5 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 Au–Cu
thin films. A relationship is established between the as-grown Au–Cu
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 Au–Cu
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 gold–copper (Au–Cu)
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, yolk–shell, 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
KCN Chemical Etch for Interface Engineering in Cu<sub>2</sub>ZnSnSe<sub>4</sub> Solar Cells
The removal of secondary phases from
the surface of the kesterite
crystals is one of the major challenges to improve the performances
of Cu<sub>2</sub>ZnSnÂ(S,Se)<sub>4</sub> (CZTSSe) thin film solar cells.
In this contribution, the KCN/KOH chemical etching approach, originally
developed for the removal of Cu<sub><i>x</i></sub>Se phases
in CuÂ(In,Ga)Â(S,Se)<sub>2</sub> thin films, is applied to CZTSe absorbers
exhibiting various chemical compositions. Two distinct electrical
behaviors were observed on CZTSe/CdS solar cells after treatment:
(i) the improvement of the fill factor (FF) after 30 s of etching
for the CZTSe absorbers showing initially a distortion of the electrical
characteristic; (ii) the progressive degradation of the FF after long
treatment time for all Cu-poor CZTSe solar cell samples. The first
effect can be attributed to the action of KCN on the absorber, that
is found to clean the absorber free surface from most of the secondary
phases surrounding the kesterite grains (e.g., Se<sup>0</sup>, Cu<sub><i>x</i></sub>Se, SnSe<sub><i>x</i></sub>, SnO<sub>2</sub>, Cu<sub>2</sub>SnSe<sub>3</sub> phases, excepting the ZnSe-based
phases). The second observation was identified as a consequence of
the preferential etching of Se, Sn, and Zn from the CZTSe surface
by the KOH solution, combined with the modification of the alkali
content of the absorber. The formation of a Cu-rich shell at the absorber/buffer
layer interface, leading to the increase of the recombination rate
at the interface, and the increase in the doping of the absorber layer
after etching are found to be at the origin of the deterioration of
the FF of the solar cells
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 150–600 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