A non-planar organic molecule with non-volatile electrical bistability for nano-scale data storage

Article abstract of DOI:10.1039/b703332j

A new non-planar organic molecule with electron donor and acceptor capabilities was synthesized for nano-scale data storage. Macroscopic I-V characteristics of organic crystalline thin films indicate that the non-planar molecule possesses good electrical

Full text of DOI:10.1039/b703332j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A non-planar organic molecule with non-volatile electrical bistability for
nano-scale data storage{
ab
Junping Hu, Yingfeng Li, Zhuoyu Ji, Guiyuan Jiang, Lianming Yang, Wenping Hu, Hongjun Gao,
a
ab
c
a
a
d
a
a
a
Lei Jiang, Yongqiang Wen, Yanlin Song* and Daoben Zhu
a
Received 5th March 2007, Accepted 4th June 2007
First published as an Advance Article on the web 21st June 2007
DOI: 10.1039/b703332j
A new non-planar organic molecule with electron donor and acceptor capabilities was synthesized
for nano-scale data storage. Macroscopic I–V characteristics of organic crystalline thin films
indicate that the non-planar molecule possesses good electrical bistability. Nano-scale recording
dots with an average diameter of 2.5 nm were realized by scanning tunneling microscopy.
threshold between the two states is necessary. Plenty of works
have demonstrated that external electric stimulation induces
11
Introduction
With the exponential growth of information, high-density data
1
recording media have become the focus of attention.
two states that originate from the structural transition,
14
redox, or the charge storage
15,16
mechanism. For instance,
Traditional magnetic materials are widely used today and
22 2
Chen et al. successfully constructed a polymer/ionic conductor
14
memory device based on electrochemical redox. Yang’s
their density growth is expected to be beyond 100 Gbit in
.
However, when the magnetic recording domain turns as small
as nano-scale, problems such as data loss may take place
3
because of the super-paramagnetic effect. With typical optical
group has done several pioneering works in organic/metal
devices and pointed out that the memory effect was ascribed to
15
charge storage. Majumdar et al. and Pradhan et al. have
recording media except for near-field optics, the density of the
data stored is restrained by the diffraction limit of the laser
reported works to control the degree of bistability by the
concentration of C60 or carbon nanotubes in the polymers, and
16
1
,4
beam.
Herein, exploring new recording materials and
the charge storage mechanism also has been proposed. As for
advanced recording technologies would be of great significance
to satisfy the demand of expansive storage capacity in the
future.
nano-scale data storage, the traditional organic functional
films are often made of two components that act as donors and
17
acceptors, respectively. But the problem of inhomogeneity of
Recently, organic materials have received much attention in
5
molecular electronics. Their advantages of designability, low
thin film is difficult to avoid, which would seriously affect the
electrical properties. Then, single molecules containing both
electron donor (D) and acceptor (A) moieties were used as the
binary system. Taking the quality of the crystalline film into
cost and easy manipulation as well as the property of switch or
electrical bistability make organic materials attractive. On the
3
other hand, scanning probe microscopies (SPMs), including
,6
account, co-planar molecules were preferred for the applica-
11,12
7
scanning tunneling microscopy (STM), scanning near-field
tion.
However, the intramolecular and intermolecular
8
optical microscopy (SNOM) and atomic force microscopy
charge transfer may interfere with each other, which is related
to the quality of the recording media in the aspect of both
9
AFM), have been explored extensively in recent years. It is
(
expected that the combination of organic molecules (one or
several molecules as the functional unit) and SPM (manipula-
tion on the nano/atom-scale) will throw light on driving the
‘
write’ and ‘read-many-times’. Furthermore, Bandhopadhyay
and Pal verified that D–p–A molecules usually have lower
on/off current ratio because of the electron conjugation in the
molecule, which restrains the usage in high-frequency binary
1
0–13
data storage to ultrahigh density stage. Our group
has
employed a series of organic molecules for information storage
with the size of recording marks below 5 nm by STM.
As known, organic molecules serving as a binary system
should have two stable states, which indicates that a barrier or
18
computation. In order to improve the performance of the
recording media, in this paper, a strategy is demonstrated to
take advantage of both dual-components and sole component;
namely, a molecule is designed to contain two separate planar
moieties including D and A, respectively, while the two
moieties are in a relatively non-planar conformation as an
obstacle to avoid the complex interference of intermolecular
and intramolecular interaction. As a result, this non-planar
configuration would suppress intramolecular coupling for the
energy barrier induced by the torsion angle. Meanwhile, the
intermolecular charge transfer would be promoted and charge
recombination becomes difficult. Hence, the memory would be
more stable. Moreover, the non-planar configuration will
contribute to a large current ratio of on/off because it
perturbs conjugation in the molecular backbone and results
a
Key Laboratory of Organic Solids and Laboratory of New Materials,
Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R.
China. E-mail: ylsong@iccas.ac.cn; Fax: (+86) 10-82627566
Graduate University of Chinese Academy of Sciences, Beijing, P. R.
b
China
State Key Laboratory of Heavy Oil Processing, University of
c
Petroleum, Beijing, 102249 P. R. China
Nanoscale Physics & Devices, Institute of Physics, Chinese Academy of
Sciences, Beijing, 100080 P. R. China
d
{ Electronic supplementary information (ESI) available: The macro-
scopic I–V characteristics of (ITO/CPMAB/Au) and the long-term
response of the ‘on’ state under an electric field of 1.80 V. See DOI:
10.1039/b703332j
3
530 | J. Mater. Chem., 2007, 17, 3530–3535
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1
8
in a comparatively low conductance state (off). Accord-
ingly, 4-3,4-dicyanophenoxy-49-dimethyl-aminoazobenzene
CPMAB) was synthesized (see Chart 1). The molecule
contains both a D moiety (amino group) and an A moiety
cyano groups), and possesses good thermal stability (without
Measurements
X-Ray diffraction of the bulk single crystal was measured with
a Rigaku RAXIS RAPID imaging plate diffractometer
equipped with graphite-monochromated Mo Ka radiation
(
(
(l = 0.071073 nm) at 20 uC. The sample was kept in a vacuum
decomposition up to 320 uC). It is noteworthy that an oxygen
bridge connects the D and A moieties, which is intended as a
barrier between them and presumed to partly contribute to the
electrical bistability and the stability. Crystalline films of
CPMAB molecules were prepared by vacuum deposition.
Macroscopic I–V characteristics of the thin films indicate that
the non-planar D–A molecules do possess good electrical
bistability. Moreover, nano-scale data storage on the thin film
was realized by STM. The results suggest that CPMAB may be
a promising candidate for non-volatile recording materials.
oven at 40 uC overnight to eliminate solvent residuals before
the measurement. The organic compound CPMAB was
2
4
deposited onto the substrates under vacuum (y4 6 10 Pa)
2
1
˚
at a rate of 0.1 A s using VPC-260F Vacuum Deposition
Equipment (ULVAC), and strict gas elimination prior to
deposition. The transmission electron microscope (TEM)
image and the selected-area electron diffraction (SAED)
pattern were obtained using a Hitachi H-8100 system. The
macroscopic I–V, I–t characteristics were recorded by a
Keithley 4200 Semiconductor System and a Micromani-
pulator 6150 probe station in a clean and shielded box under
atmospheric conditions at room temperature. Nano-scale data
recording experiments and local I–V STS were carried out with
a NT-MDT P47 STM under ambient conditions. STM images
were acquired continuously in constant current mode, using
tips made of tungsten wires (0.3 mm in diameter) by electro-
chemical etching. Micro-reflection-absorption infrared spectra
were measured with a Nicolet Magna IR 750 spectrometer
Experimental
Materials
N,N-Dimethylaniline, 4-aminophenol and 4-nitrophthaloni-
trile from Acros were used without further purification.
4
-Hydroxyl-49-(N,N-dimethylamino)azobenezene was synthe-
sized from N,N-dimethylaniline and 4-aminophenol by typical
1
9
21
diazotization and coupling reactions. Mp: ca. 199–200 uC.
equipped with a microscope (the resolution is 0.125 cm , the
1
H NMR (CDCl
3
, 400 MHz): d (ppm) 7.84 (d, J = 8.9 Hz, 2H),
diameter of the light spot is about 200 mm) and the UV-Vis
spectrum was obtained with a Hitachi U-4100 UV-Vis
spectrometer. Cyclic voltammogram (CV) was recorded on a
Zahner IM6e electrochemical work station using a Pt plate as
the working electrode, Pt wire as the counter electrode, and an
7
.79 (d, J = 8.8 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 6.76 (d, J =
8
.9 Hz, 2H), 4.97 (s, 1H), 3.08 (s, 6H). Anal. calcd for
C
14
H
15
N
3
O: C, 69.69; H, 6.27; N, 17.41. Found: C, 69.35; H,
6
.13; N, 17.24.
+
Ag/Ag electrode as the reference. 0.1 M tetra-n-butylammo-
Synthesis of the target compound CPMAB
Under the protection of dried DMF (20 mL),
-nitrophthalonitrile (1.44 g, 8.3 mmol), and 4-hydroxyl-49-
N,N-dimethylamino)azobenezene (2 g, 8.3 mmol) were in turn
added to a flask equipped with a stirrer. Then 5 g of K CO
were added in portions to the above mixture with stirring over
h. Subsequently, the reaction was continued at 25 uC for 8 h.
nium hexafluorophosphate (Bu NPF ) dissolved in acetonitrile
4
6
was employed as the supporting electrolyte.
2
N ,
4
Results and discussion
(
2
3
17
Distinguished from organic complex materials, CPMAB
contains the donor and acceptor moieties in a single molecule,
which could improve the homogeneity of the film. Fig. 1 shows
the morphology of the film with a thickness of ca. 30 nm by a
vacuum deposition method on a Cu grid with amorphous
carbon supporting film. The TEM image shows that the
organic molecules could form flat uniform films under our
experimental conditions. The SAED pattern (Fig. 1, insert)
demonstrates the single crystal nature of the thin film. It could
be inferred that the CPMAB molecules are stacked in an
orderly manner in the thin film by a vacuum deposition
method.
3
The reaction mixture was then poured slowly into 400 mL of
aqueous NaOH (10%, w/w) with stirring. The precipitate was
filtered off, washed three times with water and re-crystallized
from ethanol to afford a light yellow sheet-like solid of the
1
product CPMAB (2.6 g, yield 85%). Mp: 198 uC. H NMR
(
CDCl , 400 MHz): d (ppm) 7.95 (d, J = 8.6 Hz, 2H), 7.91 (d,
3
J = 8.8 Hz, 2H), 7.74 (d, J = 8.8 Hz, 1H), 7.35 (s, 1H), 7.28 (d,
J = 8.8 Hz, 1H), 7.16 (d, J = 8.6 Hz, 2H), 6.79 (d, J = 8.8 Hz,
+
H), 3.12 (s, 6H). MS (EI, 70 eV) m/z: 367 [M ]. Anal. calcd
2
for C H N O: C, 71.92; H, 4.66; N, 19.06. Found: C, 71.64;
2
2
17
5
To investigate the interaction between the molecules in
detail, a red bulk crystal grown from a methylbenzene–
dichloromethane (5 : 1) solution was determined by X-ray
diffraction{ after the elimination of solvent residuals. Fig. 2
shows the configuration of CPMAB and the packing diagram
H, 4.35; N, 19.16.
{
crystal system: orthorhombic, space group: Pbca, a = 7.5300(15) A, b =
Crystal structure analysis of CPMAB. C22
17 5
H N O, M = 367.41,
˚
˚
˚
1
3
2.173(2) A, c = 40.996(8) A, a = 90u, b = 90u, c = 90u, V =
3
757.8(13) A , T = 293 K, Z = 8. R (reflections) = 0.0476 (1626), wR2
˚
(
reflections) = 0.1331 (3010). CCDC reference numbers 624824. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b703332j
Chart 1 The molecular structure of CPMAB.
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Fig. 1 TEM image and SAED pattern (insert) of the CPMAB film.
of the molecules, which exhibits the relative position of the
molecular moieties in the single crystal. As expected, the D and
A moieties of CPMAB are in the different planes. By the
oxygen bridge, the torsion angle of the D moiety and the A
…
moiety is about 68.7u. C–H p interaction between C6–H6 of
one molecule and the centroid (Cg*) of the C13–C18 ring at
…
˚
1 2 x, 1 2 y, 2z) has the dimensions of H6 Cg* 2.76 A and
(
…
C6–H6 Cg* 152u. This then leads to the formation of
centrosymmetric dimers about the inversion centre at (0.5,
0
.5, 0). Although the whole molecule is non-planar, the D
moiety of a molecule tends to be close and parallel with the A
˚
moiety of an adjacent molecule with a distance of about 3.5 A
between them, while the distance between the D group and the
˚
A group in the same molecule is larger than 10 A. Herein, the
stack of molecules is favorable for the overlap of intermole-
cular orbitals in parallel moieties. The result of single crystal
X-ray diffraction is in accordance with the SAED indices of
thin film shown in Fig. 1.
Fig. 3 (a) The macroscopic I–V characteristics of the CPMAB film
by the first scan (curve I, ‘off’ state) and the second scan (curve II, ‘on’
2
state) with an active area 10 6 10 mm . The threshold voltage is 2.82 V.
Insert shows the logarithm of the ‘on’ and ‘off’ state current ratio as a
function of the applied voltage. (b) Long-term response of the ‘on’
state under an electric field of 1.50 V.
The stable and ordered crystalline thin film paves the way
for the application of the CPMAB molecule in electrical
1
2,13,20
devices.
A CPMAB thin film with a thickness of ca.
5
0 nm was deposited on an indium tin oxide (ITO)-coated
glass with the sandwich structure ITO/CPMAB(50 nm)/
Al(75 nm) and the macroscopic I–V characteristics were
measured with a standard two-terminal system (see the
Experimental part). In steps of 0.02 V, the voltage was
scanned from 0.00 V to 3.00 V (the bias was applied to the Al
electrode). Fig. 3(a) shows I–V characteristics of the first and
the second bias scan, respectively. It can be seen that before the
first bias scan, the film shows an insulating behavior (‘off’
state). The current undergoes a drastic increase when the
voltage exceeds the critical value of 2.82 V. In the second bias
scan, the current is obviously bigger than in the first scan and
the state is termed as the ‘on’ state. Based on the current
values, the conductivity in the ‘on’ state is about 3 orders of
Fig. 2 Viewed down the vectorial (a–b) direction, it clearly shows that
the two planar moieties are approximately parallel with (110) and
(
2110), respectively.
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magnitude larger than in the ‘off’ state [insert in Fig. 3(a)].
Furthermore, the film was found to retain the ‘on’ state non-
destructively for more than 5 h under the electric field of 1.50 V
the experimental results confirm that the organic thin films are
in the low conductivity state before the application of the
threshold voltage and in the high conductivity state after that.
Therefore, the recording mechanism should be ascribed to the
alternation of electrical conductivity before and after the
application of the threshold electric field.
[
Fig. 3(b)]. In addition, negative bias voltage has also been
added to the ‘on’ state film and it would not turn to the ‘off’
state even after 5 times scanning from 0.00 to 24.00 V or the
application of pulse voltage of 210.00 V for several seconds.
Taking the different work function of electrodes into account,
an analogous structure of ITO/CPMAB/Au was fabricated,
and similar results were obtained (see ESI{). The electrical
results indicate that the insulating thin film is switched to a
conducting one when the voltage reaches a threshold, and
the ‘on’ state is very stable in case the power is off or even the
much larger inversed pulse voltage is applied. Thus, the
material has good electrical bistability.
Further investigations were carried out to elucidate the
mechanism of the conductivity transition. Micro-reflection-
absorption FTIR spectra were recorded before [Fig. 5(a)] and
after [Fig. 5(b)] the application of a threshold voltage on the
thin film. The stretch vibration of –CMN exhibits a red-shift
2
1
21
(
from 2233.4 cm to 2231.0 cm ), which indicates that the
cyano group was changed from a neutral to a negatively
1
2,21
charged state in the molecule.
Compared with the
literature, the shift of the cyano stretch vibration in FTIR is
minor, which suggests a minor change in the electron density
of the cyano group. This phenomenon is attributed to the
charge delocalization from the cyano group to its connected
conjugation moiety. In addtion, the relatively strong vibration
Based on the electrical bistability of the CPMAB thin film,
data recording experiments were performed with a P47 STM
under ambient conditions using electrochemically etched
tungsten tips. CPMAB thin film with a thickness of about
2
1
at 1602 cm might be the result from the lower electron-
withdrawing ability of the cyano group towards the benzene
3
0 nm was deposited on the surface of a freshly cleaved highly
oriented pyrolytic graphite (HOPG) substrate. To induce the
recording dots, voltage pulses were applied between the STM
tip and the HOPG substrate. Fig. 4(a) shows a typical STM
image of the recorded pattern on the thin film in a scanning
2
2
ring after the application of voltage.
For organic materials with D and A moieties, intramole-
2
cular and intermolecular charge transfer (CT) are two
3
2
competitive processes. As mentioned above, preventing the
4
2
area of 20 6 20 nm . The recording dots with an average
positive and negative charges from recombination is significant
diameter of 2.5 nm were formed by applying program-
controlled voltage pulses of 2.8 V and 6 ms. The pattern was
stable during 8 h scanning except for a little thermal drift.
Meanwhile, the local conductivity of the dots and other
unrecorded places were measured by scanning tunneling
spectroscopy (STS). As illustrated in Fig. 4(b), curve I
represents the conductivity of the recording dots while curve
II shows the state of the unrecorded area. The two
conductivity curves differ. The former is in the higher
conductivity state and the latter is in the lower one. At half
hour intervals, the conductivity of the recording dots and
unrecorded area were measured over 8 h and the statistic data
were consistent. Compared with the current values shown in
Fig. 3(a), they are not in the same order of magnitude for the
different active areas and electrodes. However, the trend is
obviously consistent; namely, either in macro or micro-scale,
1
5
to the memory devices based on CT. However, the dual
operations of intermolecular CT and intramolecular CT might
make the contradictive function, i.e., the former would induce
charge separation and the latter would lead to charge
recombination. In our case, the solution of this problem is
suggested to be that the external electric field induces the ‘on’
state by intermolecular CT, and the non-planar configuration
restrains the intramolecular CT to keep the ‘on’ state stable by
preventing the charges from recombination. The appropriate
charge delocalization in the separate moieties is suggested to
create a stable environment to store the former transferred
1
6
charges even after the application of a reverse polarity bias.
The proposal is validated by the crystal structure of CPMAB,
the UV-Vis spectrum, cyclic voltammogram and the
Fig. 4 (a) A typical STM image of the recorded pattern ‘z’ on the
2
CPMAB thin film in a scanning area of 20 6 20 nm . The recording
dots, about 2.5 nm in average diameter were formed by applying
program-controlled voltage pulses of 2.8 V and 6 ms. The scan
condition is Vbias = 0.38 V, Iref = 0.043 nA. (b) Local conductivity of
the recording dots (Curve I) and unrecorded places (Curve II).
Fig. 5 The micro-reflection-absorption FTIR spectra of the CPMAB
thin film before (a) and after (b) the application of a threshold voltage.
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theoretical research as follows. On the one hand, the distance
between D and A is an important factor to determine the
charge transfer. According to the single crystal diffraction data
of CPMAB, the distance between the D moiety of one
molecule and the A moiety of the adjacent molecule is close
to the sum of the van der Waals radii, so electronic coupling
2
5
between them is dominant and efficient, thus intermolecular
CT would play an important role. On the other hand, a
theoretical study shows that the intramolecular electron
transfer is sensitive to the torsion angle that creates an
increasing potential barrier as the dihedral angle varies from
2
6
co-planar to the perpendicular position. In this case, it is the
torsion angle (68.7u) between the two different planar moieties
that decreases the conjugation degree of the molecule and
keeps stable the charge separation state induced by the
intermolecular CT. Additionally, even though the oxygen
bridge was considered to transmit the substitution effect in
Fig. 7 Cyclic voltammogram of CPMAB in an acetonitrile solution
2
1
2
7
4 6
of 0.1 M Bu NPF with a potential scan rate of 50 mV s .
some cases of photophysics,
the UV-Vis spectrum of
1
CPMAB (see Fig. 6) shows the (n, p*) and (p, p*) at
1
ratio of on/off can be explained for the reason that the non-
planar configuration might make the ‘off’ state current for the
perturbance of electron conjugation in the molecular skeleton
comparable energies, indicating the characteristics of ‘amino-
2
8
azobenzenes’ rather than those of ‘pseudo-stilbenes’. These
complementary results confirm that intramolecular CT
between the D and A in the same CPMAB molecule is
inhibited. The conformation of the two separate planes tends
to be in a perpendicular position after the application of the
electric field in order to minimize the intramolecular steric
1
8
low. Therefore, the CPMAB molecule’s configuration facil-
itates the intermolecular CT and excludes the intramolecular
CT in the majority, in other words, it is beneficial to keep the
recording information stable, which could also be verified by
the stability of macroscopic I–t characteristics in Fig. 3(b).
In order to further confirm the recording mechanism of the
CPMAB crystalline thin film, quantum chemical calculations
were performed by using the hybrid Hartree–Fock/density-
2
6
repulsion, which might stabilize the intermolecular p–p
conjugation further. Herein, for this non-planar D–A config-
uration, it is reasonable that it would be difficult for charge
recombination to take place in the molecule. Cyclic voltam-
metry (CV) of CPMAB was performed to investigate its
electrochemical behavior. As shown in Fig. 7, the onsets of Eox
3
0
functional-theory (HF/DFT) method of B3LYP with the
3
-31G* basis set. Fig. 8 shows HOMO and LUMO of the
1
6
+
CPMAB molecule. It is obvious that the amino moiety
contributes more to HOMO than to the LUMO while the
cyano moiety does the reverse. The configuration of the two
separate planar moieties in the molecule tends to a perpendi-
cular position, which is consistent with the results from X-ray
diffraction. Although the energy levels from the calculation do
not match exactly with the results of CV measurements
and Ered are 0.45 V and 21.66 V vs Ag/Ag , respectively. The
electronic energy levels of the highest occupied molecular
orbital (HOMO, 25.16 eV) and the lowest unoccupied
2
molecular orbital (LUMO, 23.05 eV) could be estimated.
9
The enegy gap is about 2.11 eV. That indicates the charge
transfer could happen with the threshold voltage. In addition,
it should be mentioned that the comparatively high current
1
8
regardless of a strong local field, the matching molecular
orbitals make it reasonable that the charge transfers from the
Fig. 8 The HOMO and LUMO molecular orbitals of CPMAB
Fig. 6 UV-Vis spectrum of CPMAB in CH
2
2
Cl
2
(y6.0
6
calculated by the HF/DFT method of B3LYP with the 6-31G* basis
set.
5
21
mol L ).
10
3
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9
G. Binnig, M. Despond, U. Drechsler, W. H a¨ berle, M. Lutwyche,
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amino moiety of one molecule to the cyano moiety of the
adjacent molecule under the application of the electric field;
and the non-planar configuration does much to prohibit the
charge recombination. Additionally, the theoretical calculation
also shows that the oxygen atom contributes little to the
molecular orbitals, which would hardly influence the charge
character of the D and A.
10 L. P. Ma, Y. L. Song, H. J. Gao, W. B. Zhao, H. Y. Chen,
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Conclusions
1
In summary, in order to combine the advantages of dual-
component and sole component donor–acceptor materials, a
new non-planar organic compound with separate electron
donor and acceptor moieties was synthesized, and its homo-
geneous crystalline films were prepared by vacuum deposition.
The thin film possesses good electrical bistability with high
current ratio of ‘on’ and ‘off’, and the ‘on’ state is very stable in
the long-term response. Nano-scale recording marks were
successfully realized on a thin film by STM and the recording
mechanism was discussed. Electrical bistability was ascribed to
intermolecular charge transfer and the non-planar configura-
tion benefits stable data storage as it can exclude the
interference of intramolecular charge transfer. The results
indicate that CPMAB is a promising candidate for nano-scale
recording materials. Further design of such non-planar
molecules and fine control of the charge transfer by the length
and rigidty of the linker between the D and A moieties are
undergoing, which might provide new strategies for develop-
ping novel functional materials for nano-scale data storage.
13 H. M. Wu, Y. L. Song, S. X. Du, H. W. Liu, H. J. Gao, L. Jiang
and D. B. Zhu, Adv. Mater., 2003, 15, 1925; Y. Q. Wen, Y. L. Song,
G. Y. Jiang, D. B. Zhao, K. L. Ding, W. F. Yuan, X. Lin,
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Acknowledgements
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The authors thank for financial support, the NSFC (Grant
Nos. 50625312, 60601027, U0634004 and 20421101), the 973
Program (No.2006CB806200, 2006CB932100) and the Chinese
Academy of Sciences. The authors also thank the reviewers for
the constructive comments and suggestions that helped
improve the manuscript greatly.
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