Study of surface and bulk electronic structure of II-VI semiconductor nanocrystals using Cu as a nanosensor

Generating Long-Lived Charge Carriers in CdS Quantum Dots by Cu-Doping for Photocatalytic CO2 Reduction

Converting CO2 into high-value-added chemicals has been recognized as a promising way to tackle the fossil fuel crisis. Quantum dots (QDs) have been extensively studied for photocatalytic CO2 reduction due to their excellent optoelectronic properties. However, most of the photogenerated charge carriers recombine before they participate in the photocatalytic reaction. It is crucial to regulate the charge carriers to minimize undesired charge recombination, thus, promoting surface photocatalysis. Herein, we report a copper-doped CdS (Cu:CdS) QD photocatalyst for CO2 reduction. Density functional theory simulations and experimental results demonstrate that Cu dopants create intermediate energy levels in CdS QDs that can extend the lifetime of exciton charge carriers. Furthermore, the long-lived charge carriers can be harnessed for the photocatalytic reaction on Cu:CdS QDs. The resultant Cu:CdS QDs exhibited a significantly enhanced photocatalytic activity toward CO2 reduction compared to the pristine CdS QDs. This work highlights the importance of charge regulation in photocatalysts and opens new pathways for the exploration of efficient QD photocatalysts.

Enhanced Charge Transfer from Coinage Metal Doped InP Quantum Dots

This paper describes coinage-metal-doped InP quantum dots (QDs) as a platform for enhanced electron transfer to molecular acceptors relative to undoped QDs. A synthetic strategy is developed to prepare doped InP/ZnSe QDs. First-principles DFT calculations show that Ag+ and Cu+ dopants localize photoexcited holes while leaving electrons delocalized. This charge carrier wave function modulation is leveraged to enhance electron transfer to molecular acceptors by up to an order of magnitude. Examination of photoluminescence quenching data suggests that larger electron acceptors, such as anthraquinone and methyl viologen, bind to the QD surface in two ways: by direct adsorption to the surface and by adsorption following displacement of a weakly bound surface cation-ligand complex. Reactions with larger acceptors show the greatest increases in electron transfer between doped and undoped quantum dots, while smaller acceptors show smaller enhancements. Specifically, benzoquinone shows the smallest, followed by naphthoquinone and then methyl viologen and anthraquinone. These results demonstrate the benefits of dopant-induced excited-state carrier localization on photoinduced charge transfer and highlight design principles for improved implementation of quantum dots in photoredox catalysis.

Electronic structure study of dual-doped II-VI semiconductor quantum dots towards single-source white light emission

Single-source white light emitting colloidal semiconductor quantum dots (QDs) is one of the most exciting and promising high-quality solid-state light sources to meet the current global demand for sustainable resources. While most of the previous methods involve dual (green-red) emissive nanostructures coated on blue LEDs to achieve white light, this work describes a single-source white light emitter of robust and superior quality using dual-doping. A modified synthesis method for intense white light emitting Cu, Mn dual-doped ZnSe QDs is engineered such that the extent of doping and concentration of ligands can alter their electronic structures. This is then customized to obtain various types of white light emissions ranging from warm white to cool white. Further, the composition-driven change in the electronic structure of the host QDs is exploited to achieve emission tunability over the entire visible range.

Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

Study of the Interface and Radial Dopant Position in Semiconductor Heterostructures Using X-ray Absorption Spectroscopy

Two questions that remain a challenge in the field of colloidal doped core/shell nanomaterials of different morphologies are the nature of the interface and the radial location of the dopant ion due to the diffusion within the lattice. Using a model system of Cu-doped CdSe/CdS quantum dots, we develop an in-depth understanding of the extended X-ray absorption fine structure (EXAFS) spectra of the dopant and host atoms to address both issues. Our findings suggest that the interface is not sharp, in agreement with the nonstructural studies in the literature. Local structure analysis around the Cu dopant ion confirms that Cu drifts out from the core toward the outer region in the absence of the shell but stays mostly in the core after the formation of a sufficiently thick interfacial barrier (∼2 monolayers). This study highlights the significance of EXAFS spectroscopy in understanding the nature of the interface in nanomaterials.

Optically Detected Magnetic Resonance Spectroscopy of Cu-Doped CdSe/CdS and CuInS2 Colloidal Quantum Dots

Copper-doped II-VI and copper-based I-III-VI₂ colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS₂ and CuInS₂/(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu²⁺ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS₂ CQDs (with and without shells) differ significantly from those of the copper-doped II-VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron-hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu²⁺ site. The electron resides near this Cu²⁺ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.

Insights into the Oxidation State of Cu Dopants in II-VI Semiconductor Nanocrystals

Luminescent Cu-doped semiconductor nanocrystals have played a pivotal role in the emergence of lighting and display applications for a long time. However, consensus regarding the Cu oxidation state and hence their emission mechanism has not been attained. Distinction between seemingly simple optically and magnetically active Cu²⁺ and inactive Cu¹⁺ has surprisingly been the subject matter of debate in the literature for more than a decade. In this Perspective, we first discuss the fundamental quantum mechanical phenomenon explaining the optical properties of the monovalent and divalent Cu dopants. We then focus down on various techniques used to differentiate between these two fundamental mechanisms, their benefits, and their pitfalls arising in large part because of the lack of spatial separation. Hence, to obtain a cohesive story consistent with all the observations, we discuss recent results from single-molecule spectroscopy to understand the optical properties and hence the oxidation state of internally doped Cu in doped nanocrystals.

Visible-Blind ZnMgO Colloidal Quantum Dot Downconverters Expand Silicon CMOS Sensors Spectral Coverage into Ultraviolet and Enable UV-Band Discrimination

Selective spectral detection of ultraviolet (UV) radiation is highly important across numerous fields from health and safety to industrial and environmental monitoring applications. Herein, a nontoxic, visible-blind, quantum dot (QD)-based sensing scheme that expands the spectral coverage of silicon complementary metal-oxide-semiconductor (CMOS) sensors into the UV, enabling efficient UV detection without affecting the sensor performance in the visible and UV-band discrimination, is reported. This scheme uses zinc magnesium oxide (ZnMgO) QDs with compositionally tunable absorption across UV and high photoluminescence quantum yield in the visible. The efficient luminescence and large Stokes shift of these QDs are exploited herein to act as an efficient downconverting material that enhances the UV sensitivity of Si-photodetectors (Si-PDs). A Si-PD integrated with the QDs results in a ninefold improvement in photoresponsivity from 0.83 to 7.5 mA W^-1 at 260 nm. Leveraging the tunability of these QDs, a simple UV-band identification scheme is further reported, which uses two distinct-bandgap ZnMgO QDs stacked in a tandem architecture whose spectral emission color depends on the UV-band excitation light. The downconverting stack enables facile discrimination of UV light using a standard CMOS image sensor (camera) or by the naked eye and avoids the use of complex optics.

Anodic near-infrared electrochemiluminescence from Cu-doped CdTe quantum dots for tetracycline detection

A sensitive anodic near-infrared electrochemiluminescence (ECL) immunosensor for the detection of tetracycline, based on Cu-doped CdTe quantum dots, was fabricated for the first time in this work. We have synthesized Cu-doped CdTe quantum dots by co-precipitation. The emission spectrum of the Cu-doped CdTe quantum dots could reach the near-infrared region at 730 nm in a short reflux time. More importantly, the ECL intensity of the CdTe quantum dots was enhanced by 2 fold after Cu element doping, which was attributed to the Cu d-orbital mixed with the conduction band and valence band of the host CdTe quantum dots. Inspired by the strong anodic ECL intensity of Cu-doped CdTe quantum dots, the anodic near infrared ECL sensor was constructed to detect tetracycline by competitive immunoassay. The detection range of the developed biosensor was 0.01-10 ng mL-1 and the detection limit was 0.0030 ng mL-1. In addition, the biosensor showed outstanding selectivity, long-term stability and high reproducibility, which has great potential in the field of analysis and detection.

Computational studies of the electronic structure of copper-doped ZnO quantum dots

Copper-doped ZnO quantum dots (QDs) have attracted substantial interest. The electronic structure and optical and magnetic properties of Cu³⁺(d⁸)-, Cu²⁺(d⁹)-, and Cu⁺(d¹⁰)-doped ZnO QDs with sizes up to 1.5 nm are investigated using the GGA+U approximation, with the +U corrections applied to d (Zn), p(O), and d(Cu) orbitals. Taking +Us parameters, as optimized in previous bulk calculations, we obtain the correct band structure of ZnO QDs. Both the description of electronic structure and thermodynamic charge state transitions of Cu in ZnO QDs agree with the results of bulk calculations due to the strong localization of Cu defect energy levels. Atomic displacements around Cu are induced by strong Jahn-Teller distortion and affect Kohn-Sham energies and thermodynamic transition levels. The average bond length of Cu-O and the defect structure are crucial factors influencing the electronic properties of Cu in ZnO QDs. The analysis of the optical properties of Cu in ZnO QDs is reported. The GGA+U results, compared with the available experimental data, support Dingle's model [Phys. Rev. Lett. 23, 579 (1969)], in which the structured green luminescence observed in bulk and nanocrystals originates from the [(Cu⁺, hole) → Cu²⁺] transition. We also examine the magnetic interaction between the copper pair for two charge states: 0 and +2, and four positions relative to the center of QDs. Ferromagnetic interaction between ions is obtained for every investigated configuration. The magnitude of ferromagnetism increases for positive charge defects due to the strong hybridization of the d(Cu) and p(O) states.

Copper Doping in II-VI Semiconductor Nanocrystals: Single-Particle Fluorescence Study

Copper doping in II-VI semiconductor nanocrystals (NCs) has sparked enormous debate regarding the oxidation state of Cu ions and their hugely differing consequences in optoelectronic applications. The identity of a magnetically active Cu²⁺ ion or a magnetically inactive d¹⁰ Cu⁺ ion has generally been probed using optical techniques, and confusion arises from the spatial clutter that is part of the technique. One major probe that could declutter the data obtained from ensemble emission is single-particle fluorescence spectroscopy. In this work, using this very technique along with X-ray absorption spectroscopy probing the local environment of dopant ions, we study Cu-doped II-VI semiconductor NCs to find conclusive evidence on the oxidation state of Cu dopants and hence the mechanism of their emission. Detailed analysis of blinking properties has been used to study the single-particle nature of the NCs.

Evaluation of dopant energy and Stokes shift in Cu-doped CdS quantum dots via spectro-electrochemical probing

Copper (Cu) doped II–VI semiconductor quantum dots (QDs) manifest high luminescent dopant emission with excellent tunability.

Cu-doped quantum dots: a new class of near-infrared emitting fluorophores for bioanalysis and bioimaging

Transition metal ion-doped quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over undoped QDs, such as larger Stokes shift to avoid self-absorption/energy transfer, longer excited-state lifetimes, wider spectral window, and improved chemical and thermal stability. Among the doped QDs emitters, Cu is widely introduced into the doped QDs as novel, efficient, stable, and tunable optical materials that span a wide spectrum from blue to near-infrared (NIR) light. Their unique physical and chemical characteristics enable the use of Cu-doped QDs as NIR labels for bioanalysis and bioimaging. In this review, we discuss doping mechanisms and optical properties of Cu-doped QDs that are capable of NIR emission. Applications of Cu-doped QDs in in vitro biosensing and in in vivo bioimaging are highlighted. Moreover, a prospect of the future of Cu-doped QDs for bioanalysis and bioimaging are also summarized.

Thermodynamics of Dual Doping in Quantum Dots

Dual doping is a powerful way to tailor the properties of semiconductor quantum dots (QDs) arising out of host-dopant and dopant-dopant interactions. Nevertheless, it has seldom been explored due to a variety of thermodynamic challenges, such as the differential bonding strength and diffusion constant within the host matrix that integrates with the host in dissimilar ways. This work discusses the challenges involved in administering them within the constraints of one host under similar conditions of temperature, time, and chemical parameters such as solubility and reactivity using CoPt-doped CdS QDs as a model system. In addition, the various forces in play, such as Kirkendall diffusion, solid- and liquid-state diffusion, hard acid soft base interaction with the host, and the effect of lattice strain due to lattice mismatch, are studied to understand the feasibility of the core to doped transformation. These findings suggest a potential approach for manipulating the properties of semiconductors by dual doping engineering.

Thermodynamic Model for Quantum Dot Assemblies Formed Because of Charge Transfer

Two initially neutral semiconductor quantum dots with appropriate band offsets can participate in a ground state charge transfer process. The charge transfer manifests itself in the form of bleaching of optical transitions and also causes the quantum dots to precipitate from solution, giving rise to assemblies with unusual properties. As this represents a postsynthetic modification of the electronic structure of quantum dots, it holds tremendous potential for improving the characteristics of quantum dot devices. Here, we study the dependencies of the properties of these assemblies on the structure of the participating quantum dots. In particular, we find that for assemblies formed out of Cu:CdS and ZnTe/CdS quantum dots, the composition of the assembly varies from 1:1.26 to 1:0.23 ZnTe/CdS to Cu:CdS as the shell thickness of CdS in ZnTe/CdS is increased. In contrast, the composition changes from 1:1.1 to 1:15 for PbSe/CdSe and Cu:CdS quantum dots, as the size of the PbSe core is increased. These observations are explained on the basis of a phenomenological thermodynamic model. The applicability of thermodynamics to this example of self-assembly is verified empirically.

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 n-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 via photodoping. Time-resolved photoluminescence measurements reveal that the ML_CBCT excited state is still formed upon photoexcitation of the n-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.

Frontier challenges in doping quantum dots: synthesis and characterization

Impurity doping in semiconductor quantum dots (QDs) has numerous prospects in implementing and altering their properties and technologies. Herein, we review the state-of-the-art doping techniques arising from colloidal synthesis methods. We first discuss the advantages and challenges involved in doping; we then discuss various doping techniques, including clustering of dopants as well as expulsion out of the lattice due to self-purification. Some of these techniques have been shown to open up a new generation of robust doped semiconductor quantum dots with cluster-free doping which will be suitable for various spin-based solid-state device technologies and overcome the longstanding challenges of controlled impurity doping. Further, we discuss inhibitors such as defects, clustering and interfaces, followed by current open questions. These include pathways to obtain uniform doping in the required radial position with unprecedented control over the dopant concentration and the size of the QDs.

We discuss state-of-the-art doping strategies for colloidal quantum dots, their principle, advantages and challenges in implementing the strategies.

Charge Carriers Modulate the Bonding of Semiconductor Nanoparticle Dopants As Revealed by Time-Resolved X-ray Spectroscopy

Understanding the electronic structure of doped semiconductors is essential to realize advancements in electronics and in the rational design of nanoscale devices. Reported here are the results of time-resolved X-ray absorption studies on copper-doped cadmium sulfide nanoparticles that provide an explicit description of the electronic dynamics of the dopants. The interaction of a dopant ion and an excess charge carrier is unambiguously observed via monitoring the oxidation state. The experimental data combined with DFT calculations demonstrate that dopant bonding to the host matrix is modulated by its interaction with charge carriers. Furthermore, the transient photoluminescence and the kinetics of dopant oxidation reveal the presence of two types of surface-bound ions that create midgap states.

Near-Unity Emitting Copper-Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators

Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.

Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in I-III-VI2 Nanocrystals

Ternary CuInS₂ nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal-based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential enables us to reversibly tune the NC Fermi energy and thereby control the occupancy of intragap defects involved in exciton decay. The results indicate that, in analogy to copper-doped II-VI NCs, emission occurs via radiative capture of a conduction-band electron by a hole localized on an intragap state likely associated with a Cu-related defect. We observe the increase in the emission efficiency under reductive electrochemical potential, which corresponds to raising the Fermi level, leading to progressive filling of intragap states with electrons. This indicates that the factor limiting the emission efficiency in these NCs is nonradiative electron trapping, while hole trapping is of lesser importance. This observation also suggests that the centers for radiative recombination are Cu²⁺ defects (preexisting and/or accumulated as a result of photoconversion of Cu¹⁺ ions) as these species contain a pre-existing hole without the need for capturing a valence-band hole generated by photoexcitation. Temperature-controlled photoluminescence experiments indicate that the intrinsic limit on the emission efficiency is imposed by multiphonon nonradiative recombination of a band-edge electron and a localized hole. This process affects both shelled and unshelled CIS NCs to a similar degree, and it can be suppressed by cooling samples to below 100 K. Finally, using experimentally measured decay rates, we formulate a model that describes the electrochemical modulation of the PL efficiency in terms of the availability of intragap electron traps as well as direct injection of electrons into the NC conduction band, which activates nonradiative Auger recombination, or electrochemical conversion of the Cu²⁺ states into the Cu¹⁺ species that are less emissive due to the need for their "activation" by the capture of photogenerated holes.

"Quantized" Doping of Individual Colloidal Nanocrystals Using Size-Focused Metal Quantum Clusters

The insertion of intentional impurities, commonly referred to as doping, into colloidal semiconductor quantum dots (QDs) is a powerful paradigm for tailoring their electronic, optical, and magnetic behaviors beyond what is obtained with size-control and heterostructuring motifs. Advancements in colloidal chemistry have led to nearly atomic precision of the doping level in both lightly and heavily doped QDs. The doping strategies currently available, however, operate at the ensemble level, resulting in a Poisson distribution of impurities across the QD population. To date, the synthesis of monodisperse ensembles of QDs individually doped with an identical number of impurity atoms is still an open challenge, and its achievement would enable the realization of advanced QD devices, such as optically/electrically controlled magnetic memories and intragap state transistors and solar cells, that rely on the precise tuning of the impurity states (i.e., number of unpaired spins, energy and width of impurity levels) within the QD host. The only approach reported to date relies on QD seeding with organometallic precursors that are intrinsically unstable and strongly affected by chemical or environmental degradation, which prevents the concept from reaching its full potential and makes the method unsuitable for aqueous synthesis routes. Here, we overcome these issues by demonstrating a doping strategy that bridges two traditionally orthogonal nanostructured material systems, namely, QDs and metal quantum clusters composed of a "magic number" of atoms held together by stable metal-to-metal bonds. Specifically, we use clusters composed of four copper atoms (Cu₄) capped with d-penicillamine to seed the growth of CdS QDs in water at room temperature. The elemental analysis, performed by electrospray ionization mass spectrometry, X-ray fluorescence, and inductively coupled plasma mass spectrometry, side by side with optical spectroscopy and transmission electron microscopy measurements, indicates that each Cu:CdS QD in the ensemble incorporates four Cu atoms originating from one Cu₄ cluster, which acts as a "quantized" source of dopant impurities.

Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy

The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Fröhlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.

Demystifying Complex Quantum Dot Heterostructures Using Photogenerated Charge Carriers

The success of heterostructure quantum dots in optoelectronic and photovoltaic applications is based on our understanding of photogenerated charge carrier localization. However, often the actual location of charge carriers in heterostructure semiconductors is quite different from their predicted positions leading to suboptimal results. In this work, photoluminescence of Cu doped heterostructures has been used to study the charge localization of alloys, inverse type I, type II, and quasi type II core/shell structures and graded alloys. Specifically, the adeptness of this method has been assessed over a range of widely studied heterostructures like CdSe/CdS, CdS/CdSe, CdSe/CdTe, Zn_1-xCd_xSe and Zn_1-xCd_xS quantum dots systems by doping them with a small percentage of Cu. The electron and hole localization obtained from this method concurs with the pre-existing understanding in cases that have been explored before, while the internal structure of previously unknown heterostructures have been predicted.

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminescent solar concentrators; and control of intraband relaxation for enhanced carrier multiplication in advanced QD photovoltaics. We examine the considerable recent progress on these multiple fronts of nanocrystal research, which has resulted in the first commercialized QD technologies. These successes explain the continuing appeal of this field to a broad community of scientists and engineers, which in turn ensures even more exciting results to come from future exploration of this fascinating class of materials.

Diffusion doping in quantum dots: bond strength and diffusivity

Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M²⁺ (Fe²⁺, Ni²⁺, Co²⁺, Mn²⁺). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.

Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2

Linear aminoalkanoic acids (AAAs) and mercaptoalkanoic acids (MAAs) were characterized as bifunctional ligands to tether CdSe QDs to nanocrystalline TiO2 thin films and to mediate excited-state electron transfer (ET) from the QDs to TiO2 nanoparticles. The adsorption of 12-aminododecanoic acid (ADA) and 12-mercaptododecanoic acid (ADA) to TiO2 followed the Langmuir adsorption isotherm. Surface adduct formation constants (Kad) were ∼10(4) M(-1); saturation amounts of the ligands per projected surface area of TiO2 (Γ0) were ∼10(-7) mol cm(-2). Both Kad and Γ0 differed by 20% or less for the two linkers. CdSe QDs adhered to ADA- and MDA-functionalized TiO2 films; data were well modeled by the Langmuir adsorption isotherm and Langmuir kinetics. For ADA- and MDA-mediated assembly values of Kad were (1.8 ± 0.4) × 10(6) and (2.4 ± 0.4) × 10(6) M(-1), values of Γ0 were (1.6 ± 0.3) × 10(-9) and (1.2 ± 0.1) × 10(-9) mol cm(-2), and rate constants were (14 ± 5) and (60 ± 20) M(-1) s(-1), respectively. Thus, the thermodynamics and kinetics of linker-assisted assembly were slightly more favorable for MDA than for ADA. Steady-state and time-resolved emission spectroscopy revealed that electrons were transferred from both band-edge and surface states of CdSe QDs to TiO2 with rate constants (ket) of ∼10(7) s(-1). ET was approximately twice as fast through thiol-bearing linker 4-mercaptobutyric acid (MBA) as through amine-bearing linker 4-aminobutyric acid (ABA). Photoexcited QDs transferred holes to adsorbed MBA. In contrast, ABA did not scavenge photogenerated holes from CdSe QDs, which maximized the separation of charges following ET. Additionally, ABA shifted electron-trapping surface states to higher energies, minimizing the loss of potential energy of electrons prior to ET. These trade-offs involving the kinetics and thermodynamics of linker-assisted assembly; the driving force, rate constant, and efficiency of ET; and the extent of photoinduced charge separation can inform the selection bifunctional ligands to tether QDs to surfaces.

Uniform Doping in Quantum-Dots-Based Dilute Magnetic Semiconductor

Effective manipulation of magnetic spin within a semiconductor leading to a search for ferromagnets with semiconducting properties has evolved into an important field of dilute magnetic semiconductors (DMS). Although a lot of research is focused on understanding the still controversial origin of magnetism, efforts are also underway to develop new materials with higher magnetic temperatures for spintronics applications. However, so far, efforts toward quantum-dots(QDs)-based DMS materials are plagued with problems of phase separation, leading to nonuniform distribution of dopant ions. In this work, we have developed a strategy to synthesize highly crystalline, single-domain DMS system starting from a small magnetic core and allowing it to diffuse uniformly inside a thick CdS semiconductor matrix and achieve DMS QDs. X-ray absorption fine structure (XAFS) spectroscopy and energy-dispersive X-ray spectroscopy-scanning transmission electron microscopy (STEM-EDX) indicates the homogeneous distribution of magnetic impurities inside the semiconductor QDs leading to superior magnetic property. Further, the versatility of this technique was demonstrated by obtaining ultra large particles (∼60 nm) with uniform doping concentration as well as demonstrating the high quality magnetic response.

Implications of orbital hybridization on the electronic properties of doped quantum dots: the case of Cu:CdSe

This paper investigates how chemical dopants affect the electronic properties of CdSe quantum dots (QDs) and why a model that incorporates the concepts of orbital hybridization must be used to understand these properties. Extended X-ray absorption fine structure spectroscopy measurements show that copper dopants in CdSe QDs occur primarily through a statistical doping mechanism. Ultraviolet photoemission spectroscopy (UPS) experiments provide a detailed insight on the valence band (VB) structure of doped and undoped QDs. Using UPS measurements, we are able to observe photoemission from the Cu d-levels above VB maximum of the QDs which allows a complete picture of the energy band landscape of these materials. This information provides insights into many of the physical properties of doped QDs, including the highly debated near-infrared photoluminescence in Cu doped CdSe QDs. We show that all our results point to a common theme of orbital hybridization in Cu doped CdSe QDs which leads to optically and electronically active states below the conduction band minimum. Our model is supported from current-voltage measurements of doped and undoped materials, which exhibit Schottky to Ohmic behavior with Cu doping, suggestive of a tuning of the lowest energy states near the Fermi level.

Photoinduced electron transfer from quantum dots to TiO2: elucidating the involvement of excitonic and surface states

CdSe QDs transfer electrons from band-edge and surface states to TiO₂; core/shell CdSe/ZnS QDs transfer electrons exclusively from band-edge states.

Dually enriched Cu:CdS@ZnS QDs with both polyvinylpyrrolidone twisting and SiO2 loading for improved cell imaging

Through harvesting of the increased Stokes shift of CdS QDs via Cu-doping, the concentration-quenching or aggregation-quenching of CdS QDs was largely alleviated. A dually-enriched strategy with both polyvinylpyrrolidone (PVP) twisting and SiO2 loading was developed for generating a highly luminescent doped-dots (d-dots) assembly for improved cell imaging.

Singlet-Triplet Splittings in the Luminescent Excited States of Colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons

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.

Photoluminescence Blinking and Reversible Electron Trapping in Copper-Doped CdSe Nanocrystals

Single-particle photoluminescence blinking is observed in the copper-centered deep-trap luminescence of copper-doped CdSe (Cu(+):CdSe) nanocrystals. Blinking dynamics for Cu(+):CdSe and undoped CdSe nanocrystals are analyzed to identify the effect of Cu(+), which selectively traps photogenerated holes. Analysis of the blinking data reveals that the Cu(+):CdSe and CdSe nanocrystal "off"-state dynamics are statistically identical, but the Cu(+):CdSe nanocrystal "on" state is shorter lived. Additionally, a new and pronounced temperature-dependent delayed luminescence is observed in the Cu(+):CdSe nanocrystals that persists long beyond the radiative lifetime of the luminescent excited state. This delayed luminescence is analogous to the well-known donor-acceptor pair luminescence of bulk copper-doped phosphors and is interpreted as revealing metastable charge-separated excited states formed by reversible electron trapping at the nanocrystal surfaces. A mechanistic link between this delayed luminescence and the luminescence blinking is proposed. Collectively, these data suggest that electron (rather than hole) trapping/detrapping is responsible for photoluminescence intermittency in these nanocrystals.

Dual-emitting quantum dot nanohybrid for imaging of latent fingerprints: simultaneous identification of individuals and traffic light-type visualization of TNT

A nanohybrid was employed for fingerprint imaging that was capable of simultaneous identification of individuals and TNT visualization in a “traffic-light” manner.

Fingerprints are a unique characteristic of an individual. Recently, it has been realized that fingerprints carry more information about individuals than just their identity, for example, they may identify potential addicts and terrorists carrying explosives. Therefore, the development of imaging moieties capable of both fingerprint staining and drug/explosive visualization is of significant importance for forensic chemistry. Here we developed a nanohybrid comprising green- and red-emitting QDs for simultaneous fingerprint imaging and TNT visualization in fingerprints. The red-emitting Cu-doped ZnCdS (Cu–ZnCdS) QDs were embedded into silica nanoparticles and the green-emitting ZnCdS QDs were anchored onto the surface of the silica nanoparticles and further functionalized with polyallylamine (PAA). Both components of the nanohybrid, i.e., the PAA-functionalized green QDs and red QD-doped silica nanoparticles, could be explored for fingerprint imaging. Due to the formation of a Meisenheimer complex between TNT and PAA, the green-emitting QDs could be quenched by TNT, meanwhile the red-emitting QDs were inert. Therefore, the nanohybrid exhibited a traffic light-type fluorescence color change (green-yellow-red) to TNT concentration in the range of 40–400 μM. This method is promising for potential applications in security-screening needs in public areas such as airports and train stations.

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-x)Cu(x)Se, that outperforms all other leading NCs by a significant margin in both small- and large-scale LSCs under full-spectrum conditions.

Long-decay near-infrared-emitting doped quantum dots for lifetime-based in vivo pH imaging

Long-decay near-infrared-emitting doped quantum dots were synthesized for lifetime-based in vivo pH imaging.

Near-infrared-emitting two-dimensional codes based on lattice-strained core/(doped) shell quantum dots with long fluorescence lifetime

Lattice-strained CdTe/CdS:Cu quantum dots (QDs) with a widely tunable near-infrared (NIR) fluorescence emission spectrum (700-910 nm) and long lifetime (up to 1 μs) are synthesized. Based on the multiemission and multi-lifetime of the well-defined QDs, NIR-emitting two-dimensional (2D) codes are achieved by embedding as-prepared QDs into agarose beads. This provides a new strategy for fluorescent 2D codes.

Cu Doping in Ligand Free CdS Nanocrystals: Conductivity and Electronic Structure Study

Ligand-free Cu-doped CdS nanocrystals (NCs) have been synthesized to elucidate their surface electronic structure. The Cu-doped ligand-free NCs unlike their undoped counterparts are shown to be luminescent. We used this Cu-related emission as a probe to study the nature of the surface trap states that results in negligible luminescence in the undoped NCs. The concentration of the sulfide ligands is shown to play a crucial role in the surface passivation of the NCs. Electrical conductivity of these NCs was also studied, and they were shown to exhibit significant conductivity of ∼10(-4) S cm(-1). Further we have shown that the electrical conductivity is closely correlated to the surface charge and hence the trap states of the individual NCs have far-reaching consequences in the device optimization.

Synthesis of uniform disk-shaped copper telluride nanocrystals and cation exchange to cadmium telluride quantum disks with stable red emission

We present the synthesis of novel disk-shaped hexagonal Cu2Te nanocrystals with a well-defined stoichiometric composition and tunable diameter and thickness. Subsequent cation exchange of Cu to Cd at high temperature (180 °C) results in highly fluorescent CdTe nanocrystals, with less than 1 mol % of residual Cu remaining in the lattice. The procedure preserves the overall disk shape, but is accompanied by a substantial reconstruction of the anion sublattice, resulting in a reorientation of the c-axis from the surface normal in Cu2Te into the disk plane in CdTe nanodisks. The synthesized CdTe nanodisks show a continuously tunable photoluminescence (PL) peak position, scaling with the thickness of the disks. The PL lifetime further confirms that the CdTe PL arises from band-edge exciton recombination; that is, no Cu-related emission is observed. On average, the recombination rate is about 25-45% faster with respect to their spherical quantum dots counterparts, opening up the possibility to enhance the emission rate at a given wavelength by controlling the nanocrystal shape. Finally, with a PL quantum efficiency of up to 36% and an enhanced PL stability under ambient conditions due to a monolayer of CdS formed on the nanocrystal surface during cation exchange, these flat quantum disks form an interesting enrichment to the current family of highly fluorescent, shape-controlled nanocrystals.

Tunable Infrared Phosphors Using Cu Doping in Semiconductor Nanocrystals: Surface Electronic Structure Evaluation

In this Letter, we report the study of the effect of ligands on the surface electronic structure of the nanocrystal by exploiting the mechanism of the Cu-related optical transition, obtained by coupling the nanocrystal conduction band to the Cu ion state in Cu-doped II-VI semiconductor nanocrystals. Systematic study of steady-state luminescence and lifetime decay dynamics of this Cu-related emission in cadmium-based chalcogenides shows that the role of oleic acid in surface passivation is unexpectedly quite different for various chalcogenides. Further, using these leads in Cu-doped CdS nanocrystals, we develop near-infrared-emitting phosphor materials that have tunable, high quantum yield (∼35%) emission with a single-exponential lifetime decay. Surprisingly, unlike the emission from other Cu-doped II-VI nanocrystals, emission from Cu doping in CdS nanocrystals is found to exhibit high thermal stability, being essentially unchanged up to 100 °C, making them more viable for use in various practical applications.