Sobia Sajid | National Centre for Physics, Islamabad (original) (raw)

Journal Papers by Sobia Sajid

A simple and fast coating method of honeycomb cores for microwave absorption has been described. ... more A simple and fast coating method of honeycomb cores for microwave absorption has been described. The honeycomb cores with two different thickness (5 and 20 mm) coated with thermoplastic resin ﬁlled with carbon powder as lossy ﬁller in 5, 10, 15, and 20 wt% have been tested for microwave absorption in 2–18-GHz frequency range. The 5-mm-thick honeycomb has shown absorption bandwidth of 14 GHz for maximum absorption of−6dB(75%) with 15 wt%. ﬁller content.However,the percent- age of the ﬁller was decreased to 10 wt% in 20-mm-thick honey- comb absorber for maximum absorption over a wide frequency range. The honeycomb sample with 10 wt% ﬁller has bandwidth of 18 GHz for−7dB (80%) reﬂection loss.The reﬂection loss measurements of coated honeycomb cores have also shown that use of E-glass ﬁber/epoxy composite can enhance the performance of the honeycomb absorber. The combination of a microwave absorbing nanocomposite and the coated honeycombs has been resulted in inferior absorption properties in 2–18-GHz frequency range.

Using sheet-by-sheet method, multiwalled carbon nanotubes (MWCNTs)/polyetheretherketone (PEEK) mu... more Using sheet-by-sheet method, multiwalled carbon nanotubes (MWCNTs)/polyetheretherketone (PEEK) multilayered composites are manufactured. The shielding efﬁciencies and dielectric constants of the MWCNTs/PEEK composites are investigated in the frequency range of 0.1–1.6 THz using terahertz time domain spectroscopy (THz-TDS). The MWCNTs/PEEK composite with a carbon-nanotubes fraction of 1.2% turns to be metallic. Moreover, compared with multilayered graphite/PEEK and single layered MWCNTs/PEEK composites, the multilayered MWCNTs/PEEK composites present excellent terahertz absorptive with a frequency response consistent with Drude’s free-electron model.

Nanodiamond (ND) powder was successfully activated by wet chemical method and by exposure of UV/O... more Nanodiamond (ND) powder was successfully activated by wet chemical method and by exposure of UV/O3 in a chamber followed by mixing in triethylenetetramine (TETA) solution. The reinforcement role of activated ND in the mechanical properties of epoxy matrix was studied. Both treatments, i.e. acid and UV/O3 Nanodiamond (ND) powder was successfully activated by wet chemical method and by exposure of UV/O3 provide ND surface with chemical functionalities for adhesion with epoxy resin. Fourier transform infrared spectroscopy was utilized to confirm the attachment of surface groups to the ND particles. The low content of acid and UV/O3 activated ND was dispersed ultrasonically in the epoxy matrix separately to make nanocomposites. The mechanical properties of the nanocomposites were investigated under three point bending. The strong interactions among activated ND particles and the epoxy resin provide efficient load transfer interfaces, which enhances the mechanical properties of the composites. It was found that the flexural strength, modulus, and toughness of 0.1 wt% ND loaded nanocomposites have been enhanced up to 85%, 57%, and 39%,
respectively for UV/O3 treated ND powder. It is also found that the optimum ND concentration to achieve maximum reinforcement is 0.1 wt% while higher concentrations lead to decrease in mechanical properties. The significant improvement of the mechanical properties of the ND/epoxy nanocomposites is attributed to the good dispersion of the functionalized ND in epoxy matrix.

Epoxy/nanocrystalline diamond nanocomposites composites were prepared by dispersing ultrasonicall... more Epoxy/nanocrystalline diamond nanocomposites composites were prepared by dispersing ultrasonically, 0.4, 0.7, 1.0, and 4.0 wt% acid-treated nanocrystalline diamond (NCD) powder in epoxy matrix. Fourier infrared spectroscopy was utilized to study the moieties attached to the nanodiamond particles. The trace elements present in NCD powder before and after acid treatment were analyzed by ion beam techniques. Thermomechanical properties of the nanocomposites showed that incorporation of low content (0.4 wt%) of nanodiamond powder into epoxy matrix enhanced the storage modulus, loss modulus, and hardness by 68,
55, and 86%, respectively, over neat epoxy. By increasing the concentration of modiﬁed NCD to 0.7 wt% resulted in lower values of hardness and thermomechanical properties but still remain higher than neat epoxy. An increasing trend in properties was again observed at 4 wt% concentration of modiﬁed nanoﬁller.
The glass transition temperature was up shifted to 1108C over neat epoxy. The mechanisms responsible for enhanced properties of epoxy matrix are also discussed in detail.
POLYM. COMPOS., 34:811–818, 2013 Society of Plastics Engineers

Optical, electrical and structural properties of argon (Ar) ion-irradiated buckypapers of multi-w... more Optical, electrical and structural properties of argon (Ar) ion-irradiated buckypapers of multi-walled carbon nanotube (MWCNT) at various doses prepared by a vacuum filtration method were investigated. It was found that the direct current (DC) conductivity and absorption spectra in the visible range were decreased with an increasing Ar ion irradiation dose. A subsequent heating of nanotube buckypapers at 800 K in a vacuum at each irradiation dose improved the conductivity of buckypapers, whereas optical absorption was unchanged. Moreover, the graphite structure of MWCNTs was transformed to amorphous structure with an increasing Ar ion irradiation dose. The decrease of optical absorption and electrical conductivity of MWCNT buckypaper at room temperature can be ascribed to the increase of defects in the irradiated MWCNTs.

Thin films of amorphous carbon nanowire (a-CNW) have been fabricated from crossed multi-walled ca... more Thin films of amorphous carbon nanowire (a-CNW) have been fabricated from crossed multi-walled carbon nanotube (MWCNT) thin film. The fabrication was done by means of ion beam irradiation on various substrates. It is found that the a-CNW thin films show electrical conduction behaviour, and electrical conductivity varies after annealing. In addition, the transmission spectra in the visible range reveal that the film has above 90% optical transmission. It can be ascribed to the fact that the decreased crystallinity of MWCNTs by ion beam irradiation has caused the incremental increase of optical transmission. We also report on a method for cutting or destroying a-CNWs using low-energy focused electron beam from a scanning electron microscope.

Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma ch... more Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma chemical vapour deposition (MPCVD) have been implanted with nitrogen ions. The nanorods were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The DNRs consist of single-crystalline diamond cores of 3-5 nm in diameter and several tens of nanometres in length. For purification from non-diamond contents, hydrogen plasma etching of DNRs was performed. Structural modifications of etched DNRs were studied after irradiating with 50 keV nitrogen ions under the fluence of 5 x 10(14), 1 x 10(15), 5 x 10(15) and 1 x 10(16) ions cm(-2). Nitrogen-ion implantation changes the carbon-carbon bonding and structural state of the nanocrystalline diamond (NCD). Raman spectroscopy was used to study the structure before and after ion irradiation, indicating the coexistence of diamond and graphite in the samples. The results indicated the increase in graphitic and sp(2)-related content, at the expense of decrease in diamond crystallinity, for ion implantation dose of 5 x 10(15) cm(-2) and higher. The method proves valuable for the formation of hybrid nanostructures with controlled fractions of sp(3)-sp(2) bonding.

Diamond-epoxy composites reinforced with low content of submicron diamond powder 0.1, 0.4, 0.7, a... more Diamond-epoxy composites reinforced with low content of submicron diamond powder 0.1, 0.4, 0.7, and 1.0 wt % were synthesized. As received diamond powder was acid treated to purify and functionalize diamond particles. Fourier Transform Infrared Spectroscopy was utilized to study the moieties attached to the diamond particles. The trace elemental analysis of impurities in diamond powder before and after acid treatment was performed using ion beam techniques. The mechanical properties of the epoxy matrix were enhanced with the addition of purified and functionalized diamond powder. The Dynamical mechanical analysis results revealed that storage modulus of the prepared composites has been increased by similar to 100% with diamond loading of 0.7 wt %. The Vickers's hardness of the diamond-epoxy composite was similar to 39% higher than that of pure epoxy for the loading of 1.0 wt % diamond powder. Mechanisms responsible for the enhancement of the mechanical properties are discussed. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

The effect of nitrogen addition in the feed gas on the finally incorporated amount of hydrogen in... more The effect of nitrogen addition in the feed gas on the finally incorporated amount of hydrogen in the diamond nanorods (DNRs) thin films has been investigated. The Raman spectroscopy measurements helped to understand the structural and quality changes with increasing nitrogen gas flow rate during CVD deposition. The hydrogen concentration was measured with 3.0 MeV He 2þ beam using elastic recoil detection analysis technique and it was found that with the addition of nitrogen, the hydrogen concentration was increased. The results of non-Rutherford backscattering spectroscopy (NRBS) used to measure the amount of nitrogen in the DNRs thin films have shown that the incorporated nitrogen is below the detection limit of NRBS technique. Our results suggested that the addition of nitrogen has affected the overall quality of diamond films in two ways; increasing the thickness of diamond films by increasing the non-diamond carbon content and increasing the hydrogen impurity incorporation. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed.

Nanocrystalline diamond (NCD) films were grown on silicon substrates by hot filament chemical vap... more Nanocrystalline diamond (NCD) films were grown on silicon substrates by hot filament chemical vapor deposition in Ar/N-2/CH4 gas mixtures. The effects of seeding process prior to deposition, the total gas pressure, and concentration of nitrogen on the grain size, morphology and bonding nature in HFCVD technique were investigated. The results indicated that a low total gas pressure is favorable for nanosized diamond crystallites. Films micrograph obtained from scanning electron microscopy showed diamond nanograins elongated with the addition of nitrogen in the plasma. Crystal structure investigations were carried out by X-ray diffraction measurements for deposited films. An increase in the size of crystallite is also observed from XRD measurements in NCD film when nitrogen was added in plasma. From Raman spectra, it was observed that the relative intensity of G peak increases indicating more graphite content after nitrogen added in the plasma. The effects of the nitrogen incorporation in nanocrystalline films in HFCVD are discussed. (C) 2009 Elsevier B.V. All rights reserved.

Gate modulation on angle-resolved photoabsorption spectra of zigzag-edge graphene nanoribbons J. ... more Gate modulation on angle-resolved photoabsorption spectra of zigzag-edge graphene nanoribbons J. Appl. Phys. 113, 103510 (2013) Molecular dynamics simulations of electrophoresis of polyelectrolytes in nano confining cylindrical geometries J. Chem. Phys. 138, 104905 (2013) Fabrication and optical properties of large-scale arrays of gold nanocavities based on rod-in-a-tube coaxials Appl. Phys. Lett. 102, 103103 (2013) Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2×1) Appl. Phys. Lett. 102, 083107 (2013) Additional information on J. Appl. Phys.

One dimensional C-C nanostructure, diamond-graphite nanorods, was synthesized by the argon rich m... more One dimensional C-C nanostructure, diamond-graphite nanorods, was synthesized by the argon rich microwave plasma chemical vapor deposition method. The nanostructures were characterized by scanning electron microscopy and transmission electron microscopy techniques. The diamond nanorods (DNRs) consist of single-crystalline diamond cores of 2-5 nm in diameter and several tens of nanometer in length. The DNRs are encapsulated in a graphitic shell of variable thickness. Raman and X-ray diffraction spectra also indicated the coexistence of diamond and graphite phases in the film. The addition of nitrogen is considered to be helpful for the highly efficient formation of graphite shell. The high content of methane in the gas mixture in the presence of argon rich environment is suggested to be responsible for the one dimensional growth. (C) 2009 Elsevier B.V. All rights reserved.

Diamond films consist of crystallites having nanometer grains were deposited using low methane co... more Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 10 10 cm À2 ) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained.

Diamond nanorods (DNRs) have been prepared by hydrogen plasma post-treatment of nanocrystalline d... more Diamond nanorods (DNRs) have been prepared by hydrogen plasma post-treatment of nanocrystalline diamond films in radio-frequency (RF) plasma-assisted hot-filament chemical vapor deposition. Singlecrystal diamond nanorods with diameters of 3-5 nm and with lengths up to 200 nm grow under hydrogen plasma irradiation of nanocrystalline diamond thin film on the Si substrate at high temperatures. The DNRs growth occurs from graphite clusters. The graphite clusters arises from the etching of diamond carbon atoms and from the non-diamond phase present in the parent film. The graphite clusters recrystallized to form nanocrystalline diamonds which further grow for diamond nanorods. The negative applied bias and surface stresses are suggested to support one-dimensional growth. The growth direction of diamond nanorods is perpendicular to the (111) crystallographic planes of diamond. The studies address the structure and growth mechanism of diamond nanorods.

a b s t r a c t Hydrogen Grain size dependence ERDA XPS RS In this investigation, diamond thin fi... more a b s t r a c t Hydrogen Grain size dependence ERDA XPS RS In this investigation, diamond thin films with grain size ranging from 50 nm to 1 µm deposited using hot filament chemical vapor deposition (HFCVD) have been analyzed by elastic recoil detection analysis (ERDA) for determining hydrogen concentration. Hydrogen concentration in diamond thin films increases with decreasing grain size. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that part of this hydrogen is bonded to carbon forming C-H bonding. Raman spectra also indicated the increase of non diamond phase with the decrease in crystallite size. Incorporation of hydrogen in the samples and increase of hydrogen content in nanocrystalline sample are discussed. Large separation between filament and substrate used for the synthesis of nanocrystalline film helped to understand the large incorporation of hydrogen in nanocrystalline diamond films during growth. The study addresses the hydrogen trapping in different samples and higher hydrogen concentration in nanocrystallites by considering the synthesis conditions, growth mechanisms for different grain sized diamond films and from the quality of CVD diamond films.

Papers by Sobia Sajid

Journal of Experimental Nanoscience, 2011

Journal of Experimental Nanoscience, 2012

Journal of Experimental Nanoscience, 2010

Journal of Crystal Growth, 2009

Journal of Experimental Nanoscience, 2011

Journal of Experimental Nanoscience, 2012

Journal of Experimental Nanoscience, 2010

Journal of Crystal Growth, 2009

Diamond and Related Materials, 2009

Diamond and Related Materials, 2010

Journal of Experimental Nanoscience, 2012

Hydrogen-incorporated nanocrystalline diamond thin films have been deposited in microwave plasma ... more Hydrogen-incorporated nanocrystalline diamond thin films have been deposited in microwave plasma enhanced chemical vapour deposition (CVD) system with various hydrogen concentrations in the Ar/CH4 gas mixture. The bonding environment of carbon atoms was detected by Raman spectroscopy and the hydrogen concentration was determined by elastic recoil detection analysis. Incorporation of H2 species into Ar-rich plasma was observed to markedly alter the microstructure of diamond films. Raman spectroscopy results showed that part of the hydrogen is bonded to carbon atoms. Raman spectra also indicated the increase of non-diamond phase with the decrease in crystallite size. The study addresses the effects of hydrogen trapping in the samples when hydrogen concentration in the plasma increased during diamond growth and its relation with defective grain boundary region.

Journal of Experimental Nanoscience, 2012

Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma ch... more Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma chemical vapour deposition (MPCVD) have been implanted with nitrogen ions. The nanorods were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The DNRs consist of single-crystalline diamond cores of 3–5 nm in diameter and several tens of nanometres in length. For purification from non-diamond contents, hydrogen plasma etching of DNRs was performed. Structural modifications of etched DNRs were studied after irradiating with 50 keV nitrogen ions under the fluence of 5 × 1014, 1 × 1015, 5 × 1015 and 1 × 1016 ions cm−2. Nitrogen-ion implantation changes the carbon–carbon bonding and structural state of the nanocrystalline diamond (NCD). Raman spectroscopy was used to study the structure before and after ion irradiation, indicating the coexistence of diamond and graphite in the samples. The results indicated the increase in graphitic and sp2-related content, at the expense of decrease in diamond crystallinity, for ion implantation dose of 5 × 1015 cm−2 and higher. The method proves valuable for the formation of hybrid nanostructures with controlled fractions of sp3–sp2 bonding.

Current Applied Physics, 2009

Journal of Applied Physics, 2010

The influences of the process gases, such as methane and nitrogen concentration on the morphology... more The influences of the process gases, such as methane and nitrogen concentration on the morphology of nanocrystallites diamond films are assessed. It has been found that the concentration of CH4 in the reactant gases is important to control the morphology of diamond nanocrystallites. The morphology of nanocrystallites changes from granular to rodlike shape by changing methane concentration in Ar/H2/CH4 microwave plasma. The addition of nitrogen is considered to be helpful in the formation of graphite content and for increasing the deposition rate. No considerable changes in the morphology of diamond nanocrystallites were observed by varying nitrogen concentration in the feed gases as observed by scanning electron microscopy. Although Raman spectroscopy indicated that the amount of sp2-bonded carbon increased by the addition of nitrogen in the plasma. The high concentration of methane in the gas mixture due to the presence of argon rich environment is suggested to be responsible for the formation of diamond nanorods.