Matthias Karg

A Solid‐State Plasmonic Solar Cell via Metal Nanoparticle Self‐Assembly

IO N Finding new mechanisms for photo-induced electron-hole separation is an important challenge for solar energy conversion. In the last decade, metal particles have been proposed as potential sensitizers in photovoltaics. Metal nanoparticles (NP) have very high absorption and scattering cross sections, that can be tailored throughout the visible region to the near infrared by varying their size and shape. [ 1 , 2 ] At least three different ways to implement metal particles in photovoltaic systems have been explored. In the fi rst case, the local near fi eld enhancement associated with surface plasmon excitation is used to enhance charge carrier generation. [ 3 ] Near fi eld effects have been reported to increase photocurrents in dye sensitized solar cells [ 4–8 ] and polymer based organic solar cells. [ 9–11 ] A second approach uses the high scattering cross section of metal particles at the surface plasmon resonance wavelengths to re-direct light into a solar cell substrate. [ 3 , 12–15 ] The third possibility is to use metal particles as the actual light harvesting element inducing charge separation in a photovoltaic device, which so far has received far less attention. We report on the fabrication and performance analysis of such a solid-state plasmonic solar cell: Photocurrent is generated by the excitation of gold or silver nanoparticles adsorbed to TiO 2 , while holes are transported through a hole conducting material (Spiro-OMeTAD) to the counter electrode. The nanoparticle fi lms are created via a fast and scalable, electrostatic self-assembly method yielding well-defi ned interparticle distances. These optically homogeneous fi lms absorb up to 37% of the incident photons at the respective surface plasmon resonance wavelengths. The spectral photocurrent response of the solar cells closely follows the absorption spectrum of the metal nanoparticle layers. The photocurrents are sustainable for at least 20 hours demonstrating that the charge separation process is regenerative. We propose three mechanisms for surface plasmon based charge carrier separation.

Temperature, pH, and ionic strength induced changes of the swelling behavior of PNIPAM-poly(allylacetic acid) copolymer microgels

The volume phase transition of colloidal microgels made of N-isopropylacrylamide (NIPAM) is well-studied and it is known that the transition temperature can be influenced by copolymerization. A series of poly( N-isopropylacrylamide- co-allylacetic acid) copolymers with different contents of allylacetic acid (AAA) was synthesized by means of a simple radical polymerization approach. The thermoresponsive behavior of these particles was studied using dynamic light scattering (DLS). Further characterization was done by employing transmission electron microscopy (TEM) and zeta potential measurements. TEM observations reveal the approximately spherical shape and low polydispersity of the copolymer particles. In addition, the measured zeta potentials provide information about the relative surface charge. Since these copolymers are much more sensitive to external stimuli such as pH and ionic strength than their pure PNIPAM counterparts, the volume phase transition was investigated at two different pH values and various salt concentrations. At pH 10 for the copolymer microgels with the highest AAA content, a significant shift of the volume phase transition temperature toward higher values is found. For higher AAA content, a change in pH from 8 to 10 can induce a change in radius of up to 100 nm making the particles interesting as pH controlled actuators.

General pathway toward crystalline-core micelles with tunable morphology and corona segregation.

We present a general mechanism for the solution self-assembly of crystalline-core micelles (CCMs) from triblock copolymers bearing a semicrystalline polyethylene (PE) middle block. This approach enables the production of nanoparticles with tunable dimensions and surface structures. Depending on the quality of the solvent used for PE, either spherical or worm-like CCMs can be generated in an easy and highly selective fashion from the same triblock copolymers via crystallization-induced self-assembly upon cooling. If the triblock copolymer stays molecularly dissolved at temperatures above the crystallization temperature of the PE block, worm-like CCMs with high aspect ratios are formed by a nucleation and growth process. Their length can be conveniently controlled by varying the applied crystallization temperature. If exclusively spherical micelles with an amorphous PE core are present before crystallization, confined crystallization within the cores of the preformed micelles takes place and spherical CCMs are formed. For polystyrene-block-polyethylene-block-poly(methyl methacrylate) triblock terpolymers a patch-like microphase separation of the corona is obtained for both spherical and worm-like CCMs due to the incompatibility of the PS and PMMA blocks. The structure of the patch-like corona depends on the selectivity of the employed solvent for the PS and PMMA corona blocks, whereby nonselective solvents produce a more homogeneous patch size and distribution. Annealing of the semicrystalline PE cores results in an increasingly uniform crystallite size distribution and thus core thickness of the worm-like CCMs.

Smart inorganic/organic hybrid microgels: Synthesis and characterisation

Responsive hybrid colloids containing both organic and inorganic components have been the subject of many investigations in recent years. These new materials combine the stimuli-responsiveness of some polymer-based colloids with the unique properties of inorganic nanoparticles. This article will review the different possible variations of such hybrid colloids. Moreover, synthetic approaches, methods of characterisation, and a few applications will be discussed. Due to the rather high number of recent publications dealing with hybrid materials based on a variety of polymers, we will mainly focus on responsive hybrid microgels made of poly(N-isopropyl-acrylamide) or poly(N-vinylcaprolactam) in aqueous media.

Multiresponsive Hybrid Colloids Based on Gold Nanorods and Poly(NIPAM-co-allylacetic acid) Microgels: Temperature- and pH-Tunable Plasmon Resonance

This work describes the control and manipulation of the optical properties of multiresponsive organic/inorganic hybrid colloids, which consist of thermo-responsive poly-(NIPAM-co-allylacetic acid) microgel cores and gold nanorods assembled on their surface. These composites are multifunctional, in the sense that they combine the interesting optical properties of the rod-shaped gold particles--exhibiting two well-differentiated plasmon modes--with the sensitivity of the copolymer microgel toward external stimuli, such as temperature or solution pH. It is shown that the collapse of the microgel core, induced by changes in either temperature or pH, enhances the electronic interactions between the gold nanorods on the gel surface, as a result of the subsequent increase of the packing density arising from the surface decrease of the collapsed microgel. Above a certain nanorod density, such interactions lead to remarkable red-shifts of the longitudinal plasmon resonance.

Surface Plasmon Spectroscopy of Gold-Poly-N-isopropylacrylamide Core-Shell Particles

Highly uniform, core-shell microgels consisting of single gold nanoparticle cores and cross-linked poly-N-isopropylacrylamide (PNIPAM) shells were prepared by a novel, versatile protocol. The synthetic pathway allows control over the polymer shell thickness and its swelling behavior. The core-shell structure was investigated by electron microscopy and atomic force microscopy, whereas the swelling behavior of the shell was studied by means of dynamic light scattering and UV-vis spectroscopy. Furthermore, the latter method was used to investigate the optical properties of the hybrid particles. By modeling the scattering contribution from the PNIPAM shells, the absorption spectra of the gold nanoparticle cores could be recovered. This allows the particle concentration to be determined, and this in turn permits the calculation of the molar mass of the hybrid particles as well as the refractive index of the shells.

Versatile Phase Transfer of Gold Nanoparticles from Aqueous Media to Different Organic Media

A novel, simple, and very efficient method to prepare hydrophobically modified gold particles is presented. Gold nanoparticles of different sizes and polydispersities were prepared. The diameter of the gold particles ranges from 5 to 37 nm. All systems were prepared in aqueous solution stabilized by citrate and afterwards transferred into an organic phase by using amphiphilic alkylamine ligands with different alkyl chain lengths. The chain length was varied between 8 and 18 alkyl groups. Depending on the particle size and the alkylamine, different transfer efficiencies were obtained. In some cases, the phase transfer has a yield of about 100%. After drying, the particles can be redispersed in different organic solvents. Characterization of the particles before and after transfer was performed by using UV/Vis spectroscopy, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS) techniques. The effect of organic solvents with various refractive indices on the plasmon band position was investigated.

Large-Area Organization of pNIPAM-Coated Nanostars as SERS Platforms for Polycyclic Aromatic Hydrocarbons Sensing in Gas Phase

Here, a new surface enhanced Raman spectroscopy (SERS) platform suitable for gas phase sensing based on the extended organization of poly-N-isopropylacrylamide (pNIPAM)-coated nanostars over large areas is presented. This system yields high and homogeneous SERS intensities, and simultaneously traps organic chemical agents as pollutants from the gas phase. pNIPAM-coated gold nanostars were organized into parallel linear arrays. The optical properties of the fabricated substrates are investigated, and applicability for advanced sensing is demonstrated through the detection in the gas phase of pyrene traces, a well-known polyaromatic hydrocarbon.

Multifunctional inorganic/organic hybrid microgels

This review summarizes recent research dedicated to hybrid colloids combining inorganic nanoparticles and cross-linked polymer networks. We discuss aspects of synthesis, characterization, and application of systems with different morphologies and properties. Due to the large number of works in the field of composite materials, we focus on materials with responsive polymer components, which are dispersed in aqueous media.

Plasmonic Library Based on Substrate-Supported Gradiential Plasmonic Arrays

We present a versatile approach to produce macroscopic, substrate-supported arrays of plasmonic nanoparticles with well-defined interparticle spacing and a continuous particle size gradient. The arrays thus present a “plasmonic library” of locally noncoupling plasmonic particles of different sizes, which can serve as a platform for future combinatorial screening of size effects. The structures were prepared by substrate assembly of gold-core/poly(N-isopropylacrylamide)-shell particles and subsequent post-modification. Coupling of the localized surface plasmon resonance (LSPR) could be avoided since the polymer shell separates the encapsulated gold cores. To produce a particle array with a broad range of well-defined but laterally distinguishable particle sizes, the substrate was dip-coated in a growth solution, which resulted in an overgrowth of the gold cores controlled by the local exposure time. The kinetics was quantitatively analyzed and found to be diffusion rate controlled, allowing for precise tuning of particle size by adjusting the withdrawal speed. We determined the kinetics of the overgrowth process, investigated the LSPRs along the gradient by UV–vis extinction spectroscopy, and compared the spectroscopic results to the predictions from Mie theory, indicating the absence of local interparticle coupling. We finally discuss potential applications of these substrate-supported plasmonic particle libraries and perspectives toward extending the concept from size to composition variation and screening of plasmonic coupling effects.