Decreasing Substrate Temperature Research Articles

Observation of enhanced heat transfer between a nanotip and substrate at nanoscale distances via direct temperature probing with Raman spectroscopy

Nanoscale heat transfer between two nanostructured surfaces holds paramount significance in the realms of extreme manufacturing and high-density data storage. However, experimental probing of heat transfer encounters significant challenges, primarily due to limitations in current instrumentation. Here, we report a method based on Raman spectroscopy to directly probe the temperature difference between a Si nanotip and SiC substrate. Results indicate a decrease in substrate temperature, while the temperature of the nanotip remains relatively stable as the nanotip moves away from the substrate from approximately 82.5 to 1320 nm. We trace this enhanced heat transfer to a significant augmentation, by one order of magnitude, in air conduction and thermal radiation energy exchange theoretically, with air conduction being the dominant mode over thermal radiation. This work advances the direct observation of surface temperatures with gaps smaller than 1 μm, utilizing a noncontact and nondestructive Raman technique, which can be extended to studying near-field heat transfer across various Raman-active surfaces.

Low-temperature growth at 225 °C and characterization of carbon nanowalls synthesized by radical injection plasma-enhanced chemical vapor deposition

The synthesis of carbon nanowalls (CNWs) necessitated a substrate heating temperature above 600 °C. This study describes a low-temperature growth at 225 °C and characterization of CNWs synthesized by the radical injection plasma-enhanced chemical vapor deposition. To investigate the effect of temperature on the growth process, CNWs were synthesized at various temperatures ranging from 200 °C to 700 °C. The morphology of the CNWs observed with a scanning electron microscope shows that wall density increases as substrate temperature decreases. The Raman spectroscopy analysis indicates that CNWs grown at 225 °C have higher defect levels than those grown at higher temperatures, such as 700 °C. Transmission electron microscopy confirmed the presence of multiple graphene layers in the CNWs grown at 225 °C. The water contact angle results revealed that CNWs grown at 225 °C had higher hydrophobicity than those grown at higher temperatures, opening up the potential for CNW applications.

Impact of temperature on residual stress and bonding in diamond-like carbon film: A molecular dynamics study under various deposition conditions

Diamond-like carbon (DLC) films exhibit excellent frictional properties; however, the high residual stress within the film, which causes detachment issues, limits their application in mechanical sliding surfaces. We have previously clarified via molecular dynamics (MD) simulations that the growth of sp3 clusters after the transition from sp2 to sp3 bonds during film formation causes the in-plane compressive stress in DLC films. In this study, we investigated whether this stress generation mechanism holds under various deposition conditions in the MD simulations, wherein incident energy, atomic flux, and substrate temperature were systematically changed. The results of the MD simulations consistently revealed that the compressive stress generation mechanism remains unchanged across different deposition conditions. With increasing incident energy and atomic flux and decreasing substrate temperature, in-plane compressive stress in the film increased owing to increased sp3 content. Conversely, when the incident energy was further increased, both stress and sp3 content decreased. Based on a molecular statistical calculation, it was shown that the stable bonding state changed from sp3 to sp2 at high temperatures. Because the temperature monotonically increased with increasing incident energy, it was concluded that the change in the stable bonding state caused the non-monotonic change in the stress and sp3 content at high temperatures.

ANALYSIS OF THE DYNAMICS AND FREEZING OF WATER DROPLETS ON METAL SURFACES

This article investigates the effects of substrate temperature, tilt angle, and droplet size on droplet impact dynamics and freezing using a Motionpro high-speed camera and a DSA-30 droplet surface analyzer. The temperature of the substrate was changed from the ambient temperature of 21°C to -13°C, and three droplet sizes (<i>D</i><sub>0</sub> = 2.57, 3.02, and 3.54 mm) were studied. The results show that some air gets trapped under the liquid film during the impact process due to insufficient escape time, resulting in the interior of the droplet being in an unstable state. Simultaneously, due to the low surface energy of the substrate, liquid droplets exhibit a rebound effect upon impact with the ambient temperature substrate, reaching their maximum height and forming a dumbbell-like shape. Furthermore, the rebound height decreases rapidly with the decrease in substrate temperature. A change in substrate temperature had no significant effect on the droplet spreading process, but decreasing substrate temperature increased the viscous forces, thereby suppressing the droplet retraction and oscillation processes, ultimately leading to longer droplet freezing times. Additionally, at low Weber numbers (<i>We </i>< 250), the droplet dimensionless parameters exhibited a similar trend with respect to dimensionless time or temperature.

Role of geometry and adhesion in droplet freezing dynamics

We propose a thermal and geometrical analytical model for the freezing front dynamics of a spherical drop. The growth is characterized by an effective diffusion coefficient that increases as the substrate temperature decreases and a spherical front that meets the edges of the drop perpendicularly. We compare our model with experimental data for substrate temperature ranging from -9 to -80 \ifmmode^\circ\else\textdegree\fi{}C. We highlight the importance of heat diffusion in the liquid and the adhesion of drops to the substrate that decreases at low temperature.

Use of the Cold Substrate Method for Biomaterials: The Structural and Biological Properties of the Ag Layers Deposited on Ti-6Al-4V

Silver nanofilms were vacuum-deposited on Ti-6Al-4V substrates at low (< 300 K) temperatures to improve their biocompatibility. After the surface of the Ti-6Al-4V samples was coated with Ag over the temperature range from 100 to 300 K, the structural and biological characteristics were investigated. The crystal structure of the Ag thin films was cubic with (111), (200), (220) and (311) peaks. The peak intensity of the Ag thin films increased with decreasing substrate temperature due to the formation of a closed surface structure and an increase in the Ag concentration (from 3.9 to 14.4%) and the film thickness (from 250 to 500 nm). The grain size of the Ag thin films decreased from ~ 300 to ~ 50 nm with decreasing substrate temperature. As the substrate temperature decreased, the surface morphology of the Ag films became homogeneous, and the surface roughness values decreased from 1.72 to 1.41 μm. An increased corrosion resistance was obtained for the temperature range from 175 to 225 K due to a decrease in the corrosion current (from 1.96×10−4 to 3.72×10−6 A, respectively) and an increase in the corrosion potential (from − 0.95 to − 0.64 V, respectively). The biological response of the Ag deposited at 125 K was higher than that of the other samples, and the antibacterial activity of this sample was ≥ 99.99% after 3 h.

Control of the pore size of honeycomb polymer film from micrometers to nanometers via substrate-temperature regulation and its application to photovoltaic and heat-resistant polymer films

Honeycomb porous polystyrene (PS) films with an aspect ratio of pore depth to pore diameter at approximately 1.0 were fabricated using the breath figure (BF) method. Two modes of water droplet coalescence in the pore growth were observed in real-time by optical microscopy. Pore size significantly increases with the increase in humidity and the decrease in substrate temperature. The porous pattern could emerge even at room temperature under high humidity of 80%. Boiling point and solvent density significantly influence the pore distribution and pore depth. Chloroform and tetrahydrofuran achieve more uniform hexagonal patterns than benzene and dichloromethane. Subsequently, to obtain nanometer porous PS film, the fast-evaporation BF process was designed by regulating the gradient substrate temperature and evaporation time, and porous mesoscopic PS film was obtained. The minimum pore diameter and corresponding pore depth are about 120 nm and 27 nm, respectively. Finally, the fast-evaporation BF process was applied to the honeycomb film formation of photovoltaic polymer poly(3-hexylthiophene) (P3HT), and the heat-resistant polymers polysulfone (PSF) and polyimide (PI).

Combined effect of relative humidity and substrate temperature on evaporation of methanol droplet

Understanding and manipulating the mechanisms involved in the evaporation of organic solvent droplets in a humid environment are of great importance for numerous industrial and biological applications. The combined effect of substrate temperature and relative humidity of the environment on evaporation of pure methanol droplets has been experimentally investigated. A drop shape analyzer was used to record the evolution of contact angle, base diameter, and volume of the droplet. At low relative humidity, the droplet experiences three stages of evaporation: constant, slightly increasing, and sharply decreasing constant angle. Substrate temperature has no effect on the qualitative behavior of the droplet at low relative humidity. At high relative humidity, droplet evolution is influenced by substrate temperature. At relatively high substrate temperature, the droplet undergoes two stages of evaporation where the contact angle increases to a maximum then decreases until the end of evaporation. At low substrate temperature, the droplet experiences a single stage where it reaches an equilibrium state with finite volume, diameter, and contact angle. In humid environment, water vapor gets adsorbed–absorbed and/or condensed on the droplet. Decreasing substrate temperature causes the temperature at the liquid–gas interface to fall below the dew point which enhances the water condensation. The evaporation rate diminishes as the relative humidity rises and its effect is more pronounced for lower substrate temperature. Relative humidity of the surroundings seems to have a negligible effect on the initial evaporation rate which is mainly attributed to the evaporation of methanol. A regime map is proposed based on the different evolution of droplets under different environmental conditions as well as substrate temperature.

Sticking behavior and transformation of tin droplets on silicon wafers and multilayer-coated mirrors

Silicon wafer and multilayer-coated mirror samples were exposed to impact of drops of molten tin to examine the adhesion behavior and cleaning possibilities. The sticking of tin droplets to horizontal substrates was examined for different surface conditions in a high vacuum chamber. Silicon wafers without a coating, with thick oxide top layer, and also with differently capped Mo/Si multilayer coatings optimized for reflection at a wavelength of 13.5 nm were exposed to tin dripping. Dependent on substrate temperature and coating, adhesion as well as detachment with self-peeling and self-contraction of spreaded drops was observed. The adhesion strength of solidified tin splats decreased strongly with decreasing substrate temperature. Non-sticking surface conditions could be generated by substrate super-cooling. The morphology of non-sticking tin droplets was analyzed by profilometry. Adhering deposits were converted in-situ via induction of tin pest by infection with gray tin powder and cooling of the samples. The phase transition was recorded by photographic imaging. It caused material embrittlement and detachment after structural transformation within several hours and enabled facile removal of tin contamination without coating damage. The temperature-dependent contamination behavior of tin drops has implications for the preferred operating conditions of extreme ultraviolet light sources with collection optics exposed to tin debris.

Wide-wavelength-range control of photoluminescence polarization in closely stacked InAs/GaAs quantum dots

We developed a method of growing closely stacked InAs/GaAs quantum dots (QDs) to control the photoluminescence (PL) polarization characteristics in a wide wavelength range. The emission wavelength of the closely stacked QDs redshifted with decreasing substrate temperature during stacking growth, while the PL polarization characteristic was controlled by the GaAs spacer layer thickness and the number of QD layers. A unified rule for the optimum GaAs spacer layer thickness that both enhances the transverse magnetic (TM)-polarized component and achieves a high PL intensity for all growth temperatures was revealed. 30-layer stacked QDs with the optimum spacer layer thickness grown at substrate temperatures from 430 to 480 °C exhibited TM-enhanced polarization characteristics in the 1.15–1.3 μm band. Moreover, we studied the one-dimensional electronic states in the closely stacked QDs with the optimized GaAs spacer layer thickness by time-resolved PL.

Electrochromic properties of rhodium oxide thin films prepared by reactive sputtering under an O2 or H2O vapor atmosphere

Rhodium (Rh) oxide thin films were prepared by reactive sputtering at different substrate temperatures under an oxygen or water (H2O) vapor atmosphere, and their crystal structure, density, optical properties, and electrochromic properties were investigated. The film density decreased with decreasing substrate temperature, and the lowest density of 4.2 g/cm3 was obtained for the film deposited at −20 °C under a H2O vapor atmosphere. The Rh oxide films deposited at −20 °C under a H2O vapor atmosphere showed anodic electrochromism between light yellow and olive i.e., bleached and colored states, respectively using an aqueous 1 M potassium hydroxide as the electrolyte; these films also exhibited the largest transmittance change, of 26%, at a wavelength of 600 nm.

The effect of fabrication conditions on 2DEGs transport characteristics at amorphous-LaAlO3/KTaO3 interfaces

Two-dimensional electron gases (2DEGs) formed at the interface between two oxide insulators present a promising platform for the exploration of emergent phenomena. While most of the previous works focused on SrTiO3-based 2DEGs, here we report on a systematic investigation of the 2DEGs at amorphous-LaAlO3/KTaO3 (a-LAO/KTO) interfaces, focusing on the effect of fabrication conditions on 2DEGs. We found that 2DEGs can be formed in a wide temperature range from room temperature to 750° under the oxygen pressure 1 × 10–4 Pa. Unexpectedly, its performance shows a unusual strong dependence on fabrication temperature: the Hall mobility increases rapidly with the decrease of substrate temperature. The highest extracted mobility of charge carriers coming from dXZ/dYZ subband is ∼6.6 × 103 cm2 V−1s−1, achieved under the condition of TS = 100 °C and PO2 = 3 × 10–5 Pa. This value is higher than that of the 2DEGs of a-LAO/SrTiO3 by a factor of 30, which reveals the unique character of the 2DEGs formed by 5d electrons. Two-band model is applied for the analysis of the transport behavior, from which information on carrier density and Hall mobility and their dependence on fabrication conditions are determined.

High-Throughput Experimental Study of Wurtzite Mn1-x Zn x O Alloys for Water Splitting Applications.

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn1–xZnxO wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn1–xZnxO thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of ZnO in MnO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn1–xZnxO alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn1–xZnxO compositions above x = 0.4. The wurtzite Mn1–xZnxO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 μA cm–2 for 673 nm-thick films. These Mn1–xZnxO films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn1–xZnxO materials with Ga dramatically increases the electrical conductivity of Mn1–xZnxO up to ∼1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott–Schottky and UPS/XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of mid-gap surface states. Overall, this study demonstrates that Mn1–xZnxO alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.

Adhesion force behaviors between two silica surfaces with varied water thin film due to substrate temperature studied by AFM

ABSTRACT The adhesion force between two silica surfaces was measured to investigate the effect of substrate temperature using an atomic force microscope (AFM) at various relative humidities (RH). Results show that the adhesion force behavior largely depends on the thickness of water thin film and surface conditions. At different RHs, the adhesion force decreases with the increase of substrate temperature, which was attributed to the decrease of water film thickness on surfaces due to dehydration and/or dehydroxylation. However, a hysteresis of adhesion force was observed by increasing and decreasing substrate temperature. When the substrate temperature returns back to a low value, the adhesion force becomes abnormally large, which was attributed to rehydroxylation, rehydration, and the evaporation of organic contaminants. At low RH (5 ± 1 %), the adhesion force also becomes abnormally large after cooling, which was attributed to the formation of siloxane bonds between the cantilever and sample surfaces. These results may help facilitate the anti-adhesion design of small-scale silicon-based systems under different conditions.

Modifying the nanostructure and the mechanical properties of Mo2BC hard coatings: Influence of substrate temperature during magnetron sputtering

A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings.

Characteristics of brush electrodeposited CuGaTe2 thin films

ABSTRACTIn this work, the brush electro deposition technique was employed for the first time to deposit CuGaTe2 (CGT) films at different substrate temperature with a deposition time was 20 min. X-ray diffraction diagram of CGT films formed at different substrate temperature were polycrystalline chalcopyrite structure. Composition of the films was determined by the EDAX attachment of the SEM. From the Atomic force microscopy images, shows that the band gap was found to slightly increase with decrease of substrate temperature, due to the small grain size. Photo electrochemical cell studies indicated the conversion efficiency of 10.82% after photoetching.

Formation of metastable cubic phase in SnS thin films fabricated by thermal evaporation

Metastable cubic SnS phase (π-SnS) is an emerging material having attractive properties for solar cell applications. In this study, we have investigated the thermal evaporation process of SnS with substrate temperatures of 250–350°C, and have observed the formation of π-SnS together with the stable orthorhombic phase (α-SnS) for the first time by using only SnS as a starting material. X-ray diffraction analysis shows that a single-phase α-SnS film is obtained at 350°C, and the fraction of π-SnS increases with decreasing the substrate temperature to 250°C. The grain shape of the SnS film grown at 250°C observed by scanning electron microscopy was more isotropic than 350°C, consistent with the cubic lattice of π-SnS. Electrical characterization by Hall measurement reveals the existence of the maximum point of carrier density as a function of substrate temperature, which is understood by the competition between the increases of crystal defects and the fraction of wider-gap π-SnS than α-SnS with decreasing substrate temperature. The formation of π-SnS brings about lowering of absorption coefficients below the absorption edge of π-SnS, which are analyzed from transmittance and reflectance spectra. The reason of π-SnS formation is discussed by comparing the conditions of film deposition with previous reports.

Preparation of SnS thin films with gear-like sheet appearance by close-spaced vacuum thermal evaporation

SnS thin films with gear-like sheet appearance were successfully prepared by close-spaced vacuum thermal evaporation using SnS powders as a source. The influence of substrate temperature on the surface morphology, chemical composition, crystal structure and optical property of SnS thin films was investigated by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction and ultraviolet–visible–near infrared spectroscopy. The results revealed that serration architecture appeared obviously in the edge of the SnS sheet and the strongest peak at 2[Formula: see text]=31.63[Formula: see text] was broadened and many shoulder peaks were observed with increasing substrate temperature. The atomic ratio of Sn to S increased from 1:1.08 to 1:1.20, the grain size became slightly smaller and the optical absorption edge had a blueshift in the SnS thin film with decreasing substrate temperature.

Frost spreading on microscale wettability/morphology patterned surfaces

Frost on a solid surface spreads essentially via building up ice bridges between condensed droplets. Modulation of condensate droplet distributions is thus an effective approach to control frost spreading. Here, we investigate the effects of both surface wettability and morphological patterns on the frost spreading velocity for various substrate surface temperatures. Our experimental results showed that the morphological patterned surfaces drastically retard frost spreading while the effect of the wettability patterned surfaces is not significant. The frost spreading velocity increases with decreasing substrate temperature on the smooth surfaces and the wettability patterned surfaces. The morphology patterned surface effectively resists frost spreading over a wide range of subcooled temperatures. A simple model is proposed to elaborate the effects of wettability, morphology, and temperature on the frost spreading velocity and the model is found to be in reasonable agreement with our experiments. Additionally, microphotography reveals ice bridging regimes in different cases. Our findings facilitate understanding of the frost spreading dynamics which can lead to the novel designs of frost-free surfaces.

Cu(In, Ga)Se2 thin film solar cells grown at low temperatures

Cu(In,Ga)Se2 (CIGS) thin film solar cells were grown on polyimide (PI) and soda lime glass (SLG) substrates at low substrate temperatures between 400°C and 500°C. Different material properties of the CIGS thin films and photovoltaic performances of the solar cells were systematically investigated. It is found that the (112), (220)/(204) and (116)/(312) peaks from X-ray diffraction (XRD) patterns show double-peak patterns as the substrate temperature decreases. The CIGS thin films grown on both PI and SLG substrates shows layered structures. The bottom and surficial layers of CIGS thin films display small size polycrystalline grains while the middle layers show large size polycrystalline grains. Both types of CIGS thin film solar cells exhibit similar efficiencies while CIGS thin film solar cells grown on PI substrates show lower open circuit voltage and fill factor but higher short circuit current density, as compared to those of CIGS thin film solar cells on SLG substrates. The highest efficiency of 6.14% has been achieved for the CIGS thin film solar cells on PI with the structure of PI/Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al grid here.