Session C24: Time-resolved Energy Transfer and Exciton Transport in Nanostructures

Ultrafast Single and Multiexciton Energy Transfer in Semiconductor Nanoplatelets

Photophysical processes such as fluorescence resonance energy transfer (FRET) enable optical antennas, wavelength down-conversion in light-emitting diodes (LEDs), and optical bio-sensing schemes. The rate and efficiency of this donor to acceptor transfer of excitation between chromophores dictates the utility of FRET and can unlock new device operation motifs including quantum-funnel solar cells and reduced gain thresholds. However, the fastest reported FRET time constants involving spherical quantum dots (QDs) (0.12-1 ns), do not outpace biexciton Auger recombination (0.01-0.1 ns), which impedes multiexciton-driven applications including electrically-pumped lasers and carrier-multiplication-enhanced photovoltaics. Precisely controlled, few-monolayer thick semiconductor nano-platelets with tens-of-nanometer diameters exhibit intense optical transitions and hundreds-of-picosecond Auger recombination, but heretofore lack FRET characterizations. We examine binary CdSe NPL solids and show that inter-plate FRET (\textasciitilde 6-23 ps, presumably for co-facial arrangements) can occur 15-50 times faster than Auger recombination and demonstrate multiexcitonic FRET, making such materials ideal candidates for advanced technologies.   

Spontaneous emission enhancement of colloidal CdSe nanoplatelets.

Colloidal CdS /CdSe/CdS nanoplatelets synthesized recently are high efficient nano-emitters and gain media for nanoscale lasers and other nonlinear optical devices. They are characterized as quantum well structure due to energy gap difference between core CdSe and shell CdS, of which the luminescent wavelength could be tuned precisely by their thickness of growth. However, the influence of environment on the material's optical properties and further enhancement of the emission to implement nanoscale systems remains to be investigated. Here we demonstrate spontaneous emission rate enhancement of these CdSe nanoplatelets coupled to a photonic crystal cavity. We show clearly the photoluminescent spectrum modification of the nanoplatelets emission and an averaged Purcell enhancement factor of 3.1 is achieved when they are coupled to carefully-designed nanobeam photonic crystal cavities compared to the ones on unpatterned surface in our experiment of lifetime measurement. Also the phenomenon of cavity quality factor increasing is observed when increasing intensity of pumping, which attributes to saturable absorption of the nanoplatelets. Our success in enhancement of emission from these nanoplatelets here paves the road to realize actual nanoscale integrated systems such as ultra-low threshold micro-cavity lasers.   

Interfacial charge separation and trapping in composite photocatalysts

We explore the phenomena of interfacial charge separation and trapping in composite metal-semiconductor systems and the interaction (energy and charge exchange) between optically excited nanoparticles and the surrounding medium. Disc-shaped copper nanoparticles (Cu NPs) were fabricated by hole\textbf{--}mask colloidal lithography on bare and thin titania film covered fused silica substrates. The dynamics of Cu oxide formation around the NPs were studied in water by localized surface plasmon resonance (LSPR) spectroscopy. We found that the oxidation rate is strongly enhanced under UV irradiation when the NPs are on the surface of the titania film, in comparison to NPs deposited on an inert fused silica substrate. The reason is sought in the ability of TiO2 to create hydroxyl radicals with strong oxidative potential in water under UV irradiation and the charge transfer at the interface between the Cu NPs and the TiO2. The results demonstrate the potential of using LSPR spectroscopy to monitor the oxidation of Cu NPs in situ and in different environments.   

Measuring Exciton Migration in Conjugated Polymer Films with Ultrafast Time Resolved Stimulated Emission Depletion Microscopy

Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion (STED) microscopy. STED is typically used in biology with sparse well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated the extension of STED to conjugated polymer films and nanoparticles of MEH-PPV and CN-PPV, despite the presence of two photon absorption, by taking care to first understand the material's photophysical properties. We then further adapt this approach, by introducing a second ultrafast STED pulse at a variable delay. Excitons that migrate away from the initial subdiffraction excitation volume during the ps-ns time delay, are preferentially quenched by the second STED pulse, while those that remain in the initial volume survive. The resulting effect of the second STED pulse is modulated by the degree of migration over the ultrafast time delay, thus providing a new method to study exciton migration. Since this technique utilizes subdiffraction optical excitation and detection volumes with ultrafast time resolution, it provides a means of spatially and temporally resolving measurements of exciton migration on the native length and time scales. In this way, we will obtain a spatiotemporal map of exciton distributions and migration that will help to correlate the energetic landscape to film morphology at the nanoscale.   

How Microstructure Defines Function in Organic Conjugated Materials: Insights from Modelling

Organic conjugated materials have attracted an increasing interest over the years for their use in organic opto- electronic devices such as light-emitting diodes, solar cells, or field- effect transistors as a result of their low cost, light weight and ease of processing from solution. The improvement of the device performances requires a deep understanding of the electronic processes taking place in these devices down to the molecular scale. Especially, the way organic conjugated molecules or polymer chains organize in the solid state appears as a critical parameter to control in order to fine tune the materials electronic and photophysical properties. In our laboratory, we have developed a multi-faceted modeling scheme that encompasses classical molecular dynamics, quantum-chemistry, non-adiabatic quantum dynamics and kinetic Monte Carlo simulations to assess multiple fundamental opto- electronic processes occurring in conjugated materials used in devices. Here, we will more specifically review work dealing with the modeling of charge transport in conjugated polymers as well as singlet fission and exciton transport in small molecules. In all cases, we will highlight how these processes are sensitive to the relative arrangement of the materials at the nanoscale.   

Probing surface recombination velocities in semiconductors using two-photon microscopy

We propose an analysis of the diffusion problem related to the two-photon time-resolved photoluminescence microscopy technique. We are considering a model of excess carrier diffusion in three dimensions, with recombination that is first order in carrier density (ie valid in low injection regime) and various boundary conditions that will apply to different use of the technique. First, we study a single planar boundary with enhanced recombination (parameterized with a recombination velocity). This planar boundary may represent the exposed sample surface, or any deep subsurface structure, such as a grain boundary or materials interface. Next, we assume the diffusion to be bounded by two parallel planes parameterized with different recombination velocities. This may apply to thin films where the diffusion length is higher than the sample thickness, or when the carrier generation volume axially spans the entire film. Finally, we investigate diffusion in a sphere where the spherical surface plays the role of a closed grain boundary. For all these cases we give analytical solutions for the three-dimensional diffusion problem for an excitation of arbitrary spatial or time dependence. We believe the solutions and scalings to be simple enough to enable convenient data fitting.   

Direct measurement of non-equilibrium phonon occupations in femtosecond laser heated Au films

We use ultrafast electron diffraction to detect the temporal evolution of phonon populations in femtosecond laser-excited ultrathin single-crystalline gold films. From the time-dependence of the Debye-Waller factor we extract a 4.7 ps time-constant for the increase in mean-square atomic displacements. We show from the increase of the diffuse scattering intensity that the population of phonon modes near the X and K points in the Au fcc Brillouin zone grows with timescales of 2.3 and 2.9 ps, respectively, faster than the Debye-Waller average. We find that thermalization continues within the initially non-equilibrium phonon distribution after 10 ps. The observed momentum dependent timescale of phonon populations is in contrast to what is usually predicted in a two-temperature model.   

Time-resolved spectroscopy at surfaces and adsorbate dynamics:insights from a model-system approach

We introduce a finite-system, model description of the initial stages of femtosecond laser induced desorption at surfaces. Using the exact many-body time evolution and also results from a novel time-dependent DFT description for electron-nuclear systems, we analyse the competition between several surface-response mechanisms and electronic correlations in the transient and longer time dynamics under the influence of dipole-coupled fields. Our model allows us to explore how coherent multiple-pulse protocols impact desorption in a variety of prototypical experiments.