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Benefiting from the dragging effect of yttrium, an ultrahigh overall-performance phase-change memory is reported, including low resistance drift, high data retention, low power consumption, fast operating speed, and good cycling endurance.

The concentration-sensitive nitrogen-doping effects on both the stability and electrical resistivity of GST124 are understood through various configurations of N dopants.

Three new stable semiconducting Ge–Sb–Te monolayers exhibit high visible-light absorbance (105–106 cm−1) and photovoltaic efficiency (26–30% at 0.1 μm), considerably larger than the currently dominant commercial photovoltaic semiconductor Si.

Ultrawide bandgap semiconductors are fundamentally important in solid-state lighting, transparent electrodes and power electronics, but their 2D forms are rarely reported and less studied. By means of ab initio simulations, we predict new trigonal YOBr and monolayered crystals with ultrawide bandgaps and exceptional properties. It is demonstrated that trigonal YOBr is energetically, dynamically and mechanically stable and shows lower energy compared with other known experimental phases. We present that, under hydrostatic pressure, the bulk YOBr crystal undergoes a structural transformation from Rm to P4/nmm, accompanied by an indirect–direct band transition. By further exploring relevant metal oxyhalides MOX (M = Sc/Y and X = Cl/Br), we suggest that, owing to the small exfoliation energy, the monolayers promise experimental fabrication by mechanical cleavage, as for graphene. These MOX monolayers possess excellent stability, large bandgaps and high carrier mobilities. We reveal interesting indirect–direct bandgap transitions in uniaxially strained ScOCl and trigonal YOBr monolayers. In addition, we highlight that remarkable ultraviolet light absorption and appreciable band edges render these MOX monolayers great candidates for potential applications in UV-electronics and photocatalysis. Our findings open a new avenue to explore phase transitions in rare-earth oxyhalides under pressure/strain and provide promising ultrawide-bandgap semiconductors for future optoelectronic devices.

Crystalline solids with ultralow thermal conductivity are paramount for the development of thermoelectric materials and thermal barrier coatings for efficient thermal energy management. Here, by high-throughput ab initio calculations, we predict a group of 56 novel layered semiconducting IV–V–VI (IV = Si, Ge Sn, Pb; V = As, Sb, Bi; VI = S, Se, Te) compounds that are energetically, mechanically and dynamically stable. We demonstrate that these hitherto-unknown semiconductors exhibit intrinsically ultralow lattice thermal conductivity between 0.28 and 2.02 W m−1 K−1 at room temperature, most of which fall below 1 W m−1 K−1. Such ultralow thermal conductivity can be attributed to the presence of avoided-crossing behavior between low-lying optical phonons and acoustic modes, which leads to the significant reduction of phonon group velocities and induces ultrahigh Gruneisen parameters causing strong anharmonic phonon–phonon scattering, thus short phonon mean free path and low κl. In addition, we reveal that these IV–V–VI compounds show significant anisotropy κl along different directions, arising from the anisotropic group velocity and anharmonicity due to the weak VI–VI interlayer interactions along the c-axis. Our work not only provides a large family of novel semiconductors with exceptionally low κl but also highlights the design of new low-κl materials.

Metal oxides are particularly promising high-temperature thermoelectric (TE) materials due to their excellent high-temperature stability, eco-friendliness and cost-effectiveness. Yet, their poor TE performance has become the main bottleneck for their wide application. As a result, it is imperative to find novel oxide materials with superior thermoelectric performance for applications at high temperatures. In this work, a high-throughput (HT) materials-discovery effort has been made to discover promising TE oxides using the ALKEMIE platform. A novel type of oxide MTa2O6 (M = Mg, Ca) was discovered for high-temperature TE applications. In addition, another screened oxide, SrTiO3, a well-known n-type perovskite oxide, was used as a benchmark for comparison. The calculated results indicate that CaTa2O6 possesses a similar band structure to SrTiO3 and thus superior electrical-transport performance. MgTa2O6 exhibits a peak value of the thermoelectric figure of merit, ZT, of larger than unity at 1000 K. We find that MgTa2O6 has a superior Seebeck coefficient compared with SrTiO3 or CaTa2O6. Further analysis of the electronic structure suggests that the flat conduction-band edge in MgTa2O6 produces a highly energy-dependent electronic density of states, and thus the high Seebeck coefficient. Furthermore, a significant reduction of the phonon relaxation time is the origin of the observed decrease in the calculated thermal conductivity between 300 K and 1000 K. The present work demonstrates the results of a rapid and successful HT screening of high-temperature TE materials, and can be extended to the exploration of other new materials.

Sb2Te3 phase-change material possesses the highest crystallization speed and hence the highest operating speed among investigated phase-change systems. Doping with Y or Sc has been exploited to optimize the performance of Sb2Te3, yet the substituted Y atoms are strongly clustered, while Sc is extremely expensive and thus is unfavourable for commercialization. In this work, we have successfully obtained better-performance and moderate-cost phase-change materials by co-doping Sc and Y based on ab initio calculations and ab initio molecular-dynamics simulations (AIMD). Sc can shrink the lattice while Y expands the lattice, which makes a perfect match between original and co-doped configurations and hence can benefit by maximizing the release of lattice strain. The co-doping increases the band gap to around 0.5 eV, and the concentration ratio of Sc and Y dopants provides an advantageous tool for controlling the electronic structure. Results of calculations using the BoltzTraP code show that co-doping can result in a significant reduction in the electrical conductivity at room temperature. AIMD simulation of amorphous co-doped Sb2Te3 shows that the incorporation of Sc and Y atoms can effectively improve the thermal stability of amorphous Sb2Te3. Overall, co-doping Sc and Y is a feasible way to improve the properties of Sb2Te3 for phase-change memory applications.

Single-layer 1T-TaSe2 has recently been found to host a Mott insulating ground state entwined with a commensurate charge density wave (CDW). However, the interplay between the CDW order and the Mott state remains unclear and elusive. Using first-principles calculations, we show that the CDW order with Star of David lattice distortions induces the formation of two different types of Ta-d orbitals (one is localized and the other is extended), and the Mott insulating state is brought on by the electron correlation in the localized orbital which is separated from the extended orbitals in energy. Interestingly, when a compressive biaxial strain is applied on monolayer 1T-TaSe2, the Mott insulating phase will first transform into a charge-transfer insulating phase and then into a normal metallic phase with increasing strain. The CDW always persists and is even enhanced under the strain, indicating that the Mottness collapse is unexpectedly not at the expense of suppression of the long-range CDW order. We further reveal that the realization of metallization in the CDW phase occurs because the localized orbital is immersed into the extended-orbital-spanned Fermi sea and its Hubbard splitting is quenched by orbital hybridization. Our findings may provide significant implications for fine control over correlated electronic states, possessing potential applications in ultrafast resistive switching.

Herein, the tape-casting technique was used to fabricate an Er-doped (K0.5Na0.5)NbO3 (KNN:0.02Er) laminar ceramic, which possesses a pseudo-cubic phase structure with regular cube-shaped grains. Considering the substitution behavior of the Er dopant in the KNN lattice, X-ray diffraction analysis and density functional theory simulations both verify that Er can replace K and Na sites. And Er-induced enlargement of the energy band gap of KNN:0.02Er has been obtained in the reflectance spectra and simulations. Furthermore, the ceramic exhibits dual-mode down-conversion and up-conversion photoluminescence (PL). As a lead-free luminescent ferroelectric, the PL intensity of KNN:0.02Er can be enhanced by polarization induced by electric poling. And the dual-mode PL can be obviously modulated under an electric field in an in situ, reversible, real-time and dynamical way. Our results offer an effective theoretical way to bridge the relationship between the crystal structure/chemical bonding environment and the performance of the KNN system, and also provide an opportunity to realize electrically controlled tuning of the PL response in KNN-based luminescent ferroelectrics and the corresponding optoelectronic devices.

Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-change random access memory (PCRAM), which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write–erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials, remains unclear. Using oxygen doped GeTe as an example, this study unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms will stay in the interstitial region near the Te vacancy that was originally occupied by oxygen, forming a “dumbbell-like” defect (O–VTe–Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration leads to a slower crystallization speed and hence the improved data retention of oxygen doped GeTe. Moreover, we find that the local oxygen configuration will increase the effective mass of the carrier and thus increase the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of GeTe phase-change material, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Recently, due to the unexpectedly high carrier mobility and strongly suppressed recombination of electron–hole pairs, exfoliated atomically thin InSe has exhibited potential applications in nanoscaled electronic devices. In this study, via first-principle calculations, we have systematically investigated the crystal and electronic structures of bilayer InSe with different stacking configurations. Interestingly, the five possible stacking configurations of bilayer InSe can be categorized into two groups: Group-S with a shorter vdW interlayer distance and smaller band gap and Group-L with a longer vdW interlayer distance and larger band gap. It is highlighted that the indirect band gap bilayer InSe can be transformed into its metallic type. We have unraveled that the electronic origin of the band gap transition is derived from the electric field-induced near free-electron gas. Furthermore, a prototype data storage device based on the bilayer InSe has been proposed; this study will shed light on the design and application of bilayer InSe as well as two-dimensional material-based electronic devices in the future.

The quantum spin Hall (QSH) phase is a peculiar physical phenomenon characterized by topologically protected helical edge states,} with potential applications in lower-power electronics and spintronics. Here{,} using first-principles calculations{,} we predict the QSH phase in Mo2M2C3O2 (M = Ti{,} Zr{,} or Hf){,} new members with ordered structures in the family of two-dimensional transition metal carbides (MXenes). The QSH phase which is confirmed by the nontrivial Z2 topological invariant and Dirac edge states arises from a d-d band inversion between the M-dxy{,}x2-y2 and the Mo-dz2 orbitals and a spin-orbital coupling (SOC)-induced splitting of the M-dxy{,}x2-y2 orbital at the [Gamma] point. With different M atoms{, the QSH gap of Mo2M2C3O2 ranges from 38 to 152 meV. These findings will broaden the scientific and technological impacts of both QSH materials and MXenes.

Resistance random access memory (RRAM) is known to be a promising candidate for next generation non-volatile memory devices, in which the diffusion of oxygen vacancies plays a key role in resistance switching. Based on first principles calculations and transition state theory, using SrZrO3 (SZO) as an example, we found that the diffusion energy of an oxygen vacancy strongly depends on its charge states and V2+O contributes mostly to the resistance switching due to its lowest activation energy. To adjust the performance of SZO RRAM, the effects of dopants (Y, V, Nb and Ta) were revealed according to their modifications on the diffusion of V2+O. We found that doping of Y or V has the most significant effect on the performance of RRAM devices. Furthermore, for dopants with various numbers of valence electrons and atomic radius, general design principles were proposed based on their different effects on the RRAM characteristics. Our results will guide the experimentations and pave a new way for the optimization of RRAM devices.