60 | Nature | Vol 577 | 2 January 2020
Article
Atomic imaging of the edge structure and
growth of a two-dimensional hexagonal ice
Runze Ma1,2,14, Duanyun Cao1,1 4, Chongqin Zhu3,4,14, Ye Tian1,1 4, Jinbo Peng^1 , Jing Guo^1 , Ji Chen^5 ,
Xin-Zheng Li5,6, Joseph S. Francisco^3 , Xiao Cheng Zeng4,7,8,9,10*, Li-Mei Xu1,6*, En-Ge Wang1,1 1,1 2*
& Ying Jiang1,6,1 3*The formation and growth of water-ice layers on surfaces and of low-dimensional ice
under confinement are frequent occurrences^1 –^4. This is exemplified by the extensive
reporting of two-dimensional (2D) ice on metals^5 –^11 , insulating surfaces^12 –^16 , graphite
and graphene^17 ,^18 and under strong confinement^14 ,^19 –^22. Although structured water
adlayers and 2D ice have been imaged, capturing the metastable or intermediate edge
structures involved in the 2D ice growth, which could reveal the underlying growth
mechanisms, is extremely challenging, owing to the fragility and short lifetime of
those edge structures. Here we show that noncontact atomic-force microscopy with a
CO-terminated tip (used previously to image interfacial water with minimal
perturbation)^12 , enables real-space imaging of the edge structures of 2D bilayer
hexagonal ice grown on a Au(111) surface. We find that armchair-type edges coexist
with the zigzag edges usually observed in 2D hexagonal crystals, and freeze these
samples during growth to identify the intermediate edge structures. Combined with
simulations, these experiments enable us to reconstruct the growth processes that, in
the case of the zigzag edge, involve the addition of water molecules to the existing
edge and a collective bridging mechanism. Armchair edge growth, by contrast,
involves local seeding and edge reconstruction and thus contrasts with conventional
views regarding the growth of bilayer hexagonal ices and 2D hexagonal matter in
general.Scanning tunnelling microscopy (STM) has been widely used to study
2D ices at surfaces^7 ,^9 ,^10 , but resolving edge structures is difficult because
STM is not sensitive to the position of nuclei and its tip can induce distur-
bances. Although transmission electron microscopy (TEM) can resolve
atomic lattice edges^23 , high-resolution TEM usually requires high-energy
electrons that can change or even completely decompose the edge struc-
ture of covalently bonded 2D materials^23 and are expected to damage
more weakly bonded ice edges. By contrast, noncontact atomic-force
microscopy (AFM) based on a qPlus sensor^24 ,^25 can probe interfacial
water with excellent resolution^12 ,^26 ,^27 , with use of a CO-terminated tip
ensuring that water molecules are only minimally disturbed thanks to
the ultrahigh flexibility of the tip apex and the weak higher-order elec-
trostatic force^12. Here we use this method to image various metastable
edge structures of a 2D bilayer hexagonal ice grown on a Au(111) surface
(Fig. 1a) and resolve the growth mechanisms with atomic detail.
The 2D ice was grown on a Au(111) surface at about 120 K with a thick-
ness of around 2.5 Å (see Methods, Fig. 1a), corresponding to two water
overlayers (Extended Data Fig. 1a–f ). The STM image of the 2D ice
(Fig. 1c) and the corresponding fast Fourier transform (FFT) image
(inset of Fig. 1a) both show a well ordered hexagonal structure, with
periodicity^6 Au(111)- 3× 3 -30° (Wood’s notation; Extended Data
Fig. 1g). Although the honeycomb H-bonding network of the 2D ice is
visible in the STM image, the detailed topology of the edge structures
is difficult to resolve. The AFM frequency-shift (Δf) image of the same
island exhibits much higher resolution (Fig. 1d), such that the atomic
structures of the zigzag and armchair edges can be easily identified.
The total length of the zigzag and the armchair edges are comparable,
but the average length of the former is statistically somewhat larger
(two-sided t-test, P = 1 × 10−7; Fig. 1b). Zigzag edges can grow perfectly
up to lengths of 60 Å, but armchair edges are always interrupted by
step kinks or defects that result in shorter lengths, predominantly
around 10–30 Å (Extended Data Fig. 2).
We then performed systematic AFM imaging at different tip heights
(see Methods and Fig. 2a). At a large tip height, where the AFM signals
are dominated by the higher-order electrostatic force^12 , we can distin-
guish two sets of 3× 3 sub-lattices in the 2D bilayer ice, one of whichhttps://doi.org/10.1038/s41586-019-1853-4
Received: 3 March 2019
Accepted: 19 September 2019
Published online: 1 January 2020
(^1) International Center for Quantum Materials, School of Physics, Peking University, Beijing, China. (^2) Physical Science Laboratory, Huairou National Comprehensive Science Centre, Beijing, China.
(^3) Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA. (^4) Department of Chemistry, University of Nebraska–Lincoln, Lincoln, NE, USA. (^5) School of
Physics, Peking University, Beijing, China.^6 Collaborative Innovation Center of Quantum Matter, Beijing, China.^7 Department of Chemical and Biomolecular Engineering, University of Nebraska–
Lincoln, Lincoln, NE, USA.^8 Department of Mechanical & Materials Engineering, University of Nebraska–Lincoln, Lincoln, NE, USA.^9 Department of Physics, University of Nebraska–Lincoln,
Lincoln, NE, USA.^10 Nebraska Center for Materials and Nanoscience, University of Nebraska–Lincoln, Lincoln, NE, USA.^11 Ceramics Division, Songshan Lake Materials Lab, Institute of Physics,
Chinese Academy of Sciences, Guangdong, China.^12 School of Physics, Liaoning University, Shenyang, China.^13 CAS Center for Excellence in Topological Quantum Computation, University of
Chinese Academy of Sciences, Beijing, China.^14 These authors contributed equally: Runze Ma, Duanyun Cao, Chongqin Zhu, Ye Tian. *e-mail: [email protected]; [email protected]; egwang@
pku.edu.cn; [email protected]