Session A18: Dynamics in II-VI Nanocrystals

Blinking and spectral diffusion of CdSe/ZnS nanoparticles

Even though the tunable optical properties of colloidal nanoparticles have been studied extensively, their luminescent behaviour is still not fully understood. The random emission intermittency and the power-law of on- and off-times as well as shifts in the emission wavelength still lack a comprehensive understanding [1]. We investigate the excitonic structure of CdSe/ZnS core/shell nanoparticles using a micro-photoluminescence (PL) setup with confocal as well as imaging optics. The nanoparticles are dispersed in toluene with 1\% PMMA and deposited by spin-coating on different substrates (bare Si/SiO$_{2}$ as well as Si/SiO$_{2}$ covered with different rough metallic layers). Depending on the substrate, we observe emission intermittency or nearly blinking-free emission with spectral jumps of 25 meV in the emission energy. Both can be assigned to excitonic transitions affected by additional charge inside or outside the nanoparticle [2]. Furthermore, we observe a phonon replica of 25 meV and smaller ($<$10 meV) energetic shifts of the emission lines, which are likely caused random charge variations in the environment of the nanoparticle. \\[4pt] [1] P. Frantsuzov et al., Nature 4, 519 (2008). \\[0pt] [2] A. Efros, Nature Mat. 7, 612 (2008)   

Photoluminescence blinking and carrier dynamics in giant nanocrystals with different electron confinement

Quantum dots have shown great promise as high quantum yield photon sources for applications in bioimaging, LEDs, lasers, etc. However, their photoluminescence (PL) intermittency (blinking) often complicates practical implementations. Recently, a new breed of giant nanocrystal quantum dots (gNQDs) with a large number of shell monolayers (ML) has been developed that show strongly suppressed blinking\footnote{Y. Chen \textit{et al.}, \textit{JACS}\textbf{ 130}, 5026 (2008)} and existence of multiexcitons.\footnote{Y.S. Park \textit{et al.,} \textit{Phys. Rev. Lett}. \textbf{106}, 187401 (2011)}$^,$\footnote{A. V. Malko \textit{et al.}, \textit{Nano Lett}., accepted (2011)} So far, their PL emission has been limited to around 630nm. In this work, we broadened this approach and extended gNQD emission to shorter wavelength in the visible spectrum. We investigated photostable CdSe/CdS gNQDs with small (480nm emission) core and compared them to large (625nm emission) core non-blinking gNQDs with similar shell thickness (14-17 ML). The small core dots show increased blinking behavior and shorter PL decay times in comparison to large core dots. The observed difference in blinking behavior is suggestive of different carrier confinement regimes leading to enhanced electron trapping at the dot's surface as well as modifications to non-radiative Auger recombination rates.   

Lifetime Blinking in Non Blinking Quantum Dots

Photoluminescence (PL) blinking is a common property of nanoscale light emitters. Nanocrystal quantum dots have often been used as model systems in studies of this intriguing phenomenon. Here, we use recently developed thick-shell CdSe/CdS NQDs to demonstrate a new regime of blinking where discrete fluctuations in the PL lifetime (``lifetime blinking'') occur without appreciable changes in the PL intensity. Single-dot measurements under controlled electrochemical charge injection [1] yield the PL lifetimes of neutral and charged excitons. We show that the observed ``lifetime blinking'' are due to random charging/discharging of the nanocrystal [2]. Indeed, the injection of electrons does not appreciably modify the PL quantum yield, which explains the coexistence of a nonblinking intensity with a ``blinking'' lifetime. At higher excitation power, charged excitons dominate the PL emission. We build a quantitative model showing that nanocrystal charging is caused by Auger-assisted ejection of a hole, producing negatively charged species. Importantly, Auger recombination that involves excitation of an electron is suppressed while hole-based processes remain efficient.\\[4pt] [1] Galland \textit{et al.}, Nature \textbf{479}, 203-207 (2011)\\[0pt] [2] Galland \textit{et al}., Submitted (2011)   

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

The phenomenon of fluorescence intermittency (blinking between ON/OFF states) has been observed for both naturally occurring fluorophores and artificial nanostructures. This study aims to resolve the long-standing controversy surrounding the origin of photoluminescence blinking in core/shell nanocrystal quantum dots. Researchers usually evoke the Auger, or A-type, mechanism in which photo-ionization of the dot leads to the OFF state, but recent observations have raised doubts about this explanation. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging [1]. We find that a second mechanism (called B-type) is the dominant cause for blinking. During B-type blinking, a photo-excited, ``hot'' electron is trapped in a surface state before being released to the core; the luminescence is quenched without any Auger process. By controlling the applied potential and the shell thickness, we can control the frequency and type of blinking, or suppress it completely. \\[4pt] [1] Galland \textit{et al.}, Nature \textbf{479}, 203-207 (2011).   

Ultrafast Cathodoluminesence Studies of Colloidal Nanocrystals

Despite possessing numerous desirable properties, including excellent photostability, high stopping-power, and fast, efficient fluorescence, semiconductor nanocrystal quantum dots are only mediocre gamma-ray scintillator materials. Efforts to improve performance are forestalled by a lack of understanding of how those materials respond to high energy radiation, which in turn comes from a lack of appropriate ultrafast tools for examining the relaxation of gamma-excited quantum dots. To this end, we have developed a 20keV as a surrogate for spontaneous gamma irradiation. We apply this technique to study the time-resolved response of thin films of CdSe/ZnS core/shell quantum dots. Energetic excitation produces a variety of excited states that can be separately resolved by consideration of their established energies and relaxation dynamics. We analyze the relative branching ratios of single excitonic, multi-excitonic and charged excitonic states to derive surprising conclusions regarding the physics of highly-excited quantum dots, as well as the probable source of poor gamma-scinitillating performance.   

Dynamic fluctuations in ultrasmall nanocrystals induce white light emission

Nanocrystals typically emit monochromatically at their size-dependent energy gaps. Recently, it was found that by pushing the size of a nanocrystal to its lower limits, absorption occurs at increasingly larger energies, but the expected blue to ultraviolet emission does not occur. Instead, ultrasmall nanocrystals begin to emit a broader spectrum. For instance, ultrasmall CdSe nanocrystals emit white light [1]. We have investigated small to ultrasmall CdSe nanocrystals using a combination of state-of-the-art scanning transmission electron microscopy and finite-temperature quantum molecular dynamics simulations. Our findings indicate that following excitation, partial thermalization sets the ultrasmall nanocrystals into a fluxional state, with a continuously varying energy gap, which results in white light emission when averaged over time. Furthermore, although the larger monochromatic emitting nanocrystals we have observed possess stable crystal cores, their surfaces are fluxional. Dynamic fluxionality should be taken into consideration when optimizing nanocrystals for applications. This work is supported by DOE grant DE-FG02-09ER46554 and Basic Energy Sciences Materials Sciences and Engineering Division.\\[4pt] [1] Bowers II, M.J. et al. J. Am. Chem. Soc 127, 15378 (2005).   

Dynamics of charge separation and multicarrier recombinaiton in CdS/CdTe heterodimer nanocrystals

Heterostructured semiconductor nanocrystals provide new ways to manipulate electronic wave functions and carrier recombination pathways. Recently, we developed a novel anion exchange reaction and synthesized anisotropically phase-segregated CdS/CdTe heterodimers with staggered band-edge alignment [1]. Here we report the ultrafast carrier dynamics in the heterodimers measured by transient absorption spectroscopy. While pump laser energy was tuned to create excitons only in the CdTe phase, we observed the bleaching of the CdS excitonic transition. The bleaching of CdS exciton states should be induced by electron injection from the CdTe phase to the CdS phase. Very rapid electron transfer time was evaluated to be about 400 fs. Moreover we found that temporal evolutions of CdS excitonic bleaching are almost independent of excitation intensity over a wide range, implying the suppression of Auger recombination. Our results indicate that charge separation efficiency of the heterodimers is enhanced due to their centrosymmetry-broken structures, and the designed nanocrystals are useful for next generation solar cells. [1] Saruyama et al., J. Am. Chem. Soc. 133, 17598 (2011).   

Transient Photoluminescence in Ligand-Exchanged Quantum Dot Assemblies

Improving electronic contact between nanocrystals (NCs) within self-assembled NC solids is a long standing goal, and recent work on ligand exchange using ammonium thiocyanate has demonstrated marked improvements in the charge mobility of NC films over longer, more insulating ligands [1]. Here, we use transient photoluminescence (PL) to study changes in radiative lifetime caused by ligand exchange. Our work begins by examining differences in lifetime and radiative efficiency between liquid dispersions and solid films of CdSe semiconductor NCs, and moves on to study additional changes in both types of samples correlated with ligand substitution. We use a combination of time-correlated single photon counting and non-linear optical Kerr gating to study PL on nanosecond and sub-picosecond time frames, respectively. We discuss the relationship between nanosecond and picosecond dynamics, and examine temperature dependence from 300 to 5 Kelvin. Initial data indicate a decrease in PL lifetime with ligand exchange, which we discuss in the context of increased transport mobilities and decreased interparticle separations, as well as changes in the steady-state optical spectra of these systems. [1] A.T. Fafarman, et al, J. Am. Chem. Soc., 133, 15753 (2011).   

Optically Excited Exciton Transfer of Spherical Quantum Dots via Optical Near-Field Interactions

We study a system composed of a mixture of different-sized spherical quantum dots (QDs) involving optical near-field (ONF) interactions to induce effective optical excitation transfer. Here energy transfer was explained by resonant energy transfer via the optical near-field interaction between the first excited state of small QDs and the second excited state of large QDs. The energy transfer in a film of different-sized QDs made of CdTe and CdSe were experimentally demonstrated. An analysis between the optical near field transfer rate [1] and F\"{o}rster type transfer rate was made. The proper understanding of the exciton transfer between these QDs is important for the design and implementation of near-field photonic devices employing them. [1] M. Ohtsu, et al., Principles of Nanophotonics, CRC Press (2008).   

Efficient Exciton Transfer from an Epitaxial Quantum Well to an Energy Gradient Structure Composed of Layer-by-Layer Assembled Colloidal Quantum Dots

In this work, we study exciton migration from a violet-emitting epitaxial quantum well (QW) to an energy gradient structure that consists of layer-by-layer assembled, green- and red-emitting quantum dot (QD) bilayer. In the experimental study, the energy gradient of these green and red QDs provides an increase of 64.2{\%} in the exciton transfer efficiency with respect to the bilayer of only red-emitting QDs. These results suggest that the energy difference between the QD layers significantly boosts the QW-QD exciton transfer rate compared to the mono-dispersed case. To support this experimental observation, we propose a theoretical model based on optical near field and density matrix to investigate the effects of energy difference between the QD layers. The strong exciton transfer from the epitaxial QWs to the colloidal QDs is essential to the energy efficiency of hybrid optoelectronic devices [1-3].\\[4pt] [1] A. Ruland, et al., Adv. Mater. 23, 4573--4577 (2011).\\[0pt] [2] M. Naruse, et al., Phys. Rev. 82, 125417 (2010).\\[0pt] [3] S. Nizamoglu, et al., Appl. Phys. Lett. 98, 163108 (2011).   

Anomalous photo-induced spectral changes in CdSe/ZnS quantum dots

We study photo-induced static and dynamic spectral changes in self-assembled CdSe/ZnS quantum dot (QD) thin films with varying QD concentrations under ambient conditions. Using spatially resolved scanning photoluminescence microscopy in conjunction with spectrally resolved time-correlated photon counting, we measure the variations in spectral intensity, emission wavelength, and recombination lifetimes as functions of photo-exposure time. We find that at low concentrations photo-darkening and photo-oxidation rates slow down with increasing QD density, but in the high concentration limit these rates are strongly enhanced. Our measurements lead us to conclude that the interplay of photo-induced surface trap discharging with preferential photo-oxidation of smaller QDs is further modulated by resonant energy transfer driven by strong inter-dot interactions in highly concentrated samples.~ Finally, we extend our studies to thin films with two different QD diameters to vary the ratio of donors to acceptors and modify the energy transfer efficiency   

Waveguide Integrated Colloidal Quantum Dot Emitters

High-yield II-VI colloidal quantum dots (QDs) have received considerable attention as quantum emitters for photonic device applications. Given their scalable synthesis and self-assembly, these QDs could serve as the basis for large arrays of single photon sources. Here, we present preliminary results on the integration of CdSe-based QDs into dielectric waveguides. Specifically, we will show how the local optical environment can be used to direct and enhance QD emission into specific spatial and spectral modes. First, we will present fabrication techniques to safely embed QDs into thin film dielectric waveguides. Then, we will present experimental back-focal-plane (BFP) measurements that demonstrate how thin-film interference effects can be used to the control the QD angular emission patterns. Experimental BFP data will be discussed and analyzed in terms of theoretical predictions based on the local density of optical states (LDOS), thus demonstrating how interference effects can be used to effectively direct emission from randomly oriented QDs. If time permits, we will also show how the extension of these techniques to individual QD emitters in ridge waveguides.   

Comparison of carrier multiplication yields in the PbX salts

In this talk I will present recent results of a collaborative effort investigating the quantum efficiency of carrier multiplication (CM) of the lead salt nanocrystals: PbS, PbSe and PbTe. These materials are promising candidates for exploring generation-III photovoltaic concepts that rely on carrier multiplication, the process in which a single photon generates more than one electron-hole pair. Despite the many apparent similarities of these materials in their bulk form, we find that these compounds exhibit strikingly different CM yields in their nanocrystalline form. We suggest that the difference in CM yields in these nanomaterials is the consequence of different competing relaxation rates, such as phonon emission. Indeed, we estimate the rate of energy dissipation due to phonon emission mediated by a polar Frohlich-type interaction for these three materials and find excellent qualitative agreement with the data. This approach could prove useful for predicting future materials to investigate for increasingly high CM yields.