Science - USA (2021-07-16)

of mechanically fragmented samples implies
some amount of fatigue or permanent defor-
mation in them.
We further mapped the nanoscale local
electrostatic surface potential of different
crystals using Kelvin probe force microscopy
(KPFM; Fig. 2, F to H) ( 16 ). Fresh 001ðÞ=ðÞ 00  1
faces created by fracturing crystals of 1 per-
pendicular to the needle axis showed large

remnant surface potentials; 4.7 V was the

highest average value observed in our scans

(Fig. 2H and figs. S11 to S13). Potential at the

fracture tip surface was either positive or neg-

ative for a particular fragment, confirming the

complementary nature of the charges on oppo-

site fragmented pieces (figs. S11 and S12). The

experiments at a crack junction on (100)/(010)

faces of imperfectly healed crystals also pro-

vided evidence for the residual opposite po-

tentials (e.g., ~0.2 V to–0.6 V; Fig. 2, F and G)

on either side of the crack line (fig. S14). Such

opposite surface potentials were also observed

at crack lines in piezoelectric ceramics ( 17 ).

The opposite surface charges on fracture

faces observed in KPFM experiments can be

understood from the piezoelectric nature of

the polar crystals.

324 16 JULY 2021•VOL 373 ISSUE 6552 sciencemag.org SCIENCE

Fig. 3. The mecha-
nism of generation of
surface charges
on fracture faces and
crystallographically
precise self-healing
in crystals of 1.
(A) Schematic depic-
tion of molecular
dipoles inside a pristine
crystal and generation
of opposite polarities
on freshly created

faces of 001ðÞ= 00  1

with breakage
of the polar molecular
arrays connected by
O–H···O hydrogen
bonds. The color
gradients marked on
the fractured crystal
represent negative
(red) and positive
(green) charges.
(BandC) 2D projection
of reconstructed 3D
Ewald sphere viewed
along thec* axis
for a pristine crystal
(B) and a neatly healed
crystal (C). (Dand
E) The measured 2D
diffraction pattern
(top panel) and
its corresponding 3D
intensity profile
(bottom panel) for
a neatly healed crystal
(D) and an imperfectly
healed crystal with
a visible crack line (E).

Hydrogen bonding

Hydrophilic Hydrophobic

Hydrophobic shield

E

Single Domain Domain 1

Domain 2

4117

• 26

4255

• 28

396

600

(^193)
207
x x
y y
A
B
C
Pristine D
Neatly healed
Neatly healed Imperfectly healed
(a,b)
(a,b)
Fracture
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