Ilka Kriegel

Effect of Surface Coating on the Photocatalytic Function of Hybrid CdS–Au Nanorods

Hybrid semiconductor-metal nanoparticles are interesting materials for use as photocatalysts due to their tunable properties and chemical processibility. Their function in the evolution of hydrogen in photocatalytic water splitting is the subject of intense current investigation. Here, the effects of the surface coatings on the photocatalytic function are studied, with Au-tipped CdS nanorods as a model hybrid nanoparticle system. Kinetic measurements of the hydrogen evolution rate following photocatalytic water reduction are performed on similar nanoparticles but with different surface coatings, including various types of thiolated alkyl ligands and different polymer coatings. The apparent hydrogen evolution quantum yields are found to strongly depend on the surface coating. The lowest yields are observed for thiolated alkyl ligands. Intermediate values are obtained with L-glutathione and poly(styrene-co-maleic anhydride) polymer coatings. The highest efficiency is obtained for polyethylenimine (PEI) polymer coating. These pronounced differences in the photocatalytic efficiencies are correlated with ultrafast transient absorption spectroscopy measurements, which show a faster bleach recovery for the PEI-coated hybrid nanoparticles, consistent with faster and more efficient charge separation. These differences are primarily attributed to the effects of surface passivation by the different coatings affecting the surface trapping of charge carriers that compete with effective charge separation required for the photocatalysis. Further support of this assignment is provided from steady-state emission and time-resolved spectral measurements, performed on related strongly fluorescing CdSe/CdS nanorods. The control and understanding of the effect of the surface coating of the hybrid nanosystems on the photocatalytic processes is of importance for the potential application of hybrid nanoparticles as photocatalysts.

Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives

Abstract Degenerately doped semiconductor nanocrystals (NCs) are of recent interest to the NC community due to their tunable localized surface plasmon resonances (LSPRs) in the near infrared (NIR). The high level of doping in such materials with carrier densities in the range of 1 0 21 cm − 3 leads to degeneracy of the doping levels and intense plasmonic absorption in the NIR. The lower carrier density in degenerately doped semiconductor NCs compared to noble metals enables LSPR tuning over a wide spectral range, since even a minor change of the carrier density strongly affects the spectral position of the LSPR. Two classes of degenerate semiconductors are most relevant in this respect: impurity doped semiconductors, such as metal oxides, and vacancy doped semiconductors, such as copper chalcogenides. In the latter it is the density of copper vacancies that controls the carrier concentration, while in the former the introduction of impurity atoms adds carriers to the system. LSPR tuning in vacancy doped semiconductor NCs such as copper chalcogenides occurs by chemically controlling the copper vacancy density. This goes in hand with complex structural modifications of the copper chalcogenide crystal lattice. In contrast the LSPR of degenerately doped metal oxide NCs is modified by varying the doping concentration or by the choice of host and dopant atoms, but also through the addition of capacitive charge carriers to the conduction band of the metal oxide upon post-synthetic treatments, such as by electrochemical- or photodoping. The NIR LSPRs and the option of their spectral fine-tuning make accessible important new features, such as the controlled coupling of the LSPR to other physical signatures or the enhancement of optical signals in the NIR, sensing application by LSPR tracking, energy production from the NIR plasmon resonance or bio-medical applications in the biological window. In this review we highlight the recent advances in the synthesis of various different plasmonic semiconductor NCs with LSPRs covering the entire spectral range, from the mid- to the NIR. We focus on copper chalcogenide NCs and impurity doped metal oxide NCs as the most investigated alternatives to noble metals. We shed light on the structural changes upon LSPR tuning in vacancy doped copper chalcogenide NCs and deliver a picture for the fundamentally different mechanism of LSPR modification of impurity doped metal oxide NCs. We review on the peculiar optical properties of plasmonic degenerately doped NCs by highlighting the variety of different optical measurements and optical modeling approaches. These findings are merged in an exhaustive section on new and exciting applications based on the special characteristics that plasmonic semiconductor NCs bring along.

Band gap splitting and average transmission lowering in ordered and disordered one-dimensional photonic structures composed by more than two materials with the same optical thickness

Abstract The optical properties of photonic structures made with more than two materials are very interesting for optical filtering and lighting applications. Herein, we compared the transmission properties of one-dimensional photonic crystals made with three, four and five materials, showing that, with a photonic crystal made of t different materials, the band gap splits in t -1 bands. The same optical thickness for the different materials layers results in split photonic band gaps with the same intensity. Photonic crystals with more than two materials are simple structures that could be used for multi-feature optical filters, or that could provide feedback for multi-peak distributed feedback lasers. Furthermore, we analysed the transmission properties of ternary and quaternary random photonic structures. These materials could very interesting for light trapping in photovoltaic devices.

Synthesis, optical properties, and photochemical activity of zinc-indium-sulfide nanoplates

Zinc indium sulfide is an interesting material for photocatalytical applications. Here, ZnxIn2S3+x nanoplates with varying value of x (between 0 and 1) were synthesized by means of colloidal chemistry. The nanoplates have a diameter of up to 100 nm and a relatively small thickness of 2.5 nm. The structure of the Zn-free particles could be assigned to β-In2S3 based on the powder X-ray diffraction and Raman spectroscopy data. Particles containing Zn exhibit the hexagonal ZnIn2S4 structure. The samples were studied by steady state and time resolved photoluminescence spectroscopy. Our results demonstrate that increasing amounts of Zn incorporation result in more intra-gap states that participate in radiative sub-band gap recombination and simultaneously decrease non-radiative defect states that might, for example, originate from vacancies in the structure. Furthermore, the nanoplates exhibit significantly increased photochemical activity in the degradation of rhodamine 6G, depending on their Zn content. The high surface-to-volume ratio and the intra-gap states might be responsible for this finding.

Optical properties of one-dimensional disordered multilayer photonic structures

The investigation of the differences between ordered and disordered materials (in the hundreds of nanometer lengthscale) is a crucial topic for a better understanding of light transport in photonic media. Here we study the light transmission properties of 1D photonic structures in which disorder is introduced in two different ways. In the first study, we have grouped the high refractive index layers in layer clusters, randomly distributed among layers of low refractive index. We have controlled the maximum size of such clusters and the ratio of the high-low refractive index layers (here called dilution). We studied the total transmission of the disordered structure within the photonic band gap of the ordered structure as a function of the maximum cluster size, and we have observed a valley in trend of the total transmission for a specific maximum cluster size. This value increases with increasing dilution. Furthermore, within one dilution we observe oscillations of the total transmission with increasing cluster size. In the second study, we have realized photonic structures with a random variation of the layer thickness. The structures were fabricated by radio-frequency (RF) sputtering technique. The transmission spectrum of the disordered structure was simulated by taking into account the refractive index dispersion of the materials, resulting in a good agreement between the experimental data and the simulations. We found that the transmission of the photonic structure in the range 300– 1200 nm is lower with respect the corresponding periodic photonic crystal. The studied disordered 1D photonic structures are very interesting for the modelization and realization of broad band filters and light harvesting devices.

Solution processable and optically switchable 1D photonic structures

We report the first demonstration of a solution processable, optically switchable 1D photonic crystal which incorporates phototunable doped metal oxide nanocrystals. The resulting device structure shows a dual optical response with the photonic bandgap covering the visible spectral range and the plasmon resonance of the doped metal oxide the near infrared. By means of a facile photodoping process, we tuned the plasmonic response and switched effectively the optical properties of the photonic crystal, translating the effect from the near infrared to the visible. The ultrafast bandgap pumping induces a signal change in the region of the photonic stopband, with recovery times of several picoseconds, providing a step toward the ultrafast optical switching. Optical modeling uncovers the importance of a complete modeling of the variations of the dielectric function of the photodoped material, including the high frequency region of the Drude response which is responsible for the strong switching in the visible after photodoping. Our device configuration offers unprecedented tunability due to flexibility in device design, covering a wavelength range from the visible to the near infrared. Our findings indicate a new protocol to modify the optical response of photonic devices by optical triggers only.

Black phosphorus-based one-dimensional photonic crystals and microcavities

The latest achievements in the fabrication of thin layers of black phosphorus (BP), toward the technological breakthrough of a phosphorene atomically thin layer, are paving the way for their use in electronics, optics, and optoelectronics. In this work, we have simulated the optical properties of one-dimensional photonic structures, i.e., photonic crystals and microcavities, in which few-layer BP is one of the components. The insertion of the 5-nm black phosphorous layers leads to a photonic band gap in the photonic crystals and a cavity mode in the microcavity that is interesting for light manipulation and emission enhancement.

Light-induced switching in pDTE–FICO 1D photonic structures

Abstract We propose the design of 1D photonic crystals and microcavities in which fluorine–indium codoped cadmium oxide (FICO) nanocrystal based layers and layers of diarylethene-based polyester (pDTE) are alternated or embedded in a microcavity. The irradiation with UV light results in two different behaviours: (i) it dopes the FICO nanocrystals inducing a blue shift of their plasmonic resonances; (ii) it changes the real part of the refractive index of the photochromic pDTE polymer. These two behaviours are combined in the proposed photonic structures and can be useful for switchable filters and cavities for light emission.

Dielectric multilayer structures fabricated by rf-sputtering

Optically confined structure is a fundamental topic in both basic and applied physics including potential application in information engineering, biological and medical sciences, sensing. In this scenario one-dimensional photonic crystals have been widely investigated and still remain an outstanding tool for new photonics, being the simplest system to exhibit a so-called photonic bandgap and therefore one of the easiest to handle in order to obtain tailored optical devices. RF sputtering techniques has demonstrated to be a viable technique for fabrication of 1D-photonic crystals allowing management and manipulation of the spectroscopic properties of optical and spectroscopic properties [1 21].

Tailoring the optical properties of one-dimensional (1D) photonic structures

We present the possibility to tailor the optical properties of 1D photonic structures by using more than two materials and by clustering the high refractive index (hRI) layer in the structures. In particular, we show that: i) with a photonic crystal made of i different materials, the photonic band gap splits in i-1 bands; ii) with a proper choice of the layer thickness, disordered photonic structures made with a high number of layers show periodic transmission peaks; iii) when the size of the hRI layer clusters, randomly distributed within the low refractive index layers, follows a power law distribution, the total light transmission follows a sigmoidal function. Furthermore, we discuss the fabrication aspects to realize the above mentioned photonic structures.