Iodine-assisted ultrafast growth of high-quality monolayer MoS 2 with sulfur-terminated edges

: Two-dimensional (2D) semiconductors have attracted great attention to extend Moore’s law, which motivates the quest for fast growth of high-quality materials. However, taking MoS 2 as an example, current methods yield 2D MoS 2 with a low growth rate and poor quality with vacancy concentrations three to five orders of magnitude higher than silicon and other commercial semiconductors. Here, we develop a strategy of using an intermediate product of iodine as a transport agent to carry metal precursors efficiently for ultrafast growth of high-quality MoS 2 . The grown MoS 2 has the lowest density of sulfur vacancies (~1.41×10 12 cm −2 ) reported so far and excellent electrical properties with high on/off current ratios of 10 8 and carrier mobility of 175 cm 2 V −1 s −1 . Theoretical calculations show that by incorporating iodine, the nucleation barrier of MoS 2 growth with sulfur-terminated edges reduces dramatically. The sufficient supply of precursor and low nucleation energy together boost the ultrafast growth of sub-millimeter MoS 2 domains within seconds. This work provides an effective method for the ultrafast growth of 2D semiconductors with high quality, which will promote their applications.


INTRODUCTION
Two-dimensional (2D) semiconductors have drawn significant attention for next generation electronics owing to their ultrathin body, dangling-bond-free nature, high mobility, and flexibility.Monolayer transition metal dichalcogenides (TMDCs), including MoS 2 and WS 2 , are the most extensively studied 2D semiconductors [1][2][3][4].However, the ultrafast growth of high-quality 2D TMDCs is still challenging, which restricts their practical applications in electronics and optoelectronics.Among the reported synthesis methods [5][6][7][8][9][10][11], chemical vapor deposition (CVD) is one of the most promising methods to grow 2D semiconductors with large area and high quality [12][13][14][15][16][17][18].In general, the fast growth of large single crystals of TMDCs by the CVD method requires sufficient precursor supply and low nucleation energy of products [19,20].Until now, some CVD methods have been used to grow TMDCs at fast speeds.For example, some research groups [5,21,22] used alkali metal halides to grow TMDCs, which achieved 1000 μm within 3 min.In another work, Zhang et al. [20] proposed that high growth temperature promotes the growth rate of WS 2 and MoS 2 with the reverse flow method, in which single crystals with the size of 450 μm were grown within 10 s.
Besides the growth rate, another key concern about monolayer semiconductors is the material quality.In fact, imperfections especially vacancies are inevitable in these monolayer TMDCs.These vacancies affect the electronic properties of TMDCs at different levels, which is currently under intense investigation [23][24][25][26].Until now, most CVD-grown TMDCs have a high density of sulfur vacancies of ~10 14 cm −2 , which deteriorates the electronic properties of TMDCs and limits their further applications [23,26].Many methods have been applied to decrease the density of sulfur vacancies.For example, an early report has shown that CVD-grown monolayer MoS 2 has a high density of sulfur vacancy [27], which is ~7.8×10 13 cm −2 .Recently, Feng et al. [28] have used thiol as a liquid precursor for CVD growth of high-quality MoS 2 with a low density of sulfur defects (~1.6 × 10 13 cm −2 ).In another work, Zuo et al. [29] have reported a strategy of using active chalcogen monomer supply to grow high-quality TMDCs with a much lower density of sulfur defects (~2× 10 12 cm −2 ).However, the density of defects in these TMDCs is still three to five orders of magnitude higher than that of silicon [30], which is ~10 9 cm −2 (corresponding to ~10 12 -10 13 cm −3 in bulk materials).It is therefore urgent to grow high crystalline MoS 2 with a high growth rate and low density of defects.
Here, we develop a strategy for ultrafast growth of high-quality MoS 2 with sulfur-terminated edges by iodine-assisted CVD.When MoO 3 and KI are mixed as the metal precursor, the iodine is released during the growth process.The newly formed iodine acts as a transport agent which helps deliver sufficient metal precursors onto the substrate surface to grow MoS 2 .Density functional theory (DFT) calculations show that the iodine reduces the nucleation energy by 1.68 eV for MoS 2 growth with sulfur-terminated edges, compared with growth without salt.Combining the supply of sufficient precursors and low nucleation energy of MoS 2 together, the iodine drastically boosts the growth speed of MoS 2 and MoS 2 domains with a size of ~780 μm are grown in just five seconds.Furthermore, the MoS 2 has ultrahigh crystalline quality with the lowest density of sulfur vacancies of ~1.41×10 12 cm −2 .The field-effect transistor (FET) based on MoS 2 exhibits carrier mobility of 175 cm 2 V −1 s −1 and with an on/off current ratio of ~10 8 , supporting their high quality with potential for electronic applications.

Ultrafast growth of monolayer MoS 2 with the iodine-assisted CVD method
The idea of the iodine-assisted growth method is schemed in Figure 1A and Figure S1.Briefly, the sulfur Natl Sci Open, 2023, Vol.2, 20230009 powder is put into a quartz boat upstream of the furnace, whose temperature can be controlled by the distance from the furnace center.The aged mixture of KI and MoO 3 is used as the metal precursors, which is loaded into another quartz boat and put at the center of the CVD furnace (see growth details in the EXPERI-MENTAL SECTION).First, we noticed that the color of the mixed precursors changed from gray to dark purple after 100 days in Ar, indicating the iodine is released (Figure S2).Then, when the temperature is over ~505°C, the mixed precursors react with each other and form K 2 MoO 7 and I 2 , which is confirmed by the thermogravimetric analysis/mass spectrometry (TGA-MS, Figure S3) and X-ray photoelectron spectroscopy (XPS, Figure S4) characterizations.Figures S3A and S3B show the mass-loss step of the mixture of MoO 3 and KI at ~505°C.The differential results of the mass-loss curve (see the blue line) also indicate the step.Figure S3C is the spectra of the thermos mass photo at ~530°C, indicating that iodine is released from the reaction between MoO 3 and KI. Figure S4D is the XPS spectra of I 3d before and after heating the mixture at ~550°C, which confirms the iodine element disappeared after heating.All the results confirm that the iodine is released.There are two unique roles of KI.First, similar to NaCl, the KI also reacts with metal precursors (MoO 3 ) and form active intermediate products with a low melting point.Second, similar to the chemical vapor transport method, the by-product of iodine acts as the transport agent to deliver the metal precursors onto the surface of the substrate for MoS 2 growth [31][32][33].Both two factors largely improve the growth rate of MoS 2 .
Figure 1B is a typical optical microscopy (OM) image of MoS 2 grown with an iodine-assisted CVD method with a growth time of 5 s, where the domain size can be ~780 μm, suggesting its ultrafast growth rate (also see Figure S5A).When the growth time reaches 10 s, we obtain continuous MoS 2 monolayer film (Figure S5B).In comparison, the traditional method (without KI) only grows MoS 2 with a domain size of ~15 μm after 20 min (Figure 1C and Figure S5C).We then compare the ten largest MoS 2 domains grown by two kinds of methods and find that the iodine-assisted method grown MoS 2 has two orders of magnitude larger domain sizes than that without KI.Figure 1D compares the domain size and the growth time of the MoS 2 grown by different methods [5,[34][35][36][37][38][39], showing the great advantages of this iodine-assisted CVD method.Figure 1E shows the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the MoS 2 , suggesting its high crystallinity.We also collected more STEM images of the MoS 2 with large scale (~380 nm 2 ) to show the low density of the sulfur vacancies (Figure S6). Figure 1G compares the density of sulfur vacancies with other reported work [14,28,29,[40][41][42][43][44], showing that our work has the lowest value of sulfur vacancies (1.41×10 12 cm −2 ) reported so far in CVD-grown monolayer MoS 2 .Overall, the characterizations above confirm that the as-grown MoS 2 has high crystalline quality with a very low density of sulfur vacancies.
To better understand the effect of KI on the growth of MoS 2 , a series of experiments are conducted with different weight ratios of KI/MoO 3 (Figure S7).When this ratio increases, the domain size of MoS 2 also increases, presumably because more iodine is formed as the transport agent (Figures S2 and S3).In addition, we carried out a series of experiments to analyze the effect of temperature on the growth of MoS 2 while the weight ratio of KI/MoO 3 is fixed at the optimized ratio of 1:3 (Figure S8).Interestingly, the MoS 2 is grown at a low temperature of 540°C, due to the low formation temperature of iodine is ~505°C.When the growth temperature increases, the as-grown MoS 2 domains get larger, and the largest domain with a size of ~780 μm is achieved in 5 s at 790°C (Figure S5A).When the growth temperature is over 830°C, we grow bilayer MoS 2 domains (Figure S8E), probably due to the oversupply of precursors.Moreover, the iodine-assisted CVD Natl Sci Open, 2023, Vol.2, 20230009 method shows good universality for the growth of other monolayer TMDCs like 2D WS 2 (Figure S9).Overall, our results indicate iodine-assisted CVD method provides an effective way to grow MoS 2 with a large domain size at a fast speed.

High quality and uniformity of the MoS 2
To study the quality and uniformity of the as-grown MoS 2 , we performed the Raman and photoluminescence (PL) spectroscopic characterizations.Figure 2A is the typical Raman spectrum of the monolayer MoS 2 , which exhibits the characteristic peaks of in-plane E 2g phonon mode at 384.1 cm −1 and out-of-plane A 1g mode at 404.8 cm −1 .The representative PL spectrum exhibits a sharp and narrow peak with the full width at half maximum (FWHM) of ~19.3 nm at room temperature (Figure 2B). Figure 2C is the PL of the MoS 2 with FWHM of ~76.4 meV (or 16.2 nm) measured at low temperature of 80 K, which is fitted into three peaks.The most dominant feature contains two peaks, labeled as neutral A excitons and negatively charged trions.The B excitons of the third peak are observed at ~2.01 eV.In addition to these three peaks, the lower energy peak resulting from the radiative recombination of deep level defect-bounded excitons (X B ) is not observed [45], which is a clear and strong evidence of high-quality MoS 2 with a low defect density and in agreement with our HAADF-STEM observations (Figure 1G and Figure S6).
Then, we focus on the uniformity of the MoS 2 by measuring the line-scanning Raman and PL along the dashed white line in the inset of OM image in Figure 2D, showing uniform and monolayer features of MoS 2 domains (Figures 2D and 2E).Besides, the fluorescence microscopy image shows high uniformity of the MoS 2 domains with large domain size (~200 μm) (Figure 2F).The height is around 0.8 nm of as-grown MoS 2 measured by atomic force microscopy (AFM) (Figure 2G), indicating it is monolayer [7].Figures 2H and 2I are the inverse fast Fourier transform (IFFT) pattern, taken by high-resolution lateral force microscopy (LFM) mode at the inner and edge part of the flake in Figure 2G, the insets are the corresponding FFT patterns, indicating that the MoS 2 has high uniformity and quality.All the characterizations prove that the iodine-assisted CVD grown MoS 2 has high uniformity and crystallinity quality over the whole domain.

Electrical performance
Based on the achieved high-quality monolayer MoS 2 domains, we fabricated FETs to characterize their electrical properties.The structure of the back-gated MoS 2 FETs is illustrated in the inset of Figure 3A and Figure S10. Figure 3A shows the transfer characteristics of FET based on as-grown MoS 2 as channel material.The device exhibits both high on/off current ratio of 10 8 and decent mobility of 175 cm −2 V −1 s −1 .The device shows a linear relationship in the output curves, indicating the Ohmic contact (Figure 3B).Besides, transfer curves of six more typical MoS 2 FETs with different channel lengths and widths are shown in Figure 3C.These data exhibit small device-to-device variation, reflecting the uniformity of the as-grown MoS 2 domains.In Figure 3D and Table S1, we compared the on/off ratio and mobility of our devices with the previous CVD-grown monolayer MoS 2 [5,29,[34][35][36][37][38][39][46][47][48], suggesting the good electrical performance of the iodine-assisted CVD grown MoS 2 .

Mechanism for the iodine-assisted growth of MoS 2 with sulfur-terminated edges
To shed light on the growth mechanism of ultrafast growth of MoS 2 with the iodine-assisted CVD method, we carried out DFT calculations to simulate the nucleation process with and without iodine.The energy diagrams for MoS 2 growth along the S-terminated edges were calculated with and without halogen element adsorption as shown in Figures 4A and 4B.With the incorporation of iodine, the highest nucleation barrier reduces from 3.41 to 1.73 eV for MoS 2 growth under near equilibrium S-rich conditions.The nucleation process can even become barrierless under non-equilibrium growth conditions (Figure S11).Meanwhile, the adsorption of iodine is proven to be the most favorable for the growth of MoS 2 above the halogen elements, since its nucleation energy drops off the most (Figure 4B and Figure S12).Moreover, to confirm the edge structure of terminated atoms, TEM and selective area electron diffraction (SAED) pattern are used.Figures 4C and 4D are the TEM images of the corner of the triangle domain and the corresponding SAED pattern.As is known that the lattice of monolayer MoS 2 has two types of molybdenum and sulfur sublattices, which Natl Sci Open, 2023, Vol.2, 20230009 reduces the hexagonal lattice from six-fold to three-fold symmetry [27].As a result, the six [−1100] diffraction spots are broken into two families: k a = {(−1100), (10−10), (0−110)} and k b = −k a (Figure 4D).Moreover, the intensity of diffraction spots pointed toward k a is lower than that of the spots toward k b .Figure 4E shows that the k a spots are higher in intensity (~10% for 80 kV electron beam) and spots represent the molybdenum sublattice, which matches well with the previous work [27].Combining all the results, we confirm that the edge structure of the iodine-assisted MoS 2 domain in Figure 4C is S-terminated.We also analyze other MoS 2 domains (Figure S13) and draw the diagram in Figure 4F, showing that over 60% of the triangle domains have edges with an S-terminated structure.Besides, the edge type of CVD-grown MoS 2 is highly dependent on the S/Mo ratio of precursors [49].In this regard, we can tune S/Mo ratio by using the iodine-assisted CVD method to realize fast growth of MoS 2 with Mo-or S-terminated edge structures.Such MoS 2 with specific types of edges may be a good platform to study edge structure-dependent properties and for applications in electronics, optoelectronics, and catalysis [50][51][52].Taking all the results together, the iodine boosts the ultrafast growth of MoS 2 domains with an S-terminated structure, matching well with TEM results.

CONCLUSION
In conclusion, we have developed an iodine-assisted CVD method for the ultrafast growth of high-quality monolayer MoS 2 with the lowest density of defects.Monolayer MoS 2 with large domain sizes is obtained in just seconds of growth time.These domains possess ultrahigh crystalline quality with the lowest density of sulfur vacancies (~1.41×10 12 cm −2 ) reported so far.FET based on iodine-assisted grown MoS 2 shows a high on/off ratio of 10 8 and mobility of 175 cm 2 V −1 s −1 .DFT calculations indicate that iodine benefits from fast nucleation and growth of MoS 2 and decreases the nucleation energy barrier by 1.68 eV, which is responsible for the ultrafast growth behavior.Our results add fresh knowledge for the growth of high-quality 2D semiconductors in short time scales, showing promise for electronic and optoelectronic applications.

Iodine-assisted CVD growth of MoS 2
First, sulfur powder (100 mg, 99.5%, Sigma-Aldrich, USA) and the mixed precursors of KI (99.9%,Sigma-Aldrich, USA)/MoO 3 (99.9%,Sigma-Aldrich, USA) with different weight ratios (between 0.1 and 1 of KI/ MoO 3 ) were loaded into the CVD furnace.Then the temperature increased to 790°C within 20 min.During the heating process, the sulfur powder was introduced into the growth chamber at 680°C from room temperature.By this way, when the sulfur reaches its melting point, the temperature of the furnace gets 790°C for the growth of MoS 2 .When the growth time is 5 s, we moved away from the quartz tube from the heating zone immediately.Meanwhile, a fan was used to cool down the furnace.In this way, the temperature of the growth substrate is decreased from 790°C to below 400°C within 1 min.For the MoS 2 growth without KI, sulfur powder and pure MoO 3 were loaded into the CVD furnace under the same growth conditions with iodine-assisted MoS 2 growth.

Transferring process of MoS 2
To transfer MoS 2 from SiO 2 /Si substrate to the target substrate (like TEM grid).We used the following steps.First, PMMA was spin-coated on the SiO 2 /Si substrate with MoS 2 grown at 4500 r min −1 for 4 min.The sample was heated on a hot plate at 90°C for 5 min.The sample was then submerged in an HF solution (5%) for a few seconds to etch the substrate.The PMMA/MoS 2 sample was then rinsed with deionized water and transferred to the target substrate.Finally, the PMMA was removed using acetone.

Characterization
The morphology of the samples was examined by optical microscope (Carl Zeiss Microscopy, Germany).AFM (Cypher ES, Asylum Research, USA) was used to measure the thickness and surface of the samples.SAED patterns were collected in FEI Tecnai F30 (USA) with an acceleration voltage of 80 kV.The HAADF-STEM imaging was carried out in a cold-field-emission double Cs-corrected STEM (FEI Spectra 300 with an acceleration voltage of 80 kV).Reflectance contrast spectra of the as-grown MoS 2 were conducted in a home-built optical measurement system.Raman and PL spectra and mappings were collected using a 532-nm laser excitation with a beam size of ~1 μm (Horiba LabRAB HR Evolution, Japan).

DFT calculations
All calculations were performed using the density functional theory as implemented in the Vienna Ab-initio Simulation Package [53].The core-electron interactions were described by projected augmented wave methods [54].Perdew-Burke-Ernzerhof generalized gradient approximation was adopted for the exchangecorrelation effect [55].The perfect MoS 2 nanoribbon was modeled by a 9×4×1 supercell.Mo and S 2 were introduced stepwise to the Mo-or S-terminated growth front of MoS 2 following the kink-flow scheme using a nanoreactor model [56].The situation for growth on Mo-terminated front is shown in Figure S14, from which we can see negligible effect of iodine on the growth of MoS 2 .The energy for every considered configuration

Device fabrication and measurements
The source and drain electrodes (5/50 nm Cr/Au) were fabricated on the MoS 2 using a direct laser writing system (miDALIX, DaLI, Germany) followed by e-beam evaporation and lift-off processes.The FET was measured using the semiconductor parameter analyzer (Keithley 4200A-SCS, USA) and probe station (LakeShore, USA) with a vacuum at a pressure of 10 −5 mbar at room temperature.Then, we calculated carrier mobility (μ) using the following formula where L and W are the length and width of the channel, respectively, I d is the drain-source current at the gate voltage (V g ), C ox is the gate oxide capacitance evaluated as 11.5 nF cm −2 for 300 nm SiO 2 , and V d is the source-drain voltage.

Figure 1
Figure 1 Iodine-assisted ultrafast CVD growth of high-quality monolayer MoS 2 .(A) Scheme of the ultrafast growth approach.(B) OM image of MoS 2 grown with iodine-assisted CVD method in 5 s.(C) Domain sizes of MoS 2 grown with and without iodine-assisting during growth.(D) Comparison of the growth time and domain size of as-grown MoS 2 with other work.(E) HAADF-STEM image of the MoS 2 .(F) Zoomed in image of the MoS 2 with mono-sulfur vacancy.The bottom panel is the corresponding intensity line profile along the red square.(G) Comparison of the sulfur vacancies of iodine-assisted grown MoS 2 with other work.

Figure 2
Figure 2 Iodine-assisted CVD grown MoS 2 with high quality and uniformity.(A), (B) The typical Raman and PL of the MoS 2 , respectively.(C) Low temperature PL of the MoS 2 at 80 K. (D), (E) Line-scan Raman spectra and PL spectra of the MoS 2 , and the inset in (D) is the OM image of the corresponding MoS 2 domain.(F) Typical fluorescence image of the as-grown MoS 2 with large size.(G) AFM image at the corner of a MoS 2 domain.(H), (I) IFFT HRAFM image at the inner and edge parts of the MoS 2 domain in (G).

Figure 3
Figure 3 Electrical performance of the iodine-assisted CVD grown MoS 2 .(A), (B) Typical transfer and output curves of the devices.The inset in (A) is a schematic of the back gate MoS 2 FET.(C) Transfer curves of six more MoS 2 FETs.(D) Comparisons of the mobility and on/ off ratios of MoS 2 .

Figure 4
Figure 4 Edge-structure dependent growth mechanism.(A) DFT calculations of the free energy for MoS 2 growth with and without iodine assisting under near equilibrium S-rich condition.(B) DFT calculation of the nucleation energy for different halogen elements under near equilibrium S-rich condition.(C) TEM image of the as-grown MoS 2 .(D) The corresponding SAED pattern of (C).(E) A line profile through the measured diffraction spots in (D).The higher intensity k b spots towards the Mo sublattice, as indicated by the arrows in (C) and (D).(F) Diagram of large MoS 2 domains with S-terminated edges vs. Mo-terminated edges.
2 nanoribbon with Mo m S n grown on it, and E(NR) is the energy of the initial MoS 2 nanoribbon.μ Mo and μ S are the corresponding chemical potential of Mo and S. Here, two scenarios, i.e., near equilibrium S-rich and non-equilibrium conditions were used to obtain the energies.For equilibrium S-rich condition, μ S is the chemical potential of an S atom in bulk S. Based on that, μ Mo is calculated by µ µ MoS 2 is the chemical potential gained from MoS 2 monolayer.For non-equilibrium conditions, μ Mo and μ S were taken from the chemical potentials of bulk Mo and bulk S, respectively.