Te Monolayer-Driven Spontaneous van der Waals Epitaxy of Two-dimensional Pnictogen Chalcogenide Film on Sapphire.

Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.

Tuning the Spin-Alignment of Interstitial Electrons in Two-Dimensional Y2C Electride via Chemical Pressure.

We report that the spin-alignment of interstitial anionic electrons (IAEs) in two-dimensional (2D) interlayer spacing can be tuned by chemical pressure that controls the magnetic properties of 2D electrides. It was clarified from the isovalent Sc substitution on the Y site in the 2D Y2C electride that the localization degree of IAEs at the interlayer becomes stronger as the unit cell volume and c-axis lattice parameter were systematically reduced by increasing the Sc contents, thus eventually enhancing superparamagnetic behavior originated from the increase in ferromagnetic particle concentration. It was also found that the spin-aligned localized IAEs dominated the electrical conduction of heavily Sc-substituted Y2C electride. These results indicate that the physcial properties of 2D electrides can be tailored by adjusting the localization of IAEs at interlayer spacing via structural modification that controls the spin instability as found in three-dimensional elemental electrides of pressurized potassium metals.

Effects of doping on transport properties in Cu-Bi-Se-based thermoelectric materials.

The thermoelectric properties of Zn-, In-, and I-doped Cu1.7Bi4.7Se8 pavonite homologues were investigated in the temperature range from 300 to 560 K. On the basis of the comprehensive structural analysis using Rietveld refinement of synchrotron radiation diffraction for Cu(x+y)Bi(5-y)Se8 compounds with the inherently disordered crystallographic sites, we demonstrate a doping strategy that provides a simultaneous control for enhanced electronic transport properties by the optimization of carrier concentration and exceptionally low lattice thermal conductivity by the formation of point defects. Substituted Zn or In ions on Cu site was found to be an effective phonon scattering center as well as an electron donor, while doping on Bi site showed a moderate effect for phonon scattering. In addition, we achieved largely enhanced power factor in small amount of In doping on Cu site by increased electrical conductivity and moderately decreased Seebeck coefficient. Coupled with a low lattice thermal conductivity originated from intensified point defect phonon scattering by substituted In ions with host Cu ions, a thermoelectric figure of merit ZT of 0.24 at 560 K for Cu1.6915In0.0085Bi4.7Se8 was achieved, yielding 30% enhancement compared with that of a pristine Cu1.7Bi4.7Se8 at the same temperature.

Ferromagnetic quasi-atomic electrons in two-dimensional electride.

An electride, a generalized form of cavity-trapped interstitial anionic electrons (IAEs) in a positively charged lattice framework, shows exotic properties according to the size and geometry of the cavities. Here, we report that the IAEs in layer structured [Gd2C]2+·2e- electride behave as ferromagnetic elements in two-dimensional interlayer space and possess their own magnetic moments of ~0.52 µB per quasi-atomic IAE, which facilitate the exchange interactions between interlayer gadolinium atoms across IAEs, inducing the ferromagnetism in [Gd2C]2+·2e- electride. The substitution of paramagnetic chlorine atoms for IAEs proves the magnetic nature of quasi-atomic IAEs through a transition from ferromagnetic [Gd2C]2+·2e- to antiferromagnetic Gd2CCl caused by attenuating interatomic exchange interactions, consistent with theoretical calculations. These results confirm that quasi-atomic IAEs act as ferromagnetic elements and trigger ferromagnetic spin alignments within the antiferromagnetic [Gd2C]2+ lattice framework. These results present a broad opportunity to tailor intriguing ferromagnetism originating from quasi-atomic interstitial electrons in low-dimensional materials.

Author Correction: Ferromagnetic quasi-atomic electrons in two-dimensional electride.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Creation of two-dimensional layered Zintl phase by dimensional manipulation of crystal structure.

The discovery of new families, beyond graphene, of two-dimensional (2D) layered materials has always attracted great attention. However, it has been challenging to artificially develop layered materials with honeycomb atomic lattice structure composed of multicomponents such as hexagonal boron nitride. Here, through the dimensional manipulation of a crystal structure from sp3-hybridized 3D-ZnSb, we create an unprecedented layered structure of Zintl phase, which is constructed by the staking of sp2-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enabling unlimited expansion of 2D libraries and corresponding physical properties.

Highly Oriented SrTiO3 Thin Film on Graphene Substrate.

Growth of perovskite oxide thin films on Si in crystalline form has long been a critical obstacle for the integration of multifunctional oxides into Si-based technologies. In this study, we propose pulsed laser deposition of a crystalline SrTiO3 thin film on a Si using graphene substrate. The SrTiO3 thin film on graphene has a highly (00l)-oriented crystalline structure which results from the partial epitaxy. Moreover, graphene promotes a sharp interface by highly suppressing the chemical intermixing. The important role of graphene as a 2D substrate and diffusion barrier allows the expansion of device applications based on functional complex oxides.

Graphene Substrate for van der Waals Epitaxy of Layer-Structured Bismuth Antimony Telluride Thermoelectric Film.

Graphene as a substrate for the van der Waals epitaxy of 2D layered materials is utilized for the epitaxial growth of a layer-structured thermoelectric film. Van der Waals epitaxial Bi0.5 Sb1.5 Te3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals.

Tuning electromagnetic properties of SrRuO3 epitaxial thin films via atomic control of cation vacancies.

Elemental defect in transition metal oxides is an important and intriguing subject that result in modifications in variety of physical properties including atomic and electronic structure, optical and magnetic properties. Understanding the formation of elemental vacancies and their influence on different physical properties is essential in studying the complex oxide thin films. In this study, we investigated the physical properties of epitaxial SrRuO3 thin films by systematically manipulating cation and/or oxygen vacancies, via changing the oxygen partial pressure (P(O2)) during the pulsed laser epitaxy (PLE) growth. Ru vacancies in the low-P(O2)-grown SrRuO3 thin films induce lattice expansion with the suppression of the ferromagnetic T C down to ~120 K. Sr vacancies also disturb the ferromagnetic ordering, even though Sr is not a magnetic element. Our results indicate that both A and B cation vacancies in an ABO3 perovskite can be systematically engineered via PLE, and the structural, electrical, and magnetic properties can be tailored accordingly.

Phase transitions via selective elemental vacancy engineering in complex oxide thin films.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Hierarchically Structured Hole Transport Layers of Spiro-OMeTAD and Multiwalled Carbon Nanotubes for Perovskite Solar Cells.

The low electrical conductivity of spiro-OMeTAD hole transport layers impedes further enhancements of the power conversion efficiency (PCE) of perovskite solar cells. We embedded multiwalled carbon nanotubes (MWNTs) in spiro-OMeTAD (spiro-OMeTAD/MWNTs) to increase carrier mobility and conductivity. However, direct electrical contact between CH3 NH3 PbI3 and the MWNTs created pathways for undesirable back-electron transfer, owing to the large work function of MWNTs, limiting enhancements of the PCE. A hierarchical structure of pure spiro-OMeTAD and spiro-OMeTAD/MWNTs was designed to block back-electron transfer and fully exploit the enhanced charge transport of spiro-OMeTAD/MWNTs. The enhanced fill factor, short-circuit current density, open-circuit voltage, and PCE (15.1 %) were achieved by using this hierarchical hole transport layer structure (MWNT concentration=2 wt %). The perovskite solar cells were fabricated by a low-temperature solution process, further decreasing their per-Watt cost.

Strong anisotropy of ferroelectricity in lead-free bismuth silicate.

Bismuth silicate (Bi2SiO5) was recently suggested as a potential silicate based lead-free ferroelectric material. Here, we show the existence of ferroelectricity and explore the strong anisotropy of local ferroelectricity using piezoresponse force microscopy (PFM). Domain structures are reconstructed using angle-resolved PFM. Furthermore, piezoresponse hysteresis loops and piezoelectric coefficients are spatially investigated at the nanoscale. The obtained results confirm the existence of ferroelectricity with strong c-axis polarization. These results could provide basic information on the anisotropic ferroelectricity in Bi2SiO5 and furthermore suggest its considerable potential for lead-free ferroelectric applications with silicon technologies.

Improved photovoltaic response of nanocrystalline CdS-sensitized solar cells through interface control.

Nanocrystalline CdS-sensitized solar cells (CdS-SSCs) based on mesoporous TiO(2) were fabricated by the spray pyrolysis deposition method. The energy conversion efficiency of these cells was drastically increased (156%) by modifying the junction structure through post-treatment that included soaking in a dilute TiCl(4) aqueous solution and subsequent thermal annealing. We propose that the post-treatment is responsible for an increased number of interconnections between TiO(2) and CdS, as well as surface passivation of the CdS sensitizer. The increase in the cell efficiency is attributed to the improved charge carrier transport, suppression of photoelectron recombination with holes both in the same sensitizer particle and in nearby ones, and suppression of photoelectron capture by the electrolyte.