The present work is a theoretical ab initio study of silicon (mainly) and silicon-based or “silicon-like” Nanocrystals and nanostructures, such as core/shell quantum dots and ultra-thin nanowires of Si, Ge, and Sn. The main focus is on the quantum confinement of Si quantum dots and the description of their structural, cohesive, electronic, and optical properties in terms of size, growth pattern and surface conditions. An important outcome of such study, besides the very satisfactory agreement with experimental measurements for nanocrystals (up to 32 Å in diameter), is the judicious extrapolation of the nanoscale results all the way to infinite silicon crystal, and the successful comparison with experiment (for both the energy gap and the cohesive energy of crystalline silicon). This is an additional verification for the essential correctness of our approach. Our present results, which are based on earlier findings of prof. Zdetsis’ group for spherical Si quantum dots, are in full agreement with those results and predictions. We have expanded our study to selective cases of pure C, Ge, Sn and their mixed nanocrystals and nanowires. Thus, the classes of systems studied here include: a) Silicon quantum dots terminated by hydrogen of three different growth models (spherical, elongated, and reconstructed) without and with oxygen “contamination” of four different modes (double bonds, bridging single bonds, hydroxyl formation and mixed modes). b) Analogous quantum dots, pure and mixed (core/shell) of C, Si, Ge, and Sn. c) Ultrafine silicon and germanium nanowires of various growth patterns. The majority of this work is based in density functional theory (DFT), both ground state and time-depended, using in most cases the hybrid functional of Becke, Lee, Parr and Yang (B3LYP), and in several places the PBE and PBE0 functionals. A limited number of calculations was performed with post SCF methods, such as many-body perturbation theory (MP2) or Coupled cluster CCSD(T), for comparison. For the study of Si and Ge nanowires we have also used properly selected (and tested) semiempirical methods and calculations. These theoretical methods and techniques are reviewed in considerable detail in the first three chapters (Part I) of the present thesis. The results of the calculations are discussed in Part II, divided in three Chapters (4-6). Chapter 4 is devoted to the structural, electronic, cohesive and elastic properties of ultrafine hydrogenated silicon and germanium nanowires. Chapter 5 describes the influence of the growth patterns and surface conditions on structural, cohesive, and electronic properties of silicon nanocrystals, as well as their size dependence all the way to infinity. This (very successful) size dependence, in full accord with quantum confinement, is also compared with the (poor) predictions of the BOLS correlation scheme. Finally, Chapter 6 deals with carbon, silicon, germanium, tin and their mixed core/shell quantum dots. / --

Η παρούσα διδακτορική διατριβή έχει ως κύριο αντικείμενο μελέτης την μοντελοποίηση και έλεγχο ενός μικρορομποτικού βραχίονα αναλυόμενου σε σφαιρικά πεπερασμένα στοιχεία σε περιβάλλον με εξωτερικές δυνάμεις Van Der Waals και συνυπολογίζοντας την τριβή. Τα κύρια σημεία είναι η εισαγωγή των εξωτερικών δυνάμεων στο μοντέλο του μικρορομπότ, η δημιουργία προσαρμοστικού ελέγχου για την επίτευξη ακολουθίας τροχιάς με αναγνώριση και ακύρωση των ισχυρών μεταβαλλόμενων εξωτερικών δυνάμεων, η αναγνώριση της θέσης και η αποφυγή εμποδίων σε άγνωστο περιβάλλον κλίμακας μικρομέτρων και ο καθορισμός τροχιάς για προσέγγιση σημείων στον χώρο εργασίας του μικρορομπότ. Προτείνεται επίσης ένα σύστημα επενέργησης σε διάταξη τένοντα με νανοκαλώδια και γίνεται μελέτη της αντοχής του σε σχέση με τις μέγιστες δυνάμεις-ροπές που παρουσιάζονται κατά τον έλεγχο. Για την αναγνώριση των εξωτερικών δυνάμεων δοκιμάζονται διαφορετικά είδη εκτιμητών και εξετάζεται η απόδοσή τους στο συνολικό σύστημα. / The present PhD thesis has a key object the modeling and control of a micro robotic manipulator, represented by spherical particles in an environment with external Van Der Waals forces and taking friction into account. The main points are a) the insertion of the external forces in the micro robot model, b) the adaptive control used in order to follow a desired trajectory, with identification and cancellation of the external forces, the position identification and avoidance of obstacles in an unstructured micrometer scale environment and the trajectory planning towards a target point in the task space of the microrobot. Also a tendon like actuation system is proposed, using nanowires and its mechanical properties are studied in order to determine the viability of its use in relation to the required torques during the control process. For the external force identification scheme, various types of estimators are proposed and their efficiency in the system is studied.