Close-packed Films Research Articles

Green Light from Red-Emitting Nanocrystals: Broadband, Low-Threshold Lasing from Colloidal Quantum Shells in Optical Nanocavities.

Spherical semiconductor nanoplatelets, known as quantum shells (QSs), have captured significant interest for their strong suppression of Auger recombination, which leads to long multiexciton lifetimes and wide optical gain bandwidth. Yet, the realization of benefits associated with the multiexciton lasing regime using a suitably designed photonic cavity remains elusive. Here, we demonstrate broadly tunable lasing from close-packed films of CdS/CdSe/CdS QSs deposited over nanopillar arrays on Si substrates. Wide spectral tuning of the stimulated emission in QSs with a fixed bandgap value was achieved by engaging single exciton (λX ∼ 634 nm), biexciton (λBX ∼ 627 nm), and multiple exciton (λMX ∼ 615-565 nm) transitions. The ensemble-averaged gain threshold of <N> ∼ 2.6 electron-hole pairs per QS particle and the low photonic cavity fluence threshold of ∼4 μJ/cm2 were attributed to Auger suppression. The tuning of the lasing emission closely aligns with our model predictions achieved by varying the array period while preserving mode confinement and quality (Q) factors. These results mark a notable step toward the development of colloidal nanocrystal lasers.

Amplified Spontaneous Emission from Electron-Hole Quantum Droplets in Colloidal CdSe Nanoplatelets.

Two-dimensional cadmium selenide nanoplatelets (NPLs) exhibit large absorption cross sections and homogeneously broadened band-edge transitions that offer utility in wide-ranging optoelectronic applications. Here, we examine the temperature-dependence of amplified spontaneous emission (ASE) in 4- and 5-monolayer thick NPLs and show that the threshold for close-packed (neat) films decreases with decreasing temperature by a factor of 2-10 relative to ambient temperature owing to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. Interestingly, for pump intensities that exceed the ASE threshold, we find development of intense emission to lower energy in particular provided that the film temperature is ≤200 K. For NPLs diluted in an inert polymer, both biexcitonic ASE and low-energy emission are suppressed, suggesting that described neat-film observables rely upon high chromophore density and rapid, collective processes. Transient emission spectra reveal ultrafast red-shifting with the time of the lower energy emission. Taken together, these findings indicate a previously unreported process of amplified stimulated emission from polyexciton states that is consistent with quantum droplets and constitutes a form of exciton condensate. For studied samples, quantum droplets form provided that roughly 17 meV or less of thermal energy is available, which we hypothesize relates to polyexciton binding energy. Polyexciton ASE can produce pump-fluence-tunable red-shifted ASE even 120 meV lower in energy than biexciton ASE. Our findings convey the importance of biexciton and polyexciton populations in nanoplatelets and show that quantum droplets can exhibit light amplification at significantly lower photon energies than biexcitonic ASE.

Silicon solar cell efficiency improvement employing electrostatically assembled polyelectrolyte–quantum dot multilayers

Down-shifting (DS) has proven to be an effective technique to address thermalization losses in photovoltaic devices. It involves the modification of the incident solar spectrum by luminescent species to better suit solar cell’s optimal absorption regions. Despite the number of favorable characteristics of quantum dots (QDs) as DS materials, their incorporation into highly ordered close-packed films remains the major drawback of QD-based DS applications. This limitation can be overcome by the sequential integration of QDs into a multilayered film via the layer-by-layer assembly technique, that allows the precise control of composition and thickness. The experimental results discussed herein indicate that upon optimization employing a model-assisted design approach, the spectral response of a silicon solar cell benefits from the down-shifting and antireflection capabilities of the QD film, as evinced by their quantum efficiency. For the optimum design, the incorporation of QDs triggered an increment of 26.1% in the power conversion efficiency driven by an increase of 29.2% in the short circuit current density. The controlled assembly of QD multilayers offers unique opportunities in light harvesting for new hybrid photovoltaic technologies.

Multi-Species Surface Reconstruction for High-Efficiency Perovskite Nanocrystal Light-Emitting Diodes.

Although colloidal perovskite nanocrystal (PNC) solution has exhibited near-unity photoluminescence quantum yield (PLQY), the luminance would be severely quenched when the PNC solution is assembled into thin films due to the agglomeration and fusion of NCs caused by the exfoliation of surface ligands and non-radiative Förster resonance energy transfer (FRET) from small to large particle sizes, which seriously affected the performances of light-emitting diodes (LEDs). Here, we used Guanidine thiocyanate (GASCN) and Sodium thiocyanate (NaSCN) to achieve effective CsPbI3 PNC surface reconstruction. Due to the strong coordination ability of these small molecules with the anions and cations on the surface of the PNCs, they can provide strong surface protection against PNC fusion during centrifugal purification process and repair the surface defects of PNCs, so that the original uniform size distribution of PNCs can be maintained and FRET between close-packed PNC films is effectively suppressed, which allows the emission characteristics of the films to be preserved. As a result, highly oriented, smooth and nearly defect-free high-quality PNC thin films are obtained, with PLQY as high as 95.1 %, far exceeding that of the original film, and corresponding LEDs exhibit a maximum external quantum efficiency of 24.5 %.

Simultaneously improved photoluminescence, stability, and carrier transport of perovskite nanocrystals by post-synthetic perfluorobutanesulfonic acid treatment.

We report a post-synthetic treatment method based on perfluorobutanesulfonic acid (PFBA) to ameliorate the photophysical performance of perovskite nanocrystals. By virtue of the PFBA treatment, both the photoluminescence efficiency and stability of perovskite quantum dot-based colloidal solutions and the electrical conductivity of their close-packed films are simultaneously improved.

Two-Color Amplified Spontaneous Emission from Auger-Suppressed Quantum Dots in Liquids.

Colloidal quantum-dot (QD) lasing is normally achieved in close-packed solid-state films, as a high QD volume fraction is required for stimulated emission to outcompete fast Auger decay of optical-gain-active multiexciton states. Here a new type of liquid optical-gain medium is demonstrated, in which compact compositionally-graded QDs (ccg-QDs) that feature strong suppression of Auger decay are liquefied using a small amount of solvent. Transient absorption measurements of ccg-QD liquid suspensions reveal broad-band optical gain spanning a wide spectral range from 560 (green) to 675nm (red). The gain magnitude is sufficient to realize a two-color amplified spontaneous emission (ASE) at 637 and 594nm due to the band-edge (1S) and the excited-state (1P) transition, respectively. Importantly, the ASE regime is achieved using quasicontinuous excitation with nanosecond pulses. Furthermore, the ASE is highly stable under prolonged excitation, which stands in contrast to traditional dyes that exhibit strong degradation under identical excitation conditions. These observations point toward a considerable potential of high-density ccg-QD suspensions as liquid, dye-like optical gain media that feature readily achievable spectral tunability and stable operation under intense photoexcitation.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

Off-Resonance Gold Nanobone Films at Liquid Interface for SERS Applications.

Extensive effort and research are currently channeled towards the implementation of SERS (Surface Enhanced Raman Spectroscopy) as a standard analytical tool as it has undisputedly demonstrated a great potential for trace detection of various analytes. Novel and improved substrates are continuously reported in this regard. It is generally believed that plasmonic nanostructures with plasmon resonances close to the excitation wavelength (on-resonance) generate stronger SERS enhancements, but this finding is still under debate. In the current paper, we compared off-resonance gold nanobones (GNBs) with on-resonance GNBs and gold nanorods (GNRs) in both colloidal dispersion and as close-packed films self-assembled at liquid-liquid interface. Rhodamine 6G (R6G) was used as a Raman reporter in order to evaluate SERS performances. A 17-, 18-, and 55-fold increase in the Raman signal was observed for nanostructures (off-resonance GNBs, on-resonance GNBs, and on-resonance GNRs, respectively) assembled at liquid-liquid interface compared to the same nanostructures in colloidal dispersion. SERS performances of off-resonance GNBs were superior to on-resonance nanostructures in both cases. Furthermore, when off-resonance GNBs were assembled at the liquid interface, a relative standard deviation of 4.56% of the recorded signal intensity and a limit of detection (LOD) of 5 × 10−9 M could be obtained for R6G, rendering this substrate suitable for analytical applications.

Bonding agents to mitigate interfacial strength loss during multi-layer concrete casting

Self-consolidating concrete (SCC) is very sensitive to interruptions or delays during the placement process. Current literature converges that interfacial bond strength between successive casting lifts could drop by up to 60%, compared with monolithic casting. In this study, two series of SCC mixtures were tested to assess the suitability of bonding agents to mitigate the drop in interfacial bond strength resulting from successive casting lifts. The slabs were cast in two layers separated by 60 or 120 min rest intervals; different bonding agents, including styrene–butadiene rubber (SBR), waste latex paint and cement/water and cement/SBR slurries were compared. The results showed that the application of polymeric bonding agents on top of the first cast is efficient to improve tensile strength and restore the integrity of the hardened member. This is attributed to the polymer particles that coalesce and fuse to form continuous close-packed films between the first and second SCC layers. The cement/water slurry did not lead to considerable improvement in pull-off strength between cast layers. Series of charts enabling the prediction of the drop in bond for a given SCC composition and rest interval as a function of the polymer dosage to the area of the cast slab are presented.

Electrochemical investigation on adsorption kinetics of long chain alkylsilanes and influence of solvents on their self-assembly and electron transfer behavior on indium tin oxide

In situ adsorption kinetics of hexadecyltrimethoxy silane (HDTMS) and octadecyltrimethoxy silane (ODTMS) on indium tin oxide (ITO) surface at short time intervals were measured by electrochemical impedance (EIS) spectroscopy technique at constant frequency. The studies suggest that, self-assembly of silanes on ITO proceeds through two distinct adsorption steps, a fast first step followed by a slower second step. The inhibition of electron transfer by self-assembled silanes prepared in ethanol, toluene and hexane solvents were evaluated by cyclic voltammetry (CV) and EIS using [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ redox species. It is shown that, the quality of silane films are influenced by the polarity of solvent, chain length of silane and ITO substrate–solvent interactions. The effect of annealing on electron transfer barrier property of silane monolayers after their formation were studied. The pinhole characteristics of silane films on ITO prepared from various solvents under study were measured using EIS. This study demonstrates a strategy of producing an ultra-smooth and close-packed silane films on ITO surfaces which can have potential applications towards high performance organic electronics.

Let There Be Order, in Films of Colloidal CdSe 2D Nanocrystals.

Progression from random orientation of 2D CdSe nanoplatelets to ordered close-packed thin films enables us to exploit the in-plane dipole moment of the band-edge transition on a macroscopic scale and gain control over the direction of charge and energy transport within the film. Momper et al. show how this can be achieved by tuning the solvent evaporation rate during deposition. They are able to switch from thermodynamically to kinetically controlled conditions for the film formation, resulting in ordered films with either face-down or edge-up alignment of the 2D nanoplatelets.

Efficient and sub-nanosecond resonance energy transfer in close-packed films of ZnSe quantum dots by steady-state and time-resolved spectroscopy

Abstract Quantum dots (QDs) structures have the large potential to provide new optico-electrical properties through energy transfer from small-sized QDs to large one. We study the energy transfer process and an influence of ligand on interior states QDs in ZnSe QDs films stabilized by thioglycerol. By combining a steady-state and time resolved photoluminescence spectroscopy measurements we demonstrated the high energy transfer efficiency and its effect on optical properties of random loose-packed QDs arrays. First shown that in ZnSe QDs films the energy transfer time constant can be as fast as 750 ps that is governed by the minimal inter-QDs distance (small length of the thioglycerol molecules) and strong excitons quantum confinement. We discussed also possible causes of an occurrence the so-called blocking QDs in arrays limiting the exciton transport length and energy transfer processes.

General Self-Assembly Method for Deposition of Graphene Oxide into Uniform Close-Packed Monolayer Films.

Depositing a morphologically uniform monolayer film of graphene oxide (GO) single-layer sheets is an important step in the processing of many composites and devices. Conventional Langmuir-Blodgett (LB) deposition is often considered to give the highest degree of morphology control, but film microstructures still vary widely between GO samples. The main challenge is in the sensitive self-assembly of GO samples with different sheet sizes and degrees of oxidation. To overcome this drawback, here, we identify a general method that relies on robust assembly between GO and a cationic surfactant (cationic surfactant-assisted LB). We systematically compared conventional LB and cationic surfactant-assisted LB for three common GO samples of widely different sheet sizes and degrees of oxidation. Although conventional LB may occasionally provide satisfactory film morphology, cationic surfactant-assisted LB is general and allows deposition of films with tunable and uniform morphologies-ranging from close-packed to overlapping single layers-from all three types of GO samples investigated. Because cationic surfactant-assisted LB is robust and general, we expect this method to broaden and facilitate the use of GO in many applications where precise control over film morphology is crucial.

Fabrication of large scale PS monolayer colloidal crystal film by using a novel secondary self-assembly method for nanoimprint technique

PS microspheres emulsions with different PS diameters of 207 and 378 nm were prepared by using an emulsifier-free polymerization method. A novel secondary self-assembly method which combined the spin coating and the lifting-up method was used to prepare the close-packed PS MCC films. During the process, the PS MCC film was firstly assembled on a glass substrate by a spin coating technique. Secondly, the obtained PS MCC film was transferred on the air-water interface by immersing the PS glass substrate in water using a dip-coater. Thirdly, the floating close-packed PS MCC films on the air-water interface was transferred to a new glass substrate by using a lifting-up method. During the spin-coating and the self-assembly process, some SDS solution was added in the PS films to improve the uniformity of the PS layer. Effects of the spin-coating speeds, the SDS solution concentration and the lifting-up rates on the morphology of PS MCC films have been discussed. Scanning electron microscopy was used to observe the morphological properties of the PS colloidal crystal films. Results indicate that a relatively close-packed and large scale PS MCC films can be obtained when the spinning speed is 2000 rpm for 60 s, the SDS solution concentration is 2 wt% and the lifting-up rate is 2 mm/min. Furthermore, the polydimethylsiloxane soft mold was replicated from the-fabricated PS MCC films and was then imprinted onto the photosensitive hybrid film under UV-irradiation and thus the micro-lens arrays built in the photosensitive hybrid sol–gel film were obtained.

Effect of bath composition and pulse electrodeposition condition on characteristics and microhardness of cobalt coatings

Abstract Nanocrystalline cobalt coatings were produced from cobalt sulfate based electrolytes by using pulse current electrodeposition technique. The effects of bath composition and electrodeposition condition on current efficiency, morphology, structure and hardness of the coatings were investigated and the optimum deposition condition was determined. It was found that increment of cobalt sulfate concentration and sodium dodecyl sulfate (SDS) concentration in the bath had a negligible effect on microhardness of the coatings, while they were effective on electrodeposition current efficiency. Adding saccharin to electrodeposition bath decreased crystallite size of hexagonal close-packed (hcp) cobalt films and increased their microhardness without significant effect on current efficiency. Smoother and less defective coatings were also obtained from baths containing SDS and saccharin. The results revealed that both the current efficiency and microhardness were changed by variation of peak current density and duty cycle. Besides change of smooth morphology of the coatings to needle-shaped one, crystallite sizes and preferred orientation also varied with increasing the current density and duty cycle.

Synthesis of hafnium nanoparticles and hafnium nanoparticle films by gas condensation and energetic deposition.

In this work we study the fabrication and characterization of hafnium nanoparticles and hafnium nanoparticle thin films. Hafnium nanoparticles were grown in vacuum by magnetron-sputtering inert-gas condensation. The as deposited nanoparticles have a hexagonal close-packed crystal structure, they possess truncated hexagonal biprism shape and are prone to surface oxidation when exposed to ambient air forming core–shell Hf/HfO2 structures. Hafnium nanoparticle thin films were formed through energetic nanoparticle deposition. This technique allows for the control of the energy of charged nanoparticles during vacuum deposition. The structural and nanomechanical properties of the nanoparticle thin films were investigated as a function of the kinetic energy of the nanoparticles. The results reveal that by proper adjustment of the nanoparticle energy, hexagonal close-packed porous nanoparticle thin films with good mechanical properties can be formed, without any additional treatment. It is shown that these films can be patterned on the substrate in sub-micrometer dimensions using conventional lithography while their porosity can be well controlled. The fabrication and experimental characterization of hafnium nanoparticles is reported for the first time in the literature.

Synthesis and Evaluation of Ideal Core/Shell Quantum Dots with Precisely Controlled Shell Growth: Nonblinking, Single Photoluminescence Decay Channel, and Suppressed FRET

Due to the unique optical properties, colloidal quantum dots (QDs) are excellent candidates for developing next-generation display and solid-state lighting technologies. However, some factors including photoluminescence blinking and Forster resonance energy transfer (FRET) still affect their practical applications. Herein, a series of ZnCdSe-based core/shell QDs with low optical polydispersity have been successfully synthesized by a “low-temperature injection and high-temperature growth” precisely controlled method. The alloyed ZnCdSe core with a certain ratio of Cd and Zn was presynthesized first. Followed by accurate ZnS shell growth, the as-synthesized core/shell QDs are nonblinking with the nonblinking threshold volume of ∼137 nm3. The PL decay dynamics are all single-exponential for both QDs in solutions and close-packed solid films when ZnS shell thickness varying from 2 to 20 monolayers. FRET can be effectively suppressed after growing 10 monolayers of ZnS shell. All of these superb characteristics...

Efficient Optical Amplification in the Nanosecond Regime from Formamidinium Lead Iodide Nanocrystals

Lead halide perovskite nanocrystals (NCs) were recently found to exhibit extraordinary optical amplification properties. The great majority of such studies implemented ultrashort photon pulses in the femtosecond regime to initiate the stimulated emission process. Yet the realization of practical lasing applications based on such materials is crucially dependent on their ability to sustain optical amplification at significantly longer time scales, at which major losses associated with spontaneous emission and nonradiative recombination occur. Herein we demonstrate highly efficient amplified spontaneous emission (ASE) from close-packed films of formamidinium lead iodide perovskite (FAPbI3) NCs under excitation in the nanosecond regime. Systematic optimization of the NC processing and thermal treatment yields solids that exhibit high ASE net modal gain up to 604 cm–1 and weakly temperature sensitive ASE thresholds with room-temperature values as low as 140 μJ cm–2. The efficient optical amplification using e...

Self-assembly of high-index faceted gold nanocrystals to fabricate tunable coupled plasmonic superlattices.

Herein, we present an effective bottom-up strategy to fabricate unprecedented macroscopic two-dimensional (2D) plasmonic gold superlattices composed of high-index faceted gold nanocrystal building blocks (NBBs) at the air-liquid interface. In this approach, a synergistic electrostatic and layered self-assembly technique was executed using unique icosidodecahedral gold nanocrystals. It showed that centimeter-squared areas of close-packed monolayer films were formed, and the interparticle spacing of neighbouring Au NBBs could be facilely manipulated from hundreds to several nanometers. Optical characterization demonstrated that particular plasmonic coupling could occur and enhance in a wide spectral range (visible and near-IR) as the self-assembled Au superlattices were tuned for an appropriate gap distance and specific NBB size; however, the orientation of individual NBBs remained somewhat unorganized. Thus, the well pronounced shift of localized surface plasmon resonances (LSPR) and the in-depth resonance splitting behaviors were presented in our investigations. Furthermore, corresponding electromagnetic simulations showed good agreement with the experimental results; this indicated that a new class of tunable coupled plasmonic Au superlattices was realized. This study complements the insights into the plasmonic coupling of layered Au superlattices and enables the colloidal self-assembly to extend to unconventional NBBs; thus, it may facilitate the design of novel plasmonic metamaterials or other superstructures for desired functionalities and applications in the future.

On the processing of monolayer C60 fullerene films and their mechanical properties

Hexagonal close-packed monolayer films of unmodified C60 fullerene were, for the first time, produced successfully by using the Langmuir–Blodgett technique. The processing parameters were optimized, and the structure of these films has been investigated using scanning tunneling microscopy and atomic force microscopy. These films have great potential applications in many different fields including transparent electrodes, sensor, optoelectronic devices and photovoltaics.