17 research outputs found
Supersaturating silicon with transition metals by ion implantation and pulsed laser melting
We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, v D, of Au and Zn, and put lower bounds on v D of the other metals ranging from 10² to 10⁴ m/s. Knowledge of v D is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 10¹⁹ Au/cm³ without cellular breakdown. Values of v D are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, D s(T m), and the equilibrium partition coefficient, k e, are shown to simultaneously affect v D. We demonstrate a correlation between v D and the ratio D s(T m)/k e ⁰·⁶⁷, which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that D s(T m)/k e ⁰·⁶⁷ might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM.Research at Harvard was supported by The U.S. Army
Research Office under contracts W911NF-12-1-0196 and
W911NF-09-1-0118. M.T.W. and T.B.’s work was supported
by the U.S. Army Research Laboratory and the U.S.
Army Research Office under Grant No. W911NF-10-1-0442,
and the National Science Foundation (NSF) Faculty Early
Career Development Program ECCS-1150878 (to T.B.).
M.J.S., J.T.S., M.T.W., T.B., and S.G. acknowledge a generous
gift from the Chesonis Family Foundation and support in
part by the National Science Foundation (NSF) and the
Department of Energy (DOE) under NSF CA No. EEC-
1041895. S.C. and J.S.W.’s work was supported by The
Australian Research Council. J.M. was supported by a
National Research Council Research Associateship
Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films
Despite
the recent astronomical success of organic–inorganic perovskite
solar cells (PSCs), the impact of microscale film inhomogeneities
on device performance remains poorly understood. In this work, we
study CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite films
using cathodoluminescence in scanning transmission electron microscopy
and show that localized regions with increased cathodoluminescence
intensity correspond to iodide-enriched regions. These observations
constitute direct evidence that nanoscale stoichiometric variations
produce corresponding inhomogeneities in film cathodoluminescence
intensity. Moreover, we observe the emergence of high-energy transitions
attributed to beam induced iodide segregation, which may mirror the
effects of ion migration during PSC operation. Our results demonstrate
that such ion segregation can fundamentally change the local optical
and microstructural properties of organic–inorganic perovskite
films in the course of normal device operation and therefore address
the observed complex and unpredictable behavior in PSC devices
Fundamental Insights into Nanowire Diameter Modulation and the Liquid/Solid Interface
Controlled modulation of diameter along the axis of nanowires
can
enhance nanowire-based device functionality, but the potential for
achieving such structures with arbitrary diameter ratios has not been
explored. Here, we use a theoretical approach that considers changes
in the volume, wetting angle, and three-dimensional morphology of
seed particles during nanowire growth to understand and guide nanowire
diameter modulation. We use our experimental results from diameter-modulated
InN and GaN nanowires and extend our analysis to consider the potential
and limitations for diameter modulation in other materials systems.
We show that significant diameter modulations can be promoted for
seed materials that enable substantial compositional and surface energy
changes. Furthermore, we apply our model to provide insights into
the morphology of the liquid/solid interface. Our approach can be
used to understand and guide nanowire diameter modulation, as well
as probe fundamental phenomena during nanowire growth
Mapping of Strain Fields in GaAs/GaAsP Core–Shell Nanowires with Nanometer Resolution
We
report the nanoscale quantification of strain in GaAs/GaAsP core–shell
nanowires. By tracking the shifting of higher-order Laue zone (HOLZ)
lines in convergent beam electron diffraction patterns, we observe
unique variations in HOLZ line separation along different facets of
the core–shell structure, demonstrating the nonuniform strain
fields created by the heterointerface. Furthermore, through the use
of continuum mechanical modeling and Bloch wave analysis we calculate
expected HOLZ line shift behavior, which are directly matched to experimental
results. This comparison demonstrates both the power of electron microscopy
as a platform for nanoscale strain characterization and the reliability
of continuum models to accurately calculate complex strain fields
in nanoscale systems
Heterojunction Photovoltaics Using GaAs Nanowires and Conjugated Polymers
We demonstrate an organic/inorganic solar cell architecture based on a blend of poly(3-hexylthiophene) (P3HT) and narrow bandgap GaAs nanowires. The measured increase of device photocurrent with increased nanowire loading is correlated with structural ordering within the active layer that enhances charge transport. Coating the GaAs nanowires with TiOx shells passivates nanowire surface states and further improves the photovoltaic performance. We find that the P3HT/nanowire cells yield power conversion efficiencies of 2.36% under white LED illumination for devices containing 50 wt % of TiOx-coated GaAs nanowires. Our results constitute important progress for the use of nanowires in large area solution processed hybrid photovoltaic cells and provide insight into the role of structural ordering in the device performance.
Keywords (keywords):
III-V Nanowires; conjugated polymers; bulk heterojunction solar cell; self-assembly; and molecular orderin
Phalaena plecta
Semiconducting nanowires have unique properties that
are distinct
from their bulk counterparts, but realization of their full potential
will be ultimately dictated by the ability to control nanowire structure,
composition, and size with high accuracy. Here, we report a simple,
yet versatile, approach to modulate in situ the diameter, length,
and composition of individual segments within (In,Ga)N nanowires by
tuning the seed particle supersaturation and size via the supply of
III and V sources during the growth. By elucidating the underlying
mechanisms controlling structural evolution, we demonstrate the synthesis
of axial InN/InGaN nanowire heterojunctions in the nonpolar <i>m</i>-direction. Our approach can be applied to other materials
systems and provides a foundation for future development of complex
nanowire structures with enhanced functionality
Self-Seeded Growth of GaAs Nanowires by Metal–Organic Chemical Vapor Deposition
Self-seeded
growth of semiconducting nanowires offers significant
advantages over foreign metal-seeded growth by eliminating seed-associated
impurities. However, density and diameter control of self-seeded nanowires
has proven challenging although it is required for integration of
nanowires into optoelectronic devices. We report the self-seeded growth
of GaAs nanowire arrays on GaAs (111)B, (110), and (111)A substrates
by metal–organic chemical vapor deposition. Our approach involves
two steps: the <i>in situ</i> deposition of Ga seed particles
and subsequent GaAs nanowire growth. Control of nanowire diameter
and array density is achieved via Ga seed deposition temperature and
substrate orientation; increased seed deposition temperatures or changing
substrate orientation from (111)A to (110) and (111)B yields reduced
areal density and larger nanowire diameters. The density and diameter
control approaches could be extended to other self-seeded III–V
nanowire material systems
Dimensional Tailoring of Hydrothermally Grown Zinc Oxide Nanowire Arrays
Hydrothermally
synthesized ZnO nanowire arrays are critical components
in a range of nanostructured semiconductor devices. The device performance
is governed by relevant nanowire morphological parameters that cannot
be fully controlled during bulk hydrothermal synthesis due to its
transient nature. Here, we maintain homeostatic zinc concentration,
pH, and temperature by employing continuous flow synthesis and demonstrate
independent tailoring of nanowire array dimensions including areal
density, length, and diameter on device-relevant length scales. By
applying diffusion/reaction-limited analysis, we separate the effect
of local diffusive transport from the <i>c</i>-plane surface
reaction rate and identify direct incorporation as the <i>c</i>-plane growth mechanism. Our analysis defines guidelines for precise
and independent control of the nanowire length and diameter by operating
in rate-limiting regimes. We validate its utility by using surface
adsorbents that limit reaction rate to obtain spatially uniform vertical
growth rates across a patterned substrate
Role of Au in the Growth and Nanoscale Optical Properties of ZnO Nanowires
Metallic nanoparticles play a crucial role in nanowire growth and have profound consequences on nanowire morphology and their physical properties. Here, we investigate the evolving role of the Au nanoparticle during ZnO nanowire growth and its effects on nanoscale photoemission of the nanowires. We observe the transition from Au-assisted to non-assisted growth mechanisms during a single nanowire growth, with significant changes in growth rates during these two regimes. This transition occurs through the reduction of oxygen partial pressure, which modifies the ZnO facet stability and increases Au diffusion. Nanoscale quenching of ZnO cathodoluminescence occurs near the Au nanoparticle due to excited electron diffusion to the nanoparticle. Thus, the Au nanoparticle is critically linked to the nanowire growth mechanism and corresponding growth rate through the energy of its interface with the ZnO nanowire, and its presence modifies nanowire optical properties on the nanoscale
Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics
Lead sulfide quantum
dots (PbS QDs) are an attractive material
system for the development of low-cost photovoltaics (PV) due to their
ease of processing and stability in air, with certified power conversion
efficiencies exceeding 11%. However, even the best PbS QD PV devices
are limited by diffusive transport, as the optical absorption length
exceeds the minority carrier diffusion length. Understanding minority
carrier transport in these devices will therefore be critical for
future efficiency improvement. We utilize cross-sectional electron
beam-induced current (EBIC) microscopy and develop methodology to
quantify minority carrier diffusion length in PbS QD PV devices. We
show that holes are the minority carriers in tetrabutylammonium iodide
(TBAI)-treated PbS QD films due to the formation of a p–n junction
with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction
with n-type ZnO forms a weaker n<sup>+</sup>–n junction. This
indicates that modifying the standard device architecture to include
a p-type window layer would further boost the performance of PbS QD
PV devices. Furthermore, quantitative EBIC measurements yield a lower
bound of 110 nm for the hole diffusion length in TBAI-treated PbS
QD films, which informs design rules for planar and ordered bulk heterojunction
PV devices. Finally, the low-energy EBIC approach developed in our
work is generally applicable to other emerging thin-film PV absorber
materials with nanoscale diffusion lengths