Shift Of Band Structure Research Articles

Fully Electrically Controlled van der Waals Multiferroic Tunnel Junctions.

The fully electrical control of the magnetic states in magnetic tunnel junctions is highly pursued for the development of the next generation of low-power and high-density information technology. However, achieving this functionality remains a formidable challenge at present. Here we propose an effective strategy by constructing a trilayer van der Waals multiferroic structure, consisting of CrI3-AgBiPSe6 and Cr2Ge2Te6-In2Se3, to achieve full-electrical control of multiferroic tunnel junctions. Within this structure, two different magnetic states of the magnetic bilayers (CrI3/Cr2Ge2Te6) can be modulated and switched in response to the polarization direction of the adjacent ferroelectric materials (AgBiPSe6/In2Se3). The intriguing magnetization reversal is mainly attributed to the polarization-field-induced band structure shift and interfacial charge transfer. On this basis, we further design two multiferroic tunnel junction devices, namely, graphene/CrI3-AgBiPSe6/graphene and graphene/Cr2Ge2Te6-In2Se3/graphene. In these devices, good interfacial Ohmic contacts are successfully obtained between the graphene electrode and the heterojunction, leading to an ultimate tunneling magnetoresistance of 9.3 × 106%. This study not only proposes a feasible strategy and identifies a promising candidate for achieving fully electrically controlled multiferroic tunnel junctions but also provides insights for designing other advanced spintronic devices.

Enhanced photocatalytic overall water splitting by tuning the relative concentration ratio of bulk defects to surface defects in SrTiO3

Defects such as oxygen vacancies play an important role in the photoactivation processes. However, the roles of surface and bulk defects in photocatalytic water splitting are still ambiguous. This paper would report a new method to introduce surface and bulk defects in SrTiO3 nanocrystals. Melamine was used as a reducing agent which would be carbonized under the N2 atmosphere. During thermal decomposition of melamine, reductive NH3 and C would generate in succession to reduce the SrTiO3 nanocrystals. The relative concentration ratio of bulk defects to surface defects in SrTiO3 nanocrystals would be tuned from 0.78 to 2.55 after reduction treatment. Furthermore, the band structures of surface and bulk defects in SrTiO3 were studied by DFT. The surface oxygen defects would make the band structure shift to a high energy level, while the bulk oxygen defects of SrTiO3 would restore the energy levels of the band structure. Thus, photocatalytic overall water splitting activity would improve significantly by rational relative concentration ratio of bulk defects to surface defects in SrTiO3.

Direct printing of surface-embedded stretchable graphene patterns with strong adhesion on viscous substrates

The graphene patterns with piezoresistance behaviors, originating from the structural deformation and band-structure shift under strain, represent an attractive characteristic for developing the small strain (<10%) sensors for wearable devices. However, the insufficient adhesion between the patterns and substrates has significantly limited their utility and reliability. Here, the surface-embedded graphene patterns were directly deposited on polydimethylsiloxane (PDMS) surfaces without hydrophilic treatment via the embedded droplet printing (EDP). The viscous PDMS films instead of the solid ones were used as substrates, and the graphene patterns were partly embedded onto the viscous PDMS surfaces. To assess the adhesion performance, a series of tests were performed. In the bending and tensile tests, the patterns strongly adhered to the PDMS films. Further, the patterns had a favorable increase in resistance in the tensile strain range of 0–3.5%. The resistance of the surface-embedded graphene patterns exhibited a negligible change for over 3 min in the ultrasonic bath. Finally, the relative resistance R/R0 remained constant after the first two adhesion tests using a 3M tape. The surface-embedded graphene patterns exhibited a strong adhesion to the flexible/stretchable substrates, indicating the application prospect in the field of flexible devices.

Tuning band structure of graphitic carbon nitride for efficient degradation of sulfamethazine: Atmospheric condition and theoretical calculation

Numerous approaches have been used to modify graphitic carbon nitride (g-C 3 N 4 ) for improving its photocatalytic activity. In this study, we demonstrated a facial post-calcination method for modified graphitic carbon nitride (g-C 3 N 4 -Ar/Air) to direct tuning band structure, i.e. , bandgap and positions of conduction band (CB)/valence band (VB), through the control of atmospheric condition without involving any additional elements or metals or semiconductors. The synthesized g-C 3 N 4 -Ar/Air could efficiently degrade sulfamethazine (SMT) under simulated solar light, i.e. , 99.0% removal of SMT with rate constant k 1 = 2.696 h −1 within 1.5 h (4.9 times than pristine g-C 3 N 4 ). Material characterizations indicated that the damaged/partial-collapsed structure and decreased nanosheet-interlayer distance for g-C 3 N 4 -Ar/Air resulted in the shift of band structure due to the denser stacking of pristine g-C 3 N 4 through oxidative exfoliation and planarization by air calcination. In addition, the bandgap of g-C 3 N 4 -Ar/Air was slightly shrunk from 2.82 eV (pristine g-C 3 N 4 ) to 2.79 eV, and the CB was significantly upshifted from −0.44 eV (pristine g-C 3 N 4 ) to −0.81 eV, suggesting the powerful ability for donating the electrons for O 2 to form • O 2 − . Fukui index ( f – ) based on theoretical calculation indicated that the sites of SMT molecule with high values, i.e. , N9, C4 and C6, preferred to be attacked by • O 2 − and • OH, which is confirmed by the intermediates’ analysis. The tuning method for graphitic carbon nitride provides a simple approach to regulate the charge carrier lifetime then facilitate the utilization efficiency of solar light, which exhibits great potential in efficient removal of emerging organic contaminants from wastewater. Modified graphitic carbon nitride (g-C3N4-Ar/Air) was synthesized via a facial post-calcination method and exhibited efficiently photocatalytic activity for SMT removal due to the tune of band structure.

Sequential lithium deposition on hexagonal boron nitride monolayer on Ir(111): Identifying intercalation and adsorption

Stepwise deposition of Li atoms onto hexagonal boron nitride (hBN) monolayer on Ir(111) is investigated by means of angle-resolved photoemission spectroscopy and low-energy electron diffraction. Sequential Li deposition progressively shifts the band structure of hBN to higher binding energies due to the induction of a variable electric potential originating from electronic charge donation by alkali atoms. At small coverages, Li atoms preferably intercalate under the hBN layer, where they are highly charged and give rise to a large initial shift of the band structure. Additionally, intercalated Li atoms effectively decouple hBN from the substrate and consequently reduce its moiré corrugation. As the deposition progresses further, Li atoms also adsorb on top of hBN, and the average effective charge of both intercalated and adsorbed Li atoms progressively diminishes due to the Coulomb repulsion penalty, which is partially screened by the metal substrate in the intercalated subsystem. This gives rise to a saturation of the respective electric potential and the band structure shift, and elaborates on the pathways and limitations of chemical functionalization of epitaxial hBN systems with charge donating species.

Bandgap-Tuned 2D Boron Nitride/Tungsten Nitride Nanocomposites for Development of High-Performance Deep Ultraviolet Selective Photodetectors.

This study presents a fast and effective method to synthesize 2D boron nitride/tungsten nitride (BN–WN) nanocomposites for tunable bandgap structures and devices. A few minutes of synthesis yielded a large quantity of high-quality 2D nanocomposites, with which a simple, low-cost deep UV photo-detector (DUV-PD) was fabricated and tested. The new device was demonstrated to have very good performance. High responsivity up to 1.17 A/W, fast response-time of lower than two milliseconds and highly stable repeatability were obtained. Furthermore, the influences of operating temperature and applied bias voltage on the properties of DUV-PD as well as its band structure shift were investigated.

Longitudinal elastic wave control by pre-deforming semi-linear materials.

An incremental wave superimposed on a pre-deformed hyper-elastic material perceives an elastic media with the instantaneous modulus of the current material. This offers a new route with a broadband feature to control elastic waves by purposely creating finite deformation field. This study proves that the governing equation of a semi-linear material under a symmetric pre-deformation condition maintains the form invariance for longitudinal wave, so the longitudinal wave control can be made by transformation method without the constraint condition on principle stretches; however, this is not the case for shear waves. Therefore, pre-deforming a semi-linear material provides a potential method for treating longitudinal and shear waves differently. Examples with elastic wave control and band structure shift through pre-deforming a semi-linear material are provided to illustrate this finding. Finally, a one-dimensional spring lattice is proposed to mimic a semi-linear material, and the dispersion relation for longitudinal waves in a sandwich structure with such spring lattice is shown to be invariant during elongation, confirming the result found based on a homogeneous semi-linear material. These results may stimulate researches on designing new hyper-elastic microstructures as well as designing new devices based on pre-deformed hyper-elastic materials.

First-principles study on hydrogen adsorption on nitrogen doped graphene

In this paper we have investigated the adsorption of Hydrogen on Nitrogen doped graphene in detail by means of first-principles calculations. A comprehensive study is performed of the structural, electronic and optical properties of hydrogen atoms adsorbed on dopant atoms sites and on carbon atoms neighboring dopant atoms. The effect of doping has been investigated by varying the concentration of doping atoms from 3.125%( one atom of nitrogen in 32 host atoms) to 6.25% ( two nitrogen atoms in 32 host atoms). Similarly the effect of adsorption has been investigated by varying the concentration of hydrogen atoms and also varying the adsorption sites. Band structure, partial density of states (PDOS) and optical properties of pure, nitrogen doped and hydrogen adsorbed graphene sheet were calculated using VASP (Vienna ab-initio Simulation Package). The calculated results for pure graphene sheet were then compared with nitrogen doped graphene and Hydrogen adsorbed graphene sheet. It is found that upon nitrogen doping the Dirac point in the graphene band structure shifts below the Fermi Energy level and energy gap appears at the high symmetric K-point. On the other hand, by adsorption of Hydrogen atom, there is further change in the band structure near the Fermi level and also the energy gap at the high symmetric K-point is increased. There is change in the dielectric function and refractive index of the graphene after H atoms adsorption on N-doped graphene. The overall absorption spectra is decreased in case of nitrogen doping and after adsorption process of Hydrogen atoms. However a significant red shift in absorption towards visible range of radiation is found to occur for hydrogen atoms adsorbed on nitrogen doped graphene sheet. The reflectivity peak of graphene increases in low energy region after H adsorption on N-doped graphene. The results can be used to tune the Fermi Energy level and to tailor the optical properties of graphene sheet in visible region.

The External Crystal Structure and Electronic Structure are Simulated about Ge&lt;sub&gt;0.6&lt;/sub&gt;Si&lt;sub&gt;0.4&lt;/sub&gt; Quantum Dots

The alloy Ge0.6Si0.4 quantum dots were studied by using density functional theory. The change of the electronic structure of each crystal which grown in different simulation temperature condition were investigated by molecular dynamics simulation method. The results indicate that quantum dots of silicon germanium alloy occupy the narrow band gap of each crystal face from low to high temperature conditions. Since the atomic density and crystal configuration is different, the band gap values are relatively different. The mechanism of dielectric constant transition is well explained based on the inter-band and in-band shift of band structure.

Light-induced deformation in a liquid crystal elastomer photonic crystal

Elastomer materials can undergo large, reversible elastic deformation, and offer novel possibilities for coupled optomechanical behavior when light itself is used to induce that deformation. This phenomenology is especially interesting to consider when photonic bandstructure effects and mechanical instabilities are present over the same length scales. Here we investigate a novel, coupled optomechanical material behavior whereby complex deformation, with the potential to occur cyclically, occurs in a soft photonic crystal structure due to a mechanical instability, as a result of constant, uniform illumination by normally incident light. We suppose that the base material for the structure is a material that responds to light by undergoing a microstructural change. Such a behavior is observed, for example, in a liquid crystal elastomer containing azobenzene moieties attached to the liquid crystal main-chains (Finkelmann et al., 2001) transformational strain generated by the effect of localized light energy on the isomerization of the azobenzene moieties can be calculated from an order-parameter based model (Hogan et al., 2002). Under uniform exposure to constant illumination, the interaction between the light, the material, and the deforming structure lead to a complex, reversible deformation sequence. We analyze the electromagnetic energy distribution inside this photonic crystal structure by solving Maxwells equations for the electromagnetic problem of light transmittance using finite element analysis. First, upon contraction of the structure due to isomerization in the uniformly illuminated material, the photonic bandstructure shifts, thereby significantly reducing the average illumination of material within the structure. The locally reduced illumination allows for a relaxation of the strain in some parts of the structure, due to the reversible isomerization at room temperature. Then, as a result of this relaxation, the structure is subjected to uniaxial stress, leading to a mechanical instability that triggers a geometrical pattern transformation. This in turn produces a second contractile deformation, as a result of the buckling-like deformation in the structure. Finally, the highly nonuniform local strain field that results generates a dramatic change in the photonic bandstructure of the system, leading to a new localization of the light that tends to reverse the effect of pattern transformation. This completes the transformation sequence, demonstrating the potential for cyclical deformation induced simply by uniform illumination. The coupled optomechanical material/structure behavior observed here could lead to applications in optically sensors, energy harvesters, or other reversible optomechanically active structures.

Electronic properties of boron- and nitrogen-doped graphene: a first principles study

Effect of doping of graphene either by boron (B), nitrogen (N), or co-doped by B and N is studied by density functional theory. Our extensive band structure and density of states calculations indicate that upon doping by N (electron doping), the Dirac point in the graphene band structure shifts below the Fermi level and an energy gap appears at the high symmetric K-point. On the other hand, by B (hole doping), the Dirac point shifts above the Fermi level and a gap appears. Upon co-doping of graphene by B and N, the energy gap between valence and conduction bands appears at Fermi level and the system behaves as narrow gap semiconductor. Obtained results are found to be in well agreement with available experimental findings.

First-principles calculation of transition-metal impurities in LaFeAsO

We present a systematic ab initio study based on density-functional calculations to understand impurity effects in iron-based superconductors. Effective tight-binding Hamiltonians for the d-bands of LaFeAsO with various transition-metal impurities such as Mn, Co, Ni, Zn, and Ru are constructed using maximally-localized Wannier orbitals. Local electronic structures around the impurity are quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We found that the impurities are classified into three groups according to the derived parameters: For Mn, Co, and Ni, their impurity-3d levels measured from the Fe-3d level are nearly 0.3 eV, -0.3 eV, and -0.8 eV, respectively, while, for the Zn case, the d level is considerably deep as -8 eV. For the Ru case, although the onsite-level difference is much smaller as O(0.1) eV, the transfer integrals around the impurity site are larger than those of the pure system by 20% \sim 30%, due to the large spatial spread of the Ru-4d orbitals. We also show that, while excess carriers are tightly trapped around the impurity site (due to the Friedel sum rule), there is a rigid shift of band structure near the Fermi level, which has the same effect as carrier doping.

Evidence of quantum confinement effects on interband optical transitions in Si nanocrystals

We present evidence for quantum confinement effects on optical transitions in ensembles of Si nanocrystals (NCs) in a ${\text{SiO}}_{2}$ matrix by considering simultaneously the dielectric function dispersions obtained by spectroscopic ellipsometry, the absorption edge, and the photoluminescence. We find that all these quantities blueshift in a similar manner with decreasing size of the NCs for diameters $d$ below 6 nm. The correlated behaviors of the three observed optical transitions, the optical gap ${E}_{G}$, and the critical points ${E}_{1}$ and ${E}_{2}$, indicate conclusively that their measured blueshifts are associated with the same quantum confinement effect and that the ``entire'' band structure shifts with $d$. In addition, our results show that the features of the band structure of Si NCs in the $d&gt;4\text{ }\text{nm}$ range are qualitatively similar to those of bulk Si. In particular, the indirectlike nature of the band gap and the criticality of the L and X points of the bulk Brillouin zone are retained, on the understanding that these concepts cannot be strict in nanocrystals of small dimensions.

Metal–insulator transition in sodium tungsten bronzes, [formula omitted], studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy on sodium tungsten bronzes, Na x WO 3 , which exhibit a metal–insulator transition as a function of x. We found that the near- E F states are localized in Na x WO 3 ( x ⩽ 0.25 ) due to the strong disorder caused by the random distribution of Na + ions in the WO 3 lattice, which makes the system insulating. In the metallic regime we found that the rigid shift of band structure can explain the metallic Na x WO 3 band structure with respect to Na doping.

Impurity-induced modulations inPbxNbSe3coupled to charge-density-wave formation

Very dilute amounts of Pd in Pd{sub {ital x}}NbSe{sub 3} introduce long-range electronic modulations of wavelength 7{ital b}{sub 0}, 4{ital b}{sub 0}, 3{ital b}{sub 0}, and 2{ital b}{sub 0} at room temperature as the Pd concentration increases in the range {ital x}=0.002 to {ital x}=0.02 while the low-temperature charge-density waves (CDW{close_quote}s) initially remain unchanged. For {ital x}{ge}0.02 the low-temperature CDW{close_quote}s are quenched while the NbSe{sub 3} structure remains intact, and the high-temperature modulations disappear, indicating a clear correlation between the two effects. The magnetoquantum oscillations due to magnetic breakdown first detect the band-structure shift followed by the sudden quenching of the nested Fermi surface sheets. The atomic force microscope scans show substantial charge transfer between chains caused by the Pd doping. {copyright} {ital 1996 The American Physical Society.}

Magnetic field effects to 230 kG on the magnetotransport and charge-density waves in NbSe 3

The transverse magnetoresistance and Hall resistance in NbSe 3 measured below the charge-density wave (CDW) transition at 59 K indicate magnetic field induced modifications of the Fermi surface with enhanced condensation of electrons into the CDW. At 1.1 K changes in the quantum oscillation frequency with depinning of the CDW indicate a band structure shift connected with impurity pinning. A new mode of CDW motion is also observed at high magnetic fields and low temperature.