arXiv subjects

Rahul Rao

Publications and source records attributed to Rahul Rao.

32 records · Page 2Linked to original sources

arXiv2021

Scalable synthesis of 2D van der Waals superlattices

Heterostructure materials form the basis of much of modern electronics, from transistors to lasers and light-emitting diodes. Recent years have seen a renewed focus on creating heterostructures through the vertical integration of two-dimensional materials, including graphene, hexagonal boron nitride, and transition metal dichalcogenides (TMDCs). However, fundamental challenges associated with materials processing have limited material quality and impeded scalability. We demonstrate a method to convert sub-nanometer metal films deposited on silicon and sapphire into TMDC heterostructures through vapor-phase processing. The resulting heterostructures and superlattices exhibit novel properties compared with stand-alone TMDCs, including reduced bandgap, enhanced light-matter coupling, and improved catalytic performance. This robust and scalable synthetic method provides new opportunities to generate a wide range of artificially stacked 2D superlattices with controlled morphology and composition.

cond-mat.mtrl-sci↗

arXiv2021

Ferrielectric-paraelectric phase transitions in layered CuInP2S6 and CuInP2S6- In4/3P2S6 heterostructures: A Raman spectroscopy and X-ray diffraction study

CuInP2S6 (CIPS) is an emerging layered material that exhibits ferrielectric ordering well above room temperature (Curie temperature TC ~ 315 K). When synthesized with Cu deficiencies, CIPS spontaneously segregates into CuInP2S6 and In4/3P2S6 domains (CIPS-IPS), which form self-assembled heterostructures within the individual lamellae. This re-structuring and resultant chemical pressure raises the Curie temperature and, depending on the Cu concentration, can be up to ~340 K for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the movement of Cu ions within the lattice. Here we uncover the microscopic origins underpinning the phase transitions in pure CIPS and CIPS-IPS (Cu0.4In1.2P2S6) by performing a temperature-dependent Raman and XRD study. We measured the frequencies and linewidths of various cation and anion phonon modes and compared them to the extracted atomic positions from the refinement of XRD data. Our analysis shows that in addition to the Cu cation movement, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This results in several discontinuities in peak frequencies and linewidths close to 315 K in CIPS. In the CIPS-IPS heterostructure, this process begins around 315 K and ends around 330 K.

cond-mat.mtrl-sci↗

arXiv2021

Pressure-driven phase transformations and phase segregation in ferrielectric CuInP$_2$S$_6$-In$_{4/3}$P$_2$S$_6$ self-assembled heterostructures

Layered multi-ferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized CuInP$_2$S$_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, CuInP$_2$S$_6$ spontaneously phase segregates to form ferrielectric CuInP$_2$S$_6$ (CIPS) and paraelectric In$_{4/3}$P$_2$S$_6$ (IPS) domains in a two-dimensional self-assembled heterostructure. Here, we study the effect of hydrostatic pressure on the structure of Cu-deficient CuInP$_2$S$_6$ by Raman spectroscopy measurements up to 20 GPa. Detailed analysis of the frequencies, intensities, and linewidths of the Raman peaks reveals four discontinuities in the spectra around 2, 10, 13 and 17 GPa. At ~2 GPa, we observe a structural transition initiated by the diffusion of IPS domains, which culminates in a drastic reduction of the number of peaks around 10 GPa. We attribute this to a possible monoclinic-trigonal phase transition at 10 GPa. At higher pressures (~ 13 GPa), significant increases in peak intensities and sharpening of the Raman peaks suggest a bandgap-lowering and an isostructural electronic transition, with a possible onset of metallization at pressures above 17 GPa. When the pressure is released, the structure again phase-separates into two distinct chemical domains within the same single crystalline framework -- however, these domains are much smaller in size than the as-synthesized material resulting in suppression of ferroelectricity through nanoconfinement. Hydrostatic pressure can thus be used to tune the electronic and ferrielectric properties of Cu-deficient layered CuInP$_2$S$_6$.

cond-mat.mtrl-sci↗

arXiv2021

Sequential Adaptive Design for Jump Regression Estimation

Selecting input variables or design points for statistical models has been of great interest in adaptive design and active learning. Motivated by two scientific examples, this paper presents a strategy of selecting the design points for a regression model when the underlying regression function is discontinuous. The first example we undertook was for the purpose of accelerating imaging speed in a high resolution material imaging; the second was use of sequential design for the purpose of mapping a chemical phase diagram. In both examples, the underlying regression functions have discontinuities, so many of the existing design optimization approaches cannot be applied because they mostly assume a continuous regression function. Although some existing adaptive design strategies developed from treed regression models can handle the discontinuities, the Bayesian approaches come with computationally expensive Markov Chain Monte Carlo techniques for posterior inferences and subsequent design point selections, which is not appropriate for the first motivating example that requires computation at least faster than the original imaging speed. In addition, the treed models are based on the domain partitioning that are inefficient when the discontinuities occurs over complex sub-domain boundaries. We propose a simple and effective adaptive design strategy for a regression analysis with discontinuities: some statistical properties with a fixed design will be presented first, and then these properties will be used to propose a new criterion of selecting the design points for the regression analysis. Sequential design with the new criterion will be presented with comprehensive simulated examples, and its application to the two motivating examples will be presented.

stat.ML↗

arXiv2020

Field-free deterministic switching of a perpendicularly polarized magnet using unconventional spin-orbit torques in WTe2

Spin-orbit torque (SOT) driven deterministic control of the magnetization state of a magnet with perpendicular magnetic anisotropy (PMA) is key to next generation spintronic applications including non-volatile, ultrafast, and energy efficient data storage devices. But, field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane anti-damping torque, which is not allowed in conventional spin source materials such as heavy metals (HM) and topological insulators due to the system's symmetry. The exploitation of low-crystal symmetries in emergent quantum materials offers a unique approach to achieve SOTs with unconventional forms. Here, we report the first experimental realization of field-free deterministic magnetic switching of a perpendicularly polarized van der Waals (vdW) magnet employing an out-of-plane anti-damping SOT generated in layered WTe2 which is a low-crystal symmetry quantum material. The numerical simulations confirm that out-of-plane antidamping torque in WTe2 is responsible for the observed magnetization switching in the perpendicular direction.

cond-mat.mes-hall↗

arXiv2020

Gaussian process surrogate modeling with manipulating factors for carbon nanotube growth experiments

This paper presents a new Gaussian process (GP) surrogate modeling for predicting the outcome of a physical experiment where some experimental inputs are controlled by other manipulating factors. Particularly, we are interested in the case where the control precision is not very high, so the input factor values vary significantly even under the same setting of the corresponding manipulating factors. The case is observed in our main application to carbon nanotube growth experiments, where one experimental input among many is manipulated by another manipulating factors, and the relation between the input and the manipulating factors significantly varies in the dates and times of operations. Due to this variation, the standard GP surrogate that directly relates the manipulating factors to the experimental outcome does not provide a great predictive power on the outcome. At the same time, the GP model relating the main factors to the outcome directly is not appropriate for the prediction purpose because the main factors cannot be accurately set as planned for a future experiment. Motivated by the carbon nanotube example, we propose a two-tiered GP model, where the bottom tier relates the manipulating factors to the corresponding main factors with potential biases and variation independent of the manipulating factors, and the top tier relates the main factors to the experimental outcome. Our two-tier model explicitly models the propagation of the control uncertainty to the experimental outcome through the two GP modeling tiers. We present the inference and hyper-parameter estimation of the proposed model. The proposed approach is illustrated with the motivating example of a closed-loop autonomous research system for carbon nanotube growth experiments, and the test results are reported with the comparison to a benchmark method, i.e. a standard GP model.

stat.ME↗

arXiv2019

Spectroscopic Evaluation of Charge-transfer Doping and Strain in Graphene/MoS2 Heterostructures

It is important to study the van der Waals interface in emerging vertical heterostructures based on layered two-dimensional (2D) materials. Being atomically thin, 2D materials are susceptible to significant strains as well as charge transfer doping across the interfaces. Here we use Raman and photoluminescence (PL) spectroscopy to study the interface between monolayer graphene/MoS2 heterostructures prepared by mechanical exfoliation and layer-by-layer transfer. By using correlation analysis between the Raman modes of graphene and MoS2 we show that both layers are subjected to compressive strain and charge transfer doping following mechanical exfoliation and thermal annealing. Furthermore, we show that both strain and carrier concentration can be modulated in the heterostructures with additional thermal annealing. Our study highlights the importance of considering both mechanical and electronic coupling when characterizing the interface in van der Waals heterostructures, and demonstrates a method to tune their electromechanical properties.

cond-mat.mes-hall↗

arXiv2015

Strength of the dominant scatterer in graphene on silicon oxide

A large variability of carrier mobility of graphene-based field effect transistors hampers graphene science and technology. We determine the scattering strength of the dominant scatterer responsible for the variability of graphene-based transistors on silicon oxide. The strength of the scatterer is found to be more consistent with charged impurities than with resonant impurities.

cond-mat.mes-hall↗

arXiv2014

Insights into carbon nanotube nucleation: Cap formation governed by catalyst interfacial step flow

In order to accommodate an increasing demand for carbon nanotubes (CNTs) with desirable characteristics one has to understand the origin of helicity of their structures. Here, through in situ microscopy we demonstrate that the nucleation of a carbon nanotube is initiated by the formation of the carbon cap. Nucleation begins with the formation of a graphene embryo that is bound between opposite step-edges on the nickel catalyst surface. The embryo grows larger as the step-edges migrate along the surface, leading to the formation of a curved carbon cap when the steps flow across the edges of adjacent facets. Further motion of the steps away from the catalyst tip with attached rims of the carbon cap generates the wall of the nanotube. Density Functional Theory calculations bring further insight into the process, showing that step flow occurs by surface self diffusion of the nickel atoms via a step-edge attachment-detachment mechanism. Since the fact that cap forms first in the sequence of stages involved in nanotube growth, we suggest that it originates the helicity of the nanotube. Therefore, the angular distribution of catalyst facets could be exploited as a new parameter for controlling the curvature of the cap and, presumably, the helicity of the nanotube.

cond-mat.mtrl-sci↗

arXiv2014

On the charge transfer between single-walled carbon nanotubes and graphene

It is important to understand the electronic interaction between single-walled carbon nanotubes (SWNTs) and graphene in order to use them efficiently in multifunctional hybrid devices. Here we deposited SWNT bundles on graphene-covered copper and SiO2 substrates by chemical vapor deposition and investigated the charge transfer between them by Raman spectroscopy. Our results revealed that, on both copper and SiO2 substrates, graphene donates electrons to the SWNTs, resulting in p-type doped graphene and n-type doped SWNTs.

cond-mat.mtrl-sci↗

arXiv2014

Probing the Electrical Properties of Overlapped Graphene Grain Boundaries by Raman spectroscopy

The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements(1). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.

cond-mat.mtrl-sci↗

arXiv2011

Multiphonon Raman Scattering in Graphene

We report multiphonon Raman scattering in graphene samples. Higher order combination modes involving 3 phonons and 4 phonons are observed in single-layer (SLG), bi-layer (BLG), and few layer (FLG) graphene samples prepared by mechanical exfoliation. The intensity of the higher order phonon modes (relative to the G peak) is highest in SLG and decreases with increasing layers. In addition, all higher order modes are observed to upshift in frequency almost linearly with increasing graphene layers, betraying the underlying interlayer van der Waals interactions.

cond-mat.mes-hall↗

arXiv2011

Effects of Layer Stacking on the Combination Raman modes in Graphene

We have observed new combination modes in the range from 1650 - 2300 cm-1 in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. The M band at ~1750 cm-1 is suppressed for both SLG and IBLG. A peak at ~1860 cm-1 (iTALO-) is observed due to a combination of the iTA and LO phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent in bulk graphite. Two previously unidentified modes at ~1880 cm-1 (iTALO+) and ~2220 cm-1 (iTOTA) in SLG are tentatively assigned as combination modes around the K point of the graphene Brillouin zone. The peak frequencies of the iTALO+ (iTOTA) modes are observed to increase (decrease) linearly with increasing graphene layers.

cond-mat.mes-hall↗

arXiv2010

Double resonance Raman study of disorder in CVD-grown single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) with varying degrees of disorder were investigated using multiple-excitation Raman spectroscopy. The lattice disorder was imparted into the nanotubes by the addition of varying amounts of sulfur to the iron catalyst in a thermal chemical vapor deposition process. Changes in the intensities of peaks occurring due to a double resonance Raman process were studied. The intensity of the disorder-induced D band increased with a decrease in the sulfur content. Upon post-synthesis heat treatment, the double resonance process got quenched due to defect healing. The second order G' band and iTOLA bands exhibited a two-peak structure, of which one of the peaks is relatively more sensitive to defects and decreased in intensity with heat treatment.

cond-mat.mes-hall↗