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Page 2 of 12 Zhang et al. Microstructures 2023;3:2023010 https://dx.doi.org/10.20517/microstructures.2022.39

Keywords: CuInP S -In P S , ferroelectric, elastic modulus, nanoindentation, CR-AFM
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INTRODUCTION
Van der Waals layered ferroelectric CuInP S (CIPS) has attracted intense research interest due to its unique
2 6
ferroelectric characteristics. A series of novel properties have been discovered in recent years, such as giant
negative piezoelectricity , tunable quadruple energy wells , large room temperature electrocaloric effect ,
[2]
[1]
[3,4]
and strong coupling between ferroelectric polarization and ionic conductivity . Based on these fascinating
[5-7]
attributes, large quantities of 2D device applications have shown up in the past several years, including
[11]
ferroelectric field-effect transistors [8-10] , ferroelectric tunneling junctions , negative capacitance field-effect
transistors [12,13] , memristors [5,14] and prototype neuromorphic computing [15-18] .
The crystal structure of CIPS can be described as a sulfur framework in which metal cations (Cu and In)
and P-P pairs fill the octahedral voids . It is noteworthy that when Cu is deficient, the system undergoes a
[19]
chemical phase separation into a paraelectric IPS phase and a ferroelectric CIPS phase [20,21] due to the
intentionally induced off-stoichiometry. Compared with normal CIPS, CIPS-IPS has a higher Curie
temperature (340 K > 315 K) [21-23] , a tunable dielectric property and a porous structure in which the IPS
[24]
phase is more conducive to the lateral migration of Cu ions [6,19] . In addition, the ferroelectric properties of
CIPS-IPS have sensitive response to out-of-plane mechanical stimuli . Recently, the utilization of strain
[7]
engineering to modulate the domain and phase has become a promising method [25,26] . However, there is a
lack of key parameters such as elastic coefficient to give accurate reference for the experimental and
theoretical investigation of this system [27,28] .

In this work, we use a combination of contact resonance atomic force microscopy and piezoresponse force
microscopy (CR-AFM and PFM) to achieve accurate Young’s modulus of CIPS and IPS in the nanoscale
phase separated CIPS-IPS. The results reveal that the Young’s modulus of the CIPS phase was
27.42 ± 0.05 GPa, slightly less than that of the IPS phase of 27.51 ± 0.04 GPa. Meanwhile, the two phases can
also be well-distinguished by the magnitude of the frictional force. The density functional theory was
introduced to obtain the accurate full elastic constant C of CIPS and IPS, and their respective Young’s
ij
modulus was deduced, which are in good agreement with our experimental values. In addition, we
quantified the equivalent piezoelectric coefficient for the CIPS phase, which has an exceptionally large value,
~40 pm/V, compared to previously reported values of 5-12 pm/V . Finally, we also discovered an
[29]
asymmetrical domain switching and proposed an ion-mediated domain switching model.

METHODS
Sample preparation and structure characterization
[20]
We synthesized Cu In P S single crystal through the chemical vapor transport method . The starting
x
y 2 6
materials were sealed in fused silica ampules, then heated to 750-775 °C at a rate of 30 °C/h and held at that
temperature for 4 days, followed by a rate of 20 °C/h cooling. The thin flakes were obtained by mechanical
exfoliation and transferred to the conductive Au/SiO /Si substrate. The energy dispersive spectroscopy
2
(EDS) was performed using a commercial ultra-high resolution cold-field emission scanning electron
microscopy system (Hitachi, Regulus 8230) to characterize the elemental compositions. The actual chemical
composition was determined to be Cu In P S .
0.57
1.04 2 6
DFT calculations
The density functional theory (DFT) calculation was carried out using the projector augmented wave
[30]
(PAW) scheme with the Perdew-Burke-Ernzerhof (PBE) functional of generalized gradient