Going to the nanoscale and creating very small devices has the clear advantage of making ultra high device densities possible. But another main motivation, which is perhaps even more important than increased densities, is a vast pool of new phenomena such as quantum effects that appear at reduced sizes and dimensions, and therefore the opportunity to make new types of devices and implement new ideas. We work on exploiting these new possibilities that nanostructures have to offer and use them for innovative device applications. Our main focus is on devices and structures based on carbon nanotubes, but we also work on other nanomaterials such as gallium nitride and silicon oxide nanowires.

Our experimental work involves cleanroom nanofabrication, nanotube/nanowire growth, nanoimaging (SEM, AFM), building equipment, setting up electronic and optical measurement experiments and investigating nanodevice properties. On the theoretical side, we use techniques ranging from molecular dynamics to ab-initio methods and density functional theory in order to predict, understand and explain experimental results. Currently, we are actively pursuing projects in all the areas below.

Controlled Nanofabrication

A significant challenge in research on nanostructures is the lack of sufficient control over the fabrication processes. However, in order to study a certain device systematically or use it for real-life applications, one needs to be able to fabricate the device in a reproducible fashion. Therefore, an important aspect of our research is the study of nanostructure fabrication processes with the goal of achieving higher levels of control and reproducibility. Examples include the application of electric field during the growth of carbon nanotubes to align them into one- and two-dimensional structures, or systematic study of the effect of gas flow rates on the structural characteristics of grown carbon nanotube forests. Our work in this area directly supports all the device research discussed further below.

Quantum Electronics

When electrons are confined in one or more dimensions, their wave nature and quantum mechanical effects become very apparent. Devices such as quantum wires and quantum dots operate based on this principle. However, for such effects to be visible at room temperature, the size of the confinement has to be no more than a few nanometers. Such devices are extremely difficult to fabricate using traditional lithography-based fabrication due to the limit in patterning resolution. However, nanowires and nanotubes provide natural choices for making such devices since they already have confinement in two dimensions. Moreover, they allow for new ways of creating extra degrees of confinement. For example, mechanical deformation in nanotubes can lead to significant change in electronic structure, enabling the realization of nanoscale quantum devices, potentially capable of operating at room temperature. In addition to providing a great vehicle for the study of quantum physics, other possible applications of these structures include future electronic technologies, turnstile devices and nanophotonics.

Optical Antennas

The optoelectronics properties of nanostructures represent a very interesting regime of operation. On one hand, due to typical photon energies of around an electron-volt, interband transistions play a major role in the device operation, that is the particle nature of the electromagnetic radiation is strongly pronounced, similar to the situation in traditional optoelectronic devices. On the other hand, device dimensions can be comparable to the wavelength of light: consider nanotube light emitters and detectors with lengths of a few hundred nanometers as an example. This means that the wave nature of light will also have a strong presenece and diffraction and antenna effects will play a major role in device operation. This creates new opportunities for engineering the optoelectronic properties, for instance the amount of light absorption in a device. Our target applications include miniaturized communication devices for wireless on-chip interconnect and efficient solar cells.

Electron Sources and Vacuum Nanoelectronics

The mobility of electrons moving in vacuum does not suffer from the scattering events that are present inside matter. As a result, electronic transport can be very fast in vacuum, making ultra-fast operation possible. Nanoscale devices where the conducting channel is in vacuum are thus becoming more and more attractive as we search for new technologies to make devices working in the THz regime. We are investigating highly controllable nanoscale electron emitters to make vacuum nano-electronic devices possible. Electrons can be emitted from a material into vacuum by heating the material to very high temperatures (thermionic emission), the application of a strong external field (field-electron emission), illumination by light (photo-electron emission), or a combination of these. Other than in vacuum nanoelectronics, electron sources are in high demand in applications such as vacuum tubes, electron microscopy and lithography equipment for micro/nanofabrication, field-emission displays, synchrotrons, electron holography and interferometry, electron-beam induced deposition, welding and air pollution removal.

Microscopy

High-resolution imaging, in particular using electron microscopes, is an integral part of device research. However, for nanoscale structures, artefacts such as those arising from sample charging and contamination deposition can severely affect the imaging process. For the accurate interpretation of nanostructure images, it is important to study these effects systematically and find ways to minimize them or compensate for them. The understanding developed in this research helps us in the imaging/inspection of devices built for our other projects. Moreover, the interaction of electron beams with nanostructures is in itself an interesting subject to study. For example, carbon nanotubes provide a very small interaciton area with the primary beam of a scanning electron microscope. Yet, they are quite easily visible in the microscope, which is quite puzzling based on traditional beam-bulk interaction models. However, new mechanisms can be at play in secondary electron emission from nanotubes compared to bulk samples. Also, their visibility could largely depend on surface charging phenomena.

Modeling and Simulation

Device modeling and simulation are important aspects of our work. These are necessary in making predictions before experiments are done, as well as interpreting the data after experiments. Nanoscale device modeling presents significant challenges. Often, nanostructures include a small enough number of atoms so that they cannot be studied using continuum modeling approaches or methods based on statistical, average properties. At the same time, they contain a large enough number of atoms that makes the exact, first-principles simulation of the entire structure almost impossible due to the computational load. Therefore, a combination of many levels of theory is needed in studying nanostructures. We use techniques ranging from classical, continuum modeling, to molecular dynamics, to quantum mechanical simulations using the density functional theory and first-principles techniques such as the Hartree-Fock method. We investigate the mechanical properties, electronic structure, transport characteristics and optical properties of nanodevices, in close connection to our experimental work described above.