Tuning Crystal Growth of Colloidal Cs3Bi2I9 Perovskite-Like Nanocrystals via a Solvent-Assisted Reprecipitation Approach.

Peptides have been shown to mediate the reduction and clustering of inorganic ions during biomineralization processes to build nanomaterials with well-defined shape, size, and composition. This precise control has been linked to specific amino acid sequence; however, there is a lack of information about the role of peptides during mineralization. Here, we investigate the nucleation and growth behavior of Au nanocrystals that are mediated by the engineered peptide AYSSGAPPMPPF. Unlike other nanocrystal synthesis schemes, this peptide produces Au nanocrystals from Au(III) ions at very low relative peptide concentrations, at ambient temperature, and in water at neutral pH. Our data show that (i) the peptide AYSSGAPPMPPF actually inhibits nucleation and growth of nanocrystals, (ii) HEPES plays an active chemical role as the reducing agent, and (iii) HAuCl4 accelerates the kinetics of nanoparticle nucleation and growth. Herein, we propose empirical rate laws for nucleation and growth of Au nanocrystals and compare kinetic rate laws for this peptide, citrate, and various other polymer ligands. We find that the peptide belongs to a unique class of nonreducing inhibitor ligands regulating the surface-reaction-limited growth of nanocrystals.

The growth process of germanium nanocrystals in a SiOx matrix was studied by an x-ray diffraction (XRD) technique using the grazing incidence of x rays. The conclusion based on the XRD analysis of the Ge-doped glass is in good agreement with that from Raman spectroscopy and x-ray photoelectron spectroscopy. There are two stages of crystallization: the first stage is nucleation of the Ge nanocrystals at 600 °C, and the second stage is growth of the nanocrystals at 800 °C. During the second stage, the decomposition of GeOx into Ge and O in the Ge-doped glass sample results in fast growth of Ge nanocrystals. This is the first report that studied the growth of Ge nanocrystals in an SiOx matrix by XRD.

Understanding the mechanism of nucleation and growth of nanocrystals is crucial for designing and regulating the structure and properties of nanocrystals. However, the process from molecules to nanocrystals remains unclear because of the rapid and complicated dynamics of evolution under reaction conditions. Here, the complete evolution process of solid-phase chloroplatinic acid during the electron beam irradiation triggered reduction and nucleation of platinum nanocrystals is recorded. Aberration-corrected environmental transmission electron microscopy is used for direct visualization of the dynamic evolution from H2 PtCl6 to Pt nanocrystals at the atomic scale, including the formation and growth of amorphous clusters, crystallization, and growth of clusters, and the ripening of Pt nanocrystals. At the first two stages, there exists a critical size of ≈2.0nm, which represents the start of crystallization. Crystallization from the center and density fluctuation are observed in the second stage of the crystallization of a few clusters with a size obviously larger than the critical size. The work provides valuable information to understand the kinetics of the early stage of nanocrystal nucleation and crystallization at atomic scale.

Scalable production of carbon-supported Pt-M (M=Co, Ni, and Fe) alloy nanocrystals is of great importance for their practical application as catalysts towards the oxygen reduction reaction (ORR), a process key to the operation of proton-exchange membrane fuel cells. Here we report the use of a fluidic device for the in situ nucleation and growth of Pt-M nanocrystals on a commercial carbon support in a continuous and scalable fashion. The use of dimethylformamide not only enables well dispersion of the carbon powders for the creation of a homogeneous reaction mixture but also helps reduce metal precursors for the heterogeneous nucleation and growth of nanocrystals on the carbon surface. The size, shape, and composition of the nanocrystals can all be tuned by changing the metal precursors added into the reaction mixture, resulting in Pt-M nanocrystals uniformly distributed across the surface of the carbon support. Among the nanocrystals, the carbon-supported Pt-Co nanocrystals show the highest ORR specific and mass activities at 0.9 V, demonstrating 11.4- and 8.8-fold enhancements over the state-of-the-art commercial Pt/C catalyst.

Nucleation and growth of PbS nanocrystals and simulation of X-ray diffraction patterns

The nucleation and growth of colloidal CdSe nanocrystals with a variety of elongated shapes were explored in detail. The critical size nuclei for the system were magic sized nanoclusters, which possessed a sharp and dominated absorption peak at 349 nm. The formation of the unique magic sized nuclei in a broad monomer concentration range was not expected by the classic nucleation theory. We propose that this was a result of the extremely high chemical potential environment, that is, very high monomer concentrations in the solution, required for the growth of those elongated nanocrystals. The shape, size, and size/shape distributions of the resulting nanocrystals were all determined by two related factors, the magic sized nuclei and the concentration of the remaining monomers after the initial nucleation stage. Without any size sorting, nearly monodisperse CdSe quantum structures with different shapes were reproducibly synthesized by using the alternative cadmium precursors, cadmium-phosphonic acid complexes. A reasonably large excess of the cadmium precursor, which is less reactive than the Se precursor, was found beneficial for the system to reach the desired balance between nucleation and growth. The shape evolution and growth kinetics of these elongated nanocrystals were consistent with the diffusion-controlled model proposed previously. The branched nanocrystals had to grow at very high monomer concentrations because the multiple growth centers at the end of each branch must be fed with a very high diffusion flux to keep all branches in the 1D-growth mode. The rice-shaped nanocrystals were found as special products of the 3D-growth stage. The growth of the nanocrystals in the 1D-growth stage was proven to be not unidirectional after the length of the nanocrystals reached a certain threshold. Experimental results indicate that coordinating solvents and two ligands with distinguishable coordinating abilities are both not intrinsic requirements for the growth of elongated CdSe nanocrystals.

For heterostructures composed of metal nanoparticles and 2D materials, improving the precision of electrochemical fabrication requires a deep understanding of the interplay of kinetic and thermodynamic phenomena. Here, we explore the deposition of copper (Cu) metal on a graphene electrode using liquid cell transmission electron microscopy (TEM), using the temporal and spatial resolution of this technique to explore the nucleation and growth of nanocrystals. The technique reveals an unexpected transient phenomenon under certain applied voltage conditions, and we discuss possible mechanisms.We created working electrodes from few-layer graphene by transferring the crystals onto liquid cell chips patterned with platinum (Pt) electrodes. Pt was used as counter and quasi-reference electrodes. We then performed electrochemical deposition of Cu on graphene by controlling the deposition potential in an acidified copper sulfate solution. In situ movies reveal that during application of the potential, Cu nanocrystals nucleate and grow following the expected kinetics of progressive nucleation and diffusion-limited growth: larger numbers of smaller nanocrystals grow at more negative overpotentials. When the applied potential is low (-40 mV, -60 mV, -80 mV, and -100 mV with respect to Pt reference electrode), particles immediately dissolve as the potential is turned off.However, we observe that when more negative potentials (-180 mV, -220 mV, -240 mV and -260 mV) are used for deposition, the growth of Cu nanocrystals continues after the voltage goes off. Typical results are shown in the Figure. These images were extracted from a movie recorded during and after pulse voltammetry at -260 mV for 10 s in a liquid cell filled with 0.1 M CuSO4 + 0.1 M H2SO4. In the first row of the Figure, Cu islands nucleate and grow on the graphene electrode during the ‘V on’ period, while the later images show continued growth of Cu islands after the voltage is turned off at t=10 s. Eventually all islands disappear after tens of seconds. Tracking the size of the individual particles shows that smaller particles disappear earlier while the largest particles grow to large sizes. The decrease in the number of islands with time is particularly apparent at higher applied potentials, which is consistent with an Ostwald ripening process, and the size threshold that separates crystals that grow or shrink appears is also consistent with the model.The eventual dissolution of Cu nanocrystals in all experiments would not ordinarily be expected at zero applied potential. The comparison of cyclic voltammetry curves among different configurations of graphene electrodes suggests that there is an intrinsic potential between the graphene and Pt electrodes which is high enough to dissolve the deposited Cu even when the applied potential is 0 mV.We consider several mechanisms that may be responsible for the unexpected transient deposition. One possible explanation for the additional growth could be that Cu is deposited during the potential pulse but does not form into nanocrystals until later. This could arise if Cu is intercalated in the graphene during the applied potential. However, the measured charge that flows during the potential pulse is not strongly dependent on voltage, in particular not varying enough to account for the large deposited volume at high potentials. A second possible explanation may arise from the capacitance of graphene. Graphene has high capacitance based on electrochemical double-layer capacitance (EDLC) measurements, and it has been shown that the electrochemical interfacial capacitance increases for larger (negative) applied potential, which is consistent with our observation of the islands grown at larger potential last longer. A final mechanism for the effect could be that the application of low cathodic potential may cause the reduction of various oxygen functional groups forming species such as -OH. These charged species could sustain additional growth by reducing Cu ions from the solution. We will discuss this possible role of graphene to drive morphological changes during electrochemical processes, and consider wider opportunities that may arise for controlling electrodeposition to tailor thin film and nanoscale structures. Figure 1

Understanding the growth behaviors of nanomaterials during liquid-phase synthesis will be beneficial in designing and applying many functional nanodevices. However, the growth pathways regarding the nanocrystal facet development remain largely unknown as direct observation is lacking. Herein, the in situ study of Pb3O4 nanocrystals' growth is reported by using the liquid cell transmission electron microscopy with high spatial and temporal resolution. The findings indicate that Pb3O4 nanocrystals' growth follows distinct trajectories with shape evolution when the growth pathways are varied. Three growth pathways are observed, including the monomer growth of Pb3O4 nanocrystals, the coalescence growth of four stationary Pb3O4 nanocrystals, and the oriented attachment growth of Pb3O4 nanocrystal pairs and multiple randomly dispersed Pb3O4 nanocrystals. It is the first observation that Pb3O4 nanocrystals with a regular quadrilateral shape are formed, in which nanocrystal facets preferentially grow along the [002] direction of Pb3O4. Theoretical analysis confirms in this study that the surface energy and physical driving force play key roles in the growth of nanocrystals in a liquid. Such understanding of the growth pathways and quantification of formation kinetics are important for the design of hierarchical nanomaterials and the control of nanocrystal self-assembly for functional devices.

3C-SiC nanocrystal growth on 10° miscut Si(001) surface

This paper reports that gas bubbles can be used to tailor the kinetics of the nucleation and growth of inorganic-nanocrystals in a colloidal synthesis. We conducted a mechanistic study of the synthesis of colloidal iron oxide nanocrystals using gas bubbles generated by boiling solvents or artificial Ar bubbling. We identified that bubbling effects take place through absorbing local latent heat released from the exothermic reactions involved in the nucleation and growth of iron oxide nanocrystals. Our results show that gas bubbles display a stronger effect on the nucleation of iron oxide nanocrystals than on their growth. These results indicate that the nucleation and growth of iron oxide nanocrystals may rely on different types of chemical reactions between the iron-oleate decomposition products: the nucleation relies on the strongly exothermic, multiple-bond formation reactions, whereas the growth of iron oxide nanocrystals may primarily depend upon single-bond formation reactions. The identification of exothermic reactions is further consistent with our results in the synthesis of iron oxide nanocrystals with boiling solvents at reaction temperatures ranging from 290 to 365 °C, by which we determined the reaction enthalpy in the nucleation of iron oxide nanocrystals to be -142 ± 12 kJ/mol. Moreover, our results suggest that a prerequisite for effectively suppressing secondary nucleation in a colloidal synthesis is that the primary nucleation must produce a critical amount of nuclei, and this finding is important for a priori design of colloidal synthesis of monodispersed nanocrystals in general.

According to the earlier author’s papers, the erbium/ytterbium co-doped oxyfluoride glass-ceramics fibers should demonstrate better 1550 nm emission under 488/515/980 nm excitation (the erbium Er 3+ ion transition 4 I 13/2 → 4 I 15/2 ) than corresponding glass fibers (the batch composition 48SiO 2 11Al 2 O 3 -7Na 2 CO 3 -10CaO-10PbO-12PbF 2 - 1.5/0.6YbF 3 -0.5/0.2ErF 3 ). Glass fibers provided as a core of standard multimode waveguide (the diameter of 62 μm) have been drawn with the mini-tower to the diameter between 50 μm and 80 μm, then annealed in the two-step regime (580°C/1h – nucleation of nano-crystals; 760oC/15/30 min – nano-crystals growth). This kind of heat treatment ensures the transparent glass-ceramics fibers with the microstructure of homogeneously distributed nano-crystals (lead, erbium and ytterbium enriched cubic fluorite-like crystals and hexagonal PbF 2 crystals) embedded in a glassy host. Their transmission covers the range of 80-90% and seems to be sufficient with respect to their provided limited length (~2m). The luminescence intensity for glass-ceramics fibers at 1530 nm is higher than that of respective glass fibers and the lifetimes of the erbium ion excited state 4 I 13/2 are of the same order (~5 ms). In that context the glass-ceramics fibers discussed above seem to be promising candidates for cores of fiber lasers at the 1550 nm band.

Hydroxyapatite (HA) was synthesized by using a hydrothermal method with Ca(NO3)2·4H2O and H3PO4. We use x-ray diffraction and field-emission scanning electron microscopy to investigate how pH, reaction temperature, hydrothermal-reaction time, and calcium-ion concentration affects the microstructure and the growth of HA crystals. In addition, we discuss the growth mechanism. The results show that the crystals grow more completely and that the aspect ratio tends to increase with increasing hydrothermal-reaction time, reaction temperature, and calcium-ion concentration. The pH of the system strongly impacts the growth of HA crystals. With increasing pH, the HA crystal grain size and aspect ratio decrease significantly. By using 1 mol/L calcium-ion concentration, pH = 10, and a hydrothermal reaction at 200 °C for 8 h, we obtain high crystallinity and crystal clear of the growth polarity with hexagonal 60–100-nm-long columnar HA, 30–40 nm in diameter. The mechanisms producing this growth may be the effect of growth conditions on ion concentration, thereby changing the HA crystal growth rate along the different crystal axes.

Rhenium diselenide (ReSe2) has gathered much attention due to its low symmetry of lattice structure, which makes it possess in-plane anisotropic optical, electrical as well as excitonic properties and further enables ReSe2 have an important application in optoelectronic devices. Here, we report the thickness-dependent exciton relaxation dynamics of mechanically exfoliated few-layer ReSe2 flakes by using time-resolved pump–probe transient transmission spectroscopies. The results reveal two thickness-dependent relaxation processes of the excitons. The fast one correlates with the exciton formation (i.e., the conversion of hot carriers to excitons), while the slow one is attributed to the exciton recombination dominated by defect-assisted exciton trapping besides photon emission channel. The decrease of scattering probability caused by defects leads to the increase of fast lifetime with thickness, and the increase of slow lifetime with thickness is related to the trap-mediated exciton depopulation induced by surface defects. Polarization-dependent transient spectroscopy indicates the isotropic exciton dynamics in the two-dimensional (2D) plane. These results are insightful for better understanding of excitonic dynamics of ReSe2 materials and its application in future optoelectronic and electronic devices.

Single-crystal Au microflakes modulated by amino acids and their sensing and catalytic properties

Particle-mediated nucleation and growth of solution-synthesized metal nanocrystals: A new story beyond the LaMer curve