Valence-Band Electronic Structures of Cu⁺‑Doped ZnS, Alloyed Cu–In–Zn–S, and Ternary CuInS₂ Nanocrystals: A Unified Description of Photoluminescence across Compositions

Copper-doped and copper-based colloidal semiconductor nanocrystals have attracted broad attention as phosphors in many contexts, but fundamental aspects of their electronic structures that give rise to their photoluminescence are not understood. Here, we report a detailed systematic investigation of the electronic structures of Cu⁺-doped ZnS, alloyed Cu–In–Zn–S, and CuInS₂ nanocrystals (NCs) using density functional theory. These calculations demonstrate a continuous evolution in electronic structure from lightly doped to ternary compositions. As an impurity, Cu⁺ introduces isolated midgap d orbitals above the valence-band edge, with large Cu­(3d)–S­(3p) covalency. As the Cu⁺ content is increased in Cu–In–Zn–S alloys, these orbitals evolve to become the CuInS₂ valence band in the ternary limit. The calculations further describe the highest occupied molecular orbital (HOMO) as localized and Cu­(3d)-based for all compositions from Cu⁺-doped ZnS to stoichiometric CuInS₂. The calculations predict that the Cu­(3d)-based HOMOs can only delocalize over ca. 2 or 3 adjacent Cu⁺ ions but not more, reflecting weak Cu⁺–Cu⁺ electronic coupling, attributable in large measure to the directionality of the d orbitals. HOMO localization is also sensitive to the local Cu⁺ environment, Cu⁺–Cu⁺ geometric connectivity, and electrostatics. We conclude that the Cu­(3d)-based HOMO of chalcopyrite CuInS₂ makes localization likely even in defect-free CuInS₂ NCs, placing this material in stark contrast with structurally analogous II–VI semiconductor NCs that have anion p-orbital-based HOMOs and show facile HOMO delocalization. The strong tendency for HOMO localization in both Cu⁺-doped II–VI and Cu⁺-based chalcopyrite NCs has significant implications for interpretation of the photophysical properties of such materials

Photoluminescence Brightening via Electrochemical Trap Passivation in ZnSe and Mn²⁺-Doped ZnSe Quantum Dots

Spectroelectrochemical experiments on wide-gap semiconductor nanocrystals (ZnSe and Mn²⁺-doped ZnSe) have allowed the influence of trap electrochemistry on nanocrystal photoluminescence to be examined in the absence of semiconductor band filling. Large photoluminescence electrobrightening is observed in both materials upon application of a reducing potential and is reversed upon return to the equilibrium potential. Electrobrightening is correlated with the transfer of electrons into nanocrystal films, implicating reductive passivation of midgap surface electron traps. Analysis indicates that the electrobrightening magnitude is determined by competition between electron trapping and photoluminescence (ZnSe) or energy transfer (Mn²⁺-doped ZnSe) dynamics within the excitonic excited state, and that electron trapping is extremely fast (<i>k</i>_trap ≈ 10¹¹ s^–1). These results shed new light on the complex surface chemistries of semiconductor nanocrystals

Thermal Tuning and Inversion of Excitonic Zeeman Splittings in Colloidal Doped CdSe Quantum Dots

Variable-temperature magnetic circular dichroism (MCD) spectroscopy is used to measure excitonic Zeeman splittings in colloidal Co²⁺- and Mn²⁺-doped CdSe quantum dots with low dopant concentrations. The data demonstrate that the competition between intrinsic and exchange contributions to the excitonic Zeeman splittings in doped quantum dots can be tuned using temperature, from being dominated by exchange at low temperatures to being dominated by intrinsic Zeeman interactions at room temperature, with inversion at easily accessible temperatures and fields. These results may have relevance to spin-based information processing technologies that rely on manipulating carrier spins in quantum dots

Surface Contributions to Mn²⁺ Spin Dynamics in Colloidal Doped Quantum Dots

Colloidal impurity-doped quantum dots (QDs) are attractive model systems for testing the fundamental spin properties of semiconductor nanostructures relevant to future spin-based information processing technologies. Although static spin properties of this class of materials have been studied extensively in recent years, their spin dynamics remain largely unexplored. Here we use pulsed electron paramagnetic resonance (pEPR) spectroscopy to probe the spin relaxation dynamics of colloidal Mn²⁺-doped ZnO, ZnSe, and CdSe quantum dots in the limit of one Mn²⁺ per QD. pEPR spectroscopy is particularly powerful for identifying the specific nuclei that accelerate electron spin relaxation in these QDs. We show that the spin-relaxation dynamics of these colloidal QDs are strongly influenced by dipolar coupling with proton nuclear spins outside the QDs and especially those directly at the QD surfaces. Using this information, we demonstrate that spin-relaxation times can be elongated significantly via ligand (or surface) deuteration or shell growth, providing two tools for chemical adjustment of spin dynamics in these nanomaterials. These findings advance our understanding of the spin properties of solution-grown semiconductor nanostructures relevant to spin-based information technologies

Absorption and Magnetic Circular Dichroism Analyses of Giant Zeeman Splittings in Diffusion-Doped Colloidal Cd_1–<i>x</i>Mn_<i>x</i>Se Quantum Dots

Impurity ions can transform the electronic, magnetic, or optical properties of colloidal quantum dots. Magnetic impurities introduce strong dopant-carrier exchange coupling that generates giant Zeeman splittings (Δ<i>E</i>_Z) of excitonic excited states. To date, Δ<i>E</i>_Z in colloidal doped quantum dots has primarily been quantified by analysis of magnetic circular dichroism (MCD) intensities and absorption line widths (σ). Here, we report Δ<i>E</i>_Z values detected directly by absorption spectroscopy for the first time in such materials, using colloidal Cd_1–<i>x</i>Mn_<i>x</i>Se quantum dots prepared by diffusion doping. A convenient method for decomposing MCD and absorption data into circularly polarized absorption spectra is presented. These data confirm the widely applied MCD analysis in the low-field, high-temperature regime, but also reveal a breakdown at low temperatures and high fields when Δ<i>E</i>_Z/σ approaches unity, a situation not previously encountered in doped quantum dots. This breakdown is apparent for the first time here because of the extraordinarily large Δ<i>E</i>_Z and small σ achieved by nanocrystal diffusion doping

Potentiometric Titrations for Measuring the Capacitance of Colloidal Photodoped ZnO Nanocrystals

Colloidal semiconductor nanocrystals offer a unique opportunity to bridge molecular and bulk semiconductor redox phenomena. Here, potentiometric titration is demonstrated as a method for quantifying the Fermi levels and charging potentials of free-standing colloidal <i>n</i>-type ZnO nanocrystals possessing between 0 and 20 conduction-band electrons per nanocrystal, corresponding to carrier densities between 0 and 1.2 × 10²⁰ cm^–3. Potentiometric titration of colloidal semiconductor nanocrystals has not been described previously, and little precedent exists for analogous potentiometric titration of any soluble reductants involving so many electrons. Linear changes in Fermi level vs charge-carrier density are observed for each ensemble of nanocrystals, with slopes that depend on the nanocrystal size. Analysis indicates that the ensemble nanocrystal capacitance is governed by classical surface electrical double layers, showing no evidence of quantum contributions. Systematic shifts in the Fermi level are also observed with specific changes in the identity of the charge-compensating countercation. As a simple and contactless alternative to more common thin-film-based voltammetric techniques, potentiometric titration offers a powerful new approach for quantifying the redox properties of colloidal semiconductor nanocrystals

Redox Brightening of Colloidal Semiconductor Nanocrystals Using Molecular Reductants

Chemical reductants of sub-conduction-band potentials are demonstrated to induce large photoluminescence enhancement in colloidal ZnSe-based nanocrystals. The photoluminescence quantum yield of colloidal Mn²⁺-doped ZnSe nanocrystals has been improved from 14% to 80% simply by addition of an outer-sphere reductant. Up to 48-fold redox brightening is observed for nanocrystals with lower starting quantum yields. These increases are quickly reversed upon exposure to air and are temporary even under anaerobic conditions. This redox brightening process offers a new and systematic approach to understanding redox-active surface “trap states” and their contributions to the physical properties of colloidal semiconductor nanocrystals

Photodoping and Transient Spectroscopies of Copper-Doped CdSe/CdS Nanocrystals

Colloidal Cu⁺-doped CdSe/CdS core/shell semiconductor nanocrystals (NCs) are investigated in their as-prepared and degenerately <i>n</i>-doped forms using time-resolved photoluminescence and transient-absorption spectroscopies. Photoluminescence from Cu⁺:CdSe/CdS NCs is dominated by recombination of delocalized conduction-band (CB) electrons with copper-localized holes. In addition to prominent bleaching of the first excitonic absorption feature, transient-absorption measurements show bleaching of the sub-bandgap copper-to-CB charge-transfer (ML_CBCT) absorption band and also reveal a photoinduced midgap valence-band (VB)-to-copper charge-transfer (L_VBMCT) absorption band that extends into the near-infrared, as predicted by recent computations. The photoluminescence of these NCs is substantially diminished upon introduction of excess CB electrons <i>via</i> photodoping. Time-resolved photoluminescence measurements reveal that the ML_CBCT excited state is still formed upon photoexcitation of the <i>n</i>-doped Cu⁺:CdSe/CdS NCs, but its luminescence is quenched by a fast (picosecond) three-carrier trap-assisted Auger recombination process involving two CB electrons and one copper-bound hole

Nanocrystals for Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) harvest sunlight over large areas and concentrate this energy onto photovoltaics or for other uses by transporting photons through macroscopic waveguides. Although attractive for lowering solar energy costs, LSCs remain severely limited by luminophore reabsorption losses. Here, we report a quantitative comparison of four types of nanocrystal (NC) phosphors recently proposed to minimize reabsorption in large-scale LSCs: two nanocrystal heterostructures and two doped nanocrystals. Experimental and numerical analyses both show that even the small core absorption of the leading NC heterostructures causes major reabsorption losses at relatively short transport lengths. Doped NCs outperform the heterostructures substantially in this critical property. A new LSC phosphor is introduced, nanocrystalline Cd_1–<i>x</i>Cu_<i>x</i>Se, that outperforms all other leading NCs by a significant margin in both small- and large-scale LSCs under full-spectrum conditions

Luminescence Saturation via Mn²⁺–Exciton Cross Relaxation in Colloidal Doped Semiconductor Nanocrystals

Colloidal Mn²⁺-doped semiconductor nanocrystals such as Mn²⁺:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn²⁺ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn²⁺ ion and an exciton within the same nanocrystal. A lower limit of <i>k</i> = 2 × 10¹⁰ s^–1 is established for the fundamental rate constant of the Mn²⁺(⁴T₁)-exciton cross relaxation