Science - USA (2022-02-25)

applications usually show worse mechanical
properties owing to their specific structural
properties, such as chain length, regioregular-
ity, and degree of polymerization.

Adaptable and breathable VDWTFs

In our design of VDWTFs (Fig. 1G), the dangling
bond–free nanosheets are staggered butted
up against each other to establish broad-area
plane-to-plane VDW contacts with minimum
interfacial trapping states to ensure excellent
charge transport across the intersheet grain
boundaries. With the bond-free VDW interac-
tions between the nanosheets, the VDWTFs
offer a natural mechanical match to soft bi-
ological assemblies typically characterized
by VDW interactions. When deformed, the
bonding-free VDW interfaces allow nano-
sheets to slide or rotate against each other to
accommodate the local tension or compres-
sion without breaking the broad-area VDW
interfaces and conductive pathways (movie S1),
which is essential for achieving unusual stretch-
ability and structural stability in the ultrathin

freestanding format. The mechanical defor-

mation of VDWTFs is readily transformed into

intersheet sliding or rotation to accommodate

local strains and deformations and overcome

topological limitations, thus endowing excep-

tional malleability and adaptability to irregular

and dynamically changing surface topogra-

phies. Lastly, the VDWTFs feature a perco-

lating network of nanochannels (dictated by

nanosheet thickness: ~3 nm) winding around

the staggered nanosheets for gas and/or nu-

trient permeation, which is critical for the

breathability of bioelectronics (movie S2).

This combination of electronic and mechan-

ical properties originates from the VDW inter-

actions among the staggered 2D nanosheets

and is difficult to achieve in typical chemical

vapor deposition–grown thin films (CVDTFs)

(Fig. 1H). The electrical and mechanical prop-

erties of CVDTFs—with their typical polycrys-

talline structure consisting of laterally stitched

domains—arestronglyinfluencedbythegrain

size, grain orientation, shape, and density of

grain boundary defects. The stiff and strong

covalent bonding within the grains and dis-

ordered bonds at the grain boundaries of the

CVDTFs (marked with red arrows in Fig. 1H)

can result in the formation of cracks and

ruptures that propagate along the grain

boundaries when they are deformed, thereby

causing mechanical fragmentation and elec-

tronic disintegration under minimal strain

(movie S1).

Structural and mechanical properties

of VDWTFs

Molybdenum disulfide (MoS 2 ) nanosheet ink

was prepared using an intercalation-exfoliation

process and assembled into VDWTFs using a

spin coating process (see supplementary mate-

rials). Scanning electron microscopy (SEM)

and transmission electron microscopy (TEM)

studies show a staggered nanosheet thin film

(Fig. 2, A and B) with an overall film thickness

of ~10 nm (fig. S1A). The MoS 2 nanosheets, with

a thickness of ~3 nm and lateral dimensions

ranging from less than one to several micro-

meters, are staggered butted up against each

854 25 FEBRUARY 2022•VOL 375 ISSUE 6583 science.orgSCIENCE

Fig. 2. Material characteristics
of VDWTFs and CVDTFs.
(A) SEM and (B) TEM images
showing VDWTFs assembled
from staggered 2D nanosheets.
(CandD) Photograph of the
(C) VDWTFs and (D) CVDTFs
floating on water. (E) Stress–strain
curve of a freestanding VDWTF.
Tensile loads cause 2D nanosheets
in VDWTFs to slide or rotate against
each other, resulting in unusual
stretchability. (F) Photographs of the
VDWTF at different tensile strains.
(G) Resistance–strain curve of the
VDWTF and CVDTF on a PDMS
substrate. (HtoK) SEM images
showing the contact interface
between the 4.3-mm-diameter
silica microspheres of different
configurations with [(H) and (I)]
VDWTFs or [(J) and (K)] CVDTFs.
Scale bars, 2mm. (L) Water contact
angles of a VDWTF (top) and a
CVDTF (bottom). (M) Optical
micrographs of a VDWTF suspended
over a polyimide substrate with
circular holes, confirming structural
robustness of the freestanding
VDWTFs. (N) Water vapor
transmission through VDWTFs
of different thickness versus
transepidermal water loss (TEWL).
See the supplementary materials for
a description of the“open bottle”
and“closed bottle”conditions.

ABC D

E F G

H I J K

L M N

RESEARCH | RESEARCH ARTICLES