Electron Damage to Supported Ice Investigated by Scanning Tunneling Microscopy and Spectroscopy

We study the interaction of low-energy electrons with crystalline ice (${\mathrm{D}}_2\mathrm{O}$) on Cu(111) by low-temperature scanning tunneling microscopy and spectroscopy. Electrons induce dissociation of the molecules with a threshold of $\approx 3\mathrm{eV}$. The large dissociation yield of the order of ${10}^{-8}/\mathrm{\text{electron}}$ and the extended area of dissociation are attributed to a shift in conduction band during the dissociation. Voltage dependent differences in imaging of ice and dissociated ice are reflected in the spectroscopic signature.

Figure 1

Series of IET manipulation of crystalline ice on Cu(111) starting with $(0.35\pm 0.05)\mathrm{nA}$ for 0.72 s at crosses with indicated manipulation voltage. (a) STM image of ice islands before manipulation, 2.7 pA, 0.4 V. (b) STM image after series of IET manipulation. (c) $I\mathrm{\text{−}}t$ characteristics during manipulation with 2.9 V; $I_{\mathrm{thr}}=3\mathrm{nA}$ is marked. (d) STM image after second series of manipulation as indicated in (b).

Figure 2

Imaging of ice and manipulated ice. (a) STM image before manipulation with 0.4 nA and indicated voltage for 0.72 s. (b)–(d) STM images after manipulation at different bias voltages and with 3 pA: (b) 0.4 V, (c) 1.4 V, (d) 2.4 V. (e) Apparent height $h$ vs $V$; measurement points are indicated in STM image (bottom): circles, on ice; triangles, on manipulated ice; filled (open) symbols represent highest (least highest) part within the representative structure. STM images are 2D images of (a) and (b). Note that the real height of the ice islands is $\approx 1.4\mathrm{nm}$ [20].

Figure 3

$I\mathrm{\text{−}}V$ spectra (a) with set point at 1.9 V, average of several spectra (up to 14), and (b) in exponential regime, average of 30 spectra. Spectra are taken with different set points, but renormalized to the value at 1.9 V.

Figure 4

$R$ signal of IETS spectra ($d^{2}I/d{V}^2$) for small ${\mathrm{D}}_2\mathrm{O}$ cluster on Ag(111), average of several spectra measured where indicated on STM images. Parameters: $f_{\mathrm{mod}}=361.1\mathrm{Hz}$, $U_{\mathrm{mod}}=8\mathrm{mV}$. (a) Original water cluster, 21 pA, 72 mV, and (b) manipulated cluster, 20 pA, 72 mV.

Figure 5

Threshold determination for islands on Cu(111), with fits (a) number of electrons $N$ needed for initiation of dissociation event, for $I_{\mathrm{manip}}=(0.65\pm 0.08)\mathrm{nA}$, and (b) dissociation rate $R$ per molecule for different manipulation currents.