Two-dimensional (2D) piezoelectric materials have gained considerable attention since they could play important roles in the nanoelectromechanical systems. Herein, we report a first-principles study on the piezoelectric properties of monolayer group-V binary compounds with theoretically stable honeycomb phases (α-phase and β-phase). Our calculations for the first time reveal that a majority of the monolayers possess extremely high piezoelectric coefficients d11, i.e., 118.29, 142.44, and 243.45 pm/V for α-SbN, α-SbP, and α-SbAs, respectively, comparable to those of recently reported group-IV monochalcogenides (d11 = 75–250 pm/V) with an identical mm2 symmetry. It is found that the giant piezoelectric responses of α-phase monolayers as compared to those of β-phase monolayers are induced by their flexible structures and special symmetry. Meanwhile, the piezoelectric coefficients of α-phase monolayers are found to be surprisingly anisotropic and obey a unique periodic trend which is not exactly identical to that for the β-phase monolayers. To gain a comprehensive understanding of the periodic trends in piezoelectricity, several factors which influence the piezoelectric coefficients are quantitatively determined.

•Deformation potential theory was employed to estimate relaxation time of Ba2ZnPn2.•Seebeck coefficient of Ba2ZnAs2 monotonously increases with increasing temperature.•p-type Ba2ZnAs2 shows a potentially higher thermoelectric performance than Ca5Al2Sb6.•The peak ZT value along z-direction for p-type Ba2ZnAs2 is larger than 2.•The chain-like structure may induce a high electrical conductivity along z.In our work, we calculated the relaxation times of Ba2ZnPn2 (Pn = As, Sb, Bi) based on the deformation potential (DP) theory using the first-principles method, and successfully predicted high thermoelectric performance (ZT > 2) along z-direction for p-type Ba2ZnAs2 and Ba2ZnSb2. The Seebeck coefficient (S) of Ba2ZnAs2 monotonously increases with increasing temperature, which is favorable for achieving high thermoelectric performance in high temperature range. We also find that the four approximately degenerated bands (Nv = 4) near the valence band edge mainly originating from the interaction between Zn atoms and Pn atoms, and the different strengths of ZnPn bonding lead to the different energy range spanned of the four bands. The weak ZnAs bonding decreases the dispersion of the four bands, which leads to a sharply increased total density of states near the valence band edge, and will largely increase the S of Ba2ZnAs2, while the strong ZnBi bonding increases the dispersion of the four bands near the VB edge, which reduces the effective mass of valence bands near the VB edge, and will be in favor of the carrier mobility. The coexistence of two heavy bands and two light bands near the VB edge contributes to their simultaneous high Seebeck coefficients and high electrical conductivities at the optimum carrier concentration. Moreover, their electrical conductivities along the z-direction are much higher than those along the x- and y-directions, possibly originating from the chain-like arrangement of covalent ZnPn4 tetrahedra.Ba2ZnAs2 is predicted to have a trend of increasing Seebeck coefficient at high temperature range, which may result in its higher ZT values than other promising synthesized Zintl compounds Ca5Al2Sb6 and A3AlSb3 (A = Ca and Sr).

Perovskite solar cells single-doped with Br− or Cs+ ions have been proved to be an effective approach to improve their efficiency and stability. In our work, we took advantage of co-doping with Br− and Cs+. At our studied doping levels from CH3NH3I:PbI2:CsBr = 1:1:0 (x = 0) to 0.85:1:0.15 (x = 0.15), CsBr doping does not introduce any detectable impurity, and the crystal grains grow larger with increasing CsBr doping level. Furthermore, when the CsBr doping level is less than x = 0.1, it can progressively enhance the optical absorption of the perovskite film, although the absorption begins to decrease when the doping level rises above x = 0.1. X-ray photoelectron spectroscopy measurements show that Br− has successfully replaced I− and bonds with Pb2+ after CsBr doping. At the optimized doping level of x = 0.1, the incorporation of CsBr in the reaction system can improve the morphology of perovskite films and greatly enhance the efficiency from 9.8% for undoped sample to 13.6%, better than single Br− or Cs+ doping. Our result shows that CsBr doping is an effective method to enhance the efficiency of perovskite solar cells.

By using first-principles method, we studied the relation between the arrangement of the MPn4 chains and the electronic structures for Zintl Ca5M2As6 (M = Sn and Ga) compounds. It was found that the connecting forms between the adjacent chains in Ca5M2As56 play a key role in determining their thermoelectric properties. The appearing of As–As bonding or not between adjacent covalent MAs4 chains mainly depends on the different electron configuration between Pn and Ga (or Sn). Such As–As bonding in Ca5Ga2As6 results in a sharp peak of density of states near the conduction band minimum, which will dramatically increase its n-type Seebeck effect. Moreover, the calculated band decomposed charge density demonstrates that the As–As bonding leads to a high charge accumulating along the y-direction for n-type Ca5Ga2As6. Combined with the high electrical conductivity along the covalent anion chain direction, a high electrical conductivity may exist in n-type polycrystal of Ca5Ga2As6. On the other hand, the absence of As–As bonding in Ca5Sn2As6 results in a sharp peak of density of states near the valence band maximum, which can enhance its p-type Seebeck effect. For Ca5Sn2As6, the small anisotropy of electrical conductivity may induce the high electrical value for its p-type polycrystal. Consequently, polycrystalline n-type Ca5Ga2As6 and p-type Ca5Sn2As6 may have good thermoelectric performance.

An experimentally synthesized graphene/Bi2WO6 composite showed an enhancement of the visible-light photocatalytic activity, while the underlying mechanism is not known. Here, first-principles calculations based on density functional theory were performed to explore the various properties of the graphene/Bi2WO6(010) composite aiming at gaining insights into the mechanism of its photocatalytic activity. The stability, electronic properties, charge transfer, and visible-light response were investigated in detail on the Bi2WO6(010) surface coupled with graphene. An analysis of charge distribution and Bader charge shows that there is a strong covalent bonding between graphene and the Bi2WO6(010) surface. The covalent interaction induces a small bandgap in graphene. The interband transition of graphene and the surface states of the Bi2WO6(010) surface would cause the absorption spectrum of graphene/Bi2WO6(010) to cover the entire visible-light region and even the infrared-light region. The photogenerated electrons flow to graphene from the conduction band of Bi2WO6 under the built-in electric field and band edge potential well. Thus, graphene serves as a photogenerated electron collector and transporter which significantly reduces the probability of electron–hole recombination and increases catalytic reaction sites not only on the surface of graphene but on also the surface of Bi2WO6. The decrease of charge recombination is possibly responsible for the enhancement of the visible-light photocatalytic activity of the graphene/Bi2WO6(010) nanocomposite.

The electronic structure and the thermoelectric properties of Mg2X (X = Si, Ge, and Sn) were studied using the density functional theory and the semi-classical Boltzmann transport theory. The three compounds of Mg2X (X = Si, Ge, and Sn) were found to be indirect band-gap semiconductors with gap magnitudes of 0.66, 0.63, and 0.29 eV, respectively. By studying the carrier concentration dependence of the transport properties, we find that the p-type Mg2X exhibit superior thermoelectric performance originating from a large density-of-states effective mass due to the large valley degeneracy of valence bands. In particular, a maximum ZT value of 1.1 for p-type Mg2Sn can be achieved at 800 K with a carrier concentration of 9.8 × 1019 cm−3, which is higher than that of Mg2Si (0.8) and Mg2Ge (1.0). The high ZT of Mg2Sn is mainly attributed to its low lattice thermal conductivity that is a consequence of the low velocity of the optical modes caused by the large mass density. These findings suggest that Mg2Sn is a promising mid-temperature thermoelectric material.

The formation energy, electronic properties, and photocatalytic activity of Mo, W mono-doped and Mo/W codoped BiVO4 were investigated using density functional theory plus U calculations (DFT + U). The calculated formation energies show that both Mo and W atoms prefer to substitute V atoms under the oxygen-rich condition, in agreement with previous experimental results. Mo or W atom doping on the V site can form continuum states above the conduction band edge of BiVO4, which is advantageous for the photochemical catalysis response. Moreover, we found that the W doped BiVO4 has a smaller band gap than the Mo doped one, and the effect of Mo and W doping on the electronic structure of BiVO4 is different. Mo/W/Mo and W/Mo/W co-doped BiVO4 have smaller formation energies and smaller band gaps than the other doping case, which may enhance the optical absorption. Thus, Mo/W/Mo and W/Mo/W co-doped BiVO4 is particularly suitable for visible-light photocatalysis.

Ba3Al2As4 exhibits an unusual anisotropic electrical conductivity, that is, the electrical conductivity along the chain is smaller than those along other two directions. The results is conflict with previous conclusion for Ca5M2Pn6. Earlier studies on Ca5M2Pn6 showed that a higher electrical conductivity could be obtained along the chain. The band decomposed charge density is used to explain such unusual behavior. Our calculations indicate the existence of a conductive pathway near the Fermi level is responsible for the electrons transport. Further, the Ba–As bonding of Ba3Al2As4 has some degree covalency which is novel for the Zintl compounds.

•Bicomponent NiO/TiO2 composite nanofiber was prepared by electrospinning.•The H2-production of TiO2 was enhanced in the presence of NiO.•Low-cost and earth-abundant NiO is an efficient cocatalyst for H2 production.•NiO content exhibits a great influence on H2-production activity of TiO2.Hydrogen has been regarded as an ideal candidate for the replacement of conventional fossil fuels due to its environmental friendliness and recycling possibility. In this work, mesoporous NiO/TiO2 bicomponent composite nanofibers were prepared by a electrospinning and calcination method. The H2-production activity of the prepared samples was examined by water splitting using methanol as scavenger under simulated solar Xenon lamp irradiation. The effect of NiO loading on the microstructure and photocatalytic H2-production activity of NiO/TiO2 composite samples was studied and discussed. The results demonstrated that the presence of a small amount of NiO obviously inhibited the growth of TiO2 crystallites. With increasing the NiO content, the average crystallite size further decreased. In contrast, the Brunauer–Emmett–Teller (BET) specific surface areas, pore volumes and average pore size steadily increased. Photocatalytic H2-production experiment confirmed that NiO was an efficient co-catalyst for the photocatalytic H2 production of TiO2. The optimal NiO loading was determined to be 0.25 wt.%, giving a H2-production rate of 337 μmol h−1 g−1 with apparent quantum efficiency (QE) of 1.7%, which exceeded the rate on pure TiO2 by more than 7 times. The enhanced H2-production activity was due to the deposition of NiO clusters on the surface of TiO2, which suppressed the recombination of photogenerated electron–hole pairs, reduced the overpotential of hydrogen production and catalyzed production of hydrogen. This work showed that low-cost and earth-abundant NiO could be used as co-catalyst for photocatalytic hydrogen production.

Magnetic ZnO, one of the most important diluted magnetic semiconductors (DMS), has attracted great scientific interest because of its possible technological applications in optomagnetic devices. Magnetism in this material is usually delicately tuned by the doping level, dislocations, and local structures. The rational control of magnetism in ZnO is a highly attractive approach for practical applications. Here, the tuning effect of biaxial strain on the d0 magnetism of native imperfect ZnO is demonstrated through first-principles calculations. Our calculation results show that strain conditions have little effect on the defect formation energy of Zn and O vacancies in ZnO, but they do affect the magnetism significantly. For a cation vacancy, increasing the compressive strain will obviously decrease its magnetic moment, while tensile strain cannot change the moment, which remains constant at 2 μB. For a singly charged anion vacancy, however, the dependence of the magnetic moment on strain is opposite to that of the Zn vacancy. Furthermore, the ferromagnetic state is always present, irrespective of the strain type, for ZnO with two zinc vacancies, 2VZns. A large tensile strain is favorable for improving the Curie temperature and realizing room temperature ferromagnetism for ZnO-based native semiconductors. For ZnO with two singly charged oxygen vacancies, 2V+Os, no ferromagnetic ordering can be observed. Our work points the way to the rational design of materials beyond ZnO with novel non-intrinsic functionality by simply tuning the strain in a thin film form.

Band engineering is one of the effective approaches for designing ideal thermoelectric materials. Introducing an intermediate band in the band gap of semiconducting thermoelectric compounds may largely increase the carrier concentration and improve the electrical conductivity of these compounds. We test this hypothesis by Pb doping in Zintl Ca5In2Sb6. In the current work, we have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5% (0.2 e per cell) for Ca5In2Sb6 by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca5In2Sb6, which originates from the Pb s states by weakly hybridizing with the Sb p states. Such an intermediate band dramatically increases the electrical conductivity of Ca5In2Sb6 and has little detrimental effect on its Seebeck coefficient, which may increase its thermoelectric figure of merit, ZT. Interestingly, a maximum ZT value of 2.46 may be achieved at 900 K for crystalline Pb-doped Ca5In2Sb6 when the carrier concentration is optimized. Therefore, Pb-doped Ca5In2Sb6 may be a promising thermoelectric material.

We have investigated the structural, electronic, and magnetic properties of A-site-ordered double-perovskite-structured oxides, AA′3B4O12 (A = Na, Ca, and La) with Mn and V at A′ and B sites, respectively, using first-principle calculations based on the density functional theory. Our calculation results show that the antiferromagnetic phase is the ground state for all the compounds. By changing the A-site ions from Na+ to Ca2+ and then to La3+, the transfer of charge between Mn and O ions was changed from 1.56 to 1.55 and then to 1.50, and that between the V and O ions changed from 2.01 to 1.95 and then to 1.93, revealing the cause for the unusual site-selective doping effect. Mn 3d electrons dominate the magnetic moment and are localized, with an intense hybridization with O 2p orbitals, which indicates that the magnetic exchange interaction between Mn ions is mediated through O and that the super exchange mechanism will take effect. These materials have a large one-electron bandwidth W, and the ratio of the on-site Coulomb repulsion U to W is less than the critical value (U/W)c, which leads to metallic behavior of AMn3V4O12. This is further evidenced by the large number of free electrons contributed by V at the Fermi surface. These calculations, in combination with the reported experimental data, prove that these double perovskites belong to the rare antiferromagnetic metallic oxides.

Experimentally synthesized Zn-doped Sr3AlSb3 exhibited a smaller carrier concentration than Zn-doped Ca3AlSb3, which induces a lower thermoelectric figure of merit (ZT) than Zn-doped Ca3AlSb3. We used first-principles methods and the semiclassical Boltzmann theory to study the reason for this differing thermoelectric behavior and explored the optimal carrier concentration for high ZT values via p-type and n-type doping. The covalent AlSb4 tetrahedral arrangement exhibited an important effect on the electronic structure and thermoelectric properties. p-type Ca3AlSb3 may exhibit good thermoelectric properties along its covalent AlSb4 chain due to its double band degeneracy at the valence band edge and small effective mass along its one-dimensional chain direction. Zn doping the Al site exhibited higher formation energy for Sr3AlSb3 than Ca3AlSb3, which explains the lower carrier concentration for Zn-doped Sr3AlSb3 than Zn-doped Ca3AlSb3. The double band degeneracy at the valence band edge for Ca3AlSb3 may also help to increase the carrier concentration. Sr3AlSb3 containing isolated Al2Sb6 dimers can exhibit a high thermoelectric performance via heavy p-type doping with a carrier concentration above 1 × 1020 holes per cm3. Moreover, the ZT maxima for the n-type Sr3AlSb3 can reach 0.76 with a carrier concentration of 4.5 × 1020 electrons per cm3.

Previous experimental work showed that Zn-doping only slightly increased the carrier concentration of Sr5Al2Sb6 and the electrical conductivity improved barely, which is very different from the results of Zn-doping in Ca5Al2Sb6. To understand their different thermoelectric behaviors, we investigated their stability, electronic structure, and thermoelectric properties using first-principles calculations and the semiclassical Boltzmann theory. We found that the low carrier concentration of Zn-doped Sr5Al2Sb6 mainly comes from its high positive formation energy. Moreover, we predict that a high hole concentration can possibly be realized in Sr5Al2Sb6 by Na or Mn doping, due to the negative and low formation energies of Na- and Mn-doped Sr5Al2Sb6, especially for Mn doping (−6.58 eV). For p-type Sr5Al2Sb6, the large effective mass along Γ–Y induces a large Seebeck coefficient along the y direction, which leads to the good thermoelectric properties along the y direction. For p-type Ca5Al2Sb6, the effective mass along Γ–Z is always smaller than those along the other two directions with increasing doping degree, which induces its good thermoelectric properties along the z direction. The analysis of the weight mobility of the two compounds confirms this idea. The calculated band structure shows that Sr5Al2Sb6 has a larger band gap than Ca5Al2Sb6. The relatively small band gap of Ca5Al2Sb6 mainly results from the appearance of a high density-of-states peak around the conduction band bottom, which originates from the Sb–Sb antibonding states in it.

The geometry structure, electronic structure, and band edge positions of Zn-doped Bi2WO6 have been studied using a first-principles method. Bi1.75Zn0.25WO6 has a high quantum efficiency (QE) caused by the large mobility along different orientations and the efficient photogenerated-electron trap. It exhibits an enhanced visible-light absorption capability (α) due to the large increase of the density of electrons in the valence band maximum (VBM) and decrease in the band gap. The interactions are both strengthened between layers and among atoms within (Bi2O2)n layers. These strengthened interactions induce an enhanced stereochemically active Bi lone pair effect, which is identified as the cause of its unique electronic structure. Furthermore, the valence band (VB) and conduction band (CB) edges of Zn-doped Bi2WO6 are slightly shifted upwards. This indicates that the dominant active species during photocatalytic reaction for Bi1.75Zn0.25WO6 are not only hole and electron but also ˙O2− ion and ˙OH radical.

ReB3 has been synthesized and was reported to have symmetry of P63/mmc [Acta Chem. Scand. 1960, 14, 733]. However, we find that this structure is not stable due to its positive formation energy. In 2009, IrB1.35 and IrB1.1 were synthesized and were considered to be superhard [Chem. Mater. 2007, 21, 1407; ACS Appl. Mater. Interfaces 2010, 2, 581]. Inspired by these results, we explored the possible crystal structures of ReB3 and IrB3 by using the developed particle swarm optimization algorithm. We predict that Pm2-ReB3 and Amm2-IrB3 are the ground-state phases of ReB3 and IrB3, respectively. The stability, elastic properties, and electronic structures of the predicted structures were studied by first-principles calculations. The negative calculated formation enthalpies for Pm2-ReB3 and P63/mmc-ReB3 indicate that they are stable and can be synthesized under ambient pressure. Their dynamical stability is confirmed by calculated phonon dispersion curves. The predicted P63/mmc-ReB3 has the highest hardness among these predicted structures. The calculated density of state shows that these predicted structures are metallic. The chemical bonding features of the predicted ReB3 and IrB3 were investigated by analyzing their electronic localization function.

•We have compared the transport properties and electronic structure.•The transport properties of p-type Ba3M3P5 are better than that of n-type ones.•p-type Ba3Ga3P5 shows better transport properties than p-type Ba3Al3P5.•In the upper valence band of Ba3Ga3P5 has the multiple extrema.The electronic structure and the thermoelectric properties of Zintl compounds Ba3M3P5 (M  = Al, Ga) were investigated by the density functional theory (DFT) combined with the semiclassical Boltzmann transport theory. It is found that the transport properties of p-type Ba3M3P5 are better than that of n-type one at optimum carrier concentration. By p-type doping, the maximum ZT of Ba3Al3P5 and p-type Ba3Ga3P5 can reach 0.49 at 500 K and 0.65 at 800 K, corresponding to the carrier concentration of 7.1 ×× 1019 holes per cm3 and 1.3 ×× 1020 holes per cm3, respectively. The higher thermoelectric performance of p-type Ba3M3P5 than n-type one is mainly due to the large valence band dispersion near the Fermi level. For Ba3Ga3P5, the multiple extrema on the top of valence bands will increase its electrical conductivity. The calculated partial charge density near the Fermi level of Ba3M3P5 shows that there is little charge density around the P1 atoms in Ba3Al3P5. On the contrary, the high charge density appears around all P atoms in Ba3Ga3P5, which may be the reason why Ba3Ga3P5 has multiple extrema on its top of valence bands. Meanwhile, the minimum lattice thermal conductivities of Ba3Al3P5 and Ba3Ga3P5, are small and are comparable to those of Ca5Al2Sb6 and Ca5Ga2Sb6. Compared with p-type Ba3Al3P5P5, p-type Ba3Ga3P5 shows better thermoelectric properties, which is mainly due to the multiple extrema on its top of the valence bands and its small band gap. Moreover, p-type Ba3Ga3P5 shows nearly isotropic transport behavior. Hence, good thermoelectric performance for p-type Ba3Ga3P5 can be predicted.

When elemental sulfur was used to hyperdope crystalline silicon to a supersaturated density of ~1020 cm−3, it was found to enhance the sub-bandgap light absorptance of the silicon substrate from 0 up to 70 % when combined with the antireflection properties of the surface dome structures that were formed by surface texturing. These textured sulfur-hyperdoped silicon samples were then thermally annealed at various temperatures, and the effects of the annealing on each sample’s optical and electrical properties were investigated. In the silicon sub-bandgap wavelength range, the absorptance of the textured hyperdoped silicon was attenuated more slowly than that of a non-textured sample, and the modulation of its reflectance and transmittance properties was attributed to the density damping of the optically absorbing state. In addition, the optically absorbing state can release more electrons than the optically non-absorbing state, and the former state also has a stronger ability to scatter electrons than the latter.

The thermoelectric properties and the electronic structure of Sr5Sn2As6 were studied according to the first principles and semi-classical Boltzmann theory. To elucidate the thermoelectric performance of Sr5Sn2As6, we simulated its carrier concentration, Seebeck coefficient, and electrical conductivity, and provided an estimated value for the thermoelectric figure of merit ZT. For pure Sr5Sn2As6, the largest Seebeck coefficient (S) is 248 (μV K−1) at 500 K, and the minimum S is 202 (μV K−1) at 1200 K. The large Seebeck coefficient over a wide temperature range most likely results from the appropriate band gap (0.55 eV) of Sr5Sn2As6. By studying the carrier concentration dependence of the transport properties, the ZT value for p-type doping was found to be ∼1.4 times that of n-type doping, which is mainly due to the larger effective mass of the valence band. Moreover, for n-type doping, both the Seebeck coefficient and the electrical conductivity along the z-direction are much larger than those along the other directions, due to the large band degeneracy and the light band, which results in a highest ZT value of 3.0 along the z-direction, with a carrier concentration of 9.4 × 1019 electrons per cm3 at 950 K. The highest ZT value for p-type along the z-direction is 1.7, with a carrier concentration of 1.2 × 1020 holes per cm3 at 950 K. Meanwhile, the minimum lattice thermal conductivity of Sr5Sn2As6 is small (0.47 W m−1 K−1), and is comparable to those of Ca5M2Sb6 (M = Al, Ga, In). Hence, good thermoelectric performance along the z-direction for n-type Sr5Sn2As6 was predicted.

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the material's electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

An epitaxial pseudocubic SmFeO3 thin film on (100) Nb-SrTiO3 was studied based on ferroelectric (FE) characterization and magnetic measurements. High-resolution transmission electron microscopy images clarify the nature of the epitaxial growth, the stress-induced structural distortion at the film/substrate interface, and the existence of two different orientation lattices. Clear grain boundaries can be seen, which could introduce an extra local distortion. Rectangular FE loops can be observed at room temperature, even by just applying a small voltage ranging from −1 to +1 V, indicative of the presence of FE polarization. Piezoelectric force microscopy images confirm the existence of FE domains and the switchable polarization. A strong ferromagnetic-like transition occurs around 185 K, which is much lower than the transition observed in the bulk sample. It is believed that the pseudocubic structure enhances FE polarization and decreases the magnetic ordering temperature, which is confirmed by the first-principles theoretical calculations. Meanwhile, the ferroelectricity in this thin film should originate from distortion and modification in the structural modules rather than from the exchange striction interaction that is found in the bulk SmFeO3.Keywords: interface; multiferroicity; strain;

The electronic structure and the thermoelectric properties of M2Zn5As4 (M = K, Rb) are studied by the first principles and the semiclassical BoltzTraP theory. It is determined that they are semiconductors with an indirect band gap of about 1 eV, which is much larger than that of Ca5Al2Sb6 (0.50 eV). The calculated electronic localization function indicates that they are typical Zintl bonding compounds. The combination of heavy and light bands near the valence band maximum may improve their thermoelectric performance. Rb2Zn5As4 exhibits relatively large Seebeck coefficients, high electrical conductivities, and the large “maximum” thermoelectric figures of merit (ZeT). Compared with Ca5Al2Sb6, the highest ZeT of Rb2Zn5As4 appears at relatively low carrier concentration. For Rb2Zn4As5, the p-type doping may achieve a higher thermoelectric performance than n-type doping. The thermoelectric properties of Rb2Zn5As4 are possibly superior to those of Ca5Al2Sb6.

The effects of doping ZnO nanowires with Al, Ga and Sb on their electronic structure and thermoelectric properties are investigated by first-principles calculations. We find that the band gap of ZnO nanowires is narrowed after doping with Al and Ga, while band gap broadening is observed in Sb doped ZnO nanowires. The lattice thermal conductivity of ZnO nanowires is obtained based on the Debye–Callaway model. The thermoelectric properties of ZnO nanowires were calculated using the BoltzTraP code. The results show that there exists an optimal carrier concentration yielding the maximum value of ZT for Al, Ga and Sb doped ZnO nanowires at room temperature. The maximum value of ZT, 0.147, is obtained for Ga doped ZnO nanowires, when the carrier concentration is 3.62 × 1019 cm−3. The figure of merit ZT of Sb doped ZnO nanowires is higher than that of Ga doped ZnO nanowires when the temperature is between 400 K and 1200 K. We also find that Al doped ZnO nanowires always have poor thermoelectric properties, which means that the Al dopant may not be the optimal choice for ZnO nanowires in thermoelectric applications.

A high value (1.4) of figure of merit (ZT) of CuGaTe2 has been experimentally discovered by T. Plirdpring et al. [Adv. Mater. 24, 3622 (2012)]. In order to further enhance its thermoelectric properties, we investigated its electronic structure and thermoelectric properties by first-principles study. Large band-valley number and high convergence of the bottom conduction bands induced a large Seebeck coefficient and a high electrical conductivity of n-type CuGaTe2. So, for n-type CuGaTe2, the maximum ZT values 2.1 may be found at 950 K by suitable carrier concentration tuning, which results in a 25% increment in the ZT value compared with p-type CuGaTe2. Band decomposed charge density calculations indicate that the transport properties are mainly determined by the Cu and Te atoms at the valence-band maximum, in contrast, transport properties are simultaneously affected by the three kind of atoms at the conduction-band minimum. At high temperature, ab initio molecular dynamics calculations demonstrate that Cu atoms precipitated from their crystal matrices lead to a decrease in thermopower. Along the high symmetry point M, the charge density of all atoms is centrosymmetric. Maybe this centrosymmetric electronic structure leads to the conduction band valley convergence at the M point.

•A new stable trigonal R-3m structure is uncovered for ReB4 by first-principles.•Calculated phonon and formation enthalpy confirm its stability.•Calculated elastic properties show that R-3m ReB4 should be a new ultra-incompressible hard material.Using newly developed particle swarm optimization algorithm on crystal structural prediction, we predicted a trigonal R-3m structure for ReB4. Its structure, elastic properties, electronic structure, phase stability, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that R-3m ReB4 is thermodynamically and mechanically stable. Our calculations on the enthalpy-pressure relationship and convex hulls have demonstrated R-3m ReB4 can be synthesized at ambient condition. The large bulk modulus, large shear modulus, large Young’s modulus, and small Poisson’s ratio show that ReB4 should be a new ultra-incompressible hard material. Further analysis on density of states and electron localization function demonstrate that there are strong B–B and B–Re covalent bonds in R-3m ReB4.

The high-pressure structures of molybdenum (Mo) at zero temperature have been extensively explored through the newly developed particle swarm optimization (PSO) algorithm on crystal structural prediction. All the experimental and earlier theoretical structures were successfully reproduced in certain pressure ranges, validating our methodology in application to Mo. A double-hexagonal close-packed (dhcp) structure found by Mikhaylushkin et al. (2008) [12] is confirmed by the present PSO calculations. The lattice parameters and physical properties of the dhcp phase were investigated based on first principles calculations. The phase transition occurs only from bcc phase to dhcp phase at 660 GPa and at zero temperature. The calculated acoustic velocities also indicate a transition from the bcc to dhcp phases for Mo. More intriguingly, the calculated density of states (DOS) shows that the dhcp structure remains metallic. The calculated electron density difference (EDD) reveals that its valence electrons are localized in the interstitial regions.Graphical abstractHighlights► A double-hexagonal close-packed (dhcp) structure of molybdenum is predicted. ► Calculated acoustic velocity confirms the bcc–dhcp phase transition at 660 GPa. ► The valence electrons of dhcp Mo are mostly localized in the interstitial sites.

•Transport properties of p-type CrSi2 may be better than that of n-type one.•The high anisotropic electrical conductivity of p-type doping induces this behavior.•Effective mass of holes along z direction is smaller than that along x direction.A comparative study of thermoelectric performances about CrSi2 and β-FeSi2 were performed by using density functional theory and Boltzmann transport theory. It is found that the transport properties of p-type CrSi2 could be better than that of n-type. The high thermoelectric performance of p-type CrSi2 are possibly due to the high anisotropy of its electrical conductivity with p-type doping. For CrSi2, the effective mass of holes along the z direction is smaller than that along the x direction, and consequently the hole mobility along the z direction may be higher than that along the x direction. A high thermoelectric performance of CrSi2 could be achieved by hole doping with concentration range of 1–6 × 1021 cm−3. The transport properties of n-type β-FeSi2 may be better than p-type one.

Spin-dependent thermoelectric effects in Aharonov–Bohm (AB) interferometer with four Rashba quantum dots (QDs) coupled to external two one-dimensional semi-infinite metallic leads are investigated theoretically. The basic thermoelectric transport characteristics, like charge (spin) Seebeck coefficient, and the charge (spin) figure of merits, have been calculated in terms of Green's function formalism. It is found that a pure spin-up (spin-down) Seebeck coefficient can be generated by the coaction of the magnetic flux and the Rashba spin-orbit interaction (RSOI) at room temperature.Highlights► We investigated the spin-dependent thermoelectric effects in quantum dots system. ► The thermoelectric characteristics have been calculated with Green's function method. ► We found that a pure spin Seebeck coefficient can be generated at room temperature.

Based on ab initio electronic structure calculations and Boltzmann transport theory, the size dependence of thermoelectric properties of ZnO single-walled nanotubes (SWNTs) and nanowires was investigated. There is an optimal carrier concentration yielding the maximum value of ZT at room temperature. The optimal carrier concentration and the maximum value of ZT depend on the diameters and structure of ZnO. The maximum value of ZT for ZnO SWNTs are remarkably higher than that of ZnO nanowires. The 9.60 Å ZnO SWNT possess the highest ZT value, 0.322, which is nearly 3-fold higher than that of the best experimental samples at room temperature.

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.

The structural, electronic, and thermoelectric properties of InSe nanotubes (InSeNTs) are investigated using first-principle calculations based on density functional theory (DFT). The thermoelectric transport coefficients of InSeNTs at room temperature are calculated within the semiclassical Boltzmann theory. Calculated total energies show that (2,2) InSeNT is more stable than other ones. As a consequence of high Seebeck coefficient and reasonable electrical conductivity, its thermoelectric powerfactor with respect to relaxation time is much larger than those of other studied InSeNTs and is nearly 10 times larger than that of BiSb nanotube. The light and heavily bands appear together around the Fermi level in (2,2) InSeNT, inducing its high Seebeck coefficient and reasonable electrical conductivity. (2,2), (4,0), and (6,0) InSeNTs are metallic. (3,3), (4,4), (6,6), and (8,0) InSeNTs are semiconducting. It is found that each Se atom in the semiconducting InSeNTs is coordinated by two In atoms, and that in the metallic InSeNTs is coordinated by three In atoms. The current research proposes a new type of nanotubes to design high-performance thermoelectric materials.

Small three-photon absorption (3PA) cross-section values of present nonlinear organic molecules limit their practical applications. Although electron donors and electron acceptors have a great effect on 3PA cross-section, little is known about how the strength and situation of electron acceptors influence the 3PA cross-section value of a compound. The present work reports 3PA effects of two fluorene derivatives with symmetric D-π-π(A)-π-D archetype, which are named as 2,7-bis(4-methoxyphenylacetylene)-9-fluorenone (FATT) and 2,7-bis(4-methoxyphenylacetylene)-9-thoine-fluorene (TSATL). Large 3PA cross-section and ideal 3PA-induced optical limiting effects have been found in the two fluorene derivatives. The two molecules both have a different electron acceptor on the fluorene core, by which the 3PA cross-section value for FATT is enhanced by nearly 3-fold compared with that for TSATL. The mechanism of this significant enhancement in 3PA cross-section has been investigated by density functional theory (DFT) and configuration interaction singles (CIS) method with use of 6-311+G basis set in combination with conductor polarizable continuum model (CPCM). The theoretical results show that increase of electronegative character of the electron acceptor on the core is responsible for the increase of 3PA cross-section values of the two molecules.

Structural relaxations, electronic properties, and surface energies of ReB2 (001) and (110) surfaces with various terminations are investigated with a first-principles method. It is found that the surface interatomic spacings of ReB2 (001) and (110) surfaces are different from those of the bulk structure. The vertical spacings between the first and second layers of the studied surfaces are contracted. The (001)-Re surface is likely to be stable without introducing a large relaxation. Among these surfaces, only the (110) surface has surface rumpling, and the Re atoms on its first layer are apt to move inward. After atomic relaxation, some covalent bonds formed by the outmost atoms of the relaxed surfaces are shorter than those of the bulk system, which indicates that the covalent B–B and Re–B bonds of the surface layer have been strengthened. An analysis of surface energies shows that after relaxation, the (001)-Re surface is more stable than other types of surfaces.Highlights► For ReB2 (001) and (110) surface, the spacings between the top two layers are contracted. ► The Re-B covalent interaction is strengthened. ► The relaxed (001)-Re surface is more stable than other ones.

Superhard IrB1.35 [Chem. Mater.2009, 21, 1407] and IrB1.1 film [ACS Appl. Mater. Interfaces2010, 2, 581] have been synthesized in experiment, but the structural formulas of iridium borides with integral ratio between Ir and B atoms are still undefined up to now. Here, we use a combination of particle-swarm optimization technique and first-principles calculations to explore the crystal structures of IrB and IrB2. We demonstrate that the new phase P1–IrB belongs to the orthorhombic Pnma space group, while P5–IrB2 (space group Pmmn) has a same structure type with OsB2. At the pressure of about 5 GPa, a phase transition occurs between the Pnma and anti-NiAs phases for P1–IrB. Further phonon and elastic constants calculations imply that both P1–IrB and P5–IrB2 are dynamically and mechanically stable and are potential low compressible materials because of their high bulk moduli. The analysis of density of states and chemical bonding indicates that the formation of strong covalent bonding in these compounds contributes greatly to their stabilities.

First-principles calculations were employed to investigate the effect of native defects and a hydrogen-related defect complex on ferromagnetism in an undoped ZnO semiconductor. The results show that the zinc vacancy (VZn) could lead to a moment of 1.73 μB in the undoped ZnO supercell, while the oxygen vacancy could not, but the formation energy of the zinc vacancy is much higher than that of the oxygen vacancy. When the hydrogen atom is doped in imperfect ZnO, the formation energy of VZn+HI sharply decreases, compared with that of VZn. Meanwhile, the VZn+HI defect complex can induce a 0.99(0.65) μB moment in the Zn15HIO16 supercell. Furthermore, the total energy of the ZnO supercell with two defect complexes for the ferromagnetic phase is lower than that for the antiferromagnetic phase, and the calculated results show that a strong magnetic coupling exists in the ferromagnetic phase. As an unintentionally doped element, H usually appears in ZnO prepared by many methods. So the ferromagnetism in the ZnO d0 semiconductor most probably arises from the defect complex of the zinc vacancy and H.

The electronic structure and transport properties of Ca5Al2Sb6 are investigated by using first-principles calculations and Boltzmann transport theory, respectively. The results show that the partially filled valence band induces a high carrier concentration of about n = 5 × 1019 cm−3. There is a combination of heavy and light bands at the conduction band edge, which may lead to a combination of high Seebeck coefficient and reasonable conductivity. At mid-and-high temperature, the thermoelectric powerfactor, with respect to relaxation time of p-type Ca5Al2Sb6, is higher than that of the n-type within the carrier concentration ranging from −10 × 1021 cm−3 to 10 × 1021 cm−3, without considering the kinds of doping. But with decreasing temperature, the thermoelectric powerfactor with respect to relaxation time of n-type Ca5Al2Sb6 is higher than that of the p-type. The thermoelectric coefficient is increasingly sensitive to carrier concentration with the decreasing temperature. Most strikingly, at 30 K, the thermoelectric powerfactor, with respect to relaxation time, is nearly thirty-five times larger than that of conventional n-type thermoelectric materials. At 300 K, the thermoelectric powerfactor with respect to relaxation time of Ca5Al2Sb6 is equal to that of the conventional p-type thermoelectric materials. Our theoretical calculations give valuable insight on how to improve the thermoelectric performance of Ca5Al2Sb6 under different temperature and doping conditions.

Using the density-functional approach, the geometries, stabilities, electronic properties, and magnetism of the YnSi (n = 2–14) clusters have been systematically investigated. The growth pattern for the different-sized YnSi (n = 2–14) clusters is Si-substituting Yn+1 clusters and keeps the similar frameworks of the most stable Yn+1 clusters. The Si atom substitutes the surface atom of the Yn+1 clusters for n < 8. Starting from n = 8, the Si atom completely falls into the center of the Y-frame. The Si atom substitutes the center atom of the Yn+1 clusters to form the Si-encapsulated Yn geometries for n > 8. The calculated results show that doping of Si atom contributes to strengthening the stabilities of the yttrium framework. In addition, the relative stability of Y12Si is the strongest among the investigated YnSi clusters, which might stem from its highest symmetry. Mulliken population analysis shows that charges always transfer from Y atoms to Si atom in all the YnSi (n = 2–14) clusters. Doping of Si atom decreases the magnetic moments of the most Yn clusters. In particular, the magnetic moment does quench completely after doping Si in Y13, which is ascribed to the disappearance of the itinerant 4d electron spin exchange effect. Finally, the frontier orbitals properties of YnSi are also discussed.Highlights►This paper systematically studied the electronic properties of the YnSi clusters. ► The results show that Si atom contributes to strengthening the stabilities of Yn. ► Mulliken population analyses show that charges always transfer from Y to Si atom. ► Doping of Si atom decreases the magnetic moment of the most Yn clusters.

Using first-principles calculations, we systematically studied the mechanical properties and electronic structure of the recently synthesized diamondlike BC5. Our calculated bulk modulus B, shear modulus G  , elastic constant c44c44, and theoretical hardness H confirm that BC5 is an ultraincompressible and superhard material. Also, it exhibits mechanical stability and metallic features. Electronic structures show that a strong covalent bond network through sp3sp3 hybridization is the origin of the excellent mechanical properties of BC5. Our results show that BC5 has good prospects in electronic application as a superhard material.Research highlights► This paper systematically studied and argued the crystal structure of BC5 in theory. ► We have studied the mechanical properties and electronic structure of BC5. ► Our calculated results agree well with the experimental data.

The electronic structure and optical properties of In4Sn3O12 and In4Ge3O12 are studied by the projector-augmented-wave method based on the density-functional theory within the generalized gradient approximation. The cation ordering of the two compounds is explored by means of first-principles calculations. It is found that the valence-band maximum of the materials is determined by the d states of metal elements and O-2p states; the conduction-band minimum is occupied by an admixture of the O-2p states, In-5s states, and Sn-5s or Ge-4s states, respectively. The two compounds are direct-bandgap semiconductors. The low intensity of the absorption coefficient, reflectivity, and loss function shows that they are good transparent conducting oxides.

First-principles calculations were carried out to investigate the structural stability of synthesized orthorhombic Ta2N3. It is found that the stoichiometric orthorhombic Ta2N3 is unstable below 20 GPa. However, it can be stabilized by a small amount of nitrogen vacancies or oxygen substitution into nitrogen sites. The calculated electron localization function indicates that both the formation of nitrogen vacancy and the substitution of oxygen atom can enhance the Ta−N bonding, which is essential for the structural stability. Furthermore, both oxygen substitution and nitrogen vacancy plays a similar role in stabilizing the orthorhombic lattice of Ta2N3. The results of our calculations show that nitrogen vacancies or oxygen substitution into nitrogen sites can alter the charge distribution over the unit cell, which leads to a new arrangement of atoms and enhanced Ta−N bonds.

The thermodynamic and mechanical stabilities for the Mn–B system are investigated using the first-principles calculations method with density functional theory. The negative formation enthalpies of Mn2B–Al2Cu (Mn2B–Al2Cu represents Mn2B in the Al2Cu structure type, the same hereinafter), MnB–CrB, MnB–FeB, MnB2–ReB2, MnB2–AlB2, MnB3–TcP3, and MnB4 indicate that they are thermodynamically stable at zero pressure. It is found that MnB2–ReB2 is more energetically favorable than synthetic MnB2–AlB2, which indicates that experimental synthesized MnB2 is a metastable phase. Among these studied compounds, monoclinic MnB4 has the largest shear modulus, the largest Young’s modulus, and the smallest Poisson’s ratio. The results of density of states and Mulliken overlap population reveal the strong covalent bonding, which results in the high bulk and shear moduli as well as small Poisson’s ratio of MnB2–ReB2 and MnB4. An analysis of the elastic constants, elastic moduli, formation enthalpy, electronic structure, and theoretical hardness shows that MnB2–ReB2 and MnB4 are potential superhard materials.

Iridium carbides with various stoichiometries were investigated by using the first-principles method. It is found that iridium carbides with the iridium germanide structures usually exhibit a better elastic property. Among these studied iridium carbides, tetragonal IrC2 with the RhSn2 structure has the largest bulk modulus and trigonal IrC4 with the IrGe4 structure is found to exhibit the largest shear modulus and smallest Poisson’s ratio. Moreover, Ir4C5 with the Ir4Ge5 structure is a semiconductor and has high bulk and shear moduli. We also found that the shear modulus of iridium carbides with the iridium germanide structures basically increases with the increased carbon content, except Ir4C5; this is due to the fact that only Ir4C5 with the Ir4Ge5 structure exhibits a semiconductor property, whereas other structures all present metallic. The eight structures for IrC4 are all ductile, except for the IrGe4 structure of IrC4 showing brittleness. On the basis of the comparison of the crystal structure, elastic constants, elastic moduli, and electronic structure for iridium carbides, we expect that Ir4C5 and IrC4 with the iridium germanide structures are potential ultrahard materials.

Orthorhombic OsB2 was synthesized at 1000 °C and its compressibility was measured by using the high-pressure X-ray diffraction in a Diacell diamond anvil cell from ambient pressure to 32 GPa [R.W. Cumberland, et al. (2005)]. First-principles calculations were performed to study the possibility of the phase transition of OsB2. An analysis of the calculated enthalpy shows that orthorhombic OsB2 can transfer to the hexagonal phase at 10.8 GPa. The calculated results with the quasi-harmonic approximation indicate that this phase transition pressure is little affected by the thermal effect. The calculated phonon band structure shows that the hexagonal P 63/mmc structure (high-pressure phase) is stable for OsB2. We expect the phase transition can be further confirmed by the experimental work.Graphical Abstract Legend (TOC Figure)Table of Contents FigurePressure induced structural phase transition from the orthorhombic structure to the hexagonal one for OsB2 takes place under 10.8 GPa (0 K), 10.35 GPa (300, 1000 K) by the first-principles predictions.

Equilibrium geometries, adsorption energies, and electronic properties of a hydrogen molecule on Si3On (n = 1–6) clusters have been investigated using density functional theory. The hydrogen molecule preferably dissociates and all H atoms bind to the 2-fold coordinated Si atom with dangling bonds in nearly a fixed direction for the smaller clusters. The dissociated H atoms favor binding to the terminal no-bridged O atoms to form the SiOH radical in the rhombus chain structures as the oxygen contents increase. We also report the interaction between Si3O4 cluster and multi-H2 molecules. Our results show that, at first, the added hydrogen molecules tend to dissociate and neutralize the dangling bonds. After all the dangling bonds are neutralized, the hydrogen begins to adsorb on the complex in molecules with a small adsorption energy. In addition, the infrared and Raman spectra are valuable in distinguishing among adsorption and dissociation of the H2 molecules on silicon oxide.

The geometry, electronic structure, magnetism, and adsorption properties of one CO molecule on the MnN (N = 2−8) clusters have been investigated based on the density functional theory (DFT) with the spin polarized generalized gradient approximation. It is found that the CO molecule adsorbs on the atop site for N = 2, 4, 7, 8 and on the bridge site for N = 3, 5, 6. The results of the calculated second-order energy differences of bare MnN cluster indicate that the Mn3, Mn6, and Mn8 clusters have relatively low stability. However, their corresponding CO complexes possess high adsorption ability implied by the C−O bond length, vibrational frequency, adsorption energy, and the charge transfer between the CO molecule and the clusters. Compared with bare Mn clusters, the adsorbing of a CO molecule enhances the magnetic moments of the MnN clusters for N = 4, 6−8.

Mn-doped germanium clusters have been systematically investigated by using the density-functional approach. It was found that doping of one Mn atom contributes to strengthening the stability of the germanium framework. Maximum peaks of the second-order energy differences were observed for clusters of sizes n = 5, 9, 12, and 14, implying their relative higher stability than other-sized MnGen clusters. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gaps of the MnGen clusters, with the exception of MnGe14, are generally lower than the corresponding pure germanium clusters. We also found that charge always transfers from manganese to germanium atoms in all sized MnGen clusters and the magnetic moment of the Mn atom does not quench when embedded in all sized Gen (n = 2–16) clusters.

•Acceptor energy levels are induced by O interstitials; corresponding to a transition of indirect-to-direct band gap and a narrowing of band gap.•The Fermi levels of defected and reconstructed TiO2 anatse (101) can be modulated in a wide range.In this paper, reconstructions and native defects of TiO2 anatase (101) surface are studied using the state-of-the-art theoretical method. We find that O interstitials are dominated defects at an oxidization environment. These O interstitials induce acceptor energy levels, corresponding to an indirect-direct band transition and a bandgap narrowing. And thus, the experimental result that an O-rich anatase TiO2 has the higher photocatalytic activity can be understood. The formation of O vacancies and Ti interstitials becomes feasible at a reduced condition, and reconstructed TiO2 anatase (101)-(1 × 1) structures present with increasing reduction degree. Furthermore, the Fermi levels of defected and reconstructed TiO2 anatse (101) can be modulated in a wide range (i.e., nearly the whole band gap), which are different from those of TiO2 rutile (110).

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.

The electronic structure and transport properties of Ca5Al2Sb6 are investigated by using first-principles calculations and Boltzmann transport theory, respectively. The results show that the partially filled valence band induces a high carrier concentration of about n = 5 × 1019 cm−3. There is a combination of heavy and light bands at the conduction band edge, which may lead to a combination of high Seebeck coefficient and reasonable conductivity. At mid-and-high temperature, the thermoelectric powerfactor, with respect to relaxation time of p-type Ca5Al2Sb6, is higher than that of the n-type within the carrier concentration ranging from −10 × 1021 cm−3 to 10 × 1021 cm−3, without considering the kinds of doping. But with decreasing temperature, the thermoelectric powerfactor with respect to relaxation time of n-type Ca5Al2Sb6 is higher than that of the p-type. The thermoelectric coefficient is increasingly sensitive to carrier concentration with the decreasing temperature. Most strikingly, at 30 K, the thermoelectric powerfactor, with respect to relaxation time, is nearly thirty-five times larger than that of conventional n-type thermoelectric materials. At 300 K, the thermoelectric powerfactor with respect to relaxation time of Ca5Al2Sb6 is equal to that of the conventional p-type thermoelectric materials. Our theoretical calculations give valuable insight on how to improve the thermoelectric performance of Ca5Al2Sb6 under different temperature and doping conditions.

The electronic structure and the thermoelectric properties of M2Zn5As4 (M = K, Rb) are studied by the first principles and the semiclassical BoltzTraP theory. It is determined that they are semiconductors with an indirect band gap of about 1 eV, which is much larger than that of Ca5Al2Sb6 (0.50 eV). The calculated electronic localization function indicates that they are typical Zintl bonding compounds. The combination of heavy and light bands near the valence band maximum may improve their thermoelectric performance. Rb2Zn5As4 exhibits relatively large Seebeck coefficients, high electrical conductivities, and the large “maximum” thermoelectric figures of merit (ZeT). Compared with Ca5Al2Sb6, the highest ZeT of Rb2Zn5As4 appears at relatively low carrier concentration. For Rb2Zn4As5, the p-type doping may achieve a higher thermoelectric performance than n-type doping. The thermoelectric properties of Rb2Zn5As4 are possibly superior to those of Ca5Al2Sb6.

Magnetic ZnO, one of the most important diluted magnetic semiconductors (DMS), has attracted great scientific interest because of its possible technological applications in optomagnetic devices. Magnetism in this material is usually delicately tuned by the doping level, dislocations, and local structures. The rational control of magnetism in ZnO is a highly attractive approach for practical applications. Here, the tuning effect of biaxial strain on the d0 magnetism of native imperfect ZnO is demonstrated through first-principles calculations. Our calculation results show that strain conditions have little effect on the defect formation energy of Zn and O vacancies in ZnO, but they do affect the magnetism significantly. For a cation vacancy, increasing the compressive strain will obviously decrease its magnetic moment, while tensile strain cannot change the moment, which remains constant at 2 μB. For a singly charged anion vacancy, however, the dependence of the magnetic moment on strain is opposite to that of the Zn vacancy. Furthermore, the ferromagnetic state is always present, irrespective of the strain type, for ZnO with two zinc vacancies, 2VZns. A large tensile strain is favorable for improving the Curie temperature and realizing room temperature ferromagnetism for ZnO-based native semiconductors. For ZnO with two singly charged oxygen vacancies, 2V+Os, no ferromagnetic ordering can be observed. Our work points the way to the rational design of materials beyond ZnO with novel non-intrinsic functionality by simply tuning the strain in a thin film form.

Band engineering is one of the effective approaches for designing ideal thermoelectric materials. Introducing an intermediate band in the band gap of semiconducting thermoelectric compounds may largely increase the carrier concentration and improve the electrical conductivity of these compounds. We test this hypothesis by Pb doping in Zintl Ca5In2Sb6. In the current work, we have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5% (0.2 e per cell) for Ca5In2Sb6 by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca5In2Sb6, which originates from the Pb s states by weakly hybridizing with the Sb p states. Such an intermediate band dramatically increases the electrical conductivity of Ca5In2Sb6 and has little detrimental effect on its Seebeck coefficient, which may increase its thermoelectric figure of merit, ZT. Interestingly, a maximum ZT value of 2.46 may be achieved at 900 K for crystalline Pb-doped Ca5In2Sb6 when the carrier concentration is optimized. Therefore, Pb-doped Ca5In2Sb6 may be a promising thermoelectric material.

We have investigated the structural, electronic, and magnetic properties of A-site-ordered double-perovskite-structured oxides, AA′3B4O12 (A = Na, Ca, and La) with Mn and V at A′ and B sites, respectively, using first-principle calculations based on the density functional theory. Our calculation results show that the antiferromagnetic phase is the ground state for all the compounds. By changing the A-site ions from Na+ to Ca2+ and then to La3+, the transfer of charge between Mn and O ions was changed from 1.56 to 1.55 and then to 1.50, and that between the V and O ions changed from 2.01 to 1.95 and then to 1.93, revealing the cause for the unusual site-selective doping effect. Mn 3d electrons dominate the magnetic moment and are localized, with an intense hybridization with O 2p orbitals, which indicates that the magnetic exchange interaction between Mn ions is mediated through O and that the super exchange mechanism will take effect. These materials have a large one-electron bandwidth W, and the ratio of the on-site Coulomb repulsion U to W is less than the critical value (U/W)c, which leads to metallic behavior of AMn3V4O12. This is further evidenced by the large number of free electrons contributed by V at the Fermi surface. These calculations, in combination with the reported experimental data, prove that these double perovskites belong to the rare antiferromagnetic metallic oxides.

An experimentally synthesized graphene/Bi2WO6 composite showed an enhancement of the visible-light photocatalytic activity, while the underlying mechanism is not known. Here, first-principles calculations based on density functional theory were performed to explore the various properties of the graphene/Bi2WO6(010) composite aiming at gaining insights into the mechanism of its photocatalytic activity. The stability, electronic properties, charge transfer, and visible-light response were investigated in detail on the Bi2WO6(010) surface coupled with graphene. An analysis of charge distribution and Bader charge shows that there is a strong covalent bonding between graphene and the Bi2WO6(010) surface. The covalent interaction induces a small bandgap in graphene. The interband transition of graphene and the surface states of the Bi2WO6(010) surface would cause the absorption spectrum of graphene/Bi2WO6(010) to cover the entire visible-light region and even the infrared-light region. The photogenerated electrons flow to graphene from the conduction band of Bi2WO6 under the built-in electric field and band edge potential well. Thus, graphene serves as a photogenerated electron collector and transporter which significantly reduces the probability of electron–hole recombination and increases catalytic reaction sites not only on the surface of graphene but on also the surface of Bi2WO6. The decrease of charge recombination is possibly responsible for the enhancement of the visible-light photocatalytic activity of the graphene/Bi2WO6(010) nanocomposite.

The thermoelectric properties and the electronic structure of Sr5Sn2As6 were studied according to the first principles and semi-classical Boltzmann theory. To elucidate the thermoelectric performance of Sr5Sn2As6, we simulated its carrier concentration, Seebeck coefficient, and electrical conductivity, and provided an estimated value for the thermoelectric figure of merit ZT. For pure Sr5Sn2As6, the largest Seebeck coefficient (S) is 248 (μV K−1) at 500 K, and the minimum S is 202 (μV K−1) at 1200 K. The large Seebeck coefficient over a wide temperature range most likely results from the appropriate band gap (0.55 eV) of Sr5Sn2As6. By studying the carrier concentration dependence of the transport properties, the ZT value for p-type doping was found to be ∼1.4 times that of n-type doping, which is mainly due to the larger effective mass of the valence band. Moreover, for n-type doping, both the Seebeck coefficient and the electrical conductivity along the z-direction are much larger than those along the other directions, due to the large band degeneracy and the light band, which results in a highest ZT value of 3.0 along the z-direction, with a carrier concentration of 9.4 × 1019 electrons per cm3 at 950 K. The highest ZT value for p-type along the z-direction is 1.7, with a carrier concentration of 1.2 × 1020 holes per cm3 at 950 K. Meanwhile, the minimum lattice thermal conductivity of Sr5Sn2As6 is small (0.47 W m−1 K−1), and is comparable to those of Ca5M2Sb6 (M = Al, Ga, In). Hence, good thermoelectric performance along the z-direction for n-type Sr5Sn2As6 was predicted.

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the material's electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

The effects of doping ZnO nanowires with Al, Ga and Sb on their electronic structure and thermoelectric properties are investigated by first-principles calculations. We find that the band gap of ZnO nanowires is narrowed after doping with Al and Ga, while band gap broadening is observed in Sb doped ZnO nanowires. The lattice thermal conductivity of ZnO nanowires is obtained based on the Debye–Callaway model. The thermoelectric properties of ZnO nanowires were calculated using the BoltzTraP code. The results show that there exists an optimal carrier concentration yielding the maximum value of ZT for Al, Ga and Sb doped ZnO nanowires at room temperature. The maximum value of ZT, 0.147, is obtained for Ga doped ZnO nanowires, when the carrier concentration is 3.62 × 1019 cm−3. The figure of merit ZT of Sb doped ZnO nanowires is higher than that of Ga doped ZnO nanowires when the temperature is between 400 K and 1200 K. We also find that Al doped ZnO nanowires always have poor thermoelectric properties, which means that the Al dopant may not be the optimal choice for ZnO nanowires in thermoelectric applications.