Here, we present a protocol to investigate the structure and dynamics of interfacial water at the atomic scale, in terms of submolecular resolution imaging, molecular manipulation, and single-bond vibrational spectroscopy.
Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-Ångström spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.
The interactions of water with the surfaces of solid materials are involved in various surface reaction processes, such as heterogeneous catalysis, photoconversion, electrochemistry, corrosion and lubrication et al.1,2,3 In general, to investigate interfacial water, spectroscopic and diffraction techniques are commonly used, such as infrared and Raman spectroscopy, sum-frequency generation (SFG), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), neutron scattering4,5,6,7,8. However, these methods suffer from the limitation of spatial resolution, spectral broadening, and averaging effects.
STM is a promising technique to overcome these limitations, which combines the sub-Ångström spatial resolution, atomic manipulation, and single-bond vibrational sensitivity9,10,11,12,13,14. Since the beginning of this century, STM has been extensively applied to investigate the structure and dynamics of water on solid surfaces3,15,16,17,18,19,20. Additionally, vibrational spectroscopy based on STM could be obtained from the second-derivative differential tunneling conductance (d2I/dV2), also known as inelastic electron tunneling spectroscopy (IETS). However, resolving the internal structure, i.e. the H-bond directionality, and acquiring reliable vibrational spectroscopy of water are still challenging. The main difficulty lies in that water is a close shell molecule, whose frontier orbitals are far away from the EF, thus the electrons from the STM tip can hardly tunnel into the molecular resonance states of water, leading to the poor signal-to-noise ratio of molecular imaging and vibrational spectroscopy.
Water adsorbed on the Au-supported NaCl(001) films provides an ideal system for atomic-scale investigation by STM with a Cl-terminated tip (Figure 1a), which is performed at 5 K in the ultrahigh-vacuum (UHV) environment with a base pressure better than 8×10-11 mbar. On one hand, the insulating NaCl films decouple water molecules electronically from the Au substrate so the native frontier orbitals of water are preserved and the lifetime of the electrons residing in the molecular resonant state is prolonged. On the other hand, the STM tip could effectively tune the frontier orbital of water toward the EF via tip-water coupling, especially when the tip is functionalized with a Cl atom. These key steps enable high-resolution orbital imaging and vibrational spectroscopy of water monomers and clusters. In addition, water molecules could be manipulated in a well-controlled manner, due to the strong electrostatic interaction between the negatively charged Cl-tip and water.
In this report, the preparation procedures of the sample and the Cl-terminated tip for STM investigation are outlined in detail in section 1 and 2, respectively. In section 3, we describe the orbital imaging technique, by which the O-H directionality of water monomer and tetramer are resolved. The tip-enhanced IETS is introduced in section 4, which allows the detection of vibrational modes of water molecules at single-bond limit, and determination of the H-bonding strength with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. In section 5, we show how the water tetramer can be constructed and switched by controlled tip manipulation. Based on the orbital imaging, spectroscopy, and manipulation techniques, isotopic substitution experiments can be performed to probe the quantum nature of protons in interfacial water, such as quantum tunneling and zero-point motion.
NOTE: The experiments are performed on water molecules adsorbed on the Au-supported NaCl(001) film (Figure 1a) at 5 K with an ultrahigh-vacuum (UHV) cryogenic STM equipped with Nanonis electronic controller.
1. Fabrication of Experimental Sample
2. Preparation of the Cl-Terminated Tip
3. Orbital Imaging of Water Monomer
4. Single-Molecule Vibrational Spectroscopy
5. Molecular Manipulation
Figure 1a illustrates the schematic of the STM experimental setup. First, Au(111) substrate is cleaned by sputtering and annealing cycles in the UHV chamber. The clean Au(111) sample shows 22×√3 reconstructed surface, where the atoms of the surface layer occupy both the hcp and the fcc sites forming herringbone structures (Inset of Figure 1b). The NaCl is evaporated on the Au(111) substrate, forming bilayer islands (Figure 1b). Then water molecules are dosed on the Au-supported NaCl(001) surface through the gas line (Figure 2) and isolated water monomers are visualized on the NaCl islands (Figure 1c). The Cl-functionalized STM tip is obtained by picking up a Cl atom from the NaCl surface (Figure 3), which could gate the HOMO of water to the proximity of EF via tuning tip-water coupling. Figure 4a is the STM image of a D2O monomer obtained with a Cl-tip, very closely resembling the HOMO of water monomer (Inset of Figure 4a). In such a near-resonance case (Figure 4b), the HOMO of water couples strongly with the vibration modes, resulting in resonance-enhanced IETS. Considering the key role of tip gating in enhancing the IET signals, this technique is named tip-enhanced IETS. Figure 4d is the tip-enhanced IETS of water, in which the frustrated rotational, bending, and stretching modes are all visualized and denoted as "R", "B", and "S", respectively23. In comparison with conventional IETS, the signal-to-noise ratio of tip-enhanced IETS is dramatically enhanced (up to 30% in relative conductance change), which is crucial for precisely determining the H-bonding strength.
Using the Cl-terminated tip, water molecules could be manipulated in a well-controlled fashion due to the electrostatic interaction between the Cl-tip and water. Figure 5a shows the procedure for constructing a water tetramer by dragging four water monomers along the predesigned trajectories (green dashed arrows in Figure 5a), sequentially. Such a cyclic tetramer structure contains two degenerate chiral states: clockwise and anticlockwise H-bonded loops, which could be discerned from the STM images (Figure 5b-c)22. The chirality of the tetramer can be switched once the Cl-tip closely approaches the water tetramer (Figure 6), in which region the reaction barrier for proton transfer is effectively suppressed. The reversible interconversion of the H-bonding chirality of the water tetramer can be monitored by recording the tunneling current as a function of time24. The switching rates could be extracted from the current versus time trace. As shown in Figure 7, the lifetime distribution of a clockwise tetramer could be fitted by an exponential decay y=Ae-t/τ (red curve in Figure 7), and the inverse of the time constant τ yields the switching rate of CS→AS for a chosen sample bias and tip height.
Based on the orbital imaging, molecular manipulation, and tip-enhanced IETS techniques, the quantum motion of protons of interfacial water could be probed at the atomic-scale. For instance, it is possible to directly visualize the concerted quantum tunneling of protons within the water clusters and quantify the impact of zero-point motion on the strength of a single H bond at a water/solid interface, which are discussed in detail in 23 and 24, respectively.
Figure 1: Experimental setup. (a) Schematic of the experimental setup. (b) STM image of bilayer NaCl(001) islands grown on the Au(111) surface. Step edges of the Au(111) surface are denoted by blue dotted lines. The inset shows the STM topography of Au(111) 22×√3 reconstructed surface. (c) STM image of isolated water monomers adsorbed on the NaCl surface. Herringbone structures of the underlying Au(111) substrate are highlighted by blue arrows. Set point: (b) 2V, 9 pA; inset: 100 mV, 50 pA; (c) 100 mV, 50 pA. The STM images in this report were all obtained at 5 K. Adapted with permission from 22, copyright 2014 Nature Publishing Group. Please click here to view a larger version of this figure.
Figure 2: Schematic of the gas line for dosing water molecules on the sample surface. The water was purified under vacuum by freeze-pump-thaw cycles. Then the water molecules were dosed in situ onto the sample surface through a dosing tube, which pointed toward the sample with a distance of ~6 cm. Please click here to view a larger version of this figure.
Figure 3: Preparation of a Cl-terminated STM tip. (a-c) Schematic of the procedure to acquire a Cl-terminated tip. The Cl-tip is obtained by bringing a bare STM tip close to the position of the Cl atom of NaCl surface (b), until a Cl atom transfers onto the apex of the STM tip (c). (d,e) STM images of the NaCl(001) surface (same area) acquired before and after the Cl atom adsorbed on the STM tip. The atomic resolution arising from the Cl– anions was resolved. A Cl atom is missing (sky-blue arrow in (e)) and the atomic resolution is improved, indicating that the STM tip is functionalized with a Cl atom. Set point: (d) 50 mV, 100 pA; (e) 50 mV, 50 pA. Adapted with permission from 24, copyright 2015 Nature Publishing Group. Please click here to view a larger version of this figure.
Figure 4: Tip-enhanced IETS of a D2O monomer. (a) STM image of a D2O monomer obtained with a Cl-tip (V = 100mV, I = 50 pA). The inset shows the calculated isosurface of charge density of the HOMO. (b) Schematic of the tip-enhanced IET process. The Cl-terminated tip "gates", the HOMO to the proximity of EF, thus resonantly enhancing the cross section of the IET process. (c) dI/dV (d) d2I/dV2 spectra obtained at the position of green stars on the water monomer. Red and blue curves are taken on the water monomer with the tip height offset by -120 pm and -40 pm, respectively. The gray curve is the background NaCl signal acquired at the tip height offset of -120 pm. Tip heights are referenced to the gap set with V = 100 mV and I = 50 pA. "R", "B", and "S" represent frustrated rotational, bending, and stretching vibration mode of water molecule, respectively. These curves are offset, presented in the y axes for clarity, and the zero levels of each curve are denoted by the dashed horizontal lines. Adapted with permission from 23, copyright 2016 American Association for the Advancement of Science. Please click here to view a larger version of this figure.
Figure 5: Adsorption configuration and STM topography of water tetramers on NaCL(001)/Au(111). (a) Procedure for constructing a water tetramer. Water monomers are manipulated by the Cl-tip along the predesigned trajectories (green dashed arrows) to form a water tetramer. (b,c) Adsorption configuration and STM images of water tetramers with anticlockwise (b) and clockwise (c) H-bonded loops, respectively. The STM images of water tetramer (the second column) show that the boundaries between the four lobes exhibit left-handed (b) or right-handed (c) rotation, which is more evident in the corresponding derivative images (the third column). O, H, Cl–, and Na+ are denoted by red, white, grey, and dark-cyan spheres, respectively. Set point: (a) 80 mV, 50 pA, (b,c) 10 mV, 80 pA. Adapted with permission from 22, copyright 2014 Nature Publishing Group. Please click here to view a larger version of this figure.
Figure 6: Chirality switching of a water tetramer. (a) Schematic showing the manipulation procedure of chirality switching of a water tetramer with a Cl-functionalized tip. On the left, the tetramer stays in the clockwise state (CS) at large tip height with the set point: V=5 mV and I=5 pA. In the middle, decreasing the tip height by 230 pm, the tetramer would undergo reversible interconversion between the clockwise and anticlockwise states. On the right, retracing the tip to the original tip height leaves the tetramer in the anticlockwise state (AS). (b) Tunneling current trace during chirality switching recorded at the position of green stars on the water tetramer. The higher and lower level of current correspond to the AS and CS state, respectively. The adsorption configuration and STM images of CS and AS state of tetramer are inserted in (b). O, H, Au, Cl–, and Na+ are denoted by red, white, golden, cyan and blue spheres, respectively. Adapted with permission from 24, copyright 2015 Nature Publishing Group. Please click here to view a larger version of this figure.
Figure 7: Extraction of the chirality switching rate from the current trace versus time. The lifetime distribution (bin size: 0.6 s) of the clockwise tetramer could be nicely fitted to an exponential decay (red curve) with the time constant of 1.37 s. The switching rate is the inverse of the time constant, 0.73 ± 0.016 s-1. The current trace was acquired at a sample bias of 3 mV and tip height of -295 pm, referenced to the gap set with V=5 mV and I=5 pA. Adapted with permission from 24, copyright 2015 Nature Publishing Group. Please click here to view a larger version of this figure.
To probe the internal structure, dynamics, and vibrational spectroscopy of water molecules adsorbed on the solid surfaces, paying particular attention to the degrees of freedom of hydrogen, some experimental steps are of crucial importance, which will be discussed in the following paragraphs.
The orbital imaging of water molecules is achieved based on two key steps. First, the insulating NaCl films decouple the water electronically from the Au substrate, second the orbital gating effect of the STM tip via tip-water coupling. To grow bilayer NaCl films on the Au(111) substrate, the temperature of the Au(111) substrate should stay around 290 K. When the temperature of the substrate is much lower, fractal structures form, or the size of NaCl islands are too small. Higher temperatures will lead to the formation of thicker NaCl islands, thus the conducting of the sample will be poor. It is worth mentioning that both HOMO and LUMO could be visualized with a sharp bare tip, while the orbital gating of the Cl-terminated tip is highly selective, such that only HOMO is detectable due to the strong coupling between the HOMO and the Pz orbital of the Cl tip. Since the molecular orbitals are spatially locked together with the geometric structures of molecules, the O-H directionality of water molecules are discerned through submolecular-resolution orbital imaging22.
Compared with real-space imaging, the vibrational spectroscopy of water can offer new insights into H-bonding configurations, dynamics, and H-bonding strength. However, probing reliable vibrational spectroscopy of water with conventional IETS has proven challenging due to the close-shell nature of the water molecule. With a Cl-terminated tip, the IETS signals could be significantly enhanced as the HOMO of water could be tuned to the proximity of EF via tip-water coupling, resulting in resonant-enhanced IETS23. As a matter of fact, the IETS of water monomers is very sensitive to the lateral position of the tip. Because the HOMO orbital of water molecules has a node plane in the center, where the molecular DOS is the smallest, this leads to a very small cross section for vibrational excitation. Therefore, the tip is usually positioned slightly away from the nodal plane to maximize the IET signals (green star in Figure 4a). Furthermore, tip-enhanced IETS of the water monomer is also sensitive to the tip height. The IET spectra are featureless at large tip-water distance (blue curves in Figure 4c-d). With decreasing tip height, the coupling of the tip with the water molecule is enhanced and vibrational features emerge (red curves in Figure 4c-d). However, tip-water coupling may have significant influence on the intrinsic energies of the vibrational modes. Indeed, the stretching modes will undergo red shift with decreasing tip height, which can be fitted to inversed exponential decays. To eliminate the tip effect, extrapolate these curves to infinite tip height to obtain the intrinsic vibrational energies23.
STM is not only an atomic probe for imaging and spectroscopic measurement, but can also manipulate individual atoms and molecules in a well-controlled fashion9,10. In this report, the manipulation of the water molecules on the insulating NaCl films is more controllable when the tip apex is functionalized with Cl because of the long-range electrostatic interaction between the water and the negatively charged Cl atom on the tip. The constructed water tetramer contains two degenerate chiral states: clockwise and anticlockwise H-bonded loops, which can be switched with a Cl-tip. Occasionally, more than two current levels emerge in the current trace during chirality switching arising from the structural relaxation of the Cl atom adsorbed at the tip apex. The chirality switching usually occurs at small tip height, in which region the Cl atom might have multiple metastable adsorption configurations on the tip due to the asymmetry of the tip apex. The hopping of the Cl atom from one configuration to another changes the tunneling current, but does not lead to switching of the tetramer chirality. As a result, no matter how many levels appear, they can be divided into two groups, and each group corresponds to one chiral state of the water tetramer. What's more, the switching rates are sensitive to the tip position in the xyz directions, depending on the coupling of the Cl-tip with the water tetramer. The switching rates will be quenched when the Cl-tip is too close to the water tetramer or positioned off-center relative to the tetramer24. To extract the switching rate from the current trace, the size of the time bin is critical. It is necessary to try several times to select an appropriate time bin to nicely fit the lifetime distribution to an exponential decay. In some cases, the two current levels are so close that the separation is comparable to the noise background, thus the adjacent averaging method is adopted to smooth the current trace to make the two current levels resolvable.
Although STM has been proven to be powerful for characterizing the structure, dynamics, and vibrational spectroscopy of water molecules on solid surfaces at the atomic scale, it suffers from limitations including: (1) conducting substrates are required to obtain the tunneling current, (2) poor temporal-resolution (usually in the order of a few hundred microseconds), (3) perturbation to the water molecules from the STM tip and the high-energy tunneling electrons during the IETS measurement, (4) UHV environment and low-temperature are indispensable. These limitations make STM fall short when compared with conventional methods for investigating water, such as optical spectroscopy, neutron scattering, and NMR. Nevertheless, the short-comings of STM can be overcome by combining other techniques. For instance, qPlus-based noncontact atomic force microscopy (nc-AFM) can be employed to determine the topology of H-bonded networks and even insulating crystal ice25,26. Ultra-fast laser combined STM is a promising tool to achieve both submolecular spatial resolution and femtosecond temporal resolution simultaneously27,28. Furthermore, employing the nitrogen-vacancy (NV) center as the scanning probe (NV-SPM) is expected to be a non-perturbative tool for detecting very weak magnetic signals, such as the spin fluctuations of protons in water and conducting NMR spectroscopy at nanoscale under ambient conditions29,30,31.
The authors have nothing to disclose.
This work is funded by the National Key R&D Program under Grant No. 2016YFA0300901 2016YFA0300903 and 2017YFA0205003, the National Natural Science Foundation of China under Grant No. 11634001, 11290162/A040106. Y.J. acknowledges support by National Science Fund for Distinguished Young Scholars and Cheung Kong Young Scholar Program. J. G. acknowledges support from the National Postdoctoral Program for Innovative Talents.
Au(111) single crystal | MaTeck | NA | |
NaCl | Sigma Aldrich | 450006 | |
Water, deuterium-depleted | Sigma Aldrich | 195294 | |
Deuterium oxide | Sigma Aldrich | 364312 | |
Sealed-off glass-UHV adapters | MDC vacuum products | 46300 | |
Diaphragm-sealed valve | any | NA | |
Bellows-sealed valve | any | NA | |
Leak valve | Kurt J. Lesker | NA | |
Scanning tunneling microscopy | CreaTec | NA | |
Electronic controller. | Nanonis | NA | |
Tungsten wire | any | diameter:0.3 mm; purity: 99.95% |