Supporting Information

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1 Temperature Effect on Transport, Charging and Binding of Low-Energy Electrons Interacting with Amorphous Solid Water Films Roey Sagi, Michelle Akerman, Sujith Ramakrishnan and Micha Asscher * Institute of Chemistry, Edmond J. Safra Campus, Givat-Ram, the Hebrew University of Jerusalem, Jerusalem , Israel of corresponding author: micha.asscher@mail.huji.ac.il Supporting Information I. Temperature effect on films growth: CPD measurements Figure S1: The CPD measured by a Kelvin probe during the ASW film growth on a Ru(0001) single crystal surface at the indicated substrate temperatures. Deposition of water molecules on a Ru(0001) substrate was performed by backfilling the UHV chamber with water vapor at a substrate temperature in the range of K, while recording the contact potential differences ( CPD) by an in situ Kelvin probe. In Figure S1, the CPD profile measured during the film growth is demonstrated to be strongly substrate temperature dependent, revealing a complex behavior. S1

2 For all temperatures, an initial sharp drop of the CPD signal to a minimum value (-1.65±0.02 V), regardless of T gr, is observed for film thicknesses above 10 ML. As the growth continues, the substrate temperature strongly affects the overall profile. Between 50 and 80 K the slope of the CPD vs. thickness decreases as T gr increases. Between 80 and 105 K the slope increases, and reaches its maximum. The minimum CPD value measured at 10 ML indicates the complete coverage of the Ru substrate by water molecules. The water molecules initially tend to grow on the Ru surface as islands 1-3. Therefore multilayer thickness is necessary for complete coverage of the Ru surface by water molecules, thus forming a partially aligned first layer dipole. Below 80 K, the slope decreases, possibly due to a smaller screening effect of the first layer dipole. In the range of K, the effect of the screening increases, therefore the slope increases. Above 105 K, the screening effect becomes weaker and eventually disappears (at T gr 120 K). These observations indicate that there are morphology differences between films grown at different substrate temperatures, which in turn influence the net dipole of the entire film. Bu et al. 4, 5 suggested that porosity is a major contributor for surface potential differences between films. The porous ASW/vacuum interface exposes a large number of dangling-bonds whose net dipole may influence the CPD values. The porosity is governed by the substrate temperature during the growth of the ASW film and is negligible for growth temperatures above 120 K. In addition, our data suggest that colder substrate temperatures gradually quench the local mobility (rotation) of water molecules within the ASW layers beyond the initial 10 ML. This screens the effect of polarization caused by the bottom layers. The same effect can be explained by a temperature dependent dielectric constant ε(t) that gradually increases with temperature 6. This may lead (at high growth temperatures) to partial alignment of the incoming water molecules at the vacuum interface, with the polarization that was previously defined by the molecules of the S2

3 first 10 ML. As a result the asymptotic CPD level detected at thicknesses above 500 ML is kept close to the minimum CPD, e.g V, at growth temperature of 120 K, see Figure S1. II. Charging buildup Figure S2: Irradiation of 700 ML thick ASW films by employing 5 ev electrons at the indicated e-beam currents (electron flux); T irr =T gr =80 K. (a) The measured current profiles at the indicated electron currents (flux) between 12 na and 3.4 µa. (b) The measured currents following normalization to the maximum current (top) and to the irradiation flux (bottom) while the time is normalized to maximum current time. The similarity of the various current profiles at current range of more than two orders of magnitude indicates that the measured current is practically e-beam flux independent. S3

4 Figure S3: Stability measurements of the charged 280 ML thick ASW films grown at T gr =120 K and irradiated at T irr =80 K for the indicated e-beam exposure times, t irr,, corresponding to measurements shown in Figure 2a. In section IIIA of the main text we demonstrate the charging buildup of 280 ML thick films grown at T gr =120 K and irradiated at T irr =80 K. As long as the steady-state current has not yet been established, the ASW films will continue to charge. As the time of electron irradiation extends, the developed voltage increases until saturation at a voltage corresponding to the electron kinetic energy. This occurs because the impinging electrons encounter a retarding field and are repelled from the ASW/vacuum interface back to the vacuum. As was shown in Figure 2b, the developed voltage (measured as CPD) can be fitted to Eq. 3, where β can represent the charging of a capacitor in a RC circuit, with RC=346±48 s, or a better defined physical meaning of the electron trapping cross-section σ, where σ=2.0± cm 2. The value of the trapping cross-section obtained in our study is comparable to those reported earlier by Sanche and co-workers (see Ref. 7 and references therein). The data in Figure S3 indicate that under the specific conditions for charging the ASW film, the charging is stable for hours, although measured here for only 1200 s. S4

5 III. Varying T irr over a fixed morphology (T gr) i) T gr =105 K As demonstrated in Figure S1 and in Figures 3 and 4 in the main text, the substrate temperature during growth (T gr ) plays an important role in determining numerous physical parameters such as the surface dipole, the electron transmission, the charge accumulation, and the TP- CPD and its temperature derivative (the d( CPD)/dT profiles). In this work, the above effects are attributed to morphology differences. Similar measurements to those shown in Figures 3 and 4 in the main text were carried out for a growth temperature of 105 K and are presented in figures S4 and S5. T gr of 105 K was chosen due to the maximum slope of the CPD vs. thickness profile (Figure S1, blue curve) observed for that particular growth temperature. Trends similar to those observed for the 120 K growth temperature were found. Investigation of the TP- CPD measurements (Figure S5) reveals a shift to lower temperatures of the shoulder-peak to 96 K (4 K lower than for T gr =120 K), and of the main minimum, which is obtained at 124 K. S5

6 Figure S4: 700 ML thick ASW films grown at 105 K and irradiated by 5 ev electrons at the indicated temperatures (T irr ). (a) Through-the-film electron transmission currents (I) as a function of irradiation time, normalized to the current measured on the clean Ru(0001) substrate (I 0 =1.5 µa). (b) Postirradiation CPD stability measurements at the indicated T irr. Figure S5: (a) TP- CPD spectra of 700 ML thick ASW films grown at 105 K and irradiated by 5 ev electrons for 600 s at the indicated T irr. The heating rate was 1 K/s. (b) The derivative with respect to temperature of the TP- CPD spectra (d( CPD)/dT) shown in part (a). ii) Charged vs. neutral films: d( CPD)/dT profiles The effect of charging on the ASW films can be observed by comparing the d( CPD)/dT profiles of the charged and non-charged films. This can be seen in Figure S6 for T gr =120 K (a) and T gr =105 K (b). The intensities of the main minima near 130 K noticeably increase in the spectra of the charged films (red) when compared with the neutral layers (black). While the minimum of the neutral film is observed at 137 K (127 K) for T gr =120 K (105 K), it shifts to 130 K (124 K) for the charged films. These differences may arise from the molecular S6

7 rearrangements within the films that take place in the presence of the excess electrons (internal electric fields). The existence of the peaks below desorption temperature in the neutral films suggests the onset of morphological modification at the ASW/vacuum interface which accelerates in the presence of trapped charges. Figure S6: d( CPD)/dT of neutral (black) and charged (red, T irr =50 K) ASW films (a) T gr =120 K and (b) T gr =105 K. iii) Derivative spectra (d( CPD)/dT) as a function of T irr In figures 4b and S5b one can see that the d( CPD)/dT peak is observed at a specific, T gr dependent temperature, regardless of the temperature of irradiation. We attempted to asses a statistical meaning for the change in the low temperature shoulder area of the derivative spectra. The area under the shoulder-peak of the d( CPD)/dT profiles was, therefore, calculated for both growth temperatures, 120 K and 105 K, by integrating the derivative profiles over their entire temperature range ( K) in order to exclude any contributions S7

8 from desorption peaks. Then we subtract the integrated area of the main peak (that has a fixed peak minimum). This way we could obtain the contribution to the derivative spectra of the shoulder-peak alone. Figures S7a and S7b present these results vs. the reciprocal of the irradiation temperature (1/T irr ). Both plots were fitted to a ( ) 1 ( ) saturation expression, where D(T irr ) is the derivative value at T irr, and τ and B are best fit parameters. Figure S7: Calculated area under the d( CPD)/dT peak shoulder for films grown at 120 K (a) and 105 K (b) as function of 1/T irr. In red- the best fit to an exponential saturation expression: ( ) 1 ( ) IV. Memory effect: Varying morphology (T gr ) at fixed T irr S8

9 Figure S8: 700 ML thick ASW films, grown at the indicated temperatures and irradiated by 5 ev electrons at T irr =50 K. (a) Through-the-film electron transmission currents (I) as a function of irradiation time, normalized to the current measured on the clean Ru(0001) substrate (I 0 =1.5 µa). (b) Post-irradiation CPD stability measurements (normalized) of films grown at K. From the data presented in figures 1a, 3a and S4a one can see that both T gr and T irr strongly affect the transmission current. For example, in the case where T irr and T gr were both equal to 50 K (Fig. 1a, black curve), the peak intensity of the transmitted current was much lower than if the film was grown at 120 K and irradiated at 50 K (Fig. 3a, black curve). In both cases, the maximum charging level is similar. Figure S8 presents measurements of normalized current (I/I 0 ) and of charging stability ( CPD vs. time) for 700 ML thick ASW films grown at different temperatures (T gr ), and irradiated at T irr = 50 K. The current vs. time profiles are relatively narrow (note the time scale, only 0-50 s), however they broaden slightly as T gr increases. One can see that the initial rise in current and the maximum value of the current are T gr dependent for growth temperatures below 90 K. All irradiations end with close to zero steady-state currents and with similar level of charging, see Fig. 5. As discussed in the main text, an anomaly in the charging stability exists for films grown at T gr < 60 K. A more detailed view of this anomaly is demonstrated in Figure S8b by comparing the normalized CPD profiles ( CPD (t) / CPD (t=0) ) in the growth temperature range of K. There is a rapid decay of charging (onset of discharge) as the growth temperature decreases from 60 K to 50 K, a counterintuitive behavior. It has been discussed in terms of gradually increasing degree of space charge effect as the temperature decreases since the layer thickness where the electrons are trapped become thinner as the T gr gradually decreases in the range of K. S9

10 V. Film thickness effect i) T gr =120 K, T irr =50 K At low irradiation temperatures, even relatively thin films can accumulate significant charge. This is apparent in Figure 7 (main text) for both the profiles of the electron transmission current and the CPD measurement over time. Significant charging of thin films can result in extremely high internal electric fields. A 50 ML thick film presents a 4 V contact potential difference. Considering that 1 ML ASW is equivalent to ~0.4 nm layer thickness 6, it leads to a total film thickness of approximately 20 nm. An electric field of V/m has been formed (according to Eq. 2 in the main text). ii) Temperature-Thickness correlation Figure S9: Transmission currents at two thicknesses and temperatures revealing correlation and perfect overlap between 840 ML thick ASW film grown and irradiated at 100 K (black) and 140 ML thick ASW film grown and irradiated at 55 K (red). CPD stability measurements (inset) demonstrate different relaxation times. VI. Similarities between the effects of growth and irradiation temperatures and film thickness on the evolution of the current over time are apparent. As can be seen in Figure S9, a perfect overlap of the current profiles can be obtained for two different films: one ASW S10

11 film which is 840 ML thick, grown and irradiated at 100 K, and the other which is 140 ML thick, grown and irradiated at 55 K. Both films were annealed, prior to their irradiation, to 120 K at a heating rate of 1 K/s. While the current profiles coincide, the stability measurements of the CPD are rather different, revealing different relaxation times because of the varied T irr. Exponential fitting of the decaying charging and extrapolation for the initial charging level immediately after irradiations were terminated hint for their identical initial CPD values. References 1. Doering, D. L.; Madey, T. E., The adsorption of water on clean and oxygen-dosed Ru(001). Surf. Sci. 1982, 123 (2-3), Haq, S.; Clay, C.; Darling, G. R.; Zimbitas, G.; Hodgson, A., Growth of intact water ice on Ru(0001) between 140 and 160 K: Experiment and density-functional theory calculations. Phys. Rev. B 2006, 73 (11), Maier, S.; Lechner, B. A. J.; Somorjai, G. A.; Salmeron, M., Growth and structure of the first layers of ice on Ru(0001) and Pt(111). J. Am. Chem. Soc. 2016, 138 (9), Bu, C.; Shi, J.; Raut, U.; Mitchell, E. H.; Baragiola, R. A., Effect of microstructure on spontaneous polarization in amorphous solid water films. J. Chem. Phys. 2015, 142 (13), Bu, C.; Baragiola, R. A., Proton transport in ice at K: Effects of porosity. J. Chem. Phys. 2015, 143 (7), Tsekouras, A. A.; Iedema, M. J.; Cowin, J. P., Amorphous water-ice relaxations measured with soft-landed ions. Phys. Rev. Lett. 1998, 80 (26), Naaman, R.; Sanche, L., Low-energy electron transmission through thin-film molecular and biomolecular solids. Chem. Rev. 2007, 107 (5), S11