Nanoscale

PAPER

Cite this: DOI: 10.1039/c5nr04625d

Received 10th July 2015,Accepted 21st August 2015

DOI: 10.1039/c5nr04625d

www.rsc.org/nanoscale

Sulfur vacancy activated field effect transistorsbased on ReS2 nanosheets†

Kai Xu,a Hui-Xiong Deng,b Zhenxing Wang,a Yun Huang,a Feng Wang,a

Shu-Shen Li,b,c Jun-Wei Luo*b,c and Jun He*a

Rhenium disulphide (ReS2) is a recently discovered new member of the transition metal dichalcogenides.

Most impressively, it exhibits a direct bandgap from bulk to monolayer. However, the growth of ReS2nanosheets (NSs) still remains a challenge and in turn their applications are unexplored. In this study, we

successfully synthesized high-quality ReS2 NSs via chemical vapor deposition. A high-performance field

effect transistor of ReS2 NSs with an on/off ratio of ∼105 was demonstrated. Through both electrical trans-

port measurements at varying temperatures (80 K–360 K) and first-principles calculations, we find sulfur

vacancies, which exist intrinsically in ReS2 NSs and significantly affect the performance of the ReS2 FET

device. Furthermore, we demonstrated that sulfur vacancies can efficiently adsorb and recognize oxidiz-

ing (O2) and reducing (NH3) gases, which electronically interact with ReS2 only at defect sites. Our

findings provide experimental groundwork for the synthesis of new transition metal dichalocogenides,

supply guidelines for understanding the physical nature of ReS2 FETs, and offer a new route toward tailor-

ing their electrical properties by defect engineering in the future.

Introduction

Graphene has shown many fascinating properties such as highcarrier mobility, outstanding mechanical properties, excellentthermal conductivity, and surprising molecular barrier pro-perties.1 However, the high leakage current, caused by its zerobandgap, limits its wide applications in electronics and opto-electronics. In contrast, as another 2D material, transitionmetal dichalcogenides (TMDs) possess sizable bandgaps ofaround 1–2 eV, which make them highly promising buildingblocks for high-performance electronic and optoelectronicapplications.2–7 One of the prominent properties of TMDs isthat they undergo a crossover from indirect bandgap in thebulk to direct bandgap in a monolayer, and as a result mono-layer TMDs absorb and emit light efficiently.8 Furthermore,layered TMDs display high electron mobility, possess a rela-tively large spin–orbit interaction and possess valley polariz-ation, thus opening up new prospects for electronic, spintronicand valleytronic devices.9 As a representative, MoS2 FETs have

shown relatively high channel mobility of 200–500 cm2 V−1 s−1

and high on/off ratio of ∼10.11,8,10 Single layer and few-layerTMDs structure and heterostructures have been studiedrecently. The heterostructure based on monolayer MoS2/WSe2realizes a p–n junction at the ultimate thick limit and constitu-tes the ultimate functional unit for nanoscale electronic andoptoelectronic devices.12

Motivated by the achievements in MoS2, many other 2Dmaterials, such as WS2, WSe2, GaS, Bi2Se3, and SnSe, havebeen studied extensively.13–15 Rhenium disulphide (ReS2) isanother important member of TMDs family. Moreover, frombulk to monolayer, ReS2 remains a direct bandgap semi-conductor,16 which may enable it to be a promising candidatefor building atomically layered optical and photovoltaicdevices. To date, monolayer and few-layer ReS2 have beenmainly synthesized via micromechanical exfoliation.16

However, this method fails to realize large-scale and uniformsamples with high yield and controllability. An effective syn-thesis method is highly desired, and chemical vapor depo-sition (CVD) has proven to be a successful method tosynthesize graphene and TMDs materials such as MoS2 andWSe2.

17,18 The great challenge in synthesizing large size andhigh quality ReS2 is most likely due to its complicated crystalstructure. Fig. 1a and b show the crystal structure of ReS2 withtriclinic symmetry, which is unlike most hexagonal TMDs.

It is noteworthy that point defects and grain boundaries areunavoidable when layered crystals are synthesized.19,20 Thesestructural defects in layered materials play a critical role on

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr04625d

aCAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center

for Nanoscience and Technology, Beijing 100190, China. E-mail: hej@nanoctr.cnbState Key Laboratory for Superlattices and Microstructures, Institute of

Semiconductors, Chinese Academy of Science, Beijing 100083, China.

E-mail: jwluo@semi.ac.cncSynergetic Innovation Center of Quantum Information and Quantum Physics,

University of Science and Technology of China, Hefei, Anhui 230026, China

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mechanical, thermal, electrical and optical properties. Forexample, the high mobility of >80 cm2 V−1 s−1 for monolayerMoS2 FET was achieved by developing a low-temperature thiolchemistry route to repair the sulfur vacancies.19 By suppressingthermally activated Gallium vacancies, multilayer GaTe FETwith an on/off ratio of ∼105 was obtained.20 On the other hand,the existing point defects may break the intrinsic surface states,which are free of dangling bonds, to make TMDs adsorb gaseseffectively.21–23 This not only can be applied to gas detectionand recognition, but also facilitates the modulation of the elec-trical property of TMDs by charge doping. Therefore, under-standing the defect physics in TMDs is crucial to explorevarious applications and design high performance devices.

Herein, we design the controlled growth of large-scale ReS2NSs via the CVD method. All characterizations clearly illustratethat the as-synthesized ReS2 NSs possess high crystal quality,uniform surface and large-scale crystals with high yield andcontrollability. The FET based on ReS2 NSs, with an on/off ratioof ∼105, is demonstrated successfully. Combining electricaltransport measurements at varying temperatures (80 K–360 K)with first-principles calculations, we find that sulfur vacanciesunintentionally exist in the synthesized ReS2 NSs and signifi-cantly affect the performance of devices. Furthermore, the elec-trical properties of the ReS2 NSs are measured in an atmospherewith oxidizing gas (O2) and reducing gas (NH3), and we find thatsulfur vacancies can not only efficiently adsorb and recognizeboth oxidizing gases and reducing gases, but also modulate theelectrical property of ReS2 NSs. Our findings are of scientificimportance to understand the physics of intrinsic ReS2 transis-tors and open up new exciting opportunities for tailoring theirelectrical properties by defect engineering in the future.

Results and discussion

ReS2 with triclinic crystal symmetry can be considered as a dis-tortion of the metal ions away from their ideal sites. The

driving force comes from Peierls distortion, which results in areduction of symmetry from P63/mmc of hexagonal TMDs toP1̄.16,24 To synthesize atomic layered ReS2, sulfur powder wasground as the chalcogen precursor, while rhenium oxidepowder was used as the Re precursor. Fig. S1a (ESI†) schemati-cally illustrates horizontal two-zone tube furnace prepared byus, which was used for synthesizing thin ReS2 layers. ReO3

powder (99.9% Alfa Aesar) was placed in a quartz boat andSiO2/Si substrate was faced downward and mounted on the topof the boat. A separate quartz boat with S powder was placedin the front zone. During the synthesis process, the tempera-ture of the back zone was increased to ∼500 °C in an argonenvironment. The relatively mild reaction conditions enablethe high-density deposition of ReS2 NSs on a variety of sub-strates, including rigid SiO2/Si and flexible carbon fibers, asdisplayed in Fig. S1b and c.† More details of the experimentsare shown in the experimental section in the ESI.† The corres-ponding X-ray energy dispersive spectroscopy characterizationconfirms that the atomic percent ratio of Re and S is exactly1 : 2. Typically, the thicknesses of ReS2 NSs are found to be inthe range of 4–15 nm by atomic force microscopic (AFM)measurements (Fig. S2†). Fig. 1c and d present cross-sectionalview and AFM image of an ReS2 NSs device with the thicknessof ∼4.8 nm, respectively.

A Raman spectrum of ReS2 NSs excited by a 532 nm laser inthe backscattering geometry is shown in Fig. 2a. Due to thereduced crystal symmetry, ReS2 manifests a more complexRaman spectrum than conventional TMDs with higher crystalsymmetries. The Raman spectrum for ReS2 NSs includes twovery prominent modes: 160 and 210 cm−1, which are attributedto the in-plane and mostly out-of-plane modes and are consist-

Fig. 1 Schemes and characterizations of layered ReS2 NSs. (a) Sche-matic diagram of the layer-by-layer crystal structure of ReS2. Gray balls,Re; yellow balls, S. (b) Top-down depiction of ReS2 derived from the tri-clinic P1̄ bulk form. (c) Cross-sectional view of devices based on ReS2NSs in a back-gate configuration. (d) AFM image of the ReS2 back-gatedevice used in our measurements. Scale bar, 200 nm.

Fig. 2 Characterization of the synthesized ReS2 NSs. (a) Raman spectraof ReS2 from 100 cm−1 to 400 cm−1. (b) XPS spectra of ReS2 NSs, whereRe 4f7/2, Re 4f5/2, S 2p3/2 and S 2p1/2 are identified. (c) TEM, (d) HRTEMand inset SAED images of a typical few-layered ReS2 NSs.

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ent with previous ReS2 Raman results.7,16 The complex Ramanpeaks result from the low crystal symmetry and fundamentalRaman modes coupled to each other and to acoustic phonons.In addition, the resistance to oxidation, which indicates thestability of ReS2 materials, is suitably used for photoelectronicapplications and laser thinning for manufacturing singlelayers. X-ray photoemission spectroscopy (XPS) was used tomeasure the binding energies of Re and S in the ReS2 NSs, asshown in Fig. 2b. The two peaks at 42.7 and 45.1 eV corres-pond to the Re 4f7/2 and Re 4f5/2 binding energies, and thepeaks at 162.8 and 164.1 eV are attributed to the S 2p3/2 andS 2p1/2 states, respectively. In addition, more accurate compo-sitional analyses from XPS analysis indicate that the ReS2 NSscontain ∼33 at% Re and ∼67 at% S. Fig. 2c shows the TEMimages of a typical thin ReS2 NS deposited directly on a TEMcarbon membrane. The lattice fringe with the distance of0.284 nm is obtained in Fig. 2d, which corresponds to the(−201) plane of triclinic ReS2. The corresponding selected areaelectron diffraction (SAED) pattern shown in the inset ofFig. 2d shows that ReS2 NS is highly crystalline. All the charac-terizations mentioned above clearly illustrate that the ReS2NSs, which were prepared by CVD synthesis, possess oxidationresistance, high crystal quality, uniform surface, large-scalecrystals and great application prospects for nanoscale elec-tronic and optoelectronic devices.

To evaluate the quality of the synthesized ReS2 NSs, a back-gate multilayer ReS2 FET was fabricated on a 300 nm-thickp-doped silicon substrate. Electrical contacts were made usingelectron beam lithography followed by thermal deposition ofCr/Au (8 nm/60 nm) electrodes. The device was annealed at300 °C for 3 hours to remove the resist residue and decreasecontact resistance. We first measured the electrical propertiesof ReS2 NSs in ambient conditions. The linearity of I–V curvesindicates the ohmic contacts between ReS2 NSs and Cr/Au elec-trodes, as shown in Fig. S3a.† Then, we characterized our ReS2NS FET by applying a drain source bias Vds (3 V) to the pair ofelectrodes and back-gate voltage Vbg (−50 V–50 V) to thep-doped silicon substrate. Fig. 3a shows the typical behavior ofReS2 FET devices with n-type channels and an on/off ratio of∼105. The AFM image presented in Fig. 1d indicates that theReS2 NS has a uniform thickness of ∼4.8 nm, as well as thechannel length and width of approximately 1.9 and 0.9 μm,respectively. From the data in Fig. 3a, we extract the electronfield-effect mobility of ∼1.5 cm2 V−1 s−1 using the standardmethod: μ = Lgm/W(ε0εr/d )Vds, which is comparable to∼0.1–10 cm2 V−1 s−1 for the single-layered MoS2 FET exfoliatedonto SiO2.

2,4,25 In addition, compared with tens of nanometershigh-κ dielectrics (such as HfO2, ZrO2), the thicker and low-κdielectric (SiO2) usually results in the loss of electrostaticcontrol by the gate and thus relatively poor SS. The outputcharacteristics of ReS2 FET in ambient conditions did not tendto saturate when the source drain voltage was scanned from 0to 0.5 V (Fig. S3b†).

Generally, the electrical properties of TMDs are stronglyinfluenced by environmental conditions, which include temp-erature and vacuum.26–29 Therefore, the temperature depen-

dence of electrical measurements using theoretical simulationwere conducted first. We measured the transfer characteristicsof ReS2 FET from 80 K to 360 K, as shown in Fig. 3b. With theincrease in temperature, Vth moved to the negative direction,which indicates that more electrons are generated. For atypical semiconductor, the thermally excited carrier concen-tration decreases at low temperatures. Then, we extracted the2D charge concentration using the parallel-plate capacitormodel,30 with

n2D ¼ CoxΔVbg=e

where Cox = ε0εr/dox, ε0 = 8.85 × 10−12 F m−1, εr = 3.9, dox =300 nm, e = 1.6 × 10−19 C and ΔVbg = Vbg − Vbg,th. The Vbg,th isestimated from the transfer curve in the back-gate voltage, Vbg,range of 45–50 V. The relationship between n2D and temperatureis plotted in Fig. 3c. In the high-temperature regime (180 K < T< 360 K), the electron concentration increases sharply, while inthe low temperature regime (80 K < T < 180 K), it changes verylittle. The relationship between resistance and temperature isalso shown in the inset of Fig. 3c. Here, the activated behaviorcan be well fitted by the Arrhenius equation:31

R ¼ R0eEa=kBT :

We obtained an extracted activation energy of Ea = 175 meV,which can be explained by the presence of impurity energylevels close to the conduction band. A similar activationenergy of ∼167 meV is also obtained for another ReS2 FETdevice with a thickness of 15 nm, which is presented inFig. S6.† Subsequently, we discuss the possibilities of theobserved electron doping. One possibility is the impurityatoms inside the crystal. During the process of the CVD syn-thesis of ReS2, the tube is pumped and flushed with Ar gas toremove impurity materials, and ReO3 (99.9%) and S powder

Fig. 3 Temperature dependent ReS2 FET and first-principles calcu-lations. (a) Transfer curve of ReS2 FET in ambient conditions, with an on/off ratio of ∼105. (b) Ids–Vgs curves recorded at various temperaturesranging from 80 to 360 K. Vds = 3 V. (c) Charge density as a function oftemperature at Vgs = 50 V. Inset: Arrhenius plot of the same device. (d)The calculated defects include Re vacancies, S vacancies and anti-sites.(e) The formation energy of S4 vacancies is lower than that of otherinvestigated defects, which indicates that S vacancies exist most possiblyin our samples synthesized by CVD.

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(99.5%) precursor have high purity. Therefore, we exclude thispossibility. Another possibility is formative point defects in thesynthesis process. To illuminate this possibility persuasively,first-principle total energy and electronic structure calculationswere performed in the framework of the density functionaltheory (see Methods). The calculated defects, including Revacancies, S vacancies and anti-sites, are displayed in Fig. 3d.Because of inversion symmetry in the ReS2 structure, S1 (S3)and S6 (S4), as shown in Fig. 3d, are equivalent. Thus, lines S1(S3) overlaps line S6 (S4), as presented in Fig. 3e. In addition,the formation energy of defect is possibly thickness depen-dent, especially when the thickness is only a few atomic layers.As the thickness would increase, the influence of the samplesurface on the inside defect states would be graduallyweakened considerably, and thus the formation energy on theinside should converge with that in the bulk system. Fig. 3eshows that the formation energy of the S4 vacancies is lowerthan that of the other defects, which indicates that S vacanciesexist most possibly in our CVD synthesized samples. Based onthe abovementioned experiments and discussions, thethermal activated n-type defects above 180 K in Fig. 3c largelycorrespond to the S vacancies. The theoretical simulations arein agreement with the experimental results, which show thatS vacancies exist in the synthesized ReS2 NSs intrinsically andthey significantly affect the performance of devices.

To further study the impact of S vacancies on the electricalproperty of ReS2 NS in different environmental conditions, theelectrical measurements were carried out in vacuum, oxidizinggas (O2) and reducing gas (NH3) atmospheres. The device washeld for 2 hours in vacuum (6 × 10−5 torr) to detach surfaceadsorbates. A relatively large increase of the drain currents invacuum occurred in the transfer curves, especially the off-statecurrent (Fig. S5a†), which may originate from the separation ofthe surface p-doped adsorbates. The charge transfer betweenthe ReS2 NSs and adsorbates can efficiently change the carrierdensity of the ReS2 NSs. Furthermore, we measured the trans-fer and output characteristics of the ReS2 FET in O2 and NH3

atmospheres. The drain current increases drastically, as shownin Fig. 4a and b, when NH3 molecules adsorb on the ReS2surface. This behavior results from electronic doping by NH3,which serves as a charge donor, and the gate control loss isdue to the fact that the Fermi level is close to the proximity ofthe conduction band edge. For O2 molecule, a prominentdecrease in Ids occurs, which indicates that O2 molecule servesas a charge acceptor. The experimental results manifest thatReS2 NSs can efficiently recognize oxidizing gases and redu-cing gases. Moreover, the electrical property of ReS2 NSs canbe modulated by charge doping, which is a vital stepping-stone in the process of integrating TMDs into future electronicdevices. The current of O2 adsorption versus time is fitted tothe Langmuir isotherm in the inset of Fig. 4b via

IðtÞ ¼ Iinitial þ φ=ð1þ φÞð1� e�t=τÞ

where Iinitial is the initial current, φ is a dimensionless para-meter characterizing surface coverage, and τ is the adsorption

time constant. The adsorption time τ is determined to be∼15 min, which is comparable to other physisorption FET gassensors.22

To understand the charge transfer processes for the O2 andNH3 molecules adsorbed on the monolayer ReS2 NSs near thevacancy sites, we performed the first-principles calculations ofthese systems based on the density functional theory(DFT)32,33 within generalized-gradient approximations (GGA)34

as implemented in the Vienna ab initio simulation package(VASP).35–37 In the first-principles calculations, the electronand core interaction are included based on the frozen-core pro-jected-augmented wave (PAW)38 approach. A 3 × 3 supercellcontaining a single gas molecule adsorption on the monolayerReS2 surface was constructed as the computational system. Outof the monolayer plane, a 20 Å vacuum spacing was added toprevent the interaction with its image. In the simulation ofadsorption process, all the atoms were allowed to relax untilthe quantum mechanical forces acting on them were less than0.02 eV Å−1. To analyze the charge transfer process, the chargedensity difference is a very useful quantity; it is defined asρ (A, α), ρ (ReS2, α) and ρ (A), which are the charge densities ofthe whole absorption system containing an adsorbate A (O2 orHN3 molecule), a defect α and monolayer ReS2, and monolayerReS2 containing a defect α and A molecule alone, respectively.The calculated adsorption energy for O2 molecule adsorptionon the ReS2 surface is −0.037 eV, which indicates a weak inter-action between the O2 molecules and ReS2 monolayer and it isphysisorption. Because the electronegativity of O atom islarger than that of Re and S atoms, the O2 molecule is expected

Fig. 4 (a) Transfer curves of ReS2 FET in oxidizing gas (O2) and reducinggas (NH3) atmosphere. (b) Output curves of ReS2 FET. Inset: Ids–T withthe time of O2 injection fitted by the Langmuir isotherm. (c) O2 moleculeadsorbed on the ReS2 surface and just above the vacancy site; (d) NH3

molecule adsorbed on the ReS2 surface and just above the vacancy. Theiso-surface value is 2.0 × 10−3 e Å−3 for the O2 adsorption and 2.0 ×10−4 e Å−3 for the NH3 adsorption. Yellow and blue distributions corre-spond to charge accumulation and depletion, respectively.

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to behave as the charge accumulator in the system of O2

adsorption on the ReS2 surface. In terms of Bader analysis,39,40

it is indeed found that the O2 molecule approximately receives0.01 electrons from ReS2. Therefore, it can also be expectedthat if O2 molecule is placed near n-type S vacancies, electroncarrier density of the n-type doping will probably be furtherreduced by the transfer of electrons around the O2 moleculebecause the O2 molecule functions as an electron accumulatorin the interaction between the O2 molecules and defects.Fig. 4c shows the charge density difference of the wholesystem for the adsorption of O2 molecules on the surface ofmonolayer ReS2, which possesses an S vacancy defect. It isapparent that the electrons are accumulated around the O2

molecule and are depleted near the defect. In this case, theBader analysis predicts that about 0.834 electrons are trans-ferred from the S vacancy to the O2 molecule, which is remark-ably larger than that in the case of O2 adsorbed on pure ReS2without any defect. Thus, with respect to the interactionbetween the O2 molecule and S vacancy in ReS2, the inter-action between the O2 molecule and pure ReS2 is negligible.O2 molecules electronically interact with ReS2 only at thedefect sites. In the case of the adsorption of NH3 molecule, thecalculated adsorption energy between NH3 molecule andmonolayer ReS2 is −0.031 eV, which is also attributed to physi-sorption, similar to the O2 adsorption on the ReS2 surface.However, we predict that ReS2 approximately receives 0.011electrons from the NH3 molecule, which indicates that theNH3 molecule serves as a charge donor and enhances the elec-tron carrier density of n-type ReS2. Fig. 4d shows that in thecase of ReS2 possessing an S vacancy, the electrons are depletedrather than accumulated around the NH3 molecule. The first-principles DFT calculated adsorption energy of the adsorptionof NH3 molecule on ReS2 with an S vacancy is −0.046 eV, whichis larger than that observed by the NH3 molecule on pure ReS2.This implies that by having S vacancy defects in the ReS2 mono-layer, the NH3 molecule adsorbate could significantly enhancethe n-type doping. Therefore, by combining electrical measure-ments in O2 or NH3 gas atmosphere and theoretical simu-lations, we demonstrate that S vacancies in ReS2 can recognizeoxidizing gas (O2) and reducing gas (NH3) with large electronicinteractions. Such unintended defects are expected to play a sig-nificant role in device performance.

Conclusions

In summary, we have synthesized large-scale ReS2 NSs on avariety of substrates, including rigid SiO2/Si and flexiblecarbon fibers, via CVD. These NSs were synthesized at 500 °Cusing ReO3 and S powder as precursors. By combining SEM,TEM and AFM measurements, the ReS2 NSs synthesized byCVD are shown to possess high crystal quality, uniform surfaceand large-scale crystals. The more accurate compositional ana-lyses from XPS measurements manifest that the ReS2 NSscontain ∼33 at% Re and ∼67 at% S. The high performanceFET of ReS2 NSs with an on/off ratio of ∼105 was demon-

strated. Using electrical transport measurements at varyingtemperatures (80 K–360 K) and first-principles calculations, wefound that sulfur vacancies intrinsically exist in the ReS2 NSs.Furthermore, the electrical properties of the ReS2 FET weredetermined under vacuum, O2 and NH3 atmospheres. Wedemonstrate that S vacancies can efficiently adsorb and recog-nize oxidizing gases and reducing gases. Together with calcu-lation simulations, we find that O2 and NH3 moleculeselectronically interact with ReS2 only at defect sites. Ourresearch study may provide a new scope for the synthesis ofnew TMDs, which are important to understand the physicalnature of FETs, and will open up new exciting opportunitiesfor enhancing and expanding their applications in electronicsand photoelectronics in future.

Acknowledgements

This study carried out at the National Center for Nanoscienceand Technology was supported by 973 Program of the Ministryof Science and Technology of China (no. 2012CB934103), the100-Talents Program of the Chinese Academy of Sciences (no.Y1172911ZX), the National Natural Science Foundation ofChina (no. 21373065 and 61474033) and Beijing Natural ScienceFoundation (no. 2144059). The study at Institute of Semiconduc-tors was supported by the National Natural Science Foundationof China under grants no. 11474273, no. 11104264 and no.61474116 and National Young 1000 Talents Plan.

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