Wei Zhan

• Zhan, Wei; Kosinskiy, Andrey; Vines, Lasse; Johansen, Klaus Magnus H; Carvalho, Patricia Almeida & Prytz, Øystein (2019). ZnCr2O4 Inclusions in ZnO Matrix Investigated by Probe-Corrected STEM-EELS. Materials. 12. doi: 10.3390/ma12060888. Full text in Research Archive Show summary

  The ZnCr2O4/ZnO materials system has a wide range of potential applications, for example, as a photocatalytic material for waste-water treatment and gas sensing. In this study, probe-corrected high-resolution scanning transmission electron microscopy and geometric phase analysis were utilized to study the dislocation structure and strain distribution at the interface between zinc oxide (ZnO) and embedded zinc chromium oxide (ZnCr2O4) particles. Ball-milled and dry-pressed ZnO and chromium oxide (α-Cr2O3) powder formed ZnCr2O4 inclusions in ZnO with size ~400 nm, where the interface properties depended on the interface orientation. In particular, sharp interfaces were observed for ZnO [2 1¯1¯ 3]/ZnCr2O4 [1 1¯ 0] orientations, while ZnO [1 2¯ 10]/ZnCr2O4 [112] orientations revealed an interface over several atomic layers, with a high density of dislocations. Further, monochromated electron energy-loss spectroscopy was employed to map the optical band gap of ZnCr2O4 nanoparticles in the ZnO matrix and their interface, where the average band gap of ZnCr2O4 nanoparticles was measured to be 3.84 ± 0.03 eV, in contrast to 3.22 ± 0.01 eV for the ZnO matrix.

• Zhan, Wei; Venkatachalapathy, Vishnukanthan; Aarholt, Thomas; Kuznetsov, Andrej & Prytz, Øystein (2018). Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale. Scientific Reports. 8(848), p. 1–7. doi: 10.1038/s41598-017-18949-9. Full text in Research Archive Show summary

  Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1−xCd x O/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.

• Jensen, Ingvild Julie Thue; Johansen, Klaus Magnus H; Zhan, Wei; Venkatachalapathy, Vishnukanthan; Brillson, Len & Kuznetsov, Andrej [Show all 7 contributors for this article] (2018). Bandgap and band edge positions in compositionally graded ZnCdO. Journal of Applied Physics. ISSN 0021-8979. 124(1), p. 015302-1–015302-8. doi: 10.1063/1.5036710. Full text in Research Archive Show summary

  Introducing Cd into ZnO allows for bandgap engineering, potentially with particularly interesting properties to observe in compositionally graded samples. In this work, compositionally graded Zn1–xCdxO samples with 0 ≤ x < 0.16 were made using metal organic vapour phase epitaxy. The chemical composition was studied using scanning transmission electron microscopy, while the band structure of the samples was investigated using a combination of cathodoluminescence spectroscopy and X-ray photoelectron spectroscopy (XPS). It is found that the reduction of the bandgap in our samples is caused by changes in the conduction band. The position of the Fermi level relative to the vacuum level, i.e., the workfunction, was also found to change upon addition of Cd, giving an apparent shift in the valence band when evaluated from the XPS valence spectra. © 2018 AIP Publishing

• Granerød, Cecilie Skjold; Zhan, Wei & Prytz, Øystein (2018). Automated approaches for band gap mapping in STEM-EELS. Ultramicroscopy. ISSN 0304-3991. 184(A), p. 39–45. doi: 10.1016/j.ultramic.2017.08.006. Full text in Research Archive Show summary

  Band gap variations in thin film structures, across grain boundaries, and in embedded nanoparticles are of increasing interest in the materials science community. As many common experimental techniques for measuring band gaps do not have the spatial resolution needed to observe these variations directly, probe-corrected Scanning Transmission Electron Microscope (STEM) with monochromated Electron Energy-Loss Spectroscopy (EELS) is a promising method for studying band gaps of such features. However, extraction of band gaps from EELS data sets usually requires heavy user involvement, and makes the analysis of large data sets challenging. Here we develop and present methods for automated extraction of band gap maps from large STEM-EELS data sets with high spatial resolution while preserving high accuracy and precision.

• Zhan, Wei; Granerød, Cecilie Skjold; Venkatachalapathy, Vishnukanthan; Johansen, Klaus Magnus H; Jensen, Ingvild Julie Thue & Kuznetsov, Andrej [Show all 7 contributors for this article] (2017). Nanoscale mapping of optical band gaps using monochromated Electron Energy Loss Spectroscopy. Nanotechnology. ISSN 0957-4484. 28(10). doi: 10.1088/1361-6528/aa5962. Full text in Research Archive Show summary

  Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

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• Granerød, Cecilie Skjold; Zhan, Wei & Prytz, Øystein (2017). Methods for Mapping the Band Gap with High Spatial Resolution by STEM-EELS. Full text in Research Archive
• Granerød, Cecilie Skjold; Bilden, Sindre Rannem; Zhan, Wei; Vines, Lasse; Johansen, Klaus Magnus H & Prytz, Øystein (2017). Measuring the ZnO bandgap and its temperature dependency with STEM-EELS. Full text in Research Archive
• Granerød, Cecilie Skjold; Zhan, Wei; Johansen, Klaus Magnus H; Vines, Lasse & Prytz, Øystein (2016). Band gap measurements by STEM EELS. Full text in Research Archive
• Granerød, Cecilie Skjold; Zhan, Wei; Aarseth, Bjørn Brevig; Nguyen, Phuong Dan; Johansen, Klaus Magnus H & Vines, Lasse [Show all 7 contributors for this article] (2016). Bandgap measurements by STEM-EELS. Full text in Research Archive
• Johansen, Klaus Magnus H; Zhan, Wei; Jensen, Ingvild Julie Thue; Granerød, Cecilie Skjold; Venkatachalapathy, Vishnukanthan & Brillson, Leonard J. [Show all 8 contributors for this article] (2016). Mapping of Optical band gaps using Electron Energy Loss Spectroscopy in ZnCdO-alloys. Full text in Research Archive
• Granerød, Cecilie Skjold; Zhan, Wei; Aarseth, Bjørn Brevig; Vines, Lasse & Prytz, Øystein (2015). Band gap measurements by STEM EELS. Full text in Research Archive
• Granerød, Cecilie Skjold; Zhan, Wei; Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej; Nilsen, Ola & Monakhov, Edouard [Show all 7 contributors for this article] (2015). Bandgap of Functional Oxides in STEM-EELS. Full text in Research Archive
• Granerød, Cecilie Skjold; Nguyen, Phuong Dan; Zhan, Wei; Monakhov, Edouard & Prytz, Øystein (2015). Band gap measurements by STEM-EELS. Full text in Research Archive
• Zhan, Wei; Venkatachalapathy, Vishnukanthan; Jensen, Ingvild Julie Thue; Granerød, Cecilie Skjold; Flage-Larsen, Espen & Johansen, Klaus Magnus H [Show all 9 contributors for this article] (2015). STEM-EELS Band Gap Measurement in Zn1-XCdXO Thin Films. Full text in Research Archive
• Zhan, Wei; Kosinskiy, Andrey & Prytz, Øystein (2015). Aberration-corrected High-resolution STEM and EELS on Cr-doped ZnO with Nano ZnCr2O4 Inclusion. Full text in Research Archive
• Granerød, Cecilie Skjold; Nguyen, Phuong Dan; Zhan, Wei; Monakhov, Edouard & Prytz, Øystein (2015). High Energy Resolution EELS on ZnO/Cu2O Heterostructures. Full text in Research Archive
• Prytz, Øystein; Zhan, Wei & Granerød, Cecilie Skjold (2015). Status of EELS band gap measurements in the monochromated Titan. Full text in Research Archive
• Zhan, Wei (2018). Bang gap mapping of alloyed ZnO using probe-corrected and monochromated STEM-EELS. Det matematisk-naturvitenskapelige fakultet. Full text in Research Archive
• Zhan, Wei; Prytz, Øystein; Kuznetsov, Andrej & Flage-Larsen, Espen (2018). Band gap mapping of alloyed ZnO using probe-corrected and monochromated STEM-EELS. Universitetet i Oslo. Full text in Research Archive Show summary

  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.

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