Photothermal Absorption Spectroscopyof Individual Gold Nanoparticles

and CdSe/ZnS Semiconductor NanocrystalsStéphane Berciaud, Laurent Cognet, David Lasne, Gerhard A. Blab and Brahim Lounis.

Centre de Physique Moléculaire Optique et Hertzienne CNRS & Université de Bordeaux 1, 351 cours de la Libération 33405 Talence Cedex.

Email : s.berciaud@cpmoh.u-bordeaux1.fr

Abstract: We present a newly developed photothermal imaging technique to perform absorption spectroscopyof individual nano-objects. Intrinsic size effects in the surface plasmon resonance of gold nanoparticles andCdSe/ZnS nanocrystals in the multiexcitonic regime are studied.

© 2006 Optical Society of AmericaOCIS codes: (180.0180) Microscopy; (350.5340) Photothermal effects; (260.3910) Optics of Metals;(290.4020) Mie theory; (300.1030) Absorption; 260.3800 Luminescence; 300.6470 Spectroscopy,semiconductors

In the fast evolving field of nanoscience, where size is crucial for the properties of the objects, simple and sensitivemethods for the detection and characterization of single nano-objects are needed. We recently developed an all-optical method called Photothermal Heterodyne Imaging that allows for the unprecedented detection of individualnano-objects such as gold nanoparticles with diameter down to 1.4 nm (i.e. 67 atoms) (Fig1.a) as well assemiconductor nanocrystals in the multiexcitonic regime (Fig 2.b) [1].

This method relies on the absorptive properties of the nano-object and does not suffer from the drawbacks ofluminescence-based methods (e.g. rapid photobleaching or blinking). The experimental setup consists of acombination of two laser beams: an intensity-modulated heating beam, close-to-resonance, and a cw off-resonanceprobe beam. Absorption of the heating beam by a nanoparticle induces a time modulated increase of the temperaturein its vicinity. The probe beam produces a frequency shifted scattered field as it interacts with the time modulatedvariations of the refractive index around the absorbing nano-object. In practice, the photothermal signal is extractedwith a lock-in amplifier by detecting the beatnote between the backward scattered field and the reflection of the probefield at the sample-coverslip interface (backward configuration).

The measured photothermal signal is directly proportional to the absorption cross section of the nano-objectunder study and is in good agreement with a calculation based on the scattering field theory from a photothermally-induced modulated index of refraction profile around the nanoparticle [2].

Photothermal heterodyne imaging opens new pathways for quantitative spectroscopy of individual nano-objects.As a first application, we studied the Surface Plasmon Resonance (SPR) of individual gold nanoparticles withdiameters down to 5 nm [3]. Sensitivity at the single particle level allows us to access the dispersions in the resonantpeak energy ER and in , the homogeneous red-half-width-at-half maximum of the SPR. We observed that a verysmall ellipticity of the nanoparticle can result in a significant dispersion on ER. Furthermore, for nanoparticles smallerthan 20 nm, is significantly broadened because of surface damping mechanisms leading to reduced SPRdecoherence times (Fig1.b). This observation of intrinsic size effects in the optical response of gold nanoparticles iswell described within the frame of Mie theory using a size-dependent modification of the dielectric constant.

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Figure 1: (a) 3D representation of a photothermal heterodyne image (5 × 5 µm2) containing individual 67 atoms gold clusters (1.4 nm diameter)The integration time per pixel was 10 ms and the heating intensity was 3 MW.cm 2− . (b) Normalized absorption spectra of 2 single gold

nanoparticles of respective diameters equal to 33 nm (open red circles), and 5 nm (open blue squares).The extracted red width at half maximum is shown on both spectra. The experimental values are compared with simulations based on Mie theory (solid lines) using size dependant

modification in the dielectric constant of gold.

PHI was also used to record the first room temperature photothermal absorption spectra of individual CdSe/ZnSsemiconductor nanocrystals of 2.0 nm in diameter (peak emission at 2.10 eV) [4]. These spectra were recorded in thehigh cw excitation regime, where excitons are created at average rates significantly higher than the radiativerecombination rate of the band edge exciton. In that case, luminescence from monoexcitonic recombination is veryweak and the prepared multiexcitons relax rapidly through efficient non-radiative Auger processes. The observedphotothermal absorption bands (Fig. 2.c) are assigned to such Auger recombinations involving biexciton and trionstates. Comparison with the photoluminescence spectra of the same nanocrystals leads to the measurement of spectralStokes shifts E free from ensemble averaging. Comparison of the mean value of E with ensemble measurementsallows for an estimation of the biexciton and trion binding energies, which are in good agreement with priortheoretical and experimental results.

Figure 2: Comparison of photoluminescence (a) and Photothermal Heterodyne images (b) of the same area (20x20 µm2) of a sample containingCdSe/ZnS nanocrystals. The integration time per point was 10 ms. Scale bar is 2µm. (c) Photoluminescence (gray line) and photothermal

absorption (black line) spectra recorded for a same individual CdSe/ZnS nanocrystal. For clarity a smoothing of the absorption spectra is shown(Red Line). The homogenous Strokes shift E is also shown.

References:[1] S. Berciaud, L. Cognet, G.A. Blab, and B. Lounis, Phys. Rev. Lett. 93, 257402 (2004).[2] S. Berciaud, D.Lasne, G.A. Blab, L. Cognet and B. Lounis, Phys. Rev. B., in press (2006)[3] S. Berciaud, L. Cognet, P. Tamarat, and B. Lounis, Nano Letters 5, 515-518 (2005).[4] S. Berciaud, L. Cognet, and B. Lounis, Nano Letters 5, 2160-2163 (2005).

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