Controlled Field Evaporation of Fluorinated Self-Assembled

Letter pubs.acs.org/Langmuir

Controlled Field Evaporation of Fluorinated Self-Assembled Monolayers Andreas Stoffers, Christian Oberdorfer, and Guido Schmitz* Institute of Material Physics, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany S Supporting Information *

ABSTRACT: Self-assembled monolayers of amino-undecanethiol and perfluoro-decanethiol are studied by atom probe tomography based on laserassisted controlled field desorption. In the case of hydrogenated chains the identification of detected molecular species is difficult because of residual hydrocarbons. By contrast, fractions of the fluorinated chains can be unequivocally identified. Although chemically similar, the evaporation of both chains appears in significantly different molecular fractions. For the fluorinated chains, a well-ordered evaporation sequence is determined that allows conclusions to be drawn about the strength of bonds under field conditions and may lay the basis for the future numerical reconstruction of the chemical structure of such films.

■

INTRODUCTION Atom probe tomography (APT) represents a powerful tool in thin film analysis on the subnanometer scale.1,2 Laser-assisted field evaporation has opened the method to semiconductors and nonconductive ceramic materials, but the analysis of soft and biological matter remains an area in which the method still has to be explored more intensively, before final conclusions about its capability should be drawn. This letter contributes to the latter important issue. Self-assembling monolayers (SAMs) are organic thin films that are deposited upon a variety of substrates by the selforganized chemisorption of specific molecules. SAMs attract particular attention because they may provide specific chemical functionality to arbitrary surfaces. Toward this aim, the constituent molecules consist of a specific reactive center that adheres to the respective substrate, a short polymeric chain that controls the thickness of the formed layer, and an end group that provides the desired chemical function.3 Au is a particularly suitable substrate. The low reactivity of the noble metal prevents oxidation that could otherwise compete with the desired chemisorption of the molecules. Reliable bonds to Au substrates are formed by thiol groups (S−H). The bond strength in short organic molecules and SAMs under a thermal or mechanical load has already attracted attention in recent theoretical work motivated by atomic force microscopy.4 In APT,5 samples of sharp needles of a 20 to 50 nm apex radius are prepared. Owing to a small radius of curvature, rather high electrical fields (typically 30 V/nm) are induced at the tip apex if a moderate dc voltage in the range of a few kilovolts is supplied. By careful control of the field, the activation barrier for desorption is partially reduced so that small additional activation by short, high voltage or laser pulses can trigger the desorption of a single atom or a small molecule from the © 2011 American Chemical Society

sample surface. By continuous evaporation, time-of-flight spectroscopy, and calculations of trajectories of the detected species, the spatial arrangement of the different components inside the sample is numerically reconstructed. In this way, a microscopic analysis of outstanding resolution in all three dimensions is obtained. In contrast to alternative methods of mass spectrometry, APT has the remarkable advantage of practically calibration-free analysis because all desorbed species necessarily appear in ionized form and the detection probability is independent of the respective mass. Polymeric material investigation with atom probe tomography is a rather young field.6−8 Besides, earlier work with conventional 1D atom probes on polymers, mostly on conductive ones such as polypyroles, should be credited.9−11 Instead of atomic species, a variety of larger molecular fractions are usually observed in the mass spectra of field-evaporated polymers. This imposes new challenges in identifying evaporated species, understanding the evaporation sequence, and reconstructing the original spatial arrangement of the polymers. Therefore, a meaningful spatial reconstruction of a polymeric material by atom probe tomography has not been achieved yet. Zhang et al.7 carefully analyzed a self-assembled monolayer of alkane-thiolate on Au tips. By splitting observed molecular events into individual atoms, they could demonstrate that the total amounts of detected hydrogen, carbon, and sulfur are very compatible with the expected stoichiometry. However, no internal structure of the layer or a particular desorption sequence was identified. The coverage of the substrate with Received: October 21, 2011 Revised: December 2, 2011 Published: December 2, 2011 56

dx.doi.org/10.1021/la204126x | Langmuir 2012, 28, 56−59

Langmuir

Letter

SAM molecules was about a quarter of that reported for planar Au substrates. In this letter, we present new measurements of selfassembled monolayers by atom probe tomography. SAMs of two different molecules, amino-undecanethiol and (1H,1H,2H,2H-)perfluoro-decanethiol (PFDT), are studied in direct comparison. In the latter molecules, most of the hydrogen atoms are replaced by much heavier fluorine, which decisively improves the contrast with background signals stemming from hydrocarbons cracked in the vacuum chamber. This offers the chance to study the evaporation sequence in greater detail.

■

EXPERIMENTAL SECTION

Because Au represents preferred substrates for SAM studies, we decided to use needle-shaped Au as substrates, although in atom probe work the noble metal has the drawback of a comparatively high evaporation threshold (53 V/nm) 12 and reveals, because of unfortunate mechanical properties, an irregular evaporation in explosive bursts when the tip voltage exceeds about 8 kV. Wires of 0.2 mm diameter are electropolished by convenient chemistry (hydrochloric acid/ethanol 1:1) using ac pulses of 2−8 V at room temperature to produce sharp tips via local necking of the wires. Tips are carefully field developed in a field ion microscope up to a voltage of 8 kV for cleaning and shaping to a nice hemispherical shape with a 25 ± 5 nm radius of curvature and a shaft half angle of 15 ± 7° (checked by transmission electron microscopy). With this geometry, the surface area scanned by modern wide-angle atom probes amounts to about 650−1500 nm2. Given the reported areal density of SAM molecules on planar substrates (21.5 Å2 per molecule),13 3250−7500 molecules should be comprised in a single measurement. Taking into account the finite efficiency of the detector and probable splitting in tiny molecular fractions, one can expect between 50 000 and 200 000 events, which is sufficient but at the minimum for compiling reasonable mass spectra. Field-developed Au tips were immediately dipped into dilute (1 mmol/L) ethanol solutions of SAM molecules for the duration of 12 h. Whereas the first attachment of the molecules appears within a few minutes, the final ordering at planar substrates needs a period of several hours.14 After being coated, samples were carefully washed in pure ethanol. Atom probe analysis was performed immediately after preparation with the wide-angle laser-assisted atom probe at the University of Münster15 equipped with a fiber-based laser system.16 A wavelength of 343 nm, a pulse width of 240 fs at 200 kHz, and a pulse energy of 100 nJ at a 50 μm probe size were used. During analysis, the tip sample was kept at a base temperature of 40 K. The laser intensity was adjusted to obtain an effective pulse fraction of 15%, which allows an estimation of the peak temperature during the short laser pulses to about 200 K.15,17

Figure 1. Direct comparison of mass spectra of (a) 11-amino-1undecane SAM and (b) 1H,1H,2H,2H-perfluoro-decane SAM as obtained by laser-assisted atom probe tomography. (The insets present high mass ranges.)

SAMs is obtained if peaks are assigned as indicated in Figure 1 and water species are neglected in the calculation of the composition. Table 1 presents quantitative composition data determined in this way. Stated errors reflect the scatter between different measurements and the uncertainty in peak assignments. The agreement with the expected stoichiometry is satisfying and strongly suggests that indeed hydrogen contained in water species does not belong to the SAM. Beside residual water or ethanol in the films, part of the hydrogen and also the carbon may arise from cracking the hydrocarbon background in the vacuum chamber. Because in fluorinated chains hydrogen atoms are mostly replaced by fluorine, evaluation gets decisively more reliable in this case, which is reflected in the smaller statistical scatter in the composition data. From the detected number of molecular fragments, the total number of SAM chains can be quantified. On average, we do observe 3500 ± 1000 fluorinated chains per measurement. Taking into account the finite efficiency of the detector (detection probability: 0.5) and the stated size of the analyzed area, this number is at the upper expected bound in comparison to the molecular density (21.5 Å2 per molecule) reported for planar Au substrates.13 Thus, it cannot be excluded that the density of molecules is slightly increased by the curvature of the substrate. In the case of the pure alkane, the observed density of molecules chains is significantly less (1600 ± 1000 chains per measurement). However, the uncertainty in the peak assignment of sulfur leads to a large relative error range here. A direct comparison of the mass spectra in Figure 1a,b reveals the remarkable different evaporation behavior of the two kinds of SAMs, which was not expected before. The alkane chain field desorbs exclusively in smaller hydrocarbon fractions. Sulfur, providing the bond to the substrate, appears as a single atom or mostly together with one or two carbon or Au atoms at

■

RESULTS AND DISCUSSION Figure 1 presents mass spectra of (a) the alkane chains and (b) their fluorinated variant in direct comparison. In both, the peaks are reasonably assigned except for very few minor ones. Because the alkane chain comes with an amino group, nitrogen appears in combination with hydrogen only in spectrum a and molecules containing fluorine appear exclusively in spectrum b. Water-type species (e.g., OH+, H2O+, and H3O+) appear in both. Because these may have formed at the moment of evaporation, it is not clear a priori whether their hydrogen content stems from the SAM layer. Also, some peaks assigned to sulfur-containing molecules could alternatively be attributed to oxygen (e.g., SO2 and CSCO2) whereas peaks assigned to some hydrocarbons may also comprise species with nitrogen (e.g., C2H3CNH,...C2H6CNH4). By trial and error, however, we found that a reasonable stoichiometry of the 57

dx.doi.org/10.1021/la204126x | Langmuir 2012, 28, 56−59

Langmuir

Letter

Table 1. Average Composition of 11-Amino-1-undecane SAM (Top) and (1H,1H,2H,2H) Perfluoro-decane SAM (Bottom) as Determined by Laser-Assisted Atom Probe Analysis (in Atomic Percent)a SAM C11H26NS C10H5F17S a

analysis expected analysis expected

C

H

S

N

29 ± 2.0 28.2 31.1 ± 2.0 30.3

61 ± 8.0 66.6 16.4 ± 3.0 15.2

4±3 2.6 3.0 ± 0.5 3.0

6±3 2.6

F

49.5 ± 0.3 51.5

Error margins represent statistical fluctuations among different measurements. The expected stoichiometry is also stated for comparison.

particular, no F−H bonds, it is clear that these molecules are formed only during evaporation. Although the detected fluorine unequivocally stems from the chains, the accompanying hydrogen certainly stems from the background atmosphere because otherwise the amount of hydrogen would be greater than the expected content by far. (Hydrogen bound to F has also been neglected in calculating the average composition stated in Table 1.) In this stage of the process, the chains lose 9 to 10 out of 14 F side arms before then, and in a third distinguished stage, the remaining molecule backbone desorbs completely. Finally, Au is detected. It always evaporates as separate atoms. The described sequence of evaporation becomes particularly clear if composition profiles are calculated from smaller analysis cylinders (10 nm in diameter), locally aligned perpendicular to the substrate surface. In Figure 3, such profiles are shown for

the most. Obviously, the bond strength to the Au substrate is in this case quite comparable to that to the carbon backbone. By contrast, from the fluorinated SAM, sulfur predominantly appears in very large molecules with a total mass larger than 200 amu. The accurate identification of the peaks demonstrates that sulfur preferentially sticks to a chain of nine carbon atoms with a varying small number of remaining fluorine or hydrogen atoms. Thus, except for losing a CF3 headgroup and stripping off most F side arms (both species are also clearly identified in the spectrum), the chain evaporates in most cases as a complete unit. Furthermore, sulfur is never observed in any combination with Au. Obviously, in the case of fluorinated molecules, the chemical bond of sulfur to the carbon chain is significantly stronger than that to the Au substrate, in remarkable contrast to the alkane chain. Because most of the hydrogen atoms are replaced by fluorine, we can neglect hydrogen and water events and restrict further analysis to species containing carbon, fluorine, and sulfur. The reliable identification of these fractions allows us to discover a meaningful evaporation sequence. If we assume a constant tip radius,18 then the structure of the film is spatially reconstructed on the basis of established algorithms19 as presented in Figure 2. To calibrate the presented depth scale in

Figure 3. Composition profile of various fragments as obtained during the field evaporation of fluorinated SAMs. The depth scale was calculated from the 3D reconstruction shown in Figure 2. However, in a general sense it also represents the time scale or the progress of the measurement. Figure 2. Three-dimensional reconstruction of fluorinated SAMs on top of a Au tip. Detected molecular fragments are represented by spheres of varying radius as indicated. For the proper depth scaling, volumes of the fragments were calculated from the average covalent atomic sizes.

important species in the mass spectrum. We have also included C3H5 events. Obviously, although with significantly less probability (about 30% of the molecules), part of the chains also break into smaller fractions. In agreement with this, few C2F4 groups and individual sulfur atoms are observed in the mass spectrum. From the early appearance of C3H5 in the composition profile, it is clear that this mode of molecule desorption is finished before the evaporation of the largemolecule backbone ends the desorption process in the dominant mode. It is obvious that the reconstructed layer structure must not be interpreted as the real geometric structure of the film. This becomes particularly clear when the electrical field required for evaporation is plotted versus the depth scale of the measurement as shown in Figure 4. Strong variation is seen in response to the currently evaporating dominant species. Thus, the

the reconstruction, the relative volume of each atomic species was estimated from the known covalent radii and scaled to the size of Au in the fcc crystal. The radius of curvature of the substrate and the required geometric field factor were adjusted so that the correct evaporation field strength of Au was achieved at the end of the measurement. Without any numerical postprocessing, a layer structure appears in the tomographic reconstruction. First, CF3 headgroups of the molecule predominantly evaporate. Second, an overwhelming number of FH molecules evaporate. Because the fluorinated chains themselves contain very few hydrogen bonds and, in 58

dx.doi.org/10.1021/la204126x | Langmuir 2012, 28, 56−59

Langmuir

Letter

can be quantified. By the direct comparison of alkanes and fluorinated alkanes, we have demonstrated that, although of very similar structure, both molecules reveal a principally different evaporation sequence that may correlate to different dielectric responses in high fields. The new detailed data on the evaporation sequence in the case of the perfluoro-decanethiol molecules imposes a challenge for future theoretical work. The present understanding of evaporation under high electric fields in combination with laser optical excitement must be decisively improved before a spatial reconstruction of the evaporated volume becomes possible.

■

ASSOCIATED CONTENT

S Supporting Information *

Development of the electrical field strength during the evaporation of amino-undecanethiol and composition profiles of respective molecular fragments. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. Field strength required for the field desorption of various fragments of the fluorinated SAM. The field was derived from the dc voltage supplied to the sample and calibrated to the known value of pure Au (53 V/nm) at the end of the analysis. The sequence of evaporated species is correlated to increasing field strength as indicated by labels. Important fragments are illustrated on the right.

■ ■

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

observed layer structure reflects different field-evaporation thresholds of the molecular fractions that are in this way estimated by our experiments: CF3, F, and SC9F4 evaporate at 10−12, 20−30, and 35 to 40 V/nm. (For further reference and comparison, the analogs to Figures 3 and 4 for the aminoundekanethiol chains are offered as Supporting Information. Because in this case the origin of hydrogen and hydrogencontaining molecules remains unclear, it must be noted that the reliability of these data is less convincing.) Predicting the described sequence of molecular fractions poses a challenge to our theoretical understanding of the field evaporation of organic molecules. Certainly, such an understanding is a prerequisite should a tomographic reconstruction of the spatial arrangement of the molecules become possible in the future. In this regard, the comparison between both kinds of SAMs is particularly elucidating. Because from a chemical point of view a C−F bond is even stronger than a C−H bond20 but apparently becomes evaporated more easily, it is quite obvious that the specific high-field conditions control the fragmentation of the molecules. As a possibility, one may suggest that owing to the higher polarizability21 of fluorine or of its chemical bond to carbon this species is more easily stripped off than hydrogen. After losing the fluorine, the remaining carbon chain becomes interestingly more stable than the alkane chain. Although the latter evaporates in small hydrocarbon fragments, the former desorbs predominantly as a complete unit. Potentially, the remaining carbon backbone may form stable double bonds during the process. Because double bonds increase the strength significantly,20 the carbon chain may withstand further fragmentation by the field until becoming desorbed as a complete unit. It will be an important issue to clarify this reaction scheme in theoretical simulations or further experimental studies with specifically modified molecules.

REFERENCES

(1) Kelly, T. F.; Miller, M. K. Rev. Sci. Instrum. 2007, 78, 031101. (2) Larson, D. J.; Cerezo, A.; Juraszek, J.; Hono, K.; Schmitz, G. MRS Bull. 2009, 34, 732. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151−256. (4) Konopka, M.; Turansky, R.; Reichert, J.; Fuchs, H.; Marx, D.; Stich, I. Phys. Rev. Lett. 2008, 100, 115503. (5) Schmitz, G. Nanoanalysis by Atom Probe Tomography. In Nanotechnology; Fuchs, H., Ed.; Wiley-VCH: Weinheim, Germany, 2009; Vol. 6. (6) Prosa, T. J.; Kostrna Keeney, S.; Kelly, T. F. J. Microsc. 2010, 237, 155. (7) Zhang, Y.; Hillier, A. C. Anal. Chem. 2010, 82, 6139. (8) Gault, B.; Yang, W.; Ratinac, K. R.; Zheng, R.; Braet, F.; Ringer, S. P. Langmuir 2010, 26, 5291. (9) Panitz, J. A.; Giaever, I. Ultramicroscopy 1981, 6, 3. (10) Nishikawa, O.; Kato, H. J. Chem. Phys. 1986, 85, 6758. (11) Nishikawa, O.; Tanaguchi, M. Chin. J. Phys 2005, 43, 111. (12) Miller, M. K. Atom Probe Tomography; Kluwer Academic: New York, 2000. (13) Camillone, N.; Chidsey, C. E. D.; Liu, G.-y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (14) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (15) Schlesiger, R.; Oberdorfer, C.; Würz, R.; Greiwe, G.; Stender, P.; Artmeier, M.; Pelka, P.; Spaleck, F.; Schmitz, G. Rev. Sci. Instrum. 2010, 81, 043703. (16) Laser system “Impulse” delivered by Clark. (17) Oberdorfer, C.; Stender, P.; Reinke, C.; Schmitz, G. Microsc. Microanal. 2007, 13, 342. (18) Within the range determined by TEM, the tip radius was adjusted to achieve the expected evaporation threshold of Au in the late stages of the measurement. (19) Bas, P.; Bostel, A.; Deconihout, B.; Blavette, D. Appl. Surf. Sci. 1995, 87/88, 298. (20) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255. (21) Lechner, M. D. Taschenbuch für Chemiker und Physiker; Springer: Berlin, 1992.

■

CONCLUSIONS Owing to their increased field of view, modern laser-assisted atom probes are suggested for the study of field-desorption processes of thin organic films. Besides a reliable determination of chemical composition, the method is able to elucidate fragmentation and evaporation under high-field condition in great detail. The critical field strength for different fragments 59

dx.doi.org/10.1021/la204126x | Langmuir 2012, 28, 56−59