Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial to develop lowdimensional spintronic devices. Using density functional theory, we show that single-layer chromium trihalides (SLCTs) (CrX3,X=F, Cl, Br and I) constitute a series of stable 2D intrinsic FM semiconductors. A free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low cleavage energy and a high in-plane stiffness. Electronic structure calculations using the HSE06 functional indicate that both bulk and single-layer CrX3 are half semiconductors with indirect gaps and their valence bands and conduction bands are fully spin-polarized in the same spin direction. The energy gaps and absorption edges of CrBr3 and CrI3 are found to be in the visible frequency range, which implies possible opt-electronic applications. Furthermore, SLCTs are found to possess a large magnetic moment of 3µB per formula unit and a sizable magnetic anisotropy energy. The magnetic exchange constants of SLCTs are then extracted using the Heisenberg spin Hamiltonian and the microscopic origins of the various exchange interactions are analyzed. A competition between a near 90 • FM superexchange and a direct antiferromagnetic (AFM) exchange results in a FM nearestneighbour exchange interaction. The next and third nearest-neighbour exchange interactions are found to be FM and AFM respectively and this can be understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction. Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo simulations and the values can further increase by applying a biaxial tensile strain. The unique combination of robust intrinsic ferromagnetism, half semiconductivity and large magnetic anisotropy energies renders the SLCTs as promising candidates for next-generation semiconductor spintronic applications.

A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.

Searching the novel 2D semiconductor is crucial to develop the next-generation lowdimensional electronic device. Using first-principles calculations, we propose a class of unexplored binary V-V compound semiconductor (PN, AsN, SbN, AsP, SbP and SbAs) with monolayer black phosphorene (α) and blue phosphorene (β) structure. Our phonon spectra and room-temperature molecular dynamics (MD) calculations indicate that all compounds are very stable. Moreover, most of compounds are found to present a moderate energy gap in the visible frequency range, which can be tuned gradually by in-plane strain. Especially, α-phase V-V compounds have a direct gap while β-SbN, AsN, SbP, and SbAs may be promising candidates of 2D solar cell materials due to a wide gap separating acoustic and optical phonon modes. Furthermore, vertical heterostructures can be also built using lattice matched α(β)-SbN and phosphorene, and both vdW heterostructures are found to have intriguing direct band gap. The present investigation not only broads the scope of layered group V semiconductors but also provides an unprecedented route for the potential applications of 2D V-V families in optoelectronic and nanoelectronic semiconductor devices. Keywords: monolayer compound semiconductor, electronic properties, phosphorene, firstprinciples I. INTRUDUCTIONTwo-dimensional (2D) semiconductors of group V elements including phosphorene, arsenene, and antimonene have been rapidly attracting interest on account of their significant wide-range fundamental band gap, high and anisotropic carrier mobility, linear dichroism, and anisotropic thermal conductivities.1-11 The outstanding properties make these systems as very favorable contenders for 2D electronics applications beyond graphene and transition metal dichalcogenides (TMDs).12-14 The success of group V monolayer motivates the ongoing search for related 2D materials with unusual properties. Recently, phosphorene has been extended notably by introducing isoelectronic IV-VI compounds, 15,16 it is thus intriguing to see whether the binary V-V compound monolayers can be achieved.As established in case of phosphorus, group V elements and their compounds can form interesting 2D layered structures such as black-phosphorene-like [α-phase, (No.64)] and blue-phosphorene-like [β-phase, (No.166)], in which the atomic layers are binding with vdW interaction. It's well known that graphene and phosphorene can be mechanically exfoliated from graphite and bulk black phosphorus.17 It is thus viable that the layered black-phosphorene-like (α-phase) and blue-phosphorenelike (β-phase) of AsP and SbAs structures can be made into monolayer AsP and SbAs.18 Very recently, Kou et al. have investigated monolayer arsenic and its compound SbAs, which can be seen as a single-layer of bulk compound with space group of R3m. Except SbAs, P, As and Sb can also be pairwise combined to form various binary compounds. It is expected that these binary V-V materials will exhibit unforeseen properties that present invaluable opportunities for ...