THz-Spektroskopie an zweidimensionalen Elektronengasen in Corbino-Geometrien

In dieser Arbeit wird die THz-induzierte Photo-Antwort eines zweidimensionalen elektronengases (2DEG) im Quanten-Hall-Regime untersucht. Dazu wird eine zweifach zusammenhängende Probengeometrie verwendet, die topologisch äquivalent zu einer ringförmigen Geometrie ist (Corbino-Geometrie). Diese Geometrie wird im Folgenden als Mäander-Corbino-Geometrie bezeichnet. Unter Quanten-Hall-Bedingungen wird der elektronische Transport, wie auch in einer Hall-bar-Probe, stark von den Randkanälen beeinflusst. Aufgrund der Ausbildung der Randkanäle entlang des Randes und einem semiisolierenden Volumenbereichs zwischen den Randkanälen ist es möglich, die inneren und äußeren Randkanäle getrennt von einander zu kontaktieren und die Photo-Antwort des semiisolierenden Volumenbereichs zu messen. Für Füllfaktoren ν < 2 wird in spektral aufgelösten Messungen eine Lorentz-förmige Resonanz bei der Zyklotronfrequenz beobachtet. Weiterhin zeigt die Analyse der Stromabhängigkeit der Photo-Antwort, dass sich das beobachtete Photosignal im Rahmen eines in der Literatur etablierten bolometrischen Modells beschreiben lässt. Die Situation ändert sich für Füllfaktoren oberhalb des ganzzahligen Füllfaktors (ν = 2). Hier eigen spektral aufgelöste Messungen eine asymmetrische Verbreiterung der Resonanz. Bei niedrigen Temperaturen (T = 300 mK) lassen sich zwei unabhängige Beiträge im Photosignal identifizieren. Ein Beitrag lässt sich dabei eindeutig auf das Heizen des Elektronengases durch die Zyklotronabsorption zurückführen (bolometrischer Beitrag), und der zweite Beitrag wird durch einen nicht-bolometrischen Mechanismus hervorgerufen. Die beobachtete Abhängigkeit der Amplitude der Photo-Antwort vom aufgeprägten Strom bei Füllfaktoren ν > 2 weist ebenfalls auf das Auftreten eines nicht-bolometrischen Beitrages zur Photo-Antwort hin. Außerdem werden Untersuchungen an einer so genannten Quasi-Corbino-Probe vorgestellt, die es erlaubt, zwei Randkanäle an einem Probenrand separat zu kontaktieren. Diese Quasi-Corbino-Probe ermöglicht es, die Photo-Antwort eines einzelnen inkompressiblen Streifens zwischen zwei Randkanälen (ohne den Einfluss des Volumens) zu untersuchen. Hier zeigen die Messungen, dass die Photo-Antwort aus der Erzeugung eines Photostroms innerhalb des inkompressiblen Randstreifens resultiert. Dementsprechend lässt sich der nicht-bolometrische Beitrag zur Photo-Antwort der Mäander-Corbino-Proben ebenfalls auf die Generation eines Photostroms innerhalb der inkompressiblen Streifen am Rand zurückführen. Darüber hinaus zeigt eine genauere Analyse der spektral aufgelösten Messungen an Mäander-Corbino-Proben einen dritten Beitrag zur Photo-Antwort. Dieser dritte Beitrag lässt sich dabei als eine neuartige Magnetoplasmon-Anregung am Probenrand interpretieren. Zum Abschluss dieser Arbeit wird die Anwendungsmöglichkeit der Mäander-Corbino-Proben als frequenzselektive, durchstimmbare und hochsensible THz-Detektoren vorgestellt.In this thesis, the THz-photoresponse between two separately connected edge-channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry which is topologically equivalent to a ring shape (Corbino-geometry), in the following called meander-Corbino-geometry. Under quantizing magnetic fields, electronic transport is strongly influenced by the edge channels, as in Hall-bar samples, however with two disjoint boundaries. This allows us to electrically connect the edge-states of each boundary separately and measure the photoresponse across the insulating bulk between the edges. At filling factors ν < 2, spectrally resolved photoresponse measurements show a Lorentzian resonance, centered at the cyclotron-frequency. Furthermore, we find that the source-drain-current dependence of the photoresponse amplitude at filling factors ν < 2 is in a good agreement with a bolometric model, which is well established in literature. The situation changes above the integer filling factor. Spectrally resolved photoresponse measurements show an asymmetric broadening of the resonance. At low temperatures (T = 300 mK), two independent contributions to the photoresponse signal can be resolved. One contribution clearly results from bolometric heating inside the bulk and the other one is caused by a non-bolometric mechanism. The source-draincurrent dependence of the photoresponse amplitude at filling factors ν > 2 also indicates the occurrence of a non-bolometric contribution to the photoresponse. Additionally, we investigate the photoresponse of a so-called quasi-Corbino-sample, which allows us to separately contact to edge channels at the same boundary. This quasi-Corbino-sample enables us to study the photoresponse of a single incompressible strip between the edge channels, without any bulk effects. The experimental results show that the photoresponse of such a quasi-Corbino-sample is caused by a photocurrent, generated inside the incompressible strip. These measurements suggest that the non-bolometric contribution mentioned above is also caused by a photocurrent generated inside the incompressible strips at the sample edges. Furthermore, a closer examination of the spectrally resolved photoresponse measurements on meander-Corbino-samples reveal a third contribution to the photoresponse which can be interpreted as a new kind of a magnetoplasmon excitation on the sample edge. Finally, we demonstrate the use of the meander-Corbino-samples as frequency-selective, tunable and highly sensitive THz-detectors

Terahertz photoresponse of a quantum Hall edge-channel diode

The Teraherz (THz) photoresponse of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a sample structure which is topologically equivalent to a Corbino geometry combined with a cross-gate technique. This quasi-Corbino geometry allows us to directly investigate the THz-induced transport between adjacent edge-states, thus avoiding bulk effects. We find a pronounced photo voltage at zero applied bias, which rapidly decreases when an external current bias is applied. The photo voltage and its dependence on the bias current can be described using the model of an illuminated photodiode, resulting from the reconstruction of the Landau bands at the sample edge. Using the sample as a detector in a Fourier transform spectrometer setup, we find a resonant response from which we extract a reduced effective cyclotron mass. The findings support a non-bolometric mechanism of the induced photo voltage and the proposed edge-channel diode model.Comment: 5 pages, 5 eps-figures, accepted for Phys. Rev.

Annealing of swift heavy ion tracks in amorphous silicon dioxide

The annealing kinetics of the high energy ion damage in amorphous silicon dioxide (a-SiO2) are still not well understood, despite the material's widespread application in material science, physics, geology, and biology. This study investigates how annealing temperature, duration, and ambient environment affect the recovery of irradiation damage produced along the trajectory of swift heavy ions in a-SiO2. The track-annealing kinetics and the changing ion track morphology were investigated using synchrotron-based small-angle X-ray scattering (SAXS) and etching methods. We found that track annealing proceeds quicker near the sample surface demonstrated by a changing track etch rate as a function of depth. Measurements of ion tracks using SAXS show only small changes in the radial density distribution profile of the ion tracks. Activation energy of the annealing process at different sample depths was determined and the effect of the capping layer during the annealing process was also studied. Combination of oxygen diffusion and stress relaxation may contribute to the observed behaviour of preferential and anisotropic healing of the ion track. The results add to the fundamental understanding of ion track damage recovery and may have direct implications for materials for radioactive waste storage and solid state nanopores

A graphene film interlayer for enhanced electrical conductivity in a carbon-fibre/PEEK composite

Carbon-fibre reinforced composites are seeing increased deployment, especially in the aerospace industry, and the next-generation of these materials will need to meet demanding performance requirements beyond just specific strength. The incorporation of nanomaterials such as graphene into composites has great potential for enhancing electrical, thermal, and mechanical properties, which could then enable new capabilities such as built-in lightning strike protection and electromagnetic shielding. One major challenge is successful integration of nanomaterials into the composite during the manufacturing process especially for thermoplastic based composites. This work explores the spray deposition of exfoliated graphene in liquid suspensions for the nano-enhancement of electrical properties in carbon-fibre reinforced polyether ether keytone (PEEK) composites. Developed thin films were smooth with RMS roughness of 1.06 μm on Si substrates and RMS roughness of 1.27 μm on CF-PEEK tapes. The addition of 1.3 wt% graphene into the interlayers of CF-PEEK composites resulted in bulk electrical conductivity enhancement both in plane and through thickness of ~ 1100% and 67.5% respectively. This approach allows for pre-consolidation introduction of high-performance nanomaterials directly to thermoplastic prepregs which could open simple pathways for the in-situ manufacturing of carbon-fibre reinforced polymer nanocomposites.This project was conducted within the ARC Training Centre for Automated Manufacture of Advanced Composites (IC160100040), supported by the Commonwealth of Australia under the Australian Research Council’s Industrial Transformation Research Program

Localization of Ag Dopant Atoms in CdSe Nanocrystals by Reverse Monte Carlo Analysis of EXAFS Spectra

The structure of CdSe nanocrystals doped with 0.2%−2.5% Ag cor- responding to 1.1−13.6 Ag atoms per nanocrystal is studied in detail by a combination of X-ray diffraction (XRD) and X-ray absorption spectroscopy at the Ag−K, Cd−K, and Se−K edges. X-ray absorption near-edge structure (XANES) data are compared with ab initio multiple scattering simulations. Extended X-ray absorption fine structure (EXAFS) spectra are analyzed by reverse Monte Carlo (RMC) simulations. The XANES data provide evi- dence that Ag is located inside the CdSe nanocrystals, and the EXAFS spectra show that the local structure of Ag can be described by tetrahedral interstitial sites in either wurtzite or zinc blende lattices similar to the coordination of Ag in Ag$_2$Se

Highly Rectifying Conical Nanopores in Amorphous SiO2 Membranes for Nanofluidic Osmotic Power Generation and Electroosmotic Pumps

Nanopore membranes are a versatile platform for a wide range of applications ranging from medical sensing to filtration and clean energy generation. To attain high-flux rectifying ionic flow, it is required to produce short channels exhibiting asymmetric surface charge distributions. This work reports on a system of track etched conical nanopores in amorphous SiO$_2$ membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 1 $\mu$m thick SiO$_2$ windows with 2.2 GeV $^{197}$Au ions and subsequent chemical etching. Structural characterisation is performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), ellipsometry, and surface profiling. Conductometric characterisation of the pore surface is performed using a membrane containing 16 pores, including an in-depth analysis of ionic transport characteristics. The pores have a tip radius of (5.7 $\pm$ 0.1) nm, a half-cone angle of (12.6 $\pm$ 0.1)$^{\circ}$, and a length of (710 $\pm$ 5) nm. The $pK_a$, $pK_b$, and $pI$ are determined to 7.6 $\pm$ 0.1, 1.5 $\pm$ 0.2, and 4.5 $\pm$ 0.1, respectively, enabling the fine-tuning of the surface charge density between +100 and -300 mC $m^{-2}$ and allowing to achieve an ionic current rectification ratio of up to 10. This highly versatile technology addresses challenges that contemporary nanopore systems face, and offers a platform to improve the performance of existing applications

Heat treated graphene thin films for reduced void content of interlaminar enhanced CF/PEEK composites

Abstract Graphene enhanced thermoplastic composites offer the possibility of conductive aerospace structures suitable for applications from electrostatic dissipation, to lightning strike protection and heat dissipation. Spray deposition of liquid phase exfoliated (LPE) aqueous graphene suspensions are highly scalable rapid manufacturing methods suitable to automated manufacturing processes. The effects of residual surfactant and water from LPE on thin films for interlaminar prepreg composite enhancement remain unknown. This work investigates the effect of heat treatment on graphene thin films spray deposited onto carbon fibre/polyether ether ketone (CF/PEEK) composites for reduced void content. Graphene thin films deposited onto CF/PEEK prepreg tapes had an RMS roughness of 1.99 μm and an average contact angle of 11°. After heat treatment the roughness increased to 2.52 μm with an average contact angle of 82°. The SEM images, contact angle, and surface roughness measurements correlated suggesting successful removal of excess surfactant and moisture with heat treatment. Raman spectroscopy was used to characterise the chemical quality of the consolidated graphene interlayer. Spectral data concluded the graphene was 3–4 layered with predominantly edge defects suggesting high quality graphene suitable for electrical enhancement. Conductive-AFM measurements observed an increase in conductive network density in the interlaminar region after the removal of surfactant from the thin film. Heat treatment of the Control sample successfully reduced void content from 4.2 vol% to 0.4 vol%, resulting in a 149% increase in compressive shear strength. Comparatively, heat treatment of graphene enhanced samples (~ 1 wt%) reduced void content from 5.1 vol% to 2.8 vol%. Although a 25% reduction in shear strength was measured, the improved electrical conductivity of the interlaminar region extends the potential applications of fibre reinforced thermoplastic composites. The heat treatment process proves effective in reducing surfactant and thus void content while improving electrical conductivity of the interlayer in a scalable manner. Further investigations into graphene loading effects on conductive enhancement, and void formation is needed