Transition metal dichalcogenides (TMDs) have recently gained tremendous interest for use in electronic and optoelectronic applications. Unfortunately, the electronic structure or band gap of most TMDs shows noncontinuously tunable characteristics, which limits their application to energy-variable optoelectronics. Thus, layered materials with better tunability in their electronic structures and band gaps are desired. Herein, we experimentally demonstrated that layered WSe2 possessed highly tunable transport properties under various pressures, with a linearly decreasing band gap that culminates in metallization. Pressure tuned the band gap of WSe2 linearly, at a rate of 25 meV/GPa. The high tunability of WSe2 was attributed to the larger electron orbitals of W2+ and Se2– in WSe2 compared to the Mo2+ and S2– in MoS2. WSe2 underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like bonding was observed in the valence electron localization function (ELF). Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering.

•Few MoS2/RGO hybrids with uniform size were successfully synthesized via hydrothermal method.•MoS2/RGO hybrids show excellent visible-light photocatalytic properties.•Good photocatalytic stability of MoS2/RGO for the second cycle indicates its promising practical applications.•Excellent photocatalytic capability of MoS2/RGO hybrids can be attributed to their enhanced adsorptivity and conductivity.MoS2/RGO hybrids was successfully synthesized via hydrothermal method. The MoS2 nanoflakes grown on the surface of the graphene were uniform and compact from the TEM images. Through the HRTEM images, we conclude the layer number of MoS2 nanoflakes is in the range of 2–6. Photocatalytic experiments have been performed to verify the excellent photocatic performance of MoS2/RGO hybrids. The methylene blue (MB) was completely decomposition by MoS2/RGO hybrids within 55 and 75 min under the UV and VIS irradiation, respectively. It is much better than the photocatalytic performance of P25 (nano TiO2 powder, the current commercial photocatalyst), especially under the VIS irradiation. Good photocatalytic stability of MoS2/RGO hybrids was proved through the secondary loop test, which further indicates its promising practical applications. We suggest that the excellent photocatalytic capability of MoS2/RGO hybrids can be attributed to their enhanced adsorptivity and conductivity because of the synergistic effect between the few layer MoS2 and the graphene.

There has been a long-standing search for 3D metallic, superhard carbon, stable under ambient conditions. Here, we report the discovery of a new 3D orthorhombic phase (denoted C14-diamond below) from first-principles calculations. This allotrope is metallic and superhard and is built from nano-layered sp3 carbon with the cubic diamond structure, connected by proper sp2-bonded ethene-type CC links. This unique configuration of sp2 carbons makes C14-diamond conductive and allows one-dimensional electronic conduction along the c-axis. The high hardness of C14-diamond is further confirmed by its higher density compared with other metallic carbon allotropes. It is dynamically stable and more favorable than most other theoretically predicted carbons in energy. Interestingly, the simulated x-ray diffraction pattern of C14-diamond is similar to that of the experimentally reported while yet unresolved carbon phase obtained by shock-compression of tetracyanoethylene (TCE) powder, indicating that it could be a potential candidate for this carbon phase. We further propose a possible transition process from TCE to C14-diamond, suggesting that conductive superhard material might be synthesized by this way.Download high-res image (149KB)Download full-size image

We investigated the optical properties and structural phase transitions of W-doped VO2(R) nanoparticles under pressure based on in situ synchrotron X-ray diffraction (XRD) and infrared (IR) spectroscopy. The structural transition sequence follows VO2(R)–VO2(CaCl2-type)–VO2(Mx) and VO2(Mx)–VO2(Mx′) within metallic phases, in compression and decompression processes, respectively, demonstrating that the structural transition can be decoupled from the metal-insulator transition (MIT). VO2(R) and VO2(CaCl2-type) exhibit expected behavior of increased metallicity under pressure; surprisingly, VO2(Mx) shows gradually decreased metallicity with increasing pressure and VO2(Mx′) is still metallic under ambient conditions. We find that the reduced metallicity of VO2(Mx) is attributed to W-doping induced local structure distortion in the high-pressure region, while the metallic properties of VO2(Mx′) are associated with the enhancement of electron concentration due to the presence of W donors, which shifted the Fermi level toward the conduction band. The present results demonstrate that the structural transition is not the key factor in driving the metal-insulator transition, and provide an effective method for inducing MIT in VO2(Mx′).

•The high pressure phase transitions of VO2 nanoparticles were studied.•Pressure-induced insulator-to-metal transition in the M1’ phase is attributed to the electron correlation effects.•The decompression-induced metal-to-insulator transition is dominated by the structural transitions.•The particle sizes affect the phase transition pressure and bulk modulus of VO2.We present pressure-induced phase transitions and metallization in VO2 nanoparticles characterized by synchrotron x-ray diffraction, Raman spectroscopy, and infrared reflectivity measurements. The M1-M1′-Mx phase transitions were found in VO2 nanoparticles upon compression. The results of IR reflectivity shown that pressure-induced metallization occurs in the M1′ phase with increasing pressure and the sample becomes fully metallic at the transition of M1′ to Mx. The metallic Mx phase transforms to metastable mixed phases displaying insulating properties upon decompression. We attribute the pressure-induced metallization of the M1′ phase to the strong electron correlations, while the metal-insulator transition (MIT) from the Mx to the mixed phases is found to be associated with the structural phase transitions. Both the electron-correlation-driven Mott transition and the structure-driven MIT can be achieved in VO2 by applying pressure. Compared with bulk samples, the VO2 nanoparticles exhibit larger bulk moduli and increased transition pressure due to their nanosize effects. High pressure provides an effective method to study the MIT in strongly correlated materials and paves the way for modifying electronic properties of VO2.

The size effects on the high pressure behaviors of monoclinic (MI) ZrO2 nanoparticles were studied using in situ high pressure synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). A size-dependent phase transition behavior under high pressure was found in nanoscale ZrO2. The normal phase transition sequence of MI–orthorhombic I (OI)–orthorhombic II (OII) occurs in 100–300 nm ZrO2 nanoparticles, while only the transition of MI–OI exists in ultrafine ∼5 nm ZrO2 nanoparticles up to the highest experimental pressure of ∼52 GPa. This indicates that the size effects preclude the transition from the OI to the OII phase in ∼5 nm nanoparticles. Upon decompression, the OII and OI phases are retained down to ambient pressure, respectively. This is the first observation of the pure OI phase ZrO2 under ambient conditions. The bulk moduli of the MI ZrO2 nanoparticles were determined to be B0 = 192 (7) GPa for the 100–300 nm nanoparticles and B0 = 218 (12) GPa for the ∼5 nm nanoparticles. We suggest that the significant high surface energy precludes the transition from the OI to the OII phase and the nanosize effects enhance the incompressibility in the ultrafine ZrO2 nanoparticles (∼5 nm). Our study indicates that this is a potential way of preparing novel nanomaterials with high pressure structures using nanosize effects.

Unlike bulk iodine, iodine molecular chains formed inside one dimensional (1D) nanochannels of AlPO4-5 (AFI) single crystals show unexpected PL behavior. Thanks to its unique 1D structure, the PL exhibits obvious polarization both in excitation and emission, by changing the angle between the c-axis of the channels and the polarization direction of the incident laser. As pressure increases, the PL intensity increases obviously due to the population increase of (I2)n chains upon compression. In contrast, the breaking of the (I2)n chain at high temperature leads to the decrease of PL intensity. Our theoretical calculation further points out that the PL may arise from the intrinsic band structure of (I2)n chains.

Several metastable doped C60 polymers are synthesized under high pressure and high temperature (1.5 GPa, 573 K and 2 GPa, 700 K, respectively), using C60/ferrocene(Fc, Fe(C5H5)2), C60/Ni(OEP) and C60/AgNO3 as starting materials. Raman and IR spectroscopy are used to study the polymerization of these samples after HPHT treatment. It is found that the polymerization degree is always lower than that of pure C60 treated at the same conditions, which is attributed to the space limitation by the dopants. We also find that even at the same conditions, the three doped materials form different polymeric phases of these doped materials. This is attributed to the unique initial lattice structures and the different degrees of spatial confinement provided by the dopants.

•We have found that the orientation of individual iodine molecules in the channels of AlPO4-5 (AFI) crystals prefer to lie along channel axis as temperature decreases.•Such temperature-induced orientation transformation is reversible upon heating up to room temperature.•The porous medium plays an important role in manipulating and controlling the orientation of individual iodine molecules.We have performed in situ low-temperature Raman spectroscopy studies on iodine molecules confined in the one-dimensional (1D) round channels of AlPO4-5 (AFI) crystals. Polarized Raman measurements show that the Raman intensity of individual iodine molecules parallel to channel axis (termed lying molecules) increases more than that of iodine molecules perpendicular to channel axis (termed standing molecules) upon cooling temperature down to −196 °C. This suggests that an orientation transformation of individual iodine molecules has occurred to increase the population of iodine molecules lying along channel axis as temperature decreases. Consequently, the corresponding Raman mode shows a redshift due to the enhancement of intermolecular interaction. This low-temperature induced orientation transformation is observed to recover when heated back to room temperature, indicating that orientation of iodine molecules confined in this porous medium can be modulated reversibly at a molecular level by cooling temperature down.

•The semi-insulating GaAs with diamond-like carbon film coatings were γ−irradiated.•The effect of small doses was observed with improvement of the structure transparency.•The structures with DLCF show higher radiation resistance than that without DLCF.We studied the properties of optical elements for the IR spectral range based on semi-insulating gallium arsenide (SI-GaAs) and antireflecting diamond-like carbon films (DLCF). Particular attention has been paid to the effect of penetrating γ-radiation on transmission of the developed optical elements. A Co60 source and step-by-step gaining of γ-irradiation dose were used for treatment of both an initial SI-GaAs crystal and DLCF/SI-GaAs structures. It was shown that DLCF deposition essentially increases degradation resistance of the SI-GaAs-based optical elements to γ-radiation. Particularly, the transmittance of the DLCF/SI-GaAs structure after γ-irradiation with a dose 9⋅104 Gy even exceeds that of initial structures. The possible mechanism that explains the effect of γ-radiation on the SI-GaAs crystals and the DLCF/SI-GaAs structures at different irradiation doses was proposed. The effect of small doses is responsible for non-monotonic transmission changes in both SI-GaAs crystals and DLCF/SI-GaAs structures. At further increasing the γ-irradiation dose, the variation of properties of both DLCF and SI-GaAs crystal influences on the transmission of DLCF/SI-GaAs system. At high γ-irradiation dose 1.4⋅105 Gy, passivation of radiation defects in the SI-GaAs bulk by hydrogen diffused from DLCF leads to increasing the degradation resistance of the SI-GaAs crystals coated with DLCF as compared with the crystals without DLCF.

Metallocene-filled single-walled carbon nanotubes (SWCNTs) have tunable electronic properties and a large potential in creating new structures due to their tunable electron doping and unique chemical reaction in a nanoscale confinement environment. Here we study the effect of high pressure on the transformations of ferrocene (FeCp2)-filled SWCNTs (FC@SWCNTs) by theoretical simulation and Raman spectroscopy. It is found that the filling of FeCp2 into carbon nanotube leads to higher transition pressures for the nanotube cross section changes and the intertubular bonding compared with the unfilled nanotubes. These results can be explained by the different charge distribution on the nanotubes due to charge transfer and the effect from host–guest interactions. Band structure analysis shows a pressure-induced decrease of band gap below 11 GPa, a transformation into a semimetal structure at 11 GPa, and subsequently, an abrupt increase of band gap due to the formation of intertube bonding. The increased host–guest interaction also leads to a decomposition of FeCp2 and the formation of a new 3D zeolite-like structure, which is quenchable to ambient conditions. Our preliminary experimental results confirm that the encapsulation of FeCp2 affects the transformations of nanotubes, which can be explained by the host–guest interactions, as demonstrated by our simulations.

Polymeric nitrogen, as a potential high-energy-density material (HEDM), has promising applications for energy storage, propellants, and explosives.The search for an effective method to recover polymeric nitrogen to ambient conditions is of great interest. Here, we study the confinement of polymeric nitrogen chain in boron nitride nanotube (BNNT) with different diameters, and find the polymeric nitrogen chain can be stable in BNNT (5, 5) at ambient conditions, while it can be also stabilized in other diameters of BNNTs with the application of little pressure. The polymeric nitrogen chains encapsulated in the BNNT dissociate into N2 molecules and release tremendous energy at above 1400 K. The hybrid structure is favored by a charge transfer from BNNT to N chain. The hybrid material has a smaller charge transfer between the N chains and the hosting BNNT compared with other hosting materials. More importantly, this smaller charge transfer not only stabilizes the polymeric nitrogen chain at ambient conditions but also favors the energy release at a more gentle condition, which can be taken as an obvious advantage of BNNT when used as hosting material. Our findings offer a highly promising route for harvesting nitrogen-based HEDMs under ambient or near ambient conditions.

This study describes the pressure-induced behavior of ZnS nanosheets by synchrotron angle-dispersive X-ray diffraction (ADXD) measurement up to 32.7 GPa. ZnS nanosheets transform from zinc blende structure to rock salt phase at 13.1 GPa and subsequently to a Cmcm structure at 20.3 GPa. The transition to the Cmcm structure is irreversible for ZnS nanomaterials at a much lower critical pressure than required for ZnS bulk materials. The special morphology of ZnS nanosheets plays a crucial role in the transition to Cmcm structures at comparatively low pressure. Continuous changes in lattice volume in the absence of volume collapse are observed after the transition from rock salt to the Cmcm structure occurs.

We investigated the temperature-driven structural phase transition (SPT) process of free-standing monoclinic vanadium dioxide (VO2 (M)) micron-sized rods by Raman spectroscopy. With increasing temperature, the phase transition sequence goes monoclinic M1 → monoclinic M1/monoclinic M2/rutile (R) → monoclinic M2/R → R. The fully metallic R phase is reached at 50 °C, which is 7 °C lower than the value of nanoparticles in our study and 18 °C lower than the value of early reports on VO2 (M) bulk materials. The intermediate M2 phase, which plays a critical role in the monoclinic to rutile transition (MRT), was clearly observed in both heating and cooling process of the micron-sized free-standing VO2 (M) rods. We suggest that the existence of nucleating defects in our samples, which have an important effect on the nucleation of the R phase, are responsible for the reduction of the phase transition temperature and the appearance of the M2 phase. This supplies a possible way to expand the practical applications of one dimensional VO2 materials.

Three-dimensional (3D) tin oxide/graphene aerogels (SnO2/GAs) were constructed by a simple, facile and environmentally friendly process. The small-sized SnO2 nanoparticles (6 nm) are encapsulated within graphene-based aerogels with interconnected 3D networks for the SnO2/GAs nanocomposite. When used as an anode material in lithium ion batteries, it delivers a high reversible capacity that is close to the theoretical capacities of SnO2 and graphene after 50 cycles. TEM observations of the samples before and after 50 cycles illustrate that the structures of the graphene network and SnO2 NPs are preserved, which explains well the good cyclic stability of the electrode. The excellent electrochemical performance of the nanocomposites can be explained by their unique 3D porous architecture and the combination of the advantages of both SnO2 and graphene in Li ion storage and transport.

In situ upconversion photoluminescence measurements of Y2O3/Eu3+ nanotubes under high pressure were carried out with 632.8 nm laser light excitation. The intensity of 5D0–7FJ (J = 0,1,2) transitions increased up to 8.2 GPa due to the enhanced crystal filed with pressure, while above 8.2 GPa, the photoluminescence intensity decreased rapidly. Full width of half maximum (FWHM) of 5D0–7F0,1,2 transitions and I0–2/I0–1 of the Eu3+ ion luminescence both exhibited obvious changes near 8.2 GPa, indicating the presence of short-range structural changes in the vicinity of the Eu3+-ion. The observed changes reflected a disordering of local micro-structure in the vicinity of Eu3+-ions resulting from the short-range distortion of YO6 octahedra under pressure. The luminescence of doped Eu3+ ions acted as an effective probe to reveal slight changes in structure and the local crystalline environment of Y2O3/Eu3+ nanotubes under high pressure.

•The formation enthalpy of oxygen vacancy increases with pressure, which makes the defect formatted harder under pressure.•The formation enthalpy of several native point defects is related to a fine interplay between the charges on the defects and applied external pressures.•The defect electronic transition levels strongly depend on the pressure.We investigate the formation enthalpies and transition energy levels for several native point defects in B1 phase of ZnO under applied hydrostatic pressure using density functional theory. The formation volume decreases gradually with increasing pressure, and increases linearly with the number of electrons adding to the system. In negatively charged state, the calculated formation enthalpy decreases with pressure, suggesting an increase in the equilibrium defect concentration. The behavior of the positively charged state is on the contrary, consistent with the results of the formation volume. In particular, the formation enthalpy of oxygen vacancy increases with pressure, which makes the defect formation harder under pressure. Under Zn-rich conditions, the “negative-U” phenomenon of oxygen vacancy, which appears under ambient conditions, vanishes with further increase in pressure when the Fermi enthalpy is close to the conduction band minimum.

Iodine molecules confined in the elliptical nanochannels of AlPO4-11 crystals can only rotate in the plane passing through the major axis of the elliptical cross-section due to size confinement. This leads to different dynamic behaviors of iodine from those confined in round channels of AlPO4-5 crystals under ambient conditions. In this work, we use high pressure technology to manipulate the nanoscaled iodine species confined in the elliptical channels of AlPO4-11 crystals. In situ polarized Raman measurements and theoretical simulations have been carried out to study the topological geometry of the confined iodine species upon compression. It was found that the population of iodine chains could significantly increase at the expense of standing iodine molecules under pressure up to 6 GPa, due to the pressure-induced rotation of standing iodine molecules. Besides, the contraction of the host framework along the channel axis favors the formation of iodine chains and strengthens the interaction of neighbouring molecules in a chain, consequently leading to a frequency redshift of the corresponding Raman mode. The different transformation dynamics of the confined iodine in AlPO4-11 crystals upon compression, compared to those in round channels of AlPO4-5 crystals, have been discussed in terms of the unique nanochannels that offer the quasi two-dimensional nanoscaled confinement environment.

High pressure synchrotron X-ray diffraction measurements on CaZrO3 were carried out in a diamond anvil cell up to 50.1 GPa at room temperature. It was found that the orthorhombic phase can be stable up to 30 GPa. A new pressure-induced phase transition was observed in CaZrO3 beyond 30 GPa. The high pressure structure of CaZrO3 was determined to be a monoclinic phase which is distinct from the high pressure structures that were previously reported for other perovskite oxides. Upon release of pressure, the high pressure phase transforms into the initial orthorhombic phase. A fit of the compression data to the third-order Birch–Murnaghan equation of state yields a bulk modulus K0 of 193(14) GPa. We propose that the unique distorted structure probably plays a crucial role in the high pressure behavior of CaZrO3. Especially, the distinct phase transformation may be related to the rotation or tilting of the ZrO6 octahedra.

The high pressure phase transition behaviors of anatase TiO2 nanosheets with high reactive {001} facets were studied using in situ synchrotron X-ray diffraction and Raman spectroscopy. The phase transition from starting anatase phase to a low ordered baddeleyite structure was found upon compression. Upon decompression, the low ordered baddeleyite structure transforms into an α-PbO2 phase. The obtained bulk modulus for TiO2 nanosheets is 317 (10) GPa, which shows ultralow compressibility compared with those of nanoparticles and bulks. We suggest that the enhanced bulk modulus for the TiO2 nanosheets can be attributed to their ultrafine thickness along the [001] direction with fewer “soft” empty O6 octahedra distributed in the TiO2 nanosheets than in other nanostructures and bulks. TiO2 nanosheets retain their original morphology after being released to ambient pressure. These results indicate that the sheet-like morphology with exposed {001} facets plays important roles in the high pressure phase transition of TiO2 nanosheets.

High-pressure behaviors of YF3:Eu3+ nanocrystals with an average grain size of 40 nm were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 31.1 GPa at ambient temperature. The pressure-induced structural phase transition starts at 11.8 GPa and completes at 23.3 GPa. YF3:Eu3+ nanocrystals with a starting phase of orthorhombic structure transform into a high-pressure phase, which is inferred to be hexagonal structure. The high-pressure structure returned to the orthorhombic phase after release of pressure. The transition pressure is enhanced in nanosized YF3:Eu3+ as compared to submicrometer size samples, which is due to the surface energy differences between submicrometer size and nanosized materials. The nanosized samples of high-pressure phase were easier to compress with smaller bulk modulus than the submicrometer size samples. The photoluminescence properties of YF3:Eu3+ have also been studied in the pressure range from ambient pressure to 25.0 GPa at room temperature. Accompanied by the structure transformation, the Eu3+ ion luminescence from 5D0 → 7F1,2,3,4 transition in YF3:Eu3+ nanocrystals emerges obvious changes, which indicate the variation of the local symmetry of Eu3+ ions.

High pressure Raman, IR and X-ray diffraction (XRD) studies have been carried out on C70(Fe(C5H5)2)2 (hereafter, “C70(Fc)2”) sheets. Theoretical calculation is further used to analyze the Electron Localization Function (ELF) and charge transfer in the crystal and thus to understand the transformation of C70(Fc)2 under pressure. Our results show that even at room temperature dimeric phase and one dimensional (1D) polymer phase of C70 molecules can be formed at about 3 and 8 GPa, respectively. The polymerization is found to be reversible upon decompression and the reversibility is related to the pressure-tuned charge transfer, as well as the overridden steric repulsion of counter ions. According to the layered structure of the intercalated ferrocene molecules formed in the crystal, we suggest that ferrocene acts as not only a spacer to restrict the polymerization of C70 molecules within a layer, but also as charge reservoir to tune the polymerization process. This supplies a possible way for us to design the polymerization of fullerenes at suitable conditions.

We investigate high-pressure induced phase transitions of YF3 and Eu-doped YF3 (YF3:Eu3+) by using the angular dispersive synchrotron X-ray diffraction technique at room temperature. It is found that the starting orthorhombic phase transforms into a new high-pressure phase which is identified as hexagonal structure in both YF3 and YF3:Eu3+. The high-pressure structure of YF3 and YF3:Eu3+ returned to the orthorhombic phase after release of pressure. The photoluminescence properties of YF3:Eu3+ have also been studied under high pressure up to 25 GPa. The Eu3+ ion luminescence lines of 5D0 → 7F1,2,3,4 transition originating from the orthorhombic phase transform into another group of luminescence lines of hexagonal phase under high pressure, which reveals the pressure-induced structural transition of YF3:Eu3+. The relative luminescence intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 transitions of the Eu3+ ions is found to increase with increasing pressure before phase transition and decrease after transition finished, indicating reducing and enhancing of the symmetry around the Eu3+ ions, respectively.

The morphology of CeO2 nanomaterials determines their properties and synthesis of well dispersed CeO2 nanocrystals with desired morphology is thus crucial for their applications. In this paper, we report the successful synthesis of CeO2 nanocrystals with different morphologies anchored on graphene hybrid composites by a hydrothermal method. Nanorods/graphene (NRs-G), nanoparticles/graphene (NPs-G) and nanocubes/graphene (NCs-G) have been selectively synthesized by optimizing experimental conditions. These nanocrystals show different exposed crystal planes and the size or morphology of the loaded CeO2 can be controlled by the alkali concentration and reactive temperature, as well as the adding of GO or not. In addition, significant changes in the photoluminescence (PL) spectra intensity and position (energy) have been observed in the synthesized samples, which depend on the morphology and size of the loaded CeO2. These interesting phenomena can be easily rationalized by the difference of oxygen storage capacity (OSC) of samples.

A systematic study of structure and dynamics behaviors of encapsulated C60H18 molecules in single wall carbon nanotube (SWCNT) has been performed by energetic analysis and molecular dynamics simulation. The results show that the C60H18 molecule inside SWCNT exhibit a diameter dependent orientation behavior in the range of 1.41–1.57 nm, in which the preferable tilted and parallel orientation have been found in nanotubes with diameter smaller and larger than 1.54 nm, respectively. It is found that the preferable orientations of C60H18 molecules are weakly affected by the C60H18–C60H18 intermolecular interaction, but unaffected by the tube chirality. The translation movement of C60H18 molecules along the tube axis direction in the nanotubes is restricted with diameter smaller than 1.45 nm, while spontaneous movement has been observed upon increasing the tube diameter. For the molecular dynamics simulation, various stacking arrangements of C60H18 molecules have been presented with the tube diameter (dt) in the range from 1.57 to 2.71 nm.

C70 nanotubes, nanorods and nanoparticles were produced by introducing a series of alcohols as precipitant into a C70/m-xylene solution. The effects of alcohols with different carbon chain lengths on the shape control of C70 nanocrystals were investigated. Alcohols with more than two carbon atoms in the longest chain linked to the hydroxyl groups induced the formation of C70 nanotube/rods. In contrast, alcohols containing two or fewer carbon atoms resulted in C70 nanoparticles. Structural analysis indicated that alcohol molecules exist in the C70 nanocrystals, forming solvated structures. The freshly formed C70 nanotubes and nanoparticles have orthorhombic and hexagonal solvated structures, respectively. Room temperature photoluminescence was further carried out on the solvated C70 nanocrystals to investigate their optical properties. We found that the luminescence intensities of C70 nanocrystals were significantly enhanced by the introduction of alcohols.Graphical abstractBoth morphologies and luminescence intensities of C70 nanocrystals have been tuned by the introduction of different alcohols.Highlights•C70 crystals were synthesized with different-geometry alcohols as shape tuners.•Shapes and structures of C70 crystals depended on the alcohol molecule lengths.•The introduction of alcohols highly enhanced the PL intensities of C70 crystals.

•High pressure study of graphene nanoplates is carried out by Raman spectroscopy.•The transition pressure for graphene nanoplates is found to be lower than that for graphite.•The reasons for the reduction of transition pressure for graphene nanoplates are discussed.High pressure Raman study on graphene nanoplates, graphite and micro-graphite has been carried out in a diamond anvil cell. A phase transformation has been observed in graphene nanoplates at 15 GPa in the experiments with or without pressure medium, which can be explained by the interlayer coupling with sp3 bonds formed in the material. For graphite and micro-graphite, the transition pressure is 19 GPa. Different transition pressures for them are attributed to the thickness difference. The lower transition pressure in graphene nanoplates has been discussed in the framework of the special limited-number layer structure and nucleation process in phase transition.

The high-pressure phase transformation of Er3+-doped Gd2O3 nanorods was studied by synchrotron radiation X-ray diffraction (XRD) and Raman scattering spectra. The starting cubic phase is stable up to about 9 GPa, and the Gd2O3/Er3+ nanorods start to transform into an amorphous phase related to monoclinic structure at 9.4 GPa. The phase transition is different from that direct crystal–crystal transformation observed in the bulk Gd2O3 and Gd2O3 nanomaterials. The bulk modulus for cubic Gd2O3/Er3+ nanorods is determined to be K0 = 196 ± 13.4 GPa by fitting to the second-order Birch–Murnaghan equation of state, which is larger than that of 164 ± 3 GPa of the bulk Gd2O3/Er3+. In addition, the quenched samples almost maintain their pristine nanorod shape but with amorphous form. It is proposed that the special rodike morphology and particle size probably play important roles in the high-pressure behavior of Er3+-doped Gd2O3 nanorods.

The phase transitions of one-dimensional (1D) anatase TiO2 nanowires were studied by in situ high-pressure synchrotron X-ray diffraction and Raman scattering up to 37 GPa. A direct anatase-to-baddeleyite transformation was observed at ∼9 GPa, which is clearly different from the size-dependent phase transition behaviors for nanocrystalline TiO2. We found the higher compressibility in the c-axis compared to the a-axis for anatase nanowires that may be attributed to both the crystal structural feature and the growth direction of the nanowires. The Ti–O bonds show abnormal changes during the anatase-to-baddeleyite phase transition. This phase transition of the TiO2 nanowires shows obvious morphology-tuned behaviors. Upon decompression, the baddeleyite phase transformed into α-PbO2 phase. The morphology of the TiO2 nanowires shows excellent stability and TiO2 nanowires with α-PbO2 phase were obtained at ambient conditions through a compression–decompression cycle. These results indicate that the nanoscale quasi-1D structure of TiO2 nanowires may contribute to the high-pressure phase transitions showing unique morphology-tuned behaviors.

The behavior of molecules and molecular chains confined in 1D nanochannels imposed by external interactions is a problem of fundamental interest. Here, we report structural manipulation of iodine confined inside zeolite (AFI) nanochannels by the application of high pressure. Structural transformations of the confined iodine under pressure have been unambiguously identified by polarized Raman spectroscopy combined with theoretical simulation. The length of the iodine chains and the orientation and intermolecular interaction of the confined iodine have been tuned at the molecular level by applied pressure. Almost all the confined iodine can be tuned into an axially oriented state upon compression, favoring the formation of long chains. The long iodine chains can be preserved to ambient pressure when released from intermediate pressures.

The high-pressure behavior of nanoporous rutile TiO2 was studied at room temperature using in situ synchrotron X-ray diffraction and Raman spectroscopy. It was found that the nanoporous rutile TiO2 starts to transform to the baddeleyite phase at a pressure of 10.8 GPa. The phase transition pressure is obviously different from those of rutile nanoparticles and the bulk solid. The rutile phase transforms into the baddeleyite phase completely when the pressure reaches beyond 26.1 GPa. Upon decompression, the baddeleyite phase transforms into the α-PbO2 phase. The bulk modulus obtained for nanoporous rutile TiO2 is 204(4) GPa. The nanoporous structure remains structurally intact during the compression–decompression cycle and thus shows excellent stability. We suggest that these high-pressure behavior characteristics of nanoporous rutile TiO2 are due to its unique nanoporous microstructure. Our study indicates that high pressure may be a powerful tool for researching the physicochemical properties of nanoporous materials, and also provides a potential method for preparing novel high-pressure phase nanoporous materials.

High-pressure Raman studies have been carried out on single crystalline C60(Fc)2 nanosheets up to 25.4 GPa. Our results show that the charge transfer between Fc (ferrocene) and C60 increases in the low-pressure range. Above 5 GPa, C60 molecules start to form a chainlike polymer structure, and this polymerization is reversible upon decompression, in contrast to that of pristine C60. The special layered structure of C60(Fc)2 restricts the polymerization of C60 molecules in some directions and explains the formation of the linear chainlike polymeric structure of the C60 lattice under pressure. We suggest that the reversible polymerization is related to the increased charge transfer and the overridden steric repulsion of counterions.

The size effect on structural transitions of Mn3O4 has been investigated under pressures by in situ synchrotron X-ray diffraction and Raman technique in a diamond anvil cell. Compared with bulk Mn3O4, Mn3O4 nanoparticles show an obvious elevation of phase transition pressure and different phase transformation routines with the occurrence of a new high-pressure phase at 14.5–23.5 GPa. The new phase most probably has an orthorhombic CaTi2O4-type structure, which is regarded as a metastable phase transforming to the higher pressure marokite-like structure. By the return to ambient pressure, the marokite phase is quenchable in bulk Mn3O4, whereas the coexistence of hausmannite and marokite phase is observed in the recovered Mn3O4 nanoparticles. It is proposed that the unique atomic conformation in Mn3O4 spinel structures, the cation distribution, and the higher surface energy together with the size-induced effect of nanocrystalline Mn3O4 probably play crucial roles in the high-pressure behavior of Mn3O4 nanoparticles.

C60 nanotubes with outer diameters ranging from 400–800 nm were polymerized at 1.5 GPa, 573 K and 2.0 GPa, 700 K, respectively. Raman and photoluminescence spectroscopy were employed to characterize the polymeric phases of the treated samples. Both Raman and photoluminescence spectra showed that the C60 nanotubes transformed into the dimer and orthorhombic phases under the two different conditions, respectively. The photoluminescence peaks were tuned from visible to near infrared range. Comparative studies indicated that C60 nanotubes were more difficult to polymerize than bulk C60 material under the same conditions due to the nanoscale size effect in the C60 nanotubes.

Hollow GeO2 walnuts were synthesized via a simple one-step process in an emulsion system. We investigated the growth mechanism and optical properties of the products and found that the reactive temperature and the addition of ethanol were crucial factors in controlling the morphology of GeO2 crystals. Above the ethanol boiling temperature, hollow walnuts were formed, whereas well-dispersed solid GeO2 polyhedrons and dimers were obtained below the critical point. A possible formation mechanism of the hollow interior of GeO2 walnuts is proposed suggesting that it was formed from the gas bubble released by the boiling ethanol and followed by the Ostwald ripening process of the encapsulating crystals. Photoluminescence measurement shows an enhanced emission peak at 538 nm for hollow GeO2 walnuts with blue shift compared with that of the solid structure. Our results indicate hollow GeO2 walnuts may have potential applications in light-emitting nanodevices. This method also suggests a new approach for fabricating other particles with hollow structure.

CeO2 nanosheets with (1 1 0) dominated surface were synthesized for the first time by a facile one-step hydrothermal method without the assistance of any surfactant or template. The role of NH3·H2O on tailoring the morphology of CeO2 nanocrystals was investigated. It was found the NH3·H2O not only serves as the precipitant, but also acts as structural direction agent in the formation of (1 1 0)-dominated CeO2 nanosheets. Raman and XRD spectra showed that the sample has a cubic fluorite structure. Compared with bulk CeO2 materials, the prepared CeO2 nanosheets exhibit an obvious blue-shift in UV absorbance. The increase of the direct band gap energy of the obtained sample exceeds 8%. This method provides an environmentally friendly way for preparing CeO2 nanostructures and tailoring their morphology. It may also be extended to the synthesis of other nanomaterials.Highlights► For the first time, (1 1 0)-dominated CeO2 nanosheets are obtained by a facile route. ► NH3·H2O plays a critical role in tailoring the morphology of the CeO2 nanocrystals. ► NH3·H2O serves as not only the precipitant, but also the structural direction agent. ► The precursor does not need to obtain before the synthesized process. ► Excluding the use of organic additives, the method provides a green chemistry route.

C60 microtubes were fabricated by a modified solution evaporation method, evaporating a solution of C60 in toluene in an atmosphere of m-xylene at room temperature. The C60 microtubes have outer diameters ranging from 2 to 8 μm. IR spectra, TG analysis and X-ray diffraction showed a solvated structure for the as-grown C60 microtubes. Through a gentle heat-treatment in vacuum, pure C60 microtubes with single crystalline fcc structure were obtained after the elimination of solvents. It is suggested that the C60 microtubes form through self-assembly from several individual C60 nanorods.Research Highlights► C60 microtubes were fabricated by a modified solution evaporation method. ► The sum molar of m-xylene and toluene doped in as-grown microtubes is that of C60. ► Single crystalline microtubes with pure C60 were obtained though heat-treatment. ► The microtubes were assembled with individual C60 nanorods.

C60 nanotubes have been synthesized by a solution–solution method. After degassing in a dynamic vacuum, the C60 nanotubes doped with alkali metals by means of vapor evaporation method. Different temperatures have been studied to evaporate the alkali metals for the doping experiments. Raman spectrum was further employed to analyze the doping concentration of the obtained samples. It was found that all three alkali metals (Li, Na and K) used can be efficiently doped into the C60 nanotubes, forming AxC60 nanotubes. The doping concentration of Li, Na changed from low to high level, depending on the experiment temperatures, while K doping always gave saturated doping. The melt points, the ionic sizes and vapor pressures of alkali metals were thought to affect the final doping results.Research Highlights► Li, Na and K can be efficiently doped into C60 nanotubes. ► Metals’ melt points, ionic sizes, vapor pressures affect the doping concentration. ► The doping concentration of Li and Na changed, but K doping was always saturated.

In-situ high-pressure X-ray diffraction and Raman spectroscopies have been performed on well-shaped CeO2 nano-octahedrons enclosed by eight (111) planes. The CeO2 nano-octahedrons are shown to be more stable than their bulk counterparts and some other reported CeO2 nanocrystals of smaller size. The transition pressure from cubic to orthorhombic phase is approximately 10 GPa higher than that of 12 nm CeO2 nanocrystals even though they have similar volume expansion at ambient conditions. Additionally, the phase transition to α-PbCl2 phase is very sluggish and uncompleted even up to 55 GPa. The TEM image of the sample after decompression from 55 GPa clearly shows that the nano-octahedrons preserve the starting shape. Such distinct high-pressure behaviors in CeO2 nano-octahedrons have been discussed in terms of their special exposure surface. Further analysis shows that the lower compressibility of the exposed (111) planes in the nano-octahedrons is believed to be the major factor to the elevation of the phase transition pressure and the sluggishness of the transition.

Single crystalline C70 nanotubes having a face-centered-cubic (fcc) structure with diameters on a nanometer scale were synthesized by a facile solution method. In situ high pressure Raman spectroscopy and X-ray diffraction have been employed to study the structural stability and phase transitions of the pristine sample. We show that the molecular orientation-related phase transition from the fcc structure to a rhombohedral structure occurs at about 1.5 GPa, which is ∼1 GPa higher than in bulk C70. Also, the C70 molecules themselves are more stable in the nanotubes than in bulk crystals, manifested by a partial amorphization at ∼20 GPa. The crystal structure of C70 nanotubes could partially return to the initial structure after a pressure cycle above 30.8 GPa, and the C70 molecules were intact up to 43 GPa. The bulk modulus of C70 nanotubes is measured to be ∼50 GPa, which is twice larger than that of bulk C70.

The study of pressure behavior for ZnS nanorods by X-ray diffraction measurements up to 37.2 GPa is carried out. ZnS nanorods transform from the initial wurtzite phase to rock salt phase at 19.6 GPa without undergoing the transition to zinc blende structure, and this is the first report about the direct phase transition from wurtzite to rock salt phase for ZnS. The longitudinal c-axis of ZnS nanorods exhibits more compressible behavior than that of radial a-axis direction, which is caused by the special rod morphology, and this induces the special direct phase transition to rock salt phase without zinc blende phase in the transition process. The results show the morphology of ZnS nanorods plays a crucial role in the special pressure behavior and also suggests we could explore the various pressure behaviors applying particular shaped nanomaterials under high pressure.

In situ high-pressure angle-dispersive synchrotron X-ray diffraction and high-pressure mid-infrared (IR) spectrum measurements of C60H18 were carried out up to 32 and 10.2 GPa, respectively. Our diffraction data indicated that the fcc structure of C60H18 was stable up to 32 GPa. The bulk modulus B0 was determined to be 21 ± 1.16 GPa, about 40% higher than that of C60. The C−H vibrations still existed up to 10.2 GPa, and the vibrational frequencies decreased with increasing pressure. IR-active vibrational frequencies and their corresponding eigenvectors of C60H18 were simulated by DMOL3. The effects of the hydrogen atoms attached to the fullerene molecular cage on the stability of the structure under high pressure are discussed.Keywords (keywords): C60H18; high pressure; hydrogenated fullerenes; IR;

The structural phase transitions of single-crystal TiO2−B nanoribbons were investigated in situ at high pressure using synchrotron X-ray diffraction and Raman scattering. Our results have shown a pressure-induced amorphization (PIA) occurred in TiO2−B nanoribbons upon compression, resulting in a high-density amorphous (HDA) form related to the baddeleyite structure. Upon decompression, the HDA form transforms to a low-density amorphous (LDA) form, while the samples still maintain their pristine nanoribbon shape. A high-resolution transmission electron microscopy (HRTEM) image reveals that the LDA phase has an α-PbO2 structure with short-range order. We propose a homogeneous nucleation mechanism to explain the pressure-induced amorphous phase transition in the TiO2−B nanoribbons. Our study demonstrates for the first time that PIA and polyamorphism occurred in the one-dimensional (1D) TiO2 nanomaterials and provides a new method for preparing 1D amorphous nanomaterials from crystalline nanomaterials.Keywords (keywords): high pressure; nanomaterial; phase transition; polyamorphism; TiO2−B;

Efficient upconversion photoluminescence (UCPL) has been observed from CdSe/ZnS quantum dots excited at both visible and NIR wavelength, 633 and 830 nm, respectively. The UCPL excited at 633 nm exhibits a linear power dependence, while the UCPL excited at 830 nm shows a quadratic power dependence with an energy gain as high as 660 meV. The UCPL was studied under hydrostatic pressure up to ∼8 GPa at room temperature compared with the pressure dependence of normal PL excited at 514 nm. Our investigation reveals that the UCPL excited at 633 and 830 nm originate from two different energy states with two different up-conversion processes. The UCPL excited at 633 nm originates from phonon populated surface states, which are not trapped to the band edge. The UCPL excited at 830 nm originates from the band edge, and two-photon absorption is the likely up-conversion excitation mechanism.

Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C60 and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.

Single crystalline TiO2−B nanoribbons with high-density nanocavities were successfully synthesized via a simple hydrothermal route. The as-prepared TiO2−B nanoribbons exhibited a large Brunauer, Emmett, and Teller (BET) surface area of about 305 m2/g because of the high-density nanocavities inside the thin nanoribbons. Electrochemical measurements indicated that the TiO2−B nanoribbons with dense nanocavities showed discharge specific capacity higher than those of TiO2−B nanotubes and nanowires. It was found that the dense nanocavities have an important influence on the electrochemical lithium intercalation properties.

Here, we report the first synthesis of TiO 2−B@C core−shell nanoribbons by a simple hydrothermal route. The as-prepared single-crystalline TiO 2−B nanoribbon cores are up to tens of micrometers in length and 50−200 nm in width. The nanoribbons are sheathed with a thin layer of carbon. The possible formation mechanism of the TiO 2−B@C core−shell nanoribbons has been elucidated. Furthermore, we tested the electrochemical intercalation properties with Li + of the as-prepared TiO 2−B@C nanoribbons, which showed higher discharge capacity than those of bare TiO 2−B nanowires and nanotubes. It was found that the morphology and the structure of the TiO 2−B@C nanoribbons had important influence on the electrochemical intercalation properties.

In this paper, C60 nanosheets with polymeric phases have been obtained under various high pressures and high temperatures, including orthorhombic and tetragonal polymeric phases. The structures have been identified and compared with those of nanorods by photoluminescence and Raman spectroscopies. The main fluorescence band shifted from 1.70 eV in the monomeric phase to near infrared in the polymeric phase when pressure and temperature were increased. The difference of photoluminescence and Raman spectra between nanosheets and nanorods samples treated under the same conditions is probably caused by different polymerization degree in these samples because of different shapes.

The effect of a new bimetallic catalyst Ho/Ni for synthesis of single-walled carbon nanotubes (SWNTs) by arc discharge has been studied. Long ribbons consisting of roughly-aligned SWNT bundles were obtained by a modified arc discharge apparatus. Ribbon lengths can reach as much as 20 cm. Both elements Ho and Ni play important roles in the synthesis of SWNTs with high yield and purity. Changes in the Ho and Ni concentration in the catalyst hardly affect the diameter distribution of SWNTs, but the yield and purity of SWNTs are very sensitive to the concentration. An optimal range of Ho/Ni compositions for synthesis of SWNTs with relatively high purity and yield is given.

High pressure Raman studies have been carried out on C60/AgNO3 and C60/Ni(OEP) up to 30 GPa. In both these doped C60 materials, pressure-induced metastable ordered polymers can be observed after pressure release. The results show that both the quenched materials contain chainlike polymers and dimers. We also find that the degree of polymerization is higher in these doped C60 materials than in bulk C60 materials after similar high pressure treatment and that C60/AgNO3 contains a higher fraction of chainlike polymers than C60/Ni(OEP) after decompression from same pressure. The results can be understood by considering the different initial lattice structures of these materials and the confinement effects of the dopants.

•The multiple-molecule model is constructed for this study for the first time.•Average molecular distance of linearly arrayed C10H16 molecules inside a single-walled carbon nanotube is evaluated.•The optimal orientation of confined C10H16 molecules is studied.A multiple-molecule model has firstly been constructed to study the stable orientations of encapsulated C10H16 molecules in single wall carbon nanotubes. The average molecular distance of adjacent C10H16 molecules is evaluated as 7.26 Å. The orientational analysis show that tube’s confinement effect plays a leading role for the diameter distribution from 9.5 Å to 11.75 Å, in which C10H16s exhibit optimal three- and two-fold axis orientations with the diameter smaller and larger than 11.3 Å, respectively. Then H…H repulsive interaction of adjacent C10H16 molecules plays an important role with diameter increased to 12.13 Å, which induces a slant two-fold orientation.

•A new example for high pressure studies on fullerene solvates.•The intercalated CS2 molecules promote the deformation of C60 molecules under pressure.•The stability of CS2 molecules under pressure is significantly enhanced in C60∗CS2 solvates.•The intercalated CS2 molecules can detect the pressure transformation of C60 molecules.High pressure IR study has been carried out on C60∗CS2 solvates up to 34.8 GPa. It is found that the intercalated CS2 molecules significantly affect the transformations of C60 molecules under pressure. As a probe, the intercalated CS2 molecules can well detect the orientational ordering transition and deformation of C60 molecules under pressure. The chemical stability of CS2 molecules under pressure is also dramatically enhanced due to the spacial shielding effet from C60 molecules around in the solvated crystal. These results provide new insight into the effect of interactions between intercalants and fullerenes on the transformations in fullerene solvates under pressure.

We investigate high-pressure induced phase transitions of YF3 and Eu-doped YF3 (YF3:Eu3+) by using the angular dispersive synchrotron X-ray diffraction technique at room temperature. It is found that the starting orthorhombic phase transforms into a new high-pressure phase which is identified as hexagonal structure in both YF3 and YF3:Eu3+. The high-pressure structure of YF3 and YF3:Eu3+ returned to the orthorhombic phase after release of pressure. The photoluminescence properties of YF3:Eu3+ have also been studied under high pressure up to 25 GPa. The Eu3+ ion luminescence lines of 5D0 → 7F1,2,3,4 transition originating from the orthorhombic phase transform into another group of luminescence lines of hexagonal phase under high pressure, which reveals the pressure-induced structural transition of YF3:Eu3+. The relative luminescence intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 transitions of the Eu3+ ions is found to increase with increasing pressure before phase transition and decrease after transition finished, indicating reducing and enhancing of the symmetry around the Eu3+ ions, respectively.