Single Nanocrystals Research Articles

Effect of self-trapped excitons in the optical properties of manganese-alloyed hexagonal-phased metal halide perovskite

Emission induced by self-trapped excitons (STEs) has recently been studied in several metal halide perovskites (MHPs), particularly in low-dimensional MHPs. STEs generally occur in low-dimensional MHPs owing to their soft lattices, strong electron–phonon coupling, and lattice distortion. However, the formation of STEs in all-inorganic high-dimensional MHPs, specifically in hexagonal phases, is challenging. From this perspective, we studied the formation of STEs in all-inorganic hexagonal-phase CsCdCl3 MHPs and further investigated the role of free excitons and STEs in the optical properties of CsCd(1-x)MnxCl3 MHPs. Mn2+ ion-activated CsCd(1-x)MnxCl3 MHPs were synthesized, and their crystal structures were refined. The CsCd(1-x)MnxCl3 MHPs exhibited orange emission upon 295 nm excitation with a high photoluminescence quantum yield of 74 %. Moreover, CsCd(1-x)MnxCl3 MHPs exhibited good thermal and chemical stability. We also investigated the effect of Mn2+ doping on CsCd(1-x)MnxCl3 single crystals, polycrystals, and nanocrystals. This work offers deep insights into the photophysical properties of MHPs as well as provides a promising approach to achieving high-quality, thermally, and chemically stable white-light emission.

Tracking Bacterial Nanocellulose in Animal Tissues by Fluorescence Microscopy.

The potential of nanomaterials in food technology is nowadays well-established. However, their commercial use requires a careful risk assessment, in particular concerning the fate of nanomaterials in the human body. Bacterial nanocellulose (BNC), a nanofibrillar polysaccharide, has been used as a food product for many years in Asia. However, given its nano-character, several toxicological studies must be performed, according to the European Food Safety Agency’s guidance. Those should especially answer the question of whether nanoparticulate cellulose is absorbed in the gastrointestinal tract. This raises the need to develop a screening technique capable of detecting isolated nanosized particles in biological tissues. Herein, the potential of a cellulose-binding module fused to a green fluorescent protein (GFP–CBM) to detect single bacterial cellulose nanocrystals (BCNC) obtained by acid hydrolysis was assessed. Adsorption studies were performed to characterize the interaction of GFP–CBM with BNC and BCNC. Correlative electron light microscopy was used to demonstrate that isolated BCNC may be detected by fluorescence microscopy. The uptake of BCNC by macrophages was also assessed. Finally, an exploratory 21-day repeated-dose study was performed, wherein Wistar rats were fed daily with BNC. The presence of BNC or BCNC throughout the GIT was observed only in the intestinal lumen, suggesting that cellulose particles were not absorbed. While a more comprehensive toxicological study is necessary, these results strengthen the idea that BNC can be considered a safe food additive.

Synthesis of stable core-shell perovskite based nano-heterostructures

Despite having exceptional optical and photoelectric properties, the application of organometal halide perovskites (OHP) is restricted due to the limited penetration depth of the UV excitation light and poor stability. Attempts have been made to make composite materials by mixing other materials such as upconversion nanoparticles (UCNP) with OHP. In contrast to linear absorption and emission of OHP, the nonlinear upconversion of UCNP offers numerous advantages such as deep penetration depth of the near-infrared (NIR) excitation light, minimal photodamage to biological tissues, and negligible background interference, which offer great potential in various applications such as multiplexed optical encoding, three-dimensional displays, super-resolution bioimaging, and effective solar spectrum conversion. However, it is challenging to synthesize hybrid OHP-UCNP nanocrystals due to the inherent difference in crystal structures of hexagonal phase UCNP and cubic phase OHP. In this work, we report OHP-UCNP heterostructured nanocrystals synthesized via growing cubic phase NaGdF4 UCNP over cubic phase CsPbBr3 OHP in a seed-mediated process based on a very small lattice mismatch and then converting cubic phase UCNP to hexagonal phase through heating. The juxtaposition of UCNP over OHP in a single nanocrystal facilitates efficient energy transfer from UCNP to OHP under NIR excitation and acts as a protective layer improving the stability. The stability is further enhanced by coating an inert UCNP shell on the OHP-UCNP nano-heterostructures with the same UCNP material earlier used in the heterostructures. The coating demonstrated greater stability under continuous UV exposure and in harsh environments such as high temperatures and polar solvents. These NIR excitable perovskite-UCNP nano-heterostructures with improved stability have great potential for use in new optoelectronic and biological applications.

Unraveling the Structural Sensitivity of CO2 Electroreduction at Facet-Defined Nanocrystals via Correlative Single-Entity and Macroelectrode Measurements.

The conversion of CO2 into value-added products is a compelling way of storing energy derived from intermittent renewable sources and can bring us closer to a closed-loop anthropogenic carbon cycle. The ability to synthesize nanocrystals of well-defined structure and composition has invigorated catalysis science with the promise of nanocrystals that selectively express the most favorable sites for efficient catalysis. The performance of nanocrystal catalysts for the CO2 reduction reaction (CO2RR) is typically evaluated with nanocrystal ensembles, which returns an averaged system-level response of complex catalyst-modified electrodes with each nanocrystal likely contributing a different (unknown) amount. Measurements at single nanocrystals, taken in the context of statistical analysis of a population, and comparison to macroscale measurements are necessary to untangle the complexity of the ever-present heterogeneity in nanocrystal catalysts, achieve true structure-property correlation, and potentially identify nanocrystals with outlier performance. Here, we employ environment-controlled scanning electrochemical cell microscopy to isolate and investigate the electrocatalytic CO2RR response of individual facet-defined gold nanocrystals. Using correlative microscopy approaches, we conclusively demonstrate that {110}-terminated gold rhombohedra possess superior activity and selectivity for CO2RR compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms, especially at low overpotentials where electrode kinetics is anticipated to dominate the current response. The methodology framework described here could inform future studies of complex electrocatalytic processes through correlative single-entity and macroscale measurement techniques.

Triplet–Triplet Energy Transfer inside the Single Organic Nanocrystal Revealed by Microscopic Time Resolved Spectroscopy

Triplet–Triplet Energy Transfer inside the Single Organic Nanocrystal Revealed by Microscopic Time Resolved Spectroscopy

Coherent X-ray diffraction of the M1 to M2 structural phase transition in a single vanadium dioxide nanocrystal

Correlated electronic materials are of interest due to strong coupling between lattice, spin and orbital degrees of freedom that give rise to emergent behaviour that is often of considerable utility for next-generation technologies. Vanadium dioxide is a prototypical material that undergoes a number of structural phase transitions near room temperature. Here are presented the results of coherent X-ray diffraction measurements on a single vanadium dioxide nanocrystal approximately 440 nm in size. Experimental findings are compared with ab-initio simulations to elucidate the origin of distortions that are observed in the diffraction pattern.

A General Method for Direct Assembly of Single Nanocrystals

AbstractControlled nanocrystal assembly is a pre‐requisite for incorporation of these materials into solid state devices. Many assembly methods have been investigated which target precise nanocrystal positioning, high process controllability, scalability, and universality. However, most methods are unable to achieve all of these goals. Here, surface templated electrophoretic deposition (STED) is presented as a potential assembly method for a wide variety of nanocrystals. Controlled positioning and deposition of a wide range of nanocrystals into arbitrary spatial arrangements − including gold nanocrystals of different shapes and sizes, magnetic nanocrystals, fluorescent organic nanoparticles, and semiconductor quantum dots − is demonstrated. Nanoparticles with diameters <10 nm are unable to be deposited due to their low surface charge and strong Brownian motion (low Péclet number). It is shown that this limit can be circumvented by forming clusters of nanocrystals or by silica coating nanocrystals to increase their effective size.

Photoluminescence Blinking Revealing Static and Dynamic Heterogeneity of the Hole Transfer Process in Phenothiazine-Adsorbed FAPbBr3 Single Nanocrystals

Lead halide perovskites are perhaps the most important substances for solar photovoltaic applications in recent times. The extraction or transfer of the electrons and holes produced on photoexcitation of these perovskites is a key process in this regard. Here, we investigate how hole transfer from FAPbBr3 to adsorbed phenothiazine (PTZ) influences the photoluminescence (PL) blinking dynamics of FAPbBr3-PTZ single nanocrystals (NCs) in immobilized and fluid conditions. In the immobilized state, while the single FAPbBr3 NCs show two-state (on–off) blinking, the FAPbBr3-PTZ single NCs exhibit a complex blinking pattern with several levels of PL intensity due to the hole transfer process. Analysis of the PL blinking data reveals the dispersive/heterogeneous nature of the hole transfer dynamics. Both particle-to-particle variation and time-dependent variation of the hole transfer rate of any given single NC are observed. Fluorescence correlation spectroscopy studies in the fluid state confirm this dispersive nature of the hole transfer dynamics, indicating the formation of a short-lived (<100 ns) charge-separated state, FAPbBr3–-PTZ+.

Band Alignment Engineering of Semiconductor Nanocrystal Heterostructures Toward Emerging Applications

Due to the superb tunable properties of quantum dots (QDs), they have been largely studied in the past few decades for many promising applications. However, for a single nanocrystal QD, its properties are largely limited due to limited tunability of the composition, charge carrier dynamics, band alignment, and so on. Thus, to solve the problem, and enhance their physical, chemical, electrical, and optical properties, the heterostructure nanocrystals (NCs), which combine single NCs with other materials of either semiconductor or metal, are proposed. In these heterostructures, their properties can be modified through tailoring the band alignment, and therefore providing a great potential for widespread applications. Here, an overview of strategies for band alignment engineering, synthesis methods, and the corresponding applications, such as lighting devices, lasers, photocatalysis, biological applications are offered.

Structural analysis and magnetic properties of cobalt-doped nanotitania

The study is an attempt to bridge the gap between experimental observations and the theoretical predictions on cobalt (Co) doped TiO2 nanoparticles prepared by low cost sol-gel method. Single phase anatase TiO2 nanocrystals with particle size ranging between 16 and 32 nm observed minor distortions in TiO2 unit cell, after cobalt substitution (3 and 5 at.%), due to its larger ionic radius, which leads to creation of oxygen vacancies. Doped TiO2 showed significant diminution from 3.51 to 3.25 eV in the band gap, supported by density functional theory. Presence of nonlinear magnetization within lower field region was also observed which indicate the existence of weak ferromagnetism dominated by paramagnetic behaviour at high field region, suggesting a mixed magnetic phase. Magnetic observations are well supported with bound magnetic polaron (BMP) model. The observed band gap shrinkage and magnetic behaviour both are associated to the oxygen vacancies created by Co doping.

Atomic mechanisms of hexagonal close-packed Ni nanocrystallization revealed by in situ liquid cell transmission electron microscopy

The fundamental understanding of the mechanism underlying the early stages of crystallization of hexagonal-close-packed (hcp) nanocrystals is crucial for their synthesis with desired properties, but it remains a significant challenge. Here, we report using in situ liquid cell transmission electron microscopy (TEM) to directly capture the dynamic nucleation process and track the real-time growth pathway of hcp Ni nanocrystals at the atomic scale. It is demonstrated that the growth of amorphous-phase-mediated hcp Ni nanocrystals is from the metal-rich liquid phases. In addition, the reshaped preatomic facet development of a single nanocrystal is also imaged. Theoretical calculations further identify the non-classical features of hcp Ni crystallization. These discoveries could enrich the nucleation and growth model theory and provide useful information for the rational design of synthesis pathways of hcp nanocrystals.

Nanorefrigerative tweezers for optofluidic manipulation

Plasmonic optical tweezers with the ability to manipulate nano-sized particles or molecules that are beyond the diffraction limit have been developed rapidly in recent years. However, plasmonic heat generation always limits its applications in capturing particles or biomacromolecules that are vulnerable to high temperatures. Here, we propose nanorefrigerative tweezers based on a single refrigerative nanocrystal, which can form a nanometer-sized cold-spot via anti-Stokes fluorescence. Numerical simulations are performed to compute the temperature and velocity fields. The results show that thermo-osmosis and thermophoresis play major roles in nanoparticle manipulation, while natural convection in the nanoscale is negligible. This tweezing scheme not only offers a sub-diffraction-limit way to manipulate nano-objects but also avoids possible thermal damage to the trapped targets. Therefore, it will potentially become a powerful tool in biomedical and biosensing research studies.

Band-Gap and Dimensional Engineering in Lead-Free Inorganic Halide Double Perovskite Cs4Cu1-xAg2xSb2Cl12 Single Crystals and Nanocrystals

Lead free double perovskites (DPs) are promising materials due to their non-toxic and tunable optical properties. In this work, a series of lead-free halide DP single crystals (SCs) and nanocrystals (NCs) (Cs4Cu1-xAg2xSb2Cl12) were reported. With alloying strategy, the optical band-gap engineering was realized and the dimension can be controlled between 2D and 3D. The Cu-alloyed SCs exhibit strong absorption from the UV-visible region to the near-infrared range (can even completely cover the bands of NIR-I and NIR-II). In addition, (Cs4Cu1-xAg2xSb2Cl12) NCs were synthesized via a top-down approach, which maintains similar optical properties and the dimensional transformation phenomenon to SCs. These results suggest the great potential of Cs4Cu1-xAg2xSb2Cl12 SCs and NCs for photovoltaic and optoelectronic applications.

Second to fifth harmonic generation in individual β-barium borate nanocrystals.

We have studied the nonlinear optical properties of single β-barium borate nanocrystals, with potential applications as probes in nonlinear sensing and imaging schemes. Our work demonstrates their ability to generate second, third, fourth, and fifth harmonics. The particles' polarization response is studied and compared with simulations based on the bulk nonlinear tensors, with good agreement. Furthermore, the nonlinear susceptibilities of different orders are estimated.

Defect-Mediated Electron Transfer in Pt-CuInS2/CdS Heterostructured Nanocrystals for Enhanced Photocatalytic H2 Evolution

The effective separation of photogenerated electrons and holes is the key to improve photocatalytic activity in hydrogen evolution reaction (HER). However, the mechanism of photoinduced charge transfer between the cocatalyst and the semiconducting heterojunction in a single nanocrystal is vague at present. Herein, the relationships between the crafted cocatalyst and the constructed semiconducting heterojunction in a single nanocrystal were verified. Manipulating these relationships will improve the performance of visible light-driven hydrogen evolution. As a model, we used CuInS2 (CIS) nanocrystals (NCs) modified by the cocatalyst platinum (Pt) NCs and the semiconductor cadmium sulfide (CdS), forming the ternary heterostructured NCs (HNCs). Under visible light irradiation, the photocatalytic activity in hydrogen evolution of Pt-CIS/CdS HNCs is 6.38 and 1.76 times higher than those of pristine CIS NCs and CIS/CdS HNCs, respectively. Transient absorption measurements certified that the high photocatalytic activity in the HER was attributed to the photoinduced electron transfer from the defect-mediated trap state in CIS to the cocatalyst Pt, which was superior to the direct Z-scheme heterojunction between CIS and CdS. This research paves a route for the design of a suitable cocatalyst and the formation of heterostructured semiconducting NCs for boosting the photocatalytic activity in solar energy conversion.

Well-Defined Copper-Based Nanocatalysts for Selective Electrochemical Reduction of CO2 to C2 Products

The electrochemical CO2 reduction reaction (CO2RR) holds the promise to revolutionize the chemical industry by producing value-added chemicals and fuels from CO2 and water while storing renewable energy. However, catalyst design still remains one of the major hurdles in the field. This Perspective discusses the current and future contributions of well-defined nanocatalysts to improving the CO2RR selectivity toward C2 products, particularly ethylene and ethanol. In this regard, the shape and size selectivity dependence of single metal copper nanocrystals is briefly reviewed and linked to single-crystal studies. Representative studies on Cu-based bimetallic nanocrystals are discussed to highlight the importance of composition and distribution of the metals for selectivity. Finally, a vision on design strategies in terms of shape and composition for the next generation of CO2RR nanocatalysts is proposed.

The digital printing of chromatic pattern with a single cellulose nanocrystal ink

Cellulose nanocrystals (CNCs), a type of natural photonic crystal, have been used to develop various optical materials owing to their chiral nematic organization, renewability, sustainability, and abundance. However, scaling up the production of CNC-based photonic materials remains challenging because of their long self-assembly time, inevitable assembly defects, static optical properties, and brittle nature. To address these drawbacks, the current study introduces flexible photonic hydrogels with chromatic patterns that are 3D printed using CNC-based inks. These viscoelastic inks were composed of photopolymerizable monomers and CNCs that harbored high aspect ratios. The luminance and color difference of the patterns in the photonic hydrogels were digitally controlled by varying the printing angles and the number of layers. Furthermore, our technique was validated by printing recognizable QR codes, Tai-chi, and hydrogel color cards. The current study provides a facile, fast, reproducible, accurate, and scalable strategy for fabricating photonic hydrogels from sustainable materials with chromatic patterns visible only in polarized light, thereby demonstrating their potential use in optical devices, anti-counterfeiting applications, and information concealment and dissemination.

From Khoi-San indigenous knowledge to bioengineered CeO2 nanocrystals to exceptional UV-blocking green nanocosmetics

Single phase CeO2 nanocrystals were bio-synthesized using Hoodia gordonii natural extract as an effective chelating agent. The nanocrystals with an average diameter of 〈Ø〉 ~ 5–26 nm with 4+ electronic valence of Ce displayed a remarkable UV selectivity and an exceptional photostability. The diffuse reflectivity profile of such CeO2 exhibited a unique UV selectivity, in a form of a Heaviside function-like type profile in the solar spectrum. While the UV reflectivity is significantly low; within the range of 0.7%, it reaches 63% in the VIS and NIR. Their relative Reactive Oxygen Species (ROS) production was found to be < 1 within a wide range of concentration (0.5–1000 μg/ml). This exceptional photostability conjugated to a sound UV selectivity opens a potential horizon to a novel family of green nano-cosmetics by green nano-processing.

Study of Single Gold Nanocrystals by Inelastic Light Scattering Spectroscopy

Study of Single Gold Nanocrystals by Inelastic Light Scattering Spectroscopy

High Speed Mass Measurement of a Single Metal-Organic Framework Nanocrystal in a Paul Trap.

Mass spectrometry (MS) has emerged as an excellent tool for the characterization of metal-organic frameworks (MOFs) based on the characteristic metal ions and organic ligands. Mass measurement of intact MOF nanocrystals, however, remains a challenge for MS technology. Here, we reported the development of a probe particles based charge detection-quadrupole ion trap mass spectrometry (probe CD-QIT MS) method, where charge detection and mass measurement of a single MOF nanocrystal were achieved under the assistance of probe particles of micrometer size. As a validation of the method, the masses of a series of polystyrene (PS) size standards from 493 nm to 1.6 μm were measured with 3 μm PS particles as probes, and the measured masses were found to match well with their certified masses. Then, charge detections and mass analysis of single ZIF-8 and GOx@ZIF-8 with a size around 600 nm were achieved successfully. The method presented here demonstrates simplicity, high speed, and accuracy. Notably, it allows quantitative measurement of the amount of immobilized GOx enzyme by using the mass difference between ZIF-8 and GOx@ZIF-8. In addition, based on the determined mass, the size analysis of these MOF particles with irregular shape was carried out and demonstrated to be complementary to transmission electron microscopy (TEM).