Band Structure Engineering of Layered WSe 2 via One-Step Chemical Functionalization

• Published in: ACS nano 2019-07, Vol.13 (7), p.7545-7555
• Main Authors: Park, Jun Hong, Rai, Amritesh, Hwang, Jeongwoon, Zhang, Chenxi, Kwak, Iljo, Wolf, Steven F, Vishwanath, Suresh, Liu, Xinyu, Dobrowolska, Malgorzata, Furdyna, Jacek, Xing, Huili Grace, Cho, Kyeongjae, Banerjee, Sanjay K, Kummel, Andrew C
• Format: Article
• Language: English
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.8b09351

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe ) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH ) S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe by (NH ) S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH ) S(aq) chemical treatment, the p-branch ON-currents ( I ) of back-gated few-layer ambipolar WSe FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe /contact metal interface. This (NH ) S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides.