6 research outputs found
Unusual Dealloying Effect in Gold/Copper Alloy Thin Films: The Role of Defects and Column Boundaries in the Formation of Nanoporous Gold
Understanding the dealloying mechanisms
of gold-based alloy thin films resulting in the formation of nanoporous
gold with a sponge-like structure is essential for the future design
and integration of this novel class of material in practical devices.
Here we report on the synthesis of nanoporous gold thin films using
a free-corrosion approach in nitric acid applied to cosputtered AuâCu
thin films. A relationship is established between the as-grown AuâCu
film characteristics (i.e., composition, morphology, and structure)
and the porosity of the sponge-like gold thin films. We further demonstrate
that the dealloying approach can be applied to nonhomogenous AuâCu
alloy thin films consisting of periodic and alternate Au-rich/Au-poor
nanolayers. In such a case, however, the dealloying process is found
to be altered and unusual etching stages arise. Thanks to defects
and column boundaries playing the role of channels, the nitric acid
is found to quickly penetrate within the films and then laterally
(i.e., parallel to the film surface) attacks the nanolayers rather
than perpendicularly. As a consequence to this anisotropic etching,
the Au-poor layers are etched preferentially and transform into Au
pillars holding the Au-rich layers and preventing them against collapsing.
A further exposure to nitric acid results in the collapsing of the
Au-rich layers accompanied by a transition from a multilayered to
a sponge-like structure. A scenario, supported by experimental observations,
is further proposed to provide a detailed explanation of the fundamental
mechanisms occurring during the dealloying process of films with a
multilayered structure
The Kirkendall Effect in Binary Alloys: Trapping Gold in Copper Oxide Nanoshells
In
this work, we report on the Kirkendall-induced hollowing process
occurring upon thermal oxidation of goldâcopper (AuâCu)
alloy nanowires and nanodots. Contrary to elemental metals, the oxidation
reaction results in the formation of gold nanostructures trapped inside
hollow copper oxide nanoshells. We particularly focus on the thermally
activated reshaping mechanism of the gold phase forming the core.
Using scanning transmission electron microscopy coupled to energy
dispersive X-ray spectroscopy mapping, we show that such a reshaping
is a consequence to the reorganization of gold at the atomic level.
The gold nanostructures forming the core were found to be strongly
dependent on the chemical composition of the alloy and the oxidation
temperature. By selecting the appropriate annealing conditions (i.e.,
duration, temperature), one can easily synthesize various heteronanostructures:
wire-in-tube, yolkâshell, oxide nanotubes embedding or decorated
by Au nanospheres. The advanced understanding of the Kirkendall effect
in binary alloy nanostructures that we have achieved in this work
will open a new door for the fabrication and the design of novel multifunctional
heteronanostructures for potential applications in different research
fields including nano-optics/photonics, biomedicine, and catalysis
Electron Beam Nanosculpting of Kirkendall Oxide Nanochannels
The nanomanipulation of metal nanoparticles inside oxide nanotubes, synthesized by means of the Kirkendall effect, is demonstrated. In this strategy, a focused electron beam, extracted from a transmission electron microscope source, is used to site-selectively heat the oxide material in order to generate and steer a metal ion diffusion flux inside the nanochannels. The metal ion flux generated inside the tube is a consequence of the reduction of the oxide phase occurring upon exposure to the e-beam. We further show that the directional migration of the metal ions inside the nanotubes can be achieved by locally tuning the chemistry and the morphology of the channel at the nanoscale. This allows sculpting organized metal nanoparticles inside the nanotubes with various sizes, shapes, and periodicities. This nanomanipulation technique is very promising since it enables creating unique nanostructures that, at present, cannot be produced by an alternative classical synthesis route
Planar Arrays of Nanoporous Gold Nanowires: When Electrochemical Dealloying Meets Nanopatterning
Nanoporous materials are of great
interest for various technological
applications including sensors based on surface-enhanced Raman scattering,
catalysis, and biotechnology. Currently, tremendous efforts are dedicated
to the development of porous one-dimensional materials to improve
the properties of such class of materials. The main drawback of the
synthesis approaches reported so far includes (i) the short length
of the porous nanowires, which cannot reach the macroscopic scale,
and (ii) the poor organization of the nanostructures obtained by the
end of the synthesis process. In this work, we report for the first
time on a two-step approach allowing creating highly ordered porous
gold nanowire arrays with a length up to a few centimeters. This two-step
approach consists of the growth of gold/copper alloy nanowires by
magnetron cosputtering on a nanograted silicon substrate, serving
as a physical template, followed by a selective dissolution of copper
by an electrochemical anodic process in diluted sulfuric acid. We
demonstrate that the pore size of the nanowires can be tailored between
6 and 21 nm by tuning the dealloying voltage between 0.2 and 0.4 V
and the dealloying time within the range of 150â600 s. We further
show that the initial gold content (11 to 26 atom %) and the diameter
of the gold/copper alloy nanowires (135 to 250 nm) are two important
parameters that must carefully be selected to precisely control the
porosity of the material
High-Pressure Synthesis of Novel Boron Oxynitride B<sub>6</sub>N<sub>4</sub>O<sub>3</sub> with Sphalerite Type Structure
A novel crystalline boron oxynitride
(BON) phase has been synthesized
under static pressures exceeding 15 GPa and temperatures above 1900
°C, from molar mixtures of B<sub>2</sub>O<sub>3</sub> and h-BN.
The structure and composition of the synthesized product were studied
using high-resolution transmission electron microscopy, electron diffraction,
automated diffraction tomography, energy dispersive X-ray spectroscopy
and electron energy-loss spectroscopy (EELS). BON shows a hexagonal
cell (<i>R3m</i>, <i>Z</i> = 3) with lattice parameters <i>a</i> = 2.55(5) Ă
and <i>c</i> = 6.37(13) Ă
,
and a crystal structure closely related to the cubic sphalerite type.
The EELS quantification yielded 42 at % B, 35 at % N, and 23 at %
O (B:N:O â 6:4:3). Electronic structure calculations in the
framework of Density Functional Theory have been performed to assess
the stabilities and properties of selected models with the composition
B<sub>6</sub>N<sub>4</sub>O<sub>3</sub>. These models contain ordered
structural vacancies and are superstructures of the sphalerite structure.
The calculated bulk moduli of the structure models with the lowest
formation enthalpies are around 300 GPa, higher than for any other
known oxynitride
Structural and Electrical Response of Emerging Memories Exposed to Heavy Ion Radiation
Hafnium oxide- and GeSbTe-based functional layers are
promising
candidates in material systems for emerging memory technologies. They
are also discussed as contenders for radiation-harsh environment applications.
Testing the resilience against ion radiation is of high importance
to identify materials that are feasible for future applications of
emerging memory technologies like oxide-based, ferroelectric, and
phase-change random-access memory. Induced changes of the crystalline
and microscopic structure have to be considered as they are directly
related to the memory states and failure mechanisms of the emerging
memory technologies. Therefore, we present heavy ion irradiation-induced
effects in emerging memories based on different memory materials,
in particular, HfO2-, HfZrO2-, as well as GeSbTe-based
thin films. This study reveals that the initial crystallinity, composition,
and microstructure of the memory materials have a fundamental influence
on their interaction with Au swift heavy ions. With this, we provide
a test protocol for irradiation experiments of hafnium oxide- and
GeSbTe-based emerging memories, combining structural investigations
by X-ray diffraction on a macroscopic, scanning transmission electron
microscopy on a microscopic scale, and electrical characterization
of real devices. Such fundamental studies can be also of importance
for future applications, considering the transition of digital to
analog memories with a multitude of resistance states