Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy

The overall goal of this experiment is to probe the structure and dynamics of interfacial water at the atomic scale using submolecular resolution imaging, molecular manipulation and single bond vibrational spectroscopy. This method can help answer fundamental issues about water science such as identifying the hydrogen bond and directionality of water and probing the hydrogen bond dynamics and vibrational spectroscopy of water molecules as shown on solid surfaces. The main advantage of this technique is that STM combines the capability of sub-angstrom Spatial resolution, atomic manipulation, and single bond vibrational sensitivity.

Except for providing insight into the structure, dynamics, and nuclear quantum effects of surface water, it can also be applied to more complex and realistic hydrogen bond system such as confined water, buck eyes, multilayer water, and water hydrogen systems. To begin, follow along with the accompanying text protocol to clean the gold 111 single crystal using cycles of argon ion sputtering and subsequent annealing. Deposit sodium chloride onto the surface of the gold crystal and then transfer it to the scanning stage of an STM set up.

Using standard STM techniques, check the coverage and size of the bi-layer sodium chloride 001 eye lens on the gold 111 substrateaight. The results should look similar to those shown here. Next purify water using freeze, pump, thaw cycles to remove any remaining impurities.

Pump the gas line to 10 to the minus five pascals and then freeze the liquid water with liquid nitrogen. Now close the bellows sealed valve and leave the gas line under vacuum. Then open the diaphragm sealed valve and let the water vapor fill in the gas line.

Then decrease the temperature of the sample to five kelvin. Open the leak valve slowly to make the pressure of the ultra high vacuum STM chamber increase to two times 10 to the minus 10th millibar. Next open the shutter.

Dose the water molecules onto the gold supported sodium chloride surface for one minute. Then close the shutter and the leak valve. At this point, check the coverage of water molecules on the surface using standard STM techniques.

Expect to see isolated water monomers on the sample surface. To begin, fabricate an electrochemically etched tungsten tip, etch it in three molar sodium hydroxide and then clean it using distilled water and ethanol as described in the accompanying text protocol. Next apply voltage pulses and controlled crashing procedures on the STM tip until the atomic chlorine atoms of the sodium chloride surface are resolved.

Then position the STM tip over the center of one of the chlorine atoms and bring the bare tip close to the sodium chloride surface in proximity with the set point. Next retract the tip to the original set point and scan the same area. Check that a chlorine atom has attached to the tip by visualizing both the improved resolution and the missing chlorine atom in the STM image.

To begin, set up the biospectroscopy module. Select the current, the differential conductance, the derivative of the differential conductance channels. Then adjust the setting time to 50 milliseconds and integration time as 300 milliseconds.

The spectro tunneling spectroscopy and the inelastic electro tunneling spectroscopy are acquired simultaneously using a lock in amplifier by modulating the first and second harmonics of the tunneling current respectively. Increase the integration time and sweep times as needed to obtain smooth spectra. Then tune the Z offset to take the biospectroscopy at different tip heights.

Next open lock in, modulate the bias, and demodulate the current. Set the modulation frequency as a few hundred hertz and the modulation amplitude as five to seven millivolts. After setting the modulation frequency, make sure there is no mechanical and electronic noise at the set point frequency and the corresponding second harmonic frequency.

To set the first harmonic phase, start by switching to the Z-Controller module. Set the tip lift to 10 nanometers and turn off the feedback, then switch to the lock in module and turn on the lock in button. Click on the first harmonic auto phase and record the phase.

Repeat the auto phase at least five times and take the average. Then subtract 90 degrees from the averaged phase to get the phase of the junction. Next set the second harmonic phase.

To accomplish this, position the STM tip on the gold substrate and start the biospectroscopy sweep from minus one volts to one volt. Then select the differential conductance channel LI X 1, and the function dY/dX which together show the derivative of DI over DV spectrum. Find a prominent peak feature in the spectrum and set the corresponding energy as the bias.

Next turn on the lock in and keep the STM system in tunneling mode. Click the second harmonic auto phase at least five times and take the average. To begin, scan the water monomer with the chlorine atom tip.

Then position the tip on the sodium chloride surface and take the biospectroscopy as the background signal. Next position the tip on the water monomer and start the biospectroscopy sweep. If the dI/dV and second derivative spectra of water are featureless, simply follow the background sodium chloride coated surface, then decrease the tip height by tuning the Z offset until the vibrational features emerge in the spectra.

To begin the construction of a water tetramer, first scan for an area containing four water monomers and position the chlorine tip on top of a monomer at the set point of V equals 100 millivolts and I equals 50 picoamperes. Decrease the height so the voltage is 10 millivolts and the current is 150 picoamperes. This will enhance the tip water interaction.

Next move the chlorine atom tip along the predesigned trajectories. Then retract the tip to its initial set point and rescan the same area to check that the water dimer is formed. Repeat this process until a water trimer and eventually tetramer are formed.

The tetramer contains two degenerate chiral states. Anti clockwise, and clockwise H bonded loops. As the chlorine atom terminated tip is lowered, the representative current bounces around as the tetramer changes between the clockwise and anticlockwise states.

When retracting the tip to the original height, the tetramer shown here is left in the anti clockwise state. The switching rates between clockwise and anti clockwise can be extracted from the current versus time trace to show the lifetime distribution of a tetramer. The clockwise tetramer can be fitted by an exponential decay.

To explore the mechanism of the proton transfer in the tetramer, the effect of full and partial isotopic substitution on the chirality switching is described here. Strikingly the chirality switching rate of the four water tetramer is substantially reduced by replacing a single water molecule with deuterium oxide. Almost to the same level of the four deuterium oxide tetramer.

While performing the procedure for manipulation and vibrational spectroscopy, it is important to remember to functionalize the STM tip with a single chlorine atom. This technology provides surface signs an ingenious method to explore the detailed topology of hydrogen bond network and the quantum motions of protons in water clusters at an atomic scale. After watching this video you should have a good understanding of how to identify the hydrogen bond directionality via opto imaging.

You should also understand how to push the vibrational spectroscopy down to a single bond limit and how to manipulate water molecules in a controlled manner.