The electronic structures of platinum dichalcogenides: PtS2, PtSe2and PtTe2

Terahertz and Infrared Plasmon Polaritons in PtTe2 Type-II Dirac Topological Semimetal

Surface plasmon polaritons (SPPs) are electromagnetic excitations existing at the interface between a metal and a dielectric. SPPs provide a promising path in nanophotonic devices for light manipulation at the micro and nanoscale with applications in optoelectronics, biomedicine, and energy harvesting. Recently, SPPs are extended to unconventional materials like graphene, transparent oxides, superconductors, and topological systems characterized by linearly dispersive electronic bands. In this respect, 3D Dirac and Weyl semimetals offer a promising frontier for infrared (IR) and terahertz (THz) radiation tuning by topologically-protected SPPs. In this work, the THz-IR optical response of platinum ditelluride (PtTe2) type-II Dirac topological semimetal films grown on Si substrates is investigated. SPPs generated on microscale ribbon arrays of PtTe2 are detected in the far-field limit, finding an excellent agreement among measurements, theoretical models, and electromagnetic simulation data. The far-field measurements are further supported by near-field IR data which indicate a strong electric field enhancement due to the SPP excitation near the ribbon edges. The present findings indicate that the PtTe2 ribbon array appears an ideal active layout for geometrically tunable SPPs thus inspiring a new fashion of optically tunable materials in the technologically demanding THz and IR spectrum.

Atomically precise vacancy-assembled quantum antidots

Patterning antidots, which are regions of potential hills that repel electrons, into well-defined antidot lattices creates fascinating artificial periodic structures, leading to anomalous transport properties and exotic quantum phenomena in two-dimensional systems. Although nanolithography has brought conventional antidots from the semiclassical regime to the quantum regime, achieving precise control over the size of each antidot and its spatial period at the atomic scale has remained challenging. However, attaining such control opens the door to a new paradigm, enabling the creation of quantum antidots with discrete quantum hole states, which, in turn, offer a fertile platform to explore novel quantum phenomena and hot electron dynamics in previously inaccessible regimes. Here we report an atomically precise bottom-up fabrication of a series of atomic-scale quantum antidots through a thermal-induced assembly of a chalcogenide single vacancy in PtTe2. Such quantum antidots consist of highly ordered single-vacancy lattices, spaced by a single Te atom, reaching the ultimate downscaling limit of antidot lattices. Increasing the number of single vacancies in quantum antidots strengthens the cumulative repulsive potential and consequently enhances the collective interference of multiple-pocket scattered quasiparticles inside quantum antidots, creating multilevel quantum hole states with a tunable gap from the telecom to far-infrared regime. Moreover, precisely engineered quantum hole states of quantum antidots are geometry protected and thus survive on oxygen substitutional doping. Therefore, single-vacancy-assembled quantum antidots exhibit unprecedented robustness and property tunability, positioning them as highly promising candidates for advancing quantum information and photocatalysis technologies.

Functional Two-Dimensional Materials for Bioelectronic Neural Interfacing

Realizing the neurological information processing by analyzing the complex data transferring behavior of populations and individual neurons is one of the fast-growing fields of neuroscience and bioelectronic technologies. This field is anticipated to cover a wide range of advanced applications, including neural dynamic monitoring, understanding the neurological disorders, human brain-machine communications and even ambitious mind-controlled prosthetic implant systems. To fulfill the requirements of high spatial and temporal resolution recording of neural activities, electrical, optical and biosensing technologies are combined to develop multifunctional bioelectronic and neuro-signal probes. Advanced two-dimensional (2D) layered materials such as graphene, graphene oxide, transition metal dichalcogenides and MXenes with their atomic-layer thickness and multifunctional capabilities show bio-stimulation and multiple sensing properties. These characteristics are beneficial factors for development of ultrathin-film electrodes for flexible neural interfacing with minimum invasive chronic interfaces to the brain cells and cortex. The combination of incredible properties of 2D nanostructure places them in a unique position, as the main materials of choice, for multifunctional reception of neural activities. The current review highlights the recent achievements in 2D-based bioelectronic systems for monitoring of biophysiological indicators and biosignals at neural interfaces.

Predicting defect stability and annealing kinetics in two-dimensional PtSe2using steepest entropy ascent quantum thermodynamics

The steepest-entropy-ascent quantum thermodynamic (SEAQT) framework was used to calculate the stability of a collection of point defects in 2D PtSe₂and predict the kinetics with which defects rearrange during thermal annealing. The framework provides a non-equilibrium, ensemble-based framework with a self-consistent link between mechanics (both quantum and classical) and thermodynamics. It employs an equation of motion derived from the principle of steepest entropy ascent (maximum entropy production) to predict the time evolution of a set of occupation probabilities that define the states of a system undergoing a non-equilibrium process. The system is described by a degenerate energy landscape of eigenvalues, and the entropy is found from the occupation probabilities and the eigenlevel degeneracies. Scanning tunneling microscopy was used to identify the structure and distribution of point defects observed experimentally in a 2D PtSe₂film. A catalog of observed defects includes six unique point defects (vacancies and anti-site defects on Pt and Se sublattices) and twenty combinations of multiple point defects in close proximity. The defect energies were estimated with density functional theory, while the degeneracies, or density of states, for the 2D film with all possible combinations or arrangements of cataloged defects was constructed using a non-Markovian Monte-Carlo approach (i.e. the Replica-Exchange-Wang-Landau algorithm (Vogelet al2013Phys. Rev. Lett.110210603)) with a q-state Potts model. The energy landscape and associated degeneracies were determined for a 2D PtSe₂film two molecules thick and30×30unit cells in area (total of 5400 atoms). The SEAQT equation of motion was applied to the energy landscape to determine how an arbitrary density and arrangement of the six defect types evolve during annealing. Two annealing processes were modeled: heating from 77 K (-196 ∘C) to 523 K (250 ^∘C) and isothermal annealing at 523 K. The SEAQT framework predicted defect configurations, which were consistent with experimental STM images.

Interface-Induced Seebeck Effect in PtSe2/PtSe2 van der Waals Homostructures

The Seebeck effect refers to the production of an electric voltage when different temperatures are applied on a conductor, and the corresponding voltage-production efficiency is represented by the Seebeck coefficient. We report a Seebeck effect: thermal generation of driving voltage from the heat flowing in a thin PtSe₂/PtSe₂ van der Waals homostructure at the interface. We refer to the effect as the interface-induced Seebeck effect. By exploiting this effect by directly attaching multilayered PtSe₂ over high-resistance PtSe₂ thin films as a hybridized single structure, we obtained the highly challenging in-plane Seebeck coefficient of the PtSe₂ films that exhibit extremely high resistances. This direct attachment further enhanced the in-plane thermal Seebeck coefficients of the PtSe₂/PtSe₂ van der Waals homostructure on sapphire substrates. Consequently, we successfully enhanced the in-plane Seebeck coefficients for the PtSe₂ (10 nm)/PtSe₂ (2 nm) homostructure approximately 42% compared to that of a pure PtSe₂ (10 nm) layer at 300 K. These findings represent a significant achievement in understanding the interface-induced Seebeck effect and provide an effective strategy for promising large-area thermoelectric energy harvesting devices using two-dimensional transition metal dichalcogenide materials, which are ideal thermoelectric platforms with high figures of merit.

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure and their dependence on the interlayer coupling, biaxial strain and external electric field are systematically investigated by using the first-principles...

Ab-Initio Study of Magnetically Intercalated Platinum Diselenide: The Impact of Platinum Vacancies

We study the magnetic properties of platinum diselenide (PtSe₂) intercalated with Ti, V, Cr, and Mn, using first-principle density functional theory (DFT) calculations and Monte Carlo (MC) simulations. First, we present the equilibrium position of intercalants in PtSe2 obtained from the DFT calculations. Next, we present the magnetic groundstates for each of the intercalants in PtSe2 along with their critical temperature. We show that Ti intercalants result in an in-plane AFM and out-of-plane FM groundstate, whereas Mn intercalant results in in-plane FM and out-of-plane AFM. V intercalants result in an FM groundstate both in the in-plane and the out-of-plane direction, whereas Cr results in an AFM groundstate both in the in-plane and the out-of-plane direction. We find a critical temperature of <0.01 K, 111 K, 133 K, and 68 K for Ti, V, Cr, and Mn intercalants at a 7.5% intercalation, respectively. In the presence of Pt vacancies, we obtain critical temperatures of 63 K, 32 K, 221 K, and 45 K for Ti, V, Cr, and Mn-intercalated PtSe2, respectively. We show that Pt vacancies can change the magnetic groundstate as well as the critical temperature of intercalated PtSe2, suggesting that the magnetic groundstate in intercalated PtSe2 can be controlled via defect engineering.

Recent progress in Van der Waals 2D PtSe2

As a new member in two-dimensional (2D) transition metal dichalcogenides (TMDCs) family, platinum diselenium (PtSe₂) has many excellent properties, such as the layer-dependent band gap, high carrier mobility, high photoelectrical coupling, broadband response, etc, thus it shows good promising application in room temperature photodetectors, broadband photodetectors, transistors and other fields. Furthermore, compared with other TMDCs, PtSe₂is chemical inert in ambient, showing nano-devices potential with higher performance and stability. However, up to now, the synthesis and its device applications are in its early stage. This review systematically summarized the state of the art of PtSe₂from its structure, property, synthesis and potential application. Finally, the current challenges and future perspectives are outlined for the applications of 2D PtSe₂.

Electronic and topological properties of group-10 transition metal dichalcogenides

The group 10 transition metal dichalcogenides (TMDs) (MX ₂: M = Ni, Pd, Pt; X = S, Se, Te) have attracted much attention in the last few decades because of observation of exotic phases and phenomena such as superconductivity (SC), topological surface states (TSSs), type II Dirac fermions, helical spin texture, Rashba effect, 3D Dirac plasmons, metal-insulator transitions, charge density waves (CDW) etc. In this review, we cover the experimental and theoretical progress on the physical phenomena influenced by the strong electron-electron correlation of the group-10 TMDs from the past to the present. We have especially emphasized on the SC and topological phases in the bulk as well as in atomically thin materials.

Multipurpose and Reusable Ultrathin Electronic Tattoos Based on PtSe2 and PtTe2

Wearable bioelectronics with emphasis on the research and development of advanced person-oriented biomedical devices have attracted immense interest in the past decade. Scientists and clinicians find it essential to utilize skin-worn smart tattoos for on-demand and ambulatory monitoring of an individual's vital signs. Here, we report on the development of ultrathin platinum-based two-dimensional dichalcogenide (Pt-TMDs)-based electronic tattoos as advanced building blocks of future wearable bioelectronics. We made these ultrathin electronic tattoos out of large-scale synthesized platinum diselenide (PtSe₂) and platinum ditelluride (PtTe₂) layered materials and used them for monitoring human physiological vital signs, such as the electrical activity of the heart and the brain, muscle contractions, eye movements, and temperature. We show that both materials can be used for these applications; yet, PtTe₂ was found to be the most suitable choice due to its metallic structure. In terms of sheet resistance, skin contact, and electrochemical impedance, PtTe₂ outperforms state-of-the-art gold and graphene electronic tattoos and performs on par with medical-grade Ag/AgCl gel electrodes. The PtTe₂ tattoos show 4 times lower impedance and almost 100 times lower sheet resistance compared to monolayer graphene tattoos. One of the possible prompt implications of this work is perhaps in the development of advanced human-machine interfaces. To display the application, we built a multi-tattoo system that can easily distinguish eye movement and identify the direction of an individual's sight.

Two-dimensional inorganic nanosheets: production and utility in the development of novel electrochemical (bio)sensors and gas-sensing applications

This review (with 178 references) focuses on inorganic layered materials (ILMs) and the use of their two-dimensional nanosheets in the development of novel electrochemical (bio)sensors, analytical devices, and gas-phase sensing applications. The text is organized in three main sections including the presentation of the most important families of ILMs, a comprehensive outline of various "bottom-up", "top-down," and hydro(solvo)thermal methods that have been used for the production of ILM nanosheets, and finally an evaluative survey on their utility for the determination of analytes with interest in different sectors of contemporary analysis. Critical discussion on the effect of the production method on their electronic properties, the suitability of each nanomaterial in different sensing technologies along with an assessment of the performance of the (bio)sensors and devices that have been proposed within the last 5 years, is enclosed. The perspectives of further improving the utility of 2D inorganic nanosheets in sensing applications, in real-world samples, are also discussed.

Two-Dimensional Platinum Diselenide: Synthesis, Emerging Applications, and Future Challenges

In recent years, emerging two-dimensional (2D) platinum diselenide (PtSe2) has quickly attracted the attention of the research community due to its novel physical and chemical properties. For the past few years, increasing research achievements on 2D PtSe2 have been reported toward the fundamental science and various potential applications of PtSe2. In this review, the properties and structure characteristics of 2D PtSe2 are discussed at first. Then, the recent advances in synthesis of PtSe2 as well as their applications are reviewed. At last, potential perspectives in exploring the application of 2D PtSe2 are reviewed.

2D materials beyond graphene toward Si integrated infrared optoelectronic devices

Since the discovery of graphene in 2004, it has become a worldwide hot topic due to its fascinating properties. However, the zero band gap and weak light absorption of graphene strictly restrict its applications in optoelectronic devices. In this regard, semiconducting two-dimensional (2D) materials are thought to be promising candidates for next-generation optoelectronic devices. Infrared (IR) light has gained intensive attention due to its vast applications, including night vision, remote sensing, target acquisition, optical communication, etc. Consequently, the generation, modulation, and detection of IR light are crucial for practical applications. Due to the van der Waals interaction between 2D materials and Si, the lattice mismatch of 2D materials and Si can be neglected; consequently, the integration process can be achieved easily. Herein, we review the recent progress of semiconducting 2D materials in IR optoelectronic devices. Firstly, we introduce the background and motivation of the review. Then, the suitable materials for IR applications are presented, followed by a comprehensive review of the applications of 2D materials in light emitting devices, optical modulators, and photodetectors. Finally, the problems encountered and further developments are summarized. We believe that milestone investigations of IR optoelectronics based on 2D materials beyond graphene will emerge soon, which will bring about great industrial revelations in 2D material-based integrated nanodevice commercialization.

Two-Dimensional Noble-Metal Chalcogenides and Phosphochalcogenides

Noble-metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two-dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal-to-semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long-term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble-metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.

Monolayer of PtSe2 on Pt(1 1 1): is it metallic or insulating?

Motivated by the recent synthesis of a PtSe₂ monolayer by direct selenization of a Pt(1 1 1) substrate and in order to reproduce ARPES experimental results, we investigate if the PtSe₂ film could have grown directly on top of the Pt substrate or if some buffer structure separates both of them. We calculate the electronic properties for different growth possibilities and come to the conclusion that the experimental outcome is not compatible with the growth of a PtSe₂ monolayer directly on top of the Pt(1 1 1) substrate.

Wafer-Scale Growth of 2D PtTe2 with Layer Orientation Tunable High Electrical Conductivity and Superior Hydrophobicity

Platinum ditelluride (PtTe₂) is an emerging semimetallic two-dimensional (2D) transition-metal dichalcogenide (TMDC) crystal with intriguing band structures and unusual topological properties. Despite much devoted efforts, scalable and controllable synthesis of large-area 2D PtTe₂ with well-defined layer orientation has not been established, leaving its projected structure-property relationship largely unclarified. Herein, we report a scalable low-temperature growth of 2D PtTe₂ layers on an area greater than a few square centimeters by reacting Pt thin films of controlled thickness with vaporized tellurium at 400 °C. We systematically investigated their thickness-dependent 2D layer orientation as well as its correlated electrical conductivity and surface property. We unveil that 2D PtTe₂ layers undergo three distinct growth mode transitions, i.e., horizontally aligned holey layers, continuous layer-by-layer lateral growth, and horizontal-to-vertical layer transition. This growth transition is a consequence of competing thermodynamic and kinetic factors dictated by accumulating internal strain, analogous to the transition of Frank-van der Merwe (FM) to Stranski-Krastanov (SK) growth in epitaxial thin-film models. The exclusive role of the strain on dictating 2D layer orientation has been quantitatively verified by the transmission electron microscopy (TEM) strain mapping analysis. These centimeter-scale 2D PtTe₂ layers exhibit layer orientation tunable metallic transports yielding the highest value of ∼1.7 × 10⁶ S/m at a certain critical thickness, supported by a combined verification of density functional theory (DFT) and electrical measurements. Moreover, they show intrinsically high hydrophobicity manifested by the water contact angle (WCA) value up to ∼117°, which is the highest among all reported 2D TMDCs of comparable dimensions and geometries. Accordingly, this study confirms the high material quality of these emerging large-area 2D PtTe₂ layers, projecting vast opportunities employing their tunable layer morphology and semimetallic properties from investigations of novel quantum phenomena to applications in electrocatalysis.

Structural, vibrational and electrical properties of type-II Dirac semimetal PtSe2 under high pressure

We present a high-pressure study of type-II Dirac semimetal PtSe₂ single crystals through synchrotron x-ray diffraction (XRD), electrical transport and Raman scattering measurements in diamond anvil cells with pressures up to 36.1-42.3 GPa, from which two critical pressure points associated with unusual electron-phonon coupling are unraveled. We show that both resistance and phonon linewidth of Raman modes display anomalies at the first critical pressure of P _r ~ 10 GPa, in accordance with a scenario of pressure-induced disappearance/appearance of type-II/type-I Dirac points around P _r predicted previously. The second critical pressure P _c ~ 20 GPa may correspond to a structural crossover of PtSe₂ from quasi-2D lattice to 3D network, which is revealed via detailed analysis of the structural parameters extracted from XRD refinement, Raman modes shifts as well as parameters from fitting of the low-temperature resistance. Our results demonstrate great tunability of PtSe₂ via strain engineering, thanks to the single p-orbital manifold derived electronic states that are susceptible to out-of-plane and in-plane distances.

Two-Dimensional/Three-Dimensional Schottky Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon

Two-dimensional (2D) platinum diselenide (PtSe₂) layers are a new class of near-atom-thick 2D crystals in a van der Waals-assembled structure similar to previously explored many other 2D transition-metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored, leaving their full technological potential far from being realized. Herein, we report 2D/three-dimensional Schottky junction devices based on vertically aligned metallic 2D PtSe₂ layers integrated on Si wafers. We directly grew 2D PtSe₂ layers of controlled orientation and carrier transport characteristics via a low-temperature chemical vapor deposition process and investigated 2D PtSe₂/Si Schottky junction properties. We unveiled a comprehensive set of material parameters, which decisively confirm the presence of excellent Schottky junctions, i.e., high-current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe₂/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to significantly broaden the versatility of 2D PtSe₂ layers in practical and futuristic electronic devices.

Horizontal-to-Vertical Transition of 2D Layer Orientation in Low-Temperature Chemical Vapor Deposition-Grown PtSe2 and Its Influences on Electrical Properties and Device Applications

Two-dimensional (2D) transition-metal dichalcogenides (2D TMDs) in the form of MX₂ (M: transition metal, X: chalcogen) exhibit intrinsically anisotropic layered crystallinity wherein their material properties are determined by constituting M and X elements. 2D platinum diselenide (2D PtSe₂) is a relatively unexplored class of 2D TMDs with noble-metal Pt as M, offering distinct advantages over conventional 2D TMDs such as higher carrier mobility and lower growth temperatures. Despite the projected promise, much of its fundamental structural and electrical properties and their interrelation have not been clarified, and so its full technological potential remains mostly unexplored. In this work, we investigate the structural evolution of large-area chemical vapor deposition (CVD)-grown 2D PtSe₂ layers of tailored morphology and clarify its influence on resulting electrical properties. Specifically, we unveil the coupled transition of structural-electrical properties in 2D PtSe₂ layers grown at a low temperature (i.e., 400 °C). The layer orientation of 2D PtSe₂ grown by the CVD selenization of seed Pt films exhibits horizontal-to-vertical transition with increasing Pt thickness. While vertically aligned 2D PtSe₂ layers present metallic transports, field-effect-transistor gate responses were observed with thin horizontally aligned 2D PtSe₂ layers prepared with Pt of small thickness. Density functional theory calculation identifies the electronic structures of 2D PtSe₂ layers undergoing the transition of horizontal-to-vertical layer orientation, further confirming the presence of this uniquely coupled structural-electrical transition. The advantage of low-temperature growth was further demonstrated by directly growing 2D PtSe₂ layers of controlled orientation on polyimide polymeric substrates and fabricating their Kirigami structures, further strengthening the application potential of this material. Discussions on the growth mechanism behind the horizontal-to-vertical 2D layer transition are also presented.

Temperature-dependent Raman spectroscopy and sensor applications of PtSe2 nanosheets synthesized by wet chemistry

We report on a wet chemistry method used to grow PtSe₂ nanosheets followed by thermal annealing. The SEM and TEM analysis confirms the formation of PtSe₂ nanosheets. Furthermore, XRD, Raman, XPS and SAED patterns were used to analyze the crystal structure and to confirm the formation of the PtSe₂ phase. The temperature-dependent Raman spectroscopy investigations were carried out on PtSe₂ nanosheets deposited on Si substrates in the temperature range 100-506 K. The shifts in Raman active E_g and A_1g modes as a function of temperature were monitored. The temperature coefficient for both modes was calculated and was found to match well with the reported 2D transition metal dichalcogenides. A PtSe₂ nanosheet-based sensor device was tested for its applicability as a humidity sensor and photodetector. The humidity sensor based on PtSe₂ nanosheets showed an excellent recovery time of ≈5 s, indicating the great potential of PtSe₂ for future sensor devices.

PtSe2/graphene hetero-multilayer: gate-tunable Schottky barrier height and contact type

Graphene-based two-dimensional hybrid materials are attracting significant attention because they can preserve novel characteristics of Dirac cone. Here, based on first-principles methods, we focus on the electronic characteristics of PtSe₂/graphene hetero-multilayer. The negative binding energies indicate that the hybrid materials can be fabricated easily in practice. Also, the n-type Schottky contact is formed and its barrier height is robust to the number of graphene layer. Moreover, the gate-voltage can effectively induce the Schottky barrier transformation from n-type to p-type and contact type transformation from Schottky to Ohmic in the PtSe₂/graphene hetero-multilayer. Thus, the work demonstrates that the graphene stacking configuration and gate-voltage will tune the electronic characteristics of PtSe₂/graphene-based nanodevices.

Highly Organized Epitaxy of Dirac Semimetallic PtTe2 Crystals with Extrahigh Conductivity and Visible Surface Plasmons at Edges

Platinum telluride (PtTe₂), a member of metallic noble-transition-metal dichalcogenides (MNTMDs), has emerged as an indispensable candidate for superconducting, magnetic, and other electronic phase engineering, as well as optic applications. Herein, we report the van der Waals epitaxy of high-crystalline few-layer PtTe₂ crystals on inert mica. Density functional theory calculations are used to illustrate a type-II Dirac cone along the Γ-A direction in the PtTe₂ crystal. Impressively, the PtTe₂ devices exhibit an extra-high electrical conductivity of 10⁷ S m^-1, 1000 times higher than that of metallic 1T MoS₂. Meanwhile, the magnetoresistance effect at low temperatures reaches 800% in a field of 9.0 T. Furthermore, near-field nanooptical properties are assessed on PtTe₂. Considering the subwavelength effect, the plasmonic wavelength λ_p ≈ 200 nm of 1T PtTe₂ is obtained and the carrier concentration calculated from λ_p is about 1.22 × 10¹⁵ cm^-2, which is 100-fold higher than that of MoTe₂ in the previous reports. Therefore, our work demonstrates the growth of MNTMDs and provides insights into the plasmonic properties of 2D metallic telluride compounds.

Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe2 Single Crystals and Their Thickness-Dependent Electronic Properties

The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe₂) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe₂ nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 μm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-ray-diffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies show characteristics E_g and A_1g vibration modes at ∼109 and ∼155 cm^-1, with a systematic red shift with increasing nanosheet thickness. Electrical transport studies show the 2D PtTe₂ nanosheets display an excellent conductivity up to 2.5 × 10⁶ S m^-1 and show strong thickness-tunable electrical properties, with both the conductivity and its temperature dependence varying considerably with the thickness. Moreover, 2D PtTe₂ nanosheets show an extraordinary breakdown current density up to 5.7 × 10⁷ A/cm², the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides to date.

Atomically thin noble metal dichalcogenide: a broadband mid-infrared semiconductor

The interest in mid-infrared technologies surrounds plenty of important optoelectronic applications ranging from optical communications, biomedical imaging to night vision cameras, and so on. Although narrow bandgap semiconductors, such as Mercury Cadmium Telluride and Indium Antimonide, and quantum superlattices based on inter-subband transitions in wide bandgap semiconductors, have been employed for mid-infrared applications, it remains a daunting challenge to search for other materials that possess suitable bandgaps in this wavelength range. Here, we demonstrate experimentally for the first time that two-dimensional (2D) atomically thin PtSe₂ has a variable bandgap in the mid-infrared via layer and defect engineering. Here, we show that bilayer PtSe₂ combined with defects modulation possesses strong light absorption in the mid-infrared region, and we realize a mid-infrared photoconductive detector operating in a broadband mid-infrared range. Our results pave the way for atomically thin 2D noble metal dichalcogenides to be employed in high-performance mid-infrared optoelectronic devices.

The mid-infrared technologies are essential to various applications but suffer from limited materials with suitable bandgap. Here the authors demonstrate that two-dimensional atomically thin PtSe₂ with variable bandgaps in the mid-infrared via layer and defect engineering is highly promising for mid-infrared optoelectronics.

Wide Spectral Photoresponse of Layered Platinum Diselenide-Based Photodiodes

Platinum diselenide (PtSe₂) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe₂ thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe₂ films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO₂/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1-1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe₂ in infrared allows PtSe₂ to be utilized as an absorber of infrared light with tunable sensitivity. The results of our study indicate that PtSe₂ is a promising option for the development of infrared absorbers and detectors for optoelectronics applications with low-temperature processing conditions.

Thickness-modulated metal-to-semiconductor transformation in a transition metal dichalcogenide

The possibility of tailoring physical properties by changing the number of layers in van der Waals crystals is one of the driving forces behind the emergence of two-dimensional materials. One example is bulk MoS2, which changes from an indirect gap semiconductor to a direct bandgap semiconductor in the monolayer form. Here, we show a much bigger tuning range with a complete switching from a metal to a semiconductor in atomically thin PtSe2 as its thickness is reduced. Crystals with a thickness of ~13 nm show metallic behavior with a contact resistance as low as 70 Ω·µm. As they are thinned down to 2.5 nm and below, we observe semiconducting behavior. In such thin crystals, we demonstrate ambipolar transport with a bandgap smaller than 2.2 eV and an on/off ratio of ~105. Our results demonstrate that PtSe2 possesses an unusual behavior among 2D materials, enabling novel applications in nano and optoelectronics.

Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.

Layered Noble Metal Dichalcogenides: Tailoring Electrochemical and Catalytic Properties

Owing to the anisotropic nature, layered transition metal dichalcogenides (TMDs) have captured tremendous attention for their promising uses in a plethora of applications. Currently, bulk of the research is centered on Group 6 TMDs. Layered noble metal dichalcogenides, in particular the noble metal tellurides, belong to a subset of Group 10 TMDs, wherein the transition metal is a noble metal of either palladium or platinum. We address here a lack of contemporary knowledge on these compounds by providing a comprehensive study on the electrochemistry of layered noble metal tellurides, PdTe₂ and PtTe₂, and their efficiency as electrocatalysts toward the hydrogen evolution reaction (HER). Observed parallels in the electrochemical peaks of the noble metal tellurides are traced to the tellurium electrochemistry. PdTe₂ and PtTe₂ can be discriminated by their distinct reduction peaks in the first cathodic scans. Considering the influence of the metal component, PtTe₂ outperforms PdTe₂ in aspects of charge transfer and electrocatalysis. The heterogeneous electron transfer (HET) rate of PtTe₂ is an order of magnitude faster than PdTe₂, and a lower HER overpotential of 0.54 V versus reversible hydrogen electrode (RHE) at a current density of -10 mA cm^-2 is evident in PtTe₂. On PdTe₂ and PtTe₂ surfaces, adsorption via the Volmer process has been identified as the limiting step for HER. A general phenomenon for the noble metal tellurides is that faster HET rates are observed upon electrochemical reductive pretreatment, whereas slower HET rates occur when the noble metal tellurides are oxidized during pretreatment. PtTe₂ becomes successfully activated for HER when subject to oxidative treatment, whereas oxidized or reduced PdTe₂ shows a deactivated HER performance. These findings provide fundamental insights that are pivotal to advancing the field of the underemphasized TMDs. Furthermore, electrochemical tuning as a means to tailor specific properties of the TMDs is advantageous for the development of their future applications.

Electronic Properties of Graphene-PtSe2 Contacts

In this article, we study the electronic properties of graphene in contact with monolayer and bilayer PtSe₂ using first-principles calculations. It turns out that there is no charge transfer between the components because of the weak van der Waals interaction. We calculate the work functions of monolayer and bilayer PtSe₂ and analyze the band bending at the contact with graphene. The formation of an n-type Schottky contact with monolayer PtSe₂ and a p-type Schottky contact with bilayer PtSe₂ is demonstrated. The Schottky barrier height is very low in the bilayer case and can be reduced to zero by 0.8% biaxial tensile strain.

Computational design of enhanced photocatalytic activity of two-dimensional cadmium iodide

The recent synthesis of two-dimensional cadmium iodide (CdI₂) opens up the questions of its properties and potential applications in optoelectronic and photovoltaic devices.

High-Performance Hybrid Electronic Devices from Layered PtSe2 Films Grown at Low Temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe₂), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe₂ on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe₂ as a potential candidate for next-generation sensors and (opto-)electronic devices, using fabrication protocols compatible with established Si technologies.

Preparation and thermoelectric properties of sulfur doped Ag2Te nanoparticles via solvothermal methods

In this work, n-type Ag(2)Te nanoparticles are prepared by a solvothermal approach with uniform and controllable sizes, e.g. 5-15 nm. The usage of dodecanethiol during the synthesis effectively introduces sulfur doping into the sample, which optimizes the charge carrier concentration of the nanoparticles to >1 × 10(20) cm(-3). This allows us to achieve the desired electrical resistivities of <5 × 10(-6)Ω m. It is demonstrated that Ag(2)Te particles prepared by this solvothermal process can exhibit high ZT values, e.g. 15 nm Ag(2)Te nanoparticles with effective sulphur doping show a maximum ZT value of ~0.62 at 550 K.