Strontium Titanate Single Crystals Research Articles

Tuning Van Der Waals Heterostructure of WS2/SrTiO3 Via in Situ Transmission Electron Microscopy

Monolayer two-dimensional (2D) materials have been widely researched and discussed in recent years, while multi-layers have received comparatively less attention. Nowadays, combining different materials to create heterostructures has become mainstream. In semiconductor industry, heterostructure can form devices such as lasers, light-emitting diodes, fast transistors and so on. Heterostructures can create well electronic properties by combining two different materials together. Therefore, heterostructures become a great tool to manipulate the electronic structures. In traditional heterostructures, the interfaces between two different materials are formed by covalent bonds, hence it requires similar lattice constants and structures. On the other hand, novel van der Waal heterostructures (vdW-heterostructure), formed by 2D materials, do not require the same strict similarity as traditional ones. VdW-heterostructures, which create 3D structures through van der Waals forces, provide a new perspective. In this study, we created a vdW-heterostructures by stacking tungsten disulfide (WS2) with single-crystal strontium titanate, SrTiO3 (STO), shown in Figure 1a,b. The crystalline atomic-layer WS2 and STO were synthesized by chemical vapor deposition (CVD) and pulsed laser deposition (PLD), respectively. They are analyzed by high-resolution transmission electron microscope (HRTEM) shown in Figure 1c-h. Once the vdW-heterostructures are formed, we conducted in situ heating by TEM. Through the advanced in situ technique, we observed the change of the twisted stacking angle by heating the vdW-heterostructure. Ultimately, the lattices will shift and rotate, minimizing the free energy, so that the vdW-heterostructure will be aligned and formed epitaxially. In addition, we utilized atomic resolution scanning transmission electron microscope (STEM) to identify the stacking images and the moiré pattern of the vdW-heterostructure. During the rotation, we can observed different moiré patterns by various twisted stacking angles. These different moiré patterns can produce different electrical properties, which can be applied to advanced semiconductor devices. Keywords: high-resolution TEM/STEM, in situ heating, 2D materials, van der Waal heterostructures, moiré pattern Figure 1. The vdW-heterostructure of STO/WS2. a) The diagram showing that the formation process of WS2/STO vdW-heterostructures which have different twisted stacking angles. b) The low-magnification SEM image showing that the single crystal STO thin film is transferred onto the TEM heating chips. The circles in the middle are the observation windows for transmission electrons. c-e) HRTEM images. f-g) Diffraction patterns of HRTEM. c,f) Single crystal STO thin film with d-spacing (1−10) = 0.284 nm. d,g) Monolayer single crystal WS2 with d-spacing (−110) = 0.274 nm. e,h) WS2(red)/STO(yellow) vdW-heterostructure with stacking angle 3.6°. Figure 1

Enabling resonant ultrasound spectroscopy in high magnetic fields.

Resonant ultrasound spectroscopy (RUS) is a powerful method to determine elastic constants with high accuracy and precision from a single measurement of the mechanical resonances of a sample. Conventionally, the quantitative extraction of elastic moduli with RUS assumes free boundary conditions which can often lead to the adoption of unstable sample positioning between ultrasonic transducers that is incompatible with extreme environments like high magnetic fields. We show that, under specific conditions, introducing a small amount of adhesive between a RUS sample and ultrasonic transducers introduces a perturbation to the free resonance condition which can be accounted for by a simple model. This means elastic constants can be determined to within the uncertainty of conventional RUS, but with significant improvements including sample stability and control of sample orientation. We demonstrate the efficacy of this approach with measurements on a range of materials including room temperature measurements on polycrystalline metals, temperature-dependent measurements of the structural phase transition in strontium titanate single crystals, and magnetic field-dependent measurements of magnetic phase transitions in gadolinium polycrystals up to 14 T.

Variation of (001) surface structures of strontium titanate single crystals caused by flash event under direct/alternative current electric fields

Flash events, which are electric power spikes occurring under an electric field and temperature above the threshold, can significantly accelerate mass diffusion over short periods of time. This study investigated the surface structure of the (001) surface of strontium titanate single-crystal treated by flash events caused by direct current (DC) and alternating current (AC) electric fields from a viewpoint of surface-related mass diffusion. The surface structuring caused by flash events was significantly different among DC and AC electric fields. The DC-flash event destabilized surface steps, causing them to wander and form surface mounds with a height of several tens of nanometers. In addition, numerous two-dimensional (2D) nuclei with a size of several nanometers are formed on the terraces. In contrast, the AC-flash event stabilized surface steps without the formation of surface mounds, causing them nearly straight with uniform terrace width at approximately 500 nm. 2D nuclei with a size of a few 10 nm were formed at the step edges with round shapes. Both methods were confirmed as evidenced by the formation of two-dimensional nuclei on the surface after the flash events, to enhance surface-related mass diffusion.

Effect of oxygen-ion implantation on the local electronic structures of strontium-titanate single crystals: An investigation using synchrotron-based x-ray diffraction and x-ray photoemission techniques

Effect of oxygen-ion implantation on the local electronic structures of strontium-titanate single crystals: An investigation using synchrotron-based x-ray diffraction and x-ray photoemission techniques

Facets control charge separation during photoelectrochemical water oxidation with strontium titanate (SrTiO3) single crystals

The photoelectrochemical water oxidation activity of SrTiO3 crystals is determined by the variable work functions of different facets. This produces facet dependent semiconductor–liquid junctions that can drive charge separation in photocatalysts.

Heterogeneity in La Distribution in Highly La-Doped SrTiO3 Crystals

Our paper is focused on the investigation of the dopant distribution in lanthanum-doped strontium titanate (LSTO) single crystals with a 5 wt.% doping level of La. Using X-ray diffraction analysis and pycnometric density measurement, we have found a discrepancy between the theoretical density and the experimentally determined value. The origin of this behavior could be either the creation of the voids in the matrix or the intergrowth of secondary phases with La surplus in the crystal. Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and selected area diffraction (SAD) microscopic investigation have confirmed the second hypothesis, namely, that in different regions of LSTO, the local concentration of La shows a substantial variation on the micro- and nanoscopic scale. In order to study the influence of the La dopants on the electronic structure and, therefore, on the electrical conductivity, we have used the local conductivity atomic force microscopy (LCAFM) method as a local electrical probe to map the in-plane electrical conductivity of the La-doped crystal’s surface. The LCAFM conductivity maps reveal heterogeneous conductivity (here in the form of the bands with higher conductivity than the surroundings), related to band-like inhomogeneities of the La distribution. Using LCAFM measurements with atomic resolution obtained between the conducting and nonconducting regions, we analyzed the spreading (spatial expansion) of doping on the undoped or low-doped part of the STO crystal. The found limitation of the doping effect of La on the dielectric part of the STO crystal to 4–5 lattice constants was in good correlation with ab initio studies from the literature.

Breaking of the Cubic Symmetry in Millimeter-Size SrTiO3 Crystals: ESR Manifestations

The found effect of lowering the symmetry of the crystal structure of thin (less than 1 mm) plates and bars of strontium titanate (SrTiO3) single crystals is discussed. This symmetry lowering manifests itself in the electron spin resonance (ESR) spectra of impurity centers (Fe3+and Mn4+ions, used as paramagnetic probes). It is shown that symmetry lowering is observed at temperaturesT 105 K, which generally correspond to the cubic phase of SrTiO3; it leads to the formation of a tetragonal nonpolar structure, differing from the antiferrodistortive (AFD) phaseD4ℎ18, which is characteristic of strontium titanate atT 105 K and was not observed previously in SrTiO3. The factors determining the distortion value are found to be the geometry and ratio of sample sizes, surface finishing quality, and crystallographic orientation of plates.

Annihilation and Generation of Dislocations by Irradiation by Ions and Electrons in Strontium Titanate Single Crystal

The physical and chemical properties of many oxide materials depend strongly on their defect concentration, which gives rise to unique electronic, optical, and dielectric properties. One such promising material for various applications, including energy storage, photocatalysis, and electronics, is SrTiO3 (STO). It exhibits several interesting phenomena, including a metal-to-insulator transition that can be induced by reduction. By extension, 1-D defects, such as dislocations, play a significant role in its electronic properties. Thus, we investigate the process of dislocation movement, its creation, and annihilation under two stimuli: ion thinning and electron irradiation. First, we designed and produced a lamella from a mechanically modified sample with variable thickness in the form of a wedge using a focused ion beam (FIB/Ga+) to investigate thickness-dependent dislocation movement. The lamella was investigated by transmission electron microscopy, allowing for the measurements of dislocation concentration as a function of its thickness. We have noticed a sharp decrease in the defect concentration with respect to the starting sample, showing a process of annihilation of dislocations. Second, we used an electron beam to drive a relatively large current into the STO surface. This experiment produced an electrical breakdown-like pattern. Optical and atomic force microscopy revealed that this pattern evolved due to the removal of material from the surface and local metal-insulator-transition along the dislocations network. Thus, we observe the dislocations generation and movement.

Enhanced Photoconductivity at Dislocations in SrTiO3.

Dislocations are 1D crystallographic line defects and are usually seen as detrimental to the functional properties of classic semiconductors. It is shown here that this not necessarily accounts for oxide semiconductors in which dislocations are capable of boosting the photoconductivity. Strontium titanate single crystals are controllably deformed to generate a high density of ordered dislocations of two slip systems possessing different mesoscopic arrangements. For both slip systems, nanoscale conductive atomic force microscope investigations reveal a strong enhancement of the photoconductivity around the dislocation cores. Macroscopic in-plane measurements indicate that the two dislocation systems result in different global photoconductivity behavior despite the similar local enhancement. Depending on the arrangement, the global photoresponse can be increased by orders of magnitude. Additionally, indications for a bulk photovoltaic effect enabled by dislocation-surrounding strain fields are observed for the first time. This proves that dislocations in oxide semiconductors can be of large interest for tailoring photoelectric functionalities. Direct evidence that electronic transport is confined to the dislocation core points to a new emerging research field.

Physically and chemically smooth cesium-antimonide photocathodes on single crystal strontium titanate substrates

The performance of x-ray free electron lasers and ultrafast electron diffraction experiments is largely dependent on the brightness of electron sources from photoinjectors. The maximum brightness from photoinjectors at a particular accelerating gradient is limited by the mean transverse energy (MTE) of electrons emitted from photocathodes. For high quantum efficiency (QE) cathodes like alkali-antimonide thin films, which are essential to mitigate the effects of non-linear photoemission on MTE, the smallest possible MTE and, hence, the highest possible brightness are limited by the nanoscale surface roughness and chemical inhomogeneity. In this work, we show that high QE Cs3Sb films grown on lattice-matched strontium titanate (STO) substrates have a factor of 4 smoother, chemically uniform surfaces compared to those traditionally grown on disordered Si surfaces. We perform simulations to calculate roughness induced MTE based on measured topographical and surface-potential variations on the Cs3Sb films grown on STO and show that these variations are small enough to have no consequential impact on the MTE and, hence, the brightness.

Van der Waals integration of high-\u03ba perovskite oxides and two-dimensional semiconductors

Two-dimensional semiconductors can be used to build next-generation electronic devices with ultrascaled channel lengths. However, semiconductors need to be integrated with high-quality dielectrics—which are challenging to deposit. Here we show that single-crystal strontium titanate—a high-κ perovskite oxide—can be integrated with two-dimensional semiconductors using van der Waals forces. Strontium titanate thin films are grown on a sacrificial layer, lifted off and then transferred onto molybdenum disulfide and tungsten diselenide to make n-type and p-type transistors, respectively. The molybdenum disulfide transistors exhibit an on/off current ratio of 108 at a supply voltage of 1 V and a minimum subthreshold swing of 66 mV dec−1. We also show that the devices can be used to create low-power complementary metal–oxide–semiconductor inverter circuits.

TiSr antisite: An abundant point defect in SrTiO3

We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate (SrTiO3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180–190 ps lifetime component is most likely the TiSr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the TiSr antisite related defects in SrTiO3.

The formation of oriented barium carbonate from the decomposition of yttria-doped barium zirconate films

Yttria doped barium zirconate (BZY) thin films show promise thanks to their high proton conductivities and their possibility for use in studying fundamental processes such as exsolution. This work demonstrates that highly oriented BZY thin (45 nm) films on (100) single crystal strontium titanate decompose into oriented barium carbonate rods and yttria stabilized zirconia when exposed to industrial grade Ar or pure CO2 at 800 °C. It is shown with transmission electron microscopy that the rods nucleate on the BZY surface. The causes and modes of decomposition in these thin films are discussed in detail.

Thermally stimulated relaxation and behaviors of oxygen vacancies in SrTiO3 single crystals with (100), (110) and (111) orientations

The strontium titanate (SrTiO3) single crystals with different orientations of (100), (110) and (111) were investigated using thermally stimulated depolarization current (TSDC) measurements, which has been proved to be an effective strategy to fundamentally study the relationship between relaxation phenomena and defect chemistry in dielectrics. The origins of different relaxations in SrTiO3 crystals were identified and the activation energy of oxygen vacancies was estimated from TSDC measurements. It was further found that oxygen-treated SrTiO3 crystals exhibit different relaxation behaviors. Noticeable changes of thermal relaxation associated with oxygen vacancies have taken place in relation to the crystalline anisotropy. The SrTiO3 (110) samples display higher concentration and activation energy of oxygen vacancies. First-principles calculations were carried out on SrTiO3 (110) crystals to study the effect of oxygen vacancy on different surface microstructure. From the resulting minimum formation energy of 0.63 eV, it demonstrates that the oxygen vacancies tend to form on the TiO-terminated surfaces. Considering the band structure, oxygen vacancies near the surface contribute to the transition of crystal from insulator to metallic characteristic.

CO adsorption, reduction and oxidation on Pb(Zr,Ti)O3(001) surfaces associated with negatively charged gold nanoparticles

Gold is deposited on atomically clean, inwards polarized, ferroelectric lead zirco-titanate deposited by pulsed laser deposition on strontium titanate (001) single crystal, then carbon monoxide adsorption and desorption experiments are investigated by in situ fast photoelectron spectroscopy using synchrotron radiation. Atomic force microscopy and high resolution photoelectron spectroscopy are consistent with the formation of 50–100 nm nanoparticles, and their Au 4f core levels point to a negative charge state of gold. As compared with a similar experiment performed on ferroelectric lead zirco-titanate with similar polarization state and without gold, the saturation coverage after exposure to carbon monoxide increases by about 68 %, and also most of the additional carbon is found in oxidized state. Desorption experiments with in situ follow-up by photoelectron spectroscopy are performed as function of temperature, and the neutral carbon intensity decreases when the ferroelectric polarization decreases, while the components corresponding to oxidized carbon remain unchanged. It looks that neutral carbon adsorption is strictly related to the polarization of the ferroelectric film, while carbon still found in molecular form is related to its carbonyl bonding on metal nanoparticles, independent of the polarization state of the substrate. Desorbed carbon at higher temperature uptakes oxygen from the substrate.

Structural, electric and pyroelectric properties of up and down graded PZT multilayers

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZrxTi1-xO3 (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO3, STO) substrate, using a strontium ruthenate (SrRuO3, SRO) film as buffer layer for epitaxial growth, and also as back electrode.Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: up structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).

Conductivity of iron‐doped strontium titanate in the quenched and degraded states

AbstractThe electrical behavior of iron‐doped strontium titanate (Fe:SrTiO3) single crystals equilibrated at 900°C and quenched below 400°C at various oxygen partial pressures () was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO3 annealed and quenched between 1.2 × 10−14 and 2.0 × 10−4 Pa exhibits a conduction activation energy (EA) around 0.6 eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in EA are found to either side of this range; a transition from 0.6 to 1 eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23 eV is found in reducing These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO3 provides insight into the origins of the conductivity activation energies observed in those samples.

Atomic imprinting into metallic glasses

Nanoimprinting by thermoplastic forming has attracted significant attention due to its promise of low-cost fabrication of functionalized surfaces and nanostructured devices, and metallic glasses have been identified as a material class ideally suited for nanoimprinting. In particular, their featureless atomic structure suggests that there may not be an intrinsic size limit to the material’s ability to replicate a mould. Here we demonstrate atomic-scale imprinting into a platinum-based metallic glass alloy under ambient conditions using atomic step edges of a strontium titanate single crystal as a mould. The moulded metallic glass replicates the ‘atomic smoothness’ of the strontium titanate, with identical roughness to the one measured on the mould even after multiple usages and with replicas exhibiting an exceptional long-term stability of years. By providing a practical, reusable, and potentially high-throughput approach for atomic imprinting, our findings may open novel applications in surface functionalization through topographical structuring.

Epitaxial Growth of Monolayer MoS2 on SrTiO3 Single Crystal Substrates for Applications in Nanoelectronics

Monolayer molybdenum disulfide (MoS2) crystals grown on amorphous substrates such as SiO2 are randomly oriented. However, when MoS2 is grown on crystalline substrates, the crystal shapes and orientations are also influenced by their epitaxial interaction with the substrate. In this paper, we present the results from chemical vapor deposition growth of MoS2 on three different terminations of single crystal strontium titanate (SrTiO3) substrates. On SrTiO3(111), the monolayer MoS2 crystals form equilateral triangles with two main orientations, in which they align their ⟨2110⟩-type directions (i.e., the sulfur-terminated edge directions) with the ⟨110⟩-type directions on SrTiO3. This arrangement allows near-perfect coincidence epitaxy between seven MoS2 unit cells and four SrTiO3 unit cells. On SrTiO3(110), the MoS2 crystals tend to align their edges with both the ⟨110⟩ and ⟨112⟩ directions on SrTiO3 because these both provide favorable coincidence lattice registry. This distorts the crystal shapes and...

Positive electrodes based on Ion-implanted SrTiO3

AbstractAn O2-electrode was fabricated using a metal ion implanted SrTiO3single crystal. The time resolved oxygen exchange rate of ion implanted strontium titanate (SrTiO3) single crystals was studied by means of oxygen solid electrolyte coulometry (OSEC). Transmission electron microscopy (TEM) was performed in order to determine structural changes after ion implantation. Moreover, theoretical modelling based on defect chemistry under equilibrium conditions was applied for determining of effective rate constants. OSEC measurements turn out to be a damage and calibration free method, which was used for the first time in order to characterize kinetic parameters of oxygen exchange on single crystalline surfaces.