Kinds Of Nanocrystals Research Articles

Upconversion Nano‐Composites for Orthogonal Trichromatic Luminescence Aiming at High Color Purity Volume Display

AbstractOrthogonal three‐color luminescence materials designed based on lanthanide doping multi‐component nano‐systems have exhibited great potential in volume display technology. Previous studies devoted to integrate the different color emitters within a single core‐shell nanoparticle, which suffers complexity in synthesis and difficulty in repeatability. In this work, an alternative is proposed. Three kinds of upconversion nanocrystals (UCNCs) with relatively simple configurations are prepared separately. Each kind of nanocrystals takes charge of emit a specific color upon one particular excitation (i.e., 808, 980, and 1540 nm pumps). Luminescent property of the mixture containing these nanocrystals are investigated. Excitation wavelength driven orthogonal three‐color luminescence property is found in this mixture. With the combination excitation, the emission color can be modulated, covering a gamut with its aera of 110% sRGB, superior to the previous reports. In addition, a quantized strategy to evaluate the orthogonality of the luminescence is also proposed, which will be useful for the further investigation and development of remote‐type controlled luminescent materials. Finally, the demonstration of colored 3D display application is conducted and presented in this work.

Real Impacts of Ligand-Capped Nanocrystals in Perovskite Solar Cells

Perovskite solar cells (PSCs) has been reached the certified power conversion efficiency (PCE) of 25.2 % in 2019. They are having received large attention because of their superior optoelectronic properties (such as high charge carrier mobility, tunable bandgap, high absorption coefficient) and even the cost-effective process.In the latest fabrication process for high-efficiency PSCs, surface-defect engineering of perovskite film has attracted tremendous attention. Because, the control of surface-defects can play a critical role in grain boundaries of perovskite, and interfaces between perovskite and charge transport layer, leading to PSCs with further improved photovoltaic (PV) performance and long-term stability. There are many exertion for reducing surface-defects state densities of perovskite film by applying an additional passivation layer, for instance cross-linked molecules and two-dimensional (2D) materials (2D-perovskite, graphene, and MoS2) at surface of perovskite film.Most recently, perovskite nanocrystals (NCs) are being investigated as an additional layer for high-efficiency PSCs owing to anticipation of their multifunctional properties including high light-harvesting properties, efficient charge transport by carrier cascade effect, and surface passivation effect. However, it seems to remain under question whether these functions are related to the light-harvesting, suitable band alignment, and surface passivation for PSCs, due to ligand capped NCs.In this study, we investigated that the functions of NCs layer in PSCs with various NCs capped same ligand. To confirm the effects of NC layer in PSCs, we fabricated n-i-p planar devices, which follow FTO/c-TiO2/Perovskite/NC layer/Spiro-OMeTAD/Au structure with three kinds of NCs (CsPbBr3, CsPbI3, and InP). All these NCs have different electrical and optical properties, but nevertheless the devices with each NC layer showed similarly improved PV performance.In analysis results of each function of the NC layer, there were no light-harvesting effects by all NCs layer due to high absorption coefficient and sufficient thickness of perovskite film. In addition, NC layer could not work as a carrier cascade layer at the interface between the perovskite film and hole transport layer due to poor electrical properties of capping ligand. Finally, the NC layer affected only on the surface passivation of perovskite film, and furthermore we found that Oleylamine (OLA) ligand of the NCs was a major cause of the surface passivation effect. We believe that the optimization of the surface passivation effect of fatty amine ligands can further improve the PV performance of PSCs.

Continuous Synthesis of Nanocrystals via Flow Chemistry Technology.

Modern nanotechnologies bring humanity to a new age, and advanced methods for preparing functional nanocrystals are cornerstones. A considerable variety of nanomaterials has been created over the past decades, but few were prepared on the macro scale, even fewer making it to the stage of industrial production. The gap between academic research and engineering production is expected to be filled by flow chemistry technology, which relies on microreactors. Microreaction devices and technologies for synthesizing different kinds of nanocrystals are discussed from an engineering point of view. The advantages of microreactors, the important features of flow chemistry systems, and methods to apply them in the syntheses of salt, oxide, metal, alloy, and quantum dot nanomaterials are summarized. To further exhibit the scaling-up of nanocrystal synthesis, recent reports on using microreactors with gram per hour and larger production rates are highlighted. Finally, an industrial example for preparing 10 tons of CaCO3 nanoparticles per day is introduced, which shows the great potential for flow chemistry processes to transfer lab research to industry.

Coexisting CsPbCl3:CsPbI3 perovskite nanocrystal glasses with high luminescence and stability

Many previous studies have reported that because of the large difference between the ionic radii of Cl and I, halogen exchange is difficult to occur between CsPbCl3 and CsPbI3 perovskite nanocrystals, and the interaction between CsPbCl3 and CsPbI3 NCs will lead to their dissolution. In this work, two kinds of nanocrystals, CsPbCl3 and CsPbI3, were successfully crystallized simultaneously in glass by an in-situ growth method for the first time. Because of the effective reabsorption of photons, the CsPbCl3:CsPbI3 nanocrystals displayed higher fluorescence emission intensity and higher photoluminescence quantum yield than those of CsPbI3 nanocrystals. In addition, CsPbCl3:CsPbI3 nanocrystal glasses showed excellent water resistance and thermal stability, which benefited from the effective protection of the glass matrix. In particular, the fluorescence intensity of the nanocomposites remained above 95% after immersion in water for 60 days. Light-emitting devices were prepared by combining a near-infrared-emitting CsPbCl3:CsPbI3 nanocrystal glass and 400 nm LED chip, which were applied to indoor plant growth.

Propanol-Triggered Luminescence for Rapid Screening of Crystal Facets in Noble Metal.

The rapid and effective recognition techniques for crystal facets in noble metal nanocrystals (NCs) are highly desirable in evaluation of their catalytic performances. Herein, it was found that the variation trend of cataluminescence (CTL) intensity in the presence of propanol was proportional to the crystal facets index change of LDH-supported Pd@Au NCs. The mechanism investigation for the propanol-triggered CTL on the surface of layered double hydroxide (LDH)-supported Pd@Au NCs revealed that the abundant negative charge in the high-index facets of LDH-supported Pd@Au NCs facilitated electron transfer between NCs and volatile propanol, resulting in acceleration of the CTL reactions. Accordingly, the propanol-triggered CTL can be as an indicator for rapid screening of crystal facets in NCs. Its universality had further been verified by screening crystal facets in LDH-supported Pd NCs. The crystal facets detected by propanol CTL probe were well-matched with those obtained by conventional high-resolution transmission electron microscope. The proposed CTL strategy exhibits some advantages, such as convenient operation, rapid response, long-term stability, and low cost. And it is expected crystal facets of other kinds of NCs could be screened by replaceable CTL probe.

Self-assembly patterning of ultrafine zirconia nanocrystal films fabricated on chemically patterned templates

Ultra-thin zirconia (ZrO2) nanocrystal films were fabricated by using a controlled dip-coating process. ZrO2 nanocrystals possess a cubic crystalline phase and large surface-to-volume area. The film composite with only several layers of nanocrystals were obtained by controlling the withdrawal speed and mass concentration of the colloidal solution. The optical properties of ZrO2 nanocrystal films were accessed by UV–vis spectroscopy, which indicated the dense and uniform structure of the nanocrystal films. The high reflection index suggested that the films could be used in the reflection coating industry. Furthermore, a micro-pattern of self-assembled monolayers of silane molecular was used as a chemical mold for selective deposition of ZrO2 nanocrystals. As a result, a self-assembly patterning of ZrO2 nanocrystals with a neat edge was fabricated on silicon substrate. The low-cost fabricating method is compatible with conventional very-large-scale integration processes and can be extended to other kinds of nanocrystals.

Tuning the functionalities of a mesocrystal via structural coupling.

In the past decades, mesocrystal, a kind of nanocrystals with specific crystallographic orientation, has drawn a lot of attention due to its intriguing functionalities. While the research community keeps searching for new mesocrystal systems, it is equally crucial to develop new approaches to tune the properties of mesocrystals. In this work, a self-organized two-dimensional mesocrystal composed of highly oriented CoFe2O4 (CFO) nano-crystals with assistance of different perovskite matrices is studied as a model system. We have demonstrated that the strain state and corresponding magnetic properties of the CFO mesocrystal can be modulated by changing the surrounding perovskite matrix through their intimate structural coupling. Interestingly, this controllability is more strongly correlated to the competition of bonding strength between the matrices and the CFO mesocrystals rather than the lattice mismatch. When embedded in a matrix with a higher melting point or stiffness, the CFO mesocrystal experiences higher out-of-plane compressive strain and shows a stronger magnetic anisotropy as well as cation site-exchange. Our study suggests a new pathway to tailor the functionalities of mesocrystals.

Liquid-Phase Syntheses and Material Properties of Two-Dimensional Nanocrystals of Rare Earth–Selenium Compound Containing Planar Se Layers: RESe2 Nanosheets and RE4O4Se3 Nanoplates

Synthesis of diverse two-dimensional nanostructures with unique material properties is of current interest and multidisciplinary importance but remains a challenge for trivalent rare earth (RE)-selenium (Se) compounds because of the weak affinity between hard rare earth cations and soft selenium anions. In this article, for the first time, we report a mild solution approach toward a series of two-dimensional trivalent RE-selenium compound nanocrystals, namely RESe2 nanosheets (RE = La to Nd, for EuSe2, nanobars were obtained) and RE4O4Se3 nanoplates (RE = Nd, Sm, Gd to Ho), under a high chemical potential of selenium obtained by activating SeO2 powder with oleylamine in high boiling point organic solvents. Both kinds of nanocrystals contain Se with -1 valence in planar Se layers, allowing for a great variability in their crystal structures. Satellite diffraction peaks were observed in the electron diffraction pattern of LaSe2 nanosheets, indicating the presence of Peierls distortion in the Se layers. In the RE4O4Se3 nanoplates, the interaction between Se(2-) ions and [Se-Se](2-) dumbbells in the Se layers increases when the radii of the RE(3+) ions decrease along the lanthanide series, resulting in a narrower optical band gap (from 1.96 to 1.73 eV). The LaSe2 nanosheet films fabricated by drop-casting exhibited good electrical conductivity at room temperature (about 1 Ω·cm(-1)). Further, the RE4O4Se3 nanoplates showed very high light extinction capacity in the visible region (extinction coefficient μi: 4.4 × 10(5) cm(-1) for Nd4O4Se3, and 3.1 × 10(5) cm(-1) for Gd4O4Se3), comparable to that (5 × 10(5) cm(-1)) of CuInS2 commonly used in solar cells.

CuO and Co3O4 Nanoparticles: Synthesis, Characterizations, and Raman Spectroscopy

Copper oxide and cobalt oxide (CuO, Co3O4) nanocrystals (NCs) have been successfully prepared in a short time using microwave irradiation without any postannealing treatment. Both kinds of nanocrystals (NCs) have been prepared using copper nitrate and cobalt nitrate as the starting materials and distilled water as the solvent. The resulted powders of nanocrystals (NCs) were characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements. The obtained results confirm the presence of the both of oxides nanopowders produced during chemical precipitation using microwave irradiation. A strong emission under UV excitation is obtained from the prepared CuO and Co3O4 nanoparticles. The results show that the nanoparticles have high dispersion and narrow size distribution. The line scans of atomic force microscopy (AFM) images of the nanocrystals (NCs) sprayed on GaAs substrates confirm the results of both X‐ray diffraction and transmission electron microscopy. Furthermore, vibrational studies have been carried out using Raman spectroscopic technique. Specific Raman peaks have been observed in the CuO and Co3O4 nanostructures, and the full width at half maximum (FWHM) of the peaks indicates a small particle size of the nanocrystals.

Silicon Nanocrystal Superlattices

Silicon Nanocrystal Superlattices

MnO Nanocrystals: A Platform for Integration of MRI and Genuine Autophagy Induction for Chemotherapy

AbstractHerein, a kind of novel monocomponent hydrophilic and paramagnetic manganese(II) oxide nanocrystal is prepared in polar solution by a one‐pot microwave‐assisted synthesis. This kind of nanocrystal can be taken up efficiently to serve as an excellent T1 magnetic resonance imaging (MRI) contrast agent with an enhanced r1 value of 0.81 mM−1 s−1. The key to the success of this method is that no additional capping agents are required for coating onto the surface via ligand exchange, facilitating research of their intrinsic biological activities. Furthermore, multiple lines of convincing evidence are presented to prove that MnO nanocrystals (NCs) elicit p53‐activation‐independent and authentic functional autophagy via inducing autophagosome formation. Notably, there are very few reports so far of the autophagy phenomenon induced by magnetic nanocrystals. Moreover, these results offer an indication for cancer therapy that MnO NCs combined with doxorubicin at a nontoxic concentration can have a definite synergistic effect, which is mediated through the genuine autophagy induction, on killing cancer cells in vitro and in vivo.

Phosphine-Free Synthesis and Photoluminescence Properties of ZnSe:Cu/ZnSe/ZnS Core/Shell Nanocrystals

High quality zinc blende ZnSe nanocrystals were successfully synthesized using an environmentally friendierly phosphine-free method. Using pre-synthesized ZnSe nanocrystals as core to dope Cu2+ ions, we obtained ZnSe:Cu/ZnSe and ZnSe:Cu/ZnSe/ZnS core/shell nanocrystals. Absorption spectruscopy, photoluminescence (PL) spectruscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to characterize the structures and properties of as-synthesized three kinds of nanocrystals. The results demonstrated that nanocrystals had well dispersion and narrow size-distributions, and the PL emission peak of as-synthesized ZnSe:Cu/ZnSe/ZnS core/shell nanocrystals could be easily tuned from 480 nm to 520 nm by using different sized ZnSe cores.

The influence of synthesis condition and aging process of silver nanocrystals on the formation of silver nanorods

Silver nanocrystals with controlled particle size were prepared by reduction of silver nitrate with sodium borohydride in the presence of trisodium citrate and then silver nanorods were synthesized by using the nanocrystals as seeds. The aging has obvious influence on the small nanocrystals but little influence on the large nanocrystals, implying different structural defects on the crystal surfaces of the two kinds of nanocrystals. The use of silver nanocrystals prepared by addition of NaBH4 all at once and prepared at 0°C helps produce the most silver nanorods with maximum aspect ratio. Only the smaller and slightly aged nanocrystals can be used as seeds for the formation of silver nanorods with high aspect ratio. The effects of concerned parameters of silver seeds can be applied to the synthesis of other anisotropic nanomaterials by a seed-mediated growth approach.

Visual imaging of quantum dots for in vivo malignancy cancer

Quantum dots (QD),a kind of nanocrystal,are made from Ⅱ-Ⅵ or Ⅲ-Ⅴ group elements.It has been reported that compared with current conventional fluorescent markers,QD have excellent optical properties such as strong fluorescence,photochemical stability and can be used for simultaneous multi-channel imaging.Meanwhile,as nanoparticle,QD can be easily surface modified with a variety of biological molecular and can reach cancer cells easily by penetrating the tumor angiogenesis.Therefore,QD have unique advantages in targeted real-time visual imaging of in vivo tumor and have great prospects in the individual diagnosis and treatment of tumor.However,the long-term biosafety after QD into the body still needs further study. Key words: Quantum dots ; Neoplasms ; Diagnostic imaging

SiGe superlattice nanocrystal pure and doped with substitutional phosphorus single atom: Density functional theory study

Ab initio density functional theory is used to simulate electronic structure of hydrogenated SiGe nanocrystal superlattice pure and doped with substitutional P single atom. The results of electronic structure calculations are compared to the same size silicon and germanium nanocrystals. The comparison reveals that the energy gap of the three kinds of nanocrystals is nearly the same in non-relativistic and relativistic limits. Because of large width of gap in the present small nanocrystals the relativistic corrections are not as much important as in the case of bulk crystals. The doping of SiGe nanocrystal with P single atom introduced an impurity level at 4 eV below original conduction band edge. This result is much larger than comparable silicon bulk and nanocrystal doping with P atoms. Results also show that the deep internal angles and bonds in SiGe nanocrystals reach approximately the angles and structure of bulk crystals after nearly three surface layers. A double positively charged layer is located at the Ge terminated surface of SiGe nanocrystal. This layer is enhanced and is accompanied with a large increase of the dipole moment of the nanocrystal in the case of P doped nanocrystal. Due to oscillatory lattice potential in SiGe superlattice, density of states show that bands are broken up to sub-bands in comparison with silicon nanocrystal density of states especially at the conduction band.

Charge Separation in Type II Tunneling Multilayered Structures of CdTe and CdSe Nanocrystals Directly Proven by Surface Photovoltage Spectroscopy

Charge separation and diffusion in type II multilayered structures of CdTe and CdSe nanocrystals with a polymer spacer are unambiguously proven by surface photovoltage spectroscopy. Holes accumulate in CdTe nanocrystal layers, and the electrons in CdSe nanocrystal layers. An increase of thickness of the polymer spacer strongly decreases the charge separation efficiency. Surface photovoltage transients demonstrate diffusion of the separated charges over several layers of the same kind of nanocrystals.

Hole mobility and trapping in PVK films doped with CdSe/CdS and CdSe quantum dots

Experimental study of the hole mobility in polyvinylcarbazole (PVK) films doped with two kinds of nanocrystals, on bare core CdSe and core-shell CdSe/CdS quantum dots, with concentrations ranging from 3 · 1010 to 3 · 1015 cm−3, is presented. The quantum dots investigated were made using colloidal chemistry. The hole mobility was measured using the time-of-flight technique as a function of the applied electrical field in the range 105–106 V/cm and for temperatures from 20°C to 50°C. The transient curves, being featureless on a linear plot, show on a double logarithmic scale a sharp inflection point indicating a dispersive carrier drift process. The recovered values of the mobility are in the range 3 · 10−8–10−6 cm2·V−1·s−1 and their field and temperature dependences can be analyzed formally within the framework of the Gaussian disorder model proposed by Bassler. The energetic disorder is, within the experimental accuracy, independent of the concentration and type of quantum dots for the CdSe quantum dots at all concentrations and for the CdS/CdSe quantum dots up to 1014 cm−3. The spatial disorder factors are very large (from 5.3 to 8.7) and do not depend in a systematic way upon the type and concentration of quantum dots (QDs). The experiments show that the apparent mobility does not change considerably with concentration, but it was found that the samples with CdSe/CdS quantum dots at concentrations from 1015 to 3 · 1015 cm−3 show a decreased photocurrent response. The dependence of the time-integrated transients (corresponding to the full charge value) upon the quantum-dot concentration has been determined. Differences in total photogenerated charge for pure and doped polymer films imply that the quantum dots of that type are the hole traps with capture times much more smaller than the transit time and with emission times a few orders longer than the transit time. CdSe quantum dots without a shell do not seem to exhibit the same properties as core shells and do not produce considerable changes in the charge transfer, even at a density of 1015 cm−3.

Electrical characterization of SiO2(Si) films as a medium for charge storage

AbstractAt present, the nonvolatile memory devices based on different kinds of nanocrystals are intensively developed. The important problem for further progress is connected with the optimization of gate multilayer structure. The aim of this work is the investigation of the electrical properties of thin nanocomposite SiO2(Si) films as a medium for the charge storage. The silicon enriched SiOx films were deposited by plasma enhanced chemical vapor deposition (PE CVD) method. The film composition was controlled changing the ratio of the flows of reactive gases SiH4 and N2O in a vacuum chamber. Subsequent thermal annealing led to the phase separation in SiOx films with a formation of nanocomposite SiO2(Si) structure of Si nanocrystals in dielectric matrix. Capacity‐voltage and current‐voltage measurements of MIS structures with gate nanocomposite SiO2(Si) films allowed to optimize the technological conditions of the formation of MIS structures with good charge storage properties. The value and the sign of stored charge were determined from the shift of C‐V characteristics. Analysis of the C‐V characteristics of MIS structures with nanocomposite SiO2(Si) films used equivalent circuit with the capacity of nanocrystals connected in parallel to the capacity of dielectric. The decrease of capacity in the accumulation region caused by nanocrystals was observed. The mechanism of current transport through the SiO2(Si) films was analyzed. It is determined to occur by tunneling through the SiO2 layers between the Si nanocrystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Relaxation Behavior and Breakdown Mechanisms of Nanocrystals Embedded Zr-doped HfO2 High-k Thin Films for Nonvolatile Memories

AbstractSemiconducting or metallic nanocrystals embedded high-k films have been investigated. They showed promising nonvolatile memory characteristics, such as low leakage currents, large charge storage capacities, and long retention times. Reliability of four different kinds of nanocrystals, i.e., nc- Ru, -ITO, -Si and -ZnO, embedded Zr-doped HfO2 high-k dielectrics have been studied. All of them have higher relaxation currents than the non-embedded high-k film has. The decay rate of the relaxation current is in the order of nc-ZnO > nc-ITO > nc-Si > nc-Ru. When the relaxation currents of the nanocrystals embedded samples were fitted to the Curie-von Schweidler law, the n values were between 0.54 and 0.77, which are much lower than that of the non embedded high-k sample. The nanocrystals retain charges in two different states, i.e., deeply and loosely trapped. The ratio of these two types of charges was estimated. The charge storage capacity and holding strength are strongly influenced by the type of material of the embedded nanocrystals. The nc-ZnO embedded film holds trapped charges longer than other embedded films do. The ramp-relax result indicates that the breakdown of the embedded film came from the breakdown of the bulk high-k film. The type of nanocrystal material influences the breakdown strength.

Structural characterization and mechanical properties of nanocrystal-containing Cu–Ti-based bulk metallic glass-forming alloys

Cylindrical Cu42.5Ti41.5Ni7.5Zr2.5Hf5Si1 bulk metallic glass with a diameter of 2 mm was fabricated by copper-mold casting. X-ray diffraction and differential scanning calorimetry analysis of the material showed that the alloy has a homogenous amorphous structure and high glass-forming ability. However, detailed observation by transmission electron microscopy revealed that a kind of nanocrystal with size of about 20 nm is sparsely distributed in the glass matrix. Nanobeam electron diffraction experiments indicated that the nanocrystal has a face-centered cubic crystalline structure. Room-temperature compression tests revealed that the alloy has a high fracture strength of 2250 MPa and obvious plastic strain of about 5.3%. Nanoindentation tests revealed that the as-cast alloy exhibits obviously serrated flow over a wide range of loading rate from 0.5 to 10 mN/min.