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.

Core/shell semiconductor nanocrystals with InAs cores were synthesized and characterized. III−V semiconductor shells (InP and GaAs), and II−VI semiconductor shells (CdSe, ZnSe, and ZnS) were overgrown on InAs cores with various radii using a two step synthesis. In the first step cores were prepared, and in the second step the shells were grown using high-temperature pyrolysis of organometallic precursors in a coordinating solvent. Core/shell growth was monitored by absorption and photoluminescence spectroscopy. The band gap shifts to the red upon growth of InP or CdSe shells, while for ZnSe and ZnS shells that have larger band offsets with respect to InAs, the band gap energy is maintained. This behavior is reproduced by band gap energy calculations using a particle within a spherical box model. The photoluminescence quantum yield is quenched in InAs/InP core/shells but increases substantially up to 20% for InAs/CdSe and InAs/ZnSe core/shells. For InAs/ZnS core/shells the enhancement of the photoluminescence quantum yields is smaller, up to 8%. The core/shell nanocrystals were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. X-ray photoelectron spectroscopy provides evidence for shell growth. The X-ray diffraction peaks shift and narrow upon shell growth, providing evidence for an epitaxial growth mode. Simulations of the X-ray diffraction patterns reproduce both effects, and show that there is one stacking fault present for every four to five layers in the core and core/shell nanocrystals. The stability of InAs/CdSe and InAs/ZnSe core/shells against oxidation is substantially improved compared with the cores, and the photostability is significantly better compared with a typical near-IR laser dye IR140. Core/shell nanocrystals with InAs cores are suggested as a novel type of fluorophores covering the near-IR spectral range, with high emission quantum yields and improved stability compared with traditional near-IR laser dyes.

We investigate the quantum size effects in the pressure-induced direct-to-indirect band gap transition in InP nanocrystals. Hydrostatic pressures of up to 13 GPa are applied to two different sizes of InP nanocrystal samples in a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra show that the nature of the emitting states at pressures up to 9 GPa is similar to that at ambient pressure, suggesting that no direct-to-indirect transition happens below 9 GPa. For both sizes, the PL peak energy exhibits a strong blueshift with rising pressure until approximately 9 to 10 GPa. Above this pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission dominates the PL spectra. As the pressure is released, both the luminescence intensity and the peak position recover in a fully reversible manner. The change in the sign of the band gap energy pressure dependence and the disappearance of the band edge luminescence indicate the pressure-induced direct-to-indirect band gap transition. Contrary to theoretical calculations, no substantial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure.

High-temperature colloidal synthesis of InAs/InP and InAs/CdSe core/shell nanocrystal quantum dots is reported. InP and CdSe shells with several thicknesses were grown on InAs cores ranging in diameter between 20 to 50 Å. Optical spectra, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to analyze the core/shell quantum dots and determine their chemical composition, average size, size distributions, and structures. The experimental results indicate that shell growth is uniform, expitaxial, and controllable. For both InP and CdSe shells, growth is accompanied by a red shift of the band gap energy as a result of the extension of the electron wavefunction into the shell region. An increase of the room temperature photoluminescence quantum yield by a factor of∼4 is observed with CdSe shell growth on InAs Cores. The growth of InP shells, however, quenches the photoluminescence quantum yield. The difference is assigned to outer surface effects in core/shell nanocrystals.

Localization of carrier wave functions to the quantum-well portion of the CdS/HgS quantum-dot quantum well (QDQW) is investigated. Nanosecond hole-burning (HB) spectra measure the photoinduced exciton coupling to a 250−cm−1 HgS phonon mode indicative of localization. Femtosecond pump-probe spectroscopy of these QDQW, however, show the photoinduced exciton couples to coherent 300−cm−1 CdS longitudinal optical-phonon modes, which is indicative of delocalization throughout the QDQW.  Femtosecond HB and three pulse pump-dump experiments reveal these results are dependent on the time scale of the experiment. These experiments indicate that the initially photoexcited electron and hole wave functions are weakly confined to the HgS monolayer. Only after long times (∼400 fs) will the exciton localize to the HgS well. These results indicate that the primary optical interaction excites electrons from a delocalized QDQW ground state and not from a localized HgS well state.

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as ‘artificial atoms’ (13). This analogy originates from the early work on quantum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted⁴ to exhibit atomic-like symmetries, for example ‘s ’ and ‘p ’. Spectroscopic studies of quantum dots have demonstrated discrete energy level structures and narrow transition linewidths^5,6,7,8,9, but the symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atomic-like electronic states with s and p character in a series of indium arsenide nanocrystals. These states are manifest in tunnelling current–voltage measurements as two- and six-fold single-electron-charging multiplets respectively, and they follow an atom-like Aufbau principle of sequential energy level occupation¹⁰.

The effect of the outer surface of core/shell nanocrystals on the fluorescence quantum yield was observed for InAs/InP and InAs/CdSe core/shells (see picture). For InAs/CdSe we observed substantial enhancement of the fluorescence quantum yield compared to the InAs core, and up to two times larger than the laser dye IR‐140. Such core/shell nanocrystals have potential use as biological fluorescent markers in the near IR spectral range.

A survey of caging and geminate recombination dynamics following the UV photolysis of I₃^- in a series of polar solvents is presented. Transient absorption in both the near-IR and UV was measured out to delays of 0.4 ns, probing evolution of the nascent product and recombined reactants, respectively. The fate of photolysis fragments is suggested to be determined shortly after the act of bond fission. Kinetic analysis shows caged fragments either recombine directly and vibrationally relax within a few picoseconds or produce long-lived complexes of unknown structure that decay exponentially in ∼40 ps, and both routes lead to ground-state I₃^-. The persistent complex exhibits a near-IR absorption spectrum that is broadened and red-shifted relative to free I₂^-. A very shallow and slow residual component of recombination may be associated with encounters of geminate pairs that initially escape the solvent cage. The choice of solvent strongly effects the probability and dynamics of caging, but not the decay rate of complex caged pairs. This is not altered by varying the temperature of an isobutyl alcohol solution from 5 to 45 °C. The results are discussed in an effort to illuminate the role played by the solvent in triiodide recombination in solution.

A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two‐level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using strong fields, resulting in substantial photoconversion. The emerging picture is that the impulsive excitation carves a coherent dynamical ‘‘hole’’ out of the ground surface density. A rigorous definition of the dynamical ‘‘hole’’ is constructed and used to define a measure of its coherence. In particular all photoinduced time dependence in the system can be directly related to the dynamical ‘‘hole.’’ All three techniques are used to simulate the pump probe experiment on the symmetric stretch mode of I₃⁻, including electronic and vibrational dephasing.

A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two‐level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using strong fields, resulting in substantial photoconversion. The emerging picture is that the impulsive excitation carves a coherent dynamical ‘‘hole’’ out of the ground surface density. A rigorous definition of the dynamical ‘‘hole’’ is constructed and used to define a measure of its coherence. In particular all photoinduced time dependence in the system can be directly related to the dynamical ‘‘hole.’’ All three techniques are used to simulate the pump probe experiment on the symmetric stretch mode of I₃⁻, including electronic and vibrational dephasing.

Control of coherent ground surface dynamics is achieved by varying the intensity of a resonant ultrafast pump pulse. This pulse cycles amplitude between the ground and excited electronic surfaces resulting in a momentum kick and a coordinate dependent loss of amplitude, creating a nonstationary vibrational distribution: the ‘V’ jump. A qualitative change in composition occurs for intensities above π pulse conditions. The induced dynamics is observed by a delayed pulse which is dispersed and analyzed against time or as a two-dimensional frequency plot. Such an analysis makes it possible to distinguish the contributions of high vibrational harmonics to the dynamics.

We report a study of the UV photolysis of I₃⁻ in alcohol solutions, using femtosecond time‐resolved transient transmission experiments. We interpret our results to indicate that photoexcitation leads to direct formation, within 300 fs, of diiodide ions which are vibrating coherently. The time scales for vibrational dephasing, vibrational relaxation, and reorientation of the fragment ions are 400 fs, 4 ps, and 5 ps, respectively. UV transient transmission experiments were conducted in order to characterize the time scales for recombination. Recombination takes place on several time scales. A fast component is assigned to primary recombination, followed by vibrational relaxation on the ground state of I₃⁻. The prominence of this recombination route is found to be strongly dependent on the solvent. Finally, time domain quantum simulation techniques are employed in order to gain insight into the nature of the nascent diiodide vibrational distribution, and to introduce a semiquantitative model for the stage of bond fission for isolated ions.