Advances in laser-assisted conversion of polymeric and graphitic carbon into nanodiamond films.

Nanodiamond (ND) synthesis by nanosecond laser irradiation has sparked tremendous scientific and technological interest. This review describes efforts to obtain cost-effective ND synthesis from polymers and carbon nanotubes (CNT) by the melting route. For polymers, ultraviolet (UV) irradiation triggers intricate photothermal and photochemical processes that result in photochemical degradation, subsequently generating an amorphous carbon film; this process is followed by melting and undercooling of the carbon film at rates exceeding 109K s-1. Multiple laser shots increase the absorption coefficient of PTFE, resulting in the growth of 〈110〉 oriented ND film. Multiple laser shots on CNTs result in pseudo topotactic diamond growth to form a diamond fiber. This technique is useful for fabricating 4-50 nm sized NDs. These NDs can further be employed as seed materials that are used in bulk epitaxial growth of microdiamonds using chemical vapor deposition, particularly for use with non-lattice matched substrates that formerly did not form continuous and adherent films. We also provide insights into biocompatible precursors for ND synthesis such as polybenzimidazole fiber. ND fabrication by UV irradiation of graphitic and polymeric carbon opens up a pathway for preparing selective coatings of polymer-diamond composites, doped nanodiamonds, and graphene composites for quantum computing and biomedical applications.

Formation and characterization of nano- and microstructures of twinned cubic boron nitride.

Nano- and microstructures of phase-pure cubic boron nitride (c-BN) are synthesized by employing nanosecond pulsed-laser annealing techniques at room temperature and atmospheric pressure. In a highly non-equilibrium synthesis process, nanocrystalline h-BN is directly converted into phase-pure twinned c-BN from a highly undercooled melt state of BN. By changing nucleation and growth rates, we have synthesized a wide range of sizes (90 nm to 25 µm) of c-BN. The electron diffraction patterns show the formation of twinned c-BN with [11[combining macron]1] as the twin axis. The twinning density in c-BN can be controlled by the degree of undercooling and quenching rates. The formation of twins predominantly occurs prior to the formation of amorphous quenched BN (Q-BN). Therefore, the defect density in nano c-BN formed under higher undercooling conditions is considerably larger than that in micro c-BN, which is formed under lower undercooling conditions. The temperature-dependent Raman studies show a considerable blue-shift of ∼6 cm-1 with a decrease in temperature from 300 to 78 K in nano c-BN as compared to micro c-BN. The size-effects of c-BN crystals in Raman spectra are modeled using spatial correlation theory, which can be used to calculate the correlation length and twin density in c-BN. It has also been found that the Raman blue-shift in nano c-BN is caused by anharmonic effects, and the decrease in Raman linewidth with decreasing temperature (300 to 78 K) is caused by three- and four-phonon decay processes. The bonding characteristics and crystalline nature of the synthesized c-BN are also demonstrated by using electron energy-loss spectroscopy and electron backscatter diffraction, respectively. We envisage that the controlled growth of phase-pure nano and microstructures of twinned c-BN and their temperature-dependent Raman-active vibrational mode studies will have a tremendous impact on low-temperature solid-state electrical and mechanical devices.

Direct conversion of carbon nanofibers into diamond nanofibers using nanosecond pulsed laser annealing.

Here, we show the direct conversion of carbon nanofibers (CNFs) into diamond nanofibers (DNFs) by irradiating CNFs with an ArF nanosecond laser at room temperature and atmospheric pressure. The nanosecond laser pulses melt the tips of CNFs into a highly undercooled state, and their subsequent quenching results in the formation of DNFs. This formation of DNFs is dependent on the degree of undercooling which is controlled by nanosecond laser energy density and one-dimensional heat flow characteristics in CNFs. The conversion process starts at the top and extends with the number of pulses. Therefore, our highly non-equilibrium nanosecond laser processing opens a new avenue for the synthesis of exciting pure and doped diamond structures at ambient temperatures and pressures for a variety of applications.

Enhanced mechanical properties of Q-carbon nanocomposites by nanosecond pulsed laser annealing.

Q-carbon is a metastable phase of carbon formed by melting and subsequently quenching amorphous carbon films by a nanosecond laser in a super undercooled state. As Q-carbon is a material harder than diamond, it makes an excellent reinforcing component inside the softer matrix of a composite coating. In this report, we present a single-step strategy to fabricate adherent coatings of hard and lubricating Q-carbon nanocomposites. These nanocomposites consist of densely-packed sp 3-rich Q-carbon (82% sp 3), and sp 2-rich α-carbon (40% sp 3) amorphous phases. The nanoindentation tests show that the Q-carbon nanocomposites exhibit a hardness of 67 GPa (Young's modulus âˆ¼ 840 GPa) in contrast to the soft α-carbon (hardness âˆ¼ 18 GPa). The high hardness of Q-carbon nanocomposites results in 0.16 energy dispersion coefficient, in comparison with 0.74 for α-carbon. The soft α-carbon phase provides lubrication, resulting in low friction and wear coefficients of 0.09 and 1 × 10-6, respectively, against the diamond tip. The nanoscale dispersion of hard Q-carbon and soft α-carbon phases in the Q-carbon nanocomposites enhances the toughness of the coatings. We present detailed structure-property correlations to understand enhancement in the mechanical properties of Q-carbon nanocomposites. This work provides insights into the characteristics of Q-carbon nanocomposites and advances carbon-based superhard materials for longer lasting protective coatings and related applications.

Role of Vitamin-D in the prevention and treatment of Alzheimer's disease.

Alzheimer's disease is the most common form of age related cognitive impairment. Aim of the present study was to see the effect of vitamin D on cognitive function in elderly. The study was conducted in Department of Physiology, King George's Medical University (KGMU). A total of 80 subjects were enrolled based on Mini Mental State Examination (MMSE) score < 24 and vitamin D deficiency. They were divided into two groups as Group A (case) and Group B (control), each group having 40 subjects. Intervention (Vitamin D supplementation) was given in Group A. The assessment of dementia was done by Mini Mental State Examination (MMSE) score. Every subject was clinically evaluated and estimation of vitamin D was done by direct ELISA kit. Gender, weight, height, BMI, residence and education were also similar between two groups. A significant (p=0.0001) change in MMSE score was observed in both Group A and Group B from baseline to 3 & 6 months and from 3 to 6 months, however, mean change was higher in Group A than Group B. In conclusion, vitamin D supplementation caused significant improvement in the cognitive performance in subjects with senile dementia.

Correlation of serum uric acid levels with certain anthropometric parameters in prediabetic and drug-naive diabetic subjects.

Introduction: Uric acid is produced during the metabolism of nucleotide and adenosine triphosphate and contains the final product of human purine metabolism. It acts both as an antioxidant and pro-inflammatory marker and has a positive association with visceral fat in overweight subjects. The aim of the present study is to find an association of uric acid level with certain anthropometric parameters in subjects having type 2 diabetes. Materials and Methods: The study included 124 urban drug-naive diabetic Indian subjects above 18 years of age from the general population of the city of North India. Uric acid concentrations were estimated by the uricase method. Fasting plasma glucose (FPG) concentrations were estimated by the glucose oxidase-peroxidase method. Anthropometric measurements and information on lifestyle factors and disease history were collected through in-person meeting. Results: All participants of the study subjects had a body mass index (BMI) of more than 23.5. BMI, waist-to-hip ratio (WHR), waist-to-height ratio, waist circumference, neck circumference, weight, age, sagittal abdominal diameter (SAD), skinfold thickness, and body roundness index were positively correlated with the serum uric acid level. The correlation of weight, BMI, SAD, and WHR was statistically significant. Conclusion: We found that serum uric acid level increases as body fat content increases. Statistical data show remarkable results for a significant correlation of uric acid level with BMI, WHR, SAD, and FPG. Hypertrophy occurs as a result of inflammatory processes and oxidative stress when the supply of energy starts to exceed the storage capacity of adipocytes, as a result, adipokines such as interleukin (IL)-1, IL-6, and tumor-necrosis factor-alpha are released more frequently which lead to low-grade chronic inflammation. Uric acid levels are much lean toward visceral obesity than overall body fat content.

Résumé Introduction: L'acide urique est produit lors du métabolisme des nucléotides et de l'adénosine triphosphate, et il représente le produit final du métabolisme des purines chez l'homme. Il agit à la fois comme un antioxydant et un marqueur pro-inflammatoire, et il est positivement associé à la graisse viscérale chez les sujets en surpoids. L'objectif de la présente étude est de rechercher une association entre le taux d'acide urique et certains paramètres anthropométriques chez des sujets atteints de diabète de type 2. Matériels et méthodes: L'étude a inclus 124 sujets diabétiques urbains indiens, naïfs aux médicaments, âgés de plus de 18 ans, issus de la population générale de la ville du nord de l'Inde. Les concentrations d'acide urique ont été estimées par la méthode de l'uricase. Les concentrations de glucose plasmatique à jeun (FPG) ont été estimées par la méthode glucose oxydase-peroxydase. Les mesures anthropométriques et les informations sur les facteurs de mode de vie et les antécédents médicaux ont été recueillies lors de rencontres en personne. Résultats: Tous les participants de l'étude présentaient un indice de masse corporelle (IMC) supérieur à 23,5. L'IMC, le rapport taille-hanche (WHR), le rapport taille-hauteur, la circonférence de taille, la circonférence du cou, le poids, l'âge, le diamètre abdominal sagittal (SAD), l'épaisseur des plis cutanés et l'indice de rondeur corporelle étaient corrélés positivement avec le taux d'acide urique sérique. La corrélation du poids, de l'IMC, du SAD et du WHR était statistiquement significative. Conclusion: Nous avons constaté que le taux d'acide urique sérique augmente avec l'augmentation de la teneur en graisse corporelle. Les données statistiques montrent des résultats remarquables pour une corrélation significative du taux d'acide urique avec l'IMC, le WHR, le SAD et le FPG. L'hypertrophie se produit en raison de processus inflammatoires et de stress oxydatif lorsque l'apport d'énergie dépasse la capacité de stockage des adipocytes. Par conséquent, des adipokines telles que l'interleukine (IL)-1, l'IL-6 et le facteur alpha de nécrose tumorale sont libérées plus fréquemment, ce qui entraîne une inflammation chronique de bas grade. Les niveaux d'acide urique sont davantage associés à l'obésité viscérale qu'à la teneur globale en graisse corporelle. Mots-clés: Anthropométrique, syndrome métabolique, microalbuminurie, acide urique sérique.

Association of stress and primary hypothyroidism.

Background: Hypothyroidism is the result of impaired production and secretion of thyroid hormones. The cardiovascular system is affected by fluctuations in thyroid hormone levels. Stressful events or stressors can affect the hypothalamic-pituitary-thyroid (HPT) axis and psychological and physiological responses. Stress increases thyroid hormone levels while decreasing TSH levels, which exacerbates autoimmune thyroid disease. Aim: To evaluate the relationship between stress and primary hypothyroidism. Methods: A total of 77 newly diagnosed hypothyroid patients (TSH >5.0 mIU/L) and 77 healthy adults (TSH 0.5-5.0 mIU/L) were enrolled. During a brief general physical examination, the following values were measured: height, weight, blood pressure, pulse, and pulse rate. A brief systemic examination of the cardiovascular system and lungs was also performed to rule out systemic diseases. Heart rate variability (HRV) processing and analysis were performed using Pro LabChart (PowerLab 8Pro) data analysis software from AD Instrument. Results: Mean Avg. RR was significantly higher, RM SSD and pRR50 were significantly lower in cases than in controls. Mean HF was significantly lower and LF/HF (frequency range) was significantly higher in cases than in controls. Mean PSS was significantly higher in cases (25.82 ± 2.83) than in controls (22.47 ± 2.10). The majority of cases (54.5%) had a high stress level. The TSH level showed a highly significant correlation with the LF/HF ratio and with the PSS score. Conclusion: The mean Avg. RR and HF were significantly higher, RM SSD and pRR50 and LF/HF (frequency range) were significantly lower in hypothyroid patients.

Laser-Assisted Surface Lithium Fluoride Decoration of a Cobalt-Free High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode for Long-Life Lithium-Ion Batteries.

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is a promising next-generation cathode material due to its structural stability, high operation voltage, and low cost. However, the cycle life of LNMO cells is compromised by detrimental electrode-electrolyte reactions, chemical crossover, and rapid anode degradation. Here, we demonstrate that the cycling stability of LNMO can be effectively enhanced by a high-energy laser treatment. Advanced characterizations unveil that the laser treatment induces partial decomposition of the polyvinylidene fluoride binder and formation of a surface LiF phase, which mitigates electrode-electrolyte side reactions and reduces the generation of dissolved transition-metal ions and acidic crossover species. As a result, the solid electrolyte interphase of the graphite counter electrode is thin and is composed of fewer electrolyte decomposition products. This work demonstrates the potential of laser treatment in tuning the surface chemistry of cathode materials for lithium-ion batteries.

Discovery of Double Helix and Impact on Nanoscale to Mesoscale Crystalline Structures.

Screw dislocations play a significant role in the growth of crystalline structures by providing a continuous source of surface steps which represent available sites for crystal growth. Here, we show that pure screw dislocations can become helical from the absorption of defects (e.g., vacancies) and develop an attractive interaction with another helical dislocation to form a double helix of screw dislocations. These single and double helices of screw dislocations can result in the formation of interesting nanostructures with large Eshelby twists. We have previously proposed the formation of a double helix of screw dislocations to explain large Eshelby twists in crystalline nanostructures (Mater. Res. Lett.2021, 9, 453-457). We now show direct evidence for the formation of a double helix during thermal annealing of screw dislocations. The large Burgers vectors associated with these dislocations are used to explain the presence of large Eshelby twists in PbSe and PbS (NaCl cubic structure) and InP and GeS (wurtzite hexagonal structure) nanowires. These single- and double-helix screw dislocations can also combine to create even larger super Burgers vectors. These large effective Burgers also unravel the mechanism for the formation of nanopipes and micropipes with hollow cores and nanotubes with Eshelby twists in technologically important materials such as SiC, GaN, and ZnO that are utilized in a variety of advanced solid-state devices.

Fabricating Graphene Oxide/h-BN Metal Insulator Semiconductor Diodes by Nanosecond Laser Irradiation.

To employ graphene's rapid conduction in 2D devices, a heterostructure with a broad bandgap dielectric that is free of traps is required. Within this paradigm, h-BN is a good candidate because of its graphene-like structure and ultrawide bandgap. We show how to make such a heterostructure by irradiating alternating layers of a-C and a-BN film with a nanosecond excimer laser, melting and zone-refining constituent layers in the process. With Raman spectroscopy and ToF-SIMS analyses, we demonstrate this localized zone-refining into phase-pure h-BN and rGO films with distinct Raman vibrational modes and SIMS profile flattening after laser irradiation. Furthermore, in comparing laser-irradiated rGO-Si MS and rGO/h-BN/Si MIS diodes, the MIS diodes exhibit an increased turn-on voltage (4.4 V) and low leakage current. The MIS diode I-V characteristics reveal direct tunneling conduction under low bias and Fowler-Nordheim tunneling in the high-voltage regime, turning the MIS diode ON with improved rectification and current flow. This study sheds light on the nonequilibrium approaches to engineering h-BN and graphene heterostructures for ultrathin field effect transistor device development.

Excimer Laser Patterned Holey Graphene Oxide Films for Nonenzymatic Electrochemical Sensing.

The existence of point defects, holes, and corrugations (macroscopic defects) induces high catalytic potential in graphene and its derivatives. We report a systematic approach for microscopic and macroscopic defect density optimization in excimer laser-induced reduced graphene oxide by varying the laser energy density and pulse number to achieve a record detection limit of 7.15 nM for peroxide sensing. A quantitative estimation of point defect densities was obtained using Raman spectroscopy and confirmed with electrochemical sensing measurements. Laser annealing (LA) at 0.6 J cm-2 led to the formation of highly reduced graphene oxide (GO) by liquid-phase regrowth of molten carbon with the presence of dangling bonds, making it catalytically active. Hall-effect measurements yielded a mobility of ∼200 cm2 V-1 s-1. An additional increase in the number of pulses at 0.6 J cm-2 resulted in deoxygenation through the solid-state route, leading to the formation of holey graphene structure. The average hole size showed a hierarchical increase, with the number of pulses characterized with multiple microscopy techniques, including scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The exposure of edge sites due to high hole density after 10 pulses supported the formation of proximal diffusion layers, which led to facile mass transfer and improvement in the detection limit from 25.4 mM to 7.15 nM for peroxide sensing. However, LA at 1 J cm-2 with 1 pulse resulted in a high melt lifetime of molten carbon and the formation of GO characterized by a high resistivity of 3 × 10-2 Ω-cm, which was not ideal for sensing applications. The rapid thermal annealing technique using a batch furnace to generate holey graphene results in structure with uneven hole sizes. However, holey graphene formation using the LA technique is scalable with better control over hole size and density. This study will pave the path for cost-efficient and high-performance holey graphene sensors for advanced sensing applications.

Enhanced Vapor Transmission Barrier Properties via Silicon-Incorporated Diamond-Like Carbon Coating.

In this study, we describe reducing the moisture vapor transmission through a commercial polymer bag material using a silicon-incorporated diamond-like carbon (Si-DLC) coating that was deposited using plasma-enhanced chemical vapor deposition. The structure of the Si-DLC coating was analyzed using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, selective area electron diffraction, and electron energy loss spectroscopy. Moisture vapor transmission rate (MVTR) testing was used to understand the moisture transmission barrier properties of Si-DLC-coated polymer bag material; the MVTR values decreased from 10.10 g/m2 24 h for the as-received polymer bag material to 6.31 g/m2 24 h for the Si-DLC-coated polymer bag material. Water stability tests were conducted to understand the resistance of the Si-DLC coatings toward moisture; the results confirmed the stability of Si-DLC coatings in contact with water up to 100 °C for 4 h. A peel-off adhesion test using scotch tape indicated that the good adhesion of the Si-DLC film to the substrate was preserved in contact with water up to 100 °C for 4 h.

Variation in response to experimental pain across the menstrual cycle in women compared with one month respose in men.

Gender differences in pain perception have been reported in literature. However, most such studies have ignored the role of female sex hormones in influencing pain response across menstrual cycle (MC). In this study, we have investigated the variation in pain response on different days of the menstrual cycle. Ninety subjects (60 females) were subjected to experimental pain of cold pressor task, on days 1, 7, 14, and 21 of the MC (females), and on four consecutive Mondays of a month (males). Male subjects showed no variation in pain response. Females reported higher pain sensitivity on days 7 and 14 of MC. We suggest that experimental pain studies involving female menstruating subjects should be carried out only during a particular phase of the cycle, and this phase should be reported in literature to increase the reproducibility of the experiment.

Nonequilibrium Structural Evolution of Q-Carbon and Interfaces.

Q-carbon is a densely packed metastable phase of carbon formed by ultrafast quenching of carbon melt in a super-undercooled state. After quenching, diamond tetrahedra are randomly packed with >80% packing efficiency. This discovery has opened a pathway to fabricate various interesting heterostructures following the highly nonequilibrium route of nanosecond pulsed laser annealing. In the present work, we demonstrate the evolution of Q-carbon/α-carbon and Q-carbon/diamond heterostructures with atomically sharp interfaces, controlled via varying solidification rates of the undercooled C melt. This structure consists of ultrahard Q-carbon (∼80% sp3 and rest sp2) with an overlayer of soft α-carbon (∼40% sp3) on the inert c-Al2O3 substrate. Using high-resolution scanning transmission electron microscopy and Raman spectroscopy analysis, we present the formation of the highly dense Q-carbon/α-carbon bilayer structure with distinctly different atomic and electronic structures. The laser-solid interaction simulations coupled with atomistic ab initio modeling further confirm the conversion of C melt into Q-carbon by achieving maximum undercooling near the substrate and further into α-carbon with a decrease in regrowth velocity (<6 m/s) away from the substrate. We present details of the evolution of heterointerfaces formed from carbon melt for designing heterostructures far from equilibrium for various functional applications by using pulsed laser processing.

Structure-property correlations in phase-pure B-doped Q-carbon high-temperature superconductor with a record Tc = 55 K.

Here, we report the detailed structure-property correlations in phase-pure B-doped Q-carbon high-temperature superconductor having a superconducting transition temperature (Tc) of 55 K. This superconducting phase is a result of nanosecond laser melting and subsequent quenching of a highly super undercooled state of molten B-doped C. The temperature-dependent resistivity in different magnetic fields and magnetic susceptibility measurements indicate a type-II Bardeen-Cooper-Schrieffer superconductivity in B-doped Q-carbon thin films. The magnetic measurements indicate that the upper and lower critical fields follow Hc2(0)[1 - (T/Tc)1.77] and Hc1(0)[1 - (T/Tc)1.19] temperature dependence, respectively. The structure-property characterization of B-doped Q-carbon indicates a high density of electronic states near the Fermi-level and large electron-phonon coupling. These factors are responsible for s-wave bulk type superconductivity with enhanced Tc in B-doped Q-carbon. The time-dependent magnetic moment measurements indicate that B-doped Q-carbon thin films follow the Anderson-Kim logarithmic decay model having high values of pinning potential at low temperatures. The crossover from the two-dimensional to the three-dimensional nature of Cooper pair transport at T/Tc = 1.02 also indicates a high value of electron-phonon coupling which is also calculated using the McMillan formula. The superconducting region in B-doped Q-carbon is enclosed by Tc = 55.0 K, Jc = 5.0 × 108 A cm-2, and Hc2 = 9.75 T superconducting parameters. The high values of critical current density and pinning potential also indicate that B-doped Q-carbon can be used for persistent mode of operation in MRI and NMR applications. The Cooper pairs which are responsible for the high-temperature superconductivity are formed when B exists in the sp3 sites of C. The electron energy loss spectroscopy and Raman spectroscopy indicate a 75% sp3 bonded C and 70% sp3 bonded B in the superconducting phase of B-doped Q-carbon which has 27 at% B and rest C. The dimensional fluctuation and magnetic relaxation measurements in B-doped Q-carbon indicate its practical applications in frictionless motors and high-speed electronics. This discovery of high-temperature superconductivity in strongly-bonded and light-weight materials using non-equilibrium synthesis will provide the pathway to achieve room-temperature superconductivity.

Reduced Graphene Oxide/Amorphous Carbon P-N Junctions: Nanosecond Laser Patterning.

The device integration of graphene and reduced graphene oxide (rGO) is impeded by scalability and high temperature (>2000 K) treatment required for effective reduction into high-quality rGO. In this article, we present a novel approach for direct laser writing of heavily reduced graphene oxide films by nanosecond laser melting of amorphous carbon on silicon (001) substrates under ambient conditions. Ultrafast quenching from the undercooled melt state above the melting threshold energy density (Ed) of 0.4 J/cm2 leads to the formation of large-area rGO films. The first-order phase transformation of liquid carbon into graphene is triggered by low undercooling at the C melt/silicon interface. The laser-irradiated rGO films exhibit electron mobility of 12.56 cm2/V s and charge carrier concentration of -1.2 × 1021/cm3 at 300 K. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest low disorder and charge transport via 2D Mott variable range hopping between the graphene islands for rGO films. The localization length corresponding to the size of these graphitic domains is 3 nm. The ultrafast regrowth of rGO creates an atomically sharp interface between n-type rGO and p-type amorphous carbon, resulting in p-n junction heterojunction diodes with a turn-on voltage of 0.3 V, rectification ratio of 110@±1.5 V, and activation energy of 0.13 eV under reverse bias. This unique laser processing method solves the problems of traps and defects associated with equilibrium-based rGO fabrication methods, enabling high conductivity and mobility, providing insights into the fundamental mechanism driving laser writing of graphene-based materials on silicon.

Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO2 Thin Films.

Vanadium dioxide (VO2) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO2 thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO2 films were grown on NiO(111)/Al2O3(0001) pseudomorphically, where the large misfit between NiO and Al2O3 were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies ( x ∼ 0.20 ± 0.02) into the VO2- x system via annealing under high vacuum (∼5 × 10-7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V4+ to V3+ and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO2 films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO2- x thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO2.

Electrical Transition in Isostructural VO2 Thin-Film Heterostructures.

Control over the concurrent occurrence of structural (monoclinic to tetragonal) and electrical (insulator to the conductor) transitions presents a formidable challenge for VO2-based thin film devices. Speed, lifetime, and reliability of these devices can be significantly improved by utilizing solely electrical transition while eliminating structural transition. We design a novel strain-stabilized isostructural VO2 epitaxial thin-film system where the electrical transition occurs without any observable structural transition. The thin-film heterostructures with a completely relaxed NiO buffer layer have been synthesized allowing complete control over strains in VO2 films. The strain trapping in VO2 thin films occurs below a critical thickness by arresting the formation of misfit dislocations. We discover the structural pinning of the monoclinic phase in (10 ± 1 nm) epitaxial VO2 films due to bandgap changes throughout the whole temperature regime as the insulator-to-metal transition occurs. Using density functional theory, we calculate that the strain in monoclinic structure reduces the difference between long and short V-V bond-lengths (ΔV-V) in monoclinic structures which leads to a systematic decrease in the electronic bandgap of VO2. This decrease in bandgap is additionally attributed to ferromagnetic ordering in the monoclinic phase to facilitate a Mott insulator without going through the structural transition.

Magnetic relaxation and three-dimensional critical fluctuations in B-doped Q-carbon - a high-temperature superconductor.

Dimensional fluctuations and magnetic relaxations in high-temperature superconductors are key considerations for practical applications in high-speed electronic devices. We report the creep of trapped magnetic flux and three-dimensional critical fluctuations near the superconducting transition temperature (Tc = 36 K) in B-doped amorphous Q-carbon. The superconducting phase in B-doped Q-carbon is formed by nanosecond pulsed laser melting in a super undercooled state followed by subsequent quenching. Time-dependent magnetic moment measurements in the B-doped Q-carbon follow the Anderson-Kim logarithmic decay model with the calculated value of pinning potential to be 0.75 eV at 1 T near Tc. There is also strong evidence of three-dimensional (3D) critical fluctuations near Tc in B-doped Q-carbon. The crossover from 2D to 3D critical fluctuations is seen at T/Tc = 1.01 as compared to T/Tc = 1.11 in conventional Bardeen-Cooper-Schrieffer (BCS) high-temperature superconductors. These critical fluctuations indicate moderate to strong electron-phonon coupling in B-doped Q-carbon. The isomagnetic temperature-dependent resistivity measurements reveal a broadening of superconducting transition width with increasing magnetic field. The upper critical field (Hc2(0)) is calculated to be 5.6 T using the power law. Finally, the superconducting region is determined in B-doped Q-carbon, as the three vertices of the superconducting region are calculated as Tc = 36.0 K, Jc = 2.9 × 109 A cm-2 and Hc2 = 5.6 T. The temperature-dependent magnetic moment and resistivity measurements also validate B-doped Q-carbon as a BCS type-II superconductor. B concentration in Q-carbon can be increased up to 50 at% by a nanosecond laser melting and quenching technique, thus providing an ideal platform for near room-temperature superconductivity.

An optimized sample preparation approach for atomic resolution in situ studies of thin films.

This work provides the details of a simple and reliable method with less damage to prepare electron transparent samples for in situ studies in scanning/transmission electron microscopy. In this study, we use epitaxial VO2 thin films grown on c-Al2 O3 by pulsed laser deposition, which have a monoclinic-rutile transition at ~68°C. We employ an approach combining conventional mechanical wedge-polishing and Focused Ion beam to prepare the electron transparent samples of epitaxial VO2 thin films. The samples are first mechanically wedge-polished and ion-milled to be electron transparent. Subsequently, the thin region of VO2 films are separated from the rest of the polished sample using a focused ion beam and transferred to the in situ electron microscopy test stage. As a critical step, carbon nanotubes are used as connectors to the manipulator needle for a soft transfer process. This is done to avoid shattering of the brittle substrate film on the in situ sample support stage during the transfer process. We finally present the atomically resolved structural transition in VO2 films using this technique. This approach significantly increases the success rate of high-quality sample preparation with less damage for in situ studies of thin films and reduces the cost and instrumental/user errors associated with other techniques. The present work highlights a novel, simple, reliable approach with reduced damage to make electron transparent samples for atomic-scale insights of temperature-dependent transitions in epitaxial thin film heterostructures using in situ TEM studies.