J. Phys. Chem. 1995, 99, 3259-3271
3259
Domain Structure of Self-Assembled Alkanethiol Monolayers on Gold C. Schiinenberger," J. Jorritsma, J. A. M. Sondag-Huethorst, and L. G. J. Fokkink Philips Research Laboratories, P. 0. Box 80.000, 5600 JA Eindhoven, The Netherlands Received: July 7, 1994; In Final Form: November 3, 1994@
The positional order of dodecanethiol monolayers self-assembled on Au(ll1) is investigated with scanningtunneling microscopy using ultrahigh tunneling resistances Rt L 300 GQ. We have studied two kinds of monolayers prepared (a) by immersion of the Au substrate into a methanoic thiol solution (as-adsorbed films), and (b) by an additional ex situ heat treatment (annealed films). The head-group of the molecules are found to chemisorb in the commensurate 43 x d3 overlayer of the Au( 1 11) surface. As-adsorbed monolayers can be characterized by an assembly of nearest-neighbor (nn) rows of molecules with many missing nn rows. Annealed films, on the other hand, are characterized by a 2 x 4 superstructure and zig-zag shaped rows of missing chemisorbed molecules. The two distinct structures are proposed to originate from two different lattices of the carbon-backbone orientational degree of freedom. Annealed films show, in addition, a considerable reduction of defect structures. Annealing to ZlOO "C results in the desorption of a fraction of the molecules; the remaining molecules stay positionally ordered and are arranged in a "striped domain" pattern.
1. Introduction
indistinguishablefrom the rest of the surface which has allowed us to reject the two models above. Our results lend strong support to the model first suggest by Edinger et al.? the apparent holes in STM micrographs are depressions in the Au surface originating from an etching process during the adsorption and organization process of the molecules. In the mean time this etching model has received further s ~ p p o r t . * ~ , ~ ~ The present paper is devoted to structural aspects of alkanethiol monolayers (mostly (212) prepared by immersion and imaged by STM with atomic scale resolution at room temperature in air. In addition to as-adsorbed samples, the structure of postannealed MLs has been studied. This paper is organized as follows: After the description of the experimental procedures in section 2, the apparent STM image contrast for alkanethiol monolayers is discussed in section 3. Section 4 describes the observed molecular structures starting with missing-row domain patterns of as-adsorbed films. Next, lower-density structures, the superstructure, and finally the structure of annealed films will be discussed. In section 5 the main results will be summarized together with some concluding remarks.
Self-assembled monolayers (SAMs) are crystalline chemisorbed organic monolayers (MLs) formed on a solid substrate by the spontaneous organization of molecules.' The possibility to functionalize the molecules enables the engineering of surfaces providing model systems for studies on wetting, adhesion, and p a s s i v a t i ~ n . ~Among *~ the various moleculesubstrate pairs, n-alkanethiols (SH-(CH2),-1 -CH3), abbreviated as C, in the following, adsorbed onto Au are a popular combination, since the films are easy to prepare and believed to be dense and of high structural q ~ a l i t y . ~ - ~ The self-assembly process of alkanethiols on Au is initiated by strong chemical interactions between the sulfur head-group and the Au surface which is believed to result in chemisorption of the molecules as thiolates, forcing them to adsorb commensurate with the Au l a t t i ~ e . ~ - A ' ~ crystalline film at room temperature can only be formed if the attractive tail-tail interaction due to lateral van der Waals forces is strong enough to align the tails in ~ a r a l l e l . ~This . ~ is the case for sufficiently long chains, n ;2 10. Using a variety of techniques such as helium scattering, electron and X-ray diffraction the sulfur head-groups of the 2. Experimental Section molecules were found to bind to the Au(ll1) surface in the commensurate 4 3 x 4 3 R30" overlayer ~ t r u c t u r e . ~ ~ In ~ - ~ ~ ~ - 'The ~ monolayers on Au( 111) were prepared from n-alkanethiol molecules (SH-(CH2),-1-CH3, abbreviated as C,) with n = addition, numerical mode1ing,l4-l5 as well as IR spectros3, 6, 10, 12, 14, and 18. The materials were obtained from copy5,16-17and X-ray d i f f r a ~ t i o n , ~ ~suggests J ~ J * that the tails Fluka and used as received. Atomic resolution was observed of the molecules are tilted by e30" away from the surface for n = 10 and 12, whereas no atomic structure was visible for normal. the very short ( n = 3) and very long (n = 18) molecules. In this paper we discuss the domain structure of alkanethiol Ocassionally some structure on the atomic scale was resolved MLs on Au( 111) as observed by scanning-tunneling microscopy for n = 6 and n = 14. But these results were not very (STM) with atomic scale resolution. In previous STM studies reproducible. All the STM images presented in this paper are of similar films, atomic resolution was either not obtained or obtained for dodecanthiol MLs (n = 12). The structural results only within a scan window smaller than the domain size.19-22 for as adsorbed C12 films, however, are representative for Clo All these previous studies had in common the observation of as well. unexpected and striking depressions appearing as "holes" in The Au substrates are thin films of thickness 100 nm grown STM micrograph^.^^-^^ These holes were originally attributed on freshly cleaved mica by evaporation either in high vacuum to the organic ML.19 Possible models included holes or regions (pressure p = 10-6-10-5 Pa, growth rate r = 5 &s) or in of disorder in the monolayer. In a recent paper,23we demonstrated that the molecular structure within depressions is Pa, growth rate r = 0.5 ultrahigh vacuum (pressure p = k s ) at a substrate temperature of 250 "C. This is known to @Abstract published in Advance ACS Abstracts, February 15, 1995. result in (111) oriented Au films.25 Due to the lower back 0022-3654i9.512099-3259$09.00/0
0 1995 American Chemical Society
Schonenberger et al.
3260 J. Phys. Chem., Vol. 99, No. IO, 1995 pressure and slower growth rate, the UHV grown substrates are in general much flatter. The monolayers are formed by immersion of the Au films into a solution of 3.5 mM thiols solved in methanol at a temperature of 20 "C for a duration ranging from seconds to days. In order to investigate the initial stage of adsorption, lower concentrations were sometimes used as well. Prior to immersion, the Au films were cleaned first in a UV-ozone reactor (UVP Inc, AR-100) for 15 min. After the immersion the films were carefully rinsed (in the order ethanol, 2-propanol, hexane) to remove all weakly adsorbed species and then dried in air. Such films will be termed as-adsorbed filmsin the following. Measured contact angles24for C12 monolayers as a function of immersion time ti agree with results from the literat~re.~ For water, the contact angle (112" advancing, and 92" receding) is found to be independent of the immersion time ti for ti 2 1 min which indicates that the surface coverage of molecules is already saturated after 1 min of immersion. The degree of ordering, however, is observed to show strong sample to sample variations in the range ti 1-10 min. Annealing experiments have been performed on dodecanethiol C12 MLs only. These films, termed annealed films in the following, were first prepared as described above. Then, they were placed onto a Cu block which was located in an oven at a temperature T, = 60-120 "C for a duration ta ranging from 1 min to 1 h. The temperature was regulated and measured directly on the Cu block using a thermocouple. After annealing, the samples were rapidly quenched to room temperature (RT) by placing them onto a second large Cu block kept at RT. With this procedure we attempted to freeze in the structure present at the annealing temperature. That this was indeed the case was verified by heating up two samples directly on the STM while simultaneously measuring images (in situ heating). With a simple resistor fixed on the backside of our sample holder, a temperature of 120 "C could be reached. The warming up was performed incrementally in steps of -5-10 "C. Having increased the temperature we had to wait a few minutes until the thermal drift was small enough to allow imaging the surface. Despite the thermal drift, we were able to follow the same surface area. The as-prepared monolayers were examined within in a few hours after preparation in an STM operating under ambient conditions. Afterwards, they were stored under a clean hood. The samples were found to be stable for at least 1 week as evidenced by the absence of structural changes within this periode upon reimaging. Our home-built tube scanner based STM is controlled by a commercial STM electronics.z6 All micrographs have been obtained in the constant tunneling current mode at scan rates of xO.1-1 pmh. The grayscale images represent the apparent surface topography in a conventional manner: white areas are topographically higher than black ones. We have used mechanically cut RRd wires as tips and, in general, imaged each sample with several different tips in order to distinguish real surface structures from tip-induced artifacts. Ultralow tunneling currents It (It = 1-10 PA) were used to image the monolayers. In most cases, the tip voltage Ut with respect to the sample was negative in the range of 10 mV to 1 V. The molecular structures, however, were found to be independent of the bias polarity. In the following we will always refer to the tip voltage Ut with respect to the Au substrate.
3. Image Contrast for Alkanethiol Monolayers The electrical tunnel current It observed in STM is to first order proportional to Ute(-eUt) exp(-Ks), where e(eU) is the
Figure 1. Schematics of the apparent topographic contrast 6s in STM between a thiol-covered (left) and uncovered (right) Au surface.
local density of states at energy eU above the chemical potential p of the substrate, Ut the voltage applied to the tip, s the tipsubstrate separation, and K the inverse decay length proportional to C#lI2, where C# is the mean barrier height between the substrate and the tip apex.27 In order for an adsorbed molecule to be visible in STM, it should have a nonvanishing @(-eU,) within the accessible voltage range, which for an STM operating in air is I Uti 5 2 V. The long alkane chains of the molecules are very good insulators with a large energy difference between the LUMO and HOMO states of -9 eV. Therefore, the chains are not expected to contribute to the tunneling current It. The thiol molecules HS-R chemisorb as thiolates onto the Au substrate, Le., AuS-R species are formed. In general, we can expect that the local Au-S bonds interact with the continuum of the bulk Au states resulting in a broadened interface resonance of energies near p with considerable density of states at the S atoms. Therefore, the tunnel current can flow from the tip to the conducting Au substrate either directly or via such an interface state. Hence, we expect to either image the position of the Au surface or the positions of the sulfur adatoms. The image contrast can best be studied experimentally on a Au surface that is only partially covered by thiol molecules as indicated in the schematics of Figure 1. Since the coverage is already close to saturation after very short immersion times, the partial coverage is not easily achieved using short immersion times. However, a monolayer can be prepared first and then partially removed either by etching or thermal desorption. We have used the second method since it ensures that the Au surface is not attacked. As will be shown later in section 4.6, the molecules desorb at temperature Ta x 110 "C. If the time of annealing at this temperature is not too long (I 1 h), islands of ordered thiol molecules are left on the surface. A STM micrograph of such an island of C12 molecules is shown in Figure 2a. This image was measured using Ut = -1 V and It = -2 pA which corresponds to a tunneling resistance Rt of 500 G Q . The border of the island is highlighted by solid dark curves. The two arrows point to monoatomic step edges of the Au substrate. Using high Rt the molecular island appears topographically raised (i.e., bright) against the Au terrace by as much as 6s = 3 A. On magnified images we always resolve ordered periodic structures on the atomic scale with a relatively large corrugation of -0.1 nm provided the tunneling resistance is sufficiently large. A typical example, obtained on another sample, is shown in Figure 2d (measured using Rt = 500 GQ). A hexagonal lattice is visible with a nearest-neighbor distance dnn = 4.9 f 0.15 A (averaged over several micrographs), in very good agreement with the 4 3 x 4 3 overlayer structure of dm = 5.0
A.
From helium diffraction studies?8 which probes the ordering of the alkyl end groups (the ones forming the actual surface), it is known that the end groups are disordered at room temperature
Self-Assembled Alkanethiol Monolayers on Gold
J. Phys. Chem., Vol. 99, No. 10, 1995 3261
Figure 2. Sequencc of STM i m q c s obtuined on a Au film partially covered with dodecanethiol molecules (see schematics of Figure I). Parts a-c represent ihc same 70 x 70 "mi area. While the thiol-covered area-the island highlighted by dark borderlinesrappears bright in (a) measured at high R, = 500 G Q ,it is only barely visible in (b) at low R, = 25 GR. Following the measurement of (b), image (c) is measured at high R, = 500 GR again. The original contrast is restored but the molecular island is eroded due to destructive mechanical tip-molecule interactions while measuring (b). Image (d) of size 13 x 13 nm2 demonstrates that atomic resolution is obtained on such islands if a high R, is used. The amows point to monoatomic step edges of the Au substrate.
(RT), and the d 3 x d 3 ordering is only confirmed at lower temperatures. X-ray and elechon diffraction which are more sensitive to the the molecular head groups, Le., the sulfurs, show that these are ordered at RT in the commensurate d 3 x d 3 R30" Au overlayer structure. Since we observe the d 3 x d 3 lattice for high tunneling resistances at RT, this demonstrates that the sulfur adatoms are imaged as anticipated in the beginning of this section. Whenever we will use the term "imaging of thiol molecules" in the following it is implicitly meant that the sulfur head-groups are imaged. The second micrograph Figure 2h shows the area of Figure 2a now observed using a smaller tunneling resistance R, = 25 G Q (U, = -50 mV, I, = -2 PA). The strong contrast apparent in Figure 2a at large R, has disatpeared and the molecular island is only harely visible (6s
