J. Phys. Chem. C 2010, 114, 16029–16036
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Imaging and Absolute Extinction Cross-Section Measurements of Nanorods and Nanowires through Polarization Modulation Microscopy† Christopher R. Carey,‡ Trevor LeBel,§ David Crisostomo,‡ Jay Giblin,‡ Masaru Kuno,‡ and Gregory V. Hartland* Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670, and UniVersity of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: April 13, 2010
This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45° it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics. 1. Introduction Optical studies of single particles provide a way of investigating their properties as well as how the particles interact with their environment.1,2 Traditionally, these measurements are performed by monitoring either emission for semiconductor nanostructures1,2 or Rayleigh scattering for metal nanoparticles.3,4 Both these techniques are zero-background and, therefore, very sensitive. However, there are some significant limitations: emission measurements can only be performed on materials with high emission quantum yields. On the other hand, Rayleigh scattering can only be detected from particles larger than a few tens of nanometers. This is because the efficiency of Rayleigh scattering is proportional to volume squared.5 This has led to considerable effort in developing absorption-based techniques for studying nanoparticles.5–10 Absorption scales as volume rather than volume squared and, thus, can potentially be used to interrogate smaller particles than is possible with Rayleigh scattering.5,9 Absorption is also more generally applicable than emission. Recent studies have shown that absorption measurements are capable of providing fundamental information about nanoparticles, such as the line width of the plasmon resonance for metals11,12 and absolute absorption cross sections.8,12–14 Several different approaches have been demonstrated for detecting and studying single nanoparticles via absorption. Perhaps the most sensitive is photothermal heterodyne imaging (PHI).7,15 This is an action spectroscopy: a relatively intense laser beam is used to heat the particle. Heat dissipation from the particle changes the refractive index of the surrounding † Part of the special issue “Protected Metallic Clusters, Quantum Wells, and Metallic Nanocrystal Molecules”. * Corresponding author. E-mail:
[email protected]. ‡ University of Notre Dame. § University of Waterloo.
medium, creating a thermal lens. This affects the propagation of a nonresonant probe beam. Fast amplitude modulation of the pump and lock-in detection of the probe allows very sensitive measurements: the spectra of metal particles as small as a few nanometers have been measured by PHI.11 PHI has been used to examine the absorption cross sections of single carbon nanotubes13 and CdSe semiconductor nanowires.14 In these experiments the PHI signal from the nanotubes or nanowires was compared to the signal from particles with accurately known cross sections, such as isolated gold nanoparticles from a sample with a well-defined size. However, in transmission mode (the most common way of doing these experiments), the PHI signal has an unusual dependence on the focus.14 When the probe is only affected by a change in refractive index of the sample (the case for a true thermal lens signal), the PHI signal displays a minimum at the focus, with two symmetrical peaks around the minimum separated by roughly the Rayleigh range.16,17 This arises from the Gouy phase change of a laser beam as it passes through a focus.18 When there are both absorption and refractive index contributions to the propagation of the probe, the two peaks become distorted.16 This creates an uncertainty when comparing signals from different particles.14 Furthermore, to create a significant PHI signal, a fairly intense heating beam must be used, on the order of MW/cm2.13,14,19 This is much larger than the fluences typically used for emission studies of semiconductors. Such high fluences can perturb the samples, leading to effects such as photobrightening in semiconductor nanostructures,20 and are also undesirable for biological imaging applications.21 Another successful technique is spatial modulation spectroscopy (SMS).8,12,22 In these measurements a particle is moved in and out of the focus of a light beam. This creates a modulated signal that directly gives the absorption cross-section of the particle. These measurements do not require intense laser
10.1021/jp101891a 2010 American Chemical Society Published on Web 04/28/2010
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sources, indeed they have been demonstrated using incoherent light from a lamp.23 Measurements for metal particles as small as 5 nm have been reported,8 indicating that the sensitivity is comparable to, but not quite as good as, PHI. One limiting factor for SMS is that only relatively low frequencies (0.1-1 kHz) can be used for modulating the sample position due to mass loading. Thus, these measurements suffer from 1/f noise. This is negated in PHI, because high frequency acousto-optic modulators can be used to chop the heating beam.7,15 Another disadvantage of SMS is that the particles must be fixed to a surface. In contrast, PHI can be used for particles in a liquid, allowing correlation measurements analogous to fluorescence correlation spectroscopy (FCS).24–26 Absorption measurements for nanoparticles must detect a small change in a large number of transmitted photons, which means that the signal must be modulated in some way. In this paper we demonstrate that it is possible to detect the absorption of single anisotopic nanostructures (nanorods and nanowires) by rapidly rotating the polarization of the light beam. These experiments are the single particle analog of polarization modulation microscopy (PMM).27–29 High frequency (100 kHz) polarization modulation is achieved using a photoelastic modulator (PEM). The high frequency operation represents a significant advantage compared to SMS. These experiments directly yield the extinction cross-section of the nanoparticles. Specifically, for one-dimensional nanostructures we obtain the difference between the parallel and perpendicular extinction cross sections (σ| - σ⊥). The measurements do not rely on comparison to calibration particles, which is an advantage compared to PHI. An obvious drawback to PMM is that we can only study anisotropic materials. However, there are many important onedimensional nanostructures, such as carbon nanotubes, where this technique can be used. Thus, we expect PMM to find an important niche in the field of optical studies of nanostructures. The paper is arranged as follows: we start by giving a description of the experimental setup and an analysis of the signal detected in PMM. We then present data for gold nanorods and CdSe semiconductor nanowires that demonstrate the utility of the technique. An important result is that by recording images at two different laser polarizations separated by 45° it is possible to construct an extinction cross-section image of the sample. 2. Experimental Section Two different laser sources were used for these measurements. For experiments with gold nanorods, which require excitation wavelengths in the near-IR region, a home-built CW Ti:sapphire oscillator was used as the light source. For the experiments with CdSe nanowires, the 532 nm output of a DPSS Nd:YAG laser (Spectra Physics Millenia Pro-Vs) was used. The power of the laser beams was controlled using neutral density filters and λ/2waveplate/polarizer combinations. A diagram of the experimental scheme is shown in Figure 1. The polarization of the laser beams was made to be 45° with respect to the axis of the PEM (Hinds Instruments, I/FS50), and the PEM was operated in half-wave retardation mode. This switches the polarization of the light beam from +45° to -45° at 101 kHz (the intermediate state is circular polarization).29,30 The laser beam was coupled into an inverted optical microscope (Olympus IX71) and focused at the sample using a 100×, 1.3 numerical aperture (NA) oil immersion objective. The beam was recollimated with a second 60×, 0.9 NA oil immersion objective and detected by a amplified Si photodiode (Thorlabs, PDA55). The output from the photodiode was sent to a lock-in amplifier (Stanford Research Systems, SR830), referenced to the 2f output
Carey et al.
Figure 1. Left: Diagram of the experimental apparatus for polarization modulation microscopy. The sample coverslip is mounted on the piezo stage at the focus of a 100×, 1.3 NA objective. A second objective is used to recollimate the beam and send it to a photodiode (PD). PEM ) photoelastic modulator; Pol ) calcite polarizer; λ/2 ) half-waveplate; APD ) avalanche photodiode; LWP ) long wave pass filter (passes sample fluorescence, but blocks the 532 nm excitation beam); BS ) beam splitter. A 90-10 beam splitter was used for experiments where only absorption was monitored, and a 50-50 beam splitter was used for the correlated absorption-emission measurements. Right: Coordinate system used for the analysis of the PMM experiments: i and j denote the axes of the PEM, F E i is the polarization direction of the incident laser field (the initial polarization), and e|| and e⊥ are the directions of the principal optical axes of the nanostructure.
of the PEM controller (Hinds Instruments, PEM-100). In this mode the lock-in detects a signal that corresponds to the difference in transmission between the two orthogonal laser polarizations. The majority of the data in this paper was recorded with a lock-in time constant of 3 ms and a 10 ms integration time. A half waveplate was placed between the PEM and the microscope to control the polarization of the laser beam at the sample. Note that in the images presented below, an angle of 0° corresponds to horizontal polarization of the laser when the PEM is turned off. In the following, we denote the polarization of the laser at the sample plane when the PEM is turned off as the initial polarization. The gold nanorods in these experiments were prepared according to the recipe outlined in ref 31. The average length of the rods in the sample was 51 ( 10 nm, with an average width of 13 ( 2 nm (errors ) standard deviations). The CdSe nanowires were synthesized according to ref 32. The average width of the nanowires was 11 ( 4 nm, with a wide range of different lengths and morphologies present. Transmission electron microscopy (TEM) images of the Au nanorods used in these experiments are shown in the Supporting Information. TEM images of the CdSe nanorods that show the different shapes present in the sample are presented in refs 14 and 32. Samples for analysis were prepared by spin coating onto a flamed glass coverslip. The position of the nanoparticles was controlled using a manual X-Y micrometer stage (Semprex) for course adjustment, and a closed-loop piezo stage (Physik Instrumente, P-527.3Cl) for fine adjustment and for collecting images. Typically a step size of 0.1 µm was used to record images. Correlated absorption and emission measurements were obtained by collecting emission through the focusing objective, with a single photon counting avalanche photodiode (APD, Perkin-Elmer SPCM-AQR-14) for detection. The output of the APD was monitored by a gated photon-counter (Stanford Research Systems, SR400). The laser power before the focusing objective was
