Room Temperature Substrates Research Articles

Understanding the Reactions Between Fe and Se Binary Diffusion Couples

Spurred by recent discoveries of high-temperature superconductivity in Fe–Se-based materials, the magnetic, electronic, and catalytic properties of iron chalcogenides have drawn significant attention. However, much remains to be understood about the sequence of phase formation in these systems. Here, we shed light on this issue by preparing a series of binary Fe–Se ultrathin diffusion couples via designed thin-film precursors and investigating their structural evolution as a function of composition and annealing temperature. Two previously unreported Fe–Se phases crystallized during the deposition process on a nominally room-temperature Si substrate in the 27–33 and 37–47% Fe (atomic percent) composition regimes. Both phases completely decompose after annealing to 200 °C in a nitrogen glovebox. At higher temperatures, the sequence of phase formation is governed by Se loss in the annealing process, consistent with what would be expected from the phase diagram. Films rich in Fe (53–59% Fe) crystalized during deposition as β-FeSe (P4/nmm) with preferred c-axis orientation to the amorphous SiO2 substrate surface, providing a means to nonepitaxial self-assembly of crystallographically aligned, iron-rich β-FeSe for future research. Our findings suggest that the crystallization of binary Fe–Se compounds at room temperature via near diffusionless transformations should be a significant consideration in future attempts to prepare metastable ternary and higher-order compounds containing Fe and Se.

N-fluorobenzenesulfonimide (NFSI)-mediated rapid regioselective oxyselenation of internal alkenes with diselenides

A rapid and efficient NFSI-mediated hydroxy or alkoxy-selenation of internal alkenes and chalcones is presented. This reaction proceeds smoothly at room temperature and features broad substrate scope, which employs NFSI as the oxidizing agent, affording β-hydroxy(alkoxy)selenides with excellent yields and high regioselectivity. An electrophilic mechanism is proposed for the transformation.

A facile strategy to produce monatomic tantalum metallic glass

While there has been significant pursuits and interest in single element metallic glass (MG), it has been extremely difficult to obtain. Despite the recent advances in preparing MGs, the massive production of stable monatomic MGs is of great challenge. Here, we show that Ta monatomic MG, presenting identical structural characteristics to those from liquid quenching, can be produced using conventional ion beam deposition on a room temperature substrate. The amorphous structure is retained even after being annealed at 573 K for 2 h, while it fully crystallizes subjected to annealing at 823 K for 5 min, corroborating the previous prediction that Ta MG is stable up to 600 K.

Unseeded Crystal Growth of (100)-Oriented Grain-Boundary-Free Si Thin-Film by a Single Scan of the CW-Laser Lateral Crystallization of a-Si on Insulator

Laser crystallization of a-Si film on insulating substrate is a promising technology to fabricate three-dimensional integrations (3D ICs), flat panel displays (FPDs), or flexible electronics, because the crystallization can be performed on room temperature substrate to avoid damage to the underlying devices or supporting plane. Orientation-controlled grain-boundary-free films are required to improve the uniformity in electrical characteristics of field-effect-transistors (FETs)fabricated in those films. This paper describes the recently found simple method to obtain {100}-oriented grain-boundary-free Si thin-films stably, by using a single scan of continuous-wave (CW)-laser lateral crystallization of a-Si with a highly top-flat line beam with 532 nm wavelength at room temperature in air. It was difficult to control crystal orientations in the grain-boundary-free film crystallized by the artificial modulation of solid-liquid interface, and any other trial to obtain preferential surface orientation with multiple irradiations resulted in grain boundaries. The self-organized growth of the {100}-oriented grain-boundary-free films were realized by satisfying the following conditions: (1) highly uniform top-flat line beam, (2) SiO2 cap, (3) low laser power density in the vicinity of the lateral growth threshold, and (4) single scan crystallization. Higher scan velocity makes the process window wide for the {100}-oriented grain-boundary-free film. This crystallization is very simple, because it is performed by a single unseeded scan with a line beam at room temperature substrate in air.

PbS colloidal quantum dots patterning technique with low vertical leakage current for the photodetection applications

The colloidal quantum dots (CQDs) are a promising nanometer-sized material system for optoelectronic applications due to the low cost, room temperature processing and substrate compatibility. Solution-processed technology such as spin-coating can make CQDs large-scale deposition on the substrates, whereas it inevitably brings about CQDs adherence to the edges even bottom of the substrates. As a result, the devices are not isolated and vertical leakage current occurs from the surface of CQDs to the substrates. In this work, two kinds of CQDs patterning techniques named etching-assisted patterning (EAP) and lift-off assisted patterning (LAP) have been put forward to significantly suppress the vertical current from several nanoampere to a few picoampere. Meanwhile, the CQDs photoconductor detectors have also been fabricated through these two patterning methods on the SiO2/Si substrates. Compared with un-patterning (UP) photodetectors, these two techniques both can enhance the photocurrent and promote the optoelectrical quality factors such as the responsivity and the specific detectivity, which make it possible to manufacture CQDs detector arrays and promote CQDs in optoelectronic applications.

The influence of Ar pressure on the structure and optical properties of non-hydrogenated a-Si thin films grown by rf magnetron sputtering onto room-temperature glass substrates

Seven 1.1–1.4-μm-thick hydrogen-free amorphous‑silicon (a-Si) thin films were deposited at high rates (larger than 10 Å/s), by rf magnetron sputtering (RFMS) onto room-temperature glass substrates. The influence of argon-gas pressure on the structural and optical properties of these amorphous films has been systematically studied. We have used the Wemple-DiDomenico (WD) single-effective-oscillator model, n2(ℏω) − 1 = EdE0/(E02 − (ℏω)2), in order to determine the WD parameters oscillator strength or dispersion energy, Ed, and oscillator energy, E0. The amorphous-to-crystalline mass-density ratio, ρa/ρx, in the empirical expression for Ed, proposed by Wemple, is the key factor in understanding the refractive-index behaviour of the RFMS-a-Si thin films. The value of the static refractive index, n(0), which is related to both the atomic structure and mass density, remains approximately constant up to a threshold Ar-gas pressure of nearly 1 Pa, and then it starts dropping rather abruptly. The decrease found in the Urbach energy, EU, as well as the increase observed in the Tauc gap and slope, Eg and B respectively, for Ar-gas pressures higher than the voided-structure threshold sputtering pressure, clearly suggests that the amount of intermediate-range order present within the RFMS-a-Si layers under study, is increasing in direct response to the increase in the Ar-gas pressure, during the deposition process. This is independently corroborated by the decreasing values of the Raman-intensity ratio, ITA/ITO, as the sputtering pressure increases, in the case of high Ar pressure a-Si films.

Quantifying Temperature-Dependent Substrate Loss in GaN-on-Si RF Technology

Intrinsic limits to temperature-dependent substrate loss for GaN-on-Si technology, due to the change in resistivity of the substrate with temperature, are evaluated using an experimentally validated device simulation framework. Effect of room temperature substrate resistivity on temperature-dependent coplanar waveguide (CPW) line loss at various operating frequency bands is then presented. CPW lines for GaN-on-high-resistivity Si are shown to have a pronounced temperature dependence for temperatures above 150 °C and have lower substrate losses for frequencies above the X-band. On the other hand, GaN-on-low-resistivity Si is shown to be more temperature insensitive and has lower substrate losses than even highly resistive Si for lower operating frequencies. The effect of various CPW geometries on substrate loss is also presented to generalize the discussion. These results are expected to act as a benchmark for temperature-dependent substrate loss in GaN-on-Si RF technology.

Room temperature sensitivity of ZnSe nanolayers to ethanol vapours

Layers of ZnSe (50, 110 and 160 nm thick) are deposited at room substrate temperature by thermal evaporation of ZnSe powder in vacuum. As-deposited and relaxed layers are investigated by spectroscopic ellipsometry to get information about their optical properties, porosity and surface roughness. The obtained data indicate that all layers are nanocrystalline and the lattice relaxation causes a porosity decrease and surface roughness increase. Ethanol sensing experiments are carried out at room temperature in air and results show an increase of the layers sensitivity with decreasing thickness while the relaxation causes sensitivity decrease. The sensitivity behavior is discussed considering the changes in the films porosity and the ethanol related decrease of the potential barriers at the nanocrystallite interfaces.

Performance and stability of co-evaporated vapor deposited perovskite solar cells

In this work, we report on the influence of fabrication conditions and on the performance and stability of co-evaporated vapor deposited methyl ammonium lead iodide (MAPbI3) perovskite solar cells. It is shown that as the substrate temperature increases, one has to increase the flux of MAI as indicated by higher vapour pressure in the reactor to maintain the appropriate stoichiometry of the devices. The pressure of MAI was maintained at 5.5–6 × 10−5 Torr and PbI2 rate of deposition was maintained at 0.75 A/s for a room temperature substrate deposition. For higher substrate temperature depositions at 50 °C and 75 °C we had to elevate the MAI pressure because of re-evaporation of MAI from the substrate at higher temperatures. A power conversion efficiency of > 17.4% has been achieved with a p-i-n device architecture of FTO/PTAA (polytriaryl amine)/MAPbI3/PCBM (phenyl-C61-butyric acid methyl ester)/Al through careful control of the MAI pressure and PbI2 evaporation rate. Higher substrate temperature devices showed improved grain sizes. The devices do not show any hysteresis. Devices deposited at higher substrate temperatures are significantly more photo stable over 100 h of continuous light exposure in nitrogen ambient.

Additive Manufacturing of Dielectric Microstructures

Additive, printed, 3D electrical traces are needed to achieve the desired advances in packaging. Recent advances in silver inks in some cases provide resistivities of 2× bulk silver (3.2E-6 Ohm-cm) when sintered at 120 C, and laser sintering can provide similar performance on substantially room-temperature substrates. Compared to wire bonds, printed traces have smaller heights (no loops) and the trace cross sections can be tailored to carry larger currents. In RF applications, printed traces can be shorter, have lower inductance and crosstalk, and be shaped to form substantially impedance-matched connections. Additive dielectric microstructures are often required to support the additive printing of conductors on 2.5D and 3D electronic structures. For example:A fillet, or at minimum a wall coating, is needed on the conductive sidewall of an unpackaged IC die that is to be connected to an underlying PCB by a printed trace. Large fillets (ca 350 um) are needed on typical single die, <50 um fillets on stacks of thinned die, and <20 um fillets are needed on flexible die.Filling the inter-die gap is necessary when interconnecting closely spaced die.Filling the gap between the substrate dielectric and die is needed when the die is mounted in a cavity in the substrate. Cavity mounting is sometimes used when the floor of the cavity is a thick metal layer that acts as both a ground plane and a heat sink. Dual-cure (UA and/or heat) resins are desirable for creating microstructures as the resins can be “pinned” in-situ to create structures that would normally be impossible due to the flow of the resin. A secondary heating step is normally used to cure any areas shadowed from the UV. These resins can shrink on curing by as much as a few percent causing delamination from underlying structures or stress in the final parts. Much like applying multiple thin coats of paint to avoid the cracking that results from applying one thick coat, applying multiple thin coats of dielectric with simultaneous UV or UV exposure between the coats allows the polymer to be deposited with low stress and without delamination. For example, a method for creating fillets around thick die by multiple layering has been described by Hines et al. It is also possible to create structures with low-shrinkage/CTE materials, but these materials are often compromised in other metrics. Creating micro dielectric structures by conventional syringe dispense methods is difficult due to the size of currently used dispense needles. Even the 190 um OD of a 34 ga needle, which is too small to be used with filled resins, is poorly suited to forming 45-degree fillets on a stack of 35 um-tall die. This work discusses some of the practical aspects and results of creating these micro dielectric microstructures and printed traces using Aerosol Jet® technology.

Randomly Distributed Conjugated Polymer Repeat Units for High-Efficiency Photovoltaic Materials with Enhanced Solubility and Processability.

Three structurally disordered terpolymer derivatives of PffBT4T-2OD (PCE11), prepared by replacing a varied amount of bithiophene linkers with single thiophenes, were found to exhibit reduced aggregation in solution with increasing thiophene content, while important redox and optoelectronic properties remained similar to those of PffBT4T-2OD. Solar cells based on random terpolymer-PC71BM blends exhibited average power conversion efficiencies of over 9.5% when processed with preheated substrates, with fill factors above 70%, exceeding those from PffBT4T-2OD. Thanks to increased solubility, random terpolymer devices were able to be fabricated on room-temperature substrates, reaching virtually identical performance among all three polymers despite remarkable thicknesses of ∼400 nm. Thus, we show that the random terpolymer approach is successful in improving processability while maintaining device performance.

Efficacy of Raman mapping over ellipsometric spectroscopy and XRD for characterization of structurally heterogeneous PLD nc-Si thin films

The present report demonstrates the efficacy of Raman mapping over X-Ray diffraction (XRD) and spectroscopic ellipsometry (SE) in structural characterization of nanocrystalline Silicon (nc-Si) films having spatial heterogeneity in crystallinity and structure. The Si films were deposited via pulsed-laser deposition (PLD) technique at room temperature and elevated substrate temperature (Ts) of 400 °C and 700 °C, under vacuum (∼10−6 mbar). XRD showed that all films were polycrystalline while SE studies showed that only at 700 °C, the film was nanocrystalline in nature otherwise largely amorphous at lower Ts. This discrepancy in results is well explained by Raman maps which confirmed the presence of micron sized structures composed of nc-Si embedded in otherwise uniform matrix comprised of amorphous Silicon (a-Si) for films fabricated at room temperature and 400 °C whereas at 700 °C the uniform background matrix is dominated by nc-Si. The real and imaginary parts of pseudo dielectric function (ε1,ε2) versus energy spectra obtained for films grown at RT, 400 °C, and 700 °C were modeled and fitted to substrate/film/top SiO2 layered structure. The simulated (ε1,ε2) patterns are in excellent agreement with the recorded spectra. From the SE studies the band gap and thickness of the films were estimated. Band gap energy was found to vary from 1.35 to 1.30 eV approximately with increasing Ts from room temperature to 400 °C, then gradually increases to 1.55 eV at 700 °C. The variation of other optical constants of the films as a function of Ts also reflects the structural transition from amorphous-like to nanocrystalline-like with increasing Ts.

Facile photochemical synthesis of α-ketoamides and quinoxalines from amines and benzoylacetonitrile under mild conditions

A selective protocol for the synthesis of either α-ketoamides or quinoxaline derivatives under the same reaction conditions has been achieved simply by varying substitution number of amino-groups. The method features metal-free, room temperature and broad substrate scopes as well as no extra oxidant. This process applies to various substituent groups and gives products in moderate to good yield. Finally, a rational mechanism was proposed.

Joint improvement of conductivity and Seebeck coefficient in the ZnO:Al thermoelectric films by tuning the diffusion of Au layer

Joint improvement of the Seebeck coefficient S and conductivity σ is a pressing issue to overcome the limit of the figure of merit (ZT) enhancement of thermoelectric materials. This study presents a joint improvement in the ZnO:Al films by tuning the diffusion of Au layer via substrate temperature, using a radio frequency-assisted molecular beam vapor deposition method. The phenomenon occurs in the ZnO:Al films at the room temperature (RT) substrate deposited on a 10 nm Au layer (RT-ZnO:AuAl). The films have a hexagonal wurtzite structure with an in-plane (002) preferred orientation and smooth surface. The resistivity values of the films are at the same magnitude of the transparent conducting oxide (~10−6 Ω·m). The carrier concentration reaches 2.60 × 1021 cm−3 for the RT-ZnO:AuAl film. The S is about 29.5 μV/K at 240 °C. Joint improvement of conductivity and Seebeck coefficient makes the PF of the RT-ZnO:AuAl film reaches 3.58 × 10−5 Wm−1 K−2, which is two orders of magnitude larger than that of the RT-ZnO:Au film without joint improvement. The results indicate that the higher conductivity originates from the Au layer, in-plane (002) preferred orientation, and the easily excited carriers in the ZnO:Al due to the low binding energy of the Zn 2p electrons. The large S in the film is related to the high effective masses due to the impact of the diffusion of the Au layer on electronic interactions.

Cu content dependence of Cu2Zn(SnGe)Se4 solar cells prepared by using sequential thermal evaporation technique of Cu/Sn/Cu/Zn/Ge stacked layers

The doping of Cu2ZnSnSe4 semiconductor with Ge element has demonstrated improvements to kesterite solar cell efficiency. However, the impact of different Cu concentrations on Cu2ZnSnGeSe4/CdS solar cell performance has been poorly studied. In this work, Cu2ZnSnGeSe4 thin films with different Cu contents were synthesized by selenization of sequential thermal evaporation precursors. Solar cells based on kesterite-type Cu2ZnSnGeSe4 (CZTGSe) were fabricated and the influence of the Cu thickness on the chemical composition and morphology of the layers and electro-optical properties of solar cells was studied. The stacking process was performed at room substrate temperature. Efficiency values in the range of 2.0–6.8% are reported as a function of Cu concentration. The highest efficiency of 6.8%, was achieved for solar cell with glass/Mo/CZTGSe/CdS/i-ZnO/ITO structure using the stacking of Cu (3 nm)/Sn (248 nm)/Cu (112 nm)/Zn (174 nm)/Ge (20 nm).

18F]ODIBO: a prosthetic group for bioorthogonal radiolabeling of macromolecules via strain-promoted alkyne-azide cycloaddition.

A novel prosthetic group for the efficient radiolabeling of macromolecules has been developed. [18F]oxadibenzocyclooctyne ([18F]ODIBO) is synthesized in high radiochemical yield and applied for nearly quantitative conjugation to azide-tagged peptides and proteins at room temperature and low substrate concentrations. The resulting bioconjugates are chemically and radiochemically pure and free of toxic solvents and catalysts.

Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy by Fine Particle Peening

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles can form a HAp layer on room-temperature substrates by the transfer and microstructural modification of the shot particles. In this study, FPP with HAp shot particles was applied to form a HAp surface layer and improve the fatigue properties of Ti–6Al–4V extra-low interstitial (ELI) for use in bio-implants. The surface microstructures of the FPP-treated specimens were characterized by micro-Vickers hardness testing, scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. FPP with HAp shot particles successfully formed a HAp layer on the surface of Ti–6Al–4V ELI in a relatively short period by shot particle transfer at room temperature; however, the thickness and elemental composition of the HAp layer were independent of the FPP treatment time. The original HAp crystal structure remained in the surface-modified layer formed on Ti–6Al–4V ELI after FPP. Furthermore, FPP increased the surface hardness and generated compressive residual stresses at the treated surface of Ti–6Al–4V ELI. Four-point bending fatigue tests were performed at stress ratios of 0.1 and 0.5 to examine the effect of FPP with HAp shot particles on the fatigue properties of Ti–6Al–4V ELI. The fatigue life of the FPP-treated specimen was longer than that of the un-peened specimen because of the formation of a work-hardened layer with compressive residual stress. However, no clear improvement in the fatigue limit of Ti–6Al–4V ELI occurred after FPP with HAp shot particles because of subsurface failures from characteristic facets.

Metal-Free Synthesis of Indoles from Arylhydrazines and Nitroalkenes at Room Temperature

A metal-free cyclocondensation reaction between arylhydrazines and nitroalkenes has been developed for the synthesis of substituted indoles using a combination of I2 (20 mol%) and diethyl phosphite (2 equiv.) at room temperature. Both N-protected and unprotected indoles were obtained from a variety of easily accessible nitroalkenes in good yields. The Medicinally important tetrahydrocarbazole scaffolds were also obtained by employing cyclic nitroalkenes using this strategy. Gram-scale preparation of indole derivative has been successfully demonstrated using this methodology. The reaction likely proceeds through [3, 3]-sigmatropic rearrangements of hydrazone intermediate. Use of safe catalyst, metal and solvent free conditions, room temperature and broad substrate scope have made this indolization protocol advantageous to access these compounds.

High Vacuum Deposition of Biferrocene Thin Films on Room Temperature Substrates

High Vacuum Deposition of Biferrocene Thin Films on Room Temperature Substrates

Nanoscale structure of submicron-thick sputter-deposited Pd films: Effect of the adatoms diffusivity by the film-substrate interaction

Pd films were deposited, at room temperature, on Ti and SiO2 substrates by normal (0°) and oblique (36° and 48°) incidence sputtering processes. Scanning electron microscopy was used to analyze the surface and cross morphology of the Pd films so to study the effect of the Pd adatoms surface diffusivity on the films nanostructure. In fact, even if the low homologous temperature (0.17) prescribes that the Pd film grows on both substrates in the Zone I of the structure zone model, some differences are observed: in normal incidence condition, the Pd film on the Ti substrate was found to be formed by tapered grains, while on the SiO2 substrate the Pd film exhibited a columnar morphology with the columns showing a cauliflower shape. Also in the oblique deposition configurations, notable differences in the nanoscale morphology of the Pd films grown in the same conditions are observed on the basis of the supporting substrate. These data suggest that, during depositions, the Pd adatoms diffusivity on the substrate, as determined by the film-substrate interaction both in normal and oblique deposition conditions, is crucial in determining the final film nanostructure. This aspect is semi-quantitatively discussed on the basis of the adsorption energy of the Pd atoms on Ti (0.69eV) and on SiO2 (0.27eV).