Direct Observation of SemiconductorâMetal Phase Transition in

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Direct Observation of Semiconductor-Metal Phase Transition in Bilayer Tungsten Diselenide Induced by Potassium Surface Functionalization Bo Lei, Yuanyuan Pan, Zehua Hu, Jialin Zhang, Du Xiang, Yue Zheng, Rui Guo, Cheng Han, Lianhui Wang, Jing Lu, Li Yang, and Wei Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00398 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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Direct Observation of Semiconductor-Metal Phase Transition in Bilayer Tungsten Diselenide Induced by Potassium Surface Functionalization Bo Lei1,3,4†, Yuanyuan Pan2,5†, Zehua Hu1,3,4†, Jialin Zhang3,6, Du Xiang4,6, Yue Zheng3,4, Rui Guo6, Cheng Han7, Lianhui Wang8, Jing Lu5, Li Yang2*, Wei Chen1 ,3,4,6*

1

National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Jiang Su 215123, China

2

Department of Physics and Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States 3

4

Department of Physics, National University of Singapore, Singapore 117542, Singapore

Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, 117546, Singapore 5

State Key Laboratory of Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China

6

7

Department of Chemistry, National University of Singapore, Singapore 117543, Singapore

SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, Shenzhen University, Shenzhen 518060, China

8

Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced

Materials (IAM), Jiangsu National Syngerstic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts &Telecommunications, 9 Wenyuan Road, Nanjing 210023, China †

These authors contributed equally to this work.

*

Authors to whom correspondence should be addressed. Electronic mail: [email protected] (L. Y.) and [email protected] (W. C.) 1

ACS Paragon Plus Environment

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Abstract Structures determine properties of materials and controllable phase transitions are, therefore, highly desirable for exploring exotic physics and fabricating devices. We report a direct observation of a controllable semiconductor-metal phase transition in bilayer tungsten diselenide (WSe2) with potassium (K) surface functionalization. Through the integration of in-situ

field-effect-transistor

(FET),

X-ray

photoelectron

spectroscopy,

ultraviolet

photoelectron spectroscopy measurements and first-principles calculations, we identify that the electron doping from K adatoms drives bilayer WSe2 from a 2H-phase semiconductor to a 1T’-phase metal. The phase-transition mechanism is satisfactorily explained by the electronic structures and energy alignment of the 2H and 1T’ phases. This explanation can be generally applied to understand doping induced phase transitions in two-dimensional (2D) structures. Finally, the associated dramatic changes of electronic structures and electrical conductance make this controllable semiconductor-metal phase transition of interest for 2D semiconductors based electronic and optoelectronic devices.

Keywords 2D materials, WSe2, phase transition, surface functionalization, potassium

2


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The layered two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted tremendous attention in recent years due to their unique properties when the materials are down-sized to single and few atomic layers.1-4 Unlike the zero bandgap of graphene,5 which greatly limits its application, TMDs possess a sizeable layer-dependent bandgap from 1.2 eV to 1.8 eV,6-7 exhibiting strong anisotropy in current flow and valley physics,8-9 suggesting their potential applications in future logic electronics and functional optoelectronic devices.2 Layered tungsten diselenide (WSe2), belonging to the family of 2D TMDs, exhibits interesting properties such as tunable bandgaps, ultrafast photoresponse with high responsivity and high current on/off ratio of 107-108.2 Moreover, WSe2 can achieve hole-dominated, ambipolar, and electron-dominated transport behaviors by controlling contact metal and the number of layers,10 which render WSe2 standing out from TMDs. Besides the previous characteristics, a distinctive feature of WSe2 is polymorphism, and different polymorphs usually exhibit very different characteristics.11-13 The natural WSe2 possesses the air stable semiconducting 2H phase, while the 1T’ phase WSe2 exhibits air unstable metallic characteristics.14-15 The 1T’ phase was reported to exhibit some interesting physical properties11, 16 and play a crucial role in enhancing the performance of 2D TMDs transistor devices by lowering the contact resistance.17 Up to now, there are several reported different methods to induce the semiconducting to metallic phase transition,18-28 and the most effective one is via the intercalation with alkali metal.29-32 Specifically, owing to the low electron affinities, lithium29-30 and potassium (K)31-32 can serve as strong electron donor for the bulk and 2D material. The metallic phase WSe2 was reported to transform from semiconducting phase by lithium and K intercalation, forming a compound as LixWSe2 and 3


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KxWSe2, confirmed by X-ray photoelectron spectroscopy (XPS),14-15, scanning transmission electron microcscopy (STEM)26,

34

33

Raman,14-15,

29

and electron energy-loss

spectroscopy (EELS).31 In addition to the study of the fundamental properties, the actual electronic characteristics are even more crucial for device application and the electronic properties of the WSe2 show a significant dependence on the polytypic structure,35 therefore, the

impact

of

semiconducting

to

metallic

phase

transition

on

WSe2

based

field-effect-transistors (FETs) needs detailed investigation.

Herein, we report a direct observation of K surface functionalization induced semiconducting to metallic phase transition in bilayer WSe2. It is found that the electron conductance of the bilayer WSe2 FET device after in-situ surface K functionalization is substantially increased by more than eight orders of magnitude at Vg = 0 V, revealing a metallic nature of the K decorated WSe2 and a phase transition from the semiconducting phase to the metallic phase. Such phase transition is further corroborated by in-situ XPS, ultraviolet photoelectron spectroscopy (UPS). Our first-principles calculations reveal that the semiconducting 2H phase to metallic 1T’ phase transition and the electrons transferred (doped) from adsorbed K lead to the phase transition, with the critical extra electron concentration of 0.3 e/f.u. for the bilayer WSe2.

Results and discussion Figure 1(a) displays the atomic force microscope (AFM) image of an as-made WSe2 FET device on highly p-doped silicon wafer with 300 nm SiO2, where 20 nm Ti/ 50 nm Au were deposited as metal contacts. AFM measurement revealed the WSe2 flake thickness of ~1.6 nm, corresponding to a bilayer sample [Figure 1(a)]. Figure 1(b) presents the Raman mapping by 4


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extracting the maximum of the E12g mode, indicating the high uniformity of the bilayer WSe2 flake. Inset is the Raman spectrum of the exfoliated WSe2, with the characteristic peaks at around 247 cm-1 (E12g) and 257 cm-1 (A1g) respectively. The Raman peak at around 310 cm-1 originates from the additional interlayer coupling.36 Combined with the AFM and Raman spectrum, the bilayer nature of the WSe2 flake was confirmed. To quantitatively extract the conductivity of the semiconducting phase WSe2 and metallic phase WSe2, the sample in Figure 1 was configured into a FET device for in-situ transport measurement. Figure 2(a) displays an Isd-Vg of the as-fabricated p-type bilayer WSe2 FET device under high vacuum (10-8 mbar) with the gate voltage (Vg) sweeping from -60 V to 50 V and source drain voltage (Vsd) fixed at 1V. The source drain current Isd increased along both positive and negative sweeping direction, indicating a typical hole dominated ambipolar transport behavior. The inset logarithmic plot discloses the Ion/Ioff ratio of the device is as large as ~107, in good agreement with previous report.2 K was in-situ evaporated from a SAES Getter onto the device under high vacuum (10-7 mbar), followed by in-situ electrical transport measurement. Figure 2(b) shows the logarithmic scale of the transfer characteristic curve with the increasing nominal thickness of the deposited K from 0 to 0.16 nm. The transfer curve in Figure 2(b) demonstrates that, in contrast to the typical ambipolar behavior of the pristine WSe2, the K decorated WSe2 exhibits n-type dominant transfer characteristics. Moreover, when the K thickness is above 0.11 nm, very weak gate dependence is observed for the bilayer WSe2 FET, revealing a transition from the semiconducting phase to the metallic phase. Herein, we have calculated the electron conductance (G = Isd/Vsd) at Vg = 0 V with the increased nominal thickness of K, as shown in 5


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Figure 2(c). The electron conductance of WSe2 increases by about eight orders of magnitude, i.e., from 1.31×10-6 µS to 102.8 µS with the increased thickness of K. In particular, there is an abrupt increase when the thickness of K is ~0.11 nm, indicating the critical phase transition point. Figure 2(d) shows the Isd-Vsd characteristics of the back-gated FETs for 0-1 V Vsd and the Vg ranging from 0 V to 50 V, with the black curves for the pristine WSe2 and the red curve for WSe2 functionalized with 0.16 nm K. Pristine and 0.16 nm K functionalized FET features nonlinear and linear Isd-Vsd characteristics, indicating Schottky and ohmic contact, respectively. The Isd with same gate voltages increases by about three orders of magnitude after the K modification, revealing a significant decrease of the channel and contact resistance. We have also fabricated WSe2 FET devices with different thickness, i.e., 3 layers, 5 layers (Figure S1) and thick WSe2 films with 65 nm and 74 nm film thickness (Figure S3). All these FET devices exhibit an abrupt increase in electron conductance when the K thickness reaches around 0.1 nm [Figures S2(b), S2(d) and Figures S4(b), S4(d)]. This indicates that the semiconductor-to-metal phase transition shall occur on the top a few layers of WSe2. To understand doping effect and the growth behaviors of K on WSe2, in-situ low-temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) measurements were carried out. Figure 3(a) and (b) show atomically resolved STM images of the clean WSe2 surface. The clean WSe2 shows a band gap of 1.3 eV as revealed by the dI/dV spectrum in Figure 3(c). The valance band edge is close to the zero bias voltage (Fermi level), indicating the hole dominated nature of WSe2. After 0.04 nm K deposition, identical K clusters with lateral size around 2 nm were randomly formed on the surface of WSe2, as 6


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shown in Figure 3(d) and (e). The corresponding STS shows that the band gap remains as ~ 1.3 eV, while the Fermi level shifts towards the conduction band minimum (CBM) by about 1 eV after the deposition of 0.04 nm K, revealing a strong electron doping effect. The STS spectra measured on top of the K-cluster and away from the K-cluster exhibit negligible difference as shown in Figure 3(f), demonstrating the homogeneous doping effect on WSe2. As shown in Figure S5, even increasing the K thickness to 1 nm, the WSe2 surface was fully covered by K clusters without a clear evidence for the formation of continuous K film. To investigate the mechanism of the interaction between K and WSe2, in-situ UPS and XPS measurements have been carried out on K functionalized WSe2. The evolution of the K-thickness dependent UPS spectra within the low kinetic energy region is displayed in Figure 4(a). By linearly extrapolating the low kinetic energy onset in UPS, the work function (WF) of bulk WSe2 has been measured to be 4.08 eV. After 0.56 nm K deposition, the WF sharply decreases to 2.36 eV, originating from substantial interfacial electron transfer from K to WSe2 and the formation of interface dipoles between K and WSe2. Figure 4(b) and 4(c) show the Se 3d and W 4f core levels evolved with the increased K thickness. At the initial stage, when 0.12 nm K deposited, the binding energies of Se 3d and W 4f core level slightly increase by around 0.2 eV and 0.15 eV respectively. This indicates that the Fermi level of WSe2 moves towards the conduction band, and hence confirming the effective electron doping process. With further increasing the K nominal thickness to 0.56 nm, the binding energies of Se 3d and W 4f core levels significantly decrease by around 1.20 eV and 1.25 eV, respectively. The decreasing of the core level energy was also observed in previous reports for extra electron induced phase transition of TMDs.14,

33

7


The abnormal changes for the

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binding energies of Se 3d and W 4f as a function of K thickness can also be explained by the semiconductor-to-metal phase transition, as shown in Figure 4(d) and Figure 4(e). At the initial stage, it is electron doping process from K to WSe2. The excess electrons from the K drive the Fermi level of the semiconducting WSe2 to move closer to the CBM, and hence the binding energies of W 4f and Se 3d increase.37 When thickness of the K reaches the critical thickness of 0.12 nm, the semiconducting phase WSe2 transforms to the metallic phase WSe2. As shown in Figure 4(e), by further increasing the thickness of K, more electrons will be transferred to WSe2 and evenly distributed on the metallic WSe2 layers. This makes the metallic WSe2 layers negatively charged, and hence the binding energies of the core levels are progressively reduced. As aforementioned, the semiconductor-to-metal phase transition occurs on the top a few layers of WSe2, within the detection limit of the surface sensitive XPS/UPS. Therefore, the in-situ XPS measurement of the K functionalized bulk WSe2 sample can also provide essential information for the electron doping induced semiconductor-to-metal

phase

transition,

complementing

the

in-situ

FET

device

measurements. To investigate the role of doping and the surface K functionalization in the semiconductor-metal phase transition process of bilayer WSe2, ab initio density functional theory (DFT) calculations were applied. As previous reported, alkali metal intercalation can induce the phase transition in WSe2 from the semiconducting 2H phase to the metallic 1T’ phase. We first calculate the electronic band structures of 2H and 1T’ phases of intrinsic bilayer WSe2 as shown in Figure 5(a-b) and monolayer WSe2 as shown in Figure S6. For these alkali adatoms, their main role is to provide extra electrons (n-doping) to 2D 8


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materials,38 motivating us to focus on doping effects on WSe2 structures. Particularly, we employ the rigid-band doping approach, in which those doped carriers are introduced by shifting the Fermi level. This method can avoid the costly simulations of supercells while capture the essential doping effect.39 Then we calculated the total energy of 2H and 1T’ bilayer WSe2 and plotted their energy difference (∆E = E1T’ − E2H) according to the doping density in Figure 5(c). For the intrinsic WSe2, 2H phase is more stable than the 1T’ one for both monolayer (Figure S7) and bilayer WSe2 (with an energy difference value of 0.27 and 0.33 eV/f.u., respectively). This is consistent with the well-accepted observations.14,

40

Beyond intrinsic cases, when increasing electron concentration, the sign of energy difference in Figure 5(c) is changed at the critical doping density of 0.3 e/f.u. Energetically, this indicates that 1T’ metallic phase will replace the 2H semiconducting one to be the more stable structure, and a corresponding phase transition can be induced. This is similar to the previous theoretically predicted and measured phase transition in MoS235,41 but the critical doping density (~ 0.3 e/f.u.) is smaller than that of theoretical results (~ 0.5 e/f.u.) in bilayer MoS2. From this ab initio DFT calculation results, we can also estimate the required thickness of adsorbed K with the critical electron density (0.3 e/f.u.). With the known ion radius of K and an assumption that each K adatom contribute one doped electron, we have calculated the corresponding thickness as shown by the upper horizontal axis of Figure 5(c). Our estimated critical thickness is about 0.095 nm, which is in good agreement with the experimentally measured critical thickness in Figure 2(c) (~ 0.11 nm). As the rigid-band conclusions only consider the simplified electron donating effect, a crucial question is whether the adsorption of alkali metal adatoms will induce other effects and substantially 9


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alter the rigid-band results. To assure this rigid-band approximation, we have explicitly calculated the doping effect and structural phase transition with K adatoms included (Figure S8 and Figure S9) and the results are essentially the same as that from the rigid-band approximation (Figure S6 and Figure 5). In Figure 5(c) and Figure S7, we have shown how the total energy changes with K adatoms on bilayer and monolayer WSe2, respectively. A similar switching of the lowest-energy configurations is observed: the critical density of K adatoms on bilayer WSe2 for this structural phase transition is about 0.24 e/f.u., which is in good agreement with the rigid-band results (~0.3 f/f.u.). Therefore, the electron doping can effectively induce such semiconducting 2H phase to the metallic 1T’ phase transition, corroborating our in-situ electrical transport and XPS/UPS measurements. The driving mechanism for this phase transition can be understood from the band alignment. As shown in Figure 5(d), the absolute band energy of 2H and 1T’s phases of WSe2 are presented. The noticeable character is that the Fermi level of the metallic 1T’ phase is rather low (around -5 eV). When extra electrons are doped into the 2H structure, they must occupy the higher energetic CBM at -3.7 eV; while in the 1T’ phase, they only need to fill the lower Fermi level at -5 eV. As a result, the electron doping will increase the total energy of the 2H phase faster than that of the 1T’ phase, inducing the phase transition ultimately. Interestingly, we can estimate the critical doping density by this band alignment. In Figure 5(d), the energy difference between the CBM of the 2H phase and the Fermi level of the 1T’ phase is around 1.28 eV. In Figure 5(c), the initial energy difference of these two phases is about 0.33 eV. Therefore, we can estimate that the required doping density is to be 0.33/1.28 = 0.26 e/f.u., which is in good agreement with the first-principles result (~0.3 e/f.u.) shown in Figure 5(c). 10


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Based on this picture, many 2D semiconductors are expected to exhibit this doping induced phase transition. In particular, larger the energy difference between the CBM of the semiconducting phase and the Fermi level of the metallic phase is, easier the phase transition is to happen under a lower doping density. This is also the reason for a potentially easier observation (less doped carriers) of this semiconductor-metal transition in WSe2 than other TMDs such as MoS2. We can also achieve the reversed phase transition from the metallic phase to semiconducting one by exposing the K functionalized bilayer WSe2 to oxygen (O2). As shown in Figure 6(a), after exposure to the pure O2 for 5mins, the metallic transfer characteristic of WSe2 returned back to the original semiconducting transfer characteristic. This reversed phase transition can be further verified by in-situ XPS/UPS measurements. As shown in Figure 6(b) and (c), after the O2 exposure, the binding energies and peak shapes of the Se 3d and W 4f clearly restored to their original semiconducting phase. The WF of the metallic WSe2 (Figure S9) largely increased to 3.64 eV after O2 exposure, which is close to its original value of 4.08 eV for the semiconducting WSe2. Such O2 exposure induced reverse phase transition is most likely originating from the oxidation of K and hence suppressing the electron doping effect. In contrast, the K functionalized metallic WSe2 possesses a good stability in nitrogen (N2) as shown in Figure S10. After exposure to N2 for 10 mins, the FET device still displays weak gate dependence and retains its metallic characteristic.

Conclusion In conclusion, we report a K surface functionalization induced phase transition in bilayer WSe2 from the semiconducting to the metallic phase, as revealed by in-situ FET device 11


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measurements, in-situ XPS/UPS characterizations and corroborated by DFT calculations. K surface functionalization can lead to significant electron doping (n-type doping) of bilayer WSe2; further increasing the electron concentration, a sharp phase transition from the semiconducting phase to the metallic phase was observed. The first-principle calculations demonstrate that it is a phase transition between the semiconducting 2H phase and the metallic 1T’ phase. It also reveals that the extra electron from K is the reason for driving this phase transition, and the critical extra electron concentration for the 2H→1T’ phase transition is 0.3 e/f.u. for the bilayer WSe2. Our results provide a deep insight to understand the phase transition mechanism in WSe2 and a versatile approach to control the semiconductor-to-metal phase transition via proper surface engineering for their applications in functional optoelectronic device applications.

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Methods

First-principles calculation. We have employed DFT calculations within generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) parameterization of the exchange-correlation functional,42 which is implemented in the Vienna ab initio simulation package (VASP). The pseudopotential is projector-augmented wave (PAW). The plane-wave cut-off energy is set to be 550 eV.43-44 The criteria for convergence is at least of 0.01eV/ Å for the residual force, and 1 × 10-6 eV per atom for the energy. We use a 6 × 6 × 1 k-grid for converged Kohn-Sham eigenvalues and wavefunctions. All structures are fully relaxed according to the force and stress calculated from DFT/PBE. A 2 × 2 supercell monolayer and bilayer WSe2 are built to investigate the stability of 2H and 1T’ phase WSe2. To mimic the case of WSe2 with K surface functionalization, we built models with K atoms absorbed in one side of monolayer and bilayer WSe2 with different K density. To avoid the periodical image interactions, the vacuum buffer space is set to be at least 15 Å.

Sample preparation and device fabrication. WSe2 flakes were mechanically exfoliated from bulk WSe2 crystals (hq-graphene) using a scotch tape and transferred onto degenerately p-type doped silicon wafers coated with 300 nm SiO2. Exactly after locating the exfoliated WSe2 flake by using high-resolution microscope (Nikon Eclipse LV100D), photoresist PMMA was immediately spin coated onto the sample to protect the flake from being degraded in air. The source and drain electrodes were precisely patterned on the flake using the conventional e-beam lithography technique, followed by thermal evaporation of Ti (20 nm) and Au (50 nm) as the metal contacts. After liftoff, the as-made devices were wire

13


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bonded onto a leaded chip carrier (LCC) and loaded in the custom-designed vacuum system (~10-8 mbar) for in-situ electrical measurements.

In-situ device characterization. FET device measurements for the K functionalized bilayer WSe2 were carried out in a home-made high vacuum system with optoelectronic measurements function and in-situ thin film growth capabilities, and measured by using an Agilent 2912A source measure unit at room temperature.2, 45 K was evaporated in-situ from a SAES Getter onto the devices in a high vacuum chamber. The nominal thickness of K layers was calibrated by a quartz crystal microbalance exactly located in front of the sample stage.

In-situ UPS and XPS characterization. In-situ UPS and XPS measurements on K-functionalized bulk WSe2 were carried out in a home-made ultrahigh vacuum system (10-10 mbar) with He I (21.2 eV) and Mg Kα (1253.6 eV) as excitation sources, respectively.2, 45-46 By applying a sample bias of 5 V, the sample work function was determined by the secondary electron cutoff at the low kinetic energy region. The nominal thickness of the in-situ deposited K layers was calibrated by quartz crystal microbalance and further corroborated by monitoring the attenuation of the substrate core level peaks after each K deposition.

LT-STM

Characterization.

In-situ

LT-STM

experiments

were

performed

in

a

custom-designed Unisoku LT-STM with a Nanonis controller. The pressure of the system is lower than 1.0 × 10−10 mbar. An Pt-Ir tip was used for the scanning at 77 K. All the images were obtained under constant current mode. In our system, the bias voltages were applied to the sample. A lock-in technique was used to collect the differential conductance dI/dV spectra, 14


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with a modulation voltage of 50 mV and a frequency of 625 Hz. When ramping the voltage, the feedback loop was opened.

Acknowledgement

Authors acknowledge the financial support from NSFC grant (21573156), National Key Basic Research Program of China (2015CB856505), Natural Science Foundation of Jiangsu Province BK20170005, Singapore MOE Grant R143-000-652-112, and the technical support from Centre for Advanced 2D Materials for the device fabrication. L.Y. is supported by the National

Science

Foundation

(NSF)

CAREER

Grant

DMR-1455346

and

NSF

EFRI-2DARE-1542815. Y. P. acknowledges the financial support from the China Scholarship Council. The computational resources have been provided by the Stampede of Teragrid at the Texas Advanced Computing Center (TACC). This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562.

Supporting Information Available: Additional FETs, STM, UPS and calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.  

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43. Kresse,

G.;

Joubert,

D.

From

Ultrasoft

Pseudopotentials

to

the

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Figure 1. Back-gated WSe2 FET and device characterization. (a) AFM images of a bilayer WSe2 FET device. (b) Raman intensity mapping of the E2g mode of the WSe2 (inset: Raman spectrum of the exfoliated WSe2 flakes).

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Figure 2. Transfer characteristics evolution for K-modified WSe2 FET. (a) Transfer characteristics (Isd-Vg) of the WSe2 device with Vsd=1 V. Inset: logarithmic plot of the transfer curve. (b) Logarithmic scale with increasing K thickness from 0 to 0.16 nm. (c) Electric conductance at Vg = 0 V versus K thickness. (d) Drain current (Isd – Vsd) characteristics of the back-gated FETs for 0-1 V drain-source voltage (Vsd) and gate-source voltages Vg ranging from 0 V to 50 V.

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Figure 3. (a) 10 × 10 nm and (b) the corresponding 5 × 5 nm atomically resolved STM images of clean WSe2. (Vs = -1.0 V) (c) Differential conductivity dI/dV measured on the clean 2

WSe2. (d) Large scale STM image of 0.04 nm K decorated WSe2. (Vs = -1.0 V; 80 × 80 nm ) (e) Close-up STM image showing the structure of one K cluster. The inset shows the lateral profile measured on the K-cluster, which indicates a lateral size of about 2 nm. (Vs = -1.0 V; 2

10 × 10 nm ) (f) dI/dV spectra measured on the K doped WSe2 which are away from the K cluster and on top of K cluster, respectively. After K doping, the fermi level is moved to the conduction band edge, indicating a strong electron doping effect.

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Figure 4. UPS and XPS characterizations before and after K modification on WSe2.

(a)

UPS spectra evolution at lower kinetic energy region and XPS core level spectra of (b) Se 3d and (c) W 4f as a function of K thickness on WSe2. (d) and (e) Schematic graphs for the electron doping process of (d) semiconducting phase WSe2 and (e) metallic phase WSe2.

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Figure 5. Band structures and the stable configurations of bilayer 2H and 1T’ WSe2 phase. Band structures of (a) 2H and (b) 1T’ bilayer WSe2 phases, respectively. Inset: top and side view of bilayer 2H and 1T’ WSe2 phases. (c) Energy difference per WSe2 formula unit (f.u.) between the 2H and 1T’ phase of bilayer WSe2 as a function of extra electron or K atom concentration. The red dot line is the rigid-band result while the black dot line is direct calculation including K adatoms. The white cross circle presents the estimated K atom density to induce phase transition of 0.24 K atom/f.u.. (d) Band alignment of bilayer 2H and 1T’ WSe2 phases.

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Figure 6. The electronic characteristics and XPS characterization of K functionalized WSe2 exposed to O2. (a) The transfer curve of WSe2 FET exposed to O2 in logarithmic scale. The WSe2 FET restores its initial semiconducting transport behaviour after O2 exposure. The XPS core level spectra of (b) Se 3d and (c) W 4f for pristine WSe2, K functionalized WSe2 (K/WSe2) and O2 exposed WSe2 (K/WSe2).

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Table of Contents Graphic

We report a semiconducting to metallic phase transition in bilayer WSe2 via surface functionalization with K, as revealed by in-situ field effect transistor device measurement, and corroborated by in-situ XPS/UPS characterizations. Our DFT calculations reveal that it is 2H to 1T’ phase transition of the WSe2 and the extra electron transferred from the K is dominant in initiating this structure transformation.

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