Quantum Theory of Advanced Materials

Novel Properties of Two-Dimensional Materials

The aim of our study is to develop and apply the state-of-the-art computational methodologies to predict and screen advanced materials for novel energy and device applications. One of the focused areas is on newly created two-dimensional (2D) layered structures and the heterostructures of them, with special attention to the effect of interaction at the interface in designing the system performance for particular applications.

• "Determination of Band Alignment in the Single Layer MoS2/WeS2 Heterojunction," M.-H. Chiu, C. Zhang, H.-W. Shiu, C.-P. Chuu, C.-H. Chen, C.-Y. S. Chang, C.-H. Chen, M. Y. Chou, C.-K. Shih and L.-J. Li, Nature Communications 6, 7666/1-6 (2015).

We determine a type-II alignment between MoS2 and WSe2 with a valence band offset value of 0.83 eV and a conduction band offset of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of WSe2 and MoS2 are well retained in their respective layers due to a weak interlayer coupling. Moreover, a valence band offset of 0.94 eV is obtained from density functional theory, consistent with the experimental determination. 

• "Charge density wave transition in single-layer titanium diselenide," P. Chen, Y.-H. Chan, X.-Y. Fang, Y. Zhang, M.Y.Chou, S.-K. Mo, Z. Hussain, A.-V. Fedorov, and T.-C. Chiang, Nature Communications 6, 8943/1-5 (2015).

High-quality layers of TiSe2 prepared on SiC exhibit a novel electronic effect - a charge density wave (CDW) transition with a transition temperature of TC = 232 ± 5 K, which is higher than the bulk TC = 200 ± 5 K. The gap evolution follows closely a BCS behavior, and the system remains a narrow-gap material across TC, in striking contrast to the usual Peierls model behavior. The results are rationalized in terms of first-principles calculations, symmetry breaking, and phonon entropy effects.

• "Strong Asymmetric Charge Carrier Dependence in Inelastic Electron Tunneling Spectroscopy of Graphene Phonons," F. D. Natterer, Y. Zhao, J. Wyrick, Y.-H. Chan, W.-Y. Ruan, M. Y. Chou, K. Watanabe, T. Taniguchi, N. B. Zhitenev, and J. A. Stroscio, Phys. Rev. Lett. 114, 245502/1-5 (2015).

The observation of phonons in graphene by inelastic electron tunneling spectroscopy is achieved, and the mapped total graphene phonon density of states is in good agreement with density functional calculations. An abrupt change in the phonon intensity is observed when the graphene charge carrier type is switched through a variation of the back-gate electrode potential. This sudden variation in phonon intensity is asymmetric in the carrier type, depending on the sign of the tunneling bias.

• "Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures," W. Zhang, C.-P. Chuu, J.-K. Huang, C.-H. Chen, M.-L. Tsai, Y.-H. Chang, C.-T. Liang, Y.-Z. Chen, Y.-L. Chueh, J.-H. He, M. Y. Chou, and L.-J. Li, Scientific Reports 4, 3826/1-8 (2014).

We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. The electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.