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By use of a two-band effective mass model and first-order perturbation theory, we demonstrate the capacity for complete spatial control over excitonic wave functions in core/barrier/shell nanostructures. By exploring parameter space with core size and barrier width as variables, we show that each exciton can be in a core-localized or shell-localized state (Type-I localization) and as well in a charge-separated state (Type-II localization). Furthermore, the width of each localization region in parameter space can be tuned by controlling material properties, providing design principles for nanostructures with desirable features for key applications. Depending on the localization regime of excitons, both radiative and nonradiative decay rates can be significantly enhanced or suppressed in these nanostructures, making them suitable for dual-color emission and for producing low threshold and broad bandwidth of optical gain.

Semiconductor quantum dots are of interest as optical gain media for lasing applications. Here we report on efficient, broad bandwidth optical gain in the CdSe/ZnS/CdSe quantum dot/barrier/quantum shell nanocrystal. These nanocrystals are known to support spontaneous emission from both CdSe phases, offering promise for lasing applications via wave function engineering. The CdSe/ZnS/CdSe nanocrystals were found to have enhanced optical gain characteristics relative to CdSe quantum dots, as shown using femtosecond transient absorption spectroscopy. The enhancement of gain metrics such as bandwidth and efficiency arises from stimulated emission from quantum shell-enabled excitations. These shell-enabled excitations increase gain bandwidth via emission from new transitions and increase efficiencies via tailored biexciton interactions. This unique two-color character in both spontaneous emission and optical gain is rationalized by slow exciton cooling from the core/shell states into the core localized quantum ...

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an "artificial atom" because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies. Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the "excitonics", confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have developed a state-resolved spectroscopic approach which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials. We focus on four main topics of longstanding interest and controversy: (i) hot exciton relaxation dynamics, (ii) multiexcitons, (iii) optical gain, and (iv) exciton-phonon coupling. Using this state-resolved approach, we reconcile long standing controversies related to phenomena such as exciton cooling and exciton-phonon coupling and make surprising new observations related to optical gain and multiexcitons.

By direct observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response in colloidal CdSe semiconductor quantum dots. This response is driven by the migration of charges to the surface of the quantum dot on a vibrationally impulsive time scale. Surface- and fluence-dependent studies reveal that the observed carrier capture based piezo response is controllable and is at least an order of magnitude larger than the intrinsic piezo response of wurtzite CdSe.

Single dot spectroscopy is performed on two-color CdSe/ZnS/CdSe core/barrier/shell nanostructures. Unlike quantum dots cores, these systems have two phases with which to emit and ultimately examine for blinking analysis. These particles are brighter than conventional quantum dots and also show the photoluminescence (PL) intensity and energy fluctuations characteristic of quantum dots. Single dot spectral diffusion analysis yields no measureable energy shift correlation between the core and the shell on the 200 ms time scale. In contrast, the single dot PL from the CdSe shell has narrower linewidths than the CdSe core, indicating differences in its spectral diffusion on shorter timescales.

The first observation of state-resolved exciton−phonon couplings are reported for the lowest four eigenstates in strongly confined colloidal quantum dots for both optical and acoustic phonons. State-resolved pump/probe experiments show that the excitonic eigenstate has a strong influence upon the Frohlich coupling to optical phonons and a moderate influence on the deformation potential coupling to acoustic phonons. These experiments provide a general approach to measuring exciton−phonon couplings in quantum dots with eigenstate-specificity.

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.

Excitation transfer pathways in colloidal core/barrier/shell nanomaterials were investigated in the CdSe/ZnS/CdSe system. Absorption of light in the outer CdSe shell results in emission from the band edge of the CdSe core. The CdSe quantum shell acts as a light harvester that indirectly increases the brightness of the CdSe quantum-dot core. Spectroscopic evidence suggesting that the CdSe core and shell are coupled by tunneling of excitons through the ZnS barrier is provided. Competition kinetic analysis shows that charge transport competes effectively with hole capture by pyridine at the outer CdSe shell and exciton relaxation within the outer CdSe shell.

Femtosecond multicolor pump−probe spectroscopy on the prototypical intramolecular electron-transfer compound ([(NH3)5RuNCRu(CN)5]-; RuRu) in aqueous solution is reported for the first time with sufficient time resolution and spectral coverage to monitor the complete spectral dynamics. From the dynamic absorption spectrum, constructed from the pump−probe data obtained over a wide range of probe wavelengths (570−1300 nm), accurate measurement of the electron transfer (ET) time (1/kET) have been obtained by a global analysis of the data. The data have lead to new insights on the role of solvent and solute nuclear motions in the ET process. Additionally, pump−probe experiments with variable wavelength pump pulses have revealed the presence of a pump wavelength-dependent dynamic feature at early time, which has been assigned to stimulated emission from nonequilibrium excited states.

We investigate the time scale for hydrated electron solvation as a function of the initial configuration of the electron/water system. The experiments employ various 2-pulse and 3-pulse femtosecond pulse sequences that allow for controllable preparation of the various optically excited and precursor states of the equilibrated hydrated electron. We observe that the conduction band electron, which is produced by detrapping of the hydrated electron, has the slowest time scale for electron solvation with an average solvation time constant of 400 fs. In contrast, the solvation dynamics are significantly faster for electrons that are produced in “pre-solvated” environments. These latter examples include the excited p-state of the hydrated electron and the precursor states involved in UV femtosecond multiphoton ionization of water.

Ultrafast coherent multi-dimensional spectroscopies form a powerful set of techniques to unravel complex processes, ranging from light-harvesting, chemical exchange in biological systems to many-body interactions in quantum-confined materials. Yet these spectroscopies remain complex to implement at the high frequencies of vibrational and electronic transitions, thereby limiting their widespread use. Here we demonstrate the feasibility of two-dimensional spectroscopy at optical frequencies in a single beam. Femtosecond optical pulses are spectrally broadened to a relevant bandwidth and subsequently shaped into phase coherent pulse trains. By suitably modulating the phases of the pulses within the beam, we show that it is possible to directly read out the relevant optical signals. This work shows that one needs neither complex beam geometries nor complex detection schemes in order to measure two-dimensional spectra at optical frequencies. Our setup provides not only a simplified experimental design over standard two-dimensional spectrometers but its optical readout also enables novel applications in microscopy.

We report on a setup for coherent multidimensional spectroscopy based on visible continuum generation obtained by propagating 130 fs

Femtosecond spectroscopy measurements are reported on the electron transfer, ET, reactions of the precursor states of the hydrated electron in the multiphoton ionization of water and the single-photon ionization of Fe(CN)64- in aqueous solutions. The ET reaction corresponds to an electron scavenging by various electron acceptors, such as Cd2+. Using the data reported herein, and previously published data on the scavenging kinetics of other electron precursors (e.g., from radiolysis) and optically excited states of the hydrated electron, it was shown that the rate constant of ET varies inversely with the volume of specific form of the hydrated electron. These data strongly support a unified model for the electron-transfer kinetics of many forms of delocalized electrons with localized electron acceptors in which ET rates are assumed to be proportional to the average electron density of the specific hydrated electron excited state or precursor.

Experimental information on the spatial extent of the putative p and conduction band states of the hydrated electron (eeq) has been obtained by monitoring and controlling the electron/hole spatial separation of a photogenerated eeq/hole pair using a femtosecond laser pulse sequence. Optical excitation of eeq with two photons is observed to dramatically alter its spatial distribution and geminate recombination yield with the hole. One-photon excitation, in contrast, has no effect on the spatial distribution. The results strongly confirm theoretical predictions on the size and location of excited states of eeq.

Direct 1-photon excitation of the hydrated electron from the ground state to the conduction band is achieved with 400 nm excitation. This provides a unique method for investigating the femtosecond dynamics and spectroscopy of the conduction band hydrated electron without interference from other states. The assignment of the 400 nm excited state to the conduction band is supported by the observation of >35 A spatial migration of the hydrated electron, as manifested in complete supression of geminate recombination. Similar dynamics and electron scavenging yields are observed for the 400 nm state and that produced by 2-photon 800 nm excitation.

A detailed investigation of ultrafast photoinduced electron transfer (ET) in a mixed valence compound ((NH3)5RuNCRu(CN)5-; RuRu) in formamide, ethylene glycol, and glycerol was performed using variable wavelength femtosecond pump−probe spectroscopy over a broad range of pump and probe wavelengths. The ET kinetics were monitored by observing recovery of the ground state population. The spectra are highly dynamic, indicating the need for broad spectral coverage to accurately unravel the ET kinetics from excited state and ground state relaxation dynamics. ET is observed to proceed more slowly in slower solvents in a manner consistent with the Hybrid model of solution phase electron transfer. The ET time (1/e) of RuRu in ethylene glycol is 220 fs compared to 100 fs in water. As recently reported for RuRu in water, the ET time is pump wavelength independent. A previously unobserved pump wavelength dependence of the early time dynamics, however, is observed which is assigned to stimulated emission prior to vibr...

The chemical mechanism of surface enhanced Raman scattering (SERS) is used as a probe of adsorbate–substrate charge-transfer excitations on atomically smooth, single crystal copper surfaces in ultra-high vacuum. It is demonstrated that SERS and charge-transfer excitations are supported on atomically smooth surfaces in contradiction to earlier consensus. The combination of SERS with electron energy loss spectroscopy (EELS) provides a great deal of insight into the nature of these excitations. It is found that these excitations are sensitive probes of local electronic structure at metal surfaces. The spatial extent, transition moment, and excited state potential energy surfaces of these excitations are probed using SERS, EELS and molecular spectroscopy simulations.

Electron energy loss experiments provide evidence for the two-dimensional spatial localization of the metal/molecule charge-transfer excited state of pyromellitic dianhydride adsorbed on copper single-crystal surfaces in ultrahigh vacuum. Codosing the surface with atomic cesium splits the interfacial loss peak into a doublet, whose energies are independent of cesium coverage. This behavior is in contrast to the smooth, continuous shift in energy expected from the classical picture of alkali metal adsorption in which the substrate Fermi energy increases with coverage at low coverage. Line shape analysis suggests that the localized region includes the four adsorbed molecules nearest each cesium adatom.

We have investigated the chemical mechanism of surface enhanced Raman scattering (SERS) on an atomically smooth metal surface using electron energy loss spectroscopy (EELS) and molecular spectroscopy simulations. The EEL spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(100) and Cu(111) are reported. Simulations of the surface-enhanced Raman spectra and electron energy loss spectra (EELS) of pyromellitic dianhydride adsorbed on Cu(100) and Cu(111) are reported. The surface enhanced Raman spectra [J. Chem. Soc. Faraday Trans. 92, 4775 (1996)] and the EEL spectra are shown to be sensitive to crystal face. The relevant excited state observed in the EEL spectrum is not intrinsic to molecular PMDA, but results from chemisorption. The Raman spectra are sensitive to the incident laser polarization on both the (100) and (111) surfaces but in different ways. These observations are shown to be a result of the excited state potential energy surface having different shape, and the respective transition dipole...

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.

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.

We demonstrate a dual pulse-shaper setup capable of independent polarization, phase, and amplitude control over each pulse. By using active phase stabilization, we achieve a phase stability of ~λ/314 between the two pulse shapers, making the dual-shaper setup suitable for both two-quantum and one-quantum measurements. The setup is compact and easily switchable between pump-probe and collinear geometries. We further illustrate the functionality of the dual-shaper setup by performing two-color 2D visible spectroscopy on colloidal CdSe quantum dots in pump-probe geometry.

Temperature-dependent photoluminescence spectra of a range of semiconductor nanocrystals are presented which reveal an unanticipated thermodynamic relationship between the core excitonic states and the surface of the nanocrystal. These results challenge the widely held view of the surface emission arising from an energetically broad distribution of midgap defect states at the surface. We show that a simpler model based on semiclassical electron transfer and invoking only a single surface state uniquely describes all observables.

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.

Biexciton processes in semiconductor nanocrystals, e.g. optical gain, are widely treated as arising from structureless biexcitons. We challenge this view by observing the binding energies of hot biexcitons in CdSe nanocrystals. These experiments reveal a previously unobserved spectrum in the biexciton binding energies that correlates with hole configuration. Analysis of this spectrum of binding energies shows that the onset and the decay of optical gain in nanocrystals are dictated by cooling dynamics of the hot biexciton rather than Auger recombination. In addition to the structural dynamics of the biexciton, we show that the capacity to control optical gain bandwidth in these nanocrystals arises from stimulated emission from higher multiexcitons.

In this chapter, the influence of quantum confinement upon light amplification is reviewed. Emphasis is placed upon the two broad classes of three-dimensionally confined nanostructures: self-assembled quantum dots and colloidal quantum dots. Fabrication, carrier dynamics, and state-of-the-art-device results for these classes of quantum dots are reviewed.

The elementary excitation in semiconductor quantum dots is the exciton, an excited electron-hole pair. The size and geometry of the dot confines the exciton thereby yielding quantum confinement effects. The simplest examples of size quantization effects include the spectrum of single exciton states which dominate the linear absorption spectrum and the Stokes shift for the spontaneous photoluminescence spectrum. Here, we report on our recent results exploring the structure and dynamics of both single excitons and multi-excitons in colloidal CdSe quantum dots using femtosecond state-resolved pump/probe spectroscopy. These experiments have revealed tremendous insight into the relaxation pathways of hot excitons, new aspects of exciton-phonon interactions, and in the first observation of the electronic structure of multi-excitons.

We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.

We report on the electronic structure of biexcitons in CdSe quantum dots using state-selective femtosecond pump/probe spectroscopy. The pump/probe experiments are compared to direct probing of biexcitons via two-dimensional electronic spectroscopy.

Employing real time observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response of quantum dots, that is quantized, tunable and an order of magnitude larger than their intrinsic piezo response.

Optical gain in strongly confined colloidal semiconductor quantum dots is measured using state resolved pump/probe spectroscopy. Though size tunable optical amplification has been previously reported for these materials, the influence of confinement enhanced multiexcitonic interactions has limited prior demonstrations to specific particle sizes or host media. Here we show that the influence of the interfering multiexcitonic interactions, and hence the development of optical gain, is dependent on the identity of the initially prescribed excitonic state. By maintaining a constant excitonic state in the size tunable electronic structure of these materials, we recover the predicted universal development of optical gain, reflected by size-independent occupation thresholds, and differential gains. In addition, we explicitly compare the influence of surface passivation on the development and lifetime of the optical gain. Furthermore, we introduce a general, state-resolved pumping scheme which enables control over the optical gain spectrum. The capacity to manipulate the optical gain spectra of these spherically confined systems is evident in both the measured stimulated emission and amplified spontaneous emission. We anticipate that state-resolved optical excitation will be a useful method of enabling the development and manipulation of optical gain in any quantized nanostructure.

We report on the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots using femtosecond spectroscopy and atomistic many-body pseudopotential calculations. Time-resolved spectroscopy shows that optical transitions between excitonic and biexcitonic states are distinct for absorptive and emissive transitions, leading to a larger Stokes shift for the biexciton than for the single exciton. The calculations explain the experimental results by showing that there is a previously unobserved electronic substructure to the band-edge biexciton which yields two distinct families of transitions.

Excitonic state-resolved optical pumping experiments were performed on strongly confined semiconductor quantum dots. We demonstrate for the first time that optical gain is dependent upon the initial excitonic state. By prescribing the specific multiexcitonic states which can create, block, and ultimately control optical gain spectra, we recover the theoretically predicted size independence, even in systems which previously showed zero gain. In addition, we show for the first time that stimulated emission in quantum dots can be controlled via specific multiexcitonic interactions.

Biexcitons in strongly confined, colloidal CdSe quantum dots were investigated with excitonic state selectivity combined with 10 fs temporal precision. Within the first 50 fs, the first excited state of the biexciton was observed. By 100 ps, mixed character biexcitons were observed, comprised of a core exciton and a surface trapped exciton. The size dependence of the biexciton binding energies is reported for these specific biexcitons. Analysis of the spectral signatures of each biexcitonic state yields a quantitative measure of enhanced excited state trapping rates at the surface of the quantum dots. By comparing the biexcitonic signals to the state-filling signals, we show that it is primarily the holes which are trapped at the interface on the 100 ps time scale.

We show that biexcitons in quantum dots posses a coarse and fine structure. The intrinsic fine structure of biexcitons has not been previously observed in any system and is the controlling parameter for optical gain.

Using state-selective excitation to tailor multi-exciton interactions, we have demonstrated that optical gain in strongly confined semiconductor quantum dots is a completely universal, size independent, and intrinsic property of these materials.

Coherent optical and acoustic phonons are simultaneously observed in CdSe quantum dots with excitonic state-specificity for the first time. These experiments yield the size and eigenstate dependent coupling strengths for both modes.

Size dependent electron and hole relaxation dynamics were measured in colloidal CdSe quantum dots with state-to-state specificity. These experiments reveal the electron and hole state-to-state relaxation dynamics with a precision of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, allowing quantitative evaluation of the manifold of pathways by which an exciton relaxes in strongly confined quantum dots. These experiments corroborate previously observed confinement induced femtosecond Auger relaxation channels for electrons, but with sufficient precision to quantitatively and unambiguously determine the size dependence of the Auger mechanism. These experiments also show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. We propose a confinement enhanced mechanism for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. The relative contributions of the relaxation pathways are identified for electrons and for holes. These state selective experiments produce a unified picture of the manifold of relaxation pathways available to both electrons and holes in strongly confined colloidal quantum dots.

Size dependent hole dynamics are measured in colloidal CdSe quantum dots for a specific state-to-state excitonic transition. These experiments show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. These experiments reveal a new mechanism for hole relaxation in colloidal quantum dots which circumvents the expected phonon bottleneck for holes. The data are consistent with a nonadiabatic surface channel as the dominant pathway for hole relaxation in colloidal semiconductor quantum dots.

Femtosecond carrier relaxation dynamics in colloidal CdSe quantum dots are measured which focus on the early time dynamics for different initial excitonic states. Selecting the initial excitonic states produces a clear influence upon the early time dynamics. The difference between the dynamics when initially populating different excitonic states provides a direct measurement of excitonic relaxation times in strongly confined colloidal semiconductor quantum dots with specificity to electrons and holes. These results furthermore show state specific biexciton interactions.

Femtosecond relaxation dynamics of colloidal CdSe quantum dots are measured for different initial excitonic states. These experiments show dramatic, previously unobserved dynamics at all probe wavelengths based upon preparation of the initial state.

We analyze epitaxially grown InGaN/GaN dot-in-a-wire heterostructures to relate growth and design properties to trends seen in photoluminescence (PL) and resonance Raman spectra. Temperature-dependent PL measurement of these dot-in-a-wire heterostructures illustrate an expected decrease in integrated PL emission and an unusual narrowing of peak width with increasing temperature. Information extracted from Resonance Raman spectra was utilized in a time-dependent model to analyze and to simulate PL spectra. These spectra were found to be in good agreement with the experimental PL data and provided insight into the broadening mechanisms affecting the samples. PL measurements were taken as a function of position on the sample and radial variation of peak energies was observed. This variation was attributed to the radial temperature gradient present during nanowire growth. These additional characteristics of the nanowire heterostructures will allow for increased understanding of these systems potentials for applications in white light emitting diodes.

We demonstrate a pulse-shaper capable of independent polarization, phase and amplitude control over each pulse. The set-up is compact and easily switchable between pump-probe and collinear geometries.