arXiv subjects

Peter Bøggild

Publications and source records attributed to Peter Bøggild.

At least 19 recordsLinked to original sources

arXiv2026

Terahertz s-SNOM reveals nonlocal nanoscale conductivity of graphene

As photonic and electronic technologies approach nanometre length scales and terahertz operating speeds, electrical conductivity can no longer be treated as a purely local material parameter. In this regime, charge transport becomes intrinsically nonlocal, with conductivity depending on both frequency and momentum, $σ(ω,q)$, fundamentally limiting field confinement, dispersion, and loss in nanoscale devices. Here, we directly measure the nonlocal nanoscale conductivity of graphene using terahertz scattering-type near-field optical microscopy. By combining broadband THz near-field spectroscopy with quantitative electrodynamic modelling, we extract the complex conductivity of single- and few-layer graphene with $\sim$50 nm spatial resolution. We find that nonlocal response dominates the terahertz conductivity of monolayer graphene even at length scales comparable to practical device dimensions. These results establish nonlocal conductivity as a measurable and design-relevant material property in the terahertz regime, providing a quantitative foundation for predicting performance limits in ultracompact photonic and electronic systems.

physics.optics↗

arXiv2025

Recommendations and tools to enable reproducibility in 2D materials research

Research on 2D materials has achieved significant milestones and fuelled a rapidly growing industrial sector. This progress, however, is accompanied by challenges in reproducibility, arising from the atomic thinness, fragility, and environmental sensitivity of these materials. Subtle variations in methods or materials can lead to drastically different outcomes, undermining reliability and slowing down both scientific and technological advances. At the same time, academic publishing and funding systems continue to place greater value on novelty than on efforts to improve reproducibility. This Expert Recommendation outlines concrete, actions researchers can take to improve reproducibility in 2D materials science. We introduce two tools - STEP (Standardised Template for Experimental Procedures) and ReChart (Reproducibility Charter) - designed to support rigorous documentation and transparent sharing of protocols, failure modes, and raw data. To illustrate the application of STEP, we provide three detailed examples covering key processes in 2D materials research: graphene growth by chemical vapour deposition (CVD) on copper foil, wet transfer of CVD graphene, and dry assembly of van der Waals heterostructures. We offer practical recommendations that spans the full research process and show how researchers can engage constructively with stakeholders across academia, funding, publishing, and industry to create a stronger basis for reproducibility, transparency and trust in the field.

physics.soc-ph↗

arXiv2025

Apparent Resonance Splitting in Self-Coupled Excitonic Systems

Thin films of high-refractive-index excitonic materials enable self-coupling by simultaneously supporting intrinsic excitonic transitions and optical resonances. These optical resonances take the form of Fabry-Perot resonances in thick films and absorption resonances in ultrathin films placed on metallic substrates. Here, we investigate whether these optical resonances lead to true exciton-photon hybridization. Using far-field reflectance and spectrally resolved photocurrent measurements, we study tungsten disulfide (WS$_2$) flakes on both metallic and dielectric substrates across a range of thicknesses. While reflectance spectra for ultrathin flakes exhibit resonance splitting between excitons and absorption resonances, our photocurrent measurements reveal only excitonic peaks, indicating that no polaritons are formed. In contrast, thicker flakes exhibit Fabry-Perot resonances that strongly couple to excitons, resulting in clear splitting in both reflectance and photocurrent spectra, and providing evidence of polariton formation. We further show that the polariton resonances can be tuned through the reflection phase at the WS$_2$-substrate interface by changing the substrate material. In addition to coupling with the strong A-exciton, we observe polariton formation involving the weaker B-exciton at shorter wavelengths, as well as higher-order hybridization where both excitons interact simultaneously with a single Fabry-Perot resonance. These findings clarify the distinction between apparent and true strong coupling in excitonic materials and demonstrate how reflection phase and flake thickness can be used to engineer light-matter interactions.

physics.optics↗

arXiv2025

Gradient Electronic Landscapes in van der Waals Heterostructures

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN) provide a versatile platform for quantum electronics. Experiments generally require encapsulating graphene within hBN flakes, forming a protective van der Waals (vdW) heterostructure that preserves delicate properties of the embedded crystal. To produce functional devices, heterostructures are typically shaped by electron beam lithography and etching, which has driven progress in 2D materials research. However, patterns are primarily restricted to in-plane geometries such as boxes, holes, and stripes, limiting opportunities for advanced architectures. Here, we use thermal scanning-probe lithography (tSPL) to produce smooth topographic landscapes in vdW heterostructures, controlling the thickness degree of freedom with nanometer precision. We electrically gate a sinusoidal topography to impose an electric-field gradient on the graphene layer to spatially modulate charge-carrier doping. We observe signatures of the landscape in transport measurements-resistance-peak spreading and commensurability oscillations-establishing tSPL for tailoring high-quality quantum electronics.

cond-mat.mes-hall↗

arXiv2025

The 2D Materials Roadmap

Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and development, spanning synthesis, properties and commercial applications. We specifically present roadmaps for high impact 2D materials, including graphene and its derivatives, transition metal dichalcogenides, MXenes as well as their heterostructures and moiré systems. The discussions are organized into thematic sections covering emerging research areas (e.g., twisted electronics, moiré nano-optoelectronics, polaritronics, quantum photonics, and neuromorphic computing), breakthrough applications in key technologies (e.g., 2D transistors, energy storage, electrocatalysis, filtration and separation, thermal management, flexible electronics, sensing, electromagnetic interference shielding, and composites) and other important topics (computational discovery of novel materials, commercialization and standardization). This roadmap focuses on the current research landscape, future challenges and scientific and technological advances required to address, with the intent to provide useful references for promoting the development of 2D materials.

cond-mat.mtrl-sci↗

arXiv2024

Interface engineering of van der Waals heterostructures towards energy-efficient quantum devices operating at high temperatures

Quantum devices, which rely on quantum mechanical effects for their operation, may offer advantages, such as reduced dimensions, increased speed, and energy efficiency, compared to conventional devices. However, quantum phenomena are typically observed only at cryogenic temperatures, which limits their practical applications. Two-dimensional materials and their van der Waals (vdW) heterostructures provide a promising platform for high-temperature quantum devices owing to their strong Coulomb interactions and/or spin-orbit coupling. In this review, we summarise recent research on emergent quantum phenomena in vdW heterostructures based on interlayer tunnelling and the coupling of charged particles and spins, including negative differential resistance, Josephson tunnelling, exciton condensation, and topological superconductivity. These are the underlying mechanisms of energy-efficient devices, including tunnel field-effect transistors, topological/superconducting transistors, and quantum computers. The natural homojunction within vdW layered materials offers clean interfaces and perfectly aligned structures for enhanced interlayer coupling. Twisted bilayers with small angles may also give rise to novel quantum effects. In addition, we highlight several proposed structures for achieving high-temperature Majorana zero modes, which are critical elements of topological quantum computing. This review is helpful for researchers working on interface engineering of vdW heterostructures towards energy-efficient quantum devices operating above the liquid nitrogen temperature.

cond-mat.mes-hall↗

arXiv2024

Quantitative mapping of smooth topographic landscapes produced by thermal scanning-probe lithography

Scanning probe microscopy (SPM) is a powerful technique for mapping nanoscale surface properties through tip-sample interactions. Thermal scanning-probe lithography (tSPL) is an advanced SPM variant that uses a silicon tip on a heated cantilever to sculpt and measure polymer films with nanometer precision. The surfaces produced by tSPL-smooth topographic landscapes-allow mathematically defined contours to be fabricated on the nanoscale, enabling sophisticated functionalities for photonic, electronic, chemical, and biological technologies. Evaluating the physical effects of a landscape requires fitting arbitrary mathematical functions to SPM datasets, however, this capability does not exist in standard analysis programs. Here, we provide an open-source software package (FunFit) to fit analytical functions to SPM data and develop a fabrication and characterization protocol based on this analysis. We demonstrate the benefit of this approach by patterning periodic and quasiperiodic landscapes in a polymer resist with tSPL, which we transfer to hexagonal boron nitride (hBN) flakes with high fidelity via reactive-ion etching. The topographic landscapes in polymers and hBN are measured with tSPL and atomic force microscopy (AFM), respectively. Within the FunFit program, the datasets are corrected for artefacts, fit with analytical functions, and compared, providing critical feedback on the fabrication procedure. Beyond application to tSPL, this protocol can improve analysis, reproducibility, and process development for a broad range of SPM experiments. The protocol can be performed within a working day by an inexperienced user, where fabrication and characterization take a few hours and software analysis takes a few minutes.

cond-mat.mes-hall↗

arXiv2024

Improving Electrical Contact Quality and Extraordinary Magnetoresistance in High Mobility III-V Semiconductors

Magnetometers based on the extraordinary magnetoresistance (EMR) effect are promising for applications which demand high sensitivity combined with room temperature operation but their application for magnetic field sensing requires further optimization. A key challenge is to obtain Ohmic metal/semiconductor contacts with low contact resistances in EMR devices comprising semiconductors with low carrier densities and high electron mobilities, yet, this topic remains scarcely investigated experimentally. By annealing high-mobility InSb in argon with systematically increasing temperatures, we experimentally demonstrate how the contact resistance to InSb films can be improved by two orders of magnitude by annealing to the micro-Ohm cm2 range without degrading the high mobility. We further show that lowering the contact resistance monotonously increases the room temperature magnetoresistance at 2 T from 700% to 65,000%. Lastly, we explore the origin of intrinsic magnetoresistance in high-mobility InSb thin films and suggest that it can best be explained by multiple band conduction.

physics.app-ph↗

arXiv2024

Encapsulated void resonators in lossy dielectric van der Waals heterostructures

Dielectric optical resonators traditionally rely on materials with the combination of high refractive indices and low optical losses. Such materials are scarce for operation in visible spectrum and shorter wavelengths. This limitation can be circumvented by relaxing the requirement of low losses. We demonstrate that highly lossy dielectric materials can be structured to support optical resonances that confine light in air voids. We theoretically design void resonances in the visible spectrum and identify resonant modes supported by void arrays. Experimentally, we fabricate void arrays in tungsten diselenide and characterize the confined resonances using far-field reflectance measurements and scanning near-field optical microscopy. Using van der Waals heterostructure assembly, we encapsulate the voids with hexagonal boron nitride which reduces the void volume causing a large spectral blue shift of the void resonance exceeding 150 nm. Our work demonstrates a versatile optical platform for lossy materials, expanding the range of suitable materials and the spectral range of photonic devices.

physics.optics↗

arXiv2024

First Principles Study of Electronic Structure and Transport in Graphene Grain Boundaries

Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions (NEGF). We generated 150 different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB. We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.

cond-mat.mes-hall↗

arXiv2023

Visualizing thickness-dependent magnetic textures in few-layer $\text{Cr}_2\text{Ge}_2\text{Te}_6$

Magnetic ordering in two-dimensional (2D) materials has recently emerged as a promising platform for data storage, computing, and sensing. To advance these developments, it is vital to gain a detailed understanding of how the magnetic order evolves on the nanometer-scale as a function of the number of atomic layers and applied magnetic field. Here, we image few-layer $\text{Cr}_2\text{Ge}_2\text{Te}_6$ using a combined scanning superconducting quantum interference device and atomic force microscopy probe. Maps of the material's stray magnetic field as a function of applied magnetic field reveal its magnetization per layer as well as the thickness-dependent magnetic texture. Using a micromagnetic model, we correlate measured stray-field patterns with the underlying magnetization configurations, including labyrinth domains and skyrmionic bubbles. Comparison between real-space images and simulations demonstrates that the layer dependence of the material's magnetic texture is a result of the thickness-dependent balance between crystalline and shape anisotropy. These findings represent an important step towards 2D spintronic devices with engineered spin configurations and controlled dependence on external magnetic fields.

cond-mat.mes-hall↗

arXiv2023

A High-Performance Quasi-1D MoS$_2$ Nanoribbon Photodetector

Molybdenum disulfide (MoS$_2$) nanoribbons have attracted increased interest due to their properties which can be tailored by tuning their dimensions. Herein, we demonstrate the growth of highly crystalline quasi-one-dimensional (1D)MoS$_2$ nanoribbons and aligned 3D triangular crystals with predominantly 3R or 2H stacking orientation. The synthesis method relies on the reaction between an ultra-thin MoO3-x film grown by Pulsed Laser Deposition (PLD) and NaF in a sulfur-rich environment. The quasi-1D MoS$_2$ nanoribbons can reach several micrometres in length, and feature single-layer (1L) edges aligned with the nanoribbon core, thereby forming a 1L-multilayer (ML) homojunction due to abrupt discontinuity in thickness. The 1L edges of the nanostructures show a pronounced second harmonic generation (SHG) due to the symmetry breaking, in contrast to the centrosymmetric ML structure, which is unsusceptible to the second-order nonlinear process. A pronounced splitting of the Raman spectra was observed in quasi-1D MoS$_2$ nanoribbons arising from distinct contributions from the 2D edges and a multilayer core. Nanoscale imaging reveals the blue-shifted exciton emission of the monolayer nanoribbon compared to the triangular MoS$_2$ counterpart due to built-in local strain and disorder. We further report on a versatile and ultrasensitive photodetector made of a single quasi-1D MoS$_2$ nanoribbon with a maximum responsivity of 872 A/W at the wavelength of 532 nm. The optoelectronic performance of the MoS$_2$ nanoribbon is superior to the previously reported single-nanoribbon photodetectors. Our findings can inspire the design of TMD semiconductors with tunable geometries for efficient optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Nanoscale view of engineered massive Dirac quasiparticles in lithographic superstructures

Massive Dirac fermions are low-energy electronic excitations characterized by a hyperbolic band dispersion. They play a central role in several emerging physical phenomena such as topological phase transitions, anomalous Hall effects and superconductivity. This work demonstrates that massive Dirac fermions can be controllably induced by lithographically patterning superstructures of nanoscale holes in a graphene device. Their band dispersion is systematically visualized using angle-resolved photoemission spectroscopy with nanoscale spatial resolution. A linear scaling of effective mass with feature sizes is discovered, underlining the Dirac nature of the superstructures. In situ electrostatic doping dramatically enhances the effective hole mass and leads to the direct observation of an electronic band gap that results in a peak-to-peak band separation of (0.64 $\pm$ 0.03) eV, which is shown via first-principles calculations to be strongly renormalized by carrier-induced screening. The presented methodology outlines a new approach for band structure engineering guided by directly viewing structurally- and electrically-tunable massive Dirac quasiparticles in lithographic superstructures at the nanoscale.

cond-mat.mes-hall↗

arXiv2022

Programming moiré patterns in 2D materials by bending

Moiré superlattices in twisted two-dimensional materials have generated tremendous excitement as a platform for achieving quantum properties on demand. However, the moiré pattern is highly sensitive to the interlayer atomic registry, and current assembly techniques suffer from imprecise control of the average twist angle, spatial inhomogeneity in the local twist angle, and distortions due to random strain. Here, we demonstrate a new way to manipulate the moiré patterns in hetero- and homo-bilayers through in-plane bending of monolayer ribbons, using the tip of an atomic force microscope. This technique achieves continuous variation of twist angles with improved twist-angle homogeneity and reduced random strain, resulting in moiré patterns with highly tunable wavelength and ultra-low disorder. Our results pave the way for detailed studies of ultra-low disorder moiré systems and the realization of precise strain-engineered devices.

cond-mat.mes-hall↗

arXiv2022

Fermi Level Depinning in Two-Dimensional Materials Using a Fluorinated Bilayer Graphene Barrier

Strong Fermi level pinning (FLP) - often attributed to metal-induced gap states at the interfacial contacts - severely reduces the tunability of the Schottky barrier height of the junction and limits applications of the 2D materials in electronics and optoelectronics. Here, we show that fluorinated bilayer graphene (FBLG) can be used as a barrier to effectively prevent FLP at metal/2D materials interfaces. FLBG can be produced via short exposure (1-3 min) to SF6 plasma that fluorinates only the top layer of a bilayer graphene with covalent C-F bonding, while the bottom layer remains intrinsic, resulting in a band gap opening of about 75 meV. Inserting FBLG between the metallic contacts and a layer of MoS2 reduces the Schottky barrier height dramatically for the low-work function metals (313 and 260 meV for Ti and Cr, respectively) while it increases for the high-work function one ( 160 meV for Pd), corresponding to an improved pinning factor. Our results provide a straightforward method to generate atomically thin dielectrics with applications not only for depinning the Fermi level at metal/transition metal dichalcogenide (TMD) interfaces but also for solving many other problems in electronics and optoelectronics

physics.app-ph↗

arXiv2021

Universal Non-Volatile Resistive Switching Behavior in 2D Metal Dichalcogenides Featuring Unique Conductive-Point Random Access Memory Effect

Two-dimensional materials have been discovered to exhibit non-volatile resistive switching (NVRS) phenomenon. In our work, we reported the universal NVRS behavior in a dozen metal dichalcogenides, featuring low switching voltage, large on/off ratio, fast switching speed and forming free characteristics. A unique conductive-point random access memory (CPRAM) effect is used to explain the switching mechanisms, supported by experimental results from current-sweep measurements.

cond-mat.mes-hall↗

arXiv2021

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are however compromised by nm-scale edge roughness and resolution variability issues, which especially affects the performance of 2D materials. Here we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF6) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2), molybdenum ditelluride (MoTe2). Scanning and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS2 > WSe2 > MoTe2 / MoS2. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA stacking order into account. Patterns in WS2 are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy, but shows consistent vertical etch angles, independent of crystal orientation. This is used to create super-resolution lithographic patterns with ultra-sharp corners at the base of the hBN crystal, which are transferred into an underlying graphite crystal. We find that the anisotropic SF6 reactive ion etching process makes it possible to downsize nanostructures to obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch-masks to pattern other nanomaterials.

cond-mat.mtrl-sci↗

arXiv2020

Fermi velocity renormalization in graphene probed by terahertz time-domain spectroscopy

We demonstrate terahertz time-domain spectroscopy (THz-TDS) to be an accurate, rapid and scalable method to probe the interaction-induced Fermi velocity renormalization νF^* of charge carriers in graphene. This allows the quantitative extraction of all electrical parameters (DC conductivity σDC, carrier density n, and carrier mobility μ) of large-scale graphene films placed on arbitrary substrates via THz-TDS. Particularly relevant are substrates with low relative permittivity (< 5) such as polymeric films, where notable renormalization effects are observed even at relatively large carrier densities (> 10^12 cm-2, Fermi level > 0.1 eV). From an application point of view, the ability to rapidly and non-destructively quantify and map the electrical (σDC, n, μ) and electronic (νF^* ) properties of large-scale graphene on generic substrates is key to utilize this material in applications such as metrology, flexible electronics as well as to monitor graphene transfers using polymers as handling layers.

cond-mat.mes-hall↗