We study plasmonic excitations in the contexts of soft matter and light-driven chemical reactions at nanoscale metals.
We prepare colloidal samples and investigate how the plasmon resonance can be tuned via layer-by-layer deposition of polyelectrolytes.
Schematic representation of the relevant fundamental primary processes occurring at nanoscale metals and a connection to the schematic representation of a catalytic cycle.
a) Even in the absence of light, the field enhancement amplifies electromagnetic vacuum fluctuations which modify potential energy surfaces by strong coupling. The timing of the other processes is essential: When a photon arrives, b) its electric field is enhanced in the near field. The LSP decays within few femtoseconds (fs) to c) hot electrons and holes, where initially a single electron or electron-hole pair carries the entire photon energy. Electrons equilibrate to a Fermi-Dirac distribution within <100 fs and d) electron-phonon coupling transfers the heat to vibrations within ~1 ps. e) The heat energy of electrons and phonons is conducted on all timescales. The secondary amide molecule sketched at the bottom is fragmented along two different pathways, depending on whether hot electrons as in c) or vibrational heat as in e) dominate.
When the nanoparticles are coated with molecular layers that exhibit strong absorption at the plasmon resonance of the metallic core, a strong coupling of the excitations can result in hybrid polaritonic states that exhibit a splitting of the resonances. The localized surface plasmon polaritons can be confined to very small mode volumes of nanoparticles, such that the coupling strength exceeds all dissipation channels. This is the regime of strong coupling. A related phenomenon is the bleaching of the hybrid absorption resonances by the very intense vacuum fluctuations that are present even in the absence of light.
"Plasmon"-driven chemistry is an emerging field at the interface of nanophysics and photochemistry. We have focused on the dimerization of 4-Nitrothiophenol (4-NTP) to Dimercaptoazobenzene (DMAB) at the surface of various nanoscale metal geometries. It is an open question, how the plasmon-assisted generation of hot charge carriers influences this reaction in addition to non-equilibrium vibrational heat. In a broader setting, we have applied surface-enhanced Raman scattering to nanoscale metals of various shapes and compositions to demonstrate the effect of local field enhancement.
Stete F., Bargheer M., Koopman W.
Optical Non-linearities in Plasmon-Exciton Core-Shell Particles: the Role of Heat
Strong coupling between plasmons and excitons gives rise to new hybrid polariton states with various fields of potential applications. Despite a plethora of research on plasmon--exciton systems, their transient behaviour is not yet fully understood. Besides Rabi oscillations in the first femtoseconds after an optical excitation, coupled systems show interesting non-linear features on the picosecond time scale. Here, we conclusively show that the source of these features is heat that is generated inside the particles. Until now, this hypothesis was only based on phenomenological arguments. We investigate the role of heat by recording transient spectra of plasmon--exciton core--shell nanoparticles with excitation off the polariton resonance. We present analytical simulations that precisely recreate the measurements solely by assuming an initial temperature rise of the electron gas inside the particles. The simulations combine established strategies for describing uncoupled plasmonic particles with a recently published model for static spectra. The simulations are consistent for various excitation powers confirming that indeed heating of the particles is the root of the changes in the transient signals.
Stete F., Koopman W., Henkel C., Benson O., Kewes G., and Bargheer M.
Vacuum-Induced Saturation in Plasmonic Nanoparticles
Vacuum fluctuations are a fundamental feature of quantized fields. It is usually assumed that observations connected to vacuum fluctuations require a system well isolated from other influences. In this work, we demonstrate that effects of the quantum vacuum can already occur in simple colloidal nano-assemblies prepared by wet chemistry. We claim that the electromagnetic field fluctuations at the zero-point level saturate the absorption of dye molecules self-assembled at the surface of plasmonic nano-resonators. For this effect to occur, reaching the strong coupling regime between the plasmons and excitons is not required. This intriguing effect of vacuum-induced saturation (VISA) is discussed within a simple quantum optics picture and demonstrated by comparing the optical spectra of hybrid gold-core dye-shell nanorods to electromagnetic simulations.
Koopman W., Sarhan R. M., Stete F., Schmitt C. N. Z., and Bargheer M.
Decoding the kinetic limitations of plasmon catalysis: the case of 4-nitrothiophenol dimerization
Plasmon-mediated chemistry presents an intriguing new approach to photocatalysis. However, the reaction enhancement mechanism is not well understood. In particular, the relative importance of plasmon-generated hot charges and photoheating is strongly debated. In this article, we evaluate the influence of microscopic photoheating on the kinetics of a model plasmon-catalyzed reaction: the light-induced 4-nitrothiophenol (4NTP) to 4,4′-dimercaptoazobenzene (DMAB) dimerization. Direct measurement of the reaction temperature by nanoparticle Raman-thermometry demonstrated that the thermal effect plays a dominant role in the kinetic limitations of this multistep reaction. At the same time, no reaction is possible by dark heating to the same temperature. This shows that plasmon nanoparticles have the unique ability to enhance several steps of complex tandem reactions simultaneously. These results provide insight into the role of hot electron and thermal effects in plasmonic catalysis of complex organic reactions, which is highly important for the ongoing development of plasmon based photosynthesis.
Liebig F., Sarhan R. M., Bargheer M., Schmitt C. N. Z., Poghosyan A. H., Shahinyan A. A., and Koetz J.
Spiked gold nanotriangles: formation, characterization and applications in surface-enhanced Raman spectroscopy and plasmon-enhanced catalysis
We show the formation of metallic spikes on the surface of gold nanotriangles (AuNTs) by using the same reduction process which has been used for the synthesis of gold nanostars. We confirm that silver nitrate operates as a shape-directing agent in combination with ascorbic acid as the reducing agent and investigate the mechanism by dissecting the contribution of each component, i.e., anionic surfactant dioctyl sodium sulfosuccinate (AOT), ascorbic acid (AA), and AgNO3. Molecular dynamics (MD) simulations show that AA attaches to the AOT bilayer of nanotriangles, and covers the surface of gold clusters, which is of special relevance for the spike formation process at the AuNT surface. The surface modification goes hand in hand with a change of the optical properties. The increased thickness of the triangles and a sizeable fraction of silver atoms covering the spikes lead to a blue-shift of the intense near infrared absorption of the AuNTs. The sponge-like spiky surface increases both the surface enhanced Raman scattering (SERS) cross section of the particles and the photo-catalytic activity in comparison with the unmodified triangles, which is exemplified by the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4′-dimercaptoazobenzene (DMAB).
Liebig F., Sarhan R. M., Schmitt C. N., Thünemann A., Prietzel C., Bargheer M., and Koetz J.
Gold Nanotriangles with Crumble Topping and their Influence on Catalysis and Surface-Enhanced Raman Spectroscopy
By adding hyaluronic acid (HA) to dioctyl sodium sulfosuccinate (AOT)‐stabilized gold nanotriangles (AuNTs) with an average thickness of 7.5±1 nm and an edge length of about 175±17 nm, the AOT bilayer is replaced by a polymeric HA‐layer leading to biocompatible nanoplatelets. The subsequent reduction process of tetrachloroauric acid in the HA‐shell surrounding the AuNTs leads to the formation of spherical gold nanoparticles on the platelet surface. With increasing tetrachloroauric acid concentration, the decoration with gold nanoparticles can be tuned. SAXS measurements reveal an increase of the platelet thickness up to around 14.5 nm, twice the initial value of bare AuNTs. HRTEM micrographs show welding phenomena between densely packed particles on the platelet surface, leading to a crumble formation while preserving the original crystal structure. Crumbles crystallized on top of the platelets enhance the Raman signal by a factor of around 20, and intensify the plasmon‐driven dimerization of 4‐nitrothiophenol (4‐NTP) to 4,4′‐dimercaptoazobenzene in a yield of up to 50 %. The resulting crumbled nanotriangles, with a biopolymer shell and the absorption maximum in the second window for in vivo imaging, are promising candidates for biomedical sensing.
Liebig F., Henning R., Sarhan R. M., Prietzel C., Schmitt C. N. Z., Bargheer M., and Koetz J.
A simple one-step procedure to synthesise gold nanostars in concentrated aqueous surfactant solutions
Due to the enhanced electromagnetic field at the tips of metal nanoparticles, the spiked structure of gold nanostars (AuNSs) is promising for surface-enhanced Raman scattering (SERS). Therefore, the challenge is the synthesis of well designed particles with sharp tips. The influence of different surfactants, i.e., dioctyl sodium sulfosuccinate (AOT), sodium dodecyl sulfate (SDS), and benzylhexadecyldimethylammonium chloride (BDAC), as well as the combination of surfactant mixtures on the formation of nanostars in the presence of Ag+ ions and ascorbic acid was investigated. By varying the amount of BDAC in mixed micelles the core/spike-shell morphology of the resulting AuNSs can be tuned from small cores to large ones with sharp and large spikes. The concomitant red-shift in the absorption toward the NIR region without losing the SERS enhancement enables their use for biological applications and for time-resolved spectroscopic studies of chemical reactions, which require a permanent supply with a fresh and homogeneous solution. HRTEM micrographs and energy-dispersive X-ray (EDX) experiments allow us to verify the mechanism of nanostar formation according to the silver underpotential deposition on the spike surface in combination with micelle adsorption.
Sarhan R. M., Koopman W., Pudell J.-E., Stete F., Rössle M., Herzog M., Schmitt C. N. Z., Liebig F., Koetz J., and Bargheer M.
Scaling up Nanoplasmon Catalysis: The Role of Heat DIssipation
Nanoscale heating by optical excitation of plasmonic nanoparticles offers a new perspective of controlling chemical reactions, where heat is not spatially uniform as in conventional macroscopic heating but strong temperature gradients exist around microscopic hot spots. In nanoplasmonics, metal particles act as a nanosource of light, heat, and energetic electrons driven by resonant excitation of their localized surface plasmon resonance. As an example of the coupling reaction of 4-nitrothiophenol into 4,4′-dimercaptoazobenzene, we show that besides the nanoscopic heat distribution at hot spots, the microscopic distribution of heat dictated by the spot size of the light focus also plays a crucial role in the design of plasmonic nanoreactors. Small sizes of laser spots enable high intensities to drive plasmon-assisted catalysis. This facilitates the observation of such reactions by surface-enhanced Raman scattering, but it challenges attempts to scale nanoplasmonic chemistry up to large areas, where the excess heat must be dissipated by one-dimensional heat transport.
Sarhan R. M., Koopman W., Schuetz R., Schmid T., Liebig F., Koetz J., and Bargheer M.
The importance of plasmonic heating for the plasmon-driven photodimerization of 4-nitrothiophenol
Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.