dfk

Professor David F. Kelley

Professor
Ph.D., University of Washington (1980) 
Postdoctoral Fellow, Bell Laboratories (1981-1982) 


Email: dfkelley@ucmerced.edu
Office Phone: 209-228-4354 
 
 Selected Publications


Postdoc position available:  The group has recently entered into a new collaborative research project with Nanosys Inc, and I am seeking a postdoc to work on this project.  Nanosys is located in Milpitas CA, and is a leading company involved in the development of Quantum Dot (QD) technology for displays. See www.nanosysinc.com.  This project is sponsored by a academic/industrial grant from the U. S. Department of Energy and will involve the development of new InP-based quantum dots for solid-state lighting purposes.  This application requires the development of QDs that operate at high photon fluxes and elevated temperatures.  Auger processes currently limit the applicability of QDs at high fluxes and much of the research will involve using ultrafast optical spectroscopy to elucidate how Auger processes can be controlled by varying the QD morphology and structure.  An application page on the UC Merced website is open, https://aprecruit.ucmerced.edu/JPF01109.    Interested candidates can also email me directly.


Physical Chemistry

Spectroscopy and dynamics of semiconductor nanoparticles. Dynamics of condensed phase energy, electron and proton transfer reactions. Ultrafast optical spectroscopy.
 
Research Overview

My research focuses on condensed phase spectroscopy and dynamics. We have been particularly interested in the chemical, optical and electronic properties of semiconductor nanoparticles and in electron transfer reactions involving inorganic dyes.

Exciton dynamics in InP/ZnSe quantum dots.

InP/ZnSe QDs have many similarities and some profound differences, compared to their CdSe-based counterparts.  Two of the main differences are that InP is less polar and has conduction and valence bans that are closer to the vacuum level than CdSe.  This affects several aspects of the photophysics.  One aspect is the role that indium dopants in the ZnSe shell plays in the hole relaxation rates.  We recently showed that following photoexcitation, the photoluminescence has a slow risetime, indicative of very slow hole cooling in doped InP/ZnSe QDs.

image003

This is understood in terms of the energy levels of the bands and the indium dopants, as shown in the figure below.

Energy level diagram and trap-mediated slow hole cooling mechanism.  The calculated energies of the quantum confined electrons and holes with respect to the vacuum level for the yellow (blue line), orange (black line) and red (red line) QDs are indicated as dotted lines. 

 

The role of these trap states in the biexciton dynamics is currently under investigation.  

 

Exciton dynamics in II-VI quantum dots.

Surface states can have a dramatic effect on the spectroscopy and photophysics of semiconductor nanocrystals, also called quantum dots (QDs).  The effect of these surface states depends on the Fermi level – the energy of the highest occupied state, as indicated in the figure, below. 

Fermi

 

Our research group has recently used transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopies to provide direct spectroscopic evidence for the phenomenon of thermal “surface charging” in II-VI (QDs).  We synthesize core/shell QDs with chalcogenide-rich surfaces, and following ligand exchange with oleylamine, these QDs have empty low-lying surface states that can be thermally populated from the valence band.  At room temperature, the surface charging equilibrium results in some fraction of the particles having a hole in the valence band, i.e., the surface acceptor states make the particle p-type.  Photoexcitation of the surface charged state results in what is essentially a positive trion, which can undergo a fast Auger recombination.  Both PL and TA (bleach recovery) kinetics of the CdSe/CdS QDs show a 70 ps decay component, which is assigned to Auger recombination of the surface charged QD.  The assignment is established by the different amplitudes of the TA and PL decays. 

Auger decay

The empty nonbonding surface orbitals can be passivated by ligation with a trialkylphosphine, and the fast decay component is absent when tributylphosphine is present.  The comparison of the TA and PL kinetics shows that the relative amplitude of the 70 ps component is a factor of about 1.5 greater in the TA than in the PL.  They also show that the fast component in the PL spectrum is shifted about 6 nm to the blue of the exciton luminescence. 

 

The presence of surface charging can result in “thermal quenching” of the QD luminescence with a Se-rich surface, as shown below.  Temperature dependent PL decay curves and fractions of bright particles are indicated for CdSe (left) and CdTe/CdSe/ZnSe (right).  This thermal quenching is due to an Auger mechanism, as given below. 

 

Auger

 

 

 

The important points are:

Photoexcitation of charged particles produces “trions” (two holes and one electron) which undergo rapid radiationless decay.

Trion formation is minimized by surface derivitization of core/shell particles which eliminates the empty (electron accepting) orbitals.

Thermal population of these orbitals follows Arrhenius behavior, as plotted below.  

 

Graph1Graph2

TATA plot 

 

 

We find that amines passivate the surface and eliminate the surface charging. 

 

 

 

The amine and tributyl phosphine ligands are stable up to about 80 °C. 

 

TQligands

Graph3Graph4

 

 

Spectroscopy and morphology of core/shell nanoparticles. 
  Lattice mismatch play a central role in determining the morphology and the spectroscopy of core shell particles.  In the present studies, we examine the role of lattice mismatch in determining the shell uniformity in core/shell nanocrystals.  

Basic idea:  If one starts with a semiconductor nanocrystal and grows a shell of a different semiconductor having the same crystal structure but a different lattice constant, then there is a lattice mismatch at the core-shell interface.  The strain energy associated with the lattice mismatch increases with shell thickness, but can be minimized by the formation of “islands”, rather than a continuous shell of uniform thickness.  Island formation increases the surface area and therefore the surface energy.  The morphology of the shell (the uniformity of the shell thickness) is therefore a trade-off between strain energy and surface energy.  The distribution of shell thicknesses is measured by measuring the distribution of charge tunneling times through the shell (a very sensitive measure of shell thickness).  To summarize, 

             Surface energy—proportional to surface area and controlled by the types of surface-adsorbed ligands.

             Strain energy—controlled by the radial composition profile—calculated through elastic continuum theory.

             Shell thicknesses—determined by measuring the rate of charge tunneling through the shell. 

 

The model system we have most extensively studied is ZnTe/CdSe.  This is a type-II system, similar to the extensively studied CdTe/CdSe system. However, ZnTe and CdSe have nearly identical lattice parameters.  But, upon annealing, the cations diffuse and one gets a particle best described as (Zn,Cd)Te/(Cd,Zn)Se, which has a large lattice mismatch.  The lattice mismatch and energetics of CdTe/CdSe and ZnTe/CdSe particles are depicted below.   

 

 

lattice mismatch

For ZnTe/CdSe zincblende nanocrystals, the we reach the following conclusions:  low temperature (200 °C) shell growth → very little cation diffusion → relatively small lattice mismatch → uniform shell growth to about  a thickness of three CdSe monolayers (a wetting layer), followed by island formation (Stranski-Krastanov growth). Shell annealing at 250 °C → extensive cation diffusion → larger lattice mismatch → island formation w/o wetting layer (Volmer-Weber growth).

 

When a hole acceptor (phenothiazine) is adsorbed on the particles surface, the charge tunneling dynamics, and hence quenching kinetics are a very sensitive probe to the local shell thickness.  Dramatically different hole quenching kinetics are associated with a smooth wetting layer (below, left) and Stranski-Krastanov island growth (below, right).  By analyzing the luminescence decay, the shell thickness inhomogeneity (the shell surface roughness) can be obtained.  The calculated curves correspond to a Poisson distribution of acceptors adsorbed on the particles at different phenothiazine concentrations. 

Lattice strain: elastic continuum calculations.  

The lattice strain energy can be calculated using elastic continuum theory.  It is the strain energy compared to the surface energy that controls the shell morphology.  For a finite shell with inner radius rc and outer radius R, the radial displacement in the shell is given by

        displacment eqn           where E is Young’s modulus and n is Poisson’s ratio.

 

Lattice coherency requires  

  coherence equation                                                        

The radial pressure is 

 pressure equation        where e is the ratio of lattice parameters and d is the ratio of shell thickness to core radius. The core is under hydrostatic pressure. The shell is under radial pressure and tangential tension. The core pressures are very large, and can be more than 109 Pa.

 

The radial and tangential strains are given by 

 strain equation

 The radial and tangential stresses and strain energy density are given by    

 stress equation

The surface energy is difficult to quantify, but is taken to be proportional to the surface area. displacement eqn

 

 

Two-dimensional CdSe Nanoplatelets

The exciton in spherical QDs extends over the entire sphere.  However, in extended CdSe nanoplatelets (NPLs) this may not be the case.  The spatial extents of NPL excitons are investigated using transient absorption (TA) spectroscopy.  The bleach magnitudes of a series of NPLs with varying lateral dimensions are compared with a quantum dot (QD) standard, allowing the relative magnitude of the heavy hole (HH) bleach to be determined as a function of size. The relative bleach of the HH absorbance decreases with increasing NPL area, while the excitonic sizes calculated from the bleach magnitudes are found to be independent of the lateral dimensions. This result is consistent with a model that considers the relative intensities of photoinduced absorption (PA) and stimulated emission (SE) contributed by distinct regions of the platelet occupied by either the electron, the electron and hole, or neither. Using this model gives an average excitonic area of 21.2 ± 2.5 nm2.

NPL exciton

 

Considering the small spatial extent of the relaxed NPL exciton compared with the number of oscillators participating in ground state absorption  partially accounts for the discrepancy between the HH extinction coefficients and the slow radiative rates, which deviate from the values calculated from the Einstein relations by a factor between ~ 30 – 80. The electron-hole overlap integral is estimated from the ratio of HH bleach magnitudes before and after trapping the hole by 4-methylbenzenethiol (MBT), a hole acceptor, and is then factored into calculations of the radiative lifetimes. Together with the measured singlet-triplet splitting and e-h overlap, these considerations allow the NPL radiative kinetics to be semiquantitatively reproduced.

 

Two-dimensional GaSe semiconductor nanoparticles.

Many types of semiconductors have properties which are particle size dependent. Semiconductor nanoparticles are particles which are sufficiently small that their physical and chemical properties are very different from those of bulk materials, and are dominated by quantum mechanical effects, so-called "quantum confinement". These particles are thus often referred to as "quantum dots." We have been interested in semiconductor nanoparticles because of their possible applications in regenerative photocells, photocatalysis and in electroluminescent devices. Development of quantum dots for all of these potential applications requires that we understand their size-dependent spectroscopy and photophysics. We have been primarily interested in the extremely photostable, two dimensional metal dichalcogenide semiconductors, such as GaSe and InSe. The crystal structure of bulk GaSe is shown

imageQFS
GaSe nanoparticles consist of "single tetra-layers", i.e., a single sheet of covalently bound Se-Ga-Ga-Se. These particles have diameters ranging from 2.5 to 10 nm and single sizes can be produced by controlling the synthetic chemical environment.

TEM images of 8.4 +/- .7 nm GaSe nanoparticles.                                                             GaSe Nanoparticles: 2.7 nm –  blue, 5.1 nm – light green,

                                                                                                                                             4.6 nm  - highly aggregated – dark green

                                                                                                                                             11.8 nm - red: “focused” distribution synthesized by multiple injections.

                                                                                                                                             Aggregate absorption spectra are dotted lines – monomers are solid lines

 

Photoexcitation of these particles produces conduction band electrons and valence band holes. A major pathway following photoexcitation is radiative decay, and the particles are strongly luminescent. 

The electrons and holes and undergo interfacial charge transfer and/or trapping into localized surface states. One of the main goals of the research have been to understand the optical spectroscopy of these particles. We also use time-resolved ultrafast absorption and emission spectroscopy to study electron transfer across the nanoparticle/nanoparticle and nanoparticle/liquid interfaces. We have shown that these nanoparticles form extended, somewhat disordered one dimensional aggregates. This type of behavior is unique among semiconductor nanoparticles and is due to their two dimensional, disk-like shapes; they form stacks in room temperature solutions.

Nanoparticles in liquid crystals.

Recently, we have been putting GaSe nanoparticles in organic liquid crystals. Specifically, GaSe nanoparticles are able to form a hybrid organic/semiconductor liquid crystal with the smectic-A phase of 4-octyl, 4’-cyano biphenyl, 8CB. This is a common liquid crystal molecule , and the phases of 8CB are shown below. 

liquid crystal

Incorporation of GaSe nanoparticles into the liquid crystal results in almost complete alignment of the particles – the particle’s normal line up with the liquid crystal director axis. This is seen from static polarized absorption measurements, below. The “order parameter” (0 = random orientations, 1= completely ordered) for these particles is about 0.96. 

Absorbance at several wavelengths as a function of the angle between the polarization of the light

and the liquid crystal director axis. Absorbances for 400 nm (open blue circles), 416 nm (solid black circles),

and 432 nm (solid red triangles) are shown. Also shown is a sine squared fit to the 416 nm absorbances.

 

Thus, the particles form well-ordered one-dimensional arrays in the liquid crystal host – the disk-like particles stack like Frisbees or dinner plates. The lack of disorder greatly increases the extent of particle-particle interactions, and fluorescence from these nanoparticle arrays is shifted about 50 nm to the red of that from solution phase GaSe nanoparticles.  Time resolved results indicate that the excitons travel large distances, at least microns!