Microscopic evidence of strong interactions between chemical vapor deposited 2D MoS₂ film and SiO₂ growth template

Abstract

Two-dimensional MoS₂ film can grow on oxide substrates including Al₂O₃ and SiO₂. However, it cannot grow usually on non-oxide substrates such as a bare Si wafer using chemical vapor deposition. To address this issue, we prepared as-synthesized and transferred MoS₂ (AS-MoS₂ and TR-MoS₂) films on SiO₂/Si substrates and studied the effect of the SiO₂ layer on the atomic and electronic structure of the MoS₂ films using spherical aberration-corrected scanning transition electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The interlayer distance between MoS₂ layers film showed a change at the AS-MoS₂/SiO₂ interface, which is attributed to the formation of S–O chemical bonding at the interface, whereas the TR-MoS₂/SiO₂ interface showed only van der Waals interactions. Through STEM and EELS studies, we confirmed that there exists a bonding state in addition to the van der Waals force, which is the dominant interaction between MoS₂ and SiO₂. The formation of S–O bonding at the AS-MoS₂/SiO₂ interface layer suggests that the sulfur atoms at the termination layer in the MoS₂ films are bonded to the oxygen atoms of the SiO₂ layer during chemical vapor deposition. Our results indicate that the S–O bonding feature promotes the growth of MoS₂ thin films on oxide growth templates.

Introduction

Transition metal dichalcogenides, such as MoS₂, have attracted much interest because of their remarkable electrical, mechanical, thermal, and optical properties. Therefore, they are considered novel materials and suitable for application in optoelectronic devices, water-splitting catalysts, sensors, field-effect transistors, capacitors, and energy storage devices [1,2,3,4,5,6,7,8,9,10]. Recent studies have focused on the large-scale growth of MoS₂ films, which is mainly carried out on Al₂O₃ and SiO₂/Si substrates [11,12,13,14]. Through chemical vapor deposition (CVD) and pulsed laser deposition (PLD), MoS₂ films are well deposited when the Si wafer has a SiO₂ layer on the top. In contrast, the growth of MoS₂ fails or MoSi₂ is synthesized instead of MoS₂ when CVD growth is carried out on a bare Si wafer even if Si substrate has a native oxide layer on top because the thin native oxide layer can be removed or penetrated during the high temperature synthesis process [4, 15]. Moreover, there are a few studies to show the formation of MoSi₂ through chemical reactions between MoS₂ and Si. When the Mo film is sulfurized on a bare Si wafer, the formation of a gas phase SiS₂ rather than MoS₂ is presumable. This indicates that making large-scale growth of MoS₂ directly on Si is hardly achievable. However, when Mo is sulfurized on SiO₂ substrates, S–O bonding is preferable rather than S–Si bonding, which helps Mo-S bonding followed by growth of a MoS₂ film. The different tendency of CVD MoS₂ growth on bare Si and SiO₂/Si substrates was found in previous studies [4, 15]. To date, it has been reported that some substrates such as SiO₂/Si and sapphire enable the large-scale deposition of MoS₂ films. However, microscopic evidence showing the interaction between the chemical-vapor-deposited MoS₂ and the SiO₂ growth template has not been observed. Microscopic studies are mainly focused on the atomic and electronic structures of MoS₂, using transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM), which can directly observe materials on an atomic level. Crystal and atomic structures with various defects have been discovered using a combination of ab-initio calculations [4, 11,12,13,14, 16,17,18,19,20,21,22].

To investigate the atomic and electronic structure and chemical state of the MoS₂/SiO₂ heterostructure, an atomic-scale study using Cs-corrected STEM and EELS with a cross-sectional view should be carried out. The use of Cs-corrected STEM with a probe corrector would enable the direct observation of heterointerfaces at the atomic resolution. Moreover, the changes in the electronic and chemical states would be available in unit-cell resolution. Interfaces between dissimilar compounds provide unusual properties that are attributed to lattice mismatch, strain, chemical bonds, and the formation of secondary interfacial layers [23,24,25,26,27]. The observation of the atomic and electronic structures of MoS₂/SiO₂ interfaces not only provides insights on the growth mechanisms and bonding states, but also suggests the possibility of application in various devices. Understanding the nature of the interfaces can provide insights on how the MoS₂ film is synthesized via CVD and how the MoS₂ `film on the SiO₂/Si template can be utilized in electronic devices.

In this work, we investigated the atomic and electronic structures of MoS₂ (more than 20 layers)/SiO₂ interfaces on an Si substrate. First, we observed and compared the atomic structure of the AS and TR-MoS₂ films on the SiO₂/Si substrates and plotted the (001) plane distance from the interface to the interior of the film. Second, we investigated the chemical bonding state at the AS and TR-MoS₂/SiO₂ hetero interfaces. The final aim of this study is figuring out the main reason why MoS₂ is deposited only on oxide substrates such as SiO_2, Al₂O₃ and understanding how the SiO₂ layer affects the structures of MoS₂ films when Mo is sulfurized through CVD [11,12,13]. We used SiO₂ for a growth template mainly because other oxides such as Al₂O₃ and SrTiO₃ do not dissolve in HF, disabling transfer process. For this purpose, we carried out a TEM-based analysis with Cs-corrected high annular dark field (HAADF) imaging, and annular bright field (ABF) STEM imaging and EELS with a combination of ab-initio calculations. To the best of our knowledge, this is the first reported paper investigating the influence of the SiO₂ template on the growth of MoS₂ films using Cs-corrected STEM and EELS with a cross-sectional view.

Experimental details

MoS₂ film deposition

SiO₂(300 nm)/Si wafers were cleaned with a standard piranha solution (3:1 mixture of H₂SO₄ and H₂O₂) using conventional cleaning procedures followed by ultrasonication in acetone, isopropyl alcohol, and deionized (DI) water. To obtain hydrophilic surfaces on the SiO₂/Si wafers, O₂ plasma and UV-O₃ surface treatments were sequentially performed for 15 min. A 10 nm-thick Mo thin film was deposited on a SiO₂/Si substrate using an E-beam evaporator. (Rocky Mountain Vacuum Tech, Englewood, CO, USA). The base pressure, E-beam voltage, and current were 10^–6 Torr, 7.3 kV, and 70 mA, respectively, and the deposition rate was approximately 0.1 Å/s. Sulfurization of the Mo thin film was performed by CVD at 900 °C for 30 min. After sulfurization, the thin films were annealed for the crystallization of the MoS₂ films.

TEM sample preparation

Due to the weak interaction between the MoS₂ film and SiO₂ layer, MoS₂ films are peeled off from SiO₂ when cross-section TEM specimen preparation using polishing. To minimize mechanical damage, we carried out FIB TEM sample preparation followed by nanomilling (Fischione 1040). Nanomill is similar to precision ion polishing, except that the milling area can be selected during nanomill process. Thus, nanomill is the optimal method for the preparation of the FIB TEM specimens.

Raman, XPS, TEM and STEM/EELS characterization

Raman and X-ray photoelectron spectroscopy were carried out to analyze vibration modes and surface chemical states of the MoS₂ films using LABRAM HR Evolution and AXIS-Hsi. The Mo to S ratio of the MoS₂ films are calculated by dividing intensity or the area of each elements by factor of Mo or S elements [4].

The TEM analysis was divided into 2 steps. First, we obtained bright-field TEM and high-resolution TEM images to confirm the quality and thickness of the TEM specimen. In this step, we analyzed the MoS₂ film using JEOL JEM-2100F. In the next step, to investigate the atomic structure of the interfaces, Cs-corrected high-resolution STEM images were obtained with a Cs-probe corrected TEM instrument (JEOL JEM ARM 200F). STEM imaging with a spherical aberration corrector provided clearer images with a spatial resolution of 80 pm. Thus, Cs-corrected HR-STEM imaging enabled the identification of elements as well as the location of atomic positions with high accuracy.

Theoretical calculation

For the calculations, the MoS₂-SiO₂ heterostructure supercell was composed of 8 Si layers of SiO₂, a monolayer of MoS₂, and a 15 Å vacuum layer to prevent the interaction between layers. Both sides of the SiO₂ slab were reconstructed surfaces, which have a lower surface energy compared to a pristine surface according to a previous study [29]. Before constructing the heterostructure, the unit cells of SiO₂ and MoS₂ were first relaxed; the lattice parameters were a = b = 4.896 Å for SiO₂ and a = b = 3.161 Å for MoS₂. Subsequently, along the x–y plane, SiO₂ in 2 × 2 lateral periodicity and MoS₂ in 3 × 3 lateral periodicity was stacked together. Considering that the monolayer MoS₂ is much more prone to deformation than bulk SiO₂, the x–y plane lattice parameter of 2 × 2 SiO₂ was employed as that of a heterostructure supercell. This allowed the MoS₂ layer to expand along the x and y directions with a lattice mismatch of approximately 3.4%. In addition, the position of the SiO₂ layer was fixed during the relaxation of the heterostructure for the same reason mentioned above. All structural relaxations and free energy calculations were performed with the Vienna ab initio simulation package (VASP) [31, 32] based on density functional theory (DFT) [33, 34]. For the replacement of the core electrons, a projector augmented wave (PAW) [35, 36] scheme was implemented. The exchange–correlation energy was described through the generalized gradient approximation (GGA) using the Perdew-Burke-Emzerhof (PBE) functional [37]. The kinetic cutoff for the plane-wave basis was 400 eV. The Brillouin zone for 3 × 3 × 1 k-point sampling was constructed with a gamma-centered grid. For electronic self-consistency and force tolerance, criteria of 5–10 eV and 0.01 eV/A were applied.

Results and discussion

The growth of the MoS₂ film and imaging of the MoS₂/SiO₂ interface

The growth and transfer process of the MoS₂ film is shown in Fig. 1(a). The experimental details are explained in the Sect. 2.1. The growth of the AS-MoS₂ film was demonstrated using Raman spectroscopy, as shown in Additional file 1: Figure S1. The Raman spectrum showed two characteristic Raman vibration modes, E¹_2g and A_1g, which indicated that the MoS₂ film grew laterally on the Si/SiO₂ substrate.

Fig. 1

a Schematic illustration of the MoS₂ film deposition and transfer. STEM images of MoS₂ film on SiO₂/Si. b Low-magnification HAADF STEM, c Cs-corrected ABF STEM, and d Cs-corrected HAADF STEM images of AS-MoS₂ film. e Low-magnification HAADF STEM, f Cs-corrected ABF STEM, and g Cs- corrected HAADF STEM images of TR-MoS₂ film

Due to the weak interaction between the TMD film and the substrate, which hinders the observation of the structure of the interfaces when the TEM sample is prepared using mechanical polishing, we prepared a TEM specimen using a focused ion beam (FIB) followed by nanomill using a Fischione 1040 nanomill. Additional file 1: Figure S2 depicts the result using the prepared AS and TR-MoS₂ films, which are in line with the Raman spectrum. Figure 1(b) and (c) is the HAADF STEM image and shows that the MoS₂ film was laterally aligned on the amorphous silicon oxide layer. Since the SiO₂ growth template is not atomically smooth, the AS-MoS₂/SiO₂ interface seems to be rough. Figure 1(c) is a contrast-inverted ABF STEM image. As ABF STEM imaging is efficient in detecting light elements, especially sulfur, the atomic configuration in Fig. 1(d) shows clearer image contrast than that in Fig. 1(c). The TR-MoS₂ film also showed comparatively well laterally aligned MoS₂ sheets at the surface and in the interior of the layer as shown in Fig. 1(e–g). In addition, the TR-MoS₂ film showed no significant difference in the overall lattice structure and interlayer distance.

Interlayer distance of the AS and TR-MoS₂ films depending on number of layers

To quantify the effect of the amorphous SiO₂ layer on the atomic structure of the MoS₂ film, we plotted the changes in the out-of-plane distance at 16 different positions and calculated the average interlayer distance with error bars. Each measured distance of the AS and TR-MoS₂ films is depicted in Additional file 1: Figure S3. As shown in Fig. 2(a) and (b), at the AS-MoS₂/SiO₂ interface, the interlayer distance was up to 3.7% shorter than that of the AS-MoS₂ film, whereas there was no significant change in the interlayer distance in the TR-MoS₂ film, as shown in Fig. 2(c) and (d). The huge error bar is not originated from the roughness of the of the SiO₂ substrate, but irregular S–O bond at the MoS₂/SiO₂ interface, making the shape of the MoS₂ surface wavy. Despite of this roughness, the difference in tendency of interlayer distance of AS and TR-MoS₂ films is non-negligible. This suggests that when MoS₂ films are grown on SiO₂ via the sulfurization of the Mo film, SiO₂ not only plays a significant role as a growth template but also influences the atomic structure of the AS-MoS₂ film at the MoS₂/SiO₂ interface. Considering that only van der Waals interactions occur between the MoS₂ layers in the bulk, the change in interlayer distance is attributed to the chemical bonding between AS-MoS₂ and SiO₂.

Fig. 2

Changes in the interlayer distance at the MoS₂/SiO₂ interface. a Position at which the interlayer distance values were measured in the AS-MoS₂ films. b Plot of the interlayer distance of AS-MoS₂. The black solid line represents the average value of the interlayer distance with error. c Position at which the interlayer distance values were measured in the TR-MoS₂ films. d Plot of the interlayer distance of TR-MoS₂. The black solid line represents the average value of the distance with error

Next, we carried out ab-initio calculations to suggest an atomic model based on STEM images. We compared the equivalent distance between SiO₂ (single crystal)-MoS₂ and MoS₂-MoS₂ with the assumption that the MoS₂/SiO₂ interface is S–O-terminated. As shown in Fig. 3(a), the shortest distance between SiO₂-MoS₂ was calculated to be 3.16 Å, which was slightly longer than the MoS₂-MoS₂ distance, 3.04 Å. Figure 3(b) illustrates the formation energy as a function of the distance between SiO₂ and MoS₂. Figure 3(b) shows that 3.16 Å provides the most stable formation energy in our calculation. Since the calculation assumed that SiO₂ was a single crystal and considered only the van der Waals interaction, the result of the calculation deviated from that of the experiment. This discrepancy suggests that there is a strong chemical interaction other than the van der Waals force between the AS MoS₂ film and the SiO₂ template. This is in contrast with the previous DFT study, which argued that the structure of MoS₂ is not affected by SiO₂ [28, 29].

Fig. 3

a Atomistic model of the MoS₂ layer on the single-crystal SiO₂. b Formation energy as a function of SiO₂-MoS₂ distance

Chemical bonding at the MoS₂/SiO₂ interface

MoS₂ films were analyzed using X-ray photoelectron spectroscopy (XPS) to determine the chemical composition and atomic ratios of the films on the SiO₂/Si substrate. The core level spectra of Mo 3d and S 2p were recorded for the AS- and TR-MoS₂ films (Additional file 1: Figure S4a–d). Figure 4 shows that the AS- and TR-MoS₂ films were both deposited in a stoichiometric composition (Mo:S = 1:2) without any significant chemical shift. In addition, the XPS characteristics of the AS-MoS₂ film were not significantly different from those of the transferred sample. However, since the detection depth of XPS was only a few nanometers and XPS detected the overall area of the MoS₂ films, the XPS profiles of the MoS₂ films could not clarify the electronic structure of the MoS₂/SiO₂ interfaces, which are defined in the sub-nanometer range.

Fig. 4

a Region of AS-MoS₂/SiO₂ in which EELS spectra were obtained. b S L-edge of the AS-MoS₂ film. c Comparison between the S L-edges of the AS-MoS₂ film (blue line) and MoS₂/SiO₂ interface (red line). d O K-edge of the AS-MoS₂ film. e Comparison between the O K-edges of the AS-MoS₂ film (yellow line) and MoS₂/SiO₂ interface (red line). f Region of TR-MoS₂/SiO₂ in which EELS spectra were obtained. g S L-edge of the TR-MoS₂ film h Comparison between the S L-edges of the TR-MoS₂ film (blue line) and MoS₂/SiO₂ interface (red line). i O K-edge of the TR-MoS₂ film. j Comparison between the O K-edges of the TR-MoS₂ film (yellow line) and MoS₂/SiO₂ interface (red line)

To identify the chemical bonding state at the MoS₂/SiO₂ interface, we obtained and analyzed the EELS SL and O K-edge spectra. Unlike XPS, EELS can provide chemical information in a local area with a spatial resolution in the sub-nanometer range, thus facilitating the analysis of the MoS₂/SiO₂ heterointerfaces. The TEM sample was thin enough for the noise in the EELS spectra to be minimized, and each spectrum was acquired at a distance of 0.6 nm. Figure 4(a) and (e) show the STEM HAADF image of both samples from which the EELS spectra were acquired; both films were found to be well attached to the substrate through the TEM sample preparation. Next, we compared the S L- and O K-edge spectra in the two above-mentioned samples to identify the bonding state of AS-MoS₂ on the SiO₂/Si substrate. Figure 4(b) shows the changes in the S L-edge at 10 different positions, and Fig. 4(c) shows the difference in the S L-edge spectra at the MoS₂/SiO₂ interface (red solid line) and the MoS₂ film (blue solid line). The two spectra showed differences not only in the intensity, but also in the overall edge structure. In addition, considering that the MoS₂/SiO₂ interface was S–O-terminated, the difference of the S L-edge at the interface.Please provide complete details for the References The appearance of the peak at 168 eV indicates that the sulfur atoms were partially bonded to the oxygen atoms. The deviation from the previous study is due to the defects at the AS-MoS₂/SiO₂ interface. In other words, at the MoS₂/SiO₂ interface, the sulfur atom of the MoS₂ film had four nearest-neighbor oxygen atoms from the amorphous SiO₂ growth template. Figure 4(d) shows the O K-edge at 10 different positions. The O K-edge also showed a peak shift at the AS-MoS₂/SiO₂ interface compared to that of the interior of SiO₂. Figure 4(c) shows that along with the S–O bond, several defects were also formed at the AS-MoS₂/SiO₂ interface over the range of 1.2–1.5 nm. In addition, the negative peak shifts of the SL and O K-edges at the AS-MoS₂/SiO₂ interface were due to an increase in the negative charge around the sulfur and oxygen ions.

The EELS edge spectra of the TR-MoS₂/SiO₂/Si heterostructure were also observed. Figure 4(g) and (i) present the S L- and O K-edges at 10 different positions. In contrast to those for the AS-MoS₂ sample, the sulfur and oxygen edge spectra exhibited no significant difference, as shown in Fig. 3(h) and (j). Since the TR-MoS₂ film was detached and transferred onto the SiO₂/Si substrate, only van der Waals interactions existed between the MoS₂ and SiO₂ template.

Based on the EELS spectra of the AS-MoS₂ sample, we prepared an electron transition diagram (Fig. 5). The EELS core loss spectra showed the transition of electrons from the core state to the unoccupied state [30]. As shown in Fig. 5(a) and (b), the peak positions near 540 eV and 560 eV in the O K-edge spectra are attributed to the transition of electrons from O 1 s to O 2p mixed with Si 3sp and Si d states [30]. Figure 5(a) shows negative peak shifts at the interface and the formation of an energy loss peak at 542 eV, which were due to an increase in the negative charge at the interface and the formation of the S–O bond, respectively. However, there is no peak shift in O K edge at the TR-MoS₂/SiO₂ interface, as shown in Fig. 5(b). This indicates that when sulfur atoms are chemically bonded with oxygen in SiO₂, excess negative charges accumulate at the interface, causing a negative peak shift of the S L- and O K-edges.

Fig. 5

Electron transition diagram of the SiO₂ layer a at the AS-MoS₂/SiO₂ and b TR-MoS₂/SiO₂ interface

Combining all experimental results, the summary of our study is illustrated in Fig. 6. At the AS-MoS₂/SiO₂ interface, the interlayer distance decreases due to the formation of S–O bonding, whereas there is no significant change in the interlayer distance at the TR-MoS₂/SiO₂ interface, 12 which can explain the role of oxide templates such as SiO₂ and Al₂O₃ on the large-scale growth of the MoS₂ film. In addition, our key findings play a role in enhancing the carrier mobility of the MoS₂ film, which can lead to the improved performance of devices [38,39,40,41,42,43].

Fig. 6

Schematic illustration of the phenomena at the AS and TR-MoS₂/SiO₂ interface. At the AS-MoS₂/SiO₂ interface, the interlayer distance decreases due to the formation of S–O bonding, whereas there is no significant change in the interlayer distance at the TR-MoS₂/SiO₂ interface

Conclusions

We prepared MoS₂ films on SiO₂/Si substrates and studied the effect of the amorphous SiO₂ layer on the atomic and electronic structure of the MoS₂ films. The interlayer distance of the AS-MoS₂ film exhibited a change at the AS-MoS₂/SiO₂ interface, which was attributed to the formation of S–O chemical bonding at the interface. Through theoretical calculations, we confirmed the existence of a bonding state in addition to the van der Waals force, which was the dominant interaction between MoS₂ and SiO₂. The formation of S–O bonding at the AS-MoS₂/SiO₂ interface layer suggested that during CVD, the Mo thin film was not only sulfurized, but the sulfur atoms at the termination layer were also bonded to the oxygen atoms of the SiO₂ layer, preventing the formation of Si-S bonding and MoSi₂ (Fig. 6). Our key findings in the study are consistent regardless of the deposition techniques. In other words, the formation of S–O bonding occurs and interlayer distance between the AS-MoS₂ film and the substrate is affected by the SiO₂ growth template even if MoS₂ is deposited by MOCVD or other deposition techniques.This study not only provides a guideline on the relationship between the interfacial structure and electrical properties of MoS₂ thin film-based heterostructures and explains the role of oxides on the growth of MoS₂ films, but also shows that this kind of interfacial interaction is prominent when it comes to single layer MoS₂ which is generally used for a wide variety of devices.