Porous silica films are of scientific interest because their large surface area, high transparency and regularly ordered structures have potential applications in optoelectronic based sensors and recording media. Understanding the movement of guest molecules inside the pores is important for applications such as chromatography and hosts for catalysts, but this property is difficult to monitor.

Hiroyuki Tanaka from the University of Tokyo and colleagues1 have used positronium time-of-flight spectroscopy to address this problem. Depth profiling enabled the researchers to quantitively investigate—in three-dimensions—the escape of guest particles through the porous structures, thereby gaining an insight into the behaviour of particle trajectories not available by previous methods.

Fig. 1: Schematic diagram of the experimental set up and the movement of positrons in the samples.

Positrons are the positively charged particles equivalent of electrons. The researchers bombarded porous silica samples with positron beams leading to the efficient generation of positronium atoms within the pore walls at precise depths—with a depth resolution of 10nm—and were quickly emitted into the pore cavities (Fig.1). The researchers then measured the time taken for the positronium to emerge from the porous networks.

The advantages of this method compared with other approaches include that positron depth profiling enables the study of channels standing vertically from the substrate, whereas standard molecular probe are limited to analysis of horizontal channels parallel to substrates. The data obtained in this research provides information about the connectivity and accessibility of the channels, as well as structural domains within the sample.

“Understanding the functions of porous materials requires knowledge of their structures and physical properties,” says Tanaka. “Electron microscopy is the traditional tool for studying the nanostructure of porous materials, but the method only provides information about the surfaces or cross-sections through the sample. Assessment for applications of porous materials requires information on accessibility from the surface or interconnectivity in the sample.”

Tanaka adds that, “the principle of our technique is based on tracking the diffusion of guest positronium atoms in the channels of interest. Our measurement has the potential to lead to new insights into all fine-grained terrestrial and extraterrestrial materials.”