6 research outputs found
Rainbow Emission from an Atomic Transition in Doped Quantum Dots
Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow–orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future
Rainbow Emission from an Atomic Transition in Doped Quantum Dots
Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow–orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future
Strain-Induced Hierarchy of Energy Levels in CdS/ZnS Nanocrystals
Aside of size and shape, the strain
induced by the mismatch of lattice parameters between core and shell
in the nanocrystalline regime is an additional degree of freedom to
engineer the electron energy levels. Herein, CdS/ZnS core/shell nanocrystals
(NCs) with shell thickness up to four monolayers are studied. As a
manifestation of strain, the low temperature radiative lifetime measurements
indicate a reduction in Stokes shift from 36 meV for CdS to 5 meV
for CdS/ZnS with four monolayers of overcoating. Concomitant crossover
of S- and P-symmetric hole levels is observed which can be understood
in the framework of theoretical calculations predicting flipping the
hierarchy of ground hole state by the strain in CdS NCs. Furthermore,
a nonmonotonic variation of higher energy levels in strained CdS NCs
is discussed
First-Principles Study of the Effect of Organic Ligands on the Crystal Structure of CdS Nanoparticles
We show with the aid of first-principles electronic structure
calculations that suitable choice of the capping ligands may be an
important control parameter for crystal structure engineering of nanoparticles.
Our calculations on CdS nanocrystals reveal that the binding energy
of model trioctylphosphine molecules on the (001) facets of zincblende
nanocrystals is larger compared to that on wurtzite facets. Similarly,
the binding energy of model <i>cis</i>-oleic acid is found
to be dominant for the (101̅0) facets of wurtzite structure.
As a consequence, trioctylphosphine as a capping agent stabilizes
the zincblende structure while <i>cis</i>-oleic acid stabilizes
the wurtzite phase by influencing the surface energy, which has a
sizable contribution to the energetics of a nanocrystal. Our detailed
analysis suggests that the binding of molecules on the nanocrystalline
facets depends on the surface topology of the facets, the coordination
of the surface atoms where the capping molecule is likely to attach,
and the conformation of the capping molecule
Crystal Structure Engineering by Fine-Tuning the Surface Energy: The Case of CdE (E = S/Se) Nanocrystals
We prove that CdS nanocrystals can be thermodynamically stabilized in both wurtzite and zinc-blende crystallographic phases at will, just by the proper choice of the capping ligand. As a striking demonstration of this, the largest CdS nanocrystals (∼15 nm diameter) ever formed with the zinc-blende structure have been synthesized at a high reaction temperature of 310 °C, in contrast to previous reports suggesting the formation of zinc-blende CdS only in the small size limit (<4.5 nm) or at a lower reaction temperature (≤240 °C). Theoretical analysis establishes that the binding energy of trioctylphosphine molecules on the (001) surface of zinc-blende CdS is significantly larger than that for any of the wurtzite planes. Consequently, trioctylphosphine as a capping agent stabilizes the zinc-blende phase via influencing the surface energy that plays an important role in the overall energetics of a nanocrystal. Besides achieving giant zinc-blende CdS nanocrystals, this new understanding allows us to prepare CdSe and CdSe/CdS core/shell nanocrystals in the zinc-blende structure
Determination of Internal Structures of Heterogeneous Nanocrystals Using Variable-Energy Photoemission Spectroscopy
This
article describes the determination of the internal structure
of heterogeneous nanoparticle systems including inverted core–shell
(CdS core and CdSe shell) and alloyed (CdSeS) quantum dots using depth-resolved,
variable-energy X-ray photoelectron spectroscopy (XPS). A unique feature
of this work is the combination of photoelectron spectroscopy performed
at lower X-ray energies (400–700 eV), to achieve surface sensitivity,
with bulk sensitive measurements at high photon energies (>2000
eV),
thereby providing detailed information about the whole nanoparticle
structure with a great accuracy. The use of high photon energies furthermore
allows us to investigate nanoparticles much larger than those studied
thus far. This capability is a consequence of the much-increased mean
free path of the photoelectron achieved at high excitation energies.
Our results show that the actual structures of the synthesized nanoparticles
are considerably different from the nominal, targeted structures,
which can be post facto rationalized in terms of the reactivity of
different constituents