arXiv subjects

Anand B. Puthirath

Publications and source records attributed to Anand B. Puthirath.

17 recordsLinked to original sources

arXiv2025

Anomalous properties of spark plasma sintered boron nitride solids

Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of mechanical strength, ductility, dielectric constant, thermal conductivity, and exceptional neutron radiation shielding capability. Through exhaustive characterizations we reveal that SPS induces non-basal plane crystallinity, twisting of layers, and facilitates inter-grain fusion with a high degree of in-plane alignment across macroscale dimensions, resulting in near-theoretical density and improved properties. Our findings highlight the importance of material design, via new approaches such as layer twisting and interlayer interconnections, to create novel ceramics with properties that could go beyond their intrinsic limits.

cond-mat.mtrl-sci↗

arXiv2023

Non-linear optics at twist interfaces in h-BN/SiC heterostructures

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, we suggest an alternative, simple and scalable approach where nanocrystalline two-dimensional (2D) film on three-dimensional (3D) substrates yield twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. Our work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

cond-mat.mtrl-sci↗

arXiv2023

Unravelling the room temperature growth of two-dimensional h-BN nanosheets for multifunctional applications

Room temperature growth of two-dimensional van der Waals (2D-vdW) materials is indispensable for state-of-the-art nanotechnology. The low temperature growth supersedes the requirement of elevated growth temperature accompanied with high thermal budgets. Moreover, for electronic applications, low or room temperature growth reduces the possibility of intrinsic film-substrate interfacial thermal diffusion related deterioration of functional properties and consequent device performance. Here, we demonstrated the growth of ultrawide-bandgap boron nitride (BN) at room temperature by using the pulsed laser deposition (PLD) process and demonstrated various functionalities for potential applications. Comprehensive chemical, spectroscopic and microscopic characterization confirms the growth of ordered nanosheet-like hexagonal BN. Functionally, nanosheets show hydrophobicity, high lubricity (low coefficient of friction), low refractive index within the visible to near-infrared wavelength range, and room temperature single-photon quantum emission. Our work unveils an important step that brings a plethora of applications potential for room temperature grown h-BN nanosheets as it can be feasible on any given substrate, thus creating a scenario for h-BN on demand at frugal thermal budget.

physics.app-ph↗

arXiv2023

Structural, optical, and thermal properties of BN thin films grown on diamond via pulsed laser deposition

Heterostructures based on ultrawide-bandgap (UWBG) semiconductors (bandgap >4.0 eV), boron nitride (BN) and diamond are important for next-generation high-power electronics. However, in-situ hetero-epitaxy of BN/diamond or vice-versa remains extremely challenging, due to their non-trivial growth kinetics. Here, we have grown BN thin film on (100) single crystal diamond by pulsed laser deposition and investigated its structural and magnetic properties, optical refractive index, and thermal conductivity. Structural characterizations confirm the mixed (stable hexagonal and metastable cubic) phase growth. Film shows diamagnetic behavior at room temperature. It displays anisotropic refractive index within the visible-to-near-infrared wavelength range. The room temperature cross-plane thermal conductivity of BN is ~1.53 W/(mK), and the thermal conductance of the BN/diamond interface is ~20 MW/(m2K). Our findings are useful for various device related applications based on UWBG BN/diamond heterostructures.

physics.app-ph↗

arXiv2023

VO2 Phase Change Electrodes in Li-ion Batteries

Use of electrode materials that show phase change behavior and hence drastic changes in electrochemical activity during operation, have not been explored for Li-ion batteries. Here we demonstrate the vanadium oxide (VO2) cathode that undergoes metal-insulator transition due to first-order structural phase transition at accessible temperature of 68°C for battery operation. Using a suitable electrolyte operable across the phase transition range and compatible with vanadium oxide cathodes, we studied the effect of electrode structure change on lithium insertion followed by the electrochemical characteristics above and below the phase transition temperature. The high-temperature VO2 phase shows significantly improved capacitance, enhanced current rate capabilities, improved electrical conductivity and lithium-ion diffusivity compared to the insulating low temperature phase. This opens up new avenues for electrode designs, allowing manipulation of electrochemical reactions around phase transition temperatures, and in particular enhancing electrochemical properties at elevated temperatures contrary to existing classes of battery chemistries that lead to performance deterioration at elevated temperatures.

physics.chem-ph↗

arXiv2023

Bottom-up Integration of TMDCs with Pre-Patterned Device Architectures via Transfer-free Chemical Vapor Deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) remain a topic of immense interest. Specifically, given their low operational switching costs, they find many niche applications in new computing architectures with the promise of continued miniaturization. However, challenges lie in Back End of Line (BEOL) integration temperature and time compliance regarding current requirements for crystal growth. Additionally, deleterious and time-consuming transfer processes and multiple steps involved in channel/contact engineering can cripple device performance. This work demonstrates kinetics-governed in-situ growth regimes (surface or edge growth from gold) of WSe2 and provides a mechanistic understanding of these regimes via energetics across various material interfaces. As a proof-of-concept, field effect transistors (FET) with an in-situ grown WSe2 channel across Au contacts are fabricated, demonstrating a 2D semiconductor transistor via a transfer-free method within the 450-600 C 2h-time window requirement BEOL integration. We leverage directional edge growth to fabricate contacts with robust thickness-dependent Schottky-to-Ohmic behavior. By transitioning between Au and SiO2 growth substrates in situ, this work achieves strain-induced subthreshold swing of 140 mV/decade, relatively high mobility of 107 +- 19 cm2V-1s-1, and robust ON/OFF ratios 10^6 in the fabricated FETs.

cond-mat.mtrl-sci↗

arXiv2023

Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.

cond-mat.mtrl-sci↗

arXiv2023

Unidirectional domain growth of hexagonal boron nitride thin films

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2022

Incommensurate magnetic orders and topological Hall effect in the square-net centrosymmetric EuGa$_2$Al$_2$ system

Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$ reveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In applied field, a new magnetic phase (A) is identified from magnetization and transport measurements, bounded by two of the $μ_0H$~=~0 incommensurate magnetic phases (AFM1,helical and AFM3, cycloidal) with different moment orientations. Moreover, magneto-transport measurements indicate the presence of a topological Hall effect, with maximum values centered in the A phase. Together, these results render EuGa$_2$Al$_2$ a material with non-coplanar or topological spin texture in applied field. X-ray diffraction reveals an out-of-plane (OOP) charge density wave (CDW) below $T_{CDW} \sim$ 50 K while the magnetic propagation vector lies in plane below $T_N$ = 19.5 K. Together these data point to a new route to realizing in-plane non-collinear spin textures through an OOP CDW. In turn, these non-collinear spin textures may be unstable against the formation of topological spin textures in an applied field.

cond-mat.str-el↗

arXiv2022

Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band XPS, FTIR, Raman) and microscopic (AFM and STEM) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (234 V) as compared to GaN (168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step towards bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

physics.app-ph↗

arXiv2022

Thin film growth of MAX phases as functional materials

Layered nanolaminate ternary carbides, nitrides and carbonitrides with general formula Mn+1AXn or MAX (n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 or 14 element, and X is C and/or N) has revolutionized the world of nanomaterials, due to the coexistence of both ceramic and metallic nature, giving rise to exceptional mechanical, thermal, electrical, chemical properties and wide range of applications. Although several solid-state bulk synthesis methods have been developed to produce a variety of MAX phases, however, for certain applications, the growth of MAX phases, especially in its high-quality epitaxial thin films form is of increasing interest. Here, we summarize the progress made thus far in epitaxial growth and property evaluation of MAX phase thin films grown by various deposition techniques. We also address the important future research directions to be made in terms of thin-film growth. Overall, in the future, high-quality single-phase epitaxial thin film growth and engineering of chemically diverse MAX phases may open up interesting new avenues for next-generation technology.

cond-mat.mtrl-sci↗

arXiv2021

Apparent Ferromagnetism in Exfoliated Ultra-thin Pyrite Sheets

Experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals has been missing until a bilayer Cr2Ge2Te6, and a three-atom thick monolayer CrI3 are shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals type ultra-thin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111) oriented sheets are predominant and is supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111) oriented three-atom thick pyrite sheet has a stable ferromagnetic ground state different from its diamagnetic bulk counterpart. This theoretical finding is evaluated experimentally employing low temperature superconducting quantum interference device measurements and observed an anomalous ferromagnetic kind of behavior.

cond-mat.mtrl-sci↗

arXiv2021

Soft-Pinning: Experimental Validation of Static Correlations in Supercooled Molecular Glass-forming Liquids

Enormous enhancement in the viscosity of a liquid near its glass transition is generally connected to the growing many-body static correlations near the transition, often coined as `amorphous ordering'. Estimating the length scales of such correlations in different glass-forming liquids is highly important to unravel the physics of glass formation. Experiments on molecular glass-forming liquids become pivotal in this scenario as the viscosity grows several folds ($\sim 10^{14}$), simulations or colloidal glass experiments fail to access the long-time scales required. Here we design an experiment to extract the static length scales in molecular liquids using dilute amounts of another large molecule as a pinning site. Results from dielectric relaxation experiments on supercooled glycerol with different pinning concentrations of sorbitol and the simulations on a few model glass-forming liquids with pinning sites indicate the robustness of the proposed method, opening a plethora of opportunity to study the physics of other glass-forming liquids.

cond-mat.soft↗

arXiv2021

A Reactive Molecular Dynamics Study of Hydrogenation on Diamond Surfaces

Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which is far from the experimentally observed hydrogen coverage. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (001), (013), (110), (113) and (111) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation in these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (001), (110), and (113) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity in the diamond surfaces, a key requirement for high-field, high-frequency device applications.

cond-mat.mtrl-sci↗

arXiv2020

Three-dimensional Printing of Complex Graphite Structures

Graphite, with many industrial applications, is one of the widely sought-after allotropes of carbon. The sp2 hybridized and thermodynamically stable form of carbon forms a layered structure with strong in-plane carbon bonds and weak inter-layer van der Waals bonding. Graphite is also a high-temperature ceramic, and shaping them into complex geometries is challenging, given its limited sintering behavior even at high temperatures. Although the geometric design of the graphite structure in many of the applications could dictate its precision performance, conventional synthesis methods for formulating complex geometric graphite shapes are limited due to the intrinsic brittleness and difficulties of high-temperature processing. Here, we report the development of colloidal graphite ink from commercial graphite powders with reproducible rheological behavior that allows the fabrication of any complex architectures with tunable geometry and directionality via 3D printing at room temperature. The method is enabled via using small amounts of clay, another layered material, as an additive, allowing the proper design of the graphene ink and subsequent binding of graphite platelets during printing. Sheared layers of clay are easily able to flow, adapt, and interface with graphite layers forming strong binding between the layers and between particles that make the larger structures. The direct ink printing of complex 3D architectures of graphite without further heat treatments could lead to easy shape engineering and related applications of graphite at various length scales, including complex graphite molds or crucibles. The 3D printed complex graphitic structures exhibit excellent thermal, electrical, and mechanical properties, and the clay additive does not seem to alter these properties due to the excellent inter-layer dispersion and mixing within the graphite material.

cond-mat.mtrl-sci↗

arXiv2020

U-carbon: metallic and magnetic

We report the discovery of a pristine crystalline 3D carbon that is magnetic, electrically conductive and stable under ambient conditions. This carbon material, which has remained elusive for decades, is synthesized by using the chemical vapor deposition (CVD) technique with a particular organic molecular precursor 3,3-dimethyl-1-butene (C6H12). An exhaustive computational search of the potential energy surface reveals its unique sp2-sp3 hybrid bonding topology. Synergistic studies involving a large number of experimental techniques and multi-scale first-principles calculations reveal the origin of its novel properties due to the special arrangement of sp2 carbon atoms in lattice. The discovery of this U-carbon, named such because of its unusual structure and properties, can open a new chapter in carbon science.

cond-mat.mtrl-sci↗

arXiv2019

Kondo exhaustion and conductive surface states in antiferromagnetic YbIr$_3$Si$_7$

The interplay of Kondo screening and magnetic ordering in strongly correlated materials containing local moments is a subtle problem.[1] Usually the number of conduction electrons matches or exceeds the number of moments, and a Kondo-screened heavy Fermi liquid develops at low temperatures.[2] Changing the pressure, magnetic field, or chemical doping can displace this heavy Fermi liquid in favor of a magnetically ordered state.[3,4] Here we report the discovery of a version of such a `Kondo lattice' material, YbIr$_3$Si$_7$, in which the number of free charge carriers is much less than the number of local moments. This leads to `Kondo exhaustion':[5] the electrical conductivity tends to zero at low temperatures as all the free carriers are consumed in the formation of Kondo singlets. This effect coexists with antiferromagnetic long-range order, with a Néel temperature $T\rm_N = 4.1\,{\rm K}$. Furthermore, the material shows conductive surface states with potential topological nature, and thus presents an exciting topic for future investigations.

cond-mat.str-el↗