Vertically Aligned Nanocomposite (VAN) thin films comprise of nanocolumns of one material embedded in another.
These films are typically grown in a self-assembled manner by Pulsed Laser Deposition (PLD). The target used
for PLD itself contains the two phases desired in the VAN thin films. The phases need to satisfy certain requirements
such as immiscibility, thermal stability and some degree of lattice coherency with the substrate they are to be grown on.
The growth process is shown schematically below.
Fig. 1: Schematic diagram showing growth of VAN thin films
Shown in Fig. 2. below are cross-sectional and plan-view STEM images of SrTiO3-Sm2O3
VAN thin films grown on SrTiO3 (001) single crystal substrates.
Fig. 2: Cross-sectional and plan-view STEM images; 2θ-ω out-of-plane XRD scans;
asymmetrical φ XRD scans; AFM topographical view and lattice-model showing coherence in Sm2O3
and SrTiO3
Given the natural coherence between SrTiO3 from nanocomposite film and the substrate, the SrTiO3
phase grows with the same orientation as the substrate. On the other hand, Sm2O3 phase minimises lattice mismatch
with SrTiO3 when it is grown with a 45° in-plane rotation w.r.t. SrTiO3 substrate and in (00l) out-of-plane orientation.
Thus, both SrTiO3 and Sm2O3 phases grow in (00l) orientation. This can be easily verified
from out-of-plane 2θ-ω XRD scans and asymmetrical φ XRD scans and explained using lattice-model showing coherence in
Sm2O3 and SrTiO3 (see Fig. 2).
The nanocolumns are of diameter ~15 nm and are spaced ~5 nm apart. A simple back-of-the-envelope calculation shows that this would
be ~1012 nanocolumns/in2.
Improved ferroelectric/piezoelectric properties by strain-tuning
VAN thin films can be used to enhance the ferroelectric/piezoelectric properties by strain-tuning at the vertical interface between
the matrix and nanocolumnar phase. Vastly improved ferroelectric Curie temperature in SrTiO3 was shown by making it
ferroelectric up to > 300 ° C in SrTiO3-Sm2O3 VAN thin films.
Fig. 3: Improved ferroelectric and tunable high frequency properties of
SrTiO3-Sm2O3 VAN films
As seen in Fig. 3, the improved ferroelectric properties in VAN thin films led to realisation of ferroelectricity in them, which in turn,
led to highly enhanced tunability of dielectric constant with applied DC field whilst maintaining low dielectric loss, when measured at 1 MHz.
Read further here.
Resistive random access memory (ReRAM) in VAN films
Memristor or Resistive Random Access Memory (ReRAM) is a type of computer memory technology in which the data is stored
in the form of different resistance states of matter, which can be switched to or from by application of a DC bias.
In oxides, the most commonly accepted mechanism of switching between resistance states is formation or breaking of conduction
channels formed by oxygen vacancies. Typically, the resistive switching property is 'activated' by a so-called 'forming'
process which causes soft dielectric breakdown of material by creating oxygen vacancies in it. As the process of forming
is inherently stochastic in nature, controlling the concentration of oxygen vacancies and their distribution is difficult
and not reproducible, leading to large variation in performance from point-to-point in a sample and between different samples
as well.
This problem of random generation of oxygen vacancies requiring large voltages is bypassed if the oxygen vacancies can be
engineered in number and position inside the sample. VAN thin films consisting of two oxides such as SrTiO3 and
Sm2O3 provide this unique opportunity by creating lattice discontinuities at the vertical interface.
Shown in Fig. 4 is a lattice model depicting the vertical interface between SrTiO3 and Sm2O3 phases.
We can see that every 11th oxygen plane in a Sm2O3 lattice in (00l) is in good coherence
with every 8th oxygen-containing plane in SrTiO3 lattice in (00l) direction, whereas between
these two coherent planes there is a discontinuity in lattice. As oxygen vacancies have a very low activation energy (1.1 eV),
they can get easily formed at the vertical interface extending throughout the thickness of the film.
Fig. 4: Schematic and cross-sectional HAADF STEM of SrTiO3-Sm2O3
vertical interface showing formation of oxygen vacancies and their confirmation through EELS study
The formation of oxygen vacancies is confirmed by EELS study in which discrepancy between measured and calculated oxygen
vacancies was compared.
Fig. 5: IV spectroscopy using C-AFM showing hysteresis in IV properties at the vertical interface
and its absence in the centre of the nanocolumn.
IV spectroscopy studies at the vertical interface and the nanocolumns show that hysteresis in IV characteristics can be found
at the vertical interface only, confirming oxygen-vacancy mediated resistive switching. Further, the endurance studies indicate
a constant resistive switching performance between a high and a low resistance states for >106 cycles. Click
here for further reading.
Mesoporous films from VAN films for Photoelectrochemical Water Splitting
Presence of arrays of densely packed nanocolumns in VAN films (~1012 nanocolumns/in2) provides an opportunity
to form extremely high surface area films if one of the two phases from the nanocomposite film can be selectively etched out.
We managed to selectively etch out Sm2O3 and MgO nanocolumns from their VAN films with SrTiO3,
keeping the SrTiO3 matrix intact.
Fig. 6: Selective etching of Sm2O3 and MgO from SrTiO3
matrix.
The selective etching reaction is shown in Fig. 6. Also shown are the comparison of XRD patterns, plan-view SEM and cross-sectional
STEM images of as-deposited VAN films and mesoporous films obtained after selective etching of Sm2O3 and MgO
nanocolumns. We can also see that the SrTiO3 matrix is almost completely intact.
Fig. 7: Photoelectrochemical measurements of mesoporous SrTiO3 films showing
improved water splitting performance in reduced mesoporous SrTiO3 films.
As seen from Fig. 7 above, photoelectrochemical water splitting performance is vastly improved in oxygen vacancy-doped
mesoporous films compared to Nb-doped plain SrTiO3 substrate. Click
here for further reading.
