Research

The research of the nanoelectronics group focuses on emergent and interaction driven phenomena of two-dimensional quantum materials, two-dimensional charge carrier systems, related hetero-, and hybrid structures as well as interfaces. The aim is to gain a fundamental understanding of the physical properties of those systems, and to learn – in a next step – to control and tailor them on purpose with the vision to achieve novel functionalities and at the same time to look for potential application in an interdisciplinary context. These low-dimensional systems are therefore integrated in nano- and microscale circuitries, as well as in proof-of-concept and prototypical devices to explore their potential for opto-/electronic or energy harvesting applications and to serve as test-bed structures towards quantum technologies.
We pursue our scientific goals with modern optical and electronic experimental techniques at extreme cryogenic environments reaching temperatures lower than 10 Millikelvin. Our research interests are clustered around the following directions that are detailed below:

 a. Excitons in 2D materials: From correlated ensembles to deterministic quantum light sources

 b. Mott-Hubbard physics

 c. 2D polar metals: Novel atomically thin quantum and meta materials

 d. Emergent phenomena in low-dimensional quantum systems

 e. Van der Waals hetero- and hybrid stacks for energy conversion via (photo-)catalysis

 

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a. Excitons in 2D materials: From correlated ensembles to deterministic quantum light sources

 

Exciton ensembles

 

An exciton is a pair of electron and hole bound together by Coulomb interaction. Excitons are observed in the optical spectra of semiconducting 2D materials such as MoS2 and WSe2 which provide a natural analogue to quantum wells. Due to the two-dimensional nature and reduced screening of the Coulomb forces, the excitons in 2D materials are very strongly bound. Besides the intralayer excitons which reside in one layer, interlayer excitons exist in the heterostructures of 2D materials. Here the electron and hole reside in different layers due to type-II band alignment. This makes the excitons longer lived and imparts a permanent electric dipole moment. Such excitons are suitable to study interaction-driven many body phases such as Bose-Einstein condensate. Thus, excitons in 2D materials are suitable for both fundamental studies such as exciton condensation, as well as for applications in exciton-based devices such as excitonic lasers, transistors and modulators. The aim of our group is to investigate the fundamental properties and interactions of excitons in 2D materials and their heterostructures as well as to embed them in nanoscale devices and establish control of these properties.

 

 

Quantum light sources

Excitons in 2D materials can also be localized at at the moiré lattice sites that is formed in the 2D material heterostructures or deterministically positioned defect sites or. Scalable and deterministic SPEs are desirable for applications in modern information and quantum technologies. We established 2D materials as a scalable platform for single-photon emitters with unprecedented control of position as well as photophysical properties owing to the all-interfacial nature. On-demand creation and precise positioning of mono-typical single-photon sources in atomically thin MoS2 with very high fabrication yield is achieved by focused ion beam irradiation with helium ions. Atomistic defects with anti-bunched emission lines with sub-10 nm lateral and 0.7 nm axial positioning accuracy can be produced in already processed circuitries. We aim to invest the control of these quantum emitters by external stimuli such as electric fields and to implement them in (2D) plasmonic and opto-electronic circuitries towards real-world quantum applications, e.g. on-chip quantum networks.

 

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b. Mott-Hubbard physics

 

The Hubbard model is used in theoretical physics to calculate the behavior of electrons in a solid-state material. The model is based on a highly periodic lattice with one electronic state at each lattice site. The model takes a nearest neighbor hoping and an onsite repulsion by other electrons into account. Therefore, this model is suitable to describe strongly correlated systems that show correlated phases such as magnetism, superconductivity, Mott insulator and others. The Hubbard model, however, is theoretically only solvable for some one-dimensional systems. Experimentally, the Mott Hubbard models can be simulated in real physical system like ultracold atoms trapped in an optical lattice. Better experimentally accessible systems are solid states materials. However, the required nearly perfect highly modulation of the electronic structure is difficult to achieve. We overcome this issue by preparing twisted bilayers of two-dimensional crystals. Thanks to the van der Waals bonding between the atomically thin layer, two single layers of 2D materials can be stacked on top of each other with precisely controlled twist angel. Slightly different lattice constants and the twist angle result in a geometric superlattice structure, a Moiré lattice also known in digital photography. The moiré lattice provides the required highly periodic “lattice-like” modulation of the electronic band structure to simulate Mott-Hubbard physics in experiment. By controlling the doping and hence the filling factors of the moiré bands in such systems using an electronic gate, it is possible to demonstrate different correlated phases. We aim to study correlation phenomena in twisted and gated homo- and hetero-bilayers acting as Hubbard simulators by a combination of resonant inelastic light scattering, optical interband excitations as well as transport investigations at cryogenic temperatures.

 

 

 

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c. 2D polar metals: Novel atomically thin quantum and meta materials

 

Key for optical technologies is the manipulation of the light-matter interaction to achieve a high level of control, particularly in technologically relevant solid-state nanomaterials. We study new and promising classes of atomically thin 2D polar metals such as 2D Ga, In, Ag, Sn and others, as well as their alloys. We work here in close collaboration with Prof. Robinson (Penn State) who has pioneered the growth if novel half-van der Waals materials. These quantum confined 2D metals exhibit fascinating properties like superconductivity, strong plasmonic responses, enormous nonlinear optical properties emerging by giant second harmonic generation and tunable epsilon near zero (ENZ) behavior qualifying them as metamaterials. Due to the half-van der Waals nature, we combine these 2D metals with other van der Waals materials with excitonic, spin or topological interesting properties in order to study coupling and proximity effects and to achieve novel functionalities. Their tunability and possibility for integration in van der Waals heterostacks makes 2D polar metals attractive systems for the realization of atomically thin metamaterials, quantum-confined metal films, tunable (quantum-)plasmonics and nano-photonics. We aim to explore the optical and transport behavior of those systems by a combination of spectroscopic imaging ellipsometry, transport and Raman measurements in order to explore their electronic and optical properties.

 

 

 

 

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d. Emergent phenomena in low-dimensional quantum systems

 

The physical properties of solid-state (quantum) systems are determined the particles and quasiparticles such as electrons, phonons (lattice vibrations), plasmons hosted by the solid as well as their manifold interactions. The light-matter interaction is for instance of great relevance for opto-/electronic materials, whereas electron-electron and electron lattice interactions are key in charge and heat transport processes. If the interactions strength is significantly enhanced and the quasiparticle behaves in a collective manner, those systems can show emergent behavior, e.g. superconductivity, magnetism, fractional quantum hall states, topological non-trivial states of matter and other correlated phases. Emergent phenomena are caused by interactions of many particles by intertwined degrees of freedom and are hence macroscopic phenomena that cannot be described by the properties of the individual constituent. The most important degree of freedoms in low-dimensional quantum systems that we are interested in are charge, spin, orbital, lattice and topology. We study spin-and orbital related phenomena as well as superconductivity in two-dimensional quantum materials and are interested in exotic, presumably non-abelian phases of matter of certain fractional quantum Hall effect states (FQHE) hosted in ultra-high mobility two-dimensional charge carrier systems in GaAs/AlGaAs heterostructures. Our aim is to generate a fundamental understanding of those fascinating emergent phases of matter. We research those phases by a combination of interband emission and absorption, spectroscopic ellipsometry, resonant inelastic light scattering and (magneto-)transport investigations at low temperatures.

 

 

 

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e. Van der Waals hetero- and hybrid stacks for energy conversion via (photo-)catalysis

To power our today’s life style, alternative “green” energy conversion and storage strategies are important, but the efficiency is still low. Efficient energy conversion processes including photovoltaics, (photo-)catalysis or thermoelectrics requires a fundamental understanding of the underlying mechanisms at the solid-solid and solid-liquid interface. We mainly focus on the development of a microscopic understanding of light-matter interaction, charge transfer processes, interfacial phonon coupling and heat transfer in realistic device schemes or under reaction conditions. As an example, we developed an electrochemical microcell with all optical access to study in operando optical interband transitions and phonon processes with sub-micrometer resolution identifying the dominant catalytically active sites in MoS2 mono- and multilayers to be edges and point defects on the basal plane. With this knowledge, we developed a strategy to increase the catalytic activity by additional point defects. Moreover, we use solid and liquid electrolytes as gate electrodes and are interested in the cross-plane charge and heat transport towards thermoelectric applications.

 

Methodology

a. Micromechanical Exfoliation and Deterministic Transfer of van der Waals Heterostructures

b. Fabrication of opto-/electronic circuitries

c. Millikelvin magneto optics and transport

d. Resonant Inelastic light scattering

e. Sub-kelvin Imaging Spectroscopic Ellipsometry and transport

f. Optical Characterization Labs

 

a. Micromechanical Exfoliation and Deterministic Transfer of van der Waals Heterostructures

Deterministic transfer of 2D crystals plays a key role in the fabrication of 2D heterostructures. It enables engineering of materials with new exciting electro-optical properties and paves the way for their integration into complex devices, the discovery of new phenomena and the exploration of novel applications.
The first step in the fabrication of heterostructures is the preparation of atomically thin crystals. The method used in our research group is micromechanical exfoliation and all-dry viscoelastic stamping [cite]. It provides the high-quality and rotational controlled van der Waals stacks. In more detail, bulk crystals of layered semiconductors are thinned down by an adhesive tape and afterwards placed on a piece of polymer.

 

 

 

With our home-built stacking device the desired atomically thin crystals can be deterministically transferred to a substrate.

This procedure can be performed repeatedly to stack many crystals on top of each other. Vacuum annealing between stacking steps reduces interlayer residuals such as air bubbles or trapped water, resulting in larger and smoother interfaces and thus better devices.

 

 

b. Fabrication of opto-/electronic circuitries

To access and manipulate the electronic properties of a material electric contacts are required. Therefore, we fabricate gold contacts by lithography onto our substrates. Micrometer sized atomically thin flakes are stamped directly to the gold contacts or are connected by additional graphene flakes. In addition, we use field effect structures with hBN and graphene gate or solid electrolyte gate electrodes to apply electric field or to tune the charge carrier density in 2D materials. Wires soldered to the gold contact pads connect the flakes to the electrical equipment such as source measurements units or lock-in amplifiers. All optical cryostats are equipped with wires and electrical feed throughs to enable simultaneous optical, electronic or optoelectronic measurements down to millikelvin temperatures.

 

c. Millikelvin magneto optics and transport

With our cryogen-free dilution refrigerator (Bluefors LD400) we are able to perform optical and transport measurements down to 10mK at magnetic fields up to 7T. The system is equipped with x-y-z piezo actuators and a low-temperature objective allowing lateral resolution of about 1 µm in the optical measurements.

 

 

 

Applying a magnetic field to a material can have a huge impact on its optical and electronic response. Especially at millikelvin temperatures, where the thermal broadening of the electronic bands is minimized, it is possible to probe the ground state and collective excitations of the system. The magnetic field perturbs the electronic band structure of the probed material. For example, energy degeneracies in the electronic band structure are lifted due to an interaction of the magnetic field with the spin of the electrons located in certain bands. As another example, the quantum hall effect can be observed optically in photoluminescence and resonant inelastic light scattering measurements on a GaAs quantum well with a tunable applied magnetic field.. This can then be correlated to transport measurements under varying magnetic field.

 

 

d. Resonant Inelastic light scattering

Inelastically scattered light on fundamental excitations such as phonons, plasmons or other collective electronic excitations involves exchange of energy such that the scattered light has lost or gained the energy of the excitation. By tuning the energy of the incoming or scattered light to a fundamental interband transition of the electronic bands, collective electronic excitations such as charge and spin density waves can be accessed and the intensity of phonon modes resonantly enhanced. The low-energy excitation spectra act like a finger print of a crystal structure, is sensitive to strain, defects, temperatures or unambiguously identifies correlated electronic phases and related phase transitions. Such low-lying collective excitations exist in low-dimensional structures such as two-dimensional charge carrier systems or the moiré superlattices formed by twisted TMDC homo- and heterobilayers.

 

 

 

For these experiment, our laboratory is equipped with an actively stabilized tunable near infrared Ti-Sa CW laser (Sirah Matisse 2) and visible solid-state lasers to excite the samples in resonance with their electronic or excitonic transitions. The scattered or emitted light from the sample is collected by efficient optics and is analyzed using a triple-stage spectrometer coupled to a liquid nitrogen cooled CCD camera (S&I TriVista). The spectrometer allows us to measure the scattered light spectrum down to sub-meV in energies

 

 

 

 

e. Sub-kelvin Imaging Spectroscopic Ellipsometry and transport

Ellipsometry is a non-destructive, optical method that measures the change in the polarization of collimated monochromatic light being reflected from thin films at a finite angle of incidence. We perform Spectroscopic Imaging Ellipsometry (SIE) in the visible and near-infrared range with a lateral resolution of less than 2µm. Using an optical multilayer model and regression analysis, we can determine parameters such as optical constants, dielectric functions, and thicknesses of layered thin film systems.

 

 

 

Our samples can be cooled using an adiabatic demagnetization refrigerator (ADR, Kiutra S-Type) with optical access and equipped with x-y-z and rotational piezo actuators. With this setup we perform ellipsometry at temperatures from 800 mK up to 300 K.

 

 

f. Optical Characterization Labs

Our optical characterization labs combine various powerful light analysis tools. The Leica Microscope with epi-illumination is used during sample fabrication and for basic optical characterization during the fabrication process. In the Micro-Raman and -Photoluminescence setup optical properties can be studied using various laser sources (488 nm, 532 nm, 632 nm, 640 nm etc.) in a temperature range from room temperature down 77 K using a liquid nitrogen continuous flow cryostat equipped with piezo actuators. In addition to the low noise spectrometer cameras, an intensified CCD (4 Picos - Stanford Computer Optics) enables time-resolved spectroscopy with a temporal accuracy down to 200 ps. In a new setup the photocurrent induced by the various light sources can be measured using a light chopper and a lock-in amplifier, for example to characterize the quality of semiconductor metal junctions in the fabricated optoelectronic circuitries.