Research Activities

The research activities of the Energy Harvesting and Autonomous Sensors group focus on the following areas:

– Novel materials and techniques for high efficiency energy conversion

– Design and optimization of Energy Scavengers for Autonomous Microsystems:

MEMS-based vibrational microgenerators

Nanogenerators on Flexible Substrates

– Multifunctional ZnO Nanostructures for Smart Products & Biomedical Applications

– Low-power chemical sensors on silicon and flexible substrates for safety and security.

Activities and Main Results

1. Controllable Fabrication of Bioinspired Three-dimensional ZnO/Si Nanoarchitectures

Three-dimensional (3D) nanowire (NW) networks (also referred to in the literature as branched nanowires) have recently emerged as promising architectures effectively translating the extraordinary properties of one-dimensional objects into 3D space. We developed a fabrication process capable to control and vary the shape of ZnO nanostructures grown onto arrays of Si nanowires (NWs) with tunable length, diameter, aspect ratio and density, resulting in a wide range of application-specific nanoarchitectures, from plant-like structures to complex three-dimensional interconnected networks.

By properly tuning ZnO growth parameters a variety of 3D shapes can be fabricated, such as ZnO “nanoforests” (Fig. 1a), “broccoli”-like structures (Fig. 1b), etc. Depending on the morphology of the nanostructures improved device performance can potentially be obtained, such as more efficient solar cells with “”forest”-like structures or better surface control for wettability applications and cell cultures with “broccoli”-like ones.

energy_harversting_img1 energy_harversting_img2
(a) (b)
Typical SEM images of ZnO nanoarchitectures mimicking (a) “nano-forests”, (b) “nano-broccoli”.

2. Harvesting Ambient Vibrational Energy

One of the key features for the next generation of portable wireless nanodevices and nanosystems (critical for sensing, medical science, environmental/infrastructure monitoring, defense technology, and even personal electronics and implantable devices) is for them to become battery-less and self-powered. Harnessing ambient energy from the environment and, in particular, seizing the effects of motion to provide power for these devices has been recognized as one of the most promising ways for real, sustainable and energetically autonomous devices.

Over the last years, we have developed piezoelectric microgenerators, both on Silicon substrate as well as a variety of flexible substrates (such as Kapton andPET) for harvesting ambient vibrations. The devices were fabricated by employing novel nanotextured ZnO films grown via a facile, low-cost, two-step chemical process. The present methodology –apart from its low-cost- is non-hazardous, environmentally-friendly and can be readily implemented to mass-fabrication, and has thus the potential to become the basis for the production of piezoelectric microgenerators for cost-efficient applications.

energy_harversting_img3 energy_harversting_img4
Photograph showing the actual size of single die containing 4 microgenerators. ZnO nanotextured film
energy_harversting_img5 energy_harversting_img6
Photograph of piezoelectric microgenerators on kapton and PET. Output voltage as a function of time for a flexible microgenerator on a Kapton substrate

3. Low-cost multi-functional bioinspired wood coatings based on ZnO nanostructures*

Wood is a naturally-occurring material that has been used for millennia in buildings not only for structural but also as a covering material and for aesthetic reasons. Despite its wide-spread use, wood has always been confronted with three external factors that deteriorate its structural properties: humidity, UV radiation and fungi. On the other hand, among inorganic materials, ZnO nanostructures have been the leading star in a variety of applications from optoelectronics, nanopiezotronics, sensing, controlling of wettability to name a few. In this work – inspired by the Namibian plant Quiver- a low-cost, solution-based route has been explored to decorate three different types of wood (balsa, bamboo and birch) with ZnO nanostructures that may serve as UV radiation and humidity protective coatings. The structural properties of the ZnO nanostructures as determined by SEM, are correlated to the UV and humidity resistance of the coated samples measured according to DIN 52184 and EN 927-6 respectively.

energy_harversting_img7 energy_harversting_img8
(a) (b)
ZnO decorated wood: (a) Fir and (b) Beech

*In collaboration with the Department of Wood & Furniture Design and Technology, TEI of Thessaly

4. Low Power Chemical Sensors

Low power or room temperature micromachined chemical sensors are a requirement for the development of portable systems or for miniaturized nodes of Wireless Sensors Networks for a variety of applications including food safety and quality, environmental monitoring, detection of hazardous or explosive materials, etc.

Over the last year, our group has been working on the development of low power micro machined sensors in two directions: (a) Microhotplate based Metal Oxide Sensors and (b) FET-type based chemical sensors. Various metal oxides have been evaluated as sensitive layers including doped and undoped SnO2, WO3, ZnO

energy_harversting_img9 energy_harversting_img10
(a) (b)
SEM image (a) of a SnO2:Pd deposited on a micro-hotplate using micro-drop technique and (b) of a sensor array with micro-dropped nanostructured sensitive materials SnO2:Pd and WO3:Cr, mounted on a package.
energy_harversting_img11 energy_harversting_img12
(a) (b)
(a) SEM images showing the interdigitated source and drain electrodes of the bottom gate FET sensors. (b) AFM image picture of as-deposited thin ZnO film grown on oxidized silicon wafer.

For more information about the group activities please contact:

Dr Christos Tsamis (