Transformation Of Thin Films Research Articles

Nanopattern transformation of ABC triblock copolymer thin films induced by strong solvent selectivity and annealing

Nanopattern transformation behaviors of polyisoprene-block-polystyrene-block-poly(2-vinylpyridine) (PI-b-PS-b-P2VP) asymmetric ABC triblock copolymer were investigated systematically with various control parameters, including different solvents for polymer solution and annealing conditions in this paper. Ordered nanopattern of PI-b-PS-b-P2VP with hexagonal cylinders could be obtained when PI-b-PS-b-P2VP toluene solution was spin-coated on silicon substrate followed by toluene vapor annealing process. When the film with hexagonal and cylindrical nanopattern was exposed to saturated toluene vapor, the order-order transition of cylindrical nanopattern to parallel nanopattern was observed due to the strong selectivity of toluene to PS and PI blocks. Furthermore, fingerprint nanopattern could also be obtained by solvent annealing in tetrahydrofuran vapor. The nanopattern transformation was due to different selectivity of solvents and incompatibilities of the three blocks of PI-b-PS-b-P2VP under various solvent annealing conditions.

On the structural, morphological and electrical properties of tantalum oxy nitride thin films by varying oxygen percentage in reactive gases plasma

Tantalum oxy nitride (Ta-ON) thin films possess unique properties due to combined beneficial aspects of tantalum nitride and tantalum oxide. In the present work, Ta-ON thin films are synthesized using energetic ions and electrons beams emanated from the hot and decaying plasma in dense plasma focus (DPF) device. Oxygen percentage in the reactive gas admixture [O/(O+N)]% is varied from 10 to 60 for thin film synthesis. Influence of oxygen percentage on structural, morphological, compositional and electrical properties of thin films is also analyzed. Increase of oxygen percentage in the gas admixture (upto 40%) resulted in transformation of thin film from tantalum nitride to tantalum oxy nitride; however; higher oxygen percentages (≥ 50%) caused amorphization in synthesized thin film. Morphological analysis revealed that pure nitrogen environment yields the granular structure of film in nano-meter range whereas escalation of grains is observed with the increase in oxygen percentage. Cross sectional SEM showed better film growth rate at lower oxygen percentages. Compositional profiles exhibited improvement of oxygen content in thin film by increasing oxygen percentage in the admixture. The electrical resistivity of films shifted from conducting (for 0% oxygen) to semiconducting-insulating (for 60% oxygen) material range for maximum oxygen percentage.

Tunable Shape Memory Polymers from α-Amino Acid-Based Poly(ester urea)s

The thermal shape memory behavior of poly(ester urea)s (PEUs) composed of varying α-amino acids and linear diols has been explored. The thermal, mechanical, and shape memory properties of PEUs are shown to be controlled by changing the amino acid and diol components of the polymer, without negatively affecting the shape memory performance of the polymer in most cases. These materials display triple-shape memory behavior and temperature memory properties due to a broad glass transition temperature interval. The versatility of the shape memory behavior of PEUs was explored by demonstrating shape transformations of thin films, salt leached scaffolds, and larger constructs. Overall good shape memory behavior in combination with the tunable properties of PEU materials makes them prime candidates for use as shape memory materials in biomedical applications.

Synthesis, characterization and thermally induced structural transformation of Au-C70 nanocomposite thin films

Au-C70 nanocomposite thin films are deposited using thermal co-evaporation technique. Low atomic concentration of Au (∼3%) are used to avoid any deterioration of the properties of fullerene C70 when used with Au. These thin films are annealed in inert atmosphere at different temperatures for 30 min in order to promote the formation of nanocomposite material. Both the Au nanoparticles and fullerene C70 matrix are evolved by thermal treatment. Spherical shaped Au nanoparticles with a diameter of 3.8 ± 0.1 nm are clearly observed in the TEM images at 350 °C. The characteristic D and G bands of amorphous carbon are evidenced by Raman scattering spectroscopy in the film annealed at 350 °C. The surface plasmon resonance (SPR) absorption band of Au nanoparticles in carbon is obtained at 565 nm after treatment at 300 °C and it is blue shifted to 549 nm for the sample annealed at 350 °C.

Transformation of amorphous alloy surface and thin film under impact of slow heavy ions

In this work thin foils of amorphous alloys VP800 (Fe73Si16B7Cu1Nb3) and VV8025X (Fe4Co66B14Cu1Nb2Mo1) maintained at the onset point temperature for primary crystallisation were irradiated with slow heavy ions (100–300keV Ar and Xe) at the fluence changing from 1010 to 1013ions/cm2. The preferential surface modification during ion implantation-sputtering was analysed with SRIM and PIXE. With the use of CEMS Fe and Fe(Si) clusters accompanied by Fe3Si and even Fe23B6 nanocrystals were found in the films irradiated at a lower fluence, whereas rather amorphous structure was found in surfaces more heavily implanted.

Low-temperature phase transformation of CZTS thin films**Project supported by the Natural Science Foundation for Youth Fund of Hebei Province, China (Grant No. A2016201087), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20131301120003), and the National Natural Science Foundation of China (Grant Nos. 11504078 and 61504054).

The low temperature phase transformation in the Cu2ZnSnS4 (CZTS) films was investigated by laser annealing and low temperature thermal annealing. The Raman measurements show that a-high-power laser annealing could cause a red shift of the Raman scattering peaks of the kesterite (KS) structure and promotes the formation of the partially disordered kesterite (PD-KS) structure in the CZTS films, and the low-temperature thermal annealing only shifts the Raman scattering peak of KS phase by several wavenumber to low frequency and the broads Raman peaks in the low frequency region. Moreover, the above two processes were reversible. The Raman analyses of the CZTS samples prepared under different process show that the PD–KS structure tends to be found at low temperatures and low sulfur vapor pressures. Our results reveal that the control of the phase structure in CZTS films is feasible by adjusting the preparation process of the films.

Effect of Al2O3 Buffer Layers on the Properties of Sputtered VO2 Thin Films

VO2 thin films were grown on silicon substrates using Al2O3 thin films as the buffer layers. Compared with direct deposition on silicon, VO2 thin films deposited on Al2O3 buffer layers experience a significant improvement in their microstructures and physical properties. By optimizing the growth conditions, the resistance of VO2 thin films can change by four orders of magnitude with a reduced thermal hysteresis of 4 °C at the phase transition temperature. The electrically driven phase transformation was measured in Pt/Si/Al2O3/VO2/Au heterostructures. The introduction of a buffer layer reduces the leakage current and Joule heating during electrically driven phase transitions. The C–V measurement result indicates that the phase transformation of VO2 thin films can be induced by an electrical field.

Nb-H Thin Films: On Phase Transformation Kinetics

It was recently shown that phases forming in thin films undergo a coherency state change depending on the film thickness. For Nb-H thin films, the coherency state was reported to change at about 38 nm. In this study the impact of the coherency state on the phase transformation kinetics is investigated for Nb films of two different film thicknesses (25 nm and 80 nm), below and above the state change thickness. The phase transformation in thin metal-hydrogen films can be studied by surface topography analyses via scanning tunneling microscopy (STM) because of the strong local lattice expansion of the hydride precipitates. STM on Nb-H reveals fast phase transformation kinetics for the 25 nm Nb-film, and much slower kinetics for the 80 nm film. This is suggested to be related to the change in the coherency between the Nb-matrix and the hydride precipitates.

Single-pulse transformation of Ag thin film into nanoparticles via laser-induced dewetting

In this study, we show that Ag thin films deposited on glass can be transformed into nanoparticles by laser-induced dewetting using a nanosecond-pulsed Nd:YAG laser. The film could be completely dewetted by a single pulse and the pulse energy density required for a 10 nm-thick Ag film was 86mJ/cm2 at λ=1064nm. This made it possible to dewet a film area of ∼10cm2 by a single pulse with energy of 850mJ. The produced particles exhibited a monomodal size distribution and the mean particle size increased as the initial film thickness increased. Repeated exposure to pulses induced no noticeable change in the particle size distribution. The initial film thickness was the only factor that determined the mean particle size. The absorption spectra of dewetted films were well consistent with the surface plasma resonance behaviors of metal nanoparticles. This process provides a facile and scalable method of forming metal nanoparticle arrays for plasmonic and other applications.

Martensite transformations in Mn2NiGa thin films grown on GaAs substrates

The purpose of this work is to investigate the correlation between magnetism and crystallographic structures of Mn2NiGa thin films grown by molecular beam epitaxy on GaAs(1 1 1) and GaAs(0 0 1) surfaces. The films present themselves with thermoelastic martensitic transformations upon cooling, and heating with high-temperature leads to austenite structures exhibiting a preferable (1 1 0) texture. X-ray diffraction measurements performed as a function of temperature reveal three different types of domain variants in the films within a large interval of temperatures. The austenite structures with lattice parameters ranging from 0.574 nm to 0.601 nm undergo volume conserving structural transitions to martensite with a c/a ratio of 1.2. The coexistence of variants with different domain configurations is induced on each GaAs substrate. Although the Curie temperatures (~360 K) are similar for films grown on GaAs(1 1 1) and GaAs (0 0 1) substrates, their saturation magnetizations are respectively 18 kA m−1 and 8 kA m−1 at room temperature and exhibit quite different magnetic irreversibility behaviors. Our results indicate that a multiplicity of possible equivalent variant domains on the GaAs surfaces makes it difficult to stabilize epitaxial films on these substrates.

Thin-Film Transformation of NH4 PbI3 to CH3 NH3 PbI3 Perovskite: A Methylamine-Induced Conversion-Healing Process.

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

Structural origin of surface transformations in arsenic sulfide thin films upon UV-irradiation

Photostructural transformations within AsxS100-x (x=30, 33, 35, 40) thin films upon exposure to LED light of different wavelengths, in both air and argon environments have been studied by high resolution XPS, Raman spectroscopy and LEIS methods. These complementary results show that light of energies close to the band gap does not modify chemical composition of the surface, but induces simple photopolymerization reactions. Superbandgap UV light, however, significantly increases S/As ratio on the surface due to formation of S-rich layer under both environmental conditions. It is proposed that photovaporization of both oxide and non-oxide cage-like molecules is responsible for the observed effect.

Phase-dependent ultrafast third-order optical nonlinearities in metallophthalocyanine thin films

We present a comprehensive study on the impact of phase transformations of metallophthalocyanine thin films on their third-order nonlinear optical (NLO) properties. The metallophthalocyanine thin films are prepared by thermally evaporating the commercially available Copper(II)2,9,16,23-Tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) and Zinc(II) 2,9,16,23-Tetra-tert-butyl-29H,31H-phthalocyanine (ZnPc) powder on glass substrate. Thermal annealing causes a phase transformation which has a distinct signature in powder X-ray diffraction and UV-Vis-NIR spectroscopy. The NLO characteristics which include nonlinear refractive index n2, as well as nonlinear absorption coefficient (βeff), were measured by using a single beam Z-scan technique. An ultrashort pulsed fiber laser emitting femtosecond pulses (Δτ ≈ 250 fs) at 1064 nm central wavelength is used as a source for the Z-scan experiment. The βeff values in as prepared thin films were ascertained to be smaller as compared to the annealed one due to the smaller value of saturation intensity (Is) which, in turn, is a consequence of ground-state bleaching in the thermally unstable amorphous state of the molecule. Interestingly, the nonlinear refractive indices bear opposite sign for CuPc and ZnPc. The variations in the third-order nonlinearity in CuPc and ZnPc are discussed in terms of molecular packing and geometries of metallophthalocyanine molecules.

Ferroelectric photovoltaic response to structural transformations in doped BiFeO3 derivative thin films

We report an investigation on crystallographic texturing, optical bandgap modulation and ferroelectric photovoltaic (FE-PV) response of semitransparent Au/BiFeO3 derivatives/Sn:In2O3 (ITO) thin film capacitors. Optimized A/B-site doping of respective Ce3+ and Mn2+ drives large-amplitude structural changes in BiFeO3 (BFO) films with partial tetragonal transformation and upsurge in extent of texture of (110) preferred orientation. Spin coated BFO, doped BiFe0.9Mn0.1O3, Bi0.88Ce0.12FeO3 and codoped Bi0.88Ce0.12Fe0.9Mn0.1O3 films of 350nm thickness on ITO coated corning glass exhibited optical transmittance of 65–80% in visible light range and revealed blue shift in optical band gap from 2.53 to 2.81eV due to splitting of t2g and eg orbital. Influence of single and co-doping on FE-PV response demonstrated crystallographic orientation and depolarizing field dependent behavior of short circuit current (JSC) and open circuit voltage (VOC). Current-voltage (J-V) characteristics of semitransparent Au/Bi0.88Ce0.12Fe0.9Mn0.1O3/ITO capacitors possessed large JON/JOFF ratio of approx. 104 at zero bias. Temporal dependence of photocurrent showed highly repeatable and stable response for consecutive ON/OFF cycles of illumination under solid state violet laser (405nm) of 160mW/cm2 intensity, a forward step towards non-destructive memory combining electronic and optical functionalities.

Influence of annealing treatment on phase transformation of Ga15Se77Tl8 thin films

The present work describes the first order phase transformation in chalcogenide Ga15Se77Tl8 thin films. Simple melt quenching method was applied to prepare the bulk Ga15Se77Tl8 samples. Ga15Se77Tl8 thin films were prepared by the thermal evaporation on glass and silicon wafer substrates under the vacuum condition of 10−5 Torr. The amorphous and glassy nature of synthesized sample was verified by differential scanning calorimeter. The phase transformation was studied at different temperatures of 343, 358 and 373 K for 2 h. The crystalline, morphological and optical properties were analysed to explain the phase transformation in as grown and thermally annealed films at different temperatures. It was found that as grown film revealed the amorphous nature, while the annealed films possessed the polycrystalline nature. From optical properties, the values of extinction (k) and absorption (α) coefficient for Ga15Se77Tl8 thin films enhanced with the increase of the incident photon energy as well as annealed temperature. The decrease in the optical band gap with respect to annealed temperatures was described on the basis of phase transformation from amorphous to polycrystalline state.

Ion irradiation induced phase transition of Co in Co/Au multilayers

We have studied the structural phase transformation in thin Co films using moderate energy Au ion irradiation. We found that local energy transfer along the track of the ion beam is an effective method for transforming fcc phase of Co into technologically appealing hcp phase. As a function of the irradiation fluence, a gradual crossover to the hcp phase was observed in X-ray diffraction data. Such phase transition was explained on the basis of evolution of grains under increasing ion fluence. Bloch-Grüneisen simulations to the four contact resistivity data showed a decrease in Debye temperature with evolution of hcp-phase Cobalt, consistent with the reduced modal freedom for phonons in the hcp structure compared to the fcc structure.

Shock wave induced martensitic transformations and morphology changes in Fe-Pd ferromagnetic shape memory alloy thin films

Combining experimental methods and classical molecular dynamics (MD) computer simulations, we explore the martensitic transformation in Fe70Pd30 ferromagnetic shape memory alloy thin films induced by laser shock peening. X-ray diffraction and scanning electron microscope measurements at shock wave pressures of up to 2.5 GPa reveal formation of martensitic variants with preferred orientation of the shorter c-axis of the tetragonal unit cell perpendicular to the surface plane. Moreover, consequential merging of growth islands on the film surface is observed. MD simulations unveil the underlying physics that are characterized by an austenite-martensite transformation with a preferential alignment of the c-axis along the propagation direction of the shock wave, resulting in flattening and in-plane expansion of surface features.

Investigation of radiation-induced transformations in thin NbN films by analytical electron microscopy

This work demonstrates implementation of low energy electron energy loss technique (EELS) in scanning transmission electron microscopy (STEM) to investigate the changes of free electron density at room temperature in ultra-thin NbN films under composite ion beam irradiation up to the deses of ∼3 d.p.a. for nitrogen atoms. It was found the constant value of the free electron density ∼1.6 ·1029 m-3 in this dose range while the irradiated material was characterized by metal type of electrical conductivity.

Influence of the microstructure on the resulting 18R martensitic transformation of polycrystalline Cu[sbnd]Al[sbnd]Zn thin films obtained by sputtering and reactive annealing

We report the influence of the microstructure on the martensitic transformation in polycrystalline CuZnAl thin films with 18R structure. The films are grown in two steps. First, CuAl thin films are obtained by DC sputtering. Second, the Zn is introduced in the CuAl thin films by the annealing them together with a bulk CuZnAl reference. The crystalline structure of the films was analyzed by X-ray diffraction and transmission electron microscopy. The martensitic transformation temperature was measured by electrical transport using conventional four probe geometry. It was observed that temperatures above 973K are necessary for zincification of the samples to occur. The resulting martensitic transformation and its hysteresis (barrier for the transformation) depend on the grain size, topology and films thickness.

Phase Transformations in Au-Fe Particles and Thin Films: Size Effects at the Micro- and Nano-scales

Thin Au-Fe bilayers (3–30 nm in total thickness) were deposited on sapphire substrates. Annealing in a temperature range of 600–1100°C resulted in solid-state dewetting and the subsequent formation of micro- and nano-particles. Electron microscopy, atomic force microscopy and in situ x-ray diffraction were employed to systematically study two phase transformations in the Au-Fe system: (1) precipitation of α-Fe from supersaturated Au-Fe solid-solution particles; and (2) α↔γ transformation in Fe and Au-Fe thin films and particles. In both cases, the transformations proceeded differently than in the bulk already for sub-micron (100 nm to 1 μm) particles. These results were explained by the low defect concentration in the particles, nucleation difficulties, slow diffusivity on facets, and Au segregation. A “reverse size effect” was observed in thin Fe films, and discussed in terms of nucleation model taking into account the small size of the parent phase. The main conclusion is that phase transformations in the particles and in the bulk proceed differently, not only for nano-sized particles as was customarily believed but also for particles of sub-micrometer size. We suggest that this size effect is governed by two different length scales: the inter-defect spacing (upper limit) and the bulk critical nucleus size (lower limit).