Browsing by Subject "Thin films"

Now showing 1 - 9 of 9
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    Data for Boundary Frustration in Double-Gyroid Thin Films
    (2024-02-29) Magruder, Benjamin R; Morse, David C; Ellison, Christopher J; Dorfman, Kevin D; dorfman@umn.edu; Dorfman, Kevin D; Dorfman Group, UMN CEMS
    We have used self-consistent field theory to predict the morphology and preferred orientation of the double-gyroid phase in thin films of AB diblock polymers. A manuscript has been submitted containing this data, and is expected to appear shortly. The data were generated using the C++ version of the open-source software PSCF (https://pscf.cems.umn.edu/). All input and output files from PSCF used to generate the data in the paper are included in this dataset, as well as the code used to process the data and generate the figures.
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    Electrochemical control of oxygen stoichiometry and materials properties in ion-gel-gated cobaltite thin films
    (2023-12) Postiglione, William
    Wide-ranging control of materials properties using applied voltages represents a longstanding goal in physics and technology, particularly for low-power applications. To this end, substantial interest has developed around electric-double-layer transistors (EDLTs) based on functional materials. More recently, electrochemical EDLTs, where ions such as O2-, H+, Li+, etc., are driven into / out of a channel material via voltage, have proven capable of offering unique benefits (including non-volatility) for a variety of novel applications. Cobaltites, such as SrCoO3-δ (SCO) have recently emerged as an archetypal example of electrochemical control of materials properties in electrolyte-gate devices. This is accomplished by voltage-driven redox cycling between two distinct phases: fully oxygenated perovskite (P) (δ ≈ 0) and oxygen-vacancy-ordered brownmillerite (BM) (δ = 0.5). To date, SCO has received the most attention in this regard, despite significant issues with air stability in the P phase, and few alternatives have been considered. Additionally, critical issues of voltage hysteresis and fundamental limits on reversibility and cycling endurance remain unaddressed.To address this, using EDLTs based on epitaxial La1-xSrxCoO3-δ (LSCO) thin films, we first investigate the electrochemical reduction that is known to occur at positive gate voltages (Vg) in such systems, establishing that the P → BM transformation occurs in LSCO over a wide doping range. Importantly, both the P and BM phase of x = 0.5 LSCO are robustly air stable, and the electrochemical reduction behavior was found to be voltage-tunable with both doping and strain. We then leverage this voltage-tuned P → BM transformation to demonstrate large property modulations in electronic transport, magnetism, thermal transport, and optical properties, achieving similar or greater ranges of control than in SCO. Next, to explore the reversibility of the transformation, we performed detailed analysis of Vg hysteresis loops, revealing a wealth of new mechanistic findings, including asymmetric transformations due to differing oxygen diffusivities in the P vs. the BM phase, non-monotonic transformation rates due to the first-order nature of the P-BM transformation, and limits on reversibility due to first-cycle structural degradation. Additionally, using minor hysteresis loops, we demonstrate the first rational design of an optimal Vg cycle, leading to state-of-the-art cycling of electronic and magnetic properties, encompassing >105 transport ON/OFF ratios at room temperature, reversible and non-volatile metal-insulator-metal and ferromagnet-nonferromagnet-ferromagnet cycling, all at ultrathin 10-unit-cell thickness. Finally, to further investigate the magnetic properties of the BM nonferromagnet “OFF” state, we performed neutron diffraction experiments, finding the first direct evidence of antiferromagnetic order in BM-SCO films and identifying weak ferromagnetism in x = 0.5 BM-LSCO. These findings thus significantly advance the understanding of voltage-induced P ↔ BM transformations in cobaltite films and pave the way for future work establishing the ultimate cycling frequency and endurance in such electrolyte-gated devices.
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    Electronic transport properties of hydrogenated amorphous silicon-germanium thin films
    (2022-01) Stolik Valor, Lis
    Interest in amorphous semiconductors stems in part from their use in large-area thin-film applications, including photovoltaics, light-emitting diodes, thin film transistors, non-volatile memories and thermoelectrics. Furthermore, alloyed amorphous semiconductors have emerged as promising materials, as their optical bandgap can be easily engineered by controlling their chemical composition. Alloyed a-Si_{1-x}Ge_{x}:H thin film samples are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system, and a series of such films with Ge content raging from (0-100)% are obtained. The Ge content is determined through X-ray photoelectron spectroscopy and qualitatively corroborated through measurements of their Raman spectra. Measurements of their dark conductivity, photoconductivity, and thermopower reveal a dual-channel conduction through the dangling bond states. Alloys with concentrations of Ge below 20% exhibit anomalous hopping conduction, while the dark conductivity of alloys with higher Ge concentrations are best fit by a combination of anomalous hopping at high temperatures and power-law temperature dependence for the low to mid-ranges, characteristic of multi-phonon hopping transport. The samples' photoconductivies show evidence of high defect state densities in the mid-gap. Corresponding measurements of the thermopower find that conduction is n-type for the purely a-Si:H and a-Ge:H samples but that the Seebeck coefficient exhibits a transition from negative to positive values as a function of Ge content and temperature. A conduction model involving the parallel contributions of the two distinct conduction mechanisms is shown to describe both the conductivity and the thermopower data to a high degree of accuracy. The clear experimental evidence of hopping conduction reported here provides important information concerning the nature of electronic conduction in amorphous semiconductors, and suggests that the concept of a mobility edge, accepted for over four decades, may not be necessary to account for charge transport in certain amorphous semiconductors.
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    Electrostatic effects in coating and printing processes
    (2015-01) Ramkrishnan, Aruna
    Coating and printing are interfacial processes that are highly relevant in industry. Precision coatings impart functionalities and boost the performance of products. On the other hand, high-resolution roll-to-roll printing is being increasingly explored for creating dense and flexible printed electronics at high speeds. Electrostatic effects often significantly influence both these processes. However, in industry, much of the current understanding of these effects is empirical and has not received a rigorous treatment. This thesis discusses how electrostatics and hydrodynamics couple in coating and printing applications, and presents different modes of investigation: simplified thin-film models and flow visualization experiments, to understand the underlying physics of these processes. Throughout this work, the electric response of liquids has been described by the perfect (non-conducting) and leaky dielectric (partially conducting) models, which are representative of many liquids used in industry. In coating processes, electrostatic charges are known to accumulate on the substrate due to various upstream operations (e.g. corona treatment, friction in roll-to-roll equipment). This leads to the buildup of an electric field in the subsequently coated film, which in turn causes the formation of defects due to electrostatically driven flows. Thus, in order to obtain high quality coatings, it is desirable to keep them resistant to electrostatic destabilization. We have carried out a systematic study via the construction of electrohydrodynamic lubrication models to understand the influence of charged substrates and charged interfaces on the leveling of liquid coatings. Based on our findings, we develop simple heuristics that can be used to design coatings that are stable to substrate charging and charge contamination. Electric fields are also present in some printing processes. Developed in the late1960s, electrostatic assist (ESA) has been long used to remove printing defects and enhance image quality in gravure printing, a high-resolution roll to-roll process. ESA involves the application of an electric field to pull ink out of cavities and transfer it onto the desired substrate. However, there is limited understanding of how this process works, which hinders its development as a tool for printed electronics. In order to address this issue, we develop a model for electrostatically assisted meniscus deformation near a cavity (this describes the first stage of electrostatic assist). Our calculations show that electric fields pull up the ink meniscus either at the edges or at the center of the cavity, depending on the ink conductivity. This suggests that ink contact with the substrate will be improved during ESA but air entrapment occurs for a certain range of conductivities, which would be detrimental to print quality. Our model also enables us to investigate the effect of cavity shape and spacing on the mode of deformation of the ink surface. In order to validate the findings from our electrohydrodynamic model, we have carried out flow visualization experiments to track the deformation of liquids contained in cavities, and these corroborate the qualitative trends of meniscus deformation predicted by the model.
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    Enhanced crystallization of amorphous silicon thin films by nano-crystallite seeding
    (2013-12) Trask, Jason
    Polycrystalline silicon (poly-Si) has become popular in recent years as a candidate for low cost, high efficiency thin film solar cells. The possibility to combine the stability against light degradation and electronic properties approaching melt-grown, wafer-based crystallline silicon, with the cost advantage of Silicon thin films is highly attractive. To fully realize this goal, efforts have been focused on maximizing grain size while reducing the thermal input involved in a critical ``annealing'' step. Of the variety of processes involved in this effort, studies have shown that poly-Si films obtained from solid-phase-crystallization (SPC) of hydrogenated amorphous silicon (a-Si:H), grown from non-thermal plasma-enhanced chemical vapor deposition (PECVD), exhibit the potential to achieve the highest quality grain structures. However, reproducible control of grain size has proven difficult, with larger grains typically requiring longer annealing times. In this work, a novel technique is demonstrated for more effectively controlling the final grain structure of SPC-processed films while simultaneously reducing annealing times. The process utilized involves SPC of a-Si:H thin films containing embedded nanocrystallites, intended to serve as predetermined grain-growth sites, or grain-growth ``seeds'', during the annealing process. Films were produced by PECVD with a system in which two plasmas were operated to produce crystallites and amorphous films separately. This approach allows crystallite synthesis conditions to be tuned independently from a-Si:H film synthesis conditions, providing a large parameter space available for process optimization, including the effects of particle size, shape, quantity, and location within the film. The work contained here-in outlines the effects of select parameters on the both grain size control and thermal budget. Reproducible control of both grain size and crystallization rate were demonstrated through varying initial seed crystal concentrations. Significant reductions in annealing times were demonstrated to occur in seeded films relative to unseeded films, with both seed crystal concentration and seed crystal geometry demonstrating significant effects on crystallization rate. Furthermore, the development of this technique has resulted in potentially new insights on the material system involved, with the observation of a potentially unique phase-transformation mechanism.
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    Liquid-film coating on rotating discrete objects
    (2018-01) Li, Weihua
    The flow of liquid films on discrete objects is encountered in coating processes for a wide range of products such as biomedical devices, automobiles, and food. Describing the shape of liquid films as they flow over discrete objects is a challenging task due to the large number of forces at play. These include gravitational, inertial, viscous, surface-tension, and centrifugal forces, and the complex interplay among them may lead to the growth of instabilities that degrade the quality of the final product. Motivated by the need to improve fundamental understanding of coating flows on discrete objects, we pick cylinders that rotate about their horizontal axes as model discrete objects and investigate four model problems highly relevant to industrial coating processes for rotating discrete objects. In each model problem, the interplay among all the forces is systematically examined to reveal the critical conditions for which a smooth coating can be obtained. For coating of surfactant-laden liquids on rotating cylinders, we applied lubrication theory to derive coupled nonlinear evolution equations to describe the variation of the film thickness and surfactant concentration as a function of time, the angular coordinate, and the axial coordinate. In the absence of gravitational effects, linear stability analysis reveals that surfactant-induced Marangoni stresses suppress the growth rate of instabilities driven by centrifugal effects and hinder the leveling of perturbations to the film thickness in both the angular and axial directions. When gravitational effects are present, Marangoni stresses lower the critical rotation rate needed to cause a liquid lobe to form and rotate in the angular direction. These stresses also lead to faster damping of this lobe, giving rise to a more axisymmetric coating. With the growth of axial instabilities at long times, Marangoni stresses significantly weaken the stabilizing effect of surface-tension forces, which are found to be responsible for keeping the coating axially uniform in a stable speed window. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and suppress the growth rate of rings that form at high rotation grates. Flow visualization experiments yield observations that are qualitatively consistent with our simulation results. For cylinders with complex surface geometries (i.e., topographically patterned cylinders and elliptical cylinders), the Galerkin finite-element method is used to solve the Stokes equations, augmented with a term accounting for centrifugal forces, in a rotating frame of reference. For rapidly rotating cylinders where gravitational forces are negligible, surface-tension forces tend to drive liquid to the low-surface-curvature areas (e.g., pattern troughs) leading to the formation of liquid pools, while centrifugal forces tend to drive liquid in the opposite direction, giving rise to liquid droplets. The number of droplets or pools at steady state depends on the rotation rate, strength of surface tension, pattern frequency, and cylinder aspect ratio. When gravitational forces become significant, it is possible to obtain a coating that closely conforms to the cylinder surface in the patterned-cylinder case. With an increase in the pattern amplitude, recirculation regions start to form inside the troughs, which may strongly influence mixing, mass transport, and heat transport. These reciprocation regions can appear and vanish as the cylinder rotates due to the variation of gravitational forces around the cylinder surface. In the elliptical-cylinder case, simulation results show that smaller aspect ratio corresponds to less liquid that can be supported on the cylinder and also larger gradients in film thickness. A suitably chosen time-dependent rotation rate can greatly improve coating smoothness relative to the constant-rotation-rate case. For cylinders with sufficiently small aspect ratio, film rupture and liquid shedding may occur over the cylinder tips, so simultaneous drying and rotation along with the introduction of Marangoni stresses will likely be especially important for obtaining a smooth coating.
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    Supporting data for Temperature-dependent thermal conductivity of MBE-grown epitaxial SrSnO₃ films
    (2023-11-06) Zhang, Chi; Liu, Fengdeng; Guo, Silu; Zhang, Yingying; Xu, Xiaotian; Mkhoyan, Andre; Jalan, Bharat; Wang, Xiaojia; wang4940@umn.edu; Wang, Xiaojia; Materials Research Science & Engineering Center
    This work studies the temperature-dependent thermal properties of a single crystalline SSO thin film prepared with hybrid molecular beam epitaxy. By combining time-domain thermoreflectance and Debye–Callaway modeling, physical insight into thermal transport mechanisms is provided. At room temperature, the 350-nm SSO film has a thermal conductivity of 4.4 W m¯¹ K¯¹ , ∼60% lower than those of other perovskite oxides (SrTiO₃, BaSnO₃) with the same ABO₃ structural formula. This difference is attributed to the low zone-boundary frequency of SSO, resulting from its distorted orthorhombic structure with tilted octahedra. At high temperatures, the thermal conductivity of SSO decreases with temperature following a ∼T ¯⁰∙⁵⁴ dependence, weaker than the typical T¯¹ trend dominated by the Umklapp scattering. Corresponding author for STEM data is K. Andre Mkhoyan. Corresponding author for film growth and XRD data is Bharat Jalan. Corresponding author for TDTR data is Xiaojia Wang.
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    Synthesis and characterization of copper zinc tin sulfide nanoparticles and thin films.
    (2012-07) Khare, Ankur
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. CZT(S,Se) solar cells have already reached power conversion efficiencies >10%. One of the ways to improve upon the CZTS power conversion efficiency is by using CZTS quantum dots as the photoactive material, which can potentially achieve efficiencies greater than the present thin film technologies at a fraction of the cost. However, two requirements for quantum-dot solar cells have yet to be demonstrated. First, no report has shown quantum confinement in CZTS nanocrystals. Second, the syntheses to date have not provided a range of nanocrystal sizes, which is necessary not only for fundamental studies but also for multijunction photovoltaic architectures. We resolved these two issues by demonstrating a simple synthesis of CZTS, Cu2SnS3, and alloyed (Cu2SnS3)x(ZnS)y nanocrystals with diameters ranging from 2 to 7 nm from diethyldithiocarbamate complexes. As-synthesized nanocrystals were characterized using high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive spectroscopy to confirm their phase purity. Nanocrystals of diameter less than 5 nm were found to exhibit a shift in their optical absorption spectra towards higher energy consistent with quantum confinement and previous theoretical predictions. Thin films from CZTS nanocrystals deposited on Mo-coated quartz substrates using drop casting were found to be continuous but highly porous. Annealing CZTS nanocrystal films at temperatures as low as 400°C led to an intense grain growth; however, thin films from CZTS nanocrystals cracked on annealing due to their high porosity. Although quantum confinement in CZTS is only accessible in nanocrystals of diameters less than 5 nm, the high volume of the ligands as compared to the volume of the nanocrystals makes it a challenge to deposit continuous compacted thin films from small nanocrystals. Films deposited from thermal decomposition of a stoichiometric mix of metal dithiocarbamate complexes were found to be predominantly CZTS. These films from complexes were found to be continuous but microporous. The diameter of the spheres making up the microporous structure could be changed by changing the anneal temperature. The structural composition of the final film could be altered by changing the heating rate of the complexes. CZTS exists in three different crystal structures: kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures. Due to the similarity in the crystal structures, it is extremely difficult to distinguish them based on X-ray diffraction. We computed the phonon dispersion curves for the three structures using ab-initio calculations, and found characteristic discontinuities at the Γ-point which can potentially be used to distinguish the three. In addition, the Γ-point phonon frequencies, which correspond to the Raman peak positions, for the three structures were found to be shifted from each other by a few wavenumbers. By deconvoluting the experimental Raman spectra for both CZTS and Cu2ZnSnSe4 (CZTSe) using Gaussian peaks, we observed that the most intense Raman scattering peak in both CZTS and CZTSe is a sum of two different peaks which correspond to scattering from their respective kesterite and stannite phases. The electronic, structural, and vibrational properties of a series of CZTS-CZTSe alloys (CZTSSe) were studied using ab-initio calculations. The S-to-Se ratio and the spatial distribution of the anions in the unit cell were found to determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. X-ray diffraction patterns and phonon distribution curves were found to be sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.
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    Synthesis, deposition, and microstructure development of thin films formed by sulfidation and selenization of copper zinc tin sulfide nanocrystals
    (2014-08) Chernomordik, Boris David
    Significant reduction in greenhouse gas emission and pollution associated with the global power demand can be accomplished by supplying tens-of-terawatts of power with solar cell technologies. No one solar cell material currently on the market is poised to meet this challenge due to issues such as manufacturing cost, material shortage, or material toxicity. For this reason, there is increasing interest in efficient light-absorbing materials that are comprised of abundant and non-toxic elements for thin film solar cell. Among these materials are copper zinc tin sulfide (Cu2ZnSnS4, or CZTS), copper zinc tin selenide (Cu2ZnSnSe4, or CZTSe), and copper zinc tin sulfoselenide alloys [Cu2ZnSn(SxSe1-x)4, or CZTSSe]. Laboratory power conversion efficiencies of CZTSSe-based solar cells have risen to almost 13% in less than three decades of research. Meeting the terawatt challenge will also require low cost fabrication. CZTSSe thin films from annealed colloidal nanocrystal coatings is an example of solution-based methods that can reduce manufacturing costs through advantages such as high throughput, high material utilization, and low capital expenses. The film microstructure and grain size affects the solar cell performance. To realize low cost commercial production and high efficiencies of CZTSSe-based solar cells, it is necessary to understand the fundamental factors that affect crystal growth and microstructure evolution during CZTSSe annealing. Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The ~2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of ~40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks. We showed the applicability of these nanocrystal coatings for thin film solar cells by demonstrating a CZTS thin film solar cell using coatings annealed in a sulfur atmosphere. We conducted a systematic study of the factors controlling crystal growth and microstructure development during sulfidation annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 °C to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 µm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700-800 oC) and sulfur pressures (500 Torr) normal grains quickly grow to ~500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ~500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr). Moreover, carbon is removed by volatilization from films where normal crystal growth is fast. There are significant differences in the chemistry and in the thermodynamics involved during selenization and sulfidation of CZTS colloidal nanocrystal coatings to form CZTSSe or CZTS thin films, respectively. To understand these differences, the roles of vapor pressure, annealing temperature, and heating rate in the formation of different microstructures of CZTSSe films were investigated. Selenization produced a bi-layer microstructure where a large CZTSSe-crystal layer grew on top of a nanocrystalline carbon-rich bottom layer. Differences in the chemistry of carbon and selenium and that of carbon and sulfur account for this segregation of carbon during selenization. For example, CSe2 and CS2, both volatile species, may form as a result of chalcogen interactions with carbon during annealing. Unlike CS2, however, CSe2 may readily polymerize at room temperature and one atmosphere. Carbon segregation may be occurring only during selenization due to the formation of a Cu-Se polymer [i.e., (CSe2-x)] within the nanocrystal film. The (CSe2-x) inhibits sintering of nanocrystals in the bottom layer. Additionally, a fast heating rate results in temperature variations that lead to transient condensation of selenium on the film. This is observed only during selenization because the equilibrium vapor pressure of selenium is lower than that of sulfur. The presence of liquid selenium during sintering accelerates coarsening and densification of the normal crystal layer (no abnormal crystal layer) by liquid phase sintering. Carbon segregation does not occur where liquid selenium was present.