Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced

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Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced Raman Spectroscopy In Tunneling Mode on Thiol Self-Assembled Monolayers Chiara Toccafondi, Gennaro Picardi, and Razvigor Ossikovski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03443 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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The Journal of Physical Chemistry

Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced Raman Spectroscopy in Tunneling Mode on Thiol Self-Assembled Monolayers Chiara Toccafondi, 1,* Gennaro Picardi, 1,2 Razvigor Ossikovski 1 1

LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau FRANCE

2

Laboratoire CSPBAT, Université Paris 13, 74 rue Marcel Cachin, 93017 Bobigny FRANCE

* +33 (0)1 69 33 43 51 ; [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: The sharp and intense Raman features associated with hexil azobenzene thiol self-assembled monolayers on gold(111) make these molecules ideal probes for Tip enhanced Raman spectroscopy (TERS) investigations. Spectroscopic and electrical measurements at the molecular junction were performed by combing TERS with scanning tunneling microscopy (STM). The azobenzene TERS signal was monitored while changing the tip-surface distance and/or increasing the bias applied to the tip. The TERS signal decreases with increasing bias faster than what expected if considering only the effect of the tip distance from the monolayer and the related drop of the optically enhancing near-field intensity. We develop a simple geometrical model accounting for the molecular bending induced by the increasingly higher local field close to the tip apex, which brings an additional modification of the TERS intensity and is able to accurately describe the experimental data.


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INTRODUCTION In recent years, the comprehension and ability to tune the interfacial and surface properties of materials has become crucial.1 The progressive miniaturization of devices has pushed technology to the nano domain, thus requiring investigation techniques with improved sensitivity and spatial resolution. Raman spectroscopy is a powerful technique which allows the identification of molecules by detecting their vibrational modes. Nevertheless, since the cross section of the Raman scattering process is very low, its application at the nanoscale level is not feasible, unless enhancing the technique sensitivity. The plasmonic effect occurring in metal nanostructures offers two solutions to this issue, either in terms of surface enhanced Raman scattering (SERS)2,3 or of tip-enhanced Raman scattering (TERS).4–7 In the latter case, the coupling with scanning probe microscopy (SPM) techniques allows to detect Raman signal with sensitivity down to single-molecule detection and with spatial resolution far below the diffraction limit.7,8 This is achieved due to a strong electromagnetic field enhancement related to localized surface plasmon resonances excited near the apex of the SPM metallic tip, when this is illuminated by focused laser light. Extremely high enhancements of Raman features were achieved using TERS on various organic molecules7–10 and carbon-based materials,4,11,12 allowing the investigation of their chemical properties at the nanoscale. The tendency towards device miniaturization also greatly stimulated the research concerning molecular scale junctions and the investigation of the electronic properties at the nanoscale level.13–16 A useful approach uses the scanning tunneling microscopy (STM) to record the electronic conductance of the junction created between the STM tip and a substrate covered by a molecular monolayer.16–18 The combination of the TERS and STM techniques,19–23 allowing co-localized measurement of the chemical and electronic properties at the nanoscale, paves the way to a 3 ACS Paragon Plus Environment


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better understanding of light-driven transport phenomena23–25 and the development of optoelectronic devices. Recently, Pal et al. attributed to plasmonic effects the origin of a lightinduced increase in electron transport,19 while Braun et al. reported a bias-driven nonlinear optical amplification of the luminescence signal of mercaptobenzothiazole molecules inside a laser-pumped tunnelling junction 23. Interface properties are critical in organic electronic devices: matching the energy level of the semiconductor and the electrode is necessary for improving the device efficiency.14,26 Self-assembled monolayers (SAMs)27 are ordered assemblies spontaneously formed at metal surfaces which are extensively used whenever a modification of the surface properties is required, especially in molecular electronics applications.28–30 The chemisorption of a molecule on a metal can alter its work function and can be used to lower the energy barrier for charge injection and increase device performances.31–33 Among many organic molecules, azobenzene derivatives stands out due to the reversible photo-switching occurring between two stereoisomers,34–36 which makes them promising candidates for memory devices, optical switches, tunable filters, holographic gratings and other optical devices.37–40 In this work, hexil azobenzene thiol SAMs on gold were investigated by STM-TERS following the Raman signal intensity while changing the applied bias and/or the tip-surface distance. By combining STM current-voltage and STM-TERS measurements, we show that the near-field Raman response of the molecules is affected not only by the tip-sample distance, but also by the applied voltage. The observed behavior has been modeled considering a voltage-induced bending of the molecules in the tip apex - substrate nano gap.

MATERIALS AND METHODS


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SAMS

Preparation.

The

6-[4-(Phenylazo)phenoxy]-hexane-1-thiol

molecules

(“azobenzene”) were purchased from ProChimia, Poland. The molecules were dissolved in pure ethanol (Sigma-Aldrich, France) to a final concentration of 5 mM. Gold (111) layers on mica substrates were purchased from Phasis, Switzerland.

The

substrates were flame annealed to ensure large flat domains and then incubated overnight in the azobenzene thiol solution at room temperature. The samples were then rinsed with fresh solvent to remove the excess of molecules and dried under nitrogen. Scanning Tunneling Microscopy.

Measurements were performed in air using a SPM

SmartSPM™-1000 system (AIST) equipped with the STM head. Gold tips were chemically etched following a recipe reported in Ref.

41

.

STM mapping measurements have been

performed using a constant tunneling current of 100pA and a bias voltage of 0.1 V. Tip-Enhanced Raman Spectroscopy. TERS experiments were performed by coupling the SPM SmartSPM™-1000 system (AIST) to a confocal Raman spectroscopy setup (Labram HR800 from Horiba Jobin Yvon). A red laser (λ=633 nm) is focused at the apex of the gold tip. Measurements were acquired using integration times between 0.5 and 2 seconds and laser power below 0.5 mW to minimize the photo-degradation of the SAMs.

RESULTS AND DISCUSSION An azobenzene-modified Au surface was imaged by STM (Figure 1a) and TERS spectra were acquired at different points using a 0.5 mW laser at 633 nm (Figure 1 b) with the gold tip tunneling at a setpoint current of 100 pA with an applied voltage of 0.1 V. All the spectra show the molecule characteristic peaks: a sharp signal around 1142 cm-1, assigned to inphase stretching of the two C–N bonds, the 1436 cm-1 N=N stretching mode and other strong bands at 1418, 1459 and 1593 cm-1 mostly corresponding to ring deformations35. Those



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peaks readily disappear upon tip retraction. TERS mappings were also acquired to check the uniformity of the molecular layer over the surface. The influence of the applied voltage on the TERS response can be observed in Figure 1 c. The plot displays the Raman intensity (waterfall plot) as a function of the Raman shift and the applied voltage, varying between −0.5 V and +0.5 V, while the current was kept constant at 100 pA. Under constant current tunneling, the vertical tip position with respect to the surface deviates from the original landing position (z0) to maintain the current at the setpoint. In the following, we will consider the actual distance of the tip from the substrate using z0 as starting position; thus, any change z in the tip position will result in a tip-substrate distance Z = z + z0. The azobenzene thiol SAM is reported to be ~2 nm thick.34,42 The spectra were acquired at a voltage step of 0.05 V (0.5 mW laser power and 2 s acquisition time). On the right of the waterfall plot, the intensity of the sharpest peak at 1142 cm-1 with varying voltage is reported. The peak intensity displays a maximum located near zero bias and progressively diminishes going towards higher voltages, for both polarities. The curve was reproduced in many experiments, performed with the tip on flat gold terraces previously localized in the STM topography. The average (and normalized) curve is shown in Figure 2 (dots); since the observed behavior is symmetric, the decay was also further averaged between both polarities. The TERS signal decreases rapidly as the module of the voltage increases and almost vanishes already beyond 0.5 V. This behavior might be at first related to a decrease in the field enhancement at the molecules location as the enhancing tip moves away. Under constant current mode, an increase of the tunneling bias results in an increase in the tip-to-surface gap. We first try to model this effect, accounting for the near-field decay with the probe tip distance from the surface controlled via the tunneling bias. Starting from the dipole model discussed in references 6,11,12,


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=

= 1 +

≈ = 1 +

(1)

where z0 is the tip landing position and ρ is the dipole radius (typically, ρ » z0, so that ρ + z0 ≈

ρ), we can predict the behavior of the near-field Raman signal (Int) when changing z. When the frequency of an incident external field matches the resonance frequency window of the surface plasmon modes localized at the tip apex, collective electron oscillations are excited. In the dipole model, the gold tip apex acts as a metal sphere whose center is located at the distance + + from the surface. The oscillating field induced at the surface of the sphere by the external field can be described outside the sphere as the field generated by an oscillating dipole located in the center of the sphere. The power p equals 8 for fully coherent scattering, 10 for fully incoherent one and doubles its value if dipole image contribution from the gold substrate is considered.12 We still need to define the relationship between z and the experimental tunneling voltage and current values. In the simplest case, electronic transport through molecules is described by non-resonant coherent tunneling.13,17 Taylor series expansion of Simmons’ expression43 for electron tunneling through an insulator enclosed between two identical electrodes yields for the tunneling current density J at the landing position z0

= =

+ !

(2)

where "=

#$ %

&2(),

+,

+,

-

* = %, &2() = #$% " = /$. "

and

+,

0 = 123,

4, , 1

− "

m and e are the electron mass and charge respectively, h is Planck’s constant, β is the tunneling decay constant, Gq is the quantum of conductance (Gq ≈ 77 µS), ) is the work function (the height of the potential barrier ; ) ≈ 5.3 eV for gold) and z is the gap between the



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two electrodes. I and S are the tunneling current and surface respectively. (Note that positive experimental values for γ necessarily entail βz > 3.) From (2), we can also deduce that the junction resistance 67 depends exponentially on the separation between contacts ( + ); indeed, by neglecting the cubic term at low voltages, one obtains the equation17,18,44 6 7 =

8

9

=

:;

< 4 =

+ => :;

+

+ => :;

< 4 = 6 1 + < 4 ≈ 6 < 4 (3)

where 6 =

+ => :;

=

/$ + => -. :4

In the last expression of (3), following the common practice, we neglected the linear term with respect to the exponential one; this approximation is valid for sufficiently large β values and small z values. It is easy to see that R(z0) grows with z0, decreases with increasing β when 0 < β z0 < 1, and grows with β when β z0 > 1. By substituting z from Eq. (3) into Eq. (1) and reintroducing the voltage through Ohm’s law, we can express the variation of the TERS peak intensity as a function of voltage like ?@

?@

≈ A +

A

B

CD

E F

E

= A +

A

B

CD

G@ E

F

(4)

where Iset is the current setpoint and R(z0) is the junction resistance at the landing position. Note that R(z0) differs from the contact resistance R(−z0),whose value is often reported as R0 in molecular junctions experiments.14,18,33,44 We assumed ρ = 35 nm (as in Ref

11

which is

compatible with the radius of curvature of the tips, ~ 20 to 30 nm, as seen by SEM), β = 0.4 Å-1 as reported for other conjugated molecules,18 Iset = 100 pA and p was taken equal to 8 for fully coherent scattering. R(−z0) literature values obtained contacting similar molecules (0.1 50 MΩ )14,18 were initially considered for R(z0), since we had no reference for this parameter. 8 ACS Paragon Plus Environment

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The resulting calculated curves are shown in Figure 2 as red lines and the enclosed shadowed area accounts for different R(z0) within the mentioned limiting values. Still, the deviation of the experimental data from the calculated ones is remarkable and it points at the inadequacy of the assumptions made. Possibly, the actual chemical and structural properties of the azobenzene thiol molecules might result in a significant deviation from the initially assumed values. Thus, we performed a more in-depth investigation of the tunneling process. Current-voltage curves were recorded while tunneling with increasing voltage at the fixed tip landing position z0. The actual z0 values are defined by the tunneling bias and current values set for the landing, and were kept constant during the curve acquisition while the current feedback was disabled. The mean I-V curve, averaged over 20 curves recorded after landing with Vset= 0.1 V and Iset = 100 pA, is displayed in Figure 3. In the inset, other curves are shown relative to different landing positions. In the low bias region (ǀVǀ < 0.05V), the curves are almost linear, as expected in coherent tunneling regimes.17,43 Similar measurements performed on bare gold also display a linear behavior but with different slopes (Supplementary material). The observed linear behavior in the selected low bias range is in agreement with conductive AFM experiments performed on comparable molecular systems.18,44,45 The data points can be indeed well fitted using equation (2). From the fitting of the experimental data (red curve in Figure 3), we estimate the junction differential resistance in the tunneling position R(z0) = 694 ± 8 MΩ for the azobenzene SAM. Our value obtained by STM measurements is considerably higher than the R(−z0) ones found in literature for both saturated and aromatic molecules of comparable thickness, obtained by conductive AFM.

18,33,38,44

This appears reasonable since R(−z0) is the estimated contact

resistance assuming a molecule of null length between the two electrodes, while R(z0) is the resistance measured at the landing point. More generally, conductive AFM and STM results



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are not always directly comparable: for instance, in the former case, tip and sample are always in physical contact, while in the latter case, the tip is very close to, but not in direct contact with the sample (at least, when tunneling above the molecular layer). We also observe that IV curves acquired after landing at increasingly higher voltages (0.05 - 1.5 V) therefore, at increasingly higher z0 positions, display progressively lower slope (see inset in Figure 3), meaning a higher resistance R(z0), as intuitively expected as the tip-sample gap increases. The change in the tunneling current while modifying the tip-surface distance at a constant voltage has been also measured, allowing for the estimation of the decay constant β, previously assumed to be 0.4 A-1. In Figure 4, the tunneling current through an azobenzene SAM is reported for increasing tip-to-surface distance. The bias voltage was kept constant at 0.1 V and, starting from the landing position , the tip was progressively retracted from the surface. As expected, the current drops from the set point value as the distance increases, and quickly reaches zero. According to the coherent tunneling model, the current density exponentially decreases with the increasing thickness of the barrier, with a decay constant β whose value is characteristic of the molecule within the junction 33,44,46. Fitting the data in Figure 4 with the last expression of (3) while using the R(z0) value derived above from I-V experiments, we estimate the exponential decay constant β to be equal to 0.27 HIJ for the azobenzene monolayer. The value is slightly lower than the values found in literature for other conjugated molecules,18,38,46–48 even though there is quite a variability in the reported values. This discrepancy is possibly to be ascribed to the different experimental technique employed or to other factors influencing the estimation of R(z0) and β in the simplified Simmons equation, as for instance differences in ordering and packing density of the film. As a matter of fact, it is difficult to compare measurements performed in different conditions (STM or conductive AFM), and different molecules, since small modifications in the chemical structure or orientation of the molecule may greatly affect the


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electron transport mechanism.15,49,50 Noticeably, comparable decay values were obtained using the same fitting procedure on current-distance curves obtained at increasing landing voltage (see supplementary material), confirming that β is almost bias independent in the lowbias regime.51 Now, Eq. (4) can be updated with the values of the tunneling resistance at the landing point R(z0) and the exponential decay factor β as derived from the above electrical measurements. However, before checking whether the fitting of TERS-versus-voltage data has improved, an additional cross-check was performed for the employed size of the tip apex. Tip-approach curves were acquired detecting the azobenzene TERS signal while progressively increasing the distance z between tip and surface at constant junction voltage (0.1 V) starting from the position defined upon landing. In this case, the TERS signal intensity is monitored rather the current (as done in Figure 4). The results are presented in Figure 5: all the Raman peaks show a decrease in intensity with increasing tip-surface separation (waterfall plot). The top graph displays the Raman spectrum recorded at 1 nm from the landing position = + 1 nm . The side graph represents the intensity decay of the 1142 cm-1 peak with z. The peak intensity drops to almost zero already beyond 5 nm and it was reproducible over many curves acquired in different areas of the sample. A similar curve is registered if the tip is progressively moved towards the surface (from z0 + 10 nm to z0); we can therefore assume that the observed intensity decay is not related to the photo-degradation of the molecular layer upon irradiation. As mentioned above, the decrease of the near-field Raman signal with increasing z can be predicted considering the dipole model. Figure 5 b shows the mean tip-approach curve averaged over 20 acquisitions (black dots) and the calculated curve (red line) obtained by fitting the data with Eq. (1) and assuming fully coherent scattering (p = 8). We could therefore determine a dipole radius ρ = 23 ± 2 nm in the case of coherent scattering (29 ± 2 nm for



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uncoherent scattering), quite close to our previous (somewhat conservative) assumption of 35 nm. The experimental data presented so far provide updated and reliable estimates of the parameters used in Eq. (4) to fit the experimental TERS decay with increasing applied bias. The curve calculated from Eq. (4) with the retrieved values of R(z0), β and ρ are expected to provide a better agreement with the experimental data, as well as a consistency check between all measurements. As shown in Figure 6 (red curve) the agreement with the experimental points is still rather poor. The fitting routine would require a much lower ρβ product, which wouldn’t be reasonable at this stage, also considering that the β is most likely underestimated. It is particularly disappointing that the same model can be used to reasonably fit the TERS decay with increasing distance from the surface (Figure 5) but not the decay with increasing tunneling voltage (Figure 6). Apart from the decrease in the magnitude of the near-field enhancement, an additional voltage-induced effect on the molecular layer needs to be considered to account for the observed faster decay of the TERS signal with increasing the tunneling bias. An isomerization from the trans- to the cis- form of the azobenzene group resulting in the fading of the trans- spectral features is to be excluded: no evidence of Raman peaks assignable to the cis- form, as the ones reported for SERS measurements22 or predicted by calculations,52 was found. In our case, the continuous irradiation with the red light prevents the switching to the cis- phase; in other words, the equilibrium position of the reaction is constantly moved towards the trans- form side. Hence, we have supposed that the increasingly higher electric field existing between the tip apex and the metal surface when the bias is increased might induce a molecular realignment, i.e., a change in the orientation of the azobenzene tethered moiety with respect to the surface normal in the molecules located below the tip apex. According to previous studies34,42 the


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azobenzene group assumes in the SAM a nearly perpendicular orientation with respect to the surface. This orientation is very favorable for profiting from the predominant enhancement of the electromagnetic field component parallel to the tip axis, therefore to observe the TERS bands due to in-plane vibrations of the planar azobenzene group. A strong electric field exists in the tip-substrate tunneling junction, inducing a dipole moment in the SAM molecules according to their polarizability. The electric field not only exerts a (small) horizontal force on the induced molecular dipole, but also applies a torque on it if the azobenzene moites are slightly tilted with respect to the vertical field direction thus acting as a bending moment. Under the action of the electrostatic torque the (originally) nearly vertical azobenzene planes may bend. The spontaneous ordering within the azobenzene SAM would then be locally disrupted for large enough tunneling voltages and a tilting of the azobenzene plane(s) would then result in a decreased optical coupling and weaker TERS signal. We have developed a model to account for this supposed molecular tilting affecting our TERS intensity vs tunneling bias data and the values of the electrostatic force and electrostatic torque acting on the molecules are derived in Appendix 2. In oblique (off-axis) backscattering configuration light is impinging at an angle θ0 (with respect the z axis) on the tip-sample gap. The detected TERS intensity I for a given normal mode in the ‘unanalyzed’ (p and s polarization of the Raman scattered photons) measurement configuration is given by I = I p + I s ∝ e Tp R e e i

2

2

+ e Ts R e e i ,

(5)

where e p = e i = [0

− cos θ 0

sin θ 0 ]

T

and

e s = [1 0 0]


T


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ei is the incident (laser) electric field and ep and es are the p and s detection polarizations, respectively parallel and perpendicular to the scattering plane, defined by light beam and the z axis. We have further assumed that the incident field is p-polarized, a necessary condition for maximal TERS enhancement. Re is the (3×3) matrix representation of the Raman polarizability tensor of the mode, modified to account for the local electromagnetic field enhancement (see Appendix 1 for the explicit form of the (modified) Raman tensor and tip amplification tensor). From (5), keeping terms containing only the highest powers of the tip amplification coefficient a (since a » b)53 results in I ∝ a 4 sin4 θ0 r332

(6)

that is, the TERS signal originates essentially from the r33 matrix element of R (in this specific oblique incident configuration). In the case of a tilting of the azobenzene planes away from the direction normal to the surface, one obtains for the rθ33 matrix element of Rθ, the tensor tilted by the angle θ from the vertical (see Appendix 1), rθ 33 =

1 2

(r

11 sin

2

) [r

θ + r22 sin 2 θ + r33 cos2 θ =

1 4

11

+ r22 + r33 + (r33 − r11 − r22 ) cos 2θ ]

(7)

so that the TERS intensity Iθ is Iθ ∝ rθ233 ∝ [r11 + r22 + r33 + (r33 − r11 − r22 ) cos 2θ ]

2

(8)

and the normalized TERS intensity In becomes 2

 r + r + r + (r33 − r11 − r22 )cos 2θ  I 2 I n = θ =  11 22 33  = [t + (1 − t ) cos 2θ ] I0  2r33  where 14 ACS Paragon Plus Environment

(9)

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t=

r11 + r22 + r33 2r33

Note that, since both the r33 element and the tensor trace (r11 + r22 + r33) are invariant with respect to rotations about the z-axis, the normalized TERS intensity is insensitive to the specific molecular in-plane arrangement within the SAM lattice. 54 Finally, if we assume direct proportionality between the molecular tilt angle θ and the tip bias V, then the change in the normalized TERS intensity due to the molecular realignment effect under the action of the applied bias becomes I n (V ) == [t + (1 − t ) cos(V V0 )]2

(10)

where the parameter V0 defines the facility of the applied bias to realign the molecules along the electric field lines (the smaller the value of V0 the easier the realignment). V0 is introduced in analogy with the threshold voltage in nematic liquid crystals, defining the minimum voltage required to overcome the intermolecular interactions in order to realign the molecules with the applied electric field. Indeed, the azobenzene thiol molecules possess the required physico-chemical anisotropies to act as a mesogen: a rigid, aromatic and thus, rich in highly polarizable electrons half, and a flexible, linear, apolar other half. The mentioned intermolecular forces are those allowing for the spontaneous, molecular self-assembling in absence of the electric field. The observed independence between TERS intensity and bias sign is compatible with the non-polarity of the aliphatic chain, meaning that the same molecular re-alignement takes place for both bias polarities. The simulated intensity decay with increasing bias voltage using Eq. (10) is reported in Figure 6 (blue line). The model reproduces quite accurately the experimental points with a considerable improvement as compared to the previous model based on Eq. (4) (red curve), supporting the hypothesis of a molecular realignment under the action of the bias voltage.



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However, it is more likely that the molecular realignment effect superimposes on the effect arising from the change in tip-sample distance (again due to the change in applied bias). The combination of these two effects can be expressed as the product of Eq. (4) and (10):

= 1 +

J

4

ln

8

NOP Q

X

× ST + 1 − T cos Z X0

2

(11)

where t and V0 are two fitting parameters. The green curve in Figure 6 is calculated from (11) and is in better agreement with the experimental data as compared to the blue one. From the fit to the experimental data we obtain t ≈ 0.67 and V0 ≈ 0.17 V (still using the previously determined values of ρ, β, R(z0), below this V0 value no change takes place in the molecular layer. Knowing that r22 = 0 for the azobenzene molecule,55 one then gets r33 ≈ 3 r11 or, equivalently, for the Raman tensor R normalized to r33 = 1, r11 ≈ 0.33. This is in fair agreement with the value r11 ≈ 0.5 assumed to describe the polarization-dependent TERS response of azobenzene SAMs.56 Under the action of the tip electric field, the Raman tensor is tilted up to an angle θ ~80° when the applied voltage is (±) 0.5 V, indicating a tilt of the azobenzene planes. Actually, an 80° tilt of the molecule in a densely packed film is hardly conceivable from the steric point of view. However, the two angle values may possibly slightly differ, namely if the molecular tilt is associated with a partial loss of the planarity of the azobenzene system. Moreover, the molecular bending results in a change in the SAM thickness and, under constant tunneling current feedback loop, in a change of the tip-substrate distance. This should further modify the retrieved tunneling parameters, such as R(z0). These considerations are likely to affect the obtained tilt angle, even though they are not taken into account in the model at this stage. According to the model a reasonable explanation for the reversible and progressive disappearance of the TERS peaks with increasing applied bias is the tilting of the azobenzene


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plane(s). As mentioned above, the electric field in the tip-sample cavity applies a torque on the molecules, which acts as bending moment on the almost vertical azobenzene moieties (see Appendix 2). The maximum torque value has been estimated to be in the range of 10-23 N·m, a value higher than the one deduced for nematic liquid crystals (see Appendix 2) and therefore fully capable of bending the molecule.

CONCLUSIONS The tip enhanced Raman response of the azobenzene thiol molecules assembled on a gold surface can be regulated both by the tip-sample distance and the applied bias voltage under STM feedback. When increasing the bias applied between the tip and the sample, the TERS signal decays at a faster rate than expected based only on a change in the tip-sample physical distance. We proposed a voltage-induced bending of the molecules located in the tip-substrate region due to the increased local electric field and accordingly, a model which provides an improved fit to the experimental data. The model is still relatively simple: more advanced refinement should consider a (cylindrically symmetric) distribution of possible tilt angles with the distance from the tip vertical axis. Also, one should include a change in the SAM thickness, and the related modification of R(z0), as a consequence of the molecular realignement. The weaknesses are indeed the likely overestimated tilt angle and the low values of the obtained beta value compared to the literature values, even though the latter has moderate influence in the model. Our main purpose here was to show an unexpected influence of the STM bias voltage on the TERS signal intensity and mention the possibility of a subtle modification of the specimen under the measuring conditions. The molecular realignment is a possibility originally suggested by a comparison of the molecular ordered monolayer sandwiched between two electrodes to physically related liquid crystal devices.



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APPENDIX 1 Consider the two reference frames shown in Figure 7, xyz and x’y’z’, where the frame x’y’z’ has its x’ axis lying in the xy plane and making an (azimuthal) angle ϕ with the x axis, whereas its z’ axis is tilted at the (polar) angle θ with respect to the z axis. The orthogonal matrix T transforming xyz into x’y’z’ is given by cosϕ − cosθ sin ϕ − sin θ sin ϕ  T =  sin ϕ cosθ cosϕ sin θ cosϕ     0 − sin θ cosθ 

The general Raman tensor R of an individual molecule (or a SAM of identically oriented molecules) in the xyz frame,  r11 r12 r13  R = r21 r22 r23    r31 r32 r33 

becomes in the the x’y’z’ frame

R' = T −1R T = TT R T (the superscript ‘T’ stands for transpose). If the planes of the azobenzene groups within a cylindrical region with z-axis rotational symmetry are tilted at the polar angle θ under the action of the tip electric field, then the Raman tensor Rθ of the molecules will be given by the average of R’ over the azimuthal angle ϕ,

1 Rθ = 2π

2π

∫T

T

R ' T dϕ

0


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We have further assumed that the Raman scattering from the individual molecules remains totally coherent, so that the individual tensors add. This assumption has been experimentally proven to hold for azobenzene SAMs.56 On the other hand, the TERS Raman tensor Re of the sample is given by (in the xyz frame) Re = AT R A

where b 0 0  A = 0 b 0    0 0 a 

is the tip amplification tensor (typically with a » b), assuming that the tip is perfectly rotationally symmetric and its axis is aligned along the z axis.57 APPENDIX 2 Electric field in the tip-sample cavity and electrostatic force acting on a SAM molecule The electric field in the tip-substrate cavity can be evaluated by using the conformal mapping method from electrostatics.58 Consider the complex potential defined by the inverse hyperbolic sine transformation

w = A arg sinh

s B

mapping the points s = x + iy from the complex plane into w = u + iv . Separation of real and imaginary parts shows that this plane potential defines electric force lines u ( x , y ) = const . and equipotential lines v ( x , y ) = const . given by



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x2 u B sinh A 2

2

+

z2 u B cosh A 2

=1

and

2

z2 v B sin A 2

2

Page 20 of 41

−

x2 v B cos A 2

=1

2

geometrically corresponding to confocal ellipses and hyperbolas, respectively. The above families of lines physically describe the electric field in the cavity between two conducting hyperbolic tips of opposite charges or, in virtue of the image charge principle, that of a charged conducting hyperbolic tip and a conducting mirror plane that is grounded (see Figure 8). If we parameterize geometrically the hyperbolic tip by specifying z0, the distance between the tip apex and the ground plane (the gold substrate), and ρ, the distance between the focus of the hyperbola and the tip apex (that can be physically identified with the electric dipole radius), then it follows from the properties of the hyperbola that z02 v B sin A 2

− =1

and

2

(ρ + z0 )2 = B 2 sin 2 v + B 2 cos2 v A

A

= B2

If the tip bias (with respect to the ground plane) equals v = V, then one easily determines the two unknown parameters A and B,

A=

V

and

z arcsin 0 ρ + z0

B = ρ + z0

The two components Ex and Ez of the electric field E ( x , z ) are obtained by separating the real and imaginary parts of the gradient of the complex potential w,

E z + iE x =

dw A = = 2 ds s + B2

A

(x + iz )2 + B 2

However, since at the ground plane Ex = 0 and E = Ez, instead of separating the two parts, it is enough to set y = 0 in the above expression, 20 ACS Paragon Plus Environment

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E=

A 2

x + B2

As expected on physical grounds, the field attains its maximum value E m = A B at the point of the plane located right below the tip (x = 0). The electric field induces a dipole moment

p = α E in a SAM molecule where α is the molecular polarizability tensor. The dipole potential energy is then W = −p E or, in our case where E = Ez,

W = −α zz E 2 = −α zz

A2 x2 + B2

in which αzz is the respective polarizability tensor element. The electrostatic force acting on the molecule is given by the gradient of the potential energy W,

F =−

A2 x ∂W = −2α zz ∂x x2 + B2

(

)

2

and is directed always towards of the tip axis, towards the region where the density of field lines is higher. F vanishes right below the tip and decays like 1 x3 at large x values. The maximum value Fm of its magnitude, attained at the position xm = B = ρ + z 0 , is

Fm =

α zz A 2 2 B3

=

α zz 2

V2

(ρ + z 0 )3 arcsin 2

z0 ρ + z0

In our case, one typically has ρ >> z0 . Indeed, Simmons’ expression for the tunneling junction resistance allows one to estimate the z0 value, given the tunneling resistance R(z0), junction surface area S, and decay constant β. By using the experimentally determined R(z0) and β values for the SAM-covered substrate, one readily finds that 2.7 nm < z0 < 3.4 nm for “reasonable” values of the (unknown) surface area S, assumed to range from 0.1 nm2 to 1



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nm2. On the other hand, the ρ value was found experimentally to be about 30 nm, so that z0/ρ

≈ 0.1 « 1. In virtue of the inequality ρ >> z0 , the expression for Fm simplifies to

Fm ≈

α zz V 2 2 ρ z 02

valid to the first order in the ratio z 0 ρ . If we take the value of the polarizability of trans azobenzene (in SI units) 55, 6×10-39 F.m2 for αyy and put z0 = 3 nm, ρ = 30 nm and V = 1 V, then we get for the maximum force Fm ≈ 12 fN. This value is several orders of magnitude smaller that the forces typically employed for single atom manipulation (~100 pN)59 and will therefore have no noticeable mechanical effect on the SAM molecules. However, the electric field E not only exerts the horizontal force F on the induced molecular dipole p, but also applies a torque (a moment of force) τ = p × E on the latter. For an electric field directed along the z axis the components of the torque are τ x = −α yz E 2 , τ y = α xz E 2 and

τ z = 0 . Therefore, the torque rotation axis is lying in the xy horizontal plane, i.e. the torque acts as a bending moment on the nearly vertical azobenzene groups. The torque magnitude is

2 xz

τ = α +α

2 yz

2

2 xz

E = α +α

2 yz

ρ 2V 2 A2 2 2 ≈ α xz + α yz 2 2 x2 + B2 z0 x + ρ 2

(

It achieves its maximum value

τ m ≈ α xz2 + α yz2

V2 z 02

right below the tip and decays like 1 x 2 far from the tip (see Figure 8).


)

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For the torque to exist at least one of the polarizability tensor off-diagonal elements αxz and

αyz must not vanish. This is possible owing to the azobenzene moieties of the SAM molecules making a small tilt angle θ with the sample normal,56,60 that is still compatible with the SAM internal disorder or the non-perfect planarity of the underlying gold surface. Then

α xz = (α zz − α xx )sin θ cosθ cos α

and α yz = (α zz − α xx )sin θ cosθ sin α

azimuthal angle of the molecule, so that

where α is the

α xz2 + α yz2 = (α zz − α xx )sin θ cos θ ≈ ∆ α θ (we have

introduced the polarizability anisotropy ∆α = α zz − α xx and have taken into account the fact that θ is small). Assuming V = 1 V, z0 = 3 nm, θ ≈ 0.2 rad, as deduced from Tamada et al.,60 and ∆α ≈ 5×10-39 F·m2 (from Pedersen et al.55) one obtains τm ≈ 1.1×10-23 N·m for the maximum torque value. This value is to be compared to that for a nematic liquid crystal in a cell. The initially aligned molecules of a typical liquid crystal cell of thickness ~ 1 µm, used as variable retarder in polarization optics, rotate from 0° to 90° at voltages of about 25 V. The polarizability anisotropy of nematic liquid crystals61 is ∆α ≈ 2×10-39 F.m2 whereas, from the typical order parameter value C = 0.55 where C =

(

)

1 3 cos 2 θ − 1 found in Ref. 62, one can deduce a mean 2

value of θ of about 0.58 rad. Consequently, one gets for the torque, fully rotating a liquid crystal molecule, τLC ≈ 7.2×10-25 N·m. This torque value is 150 times smaller than that applied to the azobenzene thiol SAM; this large ratio is essentially due to the much higher electric field in tip - substrate gap, as compared to that in a liquid crystal. As a consequence, the torque induced by the tip - substrate electric field is fully capable of bending the SAM molecules.



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SUPPORTING INFORMATION. Current-voltage curves on bare gold substrates; Current-

distance curves at different landing voltages. This material is available free of charge via the Internet at http://pubs.acs.org. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT C.T. gratefully acknowledges financial support from the Chaire de Recherche CAP sponsored by TOTAL at the Ecole Polytechnique.


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Figure 1 a) STM topography of an azobenzene SAM on Au(111) (map 0.8 x 0.8 µm2, bias 0.1 V, current setpoint 100 pA); b) Azobenzene TERS spectrum acquired with the tip tunneling (in the position marked by a cross in panel a); c) TERS spectra (waterfall plot) acquired while changing the applied bias voltage in the 0.5 V ÷ 0.5 V range; the current is set constant at 100 pA. Top: TERS spectrum obtained at applied voltage of -0.1 V (blue line), marking the characteristic azobenzene peaks; Side: intensity profile of the azobenzene peak at 1142 cm-1 (red line) with changing bias. Figure 1 153x136mm (300 x 300 DPI)

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Figure 2 Mean normalized TERS decay versus applied bias (black dots), obtained by averaging the curves from 10different experiments (both polarities). The red curves are calculated from equation (4). The shaded area represents the dispersion of the calculated curves obtained for R(z0) values ranging from 0.1 to 50 MΩ. Figure 2 296x209mm (300 x 300 DPI)



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Figure 3 Mean I-V curve on the azobenzene SAM (blue dots). The red curve is calculated from equation (2). Inset: I-V curves acquired after landing at increasing set tunneling voltage (from 0.05 V to 0.5 V). Figure 3 296x209mm (300 x 300 DPI)


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Figure 4 Mean tunneling current through an azobenzene SAM versus tip-surface distance, starting from the landing position z_0 (z = 0). Figure 4 247x163mm (300 x 300 DPI)



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Figure 5 a) Azobenzene TERS spectra as a function of the distance between tip and surface (0-10 nm, step 1 nm, constant bias 0.1 V). Top: TERS spectrum acquired at 1 nm away from the landing point (blue line); Side: tip-approach curve showing the intensity profile of the azobenzene peak at 1142 cm-1 (red line) with increasing distance between tip and surface; b) Mean normalized intensity (dots) of the azobenzene peak at 1142 cm-1 as a function of the increasing separation between tip and surface and fit curve (red line) obtained from Eq. (1). Figure 5 175x192mm (300 x 300 DPI)


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Figure 6 Mean normalized TERS decay versus increasing applied bias (black dots), obtained by averaging the curves in both polarities. The curves are obtained using Eq. (4) with the parameters derived from electrical measurements (red), Eq. (10) (blue) and a combination of the two as in Eq. (11) (green). Figure 6 296x209mm (300 x 300 DPI)



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Reference frames xyz and x’y’z’, where the x’ axis makes an azimuthal angle φ with the x axis and the z’ axis is tilted at the polar angle θ with respect to the z axis. Figure 7 114x111mm (72 x 72 DPI)


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Simple sketch showing th tip-substrate cavity and the electric force lines u(x,z) = const (dashed gray) and equipotential lines v(x,z) = const (continuous blue), the force magnitude (continuous red line) and the torque magnitude (dash-dotted green line) along x direction. Figure 8 79x86mm (300 x 300 DPI)


Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced

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