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.

The present invention provides a new method for the production of inorganic semiconductor nanocrystals having a rod-like shape. More specifically the present invention provides a method of synthesizing rod shaped Group III-V semiconductor nanocrystals. The method comprises: reacting, in a high-boiling point organic solvent, a two-source precursor solution comprising at least one metal source and at least one nonmetal source, or a single-source precursor solution, with a metal catalyst or an agent capable of producing said metal catalyst, said high-boiling point organic solvent having a temperature above 200° C., thereby forming a reaction product comprising semiconductor nanocrystals of various shape; cooling the reaction product, and subsequently exposing said cooled reaction product to at least one centrifugal step so as to obtain semiconductor nanocrystals having substantially rod-like shape.

The rod-shaped nanocrystals obtained by the method of the invention usually have organic ligands as a coating on their outer surfaces. Such organic ligands affect the solubility of the particles and may be substituted or removed, according to the application intended for said particles after the reaction is completed.

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.

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.

Lasers based on colloidal semiconductor nanostructures can benefit from the remarkable spectral coverage afforded through the quantum confinement effect. The first observation of lasing for colloidal CdSe/ZnS quantum rods in solution using a cylindrical microcavity is reported here (see also inside front cover). Lasing in the same configuration was also observed for spherical nanocrystal quantum dots. For the quantum dots lasing is not polarized, but in quantum rods the laser emission is highly linearly polarized, a desirable feature for laser and photonic applications.

Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.

New cadmium chalcogenide cluster molecules [Cd₁₀E₄(E'Ph)₁₂(Pⁿ Pr₃)₄], E = Te, E' = Te (1 ) and [Cd₁₀E₄(E'Ph)₁₂ (Pⁿ Pr₂Ph)₄] E = Te, E' = Se (2 ); E = Te E' = S (3 ); E = Se, E' = S (4 ) have been synthesized and structurally characterized by single crystal X‐ray structure analysis. The influence of the variation of the chalcogen atom is investigated by structural means and by optical spectroscopy. All cluster‐molecules have a broad emission in the blue‐visible range at low temperature as indicated by photo luminescence (PL) measurements. A clear classification of the emission peak position can be made based on the E' species suggesting that the emission is assigned to transitions associated with the cluster surface ligands. Photoluminescence excitation and absorption measurements display a systematic shift of the band gap to the higher energies with the variation of E and E' from Te to Se to S, as also occurs in the respective series of the bulk semiconductors.

Conjugated polymers and indium arsenide–based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers—a range that effectively covers the short-wavelength telecommunications band—by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is ∼0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer.

Tunneling spectroscopy of InAs nanocrystals deposited on graphite was measured using scanning tunneling microscopy, in a double-barrier tunnel-junction configuration. The effect of the junction symmetry on the tunneling spectra is studied experimentally and modeled theoretically. When the tip is retracted, we observe resonant tunneling through the nanocrystal states without charging. Charging is regained upon reducing the tip–nanocrystal distance, making the junction more symmetric. The effect of voltage distribution between the junctions on the measured spectra is also discussed.

The surface and bulk contributions to the second-order nonlinear optical response of CdSe nanocrystals is studied. The first hyperpolarizability, β_n, was measured for the nanocrystals in solution using the hyper-Rayleigh scattering method. Tri-n-octylphosphineoxide-capped nanocrystals show an enhancement in the value of the second hyperpolarizibility per unit cell, β, with reduced size. The two-state model can explain the enhancement for nanocrystals with radius down to about 1.7 nm, related with the concentration of oscillator strength, but for smaller particles the enhancement is larger than the prediction. This additional enhancement is assigned to a surface response. The contribution of surface ligands to the second harmonic signal for the nanocrystals was investigated by exchanging the tri-n-octylphosphineoxide ligands with the nonlinear chromophore nitrothiophenol. Surface exchange was evidenced through the change in particle solubility, by a substantial reduction of the fluorescence intensity, and by the vibrational spectra. The substitution to nitrothiophenol ligands leads to a size-dependent enhancement of β_n compared to the original particles. The difference in β_n between the surface-substituted and nonsubstituted particles scales with the number of surface sites for nanocrystals of different sizes. Surface exchange also leads to an enhancement in β_n for a rod-shaped sample (aspect ratio 3.3:1). The contributions to β_n for such nanocrystals can therefore be attributed to a combination of a bulk-like part arising from the nonlinear electronic response of Cd−Se bonds and a surface part effected by the nature of the ligands.

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X‐ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X‐ray diffraction shows that the nanocrystals are of the thermodynamically stable α‐MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80‐, 40‐, and 20‐nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T _N = 130 K, of bulk α‐MnS. Below T _N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T _N with reduced particle size, indicating size dependent magnetic ordering.