Carboranedithiols: Building Blocks for Self-Assembled Monolayers on

Article pubs.acs.org/Langmuir

Carboranedithiols: Building Blocks for Self-Assembled Monolayers on Copper Surfaces Tomás ̌ Baše,*,† Zdeněk Bastl,*,‡ Vladimír Havránek,§ Jan Machácě k,† Jens Langecker,† and Václav Malina∥ †

Institute of Inorganic Chemistry and §Nuclear Physics Institute of the Academy of Sciences of the Czech Republic, v.v.i., 250 68 Husinec-Ř ež, Czech Republic ‡ J. Heyrovský Institute of Physical Chemistry of the Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, 182 23 Prague 8, Czech Republic ∥ Institute of Photonics and Electronics of the Academy of Sciences of the Czech Republic, v.v.i., Chaberská 57, 182 51 Prague, Czech Republic S Supporting Information *

ABSTRACT: Two different positional isomers of 1,2-dicarbacloso-dodecaboranedithiols, 1,2-(HS)2-1,2-C2B10H10 (1) and 9,12-(HS)2-1,2-C2B10H10 (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.

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INTRODUCTION

the metal surfaces as thiolates, but this interaction is still a matter of intense experimental and computational investigation.7,8 This functional group has nevertheless been extensively used for immobilization and investigation of various molecular architectures assembled in two-dimensional surface arrays on metal surfaces.1−3 The portfolio of molecules that have been investigated for SAMs has been, since the seminal study of organic sulfurous compounds on gold films in the 1980s,9 significantly extended to molecules with various added values such as rigid molecular architectures.10,11 Dicarba-closo-

Self-assembled monolayers (SAMs) have proved to be an effective tool for tuning physical and chemical properties of metal surfaces, and these materials stay in the center of scientific interest due to their molecular dimensions, adjustable chemical composition, and two-dimensional, pseudo-crystalline character reflecting a transitional phase between the molecular and bulk levels.1−3 This research area has attracted great interest from scientists and engineers ranging from technical to biological sciences, and there is currently a high interest in the fabrication of new materials based on SAMs for electronic, medical, and materials applications.4−6 Most of the SAMs that have been studied so far comprise organic thiols tethered to coinage metal surfaces. The SH groups are supposed to bind to © 2012 American Chemical Society

Received: June 8, 2012 Revised: July 23, 2012 Published: August 3, 2012 12518

dx.doi.org/10.1021/la302334x | Langmuir 2012, 28, 12518−12526

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dodecaboranethiols, often called just carboranethiols or mercaptocarboranes, have been introduced as a new class of cluster molecules for self-assembled monolayers and monolayer-protected colloids by our laboratory in 2005.12 Their use for SAMs has attracted further attention for their advantageous features such as high stability against degradation, icosahedral molecular architecture, or relatively high dipole moments.10,12−18 The previously reported use of carboranethiols as versatile ligands with tunable electron-donating and -withdrawing properties in coordination chemistry provides an interesting basis for potential comparison with the properties of their SAMs on flat metal surfaces.19−22 Most of the SAMs have been studied on gold as a convenient, inert substrate with no tendency to oxidation. The lower stability of silver and copper to corrosion led to investigating capacities of SAMs to inhibit this undesired process.23−28 Copper surfaces with adsorbed oxygen and other factors such as solvent or surfactant concentration have been shown to determine the quality of SAMs.29−32 The issue of adsorbed oxygen is of practical relevance due to the relatively high sensitivity of freshly prepared copper surfaces to oxygen and their exposure to ambient atmosphere when transferred from the preparation chamber to the solution.23 The thiol derivatives actually adsorb on oxidized Cu films. Aerobic conditions, in contrast to anaerobic, have also been shown to lead to the formation of multilayers of thiol species.33−35 With respect to the solvent, ethanol is often used as a convenient environment for the preparation of SAMs on gold films. While no serious problems have been reported on gold, copper substrates seem to be more sensitive to this solvent and some reproducibility problems with the preparation of well-organized and densely packed copper surfaces have been observed.36 In comparison, nonpolar solvents such as isooctane proved to be more suitable for this purpose. Reaction between surface Cu2O and the SH group of thiol derivatives resulting in a copper(I) thiolate and molecules of water has recently been addressed by use of density functional theory (DFT) calculations too.37 Herein we report on contrasting behavior of two 1,2-dicarbacloso-dodecaboranedithiol positional isomers, 1,2-(HS)2-1,2C2B10H10 (1) and 9,12-(HS)2-1,2-C2B10H10 (2), on copper surfaces under both aerobic and anaerobic conditions. Each of these two isomers interacts with a polycrystalline copper film in a different way. Isomer 1, dissolved in an oxygen-containing solvent such as ethanol, exhibited the ability to etch polycrystalline copper film similarly to, for example, organic carboxylic acids.38,39 In a dry and oxygen-free solvent such as CH2Cl2, isomer 1 removed the surface copper(I) oxide layer and left a sparsely covered bare Cu(0) surface. The adsorption of this isomer from its vapor formed a self-assembled monolayer in vacuum, while it formed a multilayer under ambient conditions. In comparison, isomer 2 formed a selfassembled monolayer when deposited from a CH2Cl2 solution under inert atmosphere. The major aims of this contribution are to report on the difference between the interaction of isomers 1 and 2 with a polycrystalline copper surface in aerobic and anaerobic environments and to show the optimized conditions for preparation of their self-assembled monolayers. This study additionally provides an interesting experimental proof for the chemical dichotomy of carborane-derived ligands discussed recently in the literature.21

Article

EXPERIMENTAL SECTION

Carboranethiol Derivatives and Other Materials. Carboranedithiols 1,2-(HS)2-1,2-C2B10H10 and 9,12-(HS)21,2-C2B10H10 were purchased from Katchem (Czech Republic) and additionally crystallized from hot hexane or hexane/ dichloromethane solutions before use in experiments described in this study. Their purity was checked by GC-MS and 11B and 1 H NMR spectra. Ethanol, dichloromethane, and toluene (all of p.a. grade) were purchased from Penta (Czech Republic). Ethanol was either used as received or freshly distilled from sodium, and deoxygenated under low pressure at the temperature of liquid nitrogen (specified in the text). Dichloromethane was dried over K2CO3 and freshly distilled. Toluene was dried with a mixture of sodium metal and benzophenone and freshly distilled immediately before use. Copper Films. Copper films were prepared by evaporation of copper (99.98%, Cu foil, Aldrich) onto borosilicate glass wafers (15 × 10 × 2 mm) at temperature of ∼30 °C under a vacuum of 3 × 10−8 mbar. Copper was evaporated from a resistively heated tungsten boat with a deposition rate of 2.5−3 Å/s. Adhesion of the Cu film to the glass substrate was improved by deposition of a Cr (∼2 nm) interlayer. The thickness of the copper films, monitored with a quartz crystal oscillator, was typically 500 nm. The deposition was carried out in a Pfeiffer PLS 570 high-vacuum evaporator equipped with an oil-free pumping system and a liquid N2 Meissner trap (with a normal base pressure of about 1 × 10−8 mbar). After deposition, the vacuum chamber was filled with Ar. The freshly deposited copper films were used for further experiments within 1 h after their preparation. Modification of Cu Films. There are two procedures that we used to study the interaction between Cu films and the carboranedithiol derivatives 1,2-(HS)2-1,2-C2B10H10 and 9,12(HS)2-1,2-C2B10H10. (1) The freshly prepared copper films were exposed to 7.5 mM ethanol, dichloromethane, or toluene solution (10 mL) of the dithiol derivative for different periods (up to 6 h) and subsequently washed by rinsing with an excess of pure solvent. (2) The freshly prepared copper films were exposed to vapor of the volatile dithiol derivatives 1 or 2, either in an ambient atmosphere or in a vacuum, for different periods (3, 6, and 12 h). The exposure in ambient atmosphere was done according to the following procedure: 1 g of derivative 1 (or 2) was placed at the bottom of a glass container (0.5 L). Freshly prepared copper films were put on a glass holder approximately 1 cm above the bottom of the glass container (0.5 L) to avoid direct contact with the solid derivative. The atmosphere inside the container was stirred with a magnetic bar. Modified copper films were removed at periods specified in the discussion and analyzed without any subsequent washing. Reaction between Bulk Cu2O and Isomer 1 in EtOH. Cu2O (0.5 g, 3.5 mmol) was suspended in ethanol as received (80 mL), and 1,2-(HS)2-1,2-C2B10H10 (0.73 g, 3.5 mmol) was added at once. The mixture was stirred at laboratory temperature (∼21 °C) for 24 h. A yellowish solution was filtered from the undissolved black powder, and solvent was evaporated under reduced pressure on a rotary evaporator. The yellow solid was then dissolved in ∼10 mL of EtOH and chromatographed twice on a silica gel column with EtOH as an eluting solvent. Yellow clear solution was collected, solvent was evaporated, and 0.3 g of yellow solid product was obtained. Electrospray ionization mass spectrometry (ESI MS) (1 mg/ mL in EtOH): m/z (relative abundance) 603 ([Cu6(L)4]2−, 12519


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counterelectrode, and a working electrode. Cu films (bare or modified with 1 or 2) deposited on borosilicate glass wafers (1 × 2.5 cm) were used as a working electrode and they were connected to the potentiostat with a crocodile clip; the measurements were performed in an aqueous solution of potassium hydroxide (0.5 M) bubbled with O2(g) for 10 min before use. The scanning was started from −1.3 or −0.7 V (with a 60 s period at these potentials as a sample pretreatment) toward positive potential and cycled between −1.3 (−0.7) and +0.2 V/SCE with linear polarization at a scan rate of 50 mV/s. Contact-Angle Measurement. The dynamic contact angles of water on freshly prepared and modified copper surfaces were measured on an OCA20 instrument (DataPhysics Instruments GmbH, Filderstadt, Germany) equipped with 6fold zoom lens (0.5−4.5-fold magnification) with an integrated continuous fine focus [+(−) 6 mm], high light-transmitting capacity charge-coupled device (CCD) camera with a resolution of maximum 768 × 576 pixels, and a highperformance image processing system with 132 Mbytes/s data transfer rate. The system is equipped with SCA 20 software. The following technique was used: A drop(∼2 μL) was formed at the end of a needle attached to a syringe. The drop was lowered until it touched the surface, followed by slow rising of the needle. The drop detached from the needle tip and the static angle was determined by use of the Laplace−Young equation. Dynamic contact angles were measured for these static drops by adding distilled water (0.5 μL/s) via a needle inserted into the drop from above until the drop reached a volume of ∼10 μL. An average of nine measurements of static and dynamic contact angles was done for all samples. Mass Spectrometry. MS measurements were performed on a Thermo-Finnigan LCQ-Fleet ion trap instrument with electrospray ionization (ESI) and detection of negative ions. Samples dissolved in ethanol (concentrations approximately 100 ng/mL) were introduced to the ion source by infusion (5 μL/min) with the following setup: sheath gas flow 6 μL/min, auxiliary gas flow 2 μL/min, source voltage 5.28 kV, capillary temperature 200 °C, capillary voltage −16.96 V, tube lens voltage −84.72 V, and mass range set from 50 to 2000. The negative ions corresponding to the molecular ion were observed with 100% abundance for the highest peak in the isotopic distribution plot. Agreement of the experimental and calculated isotopic distribution patterns was observed for all the assigned peaks. Data are presented for the most abundant mass in the boron distribution plot (100%) and for the peak corresponding to the m/z value. Acid−Base Titration. 1,2-Dicarba-closo-dodecaborane-dithiol 1 or 2 (typically about 100 mg) was dissolved in 50 mL of EtOH (Penta, p.a. grade) and mixed with 50 g of deaerated distilled water. The mixture was stirred and titrated with an ethanol/water (1:1) solution of NaOH (0.1 M). This mixture of solvents was necessary for dissolving the samples that are otherwise not dissolvable in pure H2O. Computational Details. The structures of the dithiols and their dianions were optimized by use of the quantum chemistry package Dalton 2.0.43 MP2 level of theory44 was chosen with 631G* basis.45,46 Since systems of identical stoichiometry were examined (an isolated proton in vacuum has neither electronic energy nor nuclear repulsion), the differences among the calculated energies were directly used as a measure of their relative stabilities.

100), Collision-induced dissociation (CID) and a comparison of the experimental and theoretical isotopic distribution patterns are shown in the Supporting Information. IR (KBr) ν/cm−1 3633 (s), 3440 (vs), 2971 (w), 2571 (BH, vs), 1610 (vs), 1385 (m), 1251 (w), 1172 (w), 1128 (w), 1065 (s), 1035 (m), 1008 (m), 967 (m), 918 (w), 899 (w), 882 (s), 863 (s), 793 (m), 753 (w), 724 (vs), 658 (m), 632 (w), 616 (w), 582 (w), 560 (w). X-ray photoelectron scectroscopy (XPS) data for this sample are shown and discussed in the Results and Discussion section. Analytical Methods. X-ray Photoelectron Spectroscopy. XPS measurements were carried out on an ESCA 3 Mk II instrument with nonmonochromated Al Kα radiation, operated at 200 W (10 kV × 20 mA). The pressure in the XPS analysis chamber during spectra acquisition was ∼5 × 10−9 mbar. The spectra were collected at a takeoff angle of 45° with respect to the surface normal. The spectra of Cu 2p, S 2p, B 1s, C 1s, and O 1s photoelectrons and Cu L3M45M45 Auger electrons were measured. Survey scan spectra (0−1000 eV) and highresolution narrow spectra were acquired with analyzer pass energies of 50 and 20 eV, respectively. The overlapping peaks were resolved into individual components by use of the lines of Gaussian−Lorentzian shape and the damped nonlinear leastsquares technique. Spectra of the S 2p photoelectrons were fitted with a doublet with spin−orbit splitting 1.18 eV and intensity ratio 2p3/2:2p1/2 = 2:1. Atomic concentration ratios were calculated from XP spectra integrated intensities after subtraction of a Shirley-type background40 and a homogeneous sample was assumed. The accuracy of the measured electron energies was ±0.1 eV. Proton-Induced X-ray Emission and Rutherford Back Scattering. Proton-induced X-ray emission (PIXE) and Rutherford back scattering (RBS) analyses were conducted in a vacuum target chamber with a 2.92 MeV proton beam of 3 mm diameter using a 3.5 MV van de Graaff accelerator at the Nuclear Physics Institute in Ř ež near Prague. While PIXE was used to determine the thickness of etched copper films and total concentration of sulfur atoms per surface area, the RBS method was used for an independent verification of the PIXE results and to measure also the total concentration of boron atoms per surface area on Cu films modified from a vapor phase of the studied carboranedithiols. The measurement of one sample lasted typically 500 s. The measurement geometry was set up with an ion beam perpendicular to the substrate surface. Two PIXE detectors were placed at 135° and 125° scattering angles; the RBS detector was at 160° scattering angle.41 RBS spectra were evaluated with the SIMNRA program.42 The accuracy of experimental concentrations as determined from PIXE analysis is estimated to be about 5%, which combines the uncertainty of the spectra fitting and quantitative calibration. The errors of RBS data are analyzed within the SIMNRA program data processing. Atomic Force Microscopy. Measurements were performed in the contact mode on a scanning probe microscope (Digital Instruments Nano-Scope III, Santa Barbara, CA) equipped with a silicon nitride cantilever (NP-S Veeco, Camarillo, CA) with spring constant 0.12 N/m. The images were taken under ambient conditions in air. The scan rate was set to 1 Hz, and the operating set point was 2 V. Electrochemical Measurements. Voltammetric curves were obtained with a Autolab potentiostat (EcoChemie, The Netherlands). A three-electrode cell was used that consisted of a saturated calomel reference electrode (SCE), Pt plate 12520


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RESULTS AND DISCUSSION Copper films were modified with the positional isomers 1 and 2 by two different experimental procedures: first, by exposing the substrate surface to a solution of the respective isomer, and second, by exposing the substrate surface to vapor of the volatile isomers. Both procedures, when applied to copper surfaces, afford different products and are discussed in this paper with respect to their complementary character to each other. Copper Etching. Polycrystalline copper films showed etching when exposed to 1 in ethanol at ambient conditions. Figure 1 shows the thickness of a copper film, originally about

later after experimental and theoretical analysis of the acidities of both isomers. Acidity of Isomers 1 and 2. With respect to the ability of isomer 1 to etch a polycrystalline copper film in contrast to isomer 2, we analyzed the acidic character of SH groups in both isomers and computationally investigated the relative stabilities of their deprotonated forms. Both derivatives were titrated with an aqueous solution of sodium hydroxide as a strong base, and the titration curves are displayed in Figure 3. The results show

Figure 3. Acid−base titration curves of 1, 2, and benzenethiol for comparison. Figure 1. Thickness of copper films exposed to a 7.5 mM solution of 1 in toluene, dichloromethane, ethanol as received, and ethanol dry and free of oxygen for different periods (1−6 h) as determined from PIXE.

that isomer 1 is a strong diprotic acid while isomer 2 is a weak acid. Benzenethiol is shown for comparison as a representative of aromatic organic thiols. Acidities of both isomers 1 and 2 can be explained by different electronic effects of the carboranyl moieties.47 In 1, the SH groups are attached to the carborane cluster so that the carboranyl moiety shows electron-withdrawing effect to the SH groups and makes them more acidic. In 2, the effect of the carboranyl moiety is opposite (electrondonating), which makes the SH groups less acidic. The greater acidity of 1 in comparison to 2 is represented with the relative stabilities of their deprotonated forms, 12− and 22−, shown in Figure 4. Isomer 1 is less stable than isomer 2 in its free

480 nm thick, exposed to 1 in different solvents for periods of 1−6 h. The etching rate in EtOH was approximately 70 nm/h, and the film completely dissolved in approximately 7 h. Figure 2 compares atomic force microscopy (AFM) images of a

Figure 2. AFM images (5 × 5 μm) of a 500 nm thick copper film, (A) as-prepared and (B) etched in an ethanol solution of 1,2-(HS)2-1,2C2B10H10 for 3 h.

copper film as prepared and exposed to an ethanol solution of 1 at ambient conditions for 3 h. These images demonstrate a nonuniform etching of the polycrystalline copper film, resulting in an increase of the surface roughness (rms) from 1.5 to 11.8 nm. Supporting Information shows in more details the topographical changes investigated via AFM. In other solvents, including dry and deoxygenated EtOH, Cu films did not dissolve. Analogous properties have not been reported for aliphatic and aromatic thiols that have otherwise been largely studied for SAMs with ethanol as a solvent of choice.25 Further investigation of the experimental conditions for the preparation of a self-assembled monolayer (1-SAM) on a copper surface proved that the presence of oxygen is one of the key factors determining the course of the reaction. Therefore, the adsorption of both isomers 1 and 2 on a copper surface from their vapor in an ambient atmosphere with excess oxygen as well as in vacuum was carried out and the results are discussed

Figure 4. Relative stabilities of 1 and 2 and of their deprotonated forms 12− and 22−. All values are in hartrees. Hydrogen atoms in vertices of the icosahedra are omitted for clarity, and the 1 and 2 positions of carbon atoms in the skeleton are marked with larger black dots.

(protonated) form and deprotonates more easily to 12−, which exhibits a lower energy than 22−. This is in agreement with the greater mesomeric stabilization of the negative charge on thiolate sulfur atoms attached to the carbon atoms in isomer 1 than to the boron atoms opposite carbons in isomer 2.13,48 The schematics of protonated and deprotonated forms of both isomers are shown also in Figure 4. Positions of carbon atoms are depicted by larger black dots. Greater acidity of 1 than 2 can also be deduced from the chemical shift of SH in the 1H NMR 12521


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independently verified by PIXE and RBS analyses of copper films exposed to 1 in ambient atmosphere for different periods.

spectrum. While SH groups in 1 exhibit a chemical shift of SH proton at 3.77 ppm, isomer 2 shows a value of 0.61 ppm. These NMR data are in agreement with those previously published for monothiolated isomers 1-SH-1,2-C2B10H11 and 9-SH-1,2C2B10H11, 3.95 and 0.45 ppm.48 Copper Films Modified with 1 and 2 from Vapor Phase. XPS analysis of a freshly prepared copper film displays the presence of a surface copper(I) oxide layer (thickness ≤1 nm) as a consequence of its exposure to ambient atmosphere during the transfer from evaporation chamber to argon atmosphere. Exposure of this film to vapor of 1, in ambient atmosphere as well as in vacuum, leads to a complete replacement of oxygen atoms from the surface copper(I) oxide. The results of elemental analysis as determined from XPS are displayed in Table 1 and show that a copper surface

Cu 2O + 1,2‐(HS)2 ‐1,2‐C2B10H10 → 1,2‐(CuS)2 ‐1,2‐C2B10H10 + H 2O

(1)

PIXE and RBS methods were used to analyze sulfur and boron concentrations per surface area on copper films exposed to vapor of 1 for different periods (1, 2, 5, 7, 8, and 12 h) in ambient atmosphere. The results are shown in Figure 5 and

Table 1. Atomic Concentrations of Elements on Cu Films as Prepared and Modified with 1 and 2a sampleb

S

B

Cu

CCB

CCHc

O

A B C D E F G H

2.0 2.0 2.0 2.0 2.1 2.0 2.1 1.3

10.3 10.1 9.9 10.0 10.0 10.0 10.0 6.6

1.9 4.0 51.3 11.1 2.7 25.0 4.5 1.0

2.0 2.2 2.3 1.9 1.9 2.0 2.0 1.3

0.9 3.2 32.5 2.8 2.9 7.1 3.3 1.2

0.3 1.7 35.2 1.3 1.4 8.1 2.0

Figure 5. Atomic concentrations of sulfur and boron atoms determined from PIXE and RBS.

a

Determined from XPS analysis; a homogeneous sample was assumed. (A) Cu film exposed to vapor of 1 at ambient conditions for 3 h. (B) Cu film exposed to vapor of 1 in vacuum for 3 h. (C) Cu film exposed to vapor of 2 at ambient conditions for 3 h. (D) Cu film exposed to a CH2Cl2 solution of 1 for 3 h. (E) Cu film exposed to a CH2Cl2 solution of 2 for 3 h. (F) Cu film exposed to a CH2Cl2 solution of 1 for 5 h. (G) Cu film exposed to a CH2Cl2 solution of 1 for 5 h and additionally to a CH2Cl2 solution of 2 for 1 h. (H) Copper complex prepared by dissolution of bulk Cu2O in an ethanol solution of 1. c Adventitious carbon.

prove, concordantly with XPS, that a thicker layer of copper film is affected by this exposure. All samples exhibit the B:S ratio of approximately 5:1 which fits the nominal stoichiometry of the molecule. The plots in Figure 5 additionally show that the concentration of both elements increases nonlinearly due to a diffusion barrier made by molecules of 1. Concentrations of boron and sulfur atoms are displayed in number of atoms per square centimeter, and for better understanding of these results we estimate the number of boron and sulfur atoms in a densely packed monolayer per square centimeter to be approximately 2.5 × 1015 and 0.5 × 1015, respectively. This raw estimation is calculated with respect to previously published data for densely packed monolayers.12 The number of boron and sulfur atoms on a copper film exposed to vapor of 1 for 3 h at ambient conditions then corresponds to an equivalent of approximately 100 monolayers. Exposure of a copper surface to 1 in vacuum avoided oxygen and afforded a densely packed self-assembled monolayer, 1SAM, with the corresponding surface composition and oxidation states. Similarly to the sample prepared at ambient conditions, the copper surface exhibited a complete replacement of oxygen atoms from the surface copper(I) oxide, but in contrast, this sample was dominated by bulk copper, Cu(0). Cu film modified from vapor of 2 in ambient atmosphere exhibited significantly lower concentrations of boron and sulfur atoms than a Cu film modified under the same conditions with 1 and also displayed carbon and oxygen contamination characteristic of the as-prepared copper film (see Table 1). Unlike 1, the vapor of 2 does not promote the air oxidation of copper, and no thick layer containing the species is formed. The concentration of 2 adsorbed on a copper surface from the vapor phase in ambient atmosphere is even lower than would correspond to a monolayer; that can be explained by the lower volatility of 2, inferred from its stronger polarity compared to 1.13,15

b

exposed to vapor of 1 in ambient atmosphere for 3 h is dominated by B and S with mutual ratio 5:1, which fits the nominal stoichiometry of the molecule, 1,2-(HS)2-1,2C2B10H10. Only a negligible amount of oxygen with the binding energy value typical of organic -CO- moieties remains on the surface as a minor contamination originating probably from the ambient atmosphere. An interesting aspect of the XPS analysis of this sample is the relatively low intensities of the Cu peaks in the spectrum in comparison with the sample prepared in vacuum. The value of Auger parameter, 1848.5 eV, corresponds to a monovalent copper49 that dominates the sample. Equation 1 shows an idealized reaction between isomer 1 and copper monoxide (Cu2O). Stoichiometry of the reaction product is in agreement with the elemental composition of the copper surface exposed to vapor of 1 at ambient conditions for 3 h as determined from XPS analysis. In comparison, a copper surface exposed to vapor of this isomer in vacuum leads to higher intensities of Cu peaks, and the Auger parameter (1851.5 eV) shows that the sample is dominated by bulk copper, Cu(0). This experiment yields a densely covered copper surface and its composition corresponds to a self-assembled monolayer of 1, 1SAM. The experiment conducted in an ambient atmosphere with an excess of oxygen provides conditions for continuous oxidation of copper and its subsequent reaction with 1 to yield a thicker modified layer of the copper film. This was 12522


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Table 2. Measured Core Level Binding Energies and Full Width at Half-Maximuma of Spectral Lines for Cu Films as Prepared and Modified with 1 and 2 sampleb

S 2p3/2

B 1s

Cu 2p3/2

C 1s

O 1s

A B C

162.7 (1.7) 162.7 (1.7) 162.5 (1.5)

188.7 (1.9) 188.7 (1.8) 189.6 (2.0)

932.8 (1.6) 932.7 (1.6) 932.4 (1.5)

284.4 (1.7)

532.5 (2.4) 530.4 (1.5)

D

162.8 (1.9)

189.3 (2.1)

932.5 (1.5)

E

189.5 (1.8)

932.5 (1.6)

F

162.3 (2.0) 167.2 (2.0) 162.9 (2.2)

189.1 (1.9)

932.5 (1.5)

G

162.4 (1.7)

189.6 (1.9)

932.5 (1.6)

H

162.3 (1.8)

188.6 (2.0)

931.7 (1.8)

284.7 286.1 288.2 284.5 286.5 284.5 286.5 284.5 286.3 284.6 286.6 284.8 286.4

(1.6) (1.6) (1.6) (1.5) (1.5) (1.7) (1.7) (1.7) (1.7) (1.6) (1.6) (1.7) (1.7)

529.9 (2.0) 531.6 (2.0) 530.5 (2.1) 530.4 532.1 530.5 531.9

(1.5) (1.6) (2.3) (2.3)

Given in electronvolts. b(A) Cu film exposed to vapor of 1 at ambient conditions for 3 h. (B) Cu film exposed to vapor of 1 in vacuum for 3 h. (C) Cu film exposed to vapor of 2 at ambient conditions for 3 h. (D) Cu film exposed to a CH2Cl2 solution of 1 for 3 h. (E) Cu film exposed to a CH2Cl2 solution of 2 for 3 h. (F) Cu film exposed to a CH2Cl2 solution of 1 for 5 h. (G) Cu film exposed to a CH2Cl2 solution of 1 for 5 h and additionally to a CH2Cl2 solution of 2 for 1 h. (H) Copper complex prepared by dissolution of bulk Cu2O in an ethanol solution of 1. a

Copper Films Modified with 1 and 2 from Solution. Adsorption of 1 and 2 on a Cu surface from oxygen-free solutions represents an alternative approach to their adsorption from gas phase in vacuum. Both procedures avoid the exposure of the surface to an excess of oxygen during the adsorption process. Although we showed earlier that a solution of 1 in dry and oxygen-free ethanol does not etch copper film, we avoided further use of this solvent with respect to its ability to easily dissolve oxygen. Instead, we used dichloromethane, which, as shown in Figure 1, did not facilitate copper film etching either. A point of interest related directly to the adsorption experiments conducted from a solution is that the effect of different volatility of the two isomers, which played a role in the adsorption from vapor, is eliminated when two solutions of equal concentration are used. Results of XPS analysis of these samples are displayed in Tables 1 and 2. Both isomers effectively replaced oxygen from the surface copper(I) oxide, and the binding energy values of S 2p electrons suggests that there is no difference in binding from solution and from vapor phase. Surprisingly, a significantly higher concentration of boron and sulfur atoms was observed on a copper surface exposed to the solution of isomer 2 in comparison to isomer 1. The results of the copper film modification by the solution of 1 deserve a separate discussion. The surface of a copper film exposed to a CH2Cl2 solution of 1 exhibited only bulk copper, Cu(0), as shown in the Auger CuLMM spectra displayed in Figure 6. This can be explained by the complete removal of the surface copper(I) oxide as well as by a surprisingly low density of molecules of 1 on the surface. These results suggest that isomer 1 reacted with the surface copper(I) oxide to form a product that dissolves in the CH2Cl2 solution. This leads to an almost bare copper(0) surface as indicated by the Auger spectrum (Figure 6) and we verified this result in an experiment with exposure time of 5 h, after which even smaller concentrations of B and S atoms were indicated on the surface. With respect to a relatively high concentration of molecule 1 in the 7.5 mM solution, an interesting issue of the adsorption ability of this isomer on an oxide-free copper(0) surface arises. For the purpose of comparison with the adsorption of isomer 2

Figure 6. CuLMM spectra of copper films: (1) exposed to vapor of 1 in vacuum for 3 h; (2) exposed to CH2Cl2 solution of 1 for 3 h; (3) exposed to CH2Cl2 solution of 2 for 3 h; (4) as-prepared Cu film; and (5) a sample of bulk Cu2O.

on a oxide-free Cu(0) surface, a sample of copper film exposed to a CH2Cl2 solution of 1 for 5 h was subsequently washed and immediately exposed to a CH2Cl2 solution of 2 for 1 h. This period proved to be sufficient to densely cover the surface with molecules of 2. Reaction between 1 and Bulk Cu2O. To better understand the reaction between the oxidized copper surface and isomer 1, we dissolved bulk Cu2O in an EtOH solution of 1 to obtain a yellowish solution, which was filtered and an excess of the solvent was evaporated under a reduced pressure to yield a yellowish solid. More details are provided in the Experimental Section. The product was characterized by XPS, MS, IR, and Raman spectroscopy and boron elemental analysis. Electrospray ionization (ESI) mass spectrometry (MS) showed a peak at 603, which corresponds to a dianionic complex of the formula [Cu6(L)4]2−, where L is 1,2-(S−)2-1,2-C2B10H10. Experimental ESI mass spectrum and fragmentation of the complex are shown in the Supporting Information together with a comparison of the experimental and calculated isotopic 12523


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contact angles assesses the hydrophobic or hydrophilic character of the substrate surface. In the present study, this method was used to follow the changes on copper films exposed to vapor of volatile derivatives 1 and 2 for different periods (3 and 6 h). Both static and dynamic contact angle values are summarized in Table 3. The data independently

distribution patterns. Elemental analysis of boron (experimental value 30%, calculated 32%) fits the summary formula of Cu2[Cu6(L)4]. XPS analysis showed that the sample is composed of Cu(I) only. Although we were unable to grow crystals suitable for single-crystal X-ray diffraction analysis and 11 B NMR showed only a broad peak at −7.65 ppm (in ethanold6 anhydrous) the idealized equation 1 can be refined as follows:

Table 3. Static and Dynamic Contact Angles of Water on Copper Films, As-Prepared and Modified with Carboranethiolsa

4 Cu 2O + 4 1,2‐(HS)2 ‐1,2‐C2B10H10 → Cu 2[Cu6(1,2‐(S−)2 ‐1,2‐C2B10H10)4 ] + 4 H 2O

Cu film

(2)

as-prepared exposed to 1, exposed to 1, exposed to 1, first drops second drops third drops exposed to 2,

Electrochemical Characterization of Cu Films, AsPrepared and Modified with 1 and 2. The investigations described so far in this paper have provided basic information on the interactions between polycrystalline copper surfaces and carboranedithiols 1 and 2 and on optimized conditions for preparation of SAMs of both isomers. Cyclic voltammetry is another useful tool for investigating the properties of modified metal surfaces. We used an aqueous solution of potassium hydroxide bubbled with oxygen as a convenient and previously reported system50−52 for testing the stability of a copper film modified with 1 and 2 against oxidation. The cyclic voltammetric current−potential (i−E) curves are shown in Figure 7. The voltammogram for as-prepared copper film

ambient, 3 h ambient, 6 h vacuum, 1 hb

CH2Cl2, 3 h

Θstat, deg

Θadv, deg

Θrec., deg

72.7 (0.9) 69.9 (0.9) 70.7 (1.8)

80.7 (0.5) 85.4 (0.9) 84.5 (1.7)

22.8 (1.0) 28.4 (1.0) 28.4 (2.2)

49.1 65.6 74.3 59.2

72.7 79.7 82.3 73.4

19.1 24.8 25.2 17.3

(2.8) (2.5) (2.4) (1.4)

(3.7) (1.8) (1.1) (1.2)

(1.5) (2.0) (0.5) (0.9)

Θstat, Θadv, and Θrec are static, advancing, and receding contact angles of water. Standard deviations are shown in parentheses. bCu surface modified with isomer 1 from vapor phase in vacuum exhibited instability in air. a

support results of XPS, PIXE, and RBS analyses. Copper surfaces exposed to vapor of 1 for 3 and 6 h exhibit effectively the same values of contact angles of water, which indicates constant interface structure and composition. This interestingly complements the XPS, PIXE, and RBS analyses that show a greater modified layer of copper film after exposure to vapor of 1 at ambient conditions. Contact angles of water on a freshly prepared Cu surface are shown for comparison but might be influenced by organic contaminants and by partial oxidation of the surface in ambient atmosphere. Samples of 1-SAM and 2SAM have been prepared under optimized conditions: from gas phase in vacuum in the case of isomer 1 and from CH2Cl2 solution in the case of isomer 2. Both values are shifted significantly lower in comparison to the reference copper surface, which might be a manifestation of successful replacement of oxygen atoms from the surface copper(I) oxide as well as of hydrocarbon contaminants. On the other hand, we note that isomer 1 assembled on a copper surface from vapor phase in vacuum exhibited instability in air. This is demonstrated by the changes of contact angles of water measured immediately after sample removal from an inert atmosphere (first drops), and two subsequent measurements (second and third drops) at different places on the sample surface but exposed to air over the period of the previous measurement.

Figure 7. Cyclic voltammetric current response vs applied potential for copper films: () as-prepared, (---) modified with 1, and (···) modified with 2. The scanning was started from (A) −1.3 V or (B) −0.7 V, with 60 s period at this potential as a sample pretreatment, toward positive potential and cycled between the starting potential and +0.2 V/SCE by use of linear polarization at a scan rate of 50 mV/s. The region from −0.7 to 0.2 V, which covers the oxidation peaks of copper, is shown in both A and B for comparison.

significantly differs from those for films modified with 1 and 2. The as-prepared copper film displays peaks typical of polycrystalline copper in an alkaline solution.50−52 The first oxidation peak for a copper surface modified with 1 shows significantly higher intensity than for as-prepared copper film, which demonstrates that isomer 1 promotes oxidation of a copper surface. It is worth noting this aspect especially with respect to previously discussed copper film etching in an oxygen-containing ethanol solution of 1. In comparison, isomer 2 showed suppression of copper oxidation as a consequence of the surface passivation; the currents were significantly lower as shown in Figure 7. Wetting Angles on Copper Films, As-Prepared and Modified with 1 and 2. Measurement of static and dynamic

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CONCLUSIONS In conclusion, this study shows and elucidates an unprecedentedly different behavior of two positional isomers of dicarba-closo-dodecaborane dithiols, 1,2-(HS)2-1,2-C2B10H10 (1) and 9,12-(HS)2-1,2-C2B10H10 (2), on polycrystalline copper films. Isomer 1 shows etching of copper films, and two factors were determined as crucial for this reaction: first, a relatively strong acidic character of the SH groups in this diprotic acid, and second, the presence of oxygen. Copper films exposed to vapor of 1 at ambient conditions exhibit formation of a greater modified layer than only the copper surface. This was demonstrated with boron and sulfur atomic concentrations 12524


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determined from PIXE and RBS. Exposure of copper film to vapor of 1 in vacuum, avoiding the presence of oxygen, afforded a densely packed self-assembled monolayer that exhibited instability in air and promoted copper surface oxidation. Additionally, a CH2Cl2 solution of isomer 1 effectively removed oxygen atoms from the copper surface and left it sparsely covered, which raises an interesting question of its adsorption properties on a clean Cu(0) surface. In comparison, isomer 2 required a deposition from a CH2Cl2 solution to achieve a densely covered copper surface. This isomer, in contrast to 1, suppressed copper surface oxidation as demonstrated by cyclic voltammetry measurements. This study shows two positional isomers and their different reactivity with copper surfaces under aerobic and anaerobic experimental conditions and when adsorbed from a solution or from a gas phase.

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(10) Hohman, J. N.; Claridge, S. A.; Kim, M.; Weiss, P. S. Cage molecules for self-assembly. Mater. Sci. Eng. R. 2010, 70 (3−6), 188− 208. (11) Bould, J.; Machácě k, J.; Londesborough, J.; Macías, R.; Kennedy, J. D.; Bastl, Z.; Rupper, P.; Baše, T. Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces. Inorg. Chem. 2012, 51 (3), 1685−1694. (12) Baše, T.; Bastl, Z.; Plzák, Z.; Grygar, T.; Plešek, J.; Carr, M. J.; Malina, V.; Šubrt, J.; Boháček, J.; Večerníková, E.; Kříž, O. Carboranethiol-modified gold surfaces. A study and comparison of modified cluster and flat surfaces. Langmuir 2005, 21 (17), 7776− 7785. (13) Baše, T.; Bastl, Z.; Šlouf, M.; Klementová, M.; Šubrt, J.; Vetushka, A.; Ledinski, M.; Fejfar, A.; Machácě k, J.; Carr, M. J.; Londesborough, M. G. S. Gold micrometer crystals modified with carboranethiol derivatives. J. Phys. Chem. C 2008, 112 (37), 14446− 14455. (14) Hohman, J. N.; Zhang, P. P.; Morin, E. I.; Han, P.; Kim, M.; Kurland, A. R.; McClanahan, P. D.; Balema, V. P.; Weiss, P. S. Selfassembly of carboranethiol isomers on Au{111}: Intermolecular interactions determined by molecular dipole orientations. ACS Nano 2009, 3 (3), 527−536. (15) Lübben, J. F.; Baše, T.; Rupper, P.; Künniger, T.; Machácě k, J.; Guimond, S. Tuning the surface potential of Ag surfaces by chemisorption of oppositely-oriented thiolated carborane dipoles. J. Colloid Interface Sci. 2011, 354 (1), 168−174. (16) Kennedy, D. C.; Duguay, D. R.; Tay, L.-L.; Richeson, D. S.; Pezacki, J. P. SERS detection and boron delivery to cancer cells using carborane labelled nanoparticles. Chem. Commun. 2009, No. 44, 6750−6752. (17) Cioran, A. M.; Musteti, A. D.; Teixidor, F.; Krpetic, Ž .; Prior, I. A.; He, Q.; Kiely, C. J.; Brust, M.; Viñas, C. Mercaptocarboranecapped gold nanoparticles: Electron pools and ion traps with switchable hydrophilicity. J. Am. Chem. Soc. 2012, 134 (1), 212−221. (18) Juárez-Pérez, E. J.; Granier, M.; Viñas, C.; Mutin, H.; Núñez, R. Grafting of metallacarboranes onto self-assembled monolayers deposited on silicon wafers. Chem. Asian J. 2012, 7 (2), 277−281. (19) Bregadze, V. I. Dicarba-closo-dodecaboranes C2B10H12 and their derivatives. Chem. Rev. 1992, 92 (2), 209−223. (20) Jin, G.-X. Advances in the chemistry of organometallic complexes with 1,2-dichalcogenolato-o-carborane ligands. Coord. Chem. Rev. 2004, 248 (7−8), 587−602. (21) Spokoyny, A. M.; Machan, C. W.; Clingerman, D. J.; Rosen, M. S.; Wiester, M. J.; Kennedy, R. D.; Stern, C. L.; Sarjeant, A. A.; Mirkin, C. A. A coordination chemistry dichotomy for icosahedral carboranebased ligands. Nat. Chem. 2011, 3 (8), 590−596. (22) See for example: Crespo, O; Gimeno, M. C.; Jones, P. G.; Laguna, A. Gold(III) complexes with 1,2-disulfanyl-1,2-dicarba-closododecaborane. Synthesis and partial degradation reactions of [Au(S2C2B10H10)2]− salts. J. Chem. Soc., Dalton Trans. 1997, 6, 1099− 1102. (23) Laibinis, P. E.; Whitesides, G. M. Self-assembled monolayers of n-alkanethiolates on copper are barrier films that protect the metal against oxidation by air. J. Am. Chem. Soc. 1992, 114 (23), 9022−9028. (24) 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 (5), 1483−1487. (25) Caprioli, F.; Beccari, M.; Martinelli, A.; Di Castro, V.; Decker, F. A multi-technique approach to the analysis of SAMs of aromatic thiols on copper. Phys. Chem. Chem. Phys. 2009, 11 (48), 11624−11630. (26) Baše, T.; Bastl, Z.; Hávranek, V.; Lang, K.; Bould, J.; Londesborough, M. G. S.; Machácě k, J.; Plešek, J. Carborane−thiol− silver interactions. A comparative study of the molecular protection of silver surfaces. Surf. Coat. Technol. 2010, 204 (16−17), 2639−2646. (27) Di Castro, V.; Beccari, M.; Bruni, F.; Caprioli, F.; Decker, F. Comparison of the protective effect of aromatic thiols adsorbed on copper. Surf. Interface Anal. 2010, 42 (6−7), 601−604.

ASSOCIATED CONTENT

* Supporting Information S

Six figures showing a series of AFM images of copper films etched for different periods (1−3 h) and ESI MS analysis of the new [Cu6L4]2− complex. This material is available free of charge via the Internet at http://pubs.acs.org..

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AUTHOR INFORMATION

Corresponding Author

*(T.B.) Telephone +420 2 6617 3118, fax +420 2 20941502, email [email protected]. (Z.B.) Telephone +420 2 6605 3456, fax +420 2 8658 2307, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Grant Agency of the Czech Republic (Grant P205/10/0348) and by the Academy of Sciences of the Czech Republic (Grant KAN100400702).

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