Michael M. Krause

Chemical and Thermodynamic Control of the Surface of Semiconductor Nanocrystals for Designer White Light Emitters

Small CdSe semiconductor nanocrystals with diameters below 2 nm are thought to emit white light due to random surface defects which result in a broad distribution of midgap emitting states, thereby preventing rational design of small nanocrystal white light emitters. We perform temperature dependent photoluminescence experiments before and after ligand exchange and electron transfer simulations to reveal a very simple microscopic picture of the origin of the white light. These experiments and simulations reveal that these small nanocrystals can be physically modeled in precisely the same way as normal-sized semiconductor nanocrystals; differences in their emission spectra arise from their surface thermodynamics. The white light emission is thus a consequence of the thermodynamic relationship between a core excitonic state and an optically bright surface state with good quantum yield. By virtue of this understanding of the surface and the manner in which it is coupled to the core excitonic states of these nanocrystals, we show both chemical and thermodynamic control of the photoluminescence spectra. We find that using both temperature and appropriate choice in ligands, one can rationally control the spectra so as to engineer the surface to target color rendering coordinates for displays and white light emitters.

A microscopic picture of surface charge trapping in semiconductor nanocrystals.

Several different compositions of semiconductor nanocrystals are subjected to numerous spectroscopic techniques to elucidate the nature of surface trapping in these systems. We find a consistent temperature-dependent relationship between core and surface photoluminescence intensity and marked differences in electron-phonon coupling for core and surface states based on ultrafast measurements and Resonance Raman studies, respectively. These results support a minimal model of surface charge trapping applicable to a range of nanocrystal systems involving a single surface state in which the trapped charge polarization leads to strong phonon couplings, with transitions between the surface and band edge excitonic states being governed by semiclassical electron-transfer theory.

Toward Ratiometric Nanothermometry via Intrinsic Dual Emission from Semiconductor Nanocrystals

Semiconductor nanocrystals have been synthesized that support intrinsic dual emission from the excitonic core as well as the surface. By virtue of chemical control of the thermodynamics of the core/surface equilibria, these nanocrystals support ratiometric temperature sensing over a broad temperature scale. This surface-chemistry-based approach for creating intrinsic dual emission enables a completely new strategy for application of these nanocrystals in optical nanothermometry.

Ultrafast Electron Trapping at the Surface of Semiconductor Nanocrystals: Excitonic and Biexcitonic Processes

Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory.

Terahertz Bandwidth All-Optical Modulation and Logic Using Multiexcitons in Semiconductor Nanocrystals

Optical pumping of semiconductor nanocrystals with femtosecond pulse sequences was performed in order to modulate multiexciton populations. We show for the first time that control of multiexciton populations produces high speed modulation of stimulated emission. Upon the basis of the speed of multiexcitonic processes in nanocrystals, we show modulation rates approaching 1 THz by virtue of strong quantum confinement effects. Employing femtosecond optical pulse sequences, we demonstrate all-optical logic using these nanocrystals in two forms: an AND gate, and an inverter, a key step toward all optical signal processing.

Ligand Surface Chemistry Dictates Light Emission from Nanocrystals.

There are several contradictory accounts of the changes to the emissive behavior of semiconductor nanocrystal upon a ligand exchange from trioctylphosphine/cadmium-phosphonates passivation to N-butylamine. This communication explains the contradictory accounts of this reaction using new insights into ligand chemistry. Also, a previously unknown link between surface emission and cadmium-phosphonate (Z-type) ligands is shown.

The Effect of Exciton-Delocalizing Thiols on Intrinsic Dual Emitting Semiconductor Nanocrystals.

The emissive properties of thiol-capped CdSe nanocrystals (NCs) with intrinsic dual emission are investigated through temperature-dependent photoluminescence (PL) measurements. We demonstrate the influence of thiols on the relative PL intensities of the core and surface emissive states, as well as on the observed Stokes shifts. A redshift of both the core and surface PL in comparison with phosphonate-capped NCs is consistent with recent work exploring the effect of thiols as excitonic hole-delocalizing ligands. This observation is consistent with prior reports suggesting that surface excitons originate from electrons bound to cadmium trap states.

Control of phonons in semiconductor nanocrystals via femtosecond pulse chirp-influenced wavepacket dynamics and polarization.

The realistic electronic structure of semiconductor nanocrystals is characterized by excitonic fine structure and atomistic symmetry breakings that are challenging to resolve experimentally. Exciton-phonon coupling is one of the most sensitive measures of the excitonic wave functions of the nanocrystals. Here, we exploit this sensitivity via chirped pulse and polarization resolved femtosecond pump/probe spectroscopy of colloidal CdSe nanocrystals. Pulse chirp measurements and simulations are used to explore the contributions of excited- and ground-state vibrational wavepackets to the observed coherent phonons in the pump/probe signals. Polarization resolved pump/probe spectroscopy is used to explore electronic and vibrational polarization anisotropies. We find no electronic polarization anisotropy, whereas vibrational anisotropy is preserved.

Investigating the influence of ligands on the surface-state emission of colloidal CdSe quantum dots

Semiconductor based light generation is of enormous contemporary interest, given that a large fraction of global energy is used for lighting. White-light semiconductor colloidal quantum dots may find application in future solid state lighting technologies. These dots possess two inherent emission bands, a narrow emissive band attributed to a quantum confined exciton, and a broad emission associated with surface trapping. White light CdSe colloidal semiconductor nanocrystals passivated with phosphonic acids were synthesized by a hot-injection method. Aliquots of this sample are then ligand exchanged with amine and thiol ligands. These samples are embedded in polystyrene films, and a series of temperature dependent photoluminescence measurements are performed. The spectral width as a function of temperature is plotted for all samples. These data are then analyzed in terms of three models. The results suggest that surface line shape broadness may be tied to strong electron-phonon coupling and is largely ligand dependent. The amine and phosphonic acid passivated samples showed large temperature dependence over the range studied, whereas the thiol passivated sample had a lower dependence. This is tentatively explained in terms of hole delocalization in the case of thiol passivation.