60th Annual Report on Research 2015

2D radioactive ¹²⁵I monolayers are a recent development that combines the fields of radiochemistry and nanoscience. These Au supported monolayers (Figure 1) show great promise for understanding the local interaction of radiation with 2D molecular layers, offer new directions for surface patterning, and enhance the emission of chemically and biologically relevant low energy electrons. The elemental composition of these monolayers is in constant flux due to the nuclear transmutation of ¹²⁵I to ¹²⁵Te, which occurs with a half-life of 60 days. Unlike I, which is stable and unreactive when bound to Au, the newly formed Te atoms would be expected to be more reactive. We have used electron emission and X-ray photoelectron spectroscopies (XPS) to quantify the emitted electron energies, track the film composition in vacuum and the effect of exposure to ambient conditions (Figure 2). Our results reveal that the Auger electrons emitted during the ultra-fast radioactive decay process have a kinetic energy corresponding to neutral Te. XPS spectra indicate that the Te initially formed in its zero oxidation state upon exposure to air becomes oxidized to TeO₂, which density functional theory (DFT) calculations confirm is exothermic by ca. 1.2 eV. Scanning tunneling microscopy (STM) allows us to visualize the surface composition and DFT simulated STM images enable assignment of both newly formed Te atoms as well as TeO₂. Interestingly, we observe dimerization of the TeO₂ into Te₂O₄ units, which DFT indicates are ca. 80 meV more stable than isolated TeO₂. The Te in the I layer serves as a marker that allows us to demonstrate the facile mobility of the I films with time-lapse STM imaging. The fact that the Te₂O₄ units stay intact during major lateral rearrangement of the monolayer illustrates their stability (Figure 3). These results provide an atomic-scale picture of the composition and mobility of surface species in a radioactive monolayer as well as an understanding of the stability of the films under ambient conditions, which is a critical aspect in their future applications.

Figure 1 (A) Photograph of the deposition of ¹²⁵I on Au(111) using an ambient dropcasting method (B) The resulting ¹²⁵I monolayer as imaged using scanning tunneling microscopy (V = -0.4 V, I = 0.1 nA, scale bar = 2 nm).

Figure 2 XPS spectra showing nuclear transmutation of I to Te and the effect of exposure to air. The upper plot shows the I 3d core levels decreasing and the Te⁰⁺ core levels increasing over time as the ¹²⁵I decays into ¹²⁵Te by electron capture. The bottom plot shows the formation of Te⁴⁺ caused by exposing a surface of Te⁰ formed in vacuum to ambient conditions for a number of hours.

Figure 3 Diffusion of TeO₂ dimer through the I monolayer. Shown is a series of consecutive STM images taken over a constant location;