Kjeld Pedersen

Surface-enhanced Raman spectroscopy (SERS) enables the detection of molecular signatures at extremely low concentrations, even down to a single molecule, by using plasmonic nanostructures that support localized surface plasmon resonances (LSPRs). Nonetheless, fabricating highly sensitive nanostructures typically requires complex, time-consuming, and costly fabrication processes with limited scalability. In this work, we present a simple and universally applicable single-step method to fabricate plasmonic nanocrystals (NCs) from metals, such as Al, Ag, and Au. These NCs are directly grown over large surface areas on various substrates through heat-assisted growth from e-beam or magnetron sputter depositions. The LSPRs of the NCs can be tailored by selecting the metal, nanocrystal size, and surface morphology. The heat-assisted growth strategy enables the formation of NC matrices capable of detecting analyte molecules at ultralow, down to zeptomolar, concentrations via SERS, particularly when using Ag NCs. This versatile, cost-effective fabrication approach opens new opportunities for large-area plasmonic platforms in sensing, photocatalysis, and optoelectronic applications.

In the pursuit of solving the energy storage issues of leadless pacemakers, this study presents a piezoelectric energy harvester (PEH) for self-powering. The mismatch in natural frequencies between the cardiac cycles and the piezoelectric structure affects energy generation. In this research, a groundbreaking hexa-fold PEH (HF-PEH) with contact impacts was proposed to generate high energy density from cardiac cycles. The HF-PEH was manufactured and tested in vivo inside a living pig’s heart. Subsequently, we subjected this HF-PEH prototype to laboratory cardiac acceleration for over 35 million cardiac cycles to investigate its long-term performance. Post-test material analysis using SEM and x-ray energy-dispersive spectroscopy was conducted to investigate the material structure. The HF-PEH generated maximum voltage, current, and power of 1.4 V, 458.5 µA, and 367.2 µW at in-vivo animal trial with 71 beats per minute, which are the same or higher than the values a leadless pacemaker paces into the body. Post-test material analyses showed that the piezoelectric ceramic remained intact while the electrode condition changed. A verified finite element model was used to study the long-term electrode layer erosion condition with respect to the electric displacement field. Since the electrode erosion is limited to the contact-based impact region, limited power degradation in long-term performance was observed. This study also highlights the roles of electrode materials and presents potential protective methods and materials to mitigate electrode performance degradation. Our findings pave the way for practical energy harvesting applications for leadless pacemakers and underscore the need for advanced piezoelectric coatings in contact-based energy harvesting systems.

Fabrication of tall Cr nanostructures of different shapes by lithography and lift-off processes is demonstrated. By varying resist thickness, metal layer thickness, and diameter of holes in the resist mask, it is demonstrated that metal structures with shapes ranging from sharp-tipped conical over flat-top cones to nearly cylindrical can be fabricated. A comparison of resist layer dissolution in acetone, covered by Ag and Cr films, reveals that Cr films grow with an open structure of particles that allow rapid solvent diffusion through Cr layers that are several hundred nanometers thick. On the other hand, the 2D growth of Ag on the resist forms a barrier against acetone diffusion. The open structure of Cr enables the lift-off process to fabricate several-μm-high nanostructures using a single resist layer. As an example, high-aspect-ratio Si structures are demonstrated by reactive ion etching using thick Cr layers as a mask, fabricating nanopillars with 3 μm height at room temperature.

Electrochemical impedance spectroscopy (EIS), a conventional and alternating‐current‐(AC)‐based technique for impedance measurement, is commonly used in battery diagnosis. However, it requires expensive equipment and demanding operating conditions and is complex and model‐dependent in data analysis. Recently, novel direct current (DC) analytics have emerged as an alternative to EIS. They are simple yet powerful, being capable of revealing impedance information that traditionally could only be obtained through EIS and determining Li‐ion diffusion coefficient. Besides, a complete EIS spectrum can be predicted based on constant current charging curves in the support of machine learning methods. This work highlights the similarities and discrepancies between DC techniques and EIS in the electrochemical analysis of Li‐ion batteries. Looking ahead, DC techniques may be a promising substitute for EIS in future battery diagnosis, requiring simplified equipment while offering a deep understanding of battery impedance and its underlying electrochemical processes.

Loss of Li-ions, which results in capacity loss, mainly occurs on the negative electrode in the form of Li plating or a surface film. Quantifying the loss of lithium in graphite anode is essential for studies such as waste battery recycling, lithium plating on negative electrodes, and interfacial film composition, which can guide the manufacturing and application chain of commercial lithium-ion batteries. Various methods for quantifying lithium losses have been proposed. You et al.^[1] proposed the use of inductively coupled plasma (ICP) method to quantify lithium loss in graphite. This is a low-cost method that is easy to apply on a large scale, but it requires large sample amount and high instrumentation precision. Fang et al. ^[2] used water titration in conjunction with gas chromatography to quantify lithium loss from lithium metal batteries. This method is more subtle, but the produced trace amounts of gas are often too difficult to collect. In this study, we used water to quantify the lithium loss in graphite based on the method of Fang et al. We then combined pH and X-ray photoelectron spectroscopy (XPS) analysis to quantify the total lithium loss in graphite. To perform the quantify method, we washed the middle part of the negative electrodes of fresh and aged cells with DEC. We then cut out 2 cm² the negative electrodes and placed them in two volumetric flasks filled with 20 mL of deionized water each. After stirring, we sealed the flasks and placed them in a 60 °C oven for a week to ensure that the lithium compounds were fully reacted and dissolved with water. We added BaCl₂ to remove CO₃ ^2- and F^- ions, which can hydrolyze to produce OH^- and affect pH. We then performed a pH test to quantify the LiOH content in solution. As reported, LiOH in solution is mainly from Li and Li₂O compounds^[2], so we were able to quantify the Li in graphite in the form of Li/Li₂O. To analyze the surface composition of graphite materials, we conducted an XPS test. We quantified the total lithium loss based on the percentage of interfacial lithium compounds occupied by Li/Li-O. It should be noted that this quantification method is based on the assumption that the surface material is homogeneous. There are three fitted peaks for the aged negative electrode, which belong to LiF, Li₂CO₃, and Li/Li–O compounds, corresponding to an amount of 73.66%, 19.79%, and 6.55% Li, respectively. Here, the Li/Li–O compounds correspond to the pH quantified part (1.10%). Therefore, the lithium loss in the aged negative electrode can be quantified as 16.82%. In conclusion, we proposed a water reaction method to quantify the loss of lithium in graphite, which was then combined with pH and XPS analysis to quantify the total lithium loss. This method offers a subtle yet effective way to quantify lithium loss. By quantifying the total lithium loss, we can gain insights into the interfacial film composition, lithium plating on negative electrodes, and waste battery recycling, which can guide the manufacturing and application of commercial lithium-ion batteries. Reference [1] H. You, B. Jiang, J. Zhu, X. Wang, G. Shi, G. Han, X. Wei, H. Dai, Journal of Power Sources 2023, 564, 232892. [2] C. Fang, J. Li, M. Zhang, Y. Zhang, F. Yang, J. Z. Lee, M. H. Lee, J. Alvarado, M. A. Schroeder, Y. Yang, B. Lu, N. Williams, M. Ceja, L. Yang, M. Cai, J. Gu, K. Xu, X. Wang, Y. S. Meng, Nature 2019, 572, 511. [3] J. Guo, S. Jin, X. Sui, X. Huang, Y. Xu, Y. Li, D. W. Peter Kjær Kristensen, D.-I. Stroe, Journal of Materials Chemistry A 2023.

Aluminum nitride (AlN) is a material of growing interest for power electronics, fabrication of sensors, micro-electromechanical systems, and piezoelectric generators. For the latter, the formation of nanowire arrays or nanostructured films is one of the emerging research directions. In the current work, nanostructured AlN films manufactured with normal and glancing angle magnetron sputter depositions have been investigated with scanning and transmission electron microscopy, X-ray diffraction, atomic force microscopy, and optical spectroscopy. Growth of the nanostructures was realized utilizing metal seed particles (Ag, Au, and Al), allowing the control of the nucleation and following growth of AlN. It was demonstrated how variations of seed particle material and size can be used to tune the parameters of nanostructures and morphology of the AlN films. Using normal angle deposition allowed the growth of bud-shaped structures, which consisted of pillars/lamellae with wurtzite-like crystalline structures. Deposition at a glancing angle of 85° led to a film of individual nanostructures located near each other and tilted at an angle of 33° relative to the surface normal. Such films maintained a high degree of wurtzite-like crystallinity but had a more open structure and higher roughness than the nanostructured films grown at normal incidence deposition. The developed production strategies and recipes for controlling parameters of nanostructured films pave the way for the formation of matrices to be used in piezoelectric applications.

The long‐term operation of refractory‐metal‐based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3‐layer‐system, consisting of HfO₂/Ir/HfO₂, by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10⁻⁶ mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO₂ layers. An in situ X‐ray diffraction experimental comparison between 1D multilayered Ir/HfO₂ and W/HfO₂ selective emitters annealed at 1000 °C, 2 × 10⁻⁶ mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W‐based metamaterials are not long‐term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.

A variety of nanostructures capable of generating strong local electromagnetic fields (hot spots) in interaction with radiation have been under intensive investigation towards plasmonic applications in surface-enhanced Raman scattering (SERS), biosensing, broadband absorbers, thermophotovoltaics, photocatalysis, etc. In many cases, these nanostructures are formed on a surface or embedded into a near-surface layer of the dielectric substrate, making some part of the field dissipate into the bulk and not contribute to the desired plasmonic functionality. To reduce such losses, the interface between the metallic nanostructures and the dielectric environment should be engineered. In the current work, Au nanodiscs are fabricated on Si posts of very small diameter (pin-shaped structures), enabling them to decouple the strong optical near fields localized at the nanodiscs from the bulk Si substrate. The Si post diameter is optimized by adjusting the gas flow rates in reactive-ion etching, resulting in a minimum post diameter of 20 nm at the nanodisc interface. The effect of this diameter on the localized surface plasmon resonance of the nanodisc is investigated with linear optical spectroscopic measurements, where a significant spectral blue shift of the resonance band is noticed compared to similar discs formed on bulk Si surface. The experimental results are compared with modelling where a 3-fold increase in the electric field enhancement is demonstrated. The fabricated pin-shaped nanostructures are tested in SERS measurements showing a significant increase in the enhancement factor in the order of 10⁶. Thus, this work suggests a way of engineering 3D morphology to tune the substrate influence on the plasmonic properties of nanostructures and to develop efficient nanofabrication technologies.