# Welding 2D Semiconducting Crystals by Covalent Stitching Grain Boundaries in WS2

Bin Han, Chun Ma, Ke Jiang, Maria Batuk, Ahin Roy, Lucia Hughes, Hanlin Wang, Verónica Montes-García, Can Wang, Fanny Richard, et al.

► To cite this version:

Bin Han, Chun Ma, Ke Jiang, Maria Batuk, Ahin Roy, et al. Welding 2D Semiconducting Crystals by Covalent Stitching Grain Boundaries in WS2. Journal of the American Chemical Society, 2025, 147 (27), pp.24162-24170. 10.1021/jacs.5c08763. hal-05157467

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Submitted on 11 Jul 2025

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# Welding 2D Semiconducting Crystals by Covalent Stitching Grain Boundaries in WS₂

Bin Han,<sup>a#</sup> Chun Ma,<sup>a#</sup> Ke Jiang,<sup>b</sup> Maria Batuk,<sup>c</sup> Ahin Roy,<sup>d</sup> Lucia Hughes,<sup>d</sup> Hanlin Wang,<sup>a</sup> Verónica Montes-García,<sup>a</sup> Can Wang,<sup>a</sup> Fanny Richard,<sup>a</sup> Devu Prasad,<sup>c</sup> Antonio Gaetano Ricciardulli,<sup>a</sup> Sara Bals,<sup>c</sup> Valeria Nicolosi,<sup>d</sup> Yumeng Shi,<sup>b</sup> Paolo Samorì<sup>a*</sup>

KEYWORDS: TMDCs, conjugated molecules, defect engineering, molecular stitching, mobilities

**ABSTRACT:** The advent of transition metal dichalcogenides (TMDCs) has revolutionized the field of optoelectronics. In this context, recent advances in large-area synthesis of monolayer WS₂ opened the door to potential optoelectronic applications, because of its inherently high photoluminescence (PL) yield and superior electron mobility. However, randomly distributed point and line defects are key bottlenecks for efficient charge transport, hindering further development of system-on-chip (SoC) technologies. Herein, we report a molecular welding strategy using benzene-1,4-dithiol (BDT) to simultaneously passivate sulfur vacancies and bridge grain boundary (GB) fissures in monolayer WS₂. The GB welding was monitored on the atomic scale by high-angle annular dark-field scanning transmission electron microscopy. This treatment yields uniform PL emission, ~200-fold enhancement in electron mobility, a three-order-of-magnitude increase in both on-state current and I<sub>on</sub>/I<sub>off</sub> ratio for transistors across GBs, comparable to intragrain characteristics. Additionally, temperature-dependent PL spectroscopy was employed to identify the defect types and the activation energy of GBs. Our approach, utilizing facile vapor deposition of ad-hoc molecules to repair line defects in 2D crystals, offers a scalable and effective solution to repair extended line defects in two-dimensional (2D) semiconductors, advancing the development of high-performance, uniform 2D optoelectronic systems.

## Introduction

The invention of field-effect transistors (FETs) has represented a veritable cornerstone in modern science and technology, forming the foundation of semiconductor industry. Yet, over the past decades, silicon-based FETs have been approaching their fundamental limit, leading to an inevitable deviation from the trajectory predicted by Moore's Law<sup>1-2</sup>. At the same time, the discovery of graphene's remarkable properties has ushered in a new era in low-dimensional electronics<sup>3-6</sup>. Such an electronic revolution has found in atomically thick two-dimensional transition metal dichalcogenides (TMDCs) the key enablers, owing to their exceptional electronic properties<sup>6-8</sup>, including high carrier mobilities (up to 100 cm²V<sup>-1</sup>s<sup>-1</sup>), high I<sub>on</sub>/I<sub>off</sub> ratios (up to 10<sup>7</sup>) and rapid operation frequencies (~1 THz). Among these materials, tungsten disulfide (WS₂) exhibits extraordinary optoelectronic properties, such as a high photoluminescence (PL) quantum yield (~11%)<sup>9</sup>.

Bulk WS₂ crystals consist of layers stacked through weak van der Waals interactions and can be separated by mechanical or liquid-phase exfoliation<sup>10</sup>. However, the resulting 2D flakes exhibit numerous defects and considerable variability in thickness, shape, and size, hindering their applicability in electronic devices. Conversely, monolayers synthesized via bottom-up methods, such as chemical vapor deposition (CVD), provide greater control over position, shape, lateral size, and the number of layers<sup>11</sup>. Despite this, artificial growth methods introduce a higher density of defects, resulting in reduced carrier mobility<sup>12-13</sup>. For example, CVD-grown WS₂ FETs exhibit an I<sub>on</sub>/I<sub>off</sub> ratio of 10<sup>5</sup> and a mobility of 0.01 cm²V<sup>-1</sup>s<sup>-1</sup>, both of which deviate significantly from theoretical prediction and from the performance of mechanically exfoliated devices<sup>14</sup>. Therefore, achieving precise control over defects and their healing in WS₂ is of critical importance.

<footnote><sup>a</sup> Université de Strasbourg and CNRS, ISIS, 8 allée Gaspard Monge, F-67000 Strasbourg, France, E-mail: samo-ri@unistra.fr</footnote>

<footnote><sup>b</sup> International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China</footnote>

<footnote><sup>c</sup> Electron Microscopy for Materials Science (EMAT) and NANOlight Center of Excellence, University of Antwerp, Antwerp 2020, Belgium</footnote>

<footnote><sup>d</sup> CRANN & AMBER Research Centres and School of Chemistry, Trinity College Dublin, Dublin D2, Ireland</footnote>

<footnote><sup>#</sup>These authors contributed equally to this work.</footnote>
The primary intrinsic defects in CVD-grown TMDC monolayers have been identified as sulfur vacancies, with densities exceeding $8.6 \times 10^{13}$ cm$^{-215}$. These vacancies give rise to localized surface states and Coulomb scattering of carriers in 2D materials¹⁶. Considerable efforts have been dedicated to passivate these defects, both during synthesis and through post-treatment methods, with promising outcomes¹⁷⁻²⁰. For instance, Pam et al. found precursor pre-treatment on WO₃ induces a higher oxygen vacancy concentration, resulting in improved crystallinity of WS₂²¹. Zhao et al. demonstrated the molecular functionalization with thiophenol molecules can repair chalcogen vacancy defects in WSe₂¹⁹. However, beyond point defects, carrier transport in CVD-grown TMDCs is also strongly hindered by randomly distributed line defects, particularly grain boundaries (GBs)²²⁻²⁴. Tsen et al. demonstrated that fissures at grain boundaries substantially increase carrier scattering in graphene, thereby deteriorating device performance²⁵. Arend et al. reported that mirror twin boundaries in MoS₂ increase in-plane conductivity, whereas tilt boundaries lead to a reduction²⁶. Zhou et al. found that grain boundaries oriented perpendicular to the carrier transport direction severely hinder electrical conductivity²⁷. Grain boundaries inevitably appear during the synthesis of two-dimensional crystals. For instance, large triangular crystals often exhibit a center-to-vertex grain boundary structure, or a butterfly-shaped grain boundary may form when two grains meet in a staggered manner, whose shape reflects whether the formation is governed by thermodynamic or kinetic factors²⁸⁻³⁰. Furthermore, grain boundary characteristics are preserved during any transfer process onto dielectric substrates, ultimately being highly detrimental to device operation and performance. Additionally, large-scale synthesized WS₂ is typically polycrystalline, composed of domains separated by defective GBs. Despite their significance, current research endeavours are focused on the structural characterization of GBs, with limited attention being paid to their influence on the material's electrical characteristics, especially for the case of fissure line defects on charge transport³¹⁻³³. These defects are a primary cause of the non-uniform performance observed in large-area device arrays, significantly increasing carrier scattering and degrading the optical and electrical properties of 2D semiconductors²⁵, ³⁴⁻³⁷. Therefore, gaining an in-depth understanding and achieving precise control over line defects is crucial in view of their key fundamental and technological implications.

Here, we reported a molecular functionalization approach to weld adjacent grain boundaries in 2D TMDC crystals. A simple and short conjugated molecule, i.e. benzene 1,4-dithiol (BDT), serves as a molecular "glue" to create additional percolation pathways for charge transport within atomically thin films. This molecular welding approach, which was monitored on the atomic scale by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), significantly enhances the PL intensity of WS₂ monolayer at GBs, resulting in brighter and more homogeneous PL emission. The electrical performance of WS₂-FETs with molecularly welded GB is substantially improved, restored to levels

comparable to intragrain regions. Simultaneously, BDT molecules anchor to sulfur vacancies on the basal plane of WS₂ crystals, effectively passivating interfacial traps and thus suppressing the electrical hysteresis. Furthermore, temperature-dependent optoelectronic measurements reveal the activation energy of line defects in twisted twin domains, providing valuable insight into carrier conduction across grain boundaries.

## Results and Discussion

WS₂ monolayers were synthesized on a C-plane (0001) sapphire substrate using a tube furnace under controlled temperature conditions. Detailed experimental procedures are provided in the experimental section. The samples typically consist of isolated WS₂ monolayer-thick triangles, which occasionally merge to form butterfly-shaped features. The inset of Figure 1a shows an optical microscopy image of a typical triangular crystal of CVD-grown WS₂, wet transferred onto a SiO₂ substrate. Figure S1 displays an atomic force microscope (AFM) image of a representative WS₂ triangular crystal with a thickness of ca. 0.9 nm, corresponding to monolayer. The topographical surface morphology of the WS₂ flake, shown in Figure S1c, indicates an atomically flat surface with a root-mean-square roughness of 0.49 nm. Photoluminescence image in Figure 1a highlights the presence of line defects in WS₂, in line with GB defects reported in large-scale CVD-grown WS₂²⁸, ³⁸⁻³⁹. The color-coded SEM image overlay in Figure 1b shows two independent triangular crystals growing together, with the tilted grain boundary forming a faceted line (Z-geometry) connecting the triangles. The inset displays a photoluminescence image of the crystal. Figures 1c and 1g display CVD-grown WS₂ monolayers, examined by means of high-resolution HAADF-STEM. Figure 1c depicts the highly ordered WS₂ hexagonal lattice, with monosulfur vacancies observed in the regions highlighted by frame lines, or marked with yellow and orange circles. Figures 1d and 1e provide magnified views of such monosulfur vacancies, which are further analyzed through intensity line profiles in 1f acquired along the white frame lines in 1c. Furthermore, Figure S2 shows scanning tunneling microscopy (STM) images of the WS₂ lattice structure and point defects, with unit cell parameters (a = b = 0.31 ± 0.02 nm, γ = 120° ± 2°) estimated and compared to theoretical calculation. Figures 1g and 1h display a HAADF-STEM image and corresponding reciprocal space image of a twin grain boundary with a twist angle of 21°, where two crystal domains converge³⁶. The GB region, marked by a yellow dashed box, exhibits a pronounced fissure due to lattice mismatch during crystal growth. An enlarged view within the white dashed box is shown in the inset. Notably, the GB fissure appears empty, and this structural defect significantly increases the carrier scattering and impedes charge transport⁴⁰. Additional details on twinning and different twist angles are provided in Figures S3-S5. Theoretically, as illustrated in Figure S6, the three-fold rotational symmetry of WS₂ lattice maps the twin-boundary twist angles in the range between 0° (no grain boundaries) to a maximum of 60°. Grain boundaries differ from crystal wrinkles or folds, which typically appear during crystal transfer and
can be distinguished by topographic maps, as displayed in **Figure S7**.

To gain further insight into flakes, we performed Raman spectroscopy analysis, a well-established method for analyzing the chemical bonding structures of TMDCs and estimating flake thickness. **Figure 1i** reveals that the peak associated with the resonance longitudinal acoustic phono mode 2LA(M) is more intense than the A₁g(Γ) mode, with a Raman shift difference of 67.73 cm⁻¹, confirming that CVD-grown WS₂ flakes are monolayer-thick. Compared to mechanically exfoliated monolayers⁴¹, the 2LA(M) of CVD-

grown WS₂ exhibits lower intensity and a broader full width at half maximum (FWHM), indicating higher defect density in the CVD-grown WS₂ monolayer. **Figure 1j** exhibits the PL spectrum of the CVD-grown WS₂ monolayer under green laser irradiation (λ = 532 nm, power = 3.45 mW). A Lorentzian fitting was applied to resolve the exciton (X⁰) and trion (X⁻) emission peaks. The samples show a strong emission peak at 2.0 eV, corresponding to the bandgap of WS₂⁴².

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Characterization of CVD-grown WS₂ monolayers. a) Photoluminescence image of a typical triangular WS₂ monolayer crystal (excited at λ = 460 nm), highlighting the presence of a grain boundary. The inset shows the corresponding optical image of the crystal on a SiO₂ substrate. b) Colour-coded overlay of SEM image reveals a tilted grain boundary, represented by a faceted line connecting twin crystals. The inset shows the corresponding photoluminescence image. c) High-resolution HAADF-STEM image of the WS₂ monolayer with monosulfur vacancies, highlighted with frame lines, yellow and orange circles. d-e) Magnified view of the monosulfur vacancies in Figure 1c. Scale bar: 0.5 nm. f) Intensity line profiles were acquired along the frame lines in Figure 1c. g) High-resolution HAADF-STEM image of a twin grain boundary in the WS₂ monolayer, with the grain boundary region enclosed by a yellow dashed box. The upper-right panel provides an enlarged view of the area within the white dashed box. Scale bar, 1 nm. h) Reciprocal space images of the twin grain boundary at a 21° twist angle in Figure 1g. i) Raman spectra comparing mechanical exfoliated (ME) WS₂ monolayer (red) and CVD-grown WS₂ monolayer (navy). j) Photoluminescence (PL) spectrum of the WS₂ monolayer (excited at λ = 532 nm). Lorentzian fitting was applied to resolve the exciton (X⁰) and trion (X⁻) peaks.</figcaption></figure>

To further investigate and compare the defects within grain and at GBs, temperature-dependent PL spectra were recorded. **Figure 2** shows the normalized PL spectra of pristine WS₂ monolayer measured at 78 K at the inner grain and GBs, respectively. At 78K, a distinct peak at 1.93 eV appears in the PL spectrum of the WS₂ GB. For a better comparison, Lorentzian fitting was applied to deconvolute the PL spectra at 78 K. **Figure 2b** shows the Lorentzian fitting for the pristine WS₂ monolayer recorded at the interior point, distinguishing three peaks: the defect emission peak (D) at 1.8 eV, the trion emission peak (X⁻) at 2.01 eV, and the exciton emission peak (X⁰) at 2.05 eV. In contrast, the Lorentzian fitting at the GB is plotted in **Figure 2c**, revealing an additional peak at 1.93 eV, attributed to defect-associated biexciton emission (XX/D). The deconvoluted integrated areas of the peaks offer quantitative in-

formation about the defects. The weights of the XX/D and D peaks within the crystal interior are 1.1% and 8.3%, respectively. These values increase to 7.4% and 10.0% at the GB, indicating a higher defect density in those regions. Furthermore, the relative deconvoluted areas between the peaks also provide additional insights into the nature of these defects. In particular, D/X⁻ ratio indicates relative defect density, while the X⁻/X⁰ ratio reflects the degree of n-doping. At the interior point, the D/X⁻ and X⁻/X⁰ amount to 0.182 and 0.953, respectively, whereas at the GB, these ratios increase to 0.359 and decrease to 0.511, respectively, indicating that line defects at GB exhibit a stronger electron-withdrawing character compared to those inner grain sites. To bestow the differences between point defects within the grain and line defects at GB, in terms of activation energy, temperature-dependent PL spectroscopy measurements were conducted (**Figure 2d-e**). The gradual
changes in PL spectra with increasing temperature indicate the absence of phase transitions within the temperature range of 78 K to 298 K, with the defect emission peak (D) serving as a fingerprint of bound excitons (BE) rather than free excitons (FE), and the intensity decreases with the temperature increases$^{43-44}$. Since the phase remains stable across this temperature range, important physical parameters of the defects, such as thermal activation energy, exciton binding energy, and exciton-phonon interaction, can be estimated from the PL peak evolution. The temperature-dependent BE emission intensity is plotted in Figure 2f and follows the equation$^{45}$:

$$ \gamma(T) \propto I_D^{-1} = \tau^{-1} + Ce^{-E_A/k_B T} \quad (1) $$

in which, $\gamma$ is inversely proportional to the PL intensity of the bound exciton in steady state ($I_D$), $\tau$ is the lifetime of the bound exciton, C is a factor, $E_A$ is the activation energy, and $k_B$ is the Boltzmann constant. As shown in Figure 2d, the defect emission peak (D) at 1.8 eV is relatively low and disappears at temperatures above 178 K, being in line with the previous study on point defect in the interior regions of

$WS_2^{46}$. The extracted thermal activation energy of the defect-bound exciton is ca. 35 meV, corresponding to negatively charged sulfur vacancies. In contrast, the D peak at grain boundaries is more pronounced and disappears at temperatures exceeding 218 K. Figure 2f shows the normalized inverse PL intensity ($I_D^{-1}$) at grain boundary as a function of inverse temperature ($T^{-1}$) along with the equation (1). The extracted thermal activation energy ($E_A$) of the defect-bound exciton at the grain boundary is 155 ± 15 meV. Notably, compared to well-established sulfur vacancies with $E_A = 36 \pm 6$ meV$^{45}$, the grain boundaries possess higher activation energy, indicating a much stronger binding between excitons and the line defects. Therefore, an electron-rich molecule was designed to “weld” line defects, facilitating charge carrier movement across grain boundaries.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Temperature-dependent PL spectra to extract the activation energy of line defects. a) Normalized PL spectra of pristine $WS_2$ monolayer measured at 78 K at the inner grain (red) and the grain boundary (navy) (irradiated at $\lambda = 532$ nm). b) PL spectrum recorded at 78 K in the inner grain area, fitted with a Lorentzian function to resolve the defect-associated biexciton emission peak (XX/D). Defect emission (D), defect associated biexciton emission (XX/D), and trion (X-) and exciton (X°) were considered. c) PL spectrum recorded at 78 K at the grain boundary, fitted with a Lorentzian fitting. d) Temperature-dependent PL spectra for the inner grain area from 78 K to 298 K, with a 20 K interval. The inset shows an enlarged view of the defect emission peak (D). e) Temperature-dependent PL spectra for the inner grain area from 78 K to 298 K, with a 20 K interval. The inset shows an enlarged view of the defect emission peak (D). f) Inverse normalized defect emission peak intensity ($1/I_D$) as a function of inverse temperature ($1/T$), along with the corresponding fit to the data.</figcaption></figure>

While thiolated molecules are well-known for their ability to heal sulfur vacancies and reduce charge-scattering centers in TMDCs. Recent studies have demonstrated that dithiolated molecules can enhance inter-flake electronic connectivity in liquid-phase exfoliated nanosheets by forming networks that covalently bridge adjacent flakes$^{47-48}$. In this work, we employed BDT to weld the GBs in CVD-

grown $WS_2$ monolayers, leveraging its electron-donating nature and robust attachment to defect-abundant edges at GBs. Experimental details are given in the Methods section. For $WS_2$ crystals with GBs, the areal density of single S-vacancy defects at the edges and GBs ($0.92 \pm 0.45$ nm$^{-2}$) is much higher than in the interior ($0.33 \pm 0.11$ nm$^{-2}$)$^{45}$. Therefore, BDT molecules preferentially anchor at vacancies.
cies along edges and GBs, while the two terminal S atoms of organic disulfide molecules bridge the twin WS₂ crystals, effectively stitching and welding them together, as illustrated in **Figures 3c and 3d**. **Figures 3a and 3b** display a high-resolution HAADF-STEM image and corresponding reciprocal space pattern of a BDT-treated twin boundary, with additional images provided in **Figures S8 and S9**. Compared to the original GB fissures (**Figures 1g, S3-S5**), the BDT-treated regions exhibit distinct contrast changes, the void defects are significantly reduced with fissures that seem to be filled following molecular treatment. However, the molecular welding of GBs cannot be unambiguously visualized by the HAADF-STEM imaging due to the possible co-phys sorption of BDT molecules and polymer residuals in these areas. Control experiments and discussions are described in a subsequent section.

To assess the effect of BDT treatment on the chemical composition of CVD-grown WS₂ monolayers, we performed X-ray photoelectron spectra (XPS) analyses. **Figure 3e-f** and **Figure S10** show the XPS spectra of the pristine WS₂ monolayer before and after BDT treatment. The binding energy peak at 37.7 eV corresponds to the oxidation state of W 5p₃/₂ in WS₂, induced during the CVD process. Peaks at 35.1 eV and 32.9 eV correspond to the W 4f₅/₂ and W 4f₇/₂ doublets, respectively. The binding energy around 161.5 eV, corresponds to S-defects in WS₂, commonly interpreted as S atom vacancies. Following BDT treatment, the oxidized component of W 5p₃/₂ peak at 37.7 eV decreased from 6.83% to 5.29%, suggesting a reduction of W oxidation state and a more uniform chemical environment of W atoms. At the same time, S-defect component decreased from 2.59% to 1.06%, and the S/W stoichiometric ratio increased from 1.85 in the pristine state to 1.96 after

BDT treatment. This provides strong evidence for the effective functionalization of WS₂ monolayers with BDT molecules and their successful attachment to the TMDC's surface¹⁸. Grain boundaries are easily recognized since defects quench PL emission, as shown in **Figure 3g**, with the corresponding optical image in **Figure S11**. Significantly, after BDT treatment, the PL emission becomes brighter and more uniform (**Figure 3h**), providing unambiguous evidence of the efficient healing of both sulfur vacancies and GBs⁴⁹. Consequently, the PL intensity of WS₂ monolayer after BDT treatment features a 3.5-fold increase at the GB compared to the pristine sample at the same position (**Figure 3i**). **Figure 3j-i** illustrates an increase in the topographical roughness of the WS₂ GB region following molecular treatment, rising from 0.82 nm to 1.77 nm. The observed height variation further confirms the presence of molecular adsorption. Low-temperature PL measurements provide detailed information about defects. **Figure S12** compares the PL spectra of the pristine WS₂ monolayer at 78 K at the GB before and after the BDT welding. Lorentzian fitting was applied to resolve the emission associated with defects. The XX/D peak at 1.96 eV, observed in the pristine sample, nearly disappears upon BDT treatment, confirming effective healing of defects at GB by the conjugate molecule.

<figure><img src="image_4.png" /></figure>
**Figure 3.** Characterization of BDT welded grain boundaries. a) High-resolution HAADF-STEM image of twin grain boundary in BDT-treated WS₂ monolayer, with the grain boundary region enclosed by a yellow dashed box. b) Reciprocal space images of the twin grain boundary at a 13° twist angle in Figure 3a. c-d) Schematic representation of the molecular stitching process at the grain boundary of WS₂. e-f) XPS spectra (S 2p) of the pristine (e) and BDT-treated (f) WS₂ monolayer. The binding energy around 161.5 eV corresponds to S-defects in WS₂, commonly interpreted as S atom vacancies. g-h) Photoluminescence microscope images of a crystal with a grain boundary before (g) and after (h) BDT molecule treatment. i) Static PL spectra at the WS₂ grain boundary before and after BDT treatment. j-k) AFM topographical images of the pristine WS₂ twins before (j) and after (k) BDT treatment. The white dashed box indicates the 3.5×3.5 µm² region in which the root-mean-square roughness (RRMS) has been estimated as 0.82 nm (j), and 1.77 nm (k). l) Topographical profiles across black and red dashed lines in (j) and (k).

To gain further insight into the impact of GBs and molecular welding on the electronic conductance of WS₂ monolayers, FETs before and after functionalization with BDT molecules were studied, both within grains and across intra-channel GBs. The scheme of the back-gate WS₂ monolayer FET is displayed in **Figure 4a**. **Figure 4b** presents a bright-field optical image of the device alongside a photoluminescence image of the BDT-treated device, which comprises two identical FETs connected back-to-back with comparable channel length and width. The left device contains a GB within the channel, requiring carrier transport between the source and drain electrodes to cross this GB. In contrast, the right device involves carrier transport solely within the grain. **Figures 4c-e** show the logarithmic and linear transfer characteristics of both pristine and BDT-treated WS₂ FETs, comparing carrier transport within the grain (scatter line) and across the grain boundary (solid line). The on-state current within the grain is four orders of magnitude higher than across the GB, and electron mobility ($\mu_{FE}$) within the grain (4.4 cm² V⁻¹ s⁻¹) far exceeds that across the GB (0.002 cm² V⁻¹ s⁻¹). This disparity arises from the presence of fissures at the GB between twin crystals (**Figures 1g** and **3c**), which increases carrier scattering and restricts charge transfer, resulting in high sheet resistance and significantly limited current²⁵, ⁵⁰⁻⁵¹. These observations suggest that the existence of GBs greatly hinders carrier transport, consistent with electron microscopy results and spectroscopy analysis above.

The electron mobility is extracted in the linear region with the following equation⁶,

$$I_{DS} = \mu_{FE} C_{i} \frac{W}{L} \left( (V_{gs} - V_{TH})V_{DS} - \frac{V_{DS}^{2}}{2} \right) \quad (2)$$

Where $I_{DS}$ is the source-drain current, $C_i$ is the capacitance per unit area, $W$, $L$ are the channel width and length of device, respectively. $V_{gs}$, $V_{TH}$, $V_{DS}$ are the gate voltage, threshold voltage, and source-drain voltage, respectively. It is noteworthy that while BDT treatment enhances electronic conduction in both channel types, the effect is significantly more pronounced across the GB than within the interior. **Figure 4f** illustrates the statistical enhancement in device mobility and the negative shift in threshold voltage ($\Delta V_{TH}$) upon BDT treatment. The $\mu_{FE}$ of the FETs across the GB

was enhanced by approximately 200-fold, while $V_{TH}$ decreased by 22.8 V following molecular treatment. Additionally, the on-state current and the $I_{on}/I_{off}$ ratio increased by more than three orders of magnitude. This improvement can be attributed to two combined effects. First, the healing of laterally placed adjacent crystals, by stitching them with BDT, the sheet resistance is significantly reduced, enabling smooth carrier tunneling across the GBs bridged by the molecules, thereby restoring the conductivity to normal levels. Second, sulfur vacancies on the basal plane are passivated when one end of the BDT molecule binds to the defect site, while the other thiol group remains free, with its lone pair extending out of the surface. This configuration introduces n-type doping, which further boosts the current. As a result, both the carrier concentration and electrical current in the channel become comparable to those observed in the devices within the grains. In comparison, the FETs within the grain exhibited a mobility increase of less than 2-fold and a 7.6 V reduction in $V_{TH}$, with only limited improvements in both the on-state current and $I_{on}/I_{off}$ ratio. This limited enhancement is attributed to n-doping induced by the repair of the interior sulfur vacancy point defects by BDT molecules, resulting in minimal current enhancement. This behavior was further confirmed using mechanically exfoliated flakes, as shown in **Figure S13**. Furthermore, the threshold voltage ($V_{TH}$) within the grain was much lower than that across the grain boundary, underscoring the defective nature of the grain boundaries. The strong interaction between BDT molecules and S vacancies in WS₂ results in the elimination of defect states and n-type doping, which causes the $V_{TH}$ of both devices to decrease. These results demonstrate that molecular welding is a powerful and highly effective strategy for repairing GB in TMD crystals.

The molecule induced carrier density change ($\Delta n$) can be estimated by the following equation⁵²⁻⁵³.

$$\Delta n = C_i \frac{\Delta V_{TH}}{q} \quad (3)$$

where $q$ is the elementary charge. The calculated $\Delta n$ is (0.61 ± 0.27) × 10¹² cm⁻² within the grain, and (1.82±0.67)×10¹² cm⁻² at the grain boundary. This suggests that BDT molecules preferentially anchor at GBs, which are defect-rich, leading to a significant increase in the carrier concentration in the FET straddling the GB.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Electrical performance of BDT treatment. a) Schematic representation of back-gate WS₂ monolayer FET across a grain boundary functionalized with the BDT molecules. b) Top-view optical image of back-gate FETs, showing electrodes labeled "12" straddling the grain boundary (left) and electrodes labeled "23" within the grain (right), along with the corresponding PL image after BDT molecular treatment. Scale bar, 30 µm. c) Logarithmic-scale transfer characteristics of WS₂ monolayer FETs across the grain boundary, comparing performance before (black line) and after (red line) BDT molecular treatment, as well as the FETs within the grain before (brown scatter line) and after (blue scatter line) BDT molecular treatment. d) Linear-scale transfer characteristics comparing the device's performance across the GB before and after BDT treatment, with an inset schematic showing a top view of the device. e) Linear-scale transfer characteristics comparing the device's performance within the grain before and after BDT treatment, with an inset schematic showing a top view of the device. f) Statistical analysis of mobility enhancement and decreased threshold voltage after BDT treatment. The µ₀ and µBDT represent the device mobility before and after BDT treatment, respectively.</figcaption></figure>

In addition to carrier mobility, high trap density in the device contributes to hysteresis during device switching. **Figure S14** shows representative transfer characteristics with forward and backward scans (in linear scale) for BDT-treated WS₂ monolayer FETs across grain boundaries and within grains. The trap density (N<sub>TT</sub>) can be extracted from the hysteresis in the transfer curves using the following equation<sup>54</sup>,

$$N_{TT} = C_i \frac{\Delta V_T}{q} \qquad (4)$$

where ΔV<sub>T</sub> represents the hysteresis, defined as the change in threshold voltages between forward and backward scans. The pristine WS₂ within the grain shows a hysteresis of 15.2±3.4 V, and the corresponding trap density (N<sub>TT</sub>) is estimated to be 1.21 × 10¹² cm⁻², consistent with previous reports, primarily attributed to the interface between SiO₂ and WS₂¹³. However, the device across the GB exhibits a large hysteresis of 18.3±3.9 V, with a trap density of 1.46 × 10¹² cm⁻², indicating that GBs contribute to higher trap density, thereby impeding charge flow within the channel. This comparison reveals the significance of trap density and confirms the critical role played by both GBs and interfacial traps. Interestingly, both devices, within grain and cross GBs, show a clear reduction in hysteresis after BDT

treatment. Remarkably, the hysteresis of the device across the GB decreased to 10.4±2.7 V, which is even lower than that of the pristine WS₂ within the grain (13.2±1.7 V). Consequently, the trap density in the FET across the GB reaches its lowest value (0.83 × 10¹² compared to 1.05×10¹² within the grain). This suggests that grain boundaries, with their higher vacancy densities, provide more active sites for BDT molecules to anchor, leading to significant passivation of interfacial traps. This molecularly welded WS₂ device across the GB exhibited high stability and no obvious degradation even after one month when stored in N₂ filled glovebox (Figure S15). Moreover, molecular welding is achieved by repairing sulfur vacancies at both ends of the WS₂ twin grain boundaries. **Figures S16 and S17** further prove that possible polymer residues or the molecule with a single thiol group (thiophenol) cannot produce the same welding effect. Our molecular welding approach for grain boundaries is not limited to WS₂, but can also be applied to other TMDs. **Figure S18** demonstrates the application of this method to MoS₂ FETs across GB, compared to back-to-back FETs within grains, which shows similar GB repair effects as observed in WS₂. This confirms the broad applicability of our molecular welding technique in TMDs.

## Conclusion
In conclusion, we have demonstrated an unprecedented strategy for the molecular functionalization of 2D semi-conducting crystals, enabling the simultaneous healing of point defects and welding of grain boundaries using semi-conducting molecules. The functionalization with the conjugated BDT molecule, which was monitored on the atomic scale by high-resolution HAADF-STEM imaging, determined a significant improvement of the photoluminescence intensity of WS₂ monolayers at the grain boundary, resulting in a brighter and uniform PL emission. In addition, the BDT welding results in a remarkable 200-fold enhancement in electron mobility in WS₂ FETs, a three orders of magnitude improvement in both on-current and I<sub>on</sub>/I<sub>off</sub> ratio at GBs, exhibiting comparable levels to that of intra-grain FETs. The anchoring of BDT molecules on the WS₂ monolayer effectively passivates intrinsic interfacial traps, thereby suppressing electrical hysteresis. Furthermore, temperature-dependent optoelectronic measurements reveal the defective nature of the material (e.g., activation energy), providing valuable insights into carrier conduction across grain boundaries. This approach is not limited to WS₂, but can also be applied to other TMDs such as MoS₂, etc. Overall, our work offers a universal route for healing line defects in monolayer TMDs through a facile molecular vapor annealing process, enabling seamless electron transport across grain boundaries.

## ASSOCIATED CONTENT

**Supporting Information.**

Chemicals; Materials; Morphology characterization; Preparation of device; Electrical performance characterization; And the additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

## AUTHOR INFORMATION

**Corresponding Author**

Paolo Samorì – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France; E-mail: samori@unistra.fr

**Authors**

Bin Han – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Chun Ma – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Ke Jiang – International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China

Maria Batuk – Electron Microscopy for Materials Science (EMAT) and NANOLight Center of Excellence, University of Antwerp, Antwerp 2020, Belgium

Ahin Roy – CRANN & AMBER Research Centres and School of Chemistry, Trinity College Dublin, Dublin D2, Ireland

Lucia Hughes – CRANN & AMBER Research Centres and School of Chemistry, Trinity College Dublin, Dublin D2, Ireland

Hanlin Wang – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Verónica Montes-García – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Can Wang – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Fanny Richard – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Devu Prasad – Electron Microscopy for Materials Science (EMAT) and NANOLight Center of Excellence, University of Antwerp, Antwerp 2020, Belgium

Antonio Gaetano Ricciardulli – Université de Strasbourg, CNRS, ISIS UMR 7006, F-67000 Strasbourg, France

Sara Bals – Electron Microscopy for Materials Science (EMAT) and NANOLight Center of Excellence, University of Antwerp, Antwerp 2020, Belgium

Valeria Nicolosi – CRANN & AMBER Research Centres and School of Chemistry, Trinity College Dublin, Dublin D2, Ireland

Yumeng Shi – International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China

**Present Addresses**

Hanlin Wang – National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry Chinese Academy of Sciences, Beijing, 100190, P. R. China

Can Wang – State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, P. R. China

Ahin Roy – Materials Science Centre, Indian Institute of Technology, Kharagpur; Kharagpur, West Bengal, India 721302

**Author Contributions**

B.H. and C.M. contributed equally. All authors have given approval to the final version of the article.

**Notes**

The authors declare no competing financial interest.

## ACKNOWLEDGMENT

This work was financially supported by the European Commission through the Horizon Europe research and innovation program HORIZON-EIC-2023-PATHFINDEROPEN-01-01 project HYPERSONIC (GA-101129613) as well as the Agence Nationale de la Recherche through the Interdisciplinary Theoretical Institute SysChem via the IdEx Unistra (ANR-10-IDEX-0002) within the program Investissement d'Avenir, the Foundation Jean-Marie Lehn and the Institut Universitaire de France (IUF), the Chinese Scholarship Council, FWO-SB (S008721N), and the Department of Science and Technology of Jilin Province 20240602063RC.

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