Two-photon Absorption in ZnO/ZnS Core-shell Quantum Dots

LIU Shu-yu，ZHONG Mian-zeng，MENG Xiu-qing，LI Jing-bo，3，WANG Yuan-qian，XIAO Si，HE Jun*

(1.Hunan Key Laboratory for Super-microstructure and Ultrafast Process，School of Physics and Electronics，Central South University，Changsha 410083，China;

2.Research Center for Light Emitting Diodes(LED)，Zhejiang Normal University，Jinhua 321004，China;3.State Key Laboratory of Superlattices and Microstructures，Institute of Semiconductors，Chinese Academy of Sciences，Beijing 100083，China)

*Corresponding Author，E-mail:junhe@csu.edu.cn

1 Introduction

In the past decade，there has been great interest in the nonlinear optical and photoluminescence(PL)properties of nanocrystals［1-10］.Two-photon absorption(TPA)properties of quantum dots have received widespread attention，whichmay have technological applications such as bio-imaging，opticalswitching devices， quantum-dot lasers， or light emitters［11］.While multi-photon absorption properties in ZnO and ZnSquantum dots(QDs)have been observed by Z-scan technique and pump-probe measurements［12-16］，there is little research on ZnO-based core/shell nanocomposite quantum dots［17］.Compared to pure ZnO and ZnS QDs，ZnO-based core/shell structures usually have an improvement in physical and chemical properties for electronics，magnetism，optics，catalysis，mechanics，and others［18-19］.Although theoretical and experimental TPA research hasmade great progress，there still has the limitation of existing nonlinear opticalmaterials and further studies are needed［2］.In this paper，we report an investigation on TPA in ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs，which was determined by femtosecond Z-scan technique and pump-probe measurements at the non-resonantwavelength of 660 nm.The observed TPA in ZnO/ZnSnanocomposites is an instantaneous nonlinear process and is greatly enhanced compared to that in ZnS，ZnSe，and CdS QDs［11-13］， which demonstrates that ZnO-based nanocompositesmight be a promising candidate for photonic device applications at room temperature.

2 Experiments

The preparation procedures of ZnO/ZnS/Ag nanocomposite QDs are briefly described as follows.Zinc acetate and sodium hydroxide were dissolved in methyl alcohol，respectively，and stirred bymagnetic stirrers.The two solutions were homogeneously mixed and the resulting reaction mixture was grown at90℃for 6 h.After centrifuged with distilled water and ethanol several times to remove the impurities，the precipitateswere dried under vacuum at60℃.The as-prepared ZnO/ZnS and ZnO/ZnS/Ag QDs were obtained with a similar procedure except that thioacetamide and silver nitrate were added into the solution respectively.Four ZnO-based nanocomposite samples have been investigated in this work.In Sample 1 and Sample 2，the ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs were separately dissolved into the dimethy formamide(DMF)solventand both concentrations are 0.1 g/L.For Sample 3 and Sample 4，ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs were deposited on sapphire substrates to prepare for thin filmswith spin-coating technique.Themorphology and size distribution of ZnO/ZnSand ZnO/ZnS/Ag core/shell QDswere inspected with a field emission scanning electronmicroscope(SEM)(Hitachi，S-4800)and a field emission high-resolution transmission electron microscope(HRTEM)(JEOL，JEM-2100F).The one-photon absorption spectra of the samples were recorded on a UV-visible spectrophotometer(Shimadzu，U-2800)in the range from 200 to 1 000 nm.The PL spectra of the samples were measured by a fluorescence spectrophotometer(Perkin-Elmer，LS-55)using a 450W monochromatized xenon lamp with excitation wavelength of 295 nm.

3 Results and Discussion

SEM and HRTEM images as well as electron diffraction patterns of the ZnO-based nanocomposite core/shell QDs are shown in Fig.1.The SEM pictures indicate that the clustered ZnO/ZnS/Ag QDs are beaded with some bright nano-dots，which is obviously different from ZnO/ZnS QDs.The HRTEM images clearly reveal that the ZnO/ZnS core/shell QDs are crystalline with an average diameter of～9 nm.The obtained energy dispersive X-ray(EDX)spectroscopy and TEM electron diffraction patterns also confirm the ZnO/ZnS and ZnO/ZnS/Ag core/shell structures.The TEM electron diffraction pattern wasmatched against a simulated diffraction pattern generated using TEM simulator Java electron microscopy simulation(JEMS)software.With the experimental and the simulated diffraction patterns，it can be deduced that the ZnO/ZnS/Ag core/shell nanoparticles consist of ZnO core and ZnS shellwith hexagonal structure aswell as Ag nano-dotswith cubic structure.The TEM results are in good agreementwith that of XRD，as shown in Fig.2.

Fig.1 SEM micrographs，HRTEM images and electron diffraction patterns of ZnO/ZnS(a)，(c)，(e)and ZnO/ZnS/Ag(b)，(d)，(f)nanocomposite QDs.

Fig.2 XRD pattern of ZnO/ZnS/Ag nanocomposite QDs

The one-photon absorption spectra of the ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs in DMF solution are shown in Fig.3(a).The lowest excitonic transition is located at～350 nm and the size of ZnO QDs is estimated to be ～ 5 nm［20］，which is consistent with the TEM measurement.Broadening of the excitonic transition is primarily due to the inhomogeneity arising from size dispersion.Fig.3(b)displays the PL spectra for the ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs.The pure ZnO QDs were also characterized for comparison.The PL emission peak(～375 nm)is red-shifted compared to the excitonic transition(～350 nm)due to the Stokes shift.The PL intensity of ZnO/ZnS core/shell QDs is greatly enhanced compared to that of pure ZnO QDs while largely decreased as beaded with Ag nano-dots.The strong emission band(450～650 nm)in pure ZnOQDs could be attributed to the carrier recombination of the surface defect states，which aremostly suppressed in core/shell QDs due to the surface passivation.

Fig.3 UV-visible absorption spectra(a)and PL spectra(b)measured with excitation wavelength of 295 nm for ZnO/ZnS and ZnO/ZnS/Ag nanocomposite QDs.The inset shows the normalized PL spectra for comparison.

The room-temperature TPA of ZnO-based nanocompositeswas investigated at the wavelength of660 nm with a standard Z-scan technique［21］.The femtosecond laser pulse was produced by an optical parametric amplifier(TOPAS，USF-UV2)，which was pumped by a Ti∶Sapphire regenerative amplifier system (Spectra-Physics， Spitfire ACE-35F-2KXP，Maitai SP and Empower 30).The pulse repetition rate is 2 kHz and the minimum beam waist is about 40μm.For comparison，similar TPA measurements were conducted on a 1.0 mm-thick hexagonal ZnO bulk crystal with laser polarization perpendicular to its〈0001〉axis.Pure DMF solvent was examined under the same conditions and the nonlinear response was insignificant.All the Z-scans reported here were performed with excitation irradiances below the damage threshold.

Fig.4(a)and 4(b)illustrate the open-aperture(OA)Z-scan curves for the ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs in DMF solvent at different excitation irradiances(I00)，where I00is denoted as the peak，on-axis irradiance at the focal point(z=0)within the sample.I00is related to the incident irradiance by taking Fresnel's surface reflection into consideration.Based on a spatially and temporally Gaussian pulse，the normalized energy transmittance，TOA(z)，is given by［22］

where q0= β2I0Leff，β2is TPA coefficient，Leff=［1-exp(-αl)］/α，α is linear absorption coefficient，and l is the sample path length.The TPA coefficientβ2can be extracted by fitting the above equation to the OA Z-scan curves.Fig.4(a)and 4(b)indicate that theoretically fitting curves(solid lines)match well with the experimental data(symbols).The intrinsic TPA coefficient(β2QD)and TPA cross section(σ2)have been evaluated and summarized in Table 1.It is known that the nonlinear absorption coefficients are related to the thirdorder imaginary susceptibilities by［22］

Fig.4 Open-aperture Z-scans for ZnO/ZnS(a)and ZnO/ZnS/Ag(b)core/shell QDs in DMF solution at660 nm at different excitation irradiances.The symbols denote the experimental data while the solid lines are the theoretically fitting curves.Degenerate，transient transmission measurements on ZnO/ZnS(c)and ZnO/ZnS/Ag(d)nanocomposite films at 660 nm.

where n0is the linear refractive index，λ is the laser wavelength，c is the speed of light in vacuum，and ε0is the dielectric constant in vacuum.Thus，the intrinsic third-order susceptibilities of ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs can be obtained by the use of Eq.(2)with the relation of Imwhere fvis the volume fraction of nanocom posite QDs in the DMF solution and f is the local field correction that depends on the dielectric constant of DMF solventand QDs.The intrinsic TPA coefficient of QDs(β2QD)can be deduced asThe β2QDobtained for ZnO/ZnS core/shell QDs is～960 cm/GW，which is～1 000 times larger than that of the bulk ZnO and ZnS［16］.This enhancement can be attributed to the quantum confinement and optical Stark effects［1］since the average radius of ZnO core(～2.5 nm)is comparable to the Bohr exciton radius(～1.8 nm).We can convert the TPA coefficient(β2)into the TPA cross-section(σ2)by the definition ofwhere hv is the photon energy，and N0is the density of nanocomposite QDs in the solution.By the use of calculated N0=6.2×1013cm-3，the TPA cross-section of ZnO/ZnS core/shell QDs is found to be 4.3×10-44cm4·s/photon=4.3×106GM，which is at least two orders ofmagnitude higher than that of ZnS，ZnSe，CdSQDs［11-13］.As presented in Table 1，the TPA in ZnO/ZnS core/shell QDs is improved as beaded with Ag nano-dots，which is attributable to local field enhancement［22-23］.The obtained TPA coefficient is intensity independent，which manifests a third-order nonlinear process.The TPA saturation or TPA-excited free carrier absorption is insignificant under the current experimental conditions［23-24］.In addition，we also determine the TPA and three-photon absorption(3PA)coefficients of the ZnO-based core-shell QDs unambiguously with the use of a Z-scan theory developed formaterials that possess TPA and 3PA simultaneously［25］.Details of the calculation are not presented here but it verifies again that the TPA is dominant in the ZnO-based nanocomposite QDs.

Table 1 Crystallite size，one-photon absorption，intrinsic TPA，and TPA cross section of nanocomposite QDs

In the pump-probe experiments，we employed a cross-polarized，pump-probe configuration［20］with 660-nm，35-fs laser pulses from the same laser system used for the Z-scans.The intensity ratio of the pump to the probe was kept at least20∶1.With the cross-polarized configuration，any“coherent artifact”on the transient signal was eliminated.It has been examined that the probe beam alone could not influence any nonlinear effect in our experiments.Fig.4(c)and(d)illustrate the degenerate transient transmission signals(-ΔТ)as a function of the delay time for ZnO/ZnS and ZnO/ZnS/Ag core/shell nanocomposite films，respectively.For the ZnO bulk crystal，the transient transmission signals are mainly dominated by the autocorrelation function of the pump and probe pulses，which reveal that the TPA plays a key role in the observed nonlinear absorption since TPA is an instantaneous nonlinear process.When the excitation pump irradiance is increased to ～ 40 GW/cm2，there is a long absorption tail with a characteristic time of～100 ps or longer.This slow recovery process can be attributed to the absorption of TPA-excited free carriers in the bulk ZnO since the amplitude of the absorption tail grows proportionally to the square of the excitation pump irradiance(not shown in Fig.4)［21］.However，the slow recovery processes do not manifest themselves in the ZnO-based nanocomposite films with the excitation irradiance up to ～300 GW/cm2，which is the photo-induced damage threshold.The magnitude ofmain peaks in the measured dynamics increases proportionally with the excitation power，which confirms that TPA is dominant.

4 Conclusion

In summary，the TPA coefficients of ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs have beenmeasured using femtosecond Z-scan and pump-probe techniques，and compared to thatof bulk ZnO.The TPA cross-sections of ZnO-based core-shell nanocomposites are compared favorably to thatof ZnS，ZnSe and CdSQDs.All the above-discussed merits demonstrate that ZnO-based core-shell QDs are promising for multi-photon excitation imaging applications.

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劉姝妤(1989-)，女，湖南邵陽人，碩士研究生，主要從事超快激光光譜學的研究。

E-mail:278251380@qq.com

何軍(1974-)，男，湖南衡東人，教授，博士生導師，2004年于新加坡國立大學獲得博士學位，主要從事非線性光學、超快光子學等方面的研究。

E-mail:junhe@csu.edu.cn