We studied the optical gain characteristics of CdSe/ZnS core/shell colloidal quantum rods, investigated their temperature dependence, and compared the gain properties with quantum dots (QD). The gain was measured systematically for close-packed films of rods and dots under quasi-CW nanosecond optical pumping, using the variable stripe length method measuring the amplified spontaneous emission (ASE). Tunable ASE can be achieved by changing the rod diameter. Optical gain factors of up to 350 cm^-1 at a temperature range of 10−120 K were measured for quantum rods. Above 120 K, the gain decreased sharply, but by increasing the pump power, ASE was easily achieved also at room temperature. The temperature dependence was assigned to the Auger heating process and phonon assisted thermal relaxation. QD of similar diameters as the rods showed much smaller gain values (∼50 cm^-1) and a sharp decrease in gain at lowered temperatures (∼50 K), and ASE could not be detected at room temperature even at high pump powers. The significantly improved gain values in quantum rods as compared with dots were attributed to the slower Auger relaxation rates, the higher absorption cross-section, and the reduced self-absorption due to the larger Stokes shift. The temperature dependence of the threshold power for the quantum rods, used to characterize the thermal insensitivity of the system, showed two distinct temperature regions. In the low-temperature region, a very high T₀ value of 3500 K was measured, as predicted for a low-dimensional quantum confined system.

The spectroscopy and dynamics of multiple excitations on colloidal type-II CdTe∕CdSe 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 30meV 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.

The invention comprises a composite material comprising a host material in which are incorporated semiconductor nanocrystals. The host material is light-transmissive and/or light-emissive and is electrical charge-transporting thus permitting electrical charge transport to the core of the nanocrystals. The semiconductor nanocrystals emit and/or absorb light in the near infrared spectral range. The nanocrystals cause the composite material to emit/absorb energy in the near infrared (NIR) spectral range, and/or to have a modified dielectric constant, compared to the host material. The invention further comprises electro-optical devices composed of this composite material and a method of producing them. Specifically described are light emitting diodes that emit light in the NIR and photodetectors that absorb light in the same region.

A blue solid-state laser material based on 4,4′4′ dibenzyl carbamido stilbene-2,2′2′ disulfonic acid incorporated into solgel zirconia and inorganic–organic hybrid matrices is presented. The absorption maxima of the dye in various matrices are around 339–361 nm, and the broad fluorescence peaks are at 411–413 nm. Optical gain measurements using the variable stripe method show amplified spontaneous emission peaking at 437 nm.

Gold-tipped CdSe rods (nanodumbbells) were solubilized in an aqueous phase and self-assembled in a head-to-tail manner using biotin disulfide and avidin. The disulfide end of the biotin molecule attaches to the gold tip of the nanodumbbell, and the biotin end of the molecule is able to conjugate to an avidin protein. The avidin can strongly conjugate up to four biotin molecules. Changing the ratios of biotin to nanodumbbells leads to the formation of dimers, trimers, and flowerlike structures. To further improve the distribution of chain lengths, a separation method based upon weight was applied using a concentration gradient. The gold tips provide effective anchor points for constructing complex nanorod structures by self-assembly.

The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.

We present a method for performing multiexciton spectroscopy in colloidal semiconductor nanocrystals. In this regime multiexcitonic states are generated sequentially via a “ladder-climbing” mechanism. The distribution of multiexcitonic states, determined by a steady-state rate equation, adiabatically follows the illumination intensity, allowing for “slow” detection of multiexcitonic spectra. Emission spectra of multiexcitonic states with a small number of excitons are obtained without passing through states with a large number of excitons. This is in contrast with impulsive excitation schemes utilizing picosecond pulses, where in order to significantly populate a given multiexcitonic state, many of the dots necessarily pass through states with a larger number of excitons due to the Poissonian distribution of the number of absorbed photons. In particular, we observe directly the order of appearance of the various multiexcitonic peaks. This enables us to determine the threshold conditions for Auger ionization, shedding more light on the nature of this process. We are also able to observe the short-lived excitations at higher energies than triexcitons in CdSe quantum dots. Finally, we demonstrate bi- and triexcitonic optical gain in a close-packed film under quasi-continuous-wave pumping.

CdSe/ZnS QDs enable the optical probing of the biocatalytic oxidation of tyrosine derivatives and of the scission of peptides by thrombin. CdSe/ZnS QDs were modified with tyrosine methyl ester or with a tyrosine-containing peptide. The tyrosine units were reacted with tyrosinase/O₂ to yield the respective l-DOPA and quinone derivatives. The luminescence of QDs modified by the enzyme-generated quinone units is quenched. The quinone-functionalized peptide associated with the QDs was cleaved by thrombin, a process that restored the luminescence of the QDs.

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron−hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in the proximity of metal and semiconducting atomic force microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient nonradiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity as a function of its distance from the interacting tip. We take advantage of this highly distance-dependent effect to realize a contrast mechanism for high-resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high-resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement with calculations made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

A synthesis of soluble III–V semiconductor quantum rods using gold nanoparticles to direct and catalyze one-dimensional growth isdeveloped. The growth takes place via the solution–liquid–solid (SLS) mechanism where proper precursors are injected into a coordinatingsolvent. We report the synthesis of InP nanorods using indium acetate and myristic acid with gold nanoparticles as the catalysts in the SLS growthmode. A similar route was successfully developed for the growth of InAs nanorods. We find that the amount of Au catalyst in the reaction is animportant parameter to achieve shape control. Transmission electron microscope (TEM) images of InP and InAs nanocrystals revealed that thecrystals are mostly rod-shaped, and provide strong evidence for Au presence in one edge. The rods were characterized structurally using X-raydiffraction and high-resolution TEM and optically by absorption and photoluminescence.D2005 Elsevier B.V. All rights reserved.

We explore the growth mechanism of gold nanocrystals onto preformed cadmium sulfide nanorods to form hybrid metal nanocrystal/semiconductor nanorod colloids. By manipulating the growth conditions, it is possible to obtain nanostructures exhibiting Au nanocrystal growth at only one nanorod tip, at both tips, or at multiple locations along the nanorod surface. Under anaerobic conditions, Au growth occurs only at one tip of the nanorods, producing asymmetric structures. In contrast, the presence of oxygen and trace amounts of water during the reaction promotes etching of the nanorod surface, providing additional sites for metal deposition. Three growth stages are observed when Au growth is performed under air:  (1) Au nanocrystal formation at both nanorod tips, (2) growth onto defect sites on the nanorod surface, and finally (3) a ripening process in which one nanocrystal tip grows at the expense of the other particles present on the nanorod. Analysis of the hybrid nanostructures by high-resolution TEM shows that there is no preferred orientation between the Au nanocrystal and the CdS nanorod, indicating that growth is nonepitaxial. The optical signatures of the nanocrystals and the nanorods (i.e., the surface plasmon and first exciton transition peaks, respectively) are spectrally distinct, allowing the different stages of the growth process to be easily monitored. The initial CdS nanorods exhibit band gap and trap state emission, both of which are quenched during Au growth.

The exceptional fluorescence properties of single CdSe quantum rods (QRs) arising from internal and external electric fields are studied. Reversible external field induced switching of the emission in single QRs is reported for the first time. This effect was correlated with local field induced emission intensity reduction and newly observed darkening mechanism. Bimodal spectral jumps under a zero field were also observed and assigned to charged exciton emission, a phenomenon that was likewise directly controlled through an external field. These phenomena point to the use of single QRs as spectrally tunable charge sensitive fluorophores with polarized emission in fluorescence tagging and optical switching applications.

Multicomponent nanoparticles represent a new approach for creating smart materials, requiring the development of the growth of different material types on one particle. Here, we report the synthesis of asymmetric metal–semiconductor heterostructures where gold is grown on one side of CdSe nanocrystal quantum rods and dots, creating nanostructures offering intrinsic asymmetry for diverse device functionalities such as diode elements, along with one-sided chemical accessibility through the gold tips. Surprisingly, one-sided growth is preceded by two-sided growth and is generally observed in different particle shapes. Theoretical modelling in a lattice-gas model and experimental analysis show that a ripening process drives gold from one end to the other, transforming two-sided growth to one-sided growth. Ripening is therefore occurring on the nanostructure itself, leading to a phase-segregated structure. This thereby extends the realm of ripening phenomena and their significance in nanostructure synthesis, in particular for nanocrystals composed of different materials.

A new material platform is described that enables inclusion of nanocrystalline quantum dots into a polymer. This technology is compatible with semiconductor processing and may enable integration of active materials into current waveguide technologies. We describe the steps preformed to fabricate a waveguide chip that contains IR-emitting quantum dots. Optical tests demonstrate guiding and preservation of the quantum dots optical properties through the processing steps. Time resolved optical measurements indicate presence of gain in the InAs quantum dot impregnated polymer.

We report on the observation of optical gain from InAs nanocrystal quantum dots which emit at 1.55microns1.55microns and are imbedded in a novel polymer platform. The measurements are based on a three-beam time resolved pump-probe technique, which enables extracting the intrinsic gain cross section, lifetime, and recovery time. These experiments are another step toward the realization of active optical devices based on InAs nanocrystals.

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared (IR) emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs/ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high (66%) energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology (nanocrystal distribution) control.

Nucleic-acid-functionalized CdSe/ZnS quantum dots (QDs) were hybridized with the complementary Texas-Red-functionalized nucleic acid. The hybridization was monitored by following the fluorescence resonance energy transfer from the QDs to the dye units. Treatment of the QD/dye DNA duplex structure with DNase I resulted in the cleavage of the DNA and the recovery of the fluorescence properties of the CdSe/ZnS QDs. The luminescence properties of the QDs were, however, only partially recovered due to the nonspecific adsorption of the dye onto the QDs. Similarly, nucleic-acid-functionalized Au nanoparticles (Au NPs) were hybridized with the complementary Texas-Red-labeled nucleic acid. The hybridization was followed by the fluorescence quenching of the dye by the Au NPs. Treatment of the Au NP/dye DNA duplex with DNase I resulted in the cleavage of the DNA and the partial recovery of the dye fluorescence. The incomplete recovery of the dye fluorescence originated from the nonspecific binding of the dye units to the Au NPs. The nonspecific binding of the dye to the CdSe/ZnS QDs and the Au NPs is attributed to nonprotected surface vacancies in the two systems.

Multicomponent nanoparticles represent a new approach for creating smart materials, requiring the development of the growth of different material types on one particle. Here, we report the synthesis of asymmetric metal–semiconductor heterostructures where gold is grown on one side of CdSe nanocrystal quantum rods and dots, creating nanostructures offering intrinsic asymmetry for diverse device functionalities such as diode elements, along with one-sided chemical accessibility through the gold tips. Surprisingly, one-sided growth is preceded by two-sided growth and is generally observed in different particle shapes. Theoretical modelling in a lattice-gas model and experimental analysis show that a ripening process drives gold from one end to the other, transforming two-sided growth to one-sided growth. Ripening is therefore occurring on the nanostructure itself, leading to a phase-segregated structure. This thereby extends the realm of ripening phenomena and their significance in nanostructure synthesis, in particular for nanocrystals composed of different materials.

Tunneling and optical spectroscopy performed on InAs nanorods 7–25 nm long, reveal a clear dependence of the band gap on length. This (zero-dimension like) behavior is different from that of CdSe rods, where the band gap is nearly independent of length, a signature of quasi-one-dimensionality. The transition between these two regimes is governed by the ratio between the Bohr radius and the nanorods length. The gaps measured by tunneling spectroscopy are larger than the optical gaps by a factor that depends on the tunneling configuration. This is attributed to a combination of the Coulomb interaction and the voltage division between the two tunnel junctions in the STM experiment. However, the tunneling gaps were found to reduce in dense aggregates of rods.