- Email: [email protected]
Growth and characterization of large, high quality MoSe2 single crystals
Journal of Crystal Growth 363 (2013) 122–127
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of large, high quality MoSe2 single crystals Moussa Bougouma a,b,c,n, Abdelkrim Batan b, Boubie´ Guel a, Tiriana Segato c, Jean B. Legma a, Francois Reniers b, Marie-Paule Delplancke-Ogletree c, Claudine Buess-Herman b, Thomas Doneux b,nn a
Laboratoire de Chimie-Physique et d’Electrochimie, UFR/SEA, Universite´ de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso Chimie Analytique et Chimie des Interfaces, Faculte´ des Sciences, Universite´ Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium c 4 MAT, Ecole Polytechnique de Bruxelles, Universite´ Libre de Bruxelles, Avenue F. D. Roosevelt 50, CP 165/63, B-1050 Bruxelles, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2012 Received in revised form 9 October 2012 Accepted 10 October 2012 Communicated by K.W. Benz Available online 17 October 2012
MoSe2 single crystals were grown by chemical vapor transport using TeCl4 as transport agents in the temperature gradient 1020–980 1C. They were characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The characterizations showed that single crystals are perfectly homogeneous, stoichiometric and have very few defects and clean surfaces with areas in the range of 35–100 mm2. Single crystals grown by TeCl4 showed a high electrical conductivity. Their properties were highly dependent on the quality of the polycrystalline powders used for the growth. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Impurities A2. Single crystal growth B1. Transition metal dichalcogenides B2. Semiconducting materials B3. Solar cells
1. Introduction Molybdenum diselenide (MoSe2) is a layered semiconductor compound belonging to the transition metal dichalcogenide family MX2 (M¼Mo, W, X ¼S, Se), which crystallizes in the 2H-MoS2 structures. Because of its great potentialities for solar energy conversion, MoSe2 has been the subject of many investigations [1], which have shown that surface defects play a major role in the corrosion and photocorrosion of the material [2,3]. Consequently, some strategies have been devoted to surface treatment of this material in order to suppress the undesired surface limitations or surface defects as much as possible [2,3]. Unfortunately, the improvements observed with such processes vanish under prolonged illumination. Thus the best way to increase the photoconversion efficiency is the growth of single crystals with surfaces free of defects, as much as possible, and that is the aim of this work. Even though the difficulty to obtain large surfaces of MX2 single crystals by chemical vapor transport method has led some researchers to engage in
n Corresponding author at: Chimie Analytique et Chimie des Interfaces, Faculte´ des Sciences, Universite´ Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium. Tel.: þ226 70325175. nn Corresponding author. Tel.: þ 32 2 650 35 80; fax: þ 32 2 650 29 34. E-mail addresses: [email protected] (M. Bougouma), [email protected] (T. Doneux).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.10.026
electrocrystallization of binary [4–6], mixed/alloyed and ternary [7,8], semiconducting compounds thin films on conductive support, studies on MX2 single crystals remain valuable references for improving the performance of this type of conductive substrate/ thin film/electrolyte junction. Another major interest of transition metal dichalcogenides crystals resides in their layered structure. They have attracted recently a considerable attention as starting materials for the isolation of two-dimensional objects consisting of free standing crystals made of a single or a few atomic layers, in strong analogy with the graphene layer(s) obtained from graphite [9–11]. As low dimensional compounds, they exhibit unique electrical, mechanical and optical properties [10,12,13]. These 2D crystals are obtained by exfoliation or mechanical cleavage of the 3D parent compound [9–11,14]. Improving the crystalline quality of this latter will result in 2D systems of larger lateral size, reaching the macroscopic scale, and there is therefore a direct interest in the synthesis of single crystals of large areas and high quality. This work describes the growth and characterization of perfectly crystallized single crystals that may have fewer defects. Indeed, there is good evidence that the quality of the polycrystalline powders used as the starting material for the preparation of single crystals has a significant impact on the quality and properties of these single crystals [15]. The reproducible syntheses of various powders with homogeneous, well crystallized structure and well-defined microstructure have been the subject of a previous study [16]. Two types of
M. Bougouma et al. / Journal of Crystal Growth 363 (2013) 122–127
polycrystalline powders were synthesized using two different experimental protocols (protocol I and protocol II). Single crystals obtained by chemical vapor transport from protocol I polycrystalline powders have been extensively characterized in our previous works [17–20]. Studies showed that TeCl4 and SeCl4 are the best transport agents. Photocurrent gain was 390 A m 2 for single crystal grown by SeCl4 and 440 Am 2 for single crystals grown by TeCl4. Our goal is to increase this value by improving the single crystals quality. So, single crystals were grown from protocol II polycrystalline powders using TeCl4 as transport agents. The single crystals have been characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The results are compared with those of previously made single crystals from protocol I. It appears from the comparative study that the present single crystals are much better than those previously studied. The impact of polycrystalline powders on single crystals properties is discussed.
2. Experimental details 2.1. Synthesis of polycrystalline powders Polycrystalline powders have been synthesized from elements in powder (Mo: Aldrich Chemical Company, 3N) or granular forms (Se: Aldrich Chemical Company, 4N), using silica glass tubes sealed under secondary vacuum of 10 4 Pa. The tubes were carefully degreased with Ajaxs, etched (HF, 10%) and dried before introducing the reactants. After sealing, the ampoule (sealed tube) was set in an isothermal zone furnace (CARBOLYTE type 201). During synthesis (protocol II), the temperature was increased progressively up to 750 1C in 48 h, in order to avoid all explosion risks due to rapid selenium vaporization. Then, the system has been kept at a constant temperature (750 1C) for 120 h, then slowly cooled down to room temperature in 12 h. The products were recovered after opening the ampoule. After fine milling, samples were reintroduced in a new tube and sealed under secondary vacuum and were annealed at 1050 1C for up to 168 h, and then slowly cooled in the furnace to room temperature. The characterizations of polycrystalline powders obtained by different methods showed that they are homogeneous and well crystallized [16]. Results are consistent with those found in the literature [21]. As a brief recall of protocol I [16], the samples were heated to 600 1C over 48 h in steps to avoid explosions due to sudden selenium vaporization. They were kept at this temperature for up to 72 h, and then air-quenched. After recovery and fine milling, samples were reintroduced in new tube and sealed under secondary vacuum, then annealed at 1000 1C for 96–120 h and slowly cooled in the furnace to room temperature [16]. 2.2. Crystals growth conditions and characterization methods The synthesized polycrystalline powder is introduced in the bottom of transport tube, carefully avoiding tube walls soiling which would act as undesired nucleation spots. The mass of polycrystalline product weighed does not exceed 1 g. Once the transport agent has been added to the polycrystalline powder, the transport tube is then immediately connected to the high vacuum pump system, and sealed after reaching a secondary vacuum of 10 4 Pa. The extremity of the ampoule containing the product is placed in the source zone temperature (TS ¼1020 1C) and the other end in the growth zone temperature (TG ¼980 1C) [19,20,22] of a three zone isothermal furnace (LENTON Thermal Designs Ltd.) for five days.
123
Scanning electron microscopy (SEM JEOL JSM-6100) and optical microscopy (JENA POL) which use computer program Motic Images Plus V2.0ML were used for morphological characterization of single crystals. Se/Mo ratios of crystals were analyzed by Inductively Coupled Plasma (ICP) using Vista ICP-OES spectrometer and by energy dispersive analysis of X-ray coupled with SEM (EDAX-SEM). For ICP analysis, Milli-Q water (Millipore) was used in the preparation of all solutions. MoSe2 solutions were prepared by dissolving 0.3 g of MoSe2 crystals in 100 ml of hydrogen peroxide (H2O2) and boiling for 30 min. Working solutions were prepared by daily dilution. Crystal surfaces and crystal chemical composition were also characterized by image analysis coupled with SEM (mapping), and X-rays fluorescence ¨ which use spectrometer XRF: SRS 3000 S apparatus (Bruker), and the computer program Spectra Plus evaluation. X-ray photoelectron spectroscopy (XPS) was performed by means of Physical Electronics 5500 photoelectron spectrometer. All spectra were collected using Mg Ka X-ray operating at 300 W. The binding energy scale was calibrated by setting the main component of the C1s peak at 284.6 eV. Finally, electrical conductivity measurements were performed using a four-probe method and homemade software from Labview. Each measurement took 16 h and was done in the temperature range 77–300 K. Electrical connections to the crystal (cut into a rectangular shape) were made by means of four parallel copper wires laid across the basal surface of the crystal and attached to the crystal surface by means of conducting silver paint. The wires near each end of the rectangular crystal acted as current leads, while the two contact wires on either side of the central line were used to measure the potential drop across the crystal. The resistivity (r) measured is related to the characteristics of the sample by the relation:
r¼
V S n I d
ð1Þ
where V is the potential, d is the distance between two contacts, S is the surface of the section crossed by the current and I is the current intensity.
3. Results and discussion 3.1. Characterization and comparative study of single crystals grown by TeCl4 The growth of single crystals from both protocol I and protocol II polycrystalline powders has been successful in the chosen temperature gradient (1020–980 1C) in a furnace with three temperature zones. The transport over 120 h with a transport agent concentration of 3 g L 1 leads to single crystals growth in the cold or growth zone (TG ¼980 1C). There was sometimes crystal formation in the intermediate zone or even in the source zone (TS ¼1020 1C) (Fig. 1A). All single crystals are well developed, with surfaces usually between 35–100 mm2, and have a strong metallic shine (Fig. 1B). Various photographs obtained with optical microscopy or scanning electron microscopy (Fig. 2) clearly show the growth of lamellar crystals with hexagonal structure and the growth by layering sheets. Scanning electron microscopy was employed to compare the morphologies of single crystals grown from slowly cooled powders (protocol II, Fig. 2A,B) and those cooled by air-hardening (protocol I, Fig. 2C,D). Several cleaved and uncleaved single crystals were studied. On the whole, it appears that single crystals from protocol II have surfaces with very few irregularities and cavities (Fig. 2A), especially when exposing a freshly cleaved surface (Fig. 2B). However, their counterparts obtained from protocol I show many irregularities and bright areas before cleavage (Fig. 2C) and, even cleaved, often
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present irregularities (Fig. 2D). EDX microanalyses on these irregularities and bright areas nevertheless show that these are made of molybdenum and selenium, not of impurities. SEM images obtained by Legma et al. [19] on uncleaved single crystals of layered WSe2 single crystals showed similar morphologies. The comparison of photographs seems also to show a well ordered crystal growth in single crystals from protocol II as compared to single crystals from protocol I. The presence of more defects on the crystallites from protocol I powders than those from protocol II powders is due to the quality of the polycrystalline powders used for
Fig. 1. (A) Transport tube showing the growth of single crystals from protocol II in source area (TG ¼ 1020 1C), intermediate area and growth area (TS ¼980 1C) and (B) pictures of MoSe2 single crystals transported by TeCl4 surfaces ranging 35–100 mm2.
the growth. The polycrystalline powders of protocol II would significantly reduce such types of defects than those of protocol I. The growth of single crystals from polycrystalline powders with many defects would have the effect of inducing additional defects. The comparison of powders from protocol I and protocol II (Fig. 3) and of the corresponding single crystals (Fig. 2) shows a close similarity between the surface morphologies of single crystals and of the crystallites of polycrystalline powders used to grow them. This means that the polycrystalline powders quality strongly influences the crystal morphology, in agreement with the works of Klein et al. [15] who suggested that the properties of single crystals are strongly influenced by the quality of the raw materials used for the growth. EDX microanalyses allow to determine the actual stoichiometry of the crystals. A ratio Se/Mo between 1.98 and 2.06 was obtained through analyses conducted here and there or on the whole on some random areas. These results were confirmed by ICP analysis which gave an average ratio Se/Mo ¼2.02 for the same crystals. The single crystals are thus very close to stoichiometry. However, single crystals grown from protocol I polycrystalline powders give a ratio of Se/Mo ¼2.21 by ICP analysis. It is a clear evidence that single crystals prepared from protocol II polycrystalline powders are closer to stoichiometry than their counterparts grown from protocol I polycrystalline powders. According to the studies of Kline et al. [23] on transition metal dichalcogenides single crystals by microprobe technique, deviations from stoichiometry lead to significant variations in the electronic properties of these materials. The best results are obtained with single crystals closer to stoichiometry. In partial conclusion, it seems that protocol II polycrystalline powders are better suited for crystal growth than those of protocol
Fig. 2. SEM photographs of MoSe2 single crystals grown by TeCl4: (A) uncleaved surface from protocol II polycrystalline powders showing the growth of lamellar crystals in hexagonal structure and the growth by layering sheets; (B) cleaved surface from protocol II polycrystalline powders; (C) uncleaved surface from protocol I polycrystalline powders; and (D) cleaved surface from protocol I polycrystallines powders.
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Fig. 3. SEM photographs of MoSe2 polycrystalline powders: (A) protocol II polycrystalline powder; and (B) protocol I polycrystalline powder.
Fig. 4. (A) SEM mapping of MoSe2 single crystals (from protocol II) grown by TeCl4 uncleaved surface. (B) Mo, Si; (C) Se, Si; (D) Se, Mo; (E) Si, Cl; (F) Mo, Se, Si, Cl. The presence of silicon (blue) at the surface is clearly evidenced in panels B, C, E and F. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
I, as inferred by comparison of the morphologies and their stoichiometries of the single crystals obtained from the two kinds of powders.
3.2. Surface analyses During the transport, a systematic attack of the silica tube is observed with both transport agents [19] and it is thus a matter of concern whether silicon might enter in the final composition of the sample. Additional characterizations were thus conducted to check the possible presence of silicon in the prepared compounds. An image analysis coupled with SEM (mapping) study performed on non-cleaved surfaces confirms the attack of the tube by revealing the presence of silicon at the surface of certain samples (Fig. 4). Examination of the maps shows that silicon (Si) is deposited locally on the surface as large grains. It seems not to react with one or the other element present (Se, Mo, Cl). A simple cleavage is enough to eliminate it. As for chlorine, it is spread over the entire sample surface. Analyses by X-ray fluorescence–EDX microanalysis on non-cleaved single crystals show that for most samples, the surface contains silicon impurities, up to 10% for certain samples. They also show that the surface of non-cleaved single crystals contains many of the elements (Mo, Se, Cl, Si, O) used in the transport process (Fig. 5A). By contrast, EDX microanalyses (Fig. 5B) and image analyses coupled with SEM show that cleaved single crystals contain neither silicon nor chlorine and tellurium. These elements were not detected by ICP measurements conducted samples freshly cleaved and dissolved. TeCl4 do
not contaminate single crystals during crystal growth, or at very low levels, undetectable with the employed methods. Photoelectron spectroscopy (XPS) which is more sensitive than the methods previously used was considered to characterize the surface of cleaved single crystals. Fig. 6 presents a typical XPS spectrum obtained with a cleaved surface. Molybdenum and selenium peaks are observed at binding energy positions in the spectra consistent with those found in the literature [24,25]. The characterizations showed that silicon does not contaminate single crystals during the chemical growth process, because its characteristic peak (Si2p) which normally would appear at 103.4 eV (in SiO2) [25] was not revealed in any spectrum. Tellurium and chlorine characteristic peaks appear at binding energies of 572.7 eV and 200 eV, respectively. These regions are enlarged in the insets of Fig. 6, and it can be seen that none of these two elements could be detected, confirming that they do not contaminate the single crystals during chemical growth. XPS analysis also shows that cleaved surfaces are homogeneous and perfectly clean. 3.3. Electrical conductivity measurements The electrical conductivity was measured on freshly cleaved single crystals. Fig. 7 presents the conductivity curves Logs ¼ f 1=T , from which the the thermal activation energy was calculated according to the law [17,24]: e ð2Þ s ¼ s0 exp kT
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Fig. 7. Temperature dependence of the electrical conductivity for MoSe2 single crystals from protocol II, grown by TeCl4.
to 38 meV. The comparison of these results with the performances of single crystals grown with protocol I polycrystalline powders shows that single crystals grown with protocol II polycrystalline powders have better charge transport properties, since in the former case, a value of 1.4 10 3 O 1 cm 1 was obtained [26]. Single crystals from protocol II also compare favorably with other studies on MoSe2 single crystals, for which conductivity values between 0.25 O 1 cm 1 [27] and 1.08 O 1 cm 1 [22] were reported.
4. Conclusions
Fig. 5. EDX microanalysis spectrum of MoSe2 single crystal from protocol II polycrystalline powders grown by TeCl4: (A) uncleaved surface and (B) cleaved surface.
(1) Over this study, the growth conditions allowed us getting fairly massive single crystals (35–100 mm2) which may be used as electrodes in photoelectrochemical cells. (2) SEM, EDX microanalysis, optical microscopy and photoelectron spectroscopy (XPS) showed that cleaved crystals are very clean and homogeneous. Analyses by ICP showed that they are almost stoichiometric with a ratio Se/Mo¼2.02. Electrical conductivity measurements have confirmed the semiconducting character of single crystals. Moreover, these measures also showed that TeCl4 is the best transport agent in single crystal growth. (3) The comparative study with previously studied single crystals, i.e. those obtained by crystal growth from polycrystalline powders cooled by air-quenching (protocol I) shows that those prepared from slowly cooled polycrystalline powders (protocol II) give better parameters, notably electrical conductivity, surface morphology and stoichiometry. (4) The electrical conductivity, surface morphology, surface homogeneity and stoichiometry of single crystals seem highly dependent on the quality of polycrystalline powders used for the growth. Their properties seem also strongly influenced by polycrystalline powders preparation. However, photoconversion studies on these single crystals will allow a better assessment. These studies are under investigation in our laboratory.
Fig. 6. XPS spectrum of a cleaved MoSe2 single crystal from protocol II polycrystalline powders grown by TeCl4. Insets present enlarged areas of the spectrum around the characteristic binding energies for Te (573 eV) and Cl (200 eV).
where s is the conductivity, s0 a parameter which depends on the sample characteristics (thickness, structure), e the thermal activation energy, T the absolute temperature and k the Boltzmann constant. The value of electrical conductivity calculated at room temperature for MoSe2 is 1.61 O 1 cm 1,and the activation energy amounts
Acknowledgments This work was supported by the Commission Universitaire pour le De´veloppement (CUD)-Communaute´ Francaise de Belgique. Their financial support is gratefully acknowledged. Moussa Bougouma would like to thank the CUD-CIUF for receiving Fellowships to stay
M. Bougouma et al. / Journal of Crystal Growth 363 (2013) 122–127
at the Universite´ Libre de Bruxelles within the framework of the International cooperation project between the Communaute´ francaise de Belgique and the University of Ouagadougou.
[13] [14]
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Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of large, high quality MoSe2 single crystals Moussa Bougouma a,b,c,n, Abdelkrim Batan b, Boubie´ Guel a, Tiriana Segato c, Jean B. Legma a, Francois Reniers b, Marie-Paule Delplancke-Ogletree c, Claudine Buess-Herman b, Thomas Doneux b,nn a
Laboratoire de Chimie-Physique et d’Electrochimie, UFR/SEA, Universite´ de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso Chimie Analytique et Chimie des Interfaces, Faculte´ des Sciences, Universite´ Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium c 4 MAT, Ecole Polytechnique de Bruxelles, Universite´ Libre de Bruxelles, Avenue F. D. Roosevelt 50, CP 165/63, B-1050 Bruxelles, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2012 Received in revised form 9 October 2012 Accepted 10 October 2012 Communicated by K.W. Benz Available online 17 October 2012
MoSe2 single crystals were grown by chemical vapor transport using TeCl4 as transport agents in the temperature gradient 1020–980 1C. They were characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The characterizations showed that single crystals are perfectly homogeneous, stoichiometric and have very few defects and clean surfaces with areas in the range of 35–100 mm2. Single crystals grown by TeCl4 showed a high electrical conductivity. Their properties were highly dependent on the quality of the polycrystalline powders used for the growth. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Impurities A2. Single crystal growth B1. Transition metal dichalcogenides B2. Semiconducting materials B3. Solar cells
1. Introduction Molybdenum diselenide (MoSe2) is a layered semiconductor compound belonging to the transition metal dichalcogenide family MX2 (M¼Mo, W, X ¼S, Se), which crystallizes in the 2H-MoS2 structures. Because of its great potentialities for solar energy conversion, MoSe2 has been the subject of many investigations [1], which have shown that surface defects play a major role in the corrosion and photocorrosion of the material [2,3]. Consequently, some strategies have been devoted to surface treatment of this material in order to suppress the undesired surface limitations or surface defects as much as possible [2,3]. Unfortunately, the improvements observed with such processes vanish under prolonged illumination. Thus the best way to increase the photoconversion efficiency is the growth of single crystals with surfaces free of defects, as much as possible, and that is the aim of this work. Even though the difficulty to obtain large surfaces of MX2 single crystals by chemical vapor transport method has led some researchers to engage in
n Corresponding author at: Chimie Analytique et Chimie des Interfaces, Faculte´ des Sciences, Universite´ Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium. Tel.: þ226 70325175. nn Corresponding author. Tel.: þ 32 2 650 35 80; fax: þ 32 2 650 29 34. E-mail addresses: [email protected] (M. Bougouma), [email protected] (T. Doneux).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.10.026
electrocrystallization of binary [4–6], mixed/alloyed and ternary [7,8], semiconducting compounds thin films on conductive support, studies on MX2 single crystals remain valuable references for improving the performance of this type of conductive substrate/ thin film/electrolyte junction. Another major interest of transition metal dichalcogenides crystals resides in their layered structure. They have attracted recently a considerable attention as starting materials for the isolation of two-dimensional objects consisting of free standing crystals made of a single or a few atomic layers, in strong analogy with the graphene layer(s) obtained from graphite [9–11]. As low dimensional compounds, they exhibit unique electrical, mechanical and optical properties [10,12,13]. These 2D crystals are obtained by exfoliation or mechanical cleavage of the 3D parent compound [9–11,14]. Improving the crystalline quality of this latter will result in 2D systems of larger lateral size, reaching the macroscopic scale, and there is therefore a direct interest in the synthesis of single crystals of large areas and high quality. This work describes the growth and characterization of perfectly crystallized single crystals that may have fewer defects. Indeed, there is good evidence that the quality of the polycrystalline powders used as the starting material for the preparation of single crystals has a significant impact on the quality and properties of these single crystals [15]. The reproducible syntheses of various powders with homogeneous, well crystallized structure and well-defined microstructure have been the subject of a previous study [16]. Two types of
M. Bougouma et al. / Journal of Crystal Growth 363 (2013) 122–127
polycrystalline powders were synthesized using two different experimental protocols (protocol I and protocol II). Single crystals obtained by chemical vapor transport from protocol I polycrystalline powders have been extensively characterized in our previous works [17–20]. Studies showed that TeCl4 and SeCl4 are the best transport agents. Photocurrent gain was 390 A m 2 for single crystal grown by SeCl4 and 440 Am 2 for single crystals grown by TeCl4. Our goal is to increase this value by improving the single crystals quality. So, single crystals were grown from protocol II polycrystalline powders using TeCl4 as transport agents. The single crystals have been characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The results are compared with those of previously made single crystals from protocol I. It appears from the comparative study that the present single crystals are much better than those previously studied. The impact of polycrystalline powders on single crystals properties is discussed.
2. Experimental details 2.1. Synthesis of polycrystalline powders Polycrystalline powders have been synthesized from elements in powder (Mo: Aldrich Chemical Company, 3N) or granular forms (Se: Aldrich Chemical Company, 4N), using silica glass tubes sealed under secondary vacuum of 10 4 Pa. The tubes were carefully degreased with Ajaxs, etched (HF, 10%) and dried before introducing the reactants. After sealing, the ampoule (sealed tube) was set in an isothermal zone furnace (CARBOLYTE type 201). During synthesis (protocol II), the temperature was increased progressively up to 750 1C in 48 h, in order to avoid all explosion risks due to rapid selenium vaporization. Then, the system has been kept at a constant temperature (750 1C) for 120 h, then slowly cooled down to room temperature in 12 h. The products were recovered after opening the ampoule. After fine milling, samples were reintroduced in a new tube and sealed under secondary vacuum and were annealed at 1050 1C for up to 168 h, and then slowly cooled in the furnace to room temperature. The characterizations of polycrystalline powders obtained by different methods showed that they are homogeneous and well crystallized [16]. Results are consistent with those found in the literature [21]. As a brief recall of protocol I [16], the samples were heated to 600 1C over 48 h in steps to avoid explosions due to sudden selenium vaporization. They were kept at this temperature for up to 72 h, and then air-quenched. After recovery and fine milling, samples were reintroduced in new tube and sealed under secondary vacuum, then annealed at 1000 1C for 96–120 h and slowly cooled in the furnace to room temperature [16]. 2.2. Crystals growth conditions and characterization methods The synthesized polycrystalline powder is introduced in the bottom of transport tube, carefully avoiding tube walls soiling which would act as undesired nucleation spots. The mass of polycrystalline product weighed does not exceed 1 g. Once the transport agent has been added to the polycrystalline powder, the transport tube is then immediately connected to the high vacuum pump system, and sealed after reaching a secondary vacuum of 10 4 Pa. The extremity of the ampoule containing the product is placed in the source zone temperature (TS ¼1020 1C) and the other end in the growth zone temperature (TG ¼980 1C) [19,20,22] of a three zone isothermal furnace (LENTON Thermal Designs Ltd.) for five days.
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Scanning electron microscopy (SEM JEOL JSM-6100) and optical microscopy (JENA POL) which use computer program Motic Images Plus V2.0ML were used for morphological characterization of single crystals. Se/Mo ratios of crystals were analyzed by Inductively Coupled Plasma (ICP) using Vista ICP-OES spectrometer and by energy dispersive analysis of X-ray coupled with SEM (EDAX-SEM). For ICP analysis, Milli-Q water (Millipore) was used in the preparation of all solutions. MoSe2 solutions were prepared by dissolving 0.3 g of MoSe2 crystals in 100 ml of hydrogen peroxide (H2O2) and boiling for 30 min. Working solutions were prepared by daily dilution. Crystal surfaces and crystal chemical composition were also characterized by image analysis coupled with SEM (mapping), and X-rays fluorescence ¨ which use spectrometer XRF: SRS 3000 S apparatus (Bruker), and the computer program Spectra Plus evaluation. X-ray photoelectron spectroscopy (XPS) was performed by means of Physical Electronics 5500 photoelectron spectrometer. All spectra were collected using Mg Ka X-ray operating at 300 W. The binding energy scale was calibrated by setting the main component of the C1s peak at 284.6 eV. Finally, electrical conductivity measurements were performed using a four-probe method and homemade software from Labview. Each measurement took 16 h and was done in the temperature range 77–300 K. Electrical connections to the crystal (cut into a rectangular shape) were made by means of four parallel copper wires laid across the basal surface of the crystal and attached to the crystal surface by means of conducting silver paint. The wires near each end of the rectangular crystal acted as current leads, while the two contact wires on either side of the central line were used to measure the potential drop across the crystal. The resistivity (r) measured is related to the characteristics of the sample by the relation:
r¼
V S n I d
ð1Þ
where V is the potential, d is the distance between two contacts, S is the surface of the section crossed by the current and I is the current intensity.
3. Results and discussion 3.1. Characterization and comparative study of single crystals grown by TeCl4 The growth of single crystals from both protocol I and protocol II polycrystalline powders has been successful in the chosen temperature gradient (1020–980 1C) in a furnace with three temperature zones. The transport over 120 h with a transport agent concentration of 3 g L 1 leads to single crystals growth in the cold or growth zone (TG ¼980 1C). There was sometimes crystal formation in the intermediate zone or even in the source zone (TS ¼1020 1C) (Fig. 1A). All single crystals are well developed, with surfaces usually between 35–100 mm2, and have a strong metallic shine (Fig. 1B). Various photographs obtained with optical microscopy or scanning electron microscopy (Fig. 2) clearly show the growth of lamellar crystals with hexagonal structure and the growth by layering sheets. Scanning electron microscopy was employed to compare the morphologies of single crystals grown from slowly cooled powders (protocol II, Fig. 2A,B) and those cooled by air-hardening (protocol I, Fig. 2C,D). Several cleaved and uncleaved single crystals were studied. On the whole, it appears that single crystals from protocol II have surfaces with very few irregularities and cavities (Fig. 2A), especially when exposing a freshly cleaved surface (Fig. 2B). However, their counterparts obtained from protocol I show many irregularities and bright areas before cleavage (Fig. 2C) and, even cleaved, often
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present irregularities (Fig. 2D). EDX microanalyses on these irregularities and bright areas nevertheless show that these are made of molybdenum and selenium, not of impurities. SEM images obtained by Legma et al. [19] on uncleaved single crystals of layered WSe2 single crystals showed similar morphologies. The comparison of photographs seems also to show a well ordered crystal growth in single crystals from protocol II as compared to single crystals from protocol I. The presence of more defects on the crystallites from protocol I powders than those from protocol II powders is due to the quality of the polycrystalline powders used for
Fig. 1. (A) Transport tube showing the growth of single crystals from protocol II in source area (TG ¼ 1020 1C), intermediate area and growth area (TS ¼980 1C) and (B) pictures of MoSe2 single crystals transported by TeCl4 surfaces ranging 35–100 mm2.
the growth. The polycrystalline powders of protocol II would significantly reduce such types of defects than those of protocol I. The growth of single crystals from polycrystalline powders with many defects would have the effect of inducing additional defects. The comparison of powders from protocol I and protocol II (Fig. 3) and of the corresponding single crystals (Fig. 2) shows a close similarity between the surface morphologies of single crystals and of the crystallites of polycrystalline powders used to grow them. This means that the polycrystalline powders quality strongly influences the crystal morphology, in agreement with the works of Klein et al. [15] who suggested that the properties of single crystals are strongly influenced by the quality of the raw materials used for the growth. EDX microanalyses allow to determine the actual stoichiometry of the crystals. A ratio Se/Mo between 1.98 and 2.06 was obtained through analyses conducted here and there or on the whole on some random areas. These results were confirmed by ICP analysis which gave an average ratio Se/Mo ¼2.02 for the same crystals. The single crystals are thus very close to stoichiometry. However, single crystals grown from protocol I polycrystalline powders give a ratio of Se/Mo ¼2.21 by ICP analysis. It is a clear evidence that single crystals prepared from protocol II polycrystalline powders are closer to stoichiometry than their counterparts grown from protocol I polycrystalline powders. According to the studies of Kline et al. [23] on transition metal dichalcogenides single crystals by microprobe technique, deviations from stoichiometry lead to significant variations in the electronic properties of these materials. The best results are obtained with single crystals closer to stoichiometry. In partial conclusion, it seems that protocol II polycrystalline powders are better suited for crystal growth than those of protocol
Fig. 2. SEM photographs of MoSe2 single crystals grown by TeCl4: (A) uncleaved surface from protocol II polycrystalline powders showing the growth of lamellar crystals in hexagonal structure and the growth by layering sheets; (B) cleaved surface from protocol II polycrystalline powders; (C) uncleaved surface from protocol I polycrystalline powders; and (D) cleaved surface from protocol I polycrystallines powders.
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Fig. 3. SEM photographs of MoSe2 polycrystalline powders: (A) protocol II polycrystalline powder; and (B) protocol I polycrystalline powder.
Fig. 4. (A) SEM mapping of MoSe2 single crystals (from protocol II) grown by TeCl4 uncleaved surface. (B) Mo, Si; (C) Se, Si; (D) Se, Mo; (E) Si, Cl; (F) Mo, Se, Si, Cl. The presence of silicon (blue) at the surface is clearly evidenced in panels B, C, E and F. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
I, as inferred by comparison of the morphologies and their stoichiometries of the single crystals obtained from the two kinds of powders.
3.2. Surface analyses During the transport, a systematic attack of the silica tube is observed with both transport agents [19] and it is thus a matter of concern whether silicon might enter in the final composition of the sample. Additional characterizations were thus conducted to check the possible presence of silicon in the prepared compounds. An image analysis coupled with SEM (mapping) study performed on non-cleaved surfaces confirms the attack of the tube by revealing the presence of silicon at the surface of certain samples (Fig. 4). Examination of the maps shows that silicon (Si) is deposited locally on the surface as large grains. It seems not to react with one or the other element present (Se, Mo, Cl). A simple cleavage is enough to eliminate it. As for chlorine, it is spread over the entire sample surface. Analyses by X-ray fluorescence–EDX microanalysis on non-cleaved single crystals show that for most samples, the surface contains silicon impurities, up to 10% for certain samples. They also show that the surface of non-cleaved single crystals contains many of the elements (Mo, Se, Cl, Si, O) used in the transport process (Fig. 5A). By contrast, EDX microanalyses (Fig. 5B) and image analyses coupled with SEM show that cleaved single crystals contain neither silicon nor chlorine and tellurium. These elements were not detected by ICP measurements conducted samples freshly cleaved and dissolved. TeCl4 do
not contaminate single crystals during crystal growth, or at very low levels, undetectable with the employed methods. Photoelectron spectroscopy (XPS) which is more sensitive than the methods previously used was considered to characterize the surface of cleaved single crystals. Fig. 6 presents a typical XPS spectrum obtained with a cleaved surface. Molybdenum and selenium peaks are observed at binding energy positions in the spectra consistent with those found in the literature [24,25]. The characterizations showed that silicon does not contaminate single crystals during the chemical growth process, because its characteristic peak (Si2p) which normally would appear at 103.4 eV (in SiO2) [25] was not revealed in any spectrum. Tellurium and chlorine characteristic peaks appear at binding energies of 572.7 eV and 200 eV, respectively. These regions are enlarged in the insets of Fig. 6, and it can be seen that none of these two elements could be detected, confirming that they do not contaminate the single crystals during chemical growth. XPS analysis also shows that cleaved surfaces are homogeneous and perfectly clean. 3.3. Electrical conductivity measurements The electrical conductivity was measured on freshly cleaved single crystals. Fig. 7 presents the conductivity curves Logs ¼ f 1=T , from which the the thermal activation energy was calculated according to the law [17,24]: e ð2Þ s ¼ s0 exp kT
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Fig. 7. Temperature dependence of the electrical conductivity for MoSe2 single crystals from protocol II, grown by TeCl4.
to 38 meV. The comparison of these results with the performances of single crystals grown with protocol I polycrystalline powders shows that single crystals grown with protocol II polycrystalline powders have better charge transport properties, since in the former case, a value of 1.4 10 3 O 1 cm 1 was obtained [26]. Single crystals from protocol II also compare favorably with other studies on MoSe2 single crystals, for which conductivity values between 0.25 O 1 cm 1 [27] and 1.08 O 1 cm 1 [22] were reported.
4. Conclusions
Fig. 5. EDX microanalysis spectrum of MoSe2 single crystal from protocol II polycrystalline powders grown by TeCl4: (A) uncleaved surface and (B) cleaved surface.
(1) Over this study, the growth conditions allowed us getting fairly massive single crystals (35–100 mm2) which may be used as electrodes in photoelectrochemical cells. (2) SEM, EDX microanalysis, optical microscopy and photoelectron spectroscopy (XPS) showed that cleaved crystals are very clean and homogeneous. Analyses by ICP showed that they are almost stoichiometric with a ratio Se/Mo¼2.02. Electrical conductivity measurements have confirmed the semiconducting character of single crystals. Moreover, these measures also showed that TeCl4 is the best transport agent in single crystal growth. (3) The comparative study with previously studied single crystals, i.e. those obtained by crystal growth from polycrystalline powders cooled by air-quenching (protocol I) shows that those prepared from slowly cooled polycrystalline powders (protocol II) give better parameters, notably electrical conductivity, surface morphology and stoichiometry. (4) The electrical conductivity, surface morphology, surface homogeneity and stoichiometry of single crystals seem highly dependent on the quality of polycrystalline powders used for the growth. Their properties seem also strongly influenced by polycrystalline powders preparation. However, photoconversion studies on these single crystals will allow a better assessment. These studies are under investigation in our laboratory.
Fig. 6. XPS spectrum of a cleaved MoSe2 single crystal from protocol II polycrystalline powders grown by TeCl4. Insets present enlarged areas of the spectrum around the characteristic binding energies for Te (573 eV) and Cl (200 eV).
where s is the conductivity, s0 a parameter which depends on the sample characteristics (thickness, structure), e the thermal activation energy, T the absolute temperature and k the Boltzmann constant. The value of electrical conductivity calculated at room temperature for MoSe2 is 1.61 O 1 cm 1,and the activation energy amounts
Acknowledgments This work was supported by the Commission Universitaire pour le De´veloppement (CUD)-Communaute´ Francaise de Belgique. Their financial support is gratefully acknowledged. Moussa Bougouma would like to thank the CUD-CIUF for receiving Fellowships to stay
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at the Universite´ Libre de Bruxelles within the framework of the International cooperation project between the Communaute´ francaise de Belgique and the University of Ouagadougou.
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