Gate-Tuned Insulator-Metal Transition in Electrolyte-Gated Transistors Based on Tellurene

Continuous Template Growth of Large-Scale Tellurene Films on 1T'-MoTe2

Use of a template triggers an epitaxial interaction with the depositing material during synthesis. Recent studies have demonstrated that two-dimensional tellurium (tellurene) can be directionally oriented when grown on transition metal dichalcogenide (TMD) templates. Specifically, employing a T-phase TMD, such as WTe2, restricts the growth direction even further due to its anisotropic nature, which allows for the synthesis of well-oriented tellurene films. Despite this, producing large-area epitaxial films still remains a significant challenge. Here, we report the continuous synthesis of a 1T'-MoTe2 template via chemical vapor deposition and tellurene via vapor transport. The interaction between helical Te and the 1T'-MoTe2 template facilitates the Te chains to collapse into ribbon shapes, enhancing lateral growth at a rate approximately 6 times higher than in the vertical direction, as confirmed by scanning electron microscopy and atomic force microscopy. Interestingly, despite the predominance of the lateral growth, cross-sectional transmission electron microscopy analysis of the tellurene ribbons revealed a consistent 60-degree incline at the edges. This suggests that the edges of the tellurene ribbons, where they contact the template surface, are favorable sites for additional Te absorption, which then stacks along the incline angle to expand. Furthermore, controlling the synthesis temperature, duration, and preheating time has facilitated the successful synthesis of tellurene films. The resultant tellurene exhibited hole mobility as high as ∼400 cm2/V s. After removing the underlying metallic template with plasma treatment, the film showed a current on/off ratio of ∼103. This ratio was confirmed by two-terminal field-effect transistor measurements and supported by near-field terahertz (THz) spectroscopy mapping.

Phase Engineering of 2D Materials

Polymorphic 2D materials allow structural and electronic phase engineering, which can be used to realize energy-efficient, cost-effective, and scalable device applications. The phase engineering covers not only conventional structural and metal-insulator transitions but also magnetic states, strongly correlated band structures, and topological phases in rich 2D materials. The methods used for the local phase engineering of 2D materials include various optical, geometrical, and chemical processes as well as traditional thermodynamic approaches. In this Review, we survey the precise manipulation of local phases and phase patterning of 2D materials, particularly with ideal and versatile phase interfaces for electronic and energy device applications. Polymorphic 2D materials and diverse quantum materials with their layered, vertical, and lateral geometries are discussed with an emphasis on the role and use of their phase interfaces. Various phase interfaces have demonstrated superior and unique performance in electronic and energy devices. The phase patterning leads to novel homo- and heterojunction structures of 2D materials with low-dimensional phase boundaries, which highlights their potential for technological breakthroughs in future electronic, quantum, and energy devices. Accordingly, we encourage researchers to investigate and exploit phase patterning in emerging 2D materials.

A two-dimensional Te/ReS2 van der Waals heterostructure photodetector with high photoresponsivity and fast photoresponse

Two-dimensional (2D) semiconductors are the building blocks for high-performance optoelectronic devices. However, the performance of photoconductive photodetectors based on 2D semiconductors is hampered by low photoresponsivity and large dark current. Herein, a van der Waals heterostructure (vdWH) composed of rhenium disulfide (ReS₂) and tellurium (Te) is fabricated. The Te/ReS₂ vdWH photodetector exhibits a sensitive and broadband photoresponse and has high photoresponse on/off ratios under ultraviolet and visible light illumination, especially over 10² in visible light. The Te/ReS₂ vdWH photodetector achieves the responsivity of 7.9 A W^-1 at 365 nm, 3.02 A W^-1 at 450 nm, 2.37 A W^-1 at 532 nm, and 2.45 A W^-1 at 660 nm. In addition, the device achieves a high specific detectivity of 10¹¹ Jones and a fast photoresponse speed of 11.9 μs. Such high responsivity could be attributed to the efficient absorption of phonons by the Te/ReS₂ vdWH and the high-quality heterostructure interfaces with a small amount of trap states. The highly crystalline structure of Te/ReS₂ with a low density of defects reduces the grain boundary scattering, leading to the rapid diffusion of charge carriers. Moreover, the Te/ReS₂ vdWH device exhibits a photovoltaic effect and can be employed as a self-powered photodetector (SPPD), which is sensitive to visible light of 450 nm, 532 nm, and 660 nm. Our findings demonstrate that the Te/ReS₂ vdWH photodetector is an ideal building block for the next-generation electronic and optoelectronic devices in practical applications.

Van der Waals nanomesh electronics on arbitrary surfaces

Chemical bonds, including covalent and ionic bonds, endow semiconductors with stable electronic configurations but also impose constraints on their synthesis and lattice-mismatched heteroepitaxy. Here, the unique multi-scale van der Waals (vdWs) interactions are explored in one-dimensional tellurium (Te) systems to overcome these restrictions, enabled by the vdWs bonds between Te atomic chains and the spontaneous misfit relaxation at quasi-vdWs interfaces. Wafer-scale Te vdWs nanomeshes composed of self-welding Te nanowires are laterally vapor grown on arbitrary surfaces at a low temperature of 100 °C, bringing greater integration freedoms for enhanced device functionality and broad applicability. The prepared Te vdWs nanomeshes can be patterned at the microscale and exhibit high field-effect hole mobility of 145 cm²/Vs, ultrafast photoresponse below 3 μs in paper-based infrared photodetectors, as well as controllable electronic structure in mixed-dimensional heterojunctions. All these device metrics of Te vdWs nanomesh electronics are promising to meet emerging technological demands.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm^-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (V_G) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied V_G without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which V_G is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 10¹³ cm^-2 for electrons and 1.4 × 10¹³ cm^-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO₄, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Topological phase change transistors based on tellurium Weyl semiconductor

Modern electronics demand transistors with extremely high performance and energy efficiency. Charge-based transistors with conventional semiconductors experience substantial heat dissipation because of carrier scattering. Here, we demonstrate low-loss topological phase change transistors (TPCTs) based on tellurium, a Weyl semiconductor. By modulating the energy separation between the Fermi level and the Weyl point of tellurium through electrostatic gate modulation, the device exhibits topological phase change between Weyl (Chern number ≠ 0) and conventional (Chern number = 0) semiconductors. In the Weyl ON state, the device has low-loss transport characteristics due to the global topology of gauge fields against external perturbations; the OFF state exhibits trivial charge transport in the conventional phase by moving the Fermi level into the bandgap. The TPCTs show a high ON/OFF ratio (10⁸) at low operation voltage (≤2 volts) and high ON-state conductance (39 mS/μm). Our studies provide alternative strategies for realizing ultralow power electronics.

Formation of stable polonium monolayers with tunable semiconducting properties driven by strong quantum size effects

Elemental two-dimensional (2D) materials have attracted extraordinary interest compared with other 2D materials over the past few years. Fifteen elements from group IIIA to VIA have been discussed experimentally or theoretically for the formation of 2D monolayers, and the remaining few elements still need to be identified. Here, using first-principles calculations within density functional theory (DFT) and ab initio molecular dynamics simulations (AIMDs), we demonstrated that polonium can form stable 2D monolayers (MLs) with a 1T-MoS₂-like structure. The band structure calculations revealed that polonium monolayers possess strong semiconducting properties with a band gap of ∼0.9 eV, and such semiconducting properties can well sustain up to a thickness of 4 MLs with a bandgap of ∼0.1 eV. We also found that polonium monolayers can be achieved through a spontaneous phase transition of ultrathin films with magic thicknesses, resulting in a weaker van der Waals interaction of ∼32 meV Å^-2 between each three atomic layers. Also, the underlying physics comes from layered Peierls-like distortion driven by strong quantum size effects. Based on these intriguing findings, a suitable substrate on which the polonium monolayer can be grown through an epitaxial growth technique is proposed for further experiments. Our work not only extends completely the puzzle of elemental 2D monolayer materials from group IIIA to VIA, but also presents a new formation mechanism of 2D materials beyond the database of bulk materials with layered van der Waals interactions.

Sub-Band Filling, Mott-like Transitions, and Ion Size Effects in C60 Single Crystal Electric Double Layer Transistors

Electric double layer transistors (EDLTs) based on C₆₀ single crystals and ionic liquid gates display pronounced peaks in sheet conductance versus gate-induced charge. Sheet conductance is maximized at electron densities near 0.5 e/C₆₀ and is suppressed near 1 e/C₆₀. The conductance suppression depends markedly on the choice of ionic liquid cation, with small cations favoring activated transport and essentially a complete shutdown of conductance at ∼1 e/C₆₀ and larger cations favoring band-like transport, higher overall conductances at all charge densities up to 1.7 e/C₆₀, and weaker suppression at 1 e/C₆₀. Displacement current measurements on C₆₀ EDLTs with small cations show clear evidence of sub-band filling at 1 e/C₆₀, which correlates very well with the minimum in the C₆₀ sheet conductance. Overall, the data suggest a significant Mott-Hubbard-like energy gap opens up in the surface density of states for C₆₀ crystals gated with small cations. The causes of this energy gap may include both electron-electron repulsion and electron-cation attraction at the crystal/ionic liquid interface. The energy gap suppresses the insulator-to-metal transition in C₆₀ EDLTs, but it can be manipulated by choice of electrolyte.

Ionic gating in metallic superconductors: A brief review

Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range ‘bulk’ modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework—based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity—by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles.

A high performance self-powered photodetector based on a 1D Te-2D WS2 mixed-dimensional heterostructure

One-dimensional (1D)-two-dimensional (2D) van der Waals (vdWs) mixed-dimensional heterostructures with advantages of an atomically sharp interface, high quality and good compatibility have attracted tremendous attention in recent years. Herein, a mixed-dimensional vertical heterostructure is constructed by transferring mechanically exfoliated 2D WS₂ nanosheets on epitaxially grown 1D tellurium (Te) microwires. According to the theoretical type-II band alignment, the device exhibits a photovoltaic effect and serves as an excellent self-powered photodetector with a maximum open-circuit voltage (V _oc) up to ∼0.2 V. Upon 635 nm light illumination, the photoresponsivity, external quantum efficiency and detectivity of the self-powered photodetector (SPPD) are calculated to be 471 mA W^-1, 91% and 1.24 × 10¹² Jones, respectively. Moreover, the dark current of the SPPD is highly suppressed to the sub-pA level due to the large lateral built-in electric field, which leads to a high I _light/I _dark ratio of 10⁴ with a rise time of 25 ms and decay time of 14.7 ms. The abovementioned properties can be further enhanced under a negative bias of -2 V. In brief, the 1D Te-2D WS₂ mixed-dimensional heterostructures have great application potential in high performance photodetectors and photovoltaics.

Study of the Growth Mechanism of Solution-Synthesized Symmetric Tellurium Nanoflakes at Atomic Resolution

As a new member of 2D materials, 2D tellurium (Te) has recently attracted much attention due to its intriguing properties. Through hydrothermal processing, 2D Te with tunable thickness and size has been realized, and its growth mechanism has also been studied. However, the tailored growth of 2D Te nanoflakes with symmetrical morphologies and interfacial moiré fringes has never been reported. Here, 2D Te nanoflakes have been prepared using the hydrothermal method, and mirror-symmetrical shapes (including "V-shape," "heart-shape," and "paper airplane-shape") with obvious moiré fringes in the middle of the nanoflakes are observed. Comprehensive transmission electron microscopy (TEM) techniques are utilized for structural characterization of these nanoflakes, especially the moiré fringes in the symmetry axis region of the nanoflakes. The systematic analyses of the moiré fringes and the observation of obvious overlapping edges of the composing nanoflakes from the cross-sectional samples reveal the possible mechanism of morphological evolution for these symmetrical nanoflakes. These details may fill the research gap in the controllable growth of 2D Te nanomaterials, pave the way for the fabrication of 2D Te moiré superlattices and in-plane homojunctions, and promote their future versatile applications.

Recent progress, challenges, and prospects in emerging group-VIA Xenes: synthesis, properties and novel applications

The discovery of graphene (G) attracted considerable attention to the study of other novel two-dimensional materials (2DMs), which is identified as modern day "alchemy" since researchers are converting the majority of promising periodic table elements into 2DMs. Among the family of 2DMs, the newly invented monoelemental, atomically thin 2DMs of groups IIIA-VIA, called "Xenes" (where, X = IIIA-VIA group elements, and "ene" is the Latin word for nanosheets (NSs)), are a very active area of research for the fabrication of future nanodevices with high speed, low cost and elevated efficiency. Currently, any novel structure of 2DMs from the typical Xenes will probably be applicable in electronic technology. Analysis of their possible highly sensitive synthesis and characterization present opportunities for theoretically examining proposed 2D-Xenes with atomic precision in ideal circumstances, thus providing theoretical predictions for experimental support. Several theoretically predicted and experimentally synthesized 2D-Xene materials have been investigated for the group-VIA elements (tellurene (2D-Te), and selenene (2D-Se)), which are similar to topological insulators (TIs), thus potentially rendering them suitable materials for application in upcoming nanodevices. Although the investigation and device application of these materials are still in their infancy, theoretical studies and a few experiment-based investigations have proven that they are complementary to conventional (i.e., layered bulk-derived) 2DMs. This review focuses on the synthesis of novel group-VIA Xenes (2D-Te and 2D-Se) and summarizes the current development in understanding their basic properties, with the current advancement in signifying device applications. Lastly, the future research prospects, further advanced applications and associated shortcomings of the group-VIA Xenes are summarized and highlighted.

Ambipolar transport in van der Waals black arsenic field effect transistors

Black arsenic (BAs) is an elemental van der Waals semiconductor that is promising for a wide range of electronic and photonic applications. The narrow bandgap and symmetric band structure suggest that ambipolar (both n- and p-type) transport should be observable, however, only p-type transport has been experimentally studied to date. Here, we demonstrate and characterize ambipolar transport in exfoliated BAs field effect transistors. In the thickest flakes (∼ 80 nm), maximum currents, I _max, up to 60 μA μm^-1 and 90 μA μm^-1are achieved for hole and electron conduction, respectively. Room-temperature hole (electron) mobilities up to 150 cm² V^-1 s^-1 (175 cm² V^-1 s^-1) were obtained, with temperature-dependence consistent with a phonon-scattering mechanism. The Schottky barrier height for Ni contacts to BAs was also extracted from the temperature-dependent measurements. I _max for both n- and p-type conductivity was found to decrease with reduced thickness, while the ratio of I _max to the minimum current, I _min, increased. In the thinnest flakes (∼ 1.5 nm), only p-type conductivity was observed with the lowest value of I _min = 400 fA μm^-1. I _max/I _min ratios as high as 5 × 10⁵ (5 × 10²) were obtained, for p- (n-channel) devices. Finally, the ambipolarity was used to demonstrate a complementary logic inverter and a frequency doubling circuit.

Emerging Group-VI Elemental 2D Materials: Preparations, Properties, and Device Applications

Due to the ultrathin thickness and dangling-bond-free surface, 2D materials have been regarded as promising candidates for future nanoelectronics. In recent years, group-VI elemental 2D materials have been rediscovered and found superior in electrical properties (e.g., high carrier mobility, high photoconductivity, and thermoelectric response). The outstanding semiconducting properties of group-VI elemental 2D materials enable device applications including high-performance field-effect transistors and optoelectronic devices. The excellent environmental stability also facilitates fundamental studies and practical applications of group-VI elemental 2D materials. This Review first focuses on the crystal structures of group-VI elemental 2D materials. Afterward, preparation methods for nanostructures of group-VI materials are introduced with comprehensive studies. A brief Review of the electronic structures is then presented with an understanding of the electrical properties. This Review also contains the device applications of group-VI elemental 2D materials, emphasizing transistors, photodetectors, and other appealing applications. Finally, this Review provides an outlook for the development of group-VI elemental 2D materials, highlighting the challenges and opportunities in fundamental studies and technological applications.

Single- versus Dual-Ion Conductors for Electric Double Layer Gating: Finite Element Modeling and Hall-Effect Measurements

Electric double layer (EDL) gating using a single-ion conductor is compared to a dual-ion conductor using both finite element modeling and Hall-effect measurements. Modified Nernst-Planck Poisson (mNPP) equations are used to calculate the ion density per unit area in a parallel plate capacitor geometry with a bulk ion concentration of 215 ≤ c_bulk ≤ 1782 mol/m³. With electrodes of equal size at a 2 V potential difference, the EDL ion density of the single-ion conductor is ∼7 × 10¹³ ions/cm², which is approximately 50% of the ion density induced in the dual-ion conductor. However, this difference is reduced to 8% when the electrode at which the cationic EDL forms is 10 times smaller than the counter electrode. Thus, for a field-effect transistor gated by a single-ion conductor, it is especially important to have a large gate-to-channel size ratio to achieve strong ion doping. The modeled ion densities are validated by Hall-effect measurements on graphene Hall bars gated by a polyethylene oxide (PEO)-based single-ion conductor. The sheet carrier density, n_S, is ∼2 × 10¹³ cm^-2 at V_g = 2 V, which is 3.5 times smaller than the predicted value and has the same order of magnitude as the n_s measured for a PEO-based, dual-ion conductor on the same graphene. The numerical modeling results can be approximated by a simple analysis of capacitors in series, where the EDLs are modeled as capacitors with thickness estimated by the sum of the Debye screening length and the Stern layer. The series of capacitor estimate agrees with the numerical modeling of the dual-ion conductor to within 10% and the single-ion conductor to within 30% from 0.25 to 2 V (c_bulk = 925 mol/m³); similar agreement is observed in the concentration range of 353-1650 mol/m³ for both single- and dual-ion conductors.

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.

Quantum Hall effect of Weyl fermions in n-type semiconducting tellurene

Dirac and Weyl nodal materials can host low-energy relativistic quasiparticles. Under strong magnetic fields, the topological properties of Dirac/Weyl materials can directly be observed through quantum Hall states. However, most Dirac/Weyl nodes generically exist in semimetals without exploitable band gaps due to their accidental band-crossing origin. Here, we report the first experimental observation of Weyl fermions in a semiconductor. Tellurene, the two-dimensional form of tellurium, possesses a chiral crystal structure which induces unconventional Weyl nodes with a hedgehog-like radial spin texture near the conduction band edge. We synthesize high-quality n-type tellurene by a hydrothermal method with subsequent dielectric doping and detect a topologically non-trivial π Berry phase in quantum Hall sequences. Our work expands the spectrum of Weyl matter into semiconductors and offers a new platform to design novel quantum devices by marrying the advantages of topological materials to versatile semiconductors.

The superconductivity and topological surface state of type-II Dirac semimetal NiTe2

NiTe₂ is a type-II Dirac semimetal with the Dirac point very close to the Fermi level. In this paper, its electronic structure, phonon structure and electron-phonon interaction are studied via first-principles calculations. The noteworthy result is that the nontrival bands around the type-II Dirac point are strongly coupled with phonon modes, suggesting that they play an important role in superconductivity. Furthermore, the topological surface states on the (0 0 1) cleavage plane originated from the nontrivial Z ₂ are well separated from the bulk states and can be tuned to approach the Fermi level by adding holes or by V substitution. The possible topological superconductivity in type-II Dirac semimetal NiTe₂ is discussed.

Structure of the Electrical Double Layer at the Interface between an Ionic Liquid and Tungsten Oxide in Ion-Gated Transistors

The structure of electrical double layers at electrified interfaces is of utmost importance for electrochemical energy storage as well as printable, flexible, and bioelectronic devices, such as ion-gated transistors (IGTs). Here we report a study based on atomic force microscopy force-distance profiling on electrical double layers forming at the interface between the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and sol-gel films of mesoporous tungsten oxide. We successfully followed, under in operando conditions, the evolution of the arrangement of the ions at the interface with the tungsten oxide films used as channel materials in IGTs. Our work sheds light on the mechanism of operation of IGTs, thus offering the possibility of optimizing their performance.

Two-Dimensional Tellurium: Progress, Challenges, and Prospects

Since the successful fabrication of two-dimensional (2D) tellurium (Te) in 2017, its fascinating properties including a thickness dependence bandgap, environmental stability, piezoelectric effect, high carrier mobility, and photoresponse among others show great potential for various applications. These include photodetectors, field-effect transistors, piezoelectric devices, modulators, and energy harvesting devices. However, as a new member of the 2D material family, much less known is about 2D Te compared to other 2D materials. Motivated by this lack of knowledge, we review the recent progress of research into 2D Te nanoflakes. Firstly, we introduce the background and motivation of this review. Then, the crystal structures and synthesis methods are presented, followed by an introduction to their physical properties and applications. Finally, the challenges and further development directions are summarized. We believe that milestone investigations of 2D Te nanoflakes will emerge soon, which will bring about great industrial revelations in 2D materials-based nanodevice commercialization.

Electric Double-Layer Gating of Two-Dimensional Field-Effect Transistors Using a Single-Ion Conductor

Electric double-layer (EDL) gating using a custom-synthesized polyester single-ion conductor (PE400-Li) is demonstrated on two-dimensional (2D) crystals for the first time. The electronic properties of graphene and MoTe₂ field-effect transistors (FETs) gated with the single-ion conductor are directly compared to a poly(ethylene oxide) dual-ion conductor (PEO:CsClO₄). The anions in the single-ion conductor are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the negative electrode and a corresponding cationic depletion layer at the positive electrode. Because the cations are mobile in both the single- and dual-ion conductors, a similar enhancement of the n-branch is observed in both graphene and MoTe₂. Specifically, the single-ion conductor decreases the subthreshold swing in the n-branch of the bare MoTe₂ FET from 5000 to 250 mV/dec and increases the current density and on/off ratio by two orders of magnitude. However, the single-ion conductor suppressed the p-branch in both the graphene and the MoTe₂ FETs, and finite element modeling of ion transport shows that this result is unique to single-ion conductor gating in combination with an asymmetric gate/channel geometry. Both the experiments and modeling suggest that single-ion conductor-gated FETs can achieve sheet densities up to 10¹⁴ cm^-2, which corresponds to a charge density that would theoretically be sufficient to induce several percent strain in monolayer 2D crystals and potentially induce a semiconductor-to-metal phase transition in MoTe₂.