Improvement of the Thermal Stability of Self-Assembled Monolayers of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Improvement of the Thermal Stability of Self-Assembled Monolayers of Isocyanide Derivatives on Gold Azuho Tsunoi, Ganchimeg Lkhamsuren, Evan Angelo Quimada Mondarte, Syifa Asatyas, Masahiro Oguchi, Jaegeun Noh, and Tomohiro Hayashi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02256 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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The Journal of Physical Chemistry

Improvement of the Thermal Stability of Self-assembled Monolayers of Isocyanide Derivatives on Gold Azuho Tsunoi,1 Ganchimeg Lkhamsuren,1 Evan Angelo Quimada Mondarte,1 Syifa Asatyas,1 Masahiro Oguchi,1 Jaegeun Noh2,* and Tomohiro Hayashi1,3,4,*

1

Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan 2

Department of Chemistry and Institute of Nano Science and Technology, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Korea

3

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 3510198, Japan 4

JST-PRESTO, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, JAPAN

*Corresponding author: [email protected] and [email protected]

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ABSTRACT: We report that the thermal stability of self-assembled monolayers (SAMs) of two isocyanide derivatives (1-pentyl isocyanide and benzyl isocyanide) on a gold surface was drastically improved by their preparation at high temperature (373 K). In the case of conventionally prepared isocyanide SAMs, thermal desorption spectroscopy (TDS) revealed the isocyanides changed their adsorption states with a corresponding increase in binding energy. The results of surface-enhanced Raman scattering spectroscopy (SERS) measurements also clearly indicated the change in adsorption states at 373 K during heating. Theoretical calculations using density functional theory (DFT) revealed that there are two stable adsorption states (atop and adatom configurations) and that the calculated vibrational energies are in good agreement with those observed in Raman spectra.

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INTRODUCTION Surface modification of solid materials with organic molecules is one of the essential techniques in the fields of surface and interfacial science, biomedicine, catalysis, and molecular electronics. A variety of approaches have been used to construct well-defined and functional organic surfaces such as coatings with polymeric materials, LangmuirBlodgett films, and self-assembled monolayers (SAMs).1-2 In particular, SAMs have been widely utilized as model organic surfaces in fundamental research.3 They have also been employed for biosensors, lubrication films, and electron transport materials in light emitting and solar cell devices3-4 because of their well-defined and highly ordered structures as well as their ease of preparation. The most popular SAM is the thiol-gold system, in which Au makes a strong bridged structure with two thiolate molecules (-RS-Au(I)-SR-) on the Au (111) surface as proposed by a model by Yates et al., confirmed by scanning tunneling microscopy, and also supported by theoretical calculations.5-6 However, Torrelles et al. recently reported that the adatom and atop configurations coexist in the case of thiols with long alkyl chains (C16), indicating that the detail of the Au-thiol interface is still under intense debate.7 Nonetheless, the information on the adsorption states of thiol molecules provided a better understanding of the system’s formation process, factors that affect its stability, and its electron transport properties.2, 4, 8-16 Despite many successes of thiol-gold SAMs in the field of biosensing and tribology, applications to electronic molecular devices has not gained much attention because of their poor electron conductivity.17-18 Addressing this issue, molecules with an isocyanide group (-NC) possess one order of magnitude greater electron conductivity than 3



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thiol molecules.19-20 The interaction between the isocyanide group and gold was understood as synergetic σ-forward- and π-back-donation interactions, and the molecule with an isocyanide group indeed showed conductivity that was one order of magnitude higher than thiol groups.20 After these reports by Chu et al.,20 many research groups have investigated the electron transport of isocyanide SAMs20-29 and indeed indicated the potential of isocyanide SAMs for electronic device applications. Another concern that has arisen is the thermal stability against local heating during the operation of devices. Unfortunately, in contrast with thiol-gold systems, there are only a few reports on the adsorption state that determines the thermal stability of isocyanide, and the conclusions of these reports are contradictory. Sohn et al.30 reported that 4-methylphenyl isocyanide molecules on Au(111) desorb at 305 K and 375 K as measured by thermal desorption spectroscopy (TDS). On the other hand, Tysoe et al.31 reported that 1,4-diisocyanobenzene molecules remain on Au(111) even at 600 K as observed by scanning tunneling microscopy (STM). Together with the lack of the understanding of the adsorption states of these molecules, the contradiction in thermal stability motivated us to elucidate via detailed analysis the interaction between isocyanide molecules and gold surfaces. In this work, we attempted to elucidate the thermal stability of SAMs of two isocyanide derivatives (1-pentyl isocyanide and benzyl isocyanide) by TDS and performed surface-enhanced Raman scattering (SERS) measurements of the same samples to investigate the adsorption states of the isocyanide derivatives on gold. In particular, we focused on the change in adsorption state by heating of the monolayers. To clarify the adsorption states of isocyanide, we focused on the desorption process and the change of adsorption state of isocyanide derivatives by heating, and we investigated the 4


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relationship between thermal stability and adsorption states of the isocyanide molecules. Finally, we demonstrated the formation of isocyanide SAMs with high thermal stability by combining the above findings and discuss the reasons for the contradictory conclusions in previous findings on their thermal stability.

EXPERIMENTAL AND THEORETICAL METHODS Preparation of Substrates and Self-Assembled Monolayers. Au substrates were prepared by thermal evaporation of Au (99.999%, Furuuchi Chemical Co., Japan) with a thickness of 10 nm onto glass substrates (roughness is smaller than 1 nm in root mean square. 22 × 26 mm2 and 10 × 10 mm2 in size for SERS and TDS measurements, respectively; Matsunami Glass Ind., Ltd., Japan) at room temperature under a pressure of less than 3×10-5 Pa. The evaporation rate of Au was 0.2 Å/s, which was monitored using a quartz crystal microbalance (QCM: XTM/2, INFICON Holding AG, Switzerland).32 SAMs of 1-pentyl isocyanide (PIC: 97%, Sigma-Aldrich, Japan) and benzyl isocyanide (BIC: 98%, Sigma-Aldrich, Japan) were prepared by immersing Au(111) substrates in ethanol solution containing the isocyanides (concentration of 1 mM) at room temperature for 24 h. After removal of the substrates from solution, they were rinsed with ethanol (99.5%, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and dried by a gentle flow of nitrogen gas. Thermal

Desorption

Spectroscopy

(TDS)

Measurements.

TDS

measurements were carried out in an ultrahigh vacuum system (EMD-WA1000S, ESCO, Ltd., Japan) quipped with a quadrupole mass spectrometer (QMG422, Balzers, Zurich). The sample temperature was monitored with a thermocouple mounted on the surface of the sample. First, mass spectra were measured during heating to identify desorbed species. 5



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Next, mass numbers corresponding to the monomers of PIC and BIC were monitored while heating at a rate of 1 K/s. Surface-enhanced Raman Scattering (SERS) Spectroscopy. SERS measurements were performed with a home-made Raman microscope system consisting of an inverted microscope (IX-71, Olympus, Japan) equipped with an objective lens (NA = 0.95, ×100, Olympus, Japan), a continuous wave He-Ne laser (λ = 632.8 nm, NEO50MS, NEOARK) and a spectrometer (HR-640, HORIBA) combined with a CCD detector (SPEC-10, Princeton Instruments).32-34 The laser irradiated the back side of the substrate, and the back-scattered signal was collected by the same objective and introduced to the spectrometer. All SERS measurements were carried out at room temperature (23 °C). Theoretical Calculations using Density Functional Theory. Theoretical calculation of adsorption and molecular vibrational energies of PIC and BIC molecules on Au(111) were performed using a program package CASTEP,35 which has been widely and successfully applied to semiconductors as well as metal surfaces. Calculations were based on density functional theory (DFT) within a generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) exchange-correlation energy functional. Conditions for the calculations were set as similar as possible to those employed by Gilman et al.36 to check the validity of the calculated adsorption energy. In all calculations, the energy cutoff was set to 400 eV. Norm-conserving pseudopotentials were used, generated according to the scheme of Troullier and Martins37 with relativistic corrections added for the Au atoms. Adsorption of PIC and BIC on an Au surface was modeled with a periodically repeated slab geometry consisting of four layers of Au atoms as shown in Figure 1. A 6


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single HNC molecule was adsorbed on the slab surface. The thickness of the vacuum layer between two slabs was set to 11.4 Å. For all calculations, atoms constituting the isocyanide molecules, Au adatom, and Au atoms at the first layer were relaxed, whereas the other Au atoms in the slab were frozen at their bulk positions. In the calculations, the maximum force criterion was set to 0.01 eV/Å. The isocyanide molecules were placed in the atop and adatom configurations. For the atop site, the molecules were placed on the center of a 2 × 2 periodic cell. For the hexagonal close-packed (hcp) adatom site, the molecules were placed on a single Au atom that was situated on the periodic cell, followed by optimization of the structure. In this work, we tested atop, adatom, hollow (hcp and fcc), bridge configurations. In the cases of the hollow and bridge configurations, the isocyanide molecules moved towards the atop site. This indicates that the hollow and bridge sites are not stable (not even local minimum) Adsorption energies were calculated by subtracting the total energy of the system in the adsorbed state from the sum of the total energies of the molecules and the substrates by themselves. The energies of these systems were calculated using the same conditions (size of the unit cell and cutoff energy).

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Figure 1 Optimized structures of PIC and BIC molecules in atop and adatom configurations in DFT calculations.

RESULTS AND DISCUSSION TDS Analysis of the Desorption Process of PIC and BIC SAMs Prepared at Room Temperature.

Figures 2(a) and (b) show thermal desorption spectra of PIC

(m/z =97) and BIC (m/z = 117) molecules adsorbed on the Au substrate, respectively. For both PIC and BIC molecules, three desorption peaks were observed (396, 427, and 506 K for PIC, and 339, 470, and 557 K for BIC). We also measured “unrinsed” samples for which the final process of rinsing with ethanol was omitted. We observed an increase in the intensity of the first peaks (at 396 K and 339 K for PIC and BIC respectively) compared with the normal sample, whereas the second and third desorption peaks remained the same (data not shown). This finding strongly indicates that the first peak is attributed to the desorption of weakly physically bound isocyanide molecules. It should 8


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be noted that these weakly bound molecules still originate from relatively strong physisorption probably due to the strong intermolecular interaction between the polar isocyanide groups and electrostatic interactions between the molecules and their image charges,

making

them

difficult

to

remove

by

rinsing

with

ethanol.

9



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Figure 2. Thermal desorption spectra of (a) PIC (m/z = 97) and (b) BIC (m/z = 117) species from the Au substrate.

There are two possibilities for the origin of the second and third peaks. One is the coexistence of two kinds of adsorption states with different strengths of moleculesubstrate interaction in the fabrication of the SAMs. The other possibility is that heating induced the transition of the adsorption state for a portion of the molecules on the surface. SERS Measurements of PIC and BIC Adsorbed on Au. For further clarification, we performed SERS measurements to explore the adsorption states of PIC and BIC on Au. For the SERS measurements, we prepared two kinds of samples for both of the ν(N-C) mode. The vibrational energy obtained by DFT calculations using models shown in Figure 1 is indicated by bars. 10


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PIC and BIC. First, “normal” samples containing weakly and strongly bound species prepared by the procedure described in the experimental section for both PIC and vacuum (in the vacuum chamber of TDS) to leave only strongly bound species, and SERS

measurements of “heated” samples were performed.

Figure 3. Raman spectra of (a) PIC and (b) BIC molecules on Au surfaces in the region of the ν(N-C) mode. The vibrational energy obtained by DFT calculations using models shown in Figure 1 is indicated by bars.

Figures 3 (a) and (b) show SERS spectra of PIC and BIC SAMs, respectively. In both cases, two N-C stretching modes: [ν(N-C)] at 2229 and 2130 cm-1 were observed for the normal samples. DFT calculations were also performed to evaluate the vibrational frequencies of PIC and BIC molecules in free (isolated) and adsorbed (atop site) states. 11



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From the results of the calculations (gray and blue bars indicated in the spectra), vibrational peaks of the ν(N-C) for the normal sample were assigned to chemically and physically bound molecules on the Au surface. Isocyanides are isoelectronic functional moieties with carbon monoxide (CO)38-39 and its bonding to metals is considered to occur in a similar manner.40 The higher ν(N-C) assigned to chemisorption at 2229 cm-1 is due to electron donation from the antibonding σ* orbital to the Au d-band resulting in a stronger N-C bond.41-43 Note that there is a discrepancy between the vibrational energy of the isolated (theoretical value) and physisorbed (experimental) molecules. We think that there are two reasons for this. One is that DFT using GGA underestimates interatomic bonding energies, resulting in lower vibrational energies. The other is that intermolecular interaction also lowers the vibrational energies. The strong dipole-dipole interaction between CN groups may reduce vibrational energy. On the other hand, after heating the substrates, a single peak was observed at 2195 cm-1 for both PIC and BIC. Gilman et al. have reported that the adsorption energy of an isocyanide molecule is 86.8 kJ/mol higher on an adatom site of the Au surface than on the atop site.36 This result is in agreement with our calculations (81.6 kJ/mol). Furthermore, we calculated the vibrational energy of ν(N-C) modes for PIC and BIC (purple lines in the spectra). Although our calculation method is known to underestimate vibrational energy,44 the vibrational energies for the adsorbed (atop) and adsorbed (adatom) are similar to the experimental results. In our investigation, other stable configurations were not found. Therefore, we assigned the peak observed in the heated sample to the ν(N-C) modes of PIC and BIC molecules adsorbed in the adatom configuration. From the collected experimental and theoretical data, the peaks for desorption 12


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of the monomers of PIC (BIC) at 427 and 506 K (470 and 557 K) [Figure 2(a) and (b)] are assigned to molecules adsorbed on the atop site and adatom site, respectively. Comparison of Energy Diagrams for Adsorption and Desorption of Thioland Isocyanide-Gold Systems. To summarize our findings and discuss the comparison with thiol-gold systems, we constructed energy diagrams for adsorption, desorption, and change in adsorption states. Here we compare the cases of thiol and isocyanide molecules with normal alkane chains (n-hexanethiol and PIC). The expected energy diagrams of the adsorption to desorption processes for the thiol and isocyanide systems are summarized in Figure 4 (a) and (b). Adsorption energy was calculated by DFT calculations, and the activation energy for desorption was obtained using the Redhead equation, which calculates activation energy from temperature for desorption. It should be noted here that we confirmed no effect of surface morphology of Au surfaces [highly crystalline Au(111) surface and the Au substrates used in this work] on the adsorption states the desorption processes of the isocyanides using X-ray photoelectron spectroscopy (XPS) and TDS. This trend is different from the trend for Au-thiol systems.12 Therefore, we concluded that the contribution of surfaces with other plane orientations [e.g. Au(100) and Au(110)] surface to the experimental results is small we here directly compare the theoretical results obtained with the assumption of an ideal Au(111) surface and experimental results obtained with the substrates used in this work. For thiol-gold systems, in the usual preparation (immersion of gold substrates into an organic solvent containing thiols at room temperature), the thiol molecules immediately chemisorb on the gold surface with the adatom configuration45 because of the small reaction barrier.46 The stability of the monolayers is mainly governed by the gold-sulfur bond and is partially affected by intermolecular interactions. During the 13



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thermal desorption of alkanethiols, dimerization of the molecules occurs through the formation of disulfide bonds. One possibility to improve their thermal stability would be the introduction of bulky terminal groups to the molecules to suppress dimerization. In the case of isocyanides, the situation is completely different. The isocyanide molecules adsorb on the gold surface (mainly in atop configuration) accompanied by the formation of a second layer (physisorption) due to strong intermolecular interactions. Through heating, the physisorbed molecules desorb first, then a part of the chemisorbed (atop configuration) species leave the surface, and the remainder of the molecules change their adsorption state from the intermediate atop to more stable adatom configuration.

14


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Figure 4 Adsorption and desorption energy diagrams for thiol (a) and isocyanide (PIC) (b) molecules on Au(111).

Fabrication of Thermally Stable Isocyanide SAMs. To prepare isocyanide SAMs with higher thermal stability, we performed a fabrication in which the molecules 15



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adsorb on the substrate only in the adatom configuration. In this process, first “normal” SAMs were prepared as mentioned in the methodology part, and then the SAMs were heated at 373 K in the vapor of the isocyanides for 15 min to maximize the molecular packing density in the monolayers. In this process, the substrate and neat liquid (5 µL) of PIC and DIC are placed in a sealed glass dish and heated. Figure 5 (a) shows TD spectra of monomer and dimer from the fabricated PIC SAM. Only a strong desorption peak assigned for the adatom configuration was observed at 540 K, indicating that we succeeded in fabricating a thermally stable PIC SAM. For the BIC SAM, desorption peaks were still observed at low temperature (340 K). However, the intensity of the peaks in the TD spectra suggests that the packing density of the molecules including molecules in the adatom configuration increased (about 150%) compared with the BIC SAM prepared by the normal procedure. The formation of the layer of weakly bound layer critically depends on the molecule, temperature and environment (in vapor or organic solvents). We expect that it should be possible to fabricate thermally stable SAMs of BIC without weakly bound molecules by optimizing the temperature and time for the heating process in the future.

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Figure 5 Thermal desorption spectra measured for (a) PIC and (b) BIC SAMs prepared by the new approach to improve their thermal stability.

CONCLUSION In this work, we studied the thermal stability and adsorption states of PIC and BIC molecules adsorbed on gold. TDS and SERS measurements revealed that the molecules changed their adsorption states with different adsorption energies by heating at temperatures higher than 373 K. DFT calculations also revealed that there are two stable configurations for both PIC and BIC (atop and adatom configurations) and that the adsorption in the adatom configuration is more stable than that in the atop configuration by 81.6 kJ/mol in adsorption energy, in agreement with experimental results. Considering both the experimental and theoretical findings, we succeeded in fabricating thermally stable SAMs. The fabrication process included heating at 373 K to switch the adsorption states from the atop to adatom configuration under the vapor of the molecules. This process dramatically improved the thermal stability of the monolayers (from 430 to 540 K in terms of desorption temperature). In the future, further optimization and improvement in the stability of isocyanide-based SAMs may be accomplished by 17



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combining other analytical techniques such as scanning tunneling microscopy to investigate the molecular packing configurations at molecular level resolution.

Acknowledgment The author (T. H.) acknowledges the financial supports by KAKENHI (19H02565, 17K20095 and 15KK0184) and JST- PRESTO. The authors appreciate the help of Ms. Kazue Taki for the administration of this project.

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Improvement of the Thermal Stability of Self-Assembled Monolayers of

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