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

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 NaYF₄ Nanocrystals using Synchrotron-Based Energy-Dependent X‑ray Photoelectron Spectroscopy

Understanding the structure and chemical speciation of the synthesized lanthanide-doped NaYF₄ 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 NaYF₄ nanocrystals. These measurements revealed that there are two doublets for the yttrium ions in the nanocrystal instead of the single doublet in case all Y³⁺ ions have the same chemical environment, generally observed around a binding energy of 160 eV (3d photoelectrons of Y³⁺). 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 NaYF₄ 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 NaYF₄ nanocrystal. To confirm our hypothesis, a shell of NaYbF₄ or NaTmF₄ was grown over the NaYF₄ 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 NaYF₄ 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 NaYF₄ 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