Synthesis of MgO Nano-Cubes on Si (001) by Thermal Evaporation
Siti Nuurul Fatimah Hasim1, Muhammad Azmi Abdul Hamid*1, Roslinda Shamsudin1 &
Azman Jalar2
1
School of Applied Physics, Faculty of Sciences and Technology,
Institute of Microengineering and Nanoelectronics,
Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor D. E., Malaysia.
*Corresponding author: e-mail: azmi@ukm.my ,Phone: +(03) 89253404, Fax: +(03) 8925 2415
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ABSTRACT
MgO nanocubes were successfully deposited on Si (001) substrates using a thermal evaporation
method at 800ºC, under the flow of different percentages of argon and oxygen gases. The assynthesized products were characterized by X-ray diffraction (XRD), variable pressure
scanning electron microscopy (VPSEM), energy dispersive X-Ray spectroscopy (EDS) and
UV-Visible spectroscopy. The aim of this paper is to study the influence of the oxygen
percentage on the structural and morphological properties of MgO nanocubes. VPSEM results
show that very thick sharp-edged three dimensional cubes of MgO were produced at high
oxygen percentages. EDS results revealed that only Mg and O are present in the sample,
indicating a composition of pure MgO. The sharp peaks in the XRD pattern indicated high
crystallinity with a face centered-cubic MgO structure. UV-Visible spectroscopy results
indicated that the optical band gap energies from the transmission spectrum are between 5.056
eV and 5.080 eV for the MgO.
Keywords: Diffraction; electron microscopy; evaporation; magnesium oxide
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1.
INTRODUCTION
Research on metal oxide structures on metal substrates has received considerable
attention of late due to their important applications in both catalysis and microelectronics [1, 2].
Among the metal oxides studied, magnesium oxide (MgO), in particular, has received a large
amount of attention. MgO is a typical wide band gap (7.8 eV) insulator. Its electronic and
optical properties are very attractive because its low heat capacity and high melting point make
it an ideal candidate for insulation applications [3]. MgO nanostructures have also been used as
protective layers for dielectrics in AC circuits to improve discharge characteristics and panel
lifetime as a result of their anti-sputtering properties, high transmittance, and secondary
electron emission coefficient [4].
Nowadays, various morphologies of MgO nanostructures such as nanorods, nanowires,
fishbone and nanobelts [5-7], have been successfully synthesized by various methods, such as
chemical vapor deposition (CVD) [8], domestic microwave oven [9], carbothermal reduction
[10], sol-gel [11], dual magnetron sputtering [12], hydrothermal synthesis [13] and thermal
evaporation [7, 14 -16]. However, thermal evaporation was favored among researchers due to it
being an economical and simple method. Kim et al. successfully obtained MgO nanostructures
by evaporating MgB2 powders at 900ºC, in which MgO nanowires grown on Au-coated
substrates. Yang et al. also obtained MgO nanostructures with various morphologies such as
nanocone, nanotubes, nanowires and nanocubes by thermal evaporation method at 900ºC.
Meanwhile, MgO nanostructures like nanoflowers and nanotrees have been achieved
successfully by Hao et al. through investigating products at different oxygen partial pressures.
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Previous researchers have reported the use of experimental setups for the production of MgO
nanostructures which was similar to our work [1, 2, 7]. However, only a little information was
reported regarding the effect of oxygen concentration on the morphology of MgO thin films
and nanostructures. Under these circumstances, it is worthwhile to conduct this investigation.
2.
EXPERIMENTAL
The synthesis of MgO nanocubes was performed in a horizontal tube furnace (as shown
in Fig. 1) under atmospheric pressure, with similar setup reported elsewhere [1, 2, 7]. Metal Mg
powders (1g, 200 mesh) and the Si substrate (15 mm x 15 mm), separated by 10 mm in ceramic
boat, were loaded into the center of the horizontal alumina tube in the furnace. After that, the
alumina tube was purged with argon gas. Then, the furnace was heated to 800ºC at a rate of
6ºC/min under a nitrogen flow to create an inert environment. During evaporation, the
temperature inside the alumina tube was measured by a sheathed thermocouple which was
inserted into the tube and could be moved freely along the tube axis. When the temperature
reached 800ºC, a mixture of oxygen and argon (ratio 1: 4) gas was flowed into the alumina
tube. The flow was introduced in the tube with a fixed rate of 20 ml/min. The furnace
temperature was kept constant for 30 minutes. Finally, the power was switched off and the
furnace was allowed to cool down to room temperature under an argon flow at a rate of
6ºC/min. After the samples were taken out of the furnace, some white substance layer was
observed on the surface of the Si substrate.
The morphology of the synthesized products was examined by variable pressure
scanning electron microscopy (VPSEM) using a LEO 1450 microscope. The chemical
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composition and crystal structure were examined by energy dispersive X-Ray spectroscopy
(EDS) and X-ray diffraction (XRD) SIEMEN D-5000 with Cu Kα radiation, respectively. The
optical band gap energy of the samples was determined with an UV-Visible Spectrophotometer
Lambda 900 in the wavelength 250 nm to 800 nm at room temperature using ethanol as a
solvent. The samples were diluted with pure ethanol to obtain absorbance readings on an
appropriate scale.
3.
RESULTS AND DISCUSSION
Figure 2 illustrates the X-ray diffraction pattern of the sample recorded using Cu Kα
radiation (0.154 nm). The pattern was recorded between 20º to 60º at a grazing angle. All peaks of
the sample can be well indexed to the face centered-cubic MgO structure with a lattice constant of
a = 0.421 nm, which is consistent with that of bulk MgO according to JCPDS Card No. 04-0829.
The sharp diffraction patterns indicated that the structure possessed good crystallinity. No
characteristic peak of impurities was detected in the pattern, indicating the high purity of the
obtained product.
VPSEM micrographs of the synthesized product prepared under different percentages of
oxygen are shown in Fig. 3(a-f). It can be seen that a different morphologies of MgO structures
were grown on the silicon substrate. Fig. 3(a) shows that MgO nanocubes formed over the
substrate with the diameter about 20-50 nm when 1.6% O2 was flowed. Fig. 3(b) shows MgO
nanocubes structures with the diameter about 50-80 nm in the presence of 2% O2. In the presence
of 4% O2, non-uniform MgO nanocubes with diameters in the range of 60-100 nm were formed
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over the substrate (Fig. 3c). Fig. 3(d) show non-uniform MgO nanocubes structures with diameters
about 80-190 nm formed in the presence of 6% O2. It shows bigger diameters compared to 4% O2.
MgO nanocubes with diameters in the range of 160-190 nm were formed in the presence of
8% O2 (Fig. 3e). At this concentration, some portion of MgO has not completely formed cube
structures due to agglomeration. Fig. 3(f) shows three-dimensional cube-shaped MgO with rather
sharp edges and diameters about 200-400 nm were observed when oxygen increased to 10%. These
VPSEM results indicate that the average width or diameter of the structures changed with oxygen
percentage in the range of 1.6 – 10 %.
MgO structures using 1.6% oxygen (Fig. 3a) revealed smaller in diameters compared to
10% oxygen (Fig. 3f). The variation of the width and diameter with the oxygen content indicates
that oxygen gas plays an important role in controlling the nucleation and growth of MgO
nanostructures. Higher oxygen percentages provide additional oxygen that may facilitate a high
degree of supersaturation of MgO in its gaseous state, resulting in wider or thicker structures. This
finding is in good agreement with the relationship of gas-phase supersaturation with the final
morphology of the nanostructures as suggested by previous researchers [17, 18].
To understand the growth of a nanostructure, we propose a possible mechanism that may
occur in nanocubes growth. Growth mechanism of the MgO nanocubes might be explained by
means of combination of the growth temperature and the supersaturation ratio in the kinetics of
crystal growth. MgO has a face-centered cubic crystal [7], possessing relatively isotropic structure
in nature. The formation of anisotropic MgO nanostructures is found to be steered by defects, such
as edges, steps and kinks, which guide the axial growth. Meanwhile, supersaturation ratios in the
reaction systems also make a remarkable effect on the final structures. According to Sears [19] the
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growth of various nanostructures depends principally on an appropriate supersaturation ratio and
the existence of high surface energy sites.
The Mg vapor is assumed to be vaporized into molecular species at high temperature, and
the molecules are composed of stoichiometric cation-anion coordination is preserved to balance the
local charge and structural symmetry, forming a small nucleus. The chemical reaction involved can
be expressed as:
2Mg (vapor) + O2 (gas) = 2MgO (solid) [1, 2]
At the minimal oxygen partial pressure, namely, the lowest supersaturation ratio, reactant
atoms adsorbed on MgO nucleus of nearly cubic shapes (owing to the lowest surface energy of
{100} facets) could find energetically favorable sites after sufficient surface diffusion [16]. Newly
arriving molecules will continue to deposit on the formed nucleus, while the surfaces that have
lower energy start to form, such as the side surfaces. Because the growth temperature is high
(800ºC), the mobility of the molecules is high enough that the low-energy surfaces tend to be flat,
thus preventing the accumulation of newly arriving molecules on the surface, resulting in its
expansion in surface area as more molecules stick onto the rough growth front. The rough structure
leads to a rapid accumulation of incoming molecules, resulting in the fast formation of perfect
single-crystalline nanocubes, suggesting the crystallographic symmetry of MgO. An atom adsorbed
on a low-energy surface has low binding energy and a high probability of desorption [20].
Competition between the supersaturation and surface energy is responsible for the formation of the
low-index crystal planes enclosing the nanostructure. The temperature and the supersaturation ratio
are two parameters controlled by processing conditions. Higher temperature and larger
supersaturation ratio facilitate the nucleation, resulting in the formation of the cube-like structures.
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Energy dispersive X-ray spectroscopy (EDS) analysis as shown in Fig. 4 was carried out to
determine the chemical composition of the grown products. The results indicated that the
nanocubes were composed of Mg and O with an atomic ratio of approximately 1:1, which is in
accord with the stoichiometric ratio of MgO. The Au signal originated from the gold coating used
for VPSEM characterization.
Fig. 5 shows the UV-visible absorption spectrum in the wavelength range from 260 nm to
780 nm of MgO nanocubes synthesized by thermal evaporation at 800ºC. Relatively clear
absorption edges of the MgO nanocubes were observed clearly. The fundamental absorption,
which corresponds to the transition from valence band to conduction band, can be used to
determine the band gap of the material. The optical band gap Eg was deduced from the absorption
spectrum using the relation [21],


A h  E g
h
n
,
where α is the absorption coefficient, A is a constant, Eg is the band gap of the material, and the
exponent n depends on the type of transition. Here, n may have values of 1/2, 2, 3/2 and 3
corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect
transitions, respectively. The value of the optical gap is calculated by extrapolating the straight line
portion of the (αhν) 1/n vs. hν graph to the hν axis taking n = 1/2 [22].
Fig. 6 shows the plots of (αhν) 2 vs. hν for MgO nanocubes deposited at different
percentages of oxygen. The correct values of the optical gap calculated from the figure are
(5.080 ± 0.01) eV, (5.076 ± 0.01) eV, (5.064 ± 0.01) eV, (5.063 ± 0.01) eV, (5.052 ± 0.01)
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eV and (5.056 ± 0.01) eV for the MgO nanocubes structures deposited at 1.6%, 2%, 4%,
6%, 8% and 10%, oxygen, respectively. The values of the optical band gap was found to
decrease with increasing oxygen percentages. Clearly, the observed value of Eg is relatively
lower than those of MgO single crystals (7.8 eV). Such a lower band gap has also been
reported by Raj and his co-workers [23], which may be due to varied extent of nonstoichiometry of the deposited structures. In our work, high temperature (800ºC)
evaporation process may generate various structural defects, e.g. oxygen vacancies because
of partially incomplete oxidation and crystalline. These oxygen vacancies would induce the
formation of new energy levels in the bandgap of the MgO nanostructures and leading to a
smaller band gap compared to bulk. The lower band gap also caused of nanosize effect,
density of the defects and the short-range repulsion energy between atoms [23].
4.
CONCLUSION
In summary, MgO nanocube structures have been successfully prepared using the
thermal evaporation method at 800°C under different percentages of oxygen. The resulting
MgO structures are highly crystalline. MgO nanocube structures grown with an oxygen
percentage of 1.6% revealed smaller diameters compared to those grown with 10% oxygen.
Larger diameter of MgO nanocubes was due to supersaturation. From the analyzed results,
it was proved that oxygen percentage is the important factor in controlling the
microstructure of MgO. The band gap value of 1.6% oxygen MgO prepared sample was the
widest obtained at 5.080 eV in comparison to other oxygen percentage environment.
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Acknowledgments
The authors would like to thank the Universiti Kebangsaan Malaysia for facilities and financial
assistance under UKM-OUP-NBT-27-121-2010
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Figures
Fig. 1: Schematic diagram of the experimental setup for the growth of MgO structures on
silicon substrates.
Fig. 2: XRD pattern of MgO nanocubes. The indexed peaks correspond to those of the
typical face centered-cubic structure for MgO nanocubes.
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(a)
(c)
(e)
(b)
(d)
(f)
Fig. 3: VPSEM micrographs of MgO nanocubes deposited in the presence of: a) 1.6% O2,
98.4% Ar, b) 2% O2, 98% Ar, c) 4% O2, 96% Ar, d) 6% O2, 94% Ar, e) 8% O2, 92% Ar,
f) 10% O2, 90% Ar.
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Fig. 4: A typical energy dispersive X-ray spectroscopy (EDS) spectrum of the MgO
nanocubes.
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Absorption (Abs)
Wavelength (nm)
Fig. 5: Optical absorption spectrum (Abs) of MgO nanocubes deposited in different
percentages of oxygen: (a) 10%, (b) 8%, (c) 6%, (d) 4%, (e) 2% dan (f) 1.6%
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(αhν x 105 , eV.cm-1)½
Photon energy (eV)
Fig. 6: (αhν)2 versus photon energy (hν) of MgO nanocubes deposited in different percentages of
oxygen.
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