Topographical and Electrochemical Characterization of Optically Smooth Zn Films Prepared by Physical Vapor Deposition Yu Luo, a, * Nelson Yee, a Qingfang Shi, a Baoxin Zhang, a Yibo Mo, a, * Gary S. Chottiner, b and Daniel A. Scherson a, ** ,z a Department of Chemistry and b Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA Optically smooth Zn ﬁlms supported on Cu-coated glass and quartz substrates have been obtained by physical vapor deposition of Zn in metallic form. The method employed involves resistive heating of a Mo boat ﬁlled with high purity Zn shot in an Ar atmosphere at pressures of about 3-5 mTorr. Atomic force microscopy images revealed that the resulting Zn deposits consist of smooth features rms roughness ca. 0.3 nm with dimensions on the order of 200 nm. Preliminary results indicate that the electrochemical behavior of these ﬁlms in strongly alkaline solutions is somewhat different than that observed for Zn in bulk commercial form. © 2001 The Electrochemical Society. DOI: 10.1149/1.1376120 All rights reserved. Manuscript submitted November 27, 2000; revised manuscript received March 12, 2001. Available electronically May 31, 2001. Efforts in this laboratory have focused recently on the study of the kinetics and mechanism of Zn electrodissolution in strongly al- kaline aqueous solutions. Of primary interest is to gain insight into the factors that control the formation and properties of passive ﬁlms on Zn surfaces. This phenomenon is believed to be responsible for losses in capacity of Zn/MnO 2 primary batteries discharged at high currents, which renders part of the Zn powder electrochemically inert and, thus, unavailable for discharge. 1 Our strategy involves the simultaneous use of a variety of spectroscopies, including normal incidence UV-visible reﬂectance and Raman, in conjunction with microgravimetric techniques to examine Zn electrodes during dis- charge. Such experiments require deposition of optically reﬂecting ﬁlms on a quartz crystal microbalance QCM. Initial attempts to produce Zn ﬁlms by conventional high-vacuum, physical vapor deposition PVD techniques, yielded bluish white layers lacking mechanical adherence. A thorough survey of the pertinent literature revealed a single brief statement made in an old article, 2 in which reference was made to the need of using relatively high pressures to grow Zn ﬁlms. Unfortunately, no details were given on the precise conditions under which deposition was achieved, nor of the quality of the ﬁlms produced. Also of relevance to this work are the reports of Sale ´m-Vasconcelos et al., 3 who prepared very thin Zn ﬁlms ca. 10 mg/cm 2 , i.e., 14 nm thick supported on a carbon-coated glass substrate by heating a mixture of ZnO mixed with Ta powder as a reducing agent. Although other physical vapor deposition methods have been reported more recently, the resulting Zn ﬁlms seem to lack the desired optical characteristics. This paper describes a method for producing high optical quality Zn ﬁlms on Cu-coated glass surfaces, which is based on PVD under an Ar atmosphere. Also provided in this brief note are atomic force images as well as preliminary electrochemical data obtained with these ﬁlms in 1 M KOH aqueous solutions. Cu appears to be espe- cially well suited for this type of application, as it is the only sub- strate found to date onto which Zn ﬁlms of a thickness of the order of micrometers displaying a high degree of smoothness at room temperature could be produced by PVD. It should be stressed that only in one other instance have workers succeeded in obtaining smooth Zn deposits by PVD in high vacuum by keeping, in their case, the substrate at liquid air temperature; 4 however, their ﬁlms turned bluish and discontinuous as the temperature was raised to ca. 300 K. Experimental Zn ﬁlms on Cu-coated glass or quartz substrates see below were deposited in a glass bell-jar sealed via a Viton gasket to an ion turbo-pumped sump that houses all the components required for metal vapor deposition. This chamber allows pressures in the range 10 -7 -10 -8 Torr to be routinely achieved. The evaporation source was a Mo boat R. D. Mathis 99.98%, ME21 ﬁlled with Zn shot Alfa 99.9999%. Zn evaporations were performed under Ar at a pressure of 3-5 mTorr passing currents through the source of about 60 A. A shutter was interposed between the source and the substrate during degassing of the Zn source to prevent deposition of undesir- able material prior to Zn evaporation. The growth rates attained under these conditions, as determined following deposition, were about 0.8 m/h. Cu ﬁlms were vapor deposited onto either quartz or glass substrates by conventional means in a different high vacuum chamber using a mask to generate the desired patterns for the QCM substrates. All of the data in this paper were obtained with Zn/Cu/ glass specimens. A Dektak IID surface proﬁlometer Sloan Technology Co. was used to measure the thickness of the Cu and Zn ﬁlms. High- resolution topographical images of Cu and Zn surfaces were ob- tained in air with an atomic force microscope AFM, Nanoscope II, Digital Instruments, using microfabricated silicon nitride (Si 3 N 4 ) cantilevers with a force constant of 0.06 N/m. Zn ﬁlms evaporated on Cu/glass substrates see above were ex- amined by cyclic voltammetry CV in 1 M KOH solution using a PAR 173 potentiostat and an EG&G PARC 175 universal program- mer with a HgO/Hg, OH - 1 M KOH as a reference and a high area carbon as a counter electrode. The Zn/Cu/glass electrode 1 cm 2  was immersed in the electrolyte with the potentiostat activated to achieve immediate potential control once the surface came in con- tact with the solution. Based on visual inspection, the amount of Cu exposed to the electrolyte, which was restricted only to the very edge on the side of the glass substrate, was extremely small. Note that the voltammetry of Cu ﬁlms prepared by the methods described above carried out in the same electrolyte were found to be feature- less in the potential range of Zn dissolution/passivation; hence, even * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z E-mail: dxs16@po.cwru.edu Journal of The Electrochemical Society, 148 7 E295-E297 2001 0013-4651/2001/1487/E295/3/$7.00 © The Electrochemical Society, Inc. E295