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Morphology evolution of MoS2 nanorods grown by chemical vapor deposition
Journal of Crystal Growth 430 (2015) 1–6
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Morphology evolution of MoS2 nanorods grown by chemical vapor deposition Shuming Han a, Xingfang Luo a, Yingjie Cao a, Cailei Yuan a,n, Yong Yang a, Qinliang Li a, Ting Yu a,n, Shuangli Ye b a Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Electronics and Communication, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b School of Printing and Packaging, Wuhan University, Wuhan 430072, Hubei, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 May 2015 Received in revised form 29 June 2015 Accepted 12 August 2015 Communicated by: J.M. Redwing Available online 20 August 2015
We observed the regular morphology evolution of chemical vapor deposition (CVD) grown MoS2 nanorods along the gas flow direction on the SiO2/Si substrate. It can be attributed to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. The MoS2 domains experience a regular morphology transformation as well as a size change as the deposition location moves closer to the MoO3 powder. It paves the way for the development of CVD method with controlled growth parameters and opens up new venues for the tunable morphology evolution of MoS2 nanorods. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A1. Low dimensional structures A2. Growth from vapor A2. Chemical vapor deposition processes
1. Introduction As the shape and size of the nanostructures can greatly influence their physical and chemical properties [1,2], the rational design and control of the architecture and size of nanostructures has been one of the hottest topics in material science for the past few years. On the other hand, one dimensional nanostructure such as nanorods, nanowires, nanobelts and nanotubes have gained increasing attention because of its potential as the building blocks for varied industrial applications [3]. Therefore, it is necessary to control the growth of one dimensional nanostructure, since the nanomaterials with well controlled size, morphology and chemical composition may reveal more opportunities in exploring new and enhanced properties [4]. Recently, MoS2 has received much attention in many fields including electrochemical devices, hydrogen storage, catalysis, capacitors, solid lubricant, and intercalation host [5–10]. The interest in exploration of generating nanoscale MoS2 with specific morphologies and unique properties by using different approaches keeps increasing. Subsequently, many preparation methods for MoS2 nanostructures have been explored to produce nanorods,
n
Corresponding authors. Tel./fax: þ86 791 88120370. E-mail addresses: [email protected] (C. Yuan), [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jcrysgro.2015.08.005 0022-0248/& 2015 Elsevier B.V. All rights reserved.
e.g., the sonication leading to MoS2 nanorod formation [11], synthesizing nanotubes and nanorods of MoS2 by utilizing a hydrothermal method [12], a redox rout for synthesis of MoS2 nanorods using (NH4)6Mo7O24 4H2O and Na2S 9H2O as precursors at 90 1C by Wang et al. [13]. Although many kinds of methods have been developed to synthesis MoS2 nanorods, manipulating the morphology and size of MoS2 nanorods is still a challenge. In this paper, a simple and precise method to synthesize MoS2 nanorods with different thickness and length was proposed. We observed the regular morphology evolution of chemical vapor deposition (CVD) grown MoS2 nanorods along the gas flow direction on the SiO2/Si substrate. It can be attributed to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. As results, MoS2 nanorods with different thickness and length were successfully synthesized.
2. Experimental The MoS2 nanorods were grown by a CVD method [14] with solid MoO3 and sulfur precursors, as shown in Fig. 1. Briefly, a two-zone tube furnace was used to provide accurate temperature control of MoO3 and sulfur powders separately. An alumina boat with 100 mg sulfur (99.9% purity) was placed upstream in the low-temperature zone. Another alumina boat containing 20 mg (99.9% purity) of
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Fig. 1. Schematic illustration of the MoS2 nanorods CVD system.
MoO3 was placed downstream in the high-temperature zone. In order to investigate the dependence of the morphology evolution of MoS2 on the distance between the MoO3 precursor and the substrates, four pieces of Si wafer with 300 nm SiO2 layer substrates were aligned and placed face down to the MoO3 powder along the gas flow direction, which were labeled as S1, S2, S3 and S4, respectively. The S1 sample locates at the farthest position away
Fig. 2. (a) Optical microscope image MoS2 nanoparticles deposited on SiO2/Si substrate in sample S1; (b) A typical AFM image MoS2 nanoparticle in sample S1; (c) Height profile of MoS2 nanoparticle in the sample S1; (d) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S2; (e) A typical AFM image MoS2 nanorod in sample S2; (f) Height profile of MoS2 nanorod in sample S2.
S. Han et al. / Journal of Crystal Growth 430 (2015) 1–6
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Fig. 3. (a) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S3; (b) and (c) A typical AFM image MoS2 nanorod in sample S3; (d) Height profile of MoS2 nanorod in sample S3.
from the MoO3 powder, while the S4 sample locates at the closest position of the MoO3 powder. The distance between adjacent substrate is 1 cm and the size of the substrate is 1 cm. The distance between the two alumina boats is 17 cm. The temperature of the sulfur and the substrate was increased concurrently to 200 1C and 750 1C, respectively. The deposition process was conducted by evaporating sulfur and MoO3 powders simultaneously in an Ar environment with a flow rate of 30 sccm. After 5 min, the furnace was cooled down naturally to room temperature. The optical microscope image was performed using a optical microscope with an 50 objective lens adapted to a video camera and connected to a computer. The sample structure was examined by using a a Rigaku x-ray diffractometer (XRD) with Cu Kα radiation. The morphology of MoS2 nanorods were measured by using Park system atomic force microscopy (AFM). Raman spectra were collected in a Horiba Jobin Yvon Raman microscopic system. The solid-state excitation laser has a wavelength of 532 nm. A 100 objective was used to focus the laser beam. The photoluminescence (PL) measurements were also performed with the same laser by using the PL mode of the Raman microscopic system.
3. Results and discussion Fig. 2(a) shows the optical images of sample S1, which locates at the farthest position away from the MoO3 powder. It can be clearly seen that there are lots of MoS2 nanoparticles with irregular shapes on the SiO2/Si substrate. The size of MoS2 nanoparticles is about
300 nm. It should be noted that there are some relatively very short rod-like structures MoS2 nanoparticles on the SiO2/Si substrate, which indicated the early stages of MoS2 nanorods growth. Fig. 2 (b) shows the planar view of the AFM image of a MoS2 nanoparticle in the sample S1. The height profile of the MoS2 nanoparticle is shown in Fig. 2(c). The MoS2 nanparticle in the sample S1 has thickness of about 10 nm and width of about 328 nm. Fig. 2 (d) presents the optical image of sample S2, which is closer to the MoO3 powder than the sample S1. Some short MoS2 nanorods have been successfully fabricated on SiO2/Si substrate. The morphology of MoS2 nanorod is checked with AFM shown in Fig. 2(e). Obviously, a well-defined rod-like structure can be observed. The height profile of this MoS2 nanorod is shown in Fig. 2(f). From the height profile, it can be found that the MoS2 nanorod has thickness of about 98 nm and width of about 680 nm. In this paper, the MoS2 nanorods were synthesized in high S sufficient atmosphere. The higher partial pressure of gaseous S greatly increases the volatilization speed of MoO3, forming a large amount of gaseous intermediate MoO3-x in a short time [15]. Therefore, the high S sufficient atmosphere may promote the mass transfer process, which contributes to the enhancement in the crystal growth. Under the S sufficient atmosphere condition, the MoS2 crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones, which is typical for nanocrystals with high precursor feedstock. Therefore, the increase in precursor concentration has an effect of enhancing the crystal growth, resulting in the formation of MoS2 nanorods, which are not favorable for the production of highquality two-dimensional crystal [16]. Moreover, the main reason for
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Fig. 4. (a) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S4; (b) and (c) A typical AFM image MoS2 nanorod in sample S4; (d) Height profile of MoS2 nanorod in sample S4.
the morphology evolution phenomenon between S1 and S2 can be attributed to the change in the MoO3 concentration along the SiO2/ Si substrate surface. Because the distance between the substrates and the MoO3 is very small, there can be an obvious concentration gradient on the surface of substrates. Along the gas flow direction, the MoO3 concentration decreases as the substrate moves farther from the MoO3 powder, which influences the average growing rate of the MoS2 crystals. As the growing rate is slowest, the sample S1 can be considered at the early stages of MoS2 crystal growth. Therefore, many MoS2 nanoparticles with the size of about 300 nm are obtained on the first SiO2/Si substrate because of the initial nucleation process. Some relatively very short rod-like structures MoS2 nanoparticles also indicated the early stages of the MoS2 nanorods growth. When the deposition location moves closer to the MoO3 powder, MoO3 concentration increases, and thus the growing rate of the MoS2 crystals also increases. Some short MoS2 nanorods appear on the second SiO2/Si substrate. This can be attributed that the MoS2 nanoparticles start to assemble together and spontaneously aggregate into rod-like MoS2 structures in order to reduce the high surface energy through the process known as oriented aggregation [17]. Figs. 3(a) and 4(a) show the optical images of samples S3 and S4. It can be found that, with the substrate location moves closer to the MoO3 powder, the length of MoS2 nanorods becomes longer, and the crystal size also experiences a regular change. Figs. 3(b) and 4 (b) show the planar view of the AFM images of MoS2 rods in the samples S3 and S4. Figs. 3(c) and 4(c) show cross sectional AFM
images of MoS2 rods in the samples S3 and S4. The height profile of MoS2 nanorods in the samples S3 and S4 is shown in Figs. 3(d) and 4(d). The MoS2 nanorod in the sample S3 has thickness of about 348–365 nm and width of about 1.17 mm. While, the thickness and width of the MoS2 nanorod in the sample S4 increase to about 400– 502 nm and 1.525 μm, respectively. Obviously, with the decrease in the distance between the precursor and the growth location, MoS2 domains experience a regular morphology transformation as well as a size change. The length and size of MoS2 nanorods gradually increase as the deposition location moves closer to the MoO3 powder. As discussed previously, this may relate to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. The increase in precursor concentration has an effect of enhancing the crystal growth rate. Therefore, with the deposition sample moves closer to the MoO3 powder, well-defined MoS2 nanorods with smooth surface are formed from the scabbled MoS2 rod structures through an Ostwald ripening process [18]. The MoS2 nanorods in the sample S4 are further examined by Xray diffraction patterns, Raman and PL spectra. Fig. 5(a) shows the XRD pattern of sample S4 with MoS2 nanorods deposited on SiO2/Si substrate. All the diffraction peaks in the pattern can be indexed as MoS2 (ICCD Card no: 77-1716). No diffraction peaks from impurities are observed in the XRD pattern. Raman spectroscopy has been wide used to study two-dimensional materials and to identify their thickness [19–22]. The Raman spectrum of MoS2 is shown in Fig. 5(b). Two main Raman features are clearly presented, the E12g mode representing
S. Han et al. / Journal of Crystal Growth 430 (2015) 1–6
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Fig. 5. (a) XRD patterns of sample S4; (b) Raman spectrum of sample S4; (c) PL spectrum of sample S4.
the in-plane vibration of molybdenum and sulfur atoms and the A1g mode related to the out-of-plane vibration of sulfur atoms [23]. The as-made MoS2 plates showed Raman peaks in the range of 300– 480 cm 1 under 532 nm excitation assigned to MoS2 (E12g mode 383 cm 1 and A1g mode 407 cm 1). Note that the peak frequency difference between A1g and E12g modes (Δ) can be used to identify the layer number of MoS2. The value of Δ(24 cm 1) evidences the existence of MoS2 nanorods [19–22]. Fig. 5(c) shows the PL spectrum collected from the MoS2 nanorods in the forth sample. In contrast to the silent PL effect of the bulk MoS2 indirect semiconductor, the PL spectrum of MoS2 nanorods exhibited two peaks at 641 and 688 nm. The PL emission of the MoS2 nanorods could be related to the switch of PL emission process from trion recombination to exciton recombination, which is assisted with the defects in the materials [24]. Because the MoS2 nanorods were synthesized in high S sufficient atmosphere, the MoS2 crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones. In this case, instability may occur as atoms do not have enough time to move into the right lattice locations, where crystal domains could have the lowest surface free energy, and the probability of defect formation increases. Therefore, there could be many defects formed in the MoS2 nanostructures. As the excitons localize at the defect sites generally have much larger binding energy [25,26], this may suppress the thermally activated nonradiative recombination including defect trapping, and result in the PL properties.
4. Conclusion In summary, MoS2 nanorods with different thickness and length were successfully synthesized via the CVD method. The
MoS2 domains experience a regular morphology transformation as well as a size change with the decrease in the distance between the precursor and the growth location. This may relate to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. It paves the way for the development of the CVD method with controlled growth parameters and opens up new venues for the tuning the morphology evolution of MoS2 nanorods.
Acknowledgment This work is supported by National Natural Science Foundation of China (Grant nos. 51561012, 11164008, 51461019, 51361013, 11174226, and 51371129), the Project for Young Scientist Training of Jiangxi Province (Grant no. 20153BCB23016), the Natural Science Foundation of Jiangxi Province (Grant no. 20151BAB202004) and the research fund of Jiangxi Normal University (Grant no. 6628).
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Morphology evolution of MoS2 nanorods grown by chemical vapor deposition Shuming Han a, Xingfang Luo a, Yingjie Cao a, Cailei Yuan a,n, Yong Yang a, Qinliang Li a, Ting Yu a,n, Shuangli Ye b a Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Electronics and Communication, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b School of Printing and Packaging, Wuhan University, Wuhan 430072, Hubei, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 May 2015 Received in revised form 29 June 2015 Accepted 12 August 2015 Communicated by: J.M. Redwing Available online 20 August 2015
We observed the regular morphology evolution of chemical vapor deposition (CVD) grown MoS2 nanorods along the gas flow direction on the SiO2/Si substrate. It can be attributed to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. The MoS2 domains experience a regular morphology transformation as well as a size change as the deposition location moves closer to the MoO3 powder. It paves the way for the development of CVD method with controlled growth parameters and opens up new venues for the tunable morphology evolution of MoS2 nanorods. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A1. Low dimensional structures A2. Growth from vapor A2. Chemical vapor deposition processes
1. Introduction As the shape and size of the nanostructures can greatly influence their physical and chemical properties [1,2], the rational design and control of the architecture and size of nanostructures has been one of the hottest topics in material science for the past few years. On the other hand, one dimensional nanostructure such as nanorods, nanowires, nanobelts and nanotubes have gained increasing attention because of its potential as the building blocks for varied industrial applications [3]. Therefore, it is necessary to control the growth of one dimensional nanostructure, since the nanomaterials with well controlled size, morphology and chemical composition may reveal more opportunities in exploring new and enhanced properties [4]. Recently, MoS2 has received much attention in many fields including electrochemical devices, hydrogen storage, catalysis, capacitors, solid lubricant, and intercalation host [5–10]. The interest in exploration of generating nanoscale MoS2 with specific morphologies and unique properties by using different approaches keeps increasing. Subsequently, many preparation methods for MoS2 nanostructures have been explored to produce nanorods,
n
Corresponding authors. Tel./fax: þ86 791 88120370. E-mail addresses: [email protected] (C. Yuan), [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jcrysgro.2015.08.005 0022-0248/& 2015 Elsevier B.V. All rights reserved.
e.g., the sonication leading to MoS2 nanorod formation [11], synthesizing nanotubes and nanorods of MoS2 by utilizing a hydrothermal method [12], a redox rout for synthesis of MoS2 nanorods using (NH4)6Mo7O24 4H2O and Na2S 9H2O as precursors at 90 1C by Wang et al. [13]. Although many kinds of methods have been developed to synthesis MoS2 nanorods, manipulating the morphology and size of MoS2 nanorods is still a challenge. In this paper, a simple and precise method to synthesize MoS2 nanorods with different thickness and length was proposed. We observed the regular morphology evolution of chemical vapor deposition (CVD) grown MoS2 nanorods along the gas flow direction on the SiO2/Si substrate. It can be attributed to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. As results, MoS2 nanorods with different thickness and length were successfully synthesized.
2. Experimental The MoS2 nanorods were grown by a CVD method [14] with solid MoO3 and sulfur precursors, as shown in Fig. 1. Briefly, a two-zone tube furnace was used to provide accurate temperature control of MoO3 and sulfur powders separately. An alumina boat with 100 mg sulfur (99.9% purity) was placed upstream in the low-temperature zone. Another alumina boat containing 20 mg (99.9% purity) of
2
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Fig. 1. Schematic illustration of the MoS2 nanorods CVD system.
MoO3 was placed downstream in the high-temperature zone. In order to investigate the dependence of the morphology evolution of MoS2 on the distance between the MoO3 precursor and the substrates, four pieces of Si wafer with 300 nm SiO2 layer substrates were aligned and placed face down to the MoO3 powder along the gas flow direction, which were labeled as S1, S2, S3 and S4, respectively. The S1 sample locates at the farthest position away
Fig. 2. (a) Optical microscope image MoS2 nanoparticles deposited on SiO2/Si substrate in sample S1; (b) A typical AFM image MoS2 nanoparticle in sample S1; (c) Height profile of MoS2 nanoparticle in the sample S1; (d) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S2; (e) A typical AFM image MoS2 nanorod in sample S2; (f) Height profile of MoS2 nanorod in sample S2.
S. Han et al. / Journal of Crystal Growth 430 (2015) 1–6
3
Fig. 3. (a) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S3; (b) and (c) A typical AFM image MoS2 nanorod in sample S3; (d) Height profile of MoS2 nanorod in sample S3.
from the MoO3 powder, while the S4 sample locates at the closest position of the MoO3 powder. The distance between adjacent substrate is 1 cm and the size of the substrate is 1 cm. The distance between the two alumina boats is 17 cm. The temperature of the sulfur and the substrate was increased concurrently to 200 1C and 750 1C, respectively. The deposition process was conducted by evaporating sulfur and MoO3 powders simultaneously in an Ar environment with a flow rate of 30 sccm. After 5 min, the furnace was cooled down naturally to room temperature. The optical microscope image was performed using a optical microscope with an 50 objective lens adapted to a video camera and connected to a computer. The sample structure was examined by using a a Rigaku x-ray diffractometer (XRD) with Cu Kα radiation. The morphology of MoS2 nanorods were measured by using Park system atomic force microscopy (AFM). Raman spectra were collected in a Horiba Jobin Yvon Raman microscopic system. The solid-state excitation laser has a wavelength of 532 nm. A 100 objective was used to focus the laser beam. The photoluminescence (PL) measurements were also performed with the same laser by using the PL mode of the Raman microscopic system.
3. Results and discussion Fig. 2(a) shows the optical images of sample S1, which locates at the farthest position away from the MoO3 powder. It can be clearly seen that there are lots of MoS2 nanoparticles with irregular shapes on the SiO2/Si substrate. The size of MoS2 nanoparticles is about
300 nm. It should be noted that there are some relatively very short rod-like structures MoS2 nanoparticles on the SiO2/Si substrate, which indicated the early stages of MoS2 nanorods growth. Fig. 2 (b) shows the planar view of the AFM image of a MoS2 nanoparticle in the sample S1. The height profile of the MoS2 nanoparticle is shown in Fig. 2(c). The MoS2 nanparticle in the sample S1 has thickness of about 10 nm and width of about 328 nm. Fig. 2 (d) presents the optical image of sample S2, which is closer to the MoO3 powder than the sample S1. Some short MoS2 nanorods have been successfully fabricated on SiO2/Si substrate. The morphology of MoS2 nanorod is checked with AFM shown in Fig. 2(e). Obviously, a well-defined rod-like structure can be observed. The height profile of this MoS2 nanorod is shown in Fig. 2(f). From the height profile, it can be found that the MoS2 nanorod has thickness of about 98 nm and width of about 680 nm. In this paper, the MoS2 nanorods were synthesized in high S sufficient atmosphere. The higher partial pressure of gaseous S greatly increases the volatilization speed of MoO3, forming a large amount of gaseous intermediate MoO3-x in a short time [15]. Therefore, the high S sufficient atmosphere may promote the mass transfer process, which contributes to the enhancement in the crystal growth. Under the S sufficient atmosphere condition, the MoS2 crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones, which is typical for nanocrystals with high precursor feedstock. Therefore, the increase in precursor concentration has an effect of enhancing the crystal growth, resulting in the formation of MoS2 nanorods, which are not favorable for the production of highquality two-dimensional crystal [16]. Moreover, the main reason for
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Fig. 4. (a) Optical microscope images of MoS2 nanorods deposited on SiO2/Si substrate in sample S4; (b) and (c) A typical AFM image MoS2 nanorod in sample S4; (d) Height profile of MoS2 nanorod in sample S4.
the morphology evolution phenomenon between S1 and S2 can be attributed to the change in the MoO3 concentration along the SiO2/ Si substrate surface. Because the distance between the substrates and the MoO3 is very small, there can be an obvious concentration gradient on the surface of substrates. Along the gas flow direction, the MoO3 concentration decreases as the substrate moves farther from the MoO3 powder, which influences the average growing rate of the MoS2 crystals. As the growing rate is slowest, the sample S1 can be considered at the early stages of MoS2 crystal growth. Therefore, many MoS2 nanoparticles with the size of about 300 nm are obtained on the first SiO2/Si substrate because of the initial nucleation process. Some relatively very short rod-like structures MoS2 nanoparticles also indicated the early stages of the MoS2 nanorods growth. When the deposition location moves closer to the MoO3 powder, MoO3 concentration increases, and thus the growing rate of the MoS2 crystals also increases. Some short MoS2 nanorods appear on the second SiO2/Si substrate. This can be attributed that the MoS2 nanoparticles start to assemble together and spontaneously aggregate into rod-like MoS2 structures in order to reduce the high surface energy through the process known as oriented aggregation [17]. Figs. 3(a) and 4(a) show the optical images of samples S3 and S4. It can be found that, with the substrate location moves closer to the MoO3 powder, the length of MoS2 nanorods becomes longer, and the crystal size also experiences a regular change. Figs. 3(b) and 4 (b) show the planar view of the AFM images of MoS2 rods in the samples S3 and S4. Figs. 3(c) and 4(c) show cross sectional AFM
images of MoS2 rods in the samples S3 and S4. The height profile of MoS2 nanorods in the samples S3 and S4 is shown in Figs. 3(d) and 4(d). The MoS2 nanorod in the sample S3 has thickness of about 348–365 nm and width of about 1.17 mm. While, the thickness and width of the MoS2 nanorod in the sample S4 increase to about 400– 502 nm and 1.525 μm, respectively. Obviously, with the decrease in the distance between the precursor and the growth location, MoS2 domains experience a regular morphology transformation as well as a size change. The length and size of MoS2 nanorods gradually increase as the deposition location moves closer to the MoO3 powder. As discussed previously, this may relate to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. The increase in precursor concentration has an effect of enhancing the crystal growth rate. Therefore, with the deposition sample moves closer to the MoO3 powder, well-defined MoS2 nanorods with smooth surface are formed from the scabbled MoS2 rod structures through an Ostwald ripening process [18]. The MoS2 nanorods in the sample S4 are further examined by Xray diffraction patterns, Raman and PL spectra. Fig. 5(a) shows the XRD pattern of sample S4 with MoS2 nanorods deposited on SiO2/Si substrate. All the diffraction peaks in the pattern can be indexed as MoS2 (ICCD Card no: 77-1716). No diffraction peaks from impurities are observed in the XRD pattern. Raman spectroscopy has been wide used to study two-dimensional materials and to identify their thickness [19–22]. The Raman spectrum of MoS2 is shown in Fig. 5(b). Two main Raman features are clearly presented, the E12g mode representing
S. Han et al. / Journal of Crystal Growth 430 (2015) 1–6
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Fig. 5. (a) XRD patterns of sample S4; (b) Raman spectrum of sample S4; (c) PL spectrum of sample S4.
the in-plane vibration of molybdenum and sulfur atoms and the A1g mode related to the out-of-plane vibration of sulfur atoms [23]. The as-made MoS2 plates showed Raman peaks in the range of 300– 480 cm 1 under 532 nm excitation assigned to MoS2 (E12g mode 383 cm 1 and A1g mode 407 cm 1). Note that the peak frequency difference between A1g and E12g modes (Δ) can be used to identify the layer number of MoS2. The value of Δ(24 cm 1) evidences the existence of MoS2 nanorods [19–22]. Fig. 5(c) shows the PL spectrum collected from the MoS2 nanorods in the forth sample. In contrast to the silent PL effect of the bulk MoS2 indirect semiconductor, the PL spectrum of MoS2 nanorods exhibited two peaks at 641 and 688 nm. The PL emission of the MoS2 nanorods could be related to the switch of PL emission process from trion recombination to exciton recombination, which is assisted with the defects in the materials [24]. Because the MoS2 nanorods were synthesized in high S sufficient atmosphere, the MoS2 crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones. In this case, instability may occur as atoms do not have enough time to move into the right lattice locations, where crystal domains could have the lowest surface free energy, and the probability of defect formation increases. Therefore, there could be many defects formed in the MoS2 nanostructures. As the excitons localize at the defect sites generally have much larger binding energy [25,26], this may suppress the thermally activated nonradiative recombination including defect trapping, and result in the PL properties.
4. Conclusion In summary, MoS2 nanorods with different thickness and length were successfully synthesized via the CVD method. The
MoS2 domains experience a regular morphology transformation as well as a size change with the decrease in the distance between the precursor and the growth location. This may relate to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. It paves the way for the development of the CVD method with controlled growth parameters and opens up new venues for the tuning the morphology evolution of MoS2 nanorods.
Acknowledgment This work is supported by National Natural Science Foundation of China (Grant nos. 51561012, 11164008, 51461019, 51361013, 11174226, and 51371129), the Project for Young Scientist Training of Jiangxi Province (Grant no. 20153BCB23016), the Natural Science Foundation of Jiangxi Province (Grant no. 20151BAB202004) and the research fund of Jiangxi Normal University (Grant no. 6628).
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