CdSe Nanocrystal Quantum Dots Research Articles

Picosecond Electron Transfer from Quantum Dots Enables a General and Efficient Aerobic Oxidation of Boronic Acids

General visible light-mediated aerobic oxidation of boronic acids is unveiled using CdSe nanocrystal quantum dots (QDs) as the photoredox catalyst. This protocol requires mild reaction conditions and low catalyst loading (down to 10 ppm), and tolerates various functional groups. The resulting phenols and aliphatic alcohols are produced in good to high yield with turnover numbers as high as >62000. The reaction mechanism is probed using ultrafast transient absorption and luminescence spectroscopy. The existence of a rapid 350 ps initial electron transfer followed by a hole transfer is demonstrated.

Exciton diamagnetic shift and optical properties in CdSe nanocrystal quantum dots in magnetic fields

The magnetic field dependence of the optical properties of CdSe nanocrystal quantum dots (NQDs) is investigated theoretically using a perturbation method within the effective-mass approximation. The results show that the magnetic field lifts the degeneracy of the electron (hole) states. A blue-shift in the absorption spectra of m ≥ 0 exciton states is observed while the absorption peak of m < 0 exciton states is first red-shifted and then blue-shifted with increasing the magnetic field strength B. This is attributed to the interplay of the orbital Zeeman effect and the additive confinement induced by the magnetic field. The excitonic absorption coefficient is almost independent of B in the strong confinement regime. The applied magnetic field causes the splitting of degenerated exciton states, resulting in the new absorption peaks. Based on the first-order perturbation theory, we propose the analytical expressions for the exciton binding energy, exciton transition energy and exciton diamagnetic shift of 1s, 1p-1, 1p0, 1p1, 1d-2, 1d-1, 1d0, 1d1, 1d2 and 2s exciton states on the applied magnetic field in the strong confinement regime.

Spin-Pumping Efficiency in Room-Temperature CdSe Nanocrystal Quantum Dots

To understand and optimize optical spin initialization in room-temperature CdSe nanocrystal quantum dots (NCQDs), we studied the dependence of the time-resolved Faraday rotation signal on pump energy (Ep) in different sizes of NCQDs. The resulting spin-pumping efficiency (SPE) spectrum differs significantly from the absorption spectrum. In larger NCQDs, we observe two peaks in the SPE spectrum, whereas in smaller NCQDs, there is only one peak before a low, broad plateau. On the basis of our effective mass calculations, the Ep dependence of SPE arises from the competition between the heavy hole (hh), light hole (lh), and split-off (s.o.) band contributions to the optical transitions. At lower Ep, hh transitions dominate and give rise to the low-energy peaks. Because of the increasing contributions from the lh and s.o. bands which produce spin polarization in the opposite direction, SPE is reduced at higher Ep. The different number of peaks in larger and smaller NCQDs is attributed to size-dependence of the...

Site-Selective Ion Beam Synthesis and Optical Properties of Individual CdSe Nanocrystal Quantum Dots in a SiO2 Matrix

Cadmium selenide nanocrystal quantum dots (NC-QDs) are site-selectively synthesized by sequential ion beam implantation of selenium and cadmium ions in a SiO2 matrix through submicrometer apertures followed by a rapid thermal annealing step. The structural and optical properties of the NC-QDs are controlled by the ion fluence during implantation and the diameter of the implantation aperture. For low fluences and small apertures the emission of these optically active emitters is blue-shifted compared to that of the bulk material by >100 meV due to quantum confinement. The emission exhibits spectral diffusion and blinking on a second time scales as established also for solution-synthesized NC-QDs.

Synthesis of bright CdSe nanocrystals by optimization of low-temperature reaction parameters

The reaction parameters of low-temperature synthesis of colloidal nanocrystals were investigated. It was found the structural and optical properties of CdSe nanocrystal quantum dots synthesized at 130 °C are particularly sensitive to alkyl-chain lengths of ligands, the cadmium source, and ligand to precursor ratios. Under optimized synthesis conditions CdSe quantum dots with narrow ensemble size distribution and excellent photoluminescence quantum efficiencies of up to 40 percent were obtained.

Synthesis and Characterization of CdSe Nanocrystal Quantum Dots by Solution-Liquid-Solid Method

The present work describes a solution-liquid-solid (SLS) synthesis route for the preparation of CdSe nanocrystal quantum dots using the low-melting-point Bi nanoparticles as catalysts. UV-vis absorption spectra were used to determine the growth of CdSe cluster. The shape and size distribution of the CdSe QDs were determined by analysis of Transmission electron microcopy (TEM) images. A peak wavelength of 399nm in the absorption spectra on day 5 indicated that the CdSe cluster formed. TEM micrographs showed that CdSe QDs had a relatively narrow distribution of QD size, and the diameter of CdSe QDs was about 4 nm. Our preliminary results illustrated that smaller size of CdSe QDs could be prepared by SLS method and it was small enough for bioimaging applications.

Picosecond Kinetics of Strongly Coupled Excitons and Surface Plasmon Polaritons

Coupling between excitons of CdSe nanocrystal quantum dots (NQDs) and surface plasmon polaritons (SPPs) of an Ag film attached to a prism have been studied by steady-state and transient reflectivity measurements in the Kretschmann geometry. In these experiments, the angle of incidence of the probe beam selects hybrid exciton/SPP states with different wavevectors and exciton/SPP compositions. The dynamics measured in the transient reflectivity experiments are sensitive to the composition of the hybrid states. Specifically, fast dynamics are observed at probe wavevectors where the lower hybrid state has predominant SPP character. In contrast, at probe wavevectors where the lower hybrid state is predominantly excitonic, the dynamics are similar to that measured for CdSe NQDs on glass.

Role of Solvent–Oxygen Ion Pairs in Photooxidation of CdSe Nanocrystal Quantum Dots

Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent-oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent-oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix.

Size Selective Excitonic Transition Energies in Strongly Confined CdSe Quantum Dots

We report on the synthesis of CdSe nanocrystal quantum dots (QDs) of different radii (R). Size dependent optical properties like increase in the confinement energy with decreasing radius for different excitonic transitions are studied. Different excitonic transitions are calculated from the second derivative of UV-vis absorption spectra of as synthesized CdSe QDs. The transitions are assigned to specific states by calculating the transition energies using effective mass approximation. A close matching of the transition energies with the experiment suggesting that the second derivative of the absorption spectra could provide a direct knowledge of the electronic transition for the direct band gap semiconductor quantum dots.

Electronic Properties and Structure of Assemblies of CdSe Nanocrystal Quantum Dots and Ru‐Polypyridine Complexes Probed by Steady State and Time‐Resolved Photoluminescence

AbstractChemical and electronic interactions between CdSe nanocrystal quantum dots (NQDs) and Ru‐polypyridine complexes are studied in solution. It is shown that photoluminescence (PL) can be used to effectively monitor the formation of NQD‐complex assemblies in real time. It is also shown that with the aid of Langmuir isotherm modeling, the PL studies can be used to quantitatively characterize the composition of the assemblies and the strength of electronic interactions between their components. The approach demonstrated here is general and can be applied to other systems that combine semiconductor NQDs and appropriately functionalized organometallic or organic molecules interacting with NQDs via energy transfer, charge transfer, or other mechanisms leading to quenching of NQD emission.

Radiative decay of surface-trapped carriers and quantum yield in CdSe nanocrystal quantum dots

The emission dynamics in nanocrystal quantum dots (NCQDs) is very sensitive to the surface conditions because a big portion of total atoms in NCQDs exists at surfaces. Two sets of CdSe NCQDs forming the wurtzite structure but having different surface conditions are prepared by using different solvents such as octadecene (ODE) and hexadecylamine (HDA) in the synthesis. Compared to CdSe NCQDs prepared with ODE, CdSe NCQDs prepared with HDA have higher quantum yield (QY) owing to the surface passivation which reduces surface energy traps. Time-resolved photoluminescence spectra exhibit a fast decay of initially populated elecron–hole (e–h) recombination and a slow decay of surface-trapped carriers’ radiative relaxation (surface-related emission). The relative intensity of surface-related emission to initially populated e–h recombination is studied in the two sets of CdSe NCQDs, and higher quantum yielding CdSe NCQDs have the stronger surface-related emission in both sets. It is found that the radiative decay of surface-trapped carrier is important in the enhancement of the optical emission of NCQDs. The comparison of surface-related emissions in the two sets of CdSe NCQDs demonstrates that the surface passivated-NCQDs achieve high optical efficiency.

CdSe Quantum-Dot-Sensitized Solar Cell with ∼100% Internal Quantum Efficiency

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

Insights into Reversible Dissolution of Colloidal CdSe Nanocrystal Quantum Dots

The dissolution behavior of colloidal CdSe nanocrystal quantum dots is investigated. A series of experiments is conducted to study the stability of nanocrystals based on a simple reversible dissolution-growth reaction model. Immersing purified CdSe quantum dots in a fresh solvent at elevated temperatures resulted in their dissolution with rates determined by their concentration, size, type and amount of ligands present in solution, and temperature. In general, highest dissolution rates were observed for diluted quantum dot solutions in the presence of phosphonic acids and carboxylic acids ligands, at temperatures below 200 °C. Importantly, the dissolution process is reversible and regrowth of quantum dots occurs at temperatures above 220 °C. It is concluded that the most stable state of nanocrystal quantum dots in solution is an equilibrium between formed crystals and free molecular precursors in solution.

ChemInform Abstract: Photoluminescence from Single Semiconductor Nanostructures

We review some recent results in the spectroscopy of single CdSe nanocrystal quantum dots. By eliminating the effects of inhomogeneous broadening and ensemble averaging, single nanocrystal spectroscopy has revealed many new and previously unexpected physical phenomena. Among those discussed in this review are ultra-narrow emission lineshapes (∼600× narrower than ensemble spectra), a highly polarizable emitting state in the presence of strong local electric fields, line broadening as a result of environmental fluctuations, and shifting of the emission spectra over a wide range of energies (from less than 300 μeV to 80 meV). In addition, polarization spectroscopy of single nanocrystals has revealed the presence of a theoretically predicted two-dimensional transition dipole moment oriented in the xy plane of the nanocrystals. As a result, it is, in principle, possible to use polarization spectroscopy to determine the three-dimensional orientation of individual nanocrystals. These and other studies of single quantum dots have provided us with significant insight into the detailed physics and dynamics of this unique and fascinating physical system.

카드뮴 셀레나이드 양자점과 단일벽 탄소나노튜브로 구성된 이종 나노 소재를 기반으로 한 광전소자의 제작 및 특성평가

In this paper, we fabricated the optoelectronic device based on Cadmium selenide(CdSe) nanocrystal quantum dots (NQDs)/single-walled carbon nanotubes(SWNTs) heterostructure using dieletrophoretic force. The efficient charge transfer phenomena from CdSe to SWNT make CdSe-Pyridine(py)-SWNT unique heterostructures for novel optoelectronic device. The conductivity of CdSe-py-SWNT was increased when it was exposed at ultra violet(UV) lamp, and varied as a function of wavelength of incident light.

Sub-picosecond All-optical Switching of Exciton Spin Polarization in Nanocrystal Quantum Dots Toward Terabit Communication

We report all-optical polarization switching based on resonant excitation of spin-polarized e-h pairs in CdSe nanocrystal quantum dots at room temperature. Pump-probe experiments have been performed using the same and the opposite configurations of circular polarizations. Comparing the co- and the cross-circular polarization cases, it is quite evident that selectively-excited spins are flipped to a balanced state within 1 ps and that during this time, the two spin states are equally populated. The all-optical switching results are found to be in close agreement with pump-probe experimental results. The optical switching technique proposed in the present investigation should be extremely useful for future ultrafast terabit communication systems.

Effect of Surface Ligands on Optical and Electronic Spectra of Semiconductor Nanoclusters

We investigate the impact of ligands on the morphology, electronic structure, and optical response of the Cd(33)Se(33) cluster, which overlaps in size with the smallest synthesized CdSe nanocrystal quantum dots (QDs). Our density functional theory calculations demonstrate significant surface reorganization for both the bare cluster and the cluster capped with amine or phosphine oxide model ligands. We observe strong surface-ligand interactions leading to substantial charge redistribution and polarization effects on the surface. These effects result in the development of hybridized states, for which the electronic density is spread over the cluster and the ligands. The loss of one of the passivating ligands leads to either optically dark or bright additional states inside of the band gap, depending on the position of the leaving ligand on the QD surface. However, for fully ligated QDs, neither the ligand-localized nor hybridized molecular orbitals appear as trap states inside or near the band gap of the QD. Instead, being mostly optically dark, dense hybridized states could open new relaxation channels for high-energy photoexcitations. Comparing QDs passivated by different ligands, we also found that hybridized states are denser at the edge of the conduction band of the cluster ligated with phosphine oxide molecules than that with primary amines. Such a different manifestation of ligand binding may potentially lead to faster electron relaxation in QDs passivated by phosphine oxide than by amine ligands.

Modal gain enhancement by cylindrical waveguide and gain saturation in CdSe nanocrystal quantum dots

Optical modal gain enhancement of CdSe nanocrystal quantum dots in a cylindrical glass-waveguide has been observed by using a variable stripe length method. While poor modal gain was observed near the threshold excitation energy (3.5 mJ) for a thin film structure, significant modal gain (∼ 500 cm−1) was seen in the cylindrical structure due to the waveguide effect. Modal gain saturation for a stripe length was also investigated with a new gain analysis (Kyhm et al 2001 Appl. Phys. Lett. 79 3434). The stripe length dependence of gain saturation is also suppressed in the waveguide structure above the threshold excitation energy (5.5 mJ). We suggest the modal gain enhancement is involved with the intrinsic two electron–hole pair excited states (anti-bonding biexcitons) and the extrinsic waveguide effect.

Lateral heterojunction photodetector consisting of molecular organic and colloidal quantum dot thin films

We demonstrate a heterojunction photodetector of lateral geometry that utilizes an evaporated film of the hole-transporting molecular material N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD) as a charge transport layer and that is sensitized across visible wavelengths by a thin film of colloidal CdSe nanocrystal quantum dots (QDs). High photon-to-electron quantum conversion efficiencies are obtained at room temperature as a result of photoconductive gain. With an electric field of 3.0×105 V/cm applied across the electrodes, we measure the external quantum efficiency at the first QD absorption peak (at wavelength λ=590 nm) to be 13%, corresponding to an internal quantum efficiency of approximately 80%. The operating mechanism of these devices is discussed, noting that the optical response is dominated by the QD absorption spectrum while the charge transport nearly exclusively takes place in the spiro-TPD.

Enhanced ripening behavior of Mg-doped CdSe quantum dots

Pure CdSe and Mg-doped CdSe nanocrystal quantum dots were synthesized into the zinc-blende structure at a low temperature by the inverse micelle technique using paraffin oil and oleic acid as surface capping agents. The ripening behavior of the nanocrystals was monitored using the red shift in ultraviolet (UV)-visible light absorption peaks, and their size variation was estimated using the so-called, quantum confinement theory. The Lifshitz–Slyozov–Wagner (LSW) kinetics analyses were performed based on the variation in size according to the ripening temperature and time period. The activation energy (Q) and reaction rate constant (Ko) were determined for the ripening reaction using Arrhenius-type plots. The kinetics analyses reveal that the volume diffusion through the liquid-phase solution is the governing mechanism for the ripening of both nanocrystals. The Mg-doped CdSe nanocrystals showed enhanced ripening kinetics due to the low activation energy for the volume diffusion.