Neeraj Dwivedi

Methods and strategies for the synthesis of diverse nanoparticles and their applications: a comprehensive overview

Ongoing advances in nanotechnology research have established a variety of methods to synthesize nanoparticles (NPs) from a diverse range of materials, including metals, semiconductors, ceramics, metal oxides, polymers, etc. Depending upon their origin and synthesis methods, NPs possess unique physicochemical, structural and morphological characteristics, which are important in a wide variety of applications concomitant to electronic, optoelectronic, optical, electrochemical, environment and biomedical fields. This review provides a comprehensive overview on various physical, chemical and bio-assisted methods largely employed to synthesize and fabricate NPs of varying size, surface characteristics, functionalities and physicochemical behavior. The key applications of nanoparticles have also been discussed.

Nanostructured titanium/diamond-like carbon multilayer films: deposition, characterization, and applications.

Titanium/diamond-like carbon multilayer (TDML) films were deposited using a hybrid system combining radio frequency (RF)-sputtering and RF-plasma enhanced chemical vapor deposition (PECVD) techniques under a varied number of Ti/diamond-like carbon (DLC) bilayers from 1 to 4, at high base pressure of 1 × 10(-3) Torr. The multilayer approach was used to create unique structures such as nanospheres and nanorods in TDML films, which is confirmed by scanning electron microscopy (SEM) analysis and explained by a hypothetical model. Surface composition was evaluated by X-ray photoelectron spectroscopy (XPS), whereas energy dispersive X-ray analysis (EDAX) and time-of-flight secondary ion mass spectrometer (ToF-SIMS) measurements were performed to investigate the bulk composition. X-ray diffraction (XRD) was used to evaluate the phase and crystallinity of the deposited TDML films. Residual stress in these films was found to be significantly low. These TDML films were found to have excellent nanomechanical properties with maximum hardness of 41.2 GPa. In addition, various nanomechanical parameters were calculated and correlated with each other. Owing to metallic interfacial layer of Ti in multilayer films, the optical properties, electrical properties, and photoluminescence were improved significantly. Due to versatile nanomechanical properties and biocompatibility of DLC and DLC based films, these TDML films may also find applications in biomedical science.

Understanding the Role of Nitrogen in Plasma-Assisted Surface Modification of Magnetic Recording Media with and without Ultrathin Carbon Overcoats

A novel scheme of pre-surface modification of media using mixed argon-nitrogen plasma is proposed to improve the protection performance of 1.5 nm carbon overcoats (COC) on media produced by a facile pulsed DC sputtering technique. We observe stable and lower friction, higher wear resistance, higher oxidation resistance, and lower surface polarity for the media sample modified in 70%Ar + 30%N2 plasma and possessing 1.5 nm COC as compared to samples prepared using gaseous compositions of 100%Ar and 50%Ar + 50%N2 with 1.5 nm COC. Raman and X-ray photoelectron spectroscopy results suggest that the surface modification process does not affect the microstructure of the grown COC. Instead, the improved tribological, corrosion-resistant and oxidation-resistant characteristics after 70%Ar + 30%N2 plasma-assisted modification can be attributed to, firstly, the enrichment in surface and interfacial bonding, leading to interfacial strength, and secondly, more effective removal of ambient oxygen from the media surface, leading to stronger adhesion of the COC with media, reduction of media corrosion and oxidation, and surface polarity. Moreover, the tribological, corrosion and surface properties of mixed Ar + N2 plasma treated media with 1.5 nm COCs are found to be comparable or better than ~2.7 nm thick conventional COC in commercial media.

Influence of silver incorporation on the structural and electrical properties of diamond-like carbon thin films.

A simple approach is proposed for obtaining low threshold field electron emission from large area diamond-like carbon (DLC) thin films by sandwiching either Ag dots or a thin Ag layer between DLC and nitrogen-containing DLC films. The introduction of silver and nitrogen is found to reduce the threshold field for emission to under 6 V/μm representing a near 46% reduction when compared with unmodified films. The reduction in the threshold field is correlated with the morphology, microstructure, interface, and bonding environment of the films. We find modifications to the structure of the DLC films through promotion of metal-induced sp2 bonding and the introduction of surface asperities, which significantly reduce the value of the threshold field. This can lead to the next-generation, large-area simple and inexpensive field emission devices.

Studies of pure and nitrogen-incorporated hydrogenated amorphous carbon thin films and their possible application for amorphous silicon solar cells

Hydrogenated amorphous carbon (a-C:H) and nitrogen-incorporated a-C:H (a-C:N:H) thin films were deposited using radio frequency–plasma-enhanced chemical vapor deposition technique and studied for their electrical, optical, and nano-mechanical properties. Introduction of nitrogen and increase of self bias enhanced the conductivity of a-C:H and a-C:N:H films, whereas current-voltage measurement reveals heterojunction formation due to their rectifying behavior. The bandgap of these films was changed over wide range from 1.9 eV to 3.45 eV by varying self bias and the nitrogen incorporation. Further, activation energy was correlated with the electronic structure of a-C:H and a-C:N:H films, and conductivity was discussed as a function of bandgap. Moreover, a-C:N:H films exhibited high hardness and elastic modulus, with maximum values as 42 GPa and 430 GPa, respectively, at −100 V. Observed fascinating electrical, optical, and nano-mechanical properties made it a material of great utility in the development of o...

Bio-inspired in situ crosslinking and mineralization of electrospun collagen scaffolds for bone tissue engineering.

Bone disorders are the most common cause of severe long term pain and physical disability, and affect millions of people around the world. In the present study, we report bio-inspired preparation of bone-like composite structures by electrospinning of collagen containing catecholamines and Ca(2+). The presence of divalent cation induces simultaneous partial oxidative polymerization of catecholamines and crosslinking of collagen nanofibers, thus producing mats that are mechanically robust and confer photoluminescence properties. Subsequent mineralization of the mats by ammonium carbonate leads to complete oxidative polymerization of catecholamines and precipitation of amorphous CaCO3. The collagen composite scaffolds display outstanding mechanical properties with Youngs modulus approaching the limits of cancellous bone. Biological studies demonstrate that human fetal osteoblasts seeded on to the composite scaffolds display enhanced cell adhesion, penetration, proliferation, differentiation and osteogenic expression of osteocalcin, osteopontin and bone matrix protein when compared to pristine collagen or tissue culture plates. Among the two catecholamines, mats containing norepinephrine displayed superior mechanical, photoluminescence and biological properties than mats loaded with dopamine. These smart multifunctional scaffolds could potentially be utilized to repair and regenerate bone defects and injuries.

Structural and Electronic Characterization of Nanocrystalline Diamondlike Carbon Thin Films

The origin of low threshold field-emission (threshold field 1.25 V/μm) in nanocrystalline diamond-like carbon (nc-DLC) thin films is examined. The introduction of nitrogen and thermal annealing are both observed to change the threshold field and these changes are correlated with changes to the film microstructure. A range of different techniques including micro-Raman and infrared spectroscopy, X-ray diffraction, electron microscopy, energy-dispersive X-ray analysis and time-of-flight-secondary ion mass spectroscopy are used to examine the properties of the films. A comparison of the field emission properties of nc-DLC films with atomically smooth amorphous DLC (a-DLC) films reveals that nc-DLC films have lower threshold fields. Our results show that nc-DLC can be a good candidate for large area field emission display panels and cold cathode emission devices.

Probing the Role of an Atomically Thin SiNx Interlayer on the Structure of Ultrathin Carbon Films

Filtered cathodic vacuum arc (FCVA) processed carbon films are being considered as a promising protective media overcoat material for future hard disk drives (HDDs). However, at ultrathin film levels, FCVA-deposited carbon films show a dramatic change in their structure in terms of loss of sp3 bonding, density, wear resistance etc., compared to their bulk counterpart. We report for the first time how an atomically thin (0.4 nm) silicon nitride (SiNx) interlayer helps in maintaining/improving the sp3 carbon bonding, enhancing interfacial strength/bonding, improving oxidation/corrosion resistance, and strengthening the tribological properties of FCVA-deposited carbon films, even at ultrathin levels (1.2 nm). We propose the role of the SiNx interlayer in preventing the catalytic activity of Co and Pt in media, leading to enhanced sp3C bonding (relative enhancement ~40%). These findings are extremely important in view of the atomic level understanding of structural modification and the development of high density HDDs.

Superhard behaviour, low residual stress, and unique structure in diamond-like carbon films by simple bilayer approach

Simple bilayer approach is proposed for synthesizing hard and superhard diamond-like carbon (DLC) coatings with reduced residual stress. For this, M/DLC bilayer (M = Ti and Cu) structures are grown using hybrid system involving radio frequency (RF)-sputtering and RF-plasma enhanced chemical vapor deposition techniques. Ti/DLC bilayer deposited at negative self bias of 100 V shows superhard behaviour with hardness (H) as 49 GPa. Cu/DLC bilayer grown at self bias of 100 V exhibits hard behaviour with H as 22.8 GPa. The hardness of Ti/DLC (Cu/DLC) bilayer gets changed from superhard (hard) to hard (moderate hard) regime, when the self bias is raised to 300 V. Residual stress in Ti/DLC (Cu/DLC) bilayer is found to be significantly low that varies in the range of 1 GPa–1.65 GPa (0.8 GPa–1.6 GPa). The microstructure and morphology are studied by Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). SEM and AFM pictures reveal the creation of nanostructured features in the de...

Photoconductivity and characterization of nitrogen incorporated hydrogenated amorphous carbon thin films

The observation and origin of photoconductivity in high base pressure (∼10−3 Torr) grown nitrogen incorporated hydrogenated amorphous carbon (a-C:H:N) thin films is reported. The magnitude of conductivity at room temperature was measured to increase by nearly two orders of magnitude and exhibits a maximum ratio of photoconductivity to dark conductivity of 1.5 as the nitrogen content increased to 15.1 at. %. X-ray photoelectron spectroscopy, micro-Raman spectroscopy, and Fourier transform infrared spectroscopy reveal enhanced sp2 bonding at higher nitrogen contents. Residual film stress, Tauc band gap, hardness, and elastic modulus are all found to decrease with addition of nitrogen. The electrical characteristics suggest the creation of a-C:H:N/p-Si heterojunction diodes having rectifying behavior. The conductivity and electrical characteristics are discussed in term of band model, and the results show that high quality a-C:H:N films can be grown at high base pressures with properties comparable to those grown at low base pressures.