Erbium-doped TiO2 Research Articles

The role of the dopant in the electronic structure of erbium-doped TiO2 for quantum emitters

Erbium-doped TiO2 materials are promising candidates for advancing quantum technologies, necessitating a thorough understanding of their electronic and crystal structures to tailor their properties and enhance coherence times. This study explored epitaxial erbium-doped rutile TiO2 films deposited on r-sapphire substrates using molecular beam epitaxy. Photoluminescence excitation spectroscopy demonstrated decreasing photoluminescence lifetimes with erbium doping, indicating limited optical coherence times. Lattice distortions associated with Er3+ were probed by x-ray absorption spectroscopy, indicating that erbium primarily occupies Ti4+ sites and influences oxygen vacancies. Significant lattice distortions in the higher-order shells and apparent full coordination around erbium suggest that additional defects are likely prevalent in these regions. These findings indicate that defects contribute to limited coherence times by introducing alternative decay pathways, leading to shorter photoluminescence lifetimes.

Optical and microstructural studies of erbium-doped TiO2 thin films on silicon, SrTiO3, and sapphire

Rare-earth ion doped oxide thin films integrated on silicon substrates provide a route toward scalable, chip-scale platforms for quantum coherent devices. Erbium-doped TiO2 is an attractive candidate: the Er3+ optical transition is compatible with C-band optical fiber communications, while TiO2 is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped TiO2 thin films grown via molecular beam deposition on silicon, SrTiO3, and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline TiO2(rutile)/Si with single-crystalline TiO2(rutile)/r-sapphire and polycrystalline TiO2(anatase)/Si with single-crystalline TiO2(anatase)/SrTiO3, we observe that the inhomogeneous linewidth (Γinh) of the most prominent peak in the Er spectrum (the Y1–Z1 transition, 1520 and 1533 nm in rutile and anatase TiO2) is significantly narrower in the polycrystalline case. This implies a relative insensitivity to extended structural defects and grain boundaries in such films (as opposed to, e.g., point defects). We show that the growth of an undoped, underlying TiO2 buffer on Si can reduce Γinh by a factor of 4–5. Expectedly, Γinh also reduces with decreasing Er concentrations: we observe a ∼2 order of magnitude reduction from ∼1000 ppm Er to ∼10 ppm Er. Γinh then gets limited to a residual value of ∼5 GHz that is insensitive to further reduction in the Er concentration. Based upon the above results, we argue that the optical properties in these thin films are limited by the presence of high “grown-in” point defect concentrations.

Dominant co-exposed {1 0 1}/{1 1 1} facet of Er-doped rutile TiO2 film via hydrothermal doping

The origin of the improved performance of erbium-doped TiO2 film via the hydrothermal doping process is discussed. Facet engineering of rutile TiO2 on the surface atomic structure is known but the electronic structure has been rarely studied to date. Particularly, the dominant facet of rutile TiO2 has changed upon doping resulting with co-exposed {101}/{111} facet as shown in TEM results. The surface energy levels of the conduction band (CB) and valence band (VB) on different crystal surfaces change, and these variations in energy levels will cause the electrons and holes to gravitate toward different crystal faces and inhibit the electron-hole recombination.

Fabrication of TiO2 based Dye-Sensitized Solar Cell using Nerium oleander as a sensitizer

The globe has already started using renewable energy sources, mainly solar energy, to meet the rising need for energy. The Dye-Sensitized Solar Cell (DSSC), one of many varieties of solar cells, is now popular because of its low manufacturing cost and ability to function in low light. In this work, TiO2 and erbium-doped TiO2 are chosen as photoanode and Nerium oleander as a sensitizer. The photoanode materials are prepared by one-step microwave assisted hydrothermal method. These nanomaterials and sensitizer are characterized by various characterization techniques that show significant results. The designed DSSC is evaluated by the current-voltage properties study. The efficiency of the fabricated TiO2 and Er-doped TiO2 based DSSCs is found to be 0.95 % and 2.95 % respectively.

Electroluminescence from light-emitting device with erbium-doped TiO2 film sputtered onp+-Si substrate: Enhancement effect of codoping zirconium

In our previous report, the erbium (Er)-related visible and near-infrared (NIR, ∼1.54 μm) electroluminescence (EL) from the light-emitting device (LED) with Er-doped TiO2 (TiO2:Er) film sputtered on the heavily boron-doped p-type silicon (p+-Si) substrate thus forming the so-called TiO2:Er/p+-Si heterostructure was realized. However, it is still a challenge to enhance the Er-related emissions from the above-structured LED. In this work, we have adopted the strategy of codoping zirconium (Zr) into TiO2 host to significantly enhance the Er-related visible and NIR EL from the TiO2:Er/p+-Si heterostructured LED. It is revealed that the oversize Zr4+ ions substantially substitute for Ti4+ ions in TiO2 lattice. Such substitution results in somewhat distorted crystal field of Er3+ ions incorporated into TiO2 host, which increases the intra-4f transition probabilities of Er3+ ions thus enhancing the characteristic visible and NIR emissions.

Improved photoelectrochemical properties of TiO2 nanotubes doped with Er and effects on hydrogen production from water splitting

Erbium-doped TiO2 nanotubes (Er–TiO2 NTs) are prepared with a combination of anodization and electrochemical deposition using various proportions of erbium and adjusting the time of the process. The surface characterization techniques and electrochemical analysis are applied to study the physicochemical and photoelectrochemical (PEC) properties of the as-prepared photocatalysts. Er–TiO2 NTs have crystal sizes of about 24–30 nm, smaller than those of pure TiO2 NTs, and contain only the anatase phase. Er–TiO2 NTs exhibit an effective photo-conversion efficiency (PCE) of 1.58% and a photosensitivity of 115.06. The modified sample are also more efficient (photocurrent density of 6.64 mAcm−2 at a bias potential of 1.5 V vs. Hg/HgO) compare to pure TiO2 NTs. The photocatalytic activity of the Er–TiO2 NTs are evaluated in a hydrogen generation reaction, and the results show hydrogen production of ∼17.39 μmolhr−1cm−2. Further experiments demonstrate that Er–TiO2 NTs successfully degrade methylene blue, with the most active sample reaching 85% photocatalysis after 180 min. This study shows that doping conditions significantly affect the optical and electrical properties of the resulting material, and that the current electrochemical approach to metal doping can be used for efficient and stable PEC water splitting.

Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: Toward high-efficient silicon-based photovoltaics

Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500nm to 980nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO2 thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500nm for both s- and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913±51 is observed at 1.7Wcm−2. We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion.

Investigation of optical and electrical properties of erbium-doped TiO2 thin films for photodetector applications

We have investigated the electrical and optical properties of erbium (Er3+) doped TiO2 thin films (Er:TiO2 TFs) grown by sol–gel technique on glass and silicon substrates. The samples were characterized by field emission gun–scanning electron microscopes (FEG–SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL) and current–voltage measurement techniques. FEG–SEM and AFM images showed the morphological change in the structure of Er:TiO2 TFs and EDX analysis confirmed the Er3+ doped into TiO2 lattice. Broad PL emissions in visible and infrared regions were observed in undoped TiO2 samples and associated to different mechanisms due to the anatase and rutile phases. PL spectra revealed sharp peaks at 525 nm, 565 nm, 667 nm and 1.54 µm which are related to Er3+ emissions in Er:TiO2 samples. The undoped TiO2 and Er:TiO2 TFs based UV-photodetectors were fabricated, and various device parameters were investigated. The doped devices exhibit high photoresponse upon illuminating 350 nm UV light at 2 V bias with faster response time compared to undoped device.

Field-enhancing photonic devices utilizing waveguide coupling and plasmonics - a selection rule for optimization-based design.

This paper describes a systematic design study of periodic gold-nanostrip arrays placed on a thin film aimed at enhancing the electric field inside the film when irradiated by light. Based on the study, a "selection rule" is proposed, which provides optimization-based design methods with an a priori choice between field-enhancement dominated by coupling to guided modes, by plasmonic near-field enhancement or by a mix hereof. An appropriate choice of wavelength and grating period is shown to selectively suppress or include waveguiding effects for the optimized designs. The validity of the selection rule is demonstrated through a numerical topology optimization study in which gold nanostrips are optimized for electric-field enhancement in an erbium-doped TiO2 thin film, targeting increased spectral upconversion in the erbium ions. The obtained designs exhibit waveguide excitation within the predicted intervals and, for light polarized perpendicularly to the strips, plasmonic response outside.

Electroluminescence from silicon-based light-emitting devices with erbium-doped TiO2 films annealed at different temperatures

We have previously developed silicon-based light-emitting devices (LEDs) with luminescent erbium (Er)-doped TiO2 (TiO2:Er) films [Yang et al., Appl. Phys. Lett. 100, 031103 (2012)]. In an LED therein, the TiO2:Er film is sandwiched between the ITO film and heavily boron-doped p-type silicon (p+-Si). In this work, we have investigated the electroluminescence (EL) from two LEDs with the TiO2:Er films annealed at 650 and 850 °C, respectively. It is revealed that between the TiO2:Er film and p+-Si, there is an intermediate silicon oxide (SiOx, x ≤ 2) layer and its thickness increases from ∼4 to 8 nm with the increase of annealing temperature from 650 to 850 °C. Interestingly, the thickness of the intermediate SiOx layer is found to exhibit a profound impact on the EL from the LED with the TiO2:Er film on p+-Si. The EL from the LED with the 650 °C-annealed TiO2:Er film is activated only under the forward bias with the positive voltage connecting to the p+-Si substrate. Such EL consists of the oxygen-vacancy-related emissions from TiO2 host and the characteristic visible and ∼1540 nm emissions from the Er3+ ions, while the EL from the LED with the 850 °C-annealed TiO2:Er film can only be enabled by the reverse bias with the negative voltage applied on the p+-Si substrate. Such EL features only the visible and ∼1540 nm emissions from the Er3+ ions. The difference in the EL behaviors of the two LEDs as mentioned above is found to be ascribed to the different electrical conduction mechanisms.

Performance of Erbium-doped TiO2 thin film grown by physical vapor deposition technique

Undoped and Erbium-doped TiO2 thin films (Er:TiO2 TFs) were fabricated on the n-type Si substrate using physical vapour deposition technique. Field emission scanning electron microscope showed the morphological change in the structure of Er:TiO2 TF as compared to undoped sample. Energy dispersive X-ray spectroscopy (EDX) confirmed the Er doping in the TiO2 thin film (TF). The XRD and Raman spectrum showed the presence of anatase phase TiO2 and Er2O3 in the Er:TiO2 TF. The Raman scattering depicted additional number of vibrational modes for Er:TiO2 TF due to the presence of Er as compared to the undoped TiO2 TF. The UV–Vis absorption measurement showed that Er:TiO2 TF had approximately 1.2 times more absorption over the undoped TiO2 TF in the range of 300–400 nm. The main band transition, i.e., the transition between the oxygen (2p) state and the Ti (3d) state was obtained at ~3.0 eV for undoped TiO2 and at ~3.2 eV for Er:TiO2 TF, respectively. The photo responsivity measurement was done on both the detectors, where Er:TiO2 TF detector showed better detectivity (D *), noise equivalent power and temporal response as compared to undoped detector under ultra-violet illumination.

High-performance Er3+–TiO2 system: Dual up-conversion and electronic role of the lanthanide

Erbium-doped TiO2 materials are synthesized by means of a surfactant-free hydrothermal method having good photoactivities for the liquid-phase degradation of phenol and MB and the gas phase of toluene. From the structural and morphological characterization, it has been stated that the presence of Er3+ induces a progressive anatase cell expansion due to its incorporation in the TiO2 lattice. The best photocatalytic performance was attained for the samples with 2at% of Er3+ irrespective of the chemical degradation reaction essayed. From activity and optical studies under different irradiation excitation conditions, a dual-type mechanism is proposed to be at the origin of the photocatalytic activity enhancement. On one hand, the improvement observed under UV irradiation occurs by the effective charge separation promoted by Er3+ species which would act as electron scavenger. Besides, the up-conversion luminescence process of Er3+ allows profiting the NIR range of the lamp and transferring energy in the UV range to the TiO2. The dual action of Er ions located at anatase networks will open up a wide roadway for the developing of an integral solar active photocatalyst.

Light-emitting devices based on erbium-doped TiO2/p+-Si heterostructures: Engineering of electroluminescence via aluminum co-doping

We have recently reported erbium (Er)-related visible and infrared (∼1540 nm) electroluminescence (EL) from the light-emitting device (LED) based on Er-doped TiO2 (TiO2:Er)/p+-Si heterostructure, triggered by the energy transferred from oxygen-vacancy-related self-trapped excitons (STEs) to Er3+ ions in anatase TiO2. Herein, we further co-dope aluminum (Al) into the TiO2:Er film, which is also used to form heterostructure with p+-Si. The LED based on such heterostructure features the Er-related EL with the substantially suppressed visible emissions and the remarkably enhanced ∼1540 nm emission. The Al co-doping is proved not to substantially affect the amounts of oxygen-vacancy-related STEs and Er3+ ions in anatase TiO2. In this context, the above-mentioned engineering of Er-related EL is tentatively ascribed to the modification of crystal field around the Er3+ ions in anatase TiO2 by the Al co-doping.

Improvement of performance of dye-sensitized solar cells by doping Er2O3 into TiO2 electrodes

Erbium-doped TiO2 electrodes for solar energy cells were prepared and characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). I–V properties of the cells were measured using a Linear Sweep Voltammetry system. The results indicated that the performance of the cells has been improved remarkably due to Er3+ doping. Photoelectrical conversion efficiency was enhanced from 4.83% to 6.05% by Er3+ doping under AM=1.5 illumination, in which the effective area of the DSSC is 0.7×0.7cm2.

Change in photoluminescence from Er-doped TiO2 thin films induced by optically assisted reduction

Erbium-doped TiO2 (TiO2:Er) thin films with the anatase structure have been prepared on Si substrate by laser ablation. Sharp and intense Er-related emission in the visible region as well as in the IR region has been observed under over-band-gap excitation. The broad photoluminescence (PL) peaking at about 530 nm newly appears at low temperature. It has been understood that the broad PL is induced by an optically assisted reduction effect that is caused by both the H2O adsorption and the reduction process of TiO2 to Ti2O3 by UV illumination. In the IR region, Er-related emission consisted of one main peak located at 1.534 μm and many subpeaks located at around 1.54 μm can be observed even at room temperature. The drastic thermal quenching of the Er-related 1.54 μm emission is also considered due to the optically assisted reduction effect.

Effects of yttrium codoping on photoluminescence of erbium-doped TiO2 films

Er 3+ – Y 3+ codoped TiO2 films were prepared on a fused silica substrate by the sol–gel process. The effect of Y3+ codoping on the ∼1.54 μm photoluminescence (PL) properties of Er3+-doped TiO2 films are investigated. Enhancement of PL properties due to Y3+ codoping by a factor of 10 for intensity and of 1.5 for the full width at half maximum in comparison with the Er3+–Al3+ codoped SiO2 system has been observed in the film annealed at Er3+:Y3+:Ti4+=5:30(∼50):100. Extended x-ray absorption fine structure measurements show that the local chemical environment of Er3+ ions in the Er3+–Y3+ codoped TiO2 films is similar to that in Er2O3. The average spatial distance between Er3+ ions is enlarged due to the partial substitution of Y3+ for Er3+ ions in the Er2O3-like local structure. It is believed that the more intense PL emission of the Er3+–Y3+ codoped TiO2 films can be attributed to the better dispersion and distorted local structure of Er3+ ions in the TiO2 host matrix by yttrium codoping.

Optical and Structural Characterization of Erbium-Doped TiO2 Xerogel Films Processed on Porous Anodic Alumina

Titania films, doped with erbium, were fabricated on porous anodic alumina from a precursor. The samples, subjected to thermal processing up to 1270 K, exhibited strong luminescence at 1.53 μm associated with transitions of ions in the xerogel. The intensity of photoluminescence increased with the number of xerogel film depositions onto alumina. Secondary ion mass spectrometry analyses show that the pores of anodic alumina are filled by the xerogel after sequential spinning of ten layers. Cooling of the samples to 4.2 K gives enhancement of the intensity of the Er-related band at 1.53 μm and narrowing of the full width at half maximum from 20 to 10 nm. © 2001 The Electrochemical Society. All rights reserved.