Metal-like Behavior Research Articles

Integration of 2D borophene into semiconducting N, P, S, and F-doped 1D carbon for energy storage and conversion devices

Innovative energy storage and high-performing conversion devices are urgently required to meet global energy issues. In this study, we synthesized and characterized a nitrogen, phosphorus, sulfur, and fluorine-doped one-dimensional carbon (NPSF@C) integrated with two-dimensional borophene (Bph) nanosheets, forming a nanocomposite (NPSF@C/Bph). The physicochemical and structural characterization confirmed borophene’s successful doping and integration into the NPSF@C matrix. Density Functional Theory (DFT) analysis indicated that the doping of heteroatoms and the integration of Bph increased the number of conduction bands in NPSF@C/Bph, effectively reducing the band gap of the material from 1.99 eV (MWCNT) and 1.43 eV (Bph) to 0.067 eV. This transformation was attributed to the introduction of localized states near the Fermi level and an enhanced density of states, resulting in a metal-like behavior for the NPSF@C/Bph composite. The NPSF@C/Bph composite demonstrated a specific surface area of 1962.78 m2/g and a pore volume of 1.2423 cm3/g, significantly enhancing its electrochemical performance. The composite showed a high specific capacitance of 837F/g at a current density of 1 A/g and retained 92.72 % of its initial capacitance after 10,000 cycles. It also exhibited a high energy density of 78.28 Wh/kg and a power density of 4545 W/kg in a symmetric supercapacitor device. Additionally, electrochemical tests revealed that the NPSF@C/Bph composite exhibited a lower overpotential and higher current density for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) compared to its counterparts. Specifically, the overpotential for OER is 283 eV, and HER is 73 mV at a current density of 10 mA/cm2. The Tafel slopes were 63 mV/dec for OER and 67 mV/dec for HER, indicating superior reaction kinetics.

Low temperature dependence of electrical resistivity in obliquely sputter-deposited transition metal thin films

Transition metals exhibiting hcp (Ti, Zr, Hf) and bcc (V, Nb, Ta, Cr, Mo, W) crystalline structures are DC sputter-deposited by oblique angle deposition. A constant film thickness of 400 nm is prepared, whereas the deposition angle α is systematically changed from 0 to 85° A columnar structure is produced with column angle reaching β = 50° for the highest deposition angle. Crystallinity and grain size are both reduced with an increasing deposition angle, especially for α higher than 60° DC electrical resistivity vs. temperature in the range 7–300 K shows a typical metallic-like behavior with films becoming more resistive for high deposition angles. For temperatures higher than 100 K, the linear temperature dependence of resistivity is obtained for films prepared with deposition angles lower than 60° The electron-phonon is the main interaction acting on electronic transport mechanism. Oblique deposition angles give rise to an enhancement of electron-phonon interactions with a saturation effect of electrical resistivity for some metals. Resistivity measurements at low temperatures (down to 7 K) show the predominance of electron-defect interactions. Electron-phonon-defect interaction effect is particularly investigated as a function of the deposition angle and a shift of the crossover temperature is brought to the fore.

Unveiling the structural and optoelectronic properties of (P, Bi, Sb)-doped GaAs by first- principles calculations

In this work, we investigate the influence of M substitutions (M = P, Bi, and Sb) on the structural, electronic, and optical properties of gallium arsenide, GaM x As1-x for x=0,0.25,0.50,0.75,1.0 using density functional theory (DFT). We have observed that the cubic symmetry of the semiconductors in conserved, and lattice constants show a linear behavior with dopant concentration, according to Vegard’s law. Specifically, the lattice constants increased for Bi and Sb dopants, whilst decreased for P dopants. Furthermore, the bulk modulus decreased with an increase in M-concentration for all M-doped systems. The energy analysis showed that the P-doped system was the most stable system. In terms of the electronic band structure, all M-doped compounds exhibited a direct electronic band gap. The P-doped system showed a wide band gap, while the Sb-doped systems displayed a narrow band gap, both compared to pristine GaAs. The Bi-doped system showed metallic-like behavior. To gain insights into the linear optical properties of the GaM x As1-x compounds, we calculated the real and imaginary parts of the dielectric function, as well as other optical parameters such as the absorption coefficient, refractive index, extinction coefficient, and reflectivity. The results showed that P doping leads to a blue-shift in the optical absorption coefficient, while Bi and Sb doping lead to a red-shift. These findings provide valuable theoretical insights for the potential application of GaM x As1-x semiconductors in photovoltaics and optoelectronic devices.

Novel Superhard Tetragonal Hybrid sp3/sp2 Carbon Allotropes Cx (x = 5, 6, 7): Crystal Chemistry and Ab Initio Studies

Novel superhard tetragonal carbon allotropes C5, C6, and C7, characterized by the presence of sp3- and sp2-like carbon sites, have been predicted from crystal chemistry and extensively studied by quantum density functional theory (DFT) calculations. All new allotropes were found to be cohesive, with crystal densities and cohesive energies decreasing along the C5-C6-C7 series due to the greater openness of the structures resulting from the presence of C=C ethene and C=C=C propadiene subunits, and they were mechanically stable, with positive sets of elastic constants. The Vickers hardness evaluated by different models qualifies all allotropes as superhard, with Hv values ranging from 90 GPa for C5 to 79 GPa for C7. Phonon band structures confirm that the new allotropes are also dynamically stable. The electronic band structures reveal their metallic-like behavior due to the presence of sp2-hybridized carbon.

Study of direct current charge transport mechanism in a series of iridium metal complexes based on varying ancillary and cyclometalated ligands

The mechanism of charge transport in a series of iridium metal complexes prepared with varying ancillary and cyclometalated ligands is reported here. The structural conformation of the synthesized complexes is acquired via NMR spectroscopy and elemental analysis. The photophysical properties of the samples are investigated through UV–visible and photoluminescence spectroscopy. The investigation of the temperature-dependent dc electrical conductivity of the samples is conducted within the temperature range of 280–473 K. The room temperature conductivity of all samples is determined to be of the order ∼10−8 S/cm. All the complexes exhibit semiconducting behavior in a certain temperature range above room temperature. A continued elevation in temperature leads to reduction in the conductivity of all the samples resulting in metal-like conduction behavior above a certain temperature. The transition temperature of the iridium metal complexes is attributed to the removal of the coordinated water molecule from the samples as suggested by thermal analysis of the complexes. The removal of water molecule from the sample is expected to localize the electron densities which consequently results in a fall of the conductivity in the samples. The conduction mechanism in the samples is analyzed in the light of band conduction mechanism and Mott’s variable range hopping model. All the samples are found to obey 1-D VRH mechanism. The activation energy and the corresponding Mott’s parameters are derived from the ln σ vs 1/T curves.

Structural and electrical conductivity studies of Polyaniline - WO3 hybrid nanocomposites for gas sensing applications

Abstract Conducting polymer – metal oxide based hybrid nanocomposites are a fascinating class of materials for miniaturized and flexible gas sensor devices. They exhibit enhanced physiochemical properties such as sensitivity, selectivity towards various volatile and hazardous chemical and bioanalytes. Our study focuses on conducting polyaniline (PANI) and WO3 nanocomposites, where different weight percentages (wt.%) of WO3 nanoparticles are embedded within the conducting PANI matrix using an in-situ oxidation polymerization synthesis technique. The surface morphology analysis indicated that the WO3 nanoparticles with an average grain size of ~200 nm are homogeneously distributed within the PANI nanofibers. The Fourier Transform Infra-Red (FTIR) spectrum analysis showed that the absorption peaks at 1111,1291, 1385, 1474, and 1560 cm−1 are typical of the conducting PANI emeraldine phase. We attribute the additional broad peak ranging between 840 to 720 cm−1 in the spectrum to WO3 phase, wherein, the intensity of the peak increases with WO3 content in case of hybrid composites. Current-voltage (I-V) characteristics for all our samples showed linear behaviour up to 1.2 volts. Temperature-dependent DC electrical conductivity (σ) studies measured from room temperature to 120°C for pure PANI, and PANI-WO3 composites showed an enhanced electrical conductivity of values up to 0.12 S/cm for PANI as compared to WO3 with σ ~ 1.4 x 10−3 S/cm. Pure PANI exhibits semiconducting behavior with an increase in electrical conductivity with temperature due to the charge carrier delocalization within the dispersed PANI backbone. The addition of higher concentrations of WO3 in composites leads to a metallic-like behavior, characterized by a decrease in electrical conductivity with temperature. These observations are attributed to the field-assisted band bending effects at the interfaces of PANI and WO3. Our composites show desired electrical characteristics suitable for gas sensing applications.

Borides Mn3-x{Rh,Ir}5B2 with Ti3Co5B2-type: X-ray single crystal and TEM data, physical properties

A cursory investigation of the phase relations in the Mn-{Rh,Ir}-B systems prompted for each system a ternary compound, Mn3-x{Rh,Ir}5B2, the crystal structure of them was determined from X-ray single crystal data to be isotypic with the Ti3Co5B2-type (space group P4/mbm, No. 127). In both cases the Mn-site in 2a at the origin of the unit cell exhibits a significant defect (or Mn/B substitution). Transmission electron microscopy studies confirm the absence of a superstructure related to these defects/disorder. The two phases, Mn3-xRh5B2 (x∼0.34) and Mn3-xIr5B2 (x∼0.85) at 950 °C show rather limited homogeneity regions pointing towards higher Mn-contents. Whereas temperature dependent magnetization and specific heat measurements of Mn2.15Ir5B2 do not reveal any indication for a magnetic phase transition in the temperature range from 3 to 300 K, a broad specific heat anomaly at around 200 K and a tilde shape of the temperature dependent magnetization of Mn2.66Rh5B2 appears indicative of an antiferromagnetic phase transition. The latter is also reflected by an anomaly of the electrical resistivity (ρ; 4–300 K) of Mn2.66Rh5B2 which displays a non-monotonous temperature dependence, whereas ρ(T) of Mn2.15Ir5B2 displays a simple metallic-like behavior dominated by scattering from defects. Elastic moduli and Poisson's ratio were determined at room temperature from Resonant Ultrasonic Spectroscopy (RUS) data yielding Young's moduli of E ∼ 170 GPa for Mn2.66Rh5B2 and E ∼ 210 GPa for Mn2.15Ir5B2. Vickers hardness HV is lower for Mn2.66Rh5B2 (HV = 590 ≅ 5.79 GPa) than for Mn2.15Ir5B2 (HV = 655 ≅ 6.42 GPa). The indentation fracture toughness for Mn2.15Ir5B2 was IKC = 0.71 ± 0.5 MPa m1/2.

Thermal dependence of the current in TiN/Ti/HfO2/W memristors at different intermediate conduction states

The dependence of the current in TiN/Ti/HfO2/W devices on the temperature is investigated in the range from 78 K to 340 K. Resistive switching cycles at 78 K are conducted to explore the thermal dependence in filament configurations with different intermediate resistance states. The less conductive states show an increase of the current as the temperature rises, while the fully formed filament displays a metallic-like behavior. A comprehensive model, based on the Stanford Model including a series resistance, is proposed and successfully validated by experimental data. The interplay between the ohmic and non-linear components in the model for different filament states is analyzed, emphasizing the dominance of the non-linear component (and its corresponding thermal dependence) in partially formed filaments and the prevalence of the ohmic component in the fully formed filament, which shows a decreasing current as the temperature rises. A complete compact model for simulation of circuits including the thermal dependence of these devices is developed.

Enhanced conductivity in Sr doped La3Ni2O7-δ with high-pressure oxygen annealing

The structural, electrical and magnetic properties of a series of La3-xSrxNi2O7-δ polycrystalline samples have been systematically investigated with and without high-pressure oxygen annealing. Without annealing, the introduction of Sr at La site leads to a moderate enhancement of conductivity. The high-pressure oxygen annealing leads to an increase of oxygen content by ∼1.08 %. Consequently, the conductivity of the annealing samples is significantly enhanced comparing to the untreated samples. The La3-xSrxNi2O7-δ samples exhibit metal-like behavior in the whole temperature range with annealing, which is due to the hole doping effect. The present results could contribute to the exploration of possible ambient superconductivity in La3Ni2O7-δ system.

Extrinsic n-type semiconductor transition in ZrSe2 with the metallic character through hafnium substitution

Two-dimensional (2D) layered materials exhibit versatile electronic properties in their different phases. The intrinsic electronic properties of these materials can be modulated through doping or intercalation. In this study, we investigated the electronic properties of Hf-doped ZrSe2 single crystals using angle-resolved photoemission spectroscopy (ARPES) combined with first-principles density functional theory (DFT) calculations. It is observed that the valence band maxima of ZrSe2, located below the Fermi level, undergo a significant change with the introduction of Hf substitution. Hf can introduce extra charges into the conduction band, rather than making a mixed structure of HfSe2 and ZrSe2 band structure, which can cross the Fermi level. Compared to the semiconducting band structure of ZrSe2, we observed that the conduction band crosses the Fermi level at the high symmetry M point in Hf-doped ZrSe2. This suggests an increase of electron-type carriers around the Fermi level, resulting in an extrinsic charge carrier density in the conduction band, which can form a metallic behaviour. It can be noticed that the Hf cations can create disorder in the form of excess atoms of Zr, which yields more carriers in the conduction band in the shape of “smeared bands”. The tails of the “smeared band” occupied the d-orbitals extended into the Fermi level and left the d-band below. Similarly, the electrical resistance measurements further confirm the metallic-like character of Hf-doped ZrSe2 compared to the semiconductor ZrSe2, indicating increased carriers. This metallic-like behavior is suggested to be predisposed by the extrinsic electrons induced by the substitutional disorder. This study further demonstrates the possibility of band gap engineering through heavy metal doping in 2D materials.

Tunable magneto-transport properties in ultra-high Bi-doped Si prepared by liquid phase epitaxy

During the past decades, excavating the novel functional materials towards CMOS-compatible spin-transport devices has been treated as one of the most urgent tasks to revolutionize the current Si-based nonmagnetic spintronics. Among various strategies, triggering the exotic physics in Si via utilizing heavy-element hyperdoping is the most straightforward way to achieve the abovementioned expectation. In this work, by using liquid phase epitaxy growth, we have achieved the highest Bi doping level in Si up to 2.0 × 1021 cm−3. The resulting Bi-hyperdoped Si layers are confirmed to be single-crystalline and epitaxially regrown without extended defects or Bi agglomerates via various microstructural analysis. The Bi-hyperdoped Si layers exhibit a metallic-like behavior with the carrier densities up to 1.55 × 1015 cm−2 and a prominent electrical activation yield around 50 %. Negative magnetoresistance due to magnetic-field suppressed weak localization at both sides of the insulator-to-metal transition is observed. Tunable magneto-transport properties are realized in ultra-high Bi doped Si epilayers by controlling the donor concentration. This provides new possibilities for designing Si devices that integrate spin-charge conversion and spin transport. This work promises to enable heavy-element hyperdoped semiconductor epilayers as novel spin-based electronics material platform via a chip-technology compatible approach.

Morphology-electronic effects in ultra-model nanocatalysts under the CO oxidation reaction: the case of ZnO ultrathin films grown on Pt(111).

The study of the surface morphology and interface of metal-oxides is crucial for understanding the behavior of these model systems as nanocatalysts. Besides, understanding the interplay between morphology, stability, and reactivity is crucial for designing efficient catalysts. Here, we investigated the stability and dewetting of ZnO ultrathin films on Pt(111) under CO oxidation conditions. For films <1 monolayer (ML), CO-induced dewetting occurs at the metal-oxide interface or defects. The morphology, dependent on thickness, influences reactivity. (6 × 6) structures show greater CO binding and structural changes compared to (4 × 4) structures, which exhibit resilience due to Zn-OH formation. ZnO electronic properties, as revealed by Auger spectroscopy and scanning tunneling spectroscopy (STS) investigations, vary with thickness. Low-thickness films [<2 monolayers (ML)] exhibit metallic-like behavior, possibly due to Zn-Pt interaction, while thicker films show n-type semiconductor behavior with a bandgap opening (EBG = 0.9 eV at 2 ML). DFT calculations of the local density of states (LDOS) as a function of ZnO thickness confirm the thickness-dependent electronic structure, with 0.3 ML films having a higher LDOS near the Fermi level than 1 ML films. These findings highlight the critical role of ZnO morphology in determining its stability and reactivity which opens up avenues for designing efficient and more stable ZnO-based nanocatalysts for a wide range of chemical reactions, including CO oxidation and CO2 hydrogenation.

Flexible Copper Nanowire/Polyvinylidene Fluoride Membranous Composites with a Frequency-Independent Negative Permittivity.

With the increasing popularity of wearable devices, flexible electronics with a negative permittivity property have been widely applied to wearable devices, sensors, and energy storage. In particular, a low-frequency dispersion negative permittivity in a wide frequency range can effectively contribute to the stable working performance of devices. In this work, polyvinylidene fluoride (PVDF) was selected as the flexible matrix, and copper nanowires (CuNWs) were used as the conductive functional filler to prepare a flexible CuNWs/PVDF composite film with a low-frequency dispersion negative permittivity. As the content of CuNWs increased, the conductivity of the resulting composites increased sharply and presented a metal-like behavior. Moreover, the negative permittivity consistent with the Drude model was observed when CuNWs formed a percolative network. Meanwhile, the negative permittivity exhibited a low-frequency dispersion in the whole test frequency range, and the fluctuation of the permittivity spectra was relatively small (-760 to -584) at 20 kHz-1 MHz. The results revealed that the high electron mobility of CuNWs is reasonable for the low-frequency dispersion of negative permittivity. CuNWs/PVDF composite films with a frequency-independent negative permittivity provide a new idea for the development of flexible wearable electronic devices.

Synthesis of polyaniline/spinel ferrites core-shell nanocomposites via sucrose auto-combustion and in situ polymerization

Herein, we prepared new nanocomposites from polyaniline (PANI) as a conductive polymer, and ferrites, MFe2O4 (M = Co2+, Cu2+, Mg2+, Ni2+, and Zn2+), as metal-oxide ceramics using sucrose sol–gel auto-combustion and in situ of polymerization. The prepared nanocomposites’ structural, thermal, magnetic, and electrical/dielectric characteristics were investigated. X-ray diffraction (XRD) revealed the formation of the metal-oxide ceramics with weak crystallinity due to their coating in the amorphous structure of the PANI matrix. Transmission electron microscopy (TEM) images exhibited the wrapping of the entire metal-oxide ceramic particles by the PANI matrix, thus confirming the core–shell structure formation. Vibrating sample magnetometer (VSM) results showed drastic reductions in all ferrites’ magnetization by their inclusion in the non-magnetic PANI matrix. The measured coercivity values showed a noticeable lowering in the presence of PANI. Besides, the thermogravimetric analysis (TG) technique indicated apparent increases in the thermal stability of the prepared nanocomposites compared to the pure PANI. Moreover, the electrical conductivity results indicated a change from metallic-like behavior for the pure PANI to the semiconductor by including the ferrites. Furthermore, by comparing the pure PANI, the dielectric properties of nanocomposites increased by the presence of ferrites, suggesting the suitability of the entire nanocomposites to be used in energy storage devices such as capacitors.

Manipulation of quantum spin transport of TM (Mn, Fe, Co, and Ni) (II) phthalocyanines coupled to carbon nanotube leads applicable in spintronic devices

We conducted a theoretical scrutiny on the quantum spin transport characteristics of a class of open-shell 3d-transition metal (TM = Mn, Fe, Co, and Ni) (II) phthalocyanines (TMPcs) molecules sandwiched between two semi-infinite armchair single-walled carbon nanotube probes. Herein, the spin density functional formalism is utilized in conjunction with state-of-the-art non-equilibrium Green's function technique. Molecular junctions composed of MnPc and FePc have higher local magnetic spin moments than CoPc and NiPc because of the strong exchange spin-splitting between the 3d-states of the Mn or Fe cation atoms and the p-states of the N anion atom. This indicates that TMPc molecules (TM = Mn, and Fe) bridged between two single-walled carbon nanotube (SWCNT) electrodes with a π-type geometry demonstrate a pronounced transmission near the Fermi level. Furthermore, the sign and robustness of spin filter efficiency are tailored by manipulating the central atom of 3d-transition metal and the contact SWCNT electrodes. Subsequently, FePc and MnPc molecular junctions have the advantage of operating practically as ideal spin filter devices compared to CoPc and NiPc molecular structures. Moreover, the tunneling magnetoresistance (TMR) of these molecular devices is assessed. For MnPc and FePc molecular junctions, the estimated total conductance exhibited a robust trend at zero bias voltage, whereas the magnitudes of spin-up and spin-dn quantum conductances increased significantly in CoPc and NiPc molecular configurations with bias voltages range between ±0.45 and ±0.95 Volt. Spin-down carriers essentially govern the quantum conductance of TMPc (Mn, Fe, Co, and Ni) molecular junctions at bias voltage around ±0.75 Volt. By examining their I-V characteristics, it has been established that MnPc, and FePc molecular junctions exhibit metal-like behavior, whereas CoPc and NiPc molecular devices are subject to negative differential resistance. As a result of our theoretical findings, it is found that TMPcs (TM = Fe, Mn) coupled to SWCNT probes yield a robust spin filter efficiency that may be of interest for prospective technological realizations in organic-inorganic molecular spintronic devices.

Effect of induced defects on conduction mechanisms of noble-gas-implanted ScN thin films

Noble-gas implantation was used to introduce defects in n-type degenerate ScN thin films to tailor their transport properties. The electrical resistivity increased significantly with the damage levels created, while the electron mobility decreased regardless of the nature of the ion implanted and their doses. However, the transport property characterizations showed that two types of defects were formed during implantation, named point-like and complex-like defects depending on their temperature stability. The point-like defects changed the electrical conduction mode from metallic-like to semiconducting behavior. In the low temperature range, where both groups of defects were present, the dominant operative conduction mechanism was the variable range hopping conduction mode. Beyond a temperature of about 400 K, the point-like defects started to recover with an activation energy of 90 meV resulting in a decrease in resistivity, independent of the incident ion. The complex-like defects were, therefore, the only remaining group of defects after annealing above 700 K. These latter, thermally stable at least up to 750 K, introduced deep acceptor levels in the bandgap resulting in an increase in the electrical resistivity with higher carrier scattering while keeping the metallic-like behavior of the sample. The generation of both types of defects, as determined by resistivity measurements, appeared to occur through a similar mechanism within a single collision cascade.

Structural Characterization and Thermoelectric Properties of Br-Doped AgSnm[Sb0.8Bi0.2]Te2+m Systems.

Herein, we report the synthesis, structural and microstructural characterization, and thermoelectric properties of AgSnm[Sb0.8Bi0.2]Te2+m and Br-doped telluride systems. These compounds were prepared by solid-state reaction at high temperature. Powder X-ray diffraction data reveal that these samples exhibit crystal structures related to the NaCl-type lattice. The microstructures and morphologies are investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). Positive values of the Seebeck coefficient (S) indicate that the transport properties are dominated by holes. The S of undoped AgSnm[Sb0.8Bi0.2]Te2+m ranges from +40 to 57 μV·K-1. Br-doped samples with m = 2 show S values of +74 μV·K-1 at RT, and the Seebeck coefficient increases almost linearly with increasing temperature. The total thermal conductivity (κtot) monotonically increases with increasing temperature (10-300 K). The κtot values of undoped AgSnm[Sb0.8Bi0.2]Te2+m are ~1.8 W m-1 K-1 (m = 4) and ~1.0 W m-1 K-1 (m = 2) at 300 K. The electrical conductivity (σ) decreases almost linearly with increasing temperature, indicating metal-like behavior. The ZT value increases as a function of temperature. A maximum ZT value of ~0.07 is achieved at room temperature for the Br-doped phase with m = 4.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Electronic Structure- and Entropy-Driven Design of Thermoelectric Chalcogenide Cu5Sn2Se7 Leading to the Optimization of Carrier Concentration and Reduction in Thermal Conductivity

Cu5Sn2Se7 (CSS) has a potential application in thermoelectrics in that it consists of affordable, non-toxic, and earth-abundant elements. However, it is less reviewed in thermoelectrics in recent years because it exhibits a metallic-like behavior with a high carrier concentration (nH) (nH ∼ 3.0 × 1021 cm–3). To improve its thermoelectric (TE) performance, an electronic structure- and entropy (ΔS)-driven design of CSS is proposed. By analyzing the electronic structures and ΔS values of CSS alloying with three different species (In, Te, or In2Te3), we determine that the In2Te3-incorporated CSS favors performance optimization. The Hall measurement reveals that the nH of (Cu5Sn2Se7)1–x(In2Te3)x (x = 0.1) reduces to the optimal value (∼8.3 × 1020 cm–3), while the mobility (μ) increases with an increase in x so that the power factor (PF) reaches 12.0 (μW/cm K2), about 20% enhancement. At the same time, the lattice thermal conductivity (κL) reduces to 0.46 W K–1 m–1 at x = 0.1. As a consequence, the ZT value increases to 0.7 at ∼770 K, which is about 4.7 times that of the pristine Cu5Sn2Se7. The principles applied here can be used as a guidance to design other thermoelectric materials.

Graphene Nano-Optics in the Terahertz Gap

Graphene nano-optics at terahertz (THz) frequencies (ν) is theoretically anticipated to feature extraordinary effects. However, interrogating such phenomena is nontrivial, since the atomically thin graphene dimensionally mismatches the THz radiation wavelength reaching hundreds of micrometers. Greater challenges happen in the THz gap (0.1-10 THz) wherein light sources are scarce. To surpass these barriers, we use a nanoscope illuminated by a highly brilliant and tunable free-electron laser to image the graphene nano-optical response from 1.5 to 6.0 THz. For ν < 2 THz, we observe a metal-like behavior of graphene, which screens optical fields akin to noble metals, since this excitation range approaches its charge relaxation frequency. At 3.8 THz, plasmonic resonances cause a field-enhancement effect (FEE) that improves the graphene imaging power. Moreover, we show that the metallic behavior and the FEE are tunable upon electrical doping, thus providing further control of these graphene nano-optical properties in the THz gap.