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Highly Ordered Self-Assembled Monolayers of Carboxy- and Ester-Terminated Alkanethiol on Au(111): Infrared Absorption and Hyperthermal-Deposition Experiments with Cr(benzene) Ions 2

Naoyuki Hirata, Shoichiro Suga, Yuji Noguchi, Masahiro Shibuta, Hironori Tsunoyama, Toyoaki Eguchi, and Atsushi Nakajima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00292 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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The Journal of Physical Chemistry

Highly Ordered Self-Assembled Monolayers of Carboxy- and Ester-Terminated Alkanethiol on Au(111): Infrared Absorption and Hyperthermal-Deposition Experiments with Cr(benzene)2 Ions Naoyuki Hirata,†,‡ Shoichiro Suga,† Yuji Noguchi,† Masahiro Shibuta,‡,§ Hironori Tsunoyama,†,‡ Toyoaki Eguchi,†,‡ and Atsushi Nakajima*,†,‡,§

†Department

of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

‡JST,

ERATO, Nakajima Designer Nanocluster Assembly Project, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan

§Keio

Institute of Pure and Applied Science (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

Received: ABSTRACT The physical and chemical properties of carboxy- and ester-terminated alkanethiolate self-assembled monolayers (SAMs) on a gold (Au) substrate (Au(111)), which were prepared in a one-pot reaction, were investigated by means of infrared reflection absorption spectroscopy (IRAS) and hyperthermal deposition experiments with Cr(benzene)2 ions. The IRAS spectra showed that the vibrational modes of hydrogen bonded COOH and CH2 wagging are sensitive to the flatness and cleanness of Au substrates for COOH-SAM and COOMe-/COOEt-SAM, respectively. The intensities of CH2 wagging modes are temperature dependent, which is relevant to the alkyl chain ordering in the highly ordered SAM. The terminal groups of the highly ordered SAM are dissociated by hyperthermal collision events of gas-phase synthesized Cr(benzene)2 sandwich complex ions, and the adsorption properties of deposited Cr(benzene)2 are discussed in relation to IRAS and temperature programmed desorption (TPD) measurements. * Author to whom correspondence should be addressed. E-mail: [email protected]; FAX: +81-45-566-1697. 1

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1. Introduction Self-assembled monolayers (SAMs) of organic chain molecules have attracted much attention with respect to the easy fabrication of a uniform functional surface over a wide area.1-5 They include organic thiols on gold (Au) and silver (Ag) (Au/Ag-SR),6 organosilanes on silicon (Si) (Si-R),7 organic carboxylates on Ag (Ag-O2CR),8 and organic alkynes on Au (Au-CCR),9 where alkyl chains form well-packed and ordered SAMs on a substrate, constructed via spontaneous bond formation between adsorbate and surface atoms and by van der Waals interactions between adjacent alkyl chains. SAMs have been applied to corrosion inhibition, biosensors, lithography, and molecular electronics, as already described in several reviews,1-3,10-12 and the range of applications is broadening in surface and interface technology. Among the various SAMs, alkanethiols on Au substrates are extensively studied because of the oxidative stability of Au metal under ambient conditions and the ease of the preparative procedures. For advanced SAM applications, in particular, chemical modifications of the topmost SAM are attractive with respect to enhancing physical and chemical functionality. In the chemical modifications of alkanethiolate, carboxy- and ester-terminated alkanethiols are often used; -COOH, -COOMe, and -COOEt terminals are introduced because these provide a sufficient opportunity to modify the end group with other functionalized groups.13 Indeed long alkane thiols (n>6) form well-ordered and closely packed monolayers on Au(111), exteriorly exposing the functional terminals. Although the conceptual frameworks for the SAMs are schematically drawn to have a uniform and flat surface, SAMs usually include defects and disordered areas, which might reduce the ideal functionality of the topmost SAM. Since they are used as precursors for further chemical modifications,14-18 a detailed understanding of the properties and chemical stability of ordered SAMs is indispensable. As well as the cleanness of the Au substrate, its flatness is important in forming ordered and uniform SAMs, resulting in reproducible functional interfaces. In general, au Au-deposited substrate is often used to fabricate SAMs; however, its flatness is inferior to that of a single crystalline Au substrate.19 Although a SAM is formed in any case, even with a more uneven Au substrate, it should be examined to evaluate the ordering quality of the alkyl chains on the Au substrate. The ordering of the alkyl chains can be evaluated by infrared (IR) absorption,20 including sum frequency generation 2


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spectroscopy.21,22 More sensitively, chemically modified terminals exhibit ordering of alkyl chains with IR spectroscopy owing to IR activity with polar end groups .23 In this study, we focus on the quality of chemically modified SAMs; SAMs of carboxy- and ester-terminated alkanethiol are prepared on a cleaned Au(111) single crystal in a one-pot reaction, and the uniformity of the SAM is evaluated by means of infrared reflection absorption spectroscopy (IRAS). As stepped Au surfaces contain far fewer grains, some vibrational modes attributable to the SAMs are sensitive to the uniformity of the SAM, and their surface dependent behaviors are discussed as well as their temperature dependence. Indices of highly ordered SAMs are the vibrational modes of hydrogen bonded COOH for COOH-SAM,6,24 and those of CH2 wagging for COOMe- and COOEt-SAMs.20,23 For terminals of -COOH, a hydrogen-bond network on a flat Au substrate can be extensively formed, because a SAM with a uniform height on the substrate causes COOH terminal layers. Then, it is expected that the intensity of the modulated ν(C=O) mode with hydrogen bonding should be a good index for the surface flatness. For the terminals of -COOMe and -COOEt, on the other hand, since the methylene wagging mode has been found to be enhanced by polar end groups,23 the resulting prominent progressions of methylene reflect the ordering of the SAMs. The IR peak of methylene wagging is regarded as a metric to gauge the ordering of the alkyl chains. In addition, hyperthermal deposition experiments with Cr(benzene)2 ions were performed to investigate the surface reaction dynamics, where chemical modifications of the COOX SAMs cause the dissociative release of -COOX terminals and the soft-landing of Cr(benzene)2.

2. Experimental Section 2.1. SAM Preparation In the SAM preparation, two kinds of gold (Au) substrates were used: (1) a polycrystalline Au coated substrate (aurosheet(111); 10 × 10 × 1 mm3, 100 nm thickness of Au/Ti/Silica, Tanaka Precious Metals Co. Ltd., having more than 80% Au(111) facet on the surface) and (2) an Au(111) single crystal (SC) (10 × 10 × 1 mm3, orientation accuracy of 0.5 × 1014 cation complexes. The positions of these five peaks are agreement with the IR spectrum for the neutral Cr(benzene)2 complexes measured in a KBr pellet.44 These peaks are assignable to CH out-of-plane bending at 795 cm-1 (A2u), ring breathing at 972 cm-1 (A2u), CH in-plane bending at 996 cm-1 (E1u), in-plane CC ring stretching and deforming at 1429 cm-1 (E1u), and CH in-plane stretching at 3043 cm-1 (E1u), respectively. This result shows that Cr(benzene)2 cations were non-destructively soft-landed onto the substrate, losing their charge via electron transport (tunneling) from the Au(111) surface through the polymethylene chains within the SAM.24,44

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Electric Field

0.001

E1u

Molecular Axis

A2u Cr(benzene)2

(a)

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14 * * 0.5×10

(b) *

1.0×1014

* (c)

*

1.5×1014

*

1716 1742

(d) *

3043 * (E1u)

2.0×1014 1429 (E1u)

996 (E1u)

972 795 (A2u) (A2u)

3000 2800 1800 1600 1400 1200 1000 800

Wavenumber (cm-1)

Figure 8. IRAS spectra in the 700-1900 and 2700-3100 cm−1 regions at 180 K after the soft-landing of Cr(benzene)2 complexes onto C15COOH-SAM at different coverages of (a)-(d). The coverage (the number of ions) is noted in the figure. The upper right part of the figure shows the symmetric mode of Cr(benzene)2 complexes.

Strictly speaking, however, the deposition rate dependence of the IR peak intensities show that the Cr(benzene)2-derived peaks were considerably weak for the initial deposition up to 0.5 × 1014 Cr(benzene)2 complexes, which implies fragmentation of the complexes by collision or reaction with hydrogen networks of terminated carboxy groups. In fact, upward peaks at 1716 and 1742 cm-1 were observed after the deposition of Cr(benzene)2 complexes, and also the intensities of these upward peaks were rather strong for the first 0.5 × 1014 deposition of Cr(benzene)2 complexes, while the intensities increased with the complex deposition. The peaks originated from the ν (C=O) mode of the terminated carboxy groups of C15COOH-SAM, which indicates the collision-induced dissociation of COOH terminals particularly during the initial deposition events. On the other hand, the peaks derived from alkyl chains at ~2850 cm-1 (νs (C-H)) and ~2920 cm-1 (νas (C-H)) were not observed as upward peaks, although there are contaminations of adsorbed residual gases. When Cr(benzene)2 complexes penetrated into the alkyl chains of the SAM, the vibrational peaks of the CH stretching modes for alkanethiolate molecules became upward due to 18


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structural disordering of the SAM with the penetration of the projectile complexes into its interior.44 In fact, within the C17CH3-SAM the CH stretching modes for alkanethiolate molecules, including polymethylene chains, could be observed as upward peaks.24 These IR behaviors can be explained by the dissociative release of the terminated carboxy groups with the hyperthermal collisions of the Cr(benzene)2 complexes, as the hydrogen-bonded network of terminated carboxy groups would result in the dissociation rather than the soft-landing of the complexes owing to surface stiffness. Furthermore, the Cr(benzene)2 complexes are immobilized around the top of the SAM without penetrating into the alkyl chains. The molecular orientation of the deposited Cr(benzene)2 complexes was evaluated by the integrated peak intensity ratio (η) of the IR band at 972 cm-1 (A2u) to the band at 996 cm-1 (E1u).24 In the case of the ideal D6h-symmetric Cr(benzene)2 sandwich complex, as shown in the inset of Fig. 8, vibrational modes with A2u symmetry have their transition dipole moment along the molecular axis, while the modes with E1u symmetry have the dipole perpendicular to the molecular axis. From the IRAS results shown in Fig. 8, the value of η was 0.9-1.5, suggesting that the Cr(benzene)2 complexes soft-landed on C15COOH-SAM, preferentially orienting their molecular axes rather close to the surface normal direction compared to the condensed-phase complexes (η = 0.89 ± 0.07)24 although the distribution of the η value was rather wide. In the case of the soft-landing onto C15COOH-SAM prepared on an Au aurosheet, the η value was 1.96±0.59, where the deposited Cr(benzene)2 complexes were immobilized through “OH-π interactions” with the remaining carboxy groups.24,61 This result suggests that for well-ordered C15COOH-SAM on a cl-AuSC, Cr(benzene)2 complexes are immobilized with different orientations from the case with the aurosheet, owing to terminal COOH dissociation. 3.3.2 Infrared spectroscopy for Cr(benzene)2 on C15COOMe-SAM Figure 9 shows typical IRAS spectra after the hyperthermal landing of Cr(benzene)2 complexes onto C15COOMe-SAM on a cl-AuSC at 180 K. As with the deposition onto C15COOH-SAM, the peaks derived from Cr(benzene)2 complexes (795, 972, 997, 1429, and 3043 cm-1) were observed in the downward direction, while the peaks derived from the terminal group of the SAM (1444, 1744, and 2958 cm-1) were observed as upward peaks. As well as the three upward peaks assignable to the 19



terminal COOMe group, upward peaks were also observed in the 1150-1350 cm-1 region, which corresponded to the out-of-plane bending (wagging) mode of methylene groups.6,26 As discussed above, the peaks could be observed through the coupling of the polar group (C-O stretching) with the wagging motions of methylene groups, and thus the dissociation of the terminated polar ester groups lessened their peak intensities. The peaks derived from alkyl chains at ~2850 cm-1 (νs C-H) and ~2920 cm-1 (νas C-H) were not observed as upward peaks, which implies that few Cr(benzene)2 complexes penetrated into the alkyl chains of the SAM.

(a) * *

0.5×1014

(b)

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0×1014 * *

(c) * *

1.5×1014 1300-1150

2958

* * 3043 (E1u)

1744

1444

(d) 2.0×1014 0.001

1429 (E1u)

997 (E1u) 972 795 (A2u) (A2u)

3000 2800 1800 1600 1400 1200 1000 800

Wavenumber (cm-1)

Figure 9. IRAS spectra in the 700-1900 and 2700-3100 cm−1 regions at 180 K after the soft-landing of Cr(benzene)2 complexes onto C15COOMe-SAM at different coverages of (a)-(d). The coverage (the number of ions) is noted in the figure.

As with the deposition onto C15COOH-SAM, for deposition of the initial 0.5 × 1014 ions, the intensities of the downward Cr(benzene)2-derived peaks were rather weak, while those for upward SAM-derived peaks were quite strong. The IR behavior can be explained by the dissociative collisions between the Cr(benzene)2 complexes and the terminated ester groups, where terminal ester groups resulted in dissociation rather than soft-landing of the complexes. As for the molecular orientation of the deposited Cr(benzene)2 complexes, the value of η was 0.4-0.5, which indicates that the complexes tend to orient their molecular axes close to the surface 20


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parallel direction. This may be due to the CH-π interaction62 between the capping benzene rings of the complex and the hydrogen atoms of methyl groups of the SAM, close to the perpendicular direction of the alkyl chain axes of the SAM. This direction is also close to the surface parallel direction. Here two types of terminated methyl groups are considered to exist, one in the remaining ester groups without destruction, and the other being newly generated group by desorption of the methyl ester groups. 3.3.3 Infrared spectroscopy for Cr(benzene)2 on C15COOEt-SAM Figure 10 shows typical IRAS spectra after the hyperthermal landing of Cr(benzene)2 complexes onto C15COOEt-SAM at 180 K. As with the results described in sections 3.2.1 and 3.2.2, Cr(benzene)2-derived peaks (794, 972, 997, 1428, and 3044 cm-1) were observed in the downward direction, and SAM-derived peaks (1150-1350, 1741, and 2987 cm-1) were observed in the upward direction. These spectral behaviors show that the terminated ester groups of the SAM were desorbed by the collision of the Cr(benzene)2 complexes.

(a) *

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5×1014

(b) 1.0×1014 * *

*

(c) * 1.5×1014 1300-1150

1741 2987

(d)

2.0×1014 ** 0.001

3044 (E1u)

1428 (E1u)

997 (E1u)

794 972 (A2u) (A2u)

3000 2800 1800 1600 1400 1200 1000 800

Wavenumber (cm-1)

Figure 10. IRAS spectra in the 700-1900 and 2700-3100 cm−1 regions at 180 K after the soft-landing of Cr(benzene)2 complexes onto C15COOEt-SAM at different coverages of (a)-(d). The coverage (the number of ions) is noted in the figure.

21



As for the molecular orientation of the deposited Cr(benzene)2 complexes, the value of η was 0.7-1.0, which is different from that for C15COOMe-SAM, and close to that in the case of condensed-phase complexes.24 This suggests that the Cr(benzene)2 complexes are difficult to adsorb with specific orientation due to the larger steric effect or lower orientation of the terminal ethyl ester groups. 3.3.4 Temperature programmed desorption study Figure 11 shows TPD spectra for Cr(benzene)2 complexes (m/z = 208) on (a) C15COOH-, (b) C15COOMe-, and (c) C15COOEt-SAMs prepared on a cl-AuSC with a collision energy of ~20 eV. In order to reduce the aggregation and lateral interactions between the adsorbed complexes on the substrate, the number of deposited complexes was restricted to a relatively low coverage of 0.4 × 1014 complexes/cm2 (~0.2 ML). In all the TPD spectra, the thermal desorption of the supported complexes start at ~240 K and the desorption rate reaches a maximum at 260-267 K. This common behavior indicates that Cr(benzene)2 was adsorbed similarly for all the SAMs. Moreover, another desorption signal was observed at ~315 K for C15COOMe-SAM, as shown in Fig. 11(b), which may have originated from more rigidly adsorbed complexes. In order to determine the desorption process and the desorption activation energy of Cr(benzene)2 complexes soft-landed onto COOX-SAMs, Arrhenius plots63 for the TPD spectra were taken (see Fig. S6 in the Supporting Information). Linear plots were obtained only for reaction orders (a) m = 1.5, (b) m = 2, and (c) m = 3, while plots obtained by using other values were nonlinear especially in the high~263 K

Mass 208 intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) COOH-SAM ~260 K

(b) COOMe-SAM

~267 K

(c) COOEt-SAM

200

250

300

350

Temperature (K)

22


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Figure 11. TPD spectra obtained for Cr(benzene)2 complexes (m/z = 208) after the soft-landing of 4 × 1013 cations onto (a) C15COOH-, (b) C15COOMe-, and (c) C15COOEt-SAMs at 195 K. temperature range. The results showed that the Cr(benzene)2 complexes were desorbed with greater than first-order kinetics. According to the previous soft-landing studies,24,44 second-order or higher-order desorption kinetics indicate that the adsorbed Cr(benzene)2 complexes firstly begin to diffuse on the SAM substrate as the temperature is raised, followed by desorption via collision and/or gathering processes. On the other hand, the first-order desorption kinetics indicate that the complexes are desorbed independently without collision and/or gathering processes, which have been observed mainly when complexes are deeply trapped inside the SAM matrix.44 Therefore, most of the Cr(benzene)2 complexes were desorbed from near the SAM topmost outside of the alkyl chains in all the case of C15COOX-SAMs. In fact, these plots are supported by the IRAS results; the upward peaks derived from the vibrational modes of alkyl chains were barely observed for all of the SAMs, as shown in Figs. 8-10. Furthermore, the slopes of the linear fits of the Arrhenius plots yielded values of (a) ~99 kJ/mol, (b) ~76 kJ/mol, and (c) ~107 kJ/mol for the desorption activation energies of the Cr(benzene)2 complexes supported on C15COOH-, C15COOMe-, and C15COOEt-SAMs, respectively. Considering the value of ~70 kJ/mol for the desorption activation energy for the Cr(benzene)2 complexes thermally deposited via physical vapor deposition onto C18-SAM,44 the values for (a) and (c) are rather large in spite of the adsorption near the topmost surface of the SAMs. For the Cr(benzene)2 complexes soft-landed onto C15COOH-SAM, the reaction order of m = 1.5 is close to first-order desorption, implying that the Cr(benzene)2 complexes diffuse individually with difficulty on the SAM substrate due to the local OH-π interaction. For the complexes deposited onto C15COOEt-SAM, the higher activation energy (~107 kJ/mol) and the reaction order of m = 3 suggests that they were mainly desorbed from adsorption sites between the remaining terminal ester groups. Indeed, the TPD spectra consistently show that the diffusion and/or collision processes occur at higher desorption temperatures compared to the deposition of the complexes onto C15COOMe-SAM. According to all the above results, the adsorption properties of Cr(benzene)2 23



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complexes were found to vary as a result of changing the terminal groups of the SAMs in this study.

4. Conclusions SAMs of carboxy- and ester-terminated alkanethiols were prepared on a cleaned Au(111) single crystal in a one-pot reaction. Focusing on the quality of the chemically modified SAM, we investigated the uniformity of the SAM with IRAS. The vibrational modes of hydrogen bonded COOH and CH2 wagging are sensitive to the flatness and cleanness of the Au substrates for COOH-SAM and COOMe-/COOEt-SAM, respectively. Also the progression of wagging modes exhibited prominent temperature dependence, and the progression band spread out when the substrate temperature was lowered and/or when the substrate flatness was improved. The spectral results revealed the formation of highly ordered alkyl chains in the SAM on a cleaned Au(111) single crystal. The properties of the highly ordered COOX-SAM were examined by hyperthermal deposition experiments with Cr(benzene)2 ions and the dissociative release of COOX terminals was observed together with the soft-landing of Cr(benzene)2, which provided a fundamental understanding of the chemical modifications of the topmost SAM. ASSOCIATED CONTENT Supporting Information Full citation of Ref. 60, Atomic force microscopy (AFM) images (Fig. S1), IRAS spectra for C18-SAM (Fig. S2), the temperature dependence of IRAS peak intensities (Fig. S3), calculated motions for vibrational modes for CH2 wagging for a MeOOC-C15H30SH molecule (Fig. S4), calculated IR spectra for the monomer and the dimer of MeOOC-C15H30SH at B3LYP/6-31G* (Fig. S5), and Arrhenius plots taken from the TPD spectra of Cr(benzene)2 complexes (Fig. S6). The Supporting Information is available free of charge on the ACS Publications website at DOI: ***/acs.jpcc.****. Acknowledgements We are grateful to Prof. T. Ishibashi (University of Tsukuba) for fruitful discussion on wagging vibrations. This work is partly supported by JSPS KAKENHI of Grant-in-Aids for Scientific Research (A) Grant Number 15H02002. 24


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; FAX: +81-45-566-1697. Notes The authors declare no competing financial interest.

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Table 1. Vibrational mode assignments for (a) C15COOH-, (b) C15COOMe-, and (c) C15COOEt-SAMs decorated on the freshly cleaned Au(111) single crystal. observed frequency (cm-1) (a)

(b)

mode assignmenta

(c)

νas C-H (-CH3, ethyl ester) [28] νas C-H (-CH3, methyl ester) [6] νas C-H (-CH2-, alkyl) [6,28] νs C-H (-CH2-, alkyl) [6,28] ν C=O (non-hydrogen bonded) [26,28] ν C=O (hydrogen bonded) [26,28] δsc CH2 (alkyl) [6]

2987 2958 2920

2919

2920

2849

2850

2851

1740

1744

1741

1473

1466

1718 1474

1444

a

bend C-O-C (CH3OCOCH2-) [6]

δs CH3 [28]

1384

1383

1350-1150

1350-1150

1208

1209

1177

1179

progression bands (-CH2-, wagging) [6,26]

ν C-O (ester) [6,28] ν C-O (ester) [6,28]

Mode assignments according to Refs. 6, 26, and 28.

νs/as = symmetric/asymmetric stretching mode, δsc = scissors deformation mode

Table 2. Orientation angle (θ, deg.), adsorption heat (Ed, kJ/mol), reaction order (m), desorption threshold temperature (Tth, K), and desorption peak temperature (Tpeak, K) of the Cr(benzene)2 complexes soft-landed on each SAM substrate.

a

substrate

θ (deg.)a

Ed (kJ/mol)

m

Tth (K)

Tpeak (K)

C15COOH-SAM

43

99

1.5

240

263

C15COOMe-SAM

60

76

2

233

260

C15COOEt-SAM

47

107

3

244

267

14

The coverage of Cr(benzene)2 is 2.0 × 10 complexes.

31



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