Patterned aluminum nanowires produced by electron beam at the surfaces of AlF3 single crystals

Article abstract of DOI:10.1016/j.ssc.2003.08.015

Cubic- and rectangular-shape single crystals of α-AlF₃ in sizes of 5-50 μm have been synthesized by a solid-vapor phase process. Using the electron beam induced decomposition of AlF₃, a method is demonstrated for fabricating patterned aluminum nanowires in AlF₃ substrate in a scanning electron microscope. By controlling the accelerating voltage, the beam current and scanning time, it is possible to fabricate metallic nanowires of different sizes. The aluminum nanowires may act as nano-interconnects for nanoelectronics. This work demonstrates a potential technique for e-beam nanofabrication.

Full text of DOI:10.1016/j.ssc.2003.08.015

Solid State Communications 129 (2004) 681–685
www.elsevier.com/locate/ssc
Patterned aluminum nanowires produced by electron beam at the
surfaces of AlF single crystals
3
*
C. Ma, Y. Berta, Z.L. Wang
School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Dr, Atlanta, GA 30332-0245, USA
Received 24 July 2003; accepted 19 August 2003 by D.E. Van Dyck
Abstract
Cubic- and rectangular-shape single crystals of a-AlF
process. Using the electron beam induced decomposition of AlF
3
in sizes of 5–50 mm have been synthesized by a solid–vapor phase
, a method is demonstrated for fabricating patterned aluminum
3
nanowires in AlF substrate in a scanning electron microscope. By controlling the accelerating voltage, the beam current and
3
scanning time, it is possible to fabricate metallic nanowires of different sizes. The aluminum nanowires may act as nano-
interconnects for nanoelectronics. This work demonstrates a potential technique for e-beam nanofabrication.
q 2003 Elsevier Ltd. All rights reserved.
PACS: 61.46. þ w; 81.07. 2 b
Keywords: A. AlF ; A. Nanostructures
3
One-dimensional nanostructures have been extensively
investigated in recent years due to their quantum properties
and potential applications in materials research, opto-
electronics, physics, chemistry, and biomedical science
catalyst in heterogeneous reactions [8]. It is also potentially
useful as a resist [9] or storage material for atomic hydrogen
[10]. The existing literature is mainly about polycrystalline
films of AlF
simple technique for growing large-size AlF
crystals. Then, the AlF crystals are used for creating
. In this paper, we first present a novel and
3
[
1–3]. If one-dimensional nanostructures are to be used in
3
single
large-scale commercial applications, then these novel
structures must be aligned or patterned either during or
post-synthesis. There are a few approaches that have been
successfully demonstrated for aligning one-dimensional
nanostructures through natural growth [4], pattern guided
hydrofluid process [5], shadowed deposition of metals [6],
or self-assembly through surface functionalization [7].
Direct electron beam writing for creating patterned one-
dimensional nanowire structures is of great interest for
applications in nanoelectronics. The objective of this paper
is to demonstrate a novel approach or creating metallic
nanowires in AlF3 insulator substrate. It is known that
3
metallic aluminum nanowires using a fine electron beam in a
scanning electron microscope. This technique provides a
possible approach for developing complex metallic patterns
by electron beam.
The AlF films prepared by chemical method are mostly
3
rough at the surface and have a polycrystalline structure
[11]. The first task of our study is to synthesize single
crystalline AlF
phase process used for synthesis of nanobelts [12]. Synthesis
of AlF was based on a thermal evaporation of commercial
grade AlF (purity: 99.5%) powder under controlled
. Our approach is based on a solid–vapor
3
3
3
conditions (Fig. 1). The powder was placed in the center
3
aluminum trifluoride (AlF ) is of potential applications as a
of a tube furnace and evacuated for several hours to a
2
2
pressure of ,10 mbar in order to purge the oxygen in the
system. After evacuation, the temperature of the system was
*
Corresponding author. Tel.: þ1-404-894-8008; fax: þ1-404-
8
94-9140.
2
raised to 1350 8C at a rate of 15 8C min and held there at a
1
E-mail address: zhong.wang@mse.gatech.edu (Z.L. Wang).
0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2003.08.015

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C. Ma et al. / Solid State Communications 129 (2004) 681–685
fluorination reaction with nitrogen trifluoride [11] and
thermal decomposition of hexafluoroaluminate [13]. In
those studies, porous a-AlF3 was synthesized and the
surface morphology is rather rough. With our technique, a
cube or rectangular shape has been produced, which is
single crystal and with flat surfaces. To the best of our
knowledge, this is the first demonstration of the well-faceted
3
a-AlF structure that is suitable for electron beam fabrica-
tion of nanostructures. Dimensionality of the cubes and
rectangles can be tailored by controlling synthesis tempera-
ture and deposition time, and their sizes range from 5 to
5
0 mm. In general, material collected further away from the
source had finer features than those collected near the source
material, i.e. higher temperature region and higher concen-
tration of AlF vapor are the major factors in controlling
3
Fig. 1. Schematic of the tube furnace apparatus for the synthesis of
single crystal AlF
dimensionality.
AlF is very sensitive to electron beam illumination, and
3
.
3
the structural transformation occurs very quickly under the
beam, thus, the application of transmission electron
microscopy to this material is limited. Wang and Cowley
constant pressure of 300 mbar for 1–2 h. A steady flow of
argon gas was sent through the tube at a rate of 50 sccm,
which acted as a carrier gas to transport the AlF
3
vapor to
3
[14] used AlF to produce metallic nanoparticles by electron
beam illumination in TEM. Chen et al. [15] used electron
beam to induce structural transformation from amorphous
cooler temperature regions, where it might deposit onto
alumina plates located downstream from the source
material. The as-deposited material was characterized and
analyzed by X-ray diffraction (XRD: Philips PW 1800 with
Cu Ka radiation) and scanning electron microscopy (SEM:
LEO 1530 FEG).
AlF
effects of electron beam on FeF
beam illumination is to transform AlF
radiation damage of the beam, resulting in the release of the
3
to crystallized a-AlF
3
. Saifullah et al. [16] studied the
. A consequence of electron
into Al due to the
3
3
2
F
ions from the lattice. By using the electron beam in an
SEM, we may be able to produce patterned lines of
aluminum in the well-faceted AlF crystals. Instead of
Since AlF
3
has several phases, it is essential to determine
by XRD [13]. An
the phase of the as-produced AlF
3
examination of the sample with XRD indicates that it has a
¯
rhombohedral crystal structure with space group R3(148)
3
illuminating the entire sample with the electron beam, we
made only line scans. A schematic representation of the
fabrication process of the aluminum nanowires is given in
Fig. 4(a). From the SEM images (Fig. 4(b) and (c)) acquired
prior and post illumination by an electron beam, it is
apparent that line features have been created by the electron
beam, as shown by the arrowheads in Fig. 4(d). The most
important thing is that the nanowires are created only at the
area scanned by the electron beam, provided the scanning
time is long enough.
(
3
Fig. 2). This corresponds to the a-AlF structure, the most
stable form of aluminum trifluoride. The peaks in the XRD
pattern are in good agreement with the standard data for a-
3
AlF (JCPDS No. 44-0231). The sample is pure and without
the presence of other phases.
SEM images reveal the well-faceted morphology of
the as-deposited material (see Fig. 3). These results differ
from the morphology previously reported by other groups
who have synthesized a-AlF
3
through plasma-assisted
3
To induce the transformation from AlF to Al in the
sample, an accelerating voltage of 14 kV was used for a
scanning period of 1–5 min. Line scans of varying voltages
at different time scales were methodically conducted on an
3
a-AlF crystal in order to determine the parameter ranges
that produced Al nanowires (see Fig. 5). Beginning with low
accelerating voltage, a 1 kV incremental step at a constant
scanning time of 3 min were used to determine the
2
minimum voltage required to volatilize the F atoms. Our
observations revealed that 12 kV had sufficient energy to
produce an aluminum nanowires. Increasing the scanning
time of the electron beam from 3 to 7 min made the Al
nanowires more visible. This is expected since a longer
scanning time results in more F
The aluminum nanowires are formed due to the
electron beam induced decomposition following AlF
2
gas being evolved.
Fig. 2. XRD pattern recorded from the as-produced AlF
3
.
3
!

C. Ma et al. / Solid State Communications 129 (2004) 681–685
683
3
Fig. 3. SEM images of the a-AlF at low magnification. Images (a) and (b) depict the well-faceted cube morphology, while image (c) depicts the
well-faceted rectangular morphology collected from a higher temperature region.
Al þ 3/2F . The size of the Al nanowires produced by
In summary, single crystal cubic-shape AlF has been
3
2
the electron beam is determined by the voltage,
scanning time (or illumination dosage) and the probe
size. Manipulating these parameters could produce
different sizes of nanowires, as shown in Fig. 5. It is
apparent that the scan time affects the line width and
the operation voltage can also affect the line width. The
data demonstrate that the technique allows, in principle,
size-controlled fabrication of nanowires in specified
lengths at specified locations.
synthesized by a solid–vapor phase process. A method is
demonstrated for fabricating patterned aluminum nanowires
3 3
in AlF using the electron beam in SEM. Since AlF is
electrically insulating, the aluminum nanowires may act as
nano-interconnects for nanoelectronics. By controlling the
accelerating voltage, the beam current and scanning time, it
is possible to fabricate metallic nanowires of different sizes.
This demonstrates a potential technique for e-beam
nanofabrication.

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C. Ma et al. / Solid State Communications 129 (2004) 681–685
3
Fig. 4. (a) Schematic of process used to fabricate patterned Al nanowires. (b,c) SEM images of the a-AlF crystal before and after scanning by
the electron beam along the lines, respectively, showing the formation of nanowires along the beam scanning direction. (d) High magnification
SEM image of the cube after it has been scanned by an electron beam. The background feature is produced by the discharge of the substrate
during SEM imaging. The beam energy used for the writing was 14 kV.

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Research supported by the NASA URETI program.