Preparation and Characterization of Epitaxial Conductive Nb-SrTiO<Subscript>3</Subscript> Thin Films on Si Substrates

Epitaxial Nb-doped SrTiO₃ (001) thin films have been integrated on Si(001) single-crystal substrates with a TiN template layer by radio-frequency (RF) sputtering. The epitaxial growth of Nb-SrTiO₃ was achieved in an Ar environment at a substrate temperature of 540°C. The orientation relationship was determined to be simply cube-on-cube as Nb-SrTiO₃ (001)[110]||TiN(001) [110]||Si(001)[110]. The deposited Nb-SrTiO₃ films show a smooth and featureless surface with roughness below 1 nm. These smooth Nb-doped SrTiO₃/TiN/Si films have a conductivity of 500 S/cm and could be promising electrodes for subsequent ferroelectric thin films.     

Nb-doped Ga2O3 as charge-trapping layer for nonvolatile memory applications

The charge-trapping properties of Nb-doped Ga₂O₃ are investigated by using an Al/Al₂O₃/GaNbO/SiO₂/Si structure. Compared with the memory capacitor with pure Ga₂O₃, the one with lightly Nb-doped Ga₂O₃ shows better charge-trapping characteristics, including larger memory window (5.5 V at ± 6 V sweeping voltage), higher programming and erasing speeds due to its higher trapping efficiency resulted from increased trap density induced by the Nb doping. The sample with heavily Nb-doped Ga₂O₃ also shows improvements compared with the one with pure Ga₂O₃, but not as large as those for the lightly Nb-doped sample due to defects generated by excessive Nb doping. Erase saturation phenomenon is observed in all the samples, and can be suppressed by hole traps created by the Nb doping.     

Controlling Interface Intermixing and Properties of SrTiO3‐Based Superlattices

By combining state‐of‐the‐art microscopy, spectrosccopy, and first‐principles calculations, atomic‐scale intermixing behavior at heterointerfaces in SrTiO₃‐based superlattices is investigated. It is found that Nb is confined to a unit‐cell thickness without intermixing, whereas Ba diffuses only to the adjoining Nb‐doped SrTiO₃ layer. It is revealed that the intermixing behaviors at the heterointerfaces are determined by not only the migration energy, but also by the vacancy‐formation energy and the Fermi energy of each layer. Based on these results, we find a method to control the atomic‐scale intermixing at the nonpolar heterointerfaces and clearly demonstrate the property improvements obtained by constructing an abrupt heterointerface.     

Electron transport behaviour in Nb-doped SrTiO3 bicrystals

In order to elucidate the relationship between electron transport behaviour and defect chemistry, grain boundary structure and current-voltage characteristics across the boundary were investigated for Nb-doped SrTiO3 bicrystals. Two kinds of boundaries, i.e. small angle and random boundaries, were prepared for 0.2at% and 1.0at% Nb-doped SrTiO3. The bicrystals were prepared by joining two single crystals at 1400 degrees C for 10 h under a pressure of 0.4 MPa in air. High-resolution transmission electron microscopy (HRTEM) study revealed that all of the joined boundaries are free from any secondary phases or amorphous films. On the other hand, it was found that non-linearity in current-voltage dependence becomes remarkable by reduction of cooling rate after joining in small angle boundaries of 0.2at% Nb-doped SrTiO3 bicrystal. In addition, the random boundary of 1.0at% Nb-doped SrTiO3 bicrystal exhibits clear alpha = 2 I-V relation, which appears across a contact of semiconductor-insulator-semiconductor (n-i-n). From the results of HRTEM study and I-V behaviours, it could be concluded that the electron transport mechanism is controlled mainly by defect chemistry and not by the grain boundary structure.     

Phosphorene field-effect transistors using high-k gate dielectrics of epitaxial SrTiO3 layers grown on Nb-doped SrTiO3 substrates

Epitaxial SrTiO₃ (STO) thin film as a gate dielectric layer was grown on single crystalline (100) Nb-doped SrTiO₃ substrate. On the 100-nm-thick STO gate dielectric layer, a 5-nm-thick phosphorene sheet channel layer was exfoliated from a bulk crystal. A phosphorene field-effect transistor (P-STO-FET) was prepared by the formation of 90-nm-thick Au source/drain (S/D) contacts. The P-STO-FET exhibited the transport characteristics of a p-type transistor with a mobility of approximately 376 cm⁻²/Vs and an on/off ratio of approximately 10³. Furthermore, it was experimentally confirmed that the mobility of the P-STO-FET was significantly influenced by the flatness of the phosphorene sheet.     

First principle study of Nb defects in anatase (101) TiO2 surface

In this work, effects of Niobium (Nb) defects on TiO₂ surface using density functional theory (DFT) are investigated. Based on formation energy of the defects, their occurrences in two different extreme conditions, O-rich and O-poor conditions, are evaluated. Effects of Nb defects on surface and its electronic structure are studied and it is demonstrated that Nb doping widens valence band in deep energy level leaving the band gap without any change and it also lowers oxygen vacancy defect concentration due to the stronger bonding of Nb_sub defect with oxygen atoms specially bridging oxygen (most probable defect site for Oxygen vacancy). Higher density of Nb substitutional defects (Nb_sub) are examined and it is shown that higher density doping of TiO₂ surface leads to uniform distribution of defects over the anatase structure as a result of interaction of Nb defects when they are close and this fact prevents segregation of Nb atoms in Nb-doped TiO₂.     

N and p-type properties in organo-metal halide perovskites studied by Seebeck effects

Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH₃NH₃PbI₃) and mixed-halide structures (CH₃NH₃PbI_xCl_3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.     

Effects of YSZ Additions on Thermoelectric Properties of Nb-Doped Strontium Titanate

The effect of yttria-stabilized zirconia (YSZ) with a low thermal conductivity on the thermoelectric properties of Nb-doped SrTiO₃ bulk materials, which were fabricated by the conventional normal pressure sintering method in an Ar atmosphere, was examined. YSZ additions reduced the thermal conductivity but significantly enhanced the electrical conductivity. However, the Seebeck coefficient was nearly independent of YSZ content. Thus, the ZT value was enhanced, and a sample with 3 wt.% YSZ displayed the maximum ZT value, 0.21, at 900 K. Additionally, the reason for the reduced thermal conductivity and enhanced electrical conductivity by YSZ additions was investigated in detail.     

Mechanistic Understanding of Alkali-Metal-Ion Effect on Defect State in SrTiO3 During the Defect Engineering for Boosting Solar Water Splitting

Decreasing the Ti³⁺ defect concentration in SrTiO₃ is recognized as the focus of seeking a perfect photocatalytic activity. However, the defect-engineered SrTiO₃ by doping different low-valence metal ions have no similarly good activity in overall water spitting, of which the reason has not been determined. Here, Na⁺, K⁺, and Cs are deliberately doped in SrTiO₃ to obtain a close low-level concentration of Ti³⁺ defect for comparison. An interesting phenomenon is found that the K-doped SrTiO₃ has a higher Ti³⁺ concentration than others but with a better intrinsic photocatalytic activity. The analysis of band structure and charge-carriers behavior indicates the formation of a deep-state defect in Na or Cs-doped SrTiO₃, leading to a decrease in photocatalytic activity. With the density functional theory calculation and synchrotron radiation characterization, the differences in A-site-metal ionic polarization are found to localize some defect charge, accompanied by uneven structural relaxation. As a result, the deep-state defect can form in the area containing some group of atoms around the higher-polarization metal ions, limiting the activity. This study further complements the important effect of doping ions on defect state in addition to the Ti³⁺ defect concentration in theory, for designing defect engineering in SrTiO₃.     

氮掺杂浓度对氮掺杂PbTiO3电子结构和光学性能的影响

用第一性原理计算研究了不同N掺杂浓度的P型N掺杂PbTiO3的电荷密度差分、能带结构、态密度和光学性质。用N原子取代PbTiO3中的O原子后,PbTiO3的价带向高能级发生移动,费米能级进入价带顶部。随着N掺杂浓度的增加,能带带隙值变窄,结构稳定性变差。掺杂浓度2.5 at%时,N掺杂PbTiO3显示最好的P型导电性和最强的可见光吸收。N掺杂PbTiO3在半导体光电器件和光催化领域具有重要的应用价值。     

A First Principles Study of Ferroelectricity and Magnetism Coexisting in Fe-Doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

First-principles calculations were performed to investigate the coexistence of ferroelectricity and magnetism in Fe-doped tetragonal K_0.5Na_0.5NbO₃ (KNN) with different Fe concentrations. It was found that all three systems can possess both magnetism and ferroelectricity, and the magnetism becomes stronger with an increase in the concentration of Fe. The magnetic properties are due to ferromagnetism of Fe, and the Fe-Nb and Fe-O bonds induce magnetic moments on Nb and O atoms. Ferroelectricity comes from the hybridization of Nb and O, as in the case of pure KNN. The predicted results provide insight into the origin of the induced magnetic moments and an approach for obtaining multiferroic materials.     

Correlated Lattice Instability and Emergent Charged Domain Walls at Oxide Heterointerfaces

Charged domain walls provide possibilities in effectively manipulating electrons at nanoscales for developing next‐generation electronic devices. Here, using the atom‐resolved imaging and spectroscopy on LaAlO₃/SrTiO₃//NdGaO₃ heterostructures, the evolution of correlated lattice instability and charged domain walls is visualized crossing the conducting LaAlO₃/SrTiO₃ heterointerface. When increasing the SrTiO₃ layer thickness to 20 unit cells and above, both LaAlO₃ and SrTiO₃ layers begin to exhibit measurable polar displacements to form a tail‐to‐tail charged domain wall at the LaAlO₃/SrTiO₃ interface, resulting in the charged redistribution within the 2‐nm‐thick SrTiO₃ layer close to the LaAlO₃/SrTiO₃ interface. The mobile charges in different heterostructures can be estimated by summing up Ti³⁺ concentrations in the conducting channel, which is sandwiched by SrTiO₃ layers with interdiffusion and/or oxygen octahedral rotations. Those estimated mobile charges are quantitatively consistent with results from Hall measurements. The results not only shed light on complex oxide heterointerfaces, but also pave a new path to nanoscale charge engineering.     

Measure the barrier height of manganite p–n heterojunction by activation energy measurement

The barrier height of the manganite based p–n heterojunction is identified from the activation energy. The La_0.35Pr_0.32Ca_0.33MnO₃/Nb-doped SrTiO₃ p–n heterojunction is fabricated by the pulse laser deposition technology. The junction shows good rectifying behavior which can be well described by the Shockley equation. A satisfactorily logarithmic linear dependence of resistance on temperature is observed, and also the relation between bias and activation energy (E_A) deduced from the R−1/T curves is linear. As a result, the interfacial barrier of the heterojunction is obtained by extrapolating the Bias –E_A plot to Y axis, which is 0.95 eV.     

The syntheses and microstructures of tabular SrTiO3 crystal

In the present paper, the microstructures of SrTiO₃ particles obtained in two different methods were examined by scanning electron microscopy (SEM) and X-ray diffraction pattern (XRD). The two synthesis techniques included a conventional mixed solid method and a molten salt synthesis method (MSS), which proceeded through two steps. In the first, precursor Sr₃Ti₂O₇ particles were synthesized. Tabular SrTiO₃ crystals were synthesized via the superposition of the SrTiO₃ basic cell on the interface of Sr₃Ti₂O₇ particles. The microstructures of SrTiO₃ particles synthesis in those two ways were quite different. The microstructures of SrTiO₃ obtained by MSS method were high purity and obviously tabular in structure. Oriented growth faces included typical (0 0 1), (1 0 0), (1 1 0), etc. The mechanism of the oriented growth of tabular SrTiO₃ could be considered as the superposition of coordination polyhedron Ti–O₆ octahedron basic cell on the interface of Sr₃Ti₂O₇ particles.     

Size effect on polarization switching kinetics of P(VDF-TrFE) copolymer nanodots

We demonstrate polarization switching kinetics of poly(vinylidene fluoride-ran-trifluoroethylene) P(VDF-TrFE) nanodots depending on their sizes. Ferroelectric copolymer P(VDF-TrFE) nanodots, ranging from 20 to 250 nm in lateral size, were deposited on a Nb-doped SrTiO₃ substrate by a position-controlled dip-pen nanolithography technique. Ferroelectric switching characteristics of P(VDF-TrFE) nanodots was investigated by piezoresponse force microscopy. Asymmetric switching behavior was observed during switching process, indicating that the threshold electric fields for polarization reversal were measured differently under the same conditions according to the switching polarity. Significantly, when the size of P(VDF-TrFE) nanodot approached around 250 nm, electric fields for polarization switching were compatible with those of the P(VDF-TrFE) thin film. We suggest that the electric fields required for polarization reversal of the P(VDF-TrFE) nanodots present asymmetric switching behaviors, and asymptotic behavior be observed as the nanodots increase in lateral size.     

The electronic structures and optical properties of BaTiO3 and SrTiO3 using first-principles calculations

The electronic-energy band structures and total density of states (TDOS) for bulk BaTiO₃ and SrTiO₃ were calculated by the first-principles calculations using density-functional theory and local-density approximation. The calculated band structure of BaTiO₃ and SrTiO₃ show the energy band gaps of 1.81 and 1.92 eV at the Γ point in the Brillouin zone, respectively. The optical properties of the both perovskites in the core-level spectra are investigated by the first principles under scissor approximation. The optical constants like refractive index and extinction coefficient of both BaTiO₃ and SrTiO₃ were derived from the calculated real and imaginary parts of the dielectric function. The calculated spectra were compared with the experimental results for BaTiO₃, SrTiO₃ in good agreement.     

Modulating Electronic Structure of Monolayer Transition Metal Dichalcogenides by Substitutional Nb-Doping

Modulating electronic structure of monolayer transition metal dichalcogenides (TMDCs) is important for many applications, and doping is an effective way toward this goal, yet is challenging to control. Here, the in situ substitutional doping of niobium (Nb) into TMDCs with tunable concentrations during chemical vapor deposition is reported. Taking monolayer WS₂ as an example, doping Nb into its lattice leads to bandgap changes in the range of 1.98–1.65 eV. Noteworthy, electrical transport measurements and density functional theory calculations show that the 4d electron orbitals of the Nb dopants contribute to the density of states of Nb-doped WS₂ around the Fermi level, resulting in an n- to p-type conversion. Nb-doping also reduces the energy barrier of hydrogen absorption in WS₂, leading to an improved electrocatalytic hydrogen evolution performance. These results highlight the effectiveness of controlled doping in modulating the electronic structure of TMDCs and their use in electronic related applications.     

Thermoelectric Properties of Nb-Doped Ordered Mesoporous TiO<Subscript>2</Subscript>

Mesoporous materials have pores with diameters between 2 nm and 50 nm, the presence of which generally decreases the thermal conductivity of the material. By incorporating mesoporous structures into thermoelectric materials, the thermoelectric properties of these materials can be improved. Although TiO₂ is an ordinary insulator, reduced TiO₂ shows better electrical conductivity and is therefore a potential thermoelectric material. Furthermore, the addition of a dopant to TiO₂ can improve its electrical conductivity. We hypothesized that, by doping ordered mesoporous TiO₂ films with niobium, we would be able to minimize the thermal conductivity and maximize the electrical conductivity. To investigate the effects of Nb doping and a mesoporous structure on the thermoelectric characteristics of TiO₂ films, Nb-doped mesoporous films were investigated using x-ray diffraction, ellipsometry, four-point probe measurements, and thermal conductivity analysis. We found that Nb doping of ordered mesoporous TiO₂ films improved their thermoelectric properties.     

Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO3 Films

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO₃ in Si\SiO₂\Me\SrTiO₃\Me, (Me = Cr or W), Si\SrRuO₃\SrTiO₃, and Si\SrTiO₃ layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO₃, Si\SiO₂\Me\SrTiO₃\Me, M = Cr or W) and previous works (Si\BaTiO₃, Si\BaZrO₃), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO₃\SrTiO₃ structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO₃ layer in the presence and absence of SrRuO₃, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO₃ layer while pulling through the temperature gradient.     

Self-Powered Light-Temperature Dual-Parameter Sensor Using Nb-Doped SrTiO3 Materials Via Thermo-Phototronic Effect

Multifunctional sensors that can simultaneously detect light and thermal stimuli play an essential role in the research of portable devices and complex arrays of electronics. Lots of effort has been made to realize the simultaneous detection of light and temperature in a standalone device. However, since there is a strong interference of electric signals, accurate simultaneous sensing remains a challenge. Here, a self-powered dual-parameter sensor based on thermo-phototronic effect in Nb-doped SrTiO₃ (NSTO) single crystal to achieve decoupled light and temperature sensing is developed. High light- and temperature-sensing sensitivities of 0.201 µA mW cm⁻² and 16.77 µA K⁻¹ are realized, respectively. The excellent simultaneous sensing capability may be attributed to effective light-to-electrical and thermal-to-electrical energy conversion in the NSTO crystal. The excellent simultaneous sensing performance of the dual-parameter sensors, together with their ease of fabrication, pushes forward the development of highly sensitive multifunctional sensors.