High-Resolution Identification of Chemical States in Individual Metal Clusters in an Insulating Amorphous Polymer

Abstract

The effectivity of cryo-scanning transmission electron microscopy-electron energy loss spectroscopy was demonstrated for nanoscale analysis of the cross-section of the Cu/polyimide interface. The nanoscale Cu/Cu2O/CuO layer structure at the interface was clearly observed for the first time. In addition, a Cu atom was identified, embedded in the polyimide matrix, and the average valence of diffusing Cu atoms or nanoclusters was determined using (cryo-)scanning transmission electron microscopy-electron energy loss spectroscopy. On the basis of these results, we have proposed a mechanism for the diffusion of Cu atoms in polyimide. To the best of our knowledge, this is the first report of the observation of a metal atom embedded in an insulating amorphous polymer.

Fabrication of a Bilayer Structure of Cu and Polyimide To Realize Circuit Microminiaturization and High Interfacial Adhesion in Flexible Electronic Devices

“…According to the shape of the EELS spectra in Figure a and with reference to those obtained for Cu, Cu 2 O, and CuO standard bulk or powder samples (Figure S6), the valence of the Cu diffused in the PI layer of MA and MB-30 is close to that of Cu 2 O or Cu + . Here, we should be careful because electron beam irradiation during EELS measurements can cause reduction of Cu ions (i.e., Cu 2+ → Cu + or Cu 0 , or Cu + → Cu 0 ). , The EELS measurements (see Section S7, Supporting Information) were performed using very low dose amounts of electrons and a cryo-setup to avoid this effect; however, it is difficult to completely prevent the reduction of Cu ions. We postulate at this stage that Cu atoms or clusters can exist in the form of both Cu 2 O (Cu + ) and CuO (Cu 2+ ).…”

“…(1) Formation of oxidation layer on the Cu surface: After Figure a step 2, the surface of the Cu layer is naturally oxidized in the atmosphere. (2) Cu atom diffusion into PAA (reported in our previous study): See Section S10, Supporting Information. PAA, a strong acid, simultaneously dissolves a part of the oxidation layer on the Cu substrate.…”

“…HAADF-STEM observations and STEM-EELS analyses in our previous study revealed that the particles in the MB PI samples were CuO/Cu 2 O/Cu core−shell particles. 34 The Cu particles are scattered over a region ca. 1.1 μm from the Cu/PI interface in the PI layer of, for instance, MB-30 (Figure S2).…”

“…We could also distinguish the phases for Cu and Cu oxides by the difference of the HAADF contrast in this study. In our previous study, 34 we reported that the Cu/ PI interface fabricated by a similar method to that for the MB samples consists of a Cu/Cu oxide nanolayer structure. This result was based on a combination of HAADF observations and STEM-EELS analyses with nanometer-scale spatial resolution.…”

“…The O K intensity increases at the interface for both the samples (black solid arrows), where the Cu L signal is also present. The increase is understood as follows: 34 the number of oxygen atoms in PI is 0.019 × Avogadro's number per cubic centimeter, which is calculated from the density and molecular weight, whereas those in the possible oxides, Cu 2 O and CuO, are calculated by 0.042× and 0.079 × Avogadro's number per cubic centimeter, respectively. The smaller increase in the O K signal for MB-30 than that for MA-30 is consistent with the STEM observations (Figure 2).…”