Deformation Potential Approach Research Articles

B2N2O2: A wide-bandgap two-dimensional semiconductor featuring a zigzag nitrogen array of bonds

In this work, we apply a combination of theoretical techniques to characterize a two-dimensional material with formula B2N2O2, featuring a zigzag array of nitrogen atoms. We predict its energetic, thermal, and dynamic stability and determine its electronic properties, including band structure and mobility evaluation for a phonon-mediated mechanism. We show that the compound is a wideband-gap semiconductor, with parabolic band edges and with large electron and hole mobilities within the deformation potential approach. We ascribe this result to the existence of electronic channels defined by the zigzag array of nitrogen bonds, which define the edges of both conduction and valence bands. We also propose a mechanism to synthesize the compound based on oxygen functionalization and application of pressure. Finally, we show that the results can be generalized to represent a family of 2D compounds.

Strain dependence of the thermoelectric properties of perovskite MgSiO3: A first principles study

Using first-principles density functional theory based approach, we investigate the tunability of the thermoelectric properties of perovskite MgSiO3. We find that the thermoelectric properties are strongly correlated with the electronic structure and can, therefore, be modulated with uni-axial tensile and compressive strains. Key thermoelectric parameters, such as the Seebeck coefficient (S), show non-monotonic behavior, displaying large shifts with the application of 1% uni-axial tensile strain. The relevance of this small strain value also appears in structural distortion of the oxygen octahedra and band convergence in the electronic band structure. The band structure modifications also introduce a strong strain dependence of the relaxation time, as defined within the deformation potential approach. As a result, an improvement of up to 35% is observed in zT with the application of 1% tensile strain. Our results highlight a possible new route to improving thermoelectric performance in perovskite oxide thermoelectrics.

Ultrahigh on/off-current ratio γ-graphyne-1 nanotube-based sub-10-nm TFET modeling and simulation

The use of γ-graphyne-1 nanotubes (GyNTs) in tunneling field effect transistors (TFETs) suppresses ambipolarity and enhances the subthreshold swing (\(\mathrm{SS}\)) of TFETs, due to the large energy band gap and high electron effective mass of GyNTs. In this research, the analysis of the structural, electronic and thermoelectric properties of the γ-graphyne-1 family under the deformation potential (DP) approach reveals that the electron–phonon mean free path (MFP) of an armchair GyNT (3AGyNT) and zigzag GyNT (2ZGyNT) are \(24\) and \(279\) nm, respectively. Therefore, ballistic transport of sub-10-nm 3AGyNT-TFETs and 2ZGyNT-TFETs in different channel lengths is investigated utilizing the non-equilibrium Green’s function (NEGF) formalism in the DFTB platform. An ultrahigh \(\mathrm{on}/\mathrm{off}\)-current ratio (\(\mathrm{OOCR}\)) value of 1.6 × 1010 at \(V_{DD} = 0\). \(2 \;{\text{V}}\) and very low point \({\text{SS}}\) of \(5 \;{\text{mV}}/{\text{dec}}\) were demonstrated by the 3AGyNT-TFET with a channel length of 9.6 nm. 2ZGyNT-TFETs show higher on-state current and \({\text{SS}}\) and lower \({\text{OOCR}}\) than those of 3AGyNT-TFETs. A linear relationship was found between channel length and logarithmic off-state current that is consistent with the WKB approximation. The obtained results along with the ultralow power consumption of the proposed GyNT-TFETs make them candidates to replace digital silicon MOSFETs in next-generation nanoelectronic devices.

An optimum thermoelectric figure of merit using Ge2Se2 monolayer: An ab-initio approach

It is found that many two-dimensional chalcogenides are exhibiting a superlative thermoelectric behavior because of their versatile characters in terms of stability, conductivity, bandgap tunability, etc. In this paper, the thermoelectric properties of monolayer Ge2Se2 have been calculated using first-principles calculations with all involved electrical and thermal transport properties in the parameter-free framework. It has been found that the Ge2Se2 offers a bandgap of 1.13 eV with a sufficiently high thermoelectric coefficient. The thermoelectric transport parameters have been calculated by solving the linearized Boltzmann Transport Equation and found optimum to qualify for good thermoelectric materials. The investigated material exhibit outstanding thermoelectric performance in terms of an ultra low lattice thermal conductivity of the order of 0.5 Wm−1K−1 and TE Figure of merit around two (2) at 1000 K. For better accuracy in results, we employed hybrid exchange–correlation (HSE06) throughout the calculations and, instead of approximating the relaxation time, computed the accurate relaxation time using the deformation potential approach. The research findings prove its excellent behavior and worthy guideline for the experimentalists looking for materials with high FoM to design thermoelectric devices.

Photoconductivity calculations of bilayer graphene from first principles and deformation-potential approach

We report first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon $\Gamma$ point, electrons with energies less than 200 meV are scattered predominantly by LA$^\prime$ and TA$^\prime$ modes while higher-energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters into K-point phonon optical modes due to their strong coupling with electrons and their high energies. A Drude model is used to calculate photoconductivity for bilayer graphene with different doping levels. Good agreement with prior experimental trends for both the real and imaginary components of photoconductivity confirms the model's applicability. The effects of doping levels and electron-phonon scattering on photoconductiviy are analyzed. We also extract acoustic and optical deformation potentials from average scattering rates obtained from density functional theory (DFT) calculations and compare associated photoconductivity calculations with DFT results. The comparison indicates that momentum-dependent electron-phonon scattering potentials are required to provide accurate predictions.

First principles study of electronic structure and thermoelectric transport in tin selenide and phase separated tin selenide–copper selenide alloy

The electronic structure and thermoelectric transport in SnSe and its alloy with Cu2Se have been studied using the first principles technique and semi classical Boltzmann transport theory. Our study reveals that SnSe is p-type with indirect band gap of 0.66 eV, while the alloy is phase separated and n-type with negligible indirect band gap of 0.064 eV. In both cases, two fold degeneracy in band extrema have been observed within the range of 25 meV. Delocalization of Se lone pair has been observed due to Cu substitution in Sn sites, which is supposed to lower its lattice thermal conductivity. A chemical potential map has been generated obeying thermodynamic restrictions to predict the possible existence of secondary phases. Our study shows the existence of SnSe2 as a secondary phase, while the possibility of Cu2Se as a secondary phase is negligible due to its higher formation energy. We calculated the transport coefficients as a function of carrier concentration and temperature to understand the range of optimized thermoelectric performance. The transport coefficients are similar along in plane direction whereas significant deviation is observed along the cross plane direction due to anisotropy in effective masses in SnSe. The effective masses are more isotropic in alloy than SnSe, thus transport properties show less anisotropy along three directions. Significant contribution of bipolar transport is observed in SnSe, while that is not noticed in the alloy. The behaviors of the Seebeck coefficients in both cases are discussed in terms of Mott’s theory and density of states modification near Fermi energy. Electron mobilities limited by acoustic phonon, ionized impurities, alloy scattering and inter carrier scattering have been examined relying on deformation potential approach and effective mass theory. The results indicate that acoustic phonon scattering is dominant scattering mechanism in SnSe over inter carrier scattering, whereas for the alloy the former contribute very weakly. Ionized impurity scattering and inter carrier scattering are most dominant in the alloy. Alloy scattering with U = 2 eV also contribute significantly.

Impact of grain boundary characteristics on thermal transport in polycrystalline graphene: Analytical results

The effect of grain boundary (GB) structure, size, and shape on the thermal conductivity of polycrystalline graphene is studied in the framework of the deformation potential approach. Precise analytical expressions for the phonon mean free path (MFP) are obtained within the Born approximation. We found exactly two types of behavior in the long-wavelength limit: MFP varies as ω−1 for open GBs of any shape, while it behaves as ω−3 for closed configurations (loops). In the short-wavelength limit, MFP tends to a constant value for any configuration. Oscillatory behavior is observed for all GBs, which indicates that they serve as diffraction grating for phonons. This property is also inherent in GBs with irregularities caused by partial disclination dipoles. The thermal conductivity is calculated in the framework of Callaway’s approach, with all main sources of phonon scattering taken into account. Reduction of the heat conductivity with decreasing grain size is obtained in a wide temperature range. What is most interesting is that we found a marked decrease in the thermal conductivity of polycrystalline graphene containing GBs with changes in their misorientation angles.

Intrinsic Charge Transport in Stanene: Roles of Bucklings and Electron–Phonon Couplings

AbstractThe intrinsic charge transport of stanene is investigated by using density functional theory and density functional perturbation theory coupled with Boltzmann transport equations at the first‐principles level. The Wannier interpolation scheme is applied to calculate the charge carrier scatterings with all branches of phonons considering dispersion for the whole range of the first Brillouin zone. The intrinsic electron and hole mobilities are calculated to be (2–3) × 103 cm2 V−1 s−1 at 300 K. It is found that the intervalley scatterings from the out‐of‐plane and the transverse acoustic phonon modes dominate the carrier transport process. By contrast, the mobilities obtained by the conventional deformation potential approach are found to be as large as (2–3) × 106 cm2 V−1 s−1 at 300 K, in which the longitudinal acoustic phonon scattering in the long wavelength limit is assumed to be the dominant scattering mechanism. The inadequacy of the deformation potential approximation in stanene is attributed to the buckling in its honeycomb structure, which originates from the sp2–sp3 orbital hybridization and breaks the planar symmetry. This paper further proposes a strategy to enhance carrier mobilities by suppressing the out‐of‐plane vibrations through clamping by a substrate.

Electronic property and charge carrier mobility of one-dimensional nanowires consisting of C20 cages

This work presented theoretical studies on the one-dimensional (1D) nanowires constructed from fullerene C20 cages based on first-principle calculations. The relative energies, electronic, charge transport, and mechanical properties of the 1D nanowires were investigated systemically and in detail. It is found that formations of the C20 nanowires built from isolated cages were all energetically favorable. They also exhibit high kinetic stability according to molecular dynamics simulations. Although they were all constructed with C20 cages as building blocks, NW-2–NW-6, and NW-9 are semiconductors, whereas NW-1, NW-7, and NW-8 exhibit metallic property. Thus the metallic/semiconducting properties of the 1D C20 nanowires can be mainly determined by the connecting patterns. High charge mobility was revealed for the 1D C20 nanowires based on the deformation potential theory and effective mass approach. Further understanding of the charge mobility is achieved with the aid of crystal orbital analyses. Moreover, the mechanical property of the 1D C20 nanowires was also studied based on the results of Young’s modulus.

Globalk-space analysis of electron-phonon interaction in graphene and application toM-point spectroscopy

Recently, optical probes have become available that can access and observe energy renormalization due to electron-phonon interaction in graphene away from the well-studied Dirac $K$ point. Using an expanded deformation potential approach, we present a theoretical study of the electron-phonon self-energy and scattering matrix elements across the entire Brillouin zone. We elucidate the roles of modulated hopping and conventional deformation potential coupling, parameterized via standard deformation potentials, the in-plane phonon modes, intra- and interband contributions, and umklapp processes. Applying the theory to nonlinear optical transmission spectroscopy in the vicinity of the $M$ point, we find very good agreement with recently published experimental data.

Effect of Stone–Wales defects on the thermal conductivity of graphene

The problem of phonon scattering by strain fields caused by Stone–Wales (SW) defects in graphene is studied in the framework of the deformation potential approach. An explicit form of the phonon mean free path due to phonon-SW scattering is obtained within the Born approximation. The mean free path demonstrates a specific q-dependence varying as q−3 at low wavevectors and taking a constant value at large q. The thermal conductivity of graphene nanoribbons (GNRs) is calculated with the three-phonon umklapp, SW and rough edge scatterings taken into account. A pronounced decrease of the thermal conductivity due to SW defects is found at low temperatures whereas at room temperatures and above the phonon–phonon umklapp scattering becomes dominant. A comparison with the case of vacancy defects shows that they play more important role in the reduction of the thermal conductivity in GNRs over a wide temperature range.

Electronic property and charge carrier mobility of extended nanowires built from narrow graphene nanoribbon and atomic carbon chain

This work presented theoretical studies on the extended nanowires constructed from narrow graphene nanoribbons and atomic carbon chains by using self-consistent field crystal orbital method based on density functional theory. The size and connection effects on geometrical structure, relative stability and electronic property of these one-dimensional (1D) nanowires have been addressed systemically and in details. It is found that the ratio of narrow graphene nanoribbons and CC units may be the key factor to the relative thermodynamic stability of these 1D wires based on the obtained Gibbs free energy. All the extended nanowires studied have a direct band gap according to the calculated electronic band structures. The obtained band gaps of the nanowires are in the range of 0.918–2.015eV. These flexible band gaps imply the possibility to modulate the electronic properties of these extended wires by turning the sizes and connections. Furthermore, a quantitative relation between the band gap and the ratio of phenylene and CC units is obtained. Moreover, we calculate the charge carrier mobility of the extended nanowires based on the deformation potential theory and effective mass approach. The frontier crystal orbital analyses are performed to get a better understanding of the charge carrier mobility for the 1D nanowires with various sizes and connections.

Structures, stabilities and electronic properties of graphdiyne nanoribbons

The one-dimensional graphdiyne nanoribbons are studied using the self-consistent field crystal orbital method based on density functional theory in this paper. The structures, stabilities, electronic, elastic and transport properties of these nanoribbons with different edges and widths are investigated. These graphdiyne strips can be obtained from cutting the graphdiyne sheet or the carbomerization of the graphene strips. It is found that the carbomerization not only expands the structures, but also alters the stabilities, electronic, elastic and transport properties of the original systems. Here the graphdiyne nanoribbons studied are all more stable than the graphdiyne monolayer in the view of energy. Different from the graphene nanoribbons, the graphdiyne strips are all semiconductors. According to our calculations, the band gaps of the graphdiyne strips decrease monotonically as the widths increase. A quantitative relation between the band gaps and the widths of the graphdiyne nanoribbons is obtained. Moreover, we also calculate the mobilities of charge carriers for these strips based on the deformation potential theory and effective mass approach. The calculated mobilities are in the range of 102–106 cm2 V−1s−1 at room temperature. The relationship between mobilities and nanoribbon widths is different for electron and hole charge carriers. The mobilities of electrons are always larger than those of holes for these graphdiyne nanoribbons studied. Hence, the graphdiyne strips are possibly more favorable for electron transportation.

Evaluation of Charge Mobility in Organic Materials: From Localized to Delocalized Descriptions at a First‐Principles Level

The carrier mobility for carbon electronic materials is an important parameter for optoelectronics. We report here some recently developed theoretical tools to predict the mobility without any free parameters. Carrier scatterings with phonons and traps are the key factors in evaluating the mobility. We consider three major scattering regimes: i) where the molecular internal vibration severely induces charge self-trapping and, thus, the hopping mechanism dominates; ii) where both intermolecular and intramolecular scatterings come to play roles, so the Holstein-Peierls polaron model is applied; and, iii) where charge is well delocalized with coherence length comparable with acoustic phonon wavelength, so that a deformation potential approach is more appropriate. We develop computational methods at the first-principles level for the three different cases that have extensive potential application in rationalizing material design.

Theoretical studies of one‐dimensional C36 coplanar polymers

AbstractThe structures and electronic, elastic and transport properties of the one‐dimensional coplanar polymers constructed by sharing pentagons or hexagons of C36 cages are investigated using a self‐consistent field crystal orbital method based on density functional theory. It is found that these C36 coplanar polymers are all energetically favourable. The stability of the coplanar polymers is related to the atomic positions in the coalescent region and the size of retained aromatic domains. They are softer than carbon nanotubes due to the smaller Young's moduli. The calculated band structures show that these coplanar polymers can be a metal or a semiconductor. Moreover, the mobility of charge carriers is also calculated based on the deformation potential approach.

Crystal orbital study on the zigzag single-walled carbon nanotubes with multi-dichlorocarbene addition

Structure and electronic properties of zigzag single-walled carbon nanotubes with multi-dichlorocarbene addition are investigated using self-consistent field crystal orbital method. It is found that the addition can cause large deformation of the tubes and significantly modify the band structures. The addition can even cause semiconductor–metal phase transition. Furthermore, the mobility and conductivity are also calculated using the deformation potential approach for the addition systems.

Nanoengineering of lateral strain modulation in quantum well heterostructures

We have developed a method to design a lateral band-gap modulation in a quantum well heterostructure. The lateral strain variation is induced by patterning of a stressor layer grown on top of a single quantum well which itself is not patterned. The three-dimensional (3D) strain distribution within the lateral nanostructure is calculated using linear elasticity theory applying a finite element technique. Based on the deformation potential approach the calculated strain distribution is translated into a local variation of the band-gap energy. Using a given vertical layer structure we are able to optimize the geometrical parameters to provide a nanostructure with maximum lateral band-gap variation. Experimentally such a structure was realized by etching a surface grating into a tensile-strained InGaP stressor layer grown on top of a compressively strained InGaAs-single quantum well. The achieved 3D strain distribution and the induced band-gap variation are successfully probed by x-ray grazing incidence diffraction and low-temperature photoluminescence measurements, respectively.

Effect of electron-phonon interaction on the resistivity of metallic nanowires

Using a tight-binding approach for the electronic structure, deformation potential approach for the electron-phonon interaction, and the Kubo formula for the conductivity, an expression has been obtained for the resistivity of nanowire systems. The resulting expression involves (1) vertex corrections approximated by the sum of ladder diagrams, and (2) a self-energy approximated by the one-phonon processes. The dependence of the resistivity on the wire width, temperature and Fermi energy has been investigated by performing numerical calculations in terms of typical values of different parameters. It has been found that the resistivity increases with decreasing wire width. It has been further found that the resistivity increases with the temperature such that the resistivity slope increases with decreasing wire width. The temperature dependence shows a superlinear behavior for wires of width \ensuremath{\sim}20 nm. For wires of larger widths a linear behavior has been found. The resistivity is found to increase slowly with the Fermi energy. The relevance of the calculated results has been discussed in light of the existing experimental studies.

Specific features of low-temperature phonon scattering in materials with tilted disclination loops

Phonon scattering by static stress fields of circular wedge disclination loops is investigated in the framework of the deformation potential approach. Numerical calculations of the mean free path l and thermal conductivity κ demonstrate that the temperature dependence of κ exhibits a minimum at a certain temperature T* in the low-temperature range. The thermal conductivity κ sharply increases as T−3 with a decrease in temperature (T T*). The results obtained for the wedge disclination loop are compared with the available data for uniaxial disclination dipoles. It is shown that the properties of uniaxial disclination dipoles serving as sources of phonon scattering are similar to those of wedge disclination loops.

The effect oflong-range strain fields on transport properties of disclinatedmaterials

The problem of both phonon and electron scattering by long-rangestrain fields caused by wedge disclination dipoles (WDD) is studiedin the framework of the deformation potential approach. The exactanalytical results for the mean free path are obtained within theBorn approximation. The WDD-induced contribution to the residualresistivity in nanocrystalline metals is estimated. Phononscattering due to randomly distributed WDDs is shown to result in aclear crossover from T3- to T2-behaviour in the thermalconductivity, κ, at low temperatures. A combination of twoscattering processes, the phonon scattering due to biaxial WDD andthe Rayleigh-type scattering, is suggested to be of importance foramorphous dielectrics. Our results are in good agreement with theexperimentally observed κ for a-SiO2, a-GeO2, a-Se, andpolystyrene. Numerical calculations show that κ is verysensitive to the size of the dipole separation, which is fixed near20 Å.