Effects of experimental conditions on the morphologies, structures and growth modes of pulsed laser-deposited CdS nanoneedles
© Li et al.; licensee Springer. 2014
Received: 13 January 2014
Accepted: 13 February 2014
Published: 22 February 2014
CdS nanoneedles with different morphologies, structures, and growth modes have been grown on Ni-coated Si(100) surface under different experimental conditions by pulsed laser deposition method. The effects of catalyst layer, substrate temperature, and laser pulse energy on the growth of the CdS nanoneedles were studied in detail. It was confirmed that the formation of the molten catalyst spheres is the key to the nucleation of the CdS nanoneedles by observing the morphologies of the Ni catalyst thin films annealed at different substrate temperatures. Both the substrate temperature and laser pulse energy strongly affected the growth modes of the CdS nanoneedles. The secondary growth of the smaller nanoneedles on the top of the main nanoneedles was found at appropriate conditions. A group of more completed pictures of the growth modes of the CdS nanoneedles were presented.
KeywordsCdS nanoneedles Substrate temperature Laser pulse energy Growth mode 61.46.-w 61.46.Km 68.37.Lp
Nowadays, semiconductor nanomaterials like nanowires, nanorods, and nanotubes, have aroused great interest in material science and applications owing to their unique characteristics different from film or bulk materials. CdS, as a direct bandgap (2.4 eV) II-VI compound semiconductor, has good optical and electrical properties, which give it potential applications in light-emitting diodes, light sensors, photocatalysts, windows of thin film solar cells as well as absorbers and electrodes of hybrid solar cells [1–7]. Compared to CdS thin films, the CdS nanostructures such as nanoparticles, nanowires, and nanoneedles have higher optoelectronic sensitivities and efficiencies for these devices due to their large surface areas and possible quantum confinement effects [4–7]. There have been many methods for preparing CdS nanowires like electrochemical deposition [8, 9], solution process [10, 11], chemical and physical vapor deposition, etc. Among the methods, pulsed laser deposition (PLD) is a simple and efficient way to synthesize multicomponent compounds such as II-VI semiconductors [12–14]. It is easy to control the growth rate and avoid materials from pollution as a result of the adjustable frequency of pulsed laser and the good directivity of laser-ablated plasma [13, 14]. In our previous work, the CdS nanoneedles have been grown successfully using the PLD method  and the growth modes of vapor-liquid-solid (VLS) and vapor-solid (VS) have been suggested [15, 16]. In this article, the effects of the substrate temperature and the laser pulse energy on the growth of CdS nanoneedles were studied in detail. Both the VLS and VS growth modes of CdS nanoneedles were further confirmed experimentally. The transformation from VLS to VS growth modes along with the growth of the CdS nanoneedles was discussed.
The CdS nanoneedles were deposited on Si(100) substrates using Ni as catalysts by a PLD method. The experimental setup mainly consists of a Nd:YAG laser with a wavelength of 532 nm and a deposition chamber with rotating multitargets and a base pressure of 10-3 Pa. High-purity Ni and hot-pressed CdS targets (purchased from Beijing Founde Star Science & Technology Co., Ltd.) with diameter and thickness of 1.5 and 0.5 cm, respectively, were used as sources of precursors of Ni catalyst layer and CdS nanoneedles. Prior to the deposition, substrates were ultrasonically cleaned in acetone and ethanol, etched in HF solution and rinsed in deionized water, successively. To prepare the CdS nanoneedles, there were two steps involved. Firstly, Ni catalysts were deposited on the substrates by PLD with a laser pulse energy of 50 mJ and a repetition rate of 5 Hz for 10 min (without substrate heating). Secondly, the CdS nanoneedles were grown by PLD at different substrate temperatures of 200°C to 500°C, different laser pulse energy of 50 to 80 mJ and a repetition rate of 10 Hz for 30 min. In order to understand the growth mechanism of the CdS nanoneedles, the morphologies of the prepared Ni catalyst-covered substrates were observed after annealing 5 min at the different substrate temperatures of 200°C to 500°C. The morphology of all the samples was examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The crystalline structures of the CdS nanoneedles were characterized by selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The composition of the CdS nanoneedles was analyzed by energy-dispersive spectroscopy (EDS) fitted on the TEM.
Results and discussion
In order to better understand the effects of experimental conditions on the growth mechanism of the CdS nanoneedles, the laser pulse energy was changed in a series of experiments for preparation of CdS nanoneedles. In the experiments, the conditions of Ni deposition (50 mJ, 5 Hz, and 10 min) and the substrate temperature of CdS deposition (475°C) were kept unchanged, and the laser pulse energy was set from 50 to 80 mJ by every step of 10 mJ. The influence of the laser energy on the growth of the CdS nanoneedles is shown in Figure 4. In Figure 4, as the laser pulse energy is 50 mJ, there are many crooked and straight nanoneedles grown on the polycrystalline background with catalyst balls on the tops, which accords with the VLS growth mode. When the pulse energy increases to 60 mJ, almost no nanoneedles grow on the polycrystalline background film. Continuing to increase the laser pulse energy to 70 mJ, some nanoneedles grow out again, but they have some bent and poor shapes without catalyst balls on the tops. If the laser pulse energy is increased to 80 mJ, not only the size and density of the as-grown nanoneedles increase but also they have intact nanoneedle shapes, which is the typical VS growth mode. From Figure 4a,b,c,d, it could be found that the growth modes of the CdS nanoneedles change from the VLS mode to the VS mode with the increase of the laser pulse energy from 50 to 80 mJ, which reveals that the laser pulse energy strongly affected the growth of the CdS nanoneedles. With the increase of the laser pulse energy, the kinetic energy and density of the laser-ablated plasma increase and the CdS thin films are deposited faster, which would lead to that the incipient CdS nanoneedles are covered by the growing base thin films and the CdS nanoneedles grown in the VLS mode cannot grow out. This may be also related to the sputtering-off effect of the laser-ablated plasma on the catalysts, i.e., that the bombardments of plasma on the tops of the incipient CdS nanoneedles restrain the VLS growth of the CdS nanoneedles. In Figure 4c, the as-grown CdS nanoneedles have no catalyst balls on the tops, which may be due to such plasma bombardment. The growth mode of these CdS nanoneedles may have been converted to the VS mode at certain laser pulse energy (for example, above 70 mJ). In this case, the kinetic energy and density of the laser-ablated plasma will satisfy the VS growth conditions of CdS nanoneedles and make the incipient CdS nanoneedles grow faster without catalyst-leading than the base thin films as shown in Figure 4d.
In conclusion, the substrate temperature and the pulse laser energy affect the growth mode of the CdS nanoneedles, but the influenced factors are interacted. The formation of the molten catalyst spheres is confirmed to be the key to the nucleation of the CdS nanoneedles by observing the morphologies of the Ni-catalyst thin films annealed at different substrate temperatures. Under the certain conditions, changing the substrate temperature or the pulse laser energy may cause the changes of the growth modes of the CdS nanoneedles. In our experiments, under the same laser energy, the growth mode of the CdS nanoneedles is VS at a substrate temperature of 400°C, but it turns into VLS at a substrate temperature of 450°C. Also, altering the pulse laser energy from 50 to 80 mJ may also change the growth modes of the CdS nanoneedles from VLS to VS. Besides, the secondary growth of the smaller CdS nanoneedles is found on the tops of the main CdS nanoneedles. In secondary growth mode, the main CdS nanoneedles grow in VLS mode with catalysts leading, and the secondary CdS nanoneedles grow in VS mode without catalysts leading due to the decrease of the temperature of the Ni spheres on the tops of the main nanoneedles.
field emission scanning electron microscopy
fast Fourier transform
high-resolution transmission electron microscopy
pulsed laser deposition
selected area electron diffraction
transmission electron microscopy
This work is supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB934303) and National Natural Science Foundation of China.
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