45 research outputs found
Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS core/shell nanocrystals: Utilizing IV/II-VI semiconductor epitaxy
Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots. Here, we use relatively unexplored IV/II-VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II-VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II-VI nanocrystals are reproducibly 1-3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II-VI nanocrystals. We expect this synthetic IV/II-VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials
Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy
Semiconductor Nanocrystal Quantum Dot Synthesis Approaches Towards Large-Scale Industrial Production for Energy Applications
A Dual‐Excitation Decoding Strategy Based on NIR Hybrid Nanocomposites for High‐Accuracy Thermal Sensing
Crystal structures of three sterically congested disilanes
In the three sterically congested silanes, C24H38Si2 (1) (1,1,2,2-tetraisopropyl-1,2-diphenyldisilane), C24H34Br4Si2 (2) [1,1,2,2-tetrakis(2-bromopropan-2-yl)-1,2-diphenyldisilane] and C32H38Si2 (3) (1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane), the Si—Si bond length is shortest in (1) and longest in (2), with (3) having an intermediate value, which parallels the increasing steric congestion. A comparison of the two isopropyl derivatives, (1 and 2), shows a significant increase in the Si—C(ipso) distance with the introduction of bromine. Also, in the brominated compound 2, attractive intermolecular Br...Br interactions exist with Br...Br separations ca 0.52 Å shorter than the sum of the van der Waals radii. In compound 2, one of the bromoisopropyl groups is rotationally disordered in an 0.8812 (9):0.1188 (9) ratio. Compound 3 exhibits `whole molecule' disorder in a 0.9645 (7):0.0355 (7) ratio with the Si—Si bonds in the two components making an angle of ca 66°
Long-Term Colloidal Stability and Photoluminescence Retention of Lead-Based Quantum Dots in Saline Buffers and Biological Media through Surface Modification
Lead-based
quantum dots (QDs) can be tuned to emit in the transparent region
of the biological tissue (700 to 1100 nm) which make them a potential
candidate for optical bioimaging. However, to employ these QDs as
biolabels they have to retain their luminescence and maintain their
colloidal stability in water, physiological saline buffers, different
pH values, and biological media. To achieve this, four different surface
modification strategies were tried: (1) silica coating; (2) ligand
exchange with polyvinylpyrrolidone; (3) polyethyleneglycol-oleate
(PEG-oleate) intercalation into the oleate ligands on the surface
of the QDs; and (4) intercalation of poly(maleicanhydride-<i>alt</i>-1-octadecene) (PMAO) into the oleate ligands on the
surface of the QDs and further cross-linking of the PMAO. The first
two methods exhibited excellent dispersion stability in water, but
did not retain their photoluminescence. On the other hand, the intercalation
strategy with PEG-oleate helped the QDs retain their luminescence
but with poor colloidal stability in water. The fourth and final strategy
involving intercalation and cross-linking of the amphiphilic polymer
PMAO provided the QDs with colloidal stability in water but also resulted
in the QDs retaining their luminescence as well. This process involved
two steps; (1) the intercalation between octadecene chains of PMAO
with the oleates on the surface of the QDs with some of the anhydride
rings opened with PEG-amine; (2) the anhydride rings were cross-linked
with bis(hexamethylene)triamine (BHMT) to avoid detachment of the
polymer from the surface of QDs because of the polymer’s dynamic
nature in solvents. The presence of PEG molecules potentially improves
the biocompatibility of the QDs and the presence of carboxylic acids
after reaction with BHMT makes them suitable for further surface functionalization
with antibodies, proteins, and so forth. The surface-modified QDs
have excellent dispersibility in water, saline buffers, and in various
pH conditions for more than 7 months and more than 20 days in serum-supplemented
growth media. In addition to the colloidal stability, the QDs retained
their photoluminescence even after 7 months in the aforementioned
aqueous media. The intercalation and cross-linking process have also
made the QDs resistant to oxidation when exposed to ambient atmosphere
and aqueous media
Probing the Structure of NaYF<sub>4</sub> Nanocrystals using Synchrotron-Based Energy-Dependent X‑ray Photoelectron Spectroscopy
Understanding the structure and chemical
speciation of the synthesized
lanthanide-doped NaYF<sub>4</sub> nanocrystals is of paramount importance
to improve and optimize their physical and chemical properties. Hence
in this work we employ synchrotron-based high-resolution X-ray photoelectron
spectroscopy (XPS) measurements to analyze lanthanide-doped and undoped
NaYF<sub>4</sub> nanocrystals. These measurements revealed that there
are two doublets for the yttrium ions in the nanocrystal instead of
the single doublet in case all Y<sup>3+</sup> ions have the same chemical
environment, generally observed around a binding energy of 160 eV
(3d photoelectrons of Y<sup>3+</sup>). This second doublet (binding
energy ∼ 157.5 eV) was convoluted with the first doublet (binding
energy ∼ 160 eV), and the intensity of this doublet increased
with a decrease in excitation X-ray energy. The second doublet was
confirmed to belong to the yttrium ions as doped and undoped NaYF<sub>4</sub> nanocrystals exhibit this second peak. The peaks were deconvoluted
showing that the second peak is also a doublet with the ratio of the
peaks being 2:3. This is exactly the same as what we have observed
for the first doublet of the 3d photoelectrons of yttrium ions. In
addition, we observe an increase in intensity of the second doublet
in comparison to the original 3d doublet of the yttrium ions as the
excitation energy is decreased. This suggests that the second doublet
is from surface yttrium ions in the NaYF<sub>4</sub> nanocrystal.
To confirm our hypothesis, a shell of NaYbF<sub>4</sub> or NaTmF<sub>4</sub> was grown over the NaYF<sub>4</sub> nanocrystal and the second
doublet for the yttrium ions was not observed. This is an additional
confirmation that the second doublet is indeed from the surface yttrium
ions. This implies that the yttriums on the surface of the nanocrystals
have a (slightly) different chemical speciation than their counterparts
inside the nanocrystals. We attribute the new chemical speciation
of surface yttrium ions to the different chemical environment they
encounter than their counterparts inside the nanocrystal
An Effective Polymer Cross-Linking Strategy To Obtain Stable Dispersions of Upconverting NaYF<sub>4</sub> Nanoparticles in Buffers and Biological Growth Media for Biolabeling Applications
Ligands on the nanoparticle surface provide steric stabilization,
resulting in good dispersion stability. However, because of their
highly dynamic nature, they can be replaced irreversibly in buffers
and biological medium, leading to poor colloidal stability. To overcome
this, we report a simple and effective cross-linking methodology to
transfer oleate-stabilized upconverting NaYF<sub>4</sub> core/shell
nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term
dispersion stability in buffers and biological medium. Amphiphilic
poly(maleic anhydride-<i>alt</i>-1-octadecene) (PMAO) modified
with and without poly(ethylene glycol) (PEG) was used to intercalate
with the surface oleates, enabling the transfer of the UCNPs to water.
The PMAO units on the phase transferred UCNPs were then successfully
cross-linked using bis(hexamethylene)triamine (BHMT). The primary
advantage of cross-linking of PMAO by BHMT is that it improves the
stability of the UCNPs in water, physiological saline buffers, and
biological growth media and in a wide range of pH values when compared
to un-cross-linked PMAO. The cross-linked PMAO–BHMT coated
UCNPs were found to be stable in water for more than 2 months and
in physiological saline buffers for weeks, substantiating the effectiveness
of cross-linking in providing high dispersion stability. The PMAO–BHMT
cross-linked UCNPs were extensively characterized using various techniques
providing supporting evidence for the cross-linking process. These
UCNPs were found to be stable in serum supplemented growth medium
(37 °C) for more than 2 days. Utilizing this, we demonstrate
the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer
cell line), showing their utility as biolabels