Search

Density functional theory calculations are performed to explore the nature of magnetization in group-VI (O, S, Se, Te) and group-VII (F, Cl, Br, I) elements doped phosphorene monolayers. All these dopants, except for F, can introduce localized spin-polarized states in the gap of phosphorene, resulting in the magnetization of these systems. Orbital coupling analysis suggests that the distinct spin-polarization behaviors between group-VI-substituted and group-VII-substituted phosphorene result from the different orbital coupling between the dopants and neighboring P atoms. It is also demonstrated that the stability of magnetization, characterized by polarization energy, increases with the localization of spin-polarized bands. Our predictive results may inspire further experimental and theoretical exploration on the potential applications of doped 2D materials in spintronics. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials offer a promising platform for next-generation data-storage devices due to their unique planar structure, as well as brilliant electronic properties. However, the reported 2D materials-based nonvolatile memory devices have complicated architectures with multilayer stacking of 2D materials, metals, organics or oxides, which limits their capacity for device miniaturization, scalability and integration functionality. In this work, we propose a nonvolatile charge memory in pristine α-In2Se3 nanosheets with a two-terminal configuration. We present a programmable nonvolatile charge memory via applying gate voltage pulses. The devices show superb memory properties at room temperature with a large memory window, long retention time and robust endurance, which are comparable with 2D material heterostructures. Further, we demonstrate an optical manipulation of charge storage with laser power-controlled numbers of stored charges and on/off ratio. Supported by a theoretical model, we find the nonvolatile charge storage originates from the surficial/interfacial trapped electrons, which are removed via photo-generated holes. Our photo-tunable nonvolatile charge memory devices with simple structure pave the way towards large-scale integration and high-speed intelligent electronics, such as ultrafast remote operation on data coding, artificial synapse and neurons. [ABSTRACT FROM AUTHOR]

Since the Janus MoSSe monolayer was fabricated in 2017, it has attracted increasing attention because the out-of-plane mirror symmetry is broken. Here, based on first-principles calculations, we construct theoretically 2D Janus MoSSe/GaN van der Waals (vdW) heterobilayers, and demonstrate that it has direct band structure and a broad optical absorption spectrum from visible to the UV region. Interestingly, the S- and Se-interface can induce straddling type-I and staggered type-II band alignments, and the gap values are modified. Moreover, the interlayer coupling and electronic field effects on electronic structures depend on the interface characteristics in Janus MoSSe/GaN heterobilayers. The studies provide the idea to modify the electronic characteristics using interface characteristics in the 2D materials-based vdW heterostructures. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials-based van der Waals heterostructures (vdWHs) provide a platform to realize novel physical phenomena for future nanoelectronic devices. Here, our theoretical results indicate that the 2D GeSe/SnS2(SnSe2) vdWHs possess the broken-gap band alignment and can be potentially utilized in the tunnel field-effect transistors (TFETs). Moreover, negative electric field can keep type-III band alignment and enlarge the tunneling window, while the positive electric field can transform the type-III to type-I or type-II band alignments for the GeSe/SnS2(SnSe2) vdWHs. Thus, these studies open the way to fabricate the 2D materials-based TFETs and the possibility of multi-functional nanodevices applications. [ABSTRACT FROM AUTHOR]

Vertically standing SnS2 film on transparent quartz was prepared by sulfuration of Sn film deposited by magnetron sputtering method. X-ray diffraction pattern and Raman spectrum of the film indicate its good crystallinity with a layered structure. Scanning electron microscope images of the film clearly demonstrate its vertically standing structure. The thickness of a single SnS2 nanosheet is measured to be 18.4 nm according to the cross-section height profile of the atomic force microscope image. The nonlinear optical response of SnS2 nanosheet film is investigated by using the open aperture Z-scan technology with a femtosecond pulsed laser (400 fs) working at 1064 nm. The modulation depth (αs) and saturable absorption intensity (Isat) of SnS2 film are measured to be 11.7% and 1.9 MW cm−2, respectively. The Z-scan results reveal that the SnS2 nanosheet film exhibits strong saturable absorption behavior to the femtosecond laser, making it have potential applications in mode-locked lasers for femtosecond pulses generation. [ABSTRACT FROM AUTHOR]

First principles calculations are performed to explore the effect of applied vertical press on the electronic structure of monolayer MoS2. It is demonstrated that MoS2 monolayer exhibits significant modulation of its band gap and band edges by the applied vertical press. Monolayer MoS2 homojunction with type-I band alignment can be formed by applying vertical press partially on the MoS2 monolayer. The effect of the vertical press on the electronic structure of MoS2 monolayer homojunction can be significantly enhanced by enlarging the pressed area. A semiconductor-to-metal transition occurs when the MoS2 homojunction is subjected to large presses. The varying band diagram including the band edges and band gap leads to a funnelling effect with both holes and electrons concentrate on the pressed region. Our results will provide intriguing opportinuities for designing optoelectronic and photovoltaic devices with good performance based on 2D materials. [ABSTRACT FROM AUTHOR]

Through first-principles calculations, we study the stability, band structures, band alignment, and interlayer charge transfer of SnS2/BN hetero-multilayers, considering quantum size and electric field effects. We find that SnS2/BN hetero-multilayers possess the characteristics of direct band structures and type-II band alignment. Moreover, increasing the BN layer number can decrease the band gap value and work function. Additionally, type-II can be tuned to type-I band alignment in the presence of an electric field. These results indicate that the SnS2/BN system is different from that of other BN-based hybrid materials, such as MoS2/BN with type-I band alignment, which is promising for optoelectronic device applications. [ABSTRACT FROM AUTHOR]

Motivated by the recent synthesis of 2D alkaline-earth-metal hydroxide nanosheets, through first-principles calculations, we study the layer number and electric field modulations of the structural, electronic and mechanical properties of 2D X(OH)2 (X  =  Ca, Mg) multilayers. Increasing the layer number from one to ten, the Young’s modulus (Poisson’s ratio) slightly increases (decreases) and then remains saturated when the thickness is larger than four layers; while the characteristics of direct wide-gaps and work functions are robust to the layer number. In addition, the large elastic deformation (~20% biaxial stretch strain) indicates that 2D X(OH)2 possesses excellent mechanical flexibility. In particular, unlike conventional 2D materials, the electric field modulations of the band gap are very remarkable, especially for the X(OH)2 monolayer. [ABSTRACT FROM AUTHOR]

van der Waals heterobilayers have been considered to be an ideal option to enhance the electronic properties of original two-dimensional (2D) materials. We predicate theoretically that the 2D arsenene/Ca(OH)2 heterobilayers possess the characteristics of the indirect gap of 2.28 eV and type-I band alignment. Moreover, the electric field can induce the indirect–direct band gap transition of arsenene/Ca(OH)2 heterobilayers. Interestingly, the band alignment transition from type-I to type-II and type-III can be also tailored using the strength and direction of the electric field. These results provide the possibility of realizing the 2D materials-based multi-functional optoelectronic devices by applying electrostatic gating. [ABSTRACT FROM AUTHOR]

Thermally stimulated luminescence spectroscopy has been applied to study the deep centres in unintentionally doped high resistivity GaN epilayers grown by the metal organic chemical vapour deposition method on c-sapphire substrates. Two trap states with activation energies of 0.12 and 0.62 eV are evaluated from two luminescence peaks at 141.9 and 294.7 K in the luminescence curve. Our spectroscopy measurement, in combination with more accurate first-principles studies, provided insights into the microscopic origin of these levels. Our investigations suggest that the lower level at 0.12 eV might originate from CN, which behaves as a hole trap state; the deeper level at 0.62 eV can be correlated with VGa that corresponds to the yellow luminescence band observed in low-temperature photoluminescence spectra. [ABSTRACT FROM AUTHOR]