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.

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.

An efficient method for preparation of semiconductor quantum rod films for robust lasing in a cylindrical microcavity is reported. A capillary tube, serving as the laser cavity, is filled with a solution of nanocrystals and irradiated with a series of intense nanosecond laser pulses to produce a nanocrystal film on the capillary surface. The films exhibit intense room‐temperature lasing in whispering‐gallery modes that develop at the film–capillary interface as corroborated from the spacing detected for the lasing modes. Good lasing stability is observed at moderate pump powers. The method was applied successfully to several quantum‐rod samples of various sizes.

Electrical transport measurements through single InAs and CdSe semiconductor nanocrystals embedded in a thin polymer film were performed using conductance atomic force microscopy. The current and topography images showed excellent correlation, where current was detected only over the nanocrystals. A rapid current decay in consecutive scans was observed for positive sample bias, while remaining intact at negative bias. This current decay was accompanied by bias-dependent changes in the height of the nanocrystals. These phenomena, which were not observed for gold nanocrystals, are attributed to long-sustained charging of the nanocrystals.

Two-photon polarization fluorescence microscopy is used to study the nature of the emission and nonlinear absorption dipole of single CdSe/ZnS quantum rods. Rods showed strongly polarized nonlinear excitation with sharp angular dependence, following a cos⁴(φ) functional form, in agreement with the predicted two-photon absorption process. The two-photon absorption is parallel to the emission polarization and allows high orientation selectivity in excitation to be achieved. This further demonstrates the role of single molecule measurements in unraveling basic principles of light−matter interactions otherwise masked by ensemble averaging.

Scanning tunneling microscopy (STM) and optical spectroscopy measurements were performed on InAs nanorods 7 to 25 nm long. Both methods reveal a clear dependence of the band-gap on length, with a red shift for longer rods. This (zero-dimension like) behavior is different than that of CdSe rods, where the band-gap is nearly insensitive to length, a signature of quasi one-dimensionality. The transition between these two regimes is governed by the ratio between the Bohr radius and the nanorod 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.

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.

We summarize our correlated scanning tunnelling microscopy and optical spectroscopy investigations of the electronic level structure and single-electron charging effects in CdSe quantum rods. Both optical and tunnelling spectra show that the level structure depends primarily on rod diameter and not on length. With increasing diameter, the bandgap and the excited state level spacings shift to the red. The level structure is assigned using a multi-band effective-mass model. The tunnelling spectra also exhibit, depending on the tunnel-junction parameters, single-electron charging effects that yield information on the degeneracy of the electronic states.

CdSe−ZnS core−shell quantum dots (QDs) act as photochemical centers for lighting-up the dynamics of telomerization or DNA replication.

A method for the synthesis of CdSe/ZnS core/shell nanorods is reported. In the first step rods are grown, and in a second step a shell of ZnS is overgrown at moderate temperatures in a mixture of trioctylphosphine-oxide and hexadecylamine. Structural and chemical characterization using transmission electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy were performed providing direct evidence for shell growth. The emission quantum yield significantly increases by over 1 order of magnitude for the core/shell nanorods compared to the original rods because of the improved surface passivation. Rods with lengths up to ∼30 nm were investigated, and in this size regime the maximal achievable QY showed little dependence on length and strong dependence on rod diameter, with increased QY in smaller diameters. Color tunability is available via tuning of the rod diameter. The stability against photooxidation was significantly improved in core/shell nanorods compared with rods coated by organic ligands.

Dimensionality and size are two factors that govern the properties of semiconductor nanostructures^1,2. In nanocrystals, dimensionality is manifested by the control of shape, which presents a key challenge for synthesis^3,4,5. So far, the growth of rod-shaped nanocrystals using a surfactant-controlled growth mode, has been limited to semiconductors with wurtzite crystal structures, such as CdSe (ref. 3). Here, we report on a general method for the growth of soluble nanorods applied to semiconductors with the zinc-blende cubic lattice structure. InAs quantum rods with controlled lengths and diameters were synthesized using the solution–liquid–solid mechanism⁶ with gold nanocrystals as catalysts⁷. This provides an unexpected link between two successful strategies for growing high-quality nanomaterials, the vapour–liquid–solid approach for growing nanowires^8,9,10,11,12, and the colloidal approach for synthesizing soluble nanocrystals^13,14,15. The rods exhibit both length- and shape-dependent optical properties, manifested in a red-shift of the bandgap with increased length, and in the observation of polarized emission covering the near-infrared spectral range relevant for telecommunications devices^16,17.