Uniaxial Strain Engineering via Core Position Control in CdSe/CdS Core/Shell Nanorods and Their Optical Response

Abstract

Anisotropic strain engineering has emerged as a powerful strategy for enhancing the optoelectronic performance of semiconductor nanocrystals. Here, we show that CdSe/CdS dot-in-rod structures offer a platform for fine-tuning the optical response of CdSe quantum dots through anisotropic strain. By controlling the spatial position of the CdSe core within a growing CdS nanorod shell, varying degrees of uniaxial strain can be introduced. Placing CdSe cores at the end of the CdS nanorod induces strong asymmetric compression along the c-axis of the wurtzite CdSe core, dramatically altering its absorption and emission characteristics, whereas CdSe cores located near the middle of the nanorod experience a comparatively weak uniaxial strain field. The change in absorption and emission spectra and dynamics for highly strained end-position CdSe/CdS nanorods is explained by (1) relative shifting of the valence band light hole and heavy hole levels and (2) introduction of a strong piezoelectric potential, which spatially separates the electron and hole wave functions. The ability to tune the degree of uniaxial strain through core position control in a nanorod structure creates opportunities for precisely modulating the electronic properties of CdSe nanocrystals while simultaneously taking advantage of dielectric and optical anisotropies intrinsic to 1D nanostructures.

Design Principles of Colloidal Nanorod Heterostructures

“…In addition to the well-known size-tunable optical properties of colloidal quantum dots, NRHs can impart multiple dimensions to tailoring materials’ properties. Reduced symmetry can bring about optical anisotropy, polarized light emission, ,, and lifting of energy state degeneracy. , It also leads to directionality useful for charge separation and extraction. Diameter modulation along the length of nanorods, which may be achieved through ligand choice as in the dual-diameter dot-in-rods, through tapering effect in Cu 2– x S-mediate growth, or through graded alloying, can provide added advantages in directing charges or excitons.…”

“…That is, the crystal structure and the ligand binding affinity determine the surface energies of terminating facets for both the seed and the growing phases, which in turn vary the likelihood of certain facets undergoing heterogeneous nucleation and growth over others, imparting regioselectivity. Lattice strain can then dictate the morphology of the growing phase and, in some cases, even alter that of the seed. − …”

Synthesis and Assembly of Core–Shell Nanorods with High Quantum Yield and Linear Polarization

“…Therefore, both dot-in-rod and rod-in-rod core-shell NRs have been studied. [24][25][26][27][28][29][30][31] Although the degree of polarization of rod-in-rod NRs is up to 1.5 times higher than that of dot-in-rod NRs, [14] most studies have focused on dot-in-rod core-shell NRs due to the difficulties to obtain uniform CdSe NRs. [13,17,22,[32][33][34][35] In addition, although CdSe/CdS core-shell NRs have been widely studied, [14,24,27,28,[36][37][38] the main drawback with these systems is that the quasi type-II band alignment between CdSe and CdS leads to the delocalization of the electrons in both the CdSe core and the CdS shell, which significantly limits the performance of core-shell NRs in luminescent devices.…”

Spectroelectrochemical Characterization of Energetics in Type I vs Quasi-Type II Heterojunctions in CdSe@CdS Nanorod Films

“…For core–shell QDs and core–shell NRs, the size of the interior “seed” generally controls the energetic offsets in both E CB and E VB across the core–shell interface, and E CB and E VB are tuned by changes in seed diameter to create either Type I or Type II (or “quasi-Type II”) heterojunctions. ,,,,,, There are multiple variations on the choices of partner materials in these core–shell constructs, including small seeds in otherwise uniform diameter nanorods, ,,,,,,− larger seeds that form “bulbs” in these same nanorod materials, , core–shell nanomaterials with graded compositions and shapes as a function of length away from the seed location, decoration of the tips and body of the nanorod or tetrapod with catalytic (metal) sites, and even variations in the nature of the interfaces between the core and the shell semiconductors that range from atomically sharp to somewhat disordered. , In Type I heterojunctions, E CB / E VB levels in the seed are typically bracketed by these same energy levels in the shell (or rod) material, so that illumination above the band gap energy, E BG , for the shell or rod leads to charge transfer of either or both hole and electron into the seed material, i.e., charge trapping, typically followed by luminescence decay of the excited state on the seed. ,,,, In quasi-Type II heterojunctions, the E CB for both the seed and the shell/rod can be quite close in energy while offsets in E VB are maintained; thus, efficient charge separation and localization of the hole in the seed and the electron in the rod can occur across the heterostructure nanomaterial. ,,,− ,,, …”