Dielectric and Electrooptic Investigations of Liquid Crystals ......Dielectric and Electrooptic...

Dielectric and Electrooptic Investigations of Liquid Crystals and Liquid Crystalline

Nanocolloids between Subhertz and 100 Gigahertz Region

Wolfgang Haase

Eduard-Zintl-Institut für Anorganische und Physikalische Chemie,

Technische Universität Darmstadt

[email protected]

Outline of the Tutorial

• 1.) Introduction

• 2.) Basic dielectrics

• 3.) Some about electrooptical methods

• 4.) LC-Nanocolloids and their characterization

• 5.) Few selected examples

-Why such Topic? It’s timely. -Why such broad frequency Range? We will receive a broad bundle of information. -Why dielectric and electrooptic Investigations? Both fields bring in their own input. -Limitations/Restrictions Examples presented are for nematics and smectics/chiral smectics only. No agglomerations of nanoparticles are assumed → We deal with ‘Nanocolloids’. -Problems for LC-Nanocolloid Research For reproducible data one must use adequate experimental conditions. -About this tutorial Not all envisaged topics can be discussed in detail → Use these notices.

Motivation

What we can see?

„It‘s not solid, it‘s not liquid?

It‘s something in between.“

How Electric Field interact with given Medium ?

E D → E = D /ε D Dielectric Displacement [ C/m2] →related to charge densities ε Electric Permittivity [ F/m] E Electric Field Strength [ V/m] →related to forces Dielectric Displacement is informing on how the applied field organizes the electrical charges, charge migration, dipole reorganization. Permittivity is related to materials ability to transmit an electric field; ε high or low: less or more electric field generated. ε0 Vacuum permittivity = 8,85 x 10

-12 [ F /m] ε0 = 1/c0 µ0 c0 Speed of light µ0 Vacuum permeability εr Relative permittivity εr = ε / ε0 = 1 + χ χ Electric susceptibility; χ and εr dimensionless

What is the induced dielectric Polarization density P ? P = ε E – εo E = D – εo E = εo χ E

χ is the constant of proportionality between P and E

empty cell under E field filled cell without E field

filled cell under E field

The electric Displacement D

D = P + εo E = εo εr E: εr is the const. of proport. between D and E

Again: What is Polarization P ?

P ind = P α + P µ = εo χ E

electronic Polarization P α = N α Ei orientational Polarization

N particle density, α electronic Polarizability, Ei averaged internal field, µ dipole Moment, Eo orientational field

Time dependence of Polarization P P ( t ) = εo χ ( t ) E ( t )

Polarization is a time dependent convolution of the electric field. And Displacement D ?

D ( t ) = εo εr ( t ) E ( t ) It follows

If one consider χ and εr as time independent one can describe the orientational convolution or deconvolution following kinetics first order (electronic part is very fast and therefore not considered here)

Step function Response function Decay function

Relaxation time τ

d P ( t ) / d t = [ Po – P ( t ) ] / τ

Po initial polarization;

steady state condition assumed;

τ has dimension of time→ Relaxation time

Temperature dependence usually via Arrhenius-Equation:

τ = A exp (Ea /RT)

Ea Activation Energy, A constant

Glass forming liquid crystals and LC polymers

In this case the Arrhenius equation is not valid Vogel-Fulcher-Tammann-equation

τ = τ0 exp ( - B/ T-T0 )

τ0 Relaxation time in high temperature limit; B Activation parameter; T0 Ideal glass temperature Williams-Landel-Ferry equation (theoretically founded in Adams-Gibbs-DiMarzio theory): Exceptionally valid above Tg +10K

ln [tm( T ) / τ ( Tg ) ]= - C1 ( T – Tg )/ C2 + ( T-Tg )

τ(Tg ) Relaxation time at glass temperature Tg; tm(T) Relaxation time at given temperature; C1, C2 material parameter

Something to remind you

Static dielectric permittivity εs

Lars Onsager modified the well known Clausius-Mossotti equation by introducing

- cavity field factor h - reaction field factor F

Wilhelm Maier and Gerhard Meier (Mayer-Meier equation) calculated

the two principal dielectric permittivity components εII and ε for anisotropic liquids

∆ εs = ε||,s - ε ,s the (static) dielectric anisotropy For rod like molecules: - Strong longitudinal dipoles: dielectric positive - Strong transversal dipoles : dielectric negative

Frequency dependence of Polarization

P ( ω ) = εo χ ( ω ) E ( ω ) = ε ( ω ) E ( ω ) – εo E ( ω ) This lead to the frequency dependence of the permittivity ε → ε ( ω ) : εo = εs = lim ε ( ω ) static permittivity-static dielectric ω → 0 constant ε∞ = lim ε ( ω ) high frequency limit → n

2 ω → ∞

Definition: ∆ ε = εs - ε ∞ dielectric strength

Pay attention: Some times some confusion exists (see before): ∆ ε = ε|| - ε dielectric anisotropy

Representation of the Debye equation

Dielectric Permittivity as complex function: Debye Equation

The response arises after application of the field: → Phase retardation.

→ Specification of magnitude and phase is possible.

By applying alternating (oscillating ) field:

→ Response is characterized by complex permittivity

ε* ( ω ) = ε‘ ( ω ) + i ε‘‘ ( ω )

real imaginary

stored energy energy dissipation

ω = 2 π f ω - circular frequency f - frequency

Relaxation time distribution

Cole-Cole equation and Cole-Cole plot (by use of equivalent magnetic quantities it’s Argand-plot)

100

101

102

103

104

105

106

107

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0

0.1

0.2

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0.4

'

Frequency, Hz10

010

110

210

310

410

510

610

7

0

1

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3

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5

''

Frequency, Hz

0

0.1

0.2

0.3

0.4

‘ = 1/2p(1-a)

Cole-Davidson Havriliak-Negami

Fuoss-Kirkwood

, b, and n are distribution parameters, in case of =0 , b=1 and n =1 all distributions become Debye like

Example of Havriliak-Negami function

More as Cole-Cole ….

Kohlrausch-Williams-Watts: multi-exponential

Nonlinear dielectric Spectroscopy for LCs

High electric field leads to nonlinearity between Polarization and Electric Field.

Linear P = 0 c E and D = 0 r E Nonlinear P = 0 c E + b E

2 E and D = 0 r E + b E2 E

See e.g. J. Jadzyn, P. Kedziora and L. Hellemans, ‘Relaxation Phenomena’, Edits. W. Haase, S. Wrobel, Springer Publish. 2003, pp. 51-71

Apart from saturation effects of the orientation, nonlinearity can stem e.g. from - Change of the intermolecular interaction due to forming of aggregations - Conformational processes in forming new dipoles (chemical effect) First aspect might be relevant for LC-nanocomposites in case of aggregations .

Complex quantities

ε* ( ω ) = ε‘ ( ω ) + i ε‘‘ ( ω ) dielectric permittivity

Z * ( ω ) = Z‘ (ω ) - i Z‘‘ ( ω ) impedance

σ* ( ω ) = σ‘ ( ω ) + i σ‘‘ ( ω ) conductivity

M* ( ω) = M‘ ( ω ) - i M‘‘ ( ω ) electric modulus

Relations:

1/ ε* ( ω ) = M* ( ω ) = I ω C0 Z* ( ω ) = i ω ε0 / σ* ( ω )

Those relations are important for performing the experiments

Experiments in the range 10-5 Hz – 100 GHz

Electrooptical frequency range

Source: C.N. Banwell, Molecular Spectroscopy, Mac Graw Hill, p.8

Comparison Dielectric-Electrooptic

based on dielectric techniques based on electrooptical techniques

Frequency and Time domain spectroscopy

In the frequency domain the impedance analyzer measure the capacity C ( ω ) and the loss tangent. From this one obtain permittivity

tan δ = C ( ω ) – C s / C 0 ; tan δ = ε|| ( ω ) / ε

( ω ) = 1 / ω R C ( ω ) Cs static capacity, C0 Capacity in vacuum. Because of some technical problems above about 10 MHz (e.g. stray capacitance) the reflections or transmission coefficient will be extracted. Around 1 MHz the ITO mode is dominant→ use Gold cells above 0,1 MHz. Up to 500 MHz → wave guide. Our apparatus: Solartron FRA 1250 + self made Chelsea Interface, HP 4192 A, HP 4191 A In the time domain a pulse propagates in a coaxial line reflected from the sample at the end of the line. Via Fourier transform one can obtain the complex permittivity.

10 kHz - 1 GHz range for LCs

1 mHz – 100 Hz : Ions, interfacial processes to be done 10 Hz – 1 MHz : Collective modes of FLCs or AFLCs to be done 10 kHz – 1 GHz : Molecular modes will be done now 1 GHz – 100 GHz : Microwave range- microwave applications to be done 300 GHz – 1 THz : Complex vibrations of LC molecules not part of tutorial 1 THz – near IR : Vibrational processes corresponding to not part of tutorial functional groups and bonds

5

Molecular processes

For rod shaped liquid crystals at least two rotational processes are observable: -diffusive end-over-end reorientation around the short axis: ε || for nematics at about 1 -10 MHz. -diffusive reorientation around the long axis: ε for nematics at about 0,1 - 1 GHz. (Sometimes interpreted as precessional motion of the long axis around the director.) Four microscopic rotational motions see Nordio et al., Mol Phys. 25, 129 (1973), and Coffey et al., Adv. Chem. Phys. 113, 487 (2000).

Relaxation Processes of LCs/FLCs

Temperature dependence of the molecular (and collective) Relaxation processes in Liquid Crystals

Can be modeled using the Arrhenius equation. Ea activation energy, kb Boltzmann Constant, A frequency factor

Goldstone Mode

Domain Mode

Soft Mode

Relaxation around short axis

Relaxation around long axis

Freq

uen

cy.

Hz

Temperature

TC*/A*

One of our FLC example:

Temperature and pressure dependence of the molecular Relaxation Processes in Liquid Crystals

Data for the isotropic and nematic phases of 5CB* - By increasing the temperature, relaxation process is faster - By increasing the pressure, relaxation process is slower - There is a jump at phase transition isotropic-nematic

-From temperature-pressure dependent measurement one can obtain activation parameters as Volume D#V, Enthalpy D#H or Energy D#U. *S. Urban and A. Würflinger in ‚Relaxation Phenomena‘ , p. 189

10 Hz - 1 MHz range for LCs

1 mHz – 100 Hz : Ions, interfacial processes to be done 10 Hz – 1 MHz : Collective modes of FLCs or AFLCs will be done now 10 kHz – 1 GHz : Molecular modes done 1 GHz – 100 GHz : Microwave range- microwave applications to be done 300 GHz – 1 THz : Complex vibrations of LC molecules not part of tutorial 1 THz – near IR : Vibrational processes corresponding to not part of tutorial functional groups and bonds

5

Collective processes in Ferroelectric Liquid Crystals

Goldstone mode: Azimuthal angle φ – fluctuations: Phason mode Soft mode: Tilt angle θ – fluctuations: Amplitudon mode

But some other modes exists in FLCs, e.g. -Thickness mode in a twisted structure and pinned Goldstone mode in a helicoidal structure . See M. Glogarova and I. Rychetsky in ‚Relaxation Phenomena‘, pp.309-332 - Surface and Bulk Dislocation Domain mode , see for instance S. Pikin in ‚Relaxation Phenomena‘, pp. 274-309

Collective processes in antiferroelectric Phases

Phase sequence by cooling: SmA* -> SmC* -> SmC* -> SmC*b -> SmC*g -> SmC*A (-> SmI*A -> SmF*A): * The dielectric response is of different intensity: - SmC* very strong - SmC*α weak - SmC*b weak - SmC*g weak - SmC*A very weak - SmI*A very weak - SmF*A very weak *One example for this reach phase sequence with the abbreviation 9HBi is a MHBOBC homologous, synthesized in the R. Dabrowski group at Warsaw and dielectrically studied in the S. Wrobel group at Cracow

AFLC-Relaxation processes

SmA* -> SmC* -> SmC* -> SmC*b -> SmC*g -> SmC*A (-> SmI*A -> SmF*A):

M. Marzec et al., Dielectric Properties of Liquid Crystals, Edits. Z. Galewski, L.Sobczyk, Transworld Research Network, 2007, p. 89

Cole-Cole plots of the AFLCs

SmA* -> SmC* -> SmC* -> SmC*b -> SmC*g -> SmC*A (-> SmI*A -> SmF*A):

M. Marzec et al., Dielectric Properties of Liquid Crystals, Edits. Z. Galewski, L. Sobczyk, Transworld Research Network, 2007, p. 91

Bent-shaped molecules Several antiferroelectric B phases exists

A symmetric molecule is

- Molecular processes appearing at about 100 kHz, maybe some more in the MHz region. -Collective processes appearing between 50 Hz and 100 Hz.

A non symmetric molecule is

H.Kresse, in ‚Relaxation Phenomena‘ pp. 400-422

S. Wrobel et al. Ferroelectrics 243 , 277 (2000)

1 mHz - 100 Hz range for LCs

1 mHz – 100 Hz : Ions, interfacial processes will be done now 10 Hz – 1 MHz : Collective modes of FLCs or AFLCs done 10 kHz – 1 GHz : Molecular modes done 1 GHz – 100 GHz : Microwave range- microwave applications to be done 300 GHz – 1 THz : Complex vibrations of LC molecules not part of tutorial 1 THz – near IR : Vibrational processes corresponding to not part of tutorial functional groups and bonds

5

1 mHz-100Hz range: Ions, Interfacial processes Ions/Charges are present in LCs to some extent or created while applying the E-field. -Slow motion of Ions due to external field → low frequency processes. τi = mi /ξ relaxation time for ions of mass mi with ξ the friction coefficient. Effect contribute only to εII. -Space charge Relaxation due to locally aggregated ions . Under external E-field →double electric layers. τMW = ε /σ Maxwell-Wagner (Sillars) relaxtion time. σ conductivity. Effect contribute to ε and εII. In detail: Karl Willy Wagner (1914) described based on the James Clerc Maxwell (1883) theory the inner dielectric boundary layer and the layers between the electrode-sample interface. For inhomogeneous systems like Nanocolloids the Maxwell-Wagner effect is not an artifact → information about LC-Nanoparticle interaction can be obtained. If the range to lower frequencies is limited, you can detect the loss tangent tanδ = εII / ε because of the shift to higher frequencies.

1 GHz - 100 GHz range for LCs

1 mHz – 100 Hz : Ions, interfacial processes done 10 Hz – 1 MHz : Collective modes of FLCs or AFLCs done 10 kHz – 1 GHz : Molecular modes done 1 GHz – 100 GHz : Microwave range- microwave applications will be done now 300 GHz – 1 THz : Complex vibrations of LC molecules not part of tutorial 1 THz – near IR : Vibrational processes corresponding to not part of tutorial functional groups and bonds

5

Liquid Crystals and microwaves

Dielectric permittivity of LC affects speed of microwave.

By changing dielectric permittivity of LC, phase shift of microwave occurs.

The phase shift depends on the dielectric anisotropy at GHz frequencies.

The power consumption depends on the losses.

Possibility to create phase shifters, varactors etc. on the base of liquid

crystals for applications in microwave region

||

||

'

''

t

maxtan

t Tunability Figure of Merit

Cavity perturbation method

Based on the shift of resonance frequency because of the different values of . It assumes that a resonant cavity with a complex angular resonance frequency ωc is perturbed by a small material sample which causes the resonance frequency to shift by Δω to ωs.

Structure of Microstrip-line (MSL) Type Phase Shifter

The phase shifter efficiency is defined as the differential phase shift in degree divided by the maximum insertion loss (from measured S21 parameters and the losses)

Structure of Coplanar Waveguide (CPW) Type Phase Shifter

(a) Schematic construction

(b) Cross section

Taken from H. Moritake, presentation, Darmstadt 2010

Electrooptical studies for detecting relaxation processes in FLCs

-Coupling of the electric field with the spontaneous polarization. -Separation between collective modes like Goldstone and Soft mode and molecular reorientation processes is easy. -Measurements at lower frequencies and even at static field are possible. -Electrooptical methods are very sensitive: Three orders higher as dielectric methods. -Observation in arbitrary direction is possible whereas by dielectric investigation the planar arrangement is preferable. -One can easily obtain higher order harmonics what is useful for some antiferroelectrics. -Dielectric and electrooptic studies are for FLCs complementary. See: W. Kuczynski, ‘Electrooptical studies of relaxation processes in ferroelectric liquid crystals’ in ‘Relaxation Phenomena, pp. 422-444

Comparison Electroptical and Dielectric methods:

Comparison Dielectric-Electrooptic

based on dielectric techniques based on electrooptical techniques

Setup used in our lab

Determination of Switching times

According to the definition, the response time is the time difference between 10% and 90 % of the transmitted intensity

Spontaneous polarization

The measurement of the spontaneous polarization is based on the investigation of the repolarization current flowing through the cell upon application of the low frequency triangular voltage

A is the peak area, R is resistivity and S is the area of electrodes

Tilt angle

- One of the extreme position of the cone (say θ) coincides with the transmission axis of the analyzer - The cell is rotated into the opposite position (-θ) coinciding with a polarizer; the electrooptical response changes the polarity - On the end the cell has been turned at angle 2θ





• 4.) LC-Nanocolloids and their characterization*


* Slides 52-61 have been graphically designed by Dr. F. Podgornov

Particles pseudo-stabilized

Inorganic balls falling down

but stabilized due to some red berrys

Nanoparticles

Metallic nanoparticles

Semiconducting nanoparticles

Dielectric nanoparticles

Properties of Nanoparticles

• Surface plasmons • Delocalized electrons • Electrostatic blockade • Modification of host dielectric properties • Interfacial effects • Enhancement of local E field

Carbon nanotubes

• π-electrons • Phenyl-phenyl interaction • Orientational ordering of anisotropic host molecules

• Dipole-dipole interaction • Ordering of anisotropic host molecules

Ferroelectric nanoparticles

Nanoparticles in liquid crystals

Visco-elastic

properties

Dielectric properties

Interface triggered

effects

Liquid crystals Nanoparticles

Maxwell-Wagner Effect Maxwell (1873), Wagner (1914) and Fricke (1953)

Electric current passes through interfaces between two materials surface charges pile up at the interfaces, due to their different conductivities and dielectric permittivity

Boundary conditions

This interface single layer surface charge must not be confused with the double layer charge formed at a wet interface.

Necessary condition for charge accumulation

Relaxation time

Equivalent electrical circuit

Wet host

Small particles at

high volume fraction

Surface effects from counter-ions and double layers

dominate over Maxwell -Wagner

effect

Adsorbed Counter-Ions and Lateral Diffusion Effect

Electrical Double Layers Conception

• Surface charge is continuous and uniform

• Ions in the solution are point charges • Exchange of counterions between

the double layer and the bulk solution

• Finite size of the counterions • The diffuse layer is divided into an

inner layer (the Stern layer) and an outer layer (the Gouy layer)

• Counterion atmosphere near charged surface

• Valid only for rather high concentration of electrolyte solutions

Electrical Double Layers (EDLs) in LCs Polar LC molecules

Electric field in Diffuse Layer can align LC’s molecules

Influence of EDL impedance on actual voltage drop on LC layer in a cell

Liquid crystal layer

In LC cells

• EDL layers behaves like a capacitor with a non-uniform charge density • When most of the applied voltage is dropped across EDL. The voltage in the LC layer may only be a fraction of that applied to the electrode

LC 0 BEDL 2 2

ions

ε ε k T= 10 30

8πn z el nm nm

Counterion Diffusion, Schwarz ’s Theory

Schwarz model

Ions are bound to the surface

Lateral motion of ions within the electric

double layer

In modified theories, the ions can enter and leave

electric double layers

Schwarz polarization in LC host

Due to lateral motion of counter ions, the polarization appears

•The re-establishment of the original counterion atmosphere after the E field is switched off will be diffusion controlled

Relaxation time - - Diffusion coefficient

Raleigh Model

• Cylindrical particles

Two phase colloidal dispersion

Maxwell– Garnett -Wagner Model

• Spherical particles

Fricke Model

• Ellipsoidal particles

- form factor

Equivalent circuit model (by S. Kobayashi*)

• Cubic nanoparticles • Uniform distribution of nanoparticles

Electrical circuit

Debye type relaxation

Relaxation time Dielectric strength

Dispersion of nanoparticles leads to the significant change of the dielectric strength and relaxation frequency

*S. Kobayashi, Y. Sakai, T. Miyama, N. Nishida, N. Toshima, J. Nanomat. 2012, 460658

Example 1: FLCs, FLC-Nanocolloids with Silver spheres and Ions*

• FLC-mixture LASH 9: Cr 5.9 °C SmC* 61.50 °C SmA* 69.5 °C Iso,

• Ps = 65 nC/cm2 (20°C)

• 0.1 wt % Silver Nanospheres, thiol-capped, 3-7 nm or 5-15 nm

Electrooptical parameters:

Reduction of Ps down to 53 nC/cm2 (20 °C) Remarkable reduction of switching time

Practically unchanged tilt angle

Dielectric parameters:

*P.K. Mandal, A. Lapanik, R. Wipf, B. Stühn, W. Haase, APL 100, 073112, (2012)

45 °C: SmC*, 68 °C: SmA*

0

5

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15

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25

30

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40

45

50

10e-3 10e-2 10e-1 10e0 10e1 10e2 10e3 10e4 10e5 10e6

e''

Frequency, Hz

a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)a)

e''

Fitting

Goldstone Mode

Unknown process

SHF

Conductivity

0

20

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200

10e-3 10e-2 10e-1 10e0 10e1 10e2 10e3 10e4 10e5 10e6

Frequency, Hz

b)

e''

Fitting

SHF

Conductivity

Fitting formula

64

Low frequency processes continue for nematic and isostropic phase in general, hence they are not typically for FLCs or smectics only!

a.) Non doped FLC, SmC* Phase b.) Non doped FLC, SmA* Phase

g – parameter which describes the influence of EDL

Example 1: FLCs, Silver-FLC-Nanospheres and Ions*

10-4

10-3

10-2

10-1

100

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0,0

2,0x1013

4,0x1013

6,0x1013

8,0x1013

1,0x1014

1,2x1014

1,4x1014

1,6x1014

1,8x1014

Frequency (Hz)

Re

sis

tivity (

cm

)

Pure

3-5nm

10-4

10-3

10-2

10-1

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''

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45°C

62°C

68°C

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Frequency, Hz

25°C

45°C

62°C

68°C

a)

Non doped FLC 3-5nm

Example 1: FLCs, Silver-FLC-Nanospheres and Ions*

Example 2: FLCs, Gold nanorods and spheres*

• FLC CHSI: SmC* -74 0 C-SmA*-84 0 C; helical pitch 3.5 µm at 30 0 C

• Gold Nanorods (Nanopartz): diameter 10 nm, axial length 45 nm

• Gold nanospheres (Nanopartz): diameter 20 nm

• Are the properties for the different geometries comparable or not? • Properties of Nanoparticles in the electric double layer near alignment layer?

• *In part: F.P. Podgornov, A.V. Ryzhkova, W. Haase, APL 91 1 (2010) • Slides 67-70 have been prepared by Dr. F.P. Podgornov

Fig. a: Non doped FLC Fig. b: FLC/Gold Nanorods Fig. c: FLC/Gold Nanospheres


20 40 60 800

20

40

60

80 Non-doped FLC, experiment

Goldstone mode, fitting

Ultraslow mode, fitting

T=55 oC

"

'

Goldstone mode

Shunting of EDL leads to the increase of voltage dropping in LC layer and decrease of response time

Influence of gold nanorods and nanospheres on switching time of FLCs

→

←


Non-doped LC curcuit

Example 3: Electrooptic switching of FLCs* V- and W-shaped response: Dynamic voltage divider model

CP

RP

CLC

RLC

ULC

Utot

-4 -2 0 2 40,00

0,05

0,10

0,15

0,20

USat

Inte

nsi

ty, ar

b. un.

Voltage

Equivalent curcuit

Coersive voltage

Saturation voltage

)cos(])(1[

02/1222

tU

CCR

CRU

LCpLC

pLC

LC

tan= RLC(Cp+CLC).

Applied voltage (black line), voltage dropped on the FLC layer (red line) and repolarization current (blue line)

W-shaped electrooptical response. Bistable switching

Growth of dielectric constant

Growth of coersive voltage

Increase of

inversion frequency

Example 3: Electrooptic switching of FLCs*

10 100 10000

1

2

3

4

5

6

7

Co

ers

ive

vo

lta

ge

, V

Frequency, Hz

Non-doped LAHS9

gold nanorods +LAHS9

-60 -50 -40 -30 -20 -10 0 100

20

40

60

80

100

120

LNSM6 pure

LNSM6 + BaTiO3 (26nm) 0.013%

LNSM6 + BaTiO3 (9nm) 0.013%

Ps

, n

Cc

m-2

T-Tc

SmC* Sm

A*

Example 4: FLCs, Milled non harvested and harvested BaTiO3

Non harvested particles Harvested particles

Size of BaTiO3 28 nm Size of BaTiO3 26nm and 9nm

What is the difference in parameters, for example for Ps ? (Used FLC is in our case different) Can one see differences in size?

A. Mikulko et al., EPL 87 27009 (2009) A. Rudzki et al., to be submitted

A dc potential of +20 000 V is applied to the inner wire electrode while the outer foil electrode is grounded. The harvested nanoparticles accumulate on the inner wire electrode after the field has been applied for 30–60 min Nanoparticles without dipole moments or induced charge from the applied field are either rejected and accumulate on the outer glass wall or remain in suspension within the fluid.

* G. Cook, J. L. Barnes, S. A. Basun, D. R. Evans, R. F. Ziolo, A. Ponce, V. Yu. Reshetnyak, A. Glushchenko, and P. P. Banerjee, J. Apl. Phys. 108, 064309 (2010)

Hoew goes harvesting? *

Example 4: FLCs, Milled non harvested and harvested BaTiO3

Example 5: Random lasing in Cholesteric LC/TiO2 nanodispersion *

Background: Helical superstructure leads to selective reflection of the incident electromagnetic irradiation (stop band zone). Material: Nematic MLC 2463 + 35 wt % dopant ZLI 811 (both Merck) → Stop band between 532-610 nm; Pumping beam wavelength 532 nm. TiO2 from Aldrich (100 nm) added to above chiral mixture at 0.1 wt%. Dye DCM added to the CLC-TiO2 -Nanocolloid at 1.5 wt %. Experimental

•W. Haase, F. Podgornov, Y. Matsuhisa, M. Ozaki, Phys.Stat.Solid. A 204 , 3768 (2007) •F.V. Podgornov, W. Haase, K. Yoshino, J.Soc.Elect.Mat Eng. 20,35 (2011)

CLC/DCM

CLC/TIO2/DCM

Wavelength of pump light – 532 nm Pulse duration – 6 ns, Repetition rate -1 kHz Pulse energy - 5 μJ/pulse

Nanoparticles trigger the random lasing in cholesteric liquid liquid crystals

Example 5: Random lasing in Cholesteric LC/TiO2 nanodispersion *

Example 6: Experiments in the Microwave region

• Nematic Liquid Crystals are good candidates for Antennas, Varactors, Phase Shifters

• High Birefringence in the microwave range is needed

• Some wings of the reorientational processes around the long axis might contribute to the losses if molecules with strong lateral dipols are under investigation (maximum at 0,1-1 GHz, see slide 23).

• Spherical particles don‘t influence the losses and only slightly the anisotropy.

• Anisotropic particles like nanorods strongly increase the losses and therefore reduces the tunability and the quality factor.

• This are first experimental results in cooperation with R. Jakoby et. al., TU Darmstadt; more experiments have been started already.

Tunability τ and Loss Factor tanδ related to the Figure of Merit η for nematic mixtures (LHB) compared to BST

10-3

10-2

10-1

1

10

100

LHB

BST

thin-film

BST

thick-film

=10 =20

t

tanmax

=40

*F. Goelden, A. Lapanik, A. Gaebler, S. Mueller, W. Haase, R. Jakoby, Frequenz 62, 57-61 (2008)

19

LHBM4 mixture n=3, m=2 – 25% n=3, m=4 – 33% n=4, m=4 – 42%

Example 6: Experiments in the Microwave region

phenylpyrimidines

I‘m happy to answer your questions, see my e-mail

Thank you !

Dielectric and Electrooptic Investigations of Liquid Crystals ......Dielectric and Electrooptic...

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