Strain-Dependent Photoluminescence Behavior of CdSe/CdS Nanocrystals with Spherical, Linear, and Branched Topologies

Abstract

In recent years, a new generation of quantum confined colloidal semiconductor structures has emerged, with more complex shapes than simple quantum dots 1, 2 . These include nanorods 3 and tetrapods 4 . Beyond shape, it is also now possible to spatially vary the electron and hole potentials within these nanoparticles by varying the composition. Examples of these new structures include seeded dots, rods, and tetrapods, which contain a CdSe core embedded within a CdS shell [5][6][7][8] . These structures may have many uses beyond those envisioned for simple quantum dots, which are frequently employed in luminescent applications 9 . This paper is concerned with changes in the optoelectronic properties of tetrapods when the arms are bent. We demonstrate that seeded tetrapods can serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications ranging from sensitive optomechanical devices to biological force investigations.A tetrapod nanocrystal consists of a central core with four arms branching out at the tetrahedral angle 4 . We have previously shown a few ways in which tetrapod arms can be bent. For instance, when a CdTe tetrapod (4 nm wide and 100 nm long arms) is deposited on a substrate through solvent evaporation, the fluid exerts a capillary force which pulls the tetrapod towards the substrate, in some cases permanently deforming the arms 10 . Salmeron and coworkers used an atomic force microscope to press on the outward-projecting arms of surface-immobilized CdTe tetrapods, and have shown that for forces below 100 nN, the tetrapod flexes elastically 11 . Motivated by these observations, Wang and coworkers calculated the electronic level structure of a CdSe tetrapod with different degrees of arm bending, induced by nanonewton forces, and predicted a red-shift of the energy gap with increasing strain 12 .Seeded tetrapods consisting of a CdSe core with CdS arms are highly luminescent 6 and are very symmetric objects. When placed under an anisotropic stress, we expect a reduction in symmetry, which will influence the electronic level structure. To fully quantify such effects, we have examined a series of samples under diverse conditions of stress and strain. Specifically, we have examined the luminescence from seeded dots, rods, and tetrapods placed in a diamond anvil cell (DAC) as a function of applied pressure in a highly hydrostatic medium, which transmits pressure nearly isotropically, as well as a non-hydrostatic medium, which transmits pressure anisotropically. We compare the tetrapods to rods to separate out stress-induced strain effects in a single rod from effects specifically arising in a tetrapod consisting of connected *apalivisatos@lbl.gov. Supporting Information Available. Experimental methods for nanocrystal characterization, diamond anvil cell studies, and spectral analysis; emission peak maxima for CdSe/CdS rods of different lengths in toluene in a diamond anvil cell (Fig....

Outermost tensile strain dominated exciton emission in bending CdSe nanowires

“…The energy band structures of CdSe crystal under elastic strain effect have been widely investigated [26][27][28][29]. It has been demonstrated that compressive and tensile strains will result in wi dening and narrowing bandgap in CdSe crystal, respectively.…”

Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory

“…4 Attaining such control at the nanoscale holds the promise of novel technological applications such as tunable photovoltaic devices, shock-absorbers, 7 and nanoscale stress sensors. 8,9 The additional surface effects also open the door to transformation pathways that are not available to the bulk material, potentially allowing a system to become trapped in metastable states with novel properties. [10][11][12] Moreover, sufficiently small nanocrystals can be synthesized with few or no defects and are thus ideal models to study the kinetics of solid-solid phase transitions.…”

Nanoheterostructure Cation Exchange: Anionic Framework Conservation

“…For instance, backexchange of the Cu 2 Se/Cu 2 S structure could result in a strained CdSe/CdS interface, especially in the case of the larger seed, leading to a blue-shift in the recovered PL. 17,23 In addition, the 3.9-nm seed is exposed to the surface/ligand environment, making it sensitive to dielectric changes in the ligand shell and increase in surface traps resulting from the cation exchange process, 1 possibly …”