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<jats:p>We perform first principles calculations to investigate the catalytic behavior of C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> nanosheet for water splitting. For the pristine C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub>, we find that, at different hydrogen coverages, two H atoms adsorbed on the 12-membered ring and one H atom adsorbed on the 9-membered ring show excellent performance of hydrogen evolution reaction (HER). Tensile strain could improve the catalytic ability of C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> and strain can be practically introduced by building C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub>/BiN, and C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub>/GaAs heterojunctions. We demonstrate that the HER performance of heterojunctions is indeed improved compared with that of C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> nanosheet. Anchoring transition metal atoms on C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> is another strategy to apply strain. It shows that Rh@C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> exhibits superior HER property with very low Gibbs free energy change of –30 meV. Under tensile strain within ∼2%, Rh@C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> could catalyze HER readily. Moreover, the catalyst Rh@C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> works well for oxygen evolution reaction (OER) with an overpotential of 0.58 V. Our results suggest that Rh@C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> is favorable for both HER and OER because of its metallic conductivity, close-zero Gibbs free energy change, and low oneset overpotential. The outstanding performance of C<jats:sub>9</jats:sub>N<jats:sub>4</jats:sub> nanosheet could be attributed to the tunable porous structure and electronic structure compatibility.</jats:p>

<jats:p>Hill-like polycrystalline diamond grains (HPDGs) randomly emerged on a heavy boron-doped p<jats:sup>+</jats:sup> single-crystal diamond (SCD) film by prolonging the growth duration of the chemical vapor deposition process. The Raman spectral results confirm that a relatively higher boron concentration (∼ 1.1 × 10<jats:sup>21</jats:sup> cm<jats:sup>−3</jats:sup>) is detected on the HPDG with respect to the SCD region (∼ 5.4 × 10<jats:sup>20</jats:sup> cm<jats:sup>−3</jats:sup>). It demonstrates that the Au/SCD interface can be modulated from ohmic to Schottky contact by varying the surface from hydrogen to oxygen termination. The current–voltage curve between two HPDGs is nearly linear with either oxygen or hydrogen termination, which means that the HPDGs provide a leakage path to form an ohmic contact. There are obvious rectification characteristics between oxygen-terminated HPDGs and SCD based on the difference in boron doping levels in those regions. The results reveal that the highly boron-doped HPDGs grown in SCD can be adopted as ohmic electrodes for Hall measurement and electronic devices.</jats:p>

<jats:p>The ultrathin <jats:italic>β</jats:italic>-Sn(001) films have attracted tremendous attention owing to its topological superconductivity (TSC), which hosts Majorana bound state (MBSs) for quantum computation. Recently, <jats:italic>β</jats:italic>-Sn(001) thin films have been successfully fabricated via phase transition engineering. However, the understanding of structural phase transition of <jats:italic>β</jats:italic>-Sn(001) thin films is still elusive. Here, we report the direct growth of ultrathin <jats:italic>β</jats:italic>-Sn(001) films epitaxially on the highly oriented pyrolytic graphite (HOPG) substrate and the characterization of intricate structural-transition-induced superstructures. The morphology was obtained by using atomic force microscopy (AFM) and low-temperature scanning tunneling microscopy (STM), indicating a structure-related bilayer-by-bilayer growth mode. The ultrathin <jats:italic>β</jats:italic>-Sn film was made of multiple domains with various superstructures. Both high-symmetric and distorted superstructures were observed in the atomic-resolution STM images of these domains. The formation mechanism of these superstructures was further discussed based on the structural phase transition of <jats:italic>β</jats:italic> to <jats:italic>α</jats:italic>-Sn at the atomic-scale thickness. Our work not only brings a deep understanding of the structural phase transition of Sn film at the two-dimensional limit, but also paves a way to investigate their structure-sensitive topological properties.</jats:p>

<jats:p>The orientation switching of a single azobenzene molecule on Au(111) surface excited by tunneling electrons and/or photons has been demonstrated in recent experiments. Here we investigate the rotation behavior of this molecular rotor by first-principles density functional theory (DFT) calculation. The anchor phenyl ring prefers adsorption on top of the fcc hollow site, simulated by a benzene molecule on close packed atomic surface. The adsorption energy for an azobenzene molecule on Au(111) surface is calculated to be about 1.76 eV. The rotational energy profile has been mapped with one of the phenyl rings pivots around the fcc hollow site, illustrating a potential barrier about 50 meV. The results are consistent with experimental observations and valuable for exploring a broad spectrum of molecules on this noble metal surface.</jats:p>

<jats:p>Using hybrid density functional calculation, we study the atomic and electronic structures of p-type dopants, B, Al and Ga, in 4H-SiC. For B, depending on the growth condition, it can occupy both Si and C sites. In contrast, Al and Ga on the C sites exhibit too high formation energy to exist in a significant amount. In 4H-SiC, there exist two types of Si sites in wurtzite-like and zincblende-like local coordination, respectively. Our calculations suggest that the dopant atoms have negligible preference occupying the two sites. In neutral charge state, all the dopants exhibit significant distortions from the structure in the negatively charged state. For most cases, our calculations yield three distorted structures, in which the most stable one has the dopant atom displaced along its bond with one of the surrounding equatorial Si or C atoms, lowering the <jats:italic>C</jats:italic> <jats:sub>3<jats:italic>v</jats:italic> </jats:sub> symmetry to <jats:italic>C<jats:sub>s</jats:sub> </jats:italic> symmetry (i.e., a mirror symmetry only). Among the three dopant elements, Al on Si sites exhibits overall the lowest formation energy and the shallowest acceptor level. Nevertheless, it is not a hydrogenic dopant with the acceptor level 0.12 eV above the valence band maximum based on calculation using a 400-atom supercell. Its corresponding defect state exhibits apparent localization along the [0001] direction, but it is relatively delocalized in the (0001) plane.</jats:p>

<jats:p>It is well known that in the process of thermal oxidation of silicon, there are <jats:italic>P</jats:italic> <jats:sub>b</jats:sub>-type defects at amorphous silicon dioxide/silicon (a-SiO<jats:sub>2</jats:sub>/Si) interface due to strain. These defects have a very important impact on the performance and reliability of semiconductor devices. In the process of passivation, hydrogen is usually used to inactivate <jats:italic>P</jats:italic> <jats:sub>b</jats:sub>-type defects by the reaction <jats:italic>P</jats:italic> <jats:sub>b</jats:sub> + H<jats:sub>2</jats:sub> → <jats:italic>P</jats:italic> <jats:sub>b</jats:sub>H + H. At the same time, <jats:italic>P</jats:italic> <jats:sub>b</jats:sub>H centers dissociate according to the chemical reaction <jats:italic>P</jats:italic> <jats:sub>b</jats:sub> H → <jats:italic>P</jats:italic> <jats:sub>b</jats:sub> +H. Therefore, it is of great significance to study the balance of the passivation and dissociation. In this work, the reaction mechanisms of passivation and dissociation of the <jats:italic>P</jats:italic> <jats:sub>b</jats:sub>-type defects are investigated by first-principles calculations. The reaction rates of the passivation and dissociation are calculated by the climbing image-nudged elastic band (CI-NEB) method and harmonic transition state theory (HTST). By coupling the rate equations of the passivation and dissociation reactions, the equilibrium density ratio of the saturated interfacial dangling bonds and interfacial defects (<jats:italic>P</jats:italic> <jats:sub>b</jats:sub>, <jats:italic>P</jats:italic> <jats:sub>b0</jats:sub>, and <jats:italic>P</jats:italic> <jats:sub>b1</jats:sub>) at different temperatures is calculated.</jats:p>

<jats:p>The ferromagnetism of two-dimensional (2D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2D transition metal-organic framework materials (TM-NH MOF, TM = Sc–Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and <jats:italic>ab-initio</jats:italic> molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni- and Zn-NH MOFs are nonmagnetic semiconductors (SM) with band gaps of 0.41 eV and 0.61 eV, respectively. Co- and Cu-NH MOFs are bipolar magnetic semiconductors (BMS), while Fe-NH MOF monolayer is a half-semiconductor (HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3d states. This work predicts several new 2D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.</jats:p>

<jats:p>To study the electron transport properties in InGaN channel-based heterostructures, a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier. Various scattering mechanisms, such as dislocation impurity (DIS) scattering, polar optical phonon (POP) scattering, piezoelectric field (PE) scattering, interface roughness (IFR) scattering, deformation potential (DP) scattering, alloy disorder (ADO) scattering from InGaN channel layer, and temperature-dependent energy bandgaps are considered in the calculation model. A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure (DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure (SH) is made and analyzed with a full range of barrier alloy composition. The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is <jats:underline>0.04</jats:underline> for higher mobility in DH with Al<jats:sub>0.4</jats:sub>In<jats:sub>0.07</jats:sub>Ga<jats:sub>0.53</jats:sub>N barrier. Finally, the temperature-dependent two-dimensional electron gas (2DEG) density and mobility in InGaN channel-based DH with Al<jats:sub>0.83</jats:sub>In<jats:sub>0.13</jats:sub>Ga<jats:sub>0.04</jats:sub>N and Al<jats:sub>0.4</jats:sub>In<jats:sub>0.07</jats:sub>Ga<jats:sub>0.53</jats:sub>N barrier are investigated. Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.</jats:p>

<jats:p>The mobility edges and reentrant localization transitions are studied in one-dimensional dimerized lattice with non-Hermitian either uniform or staggered quasiperiodic potentials. We find that the non-Hermitian uniform quasiperiodic disorder can induce an intermediate phase where the extended states coexist with the localized ones, which implies that the system has mobility edges. The localization transition is accompanied by the <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathscr{P}}{\mathscr{T}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="script">P</mml:mi> <mml:mi mathvariant="script">T</mml:mi> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_30_9_097202_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> symmetry breaking transition. While if the non-Hermitian quasiperiodic disorder is staggered, we demonstrate the existence of multiple intermediate phases and multiple reentrant localization transitions based on the finite size scaling analysis. Interestingly, some already localized states will become extended states and can also be localized again for certain non-Hermitian parameters. The reentrant localization transitions are associated with the intermediate phases hosting mobility edges. Besides, we also find that the non-Hermiticity can break the reentrant localization transition where only one intermediate phase survives. More detailed information about the mobility edges and reentrant localization transitions are presented by analyzing the eigenenergy spectrum, inverse participation ratio, and normalized participation ratio.</jats:p>

<jats:p>We report the strong dependence of resistance on uniaxial strain in monolayer WSe<jats:sub>2</jats:sub> at various temperatures, where the gauge factor can reach as large as 2400. The observation of strain-dependent resistance and giant gauge factor is attributed to the emergence of nonzero Berry curvature dipole. Upon increasing strain, Berry curvature dipole can generate net orbital magnetization, which would introduce additional magnetic scattering, decreasing the mobility and thus conductivity. Our work demonstrates the strain engineering of Berry curvature and thus the transport properties, making monolayer WSe<jats:sub>2</jats:sub> potential for application in the highly sensitive strain sensors and high-performance flexible electronics.</jats:p>