Fermi Level Shift Research Articles

Correlation of the Graphene Fermi-Level Shift and the Enhanced Electrochemical Performance of Graphene-Manganese Phosphate for Hybrid Supercapacitors: Raman Spectroscopy Analysis.

A high structurally stable graphene-manganese phosphate (graphene-Mn3P2O8) composite with excellent cycling stability was prepared by the facile hydrothermal method. The correlation between the high electrochemical performance of graphene-Mn3P2O8 composite and the graphene Fermi-level shift was investigated using Raman spectroscopy by monitoring the disorder in the sp2 network of the composite graphene arising from the heterogeneous atoms doping during hydrothermal synthesis. The response of the Raman signatures of graphene to the chemical doping effect correlated to the electronic band structure in the vicinity of the Dirac point showed an upshift in the graphene Fermi level with an average value of about 190 meV, indicating that the composite graphene is n-type-doped. This was confirmed by X-ray photoelectron spectroscopy data, which showed a relatively high concentration of electron-donating heterogeneous atoms in the composite graphene. The electrochemical analysis confirmed that the n-type-doped composite graphene has enhanced the electrical conductivity of the Mn3P2O8 electrode and decreased the potential barriers between the electrode surface and electrolyte highest occupied molecular orbital (HOMO) for enhanced interfacial charge transfer between the electrode surface and the electrolyte; hence, the graphene-Mn3P2O8 composite electrode exhibited a high specific capacity of 38.4 mA h g-1 compared to the pristine Mn3P2O8 electrode (7.2 mA h g-1). Due to its excellent cycling stability (∼100% capacity retention over 5000 charge-discharge cycles at 5 A g-1), graphene-Mn3P2O8 composite is a promising electrode material for hybrid supercapacitors.

Fermiology of two-dimensional titanium carbide and nitride MXenes

MXenes are a family two-dimensional transition-metal carbide and nitride materials, which often exhibit very good metallic conductivity and are thus of great interest for applications in, e.g., flexible electronics, electrocatalysis, and electromagnetic interference shielding. However, surprisingly little is known about the fermiology of MXenes, i.e., the shape and size of their Fermi surfaces, and its effect on the material properties. One reason for this may be that MXene surfaces are almost always covered by a mixture of functional groups, and studying Fermi surfaces of disordered systems is cumbersome. Here, we study fermiology of four common Ti-based MXenes as a function of the surface functional group composition. We first calculate the effective band structures of systems with explicit mixed surfaces and observe gradual evolution in the filling of the Ti-d band and resulting shift of Fermi level. We then demonstrate that these band structures can be closely approximated by using pseudohydrogenated surfaces, and also compare favorably to the experimental angle-resolved photoemission spectroscopy results. By modifying the pseudohydrogen charge we then proceed to plot Fermi surfaces for all systems and extract their properties, such as the Fermi-surface area and average Fermi velocity. These are in turn used to evaluate the electrical conductivity with the relaxation time fitted to experimentally measured conductivities.

Tuned Fermi Level and Improved Thermoelectric Performance in Bi2Te3 Nanocomposites Reinforced with g-C3N4 Nanosheets

Bi2Te3 has been recognized as an efficient and scalable thermoelectric material suitable for sustainable power generation and environment friendly cooling. Nano structuring and adding nano inclusions in the bulk thermoelectric material offers cost effective and scalable strategy to attain reduced thermal conductivity and improved figure of merit. Here, we report inclusion of g-C3N4 nanosheets in Bi2Te3 to tune the thermoelectric properties of the matrix. The g-C3N4/Bi2Te3 nanocomposites were synthesized through solid state reaction route, ball milling and hot pressing techniques. The fermi level shift is evaluated for the prepared samples by using Fritzsche model. The shift in fermi level away from the conduction band edge explains observed enhancement in seebeck coefficient. The incorporated nanosheets effectively scatters phonons leading to diminished thermal conductivity and improved ZT. ZT peaks to 0.34 at 448 K for the sample BT1CN with 1 wt.% of g-C3N4.

Relationship between statistical shift of the Fermi level and activation energy of conductivity in phase-change materials

The non-Arrhenius behaviour of conductivity of phase-change materials is analysed in terms of the statistical shift of Fermi level. To show this connection, a simplified model for the density of states in a phase-change material is proposed to simulate the temperature dependence of the Fermi level. Comparison of the temperature dependence of conductivity of Ge2Sb2Te5 with the simulation results allowed estimating the position of defect levels in the bandgap.

Tuning the electronic properties of Stone-Wales defected graphene using uniaxial longitudinal and horizontal strain: A first-principles study

We investigated the influence of horizontal and longitudinal strain on the electronic proprieties of Stone-Wales defected honeycomb graphene structure using the density functional theory. Tensile and compressive uniaxial strains were independently applied on the armchair (horizontal) and the zigzag (longitudinal) directions of a defected honeycomb graphene monolayer. We found the responses of the band gap are dependent on the applied strain direction. Likewise, the induced band gap openings show evident correlations between Stone-Wales defect orientations and the applied strain directions. Our results show that by choosing the strain values and directions the band gap values can be precisely achieved. Additionally, the effects of the applied strain were calculated and analyzed for the Fermi level, work function and the charge distributions. For both the defect-free and the defected samples, Fermi level shifts up in response to the compressive strain while it shifts down in response to the tensile strain. Our study shows the electronic properties are significantly affected by the alignment between the Stone-Wales C-C middle bond orientation and the applied stain direction. Finally, we hope providing a precise tuning for the electronic properties of defected graphene will allow for future nanodevice applications.

Tuning fermi level and band gap in Li4Ti5O12 by doping and vacancy for ultrafast Li+ insertion/extraction

AbstractLi4Ti5O12 (LTO) attracts great interest due to the “zero strain” during cycles but the poor electronic and ionic conductivity critically impede the practical application. Herein, we report a synergy strategy of tuning localized electrons to shift Fermi level and band gap by Mg/Zr co‐doping and oxygen vacancy incorporation, which significantly improves Li+ and electronic transport. More importantly, the intrinsic synergistic mechanism has been revealed by neutron diffraction, X‐ray absorption spectra, and first‐principles calculations. The “elastic effect” of lattice induced by Mg/Zr co‐doping allows LTO to accommodate more oxygen vacancies to a certain degree without a severe lattice distortion, which largely improves the electronic conductivity. Mg/Zr co‐doping and oxygen vacancy incorporation effectively enhanced the dynamic characteristics of LTO electrode, achieving the excellent rate performance (90 mAh/g at 20C) and cycle stability (96.9% after 500 cycles at 10C). First‐principles calculations confirm Fermi level shifts to the conduction band, and the band gap becomes narrowed due to the synergistic modulation, and the intrinsic mechanism of the enhanced electronic and Li‐ion conductivity is clarified. This study offers some insights into achieving the fast Li+ insertion/extraction by tuning the crystal and electronic structure with lattice doping and oxygen vacancy engineering.

Electrostatics of Single Monolayer Graphene Coated Metal Electrode in Electrolyte

AbstractUnlike metals, graphene forms a hydration layer at the electrode/electrolyte interface, which is unusual for a conducting material. Here, the electrostatic properties of the hydration layer are studied by measuring the oscillation of ions at the interface due to an applied AC potential by differential reflectivity during cyclic voltammetry (CV). The amplitude of ion oscillation at picometer range sensitivity on a partially coated Au electrode by a graphene monolayer reveals a remarkable difference between the coated and uncoated part of the electrode. The uncoated Au electrode exhibits the expected behavior, but the oscillation of ions is completely screened over the graphene coated electrode up to 0.2 M of NaCl. The “insulating” behavior is consistent with CV and DPV. The behavior attributed to the hydration layer could be reversibly disrupted to de‐screen the electric field to study the electrostatics of graphene/Au interface. The shift in Fermi level of hydration‐layer‐free graphene/Au relative to Au is quantitatively consistent with work function transparency of graphene. The cut‐off of the emanating field from the underlying electrode in an electrolyte is another example of graphene's unique passivation property that will have profound effect on applications such as, supercapacitors where the charge state of interface is critical.

Adsorption of nitrogen at AlN(000-1) surface – Decisive role of structural and electronic factors

Adsorption of atomic and molecular nitrogen at AlN(000–1) surface was investigated by ab initio calculations and thermodynamic analysis. According to earlier works{Kempisty et al. Appl. Surf. Sci.2020, 532, 147,719} in equilibrium with Al vapor, the AlN(000–1) surface is thermodynamically stable in two states: low Al coverage θAl ≤ 1/3 ML and high Al coverageθAl ≅ 1 ML. In these two cases nitrogen adsorption is completely different. At low Al-covered surface the nitrogen atom is strongly bounded to N surface atom, creates the N2 admolecule that is finally detached leaving surface vacancy VN(s). This reaction chain energy gain is positive, ΔEDFTdet(N−N2)=3.50eV. Therefore, the atomic nitrogen present in plasma assisted molecular beam epitaxy (PA-MBE) fluxes induces the surface decay. N2 is adsorbed molecularly at the bare surface with the coverage independent energy gain about 1 eV. At the fully Al-covered surface atomic nitrogen is adsorbed in T4 sites with no barrier and large energy gain ΔEDFTads−Al(N)=8.68eV. Molecular nitrogen dissociates with the energy gain, dependent on additional N coverage: ΔEDFTads−Al(N2)=7.65eV at low and ΔEDFTads−Al(N2)=2.77eV at high, respectively. This change is related to the reduction of electron transfer contribution, caused by Fermi level shift down due to electron transfer from Al to N surface states. The thermodynamic analysis shows incomplete N coverage above the adsorbed Al layer due to the above adsorption energy reduction effect. The resulting incomplete N coverage is responsible for creation of nitrogen vacancies during AlN physical vapor transport (PVT) growth and their coalescence into Al-rich inclusions.

The sensitive tunability of superconducting critical temperature in high-buckled plumbene by shifting Fermi level

As the newly member added to the graphene family, the high-buckled (HB) plumbene is a metallic two-dimensional(2D) material with the electronic structure characterized by the existence of some band edges in close proximity to the Fermi level. Hence in such a 2D material the electronic density of states (DOS) presents sharp peaks around the Fermi level. In this work, we perform first-principles calculations of the conventional superconducting critical temperature (Tc) of HB plumbene. We find that the electron-phonon (e-ph) coupling strength which measures the electron mass enhancement is proportional to the DOS at the Fermi level. As a result, Tc of HB plumbene has a sizable increase (from 5.29 K to 17.21 K) when the Fermi level is shifted to the nearby DOS peaks from its intrinsic position. For 2D materials, the adjustment of Fermi level can be readily realized by applying a gate voltage in a device structure of electronic transport, instead of a chemical carrier doping. Therefore, our study proposes a feasible way to tune Tc of those 2D metallic materials such as HB plumbene which have some band edges or flat band around Fermi level.

(Invited) Light Emission Modulation from Individual Single-Walled Carbon Nanotubes By Chromophore Encapsulation

The one-dimensional structure of single-walled carbon nanotubes (NT) display optical absorption and near-infrared emission (thanks to van Hove singularities). Chromophore encapsulation into host single-walled carbon nanotubes allows to create hybrid nano-systems with tunable opto-electronic properties. Up to now, we have been confining different kinds of chromophores,1-4 absorbing from the blue/ green (400/500 nm) range (tetracyanoquinodimethane (TNCQ), quaterthiophene derivatives (4T) and tetramethyl-paraphenylenediamine (TMPD)) to the red (700 nm) range (phthalocyanine (MPc)). In addition then can be either electron donor (4T, TMPD) or acceptor (TNCQ).In this study, we investigate, at both the macroscopic and the individual scales, the electronic and the optical properties of our hybrid systems by means of Raman and photoluminescence spectroscopies.Photoluminescence experiments clearly demonstrate changes on the emission properties after encapsulation. The intensities can be increased or reduced depending on the nature of the confined chromophores (electron donor or acceptor) and on the NT diameter.From Raman measurements, a significant charge transfer from the confined dye to the nanotube is evidenced. The main relevant parameters that govern the charge transfer are the nanotube diameter and the nature of the chromophores (electron donor or acceptor).Therefore, Raman and photoluminescence experiments strongly suggest charge transfer between the confined molecules and the nanotubes, leading to a Fermi level shift which governs the radiative de-excitation efficiency.

Graphene–polyimide-integrated metasurface for ultrasensitive modulation of higher-order terahertz fano resonances at the Dirac point

The initial Fermi level of graphene is quite close to the Dirac point. It is ultrasensitive to external stimuli in the modulation of Fano resonance. Thus, the Fermi level directly skips the Dirac point during modulation with strong external excitation. Therefore, the modulation behavior of the Fano resonance is not easy to capture at the Dirac point. Here, we applied a laser pump with micropower densities and a mains with weak voltages to the terahertz metasurface and observed the modulation behavior of higher-order Fano resonance at the Dirac point. Accompanied by Fermi level shifts between the Dirac point, conduction band, and valence band, higher-order Fano resonances exhibited the process of disappearance, appearance, and re-disappearance. Concomitantly, the transmission amplitude underwent an increasing stage and a decreasing stage. More importantly, a 0.05 V change in the bias voltage or a 7.5 mW/cm2 change in the power density enabled a modulation depth of 90%. These findings could be potentially used for designing ultrasensitive terahertz metaphotonic devices.

Theoretical investigation of quantum capacitance in the functionalized MoS2-monolayer

In this work, we investigated the electronic structure and the quantum capacitance of a set of functionalized MoS2 monolayers. The functionalizations have been done by using different ad-atom adsorption on MoS 2 monolayer. Density functional theory calculations are performed to obtain an accurate electronic structure of ad-atom doped MoS2 monolayer with a varying degree of doping concentration. Subsequently, the quantum capacitance in each functionalized system was estimated. A marked quantum capacitance above 200 μF cm−2 has been observed. Our calculations show that the quantum capacitance of MoS2 monolayer is significantly enhanced with substitutional doping of Mo with transition metal ad-atoms. The microscopic origin of such enhancement in quantum capacitance in this system has been analyzed. Our DFT-based calculation reveals that the generation of new electronic states at the proximity of the band-edge and the shift of Fermi level caused by the ad-atom adsorption results in a very high quantum capacitance in the system.

First-principles investigation of the electronic and thermoelectric properties of SiGe doped with Sn and one percent B

Silicon-Germanium (SiGe) has recently attracted much interest as a room temperature thermoelectric (TE) material for converting heat into electrical output power. With the advantage of silicon being non-toxic, cost-effective, and abundant on earth, a silicon base TE material has a promising future. A first-principle calculation based on the fully self-consistent Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) to treat several forms of chemical disorders of SiGe by Sn-doping was carried out. In SiGe1-xSnx, as the Sn content increases the Fermi level shifts to the conduction band edge. Similarly, in Si1-xSnxGe, a high Sn content (x = 0.4 to 0.9) results in the Fermi level shifting to the conduction band edge. On the contrary, a low amount of Sn content (x = 0.1 to 0.3) causes the Fermi level to fluctuate between the conduction band and the valence band states. With the addition of 1% Boron impurity to the alloys Si1-xSnxGe and SiGe1-xSnx, the number of carriers (electron and hole) states was enhanced by 0.05 states/eV. This makes the alloys Si0.3Sn0.69B0.01Ge and SiGe0.4Sn0.59B0.01 promising for application as n-type electrodes in a thermoelectric generator (TEG).

A first-principles study on electronic and optical properties for Zn31-xAlxMgO

The photoelectric properties of Mg/Al doped ZnO are investigated using the first-principles study. The lattice parameters of Zn31-xAlxMgO clearly increased. The calculations are carried out by the constant Mg doping ratio as 3.13% and different concentrations of Al with 0, 3.13%, 6.25%, 9.38% and 12.5%. The corresponding bandgaps are 3.492, 2.994, 2.448, 2.102, 1.988 eV, respectively. It is revealed that the Fermi level shifts upward when the Al concentration exceeds 6.25%. Therefore, Zn27Al4MgO is beneficial to obtain n-type ZnO with a higher electron concentration than others. Meanwhile, compared with pure ZnO, the imaginary parts of complex dielectric function, absorption, reflectivity and energy loss function of Zn31-xAlxMgO (x > 0) exhibit clear red-shifts.

Investigation of Defect‐Tolerant Perovskite Solar Cells with Long‐Term Stability via Controlling the Self‐Doping Effect

AbstractAlthough there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo‐degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo‐aging mechanism due to intrinsic defects is investigated in nitrogen‐filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self‐doping effect, have been investigated. Opto‐electronic investigations and depth profiles of the elemental constituents show that after photo‐aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb‐rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep‐traps in the perovskite leads to irreversible degradation. Thin‐film solar cells that are relatively Pb‐deficient (FA‐rich) exhibit improved long‐term stability, retaining over 90% of their initial PCE during 500 h of continuous 1‐Sun illumination. This study suggests passivation of the Pb‐I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long‐term stability.

A computational study on the novel defects of graphene quantum dot as a promising anode material for sodium ion battery

The existence of defects in the graphene quantum dots can alter their electronic properties. In this study, the interaction of sodium ion/atom with defected graphene quantum dots (dGQDs) of F3.Gr, F3H3.Gr, N3.Gr and N3H3.Gr as the anode material for the sodium ion batteries was investigated using density functional theory calculations. The amount of charge transfer from Na atom to F3H3.Gr and from F3.Gr to Na ion is about 0.23 and 0.94|e|, respectively. In the complex of Na atom with dGQDs as an n-type-adsorbed dGQD, the Fermi level shifts towards the higher energy and it can be facilitated charge transfer from Na atom to dGQDs. The estimated electrical conductivity for the complexes of Na atom with F3.Gr and F3H3.Gr is 6.51 × 10−13 and 5.26 × 10−12, respectively. This can result in a favorable condition in the processes of battery charge and discharge. The cell voltage belonging to F- doped defects or N- doped defects is nearly 1.54–1.58 V or 0.58–0.61 V. It is concluded that the F- doped defects are more promising anode material for sodium ion batteries compared to N- doped defects because of their lower recovery time of adsorption, higher voltage, higher reactivity and better charge transfer.

Thermomechanical Manipulation of Electric Transport in MoTe2

AbstractLayered semimetals such as monoclinic MoTe2 and WTe2 demonstrate superconducting, topological insulating, and Weyl semimetallic states based on their unique electronic band topology. While doping concentration, lattice constants, and spin–orbit coupling can largely modulate the quantum states of the semimetals, a puzzling issue is that their functional carrier density and magnetoresistance for practical applications critically vary by temperature, which cannot be explained by the conventional phonon effect or a structural phase transition. Here, a native doping‐mediated thermomechanical manipulation of electric transport in semimetallic MoTe2 is reported, where effective transport is controlled by temperature in an equivalent manner to electric gating. Combining X‐ray diffraction, scanning tunneling microscopy, transport measurements, and first‐principles calculations, a Fermi level shift and subsequent changes in electronic structures are revealed as the origins of the practical transport changes in MoTe2. Moreover, the initial doping state of the MoTe2, determined by the Te vacancy density in two different growth methods, reciprocally affects the thermomechanical lattice and band structure changes, which is promising for novel electronic applications such as magnetic sensors and memory devices with layered semimetals.

Influence of electric and magnetic fields andσ-edge bands on the electronic and optical spectra of graphene nanoribbons

The unusual electronic and optical properties of armchair and zigzag graphene nanoribbons (GNRs) subject to in-plane transverse electric and perpendicular magnetic fields have been systematically investigated. Our calculations were carried out within the generalized multi-orbital tight-binding model based on a Hamiltonian which takes into account hopping integrals among the (s, $p_x$, $p_y$, $p_z$) atomic orbitals as well as the external electric and magnetic fields. The electronic structure consists of $\pi$ bands arising from the $p_z$ orbital and $\sigma$ bands originating from the (s, $p_x$, $p_y$) orbitals. The energy bands and optical spectra are diversified by both the nature of the edge of the nanoribbon and strength of the external fields. Armchair GNRs display a width-dependent energy gap in addition to low-energy $\sigma$ bands while the zigzag system has the unfilled flat band with $\pi$ edge states at zero energy and partially filled wide-range $\sigma$ bands. An applied in-plane electric field leads to the splitting of energy bands and shifted Fermi level, thereby enriching the inter-band and intra-band optical conductivities. The interplay between an external magnetic field and the edge geometry gives rise to extraordinary quantized Landau levels and special optical spectra.

Influence of concentration of anthocyanins on electron transport in dye sensitized solar cells

The influence of concentration of anthocyanins in dye sensitized solar cells (DSSC) has been investigated, with focus on how concentration influence electron transport. The influence on electron transport was then linked to solar cell performance. Anthocyanins were extracted from fresh flowers of Acanthus pubscenes using methanol acidified with 0.5% trifluoracetic acid, concentrated using a rotary evaporator and partitioned against ethyl acetate. Concentration of the anthocyanins was determined using Keracyanin Chloride as a standard. DSSC were fabricated using Titanium dioxide as anode, anthocyanins as sensitizers and Platinum as counter electrode material. Titanium dioxide was deposited on Fluorine doped Tin oxide glass substrate using slot coating method. Platinum was deposited on FTO glass substrate using a brush previously dipped in plastisol precursor, and annealed at 4500C for 20 min to activate Platinum. Dye sensitized solar cells were assembled using anthocyanins at varying concentrations. Performance parameters of the solar cells were measured using a solar simulator which was fitted with digital source meter. Electron transport parameters were studied using electrochemical impedance spectroscopy (EIS). Open circuit voltage, short circuit current and fill factor were observed to increase with concentration of anthocyanins. The increase in solar cell performance was attributed to increase in charge density which led more charges being available for transported to solar cell contacts. The increased charge resulted in a negative shift in Fermi level of electrons in the conduction band of TiO2. The shift in Fermi level resulted into an increase in open circuit voltage and the overall solar cell performance. EIS studies revealed increase in recombination resistance with concentration of anthocyanins. The increase in recombination resistance was found to be related to increase in electron density, and hence the shift in the Fermi level of electrons in the conduction band of TiO2.

Suppression of non-radiative recombination to improve performance of colloidal quantum-dot LEDs with a Cs2CO3 solution treatment

We report a five-fold luminance increase of green-light-emitting CdSe@ZnS quantum-dot LEDs (QLEDs) in response to treatment with a 2-ethoxyethanol solution of cesium carbonate (Cs2CO3). The maximum luminous yield of Cs2CO3-treated QLED is as high as 3.41 cd A−1 at 6.4 V. To elucidate device-performance improvement, we model measured currents as the sum of radiative and non-radiative recombination components, which are respectively represented by modified Shockley equations. Variations in model parameters show that a shift in Fermi level, reduction of barrier heights, and passivation of mid-gap defect states are the main results of Cs2CO3 treatment. In spite of a large luminance difference, light-extraction efficiency remains the same at 9% regardless of Cs2CO3 treatment because of the similarity in optical structures.