1398
Langmuir 1992,8, 1398-1405
Static Secondary Ion Mass Spectrometry of Self-Assembled Alkanethiol Monolayers on Gold Michael J. Tarlov' Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
John G. Newman Evans Central, 5909 Baker Road, Suite 580, Minnetonka, Minnesota 55345 Received December 6,1991. In Final Form: February 21, 1992
We report a static secondary ion mass spectrometry (SSIMS)study of self-assembled monolayers (SAMs) of alkanethiols [CH&H2),SH, where n = 7,9,11,15,171 adsorbed on Au. A rich variety of molecular secondary ions are observed in the negative SSIMS spectra including (M - HI-, (AuM)-, (AuSM)-, and (Au2[M - HI)- and a relatively strong (Au[M - HI$ where M is the complete alkanethiol molecule. Sulfonates and alkanesulfonates are observed in SSIMS spectra of SAMs that have been atmosphere exposed for prolonged periods; however, sulfonate species are not detected from samples that are analyzed immediately after withdrawal from thiol-ethanol solutions. SSIMS results indicatethat sulfonatesformed by air oxidation can be displaced by reimmersion of samples in thiol adsorbatesolution. Molecular secondary ions are not observed for perfluoromercaptan and carboxylic acid-terminated SAMs, although spectra distinct from those of the alkanethiol SAMs were obtained. Damage profiles indicate that the emission of molecular secondary ions is very sensitive to extremely low primary ion beam doses. In addition, the relative intensities of Au substrate and molecular ions are strongly influenced by the energy of the primary ion beam suggesting a beam penetration depth effect. Introduction The formation and properties of self-assembled monolayers (SAMs) of organosulfur compounds on gold, silver, and copper surfaces are subjects of much current interest. Of these systems, alkanethiol SAMs formed on Au surfaces have been the most extensively 5t~died.l-l~ A plethora of complementary surface analytical methods have been used to examine the monolayer films, including infrared,3J4-16 ele~trochemical,~J~-~ ellip~ometric~*~J~ contact angle meas~ r e m e n t , ~ ~ 5 *ultrahigh 8Jl vacuum electron and ion
spectroscopic,l~~~~~~"n diffraction,B92aBmicrogravimetric,30 scanning probe,3l~3~ and computational te~hniques.9-33*3~ Here we report the use of static secondary ion mass spectrometry (SSIMS) for characterization of SAMs of n-alkanethiols chemisorbed on Au surfaces. In SSIMS low primary ion flux and total dose are employed to minimize surface damage. We were motivated to use SSIMS as a characterization tool because of several potential advantages it offers, such as high surface sensitivity, excellent detection limits, and the ability to detect all elements and isotopes. In addition, many studies have demonstrated that organic molecular adsorbates can frequently be desorbed intact as secondary ions and detected directly using SSIMS.3637 With this intriguing possibility in mind, we thought that SSIMS might prove to be a valuable technique for monitoring the products and yield of surface chemical reactions involving SAMs.
(1) Nuzzo, R.G.;Allara, D. L. J.Am. Chem. SOC.1983,105,4481-4483. (2) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. SOC.1987, 109, 2358-2368. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559-3568. (4) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111, 321-335. (5) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5,723-727. Rowntree, P.; Scoles, G. J. Chem. (6) Chidsey, C. E. D.; Liu, G.-Y.; (22) Miller, C.; Cuendet, P.; Gratzel, M. J.Phys. Chem. 1991,95,877Phys. 1989,91, 4421-4423. 886. (7) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (23) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanul. Chem. (8)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. 1991,310, 335-359. (9) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989,5,1147-1152. (24) Long, H. C. D.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, (10)Ulman, A. Introduction to Ultrathin Organic Films from 2196-2202. Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, (25) Laibinis. P. E.; Bain, C. D.: Whitesides. G. M. J. Phvs. Chem. 1991. 1991, 95, 7017-7021. (11) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R.G.; (26) King, D. E.; Czanderna, A. W. Surf. Sci. Lett. 1990,235, L329Allara, D. L.; Porter, M. D. Langmuir 1988,4, 365-385. L332. (12) Laibinis,P. E.; Whitesides,G.M.;Allara,D.L.;Tao,Y.-T.;Parikh, (27) Tarlov, M. J. Langmuir 1992,8, 80-89. A. N.; Nuzzo, R. G. J. Am. Chem. SOC.1991,113, 7152-7167. (28) Strong, L.; Whitesides, G. M. Langmuir 1988,4,546-558. (13) Dubois, L. H.; Zegarski, B. R.;Nuzzo, R. G. J. Am. Chem. SOC. (29) Samant, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1991, 7, 1990, 112, 570-579. 437-439. ~. (14) Nuzzo, R. G.;Dubois, L. H.; Allara,D. L. J.Am. Chem. SOC.1990, (30jThoma8, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 112,558-569. 1991, 7, 620-622. (15) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, (31) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC.1991, 93,767-773. 113,2805-28io. (16) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. SOC.1991, 113, (32) Sun, L.; Crooks, R. M. J. Electrochem. SOC.1991,138, L23-L25. 8284-8293. (33) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989,91, 4994-5001. (17) Finklea, H. 0.;Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, (34) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990,93, 7483-7492. D.; Bright, T. Langmuir 1986,2, 239-244. (35) Benninghoven, A.; Rudenauer, R. G.; Werner, H. W. Secondary (18) Chidsey, C. E. D. Science 1991,251, 919-922. Ion Mass Spectrometry-Basic Concepts, Instrumental Aspects, (19) Nordyke, L. L.; Buttry, D. A. Langmuir 1991, 7, 380-388. Applications and Trends; John Wiley and Sons: New York, 1987. (20) DeLong, H. C.; Buttry, D. A. Langmuir 1990,6, 1319-1322. (36) Delgass, W. N.; Cooks, R. G. Science 1987,235, 545-553. (21) Creager, S. E.; Collard, D. M.;Fox, M. A. Langmuir 1990,6,1617(37) Colton, R. J.; Campana, J. E.; Kidwell, D. A.; Ross, M. M.; Wyatt, 1620. J. R. Appl. Surf. Sei. 1986, 21, 168-198.
0743-746319212408-1398$03.00/0 @ 1992 American Chemical Society
SSIMS of Alkanethiol Monolayers on Gold
Langmuir, Vol. 8, No. 5, 1992 1399
I
I CI'
CNO'
I
I
x1
"Bare" Au
Au'
AuCI2'
Au~CI' Au2'
f
Au3'
C H3 ( C H 2) gS HI A u
SII'
Au2S-
Au[M-H]2'
X5.8 Au~IM-H]'
C H 3 ( CH2) 17sH/ A U I
0
I
I
I
200
400 Mass/Charge (m/z)
600
800
Figure 1. Negative SSIMS spectra acquired from atmosphere-exposed 'bare" Au and SAMs on Au of decanethiol [CH&Hp)dH] (molecular weight = 174)and octadecanethiol [CH~(CHZ)I,SHI(molecularweight = 286). M denotes the molecular weight of the intact thiol molecule. A Xe+ primary beam energy of 7.0 keV was used. Note that the vertical axes of the SSIMS spectra of the SAMs have been multiplied by a factor of 5.8.
Although many SSIMS investigations of LangmuirBlodgett monolayers and multilayer^^^^ and alkylsilane monolayers42have been reported in the literature, we are not aware of any SSIMS studies of alkanethiol SAMs. Li et al. recently demonstrated that the molecular constituents of alkanethiol SAMs could be identified using a related technique, laser desorption Fourier transform mass spectrometry (LD-FTMS).43 In the studies presented here, we show that SSIMS can also be used to obtain molecular information from alkanethiol SAMs via intact desorption of a variety of molecular species.
Experimental Section The samples used in this study were n-alkanethiol monolayers self-assembled on 200-nm gold films sputter-deposited on single-crystal Si wafers. The wafers were primed with a 10-nm Cr layer to improve adhesion of the gold. SSIMS was performed on a homologous series of n-alkanethiol SAMs of the general formula CH&H,),,SH, where films comprised of n = 7,9,11,15, or 17 were examined. We designate the particular n-alkanethiol (38)Wandam, J.H.; Gardella, J. A. J. Am. Chem. SOC.1986,107,61926195. (39)Wandass, J. H.; Schmitt, R. L.; Gardella, J. A. Appl. Surf. Sci. 1989.40.85-96. (40) Bolbach, G.; Beavis, R.; Negra, S. D.; Deprun, C.; Ens, W.; Lebeyec,Y.;Main,D. E.;Schueler,B.;Standing,K. G.Nucl.Instrum.Methods 1988. B14.74-82. (41)Cornelio-Clark, P. A.; Gardella, J. A. Langmuir 1991,7,22792286. (42)Hayes, T. R.;Evans, J. F. J . Phys. Chem. 1984,88,1963-1973. (43)Li, Y.;Huang, J.; McIver, R. T., Jr.; Hemminger, J. C. J. Am. Chem. SOC.1992,114,2428-2432.
used in forming the monolayer with the notation Cn, where n corresponds to the number of methylene (-CHp-) units in the hydrocarbon chain. Immediately after the thin film deposition, Au/Cr/Si samples were immersed in M thiol-absolute ethanol solutions. No attempt was made to exclude air from the adsorbate solutions. Standard methods of SAM postassembly spectroscopic characterization were used to examine other samples prepared in the same manner.27 The results from these previous studies indicate that the SAMs used in this work consist of densely packed, crystalline-like molecular arrays with fully extended alkyl chains. SAM samples were prepared at NIST and shipped overnight to Evans Central for SSIMS analysis. Samples were stored and transported completely immersed in ethanol-thiol adsorbate solution in Teflon-sealed glass vials. Unless noted otherwise, all SSIMS spectra presented were obtained from samples that were withdrawn from adsorbate solutions, rinsed with methanol, blown dry with nitrogen, and inserted immediately in the rapid sample introduction chamber of the SSIMS system. SSIMS was performed with a Physical Electronics Model 6300 quadrupole-based SIMS instrument.4 A duoplasmatron source was used to generate a primary ion beam of Xe directed at 60' with respect to the surface normal. The Xe+ beam was rastered over a 2 X 2 mm area, resulting in a current density of 3.75 x lo+ A/cm2. Total ion doses of 7 X lo1* ions/cm2 were used to acquire spectra. Spectra were obtained using primary beam voltages of 0.5, 2.5, and 7.0 keV. (44)Certain commerical products and instruments are identified to adequately specify the experimental procedure. In no case does such identification imply endorsement by the National Institute of Standards and Technology.
1400 Langmuir, Vol. 8, No. 5, 1992
Tarlov and Newman
CH~(CH~)~SH/AU CHAu~S~H' 72H' Au2S'
I II
I
I
I
I
Auz[M-HI-
AuJS'
I
CH~(CH~)~~SH/AU AuM' Auz[M-HI'
I
0
200
400
600
I 800
MassKharge ( m h ) Figure 2. Negative SSIMS spectra acquired from SAMs on Au of octanethiol [CH3(CH2),SH](molecularweight = 146),dodecanethiol [CH3(CH2)11SH](molecular weight = 202),and hexadecanethiol [CH3(CH2)15SH](molecularweight = 258). M denotes the molecular weight of the intact thiol molecule. A Xe+ primary beam energy of 7.0 keV was used. The vertical axes of the SSIMS spectra of the SAMs are referenced to that of the "bare"Au spectrum of Figure 1 and have been expanded by a factor of 4.6.
Results and Discussion Negative ion SSIMS spectra obtained from "bare" Au and SAMs of decanethiol (C9) [MW = 1741 and octadecanethiol (C17) [MW = 2863 on Au are shown in Figure 1. The most intense feature of the "bare" Au spectrum is C1-. Contamination of "bare" Au by C1has been reported previously and presumably results from atmospheric exposure.23 We believe its presence is exaggerated by the extremely high negative ion yield for C1. At higher mass, ions corresponding to Au-, (AuCl)-, (AuC12)-, (Am)-, (AuZCl)-, and (Au3)- are also detected. The C9 and C17 SAM samples exhibit dramatically different SSIMS spectra. Below mlz 100 in both the C9 and C17 spectra essentially no major impurity ions such as C1- are detected. Instead, the dominant ions are (CHI-, (C2H)-, S-, and (SH)-. More remarkable is the richvariety of molecular cluster ions detected at higher masses, particularly the Au-molecular cluster of (Au[M - HI d-, where M is the complete alkanethiol molecule, which is the most prominent peak in the C9 spectrum.45 Other molecular secondary ions are observed a t lower intensities, including (M- HI-, (AuMI-, (AuSMI-, and (AudM- HI)-. It is interesting to note that the relative intensity of the Au-molecular cluster ions in this study is much greater than that of similar negative ions observed in SSIMS of Langmuir-Blodgett films on a variety of metal substrates.38-" One factor that may contribute to the high probability of ejection of intact Au-molecular thiol clusters (45) We define molecular Secondaryor cluster ions as species containing at least the intact thiolate molecule.
is the stability of the Au-S bond where a heat of adsorption of -30 kcal/mol has been measured for thiolates on Au.I4 In addition, various gold-sulfur cluster ions such as (A&)-, (AuSzH)-, (Au~S)-,(Au~S~H)-, (Au~S)-,(Au&)-, and (Au3Sd- are observed. SSIMS spectra acquired on 3 different occasions from 3 different C9 samples showed reasonable qualitative reproducibility of the fragmentation patterns. SSIMS spectra acquired from C7, C11, and C15 nalkanethiol SAMs are shown in Figure 2. They exhibit similar fragmentation patterns of molecular and Au-molecular ions shifted by the appropriate difference in molecular weight of the alkanethiol used. The intensity of all the molecular cluster ions roughly decreases with increasing number of methylene (-CH2-) chain units, n, in the alkanethiol molecule. The trend cannot be attributed simply to a matrix effect leading to reduced negative ion yields because, surprisingly, the Au- signal increases by a factor of 3 in going from n = 7 to n = 17. Undoubtedly, part of the decrease is due to the transmission function of the quadrupole analyzer where transmission decreases with mass; however, other factors intrinsic to the secondary ion emission process such as increased molecular fragmentation or decreased secondary ion survivalprobability cannot be ruled out.& In positive SSIMS spectra, the only molecular secondary ion detected from the n-alkanethiol monolayers was a very (4s)One posaibleexplanationfor the decreasein molecular ionemhion with n is that as the mass of molecular secondaries increases, they move away from the surface at lower velocities. It is generally believed that the neutralization probability of a secondary ion increases as ita velocity in moving away from the surface decreases."
SSIMS of Alkanethiol Monolayers on Gold
Langmuir, Vol. 8, No. 5, 1992 1401
CH~(CH~)~~SH/AU Exposed to Air 11 Days n
Y
.n
5
4u-
t: E
2 s Y
I
h
Y
3aJ
Au2S'
Y
E
U
200
400 MassKharge ( d z )
600
Figure 3. Negative SSIMS spectrum of an octadecanethiol [CH&H2)17SH] (molecular weight = 286) SAM on Au exposed to air for 11 days. A Xe+ primary beam energy of 7.0 keV waa used.
so3
CH~(CH~)~~SH/AU Exposed To Air 27 Days
HSO4'
C2H' Au'
321
I
I
Y
Same Sample After 1 Hr In lom3M C17/Ethanol
I
0
100
I
200 MassKharge ( d z )
I
300
Figure 4. Negative SSIMS spectra of an octadecanethiol [CH&H2)17SH] (molecular weight = 286) SAM on Au exposed to air for 27 days and the same sample immediately followingimmersion in 10-3 M CHa(CHz)17SH/ethanol solution for 1h. A Xe+primary beam energy of 7.0 keV was used.
weak ( A d ) + signal. In addition, hydrocarbon chain fragmenta were observed in the lower mass region from mlz 0 to 100with the most abundant species being (C2H3)+, (C2Hd+, (C3H3)+,(C3Ha)+, (C3H7)+, and (CrH7)+. Because of the prominence of the (Au[M - Hl2)- peak, it is worthwhile to consider possible mechanisms for ita
origin. Strong evidence exists that upon chemisorption on Au the thiol head group loses its hydrogen to form a thiolate.5~23 It is generally believed that the preferred binding site for the thiolate head group on the Au(ll1) surface is a 3-fold hollow site because it has been established that the chemisorbed thiols form an epitaxial
1402 Langmuir, Vol. 8, No. 5, 1992
Tarlov and Newman
F'
CF3( CF2)7(CH2)2S H/Au C
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200
0
*
X33.6
600
Figure 5. Negative SSIMS spectra acquired from SAMs on Au of perfluoromercaptan,CF3(CF&(CHz)zSH(molecularweight = 480), and 16-mercaptohexadecanoicacid [HOOC(CHz)lbSH] (molecular weight = 288). Note that the F signal is off scale by a factor of 8 in the spectrum of the perfluoromercaptanSAM. The expansion factor for the 16-mercaptohexadecanoicacid SSIMS spectrum is relative to that of the perfluoromercaptanspectrum.
0.0
1 .o
2.0
Ion Dose/(lO'') (ions/cm2)
Figure 6. Damage profiles obtained from a decanethiol
[CH3(CH&SHl (molecularweight = 174) SAMon Au. Mdenotes the molecular weight of the intact thiol molecule.
( 4 3 X 43)R3O0 overlayer.6~~8*~9*3~ If this rigorously holds, then in no case willtwo thiolate head groups ever be directly bound to the same surface gold atom. Although single crystal or epitaxial Au(ll1) surfaces were not used in this study, X-ray diffraction results indicate that the deposited Au films are highly (111)textured2' and, thus, a large fraction of the thiolate head groups are probably adsorbed in 3-fold hollow sites. This implies that the formation of the (Au[M - HI& species does not result from intact ejection but rather from a recombination reaction that occurs somewhere above the surface during the secondary ion emission process. We cannot rule out the possibility that the formation of (Au[M - HI& is dominated by
emission from defect sites such as those that might be encountered at Au steps or grain boundaries. In further exploring this issue, SSIMS studies of SAMs on epitaxial Au(ll1) surfaces would be useful. It also might prove interesting to compare (Au[M - HI& emission from alkanethiol SAMs formed on Ag and Cu surfaces where it is thought that the mode of binding of the thiolate head group is different from that on Au.l2*4 It is useful to compare the SSIMS results in this study to those recently reported by Li et al. for alkanethiol SAMs on Au using the technique of LD-FTMS.43 Intact molecular desorption was achieved in the LD-FTMS studies were where strong negative ion thiolate signals, (M - H)-, observed in spectra acquired from C7, C8, C11, C15, and C17 SAMs. In contrast to the results reported here, no Au substrate or Au-molecular species were observed in the LD-FTMS spectra. This is attributable to the laser energy used for desorption in those experiments, which was just below the threshold for ablation of the gold substrate. When laser power densities above the ablation threshold are used, Au-molecular species similar to those detected in SSIMS are observed in the FTMS spectra.49 Another notable feature in the investigation by Li et al. was the presence of relatively strong signals arising from alkanesulfonate species in all of the LD-FTMS spectra. (47) Winograd, N. R o g . Solid State Chem. 1982, 13,285-375. (48) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D.J . Am. Chem. SOC.1991,113, 2370-2378. (49) Hemminger, J. C. Personal communication.
Langmuir, Vol. 8, No. 5, 1992 1403
SSIMS of Alkanethiol Monolayers on Gold
7.0 keV Xe+
A U[ M-HI 2-
x1
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500
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I.
L
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700
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Figure 7. Negative SSIMS spectra of an octanethiol [CH&H&SH] (molecular weight = 146) SAM on Au acquired at Xe+ primary beam energies of 7.0 and 0.5 keV. Experiments conducted by Li et al. indicated that the sulfonates are formed by air oxidation of the SAMs. In this study no sulfonate species were detected with SSIMS from any of the SAM samples that were stored in thiolethanol adsorbate solution and analyzed immediately following removal from solution. This was true of SAMs that were stored in adsorbate solutions for up to 2 months. Sulfonate species were observed, however, from SAMsthat were exposed to air for extended periods. Figure 3 shows a negative SSIMS spectrum acquired from a C17 SAM that was exposed to the laboratory ambient for 11 days. In comparison to the C17 spectrum of Figure 1, three new peaks are observed at mlz = 80,97, and 333 that we assign to (so&, (HSOd-, and [CH&H2)1$3031-, respectively. Similar sulfonate species were observed in the LD-FTMS spectrum of a C17 SAM in the study by Li et al. It is interesting to note that no Au-sulfonate or Au-alkanesulfonate species are observed in the spectrum of the airexposed C17 SAM. Presumably the stability of Ausulfonate species is much lower than that of the corresponding Au-thiolates. Although the sulfonate signals are prominent in the C17 spectrum of Figure 3, we believe that the extent of oxidation of the C17 SAM is relatively low. Li et al. reported very little (300) in the 0.5-keV SSIMS. The absence of substrate (Au-) and near-substrate IS-,(SH)-l derived signals suggests that SSIMS sampling at 0.5 keV is limited largely to the uppermost portion of the C17 monolayer. Conclusions In summary, we have shown that SSIMS can be used to obtain molecular and chemical information from SAMs of alkanethiols on gold. Many different molecular cluster ions are observed in the negative ion spectra of the nalkanethiol monolayers. Although the spectra are rich in detail, they also are relatively "clean" and, thus, allow unambiguous identification of molecular ion species. We believe that SSIMS may prove to be a valuable technique to monitor the progress of chemical reactions on SAM surfaces and ascertain the success of chemical modification procedures. In addition, a study is in progress where SSIMS is being used to determine whether surface segregation occurs when mixed hydroxy- and methylterminated monolayers are formed. If the thiol molecules are randomly distributed in the monolayer, then the pattern of (Au[M - HI& clusters produced as a function of mole fraction of hydroxy-terminated thiol should vary in a predictable statistical manner.50 Finally, alkanethiol SAMs may also prove to be an ideal model system for probing mechanisms of secondary ion emission of organic species because the thiol monolayers are reproducibly formed, structurally well-defined, and have easily manipulated chemical functionality and thickness. Acknowledgment. We thank Professor John Hemminger for providing us with a preprint of ref 43 and discussion of these and other results prior to their publication. We also thank Dr. James Whetstone and Professor John Evans for critical discussions concerning this work. M.J.T. thanks Dr. Chang Jho of CIBA-GEIGY for his generous gift of the perfluoromercaptan. Registry No. Au, 7440-57-5; Cr, 7440-47-3;Si, 7440-21-3; CH3(CH2)7SH, 111-88-6;CH3(CHz)&H, 111-88-6;CHa(CH2)ilSH,111-88-6; CH3(CH*)lsSH,111-88-6;CH~(CH~)I,SH, 111-886;CFS(CF2)7(CHz)zSH,34143-74-3; HOOC(CH2)16SH,69839-685.
