16 research outputs found
Guided Growth of Horizontal GaN Nanowires on Spinel with Orientation-Controlled Morphologies
We report the guided growth of horizontal
GaN nanowires (NWs) on
spinel (MgAl<sub>2</sub>O<sub>4</sub>) substrates with three different
orientations: (111), (100), and (110). The NWs form ordered arrays
with distinct morphologies on the surface of the substrates, controlled
by the interaction with the substrate. The geometry of the NWs matches
the symmetry of the spinel surfaces: on MgAl<sub>2</sub>O<sub>4</sub>(111), (100), and (110) the NWs grow in six, four, and two directions,
respectively. The epitaxial relations and morphologies of the NWâsubstrate
interface were characterized by cross-sectional transmission electron
microscopy. The substrate was found to be mobilized during the growth
and either climb up or recede on/under one or two sides of the NW,
depending on the substrate orientation. Possible reasons for the similarity
and differences between the orientations of the NWs and thin GaN films
grown on MgAl<sub>2</sub>O<sub>4</sub> are proposed. These results
demonstrate the generality and flexibility of the guided growth phenomenon
in NWs and specifically show that MgAl<sub>2</sub>O<sub>4</sub>(111)
could be a low-mismatch substrate for the growth of high-quality GaN
layers and NWs
Field-Effect Transistors Based on WS<sub>2</sub> Nanotubes with High Current-Carrying Capacity
We
report the first transistor based on inorganic nanotubes exhibiting
mobility values of up to 50 cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup> for an individual WS<sub>2</sub> nanotube. The current-carrying
capacity of these nanotubes was surprisingly high with respect to
other low-dimensional materials, with current density at least 2.4
Ă 10<sup>8</sup> A cm<sup>â2</sup>. These results demonstrate
that inorganic nanotubes are promising building blocks for high-performance
electronic applications
Bottom-Up Tri-gate Transistors and Submicrosecond Photodetectors from Guided CdS Nanowalls
Tri-gate transistors
offer better performance than planar transistors
by exerting additional gate control over a channel from two lateral
sides of semiconductor nanowalls (or âfinsâ). Here we
report the bottom-up assembly of aligned CdS nanowalls by a simultaneous
combination of horizontal catalytic vaporâliquidâsolid
growth and vertical facet-selective noncatalytic vaporâsolid
growth and their parallel integration into tri-gate transistors and
photodetectors at wafer scale (cm<sup>2</sup>) without postgrowth
transfer or alignment steps. These tri-gate transistors act as enhancement-mode
transistors with an on/off current ratio on the order of 10<sup>8</sup>, 4 orders of magnitude higher than the best results ever reported
for planar enhancement-mode CdS transistors. The response time of
the photodetector is reduced to the submicrosecond level, 1 order
of magnitude shorter than the best results ever reported for photodetectors
made of bottom-up semiconductor nanostructures. Guided semiconductor
nanowalls open new opportunities for high-performance 3D nanodevices
assembled from the bottom up
Guided CdSe Nanowires Parallelly Integrated into Fast Visible-Range Photodetectors
One-dimensional
semiconductor nanostructures, such as nanowires
(NWs), have attracted tremendous attention due to their unique properties
and potential applications in nanoelectronics, nano-optoelectronics,
and sensors. One of the challenges toward their integration into practical
devices is their large-scale controlled assembly. Here, we report
the guided growth of horizontal CdSe nanowires on five different planes
of sapphire. The growth direction and crystallographic orientation
are controlled by the epitaxial relationship with the substrate as
well as by a graphoepitaxial effect of surface nanosteps and grooves.
CdSe is a promising direct-bandgap IIâVI semiconductor active
in the visible range, with potential applications in optoelectronics.
The guided CdSe nanowires were found to have a wurtzite single-crystal
structure. Field-effect transistors and photodetectors were fabricated
to examine the nanowire electronic and optoelectronic properties,
respectively. The latter exhibited the fastest rise and fall times
ever reported for CdSe nanostructures as well as a relatively high
gain, both features being essential for optoelectronic applications
Surface-Guided CsPbBr<sub>3</sub> Perovskite Nanowires on Flat and Faceted Sapphire with Size-Dependent Photoluminescence and Fast Photoconductive Response
All-inorganic lead
halide perovskite nanowires have been the focus
of increasing interest since they exhibit improved stability compared
to their hybrid organicâinorganic counterparts, while retaining
their interesting optical and optoelectronic properties. Arrays of
surface-guided nanowires with controlled orientations and morphology
are promising as building blocks for various applications and for
systematic research. We report the horizontal and aligned growth of
CsPbBr<sub>3</sub> nanowires with a uniform crystallographic orientation
on flat and faceted sapphire surfaces to form arrays with 6-fold and
2-fold symmetries, respectively, along specific directions of the
sapphire substrate. We observed waveguiding behavior and diameter-dependent
photoluminescence emission well beyond the quantum confinement regime.
The arrays were easily integrated into multiple devices, displaying
p-type behavior and photoconductivity. Photodetectors based on those
nanowires exhibit the fastest rise and decay times for any CsPbBr<sub>3</sub>-based photodetectors reported so far. One-dimensional arrays
of halide perovskite nanowires are a promising platform for investigating
the intriguing properties and potential applications of these unique
materials
Guided Growth of Horizontal GaN Nanowires on Quartz and Their Transfer to Other Substrates
The guided growth of horizontal nanowires has so far been demonstrated on a limited number of substrates. In most cases, the nanowires are covalently bonded to the substrate where they grow and cannot be transferred to other substrates. Here we demonstrate the guided growth of well-aligned horizontal GaN nanowires on quartz and their subsequent transfer to silicon wafers by selective etching of the quartz while maintaining their alignment. The guided growth was observed on different planes of quartz with varying degrees of alignment. We characterized the crystallographic orientations of the nanowires and proposed a new mechanism of âdynamic graphoepitaxyâ for their guided growth on quartz. The transfer of the guided nanowires enabled the fabrication of back-gated field-effect transistors from aligned nanowire arrays on oxidized silicon wafers and the production of crossbar arrays. The guided growth of transferrable nanowires opens up the possibility of massively parallel integration of nanowires into functional systems on virtually any desired substrate
Surface-Guided CoreâShell ZnSe@ZnTe Nanowires as Radial pân Heterojunctions with Photovoltaic Behavior
The
organization of nanowires on surfaces remains a major obstacle toward
their large-scale integration into functional devices. Surfaceâmaterial
interactions have been used, with different materials and substrates,
to guide horizontal nanowires during their growth into well-organized
assemblies, but the only guided nanowire heterostructures reported
so far are axial and not radial. Here, we demonstrate the guided growth
of horizontal coreâshell nanowires, specifically of ZnSe@ZnTe,
with control over their crystal phase and crystallographic orientations.
We exploit the directional control of the guided growth for the parallel
production of multiple radial pân heterojunctions and probe
their optoelectronic properties. The devices exhibit a rectifying
behavior with photovoltaic characteristics upon illumination. Guided
nanowire heterostructures enable the bottom-up assembly of complex
semiconductor structures with controlled electronic and optoelectronic
properties
Photoconductive CdSe Nanowire Arrays, Serpentines, and Loops Formed by Electrodeposition on Self-Organized Carbon Nanotubes
Semiconducting nanowires frequently have enhanced properties
and
unique functionality compared to their bulk counterparts. Controlling
the geometry of nanowires is crucial for their integration into nanoscale
devices because the shape of a device component can dictate its functionality,
such as in the case of a mechanical spring or an antenna. We demonstrate
a novel synthetic method for making polycrystalline CdSe nanowires
with controlled geometries by using self-organized single-walled carbon
nanotubes as a template for the selective electrodeposition of nanowires.
Nanowires of up to hundreds of micrometers in length are formed as
high-density straight arrays, as well as in the shape of serpentines
and loops. These nanowires exhibit significant photoluminescence and
photoconductivity applicable to photodetectors and respond to illumination
up to 2 orders of magnitude faster than single crystalline CdSe
BCN Nanotubes as Highly Sensitive Torsional Electromechanical Transducers
Owing
to their mechanically tunable electronic properties, carbon
nanotubes (CNTs) have been widely studied as potential components
for nanoelectromechanical systems (NEMS); however, the mechanical
properties of multiwall CNTs are often limited by the weak shear interactions
between the graphitic layers. Boron nitride nanotubes (BNNTs) exhibit
a strong interlayer mechanical coupling, but their high electrical
resistance limits their use as electromechanical transducers. Can
the outstanding mechanical properties of BNNTs be combined with the
electromechanical properties of CNTs in one hybrid structure? Here,
we report the first experimental study of boron carbonitride nanotube
(BCNNT) mechanics and electromechanics. We found that the hybrid BCNNTs
are up to five times torsionally stiffer and stronger than CNTs, thereby
retaining to a large extent the ultrahigh torsional stiffness of BNNTs.
At the same time, we show that the electrical response of BCNNTs to
torsion is 1 to 2 orders of magnitude higher than that of CNTs. These
results demonstrate that BCNNTs could be especially attractive building
blocks for NEMS
Crystallographic Mapping of Guided Nanowires by Second Harmonic Generation Polarimetry
The
growth of horizontal nanowires (NWs) guided by epitaxial and
graphoepitaxial relations with the substrate is becoming increasingly
attractive owing to the possibility of controlling their position,
direction, and crystallographic orientation. In guided NWs, as opposed
to the extensively characterized vertically grown NWs, there is an
increasing need for understanding the relation between structure and
properties, specifically the role of the epitaxial relation with the
substrate. Furthermore, the uniformity of crystallographic orientation
along guided NWs and over the substrate has yet to be checked. Here
we perform highly sensitive second harmonic generation (SHG) polarimetry
of polar and nonpolar guided ZnO NWs grown on <i>R</i>-plane
and <i>M</i>-plane sapphire. We optically map large areas
on the substrate in a nondestructive way and find that the crystallographic
orientations of the guided NWs are highly selective and specific for
each growth direction with respect to the substrate lattice. In addition,
we perform SHG polarimetry along individual NWs and find that the
crystallographic orientation is preserved along the NW in both polar
and nonpolar NWs. While polar NWs show highly uniform SHG along their
axis, nonpolar NWs show a significant change in the local nonlinear
susceptibility along a few micrometers, reflected in a reduction of
40% in the ratio of the SHG along different crystal axes. We suggest
that these differences may be related to strain accumulation along
the nonpolar wires. We find SHG polarimetry to be a powerful tool
to study both selectivity and uniformity of crystallographic orientations
of guided NWs with different epitaxial relations