Nano-grained Films Research Articles

A study on morphology dependent nanostructured ZnO thin films: An efficient gas sensing response for acetaldehyde

In this work, kinematically grown highly oriented hexagonal ZnO nanorods and nanograined films were fabricated on silicon substrates via pulsed laser deposition method. The structural, morphological, optical properties and the gas sensing studies of ZnO nanostructured thin film were systematically studied. Structural study shows high crystalline films with preferred c-axis orientated nanorods, which are grown over (002) plane nanograined films. Morphological study shows spherical-shaped nanograins and hexagonal vertical rod like structures due to kinematic growth parameters. The optical band gaps of ZnO-NR film at Eg ∼ 3.18 eV and 3.46 eV, depicts greater grain boundary resistance to nanorod resistance. The artificial olfaction effect of ZnO nanorods in detecting acetaldehyde (CH3CHO) is found to be 40% more sensitive than ZnO nanograined films. The ZnO nanorods displays a greater sensitivity response for CH3CHO than other target gases. Transient gas Responsive Resistance (TRR) of ZnO nanorods and nanograined films shows increased sensitivity with higher concentration CH3CHO (10 to 100 ppm) at ambient temperature. The study indicates that ZnO nanorods could be a potential material for the detection of CH3CHO gas at room temperature.

Grain engineering of high energy density BaTiO3 thick films integrated on Si

Ferroelectric (FE) ceramics with a large relative dielectric permittivity and a high dielectric strength have the potential to store or supply electricity of very high energy and power densities, which is desirable in many modern electronic and electrical systems. For a given FE material, such as the commonly-used BaTiO3, a close interplay between defect chemistry, misfit strain, and grain characteristics must be carefully manipulated for engineering its film capacitors. In this work, the effects of grain orientation and morphology on the energy storage properties of BaTiO3 thick films were systematically investigated. These films were all deposited on Si at 500 °C in an oxygen-rich atmosphere, and their thicknesses varied between ~500 nm and ~2.6 μm. While a columnar nanograined BaTiO3 film with a (001) texture showed a higher recyclable energy density Wrec (81.0 J/cm3vs. 57.1 J/cm3 @3.2 MV/cm, ~40% increase) than that of a randomly-oriented BaTiO3 film of about the same thickness (~500 nm), the latter showed an improved energy density at a reduced electric field with an increasing film thickness. Specifically, for the 1.3 μm and 2.6 μm thick polycrystalline films, their energy storage densities Wrec reached 46.6 J/cm3 and 48.8 J/cm3 at an applied electric field of 2.31 MV/cm (300 V on 1.3 μm film) and 1.77 MV/cm (460 V on 2.6 μm film), respectively. This ramp-up in energy density can be attributed to increased polarizability with a growing grain size in thicker polycrystalline films and is desirable in high pulse power applications.

Effect of substrate morphology on the deposition behavior of α-Al2O3 films by room temperature granule spray in vacuum process

In this study, we investigated the influence of surface morphology of a hard substrate on the deposition behavior of α-Al2O3 films coated by a granule spray in vacuum (GSV) process at room temperature. Three types of α-Al2O3 films were prepared by chemical vapor deposition (CVD) on a WC-Co base material: (006) textured film with a uniform and small mold-type morphology, (110) and (012) textured films with large grains and a relatively flat morphology, and flat (006) textured film after polishing. These CVD α-Al2O3 films were further coated with nano-grained α-Al2O3 films by the GSV process. The deposition behavior, adhesion strength, and residual surface stress were found to be strongly influenced by the surface morphology of the hard substrate. For the (006) textured CVD α-Al2O3 film with steep crests and troughs on the surface, the GSV α-Al2O3 film was deposited uniformly, possibly because the powder can be directly anchored by the molding effect and subsequent hammering effect. Such different deposition characteristics also led to a variation in the surface residual stress.

Recyclable free-polymer transfer of nano-grain MoS2 film onto arbitrary substrates

Clean transfer of transition metal dichalcogenides (TMDs) film is highly desirable, as intrinsic properties of TMDs may be degraded in a conventional wet transfer process using a polymer-based resist and toxic chemical solvent. Residues from the resists often remain on the transferred TMDs, thereby causing a significant variation in their electrical and optical characteristics. Therefore, an alternative to the conventional wet transfer method is needed—one in which no residue is left behind. Herein, we report that our molybdenum disulfide (MoS2) films synthesized by plasma-enhanced chemical vapor deposition can be easily transferred onto arbitrary substrates (such as SiO2/Si, polyimide, fluorine-doped tin oxide, and polyethersulfone) by using water alone, i.e. without residues or chemical solvents. The transferred MoS2 film retains its original morphology and physical properties, which are investigated by optical microscopy, atomic force microscopy, Raman, x-ray photoelectron spectroscopy, and surface tension analysis. Furthermore, we demonstrate multiple recycling of the resist-free transfer for the nano-grain MoS2 film. Using the proposed water-assisted and recyclable transfer, MoS2/p-doped Si wafer photodiode was fabricated, and the opto-electric properties of the photodiode were characterized to demonstrate the feasibility of the proposed method.

Engineering grain boundaries at the 2D limit for the hydrogen evolution reaction

Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs.

Efficient Perovskite Light‐Emitting Diodes Using Polycrystalline Core–Shell‐Mimicked Nanograins

AbstractMaking small nanograins in polycrystalline organic–inorganic halide perovskite (OIHP) films is critical to improving the luminescent efficiency in perovskite light‐emitting diodes (PeLEDs). 3D polycrystalline OIHPs have fundamental limitations related to exciton binding energy and exciton diffusion length. At the same time, passivating the defects at the grain boundaries is also critical when the grain size becomes smaller. Molecular additives can be incorporated to shield the nanograins to suppress defects at grain boundaries; however, unevenly distributed molecular additives can cause imbalanced charge distribution and inefficient local defect passivation in polycrystalline OIHP films. Here, a kinetically controlled polycrystalline organic‐shielded nanograin (OSN) film with a uniformly distributed organic semiconducting additive (2,2′,2′′‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole), TPBI) is developed mimicking core–shell nanoparticles. The OSN film causes improved photophysical and electroluminescent properties with improved light out‐coupling by possessing a low refractive index. Finally, highly improved electroluminescent efficiencies of 21.81% ph el−1 and 87.35 cd A−1 are achieved with a half‐sphere lens and four‐time increased half‐lifetime in polycrystalline PeLEDs. This strategy to make homogeneous, defect‐healed polycrystalline core–shell‐mimicked nanograin film with better optical out‐coupling will provide a simple and efficient way to make highly efficient perovskite polycrystal films and their optoelectronics devices.

Controlling the phase transition in nanocrystalline ferroelectric thin films via cation ratio.

Traditionally, the ferroelectric Curie temperature can be manipulated by chemical substitution, e.g., in Ba1-xSrxTiO3 as one of the archetypical representatives. Here, we show a novel approach to tune the ferroelectric phase transition applicable for nanostructured thin films. We demonstrate this effect in nano-grained BaTiO3 films. Based on an enhanced metastable cation solubility with Ba/Ti-ratios of 0.8 to 1.06, a significant shift of the phase transition temperature is discovered. The transition temperature increases linearly from 212 K to 350 K with increasing Ba/Ti ratio. For all Ba/Ti ratios, a completely diffused phase transition is present resulting in a negligible temperature sensitivity of the dielectric constant. Schottky defects are identified as the driving force behind the off-stoichiometry and the shift of the phase transition temperature as they locally induce lattice strain. Complementary temperature dependent Raman experiments reveal the presence of the hexagonal polymorph in addition to the perovskite phase in all cases. Interestingly, the hexagonal BaTiO3 influences the structural transformation on the Ba-rich side, while on the Ti-rich side no changes for the hexagonal polymorph at the ferroelectric transition temperature are observed. This concerted structural change of both polymorphs on the Ba-rich side causes a broad phase transition region spanning over a wide range up to 420 K including the transition temperature of 350 K obtained from dielectric measurements. These findings are promising for fine adjustment of the phase transition temperature and low temperature coefficient of permittivity.

Ion valence state and magnetic origin of PbPd1−xNixO2 nanograin films with a high-temperature ferromagnetism

The PbPd1 − xNi x O2 nanograin films with different Ni-doping levels (x = 0.037–0.150) were synthesized by the sol–gel spin-coating method and an oxidation treatment. The films with a thickness of about 210 nm were found to be single phase with a body-centered orthorhombic structure and to possess a large number of Pb vacancies. High-temperature ferromagnetism discovered in these films can be maintained above 380 K. Paramagnetism was also found in the films with high Ni-doping levels. The analysis on the XANES spectra and their first derivatives offered the evidences for the facts that the film’s ferromagnetism is intrinsic and the increase in the Pb valence from 2+ towards 4+, caused by the appearance of large amount of Pb vacancies and low electronegativity of Pb2+ ion, provides magnetic moments to the film’s magnetism. A carrier-mediated mechanism bridged to the bound magnetic polaron model based on the Pb vacancies, the doped Ni ions and the Pb ions with a valence higher than 2+ was adopted to interpret the origin of the magnetism within these PbPd1 − xNi x O2 nanograin films. The variation of the specific area of the grain boundary was believed to have a great influence on the proportions of the film’s ferromagnetism and paramagnetism.

Molecular dynamics simulations of tensile deformation of gradient nano-grained copper film

Molecular dynamics simulations are performed to investigate the plastic deformation of a gradient nano-grained (GNG) copper film. An extra strengthening is observed in the GNG film which indicates there is synergetic interaction between different grain-size layers. The plastic deformation of the GNG film occurs first in small grains and then propagates into larger grains with increasing strain. This orderly plastic deformation process is attributed to the GNG structure and found independent of strain rate and temperature. Grain coarsening induced by the deformation is apparently suppressed in the GNG film due to the GNG structure.

High-temperature ferromagnetism of Cu-doped PbPdO2 nanograin films

The single-phase, body-centered orthorhombic-structured, Cu-doped PbPdO2 films with a nanograin structure were synthesized by using the sol–gel spin-coating method and an oxidation treatment. It was found that large amounts of Pb vacancies exist in the nanograin film and Pb ions possess a mixed valence state of 2+ and 4+. Besides the superparamagnetism and the charge ordering state, the magnetic studies displayed that ferromagnetism, which can persist up to 380 K, exists in the Cu-doped PbPdO2 nanograin film. As a peculiar characteristic of the spin gapless semiconductor, the ferromagnetic saturation magnetization increasing unusually with temperatures above 300 K was also discovered. The bound magnetic polarons based on the Pb vacancies, the doped Cu2+ ions, and the Pb ions with a valence higher than 2+ were believed to be the origin of such high-temperature ferromagnetism.

Microstructures and nano-mechanical properties of multilayer coatings prepared by plasma nitriding Cr-coated Al alloy

A novel surface treatment combined with depositing Cr films followed by plasma nitriding was carried out on the surface of the 2024 Al alloy. Nano-grained Cr films were firstly deposited on the substrate surface by using magnetron sputtering, then plasma nitrided at 490 °C for 8 h under a gas mixture of 25% N2+75% H2. Microstructures and mechanical properties of the as-obtained coatings were characterized by using X-ray photoelectron spectrometer (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and nanoindentation. Results showed that a nitride/aluminide multilayer coating was obtained due to N-Cr and Cr-Al diffusion reaction. The outside nitride layer consisted of mainly CrN with nano crystals. The inner aluminide layer was composed of Al4Cr with nano lath-shaped structure and Al18Cr2Mg3 with large rectangle crystals. The nitrided surface showed a remarkable increase in hardness (15.4 GPa) than that of the untreated Cr film (5.2 GPa). The high valves of H/E ratio (0.078) and H3/E2 ratio (0.094) indicated the better wear resistance and plastic deformation resistance for the nitride surface.

Size-dependent kinetics of reactive diffusion in nano-grained Ag-Sn thin films

The kinetics of lateral Ag3Sn intermetallic phase propagation during Sn diffusion into nano-grained Ag films has been studied by optical microscopy, SEM and electron microprobe in a temperature range 140 – –200°C. The Sn and Ag films of various thicknesses were deposited onto glass substrate with small overlap (5–10μm). It was detected that the rates of Ag3Sn propagation in thin film couples exceed more than two orders of magnitude the propagation rate in a bulk of massive Ag-Sn samples. Kinetics of the phase propagation depends on the grain size in Ag-film: the smaller grain size, the faster is the propagation rate. The mechanism of phase propagation is proposed in assumption that the kinetics is controlled by grain boundary (GB) diffusion through the intermetallic layer with subsequent chemical reaction between Ag and Sn atoms at the Ag3Sn -Ag interface. By comparison of experimental data on Ag3Sn propagation with the proposed theory the Arrhenius equation Diδ=8.1×10-14exp-55.4kJ/molRTm3/s was obtained for products Di δ (Di is the GB diffusion coefficient of Sn in the Ag3Sn phase, δ is the GB width).

Microstructure and thermoelectric properties of Bi2Te3 electrodeposits plated in nitric and hydrochloric acid baths

The reduction behaviors of unitary and binary bismuth cation (BiIII), tellurium cation (TeIV) in nitric acid and hydrochloric acid solutions were studied at different acid concentrations. Compared to the nitrate bath, the reduction of BiIII was retarded and the reduction of TeIV was promoted in hydrochloric acid. As a result, the difference in the reduction potential of BiIII and TeIV was markedly reduced in the presence of chloride ion. The acid concentration affected the morphologies of the Bi-Te deposit; specifically a rougher deposit was electroplated at higher acid concentrations. A 25μm Bi2Te3 film obtained from 0.7M HNO3 at −20mV versus saturated calomel electrode (SCE) had columnar structure with (110) preferred orientation and its Seebeck coefficient and power factor were −72.3μV/K and 732μW/m·K2, respectively. A 25μm smooth, compact, nano-grained Bi2Te3 film was fabricated in 0.35M HCl at −20mV versus SCE and its Seebeck coefficient and power factor were −105.0μV/K and 169μW/m·K2, respectively.

Valence state and magnetism of Mn-doped PbPdO2 nanograin film synthesized by sol-gel spin-coating method

The Mn-doped PbPdO2 nanograin film with many Pb vacancies was synthesized by using a sol-gel spin-coating method. The valence states for Pb, Pd and Mn ions were found to be 2+, 2+ and the mixed one of 3+ and 4+, respectively. The existence of Pb vacancies was believed to result in the increase of the Mn valence. The ferromagnetism and the antiferromagnetism were found to coexist in the formed Mn-doped PbPdO2 nanograin film. The ferromagnetism which can be maintained to above the room temperature was proved to be intrinsic. The carrier-mediated mechanism bridged to bound magnetic polaron model was used to explain the coexistence of the antiferromagnetism and the ferromagnetism within the Mn-doped PbPdO2 film.

High-temperature ferromagnetism of Ni-doped PbPdO2 nanograin films synthesized by sol-gel spin-coating method

The single-phase Ni-doped PbPdO2 films with a body-centered orthorhombic structure were synthesized by the sol-gel spin-coating method and an oxidation treatment. The films with a thickness of about 440nm were found to have a nanograin structure and large amount of Pb vacancies. The valence states of Ni and Pd ions within the Ni-doped PbPdO2 films were very close to 2+, while the Pb ions exhibited a mixed valence between 2+ and 4+. The existence of Pb vacancies and the low electronegativity of Pb2+ ion resulted in the increase of the valence of Pb ions. The analysis of the magnetic properties indicated that the magnetisms of the single-phase Ni-doped PbPdO2 nanograin films were all composed of the ferromagnetism and the paramagnetism. The ferromagnetism enhanced with the calcination temperature increasing and could be retained up to 380K. A carrier-mediated mechanism bridged to the bound magnetic polaron model based on the Pb vacancies, the doped Ni ions and the Pb ions with a valence higher than 2+ were used to explain the magnetic origin of these Ni-doped PbPdO2 nanograin films.

Effect of Fe doping on the magnetic properties of PbPdO2 nanograin film fabricated by sol-gel method

The single-phase Fe-doped PbPdO2 film was prepared using a sol-gel spin-coating method. The film had a nanograin structure consisting of compacted particles with an average crystallization size of about 35.2nm. Large amount of Pb vacancies were found in the film. The valences of the Pb, Pd and Fe ions of the film were confirmed to be near 2+, 2+ and 3+, respectively. The additional electron provided by Fe3+ and the high ionization energy of Fe3+ ensure the stability of the valence of doped Fe ions. The measurement of hysteresis loops and the theoretical fitting of the zero-field-cooled and field-cooled magnetization versus temperature curve indicated that the ferromagnetism and the paramagnetism coexist in the Fe-doped PbPdO2. And the ferromagnetism persisted up to 380K. A bound magnetic polaron model based on the Pb vacancies, the carriers and the doped Fe ions was utilized to account for the origin of the film's ferromagnetism. The isolated Fe ions or magnetic polarons were believed to be responsible for the paramagnetism in the film.

Microstructure and magnetism of sol–gel synthesized Co-doped PbPdO2 nanograin film

The single-phase body-centered-orthorhombic-structured Co-doped PbPdO2 thin film was obtained using the sol–gel spin-coating method and an oxidation treatment. The film had a nanograin microstructure with an average grain size of about 56nm and a thickness of about 150nm. The adoption of a moderate calcination temperature of 650°C was found to be very important for preparing the single-phase-PbPdO2-based film with a high crystalline quality. The magnetic studies exhibited that the ferromagnetism and the paramagnetism coexist within the formed Co-doped PbPdO2 film and the ferromagnetism can be maintained above room temperature. The saturation magnetization of the ferromagnetism increasing with the temperature, as the peculiar characteristic of the spin gapless semiconductor, was found within the film. The investigation on the film's X-ray absorption near-edge structure revealed that the ferromagnetism of the Co-doped PbPdO2 is intrinsic.

Microstructure and magnetism of Co-doped PbPdO2 films with different grain sizes

The dependence of the magnetism of the Co-doped PbPdO2 nanograin films with peculiar spin-gapless-related feature on the grain size was studied in detail.

Dislocation-mediated relaxation in nanograined columnar palladium films revealed by on-chip time-resolved HRTEM testing.

The high-rate sensitivity of nanostructured metallic materials demonstrated in the recent literature is related to the predominance of thermally activated deformation mechanisms favoured by a large density of internal interfaces. Here we report time-resolved high-resolution electron transmission microscopy creep tests on thin nanograined films using on-chip nanomechanical testing. Tests are performed on palladium, which exhibited unexpectedly large creep rates at room temperature. Despite the small 30-nm grain size, relaxation is found to be mediated by dislocation mechanisms. The dislocations interact with the growth nanotwins present in the grains, leading to a loss of coherency of twin boundaries. The density of stored dislocations first increases with applied deformation, and then decreases with time to drive additional deformation while no grain boundary mechanism is observed. This fast relaxation constitutes a key issue in the development of various micro- and nanotechnologies such as palladium membranes for hydrogen applications.

Bandgap variation in grain size controlled nanostructured CdO thin films deposited by pulsed-laser method

Cadmium oxide (CdO) thin films were prepared by pulsed laser deposition technique. Their structure, surface morphology, optical and electrical properties have been investigated. With a decrease in the laser energy density, the average grain size of the CdO film can be adjusted from 108 to 25 nm. High-resolution TEM observation showed that more crystalline defects like lattice distortion, dislocation and amorphous structure existed in the small grained (25 nm) CdO film, and X-ray photoelectron spectroscopy analysis confirmed that the film had more oxygen vacancies. The electrical and optical properties of the films significantly depended on the grain size. With the grain size decreasing to 25 nm, the optical band gap energy of the CdO film increased obviously from 2.82 to 3.33 eV. This change in the nature of material from semimetal to a wide band gap semiconductor, combining with its higher optical transmission (92 %) in visible light region, higher carrier concentration (1.25 × 1021 cm−3) and lower electrical resistivity (2.8 × 10−4 cm−3), makes the nano-grained CdO film very useful in optoelectronic applications.