Production of single-crystal K3C60

A simplified technique for synthesizing high-quality millimeter-sized single-crystals of K₃C₆₀ is reported. The preparation method consists of a two-step process where pure C₆₀ crystals are first grown and later doped with potassium vapor. The undoped crystals are grown using an open-tube vapor transport with a flowing helium gas carrier. Doping occurs in a highvacuum environment inside a specialized quartz tube which allows phase purity to be monitored via magnetization measurements of the superconducting state. Utilizing the aforementioned procedure, one can attain samples with magnetically determined superconducting transition widths (10–90% diamagnetic shielding) of less than 1 K.     

Intercalation of C<Subscript>60</Subscript> fullerene crystals with calcium and barium via self-propagating high-temperature synthesis

We report a process for rapid intercalation of C₆₀ fullerene crystals using self-propagating high-temperature synthesis. The process has been used to intercalate C₆₀ fullerene crystals with calcium and barium. The superconducting transition temperatures of the intercalated fullerites have been measured, and the calcium-intercalated C₆₀ fullerene crystals have been characterized by X-ray diffraction.     

Synthesis of fullerene nanowhiskers using the liquid–liquid interfacial precipitation method and their mechanical,electrical and superconducting properties

Abstract

Fullerene nanowhiskers (FNWs) are thin crystalline fibers composed of fullerene molecules, including C₆₀, C₇₀, endohedral, or functionalized fullerenes. FNWs display n-type semiconducting behavior and are used in a diverse range of applications, including field-effect transistors, solar cells, chemical sensors, and photocatalysts. Alkali metal-doped C₆₀ (fullerene) nanowhiskers (C₆₀NWs) exhibit superconducting behavior. Potassium-doped C₆₀NWs have realized the highest superconducting volume fraction of the alkali metal-doped C₆₀ crystals and display a high critical current density (J_c) under a high magnetic field of 50 kOe. The growth control of FNWs is important for their success in practical applications. This paper reviews recent FNWs research focusing on their mechanical, electrical and superconducting properties and growth mechanisms in the liquid–liquid interfacial precipitation method.     

Synthesis of fullerene nanowhiskers using the liquid–liquid interfacial precipitation method and their mechanical,electrical and superconducting properties

Fullerene nanowhiskers (FNWs) are thin crystalline fibers composed of fullerene molecules, including C₆₀, C₇₀, endohedral, or functionalized fullerenes. FNWs display n-type semiconducting behavior and are used in a diverse range of applications, including field-effect transistors, solar cells, chemical sensors, and photocatalysts. Alkali metal-doped C₆₀ (fullerene) nanowhiskers (C₆₀NWs) exhibit superconducting behavior. Potassium-doped C₆₀NWs have realized the highest superconducting volume fraction of the alkali metal-doped C₆₀ crystals and display a high critical current density (J_c) under a high magnetic field of 50 kOe. The growth control of FNWs is important for their success in practical applications. This paper reviews recent FNWs research focusing on their mechanical, electrical and superconducting properties and growth mechanisms in the liquid–liquid interfacial precipitation method.     

Study of the Superconducting Cs-doped WO<Subscript>3</Subscript> Crystal Surface by Electron Backscattered Diffraction

The crystallographic surface structure of thermal vapor grown Cs _X WO₃ 0.005≤x≤0.3 crystals was investigated locally by electron backscattered diffraction in the environmental scanning electron microscope. Monoclinic to hexagonal phase transformation was shown to take place upon Cs doping to nominal concentrations of x=0.005 and x=0.05, while monoclinic to trigonal phase transition was observed at a concentration of x=0.3. In particular, the 2D superconducting crystals, of x=0.005 nominal concentration, were of inhomogeneous crystallographic phase according to the local Cs doping. The superconducting Cs-doped regions of the hexagonal phase were shown to be epitaxially grown on the WO₃ monoclinic crystal surface, the (0001) of this phase being parallel to the (001) plane of the WO₃ crystal. Our results support previous observations in these 2D superconducting Cs _X WO₃ crystals.     

Effect of Molecular Rotation on Vapor Phase Nucleation: Fullerenes C60 and C70

Abstract

The effect of rotational free energy of fullerenes C₆₀ and C₇₀ on homogeneous nucleation of crystals from vapor phase have been studied. Classical nucleation parameters have been estimated for the case of nucleation and crystal growth of C₆₀ and C₇₀ by Physical Vapor Transport (PVT) method under different supercooling in the range of 20–150 K, keeping the source temperatures constant. The result shows that the growth of large size single crystals is more feasible in the lower supercooling range. The effect of change in interfacial tension on the critical free energy of formation of the nuclei is also studied.     

Superconductivity and morphological studies on Bi2Sr2CaCu2O8 single crystals grown from stoichiometric and nonstoichiometric melts

Single crystals of Bi₂Sr₂CaCu₂O₈ (2212) have been grown by self-flux technique using stoichiometric and non-stoichiometric melts of excess CuO and Bi₂O₃. Single-crystal and powder X-ray diffraction studies have been made on the grown crystals to confirm their single crystallinity and structure respectively. Resistivity and susceptibility measurements provide information on the superconducting nature of the crystals. The effects of fluxing agents and starting composition on surface morphology and superconducting properties have been discussed.     

Fullerene Single Crystals: Structure and Electronic Properties

Abstract

This work covers an advanced investigation of the single crystal growth process for fullerenic materials and the contribution of Raman and IR spectroscopy to the understanding of the fcc high temperature rotor phase and the low temperature sc ratchet phase of pristine C₆₀. In addition substantial research contributions to the understanding of alkali metal doped fullerenes are presented.

Alkali metal doped crystals are studied with particular attention to the line broadening in the superconducting A ₃C₆₀ phase and to the phases with stoichiometry A₁C₆₀. For the former electron-phonon coupling constants could be determined for all fivefold degenerated gerade modes and a total coupling strenght of λ = 0.90 was found. For the latter the different experimental conditions for obtaining the phase separated intermediate state or alternatively the orthorhombic polymeric state are analysed. Vibrational spectra for the polymeric state obtained from phototransformation and from mono alkali metal doping are found to be characteristically different, mainly with respect to the intensity of the Raman or IR response. Finally, research results obtained from the single crystals by various other techniques like thermal expansion or low frequency elastic deformation are summarized.     

Optical and electrical characterization of chemically and photopolymerized C60 thin films on silicon substrates

A comparative characterization of C₆₀ thin films grown on silicon substrate by Physical Vapor Deposition and polymerized by chemical reaction with 1,8-octanediamine vapor or UV Pulsed laser irradiation has been carried out by means of Atomic Force Microscopy, and optical reflectance, transmittance and photoluminescence spectroscopies. The photovoltaic response and electrical characteristics of Au/C₆₀/Si diode structures have been investigated. The greatest photoluminescence efficiency and light transmittance, and at the same time the least photocurrent of diode structure were observed for chemically polymerized C₆₀. Found differences in morphology, optical, photoelectric and electrical properties of C₆₀ films polymerized by two methods indicate a difference in their composition.     

Morphology of C60 Crystals Grown from the Vapor Phase

Abstract

The morphology of C₆₀ crystals grown from the vapor phase have been studied. In all observations, only hexagonal and rectangular shaped crystal faces were found. Very different morphology, highly faceted {111} faces and flat {100} faces were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). the highly regular shape and similar distance between all neighboring macrosteps observed for the {111} faces can be explained by taking into account that edges of two adjacent {111} and {100} planes can act as step sources.     

Chemical vapor transport growth of vanadium(IV) selenide and vanadium(IV) telluride single crystals

A process for the growth of VSe₂ and VTe₂ single crystals using VCl₃ as a transport agent is proposed and experimentally demonstrated. X-ray diffractometry and Laue X-ray photography results indicate that the crystals thus grown are identical in properties to chalcogenide crystals grown using I₂ as a transport agent. The mechanism of chemical vapor transport is discussed and the process conditions are optimized.     

Single Crystal of C60F X and the X-Ray Structural Analysis

Abstract

Single crystals of fluorinated fullerene C₆₀F _x have been grown by sublimation. The crystal, ca. C₆₀F₄₆, has an fcc lattice with the cell constant 17.158(3) Å. The rotating molecules can be regarded as two inner spherical shells of carbon with radii 3.13 and 3.83 Å and an outer spherical shell of fluorine at 5.12 Å away from the center. PM3 calculations on C₆₀F_44,46,48 support the electron distribution in the ca. C₆₀F₄₆ determined by the X-ray structural analysis.     

Optical Characteristics of C60 Single Crystals Grown in Microgravity Conditions

This work is devoted to the growing and characterization of perfect C ₆₀ single crystals with the aim of further understanding of the physical properties of this material related to the low energy excited states which determine in a considerable degree its electronic properties, which, in turn, are important for its possible application. Here we present several characterization techniques based on optical properties of C ₆₀ crystals and the first results of the investigation of the C ₆₀ samples grown at the orbital space station MIR.     

The Metal-Insulator Transition and Superconductivity in Allotropes of Carbon Intercalated with Copper: Prediction and Experiment

Abstract

The complex of studies carried out on Cu_nC₆₀ samples, including SQUID and microwave measurements clearly support the existence of a superconducting phase of Cu_1,5 C₆₀ with T_c=120K. Heating a samples of pyrolitic graphite intercalated with copper and oxygen above 77K initiates a Metal - Insulator transition. We assume the same idea that the superconducting transition temperature T_c in Cu_n C₆₀ samples changes near the M -I transition.     

Initial Stage of Condensation of C60 Films on Semiconducting Substrates of Different Chemical Nature

Abstract

Conditions of growing C₆₀ films deposited from the gas‐dynamic vapor flow on different substrates [layered GaSe substrates with inactive surface, A^IIB^VI (CdS, CdSe) crystals, silicon substrates of (1 1 1) orientations, porous Si substrate] are studied. The condensate structure and growth mechanisms for different substrate are compared. It is shown that the high‐quality epitaxial films of the fullerenes can be prepared by using GaSe layered crystals as substrates.     

Vapor-phase growth of Bi2Sr2Ca n−1Cu n O x (n= 1–3) single crystals

High-quality platelike Bi₂Sr₂Ca _n?1Cu_nO_x (n= 1–3) single crystals with superconducting transition temperatures T _c=10, 85, and 110 K, respectively, are prepared by vapor-phase growth in closed cavities produced in a melt fluxed with KCl. The effect of synthesis and growth conditions on the T _c of the crystals is analyzed. The morphology of the crystals suggests that the most perfect, well-faceted crystals grow via vapor transport with constant feeding from the charge.     

Spontaneous Stoichiometry Change in Single Crystals of Superconducting (Ba1−x K x )Fe2As2 Grown by a Rapid-Heating Sn-Flux Method

To prevent the loss of K in growing single crystals of Ba_1−x K _x Fe₂As₂ we developed a rapid-heating Sn-flux method. Large single crystals with the optimal superconducting transition temperature T _C≈38 K were obtained and their structural, chemical and superconducting properties were investigated. Additionally, the effect of post-growth annealing on these crystals at different temperatures was examined. Scanning electron microscopy microprobe studies on a crystal with the composition goal of Ba_0.25K_0.75Fe₂As₂ revealed a well defined separation of two phases with compositions that are suggestive of rational ratios of the K and Ba content.     

Vapor growth of hexagonal GeO2 needle crystals

Needle crystals of haxagonal GeO₂ were grown at 750–900°C by the oxidation of GeO vapor which was generated by the reduction of GeO₂ with graphite. The largest crystals were grown at higher temperature, and had a maximum size of 10 μm diameter and 6 mm length after growth at 900°C for 5 hours. The growth direction of the crystals was parallel to the a-axis.     

Effect of Pressure on Superconducting Properties

It is shown that the existence of superconductivity in a material, and its critical temperature, depends strongly on pressure. Several parameters are pressure-dependent: (1) structure, particularly bond distances, (2) Hubbard U, (3) coupling between sites, and (4) orbital occupation number. Eliashberg theory often leads to incorrect predictions, for example in A₃C₆₀ with A = K, Rb, and Cs. While T _C is correctly predicted to be higher for Rb₃C₆₀ than for K₃C₆₀ and decreasing with pressure in both cases, Cs₃C₆₀ is not superconducting at ambient pressure. The same is the case for pure metals such as Cs and Ca (superconducting at high pressure). A theory for electron pairs, similar to the Marcus model for single electrons, appears to agree with the experiment in most cases.     

Investigation of the Electronic Structure of Cd4GeSe6 by Photoelectrochemical and Photoluminescence Methods

The Cd₄GeSe₆ crystals were grown by the chemical vapor transport method. The crystals were characterized by photoelectrochemical and photoluminescence methods and were found to be n-type, with a 1.759 eV direct band gap.