Influence of the electrochemical deposition parameters on the microstructure of MoS2 thin films

Thin Solid Films 361±362 (2000) 223±228 www.elsevier.com/locate/tsf

In¯uence of the electrochemical deposition parameters on the microstructure of MoS2 thin ®lms A. Albu-Yaron a, b, C. LeÂvy-CleÂment a,*, A. Katty a, S. Bastide a, R. Tenne c a

LCMTR, CNRS UPR 209, 2±8 Rue Henri Dunant, 94230 Thiais, France b ARO, Volcani Center, 50250 Bet Dagan, Israel c Department of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel

Abstract Thin ®lms molybdenum dichalcogenide, MoS2, were prepared by cathodic electrochemical deposition from aqueous and non-aqueous solutions of tetrathiomolybdate ions, at different temperatures. The ®lms were X-ray amorphous as deposited. They consist of an amorphous matrix in which quantum sized nanocrystallites or clusters were embedded. Upon annealing at high temperatures, the ®lms obtained from aqueous solutions become crystalline and highly texturized having their van der Waals planes oriented parallel to the substrate, whereas, those obtained from ethylene glycol solutions kept on the amorphous matrix, with slightly larger sizes MoS2 nanoparticles embedded, than before annealing. Difference in the mechanism of the electrodeposition in aqueous and ethylene glycol solutions is discussed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Electrodeposition; Transition metal dichalcogenide; Thin ®lms; Aqueous solution; Ethylene glycol; Microstructure

1. Introduction Thin ®lms semiconducting layered molybdenum dichalcogenide, MoS2, are very attractive for numerous applications, which include solar cells [1], catalysis, solid lubricants and intercalation batteries. Depending on the type of application, the requirements for the size and orientation of the crystals composing the ®lms are diverse. For photovoltaic applications, thin ®lms constituted of large crystals, in the micrometer range, and with their van der Waals (vdW) planes (i.e. (0002) 2H-MoS2 basal planes) oriented parallel to the substrate (type II texture) are required. In contrast, for catalytic applications, thin ®lms made up of nanometer size particles, with their vdW planes oriented perpendicular to the substrate (type I texture), are desirable. The preparation of MoS2 by electrochemical deposition has recently received much attention [2±7] as this technique may provide the ®lm uniformity and coverage required for large scale photovoltaic technology. This paper presents a study of the in¯uence of the electrodeposition parameters on the microstructure of the MoS2 thin ®lms and the in¯uence

* Corresponding author. Tel.: 133-1-49-781-331; fax: 133-1-49-781203. E-mail address: [email protected] (C. LeÂvy-CleÂment)

of the substrate on the recrystallization process after annealing the ®lms at high temperatures. 2. Experimental Deposition of MoS2 thin ®lms was carried out by a twoelectron reduction of tetrathiomolybdate ions (MoS422) following the reaction 2 1 MoS22 4 1 2 e 1 4 H ) MoS2 1 2 H2 S

…1†

Two electrolytes were used: water and ethylene glycol. The tetrathiomolybdate was 0.005 M. Potassium chloride (0.1 M) was the supporting electrolyte in all solvents. Ammonium chloride (0.57 M) was supplemented as a proton donor, when the electrodepositon was from ethylene glycol. Electrodeposition was carried out at room temperature for both solvents, at 988C in aqueous solutions and at 1658C in ethylene glycol. The substrates were conductive glass (F-doped SnO2, 10 V/A). Few experiments were done using molybdenum and Ni±Cr coated conductive quartz when annealing of the deposited ®lms was performed at higher temperatures ($9008C). A three electrode set up was used. Details of the electrodeposition have been presented elsewhere [3,4]. Electron probe microanalysis (EPMA) was carried out with a CAMEBAX instrument. A Philips PW 1710 X-ray

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00838-X

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diffractometer (Cu Ka ) was used. Optical absorption of the ®lms deposited on conductive glass and quartz was calculated from transmission spectra measured with a Cary 2400 spectrometer. Scanning electron microscopy (SEM) was done with a JEOL JSM-840 microscope. Conventional medium resolution transmission electron microscopy (TEM) and selected area (SAD) electron diffraction were performed using a Philips 400T electron microscope (120 kV), and high resolution (HREM), with a JEOL 4000EX(II) microscope (400 kV). 3. Results Details of sample preparation and annealing are summarized in Table 1. During electrodeposition, brown smooth layers were formed at the electrodes. When their thickness exceeded 1.5±2 mm, the layers started to peel off from the substrate. 3.1. As deposited MoS2 thin ®lms (samples 1±4): in¯uence of the solvent and temperature SEM micrographs reveal that the ®lms deposited at room temperature are fairly continuous without visible cracks. When obtained at higher temperatures (988C in water and 1658C in ethylene glycol), cracks appear indicating occurrence of stress. All the as electrodeposited ®lms are X-ray amorphous whatever is the deposition temperature. They exhibit an atomic ratio S/Mo between 1.9 and 2.1 (EPMA) and display a gradual increase of the optical density towards increasing photon energy, in their absorption spectra (Fig. 1, curve (a,c). The featureless curves are typical of amorphous materials [3,6]. The TEM image in Fig. 2a shows that the ®lms deposited from aqueous solutions on conductive glass at 208C (sample 1) are composed of very thin, small MoS2 clusters of various lengths, embedded in an amorphous material. Most of the clusters exhibit three to ten stacks of individual S-Mo-S

Fig. 1. Absorption spectra of a MoS2 thin ®lms: deposited at 208C from aqueous solution (a) as deposited (1.3 mm thick) and (b) annealed at 5508C (1 mm thick); deposited from ethylene glycol solution (c) as deposited at 1658C (sample 4) and (d) annealed at 5508C (sample 9).

sheets (2±6 nm thick and 8±25 nm long). The separation of 0.6 nm between the S-Mo-S sheets corresponds well with the 0.615 nm spacing of the (0002) vdW basal planes in 2H-MoS2 structure. The nanocrystallites lie roughly perpendicular ((0002) planes ') to the substrate surface, indicating type I texture. The MoS2 particles in the ®lms deposited at 988C (sample 2) exhibit shorter, slightly bent (0002) lattice planes, 2±7 sheets thick, embedded in an amorphous matrix, and quite frequently displaying some dislocations (Fig. 2b). Fig. 3a shows that the MoS2 ®lms deposited from ethy-

Table 1 Specimen data a Samples

Deposition conditions

Annealing conditions

E (V)

Q (C/cm 2)

t (s)

T (8C)

d (nm)

Medium

Substrate

21.2 21.2 21.0 20.4

0.41 0.80 0.65 0.07

318 221 2029 100

20 98 20 165

155

A A EG EG

CG CG CG CG

21.2 21.2

3.08

1853 678

600

21.0 20.4

0.88 0.07

2500 100

20 20 20 20 165

A A A EG EG

CG Mo Ni±Cr/Q CG CG

T (8C)

t (h)

Gas

550 900

10 9

Vac Ar

550 550

1 1

Ar Ar

As-deposited 1 2 3 4 As-deposited and annealed 5 6 7 8 9 a

139 155

88 93

A, aqueous; EG, ethylene glycol; CG, conducting glass; Mo, molybdenum foil; Ni±Cr/Q, nickel-chromium alloy on quartz.

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Selected area electron diffraction patterns (insets in Figs. 2a,b and 3a,b) are typical of crystalline material of too small dimensions to give rise to detectable X-ray diffraction peaks. The well-de®ned rings correspond to those of hexagonal 2H-MoS2 structure. The presence of the (0002) re¯ection shows that most of the crystallites have their basal planes oriented perpendicular to the surface (edge orientation) indicating type I texture. 3.2. Annealed MoS2 thin ®lms (samples 5±9) Annealing at high temperature (550±9008C), under argon

Fig. 2. TEM images of MoS2 thin ®lms as deposited from aqueous solution: (a) at 208C (sample 1); (b) at 988C (sample 2). Insets are the corresponding SA diffraction patterns.

lene glycol at 208C (sample 3) [4] are predominantly constituted of very small plate-like crystallites, 2±4-nm thick, (3± 6 stacks of individual vdW planes) of approximately same length, and are mostly oriented with the (0002) planes perpendicular to the substrate surface (type I texture). Generally, lattice defects like dislocations were not observed. HREM of MoS2 layers obtained at 1658C (sample 4) [6] (Fig. 3b) discloses the typical image of an amorphous material embodying some single or double slightly bent sheets ,0.6 nm apart (observed under optimum defocus conditions), indicating the presence of rather small 0.6± 1.2-nm thick 2H-MoS2 nanoparticles, approximately 8±12nm long.

Fig. 3. TEM images of MoS2 thin ®lms as deposited from ethylene glycol solution: (a) at 208C (sample 3) and (b) at 1658C (sample 4). Insets are the corresponding SA diffraction patterns.

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atmosphere, for 1 to 10 h, results in a decrease in the ®lm thickness by 30±50%, which is probably due to the fact that MoS2 ®lms become more compact upon crystallization and/ or to some material loss by evaporation. Those produced in aqueous medium (samples 5±7), crystallize when annealed (grains of 20±100 nm or larger are observed in TEM), irrespective of the substrate material. An example of large, well crystallized MoS2 particles, constituting layers prepared on conducting glass substrate at 208C and annealed at 5508C (sample 5) is shown in Fig. 4. Those deposited on molybdenum substrate and annealed at 9008C (sample 6) exhibit the largest crystal size (,100 nm). X-ray diffraction (XRD) spectra reveal that the ®lms are highly texturized (type II texture) with the vdW (0002) basal planes oriented parallel to the substrate (Fig 5, spectrum (a)) [3]. This preferential orientation is retained for all electrodeposited layers independent of their thickness. Annealed ®lms produced in aqueous solution exhibit a metallic lustre. The optical density in the absorption spectra (Fig. 1, curve (b)), increases sharply between 1.6 and 2.0 eV and remains almost constant above 2.2 eV, revealing a direct transition, and the characteristic MoS2 band-gap of ,1.8 eV. The two peaks at 1.8 and 2.1 eV can be assigned to the A and B excitons [8,9]. Both the as-deposited and annealed ®lms contain appreciable amounts of oxygen [3]. Although the MoS2 crystals are formed with the desired orientation, the ®lms are not photoactive presumably due to the relatively small crystal size, and the consequent larger number of dangling bonds on the non-vdW surfaces acting as recombination centers for photogenerated carriers. The size of the MoS2 crystals was considerably increased when MoS2 layers were electrodeposited from aqueous

Fig. 4. TEM image of MoS2 thin ®lms deposited on conducting glass from aqueous solution at 208C and annealed (sample 5). Inset is the corresponding SA diffraction pattern.

solution, on quartz substrate coated with a thin Ni±Cr layer, with subsequent annealing at high temperature, in a sulfur containing atmosphere (sample 7). During annealing (at 9508C), the sulfur reacts with the Ni, forming a relatively low melting point (6358C) Ni-S eutectic 33 at.% of S, on which the MoS2 crystals grow like on a liquid phase, leading to micron size crystals (Fig. 6). The (0002) XRD Bragg peak becomes extremely intense and narrow revealing the high degree of crystallinity of the ®lm of type II texture (Fig. 5, inset). The MoS2 ®lms are photoactive with a direct band gap of about 1.7 eV (photoconductivity measurements) [5]. The TEM images of the layers produced from ethylene glycol at 20 and 1658C and annealed at 5508C for 1 h in Ar atmosphere (Fig. 7a (sample 8) and 7b (sample 9), respectively), reveal a network of small, bent, tangled interlocking structures consisting of 2±5 stacks of (0002) 2H-MoS2 planes, occasionally enclosing voids. Sometimes, particles are joined with a common crystal plane or exhibit dislocation-like or stacking fault-structures. In addition, although in small numbers, heavily bent stacks comprising 3±4 MoS2 vdW planes, which de®ne rectangular or triangular nanopar-

Fig. 5. X-ray diffraction spectra of annealed MoS2 ®lms. Films deposited at 208C from aqueous solution, (a) conductive glass substrate and annealed during 1 h at 5508C in Ar (1 mm thick), (inset) Ni±Cr/quartz substrate and annealed 6 h at 9508C in S vapor (0.9 mm thick). Films deposited on conductive glass from ethylene glycol solution, (b) deposited at 208C and annealed at 5508C (sample 8), (c) deposited at 1658C and annealed at 5508C (sample 9).

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in ethylene glycol, as a result of an additional chemical reduction reaction of ethylene glycol itself acting as a reducing agent [6,10]. The in¯uence of the chemical reduction reaction is particularly important in the case of the deposition at 1658C. However, the two processes are not independent, since otherwise complete reduction of the precursor before electrodeposition of the ®lm would take place. Therefore, a mixed reduction of the tetrathiomolybdate (by ethylene glycol)-electroreduction mechanism must be envisaged. Annealing the as deposited layers at high temperatures (Table 1), had a different impact on the control of their

Fig. 6. SEM picture of an electrochemical deposited MoS2 ®lm on Ni±Cr/ quartz substrate annealed for 1 h at 9508C in sulfur atmosphere (sample 7).

ticles are observed in the annealed sample prepared at 1658C (arrowed in Fig. 7b) [6]. The respective XRD spectrum of the layers deposited in ethylene glycol and annealed, exhibits only a very broad (0002) peak, of extremely low intensity (Fig. 5, spectra (b,c)), indicating very poor crystallinity for the obtained MoS2 small particles. 4. Discussion and conclusion A common feature of the experiments in this work is that all the deposited ®lms when grown, were found X-ray amorphous, while HREM revealed them nanostructured, with very small MoS2 nanoclusters or nanocrystallites embedded like in an amorphous matrix. The formation of these crystalline embryos, densely scattered in the disordered structure, was found to be strongly in¯uenced by the parameters of electrodeposition process (temperature, medium and composition, or substrate (cathode) material). At 208C, when deposition was from aqueous solutions, ¯at, platelike nanocrystallites, 2±6 nm-thick and 8±25-nm long sizes, lying roughly normal to the substrates surface, were observed embedded in the amorphous material. While, from ethylene glycol, quantum sized, 2±4-nm thick, smaller clusters, although with much less amorphous matrix, were obtained (Figs. 2a and 3a). Generally, at 208C deposition, lattice defects as twins or dislocations were not observed. An increase in the temperature of deposition in the both used solvents (98 and 1658C in aqueous and ethylene glycol solutions, respectively), produced smaller sizes MoS2 nanoparticles composing the as grown thin ®lms, whereas creating some defects, which probably result in bending of the vdW planes (Figs. 2b and 3b). Note in Fig. 3b that, deposition at 1658C, from ethylene glycol, resulted in single or double (0002) plane-sized 2H-MoS2 nanoplatelets (,2±3 nm) embedded in an amorphous matrix. We explain the differences in size and morphology between nanoparticles obtained in aqueous and in ethylene glycol solutions, as being due to a more rapid nucleation process taking place

Fig. 7. TEM images of MoS2 ®lms deposited on conductive glass from ethylene glycol solution and annealed at 5508C, (a) deposited at 208C and annealed (sample 8), and (b) deposited at 1658C and annealed (sample 9). Insets are the corresponding SA diffraction patterns.

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morphology. While, annealing of samples deposited from aqueous solutions, on conducting glass or on molybdenum substrates, induced highly textured ®lms, made up of large ,25±100 nm crystals, with their basal (vdW) planes oriented parallel to the substrate (Fig. 4), annealing the samples as deposited at 20 and 1658C, in ethylene glycol solutions, maintained their nanostructured character (Figs. 1,5,7) as well as, the amorphous matrix. The occurrence of some polyhedral nanostructures with triangular, (arrowed in Fig. 7b), or rectangular projection in the annealed ®lms prepared at 1658C (sample 9), was also, although barely, observed. They resemble to the closed polyhedral nanostructures (i.e. fullerenes previously reported [11]. We note that these features were not present when deposition was from aqueous solutions. These results, indeed, may suggest that more than one reaction mechanism [6] might possibly be involved which account for the various resulting morphologies. Photoactive MoS2 thin ®lms of type II texture, formed of micron sized crystals, have been obtained when the deposition is made on Ni±Cr/quartz substrate from aqueous solution and after annealing at very high temperatures under sulfur atmosphere. A vdW rheotaxy mechanism has been proposed to explain the growth of the crystals during annealing [5]. The variety of microstructures observed for the electrochemically grown MoS2 thin ®lms shows that the cathodic reduction of tetrathiomolybdate is an interesting alternative growing technique to control their structures and morphologies in a wide range (from macro to nanostructures) and create related materials with a wide range of applications (solar cells, catalysis, etc.).

Acknowledgements The authors would like to thank M. Rommeluere for the EPMA analysis of the ®lms and E.A. Ponomarev for technical assistance. Part of this work was funded within the frame of the AFIRST program (project no. 980 MENRT 3).

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