@article{gupta_narayan_2023, title={Laser-induced synthesis of cubic BN nanoneedles: a new approach to fabricating nanomaterials for advanced applications}, volume={25}, ISSN={["1572-896X"]}, DOI={10.1007/s11051-023-05897-x}, number={12}, journal={JOURNAL OF NANOPARTICLE RESEARCH}, author={Gupta, Siddharth and Narayan, Jagdish}, year={2023}, month={Dec} }
 @article{gupta_gupta_gupta_2023, title={Reimagining Carbon Nanomaterial Analysis: Empowering Transfer Learning and Machine Vision in Scanning Electron Microscopy for High-Fidelity Identification}, volume={16}, ISSN={["1996-1944"]}, DOI={10.3390/ma16155426}, abstractNote={In this report, we propose a novel technique for identifying and analyzing diverse nanoscale carbon allotropes using scanning electron micrographs. By precisely controlling the quenching rates of undercooled molten carbon through laser irradiation, we achieved the formation of microdiamonds, nanodiamonds, and Q-carbon films. However, standard laser irradiation without proper undercooling control leads to the formation of sparsely located diverse carbon polymorphs, hindering their discovery and classification through manual analyses. To address this challenge, we applied transfer-learning approaches using convolutional neural networks and computer vision techniques to achieve allotrope discovery even with sparse spatial presence. Our method achieved high accuracy rates of 92% for Q-carbon identification and 94% for distinguishing it from nanodiamonds. By leveraging scanning electron micrographs and precise undercooling control, our technique enables the efficient identification and characterization of nanoscale carbon structures. This research significantly contributes to the advancement of the field, providing automated tools for Q-materials and carbon polymorph identification. It opens up new opportunities for the further exploration of these materials in various applications.}, number={15}, journal={MATERIALS}, author={Gupta, Siddharth and Gupta, Sunayana and Gupta, Arushi}, year={2023}, month={Aug} }
 @article{joshi_shukla_gupta_joshi_narayan_narayan_2023, title={Synthesis of laser-patterned MoS2 nanoneedles for advanced electrochemical sensing}, volume={6}, ISSN={["2159-6867"]}, url={http://dx.doi.org/10.1557/s43579-023-00381-y}, DOI={10.1557/s43579-023-00381-y}, abstractNote={We describe a novel excimer laser-based route for the fabrication of crystalline MoS2 nanoneedles. Laser annealing of MoS2 thin films at a low energy density of 0.08 Jcm−2 resulted in a closed-pack structure with low defects and excellent conductivity due to melting and rapid quenching. A further increase in laser annealing energy density resulted in the formation of MoS2 nano-needles. This structure of MoS2 was found to have a remarkable reduction ability for H2O2 at − 0.14 V over a wide linear range; a low detection limit (0.45 nM (S/N = 3)) and sensitivity of 2.38 μA/mM cm−2 were demonstrated.}, number={4}, journal={MRS COMMUNICATIONS}, publisher={Springer Science and Business Media LLC}, author={Joshi, Pratik and Shukla, Shubhangi and Gupta, Siddharth and Joshi, Naveen and Narayan, Jagdish and Narayan, Roger}, year={2023}, month={Jun} }
 @article{gupta_sachan_narayan_2022, title={Emergence of orbital two-channel Kondo effect in epitaxial TiN thin films}, volume={341}, url={https://doi.org/10.1016/j.ssc.2021.114547}, DOI={10.1016/j.ssc.2021.114547}, abstractNote={The continual scaledown and stringent defect control in electronic devices are opening up frontiers in Kondo effect-a powerful testbed for probing magnetic impurity coupling to a continuum of electronic states. In disordered ferromagnets, degenerate quantum states like two-level systems (TLS) with a pseudo-half spin can couple with the continuum to mimic Kondo physics, creating the orbital two-channel Kondo (2CK) effect below the characteristic Kondo temperature (TK). By employing vacuum-annealing to inject ∼12 at.% nitrogen vacancies (VN) in TiN1-x films, we generated room-temperature ferromagnetic ordering with 13.6 emu g−1 saturation magnetization. The transport characteristics exhibited non-Fermi-liquid (NFL) behavior below TK of 20.2 K. The orbital 2CK effect is signified by a clear transition from logarithmic (T < T0∼40 K), to square-root dependence (T < TK) and subsequent deviation from it (T < TD∼10 K) in resistivity upturn behavior in three distinct low-temperature regimes. By controlling VN, we establish the necessity of ferromagnetism and ultrafast tunneling centers for generating orbital 2CK effect and NFL behavior in disordered metallic systems, extending their scope in non-magnetic nanomaterials.}, journal={Solid State Communications}, publisher={Elsevier BV}, author={Gupta, Siddharth and Sachan, Ritesh and Narayan, Jagdish}, year={2022}, month={Jan}, pages={114547} }
 @article{joshi_shukla_gupta_riley_narayan_narayan_2022, title={Excimer Laser Patterned Holey Graphene Oxide Films for Nonenzymatic Electrochemical Sensing}, volume={14}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.2c09096}, DOI={10.1021/acsami.2c09096}, abstractNote={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.}, number={32}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Joshi, Pratik and Shukla, Shubhangi and Gupta, Siddharth and Riley, Parand R. and Narayan, Jagdish and Narayan, Roger}, year={2022}, month={Aug}, pages={37149–37160} }
 @article{gupta_joshi_sachan_narayan_2022, title={Fabricating Graphene Oxide/h-BN Metal Insulator Semiconductor Diodes by Nanosecond Laser Irradiation}, volume={12}, ISSN={["2079-4991"]}, url={https://doi.org/10.3390/nano12152718}, DOI={10.3390/nano12152718}, abstractNote={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.}, number={15}, journal={NANOMATERIALS}, author={Gupta, Siddharth and Joshi, Pratik and Sachan, Ritesh and Narayan, Jagdish}, year={2022}, month={Aug} }
 @article{gupta_gupta_mandal_sachan_2022, title={Laser Irradiation-Induced Nanoscale Surface Transformations in Strontium Titanate}, volume={12}, ISSN={["2073-4352"]}, url={https://doi.org/10.3390/cryst12050624}, DOI={10.3390/cryst12050624}, abstractNote={We studied the structural transformations and atomic rearrangements in strontium titanate (SrTiO3) via nanosecond pulsed laser irradiation-induced melting and ultrafast quenching. Using scanning transmission electron microscopy, we determine that the laser-irradiated surface in single-crystalline SrTiO3 transforms into an amorphous phase with an interposing disordered crystalline region between amorphous and ordered phases. The formation of disordered phase is attributed to the rapid recrystallization of SrTiO3 from the melt phase constrained by an epitaxial relation with the pristine region, which eases up on the surface, leading to amorphous phase formation. With electron energy-loss spectroscopic analysis, we confirm the transformation of Ti+4 to Ti+3 due to oxygen vacancy formation as a result of laser irradiation. In the disordered region, the maximum transformation of Ti+4 is observed to be 16.2 ± 0.2%, whereas it is observed to be 20.2 ± 0.2% in the amorphous region. Finally, we deduce that the degree of the disorder increases from atomically disordered to amorphous transition in SrTiO3 under laser-irradiation. The signatures of short-range ordering remain similar, leading to a comparable fingerprint of electronic structure. With these results, this study addresses the gap in understanding the atomic and electronic structure modified by pulsed laser irradiation and functionalizing pristine SrTiO3 for electronic, magnetic, and optical applications.}, number={5}, journal={CRYSTALS}, author={Gupta, Ashish Kumar and Gupta, Siddharth and Mandal, Soumya and Sachan, Ritesh}, year={2022}, month={May} }
 @article{moatti_mineo-foley_gupta_sachan_narayan_2022, title={Spin Engineering of VO2 Phase Transitions and Removal of Structural Transition}, volume={14}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.1c24978}, DOI={10.1021/acsami.1c24978}, abstractNote={Vanadium dioxide undergoes a metal-to-insulator transition, where the energy of electron-electron, electron-lattice, spin-spin, and spin-lattice interactions are of the same order of magnitude. This leads to the coexistence of electronic and structural transitions in VO2 that limit the lifetime and speed of VO2-based devices. However, the closeness of interaction energy of lattice-electron-spin can be turned into an opportunity to induce some transitions while pinning others via external stimuli. That is, the contribution of spin, charge, orbital, and lattice degrees of freedom can be manipulated. In this study, spin engineering has been exploited to affect the spin-related interactions in VO2 by introducing a ferromagnetic Ni layer. The coercivity in the Ni layer is engineered by controlling the shape anisotropy via kinetics of growth. Using spin engineering, the structural pinning of the monoclinic M2 phase of VO2 is successfully achieved, while the electronic and magnetic transitions take place.}, number={10}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Moatti, Adele and Mineo-Foley, Gabrielle and Gupta, Siddharth and Sachan, Ritesh and Narayan, Jay}, year={2022}, month={Mar}, pages={12883–12892} }
 @misc{joshi_riley_gupta_narayan_narayan_2021, title={Advances in laser-assisted conversion of polymeric and graphitic carbon into nanodiamond films}, volume={32}, ISSN={["1361-6528"]}, url={https://doi.org/10.1088/1361-6528/ac1097}, DOI={10.1088/1361-6528/ac1097}, abstractNote={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 109 K 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.}, number={43}, journal={NANOTECHNOLOGY}, publisher={IOP Publishing}, author={Joshi, Pratik and Riley, Parand and Gupta, Siddharth and Narayan, Roger J. and Narayan, Jagdish}, year={2021}, month={Oct} }
 @article{gupta_gupta_sachan_2021, title={Laser Irradiation Induced Atomic Structure Modifications in Strontium Titanate}, volume={11}, ISSN={["1543-1851"]}, DOI={10.1007/s11837-021-04996-1}, journal={JOM}, author={Gupta, Ashish Kumar and Gupta, Siddharth and Sachan, Ritesh}, year={2021}, month={Nov} }
 @article{joshi_gupta_riley_narayan_narayan_2021, title={Liquid phase regrowth of (110) nanodiamond film by UV laser annealing of PTFE to generate dense CVD microdiamond film}, volume={117}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2021.108481}, abstractNote={Herein we report the conversion of polytetrafluoroethylene (PTFE) into 〈110〉 nanodiamonds via a melting route using pulsed laser annealing (PLA). The converted nanodiamond (ND) film is used as a seed layer to grow dense microdiamond coating synthesized by chemical vapor deposition. We utilize an ArF excimer laser with a photon energy of 6.4 eV to decompose PTFE (bandgap: 6.0 eV). Initial laser pulses result in photochemical decomposition of PTFE, and PTFE is converted to an amorphous carbon film. This amorphous carbon film, when subjected to additional laser pulses melts, and when this melt is quenched from an undercooled state at rates exceeding 109 K/s, it undergoes first-order phase transformation into the ND film. Notably, the obtained NDs are phase pure, exhibiting full width at half maxima (FWHM) of 1.23 cm−1 and demonstrating 〈110〉 out of plane orientation characterized by Raman spectroscopy and transmission electron microscopy, respectively. The average ND size is ~28.5 nm (range: 5-30 nm) determined by scanning electron microscopy and X-ray diffraction. The COMSOL simulations substantiate the use of nanosecond laser pulses with an energy density in the range of 0.6–0.8 J/cm2 to fully convert ~ 50% crystalline PTFE into ND film. The CVD microdiamonds grew densely on the ND seed layer as compared to reduced graphene oxide confirmed by SEM and Raman analysis. This innovative method of ND fabrication by UV irradiation of PTFE opens up opportunities for generating selective coatings of advanced polymer-diamond composites and doped nanodiamonds for quantum computing and biomedical applications.}, journal={DIAMOND AND RELATED MATERIALS}, author={Joshi, Pratik and Gupta, Siddharth and Riley, Parand R. and Narayan, Roger J. and Narayan, Jagdish}, year={2021}, month={Aug} }
 @article{narayan_bhaumik_gupta_joshi_riley_narayan_2021, title={Role of Q-carbon in nucleation and formation of continuous diamond film}, volume={176}, ISSN={["1873-3891"]}, DOI={10.1016/j.carbon.2021.02.049}, abstractNote={Formation of continuous and adherent diamond films on practical substrates presents a formidable challenge due to lack of diamond nucleation sites needed for diamond growth. This problem has been solved through the formation of interfacial Q-carbon layers by nanosecond laser melting of carbon layers in a highly undercooled state and subsequent quenching. The Q-carbon layer provides ready nucleation sites for epitaxial films on planar matching substrates such as sapphire, and polycrystalline films on amorphous substrates such as glass. Each laser pulse converts about a one-cm-square area, which can be repeated with a 100–200 Hz laser to produce potentially 100–200 cm2s-1 of diamond films. This is essentially a low-temperature processing, where substrate stays close to ambient temperature, because the total heat input is quite small. The Q-carbon layer is also responsible for improved adhesion of diamond films on sapphire and glass substrates. It is also argued that the formation of Q-carbon layer is also responsible for efficient diamond nucleation during negatively biased MPCVD diamond depositions.}, journal={CARBON}, author={Narayan, J. and Bhaumik, A. and Gupta, S. and Joshi, P. and Riley, P. and Narayan, R. J.}, year={2021}, month={May}, pages={558–568} }
 @article{joshi_haque_gupta_narayan_narayan_2021, title={Synthesis of multifunctional microdiamonds on stainless steel substrates by chemical vapor deposition}, volume={171}, ISSN={["1873-3891"]}, DOI={10.1016/j.carbon.2020.09.064}, abstractNote={We report on the synthesis of multifunctional microdiamonds by chemical vapor deposition (CVD) on 304 and 316 austenitic stainless steel (SS) substrates. The increase in wettability achieved by surface scratching and the structure of ultra-dense Q-carbon achieved high nucleation density and minimized strains in diamond films. Notably, these diamond films exhibit a high amount of twinning, leading to the formation of five-fold microdiamonds. The diamonds on scratched SS substrate and Q-carbon interlayer exhibit a full width at half maximum of 8.25 cm−1 and 11.5 cm−1, compared to 26 cm−1 on bare SS substrate. The diamond films grown on bare SS substrate exhibited cracking due to high tensile stress of 2.3 GPa, ascribed to thermal mismatch between SS and diamond. The electron backscattered diffraction investigations reveal iron inclusions in diamonds synthesized on bare SS substrates, which may create ferromagnetism in these diamonds. This route, compared to the ion beam implantation method using ferromagnetic ions, yields better samples. At 800 °C, 1012 Fe atoms/cm2s are transferred from the SS substrate into the diamonds. The dominant growth orientation for these CVD diamonds was determined to be <110> out of plane. These multifunctional microdiamonds are useful for biomedical, electronic, and tribological applications.}, journal={CARBON}, author={Joshi, Pratik and Haque, Ariful and Gupta, Siddharth and Narayan, Roger J. and Narayan, Jagdish}, year={2021}, month={Jan}, pages={739–749} }
 @article{haque_gupta_narayan_2020, title={Characteristics of diamond deposition on Al2O3, diamond-like carbon and Q-carbon}, volume={2}, url={http://dx.doi.org/10.1021/acsaelm.0c00106}, DOI={10.1021/acsaelm.0c00106}, abstractNote={We have conducted a comparative study on the deposition of diamond thin film on uncoated Al2O3, diamond-like carbon (DLC, grown by pulsed laser deposition) film-coated Al2O3, and Q-carbon (fabricat...}, number={5}, journal={ACS Applied Electronic Materials}, publisher={American Chemical Society (ACS)}, author={Haque, Ariful and Gupta, Siddharth and Narayan, Jagdish}, year={2020}, month={Apr}, pages={1323–1334} }
 @article{gupta_narayan_2020, title={Direct conversion of Teflon into nanodiamond films}, volume={8}, url={https://doi.org/10.1080/21663831.2020.1778111}, DOI={10.1080/21663831.2020.1778111}, abstractNote={We report a novel nonequilibrium approach for direct laser writing diamond by melting amorphous polytetrafluoroethylene (PTFE:(C2F4) n) in ambient conditions. The nanosecond laser pulses disintegrate PTFE, forming the undercooled molten carbon. This undercooled molten carbon regrows into diamond during the ultrafast melt quenching process, which lasts for ∼100 ns. HRTEM imaging, SAED, Raman, and EEL spectroscopy investigations confirm the first-order phase transformation of PTFE into single-crystalline <110> oriented diamond, which is associated with ultrafast unseeded crystallization. Our experimental findings open up a new pathway for the selective conversion of organic polymers into nano and microdiamonds and thin films with laser-writing. Impact statement This research represents a fundamental breakthrough in selective conversion of PTFE polymeric films into single crystalline diamond thin films by pulsed laser annealing at near ambient conditions. GRAPHICAL ABSTRACT}, number={11}, journal={Materials Research Letters}, publisher={Taylor & Francis}, author={Gupta, Siddharth and Narayan, Jagdish}, year={2020}, pages={408–416} }
 @article{gupta_joshi_narayan_2020, title={Electron mobility modulation in graphene oxide by controlling carbon melt lifetime}, volume={170}, ISSN={["1873-3891"]}, DOI={10.1016/j.carbon.2020.07.073}, abstractNote={The lack of bandgap is a fundamental issue in graphene devices, which can be solved by fabricating reduced graphene oxide (rGO). However, its device integration is impeded by the elevated reduction temperature (>2000 K) requirements. Recently, we demonstrated a new approach for laser writing heavily-reduced GO by employing the nonequilibrium approach of nanosecond laser annealing (Gupta and Narayan, 2019) [1]. Here, we report on the electron mobility modulation in the liquid phase grown graphene oxide. The process involves melting and subsequent quenching of molten carbon, which triggers the first-order phase transformation of amorphous carbon (a-C) into rGO. Laser annealing at energy density above the 0.3 J/cm2 melting threshold results in liquid-phase rGO growth on Si/SiO2. The rGO films exhibit 26 cm2/V-s room-temperature electron mobility and −4.7 × 1021/cc charge carrier concentration on annealing near melt threshold. The heavily-reduced GO films are formed on -O- creeping in the loosely-packed low undercooled carbon melt during ultrafast quenching. We establish that -O- injection is an implicit function of melt lifetime, and a rise in melt lifetime triggers GO film regrowth with increased mobility >210 cm2/V-s and 2.2 × 1019/cc carrier concentration on annealing at 0.6 J/cm2. Laser annealing resolves the fundamental issues of impurities and topological defects in rGO fabrication by equilibrium-based methods, facilitating increased electron mobility in laser patterned graphene-based materials.}, journal={CARBON}, author={Gupta, Siddharth and Joshi, Pratik and Narayan, Jagdish}, year={2020}, month={Dec}, pages={327–337} }
 @article{gupta_sachan_narayan_2020, title={Evidence of weak antilocalization in epitaxial TiN thin films}, volume={498}, ISSN={["1873-4766"]}, DOI={10.1016/j.jmmm.2019.166094}, abstractNote={Defect engineering provides a tremendous opportunity to impart novel functionalities to nanomaterials. This report is focused on TiN metallic system, where the unpaired spin structure and electron-transport are controlled by injecting nitrogen vacancies (VN). The TiN films are epitaxial, with the TiN/Al2O3 epitaxial relationship given by: (1 1 1) TiN//(0 0 0 1) Al2O3 as out-of-plane, and 〈11-0〉 TiN//〈1 0 1- 0〉 Al2O3 and 〈1 1 2-〉 TiN//〈1 1 2-0〉 Al2O3 as in-plane, after 30° rotation. Epitaxy in such a large misfit system (~9.24%) is rationalized to arise via domain matching epitaxy (DME) paradigm. Following the report of room-temperature ferromagnetism [1] in TiN1−x films formed by injecting nitrogen vacancies, we provide direct experimental evidence of weak antilocalization (WAL) effects by plugging VN using nitrogen annealing of TiN films. This evidence with simultaneous loss of magnetization in nitrogen annealed TiN films is the tell-tale sign of VN acting as magnetically active defects in TiN, as their removal facilitates Berry’s phase formation and generation of time-reversal symmetry. Through detailed EELS and Raman analysis, we have explicitly shown the absence of Ti+2 polarons in TiN films on N2 annealing. The resistivity minima in TiN films are attributed to the WAL effect with persistent log T behavior under 0–7 Tesla magnetic fields. The temperature-dependent coherence length analysis also highlights the emergence of WAL under the two-dimensional localization theory. The WAL effect in TiN is similar to topological insulators, quenching on the introduction of magnetically active defects, while stable against non-magnetic defects. Our findings demonstrate the prime importance of nitrogen vacancies in tuning the magentotransport characteristics in epitaxial nitride films for optoelectronic device applications.}, journal={JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS}, author={Gupta, Siddharth and Sachan, Ritesh and Narayan, Jagdish}, year={2020}, month={Mar} }
 @article{fabrication of ultrahard q-carbon nanocoatings on aisi 304 and 316 stainless steels and subsequent formation of high-quality diamond films_2020, url={http://dx.doi.org/10.1016/j.diamond.2020.107742}, DOI={10.1016/j.diamond.2020.107742}, abstractNote={Ideal coatings require three critical elements: hardness, toughness, and adhesion. Coatings of diamond-related materials are appealing owing to their high hardness but exhibit inadequate adhesion and toughness, especially on stainless steel substrates. Q-carbon, a newly discovered allotrope of carbon has the potential to improve hardness, toughness, and adhesion. The Q-carbon is formed on melting and quenching from super undercooled melt state. In this study, we have synthesized robust Q-carbon/α-carbon and Q-carbon/nanodiamond heterostructures on austenitic stainless steel substrates by laser annealing amorphous carbon films with nanosecond laser pulses above melt threshold (ED). Maximum melt regrowth velocity of 13 m/s corresponding to Q-carbon/nanodiamond composite was obtained by laser-solid melt interaction simulations. Subsequently, Q-carbon was used as seed layer to grow microdiamonds by HFCVD (hot filament chemical vapor deposition). The Q-carbon seed layer helped growth of better quality diamonds (50% less graphitic) with FWHM of 11.5 cm-1 and demonstrated higher nucleation density in comparision with amorphous carbon-coated and bare 316 SS substrates. Diamonds grown on Q-carbon displayed ballas type of microstructure indicative of high toughness. This study on Q-carbon nanocomposite coatings provides a new pathway for fabricating ultrahard carbon-based coatings on stainless steels for biomedical and tribocorrosive applications.}, journal={Diamond and Related Materials}, year={2020}, month={Apr} }
 @article{gupta_sachan_narayan_2020, title={Nanometer-Thick Hexagonal Boron Nitride Films for 2D Field-Effect Transistors}, volume={3}, url={https://doi.org/10.1021/acsanm.0c01416}, DOI={10.1021/acsanm.0c01416}, abstractNote={Hexagonal Boron Nitride (h-BN) is attracting significant attention as an ultimate dielectric and active layer in multifunctional 2D heterostructured devices due to the high bandgap and pliability t...}, number={8}, journal={ACS Applied Nano Materials}, publisher={American Chemical Society (ACS)}, author={Gupta, Siddharth and Sachan, Ritesh and Narayan, Jagdish}, year={2020}, month={Aug}, pages={7930–7941} }
 @article{sachan_gupta_narayan_2020, title={Nonequilibrium Structural Evolution of Q-Carbon and Interfaces}, volume={12}, url={https://doi.org/10.1021/acsami.9b17428}, DOI={10.1021/acsami.9b17428}, abstractNote={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 non-equilibrium 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 rate 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 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.}, note={PMID: 31833353}, number={1}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Sachan, Ritesh and Gupta, Siddharth and Narayan, Jagdish}, year={2020}, month={Jan}, pages={1330–1338} }
 @article{gupta_narayan_2020, title={Selective Liquid-Phase Regrowth of Reduced Graphene Oxide, Nanodiamond, and Nanoscale Q-Carbon by Pulsed Laser Annealing for Radiofrequency Devices}, volume={3}, url={https://doi.org/10.1021/acsanm.0c00609}, DOI={10.1021/acsanm.0c00609}, abstractNote={We report on the fabrication of various nanoscale carbon structures (Q-carbon, diamond, α-carbon, and reduced graphene oxide) by controlling the quenching rates of undercooled molten carbon. Laser ...}, number={6}, journal={ACS Applied Nano Materials}, publisher={American Chemical Society (ACS)}, author={Gupta, Siddharth and Narayan, Jagdish}, year={2020}, month={Jun}, pages={5178–5188} }
 @article{narayan_bhaumik_sachan_haque_gupta_pant_2019, title={Direct conversion of carbon nanofibers and nanotubes into diamond nanofibers and the subsequent growth of large-sized diamonds}, volume={11}, ISSN={2040-3364 2040-3372}, url={http://dx.doi.org/10.1039/C8NR08823C}, DOI={10.1039/C8NR08823C}, abstractNote={We report a pulsed laser annealing method to convert carbon fibers and nanotubes into diamond fibers under ambient conditions.}, number={5}, journal={Nanoscale}, publisher={Royal Society of Chemistry (RSC)}, author={Narayan, J. and Bhaumik, A. and Sachan, R. and Haque, A. and Gupta, S. and Pant, P.}, year={2019}, pages={2238–2248} }
 @article{gupta_sachan_narayan_2019, title={Evidence of weak antilocalization in epitaxial TiN thin films}, url={http://www.sciencedirect.com/science/article/pii/S0304885319328847}, DOI={https://doi.org/10.1016/j.jmmm.2019.166094}, abstractNote={Defect engineering provides a tremendous opportunity to impart novel functionalities to nanomaterials. This report is focused on TiN metallic system, where the unpaired spin structure and electron-transport are controlled by injecting nitrogen vacancies (VN). The TiN films are epitaxial, with the TiN/Al2O3 epitaxial relationship given by: (1 1 1) TiN//(0 0 0 1) Al2O3 as out-of-plane, and 〈11-0〉 TiN//〈1 0 1- 0〉 Al2O3 and 〈1 1 2-〉 TiN//〈1 1 2-0〉 Al2O3 as in-plane, after 30° rotation. Epitaxy in such a large misfit system (~9.24%) is rationalized to arise via domain matching epitaxy (DME) paradigm. Following the report of room-temperature ferromagnetism [1] in TiN1−x films formed by injecting nitrogen vacancies, we provide direct experimental evidence of weak antilocalization (WAL) effects by plugging VN using nitrogen annealing of TiN films. This evidence with simultaneous loss of magnetization in nitrogen annealed TiN films is the tell-tale sign of VN acting as magnetically active defects in TiN, as their removal facilitates Berry’s phase formation and generation of time-reversal symmetry. Through detailed EELS and Raman analysis, we have explicitly shown the absence of Ti+2 polarons in TiN films on N2 annealing. The resistivity minima in TiN films are attributed to the WAL effect with persistent log T behavior under 0–7 Tesla magnetic fields. The temperature-dependent coherence length analysis also highlights the emergence of WAL under the two-dimensional localization theory. The WAL effect in TiN is similar to topological insulators, quenching on the introduction of magnetically active defects, while stable against non-magnetic defects. Our findings demonstrate the prime importance of nitrogen vacancies in tuning the magentotransport characteristics in epitaxial nitride films for optoelectronic device applications.}, journal={Journal of Magnetism and Magnetic Materials}, author={Gupta, Siddharth and Sachan, Ritesh and Narayan, Jagdish}, year={2019}, pages={166094} }
 @article{formation of q-carbon and diamond coatings on wc and steel substrates_2019, url={http://dx.doi.org/10.1016/j.diamond.2019.107515}, DOI={10.1016/j.diamond.2019.107515}, abstractNote={Recently we reported the formation of diamond and a new phase of Q-carbon, harder than diamond, on sapphire (0001) substrates by direct conversion of amorphous carbon layers by nanosecond pulsed laser melting or annealing. Here we show that this process can be extended to other substrates, specifically WC (Co-bonded) and steels that are needed for high-speed machining and oil and gas exploration and biomedical applications. Since diamond and Q-carbon layers are formed via laser melting and quenching of carbon, these layers are quite adherent to the substrates, thus solving a critical adhesion problem associated with initial graphitic layers in CVD diamond coatings. We have used these Q-carbon, diamond and Q-carbon-diamond composites as seed layers for HFCVD (hot filament chemical vapor deposition) to grow thicker and more adherent layers of diamond.}, journal={Diamond and Related Materials}, year={2019}, month={Oct} }
 @article{gupta_narayan_2019, title={Non-equilibrium processing of ferromagnetic heavily reduced graphene oxide}, volume={153}, url={https://doi.org/10.1016/j.carbon.2019.07.064}, DOI={10.1016/j.carbon.2019.07.064}, abstractNote={Discovery of ferromagnetism with simultaneous bandgap opening in graphene[1] provides an attractive platform towards multifunctional spintronic devices. However, device integration of graphene and reduced graphene oxide (rGO) is hindered by scalability and high temperature required for reducing GO. In this paper, we present a nonequilibrium approach for direct laser writing heavily reduced GO films by melting amorphous carbon in ambient conditions. Nanosecond laser irradiation melts carbon, which regrows into rGO in low undercooling conditions. These rGO films exhibit room-temperature ferromagnetism with a high saturation magnetization of 7.0 emu/g and 40 Oe coercivity. The intrinsic ferromagnetic ordering triggers a broad negative magnetoresistance (MR) cusp from 20 to 50 K. An anomalous crossover from weak localization (WL) to weak antilocalization (WAL) is observed below 5 K, suggesting a substantial enhancement in spin-orbit coupling strength, opening a new route to access topological states in rGO. The rGO films exhibit 12.6 cm2/Vs electron mobility with n-type carrier concentration of 1.2 × 1021/cc. Raman spectroscopy and temperature-dependent transport investigations in rGO suggest low-disorder, following 2D Mott variable range hopping (VRH) mechanism with a bandgap of ∼0.22 eV and 3 nm localization length. These findings open a definitive pathway for tuning electrical and magnetic properties in graphene-based materials with laser-writing.}, journal={Carbon}, publisher={Elsevier BV}, author={Gupta, Siddharth and Narayan, Jagdish}, year={2019}, month={Nov}, pages={663–673} }
 @article{gupta_narayan_2019, title={Reduced Graphene Oxide/Amorphous Carbon P–N Junctions: Nanosecond Laser Patterning}, volume={11}, url={https://doi.org/10.1021/acsami.9b05374}, DOI={10.1021/acsami.9b05374}, abstractNote={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 report, we present a novel approach for direct laser writing of heavily reduced graphene oxide (rGO) films by nanosecond laser melting of amorphous carbon on silicon (001) substrates in 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/Vs and charge carrier concentration of -1.2x1021/cc at 300 K. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest low disorder and charge transport via 2D Mott variable range hopping (VRH) 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 a p-n junction heterojunction diodes with a turn-on voltage of 0.3 V, rectification ratio of 110 @±1.5V 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.}, number={27}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Gupta, Siddharth and Narayan, Jagdish}, year={2019}, month={Jul}, pages={24318–24330} }
 @article{gupta_moatti_bhaumik_sachan_narayan_2019, title={Room-temperature ferromagnetism in epitaxial titanium nitride thin films}, volume={166}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/J.ACTAMAT.2018.12.041}, DOI={10.1016/j.actamat.2018.12.041}, abstractNote={Localized charge injection by formation of vacancies provides an attractive platform for the development of multifunctional nanomaterials with direct implications in spintronics. However, further progress in spintronics critically depends on a deeper understanding of polaronic interactions between the localized charge states. This report is focused on TiN metallic system, which exhibits Pauli paramagnetism due to the absence of unpaired localized spin states. Here, nitrogen vacancies (VN) are used as a variable to tune the magnetic properties of epitaxial TiN thin films by thermal annealing in high-vacuum and N2 environment. Systematic introduction of VN generates robust magnetic ordering in vacuum-annealed TiN1-x films, with Curie temperature (TC) ∼700 K, and saturation magnetization (Ms) at absolute zero of 13.6 emu g−1. The signature spin-glass behavior below the irreversibility temperature (Tir ∼40 K) indicates the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling interactions between the unpaired localized spin-states. Through spatially resolved electron energy-loss spectroscopy, we have determined the generation of unpaired localized spins at Ti+2 polarons with ∼12 ± 2 at.% VN in TiN1-x films. Such a large concentration of VN results in increased spin stiffness and high TC. These findings open a definitive pathway for tuning the magnetic nature of metallic materials for spintronic applications.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Gupta, Siddharth and Moatti, Adele and Bhaumik, Anagh and Sachan, Ritesh and Narayan, Jagdish}, year={2019}, month={Mar}, pages={221–230} }
 @article{scale-up of q‑carbon and nanodiamonds by pulsed laser annealing_2019, url={http://dx.doi.org/10.1016/j.diamond.2019.107531}, DOI={10.1016/j.diamond.2019.107531}, abstractNote={Q‑carbon is a densely-packed metastable phase of carbon, which is harder than diamond. It is formed by nanosecond laser melting and ultrafast quenching of amorphous carbon films. The formation of Q‑carbon is a strictly undercooling driven phenomenon and requires uniform sp3/sp2 composition of as-deposited diamond-like carbon (DLC) films. This study illustrates the growth of wafer-scale DLC and its conversion into Q‑carbon and diamond by nanosecond pulsed laser annealing (PLA). A two-dimensional transient heat conduction model was created to analyze the melt front and regrowth conditions of PLA processing, utilizing a temporal and spatial Gaussian heat source at a laser fluence of 800 mJ/cm2. The transient temperature and melt-depth profiles obtained revealed successful melting of carbon with regrowth of Q‑carbon phase. By optimizing laser plume energetics, uniformity in sp3 content (~40%) of DLC films was achieved, which led to the formation of Q‑carbon over a large area. Here, the challenges associated with the scale-up of Q‑carbon are discussed along with analysis of the interfacial bonding between the undercooled layers using atomic-force and electron microscopy techniques. The wafer scale-up of DLC films with controlled sp3/sp2 content will be useful for making scalable Q‑carbon based longer-lasting protective coatings and Q‑carbon based electronic devices. These findings should help to further the development of Q‑carbon and diamond-related materials technology for commercialization of high-performance systems.}, journal={Diamond and Related Materials}, year={2019}, month={Aug} }
 @inproceedings{gupta_sachan_narayan_2019, title={Wafer scale-up and emergence of ferromagnetism in superhard Q-carbon coatings by nanosecond pulsed laser irradiation}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85068799491&partnerID=MN8TOARS}, booktitle={International SAMPE Technical Conference}, author={Gupta, S. and Sachan, R. and Narayan, J.}, year={2019} }
 @article{gupta_sachan_bhaumik_narayan_2018, title={Enhanced mechanical properties of Q-carbon nanocomposites by nanosecond pulsed laser annealing}, volume={29}, ISSN={["1361-6528"]}, url={https://doi.org/10.1088/1361-6528/aadd75}, DOI={10.1088/1361-6528/aadd75}, abstractNote={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 sp3-rich Q-carbon (82% sp3), and sp2-rich α-carbon (40% sp3) amorphous phases. The nanoindentation tests show that the Q-carbon nanocomposites exhibit a hardness of 67 GPa (Young’s modulus ∼ 840 GPa) in contrast to the soft α-carbon (hardness ∼ 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.}, number={45}, journal={NANOTECHNOLOGY}, publisher={IOP Publishing}, author={Gupta, Siddharth and Sachan, Ritesh and Bhaumik, Anagh and Narayan, Jagdish}, year={2018}, month={Nov} }
 @article{narayan_bhaumik_gupta_haque_sachan_2018, title={Progress in Q-carbon and related materials with extraordinary properties}, volume={6}, ISSN={["2166-3831"]}, url={https://doi.org/10.1080/21663831.2018.1458753}, DOI={10.1080/21663831.2018.1458753}, abstractNote={ABSTRACT This paper summarizes our research related to Q-carbon and Q-BN and direct conversion of carbon into diamond and h-BN into c-BN. Synthesis and processing of these materials are accomplished by nanosecond laser melting and subsequent quenching of amorphous carbon and nanocrystalline h-BN. Depending upon the degree of undercooling, molten carbon (or h-BN) can be converted into Q-carbon (or Q-BN) or diamond (or c-BN). The primary focus here is on the outstanding properties of these materials, including hardness greater than diamond, ferromagnetism, p- and n-type doping, NV nanodiamonds, high-temperature superconductivity in B-doped Q-carbon, enhanced field emission, superhard composite coatings, and future applications. IMPACT STATEMENT This research represents a fundamental breakthrough in the direct conversion of carbon into diamond at ambient temperatures and pressures in the air and their extraordinary properties. GRAPHICAL ABSTRACT}, number={7}, journal={MATERIALS RESEARCH LETTERS}, publisher={Taylor & Francis}, author={Narayan, Jagdish and Bhaumik, Anagh and Gupta, Siddharth and Haque, Ariful and Sachan, Ritesh}, year={2018}, pages={353–364} }
 @article{narayan_gupta_bhaumik_sachan_cellini_riedo_2018, title={Q-carbon harder than diamond}, volume={8}, ISSN={["2159-6867"]}, url={https://doi.org/10.1557/mrc.2018.35}, DOI={10.1557/mrc.2018.35}, abstractNote={A new phase of carbon named Q-carbon is found to be over 40% harder than diamond. This phase is formed by nanosecond laser melting of amorphous carbon and rapid quenching from the super-undercooled state. Closely packed atoms in molten metallic carbon are quenched into Q-carbon with 80-85% sp ^3 and the rest sp ^2. The number density of atoms in Q-carbon can vary from 40% to 60% higher than diamond cubic lattice, as the tetrahedra packing efficiency increases from 70% to 80%. Using this semiempirical approach, the corresponding increase in Q-carbon hardness is estimated to vary from 48% to 70% compared to diamond.}, number={2}, journal={MRS COMMUNICATIONS}, publisher={Cambridge University Press (CUP)}, author={Narayan, Jagdish and Gupta, Siddharth and Bhaumik, Anagh and Sachan, Ritesh and Cellini, Filippo and Riedo, Elisa}, year={2018}, month={Jun}, pages={428–436} }
 @article{bhaumik_sachan_gupta_narayan_nori_kumar_majumdar_2018, title={Room-Temperature Ferromagnetism and Extraordinary Hall Effect in Nanostructured Q‐Carbon: Implications for Potential Spintronic Devices}, volume={1}, url={https://doi.org/10.1021/acsanm.7b00253}, DOI={10.1021/acsanm.7b00253}, abstractNote={We report extraordinary Hall effect and room-temperature ferromagnetism in undoped Q-carbon, which is formed by nanosecond pulsed laser melting and subsequent quenching process. Through detailed structure–property correlations in Q-carbon thin films, we show the excess amount of unpaired electrons near the Fermi energy level give rise to interesting magnetic and electrical properties. The analysis of the extraordinary Hall effect in Q-carbon follows nonclassical “side-jump” electronic scattering mechanism. The isothermal field-dependent magnetization plots confirm room-temperature ferromagnetism in Q-carbon with a finite coercivity at 300 K and a Curie temperature of 570 K, obtained by the extrapolation of the fits to experimental data using modified Bloch’s law. High-resolution scanning electron microscopy and transmission electron microscopy clearly illustrate the formation of Q-carbon and its subsequent conversion to single-crystalline diamond. Further, we found n-type conductivity in Q-carbon in the e...}, number={2}, journal={ACS Applied Nano Materials}, author={Bhaumik, Anagh and Sachan, Ritesh and Gupta, Siddharth and Narayan, Jagdish and Nori, Sudhakar and Kumar, Dhananjay and Majumdar, Alak Kumar}, year={2018}, month={Jun}, pages={807–819} }
 @article{gupta_bhaumik_sachan_narayan_2018, title={Structural Evolution of Q-Carbon and Nanodiamonds}, volume={70}, ISSN={1047-4838 1543-1851}, url={http://dx.doi.org/10.1007/S11837-017-2714-Y}, DOI={10.1007/s11837-017-2714-y}, number={4}, journal={JOM}, publisher={Springer Science and Business Media LLC}, author={Gupta, Siddharth and Bhaumik, Anagh and Sachan, Ritesh and Narayan, Jagdish}, year={2018}, month={Jan}, pages={450–455} }
 @article{gupta_sachan_bhaumik_pant_narayan_2018, title={Undercooling driven growth of Q-carbon, diamond, and graphite}, volume={8}, ISSN={["2159-6867"]}, url={https://doi.org/10.1557/mrc.2018.76}, DOI={10.1557/mrc.2018.76}, abstractNote={We provide insights pertaining the dependence of undercooling in the formation of graphite, nanodiamonds, and Q-carbon nanocomposites by nanosecond laser melting of diamond-like carbon (DLC). The DLC films are melted rapidly in a super-undercooled state and subsequently quenched to room temperature. Substrates exhibiting different thermal properties-silicon and sapphire, are used to demonstrate that substrates with lower thermal conductivity trap heat flow, inducing larger undercooling, both experimentally and theoretically via finite element simulations. The increased undercooling facilitates the formation of Q-carbon. The Q-carbon is used as nucleation seeds for diamond growth via laser remelting and hot-filament chemical vapor deposition.}, number={2}, journal={MRS COMMUNICATIONS}, publisher={Cambridge University Press (CUP)}, author={Gupta, Siddharth and Sachan, Ritesh and Bhaumik, Anagh and Pant, Punam and Narayan, Jagdish}, year={2018}, month={Jun}, pages={533–540} }
 @article{moatti_sachan_gupta_narayan_2018, title={Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO2 Thin Films}, volume={11}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/ACSAMI.8B17879}, DOI={10.1021/acsami.8b17879}, abstractNote={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.}, number={3}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Moatti, Adele and Sachan, Ritesh and Gupta, Siddharth and Narayan, Jagdish}, year={2018}, month={Dec}, pages={3547–3554} }
 @article{bhaumik_sachan_gupta_narayan_2017, title={Discovery of High-Temperature Superconductivity (T-c=55 K) in B-Doped Q-Carbon}, volume={11}, ISSN={["1936-086X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040089020&partnerID=MN8TOARS}, DOI={10.1021/acsnano.7b06888}, abstractNote={We have achieved a superconducting transition temperature (Tc) of 55 K in 27 at% B-doped Q-carbon. This value represents a significant improvement over previously reported Tc of 36 K in B-doped Q-carbon and is the highest Tc for conventional BCS (Bardeen-Cooper-Schrieffer) superconductivity in bulk carbon-based materials. The B-doped Q-carbon exhibits type-II superconducting characteristics with Hc2(0) ∼ 10.4 T, consistent with the BCS formalism. The B-doped Q-carbon is formed by nanosecond laser melting of B/C multilayered films in a super undercooled state and subsequent quenching. It is determined that ∼67% of the total boron exists with carbon in a sp3 hybridized state, which is responsible for the substantially enhanced Tc. Through the study of the vibrational modes, we deduce that higher density of states near the Fermi level and moderate to strong electron-phonon coupling lead to a high Tc of 55 K. With these results, we establish that heavy B doping in Q-carbon is the pathway for achieving high-temperature superconductivity.}, number={12}, journal={ACS NANO}, author={Bhaumik, Anagh and Sachan, Ritesh and Gupta, Siddharth and Narayan, Jagdish}, year={2017}, month={Dec}, pages={11915–11922} }