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Contrast, Switching Speed, and Durability of V2O5–TiO2Film-Based Electrochromic WindowsSooyeun Kim,* Minoru Taya, and Chunye Xuz

Center for Intelligent Materials and Systems, University of Washington, Seattle, Washington 98195, USA

V2O5–TiO2 films as ion storage layers were proposed for electrochromic windows �ECWs� based on poly�3,3-dimethyl-3,4-dihydro-2H-thieno�3,4-b��1,4�dioxepine�. V2O5–TiO2 �V/Ti = 70/30� films were fabricated by a sol-gel electrodeposition tech-nique. The films were investigated by X-ray diffraction, a scanning probe microscope, and impedance spectroscopy. The cyclingbehaviors of the films were studied using cyclic voltammetry �CV� in 0.1 M LiClO4/propylene carbonate solution. Electro-chromism of the films upon Li+ intercalation was investigated by transmittance measurements during the CV process. TheV2O5–TiO2 film-based ECW was successfully processed and studied. The ECW exhibited its high electrochromic contrast, rapidswitching speed, and long-term cyclic stability. Its contrast ��%T = Tmax − Tmin� was 68%T with a minimum transmittance,Tmin = 1%, at 580 nm wavelength. The ECW took 5 s to reach a fully colored state from a fully bleached state. It took 4 s to goto a fully bleached state. The asymmetry of coloring and bleaching time was explained by modeling the ECW as a simpleequivalent circuit. The cyclic durability of the ECW was tested over 150,000 cycles. It revealed the contrast degradation of only2% at 580 nm wavelength.© 2008 The Electrochemical Society. �DOI: 10.1149/1.3031978� All rights reserved.

Manuscript submitted July 21, 2008; revised manuscript received October 27, 2008. Published December 4, 2008.

0013-4651/2008/156�2�/E40/6/$23.00 © The Electrochemical Society

The electrochromic window �ECW� is one of the most popularfields in all switching window technologies due to its potential ofblocking out sunlight in summer or transmitting the rays in winter.ECWs have been applied for architectural, vehicular, and aircraftwindows, skylights, sunroofs, eyeglasses, and displays.1,2 An ECWis composed of three primary components: an electrochromic �EC�layer, an ion conducting layer, and an ion storage layer as given inFig. 1. An EC layer functions as a working electrode that changes itsoptical properties with the presence of lithium ions. An electrolyte totransport ions is used as an ion-conducting layer. An ion-storagelayer functions as a counter electrode to balance charges. The trans-port of lithium ions from an ion storage layer into an EC layer underapplied electric potential is the essential mechanism of an ECW. Thepresence of ions in an EC layer changes its optical properties caus-ing an ECW to change to its colored state. The ECW becomestransparent when lithium ions migrate out of the EC layer. The de-velopment of ECWs with conducting polymer active materials ispromising due to the numerous benefits, such as a high electrochro-mic contrast, rapid switching speed, electrochemical stability, andopen-circuit memory.2,3 Poly �3,3-dimethyl-3,4-dihydro-2H-thieno�3,4-b��1,4�dioxepine�, called PProDOT-Me2, has been usedas a cathodic conducting material as shown in Fig. 2. PProDOT-Me2film-based ECWs have been developed in our laboratory.4,5 Threeprimary components were investigated: working electrodes, an elec-trolyte, and device assembly. The main properties of an ECW,namely, its color change and switching speed, are caused by a work-ing electrode. However, the coloration efficiency of a counter elec-trode influences the electrochromic contrast of an ECW. The counterelectrode with high coloration efficiency here is very hard to achievea large contrast of the ECW. The ion intercalation and deintercala-tion rates of a counter electrode are taken into consideration for theswitching speed of an ECW as well. The long-term electrochemicalstability of a counter electrode also enhances the cyclic durability ofan ECW. Therefore, the selection of an ion storage material for acounter electrode is critical. A vanadium pentoxide �V2O5� film wasused as a counter electrode. V2O5 is a typical Li+ intercalation com-pound due to its layered structure. It has the ability to intercalatecations between the adjacent layers. The insertion rate and capacityincrease as the distance between the adjacent layers does. Substitu-tion in V2O5 by other cations with different valance states has beenused to adjust the interaction forces between two adjacent layers inthe intercalation compound.6,7 The Li+ intercalation and deinterca-lation rates in a V2O5 film can be enhanced with the addition of

* Electrochemical Society Student Member.z E-mail: chunye@u.washington.edu

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TiO2. Moreover, TiO2 has low coloration efficiency and improvesthe cyclic fatigue resistance of V2O5.8,9 A sol-gel electrodepositiontechnique has been used for the fabrication of a counter electrodebecause it has the advantage of low cost and temperatureprocessing.10 This paper focuses on a V2O5–TiO2 counter electrodedevelopment by a sol-gel electrodeposition technique and its effectson the ECW, such as a high electrochromic contrast, rapid switchingspeed, and cyclic durability.

Experimental

Preparation of solutions.— The vanadium sol was prepared us-ing a method reported by Fontenot et al.10 V2O5 powder �99.99%metals basis; Alfa Aesar, Ward Hill, MA� was dissolved in 30 wt %H2O2 �J.T. Baker, Phillipsburg, NJ� aqueous solution with a V2O5concentration of 0.15 M.11 After stirring for 1.5 h at room tempera-ture, the excess H2O2 was decomposed by sonication for 2 h. Theresultant gel was then redispersed in deionized �DI� water andstirred overnight. The obtained red-brown V2O5 solution contained0.005 mol/L vanadium ions with pH 2.7. A titanium sol was pre-pared using a method reported by Limmer et al.12 Titanium �IV�isopropoxide �30 mL, 97%; Alfa Aesar� was dissolved in glacialacetic acid �60 mL, Fisher Scientific, Waltham, MA� and stirred for30 min at room temperature. DI water �30 mL� was added to thesolution and stirred until the precipitate disappeared. The resultantsol became a transparent liquid with pH 2. DI water was added tothe sol. The resultant solution contained 0.005 mol/L titanium ions.Mixed vanadium/titanium solutions were synthesized by combininga V2O5 solution with a TiO2 solution at room temperature.

Figure 1. �Color online� Schematic design of an ECW: a working electrode,an EC layer/transparent conducting layer/glass; an electrolyte, an ion con-ductor; and a counter electrode, an ion storage layer/transparent conductinglayer/glass.

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Preparation of counter electrodes.— In order to promote goodadhesion, an indium tin oxide �ITO�-coated glass was ultrasonicallycleaned in acetone, rinsed with DI water, washed with isopropanol,and dried at room temperature before coating. An ITO-coated glasssubstrate �6 �/�, 2.5 � 2.5 cm dimensions, Thin Film Devices,CA�, a platinum plate counter electrode, and a silver wire referenceelectrode were submerged in a V2O5–TiO2 solution. TheV2O5–TiO2 coating was deposited by a chronoamperometry pro-gram �CH 1605A Electrochemical Analyzer, CH Instruments, Aus-tin, TX�. The coating was then fired in air at 120°C in order toremove excess alcohol.

Preparation of an electrolyte.— Lithium perchlorate �LiClO4,99% anhydrous; Alfa Aesar�, polymethyl methacrylate ��PMMA�,Aldrich, Milwaukee, WI�, propylene carbonate ��PC�, 99.7% anhy-drous; Aldrich�, and EC �99% anhydrous; Aldrich� were dried beforeuse. An electrolyte based on PMMA and LiClO4 was plasticized byPC and EC.4 The mixture solution was stirred for 24 h at 60°Cunder argon. Transparent and ionic conductive gel was produced.

Preparation of working electrodes.— An ITO-coated glass�6 �/�, 2.5 � 2.5 cm dimensions� was washed with an ethanola-mine aqueous solution and rinsed with DI water. It was cleanedunder ultraviolet UV ozone �model no. 384, Jelight Company Inc.,Irvine, CA� and dried at 110°C overnight before use. An ITO-coatedglass substrate, a platinum plate counter electrode, and a silver wirereference electrode were submerged in a ProDOT–Me2�0.01 M�/LiClO4 �0.1 M�/acetonitrile solution. The PProDOT–Me2coating was deposited by a chronoamperometry program. A 0.1 Msolution of LiClO4 in propylene carbonate was used as the electro-lyte to switch colors of the coatings.

Assembly and sealing of ECWs.— The cell was assembled in anargon atmosphere glove box in order to avoid any moisture contami-nation. An electrolyte was cast onto a working electrode and pressedtogether with a counter electrode. A para film �100 �m thickness�was employed as a sealant and spacer for tightly sealing the deviceand keeping the 100 �m thickness of an electrolyte. After three daysof gelation, the edges of the device were sealed with UV curingepoxy �OG112-4, Epoxy Technology, Billerica, MA�.

Characterization of the coatings.— V2O5–TiO2 films were ana-lyzed using various characterization techniques. Optical transmit-tance spectra of V2O5–TiO2 films were investigated in the wave-length range of 380–800 nm using a UV/visible/near-infrared �UV/vis/NIR� spectrophotometer �V-570, JASCO, Easton, MD�. Thesurface morphology of coatings was examined using a scanning

Figure 2. Chemical structure of poly �3,3-dimethyl-3,4-dihydro-2H-thieno�3,4-b��1,4�dioxepine�; PProDOT–Me2

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probe microscope �Dimension 3100, Veeco, Santa Barbara, CA�.The X-ray diffraction study in Nippon Rigaku RAX ra-10 with CuK� radiation was carried out on the coatings. Thickness measure-ments were performed on a P-15 profilometer �Tencor�. The electro-chemical properties of V2O5–TiO2 films were investigated using athree-electrode cell with a Pt counter electrode and an Ag referenceelectrode. A 0.1 M solution of LiClO4 in propylene carbonate wasused as an electrolyte. Cyclic voltammetric measurements were car-ried out on an electrochemical analyzer �CHI 605A, CH Instru-ments�. The cyclic voltammetric curves were recorded after fivecycles at a scan rate of 50 mV/s. Ionic conductivity measurementsof the V2O5–TiO2 films were derived using the ac complex imped-ance technique. The cell structure used for the measurement was Ptelectrode/gel electrolyte/V2O5–TiO2/ITO/glass. The equipment usedwas a PARSTAT 2263 electrochemical system.

Results and Discussion

Characterization of V2O5–TiO2 films.— Structural and chemicalanalysis of coatings.— V2O5–TiO2 �V/Ti = 70/30� films were syn-thesized by a sol-gel electrodeposition technique. The structure ofthe film was investigated by X-ray diffraction �XRD� in NipponRigaku RAX ra-10 with Cu K� radiation. The XRD pattern of a120°C-dried film is shown in Fig. 3. Broadened and small peaksappear at 9, 24, 32, and 41°, respectively, indicating that the film ispolycrystalline. The quantitative analysis of the film was performedon an energy dispersive spectrometer identifying the chemical com-position of the film.11 The stoichiometry of the film was found to beV2O5:TiO2 with a ratio of 7:3, which coincided with the processedfilm. The surface morphology of the film was investigated by ascanning probe microscope and is seen in Fig. 4, which presents acomparison to the surface morphology of V2O5 and V2O5–TiO2�V/Ti = 70/30� films. A V2O5 film showed rodlike particles, be-cause V2O5 was known to form rodlike particles easily, while TiO2showed spherical particles with diameters of 27–30 nm. TiO2 par-ticles were inserted in V2O5 by forming a V2O5–TiO2 film, whichprovided more pores. This enhanced the Li+ intercalation/deintercalation rate of a V2O5–TiO2 film.

Optical properties.— The optical density change ��OD� was mea-sured. The coloration efficiency �CE, �� was calculated using theequation, � �cm2/C� = �ODS/�Q, where S is the active area of afilm and �Q is the injected/ejected charge as the function of time.The optical density is given by �OD = log�Tb/Tc� at 580 nm wave-length, where Tb and Tc are the transmittances of a film at bleachedand colored states.13 The CE of the V2O5–TiO2 film was0.72 cm2/C, where �Q was 4.7 mC/cm2. The low CE of theV2O5–TiO2 film provided a small optical modulation with largecharge insertion or extraction. Thus, it did not significantly degrade

Figure 3. XRD pattern of a 120°C-dried V2O5–TiO2 �V/Ti = 70/30� film.

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the contrast of the ECW. In addition, it played an ion storage layer’srole, while maintaining high contrast of the ECW. Therefore, thelow CE of the counter electrode was one of crucial parameters forthe ECWs.Electrochemical properties.— Ionic conductivity measurementswere determined from ac impedance spectroscopy. The thickness ofthe films was measured to calculate lithium conductivity��200 nm�; 200 nm thick V2O5–TiO2 films were used for all im-pedance measurements. The complex impedance measurementswere made over the frequency range of 10 mHz to 100 kHz at roomtemperature. Figure 5 shows a typical complex impedance planediagram. Zre is the real impedance and Zim is the imaginary imped-ance. It consists of a slightly depressed semicircle in the high-frequency region and a straight line in the low-frequency region. Asimple equivalent R-C circuit was used to calculate ion conductivityas given in Fig. 5. Re is the ohmic resistance of the electrode, Cd isthe double-layer capacitance of the electrode/electrolyte interface, Riis the ionic resistance arising from the diffusion of lithium ions, andCg is the geometric capacitance between the electrodes. From im-pedance analysis, the intersection of the high-frequency semicirclewith the Zre-axis is Re, the electrode resistance. Re represents Ri athigh frequencies, due to the fact that the semicircle corresponds tothe lithium diffusion into the V2O5–TiO2 counter electrode in thehigh-frequency region.14 Ionic conductivity ��� of the films wascalculated using the equation � = 1/Re �d/A�, where d is the film

Figure 5. �Color online� Impedance plots of a V2O5–TiO2 �200 nm thick�film on an ITO/glass. The inset shows the equivalent circuit used for theanalysis.

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thickness, Re is the electrode resistance, and A is the electrode area.The value of ionic conductivity was 1.3 � 10−6 S/cm forV2O5–TiO2 films at room temperature, while it was 3.5� 10−7 S/cm for V2O5 films. The value of ionic conductivity in-creased with the Li+ intercalation rate of a film. Therefore, the Li+

intercalation rate of a V2O5–TiO2 film was more rapid in compari-son to that of a V2O5 film.

Lithium intercalation in a sol-gel electrodeposited V2O5–TiO2film was investigated by cyclic voltammetry �CV� with a scan rateof 50 mV/s. The CV was performed on a V2O5–TiO2/ITO/glasselectrode in the electrolyte of 0.1 M LiClO4 in propylene carbonateat room temperature. Figure 6 shows the CV of the V2O5–TiO2 film.The film presented an electrochemically active region between 1.5and −1.2 V vs Ag. The shape of the curve was a typical diffusion-controlled CV of a highly reversible Li+ intercalation/deintercalationprocess. The CV yielded a cathodic peak at −0.77 V, correspondingto Li+ intercalation. It also exhibited an anodic peak at 0.50 V, as-sociated with Li+ deintercalation. This meant that small potentialswere required for switching the film between Li+ intercalated anddeintercalated states. Its cyclic durability was tested as given in Fig.6, which shows the CV of the film with a scan rate of 50 mV/s after5 and 150,000 switching cycles. The film exhibited long-term cy-cling efficiency and electrochemical stability. Reproducibility of theCV after 150,000 cycles showed that the electrochemical Li+

intercalation/deintercalation was reversible, even though there weresmall shifts of potentials and currents. The voltammetry indicated

Figure 4. �Color online� Surface mor-phology of �a� a V2O5 film and �b� aV2O5–TiO2 �V/Ti = 70/30� film.

Figure 6. �Color online� Cyclic voltammogram of a V2O5–TiO2 film in0.1 M LiClO /PC solution after 5 and 150,000 switching cycles.

4

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that V5+ and Ti4+ were reduced to lower valence states, V4+ andTi3+, by lithiation, which was returned to the original valencethrough delithiation. After five cycles, an intercalation peak waspresent at −0.77 V with 4.7 mA, followed by a deintercalation peakat 0.50 V with −6.3 mA. After 150,000 cycling, an intercalationpeak was at −0.62 V with 4.2 mA and a deintercalation peak wasexhibited at 0.54 V with −6.1 mA. There were slight changes inpeak positions after 150,000 switching cycles. If the structure of thefilm begins to break down, the number of available intercalationsites decreases. Thus, the operation current reduces. A little degra-dation in the capacity was also observed after 150,000 switchingcycles. The CV had the inserted charge capacity of 4.3 mC/cm2 andextracted charge capacity of 4.2 mC/cm2. The slight charge unbal-ance was believed to be an experimental error.

Optical and electrochemical properties of V2O5–TiO2 film basedECWs.— An ECW was successfully processed and subjected to anoptical spectroscopy analysis. The optical spectra of the ECW in itsfully colored and bleached states were measured by UV/vis/NIRspectroscopy. Figure 7 shows the optical transmittance spectra of anECW. The contrast is given as the difference in transmittance be-tween the bleached and colored states and reported as �%T �Tmax− Tmin�. The contrast of the ECW was 68%T at 580 nm wavelength,where Tmin = 1% in a colored state and Tmax = 69% in a bleachedstate. Figure 8 shows the transmittance changes of the ECW withtime. The transport of lithium ions into a PProDOT–Me2 film causedan ECW to change to its colored state. It took 5 s to reach a fully

Figure 7. �Color online� Optical transmittance spectra of a V2O5–TiO2 film-based ECW �2.5 � 2.5 cm dimensions� in fully colored and bleached states.

Figure 8. �Color online� Transmittance changes of a V2O5–TiO2 film-basedECW with time �2.5 � 2.5 cm dimensions�.

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colored state from a fully bleached state. The ECW changed to itsbleached state when lithium ions moved out of the PProDOT–Me2film. It took 4 s to go to a fully bleached state from a fully coloredstate; 99% of the fully colored state for the ECW was alreadyreached in 1 s and 85% of its fully bleached state was achieved in1 s when the applied potential was reversed. The switching speedsof the ECW between colored and bleached states were more rapid incomparison to other electrochromic devices.15 This mainly attrib-uted to the Li+ intercalation and deintercalation rates of aPProDOT–Me2 film as a working electrode. The rapid Li+

intercalation/deintercalation rate of a V2O5–TiO2 film as a counterelectrode also contributed to the switching speed of an ECW. Figure9 shows the �OD of an ECW vs time, which was calculated accord-ing to Fig. 8. The complete coloring time to obtain �ODmax = 1.7was 5 s, and the complete bleaching time was 4 s in the V2O5–TiO2film-based ECW. In addition, the asymmetry of coloring and bleach-ing time of the ECWs was observed in Fig. 9. This was because thebleaching process was affected by the back electromotive forcewhich accelerated the movement of Li+.16-18 Li+ intercalation waslargely governed by a potential barrier for the ions to transport at theboundary between an electrolyte and a PProDOT–Me2 film, whereasLi+ deintercalation was influenced by ion transport in the film.Deintercalation was then faster than intercalation due to no barrierfor the ions to cross. The asymmetric switching behavior of theECW can be represented by an equivalent circuit composed of aresistor, a capacitor, and a power source, as shown in Fig. 10.19,20

The resistor is the sum of resistance of a working electrode �RWE�, a

Figure 9. �Color online� Optical density change ��OD� of a V2O5–TiO2film-based ECW monitored at 580 nm wavelength; a coloring potential is−1.5 V and a bleaching potential is +1.5 V.

Figure 10. Equivalent-circuit for the ECW; RECW is the resistance of theECW and C is the capacitance of the ECW.

ECW

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counter electrode �RCE�, electrolyte �REL�, and the loading resis-tance �RL�, which consists of charge transfer resistance and masstransfer resistance

RECW = RWE + RCE + REL + RL �1�

The capacitor is composed of working electrode capacitance �CWE�and counter electrode capacitance �CCE�, which are connected inseries

1

CECW=

1

CWE+

1

CCE�2�

For simplicity, R and C represent RECW and CECW. From the equiva-lent circuit, we obtain the homogeneous solution for Q�t�21

Q�t� = CVa�1 − e−t/RC� �3�This is represented in the case of coloring for an ECW. In the case ofbleaching for an ECW, Equation 3 is rewritten as

Q�t� = Q�to� − CVa�1 − e−t/RC� �4�If the switching of the ECW takes place by the first reduction, fol-lowed by oxidation, Q�to� is equal to the saturated value at the endof coloration Qsat. Therefore

Q�t� = Qsat − CVa�1 − e−t/RC� �5�

Figure 11 shows the Q�t� for coloring and bleaching of an ECW asthe function of time. Q�t�s of coloring and bleaching are governedby the parameter RC in an exponential function, e−t/RC. If we setRC = constant, where R and C are given by Eq. 1 and 2, we mayconclude that RC is the same for both coloring and bleaching whenthe material properties �RECW and CECW� remain constant. However,the resistance of the ECW �RECW� was changed between coloringand bleaching states as follows,

RECW�bleaching� � RECW�coloring�It was experimentally determined.21 This was mainly caused by thechange of the conductivity of the PProDOT–Me2 film, �WE, be-tween coloring and bleaching states

�WE�bleaching� �WE�coloring�Therefore

tb�bleaching� � tc�coloring�

We concluded that tc �coloring time� was larger than tb �bleachingtime� as given in Fig. 9. The CV of the ECW is shown in Fig. 12.The cathodic current continuously increased up to 7.5 mA, and thelarge anodic peak was observed at 0.72 V. This CV resembled that

Figure 11. �Color online� Asymmetric electrochemical switching behaviorof the ECW with operation time �in seconds� between coloring and bleachingstates.

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of the PProDOT–Me2 film which showed the behavior of Li+

intercalation/deintercalation in the film.5 This fact indicates that noelectrochemical reactions, such as side reactions, occurred on aV2O5–TiO2 counter electrode. The counter electrode only acted asan ion storage layer. Cyclic durability was examined by a chrono-amperometry program and UV/vis spectrophotometer. TheV2O5–TiO2 film-based ECW had long-term cycling efficiency andelectrochemical stability as given in Fig. 13. The transmittance deg-radation of only 2% at 580 nm wavelength was observed after150,000 cycles. This attributed to the good cyclic durability of aPProDOT–Me2 film. A V2O5–TiO2 film also contributed to long-term cycling of the ECW. It enhanced chemical cyclic stability be-cause the presence of TiO2 in V2O5 improved the cyclic fatigueresistance of V2O5.8

Conclusion

V2O5–TiO2 �V/Ti = 70/30� films were prepared by a sol-gelelectrodeposition technique. They were used as ion storage layersfor ECWs. The films exhibited more spatially uniform pores byaddition of TiO2 in V2O5 and resulted in the enhancement of opticaland electrochemical properties. The value of ionic conductivity was1.3 � 10−6 S/cm for the V2O5–TiO2 films at room temperature,which enhanced switching speed. The low coloration efficiency ofthe film provided a small optical modulation even with large charge

Figure 12. �Color online� Cyclic voltammogram of a V2O5–TiO2 film-basedECW �2.5 � 2.5 cm dimensions� after five switching cycles.

Figure 13. �Color online� Transmittance changes of a V2O5–TiO2 film-basedECW with time �in seconds� after five �solid line� and 150,000 �dashed line�switching cycles.

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insertion or extraction. This was required by achieving high contrastof the ECW. The CV performance of the film showed that the elec-trochemical Li+ intercalation/deintercalation was stable and revers-ible. Its cyclic durability test demonstrated long-term cycling effi-ciency and electrochemical stability of the film. The CV wasreproducible even after 150,000 switching cycles. The V2O5–TiO2film-based ECW exhibited a high contrast, 68%T with Tmin = 1%.The switching speed of the ECW was fast. The asymmetry of col-oring and bleaching time of the ECWs was observed. The coloringtime �tc� was found to be longer than the bleaching time �tb�. Thiswas explained by modeling an ECW as a simple equivalent circuit.From these results, the fabrication of V2O5–TiO2 film-based ECWis useful for further electrochromic device applications.

Acknowledgments

The present work was conducted under a contract from a com-pany. We are thankful to Dr. Lu Liu and Dr. Xiangxing Kong, otherresearchers at CIMS.

University of Washington assisted in meeting the publication costs of thisarticle.

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