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Embedding Ba Monolayers and Bilayers in Boron Carbide Nanowires.

Yu Z, Luo J, Shi B, Zhao J, Harmer MP, Zhu J - Sci Rep (2015)

Bottom Line: Another form of bilayer complexion stabilized at stacking faults has also been identified.Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries.Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

View Article: PubMed Central - PubMed

Affiliation: Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China.

ABSTRACT
Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to study the distribution of barium atoms on the surfaces and in the interiors of boron carbide based nanowires. Barium based dopants, which were used to control the crystal growth, adsorbed to the surfaces of the boron-rich crystals in the form of nanometer-thick surficial films (a type of surface complexion). During the crystal growth, these dopant-based surface complexions became embedded inside the single crystalline segments of fivefold boron-rich nanowires collectively, where they were converted to more ordered monolayer and bilayer modified complexions. Another form of bilayer complexion stabilized at stacking faults has also been identified. Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries. In contrast, our study revealed the previously-unrecognized possibility of incorporating dopants and impurities inside an otherwise perfect crystal without the association to any twin boundary or stacking fault. Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

No MeSH data available.


Related in: MedlinePlus

(a,b) A pair of HAADF and bright filed (BF) images showing the structure of surface complexions on the {001} surfaces. The complexion thickness ranges from 0.9 to 1.3 nm. (c) Direct observation of the transition from a partially ordered surface segregation to an ordered Ba monolayer trapped in a single-crystal nanowire. (d) A HAADF-STEM line profile was taken along this transitional region from spot A to spot B in panel (c), showing the changes in ordering. The arrows indicate the positions of Ba columns.
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f1: (a,b) A pair of HAADF and bright filed (BF) images showing the structure of surface complexions on the {001} surfaces. The complexion thickness ranges from 0.9 to 1.3 nm. (c) Direct observation of the transition from a partially ordered surface segregation to an ordered Ba monolayer trapped in a single-crystal nanowire. (d) A HAADF-STEM line profile was taken along this transitional region from spot A to spot B in panel (c), showing the changes in ordering. The arrows indicate the positions of Ba columns.

Mentions: The addition of BaO is critical in the formation of boron-rich fivefold twinned nanowires, which are mainly enclosed by {100} ((100), (010) and (001) planes are identical for boron carbide crystal with rhombohedra symmetry) planes, as shown in Supplementary Fig. S1. Without BaO in the starting precursor, the resulting boron-rich crystal was irregular in crystal shape, suggesting that the surface energies were nearly isotropic. Hence, BaO, as a growth habit modifier, induced anisotropic growth of boron carbide crystals. The exposed {100} surfaces after Ba doping were capped by Ba-enriched, nanometer-thick surface complexions, as revealed by aberration corrected STEM (Fig. 1(a,b)). Since barium is much heavier than the lightweight matrix, Z contrast (HAADF) imaging provided valuable information on the complexion structure and its chemistry. A partially-ordered layer of barium was directly attached to the {001} surface. The first layer of Ba atoms on {001} terrace were confined in the direction vertical to {001} planes but have more freedom to relax laterally. The HAADF contrast in the complexion decreased significantly approaching the vacuum, with sparsely distributed bright spots in a dark background. This is an indication that the Ba atoms in the second layer have a much lower number density and exhibit less structural ordering.


Embedding Ba Monolayers and Bilayers in Boron Carbide Nanowires.

Yu Z, Luo J, Shi B, Zhao J, Harmer MP, Zhu J - Sci Rep (2015)

(a,b) A pair of HAADF and bright filed (BF) images showing the structure of surface complexions on the {001} surfaces. The complexion thickness ranges from 0.9 to 1.3 nm. (c) Direct observation of the transition from a partially ordered surface segregation to an ordered Ba monolayer trapped in a single-crystal nanowire. (d) A HAADF-STEM line profile was taken along this transitional region from spot A to spot B in panel (c), showing the changes in ordering. The arrows indicate the positions of Ba columns.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660277&req=5

f1: (a,b) A pair of HAADF and bright filed (BF) images showing the structure of surface complexions on the {001} surfaces. The complexion thickness ranges from 0.9 to 1.3 nm. (c) Direct observation of the transition from a partially ordered surface segregation to an ordered Ba monolayer trapped in a single-crystal nanowire. (d) A HAADF-STEM line profile was taken along this transitional region from spot A to spot B in panel (c), showing the changes in ordering. The arrows indicate the positions of Ba columns.
Mentions: The addition of BaO is critical in the formation of boron-rich fivefold twinned nanowires, which are mainly enclosed by {100} ((100), (010) and (001) planes are identical for boron carbide crystal with rhombohedra symmetry) planes, as shown in Supplementary Fig. S1. Without BaO in the starting precursor, the resulting boron-rich crystal was irregular in crystal shape, suggesting that the surface energies were nearly isotropic. Hence, BaO, as a growth habit modifier, induced anisotropic growth of boron carbide crystals. The exposed {100} surfaces after Ba doping were capped by Ba-enriched, nanometer-thick surface complexions, as revealed by aberration corrected STEM (Fig. 1(a,b)). Since barium is much heavier than the lightweight matrix, Z contrast (HAADF) imaging provided valuable information on the complexion structure and its chemistry. A partially-ordered layer of barium was directly attached to the {001} surface. The first layer of Ba atoms on {001} terrace were confined in the direction vertical to {001} planes but have more freedom to relax laterally. The HAADF contrast in the complexion decreased significantly approaching the vacuum, with sparsely distributed bright spots in a dark background. This is an indication that the Ba atoms in the second layer have a much lower number density and exhibit less structural ordering.

Bottom Line: Another form of bilayer complexion stabilized at stacking faults has also been identified.Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries.Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

View Article: PubMed Central - PubMed

Affiliation: Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China.

ABSTRACT
Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to study the distribution of barium atoms on the surfaces and in the interiors of boron carbide based nanowires. Barium based dopants, which were used to control the crystal growth, adsorbed to the surfaces of the boron-rich crystals in the form of nanometer-thick surficial films (a type of surface complexion). During the crystal growth, these dopant-based surface complexions became embedded inside the single crystalline segments of fivefold boron-rich nanowires collectively, where they were converted to more ordered monolayer and bilayer modified complexions. Another form of bilayer complexion stabilized at stacking faults has also been identified. Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries. In contrast, our study revealed the previously-unrecognized possibility of incorporating dopants and impurities inside an otherwise perfect crystal without the association to any twin boundary or stacking fault. Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

No MeSH data available.


Related in: MedlinePlus