Imaging and Spectroscopy of Single Metal Nanostructure Absorption

Invited Feature Article pubs.acs.org/Langmuir

Cite This: Langmuir XXXX, XXX, XXX−XXX

Imaging and Spectroscopy of Single Metal Nanostructure Absorption Anneli Joplin,† Wei-Shun Chang,† and Stephan Link*,†,‡ †

Laboratory for Nanophotonics, Department of Chemistry, and ‡Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States ABSTRACT: The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance often involves shifting the balance between absorption and scattering to promote desirable features of their optical properties. Scattering by single metal nanoparticles is commonly characterized using dark-field scattering spectroscopy, but routine methods to characterize pure absorption over a broad wavelength range are much more complex. This article reviews work from our lab using photothermal imaging in combination with dark-field scattering and electron microscopy to separate radiative and nonradiative properties of single nanoparticles and their assemblies. We present both initial work using different laser wavelengths to explore pure absorption free from scattering contributions based on the heat released into the environment as well as the development of photothermal spectroscopy over a broad wavelength range, making it possible to resolve details that are otherwise hidden in ensemble measurements that most of the time also do not separate radiative and nonradiative properties.

I. INTRODUCTION Improving the performance of plasmonic nanoparticles in many of their prospective applications often entails enhancing certain desired aspects of their optical properties. The optical and electronic features of metal nanoparticles are defined by their localized surface plasmon resonance, which describes the oscillation of their conduction band electrons in response to light.1,2 Consequences of the plasmon resonance include significant absorption and scattering cross sections, enhanced electromagnetic fields, localized heating, and tunable hot carrier generation.3−6 The balance between these processes can be controlled through changes to the metal nanoparticle’s composition, geometry, and environment, which can all be optimized to favor one property and hence a certain application over another.7,8 For example, applications in photocatalysis benefit from metal nanostructures that simultaneously enhance photon absorption to create hot carriers, but do not scatter too many photons from the catalyst bed.6,9−14 At the other extreme, sensing applications such as surface enhanced Raman spectroscopy require hotspots in the local near field, which are primarily related to the scattering contributions.15−19 These examples showcase the need to understand each distinct aspect of the optical response and the trade-offs between them. Because metal nanoparticles scatter light efficiently, radiative contributions have been explored first, as they can be easily isolated from their net optical response on a single particle level while contributions from nonradiative processes are disguised. After excitation, the plasmon relaxes through two overarching © XXXX American Chemical Society

channels: radiative decay (elastic scattering) and nonradiative decay (absorption) as depicted in Figure 1A.20 Extinction refers to the sum of these two processes, representing the total amount of incident light removed by the nanoparticle. Incoherent scattering (photoluminescence) also occurs, but this process is very inefficient21−23 and will not be discussed here. In terms of characterization, dark-field microscopy was first exploited to image the elastic scattering of single metal nanostructures in 2000 and since that time has grown into the most frequently utilized single particle spectroscopy method.24,25 Scattering characterization across a multitude of nanostructures, especially when correlated with electron microscopy, consistently reaffirms the plasmon resonance’s sensitivity to nanoparticle shape, size, and environment.3,26 Unlike scattering, however, measuring the pure absorption of metal nanoparticles is a much more complex task both in bulk solution and for single particles. Standard optical methods such as ultraviolet−visible (UV−vis) spectroscopy that can readily determine absorption in molecular species, for which scattering can be mostly neglected, fail in this instance because the large scattering cross section ensures that they can only report extinction (i.e., total intensity loss). The nature of a transmission experiment also means that the absorption signal must be extracted from a high photon background, presenting Received: September 6, 2017 Revised: November 17, 2017 Published: November 17, 2017 A

DOI: 10.1021/acs.langmuir.7b03154 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

spectra for Au nanospheres with diameters of 50, 75, 100, and 150 nm, respectively. As this collection of spectra emphasizes, nanoparticle dimensions dictate the balance between absorption and scattering in addition to the magnitude of each cross section. For example, the total optical response of a 50 nm Au nanosphere can be primarily attributed to absorption, while that of a 150 nm Au nanosphere is composed predominantly of scattering. Beyond simple changes to nanoparticle size, factors like shape, composition, and coupling between two or more metal nanostructures in close proximity alter their absorption to an even greater degree.27,28 An initial understanding of absorption in coupled nanostructures can be obtained through theoretical approaches.8,29,30 Experimental methods of characterizing absorption are still critical though because simulations cannot always predict the impact of morphological details such as tip curvature, crystalline defects, and chemical impurities, or charge interactions between the nanostructure and the substrate or stabilizing ligand layer.27,31−34 Refinement of simulation methods, therefore, requires experimental results as a reference, defining which perturbations must be included for an accurate representation especially in engineered nanostructures with elaborate morphologies and interacting components.

II. BACKGROUND Nanoscale Absorption Methods. Several diverse methods have already been developed to characterize nonradiative properties of single nanostructures. In direct transmission techniques, light absorbed and scattered by a single nanostructure is sensitively detected by eliminating intensity fluctuations either through modulation or interferometric detection. These methods technically probe extinction, which is essentially equivalent to absorption for small nanoparticle sizes when the absorption cross section massively overwhelms the scattering cross section. Vallée and colleagues pioneered the use of spatial modulation spectroscopy, which records the modulated intensity change in transmission resulting from moving a single nanoparticle in and out of a focused excitation beam.35−41 In a slightly different execution, the beam position can also be modulated by a galvo-mirror system instead of moving the sample.42,43 Alternatively, absorption can be detected through modulated ground state depletion as demonstrated by Xie and colleagues.44 This approach involves overlapping pump and probe beams that are both on resonance with the same absorption band.44 Absorption of the modulated pump beam depletes the ground state of the molecule, preventing absorption of the probe beam and resulting in higher transmitted probe intensity.44 Sandoghar and colleagues have demonstrated another experimental approach, which measures the direct extinction of single nanoparticles by eliminating the laser noise using a balanced photodetector.45,46 In this case, the beam is divided into two components: the intensity of the first component is monitored as a reference and compared to the intensity of the second component after it has passed through the sample.46,47 Differences in intensity between the probe and reference beams arise from absorption.46,47 Techniques that rely on direct transmission readily deliver absorption information for very small nanoparticles because scattering can be neglected. However, it is not possible to separate absorption from scattering once the nanoparticle reaches a size large enough to scatter efficiently.48 Single nanostructure absorption can also be assessed through a thermal intermediate because the absorbed energy quickly relaxes into heat. Another route to evaluating single nanoparticle

Figure 1. (A) Schematic representation of the branching between different relaxation processes following plasmon excitation. (B−E) Mie theory calculations comparing the scattering (solid line), absorption (dotted line), and extinction (black line) of Au nanospheres of different diameters: 50, 75, 100, and 150 nm, respectively. (F) Artistic depiction of the photothermal method. The sample is illuminated with a modulated heating beam (green), which excites the Au nanosphere. Energy dissipated through nonradiative relaxation following absorption alters the refractive index of the medium surrounding the nanoparticle. The probe beam is scattered by this change in refractive index resulting in photothermal signal. The outer bubble represents the medium used to enhance the photothermal signal, which originates from temperatureinduced refractive changes close to the nanostructures as highlighted by the inner bubble.

another practical difficulty associated with measuring a nonradiative response especially on the single particle level. Absorption, like scattering, is influenced by the physical morphology of the nanoparticle as well as its surroundings and often does not simply track the radiative response.1−3 The most well-known example is that even in the basic case of a single Au nanosphere the relative ratio of the absorption and scattering cross sections depends sensitively on the nanoparticle size. Figure 1B−E presents Mie theory-predicted absorption (dotted line), scattering (solid line), and extinction (dash-dotted line) B


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analytical methods, but this convenient outcome pertains only to nanoparticles that fall within strict size and shape bounds illustrating the need for complementary experimental verification. A solution to Maxwell’s equations known as Mie theory has been utilized historically to predict the absorption and scattering cross-sections of metal nanospheres with remarkable accuracy.65 As reviewed earlier in Figure 1B−E, Mie theory reveals that the absorption and scattering of simple metal nanospheres depends sensitively on their size and that the ratio of these two components of the optical response can be manipulated to favor either one within a relatively small size range. Mie theory assumes a perfectly spherical nanoparticle geometry, yet in reality metal nanospheres are highly faceted. Therefore, to determine how well Mie theory describes real colloidal nanospheres, we compared it to the experimental absorption and scattering responses from Au nanospheres of varying size. We explored the balance between absorption and scattering in Au nanospheres as a function of size, revealing that their optical properties on average track Mie theory but also can deviate strongly on an individual particle basis. We relied on a combination of photothermal imaging and dark-field scattering to quantify the absorption and scattering cross sections of various Au nanospheres at a single wavelength of 532 nm experimentally.48 Figure 2A−E displays correlated images acquired from the scattering and absorption of individual nanospheres as well as the scanning electron microscopy (SEM) micrographs verifying their physical dimensions. Corroborating the Mie theory predicted spectra in Figure 1B−E, these results show that the smallest nanospheres absorb more efficiently than they scatter, and that as their diameters grow scattering rapidly overwhelms absorption.48 We extracted the magnitude of the absorption and scattering cross sections (CS) for at least 250 single nanospheres from each sample comparing our results directly to Mie theory in Figure 2F,G. Overall, experimental characterization was consistent with the trend of Mie theory and in terms of its predictions regarding the absorption and scattering intensities at 532 nm as a function of size. However, deviations are also clearly visible for individual nanospheres, even with the smallest diameters, while the greatest disagreement between experiment and theory occurred in the largest nanospheres. We determined that inconsistencies in shape and interactions with the supporting substrate were responsible for heterogeneity in the scattering and absorption cross sections of individual nanospheres. Correlated SEM revealed that the shapes of some nanoparticles strayed far from perfect spheres, especially in nanoparticles from larger average size samples as continued colloidal nanoparticle growth can produce highly irregular shapes. Fluctuation in the absorption and scattering cross sections is therefore attributed in part to the sensitivity of the plasmon resonance to nanoparticle geometry.48 However, the wide deviation of measured signal intensities was not explained solely by shape effects for the small nanosphere samples, as these samples exhibited limited geometric variation.48 We concluded that the presence of the substrate expanded diversity in absorption and scattering cross sections through the introduction of charge interactions between the nanospheres and their support.48 Overall, these discrepancies are a reminder that the optical properties depend on much finer details than the general nanoparticle size and that a nuanced understanding is necessary to meaningfully relate their physical morphology and optical response. While higher resolution electron microscopy and darkfield spectroscopy together with numerical methods capable of simulating arbitrary nanoparticle geometries have been em-

absorption was developed by Goldsmith and colleagues and involves optical microresonators, which act as incredibly sensitive thermometers.49,50 In this detection scheme, heat released by the metal nanoparticle following absorption triggers energy shifts in the microresonator’s whispering gallery modes that can be detected with high precision.49,50 The final method to be mentioned here and the focus of this article also relies on heat to detect absorption. In photothermal heterodyne imaging, absorption is recorded through a change in refractive index induced upon nanoparticle heating, thereby offering a direct assessment of nonradiative relaxation.51−56 This technique as it is utilized now was first introduced as a method to understand the nonradiative relaxation of plasmonic nanoparticles by Lounis and colleagues in 2004.57 Photothermal Imaging. Photothermal imaging stands out as an approach for understanding pure absorption of single metal nanostructures because it directly measures heat released via nonradiative decay in a method that is background free. Figure 1 F provides an artistic interpretation of the basic concepts behind the photothermal imaging technique. In this depiction, the Au nanosphere absorbs energy from a modulated heating beam (green). Electrons excited in the nanoparticle then relax, losing almost all of the energy they absorbed as heat to the nanoparticle’s immediate surroundings.20 To enhance photothermal contrast, the nanoparticle is encased in a medium (outer bubble) with a temperature-sensitive refractive index. Commonly glycerol is used and ensures that heat released by the nanoparticle results in a modulated local change in the refractive index (inner bubble). Photothermal signal originates from interference between the probe laser (red) and the thermal nanolens originating from the long-range character of the refractive index profile.55 This thermal lens decays to half its value on the length scale of the nanoparticle,56 so lock-in detection is necessary to recover the photothermal signal from single nanostructures. However, interference with the transmitted intensity of the probe laser beam yields a background free signal,55,58 making photothermal imaging a highly sensitive single particle technique. Photothermal analysis has already uncovered a number of exceptional insights. To date, photothermal imaging has been applied to characterize nonradiative relaxation in metal nanoparticles,59,60 carbon nanotubes,61,62 semiconductor nanostructures,57,63,64 and even single molecules.54 Spectral information has even been acquired over limited ranges through incorporation of tunable dye and solid-state heating lasers;60,61,63 however, these relatively narrow spectral windows fall short of the range needed to accurately determine the nonradiative properties of complex metal nanostructures, such as hybridized plasmon modes produced through metal nanoparticle coupling.27 In this Feature Article, we showcase our application of photothermal imaging to understanding the relationship between radiative and nonradiative relaxation in single metal nanoparticles and their assemblies. We focus on transitioning from an imaging technique to absorption spectroscopy to gain access to the spectral features of coupled nanostructures as an ultimate goal because, unlike simple nanoparticle geometries, strong interactions often cause a departure from scattering, and simple theoretical predictions of idealized structures therefore require experimental corroboration and exploration.

III. RESULTS Single Au Nanospheres. The optical properties of simple metal nanoparticles can be described thoroughly using classical C


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dimensions.7,26,65 In these nanostructures electronic oscillations can occur either along their length or width, leading to two orthogonal modes that are polarized parallel to each axis. Scattering by the more dominant, longitudinally polarized mode has already been exploited to determine the orientation of metal nanorods through diffraction-limited optical methods,69,70 and therefore begs the question how polarized photothermal imaging can contribute to mapping plasmon mode oscillations. The absorption of single Au nanorods is polarized and can indeed be used to determine their orientation from the longitudinal mode but also from the transverse resonance because of its higher sensitivity. We studied the polarization dependence of the photothermal signal from single Au nanorods using linearly polarized excitation.71 Figure 3A,B displays SEM

Figure 3. (A,B) Orientational imaging using polarization sensitive photothermal imaging. When using a linearly polarized heating beam, the photothermal signal is sensitive to the nanorod orientation as illustrated in the correlated SEM (A) and photothermal images (B). (C) The ensemble extinction spectrum of the Au nanorod sample studied shows the positions of the longitudinal surface plasmon resonance (∼700 nm) as well as the transverse plasmon resonance (∼520 nm). (D) Intensity of the photothermal signal using 675 nm excitation as a function of polarization angle for the nanorods in A indicated by the red and green squares. When the Au nanorod is excited at the LSPR position, the photothermal signal provides a sensitive measure of its orientation. Adapted with permission from ref 71. Copyright 2010 National Academy of Sciences.

Figure 2. (A−E) Dark-field scattering (left column), photothermal (middle column), and scanning electron microscope (SEM) images of single Au nanospheres (excitation at 532 nm). The average sizes of each Au nanosphere sample are depicted and labeled on the right-hand side: 51, 76, 88, 155, and 237 nm. (F,G) Comparison of scattering (F) and absorption (G) cross-sections (CS) determined experimentally to Mie theory predictions. The colored circles represent single Au nanospheres (with color indicating size), and the black lines in F and G correspond to the cross sections predicted by Mie theory. The red squares represent the subensemble average of more single particle data that was not correlated with SEM. Experimental results agree overall with Mie theory, but also highlighted the significant intensity deviations possible in individual nanoparticles. Adapted with permission from ref 48. Copyright 2010 American Chemical Society.

and photothermal images of the same three nanostructures. The photothermal heating wavelength of 675 nm was selected to be on resonance with the Au nanorod longitudinal surface plasmon resonance, which was centered around 700 nm for these particular nanorods as shown by their extinction spectrum in Figure 3C. The longitudinal surface plasmon resonance is dipolar in nature and consequently can only absorb light parallel to its oscillation.72 As evidenced by the varied photothermal intensities in Figure 3D for similarly sized Au nanorods, we confirmed that the highly polarized nature of this mode is maintained in absorption.71 The orientation angle of each nanorod was extracted from the modulation of its photothermal intensity as a function of excitation polarization. Figure 3D displays the photothermal signal of the Au nanorods identified by the red and green boxes in Figure 3A as a function of excitation angle. These particular nanorods exhibited opposite responses to the same excitation polarization, which is consistent with their perpendic-

ployed to this end,26,31,66−68 the results presented in Figure 2 clearly demonstrate that photothermal imaging in combination with dark-field microscopy is capable of separating the nonradiative and radiative contributions to the surface plasmon. Single Au Nanorods. Prominent effects from asymmetry encountered in metal nanospheres inspired us to also examine the nonradiative behavior of anisotropic metal nanostructures and its implications for absorption-based imaging techniques. Anisotropic metal nanoparticle geometries such as nanorods exhibit multiple optical modes related to their different D


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ular alignment as verified by SEM. These results provide additional experimental evidence that the longitudinal surface plasmon oscillation acts as a dipole absorber and occurs parallel to the length of the nanorod.71 Taking advantage of photothermal signal’s high sensitivity and the spectral separation between Au nanorod modes, we also determined nanorod orientations from absorption of the transverse surface plasmon resonance using a second excitation wavelength. With no special modifications, photothermal imaging can detect absorption from Au nanoparticles smaller than 10 nm in diameter,53,54,58 and therefore absorption of the transverse plasmon resonance was easily detected with resonant 514 nm excitation for these nanorods having average dimensions of 25 × 73 nm.71 Nanorod orientation was then also accurately calculated from heating beam polarization-induced modulation of the transverse mode absorption, leading us to draw two important conclusions. First, the ability to characterize nonradiative properties at different wavelengths can open the door to otherwise unattainable information−in this specific case, the transverse mode can act as an orientation reporter that is independent of changes to the nanorod’s environment or length that would detune the longitudinal mode from the excitation.71 Furthermore, for single anisotropic nanostructures polarization of the nonradiative response is determined by the optical mode excited, and therefore may also be valuable in understanding the optical consequences of coupling within nanoparticle assemblies. Au Nanoparticle Ring Assemblies. Building on the insights obtained from isolated metal nanoparticles, we now turn our attention to micron-sized nanoparticle assemblies created from a collection of isotropic Au nanospheres to elucidate the influence of coupling on the intensity and anisotropy of the nonradiative response. Figure 4A displays an SEM micrograph of a ring-shaped Au nanosphere assembly that was characterized using both dark-field scattering and photothermal imaging.73 In this system, individual Au nanosphere monomers show an isotropic surface plasmon resonance in the range of ∼500−550 nm, which is represented by the Mie theory scattering spectrum in Figure 4B (green line). When the nanospheres are combined within close proximity of each other in a larger assembly, they interact strongly, producing coupled modes that scatter light of primarily much longer wavelengths (Figure 4B, red line).27,73 In this series of experiments, we evaluated the radiative and nonradiative responses of the ring assembly using three different excitation wavelengths, which are indicated by the vertical lines and shaded rectangles in Figure 4B. The shortest wavelength excites the surface plasmon resonance of an isolated nanosphere directly while the longer wavelengths excite the strongly coupled modes of the assembled structure instead. Probing absorption and scattering of this nanoparticle assembly at multiple wavelengths highlighted the different behaviors of the coupled nonradiative and radiative modes. Figure 4C−E showcases images of the ring assembly obtained at two orthogonal polarizations for each wavelength range. These images illustrate that coupled plasmon modes respond more sensitively to the local arrangement of Au nanoparticles because high intensity hot spots that do not correlate to nanoparticle density appear in both scattering and absorption.73 These hot spots are clear indicators of nonuniform electromagnetic field enhancements resulting from nanoparticle coupling.27,74 Polarization-resolved absorption and scattering images furthermore revealed that the response excited by 514/550 nm exhibited the lowest degree of polarization dependence because it represents

Figure 4. (A) SEM micrograph of a ring assembled from Au nanospheres that was characterized in absorption and scattering. (B) A normalized Mie theory simulated scattering spectrum (green, right axis) represents the scattering response of a monomer Au nanosphere with a diameter of 50 nm. This spectrum is compared to the normalized experimental scattering spectrum (red, right axis) of the ring segment identified by the cyan box in A. The calculated scattering (blue solid line, left axis) and absorption (blue dotted line, left axis) spectra of the ring segment are also included for comparison. QSCA and QABS refer to the scattering and absorption efficiencies, which are defined as the optical cross-section normalized by the geometric cross-section of the total number of nanoparticles. Also shown in B are the 3 different excitation ranges explored experimentally. Photothermal imaging was executed using 514, 675, and 785 nm heating beams (dashed vertical lines), while the range of dark-field scattering was limited with the use of band-pass filters centered at 550, 700, and 800 nm (ranges indicated by the shaded rectangles). (C−E) Polarized photothermal and scattering images of the Au nanoparticle ring assembly obtained with different excitation wavelengths. These results illustrate that the modes excited with longer wavelengths (675/700 nm and 785/800 nm) were more polarized than those excited at 514/550 nm. Adapted with permission from ref 73. Copyright 2011 National Academy of Sciences.

the combined response of single particle and potentially several orthogonal or higher order coupled modes.73 In direct contrast, excitation of coupled modes at longer wavelengths produced polarized responses that exhibited their highest intensity when aligned parallel to the ring segment.73 These strongly coupled longitudinal modes, visible in both the scattering and absorption images, appear to be more sensitive to the local nanoparticle arrangement.73,75 Furthermore, dissecting the optical response into contributions from absorption and scattering revealed that the coupled modes suffer less from ohmic losses and promote enhanced electromagnetic fields. We determined the absorption and scattering cross sections of the Au nanoparticle ring by calibrating the measured intensity with reference Au nanoparticles and simulations.73 Modeling of an entire ring containing more than 1000 nanospheres was not possible, but a 500 nm E


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Figure 5. Overview of photothermal absorption spectroscopy. (A) General instrument schematic including the heating and probe beams as well as the detection scheme. Absorption spectroscopy is achieved by incorporating a white light laser as a tunable heating beam. (B) The power distribution of different heating wavelengths selected from the white light laser by the AOTF. (C) A photothermal image of 50 nm Au nanospheres and 15 nm thick Au film. The process of acquiring an absorption spectrum includes measurement of the single nanoparticle (blue triangle) followed by measurement of a gold film standard (red square) under the same conditions. (D) The experimental Au film spectrum (red squares) is utilized in combination with a simulated Au film spectrum (green circles) to correct the photothermal signal of the Au nanosphere (blue triangles). (E) A representative Au nanosphere absorption spectrum after correction. Adapted with permission from ref 76. Copyright 2015 American Chemical Society.

Figure 6. (A−C) Photothermal absorption spectroscopy in combination with dark-field scattering for the characterization of the absorption and scattering of Au nanorods across the entire visible range. SEM correlation verified that the spectra originated from single nanorods. (D−F) FDTD simulations of Au nanorods based on the sizes extracted from SEM agree well with the experimental spectra, confirming the accuracy of our technique for this spectral range. Reprinted with permission from ref 76. Copyright 2015 American Chemical Society.

that arise as a consequence of nanoparticle coupling, reaffirming the need to understand these processes separately through experimental means. Furthermore, our conclusions demonstrate the value of characterizing absorption at multiple wavelengths, especially for complex structures, because these insights have direct implications for application-driven nanostructure design. Ideally, an entire absorption spectrum can be measured to complement scattering spectroscopy that is routinely employed by using a white light lamp and a dispersive spectrometer.24,26,31,65 Photothermal Absorption Spectroscopy. To achieve a fair and direct comparison between absorption and scattering, we restricted the scattering response to certain wavelength slices by employing band-pass filters that matched the laser excitation lines used in the photothermal microscope that requires a collimated excitation beam. Instead of restricting the capabilities of dark-field scattering spectroscopy, and to improve our ability to understand nonradiative relaxation in complex nanostructures

linear ring segment of 33 nanoparticles was sufficient to achieve good agreement between experimental and simulated scattering spectra (Figure 4B). Our results demonstrated that the ring’s absorption cross section was significantly lower at 675/700 nm and 785/800 nm than at 514/550 nm. This conclusion was supported by the simulated absorption spectrum of the ring segment (Figure 4B), which illustrated that the intensity of absorption decreased at longer wavelengths. In contrast to absorption, the scattering cross sections at 675/700 nm and 785/ 800 nm were significantly larger than that recorded at 514/550 nm especially at the peak wavelength.73 This trend is consistent with the experimental and simulated scattering spectra of the ring segment that both peak at ∼900 nm. Based on these results, we determined that the Au nanoparticle ring produced the highest local field enhancement while minimizing the loss of energy to heat when it was excited at longer wavelengths, revealing the ideal excitation window for high antenna quality.73 This work highlights the significant deviations in absorption and scattering F


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originates from differences in the near-field and far-field responses of the localized surface plasmon resonance. The peak energy and amplitude of the shift is determined by the total damping of the plasmon, i.e., larger shift with larger damping, as explained by Zuloaga et al.77 In nanorods of smaller aspect ratios the plasmon band is closer to the Au interband transition energy, which leads to more intrinsic damping and therefore a larger shift between absorption and scattering resonance energies. The spectra in Figure 6 furthermore reveal how the intensity of the transverse mode relative to the main longitudinal peak is larger in absorption and decreases as the aspect ratio increases. These results hint at the wealth of information attainable from examining the radiative and nonradiative properties of metal nanostructures separately. Single Au Nanoparticle Oligomers. We also studied Au nanoparticle oligomers that were prepared by electron-beam lithography and therefore exhibited strong coupling without the degree of disorder present in the Au nanoparticle ring assemblies. The Au nanoparticle oligomers consisted of a large central nanodisk surrounded by nine smaller nanodisks, an arrangement that supports a unique optical feature called a Fano resonance. Fano resonances were first identified in atoms,78 but have since been demonstrated in numerous plasmonic systems.27 In the plasmonic Fano cluster studied here the Fano resonance results from radiative interference between superradiant and subradiant modes and therefore produces an asymmetric line shape in scattering, but not in absorption.79 Therefore, studying the absorption of a single Fano cluster allowed us to expose the influence of coupling on the collective nonradiative properties. To adequately capture spectral features of the Fano cluster, we expanded the wavelength range of our absorption spectroscopy setup to 1000 nm allowing us to verify the absence of the Fano resonance in absorption experimentally. Similar to the Au nanoparticle ring assemblies discussed earlier, the optical response of the Fano cluster extends beyond the visible range. To address this shortcoming we employed a second AOTF to access excitation wavelengths between 700 and 1000 nm.80 Figure 7A presents the experimentally measured absorption (green) and scattering (red) spectra of a 175 nm Au nanodisk. The Au nanodisk’s scattering spectrum featured a single dipolar resonance with maximum intensity at 840 nm. The same dipolar resonance is seen in the absorption spectrum, but the two spectral features differ significantly at shorter wavelengths because of contributions from the quadrupolar mode and interband transitions as confirmed by FDTD simulations (Figure 7B).80 Calculated charge distributions showing the dipolar (left) and quadrupolar (right) modes of the Au nanodisk are included in Figure 7 C. We then examined the absorption of a single Fano cluster compared to its scattering (Figure 7D). As expected based on simulations, the Fano resonance is clearly visible in the scattering spectrum (red) and notably absent in absorption (green). Agreement between the measured spectra of the Fano cluster and FDTD simulations (Figure 7E) was only possible after refining the nanodisk diameters, structure height, interparticle spacing, and the incident and collection angles. Additionally, small discrepancies remain between the measured and simulated spectra that likely arise from finer details that were not considered in simulations such as side wall angle and interactions with the environment. Simulated charge distributions (Figure 7 F) confirmed that the dip ∼800 nm in the scattering spectrum originated from interference between the subradiant and superradiant modes.

as a function of many wavelengths, we developed photothermal imaging into a broadband spectroscopic technique covering the entire visible and near-infrared spectral ranges. This conversion was made possible by incorporating a tunable white light heating beam into our photothermal setup. Compared to previous executions of photothermal spectroscopy49,60,61,63 that provided valuable information within relatively narrow wavelength ranges, we aimed to capture absorption line shapes containing multiple spectral features. Figure 5A shows a simplified schematic of our photothermal absorption spectroscopy setup, which functions in the same manner as a single wavelength photothermal imaging system with one exception.76 In order to measure an absorption spectrum, single wavelengths were selected from a white light laser by an acousto-optic tunable filter (AOTF). Selected lines are displayed in Figure 5B, which shows how the excitation wavelength can be scanned to acquire a continuous absorption spectrum. Figure 5B also highlights the variation in power of the supercontinuum laser as a function of wavelength. As indicated in the photothermal image included in Figure 5C, we measured a 15 nm thick Au film (red square) as an internal standard to eliminate discrepancies in power and any effects from focusing including chromatic aberration that affect the heating and probe beam overlap.76 Figure 5D displays raw absorption intensities obtained from the Au film (red squares) and the Au nanoparticle (blue triangles) compared to the Au film’s simulated absorption (green circles), which we used in combination with the internal Au film standard to calculate a correction factor that was applied to the measured nanoparticle signal to yield a final absorption spectrum.76 The absorption spectrum of a ∼50 nm Au nanosphere after correction is presented as an example in Figure 5E. Agreement between experimentally measured absorption spectra and FDTD simulations for individual Au nanorods verifies the accuracy of our approach across this broad spectral range. The experimental absorption (green dots) and scattering (red lines) spectra of three Au nanorods are presented in Figure 6A−C. From the spectra in Figure 6, it is easy to identify the consequences of minor geometric differences between the nanorods on their absorption and scattering responses. All three nanorods exhibited the expected Lorenztian peak assigned to the longitudinal surface plasmon resonance in their scattering and absorption spectra at longer wavelengths. However, discrepancies in resonance energy and absorption intensity at shorter wavelengths, where the transverse mode was excited, arose from slight differences in aspect ratio between these three nanorods from the same sample batch.76 Correlated SEM micrographs, which are shown as an inset in A−C, provided the exact dimensions of each nanorod that were used as input parameters for FDTD simulations.76 FDTD simulations accounting for the experimentally determined sizes and glycerol environment (Figure 6D−F) yielded excellent agreement with our experimental results. Our ability to measure nonradiative spectral information directly allowed us to capture the shift between absorption and scattering maxima in our measurements, which is a small feature that would be masked in extinction and ensemble measurements. Examining the longitudinal plasmon resonances of the nanorod spectra in Figure 6 reveals a clear blue shift for the absorption peaks compared to scattering. The magnitude of the shift decreases with increasing aspect ratio and consequently decreasing plasmon resonance energy (left to right). The measured trend of decreasing spectral shift with larger aspect ratio also appeared in the FDTD simulated spectra. This shift G


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Figure 8. (A) Absorption of the constituent pieces of the Fano structure (central disk and outer ring) measured separately to understand the influence of coupling on the nonradiative response. (B) FDTD simulations of the disk and ring components agree well with the measured absorption spectra. (C,D) Comparison between the sum of the central disk and outer ring absorption determined separately (black) and absorption of the entire Fano structure (red) demonstrates that near field coupling influences the response and reduces the intensity of absorption. (E) FDTD calculations of the near field enhancement at positions 1 and 2 indicated in (D). These images show that the highest electric field inside of the nanostructure is found at position 1, which corresponds to the highest intensity in absorption. Both maximum absorption and maximum near-field intensity inside the nanostructure at the same wavelength yields optimum hot carrier generation. Reprinted with permission from ref 80. Copyright 2016 American Chemical Society.

Figure 7. (A,B) Experimental and FDTD simulated absorption and scattering spectra of a Au nanodisk having the same size as the central disk in the decamer Fano structure. Excellent agreement was obtained between experiment and theory, demonstrating the accuracy of our technique over this extended spectral range. (C) Simulated charge plots showing the charge distribution on the central disk at the dipolar (left) and quadrupolar (right) modes. (D,E) Absorption and scattering spectra of the Fano structure. The experimental spectra (D) show excellent agreement with FDTD predictions (E) highlighting the absence of the Fano resonance in absorption. (F) Charge plots showing the different modes in the decamer structure that interfere to create the Fano resonance. Reprinted with permission from ref 80. Copyright 2016 American Chemical Society.

generated inside of the nanostructures.83 Interestingly, the largest near-field enhancement within the Fano structure corresponds to the position where absorption is also largest. These results demonstrate that, for this nanostructure, optimal hot carrier generation occurs at the position of maximum absorption to the blue side of the Fano scattering dip. These insights are highly relevant to the incorporation of Fano clusters in applications of light harvesting,84,85 sensing,86−88 and photocatalysis.4,89

Characterization of the Fano cluster absorption in comparison to its components revealed the presence of nonradiative coupling and the wavelength for optimal hot carrier generation. To elucidate the effects of coupling on nonradiative plasmon modes, we examined the absorption of the central disk and ring of the Fano cluster separately. Figure 8A,B present absorption spectra of the isolated outer ring and central disk acquired using photothermal absorption spectroscopy (A) and FDTD simulations (B). Utilizing these separated components, we can assess differences between the sum of their absorption and the combined response of the Fano cluster (Figure 8C,D). Comparing the sum of outer ring and central disk absorption (black) to the absorption of the entire Fano cluster (red) revealed that near-field coupling between the nanodisks changes the line shape of the absorption spectrum. In contrast to scattering, the effect in absorption is less dramatic, and the absorption spectrum of the Fano cluster is primarily defined by contributions from the outer ring.80 In addition, we calculated maps of the near-field intensities generated both inside and outside of the nanodisks (Figure 8E) when the Fano cluster was excited at the position of the Fano dip (position 2, subradiant mode) or at the position corresponding to the maximum absorption intensity (position 1, superradiant mode). As has been established previously, excitation at the Fano dip produced maximum near-field intensities outside of the nanoparticles.81,82 However, the yield of hot carriers is tied to the near-field

IV. FINAL REMARKS Our lab has focused primarily on evaluating wavelength-resolved absorption in metal nanostructures, but absorption spectroscopy of other materials can also be realized, especially through strategic selection of the photothermal environment that potentially allows for structures with smaller cross sections to be studied. In 2010, Orrit and colleagues elucidated how the photothermal signal depends on the surrounding medium.54 The photothermal signal results from changes to the refractive index profile surrounding the heated object and, as a consequence, is directly related to the temperature sensitivity of the medium’s refractive index, i.e., ∂n/∂T. As part of this seminal work, Orrit and co-workers demonstrated a 5-fold increase in the signal-tonoise ratio by selecting pentane as the photothermal medium instead of glycerol.54 Through a careful optimization of all parameters that contribute to the photothermal signal, the Orrit lab was able to visualize single molecules based on their H


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absorption.54 Values of ∂n/∂T are similar for most organic solvents though, making the case to study the photothermal response in media with more favorable thermo-optical properties in order to shift from imaging to spectroscopy of small nanoparticles, polymers, and even molecules. Inspired by this revelation, we explored the use of the thermotropic liquid crystal, 5CB (4-cyano-4-n-pentlbiphenyl), as a means to enhance the photothermal signal.90 Figure 9A presents a photothermal image

approach to increased sensitivity in photothermal detection.49,50 Their detection scheme relies on whispering-gallery-mode microresonators, which provide an extremely sensitive means to record temperature changes. These miniaturized thermometers are capable of reporting on minute photothermal induced shifts and can access detailed absorption information not resolvable using other methods such as Fano resonances created from coupling between the microresonator and single Au nanorods.49,50 These recent advances illustrate potential mechanisms for improving the sensitivity of photothermal absorption spectroscopy in the future. This Feature Article reviewed the development of photothermal imaging into a broadband spectroscopic technique that we have applied to examine the detailed nonradiative responses of metal nanostructures. Future developments on photothermal spectroscopy could combine advances in detection sensitivity with spectral resolution, making it possible to understand absorption in a wide range of materials.

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Figure 9. (A) Photothermal image of 20 nm Au nanospheres obtained in a liquid crystal (5CB) environment. (B) Photothermal image of Au nanospheres obtained in glycerol, a standard medium for photothermal imaging. (C) The use of 5CB as a photothermal medium increased the signal-to-noise ratio by a factor of 20. Simulated absorption spectra, which are displayed in the inset of C, confirmed that the increase in signal arose from 5CB’s advantageous thermo-optical properties and not a change in the Au nanosphere absorption cross section induced by the change in medium. These results highlight one method for achieving enhanced photothermal sensitivity. Reprinted with permission from ref 90. Copyright 2012 American Chemical Society.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anneli Joplin: 0000-0002-7894-850X Wei-Shun Chang: 0000-0002-0251-4449 Stephan Link: 0000-0002-4781-930X Notes

The authors declare no competing financial interest.

of 20 nm Au nanospheres in 5CB obtained using only 16.5 μW of heating beam power. By contrast, a heating beam power of 330 μW was necessary to obtain an image of similar signal quality in glycerol (Figure 9B). Figure 9C showcases the distribution of signal-to-noise ratios obtained in glycerol (blue) and 5CB (purple). Simulated absorption spectra of a Au nanosphere in both solvents, as shown in the inset of C, confirm that the increase in signal does not originate from an enhanced absorption cross section in the 5CB environment. Instead, our analysis of 20 nm Au nanospheres demonstrates that the signalto-noise ratio was enhanced by an order of magnitude (∼20 times) in 5CB because of its favorable thermo-optical properties.90−92 This discovery has exciting implications for the future of photothermal absorption spectroscopy, the sensitivity of which can be practically improved through further optimization of the thermo-optical environment. Others have since shown even more impressive advances in photothermal sensitivity that may enable absorption spectroscopy of single polymers, proteins, and even molecules under the right conditions. In one recent example, Orrit and colleagues harnessed near critical Xe as a state-of-the-art medium to increase the signal-to-noise ratio in photothermal measurements of Au nanospheres by over 200 times compared to glycerol.93 This work exploits the divergence of Xe’s thermal expansion coefficient near the critical point, which acts to amplify the sensitivity of the refractive index to changes in temperature, thereby enhancing the photothermal signal.93 The work of Goldsmith and colleagues embodies another extraordinary

Biographies

Anneli Joplin: Anneli Joplin is a chemistry Ph.D. student working under the supervision of Professor Stephan Link at Rice University in Houston, Texas. She received her B.S. in chemistry from Missouri State University in 2008 and was awarded an NSF graduate research fellowship in 2014. I


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REFERENCES

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Wei-Shun Chang: Wei-Shun Chang is a research scientist working with Professor Stephan Link in the Department of Chemistry at Rice University in Houston, Texas. He received his Ph.D. from the University of Texas at Austin at 2007 under the supervision of Professor Paul F. Barbara. His current research mainly focuses on ultrafast time-resolved and steady-state spectroscopy of single nanoparticles and their assemblies for applications in catalysis and sensing.

Stephan Link: Stephan Link is Professor of Chemistry and of Electrical and Computer Engineering at Rice University in Houston. He received his Ph.D. in chemistry in 2000 from the Georgia Institute of Technology where he worked for Professor Mostafa A. El-Sayed. In 2006, he joined the Rice Chemistry Department after postdoctoral positions at Georgia Tech and the University of Texas at Austin, where he worked for Professor Paul F. Barbara. His main research interests include the optical properties of single and assembled metallic nanoparticles.

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ACKNOWLEDGMENTS

This work presented here was originally funded by the Robert A. Welch Foundation (C-1664), the Army Research Office (MURI W911NF-12-1-0407), the Air Force Office of Scientific Research (MURI FA9550-15-1-0022), the National Science Foundation (CHE-0955286, EEC-0647452), the Office of Naval Research (N00014-10-1-0989), and a DURIP equipment grant (N0001412-1·0727). A.J. acknowledges support from the National Science Foundation through a Graduate Research Fellowship (1450681). J


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