CdSe Nanocrystals Research Articles

Primary amines enhance triplet energy transfer from both the band edge and trap state from CdSe nanocrystals.

In this study, the role that primary amines play during triplet energy transfer from photoexcited CdSe nanocrystals (NCs) was examined. Colloidally synthesized CdSe NCs were placed in varying concentrations of 1-propyl- or 1-octylamine, with and without 2-anthracenecarboxylic acid transmitter ligands attached. This primary amine increases upconversion quantum yield approximately 5-fold. Further addition of amine does not improve photon upconversion, as CdSe NC photoluminescence (PL) increases at the expense of triplet energy transfer. Transient absorption measurements show that the amine plays three key roles. Primary amines enhance NC PL by decreasing the nonradiative decay rate, increase the rate of triplet energy transfer, and enable the broad trap state in these CdSe NCs to participate in triplet photosensitization.

A novel fluorescent and electrochemical dual-responsive immunosensor for sensitive and reliable detection of biomarkers based on cation-exchange reaction

Sensitive and reliable detection of biomarkers is of vital importance in tumor early detection and clinical therapy. A novel fluorescent/electrochemical dual-responsive immunosensing platform for reliable and sensitive quantification of biomarkers was designed based on cation-exchange reaction. To construct such a versatile platform, the model analyte, carcinoembryonic antigen (CEA), was captured by magnetic Fe3O4 nanoparticles bound with primary antibodies (Fe3O4-Ab1) and then recognized by the detection antibodies conjugated complex containing poly(amidoamine) (PAMAM), carbon nanotube (CNT) and carboxyl functionalized CdSe nanocrystals (NCs) (CNT-PAMAM-CdSe NCs-Ab2). Via ligand exchange, the stable CdSe nanocrystals were easily functionalized with carboxylate ion (CdSe–COO-) and showed high hydrophilicity. The CdSe–COO- was effectively and densely conjugated to CNT coated dendrimer PAMAM that possesses large specific surface area. Finally, the target CEA was detected based on cation-exchange reaction (CER) by adding Ag+ to release thousands of cations Cd2+, which were detected by fluorescence and electrochemistry simultaneously. The electrochemical measurement was performed by directly detecting Cd2+ through square wave voltammetry (SWV), which displayed an excellent correlation with CEA from 5 pg/mL to 50 ng/mL, with a limit of detection (LOD) of 1.7 pg/mL. The fluorescence detection was implemented since free Cd2+ could trigger the weak fluorescence metal-sensitive dyes (Rhod-5N) to generate extremely high fluorescence signal. The fluorescence results showed the LOD for CEA detection was 0.25 pg/mL with a calibration curve range from 1 pg/mL to 20 ng/mL. The dual signal outputs showed an attractively self-correcting ability, which provides the capability of avoiding false positive signal and making the detection result more reliable. The proposed dual-responsive platform holds great promises for biomarkers detection in clinical diagnostics and therapy.

Dual-Emitting Dot-in-Bulk CdSe/CdS Nanocrystals with Highly Emissive Core- and Shell-Based Trions Sharing the Same Resident Electron.

Colloidal CdSe nanocrystals (NCs) overcoated with an ultrathick CdS shell, also known as dot-in-bulk (DiB) structures, can support two types of excitons, one of which is core-localized and the other, shell-localized. In the case of weak "sub-single-exciton" pumping, emission alternates between the core- and shell-related channels, which leads to two-color light. This property makes these structures uniquely suited for a variety of photonic applications as well as ideal model systems for realizing complex excitonic quasi-particles that do not occur in conventional core/shell NCs. Here, we show that the DiB design can enable an unusual regime in which the same long-lived resident electron can endow trionlike characteristics to either of the two excitons of the DiB NC (core- or shell-based). These two spectrally distinct trion states are apparent in the measured photoluminescence (PL) and spin dynamics of core and shell excitons conducted over a wide range of temperatures and applied magnetic fields. Low-temperature PL measurements indicate that core- and shell-based trions are characterized by a nearly ideal (∼100%) emission quantum yield, suggesting the strong suppression of Auger recombination for both types of excitations. Polarization-resolved PL experiments in magnetic fields of up to 60 T reveal that the core- and the shell-localized trions exhibit remarkably similar spin dynamics, which in both cases are controlled by spin-flip processes involving a heavy hole.

Designing High-Performance CdSe Nanocrystal Thin-Film Transistors Based on Solution Process of Simultaneous Ligand Exchange, Trap Passivation, and Doping

We report a simple, solution-based, and postsynthetic process for simultaneous ligand exchange, surface passivation, and doping of CdSe nanocrystals (NCs) for the design of high-performance field-e...

CdSe@CdS Dot@Platelet Nanocrystals: Controlled Epitaxy, Monoexponential Decay of Two-Dimensional Exciton, and Nonblinking Photoluminescence of Single Nanocrystal

Wurtzite CdSe@CdS dot@platelet nanocrystals, dot-shaped CdSe nanocrystals encased within epitaxially grown CdS nanoplatelets, are controllably synthesized with nearly monodisperse size and shape distribution and outstanding photoluminescence (PL) properties. The excellent size and shape control with their lateral to thickness dimension ratio up to 3:1 is achieved by systematically studying the synthetic parameters, which results in a simple, tunable, yet reproducible epitaxy scheme. These special types of core/shell nanocrystals possess two-dimensional emission dipoles with the ab plane of the wurtzite structure. While their near-unity PL quantum yield and monoexponential PL decay dynamics are at the same level of the state-of-art CdSe/CdS core/shell nanocrystals in dot shape, CdSe@CdS dot@platelet nanocrystals possess ∼2 orders of magnitude lower probability for initiating PL blinking at the single-nanocrystal level than the dot-shaped counterparts do.

Synthesis, characterization and optical studies on Sm2+-doped CdSe nanocrystals: a blueshift and fixed emission with high quantum yields

CdSe:Sm nanocrystals (NCs) were synthesized by a water phase method, and their structures, shapes and optical properties were further characterized. X-ray diffraction (XRD) analysis suggested that both the CdSe and CdSe:Sm NCs contained (111), (220) and (311) lattice planes in the zinc blende structure overlapped with the (002), (110) and (112) lattice planes in the wurtzite structure, and the diameters were about 4.2, 3.3 and 2.3 nm for CdSe, CdSe:Sm(8%) and CdSe:Sm(10%) NCs, respectively. All of the CdSe:Sm NCs were monodispersed and uniform spherical nanocrystals. The CdSe:Sm(10%) NCs prepared with different reaction times exhibited constant absorption spectra at 430 nm and a fixed emission peak at 581 nm. Compared with those of pure CdSe NCs, the fluorescence emission of CdSe:Sm(10%) NCs blueshifted 20–36 nm, and the absorption peak initially redshifted and then blueshifted with the prolongation of reaction time. In addition, the Sm2+ doping decreased the fluorescence lifetime and increased the quantum yields (QYs) of CdSe NCs. The QYs of CdSe:Sm NCs increased initially and then decreased with the increase in the amount of doped Sm2+. The initial pH and charge compensator concentration also exhibited significantly enhanced fluorescence emission of CdSe:Sm NCs. CdSe and CdSe:Sm NCs were prepared via a water phase method. The structure and optical properties were investigated; the Sm2+ was doped into CdSe NCs successfully. CdSe and CdSe:Sm NCs all exhibit nearly spherical particles, doping Sm2+ increases the particle size, and the QYs are also enhanced. As the reaction time varies to 1, 2, 5, 8 and 10 h, the emission spectra of CdSe NCs present redshift, while the CdSe:Sm NCs have a fixed emission peak, which is the characteristic peak of Sm2+. As compared with pure CdSe NCs, the emission spectra of CdSe:Sm NCs appear blueshift by 20–36 nm. Doping Sm2+ into CdSe NCs can improve the optical properties.

Photoinduced Surface Charging in Iron-Carbonyl-Functionalized Colloidal Semiconductor Nanocrystals.

Organometallic surface functionalization of colloidal CdSe and CdS nanocrystals using iron tetracarbonyl moieties is demonstrated to enable study of in situ colloidal nanocrystal surface redox chemistry. Spectroscopic measurements of the surface-bound metal carbonyl C-O stretches were used to elucidate the coordination environments and local symmetry of surface sites. The C-O stretching frequencies of these fragments were correlated to the electric field induced by nanocrystal surface charges and shift in energy upon surface reduction or oxidation. These measurements revealed that CdSe nanocrystals can accumulate multiple surface electrons under supra-band gap photoexcitation, a process likely relevant to photoactivated nanocrystal processes such as photobrightening. These surface charges are stable for hours and decay extremely slowly under anaerobic conditions.

Studies on CdSe/PVK nanocomposites films for electroluminescent display applications

In the present work, the thin films of CdSe/PVK nanocomposites with various concentration of CdSe have been synthesized by chemical route. Polyvinyl carbazole (PVK) was used as a stabilizer as well as host matrix to prevent agglomeration of nanoparticles. The synthesized CdSe/PVK films were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), UV–Visible absorption spectra. The crystalline size was calculated from the XRD while FESEM images confirm the surface morphology of CdSe/PVK nanocomposites. CdSe nanocrystals have hexagonal phase and size increases with increasing CdSe content, Single PL peak at 498 nm for all the samples is caused by luminescence centres or surface/defect states. The EL spectra are quite broad as compared to PL spectra and the peaks are obtained at nearly same wavelength. The EL peak intensity increases slightly and shifts towards red by increasing concentration of CdSe in polymer matrix.

Synthesis of High-Quality Carboxyl End-Functionalized Poly(3-hexylthiophene)/CdSe Nanocomposites

Carboxyl end-functionalized poly(3-hexylthiophene) (P3HT-COOH) was grafted chemically with CdSe nanocrystals (NCs) by a phosphine-free method. The particle quality of P3HT-COOH/CdSe nanocomposites was better than that of P3HT/CdSe nanocomposites, which were synthesized using the same method. Nanocrystals with controllable particle size exhibited a wurtzite crystalline structure and showed excellent nanocrystal dispersion in the P3HT-COOH matrix. Photoluminescence (PL) characterization performed on nanocomposites suggested the efficient charge transfer at the P3HT-COOH/CdSe interface. This approach based on the phosphine-free method is not only environmentally friendly but also highly efficient.

Identification of Facet-Dependent Coordination Structures of Carboxylate Ligands on CdSe Nanocrystals.

Aliphatic carboxylates are the most common class of surface ligands to stabilize colloidal nanocrystals. The widely used approach to identify the coordination modes between surface cationic sites and carboxylate ligands is based on the empirical infrared (IR) spectroscopic assignment, which is often ambiguous and thus hampers the practical control of surface structures. In this report, multiple techniques based on nuclear magnetic resonance (NMR) and IR spectra are applied to distinguish the different coordination structures in a series of zinc-blende CdSe nanocrystals with unique facet structures, including nanoplatelets dominated with {100} basal planes, hexahedrons with only three types of low-index facets (i.e., {100}, {110}, and {111}), and spheroidal dots without well-defined facets. Interpretation and assignment of NMR and IR signals were assisted by density functional theory (DFT) calculations. In addition to the identification of facet-sensitive bonding modes, the present methods also allow a nondestructive quantification of mixed ligands.

Plasmon-Enhanced Near-Field Optical Spectroscopy of Multicomponent Semiconductor Nanostructures

Multicomponent semiconductor nanostructures were studied by local spectral analysis based on surface-enhanced Raman scattering by semiconductor nanostructures located on the surface of an array of Au nanoclusters near the metallized tip of an atomic force microscope. In the gap between the metal nanoclusters and the tip, where a semiconductor nanostructure is located, there is a strong increase in the local electric field (hot spot), resulting in a dramatic enhancement of the Raman scattering signal. An unprecedented enhancement of the Raman scattering signal by two-dimensional (over 108 for MoS2) and zero-dimensional (106 for CdSe nanocrystals) semiconductor nanostructures was achieved. The use of the method for mapping the Raman scattering response of a multicomponent system of MoS2 and CdSe made it possible to identify components with a spatial resolution far exceeding the diffraction limit.

Multilayers of Carbodithioate and Sulfide-Linked CdSe Nanocrystals: Progressive Increase of Exciton Delocalization

Multilayers of sulfur-based anchoring group linkers and CdSe nanocrystals (NCs) were realized on indium-tin oxide substrate via layer-by-layer alternation. The materials were investigated by UV–vis...

Flexoelectric and local heating effects on CdSe nanocrystals in amorphous As2Se3 films

Raman and photoluminescence (PL) spectra of amorphous As2Se3:Cd films show that laser irradiation can result in the local formation of CdSe nanocrystals (NCs). Due to a pronounced laser-induced drastic drop of the film viscosity, a strong local enhancement of diffusion of atoms in the laser spot area results in a material transfer from the laser spot (evidenced by SEM and AFM data) and simultaneously facilitates the formation of CdSe NCs. The difference of the PL maximum positions measured at a high laser power density Pexc and subsequently (from the same spot) at a lower Pexc value can be explained either by local heating of the sample by the high-Pexc illumination, or by the flexoelectric effect (additional strain in the CdSe NCs induced by the electric field gradient caused by the charging of the NC surface). Based on the spectral positions of the PL maximum, with the account of the Stokes shift, the average radii of the CdSe NCs formed are estimated.

Insights into different photoluminescence mechanisms of binary and ternary aqueous nanocrystals from the temperature dependence: A case study of CdSe and Ag-In-S

A comparative study of the temperature dependences of the photoluminescence (PL) of aqueous CdSe and silver-indium-sulfide (AIS) nanocrystals (NCs) in a broad range of 10–310 K is reported. Gelatin-incorporated CdSe (and core/shell CdSe/ZnS) NCs reveal both direct (excitonic) and trap-state-mediated PL components with different temperature dependences of the PL band maximum and spectral width. Glutathione-capped AIS (and core/shell AIS/ZnS) NCs emit a single broad PL band resembling the trap-related PL of CdSe NCs but showing nevertheless a temperature PL dependence typical for excitonic PL. In combination with previously reported results, these special features allow the PL emission of ternary AIS (AIS/ZnS) NCs to be explained based on the recombination of self-trapped excitons.

Effect of Auger recombination in ensemble of CdSe nanocrystals on their luminescence

The temperature dependence of the photoluminescence intensity of colloidal CdSe nanocrystals having 5 nm in diameter was studied. It was demonstrated that the photoluminescence intensity exhibits a hysteresis at temperatures below 220 K. This effect was explained by a model taking into account formation of an additional nonradiative relaxation channel associated with Auger recombination promoted by hole trapping on the surface of the nanocrystals. The activation energy of the hole release process obtained in the framework of the model was 0.7 eV.

High-Performance Ultra-Short Channel Field-Effect Transistor Using Solution-Processable Colloidal Nanocrystals.

We demonstrate high-mobility solution-processed inorganic field-effect transistors (FETs) with ultra-short channel (USC) length using semiconductor CdSe nanocrystals (NCs). Capping of the NCs with hybrid inorganic-organic CdCl3--butylamine ligands enables coarsening of the NCs during annealing at a moderate temperature, resulting in the devices having good transport characteristics with electron mobilities in the saturation regime reaching 8 cm2 V-1 s-1. Solution-based processing of the NCs and fabrication of thin films involve neither harsh conditions nor the use of hydrazine. Employing photolithographic methods, we fabricated FETs with a vertical overlap of source and drain electrodes to achieve a submicrometer channel length. To the best of our knowledge, this is the first report on an USC FET based on colloidal semiconductor NCs. Because of a short channel length, the FETs show a normalized transconductance of 4.2 m V-1 s-1 with a high on/off ratio of 105.

Size- and Morphology-Dependent Auger Recombination in CsPbBr3 Perovskite Two-Dimensional Nanoplatelets and One-Dimensional Nanorods.

CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs), including zero-dimensional (0D) quantum dots (QDs), one-dimensional (1D) nanorods (NRs), and two-dimensional (2D) nanoplatelets (NPLs), have shown promising performances in light-emitting diode (LED) and lasing applications. However, Auger recombination, one of the key processes that limit their performance, remains poorly understood in CsPbX3 2D NPLs and 1D NRs. We show that the biexciton Auger lifetimes of CsPbBr3 NPLs (NRs) scale linearly with the NPL lateral area (NR length) and deviates from the "universal volume scale law" that has been observed for QDs. These results are consistent with a model in which the Auger recombination rate for 1D NRs and 2D NPLs is a product of binary collision frequency in the nonquantum confined dimension and Auger probability per collision. Comparisons of Auger recombination in CsPbBr3 NCs of different dimensionalities and similar band gaps suggest that Auger probability increases in NCs with a higher number of confined dimensions. Compared to CdSe and PbSe NCs with the same dimensionalities and similar sizes, Auger recombination rates in 0D-2D CsPbBr3 NCs are over 10-fold faster. Fast Auger recombination in CsPbBr3 NCs shows their potentials for Auger-assisted up-conversion and single photon source, while suppressing Auger recombination may further enhance their performances in LED and lasing applications.

Improving the Power-Conversion Efficiency through Alloying in Common Anion CdZnX (X=S, Se) Nanocrystal Sensitized Solar Cells.

In this paper, we have investigated the possibility of utilizing CdZnS and CdZnSe alloy nanocrystals (NCs) as sensitizers in quantum-dot solar cells (QDSCs). The alloy NCs were synthesized by a high-temperature hot injection method and subsequently characterized through high photoluminescence quantum yield, along with larger size compared to binary NCs. Femtosecond transient absorption measurements revealed long-lived charge carriers in the alloy structure due to more structural rigidity and less defect states. Finally, the solar-cell efficiencies of the CdZnS (CdZnSe) NCs were found to be 3.05 % (3.69 %) as compared to 1.23 % (3.12 %) efficiencies for CdS (CdSe) NCs. Thus, common anion ternary NCs have been successfully utilized for solar-cell assembly and can be helpful for constructing tandem solar cells to harvest the high-energy portion of solar radiation.

Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks.

The goal of this work is to identify favored pathways for preparation of defect-resilient attached wurtzite CdX (X = S, Se, Te) nanocrystals. We seek guidelines for oriented attachment of faceted nanocrystals that are most likely to yield pairs of nanocrystals with either few or no electronic defects or electronic defects that are in and of themselves desirable and stable. Using a combination of in situ high-resolution transmission electron microscopy (HRTEM) and electronic structure calculations, we evaluate the relative merits of atomic attachment of wurtzite CdSe nanocrystals on the {11̅00} or {112̅0} family of facets. Pairwise attachment on either facet can lead to perfect interfaces, provided the nanocrystal facets are perfectly flat and the angles between the nanocrystals can adjust during the assembly. Considering defective attachment, we observe for {11̅00} facet attachment that only one type of edge dislocation forms, creating deep hole traps. For {112̅0} facet attachment, we observe that four distinct types of extended defects form, some of which lead to deep hole traps whereas others only to shallow hole traps. HRTEM movies of the dislocation dynamics show that dislocations at {11̅00} interfaces can be removed, albeit slowly. Whereas only some extended defects at {112̅0} interfaces could be removed, others were trapped at the interface. Based on these insights, we identify the most resilient pathways to atomic attachment of pairs of wurtzite CdX nanocrystals and consider how these insights can translate to the creation of electronically useful materials from quantum dots with other crystal structures.

Ultrahigh Hot Carrier Transient Photocurrent in Nanocrystal Arrays by Auger Recombination.

In this report, we show that a new mechanism for carrier transport in solution-processed colloidal semiconductor nanocrystal arrays exists at high excitation intensity on ultrafast time scales and allows for facile intrinsic transport between as-prepared nanocrystals over long distances. By combining a high speed photoconductive switch with an ultrafast laser excitation in a sub-40 ps photoconductor, we observed transient photocurrents with peak densities of 3 × 104 - 106 mA/cm2 in self-assembled PbSe nanocrystals capped with long native oleic acid ligands. The ratio between the transient photocurrent peak and the steady-state dark current is 10 orders of magnitude. The transient mobility at the peak current is estimated to range between 0.5-17.5 cm2/(V s) for the various nanocrystal sizes studied, which is 6 to 9 orders of magnitude higher than the dark current steady-state mobility in PbSe, CdSe, and CdTe nanocrystals capped with native ligands. The results are analyzed using a kinetic model which attributes the ultrahigh transient photocurrent to multiple photogenerated excitons undergoing on-particle Auger recombination, followed by rapid tunneling at high energies. This mechanism is demonstrated for a wide range of PbSe nanocrystals sizes (diameters from 2.7 to 7.1 nm) and experimental parameters. Our observations indicate that native ligand-capped nanocrystal arrays are promising for optoelectronics applications wherein multiple carriers are photoinjected to interband states.