Heterogeneously Alloyed Semiconductor Nanocrystals with Induced Chemical Composition Gradients

Scott Niezgoda, Joe Keen, and Noah Orfield, thank you all for believing in my work enough to sacrifice your time and resources to work with me. Chinessa Atkins, Suseela Somarajan, Sameer Mahajan and John Rigueur, thank you all, even after graduation, for your continued support. Janina Jeff and Brittany Allison, thank you for helping me keep my sanity both on and off campus.

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Overview

Increasing structural confinement usually leads to increases in the band gap energy of the material. The relative motion of electron and hole in the exciton is similar to that of the electron and proton within the hydrogen atom. The situation changes in the case of nanoscale semiconductor particles with size on the order of the Bohr exciton radius.

As the size of the nanocrystal increases, the energy of the electrons in the cavity decreases.

Figure 1.1 Bulk semiconductor materials have continuous conduction and valence energy bands separated by a fixed energy gap

Surface Passivation for Improved Optical Response

The phenomena of enhanced optical properties in these materials are believed to be due to the smoothing of the core/shell interface using similar materials, which also results in a gradual change in the energy band diagram that progresses from a smaller energy band gap due to the material core to a wider energy band gap associated with with surface structures and stoichiometry.41 These nanocrystals were fabricated in the same way as typical core/shell materials; the difference is in the choice and structural compatibility of core and shell materials. The use of similar materials, especially those with the same crystal structure and only small differences in lattice constant, should eliminate the issue of carrier trapping in core/shell nanocrystals. Carrier matching can arise from the abrupt core/shell interface of dissimilar materials and reduce non-radiative processes, which are expected to eliminate the deterrents associated with surface passivation, surface defects, and defects at the core–shell interface40.

More detailed discussions of the unique phenomena observed in gradient nanocrystals and their potential will follow in the following text.

Semiconductor Alloys

Nanocrystal Alloys with Chemical Composition Gradients

Although fabricated as core/shell nanoparticles, the multi-shell structure together with the choice of compatible materials enables gradient-like configurations, resulting in enhanced photophysical properties. There are not many reports describing the differences in the physical properties of compositionally gradient nanocrystals compared to currently used configurations such as core/shell materials. The functionality of the gradient unit can be changed not only by changing the degree of material gradation, but also by the appropriate selection of shell materials.

Just as core/shell materials are classified by many types, several classifications of gradient materials are possible.

Scope of this Work

Discussions regarding the gradient nature of CdSSe nanocrystals and the effects of varying synthetic parameters on photophysical properties are included in Chapter III. Finally, a chapter summarizing the achievements of this work is presented; also included are suggestions for his future. This work was intended to exploit future studies and discussions on the effects of inducing chemical composition gradients in alloy materials on the resulting physical properties.

Synthetic Considerations for Nanocrystals with Chemical Composition Gradients

To avoid alloy nucleation, precursor injection and growth were performed at the same temperature. In addition, to avoid temperature fluctuations due to injection, high concentration precursors were used to reduce the volume size of injection fluids and reduce temperature disturbances during the reaction. Injection solutions were prepared by mixing the desired ratio of both S and Se stock solutions.

Nanocrystal Synthesis

The reaction temperatures are varied by using a digital temperature controller, connected to the heating jacket, to set the desired temperature. A thermocouple is also attached to the temperature controller and placed in the reaction flask to monitor the temperature of the solution. A rotary evaporator and a buffer trap are placed in the central neck of the reaction flask and attached to an argon bubble to enable the synthesis of nanocrystals in an environment free of water and oxygen.

The remaining neck is provided with a rubber septum to close off the reaction to the external environment and maintain synthesis conditions.

Figure 2.1 Image of nanocrystal synthesis setup. The 3-neck flask was seated atop a stir plate inside a heating mantle, heated to the appropriate reaction temperature

Characterization Techniques

Resulting data were in terms of the number of backscattered ions (count) as a function of the energies of the backscattered ions (channel). Peaks of the XRD spectra representing compositionally graded CdSSe nanocrystals are between those of zinc-doped CdSe and zinc-doped CdS. These data also show distortions in the unit cell of the composition-graded nanocrystals with increasing S concentrations, resulting in a modified cubic zinc blende structure.

Although not a direct correspondence to bulk CdS, this peak is suspected to be due to the S-rich surface of the compositionally graded CdSSe nanocrystals and indicative of progress towards a continuously growing CdS surface. Although we observed the change of S:Se ratios with growth time, analysis of absorption spectra with growth time suggests that compositionally graded CdSSe nanocrystals stop growing before completing the reaction at 120 min. The departure of the electronic structure of compositionally classified CdSSe nanocrystals from the quadratic relationship presented for homogeneous alloyed semiconductor nanocrystals in Equation 3.2 is shown in Figure 3.1051.

Due to the heavier concentration of the more electronegative sulfur atoms towards the surface of the nanocrystal, we would expect higher concentrations of carriers in surface regions of the compositionally graded nanocrystals. Previous discussions on the absorption and photoluminescence of these materials described the optical properties of the CdSSe gradient nanocrystals as independent of particle size. This hypothesis is somewhat supported by the appearance of the (2 2 0) diffraction peak of CdS in XRD spectra (Figure 3.3b) with increased growth times.

Improvements in QY may also be due to reduced non-radiative processes and increased radiative transitions as a result of the graded structure configuration in the compound48. Data for compositionally graded CdSSe [▲] and homogeneously alloyed CdSxSe1-x [●] are compared to demonstrate the departure of the optical properties of compositionally graded CdSSe from this approximation. Here, we compare the progression of the first absorption peak with the growth time at different synthesis temperatures.

Compositionally graded CdSSe nanocrystals, synthesized at T = 220 °C, produce emissions with the highest quantum yields of the materials discussed here.

Figure 2.2 Schematic depicting RBS analysis technique.

Improved Luminescence Intensity with Changing Anionic Stoichiometries

We propose that the growth of compositionally graded CdSSe nanocrystals starts from ultra-small CdSe nanocrystals and proceeds with the gradual incorporation of S atoms, which create a radially oriented chemical composition gradient. Based on stoichiometric data and structural analysis, we can only simply conclude that binary CdSe nanocrystals are present at the beginning of synthesis and S concentrations increase with growth time. The carriers are limited at some depth below the surface of the nanocrystal by an energy barrier, which is related to the increasing charge concentrations towards the surface of the nanocrystal.

The combination of individual layers and the interaction between the layers has some undefined effect on the resulting electronic structure of the entire particle. Considering the changing quantum yields with different degrees of gradation and previous reports of flicker suppression55, semiconductor nanocrystals with chemical composition gradients may prove to be model systems for studying and identifying specific non-radiative processes related to luminescence efficiency and fluorescence intermittency. To do this with CdSSe, more detailed correlations between optical properties and gradation are needed to elucidate the true nature of the phenomena observed here and to better understand the effects of gradation on quantum confinement by nanocrystals.

These new gradation effects are possible simply because of the sensitivity of the experiment to changing parameters. Furthermore, XRD data should be carefully examined and evaluated as a complementary technique for characterizing gradient effects on the crystal structures of the materials. Currently, the technology to image the internal structure of the gradient nanocrystals on the Angstrom scale does not exist and may never exist.

Nevertheless, it is imperative to develop a working model or some visualization of the internal structure of these materials. To do this precisely, it is necessary to have some insight into the growth process and gradation effects on the physical properties of the materials.

Figure 3.22 Comparisons of the (a) first absorption peaks, (b) FWHM values and Stokes Shifts, and (c) quantum yields of compositionally graded CdSSe nanocrystals synthesized at T = 220°C for 2 hours and fabricated with varying anionic

Additional Compositionally Graded Semiconducting Nanocrystal Materials

Similar to the case of compositionally graded CdSSe nanocrystals, RBS analysis ( Figure A1 ) of CdZnSe reveals the gradient nature of the as-synthesized CdZnSe nanocrystals as the Cd:Zn atomic ratio varies with growth time. The gradual increase in zinc content with growth time indicates that these CdZnSe nanocrystals are grown with a chemical composition gradient where there is a Cd-rich core and Zn-rich shell. Graphical analysis of RBS data of CdPbSe depicts changing chemical composition of Pb with growth time.

The gradual increase in sulfur content with growth time indicates that these CdPbSe nanocrystals are grown with a chemical composition gradient where there is a Cd-rich core and Pb-rich shell. Further studies on the crystallographic effects of inducing chemical composition gradients in CdPbSe materials are needed to understand this phenomenon. Takagahara, T.; Takeda, K., Theory of the quantum confinement effect on excitons in quantum dots of indirect gap materials.

Albe, V.; Jouanin, C.; Bertho, D., Confinement and shape effects on the optical spectra of small CdSe nanocrystals. W.; Char, K.; Lee, C.; Lee, S., High efficiency green light-emitting diodes based on CdSe@ZnS quantum dots with a chemical composition gradient. Deng, Z.; Yan, H.; Liu, Y., Band Gap Engineering of Quaternary Alloyed ZnCdSSe Quantum Dots via a Facile Phosphine-Free Colloidal Method.

Peng, X., Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. Reiss, P.; Carayon, S.; Bleuse, J.; Pron, A., Low polydispersity core/shell nanocrystals of CdSe/ZnSe and CdSe/ZnSe/ZnS type: preparation and optical studies.