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Conventional mechanical or chemical exfoliation approach of 2D material synthesis is largely dependent on the inherent structure of the parent material, i.e., whether it is a layered structure or a 3D bulk structure with embedded 2D substructures. A recent experiment demonstrated that unprecedented atomically thin metal oxides without bulk layered structures can be synthesized by using liquid metals. Supported by an experimental realization of atomically thin W layers through the metal atomic layer deposition method, we propose a new type of transition metal (TM)-based 2D materials that can be stabilized at the oxide interfaces with oxide substrates and overlayers. Based on the ab initio density functional theory calculations, we show that most of the TM elements can form unprecedented atomically thin 2D materials by the surface oxygen passivation, which is available from the oxide substrate and the overlayer. The stabilized 2D TM layers show diverse electronic and magnetic properties. Our results suggest a novel way to extend 2D materials study and a possible application of those 2D TM layers embedded in oxides. [ABSTRACT FROM AUTHOR]

Hafnium–zirconium oxide (HZO) thin films are of interest due to their ability to form ferroelectric (FE) and antiferroelectric (AFE) oxide phases. Density functional theory is employed to elucidate the stabilization mechanisms of both FE HZO thin films and AFE ZrO2 films. The FE orthorhombic phase is primarily stabilized by in-plane tensile strain, which spontaneously occurs during the synthesis process, and this is more effective for HZO than HfO2. Layer-by-layer stack models and core-matrix three-dimensional models of the polymorphs reveal that the electrostatic component of interfacial free energy can play a critical role in the formation of the AFE tetragonal phase in ZrO2 and the "wake-up" effect for FE HZO. [ABSTRACT FROM AUTHOR]

The development of high-performance p-type oxides with high hole mobility and a wide bandgap is critical for the applications of metal oxide semiconductors in vertically integrated CMOS devices [Salahuddin et al., Nat. Electron. 1, 442 (2018)]. Sn2+-based oxides such as SnO and K2Sn2O3 have recently been proposed as high-mobility p-type oxides due to their relatively low effective hole masses, which result from delocalized Sn s-orbital character at the valence band edge. Here, we introduce a promising ternary Sn-O-X compound, Ta2SnO6, which exhibits strong valence band dispersion and a large bandgap. In order to evaluate the performance of this oxide as a p-type semiconductor, we perform first-principles calculations of the phonon-limited room-temperature carrier mobilities in SnO, SnO2, and Ta2SnO6. Electron relaxation time is evaluated, accounting for the scatterings from acoustic deformation potentials and polar optical phonons (POP), within the isotropic and dispersionless approximation. At room temperature, the electron/hole mobilities in a given material (SnO, SnO2, and Ta2SnO6) are found to be limited by POP scattering. SnO2 shows high room-temperature electron mobility of 192 cm2/(V s), while SnO and Ta2SnO6 exhibit impressive hole mobilities, with the upper limit at 60 and 33 cm2/(V s), respectively. We find that carrier effective mass largely accounts for the differences in mobility between these oxides with correspondingly different POP scattering rates. The theoretically predicted intrinsic mobilities of each material will provide the upper limit to the real mobilities for their device applications. Our findings also suggest a necessity of further investigation to identify even higher mobility p-type oxides with smaller hole effective masses. [ABSTRACT FROM AUTHOR]

The development of high-performance p-type oxides with high hole mobility and a wide bandgap is critical for the applications of metal oxide semiconductors in vertically integrated CMOS devices [Salahuddin et al., Nat. Electron. 1, 442 (2018)]. Sn2+-based oxides such as SnO and K2Sn2O3 have recently been proposed as high-mobility p-type oxides due to their relatively low effective hole masses, which result from delocalized Sn s-orbital character at the valence band edge. Here, we introduce a promising ternary Sn-O-X compound, Ta2SnO6, which exhibits strong valence band dispersion and a large bandgap. In order to evaluate the performance of this oxide as a p-type semiconductor, we perform first-principles calculations of the phonon-limited room-temperature carrier mobilities in SnO, SnO2, and Ta2SnO6. Electron relaxation time is evaluated, accounting for the scatterings from acoustic deformation potentials and polar optical phonons (POP), within the isotropic and dispersionless approximation. At room temperature, the electron/hole mobilities in a given material (SnO, SnO2, and Ta2SnO6) are found to be limited by POP scattering. SnO2 shows high room-temperature electron mobility of 192 cm2/(V s), while SnO and Ta2SnO6 exhibit impressive hole mobilities, with the upper limit at 60 and 33 cm2/(V s), respectively. We find that carrier effective mass largely accounts for the differences in mobility between these oxides with correspondingly different POP scattering rates. The theoretically predicted intrinsic mobilities of each material will provide the upper limit to the real mobilities for their device applications. Our findings also suggest a necessity of further investigation to identify even higher mobility p-type oxides with smaller hole effective masses. [ABSTRACT FROM AUTHOR]

We have performed first-principle calculations to explore the possibility of synthesizing atomically thin transition metal (TM) layers. Buckled structures as well as planar structures of elemental 2D TM layers result in significantly higher formation energies compared with sp-bonded elemental 2D materials with similar structures, such as silicene and phosphorene. It is shown that the TM layers can be stabilized by surface passivation with HS, C6H5S2, or O, and O passivation is most effective. The surface oxygen passivation can improve stability leading to thermodynamically stable TM monolayers except Au, which is the most non-reactive metal element. Such stabilized TM monolayers also show an electronic structure transition from metallic state of free-standing TM layer to semiconducting O-passivated Mo and W monolayers with band gaps of 0.20-1.38 eV. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) heterostructures (or heterojunctions) are formed by stacking two different 2D materials (e.g., graphene, h‐BN, or transition metal dichalcogenides) across vdW gaps. In a type‐II heterojunction, 2D semiconductors are aligned with staggered bandgaps, which can effectively separate electron and hole carriers, and enable promising high‐performance photovoltaics and photodetectors. Herein, an effective vdW‐homojunction is reported, formed by one 2D material (2H‐WS2) with vdW gap engineering leading to different electronic structures and type‐II junction formation. WS2 films are synthesized by W metal deposition and controlled sulfurization method leading to a nonuniform vdW gap strain in the film. The vdW strain gradients in multilayer WS2 films are confirmed by transmission electron microscopy analysis, and the modeling by density functional theory shows an effective type‐II homojunction formation via modulated bandgaps by the vdW gap strains. The superior performance of a broadband photodetector application is confirmed by photoluminescence and photocurrent experiments. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) WO x /ZnO stack reveals a unique carrier transport behavior, which can be utilized as a novel device element to achieve a very high on/off ratio (>10 6 ) and an off current density lower than 1 nA cm -2 . These unique behaviors are explained by a dynamic band alignment between WO x and ZnO, which can be actively modulated by a gate bias. The performance of FET utilizing the WO x /ZnO stack is comparable to those of other 2D heterojunction devices; however, it has a unique benefit in terms of process integration because of very low temperature process capability (T < 110 °C). The high on/off switching with extremely low off current density utilizing the dynamic band alignment modulation at the WO x /ZnO stack can be a very useful element for future device applications, especially in monolithic 3D integration or flexible electronics.

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe 2 ) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH 4 ) 2 S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe 2 surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe 2 valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe 2 by (NH 4 ) 2 S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe 2 surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH 4 ) 2 S(aq) chemical treatment, the p-branch ON-currents ( I ON ) of back-gated few-layer ambipolar WSe 2 FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe 2 /contact metal interface. This (NH 4 ) 2 S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides.