We report on the theoretical prediction and experimental realization of new ternary zinc molybdenum nitride compounds. We used theory to identify previously unknown ternary compounds in the Zn–Mo–N ...systems, Zn3MoN4 and ZnMoN2, and to analyze their bonding environment. Experiments show that Zn–Mo–N alloys can form in broad composition range from Zn3MoN4 to ZnMoN2 in the wurtzite-derived structure, accommodating very large off-stoichiometry. Interestingly, the measured wurtzite-derived structure of the alloys is metastable for the ZnMoN2 stoichiometry, in contrast to the Zn3MoN4 stoichiometry, where ordered wurtzite is predicted to be the ground state. The formation of Zn3MoN4–ZnMoN2 alloy with wurtzite-derived crystal structure is enabled by the concomitant ability of Mo to change oxidation state from +VI in Zn3MoN4 to +IV in ZnMoN2, and the capability of Zn to contribute to the bonding states of both compounds, an effect that we define as “redox-mediated stabilization”. The stabilization of Mo in both the +VI and +IV oxidation states is due to the intermediate electronegativity of Zn, which enables significant polar covalent bonding in both Zn3MoN4 and ZnMoN2 compounds. The smooth change in the Mo oxidation state between Zn3MoN4 and ZnMoN2 stoichiometries leads to a continuous change in optoelectronic propertiesfrom resistive and semitransparent Zn3MoN4 to conductive and absorptive ZnMoN2. The reported redox-mediated stabilization in zinc molybdenum nitrides suggests there might be many undiscovered ternary compounds with one metal having an intermediate electronegativity, enabling significant covalent bonding, and another metal capable of accommodating multiple oxidation states, enabling stoichiometric flexibility.
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In this work, we present our discovery and characterization of a new kagome prototype structure, KV3Sb5. We also present the discovery of the isostructural compounds RbV3Sb5 and CsV3Sb5. All ...materials exhibit a structurally perfect two-dimensional kagome net of vanadium. Density-functional theory calculations indicate that the materials are metallic, with the Fermi level in close proximity to several Dirac points. Powder and single-crystal syntheses are presented, with postsynthetic treatments shown to deintercalate potassium from single crystals of KV3Sb5. Considering the proximity to Dirac points, deintercalation provides a convenient means to tune the Fermi level. Magnetization measurements indicate that KV3Sb5 exhibits behavior consistent with a the Curie-Weiss model at high temperatures, although the effective moment is low (0.22μB per vanadium ion). An anomaly is observed in both magnetization and heat capacity measurements at 80 K, below which the moment is largely quenched. Elastic neutron scattering measurements find no obvious evidence of long-range or short-range magnetic ordering below 80 K. The possibility of an orbital-ordering event is considered. Single-crystal resistivity measurements show the effect of deintercalation on the electron transport and allow estimation of the Kadowaki-Woods ratio in KV3Sb5. We find that A/γ2~61μOhm cm mol2 FU K2J-2, suggesting that correlated electron transport may be possible. KV3Sb5 and its cogeners RbV3Sb5 and CsV3Sb5 represent a new family of kagome metals, and our results demonstrate that they deserve further study as potential model systems.
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Al1–x Gd x N is one of a series of novel heterostructural alloys involving rare earth cations with potentially interesting properties for (opto)electronic, magnetic, and neutron detector ...applications. Using alloy models in conjunction with density functional theory, we explored the full composition range for Al1–x Gd x N and found that wurtzite is the ground-state structure up to a critical composition of x c = 0.82. The calculated temperature-composition phase diagram reveals a large miscibility gap inducing spinodal decomposition at equilibrium conditions, with higher Gd substitution (meta)stabilized at higher temperatures. By depositing combinatorial thin films at high effective temperatures using radio-frequency cosputtering, we have achieved the highest Gd3+ incorporation into the wurtzite phase reported to date, with single-phase compositions at least up to x ≈ 0.25 confirmed by high-resolution synchrotron grazing incidence wide-angle X-ray scattering. High-resolution transmission electron microscopy on material with x ≈ 0.13 and x ≈ 0.24 confirmed a uniform composition polycrystalline film with uniform columnar grains having the wurtzite structure. Spectroscopic ellipsometry and cathodoluminescence spectroscopy measurements are employed to probe the optoelectronic properties, showing that the band gap decreases with increasing Gd content x and that this effect causes the ideal Gd substitution level for cathodoluminescence applications to be low. Expanding our calculations to other rare earth cations (Pr3+ and Tb3+) reveals similar thermodynamic stability and solubility behavior to Gd. From this and previous studies on Al1–x Sc x N, we elucidate that both smaller ionic radius and higher bond ionicity promote increased incorporation of group IIIB cations into wurtzite AlN. This work furthers the development of design rules for new alloys in this material family.
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36.
Sn-assisted heteroepitaxy improves ZnTiN photoabsorbers Mangum, John S; Ke, Sijia; Gish, Melissa K ...
Journal of materials chemistry. A, Materials for energy and sustainability,
02/2024, Volume:
12, Issue:
8
Journal Article
Peer reviewed
Sustainable production of liquid fuels from abundant resources, such as carbon dioxide and water, may be possible through photoelectrochemical processes. Zinc titanium nitride (ZnTiN
2
) has been ...recently demonstrated as a potential photoelectrode semiconductor for photoelectrochemical fuel generation due to its ideal bandgap induced by cation disorder, shared crystal structure with established semiconductors, and self-passivating surface oxides under carbon dioxide reduction operating conditions. However, substantial improvements in crystalline quality and optoelectronic properties of ZnTiN
2
are needed to enable such applications. In this work, we investigate the heteroepitaxial growth of ZnTiN
2
on
c
-plane (001) sapphire substrates. Growth on sapphire improves crystal quality, while growth on sapphire at elevated temperatures (300 °C) yields highly-oriented, single-crystal-like ZnTiN
2
films. When Sn is incorporated during these epitaxial growth conditions, notable improvements in ZnTiN
2
film surface roughness and optoelectronic properties are observed. These improvements are attributed to Sn acting as a surfactant during growth and mitigating unintentional impurities such as O and C. The single-crystal-like, 12% Sn-containing ZnTiN
2
films exhibit a steep optical absorption onset at the band gap energy around 2 eV, electrical resistivity of 0.7 Ω cm, and a carrier mobility of 0.046 cm
2
V
−1
s
−1
with n-type carrier concentration of 2 × 10
20
cm
−3
. Density functional theory calculations reveal that moderate substitution of Sn (12.5% of the cation sites) on energetically-preferred cation sites has negligible impact on the optoelectronic properties of cation-disordered ZnTiN
2
. These results are important steps toward achieving high performance PEC devices based on ZnTiN
2
photoelectrodes with efficient photon absorption and photoexcited carrier extraction.
Ambient temperature growth on Si produces a polycrystalline ZnTiN
2
film while Sn-assisted growth on sapphire at elevated temperature results in a single-crystal-like ZnTiN
2
film with significantly reduced sub-bandgap absorption.
Sn-assisted heteroepitaxy improves ZnTiN 2 photoabsorbers Mangum, John S.; Ke, Sijia; Gish, Melissa K. ...
Journal of materials chemistry. A, Materials for energy and sustainability,
02/2024, Volume:
12, Issue:
8
Journal Article
Peer reviewed
Sustainable production of liquid fuels from abundant resources, such as carbon dioxide and water, may be possible through photoelectrochemical processes. Zinc titanium nitride (ZnTiN 2 ) has been ...recently demonstrated as a potential photoelectrode semiconductor for photoelectrochemical fuel generation due to its ideal bandgap induced by cation disorder, shared crystal structure with established semiconductors, and self-passivating surface oxides under carbon dioxide reduction operating conditions. However, substantial improvements in crystalline quality and optoelectronic properties of ZnTiN 2 are needed to enable such applications. In this work, we investigate the heteroepitaxial growth of ZnTiN 2 on c -plane (001) sapphire substrates. Growth on sapphire improves crystal quality, while growth on sapphire at elevated temperatures (300 °C) yields highly-oriented, single-crystal-like ZnTiN 2 films. When Sn is incorporated during these epitaxial growth conditions, notable improvements in ZnTiN 2 film surface roughness and optoelectronic properties are observed. These improvements are attributed to Sn acting as a surfactant during growth and mitigating unintentional impurities such as O and C. The single-crystal-like, 12% Sn-containing ZnTiN 2 films exhibit a steep optical absorption onset at the band gap energy around 2 eV, electrical resistivity of 0.7 Ω cm, and a carrier mobility of 0.046 cm 2 V −1 s −1 with n-type carrier concentration of 2 × 10 20 cm −3 . Density functional theory calculations reveal that moderate substitution of Sn (12.5% of the cation sites) on energetically-preferred cation sites has negligible impact on the optoelectronic properties of cation-disordered ZnTiN 2 . These results are important steps toward achieving high performance PEC devices based on ZnTiN 2 photoelectrodes with efficient photon absorption and photoexcited carrier extraction.
Abstract
Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high ...temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO
2
thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle‐like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature ...and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO2 thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325 degrees C, whereas films deposited at 2 mTorr crystallize into significantly larger needle-like grains of Brookite and Anatase at 270 degrees C. Increasing film deposition rate by a factor of 4 results in a 10x increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270 degrees C to 225 degrees C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
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