SYNTHESIS OF METAL NANOPARTICLES AND THEIR INFLUENCE ON FLUORESCENCE OF ORGANIC MATERIAL

In this work, synthesis of spherical AgNPs and their influence on optical properties of highly fluorescent organic dye rhodamine 6G (R6G) and low fluorescent conjugated polymer (RZ11) was investigated. Fluorescence changes of R6G was studied in solutions when concentration of AgNPs was increasing and fluorescence changes of RZ11 conjugated polymer was studied in thin films when layer of AgNPs was presented. Absorption and fluorescence spectra, and fluorescence decay kinetics of solutions and thin films were recorded by using steady-state and time-resolved spectroscopy. Fluorescence decay kinetics revealed that no energy transfer occur in solutions of R6G (10-6 M) and AgNPs (2.5x10-4÷7.5x10-4 M). In thin films with conjugated polymer, quenching of fluorescence with AgNPs was detected because of the energy transfer from fluorophore to metal due to short distances

Ultrafast exciton and charge carrier dynamics in nanostructured molecular layers

Due to their unique properties organic semiconductors may be used for various applications in organic optoelectronic devices: light emitting devices, lasers, field-effect transistors and photovoltaic cells and etc. Despite high perspectives of organic semiconductors they are still upstaged by their inorganic counterpart. The efficiency of electrooptical properties of organic semiconductors is tried to increase. The main goal of this thesis is a detailed investigation of ultrafast exciton and charge carrier processes in pure organic semiconductors and their blends with fullerene derivatives. Measured organic or silicon organic semiconductors were poly-di-n-hexylsilane (PDHS), polyfluorenes F8BT and PSF-BT, merocyanine MD376. Fullerenes used in blends were C60 and its derivative PCBM. Ultrafast transient absorption, fluorescence, and integral mode photocurrent measurements were used for investigations. The investigation of PDHS revealed that PDHS nanocomposites can be used for improvement of neat PDHS films fluorescence properties. The formation of intramolecular charge transfer state was proposed for PSF-BT neat films. Charge transfer scheme of the formation of long-lived charge pair state in PSF-BT/PCBM blend was presented. The formation of charge transfer states was explored in neat merocyanine films and blends with fullerene derivatives. The scheme of generation of charge pairs and free charge carriers in merocyanine blends with fullerene derivatives was discussed in detail. Analysis of measurements showed the influence of fullerene derivatives concentration, sample preparation technology and nanostructures packed with molecules to electrooptical properties of organic semiconductors

Ultrasparti eksitonų ir krūvininkų dinamika nanostruktūrizuotuose molekulių sluoksniuose

Due to their unique properties organic semiconductors may be used for various applications in organic optoelectronic devices: light emitting devices, lasers, field-effect transistors, photovoltaic cells and etc. Despite high perspectives of organic semiconductors they are still upstaged by their inorganic counterparts. Development of organic electronics requires better understanding of electrooptical properties of organic semiconductors and relationships between their structure and functions. The main goal of this thesis is a detailed investigation of ultrafast exciton and charge carrier processes in pure organic semiconductors and their blends with fullerene derivatives. Investigated organic or silicon organic semiconductors were poly-di-n-hexylsilane (PDHS), polyfluorenes F8BT and PSF-BT, merocyanine MD376. C60 fullerene and its derivative PCBM were used in blends. Ultrafast transient absorption, fluorescence, and integral mode photocurrent measurements were used for investigations. The investigation of PDHS nanocomposites revealed that PDHS incorporation into nanopores can be used for improvement of fluorescence properties. Formation of intramolecular charge transfer state was proposed for PSF-BT neat films. Charge transfer scheme of the formation of long-lived charge pair state in PSF-BT/PCBM blend was presented. The charge pair and free charge carrier generation scheme in merocyanine blends with fullerene derivatives were discussed in detail. Analysis of measurement results revealed the influence of concentration of fullerene derivatives, sample preparation technologies and sample morphology on electrooptical properties of nanostructured organic semiconductors

SYNTHESIS OF METAL NANOPARTICLES AND THEIR INFLUENCE ON FLUORESCENCE OF ORGANIC MATERIAL

In this work, synthesis of spherical AgNPs and their influence on optical properties of highly fluorescent organic dye rhodamine 6G (R6G) and low fluorescent conjugated polymer (RZ11) was investigated. Fluorescence changes of R6G was studied in solutions when concentration of AgNPs was increasing and fluorescence changes of RZ11 conjugated polymer was studied in thin films when layer of AgNPs was presented. Absorption and fluorescence spectra, and fluorescence decay kinetics of solutions and thin films were recorded by using steady-state and time-resolved spectroscopy. Fluorescence decay kinetics revealed that no energy transfer occur in solutions of R6G (10-6 M) and AgNPs (2.5x10-4÷7.5x10-4 M). In thin films with conjugated polymer, quenching of fluorescence with AgNPs was detected because of the energy transfer from fluorophore to metal due to short distances

Effect of Ag nanocube optomechanical modes on plasmonic surface lattice resonances

Noble metal nanoparticles patterned in ordered arrays can interact and generate hybrid plasmonic–photonic resonances called surface lattice resonances (SLRs). Dispersion curves help explain how the Bragg coupling conditions and radiation patterns create dipolar and quadrupolar SLRs, but they assume that the nanoparticles are static structures, which is inaccurate at ultrafast time scales. In this article, we examine how local surface plasmon resonances (LSPRs) supported by cubic Ag nanocrystals are modulated by ultrafast photophysical processes that generate optomechanical modes. We use transient absorbance spectroscopy measurements to demonstrate how the LSPRs of the nanoparticles modulate the SLR of the array over time. Two primary mechanical breathing modes of Ag nanocubes were identified in the data and input into electromagnetic models to examine how fluctuations in shape affect the dispersion diagram. Our observations demonstrate the impact of optomechanical processes on the photonic length scale, which should be considered in the design of SLR-based devices

High Electron Mobility and Its Role in Charge Carrier Generation in Merocyanine/Fullerene Blends

Charge carrier generation and drift dynamics have been investigated in two types of dye:fullerene heterojunctions: vacuum-deposited merocyanine:C-60 and solution-processed merocyanine:PC61BM blends by combining electric-field-induced fluorescence quenching and ultrafast time-resolved carrier drift measurements. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. Interfacial CT states on large C-60 domains have low energies, while CT states on PC61BM domains have larger energies, which are weakly dependent on the domain sizes. We distinguish two interfacial CT state dissociation pathways: (i) ultrafast, weakly dependent on the electric field and (ii) slow field-assisted dissociation during entire CT state lifetime. We attribute process i to low-energy, weakly bound CT states on large fullerene domains and process ii to strongly bound CT states on small domains or single fullerene molecules. The electron mobility in films with 50% of C-60 is several times higher than in the films with PC61BM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons; thus, fast motion and extraction of electrons are crucial for efficient solar cells

ZnO UV sensor photoresponse enhancement by coating method optimization

Modern high-performance photodetector research is driven by the need to simultaneously improve multiple parameters, but also fit the decreasing size of electronics and maintain low production price.  Here, we demonstrated how our synthesized ZnO tetrapod (ZnO-T) nanostructure was deposited on electrodes with variating gap by four coating methods including drop casting, microdrop casting, spray coating and slot-die coating with the same thickness. Optimizing the inter-electrode gap and coating method the record IUV/IDark ratio per unit area value of 8.73 × 106 was obtained. The fastest rise time 0.78 s and fastest decay time 0.94 s were obtained by slot-die coated sensors. High photoresponse of ZnO-Ts, the inter-electrode gap size influences formation of ZnO-T microstructure during coating process and morphology influence on photoresponse was explained. We demonstrate that even with the same optimized ZnO-T nanostructures photoresponse can be improved by 2 orders of magnitude. Our work shows the importance of coating morphology and inter-electrode gap optimization

Self-Saturable Absorption and Reverse-Saturable Absorption Effects in Diamond-Like Carbon Films with Embedded Copper Nanoparticles

Nonlinear optical properties (self-saturable absorption and reverse-saturable absorption) of diamond-like carbon films with variable amounts of embedded copper nanoparticle (DLC:Cu) films deposited by high power pulse magnetron sputtering were studied in dependence on deposition conditions. X-ray photoelectron spectroscopy was used to study elemental composition and He ion microscopy to study changes of the size, shape, and concentration of the Cu nanoparticles deposited using different pulse currents as well as argon and acetylene gas ratios. Raman scattering spectra of DLC:Cu films studied were typical of diamond-like carbon (including the films with a high Cu atomic concentration where it was up to 60 atom % and carbon atomic concentration was less than 20 atom %). In all cases, in the absorption spectra of DLC:Cu films, the surface plasmon resonance-related peak was recorded. The position of the plasmonic peak for all the deposited nanocomposite films was in a 590⁻620 nm range. For the samples containing lower amounts of copper, reverse-saturable absorption was found. Transient optical absorption measurements have shown that the reverse-saturable absorption is typical for the samples with a lower relaxation time. The longer relaxation times recorded during transient optical absorption measurements correlated with the presence of the saturable absorption effect. For DLC:Cu samples containing 58⁻62 atom % Cu, some optical inhomogeneities were observed. The samples in different places demonstrated either saturable absorption or reverse-saturable absorption. The presence of both saturable and reverse-saturable absorption effects in the same sample is explained by the lateral inhomogeneities of the DLC:Cu films

Photovoltaic Properties and Ultrafast Plasmon Relaxation Dynamics of Diamond-Like Carbon Nanocomposite Films with Embedded Ag Nanoparticles

Abstract Ultrafast relaxation dynamics of diamond-like carbon (DLC) films with embedded Ag nanoparticles (DLC:Ag) and photovoltaic properties of heterojunctions consisting of DLC:Ag and crystalline silicon (DLC:Ag/Si) were investigated by means of transient absorption (TAS) spectroscopy and photovoltaic measurements. The heterojunctions using both p type and n type silicon were studied. It was found that TAS spectra of DLC:Ag films were dependent on the used excitation wavelength. At wavelengths where Ag nanoparticles absorbed light most intensively, only DLC signal was registered. This result is in good accordance with an increase of the DLC:Ag/Si heterojunction short circuit current and open circuit voltage with the excitation wavelength in the photovoltaic measurements. The dependence of the TAS spectra of DLC:Ag films and photovoltaic properties of DLC:Ag/Si heterostructures on the excitation wavelength was explained as a result of trapping of the photoexcited hot charge carriers in DLC matrix. The negative photovoltaic effect was observed for DLC:Ag/p-Si heterostructures and positive (“conventional”) for DLC:Ag/n-Si ones. It was explained by the excitation of hot plasmonic holes in the Ag nanoparticles embedded into DLC matrix. Some decrease of DLC:Ag/Si heterostructures photovoltage as well as photocurrent with DLC:Ag film thickness was observed, indicating role of the interface in the charge transfer process of photocarriers excited in Ag nanoparticles