Frederik Schiller

The intrinsically weak nonlinear optical response of existing materials, further constrained by symmetry‐forbidden second‐order processes in centrosymmetric media, severely limits efficient frequency conversion in deeply subwavelength, ultrathin volumes. Addressing this challenge is crucial for the development of nonlinear nanophotonics. Here, we show that atomically thin, epitaxially grown crystalline silver films circumvent these restrictions through the interplay of vertical electronic quantum confinement and lateral plasmonic enhancement. We fabricate atomically thin films that exhibit an enhanced nonlinear response associated with electronic quantum wells, and subsequently pattern them into periodic nanoribbon and nanotriangle arrays sustaining infrared localized surface plasmon resonances. Strong near‐field confinement in these structures further boosts second‐harmonic generation compared to unpatterned films. Precise control over nanostructure geometry enables spectral tuning of the plasmonic resonance, and consequently, the enhanced harmonic frequency. Our findings establish an approach for activating robust second‐order nonlinearities in quantum‐confined metals, where intrinsic size effects and plasmonic resonances act synergistically. The compatibility of high‐quality epitaxial growth with microchip fabrication technology offers a scalable route toward ultracompact nonlinear optical components for on‐chip frequency conversion, sensing, and quantum photonic applications.

The inherently weak nonlinear optical response of bulk materials remains a fundamental limitation in advancing photonic technologies. Nanophotonics addresses this challenge by tailoring the size and morphology of nanostructures to manipulate the optical near field, thus modulating the nonlinear response. Here, we explore a complementary strategy based on engineering the electronic band structure in the mesoscopic regime to enhance optical nonlinearities. Specifically, we demonstrate an increase in second-harmonic generation (SHG) from crystalline silver films as their thickness is reduced down to just a few atomic monolayers. Operating at the boundary between bulk and two-dimensional systems, these ultra-thin films exhibit a pronounced enhancement of SHG with decreasing thickness. This enhancement stems from quantum confinement effects that modify the interaction between electronic states and incident light, which we explain based on quantum-mechanical calculation. Our atomically-thin crystalline silver films provide a new means to overcome the small interaction volumes inherent to nanophotonic platforms, enabling efficient nanoscale nonlinear optics with potential applications in photonics, sensing, and quantum technologies.

Covalent organic frameworks (COFs) are a versatile class of materials whose structural, electronic and functional properties can be tailored by selecting the geometry and chemical composition of their linker and spacer moieties. We have synthesized single‐layer B ₃ N ₃ ‐linked 2D COFs with biphenyl and quaterphenyl spacers on Ag(111) and Au(111) under ultra‐high vacuum conditions. The comprehensive experimental characterization combined scanning tunneling microscopy, bond‐resolved atomic force microscopy and photoemission spectroscopy. Density functional calculations revealed differences between the two BN‐substituted COFs and their carbon‐based analogues. The conduction bands of the COFs are primarily derived from electronic states of the spacer units. Introducing B ₃ N ₃ linkers into the COFs increases the band gap and reduces frontier band dispersion, effects that can be further modulated by the length of the spacers. Additionally, the site‐selective dehydrogenation of B ₃ N ₃ nodes is shown to locally modify the COF's electronic properties. We thus demonstrate the effect of atomically precise B ₃ N ₃ substitution on the electronic structure of two distinct kagome systems, through a comparative analysis of isostructural BN and CC substituted COFs. These results establish a new strategy for developing stable, metal‐free COFs with structural diversity and programmable band structures, offering insights into the controlled BN‐doping of low‐dimensional carbon nanostructures.

The intercalation of europium (Eu) below a monolayer of hexagonal boron nitride (hBN) on an Ir(111) was systematically investigated as a function of Eu coverage and substrate temperature. The structural and electronic properties were examined using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES). Depending on Eu coverage and deposition temperature, different superstructures such as (5 × M), (5 × 2) and (√3 × √3)R30◦ were identified with respect to Ir(111) substrate by LEED. At 0.12 monolayer (ML) Eu coverage, the (5 × M) superstructure (M > 2) preserved the hBN/Ir(111) Moiré pattern showing unidirectional Eu ordering beneath hBN. At 1.6ML, a (5 × 2) superstructure emerged, where excess Eu atoms diffused into the bulk, adopted a tri-valent state. The alloy formation with Ir results in a small Ir core level shift observed by XPS. At the highest preparation temperature (1/3ML), a (√3 × √3)R30◦ superstructure formed, indicating the formation of EuIr2 surface alloy beneath the hBN, containing di-valent Eu atoms with potential ferromagnetic behavior. Air exposure was used to evaluate the protection of the hBN layer, indicating a partial protection. The hBN layer, remained intact as confirmed by ARPES analysis.

We explore the effect of Rh steps on the NO dissociation process at 0.05 mbar NO, which is a crucial step of the NO reduction reaction. By using a curved Rh(111) crystal and ambient-pressure X-ray photoemission spectroscopy, data demonstrate a lower temperature onset for NO dissociation at stepped surfaces compared to Rh(111), and suggest that dissociation occurs predominantly at the lower side of the step. We also observe that surface oxidation arises at a lower temperature for Rh(111) and its B-type vicinals, with respect to A-type stepped surfaces. Surface X-ray diffraction was employed to characterize the Rh surface oxides. At the atomic scale, these were found structurally similar in flat Rh(111), A-type Rh(223) and B-type Rh(553), but they exhibited different growth behavior depending on the substrate orientation. These results showcase the structural sensitivity of NO dissociation, which is likely to affect the reactivity of these Rh surfaces under operando NO reduction conditions.

We explore the effect of Rh steps on the NO dissociation process at 0.05 mbar NO, which is a crucial step of the NO reduction reaction. By using a curved Rh(111) crystal and ambient-pressure X-ray photoemission spectroscopy, data demonstrate a lower temperature onset for NO dissociation at stepped surfaces compared to Rh(111), and suggest that dissociation occurs predominantly at the lower side of the step. We also observe that surface oxidation arises at a lower temperature for Rh(111) and its B-type vicinals, with respect to A-type stepped surfaces. Surface X-ray diffraction was employed to characterize the Rh surface oxides. At the atomic scale, these were found structurally similar in flat Rh(111), A-type Rh(223) and B-type Rh(553), but they exhibited different growth behavior depending on the substrate orientation. These results showcase the structural sensitivity of NO dissociation, which is likely to affect the reactivity of these Rh surfaces under operando NO reduction conditions.

The implementation of graphene in semiconductor technology requires bandgap tuning, which can be achieved by chemical doping, geometric confinement in nanoribbons or with the insertion of nanopores and non‐hexagonal rings. The bottom‐up on‐surface synthesis approach in ultra‐high vacuum allows for the synthesis of atomically well‐defined graphene or biphenylene nanoribbons and nanoporous graphene (NPG) structures suitable for device applications. Here, a novel 2D carbon allotrope is synthesized in the form of a functional NPG with periodically spaced biphenylene segments. First, 12‐armchair porous graphene nanoribbons (12‐pGNRs) on Au(111) and Au(788) using 7,10‐dibromo‐1,4‐diphenyl‐triphenylene (DBDT) molecular precursor are grown. Low‐temperature scanning tunneling microscopy/spectroscopy (LT‐STM/STS) and non‐contact atomic force microscopy (nc‐AFM) measurements reveal the presence of high‐quality semiconducting 12‐pGNRs. Thermal annealing of densely packed 12‐pGNRs at 550 °C triggers their lateral fusion into diverse NPG structures featuring either graphene‐type or biphenylene‐type junctions. The structural and electronic properties are again characterized by LT‐STM and nc‐AFM in combination with density functional theory (DFT) calculations. DFT shows that while graphene‐type NPGs are direct bandgap semiconductors, biphenylene‐type NPGs manifest a smaller and indirect bandgap. The NPGs are stable upon oxygen and air exposure, and the nanopores feature a noticeable affinity to carbon monoxide, making them appealing systems for chemical sensors.

Intercalation of Eu into the h -BN/Ni(111) interface restores the dispersion of the h -BN π-band and causes doping effects, while the energy shift of the π-band at the Γ point is very sensitive to the interface structure.