Nanorods
DOI: 10.1002/smll.200701289
Two-Component Nanorod Arrays by
Glancing-Angle Deposition**
discussed below, that the growth front of the developing
nanorod maintains the initial shape, so that the total
deposition rate rA þ rB along the rod axis is independent of
the lateral position x. The deposition rate rB for material B is
controlled by cos b, where
b ¼ a ð90 gÞ
(1)
Chunming Zhou and Daniel Gall*
A scalable strategy for three-dimensional (3D) assembly of
dissimilar materials with nanometer resolution is key to
harness the full potential of physical properties expected
within the quantum-confined or interface-dominated size
regime. Chemical synthesis methods yield layered and coaxial
nanowires[1,2] and branched nanocrystals,[3] while physical
vapor deposition (PVD) techniques are particularly effective
in creating nanometer-scale structures along the growth
direction perpendicular to the substrate.[4] However, despite
various successes in nanopatterning research, lateral (inplane) nanostructuring remains a great challenge. Here we
report a method to fabricate arrays of nanorods with laterally
stacked components. This technique, termed simultaneous
opposite glancing-angle deposition (SO-GLAD), builds on
conventional glancing-angle deposition,[5,6] which uses variable deposition angles to engineer (single-component) 3D
nanostructures including nanopillars,[7–9] zigzags,[10,11] nanospirals,[12–14] nanotubes,[15] and branched nanocolumns.[16––18]
During SO-GLAD, two different materials are deposited
simultaneously from oblique angles from opposite sides onto
self-assembled regular nanosphere arrays. Atomic shadowing
causes selective deposition only on the side that is exposed to
the deposition flux, leading to nanorod growth where each rod
consists of two laterally separated components. This is
demonstrated by the growth of 220-nm-wide Si–Ta twocomponent rods which exhibit a measured vertical interface
width of 28 nm. This value is in good agreement with our
theoretical prediction based on a purely geometrical analysis,
suggesting that nanodesign at considerably smaller length
scales is possible. SO-GLAD provides a new path to build
uniquely shaped multi component nanostructures from a wide
range of materials systems that are joined in a single
deposition process.
Figure 1a illustrates the SO-GLAD technique: materials A
and B, which are in our case Si and Ta, respectively, are
simultaneously deposited from oblique deposition angles a,
onto a substrate which exhibits surface mounds that are, in our
case, defined by silica spheres. We assume, in agreement with
previous studies[19,20] and the experimental observations
[] Prof. D. Gall, Dr. C. M. Zhou
Department of Materials Science and Engineering
Rensselaer Polytechnic Institute
Troy, NY 12180 (USA)
E-mail: [email protected]
[] This research was supported by the National Science Foundation,
under grant no. DMR-0645312 and CMMI-0727413. We also
acknowledge funding from the Donors of the American Chemical
Society Petroleum Research Fund under grant no. 44226-G10. We
thank Dr. Huafang Li for her generous help with TEM.
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is the angle between the surface normal and the deposition
flux, and g is the angle between the growth front and the
horizontal axis. For a spherical rod-tip, g is determined by x
and the rod width w, yielding
2x
rB / cos b ¼ cos a arcsin
w
(2)
We note that the growth rate is zero for b 908 as well as
for g 0, due to complete shadowing by the growing rod and
the neighboring rod, respectively. In addition, rB is, in general,
also affected by (i) a factor (sing)1, since the rod growth
direction is not parallel to the local surface normal, and by
(ii) a deposition rate that decreases with increasing x, due to
shadowing from neighboring nanorods. However, by normalizing with the x-independent total rate rA þ rB, both these
corrections are accounted for. Thus, the relative rates from the
two opposite sources, which determine the concentration
A
B
profiles cA ðxÞ ¼ rArþr
and cB ðxÞ ¼ rArþr
for materials A and B,
B
B
respectively, are determined numerically using Equations (1)
and (2). The result of this calculation is shown in Figure 1b,
which is a plot of the relative concentrations versus the
normalized lateral position corresponding to the chosen
experimental geometry of this study, for which the Si and
Ta sources are positioned from left and right, respectively,
both at a ¼ 848. The transition region, where the Ta
composition increases from 0 to 1, defines the interface of
the laterally stacked nanorod. The two components are mixed
in this region, since the near-horizontal portion of the growth
front is exposed to both deposition fluxes. The interface width
D decreases with increasing azimuthal deposition angle a, as
indicated by the calculated composition profiles for a ¼ 80 and
888, plotted as dotted and dashed lines in Figure 1b,
respectively. The decrease is approximately linear for large
a (>758), and follows:
D
¼ cos a
w
(3)
For the case of 220 nm-wide Si–Ta nanorods with a ¼ 848, the
interface width is 23 nm which is 10% of the rod width.
Figure 2a is a typical plan-view scanning electron
microscopy (SEM) micrograph from an array of wellseparated Si–Ta nanorods grown on a patterned substrate
by simultaneous opposite glancing-angle sputter deposition.
The nanorods exhibit a regular hexagonal array, replicating
the close-packed pattern of the silica nanospheres, which
indicates that each nanosphere acts as a nucleation site for a
Si–Ta nanorod. The average rod width is 220 30 nm, in good
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Figure 1. a) Schematic of the SO-GLAD technique. b) Predicted Si and Ta
concentration profiles versus normalized lateral position in a nanorod.
agreement with 215 32 nm, the value obtained from crosssectional micrographs discussed below. The measured interrod (center-to-center) separation, 280 30 nm, is close to the
nominal diameter D ¼ 260 nm of the nanospheres. The
nanorods are approximately circular symmetric. This is in
contrast to Cu rods that exhibit anisotropic broadening in the
deposition plane when grown under comparable geometrical
constraints but at larger homologous temperature.[18] Statistical analyses, using large-area micrographs with > 100 rods
provide a value for the nanorod number density of 14 mm2,
which is 12% smaller than the silica nanosphere number
density of 16 mm2. This slight difference can be attributed to
‘‘missing’’ columns, as shown by dark areas in Figure 2a,
caused by vacancy defects (missing spheres) in the initial
nanosphere pattern as well as extinct nanorods, which
terminate growth prematurely. The extinction is due to a
competitive growth mode that favors the growth of larger rods
at the expense of smaller neighbors which, in turn, terminate
their growth due to the exacerbated shadowing effects during
glancing-angle deposition.[20,21] The early growth termination
of some rods is also evident in the corresponding crosssectional micrograph in Figure 2b, showing 400-nm-tall Si–Ta
nanorods grown on a monolayer of close-packed silica
nanospheres on Si(001). Four of the 33 rods in this cross-
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Figure 2. SEM micrographs from nanorods with laterally stacked Si and
Ta components grown on a hexagonal array of silica spheres. a) planview micrograph; b) and c) cross-sectional micrographs from the same
sample area using secondary and back-scattered electrons, respectively.
sectional image are considerably shorter than their neighbors,
illustrating the dramatic effect of growth competition when
rods develop unequal sizes. However, the overall morphology
is rather regular, with rod widths (215 32 nm) that remain
approximately constant with height, indicating that the column
competition is relatively weak. We attribute the absence of
strong competition to the relatively large 60-nm-wide average
gap between neighboring rods, which limits long-range interrod shadowing interactions. The rods in Figure 2b are slighty
tilted toward the right, which is attributed to a larger
deposition flux from the right.
Figure 2c is a back-scattered electron image of Si–Ta
nanorods from the same area as shown in Figure 2b. This crosssectional sample was prepared by cleavage along the
deposition axis, such that for the orientation of this
micrograph, the Si deposition source is on the left and the
Ta source on the right. The strong compositional contrast is
due to the difference in electron back-scattering yield, which is
considerably higher for Ta (Z ¼ 73) than for Si (Z ¼ 14) and O
(Z ¼ 8), causing regions of the rods that consist of Ta to appear
bright while the Si and SiO2 are darker, as labeled in the image.
All rods are bright on the right and dark on the left, with
relatively sharp vertical interfaces that separate the Si and Ta
sides, respectively. Therefore, during deposition, Si (Ta) atoms
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impinge from the left (right) and only deposit onto the left
(right) side of the growing nanorods. This is exactly the
envisioned SO-GLAD process which yields laterally stacked
two-component nanorods, as illustrated in Figure 1a.
The compositional contrast is also obvious in the
transmission electron micrograph in Figure 3a, from
225 20 nm-wide and 480 nm-tall Si–Ta nanorods. Here,
the SiO2 spheres and the Si appear bright, while the Ta is dark,
since the electron-scatter cross-section is higher for the
crystalline high-Z Ta than for the amorphous Si and SiO2.
The left half-rod (Si) has a measured average width of
100 20 nm, while the right side (Ta) is 125 16 nm wide. The
rods exhibit rounded tops, corresponding to the growth front
also discussed in Figure 1a. However, the third rod from the
left is less high and shows a more pointed top. We attribute this
to the onset of growth termination for this third rod, due to
growth competition: shadowing from neighbors reduces the
deposition rate on this rod, such that, prior to complete
shadowing, deposition occurs only at the rod center, resulting
in the characteristic pointed top.
Figure 3b is an intensity line scan obtained from Figure 3a,
at the position as indicated by the arrows. The vertical axis,
labeled fe, corresponds to the fraction of electrons that are
scattered when travelling through the TEM specimen. The
background level, fe < 10% between nanorods, is associated
with electron scattering in the amorphous C membrane of the
TEM grid. The Si half-rods scatter approximately half of the
electrons ( fe ¼ 50%), while this fraction increases to above
90% for Ta. The transition region between the Si and Ta halfrods, where fe increases from 0.5 to >0.9, provides an estimate
of the interface width D, as indicated for the second rod in
Figure 3b. From the analysis of multiple rods, we obtain an
average value for D of 28 nm, which is only slightly above
23 nm, the estimate from Equation 3. This slightly higher
measured value is attributed to experimental broadening
associated with alignment uncertainties of the interface plane
with the electron beam in the TEM, as well as TEM
micrograph resolution. Therefore, the true experimental
interface width is in good agreement with purely geometric
shadowing arguments using a spherical growth front, as
discussed above, indicating that intermixing due to surface
diffusion is negligible for our Si–Ta rods. This, in turn, suggests
that considerably sharper interfaces may be achieved by the
SO-GLAD technique by simply increasing the deposition
angle from the here-employed a ¼ 848. In addition, patterning
with faceted sharp surface mounds is expected to promote
component separation and may yield much smaller D values.
In summary, periodic arrays of Si–Ta nanorods with
laterally stacked Si and Ta components were grown by SOGLAD onto silica nanosphere monolayer patterns. SEM and
TEM characterizations show that each nanorod consists of
laterally separated Si and Ta components, with a vertical
interface width of 28 nm. This value is in good agreement
with theoretical calculations based solely on the shadowing
geometry, suggesting that considerably sharper interfaces are
possible by SO-GLAD. These results demonstrate that
SO-GLAD provides a unique path to engineer nanostructure
arrays which exhibit complexities that go well beyond
single-component GLAD nanostructures with controlled
porosity,[5–18,22] vertical multilayer nanorods,[23–25] or codeposited laterally inhomogeneous layers.[26–30]
Experimental Section
Figure 3. a) Cross-sectional TEM micrograph from an array of Si–Ta twocomponent nanorods grown on silica spheres. b) Intensity line-scan
between the two arrows in (a), plotted as the fraction of scattered
electrons fe versus position x.
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The Si substrates were patterned by self-assembly of 260-nmdiameter silica nanospheres that form close-packed monolayers
during drying of a colloidal suspension on tilted hydrophilic
surfaces in a temperature and humidity controlled environment.[31] Nanorod arrays were grown onto the nanosphere
patterns in a load-locked ultrahigh vacuum (UHV) stainless steel
dc magnetron sputter deposition system with a base pressure of
1 109 Torr (1 107 Pa).[23] The 5 cm-diameter Ta (99.95%)
and Si (99.95%) targets were positioned at 7.5 cm from the
substrate, with the plane of the substrate surface perpendicularly
intercepting both target centers. A shield between the magnetrons
prevents crosscontamination of the targets, and a collimating
plate, 3 mm above the substrate surface, inhibits nondirectional
deposition flux from impinging onto the substrate and also
determines the deposition angle a ¼ 848. A schematic of the
deposition geometry can be found in the Supporting Information.
Depositions were done in 99.999% pure Ar at 3 mTorr (0.39 Pa),
with a constant power of 300 W applied to both Ta and Si sources,
yielding a column growth rate of 7 nm min1. The morphologies
and microstructures were investigated by scanning electron
microscopy (SEM, JEOL JSM-6335 Field Emission, operated at
5 kV with an emission current of 12 mA), back-scattered electron
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imaging using a Rutherford Backscattered Electron Imaging (RBEI)
detector, and transmission electron microscopy (TEM, JEOL JEM2010, operated at 200 kV). For cross-sectional SEM, samples were
cleaved along the deposition axis. TEM samples were prepared by
scratching nanorods, including the attached silica spheres, off the
surface onto ethanol wetted Cu TEM grids with a lacy carbon
coating membrane.
Keywords:
atomic shadowing . glancing-angle deposition . nanorods .
silicon . tantalum
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Received: December 20, 2007
Published online: August 8, 2008
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