Shape-and dimension-controlled single-crystalline silicon and sige nanotubes: Toward nanofluidic fet devices

Article abstract of DOI:10.1021/ja808483t

We report here on the formation of robust and entirely hollow single-crystalline silicon nanotubes, from various tubular to conical structures, with uniform and well-controlled inner diameter, ranging from as small as 1.5 up to 500 nm, and controllable wa

Full text of DOI:10.1021/ja808483t

Published on Web 02/18/2009
Shape- and Dimension-Controlled Single-Crystalline Silicon
and SiGe Nanotubes: Toward Nanofluidic FET Devices
Moshit Ben Ishai and Fernando Patolsky*
School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel AViV
UniVersity, Tel AViV 69978, Israel
Received November 5, 2008; E-mail: fernando@post.tau.ac.il
Abstract: We report here on the formation of robust and entirely hollow single-crystalline silicon nanotubes,
from various tubular to conical structures, with uniform and well-controlled inner diameter, ranging from as
small as 1.5 up to 500 nm, and controllable wall thickness. Second, and most important, these nanotubes
can be doped in situ with different concentrations of boron and phosphine to give p/n-type semiconductor
x
nanotubes. Si Ge1-x-alloy nanotubes can also be prepared. This synthetic approach enables independent
and precise control of diameter, wall thickness, shape, taper angle, crystallinity, and chemical/electrical
characteristics of the nanotubular structures obtained. Notably, diameter and wall thickness of nearly any
size can be obtained. This unique advantage allows the achievement of novel and perfectly controlled
high-quality electronic materials and the tailoring of the tube properties to better fit many biological, chemical,
and electrical applications. Electrical devices based on this new family of electrically active nanotubular
building-block structures are also described with a view toward the future realization of nanofluidic FET
devices.
Introduction
tions, and fluid-transport devices. In this regard, silicon nano-
tubes are of great importance. Silicon has been widely recog-
Since the first report on the synthesis of carbon nanotubes
almost two decades ago, much effort has been invested in the
preparation of nanotubular materials.
nonporous membranes have been extensively used as templates
nized as the most important material of the 20th century. This
is largely due to its role as a fundamental component in
integrated circuits and consequently in the microelectronics
revolution. Although silicon nanotubes have stimulated great
interest in nanomaterials as a result of their potential applica-
1
2
-4
Carbon nanotubes and
5
,6
for the subsequent preparation of nanotubular structures.
However, the nanotubes obtained are either porous, amorphous,
or polycrystalline, and their shape and dimensions strongly
depend on the characteristics of the available membranes and
carbon nanotubes. Also, the synthesis of nonlayered GaN and
9-12
tions, they have generally been suggested theoretically,
3
because of the favorable formation of sp hybridization of silicon
in silicon nanotubes. Only recently have a few groups reported
13-19
on the synthesis of silicon nanotubes,
each using a different
SiO nanotubes based on the use of core-shell nanowire
2
growth process. However, the preparation of robust, crystalline
silicon nanotubular structures is still a great challenge. It should
be noted that none of the synthetic routes described has yet
demonstrated the ability to synthesize robust, crystalline silicon
7,8
templates has recently been published. However, none of the
publications mentioned have demonstrated the ability to syn-
thesize nanotubular structures with uniform and controllable
molecular-size inner diameters of less than 5 nm, with control
over both the geometry and the chemical/electrical composition
(
9) Fagan, S. B.; Baierle, R. J.; Mota, R.; da Silva, A. J. R.; Fazzio, A.
2
of the nanotube shell. Furthermore, the SiO nanotubes obtained
Phys. ReV. B 2000, 61, 9994–9996.
had amorphous structures. The growth of reproducible and
controllable crystalline semiconductor nanotubes with uniform
inner diameter and wall thickness, and controllable morphology,
shape, and chemical composition, would be advantageous in
potential nanoscale electronics, biochemical-sensing applica-
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10.1021/ja808483t CCC: $40.75

2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 3679–3689 9 3679

A R T I C L E S
Ishai and Patolsky
nanotubes with the aforementioned control over the properties
of the nanotubes. This control is essential to the development
of future nanoscale electronics and biochemical-sensing
applications.
a postgrowth enlargement process. (e) By altering the growth
conditions, it is possible to form germanium templates of varying
shapes and subsequently a variety of other hollow structures.
Crystalline germanium nanowire templates (as depicted in
Figure 1A) were grown on (100) silicon wafers by the
vapor-liquid-solid (VLS) mechanism, from gold nanoclusters,
followed by conformally and epitaxially overcoating the cores
with a silicon shell (and various reactants/dopants), with the
use of a chemical vapor deposition system. The CVD approach
coupled with VLS has been previously reported for the synthesis
and study of germanium nanowires and Ge-core Si-shell
This article involves important innovative achievements. First,
we demonstrate the formation of robust and entirely hollow
single-crystalline silicon nanotubes, from various tubular to
conical structures, with uniform and well-controlled inner
diameter, ranging from as small as 1.5 up to 500 nm, and
controllable wall thickness. Second, and most important, the
chemical composition of the nanotubes can be readily modu-
lated. Notably, diameter and wall thickness of nearly any size
can be obtained. This unique advantage allows the achievement
of high-quality electronic materials and the tailoring of the tube
properties to better-fit many biological, chemical, and electrical
devices applications.
2
3-30
nanowires
and has already demonstrated its ability to
control the composition and the doping of the obtained
nanostructures. Figure 1A shows a schematic outline of the
procedure that generates silicon nanotubes by the nanowire-
templating process. The first step involves the deposition of gold
nanoclusters of the desired diameter on a silicon wafer, as seeds
for the germanium-nanowire axial growth. The growth substrate
was then placed inside a chemical vapor deposition system for
the synthesis of the core (Ge)-shell (Si) nanowires. The
diameter of the nanoparticles defined the diameter of the
germanium core and the resultant inner diameter of the silicon
nanotubes. The second step refers to the formation of the
Results and Discussion
In this article, we report on the use of sacrificial, “chemically
soft” single-crystalline germanium-core nanowires as a base for
the epitaxial growth of high-quality single-crystalline silicon
nanotubes. Germanium nanowires were used as templates for
the creation of core-shell heterostructures, in an ultrahigh-
vacuum chemical vapor deposition (UHV-CVD) system. The
use of germanium as a template offers the following advantages:
germanium-core template, with the use of germane (GeH
4
) as
a precursor and H as a carrier gas in a well-known two-step
2
2
8
CVD process. The first step was carried out at 315 °C, to
form nucleation sites for the following axial core-growth step,
which occurred at a lower temperature (280 °C). The subsequent
(
a) the core templates can be etched away by two alternative
mechanisms. First, the germanium core can be selectively
removed by wet-chemical etching; the other approach is the
use of thermal oxidation of the germanium core. We would like
to point out that neither etching method affects the integrity
and the crystallinity of the resultant silicon nanotubes. The
selective etching of germanium over silicon is of great impor-
tance for silicon technology. Different etchants with this property
4 2
introduction of silane (SiH ) into a mixture of H and Ar as
carrier gases, at a temperature of 450 °C, leads to the conformal
and epitaxial formation of the silicon shell on top of the
germanium-nanowire core. The resulting core-shell nanowire
heterostructures can be sonicated off the growth substrate in a
pentanol solution for the further germanium-core-etching step,
leaving the crystalline silicon-shell nanotube intact. The etching
step of the inner germanium-nanowire core is carried out in a
solution of pentanol/30% hydrogen peroxide 3:1 v/v at 60 °C
for 2 h, leaving the resultant nanotubes, which possess hydro-
philic voids, unfilled by pentanol. After template removal, the
color of the solution turns from dark brown to brownish.
Alternatively, the core material can be etched away by the dry
2
0,21
have been reported.
In this work, the etching process was
carried out by a wet-chemical approach with the use of two
22
etchants: (i) a mild H
acting H /NH OH etching solution. In both cases, germanium
is oxidized by H , and the resultant oxide is removed as a
2 2
O solution and (ii) a stronger and faster-
O
2 2
4
2 2
O
result of its high solubility in aqueous solution. The silicon walls
of the nanotube remain intact and unetched as the oxidized
silicon (SiO ) is stable and insoluble in water, and so the etching
2
2
thermal oxidation of germanium in the presence of O at
process is terminated. (b) As the germanium core and the silicon
shell both have diamond crystal structures with similar lattice
constants (∼4% difference), silicon can grow epitaxially on the
germanium nanowires and form a single-crystalline sheath in
an unannealed condition. Application of our method leads to
the direct formation of crystalline silicon shells, without the
requirement of a postgrowth annealing process, in contrast to
previous reports on the growth of silicon shells on germanium
temperatures above 350 °C, and the simultaneous vaporization
of the germanium oxide, leading to the hollow nanostructures.
Figure 1B shows a representative HRTEM image of high-quality
single-crystalline germanium nanowires grown by CVD, with
a diameter of about 20 nm (including a native-oxide sheath of
3
1
about 2 nm). In agreement with previous studies of VLS-
grown nanowires, the diameter of the germanium nanowires
correlates well with the diameter of the gold nanocluster: a
sampling of 40 wires grown from 20 nm nanoclusters had a
diameter of 20 ( 4 nm. HRTEM studies reveal that the as-
prepared GeNWs have uniform structure and diameter along
2
3,24
nanowires.
single-crystalline silicon nanotubes can be formed. Additionally,
c) the diameter of the germanium core can be reduced by
Thus, by exploiting this epitaxial relationship,
(
postgrowth thermal oxidation to give ultrasmall nanotubes, and,
alternatively, (d) ultralarge silicon nanotubes can be formed via
(
(
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680 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009

Single-Crystalline Silicon and SiGe Nanotubes
A R T I C L E S
Figure 1. Synthesis and structural characterization of Si nanotubes. (A) Schematic illustration of the synthesis process of Si nanotubes. (1) Au nanoparticles
dispersed on a Si substrate for (2) the formation of single-crystalline Ge nanowires at 280 °C, (3) Ge nanowires coated with Si at 450 °C, and (4) nanowires
dispersed into pentanol and selectively etched by H O to form Si nanotubes, as shown in the low-resolution TEM image. Scale bar is 400 nm. (B) Representative
2 2
high-resolution TEM (HRTEM) images of a typical single-crystalline Ge nanowire and its fast Fourier transform (FFT) image (inset). (C) Typical HRTEM
image of the junction between Ge core (dark) and Si sheath (light). The interplanar distance (0.20 nm) corresponds to (220) planes of Ge. Scale bar is 5 nm.
The inset is a low-resolution TEM image of the corresponding sample. Scale bar is 50 nm. (D) HRTEM image of a typical single-crystalline Si nanotube.
The lattice spacing corresponds to (111) planes of Si. Upper inset corresponds to the FFT of the same image. Bottom inset: HRTEM of the high-quality
crystalline wall of the corresponding Si nanotube. Scale bar is 5 nm. Red boxed inset: Wide-field diffraction pattern of a representative silicon nanotube. (E)
A series of representative HRTEM images of single-crystalline Si nanotubes with inner diameter of ca. 5 nm (left), 10 nm (middle), and 20 nm (right). Scale
bars are 5 nm for left image, and 10 nm for both middle and right images. The arrows highlight the hollow-core region. Insets: Representative low-resolution
TEM of the images shows that the nanotubes are straight with uniform diameter. (F) Set of typical HRTEM images taken from the edge of the nanotubes
with uniform wall thickness of (1) ca. 5 nm, (2) 10 nm, and (3) 20 nm. Scale bars are 4, 5, and 10 nm, respectively. The arrows highlight the wall-thickness
region. (G) Variation in the Si wall thickness as a function of the shell deposition time. (H) Representative energy-dispersive X-ray spectra recorded along
the longitudinal axis of Si nanotube at different sites. The colored circles highlight the representative measured sites. Scale bar is 50 nm.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3681

A R T I C L E S
Ishai and Patolsky
their entire length (more than 10 µm) with a lattice spacing of
can be indexed to the diamond structure of silicon with a [111]
growth direction. Further confirmation of this assignment was
carried out by EDX analysis. The representative EDX spectra
(Figure 1G, left), which are common to all of the hollow
nanostructures in this work, reveal a very small residue of
germanium, less than 1%, and a well-correlated silicon signal
along the longitudinal axis of the nanotubes. This result indicates
successful preparation of the silicon nanotubes. Because the
diameter of the gold eutectic-liquid droplet is the primary factor
0
.33 nm. This corresponds to the d-spacing of the (111) crystal
planes of germanium with a cubic structure, indicating that the
germanium nanowires have preferential growth orientation in
the [111] direction. The fast Fourier transform (FFT) of the
HRTEM image (Figure 1B, inset), as well as XRD experiments
on the as-grown Ge-nanowires substrate (data not shown),
confirm that GeNWs are single-crystalline and grow with a
diamond crystal structure and along the [111] direction.
The deposition of the silicon shell at 450 °C on top of the
germanium nanowire surface results in single-crystalline Ge-Si
core-shell structures. An HRTEM image of a typical Ge-Si
core-shell structure (Figure 1C) exhibits a crystalline germa-
nium core (dark) of about 20 nm and a continuous, uniform
crystalline silicon shell (light) of about 5 nm along the entire
length of the nanowire heterostructure. Further XRD experi-
ments on the Ge/Si heterostructures reveal that the germanium
cores and silicon shells are single crystals (data not shown). It
is further noticed that the core/shell interface is compositionally
sharp and no oxide layer was detected at the interface. A layer
3
3-35
in controlling the nanowire diameter of the VLS growth,
and thus the inner diameter of the resultant nanotubes, it is
possible to form nanotubes with different inner diameters simply
by changing the size of the nanocluster. Figure 1E shows a set
of representative HRTEM micrographs of the as-prepared single-
crystalline silicon nanotubes with different inner diameters. The
contrast of the images clearly shows hollow cores of about 5,
10, and 20 nm, which are nearly the same as the initial sizes of
the gold nanocatalysts. These nanotubes are straight and smooth,
with uniform diameter and wall thickness of about 5 nm along
their longitudinal axis and with excellent crystallinity. The lattice
spacing obtained for each typical nanotube and the correspond-
ing FFTs (see Supporting Information, Figure S2A, S2B, and
S2C) agree well with the interplanar distance of silicon with a
diamond-like crystal structure. It is also feasible to control the
wall thickness of the nanotubes by adjusting the deposition time
of the silicon shell. We have studied the effect on the wall
thickness of three different shell-deposition times. This effect
can be seen in Figure 1F, which shows a series of HRTEM
images of single-crystalline silicon nanotubes, with uniform wall
thickness of about 5, 10, and 20 nm corresponding to 20, 40,
and 120 min of shell-deposition time. Figure 1G, right image,
clearly shows the monotonic increase of the shell thickness as
a function of the shell-deposition time. Furthermore, the
corresponding FFTs of the HRTEM images show that the
nanotubes obtained have the cubic diamond-crystal structure
of silicon (see Supporting Information, Figure S3A, S3B, and
S3C).
2
of amorphous SiO about 1 nm thick covers the outer surface
of the nanowires. This layer formed probably when the
nanowires were exposed to ambient environment, similar to the
32
oxide sheathing of silicon nanowires previously reported. Both
the core and the shell regions have lattice spacing of 0.20 nm,
which is in excellent agreement with the (220) planes of the
known diamond crystal structure of germanium. This observa-
tion indicates a continuous epitaxial layer of silicon on the
germanium core without the requirement of a postgrowth
2
3,24
annealing process.
In addition, energy-dispersive X-ray
spectroscopy (EDX) measurements confirm the existence of the
elements germanium and silicon at a nearly uniform atomic ratio
of 1.04:1.00. Selective etching of the germanium cores suc-
cessfully transformed the Ge-Si core-shell nanowires into
crystalline silicon nanotubes. A representative HRTEM micro-
graph of a typical silicon nanotube (Figure 1D) shows that the
obtained nanotubes have a uniform and high-quality single-
crystalline structure along their entire length (more than tens
of micrometers) and further confirms that these nanotubes are
entirely hollow with no defects along their length due to the
strain introduced on the thin-wall structures. In addition, wide-
field selected-area diffraction (SAED) measurements, Figure 1D
red boxed inset and Supporting Information Figure S1, further
confirm that the silicon nanotubes are indeed single crystalline.
Also, XRD experiments on the silicon nanotubes confirm the
tubes are single crystals regardless of the thickness of the silicon
shell (data not shown). The image contrast clearly shows a
uniform hollow core of about 20 nm and a uniform wall
thickness of about 5 nm, which are equal to the original
germanium core diameter and silicon shell thickness, respec-
tively. The lattice-plane spacing obtained for a typical nanotube,
With further optimization and control of the core-template
diameter, we have successfully synthesized nanotubes with
ultrasmall inner diameters, down to about 1.5 nm, and ultralarge
diameters of 100 and about 400 nm. It is worth pointing out
that the VLS growth of germanium nanowires from large gold
seed particles (d g 50 nm) tends to produce germanium
nanowires in low yields at low growth temperatures, or to
overproduce nanowires at high temperatures, as a result of
26
splitting of the gold seeds. On the other hand, small gold seeds
d e 5 nm) grow nanowires of diameters larger than those of
(
the initial particles, a result of simultaneous axial and radial
growth. To overcome these limitations, two strategies have been
developed in this work. The first involves the enlargement of
the as-grown germanium core leading to the synthesis of
nanotubes with ultralarge inner diameters. Figure 2A schemati-
cally describes the synthesis of nanotubes based on this
approach. The second step is the core-enlargement process. This
step involves conformal deposition of a germanium shell on
top of the original germanium core (20 nm). Note that the
growth of the germanium shell is carried out at a higher
0
.314 nm, agrees well with the interplanar distance of silicon
with a cubic structure. In contrast with this, the measured
spacing of the crystallographic planes in core-shell nanowires
(Figure 1C) matched the interplanar distance of germanium. This
observation, which is consistent with all of the results of our
studies on nanotubes, indicates that, after the etching process,
the silicon nanotubes have relaxed to their equilibrium lattice
spacing. The corresponding two-dimensional Fourier transform
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3
682 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009

Single-Crystalline Silicon and SiGe Nanotubes
A R T I C L E S
Figure 2. Synthesis and characterization of ultralarge and ultrasmall silicon nanotubes. (A) A schematic drawing depicting the postgrowth enlargement
process. (1) Ge nanowires are coated with Ge shell for different deposition times, (2) epitaxially coating with Si, and (3) nanowires dispersed into pentanol
and selectively etched by H O to form Si nanotubes. (B) Series of low-resolution TEM images of a typical single-crystalline Si nanotube with uniform inner
2 2
diameter of (1) 60 nm, (2) 100 nm, and (3) 400 nm. Scale bars are 20, 50, and 100 nm, respectively. The bottom insets are representative low-resolution
TEM images of the corresponding Ge core-template after the enlargement process. Scale bars are 300 nm, 400 nm, and 1 µm, respectively. The upper inset
in image 3 corresponds to a representative electron-diffraction pattern of the nanotubes. (4) HRTEM image of sample 3. Scale bar is 5 nm. (C) Schematic
illustration of the postgrowth thermal-oxidation process. (1) Thermal etching of the native-oxide (GeO
oxidation of the Ge core for different lengths of time. (4) Thermal etching of the GeO layer results in smaller Ge cores. (D) (1) HRTEM image of a Ge core
with a diameter of ca. 20 nm. The dashed lines highlight the native oxide (GeO ) of 1-2 nm. Images 2, 3, and 4 show a series of low-resolution TEM images
of Ge-GeO
2
) sheath covers the Ge core surface. (2, 3) Thermal
2
2
2
core-sheath at different oxidation times. Scale bars are 10 nm for images 1, 2, and 3, and 20 nm for image 4. (5) HRTEM image of a typical
Si nanotube with a minimum inner diameter of ca. 2.2 nm. Scale bar is 11 nm. The inset shows a low magnification of Si NTs with inner diameter of 1.5
nm.
temperature (360 °C) to promote the radial growth. By varying
the deposition time of the germanium shell, it is possible to
enlarge the original core diameter considerably while keeping
intact the crystallinity of the germanium core. The deposition
of the silicon shell on top of the “broadened” germanium
nanowires (third step), followed by the selective removal of the
germanium core (step 4), leads successfully to crystalline silicon
nanotubes with larger inner diameters. Low-resolution TEM
studies (Figure 2B) of nanotubes prepared in this way show
nanotubes with uniform inner diameters of about 60 nm (image
1), 100 nm (image 2), and 400 nm (image 3), and uniform
contrast along their entire length (tens of micrometers), which
are properties indicative of crystalline structures. These diam-
eters are consistent with the diameters of the germanium-core
templates after the enlargement process (lower insets of Figure
2B). Note that in all cases the enlargement process was
J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3683

A R T I C L E S
Ishai and Patolsky
Figure 3. Synthesis and characterization of nanotubes with various compositions: GeSi-alloy nanotubes and p-type doped nanotubes. (A) Low-resolution
TEM image of a typical Si0.94Ge0.06 alloy nanotube. Scale bar is 50 nm. Inset: HRTEM of the nanotube edge, shows high-quality single-crystalline structure,
with lattice spacing of 0.318 nm. (B) Representative low-resolution TEM image of in situ p-type doped Si nanotube. Scale bar is 200 nm. The inset shows
high magnification of the open-ended nanotube. (C,D) Compositional depth profile of the nanotubes, probed by time-of-flight secondary-ion mass spectroscopy
(
TOF-SIMS) measurement.
performed on the original 20 nm germanium nanowires. A
typical HRTEM of the as-prepared silicon nanotubes (Figure
Figure 2D. To the best of our knowledge, this is the first report
of crystalline silicon nanotubes of such ultrasmall diameter.
These preliminary results show that it is possible to synthesize
nanotubular structures with ultrasmall inner diameters of less
than 2 nm. Further experiments are being performed to improve
the oxidation conditions of the nanowire templates in terms of
diameter and oxidation rate.
2
B, image 4) indicates that the nanotubes are single crystalline
with the same crystal orientation along the entire length of the
nanotube and with a uniform and smooth wall thickness of about
5
nm. In addition, the selected-area electron-diffraction (SAED)
pattern (Figure 2B, inset, image 3), taken from a typical
nanotube, confirms the fact that the resultant nanotubes have
the diamond cubic structure of silicon with [111] growth
direction.
A unique advantage of our synthetic approach is that it
provides us with rational and precise control over the chemical
composition of the silicon nanotubes. By subsequent introduc-
tion of dopants and other reactants, nanotubes of different
composition can be formed. Specifically, single-crystalline
The second approach involves the thermal oxidation of the
postgrowth core. This makes it possible to prepare nanotubes
with inner diameters of less than 1.5 nm. Figure 2C shows a
schematic outline of the steps required to reduce the diameter
of the as-prepared germanium-core templates. Systematic varia-
tion of the thermal-oxidation time over the range of 1-3 h, at
Si
nanotubes (Figure 3B) were synthesized on the basis of our
template approach, with the use of germane (GeH ) and diborane
), respectively. These were fed into the CVD system
simultaneously with silane (SiH ) during the shell-growth step.
The ability to synthesize p/n-type nanowires and Si Ge1-x alloy
nanowires via the CVD-VLS method has already been
x
Ge1-x alloy nanotubes (Figure 3A) and p-type silicon
4
2 6
(B H
2
2
50 °C and 1 torr O , enables us to control reproducibly and
4
considerably reduce the diameter of the germanium-nanowire
templates obtained by the conventional synthetic procedure
x
2
4,27,28
(
Figure 2C, (3) and (4)). It should be emphasized that the first
step, which involves vaporization at 450 °C of the native-oxide
sheath (GeO ) of about 1 nm thickness (dashed lines in the inset
demonstrated;
work is the first to report the synthesis of Si
doped silicon nanotubes. Si Ge1-x alloy nanotubes were identi-
fied as Si0.94Ge0.06 by EDX measurements with the use of a k
factor of Si K and Ge K radiation in the EDX spectra. The
however, to the best of our knowledge, this
x
Ge1-x alloy and
2
x
of Figure 2D, (1)), is a crucial step. Without this step, the
germanium core cannot be oxidized even at high temperatures.
Referring to the low-resolution TEM images in Figure 2D of
germanium nanowires before oxidation (Figure 2D, (1)) and
after it (Figure 2D, (2), (3), (4)), one can clearly see the distinct
boundary between the amorphous germanium-oxide layer (light
region) and the germanium core (dark region), and the sharp
reduction in the core diameter, from about 20 nm for the original
core template (Figure 2D, (1)) to about 10 nm (Figure 2D, (2)),
about 4 nm (Figure 2D, (3)), and about 1.5 nm (Figure 2D,
R
R
Si:Ge composition, which varies by not more than 0.5% along
the length of the nanotube, suggests appropriate alloying of
silicon and germanium. We also confirmed that Si0.94:Ge0.06 alloy
nanotubes are single crystalline with the same crystal orientation
along the entire nanotube length as in Figure 2J, inset. The
observed lattice spacing (0.318 nm) corresponds to the d-spacing
of the (111) crystal planes of silicon with a cubic structure and
[
x
111] growth direction. Si Ge1-x alloy nanotubes of different
(
4)), along with the monotonic increase of the oxide-sheath
Si:Ge compositional ratio can be obtained with the use of the
proposed approach, because concentrations of germanium lower
than 60% lead to nanotube walls stable in the H O etching
2 2
solution. Higher percentages of germanium lead to nanotube
walls unstable to the etching step of the germanium core. The
thickness, which was also revealed by the low-resolution TEM.
Finally, vaporization and removal of the oxide layer at 450
°
C, followed by the growth of a conformal silicon shell and
the selective etching of the germanium core, lead to the
formation of nanotubes of inner diameter significantly smaller
than that of the initial core-template. Figure 2D, (5) shows an
HRTEM image of a typical silicon nanotube with a minimum
internal diameter of about 1.5 nm, which was formed after
thermal oxidation. The hollow interior of the small-diameter
nanotube can be clearly seen. The roughness of the walls of
these tubes is the result of the germanium-core oxidation prior
to the deposition of the silicon shell, as can be clearly seen in
presence of silicon in the alloy at concentrations higher than
36
4
0% inhibits the etching of the SiGe alloy. Future studies will
be performed on SiGe alloy nanotubes to expand the Si-Ge
(
36) Bircumshaw, B. L.; Wasilik, M. L.; Kim, E. B. Y. R.; Takeuchi, H.;
Low, C. W.; Liu, G.; Pisano, A. P.; King, T.-J.; Howe, R. T. Technical
Digest; IEEE, 17th International Conference on Micro Electro Me-
chanical Systems, 2004; p 514.
3
684 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009

Single-Crystalline Silicon and SiGe Nanotubes
A R T I C L E S
composition spectrum and to electrically characterize the
obtained nanotubes.
representative low-resolution image of a conical germanium
nanowire grown at 360 °C. A high-resolution TEM image of a
typical Ge-Si core-shell conical nanowire shown in Figure
4C, (2) reveals the high-quality single-crystalline core-sheath
structure and confirms the epitaxial growth of the capping shell
along the entire length of the conical template structure. The
lattice spacing of both the shell and the core (0.33 nm)
corresponds to the d-spacing of (111) crystal planes of germa-
nium with cubic structures. This observation was confirmed by
the FFT of the TEM image (inset of Figure 4C, (2)). Selective
extraction of the germanium core from the core-shell nanocones
results in the formation of hollow silicon cones, as shown in
Figure 4C, (3). The nanotubular structure is complementary to
the morphology of the germanium template with the interior
void determined by the dimensions of the core template, which
grows at 380 °C. A lattice-resolved image of a typical conical
nanotube and its FFT image indicate that the resultant nanotubes
have a cubic diamond crystal structure of silicon with [111]
growth direction.
For p-type silicon nanotubes, we quantitatively determined
the relative boron composition using time-of-flight secondary-
ion mass spectroscopy (TOF-SIMS) analysis combined with a
sputtering etching process. A compositional depth profile, which
was carried out on the nanotubes, as representatively shown in
Figure 3C and D, reveals the boron composition of the silicon-
nanotube wall. The B/Si atomic ratio, which was measured
sequentially after multiple etching steps of the nanotubes, was
∼
1/200. Subsequent introduction of diborane during the growth
step of the silicon shell greatly influenced the wall thickness of
the obtained nanotubes. Low-resolution TEM performed on the
p-type nanotubes (Figure 3B) shows that the wall thickness of
the nanotubes was about 20 nm, as compared to about 10 nm
for the pristine silicon nanotubes (Figure 1F, middle) for an
identical shell-growth duration of 40 min. This is in agreement
with previous reports on the growth of a silicon shell on top of
3
7
germanium nanowires. By changing the partial pressure of
the reactants, it is possible to form p/n-type silicon nanotubes
Our approach exhibits a further degree of controllability
because both the shape and the taper angle can be controlled
by adjusting the growth variables. In addition, we have
developed a synthetic procedure for creating funnel-like nano-
tubular structures (Figure 4D). Figure 4D schematically outlines
the major steps required to do this. Here, the fabrication of the
germanium template involves two steps. It will be noted that
the nanotubes consist of two structures, cone and wire, and each
step leads to the formation of one of the nanostructures. At first,
the temperature was set at 360 °C to form the cone, and then
with continued flow of germanium, the temperature was
gradually lowered to 280 °C to initiate wire growth. To further
continue the wire growth, the temperature was maintained at
280 °C. A low-resolution TEM image (upper image of Figure
4E, (1)) shows a typical funnel-like nanowire template. The
junction point between the cone and the wire, and the wire
region, are marked and magnified (lower images of Figure 4E,
(1)). The funnel-like nanowire templates were then transformed
into nanotubular structures with uniform, highly crystalline
sheath and smooth surface. Figure 4E, (2) shows a low-
resolution TEM image of a representative hybrid nanotube. The
nanotubes obtained have morphology complementary to that
of the germanium template that grows at 340 °C. Detailed
studies with the use of high-resolution TEM (Figure 4E, (3))
verified that the resulting nanotubular structures are high-quality
single-crystalline silicon and the reciprocal lattice peaks obtained
from the FFT (Figure 4E, (3), inset) can be indexed to the cubic
structure of silicon with [111] growth direction.
and Si
Ge ratio.
The shape of semiconductors at the nanoscale is another factor
x
Ge1-x nanotubes with different doping density and Si:
that influences their properties, and thus the shape-controlled
growth of semiconductors can find unique applications in
electronics and photonics. Our approach can be also extended
to produce a variety of other hollow forms, such as “hollow
cones” (Figure 4A). Nanocones have been reported for carbon,
but the intrinsic fullerene lattice strongly limited the taper angles
3
8
achievable.
Figure 4A schematically illustrates the experimental procedure
used to fabricate conical silicon nanotubular structures. The same
steps described previously were used to produce the hollow
cones, except for the nanowire-template growth. Here, the
formation of nanowire templates was carried out at higher
temperatures to produce a cone-like shape as a result of
simultaneous axial and conformal radial growth. The fabrication
of germanium-cone nanowires with different taper angles by a
28,39
CVD-VLSmethodhasbeenalreadydemonstratedandstudied.
To form conical nanotubular structures with different taper
angles, a series of nanowire templates was grown at temperatures
of 340, 360, and 380 °C. Figure 4B shows a series of scanning
electron microscopy (SEM) images of germanium nanostruc-
tures whose shape can be controllably tuned from nanowires,
which were grown at 280 °C, to nanocones with various taper
angles, at higher growth temperature, up to 380 °C. The
relationship between the taper angle and the growth temperature
is depicted in Figure 4B, (5). Figure 4B, (5) is constructed on
the basis of statistical distributions of taper angles measured
over 10 germanium-cone nanowires at each growth temperature
In addition, we have extended our research into the chemical
functionalization of the nanotubes, which is a crucial step to
many future applications, including chemical separations (selec-
tive filtering), selective transport, and sensing of chemical and
biological molecules. Pristine silicon nanotubes are insoluble
in most solvents and show a strong tendency to form aggregates.
To improve their solubility, silicon nanotubes were chemically
modified by the covalent binding of silane molecules containing
different functional groups to their outer surface. Figure 5A
schematically describes the functionalization procedure used for
the dispersion of silicon nanotubes (Figure 5B). All modifica-
tions were carried out on the surface of the core-shell
nanowires. This allows us to modify the external surface of the
subsequent nanotubes without affecting their inner surface. The
outer surface of the silicon nanowires is terminated by silanol
groups (SiOH), which are employed as anchoring sites for silane
(
340, 360, and 380 °C). We see that the taper angle strongly
depends on the growth temperature, and it increases by a factor
of 2 as the temperature is raised from 340 to 380 °C. The conical
nanowires that were grown at 380 °C were coated with a 5 nm-
thick silicon shell. Both low- and high-resolution TEM images
were used to characterize the shape, shell thickness, and
crystallinity of the conical nanowires. Figure 4C, (1) shows a
(
(
(
37) Briand, D.; Sarret, M.; Kis-Son, K.; Mohammed-Brahim, T.; Duv-
erneuil, P. Sci. Technol. 1999, 14, 173–180.
38) Krishnan, A.; Dujardin, E.; Treacy, M. M. J.; Hugdahl, J.; Lynum,
S.; Ebbesen, T. W. Nature 1997, 388, 451–454.
39) Dailey, J. W.; Taraci, J.; Clement, T.; Smith, J. D.; Drucker, J.; Picraux,
S. T. J. Appl. Phys. 2004, 96, 7556–7567.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3685

A R T I C L E S
Ishai and Patolsky
Figure 4. Synthesis and structural characterization of conical and funnel-like nanotubular structures. (A) Schematic illustration of the experimental
procedure for synthesis of hollow cones. (1) Deposition of Au nanoparticles, and (2) growth of Ge-cone nanowire at 360 °C, which were (3) epitaxially
coated with Si shell at 450 °C and (4) dispersed into pentanol and chemically etched with H O to form the Si conical nanotubular structure. (B) A
2 2
set of representative scanning electron microscopy (SEM) images of the as-synthesized Ge-cone templates. The shape varies from (1) wire at 280 °C,
with uniform diameter of 20 nm along the entire length, to conical shape with various taper angles. Scale bars are 300 nm, 150 nm, 300 nm, and 1
µm. (5) Variation in the taper angle of the Ge cone as a function of temperature. Scale bars are 300, 150, and 300 nm, respectively. (C) Structural
characterization of the Ge-Si core-shell conical nanowire and the resultant Si conical nanotubular structure. (1) Low-resolution TEM image of a
typical Ge-Si core-shell cone grown at 380 °C. Scale bar is 1 µm. (2) HRTEM image of the cone edge and its FFT (inset) showing that both Ge
core (dark) and Si shell (bright) grow with a diamond crystal structure of Ge and along [111] growth direction. Scale bar is 5 nm. (3) Low-resolution
TEM image of the conical nanotubular structure. Insets: Lattice-resolved TEM image of nanotubes and its FFT. Scale bars are 1.5 µm and 10 nm,
respectively. (D) Schematic illustration of the formation of Ge-Si core-sheath funnel-like nanowire. (1) Ge cones grow at a temperature of 360 °C,
and then (2) under continued flow of GeH
4
, the temperature was gradually lowered to 280 °C to initiate growth of the wires and then maintained at
2
80 °C to continue growth of the wires to form funnel-like nanowires, which were (3) covered with a Si shell to give Ge-Si core-shell funnel-like
nanowires. (E) (1) Low-resolution TEM image of a typical nanowire with a funnel-like shape. The junction point between the wire and the cone, and
the region of wire growth from within the yellow rectangle and dark rectangle, are expanded to the bottom. Scale bars are 600 and 300 nm, respectively.
(
2) Low-resolution TEM of the funnel-like nanotube. Scale bar is 500 nm. (3) HRTEM of the sample in image (2) and its FFT. Scale bar is 10 nm.
3
686 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009

Single-Crystalline Silicon and SiGe Nanotubes
A R T I C L E S
Figure 5. (A) Schematics of the approach used for the functionalization of the Si nanotubes by various silane derivatives. (B) Dispersions of the resultant
functionalized Si nanotubes in various solvents.
molecules to form Si-O-Si covalent bonds. Silanes are known
is a hydrophilic void and can be readily modified in later steps.
Figure 5B illustrates representative results of the functionalized
Si-nanotube dispersions. Covalent linking of aminosilane or
phosphate-silane to the surface of silicon nanotubes produces
tubes readily soluble in water. Functionalized silicon nanotubes
soluble in organic solvents include nanotubes prepared with
fluorosilane and octadecylsilane.
to facilitate the solubility of nanowires and nanotubes in different
4
0-42
solvents.
Surface modification with silanes can be carried
out in the gas phase or by wet chemistry. In the present study,
we used the latter approach. The modification process is begun
with the direct attachment of silane derivatives with different
functional groups to the silicon shell of the nanowire template.
This is done by incubating the growth substrate in a solution of
the silane derivative for 2 h. The functionalized nanowire
substrate is then rinsed with the appropriate solvents and cured
for 10 min in an oven at 110 °C. The next step involves removal
The integration of nanotubular building blocks into electrical
devices is of critical importance for most required applications.
Thus, we have fabricated FET devices from our p-doped silicon
nanotubes and carried out electrical-transport measurements to
evaluate their performance. The assessment of the electrical
properties of nanotubes is a key step in the evaluation of their
suitability for electrical applications. For the fabrication of our
p-type FETs, a liquid suspension of silicon nanotubes with inner
diameter of about 20 nm and wall thickness of 4 nm was first
dispersed on a highly doped silicon substrate with a 600 nm-
2
of the nanowires from the SiO substrate by sonication and
further etching of the core material. Next, the functionalized
silicon nanotubes are centrifuged, washed, and dispersed into
liquid suspensions (common organic solvents and water). This
results in either a hydrophobic or a hydrophilic exterior surface
of the nanotube. The interior, which was left unfunctionalized,
2
thick silicon oxide (SiO ) dielectric layer. This was followed
(
(
40) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98.
by the formation of Ni/Au source-drain electrodes/contacts by
e-beam lithography. Figure 6A shows an HRSEM image of a
typical device consisting of an individual silicon nanotube with
41) Duchet, J.; Chabert, B.; Chapel, J. P.; Gerard, J. F.; Chovelon, J. M.;
Jafferzic-Renault, N. Langmuir 1997, 13, 2271–2276.
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42) Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852–5856.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3687

A R T I C L E S
Ishai and Patolsky
respectively. This is the first example of in situ-controlled doping
of silicon nanotubes. This control over the chemical composition
opens up the possibility of achieving high-quality electronic
materials and to tune their properties to better suit various
applications.
Combined, these strategies have greatly expanded the scope
of control over critical parameters of silicon-based nanotubes
and thus suggest that our synthesized silicon nanotubular
structures are ideal candidates for various biological and
chemical applications and nanoscale-device applications. Future
work in this area will be application-oriented.
Experimental Section
Growth of Ge-Si Core-Shell Nanowires. Single-crystal
Ge-Si core-shell nanowires were synthesized inside a horizontal
quartz-tube furnace in the CVD system, with the use of gold
nanoclusters (Ted Pella, Inc.) as a catalyst, first deposited on
oxidized silicon wafers. Germanium nanowires were grown at 280
2
°C, from 10% germane in 200 sccm H and 400 torr (axial growth),
while the germanium shells for the enlargement process were
deposited at 360 °C, at 4 torr (radial growth), at different deposition
times. The conical germanium nanowires were grown at higher
temperatures (340, 360, and 380 °C) under a flow of germane (30
sccm) and 200 sccm of H
like shape, the result of axial and radial growth. Reduction of the
above temperatures to 280 °C under continued flow of GeH (30
2
at 400 torr for 3 min, to give a cone-
4
sccm) leads to funnel-like structures with various taper angles. The
temperature was maintained at 280 °C for 20 min, for the purpose
of wire growth.
The silicon shells were grown at 450 °C from silane (5 sccm) in
amixtureof5sccmArand10sccmH
alloy shells were deposited from silane (0.5 sccm) and 50 sccm of
0% germane in 200 sccm H at 1 torr. The p-type silicon shells
2
at1Torr. Silicon-germanium
Figure 6. Si nanotube-based FETs. (A) Scanning electron microscopy
image of a typical device. Scale bar is 1 µm. (B) Output characteristic of
a p-type FET. Current (I) vs voltage (V) recorded on a 20 nm internal-
diameter (10 nm wall thickness) boron-doped Si nanotube at various gate
1
2
were formed by incorporation of diborane (5 sccm) and silane (5
sccm) at 1 torr. Oxidation of the germanium cores to reduce their
original diameter was accomplished by flowing oxygen at 1 torr
and 250 °C at different oxidation times over a period of 1-3 h.
The substrate-bound Ge-Si core-shell nanowires were then
voltages (V ) of: -4 V (pink), -2 V (green), 0 V (yellow), +2 V (red),
g
and -4 V (blue). The inset shows a transconductance curve at Vsd) +1 V.
Scale bar is 1 µm.
source-drain contacts (typical separation of 1 µm). Next, an
annealing step was carried out by rapid thermal annealing (RTA)
to form a stable, conducting silicide with a low Schottky barrier.
Transport characteristics can then be studied, with the highly
doped silicon substrate serving as the back gate. Figure 6B
2 2
sonicated in pentanol. By subsequent introduction of 1:3 v/v H O :
pentanol at 60 °C for 2-3 h, the as-prepared nanowires were
transformed into high-quality single-crystalline silicon nanotubes,
without the necessity of thermal annealing. The nanotube samples
were centrifuged, washed several times with ethanol, and deposited
on oxidized degenerately doped silicon wafer or copper grids or
lacey copper grids for electrical transport and TEM measurements,
respectively.
Sample Characterization. The structure and composition of the
nanostructures were investigated with the use of a 200 kV field-
emission-gun transmission electron microscope (Technai F20).
The energy-dispersive X-ray spectra were obtained with an
HTEM with a spot size of 2 nm.
shows the current (I
characteristics) of individual p-type silicon-nanotube field-effect
transistors at various gate voltages (V ). This gate-voltage
dependence is typical of p-type FETs and is similar to that of
D D
) versus drain-source bias (V ) (output
g
4
3
24,27,44
nanowire-based FET devices.
in the inset, where the current varies with the applied gate
voltage (Isd-V ) at a constant Vsd of +1 V.
This behavior is also shown
g
To summarize, we have synthesized, for the first time, robust,
single-crystalline silicon nanotubes from various nanotubular
to conical structures, with well-controlled and uniform inner
diameters (ranging from 1.5 to 500 nm), wall thickness, taper
angle, and chemical composition. The synthesis was carried out
with the use of a nanowire-template strategy combined with
gold-catalyst-assisted chemical vapor deposition. These findings
may lead to novel nanoelectronic, nanophotonic, and nanosensor
devices. We have shown that the silicon nanotubes have
conventional crystalline silicon structures. Furthermore, these
silicon nanotubes were doped in situ with boron and germanium
to form p-type silicon nanotubes and SiGe-alloy nanotubes,
TOF-SIMS analysis was conducted by a time-of-flight secondary-
ion mass spectrometer (PHI TRIFT II), on a gold-coated silicon
wafer containing deposited silicon nanotubes. The Ga primary-
+
ion beam was operated at 25 keV and 20 nA. Positive secondary-
2
ion spectra were acquired from 50 × 50 µm areas of the surfaces.
+
Sputtering was performed by Ga ions for 1 s followed by spectrum
2
acquisition. The sputtering area is 150 × 150 µm .
Electrical characterization was carried out by a cryogenic
electrical probe station (Janis Inc. model TTP-4).
Nanotubes Surface Modification. All of the modifications were
first carried out on the surface of the nanowires. The substrate-
bound nanowires were embedded in various solutions of silane
derivatives for nearly 2 h to yield both hydrophilic and hydrophobic
surfaces: 1% aminosilane and 5% phosphate-silane in water, 2%
fluorosilane and 2% octadecylsilane in dichloromethane (CH Cl ).
2 2
The samples were then rinsed with the appropriate solvent and cured
for 10 min at 110 °C. After sonication and etching, the resulting
(
(
43) Patolsky, F.; Zheng, G.; Lieber, C. M. Nat. Protocols 2006, 4, 1711–
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1
3
688 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009

Single-Crystalline Silicon and SiGe Nanotubes
A R T I C L E S
functionalized silicon nanotubes were centrifuged, washed, and
dispersed into various liquid suspensions (organic solvents and
water). The silicon nanotubes obtained possess hydrophilic/
hydrophobic exteriors and nonfunctionalized, hydrophilic interiors,
which can be readily modified in later steps.
Supporting Information Available: High-resolution wide-field
selected-area diffraction pattern of a representative silicon
nanotube, as in Figure 1D red boxed inset, confirming the single-
crystalline nature of the obtained tubes (Figure S1); measured
interplanar distance and the FFT for Figure 1E, confirming
uniform diamond crystal structure of Si (Figure S2A, S2B, and
S2C); and lattice-resolved TEM images and their FFT for Figure
Acknowledgment. We thank Dr. Yossi Lereah for helpful
discussions on the analysis of HRTEM experiments. We also thank
Dr. Ela Strauss and Dr. Yuri Rosenberg for their help with TOF-
SIMS and XRD measurements, respectively. This study was
partially supported by the Israel Science Foundation (ISF), Legacy
Program, Israel. We thank Mr. Roni Naftali for his generous
support.
1F, showing the uniform crystalline structure of Si (Figure S3A,
S3B, and S3C). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA808483T
J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3689