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

Abstract

Chemical functionalization is demonstrated to enhance the p-type electrical performance of twodimensional (2D) layered tungsten diselenide (WSe 2 ) fieldeffect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH 4 ) 2 S-(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe 2 surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe 2 valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe 2 by (NH 4 ) 2 S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe 2 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 4 ) 2 S(aq) chemical treatment, the p-branch ON-currents (I ON ) of back-gated few-layer ambipolar WSe 2 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 pdoping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe 2 /contact metal interface. This (NH 4 ) 2 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. KEYWORDS: transition-metal dichalcogenides, tungsten diselenide, (NH 4 ) 2 S(aq) chemical treatment, scanning tunneling microscopy/spectroscopy, band structure, field-effect transistors

Contact Engineering of Layered MoS2 via Chemically Dipping Treatments

“…Although I OFF of L1, L2, and L3 is nearly constant, I ON at the n ‐branch increases with negative shift of threshold voltage; therefore, the I ON / I OFF ratios between V BG = −60 and +60 V of all treated channels are improved 4 X. It is noted that the present two‐step treatment involves vacuum annealing at 473 K; therefore, it is hypothesized that molecular adsorbates, such as HS, [ 17 ] would be mostly removed from MoS 2 surface by annealing in vacuum. Moreover, NH 3 or H 2 S have negligible effects on the electronic structure of TMDs as shown in previous report.…”

“…Moreover, NH 3 or H 2 S have negligible effects on the electronic structure of TMDs as shown in previous report. [ 17 ] Since the Raman spectra in Figure 1b reveal that the two‐step treatment does not induce structure changes in MoS 2 , the shift of threshold voltage in Figure 2c is unlikely to be the result of surface or substitutional doping of MoS 2 . Instead, improvement of carrier transport behavior with the two‐step treatment is likely one of the sources of source of the threshold voltage shift, as shown in Figure S7 (Supporting Information).…”

p-/n-Type modulation of 2D transition metal dichalcogenides for electronic and optoelectronic devices

“…metal oxides [70,121,[139][140][141], metal halide [125][126][127][128][129][130], organic molecules [125, 134-136, 146, 149, 150, 152], etc. Surface deposition/coating has the advantages of patterned doping capability, relative low doping temperature, and little crystal damage [129,133,134,153]. However, the instability and poor process compatibility of some surface charge transfer dopants would restrict its real applications [137].…”

Manipulating the Light‐Matter Interaction of PtS/MoS2 p–n Junctions for High Performance Broadband Photodetection

“…[ 13–15 ] Molecular doping, such as fluoro‐based polymer, could also realize the formation of p‐type electrical property. [ 16–19 ] However, the stability of the adsorbed molecular is still a challenge. Recently, doping with heteroatom, such as Nb, V, etc., could convert the n‐type conductive behavior to p‐type.…”