Observation of Single Tin Dioxide Nanoribbons by Confocal Raman

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2007, 111, 18839-18843 Published on Web 11/30/2007

Observation of Single Tin Dioxide Nanoribbons by Confocal Raman Microspectroscopy Fengping Wang, Xingtai Zhou, Jigang Zhou, Tsun-Kong Sham,* and Zhifeng Ding* Department of Chemistry, The UniVersity of Western Ontario, London, ON N6A 5B7 Canada ReceiVed: October 4, 2007; In Final Form: NoVember 14, 2007

Confocal Raman microspectroscopy combined with scanning electron microscopy (SEM) was used to characterize morphologies, chemical structures, and optical properties of single tin dioxide nanoribbons under ambient conditions. Raman images, depth profiles, and spectra of the ribbons were analyzed to provide new insights into the structure-property relationship. The Raman images constructed from the normal Raman bands and an additional group of energy bands with very high intensities between 1000 and 1800 cm-1 clearly discriminated the nanoribbons from the luminescent nanostructures grown on ribbon surfaces. The strong luminescent nanostructures have a stoichiometry of SnOx (1 < x < 2), as assessed by energy-dispersive X-ray (EDX) spectroscopy and Raman peaks between 100 and 300 cm-1. The emission can be attributed to the defect state resonant and exciton-phonon coupling luminescence.

Introduction As an n-type wide band gap semiconductor, one-dimension (1D) tin dioxide nanostructures have motivated great interest in recent years.1-4 These nanostructures are particularly useful in gas sensors and optical devices due to their high surface-tovolume ratio and remarkable resistivity variation in a gaseous environment. The desired optical properties are crucial for nanosized optoelectronic devices. Much progress has been made on the characterization of 1D SnO2 nanostrutures.3,5-10 In addition, owing to their unique electrical and sensing properties, applications of these nanostructures in real devices are also progressing rapidly.4,11-15 However, some fundamental issues pertaining to the optical properties of 1D SnO2 nanostrutures are still unclear.16-20 Confocal Raman spectroscopy and microscopy are efficient and powerful methods to study chemical structures and their reactivity. It is equally informative on physical properties such as crystal form, crystal size, molecular orientation, and stresses from environments.10,21,22 Confocal Raman microspectroscopy can generate color-coded (3-D, where color represents intensity) images for both the sample surface and depth profile with a resolution as high as 200 nm. Herein, for the first time, we report luminescent nanostructures grown on a single SnO2 nanoribbon as well as the ribbon itself using confocal Raman microspectroscopy combined with scanning electron microscopy/energy-dispersive X-ray analysis (SEM/ EDX). On the basis of the experimental results, we provide new insights into the relationship between the structure and properties for a single SnO2 nanoribbon. Experimental Section The SnO2 nanoribbons were grown on a silicon wafer substrate by a chemical vapor deposition using SnO powders * To whom correspondence should be addressed. E-mail: tsham@ uwo.ca, Phone: (519) 661-2111 x86297. Fax: (519) 661-3022 (T.K.S.); E-mail: [email protected]. Phone: (519) 661-2111 x86161. Fax: (519) 6613022 (Z.D.).

10.1021/jp709701s CCC: $37.00

(99.9%, Alfa Aesar) as the starting materials, which were placed in an alumina tube inserted in a horizontal tube furnace. The carrier gas, Ar, was introduced at one end of the alumina tube at a flow rate of 100 standard cubic centimeter per minute (SCCM). The temperature of the furnace was held at 1000 °C for 8 h. The details of the preparation have been described elsewhere.19 The morphologies, structures, chemical composition, and optical properties of the as-prepared SnO2 nanoribbons were examined by X-ray diffraction (XRD, Rigaku, Co K radiation, λ ) 0.179 nm), SEM (a LEO 1540XB FIB/SEM, operated at 1-3 keV with a working distance of 4-6 mm), EDX spectroscopy (10 keV), and transmission electron microscopy (TEM, JEOL 2010F). Confocal Raman microspectroscopy (Alpha SNOM, WITec, Germany) was conducted under ambient conditions. A YAG linearly polarized laser (Verdi 5, Coherent Inc., Santa Barbara, CA) with a 532 nm wavelength and an argon laser (Coherent Inc.) with a 514.5 nm wavelength were used for Raman excitation. A power of 10 mW of output from the lasers was selected for the experiments. A 50× objective with 0.75 NA (Nikon Canada, Mississauga, ON) was used to focus the laser beam onto the specimen and collect Raman signals. An edge filter (RazorEdge filters from Semrock, NY) with a band-pass >160 cm-1 and a Notch filter with a band-pass > (120 cm-1 were used to block the 532 and 514.5 nm excitation lasers, respectively. A complete Raman spectrum was recorded at each image pixel by an air-cooled (-70 °C), back-illuminated CCD camera (Model DV401-BV, ANDOR) behind a grating (300 or 1800 g mm-1) of a spectrograph (UHTS300, WITec). As a general practice, Raman images were constructed by integrating the intensities of characteristic Raman bands in a scan area in the X-Y orientation with 256 × 256 pixels, including a total of 65536 spectra. Each spectrum was integrated for 100 ms. Depth scanning images depicting the chemical distribution in the X-Z orientation were also conducted. Here, X and Y © 2007 American Chemical Society

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Figure 1. SEM image of as-grown SnO2 nanoribbons. Insets show the enlarged view of two types of ribbons; inset a demonstrates that there are nanostructures on the surfaces of some SnO2 ribbons, and inset b illustrates ribbons with a very smooth surface.

represent the dimensions on the sample surface, Z represents the normal direction, and the coordinate origin is on the center of the image. Results and Discussion Figure 1 shows a typical SEM image of the SnO2 nanoribbons, indicating that the as-prepared nanoribbons are several tens of micrometers in length, 100 nm to 1 µm in width, and several tens of nanometers in thickness. This agrees well with our previous experiments.19 Close examination of the ribbons is presented in the insets a and b of Figure 1. While inset b in Figure 1 shows ribbons with a very smooth surface, there are nanostructures on the surfaces of other SnO2 ribbons (inset a in Figure 1). The elemental composition of the two types of surfaces was analyzed by EDX, as shown in Supporting Information Figure S1. The ratio of Sn to oxygen is 1:2 for the smooth nanoribbons, Figure S1a. However, there is considerable oxygen deficiency in the second type of ribbon surfaces where the Sn/O is about 1:1.43 (Figure S1b). A high-resolution TEM image of a flat nanoribbon exhibits a uniform contrast and reveals a crystalline lattice space of 0.33 nm (Figure S2). This observation agrees well with X-ray diffraction results that the SnO2 nanoribbons of this investigation have the typical rutile crystalline structure.19 A typical Raman spectrum (Figure 2a) taken from a flat SnO2 nanoribbon sample shows three fundamental Raman peaks at 472, 633, and 775 cm-1, corresponding to the Eg, A1g, and B2g vibration modes of the rutile SnO2 structure, respectively.23 Besides these three main peaks, three weak abnormal Raman lines are observed at 501, 544, and 691 cm-1, which have been detected in the Raman spectra of nanobelts, nanocrystalline particles, and nanotube arrays, respectively.6,20,22,24,25 The A2u mode of the transverse optical (TO) phonons at 501 cm-1 and the A2u mode of the longitudinal optical (LO) phonons at 691 cm-1 are IR active. The B1u mode at 544 cm-1 is Raman forbidden. It is believed that the presence of IR modes and other forbidden Raman modes can be attributed to the breaking down of the prevailing q0 ) 0 selection rule (where q0 is the wavevector of the lattice vibration) as the degree of disorder increases (reducing crystal symmetry) or as the crystal size decreases to the nanoscale (limiting vibration to the size of the crystal and increasing the surface area).6,20,24,26 In addition, a very strong Raman signal around 200 cm-1 is observed on a ribbon with extra nanostructures, as shown in Figure 2b, using a high-resolution grating (1 cm-1 resolution). One sharp peaks

Letters at 170 cm-1 with a high intensity is observed, which can be ascribed to substoichiometric Sn2O3/Sn3O4 phases.27 Simultaneously, it shows strong and sharp double peaks at 238 and 245 cm-1(as demonstrated in the inset of Figure 2b), which were observed by K. Mcguire et al.1 They assigned these two peaks to the SnO2 nanobelts. On the basis of our observation, these two peaks should belong to a Sn3O4/Sn2O3 phase (see Raman imaging below). Because the peak position at 170 cm-1 is very close to the edge of the edge filter (which is 160 cm-1), this assignment will be supported by additional experiments. Figure 2c shows the Raman spectrum of these nanostructures between 100 and 800 cm-1, measured using a 514.5 nm laser and a Notch filter with a band-pass g (120 cm-1. Two sharp peaks at 143 and 171 cm-1 with high intensity were observed, which confirms the observation of the peak at 170 cm-1 with the edge filter. Those Raman bands were attributed to a SnOx suboxide in a Sn film at 450 °C, with x ranging from 1.33 to 1.5,27 that is, the SnOx suboxide is either Sn3O4 or Sn2O3. Figure 2c also shows the double peaks at 235 and 243 cm-1. Furthermore, an additional group of very intense bands between 1000 and 1800 cm-1 was observed, as shown in Figure 2d for the SnO2 ribbons with extra nanostructures. This group has the following characteristics: (1) The signal is extraordinarily strong, compared with normal Raman (100-1000 times); and (2) the full width at half-maximum of the peaks is as narrow as the Raman scattering peaks, as illustrated in Figure 2d. The small peaks on the top of the spectrum shown in Figure 2d are possibly temporary fluctuating in their position and intensity. Tin dioxide is the most commonly used gas sensing materials.2 The excitation light for Raman measurements might cause some photochemical reactions of the species, which gives these fluctuations. For instance, amorphous carbon is known to be quite sensitive to laser irradiation providing temporary fluctuating Raman signals, as reported by Kudelski.28 The SnOx phase might absorb some gases like hydrocarbons and carbon dioxide more easily than SnO2, leading to the observed fluctuation. Another possibility is that energy levels of the oxygen vacancies may vary, causing the slight discrepancy in luminescence. Meier et al. investigated vibrational and defect states in SnOx nanoparticle films with x changing from 1.5 to 1.7.29 Raman spectra of various sizes of Sn2O3 nanoparticles did show an extra-broad band centered at 1130 cm-1, although the authors did not elaborate.29 However, the intensity is much lower than what we observed. In addition, a Raman band with a maximum at 275 cm-1 was reported for Sn2O3 nanoparticles,29 which is different from our double peaks at 238 and 245 cm-1. On the basis of the above analysis, Raman scatterings at 238 and 245 cm-1 and the strong luminescence might be assigned to Sn3O4. We will come back to this point in the following discussion of Raman imaging. Figure 3 shows the Raman images of the SnO2 nanoribbons. Figure 3a is an overview of a collection of SnO2 nanoribbons in a 60 µm × 60 µm area, which was constructed with the total intensity of the whole Raman spectrum at each pixel. It can be seen that there are some bright sports scattered on the ribbons. Figure 3b illustrates the Raman image constructed with the normal Raman bands in the spectrum shown in Figure 2a. The ribbons are clearly visible. Figure 3c is the Raman image built with the extra bands between 1000 and 1800 cm-1 shown in Figure 2d. From images 3a-c, it is evident that the normal Raman signal and the extra bands come from different areas of the ribbons. The image created from the double peak at 238 and 245 cm-1 is shown in Figure 3d. It can be seen that the distribution of substances represented by vibration modes at 238

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Figure 2. Raman spectra of the SnO2 nanoribbon sample. (a) A typical Raman spectrum of a SnO2 nanoribbon with a smooth surface, excited by the 532 nm laser system. (b) A typical Raman spectrum of a ribbon that is covered with extra phases (SnOx); the inset shows the enlarged view of the spectrum between 200 and 800 cm-1. (c) The Raman spectrum of the sample in b, obtained by the 514.5 nm laser system. (d) The extra band between 1000 and 1800 cm-1 in the Raman spectrum of a ribbon with SnOx structures.

Figure 3. Raman images of the SnO2 nanoribbons. (a) An overview of a collection of SnO2 nanoribbons, composed of the total intensity from the whole Raman spectrum. (b) Raman image constructed with the normal Raman bands of SnO2 in the spectrum shown in Figure 2a. (c) Raman image constructed with the extra bands of the SnO2 nanoribbon in the spectrum shown in Figure 2c. (d) Raman image created from the double peak at 238 and 245 cm-1 shown by the spectrum in Figure 2d.

and 245 cm-1 is almost identical to that by the extra bands around 1000-1800 cm-1 in Figure 3c, which confirms that they are from the same phase. In order to understand the origin of the extra bands and its relationship with the ribbon surface structure, we recorded Raman images (Figure 4) from a single nanoribbon covered with surface nanostructures. In Figure 4, both the smooth surface of a single nanoribbon of this type and luminescent particle nanostructures grown on it can be clearly observed. This observation is in good agreement with the SEM result in Figure 1. The image in Figure 4a is an overview of the nanoribbon, obtained from the integration of the whole Raman spectrum at each pixel. Figure 4b was constructed from the normal Raman band at 633 cm-1, which clearly shows the SnO2 ribbon

underneath of where the bright spots are absent. Figure 4c and d were composed from the extra signal bands between 1000 and 1800 cm-1 and the double peaks at 238 and 245 cm-1, respectively. It can be seen that the images in Figure 4c and d show the same spots where the nanoparticles grown on the SnO2 nanoribbon are located. These features are a good indication that luminescent nanostructures might be assigned to Sn3O4 since Sn2O3 nanoparticles do not emit strong luminescence.29 Figure 2 confirms that the double peaks at 238 and 245 cm-1, representing the luminescent nanoparticles, do not belong to the SnO2 phase. The image created from the peak at 170 cm-1 (as presented in the inset of Figure 4d) shows the opposite contrast from the image made from the integration of double peaks at 238 and

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Figure 4. Raman images of a single SnO2 nanoribbon covered with SnOx. (a) An overview composed of the total intensity from the Raman spectrum at each image pixel. (b) Raman image constructed with the normal Raman bands of SnO2 in the spectrum. (c) Raman image built with the extra bands of the nanoribbon. (d) Raman image created from the double peaks at 238 and 245 cm-1. The inset of d shows the image created from the peak at 170 cm-1.

245 cm-1, and it is also different from the image (Figure 4b) representing the SnO2 vibration mode at 633 cm-1. This characteristic indicates that we can identify at least three phases in the single nanoribbon, SnO2, Sn3O4, and Sn2O3. The Raman band at 170 cm-1 might be assigned to Sn2O3. Unfortunately, an analysis of the intensity and frequency of the Raman-active modes for the SnOx phases is not available, to the best of our knowledge. These details can be further investigated by X-rayexcited optical luminescence and X-ray absorption near-edge structures along with the confocal Raman microspectroscopy. From the Raman depth scan image (as shown in Supporting Information Figure S3), we still can see that the ribbon structure with a normal Raman signal distributes in the center of the scanning range and the luminescent nanostructures grown on the surfaces (including the top and bottom ones) of the ribbon. It further confirms that the extra bands come from the nanoparticles grown on the SnO2 nanoribbon surface. The EDX results described in Figure S1 of the Supporting Information show that there are oxygen deficiencies in these nanostructures, which means that the composition of tin and oxygen is nonstoichiometric. This is consistent with the Raman results that SnOx (x < 2) exists in the specimen. It should be noted that, although the SnOx phases were not detected by XRD, a very strong Raman signal plus luminescence characteristics of these phases were observed. The strong signal could be attributed to resonance effects involving the absorption of the laser excitation wavelength at 532 (2.35 eV) and 514.5 nm (2.41 eV).2 Dai et al.30 suggested that the SnO2 formation in an oxygen atmosphere may proceed in two steps from SnO to Sn2O3 or Sn3O4 and then to SnO2. The Raman data obtained by K. Mcguire et al.1 and Sangaletti et al.27 exhibited that the as-prepared nanobelt samples contained at least two partially oxidized stoichiometric phases, which corresponded to Sn2O3/ Sn3O4 or some other unknown phase. As we know, many of the binary transparent conducting oxides (TCOs) already possess a high conductivity due to intrinsic defects, that is, oxygen deficiencies. This is also the case for SnO2, which is a good insulator or semiconductor in its stoichiometric form with a wide band gap. Nonstoichiometry, in particular, oxygen deficiency, makes it a conductor.2 The formation energy of oxygen vacancy Vo and tin interstitial Sni (where the interstitial means the interspaces between the lattices in the crystal.) in SnO2 is very low, and thus, defects can form readily. The introduction of causes the oxygen coordination to be replaced by Sn Sn4+ i atoms in SnO2, and the chemical composition becomes quasiSnO and quasi-SnOx (sometimes called the SnOx-like phase).2 This explains the often observed coexistence of high conductivity of pure SnO2 and nonstoichiometric SnOx.31 The same case occurred in our samples. Further, the two partially oxidized stoichiometric phases as nanoparticles grown on the as-prepared

SnO2 nanoribbons were clearly identified by Raman imaging. Moreover, the 532 nm (2.35 eV) laser is slightly above the band gap of Sn3O4, which is 2.0-2.2 eV,2 assuming that Sn2O3 grown on the nanoribbons does not emit strong luminescence as do the Sn2O3 nanoparticle films.29 The laser excited the resonant emission and some exciton-phonon coupling luminescence of the defect-phase Sn3O4 caused by the oxygen vacancy. As a result, this defect phase generated extra energy bands between 1000 and 1800 cm-1, which also explains why the luminescence band is so strong and narrow. It seems that the defect phase of SnOx is much easier to grow in particle-like structures than in ribbon-like ones, which can also be seen in related systems reported by Dai et al.3 Conclusions Confocal Raman microspectroscopy, combined with SEM/ EDX, has been used to analyze the morphologies, chemical structures, and optical properties of single tin dioxide nanoribbons under ambient conditions. The Raman spectrum of the SnO2 nanoribbon confirms its rutile structure. Three extra weak features at 501, 544, and 691 cm-1 were observed for the nanoribbon, which can be ascribed to symmetry breakage. Very interestingly, Raman images illustrate that SnOx (1 < x < 2) nanostructures grew on some single-crystal SnO2 nanoribbons. The growth was caused by the defects of an oxygen vacancy on the surface of SnO2. These nanostructures generated an additional group of very intense energy bands between 1000 and 1800 cm-1, which is attributed to a defect state SnOx-like resonant emission and exciton-phonon coupling luminescence. Acknowledgment. We appreciate the financial support of NSERC (New Discovery and Equipment Grants), OPC, CIPI, CFI, OIT, PREA, and UWO (Academic Development Fund and a Start-up Fund). The Ding and Sham groups are members of the Center for Chemical Physics and the Western Institute of Nano Science at Western. We thank Dr. Yang Song for sharing his Raman spectrometer with us. Technical assistance from John Vanstone, Jon Aukema, Sherrie McPhee, Mary Lou Hart, and Marty Scheiring is gratefully acknowledged. Supporting Information Available: Composition of nanoribbons and the luminescent nanostructures by EDX analysis; high-resolution TEM of a flat SnO2; depth scan Raman images of a single nanoribbon; and a spectrum of the luminescent nanostructures plotted in various Raman shift units. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McGuire, K.; Pan, Z. W.; Wang, Z. L.; Milkie, D.; Menendez, J.; Rao, A. M. J. Nanosci. Nanotechol. 2002, 2, 499-502.

Letters (2) Batzill, M.; Diebold, U. Prog. Surf. Sci. 2005, 79, 47-154. (3) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9-24. (4) Yang, M.-R.; Chu, S.-Y.; Chang, R.-C. Sens. Actuators, B 2007, B122, 269-273. (5) Calestani, D.; Lazzarini, L.; Salviati, G.; Zha, M. Cryst. Res. Technol. 2005, 40, 937-941. (6) Peng, X. S.; Zhang, L. D.; Meng, G. W.; Tian, Y. T.; Lin, Y.; Geng, B. Y.; Sun, S. H. J. Appl. Phys. 2003, 93, 1760-1763. (7) Zheng, M. J.; Ma, L.; Xu, W. L.; Ding, G. Q.; Shen, W. Z. Appl. Phys. A 2005, 81, 721-723. (8) Li, L.; Zong, F.; Cui, X.; Ma, H.; Wu, X.; Zhang, Q.; Wang, Y.; Yang, F.; Zhao, J. Mater. Lett. 2007, 61, 4152-4155. (9) Liu, Y.; Zheng, C.; Wang, W.; Yin, C.; Wang, G. AdV. Mater. 2001, 13, 1883-1887. (10) Sergent, N.; Epifani, M.; Pagnier, T. J. Raman Spectrosc. 2006, 37, 1272-1277. (11) Sirbuly, D. J.; Tao, A.; Law, M.; Fan, R.; Yang, P. AdV. Mater. 2007, 19, 61-66. (12) Dong, Q.; Su, H.; Xu, J.; Zhang, D. Sens. Actuators, B 2007, B123, 420-428. (13) Mosca, R.; Zha, M.; Calestani, D.; Lazzarini, L.; Salviati, G.; Zappettini, A.; Zanotti, L.; Comini, E.; Sberveglieri, G. Mater. Res. Soc. Symp. Proc. 2006, 915, 137-142. (14) Vilaca, G.; Jousseaume, B.; Mahieux, C.; Belin, C.; Cachet, H.; Bernard, M.-C.; Vivier, V.; Toupance, T. AdV. Mater. 2006, 18, 10731077. (15) Kudo, N.; Shimazaki, Y.; Ohkita, H.; Ohoka, M.; Ito, S. Sol. Energy Mater. Sol. Cells 2007, 91, 1243-1247. (16) Chiodini, N.; Paleari, A.; DiMartino, D.; Spinolo, G. Appl. Phys. Lett. 2002, 81, 1702-1704.

J. Phys. Chem. C, Vol. 111, No. 51, 2007 18843 (17) Pan, S. S.; Ye, C.; Teng, X. M.; Li, L.; Li, G. H. Appl. Phys. Lett. 2006, 89, 251911/1-251911/3. (18) Brovelli, S.; Chiodini, N.; Meinardi, F.; Lauria, A.; Paleari, A. Appl. Phys. Lett. 2006, 89, 153126/1-153126/3. (19) Zhou, X. T.; Heigl, F.; Murphy, M. W.; Sham, T. K.; Regier, T.; Coulthard, I.; Blyth, R. I. R. Appl. Phys. Lett. 2006, 89, 213109/1-213109/ 3. (20) Zhou, J. X.; Zhang, M. S.; Hong, J. M.; Yin, Z. Solid State Commun. 2006, 138, 242-246. (21) Shan, C. X.; Liu, Z.; Zhang, X. T.; Wong, C. C.; Hark, S. K. Nanotechnology 2006, 17, 5561-5564. (22) Swamy, V.; Muddle, B. C.; Dai, Q. Appl. Phys. Lett. 2006, 89, 163118/1-163118/3. (23) Peercy, P. S.; Morosin, B. Phys. ReV. B 1973, 7, 2779-2786. (24) Dieguez, A.; Romano-Rodriguez, A.; Vila, A.; Morante, J. R. J. Appl. Phys. 2001, 90, 1550-1557. (25) Liu, Y.; Liu, M. AdV. Funct. Mater. 2005, 15, 57-62. (26) Wang, J. X.; Liu, D. F.; Yan, X. Q.; Yuan, H. J.; Ci, L. J.; Zhou, Z. P.; Gao, Y.; Song, L.; Liu, L. F.; Zhou, W. Y.; Wang, G.; Xie, S. S. Solid State Commun. 2004, 130, 89-94. (27) Sangaletti, L.; Depero, L. E.; Allieri, B.; Pioselli, F.; Comini, E.; Sberveglieri, G.; Zocchi, M. J. Mater. Res. 1998, 13, 2457-2460. (28) Kudelski, A. Chem. Phys. Lett. 2006, 427, 206-209. (29) Meier, C.; Luttjohann, S.; Kravets, V. G.; Nienhaus, H.; Lorke, A.; Ifeacho, P.; Wiggers, H.; Schulz, C.; Kennedy, M. K.; Kruis, F. E. J. Appl. Phys. 2006, 99, 113108/1-113108/6. (30) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 8673-8680. (31) Kilic, C.; Zunger, A. Phys. ReV. Lett. 2002, 88, 095501/ 1-095501/4.