Article pubs.acs.org/JPCC
Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution−Graphite Interface: Nonorbital-Mediated Tunneling Xixuan Guo, Flaminia Marrucci,† Nathan Price, Elizabeth L. Stewart, J. Clayton Baum, and Joel A. Olson* Florida Institute of Technology, Department of Chemistry, 150 West University Boulevard, Melbourne, Florida 32901, United States S Supporting Information *
ABSTRACT: Semiquantitative apparent tunneling barrier measurements were collected of stearic acid adsorbed onto highly oriented pyrolytic graphite from 1-phenyloctane. A detailed investigation of the topography revealed an interesting geometric phenomenon, namely, the observation of different conformers within a single monolayer; this phenomenon has not been previously reported, despite the large volume of work on these and similar systems in the literature. Apparent barrier height images showed two separate effects. The apparent barrier height map is dominated by the distribution of surface charge (as tracked by electrostatic potential) in regions where significant polarity exists (i.e., near the carboxyl groups). Conversely, in regions where there is little variation in electrostatic potential (i.e., near the interlaced alkane chains), the manifestation of the topography dominates the apparent barrier height. The electron tunneling involved in imaging these types of monolayers is not mediated by adlayer molecular orbitals since their energies fall outside of the Fermi levels of the substrate and the tip under the tunneling conditions described. These studies provide a basis for further investigations into barrier height tunneling spectroscopy of molecular monolayers on graphite.
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INTRODUCTION First reported in 1983, atomic-scale scanning tunneling microscopy (STM) has become a useful tool for the investigation of surfaces.1−5 Originally used for imaging semiconductor and metal surfaces under ultrahigh vacuum environments, it has since shown its versatility, having been used to investigate surfaces at the solid−air and solid−solution interfaces.6−9 Additionally, STM has been utilized to image surfaces that have been chemically modified. These modifications have taken the form of chemisorbed molecules,10−15 as well as physisorbed monolayers;16−18 of particular interest to the authors is the analysis of adlayer molecules that are physisorbed onto highly oriented pyrolytic graphite (HOPG) from organic solution.19−22 This type of system is useful due to its ease of preparation as well as the fact that the adsorbed molecules tend to remain relatively unaffected by the substrate, especially with regard to their electronic structure.19,21,23−25 The ability to image molecular species on a surface, while powerful, leaves the user with only topographical information. Thus, early in the development of STM, tunneling spectroscopic methods were applied to these types of systems.5,26 This is accomplished by adjusting the tunneling junction in some way while observing the current response. Most common is the current−voltage (I−V) tunneling spectroscopy wherein the bias voltage between the sample and tip is varied, while the response in the current is measured.27−30 I−V spectroscopy is an effective tool for the unambiguous determination of the distribution of energies of the surface states associated with © XXXX American Chemical Society
electron tunneling; however it is problematic with ambient STM instruments due to thermal drift. Another form of tunneling spectroscopy is current−distance (I−z) spectroscopy. For I−z spectroscopy, the tunneling gap distance (z) is varied, while the tunneling current is monitored. Generally, I−z data are correlated to the tunneling barrier height via its influence on the exponential electron state decay.5,31 This can be accomplished experimentally in different ways. If an instrument displays very little thermal drift (such as the case for modern cryogenic UHV STMs), the tip is simply retracted in place, and the corresponding current is collected.32,33 The decay constant is then determined from a fit of the I−z data directly. However, under ambient conditions, this approach is problematic due to the high thermal drift encountered. In this case, a well-established and convenient method is to provide a small modulation (∼0.1 Å) to the tunneling gap distance while monitoring the concurrent modulation in tunneling current with a lock-in amplifier.5 The barrier height (ϕ) can then be determined via eq 1 ϕ=
ℏ2 ⎛ d ln I ⎞ ⎜ ⎟ 8m ⎝ dz ⎠
2
(1)
Here, m is the electron mass, ℏ the reduced Planck constant, and I the tunneling current. Experimentally, I is known from Received: July 13, 2015 Revised: September 29, 2015
A
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ogy.36−39 The studies described here will form the theoretical basis for subsequent studies of other adsorbed species.
the set point, and dI/dz (and thus d ln I/dz) is collected from the lock-in amplifier signal. While I−z spectroscopy is easier to collect as noted, the data it provides are more difficult to interpret, especially for systems that display variability across a surface. This is because the decay of the states into space is affected by a number of factors. Much like a macroscopic workfunction, the tunneling barrier is strongly dependent on the surface dipole. For a uniform surface, the surface dipole will be spatially constant, and thus the barrier height will be constant as well. This intrinsic barrier height is the primary factor and significantly affects the decay of the states from the surface. If a surface varies in charge (qs), for instance, due to charge inhomogeneities from an adsorbed species, then the tunneling barrier will be similarly affected. In general, if a negative charge is present at the surface, then the surface dipole is augmented, and the tunneling barrier will correspondingly increase. The states will retract into the surface (appearing darker in a topographical image) and vice versa for a positive surface charge. Thus, for molecules adsorbed onto a surface, portions of the molecule with a more negative electrostatic potential should display a greater barrier height and a lower topography and vice versa. Thus, the barrier becomes a function of the surface charge (i.e., ϕ = ϕ(qs)). Also, the topography of the surface can affect the measured barrier height.26 This is entirely a geometric effect resulting from the interplay of the decay of the states from different spatial locations toward zero, well away from the surface (see Supporting Information). Thus, if the potential barrier is invariant across a surface, then the barrier height image will include a manifestation of the topography itself. Mathematically, this can be included as a simple coefficient to the actual barrier height. Because several factors can affect the measured tunneling barrier, it is customary to instead describe it as an “apparent” barrier height (ϕap).5 If we consider the actual barrier height to be a function of surface charge (qs) and the topographical effect as a simple coefficient (T), then the apparent barrier height can be described by the simple equation ϕap = Tϕ(qs)
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EXPERIMENTAL SECTION Monolayers of stearic acid (Sigma-Aldrich, Milwaukee, WI, USA) were formed on freshly cleaved HOPG (grade ZYA, Momentive Performance Materials Quartz, Inc., Strongsville, OH, USA) by spontaneous adsorption from a saturated solution (∼10 mg/mL) in 1-phenyloctane (Sigma-Aldrich, Milwaukee, WI, USA). STM tips of Pt/Ir alloy (85:15; Alfa Aesar, Ward Hill, MA, USA) were mechanically prepared by cutting at an angle with sharp wire snips. Imaging was performed under ambient conditions at room temperature with the tips immersed directly in the solution for imaging at the solution/HOPG interface. The STM used was a Molecular Imaging PicoSPM scan head (currently Agilent, Phoenix, AZ, USA) with an RHK SPM 1000 control electronics unit (Troy, MI, USA). A custom preamplifier (gain = 2 × 108 V/A, Agillent, Pheonix, AZ, USA) was used in order to provide the bandwidth necessary for the modulated signal for the barrier height measurements. Images were collected in constant current mode (100 pA). Unless otherwise noted, the sample bias was −0.8 V. All STM images (both topographical and barrier height) have been drift corrected by applying a vertical scaling and lateral skew such that serial images (collected in alternate slow-scan mode) spatially agree. The experimental arrangement for tip-modulated barrier height measurements is described elsewhere.5 Barrier height measurements were performed by modulating the z-piezo with a Stanford Research Systems DS360 ultralow distortion function generator (Sunnyvale, CA, USA). The modulation output of the STM preamplifier was monitored with a Stanford Research Systems SR530 dual-phase lock-in amplifier (Sunnyvale, CA, USA). The scan speed of the STM and time constant of the lock-in amplifier were adjusted such that the time constant was shorter than the dwell time of the tip at each pixel location during image collection. In this manner, the barrier height measurements were collected concurrently with the topographical images and corresponded on a pixel-for-pixel basis. Dimer molecule computational calculations were performed with Spartan’14 software using B3LYP density functional theory (DFT) with the 6-31G* basis set.40−42 The models were constructed as a dimer, bonded via the carboxyl groups of the two molecules. Except where geometries were constrained (as noted), geometry optimization was performed as the initial step.
(2)
Note that eq 2 only takes into account the contributions from substrate surface states. Thus, no additional states (i.e., molecular orbitals of the adlayer) are considered to mediate the tunneling. For the present case, this is because any MO states from the adsorbates fall well outside of the energetic range of tunneling states (that is, between the Fermi levels of the tip and the sample). Thus, the tunneling described herein would be considered nonorbital mediated (i.e., the tunneling is exclusively mediated by the substrate states, in this case from the HOPG). The purpose of this work is to provide a semiquantitative description (correlated to computational DFT studies) of apparent barrier height mapping of adsorbed molecular monolayers at the solution−HOPG interface. Previously reported work of barrier height measurements of molecular monolayers at the solution−HOPG interface is sparse, and what is available is qualitative in nature.34,35 The system chosen for these studies is stearic acid (H3C-(CH2)16-COOH, 1octadecanoic acid), which has been shown to adsorb onto HOPG from 1-phenyl octane and provides a thoroughly understood system with a well-defined adsorption morphol-
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RESULTS AND DISCUSSION Figure 1 shows an STM image of a monolayer of stearic acid adsorbed onto HOPG. The image is of a region measuring 7 nm × 7 nm. The molecules form a repeating pattern of dimers, joined at the carboxylic acid groups, with the alkane chains interlaced between adjacent rows. The dimer rings appear dark. The individual methylene groups of the alkane chains are clearly visible. This image is in good agreement with other reports of imaging the same type of monolayers as referenced above. An additional consideration is the conformations of the oxygen atoms within the dimer rings. There are two conformations possible, as shown in Figure 2. Figure 2(a) shows an arrangement where the carbonyl is syn to the βB
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atoms of the carboxyl dimer ring must possess the same conformation; thus oxygen atoms with the same hybridization are always across from each other on the ring. That is, a line drawn between the carbonyl oxygens (shown with red dashed lines in Figure 2) will be roughly horizontal for the syn conformer (Figure 2(a)) and roughly vertical for the anti conformer (Figure 2(b)). The difference here is subtle. At first glance, it may seem that the two conformers are energetically equivalent. However, DFT calculations of the rigid dimer (allowing only the carboxyl ring to rotate) show that the computed difference in energies between the two conformations is 13.5 kJ/mol, with the syn confomer lower in energy (see Supporting Information). At room temperature, the Boltzmann distribution would favor the syn conformer by a ratio of greater than 99:1. Therefore, it would be expected that nearly all of the dimers on the surface (there are only 28 discernible dimer rings present in the image shown in Figure 1) would correspond to the syn conformer. However, as will be discussed below, this is generally not the case for these monolayers. To discern the two conformers in the STM images, we must first pay close attention to the electrostatic potential (ESP) distribution around the dimer ring, keeping in mind that the more negative regions will appear darker for reasons discussed above. In this case, a more negative ESP is associated with the carbonyl oxygen atoms than the hydroxyl oxygens as determined by DFT calculations of the dimer rings, as shown in Figure 3. Figure 3(a) shows a slice of the electrostatic potential (ESP) taken near where the image plane would be expected between the molecule and the top layer of substrate atoms as observed in other adlayer systems on HOPG (that is, ∼0.19 nm from the plane of the molecule, corresponding to halfway between the plane of the substrate surface and the plane of the molecule when adsorbed, and parallel to both).19,43 Here the more negative ESP isolines are portrayed with colors of yellow to red, whereas the more positive ESP isolines are portrayed with colors of aqua to blue (green corresponds to zero potential). Each oxygen atom has a negative ESP region associated with it just outside of the dimer ring. However, the negative regions for the carbonyl oxygen atoms can be seen to be more negative than for the hydroxyl oxygens; indeed, in Figure 3(a), an additional isoline of ESP (representing a difference in potential of 5 kJ/mol) can be observed within the negative potential minima associated with the carbonyl oxygen atoms. This indicates that there is a greater net negative ESP in the vicinity of the carbonyl oxygen atoms near the plane of the substrate surface. This difference may seem to be at odds with what one might infer from a simple Lewis dot structure/ hybridized orbital consideration. The hydroxyl oxygens are sp3 hybridized and have two electron lone pairs, one of which is directed obliquely toward the substrate surface due to the tetrahedral electron geometry (the other lone pair is directed away from the surface and therefore is not considered for this discussion). The carbonyl oxygens are sp2 hybridized, with their single lone pair directed parallel to the substrate surface for the idealized geometry (note that this is one argument for the reason the syn conformer has a lower energy than the anti conformer: the syn conformer’s single lone pair is directed between the hydrogen atoms of the β-carbon, whereas the anti conformer’s two lone pairs are directed exactly at the hydrogen atoms of the β-carbon, resulting in increased steric repulsion). Thus, it would seem that the carbonyl lone pair is kept further from the substrate surface than the proximal lone pairs of the
Figure 1. STM image of a monolayer of stearic acid adsorbed onto HOPG. The regular ordering of the molecules is easily apparent. The repetitive dark features correspond to the carboxyl dimer rings. The features between the dark regions correspond to interlaced alkyl chains; the hydrogen atoms of individual methylene groups are plainly apparent in the image.
Figure 2. Detail of the carboxyl dimer rings for stearic acid on HOPG. These structures are arranged in registry with their appearance on the stearic acid adlayer itself as shown in Figure 1. There are two possible conformations for these molecules when they are constrained to a 2dimensional plane. The conformation depicted in (a) shows the carbonyl of the dimer ring to be syn to the β-methylene of the alkyl chain. The arrangement depicted in (b) shows the carbonyl of the dimer ring to be anti to the β-methylene of the alkyl chain. The red dashed lines represent the axes between the carbonyl oxygen atoms. The syn conformation (a) is closer to the optimized free structure for a carboxyl group where the dihedral angle between the CO double bond and the C−C bond between the α and β carbon atoms is about 10° and is the lower energy of the two planar arrangements (see Supporting Information).
methylene of the alkane chain (hereafter referred to as the syn conformer). Figure 2(b) shows an arrangement where the carbonyl is anti to the β-methylene of the alkane chain (hereafter referred to as the anti conformer), which is a flipped conformation of that shown in Figure 2(a). These models are aligned to be consistent with the molecular arrangement on the surface as observed in Figure 1. Note that due to the regular arrangement of the molecules on the surface, opposite oxygen C
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images of this kind are dependent on the influence of the surface charge on the tunneling states, in this case the states of the HOPG substrate extending through the monolayer. In general, when the surface charge is negative, the tunneling barrier increases due to augmentation of the surface dipole; this results in a larger state decay constant and thus a retraction of the electron states toward the surface with a corresponding apparent depression in the topography. Thus, it is expected that the regions near the sp2 carbonyl oxygen atoms in the dimer rings should appear darker than the rest of the depression in the topography. Indeed, this is observed in the topographic data. The boxed region in Figure 1 is expanded and reproduced in Figure 4. Each dark region in Figure 4(a) corresponds to a
Figure 3. Cross-section views of the electrostatic potential (ESP) associated with the dimer carboxyl ring for stearic acid as calculated via DFT. The syn conformer is shown. More negative ESP values are depicted in yellow/orange/red; more positive ESP values are depicted in aqua/blue; note that the potential difference between iso-lines is not the same for the two figures. (a) A cross-section taken parallel to and ∼0.19 nm from the plane of the molecule, roughly corresponding to the image plane between the monolayer and the substrate. The ESP is more negative near the sp2 carbonyl oxygen atoms than near the sp3 hydroxyl oxygen atoms. (b) A cross-section taken perpendicular to the plane of the molecule and parallel to the O−H−O line and just outside of the dimer ring. The negative potential extends further from the plane of the molecule for the carbonyl oxygen atom (a dashed blue line is provided for reference).
Figure 4. Expanded view of the boxed region depicted in Figure 1, showing the details of two adjacent carboxyl dimer rings. (a) In the upper ring, dark spots can be seen in a roughly horizontal alignment (labeled ‘A’). In the lower ring, the dark spots assume a roughly vertical alignment (labeled ‘B’). Since the dark spots are associated with the carbonyl (sp2) oxygen atoms, then considering the alignment of the alkane chains, the upper dimer ring corresponds to a syn conformer, while the lower dimer ring corresponds to an anti conformer. (b) Stick-and-ball models of stearic acid dimers showing the detail of the carboxyl rings of the two different conformers associated with the topographies. The carbonyl oxygen atoms lie over the observed dark spots. Note that the alkyl tails of the interlaced dimers are also shown between the rings.
hydroxyl oxygen atoms. However, this assessment considers only the lone pairs and neglects the other charges present in the system. In particular, the hydrogen atom of the hydroxyl group is positively charged and thus acts to counter the negative charge of the lone pairs on the hydroxyl oxygen atoms. Thus, the ESP associated with the carbonyl (sp2) oxygen atoms is more negative than that for the hydroxyl oxygen atoms. Indeed, the cross-section in Figure 3(b) (taken perpendicular to the plane of the molecule and just outside the dimer ring) shows that the negative potential in the vicinity of the carbonyl does indeed extend further from the plane of the molecule. With these considerations in mind, these results predict that the two types of oxygen atoms in the dimer ring may be discernible in STM images. Specifically, the greater negative charge directed at the surface near the sp2-hybridized carbonyl oxygen atoms would augment the surface dipole to a greater extent than for the hydroxyl oxygen atoms; this would cause an increase in the tunneling barrier and a corresponding decrease in the topographical height in the regions of the carbonyl oxygen atoms. This variation of ESP of the dimer rings is subtly apparent in the topographical STM images themselves. It should be reiterated that the contrast associated with topographical
dimer ring; the figure shows two such adjacent dimer rings. Upon close inspection of the dark regions in Figure 4(a), darker spots can be observed within each dark region (labeled A and B in Figure 4(a)). These darker spots display distinctive alignments relative to the alkyl chains; the spots labeled “A” show a roughly horizontal alignment, and the spots labeled “B” show a roughly vertical alignment. These alignments correspond to the different conformers described above and depicted with overlaid stick-and-ball models in Figure 4(b). In particular, the horizontal (A) alignment corresponds to the syn conformer, and the vertical (B) alignment corresponds to the anti conformer, with the darker spots approximated by the location of the carbonyl oxygen atoms (see the red dashed lines in Figure 2). Thus, the two conformations can be discerned via careful inspection of the topographical STM images; this understanding will inform our consideration of barrier height maps of these surfaces. It should be noted that both of these alignments (and thus molecular conformers) appear in Figure 1 to a significant extent D
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Figure 5. 2-D enantiomers of the syn conformer of stearic acid. The view is looking down on the surface (not shown).
convert back to the lower energy syn conformer in the same manner since it would then be spatially constrained on the surface. Thus, the domain-adsorbed anti conformer would represent a local energy minimum with significant energy barriers to interconversion (either via rotation of the dimer ring itself or desorption). It should be noted that this is at best a hypothetical explanation for the presence of both conformers in the adlayer. Since the topographical images are influenced by the extent of the decay of the substrate states, which in turn is dependent upon the tunneling barrier at that location on the surface, the authors decided to investigate these adlayers using barrier height mapping. Figure 6 shows a topographical image (Figure 6(a)) and its corresponding (pixel-by-pixel) dI/dz image (Figure 6(b)). The dI/dz image is a map of the lock-in amplifier output (which is proportional to dI/dz). While not a true barrier height (which would result from eq 1), it does show an onto functional correspondence to the barrier height (for positive d ln I/dz values) in that brighter portions of the dI/dz image (i.e., Figure 6(b)) represent higher barrier heights on the surface. The images in Figure 6 show regions that are 3 nm × 3 nm. Note that the overall image quality is somewhat degraded relative to the topography image shown in Figure 1; this is due to the slower scan speed and lower resolution required of the dI/dz measurement. The images show clear correlations; the dimer rings and the alkane chain portions of the surface are easily discernible. As mentioned in the Introduction, it is interesting to note that previous studies of apparent barrier height measurements for molecular monolayers at the HOPG/solution interface have not reported actual quantitative values for the barrier heights, nor do previous studies report even semiquantitative, normalized results. Rather, they simply provide gray-scale images of the topographies and the corresponding barrier height maps. We have observed that the actual measured barrier heights for these sorts of images can vary greatly and frequently display values that are obviously outside of what could be considered physically reasonable. For instance, the apparent barrier height values for the image in Figure 6(b) range from 2.7 to 7.2 eV. Considering that, in air, the HOPG barrier height has been observed to be on the order of 0.1−0.6 eV (corroborated by our own measurements) and vacuum values of ∼4 eV are typical, it is clear that these very large barrier height values must result from an artifact of the measurements.44 Furthermore, the apparent barrier heights can vary widely from experiment to experiment. We have observed values as low as ∼2 eV and as high as ∼20 eV for these surfaces. It is possible that the variation itself is due to tip differences between experiments. However, this does not explain the very
(9 vertical (anti), 19 horizontal (syn), 10 unidentifiable; a complete evaluation of Figure 1 in this regard is available in the Supporting Information). While the lower energy syn conformer appears to be favored, this distribution is unexpected since the Boltzmann distribution based on the energy difference between the conformers would suggest a >99:1 ratio, as mentioned above. Thus, it appears that the two conformers are not at thermal equilibrium with each other when constrained to the surface. Such a deviation from thermal equilibrium would require two conditions. The first condition is that there must be a mechanism for which either conformer would be present on the surface. The second condition is that the energy barrier to rotation for each conformer must be very large. As a test of the ability of dimers within the adlayer to change conformation, the authors collected serial topographical images of stearic acid monolayers on HOPG that allowed the tracking of individual dimer rings over time (see Supporting Information). These experiments displayed no dimer rings changing conformation for the minutes-long duration over which they were in the imaging area (i.e., before the drift of the tip caused an individual dimer ring to enter, and then exit, the imaging area). Thus, it appears that the energy barrier to rotation is large enough to preclude syn to anti (or vice versa) conformation changes over the course of an experiment (about 4 h before the solution evaporates to dryness). With regard to the first condition (that there exists a mechanism by which either conformer could appear on the surface), it is instructive to first consider the adsorption of an individual dimer on the surface. Assuming that the solutionphase species are predominantly in (or near) the lower energy syn conformation, then upon adsorption, the dimer would assume one of two possible 2-D enantiomers on the surface (Figures 5(a) and 5(b)). Note that these two 2-D enantiomers could not form a packed monolayer with each other due to simple geometric constraints. One possibility for the molecules to pack together would be for one of the molecules to desorb and then readsorb as the correct 2-D enantiomer. However, this would require the dimer to overcome the energy of adsorption. The authors here hypothesize that a dimer could instead pack with its 2-D enantiomer by interconverting to the anti conformer, which could be accomplished via a route where the alkane chain flips stepwise, one portion at a time, but with the dimer ring remaining constrained to the surface; this would allow the anti conformer to pack with the 2-D enantiomer of the original syn conformer but without removing the entire molecule from the surface, thus providing a significantly lower energy barrier for packing. This mechanism of conformation interconversion would allow the opposite 2-D enantiomer to pack with an existing nonfitting surface domain. Once packed within an adsorbate domain, the dimer would not be able to E
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Figure 7. Topography image, dI/dz image, and line profiles for a dimer ring. (a) Topography of a stearic acid dimer ring. As can be seen, the dark spots are vertical; therefore, this dimer ring corresponds to an anti conformer. (b) The dI/dz image that corresponds to the topographical image. (c) Line profiles of the topography and the normalized ϕap (as calculated from eq 1); the trace of the line profiles is indicated by lines in (a) and (b). As can be seen, the ϕap shows local maxima where the topography shows local minima. This is consistent with the expected effects of the negative charge loci near the carbonyl oxygen atoms.
Figure 6. Topography and corresponding dI/dz (barrier height) images. (a) A topographic image of a stearic acid adlayer. The imaged region is 3 nm × 3 nm, which does not show a full dimer but does provide sufficient surface detail. Note that the overall image quality is somewhat degraded relative to the topography image shown in Figure 1; this is due to the slower scan speed and lower resolution required of the dI/dz measurement. (b) dI/dz image with pixel-per-pixel correlation to the topography image. This is an image of the output of the lock-in amplifier and is thus proportional to dI/dz. While not a true apparent barrier height (which would result from eq 1), this image does correspond to the barrier height in that the brighter the location in this image, the higher the barrier height. In both images, the dimer rings are easily discernible, as are the methylene hydrogen atoms in the alkane chains.
topographical height. Note that these differences in ϕap and topography are the direct result of the variation of the adlayer ESP and thus the surface dipole. Since the ESP varies across the dimer ring (as shown in Figure 3), it is expected that the tunneling barrier will vary also and thus the topography. These results indicate that in the regions of the dimer rings the changes in the charge-dependent tunneling barrier (ϕ(qs) in eq 2) dominate the variation in ϕap, rather than changes in the topography coefficient, T. Figures 8(a) and 8(b) show corresponding topography and dI/dz images of a portion of an alkane chain, respectively. The lines shown in Figure 8(a) and 8(b) depict the locations of the line profiles that are shown in Figure 8(c). The line profiles show that the topography and ϕap closely follow one another in this region. Note that this is the opposite effect than what is observed in the regions of the dimer rings (Figure 7). The reason for this is that the molecule does not show very much variation in charge across the alkane chains (this makes sense since the bonds are predominantly nonpolar). Thus, ϕ(qs) also does not vary significantly over this portion of the adlayer and can generally be considered a constant in the alkane region. Therefore, any variations in ϕap must be due to changes in the topographical coefficient, T. As described above, the topographical contribution to ϕap will follow the topography as observed here. Also, this result is different than previous descriptions of barrier height measurements on these sorts of surfaces (noted above) that were described as the barrier height
high values observed. It is not clear at this time what the origin of these large values might be. Furthermore, this might be the reason actual quantitative values have been omitted from the literature. Since we are more interested in internal variations of apparent barrier height over various surface features, we have approached this by considering the normalized barrier height by simply dividing each value by the profile average. Figures 7(a) and 7(b) show corresponding topography and dI/dz images of a dimer ring, respectively. These images correspond on a pixel-by-pixel basis. The lines shown in Figures 7(a) and 7(b) depict the locations of the line profiles that are shown in Figure 7(c). The line profiles show that there is an opposing relationship between the topography and ϕap; regions of local minima for the topography correspond to local maxima for the barrier height. This is consistent with quantum theory wherein an increased barrier results in a more rapid decay of the wave function away from the surface; thus regions of increased barrier height should correspond to a decrease in the F
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Thus, the molecular states do not directly contribute to the tunneling. Therefore, the images reported here (both topographical and dI/dz) were the result of the effects of the adlayer on tunneling to the surface states of the HOPG substrate. This being the case, there are two observed types of effects the molecules of the adlayer can have on ϕap. Where there is little variation in ESP within the adlayer (i.e., where there are only nonpolar functionalities), the topographical contribution dominates, and the dI/dz images can be seen to approximate the topographical images. Conversely, where there is significant variation in ESP (i.e., where polar functionalities are present within the adlayer), the resulting variation of the surface dipole dominates the tunneling barrier itself. In the latter case (and consistent with established theory), ϕap increases in adlayer regions of more negative ESP, and vice versa. These results now form the basis for additional semiquantitative tunneling barrier height spectroscopy studies on molecular monolayers at the solution−HOPG interface.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06726. A discussion of topography effects on images of tunneling barriers. Assignment of all conformations present in Figure 1. A description of computational experiments and results. A description of STM experiments wherein dimer rings were tracked over time to observe whether they undergo spontaneous interconversion (PDF)
Figure 8. Topography image, dI/dz image, and line profiles for a portion of the alkane chain. (a) Topography of a stearic acid alkane chain. A portion of a dimer ring can be seen to the lower right of the image. (b) The dI/dz image that corresponds to the topographical image. (c) Line profiles of the topography and the normalized ϕap (as calculated from eq 1); the trace of the line profiles is indicated by lines in (a) and (b). As can be seen, the ϕap shows general correlation with the topography. This is consistent with the expected topographical effects in regions where there is little variation in charge (i.e., variations in ϕ(qs) are much smaller than variations in T).
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AUTHOR INFORMATION
Corresponding Author
image appearing to be a negative of the topographical image. According to the results presented here, the barrier height image appears as a negative of the topography only in polar regions of the adlayer where there is significant variation in ESP; in nonpolar regions of the adlayer, the barrier height image correlates fairly closely with the topography. One item of note is that the absolute variation in the raw measured ϕap values for the dimer ring is significantly larger than the absolute variation in the raw measured ϕap values for the alkane chain (∼7 vs ∼2.5 eV). This corroborates our claim that variations in ϕ(qs) due to differences in surface charge are generally more significant than variations in T due to topographical effects.
*Phone: (321) 674-7350. E-mail: jolson@fit.edu. Present Address †
New York University, Department of Chemistry, Silver Center for Arts and Science, 100 Washington Square East, 10th Floor, New York, NY 10003. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
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The authors declare no competing financial interest.
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CONCLUSIONS Despite the large volume of STM imaging of similar alkane acids reported in the literature, the observation of surfacebound conformers has not previously been reported. For the work presented here, the authors considered different conformations due to the detailed nature of the barrier height measurements. These results show that barrier height mapping of molecular adlayers at the solution−HOPG interface is achievable to submolecular resolution. Furthermore, features in the barrier height images correlate to electronic and geometric structures within the adlayer itself. Indeed, the barrier height maps can be semiquantitatively correlated to computational DFT experiments with a high degree of agreement. It is important to keep in mind that the molecular electron states are very energetically removed from the Fermi levels of either the HOPG or the tip over the voltage ranges investigated.
ACKNOWLEDGMENTS The authors gratefully acknowledge the National Science Foundation (grant 1058427) for their generous support.
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