Antal Jákli. NSF DMR and OISE

Transcription

1 Liquid crystals for sensors and organic light-emitting diodes Antal Jákli Department of Physics and Chemical Physics Interdisciplinary Program Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, USA NSF DMR and OISE

2 I. Optical sensing (based on LC reorientation) "#$%&$'('&)$*+&,( "+-(,%."*+/(01!"#$%&''()) Science 302, 2094 (2003),%."*+/(01 '&)+2(3&.%&*-&4+/('$5'*-"*%( P. POpov E. K. Mann Popov, Mann,Jákli, Phys. Rev. Appl. 1, (2014) "#$%&$'('&)$*+&,(!

3 ! Our Experimental setup "+,,&-./+)!"# $"%&'%()&* $"# *'+,#)"-,."#/(0#(1,2%334!"#!$%$&'#("#)"#*+,,'#+,-#."#/0123'#!456"#78&"#.%%2"# 9'#:;<::;#=>:9<? 6

4 1.A. 5CB response to SDS (c<cmc)!n eff (!) =!n(!) S 8(9 8/")+,:( ;"/*&- Novelty: use of spectrophotometer and circular polarizers I = I 0!sin 2 (!! "n! d) " 7

5 I.B. Sensing phospholipids (c>cmc) 430!m 6% 7"8 9% 7"8 :% 7"8 56% 7"8 <C8>,>D Birefringence is laterally non-uniform Noticed before in mixtures of surfactants and assigned to phase separation 1-3 We Propose: Opening vesicle cause defects between homeotropic and hybrid alignments. To decrease the energy, new vesicles have to open at the boundary of existing lipid spots 1 McUmber et al, Soft Matter 2012, 8(16), Kinsinger et al, Soft Matter 2010, 6, 4095 < 3 Tan et al, Langmuir 2012, 28,

6 !"#$%&'($)*$+*,(&("*-.#(/,.*$)'$*01*#2%+3/. &"/ 20!m?"EF">%3/G0)C=%B"=7 ~1!m initial spot size &EF,(F0%0(=F)"(8%(B%="1">% -,0"#=,0 =

7 Two types of hybrid domains Splay-bend twist Splay-bend Popov, Mann, Jákli, J. Mater. Chem. B, 2017,5,

8 8CB at 37 C (N phase): water below and air above I.C. Smectic LC sensing 8CB at 25 C (smectic phase): water below and air above H66% I7 Toric focal conic domains >

9 Focal conic domain coverage as a function of concentration S = Black area Whole area 100% S [%] Measurements performed after 10 minutes C [μm] Popov, Mann, Jákli, J. Mater. Chem. B, 2017,5,

10 Smectics: Expand the sensitivity range Smectic: Focal conics coverage Nematic: Planar to homeotropi/ Smectic: Rings of focal conics Concentration (DLPC) 100 nm 1 µm 100 µm Drawback: it works only after cooling from nematic phase Popov, Mann, Jákli, J. Mater. Chem. B, 2017,5,

11 1.D. Sensing chiral lipids by N* H66%I7

12 430 μm N * response to DPPC of different chirality left handed racemic right handed p=13.6±2.4 μm p=12.2±2.0 μm p=10.8±2.1 μm L-DPPC (50 μm) D-DPPC (25 μm) L-DPPC (25 μm) D-DPPC (50 μm) Textures in 12 hours after addition of phospholipid. Popov, Mann, Jákli, J. Mater. Chem. B, 2017,5,

13 I.D. Sensing using liquid crystal fibers Motivated by J. Lagerwall s group work on non-electric Gas sensing Reyes et al, Liq. Cryst. 43 (2016) We make LC core polymer sheath fibers by phase separation J. Wang J. West Wang, Jakli, West, J. Mater Liquids,

14 Measurements between crossed polarizers (I cp ) and no polarizers (I np )

15 Response times are less than 5s

16 II. Sensors Based on shape transformations II.A. Bending of LC films on addition to lipids DLPC concentration >20µM Smectic Nematic Pressure difference between air and water side Popov, Mann, Jákli, J. Mater. Chem. B, 2017,5, 5061

17 Measurements by optical profilometer (ZYGO) N phase, 40µM SmA phase, 40µM J. Cumberland Á. Buka V. Kenderesi

18 Profiles can be equally well fitted by Catenary or parabolic functions air/glycerol 0µM 0% air/air Vertical position (µm) 17µM 33µM 66µM R~500!m 83µM!p=" lc/air /R+" lc/lipid /R ~50Pa Horizontal position (µm)

19 II.B. Spontaneously forming Microlens arrays When chiral nematic immersed in water Similar to: Compound eye of bugs. Dragon fly compound eyes ommatidia commons.wikimedia.org m.harunyahya.com 56,000 ommatidia! ~50 μm

20 Materials 5CB: I 35.5ºC N 21ºC Cr!"#!!!!!! H.T.P. (CD1)=6!m -1 H.T.P. (CD2)=8!m -1 Alignment conditions Experimental conditions P. POpov E. Mann

21 Pure 5CB Polarized Optical Microscopy images 5CB+5% CD1 5CB+5% CD1 Water/water air/air air/water 5CB+5% CD1 5CB+5% CD1 5CB+5% CD1 Water/water Water/water +green filter Water/water No polarizers

22 Rings can be seen even without polarizers è interference (Newton rings)!!! =!! + h!" 2!! + 1/2 R a Fit: R=5mm

23 Origin of lens shape h b! DK,! MN L 4 D!,K01 2a! "! #$ %! & %! ' 1.! "# $! %&'() *! "#$%" &'! ()*+ ~10-16 J! "#$%& ' ( )) * +, -. /012( )) %67! "#$% & ' (( ) *+,+* -./ ' (( ) / !"# $ #%&'#()*+),! "#$%" & ' (( ) )* +, )* + - (./ ( " # $"!!!!!!!!!"!!!!!!!!!"!!!!!"!!!!"

24 2. Wall energy, W w D W>0! b= W w D-h! " # $ % && ' ( ') * + & (,- (./ $ 01 ( 0 For a=0.1mm and D=20 µm h~1µm!!!!!!!"! h b 3. Interfacial energy, W i! DK,! MN L 4 D!,K01 2a W i1 : replace LC-wall by water-wall! LC/N "! N /w = #!!" # $"%&'( # "')*!+!"#$% & '() & *+(' W i2 : increase of LC-water area! "# $ %&'( # W i = 2@h 2 (%+6)~10-13 J!! "# $ %&'( )

25 Equilibrium condition!(# $ + # & )/!h =0 h 3$ %% 2'/h % )* 4, ' $ %% 2'/h % ) 3'% $ %% )*,0 % ' + 6 For a=0.1 mm it gives h 1µ% R=a 2 /2h è R =5 mm agreement with experiments

26 Image formation

27 Results of imaging experiments Object distance: Image distance: l o =4.9 mm l i =1.9 mm Magnification: M = l o / l i = 1/ 2.6 Focal length (f) from geometric optics:! =!!!!!! +!! 1.4!!

28 Focal length (f) from lens shape 2a Lensmaker s equation 1 " $ % $ ' 2 $ ' )! " = Refractive index of water! " ~1.7 - Refractive index of LC è! = 5 %% 1.33 ~9 %% This is 5 times larger than we got from imaging experiment! è Focal length is not determined only by the shape of the lens

29 What is happening? varying film thickness + degenerate planar anchoring of water! azimuthal angle of the director varies radially by! / 2 = 2"#D / p Hasman et al. Appl. Phys. Lett., 82 (3),328( 2003) Similar to Pancharatnam phase lens or diffractive wave plate

30 Corneal lens structure of firefly compound eye (Wolken, 1971) Y. Bouligand, Tissue and Cell, 18, 621 (1986) Spiralling director structure (no defect at center)

31 Optics of Pancharatnam-type phase lens (half wave plate) f P πa 2 / (2λβ) [K. Gao et al. Appl. Opt. 55, 1145 (2016)] β = 4πΔD / p and! " $ %& 8( ΔD = a 2 / R For R ~ 5mm and p ~ 3µm we get f p~ 3 mm è Actual focal length: combination of the geometric and Pancharatnam lenses! =! #! % /! # +! % ~2.2++

32 Significance f depends on K 2, p and γ è can be tuned by T and chiral dopant è can be used for sensors Demonstration of practical use as sensors and lensing using chiral nanoparticles is in progress Popov et al, Scientific RepoRts 7: 1603 (2017)

33 III. Using Liquid crystal semiconductors in Organic Light emitting diodes 1. Cathode (!), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+) Alq 3 poly(p-phenylene vinylene) Ir(mppy) 3, a phosphorescent dopant

34 OLED vs LCD Advantages Disadvantages LCD, too not sure OLED: 10!s, LCD:1 ms actually more expensive! in principle LCDs as well Instead of comparing them let us see, if liquid crystals can help

35 birefringence in functional layers of OLEDs has advantages [1] better charge balance and wider recombination zones improve the efficiency controlling order is key for organic electronics [1] existing techniques in solution processed organic electronics: off center spin coating; shearing; chemical tailoring[2] solution processing is hardly compatible with multiple layers [4] vacuum deposition is preferred for OLEDs (up to 10 layers), however this is not compatible with alignment liquid crystal semiconductors can be a solution can be vacuum deposited and alignment influenced by thermal annealing [5] light emitting liquid crystals have been tested in solution processed OLEDs [6,7] low efficiency and only emissive LCs were studied References: [1] Becerril, H. A., etal, Adv. Mater. 20, 2588, (2008). [2] Yuan, Y. et al. Nat. Commun. 5, 3005, (2014) [3] Diao, Y. et al. Nat. Mater. 12, 665, (2013). [4]Reineke, S. et al. Rev. Mod. Phys. 85, 1245, (2013). [5] Schmidt-Mende, L. et al. Science 293, 1119, (2001) [6] Ebata, H. et al. J. Am. Chem. Soc. 129, 15732, (2007) [7] He, D. et al. Nat. Commun. 5, 5162, (2014).

36 What we did! Tuned orientation in a vacuum processed LC semiconductor film, C8-BTBT, by a novel blading process combined with different annealing protocols.! It is shown that a change in alignment of the liquidcrystalline semiconductor profoundly influences charge transport in hole and electron only devices.! Demonstrated p-i-n type OLEDs with C8-BTBT doped with a phosphorescent emitter as an emission layer to explore how the liquid-crystalline semiconductor plays a role as host matrix. C8-BTBT: B. Lussem R.Twieg Keum et al, Scientific Reports, 8:699 (2018) C-M Keum

37 Alignment control Cross-polarized microscopic images of C8-BTBT films (60 nm) without treatment (left) and treated by heating (center) or blading (right) at 130 C. The lower images are taken after rotating the samples by 45 with respect to the original position. The yellow arrows indicate an exemplary molecular domain that exhibits a strong contrast. (b) Normalized PL spectra of as-prepared, annealed and thermally assisted bladed C8-BTBT films (40 nm).

38 Birefringence measurements Blading leads to alignment (a) Schematic of the spectroscopic ellipsometry setup. d!. (b) Refractive indices of C8-BTBT films without and with heating at 90 C. (c) Refractive indices of C8-BTBT films treated by blading at RT and 90 C.

39 p-i-n type OLED with LC matrix in emission layer is demonstrated Energy level diagram of C8-BTBT OLEDs. In the emission layer, the orange box line depicts the energy level of Ir(MDQ) 2 (acac) and the black dotted lines represent the exciton states corresponding to the light emission. (b) J-V characteristics of OLEDs comprised of undoped C8- BTBT and C8-BTBT doped with 1, 5, and 10 wt% Ir(MDQ) 2 (acac) as the emission layer (c) EL spectrum of a dopant-free C8-BTBT OLED device

40 Light emission characteristics is improved by blading process PL spectrum on neat a C8-BTBT film doped with 10 wt% Ir(MDQ) 2 and normalized EL spectrum of a C8-BTBT OLED. (b) Current density and luminance vs. anode voltage characteristics of OLEDs fabricated with different treatment protocols. Comparison of EL spectra measured with a linear polarizer parallel with (solid lines) and perpendicular to (dashed lines) the blading direction