Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films

TLDR: Scanning electron nanobeam diffraction is used to monitor the morphology of organic thin films with nanometre resolution, revealing information on the arrangement of crystalline domains useful for structure–property relationship understanding.

Abstract: The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure-characterized by sharp grain boundaries and overlapping domains-to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure-property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics.

Figures

Figure 3: Comparison of grain morphology between PBTTT samples. (a,b) Virtual dark field reconstructions of as-cast and annealed PBTTT thin films, respectively. (c,d) Orientation and (e,f) Flow line maps corresponding to (a,b), respectively. Example diffraction patterns of annealed PBTTT showing (g) overlapping grain orientations and (h) a disclination.

Figure 2. Comparison of grain morphology between DIO samples. The panels on the left (a, c, e, g) are from the sample drop-cast without DIO and the panels on the right (b, d, f, h) are from the sample drop-cast with DIO. (a,b) The background HAADF images show few, if any, of the molecular film details; the web-like features are components of the supporting lacey carbon grid. Dotted lines represent the area over which the 4D-STEM scan was performed. Within the scan areas, a virtual dark field is overlaid onto the HAADF. The orientation maps (c, d) show the direction of the brightest reflection found at that location. Flow line maps (e, f) trace the molecular backbone structure, with the T1 sample demonstrating gradual lattice rotations while the T1+DIO sample shows rigid crystalline domains with significant overlap. (g) Example diffraction patterns from the T1 sample showing disclinations of opposite sign, and an example of a small overlapping fragment. (h) Example diffraction patterns from the T1+DIO showing multiple overlapping grain orientations.

Figure 1. Schematic of the diffraction imaging technique. Molecular structure of the T1 molecule (a) and schematic of the 4D-STEM technique (b). As the convergent beam rasters over the area of interest, a full diffraction pattern is acquired for each real space probe location (x, y), with a step size large enough to prevent the beam from damaging the yet unsampled neighboring positions. The molecules stack along their π-π bonds as illustrated in (c). The data structure resulting from the technique is shown in (d), with examples of diffraction patterns obtained from T1 + DIO (concentration 0.4%).

Full text

Lawrence Berkeley National Laboratory
Recent Work
Title
Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin
films.
Permalink
https://escholarship.org/uc/item/9750j84r
Journal
Nature materials, 18(8)
ISSN
1476-1122
Authors
Panova, Ouliana
Ophus, Colin
Takacs, Christopher J
et al.
Publication Date
2019-08-01
DOI
10.1038/s41563-019-0387-3
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

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-8;.<0
2'--00-
!  +    -  '  -  .  2-2  00
2 !++-,@(0
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2-    0@.+  0  .  @    
0>0-2- 
02200-20-
8-.2      .  -  0  .    "  '+
.@:--.
0@2.2'2-2- 

!++2B8'2-.'2.2-
0--"--.

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'
  -          -  
- D+@.-00--
0-''82->.
  D-      8      22
0E0'"'.-2-2-
+-+08
-   0 0 
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02    -    .-  +@    '
-0-00-.
+0.>.02A002
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>. 0-+-
0    -2    +  +   0-  .  
-0.--'2
>.2.-2- !2..-
.'0@2.-

Frequently asked questions

Q1. What are the contributions in "Diffraction imaging of nanocrystalline structure in organic semiconductor molecular thin films" ?

In this paper, a scanning nanodiffraction ( SND ) method was used to observe the nanocrystalline structure of two organic molecular thin film systems using transmission electron microscopy ( TEM ).