Chapter 3 Experiment detail
3.2 Microstructure and optical properties analysis
3.2.1 X-ray diffraction
The crystal structures of the as-grown powder were inspected by Bede D1 diffractometer at Industrial Technology Research Institute using a CuK X-ray source (λ=1.5405Å). We used small angle diffraction. The ω was fixed at 5∘, the scanning step was 0.04∘, scanning rate was 4 degree/min and count time was 1.00 second. Figure 3-2 shown XRD ω-2θ scans geometry. The dashed lines mean the trajectory of the incident beam and the detector to be in motion.
The sizes of the nanocrystallites can be determined by X-ray diffraction using the measurement of the full width at half maximum (FWHM) of the X-ray diffraction lines. The average diameter is obtained by
θ λ cos
89 . 0
D= B , where D is the average diameter of the nanocrystallite, λ is the wavelength of the X-ray source, and B is the FWHM of X-ray diffraction peak at the diffraction angle θ.
3.2.2 Optical absorption spectra
Optical transmission or absorption measurements are routinely used by chemists to
determine the constituents of chemical compounds. They are also used in the semiconductor industry, but only for certain specialized applications. We can compute the band-gap of the semiconductors by the absorption spectra.
The excitation and emission spectra were measured using U-4001 Spectrometer (Japan Spectroscopy) at Industrial Technology Research Institute with a lamp source of 150 W xenon. Fist, we must find and take out background signal from preparation sample. Measurements were performed using a glass having a path length of 2 mm. The intensity of emitted light was detected at a right angle to the incident light. Finally, we measured preparation sample (ZnO solution dip on SiO2
glass). Judging from the above, it can be concluded that background signal was deleted as we measure preparation sample. The measurement was taken in the range from 300 nm to 400 nm.
3.2.3 Photoluminescence system
Photoluminescence (PL) provides a non-destructive technique for the determination of certain impurities in semiconductors. The shallow-level and the deep-level of impurity states were detected by PL system. It was provided radiative recombination events dominate nonradiative recombination.
In the PL measurements, the 325 nm line from a He–Cd laser was used as the excitation light. Light emission from the samples was collected into the TRIAX 320 spectrometer and detected by a photomultiplier tube (PMT). As shown in Fig. 3-3, the diagram of PL detection system includes mirror, focusing and collecting lens, the sample holder and the cooling system. We utilized two single-grating monochromators (TRIAX 320), one equipped with a CCD detector (CCD-3000), and
photon counter for detection. The normal applied voltage of PMT is 800 KV.
Moreover, we used a standard fluorescent lamps to calibrate our spectral response of spectrometer and detector. The PL signals are exposed about 0.1 sec at the step of 0.1 nm. The data are transmitted through a GPIB card and recorded by a computer.
The monochromator (TRAIX 320) has a focal length of 32 cm with an optional side exit slit and has three selective 600, 1200 and 1800 grooves/mm gratings. When the entrance and exit slits are both opened to about 50 µm, the resolution is about 0.1 nm for the monochromator with PMT. The low-temperature PL measurements were carried out using a closed cycle cryogenic system.
3.2.4 Raman system
Raman scattering is a very powerful probe for investigating the vibration properties of materials. It is also influential in understanding problems as diverse as the structure of amorphous insulators, and the conduction mechanisms in ionic conductors. The experimental setup of Raman spectroscopy consists mainly of three components: a laser system serves as a powerful, monochromatic light source and a computer controlled spectrometer for wavelength analysis of the inelastically scattered light.
Figure 3-4 show the experimental setup schematically. The Raman scattering was measured with an Ar-ion laser (Coherent INNOVA 90) as an excitation source emitting at a wavelength of 488 nm. The scattered light was collected by a camera lens and imaged onto the entrance slit of the Spex 1877C. Light passes through the entrance (S1) to be collimated by M1 onto G1 where it is dispersed onto M2. After passing through S2, which acts as the filter stage to determine the pass band, the light strikes the spatial-filter mirror (M3) and passes though a fixed slit, which eliminates much of the stray light. Again the light is collimated (M4), dispersed (G2), in an
opposing direction to cancel the effects of the initial dispersion, then focused (M5) onto the exit slit of the filter stage (S3) which controls the resolution of the next spectrograph stage. In this final stage, the light is again collimated (M6) and dispersed on whichever of the turreted gratings (G3, G4 and G5 as gratings of 600, 1200 and 1800 grooves/nm, respectively) is selected by the user. The camera mirror (M7) projects a flat image onto the focal plane where it is seen by CCD.
Chemical reagent Molecular formula Degree of purity
Source
Zinc acetate dehydrate Zn(CH3COOH)2‧2H2O 99.5% Riedel-deHaen
Diethylene glycol C4H10O3 99.5% EDTA
Table 3.1. Shows that chemical reagent was used with sol-gel experiment. process.
Figure 3.1. Experiment equipment used for fabricating ZnO quantum dots (QDS).
separating solution
dip or spin coating on SiO2/Si(001)
Clear solution Centrifuge
heating up to 160 & ℃ difference aging time White colloidal formed
varying heating rate Counter flow apparatus
varying solution concentration Diethylene-glycol (DEG)
Zn(CH3COOH)2‧2H2O
Table 3.2. A flow chart of fabricate ZnO quantum dots (QDS) by sol-gel method.
Figure 3-2 XRD ω-2θ scans geometry for ZnO nanoparticles.
Figure 3-3 PL detection system.
Figure 3-4 Raman detection system.
Chapter 4 Results and discussion
4.1 HRTEM and X-ray diffraction
measurement
Shown in Fig. 4-1(a) is a typical high-resolution transmission electron microscope (HRTEM) image of the ZnO nanoparticles. Nanoparticles aged at 160 °C for 1 h and solution concentration of 0.06M was selected for particle size determination by HRTEM. The particles shape are predominantly spherical, many also exhibit surface facetting, as shown in the inset of Figure 4-1(a) where a step of one atomic layer can be seen. The nanoparticles are clearly well separated and essentially have some aggregation. Figure 4-1(b) show the size distribution of particles after aging at 160°C for 1 h (0.06M), obtained from analysis of more than 35 particles per sample.
The average diameter of the number-weighted particles obtained from a colloid aged at 160°C for 1 h (0.06M) was determined to be 4.36± 0.3 nm.
The XRD patterns of the prepared sample (ZnO solution dip on SiO2 glass) by the sol-gel process are shown in Figure 4-2. The diffraction lines are the powder X-ray diffraction pattern of the ZnO nanoparticles prepared in a different solution concentration. The diffraction pattern and interplane spacings can be well matched to the standard diffraction pattern of wurtzite ZnO, demonstrating the formation of wurtzite ZnO nanocrystals. All of the samples present similar XRD peaks that can be indexed as the wurtzite ZnO crystal structure with lattice constants a=3.253Å and c=5.219Å by Eq. 2-8, which are consistent with the value in the standard card (JCPDS 36-1451). No diffraction peaks of other species could be detected that indicates all
the precursors have been completely decomposed and no other crystal products were formed. It should be noted here that the full width at half maximum (FWHM) of the diffraction peaks increase with decreasing the concentration of zinc precursor duo to the size effect. The mean diameter of the ZnO nanocrystallites is evaluated from the FWHM of the (110) peak from 3.5nm to 12nm by Eq. 2-9 with the range of B from 0.75 to 2.45 , θ= 47.56 and λ (wavelength of incident X∘ ∘ ∘ -ray) is 1.5406Å. This value corresponds to the tail of the particle size distribution determined from HRTEM micrographs (Figure 4-2 (b)) and is close to the value of 4.2 nm obtained from the absorption data (see next section).
4.2 Absorption and Photoluminescence
Spectra
Fig. 4-3 illustrates the room temperature optical absorption (dotted lines) and PL (solid line) spectra of different ZnO quantum dots with reacting solution concentration from 0.04M to 0.32M with aging 1h at 160 . ℃ The photoluminescence (PL) spectra of the six samples, which were excited with the laser of wavelength 325nm, are shown in Fig. 4-3 (solid line). An ultraviolet-blue (UV blue) emission occurs above the band gap energy of bulk ZnO (3.3 eV) and shifts to higher energies (3.3–3.435 eV) as the QD size decreases (12–3.5 nm). Since the Bohr diameter of the exciton in bulk ZnO is on the order of 2.34 nm, we must consider the electron hole Coulomb interaction in our samples and the particles are in the moderate to weak confinement regime.
The absorption spectra are significantly blue-shifted compared to the bulk single
that the growth of the particles depends on solution concentration. In addition, it can be seen that the spectral shift is retarded toward the end with decreasing solution concentration. These factors suggest that the solution concentration strongly affects the particle size and hence the optical properties. The absorption peaks for the six samples were then obtained as 3.437 eV [ Fig. 4-3(f)], 3.463 eV [Fig. 4-3(e)], 3.475 eV [Fig. 4-3(d)], 3.516 eV [Fig. 4-3(c)], 3.584 eV [Fig. 4-3(b)]and 3.67 eV [Fig.
4-3(a)], respectively. These values are all much larger than the absorption edge of the bulk ZnO (3.3 eV). The average particle size as a function of solution concentration was determined from the absorption spectra using the effective mass model derived by Brus [35-40]. In the weak-confinement regime, the confinement energy of the first excited electronic state can be approximated by Eq. 2-10
]
where is the bulk band gap, Ry* is exciton binding energy, a is the particle radius, M is the total mass, and µ is the reduce mass. Figure 4-4 shows the bandgap versus particle diameter obtained from Eq. 2-10. The particle size was obtained from the band gap inferred from the optical absorption spectra taking =3.4 eV, m maximum relative to the absorption maximum. This redshift increases as the particle size decreases and has been observed in other II–VI nanocrystalline systems (e.g., CdSe, CdS). Our results suggest that size dependent phonon processes may be partly responsible. In this case, it gives an empirical relationship between the electron-phonon interaction and particle size. The electron-phonon interaction energy will be examined in the Raman spectra. Ultimately, however, the origin of the size-dependent redshifted emission remains unclear at present [43-47].
In additional, the PL spectra of different aging time (same solution concentration
0.08M) are illustrated in Figure 4-6. Similar blue-shifted behaviors are also found as decreasing aging time. As decreasing the aging time, the UV emission peak undergoes a blue-shift from 3.336 eV to 3.37 eV with decreasing relative intensity.
We attributed the phenomenon of blue-shift to quantum confinement effect. The PL spectra of different heating rate appear in Fig. 4-7. The same phenomenon of quantum confinement effect applies to different heating rate. We can safely say that the ZnO particles size can be controlled by reaction solution by varying concentration, aging time and heating rate.
4.3 Raman spectra analysis
Raman spectrum of the fine ZnO powder was shown in Table 4-1 [48], where the peaks at 331, 383, 438, 549, 584, and 660 cm-1 were clearly observed in the low wave-number region. The results of Raman spectra for ZnO quantum dots (QDs) with diameters of 4.4, 4.8, 5.9 and 6.4 nm are shown in Fig.4-8. Compared with its powder counterpart, several obvious changes could be observed. First, from the figure we can see that the Raman peaks around 520 and 620 cm-1 are not shifted in frequency. We attributed these peaks (520 and 620 cm-1) to signal of substrate [49].
On the other side, Raman peaks around 438 cm-1 (E2 mode) shifts toward lower frequency and their linewidths become larger with decreasing ZnO QDs diameter in the low wave-number region. Ultimately, the peak of longitudinal (LO) phonon was not observed. From the phenomenon was observed above, we will further examine it later on.
It is well known that the phonon eigenstates in an ideal crystal are plane waves due to the infinite correlation length; therefore the k=0 momentum selection rule of
the nanometer scale, the momentum selection rule will be relaxed. This allows the phonon with the wave vector
k L
k 2π
′ ±
= to participate in the first-order Raman scattering, where is the wave vector of the incident light and L is the size of the crystal. The phonon scattering will not be limited to the center of the Brillouin zone, and the phonon dispersion near the zone center must also be considered. As a result, the shift, broadening, and the asymmetry of the first order optical phonon can be observed.
k ′
The modes originating from the photon scattering by first-, second-, and third-order LO phonons are clearly observed in the spectrum under resonant excitation by the 325 nm laser line (see Fig.4-9). The energy of this line is about 440 meV higher than the band gap of ZnO. It means that there is a case of incoming resonance, where the laser line is in resonance with an interband electronic transition. As one can see from Fig. 4-9, the intensity of the resonant Raman line were sharply and the intensity ratio (2LO/1LO) becomes weak by sizes of decreasing ZnO QOs. Considering the particle size dispersion and assuming for R a Gaussian distribution f(R), the total Raman cross section for an n-phonon process can be written as[50, 51]
σn =∫σnR(ω)f(R)dR,
where µ is the electronic dipole transition moment, E0 is the (size-dependent) energy of the electronic transition, hωand hωLO are the energies of the incident photon and the LO phonon, respectively, m denotes the intermediate vibrational level in the excited state, and is the homogeneous linewidth. The overlap integral between the ground and excited states wave functions can be written as [50, 51]
Γ
) ( ) ,
where denotes a Laguerre polynomial and ∆ is the dimensionless displacement of the harmonic oscillator in the excited state. ∆ is known to be related to the Huang-Rhys parameter S by the relation S=∆
m n
Lm−
2. Therefore, the ratio σ between two-phonon and one-phonon scattering cross sections is a very sensitive function of the electron-phonon coupling strength. However, σ is strongly dependence of the excitation photon energy and E0 is the (size-dependent) energy of the electronic transition. Nevertheless, we observed the lower intensity ratio (I2LO/I1LO) as decreasing the size of ZnO QDs.
Let us move to the origin of electron-phonon coupling in ZnO QDs. It is generally accepted that the electron phonon coupling is governed by two mechanisms:
the deformation potential and the Frohlich potential. Following Loudon[52] and Kaminow and Johnston[53], the TO Raman scattering cross section is mainly determined by the deformation potential that involves the short-range interaction between the lattice displacement and the electrons. On the other hand, the LO Raman scattering cross section includes contributions from both the Frohlich potential that involves the long-range interaction generated by the macroscopic electric field associated with the LO phonons and the deformation potential. We found that the intensity of TO phonon in ZnO QDS is almost insensitive to the incident laser wavelength, while that of E1 (LO) phonon is greatly enhanced under the resonant conditions. Therefore we believe that the electron-LO interaction at decreasing the nanocrystal size is mainly associated with the Frohlich interaction.
(a)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
0 10 20 30 40 50 60 70
Fequency(%)
Particles radius(nm)
(b)
Fig. 4-1 HRTEM image (a) and size distribution (b) of the ZnO QDs fabricated using 0.06M Zn(OAc)2.
30 40 50 60 70
(f)12nm
(e)7.4nm
(d)6.5nm
(c)5.3nm
(b)4.2nm
(a)3.5nm solution concentration: (a)0.04M(b)0.06M (c)0.08M(d)0.1M(e)0.16M(f)0.32M
(112) (103) (110)
(102) (101)
(002)
XR D Inte ns it y (a . u.)
Two Theta (degree)
(100)
Fig. 4-2 XRD profiles of the ZnO QDs prepared with various concentration of Zn(OAc)2.
2.8 3.2 3.6 4.0
Fig. 4-3 PL (solid line) and absorption (dashed line) spectra near the band edge of various ZnO QD size.
2 4 6 8 10 12 14 3.3
3.4 3.5 3.6 3.7
Photon energy(eV)
Particle size(nm )
aB=2.34nm
(a)
20 40
40 80 120
Binding energy(meV)
Particle size(nm)
Bulk binding energy
(b)
Fig. 4-4 The dependence of the band gap enlargement (a) and exciton binding energy (b) versus the ZnO QDs diameter as calculated from the effective mass model and the corresponding experimental data.
2 4 6 8 10 12 14 0.14
0.16 0.18 0.20 0.22 0.24
Stokes shift(eV)
Particle size(nm)
Fig. 4-5 The dependence of the Stokes shift versus the ZnO QDs diameter.
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 (d)
(c) (b)
Intensity(a.u.)
Photon energy(eV)
(a) Difference aging time at solution concentration 0.8M : (a)0.5h (b)1h (c)1.5h (d) 2h
Fig. 4-6 PL spectra of various aging time at 0.08M solution concentration.
2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Intensity(a.u.)
Phonton energy(eV)
(a) (b) (c) variation heating rate: (d)
(a)1C/m (b)3C/m (c)5C/m (d)7C/m
Fig. 4-7 PL spectra of various heating rate at 0.1M solution concentration.
Wave number(cm-1) Symmetry Process
331 A1 Acoust. Overton
383 A1(TO) First process
410 E1(TO) First process
438 E2 First process
540 A1(LO) First process
584 E1(LO) First process
660 A1 Acoust. Overton
776 A1,E2 Acoust. Opt. comb.
Table 4-1 The Raman spectral peaks of the fine ZnO powder.
400 500 600 700 Ranman of variation solution concentration:
(a)0.1M (b)0.08M (c)0.07M (d)0.06M
Fig. 4-8 Raman spectra of various ZnO QD.
600 800 1000 1200 1400 1600 1800 2000 (e)20nm (d)6.5nm (c)5.9nm (b)4.9nm
Intensity(a.u.)
Raman shift(cm-1)
(a)4.4nm 1LO
2LO
3LO
0 5 10 15 20 25 30 35
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
I
2LO/I
1LOdiameter of ZnO crystal (nm)
J. Phys. D 34, 3430 (2001)
Fig. 4-9 Resonant Raman Spectra of various ZnO QD.
Chapter 5 Conclusion and perspectives
5.1 Conclusion
Stable ZnO nanoparticles with 3.5-12 nm in diameter were made by a rapid and continuous process of sol-gel method. The ZnO nanoparticles show a wurzite structure with variation of diameter of ZnO nanoparticles from 3.5 nm to 12 nm by x-ray diffraction. These particles exhibit quantum size effect (blue-shift of the absorption and PL spectrum with decreasing crystallite size) and nicely followed a correlation between particle size and optical energy gap from the effective mass model of ZnO quantum dots. Furthermore, we have measured Raman spectra of ZnO quantum dots with different diameters. The shift, broadening, and the asymmetry of the optical phonons have been found for ZnO quantum dots compared with ZnO bulk. The phonon confinement effect could account for these observed results. Moreover, we have extracted the electron-phonon-coupling parameter from Raman spectra, and unambiguously demonstrate that electron-phonon-coupling increase with increasing nanocrystal size mainly due to the Frohlich interaction.
5.2 Perspectives
In the process of fabricating the ZnO quantum dots by sol-gel, the smaller size and uniform distribution nanoparticles are our major targets. To achieve better homogeneity of sample, we should upgrade the centrifugal technique and the method of chemical synthesis. For optical properties, the emission characteristic of ZnO quantum dot is going to measure by low temperature photoluminescence spectra.
The most important characteristic of the ZnO QDs is the exciton lifetime, and we will use the pump-probe experiment to analyze it.
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