Perovskite Nanostructures Research Articles

(Invited, Digital Presentation) Nanostructured Perovskite Resistive RAM for Next Generation Data Storage

Resistive RAMs (Re-RAMs) have emerged as a propitious candidate among next generation non volatile memories because of their strikingly fast speed, prolonged data retention ability and high storage density. Herein, we developed an electrochemical metallization (ECM) based Re-RAM with three-dimensionally integrated single-crystalline perovskite (methyl ammonium lead halide or MAPbX3 where X=I, Br and Cl) quantum wire (PQW)/ nanowire (PNW) array embedded in a nano-engineered porous alumina membrane (PAM) serving as the switching medium. The PQWs and PNWs were grown inside the PAM template by a vapor-solid-solid-reaction (VSSR) process and were clubbed between silver (Ag) and aluminum (Al) contacts. The nanostructured perovskite Re-RAM exhibited 100 ps switching speed which is a record for perovskite Re-RAMs and also among the fastest for all types of Re-RAMs reported. The fast switching was attributed to the increased ionic (Ag+) and electronic mobility and subsequent accelerated filament formation within the body of the monocrystalline PQWs/PNWs. The PAM scaffolding imparted material and electrical stability to the environmentally delicate perovskite. As a consequence, we were able to obtain a record long (among perovskite Re-RAMs) estimated retention time > 28 years and record-high (among perovskite Re-RAMs) uninhibited cyclic endurance of 6×106 cycles. Also, utilizing the ultra-high density (2×1011/cm2) of the PQWs, a 14 nm lateral dimension ultra-small memory cell was built. In conjunction with multi-bit switching, an effective device footprint of 76.5 nm2 was achieved for a single bit storage. Further, as a concept proof, an 8×8 Re-RAM crossbar array device was fabricated which demonstrated temporally robust alphabetic data storage, with a unique metal-semiconductor-insulator-metal (MSIM) architecture to alleviate the sneaky path problem. The MSIM scheme based on quasi-self-selecting elements, demonstrated a strong potential for unhindered scalability in the future, a problem often encountered with integrating external selecting diodes and transistors. The PQW/PNW Re-RAMs also responded to light stimuli, exhibiting optical programmability among the low resistance states. In summary, these intriguing results propel perovskite Re-RAMs to the state-of-the-art standard and demonstrate an attractive potency for PQW/PNW devices to be used as an alternative technology in future storage and computing modules.

Robust photocatalytic and photoelectrochemical functions of PrFe 1-xMnxO3 perovskite nanostructures

Perovskite nanostructures of PrFe1-xMnxO3 with and without Mn ions were hydrothermally processed and extensively studied as photocatalysts and photoelectrocatalysts. The significance of Mn ions on the intrinsic characteristics of perovskites were initially evaluated to be in orthorhombic pbnm structure. PrFe1-xMnxO3 was also examined to be made up of fine particle like structures. The effective replacement of Mn ions was affirmed through several structural characterization routes. The bonding of Mn ions in the perovskite architecture and their capacity to replace the Fe sites was additionally studied using X-ray photoelectron spectroscopic (XPS) measurements. The absorbance spectra augmented the potential of PrFe1-xMnxO3 to serve as an effective photocatalyst in treating organic pollutants. Mn ions were also found to improve the defect sites in the nanomaterials, which actually played a vital role in influencing the photocatalytic and photoelectrochemical activities. The photocatalytic efficacy of Mn based perovskites was noted to be much better than their pristine counterparts through the comparative photocatalytic studies. The density of photocurrent values studied via photoelectrochemical experiments also supported the active role of Mn ions in boosting the water splitting performance. The impedance data studied via electrochemical route suggested the aid of Mn ions in accelerating the migration of charges readily to result with the observed results.

Growth, Structural, and Electrical Properties of Spin Coated LaNiO3 Conducting Oxide

LaNiO3 (LNO) perovskite oxide thin film nanostructure has been prepared using spin coating technique for its homogenous growth, and structural and electrical properties on quartz substrate. X-ray diffraction (XRD) confirmed standard cubic perovskite symmetry. Surface morphological measurements were carried out using atomic force microscope (AFM), which shows uniform, smooth, and crack free surface with RMS roughness of 7.08 nm. This oxide based conducting layer can act as an excellent conducting bottom/top electrode providing brilliant seeding or buffer layer for subsequent ferroelectric/ferromagnetic layers. This incorporates that it can act as an excellent conducting bottom electrode. I-V characteristics of LNO demonstrated a small shift in current with the application of low perpendicular magnetic field of 0.58T. p-type conduction. Average hole concentration of 3.66 x 10+17 cm-2 was estimated from the Hall effect measurements. The magnetoresistance (MR) of the sample was found to be +48.26%.

Recent Advances in Luminescent Downconversion: New Materials, Techniques, and Applications in Solar Cells

AbstractNumerous investigations have been done in pursuing phosphors with quantum yield (QY) greater than unity in terms of downconversion (DC) strategies, as well as applications in display, lighting, and particularly in novel solar cells with efficiency exceeding the Shockley–Queisser limit (≈30%). It is of significant interest that: i) DC of one high‐energy photon to two or more low‐energy photons is widely found in lanthanide and/or transition‐metal ions activated materials; ii) broadening of absorption spanning ultraviolet/blue range is achieved by introducing sensitizers with spin‐allowed transitions and especially by combining the advancing perovskite quantum dots, nanostructures, and organic dye molecules; iii) internal QY ≫ 1 and even measured QY > 1 are claimed in many near‐infrared (NIR) DC systems; iv) applications of downconverting layer atop commercial solar cells are performed to increase the photon conversion efficiency. In this review, the development of DC in theories, concepts, and experiments is first summarized, then the breakthrough in visible‐DC is briefly highlighted, NIR‐DC with latest progresses in models, materials, techniques as well as the corresponding optimizations and practices as effective downconverting layers is elaborated, and finally concluding remarks and perspectives in advancing NIR‐DC and applications in novel solar cells are given.

Low Thresholds and Tunable Modes in Plasmon‐Assisted Perovskite Microlasers

AbstractLow‐dimensional perovskite structures are ideal materials for nano/microlasers owing to their excellent optical gain properties and high emission efficiency. But it is still a challenge to achieve single‐mode and tunable lasing behaviors in micro‐scale perovskite, especially showing ultralow thresholds in tens of µJ cm−2. In this work, CsPbBr3/Ag hemispheroids have been synthesized in a designed chemical vapor deposition (CVD) process. Benefiting from the whispering gallery resonance mode and plasmon‐enhanced light–matter interactions in well‐confined hybrid cavities, the CsPbBr3/Ag plasmonic microlasers exhibit low pump thresholds down to 35–51 µJ cm−2. Interestingly, single‐mode and two‐mode lasing behaviors are determined by the morphology of plasmonic Ag nanostructures, and lasing modes could be selectively tuned by increasing the pump fluence accompanying with dynamic competitions of photonic and plasmonic modes. Simulations of electromagnetic field have been plotted to show near‐field resonant modes for a comprehensive understanding of plasmon‐assisted lasing. These results not only provide a solution to fabricate low‐threshold plasmonic microlasers, but also advocate the prospect of all‐inorganic perovskite nanostructures as promising candidates for on‐chip integrated light sources.

Morphology-controlled BaTiO3 nanostructures with high sensitivity and fast response for respiratory monitoring

As a typical perovskite oxide, BaTiO3 has been fully proved to be an excellent impedance humidity sensing material, meanwhile its application in human respiratory monitoring needs to be further researched. In this contribution, BaTiO3 nanorods (BT-NR), nanowires (BT-NW) and micron particle (BT-MP) were successfully synthesized by the molten salt synthesis (MSS) method. Various characterization techniques were used to analyze the structure, morphology and composition of these materials. As-synthesized BaTiO3 samples had been assembled as impedimetric humidity sensors and then tested systematically under relatively humidity (RH) range of 11%–97%. All the three sensors exhibited high response values greater than three orders of magnitude, which are much higher than the commercial BaTiO3 (BT-C) sensor (S97% = 4.90 × 102). Thereinto, the BT-NR humidity sensor holds the fastest response-recovery times (1 s and 22 s) and the smallest hysteresis (ΔH = 0.67%). Excellent sensing performance of BT-NR can be attributed to synergistic effects of larger specific surface area and smaller pore size. In addition, the BT-NR humidity sensor shows considerable potential in the field of human respiratory monitoring. This work provides a new perspective to high-performance humidity sensors for human body-related detection through morphology-controlled perovskite nanostructures.

Phosphatizing Engineering of Perovskite Oxide Nanofibers for Hydrogen Evolution Reaction to Achieve Extraordinary Electrocatalytic Performance

AbstractPerovskite oxides with tailored elements and structures endow promising electrocatalytic properties for hydrogen evolution reaction (HER) due to considerable activity and satisfactory long‐term operating stability. However, the synthesis of 1D perovskite nanostructures with anion dopants remains a challenge, as well as insights into the performance origin from experimental and theoretical views. Herein, combined with electrospinning and phosphatizing strategies, P‐doped Pr0.5La0.5BaCo2O5+δ perovskite nanofibers (P‐PLBC‐F) are reported for highly active HER catalysts. The P‐PLBC‐F catalyst outperforms the classic perovskites and can be comparable to the commercial Pt/C (wtPt = 20%) benchmark, exhibiting a Tafel slope of −32.9 mV dec−1 and overpotential of −307 mV at 500 mA cm−2 disk in alkaline media. The highly catalytic activity benefits from the desired 1D nanostructure, steered electronic structures (charge transfer energy ΔCo‐P = 0.79 eV) and Gibbs free energy for the H* desorption (1.02 eV) induced by the P3− doping, as supported by density functional theory calculations. The findings described here unveil the phosphatizing mechanism of the perovskites for catalyzing the hydrogen production, and broaden horizons in terms of rational designs of novel perovskite‐based catalysts for sustainable energy.

Stable CsPbBr3 Nanoclusters Feature a Disk-like Shape and a Distorted Orthorhombic Structure.

CsPbBr3 nanoclusters have been synthesized by several groups and mostly employed as single-source precursors for the synthesis of anisotropic perovskite nanostructures or perovskite-based heterostructures. Yet, a detailed characterization of such clusters is still lacking due to their high instability. In this work, we were able to stabilize CsPbBr3 nanoclusters by carefully selecting ad hoc ligands (benzoic acid together with oleylamine) to passivate their surface. The clusters have a narrow absorption peak at 400 nm, a band-edge emission peaked at 410 nm at room temperature, and their composition is identified as CsPbBr2.3. Synchrotron X-ray pair distribution function measurements indicate that the clusters exhibit a disk-like shape with a thickness smaller than 2 nm and a diameter of 13 nm, and their crystal structure is a highly distorted orthorhombic CsPbBr3. Based on small- and wide-angle X-ray scattering analyses, the clusters tend to form a two-dimensional (2D) hexagonal packing with a short-range order and a lamellar packing with a long-range order.

Correlating Orbital Composition and Activity of LaMnxNi1-xO3 Nanostructures toward Oxygen Electrocatalysis.

The atomistic rationalization of the activity of transition metal oxides toward oxygen electrocatalysis is one of the most complex challenges in the field of electrochemical energy conversion. Transition metal oxides exhibit a wide range of structural and electronic properties, which are acutely dependent on composition and crystal structure. So far, identifying one or several properties of transition metal oxides as descriptors for oxygen electrocatalysis remains elusive. In this work, we performed a detailed experimental and computational study of LaMnxNi1–xO3 perovskite nanostructures, establishing an unprecedented correlation between electrocatalytic activity and orbital composition. The composition and structure of the single-phase rhombohedral oxide nanostructures are characterized by a variety of techniques, including X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy. Systematic electrochemical analysis of pseudocapacitive responses in the potential region relevant to oxygen electrocatalysis shows the evolution of Mn and Ni d-orbitals as a function of the perovskite composition. We rationalize these observations on the basis of electronic structure calculations employing DFT with HSE06 hybrid functional. Our analysis clearly shows a linear correlation between the OER kinetics and the integrated density of states (DOS) associated with Ni and Mn 3d states in the energy range relevant to operational conditions. In contrast, the ORR kinetics exhibits a second-order reaction with respect to the electron density in Mn and Ni 3d states. For the first time, our study identifies the relevant DOS dominating both reactions and the importance of understanding orbital occupancy under operational conditions.

Circular Polarized Light Emission in Chiral Inorganic Nanomaterials.

Chiral inorganic nanostructures strongly interact with photons changing their polarization state. The resulting circularly polarized light emission (CPLE) has cross-disciplinary importance for a variety of chemical/biological processes and is essential for development of chiral photonics. However, the polarization effects are often complex and their interpretation is dependent on the several structural parameters of the chiral nanostructure. CPLE in nanostructured media has multiple origins and several optical effects are typically convoluted into a single output. Analyzing CPLE data obtained for nanoclusters, nanoparticles, nanoassemblies, and nanocomposites from metals, chalcogenides, perovskite, and other nanostructures, it is shown here that there are several distinct groups of nanomaterials for which CPLE is dominated either by circularly polarized luminescence (CPL) or circularly polarized scattering (CPS); there are also many nanomaterials for which they are comparable. The following points are also demonstrated: 1) CPL and CPS contributions involve light-matter interactions at different structural levels; 2) contribution from CPS is especially strong for nanostructured microparticles, nanoassemblies, and composites; and 3) engineering of materials with strongly polarized light emission requires synergistic implementation of CPL and CPS effects. These findings are expected to guide development of CPLE materials in a variety of technological fields, including 3D displays, information storage, biosensors, optical spintronics, and biological probes.

Facile eco-friendly synthesis of rare-earth cobaltite-based perovskite nanostructures as electrocatalysts for oxygen evolution reaction

Perovskite (ABO3) Structured gadolinium cobaltite (GdCoO3) nanorods and lanthanum cobaltite (LaCoO3) nanoplates are prepared via an eco-friendly green synthesis approach. The Perovskite-based nanostructures show an excellent oxygen evolution reaction (OER) with an overpotential of about 340 mV (LaCoO3) and 320 mV (GdCoO3) at a current density of 10 mA cm−2 in an alkaline medium. Based on the findings, GdCoO3 could be an excellent candidate as a high-performance electrocatalyst in OER application.

Tunable Magnetic Anisotropy in Patterned SrRuO3 Quantum Structures: Competition between Lattice Anisotropy and Oxygen Octahedral Rotation

AbstractArtificial perovskite oxide nanostructures possess intriguing magnetic properties due to their tailorable electron–electron interactions, which are extremely sensitive to the oxygen coordination environment. To date, perovskite oxide nanodots with sizes below 50 nm have rarely been reported. Furthermore, the oxygen octahedral distortion and its relation to magnetic properties in perovskite oxide nanodots remain unexplored thus far. Here, the magnetic anisotropy in patterned SrRuO3 (SRO) nanodots as small as 30 nm are studied. The constituent elements, in particular oxygen ions, are directly visualized via performing atomic resolution electron microscopy and spectroscopy. It is observed that the magnetic anisotropy and RuO6 octahedra distortion in SRO nanodots are both nanodot size‐dependent but remain unchanged in the first 3‐unit‐cell interfacial SRO monolayers regardless of the dots’ size. Combined with first principle calculations, a unique structural mechanism behind the nanodots’ size‐dependent magnetic anisotropy in SRO nanodots is unraveled, suggesting that the competition between lattice anisotropy and oxygen octahedral rotation mediates anisotropic exchange interactions in SRO nanodots. These findings demonstrate a new avenue toward tuning magnetic properties of correlated perovskite oxides and imply that patterned nanodots could be a promising playground for engineering emergent functional behaviors.

Control of Hot Carrier Relaxation in CsPbBr3 Nanocrystals Using Damping Ligands.

In photon-conversion processes, rapid cooling of photo-induced hot carriers is a dominant energy loss channel. We herein report a 3-fold reduced hot carrier cooling rate in CsPbBr3 nanocrystals capped with a cross-linked polysiloxane shell in comparison to single alkyl-chain oleylamine ligands. Relaxation of hot charge carriers depends on the carrier-phonon coupling (CPC) process as an important channel to dissipate energies in nanostructured perovskite materials. The CPC strengths in the two samples were measured through cryogenic photoluminescence spectroscopic measurements. The effect of organic ligands on the CPC in CsPbBr3 nanocrystals is elucidated based on a damped oscillation model. This supplements the conventional polaron-based CPC model, by involving a damping effect on the CPC from the resistance of the ligands against nanocrystal lattice vibrations. The model also accounts for the observed linear temperature-dependence of the CPC strength. Our work enables predictions about the effect of the ligands on the performance of perovskite nanocrystals in future applications.

Control of Hot Carrier Relaxation in CsPbBr3 Nanocrystals Using Damping Ligands

AbstractIn photon‐conversion processes, rapid cooling of photo‐induced hot carriers is a dominant energy loss channel. We herein report a 3‐fold reduced hot carrier cooling rate in CsPbBr3 nanocrystals capped with a cross‐linked polysiloxane shell in comparison to single alkyl‐chain oleylamine ligands. Relaxation of hot charge carriers depends on the carrier‐phonon coupling (CPC) process as an important channel to dissipate energies in nanostructured perovskite materials. The CPC strengths in the two samples were measured through cryogenic photoluminescence spectroscopic measurements. The effect of organic ligands on the CPC in CsPbBr3 nanocrystals is elucidated based on a damped oscillation model. This supplements the conventional polaron‐based CPC model, by involving a damping effect on the CPC from the resistance of the ligands against nanocrystal lattice vibrations. The model also accounts for the observed linear temperature‐dependence of the CPC strength. Our work enables predictions about the effect of the ligands on the performance of perovskite nanocrystals in future applications.

Water-assisted synthesis of lead-free Cu based fluorescent halide perovskite nanostructures

The blue fluorescent lead-free perovskite nanostructures were synthesized at room temperature in the presence of water. These perovskites demonstrated their morphology as square nanoplates, nanorods, and rectangular nanoplates for Cs2CuCl4, Cs2CuBr4, and Cs2CuBr2Cl2, respectively.

Luminescence enhancement effects on nanostructured perovskite thin films for Er/Yb-doped solar cells.

Recent attempts to improve solar cell performance by increasing their spectral absorption interval incorporate up-converting fluorescent nanocrystals on the structure. These nanocrystals absorb low energy light and emit higher energy photons that can then be captured by the solar cell active layer. However, this process is very inefficient and it needs to be enhanced by different strategies. In this work, we have studied the effect of nanostructuration of perovskite thin films used in the fabrication of hybrid solar cells on their local optical properties. The perovskite surface was engraved with a focused ion beam to form gratings of one-dimensional grooves. We characterized the surfaces with a fluorescence scanning near-field optical microscope, and obtained maps showing a fringe pattern oriented in a direction parallel to the grooves. By scanning structures as a function of the groove depth, ranging from 100 nm to 200 nm, we observed that a 3-fold luminescence enhancement could be obtained for the deeper ones. Near-field luminescence was found to be enhanced between the grooves, not inside them, independent of the groove depth and the incident polarization direction. This indicates that the ideal position of the nanocrystals is between the grooves. In addition, we also studied the influence of the inhomogeneities of the perovskite layer and we observed that roughness tends to locally modify the intensity of the fringes and distort their alignment. All the experimental results are in good agreement with numerical simulations.

Optical Behaviour of Caffeine Mixed Perovskite Photoanode for Photovoltaic Application

Halide perovskites such as methylammonium lead triiodide (CH3NH3PbI3) are attracting attention as energy-efficient light absorber materials for photovoltaic applications owing to their solution processing-power, tunable bandgap, strong absorption coefficients and cost effectiveness. Scanning Electron Microscope (SEM) and Ultra-Violet Visible Spectroscopy (UV-vis) were used to characterize the prepared samples. The substrates were rinsed with propanol to remove impurities and later sintered before precursor deposition with different caffeine ratios via spray pyrolysis. The deposition method was used due to its homogenous ability. The SEM results indicate that the introduction of caffeine enhances nucleation, leading to an increase in the growth rate compared to the control. The UV-vis result shows a redshift to a higher wavelength, indicating an increase in visible light absorption which is attributed to the spectral overlap between the CH3NH3PbI3 absorber and caffeine. The bandgap (Eg) energy analysis further confirmed that the caffeine-modified photoanode is a potential light absorber in perovskite solar cell applications. This study provides a good understanding of the optical behaviour of caffeine-mixed perovskite photoanode nanostructures and a promising approach for photon energy management.

Förster and nanometal surface-energy transfer in CsPbCl3/Yb3+ quantum-cutting multilayer structures.

Recently, in nanophotonics, thin metal films owing to the plasmon modes they support and their perovskite nanostructures exhibit novel optical properties, which have attracted considerable interest. Both the Förster resonant energy transfer (FRET) of the dopant-induced right-angled Yb3+-VPb-Yb3+ defect state and a pair of Yb3+ ions in all-inorganic perovskite nanocrystal (PeNC) CsPbCl3:Yb3+ quantum-cutting (QC) materials and the nanometal surface-energy transfer (NSET) of the excitons of PeNC-Ag nanoparticles (NPs) were investigated experimentally in CsPbCl3:Yb3+/PMMA/Ag/Si (CYAii = 1, 2, 3, 4, 5, 6), CsPbCl3:Yb3+/PMMA/Si (CYi), and CsPbCl3/PMMA/Ag/Si (CAi), representing three species of multilayer structures. It was found that due to the mediation of the Ag film and an increase in the interaction volume of donors-acceptors, FRET efficiencies increased from 26% to 66% as the spacer (or wave-guiding layer) thicknesses decreased from 63.7 to 17.8 nm. The energy-transfer efficiencies of CAi in the NSET in the surface-surface scheme followed a d-1.6-distance dependence. This distance dependence approached the d-2-distance dependence expected of a point-to-surface or 0D-2D energy transfer (ET). The ET in quantum cutting (QC) modulated by plasmons undoubtedly paves a way for improving the FRET and NSET performances of materials.

Solution-based low-temperature CsPbI3 nanoparticle perovskite solar cells

This work reports on low-temperature inorganic CsPbI3 perovskite nanostructures synthesized as the active black phase, without the additional use of organic ligands and based only on CsI and PbI2 precursors.

A small-molecule-templated nanostructure back electrode for enhanced light absorption and photocurrent in perovskite quantum dot photovoltaics

A nanostructured spiro-OMeTAD can be fabricated via nanoimprint soft lithography and realizes back-side nanostructured perovskite quantum dot solar cells showing improved photovoltaic performance due to the enhanced light absorption and photocurrent.