UC Merced

Various semiconductor nanocrystals are currently being studied for their potential in optoelectronic devices such as photovoltaics, lasers, and lighting. Indium phosphide (InP) nanocrystals are particularly noteworthy due to their reduced toxicity and increased environmental sustainability as compared to other semiconductor nanocrystals. Indium phosphide nanocrystals are defect prone but can be passivated with a shell material such as zinc selenide (ZnSe) to mitigate this property and create stable highly luminescent QDs. Due to indium phosphide’s relatively small band gap and the aforementioned stability, InP/ZnSe QDs can be used to make efficient green, red, and near IR emitters.

Chapter 1 of this work will begin with an exploration of the structure of indium phosphide zinc selenide quantum dots (QDs). The crystal and electronic structure of InP will be used to form a framework for the interpretation of the absorption and emission properties of InP/ZnSe QDs. This will be done by considering the existing literature on cadmium selenide (CdSe) QD nanocrystals. These two semiconductor nanocrystals share a zinc-blende crystal structure, and both have band structures with two degenerate valence bands at the band edge. In semiconductor QDs charge carriers can be described using an effective mass approximation determined by the curvature of these bands. This will lead to a discussion of envelope functions and fine structure states within those envelope functions. Finally, chapter 1 will include a shortened synthesis of three variations of InP/ZnSe nanoparticles that will be discussed in future chapters.

Using the framework created in Chapter 1, Chapter 2 will relate these envelope functions and fine structure states to the absorption and emission of the corresponding transitions. This interpretation will discuss the factors that contribute to the line widths of these particles and will introduce the concept of homogeneous and inhomogeneous distributions in ensemble measurements. This will be explored through the use of the photoluminescence excitation spectrum (PLE) and polarization resolved PLE. In order to fully explain the width of the excitation spectra it will be necessary to recognize and characterize two forms of inhomogeneous broadening, size inhomogeneity and interfacial inhomogeneity. This interfacial inhomogeneity leads to an inhomogeneity in the band offsets of InP and ZnSe which complicates the spectroscopy of these QDs.

Chapter 3 will introduce time resolved spectroscopies. Direct comparison with time resolved photoluminescence (TRPL) experiments with radiative lifetimes calculated using the Einstein relations will show that a thermal Boltzmann distribution between the non-emissive, and emissive states are required to relate the two values, as is predicted by the fine structure discussed in chapter 1. Furthermore, the observed radiative lifetime varies across the emission band as predicted by the Einstein relations. This property can be used to determine the extent of aggregation in samples by looking at deviations in radiative lifetime due to Förster resonant energy transfer (FRET).

If during the shelling of InP/ZnSe QDs, the presence of indium is not carefully controlled, excess indium can become incorporated into the ZnSe shell. This leads to the formation of indium-based transient traps in the shell. Indeed, most published synthesis do not control for this variable, and thus these traps contribute significantly to the photoluminescent properties of published InP/ZnSe QDs. The indium-based shell transient traps are positioned between the valence band of the InP core, and the ZnSe shell valence band. This has the effect of trapping holes in the shell until they can tunnel into the valence band. Trapped holes have very small overlap with the electron wavefunction, and therefore do not contribute directly to the absorption or emission. Instead, they provide a reservoir state in thermal equilibrium with the emissive bright state, leading to delayed band edge emission. This increases the observed radiative lifetime by up to 30% for larger QDs. The equilibrium formed by process has a larger ratio of holes in the core to shell as opposed to the system short times after the cooling process, which has comparatively more holes in the shell.

In higher fluence conditions the InP/ZnSe QDs can absorb a second photon either simultaneously or sequentially. When this happens a biexciton forms. Biexcitons primarily recombine by Auger recombination. Auger recombination is when instead of a biexciton recombining radiatively, two carriers recombine and transfer their momentum to the two remaining carriers. This can create very high energy or “Hot” carriers. These hot carriers are of great interest because they are known to be able to perform chemical reactions that can lead to the degradation and quenching of photoluminescence in QDs. Minimizing or controlling these “hot” carriers is of utmost importance to increasing the stability of InP/ZnSe QDs at high fluences. Biexciton measurements can be measured in TRPL, but the increased time resolution of Transient Absorbance (TA) makes for easier and more accurate measurements. TRPL and TA biexciton measurements will be compared to extract information about the energetic position of the biexciton emitting state as compared to the single exciton emitting state.

As mentioned above, trapped holes exhibit minimal overlap with the electron wavefunction, and also interact weakly with the valence band. When two excitons are created if one of the holes cools into a trap state, it will be unable to efficiently undergo Auger recombination. This creates a pseudo-trion, where there are two electrons in the conduction band and a single hole in the valence band. The trapped hole, now held in a trap away from the core, induces an electric field across the QD that decreases the electron-hole interaction and slows the observed radiative lifetime compared to that of the analogous trion. This pseudo trion state that will be referred to as an XT state has an observed radiative lifetime that is a factor of four longer than the biexciton state, enabling precise determination of these rates through the comparison of both time-resolved photoluminescence (TRPL) and transient absorption (TA) measurements.

Chapter 4 will consider numerous possibilities regarding the chemical/structural identity of the indium-based transient traps. It will be shown that a zinc vacancy being charge compensated by a substitutional In3+ ion is the most likely candidate. Raman spectroscopy will be used to probe the structural characteristics of the QD and to elucidate the position of excess indium in the QD. This discussion on hole core equilibriums will be expanded to coupled thiols, which can function as hole acceptors, which can trap off holes for extended periods of time, up to hundreds of nanoseconds.

InP/ZnSe QDs hold great promise for the development of optoelectronic devices with enhanced environmental sustainability and reduced toxicity compared to other semiconductors. However, the presence of indium-based traps has significant implications for the photophysics and working lifetime of devices making use of InP/ZnSe QDs. Current literature on these particles is naïve to the contributions of these traps and the fingerprints of these traps can be seen in many key papers. This work, and the work it is based on, seeks to carefully disentangle the properties of the InP/ZnSe QD from those contributed by the traps in order to allow for accurate assignments.