QDCN

Quantum Devices based on Carbon Nanotubes

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 Obiettivo del progetto (Objective)

'The aim of this project is to take advantage of the unique properties of carbon nanotubes (CNTs) for the fabrication of innovative nanoelectronic devices. The first nanodevice is a ultra-sensitive detector, designed to probe the electrical properties of individual molecules that are exposed to external perturbations, e.g. such as electric field or light. The detection scheme is based on an original approach, on the contrary to previous experiments which aimed at contacting individual organic molecules with two electrodes. With the two-electrode technique the problems have quickly appeared due to the poor control of the electrode/molecule interfaces. Here, the molecule is attached to only one electrode, a nanotube. The resistance of the nanotube is measured as a function of a gate voltage, which should be sensitive on the energy spectrum of the molecule. Low-current detection is expected to be particularly suitable for molecular electronics, since most of the molecular systems are highly resistive. This includes organic molecules, biological molecules, and semiconducting particles. Most importantly, such an approach is expected to be mostly independent on the quality of the molecule-nanotube interface, and in addition, it allows the device to be operational in higher temperature. The second proposed design of nanodevice is a non-volatile memory, which will be achieved also in one-electrode approach by combining a single nanoparticle (with diameters up to 10nm) with a carbon nanotube transistor. In the device, CNT acts as conduction channel and the charge stored in the nanoparticle behaves as a floating-gate. Charging effects will be obtained either from an atomic force microscope or directly from the nanotube. The objective will be to determine the operability of the device at room temperature and its limitation (necessary charge on floating-gate, temperature limitation).'

Introduzione (Teaser)

Microscopic carbon tubes are being used to build tiny equipment using new techniques. This opens the door to a different class of equipment and enables applications on a quantum level not seen before.

Descrizione progetto (Article)

There is so much more than meets the eye where science is concerned, particularly on the quantum scale. Miniature devices such as sensors are being built for science and technology, thanks to the use of nanomaterials. The EU-funded 'Quantum devices based on carbon nanotubes' (QDCN) project is developing an ultra-sensitive detector that probes the electrical properties of individual molecules. Such a device is being made with tiny elements called carbon nanotubes (CNTs), each approximately 1/50 000th of a human hair.

To operate, these devices also require semiconductors, i.e. materials with specific electrical conductivity. A more recent and more powerful type of semiconductor on a minute scale is known as the quantum dot: it enhances conductivity and can be used to manufacture much better devices than those currently available.

In order to measure electrical properties of an individual molecule, quantum dots are attached to only one electrode represented by a single CNT. The advantage of employing a CNT as an electrode causes localised screening from the electrode, rendering the study of the electronic structure of the semiconducting dots more accessible.

This constitutes a new, more efficient approach developed by the project to create sensors and detectors. The novel nanodevice layout greatly simplifies the fabrication process compared to standard devices which used to use two electrodes separated by a gap of a few nanometres. This is significant, as the fabrication of such devices is quite challenging and time-consuming, and involves numerous processes and intricate equipment.

The project's next step was to characterise these fabricated nanodevices and measure their accuracy. This was achieved using a new technique called 'electron counting spectroscopy' that involves measurement at low temperatures. The technique allows researchers to probe the electronic properties of semiconducting quantum dots. Importantly, it also allows them to fill or empty any semiconducting quantum dot with many electrons, a previously onerous task.

In addition to creating the detector device, the project mastered various nanodevice fabrication techniques that involve many processes and much equipment to deal with at the quantum scale. These include suspended nanotube devices, graphene devices, four terminal gold nanotube devices and catalytic nanomotor devices.

Moreover, the project's advances allow manipulation of Fermi energy (i.e. energy at absolute zero temperature) by significant amounts. This holds promise for nanoscale or molecular electronics, since the large energy manipulation and 'separation' in such applications is limited.

Overall, QDCN has shown that single-electron detection with a CNT transistor represents a new strategy to study the separation in energy between the electronic discrete levels of the semiconducting quantum dot. In particular, it has shown that electronic levels of a quantum dot can exhibit chaotic behaviour, a phenomenon which had only been examined in theory in recent decades.