Molecular and Supramolecular Designs of Organic/Polymeric Micro-photoemitters for Advanced Optical and Laser Applications

ConspectusFor optical and electronic applications of supramolecular assemblies, control of the hierarchical structure from nano- to micro- and millimeter scale is crucial. Supramolecular chemistry controls intermolecular interactions to build up molecular components with sizes ranging from several to several hundreds of nanometers using bottom-up self-assembly process. However, extending the supramolecular approach up to a scale of several tens of micrometers to construct objects with precisely controlled size, morphology, and orientation is challenging. Especially for microphotonics applications such as optical resonators and lasers, integrated optical devices, and sensors, a precise design of a micrometer-scale object is required. In this Account, we review the recent progress on precise control of microstructures from π-conjugated organic molecules and polymers, which work as micro-photoemitters and are suitable for optical applications.After the introduction on the importance of the control of the hierarchical structures from molecular assembly, we review supramolecular methodology for assembling molecules and supramolecules to form microstructures such as spheres and polygons with precisely controlled morphology and molecular orientations. The resultant microstructures act as anisotropic emitters of circularly polarized luminescence. We report that synchronous crystallization of π-conjugated chiral cyclophanes forms concave hexagonal pyramidal microcrystals with homogeneous size, morphology, and orientation, which clearly paves the way for the precise control of skeletal crystallization under kinetic control. Furthermore, we show microcavity functions of the self-assembled micro-objects. The self-assembled π-conjugated polymer microspheres work as whispering gallery mode (WGM) optical resonators, where the photoluminescence exhibits sharp and periodic emission lines. The spherical resonators with molecular functions act as long-distance photon energy transporters, convertors, and full-color microlasers. Fabrication of microarrays with photoswitchable WGM microresonators by the surface self-assembly technique realizes optical memory with physically unclonable functions of WGM fingerprints. All-optical logic operations are demonstrated by arranging the WGM microresonators on synthetic and natural optical fibers, where the photoswitchable WGM microresonators act as a gate for light propagation via a cavity-mediated energy transfer cascade. Meanwhile, the sharp WGM emission line is appropriate for utilization as optical sensors for monitoring the mode shift and mode splitting. The resonant peaks sensitively respond to humidity change, absorption of volatile organic compounds, microairflow, and polymer decomposition by utilizing structurally flexible polymers, microporous polymers, nonvolatile liquid droplets, and natural biopolymers as media of the resonators. We further construct microcrystals from π-conjugated molecules with rods and rhombic plates, which act as WGM laser resonators with light-harvesting function. Our developments, precise design and control of organic/polymeric microstructures, form a bridge between nanometer-scale supramolecular chemistry and bulk materials and pave the way toward flexible micro-optics applications.

Pushing the Limits in Real-Time Measurements of Quantum Dynamics

Time-resolved studies of quantum systems are the key to understanding quantum dynamics at its core. The real-time measurement of individual quantum numbers as they switch between certain discrete values, well known as a "random telegraph signal," is expected to yield maximal physical insight. However, the signal suffers from both systematic errors, such as a limited time resolution and noise from the measurement apparatus, as well as statistical errors due to a limited amount of data. Here we demonstrate that an evaluation scheme based on factorial cumulants can reduce the influence of such errors by orders of magnitude. The error resilience is supported by a general theory for the detection errors as well as experimental data of single-electron tunneling through a self-assembled quantum dot. Thus, factorial cumulants push the limits in the analysis of random telegraph data, which represent a wide class of experiments in physics, chemistry, engineering, and life sciences.

Antibiotics as a silent driver of climate change? A case study investigating methane production in freshwater sediments

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2) and is inter alia produced in natural freshwater ecosystems. Given the rise in CH4 emissions from natural sources, researchers are investigating environmental factors and climate change feedbacks to explain this increment. Despite being omnipresent in freshwaters, knowledge on the influence of chemical stressors of anthropogenic origin (e.g., antibiotics) on methanogenesis is lacking. To address this knowledge gap, we incubated freshwater sediment under anaerobic conditions with a mixture of five antibiotics at four levels (from 0 to 5000 µg/L) for 42 days. Weekly measurements of CH4 and CO2 in the headspace, as well as their compound-specific δ13C, showed that the CH4 production rate was increased by up to 94% at 5000 µg/L and up to 29% at field-relevant concentrations (i.e., 50 µg/L). Metabarcoding of the archaeal and eubacterial 16S rRNA gene showed that effects of antibiotics on bacterial community level (i.e., species composition) may partially explain the observed differences in CH4 production rates. Despite the complications of transferring experimental CH4 production rates to realistic field conditions, the study indicated that chemical stressors contribute to the emissions of greenhouse gases by affecting the methanogenesis in freshwaters.

Towards field-effect controlled graphene-enhanced Raman spectroscopy of cobalt octaethylporphyrin molecules

During the last decade graphene-enhanced Raman spectroscopy has proven to be a powerful tool to detect and analyze minute amounts of molecules adsorbed on graphene. By using a graphene-based field-effect device the unique opportunity arises to gain a deeper insight into the coupling of molecules and graphene as graphene's Fermi level can be controlled by the transistor`s gate voltage. However, the fabrication of such a device comes with great challenges because of contaminations stemming from processing the device inevitably prevent direct adsorption of the molecules onto graphene rendering it unsuitable for field-effect controlled graphene-enhanced Raman spectroscopy measurements/experiments. In this work, we solve this problem by establishing two different fabrication procedures for such devices, both of which are in addition compatible with large area and scalable production requirements. As a first solution, selective argon cluster irradiation is shown to be an efficient way to remove resist residues after processing. We provide evidence that after the irradiation the enhancement of the molecular Raman signal can indeed be measured, demonstrating that this procedure cleans graphene's surface sufficiently enough for direct molecular adsorption. As a second solution, we have developed a novel stacking method to encapsulate the molecules in between two graphene layers to protect the underlying graphene and molecular layer from the harsh conditions during the photolithography process. This method combines the advantages of dry stacking, which leads to a perfectly clean interface, and wet stacking processes, which can easily be scaled up for large area processing. Both approaches yield working graphene transistors with strong molecular Raman signals stemming from cobalt octaehtylporphyrin, a promising and prototypical candidate for spintronic applications, and are therefore suitable for graphene based molecular sensing applications.

Spatial and temporal variability of methane emissions from cascading reservoirs in the Upper Mekong River

Reservoirs are an important source of atmospheric methane (CH₄), a potent greenhouse gas. The Mekong, one of the largest Asian rivers, has been heavily dammed and can be a potential hotspot for CH₄ emissions. While low diffusive CH₄ flux was previously reported from cascading reservoirs in the Upper Mekong, the contribution of ebullition (bubbling) remained unexplored. To better constrain the magnitude and drivers of ebullition from these reservoirs, automated bubble traps were deployed in four reservoirs, allowing for continuous measurement of the ebullitive flux with high temporal resolution for a period of six months. To characterize the spatial variability of CH₄ fluxes mediated by ebullition and diffusion, whole-reservoir surveys were conducted using a scientific echo sounder for bubble observations together with a gas equilibrator for mapping dissolved CH₄ concentration in surface water from which diffusive fluxes were estimated. Potential production and anaerobic oxidation rates of CH₄ were estimated in laboratory incubations of sediment cores collected near the bubble trap deployment sites. The CH₄ production potential in sediments increased strongly along the reservoir cascade, with mostly minor reduction by anaerobic oxidation. Surface CH₄ fluxes, in contrast, showed high spatial variability in both ebullitive and diffusive pathways (ranging 0.05-44 and 1.8-6.4 mg m^-2 d^-1, respectively, among all reservoirs). Ebullitive fluxes were about one order of magnitude higher than diffusive fluxes and remained significant at sites as deep as 30-45 m. The repeated spatial pattern of ebullition (higher fluxes at the dam area than in upstream sections) suggests the possible control of emission rates by sediment transport and deposition.

One-step synthesis of carbon-supported electrocatalysts

Cost-efficiency, durability, and reliability of catalysts, as well as their operational lifetime, are the main challenges in chemical energy conversion. Here, we present a novel, one-step approach for the synthesis of Pt/C hybrid material by plasma-enhanced chemical vapor deposition (PE-CVD). The platinum loading, degree of oxidation, and the very narrow particle size distribution are precisely adjusted in the Pt/C hybrid material due to the simultaneous deposition of platinum and carbon during the process. The as-synthesized Pt/C hybrid materials are promising electrocatalysts for use in fuel cell applications as they show significantly improved electrochemical long-term stability compared to the industrial standard HiSPEC 4000. The PE-CVD process is furthermore expected to be extendable to the general deposition of metal-containing carbon materials from other commercially available metal acetylacetonate precursors.

Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot

Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means.

Patterning of diamond with 10 nm resolution by electron-beam-induced etching

We report on mask-less, high resolution etching of diamond surfaces, featuring sizes down to 10 nm. We use a scanning electron microscope (SEM) together with water vapor, which was injected by a needle directly onto the sample surface. Using this versatile and low-damage technique, trenches with different depths were etched. Cross sections of each trench were obtained by focused ion beam milling and used to calculate the achieved aspect ratios. The developed technique opens up the possibility of mask- and resist-less patterning of diamond for nano-optical and electronic applications.

Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot

The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ_{↑↓}=3  ms^{-1}), even in an equilibrium transport measurement.

Contrast of 83% in reflection measurements on a single quantum dot

We report on a high optical contrast between the photon emission from a single self-assembled quantum dot (QD) and the back-scattered excitation laser light. In an optimized semiconductor heterostructure with an epitaxially grown gate, an optically-matched layer structure and a distributed Bragg reflector, a record value of 83% is obtained; with tilted laser excitation even 885%. This enables measurements on a single dot without lock-in technique or suppression of the laser background by cross-polarization. These findings open up the possibility to perform simultaneously time-resolved and polarization-dependent resonant optical spectroscopy on a single quantum dot.

Photon Noise Suppression by a Built-in Feedback Loop

Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization.

Investigation of dissolved N₂O production processes during wastewater treatment system in Ulaanbaatar

Nitrous oxide (N2O) is an increasing greenhouse gas in the troposphere and a potential destroyer of stratospheric ozone layer. Wastewater treatment plant (WWTP) is one of the anthropogenic N2O sources because inorganic and organic nitrogen compounds are converted to nitrate (NO3-, in the case of standard system) or N2 (in the case of advanced system) by bacterial nitrification and denitrifcation processes in WWTP. These major processes can be distinguished by isotopocule analysis. In order to reveal production mechanisms of N2O in a standard wastewater treatment, we made water sampling at the central WWTP in Ulaanbaatar. The water samples collected from seven stations including biological reaction tanks were measured for concentration and isotopocule ratios of dissolved N2O and other inorganic nitrogen. Dissolved N2O concentration was extremely higher than that expected under atmospheric equilibrium (about 9 nmol/l) at all stations, indicating that this system is a potential source of N2O. It showed a gradual increase with the progress of biological reaction and the highest concentration (335.7 nmol/l) was observed at station N5-4 of the aeration tank when the DO was 5.7 mg/l. Nitrification by nitrifying bacteria could actively occur by the concentration of NH4+ decreased whereas NO2- and NO3- showed a temporal and monotonic increase, respectively, under high DO concentration. Although the reported values of site preference (SP) of N2O, the difference in 15N/14N ratio between central (α) and terminal (β) nitrogen, produced via NO2- reduction (SP(ND)), including both nitrifier and denitrifier denitrification, and NH2OH oxidation (SP(HO)) ranged from -10.7‰ to 0‰ and 31.4‰ to 36.3‰, respectively, the observed SP at aeration tank was close to SP(ND) rather than SP(HO). It was ranged from 0.4‰ to 13.3‰ when N2O concentration was high, implying that the NO2- reduction made a greater contribution to N2O production. Slightly elevated SP (13.3‰) only at station N5-1 was derived from the mixing of N2O produced via NH2OH oxidation and the maximal contribution of this pathway was estimated to be about 40%. In other words, the contribution of NO2- reduction was more than 60%.

Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade

Color-tunable resonant photoluminescence (PL) was attained from polystyrene microspheres doped with a single polymorphic fluorescent dye, boron-dipyrrin (BODIPY) 1. The color of the resonant PL depends on the assembling morphology of 1 in the microspheres, which can be selectively controlled from green to red by the initial concentration of 1 in the preparation process of the microspheres. Studies on intersphere PL propagation with multicoupled microspheres, prepared by micromanipulation technique, revealed that multistep photon transfer takes place through the microspheres, accompanying energy transfer cascade with stepwise PL color change. The intersphere energy transfer cascade is direction selective, where energy donor-to-acceptor down conversion direction is only allowed. Such cavity-mediated long-distance and multistep energy transfer will be advantageous for polymer photonics device application.

Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot

Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir-in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton.

Conjugated Polymer Blend Microspheres for Efficient, Long-Range Light Energy Transfer

Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation of a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications.

Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition

In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots.

Electron-beam induced nano-etching of suspended graphene

Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices.

Self-assembled conjugated polymer spheres as fluorescent microresonators

Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2-10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.

Manipulation of electronic transport in the Bi(111) surface state

We demonstrate the controlled manipulation of the 2D-electronic transport in the surface state of Bi(111) through the deposition of small amounts of Bi to generate adatoms and 2D islands as additional scatterers. The corresponding increase in resistance is recorded in situ and in real time. Model calculations based on mean-field nucleation theory reveal a constant scattering efficiency of adatoms and of small 2D Bi islands, independent of their size. This finding is supported by a detailed scanning tunneling microscopy and spectroscopy study at 5 K which shows a highly anisotropic scattering pattern surrounding each surface protrusion.

Energy transport by neutral collective excitations at the quantum Hall edge

We use the edge of the quantum Hall sample to study the possibility for counterpropagating neutral collective excitations. A novel sample design allows us to independently investigate charge and energy transport along the edge. We experimentally observe an upstream energy transfer with respect to the electron drift for the filling factors 1 and 1/3. Our analysis indicates that a neutral collective mode at the interaction-reconstructed edge is a proper candidate for the experimentally observed effect.

Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics

The formation of stable colloidal dispersions of silicon nanoparticles (Si-NPs) is essential for the manufacturing of silicon based electronic and optoelectronic devices using cost-effective printing technologies. However, the development of Si-NPs based printable electronics has so far been hampered by the lack of long-term stability, low production rate and poor optical properties of Si-NPs ink. In this paper, we synthesized Si-NPs in a gas phase microwave plasma reactor with very high production rate, which were later treated to form a stable colloidal dispersion. These particles can be readily dispersed in a variety of organic solvents and the dispersion is stable for months. The particles show excellent optical properties (quantum yields of about 15%) and long-term photoluminescence (PL) stability. The stable ink containing functionalized Si-NPs was successfully used to print structures on glass substrates by ink-jet printing. The homogeneity and uniformity of large-area printed film was investigated using photoluminescence (PL) mapping.

XeF(2) gas-assisted focused-electron-beam-induced etching of GaAs with 30 nm resolution

We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF(2)) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF(2) as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs.

Electroluminescence from silicon nanoparticles fabricated from the gas phase

Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission.

"Artificial atoms" in magnetic fields: wave-function shaping and phase-sensitive tunneling

We demonstrate the possibility to influence the shape of the wave functions in semiconductor quantum dots by the application of an external magnetic field B(z). The states of the so-called p shell, which show distinct orientations along the crystal axes for B(z) = 0, can be modified to become more and more circularly symmetric with an increasing field. Their changing probability density can be monitored using magnetotunneling wave function mapping. Calculations of the magnetotunneling signals are in good agreement with the experimental data and explain the different tunneling maps of the p(+) and p⁻ states as a consequence of the different sign of their respective phase factors.

Tuning quantum-dot based photonic devices with liquid crystals

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

A two-dimensional electron gas as a sensitive detector for time-resolved tunneling measurements on self-assembled quantum dots

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients.

Graphene on insulating crystalline substrates

We show that it is possible to prepare and identify ultra-thin sheets of graphene on crystalline substrates such as SrTiO(3), TiO(2), Al(2)O(3) and CaF(2) by standard techniques (mechanical exfoliation, optical and atomic force microscopy). On the substrates under consideration we find a similar distribution of single layer, bilayer and few-layer graphene and graphite flakes as with conventional SiO(2) substrates. The optical contrast C of a single graphene layer on any of those substrates is determined by calculating the optical properties of a two-dimensional metallic sheet on the surface of a dielectric, which yields values between C = -1.5% (G/TiO(2)) and C = -8.8% (G/CaF(2)). This contrast is in reasonable agreement with experimental data and is sufficient to make identification by an optical microscope possible. The graphene layers cover the crystalline substrate in a carpet-like mode and the height of single layer graphene on any of the crystalline substrates as determined by atomic force microscopy is d(SLG) = 0.34 nm and thus much smaller than on SiO(2).

Electrical readout of the local nuclear polarization in the quantum Hall effect: a hyperfine battery

It is demonstrated that the now well-established "flip-flop" mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization--close to the maximum value achievable--by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a mV, which decays with a time constant typical for the nuclear spin relaxation. The surprising fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.

Coulomb-interaction-induced incomplete shell filling in the hole system of InAs quantum dots

We have studied the hole charging spectra of self-assembled InAs quantum dots in perpendicular magnetic fields by capacitance-voltage spectroscopy. From the magnetic-field dependence of the individual peaks we conclude that the s-like ground state is completely filled with two holes but that the fourfold degenerate p shell is only half filled with two holes before the filling of the d shell starts. The resulting six-hole ground state is highly polarized. This incomplete shell filling can be explained by the large influence of the Coulomb interaction in this system.

Rectification in mesoscopic systems with broken symmetry: quasiclassical ballistic versus classical transport

In suitably designed mesoscopic semiconductor structures, the phenomenon of ballistic rectification can be observed. A currently discussed microscopic model relates the observations to the interplay between fully quantized and quasiclassical current paths. We present measurements that contribute substantially to the clarification of the fascinating topic. In particular, we observe the opposite sign of the output voltage as compared to the prediction. Demonstrating the basic principle upon which the rectification is based--the asymmetry of the voltage drop in a quasiclassical wire--and extending the model to the classical transport regime, we can well explain our experiments as being caused by the interplay of quasiclassical ballistic and classical transport. Tunable ballistic rectifiers generating very large output signals and operating at room temperature raise the hope for future applications.

Wetting droplet instability and quantum ring formation

InAs islands on GaAs substrates undergo a morphological change into ring-shaped configurations upon deposition of a GaAs layer after island growth. By invoking an analogy of the InAs islands to wetting droplets on solid substrates, we suggest that this transition might be brought about by a change of the surface free-energy balance at the three-phase contact-line between GaAs, InAs, and vacuum (or As atmosphere). Our scenario can also be tested in conventional liquid systems (e.g., polymers).

Nanometer-scale resolution of strain and interdiffusion in self-assembled InAs/GaAs quantum dots

Tomographic nanometer-scale images of self-assembled InAs/GaAs quantum dots have been obtained from surface-sensitive x-ray diffraction. Based on the three-dimensional intensity mapping of selected regions in reciprocal space, the method yields the shape of the dots along with the lattice parameter distribution and the vertical interdiffusion profile on a subnanometer scale. The material composition is found to vary continuously from GaAs at the base of the dot to InAs at the top.

Spectroscopy of nanoscopic semiconductor rings

Making use of self-assembly techniques, we realize nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit. We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic field perpendicular to the plane of the rings, we find that, when approximately one flux quantum threads the interior of each ring, a change in the ground state from angular momentum l = 0 to l = -1 takes place. This ground state transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic-field dispersion of the single-electron charging energy.