Water-Ice Analogues of Polycyclic Aromatic Hydrocarbons: Water Nanoclusters on Cu(111)

Abstract

Water has an incredible ability to form a rich variety of structures, with 16 bulk ice phases identified, for example, as well as numerous distinct structures for water at interfaces or under confinement. Many of these structures are built from hexagonal motifs of water molecules, and indeed, for water on metal surfaces, individual hexamers of just six water molecules have been observed. Here, we report the results of low-temperature scanning tunneling microscopy experiments and density functional theory calculations which reveal a host of new structures for water-ice nanoclusters when adsorbed on an atomically flat Cu surface. The H-bonding networks within the nanoclusters resemble the resonance structures of polycyclic aromatic hydrocarbons, and water-ice analogues of inene, naphthalene, phenalene, anthracene, phenanthrene, and triphenylene have been observed. The specific structures identified and the H-bonding patterns within them reveal new insight about water on metals that allows us to refine the so-called "2D ice rules", which have so far proved useful in understanding water-ice structures at solid surfaces.

Figure 1

High-resolution STM images of submonolayer coverage of water on Cu(111) acquired at 5 K. Insets are zoomed-in images showing the five most prevalent ordered water clusters observed. (a) Large-area image of water clusters that form on Cu(111) after a 25 K anneal. (b) Water cluster referred to as the asymmetric dimer. (c) Smallest of the three bilobed features, referred to as the symmetric dimer. (d) Bilobed structure referred to as the long dimer. (e) Another bilobed water cluster referred to as the bent dimer. (f) Water trilobed structure. Scan conditions: +50 mV, 50 pA.

Figure 2

(a) High-resolution STM images, acquired at 5 K, of asymmetric dimers on Cu(111). An image of the underlying Cu(111) lattice is included (inset) to show the orientation of the symmetry axes of the Cu crystal used to perform all experiments. The asymmetric dimers are found in six different orientations and are aligned with the √3 direction of the underlying Cu surface. (b) Top panel is a DFT-calculated structure proposed for the asymmetric dimer, which is composed of nine water molecules (n = 9) arranged as a H-bonded pentamer and hexamer. The bottom panel shows an alternate and equally stable structure that consists of two H-bonded hexamers, composed of 10 water molecules (n = 10). The side views for both proposed structures show that water molecules common to both rings in each cluster lie flat on the surface while the outer portion of each ring is buckled. DFT-calculated STM images for each structure are shown on the far right, which match well with experimental results.

Figure 3

(a–c) High-resolution STM image of a bilobed feature, referred to as the symmetric dimer, in the three possible orientations. (d) DFT-calculated structure proposed for the symmetric dimer, which consists of 13 water molecules arranged as three interconnected H-bonded hexamers. (e) DFT-calculated STM image of the proposed n = 13 water cluster, which matches well with experimental data.

Figure 4

(a–c) High-resolution STM image, acquired at 5 K, of the bilobed structure referred to as the long dimer, which is found in three different orientations on the Cu surface. The inset is an atomic-scale image of the Cu(111) single-crystal substrate upon which the clusters are adsorbed. (d) DFT-calculated structure proposed for the experimentally observed long dimer, which is composed of three H-bonded water hexamers (14 water molecules) arranged in a linear configuration. The side view shows that the predicted structure consists of a central flat hexamer flanked by two buckled water hexamers. (e) DFT-simulated STM image of the proposed n = 14 water cluster, indicating that the DA water molecules image as two bright protrusions.

Figure 5

High-resolution STM images acquired at 5 K. (a) Atomic-scale image of the Cu(111) single crystal used to perform all experiments. (b–f) High-resolution STM image of the observed bilobe structure, with a dimmer central protrusion, referred to as the bent dimer. (g) DFT-calculated structure proposed for the bent dimer, which is an isomer of the DFT structure proposed for the long dimer in Figure 4, as it also consists of three interconnected hexamers, or 14 water molecules, but in a bent arrangement. The side view shows that the predicted configuration involves a central partially flat hexamer flanked by two buckled water hexamers. (h) DFT-simulated STM of the proposed n = 14 bent dimer, which matches well with experimental observations.

Figure 6

(a) High-resolution STM images acquired at 5 K of two chiral trilobed structures, with each chiral conformer found in an up and down orientation. (b) (Top) DFT-simulated image of the proposed trilobed structure, indicating that the three DA water molecules image as three bright protrusions. Because the positions of DA molecules can alternate within their respective hexamer rings, the trilobed clusters are chiral as they are rotated slightly from the high symmetry axes of the Cu(111) surface. (Bottom) DFT-calculated structure proposed for the trilobed structures, which consist of four H-bonded hexamers or of 18 water molecules.