Langmuir 1995,11, 2563-2571
2563
High Lateral Resolution Imaging by Secondary Ion Mass Spectrometry of Photopatterned Self-Assembled Monolayers Containing Aryl Azide C. Daniel Frisbie, Eric W. Wollman, and Mark S. Wrighton" Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received November 7, 1994. I n Final Form: March 13, 1995@ Imaging secondary ion mass spectrometry (SIMS) is used to map the distribution of molecular and elemental species in a patterned self-assembled monolayer (SAM)with 1 pm lateral resolution. The patterned SAMs are made by exposing polycrystalline Au to a solution of bis( ll-(4-azido(benzoyloxy))1-undecyl) disulfide, I, which forms a photosensitive S A M (Au-I) on the Au. Subsequent W irradiation (1 > 260 nm) of Au-I SAMs through a mask and a thin film of a secondary amine results in attachment of the amine t o the Au-I S A M only in the irradiated regions. The Au-I S A M is photosensitive by virtue of a pendant aryl azide group which reacts with secondary amines under W irradiation to form hydrazine or azepine photoadducts. A large molecular fragment ion corresponding to vinyl ferrocene (mlz 212) was mapped with 1pm lateral resolution on a Au-I S A M that had been irradiated through a mask and a thin film of (2-ferrocenylethyl)(2',2',2'-trifluoroethyl)amine. SIMS can also detect intact molecular ions M+ corresponding to the 3H-azepine and hydrazine photoadducts obtained upon irradiation of Au-I SAMs in the presence of diethyl- and dibutylamines. Smaller fragment ions characteristic of the 3H-azepine and hydrazine adducts were also observed. The mass assignments were verified by a series of isotopic labeling experiments in which the observed ions displayed the expected isotopic shifts.
Introduction Secondary ion mass spectrometry (SIMS)1-5is a high lateral resolution surface analysis technique which can image the distribution of both molecular and elemental species on a variety of solid surfaces. Since imaging SIMS provides composition maps of very small areas of a surface (eg., cm2)with high lateral resolution (-100 nm in favorable cases), the technique can be regarded as a chemically sensitive microscopy. Presently, there are other relatively more common methods of chemically sensitive surface imaging which include scanning Auger microscopy,6energy dispersive or wavelength dispersive X-ray fluorescence microscopy (EDXor WDX),7and optical fluorescence microscopy.8 These imaging techniques have been used effectively in studies of corrosion of metals, surface-catalyzed reactions, contamination in microelectronics manufacture, biological tissues and assays, and chemically patterned surfaces. Elemental mapping is now
also possible with recently developed imaging X-ray photoelectron spectrometers (XPS).9What makes imaging SIMS unique is its combination ofhigh sensitivity, capacity to detect molecules as well as elements, and high lateral resolution. None of the other techniques mentioned above have all three qualities. Auger microscopy and EDX are capable of submicrometer resolution, but they are only element s e n ~ i t i v e . ~Optical ,~ fluorescence imaging can detect surface-confined molecules with part-per-million sensitivity or better, and spatial resolution is as small a s 100 nm using near-field optics.sb,h Fluorescence imaging is also nondestructive and relatively rapid. However, for many samples elements cannot be directly detected by fluorescence, and it is not always convenient or possible to label samples with molecular fluorophores. In contrast, imaging SIMS offers the possibility of sensitively detecting all elements and molecules a t high lateral resolution without the need for ad hoc chemical tags.
* Author to whom correspondence should be addressed.
Abstract published in Advance A C S Abstracts, May 15,1995. (1)Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry:Basic Concepts, Instrumental Aspects, Applications and Trends; John Wiley and Sons: New York, 1985. (2)Vickerman, J . C.; Brown, A.; Reed, N. M. Secondary Ion Mass Spectrometry: Principles and Applications; Clarendon: Oxford, U.K., 1989. (3)(a)Benninghoven,A.Angew. Chem., Int. Ed. Engl. 1994,33,1023. (b)Benninghoven, A.;Hagenhoff, B.; Niehuis, E. Anal. Chem. 1993,65, 630A. ( c ) Schweiters, J.; Cramer, H.-G.; Juurgens, E.; Niehuis, J.; Zehnpfennig,J.;Benninghoven,A. J.Vac.Sci. Technol. 1991,A9,2864@
2871. -.
(4)Winograd, N. Anal. Chem. 1993,65,622A. (5)Gillen, G.; Simons, D. S.; Williams, P. Anal. Chem. 1990,62, 2122-30. ( 6 ) (a) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger andX-ray Photoelectron Spectroscopy;John Wiley and Sons: New York, 1983. (b) Hickman, J. J.; Ofer, D.; Zou,C.; Wrighton, M. S.; Laibinis, P. E.;Whitesides, G.M.J.Am.Chem.Soc. 1991,113,1128.(c)Browning, R.; Adebanjo, R.; Miller, A. K. J . Nucl. Mater. 1989,165 (3),297. (d) Hoflund, G. B.; Asbury, D. A.; Kirszensztejn, P.; Laitinen, H. A. Surf. Interface Anal. 1986,9,169. (e) Wirth, A.; Andreoni, I.; Gregoly, G. Surf. Interface Anal. 1986,9, 157. (0 Browning, R.; Smialek, J.; Jacobsen, N. J . Mater. Sci. Lett. 1986,5 (ll), 1122. (g) Ebel, M. F.; Ebel, H.; Wernisch, J.; Gratzl, M.; Polos, L.; Toth, K.; Pungor, E. Anal. Chem. Symp. Ser. 1986,22,349.(h) MacDonald, N. C.; Hovland, C. T.; Gerlach, R. L. Scanning Electron Microsc. 1977,10 (l), 201.
(7)(a)Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig, A. D., Jr.;Lyman,C. E.; Fiori, C.; Lifsin, E. ScanningElectronMicroscopy andX-Ray Microanalysis;Plenum Press: New York, 1992. (b) Spencer, A. J.; Hawkey, L. A.; LeFurgey, A,; Dickman, K. G.; Mandel, L. J.; Ingram, P. Microbeam Anal. 1990, 25, 393. (c) Newbury, D. E.; Marinenko, R. B.; Bright, D. S.; Myklebust, R. L. Scanning 1988,10, 213. (d) Ingram, P.; LeFurgey, A.;Davilla, S. D.; Sommer, J. R.; Mandel, (e) L. J.; Lieberman, M.; Herlong, J. R. Microbeam Anal. 1988,23,433. Baker, D.;Kupke, K. G.; Ingram, P.; Roggli, V. L.; Shelburne, J . D. Scanning Electron Microsc. 1986,2,659.(0 Gorlen, K. E.; Barden, L. K. ;Del Priore, J. S.;Fiori, C. E.; Gibson, C. C.; Leapman, R. D. Reu. Sci. Instrum. 1984,55,912. (8) (a) Ploem, J. S.; Tanke, H. J. Introduction to Fluorescence Microscopy; Oxford University Press: New York, 1987. (b) Moers, M. H. P.; Gaub, H. E.; van Hulst, N. F. Langmuir 1994,10,2774. (c) Suhrbier, A.; Sinden, R. E.; Couchman, A.; Fleck, S. L.; Kumar, S.; McMillan, D. J. Eukaryotic Microbiol. 1993,40(1) 18. (d) Adenot, P. G.; Corteggiani, E.; Geze, M.; Besombes, D.; Debey, P.J.Fluoresc. 1992, 2 (3), 181. (e) Hui, S.W.; Yu,H.; Xu,Z.; Bittman, R. Langmuir 1992, 8,2724.(0 Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7,2323. (g) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991,7, 2781. (h) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J . S.; Kostelak, R. L. Science 1991,251,1468. (9)(a) Drummond, I. W.; Ogden, L. P.; Street, F. J.; Surman, D. J. J. Vac. Sci. Technol. 1991,9,1434. (b) Gurker, N.; Ebel, M. F.; Ebel, H.; Manther, M.; Hedrich, H.; Schoen, P. Surf Interfac.Anal.1987,lO ( 5 ) , 242.
0743-746319512411-2563$09.00/0 0 1995 American Chemical Society
Frisbie et al.
2564 Langmuir, Vol. 11, No. 7, 1995
Scheme 1. Photochemistry of Aryl Azide Containing SAMs
Au-l
H"
c IU
Il-d*
IV
The use of SIMS for surface analysis in this laboratory has been motivated by our recent work in the area of patterned self-assembled monolayers (SAMs).'O There are now several procedures described in the literature for lithographically defining the distribution of molecules in a S A M , and current applications of patterned S A M technology involve microfabrication,ll chemical sensing,12 selective deposition of metals,13 polymers,lob and prot e i n ~ and , ~ ~fundamental studies of adhesion,15 tribol0gy,15J6 and wetting.17 In connection with work on patterned SAMs,it is obviously important to establish the chemical composition of the patterned surface with (10)(a) Wollman, E.W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J . Am. Chem. SOC.1994,116,4395. (b) Rozsnyai, L. F.; Wrighton, M. S. J . Am. Chem. SOC.1994,116,5993. (c) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993,9,1517. (d) Frisbie, C. D.; Wollman, E. W.; Martin, J. R.; Wrighton, M. S. J . Vac. Sci. Technol. 1993,A l l , 2368. (e) Wrighton, M. S.; Frisbie, C. D.; Gardner, T. J.; Kang, D. In Microchemistry:Spectroscopy and Chemistry in Small Domains; Masuhara, H., Ed.; Elsevier Science B.V.: Amsterdam, 1994,p. 495. (0Kang, D.; Wollman, E. W.; Wrighton, M. S. In Photosensitive Metal-OrganicSystems; Kutal, C., Serpone, N., Eds.; Advances in Chemistry Series 238; American Chemical Society: Washington, DC, 1993;p 45. (g) Kang, D.; Wrighton, M. S. Langmuir 1991,7,2169. (11)(a) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994,10, 1498. (b) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994,10,2672.(c)Abbott, N.L.; Folkers, J. P.; Whitesides, G. M. Science 1992,257,1380.(d) Kumar, A.;Biebuyck, H. A.; Abbott, N. L.;Whitesides, G. M. J.Am. Chem. SOC.1992,114,9188.(e) Dulcey, C. S.; Georger, J. H., Jr.; Krauthamer, V.; Stenger, D. A,; Fare, T. L.; Calvert, J . M. Science 1991,252,551. (12)Kumar, A.; Whitesides, G. M. Science 1994,263,60. (13)(a) Dressick, W. J.; Dulcey, C. S.; Georger, J. H., Jr.; Calvert, J. M. Chem. Mater. 1993,5, 148. (b) Calvert, J. M.; Dulcey, C. S.; Peckerar, M. C.; Schur, J . M.; Georger, J. H., Jr.; Calabrese, G. S.; Sricharoenchaikit, P. Solid State Technol. 1991,Oct. 77. (14)(a) Lbpez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. SOC.1993,115,5877.(b) Bhatia, S.K.; Hickman, J. J.; Ligler, F. S. J.Am. Chem. SOC.1992,114, 4432. (c) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J . Am. Chem. SOC.1992,114,8435.(d) Rozsnyai, L. F.; Benson, D. R.; Fodor, S. P. A.; Schultz, P. G. Angew. Chem., Znt. Ed. Engl. 1992,31,759.(e) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. T.; Solas, D. Science 1991,251,767. (15)Frisbie, C. D.;Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.;Lieber, C. M. Science 1994,265,2071.
4P-N-
&
v
VI high lateral resolution. Imaging SIMS is ideally suited for this purpose, because of its sensitivity, high lateral resolution, and ability to detect molecules as well as elements. In our work described here, we have used SIMS to identify the products of photochemical reaction in a photosensitive S A M and have generated high lateral resolution SIMS images which show the distribution of photoproducts in the photopatterned SAMs. Our results add to the growing number of reports in the literature which describe the use of SIMS to image patterned m o n o l a y e r ~ ~ J and ~~,~ toJ detect ~ intact molecular ions arising from unpatterned SAMs.18a-cJg Scheme 1summarizes the chemistry utilized in making patterned SAMs in our laboratory.loa Briefly, photosensitive SAMs, Au-I, are made by immersing polycrystalline Au films into a methylcyclohexane solution of a disulfide which contains two pendant aryl azide moieties. NearU V irradiation (A > 260 nm) of Au-I S A M s through a Cron-quartz mask and a thin film of amine results in the photoattachment of the amine to the Au surface where irradiated, a s shown in Scheme 2. Using this method, a variety of amines with different functional groups can be "written" to the Au surface in any desired pattern. Significantly, no more than one monolayer of photoat(16)(a) Overney, R.M.; Meyer, E.; Frommer, J.; Guntherodt, H.-J.; Fujihira, M.; Takano,H.; Gotoh, Y. Langmuir 1994, 10, 1281. (b) Overney, R.M.; Meyer, E .Frommer, J.; Brodeck, D.; Luthi, R.; Howald, L.; Giinthterodt, H.-J.; FGihira, M.; Takano, H.; Gotoh,Y. Nature 1992, 359,133. (17)(a)Biebuyck, H. A.;Whitesides, G. M.Langmuir 1994,10,2790. (b)L6pez, G. P.; Biebuyck, H. A.;Frisbie, C. D.; Whitesides, G. M. Science l99S,260,647. (18)(a)Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F., Jr. Anal. Chem. 1994,66,2170. (b) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J.Am. Chem. SOC.1993,115,5305.(c) Frisbie, C. D.; Martin, J. R.; Duff, R. R., Jr.; Wrighton, M. S. J. Am. Chem. SOC.1992,114, 7142. (d) Vargo, T. G.; Thompson, P. M.; Gerenser, L. J.; Valentini, R. F.; Aebischer, P.; Hook, D. J.; Gardella, J. A., Jr. Langmuir 1992,8, 130. (19)(a)Offord, D. A.;John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994,10,883.(b) Offord, D. A.; John, C. M.; Griffin, J . H. Langmuire 1994,10,761.(c)Hagenhoff, B.; Benninghoven,A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993,9, 1622. (d) Tarlov, M. J.; Newman, J. G. Langmuir 1992,8,1398. (e) Li, Y.;Huang, J.; McIver, R. I., Jr.; Hemminger, J. C. J . Am. Chem. SOC.1992,114,2428.
SIMS of Patterned SAMs
Langmuir, Vol. 11, No. 7, 1995 2565
Scheme 2. Methodology for Preparing Photopatterned SAMs" Polycryrtallinr Au
t
Patterned Irradiation
I t
nv
m w
?? ? ? ????
tpvvvvvv
a The circles labeled "PS" represent the photosensitive aryl azide functional groups. The triangles labeled "trap" correspond to a functionalized amine reagent which reacts with aryl azides under UV-irradiation, as shown in Scheme 1.
tached amine is present in the patterned areas.loa Infrared spectroscopy ofAu-I substrates irradiated in the presence of diethyl amine, 11, has established that the surface photochemistry produces two different primary photoproducts on the surface, namely the 3H-azepine and hydrazine species shown in Scheme 1. The 3H-azepine and hydrazine photoproducts are 1:lphotoadducts in that only one amine is associated with each anchored thiolate. In an earlier paper, we detailed our initial results on the analysis of photopatterned Au-I S A M s by SIMS.lod In the work described here, we have used SIMS to directly detect molecular ions, M+, with masses corresponding to the intact 3H-azepine and hydrazine photoadducts on the surface. Molecular fragment ions are also observed from the irradiated surfaces which confirm the presence of the 1:l photoadducts indicated in Scheme 1. The mass assignments in the secondary ion mass spectra are verifed by a series of isotopic labeling experiments. We also show here that imaging SIMS can be used to map the elemental and molecular species distribution in patterned SAMs with 1,um lateral resolution.
Experimental Section Materials. CsI, RbI, KI, NaI, and LiI used in calibrating the Fisons (VG) M70S SIMS instrument were obtained from Alfa Products or Mallinckrodt. Diethylamine (11)and dibutylamine (111)were purchased from Aldrich and were distilled from KOH before use. The syntheses of bis(ll-(4-azido(benzoyloxy))-lundecyl) disulfide (I),(2-(2,6-dichloro-phenyl)ethyl)propylamine (IV),(2-ferrocenylethyl)(2',2',2'-trifluoroethyl)amine 0, and 1,ldihydropentadecafluorooctylamine (VI), have been previously reported.lo8 Methylcyclohexane for derivatizing Au slides with I or I-&was used as purchased from Aldrich. Hexane for rinsing
the derivatized Au substrates was purchased from Mallinckrodt and used without further purification. Ethanol was obtained from Pharmco. Synthesis. 4-AminobenzoicAciddz. Deuteration of 4-aminobenzoic acid was accomplished by stirring the acid (3 g, 14.2 mmol) in 50 mL of DzS0&0 (20% v/v) overnight followed by heating to 90 "C for 2 h. The pH of the solution was increased to pH = 6 by addition of aqueous saturated solution of NazC03 and the solution extracted with THF. Evaporation of the solvent yielded 3 g of 4-aminobenzoic acid-dz. The off-white solid was used in the next step without further purification. lH NMR spectrum (CDsCN, 6): 8.0 ppm (br s, fwhm 15 Hz). 4-Azidobenzoic Acid-da. To a stirred solution of 4-aminobenzoic acid-dz (2.8 g, 21 mmol) in 5.5%DC1 in DzO at 0 "C was added a NaNOz solution (1.8 g, 27.5 mmol, 10 mL HzO)over 20 min. White foam formed on addition of a solution of NaN3 (1.5 g, 23 mmol, 10 mL) over 10 min. The foam was isolated by filtration, dried, and recrystallized from ethanol-dl to obtain a white solid, 2.2 g. lH NMR (CD&N, 6 ) : 8 ppm (br s, fwhm = 15Hz). 2HNMR (6, referenced to the peak at 3.58 ppm of THFds) 7.1 (br s). IR(THF, cm-l): (azide) 2120, 2097, (acid) 1719, (arene) 1590. Mass spectrum (mlz,% of m/z 138): 166 (401,138 (loo), 119 (30), 110 (30), 94 (40), 65 (75), 53 (0.41, 38 (35). 4-Azidobenzoyl Chloride-dz. Thionyl chloride (30 mL, excess) and 2 drops of dimethylformamide were added to 4-azidobenzoicacid& (1.7g, 10.3mmol)in a 50 mL round bottom flask with stirring, and the mixture was heated 2 h at reflux. Thionyl chloride was removed in uucuo, and the residue was taken up in hexane and filtered and the hexane removed in vacuo to obtain tan solids (1.7 g). IR (THF, cm-l): (azide) 2129,2113, (acid chloride) 1764, (arene) 1582. bis(ll-(4-azido(benzoyloxy))-l-undecyl) Disulfide-& (I&). Synthesis of I-d4 was accomplished by addition of bis(l1hydroxyundecyl) disulfide (1 g, 2.5 mmol) to 4-azidobenzoyl chloride42 (1.4g, 7.6 mmol) and 1equiv of triethylamine in THF and heating to 50 "C in the dark for 48 h. The product was recrystallized from hexanes and from methanol-ethanol (9:l)to obtain white flakes (1.2 g). Spectroscopicdata: IR (THF,cm-l): (azide)2125,2095, (ester) 1720,(arene) 1590. lH NMR (acetonede, 6): 8.02 (4H, br s), 4.26 (4H, t), 2.7 (4H, t), 1.85-1.6 (36H, overlapping m). Calculated mass (M + H): 701.3817. FABMS (3-NBAmatrix, positive ion, mlz): 701.3824. 1,l-Dideuterodiethylamine(II-dz). N-Ethylacetamide was reduced with LiAID4 by refluxing in diethyl ether 18 h. The reaction was filtered and distilled. Unreduced N-ethylacetamide was recovered (50%)from the residues. The amine and diethyl ether azeotroped. The amine was isolated as the ammonium chloride salt by bubbling HC1 through the distilled ethereal solution. The white solids were filtered and dried t o obtain a white hydroscopic solid. lH NMR (DzO, 6): 2.9 (4, 2H), 1.1 (superimposedbr s, t, 6H). The solid was dissolvedin aminimum of water and the solution saturated with NazC03 with an equivalent of KOH, and the oily layer was separated and distilled (bp 55-57 "C) from KOH in 30%overall yield. lH NMR (benzene, 6): 2.5 (q,2H,J = 7.5 Hz), 1.05(t,3H, J = 7.5 Hz), 1.0 (br s, 3H), 0.35 (br s, 1H). EIMS (mlz): 75 (MI, 60 (M- 15). Formation of SAMs of I and I-dz. Polycrystallipe Au films were prepared by electron beam evaporation of 50 A of Ti onto a Si3N4 coated Si wafer (100) as an adhesion layer followed by 1000 A of Au. The Au coated wafers were fractured into 1.5 x 0.75 cm2slides and these were immersed in a 1mM methylcyclohexane solution of the disulfide, I. The Au slides were allowed to soak for at least 5 h before use. The same procedure was followed to derivatize Au with I-dr. Mask Fabrication. The Cr-on-quartz mask was fabricated at the Microelectronics Research Laboratory at MIT and was originally designed for use in the fabrication of microelectrodes by standard lithographic techniques. The feature sizes on the mask range from 2 to 100 pm. An optical micrograph of the mask is shown in Figure 6a. The array of 2 pm lines at the center of the micrograph are not visible. PhotopatterningofAu-ISAMs withhines. Amines lV, V, or VI were photopatterned onto Au-I S A M s as shown in Scheme 2. Briefly, a 1.5 x 0.75 cm2Au substrate was removed from the methylcyclohexanesolution of I and rinsed with copiousamounts of ethanol, hexane, and ethanol again and then blown dry with Nz. A drop (-0.5 mL) of the desired amine was placed on the
Frisbie et al.
2566 Langmuir, Vol. 11, No. 7, 1995 Au-I substrate and the Cr side of a Cr-on-quartz mask was placed in hard contact with the slide on top ofthe amine film by clipping them together. This sandwich assembly was irradiated through the mask for 90 s with filtered light (A > 260 nm) from a highpressure Hg lamp (Bauschand Lomb SP 200). The filter consisted of a 10 cm quartz tube filled with an ethanol-ethyl acetatewater ( 1 : l : l )mixture. The Cr thin film pattern on the mask effectively blocked light from hitting the Au surface directly beneath the pattern. After 90 s the assembly was removed from the light, the mask was removed, and the Au substrate was washed sequentially in ethanol and hexane and then allowed to soak for 5 min in ethanol before it was removed and blown dry with Nz. This final soaking procedure ensured complete removal of unreacted amine from the surface. These procedures resulted in the photoattachment of one amine onto the Au-I monolayer whereever light struck the surface. Two amines were patterned onto the same surface as follows: An Au slide was irradiated through a mask and a thin film of the desired amine, in the manner described above (Scheme 2). After completion of the first irradiation, the slide was rinsed, blown dry, and placed in a quartz cuvette filled with the second amine. No mask was placed in contact with the Au, but rather the entire derivatized Au surface was again irradiated through the cuvette. After 90 s of irradiation (1> 260 nm), the Au slide was removed from the cuvette and rinsed sequentially with ethanol, hexane, and ethanol and then blown dry with Nz. This procedure photoattached the second amine selectively to the areas of the Au surface where the first amine was not present. Two amines could therefore be written t o adjacent portions of the Au surface. Preparation of Samples for Survey Analysis by SlMS. Unpatterned SAMs of the diethyl- and dibutylamine photoadducts were produced for SIMS survey analysis. Au-I or Au-I-& substrates were removed from the methylcyclohexane derivatizing solution, rinsed and blown dry as described above, and then were immersed in a quartz cuvette filled with the amine (I, I-&, or 111) and photolyzed for 90 s (1 > 260 nm). After irradiation, photolyzed substrates were removed from the cuvette and rinsed liberally with ethanol and were blown dry under Nz. Secondary Ion Mass Spectrometry. All spectra and images were acquired using a Fisons (VG)IX70S magnetic sector SIMS instrument.20 Samples were protected from light during transfer in air to the spectrometer. The 16 or 25 keV Ga+ions were used for the primary beam for all data reported here. Typical instrument operating pressure was 1 x 10-9 Torr. The instrument was calibrated using a mixture of CsI, RbI, KI, NaI, and LiI salts which were ground with a mortar and pestle, dissolved in a 3:l CH30H-H20 solution and dispersed onto a polycrystalline Au film. A positive SIMS spectrum of this sample yields a number of peaks corresponding to cluster ions with masses ranging thousands of amu.21 These cluster ion peaks were used to generate a magnet calibration curve. Survey spectra were generated in the following manner. Two scans, 0-500 amu, were collected in three different 2 x cm2 areas on each sample, for a total of six scans, and averaged together to create a composite spectrum. Each scan required 21 s, yielding a total collection time of 2 min, 6 s for the six scans. The 16 keV Ga+primary ion beam was rastered at video frequency over each 2 x 10-3 cm2 area. Primary ion current density measured at the sample was 25 &cm2 and total ion dose to each area was 6.6 x 10l2primary ions/cm2. Mass resolution (mlAm) for all spectra was 500. Imaging SIMS was done using the Ga+beam operating at 16 or 25 keV and at current densities ranging from 25 to 2500 nA/ cm2depending on the magnification used. The maps consist of 256 x 256 pixels. Dwell time was usually 400 pslpixel, so that total acquisition time for each map was 26 s. Typical maximum signal during acquisition of the F element map at 25 &cmZ primary beam current density was 500 cps. (20) For a descriptionofthe instrument, see: Bayly,A. R.;Cummings, M.; Vohralik, P.; Williams, K.; Kingham, D.R.; Waugh, A. R.; Walls, J. M. In Secondary Ion Mass Spectrometry, SIMS 6, Proceedings of the 6th International Conference;Benninghoven,A., Huber,A. M., Werner, H. W., Eds.; John Wiley and Sons: New York, 1988; p 169. (21)(a) Burlak, T. M.; Campana, J. E.; Colton, R. J.; Decorpo, J. J.; Wyatt, J. R. J. Phys. Chem. 1981,85, 3840. (b) Sato, T.;Asada, T.; Ishihara, M.; Kunihiro, F.; Kammei, Y.; Kubota, E.; Costello, C. E.; Martin, S. A.; Scoble, H. A.; Biemann, K. Anal. Chem. 1987,59, 1652.
Survey spectral data were converted to ASCII and downloaded to a Macintosh IIci where they were processed using the software package, Igor. Image data were also transferred to a Mac and opened in NIH Image. A “DCoffset”in the images generated by the SIMS acquisition software was subtracted from all element and molecular fragment maps. The vinyl ferrocene fragment map in Figure 5 was subjected to a smooth. Compositeline scans for the images in Figures 4 and 5 were produced using NIH Image and represent the sum of the 256 horizontal lines which compose the image. Coverage ratios were determined from the images by dividing the average pixel intensity in the bright portions of an image by the average pixel intensity in the dark areas.
Results and Discussion The surface photochemistry ofAu-I has been established via reflectance FT-IR studies which show the presence of both a 3H-azepine and a para-substituted hydrazine, as shown in Scheme 1, when Au-I S A M s are irradiated in the presence of diethylamine, II.loaWe are interested in establishing whether SIMS may also be used to characterize the surface photochemistry of aryl azide SAMs. In particular, detection of the molecular ion for the photoproduct (the azepine and hydrazine have identical masses) would provide further confirmation of the photochemistry depicted in Scheme 1. To facilitate the unambiguous assignment of ions in our secondary ion mass spectra of photolyzed Au-I SAMs, we synthesized a deuterated analog of I, I-dd, and used this reagent to prepare Au-I-dz
D
I-d4
SAMs. Figure 1show the positive SIMS spectra of Au-I (Figure la) and Au-I-dz (Figure lb), obtained using 16 keV Ga+ ions and a dose of 7 x 10l2primary ions/cm2. It is apparent from the figure that significant molecular fragments arising from Au-I and Au-I-dz S A M s are detected at mlz 90,118, and 146 and mlz 92,120, and 148, respectively. The structures drawn represent our best estimation of the actual fragment ion structure. The important point is that the peak assignments made in Figure l a are supported by the shift of two mass units for the analogous peaks in Figure lb. Two additional high mass peaks are of interest. The peak a t mlz 197 corresponds to Au+, arising from the Au substrate. The peak mlz 354 appears to be a contaminant in that we often find it on underivatized Au substrates. We have also observed this ion when studying SAMs other than Au-I or Au-I-dz by SIMS, but we have no assignment for it as yet. A series of photolyses of Au-I or Au-I-dz S A M s in the presence of amines 11, II-dz, and I11 was undertaken to determine whether peaks corresponding to azepine a n d or hydrazine photoproducts could be observed by SIMS. Figure 2 shows the positive SIMS spectra for Au-I S A M s irradiated in the presence of I1 (Figure 2a) and Au-I-dz SAMs irradiated in the presence of I1 (Figure 2b) and 11-dz(Figure 212). The most important finding is the clear presence of the molecular ion peak for the intact photoadduct (either the azepine or hydrazine) a t mlz 393 in Figure 2a which is shifted by 2 amu to mlz 395 in Figure 2b and shifted by 4 amu to mlz 397 in Figure 2c, as expected. Detection of the molecular ion by SIMS, in conjunction with our previous FT-IR data firmly establishes the photochemistry depicted in Scheme 1. We note
SIMS of Patterned SAMs
Langmuir, Vol. 11, No. 7, 1995 2567 too
40
20
0
100 D
200
500
mlr
A e)
400
300
‘ 0 0 . 1
4
6 400
0
100
200
300 mlZ
Figure 1. Positive SIMS spectra of Au-I and Au-I-& SAMs showing characteristic fragment ions. The assignments made in (A) are supported by the isotopic shifts observed in (B). Each spectrum was acquired using 16 keV Ga+ ions and a total dose of 7 x lo1* ions/cm2. M/AM = 500. t h a t the molecular ion peaks a t mlz 393, 395, and 397 were not observed from every sample that we studied. The spectra in Figure 2 represent a consistent set of data which were obtained after analyzing many samples by SIMS. On some samples we detected peaks corresponding to the molecular ion plus 2H instead of the molecular ion itself. These observations and the variability in our detection of the molecular ion may be related to changes in instrumental parameters, such as collection efficiency or primary ion dose, as well a s sample preparation. Nevertheless, observation of the molecular ion is important insofar as it demonstrates that SIMS analysis ofmolecular surfaces parallels conventional mass spectrometry which typically identifies the molecular ion of gas-phase molecules. Peaks corresponding to some portion of the original Au-I o r Au-I-dz S A M and the photoattached amine are also detected, a s shown in Figure 2. For instance, a major peak is observed a t mlz 163 in Figure 2a which shifts by 2 amu to mlz 165 in Figure 2b, and by 4 amu to mlz 167 in Figure 2c, indicating that the ion contains a t least a portion of the arene ring of the original Au-I S A M and the diethylamine, 11. We have assigned this peak in Figure 2 as the azepine ring plus I1 or as the hydrazine plus 11, as the two photoadducts have identical masses. For simplicity, we have only shown the azepine structures in Figure 2. Another significant fragment is observed a t mlz 209 in Figure 2a which also shows the corresponding shift of 2 amu to mlz 211 in Figure 2b and 4 amu to mlz
213 in Figure 2c. This ion likely arises from a “McLafferty 1”rearrangement which is a characteristic decomposition of esters in conventional electron impact mass spectrometry.zz We also observe an important peak a t mlz 362 in Figure 2a which is not labeled. We assign the peak at mlz 362 a s the molecular ion less S plus H. As expected, the peak at mlz 362 shifts by 2 amu upon dideuteration of the arene ring (Figure 2b) and by another 2 amu upon dideuteration of the incoming diethylamine (Figure 2c). Ions a t mlz 163, 209, 362 and their corresponding isotopically labeled analogs in parts b and c of Figure 2 were observed for every sample studied and allow the conclusion, even in the absence of the molecular ion peak, that a 1 : l photoadduct is formed upon irradiation of Au-I SAMs. Control samples in which Au-I and AuI-dz S A M s were immersed in I1 and kept in the dark yielded SIMS spectra very similar to those shown in Figure 1and not those in Figure 2, indicating that no dark reaction occurs, as has been shown previously, and confirming that the peaks observed in Figure 2a-c arise from photochemical changes in Au-I or Au-I-dz SAMs. A more qualitative inspection of the spectra in Figure 2 (and Figure 3)indicates that the majority ofions detected have relatively low masses (
