EELS / 4D-STEM Investigation of Development of Local Atomic Orderings within ALD-grown Amorphous TiO2 Films
Year: 2023
Type: article
Abstract: Hydrogen as a source of sustainable energy can be produced by photoelectrochemical (PEC) water splitting process [1]. Si photo-anode could capture photons and generate charge carriers to initiate redox reactions at the surface of electrodes, leading to hydrogen production [2]. It has been found that ultrathin amorphous TiO2 (a-TiO2) films grown by atomic layer deposition (ALD) on Si photo-anode can effectively protect Si from corrosion and stabilize the generated photocurrent density at the same time [3]. However, the formation of crystalline phases within the ultrathin films may potentially impair film properties, such as corrosion resistance, which can dramatically reduce the lifetime of the PEC cell. To control and prevent nucleation and growth of the crystalline phase, it is important to understand the development of local atomic orderings in a-TiO2, from the scales of short range ordering (SRO) to medium range ordering (MRO) in different ALD growth and post-processing conditions. In this work, we combine four-dimensional scanning transmission electron microscopy (4D-STEM) and core-loss monochromated electron energy loss spectroscopy (mono-EELS) spectra to understand the details of the evolution of SRO and MRO in ALD-grown ultrathin a-TiO2 films. During 4D-STEM experiment, thousands of nano-diffraction patterns were acquired using an Electron Microscopy Pixel Array Detector (EMPAD) from a-TiO2 films grown at different ALD temperatures. Normalized intensity variance was calculated from 4D-STEM data to extract degree and type of MRO within the films as shown in Fig. 1a, and the averaged size and number of emerging nano-crystals in a-TiO2 matrix were directly measured from STEM images as shown in Fig. 1b [4]. Ti L-edges EELS spectra were acquired from the amorphous films with energy resolution of 0.2 eV. For further interpretation of EELS data, the atomic configuration a-TiO2 simulated by StructOpt algorithm was also taken into account[5]. The optimization was driven by energy using MA potentials as well as experimental variance and pair distribution function. It is observed that degree of MRO is dependent on ALD growth condition prior to crystalline nucleation (Fig. 1). This suggests that evolution of MRO could lead to crystalline nucleation. Moreover, Ti L23 EELS spectra recorded from higher and lower limit of ALD growth temperatures at different areas are shown in Fig. 2a and 2b. The significant splitting in Ti L23 EELS spectrum from the particle and spliiting shape confirm the particle is anatase nano-crystal (Fig. 2 a and b) [6], [7]. The nature of the splitting is attributed to eg and t2g split due to the repulsion between Ti 3d and O p orbitals in anatase octahedral arrangement (Fig. 2c) [8], [9]. Interestingly, Ti L23 EELS spectrum from the amorphous matrix at lower temperature (100°C) shows insignificant degree of splitting. Considering the octahedrons in a-TiO2 matrix simulated by StructOpt (Fig. 2d), this insignificant splitting of Ti L23 could be attributed to the octahedrons’ distortion and week Ti 3d and O p repulsions. However, the Ti L23 splitting becomes more significant for the amorphous area of the film grown at higher temperature (160°C), suggesting the octahedrons become more symmetrical from the SRO point of view. This symmetry enhancement within SRO at higher ALD temperature could align with the fact that degree of MRO also enhances at higher ALD temperatures as a larger-scale atomic ordering. In summary, we demonstrated the utilization of 4D-STEM and mono-EELS to investigate local atomic orderings within a-TiO2 systems. Results suggested the enhancement of degree of SRO and MRO could lead to crystalline nucleation, implying the importance of controlling local atomic ordering to have fully amorphous thin films for PEC applications. (a) Intensity variance curves of a-TiO2 films grown at different ALD temperatures, and (b) averaged number and size of observed nano-crystals at different ALD temperatures [4]. (a) Low magnification STEM image of the film grown at 160°C ALD temperature including an inset of HR-STEM image of the particle indicated by red dashed circle. (b) Ti L23 EELS spectra from different areas of the film grown at 160°C and the film grown at 100°C. (c) Anatase TiO6 octahedron. (d) An octahedron in a-TiO2 simulated by StructOpt [5].
Source: Microscopy and Microanalysis
Institutions: The Ohio State University, University of Wisconsin–Madison
Language: English
Subfield: Structural Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W4385073102