Surface Sensitivity of Electron Energy Loss Spectroscopy and

Chapter 19

Surface Sensitivity of Electron Energy Loss Spectroscopy and Secondary-Ion Mass Spectrometry of Organic Films 1,4

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M. Pomerantz , R. J. Purtell , R. J. Twieg , S.-F. Chuang , W. Reuter , Β. N. Eldridge , and F. P. Novak 2,6

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Downloaded by GEORGETOWN UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: July 16, 1994 | doi: 10.1021/bk-1994-0561.ch019

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Chemical Physics Department, Weizmann Institute of Science, 76100 Rehovot, Israel Thomas J. Watson Research Center, IBM Research Division, Yorktown Heights, NY 10598 IBM Almaden Research Center, Almaden, CA 95120

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Electron energy loss spectroscopy (EELS) and secondary-ion mass spectrometry (SIMS) are particularly useful for the study of surfaces because they give information about a very shallow depth (of order nm) of the sample. Precisely what these depths are in organic solids has been difficult to measure because the surface needs to be highly regular. We used Langmuir-Blodgett layers on silicon wafers. The films proved to be well packed and smooth. The chemical nature of the films was arranged to place "markers" at specific depths, which aided in the analysis of the data. We derive remarkably small infor mation depths: less than 0.4 nm for EELS and 2 nm for SIMS. In order to understand effects at an interface it is necessary to determine the chemical composition of the surface. Fortunately, there are a number of analytical techniques that are sensitive, to a greater or lesser degree, to the molecules at the surface and which do not see into the bulk. In this paper we report our measurements and in terpretations of the depth sensitivities of two techniques of chemical analysis that have high surface sensitivities. Electron energy loss spectroscopy (EELS) measures the frequencies of the mo lecular vibrations, and thus may be used to identify molecules. Its surface sensitivity arises from the fact that electrons of about 5 eV in energy are used. These have a short mean free path in matter. The scattering lengths have been extensively studied in inorganic solids (1), but the information depths reported for organic solids show wide variations. Values as different as about 2.5 nm (2) and 0.4 nm (3) have been reported. We shall attempt to understand this discrepancy. 4 5

Current address: Cedar Lane Apartments, Ossining, NY 10562 No longer at this address. Current address unavailable. Current address: Brookdale Road, Mahopac, NY 10541

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0097-6156/94/0561-0216$08.00/0 © 1994 American Chemical Society

Mallouk and Harrison; Interfacial Design and Chemical Sensing ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

EELS and SIMS of Organic Films

19. POMERANTZ ET AL.

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Secondary-ion mass spectrometry (SIMS) reveals the composition of the surface by measuring the masses of the ions ejected from the surface under bombardment by ions of keV energies. The damage that is done by the incident ions can be made tolerable by the use of the highly sensitive time-of-flight analysis of the mass spectra (4, 5). This is called "static SIMS" because the damage is to less than a monolayer and the sample is effectively constant. Here, also, the reported depth sensitivities have varied widely, between 1 nm (6) and 12 nm (7, 8). We wish to understand this as well. Part of the problem in making measurements of the surface of an organic solid is in preparing a surface that is uniform, i.e., in both depth and along the surface. With organic solids this is particularly difficult because cleaving or polishing tech niques are usually not applicable. As model systems, Langmuir-Blodgett films (9) have many advantages. A single molecular layer is spread and compacted on the surface of water. It is then transferred to a solid substrate. The chemical identity of this monolayer is thus guaranteed. To produce a smooth film it is necessary to use a well-compacted film on a smooth support. This is where the previous works have had the widest variations. The film should also be free of holes in order that an intrinsic depth can be measured. This may be achieved if the films are of rather straight molecules and sufficient pressure can be applied to pack the molecules tightly without collapsing the films. This gives the desired uniformity along the surface. The smoothest commonly available substrates that we know are electronic grade Si wafers. Langmuir-Blodgett films on Si have been shown to be very smooth (10) and the films seem to bridge over the roughness and thus are smoother than the substrates. Langmuir-Blodgett films offer a natural means to achieve controlled chemical variations with depth of the film. Molecules of different types may be deposited sequentially, thus making a profile of known chemical composition on a length scale of the thickness of a molecular layer. To make a chemical profile on a smaller scale of depth, one can make substitutions within the molecule of a single layer. We utilize both of these approaches in the present work. Experimental methods In order to have layers with distinctive chemical entities at known positions, one family of molecules we chose was the fluorine substituted alkoxybenzoic acids, of the general form F-(C-F ) -(C-H ) -0-Ph-COOH where "Ph" represents a benzene ring connected to the para substituents. They can be prepared with varying numbers of carbons that arefluorinated,n, or hydrogenated, m. We refer to the compounds as FnHm. (The number of fluorines is 2n+l.) When deposited as Langmuir-Blodgett films, the fluorinated end is on the outside. For the EELS measurements we are interested in the extent to which the fluorinated segment acts as a shield between the incoming electrons and the C-H groups further down the chain. As a comparison molecule we also employed the unfluorinated analogs denoted by FOHm, i.e., H - (CH ) - Ο - Ph - COOH. For the SIMS experiments, the mass of the F is a distinctive marker. In addition, for the SIMS we used perdeuterated stearic acid layers. The mass of the deuteron is another distinctive marker in the mass spectrum. The details of the layers will be given below. 2

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INTERFACIAL DESIGN AND CHEMICAL SENSING

The semifluorinated alkoxybenzoic acids were prepared by the following general sequence of reactions: First; ethyl 4-hydroxybenzoate was reacted with an ω functionalized (I, Br, or OH) 1-alkene to give the terminal olefin alkylated benzoate. In the case of the halogen terminated olefins, the ether linkage was formed by alkylation using potassium carbonate in N-methylpyrollidinone. In the case of the alcohol terminated olefins, the ether linkage was formed by a Mitsunobu coupling reaction with diethylazodicarboxylate and triphenylphosphine in tetrahydrofuran. Second, the olefinic material was reacted with the perfluoroalkyl iodide catalyzed by AIBN, 2,2'-azobisisobutyronitrile, or triethylborane. Third, the resulting material was dehalogenated with zinc and HC1 in ethanol or Bu SnH. Last, the ester was saponified with alcoholic base to give the final semifluorinated alkoxy benzoic acid. Details of the preparation will be given elsewhere (11). Orientation of the molecules as a surface film was produced by the LangmuirBlodgett method (9). Amphiphilic molecules are spread on the surface of water. The molecules naturally align with the acid (-COOH) ends into the surface of the water. A movable barrier on the surface of the water is used to apply pressure suf ficient to obtain the closest packing possible, without collapsing the film. Under continuous pressure the film is then transferred from the water surface to the substrate by smoothly inserting and removing the substrate from the water. The di rection of the meniscus confirms that the ionic ends of the molecules attach to the Si and the hydrophobic ends face away from the surface, as usual. We believe it is an important point of technique that we used polished Si wafers (electronic grade, 0.1 Ω-cm, p-type). The smoothness of Langmuir-Blodgett films on these substrates is about 0.3 nm and seems to improve when multilayers of Langmuir-Blodgett films are deposited on them (10). The wafers were cleaned by scrubbing with detergent, thorough washing with water and spin drying. (We remark that no traces of the cleaning solution or ions from the bath, such as CI, were found by secondary ion mass spectrometry, as can be seen in the SIMS spectra shown below.) The Langmuir-Blodgett films of the alkoxybenzoic acids were all made in the same way: each of the FnHm molecules were spread in hexane solution onto a water bath containing about 10" M A1C1 , held at a pH of about 5.5. The A l reacts with the COOH to form an ionic compound (12, 13). We used Al as the counterion here because these fluorinated molecules collapsed at low surface pressures when the usual divalent ions were used in the bath. With trivalent Al they are remarkably strong; the films were capable of withstanding much more than the 30 dyne/cm. pressure applied during deposition. This is a sign that good packing may be achieved, which is important in avoiding pinhole artifacts. The stearic acid and perdeuterated stearic acid (D-(C-D ) -COOH) are the purest commercially available and were used without further purification. These molecules were spread in a concentration of 1 mg per ml hexane solution on a bath containing about 10" M MnC^ at a pH of about 6.6. The Mn replaces the carboxylic acid hydrogen. Thus the perdeuterated layer contains no hydrogen. The deuterium thus serves as a marker isotope for the SIMS. Thesefilmswere deposited at a surface pressure of about 20 dynes/cm. We shall refer to them as "MnDSt".


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POMERANTZ ET AL.


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Information Depth of EELS In our experiments, the electron energy loss spectra were measured in a LeyboldHeraeus ELS 22 spectrometer, with incident electron energy of 5 eV. We report data taken under specular conditions, at an incident angle of 60° from the normal. The current density was about 5 χ 10" amp/mm , and the pressure was about 10" Torr. The energies of the reflected electrons were analyzed with a resolution of about 7 meV. The energy lost by an incident electron, ΔΕ, is transferred to an excitation of a molecular vibration. The EELS spectrum measures the number of electrons that have a specified energy loss. We wish to measure the "information depth" to which a particular molecular vibrator can be detected, by the characteristic energy loss it creates. It should be understood that an electron may lose energy to a vibration, but not contribute to an EELS spectrum if it suffers additional scatterings that knock it out of the detection windows set for the instrument. The information depth thus depends on both the energy resolution and angular resolution of the instrument. An experimental approach to the measurement of the information depth, ID, is as follows. The molecular vibrator, whose frequency is known, is placed at a known distance below the surface. The fraction of electrons that lose energy to this, and only this, vibrator is measured as a function of the number of covering atoms. The cov ering thickness that reduces the relative intensity of this energy loss by a factor 1/e is a measure of the information depth to which this vibration can be detected. Interestingly, the information depth depends on the mode of excitation of the marker. If the marker is excited by the dipolar mechanism, which is long-range, the depth is predicted to be about 3 nm (14). If the excitation is by an impact mech anism, the information depth will be influenced by the electron meanfreepath in the solid. There is abundant evidence (15, 16), and our experiment reported here con firms, that the common C-H vibration of organics is excited by impact. The C-H vibrators are an important example for obtaining the ID in an impact scattering process. For these experiments we used the semi-fluorinated alkoxybenzoic acids as the Langmuir-Blodgett film. The entire 1 inch wafer was covered with one monolayer of film. Ellipsometry gave values of about 1.6 nm for the thickness of the monolayer, which is reasonable for molecules of this length standing on the surface. We use the excitations of the buried C-H groups as the indicator vibrations. The information depth of the scattered electrons is determined from the number of fluorinated carbons at the ends of the molecule, i.e., the coverage, which reduces the signal from the C-H group. The C-F stretch vibration excitation is at an energy of about 140 meV, which is much different from the energy of the C-H stretch, 360 meV, and so cannot be confused with it. (See Figure 1). The data in Figure 1 are for single monolayers of FnHm molecules on Si wafers. The electron energy loss spectra in this series of samples are normalized to their respective elastic peaks. Thus we are measuring the information depth by the changes in the CH stretch losses relative to the elastic peak. By using this procedure, we have neglected changes in the overall spectral intensities due to such things as roughness and ordering variations from sample to sample. These effects may change


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the information depth we deduce but we presume them to be small because the packing is determined by the bulky benzene ring, which is the same for all the molecules. Curve a is for the unfluorinated molecule F0H16. It shows a strong loss at about 360 meV, characteristic of the C-H stretch (17); this will be our marker vibration. There are also broad losses centered at about 150 meV due to a variety of C-H and C-C vibrations. When the Langmuir-Blodgett film is of the molecule F2H8, the two outer carbons arefluorinated.The EELS spectrum, curve b, shows a relatively smaller C-H stretch loss compared to curve a. This occurs because the C-H groups are now shielded by the overlying C-F groups, but perhaps also because there are fewer C-H groups. This uncertainty is decided by the measurements on films of molecules F4H6 (with 4 fluorinated C's on the outside), F6H6 (with 6 fluorinated C's on the outside) and F8H8 (with 8 fluorinated C's), cf., curves c, d, and e of Figure 1. The numbers of C-H groups are not much changed among the last three molecules, but the intensities of the C-H losses decrease noticeably. This is shown in Figure 2, in which the relative intensities of the C-H stretch losses are plotted vs. the number of fluorinated C's. From this figure we deduce that there is an attenuation of the signal from the C-H stretch by about a factor of 1/3 (about e") for each pair of C-F groups between the C-H groups and the electron beam. This immediately gives a remarkably short information depth of about 0.3 nm, the length of two C-C bonds. This is conclusive evidence that the excitation of the C-H groups is by impact scattering. In the dipolar process, the range of the interaction is ten times larger, about 3 nm (14). If the scattering were dipolar, the interposing of even eight C-F groups would not much reduce the interaction with the underlying C-H groups. The result for the information depth is to be regarded as an upper limit because there may be extrinsic effects, such as holes in the film or poor packing, etc., that could lead to the uncovering of some C-H groups. But since our result is that the excitation depth is only about 2 atoms, the extrinsic effects must be virtually nil. Variations in overall signal level in the specular direction from sample to sample would also change our result. However, as noted above, within this group of samples we do not expect a large variation in surface ordering or roughness, which would change the relative elastic specular scattering intensity. There is evidence from our computer models of the packing (18) that some C-H is exposed even with near optimal packing for the case of 2fluorinatedcarbons. This may explain why there is less effect of the twofluorinatedC in F2H8 in reducing the C-H signal than expected by comparison with longerfluorinatedchains. (See Figure 2). Our result, that the excitation of the C-H vibration is reduced e-fold by two (C-F ) groups, or ID < 0.3 nm, is in reasonable agreement with the result of Schreck, et al (3). It is remarkable that, despite being contained in a Van der Waals solid, vibrators beneath a mere two atomic layers begin to be hiddenfroman EELS measurement. 1

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Figure 1. EELS spectra of single Langmuir-Blodgett films of the Al salts of FnHm molecules on Si wafers. Curve a is F0H16, b is F2H8, c is F4H6, d is F6H6, e is F8H8, which is in the order of increasing coverage of the C-H groups by C-F. The ordinate is the number of counts normalized to the number of counts at the peak of the elastic scattering, for η = 0. The rationale for the normalization is given in the text. The abscissa is the energy loss by the electrons in the specularly scattered beam.

NO. OF FLUORINATED CARBONS

Figure 2. The relative number of electrons which lose energy to a C-H vibration, vs. the number of fluorinated carbons above the C-H groups. For the normal ization, see the caption of Figure 1.



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Information Depth in SIMS In the SIMS experiments we use a similar strategy to determine the depth from which secondary ions are detected. We cover a marker, this time a distinctive mass, with another distinctive layer of material. In SIMS, the length scale is larger than that in EELS; it is of the order of several nm rather than fractions of a nm. Thus the marker and covering masses are not chosen to be in the same molecule, but are placed in different molecular layers. One sample was constructed using the molecule F4H6, F-(C-F ) -(C-H ) -O-benz-COOH. Here fluorine is a distinctive mass marker. The covering film was per-deuterated manganese stéarate, (D-(C-D ) -COO) Mn. The deuterium serves as a marker isotope. Three layers of the F4H6 films were deposited over the entire Si surface, and two layers of MnDSt films were coated over the F4H6, but only over half of the surface (see inset of Figure 3). This small innovation in sample design allows us to avoid one of the problems with SIMS measurements, that of comparing samples made at different times and measured in different runs. With this dual sample, we measure the SIMS of the three layer part, and then with no change in the SIMS parameters, we move the sample a small distance. We can then observe the effect of the additional two covering layers on the SIMS of the continuation of the same three layer sample. Our static SIMS system (19) operated with an Ar energy of 10 KeV. UHV conditions were maintained. The resolution of the instrument is about 1 part in 17,000. In Figure 3 is shown part of the mass spectrum of positive ions ejected from the sample described above and illustrated in the inset. Therightside is taken from the area covered only with F4H6. It shows a very strong peak (>24,000 counts) for a largefragmentof F4H6 with no Al but a H added. On the left side, the same mass region is shown, as observed from the area in which the F4H6 is covered with two monolayers of MnDSt. The peak due to the fluorinated ion is only about 1000 counts, for the same counting time. Thus, the two MnDSt layers reduce the emission by a factor of > 24. The other peaks on this part of the sample correspond to fragments from the MnDSt films. One might ask whether the large attenuation of the ejection of the entire F4H6 molecule is also found for smallerfragments.In fact, the attenuation ratio is similar for lighterfragmentsas well, as illustrated in Figure 4. This shows the spatial variation of the (C F ) ion as the sample is moved to direct the beam on the covered and the uncovered F4H6 film. Noting the log scale of intensity, it is found that the (C F ) ions decrease by a factor of 18. This gives an attenuation of a factor of 4 per layer, rather similar to the factor of 5 per layer for a full F4H6 molecule which has about a four fold larger molecular weight. In the region of the MnDSt film we observe, of course, the appearance of deuterated ions, such as the (C D ) . The attenuation of even smaller fragments, such as the Si from the substrate, are also strongly attenuated by as few as two monolayers. In Figure 5 is shown the low mass part of the spectrum, from an area covered by one monolayer of (hydrogenated) MnSt, (data on the left), and an adjacent area covered in addition by two layers of MnDSt, or a total of three layers. With the single layer, the signal of Si at mass = 28 daltons saturates the detectors and is offscale at 8xl0 counts; 2

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Figure 3. The heavy mass part of a SIMS spectrum of 3 Langmuir-Blodgett films of the Al salt of F4H6, on a Si wafer, is shown to the right side. The spectrum on the left is of the same range of masses, from the samefilms,but covered with two Langmuir-Blodgettfilmsof Mn deutero stéarate.


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Figure 4. The same sample as Figure 3, but the dependence of emission of light mass fragments is depicted as a function of position as the edge of the Mn deutero stéarate film is moved under the ion beam.




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Surface Sensitivity of Electron Energy Loss Spectroscopy and

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