Directed growth of single-crystal indium wires

Article abstract of DOI:10.1063/1.2208431

Tailored electric fields were used to direct the dendritic growth of crystalline indium wires between lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions that suppress sidebranching on these structures has enabled the fabrication of arbitrarily long needle-shaped wires with diameters as small as 370 nm. Electron diffraction studies indicate that these wires are crystalline indium, that the unbranched wire segments are single-crystal domains, and that the predominant growth direction is near 〈110〉. This work constitutes a critical step towards the use of simply prepared aqueous mixtures as a convenient means of controlling the composition of submicron, crystalline wires.

Full text of DOI:10.1063/1.2208431

Directed growth of single-crystal indium wires
Ishan Talukdar, Birol Ozturk, Bret N. Flanders, and Tetsuya D. Mishima
Citation: Applied Physics Letters 88, 221907 (2006); doi: 10.1063/1.2208431
View online: http://dx.doi.org/10.1063/1.2208431
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APPLIED PHYSICS LETTERS 88, 221907 ͑2006͒
Directed growth of single-crystal indium wires
a͒
Ishan Talukdar, Birol Ozturk, and Bret N. Flanders
Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078
Tetsuya D. Mishima
Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman,
Oklahoma 73019
͑
Received 24 November 2005; accepted 12 April 2006; published online 31 May 2006͒
Tailored electric fields were used to direct the dendritic growth of crystalline indium wires between
lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions
that suppress sidebranching on these structures has enabled the fabrication of arbitrarily long
needle-shaped wires with diameters as small as 370 nm. Electron diffraction studies indicate that
these wires are crystalline indium, that the unbranched wire segments are single-crystal domains,
and that the predominant growth direction is near ͗110͘. This work constitutes a critical step towards
the use of simply prepared aqueous mixtures as a convenient means of controlling the composition
of submicron, crystalline wires. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2208431͔
Directed wire-growth techniques enable the one-step as-
sembly and interfacing of submicron interconnects com-
Electronics, F10A͒ and applied across the lithographic elec-
trodes. These electrodes are deposited on 1-mm-thick Pyrex
substrates, using standard lithographic techniques, as de-
1
–4
posed of a wide variety of materials
and, therefore, are
8
promising approaches to the fabrication of high quality pho-
tonic, mechanical, and electronic devices. To realize these
goals, precise control over the structure of the wires is nec-
essary, as their structural properties will dictate their optical,
mechanical, and electronic properties in device applications.
Recent studies have revealed the potential of dielectrophore-
sis for attaining impressive structural control over assembled
wires. In a solution-phase study, Kretschmer and Fritzsche,
produced wires composed of 30-nm-diameter gold nanopar-
scribed elsewhere. The optical micrograph in Fig. 1 depicts
the ends of a tapered electrode pair. The wire spanning the
60 ␮m electrode gap was grown by depositing a 10␮l drop
of
a
0.055M aqueous solution of indium acetate
͓In͑CH3COO͒3͔ over the gap and applying a 1.0 MHz square
wave with an amplitude of 18 V and no dc offset. The wire
grew from the alternating electrode to the grounded electrode
immediately ͑to the eye͒ after switching on the alternating
voltage. When the wire interfaced with the opposing elec-
trode, the voltage was terminated manually. The growth pro-
cess was observed on an inverted microscope ͑Leica, IRB͒
equipped with a digital camera ͑Roper, CoolSnap͒.
5
ticles that were arranged in a pearl-necklace configuration;
this outcome demonstrates the assembly of interconnects
with mesoscopic structural precision, where the organization
of the particles composing the wire is periodic. In a separate,
gas-phase study, Libbrecht and Tanusheva used dielectro-
For a given electrode gap and geometry, variation of the
voltage amplitude, its frequency, and the In͑CH COO͒ con-
3
3
6
phoretic forces to direct the growth of ice crystals; this work
centration gives rise to a wide range of wire structures. Fig-
ure 2͑a͒ depicts a concentration-voltage phase diagram of
interconnect structural types, all of which were fabricated
between electrodes spaced by 60 ␮m, with an ac frequency
of 1.0 MHz. When the voltage amplitude was less than 18 V,
interconnect formation did not occur, regardless of the
demonstrates the growth of dendrites with microscopic struc-
tural precision, where the organization of the molecules com-
posing the dendrite is periodic. Attaining such microstruc-
tural control in solution-phase directed assembly would
provide a straightforward means of growing high quality
wires of composition determined simply by choice of solute.
To this end, Cheng et al. have fabricated wires from simple,
In͑CH COO͒ concentration. For concentrations less than
3
3
7
aqueous solutions of palladium acetate. While this work
stopped short of determining the stoichiometric composition
or crystallinity of the wires, it demonstrated directed wire
growth in solutions containing only dissolved salt. The
present work builds on these previous studies to demonstrate
methodology for the electrochemical growth of single crystal
indium wires from aqueous solutions of indium acetate. This
work demonstrates the use of a simple salt solution as a
medium in which to grow single-crystal metal wires between
targeted sites in a circuit.
Figure 1 shows a diagram of the wire-growth apparatus.
The function generator ͑Hewlett Packard, 8111A͒ supplies an
alternating voltage that is passed through an amplifier ͑FLC
FIG. 1. Apparatus for directed electrochemical wire growth. The image is
an optical micrograph of a wire grown from indium acetate solution. The
scale bar denotes 20 µm.
a͒Author to whom correspondence should be addressed; electronic mail:
bret.flanders@okstate.edu
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28.235.251.160 On: Fri, 19 Dec 2014 09:31:36
0
1

221907-2
Talukdar et al.
Appl. Phys. Lett. 88, 221907 ͑2006͒
probability of successful transfer, we populated the liquid in
the interelectrode region with several wire segments, break-
ing each wire free of the electrode after its growth. Translat-
ing the TEM grid through this dispersion caused the wires to
flow onto the grid. Figure 3͑a͒ shows a TEM micrograph of
a group of wires that were mounted in this manner. The wires
are needle shaped with sparse side branching. The six
wires depicted in this image range in diameters from
3
67 to 556 nm.
Figure 3͑b͒ depicts a selected area electron diffraction
pattern collected from the wire segment indicated by the ar-
row and oriented as shown in Fig. 3͑a͒. Because the area
selected by the ϳ1-␮m-diameter aperture did not include
any portion of the carbon grid, no ring patterns due to amor-
phous carbon are evident in this figure. Figure 3͑c͒ shows a
simulated ͓111͔ diffraction pattern that was calculated using
a commercial software package ͑Virtual Laboratories, Desk-
FIG. 2. ͑a͒ Concentration-voltage phase diagram of interconnect types. The
conditions that give rise to amorphous, dendritic and needle-shaped wires
are denoted by the filled circles, the filled triangles, and the unfilled circles,
respectively. ͑b͒ A representative image of an amorphous wire. ͑c͒ A repre-
sentative image of a dendritic structure. ͑d͒ A representative image of a
needle-shaped wire. The scale bars in images ͑b͒–͑d͒ represent 20 ␮m.
top Microscopist͒, requiring as input the camera length
−3
͑
790.8 mm͒, the electron wavelength ͑2.51ϫ10 nm͒, and
the known structural parameters of crystalline indium: a te-
tragonal crystal structure, with a=b=0.3251 nm and c
11
=
0.4945 nm, belonging to symmetry group I4/mmm. The
0
.040M and voltages greater than 30 V, wires with an amor-
experimental and simulated diffraction patterns are in excel-
lent quantitative agreement with each other. For example, on
phous appearance spontaneously grew. The filled circles de-
note this region in Fig. 2͑a͒ and 2͑b͒ depicts a typical amor-
phous wire. Increasing the In͑CH COO͒ concentration to
greater than 0.040M caused dendritic wire growth. The filled
triangles denote this region in Fig. 2͑a͒ and 2͑c͒ depicts a
typical dendritic sample. A real-time movie of this dendritic
growth process is available as supplementary material ͑see
Ref. 19͒. Setting the voltage to 18 V and using a saturated
¯
¯
the TEM film the distances of the 011 and 110 spots from the
00 spot are 7.33 and 8.57 mm, respectively; the angle sub-
3
3
0
tended by these diffracted spots is 53.7° ±0.4°. The corre-
sponding calculated values are 7.31 mm, 8.64 mm, and
53.78°. The error for each of these quantities is less than
0.85%. This close agreement indicates that the wire is com-
posed of crystalline indium. Of 13 wires examined in this
manner, all were crystalline indium.
͑
0.055M͒ indium acetate solution gives rise to suppressed
sidebranching and the formation of individual, needle-shaped
wires. The unfilled circles denote this small region of phase
space in Fig. 2͑a͒ and 2͑d͒ depicts a representative wire. A
real-time movie of the needle-growth process is available as
supplementary material ͑see Ref. 19͒. The dendritic wires
presented here are similar in appearance to the dendritic
To determine the continuity of the crystalline regions of
these wires, selected area diffraction patterns were collected
from successive positions along their lengths, using an area
selection aperture with a 2.2 ␮m diameter. Figures 4͑a͒–4͑c͒
represent diffraction patterns captured from the lower,
middle, and upper sections of the ϳ5-␮m-long wire indi-
cated in Fig. 3͑a͒. For this measurement, the sample was
rotated so that the electron beam impinged upon a different
face of the wire. Analyses of the discrete patterns in Figs.
4͑a͒–4͑c͒ indicate that all three diffraction patterns result
from crystalline indium observed from the ͗001͘ direction,
confirming the structural assignment made above. The dif-
fuse rings in these images result from the amorphous carbon
strands of the TEM grid beneath the wire segment. More-
over, in each of the three images, the ͗001͘ diffraction pattern
is identically oriented, the diffracted spots are located the
structures ,10that were previously observed in ZnSO4
9
solutions;
directed.
however, in the present case, their growth is
A 200 kV transmission electron microscope ͑TEM͒
made by JEOL ͑JEM-2000FX͒ was used to obtain detailed
crystallographic information of the dendritic wires. Because
the pyrex substrates of the electrode arrays are not thin
enough to transmit the electron beam, it was necessary to
transfer the wires to TEM grids. The transfer process con-
sisted of using an actuator to push a holey carbon grid ͑Ted
Pella͒ across the wire-laden electrode gap. To increase the
FIG. 3. ͑a͒ A group of needle-shaped wires, transferred onto a TEM grid. The scale bar denotes 500 nm. ͑b͒ The measured diffraction pattern of the wire
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segment indicated by the arrow in ͑a͒. ͑c͒ The simulated diffraction pattern of crystalline indium, observed from the ͓111͔ direction.
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221907-3
Talukdar et al.
Appl. Phys. Lett. 88, 221907 ͑2006͒
rected dendritic growth of crystalline wires from a broad
range of materials.
This work establishes an innovative, electrochemical ap-
proach to growing single-crystal, metallic wires between tar-
geted sites in on-chip circuitry, demonstrating the possibility
of using simply prepared aqueous solutions to grow submi-
cron wires composed of metals of choice. As the directed
growth and interfacing of wires are not easily attainable with
other crystal-growth techniques, such as the vapor-liquid-
FIG. 4. The selected area diffraction patterns measured from the ͑a͒ lower
third, ͑b͒ middle third, and ͑c͒ upper third of the ϳ5-␮m-long wire segment
indicated by the arrow in Fig. 3͑a͒.
1
7,18
solid mechanism,
this work is expected to prove useful in
applications requiring the in situ growth of single-crystal
interconnects.
same distances from the 000 spot, and no double ͑or mul-
tiple͒ sets of diffracted spots are visible. These observations
indicate that this wire segment is a single-crystal domain of
highly pure indium. Of 13 wire segments examined in this
manner, some as long as 25 ␮m, the crystal structure was
found to be invariant along the length of each segment, in-
dicating that this growth procedure produces indium wire
segments that are single crystals.
This work was supported by the National Science Foun-
dation ͑NER-0304413͒ and Oklahoma EPSCoR ͑EPS-
1
32354͒. The authors thank Daniel R. Grischkowsky for gen-
erously sharing the cleanroom facilities used to produce the
lithographic electrode arrays.
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See EPAPS Document No.E-APPLAB-88-261622 for real-time movies of
the dendritic and needle-growth-processes. This document can be reached
via a direct link in the online article’s HTML reference section or via the
EPAPS homepage ͑http://www.aip.org/pubservs/epaps.html͒.
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dritic solidification process,
just as supercooling drives
1
6
dendritic growth in pure materials. We expect our ongoing
investigation into the growth mechanism to enable the di-
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