Sammendrag
The band gap of semiconducting ZnO can be readily tuned through alloying it with other relevant oxides, such as CdO, consequently extending the performance of the corresponding materials and devices. In this context, one of the challenges is to establish the methodology for two-dimensional band gap measurements on the nanometer scale. Here, monochromated electron energy loss spectroscopy (EELS) in combination with probe-corrected scanning transmission electron microscopy (STEM) can be applied, potentially with much greater success compared to traditional techniques with low spatial resolution. However, up to now, the EELS based band gap mapping technique has not seen widespread use, primarily due to its experimental and data processing complexities.
In this work, utilizing state-of-the-art probe-corrected and monochromated STEM-EELS platform without particular instrumental design, we developed and applied methods for acquiring large band gap maps with high spatial resolution. A newly-developed efficient computing method was employed to extract band gap maps from the EELS data after proper background subtraction. All these advances are highlighted by the band gap mapping of Zn1-xCdxO/ZnO hetero structure with a spatial resolution well below 10 nm and a high spectral precision.
Nevertheless, band gap measurement by EELS are also restricted in spatial resolution, which is fundamentally determined by the delocalization length (L50) of the inelastic scattering process. The origin of this delocalization is the long range electrostatic interactions between the atomic electrons of the sample and the incident high-energy electrons. The EELS plasmon energy map has obviously higher spatial resolution than the band gap map, and its experiment as well as data extraction is also much easier to perform. In order to push the spatial resolving power in EELS band gap analysis further, the relationship between the band gaps and plasmon energies in Zn1-xCdxO was investigated based on the fact that both depend strongly on the unit cell parameter. A robust quantitative correlation was established, providing a simple and straightforward way to calculate the band gap variations just from the easily measured plasmon energy, with improved spatial resolving ability as compared with the conventional EELS approach.
In order to further verify the success of the probe-corrected and monochromated STEM-EELS technique, it was put into application to a new system, namely separate ZnCr2O4 nano-inclusions embedded in ZnO matrix. Band gap mapping of ZnCr2O4 nanoparticles in ZnO matrix and their interface was successfully achieved, confirming the validity of this STEM-EELS approach. In addition, probe-corrected STEM enables sub-ångström imaging, from which the realistic structure can be revealed. We employed atomic-resolution images together with geometric phase analysis (GPA) to analyze the structure and strain at ZnCr2O4/ZnO interfaces, which is of critical importance for thin film growth and may affect band gap.
In this work, utilizing state-of-the-art probe-corrected and monochromated STEM-EELS platform without particular instrumental design, we developed and applied methods for acquiring large band gap maps with high spatial resolution. A newly-developed efficient computing method was employed to extract band gap maps from the EELS data after proper background subtraction. All these advances are highlighted by the band gap mapping of Zn1-xCdxO/ZnO hetero structure with a spatial resolution well below 10 nm and a high spectral precision.
Nevertheless, band gap measurement by EELS are also restricted in spatial resolution, which is fundamentally determined by the delocalization length (L50) of the inelastic scattering process. The origin of this delocalization is the long range electrostatic interactions between the atomic electrons of the sample and the incident high-energy electrons. The EELS plasmon energy map has obviously higher spatial resolution than the band gap map, and its experiment as well as data extraction is also much easier to perform. In order to push the spatial resolving power in EELS band gap analysis further, the relationship between the band gaps and plasmon energies in Zn1-xCdxO was investigated based on the fact that both depend strongly on the unit cell parameter. A robust quantitative correlation was established, providing a simple and straightforward way to calculate the band gap variations just from the easily measured plasmon energy, with improved spatial resolving ability as compared with the conventional EELS approach.
In order to further verify the success of the probe-corrected and monochromated STEM-EELS technique, it was put into application to a new system, namely separate ZnCr2O4 nano-inclusions embedded in ZnO matrix. Band gap mapping of ZnCr2O4 nanoparticles in ZnO matrix and their interface was successfully achieved, confirming the validity of this STEM-EELS approach. In addition, probe-corrected STEM enables sub-ångström imaging, from which the realistic structure can be revealed. We employed atomic-resolution images together with geometric phase analysis (GPA) to analyze the structure and strain at ZnCr2O4/ZnO interfaces, which is of critical importance for thin film growth and may affect band gap.