A Triphenylamine-Containing Donor-Acceptor Molecule for Stable,
Reversible, Ultrahigh Density Data Storage
Yanli Shang,† Yongqiang Wen,*,† Shaolu Li,† Shixuan Du,‡ Xiaobo He,‡ Li Cai,‡ Yingfeng Li,†
Lianming Yang,† Hongjun Gao,‡ and Yanlin Song*,†
Key Laboratory of Organic Solids, Laboratory of New Materials, Institute of Chemistry, and Nanoscale Physics and
DeVice Laboratory, Institute of Physics, Chinese Academy of Sciences,
Beijing 100080, China
Received June 11, 2007; E-mail: ylsong@iccas.ac.cn; wyq_wen@iccas.ac.cn
In the past few years, there has been increasing interest in the
application of organic functional molecules for next generation
information storage and future optoelectronic devices.1 With the
development of the scanning probe microscopy (SPM), ultrahigh
density data storage based on SPM technology has made great
progress in recent years.2 Among these studies, organic functional
materials have been reported extensively.2b-c
Figure 1. (a) Structure of BDOYM; (b) UV-vis spectra of BDOYM (I,
Despite the remarkable research achievement on SPM-based
storage, there are important limitations that need to be overcome.
in cyclohexane solution; II, film on ITO substrate); and (c) macroscopic
I-V characteristics of the film.
From the practical view, future developments will aim at decreasing
the error rate, improving the stability, etc.3 With these motivations,
related with the formation of molecular aggregates and/or increased
polarizability of the film.7
scientists are trying to develop SPM-based storage from both
technical and material aspects, of which the improvement of stability
and ON/OFF ratio between the two sates of the thin film would be
one of the most important routes. Among the organic recording
materials being extensively studied, donor-acceptor (D-A) type
charge-transfer complex and compounds are considered to be very
promising materials for future information storage because of the
electronic switching property induced by reversible intermolecular
charge transfer.4
A thin film (with the thickness about 50 nm) was prepared on
ITO coated glass for measurement of electrical properties. The
sweep directions and typical current-voltage (I-V) characteristics
are shown in Figure 1c. When a forward voltage was applied, the
thin film exhibited its low conducting state (OFF-state, curve I).
As the voltage approached +1.7 V, a sharp increase in the current
took place, indicating the thin film switches on to its high
conducting state (ON-state). After the transition, the thin film
remained in the ON-state during the second sweep from 0 to +2.0
V (curve II), where the current arrived at 4 orders higher than that
of the thin film before switching. Such a high ON/OFF current
ratio is crucial for the memory device to realize high-resolution
and low error rate data storage.8 As known, the charge mobility of
organic materials is often determined by an electron or hole transport
process, in which an electron or hole is transferred from one
molecule to the neighboring one.9 Triphenylamine groups may have
played an important role in this case.6 The thin film switched off
to its low conducting OFF-state as the voltage approached -0.7 V
when sweeping from 0 to -2.0 V (curve III). Then it showed a
low conducting state, as indicated by a followed reverse scan (curve
IV). In our studies, the ON-state had been proven to remain stable
for more than two weeks after the external electric field was
removed. The electrical switching behavior exhibited by the thin
film made it a promising candidate for application in electric-based
erasable data storage.
Molecules with D-A structure are a kind of important materials
for data storage owing to existing two stable conducting states. At
the initial state, the thin film shows low conductivity. Upon
application of an external field, the intermolecular CT state would
be easily accessible and the conductivity of the thin film could be
greatly enhanced, which could be proved through the analyses of
the two states.10 In our electrical investigations of the thin film, a
color transition from purple to orange was observed when the
applied voltage was higher than the threshold value, and the relevant
changes in UV-vis (Figure 2) and IR (Figure S1) spectra occurred.
As can be seen from Figure 2, after the action of an electric field,
the UV-vis spectra exhibits a peak centered at 448 nm (Figure
2b, Peak 2), which indicates the intense interaction among
molecules, especially intermolecular charge transfer.4c,10,11 This
would lead to an increased number of the carriers and a delocalized
In the previous studies, we realized nanometer scale data stor-
age on thin films using scanning tunneling microscopy (STM),
where stable storage was achieved by improving of the quality of
film.5 In this work, a new D-A molecule, 2-((Z)-2-(4-diphenyl-
amino) benzylidene)-1,2-dihydro-1-oxoinden-3-ylidene) malono-
nitrile (BDOYM, Figure 1a), with a triphenylamine unit as the
electron donor and two cyanovinyl groups as the electron acceptor,
has been designed and synthesized. The triphenylamine unit, a large
conjugated tertiary amine system, can not only act as a strong
electron donor in its initial state but also stabilize the charge-
transferred state. Furthermore, because of the good hole-transporting
ability of the triphenylamine group,6 it can be expected to modify
the electrical conductivity of the material effectively. In our studies,
two distinctive conductivity states have been observed on BDOYM
films, and the ON/OFF current ratio was determined to attain 104.
Stable, reliable, reversible data storage has been demonstrated using
STM. Thus, with this new molecule, we have further developed
STM-based storage by improving the stability and ON/OFF ratio
of the thin film. Furthermore, because of the difference of spectra
before and after the action of an electric field, the information
written on BDYOM thin film with an electrical method can be read
out using an optical one. This will be a substantial step forward
toward the development of multimode data storage.
The typical UV-vis spectrum of BDOYM (10-5 M solution in
cyclohexane) displays an intramolecular charge transfer (CT)
transition at 548 nm, as shown in Figure 1b. In the UV-vis spectra
of the film on ITO substrate prepared by vacuum vapor deposition
method, the CT transition shows an obvious red-shift compared
with that of the solution state. This red-shift between the λmax (at
573 nm) of thin film and the λmax (at 548 nm) of solution might be
† Institute of Chemistry.
‡ Institute of Physics.
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J. AM. CHEM. SOC. 2007, 129, 11674-11675
10.1021/ja074226e CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
can be seen in Figure 3e, the conductivity of unrecorded regions
on the film is in a high resistance state (curve I), while it is in a
low resistance state at the recorded regions (curve II). The
comparisons presented herein indicated that the applied voltage
pulse induced a conversion of the conductance of the thin film from
a high resistance to a low one, which is in accordance with the
macroscopic I-V characteristics. That is to say, the applied voltage
pulses have induced charge transfer among the adjacent molecules,
leading to the switching of conductivity of the thin film. This could
be further supported by theoretical studies (Figure S2).
Figure 2. The UV-vis spectra of BDOYM thin film before (a) and after
(b) application an electrical field.
In summary, a triphenylamine-containing donor-acceptor mol-
ecule was designed and synthesized to improve the intrinsic storage
performance. The thin film showed excellent bistable electronic
switching behavior with a high ON/OFF current ratio. Reversible
data storage was successfully demonstrated on the thin film by
STM. Mechanism analysis indicated that the recording dot was due
to the intermolecular charge transfer induced by electric field. This
research has made a further step toward realizing stable, reliable,
and reversible electric data storage.
Acknowledgment. The authors thank the financial support from
the NSFC (Grant Nos. 50625312, 60601027, 20421101, U0634004),
973 Program (No. 2006CB806200), and CAS.
Supporting Information Available: Experimental details, theoreti-
cal calculation. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 3. STM images of typical information dots pattern and the
corresponding I-V curves. (a) Recording pattern composed of five
information dots: pulsed voltage, +2.3 V; 3.0 ms. (b, c) Erasing the first
and the second dots at position 1 and 3, respectively: pulsed voltage, -1.4
V; 3.0 ms. (d) Rewriting one information dot at position 1: pulsed voltage,
+2.3 V; 3.0 ms. (e) Typical STM I-V curves in the unrecorded (curve I)
and recorded region (curve II).
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