Direct Imaging of Radiation-Sensitive Organic Polymer-Based Nanocrystals at Sub-Ångström Resolution

Abstract

Seeing the atomic configuration of single organic nanoparticles at a sub-Å spatial resolution by transmission electron microscopy has been so far prevented by the high sensitivity of soft matter to radiation damage. This difficulty is related to the need to irradiate the particle with a total dose of a few electrons/Ų, not compatible with the electron beam density necessary to search the low-contrast nanoparticle, to control its drift, finely adjust the electron-optical conditions and particle orientation, and finally acquire an effective atomic-resolution image. On the other hand, the capability to study individual pristine nanoparticles, such as proteins, active pharmaceutical ingredients, and polymers, with peculiar sensitivity to the variation in the local structure, defects, and strain, would provide advancements in many fields, including materials science, medicine, biology, and pharmacology. Here, we report the direct sub-ångström-resolution imaging at room temperature of pristine unstained crystalline polymer-based nanoparticles. This result is obtained by combining low-dose in-line electron holography and phase-contrast imaging on state-of-the-art equipment, providing an effective tool for the quantitative sub-ångström imaging of soft matter.

Keywords: HRTEM; HoloTEM; atomic-resolution imaging; in-line holography; polymers; radiation damage; soft matter.

Scheme 1

HoloTEM experiment layout: on the left part the high-coherence illumination source, the illumination lenses, the specimen position within the illuminating electron wave field, and the objective lens are sketched. On the right part, the experimental live low-dose hologram (magnified ten times) formed in the back-focal plane of the objective lens and the relevant live low-dose HRTEM image are reported to show what is being live-observed during the survey to find the particles on the TEM grid. The one order of magnitude contrast difference between the captured live hologram and the relevant captured live multibeam image is quantified by the intensity profile measured across the particle, pointing out how the hologram enables one to detect particles otherwise invisible via low-dose multibeam imaging.

Figure 1

HRTEM image of a cluster of crystalline pristine polymeric nanoparticles: (a) HRTEM image, with the relevant hologram in the inset, of a cluster made of CAP nanoparticles. (b) Diffractogram of (a) and list of the lattice spacing measured within each nanoparticle.

Figure 2

Atomic resolution quantitative imaging experiment on a single CAPeg nanoparticle: (a) HRTEM image in the [3, 6, $\overline{4}$] zone axis of a polymer-based cocrystal CAPeg particle; in the pale blue inset, the relevant multislice simulation is reported. (b) Magnified view of the HRTEM contrast simulation. (c) Crystal structure derived from single-crystal XRD experiments and displayed using the computer program Mercury [39]; the unit cell is viewed along the [3, 6, $\overline{4}$] zone axis, like in the experimental image. The sticks and balls represent, with different colours, the atoms and the bonding of the chemical species: hydrogen = green; fluorine = yellow; oxygen = red; carbon = grey; and nitrogen = blue. (d) Experimental diffractogram. (e) Simulated diffraction pattern in the [3, 6, $\overline{4}$] zone axis and experimental (d_exp) and calculated (d_theo) spacing, along with the relevant Miller indexes.

Figure 3

Sub-ångström imaging experiment on a CAPeg thin foil: (a) raw HRTEM image showing part of a large foil of CAPeg in the [10, 3, 2] zone axis, along with, in (b), the relevant diffractogram and experimental (d_exp) and calculated (d_theo) spacings, together with the corresponding Miller indexes. The blue square in (a) marks the area shown at a higher magnification in (c). In (c), two image simulations are superimposed onto the HRTEM raw image, fitting the experimental contrast for two different thickness values: 18 nm in the upper-left part of (c), and 27 nm in the central-lower part of (c). (d) is the diffractogram of (c), evidencing two basic vectors for the plane perpendicular to the [10, 3, 2] direction and the reflections (4, −6, −11) at 93 pm and (3, 10, 0) at 85 pm. (e) Cell of CAPeg seen along the [10, 3, 2] zone axis displayed by the Mercury software [39], highlighting in pale-red the traces of planes (4, −6, −11) and (3, −10, 0) and the relevant oxygen–carbon and carbon–carbon dumbbells, respectively. The grey rectangle marks the atomic configuration of CAPeg seen magnified in the lower-right part of (e), where the two sub-Å dumbbells are underlined by grey and red dashed lines. This atomic configuration is then also shown superimposed onto the HRTEM image and simulations in the central-lower part of (c).