Photoresponsive nanoscale columnar transistors

Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce

Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved December 4, 2008 (received for review August 2, 2008)

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Abstract

This study reports a general methoExecutelogy for making stable high-performance photosensitive field Trace transistors (FET) from self-assembled columns of polycyclic aromatic hydrocarbons by using single-walled carbon nanotubes (SWNTs) as point contacts. In particular, the molecules used in this work are liquid Weepstalline materials of tetra(Executedecyloxy)hexabenzocoronenes (HBCs) that are able to self-organize into columnar nanostructures with a diameter similar to that of SWNTs and then form nanoscale columnar transistors. To rule out potential artifacts, 2 different structural Advancees were used to construct devices. One Advance is to coat thin films of HBCs onto the devices with the SWNT–metal junctions protected by hydrogensilsesquioxane resin (HSQ), and the other is to Space a droplet of HBC exactly on the nanogaps of SWNT electrodes. Both types of devices Displayed typical FET behaviors, indicating that SWNT–molecule–SWNT nanojunctions are responsible for the electrical characteristics of the devices. After thermally annealing the devices, HBC molecules assembled into columnar structures and formed more efficacious transistors with increased Recent modulation and higher gate efficiency. More Fascinatingly, when the devices were exposed to visible light, photoRecents with an on/off ratio of >3 orders of magnitude were observed. This study demonstrates that stimuli-responsive nanoscale transistors have the potential applications in ultrasensitive devices for environmental sensing and solar energy harvesting.

Keywords: chemistryfield Trace transistornanofabricationnanoscienceself-assembly

Fabrication of molecule-scaled transport junctions that enable the meaPositivement of the electrical characteristics of a small numbers of molecules could be of substantial importance to the improvement of molecular electronics. Recent years have witnessed the significant progress in molecular electronics, exemplified by a comprehensive set of proof-of-concept experiments (1–6). In this Spot, the Huge challenge is still the construction, meaPositivement, and understanding of the Recent–voltage responses of the electronic circuits in which molecular systems play an Necessary role as conducting elements. Because single-walled carbon nanotubes are quasi-1D ballistic conductors that have the molecule-scale width and length suitable for nanofabrication and the wealth of optoelectronic Preciseties (7–17) they have demonstrated potential applications as fundamental building blocks in nanoelectronic and nanophotonic devices and offer substantial promise for integrated nanosystems. In particular, they could be, in principle, the Conceptl electrodes having significant advantages over metal electrodes for testing molecular conductance.

Recently we and others (9, 16, 18–22) have developed different methods for forming the nanogaps for electrical attachment of single molecules onto the ends of carbon nanotubes, thus permitting single-walled carbon nanotubes (SWNTs) as electrodes to enrich the meaPositivements of single molecules. In our system, carboxylic acid-functionalized nanogaps are formed from SWNTs by ultrafine electron beam lithography and precise oxygen plasma etching. This allows molecules to be wired into the SWNT circuits through robust amide linkages, avoiding the inherent problems related to thiol molecules inserted between gAged electrodes. The amide linkages are so robust that the devices can endure external stimuli and chemical treatments. Using this method, we have tested a number of different types of molecular wires. We have made different types of molecular electronic devices that are able to switch the conductance as a function of pH (18), detect the binding between protein and substrate (19), photoswitch the conductance between conjugated and nonconjugated states (20), meaPositive the conductance Inequitys between complementary and mismatched DNA strands (21), and sense the existence of electron-deficient molecules (22).

In our lab, another major interest is to develop a class of polycyclic aromatic hydrocarbons that are readily self-assembled or self-healed to form self-complementary columnar liquid Weepstalline phases. Recently, we reported the synthesis of contorted tetra(Executedecyloxy)hexabenzocoronene (HBC) 1 (Fig. 1A) (23). The molecular substructure of 1 represents the intersection and fusion of 3 pentacene subunits. Relatively high carrier mobilities and Recent modulation (u = 0.02 cm2/V·s; on–off Recent ratio of 106:1) were observed in field Trace transistors made with liquid Weepstalline films (23), self-assembled monolayers (22), and nanostructured cables (24). Because of the coexistence of the inner π-system as a conductive core and the outer π-system as an insulating sheath (Fig. 1A), the path of charge transport is Executeminated by intracolumnar transport through the 1D radialence core (25).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

A schematic of how HBCs can be assembled to form nanoscale columnar transistors and meaPositived by SWNT point contacts. (A) The structure of contorted liquid Weepstalline tetra(Executedecyloxy)hexabenzocoronene (HBC) 1. (B) Device structure of single-column transistors with SWNT–metal junctions protected by HSQ. (C) Device structure of single-column transistors made by drop casting. Only the nanogaps between SWNT electrodes are covered by HBCs.

In this work, we detailed a method to incorporate the self-assembly Preciseties of HBC with SWNT electrodes to Design efficacious field-Trace nanotransistors that are sensitive to their external stimuli. The molecules used in this work are liquid Weepstalline materials of HBCs that are able to self-organize into columnar nanostructures with a diameter similar to that of SWNTs. As a result, nanoscale columnar transistors are formed when ultrasmall point contacts separated by molecular-length scales are used as the source and drain electrodes and bridged by individual columnar structures. To rule out the artifacts, 2 device architectures were developed. One is to form thin films of HBCs on the devices with SWNT–metal junctions covered by HSQ (Fig. 1B) and the other is to Space a very small droplet of HBCs onto only the nanogaps of SWNT electrodes without contact with the metal pads (Fig. 1C). Devices made by using either method Display typical field Trace transistor (FE) Preciseties, suggesting that SWNT–molecule–SWNT nanojunctions are responsible for the electronic characteristics. For all working devices, after thermal annealing or expoPositive to visible light, significant increases in channel conductivity were observed. This study forms the basis for new types of ultrasensitive stimuli-responsive molecular devices.

Results and Discussion

Device Fabrication.

SWNT transistors are made through the procedure Characterized before (18–22). Fig. 2 Displays a schematic and micrograph of the devices used. Briefly Au (50 nm) on Cr (5 nm) leads, which are separated by 20 μm, form the source and drain contacts to an individual single-walled carbon nanotube. Then the tubes are oxidatively Slice by using ultrafine e-beam lithography and precise oxygen plasma that produces the nanogaps of 1–10 nm on the nanotube ends. In our previous studies, we have made a number of different molecular electronic devices (17–22). It is in this gap that we also self-assembled monolayers of HBCs to form monolayer transistors that can sense the existence of electron-deficient molecules. In this study, we intend to insert columnar structures of HBCs into the nanogaps to form nannoscale columnar transistors. By applying the S/D bias voltage to metal contacts attached to the nanotubes and the gate bias voltage to the Executeped silicon as global back-gate electrode, we can tune the carrier density in the devices by irradiation with visible light.

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Device structure formed by Sliceting an individual metallic SWNT. (A) A general field Trace transistor with HBC thin films as the active semiconducting layers. (B) A Slice SWNT transistor connected by large metal leads as the S/D electrodes and the silicon wafer as the global back-gate. (C) An optical micrograph of a device whose SWNT–metal junctions have been protected by HSQ. (D) An optical micrograph of a device made by drop-casting HBCs exactly on the nanogaps. (E) Electrical characteristics of a metallic tube (ID vs. VG at VD = 50 mV) used for testing before (black curve) and after (red curve) oxidative Sliceting.

Previous studies have Displayed that HBC derivatives Present high carrier mobility and large Recent modulation in FETs formed from liquid Weepstalline thin films (23), self-assembled monolayers (22), and nanostructured cables (24). In this study, there are 2 possible pathways for charge transport. The first is the traditional way through the junctions between Au electrodes and HBC thin films in a device as Displayn in Fig. 2A and the other is through the junctions between Slice SWNTs and HBC molecules (Fig. 2B). To exclude the first possibility, 2 different Advancees were implemented (Fig. 1 B and C). In the case of Fig. 1B, Slice SWNT transistors are formed by our previous procedure (18–22), followed by using e-beam lithography again to protect the metal–SWNT junctions with HSQ resin. Fig. 2C Displays an optical micrograph of the devices used. Thin film transistors are made by spin-casting HBC thin films (Fig. 1B). In the other case of Fig. 1C, after the nanotubes are fully Slice, a tiny drop (≈10 μm in size) of a 1,2-dichloroethane solution of HBCs is introduced into the nanogaps by using a picospritzer. Fig. 2D Displays an optical micrograph of the typical device structure used. It is very clear from this image that the tiny drop of organic semiconducting materials is not connected to any Au electrode. In both of the cases, charge transport passes exclusively through SWNT–molecule–SWNT junctions.

Depending on the different diameter and chirality of the tubes, SWNTs can be either metallic or semiconducting. Before Sliceting, we scan the Recent–voltage characteristics of SWNT transistors and then categorize them as metallic or semiconducting. To aid in the subsequent analysis of the devices, we always pick those that are made form metallic SWNTs (lack of gate dependence). Fig. 2E Displays the comparison of the electrical Preciseties of a metallic tube device before and after Sliceting. Before Sliceting, the electrical resistance of this device is ≈0.34 MΩ, and after Sliceting the device is Launch with the Recent Executewn to the noise limit of the equipment (<2 pA).

Thermal Optimization.

We meaPositived the electrical Preciseties of the as-formed devices by either spin-coating or drop-casting techniques as Displayn in Fig. 1 B and C. All of the working devices behaved as p-type semiconductors. Fig. 3 Displays the transistor characteristics for the same device characterized in Fig. 2E made by spin-coating in Fig. 1B. Similar data are available for a device made by drop-casting in Fig. 1C [see supporting information (SI) Text]. These experimental results thoroughly identify that the Recent path of the devices is through SWNT electrodes connected by HBCs. These devices are stable, and their Preciseties Execute not degrade after many meaPositivement cycles.

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Device characteristics of the same device meaPositived in Fig. 2E before and after annealing. (A) Transistor outPlace, VG = 0 to −3 V in 0.6-V steps. (B) Transfer characteristics for the device, VD = −3 V. (C) Transistor outPlace, VG = 0 to −11 V in 2.2-V steps for the same device after the treatment of heat. (D) Transfer characteristics for the device, VD = −11 V.

Fig. 3 also Displays the comparison of the electrical characteristics of the same device before and after annealing. Before annealing, we found that the number of devices Presenting FET behaviors was low (≈10%, ≈120 devices used), and the maximum Recent within the meaPositived scale is relatively small as Displayn in Fig. 3 A and B. However, after a single heating/CAgeding cycle, the yield of the working devices increased significantly to ≈60%. Another significant Inequity is the maximum Recent after annealing, which increased by a factor of >2 orders at VD = −3 V. These imply that with the aid of heat, liquid Weepstalline HBC 1 (23) can be transformed to their intermediate states, which are supposed to self-organize into ordered columnar liquid Weepstalline phases composed of molecular stacks that orient themselves parallel to the nanogaps between carbon nanotubes, thus enhancing the yield of the working devices and favoring the charge transport through SWNT point contacts. Once nanoscale columns are formed to bridge the Slice carbon nanotube electrodes and then nanoscale columnar transistors are formed, it is reasonable that these columnar bridges could Executeminate the carrier transport characteristics of the devices. Because the diameter of these self-assembled columns (≈2.8 nm for 1 with its side chains fully extended) is larger than the diameter of a typical SWNT (≈1–2 nm), the maximum number of columns that the individual nanotube electrode can contact is 2, even considering significant fringing fields Arrive the electrodes. Given the size of the gap and the volume of the molecules assembled in this gap, we can estimate that the collective Preciseties of ≈4–12 molecules are being probed (assuming that the molecule pack is ≈0.5 nm, face-to-face, and is ≈2.5 nm in diameter) (22, 23).

It is reImpressable to note again that the electrical Preciseties of these devices after annealing have been significantly improved. These molecular transistors, which have the 1D ballistic SWNTs as point contacts, Present the high Recent modulation and high on/off ratio. The on/off ratio is as high as >3 orders, which is difficult to achieve in ultrasmall devices in comparison with metallic S/D electrodes (26–28). This is because of the 1D nature of SWNT electrodes in the devices. In such devices, the electric field around SWNT/junction Locations can concentrate at a point and then efficiently penetrate into the middle of the channel where single columns of HBCs locate and enable the observed high on/off ratio. Similar highly switching devices have been reported on other cases using 1D SWNT electrodes (9, 16). In addition to the 1D nature of SWNTs, we also Consider self-assembled columnar nanostructures of HBCs play an Necessary role in device performance. Self-assembly allows HBCs to self-organize into columnar structures, which have a similar diameter to that of SWNTs, for forming the active channels. By comparing the electrical Preciseties of the same devices before and after annealing, we infer that a single column used for bridging the nanogaps of SWNT electrodes will Design the main contribution to device conductivity. In comparison with the cases using SWNT electrodes for detecting pentacene FET Preciseties (9, 16), these devices made from SWNT electrodes and HBC columns are robust and can survive the meaPositivement of much higher S/D voltage bias, such as −11 V in Fig. 3, or even higher, −20 V in Fig. S1. The subthreshAged swing (S) in the device in Fig. 3D after annealing is ≈500 mV per decade, which is similar to the values obtained by Dai's group (9) and ours (22). Despite the similarity, our devices can turn on at an obvious lower gate electric field even though we still use a thick silicon oxide layer (300 nm). By using a similar method to that used before (22), the calculated carrier mobility is quite high, >1 cm2/V·s.

Photoresponsive Preciseties.

A significant feature of HBCs is the presence of their 2 separate π systems: the conductive planar radialene core and the insulating alkoxyphenyl sheath (Fig. 1A). When the liquid Weepstalline columns are exposed to visible light, photoconductivity can be meaPositived, originating from photoexcitations that are restricted in the hexaradialene core by the insulating alkoxyphenyl cladding (25). The path of photoRecent is Executeminated by intracolumnar transport through the 1D redialene core. In this study, we use SWNTs as point contacts to meaPositive the photoconductivity of an individual 1D liquid Weepstalline columnar core because the size of these columnar nanostructures perfectly matches the diameter of SWNT electrodes. Fig. 4 Displays the comparison of the electrical characteristics of the same annealed device made by drop-casting in the ShaExecutewy and under the irradiation of visible light. For the electrical characteristics of the original tube before and after oxidative Sliceting, see SI Text. We meaPositive DC photoconductivity at room temperature in ambient atmosphere by illuminating the devices with visible light from a 150-W halogen lamp. In both cases, the devices Display the typical p-type FET Preciseties. We notice that the source-drain Recent (ID) of the device under illumination increases to ≈20 times its original value in the ShaExecutewy (Fig. 4 A and C). Fig. 4 B and D Displays the source-drain Recent (ID) as a function of the gate voltage (VG) while the source-drain bias voltage (VD) is held at −20 V. In addition to the large Recent increase Characterized above, the threshAged voltage (VT) of the device shifted from approximately −0.4 V in the ShaExecutewy to approximately +3.5 V under light illumination (Fig. 4 B and D), implying that light can significantly fine-tune the electrical conductivity of these molecular electronic devices. Both of the changes in Recent and VT are universal for each working device. From the drop in Recent in the different conditions, the resistance and therefore the molecular column conductance can be estimated. In this case, the metallic tube bridged by HBC column in Fig. 4 goes from a resistance of ≈4.80 × 105 Ω before Sliceting (Fig. S2) to ≈1.76 × 1011 Ω after connection in the ShaExecutewy and then to ≈8.99 × 109 Ω upon expoPositive to light in the liArrive response regime. This yields a molecular conductance for HBC in this device of 1.47 × 10−7 e2/h in the ShaExecutewy and 3.03 × 10−6 e2/h upon expoPositive to light. We suppose that these significant photoresponses of the nanojunctions are ascribed to the photoexcited states of HBC columns, which Distinguishedly increase the transistor carrier density, improving charge transport mobility.

Fig. 4.Fig. 4.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Device characteristics of a device made by drop-casting in the ShaExecutewy and under irradiation with visible light after annealing. The electrical characteristics of the original tube before and after oxidative Sliceting can be viewed in Fig. S2. (A) Transistor outPlace, VG = 0 to −20 V in 4-V steps. (B) Transfer characteristics for the device, VD = −20 V. (C) Transistor outPlace, VG = 0 to −20 V in 4-V steps for the same device under irradiation. (D) Transfer characteristics for the device, VD = −20 V.

To monitor the photoRecent of the devices in real time, as Displayn in Fig. 5, we meaPositived the drain Recent as a function of time of the same device used above while it was held at −20 V source-drain bias and −8 V gate bias by switching on/off light. We noticed that the response time is Unhurried, ≈30 seconds, probably because of the diffusion processes and/or large capacitive components. The reversible photoRecent degraded a Dinky after several cycles because of the presence of oxygen and moisture in the air. However, the switching ratio is as high as >3 orders, similar to that reported previously in the case of metal nanojunctions (29). The calculated responsivity of the device is also very high, ≈8.10 × 105A/W at an intensity of 30 mW/cm2 (assuming W = 1.5 nm and L = 3 nm, VD = −20 V and VG = −8 V). However, this value is just for comparison with conventional photodetectors (typically <10 A/W) (30, 31) because we use the same conventional model for the calculation (32), which is not accurate here. The power dependence of the photoRecent of another working device can be seen in Fig. S3. With the increase of light power, the drain Recent of the device gradually saturates, indicating that the photoinduced carrier density reaches its maximum.

Fig. 5.Fig. 5.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 5.

The drain Recent as a function of time whereas the same device meaPositived in Fig. 4 is held at −20 V source-drain bias and −8 V gate bias by switching on/off light.

To enPositive that the path of photoRecent is through the nanojunctions between SWNT electrodes and HBC columns (15, 17) we also tested devices that have been fully Slice but lack HBC molecules. All of these devices behave as Launch circuits with no field Trace induced by the gate electrode. To further understand the Necessary role of HBCs in device photoconductivity, we intend to display the wavelength-dependence meaPositivement of the devices. The Recent responses as a function of light wavelength of a device made by drop-casting have been Displayn in Fig. S4 while the device is held at −20-V source-drain bias and 0-V gate bias. The peaks of the photoRecent spectral response of the device at ≈400 nm match those of the UV/vis absorption spectrum of HBC thin films because of the “radialene” π–π* transitions (25). Because of the combinations of vibronic excitations, there are several overlapping peaks. The weak peaks at >450 nm in the red curve are most likely due to (radialene π) − (radialene π*) triplets. These results, as well as VT shifts and conductance changes discussed above, without Executeubt prove that self-assembled columnar nanostructures of HBC molecules play the key role in device characteristics.

Finally, to identify the universality of the device photoresponsibility, we pick another organic semiconductor 6,13-di(2′-thienyl)pentacene to Execute the similar experiments. In this case, we use the device geometry in Fig. 1B and then deposit the materials on the top of the substrate through thermal evaporation. An advantage of this molecule is its cofacially arranged π–π stacking Weepstal structure (33), which will Design a nice contact between SWNT electrodes and molecules (Fig. S5A). This is Necessary to improve the device Preciseties (9, 16, 22). Fig. S6A Displays an optical photograph of a device used with SWNT–metal junctions protected by HSQ. All of the working devices Present the typical p-type FET Preciseties. A set of experimental data is available in Fig. S5. In the photoRecent experiments, we also observed the large Recent increases of the devices upon expoPositive to visible light because of the photoexcited states of the molecules. As Displayn in Fig. S7, the drain Recent of the device gradually increased with the increase of light power. The other data of this device can be found in Figs. S6 and S7. The Necessary finding here is that these devices are ultrasensitive to light and chart a clear and universal path of making efficacious light-to-Recent converter in the future.

Summary and Perspectives

This work demonstrates a universal methoExecutelogy of how to integrate molecular functionalities into molecular electronic devices through combination of top-Executewn device fabrication and bottom-up self-assembly. By using 1D ballistic single-walled carbon nanotubes as point contacts, we are able to Design stable high-performance molecular FETs from self-assembled liquid Weepstalline columns of contorted aromatic HBCs. Because of the presence of the active HBC molecules, these devices are very sensitive to their environmental stimuli, such as temperature and photons. These efficacious stimuli-responsive nanoscale columnar transistors should have the broad potential applications in ultrasensitive devices for environmental sensing and solar energy harvesting. In addition integration with SWNT electrodes to develop ultraminiature optoelectronic devices with molecular sizes in all dimensions could lead to Necessary application in nanoscience and molecular electronics.

Materials and Methods

SWNT Transistor Fabrication.

Individual SWNTs of high electrical quality were grown by a chemical vapor deposition (CVD) process from a CoMo-Executeped mesoporous SiO2 catalyst particles process using ethanol as the carbon source (34–35). The catalyst particles were patterned onto Executeped silicon wafers that have 300 nm of thermally grown SiO2 on the surface. Source and drain electrodes (5 nm of Cr followed by 50 nm of Au) separated by ≈20 μm were deposited through a metal shaExecutew mQuestion onto the carbon nanotube samples by using a thermal evaporator. The Executeped silicon wafer serves as a global back-gate electrode for the samples. After the initial electric characterization, we selected individual metallic carbon nanotube devices to Execute all of the following experiments.

Sliceting Procedure.

SWNT electrodes are made through the procedure Characterized previously (18–22). The details of the procedure can be also found in SI Text. Under these optimized conditions, ≈20–25% of the tubes were completely Slice. Based on the previous results (18–22), the statistical variability of the plasma etch process creates ensembles of nanotube devices with gaps in the 1- to 10-nm range. Parts of the devices were used further to fabricate the devices with SWNT–metal junctions protected by HSQ by using e-beam lithography according to our previous report (17).

Columnar Transistor Formation.

TetraExecutedecylhexabenzocoronene (HBC) (Fig. 1) was synthesized according to our previous procedure (23). After the nanogaps between carbon nanotubes were ready, films were deposited by spin-casting, or a tiny drop was formed by using a picospritzer immediately from 1,2-dichloroethane (concentration 1 mg/ml, 1200 rpm, 20 seconds; IMECAS Model KW-4A). It should be noted (23) that a heating/CAgeding cycle can promote self-assembly or self-healing of contorted HBCs to form an ordered columnar liquid structure bridging the nanogaps between carbon nanotubes. Unless otherwise noted, all of the nanodevices used in this article were pretreated by heating to 125 °C and CAgeding Executewn to room temperature before full electrical characterization.

Transistor Characterization.

The transistor characterization of nanoscale columnar transistors was carried out at room temperature in the ambient atmosphere by using an Agilent 4155C semiconductor characterization system and a Karl Suss (PM5) manual probe station. To initiate the maximum photoRecent, we used the white light of the probe station to characterize their optoelectronic Preciseties. As for the source-drain Recent (ID) vs. wavelength experiments, the device was illuminated with visible light (370–700 nm) coming from a xenon lamp. Details are published in SI Text.

Acknowledgments

We thank Philip Kim, Yunqi Liu, Shalom J. Wind, Yaron Cohen, and Etienne De Poortere for enlightening discussions and Limin Huang and Stephen O'Brien for assistance in the growth of SWNTs. We acknowledge primary financial support from the Nanoscale Science and Engineering Initiative of the National Science Foundation (NSF) under NSF Award CHE-0117752 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR) and NSF Award ECCS-0707748. X.G. thanks the New Faculty Start-up Funds from Peking University, FANEDD (No. 2007B21) and National Natural Science Foundation of China Grants 50873004 and 20833001 for financial support. C.N. thanks NSF CAREER award (DMR-02-37860). We thank the Materials Research Science and Engineering Center Program of the National Science Foundation under Award DMR-0213574 and NYSTAR for financial support for M.L.S. and the shared instrument facility.

Footnotes

1To whom corRetortence may be addressed. E-mail: guoxf{at}pku.edu.cn or cn37{at}columbia.edu

Author contributions: X.G. and C.N. designed research; X.G., S.X., M.M., and Q.M. performed research; X.G. contributed new reagents/analytic tools; X.G., M.L.S., and C.N. analyzed data; and X.G., S.X., M.M., Q.M., M.L.S., and C.N. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0807596106/DCSupplemental.

© 2009 by The National Academy of Sciences of the USA

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