Formation and Ripening of Self-Assembled Multilayers from the Vapor

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Article Cite This: Chem. Mater. 2018, 30, 5694−5703

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Formation and Ripening of Self-Assembled Multilayers from the Vapor-Phase Deposition of Dodecanethiol on Copper Oxide David S. Bergsman,†,⊥ Tzu-Ling Liu,‡,⊥ Richard G. Closser,§ Katie L. Nardi,∥ Nerissa Draeger,∥ Dennis M. Hausmann,∥ and Stacey F. Bent*,† Department of Chemical Engineering, ‡Department of Materials Science and Engineering, §Department of Chemistry, Stanford University, Stanford, California 94305, United States ∥ Lam Research Corporation, Fremont, California 94538, United States Chem. Mater. 2018.30:5694-5703. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/09/18. For personal use only.

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S Supporting Information *

ABSTRACT: The vapor-phase reaction of dodecanethiol (DDT) with copper oxide surfaces and the molecular level composition and structure of the resulting films were examined. Atomic force microscopy, cross-sectional transmission electron microscopy, and electron energy loss/ electron dispersive spectroscopy reveal that, instead of forming self-assembled monolayers, DDT etches CuO surfaces to create ∼8 nm thick Cu-thiolate multilayers. These layers are composed of surprisingly well-ordered crystallites, oriented either parallel or perpendicular to the substrate surface. Pre-etching of the CuO to expose the underlaying copper metal is shown to prevent the formation of multilayers and instead allow for the formation of the expected monolayers. Water contact angle and Fourier transform infrared spectroscopy are further shown to be ineffective at distinguishing the multilayer and monolayer thiol films. Interestingly, the multilayer films are unstable in air, ripening into particles 20 μm wide and several hundred nanometers tall over the course of a week. Air exposure also leads to the slow oxidation of the sulfur and copper within the films at a rate similar to what has been seen before for DDT monolayers. As a result, the multilayers show no significant improvement over monolayers in the prevention of oxidation.

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ions to dissolve and be removed11 or because the Cu partially dissolves and then re-adsorbs onto the copper metal surface before complexing with thiol ligands.17 Although the structure of those multilayers and the mechanism behind their formation were not well-understood, pretreatment of the CuO with acids, such as acetic acid,20,21 nitric acid,14,22 or hydrochloric acid,16,22,23 was found to remove the CuO layer, preventing the formation of multilayers during thiol deposition. To distinguish these multilayers from their monolayer counterparts, we use the term “self-assembled monolayer” or “SAM” only in reference to known, well-packed monolayer films. In recent years, the use of SAMs to control area-selective ALD for back-end semiconductor processing has created renewed interest in the study of thiol deposition on copper, particularly through vapor-phase approaches that can be more easily incorporated into industrial semiconductor fabrication processes.7,16,24 Self-assembled monolayers are also often used to prevent copper surface oxidation25,26 and corrosion.23,27 However, only a handful of articles have studied the selfassembly of thiols on copper through the vapor phase,4,5,16,28 and fewer have reported the formation of Cu-thiolate multilayers.16

INTRODUCTION The liquid-phase self-assembly of alkanethiol monolayers onto copper has been studied over the past two decades for use in a variety of applications, such as the synthesis of nanostructures, the passivation of copper surfaces, the creation of chemical sensors, and the selective deposition of materials through atomic layer deposition (ALD).1−7 Although a majority of that effort has focused on adsorption on copper metal, the formation of thiol-based self-assembled monolayers (SAMs) on copper oxide is interesting in that thiols have been reported to etch and reduce the copper oxide surface.4,5,16,8−15 In the case of cupric oxide (CuO), the mechanism behind this etching is generally believed to involve the oxidation of thiols to disulfide by the CuO, releasing two hydrogen atoms that combine with oxygen from the oxide surface to produce water. This process simultaneously reduces the copper oxide, which is eventually coated in an adsorbed thiolate SAM. In contrast, cuprous oxides (Cu2O) do not undergo thiol-based etching11 (except through the addition of other etchants, such as an alkaline solvent12), and instead, thiols bond directly on the Cu2O to form SAMs. Some reports suggested that the thiol-based etching of CuO may enable the formation of Cu-thiolate multilayers.8,11−13,16−19 Some of those reports proposed that the etched CuO is incorporated into the thiol films as a Cu-thiol complex that can be many SAM layers thick. This multilayer is suspected to form either as a result of the inability of the Cu1+ © 2018 American Chemical Society

Received: May 21, 2018 Revised: July 25, 2018 Published: July 26, 2018 5694

DOI: 10.1021/acs.chemmater.8b02150 Chem. Mater. 2018, 30, 5694−5703

Article

Chemistry of Materials

reopened, and the samples were purged for 3 min before being removed for analysis. Non-contact mode atomic force microscopy (AFM) was performed on a Park System NX-10 with an NSC35 tip purchased from Nanoworld. X-ray photoelectron spectroscopy (XPS) was performed on a PHI Versaprobe 3 with an Al Kα X-ray source. Fine scans were collected in high resolution mode with pass energy of 55 eV, a 200 μm spot size, 50 W, and 15 kV. Three to six scans per region were taken with step sizes of 0.1 eV/step, 20 ms per step. Scanning X-ray Imaging (SXI) was performed on the same equipment with a 9 μm spot size and a beam current of 1.0 W at 15 kV. XPS was then performed on specific locations on the SXI image using a 10 μm spot size, 1.2 W, and a 55 eV pass energy. All spectra were shifted so that the carbon peaks aligned with the aliphatic carbon binding energy of 284.8 eV. Cross-sectional transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and electron dispersive spectroscopy (EDS) were prepared, imaged, and analyzed all at Nanolab Technologies, Inc., Milpitas, CA. Samples were first synthesized at Stanford and quickly sealed inside a vacuum transport system before being transferred to Nanlolab Technologies to prevent extended exposure to air. There, TEM lamella were prepared by a FEI Helios Nanolab 450S SEM/Focused Ion Beam (FIB) dual beam system and imaged on a JEOL 2010F TEM at 200 kV. The lamella were then transferred to a probe-corrected FEI Titan for EELS/EDS analysis at 200 kV. The Titan TEM used a ChemiSTEM FEI X-FEG high brightness Schottky field emission gun, a Super-X 4-SDD, windowless EDS detector system, and a Gatan Enfinium ER EELS spectrometer with a fast shutter. Simultaneous EDS/EELS spectrum images were collected to cover all the elements of interests. When performing cross-sectional atomic compositional analyses, EELS was used to detect carbon, silicon, tantalum, copper, and oxygen; EDS was used to detect sulfur. FTIR spectroscopy measurements were performed using a Nicolet iS50 FTIR spectrometer with a diamond attenuated total reflectance (ATR) plate. Spectra were averaged over 200 scans with 4 cm−1 resolution. An equivalent UV-ozone or acetic acid-treated copper-onsilicon substrate was used as a background. Grazing incidence X-ray diffraction (GIXRD) measurements were performed on beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) in a helium-purged chamber with Kapton windows, using a MAR345 imaging plate area detector with X-ray energy of 12.7 keV. The detector distance of around 300 mm and detector orientation were calibrated using a LaB6 standard. Measurements were performed at an incidence angle of 0.12° relative to the Xray beam. After preparation, samples were transported immediately to the beamline in a nitrogen-filled sealed container. Scanning electron microscopy (SEM) was performed using an FEI XL30 Sirion SEM with beam energy of 5 kV. Samples were mounted onto a conductive stage using Electrodag 502 purchased from Ted Pella. Inc. Particle size distribution analysis was performed using ImageJ software. Note that the software was generally found to undersize the particles and occasionally misrepresented one particle as two. As a result, the standard deviation in the particle area was high, although total percent surface coverage was generally found to be more consistent. WCA measurements were performed on an FTA 200 goniometer using deionized (Millipore) water. Auger electron spectroscopy (AES) mapping was performed using a PHI 700 Scanning Auger Nanoprobe with beam settings 10 kV and 10 nA. Maps were obtained using 10 scans with a pixel resolution of 128 × 128.

Here, we examine the deposition of dodecanethiols (DDTs) on copper surfaces via a vapor-phase approach and address many of the above gaps in understanding. In this study, DDTs are exposed to copper pretreated with UV-ozone and the resulting films are probed by a combination of microscopic, spectroscopic, and diffraction techniques. We show that the vapor deposition of DDT results in well-aligned multilayer Cuthiolate films when the copper substrate begins as CuO. We further show that this multilayer does not form when the copper oxide is removed prior to deposition, through treatment with acetic acid, similar to what has been seen previously. These multilayers are not distinguishable from SAMs with traditional SAM characterization techniques, like Fourier transform infrared (FTIR) spectroscopy and water contact angle (WCA) measurements, which show equal, if not better, WCAs and crystallinity in FTIR spectroscopy than do thiol monolayers. Finally, the data presented here indicate that multilayers show similar, moderate passivation against oxidation when compared to traditional monolayers. Over the course of several days’ exposure to air, the multilayers ripen into micron-scale particles. This ripening does not occur in a nitrogen-purged atmosphere. Air exposure also results in the general oxidation of the Cu and S of the films into Cu-sulfonate. Overall, these results provide insight into the growth mechanism and properties of copper-thiolate multilayers formed through the vapor-phase functionalization of copper oxide surfaces with thiols.

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EXPERIMENTAL METHODS

Dodecanethiol (DDT, >98%) was purchased from Sigma-Aldrich and used without additional purification. Blanket copper (Cu) and copper-on-silicon oxide patterned substrates (Cu/SiO2) were provided by collaborators and created as follows. Blanket Cu wafers were prepared by sequentially depositing SiO2, TaN, and Cu onto a blanket Si wafer by chemical vapor deposition (SiO2) and physical vapor deposition (TaN, Cu). Patterned Cu/SiO2 substrates were prepared by conventional lithography, etching, and thin film deposition processes, with the Cu deposited by electrochemical plating. Chemical mechanical planarization was also applied on the blank and patterned copper substrates. Before DDT deposition, substrates were cleaned using 10 min of sonication in absolute ethanol (200 proof, purchased from Fisher Scientific) and 2 min of UV-ozone treatment in a Novascan PSD Series Digital UV Ozone System to remove organic contaminants from the surface and ensure the complete conversion of any surface copper oxide species to cupric oxide.11 After the UV-ozone cleaning, select substrates were immersed into glacial acetic acid (Macron, ACS grade) for 3 min to remove the native copper oxide and then immediately transferred into a home-built vapor-phase deposition chamber for DDT exposure, as described previously.4 Other samples were transferred directly into the chamber immediately after UVozone treatment. After loading substrates into the chamber, the chamber was purged with N2 for 10 min to remove any O2 or H2O that was introduced during sample loading. The chamber temperature was maintained at 70 °C by warm-wall heating with heating tape elements. A glass vial attached to the chamber was used to store the DDT, and a valve attached between the vial and the chamber controlled the dosage. The DDT vial and the line were both heated at 55 °C to increase the DDT vapor pressure and eliminate condensation within the line. After purging for 10 min, the chamber was pumped down to base pressure (about 2 mTorr) before closing off the pump and dosing DDT for 30 s. No observable increase of pressure was observed during the dose time. After dosing, the DDT valve was closed, the pump valve was

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RESULTS AND DISCUSSION To examine the effect of DDT exposure onto cupric oxide (CuO), vapor-phase DDT was exposed to Cu/SiO2 patterned substrates and examined with AFM, XPS, and TEM. The use of patterned substrates was necessary in order to observe changes in thickness as a result of the DDT exposure; on these substrates, the silicon region, which has previously been shown 5695

DOI: 10.1021/acs.chemmater.8b02150 Chem. Mater. 2018, 30, 5694−5703

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Figure 1. (a) AFM micrograph showing a topographical map of the copper-on-silicon substrates after the vapor deposition of dodecanethiol. (b) Line scans of the DDT-coated substrate shown in (a) (black) and from a substrate etched with acetic acid without any DDT (gray). (c) XPS fine scan of the UV-ozone-treated copper surface before (gray) and after (black) dodecanethiol deposition. (d) Cross-sectional SEM image of a region of the patterned substrate in (a). The green box represents the approximate region used in (e). (e) Cross-sectional EELS/EDS of a region of the samples after vapor-phase dodecanethiol deposition.

not to form SAMs after DDT exposure,6 served as a region of fixed height for reference. Figure 1a shows a representative AFM image of the Cu/SiO2 patterned samples exposed to DDT. The square areas are Cu with DDT deposited on the surface; the other regions are SiO2. From this image, a line scan of the height was taken (Figure 1b, black). Also shown in Figure 1b for comparison is a line scan from an acetic acid etched Cu/SiO2 patterned wafer without any DDT, taken over the same area of the pattern (gray); a representative AFM micrograph of the etched sample is shown in the Supporting Information, Figure 1. Because it is known that vapor-phase thiols can etch and reduce the CuO,4,5,11 the acetic acid etched control can be used to determine the baseline elevation of the bare Cu metal underlying the DDT. The line scan from the acetic acidtreated sample shows that the copper regions sit at a depth of about 6 nm from the top surface of the sample. The line scan from the DDT-coated substrate shows that the same square regions have an average height of around 2 nm compared to the silicon regions, indicating that the thickness of the DDT layer is ∼8 nm in total, well above the expected molecular length of DDT (∼2 nm).29 The copper oxide thickness before DDT exposure was roughly 3 nm, based on AFM measurements (not shown). Figure 1c shows a Cu 2p3/2 XPS fine scan before and after DDT deposition on a blanket Cu sample. Before DDT exposure, copper peaks are observed at 932.5 eV and 934.5 eV, along with a satellite peak between 938 and 946 eV. The peak at 932.5 eV arises from Cu metal beneath the surface oxide layer. The peak at 934.5 eV and the satellite peak between 938 and 946 eV are from CuO, demonstrating that UV-ozone exposure forms Cu2+ oxide.5,30,31 However, after subsequent exposure to DDT, the oxide peaks are removed, leaving only a

single Cu peak at 932.5 eV associated with Cu0 or Cu1+ (the oxidation state between these cannot be easily distinguished by XPS).11,32 The removal of the CuO features is consistent with previous reports that thiols can reduce and etch the CuO surface.4,5,8−15 This result also demonstrates that the DDT is able to penetrate the entirety of the CuO layer and not just the outermost surface. To better understand the composition of the 8 nm thick DDT layer, cross-sectional dark field TEM images were taken of the patterned Cu/SiO2 after DDT deposition (Figure 1d). A layer of material ∼ 7 nm thick is observed on top of the copper, consistent with the thiol-layer thickness described above. EELS/EDS analysis was performed on a smaller region of the scan (Figure 1e) to probe for C, O, S, Cu, Si, and Ta (here, TaN is used as a liner to reduce copper diffusion into the silicon). The EELS/EDS analysis shows the presence of the Si substrate, the Ta barrier layer, and the Cu square region. Filling most of the void space above the substrate is a carbonbased glue layer, used to protect the surface during the FIB process. Of note is the area directly above the copper region, which shows a sulfur signal nearly 8 nm thick. This region also includes a slight change in C contrast from the background carbon-based glue. These signals are consistent with a film composed of DDT molecules. Interestingly, a weak copper signal is also present above the copper substrate in this region, suggesting that some copper has been incorporated into the thiolate film. Combined, the results of both AFM and TEM/EDX suggest that exposure of the CuO surface to DDT leads to (1) the formation of an 8 nm thick film, thicker than would be expected for a DDT monolayer, (2) the reduction of copper from Cu2+ to either Cu0 or Cu1+, along with the removal of oxygen, and (3) a film composition that includes carbon, 5696

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of these peaks suggests that they correspond to the (0k0) peaks of a lattice with spacing 2π/Δq = 35.5 Å. In addition, a peak in the qxy direction at 1.64 Å−1 (d-spacing of 3.8 Å) is present, which does not correspond to one of the (0k0) peaks and has a higher peak width. Instead, this peak may indicate a less-well-ordered lattice along the (h00) direction. The origin of these peaks is discussed further below. Previous work has reported the structure of Cu-thiolate crystallites with different carbon chain lengths,33 based on a bulk wet synthesis approach. There, Cu-thiolates liquid crystallites were assigned to a head-to-head bilayer-type structure, where two planes of copper atoms are sandwiched between two monolayers of thiolates. Although the authors do not report a crystal structure for in-plane alkyl chains, it is reasonable to assume that a typical c(2 × 2) structure34 for thiols-on-copper would form. The reported (0k0) distance between bilayers was found to be related to the equation:33 d = 7.960 + 2.439L, where d is the distance between bilayers and L is the number of CH2 groups in the thiol chain. Given the 11 methylene groups in DDT, the published model predicts a crystallite d-spacing of 34.8 Å, which agrees well with the 35.5 Å spacing measured above. On the basis of previous investigations of the copper-thiolate crystal structure,33 it is reasonable to infer that the sharp, uniformly spaced peaks represent the well-ordered spacing between laminar layers of the copper-thiolate. These lattices are well-textured toward the in-plane and out-of-plane directions, with little ordering at angles other than 0° or 90° from the surface normal, since the peaks are all oriented along either the qxy or qz directions. The broader peak at qxy = 1.64 Å−1 (d-spacing of 3.8 Å) likely arises from the chain−chain spacing between alkane groups oriented out-of-plane (typically34 around 4 Å for alkane chains). The expected analogous peak in the qz direction, corresponding to the chain−chain spacing between crystallites lying flat across the substrate surface, may simply be unobservable in GIXRD due to the experimental geometry.35 From the results presented here and previous work on the structure of copper-thiolate crystals,33 a representation of the crystal structure formed during the vapor-phase deposition of dodecanethiol on cupric oxide can be constructed, as shown in Figure 3. These multilayers take the form of well-textured

sulfur, and copper. These results support the idea of vaporphase formation of copper-thiolate multilayers, similar to that previously reported for liquid-phase thiol exposure to CuO. Of the two possible models proposed in the literature for this multilayer formation in solution phase [i.e., complexing of the thiols with copper ions from (1) copper ion dissolution into the solvent and re-adsorption,17 or (2) nondissolved copper ions of the partially reduced copper oxide11], only the second model would be applicable here in the solvent-free, vaporphase process. The structure of the Cu-thiolate multilayer, including whether or not it is composed of self-assembled regions or is largely amorphous, is not readily obtainable from AFM or TEM. Therefore, GIXRD was performed on DDT-coated blanket copper substrates in order to further elucidate the Cuthiolate structure. A representative GIXRD pattern is shown in Figure 2a. Interestingly, multiple peaks along both the qxy and qz directions are present, suggesting that the films contain ordering both parallel and perpendicular to the substrate surface. Figure 2b,c shows integrated regions along the qxy and qz directions, respectively. Many of these peaks appear uniformly spaced, with a distance between peaks of 0.177 Å−1 (Supporting Information, Figure 2). The uniform spacing

Figure 3. Illustration of the horizontally and vertically aligned multilayer structures of copper-dodecanethiolate.

crystallites likely composed of head-to-head and tail-to-tail thiol groups bound to Cu1+ ions and are oriented either horizontally or vertically relative to the substrate. The Cu ions are inferred to be in the 1+ oxidation state because XPS results above suggest they are not in the 2+ oxidation state and the thiols are expected to be bound at least once to Cu, eliminating the possibility of the Cu0 metal state. The incorporation of these copper ions is expected to occur as a result of the etching

Figure 2. (a) GIXRD of dodecanethiol-on-copper substrates immediately after deposition. (b, c) integrated intensity of the GIXRD patterns along the (b) qxy and (c) qz directions. The sharp spikes are artifacts of the detector. 5697

DOI: 10.1021/acs.chemmater.8b02150 Chem. Mater. 2018, 30, 5694−5703

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Figure 4. (a) Cu XPS after acetic acid treatment (gray) and after subsequent vapor-phase DDT exposure (black). (b) A line scan extracted from AFM micrograph of the acetic acid-treated copper-on-silicon substrates before (gray) and after (black) dodecanethiol vapor deposition. (c) GIXRD of acetic acid-treated dodecanethiol-on-copper substrates immediately after deposition. (d) FTIR spectra of a copper blanket substrate after DDT exposure with and without a pretreatment with acetic acid. Inset shows a magnified view of the C−H stretching region with the spectra renormalized. (e) Cross-sectional TEM, EELS, and EDS spectral images of the patterned substrate in (b) after vapor-phase dodecanethiol deposition.

to-end in-plane horizontal crystallites or that the out-of-plane crystallites have a lateral size no longer than around 4 nm. Previous work has suggested that multilayer formation only occurs on cupric oxide. To examine this for vapor-phase DDT, samples with the native oxide layer removed were prepared by exposing the cleaned samples to acetic acid before DDT deposition. Figure 4a shows a Cu 2p3/2 XPS high-resolution scan of an acetic acid-etched, blanket copper substrate before and after DDT deposition. The absence of any Cu oxide peaks at 934.5 or 938−946 eV indicates that the oxide layer has been successfully removed by the acetic acid etch. As expected, XPS of the sample after DDT exposure also shows no copper oxide peaks. Patterned Cu/SiO2 substrates were then prepared using a similar acetic acid treatment and examined with AFM, GIXRD, and cross-sectional TEM/EELS/EDS, as above. Figure 4b shows the AFM line scan of the acetic acid-treated Cu/SiO2 patterned substrate before (gray) and after (black) DDT deposition (a topographical micrograph is shown in the Supporting Information, Figure 3). In contrast to the deposition on CuO, DDT onto acetic acid-treated Cu shows an increase in thickness of only about 2 nm, close to the thickness expected for a monolayer of DDT.29 GIXRD patterns (Figure 4c) of the blanket copper samples also do not show any of the peaks previously seen for the multilayer samples. Although a well-formed 2D SAM would have substantial in-plane order, qxy peaks associated with that ordering may simply be too low in intensity to detect. Cross-sectional TEM and EELS/EDS mapping (Figure 4e) again show the presence of a thin layer of material on top of

of the copper oxide, which releases the copper ions and allows them to be combined with thiols to form the copper thiolate multilayers. The mechanistic origin of the two alignments (vertical and horizontal) is not immediately apparent from the data presented here. However, they may be reasonably explained through the consideration of SAM formation in the absence of multilayers. Previous studies have shown that thiols initially nucleating on Cu metals lie flat against the metal surface due to van der Waals forces between the chain and the substrate.34,36 After a critical surface concentration is achieved, thiols selfassemble on the metal into vertically aligned chains so that the sulfur atoms may bind to the substrate while the alkyl chains pack tightly into crystalline regions. Here, the crystallites may adopt similar horizontal and vertical orientations, due to the initial adsorption of chains into either horizontally or vertically aligned segments, which then act as nucleation points for the crystallites. The narrow peak widths along the qxy direction (0.038 Å−1; see the Supporting Information, Figure 2a), suggest a surprisingly well-aligned crystalline structure, with coherence of length 2π/FWHM = 165 Å, spanning many crystal planes. In contrast, the out-of-plane peak widths (0.1 Å−1; see the Supporting Information, Figure 2b) are broader and only show coherence of around 63 Å, likely because the qz direction is limited by the total film thickness. The broad in-plane chain− chain packing peak at qxy = 1.64 Å−1 has a peak width of 0.15 Å−1 and a coherence length of 42 Å, suggesting either that the chain−chain packing is slightly less well-ordered than the end5698

DOI: 10.1021/acs.chemmater.8b02150 Chem. Mater. 2018, 30, 5694−5703

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Figure 5. (a, b) SEM images of the thiol multilayer films after exposure to air for 2 weeks. (c) A higher magnification image of a thiol crystallite. Dashed guidelines are added to highlight crystallite facets. (d) AFM image of a crystallite, showing it has a height around 200 nm. (e, f) SXI/XPS scans taken of a crystallite (red) and a non-crystallite (blue) region of a sample.

experiments could include modulating the copper oxide thickness and the chain length of the exposed thiols. The SAM quality and degree of packing within SAMs is often measured with techniques like WCA and FTIR spectroscopy.4,5 For example, as the SAM reaches a dense layer with tight packing, the WCA approaches a plateau, which has been reported to occur at a value near 110° for DDT on copper.5 FTIR spectroscopy is also sensitive to the crystallinity of the packing of the alkane tails of thiols. For well-formed, highly crystalline SAMs, the ideal9,29,37 symmetric (νsym(CH2)) and asymmetric (νasy(CH2)) CH2 stretching modes are ∼2850 and 2917 cm−1. These two peaks shift to higher wavenumbers if the SAM is less ordered and more liquid-like.29 To compare against the properties of well-formed SAMs, the properties of the DDT layers on copper with and without acetic acid pretreatment were also examined using WCA measurements and FTIR spectroscopy. Despite their major differences in thickness and composition, both multilayer and monolayer samples showed the same water contact angle of 106 ± 1° (n = 4, where n is the number of separate samples measured). However, the roughness of the copper-thiolate multilayer film (1.2 nm rms, Supporting Information, Figure 4c) is much higher than the roughness of the thiol-on-copper monolayer5 (0.5 nm rms, Supporting Information, Figure 4e). Increased roughness has been reported to make hydrophilic and hydrophobic surfaces exhibit lower and higher WCAs, respectively,38 which may contribute to a higher WCA for the copper-thiolate multilayer film than would be expected from just the chemistry. FTIR spectra of the CH2 stretching region for 1 cm2 samples are shown in Figure 4d. Both spectra show the expected symmetric and antisymmetric CH2 stretching peaks (2853 and 2925 cm−1 for monolayer DDT; 2851 and 2920 cm−1 for multilayer Cu-thiolate, respectively), as well as peaks associated with the terminal CH3 group’s symmetric (νsym(CH3)) and antisymmetric (νasy(CH3)) stretching (2876 and 2965 cm−1 for monolayer DDT; 2873 and 2960 cm−1 for

the copper. However, this layer is much thinner than before, consistent with AFM measurements of the layer thickness. EELS/EDS spectra also show that this layer no longer contains copper and instead shows a thin sulfur region near the surface. Thus, in the case of DDT exposure onto copper metal, the data are consistent with a model in which thiol molecules no longer etch into the surface, preventing the formation of multilayer structures. As a result, only a monolayer of DDT is formed. It is interesting to understand what limits the ultimate thickness of the thiol-on-copper oxide multilayer film. Given that the multilayer structure appears to result from the thiol etching of copper oxide, the thickness of the oxide layer is likely to have a strong impact on the thickness of the resulting multilayer film. The laminar structure of the copper-thiolate crystallites further suggests that the thiol aliphatic tail length is likely to affect the multilayer thickness. We can roughly estimate the expected thickness of a Cu-thiolate film formed from a copper oxide layer of a given thickness if we assume that the copper atoms in the thiolate films come entirely from those contained in the copper oxide. By assuming a C−C spacing of ≈3.8−4 Å between the thiol aliphatic tail groups and a repeat bilayer thickness of 35.5 Å, we can estimate a density for the Cu-thiolates of ≈4−5 copper atoms per nm3. In comparison, the copper density in copper oxide is ∼48 copper atoms per nm3, based on a bulk density of 6.31 g/cm3. Hence, a 3 nm thick CuO film would be expected to give rise to an upper limit of 29 nm for the Cu-thiolate film thickness, if there were no loss of Cu atoms from the film. This value is within a factor of 4 of the measured film thickness of 8 nm, providing further justification that the oxide thickness plays a direct role on the resulting Cu-thiolate film thickness. The lower copper density in the Cu-thiolate could be the result of Cu lost during the etching process as a volatile product. Ultimately, additional experiments are needed to fully understand how these properties affect the resulting multilayer thickness. Such 5699

DOI: 10.1021/acs.chemmater.8b02150 Chem. Mater. 2018, 30, 5694−5703

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Chemistry of Materials

at 162.1 eV (2p3/2) and 163.2 eV (2p1/2), as well as a higher binding energy peak at 168 eV. The principal peaks therefore arise from the copper substrate and the DDT molecules, indicating that the particles as well as the film around it are largely composed of copper thiolate. However, the combination of the Cu 2p peak at 935 eV and the S 2p peak at 168 eV indicates that there is also some contribution from copper sulfonate, CuSO2C12H26, as has been assigned in the literature.25,42,43 Two weeks after crystallite formation, GIXRD also shows no major changes in the (0k0) peaks associated with the head-to-head structure, though the chain− chain stacking peak has largely disappeared (Supporting Information, Figure 6). This suggests that the conversion of the thiols to sulfonates does not disrupt the layered structure shown previously (Figure 3) but may disrupt the packing between neighboring carbon chains. To better understand the process by which the thiolate multilayer transforms into particles of copper thiolate, SEM images of three copper oxide films exposed to vapor DDT were taken 0, 1, 2, 4, 8, and 16 days after exposure to air. Representative images of the films are shown in the Supporting Information, Figure 7. Films immediately taken to the SEM after DDT exposure show no indication of particle formation. However, after a day in air, particles ∼5 μm in diameter become apparent. On the basis of these observations, a particle size analysis was performed using the ImageJ software.44,45 The resulting average particle areas and percent particle surface coverage are shown in panels (a) and (b) in Figure 6, respectively. The data show that, after the first day, the particles continue to grow in size until reaching a diameter of around 7 μm (day 4); additional air exposure beyond day 4

multilayer Cu-thiolate, respectively). The CH3 group stretching frequencies agree well with literature values of 2878 and 2965 cm−1, respectively.15 The intensity of the CH2 stretching region for the multilayer film is much higher than for the monolayer, in agreement with the expected increase in the number of CH2 groups in the multilayer film. Interestingly, the peaks of the multilayer structure are slightly less blue-shifted than the monolayer structure, indicating that the multilayer structure is “more crystalline” than the monolayer. The CH3/ CH2 peak intensity ratio for the monolayer also differs noticeably from the 1:11 ratio expected for DDT. This has been observed before for SAMs on metal and is explained as a result of the dipole moment cancellation from the metal surface, which attenuates the CH2 modes.15 The Cu-thiolate film would be expected to experience less of this effect, as the film both is thicker and has components in both horizontal and vertical orientations. These data on both monolayer and multilayer films suggest that, although both films appear to be reasonably well-ordered, care must be taken in confirming the layer thickness in the Cu-thiolate system using WCA and FTIR spectroscopy, since neither method appears to be sensitive to the film thickness. Other complementary methods must be carried out to distinguish between monolayer and multilayer films. Because several key applications of thiolate SAMs on Cu require that they serve as effective passivation layers to protect or block the underlying copper,23,25−27 the stability of the layers against various chemical agents including ambient atmosphere is important. Of particular interest is the stability of the Cu-thiolate multilayers compared to the monolayer. Here, we investigated the stability of the Cu-thiolate multilayers in air. DDT-coated copper blanket substrates were left in (a) ambient air and (b) a nitrogen purged storage container for several days and examined with SEM, Auger electron spectroscopy, GIXRD, and XPS. Samples stored in the nitrogen purging environment for 2 weeks showed negligible changes in appearance and no distinct features in SEM scans. In contrast, samples stored in air undergo large-scale coarsening into micron-sized particles of Cu-thiolate, as described below. These particles could also be seen by eye. Monolayer DDT films deposited onto acetic acid-treated copper blanket substrates did not show the presence of any particles by SEM after several weeks in air. SEM images taken of multilayer samples exposed to air for as little as 1 day (Figure 5a) developed dark patches as large as 20 μm2 corresponding to particles. Near the edge of 1 cm2 copper samples, longer dendritic particles were also visible (Figure 5b). Higher magnification of a particle is shown in Figure 5c. In this image, the edges of the particle are shown to be faceted, with several edges appearing to intersect one another at 120° angles, consistent with possible crystallite formation. AFM images taken of the particles (Figure 5d, Supporting Information, Figure 5) show that they have a thickness of a few hundred nm. This further corroborates the formation of particles rather than vacancy islands, as has been seen for alkanethiols deposited on gold.39−41 The elemental composition of these particles after 4 days in air was examined using SXI/XPS. Scans collected on top of a particle show the same composition as scans taken in regions without a particle (Figure 5e,f). In both cases, the Cu 2p fine scans show a single peak around 932.3 eV, consistent with copper metal, and the appearance of a slight shoulder around 935 eV. The S 2p fine scans show two sets of peaks: thiol sulfur

Figure 6. Plots of (a) the percent coverage and (b) the average area of the particles on the copper surface after exposure to air as a function of time. The dashed curve represents a fit of the particle area to a model (r = Bt1/n), resulting in a time constant of n = 5.2. The model only includes data through day 4 because the ripening of the film terminates once the film is fully consumed. 5700

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Chemistry of Materials does not result in a significant increase in particle size or percent surface coverage. The data suggest that copper thiolate multilayer is undergoing ripening, in which copper thiolate groups dewet from bulk regions and aggregate together into larger islands to reduce defects and minimize surface energy, as has been seen for many kinds of thin films.46 An application of the particle area data to a simple time-dependent model (r = Bt1/n) suggests that the particle sizes grow with a time constant of n = 5.2 ± 2.8 (fit is shown as the dashed line in Figure 6b). Although this is only a rough model, the time constant is within the range expected for the 2D diffusion of molecules into hemispherical crystallites47,48 (r ∼ t1/4) and the time constant expected for the dewetting of a film from a straight front (r ∼ t1/5).49,50 We postulate that the slowing of continued particle growth after 4 days results from depletion of the multilayer: as the particles continue to nucleate, the thickness of the multilayer decreases as molecules from the layers diffuse into the aggregates, eventually leading to the consumption of the film and a slowing of the particle growth. A numerical estimate of the film consumption (see the Supporting Information) suggests that the particle growth will slow by ∼4 days because a significant portion of the multilayer chains will have already aggregated into particles, consistent with the experimental time constant. However, experimental confirmation of the loss of multilayer thickness after particle ripening was not performed, since a substrate like the one shown in Figure 1b for AFM measurement, which was necessary to measure the film height relative to a fixed position, could not be obtained with a large enough copper region to fit the entirety of a particle (∼20 μm long). XPS analysis was also performed as a function of air exposure time for the copper oxide blanket substrates exposed to DDT. The copper and sulfur fine scans are shown in Figure 7a,b. Copper 2p fine scans show a single peak at 932.3 eV through day 2, indicating that the exposed copper remains primarily in the Cu1+ oxidation state during this time and is not oxidized. By day 4, a shoulder peak consistent with sulfone around 935 eV, as well as a higher binding energy peak around 943 eV (Cu2+), becomes visible, suggesting some degree of oxidation has occurred. In the S 2p fine scan, only the thiol 2p3/2 and 2p1/2 peaks at 162.1 and 163.2 eV, respectively, are visible initially. Again, by day 4, an additional sulfone peak around 168 eV becomes apparent. Thus, as the films are exposed to air, they slowly begin to degrade and become oxidized, converting the thiol to sulfone and Cu1+ to Cu2+. This rate of oxidation is comparable to what has been seen previously25 as well as measured here (Supporting Information, Figure 8) for monolayer thiols on copper. However, because the oxidation state of the underlying copper is difficult to distinguish from the oxidation state of the copper within the multilayers, it is unclear if the multilayer is able to prevent surface oxidation or not. At the very least, the multilayer is no better at preventing oxidation of the copper within the multilayers than the copper underneath a DDT monolayer. However, while exposure to air seems necessary to induce particle ripening (no particles form on samples stored in a nitrogen purging environment), the particle ripening is not driven purely by the oxidation of the multilayer, as evidenced by the following two observations: (1) the time scale of particle ripening does not coincide with the time scale of the copper oxidation, as considerable particle ripening has taken place by day 4 when very little copper or sulfur oxidation has

Figure 7. XPS fine scans of the (a) copper 2p and (b) sulfur 2s peaks of the CuO substrates exposed to DDT as a function of air exposure time between 0 and 16 days.

taken place, and (2) the particles do not show a greater degree of oxidation than the surrounding multilayer (Figure 5e,f). Further studies are thus needed to investigate the ultimate driving force toward particle formation.

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CONCLUSIONS In this work, we demonstrate that the vapor-phase deposition of dodecanethiol onto copper oxide results in the formation of several nanometer thick Cu-thiolate multilayer films, rather than self-assembled monolayers. These multilayers only form when a surface CuO is present, verifying that the etching of the copper oxide films by the thiol molecules is a key step in the multilayer formation. Given the lack of solvent present in the vapor-phase system, this further suggests that the mechanism of multilayer formation does not revolve around the dissolution and renucleation of copper ions, as has been proposed previously for solution processes,17 but involves the coordination between undissolved Cu1+ ions and thiol molecules.11 Pre-removal of the oxide with acetic acid, in contrast, prevents the formation of multilayers and instead leads to self-assembled monolayers. Multilayer films are found to be crystalline, with a lattice spacing of 35.5 Å, consistent with a bilayer thiol structure sandwiched between two layers of copper atoms. The crystallites are also well-ordered, with domains lying flat against the underlying copper surface and domains standing vertically out of the plane of the substrate. Exposure of the multilayer films to air results in a ripening of the copper-thiolate crystallites into micron-sized particles. The formation of these particles, which have similar composition to the multilayers, implies that the multilayer films have sufficient chain mobility to restructure into particles. This ripening forms on the same time scale as seen for ripening from 2D to 3D structures and is believed to stop once the multilayer film becomes depleted from the regions around a particle. The 5701

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(2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (3) Ulman, A. Formation and structure of self-assembled monolayers. Chem. Rev. 1996, 96, 1533−1554. (4) Minaye Hashemi, F. S.; Bent, S. F. Sequential Regeneration of Self-Assembled Monolayers for Highly Selective Atomic Layer Deposition. Adv. Mater. Interfaces 2016, 3, 1600464. (5) Minaye Hashemi, F. S.; Birchansky, B. R.; Bent, S. F. Selective Deposition of Dielectrics: Limits and Advantages of Alkanethiol Blocking Agents on Metal-Dielectric Patterns. ACS Appl. Mater. Interfaces 2016, 8, 33264−33272. (6) Prasittichai, C.; Pickrahn, K. L.; Minaye Hashemi, F. S.; Bergsman, D. S.; Bent, S. F. Improving Area-Selective Molecular Layer Deposition by Selective SAM Removal. ACS Appl. Mater. Interfaces 2014, 6, 17831−17836. (7) Mackus, A. J. M.; Bol, A. A.; Kessels, W. M. M. The use of atomic layer deposition in advanced nanopatterning. Nanoscale 2014, 6, 10941−10960. (8) Carbonell, L.; Whelan, C. M.; Kinsella, M.; Maex, K. A thermal stability study of alkane and aromatic thiolate self-assembled monolayers on copper surfaces. Superlattices Microstruct. 2004, 36, 149−160. (9) Dilimon, V. S.; Denayer, J.; Delhalle, J.; Mekhalif, Z. Electrochemical and Spectroscopic Study of the Self-Assembling Mechanism of Normal and Chelating Alkanethiols on Copper. Langmuir 2012, 28, 6857−6865. (10) Hosseinpour, S.; Gothelid, M.; Leygraf, C.; Johnson, C. M. SelfAssembled Monolayers as Inhibitors for the Atmospheric Corrosion of Copper Induced by Formic Acid: A Comparison between Hexanethiol and Hexaneselenol. J. Electrochem. Soc. 2014, 161, C50−C56. (11) Wang, Y.; Im, J.; Soares, J. W.; Steeves, D. M.; Whitten, J. E. Thiol Adsorption on and Reduction of Copper Oxide Particles and Surfaces. Langmuir 2016, 32, 3848−3857. (12) Calderón, C. A.; Ojeda, C.; Macagno, V. A.; Paredes-Olivera, P.; Patrito, E. M. Interaction of Oxidized Copper Surfaces with Alkanethiols in Organic and Aqueous Solvents. The Mechanism of Cu 2 O Reduction. J. Phys. Chem. C 2010, 114, 3945−3957. (13) Azzaroni, O.; Cipollone, M.; Vela, M. E.; Salvarezza, R. C. Protective Properties of Dodecanethiol Layers on Copper Surfaces: The Effect of Chloride Anions in Aqueous Environments. Langmuir 2001, 17, 1483−1487. (14) Sung, M. M.; Sung, K.; Kim, C. G.; Lee, S. S.; Kim, Y. SelfAssembled Monolayers of Alkanethiols on Oxidized Copper Surfaces. J. Phys. Chem. B 2000, 104, 2273−2277. (15) Ron, H.; Cohen, H.; Matlis, S.; Rappaport, M.; Rubinstein, I. Self-Assembled Monolayers on Oxidized Metals. 4. Superior n -Alkanethiol Monolayers on Copper. J. Phys. Chem. B 1998, 102, 9861−9869. (16) Lecordier, L.; Herregods, S.; Armini, S. Vapor-deposited octadecanethiol masking layer on copper to enable area selective Hf3N4 atomic layer deposition on dielectrics studied by in situ spectroscopic ellipsometry. J. Vac. Sci. Technol., A 2018, 36, 031605. (17) Keller, H.; Simak, P.; Schrepp, W.; Dembowski, J. Surface chemistry of thiols on copper: an efficient way of producing multilayers. Thin Solid Films 1994, 244, 799−805. (18) Schlenoff, J. B.; Li, M.; Ly, H. Stability and Self-Exchange in Alkanethiol Monolayers. J. Am. Chem. Soc. 1995, 117, 12528−12536. (19) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. Comparison of the structures and wetting properties of self-assembled monolayers of n-alkanethiols on the coinage metal surfaces, copper, silver, and gold. J. Am. Chem. Soc. 1991, 113, 7152−7167. (20) Chavez, K. L.; Hess, D. W. A Novel Method of Etching Copper Oxide Using Acetic Acid. J. Electrochem. Soc. 2001, 148, G640. (21) Closser, R. G.; Bergsman, D. S.; Ruelas, L.; Hashemi, F. S. M.; Bent, S. F. Correcting defects in area selective molecular layer deposition. J. Vac. Sci. Technol., A 2017, 35, 031509.

copper and thiol within these particles also appear to oxidize after air exposure, though on a different time scale than the particle formation. However, this oxidation occurs at the same rate as the oxidation seen for copper passivation with monolayers, suggesting that the multilayer provides no additional benefit to surface passivation than ordinary monolayer films, other than serving as a sacrificial oxidation layer. Combined, these results provide new fundamental insight into the vapor-phase formation of copper thiolate multilayers, the molecular level structure of those films, and the mechanism behind their ripening into particles during air exposure.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02150.

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AFM micrographs of patterned Cu/SiO2 surfaces and blanket copper substrates exposed to various conditions, plot of peak positions and peak widths from GIXRD patterns, AFM line scan and GIXRD pattern of particle resulting from dewetting, SEM images of multilayers after air exposure showing particle formation, XPS spectra of acetic acid-treated CuO substrates after air exposure, numerical estimation of film thickness after film dewetting (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David S. Bergsman: 0000-0002-0141-6417 Richard G. Closser: 0000-0002-8971-168X Stacey F. Bent: 0000-0002-1084-5336 Author Contributions ⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant CHE-1607339). D.S.B. would also like to acknowledge support from an NSF Graduate Research Fellowship and the Gerald J. Lieberman Fellowship provided by the Stanford Vice Provost for Graduate Education. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515.

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