REPORTS
Piezoelectric Nanogenerators Basedon Zinc Oxide Nanowire Arrays
Zhong Lin Wang1,2,3* and Jinhui Song1
We have converted nanoscale mechanical energy into electrical energy by means of piezoelectriczinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force
microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zincoxide creates a strain field and charge separation across the NW as a result of its bending. The
rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads toelectrical current generation. The efficiency of the NW-based piezoelectric power generator isestimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational,
and/or hydraulic energy into electricity for powering nanodevices.
ireless devices may allow in situ,
real-time biomedical monitoring anddetection, but such devices still require a power source. Ideally, such devices
should be self-powered and not dependent on abattery. The body provides numerous potential
power sources: mechanical energy (such as body
movement, muscle stretching, blood vessel contraction), vibrational energy (acoustic waves),
chemical energy (glucose), and hydraulic energy (body fluid and blood flow), but thechallenge is their efficient conversion into electrical energy. If accomplished on the nanoscale, such power sources could greatly reduce
the size of integrated nanosystems for optoelectronics (1), biosensors (2), resonators (3),and more.
We demonstrate an approach to converting
mechanical energy into electric power with theuse of aligned zinc oxide (ZnO) nanowires(NWs). The mechanism of the power generator
relies on the coupling of piezoelectric andsemiconducting properties of ZnO as well as
the formation of a Schottky barrier between themetal and ZnO contacts. The nanogenerator has
the potential of harvesting energy from theenvironment for self-powered nanotechnology.
Among the known one-dimensional (1D)
nanomaterials, ZnO has three key advantages.
First, it exhibits both semiconducting and piezoelectric (PZ) properties that can form the basis
for electromechanically coupled sensors andtransducers. Second, ZnO is relatively biosafe
and biocompatible (4), and it can be used forbiomedical applications with little toxicity.
Finally, ZnO exhibits the most diverse and abundant configurations of nanostructures known

W

1
School of Materials Science and Engineering, Georgia
Institute of Technology, Atlanta, GA 30332, USA. 2Department of Advanced Materials and Nanotechnology, College
of Engineering, Peking University, Beijing 100871, China.
3
National Center for Nanoscience and Technology, Beijing
100080, China.

*To whom correspondence should be addressed. E-mail:zhong.wang@mse.gatech.edu

242

so far, such as NWs (5), nanobelts (NBs) (6),nanosprings (7), nanorings (8), nanobows (9),and nanohelices (10). Although numerous studies
have demonstrated novel nanodevices and applications based on NWs and NBs, little work
has been done to address the power needs ofthese nanosystems.
Our study is based on aligned ZnO nanowires
grown on c plane–oriented a-Al2O3 substrate,using Au particles as a catalyst, by the vaporliquid-solid (VLS) process (11, 12). An epitaxial relation between ZnO and a-Al2O3 allows athin, continuous layer of ZnO to form at thesubstrate, which serves as a large electrodeconnecting the NWs with a metal electrodefor transport measurement (Fig. 1A). The NW
grows along the E0001^ direction and hasside surfaces of A0110Z (Fig. 1B). Most of theAu particles at the tips of the NWs either
evaporate during the growth or fall off during
scanning by the atomic force microscope (AFM)
tip (fig. S1). For most of the NWs, the growth
front is free of Au particles or has a small hemispherical Au particle that covers only a fractionof the top (inset, Fig. 1B). For the purpose of ourmeasurements, we have grown NW arrays thathave relatively less density and shorter length(0.2 to 0.5 mm), so that the AFM tip can exclusively reach one NW without touching another.
The measurements were performed by
AFM using a Si tip coated with Pt film, which
has a cone angle of 70-. The rectangularcantilever had a calibrated normal spring constantof 0.76 N/m (Fig. 1C). In the AFM contact mode,a constant normal force of 5 nN was maintained
between the tip and sample surface. The tip
scanned over the top of the ZnO NW, and thetip_s height was adjusted according to the surface
morphology and local contacting force. The
thermal vibration of the NWs at room temperature was negligible. For the electric contact atthe bottom of the nanowires, silver paste wasapplied to connect the (large) ZnO film on thesubstrate surface with the measurement circuit.
The output voltage across an outside load ofFig. 1. Experimental design for converting nanoscale mechanical energy into electrical energy by avertical piezoelectric (PZ) ZnO NW. (A) Scanning electron microscopy images of aligned ZnO NWs
grown on a-Al2O3 substrate. (B) Transmission electron microscopy images of ZnO NWs, showing thetypical structure of the NW without an Au particle or with a small Au particle at the top. Each NW isa single crystal and has uniform shape. Inset at center: an electron diffraction pattern from a NW.
Most of the NWs had no Au particle at the top. Inset at right: image of a NW with an Au particle.
(C) Experimental setup and procedures for generating electricity by deforming a PZ NW with aconductive AFM tip. The base of the NW is grounded and an external load of RL is applied, which ismuch larger than the resistance RI of the NW. The AFM scans across the NW arrays in contact mode.

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resistance RL 0 500 megohms was continuously
monitored as the tip scanned over the nanowires (note the defined polarity of the voltage
signal). No external voltage was applied inany stage of the experiment.

Experimentally, both the topography (feedback signal from the scanner) (Fig. 2A) and thecorresponding output voltage (VL ) images acrossthe load (Fig. 2B) were recorded simultaneously when the AFM tip was scanned over the

Fig. 2. Electromechanically coupled discharging process of aligned piezoelectric ZnO NWs
observed in contact mode. (A and B) Topography image (A) and corresponding output voltage
image (B) of the NW arrays. The discharging process was so rapid that each discharge event wascharacterized by only a couple of data points, which made it difficult to display the data by rainbow
color (see fig. S2). (C) A series of line profiles of the voltage output signal when the AFM tip
scanned across a vertical NW at a time interval of 1 min. The data show the registration of theelectric signal with the location of the NW and the reproducibility of the event over an extended
period of time. Colors represent the outputs required for a series of scans. In (A) to (C), thescanning speed of the tip was 12.081 mm/s, and the time spent to acquire and output one scanpoint was 2 ms. (D) Line profiles from the topography (red) and output voltage (blue) images
across a NW. The peak of the voltage output corresponds approximately to the maximum deflectionof the NW, indicating that the discharge occurs when the tip is in contact with the compressed sideof the NW. When the tip touches the NW, PZ charges start to accumulate but no discharge occurs.
Discharge occurs when the deflection reaches nearly the maximum ym. Note that the lateraldeflection y includes the shape and contact geometry of the tip, which must be subtracted to derive
the true deflection of the NW. (E) Line profile of the voltage output signal when the AFM tip scans
across a vertical NW at 12.394 mm/s. The time spent to acquire and output one scan point was 0.05
ms, which was achieved at the maximum scan frequency of the AFM. The inset is an equivalentcircuit of the measurement to be used for simulating the discharging process. (F) The resonancevibration of a NW after being released by the AFM tip, showing that the stored elastic energy istransferred mainly into vibrational energy after creating the PZ discharge event.
www.sciencemag.org

SCIENCEVOL 312aligned NW arrays. In contact mode, as the tip
scanned over the vertically aligned NWs, theNWs were bent consecutively. The bending distance was directly recorded in the topography
image, from which the maximum bending deflection distance and the elastic modulus of the NW
as well as the density of NWs that have beenscanned by the tip were directly derived (13).
In the VL image, many sharp output peaks(like discharge peaks) were observed. Thesepeaks, typically about 4 to 50 times the noiselevel, are rather sharp and narrow, and sometimes one or two pixels represent one voltage
peak because of the limited scanning speed of
the AFM tip, so that the color distribution in theplot is not easy to display (fig. S2). By reducing
the scan range and increasing the scan frequency, more complete profiles of the discharge peaks
were captured (fig. S3). Most of the voltage
peaks are È6 to 9 mV in height. The density of
NWs contacted by the tip is counted from Fig.
2A to be È20/mm2, and the average density of
NWs whose voltage output events had been
captured by the tip in Fig. 2B is È8/mm2, thus
È40% of the NWs were contacted.
The location of the voltage peak is directly
registered at the site of the NW. A time series ofthe voltage output line profiles across one NW
acquired at a time interval of 1 min is shown in
Fig. 2C. Because the dwell time for each data
point (or pixel) is 2 ms, which is longer than the
average lifetime of the voltage peak of È0.6 ms(Fig. 2D), the peak at which VL reached the maximum was possibly missed by the Bslow[ scanning tip, so that VL shows a chopped top (arrows
in Fig. 2C). A sharp peak can be identified continuously at the location of the NW and in the
NW output voltage in each scan of the tip. Whenthe tip started to deflect the NW, no voltage
output was observed (Fig. 2D); VL was detected
when the deflection of the NW approached its
maximum. When the NW was released by the
AFM tip, VL dropped to zero, indicating that theoutput of piezoelectricity was detected toward
the end of the AFM scan over the NW.
The shape of the discharge peak can befurther improved and analyzed by maximizing
the tip scanning frequency and reducing thescanning range. Shown in Fig. 2E is a line
profile of VL when the tip was scanned over a
single NW at a scanning velocity of 12.394 mm/s.
The full width at half maximum (FWHM) of thevoltage peak was estimated to be t È 0.6 ms.
The damping behavior of the voltage peakcan be qualitatively described with an equivalent circuit (14) (Fig. 2E). The NW is approximated as a resistor RI and a capacitor C(including the contribution from the system).
The lifetime of the output voltage VL acrossthe load RL (note the polarity of the voltage)is t 0 (RL þ RI )C (15). For the experiment wehave designed, the resistance of the NW RI
is negligible relative to RL (16, 17). Thus, theequivalent capacitance of the NW and thesystem is C , t/RL , 1.2 pF.

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REPORTS
The observed sharp voltage output for ZnO
NWs was not observed for metal film (fig. S4),aligned carbon nanotubes (fig. S5), or aligned
WO3 NWs (fig. S6) under identical or similar
experimental conditions. These data rule out thepossibility of friction or contact potential as thesource of the observed VL response.
The efficiency of the electric power generated by this process can be calculated as follows.
The output electrical energy from one NW in
one piezoelectric discharge (PZD) event is
DWPZD 0 V02C/2 (15), where V0 is the peakvoltage of the discharge output. For simplicity,
we approximately take the NW as a 2D object
for easy analytical calculation. The elastic
deformation energy (WELD) created by the
AFM tip for displacing the NW is WELD 0
3YIy 2m/2L3 (15), where Y is the elastic modulus, I is the moment of inertia, L is the length of
the NW, and ym is the maximum deflection of
the NW. Dissipation of WELD mainly occurs
in three ways: (i) mechanical resonance/
vibration after releasing the NW (Fig. 2F);(ii) PZ discharge (DWPZD) for each cycle of thevibration; and (iii) friction/viscosity from theenvironmental medium. The mechanical resonance of the NW continues for many cycles,
but it is eventually damped by the viscosity ofthe medium. Each cycle of the vibration generates DWPZD, but the AFM tip in the present
experimental design catches only the energy
generated in the first cycle of vibration. Taking
into account the energy dissipated by the NW
in the first cycle of vibration DWELD (15), theefficiency of converting mechanical energy toelectrical energy is DWPZD/DWELD. Therefore,
an efficiency of 17 to 30% has been received
for a cycle of the resonance (table S1). The
large efficiency is likely due to the extremely
large deformation that can be borne by the
nanowire (18).
The physical principle for creating the PZ
discharge energy arises from how the piezoelectric and semiconducting properties of ZnO
are coupled. For a vertical, straight ZnO NW(Fig. 3A), the deflection of the NW by an AFM
tip creates a strain field, with the outer surface
being stretched (positive strain e) and the innersurface compressed (negative e) (Fig. 3B). An
electric field Ez along the NW (z direction) isthen created inside the NW volume through thePZ effect, Ez 0 ez /d, where d is the PZ
coefficient (19) along the NW direction that isnormally the positive c axis of ZnO, with the Zn
atomic layer being the front-terminating layer(20, 21). The PZ field direction is closely parallel to the z axis (NW direction) at the outersurface and antiparallel to the z axis at the innersurface (Fig. 3C). Under the first-order approximation, across the width of the NW at the top
end, the electric potential distribution from thecompressed to the stretched side surface isapproximately between V s– to V sþ Ewith Vsm 0
m3T kymk/4Ld (15), where T is the thickness of
the NW). The electrode at the base of the NW is

244

grounded. Note that V sþ and V s– are the voltages produced by the PZ effect, which are eachtypically larger than a few tens of volts (22). The
potential is created by the relative displacement of the Zn2þ cations with respect to the O2–
anions, a result of the PZ effect in the wurtzite
crystal structure; thus, these ionic charges cannot
freely move and cannot recombine without
releasing the strain (Fig. 3D). The potential difference is maintained as long as the deformation is in place and no foreign free charges(such as from the metal contacts) are injected.
The contacts at the top and the base of the
NW are nonsymmetric; the bottom contact is to
the ZnO film in contact with Ag paste, so theeffective contact is between ZnO and Ag. The
electron affinity (Ea) of ZnO is 4.5 eV (23) andthe work function (f) of Ag is 4.2 eV; there isno barrier at the interface, so the ZnO-Ag
contact is ohmic. At the tip of the NW, Pt has
f 0 6.1 eV, and the Pt-ZnO contact is a Schottky
barrier (24, 25) and dominates the entire trans-

port process. Because the compressed side of
the semiconductor ZnO NW has negative potential V s– and the stretched side has positive
potential (V sþ), two distinct transport processes
will occur across the Schottky barrier.
We now consider the case of a ZnO NW
without an Au particle at the top. In the first step,
the AFM conductive tip that induces the deformation is in contact with the stretched surfaceof positive potential Vsþ (Fig. 3, D and E). The
Pt metal tip has a potential of nearly zero, Vm 00, so the metal tip–ZnO interface is negatively
biased for DV 0 Vm – Vs þ G 0. Because the assynthesized ZnO NWs behave as n-type semiconductors, the Pt metal–ZnO semiconductor(M-S) interface in this case is a reverse-biased
Schottky diode (Fig. 3E), and little current
flows across the interface. In the second step,when the AFM tip is in contact with the compressed side of the NW (Fig. 3F), the metal
tip–ZnO interface is positively biased for DV 0
VL 0 Vm – Vs – 9 0. The M-S interface in thisFig. 3. Transport is governedby a metal-semiconductor
Schottky barrier for the PZZnO NW (see movies S1 andS2). (A) Schematic definitionof a NW and the coordination system. (B) Longitudinal strain ez distribution
in the NW after being
deflected by an AFM tip fromthe side. The data were
simulated by FEMLAB for a
ZnO NW of length 1 mm andan aspect ratio of 10. (C) The
corresponding longitudinal
PZ-induced electric field Ez
distribution in the NW. (D)Potential distribution in the
NW as a result of the PZ effect. (E and F) Contacts between the AFM tip andthe semiconductor ZnO NW[boxed area in (D)] at two reversed local contact potentials(positive and negative), showing reverse- and forwardbiased Schottky rectifying
behavior, respectively (seetext). This oppositely biased
Schottky barrier across the
NW preserves the PZ charges
and later produces the discharge output. The inset showsa typical current-voltage(I-V) relation characteristicof a metal-semiconductor(n-type) Schottky barrier. The process in (E) is to separate and maintain the charges as well as build up
the potential. The process in (F) is to discharge the potential and generates electric current. (G and H)
Contact of the metal tip with a ZnO NW with a small Au particle at the top. The PZ potential is built up in
the displacing process (G), and later the charges are released through the compressed side of the NW(H). (I) Contact of the metal tip with a ZnO NW with a large Au particle at the top. The charges aregradually ‘‘leaked’’ out through the compressed side of the NW as soon as the deformation occurs;
thus, no accumulated potential will be created.

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case is a positively biased Schottky diode, andit produces a sudden increase in the output
electric current. The current is the result of DVdriven flow of electrons from the semiconductor ZnO NW to the metal tip. The flow of
the free electrons from the loop through the
NW to the tip will neutralize the ionic charges
distributed in the volume of the NW and thus
will reduce the magnitudes of the potential Vs –and Vsþ. Thus, VL starts to drop and reaches
zero after all of the ionic charges in the NW
are fully neutralized. This mechanism explains
why the discharge curve in Fig. 2E is nearly
symmetric. According to the model, the discharge occurs when the NW is bent nearly toits maximum deflection, so VL should have asmall offset in reference to the corresponding
topography peak along the direction of tip scan,
as is observed experimentally in Fig. 2D.
We next consider the case of a ZnO NW witha small Au particle at the top as a result of VLS
growth (Fig. 3, G and H). In the first step of
the AFM tip displacing the NW, the tip is directly in contact with the ZnO NW but not theAu particle (Fig. 3G) because of its small sizeand hemispherical shape (Fig. 1B and fig. S1).
A process similar to that described in Fig. 3E
occurs, and no output voltage will be observed.
When the tip is in contact with the Au particle,
the tip is integrated with the particle as one metal
contact, and at the local interface between the
Au particle and the negatively charged, compressed side of the ZnO NW (Vs –), a forwardbiased Schottky diode is formed; thus, thecurrent flows from the tip through the Au
particle to the interface region with negative
PZ voltage (Vs –) (Fig. 3H). A discharge processsimilar to that shown in Fig. 3F occurs, and a
sharp voltage output is produced.
In the case of a NW with a large Au particle
that fully covers its top (Fig. 3I), the metal tip is

directly in touch with the Au tip at the beginning of the forced displacement. Because of
the conducting channel at the compressed sideof the NW, the polar charges produced by thedisplaced Zn 2þ and O 2– ions caused by the PZ
effect are immediately neutralized by external
free charges as soon as they are produced by
the deformation. Therefore, no accumulative
potential profile is formed at the M-S interface,and no measurable output voltage is detectedin the experimental setup we have designed(Fig. 1C).
We also measured the PZ voltage output forthe same samples of aligned ZnO NWs in the
AFM tapping mode. In this case, the deformation occurred longitudinally and there was noside displacement. The NW was vertically compressed, so the voltage created at the top of
the NW was negative Vs– so long as the baseelectrode was grounded (Fig. 4A). There was
no voltage drop across the width of the NW.
Thus, across the metal tip–ZnO interface, apositively biased Schottky diode formed andthe electrons could freely flow across theinterface. Electrons flowed as the deformation
occurred, and there was no accumulation of net
charge in the NW volume. This type of slowly
Bleaked[ current produces no detectable signal.
Therefore, for the ZnO NWs whose AFM image in tapping mode is shown in Fig. 4B, no
output voltage VL was detected beyond the noiselevel (Fig. 4C).
We now estimate the possibility of powering
nanodevices with the NW-based power generator. From table S1, the PZ energy output by one
NW in one discharge event is È0.05 fJ, and theoutput voltage on the load is È8 mV. For a NWof typical resonance frequency È10 MHz, theoutput power of the NW would be È0.5 pW. If
the density of NWs per unit area on the substrate is 20/mm2, the output power density isFig. 4. Electromechanically coupled discharging
process of aligned piezoelectric ZnO NWs observedin tapping mode. (A) Experimental setup. (B andC) Topography image (B)and corresponding output
voltage image (C) of the
NWs. The tapping force
was 5 nN, tapping frequency 68 KHz, and tapping
speed 6 mm/s. The voltage
output contains no information but noise, proving
the physical mechanism
demonstrated in Fig. 3.

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SCIENCEVOL 312È10 pW/mm2. By choosing a NW array of size
10 mm  10 mm, the power generated may beenough to drive a single NW/NB/nanotubebased device (26–28). If we can find a way to
induce the resonance of a NW array and output
the PZ-converted power generated in each cycle of the vibration, a significantly strong powersource may be possible for self-powering nanodevices. Furthermore, if the energy produced byacoustic waves, ultrasonic waves, or hydraulicpressure/force could be harvested, electricity
could be generated by means of ZnO NW arrays grown on solid substrates or even on
flexible polymer films (29). The principle andthe nanogenerator demonstrated could be thebasis for new self-powering nanotechnologythat harvests electricity from the environment
for applications such as implantable biomedical devices, wireless sensors, and portable electronics (29).
References and Notes
1. X. F. Duan, Y. Huang, R. Agarwal, C. M. Lieber, Nature
421, 241 (2003).
2. G. F. Zheng, F. Patolsky, Y. Cui, W. U. Wang, C. M. Lieber,Nat. Biotechnol. 23, 1294 (2005).
3. X. D. Bai, P. X. Gao, Z. L. Wang, E. G. Wang, Appl. Phys.
Lett. 82, 4806 (2003).
4. J. Zhou, N. S. Xu, Z. L. Wang, unpublished data.
5. M. H. Huang et al., Adv. Mater. 13, 113 (2001).
6. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947
(2001).
7. X. Y. Kong, Z. L. Wang, Nano Lett. 3, 1625 (2003).
8. X. Y. Kong, Y. Ding, R. Yang, Z. L. Wang, Science 303,
1348 (2004).
9. W. L. Hughes, Z. L. Wang, J. Am. Chem. Soc. 126, 6703
(2004).
10. P. X. Gao et al., Science 309, 1700 (2005).
11. X. D. Wang, C. J. Summers, Z. L. Wang, Nano Lett. 3, 423(2004).
12. X. D. Wang et al., J. Am. Chem. Soc. 127, 7920 (2005).
13. J. H. Song, X. D. Wang, E. Riedo, Z. L. Wang, Nano Lett.
5, 1954 (2005).
14. A piezoelectric material can be approximately characterized by a capacitor and a resistor. The capacitor
represents the piezoelectric charges accumulated in thevolume, and the resistor represents its inner resistance.
15. See supporting material on Science Online.
16. From our recent measurements of ZnO nanowires, theresistivity is from 10j2 to 10 ohmIcm (17) depending on
the contacts at the electrodes and the concentration ofoxygen vacancies. For a NW of length 0.2 mm and diameter
40 nm, the resistance is between 16 kilohms and 16
megohms, which is much smaller than the applied externalload of 500 megohms. Here we ignored the resistanceproduced by the ZnO film at the bottom of the nanowires
because it is very large and covers the entire area ofthe substrate; thus, the inner resistance is dominated by
the NW.
17. J. H. He, C. S. Lao, L. J. Chen, D. Davidovic, Z. L. Wang,
J. Am. Chem. Soc. 127, 16376 (2005).
18. W. L. Hughes, Z. L. Wang, Appl. Phys. Lett. 86, 043106(2005).
19. M. H. Zhao, Z. L. Wang, S. X. Mao, Nano Lett. 4, 587(2004).
20. The wurtzite-structured ZnO can be described as anumber of alternating planes composed of tetrahedrally
coordinated O2– and Zn2þ ions stacked alternatively
along the c axis. The oppositely charged ions produce
positively charged (0001)-Zn and negatively charged
ð0001Þ-O polar surfaces. The Zn-terminated surface is at
the growth front (positive c-axis direction) because of its
higher catalytic activity (19).
21. Z. L. Wang, X. Y. Kong, J. M. Zuo, Phys. Rev. Lett. 91,
185502 (2003).

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REPORTSþ

22. A simple calculation indicates that the magnitudes of Vs–
and Vs are on the order of a few tens to hundreds of
volts. In practice, if we consider the polarization anddielectric screening in the calculation, the local potential
is much smaller than the numbers given by the equations
here. An accurate calculation of the potential distribution
as a result of the ionic charges introduced by the PZ
effect and the surface charges caused by boundaries
must be solved numerically and self-consistently. In our
analysis, a correct magnitude and sign of the potential issufficient for illustrating the physical model.
23. S. Hasegawa, S. Nishida, T. Yamashita, H. Asahi, J. Ceramic
Proc. Res. 6, 245 (2005).

24. R. F. Pierret, Semiconductor Device Fundamentals(Addison-Wesley, Reading, MA, 1996), chapter 14.
25. W. I. Park, G. C. Yi, J. W. Kim, S. M. Park, Appl. Phys. Lett.
82, 4358 (2003).
26. Y. Huang et al., Science 294, 1313 (2001).
27. A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science
294, 1317 (2001); published online 4 October 2001
(10.1126/science.1065824).
28. J. Chen et al., Science 310, 1171 (2005).
29. U.S. patent pending.
30. Supported by NSF grant DMR 9733160, the NASA Vehicle
Systems Program and Department of Defense Researchand Engineering, and the Defense Advanced ResearchControl of Electron Localization in
Molecular Dissociation
M. F. Kling,1 Ch. Siedschlag,1 A. J. Verhoef,2 J. I. Khan,1 M. Schultze,2 Th. Uphues,3 Y. Ni,1
M. Uiberacker,4 M. Drescher,3,5 F. Krausz,2,4 M. J. J. Vrakking1
We demonstrated how the subcycle evolution of the electric field of light can be used to control the
motion of bound electrons. Results are presented for the dissociative ionization of deuterium
molecules (D2 Y Dþ þ D), where asymmetric ejection of the ionic fragment reveals that lightdriven intramolecular electronic motion before dissociation localizes the electron on one of the two
Dþ ions in a controlled way. The results extend subfemtosecond electron control to molecules andprovide evidence of its usefulness in controlling reaction dynamics.
ew-cycle laser light with a controlled evolution of the electric field E(t) 0 a(t) cos(wt þ 8), with amplitude a(t), frequency w, and carrier envelope phase 8 (1), hasrecently allowed the steering of the motion ofelectrons in and around atoms on a subfemtosecond time scale. Manifestations of this control
include the reproducible generation and measurement of single subfemtosecond pulses (2, 3)
and controlled electron emission from atoms(4, 5). Here we address the question of whether
this control can be extended to electron wave
packets in molecules and, if so, can light-field–
driven electronic motion affect reaction dynamics?
Many of the processes in terms of whichstrong-field molecular interactions are presently
interpreted (such as bond softening and enhanced
ionization) were discovered in experimental andtheoretical work on H2 and its isotopes HD and
D2 Esee (6) and references therein^. The role ofphase control in the dissociation of hydrogenhas recently been addressed in a few theoreticalstudies (7–9). We present experiments on the
dissociation of D2þ into Dþ þ D by intense
few-cycle laser pulses with controlled fieldevolution and report a pronounced dependenceof the direction of the Dþ ejection (and henceof the localization of the electron in the system)on the waveform driving the reaction. Quantum-

F

1
FOM Instituut voor Atoom en Molecuul Fysica (AMOLF),
Kruislaan 407, 1098 SJ Amsterdam, Netherlands. 2MaxPlanck-Institut für Quantenoptik, Hans-Kopfermann-Strasse
1, D-85748 Garching, Germany. 3Fakultät für Physik,
Universität Bielefeld, Universitätsstrasse 25, D-33615
Bielefeld, Germany. 4Department für Physik, LudwigMaximilians-Universität München, Am Coulombwall 1,
D-85748 Garching, Germany. 5Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149,
D-22761 Hamburg, Germany.

246

classical computations reveal that light-field
control of molecular electron dynamics is responsible for the observed phenomenon.
The dynamics of molecules in intense laser
fields typically includes ionization and dissociation. The dissociation of D2 in intense laser fields
is known to involve several pathways whose
relative importance depends on intensity and pulse
duration (6). The formation of fragment ions oc-

Projects Agency. We thank X. Wang, W. L. Hughes,
J. Zhou, and J. Liu for their help.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/312/5771/242/DC1
SOM Text
Figs. S1 to S6Table S1
Movies S1 and S2
19 December 2005; accepted 10 March 2006
10.1126/science.1124005

curs via a two-step mechanism (Fig. 1A) in which
initially the molecule is ionized by the laser field(Fig. 1A, red arrow) and a vibrational wave
packet is launched in the 1ssgþ state. Breakupof the D2þ ion is triggered by excitation to a
repulsive state or after double ionization.
In the single-ionization pathways, excitation ofbound D2þ (such as to the 2psuþ state in Fig. 1A)
by recollision of the first electron Erecollision
excitation (RCE), green line^ or directly by the laser
field Esequential excitation (SE), blue line^ leads todissociation and the formation of a Dþ ion and a D
atom. For example, in recent molecular clockstudies, vibrational motion in D2þ was timeresolved by exploiting RCE (10, 11). Additional
dissociation mechanisms can be understood byconsidering that molecular potentials are modified by strong laser fields. Bond softening (BS,purple line) (12) occurs when energy gaps open
up at avoided crossings between adiabatic fielddressed potential energy curves.
In double-ionization pathways, the formation of D2þ is followed by a second ionization

Fig. 1. (A) Pathways for the production of Dþ ions from D2 by dissociation of the molecular ion(through BS, SE, or RCE) or by Coulomb explosion (through RCI, SI, or EI). BS occurs when the avoided
crossing between diabatic potentials that are dressed by the laser field gives rise to dissociation fromvibrational levels that were originally bound (12). (B) Dþ kinetic energy spectra for dissociation of D2
by 5- to 8-fs linearly polarized (LP) and 5-fs circularly polarized (CP) laser pulses without phasestabilization, at I 0 1.2 T 0.2  1014 W cm–2 and I 0 2.4 T 0.2  1014 W cm–2, respectively.

14 APRIL 2006

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