Conformational Molecular Switch of the Azobenzene Molecule: A Scanning Tunneling Microscopy Study

We propose to utilize azobenzene as a nanomolecular switch which can be triggered by transmitting electrons above threshold biases. The effect is explained by an electron impact trans-cis conformational change of the isolated azobenzene molecules. The molecular electronic states of both isomers have been measured with spatially resolved scanning tunneling microscopy or spectroscopy, leading to suggested transition pathways of the electron-induced isomerization.

Figure 1

(a) A schematic of a light-driven molecular switch based on the conformational change of an azobenzene (AB) molecule in contact with two metallic electrodes. (b) A concept of electron-induced isomerization of an AB molecule adsorbed onto a metallic surface with a scanning tunneling microscope.

Figure 2

(a) A large scale image ($V_s=1\mathrm{V}$, $I_t=50\mathrm{pA}$) of 3 trans-azobenzenes on Au(111). (b) A current-voltage ($I\mathrm{\text{−}}V$) curve obtained at the center of tAB (left inset) and a newly produced molecule (right inset) by sweeping the voltage twice; from 1.5 to $-2.0\mathrm{V}$ on tAB (solid line) and from $-2.0$ to 2.0 V on the new product (dashed line). Insets: Topographic images ($2.2\mathrm{nm}\times 2.2\mathrm{nm}$) before the $I\mathrm{\text{−}}V$ measurement (left) and after a current jump (right), taken at $V_s=1\mathrm{V}$ and $I_t=70\mathrm{pA}$.

Figure 3

(a) STM images ($2.4\mathrm{nm}\times 1.3\mathrm{nm}$) of tAB and an electron-induced product, attributed to cAB. The voltage pulse was $-2\mathrm{V}$ (upper left to upper right and lower left to lower right) and 2.5 V (upper right to lower left). (b) The cross sections of the molecules in (a) along the dashed arrows. “1” represents the upper left molecule and “2” the upper right. (c) The proposed adsorption geometries of tAB and cAB on Au(111). (d) A spatially controlled isomerization on a selected tAB. “$t2$” (tAB) was transformed to “$c2$” (cAB) by electron beam with voltage of $-2\mathrm{V}$, while other molecules were unchanged. “$c1$” is an existing cAB.

Figure 4

(a), (b) Three dimensional SR-STS maps along the central axes of tAB and cAB, respectively, and their topographic images (insets, $1.7\mathrm{nm}\times 0.8\mathrm{nm}$). The $z$ axis is the measured $dI/dV$, the $x$ axis is the scanned length of 1.7 nm, and the $y$ axis is the scanned energy of $-1.5–1.5\mathrm{eV}$. (c), (d) The experimental (solid circle) and theoretical (solid line) $(dI/dV)/(I/V)$ spectra of tAB and cAB at the center of the molecules. A dashed line in (d) is the theoretical LDOS spectrum shifted by $+0.5\mathrm{V}$.