Suppression of Defects and Deep Levels Using Isoelectronic Tungsten Substitution in Monolayer MoSe₂

Abstract

Defects formed during chemical vapor deposition (CVD) of two‐dimensional (2D) transition metal dichalcogenides (TMDs) currently limit their quality and optoelectronic properties. Effective synthesis and processing strategies to suppress defects and enhance the quality of 2D TMDs are urgently needed to enable next generation optoelectronic devices. In this work, isoelectronic doping is presented as a new strategy to form stable alloys and suppress defects and enhance photoluminescence (PL) in CVD‐grown TMD monolayers. The isoelectronic substitution of W atoms for Mo atoms in CVD‐grown monolayers of Mo1– x W x Se2 (0 < x < 0.18) is shown to effectively suppress Se vacancy concentration by 50% compared to those found in pristine MoSe2 monolayers, resulting in a decrease in defect‐mediated nonradiative recombination, ≈10 times more intense PL, and an increase in the carrier lifetime by a factor of 3. Theoretical predictions reveal that isoelectronic W alloying to form Mo1– x W x Se2 monolayers raises the energy of deep level defects in MoSe2 to enable faster quenching, which is confirmed by low temperature (4–125 K) PL from defect‐related localized states. Isoelectronic substitution therefore appears to be a promising synthetic method to control the heterogeneity of 2D TMDs to realize the scalable production of high performance optoelectronic and electronic devices.

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS2/MoSe2/MoS2 Trilayer van der Waals Heterostructures

“…Previous attempts at improving the optical properties of CVD-grown MoSe 2 using HBr treatment 29 or the isoelectronic impurity substitution 30 We observe a significant energy gain of 18 kJ mol −1 (180 meV) per S transfer from MoS 2 to the MoSe 2 defect, which shows that defect healing in MoSe 2 by MoS 2 is thermodynamically favored. The formation energy of a S vacancy in a MoS 2 monolayer has been theoretically estimated in the 1.3 eV-1.5 eV range.…”

“…Unfortunately, the most promising approach today, namely chemical vapor deposition (CVD) growth [23][24][25][26][27] struggles to compete with exfoliated TMDC in terms of sample quality. Low temperature PL spectroscopy of CVD-grown MoS 2 and MoSe 2 reveals broad emission from defect bound excitons, which is significantly more intense than the free exciton peak [28][29][30] and is related to chalcogen vacancies induced during the CVD growth. 29,30 Here, we demonstrate a novel approach to neutralize the intrinsic defects of CVD-grown TMDCs, using flake transfer tools routinely employed in the fabrication of van der Waals heterostructures.…”

A roadmap for electronic grade 2D materials

“…As a result, a number of successful strategies for doping 2D materials have been developed, including direct charge injection via electrostatic gating, [222][223][224][225] charge donation from physically or chemically adsorbed molecules or ions, [226][227][228][229][230][231] and covalent bonding 232,233 via edge functionalization or substituted atoms. [4][5][6][7][8][234][235][236][237][238] These doping methods impact the optical, electrical, and optoelectronic properties of 2D TMDs. For examples, physisorbed or chemisorbed molecules on 2D layers act as molecular gates by injecting charge, which can modulate PL intensity through different charge carrier polarity.…”

Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers

“…[8,10] Taking MoSeS alloy monolayer as an example,d ensity-functional-theory (DFT) calculations reveal that the charge density mainly distributes on the Mo atoms,i mplying these Mo atoms could potentially act as the catalytically active sites.M oreover,t he MoSeS alloy monolayer exhibits obviously enhanced density of states (DOS) at the conduction band edge as compared to the pristine MoS 2 or MoSe 2 monolayer ( Figure 1A,D ,G), which would contribute to ap romoted electrical conductivity. [11] Particularly,t he charge density for the pristine MoS 2 or MoSe 2 monolayer is mainly localized on the center of Mo atoms,w hile that for the MoSeS alloy monolayer partially departs from the center of Mo atoms and is close to the S atoms since Satoms have astronger intrinsic electronegativity than Se atoms, [12] which could be further manifested by the shortened Mo À Sb onds and lengthened MovSe bonds ( Inspired by the above-mentioned advantages,anovel liquid-liquid interface-mediated strategy was developed for successfully fabricating MoSeS alloy monolayers (Figure 2A). With the help of heating, Se and Sp owders would gradually react with the dissociated molybdate at the oil/ water interface and hence MoSeS alloy monolayers would be ready to be formed at this interface.T heir XRD pattern in Figure S4A could be indexed to hexagonal MoSeS.X PS spectra in Figure S5 demonstrated the presence of Mo 2+ ,S 2À and Se 2À ,w hile ICP,X RF and EDX spectra synergistically verified that the atomic ratio of S:Se was almost 1:1 ( Figures S6,7, Table S1), further demonstrating the formation of MoSeS.…”