Determination of the embedded electronic states at nanoscale interface via surface-sensitive photoemission spectroscopy

Interfacial electronic state between hexagonal ZnO and cubic NiO

The interface of two dissimilar materials gives rise to a myriad of interesting structural, magnetic, and electronic properties that may be utilized to produce novel materials with unique characteristics and functions. In particular, growing a cubic oxide film on top of a hexagonal oxide substrate results in such unique properties due to the conflict of their respective stabilization mechanisms within the interface layer. This study aims to elucidate the electronic properties of the interface between hexagonal ZnO and cubic NiO by analyzing the interface electronic states within epitaxial NiO films grown on ZnO substrates, expressed in the form of ultraviolet photoemission spectroscopy (UPS) for valence band structure and X-ray absorption spectroscopy (XAS) spectra for conduction band structure. This is accomplished through a modeling approach in which the film, substrate, and interface signals are assumed to be related to each other by a set of mathematical equations, and then rearranging and modulating the equations to obtain unique UPS and XAS spectra that depict the interface electronic states.

Local symmetry-driven interfacial magnetization and electronic states in (ZnO)n/(w-FeO)n superlattices

Superlattices constructed with the wide-band-gap semiconductor ZnO and magnetic oxide FeO, both in the wurtzite structure, have been investigated using spin-polarized first-principles calculations. The structural, electronic and magnetic properties of the (ZnO)n/(w-FeO)n superlattices were studied in great detail. Two different interfaces in the (ZnO)n/(w-FeO)n superlattices were identified and they showed very different magnetic and electronic properties. Local symmetry-driven interfacial magnetization and electronic states can arise from different Fe/Zn distributions at different interfaces or spin ordering of Fe in the superlattice. The local symmetry-driven interfacial magnetization and electronic states, originating either from different Fe/Zn distribution across interfaces I and II, or by spin ordering of Fe in the superlattice, can be identified. It was also found that, in the case of the ferromagnetic phase, the electrons are more delocalized for the majority spin but strongly localized for the minority spin, which resulted in interesting spin-dependent transport properties. Our results will pave the way for designing novel spin-dependent electronic devices through the construction of superlattices from semiconductors and multiferroics.

Strain engineering of Li+ ion migration in olivine phosphate cathode materials LiMPO4 (M = Mn, Fe, Co) and (LiFePO4)n(LiMnPO4)m superlattices

The olivine phosphate family has been widely utilized as cathode materials for high-performance lithium-ion batteries. However, limited energy density and poor rate performance caused by low electronic and ionic conductivities are the main obstacles that need to be overcome for their widespread application. In this work, atomic simulations have been performed to study the effects of lattice strains on the Li⁺ ion migration energy barrier in olivine phosphates LiMPO₄ (M = Mn, Fe, Co) and (LiFePO₄)_n(LiMnPO₄)_m superlattices (SLs). The (LiFePO₄)_n(LiMnPO₄)_m superlattices include three ratios of LFP/LMP, namely SL3 + 1, SL1 + 1 and SL1 + 3, each of which is along three typical (100), (010) and (001) orientations. We mainly discuss two migration paths of Li⁺ ions: the low-energy path A channel parallel to the b-axis and the medium-energy path B channel parallel to the c-axis. It is found that the biaxial tensile strain perpendicular to the migration path is most beneficial to reduce the migration energy barrier of Li⁺ ions, and the strain on the b-axis has a dominant effect on the energy barrier of Li⁺ ion migration. For path A, SL3 + 1 alternating periodically along the (010) orientation can obtain the lowest Li ion migration energy barrier. For path B, SL1 + 3 is the most favorable for Li⁺ ion migration, and there is no significant difference among the three orientations. Our work provides reference values for cathode materials and battery design.

Anisotropy Engineering of ZnO Nanoporous Frameworks: A Lattice Dynamics Simulation

The anisotropy engineering of nanoporous zinc oxide (ZnO) frameworks has been performed by lattice dynamics simulation. A series of zinc oxide (ZnO) nanoporous framework structures was designed by creating nanopores with different sizes and shapes. We examined the size effects of varying several features of the nanoporous framework (namely, the removal of layers of atoms, surface-area-to-volume ratio, coordination number, porosity, and density) on its mechanical properties (including bulk modulus, Young's modulus, elastic constant, and Poisson ratio) with both lattice dynamics simulations. We also found that the anisotropy of nanoporous framework can be drastically tuned by changing the shape of nanopores. The maximum anisotropy (defined by Y_max/Y_min) of the Young's modulus value increases from 1.2 for bulk ZnO to 2.5 for hexagon-prism-shaped ZnO nanoporous framework structures, with a density of 2.72 g/cm³, and, even more remarkably, to 89.8 for a diamond-prism-shape at a density of 1.72 g/cm³. Our findings suggest a new route for desirable anisotropy and mechanical property engineering with nanoporous frameworks by editing the shapes of the nanopores for the desired anisotropy.

Spectral and polarization based imaging in deep-ultraviolet excited photoelectron microscopy

Using photoelectron emission microscopy, nanoscale spectral imaging of atomically thin MoS₂ buried between Al₂O₃ and SiO₂ is achieved by monitoring the wavelength and polarization dependence of the photoelectron signal excited by deep-ultraviolet light. Although photons induce the photoemission, images can exhibit resolutions below the photon wavelength as electrons sense the response. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response quantified with classical optical theory. A close correlation between experiment and theory indicates that photoemission probes the optical interaction of UV-light with the material stack directly. The utility of this probe is then demonstrated when both the spectral and polarization dependence of photoemission observe spatial variation consistent with grains and defects in buried MoS₂. Taken together, these new modalities of photoelectron microscopy allow mapping of optical property variation at length scales unobtainable with conventional light-based microscopy.

Spiers Memorial Lecture: Prospects for photoelectron spectroscopy

An overview is presented of recent advances in photoelectron spectroscopy, focussing on advances in in situ and time-resolved measurements, and in extending the sampling depth of the technique. The future...

Inorganic Lead-Free B-γ-CsSnI3 Perovskite Solar Cells Using Diverse Electron-Transporting Materials: A Simulation Study

B-γ-CsSnI₃ perovskite solar cells (PSCs) are simulated employing diverse electron-transporting layers (ETLs, including TiO₂, ZnO, SnO₂, GaN, C₆₀, and PCBM), and a comparative study has been made. Both regular and inverted planar structures are simulated. Effects of the thickness of absorbers and ETLs, doping of ETLs, and interface trap states on the photovoltaic performance are studied to optimize the device structures. The regular structures have larger short-circuit current density (J _sc) than the inverted structures, but the inverted structures have larger fill factor (FF). All of the simulated optimal PSCs have similar open-circuit voltages (V _oc) of ∼0.96 V. The PSCs with TiO₂ ETLs have the best photovoltaic performance, and the optimum structure exhibits the highest efficiency of 20.2% with a V _oc of 0.97 V, J _sc of 29.67 mA/cm², and FF of 0.70. The optimal PSCs with ZnO, GaN, C₆₀, and PCBM ETLs exhibit efficiencies of 17.88, 18.09, 16.71, and 16.59%, respectively. The optimal PSC with SnO₂ ETL exhibits the lowest efficiency of 15.5% in all of the simulated PSCs due to its cliff-like band offset at the SnO₂/CsSnI₃ interface. Furthermore, the increase of interface trap density and capture cross section is found to reduce the photovoltaic performance of PSCs. This work contributes to designing and fabricating CsSnI₃ PSCs.