10 research outputs found
Solubility of Boron, Carbon, and Nitrogen in Transition Metals: Getting Insight into Trends from First-Principles Calculations
Efficient chemical vapor deposition
synthesis of two-dimensional
(2D) materials such as graphene, boron nitride, and mixed BCN systems
with tunable band gaps requires precise knowledge of the solubility
and mobility of B/C/N atoms in the transition metals (TMs) used as
substrates for the growth. Yet, surprisingly little is known about
these quantities either from experiments or simulations. Using first-principles
calculations, we systematically study the behavior of B/C/N impurity
atoms in a wide range of TMs. We compute formation energies of B/C/N
interstitials and demonstrate that they exhibit a peculiar but common
behavior for TMs in different rows of the periodic table, as experimentally
observed for C. Our simulations indicate that this behavior originates
from an interplay between the unit cell volume and filling of the
d-shell electronic states of the metals. We further assess the vibrational
and electronic entropic contributions to the solubility, as well as
the role of anharmonic effects. Finally, we calculate the migration
barriers, an important parameter in the growth kinetics. Our results
not only unravel the fundamental behavior of interstitials in TMs
but also provide a large body of reference data, which can be used
for optimizing the growth of 2D BCN materials
All-Graphene Three-Terminal-Junction Field-Effect Devices as Rectifiers and Inverters
We present prominent tunable and switchable room-temperature rectification performed at 100 kHz ac input utilizing micrometer-scale three-terminal junction field-effect devices. Monolayer CVD graphene is used as both a channel and a gate electrode to achieve all-graphene thin-film structure. Instead of ballistic theory, we explain the rectification characteristics through an electric-field capacitive model based on self-gating in the high sourceādrain bias regime. Previously, nanoscale graphene three-terminal junctions with the ballistic (or quasi-ballistic) operation have shown rectifications with relatively low efficiency. Compared to strict nanoscale requirements of ballistic devices, diffusive operation gives more freedom in design and fabrication, which we have exploited in the cascading device architecture. This is a significant step for all-graphene thin-film devices for integrated monolithic graphene circuits
Youngās Modulus of Wurtzite and Zinc Blende InP Nanowires
The Youngās
modulus of thin conical InP nanowires with either
wurtzite or mixed āzinc blende/wurtziteā structures
was measured. It has been shown that the value of Youngās modulus
obtained for wurtzite InP nanowires (<i>E</i><sub>[0001]</sub> = 130 Ā± 30 GPa) was similar to the theoretically predicted
value for the wurtzite InP material (<i>E</i><sub>[0001]</sub> = 120 Ā± 10 GPa). The Youngās modulus of mixed āzinc
blende/wurtziteā InP nanowires (<i>E</i><sub>[111]</sub> = 65 Ā± 10 GPa) appeared to be 40% less than the theoretically
predicted value for the zinc blende InP material (<i>E</i><sub>[111]</sub> = 110 GPa). An advanced method for measuring the
Youngās modulus of thin and flexible nanostructures is proposed.
It consists of measuring the flexibility (the inverse of stiffness)
profiles 1/<i>k</i>(<i>x</i>) by the scanning
probe microscopy with precise control of loading force in nanonewton
range followed by simulations
Atomic Layer Engineering of Er-Ion Distribution in Highly Doped Er:Al<sub>2</sub>O<sub>3</sub> for Photoluminescence Enhancement
For
the past decade, erbium-doped integrated waveguide amplifiers
and lasers have shown excellent potential for on-chip amplification
and generation of light at the important telecommunication wavelength
regime. However, Er-based integrated devices can only provide small
gain per unit length due to the severe energy-transfer between the
Er-ions at high concentration levels. Therefore, active ion concentrations
have been limited to <1% levels in these devices for optimal performance.
Here, we show an efficient and practical way of fabricating Er-doped
Al<sub>2</sub>O<sub>3</sub> with Er-concentration as high as ā¼3.5%
before concentration quenching starts to limit the C-band emission
in our material. The Er-doped Al<sub>2</sub>O<sub>3</sub> was fabricated
by engineering the distribution of the Er-ions in Al<sub>2</sub>O<sub>3</sub> with the atomic layer deposition (ALD) technique. By choosing
a proper precursor for the fabrication of Er<sub>2</sub>O<sub>3</sub>, the steric hindrance effect was utilized to increase the distance
between the Er-ions in the lateral direction. In the vertical direction,
the distance was controlled by introducing subsequent Al<sub>2</sub>O<sub>3</sub> layers between Er<sub>2</sub>O<sub>3</sub> layers.
This atomic scale control of the Er-ion distribution allows us to
enhance the photoluminescence of our Er:Al<sub>2</sub>O<sub>3</sub> material by up to 16 times stronger when compared to the case where
the Er-concentration is ā¼0.6%. In addition, long lifetime of
approximately 5 ms is preserved in the Er-ions even at such high concentration
levels. Thus, our optimized ALD process shows very promising potential
for the deposition of optical gain media for integrated photonics
structures
Fabrication of Dual-Type Nanowire Arrays on a Single Substrate
A novel
method for fabricating dual-type nanowire (NW) arrays is
presented. Two growth steps, selective-area epitaxy (SAE) in the first
step and vaporāliquidāsolid (VLS) in the second step,
are used to grow two types of NWs on the same GaAs substrate. Different
precursors can be used for the growth steps, resulting in sophisticated
compositional control, as demonstrated for side-by-side grown GaAs
and InP NWs. It was found that parasitic growth occurs on the NWs
already present on the substrate during the second growth step and
that the SAE NWs shadow the growth of the VLS NWs. Optical reflectance
measurements revealed the dual-type array having improved light trapping
properties compared to single-type arrays. Dual-type NW arrays could
be practical for thermoelectric generation, photovoltaics and sensing
where composition control of side-by-side NWs and complex configurations
are beneficial
Visualization 4: Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures
Visualization 4. 3D SHG images of the vertically-aligned semiconductor nanowires using azimuthal polarization Originally published in Optics Express on 29 May 2017 (oe-25-11-12463
Visualization 1: Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures
Visualization 1. 3D SHG images of the vertically-aligned semiconductor nanowires using linear x polarization Originally published in Optics Express on 29 May 2017 (oe-25-11-12463
Visualization 3: Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures
Visualization 3. 3D SHG images of the vertically-aligned semiconductor nanowires using radial polarization Originally published in Optics Express on 29 May 2017 (oe-25-11-12463
Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy
Black phosphorus
(BP) is a layered semiconductor that recently
has been the subject of intense research due to its novel electrical
and optical properties, which compare favorably to those of graphene
and the transition metal dichalcogenides. In particular, BP has a
direct bandgap that is thickness-dependent and highly anisotropic,
making BP an interesting material for nanoscale optical and optoelectronic
applications. Here, we present a study of the anisotropic third-harmonic
generation (THG) in exfoliated BP using a fast scanning multiphoton
characterization method. We find that the anisotropic THG arises directly
from the crystal structure of BP. We calculate the effective third-order
susceptibility of BP to be ā¼1.64 Ć 10<sup>ā19</sup> m<sup>2</sup> V<sup>ā2</sup>. Further, we demonstrate that
multiphoton microscopy can be used for rapid, large-area characterization
indexing of the crystallographic orientations of many exfoliated BP
flakes from one set of multiphoton images. This method is therefore
beneficial for samples of areas ā¼1 cm<sup>2</sup> in future
investigations of the properties and growth of BP
Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy
Black phosphorus
(BP) is a layered semiconductor that recently
has been the subject of intense research due to its novel electrical
and optical properties, which compare favorably to those of graphene
and the transition metal dichalcogenides. In particular, BP has a
direct bandgap that is thickness-dependent and highly anisotropic,
making BP an interesting material for nanoscale optical and optoelectronic
applications. Here, we present a study of the anisotropic third-harmonic
generation (THG) in exfoliated BP using a fast scanning multiphoton
characterization method. We find that the anisotropic THG arises directly
from the crystal structure of BP. We calculate the effective third-order
susceptibility of BP to be ā¼1.64 Ć 10<sup>ā19</sup> m<sup>2</sup> V<sup>ā2</sup>. Further, we demonstrate that
multiphoton microscopy can be used for rapid, large-area characterization
indexing of the crystallographic orientations of many exfoliated BP
flakes from one set of multiphoton images. This method is therefore
beneficial for samples of areas ā¼1 cm<sup>2</sup> in future
investigations of the properties and growth of BP