Controlled Fragmentation of Single Molecules with Atomic Force Microscopy by Employing Doubly Charged States

By atom manipulation we performed on-surface chemical reactions of a single molecule on a multilayer insulating film using noncontact atomic force microscopy. The single-electron sensitivity of atomic force microscopy allows us to follow the addition of single electrons to the molecule and the investigation of the reaction products. By performing a novel strategy based on long-lived doubly charged states a single molecule is fragmented. The fragmentation can be reverted by again changing the charge state of the system, characterizing a reversible reaction. The experimental results in addition to density-functional theory provide insight into the charge states of the different products and reaction pathways. Similar molecular systems could be used as charge-transfer units and to induce reversible chemical reactions.

Figure 1

(a) Schematic of the experimental arrangement. Because of the NaCl thickness being larger than 10 monolayers, electron transfer is possible only between tip and molecule. (b) DINP model. (c) Constant-height AFM image of DINP on NaCl ($A=4\AA$ and $V=2.3\mathrm{V}$). Image taken 1 Å closer than the set point of $\mathrm{\Delta }f=-0.7\mathrm{Hz}$ and $V=0.4\mathrm{V}$. Scale bar is 5 Å.

Figure 2

(a) $\mathrm{\Delta }f$ ($V$) spectra on DINP for the reversible single-electron attachment (upper spectrum) and detachment (lower spectrum). (b) $\mathrm{\Delta }f$ ($V$) spectrum for the double charging of DINP. Insets correspond to the magnified circled regions of the first and second electron attachment, respectively. Sample voltage sweep is from 1.3 to 3.5 V (black trace). The reverse sweep is shown as a red trace. (c) Constant-height AFM images before [left, same as Fig. 1] and after (right) attaching two electrons to DINP. Right image ($A=4\AA$ and $V=2.4\mathrm{V}$) is taken 4 Å closer than the set point value of $\mathrm{\Delta }f=-0.7\mathrm{Hz}$ and $V=0.4\mathrm{V}$. Scale bars are 5 Å.

Figure 3

(a),(b),(d) Sequential constant–$\mathrm{\Delta }f$ images of (a) DINP prior dissociation, (b) dissociated DINP after two-electron attachment and before the spectrum in (c) was performed, and (d) after performing the spectrum in (c). Image set points are the same ($\mathrm{\Delta }f=-0.5\mathrm{Hz}$ and $V=0.4\mathrm{V}$) and $A=6\AA$. Scale bars are 10 Å. (c) $\mathrm{\Delta }f$ ($V$) spectra on aryne and iodines. The $V$ sweep starts from 1.7 V above the dissociated DINP. First $V$ is swept to $-1.7\mathrm{V}$ (black trace). The reverse sweep is shown as a red trace and ends at 0.4 V. (e) $\mathrm{\Delta }f$ ($t$) spectrum of consecutive sample voltage ramps above DINP. The dissociation ($D$) and restoration ($R$) of the $\mathrm{C}─\mathrm{I}$ bonds are indicated in the spectrum. The different total charge states of the system are indicated by gray levels in the image. The system is composed of DINP, while neutral and negatively charged, and aryne plus two iodines after being doubly negatively charged. Black trace corresponds to sample voltage sweeps from negative to positive values, whereas the red trace corresponds to the opposite. Constant-$\mathrm{\Delta }f$ images of DINP before and after the spectrum in (e) are included in the Supplemental Material [34].

Figure 4

(a) $\mathrm{\Delta }f$ ($V$) spectra on top of a single iodine. From positive to negative sample voltage (upper panel) and the reverse sweep (lower panel). The inset displays a constant-$\mathrm{\Delta }f$ image of an iodide ($\mathrm{\Delta }f=-0.7\mathrm{Hz}$, $V=0.4\mathrm{V}$, and $A=6\AA$) with a scale bar of 5 Å. Black and white scale ranges from 0 to 2.3 Å. (b) Calculated total energies for different chemical products with varied charge-state configurations on top of NaCl. (c) Reaction pathways for different charge states. First, the stability of DINP upon single-electron attachment. Second, the fragmentation of the anionic DINP upon electron attachment. Third, the $\mathrm{C}─\mathrm{I}$ bond restoration upon doubly charging aryne and two iodines. Fourth, the $\mathrm{C}─\mathrm{I}$ restoration hypothesis where the aryne gets positively charged, then, an iodide attacks the positively charged aryne forming INP. Without the need to further decrease the voltage, INP gets positively charged. Finally, another iodide attacks the positively charged INP, forming DINP.