From Impurity Doping to Metallic Growth in Diffusion Doping: Properties and Structure of Silver-Doped InAs Nanocrystals

Tunable and Orange to Near-Infrared Doped Photoluminescence of Ag-Doped II-VI QDs

The electronic doping of colloidal semiconductor nanocrystals presents significant potential for future device concepts in optoelectronic and spin-based technologies. Ag+ is gaining recognition as a novel electronic dopant in II-VI nanocrystals as it creates intragap electronic states that participate in the recombination process of photogenerated carriers within the host materials. Herein, we report the synthesis of a series of Ag-doped II-VI quantum dots (QDs) of varying sizes via a low-temperature cation exchange strategy. A surprisingly tunable dopant emission covering the orange to near-infrared window (606-725 nm for CdS/Ag and 745-877 nm for CdSe/Ag) is achieved through the manipulation of particle sizes in the host QDs, which is derived from the quantum confinement effect. The as-prepared Ag+-doped II-VI QDs exhibit effective size-dependent tunable emission with a quantum yield (QY) up to 40.7% and strongly suppressed reabsorption due to the large Stoke shift up to 0.86 eV, demonstrating their potential use as candidates for various optoelectronic devices. Furthermore, using CdSe/Ag as a case to study, both experimental results and density functional theory (DFT) calculations indicate that the successful Ag+ doping in the matrix not only results in the formation of additional extra luminescence centers but also causes a reduction in the bandgap.

Exciton-to-Dopant Energy Transfer Dynamics in Mn2+ Doped CsPbBr3 Nanowires Synthesized by Diffusion Doping

Mn2+ doped perovskite nanocrystals have garnered significant attention in optoelectronic applications. However, the synthesis of Mn2+ doped perovskite nanowires (NWs) poses challenges, and the dynamics of energy transfer from the exciton to Mn2+ remains unexplored, which is crucial for optimizing Mn2+ luminescence efficiency. Herein, we present a method to synthesize Mn2+ doped CsPbBr3 NWs with a photoluminescence quantum yield of 52% by diffusing Mn2+ into seed CsPbBr3 NWs grown via a hot injection method. We control the solution and lattice chemical potentials of Pb2+ and Mn2+ to enable Mn2+ to diffuse into the CsPbBr3 NWs while minimizing Ostwald ripening. Variable temperature photoluminescence spectroscopy reveals that the energy transfer from the exciton to Mn2+ in Mn2+ doped CsPbBr3 NWs is temperature dependent. A dynamic competition is observed between energy transfer and backward energy transfer, resulting in stronger Mn2+ photoluminescence at 80 K. This work provides a specific synthesis pathway for Mn2+ doped CsPbBr3 NWs and sheds light on their exciton-to-Mn2+ energy transfer dynamics.

Zn-Doped P-Type InAs Nanocrystal Quantum Dots

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short-wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. The p-type doping of InAs NCs is presented with Zn - enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping is achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn²⁺ replacing In³⁺ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near infrared photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high-resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short-wave infrared region utilizing heavy-metal free nanocrystal building blocks.

NiMn-Based Bimetal-Organic Framework Nanosheets Supported on Multi-Channel Carbon Fibers for Efficient Oxygen Electrocatalysis

Developing noble-metal-free bifunctional oxygen electrocatalysts is of great significance for energy conversion and storage systems. Herein, we have developed a transformation method for growing NiMn-based bimetal-organic framework (NiMn-MOF) nanosheets on multi-channel carbon fibers (MCCF) as a bifunctional oxygen electrocatalyst. Owing to the desired components and architecture, the MCCF/NiMn-MOFs manifest comparable electrocatalytic performance towards oxygen reduction reaction (ORR) with the commercial Pt/C electrocatalyst and superior performance towards oxygen evolution reaction (OER) to the benchmark RuO₂ electrocatalyst. X-ray absorption fine structure (XAFS) spectroscopy and density functional theory (DFT) calculations reveal that the strong synergetic effect of adjacent Ni and Mn nodes within MCCF/NiMn-MOFs effectively promotes the thermodynamic formation of key *O and *OOH intermediates over active NiO₆ centers towards fast ORR and OER kinetics.

Explaining the Unusual Photoluminescence of Semiconductor Nanocrystals Doped via Cation Exchange

Aliovalent doping of CdSe nanocrystals (NCs) via cation exchange processes has resulted in interesting and novel observations for the optical and electronic properties of the NCs. However, despite over a decade of study, these observations have largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NCs. Here, electrostatic force microscopy was used to determine the static charge on individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While the NC charge was relatively insensitive to the relative amount of doped cation per NC, there was a remarkable and unexpected correlation between the average NC charge and PL intensity, for all dopant cations introduced. We conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Coulombic potential introduced by ionized cations.

Identification of dopant site and its effect on electrochemical activity in Mn-doped lithium titanate

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of X-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at. %) Mn-doped lithium titanate (Li4/3Ti5/3O4, or LTO) the dopant Mn2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s → 4p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. This dopant location explains the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result - the observed remarkable 20% decrease in electrochemical rate performance of the 1% Mn-doped LTO compared to the pristine LTO.

Synthesis and Spectroscopy of Silver-Doped PbSe Quantum Dots

Electronic impurity doping of bulk semiconductors is an essential component of semiconductor science and technology. Yet there are only a handful of studies demonstrating control of electronic impurities in semiconductor nanocrystals. Here, we studied electronic impurity doping of colloidal PbSe quantum dots (QDs) using a postsynthetic cation exchange reaction in which Pb is exchanged for Ag. We found that varying the concentration of dopants exposed to the as-synthesized PbSe QDs controls the extent of exchange. The electronic impurity doped QDs exhibit the fundamental spectroscopic signatures associated with injecting a free charge carrier into a QD under equilibrium conditions, including a bleach of the first exciton transition and the appearance of a quantum-confined, low-energy intraband absorption feature. Photoelectron spectroscopy confirms that Ag acts as a p-type dopant for PbSe QDs and infrared spectroscopy is consistent with k·p calculations of the size-dependent intraband transition energy. We find that to bleach the first exciton transition by an average of 1 carrier per QD requires that approximately 10% of the Pb be replaced by Ag. We hypothesize that the majority of incorporated Ag remains at the QD surface and does not interact with the core electronic states of the QD. Instead, the excess Ag at the surface promotes the incorporation of <1% Ag into the QD core where it causes p-type doping behavior.

Chemically Tailoring the Dopant Emission in Manganese-Doped CsPbCl3 Perovskite Nanocrystals

Doping in perovskite nanocrystals adopts different mechanistic approach in comparison to widely established doping in chalcogenide quantum dots. The fast formation of perovskites makes the dopant insertions more competitive and challenging. Introducing alkylamine hydrochloride (RNH₃ Cl) as a promoting reagent, precise controlled doping of Mn^II in CsPbCl₃ perovskite nanocrystals is reported. Simply, by changing the amount of RNH₃ Cl, the Mn incorporation and subsequent tuning in the excitonic as well as Mn d-d emission intensities are tailored. Investigations suggested that RNH₃ Cl acted as the chlorinating source, controlled the size, and also helps in increasing the number of particles. This provided more opportunity for Mn ions to take part in reaction and occupied the appropriate lattice positions. Carrying out several reactions with varying reaction parameters, the doping conditions are optimized and the role of the promoting reagent for both doped and undoped systems are compared.

Oriented Built-in Electric Field Introduced by Surface Gradient Diffusion Doping for Enhanced Photocatalytic H2 Evolution in CdS Nanorods

Element doping has been extensively attempted to develop visible-light-driven photocatalysts, which introduces impurity levels and enhances light absorption. However, the dopants can also become recombination centers for photogenerated electrons and holes. To address the recombination challenge, we report a gradient phosphorus-doped CdS (CdS-P) homojunction nanostructure, creating an oriented built-in electric-field for efficient extraction of carriers from inside to surface of the photocatalyst. The apparent quantum efficiency (AQY) based on the cocatalyst-free photocatalyst is up to 8.2% at 420 nm while the H₂ evolution rate boosts to 194.3 μmol·h^-1·mg^-1, which is 58.3 times higher than that of pristine CdS. This concept of oriented built-in electric field introduced by surface gradient diffusion doping should provide a new approach to design other types of semiconductor photocatalysts for efficient solar-to-chemical conversion.

Phonon-Plasmon Coupling and Active Cu Dopants in Indium Arsenide Nanocrystals Studied by Resonance Raman Spectroscopy

Doping of semiconductor nanocrystals is an emerging tool to control their properties and has recently received increased interest as the means to characterize the impurities and their effect on the electronic characteristics of the nanocrystal evolve. We present a temperature-dependent Raman scattering study of Cu-doped InAs nanocrystals observing changes in the relative scattering intensities of the different modes upon increased dopant concentrations. First, the longitudinal optical (LO) phonon overtone mode is suppressed, indicating weakening of the coupling strength related to the effect of screening by the free electrons. Second, the transverse optical (TO) mode is relatively enhanced compared to the LO mode, which is attributed to the appearance of a coupled phonon-plasmon mode analogous to observations for n-type doped bulk InAs. These signatures indicate that the Cu impurities serve as active dopants and occupy an impurity-related pseudo sub-band akin to the heavy doping limit.

Engineering Interfacial Charge Transfer in CsPbBr3 Perovskite Nanocrystals by Heterovalent Doping

Since compelling device efficiencies of perovskite solar cells have been achieved, investigative efforts have turned to understand other key challenges in these systems, such as engineering interfacial energy-level alignment and charge transfer (CT). However, these types of studies on perovskite thin-film devices are impeded by the morphological and compositional heterogeneity of the films and their ill-defined surfaces. Here, we use well-defined ligand-protected perovskite nanocrystals (NCs) as model systems to elucidate the role of heterovalent doping on charge-carrier dynamics and energy level alignment at the interface of perovskite NCs with molecular acceptors. More specifically, we develop an in situ doping approach for colloidal CsPbBr₃ perovskite NCs with heterovalent Bi³⁺ ions by hot injection to precisely tune their band structure and excited-state dynamics. This synthetic method allowed us to map the impact of doping on CT from the NCs to different molecular acceptors. Using time-resolved spectroscopy with broadband capability, we clearly demonstrate that CT at the interface of NCs can be tuned and promoted by metal ion doping. We found that doping increases the energy difference between states of the molecular acceptor and the donor moieties, subsequently facilitating the interfacial CT process. This work highlights the key variable components not only for promoting interfacial CT in perovskites, but also for establishing a higher degree of precision and control over the surface and the interface of perovskite molecular acceptors.