Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots

Funston A.M., Jasieniak J.J., Mulvaney P.

A study was conducted to investigate the energy transfer (ET) between CdSe-based nanocrystals and organic dye molecules. The study used cores, core/shell, and rods nanocrystals with Texas Red Cadavarine and Lissamine Rhodamine B Ethylenediamine dyes. The absorption and emission spectra showed that the nanocrystals has a large absorption cross-section in the spectral region. It was observed during the study that a single dye molecule can effectively perform quenching of exciton PL on CdSe nanocrystals. The study also found that the energy transfer can be changed by changing the surface chemistry of the CdSe nanocrystals. The study concluded that the performance of the dye-based solar cells can be improved by utilizing semiconductor nanocrystals as supersensitizers. The study used UV-vis-NIR to obtained the absorption spectra of the nanocrystals.

Huang J., Huang Z., Jin S., Lian T.

Ultrafast charge transfer from quantum dots (QDs) is essential for their application in solar cells. Photoinduced hole transfer from CdSe QDs (with first exciton peak at 462 nm and estimated radius of 1.8 nm) to phenothiazine (PTZ) was investigated by time-resolved fluorescence and transient absorption spectroscopy. Fluorescence lifetime measurements showed that the presence of PTZ reduced the exciton lifetime of CdSe QDs, and the quenching rate increased with increasing PTZ concentration. Transient absorption spectra of CdSe−PTZ complexes directly revealed the formation of PTZ cation radical, suggesting that the excitons in CdSe dissociated by hole transfer to PTZ. A kinetic model based on a Poisson distribution of the number of adsorbates on each QD was found to adequately describe the adsorbate concentration-dependent fluorescence decay kinetics. According to this model, the hole transfer time was ∼2.5 ns in 1:1 CdSe−PTZ complexes and reached ∼300 ps in samples with an average of ∼6 PTZ per QD.

Sun J., Goldys E.M.

We present a simple theoretical analysis of the linear absorption coefficient due to interband transition in quantum dots of direct bandgap semiconductor materials aimed at quantification of the quantum dot concentration in colloidal suspensions. We establish the relationship of the linear absorption coefficient thus calculated with the universally used molar extinction coefficient to be able to compare our theoretical predictions with experimental studies. On the basis of this foundation, we develop a simple approach to calculate the value of the coefficient using empirical values taken from the extinction measurements and basic physical parameter of materials. We explain the long-standing discrepancies between the theories of quantum dot absorption and experiments as well as the trends of the extinction coefficient with varying quantum dot size.

Jasieniak J. ., Fortunati I., Gardin S., Signorini R., Bozio R., Martucci A., Mulvaney P.

The process of two-photon optical pumping at 800 nm t, which allows for the generation of highly stable, amplified stimulated emission from a variety of CdSe-CdS-ZnS sized nanocyrstals was investigated. The process was found to be as efficient as conventional excitation mechanisms in the 1-photon excitation region. The CdSe cores were prepared by the method describe by Van Embden et al. The CdSe-CdS-ZnS quantum dots were prepared through adapted synthetic protocols of Li et al. A zinc rich surface was created to ensure the resulting nanoparticles,which possessed both high PL yield and photostability. The nanocrystal were rendered soluble in ethanol through 5-aminopentanol and tris-hydroxyphosphine surface exchange and further incorporated into Zirconia sol-gel matrix by using a protocol. The post synthesis of the nanocrystal surface chemistry was tailored to ensure homogenous nanocrystal, which incorporate into zirconia based waveguides.

Moreels I., Lambert K., De Muynck D., Vanhaecke F., Poelman D., Martins J.C., Allan G., Hens Z.

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The observed ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ϵ scales with the nanocrystal volume, irrespective of the Q-PbSe size. From ϵ, we calculated a size-independent absorption coefficient. Its value is in good agreement with the theoretical value for bulk PbSe. At the band gap, ϵ is size-dependent. The resulting absorption coefficient increases quadratically with decreasing Q-PbSe size. Calculations of the oscillator strength of the first optical transition are in good agreement with theoretical ...

van Embden J., Jasieniak J., Gómez D.E., Mulvaney P., Giersig M.

Passivation of CdSe semiconductor nanocrystals can be achieved by overcoating the particles with a homogeneous shell of a second semiconductor. Shell layers are grown in monolayer steps to ensure homogeneous growth of the shell. The relative band edges of the two materials determine the photoreactiveity of the resultant core-shell nanocrystals. The critical role of ligands in minimizing nucleation of the shell material during the growth of the passivating layer is emphasized. The delocalization of charge carriers into the shell layers can be followed spectroscopically during the growth processes. The relative spectral shifts are directly correlated to the relative energies of the band edges.

Jasieniak J., Mulvaney P.

We report a protocol for manipulating the surface composition of CdSe nanocrystals. By combining the successive ion layer adhesion and reaction (SILAR) method developed by Li et al. J. Am. Chem. Soc. 2003, 125, 12567 with a phosphine-free selenium precursor, the surface stoichiometry of CdSe can be tunably altered from Cd- to Se-rich. By changing the overall surface stoichiometry, we demonstrate ligand binding to specific surface sites. Tertiary phosphines produce a dramatic enhancement in photoluminescence quantum yield of CdSe particles with Se-rich surfaces but have little effect on Cd-rich surfaces. Unpassivated selenium surface sites are also shown to be a cause of the photobrightening behavior of CdSe nanocrystals.

Cademartiri L., Montanari E., Calestani G., Migliori A., Guagliardi A., Ozin G.A.

We report here on a detailed study on PbS colloidal quantum dots. A characterization via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) allowed us to reliably determine the diameter and the shape of the nanocrystals. These data, together with second-derivative analysis of the absorption spectra, allowed us to determine the size dependence of seven transitions in the absorption spectrum; some of these transitions were identified on the basis of their normalized confinement energy. The size dependence of the first excitonic transition was best modeled by a four-band envelope approach which considers the anisotropy of the band edges (Andreev, A. D.; Lipovskii, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15402-15404). The extinction coefficients were calculated using concentrations obtained from inductively coupled plasma atomic emission spectrometry (ICP-AES), and their size dependence was found to follow a power law with exponent equal to approximately 2.5. In contrast with what was expected from the effective mass approximation, the per particle absorption cross section of the lowest transition was found to be strongly dependent on the particle size.

Sun B., Greenham N.C.

We present photovoltaic devices based on a blend of the conjugated polymer poly(3-hexylthiophene) (P3HT) with cadmium selenide nanorods, where the solvent for film deposition has been carefully chosen to optimize the film morphology. Using 1,2,4-trichlorobenzene (TCB), which has a high boiling point, as solvent for P3HT it is possible to obtain a fibrilar morphology, providing extended pathways for hole transport. Blend devices fabricated using this solvent gave solar power conversion efficiencies of 2.6%. This indicates that efficient transport of electrons and holes is achieved in these films, allowing them to operate effectively at solar illumination intensities.

Gur I., Fromer N.A., Geier M.L., Alivisatos A.P.

We introduce an ultrathin donor-acceptor solar cell composed entirely of inorganic nanocrystals spin-cast from solution. These devices are stable in air, and post-fabrication processing allows for power conversion efficiencies approaching 3% in initial tests. This demonstration elucidates a class of photovoltaic devices with potential for stable, low-cost power generation.

van Embden J., Mulvaney P.

The competing effects of two ligands, oleic acid (OA) and bis-(2,2,4-trimethylpentyl) phosphinic acid (TMPPA), on the nucleation rate and growth of CdSe nanocrystals in octadecene are reported. It is found that TMPPA acts as a high boiling point "nonsolvent" or "nucleating agent". Addition of TMPPA leads to higher initial particle yields and smaller particle diameters. Conversely, oleic acid inhibits nucleation and results in a drastic increase in "early time ripening" (ETR), a phenomenon that causes a rapid reduction in the number of particles within the first minutes of reaction. By controlling the number of nuclei formed with TMPPA and tuning the rate of ETR with oleic acid, high yields of particles can be obtained with sizes between 3 and 7 nm. Furthermore, in the absence of OA, the preparation of very small nanocrystals with diameters approximately 2 nm is facilitated.

Medintz I.L., Uyeda H.T., Goldman E.R., Mattoussi H.

One of the fastest moving and most exciting interfaces of nanotechnology is the use of quantum dots (QDs) in biology. The unique optical properties of QDs make them appealing as in vivo and in vitro fluorophores in a variety of biological investigations, in which traditional fluorescent labels based on organic molecules fall short of providing long-term stability and simultaneous detection of multiple signals. The ability to make QDs water soluble and target them to specific biomolecules has led to promising applications in cellular labelling, deep-tissue imaging, assay labelling and as efficient fluorescence resonance energy transfer donors. Despite recent progress, much work still needs to be done to achieve reproducible and robust surface functionalization and develop flexible bioconjugation techniques. In this review, we look at current methods for preparing QD bioconjugates as well as presenting an overview of applications. The potential of QDs in biology has just begun to be realized and new avenues will arise as our ability to manipulate these materials improves.

Snee P.T., Chan Y., Nocera D.G., Bawendi M.G.

Xie R., Kolb U., Li J., Basché T., Mews A.

We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.

Yu P., Beard M.C., Ellingson R.J., Ferrere S., Curtis C., Drexler J., Luiszer F., Nozik A.J.

We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3.45 nm. We find excellent agreement between the measured results and calculated values based on a model of small-particle light absorption. The absorption cross-section per dot is 6.2 x 10(-16)R(3) cm(2) at 2.76 eV and 3.15 x 10(-16)R(1.28) cm(2) at the first-exciton absorption peak, with the dot radius R in nm. We find that the per-quantum-dot particle oscillator strength of the first-exciton transition is constant for all sizes studied. The radiative lifetime of the first exciton calculated from the oscillator strength increases with dot size and ranges from 4 ns for the smallest dots to 14 ns for the largest ones.

Li Y., Shuang S., Jiao X., Yu C., Tang C., Lin J., Huang Y.

Huang C., Duan Y., Makarov N.S., McDaniel H., Shim M.

Bird O.F., Drbohlav K.M., Gowdy E.K., Flinkingshelt F.A., Pellows L.M., Hammel B.F., Layne B.W., Ardo S., Yang J.Y., Miller K.A., Dukovic G.

Gan Q., Yang H., Zhang Z., Liu J., Guo Y., Luo D., Liu X.

Li Y., Yu J., Li J., Wang Y., Sun B.

AbstractUtilizing photoluminescent quantum dots (QDs) as a luminescent down‐shifting (LDS) layer to convert high‐energy photons into lower‐energy ones is a prominent approach to reducing parasitic absorption of silicon heterojunction (SHJ) solar cells. Here, a ray‐optic model is presented to gain insight into light conversion contribution on the short‐circuit current density (Jsc) of the SHJ solar cell with an LDS layer. The correlation reveals that the primary factors impacting external quantum efficiency (EQE) are the absorption coefficient at short wavelengths and the photoluminescence quantum yield (PLQY) of the LDS layer. Notably, PLQY is dominant in determining the contribution to the device efficiency if the LDS layer can harvest all the parasitic light, particularly when there is no surface reflectance change. Furthermore, the EQE spectrum of high‐efficiency SHJ solar cells is experimentally investigated with the QDs LDS layer to validate the model, revealing that it aligns well with the experiment results. Employing a MgF2/QDs LDS layer, the Jsc with 0.50 mA cm−2 is enhanced, yielding the SHJ solar cells with an efficiency of over 22.3%. The work develops a broadly applicable model that aids in screening suitable photoluminescent materials for LDS layer applications in photovoltaic devices and elucidates the theoretical contributions to EQE.

Porell R.N., Nag O.K., Stewart M.H., Susumu K., Oh E., Delehanty J.B.

Adhikari L., Todorov T.I., Yang T., Hornick J., Sawant R., Paulose T., Duncan T.V.

Lee H.G., Kwon Y.W., Jung W.H., Lee H., Kim M.S., Kim H., Kim H., Kim H.J., Lee D.C., Lim J., Cho S.

Koole R., Groeneveld E., Vanmaekelbergh D., Meijerink A., de Mello Donega C.

This chapter addresses the fundamental concepts needed to understand the impact of size reduction on the electronic structure and optoelectronic properties of semiconductor nanostructures, with emphasis on quantum confinement effects. This effect is explained by two different approaches: the “top-down” and the “bottom-up”. The impact of dimensionality and shape on quantum confinement is also discussed. Subsequently, a brief description of the optical properties of semiconductor nanocrystals is presented. This is followed by a discussion of the essential characteristics of nanocrystals consisting of two (or more) different semiconductors joined together by heterointerfaces (i.e., heteronanocrystals). Moreover, the essential differences between the impact of size reduction on semiconductors in comparison to metals and insulators is discussed.

Leippe S.C., Johst F., Hoffmann B., Krohn S., Papagrigoriou K., Asmis K.R., Mews A., Bastian B.

Paul L., Thomas E.M., Chemmangat A., Gray S.K., Thomas K.G.

Demonstrated the selective enhancement of band-edge emission over trap-state emission in CdSe QDs via frequency-specific plasmon resonance coupling, and the competing role of plasmon-assisted nonradiative energy transfer by varying its number density.

Wang L., Liu L., Zhang R., Zhou Z., Zhang X., Liu D., Liang Y., Liang G.

Owing to the long-lived decay of triplet excited state, extensive efforts have been devoted to efficient triplet generation for applications covering triplet–triplet annihilation for photon upconversion, photocycloaddition and photoredox catalysis. Among the candidates, nanocrystal-molecule complexes have received tremendous attention for triplet generation because of easier spin flip and negligible energy loss during intersystem crossing. However, the triplet energy transfer (TET) from nanocrystals (NCs) to molecules can be very complicated in actual situation due to intricate energy level alignment and inevitable defect states, which often involves various decay pathes of the excited state competing with TET. Understanding the detailed carrier dynamics in such complexes is strongly necessary for related applications. Here, a CdSe-TCA (5-tetracene carboxylic acid) complex with a Type-II like energy level alignment is synthesized through precisely adjusting the dimension of CdSe NC. Based on series of spectral measurements, especially the transient absorption (TA) spectroscopy, the results show various carrier dynamics including hole-transfer-mediated TET, Förster resonance energy transfer (FRET) and carrier trapping. Although the carrier trapping by defect states in CdSe NC is revealed not associated with the TET from CdSe to TCA, the FRET is proved to competing with the TET process. Both the FRET and defect states should be refrained for efficient TET in such complexes. This study could provide further insight for understanding the carrier dynamics competition in NC-molecule complexes for triplet generation and benefit related optoelectronics applications.

Mukherjee S., Ghosh S., Ghosal M., Dey S., Mandal S., De C.K., Debnath R., Karmakar M., Karmakar A.J., Datta P.K., Mandal P.K.

AbstractGigantic magnitude of molar extinction coefficient (ɛ) (≈2 × 108 m−1 cm−1) and PLQY (0.98) through Zn‐alloying and surface‐passivating ligand variation, make the CsPbI3 based perovskite nanocrystal (PNC) the most super‐absorbing and most super‐bright among quantum dot‐PNC family. Radiative rate got enhanced 9 times and non‐radiative rate decreased 13 times. Moreover, very high (detrapping rate/trapping rate) of electron and 90% peak of ON fraction make this PNC a nearly‐non‐blinking one. Ultrafast and comparatively shorter hot exciton/electron cooling time (536 fs) and hot exciton/electron trapping time (516 fs) can explain why this PNC exhibits hitherto unobserved excellent optical properties.

Van Duong P., Thi L.A., Toan L.D., Tung D.H., Hieu D.M., Trung L.C., Tuyen N.T., Hoa N.M.

Colloidal semiconductor nanocrystals (NCs) have been extensively studied for their tunable bandgap energy and outstanding optical properties. This research focuses on the synthesis of CdSe/CdSeS heterostructure nanocrystals (HNCs) with precisely controlled spherical morphologies. The synthesis process involved forming a CdSe core and subsequently coating it with a CdSeS shell through the gradual injection of sulfur (S) precursors, allowing the shell thickness to be finely adjusted between two and six monolayers. As the shell thickness increases, both the emission and absorption spectra systematically shift to lower-energy regions, thereby modifying the optical properties of the HNCs. A strong correlation is observed between the interfacial strain at the CdSe/CdSeS boundary and the shell thickness, which significantly affects the efficiency of HNCs. Temperature-dependent analyses reveal variations in emission energy with temperature, offering insights into the thermal behavior and resilience of these crystals. Anomalies in the temperature-dependent photoluminescence linewidth are attributed to carrier relaxation into local energy minima caused by interfacial strain. Compared with bare NCs, core-shell HNCs demonstrate enhanced coupling between excitons and both longitudinal optical phonons and acoustic phonons. This study highlights the critical role of shell thickness in fine-tuning the optical and thermal properties of CdSe/CdSeS HNCs, offering valuable insights for their potential applications in advanced technologies.

Murray M.J., Neal S.L.

Coincident photo-induced weathering processes in oleic acid capped CdSe/ZnS semiconductor quantum dots (QDs), stimulated by exposure to artificial sunlight in samples with different headspace volumes, were monitored using multichannel frequency-domain photoluminescence (PL) decay measurements. The combination of process monitoring using matrix-formatted measurements and multivariate data mining permits resolution of the spectra, frequency-domain decays and kinetic profiles of the PL emitted by distinct QD subpopulations. The data indicate that three components contribute to the PL: photodarkened QDs that emit bluer, shorter-lived spectra reflecting photoionization-induced QD charging and ligand desorption; photobrightened QDs that emit redder, short-lived spectra indicative of photooxidative QD surface passivation; and photoripened QDs that emit very weak, red-shifted, large bandwidth, very long-lived spectra reflecting increasing nanoparticle growth and polydispersity. Analysis of the kinetic traces of the QD subpopulations during photoirradiation using systems of exponentials revealed the extent of coupling between the coincident photoinduced processes. The assignment of these components was supported by their emission maxima, spectral bandwidths, average and irradiation-time specific PL decay times, irradiation-dependent kinetic profiles, TEM images and literature precedents. As expected, the results show that QD photodarkening is faster and photobrightening is slower in the samples that have more exposure to water and oxygen. They also show that the dominant photodarkening and photobrightening pathways in those samples are not as strongly coupled, indicating the importance of contributions by additional processes in that case. Transmission electron microscopy (TEM) images were consistent with the PL results, but the QD samples proved too polydisperse to be characterized by single-beam dynamic light scattering measurements.