Piezoelectric Nanogenerators Based on Zinc oxide Nanowire arrays Science Zhong Lin Wang and Jinhui Song Science312,242(2006) NAAAS Do|:10.1126/ scIence.1124005 This copy is for your personal, non-commercial use only If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here The following resources related to this article are available online at www.sciencemag.org(thisinformationiscurrentasofSeptember17,2014): Updated information and services, including high-resolution figures, can be found in the online version of this article at http://www.sciencemag.org/content/312/5771/242.fullhtml Supporting Online Material can be found at / suppl.200604/13/3125771.242Dc1html This article cites 21 articles 6 of which can be accessed free http://www.sciencemag.org/content/312/5771/242.full.htmlfref-list-1 This article has been cited by 633 article(s)on the ISI Web of Science This article has been cited by 30 articles hosted by HighWire Press, see http://www.sciencemag.org/content/312/5771/242.fullhtml#related-urls This article appears in the following subject collections Materials Science http://www.sciencemag.org/cgi/collection/matsci Science(print ISSN 0036-8075: online ISSN 1095-9203)is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2006 by the American Association for the Advancement of Science; all rights reserved. The title Science is a egistered trademark of AAAs
DOI: 10.1126/science.1124005 Science 312, 242 (2006); Zhong Lin Wang and Jinhui Song Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays This copy is for your personal, non-commercial use only. colleagues, clients, or customers by clicking here. If you wish to distribute this article to others, you can order high-quality copies for your following the guidelines here. Permission to republish or repurpose articles or portions of articles can be obtained by www.sciencemag.org (this information is current as of September 17, 2014 ): The following resources related to this article are available online at http://www.sciencemag.org/content/312/5771/242.full.html version of this article at: Updated information and services, including high-resolution figures, can be found in the online http://www.sciencemag.org/content/suppl/2006/04/13/312.5771.242.DC1.html Supporting Online Material can be found at: http://www.sciencemag.org/content/312/5771/242.full.html#ref-list-1 This article cites 21 articles, 6 of which can be accessed free: This article has been cited by 633 article(s) on the ISI Web of Science http://www.sciencemag.org/content/312/5771/242.full.html#related-urls This article has been cited by 30 articles hosted by HighWire Press; see: http://www.sciencemag.org/cgi/collection/mat_sci Materials Science This article appears in the following subject collections: registered trademark of AAAS. 2006 by the American Association for the Advancement of Science; all rights reserved. The title Science is a American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on September 17, 2014 www.sciencemag.org Downloaded from
REPORTS ubstrate, which serves as a large connecting the Nws with a metal Piezoelectric Nanogenerators Based grows along mehe coon direction side surfaces of (0110)(Fig. 1B). Most of the on Zinc Oxide Nanowire Arrays Au particles at the tips of the NWs either evaporate during the growth or fall off during Zhong Lin Wang, 2,3* and Jinhui Song scanning by the atomic force microscope(AFM) tip(fig. S1). For most of the NWs, the growth zinc oxide nanowire(NW) arrays. The aligned NWs are deflected with a conductive atomic ectric front is free of Au particles or has a small hem- We have converted nanoscale mechanical energy into electrical energy by means of piezo spherical Au particle that covers only a fraction microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc of the top (inset, Fig. 1B). For the purpose of our oxide creates a strain field and charge separation across the NW as a result of its bending. The measurements, we have grown Nw arrays that rectifying characteristic of the Schottky barrier formed between the metal tip and the Nw leads to have relatively less density and shorter length (0.2 to 0.5 um), so that the AFM tip can exclu- estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, sively reach one Nw without touching another nd/or hydraulic energy into electricity for powering nanodevices. The measurements were performed by AFM using a Si tip coated with Pt film, which has a cone angle of 700. The rectangular ireless devices may allow in situ, so far, such as NWs(5), nanobelts(NBs)(6), cantilever had a calibrated normal spring constant real-time biomedical monitoring and nanosprings(7), nanorings(8), nanobows(9), of0. 76Nm(Fig 1C). In the AFM contact mode, detection, but such devices still re- and nanohelices(10). Although numerous studies a constant nomal force of 5 nn was maintained ire a power source. Ideally, such devices have demonstrated novel nanodevices and ap- between the tip and sample surface. The tip battery. The body provides numerous potential has been done to address the power needs of tip's height was adjusted according to the surface power sources: mechanical energy(such as body these nanosystems morphology and local contacting force. The movement, muscle stretching, blood vessel con. Our study is based on aligned ZnO nanowires thermal vibration of the Nws at room temper. traction), vibrational energy(acoustic waves), grown on c plane-oriented a-AlO, substrate, ature was negligible. For the electric contact at chemical energy(glucose), and hydraulic en- using Au particles as a catalyst, by the vapor- the bottom of the nanowires, silver paste was ergy(body fluid and blood flow), but the liquid-solid (VLs)process(11, 12). An epitax- applied to connect the(large)Zno film on the hallenge is their efficient conversion into elec- ial relation between ZnO and a-AL,, allows a substrate surface with the measurement circuit. trical energy. If accomplished on the nano- thin, continuous layer of Zno to form at the The output voltage across an outside load of scale, such power sources could greatly reduce the size of integrated nanosystems for opto- electronics(1), biosensors(2), resonators(3) B We demonstrate an approach to converting mechanical energy into electric power with the use of aligned zinc oxide (Zno) nanowire (NWS). The mechanism of the power generator relies on the coupling of piezoelectric and semiconducting properties of Zno as well as the formation of a Schottky barrier between the metal and ZnO contacts. The nanogenerator has 1100m the potential of harvesting energy from the environment for self-powered nanotechnology Among the known one-dimensional (ID) nanomaterials, Zno has three key advantages. First, it exhibits both semiconducting and piezo electric(PZ) properties that can form the basis for electromechanically coupled sensors and transducers. Second, Zno is relatively biosafe RL and biocompatible(4), and it can be used for biomedical applications with little toxicity Finally, Zno exhibits the most diverse and abun- dant configurations of nanostructures knot Fig. 1. Experimental design for converting nanoscale mechanical energy into electrical energy by a vertical piezoelectric(PZ)ZnO NW.(A)Scanning electron microscopy images of aligned ZnO NWs depart: grown on a-Al20, substrate.(B) Transmission electron microscopy images of zno Nws, showing the latonaeering, peking University, Beijing 100871, China. a single crystal and has uniform shape. Inset at center: an electron diffraction pattern from a Nw ter for Nanoscience and Technology, Beijing Most of the NWs had no Au particle at the top. Inset at right: image of a Nw with an 100080. China C) Experimental setup and procedures for generating electricity by deforming a PZ NW with a *To whom nce should be addressed. E-mail: conductive AFM tip. The base of the Nw is grounded and an external load of R, is applied, which is hong wang@mse gatech. edu much larger than the resistance R, of the NW. the aFM scans across the Nw arrays in contact mode 242 14ApriL2006Vol312ScieNcewww.sciencemag.org
Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays Zhong Lin Wang1,2,3* and Jinhui Song1 We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc 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 zinc oxide 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 to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices. Wireless devices may allow in situ, real-time biomedical monitoring and detection, but such devices still require a power source. Ideally, such devices should be self-powered and not dependent on a battery. 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 the challenge 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 the use of aligned zinc oxide (ZnO) nanowires (NWs). The mechanism of the power generator relies on the coupling of piezoelectric and semiconducting properties of ZnO as well as the formation of a Schottky barrier between the metal and ZnO contacts. The nanogenerator has the potential of harvesting energy from the environment 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 and transducers. Second, ZnO is relatively biosafe and biocompatible (4), and it can be used for biomedical applications with little toxicity. Finally, ZnO exhibits the most diverse and abundant configurations of nanostructures known 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 of these nanosystems. Our study is based on aligned ZnO nanowires grown on c plane–oriented a-Al2 O3 substrate, using Au particles as a catalyst, by the vaporliquid-solid (VLS) process (11, 12). An epitaxial relation between ZnO and a-Al2O3 allows a thin, continuous layer of ZnO to form at the substrate, which serves as a large electrode connecting the NWs with a metal electrode for transport measurement (Fig. 1A). The NW grows along the E0001^ direction and has side surfaces of A0110Z (Fig. 1B). Most of the Au 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 fraction of the top (inset, Fig. 1B). For the purpose of our measurements, we have grown NW arrays that have 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 rectangular cantilever had a calibrated normal spring constant of 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 the tip_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 at the bottom of the nanowires, silver paste was applied to connect the (large) ZnO film on the substrate surface with the measurement circuit. The output voltage across an outside load of REPORTS 1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. 2 Department 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 Fig. 1. Experimental design for converting nanoscale mechanical energy into electrical energy by a vertical 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 the typical structure of the NW without an Au particle or with a small Au particle at the top. Each NW is a 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 a conductive AFM tip. The base of the NW is grounded and an external load of RL is applied, which is much larger than the resistance RI of the NW. The AFM scans across the NW arrays in contact mode. 242 14 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org
xperimentally, both the topography(fee monitored as the tip scanned over the nano- back signal from the scanner)(Fig 2A)and the scanned over the vertically aligned Nws, the wires(note the defined polarity of the voltage corresponding output e images across NWs were bent consecutively. The bending dis- signal). No external voltage was applied in the load( Fig. 2B)were recorded simultaneous- tance was directly recorded in the topography any stage of the experiment ly when the AFM tip was scanned over the image, from which the maximum bending deflec- tion distance and the elastic modulus of the nw as well as the density of Nws that have been scanned by the tip were directly derived(13) In the v image (like discharge peaks) were observed. These peaks, typically about 4 to 50 times the noise level, are rather sharp and narrow, and some- times one or two pixels represent one voltage peak because of the limited scanning speed of the AFM tip, so that the color distribution in the plot is not easy to display (fig. S2 ). By reducing the scan range and increasing the scan frequen- cy, more complete profiles of the discharge peaks y (um) 1.5 were captured (fig. S3). Most of the voltage peaks are -6 to 9 mV in height. The density of NWs contacted by the tip is counted 2A to be -20/um, and the average NWs whose voltage output events 40% of the Nws were contacted. the voltage output line profiles across one Nw acquired at a time interval of l min is shown in Fig 2C. Because the dwell time for each data int (or pixel) is 2 ms, which is longer than the age lifetime of the voltage peak of -06m Fig 2D), the peak at which V, reached the max imum was possibly missed by the"slow"scan D ning tip, so E in Fig. 2C). A sharp peak can be identified con- > tinuously at the location of the Nw and in the Woutput voltage in each scan of the tip. When R the tip started to deflect the NW, no voltage output was observed (Fig 2D): V was detected when the deflection of the Nw approached its maximum. When the Nw was released by the AFM tip, V dropped to zero, indicating that the the end of the afm scan over the nw Fig. 2. Electromechanically coupled discharging process of aligned piezoelectric Zno NWs The shape of the discharge peak can be observed in contact mode. (A and B)Topography image (A) and corresponding output voltage further improved and analyzed by maximizing characterized by only a couple of data points, which made it difficult to display the data by rainbow scanning range. Shown in Fig. 2E is a line color (see fig. $2).(C) A series of line profiles of the voltage output signal when the AFM tip profile of I, when the tip was scanned over a scanned across a vertical NW at a time interval of 1 min The data show the registration of the gle Nw at a scanning velocity of 12.394 um/s. electric signal with the location of the Nw and the reproducibility of the event over an extended The full width at half maximum (FWHM)of the period of time. Colors represent the outputs required for a series of scans. In (A) to(), the scanning speed of the tip was 12.081 um/s, and the time spent to acquire and output one scan voltage peak was estimated to be t-06ms. n be qualitatively described an of the NW, indicating that the discharge occurs when the tip is in contact with the compressed side alent circuit (l4)(Fig. 2E). The Nw is approx- the NW. When the tip touches the Nw, PZ charges start to accumulate but no discharge occurs. imated as a resistor R, and a capacitor C Discharge occurs when the deflection reaches nearly the maximum ym Note that the lateral (including the contribution from the syste deflection y includes the shape and contact geometry of the tip, which must be subtracted to derive The lifetime of the output voltage Across the true deflection of the NW.(E) Line profile of the voltage output signal when the AFM tip scans the load R, (note the polarity of the voltage) across a vertical NW at 12.394 um/s. the time spent to acquire and output one scan point was 0.05 is t=(RL Ru)C(5). For the experiment we ms, which was achieved at the maximum scan frequency of the AFM. The inset is an equivalent have designed, the resistance of the NW r, circuit of the measurement to be used for simulating the discharging process. ( F)The resonance is negligible relative to RL(16, 17). Thus, the vibration of a NW after being released by the AFM tip, showing that the stored elastic energy is equivalent capacitance of the Nw and the transferred mainly into vibrational energy after creating the pz discharge event. system is C≈tR1≈1.2pF www.sciencemag.orgscIencEVol31214ApriL2006 243
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 in any stage of the experiment. Experimentally, both the topography (feedback signal from the scanner) (Fig. 2A) and the corresponding output voltage (VL) images across the load (Fig. 2B) were recorded simultaneously when the AFM tip was scanned over the aligned NW arrays. In contact mode, as the tip scanned over the vertically aligned NWs, the NWs 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 been scanned by the tip were directly derived (13). In the VL image, many sharp output peaks (like discharge peaks) were observed. These peaks, typically about 4 to 50 times the noise level, 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 the plot 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 of the 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. When the 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 the output of piezoelectricity was detected toward the end of the AFM scan over the NW. The shape of the discharge peak can be further improved and analyzed by maximizing the tip scanning frequency and reducing the scanning 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 the voltage peak was estimated to be t È 0.6 ms. The damping behavior of the voltage peak can 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 across the load RL (note the polarity of the voltage) is t 0 (RL þ RI )C (15). For the experiment we have designed, the resistance of the NW RI is negligible relative to RL (16, 17). Thus, the equivalent capacitance of the NW and the system is C , t/RL , 1.2 pF. 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 was characterized 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 the electric 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), the scanning speed of the tip was 12.081 mm/s, and the time spent to acquire and output one scan point 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 deflection of the NW, indicating that the discharge occurs when the tip is in contact with the compressed side of 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 lateral deflection 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 equivalent circuit of the measurement to be used for simulating the discharging process. (F) The resonance vibration of a NW after being released by the AFM tip, showing that the stored elastic energy is transferred mainly into vibrational energy after creating the PZ discharge event. REPORTS www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 243
REPORTS The observed sharp voltage output for Zno grounded. Note that v. and vs are the volt port process. Because the compressed side of NWs was not observed for metal film(fig. $4), ages produced by the Pz effe are each the semiconductor Zno Nw has negative po- aligned carbon nanotubes(fig. S5), or aligned typically larger than a few tens (22). The tential /. and the stretched side has positive wO, NWs(fig. S6) under identical or similar potential is created by the displace- potential (s ), two distinct transport processes experimental conditions. These data rule out the of the Zn" cations with respect to the o will occur across the Schottky barrier possibility of friction or contact potential as the anions, a result of the Pz effect in the We now consider the case of a ZnO Nw source of the observed response. crystal structure; thus, these ionic charges cannot without an Au particle at the top. In the first step, t the efficiency of the electric power gener- freely move and cannot recombine without the AFM conductive tip that induces the de- by this process can be calculated as follows. releasing the strain( Fig. 3D). The potential dif- formation is in contact with the stretched surface The output electrical energy from one Nw in ference is maintained as long as the deforma- of positive potential V.( Fig 3, D and E). The one piezoelectric harge(PZD) event is tion is in place and no foreign free charges Pt metal tip has a potential of nearly zero, I AWpzp = VoC/h2 (15), where Vo is the peak (such as from the metal contacts)are injected. 0, so the metal tip-Zno interface is negatively voltage of the discharge output. For simplicity, The contacts at the top and the base of the biased for Al=Vm-v.0. The M-S interface in this ibration after releasing the Nw(Fig. 2F) (i) Pz discharge(AWpp) for each cycle of the vibration; and (ii) friction/viscosity from the Fig. 3. Transport is governed environmental medium. The mechanical reso- y a metal-semiconductor nance of the Nw continues for many cycles, shottky barrier for the Pz but it is eventually damped by the viscosity of TV>o 5l Zno Nw (see movies S1 and the medium. Each cycle of the vibration gen- S2). (A) Schematic definition erates AWpzp, but the AFm tip in the present of a nw and the coordi- experimental design catches only the energ tion system.(B)Longi- generated in the first cycle of vibration. Taking tudinal strain e distribution into account the energy dissipated by the nw lEco in the first cycle of vibration AWED (15),the deflected by an AFM tip from efficiency of converting mechanical energy to the side, the data were electrical energy is△Wpa/△ W.. Therefore, simulated by FEMLAB for a an efficiency of 17to30% has been received闷l≈0 Zno NW of length 1 Ht for a cycle of the resonance(table SI). The an aspect ratio of 10.(C)The large efficiency is likely due to the extremely PZ-induced electric field E large deformation that can be bome by the RL stribution in the nw nanowire(18) Potential distribution in the The physical principle for creating the Pz Nw as a result of the pz ef- discharge energy arises from how the piezo- fect. (E and F) Contacts be- electric and semiconducting properties of Zno ween the AFM tip and are coupled. For a vertical, straight Zno Nw e semiconductor zno nw (Fig 3A), the deflection of the Nw by an AFM boxed area in(D)] at two re- tip creates a strain field, with the outer surface Ag △v=vV versed local contact potentials being stretched(positive strain e)and the inner (positive and negative), show- electric field E, along the nw e direction)is G ed Schottky rectifying then created inside the nw volume through the havior, respectively (see PZ effect, E. =E_/d, where d is the ot). This oppositely biased coefficient(19)along the nw direction that is shottky barrier across the normally the positive c axis of Zno, with the Zn W preserves the Pz charges atomic layer being the front-terminating layer and later produces the dis- (20, 21). The PZ field direction is closely paral- charge output. The inset shows lel to the z axis(Nw direction) at the outer a typical current-voltag surface and antiparallel to the z axis at the in (-v relation characteristic surface(Fig. 3C). Under the first-order approx- (n-type) Schottky barrier. The process in(E is to separate and maintain the charges as well as build up imation, across the width of the Nw at the top the potential. The process in(F is to discharge the potential and generates electric current. (G and H) d,the electric potential distribution from the Contact of the metal tip with a Zno NW with a small Au particle at the top. The PZ potential is built up in ompressed to the stretched side surface is the displacing process(G), and later the charges are released through the compressed side of the Nw approximately between V. to V.* [with VH (H).(D) Contact of the metal tip with a Zno NW with a large au particle at the top. the charges are u3TlymV4Ld (15), where T is the thickness of gradually leaked" out through the compressed side of the Nw as soon as the deformation occurs the Nw). The electrode at the base of the Nw is thus, no accumulated potential will be created 244 14ApriL2006Vol312ScieNcewww.sciencemag.org
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 the possibility of friction or contact potential as the source 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 V0 2C/2 (15), where V0 is the peak voltage 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 3YIy2 m/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 the vibration; and (iii) friction/viscosity from the environmental medium. The mechanical resonance of the NW continues for many cycles, but it is eventually damped by the viscosity of the 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), the efficiency of converting mechanical energy to electrical 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 inner surface compressed (negative e) (Fig. 3B). An electric field Ez along the NW (z direction) is then created inside the NW volume through the PZ effect, Ez 0 ez/d, where d is the PZ coefficient (19) along the NW direction that is normally 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 outer surface and antiparallel to the z axis at the inner surface (Fig. 3C). Under the first-order approximation, across the width of the NW at the top end, the electric potential distribution from the compressed to the stretched side surface is approximately between Vs – to Vs þ Ewith Vs m 0 m3Tkymk/4Ld (15), where T is the thickness of the NW). The electrode at the base of the NW is grounded. Note that Vs þ and Vs – are the voltages produced by the PZ effect, which are each typically 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 the effective contact is between ZnO and Ag. The electron affinity (Ea ) of ZnO is 4.5 eV (23) and the work function (f) of Ag is 4.2 eV; there is no 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 transport process. Because the compressed side of the semiconductor ZnO NW has negative potential Vs – and the stretched side has positive potential (Vs þ), 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 surface of positive potential Vs þ (Fig. 3, D and E). The Pt metal tip has a potential of nearly zero, Vm 0 0, 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 this Fig. 3. Transport is governed by a metal-semiconductor Schottky barrier for the PZ ZnO NW (see movies S1 and S2). (A) Schematic definition of a NW and the coordination system. (B) Longitudinal strain ez distribution in the NW after being deflected by an AFM tip from the side. The data were simulated by FEMLAB for a ZnO NW of length 1 mm and an 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 and the semiconductor ZnO NW [boxed area in (D)] at two reversed local contact potentials (positive and negative), showing reverse- and forwardbiased Schottky rectifying behavior, respectively (see text). This oppositely biased Schottky barrier across the NW preserves the PZ charges and later produces the discharge output. The inset shows a typical current-voltage (I-V) relation characteristic of 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 are gradually ‘‘leaked’’ out through the compressed side of the NW as soon as the deformation occurs; thus, no accumulated potential will be created. REPORTS 244 14 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org
case is a positively biased Schottky diode, and directly in touch with the Au tip at the 10 pW/um. By choosing a NW array of size it produces a sudden increase in the output ginning of the forced displacement. Because of 10 um x 10 um, the power generated may be electric current. The current is the result of A. the conducting channel at the compressed side enough to drive a single NW/NB/nanotube driven flow of electrons from the semicon- of the Nw, the polar charges produced by the based device(26-28). If we can find a way to ductor ZnO NW to the metal tip. The flow of displaced Zn" and o ions caused by the pz induce the resonance of a Nw array and output the free electrons from the loop through the effect are immediately neutralized by external the Pz-converted power generated in each cy NW to the tip will neutralize the ionic charges free charges as soon as they are produced by cle of the vibration, a significantly strong power distributed in the volume of the Nw and thus the deformation. Therefore, no accumulative source may be possible for self-powering nano will reduce the magnitudes of the potential potential profile is formed at the M-s interface, devices. Furthermore, if the energy produced by and v. Thus, V starts to drop and reaches and no measurable output voltage is detected acoustic waves, ultrasonic waves, or hydraulic zero after all of the ionic charges in the Nw in the experimental setup we have designed pressure/force could be harvested, electricity are fully neutralized. This mechanism explains (Fig. IC) could be generated by means of Zno NW ar why the discharge curve in Fig. 2E is nearly We also measured the Pz voltage output for rays grown on solid substrates or even symmetric. According to the model, the dis- the same samples of aligned ZnO NWs in the flexible polymer films(29). The principle and charge occurs when the nw is bent nearly to AFM tapping mode. In this case, the deforma- the nanogenerator demonstrated could be the its maximum deflection, so VL should have a tion occurred longitudinally and there was no basis for new self-powering nanotechnology small offset in reference to the corresponding side displacement. The Nw was vertically com- that harvests electricity from the environment topography peak along the direction of tip scan, pressed, so the voltage created at the top of for applications such as implantable biomed- as is observed experimentally in Fig 2D the NW was negative V. so long as the base ical devices, wireless sensors, and portable elec- We next consider the case of a Zno NW with electrode was grounded(Fig 4A). There was tronics(29). a small Au particle at the top as a result of Vls no voltage drop across the width of the Nw. the AFm tip displacing the Nw, the tip is di- positively biased Schottky diode formed and 1.XFDuan, YHuang, RAgarwal,CMLieber, Nature rectly in contact with the Zno Nw but not thethe electrons could freely flow across the 421.241(2003) Au particle(Fig. 3G) because of its small size interface. Electrons flowed as the deformatio 2. G. F. Zheng, F Patolsky, Y. Cui, W. U. Wang, C. M. Lieber, Nat Biotechnol. 23, 1294(2005). and hemispherical shape(Fig. 1B and fig. SI urred, and there was no accumulation of net 3. x D Bai, P X Gao, Z L Wang, E. G. Wang, Appl. Phys. a process similar to that described in Fig. 3E charge in the Nw volume. This type of slowl Let82.4806(2003) occurs, and no output voltage will be observed." leaked"current produces no detectable signal When the tip is in contact with the Au particle, Therefore, for the ZnO NWs whose AFM im- 5. M.H. Huang et al, Ad Mater. 13, 113(200 the tip is integrated with the particle as one metal age in tapping mode is shown in Fig. 4B, (2001) contact, and at the local interface between the output voltage V, was detected beyond the noi 7. X Y. Kong, Z L Wang, Nano Lett 3, 1625(2003 Au particle and the negatively charged, com- level(Fig. 4C). B. X.Y. Kong, Y. Ding, R Yang, Z L Wang, Science 303, pressed side of the ZnO NW(s), a forward- We now estimate the possibility of powering biased Schottky diode is formed; thus, the nanodevices with the Nw-based power genera- 9. W. L. Hughes, Z L Wang, Am. Chem. Soc. 126, 6703 current flows from the tip through the Au tor. From table Sl, the Pz energy output by one 10 P X Gao et al, Science 309, 1700(2005) article to the interface region with negative Nw in one discharge event is -0.05 f, and the 11. X D Wang, C). Summers, Z L Wang, Nano Lett. 3, 423 PZ voltage (s)(Fig. 3H). A discharge process output voltage on the load is -8 mV For a Nw similar to that shown in Fig. 3F occurs, and a of typical resonance frequency -10 MHz, the A D Wang et al, 1.Am. Chem Soc. 127, 7920(2005) output power of the nw would be 0.5 pw.If 13.J. H. Song, X. D. Wang, E. Riedo, Z. L. Wang, Nano lett. arp voltage output is produced. In the case of a Nw with a large Au particle the density of NWs per unit area on the sub- 14. A piezoelectric material can be approximately charac that fully covers its top(Fig. 31), the metal tip is strate is 20/um2, the output power density is erized by a capacitor and a resistor. The capacitor olume, and the resistor represents its inner resistance. 15. See supporting material on Science On line cally coupled discharging 16. From our recent measurements of Zno nanowires, the resistivity is from 10-2 to 10 ohm-cm (17) depending c process of aligned piezo the contacts at the electrodes and the concentration of electric ZnO NWs observed oxygen vacancies. For a NW of length 0. 2 um and diameter in tapping mode. (A) Ex 40 nm. the resistance is between 16 kilohms and 16 perimental setup.(B and megohms, which is much smaller than the applied extemal C) Topography image( B) load of 500 megohms. Here we ignored the resistance nd corresponding output produced by the Zno film at the bottom of the nanowires large and covers the entire area of oltage image (O) of the the substrate; thus, the inner resistance is dominated by NWs. The tapping force was 5 nN, tapping frequen- o 17.I. H. He, C.S. Lao, L ]. Chen, D. Davidovic, Z. L. Wang. cy 68 KHz, and tapping l.Am.chem.Soc.127,16376(2005) speed 6 um/s. The voltage 18. W. L. Hughes, Z. L Wang, Appl. Phys. Left. 86, 043106 output contains no infor- 19. M. H. Zhao, Z. L Wang, S.X. Mao, Nano Left 4, 587 mation but noise, proving (2004) the physical mechanism 20. The wurtzite-structured Zno can be described as a emonstrated in Fig. 3 ong the c axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (000)°pol es. The Zn-terminated surface is at the growth front(positive c-axis direction) because of its 21. Z L Wang, x.Y. Kong. ].M 185502(2003) www.sciencemag.orgscIencEVol31214ApriL2006 245
case is a positively biased Schottky diode, and it 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 Vs þ. 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 to its maximum deflection, so VL should have a small 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 with a 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 the Au particle (Fig. 3G) because of its small size and 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, the current flows from the tip through the Au particle to the interface region with negative PZ voltage (Vs – ) (Fig. 3H). A discharge process similar 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 side of the NW, the polar charges produced by the displaced Zn2þ and O2– 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 detected in the experimental setup we have designed (Fig. 1C). We also measured the PZ voltage output for the same samples of aligned ZnO NWs in the AFM tapping mode. In this case, the deformation occurred longitudinally and there was no side displacement. The NW was vertically compressed, so the voltage created at the top of the NW was negative Vs – so long as the base electrode was grounded (Fig. 4A). There was no voltage drop across the width of the NW. Thus, across the metal tip–ZnO interface, a positively biased Schottky diode formed and the electrons could freely flow across the interface. 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 noise level (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 the output voltage on the load is È8 mV. For a NW of typical resonance frequency È10 MHz, the output 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 is È10 pW/mm2. By choosing a NW array of size 10 mm 10 mm, the power generated may be enough 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 power source may be possible for self-powering nanodevices. Furthermore, if the energy produced by acoustic waves, ultrasonic waves, or hydraulic pressure/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 and the nanogenerator demonstrated could be the basis for new self-powering nanotechnology that 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 the volume, and the resistor represents its inner resistance. 15. See supporting material on Science Online. 16. From our recent measurements of ZnO nanowires, the resistivity is from 10j2 to 10 ohmIcm (17) depending on the contacts at the electrodes and the concentration of oxygen 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 external load of 500 megohms. Here we ignored the resistance produced by the ZnO film at the bottom of the nanowires because it is very large and covers the entire area of the 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 a number 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). Fig. 4. Electromechanically coupled discharging process of aligned piezoelectric ZnO NWs observed in tapping mode. (A) Experimental setup. (B and C) 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. REPORTS www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 245
REPORTS 22. A simple calculation indicates that the magnitudes of V 24. R F. Pierret, Semiconductor Device Fundamental Projects Agency. We thank X Wang, W. L. Hughes, 1. Zhou, and ] Liu for their help 5. W. I. Park, G. C Yi, ]. W. Kim, S. M. Park, Appl Phys. Lett. dielectric screening in the calculation, the local potential 82,4358(2003) Supporting Online Material here. An accurate calculation of the potential distribution 27. A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science wwsciencemag. org/cgi/content/ull312/5771/242/D as a result of the ionic charges introduced by the Pz 294, 1317(2001): published online 4 October 2001 24) igs. SI to must be solved numerically and self-consistently. In 28. ) Chen et aL, Science 310, 1171 (2005 able 51 9. U.S. patent pending Movies s1 and 52 Supported by NSF grant DMR 9733160, the NASA Vehicle 23. 5. Hasegawa, S Nishida, T. Yamashita, H. Asahi, Ceramic Systems Program and Department of Defense Researd 19 December 2005: accepted 10 March 2006 PoC.Res6,245(2005) and Engineering and the Defense Advanced Research 10 1126/science. 1124005 Control of electron localization in curs via a two-step mechanism(Fig. 1A)in which initially the molecule is ionized by the laser field Molecular dissociation Fig. 1A, red arrow) and a vibrational wave packet is launched in the lso+state. Breakup M E. King,Ch. Siedschlag, 1 A ) Verhoef, 2]. L Khan, M. Schultze Th. Uphues, Y Ni, 1 of the D, ion is triggered by excitation to a repulsive state or after double ionization. M. Uiberacker, M Drescher, F Krausz, 2.4M.]. ] Vrakking' In the single-ionization pathways, excitation of We demonstrated how the subcycle evolution of the electric field of light can be used to control the bound D,(such as to the 2pa,t state in Fig. 1A) motion of bound electrons. Results are presented for the dissociative ionization of deuterium by recollision of the first electron [recollision molecules(D,-D++D), where asymmetric ejection of the ionic fragment reveals that light excitation(RCE) green line] or directly by the laser driven intramolecular electronic motion before dissociation localizes the electron on one of the tw eld [sequential excitation( SE), blue line) leads to D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and dissociation and the formation of a dt ion and a d provide evidence of its usefulness in controlling reaction dynamics atom. For example, in recent molecular clock studies, vibrational motion in D, was time- F ew-cycle laser light with a controlled evo- classical computations reveal that light-field resolved by exploiting RCE (10, ID). Additional lution of the electric field E(=anx control of molecular electron dynamics is re- dissociation mechanisms can be understood by s((f+o), with amplitude a(n), frequen- sponsible for the observed phenomeno considering that molecular potentials are modi y o, and carrier envelope The dynamics of molecules in intense laser fied by strong laser fields. Bond softening(Bs ly allowed the steering of the motion of fields typically inchudes ionization and dissocia- purple line)(2)occurs when energy gaps open electrons in and around atoms on a subfemto- tion. The dissociation of D, in intense laser fields up at avoided crossings between adiabatic field- second time scale. Manifestations of this control is known to involve several pathways whose dressed potential energy curves include the reproducible generation and mea- relative importance depends on intensity and pulse In double-ionization pathways, the forma- surement of single subfemtosecond pulses(2, 3) duration(6). The formation of fragment ions oc- tion of D,+ is followed by a second ionization and controlled electron emission from atom (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 which rong-field molecular interactions are present LP 8.0 fs interpreted( such as bond softening and enhanced ionization) were discovered in experimental and theoretical work on H, and its isotopes HD and 2po D++D+ D2 [see(6)and references therein]. The role of LP 6.5 fs phase control in the dissociation of hydrogen has recently been addressed in a few theoretical studies(7-9). We present experiments on the LP 5.0 fs dissociation of D,+ into D+ + d by intense few-cycle laser pulses with controlled field D++ D evolution and report a pronounced dependence CP 5.0 fs of the direction of the D+ ejection(a of the localization of the electron in the system) D+D on the waveform driving the reaction. Quantum- energy /ev FOM Instituut voor Atoom en Molecul Fysica (AMOLF Planck-lnstitut fur Quantenoptik, Hans-Kopfermann-Strasse bond distance ny. Fakultat fur Physik, Fig. 1.(A)Pathways for the production of D+ ions from D, by Universitat Bielefeld, Universitatsstrasse 25, D-33615 (through BS, SE, or RCE)or by Coulomb explosion (through RCl,SI ossing between diabatic potentials that are dressed by the las D-85748 Garching, Germany. institut for Experimen- vibrational levels that were originally bound (12).(B)D+ kinetic energy spectra talphysik, Universitat Hamburg, Luruper Chaussee 149, by 5-to 8-fs linearly polarized (LP)and 5fs circularly polarized (CP)laser D-22761 Hamburg, Germany stabilization,atl=12±0.2×1014Wcm2andl=24±0.2×1014Wcm 246 14ApriL2006Vol312ScieNcewww.sciencemag.org
22. A simple calculation indicates that the magnitudes of Vs þ and Vs – are on the order of a few tens to hundreds of volts. In practice, if we consider the polarization and dielectric 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 is sufficient 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 Research and Engineering, and the Defense Advanced Research Projects Agency. We thank X. Wang, W. L. Hughes, J. Zhou, and J. Liu for their help. Supporting Online Material www.sciencemag.org/cgi/content/full/312/5771/242/DC1 SOM Text Figs. S1 to S6 Table S1 Movies S1 and S2 19 December 2005; accepted 10 March 2006 10.1126/science.1124005 Control 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 and provide evidence of its usefulness in controlling reaction dynamics. F 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), has recently allowed the steering of the motion of electrons 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 which strong-field molecular interactions are presently interpreted (such as bond softening and enhanced ionization) were discovered in experimental and theoretical work on H2 and its isotopes HD and D2 Esee (6) and references therein^. The role of phase control in the dissociation of hydrogen has recently been addressed in a few theoretical studies (7–9). We present experiments on the dissociation of D2 þ into Dþ þ D by intense few-cycle laser pulses with controlled field evolution and report a pronounced dependence of the direction of the Dþ ejection (and hence of the localization of the electron in the system) on the waveform driving the reaction. Quantumclassical 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 occurs 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 1ssg þ state. Breakup of the D2 þ ion is triggered by excitation to a repulsive state or after double ionization. In the single-ionization pathways, excitation of bound D2 þ (such as to the 2psu þ 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 to dissociation and the formation of a Dþ ion and a D atom. For example, in recent molecular clock studies, vibrational motion in D2 þ was timeresolved by exploiting RCE (10, 11). Additional dissociation mechanisms can be understood by considering 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 D2 þ is followed by a second ionization 1 FOM Instituut voor Atoom en Molecuul Fysica (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, Netherlands. 2 MaxPlanck-Institut fu¨r Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany. 3 Fakulta¨t fu¨r Physik, Universita¨t Bielefeld, Universita¨tsstrasse 25, D-33615 Bielefeld, Germany. 4 Department fu¨r Physik, LudwigMaximilians-Universita¨t Mu¨ nchen, Am Coulombwall 1, D-85748 Garching, Germany. 5 Institut fu¨r Experimentalphysik, Universita¨t Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany. 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 from vibrational 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 phase stabilization, 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. REPORTS 246 14 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org
