Langmuir 2004, 20, 23-26
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Photoluminescence Resulting from Semiconductor-Metal Solid Solution Observed in One-Dimensional Semiconductor Nanostructures Yung-Jung Hsu and Shih-Yuan Lu* National Tsing-Hua University, Department of Chemical Engineering, Hsin-Chu, Taiwan 30043, Republic of China Received June 26, 2003. In Final Form: November 10, 2003 A narrow band photoluminescence (PL) emission peak resulting from CdS-Au solid solution was observed when growing one-dimensional nanostructures of CdS via the vapor-liquid-solid mechanism by using Au as the catalyst. This emission peak was located at 680 nm, a wavelength longer than the near band edge emission of CdS at 520 nm, and was shown not to be caused by the usual trap states of CdS which lead to a broad band emission. Here, the one-dimensional nanostructures of CdS were grown in a simple, low-temperature (360 °C) metal-organic chemical vapor deposition process with a single source precursor of CdS. Straight nanowires of diameter 50-70 nm and wormlike nanorods of diameter 100-200 nm were obtained. Both the upper and lower portions of the nanorods/nanowires possessed single crystallinity as judged from the corresponding high-resolution transmission electron microscopy images and selected area electron diffraction data. This work demonstrates the feasibility of adjusting PL emission peaks of optoelectronic semiconductors through alloying with metals.
Introduction One-dimensional nanosized semiconductor materials have drawn much research attention in recent years due to their potential applications in nanoscale optoelectronic devices and possible new fundamental phenomena arising from this particular dimensionality.1-3 High-density CdS nanowires had been prepared with electrochemical deposition in anodic aluminum oxide templates.4,5 Gas-phase routes of CdS nanowire production had also been achieved via the vapor-liquid-solid (VLS)6,7 process. In general, nanowires synthesized with electrochemical deposition processes possess polycrystallinity, while single crystallinity is commonly achieved with gas-phase routes. The VLS-based gas-phase route offers the advantage of easier end product separation and higher throughput rate but often requires a high reaction temperature (for example, above 800 °C). In this work, we successfully developed a low-temperature VLS-based gas-phase process for preparation of CdS nanorods and nanowires by using a new single-source precursor of CdS. With this development, we were able to observe a photoluminescence band resulting from the existence of CdS-Au solid solution. This finding offers an alternative route for adjusting a semiconductor’s photoluminescence band through alloying with metals. Results and Discussion The morphology of the substrate surface after growth of one-dimensional (1D) nanostructures of CdS at reaction times of 2 and 10 h is shown in Figure 1. Some points can * Corresponding author.
[email protected]. (1) Lieber, C. M. Solid State Commun. 1996, 107, 607; see also, Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (3) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (4) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037. (5) Li, Y.; Xu, D.; Zhang, Q.; Chen, D.; Huang, F.; Xu, Y.; Guo, G.; Gu, Z. Chem. Mater. 1999, 11, 3433. (6) Duan, X.; Lieber, C. M. Adv. Mater. 2000, 12, 298. (7) Wang, Y.; Meng, G.; Zhang, L.; Liang, C.; Zhang, J. Chem. Mater. 2002, 14, 1773.
Figure 1. Top view SEM images of one-dimensional CdS structures grown on Au sputtered quartz substrates at the reaction temperature of 360 °C for reaction times of (a) 2 h and (b) 10 h.
be concluded from the two top view scanning electron microscopy (SEM) graphs. There appeared formation of two distinct structures, wormlike nanorod and straight nanowire. At short reaction times, wormlike nanorods dominated on the surface and were of larger diameter, while straight nanowires were sparsely distributed over the surface. As the reaction time increased, the nanorods grew longer and thinner, while the straight nanowires became more populated and also grew thinner in diameter
10.1021/la035138k CCC: $27.50 © 2004 American Chemical Society Published on Web 12/06/2003
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Figure 2. The HRTEM image and SAED data taken at the upper stem region of a CdS nanowire, prepared with a deposition time of 2 h.
and longer in length. The nanorods were with a diameter of about 100-200 nm and grew up to be longer than 1 µm for a total reaction time of 10 h, while the nanowires were much longer and thinner with a diameter of 50-70 nm and a length of several micrometers at the same reaction time. A close-up SEM image is shown in Supporting Information. For the VLS growth mechanism, the characteristic size of Au correlates with the diameter of the resulting 1D structure.8 Although the characteristic size of the sputtered Au clusters was initially in angstrom scale, the relatively high reaction temperature of the reactor caused aggregation of Au clusters to form much larger islands. The morphology of Au sputtered on quartz substrates after annealing at 360 °C in nitrogen for 1 h shows the existence of both larger and smaller islands of Au (see Supporting Information), leading to nanorods of larger diameter and thinner nanowires, respectively. Figure 2 shows the high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) measurements, taken at the upper stem region of the CdS nanowire prepared with a deposition time of 2 h. Both the upper and lower portions of the CdS nanowire stem possessed single crystallinity, and the CdS nanowires were with the hexagonal (greenockite) crystal structures. CdS nanowires prepared at longer deposition times also showed single crystallinity (data not shown here). The optical properties of the 1D nanostructures are shown in Figure 3. The dashed curve (curve f) was for the UV-vis absorption spectrum of the CdS structures grown at a reaction temperature of 360 °C with a reaction time of 10 h. An absorption edge at around 500 nm is evident, confirming to the energy band gap of 2.5 eV of CdS. The dotted curve (curve g) represented the UV-vis absorption spectrum of Au film sputtered on a quartz substrate. The (8) Gudiksen, M. S.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 8801. Gudiksen, M. S.; Wang, J.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 4062.
Figure 3. The PL spectra of CdS deposits for reaction times of (a) 2 h, (b) 4 h, (c) 6 h, and (d) 10 h at the reaction temperature of 360 °C and (e) pure Au films sputtered on the substrate. Also, the UV-vis absorption spectra of CdS deposits (for a reaction time of 10 h) and pure Au films are included in (f) and (g), respectively.
Au film thickness was estimated to be 9.5 nm according to the sputtering conditions used. There appeared a broad surface plasmon absorption band of Au centering around 700 nm. Kalyuzhny et al.9 showed that the surface plasmon absorption band of Au film shifted from 520 nm to around 700 nm with increasing film thickness up to about 10 nm. The other curves were for the photoluminescence (PL) spectra of the corresponding CdS structures collected at the four reaction times of 2, 4, 6, and 10 h, and Au film on quartz substrate, respectively, with the excitation wavelength set at 320 nm. The PL spectrum for Au film on quartz substrate (curve e) showed no emission in the interested range. For CdS nanowires (curves a-d), two emission bands, the near band edge emission at around (9) Kalyuzhny, G.; Vaskevich, A.; Ashkenasy, G.; Shanzer, A.; Rubinstein, I. J. Phys. Chem. B 2000, 104, 8238.
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520 nm and a longer wavelength emission band at around 680 nm, were observed. With increasing reaction time from 2 to 10 h, the intensity of the near band edge emission grew much stronger, while that of the longer wavelength emission dropped substantially. The photoluminescence behavior of CdS was a well-studied subject. Usually, two emission bands are observed, one because of the excitonic emission and the other trap state emission. The excitonic emission is sharp and located near the absorption edge, while the trap state emission is broad and located at a longer wavelength.7,10-13 It was proposed that the trap state emission results from the excess sulfur or cadmium at the interface, which is known to quench radiative recombination of electron-hole pairs. In the present work, however, the longer wavelength emission band was attributed to the possible formation of CdS-Au solid solution at the tip region of the one-dimensional structures, instead of the trap state emission. For a short reaction time, the one-dimensional structures grew only a short length, and the CdS-Au solid solution region accounted for a major portion of the structures such that the longer wavelength emission dominated over the much weaker near band edge emission (curve a of Figure 3). With increase in reaction time, the proportion of the solid solution region diminished because of the growth of the pure CdS region. Additionally, the long reaction process gave time for annealing of the solid solution region to segregate into pure CdS and Au domains. Consequently, the intensity of the longer wavelength emission diminished while that of the near band edge emission strengthened. Furthermore, the narrow span of the longer wavelength emission band implied that this emission was probably not caused by the trap states, which normally give a broad emission band. To further verify our proposal, two samples, both prepared at a reaction time of 2 h, were processed with different heat treatments. One was annealed in nitrogen at 600 °C for 9 h to segregate the possible CdS-Au solid solution into distinct domains, and the other was heated to 900 °C for 3 h to ensure a complete melting and mixing of CdS and Au to form a uniform solid solution with subsequent quenching to preserve the solid solution structure. We compared the results of PL spectra of the as-deposited sample with the two heat-treated ones in Figure 4. After the annealing operation, the intensity of the near band edge emission was dramatically enhanced, while the longer wavelength emission experienced a substantial suppression. This conforms to our proposal that the solid solution separates back into distinct CdS and Au domains under the annealing process. As for the melted-and-quenched sample, the PL spectrum was almost the same with that of the as-deposited one, indicating the existence of solid solution contribution in the as-deposited sample. And for the photoluminescence quantum yield (Qs), there was a slight enhancement in quantum yield after the annealing treatment. This phenomenon indicates a successful segregation of AuCdS solid solution into distinct Au and CdS domains, which helps to boost the PL efficiency. The comparison of X-ray diffraction (XRD) data collected in the 2θ range of 37°-40° among Au film, as-deposited CdS nanowire, and the two heat-treated samples is also (10) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (11) O’Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356. (12) Butty, J.; Peyghambarian, N. Appl. Phys. Lett. 1996, 69, 3224. (13) Zhan, J.; Yang, X.; Wang, D.; Li, S.; Xie, Y.; Xia, Y.; Qian, Y. Adv. Mater. 2000, 12, 1348.
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Figure 4. The PL spectra of CdS deposits for a reaction time of 2 h: (a) as-deposited; (b) after annealing in nitrogen at 600 °C for 9 h; (c) melted and quenched sample.
Figure 5. Comparison in XRD spectra for the Au diffraction peaks, Au(111), of various CdS-Au samples: (a) pure Au film; (b) as-deposited; (c) annealed; (d) melted-and-quenched. The CdS deposits were prepared with a deposition time of 2 h.
made in Figure 5. The pure Au film (curve a) showed a strong diffraction peak (Au(111)) at around 38.28°, while the as-deposited CdS nanowires on Au-coated quartz substrate (curve b) showed a diffraction at around 38.36°, attributed to Au(111). Note that CdS alone does not give any diffraction peaks in this range. For the as-deposited CdS samples, the right shift of the Au(111) diffraction peak implies that the d spacing of Au(111) decreases upon its incorporation in CdS. After the annealing operation on the as-deposited CdS nanowire sample, the Aucontributed diffraction peak moved back to a position near that of the pure Au, as shown in curve c. This conforms to our proposal that the solid solution separates back into distinct CdS and Au domains after the annealing operation. Since the phase separation of Au and CdS achieved by the annealing operation was not complete, there remained some amount of solid solution, giving a broadened diffraction peak (curve c). As for the melted-andquenched sample (curve d), the Au diffraction peak position was almost the same with that of the as-deposited one, indicating the existence of the solid solution in the asdeposited sample. Au has been known to create deep acceptor or donor energy states when doped into semiconductor materials. For example, the deep donor levels created by doping Au into Si and Ge are located at 0.77 and 0.75 eV below the
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corresponding conduction bands, respectively.14 Au in CdS nanowires could play the role of deep-level donor and offers an extra emission route that gives longer wavelength emissions. For the present case, a shift of emission wavelength from 500 to 680 nm corresponds to a reduction in energy band gap of about 0.7 eV, which falls well within the reasonable order of magnitude of deep donor level of semiconductors doped with Au. The past research efforts in growing 1D nanostructures with the VLS process were conducted at much higher temperatures, in which the semiconductor-metal solid solution could be annealed more rapidly and separated into distinct phases. Consequently, the longer wavelength emission resulting from the semiconductor-metal solid solution was not observed. In this work, because of the low reaction temperature and suitably short reaction time, the solid solution phase was preserved and detected. Conclusions A simple low-temperature gas-phase process was developed for preparation of one-dimensional structures of CdS by using a new single-source precursor. Wormlike (14) Sze, S. M. Physics of Semiconductor Devices; Wiley: New York, 1969.
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nanorods of diameter of 100-200 nm and length of over 1 µm and straight nanowires of diameter of 50-70 nm and length up to several micrometers were grown on Ausputtered fused silica substrates. These one-dimensional CdS structures showed a usual near band edge emission and a longer wavelength emission attributable to the existence of CdS-Au solid solution instead of the possible trap states. This finding will draw research attention to the rarely discussed area of possible solid solutions of metals and semiconductors and their properties and applications. More importantly, one can adjust the PL emission of optoelectronic semiconductors through alloying with suitable metals, in addition to other semiconductor materials. Acknowledgment. The authors gratefully acknowledge the support of the National Science Council of the Republic of China under Grant NSC 90-2214-E-007-003. Supporting Information Available: Experimental details for the preparation of the samples and relevant SEM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA035138K
