Seznam abstraktů 2022: SEKCE A - NANOMATERIÁLY PRO ELEKTRONICKÉ, MAGNETICKÉ A OPTICKÉ APLIKACE. UHLÍKOVÉ NANOSTRUKTURY, KVANTOVÉ TEČKY

First principles simulations are crucial in many areas of nanoscience. In electrochemistry, however, the complexity of the electrochemical environment and the presence of the external electrode potential have precluded direct application of first principles methods like DFT. We aim to overcome these barriers by utilizing recent breakthrough advances in modelling techniques that will allow us to extend the use of DFT to the complexity of the electrochemistry processes.
We demonstrate how Non-Equilibrium Green's Functions (NEGF) techniques can be used to address, from first principles, the atomistic description of metal-electrolyte interfaces in the presence of an external bias applied to the electrodes. The NEGF method, commonly used to study electronic transport in nanoscale constrictions, is used here to describe the electrified solid/liquid interface, including the electronic charge redistribution induced by the external bias, and the resulting electrostatic profile. This allows us to study the structural and dynamic changes in the liquid (including the electrical double layer) and chemical reactions occurring at the electrified interface.
We will show preliminary DFT molecular dynamics simulations of aqueous electrolytes as a proof of concept for future realistic, atomistic first-principles simulations of electrochemical processes. Additionally, we explore the possibility of reaching much larger system sizes and longer simulation times by combining the DFT approach to deal with the electrodes (where the quantum-mechanical description is essential), with a molecular mechanics model for the liquid electrolyte (or part of it, not involved in the electrochemical reactions).

Nanocrystalline oxide of composition Yb3Ga5O12 belongs to the family of materials with garnet-related structure (space group Ia−3d). It is a promising material with potential applications in the fields of optics and photonics when doped by Er3+ or Ho3+ ions. High content of Yb3+ ions in the Yb15−xErxGa25O60 and Yb15−xHoxGa25O60 hosts results in strong absorption band observed from ≈850 to ≈1050 nm due to the Yb3+: 2F7/2 → 2F5/2 transition. Excitation using ≈980 nm laser leads to efficient energy transfer from the Yb3+ ions to the neighbouring Er3+/Ho3+ ions. Subsequent radiative transitions of Er3+/Ho3+ ions from the upper- and lower-lying excited energy levels provide emission bands observed in visible and near-infrared region, respectively.
Nanocrystals of compositions Yb15−xErxGa25O60 and Yb15−xHoxGa25O60 (with x = 0, 0.01, 0.1, 0.5, 1 and 2 at.%) were prepared by sol-gel combustion technique using citric acid as chelation agent. Phase composition and morphology of the annealed powders were studied by X-ray diffraction and by scanning electron microscopy. Optical reflectance of the powders and absorption bands of present lanthanides were investigated by diffuse reflectance spectroscopy. Photoluminescence spectroscopy was used for measurement of the steady-state and time-dependent Er3+ and Ho3+ emission spectra. Near-infrared (Stokes) and upconversion (anti-Stokes) emission mechanisms are proposed using energy level diagrams of the Yb3+, Er3+ and Ho3+ ions.

Nowadays, in sensing and energy harvesting, piezoelectric materials are widely employed because of their ability to transform mechanical energy into electrical stimuli. Piezoelectric composites combine mechanical flexibility and strong electromechanical coupling constants; however, the low dielectric constant of the polymer and the non-continuity of the ceramic may hinder polarisation. The low connection between the piezoelectric particles may be addressed using conductive carbon nanoparticles, but the low connection between the piezoelectric phase is left unanswered. Innovative, highly porous 3D carbon networks may overcome these issues. In this work, it is presented an entirely original paradigm in the design and scalable fabrication of high-performance piezoelectric flexible materials enabled by hierarchically porous graphite. A 3D graphite network was filled with barium titanate and impregnated with a flexible polymer through two distinct bottom-up methods: hydrothermal and sol-gel syntheses. With the hydrothermal method, the influence of various reaction times on the tetragonality of the particles was studied to perfect a hydrothermal (conventional and microwave-assisted) barium titanate synthesis. With the sol-gel method, the sol is infiltrated on the carbon structure, and a heat treatment capable of forming tetragonal barium titanate while not degrading the carbon template is attempted. The structural phase was assessed using X-ray diffraction and Raman spectroscopy, while the morphology was assessed using SEM. The hydrothermally synthesised particles were used in an optimised water suspension with and without voltage to imprenate barium titanate particles into the graphite foam. The final device required a chitosan/zein polymer to include the resultant apparatus with built-in electrodes to test the device’s electrical output. PFM was used to assess the particles’ piezoelectric response following impregnation. The structure : piezoelectric output relationship was exploited in both approaches.

High-power infrared lasers operating in infrared are the heart of modern equipment in telecommunications, tracking and navigation systems, etc. The increasing power of lasers places high demands on the phosphors used as active media, which must exhibit high thermal stability and luminescence efficiency. Transparent ceramics based on lanthanide yttrium titanium oxides, with formula (REExY1-x)2Ti2O7 where REE is a rare earth element, may face this challenge when having defined particle sizes and distribution. The homogeneity of prepared materials is dependent on the crystallization mechanism. This contribution clarifies the influence of nucleation and growth mechanism of Y2Ti2O7 when doped with REE ions.
We prepared ceramic luminophores with defined particles size, emitting above 2 um. We used the sol-gel approach for obtaining compounds with a nanocrystal structure of (REE0,05Y0,95)2Ti2O7, where REE=Yb, Er, Eu, Tm, Ho. The crystallization temperatures were found to be dependent on the radius of REE ion. Crystallization parameters of all (Re0.05Y0.95)2Ti2O7 and activation energy of crystallization were calculated. Based on calculated Avrami parameters nanocrystal growth for compounds doped by REE was driven by homogenous nucleation with a constant nucleation rate, while for undoped compound Y2Ti2O7 crystalization mechanisms corresponded to site-saturated nucleation. The obtained results provide information about nucleation and growth properties and crystal structure of luminophores and give necessary information for preparing pure nanocrystalline powders with tailored structural properties that are suitable for photonic applications.
ACKNOWLEDGEMENTS: This research was financially supported by the Czech Science Foundation number 22-14200S and the mobility project of the Czech Academy of Sciences number AR-20-01.

The metal nanostructures on planar substrates are widely studied for applications in optoelectronics. The nanostructures are produced using a variety of approaches, such as electron- and ion-beam lithographies, etc. These techniques suffer from a limited throughput. Therefore, replication approaches were proposed to scale up the fabrication process, such as EUV or nanoimprint lithography. Such replicated patterns are then frequently transferred to the target material (such as metal) by the lift-off process. In such a process, a sufficient resist undercut is desired. Despite the great effort, the search for a reliable fabrication process to meet these requirements continues.
In this work, we report on the novel nanofabrication process based on the bilayer nanoimprint lithography combined with the metal lift-off process. In the first phase of this process, the relief pattern is fabricated by electron-beam lithography. Subsequently, a rigid OrmoStamp mold is replicated by UV nanoimprint lithography. Then, the mold is attached to a flexible polydimethylsiloxane block and used to replicate the pattern into a bilayer of imprint resist (OrmoStamp), and underlayer lift-off resist (LOR). After imprint, LOR resist is developed to produce an undercut. Finally, the pattern is transferred to metal in the lift-off process. We demonstrate the capabilities of this process by fabricating various types of metal nanostructures supporting surface plasmons, such as arrays of nanodisk and nanostripes with lateral dimensions down to 100 nm and aspect ratio ~1, and the fabricated nanostructures are characterized by scanning electron microscopy and broad-band spectroscopy to demonstrate their morphological and optical properties.

Biomedical engineering, especially biomaterials and biosensors, plays a key role in modern society. A natural consequence of such a trend is the increasing demand for the introduction of novel materials that can offer suitable sensitivity and selectivity including new sensing principles, fast and reliable response, biocompatibility, easy fabrication, and use at low cost. Among the large family of such materials, great attention has recently been focused on carbon-based materials, especially on synthetic diamond films that exhibit an extraordinary combination of the above-mentioned properties in one platform.
In this contribution, the latest advances in chemical vapor deposition and its process conditions (plasma properties, gas composition, process pressure, temperature) on the resulting properties of the diamond coatings and their surfaces for biomedical engineering applications will be presented. The influence of diamond surface chemical terminations and (nano-) structured morphology on the patterned growth of different cell lines will be shown to be a crucial issue for further bioelectronics application of the diamond coatings. Then, the use of hydrogen-terminated diamond-based field-effect transistors for label-free monitoring of proteins or cells will be discussed. Here, the intrinsic and optically transparent diamond coating is used as a highly insulating material in which the hydrogen termination induces a surface 2D hole conductivity highly sensitive to the surrounding conditions (gas, pH, biomolecules, cells). Basic knowledge of real-time monitoring of biomolecules/cells adhered on the diamond surface in terms of sensor sensitivity, stability, and reliability will be presented. Finally, the new principles of biosensing with diamond devices (impedance, optical, mass, etc.) will be highlighted.

Diamond color centers interaction with light is reflected in their opto-electronic properties - photoluminescence (PL), photocurrents, and photovoltages (PV). Photovoltage due to SiV is affected by the thickness of the active SiV layer as well as by the surface chemical groups [Stehlik et al. 10.3390/mi9060281]. Here we combine ultrathin diamond growth [Stehlik et al. 10.1021/acsami.7b14436] with nanostructured morphology of diamond to enlarge the SiV active surface area. We characterize the effect of surface chemistry (hydrogen/oxygen termination and adsorption of perylene molecules for facilitating charge and/or energy transfer) in correlation with the thickness and morphology of active SiV layer. We use 5 x 10 mm2 Si substrates with spatially distributed nanodiamond nucleation [Miliaieva et al. 10.1038/s41598-020-80438-3]. We apply CVD diamond growth for 5-15 min using microwave plasma SEKI SDS6K reactor. We evaluate the grown thickness (10-30 nm) and surface morphology by AFM and SEM. Raman spectroscopy characterizes the material composition. We correlate the PL intensity of SiV with Kelvin probe measurements of surface charge under illumination (laser or solar simulator AM 1.5) and in dark. Energy levels and charge transfer in SiV-diamond-molecular system are also calculated theoretically by DFTB methods to corroborate the experimental data. ACKNOWLEDGEMENTS: The work was supported by the project CZ.02.1.01/0.0/15_003/0000464 - Centre of Advanced Photovoltaics.

Hydrogenated nanodiamonds (ND-H) are not only fundamentally but also practically interesting since they offer a broad range of bio-applications such as drug delivery (Ho et al. 10.1080/19475411003619736), tissue engineering (Wu et al., 10.1039/C7TB00723J) or cell signaling proteins regulation (Balek et al. 10.1016/j.biomaterials.2018.05.030). Commonly, these applications are associated with the positive zeta potential of ND-H therefore it is important to understand the origin of this unusual phenomenon (Ginés et al., 10.1039/C7NR03200E) and its relation to structural, surface chemical and spectral properties.
Here we present a study on three different types of ND-H which significantly differ namely in crystallinity: i) highly defective detonation nanodiamonds (DND-H; ~ 5 nm), ii) monocrystalline top-down high-pressure high-temperature NDs (TD_HPHT ND-H; 0-30 nm) prepared by grinding of diamond monocrystals and iii) monocrystalline bottom-up HPHT NDs prepared from molecular precursors (BU_HPHT ND-H; 1-8 nm). DND-H and TD_HPHT ND-H were first purified from sp2-C by aerobic oxidation and then hydrogenated ex post at 700 and 800°C, respectively. BU_HPHT ND-H were inherently hydrogenated during the synthesis. FTIR and also Raman spectra revealed successful hydrogenation of all three ND-H types. Dynamic light scattering (DLS) and zeta potential measurements of water-based DND-H colloids revealed a characteristic strong zeta potential of +50 mV while keeping the original size distribution after the hydrogenation process. Also, TD_HPHT ND-H showed no increase in the DLS size after hydrogenation and a relatively high zeta potential of +40 mV despite the higher degree of hydrophobicity indicated by FTIR. BU_HPHT ND-H showed also positive zeta potential values (≥ +30 mV) but they were poorly dispersible in water and so, DLS size distribution did not reflect the real particle size. Electrical conductivity measurements showed much lower conductivity of the DND-H samples compared to both TD_HPHT ND-H and BU_HPHT ND-H. In contrast to DND-H and akin to BU_ HPHT ND-H, hydrogenated TD_HPHT ND-H exhibit a transmission window in the IR region at around 1330 cm-1 (phonon frequency of diamond) that has been recently assigned to a destructive Fano-type resonance of phonons and free carriers (Kudryavtsev et al., 10.1021/acs.nanolett.1c04887) and thus can be considered as a conductivity-related IR-accessible feature in diamond. Interestingly, in the case of TD_HPHT ND-H, the conductivity, as well as the transmission window, disappeared after annealing at 250°C for 1 h while the BU_HPHT NDs kept high conductivity and the IR transmission window even after the annealing treatment. This indicates that not only the transfer doping mechanism, well known from the hydrogenated bulk diamond surface, but another conductivity mechanism insensitive to surface adsorbates is involved (Ekimov et al., 10.3390/nano12030351). The insulating nature of DND-H is explained by its defective character (bulk & surface) which results in electron trapping on the sp2-C rich surface and poor carrier mobility.

We have performed a quantum-mechanical study of thermodynamic, elastic, magnetic and structural properties of several ferrimagnetic states in Mn-rich Ni-Mn-Sn martensites. They are modeled by both orthorhombic and four-layer modulated 4O structures with Mn-excess atoms randomly distributed in Ni and Sn sublattices. As far as bulk systems are concerned, the lowest-energy configurations exhibit local magnetic moments of the Mn-excess atoms with anti-parallel orientation with respect to the orientation of the total magnetic moment, i.e., ferrimagnetic state. Interestingly, bulk magnetism in the orthorhombic state was found highly anomalous (for details see M. Friák et al., Materials Transactions 63 (2022) 430) while the modulated 4O structures did not exhibit any anomaly (see M. Friák et al., Intermetallics (2022) accepted for publication). We have, therefore, studied surface-related effects in magnetism in nano-scale layers of Ni-Mn-Sn atoms. Importantly, the magnetic anomalies were found enhanced by the existence of studied surfaces in orthorhombic Ni-Mn-Sn alloys.
ACKNOWLEDGEMENTS: The authors acknowledge the Czech Science Foundation for the financial support received under the Project No. 20-16130S entitled “Multifunctional properties of powdered Ni-Mn-Sn Intermetallics”. Computational resources were made available by the Ministry of Education, Youth and Sports of the Czech Republic under the Project of the IT4Innovations National Supercomputer Center (project “e-Infrastructure CZ-LM2018140”) within the program Projects of Large Research, Development and Innovations Infrastructures and via the CESNET (Project No. LM2015042) and CERIT-Scientific Cloud (Project No. LM2015085).

Scintillation mechanism depends on material and structure. Types of structure can be different, too, ranging from bulk material, over some heterogeneous sandwich structures to nanostructures, examples being Multiple Quantum Wells (MQW) and QD structures. We will focus on description of MOVPE InGaN/GaN MQW scintillation structures.
Scintillator works as a convertor of high-energy ionizing radiation to the radiation usually in the visible part of spectrum. For the application, the light is carried through some optical medium to the photodetector, which turns it into electrical signal. These particles can be X-ray, gamma radiation or alpha, beta or proton particles [1]. These devices can be usefull for medicine purposes.
Preparation, properties and problems of MOVPE InGaN/GaN scintillators with MQW structures will be discused. Efficient detection of high-energy ionizing radiation (for example gamma 511 keV) requires thick active region. The highest distance of two adjacent QWs ought to be lower than diffusion length of GaN exciton. This makes thick enough MQW structures infeasible because of structure polarization, strength, dislocations, other defects. We have realized alternative solution, which is a hybrid sandwich InGaN/GaN MQW + BGO (Bi4Ge3O12) detector [2]. It is composed from 12 InGaN/GaN scintillators (4 scintillators joined in the stack) and enclosed by two BGO bulk scintillators 350 μm thick.
REFERENCES: [1] NIKL M. a YOSHIKAWA A. Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection. Adv. Opt. Mater. [online]. 2015, 3(4), 463-481. ISSN 21951071. Available from: doi:10.1002/adom.201400571.; [2] VANĚK T., Nitridové polovodičové struktury a jejich charakterizace, dissertation thesis, TUL, 2022.

Covalent functionalization of graphene significantly broadens its application potential, because it enables efficient tuning its electronic, magnetic and surface properties [1]. A wide range of various approaches has been developed for graphene functionalization so far. However, despite the progress, covalent modification of graphene is still limited by its low reactivity and other conditions (support, number of layers etc.) affecting its reactivity. Due to these facts, there is an unceasing need for new strategies that permit high yielding and selective graphene functionalization under controlled conditions.
Fluorographene [2,3] (FG) is a stoichiometric (C1F1) and well-defined graphene derivative, which has attracted significant attention due to its unique properties [4]. Extensive studies on the FG chemistry showed that FG is susceptible to reductive defluorination [5] and nucleophilic attack [6,7], rendering it an advantageous precursor for the preparation of graphene derivatives [8]. Hence, the utilization of FG for the preparation of multi-functionalized graphene derivatives could be considered as a very attractive approach. In this frame, our group achieved the preparation of double functionalized graphene derivatives, synthetized by a one-pot reaction of FG with organometallic nucleophiles. The nucleophilicity governed the preference for grafting and was utilized for tuning the functionalization. Finally, new experimental studies have showed that FG can be used as a template for the preparation of asymmetrical modified graphene derivatives with different functional groups at each side.
ERC Consolidator grant (H2020, ID: 683024) 2D-Chem is gratefully acknowledged.
REFERENCES: [1] A. Stergiou, et al. Prog. Mater. Sci., 114, 100683, (2020).; [2] J. T. Robinson, et al. Nano Lett., 10, 3001, (2010).; [3] W. Feng, et al. Adv. Sci., 3, 1500413, (2016).; [4] X. Chen, et al. Adv. Mater., 34, 2101665, (2022).; [5] R. Zbořil, et al. Small, 6, 2885, (2010).; [6] M. Dubecký, et al. J. Phys. Chem. Lett., 6, 1430, (2015).; [7] K. E. Whitener, et al., J. Phys. Chem. C, 119, 10507, (2015).; [8] D. D. Chronopoulos, et al. Appl. Mater. Today, 9, 60, (2017).

Interest in electrically conductive polymers has grown in recent decades. Sectors such as sensor manufacturing, biomedical applications and the electronics industry see their potential. One way to make a polymer conductive is to mix conductive particles into it. These can form electrically conductive paths, called percolations, thereby increasing the electrical conductivity. Examples of such conductive fillers are carbon fibres and carbon nanotubes. When used together, there is a good chance that the nanotubes will interconnect the carbon fibres to form a conducting network. In the field of polymer matrix materials, there is a growing interest in polymers from renewable resources, whose applications in engineering are under-researched. Of these, polylactic acid (PLA) has come to the fore, with its favourable mechanical properties providing a good basis for engineering applications. In our research, we aim to develop conductive composites with a polylactic acid matrix containing nanotubes and carbon fibres. The composite compounds were prepared by twin-screw extrusion; the compounds were then processed by injection moulding. It was shown that the electrical conductivity increased only slightly when only carbon nanotubes were added to the PLA matrix. However, when carbon fibres were added to the system, the higher shear during melt mixing promoted uniform dispersion of the carbon nanotubes, resulting in a high conductivity reinforcement network in the composite. On the other hand, the hybrid reinforcement resulted in higher viscosity, which made the melt processing more difficult and the material also became more brittle.

Over the last ten years, silver sulphide quantum dots (Ag2S QDs) have emerged as a promising non-toxic alternative to traditional binary heavy metal containing QDs for bio-medical applications due to their ultra-low solubility product constant (Ksp = 6.3 x 10-50)1 in aqueous media, almost non-existent toxicity2, strong photoluminescence, and tuneable Vis-NIR emission3. Typically, most research focuses on the bottom-up, organic synthesis of Ag2S QDs due to the resulting favourable optical properties including, ease of size control, tuneable emission in the NIR-II region and high quantum yield4. However, recently due to the toxic chemicals and harsh conditions exploited in organic synthesis, there has been a focus on shifting towards greener, aqueous synthetic methods allowing for the synthesis of instantly biocompatible QDs at lower temperatures without the need for phase transfer and the associated diminution of their fluorescence intensity5. Utilising a simple aqueous-based hot injection approach, ultra-small (< 2 nm), red-emitting L-glutathione (GSH) capped Ag2S QDs have been synthesised. Through manipulation of both the initial pH of the solution, Ag:Ligand feed ratio and reaction time, size control at the sub 2 nm scale and quantum confinement visualised through the respective absorbance properties have been observed. PL studies revealed the QDs exhibited strong 700-nm emission, independent of size, suggesting the presence of trap-state emission at ultra-small sizes, providing an exciting opportunity for the development of new ultra-small, non-toxic bio-probes with the possibility of renal excretion and reduced risk of accumulation in the body.
REFERENCES: [1] H.-Y. Yang, Y.-W. Zhao, Z.-Y. Zhang, H.-M. Xiong and S.-N. Yu, Nanotechnology, 2013, 24, 055706.; [2] C. Lu, G. Chen, B. Yu and H. Cong, Adv. Eng. Mater., 2018, 20, 1700940.; [3] C. Ding, Y. Huang, Z. Shen and X. Chen, Adv. Mater., 2021, 33, 2007768.; [4] B. Purushothaman and J. M. Song, Biomater. Sci., 2021, 9, 51-69.; [5] G. Rotko, J. Cichos, E. Wysokińska, M. Karbowiak and W. Kałas, Colloids Surf. B Biointerfaces, 2019, 181, 119-124.

Carbon dots (CDs) display photophysical features that appear similar, if not identical, apart from the numerous synthetic protocols to obtain them. However, the photoluminescence (PL) mechanism associated with the carbon core, surface, and molecular fluorophores states remains controversial. Recently, the group of M. Prato emphasized that the properties of solvothermal-synthesized CDs could be affected by the presence of unreacted species or small organic molecules [1]. The group of M. Otyepka pointed out that the intense excitation-independent PL component of CDs is attributed to molecular fluorophores, which may be surface-bound or embedded in the CD matrix [2]. It shows the necessity to understand better how the CD synthesis process affects their structure and properties. Recently, we have reported a fast microwave-assisted hydrothermal synthesis of nitrogen doped CDs from water solution of glucose and ethylenediamine [3]. They exhibited tunable PL in a wide spectral range. Now, we discovered an unexplored spontaneous room-temperature synthesis that starts from the same reactants. We investigated the spontaneous carbon dots formation process in a time span of 12 months using a combination of analytical techniques (XPS, HRTEM, NMR, MALDI TOF MS) and measuring their photoluminiscence. It provided insight into the structure of CDs, and indicated the formation of a compact network of molecular species. The hypothesized formation of CDs is supported by density functional theory calculations of the elucidated structures.
REFERENCES: [1] B. Bartolomei et al., Angew. Chem. Int. 2022, 61, e202200038.; [2] M. Langer et al., J. Phys. Chem. C 2021, 125, 12140.; [3] I.J. Gómez et al., J. Phys. Chem. C 2021, 125, 21044.

As previously reported, arrays of Nb2O5 nanocolumns on substrates may be synthesized by anodizing Al-on-Nb bilayers in oxalic, sulfuric, malonic, or phosphoric acids. Here, for the first time, we employed a new anodizing electrolyte - selenic acid (H2SeO4) due to a unique morphology of porous anodic alumina (PAA) formed in this electrolyte featuring extremely narrow pores and therefore, the minimal possible porosity (~3%) among the PAA films prepared in the usual solutions. Such a morphology may result in distinctive structural, physical, and chemical properties of niobium-oxide nanoarrays complemented by the specific dielectric, optical, electro-optical, catalytic, and antibacterial properties in case selenium species are incorporated into the column structure during the anodizing.
The columns were prepared from magnetron-sputtered Al/Nb bilayers by anodizing at 12 or 40 V, followed by re-anodizing to 80 or 120 V, respectively, and selective dissolution of the alumina overlayer. Detailed XPS characterization confirmed that selenium species is present in the column material. The fitting of narrow-scan Se 3d spectra, complicated by overlapping with Nb 4s peaks, shows that selenium is present in the form of selenate (SeO42−) and selenide (Se2−) anions, possibly creating a core-shell structure. Quantitative analysis reveals that about 1.5-3.0% of O2− anions in the oxide structure are replaced by selenate or selenide anions. Further investigation is in progress to understand the formation-structure-morphology relationship and explore the functional properties of the arrays. ACKNOWLEDGEMENTS: This research is supported by GACR grant no. 20-25486S.

Ferrites showing natural ferromagnetic resonances (NFMR) in millimeter wave frequencies, particularly between 95 and 220 GHz, are interesting because of their potentiality in the device applications for next generation 6G and beyond wireless communication. ε-Fe2O3 is an orthorhombic phase which is typically stabilized in form of nanoparticles. It has a sizeable magnetization (MS ~ 100 emu/cm3) and a strong uniaxial magnetic anisotropy reflected in an enormous coercivity (HC ~ 20 kOe) at room temperature and NFMR at around 180 GHz. The resonance frequency can be controlled by substitution of Fe3+ with other trivalent metals, which is interesting for the above-mentioned application. In this contribution, we will discuss using several soft chemistry methods for preparing ε-Fe2O3 with different particle sizes and metal substitutions. In particular, we have developed a method to obtain in one batch large amounts of ε-Fe2O3 particles which are not affected by relaxation processes, overcoming an important limitation for its application in 6G devices. The magnetic relaxation and NFMR of ε-Fe2O3 of different particle sizes and substitution levels were assessed by quasi-optical measurements and compared to Sr0.67Ca0.33Al4Fe8O19, another high anisotropy ferrite.