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Adatoms at the Sulfur Gold Interface in 1-Adamantanethiolate Monolayers, Studied Using Reaction with Hydrogen Atoms and Scanning Tunneling Microscopy Matthew M. Jobbins, Annette F. Raigoza, and S. Alex Kandel* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: Scanning tunneling microscopy is used to study monolayers of 1-adamantanethiolate as they are exposed to gasphase atomic hydrogen. H-atom reaction results in complete removal of the organic monolayer. The relaxation of the reconstruction present at the gold sulfur interface results in the formation of gold-atom islands, as well as the addition of gold atoms to extant surface defects such as steps and pits. Characterization of these changes shows that for 1-adamantanethiolate monolayers, 0.18 ( 0.033 monolayers of gold adatoms participate in bonding with thiolate sulfur atoms. This results in a 1:1 Au:S ratio, in contrast to the 1:2 Au:S ratio reported for n-alkanethiolate monolayers. The difference in adatom density implies a qualitative difference in binding between n-alkanethiols and 1-adamantanethiols.
’ INTRODUCTION Self-assembled monolayers (SAMs) of n-alkanethiolates on gold surfaces have become one of the most comprehensively studied systems in surface chemistry since their discovery in 1983.1 Despite many experimental and theoretical studies, aspects of the system remain incompletely understood.2,3 In particular, these include the nature of the gold thiol bond and the extent to which the underlying gold surface reconstructs. Over the years, many differing models of the sulfur gold interface have been proposed. Recent literature has provided substantial new evidence showing that the gold substrate does not maintain its bulk-terminated structure, and several “adatom”-based models where additional Au atoms are drawn from the bulk or scavenged from defects to interact more directly with the thiolates’ sulfur headgroups have been proposed.4 14 Our research group has completed a series of experiments in which monolayers are completely removed through reaction with atomic hydrogen. For alkanethiols with shorter carbon chains, monolayer removal occurs mainly through hydrogenation of the thiolate sulfur, followed by molecular desorption, and has been characterized thoroughly by Fairbrother and co-workers using XPS.15 In our experiments, we have used scanning tunneling microscopy (STM) to determine the coverage of gold atoms that remain once all organic material is removed.8,9 We have found that octanethiolates create a gold reconstruction that has 0.14 ( 0.033 monolayers of gold atoms in excess of bulk termination; that is, there is an additional gold adatom for every two sulfur atoms in the monolayer. This 2-to-1 sulfur-to-adatom ratio remains constant for alkanethiolate chain lengths of 2 and 12 carbons.9 r 2011 American Chemical Society
In the current study, we extend these measurements by studying the reaction of 1-adamantanethiolate monolayers with atomic hydrogen, once again measuring the coverage of gold atoms liberated upon removal of the organic monolayer from the surface. 1-Adamantanethiolate monolayers have been well characterized, and methods for preparing structured monolayers are understood.16 18 Due to increased symmetry over alkanethiols, 1-adamantanethiolate does not show all of the same features present in most alkanethiolate self-assembled monolayers (SAM). 1-Adamantanethiolate has a significantly smaller tilt angle with respect to the surface normal and therefore, clearly defined domain boundaries normally present in alkanethiolate SAMs are absent. Rotational domain boundaries exist but the size of the resulting defects are substantially smaller than those present in long-chain alkanethiolate SAMs. Furthermore, vacancy islands appear for both systems, though they are typically smaller for 1-adamantanethiolate monolayers. In a monolayer, 1-adamantanethiolate molecules have a nearest-neighbor distance of 6.72 Å.16,17 This indicates that adamantanethiolate has about 0.55 times the packing density of alkanethiolates, which have a nearestneighbor distance of 4.99 Å.19 The reduced density of 1-adamantanethiolate monolayers compared to alkanethiolates is the primary motivator for the experiments described in this article. On the basis of the models of Gorham et al.,15 we expect the lower-density adamantanethiolate monolayer will result in greater access of H atoms to the S Au bond, and thus favor sulfur hydrogenation over hydrogen Received: October 13, 2011 Revised: November 16, 2011 Published: November 17, 2011 25437
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Figure 1. 440 240 Å image of vapor deposited 1-adamantanethiol on Au(111) taken under ambient conditions. The 7 7-(9 adamantanethiolate) primitive cell from refs 16 and 17 is shown, along with a smaller rhombus that reflects the hexagonal symmetry and ∼6.7 Å nearest-neighbor spacing of the monolayer. Multiple rotational domains appear to be present, in agreement with these references.
abstraction even more strongly than alkanethiolates. Most importantly, however, we expect that reducing the density of S Au bonds nearly 2-fold will fundamentally change the nature of the surface reconstruction at the S Au interface. On the basis of measurements of adatom coverage after monolayer removal, this is what we observe.
’ EXPERIMENTAL SECTION 1-Adamantanethiol 99% (GC) was procured from SigmaAldrich and utilized without further preparation. Au(111)on-mica samples were obtained from Agilent. 99.999% pure molecular hydrogen was purchased from Praxair. Highly ordered 1-adamantanethiolate monolayers were prepared by vapor depositing the chemical on flame-annealed gold samples for 15 h at 70 .21 Tungsten STM tips were prepared using a home-built electrochemical etching apparatus based on ref 20. Etched tips were introduced directly into vacuum without further treatment and cleaned in situ by tunneling over an annealed gold substrate at a current of approximately 750 pA with a bias voltage of 30 V for several seconds. This procedure typically created sharp, oxide-free tips. STM experiments used a home-built, ultrahigh vacuum, scanning tunneling microscope with an OS-Crack thermal gas cracker capable of dosing a sample with atomic hydrogen. A complete description of the instrument has been published previously.22,23 Temperature of the hydrogen cracker was estimated to be between 1800 and 2000 K, which would generate a 6 27% conversion of H2 into H. The dose rate was estimated to be approximately 1014 atom cm 2 s 1. Flux of hydrogen reaching the surface is highly dependent on the alignment of the H-atom beam relative to the microscope’s tunnel junction, as well as “shadowing” effects where the STM tip prevents H atoms from reaching the tunnel junction. In order to prevent tip shadowing, the STM tip is retracted and moved diagonally away from the beam approximately 2500 Å during doses. A mechanical shutter positioned between the H-atom source and the STM chamber
was closed in between doses of hydrogen to allow scanning of the monitoring area without further reaction with hydrogen. In order to prevent potential interactions between the tip and the sample from biasing measurements of surface structure, a separate area was used to monitor the progress of the reaction during the experiment. This monitoring area was at least 500 Å away from the area used to quantify adatom coverage. All STM images were acquired in constant current mode with a 0.5 V sample bias and a 10 pA tunneling current. The pressure of the STM chamber during experiments was approximately 1.2 10 8 Torr with a hydrogen back pressure of about 1.0 10 6 Torr. Low-frequency noise was removed from the images using a terraced high-pass filter in the fast-scan direction.22
’ RESULTS AND DISCUSSION Vapor-deposited samples were imaged first with a separate home-built ambient-environment STM. Figure 1 shows a typical ordered monolayer of 1-adamantanethiolate prepared in this fashion. We have found that this method of sample preparation typically creates monolayers with fewer rotational domains and larger vacancy islands. Apart from step and vacancy-island defects in the gold substrate, the organic monolayer is highly ordered and relatively defect-free, with nearly complete coverage of the surface. Reaction with hydrogen atoms resulted in significant changes in surface structure, examples of which are shown in Figure 2. These changes qualitatively similar to those we observed for alkanethiolate monolayers: small, bright features are formed that are comparable in height to nearby substrate terraces; and, additionally, substrate vacancies are filled in.8,9 We have established previously that these bright features are one-atom-high gold islands, formed when gold adatoms originally incorporated in the monolayer become mobile and agglomerate following H-atom reactions that remove thiolates from the surface. Analysis of changes to the gold surfaces is done according to the methods set out in previous publications.8,9 We count all 25438
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Figure 2. STM images showing the 1-adamantanethiolate monolayers before (A and B) and after (C and D) reaction with atomic hydrogen. Removal of the monolayer is accompanied by the appearance of bright islands, as well as filling in of vacancy defects. Panels A and C are 1250 1280 Å in size; B and D are 1280 1290 Å.
additional gold atoms that appear in the “after” images in panels C and D of Figure 2. This includes measuring the total coverage of new, one-atom-high gold islands, as well as measuring the extent to which surface vacancies fill in with additional gold atoms. We also include in this measurement the migration of step edges that occurs when additional gold atoms attach to extant steps. This measurement and calculation process is show schematically in Figure 3. Figure 3A is a difference map based on the data of Figure 2A,C: a white color denotes an area with an average height that does not change significantly as a result of H-atom exposure, while the orange color shows areas that are higher in Figure 2C than in part A. Blue indicates the very small portion of the surface where average height has decreased, and may result either from small errors in image analysis or actual annealing of surface structures (which could be effected, potentially, by mobile thiolates present during the removal of the monolayer.) Figure 3B D shows an expanded view of one portion of the surface, with experimental images both before and after reaction, as well as the corresponding difference map. Areas are marked showing gold island formation (1), filling in of surface
vacancies (2), and addition of gold atoms to a step edge (3). The extent to which vacancies fill in and steps migrate indicates that these new features do indeed result from the agglomeration of gold adatoms, as any reaction product of the adamantanethiol would— even if it accumulated in these locations—produce a contrast change that would distinguish it from the neighboring gold. In our previous work, we have used bias-voltage-dependent imaging to show that gold islands form have the same height as single-atom gold steps with a broad range of tunneling conditions,8,9 and we have repeated that measurement for the gold islands presented here. Once again, we observe that apparent island height is identical over a wide range of tip sample bias voltages. The small size of the gold islands produced in reaction with adamantanethiol makes an accurate measurement of their height difficult, though the measured heights are consistent with those of small islands formed in reaction with ethanethiol monolayers. By analyzing pairs of images taken before and after reaction, as shown in Figures 2 and3, we found that 0.18 ( 0.033 monolayers of gold were freed up upon monolayer removal; the error reported is 25439
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step defects measurable in a way fairly independent of tip shape but very small islands and vacancies likely measured with some error. For alkanethiolate-on-gold monolayers, the literature seems to be converging on a consensus that gold adatoms are present as part of the alkanethiolate-gold bond.4 14 The exact structure of the gold sulfur interface remains unclear, and there is still discussion as to whether gold adatoms bind to two thiolates or to one only; this is covered thoroughly in a recent review.3 The effect of altering thiolate density, however, is not a common consideration for either theoretical or experimental work. A recently published work shows combined experimental and theoretical data for tert-butyl mercaptan that indicates that the reconstruction of the gold for this system is fundamentally different, which the authors attribute to the decreased packing density of this molecule.24 While the details of their model do not match our current coverage measurement for 1-adamantanethiol, our result also suggests that decreased packing density is an important factor in determining the density of adatoms, and a factor that should receive increased attention in further experimental and theoretical investigations. Figure 3. Difference map illustrating the method used to estimate gold adatom coverage. Panel A is the image of the monolayer before reaction. Panel B, in comparison, shows areas where surface height is observed to increase (orange) or decrease (blue) after complete H-atom reaction. The large coverage of orange features corresponds to the formation of gold islands and the filling in of atoms at steps and vacancies. Panels C E show zoomed images and the corresponding difference map, which is marked to show gold island formation (1), filling in of surface vacancies (2), and addition of gold atoms to a step edge (3).
the standard deviation of measurements made on three adamantanethiolate samples. This matches an adamantanethiolate density of 0.184 monolayers, and we conclude that the gold surface reconstructs in a fashion that provides one additional adatom for every adamanthiolate sulfur atom. This is a distinctly different result than that observed for alkanethiolate monolayers, where a 1:2 gold-to-sulfur ratio was measured. Indeed, gold coverage for alkanethiolates was 0.14 ( 0.033 monolayers, a similar value to that measured here, despite the much higher packing density of alkanethiolate molecules. Several sources of systematic error should be considered. After the gold monolayer √ is removed, we do not observe the reforming of the 23 3 herringbone reconstruction in experimental images. Given the noise floor of the experimental images, we would expect to see the reconstruction if it were present; this is complicated, however, by tip quality that potentially varies during the course of the experiment. This is potentially important because the surface layer in herringbone-reconstructed gold is ∼4% denser than in bulk-terminated gold. The most conservative approach is to consider the measured adatom coverage given both a bulk-terminated and a reconstructed surface: 0.18 ( 0.033 in the former case, 0.22 ( 0.033 in the latter. These numbers have overlapping 1σ error bars, and even the higher number is close to 1σ from the 0.184 coverage for adamantanethiolate. We will also slightly underestimate the adatom-to-sulfur ratio to the extent that film defects result in less than complete coverage in the original monolayer, although based on our images, we expect this to be small compared to the statistical variation in our measurements. Convolution with a finite STM tip size will overestimate the adatom-to-sulfur ratio, although this varies according to the features measured, with accumulation along
’ AUTHOR INFORMATION Corresponding Author
*To whom correspondence should be addressed E-mail: skandel@ nd.edu.
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