Joint research project

Domain Manipulation in VO2 Freestanding Nanomechanical Structures

Project leaders
Luca Pellegrino, Teruo Kanki
Agreement
GIAPPONE - JSPS - Japan Society for the Promotion of Science
Call
CNR/JSPS biennio 2018-2019 2018-2019
Department
Physical sciences and technologies of matter
Thematic area
Physical sciences and technologies of matter
Status of the project
New

Research proposal

This proposal aims at consolidating the collaboration between CNR-SPIN (IT) and ISIR-Osaka University (JP) groups on the development of Micro & Nano-Electromechanical Systems (MEMS&NEMS) with multifunctional transition metal oxides. This project is centered on Vanadium Dioxide (VO2), a correlated oxide material with promising features for the development of new sensors and devices. VO2 shows an insulator to metal transition at approximately 68 °C with changes of resistivity of more than three orders of magnitude associated with a structural transition from monoclinic to rutile-type lattice cell [J. Cao et al. Nano Lett. 10, 2667 (2010)] and change of optical parameters. In single crystals this phase transition (PT) is abrupt and sharp (less than 1 °C) and occurs over domains with size above the micrometric scale, while in thin films nanosized domains appear and PT is smoother and broader (5-20 °C), mainly depending on the substrate of growth that determines the amount of stress on the film and its structural properties [J. Jeong et al. Science 339, 1402 (2013)]. PT is characterized by hysteresis of several degrees. This hysteresis has been employed to realize devices having memory capabilities [T. Driscoll et al. Science 325 1518-21 (2009)]. In thin films, within the hysteresis window of the PT, coexisting electronic metallic(M)/insulating(I) nanodomains exist. The ratio between M/I domains is determined by temperature and can be controlled by electric fields or engineered by shaping VO2 into micrometric geometries [T. Kanki et al. Appl. Phys. Lett. 101, 243118 (2012)]. Strain also plays an important role, shifting the PT temperature and changing the M/I ratio.
JP and IT developed suspended VO2 structures that can be efficiently heated by Joule self-heating or deformed by electrostatic actuation or mechanically. Within such collaboration, Joule heating was used to tune the M/I domain ratio in confined VO2 regions, realizing a multi-resistive memory [L. Pellegrino et al. Adv. Mater. 24, 2929 (2012)] and a programmable mechanical microresonator [N. Manca et al. Adv. Mater. 25, 6430 (2013)]. More recently, we fabricated a VO2 micromechanical oscillator powered by a DC voltage bias only, in which electro-thermal instability of the system triggers periodic PT of the domains with consequent mechanical oscillation of the microstructures [N. Manca et al. Adv. Mat. 2017 (in press)]. PT of VO2 domains by mechanical deformation - such as bending or stretching - to control the electrical and optical properties of VO2 freestanding thin film nanostructures can be particularly appealing for the development of new piezoresistive devices, as well as for the implementation of VO2 thin film structures in flexible and stretchable devices for epidermial electronics applications. In this framework, comprehension and optimization of the mechanisms for the active control of M/I domain ratio in micro/nano sized devices is a fundamental issue for achieving tunable VO2 devices and sensors with improved sensitivity. The realization of devices with dimensions below the microscale allows amplifying the effects of the transition of even a single VO2 domain.
The aim of this project is thus to deeply investigate VO2 nanomechanical devices whose mechanical, electrical and optical behaviors are largely determined by the PT of a single or few VO2 domains. These device will be studied for developing new prototypes of sensors having high piezoresistive coefficients, enhanced responses from external stimuli and for the development of new types of actuators. This last issue is currently part of a joint project between the two groups on "solid state actuators for micro/nano robotics" with the contribution from MAECI (www.esteri.it). The present project is synergic with the above mentioned one, as it will investigate more fundamental aspects related to the effect of deformation on the physical properties of VO2 nanomechanical structures. The two projects will share results and expertize on films growth and device fabrication with beneficial mutual cross-fertilization. A strain-sensitive nanodevice is a milestone of this project. It will be pursued by merging the competences of JP on material science of thin films and nanofabrication with those of IT on the fabrication and characterization of microdevices. The success of this research project would open new directions for the realization of electrically configurable mechanical micro/nanostructures.

RESEARCH PLAN:

1) Realization of oxide-NEMS using VO2. We will optimize nanofabrication protocols on VO2-based heterostructures deposited by Pulsed Laser Deposition using low-cost and high throughput processes like nanoimprint lithography technique. We will also study the growth of VO2 on different substrates and buffer layers and check for film quality in terms of crystalline order, mechanical strength and electronic properties. Fabrication of engineered and deformable membranes made from VO2 thin films and heterostructures will be also pursued with the aim of integrating VO2 functionalities with flexible devices made of other materials.

2) VO2 domain dynamics. Characterization of domain dynamics under deformation and electric fields. Study of the interplay between domain evolution, temperature and mechanical bending of the suspended structures. Characterization will be performed by optical probes in various configurations and under different environmental conditions.

3) Strain devices. Realization of deformable VO2 structures and characterization of their electrical and optical properties as a function of deformation induced by electric fields or external pressure. Mechanical characterization will be performed by optical techniques combined with electrical measurements in controlled environment. Modeling of device operation will be performed by Finite Element Analysis.

Research goals

OBJECTIVES: One of the objectives of this project is to foster communication between the two groups, especially to share capabilities among students and young researchers. During the project, international collaboration with foreign groups will be also undertaken. Particular attention will be devoted to undertake actions to disseminate project results towards High Tech SMEs.

The main scientific objectives of this project are:

1) Development of fabrication protocols for VO2-based NEMS and fabrication of first prototypes of thin film VO2-based NEMS (double-clamped microbridges and cantilevered structures) using low-cost and high throughput processes such as nanoimprint lithography technique. Realization of an array of free-standing nanodevices will be pursued as well as the fabrication of VO2 based membranes with high quality VO2 films to be employed for future epidermal electronics device prototypes.

2) Comprehension of strain-sensitivity and domain dynamics in VO2 films and heterostructures as a function of VO2 crystalline quality related to substrate of growth, crystalline orientation and coupling with other films (heteroepitaxy).

3) Realization of a prototype of strain sensitive VO2-based nanodevice and comprehension of its working mechanisms in the framework of domain dynamics and lattice instabilities.

Last update: 12/08/2020