Nikodem Czechowski

Fluorescence enhancement of light-harvesting complex 2 from purple bacteria coupled to spherical gold nanoparticles

The influence of plasmon excitations in spherical gold nanoparticles on the optical properties of a light-harvesting complex 2 (LH2) from the purple bacteria Rhodopseudomonas palustris has been studied. Systematic analysis is facilitated by controlling the thickness of a silica layer between Au nanoparticles and LH2 complexes. Fluorescence of LH2 complexes features substantial increase when these complexes are separated by 12 nm from the gold nanoparticles. At shorter distances, non-radiative quenching leads to a decrease of fluorescence emission. The enhancement of fluorescence originates predominantly from an increase of absorption of pigments comprising the LH2 complex.

SIL-based confocal fluorescence microscope for investigating individual nanostructures

Atomic layer deposition of ultrathin HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180 degrees C. HfO2 was deposited also in a two-step temperature process where the initial growth of about 1 nm at 170 degrees C was continued up to 10-30 nm at 300 degrees C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10-12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30-40% far away from it.We developed a fluorescence confocal microscope equipped with a hemispherical solid immersion lens (SIL) and apply it to study the optical properties of light-harvesting complexes. We demonstrate that the collection efficiency of the SIL-equipped microscope is significantly improved, as is the spatial resolution, which reaches 600 nm. This experimental setup is suitable for detailed studies of physical phenomena in hybrid nanostructures. In particular, we compare the results of fluorescence intensity measurements for a light-harvesting peridinin-chlorophyll-protein (PCP) complex with and without the SIL.

Imaging of fluorescence enhancement in photosynthetic complexes coupled to silver nanowires

Optical microscopy and spectroscopy of hybrid nanostructures composed of chlorophyll-containing photosynthetic complexes and silver nanowires reveal strong enhancement of fluorescence intensity of chlorophylls bound to the protein. This effect results from interaction between excited states of molecules embedded in the photosynthetic complex and plasmon excitations in metallic nanowires. Wide-field microscopy images reveal twofold increase of the emission intensity for complexes located at the ends of the nanowires as compared to the ones lying along the nanowires. Complementary spectrally and temporally resolved experiments indicate about 10-fold average increase of the chlorophyll fluorescence rate upon coupling with the metallic nanoparticles.

Metal-Enhanced Fluorescence of Chlorophylls in Light-Harvesting Complexes Coupled to Silver Nanowires

We investigate metal-enhanced fluorescence of peridinin-chlorophyll protein coupled to silver nanowires using optical microscopy combined with spectrally and time-resolved fluorescence techniques. In particular we study two different sample geometries: first, in which the light-harvesting complexes are deposited onto silver nanowires, and second, where solution of both nanostructures are mixed prior deposition on a substrate. The results indicate that for the peridinin-chlorophyll complexes placed in the vicinity of the silver nanowires we observe higher intensities of fluorescence emission as compared to the reference sample, where no nanowires are present. Enhancement factors estimated for the sample where the light-harvesting complexes are mixed together with the silver nanowires prior deposition on a substrate are generally larger in comparison to the other geometry of a hybrid nanostructure. While fluorescence spectra are identical both in terms of overall shape and maximum wavelength for peridinin-chlorophyll-protein complexes both isolated and coupled to metallic nanostructures, we conclude that interaction with plasmon excitations in the latter remains neutral to the functionality of the biological system. Fluorescence transients measured for the PCP complexes coupled to the silver nanowires indicate shortening of the fluorescence lifetime pointing towards modifications of radiative rate due to plasmonic interactions. Our results can be applied for developing ways to plasmonically control the light-harvesting capability of photosynthetic complexes.

Large plasmonic fluorescence enhancement of cyanobacterial photosystem I coupled to silver island films

A large, two-orders-of-magnitude enhancement of fluorescence emission from cyanobacterial Photosystem I (PSI) coupled to plasmonic excitations in silver island films was observed. Such a high value has not been reported for metal-enhanced fluorescence of photosynthetic pigment-protein complexes before. The dramatic enhancement of the PSI emission occurs when PSI is excited resonantly into the Qx and Qy bands of chlorophyll a. In contrast, excitation in the carotenoid absorption band yields ten times lower enhancement factors. We attribute these large values of enhancement factor to plasmon-induced activation of excitation and emission channels absent for isolated PSI complexes.

Silver nanowires enhance absorption of poly(3-hexylthiophene)

Results of optical spectroscopy reveal strong influence of plasmon excitations in silver nanowires on the fluorescence properties of poly(3-hexylthiophene) (P3HT), which is one of the building blocks of organic solar cells. For the structure where a conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) was used as a spacer in order to minimize effects associated with non-radiative energy transfer from P3HT to metallic nanoparticles, we demonstrate over two-fold increase of the fluorescence intensity. Results of time-resolved fluorescence indicate that the enhancement of emission intensity can be attributed to increased absorption of P3HT. Our findings are a step towards improving the efficiency of organic solar cells through incorporation of plasmonic nanostructures.

Electroluminescence from carbon nanotube films resistively heated in air

Light emission from carbon nanotube (CNT) films was explored in both the near-infrared and the infrared spectral regions upon application of external bias voltage. We obviated the need to use sophisticated vacuum apparatus by employing state-of-the-art optics and detection system. It enabled us to sensitively probe electroluminescence at relatively low temperatures (T ∼ 300 °C) in ambient conditions and investigate the character of emission from CNT assemblies in real life conditions. The observed spectral response revealed distinct features and the results strongly suggest that CNT assemblies are promising candidates for optoelectronic applications, particularly in the field of telecommunication.

Polarization control of metal-enhanced fluorescence in hybrid assemblies of photosynthetic complexes and gold nanorods

Fluorescence imaging of hybrid nanostructures composed of a bacterial light-harvesting complex LH2 and Au nanorods with controlled coupling strength is employed to study the spectral dependence of the plasmon-induced fluorescence enhancement. Perfect matching of the plasmon resonances in the nanorods with the absorption bands of the LH2 complexes facilitates a direct comparison of the enhancement factors for longitudinal and transverse plasmon frequencies of the nanorods. We find that the fluorescence enhancement due to excitation of longitudinal resonance can be up to five-fold stronger than for the transverse one. We attribute this result, which is important for designing plasmonic functional systems, to a very different distribution of the enhancement of the electric field due to the excitation of the two characteristic plasmon modes in nanorods.

Direct evidence of delayed electroluminescence from carbon nanotubes on the macroscale

Spectrally resolved and kinetic response of electroluminescence was monitored from resistively heated carbon nanotube (CNT) macroassemblies. Sensitive detection system and custom-made setup for high-speed optoelectronic measurements were employed to investigate unsorted and single chirality-enriched CNTs. By increasing the content of (7,6) or (6,5) CNTs in a sample, the E11 emission peak in the infrared region became more narrow (∼150 nm), hence approaching that of commercial emitters for this spectral range. Moreover, electroluminescence initiation in CNTs occurred very rapidly and reached its full intensity within tens of milliseconds. Interestingly, observed delay between bias voltage application and electroluminescence proved triplet-triplet annihilation in the macroscopic assembly of CNTs.

Origin of bimodal fluorescence enhancement factors of Chlorobaculum tepidum reaction centers on silver island films

We focus on the spectral dependence of plasmon‐induced enhancement of fluorescence of Chlorobaculum tepidum reaction centers. When deposited on silver island film, they exhibit up to a 60‐fold increase in fluorescence. The dependence of enhancement factors on the excitation wavelength is not correlated with the absorption spectrum of the plasmonic structure. In particular, the presence of one (or multiple) trimers of the Fenna–Matthews–Olson (FMO) protein reveals itself in bimodal distribution of enhancement factors for the excitation at 589 nm, the wavelength corresponding to bacteriochlorophyll absorption of FMO and the core of the RC. We conclude that the structure of multichromophoric complexes can substantially affect the impact of plasmonic excitations, which is important in the context of assembling functional biohybrid systems.