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Microscopy and spectroscopy study of nanostructural phase transformation from β-MoO3 to Mo under UHV – MBE conditions
•We have successfully grown β-MoO3 nanoribbon structures (NRs) and phase transition from β-MoO3 NRs to MoO2 nanoparticles (NPs) with increasing substrate temperature on cleaned Si(100) substrate via ex-situ techniques in ultra-high vacuum (UHV) condition by molecular beam epitaxy (MBE) technique. We...
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Published in: | Surface science 2019-04, Vol.682, p.64-74 |
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Main Authors: | , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | •We have successfully grown β-MoO3 nanoribbon structures (NRs) and phase transition from β-MoO3 NRs to MoO2 nanoparticles (NPs) with increasing substrate temperature on cleaned Si(100) substrate via ex-situ techniques in ultra-high vacuum (UHV) condition by molecular beam epitaxy (MBE) technique. We have also shown phase transition from β-MoO3(NRs) to MoO2(NPs) to Mo via simple heat treatment by in-situ x-ray photoelectron microscopy (XPS) experiment. Compare to the earlier phase transition results on MoO3 category, these phase transitions occur in UHV condition without using any reactant like hydrogen gas.•From electron microscopy observations we have seen that, all NRs are randomly oriented with substrate normal along preferentially growth direction of β phase MoO3. With increasing substrate temperature these NRs turned to NPs with d-spacing (d011) of MoO2 phase. With further increase of substrate temperature MoO2 phase transforms to Mo phase with bimodal nanoparticle distribution. To the best of our knowledge, this will be the first report on the phase transition starting from β-MoO3to MoO2to Mo in UHV condition from both in-situ and ex-situ studies.•For in-situ XPS study, we have taken NRs sample. After measuring XPS of NRs sample, anneal the sample at subsequent temperatures, cool down and taking XPS data. From this experiment we found that Mo 3d peak shifts from 232.09 eV (for Mo6+ 3d5/2) to 228.82 eV (for Mo4+ 3d5/2) to 227.24 eV (for Mo0+ 3d5/2) whereas lattice oxygen peak generated for MoO3 cases gradually decreasing with increasing temperature and vanishes when MoO3 converts to Mo.•It is known that work function (Φ) depends on cationic oxidation states in oxide material. For molybdenum oxide cases it is expected that with decreasing Mo oxidation states as well as removal of oxygen leads to lowering in work function. Using a non-destructive Kelvin probe force microscopy (KPFM) technique, we have analyzed the average work function of different molybdenum oxide phases. Interestingl,y we have found decreasing of work function from ≈ 5.27±0.05 eV (for β-MoO3 NRs) to ≈ 4.83±0.05 eV (for Mo bimodal NPs) due to lower electronegativity of lower cationic oxidation state as well as increase in valence donor states with decreasing oxidation state of cation.•Band gap also decreasing with lowering cationic oxidation sates in oxide materials. Using UV–Vis-NIR spectrometry we have found high band gap of 3.32 eV (for β-MoO3 NRs) to 2.55 eV (for semi metallic MoO2 |
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ISSN: | 0039-6028 1879-2758 |
DOI: | 10.1016/j.susc.2018.12.008 |