Bandgap analysis and carrier localization in cation-disordered ZnGeN2

Cordell, Jacob J.; Tucker, Garritt J.; Tamboli, Adele C.; Lany, Stephan

doi:10.1063/5.0077632

“…This behaviour was found to be consistent across various SQS supercells (from 80 to 400 atoms; see Methods), with local fluctuations of high Na + density giving rise to high-energy localised S p states just above the ‘delocalised VBM’. We note that similar formation of localised anion p states at regions of low electronic potential, namely clusters of low-valence (A I/II ) cations, have recently been reported in the related A II B IV N 2 disordered compounds (including MgSnN 2 , ZnSnN 2 , ZnGeN 2 , and others) 37 – 39 , as well as disordered kesterites (CZTS) 40 .…”

Section: Resultssupporting

confidence: 79%

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

Huang

¹

,

Kavanagh

²

,

Righetto

³

et al. 2022

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I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm−1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

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“…Introducing more octet-rule-violating Ncentered tetrahedra in supercell III, with two close swaps, we find that the DFT-PBE band gap of ZnTiN 2 is reduced by 0.47 eV (21% relative to the cation-ordered reference), intermediate between the other two supercells. The band gap reductions we computed in these three supercells are consistent with previous calculations that consider antisite defects in ZnGeN 2 54 and ZnSnN 2 . 70 From these calculations, we can conclude that the band gap reduction difference is affected by both the number of the octet-rule-violating tetrahedra and their relative spatial separation.…”

Section: Electronic Structure Of Zntinsupporting

confidence: 89%

“…2 31,70,71 and ZnGeN 2 54,72. The conduction band edges are dominated by Ti d states, in contrast to ZnSnN 2 and ZnGeN 2 where conduction band edges are dominated by N p states 31,70,71.…”

mentioning

confidence: 99%

Zinc Titanium Nitride Semiconductor toward Durable Photoelectrochemical Applications

Greenaway

¹

,

Ke

²

,

Culman

³

et al. 2022

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Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of such technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here we report on synthesis and characterization of zinc titanium nitride (ZnTiN 2 ) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN 2 thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 0.1 S/cm. The band gap of this material is reduced from the 3.5 eV theoretical value by cation site disorder, and the impact of cation antisites on the band structure of ZnTiN 2 is explored using density functional theory. Under electrochemical polarization, the ZnTiN 2 surfaces have TiO 2 -or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN 2 is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry. Broader ImpactPhotoelectrochemical fuel generation has been stymied by a lack of photoelectrode materials which are both highly active and stable under long-term operation. Searches for new photoelectrodes have typically selected either stability or activity, with the intent to improve the other characteristic after the fact. Inspired by technologies that employ designed surface transformations for operational stability, such as the precipitation strengthening of Ni-based superalloys in gas turbines, here we employ co-design principles to identify a candidate photoelectrode material which can fill both stability and activity requirements. We synthesize this promising candidate photoelectrode material, ZnTiN2, which forms stable protective oxides under electrochemical operation, providing a route to stability, while being structurally compatible with established semiconductors, enabling good optoelectronic properties. We investigate the optoelectronic properties and electrochemical stability of ZnTiN2 both experimentally and computationally. These results confirm the promise of ZnTiN2 as a photoelectrode material and point to a successful new materials design strategy for photoelectrode development.

show abstract

“…Introducing more octet-rule-violating Ncentered tetrahedra in supercell III, with two close swaps, we find that the DFT-PBE band gap of ZnTiN2 is reduced by 0.47 eV (21% relative to the cation-ordered reference), intermediate between the other two supercells. The band gap reductions we computed in these three supercells are consistent with previous calculations that consider antisite defects in ZnGeN2 54 and ZnSnN2. 70 From these calculations, we can conclude that the band gap reduction difference is affected by both the number of the octet-rule-violating tetrahedra and their relative spatial separation.…”

Section: Electronic Structure Of Zntin2supporting

confidence: 89%

“…69 Examining the projected density of states (DOS) for the cation-ordered structure, we find that the valence band edges are dominated by N p states, consistent with reports of other zinc ternary nitrides with similar structure, such as ZnSnN2 31,70,71 and ZnGeN2. 54,72 The conduction band edges are dominated by Ti d states, in contrast to ZnSnN2 and ZnGeN2 where conduction band edges are dominated by N p states. 31,70,71 The experimentally synthesized ZnTiN2 is cation-disordered by XRD; that is, the material features a high density of antisite defects, where the positions of Zn and Ti atoms are swapped relative to the cation-ordered structure (although experimental quantification of cation-site disorder is difficult, see Ref [52]).…”

Section: Electronic Structure Of Zntin2mentioning

confidence: 99%

Co-design of zinc titantium nitride semiconductor towards durable photoelectrochemical applications

Greenaway

¹

,

Ke

²

,

Culman

³

et al. 2022

Preprint

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Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of such technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here we report on synthesis and characterization of zinc titanium nitride (ZnTiN2) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN2 thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 0.1 S/cm. The band gap of this material is reduced from the 3.5 eV theoretical value by cation site disorder, and the impact of cation antisites on the band structure of ZnTiN2 is explored using density functional theory. Under electrochemical polarization, the ZnTiN2 surfaces have TiO2- or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN2 is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry.

show abstract

Bandgap analysis and carrier localization in cation-disordered ZnGeN2

Cited by 19 publications

References 78 publications

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

Zinc Titanium Nitride Semiconductor toward Durable Photoelectrochemical Applications

Co-design of zinc titantium nitride semiconductor towards durable photoelectrochemical applications

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