purdue university - chem soc revyep/papers/csr_tellurene... · 2018. 10. 12. · this ournal is '...

This journal is©The Royal Society of Chemistry 2018 Chem. Soc. Rev., 2018, 47, 7203--7212 | 7203

Cite this: Chem. Soc. Rev., 2018,47, 7203

Tellurene: its physical properties, scalablenanomanufacturing, and device applications

Wenzhuo Wu, *abcd Gang Qiu,ce Yixiu Wang,abc Ruoxing Wangabc and Peide Ye*ce

Tellurium (Te) has a trigonal crystal lattice with inherent structural anisotropy. Te is multifunctional, e.g.,

semiconducting, photoconductive, thermoelectric, piezoelectric, etc., for applications in electronics, sensors,

optoelectronics, and energy devices. Due to the inherent structural anisotropy, previously reported synthetic

methods predominantly yield one-dimensional (1D) Te nanostructures. Much less is known about 2D Te

nanostructures, their processing schemes, and their material properties. This review focuses on the synthesis

and morphology control of emerging 2D tellurene and summarizes the latest developments in understanding

the fundamental properties of monolayer and few-layer tellurene, as well as the recent advances in

demonstrating prototypical tellurene devices. Finally, the prospects for future research and application

opportunities as well as the accompanying challenges of 2D tellurene are summarized and highlighted.

Key learning points(1) Design and scalable-synthesis of 2D tellurene with controlled yield and dimensions.(2) Key physical properties of 2D tellurene.(3) Application of 2D tellurene in nanoelectronics: field-effect transistors.

1. Introduction

Group VI tellurium (Te) belongs to the chalcogen elementfamily and is chemically related to selenium and sulfur. Itsrarity in the earth’s crust is comparable to that of platinum,while tellurium is far more common in the universe. Te hasappealing properties, e.g., semiconducting,1 photoconductive,2

thermoelectric,3 topological,4 and acoustic-optic properties5 forapplications in electronics, sensors, optoelectronics, and energydevices. Te is also a crucial component of many functionalmaterials, e.g., tellurides, for numerous societally-pervasivetechnologies, such as photovoltaics, thermoelectric devices,infrared imaging, etc. Bulk Te has a trigonal crystal lattice inwhich individual helical chains of Te atoms are stacked togetherby weak bonding and spiral around axes parallel to the [0001]

direction at the center and corners of the hexagonal elementarycell6 (Fig. 1a). Each tellurium atom is covalently bonded with itstwo nearest neighbors on the same chain.

Due to the inherent structural anisotropy, previously reportedsynthetic methods predominantly yield one-dimensional (1D) Tenanostructures.7–9 Much less is known about 2D Te nanostructures,

Fig. 1 (a) The crystal structure of tellurium. (b–d) The various 1D Tenanostructures shown in previous reports through (b) solution phasesynthesis. Reproduced with permission from ref. 7. Copyright 2002, RoyalSociety of Chemistry. (c) PVP-assisted hydrothermal growth. Reproducedwith permission from ref. 9. Copyright 2006, American Chemical Society.(d) Hydrothermal reduction method. Reproduced with permission fromref. 8. Copyright 2002, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Physical vapor deposition process. Reproduced with permission fromref. 26. Copyright 2013, IOP Publishing.

a School of Industrial Engineering, Purdue University, West Lafayette,

Indiana 47907, USA. E-mail: [email protected] Flex Laboratory, Purdue University, West Lafayette, Indiana 47907, USAc Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907,

USA. E-mail: [email protected] Regenstrief Center for Healthcare Engineering, Purdue University, West Lafayette,

Indiana 47907, USAe School of Electrical and Computer Engineering, Purdue University, West Lafayette,

Indiana 47907, USA

Received 25th July 2018

DOI: 10.1039/c8cs00598b

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7204 | Chem. Soc. Rev., 2018, 47, 7203--7212 This journal is©The Royal Society of Chemistry 2018

their processing schemes, and their material properties. As oneof the newest members of the 2D materials’ family, the existenceof tellurene, in its monolayer or few-layer form, has only beenrecently confirmed by experimental work10–13 and theoreticalcalculations.14,15 Inspired by the limited number of experimentalstudies available to date, there have been a few recent theoreticalexplorations16–23 which predict the intriguing properties of 2Dtellurene, e.g., extraordinary carrier mobility, significant opticalabsorption, high stretchability, etc.

In the following sections of this review, we intend tosummarize and highlight the recent theoretical and experi-mental advances in studying 2D tellurene. Firstly, we will brieflyreview the progress in synthesizing Te nanostructures since theknowledge gained from these prior efforts provides valuableinsights and guidance for the synthesis of 2D tellurene. Secondly,we will review and discuss the up-to-date understanding of theintriguing properties of 2D tellurene. The recent demonstration

of tellurene devices, e.g., transistors, will also be discussed. Lastbut not least, we will provide our outlook and perspectives forthe future opportunities and challenges in the research andapplication of 2D tellurene. It is expected that the intriguing,versatile material properties and technological potential oftellurene will open up numerous exciting opportunities in both thefundamental exploration and technological application of tellurene.

2. Synthesis of telluriumnanostructures2.1 Synthesis of one-dimensional Te nanostructures

A number of synthetic methods have been developed to deriveTe nanostructures with various morphologies during the pasttwo decades, e.g., through solution phase reactions7–9,24,25 andvapor phase deposition.26,27 The products from these synthetic

Wenzhuo Wu

Dr Wenzhuo Wu is the Ravi andEleanor Talwar Rising StarAssistant Professor in School ofIndustrial Engineering at PurdueUniversity. He received his BS inElectronic Information Scienceand Technology in 2005 fromthe University of Science andTechnology of China (USTC),Hefei and his ME in Electricaland Computer Engineering fromthe National University ofSingapore (NUS) in 2008. Dr Wureceived his PhD from Georgia

Institute of Technology in Materials Science and Engineering in2013. Dr Wu’s research interests include design, manufacturing,and integration of 1D and 2D nanomaterials for applications inenergy, electronics, optoelectronics, and wearable devices.

Gang Qiu

Gang Qiu received his BS degree inmicroelectronics from PekingUniversity, Beijing, China in2014. He is currently workingtowards his PhD degree at Schoolof Electrical and ComputerEngineering in Purdue University,West Lafayette, Indiana, USA,under the supervision of Prof.Peide. D. Ye. His current researchinterests focus on novel low-dimensional material synthesisand characterization, potentialelectronic device applications andlow-temperature magneto-transport.

Yixiu Wang

Yixiu Wang received his MSdegree in Materials Science andEngineering from the Universityof Science and Technology ofChina (USTC) under the super-vision of Prof. Shu-Hong Yu in2014. He is currently pursuinghis PhD in the School ofIndustrial Engineering under thesupervision of Prof. Wenzhuo Wu.His main research activity focuseson low dimensional materialsynthesis and novel 2D atomiccrystals targeting nanoelectronics

and energy conversion devices together with the exploration offundamental phenomena in nanoscale systems.

Ruoxing Wang

Ruoxing Wang received her BSdegree in Chemistry in 2011 fromthe University of Science andTechnology of China (USTC). Sheis currently a PhD student inIndustrial Engineering at PurdueUniversity under the supervision ofProf. Wenzhuo Wu. Her researchinterests mainly focus on nano-manufacturing including the designof functional nanomaterials andfabrication of nanodevices forvarious applications.

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efforts are predominantly 1D nanostructures, such as nano-wires, nanotubes, and nanobelts, growing along the [0001]chiral-chain direction because of the inherent structural aniso-tropy in tellurium6 (Fig. 1).

2.1.1 Vapor-phase processes. Furuta and co-authors pioneeredthe synthesis of Te whiskers a few microns in length through thesublimation of metallic Te on solid substrates using a vapor–solid(VS) process at various temperatures, and have investigated themorphology evolution of the Te whiskers and the roles of thesubstrate temperature as well as the axial dislocation in relatedprocesses.28 They further explored the growth of Te whiskers bya vapor–liquid–solid (VLS) process using various metal catalysts,e.g., Tl, Zn, and Se.29 Later, Geng et al. demonstrated a chemicalvapor deposition process for synthesizing 1D Te nanobelts byreacting Al2Te3 powder with H2O in a horizontal tube furnace at500 1C.30 A few studies have also been reported on the growthof 1D Te nanostructures by physical vapor deposition. Forexample, Li and colleagues used the physical evaporationmethod to prepare hollow prismatic Te microtubes, showingthat the reaction temperature and the gas flow rate playedcritical roles in the growth control.27 Employing a VS mechanism,He and co-authors reported a catalyst-free physical vapor depositionprocess for growing aligned 1D Te nanostructures with certaincontrol over the nucleation density, size, and structures of theformed materials.26

2.1.2 Solution-phase processes. The requirement for highgrowth temperature and delicate control in the growth atmospherelimit the scale-up potential of the vapor-phase approaches. Incontrast, the solution-phase synthetic routes are more relevant forthe potential large-scale synthesis of Te nanomaterials andhave attracted extensive attention during the past two decades.

For example, Xia and colleagues reported the first synthesis of1D trigonal-Te (t-Te) nanowires through a solution-phase, self-seeding process where the orthotelluric acid or tellurium dioxidewas reduced by hydrazine at various refluxing temperatures.7 Qianand co-workers subsequently reported the hydrothermal synthesisof Te nanobelts through the disproportionation of sodium tellurite(Na2TeO3) in aqueous ammonia solution

8 and revealed that thereaction parameters such as pH, temperature, and precursorconcentration dictate the formation of the Te nanobelts. Yu andcolleagues reported the large-scale synthesis of Te nanowiresand nanobelts with uniform morphologies and high yieldthrough a poly(vinyl pyrrolidone) (PVP)-assisted hydrothermalprocess.9 Inspired by these early work, to date, the solution-based strategies for synthesizing 1D Te nanostructures typicallyrely on the reduction of a Te precursor in the presence of asurfactant, e.g., PVP or cetyltrimethyl ammonium bromide(CTAB). The morphology control for the product materialstrongly depends on the nucleation and growth conditions,such as the choice of surfactants, pH values, precursors, reactiontime and so on. A comprehensive review of the solution-phasesynthesis of Te nanostructures has been recently provided by Yuand colleagues.31 The morphology-selective synthesis of 1D Tenanomaterials demonstrated in these prior works suggested thatthe surfactant-assisted modulation of the growth rates of relevantcrystalline planes could be utilized to precisely control thereaction kinetics, and potentially enable the further morphologyengineering of the as-synthesized Te nanomaterials.

2.2 Prior efforts in preparing 2D Te nanostructures

In contrast to the fruitful progress achieved in synthesizing 1DTe nanostructures, much less is known about the 2D Tenanostructures, their processing schemes, and their relatedproperties. He and colleagues reported the synthesis of 2Dhexagonal Te nanoplates (Fig. 2a) on the chemically inertsurface of mica through the van der Waals epitaxy (vdWE),32

during which the grown materials and the substrate are bondedby weak vdW interactions. The derived 2D Te nanoplates inHe’s report have small lateral dimensions (6–10 mm), large thick-ness (30–80 nm), and non-uniform surface/edge roughness.32 Thesubstantial thickness of these Te nanoplates is well beyond theatomic scale and may diminish their potential material relevanceand interest for fundamental exploration and technologicalapplication in the 2D limit. Huang and colleagues recentlyreported the vdWE of monolayer and few-layer Te films on agraphene/6H-SiC(0001) substrate by molecular beam epitaxy(Fig. 2b).13 The authors identified that the epitaxy Te filmsconsist of in-plane helical Te chains, which is different from thestructures of Te nanoplates in the study of He and colleagues32 aswell as the structural models theoretically proposed for monolayertellurene.14 Chen and co-authors also demonstrated the growth ofepitaxial Te thin films on highly oriented pyrolytic graphite (HOPG)substrates (Fig. 2c)33 with a higher growth rate and a highersubstrate temperature than that used in Huang’s work.13 In additionto these efforts in the epitaxial growth of Te thin films, Zhang andcolleagues recently reported the liquid-phase exfoliation of Tenanocrystals with small lateral dimensions (B100 nm) (Fig. 2d).34

Peide Ye

Dr Peide Ye is Richard J. and MaryJo Schwartz Professor of Electricaland Computer Engineering atPurdue University, USA. Hereceived his PhD from the MaxPlanck-Institute of Solid StateResearch, Stuttgart, Germany, in1996. Before joining the Purduefaculty in 2005, he worked forNTT Basic Research Laboratory,NHMFL/Princeton University, andBell Labs/Lucent Technologies/Agere Systems. His currentresearch work is focused on

atomic layer deposition technology and its device integration onnovel channel materials including III–V, Ge, 2D materials andcomplex oxides. He has authored and co-authored more than 200peer reviewed articles and 350 conference presentations includingmany invited, keynote and plenary talks. He has also served as achairman and a program committee member on top internationalconferences and symposia. He is a Fellow of IEEE and the APS(American Physical Society).

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It should be noted that these demonstrated efforts in preparing2D Te nanostructures are limited by the difficulty in deriving large-scale monolayer or few-layer 2D Te with uniform thickness,restrictions in the growth substrates and conditions (e.g., hightemperature, vacuum), and the vague potential in scaling-up. Forinstance, top-down liquid-phase exfoliation shows potential inproducing large quantities of Te nanocrystals. Nevertheless, thepoor control in thickness uniformity and the small size of thederived materials undermine the viability of such approaches.The epitaxy approaches are also proved challenging, in particularfor device implementation, due to the process-inherent demandsfor high growth temperature, delicate control in growth atmo-sphere, pressure and the epitaxy substrates. There has been a lackof feasible synthetic strategies for the scalable, substrate-freeproduction of large-area, single-crystal 2D Te with process-tunable structural and material properties. Such fundamentalknowledge and technological capability are essential for exploringthe intriguing properties of 2D Te and implementing relateddevice technologies.

2.3 Scalable solution synthesis of large-area free-standing 2Dtellurene

To address these challenges, Wu and colleagues developed asubstrate-free solution process to synthesize for the first timelarge-area, free-standing, high-quality monolayer, and few-layertellurene crystals (Fig. 3).10 The samples were grown throughthe reduction of sodium tellurite (Na2TeO3) by hydrazinehydrate (N2H4) in an alkaline solution at temperatures from160 to 200 1C, with the presence of a crystal-face-blocking

ligand PVP. Due to the substrate-free nature of the processand the use of the aqueous solution in the reaction, the 2D Teflakes can be transferred and assembled at a large scale,through a Langmuir–Blodgett process onto various substratesfor characterization and device integration. The derived 2D Teflakes from this process have edge lengths ranging from 50 to100 mm, and thicknesses from a monolayer to tens of nm(Fig. 3a). Structural and material characterization of all the2D Te samples indicates that all samples grow laterally alongthe h0001i and h1%210i directions, with the vertical stacking alongthe h10%10i directions. This is the first experimental realization oflarge-area, free-standing, high-quality 2D Te crystals through alow-temperature, scalable substrate-free solution process.

The authors further systematically studied and revealed thesynthetic pathway for deriving the 2D tellurene through thesolution process. The controlled PVP concentration is the key toobtaining 2D tellurene. A closer examination of reactions withdifferent PVP concentrations reveals an intriguing morphologyevolution in growth products with time. For each PVP con-centration, the initial growth products are dominantly 1D nano-structures, similar to previous reports.7–9 After a certain periodof reaction, structures possessing both 1D and 2D characteristicsstart to emerge. Finally, the ratio of 2D tellurene flakes whichhave a straight {1%210} edge increases with a reduction in 1D andintermediate structures and reaches a plateau after an extendedgrowth. The observed morphology evolution suggests that thebalance between the kinetic and thermodynamic growth dictatesthe transformation from 1D structures to 2D forms. In the initialreaction, PVP is preferentially adsorbed on the {10%10} surfacesof the nucleated seeds,9 which promotes the kinetic-driven1D growth. When the growth continues, the {10%10} surfacesof the formed structures become partially covered due to theinsufficient PVP capping. Since {10%10} surfaces have the lowestfree energy in tellurium, the growth of {10%10} surfaces along the

Fig. 2 Previously reported 2D Te materials. (a) vdWE-grown hexagonal Tenanoplates. Reproduced with permission from ref. 32. Copyright 2014,American Chemical Society. (b) vdWE-grown monolayer and few-layertellurene on a graphene/6H-SiC(0001) substrate. Reproduced with permissionfrom ref. 13. Copyright 2017, American Chemical Society. (c) Epitaxial Te thinfilm on an HOPG substrate. Reproduced with permission from ref. 33.Copyright 2017, Royal Society of Chemistry. (d) Liquid-phase exfoliated Tenanocrystals. Reproduced with permission from ref. 34. Copyright 2017,WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 3 (a) AFM images of 2D Te with different thicknesses; (b) STEM imageof the few-layer tellurene lattice; (c) optical image of 2D Te. Inset: Dispersed2D Te solution; (d) 3D illustration of few-layer tellurene’s structure. Reproducedwith permission from ref. 10. Copyright 2018, Springer Nature.


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h1%210i direction significantly increases through the thermodynamic-driven assembly, leading to the 1D/2D intermediate structures.Wu and colleagues further demonstrated the engineering andoptimization over the production yield of the process, and thedimensions and thicknesses of tellurene. Such capability inmaterials synthesis enables potential modulation of deviceperformance through tuning the electronic structures, e.g., aprocess-tunable bandgap (0.3–1 eV) covering the spectral rangefrom mid-infrared to near-infrared in tellurene.

3. Physical properties of tellurene

Tellurene is an emerging member of the 2D materials’ family,and there have been only a few experimental studies10–13 andtheoretical calculations.14,16–23 More advances are expected tooccur from the efforts of the material research communityin probing the fundamental properties of the emerging 2Dtellurene and implementing novel devices.

3.1 Structural phase transition in monolayer and few-layertellurene

Recent theoretical calculations suggest that, at equilibriumunder normal conditions, the a-phase derived from the bulktrigonal structure is the most stable phase for few-layer tellurene16

(Fig. 4), and the tetragonal b-phase is more stable for monolayertellurene due to the structural relaxation14 (Fig. 4). The recentexperimental efforts in deriving atomically-thin 2D Te also provideinteresting and insightful results. The vdWE monolayer Te filmgrown by Chen and co-authors on highly oriented pyrolytic graphite(HOPG) substrates33 have a crystal structure and in-plane latticeconstants consistent with the predicted b-phase.14 Nevertheless, thevdWE monolayer Te films grown on graphene layers, as reported byHuang and colleagues,13 were characterized to have a largerlattice of 4.42 � 5.93 Å2, more consistent with the bulk trigonalconfiguration, where parallel-packed Te helical chains areflat-lying in the substrate plane. The structural characterizationin Wu and colleagues’ work10 confirmed that these solution-grown, free-standing few-layer tellurene crystals possess the bulktrigonal structure (Fig. 3). This is consistent with Ji and colleagues’calculation results that a-phase derived from the bulk trigonalstructure is the most stable phase for few-layer tellurene.16

The structural discrepancy seen in the 2D tellurium derivedfrom different processes suggests the complexity of the under-lying mechanism that drives the structure formation of mono-layer or few-layer tellurene, and necessitates the devotion ofmore efforts in advancing both the theoretical understandingand the experimental investigation of the atomically-thin 2Dtellurium, e.g., the role of growth kinetics in its fundamentalstructural phase transition. Moreover, the substrate is expectedto also play a critical role in dictating the structure and propertiesof the supported tellurene.

3.2 Electronic band structure and carrier mobility inmonolayer and few-layer tellurene

The electronic band structure in monolayer and few-layer tell-urene has been recently explored using various theoreticalschemes such as density functional theory (DFT).14,16,17 Ji andcolleagues report an indirect bandgap of 1.17 eV for bilayertellurene and an expected bandgap reduction with an increasedlayer thickness in few-layer tellurene.16 When the layer numberof 2D Te increases, the valence band maximum (VBM) signifi-cantly changes from �4.98 to �4.35 eV, roughly three timesthat of the conduction band maximum (CBM), which suggeststhe formation of a p-type contact between few-layer tellurenewith most metal electrodes. This is consistent with the devicecharacteristics of field-effect transistors made from solution-grown tellurene.10 Interestingly, the band structures of few-layertellurene are predicted to have a four-fold valley degeneracy in thefirst Brillouin zone for the valence band, likely due to the(pseudo) spin non-degeneracy as a result of the strong spin–orbitcoupling in Te.16 Such a ‘‘camel’s back’’ shaped valence band isusually found in topological insulators with band inversion andplays a key role for high-performance thermoelectrics.3 By per-forming hybrid DFT calculations, Zhu and colleagues predictedthat the monolayer a- and b-tellurene exhibit indirect bandgapsof 1.15 and 1.79 eV, respectively.14 The electronic gaps and bandprofiles for monolayer and few-layer tellurene have also beenexperimentally probed. Huang and co-workers report a gap of0.92 eV for monolayer epitaxial tellurene on graphene13 and athickness-dependent bandgap evolution consistent with thetheoretical predications.14,16,17 The smaller monolayer bandgapobserved could be due to the finite density of states of theunderneath graphene near the Fermi level.13 The scanningtunneling microscopy (STS) mapping of the real space bandprofiles shows that the epitaxial monolayer and few-layer tell-urene are p-type semiconductors with the Fermi levels locatingbelow the middle of the bandgap.13 A similar bandgap evolutionhas also been reported for the epitaxial few-layer tellurene on aHOPG substrate.33

In addition to the thickness-dependent bandgap from mid-infrared to the red range which is of great interest to manyemerging technologies in mid-infrared and terahertz applications,2D Te has also been predicted to possess extraordinarily largeroom-temperature carrier mobilities ranging from hundreds tothousands of cm2 V�1 s�1, much larger than those of 2D TMDCsand few-layer BP.14,16 Electrical characterization of the few-layertellurene transistor devices yields hole mobilities consistent with

Fig. 4 Structural allotropes for 2D Tellurium. (a–d) Crystal structures ofbulk (a) and bilayer a-Te from the top- (b) and side-views (c and d); (e and f)crystal structures of bilayer b-Te from the top- (e) and side-views (f); (g andh) crystal structures of bilayer g-Te from the top- (g) and side-views (h).Reproduced with permission from ref. 16. Copyright 2018, Elsevier.


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the predicted ranges.10 Such large room-temperature mobility in2D Te is promising for constructing high-speed, energy-efficientelectronics. An anisotropy in carrier mobility and electronic trans-port is expected for 2D Te, considering its structural anisotropyand the assumed different degrees of interaction along the Techain directions (strong, covalent-type) and that between chains(weak, vdW-like).35 However, experiments have shown only a smalldegree of anisotropy (1.13) for few-layer tellurene.10 Such anunexpected weak anisotropy in the electronic transport inspiresa more profound understanding of the nature of the inter-chaininteractions in 2D tellurene. Recent theoretical study22 suggeststhat the delocalization of Te lone-pair electrons lowers the effectivemass and changes the potential in the inter-chain region, henceenhancing the transport across the chains as well as the inter-chain interactions. The lone-pair electrons are delocalized bydepletion of the density in their original positions and enhance-ment of the density in the inter-chain region, which adds ‘‘metallicbonding’’ or ‘‘covalent-like quasi-bonding’’ characteristics into theinter-chain interaction.16,22 This could also be understood bythe relatively weak nucleus attraction and multi-valence natureof Te, which is a metalloid element with dual characteristics ofboth metal and nonmetal.14

3.3 Optical properties and Raman spectroscopy of 2D Te

Recent theoretical studies suggest a similar isotropic scenariofor light absorption in few-layer tellurene,16 showing a strongbroadband absorbance almost twice to three-times those ofblack phosphorus for normal incident light linearly polarizedalong the two principal in-plane directions. Interestingly, thecalculation results also indicate a layer-dependent absorbancewhere the absorption efficiency (i.e., the absorbance per layer)substantially increases with the decreased thickness of few-layer tellurene.16 This is thought to be due to the thickness-dependent interlayer electronic hybridization and band dispersion,both of which become stronger as the layer thickness increases.This predicated high, isotropic optical-absorbance with thehigh mobility suggests the potential of tellurene for photonicsand optoelectronics applications.

The fundamental light–matter interaction in 2D materialscan also be probed and understood using spectroscopy techniquessuch as Raman scattering. In their recent report,10 Wu andcolleagues performed room-temperature angle-resolved polarizedRaman spectroscopy for solution-synthesized tellurene crystalswith controlled thicknesses, and observed striking thickness-dependent variations (e.g., shifts, appearance, and disappearance) inthe Raman vibrational modes (Fig. 5a). For instance, the thick 2D Tesamples (thicker than 20.5 nm) exhibit three Raman-activemodes consistent with the previous observations in bulk andnanostructured tellurium,32,36 indicating the ‘‘bulk’’ symmetriccharacteristics despite their 2D morphology. For the 2D telluriumcrystals within the ‘‘intermediate’’ thickness range (e.g., 9.1 nm to20.5 nm), the E1 longitudinal (LO) phonon mode appears inthe Raman spectra because the deformation potential in thetellurene lattice increases while the electro-optic effect weakens.When the 2D Te thickness further reduces to the few-layer range(smaller than 9.1 nm), degeneracy in the E1 transverse (TO) and

longitudinal (LO) phonons occurs with peak broadening, possiblydue to the thickness-dependent intra-chain atomic displacement,electronic band structure changes and symmetry assignments foreach band. Also, significant peak shifts were also observed in theRaman spectra, e.g., blue-shifts for the A1 and E2 modes, when 2Dtellurene’s thickness decreases, which may be attributed to theenhanced interlayer interactions when thinned down. Such apeculiar behavior is thought to be closely related to the uniquechiral-chain structure of tellurene. The effects of mechanicalstrain on the Raman spectroscopy for nanobelt-like 2D Tesamples has further been studied, showing an anisotropic strainresponse along the two principal in-plane crystal directions(Fig. 5b).11 The theoretical Raman intensity of bi-layer tellureneas a function of vibrational frequency has also been recentlyreported,16 showing the splitting and anomalous shifts offrequency consistent with the experimental findings.10

3.4 Magnetotransport in 2D Te

Bulk tellurium has been a material of interest for elucidatingfundamental quantum phenomena, e.g., scattering mechanismsof carriers, surface quantum states, magnetoresistance effect,quantum oscillation, topological insulator, etc.4,37,38 Recently,Du et al. carried out magnetotransport studies for the solution-synthesized 2D Te samples with magnetic fields applied alongthree principle axes.11 The temperature-dependent phase coher-ence length extracted from the weak anti-localization effect (WAL)indicates its 2D transport behavior when an applied magneticfield is perpendicular to the [10%10] plane of 2D tellurene. TheWAL effect vanishes with the applied magnetic field parallel tothe h1%210i direction (perpendicular to the Te chains). When theapplied magnetic field is along the h0001i chain direction of 2DTe, the corresponding phase coherence length extracted from the

Fig. 5 (a) Raman spectra of 2D Te with different thicknesses. Reproducedwith permission from ref. 10. Copyright 2018, Springer Nature. (b) Ramanspectra of Te thin film for both tensile and compressive c-axis strains.Reproduced with permission from ref. 11. Copyright 2017, AmericanChemical Society.


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universal conductance fluctuations (UCF) indicates the nature ofthe 1D transport mechanism. The atomically thin 2D Te crystalwith its intriguing properties is expected to provide an attractiveplatform for exploring the quantum physics at the frontier ofcondensed matter research.

3.5 Environmental stability of 2D Te

The environmental instability of many 2D materials, e.g., blackphosphorus, has severely limited the application prospects ofrelated materials. In sharp contrast, great air-stability has beendemonstrated in solution-grown 2D Te crystals for almost theentire thickness range from thick flakes down to a few-layerthickness. No significant degradation has been observed in theelectrical performance of the prototypical 2D tellurene transistorsthat were exposed to air for two months without any encapsulation.13

Theoretical studies suggest that an energy barrier exists for therelated oxidation pathways, sufficient to prevent the few-layertellurene being oxidized under ambient conditions. Such goodenvironmental stability is critical for exploring the fundamentalproperties and realizing the technological prospects of 2D tellurene.

4. Device applications of 2D Te4.1 2D tellurene field-effect transistors

Wu et al. demonstrated the prototypical 2D tellurene field-effecttransistors (FETs) with a good all-around figure of merit comparedto known 2D materials.10 The long channel devices (channellength 3 mm) exhibit large drain current over 300 mA mm�1, highroom-temperature mobility B700 cm2 V�1 s�1, and high on/offratio on the order of B105 (Fig. 6). The thickness dependence ofthe on/off ratios and field-effect mobilities in 2D tellurene longchannel devices with thicknesses ranging from over 35 nm downto a monolayer (B0.5 nm) has also been explored to elucidate thetransport mechanism of 2D Te FETs. A benchmark comparisonwith black phosphorus shows that the solution-synthesized 2Dtellurene has B2–3 times higher mobility than black phosphoruswhen the same device structure, geometry, and mobility extraction

method are adopted. By further exploring the channel scaling andintegration with atomic-layer-deposition (ALD) grown high-kdielectric for the 2D tellurene transistors, Wu and colleaguesachieved record high drive current of over 1 A mm�1 and goodon/off ratios B103 at relatively low drain bias.10 The maximumdrain current achieved is 1.06 A mm�1, which is comparable tothat of conventional semiconductor devices and the highestvalue among all 2D material transistors. This is significantfor potential design and implementation of high-performancetellurene-based electronics. The in-plane electrical transportalong different crystal directions was also studied at roomtemperature, and the average anisotropic mobility ratio is1.43 � 0.10, which is slightly lower than the reported valuesfor bulk tellurium,35 possibly due to the enhanced surfacescattering in our ultrathin Te samples.

5. Conclusions and outlook

Despite being one of the newest members to the 2D materialsfamily, 2D tellurene has attracted extensive interest for its intriguing,versatile material properties and technological potential, usheringin new research opportunities in both theoretical and experimentalstudies. Tellurene possesses a few unique characteristics andadvantages compared to the state-of-the-art 2D materials. Forinstance, the negligible bandgap of graphene leads to compromisedpower gains and limited applications for energy-efficient electronics,while tellurene has a process-modulated bandgap (B0.35–1.2 eVfrom mid-IR to visible light); transition metal dichalcogenides’ lowroom-temperature mobilities limit their prospects for high-speeddevices, while tellurene possesses a high room-temperature carriermobility (B103 cm2 V�1 s�1); the air-instability of silicene and blackphosphorous limits their potential for practical applications, whiletellurene shows a good air-stability for almost the entire thicknessrange. In terms of synthesis control, free-standing tellurene withlarge lateral dimensions (B100 mm) and process-controlledthickness (from a monolayer to tens of nm) can be scalablyproduced with high yield (495%). Nevertheless, the majority of

Fig. 6 2D tellurene FET performance. (a) Transfer curves of a typical 2D tellurene transistor with a thickness of 15 nm and stability measured for 55 days.(Inset: False-colored SEM image of a tellurene transistor. The scale bar is 10 mm.); (b) thickness-dependent on/off ratio (orange triangles) and field-effectmobility (gray circles) for 2D Te transistors; (c) high on-state current density in a 11.1 nm-thick short-channel tellurene transistor with 300 nm channel.Reproduced with permission from ref. 10. Copyright 2018, Springer Nature.


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efforts in the state-of-the-art 2D materials are restricted by factorssuch as growth substrates, synthetic conditions, and small crystalsize of the obtained materials. For instance, top-down liquid-phase exfoliation shows potential in producing large quantitiesof various atomically-thin layered materials, e.g., graphene,transition metal dichalcogenides, and boron nitride. However,the poor control in thickness uniformity and small size of thederived materials undermine the viability of such approaches.The bottom-up chemical vapor deposition process can lead tohigh-quality crystals of graphene and MoS2 with controlledthickness over a large area. However, the requirement for highgrowth temperature and delicate control in growth atmospherelimit the potential for scale-up. Epitaxy growth of 2D materialswith exotic properties, e.g., silicene, borophene, and stanene,have also been explored, though it has proved challenging dueto the process-inherent requirements for epitaxy substrates andhigh vacuum. Still, much work remains to be done to attain acomprehensive fundamental understanding of tellurene’sproperties and to realize its full potential, with the followingopportunities and challenges.

Fundamental exploration

Recent theoretical studies suggest that few-layer tellurene ishighly stretchable along the Te chain direction.16 This couldenable high-performance flexible and stretchable devices withgood lifetime and mechanical durability using the solution-grown 2D tellurene, through scalable assembly and integrationapproaches, e.g., ink-jet printing.13 Despite being an elementalmaterial, the trigonal Te lacks the centrosymmetry in its crystalstructure and has hence been predicted to exhibit piezoelectricity.39

Nevertheless, few studies have been reported on Te’s piezoelectricproperty, possibly due to the narrow bandgap of bulk Te. Thereduction of dimensionality in tellurene could lead to strong,accessible piezoelectricity compared to its bulk counterpart.12

The exploration of tellurene’s piezoelectricity is potentiallysignificant in the sense that the origin for tellurene’s piezo-electric characteristics is expected to be fundamentally differentfrom the ion polarization process for the known piezoelectricmaterials.40,41 Furthermore, the coupling of tellurene’s piezo-electricity with its appealing semiconductor characteristics sug-gests the potential of 2D tellurene as a good candidate materialplatform for emerging areas such as piezotronics and piezo-phototronics.42–44 The absence of inversion symmetry combinedwith the strong spin–orbit interaction in manufacturable 2Dtellurene may also offer interesting opportunities in exploringthe manipulation of the valley degree of freedom for practicalvalleytronics devices.45 Moreover, due to the lack of inversionsymmetry, the spin splitting of the bulk band could give rise to acurrent-induced magnetization in the nonmagnetic bulktellurium.46 It would, therefore, be interesting to explore thefeasibility of 2D tellurene as a new class of magnetoelectricmaterials which could potentially exhibit both current-inducedspin magnetization and current-induced orbital magnetization,47

considering as well the chiral nature of 2D tellurene.3,4 A recenttheoretical work also predicted that few-layer tellurene surprisinglyexhibits in-plane ferroelectricity due to the interlayer interaction

between lone pairs.19 This coupled with the nontrivial valley-dependent spin-textures for holes in few-layer tellurene couldenable novel electronic and spintronic applications. The anisotropicand layer-dependent electron and hole pockets found for few-layertellurene16 indicate that 2D tellurene possesses more suitableelectronic structures than that of 1D Te nanostructures forhigh-performance thermoelectric devices.20,21 For electronicdevice applications, it is expected that the mobility of tellurenecan be further improved through approaches such as improvinginterface quality with high-k dielectric or h-BN encapsulation toreduce the substrate phonon scattering and charge impurity.Meanwhile, a comprehensive examination of the interfacialcharacteristics between 2D tellurene and electrode materials isnecessary for providing the fundamental understanding, e.g.,the effects of metals and electrode configurations on the carriertransport in the metal-tellurene contact.23 Such knowledge iscritical for the rational design and optimization of future tellurene-based devices. The exploration of the fundamental dopingmechanism and defect chemistry in 2D tellurium is not onlyessential for providing versatility in modulating its materialproperties by design but may also enable novel device conceptsand applications. All these intriguing properties demonstrate2D tellurene as a highly promising elementary 2D semiconductorfor diverse applications and inspire us to explore the novelphysics and unusual phenomena in the relatively uncharteredareas of 2D non-layered materials, in particular, those withstructural similarity to 2D tellurene and that consist of weaklybonded atomic chains.48,49

Technological implementation

Among all the demonstrated approaches to synthesize Te nano-structures, as have been reviewed in the previous sections,hydrothermal processing emerges as a potential economic methodfor nanomanufacturing 2D tellurene, due to its energy saving, cost-effectiveness, low working temperature, and feasibility for scale-upproduction, and the potential for deriving 2D tellurene withuniform thickness and dimensions. The room-temperature air-stability of 2D Te crystals also makes it feasible and convenientto produce, package, transport, and utilize the as-fabricated 2DTe materials. The as-fabricated tellurene could also be used astemplates for deriving large-area, free-standing 2D tellurideswith tailored dimensions and properties50 as well as a versatileclass of heterostructures for functional devices. In order to realizethe manufacturing and production potential of solution-synthesized2D Te, more efforts are required to reveal and identify the scientificnature of hydrothermal processing for 2D Te, particularly on thespatial distribution of nucleation and growth, patterning, andself-assembly into device forms, which directly impact the growthrate, assembly yield, performance uniformity, and batch-to-batchreproducibility for future practical production and application of2D tellurene.

The rapid and exciting progress achieved in many emergingand ‘‘traditional’’ disciplines, e.g., nanomanufacturing, data science,condensed matter physics, materials science, solid-state chemistry,etc., is expected to excite a confluence of collective efforts from theresearch community and lead to more theoretical/experimental


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advances in probing the fundamental properties of 2D tellureneand implementing novel devices. 2D tellurene adds a new classof nanomaterials to the large family of 2D crystals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

W. Z. W. acknowledges the College of Engineering and Schoolof Industrial Engineering at Purdue University for the startupsupport. W. Z. W. was partially supported by the Oak RidgeAssociated Universities (ORAU) Junior Faculty EnhancementAward Program. W. Z. W. was partially sponsored by theNational Science Foundation under grants CMMI-1663214 andCNS-1726865. W. Z. W. and P. D. Y. were partially supported byArmy Research Office under grant no. W911NF-15-1-0574 andW911NF-17-1-0573. P. D. Y. was supported by NSF/AFOSR2DARE Program and SRC.

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