Research article

Received: 15 February 2012 Revised: 25 April 2012 Accepted: 29 April 2012 Published online in Wiley Online Library: 11 August 2012

(wileyonlinelibrary.com) DOI 10.1002/jrs.4118

Molecular orientation analysis of organic thinfilms by z-polarization Raman microscopeToshihiro Mino,a Yuika Saito,a* Hiroyuki Yoshida,b,c Satoshi Kawataa

and Prabhat Vermaa

Polarization-dependent Raman microscopy is a powerful technique to perform both structural and chemical analyses withsubmicron spatial resolution. In conventional Raman microscopy, the polarization measurements are limited only in the direc-

tion parallel to the sample plane. In this work, we overcome the limit of conventional measurements by controlling theincident polarization by a spatially modulated waveplate. In this method, the polarization perpendicular to the sample surface(z-polarization) can be detected together with the parallel polarization (xy-polarization). Because of this unique polarizationcontrol, our Raman microscope has the ability to image the molecular orientation, together with the molecular analysis. Here,we have investigated thin films of pentacene molecules that are widely studied as an organic semiconductor material. Theorientations of pentacene molecules are imaged with a spatial resolution of 300 nm. Our results clearly indicate that the lamel-lar grains show the lower tilt angles compared to the neighboring islands, which has not been proved in conventionalmethods. The substrate effects and the thickness dependence of the film are also studied. These results provide knowledgeabout the relationship between the devise performance and the film structures, which is indispensable for future deviceexploitations. Copyright © 2012 John Wiley & Sons, Ltd.

Supporting Information may be found in the online version of this article.

Keywords: z-polarization; polarization Raman microscopy; pentacene; molecular orientation; organic thin film

* Correspondence to: Yuika Saito, Department of Applied Physics, OsakaUniversity, 2–1 Yamadaoka, Suita, Osaka, 565–0871 Japan.E-mail: yuika@ap.eng.osaka-u.ac.jp

a Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita,Osaka, 565-0871, Japan

b Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

c JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan

202

Introduction

Evaluation of the material properties from the viewpoint ofdevice performance is necessary in many commercial applica-tions. For example, organic semiconductor devices have beenattracting considerable attention because of their flexibility, light-weight and low costs.[1] The organic molecules used in thesedevices are usually in the form of thin film composed of polycrys-talline domains with average domain size ranging from sub-micrometer to a few tens of nanometer. The potential barriers atthe boundaries of the crystallites often limit the charge carriermobility,[2] which can affect the device performance significantly.Therefore, it is crucial to investigate both inter- and intra-crystallinestructures in these organic thin films.

The domain structures are usually analyzed by atomic forcemicroscope (AFM). Transmission electron microscope or scanningtunneling microscope are also used; however, a possible sampledamage due to high-energy electrons and low conductivities oforganic samples limits the application of these techniques.[3]

Further, these techniques do not have any access to the informa-tion related to the intra-crystalline structures. Amongst existingtechniques, X-ray crystallography provides the most detailedinformation about crystallographic structures and orientations.[4,5]

However, one of the disadvantages of this technique is that wecannot see the non-crystalline parts of the sample, and it is diffi-cult to perform spatially resolved measurements. On the otherhand, optical measurements, such as Raman spectroscopy, havesome advantages over the other methods for characterization oforganic thin films. In polarization-dependent Raman spectros-copy, it is possible to identify not only the molecular speciesand their orientations, but also their crystalline nature.[6–10]

J. Raman Spectrosc. 2012, 43, 2029–2034

In addition, when combined with a microscopic technique, spa-tially resolved investigations are easily realized. Because of theseadvantages, polarization-dependent Raman microscopy has beena powerful method to study the orientational distributions ofassembled molecules.[11–14] However, when incident light is tightlyfocused under microscopic measurements, the polarization islimited only to components parallel to the sample plane (x- ory-polarization), and the perpendicular component (z-polarization)is usually negligible. Reasonable amount of z-polarization compo-nent in the incident light can, however, be achieved by utilizing aspatially modulated polarization.[16,15] We have recently establisheda z-polarization sensitive detection under microscopic conditionsthrough this technique.[17] The z-polarization Raman microscopycan be ideally applied to the molecular orientation imaging of anorganic thin film, because the z-polarization is sensitive to theorientations of these quasi-one dimensional molecules.

In this paper, we have imaged the molecular orientation in pen-tacene thin films via z-polarization Raman microscopy. Pentacenehas been widely studied as a promising material for organic fieldeffect transistors.[18,19] The molecular orientation image showing

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the variation in the tilt angle of pentacene molecules obtained byour microscope revealed the inter- and intra-crystalline structures.By comparing the molecular orientation distribution and thecorresponding AFM image, relationships between the grainshapes and the crystallographic structures were clarified. Further,we examined the substrate effect and film thickness dependenceon the film growth of pentacene.

Experimental

z-polarization microscopic measurements

Figure 1 schematically shows the experimental setup of ourz-polarization Raman microscope. A continuous-wave incidentlaser light was expanded and introduced to amicroscope objective,which focused the light tightly on the sample plane. The wave-length of the incident light (l = 488 nm) was carefully chosen tomeet off resonant condition for pentacene molecule, and thepower of the incident light (150 mW at the sample) was kept lowenough to avoid any sample heating or damage. For the purposeof easy scanning, the sample was placed on a piezoelectric con-trolled stage under an AFM operation system. Raman signal wasintroduced to a spectrometer through the same objective lens,which was then detected by a liquid-nitrogen-cooled CCD camera.For polarization measurements, the incident polarization patternwas modified by a liquid crystal variable waveplate (LCW). TheLCW created either a radial or an azimuthal polarization as illus-trated in Fig. 1. The LCW device was purchased from ARC Optix.The switching speed from the radial to the azimuthal polarizationwas less than 100 ms, and this dead time was considered in our

pzt stage

CCD

mask

pin holeRamansignal

AFM probe

objective lens

depolarizeredge filter

polarizer

beam expanderPC

spectrometer

laser

radial azimuthal

sample

AFMcontrol

300 nm

LCW

Figure 1. Optical setup of z-polarization Raman microscope. The liquidcrystal variable waveplate (LCW) can create radial or azimuthal polarizations,as shown in the schematic view. Inset shows the near-field image of theincident laser intensity pattern formed on the sample plane by radiallypolarized light. The diameter of the main peak (within the first dark ring)was less than 300 nm, and the contribution of the side lobes are negligiblecompared to the intensity of the main peak. Slight intensity variation writtenthese structures can be considered as noise in the measurement.

wileyonlinelibrary.com/journal/jrs Copyright © 2012 John

spectral measurements. The effective area of LCW is 10 mm indiameter, and the illumination intensity uniformity distributed overthe area was ~1%, except for the 100-micron gap located at thecenter of the LC plate. A detailed explanation of the formation ofz and xy-polarizations through the radial and the azimuthal patterncreated by LCW is explained elsewhere.[17] The polarization wasswitched from radial to azimuthal by controlling the voltageapplied to the LCW. The motion of the sample stage, the LCWand the data acquisition by the CCD camera were synchronizedby a computer control. The radial and azimuthal polarizationmeasurements were simultaneously performed at each positionof the sample; therefore, a tilt angle image could be constructedby just one scan of the sample. Raman imaging was performed insteps of 300 nm, which was close to the spatial resolution of oursystem. For a comparison, AFM imaging of the pentacene filmwas also carried out with contact-mode operation.

The inset in Fig. 1 shows the focus spot of the radially polarizedlight. This image was constructed by detecting Rayleigh scatteredlight from the AFM probe, as the probe scanned across the focusspot. From this figure, we confirmed that the spatial resolution ofour system was better than 300 nm. Since the island size of ourpentacene sample is larger than 1 micron, the spatial resolutionof our measurements is well within the range that could analyzethe inner of the grain structures.

A proper calibration for the estimation of actual value of bothpolarization components on the sample is necessary for preciseRaman analysis. This calibration was done by utilizing alpha-quartz, the Raman tensors of which are known.[20] The experi-mental polarization components at the sample plane aresummarized in Table S1 (Supporting Information). The theoret-ical values under perfect polarization conditions are also shownfor a comparison.

Fabrication of pentacene thin films

Pentacene purified by a vacuum-sublimation technique wasobtained from Aldrich Chemical Company Inc. Pentacene thinfilms were grown under a vacuum lower than 10�4 Pa with aver-age thickness of 5, 20 and 200 nm, with a deposition rate of 1.0nm/min. Two kinds of substrates, polymethyl methacrylate(PMMA)-coated and bare glass, with hydrophobic and hydrophilicsurfaces, respectively, were used to observe the dependence ofnature of substrate on film growth. The hydrophobic/hydrophilicnature of the substrate was estimated from the water contactangle, which was measured to be 95� and 13�, respectively, forthe two substrates.

Results and discussion

Raman evaluation of pentacene thin film

Figure 2(a) shows typical Raman spectra of a pentacene film mea-sured under z- (radial after LCW) polarization and xy- (azimuthal)polarization. Since pentacene is a highly symmetric quasi-onedimensional molecule, it shows strong polarization dependencein several Raman modes. The observed Raman modes at 1371and 1596 cm�1 in Fig. 2(a) are assigned to in-plane modes Ag

and B3g, respectively, on the basis of the calculated results.[21]

We chose the mode B3g at 1596 cm�1 as an indicator to investi-gate the tilt angle of the pentacene molecule because of thez-polarization sensitivity and the simplicity of the Raman tensor.A clear dominance of Raman intensity under radial polarization

Wiley & Sons, Ltd. J. Raman Spectrosc. 2012, 43, 2029–2034

(b)b

c

a

Wavenumber/cm-1

Ram

an I

nten

sity

radialazimuthal

(a)

1000 1200 16001400 1800

(c)

tilt angle /degree

Iazi

/ Irad

20 40 80600

20

40

80

60

twitst angle = 45 °

Iazi

/ Irad

= 0.6

twits

t ang

le

/deg

ree

0

1.2

1.0

0.8

0.4

0.6

a

c(c’z

xa’

)

y(b)b’

Figure 2. (a) Raman spectra of a pentacene thin film measured under z (radial) and xy (azimuthal) polarization excitation. The arrow indicates the peakat 1596 cm�1, which we chose to analyze pentacene orientations. (b) Molecular coordinate of pentacene (a, b, c) and the definitions of the tilt angle θand the twist angle f with respect to the space axes (x, y, z). The tilt angle θ is the angle between the c-axis of the molecular coordinate and the z-axis ofthe space coordinate. The molecular coordinates are transferred from (x, y, z) to the intermediate coordinates (a’, b’, c’) by the tilt operation. The twistangle f is the angle between the b-axis of the molecular coordinate and the y-axis of the space coordinate. During the twist operation, the molecularcoordinate is transferred from the intermediate coordinates (a’, b’, c’) to the final coordinates (a, b, c). (c) The dependence of the intensity ratio Iazi/Irad onthe angles θ and f. The solid lines indicate solutions of Eqn. (1) for Iazi/Irad = 0.6. The dashed line is the solution for f = 45� , which was used to calculatethe tilt angle θ from the intensity ratio Iazi/Irad.

Molecular orientation analysis of organic thin films

indicated the z-polarization sensitivity of this mode. Though themode Ag at 1371 cm�1 was also sensitive to the polarization, interms of the simplicity of the Raman tensor, the B3g mode wasmore suitable for the tilt angle analysis. Figure 2(b) illustratesthe tilted pentacene molecular axes (a, b, c) with respect tothe space axes (x, y, z). The molecular tilt is indicated by theangle θ, while the twist is indicated by the angle f.

Analysis of z-polarized Raman spectra

The Raman tensor pattern of B3g mode for pentacene at1596 cm�1 can be written as,[21]

ff

0@

1A

:

We modified the Raman tensor pattern to match the tilt andthe twist of the pentacene film by angles θ and f, respectively.For this purpose, experimental values of Raman intensitieswere obtained by multiplying three elements – (incident polar-ization components)� (Raman tensor)� (detection efficiencies).If we express the components of the incident polarization as Ix,Iy, Iz, the experimental efficiencies as a factor A for the x- andy-polarization components and as a factor B for the z-polarizationcomponent, then the intensity of the Raman mode at 1596 cm�1

can be written as:

I1596 ¼ f 2Ix A sin2θ 4 cos2θ sin2fþ cos2f

�n

þIy A sin2θ cos2fþ B cos2θ cos2f�

þIz A sin2f cos2θ� sin2θ� �2 þ

�n

2664

J. Raman Spectrosc. 2012, 43, 2029–2034 Copyright © 2012 Joh

We obtained the values A and B as 9.45 and 1.01, respectively,for our experimental setup[22] and assumed that the incidentlaser intensity Ix and Iy have the same value. However, Ix (or Iy)and Iz are different due to the difference in the nature of polari-zation in x- (or y-) and z-directions. The tilt angle θ and the twistangle f are estimated through the measured intensities of Iaziand Irad, which indicate the Raman intensities of the B3g modeunder azimuthal and radial polarizations, respectively. We haveto note that the radial and azimuthal fields were prepared bythe LCW focused in exactly the same plane centered at the sameposition; however, the intensity distribution pattern of these twopolarization components are different within the focus area.Therefore, the molecular orientation can be determined whenthe sample properties are mostly uniform within the spot.

Even without an accurate analysis, one can conclude that theratio Iazi/Irad would give some information about the orientationof the molecular principal axis, because Irad is highly sensitive tothe z-polarization and Iazi is sensitive to the xy-polarization. WhenIazi/Irad is close to 0, the molecular axis is perpendicular to thesubstrate, and when it is close to 1, the molecular axis has a tiltof 45� against the substrate. With further analyses, we estimatedthe actual tilt angle θ from the intensity ratio Iazi/Irad. We illustratethe relationship between the intensity ratio Iazi/Irad and the anglesθ and f in Fig. 2(c), which was derived from Eqn. (1) and Table S1(Supporting Information). The intensity ratio Iazi/Irad is shown bythe variation in colors in Fig. 2(c), and the color scale is also indi-cated in the figure. The color variation along the lateral axisshows Iazi/Irad as a function of the tilt angle θ, and the color

�þ B sin2f cos2θ� sin2θ� �2o

�

cos2θ cos2f�þ 4B sin2θ cos2θ sin2f

o

3775 (1)

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variation along the longitudinal axis show Iazi/Irad as a function ofthe twist angle f. As one can notice from Fig. 2(c), there areseveral possible combinations of the angles θ and f for a givenvalue of Iazi/Irad. For example, the near-vertical solid line in the leftpart of the figure shows the possible values of the angles θ and ffor Iazi/Irad = 0.6. At the same time, the curved solid line on theupper right area of the figure also correspond to the same valueof Iazi/Irad. We know from the X-ray diffraction results,[23–25] thatthe pentacene molecules are standing near perpendicular onthe substrate; therefore, we ignore the solutions of Eqn. (1)for the large values of angles θ and f appearing at upper rightarea in Fig. 2(c). By considering only the left part of the figure,one can understand that Iazi/Irad depends mainly on the tiltangle θ, and the twist angle f contributes only little. Hence, wefixed the twist angle f at 45� to determine the angle θ. This situationis indicated by the dotted line in Fig. 2(c).Following the above-mentioned method, we constructed

Raman images of the distribution of molecular orientation inpentacene thin film by utilizing the intensity ratio Iazi/Irad. Sincepotential barriers at the boundaries of the crystallites often limitthe charge mobility,[2] inter-crystalline structures, or so-called do-main structures, are important factors for determining the carriermobility in the film. Intra-crystalline structures, which indicate thecrystallographic structures of the crystallites as well as their orien-tations, are another important factors influencing the chargecarrier mobility. Figure 3(a) shows a Raman image of the sample,where the distribution of tilt angle θ is shown by colors for apentacene thin film with an average thickness of 5 nm, preparedon a PMMA substrate. Themeasurement was performed in 7.5-mmsquare area, with 25� 25 pixels. Exposure time at each measure-ment point was 7 s. As seen in Fig. 3(a), there were non-uniformlocalized orientations of pentacene molecules in the sample.Pentacene molecules in red areas were standing near perpendic-ular to the substrate with small values of tilt angle θ, while themolecules in green areas were tilted by more than 20�. It is knownfrom X-ray diffraction experiments that the pentacene films are intwo different polymorphs, namely, the thin film phase with inter-layer distance in the (001) orientation=1.54 nm, and the bulk phasewith interlayer distance in the (001) orientation=1.44 nm. The X-raydiffraction results indicate that the tilt angle of the thin film phase isabout 6� and that of the bulk phase is about 22� .[23–25] Thus, in ourexperiments, the green areas with larger values of the tilt angleθ (~20�) correspond to the bulk phase, and the red areas withsmaller values of θ correspond to the thin film phase. We note

/degree

35

30

25

20

15

10

5

(a)

2 µm

Figure 3. (a) Raman image showing the orientation distribution for a 5constructed by z-polarization Raman microscope. (b) an AFM image of the saB is an example of dendritic grain.

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the value Iazi/Irad is a function of both f and θ, which induces ambi-guity in our estimated θ. The dependence of angle f on the ratioIazi/Irad is, however, negligible in the thin film phase where the tiltangle θ is very small. We can therefore determine θ directly fromthe ratio Iazi/Irad. In bulk phases, however, because of the slight de-pendence on f, we have an ambiguity of �3.5� in the estimatedvalues of the tilt angle θ. Areas with mixed phases are alsoobserved. Figure 3(a) reveals the domain boundaries of the thin filmphase and the bulk phase crystallites, and thus identifies the areaswhere the carrier mobility could be degraded.

Figure 3(b) is an AFM image of the corresponding area. TheAFM image clarifies the domain structures of the film. Two typesof domain structures are observed. One is lamellar grain (e.g.grain A), which is ribbon like in the shape and has slightly higherthickness in topography, and the other is dendritic grain (e.g.grain B), with shapes shown by the white lines. By comparingthe tilt angle distribution and the AFM image, we can see thatthe lamellar grains are composed of the bulk phase, and the den-dritic grains are composed of mainly the thin film phase. We canthus establish a relationship between the grain shape and the tiltangle, which has not been identified in conventional methods.

Further, we have investigated the effect of the nature of sub-strate on the molecular orientations. Figure 4(a) shows the tiltangle distributions for two samples where pentacene films withan average thickness of 5 nm were grown on a PMMA substrate,and on a glass substrate. The measurements were performed for400 points in both cases. As is clearly seen from Fig. 4(a), theaverage tilt angle θ for the glass substrate was higher (13�) thanthat for the PMMA substrate (10�). Pentacene thin film on thehydrophobic surface should exhibit higher charge carrier mobil-ity due to smaller tilt angle compared to the films grown on ahydrophilic surface. It has been reported that the pentacene ona PMMA shows better transistor performance when comparedto the pentacene on a glass.[26] Pentacene films used in an actualtransistor are in most cases poly-crystalline structure, and it hasbeen said that inter-crystalline structure is the important factorfor carrier mobility. Our results suggest that not only the inter-crystalline structure, but also the average tilt angle of the intra-crystalline structure contribute to the faster device performance.The lower tilt angle increases the overlap of frontier orbitalbetween neighboring molecules, which results in faster carriermobility. In addition, we found that the effect of substrate onaverage orientation of pentacene molecules strongly dependson the film thickness. The surface morphology of the substrate

2 µm

(b)height/nm

grain A

grain B

25

20

15

10

5

0

-nm-thick pentacene film grown on PMMA substrate. The image wasme area of the sample. Grains A are examples of lamellar grains, and grain

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15

10

5

0

PMMAGlass10 ± 3 13 ± 3

30

20

10

010 20 30 400

15 ± 2 16 ± 1

Tilt angle /degree

ratio

/in %

(a)

(b)

Figure 4. Substrate effect on pentacene orientation. The histogramsshow the variation of the tilt angle θ for (a) 5-nm and (b) 20-nm thickpentacene films grown on PMMA (solid lines) and on glass (dashed lines)substrates. The numbers in the figures show the average values and thestandard deviations of the tilt angles.

Molecular orientation analysis of organic thin films

directly affects the average tilt angle of the molecule, especiallyin the case of an ultrathin film. Since pentacene–substrate andpentacene–pentacene interaction forces determine the orienta-tions, the first few layers of pentacene molecules near the sub-strate are greatly influenced by the substrate morphology. Asthe thickness of the film increases, the effect of substratedecreases. The surface effect became negligible for thicker films,for example, for a sample with an average film thickness of20 nm, as shown in Fig. 4(b). The tilt angle histograms of thetwo substrates show almost the same shapes with comparableaverage tilt angles.

Finally, we have also investigated the relationship between thefilm thickness and the molecular orientations for samples grownon one particular kind of substrate. Apart from 5-nm and 20-nmthick pentacene films, we also measured a pentacene film grownon glass substrate with average thickness of 200 nm. Similar tothe histograms shown in Fig. 4, Fig. 5 shows normalized histo-grams for tilt angle distributions for sample with three different

20

10

0

10 20 30 400

5 nm20 nm200 nm

13 ± 3

24 ± 2

16 ± 130

ratio

/in %

Tilt angle /degree

Figure 5. Thickness dependence of pentacene orientation. The histogramsshow the variation in the tilt angle, which were extracted from Ramanmeasurements of pentacene films grown on glass substrates. The solid,dashed and dotted lines indicate the results of 5, 20 and 200 nm thicksamples, respectively.

J. Raman Spectrosc. 2012, 43, 2029–2034 Copyright © 2012 Joh

thicknesses. We can clearly see the average orientations of thepentacene films increases with the film thicknesses. The valuesof average tilt angle θ are 13�, 16� and 24� for film thickness of5, 20 and 200 nm, respectively. The estimated value of tilt anglefor the 200 nm thick film is close to the value obtained from bulksample by X-ray diffraction. We thus conclude that most of thepentacene molecules for 200-nm thick film were in the form ofbulk phase. On the other hand, in the case of 5-nm and 20-nmthick samples, the average values of θ was between those ofthe bulk phase and the thin film phases. Thus, we conclude thatthe 5-nm and 20-nm thick films contained mixtures of the twophases. This was also seen in the image shown in Fig. 3(a).

It was reported that the crystalline structure of the pentacenefilm evolves from the thin film phase to the bulk phase as the filmthickness increases to about 60 nm.[4,27] Thus, we can be assuredthat our pentacene film with a thickness of 200 nm contains onlybulk phases. The average values of θ for thinner films (5 nm and20 nm) estimated by our technique is larger than the values pre-dicted by X-ray diffraction. One of the reasons for this deviation isthe contribution of non-crystalline parts, which cannot bedetected by X-ray crystallography. Another reason is the possibleroughness of the substrate. The relationship between the spaceaxes and the molecular axes could be locally modified due tothe roughness of the substrate. These factors largely affect themolecular layers close to the substrate and hence show up moreprominently in thinner films. Another possibility is intermolecularinteractions that affect the value of the Raman tensor compo-nents. The intermolecular interactions can be evaluated by lowwavenumber Raman measurements,[6] which were not accessiblein our current setup. The deviation of the tilt angle from the X-raydata suggests that the existence of phenomena such as twisting,substrate roughness and contribution from the non-crystallineparts in thin films could be investigated by polarization-dependentRaman spectroscopy.

203

Conclusions

In conclusion, we have demonstrated orientation analyses ofpentacene thin films by the z-polarization Raman microscopewith spatial resolution of 300 nm. In this method, the polarizationperpendicular and parallel to the sample plane were selectivelyexcited under microscopic configuration. The Raman intensityratio Iazi/Irad obtained under azimuthal and radial polarizationexcitation is the key parameter of our measurements. The ratioIazi/Irad is directly related to the tilt angle of the molecular princi-pal axis with respect to the normal to the substrate. The absolutevalues of the tilt angle θ were estimated based on the Ramantensor analyses. Because of the advantage of z-polarizationmicroscopy, the non-uniformity of the crystallographic structurewas revealed, which has not been studied so far by other meth-ods such as X-ray diffraction or scanning probe techniques. Sub-strate effects on the growth of the pentacene molecules werealso studied, and we found that the average tilt angle was largerfor hydrophilic substrates, in comparison with the hydrophobicsubstrates. Our result supports that the PMMA substrate is prefer-able for devise performance in terms of the highly standingmolecular orientations (low tilt angles). Film thickness depen-dence of the molecular orientations was also confirmed. For athick film (200 nm) that consists mostly of the bulk phase, the tiltangle θ was estimated in consistent with the X-ray crystallogra-phy result. However, in the case of thinner films (5 nm, 20 nm),

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our results had deviation from the X-ray results. There are stillsome unknown parameters about the sample such as contribu-tions of non-crystalline parts, roughness of the substrates orintermolecular interactions, which can cause these ambiguities.The investigation of these parameters will be necessary for futurenano-devise fabrications. Since z-polarization is sensitive to theorientations in many molecular systems, this technique can beideally applied to the characterization of molecular thin films,which is essential for future device research.

Supporting information

Supporting Information may be found in the online version ofthis article.

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