Langmuir 1995,11, 1849-1851
1849
Preparation of Self-AssembledMonolayers on InP Y. Gu, Z. Lin, R. A. Butera, V. S. Smentkowski, and D. H. Waldeck" Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received March 28, 1995. I n Final Form: April 4, 1995@ This work describes the preparation of octadecanethiol-basedmonolayers on single crystal InP surfaces. The quality and nature of the monolayer are characterized by contact angle measurements, photocurrent measurements, ellipsometry, and X-ray photoelectron spectroscopy. The use of alkanethiol-based compounds for chemical modification of interfaces has shown a burst of activity -~ most (surface assembled monolayer or S A M S ) ; ~however of this effort has concerned metal substrates. A notable exception is the recent work by Allara and co-workers4on the chemical modification of GaAs and of Chidsey5 on the chemical modification of Si. For a variety reasons (optoelectronic devices, photovoltaic devices, photocatalysis), it is desirable to chemically modify and control semiconductor interfaces. This work reports on the preparation of a n octadecylthiol-based monolayer on InP-a direct gap semiconductor (1.35 eV)with important optoelectronic characteristics and interesting photoelectrochemical behavior.6 The aim of this report is to demonstrate the formation of this monolayer on InP and describe its preparation. The substrate is n-type InP single crystal (Crystacom, Inc.), whose carrier density is 3 x 10l8~ m - The ~ . material was purchased with the (100) face oriented and with one side polished by the manufacturer. The octadecylthiol was purchased from Aldrich (98%). The substrate was cleaned by dipping it for 5 min in concentrated HF (49%), followed by a deionized water rinse, drying with N2, and then immediately placing it into a test tube containing the molten thiol. The test tube was kept in a n oil bath at 140 "C for a period of 12 h. During this process, the molten thiol was kept under N2 to minimize its exposure to oxygen. It was found that oxidation occurred under these experimental conditions, unless the N2 purge was used. Both the temperature and time of the processing had a significant influence on the quality of the organic layer formed a t the interface. This procedure was adapted from that of Allara's for G ~ A s .Compared ~ to G A S , InP requires longer time and higher temperature. After the sample was removed from the thiol solution, it was rinsed with chloroform, followed by acetone and then methanol. This rinse left a clean, shiny, and stainless surface. The presence of the monolayer film was demonstrated with a variety of techniques: spectroscopic,electrochemical, and physical.
Physical Characteristics The high coverage of octadecyl was demonstrated by measurement of the advancing contact angle. A RameAbstract published in Advance ACS Abstracts, May 15, 1995. (1)Nuzzo, R. G.; Allara, D. L. J . Am. Chem. SOC. 1983,105, 4481. (2)Ulman, A.Introduction t o Thin Organic Films: From LungmuirBlodgett to SelfAssembly. Academic Press: New York, 1991. (3)Haran, A.;Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994,263,948. (4)(a)Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J.Am. Chem. SOC. 1992,114,1514-1515. (b) Nakagawa, 0.S.; Ashok, S.; Sheen, C. W.; Martensson, J.; Allara, D. L. Jpn. J . Appl. Phys. 1991,30(12B),3759-3762. (5)Linford, M.R.; Chidsey, C. E. D. J . Am. Chem. SOC. 1993,115, 12631. (6)Koval, C. A,; Segar, P. R. J . Am. Chem. SOC.1989,111, 20042010.
Hart, Inc., Model 100-00-115 instrument was used for measuring the angle. Films prepared in this way gave a n advancing contact angle of 105". This contact angle is much higher than that observed for the "bare" substrate (InP has a native oxide7which forms on the surface) value, ca. 62". The angle reported here is smaller than that typically found for monolayers on gold (ca. 116") but it is comparable to those reported for GaAs (ca. 103").4 In contrast, the surface modified with Na2Sl0gave a contact angle of 72". In addition to the octadecylthiol films discussed here, films have been prepared with alkanethiols ranging in length from 5 to 18 carbons. In all cases, the high coverage is found with contact angles like that reported above for the octadecanethiol. The contact angle does not degrade with exposure to air over a period of weeks. Lastly, the electrode films were exposed to solutions of CHCl3 for a period of 6 h and the film quality (as judged by contact angle measurements) did not degrade.
Spectroscopic Information A film thickness was measured by use of single frequency null ellipsometry. The instrument used for this measurement was a Gaertner L-117 production ellipsometer. The thickness measurement was performed at a 70" angle of incidence with a He-Ne laser (6328 A) as the light source. The reference surface was InP that was treated in the same manner but was not exposed to the thiol. The native oxide formed on this type of surface is reported to be less than 7 A.7b The ellipsometric angles (A, Y) were determined to be (154.65,8.425) for the bare substrate and (151.00,8.785)for the coated surface. These parameters give an average thickness of 18A for the film.8 This thickness is consistent with those reported for S A M s on goldgand on G ~ A s This . ~ result demonstrates that the film is a monolayer, not a multilayer film. The Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS)data reported here were obtained with a Physical Electronics Model 550 spectrometer. The angle resolved XPS data were obtained using a Leybold Heraeus dual anode X-ray gun and preretarding hemispherical energy analyzer (EA-11) instrument. The data were obtained under UHV condiTorr). MonochromatizedMg Ka radiation tions (
