Preparation of Atomically Smooth Germanium Substrates for Infrared Spectroscopic and Scanning Probe Microscopic Characterization of Organic Monolayers Brian W. Gregory, †,§ Sajan Thomas, † Susan M. Stephens, † Richard A. Dluhy, † and Lawrence A. Bottomley* ,‡ Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, and School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received March 17, 1997. In Final Form: July 23, 1997 X The suitability of antimony-doped germanium crystals for use as infrared attenuated total reflection wave guides and conducting substrates for scanning tunneling microscopic imaging of absorbed organic monolayers has been evaluated. We have used repetitive ion bombardment followed by resistive heating of a germanium crystal under ultrahigh vacuum conditions to produce large, atomically flat regions on a Ge(111) crystal face. Through the combined use of STM, Auger spectroscopy and low-energy electron diffraction, we have demonstrated that monolayer quantities of tellurium electrochemically deposited onto the Ge surface dramatically reduce the rate of surface oxidation and permit the use of germanium crystals as STM substrates in air. The first STM images acquired in air with Ge as the conductive substrate are reported. No degradation of IR spectral quality occurs when using the Te-covered, Sb-doped Ge crystals as ATR elements. These findings suggest that Te-coated, atomically flat, low-resistivity Ge substrates are suitable for both IR spectroscopic and STM imaging of organic monomolecular films. Introduction Crystalline germanium is a semiconductor material with the crystal structure of diamond and a lower melting point and smaller bandgap (by 0.47 eV) than silicon. The structural and electronic properties of germanium and silicon surfaces are of interest due to the importance of these materials in the semiconductor and optoelectronics industries. 1 The surface structures for the low-index planes of Ge single crystals have been thoroughly inves- tigated. 2,3 The three-fold symmetric Ge(111) surface undergoes reconstruction to the (2 × 1) surface structure upon cleavage at room temperature and to the c(2 × 8) surface structure upon annealing at 100 °C. Surface reconstructions result in decreased surface free energy and reactivity. In addition to its optoelectronics applications, germa- nium is widely used in spectroscopic applications. High- purity, optical quality germanium is readily available and has enabled Ge to be widely used as an attenuated total reflectance (ATR) crystal material in infrared (IR) spec- troscopy. In particular, Ge has been extensively used as a substrate for IR studies of monomolecular films. 4-8 The focus of our current research is the characterization of organized, two-dimensional monomolecular structures at interfaces in their native aqueous environment. 4-13 Our immediate aim is the development of an integrated approach permitting infrared spectroscopic measurements and scanned probe imaging to be performed on the same monomolecular film bound to a single substrate. Vibra- tional spectroscopy is a powerful tool for the study of ultrathin organic structures at interfaces; however, the information obtained is of a macroscopic, surface-averaged nature. Scanning probe microscopies (i.e., STM and AFM) are powerful methods for imaging the morphology and topology of surfaces at atomic resolution and provide information on the real space, two-dimensional structure of monolayer surfaces. Thus, the potential advantage in combining these two powerful methods for studying ultrathin films provides a strong incentive to find a common substrate. A necessary requirement for combining the IR/STM techniques is the selection of a substrate that is both IR transparent and electrically conductive. We propose using germanium for this purpose. The problems associated with using normal, optical quality, ATR-type germanium crystals for combined IR/ATR applications are 3-fold: (1) high purity, optical quality germanium crystals are typically highly resistive (∼50 Ω cm), rendering them unsuitable as a conducting substrate for many STM applications. Doping germanium with extra (extrinsic) charge carriers is a common method of enhancing its conductivity. (2) Germanium used for infrared spectro- * Author to whom correspondence should be addressed. § Present address: Department of Chemistry, Illinois State University, Normal, IL 61790-4160. † University of Georgia. ‡ Georgia Institute of Technology. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Becker, R. S. In Scanning Tunneling Microscopy I; Strocio, J. A., Kaiser, W. J., Eds.; Academic Press: San Diego, CA, 1994; Chapter 5.2. (2) (a) Weisendanger, R. Scanning Probe Microscopy and Spectros- copy; Cambridge University Press: Cambridge, England, 1994. (b) Stroscio, J. A., Kaiser, W. J., Eds. Scanning Tunneling Microscopy; Methods of Experimental Physics 27; Academic Press: San Diego, CA, 1993. (c) Becker, R. S.; Swartzentruber, B. S.; Vickers, J. S.; Klitsner, T. Phys. Rev. B 1989, 39, 1633. (3) Feenstra, R. M.; Slavin, A. J. Surf. Sci. 1991, 251/252, 401. (4) Cornell, D. G.; Dluhy, R. A.; Briggs, M. S.; McKnight, J.; Giersach, L. M. Biochemistry 1989, 28, 2789. (5) Dluhy, R. A.; Cornell, D. G. In Fourier Transform Infrared Spectroscopy In Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991; p 192. (6) Rana, F. R.; Mautone, A. J.; Dluhy, R. A. Biochemistry 1993, 32, 3169. (7) Rana, F. R.; Mautone, A. J.; Dluhy, R. A. Appl. Spectrosc. 1993, 47, 1015. (8) Gregory, B. W.; Dluhy, R. A.; Bottomley, L. A. J. Phys. Chem. 1994, 98, 1010. (9) Bottomley, L. A.; Haseltine, J. N.; Allison, D. P.; Warmack, R. J.; Thundat, T.; Sachleben, R. A.; Brown, G. M.; Woychik, R. P.; Jackosbon, K. B.; Ferrell T. L.; Schrick, J. J. J. Vac. Sci. Technol. A 1992, 10, 591. (10) Thundat, T.; Warmack, R. J.; Allison, D. P.; Ferrell, T. L.; Bottomley, L. A. J. Vac. Sci. Technol. A 1992, 10, 630. (11) Fowler, K.; Bottomley, L. A.; Schreier, H. J. Control. Res. Bioact. Mater. 1992, 22, 283. (12) Allison, D. P.; Bottomley, L. A.; Thundat, T.; Brown, G. M.; Woychik, R. P.; Schrick, J. J.; Jacobson, K. B.; Warmack, R. J. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 10129. (13) Bottomley, L. A.; Jones, J. A.; Ding, Y.; Allison, D. P.; Thundat, T.; Warmack, R. J. S.P.I.E. Proc. Biomed. Opt. Soc. 1993, 1891, 48. 6146 Langmuir 1997, 13, 6146-6150 S0743-7463(97)00287-4 CCC: $14.00 © 1997 American Chemical Society