Mechanical and electrical properties of CdTe tetrapods studied by atomic force microscopy

Fang, Liang; Park, Jeong Young; Cui, Yi; Alivisatos, Paul; Shcrier, Joshua; Lee, Byounghak; Wang, Lin‐Wang; Salmerón, Miquel

doi:10.1063/1.2786993

Cited by 61 publications

(73 citation statements)

References 17 publications

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“…Nevertheless, some studies have been published concerning the controlled deposition of tetrapods on substrates. [8,9] It would be desirable on the other hand to realize complex 3D networks of branched nanocrystals joined to each other via their tips, as this might lead to materials with controlled porosity, mechanical and electronic properties; such materials might even be used as scaffolds.…”

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confidence: 99%

End‐to‐End Assembly of Shape‐Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains

et al. 2009

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Welding nanocrystals for assembly: The welding of Au domains grown on the tips of shape-controlled cadmium chalcogenide colloidal nanocrystals is used as a strategy for their assembly. Iodine-induced coagulation of selectively grown Au domains leads to assemblies such as flowerlike structures based on bullet-shaped nanocrystals, linear and cross-linked chains of nanorods, and globular networks with tetrapods as building blocks

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confidence: 99%

End‐to‐End Assembly of Shape‐Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains

et al. 2009

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“…1A). Atomic force microscopy studies on CdTe tetrapods demonstrated that nanonewton forces are capable of bending these lever arms (11). Furthermore, electronic level structure calculations on CdSe tetrapods predicted that force-induced arm bending, which causes a strain in the nanocrystal, results in a red shift of the energy gap (12).…”

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confidence: 99%

Luminescent nanocrystal stress gauge

Choi

Koski²,

Olson³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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Microscale mechanical forces can determine important outcomes ranging from the site of material fracture to stem cell fate. However, local stresses in a vast majority of systems cannot be measured due to the limitations of current techniques. In this work, we present the design and implementation of the CdSe-CdS core-shell tetrapod nanocrystal, a local stress sensor with bright luminescence readout. We calibrate the tetrapod luminescence response to stress and use the luminescence signal to report the spatial distribution of local stresses in single polyester fibers under uniaxial strain. The bright stress-dependent emission of the tetrapod, its nanoscale size, and its colloidal nature provide a unique tool that may be incorporated into a variety of micromechanical systems including materials and biological samples to quantify local stresses with high spatial resolution.nanoscience | semiconductor nanocrystals | fluorescence | luminescent stress gauge L ocal microscale stresses play a crucial role in inhomogeneous mechanical processes from cell motility to material failure. Stress is a tensor representing force per unit area and is directly related to strain-a tensor that represents change in size and/or shape-via the stiffness constants of a material. Contact-probe techniques that measure stiffness such as atomic force microscopy (1, 2), indentation testing (3), and optical coherence elastography (4), and noncontact techniques that measure stress such as micro-Raman spectroscopy (5), electron backscatter diffraction (6), and polymeric post arrays (7) have been used to quantify local mechanical behavior with high spatial resolution. However, these techniques remain limited to studies in specific material systems due to spectroscopic and geometric constraints. For example, although the mechanical behavior of cells can be indicative of significant aspects of their biology, including metastatic potential (8), the stresses exerted by cells in physiologic threedimensional culture systems cannot currently be quantified. A luminescent nanocrystal probe, with its small size, bright and narrow emission, and colloidal processability is ideally suited to measure local stresses in a variety of systems without spectroscopic requirements from or excessive perturbations to the material of interest.We present here the design and implementation of a luminescent nanocrystal stress gauge, the CdSe-CdS core-shell tetrapod. The tetrapod, with a CdSe quantum dot at its core, has the same advantages as its widely used quantum dot predecessor, including tunable quantum confinement and high fluorescence quantum yields (9). Four CdS arms protruding from the CdSe core confer a branched, three-dimensional structure on the tetrapod; these arms can act as dynamic levers to torque the CdSe core and alter its optical response. The tetrapod can be incorporated into many materials, yielding a local stress measurement through optical fluorescence spectroscopy of the electronically confined CdSe core states. In this report, we calibrate the stress...

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“…We estimate that a 1.4-nm CdSe nanosheet has a yield strength of 4.4-6.8 GPa, which matches well the theoretical strength of 5.3 GPa. [Experiments show that 200-to 450-nm CdS nanocrystals and 15-nm CdTe tetrapods have an experimental yield strength of 2.2 and 2.6 GPa (35,36), and their ideal yield strengths are estimated as 4.6 GPa and 6.0 GPa by the Griffith standard of ∼E∕10 (E, Young's modulus). Extrapolating these results to CdSe, CdSe should have a yield strength close to 2.4 GPa (2.2-2.6 GPa).…”

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confidence: 99%

“…However, fracture was observed at lower pressure in large-scale two-dimensional CdSe and ZnS nanocrystals without surface-ligand bonding (32)(33)(34). Similarities in structure and properties of several related semiconductors allow one to correlate CdS and CdTe, and to estimate the elastic strengths of CdSe (35,36). We estimate that a 1.4-nm CdSe nanosheet has a yield strength of 4.4-6.8 GPa, which matches well the theoretical strength of 5.3 GPa.…”

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confidence: 99%

Reconstructing a solid-solid phase transformation pathway in CdSe nanosheets with associated soft ligands

Wang

Wen

Hoffmann

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

128

110

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Integrated single-crystal-like small and wide-angle X-ray diffraction images of a CdSe nanosheet under pressure provide direct experimental evidence for the detailed pathway of transformation of the CdSe from a wurtzite to a rock-salt structure. Two consecutive planar atomic slips [(001) h110i in parallel and (102) h101i with a distortion angle of ∼40°] convert the wurtzite-based nanosheet into a saw-like rock-salt nanolayer. The transformation pressure is three times that in the bulk CdSe crystal. Theoretical calculations are in accord with the mechanism and the change in transformation pressure, and point to the critical role of the coordinated amines. Soft ligands not only increase the stability of the wurtzite structure, but also improve its elastic strength and fracture toughness. A ligand extension of 2.3 nm appears to be the critical dimension for a turning point in stress distribution, leading to the formation of wurtzite (001)/zinc-blende (111) stacking faults before rock-salt nucleation.phase transition | semiconductor | dimensionality R ational synthesis of materials with defined structure and properties requires a reasonably complete understanding of the associated kinetic transformations and microscopic mechanisms (1). We need to know these in order to understand how associated soft-bonded organics may tune structural stability, elastic strength, and fracture toughness (1-3). The study of solid-solid phase transformations of materials, and their structural stabilization upon incorporation of soft materials at different scales, can be very helpful in this regard. For instance, application of pressure and temperature converts graphite to a hard diamond phase that persists at ambient conditions, allowing for a wide range of known applications (4). Upon addition of soft materials such as silicon and metals (5, 6), the sintered diamond-dominant nanocomposites show improved yield strength and fracture toughness (by several orders of magnitude) without significant diminution of hardness.Many fascinating natural substances, such as human bone and other biological materials, also contain exceptionally strong building blocks (2, 3) essential to their function. The subtle interplay between soft organics and embedded brittle materials is responsible not only for enhancement of the structural stability of embedded brittle materials, but also for improvement of the yield strength and fracture toughness of assembled hierarchical organizations (2, 3).Microscopic mechanisms of transformation are difficult to determine. In bulk solids, as metastable higher energy and higher density phases nucleate in multiple domains, structural features (such as defects, microstructure, and impurities) may modify significantly the kinetics and mechanism of a phase transformation. In principle, calculations could provide the energy difference between structural polymorphs at a given pressure or temperature. But putting aside problems in computing activation barriers, there is often a large discrepancy between theory and experiment. F...

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Mechanical and electrical properties of CdTe tetrapods studied by atomic force microscopy

Cited by 61 publications

References 17 publications

End‐to‐End Assembly of Shape‐Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains

End‐to‐End Assembly of Shape‐Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains

Luminescent nanocrystal stress gauge

Reconstructing a solid-solid phase transformation pathway in CdSe nanosheets with associated soft ligands

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