Electrical Characterization of Colloidal Lead Sulphide Nanocrystals

Abstract

By approaching the end of Moore’s law, many efforts have been undertaken for semiconductor technology to keep its progress pace. Finding new materials to be employed instead of the conventional materials or finding new paradigms for data processing are among them. In order to contribute to these efforts, I investigated the electrical properties of lead sulphide nanocrystals. Lead sulphide is a small bandgap material with many promising properties and applications. This material could be synthesized recently in the form of 2D nanosheets which are laterally large enough to be individually contacted. These nanosheets have been already used as field-effect transistors (FETs) and solar cells. In order to employ this material for future electronics, its transistor behavior had to be improved. As the first step, the lateral size of the nanosheets has been optimized. By reducing the width of the crystal, which is equivalent to the reduction of the transistors’ active channel, the functionality of the devices has been boosted. This became possible by employing quasi-1D stripes instead of the normal squared nanosheets. This modification has improved the on/off ratio by a few orders of magnitude. The other important aspect is the possible imperfections of the crystal, which could hinder its high performance when it is used as a functional device. One of the critical imperfections in colloidal crystals could be their surface which normally contains trap states, originating from dangling bonds. To address this problem, the capability of different halogenoalkanes has been investigated to passivate the surface of the PbS nanosheets. Electrical measurements have shown that Cl- ions, which are provided by the decomposition of chloroalkanes, are able to effectively passivate these traps. The conductivity, field-effect mobility and on/off ratio of the sheets were significantly higher, when a Cl-containing coligand had been used to synthesize them, in addition to oleic acid as the main ligand (hybrid surface passivation). The passivation effect has been further signified by increasing the amount of the used coligand. The existence of Cl- ions on the surface of the high performance nanosheets has been confirmed with X-ray photoelectron spectroscopy. Further, the contact metal and the working environment have remarkable effects on the FET behavior of the sheets. Carefully adjusting the aforementioned parameters made it possible to realize high performance n-type FETs based on PbS. For this purpose, narrow stripes with a fully Cl-passivated surface have been contacted with Ti, as a proper contact metal for n-type materials, and the obtained devices have been characterized in vacuum. The outcome of this experiment was a FET with record on/off ratio and field-effect mobility among other colloidal materials. To convert the devices to p-type, Au contacts have been employed and air has been introduced to the crystal. The applications of the PbS nanosheets can be also expanded beyond conventional electronics. For this purpose, the spin-orbit coupling of the nanosheets has been investigated by performing circular photogalvanic measurements in order to probe their potential application in spintronics. When a material has a strong spin-orbit coupling, its band structure splits based on the spin orientation of the charge carriers. By illuminating this material with circularly polarized light, the spin equilibrium of the split bands is broken, resulting in an asymmetric distribution of the carriers in momentum space. This can be detected as a helicity-dependent net current while no bias voltage is applied to the material. The amplitude of this current could be considered as a measure for the strength of the spin-orbit coupling. In order to see this effect with the PbS nanosheets, their crystalline symmetry has been broken by applying asymmetric interfaces on top and underneath (SiO2 and vacuum) or by applying a gate electric field. The observed effect, which could be tuned by altering the gate voltage or by changing the thickness of the crystal, has been also investigated theoretically by density-functional theory calculations. The obtained insight, which is the first observation of the spin transport through colloidal materials, has opened new pathways for colloidal spintronics. Eventually, to further expand the potential applications of PbS, its properties have been altered by manipulating the crystal. A certain form of PbS nanowires has been found that shows metallic behavior, in contrast to the other forms which are all semiconducting. The reason for that is the presence of the single element Pb rich {111} facets on the surface of these wires, which makes the crystal metallic. This has been done by synthesizing small octahedra during the early stages of the synthesis. By the attachment of these octahedra, zigzag shaped metallic nanowires are produced which have the Pb rich {111} surface facets. Transport measurements have been employed to prove the metallic character of the wires in addition to density-functional theory calculations. The ability to tune the electrical properties of a material only by sculpting its crystal is important since it helps to better optimize many types of crystalline systems. The results introduce new applications for PbS nanowires such as flexible interconnects.

In dieser Arbeit wurden die elektrischen Eigenschaften verschiedener PbS Nanostrukturen untersucht. Solche Nanostrukturen könnten als aktives Material für zukünftige elektronische Bauteile oder für neue Methoden der Datenverarbeitung (z. B. für Spintronik) verwendet werden. Im ersten Teil der Arbeit wurden die Nanostrukturen zur Herstellung von Feldeffekttransistoren (FETs) verwendet. Die laterale Breite des Kristalls wurde verringert, um die Leistung dieser FETs zu verbessern. Schmale PbS Nanostreifen zeigten dabei ein besseres Transistorverhalten (On/Off-Verhältnis und Feldeffektmobilität) als breite Nanoblätter. Zusätzlich wurden verschiedene Halogenalkane verwendet, um die Oberfläche der Nanoblätter zu passivieren. Wenn Chlorid-Ionen zu der Synthese hinzugefügt wurden, waren die Oberflächendefekte des Kristalls passiviert und die Nanoblätter wiesen ein höheres On/Off-Verhältnis und eine höhere Feldeffektmobilität auf. Die Existenz von Chlorid-Ionen wurde auch mit der Röntgenphotoelektronenspektroskopie (XPS) bestätigt. Hochleistungs-n-Typ-FETs wurden unter Verwendung Chlorid-passivierter schmaler Streifen, die mit Ti kontaktiert und im Vakuum vermessen wurden, erreicht. Die Leistung dieser Transistoren entspricht der höchsten, die für kolloidale Nanostrukturen beobachtet wurde. Durch Änderung des Kontaktmetalls zu Au zeigten die Nanoblätter unter Luftatmosphäre ein p-Typ-Verhalten. Im zweiten Teil der Arbeit wurden die Nanoblätter für Anwendungen jenseits herkömmlicher Elektronik untersucht. Zu diesem Zweck wurde die Rashba Spin-Bahn-Kopplung der Nanoblätter durch die zirkular photogalvanische Messungen untersucht, um ihre mögliche Anwendung in der Spintronik zu erforschen. Die Symmetrie des Kristalls wurde durch die asymmetrischen Grenzflächen oben und unten (SiO2 und Vakuum) oder durch den Einfluss eines äußeren elektrischen Feldes aufgebrochen. Durch die Beleuchtung mit zirkular-polarisiertem Licht war es möglich, elektrische Ladungen in Nanoblättern auszurichten und in elektrischen Strom umzuwandeln ohne, dass eine Spannung angelegt werden musste. Durch die Variation der Schichtdicke der Nanoblätter, der Polarisation des verwendeten Lichtes und der Intensität der elektrischen Felder ließ sich der Effekt kontrollieren. Die experimentellen Beobachtungen wurden mit Simulationen der elektronischen Struktur der Materialien unterstützt. Dies ist die erste Beobachtung des Spintransports in kolloidalen Materialien. Im letzten Teil wurde eine bestimmte Form von PbS-Nanodrähten untersucht, die metallisch ist. Diese Drähte bestanden aus Oktaedern, dessen Oberfläche durch {111} –Facetten gebildet wurde und aus Pb-Atomen bestand. Wegen dieser Pb-reichen Oberfläche waren diese Drähte metallisch. Transportmessungen und DFT-Berechnungen wurden verwendet, um den metallischen Charakter der Drähte zu erklären. Diese Ergebnisse führen zu neuen Anwendungen für PbS, z. B. zu flexiblen Verbindungen.