Engineering Optical Absorption in Late Transition-Metal Nanoparticles by Alloying

Alloying is an increasingly important handle to engineer the optical properties of metal nanoparticles that find applications in, for example, optical metamaterials, nanosensors, and plasmon-enhanced catalysis. One advantage of alloying over traditionally used particle size and shape engineering is that it, in principle, enables tuning of optical properties without a spectral shift of the localized surface plasmon resonance, which is important for applications where a specific spectral band is targeted. A second advantage is that alloying simultaneously enables adjustment of nanoparticle electronic, chemical, mechanical, and light absorption properties. However, a systematic survey of the impact of alloying on light absorption in metal nanoparticles does not exist, despite its key role in applications that include photothermal therapy, plasmonic heat generation, and plasmon catalysis. Therefore, we present here the systematic screening of the light absorption properties of binary late transition-metal alloys composed of Au, Ag, Cu, Pd, and Pt in the visible spectral range, based on a combination of experiments and finite-difference time-domain simulations, and discuss in detail the underlying physics. By studying these 10 alloy systems for 14 different nanoparticle sizes, we find that most nanoparticles experience a maximal absorption efficiency at around 80 nm particle diameter, and that most alloy systems outperform their neat constituents, with integrated absorption enhancement factors of up to 200%. This highlights the untapped potential of alloying for the engineering of light absorption in nanoparticles, and the presented material screening constitutes a resource for the rational selection of alloy systems with tailored absorption properties

Nanoplasmonic hydrogen sensing

In this review we discuss the evolution of surface plasmon resonance and localized surface plasmon resonance based hydrogen sensors. We put particular focus on how they are used to study metal-hydrogen interactions at the nanoscale, both at the ensemble and the single nanoparticle level. Such efforts are motivated by a fundamental interest in understanding the role of nanosizing on metal hydride formation processes. However, nanoplasmonic hydrogen sensors are not only of academic interest but may also find more practical use as all-optical gas detectors in industrial and medical applications, as well in a future hydrogen economy, where hydrogen is used as a carbon free energy carrier

Optical Hydrogen Nanothermometry of Plasmonic Nanoparticles under Illumination

The temperature of nanoparticles is a critical parameter in applications that range from biology, to sensors, to photocatalysis. Yet, accurately determining the absolute temperature of nanoparticles is intrinsically difficult because traditional temperature probes likely deliver inaccurate results due to their large thermal mass compared to the nanoparticles. Here we present a hydrogen nanothermometry method that enables a noninvasive and direct measurement of absolute Pd nanoparticle temperature via the temperature dependence of the first-order phase transformation during Pd hydride formation. We apply it to accurately measure light-absorption-induced Pd nanoparticle heating at different irradiated powers with 1 \ub0C resolution and to unravel the impact of nanoparticle density in an array on the obtained temperature. In a wider perspective, this work reports a noninvasive method for accurate temperature measurements at the nanoscale, which we predict will find application in, for example, nano-optics, nanolithography, and plasmon-mediated catalysis to distinguish thermal from electronic effects

Kalorimetrische Untersuchungen zu Magnetismus, Supraleitung und Nicht-Fermi-Flüssigkeits-Effekten in Systemen mit starken Elektronenkorrelationen

Die Arbeit befaßt sich mit der Messung und Analyse der spezifischen Wärme verschiedener stark korrelierter Elektronensysteme bei tiefen Temperaturen und hohen Magnetfeldern. Zunächst wird der im Rahmen dieser Arbeit verwendete, auf der Meßmethode der thermischen Relaxation beruhende Aufbau des Kalorimeters (Einsatzbereich 0.05K<T<4K und 0<B<12T) ausführlich erläutert. Danach werden die Ergebnisse von Messungen an den drei Schwere-Fermionen-Verbindungen CeCu2Si2, CeNi2Ge2 und YbRh2Si2 dargelegt. Wenngleich alle drei Systeme bei tiefen Temperaturen durch den für Schwere-Fermionen-Systeme charakteristischen, stark erhöhten elektronischen Beitrag zur spezifischen Wärme gekennzeichnet sind zeigen sich deutliche Unterschiede im beobachteten Grundzustandsverhalten. An CeCu2Si2 wird die für T<1K auftretende Konkurrenz zwischen einem supraleitenden und einem magnetischen Grundzustand ausführlich studiert. In YbRh2Si2 zeigt sich bei einer für 4f-Systeme bemerkenswert tiefen Temperatur von ca. 70mK ein Übergang in eine magnetische Phase, während der Grundzustand von CeNi2Ge2 wegen stark ausgeprägter Probenabhängigkeiten immer noch kontrovers diskutiert wird. Des weiteren zeigen alle drei Verbindungen deutliche Abweichungen vom Verhalten einer Fermi-Flüssigkeit. Die Theorie der Fermi-Flüssigkeit hat sich für metallische Verbindungen als sehr erfolgreich auch bei der Beschreibung des Verhaltens eines Systems aus stark wechselwirkenden Ladungsträgern erwiesen. Warum diese Theorie auf die untersuchten Verbindungen nicht anwendbar zu sein scheint, wird im Rahmen moderner Modellvorstellungen wie z. B. der Nähe zu einem quantenkritischen Punkt diskutiert. Die an Sr2RuO4, dem ersten Kupfer-freien Perowskit Supraleiter, durchgeführten Messungen der spezifischen Wärme dokumentieren das Auftreten von zwei Zusatzbeiträgen für T<Tc, die eine Interpretation der spezifischen Wärme des supraleitenden Zustands von Sr2RuO4 im Hinblick auf die Topologie des Ordnungsparameters deutlich erschweren

Single Particle Plasmonics for Materials Science and Single Particle Catalysis

Single particle nanoplasmonic sensing and spectroscopy is a powerful and at the same time relatively easy-to-implement research method that allows monitoring of changes in the structure and properties of metal nanoparticles in real time and with only few restrictions in terms of surrounding medium, temperature and pressure. Consequently, it has been successfully used in materials science applications to, for instance, reveal the impact of size and shape of single metal nanoparticles on the thermodynamics of metal hydride formation and decomposition. In this Perspective, we review and discuss the research efforts that have spurred key advances in the development of single particle nanoplasmonic sensing and spectroscopy as a research tool in materials science. On this background we then assess the prospects and challenges toward its application in single particle catalysis, with the aim to enable operando studies of the relationship between metal nanoparticle structure or oxidation state and catalytic performance

Plasmonic Metasurface for Spatially Resolved Optical Sensing in Three Dimensions

The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing. To date, nanoplasmonic sensors typically consist of two-dimensional (2D) nanoparticle arrays and can therefore only probe processes that occur within the array plane, leaving unaddressed the potential of sensing in three dimensions (3D). Here, we present a plasmonic metasurface comprising arrays of stacked Ag nanodisks separated by a thick SiO2 dielectric layer, which, through rational design, exhibit two distinct and spectrally separated LSPR sensing peaks and corresponding spatially separated sensing locations in the axial direction. This arrangement thus enables real-time plasmonic sensing in 3D. As a proof-of-principle, we successfully determine in a single experiment the layer-specific glass transition temperatures of a bilayer polymer thin film of poly(methyl methacrylate), PM/VIA, and poly(methyl methacrylate)/poly(methacrylic acid), P(MMA-MAA). Our work thus demonstrates a strategy for nanoplasmonic sensor design and utilization to simultaneously probe local chemical or physical processes at spatially different locations. In a wider perspective, it stimulates further development of sensors that employ multiple detection elements to generate distinct and spectrally individually addressable LSPR modes

Size-Dependent Kinetics of Hydriding and Dehydriding of Pd Nanoparticles

Using a new indirect nanoplasmonic sensing method with subsecond resolution, we have studied hydriding and dehydriding kinetics of Pd nanoparticles in the size range 1.8-5.4 nm. Strong particle-size effects are observed. The scaling of the hydriding and dehydriding time scales satisfies power and power-exponential laws. The former (with an exponent of 2.9) is in perfect agreement with Monte Carlo simulations of diffusion-controlled hydriding kinetics. The latter is explained by the effect of surface tension on hydrogen desorption from the surface layer. The approach is generalizable to other reactant-nanoparticle systems

Optimization of the Composition of PdAuCu Ternary Alloy Nanoparticles for Plasmonic Hydrogen Sensing

Alloying is a long-standing central strategy in materials science for the tailoring and optimization of bulk material properties, which more recently has started to find application also in engineered nanomaterials and nanostructures used in, among other, nanoplasmonic hydrogen sensors. Specifically, alloying Pd nanoparticles to form binaries and ternaries with the coinage metals Au and Cu has proven efficient to mitigate hysteresis in the sensor response, improve response and recovery times, boost sensitivity in the low hydrogen concentration sensing range, and reduce the detrimental impact of carbon monoxide poisoning. However, when surveying the corresponding studies, it is clear that there is a trade-off between the sensitivity enhancement and the CO-poisoning resistance effects provided by Au and Cu alloyants, respectively. Therefore, in this work, we systematically screen the impact of the Au and Cu concentration in PdAuCu ternary alloy nanoparticles used for plasmonic hydrogen sensing, to obtain a champion system with maximized sensitivity and CO-poisoning resistance based on an evaluation using the stringent ISO 26142 test protocol. As the main results, we find that the best hysteresis-free and sensitive response combined with deactivation resistance to 500 ppm CO in synthetic air is obtained for the Pd65Au25Cu10 ternary alloy system, which also exhibits good long-term stability during operation under severe CO poisoning conditions

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

Copper nanostructures are ubiquitous in microelectronics and heterogeneous catalysis and their oxidation is a topic of high current interest and broad relevance. It relates to important questions, such as catalyst active phase, activity and selectivity, as well as fatal failure of microelectronic devices. Despite the obvious importance of understanding the mechanism of Cu nanostructure oxidation, numerous open questions remain, including under what conditions homogeneous oxide layer growth occurs and when the nanoscale Kirkendall void forms. Experimentally, this is not trivial to investigate because when a large number of nanoparticles are simultaneously probed, ensemble averaging makes rigorous conclusions difficult. On the other hand, when (in situ) electron-microscopy approaches with single nanoparticle resolution are applied, concerns about beam effects that may both reduce the oxide or prevent oxidation via the deposition and cross-linking of carbonaceous species cannot be neglected. In response we present how single particle plasmonic nanospectroscopy can be used for the in situ real time characterization of multiple individual Cu nanoparticles during oxidation. Our analysis of their optical response combined with post mortem electron microscopy imaging and detailed Finite-Difference Time-Domain electrodynamics simulations enables in situ identification of the oxidation mechanism both in the initial oxide shell growth phase and during Kirkendall void formation, as well as the transition between them. In a wider perspective, this work presents the foundation for the application of single particle plasmonic nanospectroscopy in investigations of the impact of parameters like particle size, shape and grain structure with respect to defects and grain boundaries on the oxidation of metal nanoparticles

Intrinsic Fano Interference of Localized Plasmons in Pd Nanoparticles

Palladium (Pd) nanoparticles exhibit broad optical resonances that have been assigned to so-called localized surface plasmons (LSPs). The resonance's energy varies with particle shape in a similar fashion as is well known for LSPs in gold and silver nanoparticles, but the line-shape is always anomalously asymmetric. We here show that this effect is due to an intrinsic Fano interference caused by the coupling between the plasmon response and a structureless background originating from interband transitions. The conclusions are supported by experimental and numerical simulation data of Pd particles of different shape and phenomenologically analyzed in terms of the point dipole polarizability of spheroids. The latter analysis indicates that the degree of Fano asymmetry is simply linearly proportional to the imaginary part of the interband contribution to the metal dielectric function