Type of presentation: Poster

ID-5-P-1953 Low voltage electron microscope - new tools for the measurements of nanoparticles diameters

Kocová L.^{1, 2}, Bílý T.¹, Langhans J.¹, Nebesářová J.^{1, 3}

¹Biology Centre ASCR, v.v.i., Branisovska 31, 370 05 Ceske Budejovice, Czech Republic, ²Fakulty of Science, University of South Bohemia, Branisovska 31, 370 05 Ceske Budejovice, Czech Republic, ³Faculty of Science, Charles University in Prague, Vinicna 7, 128 43 Praha 2, Czech Republic

lucie_kocova@paru.cas.cz

The low voltage electron microscope (LVEM) is an unusual type of microscope dedicated for the observation of specimens composed of low atomic number elements [1].Its main advantage consists in the use of the accelerating voltage around 5 kV which causes nearly twenty times more image contrast enhancement than a routine transmission electron microscope working at the accelerating voltage 100 kV [2]. Its disadvantage is that such low energy primary electrons are able to pass only through extremely ultra-thin specimens with the thickness. Therefore a measurement of nanoparticles size distribution seems to be the convenient application for this microscope working in TEM mode.

In the study LVTEM (Delong Instruments, Brno) was used for measurements of size distributions of laboratory prepared palladium (Pd) cubes and spheres and commercially prepared quantum dots (QD) nanoparticles (Invitrogen), both with diameters below 20 nm. The aim was to compare the accuracy of measurements with results obtained by means of a routine transmission electron microscope working at the accelerating voltage 100 kV (TEM) and a high resolution scanning electron microscope using the accelerating voltage below 10 kV (HRSEM).

Suspensions of nanoparticles were dropped on copper grids covered by un-direct evaporated carbon layer [3] and air dried after removing of the solution excess by a small piece of a filter paper. Digitally recorded images from all microscopes were treated with the ImageJ freeware program [http://imagej.en.softonic.com/]. The counting and measuring of a nanoparticles size distribution was performed on thresholded images.

We confirmed that the LVTEM produces images with very high contrast (Fig. 1). However size distributions of measured nanoparticles were slightly shifted to the greater values in comparison with size distributions obtained by means of routine TEM (Fig. 2). The decrease of the accelerating voltage causes less resolution, which is still sufficient for the observation of nanoparticles, on the other hand, significantly increases effective cross section leading to a stronger electron scattering. This phenomenon appears mainly at the nanoparticles edges and makes them in LVTEM bigger. This explanation was proved by results of nanoparticles size distributions in HRSEM and Monte Carlo simulations of electron scattering in Pd nanoparticles (Fig. 3).
Based on our results we can conclude that the LVTEM is the excellent tool for the measurement of nanoparticles size distributions.

References
[1] LVEM5 Application Note. February (2013), www.lv-em.com
[2] Delong et al. Proc. EUREM 12, 197 (2000)
[3] MAISHEV et al. Combined ion-source and target-sputtering magnetron and a method for sputtering conductive and nonconductive materials [patent] US6214183 B1

This work was supported by the Academy of Sciences (Z60220518) and Technology Agency of the Czech Republic (TE01020118).

Fig. 1: Quantum dots nanoparticles in LVTEM (5 kV) in left and in TEM (80 kV) in right.	Fig. 2: Histogram of Pd cubes (15 nm) determined by LVTEM (5kV).
Fig. 3: The Monte Carlo model of electron scattering at Pd cubes (15 nm) simulated for 80 kV (right) and 5 kV (left) primary electrons.