Sequential Physical Vapor Deposition Research Articles

Sequentially PVD‐Grown Indium and Gallium Selenides Under Compositional and Layer Thickness Variation: Preparation, Structural and Optical Characterization

AbstractGroup IIIA metal chalcogenides are an auspicious material system due to their variability of properties and hence the multitude of application options, for example, in the fields of optoelectronic, thermoelectric, piezo‐, and ferroelectric devices. Indium and gallium selenide films are innovatively grown in a sequential PVD (physical vapor deposition) process starting from metal precursor layers of various thicknesses, which are then subject to chalcogenization in different selenium contents. The resulting thin films are investigated for structural and optical properties by Raman, XRD (X‐ray diffraction), and UV–Vis–NIR spectrometry, revealing that all the compounds In2Se3, InSe, In4Se3, Ga2Se3, and GaSe as well as different polytypes can be achieved depending on the metal/chalcogen ratio. Results from Raman and XRD spectroscopy are highly consistent, and also from the optical measurements changes in absorption characteristics can be correlated. The results indicate, that by fine‐tuning the selenium content, deliberately growing ultra‐thin layers of the different indium and gallium phases will be possible, thus opening up a promising route for 2D material fabrication. Given the scalability of the fabrication method, it is highly promising for large‐scale deployment of the materials.

Spectrum‐Selective High‐Temperature Tolerant Thermal Emitter by Dual‐Coherence Enhanced Absorption for Solar Thermophotovoltaics

AbstractSolar thermophotovoltaics (STPV) receive considerable attention for the record high energy transfer efficiency beyond conventional photovoltaics systems, which requires a well‐designed structured absorber/emitter and high operating temperature. However, the inherent tradeoff between increased operation temperature and material/structure stability (especially for the thermal emitters) severely hinders the development of STPV. In this work, a new design is proposed for a step‐function‐like thermal emitter based on a two‐path (quasi) coherent perfect absorption effect, which is experimentally enabled by a few‐layer Si/Mo/AlN lamellar film on a Mo substrate through a sequential physical vapor deposition process. The as‐prepared lamellar emitter exhibits maximal emissivity of ≈97% for 1.4 µm with excellent thermal suppression down to 10% across 3.4–10 µm, the performance of which is well maintained up to 973 K. When employed in the established STPV system, the dual coherent enhanced absorption (DCEA) system contributes to an increase of 20% in the total system efficiency compared to the unimodal coherent perfect absorption counterpart. The design proposed in this study provides a new methodology for improving the efficiency of the STPV system and may have significant applications in improving thermal energy regulation for compact systems.

Charge Transfer in Self-Assembled Fullerene-Tetraphenylporphyrin Non-Covalent Multilayer

Self-assembly of organic molecules is a promising method for generating multilayer systems for fabrication of functional devices. In particular, fullerene (C60) and porphyrin molecules offer a variety of binding modes, including π–π interactions, dipole electrostatic attraction, and hydrogen bonding, to tailor the charge separation and charge recombination limiting device performance. Here, we investigate multilayer systems obtained by the sequential physical vapor deposition of C60 and tetraphenylporphyrin (H2TPP) layers, focusing on the effect of the interfaces on the charge transfer processes. Absorbance spectra indicate noncovalent-like π-stacking, with the increment of fullerene interfaces shifting the porphyrin Soret band toward the blue. Similarly, surface photovoltage measurements in the multilayer systems show that as the number of interfaces increases, so does the photogeneration of charge. Charge separation follows carrier generation given that the recombination time, associated to trap states, decreases. This behavior indicates that the Donor-Acceptor nature of the fullerene-porphyrin bilayer system is conserved, and even enhanced in the multilayer film, and that the number of interfaces aids to the formation of selective paths for charge carrier collection, demonstrating its potential in optoelectronic devices.

Characterization of Thin MAPb(I1–xBrx)3 Alloy Halide Perovskite Films Prepared by Sequential Physical Vapor Deposition

Lead iodide (PbI2)-rich methylammonium lead bromide-iodide (MAPb(I1–xBrx)3) thin-films were prepared by sequential physical vapor deposition of methylammonium lead tri-bromide (MAPbBr3) on methylammonium lead tri-iodide (MAPbI3) bottom layer. The structural, optical, morphological, and electrical properties of the thin-films were studied as the thickness of methylammonium bromide (MABr) was increased from 300 to 500 nm. X-ray diffractograms confirmed transformation of tetragonal MAPbI3(x is 0.0) to the cubic-like structure of MAPbBr3 (x is 1.0) as MAPb(I1–xBrx)3 (x = 0.89–0.95) and PbI2 were formed. The bromine mole ratio x decreased as MABr thickness increased. UV-Vis absorption spectra showed that the bandgap of the thin alloy film decreased from 2.21 to 2.14 eV as x decreased. Scanning electron micrographs depicted densely packed grains that entirely covered the substrate and contained very few pinholes. The average grain size increased from 150 to 320 nm as x decreased. Electrical properties showed high charge carrier mobility that increased linearly with MABr thickness. FTO/MAPb(I1–xBrx)3/Au devices using fluorine-doped tin oxide (FTO) as substrate and gold (Au) as contacts were fabricated and current-voltage characteristics were determined. Space-charge-limited current theory was applied to charge carrier mobility and trap density of MAPb(I1–xBrx)3 thin-films. The charge carrier mobility increased as x decreased. The power conversion efficiency (PCE) of FTO/MAPbBr3/Au, FTO/MAPb(I0.11Br0.89)3/Au and FTO/MAPbI3/Au solar cells were 0.56, 0.62, and 1.15%. Devices including titanium dioxide compact layer (c-TiO2) and titanium dioxide mesoporous (m-TiO2) layer as electron transport layers were also fabricated for the application of Mott-Shottky (M-S) theory. Analyses of dark current-voltage and capacitance-voltage curves of FTO/c-TiO2/m-TiO2/MAPb(I0.11Br0.89)3 solar cells revealed a sizeable built-in voltage (Vbi) of 1.6 V and an accumulation of charge at interfaces for voltages greater than 0.2 V, respectively. Similar analyses for FTO/TiO2/MAPbI3/Au showed a small Vbi of 0.7 V and no charge carrier at interfaces. The work paves a way for reproducible growth of MAPb(I1–xBrx)3 for solar cells and sheds more light on the degree of ion migration in mixed halide and pure halide perovskites.

Stability and Exchange Processes in Ionic Liquid/Porphyrin Composite Films on Metal Surfaces.

In light of increasing interest in the development of organic–organic multicomponent heterostructures on metals, this molecular-scale study investigates prototypical composite systems of ultrathin porphyrin and ionic liquid (IL) films on metallic supports under well-defined ultrahigh vacuum conditions. By means of angle-resolved X-ray photoelectron spectroscopy, we investigated the adsorption, stability, and thermal exchange of the resulting films after sequential physical vapor deposition of the free-base porphyrin 5,10,15,20-tetraphenylporphyrin, 2H-TPP, and the IL 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], on Ag(111) and Au(111). 2H-TPP shows two-dimensional growth of up to two closed molecular layers on Ag(111) and Au(111) and three-dimensional island growth for thicker films. IL films on top of a monolayer of 2H-TPP exhibit Stranski–Krastanov-like growth and are stable up to 385 K. The 2H-TPP layer leads to destabilization of the IL films, compared to the IL in direct contact with the bare metals, by inhibiting the specific adsorption of the ions on the metal surfaces. When the porphyrin is deposited on top of [C8C1Im][PF6] at low temperature, the 2H-TPP molecules adsorb on top of the IL film at first but replace the IL at the IL/metal interfaces upon heating above 240 K. This exchange process is most likely driven by the higher adsorption energy of 2H-TPP on Ag(111) and Au(111) surfaces, as compared to the IL. The behavior observed on Ag(111) and Au(111) is identical. The results are highly relevant for the stability of porphyrin/IL-based thin film catalyst systems and molecular devices, and more generally, stacked organic multilayer architectures.

Synthesis and characterisation of methylammonium lead tri-bromide perovskites thin films by sequential physical vapor deposition

Methylammonium lead tri-bromide (MAPbBr3) thin films were grown by sequential physical vapor deposition of lead(II)bromide and methylammonium bromide (MABr). X-ray diffractograms confirmed the cubic MAPbBr3 structure with Pm3‾m space group. UV–Vis spectra revealed a red shift in absorption onset from 540 to 550 nm as the thickness of MABr increased with an optimum band gap of 2.28 eV, obtain from a Tauc Plot. Field emission scanning electron micrographs showed large pin-hole-free and densely packed grains with average grain size that increased from 217 to 302 nm with the thickness of MABr. Analysis of dark current-voltage characteristics of Au/MAPbBr3/FTO devices revealed ohmic behaviour at low voltages and trap-limited space charge limited current at high voltages. The carrier mobility increased from 1.89 × 10−2 to 1.08 × 10−1 cm2 V−1 s−1 while trap density decreased from 1.89 × 1016 to 1.40 × 1016 cm−3 as the thickness of MABr increased.

Ultra-Thin Rechargeable Lithium Ion Batteries on Flexible Polymer: Design, Low Temperature Fabrication and Characterization

In this work we developed flexible rechargeable thin film lithium ion batteries based on all-solid-state materials. The flexible batteries were fabricated using low temperature sequential physical vapor deposition techniques. A thin film battery (TFB) architecture consisting of Ti/V2O5/LiPON/Li/encapsulation multilayer was deposited on flexible polyimide substrate. To the best of our knowledge, the obtained TFBs are among the thinnest reported with an overall thickness of 50 μm including substrate, active layers and encapsulation stack. The fabricated TFBs delivered discharge capacity as high as 0.15 mAh (1.5 cm² of cathode area) upon galvanostatic cycling at 30 μA.cm−2 within 3.8–1.5 V voltage range. The capacity decreased during cycling with an average fading rate of 0.25% over 100 cycles. Electrochemical impedance spectroscopy (PEIS) measurements and modeling analysis revealed an increase of the internal resistance during cycling. Furthermore, it was shown that the internal resistance increase was induced by a progressive decrease of the electrolyte/electrodes contact area. TFBs having the same architecture and cell design were fabricated on two substrates: a flexible polyimide foil and a rigid silicon wafer. Electrochemical characterization showed a clear influence of substrate type on TFBs discharge capacity and internal resistance variations during cycling, the results are discussed within this paper

Pd Nanoparticle Formation in Ionic Liquid Thin Films Monitored by in situ Vibrational Spectroscopy.

Ionic liquids (ILs) are flexible reaction media and solvents for the synthesis of metal nanoparticles (NPs). Here, we describe a new preparation method for metallic NPs in nanometer thick films of ultraclean ILs in an ultrahigh vacuum (UHV) environment. CO-covered Pd NPs are formed by simultaneous and by sequential physical vapor deposition (PVD) of the IL and the metal in the presence of low partial pressures of CO. The film thickness and the particle size can be controlled by the deposition parameters. We followed the formation of the NPs and their thermal behavior by time-resolved IR reflection absorption spectroscopy (TP-IRAS) and by temperature-programmed IRAS (TR-IRAS). Codeposition of Pd and [C1C2Im][OTf] in CO at 100 K leads to the growth of homogeneous multilayer films of CO-covered Pd aggregates in an IL matrix. The size of these NPs can be controlled by the metal fraction in the co-deposit. With increasing metal fraction, the size of the Pd NPs also increases. At very low metal content, small Pd carbonyl-like species are formed, which bind CO in on-top geometry only. Upon annealing, the [OTf](-) anion coadsorbs at the NP surface and partially displaces CO. Co-adsorption of CO and IL is indicated by a strong red-shift of the CO stretching bands. While the weakly bound on-top CO is mainly replaced below the melting transition of the IL, coadsorbate shells with bridge-bonded CO and IL are stable well above the melting point. Larger three-dimensional Pd NPs can be prepared by PVD of Pd onto a solid [C1C2Im][OTf] film at 100 K. Upon annealing, on-top CO desorbs from these NPs below 200 K. Upon melting of the IL film, the CO-covered Pd NPs immerse into the IL and again form a stable coadsorbate shell that consists of bridge-bonded CO and the IL.

Thin films of PtAl2 and AuAl2 by solid-state reactive synthesis

The intermetallic compounds AuAl2 and PtAl2 are colored purple and yellow respectively. In the past they have been prepared by bulk melting techniques or by co-deposition in a magnetron sputterer. Here, however, we investigate films of AuAl2, PtAl2 and (Au,Pt)Al2 prepared by sequential physical vapor deposition of the elements, followed by in situ solid-state reaction. The microstructure, dielectric functions, optical properties and thermal stability of the resulting films are characterized and compared to those prepared by bulk melting or co-deposition. The (Au,Pt)Al2 films show a color gamut that stretches from purple to brassy yellow depending on composition and microstructure. High temperature synchrotron X-ray diffraction experiments show that the (Au,Pt)Al2 phase is metastable, decomposing when heated above 420°C. In contrast, the pure AuAl2 or PtAl2 phases are stable to about 580°C before they oxidize or decompose. The alternative possibility of producing the purple-to-yellow color gamut by depositing optical stacks of very thin films of AuAl2 and PtAl2 is also assessed. Either scheme will provide a range of colors lying between those of the binary compound endpoints. Calculations predict that deposition of AuAl2 onto PtAl2 will produce more intense colors than vice versa, an unexpected finding that is worth further investigation.

Design, Fabrication and Characterization of Flexible Thin Film Rechargeable Lithium Ion Batteries 

The advent of fully flexible electronics is considered as a major technological leap forward and has attracted much attention during the last decade. Compared with conventional electronics, flexible electronics are lightweight, bendable and may be wearable or implantable. Hence the rapid and continuous growth in this field has stimulated the research and development of corresponding flexible energy storage systems and more particularly rechargeable lithium ion batteries [1-3]. In this work we developed flexible thin film lithium ion batteries based on all-solid-state materials. The flexible batteries were fabricated using sequential physical vapor deposition techniques. A thin film battery (TFB) architecture consisting of Ti/V2O5/LiPON/Li/encapsulation multilayer was deposited on flexible polyimide substrate. The fabricated flexible TFBs presented an overall thickness of 70µm (figure 1). The flexible TFBs delivered discharge capacity as high as 0.15mAh (1.5 cm² of cathode area) upon galvanostatic cyling at C/5 rate within 3.8-1.5V voltage range. The capacity decreased during cycling with an average fading rate of 0.25% over 100 cycles (figure 2). Electrochemical impedance spectroscopy (EIS) measurements and modeling analysis revealed a significant and continuous increase of the internal resistance during cycling. Furthermore, it was shown that the internal resistance increase was induced by a progressive decrease of the electrolyte/electrodes contact area. A strong correlation was identified between the variation of electrolyte/electrode contact area and variation of flexible TFB discharge capacity. TFBs having the same architecture and the same design were fabricated on two substrates: a flexible polyimide foil and a rigid silicon wafer. Electrochemical characterization showed a clear influence of substrate type on TFBs discharge capacity and internal resistance variations during cycling (figure 3). The electrochemical results variation with substrate type is discussed in terms of differences of TFB multilayer mechanical behaviour during cycling in both configurations.    References 1-      Y. Hu and X. Sun, J. Mater. Chem. A, 2, 10712 (2014). 2-      L. Li, Z. Wu, S. Yuan, and X. Zhang, Energy Environ. Sci., 7, 2101 (2014) 3-      M. Osiak, H. Geaney, E. Armstrong and C. O'Dwyer, J. Mater. Chem. A, 2, 9433 (2014). Figure 1

Self-localization of mixed organophosphonic acid and organothiol monolayers on patterned Al–Cu substrates

The aim of this work is to investigate the selective adsorption of ultra-thin organic films on model heterogeneous aluminum–copper surfaces via self-assembly from mixed monomer solutions. Patterned aluminum/copper substrates were prepared via microsphere lithography by means of sequential physical vapour deposition of the respective metal films on silicon wafers. The formation of monolayers with a methylene end group on the bi-metallic surfaces was performed from a dilute cocktail solution of two organic molecules with thiol and phosphonic acid head groups. The ordering of the monolayers was analyzed by means of Polarization Modulated-Infrared Reflection Absorption Spectroscopy (PM-IRRAS). Imaging Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) experiments were performed to analyze the lateral distribution of the two mono-functional molecules. The preferred adsorption sites of the functionalities in the cocktail solution reproduced the patterned image of the model substrate with chemical contrast. The copper regions were covered with monolayers of thiol head group and organophosophonic acid monolayer was detected on aluminum regions. This self-localization of the organofunctional molecules converted the heterogeneous surface of the patterned substrate to a surface uniformly covered with methyl terminated self-assembled monolayers (SAMs).

Doubly Localized Surface Plasmon Resonance in Bimodally Distributed Silver Nanoparticles

Growth of bimodally distributed silver nanoparticles using sequential physical vapour deposition (PVD) is reported. Growth conditions of nanoparticles are defined in the following three steps: In the first step, nanoparticles are grown at a heated substrate and then exposed to atmosphere, in the second step, nanoparticles are vacuum annealed and finally re-deposition of silver is performed in the third step. This special way of deposition leads to the formation of bimodally distributed nanoparticles. It has been investigated that by changing the deposition time, different sets of bimodally distributed nanoparticles can be grown. Localized surface plasmon resonance (LSPR) of such bimodally distributed nanoparticles generates double plasmon resonance peaks with overlapped absorption spectra. Double plasmon resonance peaks provide a quick indication of the existence of two sets of nanoparticles. LSPR spectra of such bimodally distributed nanoparticles could be modeled with double Lorentz oscillator model. Inclusion of double Lorentz oscillator model indicates that there exist two sets of non-interacting nanoparticles resonating at different plasma frequencies. It is also reported that silver nanoparticles grown at a heated substrate, again attain the new shape while being exposed to atmosphere, followed by vacuum annealing at the same temperature. This is because of physisorption of oxygen at the silver surface and change in surface free energy. The re-shaping due to the adsorbed oxygen on the surface is responsible for bimodal size distribution of nanoparticles.

MOS structures containing silicon nanoparticles for memory device applications

Metal-oxide-silicon structures containing layers with amorphous or crystalline silicon nanoparticles in a silicon oxide matrix are fabricated by sequential physical vapour deposition of SiOx (x = 1.15) and RF sputtering of SiO2 on n-type crystalline silicon, followed by high temperature annealing in an inert gas ambient. Depending on the annealing temperature, 700°C or 1000°C, amorphous or crystalline silicon nanoparticles are formed in the silicon oxide matrix. The annealing process is used not only for growing nanoparticles but also to form a dielectric layer with tunnelling thickness at the silicon/insulator interface. High frequency C-V measurements demonstrate that both types of structures can be charged negatively or positively by applying a positive or negative voltage on the gate. The structures with amorphous silicon nanoparticles show several important advantages compared to the nanocrystal ones, such as lower defect density at the interface between the crystalline silicon wafer and the tunnel silicon oxide, better retention characteristics and better reliability.

Hydroxyapatite Growth on Glass/CdSe/SiO<sub>x</sub> Nanostructures

The aim of this study was to find if nanocrystal layers obtained by well-established nanotechnology are able to induce deposition of hydroxyapatite [Ca10(PO4)6(OH)2]. It is known that nanosized objects and porous structures influence biological events and they may be used to create biologically integrated multifunctional devices including biomaterials and sensors. In this work, sequential physical vapour deposition of CdSe and SiO, or SiOx film was used to modify glass substrates. To study the ability of the nanostructured surfaces to induce hydroxyapatite deposition, samples were immersed in a simulated body fluid and simultaneously irradiated with a scanning laser beam for a few minutes. This resulted in a porous sponge-like non homogeneous hydroxyapatite layer, consisting of networks of aggregates of nano dimensions on the modified surfaces. Analysis showed higher Ca and P contents in the stripes of the laser-substrate interaction, which indicated the influence of the laser energy. The method of laser-liquid-solid interaction used has led to a synergistic effect due to the simultaneous use of the nanostructured substrate, aqueous solution and laser energy.

Nanoparticle layers of CdSe buried in oxide and chalcogenide thin film matrices

Nanoparticles of CdSe embedded in amorphous SiO x , GeS 2 and polycrystalline ZnSe thin films have been produced by sequential physical vapour deposition of CdSe and one of the matrix materials. High-resolution electron microscopy has been used to prove the formation of CdSe nanoparticles. Particles with nearly spherical shape were observed whose spatial distribution follows the surface morphology of the ‘matrix’ films. The mechanism of nanoparticle formation on a rough surface has been discussed. It has been found that annealing of SiO x –CdSe films at 973 K leads to the formation of isolated CdSe nanoparticles homogeneously distributed in the matrix. Quantum-size increase in the optical band gap of CdSe nanoparticles has been observed in all matrices used.

Formation of CdSe nanoclusters in SiO x thin films

CdSe nanoclusters embedded in silicon oxide layers are produced by sequential physical vapor deposition of SiO x ( x≈1.5) and CdSe on crystalline silicon substrates at room temperature. High-resolution electron microscopy is used to prove the formation of CdSe nanoclusters as well as to study their shape, size and structure. Cross-sectional electron micrographs of the as-deposited samples reveal clusters with nearly spherical shapes, which are not arranged in a plane. The spatial distribution of the CdSe clusters follows the surface morphology of the SiO x films. The average size of the nanoclusters is about two times greater than the nominal thickness of the CdSe layers deposited. Upon annealing the samples at 670 K for 80 min, a slight size increase is observed accompanied by some improvement in crystallinity of the CdSe nanoclusters. The σ/ a ratio ( a: average size of nanocrystals, σ: half-width at half-maximum of size distribution) found for 1-nm CdSe deposited on 20-nm SiO x is 0.13–0.14, while for 2-nm CdSe deposited on 40-nm SiO x it is 0.19.

Apparatus for codeposition and layered deposition of reactive metals and volatile organic compounds with control of stoichiometry and film thickness

An effective method is reported for the simultaneous or sequential physical vapor deposition of reactive alkali metals and volatile macrocyclic complexants which react to form thin stoichiometric films of alkalides and electrides. These thermally unstable and highly reactive films, typically between 300 and 1500 Å thick, are deposited under high vacuum onto a quartz substrate which can be cooled to as low as −130 °C. Quantitative optical absorption measurements are made on these films in situ. Acquisition of the optical data and control of sample deposition rates, oven temperatures, and film thicknesses are accomplished with a computer.