Übergangsmetall-dotierte Halbleiter-Nanostrukturen aus lösungsmittelbasierter Herstellung: Von der Funktionalität zum Bauelement

Combining the outstanding electrical and optical properties of semiconductors with magnetic functionalities in a single material is a vision with the perspective to revolutionize information technology. A promising approach towards this goal lies in the partial replacement of host atoms of a ‘classical’ semiconductor by transition metal ions with nonzero spin moment, introducing novel magnetic and magneto-optical functionalities. Among these so-called diluted magnetic semiconductors (DMS), colloidal nanostructures offer strongly increased exchange interactions due to the high level of quantum confinement accessible in those materials. In this thesis, the degrees of freedom provided by the colloidal preparation of DMS nanostructures are explored. It is investigated how the optical and magneto-optical properties of DMS nanostructures can be influenced by modifying their architecture with monolayer or even atomic precision, and it is evaluated, whether their magnetic functionalities can be addressed electrically via integration in a device. Colloidal quantum wells (so-called nanoplatelets) offer the possibility to introduce transition metal dopants with monolayer precision. This potential was used to precisely control the exchange interactions and furthermore, the magneto-optical response was utilized to obtain a detailed insight into the excited electronic structure of these materials. By decreasing the size of spherical quantum dots to – in total – 26 atoms (13 cadmium and 13 selenide atoms, respectively) in magic sized clusters, it could be shown that their optical and magneto-optical properties can be modified by single atomic impurities. In manganese-doped clusters, evidence was provided that the magneto-optical functionality can be digitally controlled via doping with either one or two transition metal ions. On the other hand, alloying with isovalent zinc shifts the band edge energy in discrete steps. Observation of specific magneto-optical functionalities in manganese doped alloy clusters revealed the successful combination of four different elements in single clusters. Beyond that, the reduced size and distinct lattice structure of the magic sized clusters was found to have an impact on structure related properties. Doping with cobalt ions enabled an optical evidence of reduced cobalt-anion bond length compared to bulk, evidencing the clusters’ capability to respond to the replacement of one constituent by an atom of smaller size. Surprisingly, a twofold (as compared to bulk) enhanced temperature dependence of the bandgap energy was found, which could be traced back to the reduced cluster size, i.e., the small number of bonds and the high amount of surface atoms. In order to take the step from material development to devices, the incorporation of colloidal DMS nanostructures into an electrically driven device is demonstrated for the first time. By incorporating spherical DMS quantum dots into a solution processed device, electrically triggered magnetic ordering of the dopant spins could be achieved.Das zentrale Ziel der Spinelektronik, welches die Entwicklung neuartiger Technologien zur Informationsspeicherung und -verarbeitung verfolgt, besteht in der Vereinigung der elektronischen und optischen Eigenschaften von Halbleitern mit einer magnetischen Funktionalität in einem einzigen Materialsystem. Einen vielversprechenden Ansatz stellen verdünnt magnetische Halbleiter-Nanostrukturen dar, in denen durch Dotierung mit Übergangsmetallen eine magneto-optische Funktionalität erzeugt wird. Die lösungsmittelbasierte Synthese eröffnet hier die Möglichkeit, dotierte Nanostrukturen mit unterschiedlichster Form, Größe und Zusammensetzung herzustellen, und erlaubt außerdem deren Weiterverarbeitung aus einer Dispersion bis hin zum Bauelement. In der vorliegenden Dissertation werden diese unterschiedlichen Freiheitsgrade in mehrerer Hinsicht ausgenutzt. Zum einen wird untersucht, welche Auswirkungen eine Übergangsmetall-Dotierung mit Monolagen- oder sogar atomarer Präzision auf die Materialeigenschaften hat. Zum anderen wird erstmals erarbeitet, ob und wie die magneto-optische Funktionalität von magnetisch dotierten Halbleiternanokristallen in elektronischen Bauteilen gezielt erzeugt werden kann. Die magnetische Dotierung von kolloidalen Nanoplättchen mit komplexem Kern-Hülle Aufbau eröffnete die Möglichkeit, durch Variation der Schichtdicken und der Schichtzusammensetzung mit einer Genauigkeit von einzelnen Monolagen die magneto-optische Funktionalität gezielt zu manipulieren. Außerdem erlaubte die Dotierung einen detaillierten Einblick in die elektronische Struktur dieser neuartigen Materialklasse. Durch eine Reduzierung der Abmessungen von Nanokristallen hin zu Nanoclustern bestehend aus nur 26 Atomen (je 13 Cadmium- und Selenatome) wird es möglich, die optischen und magneto-optischen Eigenschaften durch den Austausch gar einzelner Atome zu verändern. In Mangan-dotierten Nanoclustern konnte gezeigt werde, dass die magneto-optische Funktionalität der Cluster durch den Einbau von einem bzw. zwei Mangan-Ionen in digitaler Weise kontrolliert werden kann. Durch den Nachweis von magneto-optischer Funktionalität in Mangan-dotierten Cadmium-Zink-Selenit Mischclustern gelang es, den erfolgreichen Einbau von vier verschiedenen Atomsorten in einzelne Cluster bestehend aus 26 Atomen nachzuweisen. Interessante Einflüsse der geringen Größe der Cluster auf strukturabhängige Eigenschaften zeigten sich bei den optischen Experimenten. Anhand eines internen optischen Übergangs in Kobalt-dotierten Clustern konnten Rückschlüsse auf die Bindungslängen zwischen den Kobalt- und Selen-Ionen gewonnen werden. Außerdem wurde eine im Vergleich zum Volumenhalbleiter zweifach verstärkte Abhängigkeit der Bandlückenenergie von der Temperatur beobachtet und mit Hilfe einer thermodynamischen Interpretation auf die geringe Anzahl an Bindungen in den Clustern sowie den hohen Anteil an Oberflächenatomen zurückgeführt. Zum Schluss konnte durch die Implementierung von magnetisch dotierten, kolloidalen Quantenpunkten in elektronischen Bauelementen der nächste Schritt hin zur Nutzung von verdünnt magnetischen Halbleiter-Nanostrukturen in spinelektronischen Bauelementen vollzogen werden

Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in nanoclusters

The fundamental bandgap E-g of a semiconductor-often determined by means of optical spectroscopy-represents its characteristic fingerprint and changes distinctively with temperature. Here, we demonstrate that in magic sized II-VI clusters containing only 26 atoms, a pronounced weakening of the bonds occurs upon optical excitation, which results in a strong exciton-driven shift of the phonon spectrum. As a consequence, a drastic increase of dE(g)/dT (up to a factor of 2) with respect to bulk material or nanocrystals of typical size is found. We are able to describe our experimental data with excellent quantitative agreement from first principles deriving the bandgap shift with temperature as the vibrational entropy contribution to the free energy difference between the ground and optically excited states. Our work demonstrates how in small nanoparticles, photons as the probe medium affect the bandgap-a fundamental semiconductor property. The bandgap of nanostructures usually follows the bulk value upon temperature change. Here, the authors find that in small nanocrystals a weakening of the bonds due to optical excitation causes a pronounced phonon shift, leading to a drastic enhancement of the bandgap's temperature dependence.

Quantum confinement-controlled exchange coupling in manganese(II)-doped CdSe two-dimensional quantum well nanoribbons

The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to similar to 13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.

Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters

Nanoclusters are important prenucleation intermediates for colloidal nanocrystal synthesis. In addition, they exhibit many intriguing properties originating from their extremely small size lying between molecules and typical nanocrystals. However, synthetic control of multicomponent semiconductor nanoclusters remains a daunting goal. Here, we report on the synthesis, doping, and transformation of multielement magic-sized clusters, generating the smallest semiconductor alloys. We use Lewis acid–base reactions at room temperature to synthesize alloy clusters containing three or four types of atoms. Mass spectrometry reveals that the alloy clusters exhibit “magic-size” characteristics with chemical formula of Zn_<i>x</i>Cd_{13–<i>x</i>}Se₁₃ (<i>x</i> = 0–13) whose compositions are tunable between CdSe and ZnSe. Successful doping of these clusters creates a new class of diluted magnetic semiconductors in the extreme quantum confinement regime. Furthermore, the important role of these alloy clusters as prenucleation intermediates is demonstrated by low temperature transformation into quantum alloy nanoribbons and nanorods. Our study will facilitate the understanding of these novel diluted magnetic semiconductor nanoclusters, and offer new possibilities for the controlled synthesis of nanomaterials at the prenucleation stage, consequently producing novel multicomponent nanomaterials that are difficult to synthesize

Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn²⁺-Doped CdSe Quantum Dots

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1S_e, the dominant hole character of each excitonic transition can be identified, making use of the fact that the <i>g</i>-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals

Digital Doping in Magic-Sized CdSe Clusters

Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)₁₃ clusters on the discrete number of Mn²⁺ ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn²⁺ ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn²⁺-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications

Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device

Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp–d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn²⁺-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn²⁺ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device

Co2+-doping of magic-sized CdSe clusters: Structural insights via ligand field transitions

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of similar to 10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)(13) clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small sized clusters.

Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co−Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely smallsized clusters.© 2018 American Chemical Societ

sp–d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin–orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material