Thin Film Counterparts Research Articles

CuPc nanowires transistors fabricated by vacuum thermal deposition

ABSTRACTOrganic semiconductor nanowires have received much attention because of their superior performance compared with their thin film counterparts. In this paper, we fabricated large amount of oriented CuPc nanowires by weak epitaxy growth of CuPc on p-6P bilayer. These nanowires possess diameter of 0.4∼1.2 µm and length of several micrometers. With such method, CuPc nanowire transistors can be fabricated, and the high mobility up to 0.37 cm2/Vs has been realized.

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films.

The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI3 ) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 × 10-13 cm-2 ) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures.

Film thickness dependent microstructural changes of thick copper metallizations upon thermal fatigue

Abstract

Self-Assembled Molecular Nanowires for High-Performance Organic Transistors.

While organic semiconductors provide tantalizing possibilities for low-cost, light-weight, flexible electronic devices, their current use in transistors-the fundamental building block-is rather limited as their speed and reliability are not competitive with those of their inorganic counterparts and are simply too poor for many practical applications. Through self-assembly, highly ordered nanostructures can be prepared that have more competitive transport characteristics; however, no simple, scalable method has been discovered that can produce devices on the basis of such nanostructures. Here, we show how transistors of self-assembled molecular nanowires can be fabricated using a scalable, gradient sublimation technique, which have dramatically improved characteristics compared to those of their thin-film counterparts, both in terms of performance and stability. Nanowire devices based on copper phthalocyanine have been fabricated with threshold voltages as low as -2.1 V, high on/off ratios of 105, small subthreshold swings of 0.9 V/decade, and mobilities of 0.6 cm2/V s, and lower trap energies as deduced from temperature-dependent properties, in line with leading organic semiconductors involving more complex fabrication. High-performance transistors manufactured using our scalable deposition technique, compatible with flexible substrates, could enable integrated all-organic chips implementing conventional as well as neuromorphic computation and combining sensors, logic, data storage, drivers, and displays.

An Investigation into Capacitive Performances and Impedance Contributions Due to Structural Differences of Biochar Thin Film and Monolith Supercapacitor Electrodes

Supercapacitors are an upcoming, high power density energy storage technology. Unlike in batteries, energy is stored physically through the adsorption of oppositely charged ions to a surface. Electrode materials that facilitate this process have high conductivity, extensive porosity, and high specific surface area. Porous carbon powders are used commercially, being pressed into thin films and held together with a binder material [1]. While carbon nanotubes and graphene are the focus of many researchers in this field, our group is investigating a unique alternative: biochar. Biochar is pyrolyzed biomass, and our group uses different types of wood as precursor materials. Through a controlled pyrolysis process, it is possible to preserve the internal macrostructures of wood, creating pathways throughout the carbon structure that should facilitate ion transport. By creating large, continuous monolithic pieces of carbon, devices can be constructed differently from the status quo. Using monolithic slices as electrodes simplifies the manufacturing process, reduces the number of ancillary components required per device, eliminates the need for a non-conductive binder material, and enables the construction of larger electrodes. Currently there is a lack of understanding regarding the relationships between biochar macrostructures and capacitive performance. While monolithic biochar electrodes were found to show similar capacitive performance to their thin film counterparts in [2], the study did not include an evaluation from a structural point of view. Additionally, although it was found that increasing electrode thickness of thin films resulted in an increase in device resistance [3], this relationship has not been explored for monolithic electrodes nor at a relevant scale, as it becomes possible to make electrodes hundreds of times thicker than those currently used [1]. Competitive capacitive performance of powdered biochar thin film electrodes compared to thin film alternatives has been demonstrated in [4] and [5]. The overall goal of this project is to determine if monolithic biochar electrodes can compete with the capacitive performance of powdered biochar thin film electrodes. By constructing monolithic slices and powdered thin films from the same biochar, electrochemical influences of the macrostructure are investigated. Capacitive performance metrics such as charge/discharge rate capability and self-discharge rates are explored for the two electrode structures, as well as for different electrode thicknesses. Frequency-dependent resistances and their respective contributions to total device resistance are analyzed through Electrical Impedance Spectroscopy (EIS). Mass transfer and diffusional resistances, which are believed to be highly dependent on both electrode geometry and structure will be reported. Characterization of the electrode materials using N2 and CO2 physisorption; helium pycnometry; and SEM imaging coincides with the electrochemical testing methods to help resolve the causes of performance differences and evaluate their significance. Preliminary results show that monolithic biochar electrodes up to 1mm thick have comparable specific capacitance performance to the thin films at 100mA/g, which was the highest current density employed thus far. Upcoming work will apply larger current densities to determine if the channels of the macrostructures contribute to high power performance or reduce volumetric capacitance. Monolithic electrodes up to 5mm thick are able to achieve similar specific capacitances to the thin films at a low current density (5mA/g), but their performance degrades significantly with charge rate. The experiments mentioned in the preceding paragraphs were all conducted on biochar from sugar maple wood. Research direction for early 2017 will involve analysis on biochar from different types of precursor wood. Soft and hard woods, and the variety of species within these categories have vastly different types and sizes of internal structures (Figure) [6]. The effects of these macrostructures on capacitive performance and ion transport will be assessed, and these results will be available for presentation.

Compositional Optimization of Alloy FexNiy(OH)2 Nanoparticles for Alkaline Electrochemical Oxygen Evolution

It has recently been demonstrated by Bell and co-authors [1] that intentional iron doping into a nickel hydroxide film causes significant enhancement in electrochemical performance for the oxygen evolution half reaction (OER) of alkaline electrolysis. A subsequent study by Boettcher and co-authors [2] demonstrated that unintentional iron doping of a nickel hydroxide catalyst occurs from ppm- to ppb-level iron contaminants in alkaline electrolytes; the iron incorporates into the nickel hydroxide structure, and causes at least an order of magnitude increase in current density. Freibel et al. subsequently demonstrated that iron may in fact be the active site of OER, with nickel hydroxide acting as a host [3]. The majority of the work thus far investigating how iron content in an FexNiy(OH)2 catalyst affects OER has been performed on thin film catalysts; Burke et al. specifically describe the need for catalyst architectures that reduce mass transport limitations and allow improved access to more active sites per unit mass of catalyst [4]. As a result, there is an interest in developing nanoscale catalysts that are composed of iron and nickel and result in greatly improved performance metrics for OER over their thin film counterparts. Here, we will present results for the optimization of a FexNiy(OH)2 nanoparticle catalyst. In this work, the nanoparticles are synthesized with an alloy morphology and the atomic ratio of iron to nickel is varied from 1:5 to 5:1. Once synthesized, the nanoparticles are evaluated in 1 M KOH that has been purified to remove any iron impurities. Electrochemical performance of this series of nanoparticle catalysts will be discussed as well as our efforts to characterize the nanoparticles with electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction. [1] M.W. Louie, A.T. Bell, An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen, J. Am. Chem. Soc., 135 (2013) 12329-12337. [2] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation, J. Am. Chem. Soc., 136 (2014) 6744-6753. [3] D. Friebel, M.W. Louie, M. Bajdich, K.E. Sanwald, Y. Cai, A.M. Wise, M.-J. Cheng, D. Sokaras, T.-C. Weng, R. Alonso, R.C. Davis, J.R. Bargar, J.K. Norskov, A. Nilsson, A.T. Bell, Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting, J. Am. Chem. Soc., 137 (2015) 1305–1313. [4] M.S. Burke, L.J. Enman, A.S. Batchellor, S.H. Zou, S.W. Boettcher, Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy)hydroxides: Activity trends and design principles, Chemistry of Materials, 27 (2015) 7549-7558.

Bifacial multicrystalline silicon thin film solar cells

Based on well-defined lab conditions, we have developed experimental methods to characterize bifacial laser crystallized multicrystalline silicon (mc-Si) thin film solar cells fabricated on glass. Key parameters which determine the performance of the bifacial solar cells such as light intensity and incidence angle dependence on both sides have been characterized. From these parameters and the local irradiance data, the annual power output of the bifacial solar cells can be simulated easily. In addition, bifacial measurements under well-defined conditions using a single sun simulator have been performed by means of a mirror. The best bifacial mc-Si thin film solar cell shows a “bifacial efficiency” of 12.4% compared to state of the art monofacial mc-Si thin film counterpart of 12.1%. According to our simulation, even a “bifacial efficiency” above 14% is realistic if the solar cell is installed in front of a white diffuse scattering surface with a reflectance of ~90%. A threshold of 24% additional light contribution at which the bifacial mc-Si thin film solar cells outperform the monofacial ones have been determined by simulations. This shows that the advantages of the bifacial cells can already be observed at very low level (<25%) of additional reflected light contribution.

Genesis of flake-like morphology and dye-sensitized solar cell performance of Al-doped ZnO particles: a study

In dye-sensitized solar cell (DSSC) application, the particulate morphologies of photo-anode facilitate efficient dye loading and thus lead to better photo-conversion efficiency than their thin film counterpart. However, till date, the electronic and optical properties as well as the DSSC application of Al-doped ZnO (AZO) particles as photo-anode material is studied less than thin films. Herein, phase formation behavior, morphology evolution, optical properties, and dye-sensitized solar cell performance of wet chemically prepared ZnO and AZO (dopant level: 1–4 mol%) particles are studied. It is found that Al doping modulates significantly the ZnO morphology which in turn results the maximum dye adsorption as well as best photo-conversion efficiency at optimum dopant concentration. Specifically, the nanoparticle of ZnO turns predominantly to flake-like morphology with a higher surface area when 2 mol% Al is doped. Such morphology modulation is expected, since the crystallinity, lattice parameters, and lattice strain of ZnO changes appreciably with Al doping. The variations of optical properties (absorbance, diffused reflectance, and band gap) of AZO materials as compared to primitive ZnO are also identified through UV-vis studies. An attempt is made here to correlate the structural features with the photovoltaic performances of ZnO and AZO.

Enhanced magneto-optical effect in Y1.5Ce1.5Fe5O12 thin films deposited on silicon by pulsed laser deposition

Polycrystalline Ce:YIG thin films deposited on silicon are promising material candidates for integrated nonreciprocal photonic devices. So far, the reported Faraday rotation of polycrystalline Ce:YIG thin films on silicon is much lower than that of their single crystal or epitaxial thin film counterparts, limiting the magneto-optical figure of merit, device bandwidth and fabrication tolerance. In this paper, we report the growth of Ce:YIG thin films on silicon from targets with high Ce concentration up to nominally Y1.5Ce1.5Fe5O12 by pulsed laser deposition. The polycrystalline Y1.5Ce1.5Fe5O12 thin film showed pure garnet phase, smooth surface roughness of 0.7 nm, a dominant Ce3+ valence state and a bulk-like saturation magnetization of 125 emu/cm3 at room temperature. This material shows high Faraday rotation of −6410 deg/cm at 1550 nm wavelength, exceeding that of an Y2Ce1Fe5O12 epitaxial thin film on a GGG (100) substrate. However higher loss and lower figure of merit is also observed at 1550 nm wavelength compared to Y2Ce1Fe5O12 thin films, possibly due to the low oxygen partial pressure during fabrication. Low deposition oxygen partial pressure is essential to enhance the Ce solubility and Ce3+ concentration, which results in large Faraday rotation.

Novel Direct Nanopatterning Approach to Fabricate Periodically Nanostructured Perovskite for Optoelectronic Applications

While indirectly patterned organic–inorganic hybrid perovskite nanostructures have been extensively studied for use in perovskite optoelectronic devices, it is still challenging to directly pattern perovskite thin films because perovskite is very sensitive to polar solvents and high‐temperature environments. Here, a simple and low‐cost approach is proposed to directly pattern perovskite solid‐state films into periodic nanostructures. The approach is basically perovskite recrystallization through phase transformation with the presence of a periodic mold on an as‐prepared solid‐state perovskite film. Interestingly, this study simultaneously achieves not only periodically patterned perovskite nanostructures but also better crystallized perovskites and improved optical properties, as compared to its thin film counterpart. The improved optical properties can be attributed to the light extraction and increased spontaneous emission rate of perovskite gratings. By fabricating light‐emitting diodes using the periodic perovskite nanostructure as the emission layers, approximately twofold higher radiance and lower threshold than the reference planar devices are achieved. This work opens up a new and simple way to fabricate highly crystalline and large‐area perovskite periodic nanostructures for low‐cost production of high‐performance optoelectronic devices.

Accurate Measurement of Porous Low-k Thin-Films by Nanoindentation: Densification Scaling versus Substrate Effects

Historically, the integration of insulators with decreasing dielectric constants, k, has been critical in improving devices performance. The most efficient approach to decrease k is by introducing porosity. Since porosity directly impacts the Young's modulus, the accurate determination of the mechanical properties of porous dielectrics is essential in enabling their implementation into state-of-the-art devices. Currently, nanoindentation is the technique of choice, but as technology relevant thicknesses of dielectrics keep decreasing, substrate effects are unavoidably encountered. These issues have so far been circumvented by measuring bulk materials instead of their thin-film counterparts. This approach is, however, less than ideal as physical properties can be size-dependent. Moreover, pores densification also occurs, potentially leading to overestimated Young's modulus values. In this paper, we define a novel protocol based on ultra-low load, quasi-static nanoindentation to measure mechanical properties of porous thin-films as a function of porosity (0–60%) and thickness (150–700 nm). We could isolate materials densification from substrate effect, and we demonstrated that the former can be accounted for, while the latter does not happen until indentation depths are remarkably higher than the traditionally accepted indentation limit of 10% of the film thickness. Consequently, extremely accurate mechanical characterization of porous low-k thin-films is now possible.

Thin Films to Single Crystals: Organometal Halide Perovskite Materials for Advanced Optoelectronics

The recent emergence of perovskite materials has revolutionized the photovoltaic (PV) technology and offers solutions to contemporary energy and environmental issues. Moreover, the capabilities of single crystals are far superior to the thin film counterparts. This mini review outlines the growth parameters and crystal kinetics involved in the perovskite single crystal growth process for a superior wafer-style solar cell devices. Typically, perovskite solar cells with perovskite in the film form are attractive with their higher performance but, they degrade at faster rate, suffer immensely from a high density of traps and grain boundaries, which markedly limit the potential performance in devices. This review discusses a list of factors affecting it and provide future prospects of this thriving technology.

Electrosprayed Myocet-like Liposomes: An Alternative to Traditional Liposome Production.

Although doxorubicin (DXR) has been on the market for many years as an anti-cancer drug, a number of serious dose-limiting toxicities hinder its widespread use. To reduce the known toxicities of soluble DXR, various liposomes have been designed including Doxil, Caelyx, and Myocet. Myocet, a non-PEGylated liposomal formulation containing DXR, was found to reduce the toxicities associated with soluble DXR and has been used in Europe and Canada (but not the US) as a first line therapy. While regarded as successful, Myocet does have some formulation drawbacks including stability, drug release, and an arduous formulation and remote loading method for preparation. Our lab has developed a liposomal electrospray process in which formulation and remote loading occurs continuously in one step, cutting down on the total time of production and increasing the drug retention in the liposomes with respect to more conventional methods. Electrosprayed Myocet-like liposomes were then tested in vitro for release kinetics and cytotoxicity with respect to a more conventional formulation method. Myocet-like liposomes manufactured via electrospray had similar DXR loadings, hydrodynamic diameters, morphologies, and cytotoxic profiles as their thin-film hydration counterparts, but their release profiles were drastically prolonged. Our findings indicate that electrospray is a viable manufacturing procedure to scalably produce Myocet-like liposomes that appear to be more stable than those formulated through thin-film hydration.

Deposition of quaternary sputtered CIGS nanorods via glancing angle deposition

Nanostructured materials have become an attractive alternative to their thin film and bulk counterparts in photovoltaic and photoconductivity research. This is mainly attributed to their superior optical and electrical properties. Light trapping in vertically aligned nanostructures results in high optical absorption and provides enhanced carrier collection by utilizing a fully depleted p–n‐junction between the anode and cathode via an isolated ”capping” construction. The combination of these two features can potentially lead to the development of high efficiency nanostructured devices including solar cells, photodiodes, and photodetectors. Optical absorption proper ties of nanorod arrays of CuInx Ga1–xSe2 (CIGS), a p‐type semiconductor with a wide band gap ranging from 1.0 eV to 1.7 eV, are compared to their thin film counterpart. Utilizing an RF sputtering system, a quaternary target, and glancing angle deposition (GLAD) technique, vertical arrays of CIGS nanorods were fabricated while conventional films were fabricated by normal incidence deposition. Scanning electron microscopy (SEM) images indicated a successful growth of CIGS nanorods. Optical absorption was found to be strongly altered by the presence of the nanorod structures through spectroscopic reflectometry. (© 2016 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)

Divergence-tolerant resonant bandpass filters.

Bandpass filters based on subwavelength dielectric gratings are grounded in physical principles that are totally distinct from their thin-film counterparts. Ease in fabrication, design scalability, material sparsity, and on-chip integration compatibility makes them a promising alternative especially for long-wavelength applications. Here we demonstrate the interesting attribute of resonant bandpass filters of high angular stability for fully conical light incidence. Fashioning an experimental bandpass filter with a subwavelength silicon grating on a quartz substrate, we show that fully conical incidence provides an angular full width at half-maximum linewidth of ∼9.5° compared to a linewidth of ∼0.1° for classical incidence. Slow angular variation of the central wavelength with full conical incidence arises via a corresponding slow angular variation of the resonant second diffraction orders driving the pertinent leaky modes. Moreover, full conical incidence maintains a profile with a single passband as opposed to the formation of two passbands characteristic of resonant subwavelength gratings under classical incidence. Our experimental results demonstrate excellent stability in angle, spectral profile, linewidth, and efficiency.

FerrimagneticDyCo5Nanostructures for Bits in Heat-Assisted Magnetic Recording

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare earth and a transition metal, DyCo$_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in-plane upon heating above its spin-reorientation temperature (T$_R$) of ~350 K, just above room temperature. Upon cooling back to room temperature (below T$_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with ~45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin film counterpart. Our results pave the way for novel applications of ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices near room temperature.

Spiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport via mesoscale ordering.

We report the crystal structure and hole-transport mechanism in spiro-OMeTAD [2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene], the dominant hole-transporting material in perovskite and solid-state dye-sensitized solar cells. Despite spiro-OMeTAD's paramount role in such devices, its crystal structure was unknown because of highly disordered solution-processed films; the hole-transport pathways remained ill-defined and the charge carrier mobilities were low, posing a major bottleneck for advancing cell efficiencies. We devised an antisolvent crystallization strategy to grow single crystals of spiro-OMeTAD, which allowed us to experimentally elucidate its molecular packing and transport properties. Electronic structure calculations enabled us to map spiro-OMeTAD's intermolecular charge-hopping pathways. Promisingly, single-crystal mobilities were found to exceed their thin-film counterparts by three orders of magnitude. Our findings underscore mesoscale ordering as a key strategy to achieving breakthroughs in hole-transport material engineering of solar cells.

Direct Conversion of Perovskite Thin Films into Nanowires with Kinetic Control for Flexible Optoelectronic Devices.

With significant progress in the past decade, semiconductor nanowires have demonstrated unique features compared to their thin film counterparts, such as enhanced light absorption, mechanical integrity and reduced therma conductivity, etc. However, technologies of semiconductor thin film still serve as foundations of several major industries, such as electronics, displays, energy, etc. A direct path to convert thin film to nanowires can build a bridge between these two and therefore facilitate the large-scale applications of nanowires. Here, we demonstrate that methylammonium lead iodide (CH3NH3PbI3) nanowires can be synthesized directly from perovskite film by a scalable conversion process. In addition, with fine kinetic control, morphologies, and diameters of these nanowires can be well-controlled. Based on these perovskite nanowires with excellent optical trapping and mechanical properties, flexible photodetectors with good sensitivity are demonstrated.

Enhanced near-infrared photoresponse of organic phototransistors based on single-component donor-acceptor conjugated polymer nanowires.

Single-component near-infrared phototransistors based on ambipolar organic semiconductor nanowires have been investigated and compared with their corresponding thin-film counterparts. The nanowire organic phototransistors (NW-OPTs) showed photocurrent/dark-current ratios and photoresponsivities as high as 1.3 × 10(4) and 440 mA W(-1) for the p-type channel, and 3.3 × 10(4) and 70 mA W(-1) for the n-type channel, respectively, upon near-infrared illumination with an intensity of 47.1 mW cm(-2). These were much higher values compared to their thin-film counterparts. The enhancement of the near-infrared photoresponse could be attributed to the larger trap density originating from the semiconductor/insulator interface and the semiconductor/air interface. The performance of NW-OPTs was demonstrated to open up new possibilities to improve the near-infrared photoresponse of single-component devices.

Surface modification of cadmium sulfide thin film honey comb nanostructures: Effect of in situ tin doping using chemical bath deposition

Even though nanostructures possess large surface to volume ratio compared to their thin film counterpart, the complicated procedure that demands for the deposition on a substrate kept them back foot in device fabrication techniques. In this work, a honey comb like cadmium sulfide (CdS) thin films nanostructure are deposited on glass substrates using simple chemical bath deposition technique at 65°C. Energy band gaps, film thickness and shell size of the honey comb nanostructures are successfully controlled using tin (Sn) doping and number of shells per unit area is found to be maximum for 5% Sn doped (in the reaction mixture) sample. X-ray diffraction and optical absorption analysis showed that cadmium sulfide and cadmium hydroxide coexist in the samples. TEM measurements showed that CdS nanostructures are embedded in cadmium hydroxide just like “plum pudding”. Persistent photoconductivity measurements of the samples are also carried out. The decay constants found to be increased with increases in Sn doping.