Mechanical Behavior of Nanocrystalline NaCl Islands on Cu(111)

The mechanical response of ultrathin NaCl crystallites of nanometer dimensions upon manipulation with the tip of a scanning tunneling microscope (STM) is investigated, expanding STM manipulation to various nanostructuring modes of inorganic materials as cutting, moving, and cracking. In the light of theoretical calculations, our results reveal that atomic-scale NaCl islands can behave elastically and follow a classical Hooke’s law. When the elastic limit of the nanocrystallites is reached, the STM tip induces atomic dislocations and consequently the regime of plastic deformation is entered. Our methodology is paving the way to understand the mechanical behavior and properties of other nanoscale materials.

Figure 1

(a) STM image ($I_T=0.31\mathrm{nA}$, $U_T=-1\mathrm{V}$, $175\times 175{\mathrm{nm}}^2$) of (001) oriented NaCl islands partially covering the Cu(111) surface. (b) Atomic resolution STM image ($I_T=0.31\mathrm{nA}$, $U_T=-1\mathrm{V}$, $22\times 22{\mathrm{nm}}^2$) of a bilayer NaCl island. Note that domain I is laterally compressed and rotated with respect to domain II. (c) Scheme of the three possible processes that can occur when an STM tip is approached laterally towards a NaCl nanocrystallite. (d),(e) STM images ($I_T=0.31\mathrm{nA}$, $U_T=-1\mathrm{V}$, $10\times 10{\mathrm{nm}}^2$) of the same area before (d) and after (e) cutting the NaCl crystallite by lateral manipulation with the tip in constant-current mode ($I_T=100\mathrm{nA}$, $U_T=-0.05\mathrm{V}$).

Figure 2

(a),(b) STM images ($I_T=0.31\mathrm{nA}$, $U_T=-1\mathrm{V}$, $30\times 44{\mathrm{nm}}^2$) before and after the motion of a NaCl nanoisland upon lateral manipulation. (c) Schemes of the two manipulation modes used for the dislocation of NaCl islands. (d)–(g) STM images ($I_T=0.31\mathrm{nA}$, $U_T=-1\mathrm{V}$, $44\times 44{\mathrm{nm}}^2$) of the same surface area before (d),(f) and after (e),(g) two cracking experiments in the upper and lower panel, respectively. Lateral manipulation is done in these experiments in the constant height mode using a bias voltage of $-0.05\mathrm{V}$ ($z$ $\mathrm{\text{offset}}=7\AA$ for dragging or 4 Å for pushing, manipulation speed is $10\AA /\mathrm{s}$).

Figure 3

(a) Length and width of a nanoisland, which is fixed at the left edge, are defined by $x_1$ and $y$. The STM tip, its pathway (arrow) and the cracking line are shown, whereas $x_2$ is the distance between tip and cracking edge. (b) Calculated lengths $x_2$ of cracked crystallites [$24\times 85$ NaCl pairs arranged vertically in two atomic layers adsorbed on Cu(111)] as a function of the cantilever dimensions $x_1$ for different widths $y$ ($x_3$ is kept constant at 31 Å). (c) Sequence of the cracking process for 5 particular time intervals named I–V, visualizing the spatial distribution of the strain within the islands. Atoms in white are in their initial, relaxed position, while the colored ones correspond to expanded and compressed interatomic distances in the $x$ direction with a difference larger than 0.02 Å as compared to the initial distances.

Figure 4

(a) Total energy as a function of the deformation distance $d$ for NaCl cantilever of 229 Å in length and various widths $y$. The arrows mark the regimes of elastic and permanent deformation of the cantilever for the $y=66\AA$ curve. (b) Schematic top view of a NaCl bilayer during the cracking process. The STM tip is moving from top to the bottom, causing the deformation of the bilayer that is quantified by the distance $d$ from the initial status. (c) Fitting of the total energy curve for $y=66\AA$ with a parabolic function in the regime of elastic deformations.