Temperature Effect on Transport, Charging and Binding of Low

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Temperature Effect on Transport, Charging and Binding of LowEnergy Electrons Interacting with Amorphous Solid Water Films Roey Sagi, Michelle Akerman Sykes, Sujith Ramakrishnan, and Micha Asscher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01674 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Temperature Effect on Transport, Charging and Binding of LowEnergy 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 9190401, Israel

e-mail of corresponding author: [email protected]

Abstract The charging of solid molecular films grown on grains is an important phenomenon observed in astrochemical processes that take place in interstellar space and is relevant in high altitude environmental physics and chemistry. In this work, we present temperature dependence study of both the conductivity and capacitance of Amorphous Solid Water (ASW) films (hundreds of monolayers thick) deposited on a Ru(0001) substrate. These layers subsequently interact with low-energy electrons (5 eV) at the temperature range 50-120 K under Ultra-High Vacuum (UHV) conditions. The charging of the ASW films was measured via Contact Potential Difference (CPD) detection utilizing an in situ Kelvin probe and found to be sensitive to the substrate temperature during film growth, to the substrate temperature during electron irradiation and to the film thickness. Internal electric fields exceeding 108 V/m are developed. It is shown that solid water conducts and stores electrons with “memory” of the film's thermal history. Furthermore, we propose that trapped electrons discharge during substrate annealing in a process that is driven by the formation and propagation of cracks within the molecular layer, similar to the release of gas 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

molecules embedded inside ASW films, at significantly lower temperatures than the onset of crystallization. Thermal binding energies of electrons to the ASW matrix are obtained from the discharge measurements, in the energy range of 0.26±0.08 eV. These values are one order of magnitude smaller than those obtained via photoemission studies.

I.

Introduction

The charging phenomena within molecular solids and near their surfaces have an important role in diverse processes in nature, which include astrochemical, biological and environmental systems. When interacting with condensed matter, most forms of high-energy radiation generate low-energy (typically less than 50 eV) secondary electrons. These secondary electrons are chemically active and responsible for many of the radiation-induced chemical reactions1. Relevant examples are the non-thermal reactions in water ices, with or without other co-adsorbed molecules. Amorphous Solid Water (ASW) grown by condensation of water vapor on cold solid surfaces (T < 130 K) is the most abundant form of water in the universe2-5. Therefore, lowenergy charged particles, electrons and ions, interacting with molecular solid materials, are among the fundamental and most common physical and chemical events. The condensed phases of water reveal complex properties, such as charge accumulation and transport through ice. Numerous studies have investigated the interaction of low-energy6-9 and high-energy10 positively charged ions with ASW. Similar studies with low-energy electrons9, 11-13 have been relatively neglected, despite

2 ACS Paragon Plus Environment

Page 3 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the fact that it has been shown that electrons may be transmitted through water films under certain conditions9, 13. Condensed-phase water molecules can trap low-energy electrons via the process of solvation. Its stabilization is obtained by the interaction of the injected charges with the dipole-moments of the surrounding water molecules. Excess electrons within water films exist in three characteristic states. In the first state, the electrons are confined in space in a way that provides maximum stabilization (the ground state). Nonetheless, a question of whether the wave function of these excess electrons overlaps that of the surrounding water molecules is under debate in the literature. Traditionally, it has been thought that the excess electrons reside inside a solvent cavity, e.g. Kevan structure14. However, a more recent model15 suggests the existence of a wave function overlap. This solvated electron can be excited to higher energetic level, while remaining trapped, however having exceedingly short lifetimes16. In the third state, the electrons exist transiently as delocalized waves in the water conduction band (see Ref. 17 and references therein). Another view suggests that the electrons residing in shallow traps and are only partially solvated, will become fully solvated as a result of considerable rearrangement of the surrounding water molecules18. Completion of this solvation process is characterized by a very long time scale (hours) at low temperatures. One may conclude that ASW and ice films, as other molecular films (e.g. condensed oxygen19, chloromethanes20,

21

and ammonia22), can be charged by exposure to

incident electrons at various energies using an electron source. Once charges begin to accumulate at the ASW/vacuum interface, a voltage is developed between the two



Page 4 of 40

electrodes, namely the ASW/vacuum interface and the metallic substrate. This voltage can be measured as a contact potential difference (CPD) using a Kelvin probe. It has been shown that the accumulation of low-energy positive charges on top of the ASW film reveals a linear ∆CPD (change of the CPD) dependence on the film thickness6, 8-10 and is consistent with a classical plate capacitor model, given by ∆𝑉𝑉 =

𝑄𝑄𝑄𝑄 𝐴𝐴𝜀𝜀0 𝜀𝜀(𝑇𝑇)

(1)

where Q is the accumulated charge, A is the area of the ASW exposed to the striking

charges, L is the film thickness, ε0 is the vacuum permittivity and ε(T) is the static dielectric constant of the film. At temperatures below 150 K, it is often assumed to be

constant at ε(T) ∼3.2, mostly when discussing crystalline ice23. However, for ASW films its value is under debate24,

25

. This level of charging can yield very high

electrostatic fields (E) 6, 9, 26, in the range of 1-10×108 V/m, according to 𝐸𝐸 =

∆𝑉𝑉 𝐿𝐿

(2)

where L is of the order of a few hundreds of nanometers. The electric field values

obtained by positive charge accumulation are higher than any value achievable by conventional metal plate capacitors. Thus, the charging of ASW films provides many advantages in studying the effects of electric fields on molecular processes in condensed phases. For example, it has been shown that charging of these films can lead to reorientation of co-adsorbed dipolar molecules such as acetone27 and trans to gauche conformational change of 1,2 dichloroethane28. However, one cannot neglect thermal mobility of small molecules within the ASW pores (not electric field driven) as pointed out in Ref. 29.


Page 5 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Due to the large electron exposure (relative to that of ions), a different picture is obtained in the case of low-energy electron charging of relatively thick ASW films9. Bombardment of electrons results in an asymptotic behavior of the measured ∆CPD. The maximal ∆CPD voltage is dictated by the impinging electron kinetic energy due to the generation of a retarding field developed by the accumulated charges within the water/vacuum interface9, 30. Therefore, the maximum ∆CPD value attained is strongly correlated with the incident electron kinetic energy. In this manuscript we present the time, temperature and layer thickness dependence of the transmission and accumulation of low-energy electrons interacting with ASW films.

II. Experimental methods The experiments reported here were conducted in a previously described system31 under Ultra High Vacuum (UHV) conditions with a base pressure < 2×10-10 Torr. A Ru(0001) substrate is attached at the bottom of a closed-cycle He cryostat (Janis), which cools the sample down to 35 K. The substrate surface is daily cleaned by a 12 minute sputter using 800 eV Ne+ ions, with subsequent flash-annealing to 1450 K (heating rate of 10 K/s) and stabilization at this temperature for 180 s to allow for the desorption of surface oxygen atoms. Then, the substrate undergoes controlled cooling to a desired temperature at a constant rate (3 K/s). Temperature readings are obtained via a K-type thermocouple spot welded to the side of the ruthenium single crystal (8x8x2 mm) providing accuracy of ±1 K. A pair of 0.5 mm diameter Ta wires spot welded to the edges of the sample is used in a resistive heating procedure to elevate the sample temperature. These Ta wires are attached to a pair of 3 mm diameter Ta 5 ACS Paragon Plus Environment


Page 6 of 40

rods connected to a sapphire plate at the bottom of the cryostat. A computer controlled heating algorithm (LabView) enables temperature programmed desorption (TPD) measurements. Stabilization of the sample temperature below 300 K is performed by a LakeShore 335, utilizing a Si diode attached to the bottom of the Hecryostat as a high accuracy temperature monitor. Water molecules were introduced into the system by backfilling the UHV chamber, in order to ensure identical ASW layer thickness over all cryogenically cooled conducting elements in addition to the sample. Triple distilled water, maintained in a glass ampoule, was further purified by several freeze-pump-thaw cycles. The chamber is equipped with a Kelvin probe (KP S- Besocke-Delta-Phi) for measuring ∆CPD. Combining the ∆CPD measurements with the TPD routine enables us to continuously monitor the changes in work function (∆Φ-TPD or TP-∆CPD procedures) as rearrangements of the adsorbates take place prior to any desorption. Surface cleanliness can be determined by employing a LK Technologies mini-Auger analyzer. Exposure is calculated in Langmuir units (L) where 1 L=10-6 Torr×s. Exposure of 1 L of water vapor is equivalent to the adsorption of 1 monolayer (ML) of the ASW film, as derived from the onset of the ice multilayer desorption peak near 160 K while performing TPD as a function of water coverage. A Kimball Physics ELG-2 electron gun enables the exposure of the sample to lowenergy (1-200 eV) electrons. Typically, the electron current measured on the clean Ru(0001) sample was 1.5 μA, recorded using a Keithley 6487 picoammeter.


Page 7 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


III. Results The results of electron charging of ASW films grown on a Ru(0001) substrate will be presented and discussed below. These experiments include the study of the current transmission of electron beams through ASW layers and the ∆CPD detection of the voltage resulting from charge accumulation on the film surface. The ASW films were grown at various substrate temperatures, termed Tgr, and then irradiated with electrons at various substrate temperatures, Tirr (Tirr was not necessarily kept equal to Tgr). Deposition of water molecules on the substrate was performed by backfilling the UHV chamber with water vapor at substrate temperatures in the range of 50-120 K while recording the changes of the CPD. The ∆CPD profiles, measured during the film growth, are strongly substrate temperature dependent and reveal a complex trend (see Supporting Information Fig. S1). For all growth temperatures, an initial sharp drop of the ∆CPD signal to a minimum value (-1.65 V, regardless of Tgr) is observed below 10 ML. This value indicates the complete coverage of the Ru surface (as water tend to grow on Ru(0001) surface as islands32-34). As the growth continues, the ∆CPD signal rises with strong Tgr dependence, affecting the ∆CPD value observed at the end of the film growth. These non-identical values indicate that there are morphological changes in films grown at different substrate temperatures (mostly varied degree of porosity25,

35-37

). Another interpretation might be that this strong temperature effect

occurs due to the

temperature dependent dielectric constant ε(T) that gradually

increases with increasing the growth temperature24. This would influence the net dipole of the entire film as detected by the Kelvin probe.

Most previously reported experiments regarding the charging of ASW and ice films with electrons used low-energy electron transmission (LEET) spectroscopy38 in order to calculate the cross-sections for trapping the excess electrons11-13. In those 7 ACS Paragon Plus Environment


Page 8 of 40

experiments, the water films were bombarded with electrons at low currents (femtoto nano-Amperes), 3 to 9 orders of magnitude lower than those reported here. Usually, low currents are used to prevent significant charging of the films. Here, we seek to maximize this effect. Our measurements revealed only negligible variation in the current spectra profiles while changing the electron flux in the nA to μA incident ebeam current range (see Supporting Information Fig. S2). None of these electron bombardment studies have monitored hydrogen molecules evolving due to water molecule dissociation. We cannot, however, rule out small decomposition levels below our instrumental detection limit that apparently do not affect the results discussed below. Decompositions were reported due to electron bombardment but usually at higher electron energy (threshold of 6.3 eV for Dissociative Electron Attachment mechanism39 and much higher yield at energy of 100 eV or more through ionization initiated route40). Profiles of the electron transmission currents and the corresponding charging of 700 ML thick ASW films grown and irradiated at substrate temperatures in the range of 50-100 K are shown in Fig. 1. The temperature significantly affects both the electron transmission current and the charging. The intensity, position (in time), and width of the peak current, along with the steady-state current reached after longer electron exposure, and the level and stability of the charging, are all strongly substrate temperature dependent. Irradiation and ∆CPD detection, as shown in Fig. 1, were carried out at the films’ growth temperatures (i.e. Tirr=Tgr without any temperature modifications in between). Constant electron kinetic energy of 5 eV (relative to ground level) and constant flux (equivalent to current of 1.5 µA measured on the clean Ru substrate) were used throughout this study. The electron irradiations (and current measurements) were followed by ∆CPD recording with a 7 minute delay 8 ACS Paragon Plus Environment

Page 9 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between the two measurements, dictated by the UHV chamber geometrical constraints. It is well established that the (low) conductivity of semiconductors (pure water has a band gap of ~11 eV41 and is considered an insulator) is expected to increase as the

temperature increases. Furthermore, the conductivity of a solid also depends on its morphology (thickness, porosity, pore dimensions, defects, etc.). As was previously mentioned, the morphology of the ASW films is a temperature dependent parameter, which affects mostly the degree of porosity. Water typically grows with a higher degree of porosity as the substrate temperature decreases42, 43. In addition, amorphous solid water is known to exist in one of two phases44, 45: low density amorphous water (LDA)- ρc=0.94 g/cm3 for Tgr > 50 K, or high density amorphous water (HDA)- ρc=1.17 g/cm3 for Tgr < 50 K, where ρc is the compact film density. Given the temperature range discussed in our study, we may assume that our ASW films are primarily in the low-density phase. Finally, the electric fields that arise from charging the films may lead to rearrangement of the water molecules, hence changing their electrical (average dipolar) characteristics.



Page 10 of 40

Figure 1: 700 ML thick ASW films bombarded by 5 eV electrons. Tirr=Tgr (Tirr= sample temperature during irradiation; Tgr=ASW growth temperature). (a) Through-the-film electron transmission currents (I) as a function of electrons irradiation time at the indicated film temperatures, normalized to the current measured on the clean Ru(0001) substrate (I0=1.5 μA), and (b) ∆CPD measurements vs. time following the charging period shown in Fig. 1a, at the indicated substrate temperatures.

In the coming sections we will separate between the influence of the temperature and the morphology. In section A we will discuss the charge accumulation with respect to the electron exposure time. In Section B the influence of the substrate temperature on the transport of electrons through films grown at a constant Tgr will be demonstrated. Section C will present the effect of different Tgr on films irradiated at constant Tirr 10 ACS Paragon Plus Environment

Page 11 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Tirr≠Tgr). In section D we will discuss how the layer thickness affects both the electron transmission current and the overall charging. III. A. Charging buildup Control over the amount of trapped charges within the ASW film can be obtained by varying the exposure time of the film to the e-beam, tirr, as shown in Fig. 2. Maximum charging in any given film may be obtained only if all of the traps that are accessible to the impinging electrons are occupied while the (negative) charges are stabilized for exceedingly long time (thousands of seconds, as in Fig. 1b). The results in Fig. 1 imply that one of the conditions for obtaining the maximum charging is that the transmission current will approach zero. 280 ML thick films, grown at Tgr=120 K and irradiated at Tirr=80 K were used to demonstrate the irradiation time (tirr) effect. As tirr extends beyond the maximum current intensity, the extent of charging increases, however, at a gradually lower rate, as extracted from the derivative of the red fitting curve in Fig. 2b. In other words, within the current's steady-state region (Fig. 2a, beyond 3500 s), the charge accumulation reaches its maximum value and becomes saturated (d(∆CPD)/dtirr≅0). In cases where the steady-state current drops to zero (e.g.

T=50 K in Fig. 1a, black curve), the maximum ∆CPD voltage will be correlated with the electrons' kinetic energy. As shown in Fig. 2b, plate capacitor charging voltage (∆CPD) vs. tirr follows the expression: 𝑉𝑉(𝑡𝑡𝑖𝑖𝑖𝑖𝑖𝑖 ) = 𝑉𝑉0 × [1 − 𝑒𝑒𝑒𝑒𝑒𝑒(−𝛽𝛽𝑡𝑡𝑖𝑖𝑖𝑖𝑖𝑖 )] + 𝐵𝐵

(3)

where 𝜷𝜷 = 𝟏𝟏⁄𝑹𝑹𝑹𝑹, is a typical time dependent capacitor charging model (RC circuit), and R and C are the apparent resistivity and capacitance of the film, respectively. A

better physical description looks at the expression in eq. 3 as representing the probability for an electron to be trapped19 by extracting the charge trapping cross-



Page 12 of 40

section according to 𝜷𝜷 = 𝝈𝝈𝑰𝑰𝟎𝟎 /𝑨𝑨𝒆𝒆, where σ is the charge trapping cross-section, I0 is

the current measured on the bare sample, A is the sample area and e is the elementary charge. B is a bias voltage element that is associated with the voltage arising from the polarization of the neutral water film developed during its growth (see Supporting Information Fig. S1). The bias voltage may also include contributions from the ebeam energy spread (±0.2 eV), and from other, not quantitatively understood parameters, such as the energy levels of the traps relative to the vacuum level. At the end of ∆CPD measurements the charging is stable for long periods of time, significantly exceeding 1200 s as demonstrated in the Supporting Information Fig. S3. The trapping cross-section obtained from Fig. 2b is σ=2.0±0.3×10-16 cm2. This value is comparable to cross sections obtained in LEET measurements using 10-14 A of electron flux13. The ASW films shown in Fig. 2 reveal yet another interesting characteristic of the current profile. During the first few seconds of exposure to the e-beam, an instantaneous, steep increase in the current takes place (up to 0.4 of I0). The current becomes stable for about 100 s and then it increases again until the main peak is established.


Page 13 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: (a) Through-the-film transmission currents of 5 eV electrons striking 280 ML thick ASW layers grown at 120 K and irradiated at 80 K for the indicated irradiation times. (b) The Post-irradiation ∆CPD as a function of the electron beam irradiation time, tirr. The red line is a fit to a saturating exponent, expressed in Eq. 3.

III. B. Irradiation temperature effect on films of identical morphology To avoid any morphological differences prior to electron irradiation, films were grown at 120 K and then were cooled down to a given irradiation temperature, Tirr. Fig. 3 shows the transmission current profiles (a), followed by ∆CPD measurements of the charged films for stability assessment (b). Here, the current peaks broaden and shift to longer irradiation times with increasing Tirr. In contrast to the current profiles presented in Fig. 1, no significant changes in the peak intensity exist here (0.6±0.2). 13 ACS Paragon Plus Environment


Page 14 of 40

There are however, differences in the peak profile: its width and the steady-state current (beyond 600 s). Post irradiation ∆CPD measurements (Fig. 3b) reveal a similar trend to that seen in Fig. 1: lower level of charging as Tirr increases with negligible charging at Tirr=120 K. At the 90-105 K Tirr range, where the steady-state current (I/I0) is higher than 0.5, there is only partial charging (3.0±0.5 V) at irradiation time of 600 s.

Figure 3: 700 ML thick ASW films grown at 120 K and irradiated by 5 eV electrons at the indicated Tirr. (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 (I0=1.5 μA). (b) Post-irradiation ∆CPD vs. time measurements.

The response of the charging to subsequent sample heating was examined in a temperature programmed ∆CPD mode (TP-∆CPD) at a heating rate of 1 K/s (see Fig. 4a). The ∆CPD sharply decreases in the temperature range of 110-140 K (prior to any 14 ACS Paragon Plus Environment

Page 15 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


desorption) until it reaches a minimum of -1.7 V at 150 K. At 180 K the ∆CPD increases again until the Ru surface is clean, following desorption of all the water molecules, at ~230 K.

Figure 4: (a) TP-∆CPD spectra of 700 ML thick ASW films grown at 120 K and irradiated for 600 s at the indicated Tirr. Heating rates were 1 K/s (b) The derivatives of the TP-∆CPD profiles with respect to temperature (d(∆CPD)/dT).

Previous studies46 have noted the similarity between standard TPD measurements (ΔP-TPD) and the temperature derivative of the TP-∆CPD profiles of non-charged ASW films on a Ru substrate. The derivative profiles (d(∆CPD)/dT) of the charged films (Figure 4b) proceed via a well-defined minimum, independent of Tirr, at T=130 K with a smaller, low temperature shoulder at 100 K. These two minima are practically absent in non-charged films grown at 120 K. In the neutral films, only one



Page 16 of 40

minimum with significantly lower intensity is seen (at 137 K, the black curve in Supporting Information Fig. S6a). The clear presence of the low temperature double peak in the charged films implies that there are two different principal populations of trapped electrons. Upon annealing, the trapped electrons overcome energetic barriers and flow to be neutralized on the metallic substrate. The width of the lower temperature shoulder is Tirr dependent and is observed between 50 K and 100 K (see Fig. 4b). The widest profile of this shoulder (stretched over about 50 K) is obtained at the lowest Tirr (50 K) and it gradually shrinks to zero width at Tirr=100 K. The broad temperature range of discharge indicates a wide range of binding energies for the electrons. As Tirr increases, the probability to trap an electron in the lower temperature sites gradually diminishes. Therefore, this shoulder narrows at higher irradiation temperatures. The elimination of the lower temperature sites during heating does not feed the population of the high temperature sites, as the 130 K peak intensity remains constant. The 130 K minimum is followed by a minor maximum at 157 K (possibly due to crystallization47, 48) and then three additional maxima are observed, which are attributed to desorption of the multilayer (small maximum at 163 K), to the first monolayer of water (183 K) and to water molecules that are bound to partially dissociated molecules49 (212 K). Above 150 K all measurements, regardless of Tirr, overlap each other and reflect the neutral water molecules desorbing from the ruthenium surface.


Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


III. C. Memory effect: Different Tgr at fixed Tirr The previous section has shown the effect of the irradiation temperature on the transmission currents and on the charging level of films with a morphology determined by its fixed growth temperature (120 K). In this section, the dependence of the current and charging on the morphology (Tgr) at constant Tirr will be addressed. ASW layers, 700 ML thick, were grown at different temperatures (Tgr), and were subsequently irradiated at a fixed temperature (Tirr=50 K). For this set of measurements, the current vs. time profiles broaden slightly as Tgr increases, although the profiles are relatively narrow (see Supporting Information Fig. S8a, note the time scale of only 0-50 s). In addition, the initial current rise and the maximum value of the current are Tgr dependent for growth temperatures below 90 K. All irradiations terminate with a steady-state current close to zero and with similar charging levels as shown in Fig. 5 (note the expanded ∆CPD axis between 6.0-6.8 eV). The ∆CPD behavior reveals the effect of Tgr on the stability of the charged film. ASW films grown at Tgr < 60 K exhibit an unusually fast decay of the ∆CPD signal. The rate of decay decreases as Tgr approaches 60 K (see Supporting Information Fig. S8b). At Tgr ≥ 60 K, the ∆CPD signal slightly increases with time (0 to 1000 s) to a value of 6.4±0.4 V. It seems that there are two different stabilization mechanisms for reducing the free energy of the charged films. At growth temperatures below 60 K, the system attempts to stabilize the charged capacitor by reducing the high electric field inside the film. This may occur through a proton cascade towards the top, charged, layer through the poorly coordinated water molecules network that constructs the ASW pore walls25,

35

or by pushing the trapped electrons deeper. This leads to a time



Page 18 of 40

evolved screening of the charges, and thus to a decaying ∆CPD profile. In the higher growth temperature regime (Tgr > 60 K), where less porous and more rigid water matrices form (in which the proton mobility is limited), the system is being stabilized by "pushing" the electrons towards the water/vacuum interface. This may explain the slow and small rise of the ∆CPD with time. Whatever the stabilization mechanism may be, for both ranges of growth temperature, the charging approaches its maximum possible level defined by the electron kinetic energy.

Figure 5: Post e-beam irradiation charging level via ∆CPD measurements as a function of time of 700 ML thick ASW films, grown at the indicated temperatures and bombarded for 600 s by 5 eV electrons at a fixed Tirr=50 K. Note the expanded y-axis compared to Fig. 1b.

The corresponding TP-∆CPD measurements presented in Fig. 6 reveal that the sharp drop of the ∆CPD (discharge) appears at gradually increasing temperatures as Tgr increases (Fig. 6a). A corresponding shift of the minimum of the derivative spectra (d(∆CPD)/dT) with a minor decrease in their intensity are shown in Fig. 6b. These measurements emphasize the apparent morphological differences between the layers that are kept as a “structural memory” due to the varied growth temperature. If the structure of the films was kept identical, one would expect to observe no differences 18 ACS Paragon Plus Environment

Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in both the time-dependent (charging stability) and the temperature-dependent (discharge) ∆CPD profiles.

Figure 6: (a) TP-∆CPD spectra of 700 ML thick charged ASW films grown at the indicated temperatures and irradiated at a fixed Tirr=50 K, recorded at a constant heating rate of 1 K/s (b) The derivative of the TP-∆CPD profiles shown in (a) (d(∆CPD)/dT) vs. temperature. (c) Apparent binding energies of the electrons to the ASW films as derived by a Redhead like



Page 20 of 40

analysis of the main minimum vs. the reciprocal growth temperature. In red- a best fit to a decaying exponent expression.

The derivative of the TP-∆CPD profile can be compared to the standard ∆P-TPD measurement for a given system, since desorption may modify the surface work 2 function. In Figure 6c, the outcome of a Redhead-like treatment50 (𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑 ⁄𝑅𝑅𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚 =

(𝜗𝜗⁄𝛽𝛽 ) exp{− 𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑 ⁄𝑅𝑅𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚 }) is presented for calculating the binding energies of the

electrons to the morphologically varied water films. In Figure 6b, the main minimum in the derivative spectra is shown to shift to higher temperatures as the growth temperature increases. This treatment enables us to obtain binding energies (BE) in the range of 4-8 kcal/mol. It was found that these binding energies decay exponentially when plotted vs. 1/Tgr, following the expression: 𝐵𝐵𝐵𝐵�𝑇𝑇𝑔𝑔𝑔𝑔 � = 𝐵𝐵𝐵𝐵0 𝑒𝑒𝑒𝑒𝑒𝑒�− 𝜏𝜏⁄𝑇𝑇𝑔𝑔𝑔𝑔 � + 𝐵𝐵, where τ and B are best fit parameters. The increased effective binding energy as the growth temperature increases is possibly due to the smoother structure of the pores. The presence of more corrugated pores at lower Tgr leads to an increased surface area of the film36, and thus to a larger number of traps that are more likely located at interfaces. However, the energetic barrier needed to overcome on the way to discharge is apparently lower at these morphologies perhaps due to the reduced coordination of the surface molecules together with a proximity effect the negative charges develop which leads to increased repulsion. As Tgr increases and the pores become smoother, there are less trapping sites, but the electrons are more strongly bound to these sites (less repulsion), therefore their discharge appears at higher temperature.


Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


III. D. The effect of film thickness In section III. B. we have demonstrated that as the irradiation temperature increases, the electron transmission profiles broaden, and higher steady-state currents are obtained. This means that as Tirr increases, more electrons transport through the film.

.



Page 22 of 40

Figure 7: Variation of the transmission current (a, c) and of the charging (b, d) vs. time at the indicated film thicknesses (a, b: Tirr=Tgr=120 K; c, d: Tgr=120 K, Tirr=50 K). (e) A scheme of the trapped electrons vertical distribution inside the thick ASW films for low (solid line) and high (dashed line) irradiation temperatures.

In other words, the probability to trap excess electrons is lower and the films become more "transparent" for the electrons. One way to increase the probability of trapping the electrons within the ASW layer before they reach the grounded substrate (to increase the total number of traps in the film), is to increase the film thickness. Figure 7 shows how the film thickness and Tirr affect both the transmission currents and the level of charging of the film. As expected, for both high and low Tirr, the transmission currents vs. time became narrower as the films became thicker, indicating that there are more sites available to trap the transmitted electrons. At high Tirr (120 K), thick layers (more than 10,000 ML) are necessary for significant charging to occur, while at low Tirr (50 K) thinner layers are enough to efficiently charge the film at the topmost layers.

IV. Discussion IV. A. Transmitted currents: The most nontrivial observation we report here is the fact that electron currents are recorded through hundreds of layers of solid water. The electrical properties of ice have drawn much attention throughout the years (see e.g. Ref. 51 and references therein). Water is not an ordinary band-gap material due to the nature of its hydrogen-bonded matrix. It is sometimes referred to as a 'protonic semiconductor', and no detectable conduction of electrons is expected when subjected to external electric fields. Therefore, some questions arise regarding the nature of the 22 ACS Paragon Plus Environment

Page 23 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron conduction channels through the solid ASW. Do the electrons transport through a conduction band of the water film (despite the challenging intrinsic disorder within ice or ASW films, attempts to characterize its band structure were made39) or via a conjugated energy band-like system, and whether these channels are associated with the bulk or with the internal surface (e.g. grain boundaries, pores or cracks). A network of occupied traps can also be considered as donors of charges, thus transforming the water film into an "n-type-like" semiconductor. If the density of the occupied shallow traps or defects is sufficiently high, it can lead to the formation of energy bands. As the ASW film is exposed to the e-beam and charges begin to accumulate, these energy bands can be filled. Due to the electric field that develops between the two electrodes (the top electrode is composed of electrons solvated within the top ASW/vacuum interface layers and the bottom electrode is the Ru(0001) substrate), charges will transport across the film in a similar manner to that known for ordinary doped semiconductors. The origin of negative charge conduction by ASW is currently not well understood. Note, however, that the overall current profiles are not e-beam flux dependent in the 10-8-10-6 A range, as discussed in the Supporting Information Fig. S2. To minimize artifacts, our water deposition procedure has been via backfilling the UHV chamber. This way, all of the cold and conducting sample support elements are covered with an ASW layer of the same thickness as the sample itself (assuming unity sticking probability of water over all exposed cold surfaces). This method avoids current "leaks" via clean metallic surfaces that may be exposed to the electron beam. Generally, the current measured between two contacts is linked to the applied voltage by Ohm's law, 𝐼𝐼 = 𝜌𝜌𝜌𝜌, where in this case I is the current measured between the

metallic substrate (ruthenium) and the ground, ρ is the apparent conductivity of the

23

ACS Paragon Plus Environment


Page 24 of 40

ASW film and V is the contact potential difference change (∆CPD) across the film

(see Eq. 1). However, due to the propensity of cold solid water to trap charges and due to the evolution of the voltage that develops across the film over time, Ohm's law in its simple form cannot be applied here. The increasing measured voltage (reflecting the electric field inside the film) is associated with more charges that gradually accumulate at the ASW/vacuum interface. Furthermore, variations between films grown at different Tgr or irradiated at different Tirr will introduce another parameter

that affects the electric field inside the ASW films; that is the ASW film's temperature dependent dielectric constant ε(T). The temperature dependence of the dielectric

constant of ASW is a concept which is not yet fully understood and is debated in the literature, see Eq. 1 above. Some models for determining the effective dielectric constant will take into account the interface between two environments with different dielectric constants, in this case the ASW and the vacuum. The degree of porosity affects the total surface area of the film, thus affects the amount of interaction between the two environments. Therefore, variations in the degree of porosity of the ASW film will lead to a different influence of the vacuum on the ASW film effective

dielectric constant and may alter the film capacity and conductivity. The increased surface area can also enhance defect mobility because these motions are facilitated on the surface52. The increased porosity will also lead to higher density of surface traps, thus, to a higher capacity of the film. Another indirect effect of film temperature is the formation of thinner but denser charging at the ASW/vacuum interface (electrode) at low temperatures. As a result the behavior of this "nano-capacitor" as classical plate capacitor is better defined. Indeed, the currents (I(t)) measured here reveal a complex behavior with time, comparable to the results reported in LEET studies13 (but in our case, at currents that 24 ACS Paragon Plus Environment

Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are orders of magnitude higher), as demonstrated in Figure 2. At first, as the field inside the film develops and begins to increase, the current responds, and rapidly jumps (to I/I0=0.4 and stays at this level for about 100 s under the specific experimental conditions) before increasing to a maximum. Then the current starts to decay over a larger time scale, as shown in Fig. 2. This behavior implies the simultaneous influence of three cross-linked and competing processes that describe the electron-ASW interaction: conductivity, charging and retardation. Electrons impinging the ASW/vacuum interface possess an effective penetration depth before they thermalize at the ASW film temperature. Some of these electrons will be trapped (solvated) and some will be scattered inside the film. As the electron bombardment continues, more traps are being occupied, therefore the density of unoccupied traps decreases, leading to a charged layer of thickness d (where 𝑑𝑑 ≤ 𝐿𝐿, L is the entire ASW film thickness). As already mentioned above, this charged layer

can be considered as the top electrode of a plate-capacitor; the bottom electrode being the grounded metallic substrate. Applying strong electric fields to any water film (crystalline ice or ASW) will generate three distinct processes: 1. Polarization of the individual molecules due to displacements of the electrons relative to the nuclei. 2. Polarization of the entire film due to reorientation of the (polar) molecules along the electric field vector. This process is slow and requires thermal activation and may lead to local violations of the hydrogen-bond matrix structure (i.e. formation of defects). 3. Flow of protons in accordance with the applied field. 25 ACS Paragon Plus Environment


Page 26 of 40

The dynamics of the trapping event originates from the system’s propensity to stabilize and is governed by the different processes listed above. When charging and internal electric fields are introduced into the system, water molecules will polarize and accommodate themselves to the local electric field they encounter, following process 1 mentioned above. As the temperature increases, the water molecules gain sufficient thermal energy to reorient in response to the local field created by the trapped charge, leading to reduced energy of the system. The full stabilization of a trapped electron requires the presence of defects, which facilitates the reorientation of the water molecules. The dynamics of the stabilization process apparently proceeds also through the migration of the defects (both ionic and Bjerrum type)51 according to process 3. This is a reasonable hypothesis since six shell water molecules should direct one of the O-H bonds towards the excess electron as the Kevan structure requires14. After forming the charged layer, the part of the film that is not charged will behave according to process 2 discussed above. In addition to the trapping events, the impinging electrons undergo elastic and inelastic scattering processes inside the film11, 12. In more porous ASW films (obtained at lower Tgr), the density of the traps is higher. Therefore, the mean free path for scattered electrons is shorter, and the probability of an electron becoming trapped increases. This reduces the probability that an electron will reach the counter electrode, leading to a current decrease accordingly. Eventually, a gradually more effective retardation (voltage) of the impinging electrons develops at the ASW/vacuum interface after a short irradiation time. This is shown in Figure 5 and in Supporting Information Figure S8a. Furthermore, impinging and scattered electrons may collide with occupied shallow traps, ejecting trapped electrons, and leading to an increase of the total measured 26 ACS Paragon Plus Environment

Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


current. The retardation of further impinging electrons will start to play a role when the vacant trap density drops significantly. Since slower or fewer electrons can reach the film's surface, the overall current will gradually diminish. This balance forms the current maximum. Because the charged fraction of the film (d/L) is larger (d is thicker) at higher Tirr, it takes a longer time to reach such a peak and the full charging is also a relatively slower process (see Fig. 3). A similar behavior like that observed at lower temperatures occurs as the film thickness increases. The intensity of the overall current profile diminishes as the film thickness increases since the electrons may encounter a larger number of scattering events that eventually will lead to electron trapping. This results in a suppression of the current, which was observed in thinner films. Finally, the establishment of a steady-state current should be explained. The steadystate current regime (the plateau after the peak current) is attributed to a time independent, constant resistivity (or steady-state conductivity), which reflects a constant electric field (Fig. 2) that obeys Ohm's law. This steady-state conductivity is the sum of all of the contributions discussed above, which are all temperature dependent. In the steady-state regime, equilibrium is established between the conductance, capacitance (charging) and retardation of the charged ASW layer. The steady-state conductivity decreases as the irradiation temperature decreases, and at the lowest Tirr, the steady-state conductivity approaches zero (maximum charging and retardation). The internal electric field affects not only the behavior of the electrons, but also the dipole moments of the individual water molecules and the proton-defects within the ASW film. The net dipole will approach equilibrium and the migration of defects will 27 ACS Paragon Plus Environment


Page 28 of 40

be exhausted unless the formation of new defects occurs. These counter-actions will lead to the intermittent reduction of the intensity of the internal film electric field and the resulted retardation, allowing more impinging electrons to interact with and penetrate the film. When the thermal energy is sufficiently high, the system can approach equilibrium at a non-zero steady-state current. IV. B.

Charging: The charging level, determined by the voltage across the ASW

film and measured by a Kelvin probe is very sensitive to the growth temperature (Tgr), irradiation temperature (Tirr), film thickness and irradiation duration (tirr). For the various conditions we tested (120 K and below), once the charging has been established, it is very stable over time (for thousands of seconds, usually measured for 1200 s). At low Tirr (< 60 K), the current proceeds via a sharp peak followed by a rapid drop to (almost) zero, indicating that all the negative charges got trapped and squeezed within a rather narrow fraction of the entire film (𝑑𝑑 ≪ 𝐿𝐿, as illustrated in

Figure 7e). Under these conditions the full charging (∆V > 5 eV) is completed within

a short period of time, in less than 100 seconds, and efficient retardation prevents any further electron transport from taking place. In addition, for growth temperatures below 60 K, there is an apparent space charge effect due to the high density of the trapped (solvated) negative charges within the very top fraction of the film. This results in a repulsion among these trapped electrons that leads to a relatively fast discharge (See Figure 5, the black curve for Tgr=50 K and in the Supporting Information Figure S8b). At higher Tirr, higher thermal energy and faster mobility of the water molecules result in deeper penetration of the electrons and a gradual increase in the fraction of the ASW film that becomes charged (d) (Fig. 7e, the dashed schematic line). As a result, more time is required to complete the charging (See Figures 1 and 3). At both low 28 ACS Paragon Plus Environment

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and high Tirr, in order to complete the charging process, the system has to reach the steady-state current value. While the Tirr is dominant in determining the charging behavior, the growth temperature (Tgr) has an effect of its own, which is of a morphology-memory nature. At high Tgr, the ASW film is less porous and rigid. In order to obtain a current profile similar to that recorded at low Tgr, a compensating low Tirr is necessary. A similar relationship is observed between Tirr and ASW layer thickness, where higher Tirr requires thicker layer to match the current profile observed at low Tirr (See for example Supporting Information Fig. S9). With respect to the internal electric field inside the ASW film, an e-beam striking at a low Tirr may lead to the formation of extremely high electric fields. From the data shown in Figure 7d, an internal electric field of 2±1×108 V/m is obtained (based on Eq. 2, L≈20 nm, ∆CPD=4 V) for film irradiated at 50 K.

IV. C. Discharge: Heating a fully charged ASW film in a TP-∆CPD mode and studying the TP-∆CPD derivative vs. temperature reveal the sensitivity of these measurements to the fine details regarding the morphology-memory of the film, as shown in Figures 4 and 6. It turns out, that irradiating the film at low Tirr leads to a population of more weakly bound electrons, as demonstrated in the lower temperature shoulder in the d(∆CPD)/dT spectrum shown in Fig. 4b. A remarkable observation is that the discharge temperature (130 K for Tgr=120 K, Fig. 4b) of the main population of trapped electrons is independent of the Tirr, but it is strongly dependent on the Tgr (Fig. 6). It turns out that the binding energy of this majority population of trapped electrons decreases as Tgr decreases (see Fig. 6c). It means that the more porous and internally corrugated films that form at low Tgr, lead 29 ACS Paragon Plus Environment


Page 30 of 40

to an increased amount of weakly bound electrons. Moreover, from the data shown in Figure 6b the derivative of the TP-∆CPD spectra- d(∆CPD)/dT, one may extract an apparent (thermal) binding energy of the charged species to the ASW framework under the defined conditions. This is obtained by performing a line shape analysis that mimics that of a simple ∆P-TPD, often termed Redhead TPD analysis that is used to extract activation energy for desorption50. The increased water molecules mobility upon heating the film (mostly rotational motion and defects transport), leads to the discharge of trapped electrons, namely their liberation and transport to the grounded metallic substrate. Bu et al.25 have found a strong correlation between profiles of the TP-∆CPD and IR absorbance of the OH dangling bonds (~3720 cm-1) of neutral ASW films (Tgr ≤ 30 K) during annealing. Because the dangling bonds are associated with surface sites, this correlation shows how the porosity of the film affects its spontaneous polarization and reveals that the film undergoes a dramatic change while it is heated. Yet, more information is needed in order to explain the consistent shift in temperature of the sharp drop of the ∆CPD during annealing between films grown at different temperatures (Fig. 6). One possibility is that this discharge event takes place through successive rearrangements of defects all the way to the substrate. This defect unlocking is a thermally activated process52 and perhaps is also a morphology dependent process. The morphology may affect the temperature at which defects become mobile, thus may affect the discharge temperature observed as a minimum in the d(∆CPD)/dT spectrum. The discharge may also occur through morphological changes that the film undergoes, due to either shrinkage or collapse of the pores during annealing36, which may also affect the dielectric constant24, 53.


Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Another possibility is that the electron discharge proceeds via the formation of nanometer sized cracks and structural failures upon heating the films. In the case of thin water films, trapped molecules are known to desorb abruptly at temperature slightly lower than the water molecules desorption, but usually at temperatures much higher than if it would desorb from the bare metallic substrate (known as the "caging" phenomenon54 or the "molecular volcano" effect55). However, for thick water films, trapped molecules inside the ASW matrix may also desorb at lower temperatures than the crystallization temperature of adsorbed water films and the onset for desorption of the multilayer water molecules56. This was suggested to occur due to the development of cracks and pores, propagating from the bottom of the film towards the water/vacuum interface57. A similar model may apply to the trapped electrons as well, where the release of the electrons (discharge) arises from the formation of miniature cracks propagating through the films, allowing for easier transport along the pore walls and cracks until they reach the substrate, for example at 130 K. This mechanism may explain the sharp drop of the measured ∆CPD voltage at substrate temperatures significantly lower than those required for crystallization (157 K) and for multilayer desorption (160-165 K). Whatever the mechanism may be, the role of the ASW growth temperature on the electron discharge temperature indicates that the film morphology influences the binding energy of the electrons to the traps. This is evident from the consistent shift of the main minimum of the d(∆CPD)/dT spectra with changes in Tgr (Fig. 6b), and implies a morphological "memory" effect within the ASW film on the electron trapping and discharge phenomena. There is a significant difference between the "thermal" binding energies of electrons to ASW (4-8 kcal/mol – 0.26±0.08 eV) and its growth temperature dependence 31 ACS Paragon Plus Environment


Page 32 of 40

discussed in our study and the "binding energies" of electrons obtained via photoemission measurements, typically in the range 3.0±0.5 eV17,

18

. In the

photoemission studies, the binding of electron to ASW includes the effect of the system's work functions in the presence of the water films, which is not always well characterized and also includes the ASW layer thickness, which has its own effect. This is basically the system's ionization energy. The two different kinds of measurements (photoemission and discharge) refer to different "binding energies". We believe that the thermal (discharge) definition is the more accurate among the two.

V. Conclusions The DC conductivity of electrons through ASW films and their charging characteristics were investigated. Both physical phenomena were found to be strongly temperature, morphology and layer thickness dependent. The ability of water molecules to reorient in response to the trapped charges (or in response to the internal electric field) is limited at low temperatures, thereby inhibiting the current flow while stabilizing the overall charging (except for the coldest 50-60 K temperature range due to apparent space charge effect). The morphology of the film, defined by the film growth temperature, affects the stability of the electron traps, which become energetically deeper at higher growth temperatures. Furthermore, it is proposed that the electron discharge is driven by the formation and propagation of cracks within the ASW framework upon heating the films, a mechanism similar to the release of trapped gas molecules at low temperatures. However, in this case the release of the electrons is to the opposite direction: while gas molecules are released out to the 32 ACS Paragon Plus Environment

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


vacuum, the trapped electrons flow towards the metallic substrate. We cannot rule out the possibility that the apparent static dielectric constant (εs) of the ASW film not only depends on the film temperature, but also on the film morphology. In addition, the ASW film thickness necessary for efficient electron trapping (solvation) decreases as the irradiation temperature decreases, allowing for the formation of extremely high electric fields inside the films, more than 2×108 V/m. In conclusion, solid water conducts and stores negative charges with a strong “memory” effect of its preparation. The charged films obey plate capacitor physics with some deviations, apparently because the solvated electrons are spread over a fraction of the film (not precisely defined) and not limited to only a few ASW layers near the ASW/vacuum interface. The thermal binding energies of electrons to the ASW (dictated by the discharge temperature, 0.26±0.08 eV) are very different than the values (3.0±0.5) obtained via photoemission measurements. We suggest that the thermal values are more appropriate as binding energies while the photoemission values should be better referred to as ionization energies.

Supporting Information ∆CPD measurements of ASW films during their growth at various growth temperatures; the charging buildup as a function of the beam flux and as a function of the irradiation time; the effect of varying the irradiation temperature on morphologically different films and a comparison between the d(∆CPD)/dT spectra of neutral and charged films; current measurements and ∆CPD time-stability measurements of the "memory" effect; the thickness effect and the temperaturethickness correlation. 33 ACS Paragon Plus Environment


Page 34 of 40

Acknowledgement Partial support by the Israel Science Foundation (ISF) and by the German Israel Foundation (GIF) is acknowledged. The help provided by Dr. Edvard Mastov and Marcelo Friedman is greatly appreciated.

VI. References 1.

Garrett, B. C.; Dixon, D. A.; Camaioni, D. M.; Chipman, D. M.; Johnson, M. A.; Jonah, C. D.; Kimmel, G. A.; Miller, J. H.; Rescigno, T. N.; Rossky, P. J., et al. Role of water in electron-initiated processes and radical chemistry: Issues and scientific advances. Chem. Rev. 2005, 105 (1), 355-389.

2.

Johnson, R. E.; Quickenden, T. I., Photolysis and radiolysis of water ice on outer solar system bodies. J. Geophys. Res.: Planets 1997, 102 (E5), 10985-10996.

3.

Madey, T. E.; Johnson, R. E.; Orlando, T. M., Far-out surface science: radiationinduced surface processes in the solar system. Surf. Sci. 2002, 500 (1-3), 838-858.

4.

Watanabe, N.; Kouchi, A., Ice surface reactions: A key to chemical evolution in space. Prog. Surf. Sci. 2008, 83 (10-12), 439-489.

5.

Bennett, C. J.; Pirim, C.; Orlando, T. M., Space-weathering of solar system bodies: A laboratory perspective. Chem. Rev. 2013, 113 (12), 9086-9150.

6.

Shin, S.; Kim, Y.; Moon, E.-s.; Lee, D. H.; Kang, H.; Kang, H., Generation of strong electric fields in an ice film capacitor. J. Chem. Phys. 2013, 139 (7), 074201.

7.

Wu, K.; Iedema, M. J.; Cowin, J. P., Ion penetration of the water-oil interface. Science 1999, 286 (5449), 2482-2485.


Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8.

Cowin, J. P.; Tsekouras, A. A.; Iedema, M. J.; Wu, K.; Ellison, G. B., Immobility of protons in ice from 30 to 190 K. Nature (London, U. K.) 1999, 398 (6726), 405-407.

9.

Horowitz, Y.; Asscher, M., Low energy charged particles interacting with amorphous solid water layers. J. Chem. Phys. 2012, 136 (13), 134701.

10. Shi, J.; Famá, M.; Teolis, B. D.; Baragiola, R. A., Ion-induced electrostatic charging of ice at 15-160 K. Phys. Rev. B 2012, 85 (3), 035424. 11. Simpson, W. C.; Orlando, T. M.; Parenteau, L.; Nagesha, K.; Sanche, L., Dissociative electron attachment in nanoscale ice films: Thickness and charge trapping effects. J. Chem. Phys. 1998, 108 (12), 5027-5034. 12. Michaud, M.; Wen, A.; Sanche, L., Cross sections for low-energy (1-100 eV) electron elastic and inelastic scattering in amorphous ice. Radiat. Res. 2003, 159 (1), 3-22. 13. Balog, R.; Cicman, P.; Field, D.; Feketeova, L.; Hoydalsvik, K.; Jones, N. C.; Field, T. A.; Ziesel, J. P., Transmission and trapping of cold electrons in water ice. J. Phys. Chem. A 2011, 115 (25), 6820-6824. 14. Kevan, L., Solvated electron-structure in glassy matrices. Acc. Chem. Res. 1981, 14 (5), 138-145. 15. Larsen, R. E.; Glover, W. J.; Schwartz, B. J., Does the hydrated electron occupy a cavity? Science 2010, 329 (5987), 65-69. 16. Pshenichnikov, M. S.; Baltuska, A.; Wiersma, D. A., Hydrated-electron population dynamics. Chem. Phys. Lett. 2004, 389 (1-3), 171-175. 17. Zhao, J.; Li, B.; Onda, K.; Feng, M.; Petek, H., Solvated electrons on metal oxide surfaces. Chem. Rev. 2006, 106 (10), 4402-4427.



Page 36 of 40

18. Bovensiepen, U.; Gahl, C.; Stahler, J.; Bockstedte, M.; Meyer, M.; Baletto, F.; Scandolo, S.; Zhu, X. Y.; Rubio, A.; Wolf, M., A dynamic landscape from femtoseconds to minutes for excess electrons at ice-metal interfaces. J. Phys. Chem. C 2009, 113 (3), 979-988. 19. Marsolais, R. M.; Deschenes, M.; Sanche, L., Low-energy electron transmission method for measuring charge trapping in dielectric films. Rev. Sci. Instrum. 1989, 60 (8), 2724-2732. 20. Ayotte, P.; Gamache, J.; Bass, A. D.; Fabrikant, II; Sanche, L., Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability. J. Chem. Phys. 1997, 106 (2), 749-760. 21. Bass, A. D.; Gamache, J.; Ayotte, P.; Sanche, L., Charge stabilization by chloromethane molecules on multilayer Kr films. J. Chem. Phys. 1996, 104 (11), 4258-4266. 22. Stahler, J.; Meyer, M.; Bovensiepen, U.; Wolf, M., Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales. Chem. Sci. 2011, 2 (5), 907-916. 23. Hobbs, P. V., Ice physics. Oxford : Clarendon Press: Oxford, 1974. 24. Tsekouras, A. A.; Iedema, M. J.; Cowin, J. P., Amorphous water-ice relaxations measured with soft-landed ions. Phys. Rev. Lett. 1998, 80 (26), 5798-5801. 25. 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), 134702. 26. Horowitz, Y.; Asscher, M., Electron-induced chemistry of methyl chloride caged within amorphous solid water. J. Chem. Phys. 2013, 139 (15), 154707.


Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Shin, S.; Kim, Y.; Kang, H.; Kang, H., Effect of electric field on condensedphase molecular systems. I. Dipolar polarization of amorphous solid acetone. J. Phys. Chem. C 2015, 119 (27), 15588-15595. 28. Shin, S.; Park, Y.; Kang, H.; Kang, H., Electric field effect on condensed-phase molecular systems. IV. Conformational change of 1,2-dichloroethane in a frozen molecular solid. J. Phys. Chem. C 2017, 121 (45), 25342-25346. 29. Bhuin, R. G.; Methikkalam, R. R. J.; Bag, S.; Pradeep, T., Diffusion and crystallization of dichloromethane within the pores of amorphous solid water. J. Phys. Chem. C 2016, 120 (25), 13474-13484. 30. Bader, G.; Perluzzo, G.; Caron, L. G.; Sanche, L., Elastic and inelastic mean-freepath determination in solid xenon from electron transmission experiments. Phys. Rev. B 1982, 26 (11), 6019-6029. 31. Livneh, T.; Asscher, M., The adsorption and decomposition of C2H4 on Ru(001): A combined TPR and work function change study. J. Phys. Chem. B 2000, 104 (14), 3355-3363. 32. Doering, D. L.; Madey, T. E., The adsorption of water on clean and oxygendosed Ru(001). Surf. Sci. 1982, 123 (2-3), 305-337. 33. 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 densityfunctional theory calculations. Phys. Rev. B 2006, 73 (11), 115414. 34. 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), 3145-3151. 35. Bu, C.; Baragiola, R. A., Proton transport in ice at 30–140 K: Effects of porosity. J. Chem. Phys. 2015, 143 (7), 074702.



Page 38 of 40

36. Mayer, E.; Pletzer, R., Astrophysical implications of amorphous ice- a microporous solid. Nature (London, U. K.) 1986, 319 (6051), 298-301. 37. Kimmel, G. A.; Stevenson, K. P.; Dohnalek, Z.; Smith, R. S.; Kay, B. D., Control of amorphous solid water morphology using molecular beams. I. Experimental results. J. Chem. Phys. 2001, 114 (12), 5284-5294. 38. Naaman, R.; Sanche, L., Low-energy electron transmission through thin-film molecular and biomolecular solids. Chem. Rev. 2007, 107 (5), 1553-1579. 39. Kimmel, G. A.; Orlando, T. M.; Vezina, C.; Sanche, L., Low-energy electronstimulated production of molecular hydrogen from amorphous water ice. J. Chem. Phys. 1994, 101 (4), 3282-3286. 40. Petrik, N. G.; Kimmel, G. A., Electron-stimulated reactions at the interfaces of amorphous solid water films driven by long-range energy transfer from the bulk. Phys. Rev. Lett. 2003, 90 (16), 4. 41. Kobayashi, K., Optical spectra and electronic structure of ice. J. Phys. Chem. 1983, 87 (21), 4317-4321. 42. Barnun, A.; Kleinfeld, I.; Kochavi, E., Trapping of gas-mixtures by amorphous water ice. Phys. Rev. B 1988, 38 (11), 7749-7754. 43. Stevenson, K. P.; Kimmel, G. A.; Dohnalek, Z.; Smith, R. S.; Kay, B. D., Controlling the morphology of amorphous solid water. Science 1999, 283 (5407), 1505-1507. 44. Floriano, M. A.; Handa, Y. P.; Klug, D. D.; Whalley, E., Nature of the transformation of ice-I and low-density amorphous ice to high-density amorphous ice. J. Chem. Phys. 1989, 91 (11), 7187-7192. 45. Jenniskens, P.; Blake, D. F., Structural transitions in amorphous water ice and astrophysical implications. Science 1994, 265 (5173), 753-756.


Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


46. Lilach, Y.; Romm, L.; Livneh, T.; Asscher, M., The first layers of water on Ru(001). J. Phys. Chem. B 2001, 105 (14), 2736-2742. 47. Smith, R. S.; Kay, B. D., The existence of supercooled liquid water at 150 K. Nature (London, U. K.) 1999, 398 (6730), 788-791. 48. Lee, D. H.; Bang, J.; Kang, H., Surface charge layer of amorphous solid water with adsorbed acid or base: Asymmetric depth distributions of H+ and OH- ions. J. Phys. Chem. C 2016, 120 (22), 12051-12058. 49. Held, G.; Menzel, D., Isotope effects in structure and kinetics of water adsorbates on Ru(001). Surf. Sci. 1995, 327 (3), 301-320. 50. Yates, J. T., Experimental innovations in surface science : a guide to practical laboratory methods and instruments. New York : Springer-Verlag: New York, 1998. 51. Petrenko, V. F.; Whitworth, R. W., Physics of Ice. Oxford University Press: Oxford, 2002; p 392. 52. Moon, E. S.; Lee, C. W.; Kang, H., Proton mobility in thin ice films: a revisit. Phys. Chem. Chem. Phys. 2008, 10 (32), 4814-4816. 53. Brown, D. E.; George, S. M.; Huang, C.; Wong, E. K. L.; Rider, K. B.; Smith, R. S.; Kay, B. D., H2O condensation coefficient and refractive index for vapordeposited ice from molecular beam and optical interference measurements. J. Phys. Chem. 1996, 100 (12), 4988-4995. 54. Livneh, T.; Romm, L.; Asscher, M., Cage formation of N2 under H2O overlayer on Ru(001). Surf. Sci. 1996, 351 (1-3), 250-258. 55. Smith, R. S.; Huang, C.; Wong, E. K. L.; Kay, B. D., The molecular volcano: Abrupt CCl4 desorption driven by the crystallization of amorphous solid water. Phys. Rev. Lett. 1997, 79 (5), 909-912.



Page 40 of 40

56. Barnun, A.; Dror, J.; Kochavi, E.; Laufer, D., Amorphous water ice and its ability to trap gases. Phys. Rev. B 1987, 35 (5), 2427-2435. 57. May, R. A.; Smith, R. S.; Kay, B. D., The release of trapped gases from amorphous solid water films. II. “Bottom-up” induced desorption pathways. J. Chem. Phys. 2013, 138 (10), 104502.

TOC Graphic:


Temperature Effect on Transport, Charging and Binding of Low

Recommend Documents