Published on Web 09/20/2005
Diamond-Hexagonal Semiconductor Nanocones with Controllable Apex Angle
Linyou Cao,† Lee Laim,† Chaoying Ni,‡ Bahram Nabet,§,†, and Jonathan E. Spanier*,†,§,
Department of Materials Science and Engineering, Department of Electrical and Computer Engineering,
A. J. Drexel Nanotechnology Institute, Drexel UniVersity, Philadelphia, PennsylVania 19104, and
Department of Materials Science and Engineering and the W. M. Keck Electron Microscopy Facility,
UniVersity of Delaware, Newark, Delaware 19716
Received July 6, 2005; E-mail: spanier@drexel.edu
Control of nanocrystal topology and of the formation of selected
polymorphs is an important feature for fundamental studies of
crystal growth and for investigating the shape and structural
dependence of many properties.1 Such control is also critical in
developing new pathways for materials synthesis2 and new ap-
plications of nanostructured materials.3
Conical nanostructures of carbon,4a BN,4b AlN,4c SiC,4d ZnO,4e
and Si4f-g have attracted interest recently because of their potential
uses in field emission, in nanoscaled manipulation, and as scanning
probe microscopy and near-field scanning optical microscopy
probes. Tapered nanostructures are expected to have higher bending
Figure 1. (a) SEM image of an array of silicon nanocones (SiNCs). Upper
inset: a representative TEM image of one SiNC tip showing the Au
stiffness than nanotubes and nanowires and better resistance to
thermal-induced drift than nanowires.5 Shape engineering of
properties represents an important design flexibility for these and
other applications.6
nanoparticle at the tip. Lower inset: the hexagonal cross-section of a SiNC
with a scale bar of 1 µm. (b) Distribution of conical angles and log-normal
fitting curve with fitting parameters identified in the text.
though some possess catalyst-free tips with rtip ≈ 1-2 nm (not
shown). Similar results were obtained with GeNCs (see Supporting
Information). EDS analysis performed in TEM and SEM (not
shown) confirms that each SiNC consists of Si throughout the
structure. In addition, the SiNCs possess a relatively narrow
distribution of conical angles (Figure 1b). This distribution can be
fitted by a log-normal function F(θ) ) A exp[-ln2(θ/θm)/2σ2] using
the method of least squares, where the fitting parameters A, θm,
and σ correspond to the fraction of SiNCs with the most probable
apex angle, the most probable apex angle, and the width of the
distribution, respectively; such a distribution is observed often in
the morphology of nanoscale materials.8c
Silicon-based nanostructures produced by bottom-up methods
are highly attractive as key elements in emerging nanoelectronic
device technologies due to performance superior to current top-
down fabricated Si-based commercial devices and its cost-effective-
ness for developing large-scaled electronic circuits.7 Despite the
number of polymorphs in bulk Si, nanostructured silicon materials
produced by bottom-up methods have been limited to the diamond-
cubic (DC) structure.8 Here, we report the metal-catalyzed chemical
vapor deposition synthesis of Si and Ge solid nanocones (SiNCs,
GeNCs) with control of apex angles. Significantly, we also find
that the structure of the SiNCs is not the conventional DC phase,
but rather diamond-hexagonal (DH).
Images a and b of Figure 2 show a representative TEM image
of a SiNC and the selected area electron diffraction (SAED) which
was collected from the area indicated by the circle in Figure 2a,
respectively. Shown in the inset of Figure 2b is the defocusing
diffraction-mode image of the same area where SAED was
collected. This diffraction pattern indicates hexagonal symmetry,
and it can be indexed as a superposition of [0001] and [123h0] zone
axis patterns from the DH structure. These zone axis patterns
possess common 1h21h0 reflections, and analysis demonstrates that
the axial direction of this SiNC is [1h21h0], as illustrated in Figure
2b. In addition, Figure 2c shows a HRTEM image of the [21h1h0]
projection of the SiNC, which is in good accordance with the
modeling view for DH silicon shown in its inset. (Those Si atoms
which are colored gray in the model cannot be observed clearly in
the HRTEM image due to limits of instrument resolution.) A
different type of diffraction pattern is also observed (Figure 2d).
This pattern can be indexed as superposition of [011h1] and two
201 ([202h1] and [022h1]) zone patterns of DH Si (see Supporting
Information). From these HRTEM and electron diffractions, the
lattice constants are determined to be a ) 3.82 Å and c ) 6.22 Å,
with c/a ) 1.63; these values are consistent with those reported in
DH silicon film (see Supporting Information).8a,b We believe the
formation of DH silicon phase is due to stacking faults on {111}cubic
Evaporated and annealed 2 nm thick Au films or 2-30 nm
diameter Au colloids (Ted Pella) cast on poly-L-lysine-functional-
ized SiO2-coated Si(100) substrates were exposed to flowing
precursor of 10% SiH4 in He, ∼20-200 sccm (10% GeH4 in He,
∼50 sccm), and carrier gas of N2 or 5% H2 in Ar, ∼100 sccm
(∼50 sccm) at 650 °C (400 °C) under ∼20 Torr (∼400 Torr) for
synthesis of SiNCs (GeNCs) in a quartz tube furnace for 5-150
min. The resulting SiNCs and GeNCs were characterized by
scanning electron microscopy (SEM, FEI-XL30), energy-dispersive
X-ray spectroscopy (EDS), transmission electron microscopy (TEM,
JEOL 2010F), and Raman scattering spectroscopy (Renishaw 1000).
TEM samples were prepared by sonicating the SiNCs and GeNCs
from growth substrates in ethanol and dropping onto Ni or Cu TEM
grids.
Figure 1a shows an SEM image of a representative yield of
SiNCs; the insets show a TEM image of a SiNC tip with a catalyst
particle (upper) and an SEM of the hexagonal cross-section of a
SiNC (lower). The SiNCs shown are ∼10 µm long, ∼2 µm wide
at the base, and the typical tip radius rtip of the SiNC is ∼5 nm,
† Department of Materials Science and Engineering, Drexel University.
§ Department of Electrical and Computer Engineering, Drexel University.
A. J. Drexel Nanotechnology Institute, Drexel University.
‡ University of Delaware.
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13782
J. AM. CHEM. SOC. 2005, 127, 13782-13783
10.1021/ja0544814 CCC: $30.25 © 2005 American Chemical Society