In this review, the X-ray topography results of various types of single crystal diamonds (SCDs) are reported. Dislocations and dislocation bundles are present in all types of SCDs, the only exception being type IIa high-pressure, high-temperature (HPHT) SCDs. The technology of growing HPHT type IIa SCDs has advanced to a level where the samples show almost no dislocations or dislocation bundles. However, very few groups appear to have perfected the process of HPHT growth of type IIa SCDs. There appears to be a characteristic difference in the dislocations present in type Ib HPHT and chemical vapor deposited (CVD) SCDs. The dislocations in CVD SCDs are mostly in aggregate form, while in HPHT type Ib diamonds there are line dislocations which propagate in <111> or <112> directions. The CVD SCDs growth appears to be in the early stage in terms of the control of dislocations and dislocation bundles, compared to other semiconductor wafers. The dislocations and dislocation bundles and aggregates in SCDs limit their applications in electronic and optical devices. For instance, high-power laser windows must have low dislocations and dislocation bundles. For electronic devices such as high-power diodes, dislocations reduce the breakdown voltage of SCDs, limiting their applications. The knowledge of dislocations, their identification and their origin are, therefore, of utmost importance for the applications of SCDs, be they HPHT or CVD grown.

In this study, we report the photoresponse of vertically aligned few-layered graphene (VAG) upon infra-red (IR) irradiation at room temperature. Four probe measurements showed the current–voltage (I–V) characteristic of electrical switching during pulsed IR irradiation. The photoresponse reported here for VAG was significantly higher than that reported for carbon nanotube (CNT) samples. Our investigation shows that such a photoresponse arose solely from the bolometric effect, where the conductivity changed with temperature. The resistance magnitude of the VAGs increased ~two fold for each 6 °C increase in temperature. Also, the Thermal Coefficient of Resistance (TCR) in this region was ~11%/K, which is the highest TCR value reported for any carbon nanomaterial.

Thermal characterization of polycrystalline diamond based on an infrared thermal imaging technique is proposed. The temperature mapping of polycrystalline diamond under a metal wire heating are recorded using a $15~\mu \text{m}$-resolution infrared thermal imaging system. The thermal conductivity of the polycrystalline diamond is derived from the temperature profile using numerical fitting with a 3D heat diffusion model. The thermal conductivity of the polycrystalline diamond is also determined using a $3\omega$ technique. The agreement between the thermal conductivities measured using these two techniques is within 15%.

We report homoepitaxial growth of large sized lightly boron doped CVD diamond on HPHT diamond (100) substrates. Light boron doping was achieved by using a solid boron nitride disk as a substrate holder. Colourless boron doped diamonds with no macroscopic defects such as non-epitaxial crystallites over large areas of at least 4 × 4 mm2 and thicknesses as thin as 0.77 mm to thick layers of 1.5 mm were grown. The plates were subsequently separated from the substrates by laser cutting and polished. Hall Effect measurement at room temperature indicates that the free hole concentration, hole mobility and resistivity are ~1013 cm−3, 1550–2200 cm2 V−1 s−1 and 71–160 Ωcm, respectively. Infrared absorption spectroscopy was used to estimate the effective boron concentration in the plates. Raman/photoluminescence (λex = 514.5 nm) and UV–visible spectroscopy were used to characterize the high quality CVD diamond plates and we observe that nitrogen-related optical features are absent upon boron doping.

We systematically studied the influence of optically active defects on thermal conductivity for polycrystalline diamonds (PCDs) with different colour, crystalline quality and impurity concentrations. The thermal conductivities of PCDs on the growth (top) and nucleation (bottom) surfaces were characterized with 3 ω technique. It is found that the bottom surface shows lower thermal conductivity as compared to the top surface. This could be due to the higher defect density in the bottom surface. Defects analyzed includes non-diamond carbon phase, C–H stretching vibration, Si vacancy, and substitutional nitrogen (Ns 0 ). Our results suggest that, for the top surface, the heat transport is mainly controlled by the concentration of Ns 0 . For the bottom surface, non-diamond carbon phase, Si vacancy, C–H stretch and Ns 0 defects all lead to an obvious reduction in the thermal conductivity. Most importantly, we derived a well fitted equation that estimates the thermal conductivity by optical transmittance, and the equation was demonstrated to be valid at any wavelength in visible region.

Homo-epitaxial (100) single crystal diamond films of different isotopic (C-13) compositions were synthesized by micro-wave plasma chemical vapor deposition. An increase in C-13 concentration (0 to 99%) demonstrated clear developments in microstructures. These are related to changes in: (i) morphology, (ii) residual strain and dislocations and (iii) mesoscopic defects. (i) was mainly through increased presence of steps/ledges and corresponding increase in surface roughness. This was accompanied by significantly higher residual strain and presence/energy of dislocations. C-13 concentration also determined mesoscopic defects: presence of non-(100) orientations, nature of grain boundaries and micro-segregations of C-13 and/or CH. A link has been established between various reproducible, experimental observations through stipulated increase in interfacial energy and stresses present at grain boundaries.

Microwave plasma treatment of single wall carbon nanotube (SWNT) films called bucky papers (BPs) resulted in changes in the relative proportion of different chiralities of SWNTs present in the BP and the production of vertical microstructures on the surface of BP. The plasma was created using H2 gas mixed with Ar or CH4, at a temperature of 900 °C and a pressure of 70 Torr. Radial breathing mode spectra of the BPs revealed that the preferential sputtering by plasma is not with respect to the diameter or the metallic nature of SWNTs. We propose that the lengths of SWNTs influence how they interact with plasma. Longer tubes will have higher dielectric constants and hence will be polarized more strongly by the electric field of the plasma sheath. This in turn results in greater ion bombardment and sputtering. Finite element method was used to find the strengths of the induced electric fields on model SWNT surfaces. Microscopy, Raman, and X-ray photoelectron spectroscopy were used to study the effect of plasma on the crystallinity of the surviving SWNTs. Structural integrity of SWNTs was preserved after the plasma treatment.

Polycrystalline diamond films (similar to 10 mu m thick) were grown on five different silicon (Si) substrate orientations by microwave plasma chemical vapor deposition. The selected Si substrates had a range of f(theta): 0.18-0.65. It is to be noted that f(theta) scales inversely with the packing density of the interface. As f(theta) decreased, three changes in the polycrystalline diamond microstructures were observed. (i) At the film surface <110>-fiber texture increased but <100>-fiber dropped. (ii) A novel reconstruction technique was proposed and tested for faceted microstructures. The reconstructed microstructures revealed that the observed texture changes, with a decrease in f(theta), was accompanied by elimination of very fine facets. (iii) Noticeable differences in Raman estimated stress gradients were also observed: the lowest stress gradients for more closed packed substrates.

Polycrystalline diamond films were synthesized, by microwave plasma chemical vapor deposition. Films were mechanically stable until 30 mu m thickness, while further deposition led to catastrophic film delamination and rupture. This coincided with fracture of the single-crystal silicon substrate, creating polycrystalline silicon with clear shear markings. Films grown to 12, 18, 21, and 30 mu m were subjected to detailed investigations. Larger film thickness modified intrinsic stresses (estimated from Raman shift) from mildly compressive to strongly tensile. However, normal in-plane residual stresses and dislocation densities, as estimated from X-ray diffraction, dropped. Film growth enhanced anisotropies in crystallographic texture brought changes to grain morphology and significantly increased out-of-plane residual shear stress. Though different models of film delamination/rupture were deliberated, they fail to assimilate all aspects of experimental observations. Shear anisotropy-induced lateral stresses, on the other hand, can explain film rupture and relate the same with substrate/film microstructural developments.

CHESS has a monochromatic x-ray topography capability serving continually expanding user interest. The setup consists of a beam expanding monochromator, 6-circle diffactometer, and CHESS designed CMOS camera with real time sample-alignment capability. This provides rocking curve mapping with angle resolution as small as 2 mu radians, spatial resolution to 3 microns, and field of view up to 7mm. Thus far the capability has been applied for: improving CVD-diamond growth, evaluating perfection of ultra-thin diamond membranes, correlating performance of diamond-based electronics with crystal defect structure, and defect analysis of single crystal silicon carbide. This paper describes our topography system, explains its capabilities, and presents experimental results from several applications.

Radiation detectors based on diamond are highly favored for particle physics research due to the superior radiation hardness. In this work, we investigate the influence of impurities and crystalline imperfections on the charge collection efficiency (CCE) of single crystal diamond. Seventeen (17) ultra-low fluorescent diamond samples grown by microwave plasma chemical vapor deposition method from Ha Technologies PTE LTD are preselected for this study. The measured CCE of all samples using Am-241 (alpha-particles) as ionizing source are analyzed together with the concentration of trace impurities and crystalline imperfection in the crystal. The amounts of impurities are quantified from integrated fluorescence intensity arising from the nitrogen vacancies (NV) created during different CVD growth process conditions. The crystal imperfections are assessed by X-ray rocking curves from X-ray topography images obtained at the Cornell High Energy Synchrotron Source. The CCE decays rapidly as the intensity of NV (I-Nv), phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCE(I-NV) = 100/(1 + (I-NV/1.0)(5/2)). The energy resolution, Delta E/E (ratio of the energy spectrum width to the most probable peak) highly correlates with broader rocking curve width distribution.Prime novelty statement: This work provides an understanding on the most important factors that contribute to degradation of charge collection efficiency (CCE) in diamond based detectors and sensors. The CCE decays rapidly as the intensity of nitrogen vacancy phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCE(I-NV) = 100/(1 + (I-NV/1.0)5/2). The energy resolution, that is the ratio of the energy spectrum width to the most probable peak, highly correlates with broader X-ray rocking curve width distribution. (C) 2015 Elsevier B.V. All rights reserved.

This study describes the growth of 13C-enriched (100)-oriented diamond single crystals by the MPCVD (microwave plasma chemical vapour deposition) process. All crystals are at least 6×6mm2 in area and 0.5–1.0mm in thickness. The samples with nominal 13C percentages (R=[13C]/[13C+12C]) of 0.011 (natural abundance), 0.10, 0.21, 0.24 and 0.34 were obtained by controlling the flow of the carbon-13 and carbon-12 methane feed gases. To obtain thicker and near-colourless quality 13C-enriched diamond, 190ppm of nitrogen was added into the gas mixture. The shift towards lower frequency in the Raman peak positions and decrease in the thermal conductivities of the near-colourless crystals with increasing 13C percentages are similar to previous studies of isotopically-controlled diamond grown with no nitrogen additive. The images of the structural defects associated with 13C-enrichment obtained by spatially-resolved X-ray rocking curve measurement show distinct patterned structures that runs parallel to the <100> direction. Moreover, the broadening in the line width of the nuclear magnetic resonance (NMR) peak from sp3 13C correlates with increasing R. We also expand the study by injecting 500ppm of nitrogen. Higher nitrogen concentration leads to the formation of brown crystals. The brown crystals show far greater 13C NMR peak intensity than the near-colourless. This suggests that paramagnetic nitrogen impurities in the brown crystals hasten the spin-lattice relaxation time of the 13C nuclear spin that resulted in higher intensity. The isotopic splitting observed for the localized vibrational mode of the NVH0 defect in brown crystals is attributed to the co-existence of both the 13C (3114.2cm−1) and 12C (3123.5cm−1). Unlike the isotopic splitting observed for NVH0 defect, the peak position of the Ns+ defect shifts towards lower frequency as R increases. Not only have we demonstrate the growth of bigger isotopically-controlled diamond single crystals, the results shown here have provided a framework to further investigate the interplay between 13C atoms and nitrogen impurity.

Nanoparticles (NPs) of Pt, Ni or NiO, deposited on few-layered graphene using pulsed laser ablation catalyze the in-situ formation of graphitic shells from graphene. These shells encapsulate the NPs as their cores. High substrate temperature (>450 °C) was necessary for this unique transformation. Size distribution of the core–shell NPs could be adjusted by varying the deposition parameters. The composite of graphene and core–shell Ni NPs was found to be more stable than bare unsupported Ni NPs in corrosive acid and iodine based solutions. The composite survived for about an hour inside the acid and for around 3.5 months in iodine solution. These core–shell composite materials with longer life could be instrumental in eliminating the use of precious metals in several industrial processes and commercial devices. We found them to be highly effective as catalyst in dye sensitized solar cells, surpassing conventionally used Pt thin films in some cases.

Lack of control on the chirality or diameter of single-wall carbon nanotubes (SWCNTs) during synthesisis a major impediment in the path of their widespread commercialization. We demonstrate that the humble technique of catalytic chemical vapor deposition of methane, without any sophisticated catalyst preparation, can provide significant control on the diameter of the synthesized SWCNTs. The catalyst used is a solid solution of the bimetals Fe-Mo or Co-Mo in MgO. The radial breathing modes (RBMs) in the Raman spectra of SWCNTs were used to find out the diameters. Kataura plot along with RBMs was used to study the chirality of the tubes. High concentration of the catalysts (Co: Mo: MgO = 1:0.5: 15 and Fe: Mo: MgO = 1: 0.5: 30) resulted in high yields. However, most of these carbonaceous materials were impurities. Reducing the concentration not only improved the purity and crystallinity (I-D/I-G ratio similar to 0.1), but most importantly reduced the diameter spread of the SWCNTs. Majority of the SWCNTs grown using the low concentration catalysts (Co: Mo: MgO = 1: 0.5: 300 and Fe: Mo: MgO = 1: 0.5: 200) were estimated to have diameters lying between 1.13 and 1.65 nm. This narrowing of diameter spread happened for both Fe and Co catalyst systems and depended only on the concentration of the catalyst. (C) 2014 Elsevier B.V. All rights reserved.

We report the photoresponse of vertically aligned graphene upon IR irradiation at room temperature. Four probe measurements have shown electrical switching in I-V characteristics during pulsed IR irradiation. The photoresponse reported here for vertically aligned graphene (VAG) is much higher than carbon nanotube (CNT) samples. Our investigation has shown that such photoresponse arise solely due to bolometric effect, where the conductivity changes with temperature. The magnitude of the resistance of VAGs increases by 2 fold for 6 0C increase in temperature. Also the Thermal Coefficient of Resistance (TCR) in this region is 11 TCR value reported so far for any carbon nanomaterials.

We demonstrate here the effect of electron beam induced deposited platinum on the electrical transport through multilayer graphene sheets. Platinum metal is deposited at different positions on the graphene multilayers, i.e., including as well as excluding the bottom contact sites and the change in electrical conductance of the same multilayer graphene sheets before and after platinum deposition is segregated. An improvement in electrical conductance is observed even if the metal is deposited at the part of the graphene sheets that does not touch the bottom gold electrodes, and hence this experimental approach directly demonstrates that the contact improvement is not the sole reason for the improved electrical conduction. The improvement in electrical performance of the graphene sheets is explained in terms of the doping of graphene sheets caused by the charge transfer between the deposited metal and the graphene and thereby modified density of states for electrical conduction. Metal deposition also leads to the increased interlayer interaction of the graphene sheets as revealed by the transmission electron microscopy analysis. Further, two types of breakdown behaviors viz. sharp and stepped breakdowns observed for these graphene devices are explained in terms of the effective graphene-metal contact area. These studies reveal the implications of top metal contact fabrication on graphene for electronic devices.

NiO nanoparticles were deposited homogeneously over few layered graphene platelets (GPs) by pulsed laser ablation. The material was used as an electrocatalyst for the counter electrode (CE) of a dye-sensitized solar cell. GPs were synthesized by oxidation of graphite powder followed by a thermal exfoliation and reduction process. The CE made from GPs with NiO nanoparticles attached (NiO-GP) yielded 3.06% power conversion efficiency which is comparable to a conventional platinum thin film based CE (3.57%). The fill factor and short circuit current density were 0.61 and 7.53 mA/cm2, respectively. The NiO-GP CE outperformed CEs produced using both unsupported NiO nanoparticles (2.03%) and pristine GPs (2.46%). Catalytic activities of the CEs were analyzed using electrochemical impedance spectroscopy. Charge transfer resistances for the two interfaces GP-electrolyte and NiO-electrolyte of the NiO-GP CE were 0.85 and 1.72 Ω cm2, respectively. These values were much smaller than that of the bare GP (3.96 Ω cm2) and NiO nanoparticle (20.85 Ω cm2) based CEs. Thus the catalytic ability of each component in the NiO-GP mixture is better than those of the individual components, indicating a synergistic effect. The resistance for the Pt-electrolyte interface (0.63 Ω cm2) was only slightly better than that for NiO-GP.