Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures

McBride, James R.; Treadway, Joseph A.; Feldman, L. C.; Pennycook, Stephen J.; Rosenthal, Sandra J.

doi:10.1021/nl060993k

Cited by 222 publications

(255 citation statements)

References 26 publications

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“…Only isolated ZnS islands could be grown on the surface of CdSe cores. 56 For this reason, we decided to investigate shell materials that can adopt a cubic, zinc blende structure with a similar lattice parameter to that of cubic, diamond Ge. Several IIÀVI and IIIÀV semiconductors are available for this purpose (Table 1).…”

Section: Resultsmentioning

confidence: 99%

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

et al. 2014

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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, watersoluble 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. E lemental germanium (Ge) is a relatively abundant and robust covalent semiconductor. 1 Ge has a small indirect band gap (0.661 eV or 1876 nm) and a large Bohr radius (24 nm), which together theoretically provide for a wide range of emission energies attainable via size-tunable quantum confinement. 2À4 Further, recent reports suggest that strain 5À7 and doping strategies 8 may result in direct band gap Ge nanostructures. As such, Ge is particularly interesting in the development of near-infrared (near-IR) active quantum dot fluorophores for applications in biology (imaging and tracking), telecommunications, and energy conversion (photovoltaics, photocatalysis). ABSTRACT Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or

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Section: Resultsmentioning

confidence: 99%

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

et al. 2014

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“…The growth of the CdSe tips leads to a new absorption feature near 585 nm and the corresponding PL from CdSe band edge. The double-peaked PL spectrum of CdS/CdSe nanorod heterostructures arises from a bimodal size distribution of CdSe presumably owing to the differentiation of growth rates on the two ends of the seed CdS nanorod 30,40,41 (see Supplementary Fig. 4).…”

Section: Challenges In Synthesismentioning

confidence: 99%

Double-heterojunction nanorods

et al. 2014

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As semiconductor heterostructures play critical roles in today's electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution-processable colloidal quantum dots in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising, since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here we develop doubleheterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap material. The double heterojunction can provide independent control over the electron and hole injection/ extraction processes while maintaining high photoluminescence yields. Light-emitting diodes utilizing double-heterojunction nanorods as the electroluminescent layer are demonstrated with low threshold voltage, narrow bandwidth and high efficiencies.

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“…However, large red shifts by as much as 130 nm have been observed upon capping CdTe QDs with a semiconductor shell 205 and, therefore, it is unclear whether the position of the absorption peak can be used to determine the cross section reliably. Further, these QDs are highly anisotropic and exhibit a characteristic bullet shape, 206 which complicates the direct determination of the QD diameter based on simple empirical formulas.…”

Section: Absorption Cross Section Of Qdsmentioning

confidence: 99%

Photophysics of Individual Single-Walled Carbon Nanotubes

2008

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Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not “blink” or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300–500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50–100 ps) that corresponded to electron–hole recombination. As the excitation intensity was increased, multiple electron–hole pairs were generated in the SWNT; however, these e−h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e−h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron–hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area o...

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Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures

Cited by 222 publications

References 26 publications

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

Double-heterojunction nanorods

Photophysics of Individual Single-Walled Carbon Nanotubes

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