Volume and Concentration Scaling of Magnetism in Dilute Magnetic Semiconductor Quantum Dots

Investigation of the magnetism of dilute magnetic semiconductors (DMSs) by changing particle dimensionality and doping concentration and its complete understanding is a major step toward their application in multifunctional devices. The importance of effects, such as magnetization reversal with size and magnetic invariance with respect to doping concentration for an appropriate functioning of DMS systems, is empirically well-known. However, explicit demonstration of these effects, specifically in nanomaterials, has so far not been studied mainly due to the lack of synthetic handle. In this work, we have demonstrated the prerequisites of DMS materials by isolating origins of magnetism arising due to clustering of magnetic dopant ions as well as sp–d exchange interaction with the host. We have studied magnetism with varying concentrations of dopant ions and have shown that the magnetism arising due to exchange interaction with the host is invariant up to 10% doping concentration, demonstrating the concentration scaling in DMS systems. Additionally, the study of size-dependent magnetic behavior revealed the effect of domain size and disordered spin on the surface, leading to a change in magnetization/ion as well as magnetization reversal

Magnetism at the Interface of Magnetic Oxide and Nonmagnetic Semiconductor Quantum Dots

Engineering interfaces specifically in quantum dot (QD) heterostructures provide several prospects for developing multifunctional building block materials. Precise control over internal structure by chemical synthesis offers a combination of different properties in QDs and allows us to study their fundamental properties, depending on their structure. Herein, we studied the interface of magnetic/nonmagnetic Fe₃O₄/CdS QD heterostructures. In this work, we demonstrate the decrease in the size of the magnetic core due to annealing at high temperature by the decrease in saturation magnetization and blocking temperature. Furthermore, surprisingly, in a prominently optically active and magnetically inactive material such as CdS, we observe the presence of substantial exchange bias in spite of the nonmagnetic nature of CdS QDs. The presence of exchange bias was proven by the increase in magnetic anisotropy as well as the presence of exchange bias field (<i>H</i>_E) during the field-cooled magnetic measurements. This exchange coupling was eventually traced to the <i>in situ</i> formation of a thin antiferromagnetic FeS layer at the interface. This is verified by the study of Fe local structure using X-ray absorption fine structure spectroscopy, demonstrating the importance of interface engineering in QDs

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–<i>x</i>Cd_<i>x</i>Se and Zn_1–<i>x</i>Cd_<i>x</i>S 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

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

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

Understanding the Role of Surface Capping Ligands in Passivating the Quantum Dots Using Copper Dopants as Internal Sensor

The role of ligands in passivating quantum dots has been studied in this work using Cu doping as internal sensors. This has been elucidated using the example of dodecanethiol, 3-mercaptopropionic acid, trioctylphosphine, trioctylphosphine oxide, and primary amines for the passivation of CdSe quantum dots by exchanging the original ligands. Steady state and time dependent photoluminescence spectra of the Cu emission have provided the basis for determining the role of ligands. The surface of the quantum dots with ligand exchange has been monitored using nuclear magnetic resonance spectroscopy. The results suggest that the presence of trioctylphosphine, trioctylphosphine oxide, and oleylamine ligands on CdSe quantum dot surface lead to better photoluminescence efficiency. Further, increase in the chain length of the primary amines increases the effectiveness of passivation on the CdSe quantum dot surface. We have also extended this method to the study of oleylamine capping in CdS quantum dots

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

Study of Surface and Bulk Electronic Structure of II–VI Semiconductor Nanocrystals Using Cu as a Nanosensor

Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. Despite a high demand to obtain size specific band offsets, very few studies exist where meticulous methods like photoelectron spectroscopy are used. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intragap Cu-related emission feature assigned to the transition from the conduction band to the atomic-like Cu <i>d</i> state. Using this transition we determine the relative band alignment of II–VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the excitonic band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the excitonic emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time

Optical Signatures of Impurity–Impurity Interactions in Copper Containing II–VI Alloy Semiconductors

We study the optical properties of copper containing II–VI alloy quantum dots (Cu_<i>x</i>Zn_<i>y</i>Cd_{1–<i>x</i>–<i>y</i>}Se). Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range, and the formation of secondary phases of copper selenide are observed only at <i>x</i>_Cu > 0.45. The optical absorption and emission spectra of these materials are observed to be a strong function of <i>x</i>_Cu, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply (from 1 × 10 ^–2 nm³ to 4 × 10 ^–2 nm³) at <i>x</i>_Cu = 0.12, suggesting a composition induced change in local electronic structure. These materials may serve as model systems to understand the electronic structure of I–III–VI₂ semiconductor compounds

Core–Shell to Doped Quantum Dots: Evolution of the Local Environment Using XAFS

Internal structure study at an atomic level is a challenging task with far reaching consequences to its material properties, specifically in the field of transition metal doping in quantum dots. Diffusion of transition metal ions in and out of quantum dots forming magnetic clusters has been a major bottleneck in this class of materials. Diffusion of the magnetic ions from the core into the nonmagnetic shell in a core/shell heterostructure architecture to attain uniform doping has been recently introduced and yet to be understood. In this work, we have studied the local structure variation of Fe as a function of CdS matrix thickness and annealing time during the overcoating of Fe₃O₄ core with CdS using X-ray absorption spectroscopy. The data reveals that Fe₃O₄ core initially forms a core/shell structure with CdS followed by alloying at the interface eventually completely diffusing all the way through the CdS matrix to form homogeneously Fe-doped CdS QDs with excellent control over size and size distribution. Study of Fe K-edge shows a complete change of Fe local environment from Fe–O to FeS