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Quantum Devices based on Carbon Nanotubes

Final Report Summary - QDCN (Quantum devices based on carbon nanotubes)

In order to clearly present the comprehensive summary of the project results, the following plan is proposed. First the main scientific achievement are described. Further the skills and experience acquired by Marie Curie fellow during the project are listed. Finally, the impact and conclusions of the project are discussed.

Let us start with two scientific achievement of the Marie Curie project. First of all, a novel nanodevice layout has been developed, in which the probed quantum dots are attached to only one electrode - a carbon nanotube. The advantage of employing carbon nanotube as an electrode causes significant decrease of screening from the electrode, allowing an improved access to study the electronic structure of the semiconducting dots. Besides, such nanodevice layout greatly simplifies the fabrication process compared to standard devices with two electrodes (source and drain), which are separated by a few nanometres gap. Moreover, the MC fellow has mastered various fabrication techniques of nanodevices that involve many step processes and appropriate equipment. For example: suspended nanotube devices, graphene devices, four terminal gold nanotube devices and catalytic nanomotor devices.

The main goal of the project was to develop a new technique. This techniques is called electron counting spectroscopy and it has been used to probe the electronic properties of semiconducting CdSe quantum dots. This technique allows to fill or empty a semiconducting quantum dot with many electrons. The ability to shift the Fermi energy by a large amount holds promise for nanoscale or molecular electronics, since the large energy separation between the levels often has limited access to only few levels. The detection scheme is based on an original approach where the investigated particle is attached to only one electrode, a carbon nanotube.

Essentially, this technique consists of measuring the conductance of the nanotube as a function of the voltage applied on the gate. This allows the detection of individual electrons transferred onto the quantum dot. The electron transfer occurs only when the electrochemical potential of the nanotube matches the energy levels in the particle, while sweeping the gate voltage. This study have shown that single-electron detection with carbon nanotube transistor represents a new strategy to study the separation in energy between the electronic discrete levels of the semiconducting quantum dot. In particular, for the first time it has been shown that the electronic levels of a dot can exhibit a chaotic behaviour, which was predicted theoretically decades ago.

The experience obtained by the fellow within training proposed in the Marie Curie project involved many scientific and technological aspects. First he mastered advanced nanofabrication skills in order to fabricated novel nanodevices which combine the contacted nanotube device with single molecules or quantum dots. Note, that the fabrication of such devices is quite challenging and time-consuming, because it involves many step processes and appropriate equipments. Next step aimed at characterization of fabricated nanodevices, involving transport measurement at low temperature regime. The applicant developed a new technique called 'electron counting spectroscopy'. This technique allows to probe the electronic properties of semiconducting CdSe quantum dots. Importantly, it also allows to fill or empty any semiconducting quantum dot with many electrons.

Training provided by the quantum nanoelectronic group gave the possibility to involve the young researcher into other aspects of the project. For instance, he fabricated the nanodevices consisting of gold nanotubes, which are used to demonstrate the Aharonov-Bohm effect in such small metallic structures. In this project the applicant fabricated the devices and conducted low temperature (∼50 mK) four-point transport measurements.

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