Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ Chapter OPTICAL LIMITING AND Z-SCAN STUDIES OF MONOAND MULTI-FUNCTIONAL FULLERENE (C60) INCORPORATED WITH POLYMERS 5.1. Introduction Fullerene and its derivatives have attracted a great deal of interest for their unique interaction with polymers [5.1-5.7] and their wide range of promising applications as an excellent optical limiter for eye and sensor protection from high intensity laser pulses [5.8,5.9]. Other potentials such as optical switches and a single-C60 transistor have been investigated [5.10,5.11]. Moreover, recently strong magnetic signals have been found in rhombohedral C60 polymer (Rh-C60), and studies of synthetic route to the C60H30 polycyclic aromatic hydrocarbon and its laser-induced conversion into fullerene-C60 have also received attention [5.12,5.13]. Hence, the preparation and both electronic and optical properties of fullerene composites are still an interesting research topic. C60-based polymeric materials are normally prepared by simply dispersing C60 into polymeric matrices. However, the poor adhesion between C60 and polymer prevents the well dispersion of C60 in the matrix [5.14,5.15]. To improve the affinity therein, C60 is functionalized and interacts with complementary functional groups of a polymer, and the specific interaction between C60 and the polymer plays an important role in the resulting 48 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ mechanical performance and optical properties [5.14-5.16]. It is further expected that the optical properties of C60 derivatives may change upon functionalization. So far, Sun and his colleagues [5.3,5.17,5.18] have examined a series of methano-C60 derivatives and they have found that the mono-functional C60 derivatives show similar optical limiting responses to that of parent C60 while multi-functional ones give poorer performance. In addition, since the first example of C60 involved in supramolecular phenomena was reported by Ermer in 1991 [5.19], much related research [5.20-5.23] has been focused on the formation of supramolecular buckminsterfullerene, fullerene chemistry, and assemblies and arrays held together by weak intermolecular interactions. Moreover, the recent studies deal with supramolecular C60-containing polymeric materials based on functionalized C60 and polymers possessing suitable functional groups, which successfully overcome the incompatibility between pristine C60 and polymers. However, the nonlinear optical (NLO) properties of similar supramolecular fullerene incorporated with polymers (PSI-46) have not been fully investigated yet. Consequently, the main challenge that still remains is how to improve the optical limiting and NLO properties of C60. In this chapter, we report a systematic investigation of excited state mechanisms correlated to the NLO properties of mono-functional 1,2-dihydro-1,2- methanofullerene[60]-61-carboxylic acid (FCA), multi-functional fullerenol and 1-(4methyl)- piperazinylfullerene[60] (MPF) –containing polymers in room-temperature solution measured under the same experimental conditions as those used for the fullerenes. The results show that the NLO responses of mono-functional FCA/PSVPy32 toward nanosecond laser pulses at 532 nm are better than that of the [60]fullerene, which 49 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ suggests significant contributions of triplet-triplet states absorption and a fast intersystem crossing (ISC) associated with dynamics among excited states. Our data indicate that the optical limiting performances of the fullerenes are significantly affected by the disturbance of π–electronic system in C60 cage due to multi-addends of fullerenol or MPF. Mechanistic implications of our results are discussed, and an excited state reverse saturable absorption model that consistently accounts for the NLO properties of all the samples is presented. 5.2. Materials Our C60 (99.9%) sample used in these researches was obtained from Beijing University, Beijing, China. While mono-functional 1,2-dihydro-1,2- methanofullerene[60]-61-carboxylic acid, multi-functional fullerenol and 1-(4-methyl)piperazinylfullerene[60] –containing polymers were prepared in the following steps, respectively. Firstly, 1,2-dihydro-1,2-methanofullerene [60]-61-carboxylic acid (FCA) was synthesized by the method reported by Isaacs and his co-workers [5.24,5.25]. The FCA has been prepared carefully to be [6,6]-closed isomer with 58 π-electrons. Poly (styreneco-4-vinylpyridiene) (PSVPy) and polystyrene (PS) were prepared by free-radical copolymerization initiated by an initiator for polymerization (AIBN). The molar percentage of 4-vinylpyridine unit in PSVPy32 was 32% as determined by elemental analysis. C60 and FCA were dissolved in toluene and 1,2-dichlorobenzene, respectively, into which appropriate amounts of PSVPy32 and PS were added. The six samples were prepared with the same transmittance of 65 % and concentration of ~1.6x10-3 M, as shown in Table 1. The structure of FCA [5.2] is shown below. 50 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ O H OH FCA Secondly, multi-functional fullerenol (Fol) was synthesized and characterized according to literature [5.26]. On the average, Fol has 10 to 12 hydroxy addends [5.1]. Toluene and N,N-dimethylformamide (DMF) of AR grade were purchased from Fisher Scientific Company, USA and used as received. Two poly(styrene-co-4-vinylpyridine) (PSVPy) samples containing 20 and 32 mol% of vinylpyridine, denoted as PSVPy20 and PSVPy32, respectively, were prepared by free radical copolymerization. Poly(styrene-cobutadiene) was purchased from Aldrich Company, USA, which is a kind of high-impact polystyrene (HIPS) with a melt index (200oC/5 kg, ASTM D 1238) of 2.8 g/10 min. (OH)10-12 Fol Fol was dissolved in DMF, into which an appropriate amount of PSVPy32 was added. The mixture was stirred continuously overnight and then added into diethyl ether dropwise with vigorous stirring. The precipitates (blends) were isolated by centrifugation and washed with diethyl ether for three times. The blends were dried in vacuo at 50 oC for one week. The Fol contents in various blends were determined by thermogravimetric 51 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ analysis. The brittleness of PSVPy made it difficult to prepare Fol/PSVPy samples for dynamic mechanical analysis (DMA) measurements [5.1]. Instead, a mixture of 26 parts of PSVPy20 and 74 parts of HIPS was used as the matrix. To achieve a better compatibility between PSVPy and HIPS, PSVPy20 was used in view of its higher styrene content. Appropriate amounts of Fol, PSVPy20 and HIPS were mixed in a Laboratory Mixing Molder (ATLAS, USA) at 190 oC for 30 at a speed of 120 rpm. The mixed samples were compressed into films with a thickness ca. 0.25 mm under a pressure of 12 MPa at 140 oC and then at room temperature at the same pressure for 30 using a hydraulic press (Fred S. Carver Inc., USA). The Fol contents in various samples were determined by thermogravimetric analysis. Samples used in this study are PSVPy20/HIPS polymer matrix, and polymer matrix filled with 2.4, 4.8, and 11.6 wt% of Fol, which are denoted as Fol2.4, Fol4.8, Fol11.6, respectively. Finally, supramolecular multi-functional 1-(4-Methyl)- piperazinylfullerene (MPF) - containing polymers was prepared based on the following steps. 1Methylpiperazine (98%) was purchased from Sigma-Aldrich Company, USA. (3Cyanopropyl)methyldichlorosilane and dichlorodimethylsilane were supplied by Fluka Chemika-Biochemika Company. Chlorobenzene (AR grade) was purchased from BDH, UK, and tetrahydrofuran (THF, AR grade) from Fisher Scientific, UK. 1-(4-Methyl)piperazinylfullerene, a red and powder-like multi-functional C60 derivative, was synthesized and characterized according to literature [5.27], which has an average stoichiometry of C60(HNC4H8NCH3)9 with a structure as follows: 52 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ H9 MPF (N N CH3)9 The random copolymer of dimethylsiloxane and (3-carboxypropyl)methylsiloxane was synthesized and characterized according to the method reported by Li and Goh [5.28]. It contains 45.5 mol% of (3-carboxypropyl)methylsiloxane unit as determined by 1H-NMR and has a number-average molecular weight 4,100 and polydispersity 1.08. This transparent and gel-like copolymer is denoted as PSI-46 with the following structure: CH3 CH3 (Si-O) m CH3 ( Si-O )n CH2 CH2 H2C COOH PSI-46 To prepare supramolecular multi-functional MPF/PSI-46 composites, an appropriate amount of PSI-46 was added into the THF solution of MPF. The mixture was continuously stirred overnight. Three samples were prepared, which are MPF/PSI-46 (1:2), MPF/PSI-46 (1:4), and MPF/PSI-46 (1:6), denoted as MPF(1:2), MPF(1:4), and MPF(1:6), respectively where the ratios in the parentheses refer to the ratios of nitrogen atoms of MPF over the carboxylic groups of PSI-46. 53 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ 5.3. Results and discussion 5.3.1. Ultraviolet and visible (UV-vis) absorption spectra The ultraviolet and visible (UV-vis) absorption sectra of the supramolecular mono- and multi-functional C60 - containing polymers measured in room-temperature solutions were recorded at the wavelength range ~190-820 nm on a Hewlett Packard 8452A Diode Array spectrophotometer with a Hewlett Packard Vectra QS/165 computer system. The spectral parameters are summarized in Table 5.1. For the mono-functional FCA-polymer composites, the absorption spectra are quite similar to that of C60 as shown in Figure 5.1(a). The multi-functional Fol has a marginally different absorption spectrum in comparison with its parent C60 in which the spectrum is slightly blue shifted. In contrast, the MPF has a significantly different spectrum from that of C60. The difference is not only in spectral shape but also in absorptivity (Figure 5.1(c), and Table 5.1). This indication is caused by the broken symmetry of the π–electronic system in C60 cage due to multi-addends of MPF. 5.3.2. Ground-State Absorption Parameters The ground-state absorption parameters of the supramolecular mono-functional FCA, and multi-functional Fol and MPF - containing polymers at room temperature are shown in Table 5.1. The parameters are noticeably different from that of C60. At 532 nm, the absorptivity (ε) of FCA and Fol -polymer composites is larger than that of its parent C60. This contributes to the higher ground state cross section of the supramolecular in comparison with that of C60. Similar observation has also been investigated by Sun et al. [5.3] in a series of fullerene derivatives. However, at the same wavelength the molar 54 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ absorptivity of MPF observed at higher transmittance of 75% is much poorer than that of its parent, indicating the effect of broken symmetry in the π–electronic system of C60 cage. Moreover, the ground-state absorption spectra as displayed in Figure 5.1(c) of the Absorbance (a.u) Absorbance (a.u) Absorbance (a.u) MPF-polymer composites provide no evidence for molecular aggregation. 1: C60 (a) 2: FCA 3: FCA/PSVPy32-A 4: FCA/PS-A 5: C60-c (b) 6: Fol-c 7: Fol/PS-c 8: Fol/PSVPy32-c1 : C60 (c) 10: MPF 11: MPF/PSI-46(1:6) 12: PSI-46 10 11 12 177 354 531 708 885 λ (nm) Fig. 5.1. UV-Vis absorption spectra of (a) 1: C60 in toluene, 2: FCA, 3: FCA/PSVPy-A, and 4: FCA/PS-A, wherein, 2, and are in 1,2-dichlorobenzene solutions; (b) 5: toluene solutions of C60-c, DMF solutions of 6: Fol-c, 7: Fol/PS-c, and 8: Fol/PSVPy32c1; (c) 9: C60 in chlorobenzene, 10: MPF and 11: MPF/PSI-46(1:6) in tetrahydrofuran (THF), respectively, and 12: PSI-46. All the solutions are directly used in the optical limiting and Z-scan measurements at room temperature. 55 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ Table 5.1. Ground-state absorption parameters measured at 532 nm of C60, and the supramolecular mono-functional FCA, and multi-functional Fol and MPF - containing polymers. Sample Solvent Concentration (M) Linear Transmittance at 532 nm Absorptivity ε (M-1 cm-1) C60 Toluene 1.6 x10-3 65% 940 1.6 x10-3 65% 1169 1.6 x10-3 65% 1169 1.6 x10-3 65% 1169 1.6 x10-3 65% 1169 1.6 x10-3 65% 1169 1.2dichlorobenzene 1.2FCA/PSVPy32-A dichlorobenzene 1.2FCA/PSVPy32-B dichlorobenzene 1.2FCA/PS-A dichlorobenzene 1.2FCA/PS-B dichlorobenzene FCA C60-d Toluene 1.65x10-4 70.1% 940 C60-c Toluene 1.65 x10-3 70.1% 940 Fol-d DMF 1.42 x10-4 70.5% 1069 Fol/PSVPy32-d1 DMF 1.42 x10-4 69.6% 1108 Fol-c DMF 1.42 x10-3 70.3% 1080 Fol/PSVPy32-c2 DMF -3 1.42 x10 69.5% 1113 C60 Chlorobenzene 1.32 x 10-3 75.3% 940 75.2 140 100% – MPF -3 THF 9.05 x 10 PSI-46 THF 14.0 gl -1 MPF(1:2) THF 9.05 x 10-3 75.0% 140 MPF(1:4) THF 9.05 x 10-3 74.7% 140 MPF(1:6) THF 9.05 x 10-3 74.9% 140 56 Chapter Optical limiting and Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers ________________________________________________________________________ 5.3.3. Photoluminescence performance of FCA, Fullerenol, and MPF – containing polymers versus C60 The photoluminescence (PL) measurement for each sample was carried out using a luminescence spectrometer (LS 55, Perkin-Elmer Instrument U.K.) with the excitation wavelength of 442 nm for FCA-polymer composites, and 480 nm for multi-functional Fol and MPF – polymer composites, respectively. Photoluminescence spectra of the FCA, Fol, and MPF - containing polymers were subsequently measured in room-temperature with different concentration of polymers. As shown in Figure 5.2(a), all mono-functional FCA incorporated with PSVPy or PS has poorer PL intensity than that of the multifunctional Fol and MPF – polymer composites. In general, one can also observe that most of the fullerene derivatives have an improvement of the PL by the reducing of the polymers concentration (Figures 5.2(b) and 5.2(c)). In addition, for the supramolecular multi-functional Fol and MPF, the peaks of PL spectra are apparently blue-shifted in comparison to that of its parent C60 (Figure 5.2). The results are consistent with that based on the UV-vis measurement in Fig. 5.1. To evaluate qualitatively that there is light emission from the lowest excited singlet state to the ground state, that may affect the population of excited electrons at the excited triplet states, we have performed a series of photoluminescence (PL) measurements at room temperature. Figure 5.2(a) shows that there is an increase in the PL intensity of mono-functional FCA, in comparison to its parent C60. This is due to a significant disturbance of π-electron system of C60 cage upon multi-functionalization, consistent with our UV-Vis absorption spectra (Figure 5.1). Figure 5.2(a) also shows that 57 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ have a broader band transmission characteristic compared to pure MWNTs. If these coated MWNTs are used as protective layer for an optical sensor, they would not alter the spectral responsivity of the optical sensors. Fig. 6.6. SEM images of (a) randomly aligned MWNTs; (b) MWNTs coated with 80 nm a-SiC (MWNT-a-SiC80); (c) MWNTs coated with 180 nm a-SiC (MWNT-a-SiC180); (d) MWNTs coated with 180 nm a-SixNy (MWNT-aSiN180). 100 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ Transmittance (%) 80 60 40 MWNTs MWNT-a-SiN180 MWNT-a-SiC80 λ = 532 nm 20 200 400 600 800 1000 Wavelength (nm) Fig. 6.7. Linear transmission spectra of MWNTs (solid line), MWNT-a-SiC80 (dashed line), and MWNT-a-SiN180 (dotted line) suspended in water. A more stable transmission characteristic is obtained for thin film coated MWNTs. 6.3. Results and discussion To study the optical limiting (OL) performances of the carbon nanomaterials, OL measurements were carried out. In these OL measurements, all the samples of carbon nanoparticles as well as carbon nanocomposites or carbon nanoballs, and aSixNy or a-SiC coated MWNTs were suspended in water. These measurements were 101 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ conducted with linearly polarized laser pulses of 7-ns duration from frequencydoubled Nd:YAG laser (Spectra-Physics, DCR3), which has wavelength of 532 nm and its fundamental wavelength of 1064 nm. The minimum beam waist of the focused laser beam was ~35 μm, determined by the standard Z-scan method [6.24]. The OL measurements [6.25] conducted using single shot techniques were carried out by fixing the sample at the focal point. The details of the experimental setup can be found in Chapter 2, section 2.2 [6.1,6.25]. Figure 6.8(a) shows the nonlinear transmission of C60 in toluene, and carbon nanocomposites of Ni20Graphite80, Ni20Graphite80Y0.05, and (BN)20Graphite80 in distilled water, respectively measured at 532 nm. Here, all the carbon nanocomposites have the same linear transmission of T = ~70%. This set of data obviously brings evidence that at the same composition of Graphite80 in the carbon nanocomposites, Ni20Graphite80Y0.05 has shown better optical limiting properties than that of Ni20Graphite80 and (BN)20Graphite80. Therefore, the Y0.05 added to Ni20Graphite80 during the fabrication of the carbon nanocomposites of Ni20Graphite80Y0.05 has slightly improved the optical limiting performance of Ni20Graphite80. In Fig. 6.8(b), the carbon nanocomposites of Co40Graphite60 indicates that its optical limiting behavior is marginally superior in comparison with that of C60 in toluene, and carbon nanocomposites of Ni20Graphite80Y0.05, and B50Graphite50 in distilled water, respectively. It means that from different composition of graphite in the carbon nanocomposites, Co40Graphite60 is the best choice for optical limiting nanomaterials in this group. 102 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ 0.8 0.7 (a) 0.6 Ni20Graphite80 Ni20Graphite80Y0.05 (BN)20Graphite80 Transmittance (x 100%) 0.5 0.4 0.3 C60 in Toluene 0.2 0.1 0.1 0.8 0.7 (b) 0.6 0.5 0.4 0.3 0.2 C60 in Toluene Ni20Graphite80Y0.05 Co40Graphite60 B50Graphite50 0.1 0.1 Input Fluence (J/cm2) Fig. 6.8. (a) Nonlinear transmission of C60 in toluene (filled squares), and carbon nanocomposites of Ni20Graphite80 (filled inverted triangles), Ni20Graphite80Y0.05 (open triangles), and (BN)20Graphite80 (open hexagons) in distilled water, respectively. (b) Optical limiting responses of C60 in toluene (filled squares), and carbon nanocomposites of Ni20Graphite80Y0.05 (open triangles), Co40Graphite60 (open diamonds), and B50Graphite50 (open circles) in distilled water, respectively. All the samples both in (a) and (b) have the same linear transmission of T = ~70% and the optical path of 10 mm at wavelength of 532nm. 103 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ To assess the superiority of the OL performances on Co40Graphite60, one subsequently compares such properties with that of other group of carbon family such as carbon nanoparticles of Graphite-Merck, and carbon nanoballs of CNBs-CVD suspended in distilled water solutions, respectively. Fig. 6.9(a) displays the comparison of the optical limiting performance of these samples conducted at the same linear transmission of T = ~70 %. It is obviously seen that carbon nanocomposites of Co40Graphite60 indicates superiority among the carbon nanoparticles of Graphite-Merck and CNBs-CVD. On the other hand, it is interesting to note that when the initial linear transmittance of Co40Graphite60 is increased to be ~80 % at 532 nm as shown in Fig. 6.9(b), we observed that the nonlinear transmission behaviors of Co40Graphite60 in distilled water is comparable with that of carbon nanoparticles prepared by laser ablation method denoted as carbon nanoparticles-lab. Furthermore, their OL behaviours are also much better than C60 in toluene. This result should open a window of opportunity for nonlinear-optical limiting technology. The OL mechanism of both Co40Graphite60 and carbon nanoparticles are proposed to be due to nonlinear scattering process related to micro-plasma expansion. At higher fluence of ~0.62 J/cm2, the heat in the micro-plasmas is quickly transferred to the surrounding distilled water and forms bubbles. These bubbles enhance consequently the nonlinear scattering effect. The mechanism is quite similar to that observed in carbon nanotubes [6.2,6.3]. 104 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ 0.8 0.7 (a) 0.6 Transmittance (x 100%) 0.5 Graphite-Merck CNBs-CVD Co40Graphite60 0.4 0.3 λ = 532 nm 0.2 0.1 0.01 1.0 0.8 0.1 10 (b) 0.6 0.4 0.2 0.0 0.01 C60 in Toluene Carbon nanoparticles-lab Co40Graphite60 0.1 10 Input Fluence (J/cm2) Fig. 6.9. (a) Comparison of the optical limiting performance of Co40Graphite60 (open diamond), and that of carbon nanoparticles of Graphite-Merck (filled circles), and CNBs-CVD (open squares) in distilled water solutions, respectively. All the samples have the same linear transmission of T = ~70 % and the optical path of 10 mm. (b) Nonlinear transmission behavior of carbon nanoparticles prepared by laser ablation method explained in section 6.1. (open inverted triangles), and Co40Graphite60 (open diamond) in distilled water in comparison with that of C60 in toluene. Here, the initial linear transmittance of all the samples is ~80 % at 532 nm, and the used optical path is 10 mm. 105 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ To reconcile the broadband OL properties of a-SixNy and a-SiC coated MWNTs, we have performed the OL measurements of the samples at wavelengths of 532 and 1064 nm, respectively. It should be noted that each data in this experiment was collected using a single shot mode with the time interval of 20 seconds. The aim is to eliminate the dominant thermal heating effect of high nanosecond laser intensity to the micro-plasma of MWNTs. Therefore, the pure OL performances can be obviously observed. Figure 6.10(a) shows that the OL behavior of the MWNT-aSiN180 is obviously inferior in comparison with that of the pure MWNTs, while the OL of MWNT-a-SiC80 is marginally poorer than that of its parent MWNTs at 532 nm. The reason is that a-SixNy and a-SiC films may have blocked some areas of the expansion of MWNTs micro-plasma. Therefore, a heat transfer from MWNTs to surrounding distilled-water leading to solvent bubble growth is reduced, and a phase transition from solid to gas of the MWNTs generating explosive growth of hot carbon vapour cavities is thus limited as a result in reducing the performance of OL MWNT it self. In addition, one can identify from Fig. 6.10(b) that the OL performance of MWNT-a-SiC80 is significantly improved in comparison with that of MWNT-aSiC180 at all the input fluences. Therefore, it is confirmed that the thicker the a-SiC in MWNT-a-SiC is, the poorer the OL behavior of MWNT-a-SiC is. 106 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ 0.7 0.6 (a) 0.5 MWNTs MWNT-a-SiC80 MWNT-a-SiN180 SixNy λ = 532 nm Transmittance (x 100%) 0.4 0.3 0.2 0.1 0.01 0.1 10 100 10 100 0.7 0.6 (b) 0.5 0.4 0.3 0.2 0.1 0.01 MWNTs MWNT-a-SiC80 MWNT-a-SiC180 λ = 532 nm 0.1 Input Fluence (J/cm2) Fig. 6.10. Optical limiting responses of various samples in a constant ~60% linear transmittance at 532nm. (a): MWNTs (●), MWNT-a-SiC80 (□), MWNT-a-SiN180 (Δ), and SixNy (▲) solution (b): MWNTs (●), MWNT-a-SiC80 (□), and MWNT-a-SiC180 () solution 107 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ Figure 6.11 shows the optical limiting responses of MWNTs, MWNT-aSiC80, MWNT-a-SiC180, and MWNT-a-SiN180 in a constant ~60% linear transmittance at 1064 nm. Here, one can observe the same OL characteristics of those measured at wavelength of 532 nm. The results show that coating either a-SiC or aSixNy over the whole carbon nanotube does not remove the optical limiting ability of the MWNT. This is because a-SixNy and a-SiC are essentially not optically active, as confirmed by optical limiting measurements done on bulk Si3N4 and SiC powders, which give linear transmission only. Thus the laser light is able to pass through the coating and interact with the enclosed MWNT, thereby producing the optical limiting effect. Measurements at both wavelengths show good consistency, with pure MWNT giving the best and MWNT-SiC180 the poorest optical limiting characteristics. As for MWNT-SiC80 and MWNT-SiN180, both have poorer optical limiting behavior than their parent MWNTs. This observation can be explained by the nonlinear scattering of the laser light by the MWNTs. In the nonlinear scattering, the key mechanism is the ionization and excitation of the MWNTs to form rapidly expanding micro-plasmas. When we coat MWNTs with either a-SixNy or a-SiC, we are restricting the expansion of micro-plasmas formed within the MWNTs. Since the micro-plasmas formed in the coated MWNTs cannot expand as readily as those formed in the pure MWNTs, the extent of light scattering by the microplasmas is smaller. Thus, pure carbon nanotubes have the greatest optical limiting effect among these four samples. We can also see that the MWNT-SiN180 shows slightly stronger optical limiting properties compared to MWNT-SiC180. This phenomenon can be explained by the relative hardness of aSixNy and a-SiC. Nanoindentation of standard films grown under similar deposition conditions show that a-SiC has a higher hardness value compared to a-SixNy. Thus, although the thickness of the a-SixNy and a-SiC coatings are similar, the micro- 108 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ plasmas formed by carbon nanotubes in MWNT-a-SiN180 as shown in Fig. 6.10 expands more easily and thereby scatters light to a greater extent. Transmittance (x 100 %) 0.7 0.6 0.5 0.4 0.3 0.2 0.01 MWNTs MWNT-a-SiC80 MWNT-a-SiC180 MWNT-a-SiN180 λ = 1064 nm 0.1 10 Input Fluence (J/cm2) Fig. 6.11. Optical limiting responses of MWNTs (●), MWNT-a-SiC80 (□), MWNTa-SiC180 (), and MWNT-a-SiN180 (Δ) in a constant ~60% linear transmittance at 1064 nm. 109 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ In comparing the optical limiting behavior of MWNT-a-SiC80 and MWNT-aSiC180 at both 532 and 1064 nm wavelengths, we observe that MWNTs coated with the thinner a-SiC coating shows a better optical limiting effect. This result is consistent with previous findings that optical limiting behavior is size-dependent [6.26]. The thick MWNT-a-SiC180 hinders micro-plasma expansion in the carbon nanotubes. Moreover, with a thicker layer of a-SiC deposited, thermal conduction to the surrounding water becomes poorer, suppressing bubble formation. 6.4. Conclusion In conclusion, we have studied the optical limiting properties of a series of new carbon families of carbon nanocomposites, carbon nanoparticles, carbon nanoballs, and amorphous films of SixNy, and SiC coated carbon nanotubes, respectively. It has been established that at the initial linear transmittance of ~70%, the carbon nanocomposites of Co40Graphite60 posses better OL properties than that of Ni20Graphite80Y0.05, and B50Graphite50. Moreover, when the initial linear transmittance of all the carbon nanomaterials is increased to be ~80 % at wavelength of 532 nm, we have observed that the nonlinear transmission behaviors of Co40Graphite60 in distilled water is comparable with that of carbon nanoparticles prepared by laser ablation. Such high OL performance suggests a strong potential applications in all-optical limiting devices. On the other hand, although both a-SiC and a-SixNy are optically inactive materials, as coatings they influence the optical limiting properties of MWNT by hindering the nonlinear scattering mechanism in the carbon nanotubes. Linear transmission spectra as well as OL measurements show a broader band transmission 110 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ characteristic for a-SiC or a-SixNy coated MWNTs. Such materials are ideal as protective layers on optical sensors that function in the UV optical wavelength regions. The optical limiting effects of the MWNT can be reduced by changing the thickness or material of the coating, which acts to suppress the extent of micro-plasma expansion in the carbon nanotubes. 111 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ References [6.1]. H. I. Elim, J. Ouyang, J. He, S. H. Goh, S. H. Tang, W. Ji, Chem. Phys. Lett. 369, 281 (2003). [6.2]. X. Sun, R.Q. Yu, G.Q. Xu, T.S.A. Hor, and W. Ji, Appl. Phys. Lett. 73, 3632 (1998). [6.3]. P. Chen, X. Wu, X. Sun, J. Lin, W. Ji, and K.L. Tan, Phys. Rev. Lett. 82, 2548 (1999). [6.4]. G. A. J. Amaratunga, M. Chhowalla, C. J. Kiely, I. Alexandrou, R. Aharonov and R. M. Devenish, Nature (London) 383, 321 (1996). [6.5]. A. Bezryadin, R. M. Westervelt, and M. Tinkham, Appl. Phys. Lett. 74, 2699 (1999). [6.6]. J. Yu, E. G. Wang and X. D. Bai, Appl. Phys. Lett. 78, 2226 (2001). [6.7]. L. Diederich, E. Barborini, P. Piseri, A. Podestà, P. Milani, A. Schneuwly and R. Gallay, Appl. Phys. Lett. 75, 2662 (1999). [6.8]. M. P. Siegal, D. L. Overmyer, R. J. Kottenstette, D. R. Tallant, and W. G. Yelton, Appl. Phys. Lett. 80, 3940 (2002). [6.9]. D. Li, Y. Liu, H. Yang, and S. Qian, Appl. Phys. Lett. 81, 2088 (2002). [6.10]. D. Vincent, S. Petit, S. L. Chin, Appl. Optics 41, 2944 (2002). [6.11]. Z. Jin, X. Sun, G. Xu, S. H. Goh, and W. Ji, Chem. Phys. Lett. 318, 505 (2000). [6.12]. Z. Jin, L. Huang, S. H. Goh, G. Xu, and W. Ji, Chem. Phys. Lett. 332, 461 (2000). [6.13]. G. X. Chen, M. H. Hong, T. C. Chong, H.I. Elim, G. H. Ma, and W. Ji, J. Appl. Phys. 95, 1455 (2004). 112 Chapter Optical limiting studies of new carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes _____________________________________________________________________ [6.14]. J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt and S. R. P. Silva, J. Appl. Phys. 80, 440 (1996). [6.15]. D. S. Knight and W. B. White, J. Mater. Res. 4, 385 (1989). [6.16]. Z.Y. Zhong, H. Chen, S. Tang, J. Ding, J. Lin, K.L. Tan, Chem. Phys. Lett. 330, 41 (2000). [6.17]. P. Chen, H.B. Zhang, G.D. Lin, Q. Hong and K.R. Tsai. Carbon 35, 1495 (1997). [6.18]. Z. Y. Zhong, Z. T. Xiong, L. F. Sun, J. Z. Luo, P. Chen, J. Lin, and K. L. Tan, J. Phys. Chem. B 106, 9507-9513 (2002). [6.19]. G.W. Ho, A.T.S. Wee, J. Lin, Appl. Phys. Lett. 79, 260 (2001). [6.20]. A. Gohel, K.C. Chin, K.Y. Lim, S.T. Tay, R. Liu, G.S. Chen, A.T.S. Wee, Chem. Phys. Lett. 371, 131 (2003). [6.21]. K.Y. Lim, C.H. Sow, J. Lin, F.C. Cheong, Z.X. Shen, J.T.L. Thong, K.C. Chin, A.T.S. Wee, Adv. Mater. 15, 300 (2003). [6.22]. F.C. Cheong, K.Y. Lim, C.H. Sow, J. Lin, C.K. Ong, Nanotechnology 14, 433 (2003). [6.23]. K.C. Chin, A. Gohel, H.I. Elim, W. Ji, G.L. Chong, K.Y .Lim, C.H. Sow, A.T.S. Wee, Chem. Phys. Lett. 383, 72 (2004). [6.24]. M. Sheik-Bahae, A. A. Said, T. Wei, D. J. Hagan, E. W. Van Stryland, IEEE J. Quantum Electron 26, 760 (1990). [6.25]. H.I. Elim, W. Ji, G.C. Meng, J.Y. Ouyang, and S.H. Goh, Journal of Nonlinear Optical Physics & Materials 12, 175 (2003). [6.26]. Z. Jin, L. Huang, S.H. Goh, G. Xu, and Wei Ji, Chem. Phys. Lett. 352, 328 (2002). 113 Chapter Conclusion Chapter CONCLUSION The objective of the studies presented in this thesis is to investigate nonlinear optical (NLO) properties of new nanomaterials using Z-scan, optical limiting and pump-probe techniques. The semiconductor quantum dots of PMMA-TiO2 have been observed to have large and ultrafast optical nonlinearities at wavelengths close to their excitonic transition. Moreover, the NLO properties of carbon nanotubes have also been identified for potential application in optical switching devices due to their ultrafast response. In addition, the carbon nanocomposites of Co40Graphite60 and carbon nanoparticles as well as mono-functional fullerene[60] (FCA) incorporated with polymers have been found to induce better nanosecond pulses optical limiting properties than that of C60 at wavelength of 532 nm. The present NLO studies of new quantum dots and quantum wires offer clear evidence that (i) PMMA-TiO2 quantum dot is useful for optical switching devices in optical telecommunication system; (ii) 1.3 μm thickness of aligned cabon nanotubes has potential for optical switching devices; and (iii) carbon nanocomposites of Co40Graphite60, carbon nanoparticles, and mono-functional fullerene[60] of FCA incorporated with polymers are a very good candidate for optical limiting devices. These studies have taken into account some significant findings of NLO properties in new quantum dots and quantum wires. However, it is important to emphasize that the properties have been studied only at limited wavelengths. To fully understand the NLO mechanisms, a series of experiments should be conducted at broadband wavelengths. 114 Chapter Conclusion The approach outlined in this thesis should be valuable for further studies related to the improvement of these new nanomaterials. In the practical application, some NLO devices such as optical switching and saturable absorbing devices are highly desirable and operated on broadband wavelengths. Therefore, more experiments are needed to further study of the materials in infrared wavelengths. 115 [...]... cm2) FNLO TSA σeff./σG Limiting Threshold (J/cm2) C60 Toluene 3. 62 1 .2 0 .25 3 .22 3.1 3. 62 2.0 0 .28 2. 96 4.1 3. 62 0.7 0.15 4.40 2. 3 3. 62 2.1 0 .28 2. 96 5.4 3. 62 0.7 0.15 4.40 2. 0 3. 62 0.9 0.16 4 .25 2. 9 1.2dichlorobenzene 1.2FCA/PSVPy 32- A dichlorobenzene 1.2FCA/PSVPy 32- B dichlorobenzene 1.2FCA/PS-A dichlorobenzene 1.2FCA/PS-B dichlorobenzene FCA -18 C60-d Toluene 2. 77 1.6 0. 32 3.19 >5 .2 C60-c Toluene 2. 77... 0.30 3.38 3.0 Fol-d DMF 3 .22 5.0 >0. 32 10 Fol/PSVPy 32- d1 DMF 3 .22 5.1 >0. 32 10 Fol-c DMF 3 .22 2. 0 >0. 32 10 Fol/PSVPy 32- c2 DMF 3 .22 1.9 >0. 32 6 C60 Chlorobenzene 2. 59 0.6 0 .25 4. 82 2.7 MPF THF 0.38 2. 0 >0.4 10 MPF(1 :2) THF 0.38 2. 4 >0.4 10 MPF(1:4) THF 0.38 2. 3 >0.4 10 MPF(1:6) THF 0.38 2. 2 >0.4 10 67 Chapter 5 Optical limiting and Z-scan studies... 0.6 MPF 0.4 MPF(1 :2) MPF(1:4) 0 .2 MPF(1:6) 0.0 0.01 0.1 1 10 1.0 (b) 0.8 0.6 0.4 0 .2 0.0 DMF/PSVPy 32 T = 70% Fol-d Fol-c C60-c Fol/PSVPy 32- d2 Fol/PSVPy 32- c2 0.01 1.0 (c) Input Fluence (J/cm2) Fig 5.3 Nonlinear transmission responses of (a) C60-toluene (filled circles), FCA in 1.2dichlorobenzene (open triangles), FCA/PSVPy 32- B in 1 .2- dichlorobenzene (filled diamonds), FCA/PSVPy 32- A in 1 .2- dichlorobenzene... + 2 G 1 2 3 3 (5.7) Taking the first two terms into Beer’s law and integrating it, one can obtain the equation of the transmittance input fluence-dependent T = T0 /[1 + (1 − T0 )(Fin / FNLO )] , (5.8) where T0 = exp(-σGn0L) is the sample transmittance in the limit of low light intensity, L is the sample thickness, Fin is the incoming laser fluence, and FNLO ≅ σG/μ1 is the parameter characterizing... FCA in 1 .2- dichlorobenzene (open triangles), FCA/PSVPy 32- A in 1.2dichlorobenzene (open diamonds), FCA/PS-A in 1 .2- dichlorobenzene (open hexagons), PS in 1 .2- dichlorobenzene (filled squares), and PSVPy 32 in 1 .2- dichlorobenzene (open squares) The measurements are conducted with 5-ns laser pulses at λ = 5 32 nm The solid curves are the best fits by the Z-scan theory [5 .29 ] 74 Chapter 5 Optical limiting... 5.3 Ground-state absorption measured using UV-Vis spectrometer and nonlinear optical parameters of diluted and concentrated samples of C60, Fol and Fol/PSVPy measured at the same I = 69.7 MW/cm2 (cm/GW) n2 (x 10- 12 cm2/W) Im(χ(3)) (x 10-11 esu) Re(χ(3)) (x 10-11 esu) 46 165 -0.43 -2. 06 1.7 6.0 -2. 5 -11.7 1360 1080 13 82. 1 -0 .21 -1.08 0.47 3.0 -1 .2 -6 .2 1344 11 12 12. 7 90.3 -0.19 -1.19 0.46 3.3 -1.1 -6.8... 5 .2 Photoluminescence spectra measured at room temperature of (a) 10-mm-thick C60 in toluene, and FCA, FCA/PS-B, FCA/PSVPy 32- B, FCA/PSVPy 32- A and FCA/PSA in 1 ,2- dichlorobenzene, respectively carried out at the same linear transmittance of 65% at 5 32 nm by using 440 nm as the excitation source; (b) 10-mm-thick Fol-d, Fol-c, 1: Fol/PSVPy 32- d1, 2: Fol/PSVPy 32- c1, 3: Fol/PS-d, and 4: Fol/PS-c in DMF, respectively;... containing polymers were investigated to compare the results with those of C60 Shown in Figure 5.3 are optical limiting responses of the supramolecular mono- and multifunctional C60 –containing polymers in solutions of 65, 70 and 75% linear transmittances in a cuvette with 1 or 10 mm optical path length for concentrated or diluted samples The transmittances are first linear with input fluences (Fin)... composites, in which polymers have negligible effects because of laser-induced nonlinear scattering - a completely different limiting mechanism [5.37,5.38] 5.3.6 Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers Figure 5.6(a) shows the open aperture Z-scans of C60, C60/PSVPy 32, and C60/PS in toluene They have been measured by using 5-ns laser pulses of λ = 5 32 nm at the input... input irradiance of 160 MW/cm2 (or an input fluence of 0.8 J/cm2) One can conclude that there is no significant difference among these Z-scans, which eliminates the possibility of direct interaction effects between C60 and PS or PSVPy 32 [5 .2] However, the open aperture Z-scans are different for C60 in toluene, and FCA, FCA/PSVPy 32- A, and FCA/PS-A in 1 .2- dichlorobenzene, as shown in Figure 5.6(b) One can . Fol/PSVPy 32- d 1 DMF 3 .22 5.1 >0. 32 <3.19 >10 Fol-c DMF 3 .22 2. 0 >0. 32 <3.19 >10 Fol/PSVPy 32- c 2 DMF 3 .22 1.9 >0. 32 <3.19 >6 C 60 Chlorobenzene 2. 59 0.6 0 .25 4. 82 2.7 MPF. 2. 0 0 .28 2. 96 4.1 FCA/PSVPy 32- A 1 .2- dichlorobenzene 3. 62 0.7 0.15 4.40 2. 3 FCA/PSVPy 32- B 1 .2- dichlorobenzene 3. 62 2.1 0 .28 2. 96 5.4 FCA/PS-A 1 .2- dichlorobenzene 3. 62 0.7 0.15 4.40 2. 0. containing polymers. Sample Solvent σ G (x 10 -18 cm 2 ) F NLO T SA σ eff. / σ G Limiting Threshold (J/cm 2 ) C 60 Toluene 3. 62 1 .2 0 .25 3 .22 3.1 FCA 1 .2- dichlorobenzene 3. 62 2.0