Ge(111) Surface with

Subscriber access provided by Colorado State University | Libraries

Article

Exploring Azobenzenethiol Adsorption on the Ag/ Ge(111) Surface with Surface Raman Spectroscopy Ya-Rong Lee, Yi-Hung Su, Li-Wei Chou, Jia-Ren Lee, Jiing-Chyuan Lin, and Juen-Kai Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp406298h • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on October 5, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

The Journal of Physical Chemistry

Exploring

Azobenzenethiol

Adsorption

on

the

Ag/Ge(111) Surface with Surface Raman Spectroscopy Ya-Rong Lee†, Yi-Hung Su†, Li-Wei Chou†, Jia-Ren Lee‡, Jiing-Chyuan Lin*,†, and Juen-Kai Wang*,†,# Department of Physics, National Kaohsiung Normal University, Kaohsiung, Taiwan, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan †

Academia Sinica

#

National Taiwan University

‡

National Kaohsiung Normal University

RECEIVED DATE TITLE RUNNING HEAD CORRESPONDING AUTHOR FOOTNOTE ∗Corresponding author: E-mail: Juen-Kai Wang, [email protected]; Jiing-Chyuan Lin, [email protected].

ACS Paragon Plus Environment


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

ABSTRACT Self-assembled monolayers (SAMs) formed with thiols on surfaces represent the most representative system of such kind.

Their detailed adsorption orientation and kinetics are

however rarely elucidated completely, making the development of the SAM systems mostly based on try-and-error approach. We have studied the adsorption of azobenzenethiol (azoSH) on the Ag/Ge(111)-

(

)

3 × 3 R30° surface, as an archetype of SAMs on compound surfaces, with

in-situ surface Raman spectroscopy. Two different adsorbates have been identified with their vibrational signatures and orientations. They respectively correspond to the two adsorption sites of this compound surface system, owing to distinct molecule-surface interactions, and both exhibit Langmuir adsorption behavior. These traits are compared with that on the Ge(111) surface, bearing homogeneous adsorption propensity, where one precursor of adsorption has been identified. The revelation of the detailed adsorption traits of azoSH has demonstrated that surface Raman spectroscopy is expedient in revealing complex adsorption behaviors of the SAM systems.

KEYWORDS: self-assembled monolayer, thiol, Raman spectroscopy, adsorption


Page 2 of 32

Page 3 of 32

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. INTRODUCTION Self-assembled monolayers (SAMs) have received a great deal of attention for enormous potential in tailoring surface properties for variety of important applications.1,2 Thiol molecules have been prevalently studied and used in this territory during last two decades because of their high affinity to noble-metal surfaces, making possible of generating well-defined surfaces with useful and highly alterable chemical functionalities.1-3

Azobenzene represents a prototypic

photoswitching molecule based on the trans-cis isomerization of a N=N double bond.4,5 The prominent photo-mediated structural change between its two isomeric geometries which possess distinct optical, chemical, and mechanical properties have additionally inspired diverse applications based on this moiety.6-8 Therefore, its derivatives have been anchored on various surfaces through the mercapto functional group (SH) to form a monolayer coating to modulate the physical property of the surface through the photoinduced conformational switching of the azobenzene moiety.9-14 While a lot of studies of SAMs comprising the flexible alkylthiols containing azobenzene as a terminal group have been focused on the character of photoswitching14 and electron transfer,9-13 less has been concerned on the structure change during their monolayer formation on surfaces, which plays a key role in affecting the character of SAMs. How the monolayer of alkanethiols or arenethiol is grown on the noble-metal surface has been pictured as a two-step process: it begins with condensation of low-density crystalline island, characterized by surface-aligned molecular axes, and is followed by the appearance of a denser phase by realigning the molecular axes approximately along the surface normal.15-16 Such scenario is perceived based on the competitive balance between the formation of thiolates and the intermolecular interaction. Nonetheless, such fundamental study of the thiol molecules with the azo moiety has never been pursued. In this study, we take on the pursuit of the simplest version of such system—azobenzenethiol (azoSH). For this molecule, except the strong interaction between SH group and noble metal surfaces and the intermolecular interaction, the -N=N- moiety



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

of azoSH could also favorably interact with the surfaces.17,18 It is thus imperative to understand how these fundamental interactions govern the primary adsorption step, staking the basic ground for the further revelation of the more complicated photoswitching reactions of such SAM systems. Instead of the conventional Au surface, we investigate such fundamental traits on the Ag/Ge(111)-

(

)

3 × 3 R30° surface, abbreviated as Ag/Ge(111)- 3 . This surface system has

been thoroughly investigated by our group.19 The surface

3 × 3 reconstruction is formed as a

Ge(111) surface is deposited with the full coverage of silver and followed by annealing to 850 K and is stable up to 500 K. Our previous study with scanning tunneling microscopy (STM) and theoretical calculations show that two inequivalent Ag trimers and one Ge trimer form in the toplayer unit cell of the Ag/Ge(111)- 3 surface.19 This surface represents a platform substrate that bears both noble-metal and semiconductor surface characters simultaneously—some studies have been carried out on preparing thiol-functionalized Ge surfaces for electronic applications,20,21 thus providing a wonderful opportunity to understand how the adsorption of azoSH on this wellcharacterized compound surface differs from that on the well-known homogeneous Ge(111) surface and to explore the possibility of growing SAMs on the surfaces of similar type. Although many techniques have been utilized in the adsorption studies of SAMs—including scanning probe microscopy (STM) for molecule-resolved distribution of adsorbates,22 X-ray photoemission spectroscopy (XPS) for surface chemical composition,23 sum-frequency generation spectroscopy (SFG) for surface molecular orientation and vibrational identification,24 surface-enhanced Raman scattering (SERS) for detecting diminutive molecular trace on nanostructured metal surfaces,25-27 etc.

The challenge nonetheless has always been how to

acquire physically meaningful information from a minute amount of surface adsorbates (from sub-monolayer to one monolayer).

In this study, we performed in situ polarized Raman

spectroscopic measurements of the adsorption of azoSH on the Ag/Ge(111)- 3 surface in vacuum, providing several unique characteristics delineated below that are impossible or hard to ACS Paragon Plus Environment

Page 4 of 32

Page 5 of 32

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


come by with the aforementioned ones—The advantages of using normal Surface Raman spectroscopy (NSRS) to interrogate molecular adsorption on surfaces have been explained and demonstrated in our previous publications.17,28 In brief, it is capable of non-invasively acquiring the surface composition and structure from the obtained vibrational signature; its high energy resolution enables the resolution of minutely disparate interactions happening upon adsorption; the adsorbate orientation can be deduced with polarized Raman spectroscopy; the amount of adsorbates can be quantified with the procured Raman signal strength. We have taken these advantages to study various adsorbate systems: hydrogenated Si and Ge surfaces,29-31 and silbene, distyrylbenzene, and azobenzene on Ag/Ge(111)- 3 .17 In particular, we have identified the first adsorbed layer and the adsorbates above it with their distinct Raman signatures and have utilized their respective signatures to determine the molecular coverage, confirming the potential of using NSRS as a sensitive, non-invasive tool to characterize surface adsorbates.

These unique

competencies would render the NSRS study of SAMs a brand new analytic way to reveal the intricate interactions involved in molecular scale. There have been many studies of azobenzene and its derivatives with SERS. A few relevant examples are listed below. Highly ordered SAM films of azobenzenealkanethiols on flat Au(111) terraces prepared on mica was investigated with SERS, in-plane X-ray diffraction and atomic force microscopy.12 The distance dependence of enhanced Raman scattering was revealed with use of SAMs of alkanethiol of different numbers of methylene units with an end group of azobenzene as the Raman probe.32 SERS was used as a probe to study the reversible photoswitching of azobenzene-functionalized SAMs.14 Tip-enhanced Raman scattering (TERS) was performed on azobenzene thiol SAMs on Au(111) to demonstrate the tip-Au gap mode. 33 As a note, SERS—similarly for TERS—differs from NSRS in two primordial aspects. Firstly, SERS operates with the aid of some nanostructures (mostly made of noble metals) in proximity to boost Raman scattering via plasmon-mediated local enhanced optical field, thus confounding the



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

extraction of molecular orientation from scattered Raman radiation in far zone if not impossible. Secondly, the charge-transfer process at the molecule-metal junction might alter molecular Raman polarizability and even molecular configuration, hence also complicating the interpretation of the obtained vibrational signature. In contrast, NSRS, albeit producing weak Raman signal without any enhancement, provides palpable information of adsorbed molecules from the procured Raman idiosyncrasies that can even be compared directly with simulation. In this report, we first synopsize the character and stability of azoSH on Ag/Ge(111)- 3 and assign the modes of the obtained Raman spectrum. It is followed by the presentation of the change of the Raman shifts and the molecular orientation during the molecular adsorption. The comparison is then made between the azoSH deposited on the Ag/Ge(111)- 3 surface and the bare Ge(111) surface. Lastly, we discuss the two competing orientations of azoSH anchoring on the Ag/Ge(111)- 3 surface during the SAM growth.

2. EXPERIMENTS, ANALYSIS AND CALCULATION The experiments were carried out in two separate home-built ultrahigh vacuum (UHV) chambers to study the adsorption behavior of azoSH on the Ag/Ge(111)- 3 surface with different techniques. The temperature programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) studies were carried out in one UHV chamber with a base pressure of ~2×10 -10

Torr. The procedure to prepare the Ag/Ge(111)- 3 surface has been reported previously.34

Briefly, the Ge(111) surface was firstly cleaned by several Ar-ion sputtering-annealing cycles until a sharp c(2×8) LEED pattern emerged. Silver was then evaporated from a home-made Ag doser onto the Ge surface at 100 K, followed by annealing to 700 K for 1 min., yielding a

3 × 3 LEED pattern. The surface temperature was measured by a K-type thermocouple inserted into a predrilled hole at the top–center edge of the Ge sample. The commercially acquired azoSH powder (Uniwang) was purified and re-crystalized in dichloromethane. During ACS Paragon Plus Environment

Page 6 of 32

Page 7 of 32

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


the deposition of azoSH, its crucible was kept at 395 K to produce sufficient vapor pressure, while the substrate temperature was maintained at 100 K. In the TPD experiments, the sample surface was positioned ~1 mm from an aperture with a diameter of 2 mm-diameter aperture, the evaporated gas was monitored by a quadrupole mass spectrometer (QMS, UTI) in the mass range of 1-300 amu, and a heating rate of 3 K/s was used. A Mg Kα X-ray source (1.254 keV), operating at 300 W (15 kV, 20 mA), was used along with a constant analyzer with a pass energy 10 eV for the XPS study. A series of XPS spectra obtained after 1 ML azoSH adsorbed at 100 K on the Ag/Ge(111)- 3 surface followed by subsequent flashing to the indicated temperature. All the X-ray photoelectron spectra were calibrated with the binding energy of the substrate Ag3d5/2 peak at 368.3 eV and were fitted with Voigt profiles after Shirley background subtraction. The surface Raman study of azoSH adsorbed on the Ag/Ge(111)- 3 surface and the Ge(111) surface were carried out in another UHV chamber with a routine base pressure of ~3×10-10 Torr in conjunction with a Raman spectroscopy system. More detailed description has been reported elsewhere.28 For Raman measurements, an Ar-ion laser emitting at 514.5 nm served as the excitation light source. A laser-line filter was used to eliminate plasma emission lines adjunct. The laser beam was focused with a cylindrical lens and impinged, via a glass viewport, to the sample with an incident angle of 65°. Its polarization angle was varied with a half-wave plate. The scattered radiation light, collected with a lens (f-number = 1) and polarization-analyzed with a broadband polarizer, was sent through a long-pass filter to eliminate the excitation radiation and then to a 30-cm monochromator (1200 gr/mm grating, Horiba, TRIAX 320) equipped with a liquid-nitrogen cooled charge-coupled device for spectral analysis.

The resultant spectral

resolution and reproducibility were 8 and 1 cm-1, respectively. Each spectrum was accumulated for 10 or 20 min. The laser power used for this study was 45 mW and thus the resultant irradiated power density on the sample surface is about ~200 W/cm2 which produce 10K rise in the irradiated sample area.28 ACS Paragon Plus Environment


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

Page 8 of 32

The thus recorded Raman spectra were subtracted by the background measured before the azoSH dosage and then normalized with respect to the Raman intensity of the Ge(111) phonon at 857 cm-1. The profile analysis of each isolated Raman peak was performed by numerically fitting to a Lorentzian profile. For spectra containing two overlapping peaks, they were de-composed by fitting to two Lorentzian profiles with their respective peak widths at low surface coverage and at saturated coverage. The Raman spectrum of the free azoSH molecule was calculated with Gaussian03 with PBEPBE/ 6-31+G* level.35 The molecular orientation of the adsorbed azoSH on the sample surface was extracted by the analysis of the polarization-dependent spectra with a three-layer model.36 The details of the analysis and the simulation are given in Supporting Information and Ref. 36. In brief, the azoSH molecules—being simplified as simple rod radiators36—represent as a uniform film on a surface. Namely, the molecular orientation is simplified as the tilt angle of the rod, β, with respect to the surface normal direction. The vacuum, the molecular layer, and the substrate underneath thus form a three-layer system. The electromagnetics of the Raman scattering process of this system can be described by the following equation:

 Es  ω 2  Es  t t t out in (1)  p  = 2R ⋅ Aout ⋅ Rout ⋅ α ⋅ Rin ⋅ Ain ⋅  p  ,  Eout  c  Ein  t t r r where Rin ( Rout ) is the coordinate transformation matrix between Ein ( Eout ) and the molecular

t

coordinate, α is the Raman polarizability tensor in the molecular coordinate, and ω R is the frequency of the Raman radiation.36 Ain ( Aout ) is the Fresnel factor to account for the excitation (scattering) light wave transmitting through the interfaces and can be obtained based on the standard thin-film optics of the three-layer model with an assumed dielectric constant of the

t

molecular layer. α at 1440 cm-1 was calculated with Gaussian03 of PBEPBE/ 6-31+G* level. A Gaussian distribution was assumed for the orientation of the molecules on the surface: 2 p ( β ) ~ exp  − ( β − β0 ) σ 2  ,  

(2)

where β 0 is the mean tilt angle and σ reflects the distribution width. According to the optical geometry of the polarization Raman setup, the ultimate Raman scattering intensity was then ACS Paragon Plus Environment

Page 9 of 32

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


calculated as a function of the polarization angle of the excitation light wave at different tilt angles with the consideration of the inhomogeneous angular spread. This approach to derive the relationship between the excitation light wave and the resultant light wave at a Raman-shifted frequency and its dependence on the Raman tensor of the molecular adlayer, Equation (1), can be compared with the one derived by Sourisseau37, in which both 90° and 180° geometries were adopted—The Raman radiation is collected in a direction normal to the excitation direction in a 90° geometry while it is collected in a backward direction in a 180° geometry. Nonetheless, these geometries are not applicable to the current experimental geometry.

3. RESULTS AND DISCUSSION 3.1. Adsorption Characteristics of azoSH on Ag/Ge(111)- 3 . Before presenting the Raman results, the traits of azoSH molecule adsorbed on Ag/Ge(111)- 3 are first described. For 1 ML azoSH, defined below, adsorbed on Ag/Ge(111)- 3 at 100 K, the XPS spectrum exhibits a S2p band at 163~166 eV with a typical doublet character that reflects the 2p3/2 and 2p1/2 peaks at 163.6 and 164.8 eV, respectively, after spectral de-convolution, as shown in Figure 1. According to previous investigations ,3,38,39 this XPS signature is associated with the adsorbed azoSHs on the surface with spin-orbit splitting of 1.2 eV and area ratio of 2:1. Upon increasing the annealing temperature to ~400 K, the S2p doublet band became broadened and weaker, and was red-shifted to 161.7 and 162.7 eV, respectively, as shown in Figure 1, indicating the formation of the substrate-S bond on the Ag/Ge(111)- 3 surface.38,39 Figure 2(a) shows the integrated intensities of the S2p bands of the adsorbed azoSH, the thiolate (azoS-substrate bond), and the free S atom as a function of the annealing temperature for the monolayer azoSH adsorbed on the Ag/Ge(111)3 surface. The adsorbed thiol signal undertakes a drastic drop at 350 K, decreases to 4% at 400 K, and reaches to the noise level at 500 K; the thiolate signal, on the other hand, starts increasing at 350 K and reaches a maximum at 450 K, followed by a decrease; the free S signal follows a similar trend as the thiolate signal except that its signal remains beyond 450 K. The shift in the relative strength between the adsorbed thiol and thiolate signals indicates that the formation of ACS Paragon Plus Environment


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

Page 10 of 32

thiolate starts at 350 K and is accompanied with the decrease of the surface azoSH molecules on the surface. Furthermore, notice that the maximal thiolate signal is significantly less than the maximal adsorbed thiol signal, suggesting that some of the surface azoSH molecules desorb from the surface. This finding is confirmed by the corresponding TPD spectrum of the parent ion (amu = 214)—Figure 2(b): the surface azoSH molecules desorb predominantly at 400 K.

This

behavior was similarly observed on the Ge(111) surface. The results above indicate that azoSH molecules weakly adsorb on the two sample surfaces at 100 K, setting a firm ground for the indepth study using surface Raman spectroscopy. 3.2. Identification of Raman Modes. The presentation of the Raman results starts from the mode assignment of the Raman spectra of azoSH in crystalline form at room temperature and 1 ML azoSH adsorbed on the Ag/Ge(111)- 3 and Ge(111) surfaces at 100 K.

The Raman

spectrum of crystalline azoSH, acquired with a commercial Raman microscope and shown in Figure 3(a), exhibits nine prominent peaks from 1000 to 1600 cm-1, which are similar to the ones of azobenzene 17 as expected except the two peaks at 1079 and 1396 cm-1, because azoSH merely bears an extra SH group attached to the para-position of azobenzene. The other peaks shown in Figure 3(a)—which can be assigned accordingly with the help of the calculated Raman polarizability of free azoSH molecule, shown in Figure 3(b) —corresponds to the fingerprints of in-plane molecular vibrations of N-N, C-S, and C-N stretching, and ring vibration and deformation. The assignments of the Raman-active modes40 are summarized in Table 1 and are described below. The strong peak at 1437 cm-1 is attributed to the N=N stretching combined with the inphase displacement of two phenyl rings C-C-C asymmetric stretching and bending, whereas the peak at 1582 cm-1 is similar in character but with the in-phase displacement of two phenyl rings C-C-C symmetric stretching and C-H bending. Similarly, the small peak at 1465 cm-1 and a weak shoulder at 1481 cm-1 are also attributed to the N=N stretching in conjunction with the


Page 11 of 32

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


phenyl ring vibrational mode. Another main peak at 1144 cm-1 and the small one at 1187 cm-1 are referred to the C-N symmetric stretching coupled with the C-H bending mode on phenyl ring. The mode of the C-H bending-out coupled with C-N stretching-out is 1144 cm-1, whereas that coupled with the C-N stretching-in is at 1187 cm-1. The lowest frequency at 1000 cm-1 is assigned to the trigonal ring deformation mode. All of the vibrational bands mentioned above originate from the azobenzene moiety. The two unassigned new peaks are assigned based on the calculation result: the 1079 cm-1 peak is due to the S-C stretching coupled with a breathing mode of the phenyl ring that is near to the S atom (dubbed PhS); the other peak at 1396 cm-1 is due to the N=N stretching coupled with the C-C stretching and C-H bending on the phenyl ring PhS. However, within the detection limit of the Raman instrument, the Raman peak corresponding to the S-H stretching mode at ~2567 cm-1 was not observed due to its low polarizability. Clearly from the spectra shown in Figure 3, the Raman signatures of azoSH on the Ag/Ge(111)- 3 and Ge(111) surfaces have a great resemblance to that in crystalline form. However, a small shoulder appears at the left shoulder of the 1437-cm-1 peak for the case of Ag/Ge(111)- 3 after closer examination. This small shoulder is predominated at low surface coverage (discussed further below). Besides, most of the Raman peaks of the surface adsobates exhibit different frequency shifting. This phenomenon seemingly points to another Raman peak and will be later assigned to the first-layer adsorbates, similar to our previous studies.17, 28 Based on the assignment of these characteristic peaks of the adsorbed azoSH, the coverage of the deposited azoSH as a function of the deposition time can be characterized. Within the detection limit of the Raman instrument, the Raman peak corresponding to the S-H stretching mode at ~2570 cm-1 was not observed due to its low polarizability. As a note, the possibility of detecting cis-azoSH in the Raman experimental condition is eliminated because the absorption of trans-azoSH at 514.5 nm and thus the photoisomerzed product are both negligible.9 This argument is consistent with the fact that the



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

Page 12 of 32

most prominent Raman peaks of cis-azoSH at 1134 cm-1 and in 1300-1500 cm-1 are less intense in the obtained Raman spectrum.41 3.3. Coverage Dependence. A series of Raman spectra acquired as a function of the dosage time of azoSH confer its adsorption isotherm kinetics. In the case of Ag/Ge(111)- 3 , the coverage of one monolayer (ML) adsorption is assumed when the integrated intensity of the Raman peak at 1144 cm-1 reaches saturation (Figure 4). The 1144-cm-1 peak (C-N stretch) is selected because of its relatively high intensity and little interference from adjoining peaks. Its integrated intensity thus reflects the amount of azothiol adsorbed on the surface. The data was fitted to the Langmuir isotherm model,42-44 assuming that the adsorption rate is proportional to the available adsorption sites on the surface and no multilayer adsorption takes place. Namely, the saturated Raman signal reflects the monolayer adsorption and, accordingly, the coverage (θ) can be determined according to this curve. The Raman spectra of azoSH deposited for 0.3 and 1 ML are exemplified in Figure 5.

The spectral profiles of the characteristic peaks undergo

significant change as the coverage is increased from 0.3 to 1 ML. Such profile change becomes more elucidative by spectral decomposition. Specifically, after spectral decomposition of the broad peak at 1430 cm-1, two overlapping peaks emerge at 1423 and 1437 cm-1. The intensity of the 1437-cm-1 peak for the 1 ML adsorption is larger than that for the 0.3 ML adsorption, while the intensities of 1423-cm-1 peak at the two adsorption coverages are almost equal. This result can be compared with our previous works on stilbene and azobenzene adsorbed on Ag/Ge(111)-

3 .17,28 As an instance of azobenzene, the N=N stretch undergoes a significant red shift of 14 cm-1 for submonolayer trans-azobenzene as compared to multilayer case. It is also known that the molecular conformation and adsorption orientation are expected to vary at different coverages. In this case, the red shifting of the N=N stretch is attributed to the significant interaction between the azo group and the surface. Therefore, in the case of deposited azoSH on Ag/Ge(111)- 3 , the more intense 1423-cm-1 peak, as compared to the 1437-cm-1 peak at 0.3 ML, presumably ACS Paragon Plus Environment

Page 13 of 32

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


corresponds to the azo group of most deposited azoSH interacting with the surface, while the 1437-cm-1 peak becomes stronger than the 1423-cm-1 peak at 1 ML because more azo groups interact very weakly with the surface. A sensible scenario then emerges: the deposited azoSH molecules at 0.3 ML mostly lie on the flat surface, while that at 1 ML predominantly stand up. This picture makes a good correspondence to the isotherm curve shown in Figure 4. The isotherm indicates that the adsorption of azothiol on Ag/Ge(111)- 3 at 100 K is a twostep process including a fast adsorption take place about 25 min., followed by the slow adsorption extending up to 90 min. Such two-stage growth process is qualitatively consistent with the observations of numerous previous studies.15,16 For example, UHV scanning tunneling microscopy has provided a molecular picture of how self-assembly of monolayer alkanethiols is formed on Au surface:16 It follows a two-step process that begins with condensation of lowdensity crystalline island, characterized by surface-aligned molecular axes, and is followed by the appearance of a denser phase by realigning the molecular axes approximately along the surface normal. Similarly, the peaks at 1396 and 1582 cm-1, corresponding to the N=N stretching coupled with the different phenyl ring vibrational mode, are also downshifted by 6 and 10 cm-1, respectively, at the coverage of 0.3 ML (Figure 5). It could be possibly because the interaction between the -N=N- moiety and the surface also affect those normal modes while azoSH lies down on the surface. This portrait of the deposition of azoSH on Ag/Ge(111)- 3 will be further confirmed by the polarization-dependent results. 3.4. Tilt Angle. Taking advantage of the orientation propensity of the Raman polarizability, the tilt angle of the adsorbed molecules can be revealed by the polarization-dependent Raman measurements. Figure 6 shows Raman spectra of azoSH deposited for 1 ML on Ag/Ge(111)- 3 with four different polarization schemes: PP, PS, SP and SS—the first character represents the polarization direction of the incident laser beam with respect to the sample surface, while the second one represents the polarization direction of the collected radiation. Note that the relative ACS Paragon Plus Environment


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

Page 14 of 32

intensity ratio of the 1423-cm-1 peak with respect to the 1437-cm-1 peak decreases significantly (from 0.24 to 0.07) as the polarization scheme is changed from PP to PS. On the other hand, that ratio shows similar behavior (from 0.8 to 0.53) as the polarization scheme is altered from SS to SP. This preliminary result further corroborates that the peaks at 1423 and 1437 cm-1 correspond to different molecular orientations with respect to the sample surface and is consistent with the finding in the coverage dependence above. The average title angle of the molecular orientation on the surface was obtained by gradually varying the excitation polarization direction while maintaining the collection polarization direction in the polarization-dependent Raman measurement. Figure 7 shows the integrated intensities of the peaks at 1437 and 1423 cm-1 as a function of the incident polarization angle, respectively. The retrieval of the orientation of the rod-shaped azoSH on the surface was engendered with the quantum-chemistry density function theory calculation of its Raman polarizability as well as the electrodynamic analysis of the threelayer model in polarized Raman spectroscopy. In the effort of simulating the Raman scattering intensity of the adsorbed azoSH molecular layer as a function of the polarization angle of the excitation light wave in our laboratory geometry (Figure S1 in Supporting Information), the dielectric constants of the vacuum, the azoSH layer, the Ge substrate—amounting to the three-layer system—and possibly silver involved are 1, 2.4±0.3

45

, and 15.2±22.6

46

, respectively. It is unknown how the surface Ag

adatoms alter the dielectric function of the substrate surface. The calculation done with the Ag substrate was also performed for comparison. The dielectric constant of Ag used for calculation is 10.6±0.33 47. For azoSH being considered as a simple rod shape, 1.2 nm was assumed for its major-axis length while 0.3 nm was for its minor-axis one. The two numbers were obtained by the Gaussian03 calculation for free azoSH.

The calculation also conferred the dynamical

polarizability tensor of the 1440-cm-1 vibrational mode:


Page 15 of 32

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


 0.151  0 0  α= 0 0.422 0.09  ,    0 0.09 −20.004  t

(3)

where the xyz coordinate is defined in Supporting Information (Figure S2): the molecular z axis is along the line connecting between the centers of the two phenyl rings, the y axis is on the plane constructed by the two phenyl rings, and the x axis is normal to that plane. Two idiosyncrasies emerge from this calculated tensor: (1) it is a symmetrical matrix and (2) α zz is much larger than the other tensor elements, indicating that this Raman polarizability tensor has a predominant dipole character that is parallel to the molecular z axis—namely, the principal axis of the molecular chain. This result is in agreement with the rod-shape assumption applied to the free r 2 t azoSH. This α was then used in Eq. (1) to calculate the total scattering intensity, Eout , as a function of the polarization angle of the excitation light wave at different molecular tilt angles. As shown in Figure 7, the polarization dependence of the 1423-cm-1 peak agrees with the simulated curve with a mean tilt angle of 90° with respect to the surface normal, indicating that this peak corresponds to the thiol molecules lying on the surface. Alternately, the polarization dependence of the 1437-cm-1 peak concurs with the simulated curve with a mean tilt angle of 60°. Assuming that the Ag-S bond of the adsorbed thiols is normal to the surface, the S-phenyl bond—which is parallel to the main backbond of the azobenzene moiety—would be oriented at 60° with respect to the surface normal, which is coincidently characteristic of the sp3 hybridization of the sulfur atom and is consistent with the calculated preferred Ag-S-phenyl angle (~104°)

48

. Finally, the resultant angular spread is less than 15º, indicating that the molecular

orientation is relatively uniform. In addition, the polarization experiments were also carried out for the thiolate case by annealing the low temperature adsorption layer to 500 K. The tilt angle of the standing-up thiolate is also found to the same as that of the standing-up azoSH physisorbed at 100 K (see Supporting Information). This result agrees with the result obtained in the case of benzenethiol on Au(111) or gold electrodes. 49,50 As a note, the polarization dependence of the characteristic peak at 1441 cm-1 of 1 ML azoSH on the Ge(111) surface shows a similar standing-up orientation (Figure not shown here).

This information will be helpful in the comparison between the

adsorption characteristics of the two systems stated below. ACS Paragon Plus Environment


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

Page 16 of 32

We notice that Banik et al. has tracked the orientational evolution of a CO molecule residing at the junction between two dibenzenedithiol linked silver nanospheres—nano-dumbbell—by analyzing the variation of its surface-enhanced Raman spectrum 27. The local field at the CO site altered by the Ag nano-dumbbell was modeled by two Ag7 clusters with a molecular link, although the nanospheres have a diameter of 100-200 nm. With some elaborate simulation effort, they obtained the orientation of the CO molecule at the hot spot. Furthermore, Zhang et al. recently

have

demonstrated

the

direct

observation

of

single

meso-tetrakis(3,5-di-

tertiarybutylphenyl)-porphyrin (H2TBPP) molecule on the Ag(111) surface scanning tunneling microscopy plus tip-enhanced Raman scattering.

51

with the use of

They attributed the

orientation-dependent TERS spectra to the change of Raman activity caused by altered molecular orientation, notwithstanding that the detailed local field traits (field distribution and direction) near the junction between the tip apex and the surface corrugation was not revealed. In contrast to these two recent efforts, the molecular orientation of azoSH on the smooth Ag/Ge(111)- 3 surface in this study is retrieved by taking advantage of the three-layer model in which the electromagnetic wave at the molecular site is analytically solvable. The retrieval analysis is palpable and thus refutable experimentally. 3.5. Two Adsorption Phases on Ag/Ge(111). According to the result of Figure 3, there are two types of oriented azoSH molecules on Ag/Ge(111)- 3 during the deposition.

Their

competition is shown in the respective integrated intensities of the two idiosyncratic Raman peaks as a function of the dosage time—shown in Figure 8(a). The intensity of the 1423-cm-1 peak reaches saturation at the initial 10 min., indicating that the lying-down azoSH molecules occupy their favorable adsorption sites quickly and remain stably on these sites. On the other hand, the intensity of the 1437-cm-1 peak increases to a plateau after around 35 min., indicating that the molecular affinity—and thus the adsorption free energy—of the standing-up azoSH is significantly smaller than that of the lying-down one. Furthermore, as there is no “wane-wax” ACS Paragon Plus Environment

Page 17 of 32

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


relationship between the intensities of the two peaks, their corresponding adsorption sites differ and there is no competition between them.

The stable Ag/Ge(111)- 3 surface is covered

partially with silver atoms,19 creating at least two probable adsorption sites for azoSH. Is it possible that the 1423-cm-1 and 1437-cm-1 peaks, corresponding to adsorbed molecules with different tilt angles, represent these two different adsorption sites? In the light of the strong interaction between the azobenzene moiety and the silver atoms, as revealed from our previous study,17,18 it is expected that the adsorption site for the lying-down adsorbates is the silver trimer on the Ag/Ge(111)- 3 surface.18 The answer to this question would become more illuminated when these adsorption kinetics are compared with that on the bare Ge(111) surface. Taking into consideration for the case of 1 ML azoSH adsorbed on Ge(111), there is only one Raman peak centered at 1441 cm-1, as shown in Figure 3(d). However, for very low coverage azoSH adsorbed on Ge(111), the peak centered at ~1435 cm-1 is more broaden and can be decomposed into two Lorentzian profiles at 1428 and 1441 cm-1 ( Figure not shown), similar to the case of adsorbed azoSH on Ag/Ge(111)- 3 . Hence, the 1428-cm-1 peak can be assigned to the lying-down azoSH on Ge(111). Figure 8(b) shows the integrated Raman intensities of the 1428-cm-1 and 1441-cm-1 peaks as a function of the azoSH dosage time on the Ge(111) surface. These two peaks correspond to the 1423-cm-1 and 1437-cm-1 peaks, respectively, in the case of the Ag/Ge(111)- 3 surface.

In the initial dosage on the Ge(111) surface, the lying-down

adsorbates—the integrated Raman intensity of the 1428-cm-1 peak—increase to a maximum at ~10 min. and then decreases to zero at ~25 min., while the standing-up ones—the integrated Raman intensity of the 1441-cm-1 peak—grow monotonically and is expected to reach a plateau after 100 min. The short-lived kinetic behavior of the lying-down adsorbates on the Ge(111) surface indicates that they represent the precursor of the ultimate standing-up adsorbates. In contrast, both the lying-down and standing-up adsorbates on the Ag/Ge(111)- 3 surface have their mutually independent adsorption kinetics. This drastic difference in the adsorption kinetics ACS Paragon Plus Environment


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

Page 18 of 32

of azoSH on these two surfaces could be attributed to the stronger interaction between the N=N moiety with the silver atoms than with the Ge(111) surface, engendering the adsorbates on the sites of silver atoms to be inert to the intermolecular interaction—namely, the lying-down adsorbates on that sites are energetically more favorable. In comparison, the relatively weaker interaction between the azobenzene moiety and the Ge(111) surface would allow for a wane-wax phase growth shown in Figure 8(b). This argument is supported by the fact that the integrated intensity of the 1423-cm-1 peak of the azoSH deposited on the Ag/Ge(111)- 3 surface persists after being annealed to 300 K, while that of the 1428-cm-1 peak of the azoSH deposited on the Ge(111) surface, in contrast, decreases to zero. Namely, the binding energy of the lying-down azoSH molecules on the Ag/Ge(111)- 3 surface is stronger than that on the Ge(111) surface. The second message delivered by Figure 8 is that the Langmuir-like behavior of the two adsorption kinetics of both the lying-down and standing-up adsorbates signifies no multi-layer adsorption happening on both the Ag/Ge(111)- 3 and Ge(111) surfaces. This inference is in congruence with the picture that the impinging molecules onto the surfaces do not adsorb stably on the top of the absorbed azoSH to undertake multilayer adsorption.

4. CONCLUSIONS In sum, we have presented the investigation of the adsorption kinetics of azobenzenethiol on the Ag/Ge(111)- 3 surface mainly with the use of polarized Raman spectroscopy.

The

adsorbates of different orientations have been identified by spectral deconvolution and polarization dependence of the obtained Raman spectra. Two adsorption phases are identified and correspond to lying-down and standing-up adsorbates. The lying-down adsorbates resides on the silver atoms on the Ag/Ge(111)- 3 surface, owing to the strong interaction between the N=N- moiety and the silver atoms, while the standing-up adsorbates exists on the pristine Ge surface.

Both adsorbates follow the Langmuir adsorption kinetic upon adsorption. ACS Paragon Plus Environment

These

Page 19 of 32

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


adsorption traits on the Ag/Ge(111)- 3 surface are in great contrast to that on the genuine Ge(111) surface. For the Ge(111), the lying-down adsorbates are unstable with respect to the increasing surface concentration and act as the precursor of the ultimate standing-up adsorbates. Finally, no multi-layer adsorption occurs on both surfaces.

These results demonstrate that

polarized Raman spectroscopy is a powerful tool to unravel the adsorption orientation and kinetics of different adsorbates even in the compound surface system, thanks to its non-invasive nature and high energy resolution.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support by the Academia Sinica in Taiwan and the National Science Council of Taiwan, Republic of China (Grants NSC 99-2113-M-001015 and NSC 100-2113-M-001-012) of this research.

SUPPORTING INFORMATION AVAILABLE Additional information mentioned within the text is provided. This information is available free of charge via the Internet at http://pubs.acs.org.



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

Page 20 of 32

REFERENCES (1)

Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. SelfAssembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103-1169.

(2)

Claridge, S. A.; Liao, W.; Thomas, J. C.; Zhao, Y.; Cao, H. H.; Cheunkar, S.; Serino, A. C.; Andrews, A. M.; Weiss, P. S. From the Bottom Up: Dimensional Control and Characterization in Molecular Monolayers. Chem. Soc. Rev. 2013, 42, 2725-2745.

(3)

Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Self-Assembled Monolayers of Thiols and Dithiols on Gold: New Challenges for a Well-Known System. Chem. Soc. Rev. 2010, 39, 1805-1834.

(4)

Tegeder, P. Optically and Thermally Induced Molecular Switching Processes at Metal Surfaces. J. Phys.: Condens. Matter 2012, 24, 394001.

(5)

Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Frechet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301.

(6)

Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.; Gaub, H. E. SingleMolecule Optpmechanical Cycle. Science 2002, 296, 1103-1106.

(7)

Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samor, P.; Mayor, M.; Rampi, M. A. Light-Powered Electrical Switch Based on Cargo-Lifting Azobenzene Monolayers. Angew. Chem. Int. Edit. 2008, 47, 3407-3409.

(8)

Beharry, A. A.; Woolley, G. A. Aazobenzene Photoswitches for Biomolecules. Chem. Soc. Rev. 2011, 40, 4422-4437.

(9)

Pace, G.; Ferri, V., Grave, C.; Elbing, M.; von Hanish, C.; Zharnikov, M.; Mayor, M.; Rampi, M. A.; Samori, P. Cooperative Light-induced Molecular Movements of Highly


Page 21 of 32

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


Ordered Azobenzene Self-assembled Monolayers. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9937-9942. (10)

Kumar, A. S.; Ye, T.; Takami, T.; Yu, B.; Flatt, A. K.; Tour, J. M.; Weiss, P. S. Reversible Photo-Switching of Single Azobenzene Molecules in Controlled Nanoscale Environments. Nano Lett. 2008, 8, 1644-1648.

(11)

Evans, S. D.; Johnson, S. R.; Ringsdrof, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436-6440.

(12)

Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, JA.; Durbin, M. K.; Dutta, P.; Huangt, K. G. A Highly Ordered Self-Assembled Monolayer Film of an Azobenzenealkanethiol on Au(111): Electrochemical Properties and Structural Characterization by Synchrotron in–Plane X-ray Diffraction, Atomic Force Microscopy, and Surface-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 1995, 117, 6071-6092.

(13)

Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.; Carley, R.; Reuter, K.; Weinelt, M. Structure and Excitonic Coupling in Self-Assembled Monolayers of Azobenzene-Functionalized Alkanethiols. J. Am. Chem. Soc. 2010, 132, 1831-1838.

(14)

Zheng, Y. B.; Payton, J. L.; Chung, C.; Liu, R.; Cheunkar, S.; Pathem, B. K.; Yang, Y.; Jensen, L.; Weiss, P. S.

Surface-Enhanced Raman Spectroscopy to Probe Reversibly

Photoswitchable Azobenzene in Controlled Nanoscale Environments. Nano Lett. 2011, 11, 3447-3452. (15)

Poirier, G. E.; Pylant, E. D. The Self-Assembly Mechanism of Alkanethiols on Au(111). Science 1996, 272, 1145-1148.

(16)

Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H.; Buck, M.; Woll, C. Coexistence of Different Structural Phases in Thioaromatic Monolayers on Au(111). Langmuir 2003, 19, 4958-4968.

(17)

Chou, L. W.; Lee, Y. R.; Jiang, J. C.; Lin, J. C.; Wang, J. K. Unraveling Molecular Adsorption

with

Surface

Raman

Spectroscopy:


trans-Stilbene,

trans,trans-


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

Page 22 of 32

Distyrylbenzene, and trans-Azobenzene on Ag/Ge(111). J. Phys. Chem. C 2011, 115, 516-520. (18)

Wu, H. C.; Chou, L. W.; Lee, Y. R.; Su, C.; Lin, J. C. STM Study of Azobenzene SelfAssembly on Ag/Ge(111)-

(19)

(

)

3 × 3 R30°. Surf. Sci. 2009, 603, 2935-2944.

Chou, L. W.; Wu, H. C.; Lee, Y. R.; Jiang, J. C.; Su, C.; Lin, J. C. Atomic Structure of the Ag/Ge(111)-

(

)

3 × 3 Surface: from Scanning Tunneling Microscopy Observation to

Theoretical Study. J. Chem. Phys. 2009, 131, 224705. (20)

Loscutoff, P. W.; Bent, S. F. Reactivity of the Germanium Surface: Chemical Passivation and Functionalization. Annu. Rev. Phys. Chem. 2006, 57, 467-495.

(21)

Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. Formation of Alkanethiol Monolayer on Ge(111). J. Am. Chem. Soc. 2001, 123, 2422-2425.

(22)

Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates, J. T., Jr. Gold Adatom as a Key Structural Component in Self-Assembled Monolayers of Organosulfur Molecules on Au(111). Prog. Surf. Sci. 2010, 85, 206-240.

(23)

Castner, D. G.; Hinds, K.; Grainger, D. W. X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Lamguir 1996, 12, 5083-5086.

(24)

Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. Self-Assembly of n-Alkanethiol Mnolayers. A Study by IR-Visible Sum Frequency Spectroscopy (SFG). J. Phys. Chem. B 2000, 104, 576-584.

(25)

Yu, H.; Zhang, J.; Zhang, H.; Liu, Z. Surface-Enhanced Raman Scattering (SERS) from Azobenzene Self-Assembled “Sandwiches”. Langmuir 1999, 15, 16-19.

(26)

Jung, U.; Muller, M.; Fujimoto, N.; Ikeda, K.; Uosaki, K.; Cornelissen, U.; Tuczek, F.; Bornholdt, C; Zargarani, D.; Herges, R.; Magnussen, O. Gap-mode SERS Studies of


Page 23 of 32

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


Azobenzene-Containing Self-Assembled Monolayers on Au(111). J. Colloid Interf. Sci. 2010, 341, 366-375. (27)

Banik, M.; Ei-Khoury, P. Z.; Nag, A.; Rodriguez-Perez, A.; Guarrottxena, N.; Bazan, G. C.; Apkarian, V. A. Surface-Enhanced Raman Trajectories on a Nano-Dumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse. Acs Nano 2012, 6, 1034310354.

(28)

Chou, L. W.; Lee, Y. R.; Wei, C. M.; Jiang, J. C.; Lin, J. C.; Wang, J. K. Surface Raman Spectroscopy of trans-stilbene on Ag/Ge(111): Surface-induced effects. J. Phys. Chem. C 2009, 113, 208-212.

(29)

Wang, J. K.; Tsai, C. S.; Lin, C. E.; Lin, J. C. Vibrational Dephasing Dynamics at Hydrogenated and Deuterated Semiconductor Surfaces: Symmetry Analysis. J. Chem. Phys. 2000, 113, 5041-5052.

(30)

Wang, J. K.; Tsai, C. S.; Lin, J. C. Vibrational Dynamics at C and Ge Interfaces. Surf. Sci. 2002, 502-503, 364-373.

(31)

Tsai, C. S.; Chen, C.; Lin, C. E.; Lin, J. C.; Wang, J. K. Surface Raman Scattering Study on the Hydrogenated Semiconductor Surfaces: Vibrational Dephasing Dynamics. Surf. Sci. 1999, 427-428, 318-323.

(32)

Ye, Q.; Fang, J.; Sun, L. Surface-Enhanced Raman Scattering from Functionalized

Self-Assembled Monolayers. 2. Distance Dependence of Enhanced Raman Scattering from an Azobenzene Terminal Group. J. Phys. Chem. B 1997, 101, 8221-8224. (33)

Picardi, G.; Chaigneau, M.; Ossikovski, R; Licitrab, C; Delapierreb, G. Tip Enhanced Raman Spectroscopy on Azobenzenethiol Self-Assembled Monolayers on Au(111). J. Raman Spectrosc. 2009, 40, 1407-1412.



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

(34)

Page 24 of 32

Wu, H. C.; Chou, L. W.; Wang, L. C.; Lee, Y. R.; Wei, C. M.; Jiang, J. C.; Su, C.; Lin, J. C. Adsorption and Desorption of Stilbene from the Ag/Ge(111)- 3 Surface. J. Phys. Chem. C 2008, 112, 14464-14474.

(35)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.04; Gaussian Inc.: Pittsburgh, PA, 2003.

(36)

Hines, M. A.; Chabal, Y. J.; Harris, T. D.; Harris, A. L. Measuring the Structure of Etched Silicon Surfaces with Raman Spectroscopy. J. Chem. Phys. 1994, 101, 8055-8072, and references therein.

(37)

Sourisseau, C. Polarization Measurements in Macro- and Micro-Raman Spectroscopies: Molecular Orientations in Thin Films and Azo-Dye Containing Polymer Systems. Chem. Rev. 2004, 104, 3851-3891.

(38)

Yang, Y. W.; Fan, L. J. High-Resolution XPS Study of Decanethiol on Au(111): Single Sulfur-Gold Bonding Interaction. Langmuir 2002, 18, 1157-1164.

(39)

Zharnikov, M. High-Resolution X-Ray Photoelectron Spectroscopy in Studies of SelfAssembled Organic Monolayers. J. Electron Spectrosc. 2010, 178-179, 380-393.

(40)

Armstrong, D. R.; Clarkson, J.; Smith, W. E. Vibrational Analysis of trans-Azobenzene. J. Phys. Chem. 1995, 99, 17825-17831.

(41)

Stuart, C. M.; Frontiera, R. R.; Mathies, R. A. Excited-State Structure and Dynamics of cis- and trans-Azobenene from Resonance Raman Intensity Analysis. J. Phys. Chem. A 2007, 111, 12072-12080.

(42)

Schwartz, D. K. Mechanisms and Kinetics of Self-Assembled Monolayer Formation. Annu. Rev. Phys. Chem. 2001, 52, 107-137.

(43)

Thomas, R. C.; Sun, L.; Crooks, R. M. Real-Time Measurements of the Gas-Phase Adsorption of n-Alkylthiol Mono- and Multilayers on Gold. Langmuir 1991, 7, 620-622.


Page 25 of 32

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

(44)


Schreiber, F. Structure and Growth of Self-Assembling Monolayers. Prog. Surf. Sci. 2000, 65, 151-256.

(45)

Tamada, K.; Haruhisa, A.; Wei, T. X. Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide Self-Assembled Monolayers Studied by Surface Plasmon Spectroscopy: Influences of side Chain Length and Contacting Medium. Langmuir 2002, 18, 5239-5246.

(46)

Aspnes, D. E.; Studna, A. A. Dielectric Functions and Optical Parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV. Phys. Rev. B 1983, 27, 985-1009.

(47)

Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370-4379.

(48)

Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. Structure and Binding of Alkanethiolates on Gold and Silver Surfaces: Implications for Self-Assembled Monolayers. J. Am. Chem. Soc. 1993, 115, 9389-9401.

(49)

Kafer, D.; Bashir, A.; Witte, G. Interplay of Anchoring and Ordering in Aromatic SelfAssembled Monolayers. J. Phys. Chem. C 2007, 111, 10546-10551.

(50)

Szafanski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Surface-Enhanced Raman Spectroscopy of Aromatic Thiols and Disulfides on Gold Electrodes. Langmuir 1998, 14, 3570-3579.

(51)

Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chan, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y.; Yang, J. L.; Hou, J. G. Chemical Mapping of a Single Molecule by Plasmon-Enhanced Raman Scattering. Nature 2013, 498, 82-86.



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

Page 26 of 32

Table 1. Observed and calculated azobenzenethiol Raman active vibrational frequencies (in cm-1) and their assignments. DFTa

Experiment

Powder

1 ML on Ag/Ge(111)3

1 ML on Ge(111)

999(w)

1002

1002

984

δ (CCC)

1069(m)

1079

1079

1088

ν (CS)+S-phenyl ring breathing

1144(st)

1144

1144

1136

δ (CCH)(out/in)+ ν (CN)(out/in)+ δ (CNN) + δ (CCN)

1187(m)

1187

1186

1190

δ (CCH)(out/in)+ ν (CN)(in/out)+ δ (CNN) + δ (CCN)

1315(w)

1312

1312

1350

δ (CCH)+ δ (NCC)

1395(m)

1394

1394

1411

ν (NN)+δ (CCH)+S-phenyl ring ν (CC) ν (NN)+ ν (CC)+δ (CCN)+ δ

1423

a

(CCH)

ν (NN)+ ν (CC)+ δ (CCN)+ δ

1437(st)

1437

1441

1432

1460(st)

1465

1465

1455

ν (NN)+δ (CCN)+δ (CCH)

1484(m)

1481

1481

1481

ν (NN)+ ν (CC)+ δ (CCH)

1584(m)

1582

1585

1602

ν (NN)+ ν (CC)+ δ (CCH)

(CCH)

No scaling factor for the frequencies calculated from DFT/PBEPBE/ 6-31+G*

b

c

Assignmentb,c

Ref. 40 and 41

ν = stretch and δ = in-plane bend. ν (CC), δ (CCC) and δ (CH) are motions within the

phenyl rings. These symbols represent several combinations of different atoms in phenyl rings. S-phenyl ring is the phenyl ring that bonds to the S atom.


Page 27 of 32

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


FIGURE CAPTIONS Figure 1. XPS spectrum around S2p characteristics of 1 ML azoSH deposited on Ag/Ge(111)-

3 at 100 K (top, open circles) and after flashing to 400 K (bottom, closed circles). The red, green, and blue lines represent the de-convoluted profiles for thiol, thiolate and S atom, respectively. Figure 2. (a) Integrated intensities of thiol, thiolate, and atomic sulfur atom peaks (red, green, and blue, respectively) in S2p XPS spectra vs. sample temperature. The solid lines are a guide to the eye. (b) Temperature programmed desorption spectrum (ion signal at m/e= 214) of 1 ML azoSH molecular desorption from the Ag/Ge(111)- 3 surface. Figure 3. (a) Normal Raman spectrum of crystalline azobenzenethiol; (b) calculated Raman polarizability of azobenzenethiol; (c) Raman spectrum of 1 ML azobenzenethiol deposited on Ag/Ge(111)- 3 at 100 K; (d) Raman spectrum of 1 ML azobenzenethiol deposited on Ge(111). Characteristic peaks in (c) and (d) are resolved by curve fitting to Lorentzian profiles (dashed curves) and marked numerically. Figure 4. Integrated Raman intensity of the 1142 cm-1 peak as a function of azobenzenethiol dosage time on the Ag/Ge(111)- 3 surface.

Solid red curve represents a fit to Langmuir

isotherm model. Figure 5. Raman spectra of azobenzenethiol deposited on the Ag/Ge(111)- 3 surface at 0.3 (bottom) and 1 ML (top) between 1350 and 1650 cm-1. The red and green peaks represent the decomposed peaks at high and low coverage, respectively. Figure 6. Polarized Raman spectra of 1 ML azoSH on the Ag/Ge(111)- 3 surface at four different excitation-collection polarization combinations: (a) p-polarized excitation and ppolarized collection, (b) p-polarized excitation and s-polarized collection, (c) s-polarized excitation and p-polarized collection, and (d) s-polarized excitation and s-polarized collection. The red and green peaks represent the decomposed peaks at high and low coverage, respectively. ACS Paragon Plus Environment


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

Page 28 of 32

Figure 7. Normalized integrated intensities of 1423-cm-1 and 1437-cm-1 peaks (filled and open circles, respectively) as a function of excitation polarization angle. The p-polarized excitation beam corresponds to the polarization angle of 90°. The red, black, and green lines represent simulated polarization-dependent Raman intensities, based on the three-layer model, for three tilted angles (90, 60, and 45°) with respect to surface normal direction. Figure 8. Integrated Raman intensities of 1423-cm-1 and 1437-cm-1 peaks as a function of dosage time for adsorbed azoSH (a) on Ag/Ge(111)- 3 at 100 K and (b) on Ge(111) at 100 K. The solid line is a guide to the eye.


Page 29 of 32

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


Figure 1

Figure 2

Figure 3



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

Figure 4

Figure 5

Figure 6


Page 30 of 32

Page 31 of 32

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


Figure 7

Figure 8



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

Table of Content Graphic


Page 32 of 32

Ge(111) Surface with

Recommend Documents