# Structure of Manganese Oxide Nanoparticles Extracted via Pair Distribution Functions
## 2 Experimental Details  
MnO nanoparticles were obtained by adapting the protocol of [PERSON] et al. [38] as follows. First, a manganese (II) oleate complex was produced via an exchange chemical reaction between manganese (II) chloride and sodium oleate. A quantity of 1.24 gm of the resulting manganese (II) oleate and 10 gm of 1-hexadecene were then combined in a 50 mL round-bottom flask attached to a Schlenk line. The mixture was degassed at 80 \({}^{\circ}\)C for 90 min, and then the vacuum was switched to Ar atmosphere. Next, the flask was heated to 280 \({}^{\circ}\)C and held at this temperature for 30 min, followed by cooling to room temperature. Nanoparticles were precipitated by adding 10 mL of acetone, collected by centrifugation at 9000 rpm for 10 min, re-dispersed in hexane, washed again with acetone and collected by centrifugation. Finally, the nanoparticles were re-dispersed in toluene to obtain a 10 mg\(\cdot\)mL\({}^{-1}\) stock of colloidal MnO nanoparticles. Thin Si wafer substrates measuring 1 cm \(\times\) 1 cm were first cleaned using ultrasonication in acetone, ethanol, and isopropanol, and then dried by flashing compressed air. The stock dispersion of the nanoparticles in toluene was drop-casted onto a substrate, and, then, the solvent was allowed to evaporate slowly in ambient conditions. Morphology of the synthesized nanoparticles was determined by transmission electron microscopy (TEM) using a JEOL JEM 2100 working at 200 kV TEM. The average diameter of the nanoparticles was determined by measuring the diameters of 200 particles and fitting the histograms to a Gaussian distribution function. Figure 1 shows a high resolution (HR)-TEM image of the as-synthesized colloidal MnO nanoparticles. The TEM picture was taken before deposition on the silica substrate. As discussed below, the silica substrate seems to alter the crystallinity of the sample. Fast Fourier transform (FFT) images including plane-spacing calculations are shown in the Supplementary Material. HE-XRD measurements with a photon wavelength of 0.21 A were performed at beamline BL04B2 at the Japanese synchrotron facility SPring-8. An ionization chamber was used for monitoring the intensity of the incident X-rays. Three CdTe detectors were used to measure the intensity of the scattered X-rays. The small-angle scattering part of the signal was measured with a two-axis diffractometer to cover the low-Q region (\(\approx\)0.1 A\({}^{-1}\)). A vacuum chamber at HE-XRD setup was used to suppress air scattering around the sample. The HE-XRD setup at this beamline is described in more detail elsewhere [39]. Although radiation damage to individual nanoparticles should be reduced as much as possible, nanoparticles on a metallic substrate can be healed through a healing mechanism that involves a neutralization process [40]. Experimental \(I(Q)\) data were obtained for MnO nanoparticles of approximately 10 nm size. Background intensity was determined by carrying out a measurement on a bare substrate. The proper subtraction of the background is important for capturing the diffuse scattering signal in the data properly. Figure 2 shows the experimental \(I(Q)\) results for the MnO film sample. Only one Bragg-type peak, centered at 3.3 A\({}^{-1}\), is seen. Other peaks are an order of magnitude weaker and represent diffuse scattering. It is likely that the diffraction peaks of the MnO film are too weak to be observed due to the strong and sharp diffraction peaks of the silicon substrate, as has been reported previously for MnO\({}_{\mathrm{x}}\) on stainless steel [6]. TEM analysis provides grain size information. The particle size obtained via TEM is usually larger than the crystallite size given by XRD. This is to be expected since a particle typically consists of several crystallites.