Characterization and manipulation of Bi2Te3 topological insulator surfaces by scanning probe microscopy.

Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη και Τεχνολογία Υλικών

Σύνθεση νανοσολήνων άνθρακα μέσω θερμόλυσης σε στερεά κατάσταση

155 σ.Η παρούσα μελέτη εκπονήθηκε στο πλαίσιο της διπλωματικής εργασίας που έλαβε χώρα στο Εργαστήριο Ανόργανης και Αναλυτικής Χημείας της Σχολής Χημικών Μηχανικών του ΕΜΠ. Σκοπός της εργασίας ήταν η σύνθεση νανοσωλήνων άνθρακα με θερμόλυση οργανομεταλλικής ένωσης σε στερεά κατάσταση. Μια ευρέως χρησιμοποιούμενη μέθοδος για τη σύνθεση νανοσωλήνων άνθρακα είναι η χημική εναπόθεση ατμών (CVD). Διατηρώντας τη βασική αρχή της μεθόδου αναλλοίωτη, έχουν αναπτυχθεί πολλές τεχνικές σύνθεσης νανοσωλήνων άνθρακα, οι οποίες διαφοροποιούνται ανάλογα με την επιλογή των ενώσεων που θα αποτελέσουν την πηγή άνθρακα, τον καταλύτη που επιδρά στο μηχανισμό ανάπτυξης και διάφορων άλλων παραμέτρων. Μια τέτοια τεχνική είναι η θερμόλυση στερεάς ουσίας. Στη συγκεκριμένη εργασία χρησιμοποιήθηκαν δύο διαφορετικές ενώσεις σε στερεά μορφή για τη σύνθεση των νανοσωλήνων άνθρακα, συγκεκριμένα η οργανομεταλλική ένωση Ni(DMG) και η μελαμίνη παρουσία φερροκενίου. Οι πυρολύσεις έγιναν σε δυο διαφορετικά είδη φούρνων. Πιο συγκεκριμένα, σε συμβατικό φούρνο και φούρνο μικροκυμάτων, με απότομη ή σταδιακή αύξηση της θερμοκρασίας στους 700, 800 και 900 C. Ο χαρακτηρισμός των προϊόντων έγινε με διάφορες φυσικές μεθόδους ανάλυσης, όπως η Περίθλαση Ακτίνων Χ - XRD, η Θερμοσταθμική Ανάλυση – TG -DTG, και με Ηλεκτρονιακό Μικροσκόπιο Σάρωσης – SEM. Οι αναλύσεις με τις παραπάνω μεθόδους, επιβεβαίωσαν το σχηματισμό νανοσωλήνων άνθρακα πολλαπλών τοιχωμάτων, οι οποίοι περιβάλλονται από κάποιο ποσοστό άμορφου άνθρακα.This final year project thesis was performed in the Laboratory of Inorganic and Analytical Chemistry, at the School of Chemical Engineering at NTUA. The aim of this work was the synthesis of carbon nanotubes by thermolysis of organometallic compounds in the solid state. One widely used method for the synthesis of carbon nanotubes is the chemical vapor deposition (CVD) technique. Maintaining the basic principle of the method unchanged, many techniques have been developed for the synthesis of carbon nanotubes, which vary depending on the choice of compounds that will be used as the source of carbon and the catalyst which affects the growth mechanism and various other parameters. One such technique is the thermolysis of solid compounds. In this work, we used two different compounds in solid form for the synthesis of carbon nanotubes, specifically, the organometallic compound Ni(DMG)2 and a mixture of melamine with ferrocene. The thermolysis reactions were carried out in two different types of ovens, a conventional oven and a microwave, with abrupt or gradual increase in temperature to 700, 800 and 900 0C. The characterization of the products was performed by various instrumental methods of analysis such as X-ray diffraction (XRD), Thermogravimetric Analysis (TG-DTG), and Scanning Electron Microscopy (SEM). The analysis of the results confirmed the formation of multiple walled carbon nanotubes, surrounded by a percentage of amorphous carbon.Αστεριώνα-Μαρία Η. Νέτσο

Identifying Native Point Defects in the Topological Insulator Bi2Te3

We successfully identified native point defects that occur in Bi2Te3 crystals by combining high-resolution bias-dependent scanning tunneling microscopy and density functional theory based calculations. As-grown Bi2Te3 crystals contain vacancies, antisites, and interstitial defects that may result in bulk conductivity and therefore may change the insulating bulk character. Here, we demonstrate the interplay between the growth conditions and the density of different types of native near-surface defects. In particular, scanning tunneling spectroscopy reveals the dependence on not only the local atomic environment but also on the growth kinetics and the resulting sample doping from n-type toward intrinsic crystals with the Fermi level positioned inside the energy gap. Our results establish a bias-dependent STM signature of the Bi2Te3 native defects and shed light on the link between the native defects and the electronic properties of Bi2Te3, which is relevant for the synthesis of topological insulator materials and the related functional properties.status: publishe

Two-dimensional tellurium superstructures on Au(111) surfaces

Two-dimensional (2D) allotropes of tellurium (Te), recently coined as tellurene, are currently an emerging topic of materials research due to the theoretically predicted exotic properties of Te in its ultrathin form and at the single atomic layer limit. However, a prerequisite for the production of such new and single elemental 2D materials is the development of simple and robust fabrication methods. In the present work, we report three different 2D superstructures of Te on Au(111) surfaces by following an alternative experimental deposition approach. We have investigated the superstructures using low-temperature scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy (AES), and field emission AES. Three superstructures (13 x 13, 8 x 4, and & RADIC;11 x & RADIC;11) of 2D Te are observed in our experiments, and the formation of these superstructures is accompanied by the lifting of the characteristic 23 x & RADIC;3 surface reconstruction of the Au(111) surface. Scanning tunneling spectroscopy reveals a strong dependence of the local electronic properties on the structural arrangement of the Te atoms on the Au(111) support, and we observe superstructure-dependent electronic resonances around the Fermi level and below the Au(111) conduction band. In addition to the appearance of the new electronic resonances, the emergence of band gaps with a p-type charge character has been evidenced for two out of three Te superstructures (13 x 13 and root 11 x root 11) on the Au(111) support. Published under an exclusive license by AIP Publishing

Scanning probe microscopy induced surface modifications of the topological insulator Bi2Te3 in different environments

We investigated the topological insulator (TI) Bi2Te3 in four different environments (ambient, ultra-high vacuum (UHV), nitrogen gas and organic solvent environment) using scanning probe microscopy (SPM) techniques. Upon prolonged exposure to ambient conditions and organic solvent environments the cleaved surface of the pristine Bi2Te3 is observed to be strongly modified during SPM measurements, while imaging of freshly cleaved Bi2Te3 in UHV and nitrogen gas shows considerably less changes of the Bi2Te3 surface. We conclude that the reduced surface stability upon exposure to ambient conditions is triggered by adsorption of molecular species from ambient, including H2O, CO2, etc which is verified by Auger electron spectroscopy. Our findings of the drastic impact of exposure to ambient on the Bi2Te3 surface are crucial for further in-depth studies of the intrinsic properties of the TI Bi2Te3 and for potential applications that include room temperature TI based devices operated under ambient conditions.status: publishe