Rutile Single Crystals Research Articles

Bulk Single Crystals and Physical Properties of Rutile GeO2 for High‐Power Electronics and Deep‐Ultraviolet Optoelectronics

The top‐seeded solution growth for rutile GeO2 single crystals using alkali carbonates or fluorides as flux is applied. Structural data of obtained single crystals confirm the rutile phase with a = b = 4.3966 Å and c = 2.8612 Å. The crystals with diameter of 5–15 mm are either undoped or intentionally doped with Sb5+, Sn4+, Al3+, Ga3+, and F− ions. It is found that Sb5+ is a very efficient n‐type donor enabling free electron concentration even above 1020 cm−3; thus, Sb‐doped GeO2 is a potential substrate for vertical power devices. In contrast, crystals doped with Al and Ga do not show p‐type conductivity suggested by the theory. The onset of the absorption occurs at 5.0 and 5.5 eV perpendicular and parallel to the c‐axis, respectively. Rutile GeO2 shows a very intense photoluminescence peaking at 420 nm (blue) and 520 nm (green). Raman spectra show narrow lines, in particular at high phonon energy (B1g, 170 cm−1). Prepared wafers show FWHM values of rocking curves below 30 arcsec and polishing is achieved down to RMS roughness of 0.15 nm. Transmission electron microscopy images do not show point or extended structural defects with uniform Sb distribution.

Anisotropic ferroelectric-shaped hysteresis loop and colossal permittivity in thermally treated rutile TiO2 single crystals

Space charge polarization is an unavoidable phenomenon in electronic components that leads to deterioration of electrical response. Hence manipulation and utilization of space charge polarization to improve electrical properties is a phenomenal task. In the present study, heating treatment produces oxygen vacancies, resulting in the crystal structures (i.e., modulated structure and intergrowth structure) and band gap are anisotropic, which in turn leads to anisotropic nonlinear polarization behavior and colossal permittivity. Quantitative analysis of the atomic-scale structure confirms the modulated structure and intergrowth structure are anisotropic. A modulated structure along a axis of rutile TiO2 with a width of six times that of a rutile crystal cell a is constructed through an intergrowth structure between rutile and srilankite TiO2 phases in rutile TiO2 single crystals, wherein space charges at the intergrowth structure give rise to ferroelectric-shaped hysteresis in heated TiO2 single crystals along [001] direction. Impedance spectra display three kinds of space charge polarizations (modulated structure, intergrowth structure, and electrode non-ohmic contact) contribute to the colossal permittivity along [001] direction. The space charge polarizations contribute to a colossal permittivity (∼45,000), high resistivity (∼1010 Ω cm), and ferroelectric-shaped P-E loop (remanent polarization: ∼17.1 μC/cm2, coercive field: ∼4.3 kV/cm) in the heated TiO2 single crystal along [001] direction, while a linear polarization behavior and permittivity value of 120 are obtained along [100] direction. The findings provide guidance for the utilization of space charge polarization in dielectric materials.

The hydrophilicity mechanism of anatase and rutile (110) TiO2 films based on donor-acceptor complexes

When the surface of TiO2 is exposed to ultraviolet (UV) light, its hydrophilicity increases initially and then contact angle relaxation progresses over time. We investigated the source mechanism and working principles of long-lasting hydrophilicity in an ambient environment. Previously, we reported that the amount of donor-acceptor complexes (DACs) generated by polarizable OH groups with strong charge polarity is critical for maintaining long-lasting hydrophilicity (>90 days) on an anatase surface. In less reactive TiO2 films with low surface energy, a fast relaxation occurs due to the loss of polarizable OH groups. In this research, quantitative four-peak X-ray photoelectron spectroscopy analysis with TiO2 O 1 s spectra surface analysis was used to determine the relative amount of DACs and the binding energy shifting of water adsorbates on single crystal rutile (110) and polycrystalline anatase after UV exposure or plasma treatment. Films treated with oxygen or argon plasma and/or UV were analyzed and compared to suggested mechanisms that can occur on the surface of the TiO2 phase. All treated rutile (110) films except those treated with an Ar plasma exhibited rapid contact angle relaxation during the first 3 days, and the angle increased gradually to >50° over 14 days. In contrast, all anatase films retained long-lasting hydrophilicity, demonstrating a <∼10° contact angle over 56 days. At the first measurement, the binding energies were shifted for rutile (110) treated with O2 plasma and O2 plasma+UV films, demonstrating Ti–OH and DAC values of +0.15 eV each. However, the Ti–OH and DACs of all anatase films were located in the negative binding energy position, indicating that the DACs of rutile (110) were unstable due to a relatively weak bonding. In addition, the IOH/Ibulk intensity ratio of Ti–OH for rutile (110) treated with an O2 plasma increased significantly from 20 % to 37 %. However, the intensity ratio for O2 plasma+UV increased only from 20 % to 23.8 %. This result in combination with the binding energy shifting tendency indicates that relaxed OH groups increased rapidly without UV treatment. When rutile (110) was irradiated with UV, the effect of photoinduced electron-hole pairs shown with Ti 2p spectra was much shorter than that of anatase. Differences in relaxation occurred in both phases due to the charge polarity effect between rutile (110) and anatase, the unstable DACs related to weak OH groups, and the difference in surface characteristics. Using single crystal rutile (110) with a well-defined surface, these experiments validated further the DAC mechanism and theory presented by our group. Specifically, we confirmed that both the amount of DACs and the binding energy position shifting are important for long-term hydrophilicity.

Facet-Dependent Interfacial Charge Transfer between T-Phase VS2 Nanoflakes and Rutile TiO2 Single Crystals.

The hybridizations of two-dimensional (2D) metallic materials with semiconducting transition metal oxides (TMOs) register attractive heterojunctions, which can find various applications in photostimulated circumstances. In this work, we developed an ambient-pressure chemical vapor deposition method to directly grow T-VS2 on atomically smooth rutile TiO2 single crystals with different terminations and thus successfully constructed a heterojunction model of VS2/TiO2 with a well-defined clean interface. Detailed measurements with Kelvin probe force microscopy revealed the facet-dependent charge transfer occurring at the VS2/TiO2 interfaces, seeing variations not only in the amount and direction of the transferred electrons but also in the photoinduced surface potential changes and the dynamics of photogenerated charge carriers under ultraviolet irradiation. Interestingly, ultrathin T-VS2 was found with considerable magnetism at room temperature, disregarding the charge exchange with the TiO2 substrates. These results may bring deep insights into the photoinspired functionalities of the hybridized system combining metallic transition metal dichalcogenides and TMO materials.

Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania.

One of the main challenges to expand the use of titanium dioxide (titania) as a photocatalyst is related to its large band gap energy and the lack of an atomic scale description of the reduction mechanisms that may tailor the photocatalytic properties. We show that rutile TiO2 single crystals annealed in the presence of atomic hydrogen experience a strong reduction and structural rearrangement, yielding a material that exhibits enhanced light absorption, which extends from the ultraviolet to the near-infrared (NIR) spectral range, and improved photoelectrocatalytic performance. We demonstrate that both magnitudes behave oppositely: heavy/mild plasma reduction treatments lead to large/negligible spectral absorption changes and poor/enhanced (×10) photoelectrocatalytic performance, as judged from the higher photocurrent. To correlate the photoelectrochemical performance with the atomic and chemical structures of the hydrogen-reduced materials, we have modeled the process with in situ scanning tunneling microscopy measurements, which allow us to determine the initial stages of oxygen desorption and the desorption/diffusion of Ti atoms from the surface. This multiscale study opens a door toward improved materials for diverse applications such as more efficient rutile TiO2-based photoelectrocatalysts, green photothermal absorbers for solar energy applications, or NIR-sensing materials.

Bias voltage induced-electrical conductivity at electrode interfaces of a rutile TiO[formula omitted] single crystal: Two different mechanisms depending on temperature

The electric field-induced conductivity of TiO2 is important in various emerging practical applications, including the recently proposed selective trace hydrogen sensing. In the present study, the bias voltage-induced electrical characteristics were investigated between 295 K (ambient temperature) and 600 K for a semi-insulated rutile TiO2 single crystal. The appearance of conductivity by elevating the temperature was shown to be due to the electromigration of mobile dopants including defects. It was found that there are two different mechanisms causing the conductivity depending on the temperature. At low temperature, close to room temperature, hydrogen migrating primarily along the open [001] channel triggers the conductivity. On the other hand at high temperature above 450 K, the migration of oxygen vacancy (VO) perpendicular to [001] induces the conductivity. It is proposed that VO is injected at the anode interface, which then migrates to the cathode interface and aggregates there. It was observed that the VO injection is not accompanied by the formation of the Magnèli phase. The proposed mechanism was examined by spectroscopic techniques such as time-of-flight secondary ion mass spectrometry and hard X-ray photoemission spectroscopy.

Alumina precipitate along dislocations in rutile single crystal of diffusion couple during cooling from 1200° to 1400°C

Diffusion couples of rutile and corundum single crystals were prepared and examined for the solid solution and precipitation of corundum in rutile during cooling. Alumina dissolves in rutile according to the following equation, Al2O3→2AlTi′+3OO+VO∙∙. In the sample air-quenched from 1200 °C, the supersaturated oxide ion vacancies, VO⋅⋅, reacted with the edge component of screw dislocation parallel to c axis to form helical dislocations, along which corundum was precipitated. When air-quenched from 1300 °C and 1400 °C, corundum was precipitated in the form of stacked rectangular loops. Corundum grains with eight lobes extending in the < 101 > directions were precipitated in the samples cooled at 10 °C/min from 1300 °C and 1400 °C. The solid solution reaction increased the volume of rutile single crystals and caused voids outside the reaction region at the interface. Both the corundum and rutile components of the voids also contributed to the solid solution reaction.

High Quantum Yield Sensitization of Single Crystal TiO2 Electrodes with CsPbBr3 Nanocrystals

Abstract Lead halide perovskites are promising materials for solar cells and photodetectors. Nanocrystals of this family of perovskites also have distinct properties. In this study we report the attainment of high quantum yield sensitized photocurrents from stabilized CsPbBr3 perovskite nanocrystals thermally evaporated onto doped single crystal rutile (110) TiO2 electrodes. Sensitized photocurrent stabilization was achieved by capping the nanocrystals with a thin NiOx layer and Au carrier transport and protection layers respectively as well as the judicious choice of solvent and electrolyte.

Monitoring the Lifetime of Photoexcited Electrons in a Fresh and Bulk Reduced Rutile TiO2 Single Crystal. Possible Anisotropic Propagation.

Defects (oxygen vacancies and interstitial cations) in oxide semiconductors have recently been invoked as a key property behind increased photocatalytic reaction rates. In this work, we have monitored by transient absorption spectroscopy (TAS) excited electrons in the conduction band decaying into the invoked traps to extract their lifetime using a rutile single crystal instead of the more conveniently used powder homologue. This is preferred in order to rule out grain boundary, degree of crystallinity, and size effects among other parameters that would obscure the results. It was found, in the energy region investigated (1.3-1.8 eV), that the lifetime of excited electrons is about four times shorter for the bulk defect crystal when compared to the fresh one. This indicates that the created defects (mostly oxygen defects and interstitial Ti cations) are unlikely to contribute to reaction rate enhancement.

Toward a Comprehensive Understanding of Photocatalysis: What Systematic Studies and Alcohol Surface Chemistry on TiO2(110) Have to Offer for Future Developments.

Heterogeneous photocatalytic systems are usually described based on electrochemistry, which the vast majority of interpretations and strategies for optimizing photocatalysts rely on. Charge carrier dynamics are usually in the spotlight, whereas the surface chemistry of the photocatalyst is neglected. This is unjustified, because studies on alcohol photoreforming on metal-decorated rutile single crystals revealed that the electrochemical reaction model is not generally applicable. Hence, many photocatalytic reactions may proceed in a different manner and the thermal chemistry needs to be accounted for. The new mechanism is particularly relevant for reactions in gaseous environments in the absence of solvated ionic species. Here, we compare both mechanisms and highlight their differences and consequences for photocatalysis. Based on alcohol photochemistry, we demonstrate the importance of thermal reactions in photocatalytic mechanisms and the relevance of systematic studies in different environments for a holistic understanding of photocatalysis.

Ferromagnetism and two magnetic phases in rutile (TiO&lt;sub&gt;2&lt;/sub&gt;) implanted with cobalt ions: effect of substrate temperature and orientation

Room-temperature ferromagnetism and two magnetic phases: cobalt metal nanoparticles and a solid solution of divalent cobalt ions have been observed in single-crystal rutile (TiO2) implanted with cobalt ions to high dose level. Here we present the results of our structural and magnetic studies showing an influence both the temperature and the orientation of TiO2 substrate during a cobalt ion implantation, as well the effect of post-implantation thermal annealing (in air or in vacuum) on ferromagnetism value and magnetic phase composition of Co-implanted rutile.

Rapid Growth of High-Quality Rutile TiO2 Single Crystals through a Laser Floating Zone Method

The rapid growth of rutile TiO2 single crystals through a laser floating zone (LFZ) method was demonstrated. LFZ has a higher power density, which is suitable for the growth of TiO2 crystals with a high melting point. By optimizing the crystal growth parameters, including the growth rate, gas atmosphere, and rotation rate, the crystals could achieve their largest size of φ 9 mm × 25 mm, with a growth cycle of 12 h, and no cracks appeared. The properties of the obtained crystals were close to those of the crystals grown using other schemes, with a whole transmission range of 0.41–6.56 μm, thermal expansion coefficient of 9.92 × 10−6/K, and laser damage threshold of 1.44 GW/cm2. The achieved results indicated that the crystals have high quality and good integrity when grown using LFZ and also imply a new choice for the rapid growth of rutile TiO2 single crystals.

Strong Isotopic Fractionation of Oxygen in TiO2 Obtained by Surface-Enhanced Solid-State Diffusion.

Isotopically pure semiconductors have important applications for cooling electronic devices and quantum computing and sensing. Raw materials of sufficiently high isotopic purity are expensive and difficult to obtain; therefore, a post-synthesis method for removing isotopic impurities would be valuable. Through isotopic self-diffusion measurements of oxygen in rutile TiO2 single crystals immersed in water, we demonstrate fractionation of 18O by a factor of 3 below natural abundance in a near-surface region up to 10 nm wide. The submerged surface injects O interstitials that displace lattice 18O deeper into the solid as a result of the statistics of interstitialcy-mediated diffusion combined with steep chemical gradients of O interstitials. Slightly acidic and slightly basic liquid solutions both enhance the fractionation and affect the details of isotopic profile shapes through several chemical and physical mechanisms.

Anomalous durability of (001) plane against oxygen vacancy injection at electrode/rutile-TiO[formula omitted] interfaces

We have investigated the electro-coloration of a rutile TiO2 single crystal by the voltage-bias annealing in dry air between a point cathode and a flat anode. The electro-coloration is due to the conductive VO filament that grows from the cathode/TiO2 interface to mm size. We proposed that the growth mechanism of this macroscopic size VO filament is the oxygen (O2−) transfer reaction at the anode interface followed by the thermally assisted-electric field-induced VO diffusion to the cathode. It was found that no electro-coloration occurs on (001), whereas it occurs on (110) and (100). This striking contrast between the surface crystal orientations was found to be due to the anisotropic VO diffusion that was found to be much faster perpendicular to [001] than parallel. Accordingly, VO is not injected into the anode/(001) interface in the oxygen transfer reaction. The proposed mechanism was examined by the surface-sensitive TOF-SIMS and the bulk-sensitive dynamic SIMS for analyzing the VO spatial distribution using the 18O isotope tracer. The anomalous durability of the (001) plane against the VO injection suggests that the atomic arrangement of TiO2 at the electrode interface is crucial for controlling the VO filament growth in resistive switching devices.

The Influence of Electrode Thickness on the Structure and Water Splitting Performance of Iridium Oxide Nanostructured Films.

For a safe environment, humanity should be oriented towards renewable energy technology. Water splitting (WS), utilizing a photoelectrode with suitable thickness, morphology, and conductivity, is essential for efficient hydrogen production. In this report, iridium oxide (IrOx) films of high conductivity were spin-cast on glass substrates. FE-SEM showed that the films are of nanorod morphology and different thicknesses. UV-Vis spectra indicated that the absorption and reflectance of the films depend on their thickness. The optical band gap (Eg) was increased from 2.925 eV to 3.07 eV by varying the spin speed (SS) of the substrates in a range of 1.5 × 103–4.5 × 103 rpm. It was clear from the micro-Raman spectra that the films were amorphous. The Eg vibrational mode of Ir–O stretching was red-shifted from 563 cm−1 (for the rutile IrO2 single crystal) to 553 cm−1. The IrOx films were used to develop photoelectrochemical (PEC) hydrogen production catalysts in 0.5M of sodium sulfite heptahydrate Na2SO3·7H2O (2-electrode system), which exhibits higher hydrogen evaluation (HE) reaction activity, which is proportional to the thickness and absorbance of the used IrOx photocathode, as it showed an incident photon-to-current efficiency (IPCE%) of 7.069% at 390 nm and −1 V. Photocurrent density (Jph = 2.38 mA/cm2 at −1 V vs. platinum) and PEC hydrogen generation rate (83.68 mmol/ h cm2 at 1 V) are the best characteristics of the best electrode (the thickest and most absorbent IrOx photocathode). At −1 V and 500 nm, the absorbed photon-to-current conversion efficiency (APCE%) was 7.84%. Electrode stability, thermodynamic factors, solar-to-hydrogen conversion efficiency (STH), and electrochemical impedance spectroscopies (EISs) were also studied.

Germanium dioxide: A new rutile substrate for epitaxial film growth

Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of a=4.398Å and c=2.863Å. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties.

Verneuil growth of TiO2 (rutile) crystals of large size and low dislocation density at low gas flow rate

Large (30 mm in diameter) TiO2 (rutile) single crystals with low dislocation density were grown by the Verneuil method using a single crystal furnace designed and improved by the authors. The structure of the burner was optimized by numerical simulation analysis so that the crystal could be grown at a low gas flow rate. The investigation of the growth process parameters (i.e., growth rate, outer flow rate of O2, inner flow rate of O2, and the increment of H2 flow rate) shows that the inner flow rate of O2 and the increment of H2 flow rate have the strongest influence on the crystal growth process. On this basis, other growth parameters (growth rate, outer flow rate of O2) were also optimized. Compared with the conventional Verneuil method, the crystal can be grown at a considerably low gas flow rate (40–50% lower) with the method in this work, which reduces the melt turbulence impacted by gas flow, enabling a steady and clear solid-liquid interface and improving the crystal quality. The optimum growth conditions are for the growth rate of 6 mm/h, O2 outer flow rate of 3.5 L/min, O2 inner flow rate of 5.5 L/min, and increment of H2 flow rate of 0.1 L/4 min. The etch pit density of the rutile crystals is 3.29 × 104 cm−2, an order of magnitude lower than that of the crystals grown by the conventional Verneuil method. The optical properties of the crystal are comparable to those grown by the floating zone method. Especially, it is easier to obtain a larger crystal size with lower production costs. Our results provide a possible route for industrializing the Verneuil production of large, high-quality and low-cost rutile single crystals.

Modulated Photoluminescence of Single-Layer MoS2 on Various Rutile TiO2 Surfaces: Implications for Photocatalytic Applications

Hybridization with oxide semiconductors provides a versatile strategy for tailoring physicochemical properties of two-dimensional materials (2DMs). However, the direct impacts of specific interface interactions have not yet been very well categorized, in particular at an atomic level. In the present work, through a chemical vapor deposition (CVD) method, we successfully grew monolayer MoS2 flakes on an atomically smooth rutile TiO2 single crystal with (100), (110), and (001) terminations. We found that the fabrication of comparable high-quality MoS2 on all of the TiO2 substrates can only be achieved via finely varying the growth parameters. Moreover, the photoluminescence of MoS2 also changes against the substrate terminations, showing a gradually reduced A0/A– exciton ratio following the sequence of (100) > (110) > (001). Detailed X-ray photoelectron spectroscopy measurements showed the same tendency for the binding energy shifts of both Ti and O in the MoS2/TiO2 samples, which were attributed to the varied dipole fields established at the MoS2/TiO2 interfaces. Our work not only reinforces the important role of interface charge redistribution in tailoring the properties of hybridized systems but also emphasizes that the facet effect may be applied as an efficient strategy for optimizing the photocatalytic activities of compositional systems.

Polarity reversal of resistance response to trace H2 gas in the air between asymmetrically shaped electrodes on rutile-TiO2 single crystal

We investigated the resistance response to trace hydrogen gas in the air between the asymmetrically shaped point contact- (Pt tip) and plane contact- (Al thin-film) electrodes formed in-plane on a rutile TiO2 single crystal at 673 K. It was found that the polarity of the resistance response, that is, the increase or the decrease of the resistance by exposing to hydrogen, reverses depending on the bias voltage polarity. This reversal mechanism of the resistance response was analyzed from the electrical conduction properties and the depth profile of the oxygen tracer (18O) in the electric field-induced diffusion. The central mechanism is proposed to be the oxygen transfer reaction at the Pt/TiO2 interface, accompanied by the modulation of the positively charged-oxygen vacancy concentration, thus the resistance between the two terminals. It is also proposed that the proton hinders the transfer reaction at the interface; and hence, the resistance increases by exposing to hydrogen with biasing the positive voltage to the Pt tip electrode. The application of this reversal of the resistance response for separating hydrogen and ethanol in the trace gas sensing is discussed, which is quite limited for a conventional semiconductor gas sensor.

Interfacial carrier transport properties of a gallium nitride epilayer/quantum dot hybrid structure

Electron transport layers (ETLs) play a key role in the electron transport properties and photovoltaic performance of solar cells. Although the existing ETLs such as TiO2, ZnO and SnO2 have been widely used to fabricate high performance solar cells, they still suffer from several inherent drawbacks such as low electron mobility and poor chemical stability. Therefore, exploring other novel and effective electron transport materials is of great importance. Gallium nitride (GaN) as an emerging candidate with excellent optoelectronic properties attracts our attention, in particular its significantly higher electron mobility and similar conduction band position to TiO2. Here, we mainly focus on the investigation of interfacial carrier transport properties of a GaN epilayer/quantum dot hybrid structure. Benefiting from the quantum effects of QDs, suitable energy level arrangements have formed between the GaN and CdSe QDs. It is revealed that the GaN epilayer exhibits better electron extraction ability and faster interfacial electron transfer than the rutile TiO2 single crystal. Moreover, the corresponding electron transfer rates of 4.44 × 108 s−1 and 8.98 × 108 s−1 have been calculated, respectively. This work preliminarily shows the potential application of GaN in quantum dot solar cells (QDSCs). Carefully tailoring the structure and optoelectronic properties of GaN, in particular realizing the low-temperature deposition of high-quality GaN on various substrates, will significantly promote the construction of highly efficient GaN-ETL based QDSCs.