Surface Of CdSe Quantum Dots Research Articles

Thermodynamics of Ligand Exchange with Aromatic Ligands on the Surface of CdSe Quantum Dots

Quantum dots (QDs) have great application prospects in optoelectronic devices because of their excellent optical properties. Nevertheless, the original ligands of the long carbon chain introduced in the synthesis process are not conducive to the application of CdSe QDs in optoelectronic devices due to their insulating nature. Therefore, short-chain and aromatic ligands are employed to replace the original ligands. However, the influences of conjugation structures on ligand exchange have rarely been systematically studied from a thermodynamic perspective. In this work, two types of aromatic ligands, namely, a series of benzoic acid (BA) ligands and cinnamic acid (CA) ligands that introduced electron-donating (methoxy-) and electron-withdrawing groups (fluoro-) at the meta and para positions of the benzene ring of aromatic ligands, were chosen to investigate the ligand exchanges on the surface of QDs from oleic acid (OA) to a series of aromatic ligands by 1H nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC). 1H NMR showed that the average exchange ratio of CA ligands to OA was ∼1:1, which made it possible to obtain thermodynamic parameters through the determined ligand exchange model by ITC. Furthermore, thermodynamic results showed that ligand exchange with CA ligands was endothermic (ΔH > 0) and entropy-increasing (ΔS > 0), indicating that the process was entropy-driven. In addition, the equilibrium constants (K) of ligand exchange with CA ligands were larger than those with BA ligands. However, the K values of ligand exchange with BA ligands were ∼1, which resulted in ΔG ∼ 0 kJ/mol, demonstrating that ligand exchange with BA ligands was less favorable at 298.15 K. Finally, the introduction of substituents on meta and para positions of the aromatic ligand benzene ring had no significant effect on the ligand exchange process. This work not only elaborates on the process of exchange with aromatic ligands but also has fundamental significance for the application of aromatic ligand-modified QDs in optoelectronic devices.

Formation of the Inorganic and Organic Shells on the Surface of CdSe Quantum Dots.

Embedding quantum dots (QDs) into an organic matrix of controllable order requires the identification of their structural characteristics. This analysis is necessary for the creation of anisotropic composites that are sensitive to external stimuli. We have studied the QD structures formed during the single-step synthesis of CdSe/ZnS QDs and their transformations after the initial ligand's substitution for another ligand. This single-step process leads to the formation of the core/shell structure. We detect the presence of two oleic acid residues ionically connected to Zn and Cd. At the same time, the amount of Cd oleate at the surface is very small. We observe the ligand exchange process at the surface of the core/shell QDs. The oleic acid residues are substituted by terphenyl-containing (TERPh-COOH) aromatic acid residues. The reaction between CdSe/ZnS carrying TOP and oleic acid residues ionically bound with QDs and terphenyl-containing acid leads to the coexistence of multiple ligands on the QD surface at a ratio of 11:6:33 for TOP/OA/TERPh-COOH.

Thermodynamic Implications of the Ligand Exchange with Alkylamines on the Surface of CdSe Quantum Dots: The Importance of Ligand–Ligand Interactions

Ligand exchange is a critical step for tuning the properties of quantum dots (QDs), as well as functionalization for wide applications. Previous studies have categorized X-, L-, and Z-types of liga...

Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered Layer-by-Layer Assembly toward Multifarious Photoredox Organic Transformation

Semiconductor quantum dots (QDs) have garnered tremendous attention by virtue of their substantial light-harvesting and conversion efficiencies, large number of active sites, unique quantum size confinement, and multiple exciton generation effects. In this regard, recent years have witnessed their widespread applications in photocatalysis. Nonetheless, intrinsic disadvantages of QDs including unfavorable photostability, ultrafast charge recombination rate, and sluggish kinetic of charge carriers retard the construction of high-efficiency QDs-based photocatalysts for solar energy conversion. Thus far, in-depth investigation on the visible-light-driven photoredox organic transformation over QDs has not yet been exhaustively explored, and corresponding photocatalytic mechanisms remain elusive. In this work, selecting cadmium selenide (CdSe) as a quintessential category of semiconductor QDs, we demonstrated a facile, green, easily accessible, and rather efficient electrostatic self-assembly strategy to conspicuously boost the versatile photoredox performances of CdSe QDs toward selective organic transformation under visible light irradiation by intimately integrating with graphene (GR) via judicious surface charge tuning. In this scenario, intrinsically negatively charged CdSe QDs and surface-modified positively charged GRs were utilized as the building blocks for spontaneous electrostatic self-assembly buildup, which gives rise to well-defined CdSe QDs–GR ensembles. More intriguingly, ligands capped on the CdSe QDs surface enable the alternate layer-by-layer (LbL) assembly of CdSe QDs and GR forming three-dimensional spatially multilayered heterostructures. Furthermore, it was significant to unveil that such self-assembled CdSe QDs–GR nanocomposites exhibit remarkably enhanced and multifunctional photoredox performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and selective reduction of nitroaromatics to amino compounds under visible light irradiation, which far exceeds the pristine CdSe QDs counterpart, which exhibits almost negligible photoactivities. This can be ascribed to the pivotal role of GR for conspicuously capturing and shuttling electrons from band-gap photoexcitation of CdSe QDs, intimate interfacial contact between the building blocks, and enlarged specific surface area stemming from seamless GR encapsulation and intercalation, along with the unique ligand-triggered LbL assembly integration mode between CdSe QDs and GR, hence synergistically reducing the recombination rate and prolonging the lifetime of charge carriers. Furthermore, photoredox mechanisms of the CdSe QDs–GR ensemble were elucidated. It is anticipated that our work would afford an efficacious avenue to finely modulate the charge transport over QDs for solar energy conversion.

PH-Dependent structure of water-exposed surfaces of CdSe quantum dots.

Increasing negative charge density at the surfaces of CdSe quantum dots (QDs) effects a bathochromic shift of their ground state optical spectra with increasing pH due to electrostatic and chemical modifications at the QD surface. These modifications are enabled by weakly-bound ligands that expose the surface to the aqueous environment.

Susceptible Surface Sulfide Regulates Catalytic Activity of CdSe Quantum Dots for Hydrogen Photogeneration.

Semiconducting quantum dots (QDs) have recently triggered a huge interest in constructing efficient hydrogen production systems. It is well established that a large fraction of surface atoms of QDs need ligands to stabilize and avoid them from aggregating. However, the influence of the surface property of QDs on photocatalysis is rather elusive. Here, the surface regulation of CdSe QDs is investigated by surface sulfide ions (S2- ) for photocatalytic hydrogen evolution. Structural and spectroscopic study shows that with gradual addition of S2- , S2- first grows into the lattice and later works as ligands on the surface of CdSe QDs. In-depth transient spectroscopy reveals that the initial lattice S2- accelerates electron transfer from QDs to cocatalyst, and the following ligand S2- mainly facilitates hole transfer from QDs to the sacrificial agent. As a result, a turnover frequency (TOF) of 7950 h-1 can be achieved by the S2- modified CdSe QDs, fourfold higher than that of original mercaptopropionic acid (MPA) capped CdSe QDs. Clearly, the simple surface S2- modification of QDs greatly increases the photocatalytic efficiency, which provides subtle methods to design new QD material for advanced photocatalysis.

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy.

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic Acids: Unraveling Structural Requirements for Ligand-Induced Chirality.

Functionalization of colloidal quantum dots (QDs) with chiral cysteine derivatives by phase-transfer ligand exchange proved to be a simple yet powerful method for the synthesis of chiral, optically active QDs regardless of their size and chemical composition. Here, we present induction of chirality in CdSe by thiol-free chiral carboxylic acid capping ligands (l- and d-malic and tartaric acids). Our circular dichroism (CD) and infrared experimental data showed how the presence of a chiral carboxylic acid capping ligand on the surface of CdSe QDs was necessary but not sufficient for the induction of optical activity in QDs. A chiral bis-carboxylic acid capping ligand needed to have three oxygen-donor groups during the phase-transfer ligand exchange to successfully induce chirality in CdSe. Intrinsic chirality of CdSe nanocrystals was not observed as evidenced by transmission electron microscopy and reverse phase-transfer ligand exchange with achiral 1-dodecanethiol. Density functional theory geometry optimizations and CD spectra simulations suggest an explanation for these observations. The tridentate binding via three oxygen-donor groups had an energetic preference for one of the two possible binding orientations on the QD (111) surface, leading to the CD signal. By contrast, bidentate binding was nearly equienergetic, leading to cancellation of approximately oppositely signed corresponding CD signals. The resulting induced CD of CdSe functionalized with chiral carboxylic acid capping ligands was the result of hybridization of the (achiral) QD and (chiral) ligand electronic states controlled by the ligand's absolute configuration and the ligand's geometrical arrangement on the QD surface.

Controlled Modulation of Surface Coating and Surface Charging on Quantum Dots with Negatively Charged Gelatin for Substantial Enhancement and Reversible Switching in Photoluminescence

Here, the roles of coating and charging on the surface of CdSe quantum dots (QDs) are systematically investigated in tuning their emission via a simple change of pH in solution. A very thick layer of ZnS with a wide bandgap is chemically grown first to passivate the CdSe for retaining their high emission intensity, which is further encapsulated with a layer of degraded gelatin to achieve the phase transfer of QDs from hexane into aqueous solution. Interestingly, the degraded gelatin‐capped CdSe/ZnS QDs (i.e., gelatinated QDs) greatly enhance their emission by adjusting pH from acidic to basic but greatly quench their emission by adjusting pH from basic to acidic. This highly reversible switching in photoluminescence is demonstrated for five cycles without obvious deterioration. With the increase of pH, there is a great decrease in radiative relaxation lifetime but a great increase in nonradiative relaxation lifetime as measured by time‐resolved photoluminescence decays of the gelatinated QDs. As a result, the nonradiative lifetime becomes much longer than the radiative lifetime with the increase of pH so that the radiative emission occurs earlier before the nonradiative pathways take effect, resulting in greatly increased photoluminescence. Importantly, the successful demonstrations for the phase transfer of hydrophobic QDs into aqueous solution together with the reversible switching in photoluminescence arise from the reprocessing of acidic‐degraded gelatin in weak alkaline solution, resulting in a great reduction in isoelectric point of the sequential acidic‐alkaline degraded gelatin in free versus surface‐bound forms from 7 to 4. With the exposure of more negatively charged carboxylic groups externally, the negative surface charging is greatly increased with the increase of pH while it is greatly decreased with the decrease of pH, leading to reversible switching in photoluminescence.

Influence of size and surface state emission on photoluminescence of CdSe quantum dots under UV irradiation

Under UV irradiation, photo-oxidation occurs on the surface of CdSe quantum dots (QDs), thus modifying their QD optical properties. The influence of size and surface state (SS) emission on the photoluminescence (PL) evolution of CdSe QDs was investigated. Colorimetric study of the CdSe QD emission under UV irradiation indicates that the QD size and SS emission will influence the movement of chromaticity parameters. A photo-corrosion effect reduced the QD size and caused blue shift of the band edge (BE) peak. The irradiation-induced blue shift of QD emission always moves the corresponding chromaticity coordinate counterclockwise in the chromaticity diagram. For CdSe QDs with a dominant SS peak, the different evolution between BE and SS peak under UV irradiation will substantively influence the transformation of emission color: the chromaticity coordinate moved faster when PL increased than that when it decreased. Generally, the emission color of CdSe QDs with relatively large size and weak SS peaks is more stable in irradiation process.

Coverage control of CdSe quantum dots in the photodeposition on TiO2 for the photoelectrochemical solar hydrogen generation

CdSe quantum dots (QDs) have successfully been formed on the TiO2 surface by the photodeposition of Se QDs and their subsequent transformation into CdSe QDs (CdSe/TiO2) (Fujishima et al., 2014). The addition of mercaptoacetic acid (MAA) in the second step of the two-step photodeposition process significantly decreases the CdSe particle size and the contact angle against the TiO2 surface to increase the TiO2-surface coverage by CdSe QDs with the particle size distribution sharpened. X-ray photoelectron and Raman spectroscopy measurements indicated that MAA is densely chemisorbed on the surface of CdSe QDs through CdS bond, whereas sparsely adsorbed on the TiO2 surface. Photoelectrochemical (PEC) cells using CdSe/TiO2 as the photoanode for hydrogen (H2) generation from aqueous sulfide solution were fabricated. The rate of H2 generation strongly depends on the concentration of MAA (C) added in the photoanode preparation, and the photoanode prepared at C=0.04mM affords a maximum solar-to-hydrogen conversion efficiency of 0.028%.

Effect of environment on the preparation of CdSe quantum dots capped with mercaptoacetic acid.

We report a preparation of CdSe quantum dots in the presence of capping molecules under ambient air and argon atmospheres. The growth of the quantum dots with applying an equimolar ratio of precursor of Cd2+ and Se2- in the presence of a high percentage of the mercaptoacetic acid as capping molecules is studied. Quantum dots are characterized via EDX, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis (TGA), UV-Vis optical absorption, Raman and infrared spectroscopy. Combination of EDX and Raman spectroscopy showed a graded diffusion of sulfur on the surface of CdSe quantum dots for the sample prepared under argon atmosphere, while the sample prepared under ambient air atmosphere the sulfur isn't diffused into the CdSe core. In addition, the sample prepared under air revealed a formation of oxides on the surface of the quantum dots. On the other hand, we have studied the nature of capping and how the surface of the quantum dots core is terminated for the prepared samples. In addition, we have studied the acoustic and optical phonons of the quantum dots cores.

Chemical Bath Deposition of Stoichiometric CdSe Quantum Dots for Efficient Quantum-Dot-Sensitized Solar Cell Application

Cadmium selenide (CdSe) quantum dots (QDs) have been prepared by suppressing the formation of an amorphous selenium oxide (SeO2) layer on the surface of CdSe QDs. The precursor ratio for the chemical bath deposition was optimized to minimize the SeO2 layer growth and develop high-efficiency CdSe QDs for quantum-dot-sensitized solar cell (QDSSC) applications. The morphologies and optical and electrical properties of the CdSe QDs were investigated, and the growth mechanism was also proposed for the CdSe and SeO2 layer formation. Furthermore, electrochemical impedance spectroscopy results indicated that the SeO2 layer reduced the recombination resistance of photogenerated electron–hole pairs, thus degrading the cell efficiency. Therefore, a QDSSC based on an optimized CdSe/CdS/ZnO nanowire photoanode, polysulfide electrolyte, and Au counter electrode can enhance the power conversion efficiency to 4.8% under AM 1.5 G one-sun illumination.

Surface photovoltage properties of self-assembled CdSe quantum dots arrays fabricated by slow evaporation approach

Densely packed and highly ordered CdSe quantum dots (QDs) arrays were prepared through a slow evaporation route; to study the photo-generated charges property of ordered CdSe QDs arrays, illumination induced charge separation in the CdSe QDs arrays were investigated through surface photovoltage (SPV) technology based on lock in amplifier. The significantly enhanced SPV signal of CdSe QDs arrays after post-annealing treatment could be attributed to the greatly increased escape of holes from CdSe QDs surface, as well as the decreased the inter nanocrystal distance. Moreover, much faster excess holes dominates in the frequency range of 30 Hz and 200 Hz, which results in the positive charging of the CdSe QDs arrays near the surface. Our results give a guide for its practical photoelectric applications based on nanocrystals.

A green, low-cost, and highly effective strategy to enhance the performance of hybrid solar cells: Post-deposition ligand exchange by acetic acid

We utilize an adsorption/desorption process between the excess acetic acid in solution and the bound oleate ions on the surface of CdSe quantum dots to achieve ligand exchange process in the hybrid film of poly (3-hexylthione) (P3HT): CdSe quantum dots (QDs). By this post-deposition ligand exchange method, we achieved hybrid solar cells with power conversion efficiency of nearly 2% at low annealing temperature (120°C). Compared to the thiols that are widely used in hybrid solar cells, acetic acid shows its advantages of environmental friendliness, non-toxicity and mild odors. As such, our method is attractive for future hybrid solar cell applications.

Emission enhancement of a carbazole-based fluorophore on a quantum dot surface

A novel carbazole-based fluorophore 5-(carbazol-9-yl)-5'-mercapto-2,2'-bithiophene (CBTSH) was synthesized. CBTSH exhibited emission enhancement when attached to the surface of CdSe quantum dots (QDs). The emission quantum yield of CBTSH attached to CdSe is almost 5 times larger than that of CBTSH alone. The emission enhancement of CBTSH on the CdSe surface is due to the restriction on the rotational and vibrational motions of the carbazolylbithienyl unit as a result of QD aggregation.

Effects of Metallic Ions on Photoluminescence Properties of CdSe/PAMAM Nanocomposites and their Application in Fingerprint Detection

Uniform and well-dispersed photoluminescent semiconductor CdSe(Cadmium selenium) QDs(Quantum dots) werein-situprepared stabilized by G4.0-NH2PAMAM dendrimers in water and effects of metallic ions such as Zn2+, Cd2+, Cu2+, Ag+, Pb2+, K+, Na+on the photoluminescence(PL) properties of CdSe QDs were studied using spectrophotometric titration. The results show that Zn2+and Cd2+ions enhance the PL intensity of CdSe QDs, Cu2+, Ag+and Pb2+ions quench the PL intensity, while K+and Na+ions have no obvious effects. Those are attributed to the surface modification of CdSe QDs. Excess Cd2+ions can reduce the non-combination centers, increase the passivation of PAMAM on the surface of CdSe QDs, and form a Schottky-like barrier which enhanced the PL efficiency. Excess Zn2+ions can also increase the PL intensity of CdSe QDs in the same way as Cd2+ions because of the much similar lattice parameters of CdSe and ZnSe. However, a CuSe, Ag2Se or PbSe shell on CdSe QDs emerges due to the substitution of Cd2+ions, which leads to the decrease of band-edge emmiting of CdSe QDs. And the quenching effect of Ag+is stonger than Cu2+and Pb2+because of the lower solubility of Ag2Se formed on the surface of CdSe QDs. Oil latent fingerprints deposited on tinfoil surface treated with CdSe/dendrimer NCs(Nanocomposites)、Cd2+-CdSe/ PAMAM NCs and Zn2+-CdSe /PAMAM NCs emitted bright yellow photoluminescence under ultraviolet excitation of 365nm from an UV LED in the dark., among, fingerprint treated with Cd2+-CdSe/dendrimer NCs and Zn2+-CdSe /PAMAM NCs was detected with better resolving rate.

Fabrication of carboxyl functionalized CdSe quantum dots via ligands self-assembly and CdSe/epoxy fluorescence nanocomposites

A facile approach to introduce carboxyl groups onto the surface of CdSe quantum dots (QD) was achieved via oleic acid ligands self-assembly, and then CdSe quantum dots/epoxy fluorescence nanocomposites were successfully fabricated. The surface constitution of QD, the better dispersibility of QD in epoxy matrix, and the reactivity with epoxy of carboxyl groups functionalized QD were characterized by Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC), respectively. As the result of surface modification, the carboxyl groups functionalized QD could be stably and homogenously dispersed in chloroform and acid anhydride cured epoxy matrix. The fresh fluorescence as well as the toughening behavior of the QD/epoxy nanocomposites was presented.

One-Pot Synthesis of Biocompatible CdSe/CdS Quantum Dots and Their Applications as Fluorescent Biological Labels.

We developed a novel one-pot polyol approach for the synthesis of biocompatible CdSe quantum dots (QDs) using poly(acrylic acid) (PAA) as a capping ligand at 240°C. The morphological and structural characterization confirmed the formation of biocompatible and monodisperse CdSe QDs with several nanometers in size. The encapsulation of CdS thin layers on the surface of CdSe QDs (CdSe/CdS core–shell QDs) was used for passivating the defect emission (650 nm) and enhancing the fluorescent quantum yields up to 30% of band-to-band emission (530–600 nm). Moreover, the PL emission peak of CdSe/CdS core–shell QDs could be tuned from 530 to 600 nm by the size of CdSe core. The as-prepared CdSe/CdS core–shell QDs with small size, well water solubility, good monodispersity, and bright PL emission showed high performance as fluorescent cell labels in vitro. The viability of QDs-labeled 293T cells was evaluated using a 3-(4,5-dimethylthiazol)-2-diphenyltertrazolium bromide (MTT) assay. The results showed the satisfactory (>80%) biocompatibility of as-synthesized PAA-capped QDs at the Cd concentration of 15 μg/ml.

Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines

The exchange kinetics of octylamine between a free state and a state bound to the surface of CdSe quantum dots is analyzed using nuclear magnetic resonance (NMR) spectroscopy in solution. On the basis of 1D 1H and DOSY and NOESY spectroscopy, we find that all octylamine molecules present interact with the CdSe quantum dot surface, although the octylamine resonances and diffusion coefficient resemble those of free octylamine. This indicates that these NMR observables are a weighted average of a free and a bound state, implying that the overall octylamine exchange rate is fast as compared to the intrinsic NMR time scale. A lower limit on the first-order desorption rate constant of 50 s−1 follows from DOSY measurements as a function of the diffusion delay. This result is compared with literature data based on the quenching of the CdSe photoluminescence upon alkylamine desorption.