Crystalline Thin Films Research Articles

Growth of crystalline thin films of picene on semimetallic Bi(111) surface.

We report the scanning tunneling microscopy/spectroscopy (STM/STS) studies on structural and electronic properties of picene films grown on the semimetallic Bi(111) substrate held at different temperatures. Under room-temperature deposition, the picene molecules form a crystalline (001) monolayer with the standing-up orientation, indicating the weak molecule-substrate interaction. When deposited on the Bi(111) substrate held at 150K, picene molecules form a bulk-like (211̄ monolayer with building blocks of picene trimers. High-resolution STM images reveal that each trimer consists of two tilted molecules and one side-on molecule. Further reducing the deposition temperature to 90K leads to the formation of nanostripe arrays, in which the side-on molecules adopt the π-π stacking. STS measurements demonstrate that the crystalline (001) monolayer of picene exhibits a larger gap compared with picene crystals, which can be attributed to the decoupling of the upright standing molecules from the semimetallic Bi(111) substrate.

Crystallinity improvement of physical-vapor-deposited WS2 films by controlling particle energy

Abstract High-crystallinity WS2 thin films are crucial for their applications in next-generation electronic devices, particularly in logic transistors. Consequently, a physical vapor deposition (PVD) is a promising method for achieving such films because of its scalability and precise control over deposition conditions. In this study, WS2 thin film crystallinity has been investigated by controlling particle energy during the PVD process. The scattering of particles is influenced by Ar pressure, in which a throw distance of the particles is determined. The target-substrate (T-S) distance was set in conjunction with the throw distance in terms of the composition and energy of the particles at the surface on the wafer. The crystallinity of the deposited films was assessed through Raman spectroscopy, X-ray photoelectron spectroscopy, and circular transmission line method measurements. Controlling of T-S distance and Ar pressure effectively minimized film damage during the PVD process, resulting in crystallinity improvement. Therefore PVD-WS2 films hold significant potential for various applications in next-generation logic transistors.

Thin film epitaxy of high quality ferromagnetic semiconductor EuO using pulsed laser deposition equipped with Nd:YAG laser

Abstract Pulsed laser deposition with the fourth harmonic wave of Nd-doped Y3Al5O12 laser was applied to thin film epitaxy of a ferromagnetic semiconductor EuO which contains metastable Eu2+ ions. Highly crystalline, flat, and stoichiometric EuO (001) epitaxial thin films were successfully grown on YAlO3 (110) single crystal substrates. The EuO film exhibited ferromagnetism reflecting the 4f electronic configuration of Eu2+. These results indicate that Nd-doped Y3Al5O12 laser is useful for deposition of oxide thin films requiring oxygen stoichiometry.

Preparation of Octacalcium Phosphate Thin Film with Exposing Reactive Crystalline Plane in Biological Fluid.

Octacalcium phosphate (OCP) has been used as a bone replacement material due to its higher bone affinity. However, the mechanism of affinity has not been clarified. Since the 100 crystalline plane of OCP is closely involved in the biological reactions during osteogenesis, it is important to expose the 100 crystalline plane of OCP to the biological fluid to precisely measure the interfacial reactions. In this study, the OCP plate-like crystals were fixed on a conductive substrate in the form of single-particle deposition, and the thin films with exposing 100 crystalline planes were fabricated. Then, the characteristics of hydration layers in the OCP crystals were enhanced by the exposure of 100 crystalline planes through the thin film formation, and the bioreactivity was found to be associated with the swelling and dissolution of the hydration layer in the biological fluid. Specifically, the OCP crystals were deposited on the gold sensor by electrophoretic deposition (OCP/Au-1). The results showed that the OCP plate-like crystals were selectively deposited on the gold sensor by electrophoresis. Subsequently, it was found that the ultrasonication of OCP/Au-1 resulted in the formation of an OCP crystalline thin film (m-OCP/Au-1) with the single-particle thickness on the gold sensor with exposing 100 crystalline planes. Moreover, the FT-IR spectra of m-OCP/Au-1 showed that the structure of the phosphate ions was rearranged by ultrasonication in the hydration layer, resulting in the regulation of the layered nanostructures, promoting higher crystallinity. Furthermore, the XPS spectra of m-OCP/Au-1 indicated that the hydrogen phosphate ions in the hydration layer were exposed on the 100 crystalline plane of the topmost surface. The prepared m-OCP/Au-1 was stable in citrate buffer, whereas it showed very high reactivity in phosphate buffer as the hydration layer gradually dissolved after the swelling, which was measured by the QCM-D technique. Therefore, the OCP crystalline thin films in this study were found to have higher surface reactivity due to the enhanced exposure of the hydration layer, which is assumed to be the cause of their bone-regeneration-promoting effect (i.e., higher bone affinity). The films in this study were stable at gastric acid pH and dissolved at neutral pH, which could make them useful as the orally administered drug carrier.

Atomic layer deposition of crystalline molybdenum trioxide and suboxide thin films using molybdenum(II) acetate dimer precursor

Molybdenum oxide thin films are of interest due to a large range of possible phases, high work functions, and catalytic activity. These films have applications in areas, such as sensors, chromic, and semiconductor devices. In this work, a molybdenum(II) acetate dimer precursor was used with ozone for the atomic layer deposition of molybdenum oxide thin films. The films were grown at 200–300 °C yielding highly crystalline films even at the lowest deposition temperatures. X-ray diffraction measurements showed that the as-deposited films consist of molybdenum suboxides and/or a phase-pure orthorhombic molybdenum trioxide phase depending on the deposition conditions. Time-of-flight elastic recoil detection analysis showed that the stoichiometry was close to molybdenum trioxide, and the films were exceptionally pure with main impurities being hydrogen and carbon, which were at the detection limit of the instrument (0.1 at. %). This process, allowing the deposition of very pure and highly crystalline thin films with tunable phases and oxidation states, is very promising for future industrial applications.

Comparative Optical Properties of Amorphous and Crystalline MoO3 Films by Spectroscopic Ellipsometry Study

This paper presents a comprehensive study of the optical properties of amorphous and crystalline MoO3 thin films using spectroscopic ellipsometry. MoO3 films were deposited on SiO2/Si substrates with varying oxide thicknesses using the atomic layer deposition (ALD) method. The study utilized various optical oscillator models, including Lorentz, Tauc–Lorentz and Harmonic, to analyze the dielectric functions and identify midgap states resulting from oxygen deficiencies. The findings highlight significant differences in the optical properties between amorphous and crystalline MoO3 films, demonstrating the impact of structural phases and substrate conditions. Specifically, amorphous films exhibited lower broadening and energy peaks compared to crystalline films, which showed distinct midgap states indicative of higher oxygen vacancy concentrations. These results enhance the understanding of the optical behavior of MoO3 films and their potential applications in advanced optoelectronic devices.

Influence of Deposition Temperature on Cu-BDC Surface-Anchored Metal-Organic Framework Formation.

Surface-anchored metal-organic frameworks (surMOFs) are crystalline, nanoporous, supramolecular materials mounted to substrates that have the potential for integration within device architectures relevant for a variety of electronic, photonic, sensing, and gas storage applications. This research investigates the thin film formation of the Cu-BDC (copper benzene-1,4-dicarboxylate) MOF system on a carboxylic acid-terminated self-assembled monolayer by alternating deposition of solution-phase inorganic and organic precursors. X-ray diffraction (XRD) and atomic force microscopy (AFM) characterization demonstrate that crystalline Cu-BDC thin films are formed via Volmer-Weber growth. Changes in film morphology as the deposition temperature increases are seen by AFM with more isolated nanorod-like crystallites observed at lower temperatures, while vertical nanoplatelet-like structures form at higher temperatures. At 45 °C, the nanoplatelets are observed to be composed of fused nanorod segments aligned in one direction. Ellipsometry confirms that both increasing temperature and number of deposition cycles yield more film deposition in agreement with infrared reflectance-absorbance spectroscopy (IRRAS) that was further used to characterize the chemical binding and orientation in the surface-bound Cu-BDC nanostructures. In addition to observing strong preferred orientation of the nanostructures from the enhancement of the symmetric carboxylate stretch and absence of the antisymmetric stretch, IRRAS results show the emergence of a new binding motif associated with segmented nanoplatelet formation at a higher deposition temperature as well as at high surface coverage after 12 deposition cycles. From the appearance of this higher frequency symmetric carboxylate peak alongside peak splitting in BDC deformation modes, IRRAS data support the presence of a strained configuration that accompanies the appearance of Cu-BDC segmented nanoplatelets observed by AFM.

Predicting 2D Crystal Packing in Thin Films of Small Molecule Organic Materials

AbstractThe large variety of structural morphologies realized in organic semiconductors is a big challenge for the microscopic modeling of such systems. A global computational solution is still out of reach due to prevalent molecular flexibility. However, the specific case of crystalline thin films that exhibit surface alignment of molecular π‐systems for optoelectronic applications of high technological relevance, seems to be a simpler task. This study proposes an approach for the structure prediction of two‐dimensional (2D) molecular layers as precursors for the three‐dimensional (3D) structure of deposited crystalline thin films. Based on grid search sampling for the layer's degrees of freedom, it requires only a small number of trial structures to find complex packing motifs of layered molecular materials. It facilitates parallel screening among multiple molecular conformers, which is usually very difficult and expensive, using the latest 3D‐based prediction methods. The study researches theoretically and experimentally a set of known and newly crystallized compounds of evaporable flexible molecules with interesting optoelectronic properties, predicts their packing in 2D layers, and compares them with experimentally resolved crystal structures, obtaining very good agreement in the packing of these molecules within layers. The computational costs are estimated to be several orders of magnitude lower than with 3D methods.

Heterostructure growth, electrical transport and electronic structure of crystalline Dirac nodal arc semimetal PtSn4

Topological semimetals have recently garnered widespread interest in the quantum materials research community due to their symmetry-protected surface states with dissipationless transport which have potential applications in next-generation low-power electronic devices. One such material, , exhibits Dirac nodal arcs and although the topological properties of single crystals have been investigated, there have been no reports in crystalline thin film geometry. We examined the growth of heterostructures on a range of single crystals by optimizing the electron beam evaporation of Pt and Sn and studied the effect of vacuum thermal annealing on phase and crystallinity. The electrical resistivity was fitted to a modified Bloch–Grüneisen model with a residual resistivity of 79.43(1) cm at 2K and a Debye temperature of 200K. Nonlinear Hall resistance indicated the presence of more than one carrier type with an effective carrier mobility of 33.6 and concentration of 1.41 at 300 K. X-ray photoemission spectra were in close agreement with convolved density of states and a work function of 4.7(2) eV was determined for the (010) surface. This study will facilitate measurements that require heterostructure geometry, such as spin and topological Hall effect, and will facilitate potential device incorporation in future quantum technologies.

High-Quality SnSe Thin Films for Self-Powered Devices and Multilevel Information Encryption.

As semiconductor technology advances toward miniaturization and portability, thin films with excellent thermoelectric performance have garnered increasing attention, particularly for applications in self-powered devices and temperature-responsive sensors. The high Seebeck coefficient of SnSe thin films makes them promising for temperature sensing, but their poor electrical conductivity limits their potential as thermoelectric generators. In this work, high-quality a-axis oriented SnSe thin films were deposited on quartz substrates by using magnetron sputtering. The substrate temperature was optimized to improve the crystallinity of the SnSe thin film, resulting in larger grain sizes, which subsequently contributes to the improved carrier mobility. The Seebeck coefficient is enhanced while optimizing the electrical conductivity, enabling the SnSe thin film to achieve both excellent sensing and power generation performance. The SnSe film deposited at 673 K exhibits a high power factor of approximately 346 μW m-1 K-2 at 620 K. A temperature-responsive sensing array was developed for multilevel information encryption, showing significant potential for applications in password encryption. The maximum output power density of the optimized thermoelectric generator with six SnSe legs is about 9 W m-2 at a temperature difference of 50 K.

Bioinspired single-shot polarization photodetector based on four-directional grating arrays capped perovskite single-crystal thin film.

Polarization photodetectors (pol-PDs) have widespread applications in geological remote sensing, machine vision, biological medicine, and so on. However, commercial pol-PDs use bulky and complicated optical systems with lenses, polarizers, and mechanical spools, which are complex and cumbersome, and respond slowly. Inspired by the desert ants' compound eyes, we developed a single-shot pol-PD based on four-directional grating arrays capped perovskite single-crystal thin film without other standard polarization optics. Our pol-PD has a high detectivity, two orders of magnitude greater than that of commercial photodetectors, and exhibits high polarization sensitivity. The high performance of our pol-PD is due to the highly crystalline perovskite single-crystal thin film and regular nanograting structure, made by a nanoimprinting crystallization method. Our single-shot pol-PD is a compact on-chip optoelectronic device that demonstrates excellent performance in a wide range of applications including accurate bionic navigation, sharp image restoration in hazy scenes, stress visualization of polymers, and detection of cancerous areas in tissues without histological staining.

Crystallinity and adhesiveness improvements of Ag thin films by Bi- and Sb-assisted growth

Crystallinity and adhesiveness improvements of Ag thin films by Bi- and Sb-assisted growth

The role of manganese in CoMnOx catalysts for selective long-chain hydrocarbon production via Fischer-Tropsch synthesis

Cobalt is an efficient catalyst for Fischer−Tropsch synthesis (FTS) of hydrocarbons from syngas (CO + H2) with enhanced selectivity for long-chain hydrocarbons when promoted by Manganese. However, the molecular scale origin of the enhancement remains unclear. Here we present an experimental and theoretical study using model catalysts consisting of crystalline CoMnOx nanoparticles and thin films, where Co and Mn are mixed at the sub-nm scale. Employing TEM and in-situ X-ray spectroscopies (XRD, APXPS, and XAS), we determine the catalyst’s atomic structure, chemical state, reactive species, and their evolution under FTS conditions. We show the concentration of CHx, the key intermediates, increases rapidly on CoMnOx, while no increase occurs without Mn. DFT simulations reveal that basic O sites in CoMnOx bind hydrogen atoms resulting from H2 dissociation on Co0 sites, making them less available to react with CHx intermediates, thus hindering chain termination reactions, which promotes the formation of long-chain hydrocarbons.

Electrochemical Synthesis and Optimization of CdSexTe1-X Thin-Films for Enhanced Solar Cell Performance

The considerably wide light absorption spectrum and excellent chemical stability of CdSexTe1-x alloyed thin films show its significant applicability in solar cell thin film technology. In this study, a novel, economical electrodeposition technique was utilized to synthesize the CdSexTe1-x thin films, offering a simpler setup and reduced energy consumption compared to other methods of solar cell fabrication used in industrial scale. The performance of CdSexTe1-x thin film was examined with respect to several parameters, such as electrolyte composition, electrodeposition potential, treatment solution, and annealing temperature. XRD, SEM, and EDS were employed to investigate the fabricated CdSexTe1-x thin film's crystallinity, morphology, and elemental compositions. The synthesized CdSexTe1-x thin film demonstrated promising fill factor, short circuit photocurrent density (Jsc), and open-circuit voltage (Voc). The results reveal that the synthesized CdSexTe1-x thin film exhibited enhanced physical characteristics and photoelectrochemical solar cells performances as remarkable light harvesting absorber layers.

Explaining the Voltage Hysteresis and Slow Relaxation of Silicon Nanoparticles with a Chemo-Mechanical Particle-SEI Model

Silicon is widely considered to be a promising next-generation anode material, primarily due to its remarkably high theoretical capacity. Furthermore, silicon is an abundant, cheap, and widely spread material. However, a major challenge for the commercialization of silicon anodes is the significant voltage hysteresis reducing efficiency and leading to detrimental heat generation during fast-charging. Additionally, the hysteresis causes an unclear state-of-charge (SOC) to voltage relation impeding precise SOC estimation.The voltage hysteresis behavior of silicon anodes is addressed in literature with three different arguments: phase transformations, plastic flow of silicon, and slow diffusion. These approaches can interpret the voltage hysteresis of crystalline silicon, thin films, and large anode particles, respectively. Nevertheless, we show that they cannot explain the hysteresis observed for silicon anodes consisting of amorphous nanoparticles, the relevant material for next-generation lithium-ion batteries.Our investigation highlights the chemo-mechanical interplay between the silicon nanoparticle and the covering Solid-Electrolyte Interphase (SEI) as the underlying cause for the significant voltage hysteresis. The SEI, a thin passivating layer, forms on negative electrode particles due to electrolyte decomposition [1]. In the case of silicon particles, the volume changes during lithiation and delithiation result in massive strains and plastic deformation occurring within the SEI [2]. Our chemo-mechanical particle-SEI description successfully replicates the observed open-circuit voltage hysteresis in experiments and aligns with the Plett model [3]. Moreover, our visco-elastoplastic SEI model reproduces the voltage difference between slow cycling and the relaxed open-circuit voltage. In our recent work, we show that a sophisticated mechanical model explains the slow voltage relaxation observed for silicon nanoparticles as well as the observed C rate dependence [4].To conclude, we explain the voltage hysteresis and the slow voltage relaxation of silicon nanoparticles with a visco-elastoplastic particle-SEI model and discuss options to mitigate the size of the voltage hysteresis. This detailed physical understanding can improve the performance of all-silicon anodes and contribute to their commercialization.[1] Köbbing, L.; Latz, A.; Horstmann, B. J. Power Sources 2023, DOI: 10.1016/j.jpowsour.2023.232651.[2] Kolzenberg, L.; Latz, A.; Horstmann, B. Batter. Supercaps 2022, DOI: 10.1002/batt.202100216.[3] Köbbing, L.; Latz, A.; Horstmann, B. Adv. Funct. Mater. 2024, DOI: 10.1002/adfm.202308818.[4] Köbbing, L.; Latz, A.; Horstmann, B. (in preparation). Figure 1

Atomic layer deposition of tin monosulfide thin film using Sn(acac)2 and H2S

In this study, we deposited tin monosulfide (SnS) thin film using Tin(II) 2,4-pentanedionate [Sn(acac)2] precursor and hydrogen sulfide (H2S) reactant. And we performed post annealing to improve the crystallinity of SnS thin films. The process window was 130 °C–150 °C, and the growth rate was 0.34 Å/cycle. To investigate crystallinity and the phase of SnS thin films, grazing incidence x-ray diffraction (GI-XRD) and Raman spectroscopy were performed. SnS thin films showed a single orthorhombic phase after annealing. In addition, transmission electron microscopy (TEM) was utilized to confirm the two-dimensional (2D) layered structure of SnS thin films. Post-annealed SnS thin film clearly showed a 2D layered structure. X-ray photoelectron spectroscopy (XPS) was performed to confirm the bonding state of the thin film. The results indicated that the SnS thin film only shows binding energies corresponding to the oxidation states of Sn2+ in the Sn 3d spectra and S2- in the S 2p spectra. Ultraviolet–visible (UV–vis) spectroscopy and ultraviolet photoelectron spectroscopy (UPS) were performed to confirm the optical properties and to calculate the band structure of the thin film. All of SnS thin film showed a p-type characteristic. The post-annealed SnS thin films exhibited better electric properties, confirmed by Hall measurement.

Integrated electro-optic modulator on a lead zirconate titanate-silicon nitride heterogeneous platform.

Integrated electro-optic (EO) modulators are the core components of the optoelectronic information technology, and lithium niobate is currently the most widely used crystalline thin film material; however, finite EO coefficients limit the modulation efficiency of the modulators. In this Letter, we present an integrated EO modulator using a microring resonator on the lead zirconate titanate (PZT) and silicon nitride (SiN) heterogeneous platform. The microwave attenuation is reduced by using low loss tangent and dielectric constant SiN as the electrode substrate, achieving an EO bandwidth of 33 GHz. Thanks to the high quality of the PZT film deposition and the substantial EO overlap of our structure, ultrahigh modulation efficiency with the half-wave voltage-length product of 0.7 V·cm is achieved. In addition, as a remarkable result, an 80-Gbps on-off keying signal is generated using the modulator.

Thickness dependent crystallinity, hydrophilicity, and surface microtexture in CaF2 thin films deposited via thermal evaporation method

This study investigates the deposition and characterization of CaF₂ thin films with thicknesses ranging from 40 to 320 nm, fabricated via thermal evaporation onto glass substrates. X-ray diffractometry (XRD) revealed that increased film thickness leads to enhanced crystallinity, with larger crystallite sizes and reduced lattice strain. Wettability analysis demonstrated a transition from hydrophobic behavior (contact angle ∼70° for 40 nm) to hydrophilic behavior (contact angle ∼52° for 320 nm) as film thickness increased. Surface morphology, examined via atomic force microscopy (AFM), showed a significant reduction in surface roughness in thicker films, with smoother, more coalesced surfaces observed for films over 160 nm. Monofractal analysis further confirmed the evolution of surface microtexture, with thicker films exhibiting higher uniformity and reduced surface complexity. These findings provide critical insights into optimizing CaF₂ thin films for applications in optics, electronics, and surface coatings, where improved crystallinity and surface properties are essential.

Morphology and structural properties of Sb2(SxSe1-x)3 thin film absorbers for solar cells

Using the thermal evaporation method, thin crystalline films of Sb2(Sx Se1-x)3 are produced at the substrate temperature of 300℃. The mixed powders of the Sb2S3 and Sb2Se3 is used as a source material. The influence of the S/Se component ratio on the morphology and structural characteristics of Sb2(SxSe1-x)3 thin films is investigated. As demonstrated by the results of X-ray energy dispersive spectroscopy, the formed films of Sb2(SxSe1-x)3 have a components ratio close to the stoichiometry. Besides, Morphological and structural analyses reveal significant differences in the surface morphology of Sb2(SxSe1-x)3 thin film absorbers, indicating that the properties of the films vary as a function of the S/Se composition ratio.

Ultrafast Time-Domain Spectroscopy Reveals Coherent Vibronic Couplings upon Electronic Excitation in Crystalline Organic Thin Films.

The coherent coupling between electronic excitations and vibrational modes of molecules largely affects the optical and charge transport properties of organic semiconductors and molecular solids. To analyze these couplings by means of ultrafast spectroscopy, highly ordered crystalline films with large domains are particularly suitable because the domains can be addressed individually, hence allowing azimuthal polarization-resolved measurements. Impressive examples of this are highly ordered crystalline thin films of perfluoropentacene (PFP) molecules, which adopt different molecular orientations on different alkali halide substrates. Here, we report polarization-resolved time-domain vibrational spectroscopy with 10 fs time resolution and Raman spectroscopy of crystalline PFP thin films grown on NaF(100) and KCl(100) substrates. Coherent oscillations in the time-resolved spectra reveal vibronic coupling to a high-frequency, 25 fs, in-plane deformation mode that is insensitive to the optical polarization, while the coupling to a lower-frequency, 85 fs, out-of-plane ring bending mode depends significantly on the crystalline and molecular orientation. Comparison with calculated Raman spectra of isolated PFP molecules in vacuo supports this interpretation and indicates a dominant role of solid-state effects in the vibronic properties of these materials. Our results represent a first step toward uncovering the role of anisotropic vibronic couplings for singlet fission processes in crystalline molecular thin films.