9 research outputs found
A Facile Route for Producing Single-Crystalline Epitaxial Perovskite Oxide Thin Films
We
report how a low vacuum pressure process followed by a few-minute
annealing enables epitaxial stabilization, producing high-quality,
phase-pure, single-crystalline epitaxial, and misfit dislocation-free
BiFeO<sub>3</sub>(001) thin films on SrTiO<sub>3</sub>(001) at ā¼450
Ā°C less than current routes. These results unambiguously challenge
the widely held notion that atomic layer deposition (ALD) is not appropriate
for attaining high-quality chemically complex oxide films on perovskite
substrates in single-crystalline epitaxial form, demonstrating applicability
as an inexpensive, facile, and highly scalable route
Selective Epitaxial Growth on Germanium Nanowires via Hybrid Oxide-Stabilized/VaporāLiquidāSolid Growth
The introduction low levels of oxygen during the vaporāliquidāsolid
growth (VLS) of germanium nanowires causes an oxide sheath to form
at the catalyst/nanowire/vapor interface during growth. This results
in extremely high aspect ratio nanowires due to the removal of homoepitaxial
deposition and the finite energy required for heterogeneous nucleation
of germanium on its oxide. With the removal of oxygen, the catalyzed
oxide sheath terminates and conventional growth with finite sidewall
deposition dominates subsequent growth. The successful transition
between oxide-stabilized and conventional VLS regimes can be deliberately
manipulated to grow finite conical nanowire segments with discontinuous
changes in diameter
Dynamics of Photogenerated Surface Charge on BiFeO<sub>3</sub> Films
We report on the spatial and temporal evolution of photoinduced charge generation and carrier separation in heteroepitaxial BiFeO<sub>3</sub> thin films deposited on Nb:SrTiO<sub>3</sub> as measured in ambient at room temperature with Kelvin probe and piezoresponse force microscopy. Contributions from the self-poled and ferroelectric polarization charge are identified from the time evolution of the correlated surface potential and ferroelectric polarization in films as grown and following poling, and at different stages and intensities of optical illumination. Variations in the surface potential with bias voltage, switching history, and illumination intensity indicate how both bulk ferroelectric photovoltaic and the domain wall offset potential mechanisms contribute to the photogenerated charge
Shape-Controlled Vapor-Transport Growth of Tellurium Nanowires
A vapor transport method is employed to synthesize single
crystalline
tellurium nanowires with tailorable size using tellurium powder in
an inert atmosphere. The average nanowire diameter is tunable between
50 and 3000 nm with an associated length of 1 to 22 Ī¼m. Growth
temperature and time are used to specifically control the behavior
of supersaturation, leading to nucleation and morphological control
in this vaporāsolid growth regime. Analysis of the resulting
nanowire product provides insight into the dominant reaction kinetics
involved in these growths and suggests routes by which to control
growth product. This methodology provides a practical approach to
synthesizing high-quality tellurium nanowires of various sizes and
their incorporation into two-dimensional mesh-like structures through
controlled nucleation and growth dynamics
Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide
The
perovskite phase of cesium lead iodide (Ī±-CsPbI<sub>3</sub> or
āblackā phase) possesses favorable optoelectronic
properties for photovoltaic applications. However, the stable phase
at room temperature is a nonfunctional āyellowā phase
(Ī“-CsPbI<sub>3</sub>). Black-phase polycrystalline thin films
are synthesized above 330 Ā°C and rapidly quenched to room temperature,
retaining their phase in a metastable state. Using differential scanning
calorimetry, it is shown herein that the metastable state is maintained
in the absence of moisture, up to a temperature of 100 Ā°C, and
a reversible phase-change enthalpy of 14.2 (Ā±0.5) kJ/mol is observed.
The presence of atmospheric moisture hastens the black-to-yellow conversion
kinetics without significantly changing the enthalpy of the transition,
indicating a catalytic effect, rather than a change in equilibrium
due to water adduct formation. These results delineate the conditions
for trapping the desired phase and highlight the significant magnitude
of the entropic stabilization of this phase
Subsurface Imaging of Coupled Carrier Transport in GaAs/AlGaAs CoreāShell Nanowires
We demonstrate spatial probing of
carrier transport within GaAs/AlGaAs coreāshell nanowires with
nanometer lateral resolution and subsurface sensitivity by energy-variable
electron beam induced current imaging. Carrier drift that evolves
with applied electric field is distinguished from a coupled drift-diffusion
length. Along with simulation of injected electron trajectories, combining
beam energy tuning with precise positioning for selective probing
of core and shell reveals axial position- and bias-dependent differences
in carrier type and transport along parallel conduction channels.
These results indicate how analysis of transport within heterostructured
nanomaterials is no longer limited to nonlocal or surface measurements
Direct Measurement of Band Edge Discontinuity in Individual CoreāShell Nanowires by Photocurrent Spectroscopy
Group IIIāV coaxial coreāshell
semiconducting nanowire
heterostructures possess unique advantages over their planar counterparts
in logic, photovoltaic, and light-emitting devices. Dimensional confinement
of electronic carriers and interface complexity in nanowires are known
to produce local electronic potential landscapes along the radial
direction that deviate from those along the normal to planar heterojunction
interfaces. However, understanding of selected electronic and optoelectronic
carrier transport properties and device characteristics remains lacking
without a direct measurement of band alignment in individual nanowires.
Here, we report on, in the GaAs/Al<sub><i>x</i></sub>Ga<sub>1ā<i>x</i></sub>As and GaAs/AlAs coreāshell
nanowire systems, how photocurrent and photoluminescence spectroscopies
can be used together to construct a band diagram of an individual
heterostructure nanowire with high spectral resolution, enabling quantification
of conduction band offsets
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Surface Chemically Switchable Ultraviolet Luminescence from Interfacial Two-Dimensional Electron Gas
We
report intense, narrow line-width, surface chemisorption-activated
and reversible ultraviolet (UV) photoluminescence from radiative recombination
of the two-dimensional electron gas (2DEG) with photoexcited holes
at LaAlO<sub>3</sub>/SrTiO<sub>3</sub>. The switchable luminescence
arises from an electron transfer-driven modification of the electronic
structure via H-chemisorption onto the AlO<sub>2</sub>-terminated
surface of LaAlO<sub>3</sub>, at least 2 nm away from the interface.
The control of the onset of emission and its intensity are functionalities
that go beyond the luminescence of compound semiconductor quantum
wells. Connections between reversible chemisorption, fast electron
transfer, and quantum-well luminescence suggest a new model for surface
chemically reconfigurable solid-state UV optoelectronics and molecular
sensing
BaTiO<sub>3</sub> Thin Films from Atomic Layer Deposition: A Superlattice Approach
A superlattice
approach for the atomic layer deposition of polycrystalline
BaTiO<sub>3</sub> thin films is presented as an example for an effective
route to produce high-quality complex oxide films with excellent thickness
and compositional control. This method effectively mitigates any undesirable
reactions between the different precursors and allows an individual
optimization of the reaction conditions for the BaāO and the
TiāO subcycles. By growth of nanometer thick alternating BaĀ(OH)<sub>2</sub> and TiO<sub>2</sub> layers, the advantages of binary oxide
atomic layer deposition are transferred into the synthesis of ternary
compounds, permitting extremely high control of the cation ratio and
superior uniformity. Whereas the BaĀ(OH)<sub>2</sub> layers are partially
crystalline after the deposition, the TiO<sub>2</sub> layers remain
mostly amorphous. The layers react to polycrystalline, polymorph BaTiO<sub>3</sub> above 500 Ā°C, releasing H<sub>2</sub>O. This solid-state
reaction is accompanied by an abrupt decrease in film thickness. Transmission
electron microscopy and Raman spectroscopy reveal the presence of
hexagonal BaTiO<sub>3</sub> in addition to the perovskite phase in
the annealed films. The microstructure with relatively small grains
of ā¼70 Ć
and different phases is a direct consequence
of the abrupt formation reaction. The electrical properties transition
from the initially highly insulating dielectric semiamorphous superlattice
into a polycrystalline BaTiO<sub>3</sub> thin film with a dielectric
constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing
at 600 Ā°C in air, which, together with the suppression of ferroelectricity
at room temperature, are very appealing properties for voltage tunable
devices