Quantized Electronic Doping towards Atomically Controlled “Charge-Engineered” Semiconductor Nanocrystals (original) (raw)
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ACS nano, 2017
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 w...
2014
The size dependence of the quantized energies of elementary excitations is an essential feature of quantum nanostructures, underlying most of their applications in science and technology. Here we report on a fundamental property of impurity states in semiconductor nanocrystals that appears to have been overlooked-the anticrossing of energy levels exhibiting different size dependencies. We show that this property is inherent to the energy spectra of charge carriers whose spatial motion is simultaneously affected by the Coulomb potential of the impurity ion and the confining potential of the nanocrystal. The coupling of impurity states, which leads to the anticrossing, can be induced by interactions with elementary excitations residing inside the nanocrystal or an external electromagnetic field. We formulate physical conditions that allow a straightforward interpretation of level anticrossings in the nanocrystal energy spectrum and an accurate estimation of the states' coupling strength.
Level Anticrossing of Impurity States in Semiconductor Nanocrystals
Scientific Reports, 2014
The size dependence of the quantized energies of elementary excitations is an essential feature of quantum nanostructures, underlying most of their applications in science and technology. Here we report on a fundamental property of impurity states in semiconductor nanocrystals that appears to have been overlooked-the anticrossing of energy levels exhibiting different size dependencies. We show that this property is inherent to the energy spectra of charge carriers whose spatial motion is simultaneously affected by the Coulomb potential of the impurity ion and the confining potential of the nanocrystal. The coupling of impurity states, which leads to the anticrossing, can be induced by interactions with elementary excitations residing inside the nanocrystal or an external electromagnetic field. We formulate physical conditions that allow a straightforward interpretation of level anticrossings in the nanocrystal energy spectrum and an accurate estimation of the states' coupling strength.
Multifunctional Doped Semiconductor Nanocrystals
The Journal of Physical Chemistry Letters, 2012
Multifunctional nanomaterials with combined magnetic and optical properties remain one of the most demanded materials in upcoming research. To obtain these materials, we report here several doped semiconductor nanocrystals that simultaneously show tunable emission in a visible and NIR spectral window, aboveroom-temperature ferromagnetism, and improved conductivity. These nanocrystals are designed by inserting Ni(II) as a dopant in various semiconducting hosts with binary, alloyed, and ternary composition, and the induced multifunctional properties are observed to be stable and reproducible. These semiconducting materials combined with fluorescence and magnetic properties would be useful for a wide range of applications spanning from life science to modern developing device technology.
Chemistry of Materials
The efficiency of nanocrystal (NC)-based devices is often limited by the presence of surface states that lead to localized energy levels in the bandgap. Yet, a complete understanding of the nature of these traps remains challenging. Although theoretical modeling has greatly improved our comprehension of the NC surface, several experimental studies suggest the existence of metal-based traps that have not yet been found with theoretical methods. Since there are indications that these metal-based traps form in the presence of excess electrons, the present work uses density functional theory (DFT) calculations to study the effects of charging II−VI semiconductor NCs with either full or imperfect surface passivation. It is found that charge injection can lead to trap-formation via two pathways: metal atom ejection from perfectly passivated NCs or metal− metal dimer-formation in imperfectly passivated NCs. Fully passivated CdTe NCs are observed to be stable up to a charge of two electrons. Further reduction leads to charge localization on a surface Cd atom and the formation of in-gap states. The effects of suboptimal passivation are probed by charging NCs where an X-type ligand is removed from the (100) plane. In this case, injection of even one electron leads to Cd-dimerization and trap-formation. Addition of an L-type amine ligand prevents this dimer-formation and is suggested to also prevent trapping of photoexcited electrons in charge neutral NCs. The results presented in this work are generalized to NCs of different sizes and other II−VI semiconductors. This has clear implications for n-doping II−VI semiconductor NCs without introducing surface traps due to metal ion reduction. The possible effect of these metal ion localized traps on the photoluminescence efficiency of neutral NCs is also discussed.
Chemistry of Materials, 2019
The efficiency of nanocrystal (NC)-based devices is often limited by the presence of surface states that lead to localized energy levels in the bandgap. Yet, a complete understanding of the nature of these traps remains challenging. Although theoretical modeling has greatly improved our comprehension of the NC surface, several experimental studies suggest the existence of metal-based traps that have not yet been found with theoretical methods. Since there are indications that these metal-based traps form in the presence of excess electrons, the present work uses density functional theory (DFT) calculations to study the effects of charging II−VI semiconductor NCs with either full or imperfect surface passivation. It is found that charge injection can lead to trap-formation via two pathways: metal atom ejection from perfectly passivated NCs or metal− metal dimer-formation in imperfectly passivated NCs. Fully passivated CdTe NCs are observed to be stable up to a charge of two electrons. Further reduction leads to charge localization on a surface Cd atom and the formation of in-gap states. The effects of suboptimal passivation are probed by charging NCs where an X-type ligand is removed from the (100) plane. In this case, injection of even one electron leads to Cd-dimerization and trap-formation. Addition of an L-type amine ligand prevents this dimer-formation and is suggested to also prevent trapping of photoexcited electrons in charge neutral NCs. The results presented in this work are generalized to NCs of different sizes and other II−VI semiconductors. This has clear implications for n-doping II−VI semiconductor NCs without introducing surface traps due to metal ion reduction. The possible effect of these metal ion localized traps on the photoluminescence efficiency of neutral NCs is also discussed.
Nanoscale, 2010
Relativistic density functional theory calculations have been conducted to examine the effect of atomic under-coordination on the crystal structure, binding energy, and electron configuration of cuboctahedral and Marks decahedral gold clusters. Trend consistency between calculations and experimental observations confirmed the predictions made using BOLS correlation theory, suggesting that the shorter-and-stronger bonds between under-coordinated atoms induce local structure relaxation, potential well depression, and the associated local charge and energy densification, as well as the polarization of the otherwise conducting s-electrons (valence charge) by the densely-and tightlytrapped core electrons of which the binding energy shifts positively to deeper energies. Findings are in good agreement with scanning tunneling microscopy/spectroscopy results from monomers, dimers, chain ends, and nanostructures of gold and other metals.
Gold Doping of Silver Nanoclusters: A 26-Fold Enhancement in the Luminescence Quantum Yield
Angewandte Chemie (International ed. in English), 2016
A high quantum yield (QY) of photoluminescence (PL) in nanomaterials is necessary for a wide range of applications. Unfortunately, the weak PL and moderate stability of atomically precise silver nanoclusters (NCs) suppress their utility. Herein, we accomplished a ≥26-fold PL QY enhancement of the Ag29 (BDT)12 (TPP)4 cluster (BDT: 1,3-benzenedithiol; TPP: triphenylphosphine) by doping with a discrete number of Au atoms, producing Ag29-x Aux (BDT)12 (TPP)4 , x=1-5. The Au-doped clusters exhibit an enhanced stability and an intense red emission around 660 nm. Single-crystal XRD, mass spectrometry, optical, and NMR spectroscopy shed light on the PL enhancement mechanism and the probable locations of the Au dopants within the cluster.
Journal of the American Chemical Society, 2013
In this work, we synthesized InP and InAs nanocrystals (NCs) capped with different inorganic ligands, including various molecular metal chalcogenide complexes (MCCs) and chalcogenide ions. We found that MCCs and chalcogenide ions can quantitatively displace organic ligands from the surface of III−V NCs and serve as the inorganic capping groups for III−V NC surfaces. These inorganic ligands stabilize colloidal solutions of InP and InAs NCs in polar solvents and greatly facilitate charge transport between individual NCs. Charge transport studies revealed high electron mobility in the films of MCC-capped InP and InAs NCs. For example, we found that bridging InAs NCs with Cu 7 S 4 − MCC ligands can lead to very high electron mobility exceeding 15 cm 2 /(V s). In addition, we observed unprecedented ambipolar (positive/negative) photoresponse of MCC-capped InAs NC solids that changed sign depending on the ligand chemistry, illumination wavelength, and doping of the NC solid. For example, the sign of photoconductance of InAs NCs capped with Cu 7 S 4 − or Sn 2 S 6 4− ions converted from positive at 0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation of this unusually complex photoconductivity of InAs NC solids.