PhD defence by Konstantinos Tsoukalas

Abstract

Small advances forge the path to greatness in modern technology. Nanotechnology has given birth to complex and multifunctional devices that can fit in a chip or at the tip of a finger. A modern smartphone packs computational power and sensors that would fill an entire room in the previous decades. Further miniaturization could lead to, e.g., quantum computers or even an entire lab on a chip. However, as devices become smaller, they enter the quantum regime. Quantum physics offers a probabilistic worldview where quantities are described by statistical distributions. It is not enough to describe the energy of an object by its average value but we need more information about its distribution. As a consequence, in stark contrast with the boring and empty classical vacuum, the quantum vacuum contains quantities whose average is zero but their fluctuations lead to observable effects. A class of systems called nanoelectromechanical systems (NEMS), is particularly sensitive to quantum vacuum fluctuations. NEMS are nanoscale machines with flexible mechanical structures that can be electrically controlled. Despite atoms in NEMS being neutral, their quantum motion occasionally separates the positive and negative charges between the mechanical parts, leading to the van der Waals or Casimir force. The closer the mechanical parts are, the greater the force, overpowering the mechanical restoring forces and permanently sticking them together. The stiction problem in NEMS is a major challenge towards future technologies. In this work, we provide design rules to avoid sticking parts and novel designs that leverage the non-linear behavior of materials to ameliorate the issue. Despite its destructive effect, we have further used this force in a constructive manner. We used the Casimir force between nanostructures, which is omnipresent since we cannot remove the vacuum, to enable NEMS that can remain in a fixed state without using electricity, which can be an important energy saver in large-scale systems with integrated NEMS, such as data centers. But even the sticking itself can be put to good use. As features become smaller, it becomes increasingly difficult to precisely control them. To overcome this issue, we used traditional lithography to pattern pieces of a nanocavity that subsequently self-assembled due to the Casimir force. The resulting cavity has atomic size gaps and can confine light in a region that is 100 times lower than its wavelength. In addition to the technological applications, we showed that the fluctuations in the energy of a charged particle that lies above a surface, influenced by the periodic separation of positive and negative charges, can be enough to overcome a chemical reaction barrier and enable chemical reactions even at absolute zero temperature.

Supervisors

• Principal supervisor: Professor Søren Stobbe, DTU Electro, Denmark
• Co-supervisor: Senior Researcher Philip Trøst Kristensen, DTU Electro, Denmark

Evaluation Board

• Professor Martijn Wubs, DTU Electro, Denmark
• Professor Carsten Henkel, University of Potsdam, Germany
• Professor N. Asger Mortensen, University of Southern Denmark, Denmark

Master of the Ceremony

• Senior researcher Thomas Christensen, DTU Electro, Denmark