To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/μm(2) at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

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

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe, chemical vapor deposition (CVD)-grown MoS, and MBE SnSe are compared. Both MBE-grown WSe and CVD-grown MoS display a selective air exposure response at the step edges, consistent with oxidation on WSe and adsorption of hydrocarbon on MoS, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe is decomposed to SnO and SeO, as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe being driven by the weak bonding of SnSe.

The performance of electronic/optoelectronic devices is governed by carrier injection through metal-semiconductor contact; therefore, it is crucial to employ low-resistance source/drain contacts. However, unintentional introduction of extrinsic defects, such as substoichiometric oxidation states at the metal-semiconductor interface, can degrade carrier injection. In this report, controlling the unintentional extrinsic defect states in layered MoS 2 is demonstrated using a two-step chemical treatment, (NH 4 ) 2 S(aq) treatment and vacuum annealing, to enhance the contact behavior of metal/MoS 2 interfaces. The two-step treatment induces changes in the contact of single layer MoS 2 field effect transistors from nonlinear Schottky to Ohmic behavior, along with a reduction of contact resistance from 35.2 to 5.2 kΩ. Moreover, the enhancement of I ON and electron field effect mobility of single layer MoS 2 field effect transistors is nearly double for n-branch operation. This enhanced contact behavior resulting from the two-step treatment is likely due to the removal of oxidation defects, which can be unintentionally introduced during synthesis or fabrication processes. The removal of oxygen defects is confirmed by scanning tunneling microscopy and X-ray photoelectron spectroscopy. This two-step (NH 4 ) 2 S(aq) chemical functionalization process provides a facile pathway to controlling the defect states in transition metal dichalcogenides (TMDs), to enhance the metal-contact behavior of TMDs.

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