Reversible, erasable, and rewritable nanorecording on an H2 rotaxane thin film.

Abstract

By modifying the molecular structure, we have successfully achieved reversible, erasable, and rewritable nanorecording in rotaxane Langmuir−Blodgett (LB) thin films by applying voltage pulses using scanning tunneling microscope. The recording mechanism is attributed to conductance transition of molecular thin films, and makes rotaxane a promising candidate for the application of practical nanorecording medium.

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Reversible, Erasable, and Rewritable Nanorecording on an H2 Rotaxane Thin
Film
Min Feng,†Li Gao,†Zhitao Deng,†Wei Ji,†Xuefeng Guo,‡Shixuan Du,†Dongxia Shi,†
Deqing Zhang,‡Daoben Zhu,‡and Hongjun Gao*,†
Institute of Physics and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received October 9, 2006; E-mail: hjgao@aphy.iphy.ac.cn
Reversible, erasable, and rewritable nanoscale recording on
organic thin films is of practical importance in ultrahigh density
information storage. The scanning tunneling microscope (STM) is
a powerful tool that enables nanorecording on organic films by
inducing conductance transitions of the organic molecules.1-6
Rotaxane molecules have shown significant potential to be used
as the building blocks for molecular electronics.7,8Writing high-
conductance nanoscale marks on rotaxane H1 thin films was
realized using STM in our previous studies, while, erasing of the
marks showed some difficulties.9In this Communication, we report
on the reversible, erasable, and rewritable nanorecording on the
rotaxane H2 Langmuir-Blodgett (LB) thin films. The dots written
with ∼3 nm feature size show significant stability in air at room
temperature.
Figure 1a shows the structure of the H2 molecule. It is a variation
of the H1 molecule.10An H2 molecule consists of a π-electron-
deficient ring cyclobis(paraquat-p-phenylene) (CBPQT4+) and a
dumbbell-shaped component. The dumbbell component has two
π-electron-rich recognition sites (TTF and DNP; see Figure 1a)
and is terminated by bulky “stoppers”. The ring encircles part of
the dumbbell, making them mechanically interlocked with each
other. The ring can move back and forth between the two different
π-electron-rich recognition sites in response to the external stimuli,
resulting in the switching between the two stable structures. Recent
theoretical calculations11-13have revealed that the movement of
the CBPQT4+ring is accompanied by a change in molecular
electronic structure. The difference between H1 and H2 molecules
lies in the spacer between TTF and DNP. In H2, a rigid cyclohexyl
spacer replaces the soft alkyl spacer (-(CH2)5-) in H1.10
For STM recording, the H2 rotaxane thin films with ∼20 nm
thickness were prepared on highly oriented pyrolytic graphite
(HOPG) substrates using the LB technique; For macroscopic I-V
characterization, the films with a thickness of ∼70 nm were
prepared on the indium tin oxide (ITO) coated glass substrates;
And for the micro-Raman studies, the thin films were also prepared
on ITO coated glass substrates. The thickness of the film is about
100 nm, with some protuberant islands of ∼400 nm in height. The
micro-Raman spectra were acquired on these plateaus with an
excitation wavelength of 632.8 nm.
By applying voltage pulses onto the H2 thin films through the
STM tip, we realized the repeatable and rewritable nanorecording
(Figure 1b). Nanoscale dots can be written repeatedly with the
voltage pulses (∼2.0 V, 0.1-10 ms). This case is similar to that of
the H1 thin films.9What is more interesting is that the marks written
on the H2 thin films can be erased, re-recorded, and re-erased on
the same site. In the whole recording process, the dots remain a
size of ∼3 nm and are stable in air at room temperature for more
than 12 h. The marks are directly visible in the current image in
conductive contact AFM characterizations, but invisible in the
topographic image [see Figure 1c], suggesting that appearance of
dots are indeed due to conductance transitions of the H2 molecules.
Our further studies show that the H2 films are competent for the
reversible nanorecording even after 1 month of being stored in air
at room temperature.
To verify the origin of the conductance transitions of the H2
molecules, we performed the macroscopic I-V measurements on
the H2 films using a standard I-V characterization system
(Keithley, model 4200SCS). The ITO substrate served as one
electrode. We used a freshly cleaved HOPG plate as another
electrode, which was pressed against the H2 thin film to ensure a
good contact. Figure 2 shows the I-V characteristic of the H2 film,
which manifests an electrical bi-stability with a threshold of 1.4
V. The film is initially in the high-impedance state of 108Ω‚cm,
while at 1.4 V, the film abruptly switches to a low-impedance
†Institute of Physics.
‡Institute of Chemistry.
Figure 1. (a) Molecular structure of the H2 molecule. (b) Frames 1-3
show bright marks written one by one using STM; frames 4-6 show the
erasing, rewriting, and re-erasing on the same recording site, with Vb) 0.8
V, It) 0.05 nA. The voltage pulse for recording was 2 V for 3 ms, the
pulse for erasing was -2 V for 3 ms. Scale bar is 6 nm. (c) Topographic
(left) and current (right) AFM images of two marks written by conductive
contact AFM on a 8 nm thick H2 films. Scale bar is 10 nm.
Published on Web 02/02/2007
2204 9 J. AM. CHEM. SOC. 2007, 129, 2204-2205
10.1021/ja067037p CCC: $37.00 © 2007 American Chemical Society

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(conductive) state of 106Ω‚cm (curve 1 in Figure 2). A subsequent
voltage scan from 0 to 0.5 V (curve 2) on the same region verifies
that the H2 thin film keeps the new state of low-impedance,
exhibiting a memory effect. When a reverse voltage is applied, the
film returns to its original insulating state (curve 3 and 4). The
conductance difference between the two states is 2 orders of
magnitude. Our extensive measurements showed that this conduc-
tance transition can be well repeated. No obvious degradation of
the thin film was observed after 6 cycles. These results indicate
that the H2 film possesses the reversible conductance transition
properties controlled by the external voltages.
Theoretical studies11-13have demonstrated that the conductance
switching of the rotaxane molecules is induced by the movement
of the CBPQT4+between TTF and DNP recognition sites. Micro-
Raman spectra of the H2 film, acquired before and after the
conductance transition from insulating to conductive states, show
distinct changes in two regions, 1385-1445 cm-1and 1580-1640
cm-1[see Figure 3]. Our density functional calculations reveal that
the peak at 1414 cm-1originates from the shared CdC torsional
vibration of DNP. The disappearance of 1414 cm-1peak in the
conductive state might be a result of the depressed CdC torsional
vibration of DNP when CBPQT4+moves to it, while a new peak
located at 1615 cm-1appears after the conductance transition,
ascribed to the CdC stretching vibration of TTF group when the
CBPQT4+leaves. On the basis of the experimental and theoretical
results, we suggest that the conductance transition of the H2 thin
film originates from the intramolecular motion of CBPQT4+.
We suggest that the spacers in the molecules might be responsible
for the different behaviors of nanorecording erasable in the H2 thin
film, not in the H1 thin film. The alkyl chain is relatively soft and
the steric hindrance is relatively small compared to the cyclohexyl
group. It is likely that two H1 molecules are packed very close
with a layout of (TTF-DNP-space)/(space-TTF-NDP). When
the ring moves from TTF to DNP, the TTF group in adjacent H1
molecules may stabilize the overall system. This will make it
difficult for the ring to move back to TTF site, and consequentially,
the nanoscale dots in the H1 thin films are difficult to erase. In
contrast, the spacer in H2 molecule is relatively rigid, and
correspondingly, the intermolecular interaction of H2 molecules
will not be as strong as that in H1 molecules. This makes the
CBPQT4+ring move back to the TTP from DNP sites less
toilsomely. Therefore, the relaxation of the ring from metastable
state to the ground state is enhanced, which makes it possible for
the recovery of the original state in the H2 molecules, and
consequentially, the nanodots in the H2 thin film are erasable.
Our experiments show clearly the erasable and rewritable
nanorecording behaviors in H2 thin films as a result of the
conductance transition in the H2 rotaxane molecules. This work
indicates the potential for the chemical modification of the rotaxane
molecules as a promising route toward molecular memories, which
have stable electrical properties and a long lifetime suitable for
practical applications.
Acknowledgment. We thank Lifeng Chi, Harald Fuchs at
Muenster University in Germany, and Zhihai Cheng and Xiao Lin
for experimental assistance. Work at IOP was supported by grants
from National Science Foundation of China, National “863” and
“973” projects of China, the Chinese Academy of Sciences, and
Supercomputing Center, CNIC, CAS.
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JA067037P
Figure 2. Current-voltage relations on the same region of an H2 thin
film showing the reversible conductance transition with applied voltages.
(curve 1) When initially applying the voltage from 0 to 1.4 V, I-V curve
shows the transition from high to low impedance. (curve 2) The film retains
the low impedance state when applying the voltage from 0 to 0.5 V. (curve
3) When a negative voltage from 0 to -1.0 V is applied, the film retains
the low impedance state at first, but begins to recover the original high
impedance state at -1.0 V. (curve 4) A negative voltage from 0 to -1.5 V
is applied, and the conductance is now at the original high impedance level.
Figure 3. Raman spectra of the H2 rotaxane thin film before (1) and after
(2) the conductance transition.
C O M M U N I C A T I O N S
J. AM. CHEM. SOC. 9 VOL. 129, NO. 8, 2007 2205