5 research outputs found
Self-Assembled Hierarchical Nanostructures for High-Efficiency Porous Photonic Crystals
The nanoscale modulation of material properties such as porosity and morphology is used in the natural world to mold the flow of light and to obtain structural colors. The ability to mimic these strategies while adding technological functionality has the potential to open up a broad array of applications. Porous photonic crystals are one such technological candidate, but have typically underachieved in terms of available materials, structural and optical quality, compatibility with different substrates (<i>e</i>.<i>g</i>., silicon, flexible organics), and scalability. We report here an alternative fabrication method based on the bottom-up self-assembly of elementary building blocks from the gas phase into high surface area photonic hierarchical nanostructures at room temperature. Periodic refractive index modulation is achieved by stacking layers with different nanoarchitectures. High-efficiency porous Bragg reflectors are successfully fabricated with sub-micrometer thick films on glass, silicon, and flexible substrates. High diffraction efficiency broadband mirrors (<i>R</i> ≈ 1), opto-fluidic switches, and arrays of photonic crystal pixels with size < 10 μm are demonstrated. Possible applications in filtering, sensing, electro-optical modulation, solar cells, and photocatalysis are envisioned
Giant O<sub>2</sub>‑Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures
We
demonstrate exceptionally large modulation of PL intensity in
hierarchical titanium dioxide (TiO<sub>2</sub>) nanostructures exposed
to molecular oxygen (O<sub>2</sub>). Optical responsivities up to
about 1100% at 20% O<sub>2</sub> concentrations are observed in hyperbranched
anatase-phase hierarchical structures, outperforming those obtainable
by commercial TiO<sub>2</sub> nanopowders (up to a factor of ∼7
for response to synthetic air) and significantly improving the ones
typically reported in PL-based opto-chemical gas sensing using MOXs.
The improved PL response is discussed in terms of the specific morphology
of hierarchical structures, characterized by simultaneous presence
of small nanoparticles, large surface areas, and large voids. These
characteristics guarantee an optimal interplay between photogenerated
charges, PL-active centers, and adsorbed gas molecules. The results
highlight the potentialities offered by hierarchical structures based
on TiO<sub>2</sub> or other MOXs and open interesting scenarios toward
the development of all-optical and/or hybrid (opto/electrical) chemical
sensors with improved sensitivity
Multiscale Effect of Hierarchical Self-Assembled Nanostructures on Superhydrophobic Surface
In
this work, we describe self-assembled surfaces with a peculiar
multiscale organization, from the nanoscale to the microscale, exhibiting
the Cassie–Baxter wetting regime with extremely low water adhesion:
floating drops regime with roll-off angles < 5°. These surfaces
comprise bundles of hierarchical, quasi-one-dimensional (1D) TiO<sub>2</sub> nanostructures functionalized with a fluorinated molecule
(PFNA). While the hierarchical nanostructures are the result of a
gas-phase self-assembly process, their bundles are the result of the
capillary forces acting between them when the PFNA solvent evaporates.
Nanometric features are found to influence the hydrophobic behavior
of the surface, which is enhanced by the micrometric structures up
to the achievement of the superhydrophobic Cassie–Baxter state
(contact angle (CA) ≫ 150°). Thanks to their high total
and diffuse transmittance and their self-cleaning properties, these
surfaces could be interesting for several applications such as smart
windows and photovoltaics where light management and surface cleanliness
play a crucial role. Moreover, the multiscale analysis performed in
this work contributes to the understanding of the basic mechanisms
behind extreme wetting behaviors
Hyperbranched Quasi-1D TiO<sub>2</sub> Nanostructure for Hybrid Organic–Inorganic Solar Cells
The
performance of hybrid solar cells is strongly affected by the
device morphology. In this work, we demonstrate a polyÂ(3-hexylthiophene-2,5-diyl)/TiO<sub>2</sub> hybrid solar cell where the TiO<sub>2</sub> photoanode comprises
an array of tree-like hyperbranched quasi-1D nanostructures self-assembled
from the gas phase. This advanced architecture enables us to increase
the power conversion efficiency to over 1%, doubling the efficiency
with respect to state of the art devices employing standard mesoporous
titania photoanodes. This improvement is attributed to several peculiar
features of this array of nanostructures: high interfacial area; increased
optical density thanks to the enhanced light scattering; and enhanced
crystallization of polyÂ(3-hexylthiophene-2,5-diyl) inside the quasi-1D
nanostructure
TiO<sub>2</sub> Nanotubes: Interdependence of Substrate Grain Orientation and Growth Rate
Highly
ordered arrays of TiO<sub>2</sub> nanotubes can be produced
by self-organized anodic growth. It is desirable to identify key parameters
playing a role in the maximization of the surface area, growth rate,
and nanotube lengths. In this work, the role of the crystallographic
orientation of the underlying Ti substrate on the growth rate of anodic
self-organized TiO<sub>2</sub> nanotubes in viscous organic electrolytes
in the presence of small amounts of fluorides is studied. A systematic
analysis of cross sections of the nanotubular oxide films on differently
oriented substrate grains was conducted by a combination of electron
backscatter diffraction and scanning electron microscopy. The characterization
allows for a correlation between TiO<sub>2</sub> nanotube lengths
and diameters and crystallographic parameters of the underlying Ti
metal substrate, such as planar surface densities. It is found that
the growth rate of TiO<sub>2</sub> nanotubes gradually increases with
the decreasing planar atomic density of the titanium substrate. Anodic
TiO<sub>2</sub> nanotubes with the highest aspect ratio form on Ti(−151)
[which is close to Ti(010)], whereas nanotube formation is completely
inhibited on Ti(001). In the thin compact oxide on Ti(001), the electron
donor concentration and electronic conductivity are higher, which
leads to a competition between oxide growth and other electrochemical
oxidation reactions, such as the oxygen evolution reaction, upon anodic
polarization. At grain boundaries between oxide films on TiÂ(<i>hk</i>0), where nanotubes grow, and Ti(001), where thin compact
oxide films are formed, the length of nanotubes decreases most likely
because of lateral electron migration from TiO<sub>2</sub> on Ti(001)
to TiO<sub>2</sub> on TiÂ(<i>hk</i>0)