Deposition of MoSe2 flakes using cyclic selenides

Deposition of MoSe 2 flakes using cyclic selenides† Jaroslav Charvot, a Raul Zazpe, bc Richard Krumpolec, d Jhonatan RodriguezPereira, bc David Pavliˇ n´ ak, d Daniel Pokorn´ y, a Milan Klikar, a Veronika Jel´ ınkov´ a, e Jan M. Macak bc and Filip Bureˇ s* ae The currently limited portfolio of volatile organoselenium compounds used for atomic layer deposition (ALD) has been extended by designing and preparing a series of four-, fiveand six-membered cyclic silylselenides. Their fundamental properties were tailored by alternating the ring size, the number of embedded Se atoms and the used peripheral alkyl chains. In contrast to former preparations based on formation of sodium or lithium selenides, the newly developed synthetic method utilizes a direct and easy reaction of elemental selenium with chlorosilanes. Novel 2,2,4,4-tetraisopropyl-1,3,2,4diselenadisiletane, which features good trade-offbetween chemical/thermal stability and reactivity, has been successfully used for gas-to-solid phase reaction with MoCl 5 affording MoSe 2 . A thorough characterization of the as-deposited 2D MoSe 2 flakes revealed its out-of-plane orientation and high purity. Hence, the developed four-membered cyclic silylselenide turned out to be well-suited Seprecursor for ALD of MoSe 2 . Introduction Despite having been developed more than 50 years ago, Atomic Layer Deposition (ALD), a thin-lm manufacturing technology, 1 still attracts growing attention worldwide. 2 This is due to ALD's unique advantages including high uniformity of prepared nanolayers, 3 precise thickness control 4 or possibility to cover non-planar substrates like nanotubes. 5 A reaction between a gaseous precursor and free functional groups of the selected substrate ensures accurate deposition of atomic monolayer with minimum of defects. On the contrary, exclusive gas-to-solid phase reaction is also one of the biggest ALD limitation. For such reaction, a precursor of sufficient volatility and thermal stability with persisting high reactivity is essential. Finding a compromise between the aforementioned properties is usually not a simple task. Transition metal dichalcogenides (TMDC) form layered crystal structures featuring chalcogen–metal–chalcogen units bound via covalent bonds. Stoichiometric MX 2 monolayers interacts via weak van der Waals forces dependent on the selected metal (M), chalcogen (X) and their supramolecular arrangement. 6 TMDC are oen semiconductors with a narrow band gap –an interesting ability exploitable in electronics, electrocatalysis or photocatalysis, especially in water splitting or hydrogen evolution reactions (HER). In addition to widely explored performance of MoS 2 in the HER, 7,8 MoSe 2 , 9 GaSe (ref. 10) or WSe 2 (ref. 11) showed also promising results. Bis- (trialkylsilyl)selenides 12 are currently the most favourite ALD selenium precursors used for deposition of the latter selenides. Recently, selenium dimethyldithiocarbamate was successfully used for deposition of Sb 2 Se 3 as presented by Sarkar. 13 Our research group introduced bis(trialkylstanyl)selenides 14 and cyclic silylselenides 15 as Se-precursors with decreased sensitivity towards air and moisture. Six-membered selenide containing two selenium atoms turned out to be the best precursor so far. This prompted us to explore the family of cyclic silylselenides bearing more selenium atoms further. Results and discussion Synthesis The general reaction pathway towards cyclic silylselenides is outlined in Scheme 1. The synthesis and thermal properties of 1 (ref. 16) 2(ref. 17) and 3(ref. 17) were reported earlier, see also our recent communication for comprehensive characterization. 15 The general methodology utilizes Li 2 Se, prepared from elemental Se and its reaction with Li or LiBHEt 3 and subsequent a Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentsk´ a 573, Pardubice, 53210, Czech Republic. E-mail: [email protected] b Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, N´ am. ˇ Cs. Legi´ ı565, Pardubice, 53002, Czech Republic c Central European Institute of Technology, Brno University of Technology, Purkyˇ nova 123, Brno, 61200, Czech Republic d Department of Physical Electronics, CEPLANT—R&D Center for Plasma and Nanotechnology Surface Modications, Faculty of Science, Masaryk University, Kotl´ aˇ rsk´ a 267/2, 61137 Brno, Czech Republic e The Institute of Technology and Business in ˇ Cesk´ e Budˇ ejovice, Okruˇ zn´ ı517/10, 370 01, ˇ Cesk´ e Budˇ ejovice, Czech Republic †Electronic supplementary information (ESI) available: Further synthetic details, NMR spectra, GC/MS records, DSC/TGA curves, SEM pictures, Raman spectra and XPS spectra. See DOI: 10.1039/d0ra10239c Cite this: RSC Adv.,2021,11, 22140 Received 4th December 2020 Accepted 17th June 2021 DOI: 10.1039/d0ra10239c rsc.li/rsc-advances 22140 |RSC Adv.,2021,11, 22140–22147 © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Advances PAPER Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue reaction with appropriate dichlorosilane (Method A). Compared to formerly prepared 3, the synthesis of asymmetric compound 4starts from inexpensive silanes and affords higher yield along with more volatile selenide. Novel four-membered cycle 6containing two selenium atoms was prepared in the same way in almost quantitative yield, which signicantly facilitated its isolation. Using bulkier tBu 2 SiCl 2 gave no reaction although the reaction temperature was elevated or BF 3 $OEt 2 was added. On the contrary, the reaction with Me 2 SiCl 2 gave tetramethylsubstituted four-membered cycle similarly to 6(as monitored by GC/MS). However, its very reactive nature made its isolation very difficult even when a strict inert atmosphere was maintained. Tetraethyl derivative 5showed signicantly improved stability and facile isolation but its NMR showed a mixture of two products giving almost the same signals with only small difference in chemical shis (see the ESI†). On the contrary, no other product was detected by GC/MS. The former preparation of 5employing Na 2 Se instead of Li 2 Se reported also formation of six-membered cycle containing three Se atoms in 40% yield. 18 Hence, we prepared Na 2 Se, by reacting Se with NaBHEt 3 , which was subsequently treated with Et 2 SiCl 2 or iPr 2 SiCl 2 (Method B) to afford similar mixture of 5and exclusively 6. Compounds 5 and 6of the same quality were prepared regardless using Na 2 Se or Li 2 Se and none six-membered cycle was detected in both cases. All of our attempts to isolate and identify the unknown compound prepared along with 5failed. Since our main goal is to design simple selenium precursor for ALD with easy synthesis and isolation, we have excluded 5from further studies. When comparing both reactions using Na 2 Se or Li 2 Se (Methods B or A), the latter showed better yields. Employing Method A, tetraisopropyl cyclic selenide 6can be prepared and isolated almost quantitatively. Selenium can be reduced to silylselenoles or bis(trialkyl) selenides using trialkylsilanes. This was rstly reported for Et 3 Si–SeH (ref. 19) and cHex 3 Si–SeH (ref. 20) and later also for other silanes as demonstrated in our recent work. 21 Based on these observations, we developed a new synthetic method towards four-membered cycles 6and 7, which utilizes a direct reaction of elemental selenium with dialkylchlorosilanes in the presence of amine (Method C). Compared to synthesis of former silylselenoles carried out at 250 C within 48 h, the newly developed method requires only slightly raised reaction temperature (120 C) and short reaction time (3 h). Moreover, it utilizes readily available and inexpensive starting materials, avoids reactive lithium species and even excludes solvents. Scheme 1 Reaction pathway towards cyclic silylselenides. © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11, 22140–22147 | 22141 Paper RSC Advances Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Purication of products 6and 7is simple and involves only ltration and crystallization at 78 C. Derivative 6is stable and can withstand ambient conditions for several hours or even days in case of 7. If stored under an inert atmosphere at 5C, no degradation was observed aer several months. These properties make 6and 7very promising Se-precursors for ALD. A low-yielding (11%) photochemical preparation of 7including its X-ray structure has been formerly reported by Saak et al. 22 Derivative 6was prepared for the rst time. A similar reaction with Me 2 SiHCl provided a variety of products, which amounts depend on the used temperature and cannot be easily isolated. A small amount of the aforementioned six-membered cycle was detected if the reaction was carried out under elevated temperature to 250 C (see the ESI†for more details). Structure and purity of the prepared silylselenides have been conrmed by 1 H/ 13 C/ 29 Si/ 77 Se NMR and GC/MS analysis (see the ESI†for details). Thermal properties Besides easy synthesis and facile isolation, thermal stability and volatility are crucial properties of ALD precursors. Thermal properties of selenides 4–7were studied by DSC and TG analysis at atmospheric pressure; for thermal properties of 1–3see our previous communication. 15 DSC thermograms are shown in Fig. 1 and Table 1 (for complete records including both cooling/ heating programs see Fig. S22–S25 in the ESI†). Liquid 4was rstly cooled to 100 C, while the sample solidied amorphously at around 90 C. Its reverse heating revealed cold crystallization at 62 C followed by broad melting process appearing at 21 C. The sample began to evaporate at around +150 C and was completely evaporated at +210 C. The DSC of liquid 5showed two peaks of crystallization at 0 and 64 C under cooling as well as two melting processes (broad at +2 and sharp at 63 C) under heating program. This is most probably due to aforementioned mixture. Nevertheless, the sample was completely evaporated at +265 C. Selenide 6underwent crystallization followed by melting that appeared at +3 and +32 C. It was evaporated between 220 and 285 C. Analogical thermal behavior was recorded for 7, which features higher thermal robustness (T m ¼170 C, T c ¼157 C) and complete evaporation at 315 C. The TGA shown in Fig. 1 corroborates the DSC measurements and conrmed the good volatility of target organoselenides (see also Fig. S26–S29 in the ESI†). The highest/ lowest volatility was observed for derivative 4/7substituted with methyl/tbutyl alkyl chains, which implies that thermal properties of 1–7are easily tunable by proper alkyl substitution. Moreover, zero residues were detected aer TGA indicating sufficient thermal stability during heating. Preparation of MoSe 2 akes by ALD In our recent work on MoSe 2 deposition by ALD, 15 we have identied that six-membered silylselenides bearing two selenium atoms performed better than ve-membered analogues. Especially compound 3showed outstanding performance in MoSe 2 deposition. Hence, we have selected selenide 6as a model compound for surface reaction with MoCl 5 and deposition of MoSe 2 . It features four-membered structure bearing two selenium atoms and facile preparation, easy isolation, good volatility and the highest attained yields using Methods A–C. The deposition of MoSe 2 has been carried out in a custom thermal ALD system (see Experimental part for ALD process details). Glass, annealed titanium foil (with TiO 2 surface in the anatase phase) and silicon wafer (with SiO 2 surface) were used as substrates. The ALD cycle was comprised of four steps described as follows: Se precursor (800 ms)–N 2 purge (5 s)–Mo precursor (800 ms)–N 2 purge (5 s). The number of cycles applied were 800 and the deposition temperature was 300 C (detailed description in Experimental section). The morphology and structure of the as-deposited MoSe 2 on the different substrates was characterized by means of scanning electron microscope (SEM) as displayed in Fig. 2. Therein, one can observe the MoSe 2 grew as 2D aky nanosheets, mainly oriented out-of-plane. ALD processes applying same number of cycles but different Se dose, namely 400 and 1200 ms (while keeping Mo dose 800 ms), were conducted in order to verify the self-limiting Fig. 1 TGA (top) and DSC (bottom) records of 4(green), 5(black), 6 (red) and 7(blue). Table 1 Fundamental thermal properties of studied cyclic selenides 4–7 Comp. T ca [C] T mb [C] T Ec [C] 462 21 +150 to +210 564/0 63/+2 +160 to +265 6+3 +32 +220 to +285 7+157 +170 +250 to +315 a Temperature of crystallization. b Temperature of melting. c Range of evaporation. 22142 |RSC Adv.,2021,11, 22140–22147 © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online nature of the process. Corresponding SEM top view images from annealed Ti foils and Si wafer, together with the SEM crosssection of the Si wafer, are shown in the Fig. S30.†Therein, the MoSe 2 deposited applying a Se dose of 800 and 1200 ms showed similar features, i.e. the density and size of MoSe 2 akes, indicating a saturation regime and conrming the selflimiting nature of the process. Additionally, the deposition temperature dependence was studied conducting ALD processes (800 cycles) at different temperatures, 250 and 200 C (in addition to 300 C). In contrast to 2D aky crystallites obtained at 250 C, the as-deposited MoSe 2 at 200 C exhibited granular morphology (see Fig. S31† for the corresponding SEM images). These results revealed a thermal dependence of as-deposited MoSe 2 morphology. Grazing Incident X-Ray Diffractometry (GI-XRD) characterization of as-deposited MoSe 2 at 300 C provided the corresponding GI-XRD patterns on the different substrates shown in the Fig. 3. The patterns clearly exhibited diffraction peaks at 2q 13.5, which matched well with the (002) plane of hexagonal (2H) MoSe 2 , and conrmed the presence of MoSe 2 with out-ofplane orientation in line with the SEM images. The as-deposited MoSe 2 structure was further characterized by means of Raman spectroscopy, a paramount technique for the assessment of layered materials. The corresponding Raman spectra and obtained from as-deposited MoSe 2 at 300 C (see Fig. 4) and 200 and 250 C (see Fig. S32†) exhibited characteristic 2H-MoSe 2 peaks, namely, A 1g (out-of-plane) and E 1 2g (inplane) modes at 238 and 285 cm 1 , respectively. 23 The few-layered nature of the deposited 2D MoSe 2 was conrmed by the red shiof the A 1g peak as compared to the peak position for MoSe 2 powder (242 cm 1 ), while the observed difference in the relative intensities between the A 1g and E 1 2g modes indicated the prevailing out-of-plane orientation, as observed in the SEM images and XRD results. 24 Fig. 2 SEM top images at two different magnifications of the asdeposited ALD MoSe 2 at 300 C upon 800 cycles (800 ms Se dose) on different substrates. The as-deposited MoSe 2 shows 2D flaky nanosheets morphology mainly out-of-plane oriented. Fig. 3 XRD patterns of the as-deposited ALD MoSe 2 on different substrates upon 800 ALD cycles (800 ms Se dose). The plane (002) revealed the out-of-plane orientation of the as-deposited ALD MoSe 2 at 300 C onTianium foil (top), silicon wafer (middle) and glass (down). Fig. 4 Raman spectra obtained from (top) MoSe 2 powder and asdeposited ALD MoSe 2 on titanium foil (middle) and glass (down) at 300 C upon 800 ALD cycles (800 ms Se dose). © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11, 22140–22147 | 22143 Paper RSC Advances Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online X-ray Photoelectron Spectroscopy (XPS) analysis was conducted to assess the surface chemical composition of the asdeposited MoSe 2 . Here, it is important to note that the use of adventitious carbon (284.8 eV) as a reference to adjust the binding energy scale was not reliable, basically due to the strong overlapping of C 1s signal with Se LMM. Instead, the binding energy of Mo 3d at 228.3 eV corresponding to MoSe 2 was used for this purpose. 25,26 Fig. S33†shows the XPS survey spectra for MoSe 2 deposited at 300 C on glass, annealed Ti foils and Si wafer, whereas Fig. S34†exhibits the XPS survey spectra obtained for MoSe 2 deposited at 200 and 250 C on Si wafer. The lecolumn in Fig. 5, shows the deconvolution of the XPS high resolution spectra of Mo 3d obtained from as-deposited ALD MoSe 2 at 300 C on the different substrates. The most intense doublets (orange) centered at 228.3 and 231.4 eV corresponds to Mo(IV)–Se, corroborating the growth of MoSe 2 . The peaks at 229.9 and 233.0 eV (green) were attributed to Mo–Se–O. Actually, the presence of Mo–Se–O bonds evidences both the chemical reaction of the Se precursor with the hydroxyl groups from the substrates surface, and the posterior reaction of the resulting chemisorbed counterpart with the Mo precursor. The last doublet at 232.5 and 235.7 eV (blue) was associated with Mo(VI)–O, originated upon the reaction of MoCl 5 with the hydroxyl groups from the substrate surface during the early stage of the deposition process. Regarding the deconvolution of the XPS high resolution spectra of Se 3d (right column of Fig. 5), it exhibited four components ascribed to two Se 3d 5/2 and Se 3d 3/2 spin–orbit splitting. The rst and most intense doublet located at 53.8 and 54.7 eV, corresponding to Mo–Se bonds, which unambiguously corroborated the growth of MoSe 2 . And the second doublet with the peaks centered at 55.2 and 56.1 eV, attributed to Mo–Se–O, conrming the presence of this species in Mo 3d. Regarding the MoSe 2 deposited at 250 C (see Fig. S35†) the deconvoluted XPS high resolution spectra of Mo 3d and Se 3d exhibited the same features than those described for MoSe 2 deposited at 300 C. In contrast, the MoSe 2 deposited at 200 C (Fig. S35†) displayed a doublet at 54.7 and 55.6 eV in the Se 3d deconvoluted XPS high resolution spectra ascribed to Se–Se or Se 0 . This would suggest that the chemical reaction between the Mo and Se precursors could be thermally limited at a deposition temperature of 200 C. As compared to the XPS results obtained in our previous work from the MoSe 2 deposited using selenides 1and 3, 15 one must notice relevant differences: (i) the absence of chlorine residues (mainly present by Mo–Cl species) and (ii) the welldened Se 3d doublet. Those differences suggested the absence of impurities (other chemical species) and complete ligand exchange reaction between selenide 6and MoCl 5 resulting in a high purity as-deposited ALD MoSe 2 . Conclusions In summary, we have investigated a series of four-, veand sixmembered cyclic silylselenides as potential Se-precursors for ALD. They were prepared in a straightforward manner employing elemental selenium and chlorosilanes. The solventfree Method C, which starts from Se and commercially available and inexpensive chlorosilanes, is in particular operationally very simple, avoids generation of lithium or sodium selenides and affords four-membered derivatives 6and 7in good yields of 77 and 61%, respectively. The performed DSC and TG analysis revealed that volatility can be tailored by choosing proper peripheral alkyl substituents. Especially novel silylselenide 6bearing isopropyl groups showed very good trade-off between its chemical stability and sufficient volatility. Its ALD reaction with MoCl 5 successfully provided 2D MoSe 2 nanosheets that were characterized by SEM, GI-XRD, Raman spectroscopy and XPS. Compared to previous silylselenides, precursor 6showed complete ligand exchange reaction and its ALD afforded MoSe 2 of high purity. Experimental The NMR and GC/MS spectra were recorded with a Bruker AVANCE 400 instrument and a GC/EI-MS conguration Fig. 5 XPS high-resolution spectra of Mo 3d (left) and Se 3d (right) corresponding to as-deposited ALD MoSe 2 upon 800 ALD cycles at 300 C (800 ms Se dose). 22144 |RSC Adv.,2021,11, 22140–22147 © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online including gas chromatography Agilent Technologies 6890N (HP-5MS, 30 m column, I.D. 0.25 mm, lm 0.25 mm) equipped with a mass detector Network MS detector 5973 (EI 70 eV, range 33–550 Da). Me 4 Si and Me 2 Se were used as internal standards for 1 H/ 13 C/ 29 Si and 77 Se NMR measurements (d¼0 ppm). Thermal properties of target molecules were measured by differential scanning calorimetry (DSC) with a Mettler-Toledo STARe System DSC 2/700 equipped with FRS 6 ceramic sensor and cooling system HUBER TC100-MT RC 23 or by thermogravimetric analysis (TGA) with a Mettler-Toledo STARe System TGA 2 equipped with a horizontal furnace LF (400 W, 1100 C), balance XP5 (resolution 1 mg) and cooling system HUBER Minichiller 600. DSC thermograms of the target compounds were measured in aluminous crucibles with a small hole in the lid under N 2 inert atmosphere. DSC curves were determined with a scan rate of 5 C min 1 within the range 100 Cto +400 C. The synthesis and workup were performed under Ar atmosphere or in a nitrogen-lled glovebox. All used solvents were properly dried before use. The used glassware was poured into sodium hypochlorite bath to destroy remaining organic selenides before cleaning. General method A Dry THF (40 ml) and ne selenium powder (1.0 g, 12.6 mmol) were placed to a 100 ml Schlenk ask. The suspension was cooled to 0 C and LiBHEt 3 (25.2 ml, 25.2 mmol, 1 M solution in THF) was added slowly. The mixture was stirred at 25 C for 2 h forming white suspension of Li 2 Se. The corresponding dialkyldichlorosilane (1 or 0.5 eq. for 5/6or 4) dissolved in THF (10 ml) was added dropwise and the reaction mixture was stirred at 25 C for 15 h. The solvent was evaporated in vacuo, dry hexane (30 ml) was added and the solution was ltered via a cannula. The hexane solutions of 4or 5were evaporated in vacuo and the remaining yellowish oil was puried by vacuum distillation. The hexane solution of 6was cooled to 78 Ctoafford colourless solid, while the supernatant was removed via a syringe. The nal product was dried in vacuo. General method B Dry THF (40 ml) and ne selenium powder (1.0 g, 12.6 mmol) were placed to a 100 ml Schlenk ask. The suspension was cooled to 0 C and NaBHEt 3 (25.2 ml, 25.2 mmol, 1 M solution in THF) was added slowly. The mixture was stirred at 25 C for 2 h forming dark violet suspension of Na 2 Se. The remaining procedure is similar to Method A. General method C Elemental selenium (1.0 g, 12.6 mmol) along with dialkylchlorosilane (1 eq.) and EtNiPr 2 (2.2 ml, 1.64 g, 12.6 mmol) were introduced into a pressure vessel lled with argon and containing a magnetic stirrer. The mixture was stirred at 120 C for 3 h and then was cooled to 25 C to solidify. A second stirrer was added along with hexane (15 ml) and the mixture was vigorously stirred and shook until a yellow solution with a white precipitate were formed. The mixture was ltered via cannula and cooled to 78 Ctoafford yellowish crystals, while the hexane was removed via a syringe. The crystal product was dried in vacuo. 2,2,4,4,5,5-Hexamethyl-1,3,2,4,5-diselenatrisilolane (4) The title compound was prepared from Me 4 Si 2 Cl 2 (1.2 ml, 1.2 g, 6.3 mmol) and Me 2 SiCl 2 (0.8 ml, 0.8 g, 6.3 mmol) following the Method A. Yellowish liquid (1.6 g, 75%) with bp ¼95–100 C(1 torr). 1 H-NMR (400 MHz, 25 C, C 6 D 6 ): d( 1 H) ¼0.45 (s, 12H, CH 3 ), 0.78 (s, 6H, CH 3 ) ppm. 13 C-NMR APT (100 MHz, 25 C, C 6 D 6 ): d( 13 C) ¼0.6, 9.9 ppm. 29 Si-NMR (80 MHz, 25 C, C 6 D 6 ): d( 29 Si) ¼13.9, 24.9 ppm. 77 Se-NMR (76 MHz, 25 C, C 6 D 6 ): d( 77 Se) ¼303.5 ppm. EI-MS: m/z¼319 (30), 211 (20), 73 (100). 2,2,4,4-Tetraethyl-1,3,2,4-diselenadisiletane (5) The title compound was prepared from Et 2 SiCl 2 (1.9 ml, 1.99 g, 12.6 mmol) following the Method A (1.9 g, 92%) or Method B (0.9 g, 43%). Compound 5was prepared as a mixture with inseparable impurity. Yellow liquid with bp ¼120–140 C(1 torr). EI-MS: m/z¼332 (20, M + ), 303 (100), 275 (30), 245 (20), 217 (20), 59 (10). For the NMR spectra see the ESI.† 2,2,4,4-Tetraisopropyl-1,3,2,4-diselenadisiletane (6) The title compound was prepared from iPr 2 SiCl 2 (2.3 ml, 2.3 g, 12.6 mmol) following the Method A (2.4 g 99%) or Method B (1.8 g, 72%). It was also prepared from iPr 2 SiHCl (2.2 ml, 1.9 g, 12.6 mmol) following the Method C (1.9 g, 81%). Yellowish liquid. 1 H-NMR (400 MHz, 25 C, C 6 D 6 ): d( 1 H) ¼1.15 (d, J¼ 7 Hz, 24H, CH 3 ), 1.21–1.30 (m, 4H, CH) ppm. 13 C-NMR APT (100 MHz, 25 C, C 6 D 6 ): d( 13 C) ¼17.72, 18.37 ppm. 29 Si-NMR (80 MHz, 25 C, C 6 D 6 ): d( 29 Si) ¼18.1 ppm. 77 Se-NMR (76 MHz, 25 C, C 6 D 6 ): d( 77 Se) ¼402.33 ppm. EI-MS: m/z¼388 (10, M + ), 345 (100), 303 (20), 275 (20), 231 (20), 59 (10). 2,2,4,4-Tetra-tert-butyl-1,3,2,4-diselenadisiletane (7) The title compound was prepared fromtBu 2 SiHCl (2.6 ml, 2.3 g, 12.6 mmol) following the Method C (1.7 g, 61%). Orange crystals. 1 H-NMR (400 MHz, 25 C, C 6 D 6 ): d( 1 H) ¼1.23 (s, 36H, CH 3 ) ppm. 13 C-NMR APT (100 MHz, 25 C, C 6 D 6 ): d( 13 C) ¼25.12, 29.19 ppm. 29 Si-NMR (80 MHz, 25 C, C 6 D 6 ): d( 29 Si) ¼19 ppm. 77 Se-NMR (76 MHz, 25 C, C 6 D 6 ): d( 77 Se) ¼321.9 ppm. EI-MS: m/z¼387 (100), 345 (80), 246 (20), 57 (20). Deposition of MoSe 2 The deposition of MoSe 2 was carried out in a custom-made thermal ALD system applying a deposition temperature of 300 C at a chamber pressure of 2 mbar. The Mo precursor, MoCl 5 (Strem, anhydrous 99.6%), and synthesized 6were heated up to get sufficiently high vapour pressure at 120 and 155 C, respectively. The precursors-enriched carrier gas was delivered through separate heated stainless steel lines (separate for each precursor) directly into the cylindrical deposition chamber of diameter 50 mm and length 300 mm. The substrates were placed on the stainless steel holder of dimensions 35 80 mm placed in the centre of the precisely temperature-controlled chamber. The MoSe 2 ALD process © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11, 22140–22147 | 22145 Paper RSC Advances Open Access Article. Published on 23 June 2021. Downloaded on 12/14/2021 2:05:20 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online started immediately aer 5 pulses of ultrapure water, applied to increase the number of hydroxyl active sites on the substrates surface. The ALD cycle was comprised of four steps described as follows: Se precursor (800 ms)–N 2 purge (5 s)–Mo precursor (800 ms)–N 2 purge (5 s). The number of cycles applied was 800. In parallel, ALD processes applying the same number of cycles but different Se dose, namely 400 and 1200 ms (for a xed Mo dose of 800 ms) were conducted in order to verify the selflimiting nature of process. As to the deposition temperature dependence, it was evaluated by ALD processes conducted at 200 and 250 C (in addition to 300 C). N 2 (99.999%) was used as a carrier gas. The precursors were boosted from the heated standard stainless steel bubblers (Strem, catalog no. 98-0276) at aow rate of 40 standard cubic centimeter per minute (sccm) in all processes. MoSe 2 characterization The structure and morphology of the as-deposited MoSe 2 were assessed by eld emission scanning electron microscope (FESEM JEOL JSM 7500F). X-ray diffraction (XRD) analysis was carried out using Panalytical Empyrean with Cu tube and Pixcel3D detector. Grazing incidence XRD was performed applying an incident angle of 1 degree. The patterns were recorded in range of 5 –65, step size was 0.026 degree. Raman micro-spectrometer HORIBA LabRAM HR Evolution system coupled by with a confocal microscope was to conduct Raman measurements taken by 532 nm (green) laser excitation source in the range 100–500 cm 1 . All spectra were carefully corrected by baseline correction and noise reduction. Spikes were eliminated by spectra accumulation or manually in the LabSpec 6 soware. The surface chemical composition of MoSe 2 was monitored by X-ray photoelectron spectroscopy (XPS) (ESCA2SR, ScientaOmicron) using a monochromatic Al Ka(1486.7 eV) X-ray source. Due to the strong overlapping of C 1s signal with Se LMM, the binding energy scale was referenced to the binding energy of Mo 3d at 228.3 eV corresponding to MoSe 2 . 25,26 The deconvolution of the Mo 3d spectra included the use of eight components since Mo 3d has a strong overlapping with the Se 3s signal. From those, six components corresponded to the three spin–orbit splitting of Mo 3d, Mo 3d 5/2 and Mo 3d 3/2 ,i.e. three chemical species and the remaining two correspond to Se 3s signals. Conflicts of interest There are no conicts to declare. Author contributions Funding acquisition: FB, JMM; investigation: JC, RZ, RK, JR-P, DP, DP, MK, VJ; methodology: JC, RZ; project administration: FB, JMM; writing –original dra: JC, RZ; writing –review & editing: FB. Acknowledgements This research was supported by the Czech Science Foundation (18-03881S) and the Ministry of Education, Youth and Sports of the Czech Republic (MEYS CR, projects LM2018097, LQ1601, LM2018103). We thank Mr L. Hromadko for SEM measurements. References 1 T. Suntola and J. Antson, Method for producing compound thin lms, US Pat., 4058430, 1974. 2 J. Cai, X. Han, X. Wang and X. Meng, Matter, 2020, 2, 587. 3 S. M. George, B. Yoon, R. A. Hall, A. I. Abdulagatov, Z. M. Hibbs, Y. Lee, D. Seghete and B. H. 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