液晶摻雜硒化鎘奈米柱與量子點製作可調控發光元件

國立臺灣大學理學院物理學系 碩士論文

Department of Physics College of Science

National Taiwan University Master Thesis

液晶摻雜硒化鎘奈米柱與量子點製作可調控發光元件 Color-Tunable Light Emitting Device based on the mixture

of CdSe nanorods and dots embedded in Liquid Crystal Cells

陳曉聖

Hsiao-Sheng Chen

指導教授：陳永芳 博士 Advisor: Yang-Fang Chen, Ph.D.

中華民國 98 年 7 月

July, 2009

I

Abstract

A newly designed color-tunable light emitting device based on the mixture of

CdSe nanorods and quantum dots embedded in liquid crystal cells has been developed.

The underlying working principle is derived from a large alignment energy due to the

enhanced anchoring force of liquid crystal molecules through the amplified surface

area of nanorods. After embedded into a liquid crystal cell, nanorods will align along

the orientation of liquid crystal molecules. As the external bias is applied, nanorods

would be driven in the direction according to the orientation of liquid crystal

molecules. The nanorods possess optically anisotropic property, whereas quantum

dots are spherically symmetric. Accordingly, the relative ratio of the emission

intensity between quantum dots and nanorods can be manipulated by an external bias,

and the emission color of the device is therefore tunable. Our work shown here may

pave a new route for the future development of smart optoelectronic devices.

II

摘要

不同於球形的量子點或其他形狀的奈米結構的發光，一維長棒狀

的 奈 米 柱 由 於 其 幾 何 上 的 結 構 ， 使 得 其 發 光 具 有 異 相 性

(anisotropy)。然而，卻缺乏將大量的奈米柱排列至同一方向，以及

操控奈米柱方向的方法，因此，我們研究出利用液晶來排列硒化鎘

(CdSe)奈米柱，並且利用半導體的幾何形狀和液晶分子的特性製作出

可調控發光的液晶元件，從我們的研究成果可以發現有趣的現象，對

於奈米柱的應用將更為廣泛。這些研究成果主要包括了:

1.利用液晶操控硒化鎘(CdSe)奈米柱的排列:

由於液晶分子的方向可以順著配向膜配向的方向，因而將單軸分

子作一致性的排列，我們製作液晶 cell 並將硒化鎘(CdSe)的量子點

和奈米柱摻雜於液晶 cell 內。利用光激螢光光譜來比較硒化鎘(CdSe)

量子點和奈米柱的發光偏振行為，我們比較其平行與垂直於配向膜配

向方向之發光強度，我們發現零維量子點其平行和垂直的發光強度沒

有太大的變化，然而，擁有一維結構的奈米柱具有相當強的偏振性

(Polarized PL)，除此之外，我們由理論公式去量化奈米柱在液晶

III

cell 裡的排列程度，實驗結果顯示，硒化鎘(CdSe)奈米柱的偏振度

(Polarization ratio)為 0.53，成功的證明利用液晶的分子間作用

力可以將硒化鎘(CdSe)奈米柱作同方向的排列。

2.藉由外加電壓操控硒化鎘(CdSe)奈米柱分子的方向:

液晶(E7)是一種極性分子，當有外加電壓超過臨界電壓時，液晶

分子的長軸會順著外加電場方向，藉由液晶分子的隨外加電壓的特

性，我們可以調控硒化鎘(CdSe)奈米柱分子的方向。同樣地，我們將

硒化鎘(CdSe)量子點和奈米柱摻雜在液晶 cell 裡面，觀察光激螢光

光譜在平行和垂直配向膜配向方向時的發光強度，因為量子點的球形

對稱，當無外加電壓時和加電壓之間的光譜強度維持不變；相反地，

奈米柱因其量子局限效應(Quantum confinement)，光譜強度會隨著

電場大小作改變。基於以上的特性，我們設計了一個可調控發光顏色

的液晶元件，相信這樣的成果未來在液晶顯示器與發光二極體將會有

很大的應用。

關鍵詞 : 硒化鎘，量子點，奈米柱，偏振性，液晶元件。

IV

Contents

Chapter 1. Introduction ... 1

1.1 Introduction ... 1

References4 Chapter 2. Theoretical Background...5

2.1.1 Liquid Crystals... 5

2.1.1.1 Calamitic Liquid Crystals... 5

2.1.2 Basic physical properties of Liquid crystals ... 6

2.1.2.1 Orientational order parameter... 6

2.1.2.2 Dielectric anisotropy... 7

2.1.2.3 Refractive Index... 8

2.1.2.4 Elastic constants... 9

2.1.2.5 Viscosity ... 10

2.1.3 Deformation of nematic liquid crystals by an electric field... 11

2. 2.1 Semiconductor nanowires and nanorods ... 16

References ... 19

Chapter 3. Experiment ... 30

3.1 Micro-Photoluminescence ... 30

3.1.1Principles and Applications of Micro-Photoluminescence

V

... 30

3.1.2 The Apparatus for Micro-Photoluminescence Measurement ... 34

3.2 Transmission Electron Microscope (TEM) ... 34

References ... 36

Chapter 4. Color-tunable light emitting device based on the mixture of CdSe nanorods and dots embedded in liquid crystal cells... 40

4.1Introduction... 40

4.2 Experiment ... 41

4.2.1 Sample preparation ... 41

4.2.2 Experimental setup ... 44

4.3 Results and Discussion ... 45

4.4 Summary... 48

References ... 50

Chapter 5 Conclusion ... 56

VI

List of Figures

Figure 2.1 The schematic showing of isotropic liquid phase ... 21

Figure 2.2 The schematic showing of nematic liquid crystal phase .... 21

Figure 2.3 The schematic showing of (a) smectic-A liquid crystal phase, (b) smectic-C liquid crystal phase ... 22

Figure 2.4 The schematic showing of cholesteric liquid crystal phase, where is the director ... 23

Figure 2.5 The schematic showing of smectic-C* liquid crystal phase ...

... 24

Figure 2.6 The schematic showing of (a) splay, (b) twist, and (c) bend deformations in liquid crystal ... 25

Figure 2.7 The schematic showing of three principal viscosity coefficients η

, η

and η

26

Figure 2.8 Nematic LC under an external bias (a) △ε> 0 (b) △ε< 0 . 26

Figure 2.9 The ionic charges induced by an external field in LC cell. 27

VII

Figure 2.10 The schematic showing of basic geometries of the dielectric reorientation of nematic liquid crystals (△ε>0). On the left hand side, the initial states distinguish by different starting orientations. On the right hand side, the deformed states above the threshold are illustrated... 28

Figure 2.11 Polarized excitation and emission spectra of nanowires. (A) Excitation spectra of a 15-nm-diameter InP nanowire. These spectra were recorded with the polarization of the exciting laser aligned parallel (solid line) and perpendicular (dashed line) to the wire axis. The polarization ratio, ρ, is 0.96. Inset, plot of the polarization ratio as a function of energy. (B) Emission spectra of the same wire as in (A). These spectra were taken with the excitation parallel to the wire, while a polarizer was placed in the detection optics. The polarization ratio of the parallel (solid line) to perpendicular (dashed line) emission is 0.92. The spectra were taken with integration times of 10 s. Inset, plot of the polarization ratio as a function of energy. Reprinted with permission from J. F.

Wang et al., Science 293. 1455(2001) ... 29

Figure 2.12 CL spectra from the nanowire region and substrate region taken for the p(solid spots) and s(open spots) polarization directions. The polarization ratio, ρ, is only 0.5. Reprinted with permission from Appl. Phys. Lett. 88, 153106 (2006) ...

... 29

Figure 3.1 Energy transition in (a) direct and in (b) indirect gap semiconductor between initial states and final states…….37

Figure 3.2 The schematic representation of low temperature photoluminescence (after R. A. Strading and P. C. Klipstein)

...

... 37

VIII

Figure 3.3 Illustration of different processes that can give rise to light emission in semiconductor.

(a) The band to band recombination.

(b)Excitonic recombination.

(c)Free hole-neutral donor recombination.

(d)Free electron recombines with a hole on a neutral acceptor.

(e)Donor-acceptor recombination ... 38

Figure 3.4 The schematic diagram of the experimental setup used in the optical measurement... 39

Figure 3.5 Schematic diagram of TEM ... 39

Figure 4.1 (a) and (b) show transmission electron microscopy (TEM) images of CdSe nanorods and quantum dots, respectively. (c) and (d) show the photoluminescence (PL) spectra of CdSe nanorods and quantum dots, where the maximum PL intensity is at 650 nm (1.9 eV) and 580 nm (2.1 eV) approximately ... 52

Figure 4.2 Schematic shows the structure of the fabricated sample with and without an external bias. The arrow is the rubbing direction. The external field will drive the direction of LC molecules perpendicular to the cell plane, and it will also drive nanorods along the same direction due to the interaction between LC molecules and nanorods... 53

Figure 4.3 The experimental setup for photoluminescence measurement ... 53

Figure 4.4 Dependence of photoluminescence spectra of CdSe nanorods

IX

and quantum dots on the angle of analyzer without an external bias. The inset is the variation of the emission intensity of CdSe nanorods versus analyzer angle ... 54

Figure 4.5 Absorption spectra of CdSe nanorods without an external bias in the case of analyzer parallel (red line) and perpendicular (black line) to the rubbed PI direction... 55

Figure 4.6 Photoluminescence spectra of CdSe nanorods and quantum

dots with an external bias of about 20 V. The inset is the

variation of the emission intensity of CdSe nanorods versus

analyzer angle ... 55

X

List of Tables

Table 4.1 The list of the sample fabrication processes………44

1

Chapter 1 Introduction

1.1 Introduction

One-dimensional nanostructures, such as nanorods, nanowires, and nanotubes,

etc., have become a class of attractive materials as their geometric anisotropy gives

rise to unique physical properties [1-8]. For example, the emission and absorption

spectra arising from one-dimensional semiconducting wires can be highly

anisotropic, and hence serve as an excellent candidate for the application in

polarized optoelectronic devices. On the other hand, liquid crystal (LC) is an

anisotropic fluid, which is thermodynamically between isotropic fluids and

crystalline solid. Liquid crystals (LCs), a promising material, have widely applied

to the LCD [9, 10]. They possess excellent properties such as low-voltage driven,

lightweight and cheapness. Therefore scientists use their in particular characteristics

in many fields for advanced applications. For example, some works have shown

the tunable wavelengths of photonic crystals in semiconductor materials using

infiltrated LCs [11, 12]. The most useful property of LC lies in the fact that its

molecular orientation can be easily controlled via an external bias. On this basis,

numerous applications have been established, among which one prominent case

should be ascribed to the liquid crystal display (LCD). Combining zero and

2

one-dimensional semiconductor nanostructures with the well developed LCD

technology, herein, we propose the feasibility of designing a novel color-tunable

light emitting device. We ingeniously demonstrate a color-tunable emission device

by embedding semiconductor nanorods and quantum dots in a LC cell. The

underlying mechanism is as follows. Nanorods will align along the orientation of

LC molecules due to a large alignment energy caused by the enhanced anchoring

force through amplified surface area in nanomaterials. When the orientation of LC

molecules is altered by an external bias, the reorientation of the nanorods will

follow that of liquid crystal through the minimized elastic energy of interaction via

the electric field. Because the emission of nanorods is strongly anisotropic, and that

of quantum dots is spherically symmetric, i.e. isotropic, we therefore can fine-tune

the ratio of the emission intensity between nanorods and quantum dots. If we

intentionally select nanorods and quantum dots with different emissive wavelengths,

the resulting emission color of this newly designed device could thus be

manipulated. Our result elaborated here should be very useful for the future

development of smart optoelectronic devices.

This thesis is organized in the following manner. In chapter 2, we give an

introduction to theoretical background, including the basic physical principle of

several nematic liquid crystal and one-dimensional semiconductor materials. In

3

chapter 3, the experimental setup and such as micro-PL and TEM are performed. In

chapter 4, we present a detailed description and investigation on our designed

“color-tunable light emitting device”. The polarization of PL and external electrical

experiment are carried out for the further studies. Some interesting results and

discoveries have been obtained from our analyses. Chapter 5 summarizes the

results of this thesis.

4

References

[1]Y. Murakami, E. Einarsson, T. Edamura, S. Maruyama, Phys. Rev. Lett. (2005).

[2] Jianfang Wang, et al. Science 293, 1455 (2001).

[3] H. Y. Chen, Y. C. Yang, H. W. Lin, S. C. Chang, and S. Gwo, OPTICS EXPRESS

16, 13465 (2008).

[4] M. Bashouti, W. Salalha, M. Brumer, E. Zussman, and E. Lifshitz, Chem. Phys.

Chem., 7, 102 (2006).

[5] Mikhail Artemyev, Björn Möller Ulrike Woggon, Nano Lett., 3 (4), 509 (2003).

[6] C. X. Shan, Z. Liu, and S. K. Hark, Phys. Rev. B 74, 153402 (2006).

[7] A. Lan, J. Giblin, V. Protasenko, and M. Kuno, Appl. Phys. Lett. 92, 183110

(2008).

[8] N. Yamamoto, Appl. Phys. Lett. 88, 153106 (2006).

[9] P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Clarendon, Oxford,

1993).

[10] P. C. Yeh, and C. Gu, Optics of Liquid Crystal Displays (Wiley, New York,

1999).

[11] C. Schuller, F. Klopf, J. P. Reithmaier, M. Kamp, and A. Forchel, Appl. Phys.

Lett. 82, 2767 (2003).

[12] B. Maune, M. Loncar, J. Witzens, M. Hochberg, T. Baehr-Jones, D. Psaltis, A.

Scherer and Yueming Qiu, Appl. Phys. Lett. 85, 360 (2004).

5

Chapter 2

Theoretical Background

2.1.1 Liquid Crystals [1, 2]

2.1.1.1 Calamitic Liquid Crystals

In general, the most usual kind of LCs consists of rod-like molecules with its

molecular axis longer than the other two named calamitic LCs. For example,

considering the nematic LC phase with rod-like molecules, it is found that the long

axes of the molecules prefer to direct along a sure direction when they change from

one direction to another. Figure 2.1 and Figure 2.2 show the fundamental difference

between the LC phase and isotropic liquid phase such as water at room temperature.

There are many smectic LC phases in the world. Smectic-A phase and smectic-C

phase are two important cases among those phases. The former describes the director

is parallel to the layer and the latter is at an angle to the layer. Figures 2.3 (a) and (b)

show the difference between smectic-A phase and smectic-C phase.

Another in particular LC phases have the characteristic of chirality, indicating

that those phases do not own inversion symmetry. This leads to the nematic phase and

some of the smectic phases disappear instead of chiral types with various physical

structures. For instance, in a cholesteric phase (chiral nematic phase), the director in

the in-plane layer rotates gradually along a direction perpendicular to the layer as

6

shown in Figure 2.4. The distance called the pitch is defined as the director rotates for

one full revolution also shown in Figure 2.4. The physical structure period should be

indeed half the pitch because of the symmetry between director and .

If we replaced the smectic-C phase by the chiral smectic C phase, we can obtain

the phase (smectic-C* phase) whose director keeps a fixed tilt angle in the in-plane

layer. However, the director rotates like a cone from one layer to the next layer as

shown in Figure 2.5. The pitch is the distance for one full revolution around the cone

as illustrated in Figure 2.5.

2.1.2 Basic physical properties of Liquid crystals 2.1.2.1 Orientational order parameter

As described above, the nematic phase of the LCs molecules is rod-like form

with their molecular axis longer than the other two. Therefore, we can define a vector

to state the average preferred orientation of the LCs in the discussed system. We

usually name the vector , the director. Generally speaking, in a homogeneous and

inhomogeneous nematic LCs system, the director is a constant value and depends on

the space position (x, y, z) in the system, respectively. We can define a unit vector to

exhibit the long axis of each molecule, and the director becomes the statistical

average of the unit vectors in a small volume element around the discussed LC

molecules.

7

The orientational order parameter S of a LC can be defined as

, (2.1)

where θ is the angle between the long axis of a LC molecule and the director . For a

perfect parallel alignment, S = 1, while for totally random directions, S = 0. Note that

the nematic phase of LCs usually has an intermediate value of the orientational order

parameter S ranging from 0.4 to 0.6 at low temperatures typically. In addition, the

values of the order parameter S depend on the structure of the molecules and the

temperature dramatically.

2.1.2.2 Dielectric anisotropy

The nematic and smectic LCs are uniaxial symmetry due to the orientational

ordering of the rod-like molecules. Because of the properties of uniaxial symmetry,

the dielectric constants become different in value along the preferred axis (ε^∥) and

perpendicular to this axis (ε┴), manifesting the anisotropy of LCs. The dielectric

anisotropy can be defined as

Δε=ε^∥ −ε┴, (2.2)

It is noted the dielectric anisotropy Δε is the most important properties when applied

the LCs to LC industry. In order to emphasize the benefit, here we discuss an applied

electric field in nematic LC. The induced dipole moment of the LC molecules is not

parallel to the external electric field due to the anisotropy of LC, except the molecular

8

axis parallel or perpendicular to the electric field. The effect causes a net torque to

align the LC molecules along the direction of the electric field for most rod-like LC

molecules. In the system, the electrostatic energy can be defined as

, (2.3)

where means the electric field vector. is the displacement field vector, which is

independent of the direction of the LCs in a homogeneous surrounding. The director

is parallel to the applied electric field with positive dielectric anisotropy (ε_∥> ε┴) of

LCs when minimizing electrostatic energy of the system. In general, the dielectric

anisotropy of the nonpolar LCs with rod-like molecules is positive. However, there is

an excess contribution to the dielectric constant because of the permanent dipole

moment generated in the polar LC molecules. In addition, as result of depending on

the angle between the dipole moment and the molecular axis, the dipole contribution

can result in an increase or a decrease of Δε, finally leading to a negative value of Δε.

The director is perpendicular to the applied electric field with negative dielectric

anisotropy (ε_∥< ε┴) of LCs when minimizing electrostatic energy of the system. The

dielectric anisotropy in fact can change from -2εo to 15εo, which also depends on the

temperature strongly.

2.1.2.3 Refractive Index

Nematic LCs often appear as an opaque milky fluid. The scattering of light is due

to the random fluctuation of the refractive index of the sample. The main cause of the

9

scattering leading to the milky appearance is due to the discontinuity of the refractive

index at the domain boundaries. A slab of nematic LC can be obtained with a uniform

alignment of the director. Such a sample exhibits uniaxial optical symmetry with two

principal refractive indices no and ne. The ordinary refractive index no is for light with

electric field polarization perpendicular to the director and the extraordinary refractive

index ne is for light with electric field polarization parallel to the director. The

birefringence (or optical anisotropy) is defined as △n=ne-no. If △n>0, the LC is said

to be positive birefringence, whereas if △n<0, it is said to be negative birefringence.

In classical dielectric theory, the macroscopic refractive index is related to the

molecular polarizability at optical frequencies (~10¹⁴ Hz). The existence of the optical

anisotropy is due mainly to the anisotropic molecular structures. Most LCs with

rodlike molecules exhibit positive birefringence ranging from 0.05 to 0.45. The

optical anisotropy plays an important role in changing the polarization state of light in

liquid crystal.

2.1.2.4 Elastic constants

Liquids and solids have the property of elasticity, even LCs possess the curvature

elasticity. If a LC system is disturbed from its equilibrium structure, the magnitude of

restoring torques is decided by the elastic constants of the LC. As compared with

solids, the torques in a LC system present a relatively weak value. The external

10

electric field has been applied to the control of LC reorientation in LC industry. In a

stable LC system, we can obtain the static configuration by balancing the electric

torque and the elastic restoring torque. In a stable LC system, we also can distinguish

into an association of three fundamental deformations: splay, twist, and bend, as

shown in Figure 2.6. The elastic energy in a stable LC system can be determined by a

quadratic function of the curvature strain tensor for an isothermal deformation in an

incompressible fluid. Here we present the elastic energy density of a deformed LC as

follows:

, (2.4)

The symbols of K1, K2, and K3 are the splay, twist, and bend elastic constants,

respectively. As described above such as orientational order parameter and dielectric

anisotropy, the elastic constants strongly depend on temperature.

2.1.2.5 Viscosity

The viscosity of a fluid is defined as the ratio of shearing stress to the rate of

shear. This viscosity causes from the intermolecular forces in the fluid and it presents

an internal resistance to move. In general, the viscosity increases at low temperatures

due to lower molecular kinetic energy. In nematic LCs system, it has been found the

effective viscosity depends on the angle between the director and the direction of flow

and on the direction of the velocity gradient. Here we state three conditions as shown

11

in Figure 2.7. The viscosity coefficient η2 is described as the rod-like molecules are

perpendicular to the velocity gradient, but aligned in the direction of the flow. The

viscosity coefficient η1 is described as the rod-like molecules are parallel to the

velocity gradient, but perpendicular to the direction of the flow. The viscosity

coefficient η3 is described as the rod-like molecules are perpendicular to the velocity

gradient, but perpendicular to the direction of the flow. We can measure those three

viscosity coefficients by oscillating plate viscosimeter. Using this method, we can let

the one bounding plate be fixed while the other one be moved to generate a shear as

shown in Figure 2.7, where the moving plate oscillates at very low frequency. Owing

to the damping of the oscillations via the viscous medium, the viscosity coefficients of

η1, η2, η3 can be evaluated. It is worth noting that the magnitude of the viscosity is not

only affected by attractive intermolecular forces, but also by steric parts such as

length and breadth of the molecules, lateral branches and so on. Here we must

emphasize an important parameter, the rotational viscosity coefficient γ1. It provides a

resistance to the rotational motion of the LC molecules and can be applied to the LC

display. For example, the switching time is approximately proportional to γ1d² in most

LC display, where d means the distance of the cell gap. In addition, LCs with high

values of dielectric anisotropy Δε often have higher viscosity coefficient.

2.1.3 Deformation of nematic liquid crystals by an electric field

12

Because of very widely important application via using electric field to reorient

the LCs, here we discuss in detail the affection by an electric field. The manifestation

of an electric field results in extra terms in the expression of the following free energy

density:

, (2.5)

We can cancel the first term due to independent of the direction of the director.

Considering the second term, the free energy of the nematic LCs concerned with an

electric field should be a minimum for a definite direction of the director relative to

the field direction due to the dielectric anisotropy Δε. As shown in Figure 2.8, if there

is a nematic LCs system with positive dielectric anisotropy (Δε＞0), the stable state

which should minimize the free energy is described by a director parallel to the

external field direction. In contrast, if the nematic LCs system with negative dielectric

anisotropy (Δε＜0), the stable state prefers to a director direction perpendicular to the

external field direction. In order to avoid the induced separation of charged impurities

forming electric double layer in LC cell, an alternating voltage across the LC cell. The

electric double layer model is shown in Figure 2.9. In view of an original state that the

direction of the director concerned with external field direction does not satisfy the

condition of minimum free energy, it leads to a sufficiently strong electric field to

generate a torque on the LCs which results in a reorientation of the director. We can

13

obtain a deformed state which owns lower free energy than the original state. The

equilibrium condition to cause a deformed stable state is the balance between the

electric torque and the restoring elastic torque of the LC. In addition, the stable state is

always obtained by the reorientation of director configuration when minimizing the

free energy of the system.

If we think of the electric energy in Equation (2.4), the

distribution of the director all over the system can be evaluated. According to

Equation (2.4), we can derive the critical field strength where the destabilizing electric

torque overcomes the stabilizing restoring elastic torque. Figure 2.10 are illustrated

several field-induced deformations of nematic LCs system. In Figure 2.10 (a), (b), and

(c), three fundamental situations are shown with different initial orientation of the

director. It is noted that the applied electric field is perpendicular to the director in

Figure 2.10 and the dielectric anisotropy Δε is positive so that the director prefers to

align parallel to the external field direction. Figure 2.10 (a) shows the result of a pure

splay deformation for a small director displacement. If having a higher field to the

deformation, we can obtain mainly a splay deformation with a combination of a bend

deformation. The threshold field Eo is given by

, (2.6) where d is the sample thickness.

14

Figure 2.10 (b) shows the result of a pure twist deformation. The threshold field Eo is

given by

, (2.7)

Figure 2.10 (c) shows the result of a small pure bend deformation. If having a higher

field to the deformation, we can obtain mainly a bent deformation with a little splay

deformation. The threshold field Eo is given by

, (2.8)

It must be emphasized that Equations (2.6)－(2.8) are derived under the simplifying

assumptions, where the interaction between the molecules and the surface is strong

and the electric conductivity is neglected here. According to (Uo is threshold

voltage), it indicates that the threshold voltage is independent of the sample thickness.

Similar deformations can be discussed in nematic LCs system with negative dielectric

anisotropy (Δε＜0). Then we can obtain the same critical field strength when the

electric field is perpendicular to the situation as shown in Figure 2.10. Figure 2.10 (d)

shows schematically the director alignment in a planar-twisted cell where the twist

angle is 90 degree. The threshold field Eo for a nematic LCs system with positive

dielectric anisotropy (Δε＞0) is given by

, (2.9) It should be emphasized here that a decrease of the anchoring energy leads to a

15

decrease of the threshold field Eo. In addition, for a tilted director alignment, there is

no threshold field theoretically. It means that the deformation starts at an infinite

small field strength.

We can derive theoretically the equation of motion of the director (the dynamics

of the field-induced deformations) which presents the balance of the elastic and

viscous forces and the external electric field. Considering a particular case of a pure

twist deformation, it means that the system is not accompanied by a change in

position of the centers of gravity of LC molecules (in contrast to splay and bend

deformations). The equation of motion can be written as:

, (2.10)

where θ is the angle between the local director and the substrates, z is the coordinate

perpendicular to the substrate, t is the time, and γ1 the rotational viscosity. In general,

Equation (2.10) must be solved numerically. However, for some simplifying

assumptions, we can derive the following result for the switching times of the

dielectric reorientation as follows:

, (2.11)

where U is voltage and Uo is the threshold voltage, and

, (2.12)

Equations (2.11) and (2.12) are also valid for bend and splay deformations if small

16

deviations from the initial orientation of the director. Owing to depend on the initial

orientation of the director, in Equation (2.12), different elastic constants can be

replaced K2, for a planar layer K1, for a homeotropic layer K3, and for a planartwisted

layer K1 (K3 – 2K2)/4. Finally, it worth noting that the switching times are mainly

determined by the rotational viscosity γ1 and the sample thickness d from Equations

(2.11) and (2.12). In particular, Δε and the driving voltage are the crucial factors to

affect the rise time.

2. 2.1 Semiconductor nanowires and nanorods [5]

Nanoscale materials have attracted tremendous interest and attention over the

past decade for their vast areas of applications ranging from electronics to

pharmaceuticals.

One-dimensional (1D) nanostructures such as nanowires, nanobelts and

nanotubes have become the focus of intensive research owing to their fascinating

properties, unique applications in macroscopic physics and fabrication of nanoscale

electronic and optoelectronic devices. Semiconductor nanowires have demonstrated

significant potential as fundamental building blocks for electronic and photonic

devices and offer substantial promises for integrated nanosystems. The rectifying

properties of semiconductor nanowires-based electronics demonstrated the versatility

of the nanowires-based electronics device.

17

Quantum confinement effects of the semiconductor nanowires on the other hand,

produce unique optical properties that can be applied to nanophotonic devices.

Notably, the key feature of semiconductor nanowires that has enabled much of their

success has been the growth of materials with reproducible electronic and optical

properties that can be in turn, integrated into functional nanoscale devices.

Consequently, nano-devices based on semiconductor materials such as field-effect

transistors (FETs) [6], lasers [7], light emitting diodes (LEDs) [8] and sensors [9]

have been demonstrated.

Quantum size-confinement plays an important role in determining the energy

levels of 1D nanostructures when its diameter is below the critical Bohr radius. Lu et

al. found that the absorption edge of Si nanowires display significant blue shift as

compared with the indirect bandgap (~1. 1 eV) of bulk silicon [10-12]. They observed

sharp, discrete features in the absorption spectra and relatively strong band-edge

photoluminescence (PL). These different optical features mainly resulted from

quantum-confinement effects and also the variation in growth direction for these Si

nanowires [13]. In contrast to quantum dots, light emitted from nanowires is highly

polarized along the longitudinal axes. One-dimensional nanostructures have thus far

been primarily synthesized by lithography and epitaxial techniques on semiconductor

substrates [14–16]. Free standing nanowires, on the other hand, of many

18

semiconductors have recently been formed by a growth technique using nanosized

liquid droplets of the metal solvent [17–20]. This technique has the advantage of

being able to produce heterogeneous structures along the nanowire, such as p-n

junctions [17] and superlattices [18]. Optical properties of the nanowires formed using

the above technique have been studied by photoluminescence (PL) and absorption

spectroscopy, which showed a blueshift of the peak energy due to the quantum

confinement effect and giant polarization anisotropy in PL and optical absorption.

Those properties provide the potential for new applications of the semiconductor

nanowires. For example, Wang et al. [21] showed a prominent anisotropy in the PL

intensities in the direction parallel and perpendicular to the long axes of an individual,

isolated indium phosphide (InP) nanowires (Figure 2.11), whose polarization ratio is

0.96, but that of numerous InP nanowires grown on a substrate [22] (Figure 2.12), where

some of them were inclined from nomal of substrate, is only 0.5. The magnitude of the

polarization anisotropy could be quantitatively due to the large dielectric constant

contrast between the nanowires and the surrounding environment, as opposed to

quantum mechanical effects such as mixing of valence bands.

19

References

[1] P. J. Collings and M. Hird, R. A. Stradling and P. C. Klipstein, Introduction to

Liquid crystals Chemistry and Physics (Taylor & Francis, 1997).

[2] P. J. Collings, Liquid crystals (Nature’s Delicate Phase of Matter) (Princeton

University Press, 1990).

[3] de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Clarendon (Oxford,

1993).

[4] P. C. Yeh, and C. Gu, Optics of Liquid Crystal Displays (Wiley, New York, 1999).

[5] A. A. Balandin and K. L. Wang, Handbook of Semiconductor Nanostructures and

Nanodevices 4 (American Scientific Publishers, Los Angeles, California, USA,

2006).

[6] R. Martel, T Schmidt, H. R. Shea, T. Hertel, and P. Avouris, Appl. Phys. Lett. 73,

2447 (1998).

[7] J.C. Johnson, H. J.Choi, K. R. Knutsen, R D. Schaller, R Yang, and R. J. Saykally,

Nature Mater. 1, 106 (2002).

[8] H. M. Kim, T W. Kang, and K. S. Chung, Adv Mater. 15, 567 (2003).

[9] A. Kolmakov, Y. Zhang, G. Cheng, and M. Moskovits, Adv Mater. 15, 997 (2003).

[10] X. Lu, T. Hanrath, K. P. Johnston, and B. A. Korgel, Nano Lett. 3, 93 (2003).

[11] T. Hanrath and B. A. Korgel, J. Am. Chem. Soc. 124, 1424 (2001).

20

[12] J. D. Holmes, K. P. Johnston, R. C. Doty, and B .A. Korgel, Science 287, 1471

(2000).

[13] M. V Wolkin, J.Jorne, P. M. Fauchet, G. Allan, and C. Deleme, Phys. Rev. Lett.

82, 197 (1999).

[14] Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 (1982).

[15] T. Someya, H. Akiyama, and H. Sakaki, Phys. Rev. Lett. 74, 3664 (1995).

[16] P. Ils, M. Michel, A. Forchel, I. Gyuro, M. Klenk, and E. Zielinski, Appl. Phys.

Lett. 64, 496 (1994).

[17] M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith, and C. M. Lieber, Nature

_London_ 415, 617 (2002).

[18] Y. Wu, R. Fan, and P. Yang, Nano Lett. 2, 83 (2002).

[19] T. Mokari, E. Rothenberg, I. Popov, R. Costi, and U. Bnin, Science 304, 1787

(2004).

[20] J. Goldberger, R. He, Y. Zhang, S. Lee, H. Yan, H-J. Choi, and P. Yang, Nature

(London) 422, 599 (2003).

[21] J. F. Wang, M. S.Gudiksen, X. EDuan, Y. Cui, and C. M. Lieber, Science 293,

1455 (2001).

[22] N. Yamamoto, S. Bhunia and Y. Watanabe, Appl. Phys. Lett. 88, 153106 (2006).

21

Figure 2.1 The schematic showing of isotropic liquid phase.

Figure 2.2 The schematic showing of nematic liquid crystal phase.

22

(a)

(b)

Figure 2.3 The schematic showing of (a) smectic-A liquid crystal phase,

(b) smectic-C liquid crystal phase.

23

Figure 2.4 The schematic showing of cholesteric liquid crystal phase,

where is the director.

24

Figure 2.5 The schematic showing of smectic-C* liquid crystal phase.

25

(a) (b)

(c)

Figure 2.6 The schematic showing of (a) splay, (b) twist, and (c) bend

deformations in liquid crystal.

26

Figure 2.7 The schematic showing of three principal viscosity coefficients η

, η

and η

Figure 2.8 Nematic LC under an external bias (a) △ε> 0 (b) △ε< 0.

27

Figure 2.9 The ionic charges induced by an external field in LC cell.

28

Figure 2.10 The schematic showing of basic geometries of the dielectric

reorientation of nematic liquid crystals (

. On the left hand side, the

initial states distinguish by different starting orientations. On the right

hand side, the deformed states above the threshold are illustrated.

29

Figure 2.11 Polarized excitation and emission spectraof nanowires. (A) Excitation spectra of a 15-nm-diameter InP nanowire. These spectra were recorded with the polarization of the exciting laser aligned parallel (solid line) and perpendicular (dashed line) to the wire axis. The polarization ratio, ρ, is 0.96. Inset, plot of the polarization ratio as a function of energy.

(B) Emission spectra of the same wire as in (A). These spectra were taken with the excitation parallel to the wire, while a polarizer was placed in the detection optics. The polarization ratio of the parallel (solid line) to perpendicular (dashed line) emission is 0.92. The spectra were taken with integration times of 10 s. Inset, plot of the polarization ratio as a function of energy. Reprinted with permission from J. F. Wang et al., Science 293.

1455(2001).

Figure 2.12 CL spectra from the nanowire region and substrate region taken for the p(solid spots) and s(open spots) polarization directions. The polarization ratio, ρ, is only 0.5. Reprinted with permission from Appl.

Phys. Lett. 88, 153106 (2006).

30

Chapter 3 Experiment

3.1 Micro-Photoluminescence [1]

3.1.1Principles and Applications of Micro-Photoluminescence

For a perfect semiconductor crystal, a light source with photon energy higher

than the band gap of the semiconductor crystal will excite the carriers to their excited

states. As soon as the excitation occurs, all excited electrons and holes will relax to

the bottom of the conduction band and the top of valence band, respectively, and then

the radioactive recombination may occur under the condition of momentum

conservation as shown in Figure 3.1. When the maximum of the valence band and the

minimum of the conduction band occur at the same value of the wave vector k,

transitions are direct and the material is a direct-gap semiconductor (for instance,

GaAs and GaN). For a direct-gap material, the most probable transition is across the

minimum energy gap which is between the most probably filled states at the minimum

of the conduction band and the states most likely to be unoccupied at the maximum of

the valence band. If the band extreme do not occur at the same wave vector k, the

transition is indirect. To hold the condition of momentum conservation in such an

indirect-gap material (for example, Si and Ge), the participation of phonons is

required. Such a process is also called a phonon-assisted process. Therefore, the

31

recombination of electron-hole pairs must be accompanied by the simultaneous

emission of a photon and a phonon. The probability of such a process is significantly

lower as compared with direct transitions. The radioactive recombination that is

caused by the incandescence coming from hot source is called photoluminescence

(PL). As shown in Figure 3.2, for example, if there is a multiplicity of excited states,

only transitions from the lowest excited state can generally be observed at low

temperatures because of rapid thermalization [2]. Figure 3.3 illustrates the process of

photoexcitation, as well as different processes that may cause light emission. If a

photon with its energy higher than the band gap of the sample, an electron in the

valence band will be excited to conduction band and soon dribbles down to the

bottom of conduction band by reaching thermal equilibrium with the lattice, i.e.,

emitting phonons. Figure 3.3(a) is an interband transition. In this case, a direct

recombination between an electron in the conduction band and a hole in the valence

band results in the emission of a photon of energy Eg =hν Although this recombination

occurs from states close to the corresponding band edges, the thermal distribution of

carriers in these states will lead, in general, to a broad emission spectrum. If the

semiconductor is pure, the recombination will be between electrons in the conduction

band and holes in the valence band. As the temperature is sufficiently low (e.g., less

than 25 K for GaAs), an electron and a hole will form a bound state due to the

32

Coulomb interaction. This electron-hole pair is the so-called exciton. The

recombination of an exciton will give rise to sharp-line luminescence with energy of

the band gap minus the binding energy of the exciton. Figure 3.3(b) shows an

excitonic transition. If the semiconductor contains impurities, several new

recombination paths via the impurity states open up. Electrons from the conduction

band may recombine with neutral acceptors which become negatively charged after

recombination (as shown in Fig. 3.3(c)). Neutral donors may recombine with holes in

the valence band, becoming positively charged (as shown in Fig. 3.3(d)). At higher

impurity concentrations electrons bound to donors may recombine directly with holes

bound to acceptors, giving rise to donor–acceptor pair luminescence (as shown in Fig.

3.3(e)). A third category of impurities is isoelectronic impurities, where the impurity

has the same valency as the atom it replaces in the host lattice. The isoelectronic

impurities may bind excitons, which can give luminescence, substantially below the

energy of free excitons. Luminescence studies are, in general, a very powerful method

for obtaining information about impurities, also at low concentrations. It should also

be noted that not all recombination between electrons and holes results in light

emission, since there may also be efficient nonradiative recombination paths. PL is

one of the most useful optical methods for the semiconductor industry [2, 3], with its

powerful and sensitive ability to find impurities and defect levels in silicon and group

33

III-V element semiconductors, which affect materials quality and device performance.

A given impurity produces a set of characteristic spectral features. This fingerprint

identifies the impurity type, and often several different impurities can be seen in a

single PL spectrum. In another use, the full width at half maximum (FWHM) of PL

peak is an indication of sample quality and crystallinity, although such analysis has

not yet become highly quantitative. Besides, PL is sensitive to the strain field inherent

in the semiconductor heterostructures, and can measure the magnitude and the

direction of the strain field. Photoluminescence can also determine the band gaps of

semiconductors. This is very important for binary (AxB1-x) and ternary (AxB1-xC)

alloys whose gaps vary with the compositional parameter x which must be accurately

known for applications. When the relation between gap energy and x is known, the PL

measurement of gap can be inverted to determine x. From this, a two-dimensional

map of alloy composition can be obtained as the exciting laser beam is scanned across

the surface of the sample, which is a useful tool to determine inhomogeneity.

Among the optical characterization methods, PL are probably the best developed

to carry out such spatial scanning, with commercial equipment available. An

interesting PL measurement with the aid of a polarizer can help us study the optical

anisotropy of semiconductor heterostructures. By way of this method, we are able to

investigate the microstructure of the sample and the mechanism of the radioactive

34

recombination of the electrons and the holes in detail.

3.1.2 The Apparatus for Micro-Photoluminescence Measurement

system as shown in Figure 3.4. The excitation source can be any laser whose photon

energy is higher than the band gap of the materials to be examined, and whose power

is sufficient to excite an adequate signal. In our measurements, a laser beam with 374

nm is focused into a microscope to be the excitation source. The luminescence from

sample is again focused into the microscope and pass through optical fiber then enters

a TRIAX 320 spectrometer. Finally, a high response H5783 photomultiplier tube is

employed as the detector. The signal from detector switched by SACQ2 sends to

computer. These photons can be generated at continuous powers of watts. Usually,

tens of mW are often adequate to give good signals. The intensity of the Micro-PL

signal apparently depends on the quality of the materials to be examined, the handling

ability of the system, and the sensitivity of the detector.

3.2 Transmission Electron Microscope (TEM)

A main TEM system consists of electron gun, condenser system and objective

lens. The electrons are generated and accelerated to required high energy by electron

gun. A condenser system is set up of different magnetic lenses and apertures makes it

possible to get either a parallel beam (micro probe for TEM) or a convergent beam

35

with selected convergence angles (nano probe for STEM and CBED). Furthermore,

the beam can be scanned (STEM) or tilted (DF-TEM). Most important objective lens

in the microscope since it generates the first intermediate image, the quality of which

determines the resolution of the final image. Images and diffraction pattern can

directly be observed on the viewing screen in the projection chamber or via a TV

camera mounted below the microscope column. Images can be recorded on negative

films, on slow-scan CCD cameras or on imaging plates. Schematic representation of

TEM is shown in Figure 3.5.

36

References

[1] K. J Wu, Master Thesis, N. T. U., Taiwan (2006).；K. C. Chu, Doctoral

dissertation, N.T.U., Taiwan (2005).

[2] R. A. Stradling and P. C. Klipstein, in Growth and Characterisation of

Semiconductors (Hilger, 1990).

[3] S. Perkowitz, in Optical Characterization of Semiconductors: Infrard, Raman, and

Photoluminescence Spectroscopy (Academic Press, 1993).

37

Figure 3.1 Energy transition in (a) direct and in (b) indirect gap semiconductor between initial states and final states.

Figure 3.2 The schematic representation of low temperature

photoluminescence (after R. A. Strading and P. C. Klipstein).

38

Figure 3.3 Illustration of different processes that can give rise to light emission in semiconductor.

(a) The band to band recombination.

(b)Excitonic recombination.

(c)Free hole-neutral donor recombination.

(d)Free electron recombines with a hole on a neutral acceptor.

(e)Donor-acceptor recombination.

39

Figure 3.4 The schematic diagram of the experimental setup used in the optical measurement.

Figure 3.5 Schematic diagram of TEM.

40

Chapter4

Color-tunable light emitting device based on the mixture of CdSe nanorods and dots embedded in liquid crystal cells

4.1 Introduction

One-dimensional nanostructures, such as nanorods, nanowires, and nanotubes,

etc., have become a class of attractive materials as their geometric anisotropy gives

rise to unique physical properties [1-8]. For example, the emission and absorption

spectra arising from one-dimensional semiconducting wires can be highly anisotropic,

and hence serve as an excellent candidate for the application in polarized

optoelectronic devices. On the other hand, liquid crystal (LC) is an anisotropic fluid,

which is thermodynamically between isotropic fluids and crystalline solid. The most

useful property of LC lies in the fact that its molecular orientation can be easily

controlled via an external bias. On this basis, numerous applications have been

established, among which one prominent case should be ascribed to the liquid crystal

display (LCD). Combining zero and one-dimensional semiconductor nanostructures

with the well developed LCD technology, herein, we propose the feasibility of

designing a novel color-tunable light emitting device. We ingeniously demonstrate a

41

color-tunable emission device by embedding semiconductor nanorods and quantum

dots in a LC cell. The underlying mechanism is as follows. Nanorods will align along

the orientation of LC molecules due to a large alignment energy caused by the

enhanced anchoring force through ampled surface area in nanomaterials. When the

orientation of LC molecules is altered by an external bias, the reorientation of the

nanorods will follow that of liquid crystal through the minimized elastic energy of

interaction via the electric field. Because the emission of nanorods is strongly

anisotropic, and that of quantum dots is spherically symmetric, i.e. isotropic, we

therefore can fine-tune the ratio of the emission intensity between nanorods and

quantum dots. If we intentionally select nanorods and quantum dots with different

emissive wavelengths, the resulting emission color of this newly designed device

could thus be manipulated. Our result elaborated here should be very useful for the

future development of smart optoelectronic devices.

4.2 Experiment

4.2.1 Sample preparation

Syntheses of CdSe quantum dots and nanorods have been well documented

[9-12]. In this study, CdSe quantum dots were synthesized by a previously reported

protocol [12], and CdSe nanorods were synthesized according to the reported method,

except for a slight modification regarding the usage of surfactants [13]. In brief, a

42

selenium (Se) injection solution containing 0.073 g of Se was prepared by dissolving

Se powder in 1 ml of tri-n-octyl phosphine. 0.20 g of CdO and 0.71 g of tetradecyl

phosphonic acid (TDPA) were loaded into a 50 ml three-neck flask and heated to 200

°C under Ar flow. After the CdO was completely dissolved, judging by the vanishing

of the brown color of CdO, the Cd-TDPA complex was allowed to cool down to room

temperature. Subsequently, 3.00 g of tri-n-octyl phosphine oxide (TOPO) was added

to the flask, and the temperature was raised to 320 °C to produce an optically clear

solution. At this temperature, the Se injection solution was swiftly injected into the

hot solution. The reaction mixture was maintained at 320 °C for the growth of CdSe

crystals. After 5 min, the temperature was quenched to 40 °C to terminate the reaction.

5 ml of toluene was then introduced to dissolve the reaction mixture, and a brown

precipitate was obtained by adding 5 ml of isopropanol and centrifuged at 3000 rpm

for 5 min. The precipitate was dispersed in toluene for the transmission electron

microscope (TEM) characterization. As the TEM image of CdSe nanorods shown in

Figure 4.1(a), the length and diameter of CdSe nanorods are 25 nm and 7 nm on

average, respectively. The TEM image for the studied CdSe quantum dots is revealed

in Figure 4.1(b), showing that the size of of CdSe quantum dots is about 5 nm. The

corresponding photoluminescence spectra of CdSe nanorods and quantum dots are

shown in Figure 4.1 (c) and (d), respectively.

43

A drawing of the LC cell is depicted in Figure 4.2, in which the top-view and

side-view structures of LC cell, consisting of embedded nanorods and quantum dots,

are presented. The LC cell is composed of two glass substrates with indium tin oxide

(ITO) on the surface, coated by a polyimide (PI, AL21004 (Japan Synthetic Rubber

Corp)) alignment layer. The PI layer, after being spin coated on clean ITO glass

substrate, was then soft baked at 70^o C for 110 seconds, hard baked at 240^oC for 8

minutes. Subsequently, the PI-coated ITO glass was rubbed to serve as homogeneous

alignment layer for arranging the direction of LC molecules. After preparation of

lower and upper treated substrates, a mixture composed of nematic LC E7 (Merck),

CdSe nanorods and CdSe quantum dots was injected into the cell through a shot

syringe. The distance between lower and upper substrates was about 7μm. Owing to

the capillary force, the cell could be thoroughly filled with LC and CdSe

nanocomposites. The device was then sealed for further measurement. Table 4.1

shows the list of the sample fabrication processes in this work. In this study, the

concentration of CdSe nanorods and quantum dots was about 8.5x10¹⁰ cm^-3 and 4x10¹⁰

cm^-3, respectively. As the top view shown in Figure 2, the nanorods will align with LC

molecules along the rubbing direction due to the surface coupling between LC

molecules and nanorods. When the applied external bias is large enough, the

44

orientation of LC molecules would be forced to align perpendicular to the cell plane,

and the reorientation of nanorods would follow that of LC molecules, as shown in

Figure 2.

Table 4.1 The list of the sample fabrication processes

2. Clean the ITO glasses.

3. Spin Coating the ITO glasses with PI layer (AL21004).

4. Bake the ITO glasses after spin coating (70℃ for 110sec & 240℃ for 8 mins).

5. Rub the PI substrate for an unidirection.

7. Compose two substrates (a rubbed PI substrate and a rubbed PI substrate together with AB glue (left and right side).

8. Inject liquid crystal (E7) and CdSe nanocomposites into the LC cell.

9. Seal the composite with AB glue (top and bottom side).

10. Two electric-wires connect with the LC cell.

4.2.2 Experiment setup

Photoluminescence spectra were used to analyze the emission characteristics of

the device containing LC and CdSe nanocomposites. The schematic plot of the

experimental setup is shown in Figure 4.3. A 374 nm laser was used for the pumping

source, which will stimulate the emission of CdSe nanocomposites. The emission

from CdSe nanocomposites passes through the analyzer and depolarizer, and the

signal was detected by a photomultiplier tube (PMT). The analyzer was mounted in

front of the entrance slit of the spectrometer in order to distinguish the orientation of

the polarized electric field. The depolarizer was placed between the entrance slit and

45

the analyzer of the emission signal in order to eliminate the possible error in the

detected polarization due to the measuring equipments. In order to avoid the induced

separation of charged impurities, forming electric double layer in LC cell, we apply an

alternating square wave voltage at 1 kHz frequency across the sample compartment.

4.3 Results and Discussion

Figure 4 shows the polarized behavior of the photoluminescence spectra arising

from CdSe nanorods and quantum dots embedded in LC cell. The maximal

photoluminescence intensities of nanorods and quantum dots are at 580 nm (2.1 eV)

and 650 nm (1.9 eV), respectively. Upon rotating the analyzer angle with respect to

the rubbed PI direction, the emission intensity from CdSe nanorods is then changed

accordingly, while that of CdSe quantum dots remains constant. This result indicates

that CdSe nanorods embedded in the LC cell is well aligned with the LC molecules

along the rubbed direction. It is a consequence of the minimization for the elastic

energy due to interaction between LC molecules and nanorods [19]. The inset in

Figure 4.4 shows the emission intensity of CdSe nanorods as a function of the

analyzer angle, which exhibits a periodic function and follows the cos²θ rule. It is

worth noting that this result is a fully reversible process. In order to confirm the fact

that the observed anisotropic emission indeed arises from the interaction between LC

moles and nanorods, we have fabricated CdSe nanocomposites in a rubbed PI cell

46

without LC infiltration, and the result shows the disappearance of emission

anisotropy.

To gain understanding of the emission anisotropy in a quantitative manner, the

polarization ratio of CdSe nanorods can be calculated by ρ=(I^∥-I^⊥)/( I^∥+I^⊥) [14-16],

where I^∥and I^⊥represent the intensities of emission parallel and perpendicular

to the rubbed PI direction, respectively. The large optical anisotropy could be

rationalized in terms of the differences in dielectric constant between the nanorods and its

surroundings [18]. According to the model developed previously, when the electromagnetic

field is polarized parallel to the nanorod, the electric field inside the nanorod is not reduced.

In contrast, when polarized perpendicular to the cylinder, the electric field amplitude is

attenuated by a factor δ, according to the equation given by

//,

//

i Ee

E  (4.1)

2 ,

= δ ,

s s

Ee

Ei





 

 ^ 



1 ,

1

2 2 2

// 2

2 // 2

//

//



 



 



 



 









i i

i

E E

E E

I I

I

I ⁱ

where Ei is the electric field inside the nanorod, Ee is the excitation field, and ε and εs are the dielectric constant of the nanorod and surrounding, respectively. Consequently, by applying the dielectric constant of CdSe nanorods (bulk ε=10.2) [17] and LC E7 (ε^∥=3.02), we then deduce a theoretical value of the polarization ratio of 0.65. In comparison, according to the experimental result shown in Figure 5, the polarization ratio is calculated to be 0.53. The discrepancy between theoretical and experimental values may be attributed to the fact that CdSe nanorods in our study are not an ideal infinite dielectric cylinder as proposed in the theoretical model [18].

(4.2)

(4.3)

47

In order to further examine the dielectric cylinder model for the explanation of

the optical anisotropy of CdSe nanorods, the polarization dependent absorption

spectra were performed. Recorded spectra for the absorbance of CdSe nanorods

parallel and perpendicular to the rubbed PI direction are shown in Figure 4.5. Clearly,

large absorption anisotropy was observed, and the order parameter at 620 nm is

deduced to be 0.54, which is about the same as that measured from

photoluminescence spectra.

Figure 4.6 shows the polarization dependence of the emission spectra of CdSe

nanocomposites embedded in LC cell under an ac square wave voltage of about 20V.

It is found that the emission spectra almost remain the same when the polarization is

parallel and perpendicular to the rubbed PI direction. The inset shows the emission

intensity of CdSe nanorods as a function of analyzer angle. The intensity does not

change with the analyzer angle, but slightly decreases due to fluorescence quenching.

The result implies that when the measurement of the emission intensity is

perpendicular to the cell plane, the emission spectrum is isotropic for the LC cell

under an external bias of 20 V. This behavior can be rationalized as follows. It is well

known that LC director can be driven by an external bias to minimize the electrostatic

energy of the system, and the director of nematic LC (E7) with a positive dielectric

anisotropy will be parallel to the electric field. The anchoring force between nanorods