Omar F. Farhat*1, M. Husham
2, Azeez A. Barzinjy 2,3, Abbas M. Selmanan4,
A. A. Abuelsamen5, Mohamed
Bououdina6, Asad A. Thahe7
1Physics Department, Faculty of Sciences, Alasmarya
Islamic University, Zliten, Libya
2Department of Physics Education, Faculty of
Education, Tishk International University (TIU), Erbil, KRG, Iraq
3Scientific Research center, Soran University,
Kurdistan Region, Iraq
4Department of Pharmacognosy and Medicinal plants,
Faculty of Pharmacy, University of Kufa, Najaf, Iraq
5Medical Imaging and Radiography Department, Aqaba
University of Technology, Aqaba, Jordan
6Department of Mathematics and Sciences, Faculty of Humanities and
Sciences, Prince Sultan University, Riyadh, Saudi Arabia
7Department of Medical Physics, College of applied science,
University of Fallujah, Fallujah, Iraq
*Corresponding author: omarfarhat67@yahoo.com
ABSTRACT
ZnO nanorods (NRs) have been successfully
grown onto Silicon (Si) substrates. The properties of
ZnO NRs have been characterized using field emission scanning microscopy, Photoluminescence, and X-ray diffraction. Results showed
that the growth
temperature (85, 90, 95, and 100oC) significantly affected the properties of ZnO NRs. At a temperature of 95 oC, the structural and optical properties have been significantly
enhanced, besides, well-aligned ZnO NRs have been obtained. With the rise of growth
temperature from 85 to 95oC, the crystallite size increases (52 to 94 nm),
and the near band edge emission to deep level emission ratio is enhanced.
Besides, the aspect ratio for the prepared ZnO NRs has increased significantly
reaching 19.42. This study
emphasizes the significance of growth temperature in tunning the structural and
microstructural and subsequently the physicochemical properties of ZnO NRs by fine control of
the growth temperature. Moreover, a facile and cost-efficient method for
fabricating ZnO nanorods for electronic applications based on silicon is
presented in this study.
KEYWORDS: Growth temperature; optical properties;
silicon substrates; structural, ZnO NRs.
1.
INTRODUCTION
ZnO
features a broad direct energy gap value between 3.37–3.80 eV with outstanding
physical properties such as photoconductivity and transparency (Guo et al., 2012). ZnO has garnered significant attention
due to its exceptional properties, affordability, non-toxicity, and simplicity of
production (Abdulrahman et
al., 2021). Nanostructured
materials exhibit enhanced properties, including quantum confinement effects
and a high surface-area-to-volume ratio, which surpass those of bulk materials
(Guo et al., 2012). The remarkable features of ZnO nanostructures
make them ideal candidates for a range of environmental applications, including
UV detection and gas sensing, and piezoelectric application. The high
surface-to-volume ratio provided by nanostructures offers distinguished
features for nanostructure-based devices. Various chemical and physical
methods, including chemical bath deposition, have been utilized to grow
different ZnO nanostructures (Farhat et al., 2021), thermal oxidation (Farhat et al., 2020), thermal evaporation (Bouhssira et al., 2006)
and chemical vapor deposition (Lee et al., 2004). However, the growth temperature of ZnO nanostructures is of significant
concern. Therefore, a moderate growth temperature is desirable due to factors
such as the simplicity of the preparation and cost.
Researchers
have focused on fabricating nanostructured materials on various substrates to
adapt to both flexible and non-flexible devices (Altissimo et al., 2010; Lim et al., 2022; Sherwan et al., 2023).
It has been highlighted that growth experimental parameters such as the growth temperature,
duration, and precursor concentration significantly influence the properties of
ZnO NRs (Farhat et al., 2023; Husham et al., 2015). Nonetheless, the impact
of preparation temperature on ZnO NRs properties deposited on various
substrates requires further investigation and optimization for prospective applications.
Herein, the growth temperature has been varied within a narrow range (85 ‒ 100oC) and the evolution of structure (crystallinity), microstructure
(diameter, length, and aspect ratio), and optical features of ZnO NRs prepared
onto Si substrates, have been examined.
2.
MATERIALS AND METHODS
For the preparation of ZnO NRs, the Si substrates were cleaned and prepared. The
substrates cleaning process was explained elsewhere (Farhat et al., 2017).
Then, a ZnO seed layer was applied to the surface of the cleaned Si substrates by
radio frequency (RF) sputtering. Sputtering was done under the flow of argon gas
at a chamber pressure of 5.5 mTorr and sputtering power of 150 W. Then, the deposited
seed layer was introduced to a furnace and annealed at 300 oC. ZnO
NRs grown using 0.05 M of zinc nitrate hexahydrate [Zn (NO3) 6H2O]
and hexamethylenetetramine (HTMA) (C6H12N4). Precursors
were separately dissolved in deionized water (DIW) in 100 ml beakers. After
that, solutions were mixed and stirred at room temperature for 30 min. Si substrates
were then immersed in the beaker vertically and heated by oven for 3h at
various temperatures (80, 90, 95, and 100oC). Finally, samples were removed
from the beakers and rinsed with DIW. Morphological, optical, and structural properties
of the grown NRs were characterized. The dimensions of vertically grown NRs
were estimated using origin software from the normal size distribution of NRs using
ImageJ software.
3.
RESULTS AND DISCUSSION
Morphological observations: FESEM analysis
Figure 1 shows the surface morphology properties of ZnO NRs grown onto Si substrates at
different temperatures with
the corresponding standard deviation measurements (STDV). As it can be seen, the NRs mean diameter varies as a function of the growth temperature. The average mean diameter for
the prepared ZnO NRs is found to be 130, 128, 104, and 139 nm for NRs arrays grown
at 85, 90, 95, and 100°C, respectively. Further, it is
noticed that the ZnO NRs prepared at 85, 90, and 100°C bath temperatures revealed irregular size distribution. The
average diameter varies between 60 ‒ 200 nm. Conversely, NRs prepared at a
temperature of 95°C exhibit a relatively uniform diameter distribution with the
highest aspect ratio of 19.42. The size distribution of the prepared ZnO NRs
can be attributed to variations in the rate of chemical reactions. The thermal degradation of HTMA releases
hydroxyl ions into the solution through the reaction pathways. Subsequently, Zn(OH)2
will be formed when Zn ions start to react with the hydroxyl ions as the
hydroxyl ions reach supersaturation. Afterwards, Zn (OH)2 decomposes
to become ZnO. The formation of ZnO NRs can be explained by the chemical
reactions represented by equations (1-4) (Zhang et al., 2007):
(CH2)6N4
+ 6H2O → 6HCHO + 4NH3 (1)
NH3 + H2O → _files/image001.png)
+ OH- (2)
Zn2+ + 2OH- → ZnO (nuclei) + H2O
(3)
Zn_files/image002.png)
→ ZnO + H2O + 2OH- (4)
A temperature of 95 °C seems to be
sufficient for the complete thermal decomposition of HMTA and Zn(NO)3
to provide OH- and Zn2+ ions, thereby enabling the
formation of ZnO. This indicates that an adequate temperature enhanced the decomposition
of HMTA resulting in the formation of ZnO NRs at
relatively low temperatures with controlled diameter and length. It has been reported
that, by adjusting the growth temperature, it is possible to enhance the
development of ZnO NRs (Jaqsi et al., 2023). Thus, high-density nanorods
are formed because there are sufficient starting precursors. A small amount of
Zn and OH ions lead to a slower nucleation rate, which induces a lower nanorod
growth rate with enhanced properties.
_files/image003.png)
Figure 1:
FESEM images with the corresponding STDV measurements of the grown ZnO NRs at
different temperatures.
The HMTA has a crucial role in inhibiting the growth of
the six crystalline planes of the NRs, which led to the development of ZnO NRs
along the c-axis (Feng et al., 2016). As the reaction temperature rises, the polar planes expand more
rapidly, influencing the crystal's shape. As a result, higher Miller index
planes with low surface energy are favored, encouraging the growth of
pencil-like ZnO nanorod arrays. Furthermore, the elevated surface mobility at
higher temperatures causes the ZnO molecular species deposited on the base and
surfaces to migrate to the tops of the nanorod arrays, ultimately producing a
nanopencil-like structure as the ending product (Gu et al., 2020). This
manifests that the choice of an adequate temperature helps
in enhancing the degradation of HMTA resulting in the formation of ZnO NRs along
c-axis. Figure 2 depicts the FESEM cross-sectional
view of ZnO NRs deposited onto Si substrate at different growth temperatures. As shown in the figure, well-aligned
ZnO NR arrays with high density and uniform length have been successfully grown
on the Si substrate surface, regardless of the growth temperature.
Figure 2: Cross-sectional for the grown ZnO
NRs arrays p repared on Si substrate grown at different temperatures.
The
estimated mean diameter, length, and aspect ratio of the grown NRs arrays are summarized in Table
Table 1: Variation of diameter, length, and aspect
ratio with temperature of ZnO NRs.
|
Temperature
(°C)
|
Diameter (nm)
|
length (nm)
|
Aspect ratio
|
|
85
|
130
|
1700
|
13.07
|
|
90
|
128
|
1431
|
11.18
|
|
95
|
104
|
2020
|
19.42
|
|
100
|
139
|
2100
|
15.10
|
Structural analysis: XRD characterization
Figure illustrates the XRD analysis of ZnO NRs grown
onto Si substrates at different temperatures. Characteristic
peaks corresponding to ZnO NRs and Si substrate, are observed. The prepared films a manifest pronounced
diffraction peak at 2θ = 34.4° corresponding to the reflection (0 0 2) of the hexagonal lattice structure of
ZnO phase, thus confirming that (002) is the preferred crystallographic orientation
plane. The appeared peak at 2θ = 69.30° is
assigned to the reflection (4 0 0) of the cubic lattice structure of the Si
substrate, as referred in ICSD card No.01-080-0075. Interestingly, as the growth
temperature increased from 85 to 100°C,
diffracted peaks intensity is increased and become more defined. The dominant peak (0 0 2) intensity
increases with increasing the chemical bath temperature, suggesting that the
ZnO nanorods are vertically grown on the Si substrate surface with enhanced crystallinity.
Figure 3: XRD patterns with temperatures for ZnO NRs grown on Si substrates.
Further,
a slight peak shift for lower 2θ is noted, indicating that the as-grown ZnO nanorods
exhibit residual compressive stress caused by lattice parameter compression (Mosalagae et al., 2020; Jaqsi
et al., 2023). This may
be due to a mismatch between the lattice parameters of ZnO with a hexagonal
lattice and Si with a cubic lattice.
Table
2 summarizes the structural (d-spacing
and lattice parameter c) and microstructural (strain and crystallite
size) parameters of ZnO NRs prepared at different temperatures. Lattice parameter
(c) is calculated and compared with the
unstrained lattice.
It has been found that, the c
value decreases with growth temperature as it was
increased from 85 to 95°C,
with a pronounced increase in crystallite size (from 37 to 57 nm). The minimal lattice compressive strain is found at 95°C. However, the compressive strain keeps increasing as the temperature rises to
100°C, accompanied by a notable decrease in crystallite size. This finding indicates that
the crystallinity of the grown ZnO NRs is notably enhanced when the CBD temperature
reaches 95oC. Figures 4 a and b illustrate the lattice parameters
and crystalline size for the prepared ZnO NRs as a function of the CBD
temperature, respectively. As shown in Figure 4a, the lattice parameter of ZnO
NRs gradually increases as the growth temperature rises from 85°C to 95°C, but
then unexpectedly decreases beyond 95°C. The obtained lattice parameter results
of ZnO NRs are in good agreement with (Ahmed et al., 2021). The crystallite size (D) of the synthesized ZnO NRs follows a trend similar to that of the
lattice parameters.
The crystallite increased as the growth temperatures rose from 85°C to 95°C,
indicating an enhancement in crystal quality, as shown in Figure 4b.
Table 2 presents the structural
analyses of the grown ZnO NRs. The table shows that, the highest lattice
parameter value was attained at a growth temperature of 95°C. Besides, ZnO NRs
grown at 95°C possess the lowest compressive strain. However, when the growth
temperature increased to 100°C, the compressive strain increased further, while
the crystallite size decreased to 54 nm. Thus, based on this assertion, the
crystallinity of ZnO NRs is significantly improved when the CBD temperature
reaches 95°C.
Table 2:
Structural analyses of ZnO NRs grown on Si substrates with different
temperatures
|
Temperature (°C)
|
2θ
(°)
|
d
(Å)
|
FWHM
(°)
|
c
(Å)
|
_files/image006.png)
_files/image007.png)
|
D
(nm)
|
|
Unstrained
|
|
|
|
5.2098
|
|
|
|
85
|
34.43
|
2.6028
|
0.2311
|
5.2056
|
-0.0806
|
37
|
|
90
|
34.42
|
2.6030
|
0.2101
|
5.2060
|
-0.0729
|
41
|
|
95
|
34.41
|
2.6039
|
0.1519
|
5.2078
|
-0.0383
|
57
|
|
100
|
34.42
|
2.6029
|
0.1583
|
5.2058
|
-0.0767
|
54
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
_files/image008.png)
|
_files/image009.png)
|
|
Figure 4: Evolution of (a) lattice
parameter and (b) Strain and crystallite size as function of CBD temperature.
|
Optical properties : Photoluminescence (PL) analysis Error! Reference source not found.5 demonstrates the PL measurements for ZnO NRs grown onto Si
substrates at different temperatures.
As can be seen, all PL spectra manifest
two peaks corresponding to ZnO emission bands. The dominant peak was observed to be
located at about 378 nm which was
attributed to the near-band
edge (NBE) emission of ZnO energy gap. This emission is because of the
recombination of free excitons (Wang, 2012). The broad peak observed in the 460–700 nm
range (visible region) is
attributed to the deep-level emissions (DLE) of ZnO. Interestingly, the latter is the most detected
emission in ZnO nanostructures. It is ascribed to the structural defects in ZnO
crystals such as oxygen vacancy, zinc vacancies, and other associated
structural defects (Hu et
al., 2010). When UV light illuminates
ZnO NRs, vacancies become
prone to the holes generated upon the exposure of UV. Therefore, photons could
be emitted in the visible region due to the electron-hole recombination at the
oxygen vacancies (Selman et
al., 2014). Further,
no considerable shift is observed in the NBE peak position with the increase of
the crystallite size due to grain growth accompanying the rise of CBD
temperature. However, a slight
red shift was detected
for the film prepared at 100 oC. Notably, peaks’ intensity is found to enhance as
the temperature increased from 85 to 100oC. This agrees with the XRD analysis, where the
crystallinity is enhanced, and the crystallite size increases as the
temperature in the CBD rises.
|
|
_files/image010.png) |
Figure 5: PL spectra as a faction of temperature for
ZnO NRs grown on Si substrates
Table
3 summarizes the optical analyses for ZnO NRs grown at different temperatures.
Table 3: PL spectra for the prepared ZnO NRs on Si
substrates at different temperatures.
|
Temperature
(ºC)
|
NBE Position (nm)
|
Band gap (eV)
|
FWHM
(nm)
|
(NBE/DLE) Ratio
|
|
85
|
381.13
|
3.25
|
17.93
|
0.970
|
|
90
|
379.80
|
3.26
|
13.77
|
1.772
|
|
95
|
379.80
|
3.26
|
13.94
|
2.521
|
|
100
|
381.64
|
3.25
|
15.66
|
2.293
|
As shown in Table 3, the NBE/DLE ratio increased as the growth temperature increased and attained
a maximum value of 95oC. The increase in
NBE/DLE ratio indicates that
the ZnO NR structure possessed a high-crystal quality with minimum structural
defects (Fan et al., 2010; Husham et al., 2015). The enhancement of nanostructure optical
characteristics can be attributed to the high intensity of the UV emission peak
combined with a minimal density of defect in the PL spectra (Hassan et al., 2012).
4.
CONCLUSIONS
ZnO NRs
were successfully grown onto Si substrates by a simple cost-effective CBD
technique with fine tuning of bath temperature. XRD measurements showed a well
crystalline ZnO NRs. Structural and microstructural parameters were found
sensitive to the CBD temperature. Besides, surface morphology analyses for the
grown ZnO NRs showed well aligned ZnO NRs grown perpendicularly to the substrates.
Results showed that the optical and structural properties remarkably improved with
the rise of the growth temperature. At a growth temperature of 95°C, the crystallite
size, the aspect ratio, and the strain reached the optimum values, consequently
leading to enhanced optical properties. Indeed, the ratio of NBE/DLE improved
as revealed by PL analysis. Analyses showed that the properties of ZnO NRs can
be modified by a fine control of preparation temperature. Additionally, the report
presented a simple method to prepare high crystalline ZnO NRs grown on Si substrates
for important silicon-based applications.
ACKNOWLEDGMENTS
The
authors sincerely appreciate the support from Universiti Sains Malaysia (USM) for
their assistance this work under Grant number 304/PFIZIK/6313095. The author
would like to thank Prince Sultan University for their support.
REFERENCES
Ahmed F.A., Sabah M. A., Azeez
A. B., Samir M. H., Naser M. A., Munirah A. A. (2021). Fabrication and Characterization of
High-Quality UV Photodetectors Based ZnO Nanorods Using Traditional and
Modified Chemical Bath Deposition Methods. Nanomaterials 11, 677. https://doi.org/10.3390/nano11030677
Altissimo,
M. (2010). E-Beam lithography for micro-/nanofabrication. Bio
microfluidics, 4, 026503. https://doi.org/10.1063/1.3437589
Bouhssira N., Abed S., Tomasella
E., Cellier J., Mosbah A., Aida M. S., & Jacquet M. (2006). Influence of
annealing temperature on the properties of ZnO thin films deposited by thermal
evaporation. Applied Surface Science, 252, 5594-5597.https://doi.org/10.1016/j.egypro.2012.07.008
Fan D.,
Zhang R., & Wang X. (2010). Synthesis and ultraviolet emission of aligned
ZnO rod-on-rod nanostructures. Solid state communications, 150, 824-827.
https://doi.org/10.1016/j.ssc.2010.02.013
Farhat O. F., Husham M.,
Bououdina M., Abuelsamen A. A., Oglat A. A., & Mohammed N. J. (2021).
Tape-based novel ZnO nanoaggregates photodetector. Sensors and Actuators A:
Physical, 332, 113210. https://doi.org/10.1016/j.sna.2021.113210
Farhat
O. F., Hisham M., Bououdina M., Oglat A. A., & Mohammed N. J. (2020).
Growth of ZnO nanostructures by wet oxidation of Zn thin film deposited on
heat-resistant flexible substrates at low temperature. Semiconductors, 54,
1220-1223. https://doi.org/10.1134/S1063782620100103
Farhat
O. F., Husham M., ALDelfi H. H., & Bououdina M. (2023). Tuning the diameter
and optical properties of ZnO nanorods grown onto flexible substrates at
different temperatures. Journal of Crystal Growth, 607, 127115. https://doi.org/10.1016/j.jcrysgro.2023.127115
Farhat
O. F., Halim M. M., Ahmed N. M., Oglat A. A., Abuelsamen A. A., Bououdina M.,
& Qaeed M. A. (2017). A study of the effects of aligned vertically growth
time on ZnO nanorods deposited for the first time on Teflon substrate. Applied
Surface Science, 426, 906-912. https://doi.org/10.1016/j.apsusc.2017.07.031
Feng
W., Wang B., Huang P., Wang X., Yu J., & Wang C. (2016). Wet chemistry
synthesis of ZnO crystals with hexamethylenetetramine (HMTA): Understanding the
role of HMTA in the formation of ZnO crystals. Materials Science in
Semiconductor Processing, 41, 462-469. https://doi.org/10.1016/j.mssp.2015.10.017
Gu P., Zhu
X., & Yang D. (2020). Vertically aligned ZnO nanorods arrays grown by
chemical bath deposition for ultraviolet photodetectors with high response
performance. Journal of Alloys and Compounds, 815, 152346. https://doi.org/10.1016/j.jallcom.2019.152346
Guo L.,
Zhang H., Zhao D., Li B., Zhang Z., Jiang M., & Shen D. (2012). High
responsivity ZnO nanowires-based UV detector fabricated by the
dielectrophoresis method. Sensors and Actuators B: Chemical, 166, 12-16.
https://doi.org/10.1016/j.snb.2011.08.049
Hu A., Wu
F., Liu J., Jiang J., Ding R., Li X. & Huang X. (2010). Density-and adhesion-controlled
ZnO nanorod arrays on the ITO flexible substrates and their electrochromic
performance. Journal of Alloys and Compounds, 507, 261-266. https://doi.org/10.1016/j.jallcom.2010.07.173
Hassan
J.J., Mahdi M. A., Kasim,S. J., Ahmed N. M., Abu Hassan H., & Hassan Z.
(2012). High sensitivity and fast response and recovery times in a ZnO nanorod
array/p-Si self-powered ultraviolet detector. Applied Physics Letters, 101(26).
https://doi.org/10.1063/1.4773245
Husham M.,
& Hassan Z. (2015). Synthesis of nanocrystalline CdS thin films via
microwave-assisted chemical bath deposition for highly photosensitive and rapid
response photodetectors. Journal of Nanoelectronics and Optoelectronics,10,783-789.
https://doi.org/10.1016/j.sna.2015.04.010
Husham M.,
Hassan Z., Selman A.M., Allam N.K. (2015). Microwave-assisted chemical bath
deposition of nanocrystalline CdS thin films with superior photodetection
characteristics, Sensors and Actuators A 230, 9–16. https://doi.org/10.1016/j.sna.2015.04.010
Jaqsi
M.K., Kareem A., & Abdullrahman A. (2023). Investigating the impact of growth
temperatures on the ZnO nanorods properties grown with simplest spray technique. Scientific journal of the University of
Zakho, 11,112–118. https://doi.org/10.25271/sjuoz.2023.11.1.1072
Lim Y.H.,
Choi H-J, & Lee J. (2022). Fabrication of
nanostructures on a large-area substrate with a minimized stitch error using
the step-and-repeat nanoimprint process.Materials, 15,6036. https://doi.org/10.3390/ma15176036
Mosalagae
K., Murape D. M., & Lepodise L. M. (2020). Effects of growth conditions on
properties of CBD synthesized ZnO nanorods grown on ultrasonic spray pyrolysis
deposited ZnO seed layers. Heliyon, 6, e04458. https://doi.org/10.1016/j.heliyon.2020.e04458
Mohammed H. K., Raghad Y. M., Mohammed A. I. (2023).
Influence of annealing temperature on Zinc Oxysulfide thin films properties
deposited by thermal spray technique. Scientific journal of the University of
Zakho, 11,600-605. https://doi.org/10.25271/sjuoz.2023.11.4.1192
Selman A.
M., Hassan Z., & Husham M. (2014). Structural and photoluminescence studies
of rutile TiO2 nanorods prepared by chemical bath deposition method
on Si substrates at different pH values. Measurement, 56, 155-162. https://doi.org/10.1016/j.measurement.2014.06.027
Sherwan M.I.,
Sabah M.A. (2023). The effect of calcination temperature on the properties of
ZnO Nanoparticles synthesized by using leaves extacts of Pinus Brutia tree. Scientific
journal of the University of Zakho, 11, 286–297. https://doi.org/10.25271/sjuoz.2023.11.2.1087
Woong
L., Jeong M. C., & Myoung J. M. (2004). Catalyst-free growth of ZnO
nanowires by metal-organic chemical vapour deposition (MOCVD) and thermal
evaporation. Acta Materialia, 52, 3949-3957. https://doi.org/10.1016/j.actamat.2004.05.010
Zhang
Z., Wang S. J., Yu T., & Wu T. (2007). Controlling the growth mechanism of
ZnO nanowires by selecting catalysts. The Journal of Physical Chemistry C, 111,
17500-17505. https://pubs.acs.org/doi/10.1021/jp075296a
_files/image004.png)
_files/image005.png)