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J. Phys. Chem. B 2009, 113, 1652–1654
A Large Enhancement of Photoinduced Second Harmonic Generation in CdI2-Cu Layered Nanocrystals M. Idrish Miah* Nanoscale Science and Technology Centre, Griffith UniVersity, Nathan, Brisbane, QLD 4111, Australia, School of Biomolecular and Physical Sciences, Griffith UniVersity, Nathan, Brisbane, QLD 4111, Australia, and Department of Physics, UniVersity of Chittagong, Chittagong-4331, Bangladesh ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: December 4, 2008
Photoinduced second harmonic generation (PISHG) in undoped as well as in various Cu-doped (0.05-1.2% Cu) CdI2 nanocrystals was measured at liquid nitrogen temperature (LNT). It was found that the PISHG increases with increasing Cu doping up to ∼0.6% and then decreases almost to that for the undoped CdI2 for doping higher than ∼1%. The values of the second-order susceptibility ranged from 0.50 to 0.67 pm V-1 for the Cu-doped nanocrystals with a thickness of 0.5 nm. The Cu-doping dependence shown in a parabolic fashion suggests a crucial role of the Cu agglomerates in the observed effects. The PISHG in crystals with various nanosizes was also measured at LNT. The size dependence demonstrated the quantum-confined effect with a maximum PISHG for 0.5 nm and with a clear increase in the PISHG with decreasing thickness of the nanocrystal. The Raman scattering spectra at different pumping powers were taken for thin nanocrystals, and the phonon modes originating from interlayer phonons were observed in the spectra. The results were discussed within a model of photoinduced electron-phonon anharmonicity. 1. Introduction Multiphoton spectroscopy and photoinduced second harmonic generation (PISHG) are the two important tools to investigate the higher-order nonlinear optical processes, particularly, in semiconductors and insulators.1 However, the PISHG is prevented by symmetry in a centrosymmetric material.1 Thus, in order to observe the PISHG, one needs to have a noncentrosymmetric material. Fortunately, there are various ways to enhance the PISHG. These include the reduction of the size of the crystals (the enhancement of the quantum-confined effect, or the size effect), lowering the lattice temperature, and the insertion of a suitable impurity into the crystal with an appropriate amount. Cadmium iodide (CdI2) crystals are wide-band-gap semiconductors having a layered structure, space group C46V, with highly anisotropic chemical bonds. The band structure calculations of the CdI2 crystals have also shown2-5 a large anisotropy in the space-charge density distribution, causing high anisotropy in the corresponding optical spectra. The anisotropic behavior of the CdI2 crystals favors the local noncentrosymmetry, highlighting the possibility of making them be able for the PISHG investigations. In the presence of Cu impurities, the anharmonic electron-phonon interactions in CdI2 nanocrystals are caused by power photoinducing laser light. Experimental as well as theoretical investigations performed in pure CdI2 single crystals in the past few years using multiphoton spectroscopy have shown that CdI2 processes higher-order optical nonlinearities.6-8 For example, Miah in ref 8 observed relatively large third-order nonlinearity in CdI2 single crystals at liquid nitrogen temperature (LNT). A promising investigation for the magnetic-field-induced ferroelectricity in Cu-doped CdI2 has been reported by Kityk et al.9 However, this interesting measurement was preliminarily performed a decade ago and the most recent reports for this system are * E-mail:
[email protected]. Phone: +617 3735 3625. Fax: +617 3735 7656.
rare,10,11 even if one includes the investigation of the photoinduced transformation of defects with coactivators Mn, Eu, and S.10 Here, in this paper, we study the PISHG in undoped and various Cu-doped CdI2 nanocrystals at LNT. It is found that the PISHG increases with increasing Cu doping up to ∼0.6% and then decreases almost to that for the undoped CdI2 for doping higher than ∼1%. A large enhancement in the PISHG in Cu-doped thin nanocrystals is observed. The Cu-doping dependence shows a parabolic fashion, suggesting a crucial role of the Cu agglomerates in the observed effects. The PISHG in crystals with various nanosizes is also investigated at LNT. The size dependence demonstrates the quantum-confined and size effects with a maximum PISHG for 0.5 nm and with a clear increase in the PISHG with decreasing thickness of the nanocrystal. 2. Experimental Section Measured samples were taken from nanocrystals of undoped as well as various Cu-doped CdI2 (synthesized and grown from the mixture of CdI2 and CuI using the standard Bridgman method12). The structure was monitored using an X-ray diffractometer, and the homogeneity was controlled using a polarimeter. The nanocrystal thickness was controlled using a radio frequency interferometer. The PISHG was measured using a method similar to that employed in ref 13. We used a Nd: YAG laser, as a fundamental laser for the PISHG, which generates picosecond pulses (average power 15 MW) with a repetition rate of 80 mHz. The output PISHG (λ ) 530 nm) and fundamental (λ ) 1060 nm) signals were spectrally separated by a filter (grating monochromator) with a spectral resolution of ∼5 nm mm-1. Detection of the doubled-frequency output PISHG signal (in the green spectral region) was performed by a photomultiplier (with a time resolution of about 0.5 ns), with an electronic boxcar integrator (EBI) for the
10.1021/jp807944y CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
Enhancement of PISHG in CdI2-Cu Layered Nanocrystals
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Figure 1. Schema of the experimental setup (P, polarizer; L, lens; S, sample; A, analyze; F, filter/monochromator; PMT, photomultiplier tube; ω, fundamental frequency; 2ω, doubled-frequency PISHG). The Nd:YAG laser generates picosecond pulses (15 MW) at a repetition rate of 80 mHz. The output PISHG (530 nm) and fundamental (1060 nm) signals were spectrally separated by a grating monochromator (F).
Figure 3. PISHG as a function of pumping power density for various thicknesses of the nanocrystals (0.6% Cu).
Figure 2. Typical spectral distribution of absorption coefficient measured at LNT of CdI2-Cu nanocrystal.
registration of the output. A schema of the experimental setup is shown in Figure 1. All of the measurements were performed at LNT by mounting the samples in a temperature-regulated cryostat. The Raman scattering was taken at the wavelength of argon laser at different pumping power densities.
Figure 4. Raman spectra of the Cu-doped CdI2 (1.5 nm; 0.6%), which were observed at LNT.
3. Results and Discussion The absorption spectra for a typical CdI2-Cu nanocrystal were measured, and an indirect type of energy gap (Eg) for the crystal was determined from the (REλ)1/2 versus Eλ plot.14 The determined value of Eg ) 3.7 eV agrees well with the earlier experimental results as well as the band structure calculations for the undoped CdI2.2-4,7 A typical spectral distribution of absorption coefficient is shown in Figure 2. The pumping power density dependence of the PISHG signal for various nanosize thicknesses of the CdI2-Cu crystal was measured. Figure 3 shows the results. As can be seen, the PISHG increases with increasing power density and then decreases to a value a little higher than background after reaching a maximum. The PISHG dependence also shows a beginning of slight preserve. From the size dependence, as shown in Figure 3, we see that the PISHG increases with decreasing thickness of the nanocrystals, with a maximum for 0.5 nm. However, a significant enhancement in PISHG occurs for the thin nanocrystals. This is a consequence of the contribution of the nanoconfined trapping level effectively interacting with the phonon subsystems. The increase of PISHG with decreasing thickness correlates with the increasing anharmonic interlayer phonon interaction in thin nanocrystals.15 The involvement of interlayer phonon here was confirmed by the observation of phonon modes (originating from interlayer phonons) in Raman spectra (Figure 4). As can be seen, a large increase in the low-frequency Raman
modes appeared after a GW m-2 power treatment, offering the confirmation of the contribution from electron-phonon anharmonic mechanism for the noncentrosymmetry. The qualitative and quantitative changes that occurred for the thinner nanocrystals correspond to the manifestation of the quantum-confined excitonic levels perpendicularly to the layer. The relatively large relaxation time observed in the PISHG pulsesdemonstratestheprincipalroleoflong-livedelectron-phonon states in the observed effects explained within a model of photostimulated electron-phonon anharmonicity,16-18 where the relaxation time for the thin nanolayers should be larger than that for the strong localized electron-phonon states due to the nanosized effects. In Figure 5, we plot the second-order susceptibility (SOS) determining the PISHG as a function of sample thickness for an undoped sample and samples with various Cu contents. As can also be seen, the PISHG decreases with increasing thickness of the crystal, and for thicknesses higher than 10 nm, the PSSHG reduces almost to that for the undoped crystal, demonstrating that a significant enhancement in the PISHG can be achieved by reducing the size of the nanocrystals. Figure 6 shows the second-order susceptibility as a function of Cu-doping density for samples with different thicknesses. One can see that with increasing Cu content up to ∼0.6% the PISHG significantly increases and with a further increase the PISHG decreases. The Cu-doping dependence shows a parabolic
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Miah centers, effectively contributing to the noncentrosymmetry of the output charge density,16 as well as leads to the occurrence of metallic clusters which additionally scatter light, and consequently, suppresses the effect at higher Cu content through the limitation of the enhancement of the local hyperpolarizability for the Cu agglomerate as well as the corresponding nonlinear dielectric susceptibility. From the above, one can conclude that a crucial role of the Cu agglomerates was involved in the processes and was responsible for the Cu-induced observed effects. 4. Conclusions
Figure 5. Second-order optical susceptibility as a function of sample thickness for various Cu contents.
The PISHG in undoped as well as in various Cu-doped CdI2 nanocrystals with various thicknesses was measured at LNT. It was found that the PISHG increases with increasing Cu doping up to ∼0.6% and then decreases almost to that for the undoped CdI2 for doping higher than ∼1%. The values of SOS were estimated and were found to be within a range from 0.50 to 0.67 pm V-1 for the Cu-doped nanocrystals with a thickness of 0.5 nm. The Cu-doping dependence suggested a crucial role of the Cu agglomerates in the observed effects. The nanosize dependence demonstrated the quantum-confined effect with a maximum PISHG for 0.5 nm and with a clear increase in the PISHG with decreasing thickness of the nanocrystal. A large enhancement observed in the PISHG in Cu-doped thin nanocrystals manifested the fact that the insertion of the Cu impurities favoredastrongerlocalphotoinducedanharmonicelectron-phonon interaction. This was verified by the observation of the phonon modes in the Raman scattering spectra. References and Notes
Figure 6. Second-order optical susceptibility as a function of Cudoping density for samples with different thicknesses.
fashion. For the Cu content ∼0.6%, the PISHG achieves its maximum for all of the crystal thicknesses. As shown first by Kityk et al.,9 the insertion of the Cu impurities favors a stronger local electron-phonon interaction, particularly its anharmonic part, through the alignment of the local anharmonic dipole moments by the pumping light.9,19 As the particular role of the local electron-phonon anharmonicity is described by third-order rank tensors in disordered systems,10 the PISHG is very similar to that introduced for the third-order nonlinear optical susceptibility, which has been confirmed by observing the relatively large third-order susceptibility of undoped CdI2 crystals.6-8 The local disordering of the Cu agglomerates plays an additional role in the nanosize-confined effects. As found, the PISHG decreases for relatively higher Cu density, and for densities higher than ∼1%, the PISHG reduces almost to that for the undoped sample. This decrease of PISHG with increasing Cu content is caused by agglomeration of the Cu impurities that is typical of such kinds of layered crystals.20 This effect can be understood in terms of the metallic agglomerates that formed in the process. The formation of the Cu agglomerates favors a reduction in the active electron-phonon
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