X-ray photoelectron spectroscopy of high-throughput mechanically exfoliated van der Waals materials†

Nuria Jiménez-Arévalo*^a, Carlo Mariani^ab, Fabrice Leardini^c, Francesco Pandolfi^b, Ilaria Rago^b and Riccardo Frisenda*^a
aDipartimento di Fisica, Università di Roma “La Sapienza”, 00185, Rome, Italy. E-mail: nuria.jimenezarevalo@uniroma1.it; riccardo.frisenda@uniroma1.it
^bIstituto Nazionale di Fisica Nucleare Sezione di Roma, 00185 Rome, Italy
^cDepartamento de Física de Materiales, Universidad Autónoma de Madrid, 28049 Madrid, Spain

First published on 28th August 2024

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Introduction

Van der Waals (vdW) materials, such as graphite or transition metal dichalcogenides (TMDCs), have garnered significant interest due to their outstanding optical, electrical^1–5 and mechanical^6–8 properties. Owing to their layered structure, these materials can be mechanically exfoliated, producing excellent-quality two-dimensional (2D) flakes, which are homogenous in thickness and defect-free.^9–12 Nevertheless, the typical size of exfoliated flakes is of the order of 100 μm², thus hindering the scalability and applicability of these compounds outside the laboratory. Consequently, many efforts have been directed towards scaling up the mechanical exfoliation methods to produce large quantities of 2D flakes.^13–16 A recent research study by Y. Sozen et al.¹⁵ has reported a method to produce massive parallel exfoliation of vdW materials using a roll-to-roll set-up. This technique has facilitated the production of densely packed mechanically exfoliated vdW material flakes with large lateral sizes on arbitrary substrates.

X-ray photoelectron spectroscopy (XPS) is a widely used and powerful characterisation technique for investigating the elemental abundances and chemical composition of materials.^17–19 However, in conventional laboratory settings, the spatial resolution of XPS typically exceeds 10⁻¹ mm², which is much larger than the typical size of 2D flakes. Thus, synchrotron radiation is often employed for this purpose,²⁰ even though it is less accessible to a general audience and not suitable for rapid surveys of the chemical composition of the 2D flakes. Therefore, the challenge of easily investigating the elemental composition of mechanically exfoliated 2D flakes using conventional XPS apparatus remained unsolved.

In this work, we employed a similar technique to that developed by Y. Sozen et al.¹⁵ to characterise with XPS the composition of various mechanically exfoliated vdW materials transferred on large-area substrates (of the order of 1 cm²), using minimal quantities of the chosen material (approximately 10 μg). This approach enables the possibility of assessing the purity of commercial vdW materials, investigating composition modifications induced by different techniques, or the characterisation of laboratory-produced materials that are typically produced in small quantities. Also, it can ease the comparison of the same 2D material exfoliated or produced by different research groups.

Our findings were validated through a thorough characterisation of MoS₂ as the case of study, as well as by the analysis of other vdW materials such as graphite (Gr), WSe₂, and FePS₃. Notably, the characterisation of air-sensitive FePS₃ highlights the capability of performing this transfer method under an inert atmosphere. Additionally, in our investigation, we show the fast degradation of this compound when stored in air by the observation of oxide species by XPS. This underscored the importance of carefully managing environmental factors to maintain the stability of air-sensitive vdW materials.

Overall, this research introduces a valuable tool for characterising vdW material purity and quality, without the necessity of large quantities of materials and can pave the way to a more systematic comparison of vdW materials produced in different laboratories.

Experimental method

Chemicals and substrates

For the MoS₂ and graphite exfoliations, bulk natural molybdenite (Moly Hill mine) and graphite (Pro Graphite) were used. WSe₂ was supplied by HQGraphene. FePS₃ was synthesized at the MIRE Lab from Universidad Autónoma de Madrid by chemical vapor transport method. Here, a stoichiometric mixture of Fe (Goodfellow, 99.0% purity), S (Merck, 99.99% purity) and P₂S₅ (Sigma Aldrich, 99% purity) powders was placed into a quartz ampoule inside an Ar-filled glove box (Jacomex Model Campus). The open side of the ampoule was connected to a valve with Viton fittings and taken out of the glove box. Then it was connected to a high vacuum pump (in the 10⁻⁶ mbar range) and sealed using a blowtorch. To avoid the sublimation of P₂S₅ precursor during sealing in vacuum, the bottom part of the ampoule was immersed in a liquid nitrogen bath. Afterwards, the sealed ampoule was placed in a two-zones split tube furnace (Carbolite). The two extremes of the ampoule were heated at 740 °C and 680 °C during 100 h, respectively. FePS₃ flakes were collected at the cold side of the ampoule.

Sample preparation

A few micrograms of the target vdW materials were placed on Nitto tape. Several manual exfoliations were produced until a homogeneous distribution similar to the one reported in¹⁵ was obtained. The Nitto tape with the flakes was then stuck to the Si wafer (〈111〉, p-type, 1 Ω cm, with a native SiO₂ layer on top) and annealed at 110 °C for 5 minutes. After the sample cooled down, the Nitto tape was removed. This process can be repeated to increase the coverage. See Fig. S1† for a detailed schematic of the procedure.

Morphological characterisation

The morphology of the Si sample covered by the flakes has been investigated by optical microscopy using a Motic BA 340 microscope and by scanning electron microscopy (SEM) with a field emission Zeiss Auriga 405 instrument (operated at 3 kV and 4.3 mm of working distance) at the CNIS Laboratory of Sapienza University of Rome. A further morphological characterisation has been performed by atomic force microscopy (AFM) using a Park Systems NX10 AFM in tapping mode operated in air. We used a Nanosensors PPP-NCHR AFM tip and collected micrographs of 512 × 512 pixels at a measurement rate of 0.1 Hz (that corresponds approximately to speeds of 5 μm s⁻¹).

X-ray photoelectron spectroscopy

The composition of the flakes was investigated by XPS. The characterisation was carried out in an ultra-high vacuum (UHV) chamber with a base pressure in the low 10⁻¹⁰ mbar range. Photoelectrons were excited with an Al Kα (1468.7 eV) or Mg Kα (1253.6 eV) photon source and measured with a hemispherical electron analyser (VG Microtech Clam-2) in pass energy (PE) mode, with PE = 50 eV. Binding energy was calibrated by acquiring the Au 4f_7/2 (84.0 eV) core level after each measurement using a reference gold film (cleaned by Argon ion sputtering in UHV) mounted next to the investigated sample and in good electrical contact with it. The software used to fit the single components of the core-levels was XPST (X-ray Photoelectron Spectroscopy Tools) implemented in Igor Pro (WaveMetrics, Inc.).

Samples were held to the Mo sample holder using Ta strips and were annealed at 120 °C for at least 12 h in UHV before XPS measurements to remove eventual airborne contaminants.

Results and discussion

Molybdenum disulfide (MoS₂) has been used as the first case of study of the viability of employing the large-area mechanical exfoliation and transfer method for XPS analysis. Three transfers, each one lasting 5 minutes at 110 °C, were conducted on the same Si wafer to obtain a sample of high-density tightly packed flakes. The morphological characterisation of this sample is presented in Fig. 1, with additional information available in Fig. S2† regarding the intermediate transfers.

In Fig. 1a, a photograph of the sample under research is given. The sample is approximately 1 cm² large. In the upper and lower parts of the photograph the Si substrate is revealed; meanwhile, the central part of the sample is covered with the MoS₂ flakes. Fig. 1b offers a schematic representation of the deposited MoS₂ flakes from different angles. Microscale characterisation (Fig. 1c–e) reveals uncovered spaces where the substrate remains exposed, schematically illustrated in Fig. 1b. Both optical microscopy (OM) and scanning electron microscopy (SEM) images, performed in the same sample region, show contrast between the MoS₂ and the substrates. Nevertheless, it is worth noting that the contrast obtained by SEM is higher than the OM one. Image processing Gwyddion software facilitated estimating the covered area in that region, revealing a coverage C_OM = (60 ± 5) % by OM and C_SEM = (70 ± 5)% by SEM (see Fig. S3†). The higher contrast provided by SEM enables a more accurate and precise estimation of the covered area, as compared to OM, which depends on the refractive indexes of the vdW material and the substrate and on the thickness of the flakes. These findings were also obtained for Gr flakes noticing that, in that case, the SEM image provided an inverted contrast with respect to MoS₂ due to the higher conductivity of graphite compared to the Si substrate (see Fig. S4†).

We noted that the area covered along the sample can vary among regions (see Fig. S2c†). Thus, to get a more precise estimation of the area covered, several optical images were acquired in different sample regions and analysed, yielding an average value of C_OM = (55 ± 5)% for the MoS₂ sample. Even though this value provides a lower limit for the coverage, as discussed before, it offers insights into the fraction of area covered by the MoS₂ flakes.

Finally, AFM imaging (Fig. 1e) shows that the height of the transferred flakes, with an average value of approximately 90 nm (see Fig. S5†), is in good agreement with the reported value by Y. Sozen et al.¹⁵ We noticed that this thickness is larger than the typical probing depth of XPS for the photon energies used in the research, which is in the order of 1–3 nm.^21–23

After the morphological characterisation, we proceed to analyse the MoS₂ transferred sample using XPS. Fig. 2a displays the survey spectrum of the sample after 12 hours of annealing in UHV at 150 °C, revealing distinct peaks ascribed to MoS₂ and Si/SiO₂, as labelled in Fig. 2a. Eventually, peaks ascribed to Ta are also observed, coming from the Ta strips that hold the sample in place.

By the analysis of the areas of the core levels, normalised to their cross sections, coming from the substrate (Si and O) and of the ones coming from the material under investigation (Mo, S) we were able to estimate a coverage of C_XPS = (70 ± 5)%. This result is in good agreement with the results obtained by SEM and OM. It can be then concluded that the analysis of the coverage by morphological techniques, such as SEM or OM, or compositional techniques, such as XPS, point out to a similar coverage of the MoS₂ flakes on the Si substrate. Considering the various error sources for the different techniques, the coverages estimated in the three cases are well in agreement with each other (C_OM = (55 ± 5)%, C_SEM = (70 ± 5)%, C_XPS = (70 ± 5)%).

A second MoS₂ sample obtained with a single transfer (C_OM = (17 ± 5)%) was also subjected to XPS analysis. Despite the low coverage, well-defined signals were recorded for the survey and single spectra (see Fig. S6 and S7†). In addition, the coverage by XPS was also obtained by comparing the areas of the Mo and S peaks with the ones of Si and O finding a coverage of C_XPS = (25 ± 5)%. The good agreement between the OM and XPS estimated coverage is consistent with the previously discussed MoS₂ sample showing a higher coverage. A further discussion about this aspect and about the inelastic peak shape analysis^24–26 can be found in the ESI (Fig. S8 and S9†).

A detailed examination of the Mo 3d, S 2p, and Si 2p core levels peaks is provided in Fig. 2b, c and d, respectively and in Table S1.† It is noticed the presence of the Mo⁴⁺ 3d spin–orbit split doublet (at 229.8 eV and 233.0 eV for the 3d_5/2 and 3d_3/2) and the presence of S 2s (227.1 eV) core levels,^27–31 with no Mo⁶⁺ components indicative of molybdenum oxide,²⁷ confirming material purity. The S 2p region displays its corresponding spin–orbit (2p_3/2 at 162.6 eV and 2p_1/2 at 163.8 eV),^27,30 while the Si 2p region exhibits the two Si 2p_3/2 (99.7 eV) and Si 2p_1/2 (100.3 eV) spin–orbit split doublet^32,33 along with a peak centred at 104.0 eV attributed to the SiO₂ component,^32,33 indicating the presence of surface oxide on Si.

Finally, it is noteworthy the presence of a tiny C 1s peak at around 285.5 eV in the survey spectrum (see Fig. 2a) in comparison with the core levels of the MoS₂ flakes. The energy of this peak points to some hydroxide and oxide carbon species^34,35 that could indicate the presence of residues from the Nitto tape on top of the transferred flakes or on the substrate. However, this small carbon/molybdenum ratio highlights the cleanliness of the technique employed, despite using a polymeric glue tape and high temperatures, suggesting that the presence of residue and contaminants is minimal.

To demonstrate the general nature and applicability of this technique to various vdW materials, graphite flakes and WSe₂ flakes were also deposited onto Si wafers. The morphological characterisation can be found in the ESI (Fig. S4 and S10†). Fig. 3 displays the survey spectra and the main core levels of these two compounds.

In the case of the graphite sample (Fig. 3a and b), a distinct and well-defined peak centred at 284.8 eV is observed,³⁴ along with a shoulder at 285.9 eV corresponding to the oxidised states of carbon and other functionalization groups.^34,35 The contribution of this oxide component is 18.5%. Nevertheless, the intensity of this C 1s core level is significantly higher than in the other vdW materials under investigation (as evidenced in Fig. 3a), and the main peak at 284.8 eV indicates a clear presence of sp² carbon, as expected for Gr flakes.³⁶ The coverage of this sample is C_OM = (45 ± 5)%, C_SEM = (34 ± 5)%, and C_XPS = (36 ± 5)% consistent with the previous results of coverage obtained for MoS₂.

Additionally, these results were compared with those of highly oriented pyrolytic graphite (HOPG) measured in the same XPS apparatus. HOPG is the most similar bulk system to the Gr-flakes under investigation in this research, as it is composed of layers of graphite crystallites highly oriented along the c axis. In Fig. S11a,† it can be observed that the line shapes of these two graphitic materials closely match. The analysis of the residual difference signal (Fig. S11b†) between these two fits highlights differences at BE above 285.5 eV which is compatible with the presence of functionalized groups, such as hydrocarbons, carbon hydroxides and oxides at higher binding energy,^34–36 in the case of Gr-flakes. This is presumably due to a higher density of edges caused by the finite size of the flakes, with respect to the large terraces present in HOPG. Overall, these results are a good validation of this high-throughput massive exfoliation of vdW materials to measure the XPS spectra.

Tungsten diselenide (WSe₂) was also analysed by XPS (see Fig. 3c–e). A significant number of core levels ascribed to this compound were recorded in the survey spectrum (Fig. 3c). We focused on measuring the two most intense peaks of each element with high-resolution, which are the Se 3d (Fig. 3d) and the W 4f^37–39 (Fig. 3e). Both core-levels show their characteristic spin orbit doublets Se 3d_5/2 (54.8 eV), Se 3d_3/2 (55.7 eV) and W 4f_7/2 (32.6 eV), W 4f_5/2 (34.7 eV),^37–40 with the absence of oxide components.⁴⁰

Lastly, FePS₃, a magnetic vdW material, was investigated. The peculiarity of this compound is that, unlike the others explored in this work, it was synthesised by some of the authors of the present manuscript. This made this sample of particular interest for elemental investigation and to check the presence of eventual contaminants. Moreover, FePS₃ is known in literature for its sensitivity to oxygen species and its tendency to degrade in air,^41,42 prompting us to investigate its degradation using XPS.

FePS₃ was first transferred onto a Si wafer using the technique reported in the experimental section, conducted in ambient atmosphere. After analysing the composition of the compound by XPS, we observed the signal from some oxide peaks (see ESI, Fig. S12†). However, whether the oxidation occurred during the transfer method or during the synthesis remained uncertain. To address this uncertainty, we fabricated a second sample performing the exfoliation and transfer of FePS₃ inside a gloveless anaerobic chamber⁴³ filled with an inert N₂ atmosphere. The results obtained for this last sample are reported in Fig. 4.

Fig. 4a presents the survey spectrum for the as-prepared FePS₃ sample. Peaks ascribed to this compound are labelled in the figure, evidencing the lack of contamination from unexpected elements and the lack of oxides observed in the sample prepared in air, evidencing the necessity to use inert atmospheres when transferring air-sensitive compounds.

The deconvoluted high-resolution XPS spectrum of FePS₃ shows the typical Fe 2p line shape.^44–49 Peaks at 709.2 eV and 722.7 eV correspond to the 2p_3/2 and 2p_1/2 core levels of Fe²⁺,^{44,46,48–50} respectively, and peaks at 710.9 eV and 723.7 eV correspond to the Fe³⁺ 2p_3/2 and 2p_1/2,^46,47,50 respectively. In addition, two satellite peaks are present in the recorded spectrum, already observed and reported for this compound.^45,46 The S 2p core level shows the S 2p spin–orbit split doublet at 162.4 eV and 163.5 eV,^48,50 which is slightly shifted towards higher binding energies compared to other works in literature^44,46,51 but agrees well with the results reported for electrochemically-exfoliated FePS₃⁴⁹ and mechanically exfoliated FePS₃ by mortar grinding.⁵⁰ A broad peak was also observed at around 168.5 eV due to the presence of oxides and sulfates.^44,49,52 The P 2p core level evidences the spin–orbit doublet at 131.8 eV and 132.7 eV.^{44,46–49,51}

The stability of the compound was further investigated after storing it in air at room temperature for 7 and 20 days. After 7 days, the XPS spectra remained consistent with the as-prepared samples (see Fig. S12†), therefore showing no sign of degradation in this time scale. This result is consistent with previous reports on mechanically exfoliated FePS₃ flakes that demonstrated stability for 6 days by Raman spectroscopy.⁴¹ However, degradation was observed after 20 days, with noticeable changes in the Fe 2p, S 2p and P 2p core levels (see Fig. 4b–d), also consistent with the degradation of the optoelectronic properties of devices based on FePS₃ flakes after some weeks in ambient conditions.⁴² An increase in the 168.5 eV peak in the S 2p suggests the appearance of sulfates and oxides. Furthermore, we observe a strong change in the P 2p core level with the emergence of an intense peak around 134 eV, also observed on the sample prepared in air (see Fig. S12†), which is ascribed to phosphorous oxide.^46,47,53 Finally, the Fe 2p peak increases in intensity and slightly shifts to higher binding energies, probably also due to the presence of oxygen bonds.^54,55

Overall, characterising air-sensitive FePS₃ by XPS highlights the necessity of employing inert atmospheres when transferring this type of materials through massive parallel heat-assisted exfoliation. Additionally, this work underscores the utility of evaluating the quality of exfoliated vdW materials, as their chemical composition can be modified drastically in short periods of time, thereby impacting the research results.

Conclusions

In this research, we have successfully demonstrated the use of X-ray photoelectron spectroscopy to investigate the chemical composition of van der Waals materials. VdW flakes have been deposited over Si wafers in areas of about 1 cm² using little quantities of material (around 10 μg). Our findings evidence that even with minimal coverage, XPS provides excellent resolution. The survey spectra and highly resolved core levels have been recorded to determine the chemical composition of MoS₂, graphite, WSe₂ and FePS₃, which agrees with the literature published until now. In addition, we have shown that when working with air-sensitive materials, such as FePS₃, the transfer method needs to be conducted in an inert atmosphere to prevent sample oxidation.

This work opens up the possibility of controlling the purity of commercial materials and assessing the damage caused by air exposure or other induced modifications. At the same time, it is an easy and straightforward technique for facilitating comparisons of the same 2D materials across different research groups.

Data availability

The data that support the findings of this research are available from the authors upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was partially funded by the European Union-Next Generation EU under the PRIN Grants 2D-FRONTIERS (20228879FT), 2D-PentaSensing (P2022HT3FF) and ANDROMeDA (PRIN_2020Y2JMP5) of MUR, by the INFN Ptolemy project, EC project ATTRACT (Grant Agreement 777222), PID2021-126098OB-I00/AEI/FEDER10.13039/501100011033 grant of Spanish MICINN, and by Sapienza University Ateneo and “Avvio alla Ricerca” and "Ateneo" funds.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr02882a

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