Periodic boundary condition(PBC)is a standard approximation for calculating crystalline materials properties.However,a PBC crystal is not the same as the real macroscopic crystal;therefore,if applied indiscriminately,...Periodic boundary condition(PBC)is a standard approximation for calculating crystalline materials properties.However,a PBC crystal is not the same as the real macroscopic crystal;therefore,if applied indiscriminately,it can lead to erroneous conclusions.For example,unlike other extensive observables such as total energy,the polarization of a macroscopic crystal cannot always be described by a PBC model,because polarization is inherently nonlocal and strongly dependent on surface terminations,irrespective of crystal size.Moreover,the symmetry of the macroscopic crystal can be altered when the PBC is applied to a macroscopic crystal.We demonstrate in this paper that the polarization of a macroscopic crystal receives contributions from both the repeating bulk units and the crystal surfaces,which must be treated on an equal footing.When the combined system of the bulk and its surfaces is taken into account,materials traditionally classified as nonpolar can,in fact,admit polar symmetry,thus explaining why experimentalists have observed polarization in some nominally"nonpolar"systems.Our study,thus,clarifies that polarization can only exist in polar group systems and that apparent violations of the Neumann's principle reported in some recent works originate from misinterpreting bulk PBC crystal as intrinsic macroscopic crystal,ignoring the contribution from the surfaces.We demonstrate that when the full bulk-plus-surface system is considered,the crystal polarization and symmetry are fully consistent with Neumann's principle.展开更多
Metal oxide semiconductor-based chemiresistors have been facing challenges in achieving low-temperature hydrogen sensing at the ppb level.The oxygen vacancy is considered to play a vital role in gas sensing properties...Metal oxide semiconductor-based chemiresistors have been facing challenges in achieving low-temperature hydrogen sensing at the ppb level.The oxygen vacancy is considered to play a vital role in gas sensing properties.However,a nonstoichiometric metal oxide semiconductor with efficient and controllable oxygen vacancies is not easily accessible.Herein,a facile solution reduction method of NaBH_(4)was developed to modulate the oxygen vacancies of ZnO nanosheets.The systematic characterization studies confirm the successful introduction of oxygen vacancies in ZnO samples.The gas sensing investigations illustrate that the construction of controlled oxygen vacancies greatly enhances the hydrogen sensing properties of ZnO nanosheets,especially a significantly reduced operating temperature(150℃)with a higher response(~38.2 for 200 ppm),a calculated limit of detection(55 ppb)and a quick recovery speed(only 6 s).The increased molecule adsorption and narrower band gap generated by controlled oxygen vacancies contribute to their superior sensing properties.The oxygen vacancy engineering strategy of pure metal oxide semiconductor-based materials shows great potential for creating low-temperature high-response sensors.展开更多
基金supported by the National Key Research and Development Program of China(Grant No.2024YFA1409800)the National Natural Science Foundation of China(Grant No.12088101)。
文摘Periodic boundary condition(PBC)is a standard approximation for calculating crystalline materials properties.However,a PBC crystal is not the same as the real macroscopic crystal;therefore,if applied indiscriminately,it can lead to erroneous conclusions.For example,unlike other extensive observables such as total energy,the polarization of a macroscopic crystal cannot always be described by a PBC model,because polarization is inherently nonlocal and strongly dependent on surface terminations,irrespective of crystal size.Moreover,the symmetry of the macroscopic crystal can be altered when the PBC is applied to a macroscopic crystal.We demonstrate in this paper that the polarization of a macroscopic crystal receives contributions from both the repeating bulk units and the crystal surfaces,which must be treated on an equal footing.When the combined system of the bulk and its surfaces is taken into account,materials traditionally classified as nonpolar can,in fact,admit polar symmetry,thus explaining why experimentalists have observed polarization in some nominally"nonpolar"systems.Our study,thus,clarifies that polarization can only exist in polar group systems and that apparent violations of the Neumann's principle reported in some recent works originate from misinterpreting bulk PBC crystal as intrinsic macroscopic crystal,ignoring the contribution from the surfaces.We demonstrate that when the full bulk-plus-surface system is considered,the crystal polarization and symmetry are fully consistent with Neumann's principle.
基金supported by funding from the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang(2019R01006)the National Natural Science Foundation of China(No.52172244).
文摘Metal oxide semiconductor-based chemiresistors have been facing challenges in achieving low-temperature hydrogen sensing at the ppb level.The oxygen vacancy is considered to play a vital role in gas sensing properties.However,a nonstoichiometric metal oxide semiconductor with efficient and controllable oxygen vacancies is not easily accessible.Herein,a facile solution reduction method of NaBH_(4)was developed to modulate the oxygen vacancies of ZnO nanosheets.The systematic characterization studies confirm the successful introduction of oxygen vacancies in ZnO samples.The gas sensing investigations illustrate that the construction of controlled oxygen vacancies greatly enhances the hydrogen sensing properties of ZnO nanosheets,especially a significantly reduced operating temperature(150℃)with a higher response(~38.2 for 200 ppm),a calculated limit of detection(55 ppb)and a quick recovery speed(only 6 s).The increased molecule adsorption and narrower band gap generated by controlled oxygen vacancies contribute to their superior sensing properties.The oxygen vacancy engineering strategy of pure metal oxide semiconductor-based materials shows great potential for creating low-temperature high-response sensors.
