Thickness-Dependent Nanoscale Elastic Stiffening of Chemical Vapor Deposited Atomically Thin 2H-MoS2 Films.

Understanding the nanoscale elastic-size-effects of atomically thin transition-metal dichalcogenides (TMDs) as a function of thickness underpins the avenue of flexible 2D electronics. In this work, we employed the atomic force acoustic microscopy (AFAM) technique to investigate the thickness-dependent elastic properties of CVD grown 2H-MoS2 films. The monolayer MoS2 exhibited a Young's modulus of 273 ± 27 GPa. Our systematic analysis from bulk to monolayer suggests that the 2H-MoS2 phase exhibits nanoscale elastic-stiffening behavior with decreasing number of layers (thickness). The Young's modulus increased by a factor of ∼2.7 for monolayer MoS2 when compared with the bulk. First-principle DFT calculations affirm the nanoscale elastic-stiffening behavior of MoS2 with decreasing number of layers. Our findings suggest that the observed elastic stiffening is due to the interlayer sliding, which may be facilitated by defects in MoS2 layers. The observed elastic stiffening may be of potential importance for understanding TMD based nanomechanical devices.

Interplay of piezoresponse and magnetic behavior in Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramics.

Extensive piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements in conjunction with piezoresponse spectroscopy have been carried out on pellets of Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramic samples in order to characterize their ferroelectric and magnetic nature and correlate the findings with our recent far-infrared spectroscopic studies on these samples. We are able to clearly discern the switching behavior of the 71° and 109° ferroelectric domains as distinct from that of the 180° domains in both pristine and Ba-doped bismuth ferrite samples. While substitution of Ba at the Bi site in bismuth ferrite does not affect the ferroelectric and magnetic properties to a great extent, Ca-doped samples show a decrease in their d 33 values with a concomitant increase in their magnetic behavior. These results are in agreement with the findings from our far-infrared studies.

The Role of In-Plane Oxygen Vacancy Defects in SnO2 Nanoparticles for CH4 Sensing.

The utility of different types of surface defects in SnO2 nanoparticles (NPs) for the detection of low concentration (50 ppm) of methane (CH4) at relatively low temperature of 50 °C is established. Chemically synthesized SnO2 quantum dots are annealed in air and Ar environments at 800 °C to make two different sets of SnO2 NPs. Variation in dimension, morphology and optical properties due to the annealing conditions are elaborated using X-ray diffraction, transmission electron microscopy and UV Visible spectroscopy respectively. Electron energy loss spectroscopy provides an insight of defect distribution in NPs. Detailed temperature dependent photoluminescence and the Raman studies allow understanding the interplay of in-plane oxygen and bridging oxygen vacancies in above two samples for low concentration CH4 detection at low temperature. The sensor response was about 1-2% due to low operating temperature. The decisive role of in-plane oxygen vacancy to detect low concentrations of gas and utility of bridging oxygen vacancy for improved response at high temperature are further corroborated from the analysis of sensor response and Arrhenius type plots.

Understanding the structural and chemical changes in vertical graphene nanowalls upon plasma nitrogen ion implantation.

Shallow plasma ion implantation is a versatile method for nitrogen incorporation in vertical graphene nanowalls (VGNs). However, the defects introduced by the process and the preference of nitrogen to occupy various locations in the 2D layered structure make the characterization complex. We have simplified the analysis of 2 kV nitrogen plasma ion implanted VGNs by correlating the binding energy of N1s electrons with the chemical state of nitrogen as lone-pair localized (N1), lone-pair de-localized (N2) and quaternary nitrogen (N3). This new approach helps to understand the electronic nature of implanted VGNs, based on the occupancy of structural locations by nitrogen. The C1s photoelectron spectra and G-peak intensity normalized comparison of the entire Raman spectra revealed large scale sp2C to sp3C conversion and generation of defects upon implantation. The increase in relative stiffness of implanted VGNs, as observed in atomic force acoustic microscopic studies, was correlated with the formation of graphitic CNx (N2), crosslinking of layers by nitrogen (N3) and interlayer sp3 carbon.

Physicochemical properties of nanocomposite: Hydroxyapatite in reduced graphene oxide.

Graphene oxide (GO) based nanocomposites have gained considerable attention in the field of material science due to their excellent physicochemical and biological properties. Incorporation of nanomaterials into GO sheets prevents the formation of π-π stacking bond thereby giving rise to composites that show the improved properties compared to their individual counterparts. In this work, reduced graphene oxide (rGO) - hydroxyapatite (HAP) nanocomposites were synthesized by ultrasonic method. Increasing the c/a ratio of HAP in the diffraction pattern of rGO/HAP nanocomposites indicates the c-axis oriented grown HAP nanorods interacting with rGO layers. Shift in wavenumber (15cm-1) and increase of full width at half maximum (45cm-1) of G band in Raman spectra of the rGO/HAP nanocomposites are observed and attributed to the tensile strain induced due to the intercalated HAP nanorods between the rGO layers. Atomic force microscopy (AFM) and phase imaging studies revealed the intercalation of HAP nanorod with diameter 30nm and length 110-120nm in rGO sheets was clearly perceived along with improved elasticity compared to pristine HAP. 13C-NMR results proved the synergistic interaction between both components in rGO/HAP nanocomposite. The novel properties observed and the microscopic mechanism responsible for this are a result of the structural modification in rGO layers brought about by the intercalation of HAP nanorods.

Effect of reactive atmosphere of synthesis on properties of nano-ceria.

Ceria is an important rare-earth oxide with ever increasing applications. In nanocrystalline form, ceria exhibits novel and improved properties compared to its microcrystalline counterpart. The variation in the properties was observed to be a function of lowering particle size. This was also attributable to the presence and extent of Ce in 3+ state in ceria. Thermal decomposition of a thermally less stable cerium metal salt is an useful and simple method for the synthesis of nanocrystalline ceria. As atmosphere under which the decomposition occurs is expected to alter the Ce3+ content and thus the property, the present study is to investigate the effect of reactive atmosphere of decomposition vis-à-vis the particle size on the property of nanocrystalline ceria. Nanocrystalline ceria was synthesized by controlled thermal decomposition of cerium nitrate under four different atmospheres and the products were analyzed by X-Ray Diffraction, X-ray photoelectron spectroscopy and Laser Raman Spectroscopy. The observed property variation was found to be more dependent on the Ce3+ content thus the synthesis atmosphere rather than the particle size.

Implantation induced hardening of nanocrystalline titanium thin films.

Formation of nanocrystalline TiN at low temperatures was demonstrated by combining Pulsed Laser Deposition (PLD) and ion implantation techniques. The Ti films of nominal thickness approximatly 250 nm were deposited at a substrate temperature of 200 degrees C by ablating a high pure titanium target in UHV conditions using a nanosecond pulsed Nd:YAG laser operating at 1064 nm. These films were implanted with 100 keV N+ ions with fluence ranging from 1.0 x 10(16) ions/cm2 to 1.0 x 10(17) ions/cm2 The structural, compositional and morphological evolutions were tracked using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometry (SIMS) and Atomic Force Microscopy (AFM), respectively. TEM analysis revealed that the as-deposited titanium film is an fcc phase. With increasing ion fluence, its structure becomes amorphous phase before precipitation of nanocrystalline fcc TIN phase. Compositional depth profiles obtained from SIMS have shown the extent of nitrogen concentration gradient in the implantation zone. Both as-deposited and ion implanted films showed much higher hardness as compared to the bulk titanium. AFM studies revealed a gradual increase in surface roughness leading to surface patterning with increase in ion fluence.

Temperature dependence of phonon modes in nanocrystalline La0.67ca0.33MnO3 as observed by infrared spectroscopy.

Low temperature infrared absorption measurements have been carried out on nanocrystalline La0.67Ca0.33MnO3 powder across the magnetic and the insulator-to-metal phase transitions. Interesting changes are observed in the temperature dependence of the mid-IR polaronic background and the stretching mode of the MnO, octahedron. Upon cooling below 250 K, the overall absorbance increases, but, unlike in the case of microcrystalline LCMO, the stretching mode is not completely screened even at 5 K. The temperature dependence of the stretching mode parameters points to a much reduced phonon-polaron coupling as compared to that in the bulk material.

Synthesis and luminescence properties of alumina encapsulated InN nanorods.

The present study reports a two-step procedure to synthesize InN nanorods inside the pores of an anodic alumina membrane. In the first step, pores of the membrane are filled with indium via electrodeposition. The second step involves nitridation of the as-deposited nanorods by room temperature plasma annealing. X-ray diffraction studies reveal that as-deposited nanorods consist of In, In2O3 and In(OH)3 phases which get converted to mixed hexagonal and cubic phase InN on plasma annealing. Cross sectional scanning electron microscope study reveals nanorod diameter and length to be 150 nm and 1 microm respectively. X-ray mapping results establish that uniform distribution of nitrogen throughout the length of nanorod has been achieved as a result of plasma annealing. Observation of photoluminescence peaks at 1.4 and 1.6 eV corresponding, to the absorption edges of cubic and hexagonal phases of InN show that room temperature photoemission is due to band to band recombination. The use of alumina as a template for nanorod growth prevents post-deposition agglomeration and provides mechanical strength. Possibility of total internal reflection at the InN-Al2O3 interface makes these structures ideally suitable to reduce the emitted light intensity losses.

Synthesis and characterization of nanocrystalline Cr2O3 and ZrO2 ceramic materials.

A novel methodology based on a hybrid approach has been evolved for synthesizing nearly monodisperse nanocrystalline oxides. The approach basically involves precipitation of gelatinous hydroxide in liquid phase hydrolysis and subsequent temperature programmed calcination to obtain nanocrystalline oxide. Cr2O3 and ZrO2 have been synthesized through this route. This paper describes synthesis procedures giving details of temperature windows required for this synthesis. In addition, solid state analytical technique like X-Ray Diffraction (XRD) and Photoluminescence (PL) have been used to characterize these materials. Especially PL was used to derive information on confinement effect. Transmission Electron Microscope (TEM), Scanning Probe Microscope (SPM), and Scanning Near Field Optical Microscope (SNOM) were used to derive morphology.

SPM characterization of pulsed laser deposited nanocrystalline CrN hard coatings.

Nanocrystalline CrN coatings, widely required for surface engineering application covering wear and corrosion resistance, need to be investigated for atomic scale morphology, surface roughness, local stiffness, phase uniformity, and homogeneity. Evolution of these properties as a function of thickness need to be studied. In this paper, we have attempted to address these issues through use of a multimode scanning probe microscope (SPM) equipped to carry out Atomic Force Microscopy (AFM) and Atomic Force Acoustic microscopy (AFAM) of Chromium nitride films (100-500 nm thick) on Si prepared under high vacuum by pulsed Laser Ablation using Nd-YAG Q-switched laser. Prior to SPM analysis, the coatings were annealed in N2 atmosphere at 700 degrees C for 30 minutes for improving crystallanity and coating substrate adhesion. The GIXRD patterns of these annealed specimens showed formation of nanocrystalline CrN. Also signature of amorphous phases was seen. The grain size was estimated to be less than 30 nm. Contact mode AFM imaging revealed a roughness value less than 50 nm. Local stiffness values were calculated from AFM force-distance curves. Imaging of frictional force and surface flaws are being investigated by Frictional Force Microscopy (FFM), resonance spectroscopy, and AFAM, respectively. The contrast in AFAM images is seen due to variation in surface elasticity in reference and CrN samples. Stiffness constant and elastic modulus were calculated for both the samples and compared.

Spectroscopic characterization of nanocrystalline chromium nitride (CrN).

Nanocrystalline chromiuim nitride has been synthesised by direct gas phase nitridation of nanocrystalline chromia at 1100 degrees C in ammonia-atmosphere. XRD of this material showed formation of single phase CrN with particle size around 20 nm. AFM studies showed particle distribution along with some soft agglomerated nanostructures. Nanocrystalline Cr2O3 and partially-as well as fully--converted nanocrystalline CrN were also investigated using various spectroscopic techniques like XPS, FT-IR, and Raman for gaining insight into the conversion pathways. Spectroscopic investigations of these materials clearly indicate that complete conversion of CrN occurs by nitriding at 1100 degrees C for 4 hrs. The salient spectroscopic features of these nanocrystalline materials with respect to their microcrystalline counterparts are discussed.