Science - USA (2021-07-16)

(Antfer) #1

in a perpendicular direction by bipyrazole
molecules via O–H···N and N–H···O hydrogen-
bonding interactions (fig. S7C). In addition, the
bipyrazole molecules also pack together via
N–H···N interactions parallel to the water chains
(fig. S7B). As a result, a strong 3D cooperative
hydrogen-bonding network is formed. The
methyl groups of bipyrazoles close-pack to
form hydrophobic zones that are intercalated
in the 3D hydrogen-bonded network (Fig. 2B;
see table S2 for H-bonding information).
The hardness (H, resistance to plastic de-
formation) and elastic modulus (E, resistance
to elastic deformation) of crystal 1 measured
by nanoindentation suggest that this crystal is


several orders of magnitude harder (H= 0.4 to
0.5 GPa) and stiffer [E≈8 GPa on (100)/(010);
E≈15 GPa on (001)] than most other self-
healing materials (table S1), but is typical of
ordered organic crystals (fig. S8) ( 12 ).
Piezoresponse force microscopy (PFM) was
used on (100)/(010) faces of the polar single
crystals to investigate piezoelectric property
at nanoscale (Fig. 2C) ( 13 , 14 ). The amplitude
(Fig. 2D) of the response signal in PFM is di-
rectly related to the local mechanical strain
experienced by the sample in response to
electrical bias applied on the cantilever tip.
The phase of the signal is directly related to
direction of the electric polarization of the

microscopic region of the surface monitored
under the tip. These tests allow identification
of orientation of the polarization direction of
domain structures, visualized by color contrast
in the phase image (Fig. 2E) ( 15 ). The average
vertical piezoelectric coefficient of pristine
crystals estimated from PFM is ~7.8 pm/V;
this value is lower (~5.4 pm/V) for mechan-
ically fragmented pieces (fig. S9). The average
effectived 33 values measured from piezome-
try experiments on single crystals (~8.7 pC/N)
are higher than those from pellet samples made
by mechanically compacted microcrystalline
powders (~7.6 pC/N; these units are equivalent
to pm/V) (fig. S10). The drop in the performance

322 16 JULY 2021•VOL 373 ISSUE 6552 sciencemag.org SCIENCE


1 2

0 ms 2 ms

40 μm

(001)

(010)

(100)

250 μm

250 μm

100 μm

50 μm

3

B 1

1

2

2

(^34567)
(^8910)
A
C
DEF G
0 ms 2 ms
40 μm
-17 nm
25 nm
1 μm
HN N
HN N
H 3 C CH 3
H 3 C CH 3
Fig. 1. Micrographs of crystals of 1 exhibiting self-healing.(A) A pristine crystal marked with faces and molecular structure of compound 1 .(B) Repeat
self-healing cycles are shown in panels 1 to 10. (CandD) Ultrafast self-propelling mechanical actuation and auto-recombination of fractured pieces of crystal 1.
(EtoG) SEM image (E), dark-field optical image (F), and AFM image (G) of a neatly healed crystal with no sign of cracks. Insets are video grabs of the
corresponding crystals at the cracked stage.
RESEARCH | REPORTS

Free download pdf