29 research outputs found
Atomic Layer Deposition of Aluminum Sulfide: Growth Mechanism and Electrochemical Evaluation in Lithium-Ion Batteries
This study describes the synthesis
of aluminum sulfide (AlS<sub><i>x</i></sub>) thin films
by atomic layer deposition (ALD)
using trisÂ(dimethylamido)aluminum and hydrogen sulfide. We employed
a suite of in situ measurement techniques to explore the ALD AlS<sub><i>x</i></sub> growth mechanism, including quartz crystal
microbalance, quadrupole mass spectrometry, and Fourier transform
infrared spectroscopy. A variety of ex situ characterization techniques
were used to determine the growth characteristics, morphology, elemental
composition, and crystallinity of the resultant AlS<sub><i>x</i></sub> films. This study revealed that the AlS<sub><i>x</i></sub> growth was self-limiting in the temperature range 100–250
°C, and the growth per cycle decreased linearly with increasing
temperature from ∼0.45 Å/cycle at 100 °C to ∼0.1
Å/cycle at 250 °C. The AlS<sub><i>x</i></sub> films
were amorphous in this temperature range. We conducted electrochemical
testing to evaluate the ALD AlS<sub><i>x</i></sub> as a
potential anode material for lithium-ion batteries (LIBs). The ALD
AlS<sub><i>x</i></sub> exhibited reliable cyclability over
60 discharge–charge cycles with a sustainable discharge capacity
of 640 mAh/g at a current density of 100 mA/g in the voltage window
of 0.6–3.5 V
Kinetics for the Sequential Infiltration Synthesis of Alumina in Poly(methyl methacrylate): An Infrared Spectroscopic Study
Sequential infiltration synthesis
(SIS) is a method for growing
inorganic materials within polymers in an atomically controlled fashion.
This technique can increase the etch resistance of optical, electron-beam,
and block copolymer (BCP) lithography resists and is also a flexible
strategy for nanomaterials synthesis. Despite this broad utility,
the kinetics of SIS remain poorly understood, and this knowledge gap
must be bridged in order to gain firm control over the growth of inorganic
materials inside polymer films at a large scale. In this paper, we
explore the reaction kinetics for Al<sub>2</sub>O<sub>3</sub> SIS
in PMMA using in situ Fourier transform infrared spectroscopy. First,
we establish the kinetics for saturation adsorption and desorption
of trimethyl aluminum (TMA) in PMMA over a range of PMMA film thicknesses
deposited on silicon substrates. These observations guide the selection
of TMA dose and purge times during SIS lithography to achieve robust
organic/inorganic structures. Next, we examine the effects of TMA
desorption on BCP lithography by performing SIS on silicon surfaces
coated with polystyrene-<i>block</i>-polyÂ(methyl methacrylate)
films. After etching the organic components, the substrates are examined
using scanning electron microcopy to evaluate the resulting Al<sub>2</sub>O<sub>3</sub> patterns. Finally, we examine the effects of
temperature on Al<sub>2</sub>O<sub>3</sub> SIS in PMMA to elucidate
the infiltration kinetics. The insights provided by these measurements
will help extend SIS lithography to larger substrate sizes for eventual
commercialization and expand our knowledge of precursor–polymer
interactions that will benefit the SIS of a wide range of inorganic
materials in the future
Vapor-Phase Atomic-Controllable Growth of Amorphous Li<sub>2</sub>S for High-Performance Lithium–Sulfur Batteries
Lithium–sulfur (Li–S) batteries hold great promise to meet the formidable energy storage requirements of future electrical vehicles but are prohibited from practical implementation by their severe capacity fading and the risks imposed by Li metal anodes. Nanoscale Li<sub>2</sub>S offers the possibility to overcome these challenges, but no synthetic technique exists for fine-tailoring Li<sub>2</sub>S at the nanoscale. Herein we report a vapor-phase atomic layer deposition (ALD) method for the atomic-scale-controllable synthesis of Li<sub>2</sub>S. Besides a comprehensive investigation of the ALD Li<sub>2</sub>S growth mechanism, we further describe the high performance of the resulting amorphous Li<sub>2</sub>S nanofilms as cathodes in Li–S batteries, achieving a stable capacity of ∼800 mA·h/g, nearly 100% Coulombic efficiency, and excellent rate capability. Nanoscale Li<sub>2</sub>S holds great potential for both bulk-type and thin-film high-energy Li–S batteries
Atomic Layer Deposition of Al–W–Fluoride on LiCoO<sub>2</sub> Cathodes: Comparison of Particle- and Electrode-Level Coatings
Atomic layer deposition
(ALD) of the well-known Al<sub>2</sub>O<sub>3</sub> on a LiCoO<sub>2</sub> system is compared with that of a
newly developed AlW<sub><i>x</i></sub>F<sub><i>y</i></sub> material. ALD coatings (∼1 nm thick) of both materials
are shown to be effective in improving cycle life through mitigation
of surface-induced capacity losses. However, the behaviors of Al<sub>2</sub>O<sub>3</sub> and AlW<sub><i>x</i></sub>F<sub><i>y</i></sub> are shown to be significantly different when coated
directly on cathode particles versus deposition on a composite electrode
composed of active materials, carbons, and binders. Electrochemical
impedance spectroscopy, galvanostatic intermittent titration techniques,
and four-point measurements suggest that electron transport is more
limited in LiCoO<sub>2</sub> particles coated with Al<sub>2</sub>O<sub>3</sub> compared with that in particles coated with AlW<sub><i>x</i></sub>F<sub><i>y</i></sub>. The results show
that proper design/choice of coating materials (e.g., AlW<sub><i>x</i></sub>F<sub><i>y</i></sub>) can improve capacity
retention without sacrificing electron transport and suggest new avenues
for engineering electrode–electrolyte interfaces to enable
high-voltage operation of lithium-ion batteries
Amorphous Metal Fluoride Passivation Coatings Prepared by Atomic Layer Deposition on LiCoO<sub>2</sub> for Li-Ion Batteries
Amorphous Metal Fluoride Passivation Coatings Prepared
by Atomic Layer Deposition on LiCoO<sub>2</sub> for Li-Ion Batterie
Energy Levels, Electronic Properties, and Rectification in Ultrathin p‑NiO Films Synthesized by Atomic Layer Deposition
NiO is an attractive p-type transparent semiconductor
that is being
explored for a variety of applications. We report a systematic study
of the electronic properties, relevant to hole-transporting materials
in solar energy conversion applications, of NiO synthesized by atomic
layer deposition (ALD). The acceptor concentration, flat band potential,
and valence band position were determined by electrochemical Mott–Schottky
analysis of impedance data in aqueous electrolytes for films less
than 100 nm in thickness on F:SnO<sub>2</sub> (FTO)-coated glass substrates.
The effects of postdeposition annealing and film thickness were studied.
Oxidation of the NiO was observed at temperatures as low as 300 °C
in 1 atm of oxygen. Films annealed at 400 °C and above in Ar
exhibited signs of thermal decomposition. Thinner films were found
to have a higher carrier concentration. F:SnO<sub>2</sub> and thermally
evaporated Ag were both observed to form ohmic contact to ALD-synthesized
TiO<sub>2</sub> and NiO. A p/n heterojunction diode was fabricated
from the transparent ALD TiO<sub>2</sub> and NiO layers with the structure
FTO/NiO/TiO<sub>2</sub>/Ag that showed rectification
Combining Electronic and Geometric Effects of ZnO-Promoted Pt Nanocatalysts for Aqueous Phase Reforming of 1‑Propanol
Compared with Pt/Al<sub>2</sub>O<sub>3</sub>, sintering-resistant
Pt nanoparticle catalysts promoted by ZnO significantly improved the
reactivity and selectivity toward hydrogen formation in the aqueous
phase reforming (APR) of 1-propanol. The improved performance was
found to benefit from both the electronic and geometric effects of
ZnO thin films. <i>In situ</i> small-angle X-ray scattering
and scanning transmission electron microscopy showed that ZnO-promoted
Pt possessed promising thermal stability under APR reaction conditions. <i>In situ</i> X-ray absorption spectroscopy showed clear charge
transfer between ZnO and Pt nanoparticles. The improved reactivity
and selectivity seemed to benefit from having both Pt-ZnO and Pt-Al<sub>2</sub>O<sub>3</sub> interfaces
First-Principles Predictions and <i>in Situ</i> Experimental Validation of Alumina Atomic Layer Deposition on Metal Surfaces
The atomic layer deposition (ALD)
of metal oxides on metal surfaces
is of great importance in applications such as microelectronics, corrosion
resistance, and catalysis. In this work, Al<sub>2</sub>O<sub>3</sub> ALD using trimethylaluminum (TMA) and water was investigated on
Pd, Pt, Ir, and Cu surfaces by combining <i>in situ</i> quartz
crystal microbalance (QCM), quadrupole mass spectroscopy (QMS), and
scanning tunneling microscopy (STM) measurements with density functional
theory (DFT) calculations. These studies revealed that TMA undergoes
dissociative chemisorption to form monomethyl aluminum (AlCH<sub>3</sub>*, the asterisk designates a surface species) on both Pd and Pt,
which transform into AlÂ(OH)<sub>3</sub>* during the subsequent water
exposure. Furthermore, the AlCH<sub>3</sub>* can further dissociate
into Al* and CH<sub>3</sub>* on stepped Pt(211). Additional DFT calculations
predicted that Al<sub>2</sub>O<sub>3</sub> ALD should proceed on Ir
following a similar mechanism but not on Cu due to the endothermicity
for TMA dissociation. These predictions were confirmed by <i>in situ</i> QCM, QMS, and STM measurements. Our combined theoretical
and experimental study also found that the preferential decoration
of low-coordination metal sites, especially after high temperature
treatment, correlates with the differences in free energy between
Al<sub>2</sub>O<sub>3</sub> ALD on the (111) and stepped (211) surfaces.
These insights into Al<sub>2</sub>O<sub>3</sub> growth on metal surfaces
can guide the future design of advanced metal/metal oxide catalysts
with greater durability by protecting the metal against sintering
and dissolution and enhanced selectivity by blocking low-coordination
metal sites while leaving (111) facets available for catalysis
Catalysts Transform While Molecules React: An Atomic-Scale View
We explore how the atomic-scale structural and chemical
properties
of an oxide-supported monolayer (ML) catalyst are related to catalytic
behavior. This case study is for vanadium oxide deposited on a rutile
α-TiO<sub>2</sub>(110) single-crystal surface by atomic layer
deposition (ALD) undergoing a redox reaction cycle in the oxidative
dehydrogenation (ODH) of cyclohexane. For measurements that require
a greater effective surface area, we include a comparative set of
ALD-processed rutile powder samples. In situ single-crystal X-ray
standing wave (XSW) analysis shows a reversible vanadium oxide structural
change through the redox cycle. Ex situ X-ray photoelectron spectroscopy
(XPS) shows that V cations are 5+ in the oxidized state and primarily
4+ in the reduced state for both the (110) single-crystal surface
and the multifaceted surfaces of the powder sample. In situ diffuse
reflectance infrared Fourier transform spectroscopy, which could only
achieve a measurable signal level from the powder sample, indicates
that these structural and chemical state changes are associated with
the change of the Vî—»O vanadyl group. Catalytic tests on the
powder-supported VO<sub><i>x</i></sub> revealed benzene
as the major product. This study not only provides atomic-scale models
for cyclohexane molecules interacting with V sites on the rutile surface
but also demonstrates a general strategy for linking the processing,
structure, properties, and performance of oxide-supported catalysts
W:Al<sub>2</sub>O<sub>3</sub> Nanocomposite Thin Films with Tunable Optical Properties Prepared by Atomic Layer Deposition
A systematic alteration
in the optical properties of W:Al<sub>2</sub>O<sub>3</sub> nanocomposite
films is demonstrated by precisely varying
the W cycle percentage (W%) from 0 to 100% in Al<sub>2</sub>O<sub>3</sub> during atomic layer deposition. The direct and indirect band
energies of the nanocomposite materials decrease from 5.2 to 4.2 eV
and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10
to 40. X-ray absorption spectroscopy reveals that, for W% < 50,
W is present in both metallic and suboxide states, whereas, for W%
≥ 50, only metallic W is seen. This transition from dielectric
to metallic character at W% ∼ 50 is accompanied by an increase
in the electrical and thermal conductivity and the disappearance of
a clear band gap in the absorption spectrum. The density of the films
increases monotonically from 3.1 g/cm<sup>3</sup> for pure Al<sub>2</sub>O<sub>3</sub> to 17.1 g/cm<sup>3</sup> for pure W, whereas
the surface roughness is greatest for the W% = 50 films. The W:Al<sub>2</sub>O<sub>3</sub> nanocomposite films are thermally stable and
show little change in optical properties upon annealing in air at
500 °C. These W:Al<sub>2</sub>O<sub>3</sub> nanocomposite films
show promise as selective solar absorption coatings for concentrated
solar power applications