Chapter 3 A Novel Low Temperature Growth and Characterization of Single Crystal
3.2 Experimental Section
3.2.2 Growth of ZnO nanorods
The ZnO nanoparticle colloids in ethanol solution were directly dipped onto ITO substrate and dried at room temperature. Prior to nanorod growth, the substrate was under the heat treatment in dry air at 300℃ for 12h (Figure 1b). Then the ZnO nanostuctured substrates were immersed in equimolar of zinc nitrate (Zn(NO)3·6H2O) and methenamine (C6H12N4) aqueous solutions at 90 oC for 24h (Figure 1c). The thin film was formed above the nanostructured substrate. Subsequently, the product was thoroughly washed with deionized water and allowed drying in air at room temperature.
The as-grown products were further characterized and analyzed by scanning electron microscopy (SEM) (Hitachi S-4700 FEG at 15 kV), Transmission electron microscopy (TEM) (JOEL 2000FX at 200 kV) and Philip Tecnai 20 high-resolution TEM (HRTEM) at 200 kV equipped with a GATAN digital photograph system and energy dispersion x-ray spectroscopy (EDS). The absorption spectrum was recorded on a HP8453 UV-VIS spectrometer. The Raman spectra were measured on Renishaw system 2000 micro-Raman spectrometer with a 514 Ar+ laser as excitation source. Photoluminescence (PL) spectra were also performed at room temperature using He-Cd laser line of 325 nm. An x-ray diffraction study of the samples was carried out with MAC Sience MXP18 X-ray diffractometer（30 kV, 20 mA）with copper target at a scanning rate of 4o/min.
3.
3.3.1 ZnO nanoparticles seeds
V-vis, TEM and XRD 3 Characterization
U
article colloids is shown in Figure
3-)
to ~3.6 nm, which consists with our TEM m surement. In the TEM image as shown in Figure 3-3, the nanoparticles are essentially
noparticles are almost fully well separated, indicating that CTA
The UV-vis absorbance spectrum of the ZnO nanop
3. The absorption spectrum shows a well-defined exciton band at 327 nm and significant blue shift relative to the bulk exciton absorption (373 nm) [22]. This shift phenomenon mainly corresponds to the confinement effect of the small size ZnO nanoparticle colloids (particle size ≤ 7 nm [23]. According to the experimental relationship between the absorption shoulder (λ1/2) and the particle size reported by Meulenkamp [24], the diameter difference of the nanopartices during aging at 60 oC can be monitored. After aging for 2h, the particles average diameter is estimated
ea
monodispersed. Moreover, the na
OH surfactants efficiently cap on the particle surfaces and successfully prevent the aggregation from sol-gel reaction.
3.2.2 Characterization of the as-grown ZnO nanorods SEM
Figure 3-4 shows the typical field emission SEM images of ZnO nanorods grown on nanostructured substrate at 90 C aqueous solution. As observed in Figure 3.2, these o imag
anostructured substrate, the pure ITO substrates were used without ZnO nanoparticles at the es show clearly that highly density and straight nanorods can grow over through the whole surface of these substrates. Although the nanorods are not perfectly aligned on the substrate, they have a tendency to grow toward perpendicular to the substrate. The nanorods grown with 10×10-3, 8.4×10-3 and 6.8×10-3 M Zn2+ aqueous solutions at 90 oC for 24h exhibit mean diameters about 50, 45 and 40 nm, respectively (Fig. 3−4(a-b-c). We also investigated the nanorods growth at higher concentration (17×10-3, 20×10-3 M), the diameters of nanorods were increased to about 100 nm, implying that the diameters of nanorods are dependent on aqueous solution concentration. Meanwhile, as shown in the inset of Figure 3.4a, the ZnO nanorod has a hexagonal prismatic cross-section and hemisphere appearance at ends. In order to identify the growing site of the nanorods on the n
Chapter 3 ZnO Nanorods: Characterization of ZnO Nanorods
same experiment conditions. As approach, there is barely nanorod grown on the whole ubstrates, indicating that ZnO nanoparticle really play a crucial role in ZnO nanorods
RD and Raman s
growth mechanism.
X
e of as-grown ZnO nanorods was investigated using XRD (Figure 3-5a
The crystal structur
); the [002] reflection apparently has sharpened up comparative to the [101] maximum reflection of ZnO zincite (JCPDS 36-1451). It showed that the ZnO nanorods were grown with c-axis orientation and trend towards to substrate surface. Figure 3-5b shows Raman spectra for ITO substrate and ZnO nanorods growth on the substrate. Clearly, the ZnO nanorods have only one peak at 432 cm-1 corresponding to ZnO optical phonons E2 mode [25].
TEM and EDS
In order to characterize the ZnO nanorods in advanced, TEM was employed to observe the nanorod microstructure. To prepare TEM sample, the nanorods were scraped from substrate, briefly ultrasonicated in methanol and then dispersed onto a carbon film covered copper grid. In Figure 3-6a, the bright–field TEM image shows that the nanorods are traight and have a uniform diameter. The typical diameter and length-to-diameter ratios of
notable that the individual nanorod misphere appearance at the end in Figure 3-6a as well as in Figure 3-4a, whereas the othe
s
the nanorods are about 50 nm and 25, respectively. It is has a he
r nanorod end is shaped like a flatness appearance. This observation suggests that the nanorods would be grown from the well-defined faceted end to the hemisphere end, which is similar to the previous reports on the single crystal ZnO whiskers [26,18]. In addition, the EDS analysis of nanorods showed the nanorods contain only Zn and O, indicating that there is no other metal impurity as catalyst (Figure 3-6c). The HRTEM lattice fringes image and selected area electron diffraction pattern (SAED) shown in Figure 3.4b reveal that in this selected area, the nanorods possess a single crystal hexagonal structure without dislocation and stacking fault. The image also confirms that the nanorods grow along the [0001]
direction (indicated with an arrow).
Photoluminescence
Photoluminescence (PL) of the ZnO nanorods were measured in room temperature and the spectra were shown in Figure 3-7. In Figure 3-7a, a sharp near band-edge emission at 378 nm and a broad green emission at ~580 nm were observed. The near band-edge
pecific defect [28]. In addition, the heat-treatment step could cause a rther decrease intensity of the green emission and increase the intensity of band-edge -7b). This result can be attributed to the decrease of the amount of the singl
emission is attributed to a well-known recombination of free excitons [27], and the green emission is resulted from the recombination of photogenerated hole with a singly ionized charge state of the s
fu
emission (Figure 3
y ionized oxygen vacancies in the ZnO and acquired high crystal quality of ZnO nanorods with increasing the heat-treatment temperature.
3.2.3 Influence parameter of growth one-dimensional ZnO Concentration of ZnO nanoseeds
In order to realize the influence of self-seeds on the nucleation and growth mechanism f ZnO nanorods, more detailed experiments were carried out. First, the effects of various the nanoparticles seeds precursor were investigated. It was found that the aver
o
concentrations in
age diameter of the nanorod increase to about 70± 25 nm, and the length increase to about ~3 µm when the precursor concentration increase to about ~0.05M (Figure 3-8a).
This increase could be attributed to the seeds aggregation because high concentration precursor could form bigger nanoclusters and large size seeds, subsequently favored the growth of coarser nanorods. Figure 3-8b shows the TEM image of such nanorods, indicating the nanorods were straight along the longitudinal axis.
Heat-treatment temperature of nanoseeds
Nevertheless, we also found that low heat treatment temperature for preparing the nanostructured substrate can decrease the diameter of nanorods. As shown in Figure 3-9, when the substrate temperature was heated at ~250 C, the bundle-like aggregation of ZnO nanorods about 30 nm in diameter and length in excess of 1 µm were produced.
Notably, when the heat treatment temperature of ZnO nano-seeded substrate was stetted
o
products after the hydrolysis-condensa
Chapter 3 ZnO Nanorods: Influence Parameter of Growth ZnO nanorods
about 200 oC, the morphology of ZnO nanostructures became significant high ratio of the final tion of Zn2+ salt aqueous solution on seeded substrates.
s shown in Figure 3.10a and Figure 3.10b, the morphology of final products is definitely ifferent from the original ZnO nanoparticles and looks like 1D nanowire. The average density nanowire-like materials is around 30 nm and the length is up to seve
wire-like appearances. Figure 3-10 shows the low-magnification SEM pictures of
A d
diameter of the
high-ral micrometers. Thus it is suggested that the ZnO NPs could serve as seeds at the hydrolysis-condensation process and enhance the anisotropic growth of 1D nanomaterials.
TEM
Figure 3-11 displays TEM pictures of the final products, for which the sample is prepared by scratching and dispersing the final products on copper grid. As shown in Fig.
3-11(a) and Figure 3-11(b), there are several nanowire-like materials in this micrograph.
Moreover, it is found some of them have different contrast in the central part and outside part (as indicated by the arrows in this picture). This result implies that some 1D ZnO nanomaterials have hollow and tubular nanostructures. Figure 3-11(b) the HRTEM image nd corresponding selected area electron diffraction (SAED) pattern (inset) of tubular ZnO anomaterials. The thickness of the nanotubes wall is approximately 6.5 nm. The
nanotube is a single crystal
y smaller than that of -ZnO/solution. Thus the nuclei will be favored to create on the ITO substrate than in the
e substrate seem to be able to further lower the a
n
selected area electron diffraction pattern (inset) implies the ZnO with growth direction along [001].
Based on the above observations, ZnO nanoparticle seeds have been demonstrated to have the essential effect on ZnO nanorod formation. In the present method, the nanorod growth mechanism can not be explained by using the well-known vapor-liquid-solid growth mechanism [11], in which a liquid metal droplet is located at the growth front. As a result, a possible mechanism for the hexagonal ZnO (h-ZnO) crystal growth in this case was believed to be related to common crystal growth as solution chemistry. For the metal oxide (h-ZnO) growth, nucleation always needs to overcome higher activation energy barrier, and the interface energy between h-ZnO/ITO substrate is usuall
h
solution. Moreover, the nanoparticles on th
h-ZnO/substrate interface energy so the nucleation of ZnO nanorods would take place in
(binding) could promote to the metal organic molecule anchored on the nanoparticl
lower saturation ratio than those without. Because the ZnO nanoparticle have the hydroxyl sorption es surface.
as uclei for the directly epitaxial growth of ZnO nanorods because they have the same crystal
atalyst. XRD, Raman, SEM, TEM and HRTEM analysis indicated that the pure single ZnO nanorods have the uniform size distribution and hexagonal wurtzite structure.
The
group on the surface as ZnO/Zn(OH)2 core-shell nanostructure [29], electrostatic ab
Upon the hydrolysis-condensation reaction, the as-prepared ZnO nanoparticles can serve n
structure and lattice parameter. Compared with other reports [11-17], it should be emphasized that the much lower growth temperature in this research is of particular significance and can be very helpful to complex advanced nanodevice integration where the conducting layer such as metal can not survive at a high temperature.
3.4 Conclusions
In summary, we have demonstrated a novel two-step procedure for preparing large-scale growth of single crystal ZnO nanorods by a soft solution method without metal c
crystal
low-temperature growth can be achieved due to the help of ZnO self-seeds and it offers a desirable route for large-scale ZnO nanorods growth. The room temperature PL spectra of the ZnO nanorods exhibit a strong UV emission of 378 nm and a weak green emission of 580 nm. It should be noted that the low-temperature growth process requires no expensive and precise vacuum equipment so as to permit large-scale fabrication at low cost.
Furthermore, the highly optical transparency and electrical conductivity of the ITO glass substrate can provide a great application potential in future optoelectronic nanodevice.
Figure 3-1 S m
EM image of ZnO columns synthesized based on the previously reported ethod. [From ref. 21]
Chapter 3 ZnO Nanorods: Figure section
(a) (b)
(c)
Figure 3-2 A schematic illustration for ZnO nanorod growth on nanostructured substrate by soft chemical method. (a)-(b) formation of ZnO nanoparticle colloids through sol-gel reaction and dispersion on ITO substrate; (c) the ZnO nanorods directly grow from the nanoparticles via hydrolysis-condensation process.
Chapter 3 ZnO Nanorods: Figure section
(c)
Figure 3-3 (a) Absorption spectra, (b) TEM image and (c) XRD pattern of CATOH-capped ZnO nanoparticles.
(b)
Figure 3-4 SEM image of ZnO nanorods grown in aqueous solution on the nanostructured substrates. The corresponding concentration of Zn2+ aqueous solutions is (a) 10×10-3, (b) 8.4×10-3 and (c) 6.8×10-3 M. The inset in Fig. 3-4(a) exhibits hexagonal prismatic cross-section and a hemispherical end.
Chapter 3 ZnO Nanorods: Figure section
Figure 3-5 (a) XRD pattern and (b) Raman spectrum of the ZnO nanorods on ITO substrate.
(c)
Figure 3-6 (a) TEM and select area diffraction image of single crystal ZnO nanorods grown in 10×10-3 M Zn2+ aqueous solution. The diffraction pattern shows that the nanorod grows along [0001] direction. (b) High resolution TEM image obtained from the edge of an individual nanorod and its corresponding SAED diffraction pattern (inset) (c) EDS analysis of ZnO nanorod.
Chapter 3 ZnO Nanorods: Figure section
Figure 3-7 PL spectra of (a) the as-grown ZnO nanorods and (b) heat treated at 350 oC for 12 h.
Fig. 3-8 (a) SEM and (b) TEM images of the ZnO nanorods grown under same condition but using precursor concentration of nanoparticles seeds about 0.05M.
Figure 3-9 SEM image of the ZnO nanorods grown at the same condition but using the nostructured substrate with low heat treatment temperature of ~250
na oC.
Chapter 3 ZnO Nanorods: Figure section
Figure 3-10 SEM image of the ZnO nanostructure grown at the same condition but using the nanostructured substrate with low heat treatment temperature of ~200 oC.
(a) (b)
(c) (d)
wall tube
Figure 3-11 (a) A low magnification TEM image of as-synthesis ZnO nanotubes, (b,c) high magnification TEM images of a single nanotube, and (d) a HRTEM image of ZnO nanotube.
3.5 Reference ] Morales, A. M.; Libber, C. M. Science 279 (1998) 208.
] H. Dai, E. W. Wong, Y. Z. Lu, F. Shoushan, C. M. Libber, Nature 375 (1995) 769.
D. Hu, Science 277 (1997) 1287.
L. Vayssieres, K. Keis, A. Hagfeldt, S. –E. Lindquist, Chem. Mater. 13 (2001) 4386.
] K. Hara, T. Horiguchi, T. Kinoshita, K. Saya Mater. Sol. Cells 64 (2000) 115.
[20] O. Milosevic, D. Uskokovic, Mater. Sci. Eng. A A168 (1993) 249.
[21] L. Vassieres, K. Keis, S. –E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3350.
[8 ma, H. Sugihara, H. Arakawa, Sol. Energy
3] U. Koch, A. Fojtik, H. Weller, A. Henglein, Chem. Phys. Lett. 122 (1985) 507.
4] E. A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566.
5] X. L. Xu, S. P. Lau, J. S. Chen, G. Y. Chen, B. K. Tay, J. Cryst. Growth 223 (2001) 201.
6] J. Q. Hu, Q. Li, N. B. Wong, C. S. Lee, S. T. Lee, Chem. Mater. 14 (2002) 1216.
7] D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, M. Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038.
8] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J.
Appl. Phys. 79 (1996) 7983.
9] H. Zhou, H. Alves, D. M. Hofmann, W. Kriegseis, B. K. Meyer, G. Kaczmarzyk, A.
Hoffmann, Appl. Phys. Lett. 80 (2002) 210.
[2 [2 [2 [2 [2
[2
[2
ZnO Nanotip Arrays
4
Low Approach Toward Highly Aligned ZnO Nanotip
tracted ing tip for the high resolution imaging atomic force microscopy [1], the photonic crystal for waveguide and the
s technologies, the
on materials with low work function and thermal stability under the high vacuum environment, such as
7]. Among rge exciton binding energy (60 meV) semiconductor compound, which is useful for light emitting diodes, room
erature nanolasers, gas sensors, piezoel
Chapter
-temperature Solution
Arrays
4.1 Introduction
Owing to the unique geometries of small radii of curvature, the nanotips have at significant interests for the potential applications such as the prob
field emitter for flat-plane displays [2]. For the field emitter array
manipulation of high density, vertical aligned and well-ordered fine tips are particularly desirable. Currently, developing high-performance field emitter mainly relies
carbon-based (carbon nanotubes, diamond-like carbon) [3], silicon-based (Si, SiN, SiCN) [4], oxide-based (MoO2, MoO3, ZnO) [5,6], and others semiconductor materials [
these materials, zinc oxide (ZnO) is a wide band-gap (3.4 eV) and la
temp ectric devices, and solar cells [8], etc.
Recently, one dimensional (1D) ZnO nanostrustures, such as nanowires, nanotubes and nanorods, have attracted great attentions due to the excellent physical and chemical
These properties are changed
anodic alumina template(AAO) [9], metal catalyst-assisted vapor phase transport (MCVPT) [10], xy (MOVPE) [12] and common thermal evaporation method [13]. Much efforts have been
, which are crucial for developing the next generation of nanoscale electronic and optoe rfectly aligned ZnO nanowire arrays are achieved via
nO
nano hire substrate by using MCVPT technique
with the gold catalyst at 900−925 oC [8]. However, the necessary of relative high
of 1D ZnO arrays. In addition, the MCVPT technique is not suitable for growing sharp tips since the metal catalyst droplets would form at the terminating growth front. Notably, the soft chemical route greatly facilitates the approach to scaled-up fabricated 1D ZnO nanostructures with relative low-cost, which also has been demonstrated to be very efficient in synthesizing single crystal structure at a remarkably low temperature [15]. Vayssieres et al. reported the aligned ZnO microsize rod arrays growing on conducting tin oxide glass by hydrothermal procedure at 95 oC aqueous solution [16]. Liu et al. utilized the well-defined (001) facet microrods as the substrates to grow helical structures of ZnO nanorod and column arrays [17]. Recently, we proposed the using of a layer of nanoparticles with self-seeding assistance to attain ZnO nanorods on ITO substrate [18]. However, the polycrystalline nano-seeds on the substrate generated the random orientations of our nanorods, so the nanorod alignment was unsatisfied. To date, fabricating perfectly aligned and well-isolated 1D ZnO nanoarrays on the Si substrates with soft chemical method is a valuable challenge, which would benefit in the electronic device integrations due to the mild growth conditions. To our knowledge, growing well-aligned ZnO nanotip (nanoneedle) arrays on Si substrate rarely succeeded except using the MOCVD-based method at temperature 400−500 oC [19,20].
properties that are highly different from the bulk materials.
by quantum effects.
Generally, 1D ZnO nanostructures are synthesized by the following methods:
metal-organic chemical vapor deposition (MOCVD) [11], metal-organic vapor epita
devoted to obtain highly aligned and well-separated (i.e. isolated standing) 1D ZnO arrays
lectronic devices. Such pe
MCVPT or MOVPE techniques [8,10,14]. Huang et al. demonstrated the aligned Z wire arrays epitaxially grown on (0001) sapp
temperature and expensive sapphire substrates restrict the production and wide applications
In the chapter 4, we grew vertical and isolated single-crystal ZnO nanotip arrays on a ZnO thin film by soft chemical technique at a low temperature 95 oC without any metal catalyst or template. The field emission (FE) of our ZnO nanotip arrays shows a turn-on field of 10.8 V/µm at a current density of 0.1 µA/cm-1. Moreover, we also successfully fabricated the self-oriented nanotip arrays on ZnO microrod by soft chemical route. This fascinating morphology suggested that our new strategy might provide an insight for building a controllable well-ordered multiple nanostructures at a significant low temperature.
.2 Experimental Section
All the chemicals from Tokyo Chemical Industry were used as received without further ZnO nanotip arrays growth was prepared by the wet chemical technique, as ayssieres et al [16]. In this study, 0.1 g of zinc nitrate (Zn(NO)3·6H2O, 3×10-3 M) and 0.1 g of methenamine (C6H ere dissolved in 100 ml aqueous solution as the precursor solution. Prior to growing ZnO nanotips, a thin ZnO layer (ca.
was then put into a bottle filled with pr ueous solution in the oven at 95 oC for several hours. After the crystal growth, the final products were rinsed with distilled water several times, and then dried in oven at 100 C for 4h. The morphology of as-synthesis characterized and analyzed by scanning electron microscopy (SEM) (JOE
Chapter 4 ZnO Nanotip arrays: Experimental Section
4
purification.
reported by V
12N4 , 6×10-3 M) w
200 nm) was deposited on the Si substrate by radio frequency sputtering. The substrate ecursor aq
o
products was further
L JSM-6500F at 10 kV) and high-resolution transmission electron microscopy (HR-TEM) (JOEL 2010F at 200 kV). Photoluminescence spectrum was also measured at room temperature using He-Cd laser with a wavelength 325 nm as excitation source.
4.3 Result and Discussion
SEM and TEM
The surface morphology of as-grown ZnO nanotip arrays on the ZnO film substrate was studied by the field-emission scanning electron microscope (FE-SEM). Figure 4-1a and Figure 4-1b show the top and tilted views of low-magnification SEM images. The numerous nanotips appear as bright dots and are well-separated across the entire substrate, implying that a large density of ZnO nanotips preferentially grew normally to the substrate
surfa
-grained film and the spacing between well-oriented nanotips estimates abou
ce, and almost isolated from each other with a density of approximately 1.22 ×1010 tips/cm2. The higher-magnification image (Fig. 4-1c) further shows the formation of a regular nanotip arrays with a perfectly vertical alignment. The nanotips are found to grow from the tops of nano
t 100 nm. The detail morphology can be observed by high magnification side-view image, as shown in Figure 4-1d. The nanotips are parallel to each other and perpendicular to ZnO thin film. The base diameter of a nanotip is around 70 nm and the length is around 410 nm. The length could be further controlled by changing the growth parameters, such as
t 100 nm. The detail morphology can be observed by high magnification side-view image, as shown in Figure 4-1d. The nanotips are parallel to each other and perpendicular to ZnO thin film. The base diameter of a nanotip is around 70 nm and the length is around 410 nm. The length could be further controlled by changing the growth parameters, such as