Growth of Al2O3 nanowires on the Cu-9 at.%Al(111) single crystal surface

Article abstract of DOI:10.1111/j.1551-2916.2011.04704.x

Aluminum oxide structures were grown on Cu-9 at.%Al(111) single crystals. Usually alumina grows in the form of an epitaxial thin film. The process is based on Al atom segregation and subsequent oxidation of the Al topmost surface. Strong temperature depen

Full text of DOI:10.1111/j.1551-2916.2011.04704.x

J. Am. Ceram. Soc., 94 [11] 4084–4088 (2011)
DOI: 10.1111/j.1551-2916.2011.04704.x
©
2011 The American Ceramic Society
ournal
J
Growth of Al O Nanowires on the Cu-9 at.%Al(111) Single Crystal Surface
2
3
‡,†
§
‡
‡
¶
Iva Matolı
´
nova
´
,
Slavomı
´
r Nems
ˇ
a
´
k, Milos
ˇ
Cabala, Nataliya Tsud, Jana Poltierova-Vejpravova,
ˇ ´ ´
§
‡
Katerina Veltruska, Michiko Yoshitake, and Vladimır Matolın
‡
´
´
´
‡
Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University in Prague,
8000, Prague, Czech Republic
1
§
Advanced Electronic Materials Center, National Institute for Materials Science,
Tsukuba, Ibaraki, 305-0003, Japan
¶
Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague,
2116, Prague, Czech Republic
1
Aluminum oxide structures were grown on Cu–9 at.%Al(111)
single crystals. Usually alumina grows in the form of an epi-
taxial thin film. The process is based on Al atom segregation
and subsequent oxidation of the Al topmost surface. Strong
temperature dependence of the alumina growth mode was
observed during the oxidation of a Cu–9 at.%Al single crystal.
In this work we have reported for the first time the formation
of alumina wire-like nanostructures on the substrate surface
during high-temperature oxidation at low pressure. The nano-
wire growth was observed during substrate annealing at
to prepare γ-Al O nanofibers. Alumina nanoribbons and
2 3
nanorings were prepared via high temperature route using
aluminum foil and SiO as a precursor.
1
1
Several groups have been interested in the formation of
alumina during high temperature oxidation processes of alu-
1
2–14
mina-forming alloys.
Studies of thin alumina film growth
have pointed to the formation of γ- and
layers on NiAl(001) and NiAl(110) single crystals.
By oxidizing NiAl surfaces vicinal to the (100) plane tilted
1
3–15
and structures
h-Al
2
O
3
1
6
along crystallographic direction [010], Luo et al. have pre-
pared h-Al strips along [001] direction. The formation of
the ultra-thin well-ordered stoichiometric γ-Al layer on
the Cu–9 at.%Al(111) single crystal was reported with an
1
070 K while annealing at 910 K led to the formation of a flat
2 3
O
and continuous thin alumina layer. Morphology, chemical com-
position, and structure characterization of the prepared nano-
structures was done by means of scanning electron microscopy,
energy-dispersive X-ray spectroscopy, X-ray photoelectron
spectroscopy, and X-ray diffraction. The experimental results
point to the formation of Θ alumina phase nanowires due to
surface segregation of Al atoms. Mechanism of alumina nano-
structure formation is discussed. The experiment clearly
showed that nanowire oxide structure growth can be obtained
by a bottom-up process with a mass transfer from the substrate
to the assembled nanostructure.
2 3
O
1
4,17–19
optimal oxidation procedure by Yoshitake et al.
dosing of oxygen at 910 K the well-ordered γ-Al
(7/√3 9 7/√3)R30° structure and thickness up to 3–4 nm has
. By
2
O film of
3
2
0
been obtained. A thin alumina film grown by Varga et al.
exhibited a different structure due to lower preparation
temperature.
In this article, we report strong structural dependence of
alumina growth on Cu–9 at.%Al(111) on oxygen exposure
and temperature. For the first time we show the low pressure
growth of alumina nanowires based on aluminum diffusion
from the substrate.
I. Introduction
LUMINUM oxide has been extensively studied in recent
years due to its wide potential of applications in elec-
II. Experimental Procedure
A
devices.
tronics, catalysis, sensors, fuel cells, and data storage
Since the beginning of the electronic industry, the
A Cu–9 at.%Al(111) single crystal was introduced into the
chamber equipped with an X-ray photoelectron spectroscopy
subsystem (XPS). The sample was first cleaned by cycles of
1–3
trend of reducing the size, cost, and power consumption is
permanently present, and nanostructured alumina has there-
fore attracted much attention. Recently, alumina nanostruc-
tures have been prepared by various methods, e.g. by
chemical etching of highly ordered porous anodic aluminum
+
Ar sputtering at room temperature and annealing at 770 K
1
7
until no trace of impurity was detected. Then the sample
was oxidized at different temperatures under a partial oxygen
pressure of 8 9 10 Pa.
À6
4
–6
oxide membranes,
boehmite (AlOOH) or pseudo boehmite synthesized by
by calcination of nanocrystalline
Chemical composition and cleanness of the sample surface
was checked by XPS. XPS measurements were performed with
7
,8
sol–gel, hydrothermal or solvothermal methods.
et al. synthesized mezoporous alumina nanostructures with
Gan
a
conventional X-ray source using the AlKa line
9
(hm = 1486.6 eV) and a multichannel energy analyzer (SPECS
Phoibos MDC 9) at a total resolution DE = 1 eV. All XPS
experiments were performed in situ in an ultra-high vacuum
tunable morphologies through the template-free solvothermal
approach. Liu et al. applied chemical precipitation method
10
À8
experimental chamber operating at base pressures < 10 Pa.
Surface morphology was observed ex-situ by means of
scanning electron microscopy (SEM, TESCAN – MIRA) at
electron beam energy of 15–30 keV. The microscope was
equipped with a detector for energy dispersive X-ray spec-
troscopy (EDX, Bruker XFlash 4010).
For structure characterization the X-ray diffraction (XRD)
patterns were recorded on a diffractometer (X’pert Pro Mate-
rial Research Diffractometer) using monochromatized CuKa
source (k = 1.54056 Ǻ) at 3° incidence angle.
W. Mullins—contributing editor
Manuscript No. 28607. Received September 14, 2010; approved May 20, 2011.
This work was supported by the Ministry of Education of the Czech Republic,
under the Research Programs No. MSM 0021620834 and No. LC08056, and by the
Czech Grant Agency, under Grant No. 202-09-H041.
†
Author to whom correspondence should be addressed. e-mail: imatol@mbox.troja.
mff.cuni.cz
4084

November 2011
III. Results and Discussion
Growth of Al O Nanowires
2
4085
3
Therefore, we consider the nanostructures to be composed of
aluminum oxide.
Chemical composition of the sample was further investi-
gated by XPS. The O 1s spectrum of the oxidized Cu–9 at.%
Al(111) sample with nanostructures on the surface marked
Annealing of the Cu–9 at.%Al(111) single crystal in oxygen
atmosphere at 910 K leads to the formation of a continuous
1
4,17–19
single-crystalline alumina thin film.
In Fig. 1 we pres-
ent the result of such process after 9 h and 16 min of anneal-
ing corresponding to a total oxygen exposure of 2000 L.
SEM image shows residual holes in the alumina film. It
points out that the oxidation process was stopped before the
completion of the alumina film which is grown in Frank–
“
nanowires” is shown in Fig. 6(a). For comparison, we
added the O 1s spectrum acquired from the reference oxide
layer presented in Fig. 1. The O 1s reference peak intensity
was ~10 times higher and, for better clarity, we divided its
intensity in Fig. 6(a) by a factor of 10. The spectrum (see
dashed line) is characterized by a peak at 532.3 eV corre-
2
1
Van der Merwe mode. Hexagonal symmetry of hole bor-
ders, highlighted with bright edges in Fig. 1 due to the edge
effect (more secondary electrons escape from areas with
23
sponding to oxygen from aluminum oxide. Because of
small thickness of the oxide film, no charging effect was
observed. The O 1s spectrum of the sample with nanowires is
composed of two components. The main O 1s peak appears
at an unusually high binding energy of 536.1 eV, which can
be explained by O 1s photoemission from charged alumina
2
2
sharp edges than from flat ones), reflects the hexagonal
symmetry of the substrate. Holey character of hexagonal
structures was confirmed by SEM observations of hole
borders at higher magnifications (not shown).
By increasing the annealing temperature we obtained
completely different mode of aluminum oxide growth charac-
terized by the formation of nanowires. After heating of the
(
by 3.8 eV) due to worse charge transfer to the substrate.
The photoemission intensity at 532.1 eV corresponds to
charge-free aluminum oxide ultra-thin film covering the
Cu–9 at.%Al(111) single crystal at 1070 K for 18 h 30 min
À6
(
i.e. oxygen exposure of 4000 L at 8 9 10 Pa) the surface
was covered by wire-like structures as can be seen in Fig. 2.
A detailed view of typical nanowires presented in Fig. 3
reveals a predominantly bent shape; the wires are 2–5 lm
long and 50–200 nm thick. However, some parts of the sam-
ple exhibit well-shaped straight whisker-like structures, as
can be seen in Fig. 4(a), suggesting single-crystalline growth.
It seems that this is due to overheating of these parts of the
Cu–9 at.%Al(111) single crystal. Figure 5 presents a side
view showing wire bases. The nanowires are ended with
thicker termination at the Cu–9 at.%Al alloy substrate that
was observed for most the structures.
The chemical analysis was done by performing EDX that
provided a quantitative estimation of the concentration ratios
of elements present in the sample. X-ray emission spectra
were analyzed after subtracting a contribution from the
bremsstrahlung background. Spectra were recorded at three
points, at the wire base (point 1), at the central part, (point 2)
and at the Cu–Al alloy surface out of the nanostructures
(
point 3), as can be seen in Fig. 3. Results of EDX point anal-
ysis are summarized in Table I. One can see that the nano-
structure contains higher concentration of aluminum and
oxygen than the substrate; the highest content of aluminum
was detected at the wire root. The EDX element map of the
bottom part of the crystalline nanowire [Fig. 4(a)] presented
in Fig. 4(b) confirms the distribution of Cu, Al, and O atoms.
Fig. 2. SEM image of the Cu–9 at.%Al(111) surface covered by
alumina nanowires. The image was acquired in combination with
backscattered-electron (BSE) and secondary-electron modes (SE) to
obtain atomic-number contrast together with topographical contrast
and shadowing effects.
Fig. 1. SEM image of the holey structure of the alumina film
grown on Cu–9 at.%Al(111). To display topographical contrast the
secondary-electron image was acquired. It results in appearing as a
bright outline of the holes due to the edge effect.
Fig. 3. Detailed view of alumina nanowires. The image was
acquired in secondary-electron mode (SE). In centers of the marked
red circles EDX point analysis was done, the results are presented in
Table I.

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Journal of the American Ceramic Society—Matolı ´n ova´ et al.
Vol. 94, No. 11
(
a)
(
b)
Fig. 5. Side view of wire bottom terminations. The image was
acquired in secondary-electron mode (SE).
(
a)
Fig. 4. Secondary-electron image of well-shaped straight whisker-
like structures (a). EDX element map of the bottom part of the
structure showing distribution of Al, O, and Cu atoms (b).
(
b)
Table I. EDX Point Analysis from Points Indicated in Fig. 3
Point
Cu (at.%)
Al (at.%)
O (at.%)
1
2
3
43.8
54.4
67.0
20.2
14.9
8.2
36.0
30.7
24.8
substrate crystal surface similar to the case of the reference
oxide layer. It showed clearly that higher oxidation tempera-
ture did not lead to a continuous oxide formation because O
1s intensity of the non-shifted peak was smaller by two
orders of magnitude relative to the reference sample.
In Fig. 6(b) we compared the Cu 3p + Al 2p spectra of the
clean non-oxidized substrate (bottom spectrum) and the oxi-
dized samples‐the surface covered by the nanowires (middle
spectrum) and the reference alumina layer (upper spectrum).
Main peaks at 75.2 and 77.6 eV in Fig. 6(b) correspond to
Cu 3p3/2 and Cu 3p1/2 photoemission. Shoulder at 72.9 eV
corresponds to the non-resolved Al 2p states from the Cu–Al
Fig. 6. XPS spectra of the nanowires and the reference alumina
layer covering the Cu–9 at.%Al(111) surface: O 1s (a), Cu 3p + Al
2p (b).
2
4
alloy. Al 2p state of the reference aluminum oxide appears
24
at 75.0 eV. In the case of the nanowires, new state appears
at 78.8 eV showing again a charge-induced shift of the peak
of oxidized aluminum by 3.8 eV, approximately. However, it
is overlapped by more intense Cu 3p feature.
To understand better the nanowire structure, X-ray dif-
fraction (XRD) analysis was performed. Figure 7 shows the
XRD pattern in the 2Θ range from 20 to 70° that was identi-
86-1410). The mean crystallite size of h-Al
2
O
3
was estimated
2
5
by using the Sherrer’s formula
(crystallite size = 0.9k/
B cos h, where k = 1.54056 Ǻ, B = FWHM in radians, and
h = the Bragg angle). The (202) and (111) reflection peaks
were applied. The size dimensions were 55–65 nm.
The SEM, EDX, XPS, and XRD experiments showed that
alumina nanowires can be formed by heating the Cu–9 at.%
2 3
fied as the reflections of monoclinic h-Al O (JCPDS #

November 2011
Growth of Al O Nanowires
2
4087
3
hot surface. The difference between growth of continuous
films and nanowires is in substrate annealing temperature.
Higher temperature assures higher aluminum atom mobility
2
7,28
and lower sticking probability of oxygen at the surface,
i.e. the conditions less favoring the aluminum oxide thin film
formation. It results probably in formation of larger alumi-
num nuclei, which are oxidized with higher probability than
single Al atoms interacting with the Cu surface. The hypoth-
esis is supported by Fig. 8 where a SEM side view and ele-
ment maps of aluminum rich surface region together with
nanowire base formation are presented. It is likely that Al
atoms are diffusing from Al rich bottom over the nanowire
surface toward its end where Al is oxidized due to higher
impinging flux of oxygen because of higher accessibility of a
small 3-dimensional structure to adsorption from the gas
phase. This process results in formation of Al gradient favor-
ing subsequent diffusion of Al atoms from bottom up to the
nanowire termination.
Fig. 7. XRD pattern of the alumina nanowires.
IV. Conclusion
In previous studies the formation of ultra-thin monocrystalline
alumina layer with the perfect flatness on single crystal
Cu–9 at.%Al(111) surface in low pressure of oxygen was
reported. In this work we present for the first time a low pres-
sure growth of aluminum oxide nanowires from the Al-con-
taining alloy Cu–9 at.%Al(111). The segregated Al atoms
diffusing on the surface form alumina nanostructures at higher
annealing temperature compared to the condition of the alu-
mina film preparation. At higher temperature, high mobility
of Al atoms and low sticking probability of oxygen result in
growth conditions which are not more favored to growth of
continuous alumina films. The increase of annealing tempera-
ture leads also to changes in alumina crystallographic struc-
ture. It clearly shows that a variety of alumina forms can be
prepared controlling the annealing time and the substrate tem-
perature: from the continuous well-ordered film to the nano-
wires.
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