element which diffracts the incident wave-length at zero electric field, and applying afield results in an optically transparent state.Parameters such as the thickness of the thinfilm, its morphological features, the interfacialroughness between the polymer and LC layers,and the LC orientation in the droplets areimportant considerations for the electrical andoptical performance of the thin films.

TECHNIQUES FOR THE CHARACT-ERIZATION OF THIN FILM HPDLCSMultiple characterization techniques are usedfor the analysis of thin film HPDLCs. Spec-troscopy, wavefront sensing, interferometryand microscopy are some of the more impor-tant tools used. A brief review on the use ofsome of these techniques is given here.

Understanding the LC droplet orientationaids in interpreting the behavior of light inci-dent on HPDLCs. Nuclear magnetic resonancespectroscopy has been shown to be an impor-tant technique to study LC alignment anddynamics in the confining polymer matrix [6].Typically for these studies deuterated protoncomponents in the LC are analyzed to predicttheir orientation.

Wavefront-sensing techniques are used todetermine the quality of the wavefront trans-mitted through the HPDLC. For example, thewavefront analyzer from AOA, Inc. which isbased on Shack Hartmann wavefront analysis[7] compares the deviation of a test wavefronttransmitted through the HPDLC to an idealreference wavefront. Software analysis of thetwo wavefronts enables accurate computingof the aberrations in the test wavefront. A flatwavefront traversing through an HPDLC struc-ture encounters interfacial roughness bet-ween the LC-rich and the polymer-rich layers.Also within the LC-rich layer RI inhomo-geneities exists due to variation in droplet sizeand distribution [8]. Thus the wavefront trans-mitted through the HPDLC structure has an

B I O G R A P H YKashma Rai is workingfor her PhD in electricalengineering at DrexelUniversity. Her researchinterests include devel-oping applications forthin-film holograms andthe fundamental understanding of liquidcrystal (LC) polymer interactions using spec-troscopical and microscopical techniques.She has been using scanning electronmicroscopy to study LC/polymer compositesfor the last four years.

A B S T R A C TFundamental research on the interaction oflight with materials such as liquid crystals(LCs) and polymers has led to innovativeapplications such as the omnipresent LCDs.Here we briefly review traditional tech-niques used for the analysis of holographicpolymer dispersed liquid crystals (HPDLCs)and discuss new techniques. Scanning elec-tron microscopy of samples under high vac-uum is a well-known technique used forHPDLC structure analysis where the LC isremoved prior to imaging. In this work thegrating structure is imaged for the first timeusing variable pressure SEM without LCremoval and compared with the conven-tional high-vacuum SEM technique. The useof environmental SEM was also explored forHPDLC imaging.

K E Y W O R D Svariable pressure scanning electronmicroscopy, environmental scanning elec-tron microscopy, liquid crystal polymer,holography, morphology, Bragg gratings

A C K N O W L E D G E M E N T SWe acknowledge Drexel CRF and specificallyEd Basgall and Dee Breger at CRF for theirvaluable input.

A U T H O R D E TA I L SKashma K. Rai,Electrical and Computer EngineeringDepartment,Drexel University,3141 Chestnut Street,Philadelphia, PA 19104, USAPhone: +1 215 939 0924Email: kashma@gmail.com

Microscopy and Analysis 23(5):17-19 (AM),2009

SEM O F HPDLCS

I N T R O D U C T I O NElectric-field controllable, holographicallyformed, polymer-dispersed liquid crystals(HPDLCs) consist of periodic layers of liquidcrystal (LC) droplets inside a polymer matrix.Their electro-optical applications include pho-tonic crystals [1,2], reconfigurable mirrors [3]and displays [4].

Based on their fabrication method, eitherreflection mode HPDLCs or transmission modeHPDLCs can be created [5]. In reflection typeHPDLCs a wavelength proportional to the peri-odicity of the layers is reflected at zero electricfield bias. Applying an electric field results inan optically transparent state, allowing all inci-dent wavelengths to be transmitted. The oper-ation of a reflection type HPDLC is shown inFigure 1. The HPDLC film is sandwiched bet-ween conductive coated substrates. Glass iscommonly used as a substrate, coated withconductive indium tin oxide (ITO). The aver-age orientation of the LC droplets is random atzero field bias. A refractive index (RI) mismatchexists between the randomly oriented LCs andthe polymer RI np. The unit vector n representsthe orientation of the LC globules, as shown inFigure 1a. The periodically varying RI due tothe alternating LC and polymer regions cre-ates a Bragg grating which reflects a specificwavelength of light. This is represented by thereflectance of green wavelengths and thetransmission of remnant incident wavelengthsfrom the HPDLC hologram, as shown in Figure1a. Applying an electric field across the HPDLCvia the conductive coating reorientates the LCsalong the direction of the field (Figure 1b). TheLC is chosen such that the RI of the reorientedLC is the same as the RI np of the polymer. Thusa uniform RI in the HPDLC due to RI matchbetween the polymer and LC layers erases theBragg grating. Thus the entire incident wave-length spectrum is transmitted without wave-length selectivity, as shown in Figure 1b.

A transmission type HPDLC is a dispersive

Variable Pressure and Environmental SEM ofThin-Film Liquid Crystal/Polymer CompositesKashma K. Rai, Sameet K. Shriyan and Adam K. Fontecchio Electrical and Computer Engineering Department, Drexel University, Philadelphia, PA, USA

MICROSCOPY AND ANALYSIS JULY 2009 17

Figure 1: Schematics showing the optical responses of a reflection mode HPDLC to the visible wavelength spectrum (a) when no electric field is applied acrossit and (b) when an electric field is applied across it. See text for further details.

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RMS profile in terms of the phase variationsfrom each of the interfaces. These wavefrontsurface phase variations carry informationregarding the interfacial morphology of theHPDLC. By looking at the final RMS wavefrontaberration values and the Strehl ratios [9] indi-cated by the wavefront analyzer, a generalprediction of the HPDLC morphology can bemade. Low RMS variations and higher Strehlratios are indicative of smoother interfacialroughness for an HPDLC.

It is crucial to measure the critical dimensionsof an HPDLC, such as its thickness, to ensure itsuniformity during holographic fabrication.This guarantees consistent optical and electri-cal performance. Optical profilometers such asZygo’s New View series, which use an opticalphase-shift interferometry technique, accu-rately determine the HPDLC 3D profile metrol-ogy. This is a quick non-contact method, whichis non-destructive to the sample and also doesnot require sample preparation.

Polarizing optical microscopy (POM) is themost basic technique used for imaging the sur-face topography of thin film HPDLCs. Here alight microscope images the sample cell placedbetween crossed polarizers. POM is an instantway of analyzing the surface of the HPDLCthin film on the micrometer scale, such as thegrating pitch of transmission mode HPDLCs.However due to lack of high-resolution imag-ing capabilities this method cannot be used forextensive morphological analysis.

Atomic force microscopy (AFM) has beenused for the analysis of the dimpled polymer

structure at the LC-polymer interface ofHPDLCs [10]. Tapping mode was used here toprevent damage to the soft polymer interface.There is a limit on the maximum height andmaximum area this technique can imagewhich is the range of several micrometers. Alsothe image scanning process can take severalminutes with increased likelihood of artifactsin the image.

SEM and transmission electron microscopy(TEM) are two high-resolution techniquesused for the nanometer-scale structural analy-sis of HPDLCs [5]. They are widely used for thevisual inspection of phase separation betweenLCs and polymer, the LC droplet diameter,droplet structure, grating pitch, droplet dis-persion, and droplet density determination.

The conventional SEM technique requiresthe test sample be placed in a vacuum cham-ber. The sample preparation involves freezingits surface to preserve the grating structureand fracturing the appropriate section forimaging. The LC is removed from the sampleand a thin conductive coating is applied on theremaining surface, typically by sputter coating,to render it electrically conductive to preventaccumulation of charges during imaging. Thecoating should be of the right thickness toenable accurate imaging of the surface with-out obscuring finer details of the morphology.However, the removal of the LC during samplepreparation is disadvantageous since it resultsin variation of the grating profile.

The use of TEM morphological analysis typi-cally requires an embedding process followed

MICROSCOPY AND ANALYSIS JULY 200918

by sectioning. The embedding process resultsin leaching of impurities into the grating struc-ture leading to alteration of the grating pitch.The preparation of samples for both SEM andTEM are labor intensive. In order to determinethe accurate high-resolution features of thegrating structure it is essential that the LC isnot removed from the grating structure andthat the sample surface is not chemicallytreated, which could result in possible distor-tion of the features.

Cryo-SEM has been used recently to preservethe grating structure for imaging without LCremoval. Here the sample was frozen andimaged without LC removal [11]. Cryo-SEMsample preparation requires initial freeze frac-turing of the surface using liquid nitrogen at -182oC. To apply a conductive coating onto thesample the temperature was increased toabout -90oC and cooled back to -182oC prior toimaging. The cryogenically fixed sample wasthen imaged in the SEM on a cryo-stage.

Variable-pressure SEM (VPSEM) and envi-ronmental SEM (ESEM) are two techniquesused here for the first time for the morpho-logical study of HPDLCs at high resolutionwithout the need for LC removal or conductivecoating. The specimen preparation processinvolves freeze fracturing the sample followedby imaging in the SEM chamber. In this studythe HPDLC grating structure was imaged inconventional HV-SEM and VPSEM and thegrating structure was compared. The poten-tial of the ESEM technique for imaging theHPDLC structure was also explored.

Figure 2: SEM images of cross-sections of HPDLCs. (a) HV-SEM image of reflection mode HPDLC. (b) VPSEM image of reflection mode HPDLC. (c) HV-SEM image of transmission mode HPDLC. (d) VPSEM image of transmission HPDLC.

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SEM O F HPDLCS

MICROSCOPY AND ANALYSIS JULY 2009 19

Figure 3: Environmental SEM images of the transmission mode HPDLCs. (a) Image of the surface features. (b) Image of a cross-section.

M AT E R I A L S A N D M E T H O D SHPDLC FabricationAn acrylate formulation was used for thephoto-initiated polymerization method offabrication of HPDLCs [12]. The constituents ofthe recipe were tri- and hexa-functionaloligomers EBECRYL 4866 and EBECRYL 8301(Cytec, NJ, USA), both at 22.5% w/w, 32.4%w/w of LC BL038 (EM Industries, NY, USA),12.6% w/w photoinitator (4% w/w Rose Ben-gal, 10% w/w of coinitiator N-phenyl glycineand 86% w/w of N-vinyl pyrrolidone) and 10 %w/w Tween 80 surfactant.

A sample cell was prepared by placing thiswell-blended prepolymer recipe between twoITO coated glass slides separated by 5 µm usingglass spacers. A holography setup using a 532nm Verdi laser generating standing wave pat-terns was used to fabricate the HPDLC fromthe prepolymer cell. During the HPDLC forma-tion the oligomer phase separated and poly-merized into periodic layers alternating withLC droplets layers.

Sample Preparation Technique for SEMTo prepare the HPDLC sample for HV-SEMimaging, the sample surface was frozen usingliquid nitrogen and fractured to obtainrequired cross-sections. The LC was removedwith methanol. The sample was then sputtercoated with a 3-nm thick platinum-palladiumcoating to enable good contrast, high resolu-tion and prevent charge accumulation in thespecimen. For the VP/ESEM imaging the sam-ple was freeze fractured and placed directly inthe SEM chamber.

Scanning Electron Microscopy A Zeiss Supra 50VP scanning electron micro-scope was used for both HVSEM and VPSEMimaging. An FEI XL30 SEM was used for theenvironmental SEM imaging.

The HV-SEM imaging was performed at anaccelerating voltage of 2 kV under high vac-uum. The reflection HPDLC cross-section wasimaged using an SE detector and the transmis-sion HPDLC cross-section was imaged withZeiss’s patented In-lens SE detector. The VPSEMwas performed at 4 kV and a VPSE detectorwas used for both reflection mode and trans-mission mode HPDLC imaging. The chamberpressure was set at 20 Pa.

A GSE detector was used for ESEM imagingat 10 kV. The pressure inside the SEM chamberwas set at 400 Pa.

R E S U LT S A N D D I S C U S S I O NFigure 2 shows SEM images of the HPDLC grat-ing structure imaged using the HVSEM andVPSEM. Figures 2a and 2b are images of reflec-tion mode HPDLCs. Figures 2c and 2d are thetransmission mode HPDLC images.

Here the black voids are the LC dropletsregions and the area surrounding them is thepolymer matrix, as shown in Figure 2a. The LClayered structure is illustrated by drawing linesat the centre of two adjacent LC layers shownin 2a-d. Figure 2a also illustrates the gratingpitch, which is the thickness of one layer ofpolymer and liquid crystal or the distancebetween the centers of two adjacent LC layers.

As seen in the SEM images, the grating pitch ofthe reflection HPDLCs was lower than that ofthe transmission HPDLCs and had smaller LCdomains.

The LC droplets varied in size, shape andwere interconnected in the acrylate recipe[12]. These SEM images provided morphologi-cal details from the HPDLC cross-section suchas the LC size and shape distribution. Forreflection mode HPDLCs, shown in Figures 2aand 2b, there was polydispersity of the dropletsize and shape and the droplets on an averagewere elongated along the LC layer. The trans-mission HPDLCs had larger LC droplet regionsand the interconnectivity between them madethem appear more as a continuous layer ratherthan discrete droplet regions, as shown in Fig-ure 2c and 2d.

A comparison of the difference in the grat-ing spacing between the two types of imagingusing a image analysis tool showed an averagedecrease of grating pitch by 18.6% for reflec-tion HPDLC imaged in HV compared to VP. Adecrease of grating spacing was observed fortransmission HPDLCs by 17.5% for these tworespective modes.

The reduction in the grating pitch measuredby conventional mode HV-SEM imaging is dueto the removal of LCs. Air replaces the voidscreated by LC elimination and the polymermatrix contracts around these regions, reduc-ing the average pitch of the grating.

Figure 3 shows the HPDLCs samples imagedin an ESEM using ambient conditions. How-ever due to the decreased resolution obtainedfrom this technique we were able to imageonly the transmission HPDLC with discernablefeatures. A noticeable contrast was observedbetween the LC and the polymer layers to dis-tinguish the HPDLC surface feature seen in Fig-ure 3a and the jagged interface between theLC/polymer interfaces imaged in Figure 3b.

C O N C L U S I O N SIn this study three SEM imaging techniqueswere used for the analysis of the morphologyof HPDLCs. The significant advantage ofVPSEM and ESEM compared to conventionalSEM is that they represent the true morphol-ogy profile without the need for LC removal orconductive coating. Comparing VPSEM images

with HVSEM images revealed the variation ofthe grating spacing for acrylate recipe HPDLCsafter LC removal. Thus VPSEM was used for thefirst time to quantify the effect of LC removalon the true morphology of acrylate formula-tion based HPDLCs imaged and quantify thedifference. ESEM has lower resolution com-pared to HV-SEM mode but can be used toanalyze grating morphology of transmissionmode HPDLCs with discernable contrastbetween the polymer and LC-rich regions.

R E F E R E N C E S 1. Escuti, M. J. et al. Two-dimensional tunable photonic crystal

formed in a liquid-crystal/polymer composite: thresholdbehavior and morphology. Applied Physics Letters83(7):1331-1333, 2003.

2. Liu, Y. J., Sun, X. W. Electrically tunable three-dimensionalholographic photonic crystal made of polymer-dispersedliquid crystals using a single prism. Japanese Journal ofApplied Physics Pt 1 46(10A): 6634-6638, 2007.

3. Ermold, M. L., Fontecchio, A. K. Electrically switchableholographic parabolic mirrors. Journal of Applied Physics 99(9): 93111-1-93111-5, 2006.

4. Qi, J., Crawford, G. P. Holographically formed polymer dis-persed liquid crystal displays. Displays 25(5):177-186, 2004.

5. Bunning, T. J. et al. Holographic polymer-dispersed liquidcrystals (H-PDLCs). Annual Review of Materials Science30:83-115, 2000.

6. Vilfan, M. et al. Deuteron NMR study of molecular orderingin a holographic-polymer-dispersed liquid crystal. PhysicalReview E 66(2):021710-021711, 2002.

7. Platt, B. C., Shack, R. History and principles of Shack-Hartmann wavefront sensing. Journal of Refractive Surgery17(5):S573-S577, 2001.

8. Sutherland, R. L. et al. Coherent diffraction and randomscattering in thiolene-based holographic polymer-dispersedliquid crystal reflection gratings. Journal of Applied Physics99(12):123104-1-123104-12, 2006.

9. Wyant, J. C., Creath, K. Basic Wavefront Aberration Theoryfor Optical Metrology. In: Applied Optics and OpticalEngineering, Volume XI. , Eds. Shannon, R. R. and Wyant, J.C., Academic Press, pp 2-53, 1992.

10. Fontecchio, A. K. et al. Cavity Morphology of PolymerDispersed System Utilizing Atomic Force Microscopy. In:Advances in Liquid Crystalline Materials and Technologies.Symposium, Materials Research Society, Boston, CC6.5.1-CC6.5.6, 2002.

11. Lloyd, P. F. et al. Cryo-SEM Study of Liquid Crystals in a Thiol-ene Polymer. Microscopy and Microanalysis 13(Suppl 2):1464-1465 CD, 2007.

12. Drzaic, P. S. In: Liquid Crystal Dispersions. World ScientificPublishing Co., NJ, USA, pp 11-97, 1995.

©2009 John Wiley & Sons, Ltd

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