Showing papers on "Exciton published in 2007"

Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals

Abstract: Because of the strong spatial confinement of electronic wave functions and reduced dielectric screening, the effects of carrier-carrier Coulomb interactions are greatly enhanced in semiconductor nanocrystals (NCs) compared with those in bulk materials. These interactions open a highly efficient decay channel via Auger recombination, which represents a dominant recombination pathway for multiexcitons in NCs. Furthermore, strong Coulomb coupling between charge carriers leads to extremely efficient direct photogeneration of multiexcitons by single photons via carrier (or exciton) multiplication. This review focuses on spectral and dynamical properties of multiexcitons in semiconductor NCs. The specific topics discussed here include the structure of NC electronic states, spectral signatures of multiexcitons in transient absorption and photoluminescence, exciton-exciton interaction energies, Auger recombination, and carrier multiplication. This chapter also briefly reviews the implications of multiexciton effects for practical technologies, such as NC lasing and photovoltaics.

Multiple Exciton Generation in Colloidal Silicon Nanocrystals

Abstract: Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap E g ) 1.20 eV) to be 2.4 ± 0.1Eg and find an excitonproduction quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth’s crust, and poses no significant environmental problems regarding toxicity.

Excitations in a nonequilibrium Bose-Einstein condensate of exciton polaritons.

Abstract: We develop a mean-field theory of the dynamics of a nonequilibrium Bose-Einstein condensate of exciton polaritons in a semiconductor microcavity. The spectrum of elementary excitations around the stationary state is analytically studied by means of a generalized Gross-Pitaevskii equation. A diffusive behavior of the Goldstone mode is found in the spatially homogeneous case and new features are predicted for the Josephson effect in a two-well geometry.

Stepwise quenching of exciton fluorescence in carbon nanotubes by single-molecule reactions.

Abstract: Single-molecule chemical reactions with individual single-walled carbon nanotubes were observed through near-infrared photoluminescence microscopy. The emission intensity within distinct submicrometer segments of single nanotubes changed in discrete steps after exposure to acid, base, or diazonium reactants. The steps were uncorrelated in space and time and reflected the quenching of mobile excitons at localized sites of reversible or irreversible chemical attack. Analysis of step amplitudes revealed an exciton diffusional range of about 90 nanometers, independent of nanotube structure. Each exciton visited about 10,000 atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations.

Exciton–plasmon interactions in molecular spring assemblies of nanowires and wavelength-based protein detection

Abstract: Electronic interactions at the nanoscale represent one of the fundamental problems of nanotechnology. Excitons and plasmons are the two most typical excited states of nanostructures, which have been shown to produce coupled electronic systems. Here, we explore these interactions for the case of nanowires with mobile excitons and nanoparticles with localized plasmons and describe the theoretical formalism, its experimental validation and the potential practical applications of such nanoscale systems. Theory predicts that emission of coupled excitations in nanowires with variable electronic confinement is stronger, shorter and blue-shifted. These predictions were confirmed with a high degree of accuracy in molecular spring assemblies of CdTe nanowires and Au nanoparticles, where we can reversibly change the distance between the exciton and the plasmon. The prepared systems were made protein-sensitive by incorporating antibodies in the molecular springs. Modulation of exciton-plasmon interactions can serve as a wavelength-based biodetection tool, which can resolve difficulties in the quantification of luminescence intensity for complex media and optical pathways.

Molecular plasmonics with tunable exciton-plasmon coupling strength in J-aggregate hybridized Au nanorod assemblies.

Abstract: Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.

Carrier multiplication in InAs nanocrystal quantum dots with an onset defined by the energy conservation limit

Abstract: Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron-hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2Eg). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III-V semiconductors should exhibit a CM threshold near the apparent 2Eg limit. Further, we discuss the possibility of achieving sub-2Eg CM thresholds through strong exciton-exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2Eg.

Exciton-plasmon-photon conversion in plasmonic nanostructures.

Abstract: A silver-nanowire cavity is functionalized with CdSe nanocrystals and optimized towards cavity quantum electrodynamics by varying the nanocrystal-nanowire distance d and cavity length L. From the modulation of the nanocrystal emission by the cavity modes a plasmon group velocity of v (gr) approximately 0.5c is derived. Efficient exciton-plasmon-photon conversion and guiding is demonstrated along with a modification in the spontaneous emission rate of the coupled exciton-plasmon system.

Organic Molecular Solids

Abstract: 1 Introduction. 1.1 What are Organic Solids? 1.2 What are the Special Characteristics of Organic Solids? 1.3 Goals and Future Outlook. Problems for Chapter 1. Literature. 2 Forces and Structures. 2.1 Forces. 2.1.1 Inductive Forces. 2.1.2 Van der Waals Forces. 2.1.3 Repulsive Forces. 2.1.4 Intermolecular Potentials. 2.1.5 Coulomb Forces. 2.2 Structures. 2.2.1 Crystals of Nonpolar Molecules. 2.2.2 Crystals of Molecules with Polar Substituents. 2.2.3 Crystals with a Low Packing Density, Clathrates. 2.2.4 Crystals of Molecules with Charge Transfer, Radical-ion Salts. 2.3 Polymer Single Crystals: Diacetylenes. 2.4 Thin Films. 2.5 Inorganic-Organic Hybrid Crystals. Problems for Chapter 2. Literature. 3 Purification of Materials, Crystal Growth and Preparation of Thin Films. 3.1 Purification. 3.2 Highest Purity. 3.3 Crystal Growth. 3.4 Mixed Crystals. 3.5 Epitaxy, Ultrathin Films. Problems for Chapter 3. References. 4 Impurities and Defects. 4.1 Foreign Molecules, Impurities, and X traps. 4.2 Structural Defects. 4.2.1 Point Defects. 4.2.2 Dislocations. 4.2.3 Grain Boundaries. 4.2.4 Dipolar Disorder. 4.3 Characterisation and Analysis of Impurities. 4.4 Characterisation of Defects. Literature. 5 Molecular and Lattice Dynamics in Organic Molecular Crystals. 5.1 Introduction. 5.2 Intramolecular Vibrations. 5.3 Phonons. 5.3.1 The Eigenvector. 5.3.2 The Wavevector. 5.3.3 The Frequencies (K). 5.3.4 Excitations. 5.4 Experimental Methods. 5.4.1 Inelastic Neutron Scattering. 5.4.2 Raman Scattering and Infrared Absorption. 5.5 The 12 External Phonons of the Naphthalene Crystal. 5.5.1 Dispersion relations. 5.5.2 Pressure and Temperature Dependencies. 5.6 Analytic Formulation of the Lattice Dynamics in Molecular Crystals. 5.7 Phonons in other Molecular Crystals. 5.8 Hindered Rotation and Diffusion. 5.8.1 Nuclear Magnetic Resonance. 5.8.2 Benzene Crystals. 5.8.3 Methyl Groups. 5.8.4 Diffusion. Problems for Chapter 5. References. 6 Electronic Excited States, Excitons, Energy Transfer. 6.1 Introduction. 6.2 Some historical remarks. 6.3 Optical Excited States in Crystals. 6.4 Davydov Splitting and Mini-Excitons. 6.5 Frenkel Excitons. 6.5.1 Excitonic States, Fundamental Equations. 6.5.2 Polarisation and Band Structure. 6.5.3 Coherence. 6.6 Charge Transfer (CT) Excitons. 6.7 Surface Excitons. 6.8 Excimers. 6.9 Exciton Processes, Energy Conduction. 6.9.1 Sensitised Fluorescence. 6.9.2 Delayed Fluorescence by Triplet Excitons. 6.9.3 Excitonic Processes. 6.10 Excitonic Processes in other Systems. 6.11 Future Developments. Problems for Chapter 6. Literature. 7 Structure and Dynamics of Triplet States. 7.1 Introduction and Historical Remarks. 7.2 Spin Quantisation in Triplet States. 7.3 The Dipole-Dipole Interaction, Fine Structure. 7.3.1 Zero Field (B0 = 0). 7.3.2 Zeeman Splitting (B0 = 0). 7.3.3 Powder Spectra. 7.4 Mini-Excitons. 7.5 Triplet Excitons. 7.5.1 Anthracene and Naphthalene Crystals: Two-dimensional Triplet Excitons. 7.5.2 Dibromonaphthalene Crystals: coherent, one-dimensional Triplet Excitons. 7.6 Optical Spin Polarisation (OEP). 7.7 Optical Nuclear-Spin Polarisation (ONP). 7.8 Perspectives. Problems for Chapter 7 Literature. 8 Organic Semiconductors. 8.1 Preliminary Historical Remarks. 8.2 Conductivity and Mobility of nearly-free Charge Carriers. 8.3 Charge Carriers in Organic Semiconductors: Polarons, Shallow Traps and Deep Traps. 8.4 Generation of Charge Carriers and Charge Transport: Experimental Methods. 8.4.1 The TOF Method: Gaussian Transport. 8.4.2 Photogeneration of Charge Carriers. 8.4.3 Contacts, Injection, Ejection, and Dark Currents. 8.4.4 Space-Charge Limited Currents. 8.5 Charge-Carrier Mobilities in Organic Molecular Crystals. 8.5.1 Band- or Hopping Conductivity? 8.5.2 Temperature Dependence and Anisotropy of the Mobilities. 8.5.3 Electric-field Dependence. 8.5.4 Band Structures. 8.5.5 Charge-Carrier Traps. 8.6 Charge Transport in Disordered Organic Semiconductors. 8.6.1 The Bassler Model. 8.6.2 Mobilities in High-Purity Films: Temperature, Electric-Field, and Time Dependence. 8.6.3 Binary Systems. 8.6.4 Discotic Liquid Crystals. 8.6.5 Stationary Dark Currents. Problems for Chapter 8. Literature. 9 Organic Crystals of High Conductivity. 9.1 Donor-Acceptor Systems. 9.2 Strong CT Complexes, Radical-ion Salts. 9.3 The Organic Metal TTF-TCNQ - Peierls Transition and Charge-Density Waves. 9.4 Other Radical-ion Salts and CT Complexes. 9.5 Radical-Anion Salts of DCNQI. 9.6 Radical-Cation Salts of the Arenes. 9.6.1 Direct-current Conductivity. 9.6.2 X-Ray Scattering. 9.6.3 Optical Reflection Spectrum. 9.6.4 Magnetic Susceptibility. 9.6.5 Spin Resonance of the Conduction Electrons (ESR). 9.6.6 Charge-Density-Wave Transport. Problems for Chapter 9. Literature. 10 Organic Superconductors. 10.1 Introduction. 10.2 Mainly One-dimensional Charge-Transfer Salts as Superconductors Bechgaard Salts. 10.3 Quasi-Two-dimensional Charge-Transfer Systems as Superconductors. 10.4 The Nature of the Superconducting State in Organic Salts. 10.5 Three-dimensional Superconductivity in Fullerene Compounds. Literature. 11 Electroluminescence and the Photovoltaic Effect. 11.1 Electroluminescence: Organic Light-Emitting Diodes (OLEDs). 11.1.1 Historical Remarks. 11.1.2 The Principle of the OLED. 11.1.3 Multilayer OLEDs. 11.1.4 Electro-optical Properties. 11.2 Photovoltaic Effect: Organic Photovoltaic Cells. 11.2.1 Exciton Dissociation. 11.2.2 Photovoltaic Characteristics. 11.2.3 CuPc/C60 Solar Cells. Literature. 12 Towards a Molecular Electronics. 12.1 What is Molecular Electronics and What Will it Do? 12.2 Molecules as Switches, Photochromic Effects. 12.3 Molecular Wires. 12.4 Light-Induced Phase Transitions. 12.5 Molecular Rectifiers. 12.6 Molecular Transistors. 12.7 Molecular Storage Units. Appendix: Coloured Plates. Index.

Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots

Abstract: We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at ∼4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.

Room-temperature stimulated emission of ZnO: Alternatives to excitonic lasing

Abstract: ZnO is a wide gap semiconductor which has possible applications in blue light emitting diodes and lasers, devices, which are currently based on GaN. One advantage of ZnO compared to GaN is the much higher exciton binding energy of $60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ compared to $26\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in GaN. Due to this exciton binding energy many authors ascribe stimulated emission at room temperature (RT) to excitonic processes with presumably very low thresholds. In this contribution we investigate the temperature dependence of the band gap and of the homogeneous width of the free exciton resonance. Together with new and previous calculations and experimental data, these findings cast some doubt on the above claim and we present alternative interpretations for RT stimulated emission in ZnO.

Exciton fission and fusion in bis(tetracene) molecules with different covalent linker structures

Abstract: Bichromophoric molecules can support two spatially separated excited states simultaneously and thus provide novel pathways for electronic state relaxation Exciton fission, where absorption of a single photon leads to two triplet states, is a potentially useful example of such a pathway In this paper, a detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented Their spectroscopy is analyzed in terms of a three-state kinetic model in which the singlet excited state can fission into a triplet pair state, which in turn undergoes recombination on a time scale longer than the molecule's radiative lifetime This model allows us to fit both the prompt and delayed fluorescence decay data quantitatively The para-phenylene linked bis(tetracene) molecules 1,4-bis(tetracen-5-yl)benzene (1) and 4,4‘-bis(tetracen-5-yl)biphenylene (2) show intramolecular exciton fission with yields of ∼3%, whereas no delayed fluorescence is observed for tetracene or the meta-linked molecu

Solar Cells Based on Quantum Dots: Multiple Exciton Generation and Intermediate Bands

Abstract: Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of multiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the con fined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.

Excitonic effects in the optical spectra of graphene nanoribbons.

Abstract: We present a first-principles calculation of the optical properties of armchair-edged graphene nanoribbons (AGNRs) with many-electron effects included. The reduced dimensionality of the AGNRs gives rise to an enhanced electron−hole binding energy for both bright and dark exciton states (0.8−1.4 eV for GNRs with width ∼1.2 nm) and dramatically changes the optical spectra owing to a near complete transfer of oscillator strength to the exciton states from the continuum transitions. The characteristics of the excitons of the three distinct families of AGNRs are compared and discussed. The enhanced excitonic effects found here are expected to be of importance in optoelectronic applications of graphene-based nanostructures.

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

Abstract: The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization- entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

Influence of Nanoscale Phase Separation on the Charge Generation Dynamics and Photovoltaic Performance of Conjugated Polymer Blends: Balancing Charge Generation and Separation

Abstract: Through controlled annealing of intimately mixed blends of the polyfluorene copolymers poly(9,9‘-dioctylfluorene-co-bis(N,N‘-(4,butylphenyl))bis(N,N‘-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9‘-dioctylfluorene-co-benzothiadiazole) (F8BT) we observe the change in charge generation dynamics and photovoltaic performance as the length of nanoscale phase separation is varied from 5 nm or less to greater than 40 nm. We find that device efficiency is optimized for a phase separation of ∼20 nm, significantly larger than the exciton diffusion length of ∼5−10 nm. Femtosecond time-resolved transient absorption measurements confirm that the charge generation time is longer and charge generation efficiency is lower in films with a more evolved morphology. Photoluminescence quantum efficiency is also observed to monotonically increase with annealing temperature consistent with a decrease in exciton dissociation resulting from a coarsening of phases. Using a Monte Carlo model of exciton diffusion and dissociation ...

Effect of quantum and dielectric confinement on the exciton-exciton interaction energy in type II core/shell semiconductor nanocrystals.

Abstract: We study theoretically two electron-hole pair states (biexcitons) in core/shell hetero-nanocrystals with type II alignment of energy states, which promotes spatial separation of electrons and holes. To describe Coulomb interactions in these structures, we apply first-order perturbation theory, in which we use an explicit form of the Coulomb-coupling operator that takes into account interface-polarization effects. This formalism is used to analyze the exciton-exciton interaction energy as a function of the core and shell sizes and their dielectric properties. Our analysis shows that the combined contributions from quantum and dielectric confinement can result in strong exciton-exciton repulsion with giant interaction energies on the order of 100 meV. Potential applications of strongly interacting biexciton states include such areas as lasing, nonlinear optics, and quantum information.

Chirality dependence of exciton effects in single-wall carbon nanotubes: Tight-binding model

Abstract: We have studied the exciton properties of single-wall carbon nanotubes by solving the Bethe-Salpeter equation within tight-binding models. The screening effect of the $\ensuremath{\pi}$ electrons in carbon nanotubes is treated within the random phase and static screened approximations. The exciton wave functions along the tube axis and circumference are discussed as a function of $(n,m)$. A $2n+m=\mathrm{const}$ family behavior is found in the exciton wave function length, excitation energy, binding energy, and environmental shift. This family behavior is understood in terms of the trigonal warping effect around the $K$ point of a graphene layer and curvature effects. The large family spread in the excitation energy of the Kataura plot is found to come from the single-particle energy.

Photoluminescence spectroscopy of carbon nanotube bundles: Evidence for exciton energy transfer

Abstract: We investigate photoluminescence of nanotube bundles. Their spectra are explained by exciton energy transfer between adjacent tubes, whereby excitation of large gap tubes induces emission from smaller gap ones. The consequent relaxation rate is faster than nonradiative recombination, leading to enhanced photoluminescence of acceptor tubes.

Multiexcitons in type-II colloidal semiconductor quantum dots

Abstract: The spectroscopy and dynamics of multiple excitations on colloidal type-II $\mathrm{Cd}\mathrm{Te}∕\mathrm{Cd}\mathrm{Se}$ core-shell quantum dots (QDs) are explored via quasi-cw multiexciton spectroscopy. The charge separation induced by the band offset redshifts the exciton emission and increases the radiative lifetime. In addition, we observe a significant modification of multiexciton properties compared with core-only or type-I QDs. In particular, the Auger recombination lifetimes are significantly increased, up to a nanosecond time scale. While in type-I QDs the Auger lifetime scales with the volume, we find for type-II QDs a scaling law that introduces a linear dependence also on the radiative lifetime. We observe a blueshift of the biexciton emission and extract biexciton repulsion of up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in type-II QDs. This is assigned to the dominance of the Coulomb repulsion as the positive and negative charges become spatially separated, which overwhelms the correlation binding term. Higher electronic excited states can remain type I even when the lowest transition is already type II, resulting in a different size dependence of the triexciton emission. Finally, we discuss the possibilities of ``multiexciton band gap engineering'' using colloidal type-II QDs.

Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes

Abstract: The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes (PhOLEDs) is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers (D-EMLs) by confining the exciton formation near the interface between the emitting layers The external quantum efficiency was maintained almost constant up to 22mA∕cm2 (10000cd∕m2) by adopting the D-EMLs in Ir(ppy)3 based PhOLEDs, resulting in high external quantum efficiency (ηext=131%) at high luminance

Carrier Multiplication and Its Reduction by Photodoping in Colloidal InAs Quantum Dots

Abstract: Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core−shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ∼1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

Manipulating exciton fine structure in quantum dots with a lateral electric field

Abstract: The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero.

Carrier multiplication yields in CdSe and CdTe nanocrystals by transient photoluminescence

Abstract: Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. Although the technique is particularly sensitive due to enhanced biexciton radiative rates relative to the exciton, kradBX > 2 kradX, we find no evidence for CM in CdSe and CdTe NCs up to photon energies E > 3 Eg, well above previously reported relative energy thresholds.

Electrically driven thermal light emission from individual single-walled carbon nanotubes

Abstract: Light emission from nanostructures exhibits rich quantum effects and has broad applications. Single-walled carbon nanotubes (SWNTs) are one-dimensional metals or semiconductors in which large numbers of electronic states in narrow energy ranges, known as van Hove singularities, can lead to strong spectral transitions1,2. Photoluminescence and electroluminescence involving interband transitions and excitons have been observed in semiconducting SWNTs3,4,5,6,7,8,9, but are not expected in metallic tubes owing to non-radiative relaxations. Here, we show that, under low bias voltages, a suspended quasi-metallic SWNT (QM-SWNT) emits light owing to Joule heating, displaying strong peaks in the visible and infrared, corresponding to interband transitions. This is a result of thermal light emission in a one-dimensional system, in stark contrast with featureless blackbody-like emission observed in large bundles of SWNTs or multiwalled nanotubes10,11,12. This allows for probing of the electronic temperature and non-equilibrium hot optical phonons in Joule-heated QM-SWNTs.

Unified picture of electron and hole relaxation pathways in semiconductor quantum dots

Abstract: Size dependent electron and hole relaxation dynamics were measured in colloidal CdSe quantum dots with state-to-state specificity. These experiments reveal the electron and hole state-to-state relaxation dynamics with a precision of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, allowing quantitative evaluation of the manifold of pathways by which an exciton relaxes in strongly confined quantum dots. These experiments corroborate previously observed confinement induced femtosecond Auger relaxation channels for electrons, but with sufficient precision to quantitatively and unambiguously determine the size dependence of the Auger mechanism. These experiments also show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. We propose a confinement enhanced mechanism for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. The relative contributions of the relaxation pathways are identified for electrons and for holes. These state selective experiments produce a unified picture of the manifold of relaxation pathways available to both electrons and holes in strongly confined colloidal quantum dots.

Observation of excitons in one-dimensional metallic single-walled carbon nanotubes.

Abstract: Excitons are generally believed not to exist in metals because of strong screening by free carriers. Here we demonstrate that excitonic states can in fact be produced in metallic systems of a one-dimensional character. Using metallic single-walled carbon nanotubes as a model system, we show both experimentally and theoretically that electron-hole pairs form tightly bound excitons. The exciton binding energy of 50 meV, deduced from optical absorption spectra of individual metallic nanotubes, significantly exceeds that of excitons in most bulk semiconductors and agrees well with ab initio theoretical predictions.

Temperature and size dependence of nonradiative relaxation and exciton-phonon coupling in colloidal CdTe quantum dots

Abstract: We report on the temperature and size dependence of the photoluminescence of core CdTe colloidal quantum dots (QDs). We show that at temperatures lower than 170 K a thermally activated transition between two different states separated by about 12−20 meV takes place. At temperatures higher than 170 K, the main nonradiative process is thermal escape assisted by multiple longitudinal optical (LO) phonons absorption. Moreover, we show that quantum confinement affects both the exciton−acoustic phonons and the exciton−LO phonons coupling. The coupling constant with acoustic phonons is strongly enhanced in QDs (up to 31μeV/K) with respect bulk CdTe (0.7μeV/K). On the contrary, the exciton-LO phonons coupling constant decreases as the dot size decreases (down to 14 meV with respect 24.5 meV in the bulk).