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1 Real-space imaging of interfacial water with submolecular resolution Jing Guo, Xiangzhi Meng, Ji Chen, Jinbo Peng, Jiming Sheng, Xinzheng Li, Limei Xu, Junren Shi, Enge Wang *, Ying Jiang * International Center for Quantum Materials (ICQM) and School of Physics, Peking University, Beijing , P.R.China Contents: I. Adsorption configurations of water monomers and tetramers II. III. IV. Background removal in di/dv spectra Influence of the tip apex on the tip-water coupling Tip-water interaction versus water-substrate interaction V. Bias and current dependent orbital imaging of a water monomer VI. VII. VIII. IX. Orbital imaging of an asymmetric water monomer Disturbance of the tip on the water monomer at small tip heights Procedure for constructing a water tetramer Tip height dependent PDOS and di/dv spectra of a water tetramer These authors contributed equally to this work. * egwang@pku.edu.cn; yjiang@pku.edu.cn NATURE MATERIALS 1

2 I. Adsorption configurations of water monomers and tetramers: Adsorption energy was calculated by subtracting the total energy of a nh 2 O/NaCl(001)/Au structure from the sum of the energies of a relaxed bare NaCl(001)/Au substrate and n isolated water molecules in gas phase: 1 Eads = { E[( NaCl(001) / Au) relaxed ] + n E[( H 2O) gas ] E[( nh 2O / NaCl(001) / Au) relaxed ]} (1) n To study the adsorption configurations of water monomers on the NaCl(001) surface using DFT geometry optimization, we have searched dozens of adsorption models including the ones reported previously S1,S2. Fig. S1 shows the five most stable adsorption configurations of water monomers on the NaCl(001) surface. Figure S1 Side (top row) and top (bottom row) views of the five most stable adsorption structures of water monomers. The heights of oxygen atoms above the top layer of the NaCl surface are 2.35Å (a), 2.36Å (b), 2.28Å (c), 2.35Å (d), and 2.27Å (e). The lengths of OH bonds are (a): 0.98Å (downward) and 0.97Å (upward), (b): 0.99Å (downward) and 0.97Å (upward), (c): 0.98Å (both), (d): 0.99Å (downward) and 0.97Å (upward), and (e): 0.97Å (both). The HOH angles are (a), (b), (c), (d), and (e). The adsorption energies of these five configurations are listed in Table S1 for comparison. With the Au substrate included, (a) and (b) are the most stable configurations 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION with similar adsorption energy. The water monomer (b) is an asymmetric counterpart of (a), showing a rotation of about 37 along the flat OH bond of the monomer (a). If the Au substrate is removed, configuration (c) is found to be the most stable, which agrees with the earlier studies for water on NaCl (001) surface S1,S2. It is thus clear that long-range dispersion forces from the Au substrate have different effects on the different configurations, which stabilizes the configurations (a) and (b) against (c). Adsorption Energy (mev) a b c d e E ad (with Au substrate) E ad (without Au substrate) Table S1 Adsorption energies E ad of five different configurations of water monomers. Nudged elastic band (NEB) calculations show that there is no barrier between the configurations (a) and (b). However, we found that the energy difference between these two states can vary up to 8 mev when we put the water monomer at different Na + sites on the NaCl(001) surface. These two states have significantly larger adsorption energies than other configurations (c-e) regardless of the adsorption site. Such a variation of the relative energy difference may originate from the influence of the Au substrate: we constructed the NaCl/Au coincidence structure in the DFT calculation by superposing a NaCl (2 2) unit cell on a superstructure of the Au(111) substrate with a residual strain of about 5%. Further considering that the NaCl(001) island is incommensurate with the Au(111) substrate and the Au(111) surface has herringbone reconstruction in reality, it is reasonable that configurations (a) and (b) might be stable against each other at different sites of the NaCl(001) surface, which is consistent with the experimental observation (see Fig. S8 and S9). Therefore, the configurations (a) and (b) are actually not two distinct states of a single water molecule but corresponding to two different types of molecules whose relative stability is site-dependent. NATURE MATERIALS 3

4 For the adsorption configurations of water tetramers, two candidate structures were found to be energetically stable among all the structures we tested. One is the flat tetramer observed in our experiments (Fig. S2a and b) and the other is the buckled tetramer proposed by Yang et al. (Fig. S2c and d) S2. The adsorption energy of the flat tetramer is significantly larger than that of the buckled tetramer no matter whether the Au substrate is included or not (see Table S2). For the flat tetramer, the oxygen atoms of water reside on top of Na + with the same height of 2.35 Å, but shift slightly toward the central Cl -. A square H-bonded network forms among the four water molecules with the O-O separation of 2.78 Å. Each water molecule donates and accepts just one H-bond yielding a cyclic water tetramer. The other four free OH-bonds point obliquely upward along the Na + -Cl - line (about 50 with respect to the surface plane). The lengths of OH bonds are 0.99 Å (flat OH) and 0.97 Å (upward OH). The HOH angles are For the buckled tetramer, the heights of the oxygen atoms are 2.46 Å (lower molecule) and 3.00 Å (upper molecule). The lengths of OH bonds are 1.02 Å (flat OH of upper molecule), 0.97 Å (downward OH of upper molecule), and 0.99 Å (both OH of lower molecule). The HOH angles are (upper molecule) and (lower molecule). Figure S2 Top and side views of the adsorption configurations of the flat (a and b) and buckled (c and d) water tetramers. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Adsorption energy (mev) flat buckled E ad (with Au substrate) E ad (without Au substrate) Table S2 Adsorption energies per water molecule (E ad ) of the flat and buckled water tetramers. II. Background removal in di/dv spectra: Figure S3 Background removal in di/dv spectra. a, Sketch of the measurement geometry of on and off spectra and assignment of the symbols. Two different tunneling paths are depicted as 1 and 2 in on state. In off state, the electron tunneling path 3 is similar to the path 2 in on state. The tip-water hybridization is denoted by a double-ended green arrow. b, Exponential fitting of F on /F off as a function of the tip height at NATURE MATERIALS 5

6 the zero bias, which shows a nonzero offset. F on and F off are the spectra taken in on and off states, respectively. c, di/dv spectra recorded on a water monomer (blue) and the NaCl surface (red) at a tip height of -60 pm relative to the set point: V=100 mv, I=50 pa. d, The di/dv spectrum after the background removal following Eq. (8). The di/dv spectra taken in on state (F on ) contain contributions from two different tunneling processes as shown in Fig. S3a S3. In the first process, the electron tunneling occurs between the tip-coupled water molecule and the Au substrate (path 1); while in the other process, the electrons from the tip tunnel directly into the Au substrate without going through the water (path 2). The conductance contributions from the two paths 1 and 2 are denoted as F 1 and F 2, respectively. DFT calculations reveal that the coupling of water with the noble metals (Pt, Ag, Au ) is mainly through the metal d-states S4,S5, which means that the paths 1 mainly involves d orbital channels of the Au tip. Considering the large separation between the tip and the Au substrate, the tunneling path 2 should be mainly contributed by the sp orbitals of the tip due to the more delocalized character of the sp electrons than the d electrons. Hence, the interference between these two tunneling paths should be negligible under the first-order approximation. We then have the simple form: F on = F 1 +F 2. F 1 contains the information of the hybridized water DOS. In order to extract F 1, we need to remove the background signal F 2 arising from the direct tunneling between the tip and the Au substrate. We assume that the DOS of the Au substrate is sufficiently flat in the investigated bias range ( V <350mV) and could be considered as a constant. Therefore, the contributions of the two tunneling processes in on state are simply given by S6 : F s V e κ 1s0 1 ρho( 2 1, ) ρ s (2) F2 ( V) e κ 2s1 ρt ρ s (3) 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION where s 0 is the thickness of the bilayer-nacl film. s 1 is the tip-au separation in on state; e κ e κ 1s0 and 2s1 are transmission coefficients of tunneling barriers of water-au and tip-au, respectively; ρ ( s, V ) is the water DOS hybridized with the tip; ρ s and ρ t ( V ) are HO 2 1 the DOS of the Au substrate and the tip, respectively. F on can be then written by: F = C ρ ( s, V) ρ e + C ρ ( V) ρ e (4) κ 1s0 κ2s1 on 1 H2O 1 s 2 t s DFT calculations show that ρ ( s, V ) exhibits an exponential behavior as a HO 2 1 function of the tip-water separation, thus it could be rewritten by: ρ ( s, V) ρ ( V) e κ HO 2 1 HO 2 31 s, where κ 3 could be energy dependent and relies on the details of the tip-water coupling. Since the bias voltage is small compared to the work functions of the tip and the sample, κ 1 and κ 2 are both bias independent. Therefore, Eq. (4) can be rewritten by: F = C ρ ( V) e + C ρ ( V) e (5) κ31 s κ2s1 on 3 H2O 4 t 1s0 where e κ, s ρ and C 1 are absorbed in C 3 ; ρ s and C 2 are absorbed in C 4. For the tip in off state (Fig. S3a), the electron tunneling path 3 is similar to the tunneling path 2 in on state. If we denote the tip-au separation in off state as s 2, the di/dv spectra F off can be given by in analogy to F 2 : F C V e κ 2s2 off = 5 ρt ( ) (6) The ratio between Eq. (5) and Eq. (6) gives: F ( ) on C C ρ V (, ) = + (7) F C C V off where s = s 1 - s 2. 4 κ2 Δs 3 HO 2 κ3δs ( κ3 κ2) s2 s2 V e e e 5 5 ρt ( ) C4 2 s can be kept as a constant when varying s 1 and s 2 such that e κ C 5 Δs becomes tip NATURE MATERIALS 7

8 C4 2 height independent. The value of e κ C 5 Δs is then derived from Eq. (7) by an exponential Fon fitting of ( s2, V) F off as a function of the tip height at a certain bias. Combining Eq. (5) with Eq. (6), we finally have: C = (8) 4 2 s F1 Fon e κ Δ Foff C5 Fig. S3b-d demonstrates the di/dv background removal of a water monomer. Following Eq. (8), the di/dv spectrum (F 1 ) containing the hybrid water DOS (Fig. S3d) was obtained from the di/dv spectra recorded on the water monomer (F on ) and the NaCl C4 2 surface (F off ) (Fig. S3c), using the prefactor e κ C 5 Δs as determined from the exponential fitting of F on /F off (Fig. S3b). III. Influence of the tip apex on the tip-water coupling: Figure S4 Side (top row) and bottom views (bottom row) of different Au tip models. The bilayer NaCl(001) slab and the Au(111) substrate are removed for clarification. The water monomer is fully relaxed during the geometry optimization. a, [111] cleaved face with one atom at the end (111_1Au). b, [111] cleaved face with three atoms at the end 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION (111_3Au). c, [111] cleaved face with three atoms at the end plus one layer of Au (111_3Au+slab). d, [100] cleaved face with four atoms at the end (100_4Au). e, [110] cleaved face with four atoms at the end (110_4Au). To explore the influence of the tip apex on the tip-water coupling, we constructed five different types of Au tips (Fig. S4) and calculated the PDOS of a water monomer on NaCl(001) with a tip-water separation of 3.0 Å (Fig. S5). The water monomer is fully relaxed during the geometry optimization. Note that the adsorption configuration of the monomer is barely affected by the presence of the tip. As shown in Fig. S5, different tip apexes not only lead to the variation of the energy scale of LUMO/HOMO within about 0.5 ev, but also selectively enhance the HOMO states (tip 100_4Au) or the LUMO states (tip 111_1Au) around E F. Although the absolute energy positions and intensity of LUMO/HOMO states rely on the tip termination, the shape of those orbitals keeps almost the same, as determined from inspection of the real-space distribution of the charge density within corresponding energy windows. This is consistent with the experimental observation. Figure S5 Calculated PDOS projected onto the water molecule for a water monomer on NaCl(001) using different types of Au tip models at a tip-water separation of 3.0 Å. NATURE MATERIALS 9

10 Fig. S6 shows the STM images of water monomers acquired by three typical types of STM tips which appear most frequently in the experiments. The tip in Fig. S6a is sensitive both to the LUMO and HOMO states. In contrast, tips in Fig. S6b and S6c selectively enhance the LUMO and HOMO states within the bias range investigated, respectively. The STM images (Fig. 3) and the di/dv spectra (Fig. 2f) of water shown in the main manuscript were acquired with the same type of tip as shown in Fig. R3a, but had a different energy scale. In spite of the variation of the energy scale and the selective sensitivity to the frontier orbitals, different STM tips yield similar HOMO and LUMO features: the HOMO appears as a double-lobe structure with a nodal plane in between, while the LUMO corresponds to an egg-shaped lobe developing between the two HOMO lobes. We found that the best agreement between the experimental di/dv spectra (Fig. 2f) and the calculated PDOS (Fig. 2e) of water was achieved with a (111_3Au+slab) tip shown in Fig. S4c. Figure S6 Bias-dependent orbital imaging of water monomers obtained with three different types of tips a, b, and c. Tunneling current of a: I=250 pa; b: I=50 pa; c: I=50 pa. The bias voltage is inserted in the upper left corner of each image. Tip a is sensitive both to the LUMO and HOMO. Tips b and c are sensitive to LUMO and HOMO, 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION respectively. We note that the energy scale of the calculated PDOS (Fig. 2e) is still about two times larger than that of the di/dv spectra (Fig. 2f), even after taking into account the variation of LUMO/HOMO states due to the changes in the tip apex. Such a discrepancy may result from the non-local electron correlation induced by instantaneous polarization interactions. The image-charge effect in the transport measurement of a molecule between two electrodes is such an example S7. In this recent work, upon decreasing the lead-molecule distance by just a few Ångström, the HOMO and LUMO levels of a single porphyrin-type molecule could shift towards E F as high as several hundreds of mev because of electron interaction with image charges in the metal leads. In spite of its apparent simplicity in electrostatic terms, the image-charge effect is the consequence of non-local electron correlation which can not be captured in DFT calculations as a matter of principle. IV. Tip-water interaction versus water-substrate interaction: 500 Energy (mev) Tip-water interaction Substrate-water interaction Water diffusion barrier Tip Height ( Å) Figure S7 Calculated tip-water and substrate-water interaction energies as a function of tip height. The blue spheres represent the interaction energy between the Au tip and the water monomer. The red triangles are the interaction energy between the Au-supported NaCl(001) surface and the water monomer. Two different kinds of tips (Fig. S4a and b) are NATURE MATERIALS 11

12 used and the results are the average of them. The gray dashed line denotes the calculated diffusion barrier of the upright water monomer (Fig. S1a) from Na + site to its nearest-neighbor, using the nudged elastic band (NEB) method. The tip height is defined as the vertical distance between the endmost atom of the STM tip and the oxygen atom of water. The interaction between the tip and the water is much stronger than that between the water and the Au substrate, raising the question on the stability of water under the tip-water coupling. In fact, the water is adsorbed on the NaCl films supported by the Au substrate. The tip-water interaction should be much weaker than the water-nacl interaction, considering that the NaCl(001) surface is highly hydrophilic. To compare the tip-water interaction with the water-substrate interaction in a systematic and quantitative way, we performed detailed DFT calculations and plotted out the interaction energies of tip-water and water-nacl(001)/au(111) as a function of tip height (Fig. S7). It is obvious that for the tip height above 3 Å the water-substrate interaction is far stronger than the tip-water interaction, which ensures that the water molecules can remain stable on the NaCl surface in the presence of tip-water coupling. Furthermore, the tip-water interaction energy is also substantially smaller than the calculated diffusion barrier of water on the NaCl surface (Fig. S7). Therefore, the tip can neither pick up the water molecule nor drag it away with it during the scanning as long as the tip-water separation remains above 3 Å. All the orbital images of water shown in the paper are reproducible. V. Bias and current dependent orbital imaging of a water monomer: The imaging of water monomers was studied in a systematic way as functions of the bias and the tunneling current (Fig. S8a). With the bias fixed, the current dependent images show that the HOMO/LUMO features only emerge at small tip heights and become prominent as the tip height decreases (upper two rows of Fig. S8a), which is consistent with 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION the tip height dependent PDOS (Fig. 2e) and di/dv spectra (Fig. 2f). With the tunneling current fixed, the bias dependent images show that the HOMO gradually evolves into the LUMO when the polarity of the bias voltage changes from positive to negative (bottom row of Fig. S8a). In addition, we notice that the orbital images around the zero bias are dominantly contributed by the HOMO, whereas the LUMO becomes overwhelming when the bias voltage drops below -100 mv. This behavior agrees well with the measured di/dv spectra which exhibit a long HOMO-like tail across the Fermi level and a broad LUMO-like peak around V (Fig. 2f). Figure S8 Bias and current dependent orbital imaging of a water monomer. a, Orbital imaging of a symmetric water monomer as functions of the bias (bottom row) and the tunneling current (upper two rows). The relative tip height (z) at the center of the monomer, referenced to the gap set with: V=10 mv and I=50 pa, is inserted in the upper right corner of each image. b and c, Top and side views, respectively, of the calculated adsorption NATURE MATERIALS 13

14 configuration of a monomer. d and e, Calculated isosurfaces of the charge density of HOMO and LUMO, respectively. Such detailed imaging of HOMO and LUMO enables us to discriminate the bond orientation of water monomers with unprecedented precision. The mirror symmetry of the HOMO image with respect to the nodal plane suggests that the HOH plane of the water monomer is perpendicular to the surface (see Fig. S8b and d). The egg shape of the LUMO lobe is readily distinguishable in the image acquired at V= -300 mv and I=250 pa, based on which the directionality of the flat OH bond of the water monomer is determined to be along the [010] direction of the NaCl(001) surface (Fig. S48 and e). Interestingly, the LUMO feature does not develop from the center but towards the upper edge of the HOMO (see bottom row of Fig. S8a). This deviation implicates the directionality of the upright OH bond of the water monomer, which is not strictly vertical but tilts slightly away from the surface normal towards the [010] direction (Fig. S8c). VI. Orbital imaging of an asymmetric water monomer: Figure S9 Orbital imaging of an asymmetric water monomer. a-d, STM images of HOMO (a), HOMO+LUMO (b and c), LUMO (d) of an asymmetric water monomer. The arrows in a and d highlight the nodal plane of the HOMO and the orientation of the LUMO, 14 NATURE MATERIALS

15 SUPPLEMENTARY INFORMATION respectively. Set point of a: V=10 mv and I=100 pa; b: V=-200 mv and I=100 pa; c: V=-250 mv and I=100 pa; d: V=-300 mv and I=100 pa. e and f, Top and side views, respectively, of the calculated adsorption configuration of an asymmetric water monomer. g and h, Calculated isosurfaces of the charge density of HOMO and LUMO, respectively. In addition to the symmetric water monomers (Fig. S1a) as discussed in Fig. 3 and Fig. S8, we also observed asymmetric monomers (Fig. S1b). Fig. S9 shows the orbital imaging of such an asymmetric monomer. Similar to the symmetric monomer, the HOMO of the asymmetric monomer gradually evolves into the LUMO when the polarity of the bias voltage changes from positive to negative (Fig. S9a-d). However, the absence of the mirror symmetry with respect to the nodal plane of the HOMO (see Fig. S9a) suggests that the HOH plane of water is no longer perpendicular to the surface but tilts towards the [100] direction of the NaCl(001) surface (see Fig. S9e and g). The tilt of the HOH plane can be also evidenced in the LUMO image (Fig. S9d), where the LUMO lobe does not align with the [010] direction in clear contrast to the case of the symmetric monomer. In addition, the orientation of the LUMO lobe with respect to the [010] direction allows us to determine that the flat OH bond of the asymmetric monomer is oriented along the [010] direction of the NaCl(001) surface (see Fig. S9e and h). By carefully comparing the STM orbital images (Fig. S9a and d) with the calculated isosurfaces of HOMO and LUMO (Fig. S9g and h), one can readily discriminate the bond orientation of different asymmetric monomers. VII. Disturbance of the tip on the water monomer at small tip heights: We observed the disturbance of the tip on the water monomer when the tip-water separation became too small (possibly < 3 Å). As shown in Fig. S10, the tip-water coupling has negligible influence on the HOMO image of an asymmetric water monomer when the relative tip height is above -45 pm (Fig. S10a-c). Upon decreasing the relative tip height to NATURE MATERIALS 15

16 about -78 pm, the double-lobe structure of the HOMO starts to change from asymmetric to symmetric (Fig. S10d-f), corresponding to the transition from the tilting configuration (Fig. S1b) to the upright configuration (Fig. S1a). The transition of the orbital image does not show any dependence on the bias polarity, suggesting that the electric field between the tip and the sample has a negligible impact on the orientation of the water monomer. Figure S10 HOMO imaging of an asymmetric water monomer as a function of the tunneling current at V=100 mv. Tunneling current of a: I=10 pa; b: I=30 pa; c: I=50 pa; d: I=100 pa; e: I=200 pa; f: I=400 pa. The relative tip height (z) at the center of the monomer, referenced to the gap set with: V=100 mv and I=10 pa, is inserted in the upper right corner of each image. With the tip further approaching the water molecule, it can significantly distort the orbital shape of water and finally drag away the water with the tip as shown in Fig. S11. Fig. S11a exhibits a perfect double-lobe structure corresponding to the nearly unperturbed HOMO. However, the HOMO structure gradually distorts as the tip height decreases as shown in Fig. S11b-d. The distortion of the HOMO may arise from formation of the covalent bond between the d states of the Au tip and the 1b 1 orbital of water at small tip-water separations S5. When the interaction energy between the tip and the monomer exceeds the diffusion barrier of water on NaCl surface (Fig. S7), the water monomer can be 16 NATURE MATERIALS

17 SUPPLEMENTARY INFORMATION easily disturbed by the tip and became unstable as shown in Fig. S11d. DFT calculations reveal that the adsorption configuration and spatial orbital structure of water remain nearly unperturbed as long as the tip-water separation is above about 3 Å. Therefore, it is important in the experiments to carefully tune the tip height into an optimal range, where the influence of the tip on the original adsorption configuration and the orbital shape of water is negligible while the molecular DOS around the E F enhanced by the tip-water coupling is still sufficient for high-resolution imaging. All the images shown in the paper are disturbance-free and reproducible. Figure S11 HOMO images of a water monomer acquired at different tip heights, showing the orbital distortion under the strong tip-water coupling. Set point of a: V=30 mv and I=200 pa; b: V=30 mv and I=250 pa; c: V=30 mv and I=300 pa; d: V=30 mv and I=350 pa. The relative tip height (z) at the center of the monomer, referenced to the gap set with: V=30 mv and I=200 pa, is inserted in each image. VIII. Procedure for constructing a water tetramer: The water molecule has very low mobility on the NaCl(001) surface at 5 K, such that only isolated water monomers can be found. The formation of tetramers is thus kinetically forbidden. Therefore, we have to use STM tip to manipulate the isolated monomers to build tetramers. All the tetramers constructed in this way have the identical adsorption configuration. Alternatively, the tetramers can form spontaneously by heating up the sample to 50 K-80 K, at which the water monomers can gain enough thermal energy to overcome the diffusion barrier. Such tetramers have exactly the same structure as the ones built by NATURE MATERIALS 17

18 manipulation at 5 K. In this paper, we chose to construct tetramers by tip manipulation simply for the sake of convenience. Figure S12 Formation sequence of a water tetramer by manipulating four water monomers with a Cl-terminated tip. The blue dashed arrows in (a-d) highlights the trajectories along which the water monomers were manipulated by the tip. Set point of (a-e): V=80 mv and I=50 pa. f, Zoom-in STM image of the constructed water tetramer highlighted in e. Set point: V=20 mv and I=80 pa. Individual water monomers can be manipulated on the NaCl(001) surface with a well-controlled manner by a Cl-terminated tip, which offers an efficient way to construct water clusters such as tetramers. The Cl-terminated tip was obtained by scanning the NaCl surface in close proximity (below V=5 mv and I=2.5 na) until the resolution was suddenly improved. Fig. S12 illustrates the sequence of constructing a water tetramer from four monomers. First, the Cl-tip was positioned on top of the monomer at the set point (V=100 mv, I=50 pa); Second, the tip height was decreased to the set point (V=10 mv, I=150 pa) 18 NATURE MATERIALS

19 SUPPLEMENTARY INFORMATION to increase the tip-water interaction; Third, the tip was moved along the predesigned trajectories as highlighted by blue dashed arrows in Fig. S12a-d; Finally, the tip was retracted to the initial set point (V=100 mv, I=50 pa), and the image was rescanned. Fig. S12a-e show a complete manipulation sequence of assembling the monomers to form a dimmer (Fig. S12c), a trimmer (Fig. S12d), and a tetramer (Fig. S12e). We note that the water dimmer and trimmer are not very stable such that they can be easily disturbed by the tip during the scanning. However, the tetramer is quite stable once formed, allowing long-term imaging and spectroscopic measurements. IX. Tip height dependent PDOS and di/dv spectra of a water tetramer: Figure S13 Tip height dependent PDOS and di/dv spectra of a water tetramer. a, Calculated PDOS of a water tetramer on NaCl(001) at different tip-water separations, projected onto the four water molecules. The tip is positioned at the center of the tetramer. The energy zero is the Fermi level. b, Experimental di/dv spectra of a water tetramer acquired at different tip heights. Reference point (0 Å): V= 100 mv and I=50 pa. The calculated PDOS of a water tetramer adsorbed on NaCl(001) shows that the tip-water coupling leads to a broadening and shift of the HOMO towards the Fermi level, whereas the LUMO is less affected and stays well above the Fermi level (Fig. S13a). A NATURE MATERIALS 19

20 prominent HOMO tail extends over the Fermi level up to 1 ev. As the tip approaches the molecule, the HOMO states near the Fermi level are enhanced due to the increased tip-water coupling. Using the method of di/dv background removal as described in Fig. S3, we obtained tip height dependent di/dv spectra of a water tetramer (Fig. S13b). Similar to the calculated PDOS, a tail feature across the Fermi level develops from the positive bias. By comparing the STM images acquired within the tail region with the calculated isosurfaces of frontier molecular orbitals, the tail feature is attributed to HOMO-like states. As the tip approaches the water tetramer, the magnitudes of the di/dv spectra increase dramatically, in accordance with the calculated PDOS (Fig. S13a). Therefore, the enhanced DOS of water near the Fermi level mainly arises from the development of the HOMO-like states due to the tip-water coupling. We did not observe any LUMO-like features throughout the accessible bias range (from -400 mv to 400 mv). Supplementary references: S1. Park, J. M., Cho, J. H. & Kim, K. S. Atomic structure and energetics of adsorbed water on the NaCl(001) surface. Phys. Rev. B 69, (2004). S2. Yang, Y., Meng, S. & Wang, E. G. Water adsorption on a NaCl (001) surface: A density functional theory study. Phys. Rev. B 74, (2006). S3. Sautet, P. Atomic adsorbate identification with the STM: a theoretical approach. Surf. Sci. 374, (1997). S4. Michaelides, A., Ranea, V. A., Andres, P. L. de & King, D. A. General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces. Phys. Rev. Lett. 90, (2003). S5. Carrasco, J., Michaelides, A. & Scheffler, M. Insight from first principles into the nature of the bonding between water molecules and 4d metal surfaces. J. Chem. Phys. 130, (2009). S6. Chen, C. J. Introduction to Scanning Tunneling Microscope (Oxford Univ. Press, New 20 NATURE MATERIALS

21 SUPPLEMENTARY INFORMATION York, 2008). S7. Perrin M. L. et al. Large tunable image-charge effects in single-molecule junctions. Nature Nano. 8, 282 (2013). NATURE MATERIALS 21