Unusual Selectivity of Metal Deposition on Tapered Semiconductor Nanostructures

We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS nanoheterostructures with conelike, tapered geometries in order to examine the effects of shape on the location-specific deposition of Au under ambient conditions. Although preferential metal deposition at surface defect sites are generally expected, we found suprisingly that Au growth at the side facets of tapered linear and branched structures was significantly suppressed. Further investigation revealed this to be due to a highly efficient electrochemical Ostwald ripening process which was previously thought not to occur in branched nanostructures such as tetrapods. We exploited this phenomenon to fabricate uniform asymmetrically tipped CdSe-seeded CdS tetrapods with conelike arms, where a solitary large Au tip is found on one of the arms while the other three arms bear Ag₂S tips. Importantly, this work presents a synthetic route toward the selective deposition of metals onto branched semiconductor nanostructures whose arms have nearly symmetric reactivity

Hierarchical Multicomponent Nanoheterostructures via Facet-to-Facet Attachment of Anisotropic Semiconductor Nanoparticles

As performance and functionality requirements for solution-processed nanomaterials become more stringent and demanding, there is an ever-growing need for hierarchical nanostructures with sophisticated architecture and complex composition. However, the production of structurally complex nanomaterials is often not possible by direct synthesis. In this work, we describe synthetic methodology to covalently link presynthesized anisotropic semiconductor nanoparticles of different composition in a stoichiometrically controlled manner via specific facet sites at room temperature. We demonstrate that CdSe nanorods can be cojoined with CdTe tetrapods via a competitive cation-exchange process with Ag⁺ that results in linking between the tips of the tetrapod arms with only one end of each nanorod via a Ag₂Se–Ag₂Te interface. This selective linking was engineered by having a large fraction of CdSe nanorods present in the reaction, which sterically hindered homolinking between Ag₂Se-tipped CdSe nanorods and Ag₂Te-tipped CdTe tetrapods with themselves. Cation back-exchange with Cd²⁺ and a size-selective purification to remove unlinked products yields samples enriched in heterolinked CdTe tetrapod–CdSe nanorod structures. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the structure and composition of the nanorod-linked tetrapods, while time-resolved and pump-dependent photoluminescence data were consistent with a type II band offset at the CdTe–CdSe interface. The synthetic approach to colloidal nanoheterostructures described here is highly distinct from traditional methods involving a series of nucleation and growth steps at elevated temperature

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump–probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born–Oppenheimer regime

Promoting 2D Growth in Colloidal Transition Metal Sulfide Semiconductor Nanostructures via Halide Ions

Wet-chemically synthesized 2D transition metal sulfides (TMS) are promising materials for catalysis, batteries and optoelectronics, however a firm understanding on the chemical conditions which result in selective lateral growth has been lacking. In this work we demonstrate that Ni₉S₈, which is a less common nonstoichiometric form of nickel sulfide, can exhibit two-dimensional growth when halide ions are present in the reaction. We show that the introduction of halide ions reduced the rate of formation of the nickel thiolate precursor, thereby inhibiting nucleation events and slowing growth kinetics such that plate-like formation was favored. Structural characterization of the Ni₉S₈ nanoplates produced revealed that they were single-crystal with lateral dimensions in the range of ∼100–1000 nm and thicknesses as low as ∼4 nm (about 3 unit cells). Varying the concentration of halide ions present in the reaction allowed for the shape of the nanostructures to be continuously tuned from particle- to plate-like, thus offering a facile route to controlling their morphology. The synthetic methodology introduced was successfully extended to Cu₂S despite its different growth mechanism into ultrathin plates. These findings collectively suggest the importance of halide mediated slow growth kinetics in the formation of nanoplates and may be relevant to a wide variety of TMS

Facet to Facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control

Nonclassical growth mechanisms such as self-assembly and oriented attachment are effective ways to build complex nanostructures from simpler ones. In the latter case, the nanoparticle components are electronically coupled; however, control over the attachment between nanoparticles is highly challenging and generally requires a delicate balance between dipole-, ligand-, and solvent-based interactions. To this end, we perform incomplete cation exchange with Ag⁺ (Cu⁺) on CdSe-seeded CdS nanorods and tetrapods to exclusively convert their tips into small Ag₂S (Cu₂S) domains. Selective removal of the ligands from these inorganic domains results in spontaneous, site-specific bridging of the nanoparticles. Using this method, we demonstrate the fabrication of polymer-like linear and branched nanoparticles with enhanced electrical properties, as well as the stoichiometric formation of nanoparticle homo- and heterodimers and tetramers. We show that linked structures can then be completely cation exchanged with Pb²⁺ to generate PbSe/PbS-based nanostructured photodetector media with enhanced properties

Solution-Processed 2D PbS Nanoplates with Residual Cu₂S Exhibiting Low Resistivity and High Infrared Responsivity

We report the synthesis of colloidal 2D PbS nanoplates with residual Cu₂S domains via a partial cation-exchange process involving Pb²⁺ and presynthesized hexagonal Cu₂S nanoplates with an average thickness of ∼3 nm and edge lengths of ∼150 nm. Different from previously reported PbS nanosheets whose basal planes are ±{100}_PbS, our approach yields nanoplates whose basal planes are ±{111}_PbS, which was previously theoretically predicted to have better surface ligand passivation. Subsequently, we found that the PbS nanoplates showed improved colloidal stability and did not suffer from severe aggregation despite numerous solvent wash steps. We further incorporated a film of nanoplates into a planar photodetector device with lateral Au electrodes. The amount of residual Cu₂S in the PbS nanoplates, which can be tuned by adjusting the reaction time of the cation-exchange process, was found to play a crucial role in determining the in-plane conductivity of the film and therefore its photodetection efficiency. For PbS nanoplates with 7.8% residual Cu⁺, the responsivity and specific detectivity at 808 nm was ∼1739 A/W and ∼2.55 × 10¹¹ Jones, respectively. The high responsivity was attributed to the very low PbS nanoplate film resistivity of 8.04 ohm·cm, which is comparable to commercial doped semiconductors

Gene Detection in Complex Biological Media Using Semiconductor Nanorods within an Integrated Microfluidic Device

The salient optical properties of highly luminescent semiconductor nanocrystals render them ideal fluorophores for clinical diagnostics, therapeutics, and highly sensitive biochip applications. Microfluidic systems allow miniaturization and integration of multiple biochemical processes in a single device and do not require sophisticated diagnostic tools. Herein, we describe a microfluidic system that integrates RNA extraction, reverse transcription to cDNA, amplification and detection within one integrated device to detect histidine decarboxylase (HDC) gene directly from human white blood cells samples. When anisotropic semiconductor nanorods (NRs) were used as the fluorescent probes, the detection limit was found to be 0.4 ng of total RNA, which was much lower than that obtained using spherical quantum dots (QDs) or organic dyes. This was attributed to the large action cross-section of NRs and their high probability of target capture in a pull-down detection scheme. The combination of large scale integrated microfluidics with highly fluorescent semiconductor NRs may find widespread utility in point-of-care devices and multitarget diagnostics

Dual Wavelength Electroluminescence from CdSe/CdS Tetrapods

We fabricated a single active layer quantum dot light-emitting diode device based on colloidal CdSe (core)/CdS (arm) tetrapod nanostructures capable of simultaneously producing room temperature electroluminesence (EL) peaks at two spectrally distinct wavelengths, namely, at ∼500 and ∼660 nm. This remarkable dual EL was found to originate from the CdS arms and CdSe core of the tetrapod architecture, which implies that the radiative recombination of injected charge carriers can independently take place at spatially distinct regions of the tetrapod. In contrast, control experiments employing CdSe-core-seeded CdS nanorods showed near-exclusive EL from the CdSe core. Time-resolved spectroscopy measurements on tetrapods revealed the presence of hole traps, which facilitated the localization and subsequent radiative recombination of excitons in the CdS arm regions, whereas excitonic recombination in nanorods took place predominantly within the vicinity of the CdSe core. These observations collectively highlight the role of morphology in the achievement of light emission from the different material components in heterostructured semiconductor nanoparticles, thus showing a way in developing a class of materials which are capable of exhibiting multiwavelength electroluminescence

Mitochondria Targeted Protein-Ruthenium Photosensitizer for Efficient Photodynamic Applications

Organelle-targeted photosensitization represents a promising approach in photodynamic therapy where the design of the active photosensitizer (PS) is very crucial. In this work, we developed a macromolecular PS with multiple copies of mitochondria-targeting groups and ruthenium complexes that displays highest phototoxicity toward several cancerous cell lines. In particular, enhanced anticancer activity was demonstrated in acute myeloid leukemia cell lines, where significant impairment of proliferation and clonogenicity occurs. Finally, attractive two-photon absorbing properties further underlined the great significance of this PS for mitochondria targeted PDT applications in deep tissue cancer therapy