Modification Of Band Structure Research Articles

Tailoring the chemical bonding of GeTe-based alloys by MgB2 alloying to simultaneously enhance their mechanical and thermoelectric performance

As a promising mid-temperature thermoelectric material, GeTe-based alloys have been intensively studied recently. Besides high figure of merit (ZT), good service stability is also significant for the practical applications of thermoelectric materials, which is especially a challenge for GeTe-based alloys having phase-transition behaviors. In this work, we have demonstrated a MgB2-alloying strategy to simultaneously enhance the mechanical and thermoelectric performance of GeTe-based alloys, owing to the tuning of chemical bonding. The negative coefficient of thermal expansion (CTE) of GeTe near the phase-transition temperature can be fully eliminated through co-alloying of Sb and MgB2. The enhancement of compressive strength and hardness by MgB2 alloying can be mainly ascribed to the solid-solution effect and the tuning of grain structures. Moreover, the Mg-substitution induced delocalization of electrons and modulation of band structures can facilitate the transport of charge carriers. Combining these MgB2-alloying effects, we can obtain (Ge0.9Sb0.1Te)1-x(MgB2)x samples with a peak ZT of 1.92 at 773 K and average ZT of 1.1 within 300–773 K, smooth CTE in the whole working temperatures, high compressive strength of ∼271 MPa and Vickers hardness of ∼236 Hv. The chemical-bonding mechanism developed in this work should also be potential for enhancing the overall performance of other thermoelectric materials.

Layer-dependent charge density wave phase transition stiffness in 1T-TaS2 nanoflakes evidenced by ultrafast carrier dynamics

Novel physical properties emerge when the thickness of charge density wave (CDW) materials is reduced to the atomic level, owing to the significant modification of the electronic band structure and correlation effects. Here, we investigate the layer-dependent CDW phase transition and evolution of the nonequilibrium state of 1T-TaS2 nanoflakes using pump-probe spectroscopy. Both the low-energy single-particle and collective excitation relaxations exhibit sharp changes at ~ 210 K, indicating a phase transition from commensurate CDW to nearly commensurate CDW state. The single particle process reveals that the phase transition stiffness (PTS) is thickness-dependent. Moreover, a small PTS is observed in thin nanoflakes, which is attributed to the reduced thickness that increases the fluctuation and inhibits the nucleation and growth of discommensurations. In addition, the phase mode vanishes when the discommensuration network appears. Our results suggest that the carrier dynamics could be an efficient operational approach to measuring the quantum phase transition in correlated materials.

Infrared photodetectors based on multiwalled carbon nanotubes: Insights into the effect of nitrogen doping

Multiwalled carbon nanotubes (MWCNTs) and their nitrogen-doped counterparts (N-MWCNTs) were synthesized by the pyrolysis technique. For infrared detection, Ag-MWCNTs-Ag structured devices have been fabricated and their infrared photoresponses are assessed. The devices show stable photoresponses over multiple cycles. The observed responsivity values of the devices are in the range of 0.2–0.4 AW−1 (at a low bias voltage of 0.5 V) and are superior to many other infrared detectors using similar materials. In both the devices, the photocurrents originate predominantly due to photoconductive effect of the semiconducting MWCNTs. External quantum efficiency (EQE) of the N-doped MWCNTs based device is much higher than that of the undoped one. The better performance of the N-doped MWCNTs based device is due to higher infrared absorption and trap assisted gain influenced by the band structure modification of the nanotubes by doped heteroatoms.

The influence of shape and orientation of scatters on the photonic band gap in two-dimensional Bravais-Moiré lattices

We perform a theoretical study of light propagation properties in two-dimensional square photonic crystals following Bravais-Moiré patterns, paying particular attention to the influence of the transversal shape and the orientation of the dielectric scatters onto the width and position of photonic band gaps. In this sense, we have considered both square and triangular transversal geometries for the dielectric scatters, together with the possible rotation of either all the elements or of one half of them, within the unit cell. Results for the photonic dispersion relations and band gaps are compared with those arising from the analysis of structures with simple bi-atomic Bravais unit cells. It comes out that wider photonic gaps appear when using square-shaped scatters. The use of Bravais-Moiré cells with the same kind of cores enhance the width of these gaps but shift them towards higher frequencies. Rotation of all elements within the cell in angles of 0.23 rad and 0.46 rad causes very small, if not null, changes in the photonic gap widths. However, the rotation of one half of the scatters in the cell, leaving the other half unrotated does produce noticeable modifications in the photonic band structure: For crystals made of square-shaped dielectric cores and simple cubic cells, this rotation strongly modifies the photonic structure, whilst for Bravais-Moiré crystals the same kind of change takes place for cells made of triangular-shaped cores.

Shaping the role of germanium vacancies in germanium telluride: metastable cubic structure stabilization, band structure modification, and stable N-type conduction

Understanding and controlling point defects in semiconductors are essential for developing advanced electronic and optoelectronic devices. Germanium telluride (GeTe), a semiconductor with a rhombohedral-to-cubic structural phase transition and a high concentration of intrinsic vacancies on the Ge sublattice, has recently attracted much interest for thermoelectric applications. However, the role of Ge vacancies in structural change and performance optimization remains obscure. Herein, we first unraveled the importance of Ge vacancies by combining first-principles calculations and Boltzmann transport theory. It is revealed that (1) Ge vacancies are more likely to spontaneously form in cubic GeTe, addressing its defective character; (2) Ge vacancies play a vital role in stabilizing cubic GeTe; and (3) Ge vacancies produce unfavorable band structure modification, leading to a reduced power factor. The following experiment found that AgInTe2 alloying promotes a symmetry change from rhombohedral to cubic and deteriorates the thermoelectric performance, in good agreement with the abovementioned conclusions. More importantly, a single-phase cubic GeTe-based material with stable n-type conduction was first discovered based on the defect chemistry approach. Our findings shed new light on the critical role of Ge vacancies in the structure-property relationship and stimulate the strategy of point defect engineering for future thermoelectric applications.

Synthesis of oxygen-doped-g-C3N4/WO3 porous structures for visible driven photocatalytic H2 production

Noble-metal-free sustainable energy conservation has been attracting considerable attention for environmental protection and remediation. Herein, we present a low-temperature (<100 °C) procedure for fabricating porous oxygen-doped g-C3N4/WO3 structures with H2O2, oxygen doping creates the trap energy levels which are more sensitive towards visible light and studied their photocatalytic H2 evaluation under visible light irradiation. The oxygen generates CO, N–O, C–O–W, and C–O–W–N, which can alter the oxidation state of tungsten and boost-up the photocatalytic properties of O-g-C3N4/WO3. The optimized photocatalyst (O-g-CNW-4) produces higher H2 evaluation (15,142 μmol/g), which is 4.7 times higher than that produced by mechanical mixing samples. The prepared samples have highly porous structures with pore diameters of 20–30 nm. The CO, N–O, C–O–W, and C–O–W–N linkage vibrations of the prepared samples are analyzed, and their chemical confirmations and elemental oxidation states are verified. The oxygen-doped g-C3N4/WO3 porous structures exhibit large surface-areas with a huge quantity of active surface sites, band-structure modulation, and effective visible-light-harvesting of the photoinduced charge carriers, which is useful for enhancing photocatalytic H2 production.

How defects influence the photoluminescence of TMDCs

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, a class of ultrathin materials with a direct bandgap and high exciton binding energies, provide an ideal platform to study the photoluminescence (PL) of light-emitting devices. Atomically thin TMDCs usually contain various defects, which enrich the lattice structure and give rise to many intriguing properties. As the influences of defects can be either detrimental or beneficial, a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring. Herein, recent advances in the defect influences on PL emission are summarized and discussed. Fundamental mechanisms are the focus of this review, such as radiative/nonradiative recombination kinetics and band structure modification. Both challenges and opportunities are present in the field of defect manipulation, and the exploration of mechanisms is expected to facilitate the applications of 2D TMDCs in the future.

(Invited) Engineering Strain, Defects, and Electronic Properties of (110)-Oriented Strained Si

Strain engineering of group IV semiconductors has been extensively investigated with an aim to produce high mobility platform for high performance electronic devices. The lattice strain significantly affects the carrier mobility via modulation of the energy band structure and its effect on the crystalline defect formation. The lattice structure of the strained crystal is determined by the amount of the strain and the boundary condition. In this study, the morphological aspects and the electronic properties of the (110)-oriented strained Si/SiGe/Si heterostructures grown by molecular beam epitaxy (MBE) have been studied. Potential of this material system as a high hole mobility semiconductor platform is discussed. It has been found that highly anisotropic lattice strain and defect configuration can be realized in this heterostructure and that this anisotropic feature plays important roles in the enhancement of the hole mobility.

The effect of MoS2 modulated doping with molybdenum-oxide on the photovoltaic performance for MoS2/n-Si heterojunction solar cells

Achieving large Fermi energy splitting under illumination is critical for improving the performance of photovoltaic devices. In this work, we conducted the energy band structure modulation of MoS2 thin films via MoOx doping scheme to realize its photovoltaic operation on n-type c-Si by magnetron sputtering. It is found that, by capping a MoOx overlayer, the MoS2 electrons density decreased and the Fermi level shifted ~0.4 eV towards valence band, consequently the MoS2 conductive type changing. With this doped MoS2 layer, the built-in electric field of fabricated MoS2/n-Si heterojunction devices greatly improved and exceeded 400 mV owing to the larger interface energy-level differences. Meanwhile, the defects recombination losses reduced benefiting from the holes-selective behavior of O-deficiencies in MoOx, so that the photo-generated carriers extraction process were promoted. As a result, the doped MoS2/n-Si devices showed excellent rectification behavior with rectifying ratio to 105 and ideality of 1.12. The TRPL spectrums corroborated the enhanced carrier transport mechanism at MoS2/n-Si interface by MoOx modulation doping effect. Correspondingly, the significant improvement of doped MoS2/n-Si solar cells performance (Voc of 289 mV, FF of 60.72% and Jsc of 31.25 mA/cm2) was achieved, exhibiting an optimized conversion efficiency of 5.47%. The UPS analysis revealed that the doping mechanism of MoOx/MoS2 stacks lies in energy band overlap and electrons contact transfer between these two materials. This observation of MoOx modulation doping effect on large-area sputtering MoS2 could help in building a better interface carrier transport in photovoltaic and optoelectronic applications of 2D materials.

Out-of-plane magnetic anisotropy engineered via band distortion in two-dimensional materials

We unveil the connection between magnetic anisotropy and band structures in $\mathrm{\ensuremath{\Gamma}}$-specialized ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}/{\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ heterostructure, through first-principles calculations. A significant band distortion occurs near the $\mathrm{\ensuremath{\Gamma}}$ point with a formed valley. As the band-distortion-caused valley moves relative to the quasivalence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such a rule applies to either the strained or polarization-switched ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}/{\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ heterostructure. These findings demonstrate the feasibility of predicting the magnetic anisotropy by band structure modification in two-dimensional magnetic systems.

Can doping of transition metal oxide cathode materials increase achievable voltages with multivalent metals?

AbstractThe use of substitutional p‐doping as a means to enhance the insertion energies of multivalent metals in transition metal oxides, and therefore the resulting voltages in an electrochemical cell, due to band structure modulation is investigated using first principles calculations. The investigations reveal the formation of n‐hole polarons (with n &gt; 1) in the form of oxygen dimers in p‐doped charge‐transfer insulating transition metal oxides, caused by localized p holes on oxide ions in agreement with previous findings. It is found that the oxygen dimer formation has an adverse effect on adsorption energetics compared to the single‐hole case without dimerization. On the other hand, strained systems or Mott insulators with qualitatively different valence band composition do not exhibit oxygen dimerization with multihole doping. The results demonstrate the advantages and limitations of transition metal oxide electrode p‐doping and show a path to possible strategies to overcome detrimental effects.

Nitrogen doped FeS2 nanoparticles for efficient and stable hydrogen evolution reaction

Abstract Performance breakthrough of electrocatalysts highly relies on the regulation of internal structures and electronic states. In present work, for the first time, we successfully synthesized nitrogen doped FeS2 nanoparticles (N-FeS2) as the electrocatalysts for hydrogen evolution reaction (HER). The band structure and electronic state of FeS2 are modulated by a nitrogen doping strategy, as confirmed by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Owing to the band structure and electronic state regulation as well as the weakening of H−S interaction, the designed N-FeS2 electrocatalyst exhibits superior catalytic performance with a low overpotential (~126 mV at 10 mA cm−2) and excellent activity stability under alkaline conditions, which is substantially improved as compared with that of the pure FeS2 counterpart. Our work demonstrates that the modulation of electron state and band structure of an electrocatalyst, which can provide a valuable guidance for designing excellent catalysts for hydrogen evolution reaction and beyond.

High Curie Temperature Ferromagnetism and High Hole Mobility in Tensile Strained Mn‐Doped SiGe Thin Films

AbstractDiluted magnetic semiconductors based on group‐IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn‐doped SiGe thin films are first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples become ferromagnetic semiconductors, in which the Curie temperature increases with increasing Mn doping concentration and reaches 280 K with 5% Mn concentration. The data suggest that the ferromagnetism comes from the hole‐mediated process and is enhanced by the tensile strain in the SiGe crystals. Meanwhile, the Hall effect measurement up to 33 T to eliminate the influence of anomalous Hall effect reveals that the hole mobility of the annealed samples is greatly enhanced and the maximal value is ≈1000 cm2 V−1 s−1, owing to the tensile strain‐induced band structure modulation. The Mn‐doped SiGe thin films with high Curie temperature ferromagnetism and high hole mobility may provide a promising platform for semiconductor spintronics.

Decomposition of highly persistent perfluorooctanoic acid by hollow Bi/BiOI1-xFx: Synergistic effects of surface plasmon resonance and modified band structures

Perfluorooctanoic acid (PFOA) is highly stable due to the strong CF bond and extremely difficult to be removed by conventional photocatalysts. In this study, Bi doped BiOI1-xFx solid solutions with hollow microsphere structure were prepared through a facile one-step hydrothermal method. Compared with pure BiOI and BiOF, the band gap of the Bi/BiOI1-xFx solid solutions was significantly reduced, thus promoting the visible light absorbance. The cavity structure of the BiOI1-xFx solid solutions enhanced the surface areas and active sites for reaction. The local electromagnetic field dominated by surface plasmon resonance (SPR) effect of Bi metal on the surface favored the separation of the photoinduced charge pairs. As a consequence, Bi/BiOI0.8F0.2 (x = 0.20, the doping amount of fluorine was 20 %) composite displayed the best photocatalytic performance for decomposing PFOA, and 40 mg/L PFOA could be removed within 2 h illumination. The degradation rate constant (k = 0.0375 min−1) of PFOA by Bi/BiOI0.8F0.2 was about tenfold of that by pure BiOI and BiOF. Superoxide radical (·O2-) predominated in the degradation of PFOA by Bi/BiOI0.8F0.2, and the possible degradation pathway of PFOA by Bi/BiOI0.8F0.2 was proposed. This work provides a highly efficient catalyst for the practical application in removal of highly persistent PFOA.

Optimal performance of Cu1.8S1\u2212xTex thermoelectric materials fabricated via high-pressure process at room temperature

Cu1.8S has been considered as a potential thermoelectric (TE) material for its stable electrical and thermal properties, environmental benignity, and low cost. Herein, the TE properties of nanostructured Cu1.8S1−xTex (0 ⩽ x ⩽ 0.2) bulks fabricated by a facile process combining mechanical alloying (MA) and room-temperature high-pressure sintering (RT-HPS) technique were optimized via eliminating the volatilization of S element and suppressing grain growth. Experimentally, a single phase of Cu1.8S was obtained at x = 0, and a second Cu1.96S phase formed in all Cu1.8S1−xTex samples when 0.05 ⩽ x ⩽ 0.125. With further increasing x to 0.15 ⩽ x ⩽ 0.2, the Cu2−zTe phase was detected and the samples consisted of Cu1.8S, Cu1.96S, and Cu2−zTe phases. Benefiting from a modified band structure and the coexisted phases of Cu1.96S and Cu2−zTe, the power factor is enhanced in all Cu1.8S1−xTex (0.05 ⩽ x ⩽ 0.2) alloys. Combining with a drastic decrease in the thermal conductivity due to the strengthened phonon scatterings from multiscale defects introduced by Te doping and nano-grain boundaries, a maximum figure of merit (ZT) of 0.352 is reached at 623 K for Cu1.8S0.875Te0.125, which is 171% higher than that of Cu1.8S (0.130). The study demonstrates that doping Te is an effective strategy to improve the TE performance of Cu1.8S based materials and the proposed facile method combing MA and RT-HPS is a potential way to fabricate nanostructured bulks.

Ammonia Sensing by Sn1–xVxO2 Mesoporous Nanoparticles

Chemiresistive gas sensing by metal oxide based materials has been usually explained in terms of surface chemistry and band structure modifications due to factors such as chemical composition, part...

Large positive magnetoresistance with high Hall mobility and intercalation of Fe dopants in the quenched Bi2Te3 single crystals

A study of Fe-ion intercalation in Bi2Te3 probed by the x-ray diffraction, magnetization, Hall measurement, and spectra of synchrotron x-ray photoemission spectroscopy is reported, in which a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed in the quenched FexBi2Te3 single crystals with x = 0.01 and 0.15. Large positive MR of ∼470% with a Hall mobility of ∼44000 cm2/Vs at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample with a quenching temperature Tq of 400 °C, where the electrical parameters can be tuned by Tq. Larger c-axis lattice constants and electron-dominant carriers in the quenched samples lead to the inference that the doped Fe atoms are mostly located at the intercalated van der Waals gap positions and exist as a metallic state of Fe0, which is confirmed by the synchrotron x-ray photoemission spectroscopy (XPS) spectra. Combining the x-ray diffraction, magnetization, and Hall-coefficient studies, here we pave a simple way for probing into the location of Fe dopants in Bi2Te3. In addition, novel magnetotransports of Fe-doped Bi2Te3 associated with the absence of ferromagnetism have been presented, and satisfy the scenario that the topological surface state persists without the loss of topological insulating characteristics of the linear band dispersion, and the large MR may originate from the surface magnetism, even in the presence of Fe impurities. This study also discusses the huge enhancement of Hall mobility arising from the surface Dirac electrons that are accompanied with a modified band structure in quenched Fe0.01Bi2Te3.

Defect-modified reduced graphitic carbon nitride (RCN) enhanced oxidation performance for photocatalytic degradation of diclofenac

Hydroxyl radicals (OH) have robust non-selective oxidizing properties to effectively degrade organic pollutants. However, graphitic carbon nitride (g-C3N4) is restricted to directly generate OH due to its intrinsic valence band. In this study, we report a facile environmental-friendly self-modification strategy to synthesize reduced graphitic carbon nitride (RCN), with nitrogen vacancies and CN functional groups. The incorporation of CN enabled to downshift the valence band level, which endowed RCN with the capacity to directly generate OH via h+. Experimental and instrumental analyses revealed the critical roles of nitrogen vacancies and CN groups in the modification of the RCN band structure to improve its visible light absorption and oxidizing capacity. With these superior properties, the RCN was significantly enhanced for the photocatalytic degradation of DCF under visible light irradiation. The self-modification strategy articulated in this study has strong potential for the creation of customized g-C3N4 band structures with enhanced oxidation performance.

Preparation of C3N5 nanosheets with enhanced performance in photocatalytic methylene blue (MB) degradation and H2-evolution from water splitting

Ultrathin C3N5 nanosheets with enhanced photocatalytic methylene blue (MB) degradation and H2-evolution performance were prepared from thermal treatment of 3-amino-1,2,4-triazole (3-AT) and NH4Cl followed with a protonate procedure. The characterization results revealed that the protonating process could contribute to the exfoliation of C3N5 with large surface area, the effective charge transfer capability and the modified band structure. The as-prepared C3N5 nanosheets exhibited enhanced properties in photocatalytic reactions such as MB photodegradation and H2-evolution from water splitting. This study offered a feasible route to prepare highly-efficient two-dimensional photocatalyst, which could be applied potentially for implementation in wide range of energy generation and environmental applications.

Topological band structure transitions and goniopolar transport in honeycomb antimonene as a function of buckling

The electronic band topology of monolayer $\beta$-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the $p_z$-orbitals, and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure becomes a trivial semiconductor. Spin-orbit coupling and edge states are discussed.