Angle resolved vibrational properties of anisotropic transition metal trichalcogenide nanosheets

Axially-Polarized Excitonic Series and Anisotropic van der Waals Stacked Heterojunction in a Quasi-1D Layered Transition-Metal Trichalcogenide

Anisotropic optical 2D materials are crucial for achieving multiple-quanta functions within quantum materials, which enables the fabrication of axially polarized electronic and optoelectronic devices. In this work, multiple excitonic emissions owning polarization-sensitive orientations are clearly detected in a multilayered quasi-1D ZrS3 nanoribbon with respect to the nanostripe edge. Four excitons denoted as AS1, AS2, AS, and A2 with E ⊥ b polarized direction and one prominent A1 exciton with E || b polarized emission are simultaneously detected in the polarized micro-photoluminescence (µPL) measurement of 1.9-2.2 eV at 10 K. In contrast to light emission, polarized micro-thermoreflectance (µTR) measurements are performed to identify the polarization dependence and verify the excitons in the multilayered ZrS3 nanoribbon from the perspective of light absorption. At 10 K, a prominent and broadened peak on the lower-energy side, containing an indirect resonant emission (DI) observed by µPL and an indirect defect-bound exciton peak (AInd) observed by both µPL and µTR, is simultaneously detected, confirming the existence of a quasi-direct band edge in ZrS3. A van der Waals stacked p-GaSe/n-ZrS3 heterojunction solar cell is fabricated, which demonstrates a maximum axially-polarized conversion efficiency up to 0.412% as the E || b polarized light incident onto the device.

Laser irradiation induced structural transformation in layered transition metal trichalcogenide nanoflakes

Laser irradiation is a powerful tool in inducing changes in lattice structures and properties of two-dimensional (2D) materials through processes such as heating, bleaching, catalysis, etc. However, the underlying mechanisms of such transformations vary dramatically in different 2D materials. Here, we report the structural transformation of layered titanium trisulfide (TiS3) to titanium disulfide (TiS2) after irradiation. We systematically characterized the dependence of the transformation on laser power, flake thickness, irradiation time, and vacuum conditions using microscopic and spectroscopic methods. The underlying mechanism is confirmed as the heat-induced materials decomposition, a process that also occurs in many other transition metal trichalcogenide materials. Furthermore, we demonstrate that this spatial-resolved method also enables the creation of in-plane TiS3-TiS2 heterostructures. Our study identifies a new family of 2D materials that undergo a structural transformation after laser irradiation and enriches the methods available for developing new prototypes of low-dimensional devices in the future.

Thermodynamics and Kinetics in Anisotropic Growth of One-Dimensional Midentropy Nanoribbons

One-dimensional (1D) materials demonstrate anisotropic in-plane physical properties that enable a wide range of functionalities in electronics, photonics, valleytronics, optoelectronics, and catalysis. Here, we undertake an in-depth study of the growth mechanism for equimolar midentropy alloy of (NbTaTi)_0.33S₃ nanoribbons as a model system for 1D transition metal trichalcogenide structures. To understand the thermodynamic and kinetic effects in the growth process, the energetically preferred phases at different synthesis temperatures and times are investigated, and the phase evolution is inspected at a sequence of growth steps. It is uncovered that the dynamics of the growth process occurs at four different stages via preferential incorporation of chemical species at high-surface-energy facets. Also, a sequence of temperature and time dependent nonuniform to uniform phase evolutions has emerged in the composition and structure of (NbTaTi)_0.33S₃ which is described based on an anisotropic vapor-solid (V-S) mechanism. Furthermore, direct evidence for the 3D structure of the charge density wave (CDW) phase (width less than 100 nm) is provided by three-dimensional electron diffraction (3DED) in individual nanoribbons at cryogenic temperature, and detailed comparisons are made between the phases obtained before and after CDW transformation. This study provides important fundamental information for the design and synthesis of future 1D alloy structures.

Lattice Transformation from 2D to Quasi 1D and Phonon Properties of Exfoliated ZrS2 and ZrSe2

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS₂ ), zirconium diselenide (ZrSe₂ ), zirconium trisulfide (ZrS₃ ), and zirconium triselenide (ZrSe₃ ). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX₂ (where X = S or Se) are observed. For 2D-ZrX₂ , Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX₂ , Raman modes are identical to exfoliated ZrX₃ nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX₂ are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS₂ , a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX₂ are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX₂ compared to 2D-ZrX₂ , which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.

Graphene Oxide Nanosurface Reduces Apoptotic Death of HCT116 Colon Carcinoma Cells Induced by Zirconium Trisulfide Nanoribbons

Due to their chemical, mechanical, and optical properties, 2D ultrathin nanomaterials have significant potential in biomedicine. However, the cytotoxicity of such materials, including their mutual increase or decrease, is still not well understood. We studied the effects that graphene oxide (GO) nanolayers (with dimensions 0.1-3 μm and average individual flake thickness less than 1 nm) and ZrS₃ nanoribbons (length more than 10 μm, width 0.4-3 μm, and thickness 50-120 nm) have on the viability, cell cycle, and cell death of HCT116 colon carcinoma cells. We found that ZrS₃ exhibited strong cytotoxicity by causing apoptotic cell death, which was in contrast to GO. When adding GO to ZrS₃, ZrS₃ was significantly less toxic, which may be because GO inhibits the effects of cytotoxic hydrogen sulfide produced by ZrS₃. Thus, using zirconium trisulfide nanoribbons as an example, we have demonstrated the ability of graphene oxide to reduce the cytotoxicity of another nanomaterial, which may be of practical importance in biomedicine, including the development of biocompatible nanocoatings for scaffolds, theranostic nanostructures, and others.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Quasi-One-Dimensional van der Waals Transition Metal Trichalcogenides

The transition metal trichalcogenides (TMTCs) are quasi-one-dimensional (1D) MX3-type van der Waals layered semiconductors, where M is a transition metal element of groups IV and V, and X indicates chalcogen element. Due to the unique quasi-1D crystalline structures, they possess several novel electrical properties such as variable bandgaps, charge density waves, and superconductivity, and highly anisotropic optical, thermoelectric, and magnetic properties. The study of TMTCs plays an essential role in the 1D quantum materials field, enabling new opportunities in the material research dimension. Currently, tremendous progress in both materials and solid-state devices has been made, demonstrating promising applications in the realization of nanoelectronic devices. This review provides a comprehensive overview to survey the state of the art in materials, devices, and applications based on TMTCs. Firstly, the symbolic structure, current primary synthesis methods, and physical properties of TMTCs have been discussed. Secondly, examples of TMTC applications in various fields are presented, such as photodetectors, energy storage devices, catalysts, and sensors. Finally, we give an overview of the opportunities and future perspectives for the research of TMTCs, as well as the challenges in both basic research and practical applications.

High-Responsivity Multiband and Polarization-Sensitive Photodetector Based on the TiS3/MoS2 Heterojunction

Two-dimensional (2D) material photodetectors have received considerable attention in optoelectronics as a result of their extraordinary properties, such as passivated surfaces, strong light-matter interactions, and broad spectral responses. However, single 2D material photodetectors still suffer from low responsivity, large dark current, and long response time as a result of their atomic-level thickness, large binding energy, and susceptibility to defects. Here, a transition metal trichalcogenide TiS₃ with excellent photoelectric characteristics, including a direct bandgap (1.1 eV), high mobility, high air stability, and anisotropy, is selected to construct a type-II heterojunction with few-layer MoS₂, aiming to improve the performance of 2D photodetectors. An ultrahigh photoresponsivity of the TiS₃/MoS₂ heterojunction of 48 666 A/W at 365 nm, 20 000 A/W at 625 nm, and 251 A/W at 850 nm is achieved under light-emitting diode illumination. The response time and dark current are 2 and 3 orders of magnitude lower than those of the current TiS₃ photodetector with the highest photoresponsivity (2500 A/W), respectively. Furthermore, polarized four-wave mixing spectroscopy and polarized photocurrent measurements verify its polarization-sensitive characteristics. This work confirms the excellent potential of TiS₃/MoS₂ heterojunctions for air-stable, high-performance, polarization-sensitive, and multiband photodetectors, and the excellent type-II TiS₃/MoS₂ heterojunction system may accelerate the design and fabrication of other 2D functional devices.

The synthesis of competing phase GeSe and GeSe2 2D layered materials

We demonstrate the synthesis of layered anisotropic semiconductor GeSe and GeSe₂ nanomaterials through low temperature (∼400 °C) and atmospheric pressure chemical vapor deposition using halide based precursors. Results show that GeI₂ and H₂Se precursors successfully react in the gas-phase and nucleate on a variety of target substrates including sapphire, Ge, GaAs, or HOPG. Layer-by-layer growth takes place after nucleation to form layered anisotropic materials. Detailed SEM, EDS, XRD, and Raman spectroscopy measurements together with systematic CVD studies reveal that the substrate temperature, selenium partial pressure, and the substrate type ultimately dictate the resulting stoichiometry and phase of these materials. Results from this work introduce the phase control of Ge and Se based nanomaterials (GeSe and GeSe₂) using halide based CVD precursors at ATM pressures and low temperatures. Overall findings also extend our fundamental understanding of their growth by making the first attempt to correlate growth parameters to resulting competing phases of Ge–Se based materials.

We report the synthesis of layered anisotropic semiconductor GeSe and GeSe₂ nanomaterials through low temperature and atmospheric pressure chemical vapor deposition using halide based precursors. The crystal phase is controlled by simply changing selenium vapor pressure.

Anisotropic quasi-one-dimensional layered transition-metal trichalcogenides: synthesis, properties and applications

The strong in-plane anisotropy and quasi-1D electronic structures of transition-metal trichalcogenides (MX3; M = group IV or V transition metal; X = S, Se, or Te) have pronounced influence on moulding the properties of MX3 materials. In particular, the infinite trigonal MX6 prismatic chains running parallel to the b-axis are responsible for the manifestation of anisotropy in these materials. Several marvellous properties, such as inherent electronic, optical, electrical, magnetic, superconductivity, and charge density wave (CDW) transport properties, make transition-metal trichalcogenides (TMTCs) stand out from other 2D materials in the fields of nanoscience and materials science. In addition, with the assistance of pressure, temperature, and tensile strain, these materials and their exceptional properties can be tuned to a superior extent. The robust anisotropy and incommensurable properties make the MX3 family fit for accomplishing quite a lot of compelling applications in the areas of field effect transistors (FETs), solar and fuel cells, lithium-ion batteries, thermoelectricity, etc. In this review article, a precise audit of the distinctive crystal structures, static and dynamic properties, efficacious synthesis schemes, and enthralling applications of quasi-1D MX3 materials is made.

Effect of Band Symmetry on Photocurrent Production in Quasi-One-Dimensional Transition-Metal Trichalcogenides

Photocurrent production in quasi-one-dimensional (1D) transition-metal trichalcogenides, TiS₃(001) and ZrS₃(001), was examined using polarization-dependent scanning photocurrent microscopy. The photocurrent intensity was the strongest when the excitation source was polarized along the 1D chains with dichroic ratios of 4:1 and 1.2:1 for ZrS₃ and TiS₃, respectively. This behavior is explained by symmetry selection rules applicable to both valence and conduction band states. Symmetry selection rules are seen to be applicable to the experimental band structure, as is observed in polarization-dependent nanospot angle-resolved photoemission spectroscopy. Based on these band symmetry assignments, it is expected that the dichroic ratios for both materials will be maximized using excitation energies within 1 eV of their band gaps, providing versatile polarization sensitive photodetection across the visible spectrum and into the near-infrared.

Low-Temperature Phase-Controlled Synthesis of Titanium Di- and Tri-sulfide by Atomic Layer Deposition

Phase-controlled synthesis of two-dimensional (2D) transition-metal chalcogenides (TMCs) at low temperatures with a precise thickness control has to date been rarely reported. Here, we report on a process for the phase-controlled synthesis of TiS₂ (metallic) and TiS₃ (semiconducting) nanolayers by atomic layer deposition (ALD) with precise thickness control. The phase control has been obtained by carefully tuning the deposition temperature and coreactant composition during ALD. In all cases, characteristic self-limiting ALD growth behavior with a growth per cycle (GPC) of ∼0.16 nm per cycle was observed. TiS₂ was prepared at 100 °C using H₂S gas as coreactant and was also observed using H₂S plasma as a coreactant at growth temperatures between 150 and 200 °C. TiS₃ was synthesized only at 100 °C using H₂S plasma as the coreactant. The S₂ species in the H₂S plasma, as observed by optical emission spectroscopy, has been speculated to lead to the formation of the TiS₃ phase at low temperatures. The control between the synthesis of TiS₂ and TiS₃ was elucidated by Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution electron microscopy, and Rutherford backscattering study. Electrical transport measurements showed the low resistive nature of ALD grown 2D-TiS₂ (1T-phase). Postdeposition annealing of the TiS₃ layers at 400 °C in a sulfur-rich atmosphere improved the crystallinity of the film and yielded photoluminescence at ∼0.9 eV, indicating the semiconducting (direct band gap) nature of TiS₃. The current study opens up a new ALD-based synthesis route for controlled, scalable growth of transition-metal di- and tri-chalcogenides at low temperatures.

In-Plane Anisotropic Raman Response and Electrical Conductivity with Robust Electron-Photon and Electron-Phonon Interactions of Air Stable MoO2 Nanosheets

The monoclinic m-MoO₂ shows strong in-plane anisotropy that is essential to design anisotropic nanodevices. However, there is not yet detailed study on the anisotropy of m-MoO₂ nanostructures. Here, the in-plane anisotropic Raman spectra and electrical conductivity of single-crystal m-MoO₂ nanosheets were investigated systematically for the first time. Angle-resolved polarized Raman spectroscopy (ARPRS) shows distinct dependence on the excitation laser wavelength and sample thickness, demonstrating the robust wavelength-dependent optical absorption, electron-photon/electron-phonon interactions, and anisotropic phonon polarization in m-MoO₂ nanosheets. Such results agree well with the semiclassical Placzek model. Moreover, the angle-dependent electrical conductivity of starburst-like m-MoO₂ nanosheet devices shows a strong anisotropy with a conductivity ratio (σ_max/σ_min) of up to 10.1, which is the largest value in the previously reported 2D materials. This work underscores the importance of understanding the in-plane anisotropic light-matter interactions for the application of m-MoO₂ nanosheets in anisotropic plasmonics and artificial synapse.

Probing the Domain Architecture in 2D α-Mo2 C via Polarized Raman Spectroscopy

MXenes are emerging 2D materials with intriguing properties such as excellent stability and high conductivity. Here, a systematic study on the Raman spectra of 2D α-Mo₂ C (molybdenum carbide), a promising member in MXene family, is conducted. Six experimentally observed Raman modes from ultrathin α-Mo₂ C crystal are first assigned with the assistance of phonon dispersion calculated from density functional theory. Angle-resolved polarized Raman spectroscopy indicates the anisotropy of α-Mo₂ C in the b-c plane. Raman spectroscopy is further used to study the unique domain structures of 2D α-Mo₂ C crystals grown by chemical vapor deposition. A Raman mapping investigation suggests that most of the α-Mo₂ C flakes contain multiple domains and the c-axes of neighboring domains tend to form a 60° or 120° angle, due to the weak MoC bonds in this interstitial carbide and the low formation energy of the carbon chains along three equivalent directions. This study demonstrates that polarized Raman spectroscopy is a powerful and effective way to characterize the domain structures in α-Mo₂ C, which will facilitate the further exploration of the domain-structure-related properties and potential applications of α-Mo₂ C.

Highly Sensitive Polarization Photodetection Using a Pseudo-One-Dimensional Nb(1- x)Ti xS3 Alloy

Low-symmetry layered two-dimensional (2D) materials with strong in-plane optical anisotropy can potentially be applied for polarization photodetection. This is especially true for those 2D materials with a direct band gap, which can efficiently absorb light with specific axial polarization. However, discovering such new anisotropic 2D materials with a direct band structure is still extremely challenging. Here, we fabricate a photodetector using a pseudo-one-dimensional (pseudo-1D) Nb_{(1- x)}Ti _xS₃ alloy device and demonstrate that it is highly sensitive to the polarized light because of the strong in-plane optical anisotropy and direct band gap of the alloy by combining angle-resolved polarization Raman spectroscopy, azimuth-dependent reflectance difference microscopy, polarization-dependent absorption spectroscopy, and hybrid functional theory calculations. As a consequence, the polarization photodetector of the Nb_{(1- x)}Ti _xS₃ alloy shows a large photocurrent anisotropic ratio and a high photoresponse. The choice of a low-symmetry layered pseudo-1D Nb_{(1- x)}Ti _xS₃ alloy in polarization photodetection might open up new functionalities for novel optoelectronic device applications.

Gate-Controlled Metal-Insulator Transition in TiS3 Nanowire Field-Effect Transistors

We explore the electrical characteristics of TiS₃ nanowire field-effect transistor (FETs), over the wide temperature range from 3 to 350 K. These nanomaterials have a quasi-one-dimensional (1D) crystal structure and exhibit a gate-controlled metal-insulator transition (MIT) in their transfer curves. Their room-temperature mobility is ∼20-30 cm²/(V s), 2 orders of magnitude smaller than predicted previously, a result that we explain quantitatively in terms of the influence of polar-optical phonon scattering in these materials. In the insulating state (<∼220 K), the transfer curves exhibit unusual mesoscopic fluctuations and a current suppression near zero bias that is common to charge-density wave (CDW) systems. The fluctuations have a nonmonotonic temperature dependence and wash out at a temperature close to that of the bulk MIT, suggesting they may be a feature of quantum interference in the CDW state. Overall, our results demonstrate that quasi-1D TiS₃ nanostructures represent a viable candidate for FET realization and that their functionality is influenced by complex phenomena.

Fast and controlled growth of two-dimensional layered ZrTe3 nanoribbons by chemical vapor deposition

We first demonstrated ZrTe₃ nanoribbons can be grown directly by chemical vapor deposition method, which exhibit intriguing magnetic properties.

Quasi-1D TiS3 Nanoribbons: Mechanical Exfoliation and Thickness-Dependent Raman Spectroscopy

Quasi-one-dimensional (quasi-1D) materials enjoy growing interest due to their unusual physical properties and promise for miniature electronic devices. However, the mechanical exfoliation of quasi-1D materials into thin flakes and nanoribbons received considerably less attention from researchers than the exfoliation of conventional layered crystals. In this study, we investigated the micromechanical exfoliation of representative quasi-1D crystals, TiS₃ whiskers, and demonstrate that they typically split into narrow nanoribbons with very smooth, straight edges and clear signatures of 1D TiS₃ chains. Theoretical calculations show that the energies required for breaking weak interactions between the two-dimensional (2D) layers and between 1D chains within the layers are comparable and, in turn, are considerably lower than those required for breaking the covalent bonds within the chains. We also emulated macroscopic exfoliation experiments on the nanoscale by applying a local shear force to TiS₃ crystals in different crystallographic directions using a tip of an atomic force microscopy (AFM) probe. In the AFM experiments, it was possible to slide the 2D TiS₃ layers relative to each other as well as to remove selected 1D chains from the layers. We systematically studied the exfoliated TiS₃ crystals by Raman spectroscopy and identified the Raman peaks whose spectral positions were most dependent on the crystals' thickness. These results could be used to distinguish between TiS₃ crystals with thickness ranging from one to about seven monolayers. The conclusions established in this study for the exfoliated TiS₃ crystals can be extended to a variety of transition metal trichalcogenide materials as well as other quasi-1D crystals. The possibility of exfoliation of TiS₃ into narrow (few-nm wide) crystals with smooth edges could be important for the future realization of miniature device channels with reduced edge scattering of charge carriers.

Anomalous isoelectronic chalcogen rejection in 2D anisotropic vdW TiS3(1-x)Se3x trichalcogenides

Alloying in semiconductors has enabled many civilian technologies in electronics, optoelectronics, photonics, and others. While the alloying phenomenon is well established in traditional bulk semiconductors owing to a vast array of available ternary phase diagrams, alloying in 2D materials still remains at its seminal stages. This is especially true for transition metal trichalcogenides (TMTCs) such as TiS3 which has been recently predicted to be a direct gap, high carrier mobility, pseudo-1D semiconductor. In this work, we report on an unusual alloying rejection behavior in TiS3(1-x)Se3x vdW crystals. TEM, SEM, EDS, and angle-resolved Raman measurements show that only a miniscule amount (8%) of selenium can be successfully alloyed into a TiS3 host matrix despite vastly different precursor amounts as well as growth temperatures. This unusual behavior contrasts with other vdW systems such as TiS2(1-x)Se2x, MoS2(1-x)Se2x, Mo1-xWxS2, WS2(1-x)Se2x, where continuous alloying can be attained. Angle-resolved Raman and kelvin probe force microscopy measurements offer insights into how selenium alloying influences in-plane structural anisotropy as well as electron affinity values of exfoliated sheets. Our cluster expansion theory calculations show that only the alloys with a small amount of Se can be attained due to energetic instability above/below a certain selenium concentration threshold in the ternary phase diagrams. The overall findings highlight potential challenges in achieving stable Ti based TMTCs alloys.

Large birefringence and linear dichroism in TiS3 nanosheets

TiS3 nanosheets have proven to be promising candidates for ultrathin optoelectronic devices due to their direct narrow band-gap and the strong light-matter interaction. In addition, the marked in-plane anisotropy of TiS3 is appealing for the fabrication of polarization sensitive optoelectronic devices. Herein, we study the optical contrast of TiS3 nanosheets of variable thickness on SiO2/Si substrates, from which we obtain the complex refractive index in the visible spectrum. We find that TiS3 exhibits very large birefringence, larger than that of well-known strong birefringent materials like TiO2 or calcite, and linear dichroism. These findings are in qualitative agreement with ab initio calculations that suggest an excitonic origin for the birefringence and linear dichroism of the material.

Spotting the differences in two-dimensional materials – the Raman scattering perspective

This review discusses the Raman spectroscopic characterization of 2D materials with a focus on the “differences” from primitive 2D materials.

Influence of chalcogen composition on the structural transition and on the electronic and optical properties of the monolayer titanium trichalcogenide ordered alloys

Based on first-principles spin-polarized density functional theory, we investigate the effects of chalcogen composition on the structural, electronic, and optical properties of monolayer (where X and X′ = S, Se, Te) ordered alloys with values of x of 0, 0.167, 0.333, 0.500, 0.667, 0.833, and 1.

Novel Surface Molecular Functionalization Route To Enhance Environmental Stability of Tellurium-Containing 2D Layers

Recent studies have shown that tellurium-based two-dimensional (2D) crystals undergo dramatic structural, physical, and chemical changes under ambient conditions, which adversely impact their much desired properties. Here, we introduce a diazonium molecule functionalization-based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently, gallium telluride (GaTe), zirconium telluride (ZrTe₃), and molybdenum ditelluride (MoTe₂) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show that functionalizing molecules reduce surface reactivity of Te-containing 2D surfaces by chemical binding followed by an electron withdrawal process. While pristine surfaces structurally decompose because of strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive atomic force microscopy, photoluminescence, and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties.

Environmental stability of 2D anisotropic tellurium containing nanomaterials: anisotropic to isotropic transition

We report on the vibrational (Raman) spectrum and structural transformation of semiconducting pseudo-1D GaTe and ZrTe₃ nanomaterials driven by ambient molecular interactions at the nanoscale by angle-resolved Raman spectroscopy, atomic force microscopy (AFM), and environmental X-ray photoelectron (XPS) measurements. The results show that tellurium containing pseudo-1D materials undergo drastic structural and physical changes within a week. During this process, new Raman peaks start to emerge and surface roughness increases substantially. Surprisingly, aged Raman spectra of GaTe, ZrTe₃, and α-TeO_x show striking similarities suggesting that oxidation of tellurium takes place. Careful, environmental tests reveal that the interaction between GaTe and H₂O molecules forms Te-O bonds at the outermost layers of GaTe which leads to newly emerging Raman peaks, a much reduced Schottky junction current density, and an anisotropic to isotropic structural transition. These findings offer fresh interpretation of the aging mechanisms for these material systems, provide new interpretation of the Raman spectrum of aged GaTe which was previously presumed to be of the hexagonal phase, and introduce an anisotropic to isotropic transformation effect induced by molecular interactions on the surface.