Research

Polarization Anisotropy and Photophysics of Colloidal II-VI Core-shell Quantum Dots

Advisor: Prof. David F. Kelley, Department of Chemistry, University of California Merced, California, US

Ligand field as a source of anisotropy in II-VI quantum dots

Colloidal nanocrystals find applications in display technology and photovoltaics, and can be synthesized in a variety of shapes, commonly a quasi-spherical shape. The optical properties of these dots are connected to the angular momentum fine structure and are sensitive to the extent of anisotropy in the system. Anisotropy is commonly considered to result from either the crystal field (asymmetry in the unit cell) or deviation from spherical symmetry in the morphology of the dots. We hypothesized that the electric field produced by the charged ligands on the dots' surfaces is an additional source of anisotropy. Therefore, the degree of anisotropy can be tuned by choosing the charged nature of the ligation, changing the fine structure splitting, and the degree of mixing in the different states, resulting in a measurable change in the spectroscopy and dynamics of the sample. We demonstrated the same by synthesizing quantum dots with a strongly charged ligand (octadecylphosphonic acid) and performing a ligand exchange to a weakly charged ligand (octanethiol), showing a measurable change in the spectroscopy of the dots – decrease in Stokes shift, polarization anisotropy (middle panel), and radiative lifetimes (left panel). We also noticed that certain synthetic methods that are considered equivalent produced quantum dots that were more sensitive to the ligand fields than others. We hypothesized that the nature of faceting that appears on the surface of the quantum dots changes based on the ligation and growth temperatures. A more detailed description can be found here.

Polarization anisotropy in PLE spectroscopy to study the fine structure in CdTe quantum dots

The excitonic fine structure of II-VI  quantum dots is well understood since Efros described it in terms of the isotropic exchange interaction (η), anisotropic factor (∆), and other material parameters. The fluorescence properties of these dots are closely related to their excitonic fine structure and, hence, are the center of a vast majority of research in the area. This fine structure splitting is often too small to be resolved spectroscopically in ensemble emission or absorption spectra. Several studies have examined the fine structure of II-VI and III-V colloidal dots. A good proportion of these studies are single particle studies where the individual emission lines from the delicate structure states are easily resolvable. However, it is reasonable to assume that one starts to produce charged excitons in millions of excitation events that occur every second. This means the study may be tainted by contributions from charged excitonic states that may get miss-assigned as fine structure splittings, making these studies unreliable. Our group has focused on employing polarization-selective Photoluminescence Excitation (PLE) spectroscopy of ensembles to study the fine structure. It does not have the same reliability issues faced by single-particle studies.

The static PLE, PLE emission, and polarization anisotropy can be fit to obtain η and ∆. The parameters obtained with this fit serve as a starting point to fit the PLE anisotropy data. The slope of the PLE anisotropy data around the first zero (0 − 100 meV region), the x-intercept, and the peak value on either side are all dependent on the splitting between ±1U and 0U states and their linewidths. The figure below shows the experimental PLE anisotropy data (blue circle) and the calculated PLE anisotropy data (red line). The final set of parameters obtained upon fitting both the static PLE and PLE anisotropy are σ_U = 51 meV, η = 16 meV, and ∆ = 18 meV. The splitting between ±1U and 0U states obtained from the fitting is 12.4 meV, and a calculated Stokes shift of 63.4 meV.

Anisotropy also affects the radiative lifetimes of quantum dots. The mixing of ±1U state with ±1L due to the breaking of spherical symmetry redistributes the oscillator strength in the lowest emitting ±1L state. The energy difference between the slightly allowed ±1L and ±2 states determines the fraction of the population that will end up in the ±1L states. Both of these factors combined determine the radiative lifetime of a quantum dot. Consequently, any change to the fine structure should result in a change in the measured radiative lifetime of the sample.

Silver-Inidum-Galium-Sulfide (AIGS) photophysics & radiative dynamics

AIGS belongs to the I-III-VI and has been of interest for display and LED technology applications because of its non-toxic composition and featureless absorbance spectra. Several studies have shown that producing high-quality samples is challenging due to their susceptibility to quenching due to defects. Nanosys managed to synthesize high PLQY samples (~90%), which gives us an opportunity to look into the photophysics of these nanocrystals. The radiative dynamics and PLE anisotropy show signatures of possible inhomogeneous metal concentration and possible trap formation.

Figure: Schematic of the In(Ga)As coupled quantum dots in a Schottky diode (left panel), center panel (a) shows the direct and indirect exciton states interacting with a charge in the vicinity of the QDM (b) shows the calculated spectra of the neutral exciton under the influence of the charge and right panel shows PL spectra of a QDM with positive-trion and neutral exciton without (a) and with interacting charges (b).