Kristian Sommer Thygesen

Polycyclic aromatic hydrocarbons (PAHs) on hexagonal boron nitride (hBN) are an emerging molecular platform for single-photon emitters, but predictive design strategies across chemical space have been lacking. Here we establish two general principles governing PAH/hBN emitters. The zero-phonon-line energy is molecule-controlled and robust to the substrate environment, whereas the Huang–Rhys (HR) factor is environment-governed and exhibits two distinct regimes. Pristine hBN modestly enhances HR through soft interfacial modes. In contrast, the NB antisite defect, introduced to immobilize the molecules, produces a molecule-specific HR response that cannot be predicted from gas-phase or pristine-hBN data alone. Strikingly, the defect can either partially restore the HR factor toward its intrinsic gas-phase value or amplify it by up to 3.7×, depending on the molecule. This positions defect engineering as a tunable design knob for vibronic coupling in molecule/hBN emitters. Combining high-throughput hybrid density functional theory (DFT) screening of 662 PAHs identifies 47 gas-phase candidates with low HR factors and high radiative rates spanning the near-infrared to ultraviolet. Among five representative candidates explicitly tested on NB@hBN, two retain HR factors below unity in gas-phase, pristine hBN, and defect-bound environments. However, no PAH simultaneously exhibits favorable single-photon-emitter properties and an excited-state singlet–triplet splitting below 0.5 eV, suggesting that PAH-based spin–photon interfaces will require further molecular design. All data are released through the open MolExDB platform.

The bulk and surface electronic structure of MoAlB(010) is studied by a combination of angle-resolved photoemission spectroscopy and density functional calculations. The observed bulk Fermi-level crossings agree with the previously reported bulk Fermi surface of the material. Additionally, we find several surface states in the wide projected bulk band gaps around the Fermi energy. The surface states differ in their stability under residual-gas exposure in the vacuum system and in the magnitude of their Rashba-type spin-orbit splitting. We explain this in terms of their elemental and orbital character. A surface state arising from Al dangling bonds is sensitive to surface contamination, whereas a mainly Mo-derived surface state exhibits the stronger spin-orbit splitting.

The past decade has seen rapid growth in the number of experimentally realized two-dimensional (2D) materials with diverse chemical and physical properties. However, information on their crystal structure, synthesis routes, and measured or predicted properties remains scattered across thousands of publications. Here, we consolidate this fragmented knowledge by establishing X2DB─an open infrastructure that integrates experimental and computational data on 2D materials. Using extensive literature mining and direct community uploads, we identify 370 unique 2D materials that have been realized in monolayer or few-layer form and link them to their digital counterparts in computational databases, enabling consistent ab initio characterization of their properties across monolayer, bilayer, and bulk forms. We describe the structure and content of the database, highlight its support for community uploads, illustrate how it can be used to generate scientific insight, and introduce a hierarchical classification of the known set of 2D materials. Our work supports the integration and cross-fertilization of experimental and theoretical knowledge and contributes to data-driven and predictive synthesis of 2D materials.

Compared to bulk solids, defects in low-dimensional materials and, specifically, 2D systems are expected to have a stronger effect, detrimental or beneficial, on their properties. Owing to their geometry, defects in 2D materials can easily be formed due to the interaction with the environment or under impacts of energetic particles, such as ions and electrons. At the same time, many concepts of defect production under irradiation in bulk systems are not applicable for 2D materials or require substantial modifications. Various aspects of the physics and chemistry of defects in 2D materials have been addressed, and the results of these investigations are presented in hundreds of research papers and review articles. However, the challenges and open questions that still remain in the field have received relatively little attention. These topics were recently addressed at the symposium “Defect-mediated engineering of nanomaterials for energy and quantum applications” organized by the Beilstein-Institut. Following the discussions at the symposium, here, we present the challenges and open questions in our understanding of the behavior of defective 2D materials, interaction of energetic particles with low-dimensional targets, and defect-mediated engineering of the properties of 2D systems. We further discuss possible solutions to these problems or suggest “work-arounds”, which should accelerate the progress in the field.

Photonic devices depend critically on the dielectric materials from which they are made, with higher refractive indices and lower absorption losses enabling new functionalities and higher performance. However, these two material properties are intrinsically linked through the empirical Moss rule, which states that the refractive index of a dielectric decreases as its band gap energy increases. Materials that surpass this rule, termed super-Mossian dielectrics, combine large refractive indices with wide optical transparency and are therefore ideal candidates for advanced photonic applications. This Review surveys the expanding landscape of high-index dielectric and semiconductor materials, with a particular focus on those that surpass the Moss rule. We discuss how electronic band structures with a large joint density of states near the band edge give rise to super-Mossian behavior and how first-principles computational screening can accelerate their discovery. Finally, we establish how the refractive index sets the performance limits of nanoresonators, waveguides, and metasurfaces, highlighting super-Mossian dielectrics as a promising route toward the next performance leap in photonic technologies.

We introduce a method to calculate defect-assisted Shockley-Read-Hall (SRH) recombination rates in imperfect semiconductors from first principles. The method accounts for the steady-state recombination dynamics under given nonequilibrium conditions (split quasi-Fermi levels) by invoking a full solution to the rate equations describing transitions across the band gap via all possible charge states of the defect. Transition rates due to radiative and nonradiative multiphonon emission processes are calculated from first principles. The method is used to evaluate the effect of selected defects on the photovoltaic device parameters of seven emergent photovoltaic semiconductors. These examples clearly highlight the limitations of commonly employed approximations of the recombination dynamics. Our work advances the description and understanding of defect-induced losses in photovoltaics and provides a basis for developing the important concept of defect-tolerant semiconductors and discovering high-performance photovoltaic materials computationally.

The advent of 2D materials has revolutionized condensed matter physics and materials science, offering unprecedented opportunities to explore exotic physical phenomena, engineer novel functionalities, and address critical technological challenges across diverse fields. Over the past two decades, the exploration of 2D materials has expanded beyond graphene, encompassing a vast library of atomically thin crystals and their heterostructures. These materials exhibit extraordinary electronic, optical, thermal, mechanical, and chemical properties, and hold promise for breakthroughs in electronics, optoelectronics, quantum technologies, energy storage, catalysis, thermal management, filtration and separation, and beyond. Many exciting new physics and phenomena continue to emerge, while select 2D materials, such as graphene, h-BN, and the semiconducting transition metal dichalcogenides (TMDCs), are transitioning from laboratory-scale demonstrations to industrial applications. In this context, a holistic understanding of synthesis, structure-property relationships, integration, and performance optimization is essential. This roadmap reviews the multifaceted challenges and opportunities in 2D materials research, focusing on the synthesis, properties and applications of representative systems including graphene and its derivatives, TMDCs, MXenes as well as their heterostructures and moiré systems.

We introduce a computational framework leveraging universal machine learning interatomic potentials (MLIPs) to dramatically accelerate the calculation of photoluminescence (PL) spectra of atomic or molecular emitters with ab initio accuracy. By replacing the costly density functional theory (DFT) computation of phonon modes with much faster MLIP phonon mode calculations, our approach achieves speed improvements exceeding an order of magnitude with minimal precision loss. We benchmark the approach using a dataset comprising ab initio emission spectra of 791 color centers spanning various types of crystal point defects in different charge and magnetic states. The method is also applied to a molecular emitter adsorbed on a hexagonal boron nitride surface. Across all the systems, we find excellent agreement for both the Huang-Rhys factor and the PL lineshapes. This application of universal MLIPs bridges the gap between computational efficiency and spectroscopic fidelity, opening pathways to high-throughput screening of defect-engineered materials. Our work not only demonstrates accelerated calculation of PL spectra with DFT accuracy, but also makes such calculations tractable for more complex materials.