We report an efficient synthesis of copper indium sulfide nanocrystals with strong photoluminescence in the visible to near-infrared. This method can produce gram quantities of material with a chemical yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepared nanocrystals with a few monolayers of CdS or ZnS increases the photoluminescence quantum efficiency to > 80%. On the basis of time-resolved spectroscopic studies of core/shell particles, we conclude that the emission is due to an optical transition that couples a quantized electron state to a localized hole state, which is most likely associated with an internal defect.

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

We have developed a new class of colloidal nanocrystals composed of Cu-doped ZnSe cores overcoated with CdSe shells. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic +2 species and serve as a source of permanent optically active holes. This implies that the Fermi level is located below the Cu(2+)/Cu(1+) state, that is, in the lower half of the forbidden gap, which is a signature of a p-doped material. It further suggests that the activation of optical emission due to the Cu level requires injection of only an electron without a need for a valence-band hole. This peculiar electron-only emission mechanism is confirmed by experiments in which the titration of the nanocrystals with hole-withdrawing molecules leads to enhancement of Cu-related photoluminescence while simultaneously suppressing the intrinsic, band-edge exciton emission. In addition to containing permanent optically active holes, these newly developed materials show unprecedented emission tunability from near-infrared (1.2 eV) to the blue (3.1 eV) and reduced losses from reabsorption due to a large Stokes shift (up to 0.7 eV). These properties make them very attractive for applications in light-emission and lasing technologies and especially for the realization of novel device concepts such as "zero-threshold" optical gain.

Nanoscale materials have been investigated extensively for applications in memory and data storage. Recent advances include memories based on metal nanoparticles, nanoscale phase-change materials and molecular switches. Traditionally, magnetic storage materials make use of magnetic fields to address individual storage elements. However, new materials with magnetic properties addressable via alternative means (for example, electrical or optical) may lead to improved flexibility and storage density and are therefore very desirable. Here, we demonstrate that copper-doped chalcogenide nanocrystals exhibit not only the classic signatures of diluted magnetic semiconductors--namely, a strong spin-exchange interaction between paramagnetic Cu(2+) dopants and the conduction/valence bands of the host semiconductor--but also show a pronounced and long-lived photoinduced enhancement of their paramagnetic response. Magnetic circular dichroism studies reveal that paramagnetism in these nanocrystals can be controlled and increased by up to 100% when illuminated with above-gap (blue/ultraviolet) light. These materials retain a memory of the photomagnetization for hour-long timescales in the dark, with effects persisting up to ∼80 K.

Dielectric constants of MAPbX3 (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX3 in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI3 that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation.

Hot electrons are created in core/shell CdSe/ZnSe colloidal quantum dots by mid-infrared intraband (4 μm) excitation and are probed by time-resolved visible spectroscopy. The hot electron, in the first excited conduction state 1Pe of the CdSe core, is efficiently extracted by tunneling through the ZnSe shell. Electron extraction times are temperature-independent. They range from ∼100 ps for thick, ∼3 nm, uniform ZnSe shells to <4 ps for high-surface-area irregular ZnSe shells, and they compete favorably with intraband relaxation. The hot electron extraction leads to a quench of the visible photoluminescence. This is a first step toward infrared detection using the intraband transitions of colloidal quantum dots.

The rates of multicarrier recombination are measured in colloidal quantum dots with and without hole surface trapping and with hole extraction in type II core/shell systems. We report that hole trapping or the physical separation of electronic and hole wavefunctions into different semiconductor domains of a type II system have an insignificant effect on the multicarrier recombination dynamics. These observations are inconsistent with the accepted Auger transition mechanism.

The interband and intraband spectra of colloidal II-VI CdS and CdSe quantum dot cores and CdSZnSe, CdSCdSe, CdSeCdS, and CdSeZnSe core/shell systems are reported. Infrared absorption peaks between 0.5 and 0.2 eV are observed. The slope of the intraband energy versus the first interband absorption feature is characteristic of the relative band alignments of the materials constituting the core and the shell and it is analyzed within an effective mass model. The analysis provides a new estimate of the band gap of zinc blende CdSe as well as the band offsets in zinc blende and wurtzite CdSe, CdS, and ZnSe.