NFML | Research Areas | Nanostructured Functional Materials Laboratory

Research Overview

Vertically Aligned Nanocomposite (VAN) thin films

VAN thin films

Vertically Aligned Nanocomposite (VAN) thin films comprise 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.

VAN growth process schematic — Fig. 1: Schematic diagram showing growth of VAN thin films.

Shown below are cross-sectional and plan-view STEM images of SrTiO₃-Sm₂O₃ VAN thin films grown on SrTiO₃ (001) single crystal substrates.

STO-Sm2O3 VAN thin films characterization — 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 the 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 the lattice-model showing coherence in Sm₂O₃ and SrTiO₃.

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.

Ferro & Piezoelectric

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.

STO-Sm2O3 VAN TMD — 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 the 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.

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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 the 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.

This problem of random generation of oxygen vacancies 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 below is a lattice model depicting this interface. We can see that every 11^th oxygen plane in a Sm₂O₃ lattice in (00l) is in good coherence with every 8^th oxygen-containing plane in SrTiO₃ lattice, whereas between these two coherent planes there is a discontinuity.

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, endurance studies indicate a constant resistive switching performance between a high and a low resistance states for >10⁶ cycles.

Advanced Materials

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.

Selective etching process — Fig. 6: Selective etching of Sm₂O₃ and MgO from SrTiO₃ matrix. The matrix is almost completely intact as seen from plan-view SEM and cross-sectional STEM.

PEC measurements — Fig. 7: Photoelectrochemical measurements of mesoporous SrTiO₃ films showing improved water splitting performance in reduced mesoporous SrTiO₃ films compared to Nb-doped plain SrTiO₃ substrate.

Read Nano Letters Paper

Epitaxial Free-standing Membranes

We are exploring the fabrication of epitaxial free-standing membranes. The process typically involves thin film growth, support application, selective chemical etching of a sacrificial buffer layer, and final device integration.

Fig. 8: Schematic process flow for free-standing epitaxial thin film formation.

Printed Electronics

Our research in printed electronics spans the entire device fabrication lifecycle. This includes the synthesis and characterization of functional materials, the formulation and rheological optimization of bespoke inks, and the deployment of these inks using advanced printing processes such as aerosol jet and dispenser printing onto flexible substrates.

We aim to develop a range of applications such as energy harvesting devices, pressure sensors, temperature monitors, and motion sensors.

Development of printed electronic devices from bespoke inks of functional materials — Fig. 9: Comprehensive workflow for the development of printed electronic devices. The process encompasses functional material synthesis, bespoke ink formulation and rheology optimization, various printing processes (e.g., dispenser and aerosol jet printing) onto flexible substrates, and final device applications in energy harvesting and sensing.

Piezoelectric Microelectromechanical Systems (MEMS) Devices

Research in Progress

Details about our work on piezoelectric MEMS devices will be updated here soon.