Mixed-Matrix Quantum Dot Phosphors

Previous research demonstrated that the intensity of fluorescent emission from cadmium sulfide quantum dots (Q-CdS) is related to the degree to which the surface states are isolated from the core of the quantum dot and that treatment of the quantum dot surface passivated by inorganic salts as a Schottky-like barrier (decreasing the frequency of surface-based nonradiative conversion) yields an effective model for emission intensity (see PREVIOUS PROJECTS below).

A similar model might have been be applied to the phosphorescent trap state in doped cadmium/zinc sulfide mixed-matrix semiconductor quantum dots. Particularly, manganese (II)-doped phosphors (bulk or quantum dot) of the type Cd_xZn_1-xS:Mn exhibit decreasing phosphorescent quantum yield with increasing cadmium composition of the matrix. This phenomenon might correspond to the lowering of the matrix conduction band and concomitant increasing frequency of escape from the phosphorescent trap state as cadmium composition increases. Confounding influences on the quantum dot electronic structure from both matrix composition and quantum confinement complicate experiments to test this hypothesis for mixed-matrix quantum dot phosphors. However, these influences can be unraveled by independent determination of matrix composition: It has been demonstrated that the interstitial-dopant-based emission wavelength of Cd_xZn_1-xS:Ag is determined by the matrix composition. Also, the band gap energy of cadmium/zinc sulfide mixed-matrix quantum dots has been empirically modeled as a function of matrix composition. Therefore, analysis of silver (I)-doped mixed-matrix quantum dots [Q-Cd_xZn_1-xS:Ag] should yield both matrix composition (from the emission wavelength) and quantum confinement due to quantum dot size (from separating the effective band gap energy into composition-based and quantum-confinement-based components). Replacing the silver (I) dopant with manganese (II) dopant in the identical quantum dot preparation [Q-Cd_xZn_1-xS:Mn] should provide quantum dot phosphors through which the effect of lowering the matrix conduction band on the phosphorescent emission intensity can be quantitatively compared to a simple model of trap state escape frequency determined by trap depth relative to the matrix conduction band.

Katie Burgan and Craig Lott (2009 graduates of Mercer University, chemistry majors) worked on this project in 2008-09. They created a protocol for preparing Q-Cd_xZn_1-xS:Ag and Q-Cd_xZn_1-xS:Mn, used the variable silver-based emission to determine quantum dot composition (and size), and probed the variation in manganese-based emission intensity as function of quantum dot composition (and size). But they found that the phosphorescence intensity variation wasn’t consistent with escape from phosphorescent state to the matrix conduction band. Rather, their Q-Cd_xZn_1-xS:Mn appeared to behave as though escape was occurring to a near-band-edge defect state and that the rate of the escape process varied with matrix composition in a way consistent with the Marcus theory of electron transfer rates.

This is a very interesting result that could illuminate electronic processes occurring in excited quantum dots.

[More information will be posted as results are available.]

CONTINUING RESEARCH PROJECT

Polypyrrole Oligomers and Bipolaron Formation

Aubrey Bauch (2008 graduate of Mercer University, chemistry major) continued the research on polypyrrole and bipolaron formation started previously (see PREVIOUS PROJECTS below). By replacing the interrupting co-monomer (in a strategy of co-polymerization with pyrrole) with a terminating co-monomer, Aubrey prepared substituted polypyrrole oligomers with length selected by the relative amounts of substituted pyrrole and pyrrole-based terminating monomer combined. The formation of bipolaron-like structure in the polypyrrole oligomers resulting from oxidative co-polymerization was evaluated relative to the minimum required length of the oligomer (determined by MALDI-TOF mass spectrometry) to learn more about the bipolaron formation process (or about, in other words, the conductivity of polypyrrole and similar semiconducting polymers). Aubrey found strong evidence for bipolaron-like structure formation limited by oligomer size, but oxidative polymerization caused some problems.

This project is now continuing via attempts at more controlled synthesis of length-selected substituted polypyrrole oligomers. All of our experiments with polypyrrole have shown no bipolaron formation for very short polypyrrole segments and easily measurable (and visible) bipolaron formation for very long polypyrrole segments. We hope to find a well-behaved system through which to observe a discrete transition from charge-delocalizing to non-charge-delocalizing behavior as a function of polypyrrole segment length and ability to form bipolaron-like structure.

[More work remains to be done on this project.]

PREVIOUS PROJECTS

I. Interrupted colloidal polypyrrole and bipolaron formation. Umar Kkokhar's (2004 graduate of Mercer University, chemistry major) senior research project involved the conducting polymer polypyrrole and related copolymers. Sarah Wright (2006 graduate of Mercer University, chemistry major) prepared an "interrupted polypyrrole" (shown in Figure 1 below) by copolymerization and analyzed how varying the number of pyrrole units between interruptions affected the properties of the polymer. Experimental results suggest that a minimum number of pyrrole units between the "interruptions" is required for the formation of the bipolaron-like structure resulting from oxidation of the polypyrrole. This minimum number of pyrrole units might be similar to the minimum size of the bipolaron (or the minimum amount of separation between positive charges in the oxidized polypyrrole). While we have a good approximation of this "minimum bipolaron size," research continues until we determine a more definitive value.

Figure 1. Interrupted Polypyrrole

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II. Luminescence properties of colloidal semiconductors. Colloidal cadmium sulfide (Q-CdS) exhibits intense near-band-gap photoluminescence (PL; ~450 nm), and the introduction of some salts (such as zinc chloride) dramatically enhances this PL intensity. If the adsorption of zinc cations into the compact layer about the CdS Q-particles (figure below) induces charge separation within the particles, then this charge separation might contribute to the retention of photo-generated charge carriers within the core of the Q-particles with concomitant increase in PL intensity.

wpe1.jpg (17607 bytes) Figure 2. Aqueous Q-particle and compact-layer ions.

We have analyzed the introduction of a variety of soluble salts to aqueous Q-CdS and have constructed a quantitative charge shell model (or Schottky-like barrier model) to connect PL intensity with induced charge separation in the Q-particles. Treating the introduction of charge separation as formation of a Schottky-like surface barrier, our model predicts: A critical surface charge density is accompanied by dramatic rise in PL intensity, and a maximum surface charge density exists beyond which further induced charge separation has no effect on PL intensity.

Kaushik Patel (2002 graduate of Mercer University, chemistry major) performed the fluorometric titrations of aqueous cadmium sulfide Q-particle sols against several luminescence-enhancing salts, yielding results in agreement with the Schottky-like barrier model. These results have been published:

D. E. Moore, K. Patel, "Q-CdS Photoluminescence Activation on Zn²⁺ and Cd²⁺ Salt Introduction," Langmuir, 2001, 17, 2541.

Project Extenstion: The investigation of Q-particle luminescence included also phosphorescence, the much longer lifetime PL associated with dopant ions in the sulfide Q-particle matrix. Kensley Nichols (May 2001 graduate of Mercer University, chemistry major) initiated these experiments with mixed sulfide matrices, Q-CdxZn1-xS, doped with manganese (II) and Blake Wester (former Mercer chemistry major) continued this project by looking at the impact of matrix composition on sulfide Q-particle phosphorescence. Ken and Blake both tested the activation of phosphorescence (~590 nm) intensity against the Schottky-like barrier model (described above). They found that the Q-particle phosphorescence intensity for Q-Cd_xZn_1-xS:Mn is very sensitive to matrix composition, and that matrix band gap energy changes of less than 0.10 eV are enough to "switch" manganese (II)-based phosphorescence on-to-off. While small, this energy change is similar in magnitude to the Schottky-like energy barrier that we propose leads to the dramatic activation of Q-particle PL intensity (see Langmuir paper referenced above). Work on the relationship between matrix composition and phosphorescence intensity is continuing. (There’s more information on this project extension under CONTINUING RESEARCH PROJECTS above.)

III. Mechanism of Q-particle aggregation and pattern precipitation. Cadmium sulfide is precipitated in gels (inorganic TEOS gels, AOT-based organogels, and polyacrylaminde gels) as "free-surface" nanoscopic colloidal particles (Q-particles). Under appropriate conditions, these Q-particles are observed to aggregate in regular patterns, parallel bands in test tubes, concentric rings in petri dishes -- the so-called Liesegang phenomenon (see figure below). [Liesegang, R.E., Naturiss. Wochenshr., 1896, 11, 353.] Currently, this aggregation process is being kinetically studied in an effort to elaborate on the accepted mechanism(s) by which the pattern precipitation phenomenon occurs.

The results of Josh Rubin's chemistry research project (2001) support the colloid (Q-particles) model for the Liesegang phenomenon and a mechanism of Q-particle formation. Kinjal Vakil and Stephanie Godfrey's joint research project (2002) expanded this work to include the general kinetic investigation of Q-particle growth, which seems to be one component of the pattern precipitation process. The colloid formation rate law appears to follow the form below with a rate constant consistent with diffusion-controlled reaction:

d[Q-CdS]/dt = k[Cd²⁺][S^2-]^1/2

Figure 3. Liesegang phenomenon. This image shows cadmium sulfide bands in test tube of polyacrylamide gel. Experiment by Wendy Joyner; digital photo by Vinnie Pham. (Both are Mercer University graduates and chemistry majors.)

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IV. Ion-dye associates and chemical sensing applications. The immobilization of chemoresponsive dyes on solid supports is one potential method for constructing chemical sensor elements. The immobilization can be through non-covalent interactions, or associations. To study this potential method, we have been investigating the association (in water) between bromothymol blue (a pH-sensitive dye) and ammonium-bearing species - both small molecules and polymers. Interestingly, the association between the dye and small-molecule ammonium species is very weak, whereas the association between the dye and insoluble, ammonium-bearing (bulk) polymers is considerably stronger. This comparison is based on the results from Martin Sarkar's (2000 Mercer University graduate, chemistry major) senior research project. Now, I'm interested in probing the association between bromothymol blue and mid-sized (soluble) ammonium-bearing oligomers… and wondering if some cooperative effect leads to the dramatic increase in the strength of ion-dye associations for polymers, relative to that for smaller molecules.

DEM · Current Student Research Projects in Chemistry · August 2009