Szu-Hsueh Lai

Adrien Reynaud

Ning-Ning Zhang

Minjeong Jeong Kwak

Bogdan Vysotskyi

Sergio Dominguez-Medina

Thomas Fortin

Martial Defoort Kavya Clement

Tae Geol Lee

Geol Lee

Kun Liu

Sébastien Hentz

Christophe D Masselon
email: christophe.masselon@cea.fr

Characterizing Nanoparticle Mass Distributions Using Charge-Independent Nanoresonator Mass Spectrometry

Keywords: Nanoelectromechanical sensor, Mass spectrometry, Nanoparticle, Single-particle characterization, Intact mass Nanoparticle, NEMS: Nano-Electro-Mechanical Sensor, MS: Mass Spectrometry, TEM: Transmission Electron, SEM: Transmission Electron Microscopy, DMA: Differential Mobility Analyzer, NTP: (Gold) Nanotetrapods, PEG: Polyethylene Glycol, NMR: Nuclear Magnetic Resonance, DLS: Dynamic Light Scattering, NTA: Nanoparticle Tracking Analysis, SAXS: Small Angle X-Ray Scattering, AFM: Atomic Force Microscopy, PMD: Particle Mass Distribution, ICP: Inductively Coupled Plasma, PSD: Particle Size Distribution, GPC: gel permeation chromatography

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Thanks to their tunable physical, chemical, and biological properties, nanoparticles (NPs) have attracted tremendous interest in industrial as well as biomedical applications. [START_REF] Jeevanandam | Review on Nanoparticles and Nanostructured Materials: History, Sources, Toxicity and Regulations[END_REF] As an example, synthetic gold NPs have been widely used in industry and academia in sensing, [START_REF] Zhu | Gold Nanoparticle Based Biosensors[END_REF] electron microscopy, [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] materials science, [START_REF] De Cicco | Use of Nanoparticles for Enhancing the Interlaminar Properties of Fiber-Reinforced Composites and Adhesively Bonded Joints-a Review[END_REF] and biomedical research applications. [START_REF] Cardoso | Advances in Magnetic Nanoparticles for Biomedical Applications[END_REF] Likewise, non-metallic particles such as silica NPs have widespread applications in polishing and as additives to drugs, cosmetics, printer toners, and foodstuffs. [START_REF] Katz | Nanotechnology in Cosmetics[END_REF][START_REF] Peters | Nanomaterials for Products and Application in Agriculture, Feed and Food[END_REF] The rapid increase in production of synthetic NPs has been accompanied by inevitable issues related to concerns surrounding environmental contamination and human toxicity. [START_REF] Jeevanandam | Review on Nanoparticles and Nanostructured Materials: History, Sources, Toxicity and Regulations[END_REF][START_REF] Jia | The in Vitro and in Vivo Toxicity of Gold Nanoparticles[END_REF] Therefore, it is particularly important that appropriate scientific tools be developed to characterize both synthetic and natural NPs.

As nanomaterials exhibit size-dependent properties that determine their physical characteristics and how they interact with their environment, size is considered the critical parameter directly related to their practical applications. [START_REF] Mahmoudi | The Need for Robust Characterization of Nanomaterials for Nanomedicine Applications[END_REF] Therefore, several technologies have been used to characterize NPs, including light scattering approaches (e.g., DLS, nanoparticle tracking analysis, small-angle x-ray scattering) [START_REF] Carvalho | Application of Light Scattering Techniques to Nanoparticle Characterization and Development[END_REF] and imaging technologies (e.g., atomic force microscopy, SEM, TEM) [START_REF] Mourdikoudis | Characterization Techniques for Nanoparticles: Comparison and Complementarity upon Studying Nanoparticle Properties[END_REF] as a means to estimate size or glean morphological details. In practice, the average size of an NP sample is obtained by measuring either a group of particles (i.e., ensemble measurement), or a large number of individual particles and determining a distribution (i.e., single-particle measurement). Thus, particle size is always represented by a distribution of values, which can be fitted to a probability distribution providing an average particle size and its associated uncertainty, combining instrumental error and the intrinsic size distribution of the sample. Unfortunately, most of the methods used to characterize NPs have specific limitations due to limited descriptors (1D-or 2D-rendition of 3D particles), or to an inability to characterize particles that are either non-spherical or of heterogeneous composition. Consequently, different methods applied to the same sample often provide discordant size descriptions. For instance, characterization of NIST 10-nm gold nanoparticles yielded sizes ranging from 8.5 ± 0.3 nm to 11.3 ± 0.1 nm depending on the method used. 12 These discrepancies may result from various biases in size estimations due to lateral resolution, and/or from invalid sphericity assumptions.

In an attempt to overcome these difficulties, particle mass distribution (PMD) can be used in complement to other NP characterization methods. Mass is a fundamental parameter that is used to identify and classify molecules as well as particles. Unlike size determination, mass measurements explicitly account for the three-dimensional nature of NPs and their associated density, even when these characteristics are not uniform across the whole particle, such as in the case of hollow or composite nanoparticles. [START_REF] Olesik | Considerations for Measurement of Individual Nanoparticles or Microparticles by ICP-MS: Determination of the Number of Particles and the Analyte Mass in Each Particle[END_REF] Mass spectrometry (MS) is a powerful tool used in many fields to weigh molecules and characterize their structures through fragmentation. In 2016, Zhang et al. [START_REF] Zhang | Mass Measurement of Single Intact Nanoparticles in a Cylindrical Ion Trap[END_REF] reported mass measurements of individual NPs using a specially designed ion-quadrupole-ion trap combined with particle detection by light scattering microscopy. In their setup, particles were irradiated by a UV lamp to induce charge variation through photoelectric effects, and the mass-to-charge ratio (m/z) and the number of elementary charges (z) carried by individual NPs were repeatedly determined. Although promising, the method suffered from inadequate statistics as each particle was monitored multiple times, and from a complex mass determination scheme based on the Mathieu equation. In addition, the authors reported extensive aggregation issues. To measure the mass of intact macromolecular ions, the leading technology is charge detection mass spectrometry (CDMS). [START_REF] Keifer | Charge Detection Mass Spectrometry: Weighing Heavier Things[END_REF] Like Zhang et al. [START_REF] Zhang | Mass Measurement of Single Intact Nanoparticles in a Cylindrical Ion Trap[END_REF] , CDMS also measures m/z and z for individual particles in order to derive their mass. However, for NP analysis -especially for particles that ionize poorly -the reliance of CDMS on ionization limits its applicability. Alternatively, commercially available inductively coupled plasma mass spectrometry (ICP-MS) operated in single-particle mode can be used to determine the elemental composition of NPs by quantifying specific atomic ions. [START_REF] Scheffer | ICP-MS as a New Tool for the Determination of Gold Nanoparticles in Bioanalytical Applications[END_REF] Analytes are ionized in plasma and dissociated into elementary ions, which are sorted based on their atomic masses. Therefore, the weight of individual particles can be determined following appropriate quantitative calibration of ion intensities, and particle size derived if the corresponding density is known. Nevertheless, several types of NP are not amenable to ICP-MS due to its intrinsic low sensitivity for some elementary ions (e.g., C, Si). Hence, ICP-MS analysis of silica NP, as well as organic or polymer-coated inorganic NPs present intractable challenges.

A new type of mass spectrometry technology based on nano-electromechanical sensors (NEMS) is emerging that allows charge-independent mass analysis in the mega-to giga-Dalton (~attogram to femtogram) range. [START_REF] Dominguez-Medina | Neutral Mass Spectrometry of Virus Capsids above 100 Megadaltons with Nanomechanical Resonators[END_REF] This technology has specific advantages over ICP-MS and CDMS, such as its ability to characterize the inertial mass of intact individual particles, regardless of their elemental composition or ionization efficiency. In the study presented here, we investigated the usefulness of NEMS-based mass spectrometry for NP characterization. For this purpose, we analyzed various NP standards, such as synthetic sphere-like gold NPs and silica NPs with characteristic sizes ranging from ~15 to 50 nm in diameter. Furthermore, we measured the PMD of non-spherical gold nanotetrapods (NTPs), which are impossible to adequately describe using a single-dimensional parameter. Our results demonstrate the potential of inertial mass measurement of intact NPs using NEMS-MS as a complement to size and morphological characterization.

Results and Discussion

Particle size distribution (PSD) is one of the key factors revealing the relationship between NP size and physical properties. [START_REF] Caputo | Measuring Particle Size Distribution of Nanoparticle Enabled Medicinal Products, the Joint View of EUNCL and NCI-NCL. A Step by Step Approach Combining Orthogonal Measurements with Increasing Complexity[END_REF] Currently, it is also the major parameter used to evaluate the polydispersity of NPs. However, using a 1D descriptor such as the diameter to represent 3D structures, especially for non-spherical and irregularly shaped particles, can present intractable challenges. An alternative complementary strategy is to measure the PMD. Nanomechanical mass sensors have been used in the past to characterize individual NPs [START_REF] Dohn | Mass and Position Determination of Attached Particles on Cantilever Based Mass Sensors[END_REF] , but the capacity to directly derive NP inertial mass distributions using NEMS-based MS has only recently become available. 17

Benchmarking NEMS mass measurements with sphere-like gold NPs

To benchmark the performance of NEMS-MS for PMD determination, we first analyzed commercial 15-nm and 30-nm sphere-like gold NPs with masses anticipated to lie in the 15-20 MDa and 100-250 MDa ranges, respectively. Results were plotted and are summarized in Figure 1 and Table 1,Error! Reference source not found. respectively. The inset in Figure 1a shows the mass distribution for 15-nm gold NPs based on a series of measurements made under the same operating conditions, repeated on multiple non-consecutive or consecutive days over a week. Importantly, the measurement on the last day was performed after changing the nanoresonator array. The number of particles detected in these measurements ranged from ~400 to 1300 depending on the total duration of the experiment. As illustrated by the box and violin plots, only minor differences were found across days and arrays. Slight day-today variations could be attributed to uncontrolled variations in sampling, aggregation, ESI conditions, or partially incomplete desolvation. When these four populations were fitted to a Gaussian distribution model using non-linear least-squares fitting, variations in the average and standard deviation did not exceed 1.3 MDa and 0.5 MDa, respectively. Notably, taking these measurements into account, the standard uncertainty for the 15-nm gold PMD (~0.3 MDa) corresponded to a ±0.08-nm deviation in diameter for perfectly spherical particles (see Figure 1b). This level of measurement uncertainty compares favorably with current size-characterization technologies such as TEM. [START_REF] Mourdikoudis | Characterization Techniques for Nanoparticles: Comparison and Complementarity upon Studying Nanoparticle Properties[END_REF] However, it should be noted that the uncertainty with NEMS measurements is density-dependent, and in this particular case is very low due to the high density of gold. The limit of detection (LOD) of our NEMS devices, which was previously estimated from the resonator's quality factor and frequency noise level, is approximately 0.5 MDa. [START_REF] Clement | Requirements and Attributes of Nano-Resonator Mass Spectrometry for the Analysis of Intact Viral Particles[END_REF] The mass uncertainty over multiple measurements was close to the LOD. These results indicate the stability and consistency of NEMS-MS measurements over several days or when using distinct arrays.

A spectrum consisting of 3,437 mass detection events was obtained by combining the four datasets from 15-nm gold NP measurements. This combined spectrum improved precision due to its greater number of points, and thus more robust statistics. The overall mean mass was 18.0 MDa with a standard deviation of 5.7 MDa (Figure 1a). Incidentally, this happens to be the smallest mass standard reported using our current NEMS-MS system design. Using the density of gold (19.3 g/cm 3 ) and applying the spherical particle hypothesis, a mean diameter of 14.4 nm was calculated. This value was in excellent agreement with the average size reported from TEM images (14.2 nm).

Figure 1 -Overlayed mass distributions for two commercial gold nanoparticle samples (a) and overlayed size distributions derived from the mass distributions, assuming uniform density and perfectly spherical shape (b). The inset in (a) shows the results from repeated measurements of the 15-nm gold NP sample on multiple consecutive and non-consecutive days over a week, with two distinct nanoresonator arrays.

We performed similar analyses on 30-nm sphere-like gold NPs to assess the upper mass range (> 150 MDa). For these NPs, 653 individual particles were recorded, the mass determined, 157.0 MDa, corresponded to a computed average diameter of 29.6 nm based on the density of gold and the hypothesis that the particles are perfectly spherical (Figure 1b). In line with the results for the 15-nm NP, the average size of the 30-nm gold NPs computed from mass measurements was about 1.4% larger than the TEM-based size indicated by the supplier. All these measurements of sphere-like gold NP standards substantiate the suitability of NEMS-MS for the characterization of intact NPs using mass measurement.

Density characterization of amorphous silica nanoparticles

Korea Research Institute of Standards and Science (KRISS) manufactured a series of sizecontrolled spherical silica nanoparticles (diameter ranging from 20 to 90 nm) displaying narrow size distributions (standard deviation < 3.2 nm). [START_REF] Kim | Corrigendum: A Reliable Approach for Assessing Size-Dependent Effects of Silica Nanoparticles on Cellular Internalization Behavior and Cytotoxic Mechanisms[END_REF] These particles have been thoroughly characterized using a broad variety of methodologies, and constitute a suitable alternative to polystyrene nanospheres as standards for instrument calibration. Although the size of these particles could be readily determined using (e.g.) electron microscopy, other physical properties such as density and weight are more challenging to access, and application of several methods produced non-convergent estimates (Certificated Reference Material: 301-04-001).

Here, we investigated the density of ~50 nm SiO2 NPs using electron microscopy and NEMS-MS technology to measure size and mass, respectively. Size measurements according to TEM (FEI Tecnai F-30) and SEM (FE-SEM; HITACHI S-4800) analysis yielded diameters of 51.9 ±1.2 nm and 53.8 ±0.8 nm, respectively. The distribution of mass measurements acquired by NEMS-MS could be fitted by a Gaussian peak (Figure 2a). The resulting mean mass and standard uncertainty were 87.9 ±0.7 MDa (Table S2.1) and the standard deviation of the PMD was 11.1 MDa. From these assessments, we computed average densities of 1.79 and 1.99 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] , depending on the method used to determine size (Figure 2b). SiO2 exists in a large variety of crystalline forms with density values ranging from ~ 2.27 to 2.65 g/cm 3 (Handbook of Chemistry and Physics Ed 102 Online). Considering the uncertainties associated with the size and mass estimations, the density of the NPs analyzed here could be as high as 2.16 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] (based on size measured by TEM), which is still lower than that of the least dense crystalline SiO2 forms. As shown in Figure 2b, the contribution of mass uncertainty to density estimates was minor compared to the contribution of size uncertainties (Table S2.2 and Table S2.3). It has been reported that silica NPs appear ~3% smaller in TEM than in SEM due to their poor edge contrast. [START_REF] Tuoriniemi | Intermethod Comparison of the Particle Size Distributions of Colloidal Silica Nanoparticles[END_REF] Our observations appear consistent with these reports. This supports the range determined from SEM measurements (1.79 ±0.12 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] ). Although TEM and SEM results tended to diverge, there was a narrow size range (53.0-53.1 nm) over which estimates from the two methods overlapped. Based on these results, the density of silica NPs is certainly lower than that of SiO2 crystalline forms, and is most likely in the range of 1.83 to 1.91 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] . Interestingly, a low average density of ~1.87 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] was previously reported based on differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) measurements of silica NP size standards (40-200 nm) synthesized by a sol-gel process. [START_REF] Kimoto | Characterization of Nanosized Silica Size Standards[END_REF] Our results were in very good agreement with this density, which provides further evidence that silica NPs produced by sol-gel processes have an amorphous nature, with a corresponding lower density than crystalline forms -according to the Handbook of Chemistry and Physics (Ed 102 Online), the density of amorphous silica is in a range between 1.9 and 2.2 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] . These results illustrate the relevance of NEMS-MS mass measurements to derive important properties of NPs that cannot be determined by ICP-MS (as is the case for silica NP).

Mass characterization of non-spherical gold nanotetrapods

Nanobranched gold particles have drawn great interest for various biological and biomedical applications due to their excellent NIR plasmon resonance properties. [START_REF] Cheng | Construction and Validation of Nano Gold Tripods for Molecular Imaging of Living Subjects[END_REF] For instance, monodisperse gold nanotetrapods (NTPs) with tunable and ultra-narrow plasmonic bands were produced using a facile seed-mediated growth method. Upon laser irradiation, these PEGylated NTPs possess remarkable photothermal conversion efficiencies and photoacoustic imaging properties. [START_REF] Chang | Gold Nanotetrapods with Unique Topological Structure and Ultranarrow Plasmonic Band as Multifunctional Therapeutic Agents[END_REF] Although transmission electron microscopy (TEM) can be used to monitor the size and shape of synthesized NTPs, it has been difficult to identify appropriate statistical descriptors to characterize the heterogeneity of these non-spherical particles. In addition, it is not currently feasible to quantify the grafting of polymer chains (e.g., PEG) onto individual NTP through size estimations. In this context, a method to determine the inertial mass of PEGylated NTPs seems particularly appealing.

As shown above, NEMS-MS technology demonstrated its ability to analyze monodisperse spherelike NPs, however, polydisperse particles or NPs with a non-unitary aspect ratio constitute distinct challenges. To investigate this aspect, we determined the PMD for monodisperse single-crystalline gold NTPs with unique topological structures (Figure 3a). A growth mechanism based on kinetically-controlled deposition and diffusion of adatoms was used to produce gold NTPs with ~85% purity. [START_REF] Chang | Gold Nanotetrapods with Unique Topological Structure and Ultranarrow Plasmonic Band as Multifunctional Therapeutic Agents[END_REF] PEGylation of NTPs has been shown to significantly improve their thermal and colloidal stability, consequently, the NTPs analyzed here were PEGylated by addition of monomethoxy-poly(ethylene glycol)-thiol (mPEG-SH).

Based on TEM imaging results, a non-negligible proportion (up to 13%) of the particles produced appeared to be potentially defective (e.g., irregularly shaped, missing arms), or to be pseudospherical (Figure 3b). Unfortunately, these minor species remain challenging to characterize and quantify because of the difficulty in using 2D morphology to assess underlying 3D structures. For instance, a 3-armed structure could appear 2-armed in the image if the third arm lies perpendicular to the imaging plane. Likewise, a bona fide tetrapod could be mistaken for a 3armed structure because of a hidden arm.

Faced with these uncertainties, measurement of the mass of intact NPs is complementary to imaging, in that it reveals the true polydispersity of the samples. Figure 3c shows the PMD of PEGylated gold NTPs using NEMS-MS technology, with a Gaussian fit over its major mode. An asymmetric distribution extending over the mass range 40-68 MDa was observed, featuring a minor mode at about 44 MDa and a major mode at ~57 MDa. Interestingly, the right side of the PMD displayed an abrupt edge, whereas the aspect on the left side was more irregular, indicating the presence of incomplete structures. In addition, low-abundance signals in the ranges 20-40 MDa and 100-130 MDa were most likely the result of damaged NPs and multimers, respectively.

The average volume of a gold NTP can be computed based on the size of its core particle, and the number, length, and width of its arms (See Figure S3). TEM analysis revealed that fully formed NTPs had an average arm length of 22.2 ± 0.6 nm and width of 8.0 ± 0.5 nm (Figure 3a). Furthermore, nuclear magnetic resonance (NMR) measurements using a grafting density analysis approach (Figure S4 and Table S4) revealed an average of ~1954±25 PEG chains on each gold NTP. [START_REF] Lu | A Non-Sacrificial Method for the Quantification of Poly(Ethylene Glycol) Grafting Density on Gold Nanoparticles for Applications in Nanomedicine[END_REF] Based on its average molecular weight (5000 g/mol), the total mass of PEG on a gold NTP is thus 9.8±0.2 MDa. As a result, the intact mass of a PEGylated gold NTP could be anticipated to lie between 54 and 60 MDa.

Taking into account all these considerations, we computed the theoretical volume, mass, and fractional amounts for a variety of apparent particle shapes, yielding mass estimates in the range 44.3 to 49.8 MDa. (See Table 2 and Supporting Information S3). The mass computed for the gold content of integral NTPs was close to the mass (48.5±1.0 MDa) determined from ICP-MS measurement (iCAP Q, Thermo Scientific) of the number of gold atoms in individual particles (Table S5).

We also confirmed the thermal stability of PEG chains on a gold NTP by thermogravimetric analysis. Unlike citrate-capping ligands, thiol-terminated PEG chains form strong covalent S-Au bonds that are stable up to 250 °C. [START_REF] Lu | A Non-Sacrificial Method for the Quantification of Poly(Ethylene Glycol) Grafting Density on Gold Nanoparticles for Applications in Nanomedicine[END_REF] In this study, the heated capillary temperature was set to 200 °C to allow desolvation of nebulized particles from the NTP solution. Thus, we expected NEMS-MS to measure an intact mass for NTP surrounded by PEG chains.

The results held out this expectation as the dominant peak at ~57 MDa agreed well with the intact mass of the NTP structure -as determined by ICP-MS -incremented by the number of PEG sidechains. Moreover, an estimate of the proportion of integral gold NTPs from NEMS-MS measurements was about 88% (taking into account the particle mean mass ± 2 standard deviations). This proportion is close to the relative fraction derived from TEM observations. It is also interesting to note that the calculations of the dominant side product masses converged to a value ranging from 43 to 48 MDa. Although it remains difficult to distinguish between 3-armed, 1-armed, or immature irregularly shaped particles, the coincidence in mass suggests a kineticallycontrolled pathway in which mass growth continues regardless of shape. Considering the possible volume bias of the particle due to the TEM analysis and the mass deviations of the number of PEG chains on each NP, a precision within 2 MDa (~3%) between predicted mass, ICP-MS result, and NEMS-MS can be considered quite comparable. In summary, NEMS-MS technology not only characterized the PMD of non-spherical gold NTP with PEG side-chains, but also reflected the presence of side products, which is still a challenge when using other technologies.

Conclusion

The aim of this study was to determine the capacity of NEMS-MS to caracterize NPs, whatever their shape and size. The characterization of sphere-like gold NPs of two different diameters (15 nm and 30 nm) using NEMS-MS technology returned particle masses ranging between 5 MDa and 250 MDa. Measurements of 15-nm gold NPs across experiments performed on different days and with distinct NEMS arrays showed good consistency. Based on the spherical shape hypothesis and the isotropic density of gold, the NP size computed from the NEMS-MS result deviated by a maximum of 1.4% from the value provided by the manufacturer. The density of silica NPs was also characterized by combining size investigations (from SEM and TEM images) and NEMS-MS mass measurements. The average density determined from these measurements was about 1.9 g/cm [START_REF] Kammers | Self-Assembled Nanoparticle Surface Patterning for Improved Digital Image Correlation in a Scanning Electron Microscope[END_REF] , which is similar to the value reported previously based on DMA and APM analyses. Finally, the PMD determined for non-spherical gold nanotetrapods (NTPs) not only confirmed the approximate number of PEG chains coating particles (as estimated by NMR) but also revealed the polydispersity of these particles. Both of these figures of merit are difficult to efficiently and precisely evaluate based on size descriptors alone. In summary, this first assessment of the NEMS-MS technology demonstrated that it is suitable for characterizing NPs nebulized from a condensed phase to the gas phase via the process of electrospray ionization (ESI). This novel addition to the nano-characterization toolbox can be applied in a straightforward manner to determine the PMD of non-spherical or heterogeneous NPs, including soot particles, or hybrid NPs.

Material and methods

Material and sample preparation

Gold nanoparticles measuring 15 and 30 nm in diameter (EM.GC20) suspended in citrate buffer were purchased from BBI Solutions (Crumlin, United Kingdom). They were diluted in 50% deionized water/methanol (Sigma Aldrich, Saint-Louis, Missouri, USA) to a final concentration of 7 × 10 11 and 1 × 10 11 particles/mL, respectively, before performing mass measurements. Amorphous silica nanoparticles (ASiNPs) measuring about 50 nm diameter were provided by the Korea Research Institute of Standards and Science (KRISS) [START_REF] Kim | Corrigendum: A Reliable Approach for Assessing Size-Dependent Effects of Silica Nanoparticles on Cellular Internalization Behavior and Cytotoxic Mechanisms[END_REF] . They were implemented using methods modified from the protocols proposed by Hartlen et al. [START_REF] Hartlen | Facile Preparation of Highly Monodisperse Small Silica Spheres (15 to >200 Nm) Suitable for Colloidal Templating and Formation of Ordered Arrays[END_REF] The solution was first passed through a 0.22-micron syringe filter and then diluted 100-fold with double-distilled water to a final concentration of about 5 × 10 11 particles/mL. Gold nanotetrapods (NTPs) were synthesized according to the method reported previously. [START_REF] Chang | Gold Nanotetrapods with Unique Topological Structure and Ultranarrow Plasmonic Band as Multifunctional Therapeutic Agents[END_REF] Freshly prepared gold NTPs were centrifuged and re-dispersed in an aqueous mPEG-SH (JenKem Technology Ltd.) solution (0.10 mg/mL). The average Mw of mPEG-SH was 5000 g/mol, with a polydispersity of less than 1.02 as determined by gel permeation chromatography (GPC). After ligand exchange, free PEG was removed by centrifugation, and the sample was lyophilized to remove water. Lyophilized mPEG-NTPs were resuspended in 50% water-ethanol to a final concentration of about 5×10 11 particles/mL before performing mass measurements. Nebulized droplets containing particles then pass through a heated capillary (175-200 °C) where desolvation occurs. The particle beam is subsequently focused through an aerodynamic lens onto an array of nanomechanical sensors. The aerodynamic lens is composed of a pressure-limiting orifice followed by a series of specifically calibrated apertures and expansion chambers. The lens is designed to take advantage of a trade-off between particle inertia and Brownian diffusion. The specifications include geometric parameters (spacers between orifices, orifice diameters) and the inlet and outlet pressures (Figure S7). The parameters for our architecture were optimized for 100-nm diameter particles with close to unit density, in line with the guidelines presented by Wang et al. [START_REF] Wang | Aerodynamic Focusing of Nanoparticles: I. Guidelines for Designing Aerodynamic Lenses for Nanoparticles[END_REF] . As reported in a previous communication, the focusing system produces a ~1.5-mm crosssection beam at the chip position.

NEMS-MS architecture

Nano-resonators and mass calculation

The crux of the NEMS-MS architecture is the sensitive element used to measure the mass of incoming nanoparticles. In the current implementation, an electrostatically-actuated nanoresonator detects mass addition events in real-time. Thus, when a single particle lands on a nanoresonator's beam at a time 𝑡, the added mass causes the resonance frequency 𝑓 𝑛 to shift down by ∆𝑓 𝑛 . The relative resonance frequency shift ∆𝑓 𝑛 /𝑓 𝑛 depends both on the added Δ𝑚 and on the landing position 𝑥, as expressed in the following equation:

Δ𝑚 = 2𝑀 Δ𝑓 𝑛 𝑓 𝑛 𝛼 𝑛 𝜙 𝑛 2 (𝑥)
Where 𝑀 is the mass of the resonator beam, the function 𝜙 𝑛 (𝑥) is the mode shape of the 𝑛-th mode, and 𝛼 𝑛 is a constant 𝛼 𝑛 = -2 ∫ 𝜙 𝑛 2 (𝑥) 𝑥=1 𝑥=0

𝑑𝑥. This equation provides a single relationship between two unknowns: mass and position. Thus, to determine the added mass, the beam must be actuated with the two first modes 𝑛 = 1,2 and the frequency shifts tracked for both. A 20nanoresonator array was used to increase the detection cross-section. The resonance frequency of each resonator was tuned by varying its length. Resonators in the array ranged from 7.61 µm to 10 µm in length. They were 160 nm thick and 300 nm wide (Figure S8).

In previous work, the phenomena that influence mass measurements were listed. These include frequency noise and fabrication defects [START_REF] Clement | Requirements and Attributes of Nano-Resonator Mass Spectrometry for the Analysis of Intact Viral Particles[END_REF] . The influence of frequency noise is characterized, and a calibration method has been proposed to correct for fabrication defects. With the current design, the lower mass LOD was approximately 5 MDa, and the overall mass uncertainty was conservatively estimated to be within ± 0.5 MDa.

When frequency shifts are translated into the mass-position domain, a mass spectrum can be plotted. However, gold and silica nanoparticles were also characterized by average diameter and the standard deviation of their distribution. Thus, the mass distributions were converted into diameter distributions by computing the mass equivalent diameter for each particle, assuming the density of gold to be 1.93 × 10 4 kg/m 3 .

Figure 2 -

 2 Figure 2 -NEMS-MS derived mass distribution of ~50 nm silicon dioxide nanoparticles and inset showing the derived size distribution assuming a density of 1.9 g/cm 3 (a). Comparison of average density values and standard errors derived from NEMS-MS mass and electron microscopy (TEM or SEM) size measurements (b). (N: number of acquired events,  location, : standard deviation of Gaussian fit)

Figure 3 (

 3 Figure 3 (a) Three-dimensional representation of a gold nanotetrapod (yellow) surrounded by PEG chains (blue fibers). (b) TEM image of gold nanotetrapods. Scale bar: 200 nm. (c) PMD of gold nanotetrapods according to NEMS-MS measurements. Data were binned to 1 MDa when plotting the mass distribution histogram which was fitted with a Gaussian distribution curve.

A

  schematic diagram of the NEMS-MS system is shown in Figure S6. It is composed of three successive stages: first, a nanoelectrospray ionization (nanoESI) source is used to generate airborne particles from a colloidal solution. The liquid sample flows through a 30-µm i.d. fused silica tip emitter (FS360-50-30-N-20-C12, PicoTipTM), and then a 2290E-5 Tektronix HV power supply (Tektronix Inc., Beaverton, United-States) nebulizes the flow by applying high voltages (2-4 kV).

Table 1 -

 1 NEMS-MS-deruved diameter and mass compared to supplier's specifications.

	Supplier specifications (TEM)	NEMS-MS	Relative differences
	Measured mean	Computed	Computed	Measured mean	Diameter	Mass
	diameter ± S.D.	mean mass*	mean diameter	mass
			± S.D.
	(nm)	(MDa)	(nm)	(MDa)	%	%
	14.2±0.9	17.4	14.4±1.5	18.0	1.4	2.9
	29.2±2.1	151.7	29.6±1.6	157.0	1.4	3.5
	* Using the density of gold (19.3 g•cm -3 ) and assuming perfect sphericity.

Table 2 -

 2 Mass estimation of integral gold NTP and possible side products. -Computed particle volume, mass, and proportions for a variety of apparent particle shapes.

	Shape	Integral	2 or 3 arms	1-arm	Sphere	Immature
		NTPs	3 arms	2 arms	(big head)	(15.1 nm diameter)	(irregular)
	Volume (nm 3 )	3804 -	2985 -	2166 -	3392	1803	3349
		4283	3464	2645
	Surface area (nm 2 )	2101	~1737	~1265	~1536	716	~1072
	Mass gold atoms (MDa)	44.3 -	34.8 -40.3 25.3 -30.8	39.5	21.0	39.0
		49.8*
	Mass PEG chains (MDa)	9.8	8.1	5.9	6.7	3.3	5.0
	Mass whole particle	54.1-59.6	42.9 -48.4 31.2 -36.7	46.2	24.3	44.0
	(MDa)
	Relative fraction (%) #	87.1	4.7		2.0	1.6	4.7

*48.5 MDa according to ICP-MS measurements # Based on TEM result for 255 particles (human evaluation)

Acknowledgments

The authors acknowledge support from the European Union through the ERC project Enlightened (Grant agreement # 616251), the LETI Carnot Institute NEMS-MS project, and the DGA Astrid NEMS-MS project. We thank M. Sansa for helpful discussions, and V. Brun for her feedback and support.

Funding

This work was funded in part by the European Union through the ERC Enlightened project (GA #616251) and by CEA PTC Instrumentation/Detection (VIA-NEMS). In addition, K.L. thanks the National Natural Science Foundation of China (21975094) for financial support and the Jilin Province International Collaboration Base of Science and Technology (YDZJ202102CXJD004).

Supporting Information Available:

Repeatability study data, Silica density characterization, NTP (NanoTetraPods) mass calculation, NMR-derived mass of grafted PEG on NTPs, NTP mass measured by ICP-MS, NEMS-MS architecture and NEMS particle sensor, information on aerodynamic focusing.

Declaration on conflict of interest

C.M. and S.H. are co-inventors of patents US9506852B2, EP2779209A1, and JP6352004B2.


Elena Solana-Madruga

Olivier Mentré

Eugenia P Arévalo-López

Dmitry Khalyavin

Francois Fauth

Alexandr Missiul

Angel M Arévalo-López
email: angel.arevalo-lopez@univ-lille.fr.b.dpto.

High-pressure ilmenite-type MnVO 3 : crystal and spin structures in the itinerant-localized regimes

come

Introduction

Transition metal oxides with ABO3 stoichiometry are intensively studied due to their chemical, structural and functional versatility. A wide variety of crystal structures accepting different degrees of distortion can be stabilized, mainly depending on the relative ionic radii of A-and B-site cations, as defined by the Goldschmidt relation. 1 Synthesis conditions can also induce notable effects on the preferred crystallization of the compounds with different structures. In particular, high pressure and high temperature synthesis provides accessibility to metastable compounds with unusual oxidation states and metal coordinations. Small cations can also be stabilised into large voids. 2,3 This is the case, for instance, of A-site manganites, where perovskite and corundum related structures are reported with a rich variety of cation ordered superstructures. 4 Two high pressure polymorphs of MnVO3 were first reported in 1970. The initial structural characterization revealed a triclinic distortion 1 of an ilmenite-type structure (MnVO3-I) and an orthorhombic (Pnma) perovskite (MnVO3-II) for the low-and the high-pressure phases below and above 4 GPa respectively. 5 It was not until 2011 when MnVO3-II, the only quenchable simple perovskite with Mn 2+ in the A site, was fully understood.

It showed a complex behaviour with spin polarized conduction electrons and an incommensurate magnetic structure in a halfmetallic state. 6 MnVO3-I however, has not been fully understood. It belongs to the AVO3 with A = Mg, Mn, Co, Ni and Cu high-pressure triclinic ilmenites. 7,8,9 MgVO3 and CoVO3 exhibit a ladder-like V-V dimerization and non-magnetic singlet ground states with a concomitant triclinic to rhombohedral phase transformations at 500 and 550 K respectively. 10,11 NiVO3 and CuVO3 have not been fully studied. 9,12,13 MnVO3-I has been recently obtained in single-crystal form from a solid-state recrystallization under high-pressure and hightemperature conditions. 14 The ladder-like V -V dimerization pattern is also observed in this phase. Here we present a complete characterization of bulk MnVO3-I, including the thermal evolution of its physical properties and ground magnetic structure. A comparative study against the related MgVO3 and CoVO3 is performed to discuss the observed behaviour. Unprecedented magnetic order into the V sublattice is observed despite the presence of V -V dimers, which is justified in terms of a combination of correlation-(Mott) and band-effects (Peierls) in the low temperature triclinic semiconducting phase against the metallic-like high temperature rhombohedral polymorph, similar to the VO2 scenario. [15][16][17][18][19]

Experimental

Pollycrystalline MnVO3-I was obtained at 4 GPa and 1273 K for 30 minutes using a Walker-type multianvil press, further details concerning this technique are given in the SI. Stoichiometric amounts of MnO (99.9% Sigma-Aldrich) and VO2 were placed in a Pt capsule and into the press. VO2 was prepared from a 1:1 V2O3:V2O5 mixture in vacuum at 700 ºC for 24 h, and V2O3 was previously obtained via reduction of V2O5 (99.9% Sigma-Aldrich) under hydrogen at 700 ºC. Synchrotron X-ray radiation powder diffraction (SXRPD) was performed at the BL04 MSPD beamline of ALBA synchrotron in Spain. Data were collected using a λ = 0.4142 Å wavelength in the 0º < 2θ < 40º angular range with a 0.006º effective step size. [20][21] High resolution neutron powder diffraction (NPD) data were collected in a combination of several high-pressure experiment runs (63 mg in total) in the WISH diffractometer at the ISIS facility. 22 Long scans were obtained at 2 K and 100 K and shorter scans were collected every 3 K on warming. Rietveld refinements were performed using Fullprof Suite. 23 Single-crystal data from isolated small crystals was collected on a Bruker DUO diffractometer (Mo Kα radiation) and treated using the JANA 2006 software. 24 X-ray photoelectron spectroscopy (XPS) analyses were carried out in an ultra-high vacuum (UHV) system Scanning XPS microprobe PHI 5000 VersaProbe II, with an Al Kα X-ray source (hν= 1486.6 eV) monochromatic with 200 µm beam diameter, and a Multi-Channel Detector (MCD) analyzer. EDX analysis were also performed in a FLEXSEM 1000, see SI for further details. Magnetic properties were measured using a PPMS 9T Dynacool from Quantum Design. Magnetization was measured in ZFC-FC mode under 0.1 T and hysteresis loops collected at several temperatures. Heat capacity was measured on an as-obtained pellet between 2 and 300 K and corrected with an addendum. Resistivity measurements were performed using a 4-probe system with a Keithley 6221 source and a 2182A Nanovoltmeter installed in a Linkam high temperature station.

Extended Hückel tight-binding calculations (EHTB) have been performed using the SAMOA package. [START_REF] Dai | SAMOA (Structure and Molecular Orbital Analyzer[END_REF] DFT+U calculations were performed with the general gradient approximation (GGA) using the popular Perdew-Burke-Ernzerhof (PBE) approach in the Vienna ab initio simulation package (VASP) [START_REF] Kresse | [END_REF] . Extra on-site electron-electron Coulomb interactions were also considered with the GGA+U approach implemented in the code. Crystal structures were optimized by minimizing the total energy, phonon dispersions calculations were not performed to test dynamical stability of the optimized structures. The lattices and atomic positions were relaxed starting from the 300 K and the 560 K MnVO3-I refined models, within a convergence force criterion of 3.10 -2 eV/Å. The relaxed lattice contraction ∆V/V0 are 6.5% and 4.9% for the rhombohedral and triclinic cells respectively, as commonly found for DFT. It was checked that the experimental low temperature triclinic phase is most stable by ∼40 meV per formula unit, compared to the rhombohedral form. For electronic calculations, we assume the most stable AFM state with U = 1 eV for Mn and V atoms. The electrons Mn, V (3s, 3p, 3d, 4s) and O (2s, 2p) were treated as valence states. The cutoff energy for the plane-wave-basis is 400 eV, and the total energy of the system was converged with respect to the plane-wave cutoff energy and reciprocal space samplings. The free-energy convergence criterion was set to10 -10 eV using a Γcentered 8 × 8 × 8 Monkhorst-Pack k-point mesh. A systematic revision of all possible spin configurations was not pursued, but can be performed in future theoretical studies aiming a further understanding of the transition mechanism.

Results and discussion

MnVO3-I was synthesized under high-pressure and high-temperature conditions as described in the experimental section above. Small single crystals were successfully isolated for structural determination at room temperature (CCDC 2259432) in good agreement with the recently reported MnVO3 crystal structure. 14 The model obtained by single crystal (Table S1) was subsequently used in the Rietveld refinements of SXRPD and NPD data and results are summarized in SI. MnVO3-I crystallizes in the 1 space group with a = 5.0177(7) Å, b = 5.0513(7) Å, c = 5.5210(8) Å, α = 116.679(6) °, β = 90.044(6) ° and γ = 118.924(5) ° cell parameters at 300 K. The structure is shown in Figure 1a andb. It presents a distorted ilmenite-type structure, as the MgVO3 and CoVO3. 10,11 The low symmetry in MnVO3-I arises from the distortion of the Mn and V honeycomb lattices. Three different V -V lengths are observed, two long (3.025(1) Å and 3.00(1) Å) and one short (2.849(1) Å) at 300 K. The short length of the V -V pair is still comparable to those of the related MgVO3 (~2.78 Å) and CoVO3 (~2.74 Å). This agrees with the ionic radius of the different A cations, 0.72, 0.745 and 0.83 Å for Mg, Co and Mn respectively. Moreover, the short V -V bond is below the critical interatomic distance (Rc ~2.94 Å) for which direct metal -metal bond may be formed in vanadium oxides, 27,[START_REF] Goodenough | Magnetism and the chemical bond[END_REF] thus structural clues for V -V dimerization exist in MnVO3-I. The Mn sublattice shows no formation of dimerized features exhibiting a narrow distribution of Mn -Mn distances (3.065(1), 3.017(1) and 2.964(1) Å at 300 K). The calculated bond valence sum (BVS) [START_REF] Brown | [END_REF] results in ~ 2.08+ for Mn and ~ 3.47+ for V. This reflects the stressed structure obtained in a high-pressure metastable phase. EDX and XPS measurements respectively support the expected composition and formal oxidation states Mn 2+ V 4+ O3 2-, see SI. Figure 1d shows a small angular range for the thermal evolution of the SXRPD patterns between 300 and 600 K. A reversible first order structural phase transition from triclinic ( 1 ) to rhombohedral ( 3 ) ilmenite-type structure occurs at 475 K with a coexistence of both phases up to 525 K. Refined atomic positions confirm the presence of V -V dimers as revealed by interatomic distances in Figure 1e. In the rhombohedral phase the dimerization is vanished by symmetry, resulting in a V -V and Mn -Mn distances of 2.984(9) Å and 2.960(6) Å respectively at 560 K. The transformation matrix between the triclinic 1 and rhombohedral 3 polymorphs used in here is: The cell parameters and the volume (SI) exhibit a change at the transition and both structures coexist in the 475-525 K temperature range proving that it is of first order. Figure 1c shows the electrical resistivity measurements at high temperatures with a clear change from a semiconductor to a metal-like transition at 450 K on heating. However, one should be careful, since this could just mean that the Please do not adjust margins

(a',b',c')R = (a,b,c)T [1 0 1, 0 1 2, 0 0 3].
Please do not adjust margins carrier mobility is strongly affected by scattering with phonons and/or defects as temperature increases. It is however 25 K below the structural transition, suggesting that the electronic correlations (Mott) may need to be lost before the structural change can occur (Peierls). This somehow resembles the dynamical dimer state in MgVO3, where a difference of ~100 K between the magnetic and the structural transition was reported. 10 Moreover, MgVO3 shows a change from a Pauli-paramagnetic (metal) to a non-magnetic dimerized state (semiconductor). Thus, by analogy, suggesting the observed electronic transition at 450 K in MnVO3-I involves metallization. It is also similar to the metal-insulator transition in VO2, where the dimerization and the electron correlations break at the transition. 16 Low temperature dependent magnetization measurements for MnVO3-I in 0.01 T external field (Fig. 2a) show a ferrimagnetic transition at TN = 77(1) K. The Curie-Weiss fit of the inverse susceptibility above 250 K shows a paramagnetic moment of 5.7(1) µB / f.u., close to the theoretical value of 5.92 µB when considering only Mn 2+ . Weiss temperature of θW = -233(1) K demonstrates that antiferromagnetic spin-spin interactions are dominant but partially frustrated (|θ| / TN ≈ 3). Magnetic hysteresis loops below TN developed a weak ferromagnetic (WFM) component with a remnant magnetization of 0.07 μB at 2 K. Heat capacity measurements present a lambda-like peak at 77 K in agreement with the WFM transition, see Figure 2c. Assuming the vanadium atoms are in a dimerized singlet state and after the phonon contribution was subtracted, the magnetic entropy released of 11(1) J mol -1 K -1 accounts for 3/4 of the theoretical value S = Rln(2S+1) = 14.9 J mol -1 K -1 for MnVO3-I.

Mn3V2O8 minority phase presents a transition at 21 K, 30 which appears below the ferrimagnetic transition of MnVO3-I (TN = 77 K). Thus, the observed features in our experimental data are intrinsic to our sample. Refinements of neutron diffraction data consider the impurity phase, thus ensuring its presence does not affect the interpretation of the magnetic properties.

The temperature dependence of the MnVO3-I NPD data reveals the appearance of magnetic peaks below TN = 77 K, see SI. These extra reflections coincide with the crystal structure and can be indexed with propagation vector k0 = [0 0 0] (Γ point of the first Brillouin zone).

ISODISTORT was used to determine the possible magnetic structures and space group. [START_REF] Stokes | ISODISTORT[END_REF]32 The final Rietveld fit was obtained with Fullprof with the mΓ1 -irreducible representation with the triclinic 1 ' magnetic space group (2.6). The magnetic structure, depicted in Figure 3, can be described as AFM layers of Mn and V atoms. The refined ordered magnetic moments (Rmag = 2.71%, χ 2 = 1.49) at 2 K converge to 3.87(2) and 0.44(5) for Mn and V respectively. These are confined in the bc plane with refined components mMn = (0, 3.27(2), 3.85(1)) μB and mV = (0, 0.18(1), -0.17(1)) μB. Refinement of the magnetic structure considering only Mn moments was attempted, but resulted in fits of much lower quality (Rmag = 8.32%, χ 2 = 3.04), thus proving the necessity to include the vanadium contribution into the model. The slightly reduced moment values obtained for both sublattices reflect the discussed frustration in the system, previously observed from susceptibility data. Moreover, Mn and V spin sublattices are almost perpendicular, with a relative angle of 87.7(3) ° between them. The WFM component observed in the M vs H loops cannot be originated from antisymmetric Dzyaloshinskii-Moriya exchange interaction, since Mn (or V) atoms are centrosymmetric by neighboring pairs. Thus, one needs to consider the mΓ1 + irreducible representation which would result in a ferromagnetic component along the x direction. This may also imply a lower symmetry. However, due to its small value of 0.07 μB no attempts of refinement were pursued. The temperature variation of the Mn and V moments along with the fit to critical laws μ(T) = μ(0)[1-(T/TM) α ] β are shown in Figure 3c. They result in TM = 84.8(4) K and 31(2) K for Mn and V respectively, and α = 1.68 (24) and β = 0.35(3) shared. The critical exponent β is close to the theoretical value of β = 0.34 for a three-dimensional XY magnet, appropriate to S = 5/2 Mn 2+ and S = ½ V 4+ spins laying in the bc plane but connected along z by Mn -V super-exchanges. Independent transitions for Mn and V are in good agreement with low temperature features in our bulk magnetic susceptibility measurements. 5 The appearance of localized spins in the vanadium site of MnVO3-I is somehow unexpected when compared with the other well studied triclinic distorted ilmenite-type vanadates AVO3 (A = Mg and Co).

MgVO3, with a V -V dimer distance of ~2.78 Å, shows a non-magnetic nature due to the 3dxy orbital pairing below 500 K. 10 CoVO3, with a ~2.74 Å V -V distance, presents dimerized vanadium and is considered to be magnetic solely due to cobalt cations with a S = 3/2 state. 11 MnVO3-I has a ~2.85 Å V -V dimer bond, also below the critical distance for direct exchange (Rc ~ 2.94 Å). 30,[START_REF] Stokes | ISODISTORT[END_REF] Along with the metal-like -semiconductor transition observed at 450 K, it is thus expected, by analogy, to be magnetic exclusively due to the manganese cations. However, due to ionic radii size of the Mn these bonds are longer than for MgVO3 and CoVO3, as well as the stronger magnetic interactions between d 5 -d 1 Mn 2+ -V 4+ spins, results in low temperature localization of the V magnetic moments and the idea of a partial V -V dimerized state emerges. A resembling scenario appears in the GaV4O8, where besides the charge ordering behavior, a trimerized state is observed along with reduced spin localization at low temperature in the so-called "hybrid electrons". 33 From extended Hückel tight-binding calculations (EHTB) in distorted V6O24 honeycomb ring clusters isolated from AVO3 (A = Mg, Co and Mn) we can clearly observe a bonding molecular orbital formation for the Mg and Co compounds. However, this is not visible for MnVO3-I and it thus supports the conclusion that although there is a shortening in one of the V -V bonds, it is not sufficient to prevent the vanadium to magnetically order at low temperature. This implies that the Rc ~ 2.94 Å critical value for direct metal-metal bonding should be used as reference only. It also rises the question of whether the simple image of a Peierls-like transition is occurring in MnVO3-I, where the metallic-like state should be ruled by the vanadium cations without intervention of the manganese, like in the MnVO3-II perovskite analogue. 6 However, the observation of a first order structural transition at high-temperature and spin ordering in both Mn and V cations points toward an electronic correlation scenario. Besides these, the difference of 25 K between the metallike -semiconductor and the structural transitions may indicate all together that a combination of both effects is taking part. The transition is in fair agreement with the calculated density of states (DOS) for the high-(trigonal) and low-temperature (triclinic) MnVO3-I using our experimental models in the AFM (k = [0 0 0] state), see Figure 4b. The high spin local Mn 2+ is well observed by half- Please do not adjust margins Please do not adjust margins occupied t2g and eg 3d bands split by ~1 eV. They show large exchange splitting between the majority and minority spins (~3 eV) for both structures. The calculated moment of ~4 μB for Mn (U = 1 eV) suggest a partial delocalization and Mn-O valence effects of the Mn d electrons. Moreover, the majority V 4+ 3d bands (t2g orbitals) mainly participate to the upper valence and lower conduction bands, with weak Mn-O-V hybridization. In the low temperature structure, a narrow gap of 0.2 eV opens in the t2g manifold, in agreement with the semiconducting behavior observed. The V -V pairing accounts for the reduced localized moment on V site (∼0.66 µB/V). At hightemperature in the rhombohedral structure, the vanadium t2g manifold degenerates and crosses the Fermi level within a metal-like state. However, an important localized moment of ∼1 µB per V is calculated, which picture the main contribution of V atoms compared to O and Mn atoms in the narrow band crossing at the Fermi level. The different electronic behaviour of the triclinic and the trigonal phases show important coulombic effects, thus supporting the mixed scenario between electronic correlation and band effects discussed above, somehow similar to VO2. Further studies of the accurate electronic structure should clarify what is the exact mechanism of the transition in MnVO3-I. Moreover, the realization of new systems similar to VO2 is of current interest through the realization of thin film devices that may stabilize the high-pressure phase of MnVO3-I. 34,35

Conclusions

A metal-like -semiconductor transition was observed in the MnVO3-I high pressure phase at 450 K. This is accompanied by a 3 to 1 structural transition where a Peierls-like V -V bond shortening occurs resembling that of VO2. MnVO3-I orders antiferromagnetically at TN = 77(1) K with a small ferromagnetic component of 0.07 μB at 2 K. NPD reveals a k = [0 0 0] magnetic propagation vector. The magnetic structure exhibits Mn and V spin components of 3.87(2) and 0.44(5)μB respectively. Contrary to the MgVO3 and CoVO3 analogues, EHTB calculations highlight that the dimerization is not full in MnVO3-I and point towards a more complex electronic behaviour that may be clarified by RIXS experiments and more detailed calculations.

Figure 1 .

 1 Figure 1. a) Crystal structure of MnVO3-I viewed along the [110] in the rhombohedral high-temperature cell (green axes). b) Cobalt and vanadium honeycomb-layers viewed along the cT* direction of the triclinic (blue) cell, identifying the three different V -V bonds with a ladder pattern and their lengths at 300 K. c) Resistivity of a MnVO3-I pellet fragment at high temperature in a semi-log representation. It shows a transition from semiconducting to a metallic state on heating at 450 K. d) Thermal evolution of the SXRD patterns on warming between 300 and 600 K. e) Temperature dependence of V-V lengths obtained from the refined crystal structure with a coexistence of both phases in the shaded area.

Figure 2 .

 2 a) Temperature dependence of the zero-field (ZFC, red) and field cooling (FC, blue) magnetic susceptibilities of MnVO3-I in a 0.1 T external applied field. The inverse susceptibility shows the results of a Curie-Weiss fit. b) Field dependent magnetization at different temperatures. c) Specific heat divided by temperature (blue dots) along with the lattice contributions (dashed purple line). Inset shows the calculated entropy.

Figure 3 . 5

 35 a) Spin order in the honeycomb layers with Mn and V moment components at 2 K, vanadium moments are scaled three times. b) Difference between 2 K and 100 K neutron powder diffraction data with the refined k = [0 0 0] magnetic model for MnVO3-I. Small secondary magnetic signals from Mn3V2O8 and MnO where considered as second and third phases respectively. c) Temperature variation of the ordered Mn and V moments from neutron diffraction showing the critical law fit described in the text. The color intensity at the bottom shows the thermal evolution of the (-1 0 1) + k magnetic reflection This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | Please do not adjust margins Please do not adjust margins

Figure 4 .

 4 Figure 4. a) Extended Hückel tight-binding calculations of isolated V6O24 units from P-1 MgVO3 (identical to CoVO3) and MnVO3 where the dimerization is clear for the former. b) and c) Total and atomic projected DOS for MnVO3-I in the low-and high-temperature polymorphs respectively (DFT, U = 1 eV, AFM spin configuration).

Acknowledgements

We thank support from ANR AMANTS project (19-CE08-0002-01). The ALBA synchrotron (ID proposal 2022025657) and the ISIS muon and neutron source are acknowledged for access to beam time. The Chevreul Institute (FR 2638), Region Hauts-de-France, and FEDER are acknowledged for funding the X-ray diffractometers, the "LEGO" multianvil-press and the PPMS magnetometer. E.P.A.L. thanks financial support from DGAPA-UNAM Project IA103923 and L. Huerta for technical support in XPS measurements.

Author Contributions

The study was designed by AMAL. Synthesis was performed by ESM and AMAL. Bulk magnetism and electric conductivity data were measured and analysed by ESM and AMAL. XPS experiments and analysis were performed by EPAL. Synchrotron diffraction data were collected and analysed by AM, FF, ESM and AMAL. Neutron diffraction data were collected and analysed by DK, ESM and AMAL. EHTB calculations were done by OM and AMAL. The paper was written with contributions from all authors.

Conflicts of interest

There are no conflicts to declare.