Langmuir 2008, 24, 8393-8394
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Comments Can t-Te Nanowires Really Give Blue-Violet Emission? Reply to Comment on High-Quality Luminescent Tellurium Nanowires of Several Nanometers in Diameter and High Aspect Ratio Synthesized by a Poly(Vinyl Pyrrolidone)-Assisted Hydrothermal Process
The photoluminescence of trigonal tellurium (t-Te) nanowires has rarely been studied previously, but it has attracted increasing attention in recent years. Room-temperature photoluminescence spectroscopy studies indicated that nanobelts with a width of 250-800 nm do not give emission with an excitation wavelength of 365 nm as reported by Rao et al.1 Gray t-Te microbelts with a width ranging from hundreds of nanometers to several micrometers, which were synthesized by our group, also do not show emission (insets in Figure 9).2 However, we have found that two strong photoluminescence emission peaks in the blueviolet region (390-550 nm) with an excitation wavelength of 365 nm were observed in samples of dark-blue ultrathin t-Te nanowires (Figure 11b).2 Later on, blue t-Te nanowires/nanorods were also been synthesized by Xi et al.,3 Xie et al.,4 and Chang et al.5 Similar emission features have also been observed by these three groups.3–5 In a recent comment published in this journal, Roy and Tyagi et al.6 claimed that they observed a similar emission from the sample holder itself and not from the tellurium (t-Te) nanowire, where they recorded the spectrum in a spectrometer (Hitachi model F-4500) using the standard solid sample holder (a springloaded metallic sample holder with a black polymer cushion support for holding the samples) provided with the instrument. Their results showed that the photoluminescence (PL) spectrum of the sample holder only (without any sample on it) is surprisingly similar to that reported by our group1 for their ultrathin nanowire samples. They also claimed that the samples with nanowires usually have gaps between the wires, thus the PL contribution from the holder (if it emits) can always arise because light can pass through from the glass slides.6 In this reply, we present our new results and confirm that the ultrathin t-Te nanowires indeed give their intrinsic blue-violet emission, but not from the sample holder itself or partially contributed by the sample holder. Our measurement was performed on the same Perkin-Elmer LS55 luminescence spectrometer as we reported for measurement of ultrathin t-Te nanowires previously.2 Our results indicated that the blank Si wafer on the sample holder did not give any emission (Figure 1a). However, if the ultrathin Te nanowires were put on this thick Si wafer at first and then load on the sample holder, the strong emission peaks in blue-violet region was observed with excited wavelength at 365 nm (Figure 1b). * Corresponding author. E-mail:
[email protected]. (1) Gautam, U. K.; Rao, C. N. R. J. Mater. Chem. 2004, 14, 2530. (2) Qian, H.-S.; Yu, S.-H.; Gong, J.-Y.; Luo, L.-B.; Fei, L.-F. Langmuir 2006, 22, 3830. (3) Xi, G. C.; Liu, Y. K.; Wang, X. Q.; Liu, X. Y.; Peng, Y. Y.; Qian, Y. T. Cryst. Growth Des. 2006, 6, 2567–2570. (4) Zhang, B.; Hou, W. Y.; Ye, X. C.; Fu, S. Q.; Xie, Y. AdV. Funct. Mater. 2007, 17, 486–492. (5) Lin, Z. H.; Yang, Z.; Chang, H. T. Cryst. Growth Des. 2008, 8, 351–357. (6) Roy, M.; Sen, S.; Gupta, S. K.; Tyagi, A. K. Langmuir 2007, 23, 10873.
Figure 1. Room-temperature PL spectra taken on (a) blank thick Si wafer and (b) 4-9 nm tellurium nanowires on the Si wafter. The excitation spectrum of the ultrathin Te nanowires was obtained under an emission wavelength of 365 nm.
These two samples were measured on the same Perkin-Elmer LS55 spectrophotometers and used the same sample holder. If the sample holder itself gave emission in our case, then the equipment should also detect the emission from the sample holder itself; however, we do not observe such blue-violet emission from the blank sample holder only (without any sample on it) in this case. In addtion, we noticed that Roy and Tyagi et al.6 used a Hitachi F-4500 spectra to measure the emission property of Te samples. In our work, we used a Perkin-Elmer LS55 luminescence spectrometer to measure the PL spectra of the ultrathin Te nanowires. The standard commercial sample holder we used is different from that used in their study. To confirm whether the sample holder of the Perkin-Elmer LS55 (the equipment used in our study) gives blue-violet emission in our case or not, the blank sample holder was examined. The commercial sample holder itself is made of aluminum and with a layer of thin films whose composition is still unknown. Figure 2b shows that the sample holder (without any sample on it) that we used indeed gives relatively weak broadening emission and does not show two distinguishable peaks such as those observed for the t-Te nanowires covered on the sample holder (curve a in Figure 2). The emission properties of the Te nanostructures is indeed related to the particle size. After our study on the emission properties of ultrathin nanowires, several reports on the emission properties of Te nanowires/nanorods have also been published.3–5 For example, blue Te nanrods or nanowires with emission properties similar to those that we have reported have also been synthesized though different solution approaches by different groups.3–5 The dispersion of ultrathin t-Te nanowires in water gives a deep blue color, which is dramatically different from the gray color usually observed for t-Te nanostructures reported previously.,2 This is another indication of the possible unique optical properties of the ultrathin t-Te nanowires. It has been pointed out that the gray t-Te microbelts with a width ranging from hundreds of nanometers to several micrometers, a length ranging from tens of micrometers to hundreds of micrometers, and a
10.1021/la800565a CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
8394 Langmuir, Vol. 24, No. 15, 2008
Figure 2. PL spectra measured at room temperature on the PerkinElmer LS55. (a) Ultrathin nanowires loaded into the sample holder and (b) the blank sample holder itself. The excitation wavelength is 365 nm.
thickness of a few nanometers had no emission, which is consistent with the data reported by Rao et al.1 However, ultrathin t-Te nanowires indeed give intrinsic emission under an excitation of 365 nm, and this blue-violet emission has also been observed by other groups.3–5 Xi et al. used CTAB surfactant instead of poly(vinyl pyrrolidone)(PVP) to synthesize the Te nanowires with an average diameter of 7 nm, showing emission similar to what we observed.3 In addition, Xie’s group has reported similar photoluminescence spectra of the ultrathin Te-Pt nanowires in the quartz cell (not the solid sample holder) by dispersing their Te-based samples in ethanol,4 which was also recorded on the Perkin-Elmer LS55 luminescence spectrometer. Very recently, Chang et al. reported that Te nanorods with different aspect ratios prepared by using sodium dodecyl sulfate (SDS) as a surfactant also displayed a broad emission band in the range of 250 to 650 nm using 210 nm as the excitation wavelength.5 Even though their spectra were recorded on the Hitachi F-4500 spectrophotometer, the emission band could be deconvoluted into four Gaussian peaks centered at 334, 397, 460, and 507 nm (Figure 7B).5 The sample holders from different commercial sources could have different surfaces (usually covered with a thin film); however,
Comments
we did not observe blue-violet emission in our another microbelt sample (which was large compared with the ultrathin nanowires), which is also measured on the Perkin-Elmer LS55 luminescence spectrometer.2 If only the sample holder itself can give such blue-violet emission with two distinguishable peaks, it should also be observed in the PL spectra for microbelts and the blank Si wafer; however, we do not observe such emission in either case. Regarding to the origin of the emission lines observed by Roy and Tyagi et al., they argued that it could be either the black polymer cushion used in the sample holder or the glue used to fix the sample holder.6 Their observation (i.e., no blue-violet emission for their samples when they were recorded using a thick glass and/or metal separator between the holder and the glass slide) can only indicate that their Te samples do not emit, which is similar to our observation of the sample of Te microbelts2 and also that reported by Rao et al.1 In conclusion, our results clearly demonstrated that ultrathin Te nanowires indeed display intrinsic blue-violet emision, which is not due to the contribution from the sample holder of the PL spectrophotometer (Perkin-Elmer LS55 luminescence spectrometer), even though the sample holder for the Hitachi model F-4500 (a spring-loaded metallic sample holder with a black polymer cushion support for holding the samples) shows a similar weak, broad emission band as argued by Roy and Tyagi et al.6 The similar emission band of the blank sample holder detected by Roy and Tyagi et al.6 to the one that we reported for blue-violet emission from ultrathin nanowires2 and by other groups3–5 could be just a coincidence. Acknowledgment. We thank Dr. Bin Zhang and Professor Yi Xie in the Department of Chemistry, University of Science and Technology of China, for discussions of the luminescence data of their ultrathin Te-Pt and Te nanowires dispersed in ethanol. We thank the financial support by the National Science Foundation of China (NSFC) (Grants nos. 50732006, 20325104, 20621061, 20671085). Hai-Sheng Qian and Shu-Hong Yu*
ReceiVed February 21, 2008 LA800565A
