Haiguang Zhao

Towards controlled synthesis and better understanding of highly luminescent PbS/CdS core/shell quantum dots

A two-step cation exchange procedure has been developed for synthesizing PbS/CdS core/shell quantum dots (QDs) with a much thicker shell than previously reported, which expands the flexibility of the current cation exchange approach. The thick-shell QDs allow relatively easy observation of the core/shell morphology by transmission electron microscopy as well as exhibiting characteristic absorption and emission of CdS when the shell thickness reaches 1.8 nm. X-ray diffraction patterns show gradual transformation from a rock salt PbS pattern to a zinc blend CdS pattern with increasing shell thickness and the overall diffraction pattern is indeed the same as that of the CdS standard when the shell is thicker than 3.6 nm. The thick-shell QDs were further analyzed by energy dispersive X-ray spectrometry. It is found that Pb only exists in the core region and is essentially absent in the shell region. All of these results consistently suggest that the shell is made of CdS, instead of ternary PbxCd1−xS alloy in thick-shell QDs. As direct experimental identification of the shell composition in thin-shell QDs is difficult, experimental data and calculations are combined to indirectly probe this issue. The comparison of band gap versus core size plots of different compositional models indicates that it is highly likely that the thin shell is also made of CdS. Importantly, these core/shell PbS/CdS QDs not only show significantly increased quantum yield up to 67% at the optimal shell thickness of about 0.7 nm, they are also much more photostable and thermally stable than the shell-free PbS QDs.

Effect of CdS shell thickness on the optical properties of water-soluble, amphiphilic polymer-encapsulated PbS/CdS core/shell quantum dots

Cation exchange, recently explored for synthesizing core/shell quantum dots (QDs), causes continuous core size change during shell formation. By carefully varying parent PbS QD size and cation exchange conditions, we have synthesized PbS/CdS core/shell QDs with a similar PbS core size of ∼4.5 nm yet a different CdS shell thickness. This enables us to study the effect of shell thickness on the properties of PbS QDs after their transfer from chloroform into watervia poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). It was found that the quantum yield (QY) of PbS cores in water firstly increases with shell thickness up to ∼0.7 nm, reaching 33%, owing to better surface passivation and then decreases to 1.7% when the shell thickness reaches 2.3 nm. Such decline is due to the formation of new defects with shell deposition. In contrast, the variation amplitude of QY during water transfer monotonically decreases and QD photostability monotonically improves with shell thickness. It is clear that although newly introduced defects play a fundamental role in the absolute QY, they do not show any overwhelmingly negative effects on the variation of QY with environments and photostability. The colloidal stability of QDs in buffers containing different salt concentrations seems to be not affected by the shell thickness, possibly due to the same steric stabilization effect of the amphiphilic polymer in all samples. Further investigation on a series of core/shell samples confirms that ∼0.7 nm is an optimal shell thickness for various core sizes investigated herein, consistently yielding the maximum QY and reasonably good photostability.

Controlling photoinduced electron transfer from PbS@CdS core@shell quantum dots to metal oxide nanostructured thin films.

N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent a promising alternative to traditional photovoltaic devices. However, colloidal PbS QDs capped with pure organic ligand shells suffer from surface oxidation that affects the long term stability of the cells. Application of a passivating CdS shell guarantees the increased long term stability of PbS QDs, but can negatively affect photoinduced charge transfer from the QD to the oxide and the resulting photoconversion efficiency (PCE). For this reason, the characterization of electron injection rates in these systems is very important, yet has never been reported. Here we investigate the photoelectron transfer rate from PbS@CdS core@shell QDs to wide bandgap semiconducting mesoporous films using photoluminescence (PL) lifetime spectroscopy. The different electron affinity of the oxides (SiO2, TiO2 and SnO2), the core size and the shell thickness allow us to fine tune the electron injection rate by determining the width and height of the energy barrier for tunneling from the core to the oxide. Theoretical modeling using the semi-classical approximation provides an estimate for the escape time of an electron from the QD 1S state, in good agreement with experiments. The results demonstrate the possibility of obtaining fast charge injection in near infrared (NIR) QDs stabilized by an external shell (injection rates in the range of 110-250 ns for TiO2 films and in the range of 100-170 ns for SnO2 films for PbS cores with diameters in the 3-4.2 nm range and shell thickness around 0.3 nm), with the aim of providing viable solutions to the stability issues typical of NIR QDs capped with pure organic ligand shells.

Enhanced photovoltaic properties in dye sensitized solar cells by surface treatment of SnO2 photoanodes

We report the fabrication and testing of dye sensitized solar cells (DSSC) based on tin oxide (SnO2) particles of average size ~20 nm. Fluorine-doped tin oxide (FTO) conducting glass substrates were treated with TiOx or TiCl4 precursor solutions to create a blocking layer before tape casting the SnO2 mesoporous anode. In addition, SnO2 photoelectrodes were treated with the same precursor solutions to deposit a TiO2 passivating layer covering the SnO2 particles. We found that the modification enhances the short circuit current, open-circuit voltage and fill factor, leading to nearly 2-fold increase in power conversion efficiency, from 1.48% without any treatment, to 2.85% achieved with TiCl4 treatment. The superior photovoltaic performance of the DSSCs assembled with modified photoanode is attributed to enhanced electron lifetime and suppression of electron recombination to the electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS) carried out under dark condition. These results indicate that modification of the FTO and SnO2 anode by titania can play a major role in maximizing the photo conversion efficiency.

Functionalized multi-wall carbon nanotubes/TiO2 composites as efficient photoanodes for dye sensitized solar cells

We report on the effects of incorporation of different concentrations of carboxyl group (COOH)-functionalized multi-wall carbon nanotubes (F-MWCNTs) into TiO2 active layers for dye-sensitized solar cells (DSSCs). Standard DSSCs with bare TiO2 exhibit a photo-conversion efficiency (PCE) of 6.05% and a short circuit current density (Jsc) of 13.3 mA cm−2. The presence of 2 wt% F-MWCNTs in the photoanodes increases the PCE up to 7.95% and Jsc up to 17.5 mA cm−2. The photoanodes were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The electrochemical behaviour of the solar cells was investigated by electrochemical impedance spectroscopy (EIS). We attribute the improved performances to the combined effect of increased dye loading and reduced charge recombination (as clarified by dye loading and EIS measurements), due to the conformal coverage of F-MWCNTs, which allows fast and efficient charge collection in operating solar cells. These results can help in improving the PCE in DSSCs in an elegant and straightforward way, minimizing the need of additional steps (e.g. pre- and post-treatment with TiCl4) for photoanode preparation.

Lanthanide Ion Doped Upconverting Nanoparticles: Synthesis, Structure and Properties.

Lanthanide doped upconverting nanoparticles (UCNPs) have emerged as a new class of luminescent materials, with major discoveries and overall significant progress during the last decade. Unlike multiphoton absorption in organic dyes or semiconductor quantum dots, lanthanide doped UCNPs involve real intermediate quantum states and convert infrared (IR) into visible light via sequential electronic excitation. The relatively high efficiency of this process even at low radiation flux makes UCNPs particularly attractive for many current and emerging areas of technology. The aim of this article is to highlight several recent advances in this rapidly growing field, emphasizing the relationships between structure and properties of UCNPs. Additionally, various strategies developed for the synthesis of UCNPs with a focus on the various synthetic approaches that yield high-quality monodisperse samples with controlled size, shape and crystalline phase are reviewed. Emerging synthetic approaches towards designed structure to improve the optical and electronic properties of UCNPs are discussed. Finally, recent examples of applications of UCNPs in biomedical and optoelectronics research, giving our own perspectives on future directions and emerging possibilities of the field are described.

Self-selective recovery of photoluminescence in amphiphilic polymer encapsulated PbS quantum dots

Self-selected recovery of the photoluminescence (PL) of amphiphilic polymer encapsulated PbS quantum dots (QDs) was observed in water for the first time and possible mechanisms were proposed based on investigations by means of transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction and fluorescence spectroscopy. Water-soluble PbS QDs were synthesized by transferring monodispersed QDs capped with hydrophobic ligands of oleylamine from an organic solvent into water via amphiphilic polymers poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). The water transfer process leads to a double size distribution (5.6 ± 0.9 nm and 2.7 ± 0.4 nm), attributed to ligand etching together with Ostwald ripening, as well as the fast decay of PL. The automatic recovery of the PL in PbS QDs stored in water in the dark for 3 months was only observed for the subset of smaller QDs and is largely due to the removal of surface defects with aging, as evidenced by the decreased percentage of unpassivated surface atoms from XPS studies. In contrast, the PL of the subset of larger QDs in the same sample does not self-recover in water and can only be slightly recovered by transferring them into environments with less external quenches. The results strongly suggest that it is the surface defect in the larger QDs themselves, introduced during Ostwald ripening, that is primarily responsible for their non-emitting status or rather low PL intensity under different conditions. The increase of unpassivated Pb atoms in larger PbS QDs after the 3 month aging has been confirmed by XPS, which explains their non-recovery behavior in water. The PL-recovered QD sample in water is very stable and shows comparable photostability to the initial QDs dispersed in an organic phase.

Ultrasensitive, Biocompatible, Self‐Calibrating, Multiparametric Temperature Sensors

Core-shell quantum dots serve as self-calibrating, ultrasensitive, multiparametric, near-infrared, and biocompatible temperature sensors. They allow temperature measurement with nanometer accuracy in the range 150-373 K, the broadest ever recorded for a nanothermometer, with sensitivities among the highest ever reported, which makes them essentially unique in the panorama of biocompatible nanothermometers with potential for in vivo biological thermal imaging and/or thermoablative therapy.

Dynamics of semiconducting nanocrystal uptake into mesoporous TiO2 thick films by electrophoretic deposition

Electrophoretic deposition (EPD) is a simple technique for the uptake of nanoparticles into mesoporous films, for example to graft semiconducting nanocrystals (quantum dots, QDs) on mesoporous oxide thick films acting as photoanodes in third generation solar cells. Here we study the uptake of colloidal QDs into mesoporous TiO2 films using EPD. We examined PbS@CdS core@shell QDs, which are optically active in the near infrared (NIR) region of the solar spectrum and exhibit improved long-term stability toward oxidation compared to their pure PbS counterpart, as demonstrated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. We applied Rutherford backscattering spectrometry (RBS) to obtain the Pb depth profile into the TiO2 matrix. EPD duration in the range from 5 to 120 min and applied voltages from 50 to 200 V were considered. The applied electric field induces the fast anchoring of QDs to the oxide surface. Consequently, QD concentration in the solution contained in the mesoporous film drastically decreases, inducing a Fick-like diffusion of QDs. We modelled the entire process as a QD diffusion related to the formation of a QD concentration gradient, and a depth-independent QD anchoring, and were able to determine the electric field-induced diffusion coefficient D for QDs and the characteristic time for QD grafting, in very good agreement with the experiment. D increases from (1.5 ± 0.4) × 10−5 μm2 s−1 at 50 V to (1.1 ± 0.3) × 10−3 μm2 s−1 at 200 V. The dynamics of EPD may also be applied to other different colloidal QDs and quantum rod materials for the sensitization of mesoporous films. These results quantitatively describe the process of QD uptake during EPD, and can be used to tune the optical and optoelectronic properties of composite systems, which determine, for instance, the photoconversion efficiency in QD solar cells (QDSCs).

Green synthesis of near infrared core/shell quantum dots for photocatalytic hydrogen production.

Quantum dots (QDs) are attractive systems for potential applications in future solar energy technologies, due to their optical properties which are tunable as a function of size and composition. In this study, we synthesized PbS QDs with first excitonic peak in the range 1060 to 1300 nm using a PbCl2/sulfur molar ratio of 10:1. The first excitonic absorption peak from 1300 to 950 nm of the PbS/CdS core/shell QDs can be further synthesized via the cation exchange approach. Our method resulted in high quantum yield, good stability, monodisperse QD solutions with a full surface coverage by excess Cd cations. In addition, we used our core/shell QDs in a photoelectrochemical cell for hydrogen generation. This heterostructure exhibited a saturated photocurrent as high as 3.3 mA cm-2, leading to ∼29 ml cm-2 d-1 hydrogen generation, indicating the strong potential of our core/shell QDs for applications in water splitting.