Chapter 3. Experiment
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 70o C for 110 seconds, hard baked at 240oC 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
1. Cut the big ITO glasses into a small one (2x2 cm2).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
//, 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
48
and LCs can provide a large alignment energy [19], which will drive nanorods along
the orientation of LC molecules. If the nanorods are now well aligned perpendicular
to the LC cell plane, the emission detected in front of the cell plane, as in our
experiment, should be isotropic. Consequently, the reduced optical anisotropy is
mainly due to the reorientation of CdSe nanorods driven by biased LC molecules.
Quite interestingly, it is found that the ratio of the relative emission intensity between
CdSe nanorods and quantum dots can be manipulated by an external bias. When there
is no external bias, as shown in Figure 4, the emission intensity from CdSe nanorods
is larger than that of CdSe quantum dots for the polarization parallel to the rubbed PI
direction. In contrast, the emission intensity from CdSe nanorods is smaller than that
of CdSe quantum dots when the external bias is applied, as shown in Figure 6. The
results establish the external bias fine-tuning emission color in this newly developed
device.
4.4 Summary
We have successfully demonstrated a color-tunable light emitting device by
incorporating semiconducting nanorods and quantum dots in a liquid crystal cell. The
underlying mechanism is based on the large alignment energy resulting from the
enhanced anchoring force of LC molecules through the amplified surface area of
nanorods. In view of the well established liquid crystal technology, our approach
49
elaborated here may be very useful for the development of smart optoelectronic
devices in the near future.
50
References
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Chou, and H. T. Chiu, J. Phys. Chem. B 108, 10687 (2004).
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52
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.
53
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.
Figure 4.3. The experimental setup for photoluminescence
measurement.
54
Figure 4.4. Dependence of photoluminescence spectra of CdSe nanorods 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.
540 560 580 600 620 640 660 680 700 720
0.0
55
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.
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.
540 560 580 600 620 640 660 680 700
0.0
56
Chapter 5 Conclusion
In general, owing to thermal and mechanical characteristics, one dimensional
semiconductors like nanorods, nanowires hardly make use in integrated photonic
systems and other optoelectronics devices, such as optical switch. In this work, we
have successfully demonstrated an approach to design a color-controllable device
consisting of nanorods and quantum dots and nematic liquid crystals. By means of
unique optical anisotropy of nanorods and spherical symmetry of quantum dots, the
color of the emission arising from our designed light emitting device can be controlled
by an external bias. In the future, the controllable two color emissions can be
extended to three colors (RGB) emissions, or by incorporating surface plasmon
resonance in our newly designed device, it may lead to the development of many
novel smart optoelectronic devices. Because liquid crystal display technology is
well-developed, controlling the color of emission from a display as shown here may
be realized in the near future.