Optical Second Harmonic Generation from ZnO NanofluidsâA Tight

Article pubs.acs.org/JPCC

Optical Second Harmonic Generation from ZnO NanofluidsA Tight Binding Approach in Determining Bulk χ(2) Christopher B. Nelson,† Kevin E. Shane,† Amani A. Al-Nossiff,‡ and Mahamud Subir*,‡ †

Department of Physics, Ball State University, Muncie, Indiana 47306, United States Department of Chemistry, Ball State University, Muncie, Indiana 47306, United States

‡

S Supporting Information *

ABSTRACT: ZnO nanomaterials exhibit attractive optical properties that are important in the realm of catalysis and nanotechnology involving the developments of solar cells, chemisensors and other optoelectronic devices. Herein, we provide experimental evidence of resonantly enhanced optical second harmonic generation (SHG) from a ZnO nanofluid. Moreover, we develop a simple theoretical model, based on tight binding theory, to calculate the bulk static second order susceptibility components, χ(2) ijk , of ZnO nanoparticles (NPs). We show that a tight binding approach, which is sensitive to local structure, can be (2) (2) used to determine χ(2) ijk of polar semiconducting crystals. The ratio, χzzz/χzyy = 3.07, obtained based on this approach is in excellent agreement with results for bulk ZnO crystals previously reported. However, our finding shows that there is a discrepancy between the theoretical prediction and experimental observation for the ZnO nanofluid. By modifying the ZnO nanoparticle surface via adsorption of an organic dye we further demonstrate that this discrepancy is due to a surface contribution to the overall SHG signal. Lastly, while the applicability of the tight binding method to calculate bulk χ(2) ijk for ZnO is demonstrated here, the potential advantage of this method to characterize surface second order susceptibility is also discussed.

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INTRODUCTION Metal oxide nanoparticles (NPs)1,2 have become popular due to their wide range of applications in optoelectronics,3−6 catalysis,7,8 and environmental remediation.9,10 Nanoparticles, in general, are of fundamental scientific interest because they represent an intermediate state of matter bridging the electronic and optical properties between those of the bulk materials, and those of molecules and small clusters.11,12 Nanoparticles exhibit a large surface area to volume ratio, and therefore, surface chemistry plays a key role in various applications of NPs. Most common applications of metal oxide NPs are in the realm of nanotechnology and pollution remediation. At the heart of nanotechnology is the development of dye sensitized solar cells (DSSCs)3−5 and sensors13−15 using semiconducting NPs as the building blocks. An important example of metal oxides for this purpose is ZnO NPs. Zinc oxide exhibits numerous optoelectronic properties that are attractive for device performances.2 These include a direct and wide band gap (3.37 eV at room temperature), large exciton binding energy (∼60 meV), large piezoelectric constants, and of particular interest, which provides the basis of this scientific exploration, a large nonlinear optical (NLO) response. While an extensive amount of research with respect to synthesis and design9,16,17 of ZnO nanomaterials are on the rise, demand1,2 for obtaining fundamental knowledge related to the optoelectronic properties of nanomaterials is at its apex. An interesting and useful aspect of ZnO NPs is that they are noncentrosymmetric and thus, exhibit strong NLO properties. This entails frequency doubling; i.e., a second harmonic © 2015 American Chemical Society

generation (SHG) capability characterized by the material’s second order susceptibility, χ(2). The utility of SHG in nanomaterials is manifold because the NLO response can be used in integrated nonlinear optical devices2 (e.g., frequency converter, lasers, LEDs, etc.), and the NPs can be used as nanoprobe for chemical sensing and bioimaging. There are several reports18−24 of SHG from a ZnO single crystal and nanomaterials, including nanorods, -wires, and -crystals, either deposited on a substrate or as thin-films. Herein, we show that a ZnO nanofluid yields a strong SHG signal. A nanofluid is defined as a stable suspension of NPs less than 100 nm in diameter in a liquid medium. At the nanoscale Brownian motion overcomes any sedimentation due to gravity. The motivation to study SHG from a ZnO nanofluid instead of deposits has been 2-fold: (1) the transmission geometry provides a facile experimental configuration to characterize NLO properties of NPs and (2) the suspended NPs; i.e., liquid−solid interface, represent a practical chemical system that is relevant in various applications3−10 of these metal oxide NPs. In this article, we report measurement of an SHG spectrum which displays resonantly enhanced SHG from ZnO nanofluids in the vicinity of the direct band gap and present SHG polarization anisotropy data in the wavelength range of 370 to 440 nm. In light of these experimental results, the objective of Received: November 25, 2014 Revised: January 12, 2015 Published: January 13, 2015 2630

DOI: 10.1021/jp5117992 J. Phys. Chem. C 2015, 119, 2630−2636

Article

The Journal of Physical Chemistry C

Figure 1. Schematic diagram (A) depicting forward geometry of SHG detection from ZnO nanofluid. SHG response as a function of incident power of the laser at 800 nm (B) and number density of ZnO NPs suspended in acetonitrile at a fixed average power (C).

number density of ZnO NPs was fixed at 1 × 1012 mL−1. For the experiment with ZnO NPs and coumarin (C343) dye, the final concentration of the dye in the ZnO NP suspension (1 × 1012 mL−1) was 73 uM. An UV−vis spectrum of the ZnO nanofluid and the C343 dye is shown in Figures A.1 and A.2 in the Supporting Information, respectively. Experimental Setup. The experimental configuration for SHG measurements consisted of a Nd:YVO4 solid state laser (Spectra-Physics, Millennia PRO 15sJ) pumped Ti:sapphire Tsunami oscillator (Spectra-Physics, 3941-X1BB), which provided 70 fs pulses at a repetition rate of 80 MHz. The femtosecond laser pulse train was passed through a GlanThompson polarizer (GTH10M), a half-wave plate (WPH10M-808) and then focused at the center of a standard 1.0 cm quartz cuvette (Starna Cells) containing the nanofluid sample (Figure 1.A). All the optical components were purchased from Thorlabs, Inc. A red filter was used between the sample and the lens to block all the stray light at twice the frequency of the laser light. The focal length of the focusing lens (LA4148) was 5.0 cm and the ZnO nanofluid sample was stirred and kept at a constant temperature of 22 °C using a Flash 300 temperature-controlled cuvette holder (Quantum Northwest). The generated SHG signal was collected in forward direction using a 5.0 cm lens (LA4148). It was passed through a blue filter to block the residual laser beam and focused into a monochromator (Acton SP2500, Princeton Instruments). Before the monochromator, an analyzer (GLB10) was used to select the polarization of the SHG signal. A typical energy of 4.1 nJ per pulse at 800 nm was used to perform the experiments. The generated light was detected using a PMT (H11461, Hamamatsu), which was then amplified (SR445A) and processed using a gated photon counter (SR400) (Stanford Research Systems). Using a home-built Labview program (National Instruments) the SHG intensity was recorded. For the polarization anisotropy data the SHG

the work has been to develop a simple theoretical framework to determine χ(2) ijk of ZnO nanoparticles in a liquid suspension. The index i refers to the components of the SHG field, whereas j and k correspond to the Cartesian components of the fundamental laser fields. While earlier investigations with deposited samples highlighted NLO properties of ZnO there are only a limited number of theoretical models25−28 focusing on the determination of χ(2). Here we develop a simple tight binding model based on the linear combination of atomic orbitals (LCAO) approach to calculate the static bulk χ(2) ijk components of ZnO nanoparticles. To our knowledge, this is the first attempt to calculate χ(2) ijk in such a manner. We present the experimental results and then describe the theoretical aspects of the tight binding model developed to determine bulk χ(2) ijk for ZnO. The results obtained using this method is compared to previous experimental values for ZnO crystals.18,19 We further demonstrate that for ZnO nanofluid there is a surface component to the overall SHG signal and provide a brief discussion on the capability of the tight binding model to characterize the surface contribution.

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EXPERIMENTAL METHODS AND RESULTS Chemicals and Sample Preparation. High purity (99.95%) ZnO nanoparticles (average particle size, 18 nm) were purchased from US Research Nanomaterials, Inc. and were used as received. The morphology of the particles is reported as nearly spherical with crystal phase as single crystal. To prepare the nanofluid sample, a small quantity (few milligrams) of the milky white solid ZnO NPs were weighed and dispersed in acetonitrile (Chromasolv Plus, for HPLC, ≥ 99.9%, Sigma-Aldrich). The suspension was sonicated and diluted to achieve the desired particle concentration, the number density of which was determined using the true density of the material (5.606 g/cm3) and assuming spherical particle volume. In the polarization anisotropy measurements, the 2631


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Figure 2. Normalized SHG intensity vs SHG wavelength (A) and SHG polarization anisotropy data, normalized SHG intensity versus γ, at four distinct wavelengths above, at, and below the band gap (B).

signal was collected at a fixed wavelength and output polarization (vertical; p-polarized) (Figure 1A). Each data point corresponds to an average of at least 5 counts per second. Alternatively, the same result was obtained when data were collected using a CCD camera (PIXIS 400B, Princeton Instruments) and then processed using the WinSpec software provided by Princeton Instruments. Experimental Results. The detected signal at 2ω in the forward direction from ZnO nanofluid in acetonitrile, the solvent of choice for most DSSC applications, for a range of number density of NPs, exhibit a quadratic behavior as a function of incident laser power (Figure 1B). The markers correspond to experimental data and the solid lines represent a fit to a quadratic function, I2ω ∝ Iω2, depicting the second order process.29 In accordance with theoretical predictions,30 the SHG signal from the noncentrosymmetric particles is found to be strongest in the forward direction as compared to measurements made with a 90 deg detection geometry. Figure 1C shows the linear range of the SHG output with respect to the NP number density. The SHG signal deviates from linearity and diminishes due to turbidity at particle density higher than 2 × 1012 mL−1. To ensure scattering does not influence the SHG signal, the SHG spectrum and polarization anisotropy data (Figure 2, parts A and B, respectively) were collected at a number density of 1 × 1012 mL−1, which is within the linear range (Figure 1C). Figure 2A shows the normalized SHG intensity (red markers), corrected for turbidity31 at ω and 2ω, as a function of SHG wavelength. The UV−vis spectrum of ZnO nanofluid (see Supporting Information, Figure A.1) shows the attenuation of light along a path length of 1.0 cm, as a function of wavelength. To correct for the turbidity at both the fundamental and SHG wavelengths, the raw SHG intensity from ZnO nanofluid was divided by a transmittance factor obtained from the measured ZnO nanofluid optical density. The hyper-Rayleigh (HR) intensity from the solvent, acetonitrile, which is wavelength insensitive, has been used to normalize the data to take into account of the efficiency of the optical components and the detector at different wavelengths. The spectrum (Figure 2A) was collected with both the fundamental and the detected SHG field polarization in z

direction. Clearly, Figure 2A shows an enhancement in SHG toward the exciton resonance, determined to be 376 nm based on UV−vis spectrum of the nanofluid (see Supporting Information). The polarization anisotropy data (Figure 2B) were collected by detecting SHG intensity at a fixed polarization (Pz) and turning the half-wave plate (HWP) at an increment of 5 degrees. This corresponds to a 10 degrees change in the polarization angle (γ) of the incident beam. The SHG intensity at 2ω from ZnO nanofluid at a given γ contains HR scattering from the bulk acetonitrile solvent. Because HR is incoherent, we subtracted the HR intensity obtained separately from neat acetonitrile. The HR intensity detected from the isotropic acetonitrile solvent is in the order of 10−2 fold weaker compared to the magnitude of the SHG intensity from ZnO nanoparticles, for all polarization angles (see Supporting Information, Figure A.3(a)). Previous studies have shown32 that the HR scattering from liquid acetonitrile is predominantly from reorientation of the molecules. The HR corrected intensity was then divided by the SHG intensity from ZnO nanofluid collected (also corrected for HR) at an incident polarization angle of zero. The SHG intensity from the ZnO nanoparticles at γ = 0 serves as an internal reference and (2) provides a simple way to obtain and compare χ(2) zzz/χzyy ratios. The polarization anisotropy data reveal an oval feature along the z-axis that is invariant of the excitation wavelength. In the next section, a simple quantum mechanical calculation that allows us to determine the static bulk χ(2) ijk of the ZnO crystals, which in turn explains part of the observed polarization anisotropy data (Figure 2B), is described.

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THEORETICAL METHOD. TIGHT BINDING CALCULATION OF BULK χ(2) IJK As mentioned earlier, much research is being done on the nonlinear optical properties of metal oxide nanoparticles.1−10 Because they have a larger surface to volume ratio than macroscopic objects, both the bulk and surfaces of nanoparticles can act as sources for SHG in ZnO nanostructures.19,30,33 Theoretical work has focused on calculating χ(2) ijk for bulk semiconductors.26 The agreement between theory and experiment for first order susceptibilities for most semi2632


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2 Figure 3. (A) Tetrahedral bonding with oxygen (red) at the center of four Zinc (purple) atoms. (B) Plot of normalized |P(2) z | based on eq 4 (solid black), superimposed onto the experimental data: ZnO NPs only (red circles) and ZnO NPs with C343 (blue circles). The fundamental wavelength used in this experiment is 800 nm and the plotted SHG intensity was detected at 400 nm.

conductors is well established; however, the second order susceptibility presents a different situation. One pressing issue has been an old disagreement between experiment and theory27,28 concerning SHG from static sources. A static source of SHG is the stationary charge distribution associated with bound state electrons which are not excited across the band gap. The interaction Hamiltonian used for this approach is HI1 = er·⃗ E⃗ . Currently a model proposed by Aversa and Sipe34 using this approach has been used to calculate terms of χ(2) ijk for bulk wide gap semiconductors. The theoretical approach considered here has been motivated by the need to develop a generic method for calculating the static components of χ(2) ijk for the bulk SHG sources of ZnO NPs that also has the potential to characterize the surface contribution from the nanoparticles. To this end, we applied a tight binding method. We show that this facile approach produces χ(2) ijk values that are in good agreement with (2) experimental χijk values for bulk ZnO crystals reported earlier.18,19 Such a method in principle can be used to calculate χ(2) ijk for a bulk solid with a regular crystalline structure as well as for a surface comprised of localized structures. As mentioned, a localized structure may have group symmetry or be amorphous. Tight binding35−37 has demonstrated applicability in terms of modeling the valence electronic structure for both situations. Since it is known29 that bulk ZnO has 6mm group symmetry we consider that it is reasonable to model the bulk contribution to the SHG. One approach used for bulk crystals is the socalled Kleinman symmetry.29 The Kleinman symmetry is only valid when the laser wavelength is sufficiently off resonance as to render χ(2) ijk independent of applied frequency, which is equivalent to no dispersion. As shown (Figure 2B), the polarization anisotropy data are collected at below, at, and above the band gap. The individual terms χ(2) ijk are scalars, if dispersion were present we would see a change in the aspect ratio between the polar amplitudes as a function of frequency. However, the polarization anisotropy data in Figure 2B is seen to be independent of frequency, so we assume using the Kleinman symmetry is acceptable for modeling the bulk contribution to the data. For this experiment the bulk source components, associated with the 6mm group in the Kleinman index scheme are shown in eq 1.

(2) 2 (2) 2 (2) 2 Pz(2) = χzxx Ex + χzyy Ey + χzzz Ez

(1)

The term P(2) z is the second order induced polarization that leads to generation of detected SHG field at 2ω. Here we assume that the interior of each NPs can be modeled as a bulk source, and that the ẑ axis of the ith nanoparticle makes an angle θi with the polarization, P⃗laser, of the fundamental laser beams. Owing to the experimental setup the direction of laser propagation is x̂, so this reduces to, (2) 2 2 (2) 2 2 Pz(2) (i) = χzyy E0 sin (θi) + χzzz E0 cos (θi)

(2)

The polarizations of each nanoparticle have to be averaged over. This can be done using a three polarizer approach. Assuming each nanoparticle has a ZnO crystalline structure comprising its bulk, this internal structure has a preferred ẑnanoparticle axis with respect to the orientation of the nanoparticle. To average over all possible orientations of the nanoparticles we assume that each particle ẑnanoparticle axis makes an angle αi with the lab ẑ axis. Thus, we can write γ = θ + αi, where γ is the angle between the polarization of the fundamental beam, P⃗laser, and the lab ẑ axis (Figure 1A) and the index on θ is dropped. We can write the polarization due to the ith nanoparticle in the lab frame as, (2) 2 2 (2) 2 2 2 Pz(2) (i), lab = (χzyy E0 sin (θ ) + χzzz E0 cos (θ )) cos (αi)

(3)

The angle αi averages to α̅ = π, and using double angle formulas we obtain, (2) 2 2 (2) 2 2 Pz(2) , lab = χzyy E0 sin (γ ) + χzzz E0 cos (γ )

(4)

Another consideration is the polarity of the molecule. ZnO is a polar covalent solid in the Wurtzite structure. According to Ivanov and Pollman38 65% of the valence charge resides on the O atom. We assume that the static contribution to the SHG comes from the fact that the laser field is distorting the charge density of a given ZnO molecule with the bulk of the distortion around the O atom and thus, restrict our calculation to considering the contribution to the SHG due to distortions in the valence structure of the molecular orbitals. To calculate the induced molecular dipole moments we need first to find the valence charge densities around the O atom. We assume a given 2633


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The Journal of Physical Chemistry C O atom is surrounded by four Zn atoms in a tetrahedral configuration determined by the Wurtzite structure. This is shown in Figure 3A. To calculate the SHG originating from a configuration as shown above we form a 8 × 8 Hamiltonian matrix with 4 zinc hybid orbitals pointing at the O atom. To calculate the molecular charge density we will orthogonalize these hybrids with respect to the valence |2s⟩, |2p⟩ orbitals on the O atom. Details of the calculation of the |p⟩ orbitals for both Zn and O can be found in Harrison.39 The hybrid-S orbital matrix elements for the Hamiltonian are calculated as,

Here we have neglected dipole moments between the hybrids, which are assumed to be orthogonal. The molecular dipole moment given in eq 9b can be written compactly as, μsj′ h ′ = e{CssChmp ⟨s| r |⃗ pj ⟩ + ChmsCsp ⟨pj | r |⃗ s⟩} m

μpk ′ h ′ = e{C p sc hlp ⟨s| r |⃗ pk ⟩ + ChlsC p p ⟨pk | r |⃗ s⟩} m

3 Vppσ cos(θix) 2 2 And similarly for the other p−p matrix elements; Vspσ

+

(6)

x ,y,z

|s′⟩ = Css|s⟩ +

4

∑ Csp |pj ⟩ + ∑ Csh |hl⟩ l

j

j

l=1

x ,y,z

|p′⟩j = C p s|s⟩ + j

j k

j i

i

k=1

x ,y,z

|h′⟩k = Chks|so⟩ +

(8a)

4

∑ C p p|pi ⟩ + ∑ C p h |hk⟩

(8b)

4

∑ ch p|pi ⟩ + ∑ Ch h |hl⟩ k i

k l

i

l=1

(8c)

Here the C terms are components of the eigenvectors, which depend on the choice of tight binding overlap parameters, ηssσ, ηspσ, ηppσ. Here we show that one choice of these introduced by Harrison41 gives results which agree well with established χ(2) ijk values18,19 for bulk ZnO. These molecular wave functions will be used to construct the molecular dipole moments which compose the second order susceptibility. The only dipole matrix elements that can be formed from these are μsi′ p ′ = e⟨S′| r |⃗ pi′⟩i

(9a)

μsj′ h ′ = e⟨S′| r |⃗ hl′⟩ j

(9b)

μpk′ h ′ = e⟨pi ′| r |⃗ hm′ ⟩k

(9c)

i

l

i

m

m

m i

i

i

k

m k

(12)

⎡ ⎤ μsi′ p ′μpj′ h ′μsk′ h ′ Nb ⎢ ⎥ ∑ ℏ2 ⎢⎣ h ′ , p ′ (ωsp − 2ω0)(ωsp − ω0) ⎥⎦

(13)

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DISCUSSION First, we compare the theoretical χ(2) ijk values obtained using the tight binding model developed in this work with the previously reported values for bulk ZnO crystals. Previously measured18,19 (2) −8 −8 values for χ(2) zyy and χzzz are 2.9 × 10 esu and 8.7 × 10 esu, respectively. The calculated tight binding values are within 3% of the measured χ(2) ijk values. This suggests that the simple quantum mechanical model developed here, which is sensitive to the local structure, is indeed applicable in calculating second order susceptibilities of ZnO and bear the potential to be extended to other semiconducting materials with known crystal (2) structure. Now, we compare the χ(2) zzz/χzyy ratio based on the theoretical calculation with that of the experimental ratio obtained from the polarization anisotropy measurements for (2) the ZnO nanofluids. The experimental and theoretical χ(2) zzz/χzyy ratios are 1.17 ± 0.01 and 3.07, respectively. In practice, electronic quantities calculated using the tight binding method should be sufficiently sensitive to the local geometry to allow for a fit accurate to within 10% of experiment.29,35,36 The

where i, j, k refer to x, y, z spatial indices. Here the dipole equation, eq 9a, can be written in terms of the eigenvector components, μsi′ p′ = {CssC p p⟨s| r |⃗ pi ⟩ + CspC ps⟨pi | r |⃗ s⟩}

m

Here the sum is over all triple dipole matrix products s′, p′, h′ and N is the number density of tetrahedral configurations in the nanoparticle sample. The laser wavelength is taken as λ = 800nm with ω0 the laser frequency. The components χ(2) ijk for a ZnO molecule in the tetrahedral configuration shown in Figure 3A can now be calculated using hybrid tetrahedral angles and interatomic distances di = 1.98 Å from the literature.36 The magnitude of χ(2) ijk depends on the number density N of these in a given sample. To determine this we first assume a nanoparticle with a radius Rnp = 9.0 × 10−9m, which yields a volume Vnp = (4/3)πR3np = 3.05 × 10−24m3. The configuration shown above has a square volume, Vcb ≈ 3.8 × 10−29m3. This gives a number density of N = 2.62 × 1028m−3. So the number of SHG source configurations in a 18 nm diameter nanoparticle is Nb = Vnp/Vcb = 8.02 × 104. Lastly, we use the set of tight binding parameters introduced by Harrison,41 ηssσ = −1.32, ηspσ = 1.44, ηppσ = 2.22. This has been shown to be more accurate for polar covalent materials. 37 The model yields the −8 −8 susceptibilities χ(2) esu, χ(2) zyy = 2.91 × 10 zzz = 8.94 × 10 esu, which substituted into eq 4 results in the pattern shown in Figure 3B (solid black line).

+

2

l

χijk(2) (2ω0 , ω0) =

3 Vppσ cos(θiy) and 2 Vspσ 3 ⟨Hi − O⟩pz = + Vppσ cos(θiz) (7) 2 2 The angles can be calculated from the geometry of the bulk. This Hamiltonian matrix can be diagonalized to obtain eigenvalues and eigenvectors. Once this is done we are left with the following wave functions, ⟨Hi − O⟩py =

(11)

All of these terms involve expectation values of the form ⟨pi|r|⃗ s⟩ which were evaluated using pseudowave functions.39 Here r ⃗ is a radius on the O atom. These are computed using Maple V15. The atomic energy levels in the above matrix elements have a difference frequency ωsp defined as, (ωsp = (εPO − εSO)/ℏ). This frequency is sufficiently different from the resonant frequency of the molecule so as to allow the second order susceptibility to be written as,

Vssσ 3 + Vspσ (5) 2 2 The overlap integrals are calculated using interatomic Zn−O distances taken from the literature.40 The hybrid-P orbital matrix elements are calculated as, Vspσ

j

And eq 9c can be written as

⟨Hi − O⟩s = ⟨hZni|H |So⟩ =

⟨Hi − O⟩px = ⟨hZni|H |px ⟩ =

j

(10) 2634


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definite symmetry. Until now, most of the models developed for bulk semiconducting materials used Bloch functions34 to model the electrons which interact with the laser field. However, the use of Bloch functions requires periodic boundary conditions.46 The electron wave functions in each region would therefore have to satisfy nonperiodic boundary conditions,47 which excludes the use of Bloch functions on a surface with different localized and/or amorphous structures. Accordingly, the tight binding model can be very useful to characterize the surface component of the SHG, and is indeed the subject of our ongoing research.

comparison of the two ratios after considering the 10% margin of error, suggest there is a discrepancy between the theoretical and the experimental values obtained for ZnO nanofluids. This we attribute to the fact that the tight binding calculation models the bulk SHG source alone and that the surface of ZnO nanoparticles acts as a significant source of the SHG. We note that for nanoparticle suspended in a liquid medium both the bulk and surfaces of nanoparticles can act as sources for SHG19,30,33 because NPs have a larger surface to volume ratio than macroscopic objects. It is known that surfaces on ZnO nanostructures have localized features which may not be evenly distributed.42 This means, among other possibilities, there could be a boundary between regions with different crystal structure, which contributes to the overall SHG. Because the SHG intensity in the polarization data does not vanish and the residual intensity is not aligned with bulk χ(2) ijk components, it is reasonable to ascertain that the surface SHG source is not 6mm. To further prove the hypothesis that there is a surface contribution we opted to modify the ZnO nanoparticle surface by having an organic dye, Coumarin 343 (C343), adsorbed onto its surface. The C343 dye is known to adsorb on metal oxide nanoparticle surface and has been used to sensitize TiO2 and ZnO nanostructures for the development of dye sensitized solar cells.43,44 It absorbs light in the vicinity of the generated SHG wavelength (400 nm) with its maximum peak at 444 nm in acetonitrile (see Supporting Information). The C343 dye solution in acetonitrile does not yield coherent SHG signal; however, there is a weak fluorescence background at 2ω. This was subtracted from the overall SHG intensity from C343− ZnO NP suspension. The raw intensities are shown in Figure A.3(b) in the Supporting Information. To normalize the data, the corrected intensity was divided by the SHG intensity from C343−ZnO NP suspension at γ = 0. The polarization anisotropy data (Figure 3B, blue circles) with C343 adsorbed onto the ZnO NPs clearly indicates a reduction of the SHG intensity, in comparison to the dye-free ZnO NPs, when γ = 90° and 270°; i.e., when the polarization of the incident beam is orthogonal to the polarization of the detected SHG light. The simplest explanation for the reduction is that the C343 molecules act to shield some part of the ZnO nanoparticle surface from the laser field, so that this part does not act as a source of SHG. Experimental evidence42 suggests that the ZnO surface may have local structures that are either amorphous or have definite group symmetry. On the basis of this there are two possibilities in terms of a microscopic explanation of the screening effect seen: (1) the C343 adsorbs to the surface in a van der Waals type interaction with little surface modification or (2) a charge transfer45 occurs between the C343 molecule and the surface, thus altering the shape of the surface locally and changing the frequency and/or the intensity of the SHG signal. Irrespective of the mechanism, the experimental data provides direct evidence that the surface of the ZnO NPs can and does act as a source of the observed SHG signal which can be modulated by the adsorption of a dye. Thus, we attribute the surface effect as the source of the discrepancy between the theory and the experimental polarization anisotropy data for the ZnO nanofluids. Although the focus of this work has been to show the applicability of the tight binding model to predict bulk χ(2) ijk , the method developed also has the potential to treat localized surface structures that can be a source of SHG. This is because the tight binding model is sensitive to the local structure with

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CONCLUSION We demonstrated that an 18 nm ZnO nanofluid yields resonantly enhanced SHG signal and shows polarization response that is independent of wavelength in the vicinity of ZnO band gap. This finding is significant for the development of optoelectronic devices and sensors based on the nonlinear optical properties of semiconducting nanomaterials. Moreover, we have developed a simple tight-binding approach to calculate the bulk static χ(2) ijk components of ZnO. The calculated values are in excellent agreement with previous results18,19 for bulk ZnO crystals. However, comparison of the theoretical result with that of the experimental data obtained for ZnO nanofluids show that only a part of the experimental observation is explained by the theory. There is a surface contribution, in addition to the bulk source, to the overall SHG from the ZnO nanoparticles suspended in a liquid medium. This is demonstrated by the experimental evidence in which the surface of the nanoparticle is modified via adsorption of an organic dye. Finally, the development of the tight binding approach to calculate χ(2) ijk presented here will open up the possibility to characterize the surface susceptibilities of semiconducting nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

The UV−vis spectra of the ZnO nanofluid and the C343 dye in acetonitrile and a plot of uncorrected and un-normalized raw SHG intensity vs polarization angle γ data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.S.) Telephone: 765-285-8306. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are greatly indebted to the valuable comments and suggestions provided by Dr. Jerry I. Dadap and Dr. Walter A. Harrison. The authors also wish to thank Dr. Antonio Cancio and Dr. Tykhon Zubkov for useful discussions. We also thank Ball State University for their financial support and resources.

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REFERENCES

(1) Taylor, R.; Coulombe, S.; Otanicar, T.; Phelan, P.; Gunawan, A.; Lv, W.; Rosengarten, G.; Prasher, R.; Tyagi, H. Small Particles, Big Impacts: A Review of the Diverse Applications of Nanofluids. J. Appl. Phys. 2013, 113, 011301(1−19). (2) Janotti, A.; Walle, C. G. V. d. Fundamentals of Zinc Oxide as a Semiconductor. Rep. Prog. Phys. 2009, 72, 126501(1−29).

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