In-plane anisotropy in biaxial ReS2 crystals probed by nano-optical imaging of waveguide modes. (arXiv:2302.02649v1 [physics.optics])

2023-02-07T02:29:41+00:00February 7th, 2023|Categories: Publications|Tags: |

Near-field imaging has emerged as a reliable probe of the dielectric function of van der Waals crystals. In principle, analyzing the propagation patterns of subwavelength waveguide modes (WMs) allows for extraction of the full dielectric tensor. Yet previous studies have mostly been restricted to high-symmetry materials or narrowband probing. Here, we resolve in-plane anisotropic WMs in thin rhenium disulfide (ReS2) crystals across a wide range of near-infrared frequencies. By tracing the evolution of these modes as a function of crystallographic direction, polarization of the electric field and sample thickness, we have determined the anisotropic dielectric tensor including the elusive out-of-plane response. The excitonic absorption at ~1.5 eV manifests itself as a clear backbending feature in the WM dispersion and a reduction of the quality factors as fully supported by numerical calculations. Our results extend the sensitivity of near-field microscopy towards biaxial anisotropy and provide key insights into the optoelectronic properties of ReS2.

Published in: "arXiv Material Science".

Possible realization of hyperbolic plasmons in a few-layered rhenium disulfide. (arXiv:2301.06521v1 [cond-mat.mtrl-sci])

2023-01-18T02:29:49+00:00January 18th, 2023|Categories: Publications|Tags: |

The in-plane structural anisotropy in low-symmetric layered compound rhenium disulfide ($text{ReS}_2$) makes it a candidate to host and tune electromagnetic phenomena specific for anisotropic media. In particular, optical anisotropy may lead to the appearance of hyperbolic plasmons, a highly desired property in optoelectronics. The necessary condition is a strong anisotropy of the principal components of the dielectric function, such that at some frequency range, one component is negative and the other is positive, i.e., one component is metallic, and the other one is dielectric. Here, we study the effect of anisotropy in $text{ReS}_2$ and show that it can be a natural material to host hyperbolic plasmons in the ultraviolet frequency range. The operating frequency range of the hyperbolic plasmons can be tuned with the number of $text{ReS}_2$ layers.

Published in: "arXiv Material Science".

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure. (arXiv:2110.03492v1 [cond-mat.mtrl-sci])

2021-10-08T02:30:12+00:00October 8th, 2021|Categories: Publications|Tags: , , , , |

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe2 and ReS2, nonradiative energy transfer (ET) from higher to lower work function material (ReS2 to MoSe2) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe2 photoluminescence (PL) enhancement as compared to the MoSe2 area alone. After completely blocking the CT process, more than one order of magnitude higher MoSe2 PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS2 and WSe2, CT dominates over ET, resulting in a severely quenched WSe2 PL. This study not only provides significant insight into the competing interlayer processes, but also shows an innovative way to increase the PL quantum yield of the desired TMD material using ET process by carefully choosing the right material combination for HS.

Published in: "arXiv Material Science".

Compact Modeling of pH-Sensitive FETs Based on Two-Dimensional Semiconductors. (arXiv:2109.06585v1 [physics.app-ph])

2021-09-15T04:30:19+00:00September 15th, 2021|Categories: Publications|Tags: , |

We present a physics-based circuit-compatible model for pH-sensitive field-effect transistors based on two-dimensional (2D) materials. The electrostatics along the electrolyte-gated 2D-semiconductor stack is treated by solving the Poisson equation including the Site-Binding model and the Gouy-Chapman-Stern approach, while the carrier transport is described by the drift-diffusion theory. The proposed model is provided in an analytical form and then implemented in Verilog-A, making it compatible with standard technology computer-aided design tools employed for circuit simulation. The model is benchmarked against two experimental transition-metal-dichalcogenide (MoS2 and ReS2) based ion sensors, showing excellent agreement when predicting the drain current, threshold voltage shift, and current/voltage sensitivity measurements for different pH concentrations.

Published : "arXiv Mesoscale and Nanoscale Physics".

Unraveling the Photophysics of Liquid-Phase Exfoliated Two-Dimensional ReS2 Nanoflakes. (arXiv:2105.09704v1 [cond-mat.mtrl-sci])

2021-05-21T02:29:44+00:00May 21st, 2021|Categories: Publications|Tags: , |

Few-layered transition metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Amongst the various types of TMDs available today, rhenium-chalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct bandgap behavior throughout multi-layered stacks. In this letter, we have analyzed the nature and dynamics of charge carriers in highly crystalline liquid-phase exfoliated ReS2, using a unique combination of optical pump-THz probe and broadband transient absorption spectroscopy. Two distinct time regimes are identified, both of which are dominated by unbound charge carriers despite the high exciton binding energy. In the first time regime, the unbound charge carriers cause an increase and a broadening of the exciton absorption band. In the second time regime, a peculiar narrowing of the excitonic absorption profile is observed, which we assign to the presence of built-in fields and/or charged defects. Our results pave the way to analyze spectrally complex transient absorption measurements on layered TMD materials and indicate the potential for ReS2 to produce mobile free charge carriers, a feat relevant for photovoltaic applications.

Published in: "arXiv Material Science".

Two-dimensional ReS2: Solution to the Unresolved Queries on Its Structure and Inter-layer Coupling Leading to Potential Optical Applications. (arXiv:2103.14382v1 [cond-mat.mtrl-sci])

2021-03-29T02:29:32+00:00March 29th, 2021|Categories: Publications|Tags: , |

Over the last few years, ReS2 has generated a myriad of unattended queries regarding its structure, the concomitant thickness dependent electronic properties and apparently contrasting experimental optical response. In this work, with elaborate first-principles investigations, using density functional theory (DFT) and time-dependent DFT (TDDFT), we identify the structure of ReS2, which is capable of reproducing and analyzing the layer-dependent optical response. The theoretical results are further validated by an in-depth structural, chemical, optical and optoelectronic analysis of the large-area ReS2 thin films, grown by chemical vapor deposition (CVD) process. Micro-Raman (MR), X-ray photoelectron spectroscopy (XPS), cross-sectional transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDAX) have enabled the optimization of the uniform growth of the CVD films. The correlation between the layer-dependent optical and electronic properties of the excited states was established by static photoluminescence (PL) and transient absorption (TA) measurements. Sulfur vacancy-induced localized mid-gap states render a significantly long life-time of the excitons in these films. The ionic gel top-gated photo-detectors, fabricated from the as-prepared CVD films, exhibit a large photo-response of ~ 5 A/W and a remarkable detectivity of ~ 1011 Jones. The outcome of the present work will be useful to promote the application of vertically grown large-area films in the field of optics and opto-electronics.

Published in: "arXiv Material Science".

Unidirectional Alignment of AgCN Microwires on Distorted Transition Metal Dichalcogenide Crystals. (arXiv:2102.08006v1 [cond-mat.mtrl-sci])

2021-02-17T02:29:39+00:00February 17th, 2021|Categories: Publications|Tags: , , , |

Van der Waals epitaxy on the surface of two-dimensional (2D) layered crystals has gained significant research interest for the assembly of well-ordered nanostructures and fabrication of vertical heterostructures based on 2D crystals. Although van der Waals epitaxial assembly on the hexagonal phase of transition metal dichalcogenides (TMDCs) has been relatively well characterized, a comparable study on the distorted octahedral phase (1T’ or Td) of TMDCs is largely lacking. Here we investigate the assembly behavior of one-dimensional (1D) AgCN microwires on various distorted TMDC crystals, namely 1T’-MoTe2, Td-WTe2, and 1T’-ReS2. The unidirectional alignment of AgCN chains is observed on these crystals, reflecting the symmetry of underlying distorted TMDCs. Polarized Raman spectroscopy and transmission electron microscopy directly confirm that AgCN chains display the remarkable alignment behavior along the distorted chain directions of underlying TMDCs. The observed unidirectional assembly behavior can be attributed to the favorable adsorption configurations of 1D chains along the substrate distortion, which is supported by our theoretical calculations and observation of similar assembly behavior from different cyanide chains. The aligned AgCN microwires can be harnessed as facile markers to identify polymorphs and crystal orientations of TMDCs.

Published in: "arXiv Material Science".

ReS2/h-BN/Graphene Heterostructure Based Multifunctional Devices: Tunnelling Diodes, FETs, Logic Gates & Memory. (arXiv:2012.03219v1 [cond-mat.mes-hall])

2020-12-08T04:30:36+00:00December 8th, 2020|Categories: Publications|Tags: , , |

We investigate a two-dimensional (2D) heterostructure consisting of few-layer direct bandgap ReS2, a thin h-BN layer and a monolayer graphene for application to various electronic devices. Metal-insulator-semiconductor (MIS)-type devices with two-dimensional (2D) van-der-Waals (vdW) heterostructures have recently been studied as important components to realize various multifunctional device applications in analogue and digital electronics. The tunnel diodes of ReS2/h-BN/graphene exhibit light tuneable rectifying behaviours with low ideality factors and nearly temperature independent electrical characteristics. The devices behave like conventional MIS-type tunnel diodes for logic gate applications. Furthermore, similar vertical heterostructures are shown to operate in field effect transistors with a low threshold voltage and a memory device with a large memory gate for future multifunctional device applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

Crystallographic Reconstruction Driven Modified Mechanical Properties in Anisotropic Rhenium Disulfides. (arXiv:2012.01045v1 [cond-mat.mtrl-sci])

2020-12-03T02:29:40+00:00December 3rd, 2020|Categories: Publications|Tags: |

Atomic-scale investigation on mechanical behaviors is highly necessary to fully understand the fracture mechanics especially of brittle materials, which are determined by atomic-scale phenomena (e.g., lattice trapping). Here, exfoliated anisotropic rhenium disulfide (ReS2) flakes are used to investigate atomic-scale crack propagation depending on the propagation directions. While the conventional strain-stress curves exhibit a strong anisotropy depending on the cleavage direction of ReS2, but our experimental results show a reduced cleavage anisotropy due to the lattice reconstruction in [100] cracking with high resistance to fracture. In other words, [010] and [110] cracks with low barriers to cleavage exhibit the ultimate sharpness of the crack tip without plastic deformation, whereas [100] cracks drive lattice rotation on one side of the crack, leading to a non-flat grain boundary formation. Finally, crystallographic reconstruction associated with the high lattice randomness of two-dimensional materials drives to a modified cleavage tendency, further indicating the importance of atomic-scale studies for a complete understanding of the mechanics.

Published in: "arXiv Material Science".

Anomalous fracture in two-dimensional rhenium disulfide

2020-11-18T20:36:37+00:00November 18th, 2020|Categories: Publications|Tags: |

Low-dimensional materials usually exhibit mechanical properties from those of their bulk counterparts. Here, we show in two-dimensional (2D) rhenium disulfide (ReS2) that the fracture processes are dominated by a variety of previously unidentified phenomena, which are not present in bulk materials. Through direct transmission electron microscopy observations at the atomic

Published in: "Science Advances".

Falttening van der waals heterostructure interfaces by local thermal treatment. (arXiv:2010.04783v1 [cond-mat.mtrl-sci])

2020-10-13T02:29:40+00:00October 13th, 2020|Categories: Publications|Tags: , , |

Fabrication of custom-built heterostructures based on stacked 2D materials provides an effective method to controllably tune electronic and optical properties. To that end, optimizing fabrication techniques for building these heterostructures is imperative. A common challenge in layer-by-layer assembly of 2D materials is the formation of bubbles at the atomically thin interfaces. We propose a technique for addressing this issue by removing the bubbles formed at the heterostructure interface in a custom-defined area using the heat generated by a laser, equipped with raster scanning capabilities. We demonstrate that the density of bubbles formed at graphene-ReS2 interfaces can be controllably reduced using this method. We discuss an understanding of the flattening mechanism by considering the interplay of interface thermal conductivities and adhesion energies between two atomically thin 2D materials.

Published in: "arXiv Material Science".

Graphene-Rhenium Disulfide vertical heterostructures visualized at the atomic scale. (arXiv:2010.04697v1 [cond-mat.mtrl-sci])

2020-10-12T02:29:23+00:00October 12th, 2020|Categories: Publications|Tags: , , |

Vertical stacking of atomically thin materials offers a large platform for realizing novel properties enabled by proximity effects and moir’e patterns. Here we focus on mechanically assembled heterostructures of graphene and ReS$_2$, a van der Waals layered semiconductor. Using scanning tunneling microscopy and spectroscopy (STM/STS) we image the sharp edge between the two materials as well as areas of overlap. Locally resolved topographic images revealed the presence of a striped superpattern originating in the interlayer interactions between graphene’s hexagonal structure and the triclinic, low in-plane symmetry of ReS$_2$. We compare the results with a theoretical model that estimates the shape and angle dependence of the moir’e pattern between graphene and ReS$_2$. These results shed light on the complex interface phenomena between van der Waals materials with different lattice symmetries.

Published in: "arXiv Material Science".

Additional excitonic features and momentum-dark states in ReS2. (arXiv:2009.01421v2 [cond-mat.mes-hall] UPDATED)

2020-10-02T02:29:41+00:00October 2nd, 2020|Categories: Publications|Tags: |

Unidirectional in-plane structural anisotropy in Rhenium-based transition metal dichalcogenides (TMDs) introduces a new class of 2-D materials, exhibiting anisotropic optical properties. In this work, we perform temperature dependent, polarization-resolved photoluminescence and reflectance measurements on several-layer ReS$_{2}$. We discover two additional excitonic resonances (X$_{3}$ and X$_{4}$), which can be attributed to splitting of spin degenerate states. Strong in-plane oscillator strength of exciton species X$_{1}$ and X$_{2}$ are accompanied by weaker counterparts X$_{3}$ and X$_{4}$ with similar polarization orientations. The in-plane anisotropic dielectric function has been obtained for ReS$_{2}$ which is essential for engineering light matter coupling for polarization sensitive optoelectronic devices. Furthermore, our temperature dependent study revealed the existence of low-lying momentum-forbidden dark states causing an anomalous PL intensity variation at 30 K, which has been elucidated using a rate equation model involving phonon scattering from these states. Our findings of the additional excitonic features and the momentum-dark states can shed light on the true nature of the electronic band structure of ReS$_{2}$.

Published in: "arXiv Material Science".

Heterostructures of hetero-stack of 2D TMDs (MoS2, WS2 and ReS2) and BN. (arXiv:2009.11495v1 [cond-mat.mtrl-sci])

2020-09-25T02:29:27+00:00September 25th, 2020|Categories: Publications|Tags: , , , |

In this manuscript, we describe optical emission of heterostructure of hetero-stack between 2D TMDs (MoS2, WS2, and ReS2) and BN. Similar to our previous results on the stack of similar type of TMDs, intense PL emission peak is observed around 2.13 eV but is split around 2.13eV into two or more peaks depending on the different stack of TMDs with BN. The transitions from the valence band of BN to conduction bands of different TMD stacks due to quantum coupling and specific orientation explain the strong peak in the PL spectra.

Published in: "arXiv Material Science".

Laser-Assisted Multilevel Non-Volatile Memory Device Based on 2D van-der-Waals Few-layer-ReS2/h-BN/Graphene Heterostructures. (arXiv:2009.00190v1 [cond-mat.mtrl-sci])

2020-09-02T02:29:27+00:00September 2nd, 2020|Categories: Publications|Tags: , , |

Few-layer ReS2 field-effect transistors (FET) with a local floating gate (FG) of monolayer graphene separated by a thin h-BN tunnel layer for application to a non-volatile memory (NVM) device is designed and investigated. FG-NVM devices based on two-dimensional (2D) van-der-Waals (vdW) heterostructures have recently been studied as important components to realize digital electronics and multifunctional memory applications. Direct bandgap multilayer ReS2 satisfies various requirements as a channel material for electronic devices as well as being a strong light-absorbing layer, which makes it possible to realize light-assisted optoelectronic applications. The non-volatile memory operation with a high ON/OFF current ratio, a large memory window, good endurance (> 1000 cycles) and stable retention (> 104 s) have been observed. We demonstrate successive program and erase states using 10 millisecond gate pulses of +10 V and -10 V, respectively. Laser pulses along with electrostatic gate pulses provide multi-bit level memory access via opto-electrostatic coupling. The devices exhibit the dual functionality of a conventional electronic memory and can store laser-pulse excited signal information for future all-optical logic and quantum information processing.

Published in: "arXiv Material Science".

Investigating phase transitions from local crystallographic analysis based on machine learning of atomic environments. (arXiv:2006.10001v1 [cond-mat.mtrl-sci])

2020-06-18T02:30:05+00:00June 18th, 2020|Categories: Publications|Tags: , |

Traditionally, phase transitions are explored using a combination of macroscopic functional characterization and scattering techniques, providing insight into average properties and symmetries of the lattice but local atomic level mechanisms during phase transitions generally remain unknown. Here we explore the mechanisms of a phase transition between the trigonal prismatic and distorted octahedral phases of layered chalogenides in the MoS2 ReS2 system from the observations of local degrees of freedom, namely atomic positions by Scanning Transmission Electron Microscopy (STEM). We employ local crystallographic analysis based on machine learning of atomic environments to build a picture of the transition from the atomic level up and determine local and global variables controlling the local symmetry breaking. In particular, we argue that the dependence of the average symmetry breaking distortion amplitude on global and local concentration can be used to separate local chemical and global electronic effects on transition. This approach allows exploring atomic mechanisms beyond the traditional macroscopic descriptions, utilizing the imaging of compositional fluctuations in solids to explore phase transitions over a range of realized and observed local stoichiometries and atomic configurations.

Published in: "arXiv Material Science".

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2. (arXiv:2001.07308v1 [cond-mat.mtrl-sci])

2020-01-22T02:29:46+00:00January 22nd, 2020|Categories: Publications|Tags: , |

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

Published in: "arXiv Material Science".

Inlaid ReS2 Quantum Dots in Monolayer MoS2. (arXiv:1912.08746v1 [cond-mat.mes-hall])

2019-12-19T02:29:59+00:00December 19th, 2019|Categories: Publications|Tags: , |

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are prospective materials for quantum devices owing to their inherent 2D confinements. They also provide a platform to realize even lower-dimensional in-plane electron confinement, e.g., 0D quantum dots, for exotic physical properties. However, fabrication of such laterally confined monolayer (1L) nanostructure in 1L crystals remains challenging. Here we report the realization of 1L ReS2 quantum dots epitaxially inlaid in 1L MoS2 by a two-step chemical vapor deposition method combining with plasma treatment. The lateral lattice mismatch between ReS2 and MoS2 leads to size-dependent crystal structure evolution and in-plane straining of the 1L ReS2 nanodots. Optical spectroscopies reveal the abnormal charge transfer between the 1L ReS2 quantum dots and the MoS2 matrix, resulting from electron trapping in the 1L ReS2 quantum dots. This study may pave the way for realizing in-plane quantum-confined devices in 2D materials for potential applications in quantum information.

Published in: "arXiv Material Science".

Extending the Colloidal Transition Metal Dichalcogenide Library to ReS2 Nanosheets for Application in Gas Sensing and Electrocatalysis

2019-12-12T06:33:26+00:00December 12th, 2019|Categories: Publications|Tags: , , , |

Colloidal synthesis of rhenium disulfide nanosheets enables a simple and cost‐effective exploitation of its peculiar layer‐independent properties for gas‐sensing and electrochemical H2 production. The surface functionalization of the nanosheets leads to sensitive and fast response gas sensors, while their assembly with carbon nanotubes enhances its electrocatalytic activity, making both device performances competitive with chemical vapor deposition rhenium disulfide. Abstract Among the large family of transition metal dichalcogenides, recently ReS2 has stood out due to its nearly layer‐independent optoelectronic and physicochemical properties related to its 1T distorted octahedral structure. This structure leads to strong in‐plane anisotropy, and the presence of active sites at its surface makes ReS2 interesting for gas sensing and catalysts applications. However, current fabrication methods use chemical or physical vapor deposition (CVD or PVD) processes that are costly, time‐consuming and complex, therefore limiting its large‐scale production and exploitation. To address this issue, a colloidal synthesis approach is developed, which allows the production of ReS2 at temperatures below 360 °C and with reaction times shorter than 2h. By combining the solution‐based synthesis with surface functionalization strategies, the feasibility of colloidal ReS2 nanosheet films for sensing different gases is demonstrated with highly competitive performance in comparison with devices built with CVD‐grown ReS2 and MoS2. In addition, the integration of the ReS2 nanosheet films in assemblies together with carbon nanotubes allows to fabricate electrodes for electrocatalysis for H2 production in both acid and alkaline conditions. Results from proof‐of‐principle devices show an electrocatalytic overpotential competitive with devices based on ReS2 produced by

Published in: "Small".

Grain Boundaries: Nanoassembly Growth Model for Subdomain and Grain Boundary Formation in 1T′ Layered ReS2 (Adv. Funct. Mater. 49/2019)

2019-12-08T12:31:48+00:00December 8th, 2019|Categories: Publications|Tags: |

In article number https://doi.org/10.1002/adfm.2019063851906385, Jinhua Hong, Feng Ding, Hua Xu, and co‐workers reveal a nanoassembly growth model of low symmetry 2D materials to understand the formation mechanism of grain boundaries and subdomains in CVD grown 1T’ ReS2.

Published in: "Advanced Functional Materials".

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