Shift Of Conduction Band Edge Research Articles

Band gap correction via GGA[formula omitted]mBJ function and strained modulated physical properties of 2D-like DyOBr oxybromide

Two-dimensional (2D) oxyhalides offer unique and fascinating properties due to long-range magnetic ordering in a low dimensional limit. These have potential applications in spintronics and low-dimensional data storage devices. Herein, we demonstrate the biaxial ([110]) strain impact on the electronic and magnetic properties of thermodynamically stable 2D like DyOBr oxybromide based on the first-principles calculations including Hubbard parameter (U) and U+mBJ (modified-Becke-Johnson) methods as well as spin–orbit coupling (SOC) effects. Our calculations predict that an unstrained system exhibits an anti-ferromagnetic (AFM) insulating state with a direct wide energy band gap (Eg) of 5.404 eV within GGA+U+mBJ scheme and the estimated Neel temperature (TN) using the Ising model is 13 K, which is in excellent agreement with the experimentally observed values of 5.41 eV and 9.5 K, respectively. Moreover, it is revealed that Dy ions displayed a large partial spin magnetic moment (ms) of 4.969 μB/f.u. (per formula unit) with S = 2.5 as they lie in ＋ 3 state having the electronic configuration of [fx(x2−3y2)+fy(3x2−y2)]↑2↓1, [fxz2+fyz2]↑2↓1, [fz(x2−y2)]↑1↓0, [fxyz]↑1↓0, and [fz3]↑1↓0. Interestingly, the system displays a large magnetic anisotropy energy of 4.31 meV/atom with a magnetic easy axis of [100] (a-axis), which enhances its utilization in magnetic memory devices. Further, it is established that the AFM insulating behavior of the structure remains robust against biaxial ([110]) strain for the considered range of ±5%. However, a well-ordered shift in conduction band edge (CBE) position to higher and lower energy values is predicted for compressive and tensile strains, respectively. Strikingly, it is found that the TN increases to 265% for ＋5% strain.

Magnetism-Induced Band-Edge Shift as the Mechanism for Magnetoconductance in CrPS4 Transistors.

Transistors realized on the 2D antiferromagnetic semiconductor CrPS4 exhibit large magnetoconductance due to magnetic-field-induced changes in the magnetic state. The microscopic mechanism coupling the conductance and magnetic state is not understood. We identify it by analyzing the evolution of the parameters determining the transistor behavior─carrier mobility and threshold voltage─with temperature and magnetic field. For temperatures T near the Néel temperature TN, the magnetoconductance originates from a mobility increase due to the applied magnetic field that reduces spin fluctuation induced disorder. For T ≪ TN, instead, what changes is the threshold voltage, so that increasing the field at fixed gate voltage increases the density of accumulated electrons. The phenomenon is explained by a conduction band-edge shift correctly predicted by the ab initio calculations. Our results demonstrate that the band structure of CrPS4 depends on its magnetic state and reveal a mechanism for magnetoconductance that had not been identified earlier.

Triphenylphosphine assisted phosphorization of g-C3N4 for enhanced photocatalytic activity

It is an effective strategy to regulate the structure of photocatalysts for enhanced photocatalytic hydrogen evolution. In this work, a facile method was developed to introduce P atoms into g-C3N4via the thermal calcination treatment. In contrast to pristine g-C3N4, the obtained P-CN(PPh3) exhibited a negative shift of conduction band edge with enhanced light absorption after the introduction of P atoms into g-C3N4, thus benefiting the photocatalytic hydrogen evolution. Moreover, the introduction of P element in g-C3N4 can also improve the charge separation during the photocatalytic hydrogen reaction, significantly accelerating the photocatalytic hydrogen production rate of P-CN(PPh3).

Lattice strain and band overlap of the thermoelectric composite Mg2Si1−xSnx

${\mathrm{Mg}}_{2}{\mathrm{Si}}_{1\text{\ensuremath{-}}x}{\mathrm{Sn}}_{x}$ solid solutions show enhanced thermoelectric performance when the Sn mole fraction $x$ is approximately $x=0.7$. This has been discussed in terms of complexity of the electronic structure arising from the crossover of two bottom conduction bands, but direct detailed understanding of the origins and the precise nature of this band convergence is limited. Here, we report first-principles calculations of the band edge changes analyzed by band unfolding and crystal orbital Hamilton population techniques. We find that strain is particularly important at this crossing. ${\mathrm{Mg}}_{2}\mathrm{Si}$ and ${\mathrm{Mg}}_{2}\mathrm{Sn}$ show opposite trends in the conduction band edge shifts leading to a band crossover at $x\ensuremath{\sim}0.625$. However, there are also important effects due to disorder. Transport calculations show that enhancement of the figure of merit $ZT$ value owes to the combined effects of lattice strain, band edge overlap, composition change, and disorder.

Investigation of electronic and optical properties of PbxSn1-xO2 for optoelectronic applications: A TB-mBJ DFT approach

A comprehensive study of electronic and optical properties of PbxSn1-xO2 (x = 0.125, 0.25 and 0.375) have been performed utilizing PBE-GGA + TB-mBJ scheme employing WIEN2K package under DFT formalism. The lattice constant parameters obtained for PbxSn1-xO2 are observed to be invariant with rutile SnO2. The DoS (Density of States) profile of PbxSn1-xO2 states a dominant impact of partial states of Pb along the conduction band. A red shift of conduction band edge has been observed with increasing Pb incorporation in PbxSn1-xO2 supercells. The contribution due to partial states of O is supreme as compared to Pb along the valence band, as also encountered in SnO2. The nature of bandstructure is in parity with DoS profile. With increased incorporation of Pb atoms in PbxSn1-xO2 supercells, a significant lowering of conduction band minimum occurs thereby resulting in reduced band gap. The nature of reduction of optical band gap values is in parity with the electronic band structure. The optical parameter characteristic curves undergo redshift due to increased Pb incorporation in PbxSn1-xO2 supercells. Distinct peak values in absorption characteristic curve claims the material to have direct band gap. Higher value of static refractive index causes dispersion in the material. The real (imaginary) part of dielectric constant is consistent with the refractive index (extinction coefficient) characteristic curves. The reflection coefficient curve indicates lossy nature of the material. The reflection coefficient curve contradicts the loss coefficient characteristic curve. Hence, the consequences of this investigation claim the competency of PbxSn1-xO2 material for optoelectronic applications.

Dual-defects induced band edge reconstruction of tin dioxide via cobalt and nitrogen Co-Doping for wearable supercapacitor application

Dual-defects induced band edge reconstruction of tin dioxide (SnO2) via metal and non-metal ions co-doping strategy is applied to synthesize cobalt and nitrogen co-doped tin dioxide (Co, N-SnO2) on activated carbon fiber (ACF) for supercapacitor. N-doping contributes to upward shift of valence band edge through the hybridization of N-2p and O-2p and Co-doping contributes to downward shift of conduction band edge through the hybridization of Co-3d and O-2p. The bandgap is highly narrowed from 2.29 eV for SnO2 to 0.68 eV for Co, N-SnO2. The Co, N-SnO2/ACF electrode achieves specific capacitance of 361.2 F g−1 at 1 A g−1, rate capability of 61.1% from 1 to 10 A g−1 and cycling stability of 103.3% for 10,000 cycles. The density functional theory proves that Co-doping mainly promotes the capacity due to its easier deprotonation process in acid electrolyte, whereas N-doping mainly promotes the rate capability due to its narrower bandgap and thus facile electron transport. The wearable bracelet-supercapacitor based on Co, N-SnO2/ACF electrode delivers an energy density of 70.7 Wh Kg−1 under a wide potential window of 2.0 V. This finding provides a feasible route to regulate the band edge of metal oxide electrode for the promising energy storage.

The effect of strain and spatial Bi distribution on the band alignment of GaAsBi single quantum well structure

The band line-up and band offset calculations of GaAs0.978Bi0.022/GaAs single quantum well with spatial changes of Bi composition were reported. The spatial Bi profile and a certain amount of the Bi composition in the barrier layer were determined by HR-XRD measurements. Virtual Crystal Approximation and Valence Band Anti-Crossing models were used including strain effects to obtain conduction and valence band edge shifts with Bi incorporation. Photoluminescence (PL) measurements were performed at a low temperature of 8 K as a function of excitation intensity. The PL spectra have shown asymmetric line shapes, which were fitted with different Gaussian functions. Comparing experimental PL results with calculated band edge energies, it was found that optical transition is a type I under low intensity excitation while the optical transition is switched from type I to type II due to the spatial changes in Bi concentrations. The band offsetsΔEc/ΔEv were also determined.

Effects of structure and electronic properties of D-π-A organic dyes on photovoltaic performance of dye-sensitized solar cells

Abstract Herein, we examine the performance of dye-sensitized solar cells containing five D-π-A organic dyes designed by systematic modification of π-bridge size and geometric structure. Each dye has a simple push-pull structure with a triarylamino group as an electron donor, bithiophene-4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M11), 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene-thiophene (M12), thiophene-4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M13), 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene-benzene (M14), and 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M15) units as π-bridges, and cyanoacrylic acid as an electron acceptor/anchor. The extension of the π-bridge linkage favors wide-range absorption but, because of the concomitant molecular volume increase, hinders the efficient adsorption of dyes on the TiO2 film surface. Hence, higher loadings are achieved for smaller dye molecules, resulting in (i) a shift of the TiO2 conduction band edge to more negative values, (ii) a greater photocurrent, and (iii) suppressed charge recombination between the photoanode and the redox couple in the electrolyte. Consequently, under one-sun equivalent illumination (AM 1.5 G, 100 mW/cm2), the highest photovoltage, photocurrent, and conversion efficiency (η = 7.19%) are observed for M15, which has the smallest molecular volume among M series dyes.

Role of the surface density of states in understanding size-dependent surface band bending in GaN nanowires

In semiconductor nanostructures, surface states play a very important role in determining the surface potential or contact potential difference (CPD). We measure the CPD value of individual GaN nanowires (NWs) using Kelvin probe force microscopy (KPFM). The corresponding surface band bending (SBB) is calculated from the measured CPD value and is found to increase with a decrease in the NW size. In order to investigate the size dependence of the SBB, the electronic density of states of the surface atoms of the nanowires at different values of the diameter are calculated using the first-principle density functional theory (DFT). The calculated density of states of the nanowires revealed size-dependent occupancy and shift of conduction band edge. The relative position of the conduction band edge corroborates with size-dependent SBB values obtained from KPFM measurements.

Probing the effect of selenium substitution in kesterite-Cu2ZnSnS4 nanocrystals prepared by hot injection method

In this paper, we report the effect of sulfur (S) substitution with selenium (Se) in CZTS nanocrystals prepared by hot injection method. The formation of kesterite-copper zinc tin sulfide (Cu2ZnSnS4, CZTS) and copper zinc tin selenide (Cu2ZnSnSe4, CZTSe) nanocrystals is confirmed by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) analysis. The XRD, TEM and atomic force microscopy (AFM) analysis shows an overall increase in average crystallite size upon Se substitution. AFM images revealed an increase in root mean square surface roughness (Sq) and average surface roughness (Sa) when S in CZTS is replaced by Se. The substitution of S by Se in host CZTS narrows the optical band gap from 1.56 to 1.03 eV. The ultraviolet photoelectron spectroscopy (UPS) analysis shows shift in valence band and conduction band edge in CZTSe compared to CZTS. The photocurrent density measurement in synthesized CZTSe thin films is ~ 4 to 5 times higher than CZTS thin films. The obtained results show that CZTSe can be a promising candidate as absorber material in photovoltaic applications.

Theoretical design of porphyrin dyes with electron-deficit heterocycles towards near-IR light sensitization in dye-sensitized solar cells

We develop a series of porphyrin sensitizers with electron-deficit heterocycles based on the well-known YD2-o-C8 dye for application in dye-sensitized solar cells with the help of ab initio density functional theory calculations and quantum dynamics simulation based on the tight-binding extended Hückel Hamiltonian at the semiempirical level. The calculation results show that introduction of electron-deficit heterocycles into the porphyrin dyes can remarkably red-shift and broaden the Q band of the absorption spectrum and extend the coverage into near-infrared region, improving the light-harvesting ability. The key parameters influencing the photocurrent and photovoltage performance such as maximum short-circuit photocurrent (JSCmax), intramolecular charge transfer, interface electron injection, and the shift of TiO2 conduction band edge (ΔECB) are superior to YD2-o-C8 dye. Therefore, the designed sensitizers would be promising candidate for utilization in dye-sensitized solar cells.

Light Harvesting and Optical-Electronic Properties of Two Quercitin and Rutin Natural Dyes

The photovoltaic properties of two dyes (quercitin (Q) and rutin (R)) were experimentally investigated. The results showed that Q had excellent photoelectric properties with J s c of 5.480 mA·cm−2, V o c of 0.582 V, η of 2.151% larger than R with J s c of 1.826 mA·cm−2, V o c of 0.547 V, and η of 0.713%. For a better understanding of the photoelectric properties of two molecules and illustrating why the performances of Q is better than R from the micro-level, the UV-VIs spectrum, Fourier transforms infrared (FT-IR) spectrum, and cyclic voltage current characteristics were experimentally investigated. What is more, density functional theory (DFT) and time dependent density functional theory (TD-DFT) have been implemented in theoretical calculation. Based on the calculated results, frontier molecular orbitals (FMOs), charge differential density (CDD), infrared vibration, first hyperpolarizability, projected density orbital analysis (PDOS), electrostatic potential (ESP), and natural bond orbital (NBO) were analyzed. Hole/electron reorganization energies ( λ h / λ e ), light harvesting efficiency (LHE), fluorescent lifetime (τ), absorption peak, and the vertical dipole moment ( μ n o r m a l ) were calculated, and the shift of conduction band edge of a semiconductor (ΔECB) has been analyzed, which has a close relationship with J s c and V o c . The results demonstrated that, due to the higher LHE, τ, μ n o r m a l , and red-shifted absorption peak, Q has better photoelectric properties than R as a promising sensitizer.

Напряженно-деформируемое состояние гетероструктуры In1-xGaxAs/GaAs

There are no models in use that would take into account the influence of internal deformations in strained structures on the key parameters of heterojunctions depending on the concentration of layer's components and thickness. The aim of the present paper is to determine the dependence of elastic strains and deformations and define changes in zone diagram of In1-xGaxAs/GaA structure depending of the concentration of components and thicknesses of layer and substrate. The paper features calculation of strains in In1-xGaxAs/GaA heterostructure at different correlations between the thicknesses of contacting semiconductors. The calculated values of conduction band edge shift and valence-band splitting in substrate and layer allow the authors to define the dependence between the alteration of band gap width in presence of deformation produced by the change of the thicknesses of layer and substrate and concentration of components. The authors arrive at the definition, according to which the stress-strained state of In1-xGaxAs/GaA heterojunction leads to remarkable changes in its energy band structure. The paper includes equation of the dependence of main parameters of the energy band zone diagram of heterojunction on concentrations of components and proportion of thicknesses. The selection of correlations between substrate and layer thicknesses gives possibilities for a forecast and, further, management of parameters of the zone diagram and, consequently, of electronic properties of In1-xGaxAs/GaA heterojunctions. As a result of the research, the calculation formula of band gap width changes can be seen as a basis for analysis of changes being subject to alterations in concentration of components and correlation between layer and substrate thicknesses in elastically strained heterostructure.

Enhancement of solar cell performance through the formation of a surface dipole on polyacrylonitrile-treated TiO2 photoelectrodes

Polyacrylonitrile (PAN) was adsorbed onto the TiO2 surface, and the resulting photoelectrodes were applied in dye-sensitized solar cells (DSSCs). The DSSC with the PAN-modified TiO2 showed an increase in its short-circuit current (Jsc) and a decrease in its open-circuit voltage (Voc), resulting in a 17% enhancement of its photovoltaic performance when compared to the reference cell. By incorporating PAN onto the surface of the TiO2, a surface dipole was formed, which induced a conduction band edge shift of the TiO2 in a positive direction. This shift led to a considerable improvement in the electron injection efficiency, and hence the Jsc.

Performance Regulation of Thieno[3,2-b]benzothiophene π-Spacer-Based D-π-A Organic Dyes for Dye-Sensitized Solar Cell Applications: Insights From Computational Study.

Dye-sensitized solar cells (DSSCs) have been widely investigated; however, the development of promising dye sensitizers is still appealing. In this work, we perform a detailed theoretical search for high-efficiency D-π-A organic dyes using density functional theory and time-dependent density functional theory calculations. Specifically, we perform geometric optimization, and electronic structure and absorption spectra calculations for isolated dyes for two thieno[3,2-b]benzothiophene π-spacer-based D-π-A organic dyes SGT129 and SGT130, which show significant efficiency difference, before and after binding to a TiO2 semiconductor. The calculation results reveal that the coplanar configuration between the electron donor and the π-spacer can enhance electronic communication efficiently, thus facilitating intra-molecular charge transfer from the electron donor to the acceptor groups in SGT130. The absorption spectrum of SGT130 broadens and is red-shifted owing to the decreased bandgap. The higher light-harvesting efficiency, favorable intra-molecular charge transfer, larger shift of the conduction band edge in the TiO2 semiconductor, and slower charge recombination between the injected electrons in the TiO2 conduction band and the electrolyte explain the superior efficiency of SGT130 over that of SGT129. Using SGT130 as the reference dye, we further design four novel dyes 1–4 by modifying the π-spacer with electron-rich and electron-withdrawing moieties. Judging from the theoretical parameters influencing the short-circuit current and open-circuit voltage, we found that all dyes would perform better than SGT130 in terms of the favorable interfacial charge transfer (ICT) and light-harvesting efficiency, as well as the larger shift of the TiO2 conduction band edge. Our theoretical research is expected to provide valuable insights into the molecular modification of TBT-based D-π-A organic dyes for DSSC applications.

Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns.

The moiré pattern formed between a two-dimensional (2D) material and the substrate has played a crucial role in tuning the electronic structure of the 2D material. Here, by using scanning tunneling microscopy and spectroscopy, we found a moiré-pattern-dependent band gap and work function modulation in hexagonal boron nitride (hBN)/Cu(111) heterostructures, whose amplitudes increase with the moiré pattern wavelength. Moreover, the work function modulation shifts agree well with the conduction band edge shifts, indicating a spatially constant electron affinity for the hBN layer. Density functional theory calculations showed that these observations in hBN/Cu(111) heterostructures mainly originated from the hybridization of the N 3p z orbital and Cu 4s orbital in different atomic configurations. Our results show that the twist-angle dependence of moiré patterns in hBN/Cu(111) heterostructures can be used to tailor the electronic properties including band gap and work function.

Control of open circuit voltage by base treatment of photoelectrode surface in solar cells

ABSTRACTWe investigated effects of conjugate bases as surface modifiers on the photovoltage of dye-sensitized solar cells (DSSCs). TiO2 films were soaked in aqueous solutions containing PO43−, CO32− and C2O42− ions to prepare Ph-TiO2/FTO, Ca-TiO2/FTO and Ox-TiO2/FTO electrodes, respectively. For a reference cell, Wa-TiO2/FTO electrode treated with pure water (H2O) was also prepared. With increasing basicity of the surface modifiers, open circuit voltage (Voc) of DSSCs with the surface-modified photoelectrodes was improved in the order of Ph-TiO2/FTO ≈ Ca-TiO2/FTO > Ox-TiO2/FTO > Wa-TiO2/FTO. Dark current measurements revealed that the enhancements in Voc were attributed to a negative shift of conduction band edge of TiO2 by base treatment such as PO43−, CO32− and C2O42−.

Revised Analysis of Design Options and Minimum Subthreshold Swing in Piezoelectric FinFETs

The theoretical limit of sub-threshold swing ( SS ) in the n-type piezoelectric FinFET (Piezo-FinFET) is thoroughly investigated. Gate voltage modulated strain, conduction band edge shift, and the SS are evaluated utilizing physics-based modeling and numerical simulation, indicating a strong dependence of SS on the fin width, crystalline orientation, and physical properties of the piezo-material used in the gate stack. For [001] Si and [111] Ge Piezo-FinFETs with 5-nm fin width and 3-nm PZT-5H thickness, SS values down to 40 mV/decade are achievable after design optimizations. A figure of merit, ${E}_{\text{cr}} {d}_{z3}/{S}_{\text{pie}, 33}$ , is proposed for the piezo-materials in terms of the critical electric field ( ${E}_\text{cr}$ ), the piezoelectric strain constant ( ${d}_{z3}$ ), and the elastic compliance constant ( ${S}_{\text{pie}, 33}$ ), to guide the Piezo-FinFET designs.

Surface NH2-rich nanoparticles: Solidifying ionic-liquid electrolytes and improving the performance of dye-sensitized solar cells

The surface properties of nanoparticles have a significant influence on the properties of the gel electrolytes. Herein, the surface NH2-rich nanoparticle (A-SiO2), with a tightening network, is synthesized by silanizing SiO2 nanoparticles with pre-polymerized aminopropyltriethoxysilane, which is further employed to prepare ionic-liquid gel electrolytes for dye-sensitized solar cells. The addition of a small amount of A-SiO2 can effectively solidify the ionic-liquid, whereas a large number of NH2 groups on the SiO2 surface leads to a large negative shift of the TiO2 conduction band edge, and can react with I3− in the form of a Lewis complex, resulting in an increase in the concentration of I− and a decrease in the concentration of I3− in the electrolyte. In addition, the ionic-liquid gel electrolyte possesses thixotropic behavior, which allows it to easily penetrate into the inner part of the TiO2 mesoporous film. As a result, large improvements of the photovoltage from 695 mV to 785 mV and of the photocurrent from 13.3 mA cm−2 to 14.9 mA cm−2 are achieved. This leads to significant enhancement of the power conversion efficiency, from 6.2% to 8.1%, for the cell with A-SiO2 compared to that of the pristine ionic-liquid electrolyte.

Molecular Control of the Band Edge Movement and the Recombination Process in Donor–Acceptor Hemicyanine-Sensitized Solar Cells

The presence of downward shift in the band edge and the recombination reactions in the hemicyanine-sensitized solar cell reduces the open-circuit potential (VOC) and the short-circuit current density (JSC), which in turn decreases the dye cell performance. Choosing either an electrolyte possessing minimum overpotentials or a systematic dye design which can efficiently suppress the diffusion of charged species toward the TiO2 can improve the overall power conversion efficiency (PCE). Here, a series of donor–acceptor (D-A) hemicyanine dyes were synthesized utilizing a planar heterotriangulene (HT) or triphenylamine (TPA) donor and alkyl-functionalized indolium carboxylic acid acceptor unit. By introducing strong HT donor instead of TPA, the photophysical, and electrochemical properties of D-A dyes are significantly modulated. The strong donor nature of HT and effective passivation of surface by hydrophobic alkyl chains close to the anchoring group for NC3 dye exhibits an average PCE of 4.34% with a VOC of 0.416 V, JSC of 20.04 mA cm–2, and fill factor (ff) of 52.03% under simulated AM 1.5G illumination (100 mW cm–2) without 3α,7α-dihydroxy-5β-cholic acid coadsorbent (CDCA). The intrinsic dipole of the hemicyanine dye and the presence of Li+ ions in iodide/triiodide redox couple without tert-butylpyridine (TBP) additive induces a downward shift in conduction band edge (ECB) of TiO2. By rational molecular design, the extend of shift in ECB is controlled and enhanced the VOC. Electrochemical impedance spectroscopy (EIS) studies revealed the high charge transfer resistance (Rct) and long lifetime (τ) of injected electrons in HT-based dyes than that of TPA derivatives, which provide insight into the passivation of Li+ and I– ions by current D-A dye design possessing alkyl functionalities to increase both the JSC and VOC.