Ahmad Ranjbar

Technische Universität Dresden | TUD · Materials Sceinec and Engineering

Two-dimensional (2D) topological insulator (TI) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure topology identified by the $Z_2$ number as protected by time-reversal symmetry (TRS). 2D TIs have intriguing spin-velocity locked conducting edge states...

2H-NbSe2 is a prototypical charge-density-wave (CDW) system, exhibiting such a symmetry-breaking quantum ground state in its bulk and down to a single-atomic-layer limit. However, how this state depends on dimensionality and what governs the dimensionality effect remain controversial. Here, we experimentally demonstrate a robust 3 × 3 CDW phase in...

The family of III–V element compounds (i.e., XY compounds; X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) have been intensively investigated for several decades because of their enormous applications for many optoelectronic devices. Here, by employing first‐principles calculations, the electronic structures of bulk XY haeckelite compounds are exami...

Optical second-harmonic generation (SHG) can only occur in noncentrosymmetric crystals in the leading electric-dipole approximation. Transition metal dichalco genides with the 1T octahedral coordination is centrosymmetric, hence precluding SHG. Here we report the surprising observation of SHG in atomically thin1T−TiSe2, a prototypical charge densit...

We performed virtual materials screening to identify promising topological materials for photocatalytic water splitting under visible light irradiation. Topological compounds were screened based on band gap, band edge energy, and thermodynamics stability criteria. In addition, topological types for our final candidates were computed based on electr...

MXenes have triggered great interests because of their potential applications. Depending on the exfoliation, MXenes can be terminated by O, F and OH groups. The properties of MXenes can be designed. Some F and O-terminated MXenes are predicted to be topological insulators (TIs). In OH-MXenes, the work-functions are significantly reduced. The image...

The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics. For example, it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH) functionalized MXenes. Most of these OH-terminated MXene are metallic, but some of them, e.g. Sc2C...

The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics and offer many potential applications in electronic devices and sensors. Using density functional theory (DFT), it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH)...

Chemical exfoliation of layered MAX phase compounds into novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has opened new opportunities in materials science and technology. In recent extensive theoretical studies, it has been demonstrated that MXenes containing transition metals with open d orbital shells exhibit a...

Considering the recent breakthroughs in the synthesis of novel two-dimensional (2D) materials from layered bulk structures, ternary layered transition metal borides, known as MAB phases, have come under scrutiny as a means of obtaining novel 2D transition metal borides, so-called MBene. Here, based on a set of phonon calculations, we show the dynam...

Recently, a group of MAX phases, (Mo$_{2/3}$Y$_{1/3}$)$_2$AlC, (Mo$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Y$_{1/3}$)$_2$AlC, and (V$_{2/3}$Zr$_{1/3}$)$_2$AlC, with in-plane ordered double transition metals, named iMAX phases, have been synthesized. Experimentally, some of these MAX phases can be chemically exfoliated...

Recently, a group of MAX phases, (Mo$_{2/3}$Y$_{1/3}$)$_2$AlC, (Mo$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Y$_{1/3}$)$_2$AlC, and (V$_{2/3}$Zr$_{1/3}$)$_2$AlC, with in-plane ordered double transition metals, named iMAX phases, have been synthesized. Experimentally, some of these MAX phases can be chemically exfoliated int...

Chemical exfoliation of MAX phases into two-dimensional (2D) MXenes can be considered as a major breakthrough in the synthesis of novel 2D systems. To gain insight into the exfoliation possibility of MAX phases and to identify which MAX phases are promising candidates for successful exfoliation into 2D MXenes, we perform extensive electronic struct...

Chemical exfoliation of MAX phases into two-dimensional (2D) MXenes can be considered as a major breakthrough in the synthesis of novel 2D systems. To gain insight into the exfoliation possibility of MAX phases and to identify which MAX phases are promising candidates for successful exfoliation into 2D MXenes, we perform extensive electronic struct...

Recently, two-dimensional (2D) transition metal carbides and nitrides, namely, MXenes have attracted lots of attention for electronic and energy storage applications. Due to a large spin-orbit coupling (SOC) and the existence of a Dirac-like band at the Fermi energy, it has been theoretically proposed that some of the MXenes will be topological ins...

The recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has brought a new opportunity to materials science and technology. This review highlights the computational attempts that have been made to understand the physics and chemistry of this very promising...

The family of two-dimensional transition metal carbides, so called MXenes, has recently found new members with ordered double transition metals M$'_2$M$"$C$_2$, where M$'$ and M$"$ stand for transition metals. Here, using a set of first-principles calculations, we demonstrate that some of the newly added members, oxide M$'_2$M$"$C$_2$ (M$'$= Mo, W;...

Using a set of first-principles calculations, we studied the electronic structures of two-dimensional transition metal carbides and nitrides, so called MXenes, functionalized with F, O, and OH. Our projected band structures and electron localization function analyses reveal the existence of nearly free electron (NFE) states in variety of MXenes. Th...

MXenes are a set of two-dimensional transition metal carbides and nitrides that offer many potential applications in energy storage and electronic devices. As an important parameter to design new electronic devices, we investigate the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principle...

Using a generalized Hubbard Hamiltonian, many-electron wavefunctions of negatively charged (NV−) and neutral nitrogen-vacancy (NV0) centers in diamond were calculated. We report the effect of symmetric relaxation of surrounding atoms on the spin density, calculated from the many electron wavefunctions in the ground and excited states. We evaluated...

Using a generalized Hubbard Hamiltonian, many-electron calculations of energy levels and corresponding wave functions of negatively charged and neutral nitrogen-vacancy centers in diamond were reported. The energies, orbital, and spin symmetries of the ground and excited states are in good quantitative agreement with available optical and electron...

Many-body wavefunctions were utilized to calculate von Neumann’s entropy as an entanglement measurement for neutral and negatively charged nitrogen vacancy (NV) centers in diamond. A generalized Hubbard Hamiltonian which considers e-e interaction terms completely was used to calculate many-electron wavefunctions of the ground and excited states. Co...

Using first-principles density functional theory, we investigated the hydrogen storage capacity of Li-functionalized adamantane. We showed that if one of the acidic hydrogen atoms of adamantane is replaced by Li/Li + , the resulting complex is activated and ready to adsorb hydrogen molecules at a high gravimetric weight percent of around ∼7.0%. Due...

Using first-principles electronic-structure calculations, we studied the structural and magnetic properties of various hydrogen clusters, including hydrogen monomer, dimers, trimers, tetramers, and hexamers adsorbed on a graphene surface. The magnetic behaviors of such defective systems were shown to strongly depend on the geometrical configuration...

Density functional theory (DFT) calculations were done to obtain the energies of two perfluorosulfonic acid membranes at low humidity conditions. For the first time, an artificial neural network (ANN) approach along with statistical methods were employed for modeling, prediction, and analysis of the energies derived by the DFT method. The ANN metho...

Recent scanning tunneling microscopy (STM) experiments display images with star and ellipsoidal like features resulting from unique geometrical arrangements of a few adsorbed hydrogen atoms on graphite. Based on first-principles STM simulations, we have found that the model with three hydrogen atoms, in which the hydrogen atoms are symmetri-cally p...

The relationship between unpaired electron delocalization and nearest-neighbor atomic relaxations in the vacancies of diamond has been determined in order to understand the microscopic reason behind the neighboring atomic relaxation. The Density Functional Theory (DFT) cluster method is applied to calculate the single-electron wavefunction of the v...

Ab initio density functional theory calculations were used to investigate cohesive, electronic, and structural properties of cagelike and spherical hydrogen terminated nanoparticles of diamond. Unlike cagelike nanodia-mond particles, the variation of calculated energies of spherical ones is not monotonic. The variation range of the calculated energ...