Copper-doped semiconductors are classic phosphor materials that have been used in a variety of applications for many decades. Colloidal copper-doped semiconductor nanocrystals have recently attracted a great deal of interest because they combine the solution processability and spectral tunability of colloidal nanocrystals with the unique photoluminescence properties of copper-doped semiconductor phosphors. Although ternary and quaternary semiconductors containing copper, such as CuInS2 and Cu2ZnSnS4, have been studied primarily in the context of their photovoltaic applications, when synthesized as colloidal nanocrystals, these materials have photoluminescence properties that are remarkably similar to those of copper-doped semiconductor nanocrystals. This review focuses on the luminescent properties of colloidal copper-doped, copperbased, and related copper-containing semiconductor nanocrystals. Fundamental investigations into the luminescence of copper-containing colloidal nanocrystals are reviewed in the context of the well-established luminescence mechanisms of bulk copper-doped semiconductors and copper(I) molecular coordination complexes. The use of colloidal copper-containing nanocrystals in applications that take advantage of their luminescent properties, such as bio-imaging, solid-state lighting, and luminescent solar concentrators, is also discussed. a Measured at 20 K b Measured in a PV device 2. Synthesis and Structural Characterization of Colloidal Copper-Containing Nanocrystals Progress in the synthesis of luminescent copper-doped colloidal semiconductor NCs mirrors that in the synthesis of colloidal semiconductor NCs in general. The earliest reports from the mid-1980s through the 1990s describe the synthesis of copper-doped CdS and ZnS colloidal NCs by simply adding small amounts of copper salts to the aqueous-phase arrested precipitation reactions

The electronic and magnetic properties of the luminescent excited states of colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet-triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu(+)-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu(+)-doped semiconductor nanocrystals.

The complex [Co(dmgH)2pyCl]2+ (1, dmgH = dimethylglyoximate, py = pridine) has been used as a molecular catalyst for visible light driven hydrogen production in the presence of [Pt(tolylterpyridine)(phenylacetylide)]+ (3) as a photosensitizer and triethanolamine (TEOA) as a sacrificial reducing agent. Complex 3 is quenched oxidatively by [Co(dmgH)pyCl]2+ (1) with a rate constant kq of 1.27 x 10(9) M(-1) s(-1). Photogeneration of H2 is only seen when 1 + 3 + TEOA are all present. H2 production is maximized for this system at pH 8.5 and declines to very low levels at pH < 7 and pH > 12. Irradiation of the reaction solution initially containing 1.61 x 10(-2) M TEOA, 1.11 x 10(-5) M of 3, and 1.99 x 10(-4) M of Co catalyst 1 in MeCN/water (3:2 v/v) at pH = 8.5 for 10 h with lambda > 410 nm yields 400 turnovers of H2. When TEOA is 0.27 M, approximately 1000 turnovers are obtained after 10 h of irradiation. Spectroscopic study of the photolyses solutions suggests that H2 formation proceeds via Co(I) and protonation to form Co(III) hydride species.

This paper describes a quantitative analysis of the chemical composition of organic/inorganic interfaces of colloidal 3.1-nm CdSe quantum dots (QDs) synthesized with trioctylphosphine oxide (TOPO) as the coordinating solvent and purified by successive precipitations from a chloroform/methanol solvent/nonsolvent system. A combination of X-ray photoelectron spectroscopy, inductively coupled plasma-atomic emission spectroscopy, and NMR (both 1H and 31P) reveals that the only ligands that form a stable population on the surface of the QDs are X-type alkylphosphonate and carboxylate ligands. n-Octylphosphonate (OPA), a known impurity in technical-grade (90%) TOPO, and P′-P′-(di-n-octyl) pyrophosphonate (PPA), the self-condensation product of OPA, cover ∼84% of the atoms on the surface of the QDs, whereas few of the L-type (datively bound) ligands hexadecylamine (HDA), TOPO, and trioctylphosphine selenide (TOPSe) are present as bound ligands once the excess free surfactant is removed from the reaction mixture. Purified QDs synthesized in 99% TOPO (with no alkylphosphonates present) have no phosphorus-containing ligands on the surface. Despite the approximately constant surface coverage of phosphorus-containing ligands, the photoluminescence quantum yield of the solution of QDs steadily decreases during purification from ∼15% to less than 1%. Proton NMR analysis of the QD samples and photoluminescence spectra of QDs exposed to various concentrations of methanol suggest that this decrease is due to a combination of progressive loss of small amounts of HDA and adsorption of methanol to the surface of the QDs during purification.

A combination of transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopies performed on solution-phase samples of colloidal CdSe quantum dots (QDs) allows the construction of a time-resolved, charge carrier-resolved map of decay from the first excitonic state of the QD. Data from TA and TRPL yield the same six exponential components, with time constants ranging from ∼1 ps to 50 ns, for excitonic decay. Comparison of TA signals in the visible and near-infrared (NIR) spectral regions enables determination of the relative contributions of electron and hole dynamics to each decay component, and comparison of TA and TRPL reveals that each component represents a competition between radiative and nonradiative decay pathways. In total, these data suggest that the QD sample comprises at least three distinct populations that differ in both the radiative and nonradiative decay pathways available to the excitonic charge carriers, and provide evidence for multiple emissive excitonic states in which the hole is not in the valence band, but rather a relaxed or trapped state.