Session P15: Focus Session: Relaxation and Phonons in Nanostructures

Properties of the LO-Phonon in GaN Nanocrystallites

Resonant Raman scattering in wurtzite structured GaN nanocrystallites of various morphologies were studied. The LO polar mode exhibited Fr\"{o}hlich-type resonant Raman scattering whose characteristics were found to depend weakly on the morphology of the crystallites. In contrast, the UV-laser heating and heat retention in the porous media of a crystallite ensemble were discovered to drastically modify the Raman properties. An ensemble temperature on the order of 550 K was inferred from the electron-phonon interaction model, a result that was verified via Raman scattering experiments at the elevated temperature regime. Complementary photoluminescence investigations concur with the Raman findings. The LO behavior of the GaN nanocrystallites, at temperature range 77 K- 900 K was investigated as well; the behavior is discussed in terms of the anharmonic decay mechanisms and the phonon dispersion curve of GaN of the wurtzite structure   

Intraband carrier relaxation in semiconductor quantum rods: Competition between phonon-assisted cooling and Auger heating

Quantization of electronic and phonon energies and large surface-to-volume ratios significantly modify energy relaxation mechanisms in nanoscale semiconductors compared to bulk materials. In the case of ultrasmall, semiconductor nanocrystals (NCs), strong quantum confinement leads to greatly enhanced carrier-carrier interactions that open new NC-specific energy relaxation and recombination channels. Here, we analyze the effect of Auger heating on the energy relaxation dynamics in elongated CdSe nanocrystals [quantum rods (QRs)]. At low carrier densities, less than 2-3 photoexcited electron-hole (e-h) pairs per QR on average, we observe bulk-like, fast, phonon-assisted carrier cooling with a time constant of around 0.5 ps. At high pump-intensities (more than 2-3 e-h pairs per QR), we detect a dramatic, orders-of-magnitude reduction in the energy relaxation rate resulting from efficient Auger heating. In this regime, energy relaxation directly correlates with recombination dynamics, which is an effect that has never been observed either in bulk or low-dimensional materials. Furthermore, we find that Auger heating differs in short and long QRs that can be explained by the difference in the scaling of Auger rates with respect to the carrier density in zero-dimensional (0D) and 1D semiconductors.   

Confined optical phonons in PbSe quantum dots

We characterized the confined optical phonons of PbSe quantum dots (QDs) by Raman scattering and far-infrared (FIR) absorption spectroscopy. The size dependence of the Raman spectrum is consistent with theoretical calculations within a dielectric continuum model, considering the phonon dispersion in the bulk material. The electron-phonon coupling strengths inferred from overtones in the Raman spectra is three orders of magnitude larger than expected from the calculated electron wave functions. The FIR absorption spectra exhibit an asymmetric broad peak near the bulk LO phonon frequency. The measured spectra are inconsistent with the ?hard-boundary? condition that has been used successfully in prior work on lead-salt QDs. We will discuss the size-dependence of the confined optical phonon and the possible mechanism to explain the FIR absorption spectra of PbSe QDs.   

Effects of Band Structure on the Electronic and Optical Properties of Semiconductor Nanocrystals: Lead Selenide vs. Cadmium Selenide

PbSe and CdSe nanocrystals (NCs) can be synthesized with high monodispersity, have a size-tunable band gap, and can exhibit near unity photoluminescence quantum yields. These two materials have significant differences with respect to bulk band gap, crystal structure, Bohr radius, and carrier effective masses that result in very distinct energy structures. Here we perform a side-by-side comparison of the optical and electronic properties of these two materials in both the single and multiexciton regimes for NCs of comparable sizes. Femtosecond transient absorption (TA) spectroscopy is used to study inter- and intraband relaxation of photo-generated carriers including various types of Auger-relaxation processes. We have discovered that for PbSe NCs, photo-generated single excitons with sufficient energy in excess of the band gap are able to relax by producing multiple excitons (carrier multiplication). In carrier multiplication, intraband excess energy is transferred to a valence band electron that is excited into the conduction band, resulting in the formation of two or more excitons per initially photo-excited exciton [Phys. Rev. Lett. 2004, v.92, 186601/1-4]. We have found that this effect of multiexciton generation, which has never been found to occur with significant efficiency in bulk semiconductors, can occur with up to 100{\%} efficiency in PbSe NCs depending upon the absorbed photon energy and occurs at wavelengths that are relevant to solar energy conversion. This process, which is an enabler of Generation III solar cells, has the potential to considerably increase the power conversion efficiency of NC-based photovoltaics. Pump-power dependent TA studies performed with a probe pulse tuned to near the photoluminescence maximum have revealed that both PbSe NCs and CdSe NCs can exhibit optical gain and, when incorporated into high optical quality sol-gel waveguides, are capable of producing size-tunable amplified spontaneous emission [J. Phys. Chem. B 2003, v.107, 13765-8]. Similar TA studies performed with a probe pulse tuned to the band-edge absorption feature reveal that differences in crystal structure cause the two materials to have significantly different exciton degeneracy, which directly results in different thresholds for gain.   

Investigation of the Phonon Frequency Shifts in ZnO Quantum Dots

Nanostructures made of ZnO have recently attracted attention due to their proposed applications in low-voltage and short-wavelength electro-optical devices. However, the origin of the observed phonon frequency shifts in such nanostructures is not always understood. We carried out both resonant and non-resonant Raman measurements for 20 nm-diameter ZnO quantum dots (QDs) and bulk ZnO reference samples [1]. A comparison with a recently developed theory [2], allowed us to clarify the origin of the phonon frequency shifts in ZnO QDs. It was found that the phonon confinement results in phonon frequency shifts of only few cm$^{-1}$. At the same time, the UV laser heating of the QD ensemble was found to induce a large red shift of phonon frequencies for up to 14 cm$^{-1}$. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] K.A. Alim, V.A. Fonoberov, and A.A. Balandin, Appl. Phys. Lett., in review (2004). [2] V.A. Fonoberov and A.A. Balandin, Phys. Stat. Solidi C \textbf{1}, 2650 (2004); cond-mat/0405681; cond-mat/0411742.   

Ultrafast flipping of exciton spin orientation in colloidal CdSe quantum dots

A nonlinear optical spectroscopy is demonstrated that retrieves exciton spin orientation dynamics using linearly polarized excitation. Rotationally averaged optical selection rules for quantum dots dictate that the sign of the signal is reversed when the spin state flips. Results are reported for CdSe nanocrystal samples with mean diameters from 3.1 nm to 5.0 nm. Ultrafast exciton spin flip times correspondingly range from 236 fs to 1.2 ps. Implications for quantum computation and spintronics applications are that exciton transitions can be used to induce long-lived spin polarization, but memory of exciton spin orientation decays on times less than 1 ps.   

Ultrafast carrier dynamics and carrier localization in thermally annealed Si nanoclusters

We present results of time-resolved terahertz pulse spectroscopy experiments on thermally deposited and annealed Si nanoclusters embedded in a SiO$_{2}$ matrix. These clusters range in size but are on the order of 5 nm in diameter, which is comparable to the Bohr radius in Si where carriers are expected to be strongly localized. The frequency-resolved conductivity of these samples after excitation by a 400 nm, 100 fs pump pulse is non-Drude like with a real component that increases with frequency and a negative imaginary component indicative of carrier localization. A series of samples is investigated as a function of anneal temperature, showing transient absorption decays ranging from a few picoseconds to several hundred picoseconds as the anneal temperature increases. Several models to explain the observed response are discussed. The authors acknowledge financial support from NSERC, CIPI and iCORE.   

A Model for the Powerlaw Behavior of Blinking Dynamics of Single Quantum Dots

Quantum dots (QDs) are a new generation of fluorescent markers for biological labeling with exceptional optical properties. However, single QD fluorescence blink due to a photo-induced electron transfer (ET) process, where the QD core loses an electron to a surface based trap site generating a non-fluorescent core-shell charge-separated state. This QD blinking behavior complicates their imaging applications. We observed that CdSe/ZnS QD blinking is largely suppressed in cysteine and histidine solutions and provided a mechanism for the blinking suppression. Cysteine and histidine act as small molecule ligands and bind to the Zn$^{II}$ based surface trap sites. The electronic interaction between the ligand and Zn$^{II}$-based trap sites raises the energy of the non-fluorescent core-shell charge-separated state and thus shuts down the photo-induced ET process, leading to blinking suppression. Based on ligand binding mechanism, we developed a model that accounts for the observed power law behavior of QD blinking kinetics.