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In this work, we study systematically low-energy bands, optical absorbance and spin/valley-Hall conductivity of silicene and germanene in the presence of a perpendicular electric field. Our analytical calculations indicate that both silicene and germanene are semiconductors with a tiny energy gap and we can control their energy gap by the perpendicular electric field. Our calculations also demonstrate that the low-frequency optical absorbance of silicene is much greater than that of germanene and the external electric field plays an important role in determining the optical absorption peaks. When the Fermi level is in the forbidden band, the Hall conductivity is quantized, while spin/valley-Hall conductivities of both silicene and germanene depend strongly on the Fermi energy when the Fermi level is in the conduction band. Analytical results for spin/valley-Hall conductivities of silicene and germanene are presented in detail in this work.

At present, physicochemical properties of amino acid molecular crystals are of the utmost interest. The compounds where molecules have different chirality are the focus of particular interest. This paper, presents a study on the structural and electronic properties of crystalline L- and DL-valine within the framework of density functional theory including van der Waals interactions. The results of this study showed that electronic properties of the two forms of valine are similar at zero pressure. Pressure leads to different responses in these crystals which is manifested as various deformations of molecules. The pressure effect on the infrared spectra and distribution of electron density of L- and DL-valine has been studied.

This work is based on a recent theoretical study of how the hydrostatic pressure and core/shell sizes affect the optical properties associated with the transition from the ground state to first excited state (1s–1p), of an exciton confined in spherical core/shell quantum dots (SCSQDs). We have computed under an effective mass framework, linear, third-order nonlinear, and total absorption coefficients (AC) and refractive index (RI) as functions of photon energy for different sizes of SCSQDs with varying hydrostatic pressure. Our results show that the optical absorption is deeply dependent on the incident light intensity. Both AC and RI significantly influenced by the confinement and pressure effects.

Using the augmented plane wave + local orbitals method with different approximation functionals, we investigate systematically the electronic, optical and elastic properties of stannite-type Cu2CdGeSe4. Among different approximation functionals, the modified Becke–Johnson (mBJ) potential with Hubbard-corrected parameter U (mBJ + U) gives the most reliable results on the electronic properties of Cu2CdGeSe4 in comparison with the experimental data. Elastic modulus, elastic constants and the Poisson’s ratio of Cu2CdGeSe4 were calculated using the Elastic software package. Optical properties such as wide spectrum of absorbed photon energy, namely 1.3–27.2 eV, high absorption coefficient (above 104 cm−1) and optical anisotropy suggest the application of Cu2CdGeSe4 in solar cells.

Using density functional theory, we have studied the structural, electronic and optical properties of two-dimensional graphene-like $$\hbox {C}_2\hbox {N}$$ nanosheet under in-plane strains. Our results indicate that the $$\hbox {C}_2\hbox {N}$$ nanosheet is a semiconductor with a direct band gap of 1.70 eV at the equilibrium state opening between the highest valence band and lowest conduction band located at the $$\varGamma $$ point. The band gap of the $$\hbox {C}_2\hbox {N}$$ nanosheet decreases with the increasing of both uniaxial/biaxial strains. In the presence of the strain, we found band shift and band splitting of the occupied and unoccupied energy states of the valence and conduction bands, resulting in a decrease of the band gap. Furthermore, the absorption and reflectance spectra for the $$\hbox {C}_2\hbox {N}$$ nanosheet have a broad peak around 2.6 eV, where a maximum absorption value is up to $$3.2 \times 10^{-5}\,\hbox {cm}^{-1}$$ and reflectance is about 0.27%. Moreover, our calculations also show that the optical properties of the $$\hbox {C}_2\hbox {N}$$ nanosheets can be controlled by applying the biaxial and uniaxial strains. The obtained results might provide potential applications for the $$\hbox {C}_2\hbox {N}$$ nanosheets in nanoelectronics and optoelectronics.

In this work, we consider the effect of out-plane strain on the electronic properties of AB-stacked bilayer armchair graphene nanoribbons (BAGNRs) using density functional theory. At equilibrium, the interlayer distance of BAGNRs is $$d_0=3.326$$ A. Our DFT calculations show that while the dependence of the band gap of 11-BAGNR on interlayer distance d is insignificant, especially in the case of the $$d

In this work, the effect of Peierls distortion on the electronic properties of armchair graphene nanoribbons is theoretically studied in a tight-binding approximation. As a consequence of Peierls distortion, when the Kekule-type bond alternation is introduced, the band gap of armchair nanoribbons strongly depends on the difference in bond length between short and long bonds in the honeycomb lattice. We can control the band gap of a nanoribbon by its uniaxial strain and a semiconductor–metal phase transition can occur at certain elongations.

Graphene-based two-dimensional van der Waals heterostructures (vdWH) have recently shown a great potential for high-performance nanodevices. Here, we design a novel vdWH, consisting of the graphene and a noble transition metal dichalcogenide $${\mathrm{{PtS}}_2}$$ monolayer and investigate its electronic properties as well as the effect of electric field. We find that the key intrinsic characteristics of both the graphene and $${\mathrm{{PtS}}_2}$$ monolayers are well preserved due to the weak vdW interactions. The graphene is found to form an n-type Schottky contact with the $${\mathrm{{PtS}}_2}$$ monolayer with a small barrier height of 0.11 eV. Moreover, this small barrier height of the $$\mathrm{{G}}/{\mathrm{{PtS}}_2}$$ vdWH is known to be very sensitive to the external electric field. It can be controlled and turned to the Ohmic contact under electric field. These findings could provide significant knowledge for designing novel electronic and optoelectronic devices of such heterostructure.

We study the two-photon-induced cyclotron–phonon resonance (CPR) effect resulting from the interaction between electrons and phonons in a quantum well with the modified-Poschl–Teller (MPT) potential. The CPR is considered via the magneto-optical absorption coefficient (MOAC) and the full-width at half-maximum (FWHM). Numerical results are presented for a typical GaAs MPT quantum well. It is found that the quantum well parameters, such as well-depth and well-width, the magnetic field, and the temperature affect significantly the energy separation, the MOAC, and the FWHM. Obtained results agree well with previous theoretical and experimental results. Especially, we found two new rules of the well-depth and well-width-dependent FWHM, which should be tested by experimental works to examine their validities.

The pressure and temperature effects on the optical responses involving the $$1s-1p$$ intersubband transition of an exciton in a spherical quantum dot are investigated. Calculations are performed in the framework of the effective mass approximation and the energies are obtained by using a Ritz variational method. Our approach is based on the Hylleraas formalism were the correlations between the electron and hole are taken into account. Temperature, pressure and the size effects on the linear and third nonlinear optical properties are analyzed. Our results show that the temperature and pressure provide important effects on linear and nonlinear parts of the absorption coefficient (AC) and the relative refractive index change (RI) associated to the $$1s-1p$$ transition. We found that by increasing the temperature and pressure the AC and RI curves shift to lower and higher energies respectively. Calculations show also that the dot size affects considerably the AC and RI and their corresponding amplitude.

In this work, we investigate band-gap tuning in bilayer MoS2 by an external electric field and by applied biaxial strain. Our calculations show that the band gaps of bilayer MoS2 can be tuned by the perpendicular electric field or biaxial strain. The band gaps of bilayer MoS2 decrease with increasing applied electric field or biaxial strain. When the electric field was introduced, electronic levels are split due to the separation of the valence sub-band and the conduction sub-band states. Our calculations also show that the change in the band gap of bilayer MoS2 is due to the separation of electronic levels by electric field via the Stark effect. At the electric field $$E_{\mathrm{Field}} = 5.5$$ V/nm or biaxial strain $$\varepsilon = 15\%,$$ bilayer MoS2 becomes metallic. The semiconductor–metal phase transition in bilayer MoS2 plays an important role in its application for nanodevices.

In this work, the phonon-assisted cyclotron resonance in a special symmetric quantum well is studied in detail by evaluating the magneto-optical absorption coefficient (MOAC) and the full width at half maximum (FWHM). The results include the two-photon absorption process and the electron–phonon interaction as well. The influence of the magnetic field, the quantum well parameter, and the temperature on the MOAC and the FWHM is illustrated plainly. Our results are in good agreement with those accessible to the theoretical and experimental works of literature. In addition, we found a new rule of the temperature-dependent FWHM for the phonon absorption process, which should be examined by an experiment work in the future.

Hot electrons cooling by phonons in GaAs/AlAs cylindrical quantum wire (CQW), under the influence of an intense electromagnetic wave (EMW), is studied theoretically. Analytic expression for the electron cooling power (CP) is derived from the quantum transport equation for phonons, using the Hamiltonian of interacting electron–optical phonon system. Both photon absorption and emission processes are considered. Numerical results show that the CP reaches maximum when the energy difference between electronic subbands equals the energy of an optical phonon plus the photon energy. Under the influence of the EMW, the negative CP is observed showing that electrons gain energy from phonon and photon instead of losing their energy. Also, the CP increases with increasing the EMW amplitude. Our results theoretically clarify the mechanism of the electron cooling process by phonons in the GaAs/AlAs CQW under the EMW, which is of significance for designing and fabricating high-speed nanoelectronic devices based on this material.