Recent experimental studies showed that formation of a two-dimensional crystal structure of nitrogenated holey graphene (NHG) is possible. Similar to graphene, NHGs have an atomically thin and strong ...crystal structure. Using first-principles calculations, we investigate the structural, phononic, and thermal properties of monolayer NHG crystal. Our charge analysis reveals that the charged holey sites of NHG provide a reactive ground for further functionalization by adatoms or molecules. We also found that similar to graphene, the NHG structure has quite high-frequency phonon modes and the presence of nitrogen atoms leads to the emergence of additional vibrational modes. Our phonon analysis reveals the presence of three characteristic Raman-active modes of NHG. Furthermore, the analysis of constant-volume heat capacity showed that the NHG structure has a linear temperature dependence in the low-temperature region. The strong lattice structure and unique thermal properties of the NHG crystal structure are desirable in nanoscale device applications.
The adsorption characteristics of alkali, alkaline-earth, and transition metal adatoms on silicene, a graphene-like monolayer structure of silicon are analyzed by means of first-principles ...calculations. In contrast to graphene, interaction between the metal atoms and the silicene surface is quite strong due to its highly reactive buckled hexagonal structure. In addition to structural properties, we also calculate the electronic band dispersion, net magnetic moment, charge transfer, work function, and dipole moment of the metal adsorbed silicene sheets. Alkali metals, Li, Na, and K, adsorb to hollow sites without any lattice distortion. As a consequence of the significant charge transfer from alkalis to silicene, metalization of silicene takes place. Trends directly related to atomic size, adsorption height, work function, and dipole moment of the silicene/alkali adatom system are also revealed. We found that the adsorption of alkaline-earth metals on silicene is entirely different from their adsorption on graphene. The adsorption of Be, Mg, and Ca turns silicene into a narrow gap semiconductor. Adsorption characteristics of eight transition metals Ti, V, Cr, Mn, Fe, Co, Mo, and W are also investigated. As a result of their partially occupied d orbital, transition metals show diverse structural, electronic, and magnetic properties. Upon the adsorption of transition metals, depending on the adatom type and atomic radius, the system can exhibit metal, half-metal, and semiconducting behavior. For all metal adsorbates, the direction of the charge transfer is from adsorbate to silicene, because of its high surface reactivity. Our results indicate that the reactive crystal structure of silicene provides a rich playground for functionalization at nanoscale.
First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, ...honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their pi and pi(*) bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.
Ultrathin two-dimensional Janus-type platinum dichalcogenide crystals formed by two different atoms at opposite surfaces are investigated by performing state-of-the-art density functional theory ...calculations. First, it is shown that single-layer PtX2 structures (where X = S, Se, or Te) crystallize into the dynamically stable 1T phase and are indirect band gap semiconductors. It is also found that the substitutional chalcogen doping in all PtX2 structures is favorable via replacement of surface atoms with a smaller chalcogen atom, and such a process leads to the formation of Janus-type platinum dichalcogenides (XPtY, where X and Y stand for S, Se, or Te) which are novel single-layer crystals. While all Janus structures are indirect band gap semiconductors as their binary analogues, their Raman spectra show distinctive features that stem from the broken out-of-plane symmetry. In addition, it is revealed that the construction of Janus crystals enhances the piezoelectric constants of PtX2 crystals significantly both in the in-plane and in the out-of-plane directions. Moreover, it is shown that vertically stacked van der Waals heterostructures of binary and ternary (Janus) platinum dichalcogenides offer a wide range of electronic features by forming bilayer heterojunctions of type-I, type-II, and type-III, respectively. Our findings reveal that Janus-type ultrathin platinum dichalcogenide crystals are quite promising materials for optoelectronic device applications.
Ab initio calculations within the density-functional theory formalism are performed to investigate the chemical functionalization of a graphene-like monolayer of silicon-silicene-with B, N, Al, or P ...atoms. The structural, electronic, magnetic, and vibrational properties are reported. The most preferable adsorption sites are found to be valley, bridge, valley and hill sites for B, N, Al, and P adatoms, respectively. All the relaxed systems with adsorbed/substituted atoms exhibit metallic behavior with strongly bonded B, N, Al, and P atoms accompanied by an appreciable electron transfer from silicene to the B, N, and P adatom/substituent. The Al atoms exhibit opposite charge transfer, with n-type doping of silicene and weaker bonding. The adatoms/substituents induce characteristic branches in the phonon spectrum of silicene, which can be probed by Raman measurements. Using molecular dynamics, we found that the systems under study are stable up to at least T = 500 K. Our results demonstrate that silicene has a very reactive and functionalizable surface.
By performing first-principles calculations, we propose a stable direct band gap semiconductor Janus single-layer structure, In2SSe. The binary analogs of the Janus structure, InS and InSe single ...layers are reviewed to evince the structural and electronic relation with In2SSe. The structural optimization calculations reveal that a Janus In2SSe single layer has hexagonal geometry like the InS and InSe single layers, which are also its structural analogs. The Janus single layer is dynamically stable, as indicated by the phonon spectrum. The electronic band diagram of the Janus structure shows that an In2SSe single layer is a direct band gap semiconductor, in contrast to its analogs, InS and InSe single layers, which are indirect band gap semiconductors. Nevertheless, it is found that the strain effect on electronic properties of the InS and InSe single layers designates the electronic structure of the Janus single layer. A rough model for the construction of the electronic band diagram of the Janus structures is discussed, and it is indicated that the difference in work functions of chalcogenide sides in the Janus structure determines the construction of the electronic structure. It is found that the Janus structure is a robust direct gap semiconductor under tolerable strain; for that reason, the Janus In2SSe single layer is a candidate for optoelectronic nanodevice applications.
We present our study on atomic, electronic, magnetic, and phonon properties of the one-dimensional honeycomb structure of molybdenum disulfide (MoS2) using the first-principles plane wave method. ...Calculated phonon frequencies of bare armchair nanoribbon reveal the fourth acoustic branch and indicate the stability. Force constant and in-plane stiffness calculated in the harmonic elastic deformation range signify that the MoS2 nanoribbons are stiff quasi one-dimensional structures, but not as strong as graphene and BN nanoribbons. Bare MoS2 armchair nanoribbons are nonmagnetic, direct band gap semiconductors. Bare zigzag MoS2 nanoribbons become half-metallic as a result of the (2 × 1) reconstruction of edge atoms and are semiconductor for minority spins, but metallic for the majority spins. Their magnetic moments and spin-polarizations at the Fermi level are reduced as a result of the passivation of edge atoms by hydrogen. The functionalization of MoS2 nanoribbons by adatom adsorption and vacancy defect creation are also studied. The nonmagnetic armchair nanoribbons attain net magnetic moment depending on where the foreign atoms are adsorbed and what kind of vacancy defect is created. The magnetization of zigzag nanoribbons due to the edge states is suppressed in the presence of vacancy defects.
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