Nonmetallic State Research Articles

Structural evolution and metallicity of lead clusters.

The evolution of the metallic state in lead clusters and its structural implications are subject to ongoing discussions. Here we present molecular beam electric deflection studies of neutral PbN (N = 19-25, 31, 36, 54) clusters. Many of them exhibit dipole moments or anomalies of the polarizability indicating a non-metallic state. In order to resolve their structures, the configurational space is searched using the Pool Birmingham Cluster Genetic algorithm based on density functional theory. Spin-orbit effects on the geometries and dipole moments are taken into account by further relaxing them with two-component density functional theory. Geometries and dielectric properties from quantum chemical calculations are then used to simulate beam deflection profiles. Structures are assigned by the comparison of measured and simulated beam profiles. Energy gaps are calculated using time-dependent density functional theory. They are compared to Kubo gaps, which are an indicator of the metallicity in finite particles. Both, experimental and theoretical data suggest that lead clusters are not metallic up to at least 36 atoms.

Magnetic and structural properties of Mn-doped Bi2Se3 topological insulators

A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2K to 300K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.

Variable emissivity property of magnetron sputtering thermochromic film

Thermochromic films are prepared on the substrate of yttria stabilized zirconia by radio frequency magnetron sputtering technique. Crystalline structure and surface morphology are characterized. Characterization result shows that the films are of perovskite structure exhibiting a dense and smooth surface morphology. Composition analysis is performed and the result indicated that the element composition of films can be adjusted to close its stoichiometric ratio by controlling the oxygen flow ratio. Temperature-dependent reflectivity and emissivity are studied. Reflectivity spectra show that the film undergoes a transition from a metallic state to a non-metallic state with increasing temperature. Emissivity of the films is large above the transition temperature and it decreases sharply below the temperature. The emissivity increment at 97–373K can approach 0.39 by controlling sputtering pressure, working gas and film thickness.

Metallic and Mott insulating spin-frustrated antiferromagnetic states in ionic fullerene complexes with a two-dimensional hexagonal C60·- packing motif.

(MDABCO(+))(C60(·-))(TPC) (1), in which MDABCO(+) is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m0 =6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ(+))(C60(·-))(TPC) (2) with a hexagonal arrangement of C60(·-) was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ(+)) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results.

Electrical and optical properties of fluid iron from compressed to expanded regime

Using quantum molecular dynamics simulations, we show that the electrical and optical properties of fluid iron change dramatically from compressed to expanded regime. Along isochores, the main trend of the electrical resistivity, which is in good agreement with experimental data, is found to be well reproduced by our calculations. Near low densities, where the constant volume derivative of the electrical resistivity on internal energy becomes negative, the transition of iron from the metallic to the non-metallic state takes place. The study of the optical conductivity, absorption coefficient, and Rosseland mean opacity shows that, quantum molecular dynamics combined with the Kubo-Greenwood formula provide powerful tools to explore the electrical and optical properties of fluid iron.

Crystal Growth, Structure, and Transport Properties of the Charge-Transfer Salt Picene/2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane

Single crystals of the charge-transfer salt picene/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane have been grown using physical vapor transport. The crystal structure was determined using single-crystal X-ray diffraction. It was found that the crystals grow in a 1:1 molecular ratio and adopt a monoclinic structure with alternate stacking. Both X-ray data and Raman measurements show that the grown crystals are of good quality. From structure and infrared data, the charge transfer between acceptor and donor molecules was estimated to be approximately 0.14–0.19 electron. Transport measurements indicate a nonmetallic ground state with an activation energy of 0.6 eV. The supporting density functional theory calculations on molecular model systems as well as on crystalline structures confirm the amount of charge transfer and provide first insights into the electronic structure of the new material.

75As NQR and NMR Studies on the Superconducting State of Ca10Pt4As8(Fe1−xPtxAs)10

75As NQR and NMR measurements of Ca10Pt4As8(Fe1−xPtxAs)10 with the superconducting transition temperature Tc = 33.4 K have been carried out. The observed 75As NQR spectra can be divided into those from As sites in (Fe1−xPtx)As and PtAs2 layers. From the NQR spectra of 75As in the (Fe1−xPtxAs) layers, x is found to be ~20%, indicating that the superconductivity is robust against nonmagnetic impurities. This contradicts the existence of the sign reversal of the order parameter in the system. The 75As nuclear spin–lattice relaxation rates 1/T1’s from the (Fe1−xPtx)As and PtAs2 layers indicate that the PtAs2 layers are in a nonmetallic state and do not contribute to the occurrence of the superconductivity.

Comparing Pt/SrTiO3 to Rh/SrTiO3 for hydrogen photocatalytic production from ethanol

Photocatalytic hydrogen production from ethanol as an example of biofuel is studied over 0.5 wt% Rh/SrTiO3 and 0.5 wt% Pt/SrTiO3 perovskite materials. The rate of hydrogen production, rH2, over Pt/SrTiO3 is found to be far higher than that observed over Rh/SrTiO3 (4 × 10−6 mol of H2 g catal. −1 min−1 (1.1 × 10−6 mol of H2 m catal. −2 min−1) compared to 0.7 × 10−6 mol of H2 g catal. −1 min−1 (5.5 × 10−8 mol of H2 m catal. −2 min−1), respectively, under UV excitation with a flux equivalent to that from the sun light (ca. 1 mW cm−2). Analyses of the XPS Rh3d and XPS Pt4f indicate that Rh is mainly present in its ionic form (Rh3+) while Pt is mainly present in its metallic form (Pt0). A fraction of the non-metallic state of Rh in the catalyst persisted even after argon ion sputtering. The tendency of Rh to be oxidized compared to Pt might be the reason behind the lower activity of the former compared to the later. On the contrary, a larger amount of methane are formed on the Rh containing catalyst compared to that observed on the Pt containing catalyst due to the capacity of Rh to break the carbon–carbon bond of the organic compound.

Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se

We study the nature of (001) surface states in Pb_{0.73}Sn_{0.27}Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above the band-gap-inversion temperature. Our calculations predict not only metallic surface states with a nontrivial chiral spin structure for the TCI case, but also nonmetallic (gapped) surface states with nonzero spin polarization when the system is a normal insulator. For both phases, angle- and spin-resolved photoelectron spectroscopy measurements provide conclusive evidence for the formation of these (001) surface states in Pb_{0.73}Sn_{0.27}Se, as well as for their chiral spin structure.

Environmental durability of electrochromic switchable mirror glass at sub-zero temperature

Electrochromic switchable mirrors are expected to be used in energy-saving windows because their optical properties can be changed by application of a voltage. For practical use, the negative impact of environmental conditions on the optical switching properties of the device should be minimized. In this work, the durability of an electrochromic device was investigated in a thermostat/humidistat chamber at −5°C. After devices were stored at sub-zero temperatures, the switching speed and maximum transmittance decreased only slightly with increasing exposure time. In other words, degradation of the device did not significantly occur at sub-zero temperatures. In comparison, the optical switching properties of a device stored at 40°C and 60% relative humidity severely deteriorated. This deterioration was attributed to the degradation of the surface of the Mg4Ni optical switching layer. This layer was affected by the environmental conditions, and in particular changed from the metallic magnesium state to a non-metallic state. In future work, we will investigate suitable measures for preventing environmental damage to the device.

Degradation Analysis of Electrochromic Switchable Mirror Glass Based on Mg–Ni Thin Film at Constant Temperature and Relative Humidity

An electrochromic mirror glass that can be switched between reflective and transparent states by voltage application is presented. For the practical applications of such a device such as in energy-saving windows, optical devices, and electronic devices, the effects of environmental factors such as temperature and humidity on the optical switching properties of the device should be investigated in detail. In this work, the effects of constant temperature and relative humidity conditions on the device properties were studied. In particular, surface analysis studies were conducted. When these devices were kept at 40 °C and 60% relative humidity (RH), the speed of switching from the reflective state to the transparent state decreased with increasing time of storage in the bath. This reduced switching speed was related to the degradation of the surface metallic layers of the device, which could be observed by transmission electron microscopy (TEM) as an increase in surface roughness (Ra = 19.8 nm). Although the as-prepared device had a Mg4Ni optical switching layer approximately 40 nm in typical thickness, the degraded device exhibited an expanded layer of approximately 78 nm thickness. This increased thickness was associated with the change to the nonmetallic states of oxide and hydroxide from the metallic states of magnesium and nickel in the layer because of the adverse effects of the atmosphere.

Degradation Analysis of Electrochromic Switchable Mirror Glass Based on Mg–Ni Thin Film at Constant Temperature and Relative Humidity

An electrochromic mirror glass that can be switched between reflective and transparent states by voltage application is presented. For the practical applications of such a device such as in energy-saving windows, optical devices, and electronic devices, the effects of environmental factors such as temperature and humidity on the optical switching properties of the device should be investigated in detail. In this work, the effects of constant temperature and relative humidity conditions on the device properties were studied. In particular, surface analysis studies were conducted. When these devices were kept at 40 °C and 60% relative humidity (RH), the speed of switching from the reflective state to the transparent state decreased with increasing time of storage in the bath. This reduced switching speed was related to the degradation of the surface metallic layers of the device, which could be observed by transmission electron microscopy (TEM) as an increase in surface roughness (Ra = 19.8 nm). Although the as-prepared device had a Mg4Ni optical switching layer approximately 40 nm in typical thickness, the degraded device exhibited an expanded layer of approximately 78 nm thickness. This increased thickness was associated with the change to the nonmetallic states of oxide and hydroxide from the metallic states of magnesium and nickel in the layer because of the adverse effects of the atmosphere.

Mg–Ni thin-film composition dependence of durability of electrochromic switchable mirror glass in simulated environment

Mg–Ni thin films as a switchable mirror material are promising for application in energy-saving windows because they can change their state between reflective and transparent by hydrogenation and dehydrogenation. In our previous work, we found that electrochromic switchable mirror glass based on a Mg 4Ni thin film is affected by environmental factors such as temperature and humidity. In this work, we investigated the effects of the environment on the optical switching properties of electrochromic switchable mirrors with Mg–Ni thin films of various compositions suitable for a broad range of applications and operating environments. When the mirror devices were kept in a simulated environment with a constant temperature 40 °C and a constant relative humidity of 80%, controlled by a thermostat/humidistat bath, their optical switching properties degraded. The degradation was found to be related to the change in the Mg–Ni thin films into nonmetallic states of oxides and hydroxides. The device with a Mg 6Ni thin film kept in the bath for 7 days showed no optical switching property. The mechanism of degradation in the bath was strongly affected by the composition of the Mg–Ni thin-film optical switching layer. The device constructed with an optical switching layer having high magnesium content degraded more rapidly in the test.

Transition of expanded liquid iron to the nonmetallic state under supercritical pressure

A transition of expanded liquid iron to the nonmetallic state under high pressures (30–100 kbar) at high temperatures (of about 1 eV) is discovered. The result is obtained by direct measurement of the dependence of resistivity on the specific internal energy and volume. Measurements are taken in the specific volume range from the melting curve to values six times higher than the normal specific volume V 0 in the solid state. It is shown that iron remains in the metallic state up to a relative volume of V/V 0 = 3–4, at which the resistivity attains a value of about 3–4 μΩ m and becomes almost independent of temperature, while the conduction electron mean free path decreases to the atomic spacing. For V/V 0 = 4–5, a transition to the nonmetallic state takes place, for which the temperature coefficient of resistance becomes negative and its absolute value becomes much higher than in the metallic state.

Two melts phase separation in the liquid Sb-Sb2S3system: critical sound wave propagation and metal-non-metal transition

The sound velocity and magnetic susceptibility as a function of temperature and composition were measured to investigate critical sound wave propagation and metal-non-metal transition in the liquid Sb-Sb2S3 system. The sound velocity in a homogeneous alloy around 60 at.% of Sb decreases very rapidly and the rate of decrease increases as the two melts phase is approached, which is the typical temperature dependence of the sound velocity in a liquid with a miscibility gap. Below the critical point, the sound velocity was measured along the phase boundary. Using those data, the phase boundary was precisely determined. The critical point is located at 901±2 o C and 41.5 ±0.5 at.% S, and the critical exponent of the phase boundary is about 1/3. On the other hand, the magnetic susceptibility as a function of temperature and composition indicates that the electronic state is metallic in liquid Sb and non-metallic in molten Sb2Se3, and crossover form the metallic to non-metallic state occurs around the critical composition.

Surface Analysis of Electrochromic Switchable Mirror Glass Based on Magnesium-Nickel Thin Film in Accelerated Degradation Test

With the capability to change between reflective and transparent states, electrochromic switchable mirrors are expected to have numerous applications in optical devices, electronics devices and new energy-saving windows. If conventional windows can be adapted to incorporate switchable mirror technology, the solar radiation coming into a room could be effectively controlled, owing to features of the reflective state. Conventional windows are often subjected to environmental conditions such as high temperature and humidity. Considering practical use, we investigated the effects of the environment on the optical switching properties of the device in accelerated degradation tests using a thermostat/ humidistat bath at a constant temperature of 313 K and constant relative humidity of 80%. When the device was kept in the bath for 950.4 ks, it lost its optical switching properties. This result was associated with the degradation of the surface layer of the Mg4Ni thin film, which became rougher with increasing bath duration. We confirmed that the layer contained species in non-metallic states of oxide and hydroxide. Furthermore, the degradation mechanism to form the mixture state depended on the holding time in the bath. [doi:10.2320/matertrans.MBW201010]

Metal–nonmetal transition in the copper–carbon nanocomposite films

We prepared Cu nanoparticles in a-C:H thin films by co-deposition of RF-sputtering and RF-PECVD methods at room temperature. By increasing Cu content in these films a nonmetal–metal (N–M) transition is observed. This transition is explainable by the power law of percolation theory. The critical metal content is obtained 56% and the critical exponent is obtained 1.6, which is larger than the exponent for 2 dimension systems and smaller than the one for 3 dimension systems. The electrical conductivity of dielectric samples was explained by tunneling. Activation tunneling energy that was obtained from temperature dependence of electrical resistivity correlates with near infrared absorption peak of samples and both of them depend on Cu content of thin films. In the early stage of N–M transition, by increasing metal content, a peculiar effect of metallic to nonmetallic state occurs.

Accelerated degradation studies on electrochromic switchable mirror glass based on magnesium–nickel thin film in simulated environment

Electrochromic switchable mirror glass is expected to be implemented in new energy-saving windows, as it can switch between reflective and transparent states simply through an applied voltage. In its reflective state, it can effectively control the solar radiation coming into a room. However, conventional windows are often affected by environmental factors such as temperature and humidity. In this work, we investigated the effects of the environment on the optical switching properties of the mirror. In accelerated degradation studies, when the mirror was kept in a simulated environment with high temperature and high relative humidity controlled by a thermostat/humidistat bath, its optical switching properties degraded. After being kept for 34 days at a temperature of 30 °C and a relative humidity of 80%, the mirror never showed its optical switching property. Moreover surface roughness increased up to R a=13.1 nm, due to the degradation of the surface layer of the Mg 4Ni thin film. The layer changed into a non-metallic state of oxide and hydroxide, as determined by the X-ray photoelectron microscopy. We confirmed that the mirror was significantly affected by the environment, especially by high relative humidity in the atmosphere, which caused rapid degradation of the mirror.

Source of the catalytic activity of gold nanoparticles

Analysis of literature results for the effect of particle size in Au/TiO2 catalysts on their activities and activation energies (E) for CO oxidation by constructing ’Compensation’ plots of E vs. ln A (A = pre-exponential factor) leads directly to the conclusion that high activity is primarily due to particles so small as to be in the non-metallic state.

Direct measurements andab initiosimulations for expanded fluid aluminum in the metal-nonmetal transition range

We have performed direct measurements and quantum molecular-dynamics simulations for expanded aluminum at densities two to nine times lower than the normal solid density and internal energies ranging from 7 to 70 kJ/g. The simulation results were found to be in good agreement with the experimental data and reproduce well the main trends observed in the measured dependencies of the electrical resistivity and pressure versus internal energy along isochores. A systematic study of the optical conductivity spectra, one-particle density of states, and the distributions of the electronic charge over supercell shows that the transition of expanded aluminum to a nonmetallic state takes place close to the density at which the constant volume derivative of the electrical resistivity on internal energy becomes negative.