Direct Laser Writing of Ferromagnetic Nickel Utilizing the Principle of Sensitized Triplet-Triplet Annihilation Upconversion

Abstract

Direct laser writing of ferromagnetic microstructures is of great interest for sensing and data storage in compact three-dimensional architectures. However, reliable direct laser writing of metallic and even more so ferromagnetic materials remains a major challenge. Here, we present a novel photoresist suitable to direct laser write ferromagnetic nickel based on sensitized triplet-triplet annihilation upconversion. By combining an in-situ photochemical deoxygenation process with a sensitized triplet-triplet annihilation upconversion process as well as a photoreduction of Ni2+ ions, the deposition of metallic nickel is enabled under ambient conditions. Using this approach, nickel structures are fabricated as a proof of concept. Scanning electron microscopy and EDX analysis confirm the spatially confined deposition of nickel, while magnetic characterization by vibrating sample magnetometry and scanning NV magnetometry demonstrate the ferromagnetic nature of the printed structures. This work presents a major step forward in extending the possibilities of direct laser writing to metallic and ferromagnetic materials.

Figures (5)

• Figure 1: Graphical representation of the cooperating chemical processes. The fundamental process is the photochemical deoxygenation performed by excitation of the photosensitizer (PS), generating singlet state oxygen (${\text{O}_{2_{\text{S}_1}}}$) by energy transfer. The excited state of molecules is symbolized by a yellow halo. This ${\text{O}_{2_{\text{S}_1}}}$ is permanently removed by oxidation of the solvent, symbolized by bars (left part). This provides a local deoxygenated volume which enables a sTTA-UC process generating the annihilator (A) in its first excited singlet state S_1. The annihilator S_1 enables the photoreduction of metal ions into neutral atoms that precipate into bulk metal (M) via electron transfer by an electron donor (ED) (right part).
• Figure 2: Energy level diagram showing the steps of the triplet-triplet annihilation upconversion process. First, light gets absorbed by the sensitizer (green arrow). After internal conversion and vibrational relaxation (pink arrow) to its first singlet state, the triplet state $\text{T}_1$ is formed via intersystem crossing (dashed dark blue arrow). Next, TTET occurs between the emitter and sensitizer (dashed orange arrow). When two annihilator molecules in the $\text{T}_1$ state collide, triplet-triplet annihilation upconversion occurs exciting one molecule to a higher energy singlet state with the second relaxing to the ground state (red arrows). The bright blue arrows represent potential non-radiative decay during the process of sTTA-UC. Adapted from bennison2021organic.
• Figure 3: Latimer diagram of a photocatalytic reduction of nickel ions by perylene (per) with DIPEA as an electron donor. The central vertical arrow indicates excitation of perylene to the $\mathrm{S}_1$ state with the corresponding excitation energy. Apart from this one, each arrow represents a redox half-reaction between two adjacent oxidation states. Red numbers indicate reduction and blue numbers oxidation potentials, both referenced to SCE. Electron transfer is thermodynamically favorable when the reduction potential of the acceptor exceeds that of the donor. For the electron donor DIPEA, the oxidation potential is given. For consistency, it is converted to the corresponding reduction potential (i.e. with inverted sign) when evaluating the energy balance.
• Figure 4: SEM images of 2.5D nickel structures printed using the proposed resist. a) Our university's logo. b) A close up of an array of ring segments. c) EDX obtained countmap visualizing the distribution of nickel matched to the sample area in b). d) Cross section of a structure generated using a focused ion beam.
• Figure 5: Magnetic characterization of nickel microdots. a) Room-temperature magnetic hysteresis loop of a nickel dot array of elements analogous to the structure shown in c. b) Schematic of the scanning NV magnetometer utilized for single-structure characterization. The upper-left inset schematically displays an optically detected magnetic resonance (ODMR) spectrum, where the resonances $f_-$ ($m_\mathrm{S}=0 \leftrightarrow m_\mathrm{S}=-1$) and $f_+$ ($m_\mathrm{S}=0 \leftrightarrow m_\mathrm{S}=+1$) of the NV electronic ground state are visible. Within the small-field approximation, the Zeeman splitting around the zero-field splitting $D$ is proportional to the magnetic field projection onto the NV quantization axis, $\mu_0H_\mathrm{NV||}$. c) Top left: SEM image of the investigated microdot. The main panel shows the quantitative stray field map ($\mu_0H_\mathrm{NV||}$) recorded scanning the NV at a height of 500nm above the nickel structure. The structure was saturated at 300mT and measured in remanence, using a small bias field to ensure clear splitting of the NV resonances.