Effects of Dielectric Stoichiometry on the Photoluminescence Properties of Encapsulated WSe2 Monolayers

Two-dimensional transition-metal-dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact performances. However atomically thin materials are highly sensitive to surrounding dielectric media, which imposes severe limitations to their practical applicability. Hence for their suitable integration into devices, the development of reliable encapsulation procedures that preserve their physical properties are required. Here, the excitonic photoluminescence of WSe2 monolayer flakes is assessed, at room temperature and 10 K, on mechanically exfoliated flakes encapsulated with SiOx and AlxOy layers employing chemical and physical deposition techniques. Conformal flakes coating on untreated - non-functionalized - flakes is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. No significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques. Remarkably, our results evidence that the flakes' optical emission is strongly influenced by the quality of the encapsulating oxide - stoichiometry -. When the encapsulation is carried out with slightly sub-stoichiometric oxides two remarkable phenomena are observed. First, there is a clear electrical doping of the monolayers that is revealed through a dominant trion - charged exciton - room-temperature photoluminescence. Second, a strong decrease of the monolayers optical emission is measured attributed to non-radiative recombination processes and/or carriers transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation.Comment: 30 pages, 6 figure

Nanoscale measurement of the dielectric constant of supported lipid bilayers in aqueous solutions with electrostatic force microscopy

We present what is, to our knowledge, the first experimental demonstration of dielectric constant measurement and quantification of supported lipid bilayers in electrolyte solutions with nanoscale spatial resolution. The dielectric constant was quantitatively reconstructed with finite element calculations by combining thickness information and local polarization forces which were measured using an electrostatic force microscope adapted to work in a liquid environment. Measurements of submicrometric dipalmitoylphosphatidylcholine lipid bilayer patches gave dielectric constants of εr ∼ 3, which are higher than the values typically reported for the hydrophobic part of lipid membranes (εr ∼ 2) and suggest a large contribution of the polar headgroup region to the dielectric response of the lipid bilayer. This work opens apparently new possibilities in the study of biomembrane electrostatics and other bioelectric phenomena

Dynamic electrostatic force microscopy in liquid media

We present the implementation of dynamic electrostatic force microscopy in liquid media. This implementation enables the quantitative imaging of local dielectric properties of materials in electrolyte solutions with nanoscale spatial resolution. Local imaging capabilities are obtained by probing the frequency-dependent and ionic concentration-dependent electrostatic forces at high frequency (>1 MHz), while quantification of the interaction forces is obtained with finite-element numerical calculations. The results presented open a wide range of possibilities in a number of fields where the dielectric properties of materials need to be probed at the nanoscale and in a liquid environment

The Marsh

A descriptive narration of a project to create a short 2-D/3-D computer animated film, The Marsh. The film is a non-character-based narrative progression showing a marsh during three phases of human history. Includes original thesis proposal, original storyboard, color stills, and the original poem that inspired the film

Near-field microwave techniques for micro – and nano - scale characterization in materials science

In this paper, the basic principles of Near-Field Microscopy will be reviewed with focus on the micro- and nano-scale resolution configurations for material science measurements. Results on doping profile, dielectric and magnetic properties will be presented, with details on the calibration protocols needed for quantitative estimation of the dielectric constant and of the permeability

Calibrated Microwave Reflectance in Low-Temperature Scanning Tunneling Microscopy

We outline calibrated measurements of the microwave reflection coefficient from the tunnel junction of an ultra-high vacuum low temperature scanning tunneling microscope. The microwave circuit design is described in detail, including an interferometer for enhanced signal-to-noise and a demodulation scheme for lock-in detection. A quantitative, in-situ procedure for impedance calibration based on the numerical 3-error-term model is presented. Our procedure exploits the response of the microwave reflection signal due to the change of the tunneling conductance caused by sub-nm variation of the tunneling distance. Experimental calibration is achieved by a least-squares numerical fit of simultaneously measured conductance and microwave reflection retraction curves at finite conductance. Our method paves the way for nanoscale microscopy and spectroscopy of dielectric surface properties at GHz frequencies and cryogenic temperatures. This opens a promising pathway even for dielectric fingerprinting at the single molecule limit.Comment: The manuscript has been improved in response to reviewer comments. Changes include addition of extra details and verification, updated figure layout to improve clarity, and additional context added to the introduction and conclusion. The conclusions and the underlying data remain the same. 12 pages, 4 figures, submitted to Review of Scientific Instrument

Design of a modular data logging system with the aid of the graphical LabVIEW development platform

Die Arbeit gliedert sich in einen Textteil und einen Softwareteil. Die ersten Kapitel des Textteils zeigen den theoretischen Hintergrund der Messtechnik und der verwendeten Programmiersprache LabVIEW. Danach schließt sich eine Erläuterung des in LabVIEW erstellten Programmsystems an. Im siebten Kapitel wird auf die durchgeführten Kalibrierungsmessungen eingegangen. Das achte Kapitel gibt eine kurze Zusammenfassung der gewonnenen Erfahrungen. Im Anhang finden sich zwei Beispielprotokolle der Aufnehmerkalibrierung, erstellt mit dem LabVIEW-Programmsystem sowie das Blockdiagramm des Aufnehmerkorrektur-VIs. Das eigentliche Softwaresystem steht inklusive aller Hilfsprogramme als LabVIEW-Projekt, ausführbare Exe-Datei und als Setup-Version auf der beilegten CD zur Verfügung. Für die Ausführung wird die LabVIEW-Runtime 8.0.1 benötigt. Der Datenträger enthält außerdem die vorliegende Arbeit im PDF und Latex-Format, Protokolle der Aufnehmerkalibrierungen die Messprogramm-Hilfe und die benötigten Steuerdateien für Aufnehmer und Messprogramm