Band Structure Engineering of Layered WSe2 via One-Step Chemical Functionalization

Jun Hong Park; Amritesh Rai; Jeongwoon Hwang; Chenxi Zhang; Iljo Kwak; Steven F Wolf; Suresh Vishwanath; Xinyu Liu; Malgorzata Dobrowolska; Jacek Furdyna; Huili Grace Xing; Kyeongjae Cho; Sanjay K Banerjee; Andrew C Kummel

doi:10.1021/acsnano.8b09351

Band Structure Engineering of Layered WSe₂ via One-Step Chemical Functionalization

ACS Nano. 2019 Jul 23;13(7):7545-7555. doi: 10.1021/acsnano.8b09351. Epub 2019 Jul 1.

Authors

Jun Hong Park^{1

2}, Amritesh Rai³, Jeongwoon Hwang^{4

5}, Chenxi Zhang⁴, Iljo Kwak², Steven F Wolf², Suresh Vishwanath⁶, Xinyu Liu⁷, Malgorzata Dobrowolska⁷, Jacek Furdyna⁷, Huili Grace Xing^{6

8}, Kyeongjae Cho⁴, Sanjay K Banerjee³, Andrew C Kummel^{2

9}

Affiliations

¹ School of Materials Science and Engineering , Gyeongsang National University , Jinju 52828 , Republic of Korea.
² Materials Science and Engineering Program , University of California, San Diego , La Jolla , California 92093 , United States.
³ Microelectronics Research Center, Department of Electrical and Computer Engineering , The University of Texas at Austin , Austin , Texas 78758 , United States.
⁴ Department of Materials Science and Engineering , The University of Texas at Dallas , Dallas , Texas 75080 , United States.
⁵ Department of Physics Education , Chonnam National University , Gwangju 61186 , Republic of Korea.
⁶ School of Electrical and Computer Engineering , Cornell University , Ithaca , New York 14853 , United States.
⁷ Physics Department , University of Notre Dame , Notre Dame , Indiana 46556 , United States.
⁸ Department of Materials Science and Engineering , Cornell University , Ithaca , New York 14853 , United States.
⁹ Departments of Chemistry and Biochemistry , University of California, San Diego , La Jolla , California 92093 , United States.

PMID: 31260257
DOI: 10.1021/acsnano.8b09351

Abstract

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe₂) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH₄)₂S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe₂ surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe₂ valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe₂ by (NH₄)₂S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe₂ 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 (NH₄)₂S(aq) chemical treatment, the p-branch ON-currents (I_ON) of back-gated few-layer ambipolar WSe₂ 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 WSe₂/contact metal interface. This (NH₄)₂S(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.

Keywords: (NH)S(aq) chemical treatment; band structure; field-effect transistors; scanning tunneling microscopy/spectroscopy; transition-metal dichalcogenides; tungsten diselenide.