Jun Kang

Band offsets and heterostructures of two-dimensional semiconductors

The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) are investigated from first principles calculations. The band alignments between different MX2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering.

Tuning Interlayer Coupling in Large-Area Heterostructures with CVD-Grown MoS2 and WS2 Monolayers

Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS2 and MoS2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS2 and MoS2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical effects.

Electronic structural Moiré pattern effects on MoS2/MoSe2 2D heterostructures

The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moiré pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moiré pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other electronic and optical properties.

Monolayer semiconducting transition metal dichalcogenide alloys: Stability and band bowing

expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8eV to 1.0eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics. V C 2013 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4799126]

Symmetry-dependent transport properties and magnetoresistance in zigzag silicene nanoribbons

First principles calculations are performed to study the transport properties of zigzag silicene nanoribbons (ZSiNRs). ZSiNRs show symmetry-dependent transport properties similar to those of zigzag graphene nanoribbons, although the σ mirror plane is absent. Even-N and odd-N ZSiNRs have very different current-voltage relationships, which can be attributed to the different parity of their π and π* bands under c2 symmetry operation with respect to the center axis. Moreover, magnetoresistance effect is observed in even-N ZSiNRs, and the order can reach 1 000 000%. On the basis of these interesting transport properties, ZSiNR-based logic devices, such as not, and, and or gates, are proposed.

Doping induced spin filtering effect in zigzag graphene nanoribbons with asymmetric edge hydrogenation

The magnetic and spin dependent transport properties of asymmetrically hydrogenated zigzag graphene nanoribbons, which are C–H2 bonded at one edge while C–H bonded at the other, are investigated from first-principles calculations. Due to their special distributions of the density of states near Fermi level, a perfect (100%) spin filtering effect can be achieved in such graphene nanoribbons through p-type or n-type doping. Moreover, a negative differential resistance effect is observed in both doping case, which results from the reducing of conductance near Fermi level with increasing bias voltage.

Two-dimensional semiconductor alloys: Monolayer Mo1-xWxSe2

Monolayer Mo1−xWxSe2 (x = 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo1−xWxSe2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo1−xWxSe2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors.

First-principles study of magnetic properties in Mo-doped graphene

The geometric structure, electronic structure and magnetic properties of substitutionally Mo-doped graphene are studied based on first-principles calculations. Mo introduces a magnetic moment of 2 μB in graphene. The magnetic properties and band structure can be well understood using a hybridization model. Magnetic coupling between two Mo impurities is also discussed. Depending on the relative position of the two Mo impurities, the ground state of the system can be ferromagnetic, antiferromagnetic or paramagnetic. A Ruderman-Kittel-Kasuya-Yosida (RKKY)-like behavior is observed when the distance between Mo atoms is relatively large. However, when the distance between Mo atoms is rather small, the RKKY model is not suitable to describe the magnetic ordering due to their non-neglectable direct interactions.

Pentagonal monolayer crystals of carbon, boron nitride, and silver azide

In this study we present a theoretical investigation of structural, electronic and mechanical properties of pentagonal monolayers of carbon (p-graphene), boron nitride (p-B

Modulating the bandgaps of graphdiyne nanoribbons by transverse electric fields

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