1T' MoTe2 in-plane Heterostructure

Subscriber access provided by UNIV OF TEXAS DALLAS

Functional Nanostructured Materials (including low-D carbon)

Low-contact Barrier in 2H/1T’ MoTe2 in-plane Heterostructure Synthesized by Chemical Vapor Deposition Xiang Zhang, Zehua Jin, Luqing Wang, Jordan Adam Hachtel, Eduardo Villarreal, Zixing Wang, Teresa Ha, Yusuke Nakanishi, Chandra Sekhar Tiwary, Jiawei Lai, Liangliang Dong, Jihui Yang, Robert Vajtai, Emilie Ringe, Juan-Carlos Idrobo, Boris I. Yakobson, Jun Lou, Vincent Gambin, Rachel Koltun, and Pulickel M. Ajayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00306 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Low-contact Barrier in 2H/1T’ MoTe2 in-plane Heterostructure Synthesized by Chemical Vapor Deposition Xiang Zhang1,‡, Zehua Jin1,‡, Luqing Wang1, Jordan A. Hachtel2, Eduardo Villarreal1, Zixing Wang3, Teresa Ha4, Yusuke Nakanishi1, Chandra Sekhar Tiwary1, Jiawei Lai1, Liangliang Dong3, Jihui Yang1, Robert Vajtai1, Emilie Ringe1,3, Juan Carlos Idrobo2, Boris I. Yakobson1, Jun Lou1, Vincent Gambin4, Rachel Koltun4, Pulickel M. Ajayan1,3,* 1. Department of Materials Science & NanoEngineering, Rice university, Houston, TX 77005, USA 2. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3. Department of Chemistry, Rice University, Houston, TX 77005, USA 4. NG Next, Northrop Grumman Corporation, Redondo Beach, CA, 90278, USA ‡ These authors contributed equally to this work

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

* Email: [email protected]

KEYWORDS: Two-dimensional materials, chemical vapor deposition, MoTe2, in-plane heterostructure, metal-semiconductor junction, contact resistance

Abstract Metal-semiconductor contact has been a critical topic in the semiconductor industry because it influences device performance remarkably. Conventional metals have served as the major contact material in electronic and optoelectronic devices, but such selection becomes increasingly inadequate for emerging novel materials such as twodimensional (2D) materials. Deposited metals on semiconducting 2D channels usually form large resistance contacts due to the high Schottky barrier. A few approaches have been reported to reduce the contact resistance but they are not suitable for large-scale application or they cannot create a clean and sharp interface. In this study, a chemical vapor deposition (CVD) technique is introduced to produce large-area semiconducting


2

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2D material (2H MoTe2) planarly contacted by its metallic phase (1T’ MoTe2). We demonstrate the phase-controllable synthesis and systematic characterization of largearea MoTe2 films, including pure 2H phase or 1T’ phase, and 2H/1T’ in-plane heterostructure. Theoretical simulation shows a lower Schottky barrier in 2H/1T’ junction than in Ti/2H contact, which is confirmed by electrical measurement. This one-step CVD method to synthesize large-area, seamless-bonding 2D lateral metal-semiconductor junction can improve the performance of 2D electronic and optoelectronic devices, paving the way for large-scale 2D integrated circuits.

Introduction Atomically thin 2D materials have drawn significant attention since the discovery of graphene.1,2 The 2D materials family consists of a variety of members that carry a wide range of novel properties.2–6 Interesting optical, mechanical and electrical phenomena have been frequently reported.7–12 In addition, 2D materials can be further combined to form vertical or lateral heterostructures with structural complexity as well as property diversity. These heterostructures include semiconductor-semiconductor junctions, such


3


Page 4 of 31

as WS2/MoS2,13–15 WSe2/MoSe2,16,17 MoTe2/MoS2,18,19 metal-insulator junctions, such as graphene/hexagonal boron nitride (hBN),20–22 and metal-semiconductor junctions.23– 27

Among them, 2D metal-semiconductor junctions have been recently extensively

studied. Because they show potential to reduce contact resistance between the 2D semiconducting material and the deposited metal electrodes like Au or Ti due to the Schottky barrier.28–32 Low contact resistance is essential for improving the performance of 2D electronic devices. A few approaches have been proposed to reduce the contact resistance such as using graphene contacts,33–39 contact doping,25,40,41 and phase-engineering.42–49 Graphene contact has proven difficult to be directly synthesized with transition metal dichalcogenides (TMDs) due to its lattice mismatch and incompatible synthesis methods. Physical transfer is the most common fabrication method, yet it requires complicated procedure and it can be difficult to create a clean and sharp interface. The latter two chemical methods remain insufficient for large-scale application and chemical stability. Therefore, direct growth of large-area, seamless-bonding 2D metal-semiconductor junctions with industrial compatibility plays a critical role for the future of vdWs integrated circuits. Recent research efforts in 2D materials have shown an increasing focus on molybdenum ditelluride (MoTe2). MoTe2 has two stable phases, hexagonal 2H phase and monoclinic 1T’ phase (illustrated in Figure 1a), both of which can be synthesized directly.50–56 Monolayer 2H MoTe2 is a semiconductor with a direct band gap of 1.1 eV, while bulk 2H MoTe2 has an indirect


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


band gap of 1 eV.57 These numbers are close to that of Si, which makes 2H MoTe2 promising for electronic devices and optoelectronic applications.58–61 1T’ MoTe2 is metallic and exhibits a large magnetoresistance and pressure-driven superconductivity at low temperature.62,63 Due to the small energy difference between these two phases,64 the 2H-1T’ phase transition has been achieved by using laser irradiation,43 or tensile strain.65 It is worth noting that 1T’ phase and 2H phase MoTe2 can form a metal-semiconductor junction, where 1T’ MoTe2 can be used as a contact material to solve the contact issue. MoTe2 thus not only possesses a myriad of physical properties to further unravel, but also shows great potential towards various industrial applications such as analog circuits and spintronics. Previous reports have shown various methods to synthesize MoTe2: mechanical exfoliation,43,57,58 chemical vapor transport (CVT),66,67 and chemical vapor deposition (CVD)50,51,54,55,68 have been the most studied. Compared to other methods, CVD method is the most promising for large-scale synthesis and thickness control, making MoTe2 a candidate for wafer-scale integration of devices. It has been reported that monolayer MoTe2 tends to oxidize in air.69 Thus, few-layer MoTe2 films should be more suitable for practical applications. Here we report the phase-controlled synthesis of large-area MoTe2 films by tellurizing thin Mo films. Pure 2H phase, pure 1T’ phase, and 2H/1T’ heterostructure MoTe2 films are obtained by controlling the reaction time and temperature. A series of techniques are used to characterize the synthesized MoTe2 films including Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) spectrophotometry, and transmission electron microscopy (TEM). The micro-scale optical transmission and absorption spectra of CVD-grown MoTe2 film is investigated by Microextinction spectroscopy (MExS), which has not been reported before. Several types of MoTe2


5


Page 6 of 31

based devices are fabricated and measured. In a field-effect transistor (FET) device, 2H phase MoTe2 channel shows p-type semiconducting behavior and exhibits exponentially higher sheet resistance than 1T’ MoTe2. We also demonstrate a decreased contact resistance through using the 1T’ phase as the contact electrodes for 2H phase-based transistor by observing improved drainsource current compared to Ti/Au metal contacts. Theoretical simulations further confirm that the contact barrier of 2H/1T’ MoTe2 in-plane heterostructure is lower than that of 2H MoTe2/Ti vertical junction.

Results and Discussions Two-dimensional MoTe2 films are synthesized by tellurizing Mo thin film in a tube furnace, as illustrated in Figure 1b. By employing different reaction temperatures and times, we have synthesized large-area, high-quality MoTe2 samples and achieved pure 2H phase, pure 1T’ phase as well as mixed 2H/1T’ phase. The detailed experimental method can be found in the Supporting Information. The optical image in Figure 1c shows a Mo film, 2H MoTe2, 1T’ MoTe2, and 2H/1T’ mixed-phase MoTe2 from left to right respectively, all on SiO2/Si substrate. Unlike previous works that reported only 1T’ MoTe2 obtained from Mo film,50,56 we demonstrate that 2H MoTe2 and 2H/1T’ mixed-phase MoTe2 can be also formed by adjusting growth temperature and time. Moreover, large-area 2H and 1T’ phase can be clearly identified on the MoTe2 films, indicating that our method is more suitable for studying the junction between these two phases.52,55,70 As-synthesized MoTe2 is characterized by Raman spectroscopy using a 532 nm excitation laser. 2H and 1T’ phases of MoTe2 give two distinct Raman patterns, which are presented in Figure 1d. The Raman-active modes of E1g (~118 cm-1), A1g (~172 cm-1), E12g (~232 cm-1), and B12g (~287


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cm-1) are observed in 2H MoTe2. E12g and E1g are in-plane modes, while A1g and B12g are out-ofplane modes.71 For 1T’ MoTe2, the Raman-active modes of Au (~108 cm-1), Ag (~127 cm-1), Bg (~161 cm-1), and Ag (~256 cm-1) are observed. All the peaks observed are consistent with previous reports,50,51,71,72 suggesting both 2H and 1T’ MoTe2 films have been successfully synthesized. E12g (~232 cm-1) and Bg (~161 cm-1) are the prominent peaks of 2H MoTe2 and 1T’ MoTe2, respectively. The dominant peaks from one phase are fully suppressed in the Raman spectra of the other phase, indicating MoTe2 can be grown phase pure. In addition, XRD and XPS are also used to characterize the 2H phases of MoTe2, which are displayed in Figure S3. To investigate the in-plane heterostructure of 2H/1T’ MoTe2, Raman intensity mapping is conducted near the interface. A square region of 50 × 50 µm is selected for Raman mapping in Figure 2a. Raman spectra collected from the left area shows only 2H MoTe2 peaks, while the right area shows only 1T’ MoTe2 peaks. Raman intensity mapping using the 2H MoTe2 E12g mode at 232 cm-1 (Figure 2b) and the 1T’ MoTe2 Bg mode at 161 cm-1 (Figure 2c) demonstrates the spatial distribution of 2H/1T’ MoTe2 heterostructure. The match between Raman mapping and optical image suggests that we can distinguish these 2H and 1T’ phases by their optical contrast. SEM images of the heterostructure are illustrated in Figure S4, which also imply that a sharp interface has been formed. The contrast between 2H and 1T’ MoTe2 can be attributed to their different electrical conductivities. Figure 2d shows an optical image of a region containing both 2H and 1T’ phase. AFM measurements are conducted near the interface as displayed in Figure 2e and 2f. No significant difference between these two phases is observed, which confirms that 2H and 1T’ MoTe2 have a very similar thickness and surface morphology. To study the optical properties of MoTe2, Mo films are deposited on sapphire substrates by ebeam evaporation, followed by the synthesis process described previously. Sapphire substrates


7


Page 8 of 31

are used for optical measurement because they transmit a high proportion of visible and infrared light. The Raman spectra and the optical spectra from the visible to the near-infrared of 2H MoTe2 and 1T’ MoTe2 are displayed in Figure S5 and S6, respectively. The absorption measurement indicates that CVD-grown 2H MoTe2 has an optical gap of 1.05 eV, while 1T’ MoTe2 does not show any absorption peak from 400 nm to 1400 nm. Furthermore, we explored the micro-scale optical transmission and absorption spectra of MoTe2 samples by using MicroExtinction Spectroscopy (MExS),73 which has not been reported before. Figure 2g shows an optical image of MoTe2 on sapphire, where 2H phase, 1T’ phase and sapphire regions are indicated. The total integrated transmission intensity mapping of the same regions acquired by MExS is displayed in Figure 2h. The 1T’ phase has around 20% larger total integrated transmission intensity than the 2H phase, while sapphire has the highest transmission. All the regions demonstrate uniform transmission strength throughout, which can be easily distinguished by the significant intensity differences among them. An absorbance study is conducted across the 2H/1T’ MoTe2 interface, where the spectrum at each pixel along the arrow in Figure 2h is retrieved from the MExS datacube output. Figure 2i shows the evolution of the absorbance spectra collected from 1T’ phase to 2H phase, where transmission intensity has been converted to absorbance. The first pixel is in the 1T’ phase region and no absorption peak is observed (blue curve). The last spectra (red curve in Figure 2i) is collected in the 2H phase region, which has two peaks and higher absorbance intensity than the spectra taken from the 1T’ region. The two peaks are located at 495 nm and 715 nm, which are close to the previous measurement on pure 2H MoTe2.57 The evolution of absorbance spectra collected from 1T’ phase to 2H phase clearly shows the different optical properties between these two phases. Our results provide valuable knowledge about optical properties of MoTe2, facilitating its application in optical devices.


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The crystallographic configuration of MoTe2 films are characterized by electron microscopy. MoTe2 films grown on SiO2/Si are transferred onto lacey carbon TEM grids using a PMMA assisted transfer method with hydrofluoric acid (HF) as the etchant.

High-resolution

transmission electron microscopy (HRTEM) images of the MoTe2 films and their corresponding fast Fourier transforms (FFTs) are shown in Figure 3, with the hexagonal 2H phase in Figure 3a and 3b and the monoclinic 1T’ phase in Figure 3c and 3d. The interface between the two phases in the lateral heterostructures is investigated further with aberration-corrected scanning transmission electron microscopy (STEM). Figure 3e show a defocused (low-magnification) STEM Ronchigram of the interface between 2H and 1T’ MoTe2 regions. The difference between the two samples can be observed even at low magnifications, due to the fact that in the transferred one a large number of breaks and discontinuities are present in the 1T’ region while the 2H region stays relatively continuous. A high magnification STEM image of the 2H/1T’ interface is shown in Figure 3f. While the thickness of the heterostructures (~10 nm) and carbon contamination on the surface prevent direct imaging of the atomic structure of the interface, the spatial resolution is high enough to observe the interface region and the crystal structure on either side of the interface. Figure 3g shows the FFT from the region on the 2H side highlighted by the blue box, and Figure 3h shows the FFT from the 1T’ region highlighted by the red box. The two FFTs show the same hexagonal and monoclinic structure observed in the reference images in Figure 3b and 3d, indicating that the heterostructure is single crystal on either side of the interface, and that the two crystal phases interface directly. To study the electrical properties of the large MoTe2 films, several types of electrical devices have been fabricated. Figure 4a shows a schematic of the two types of MoTe2 devices measured and Figure 4b shows the optical image of a typical fabricated device. CVD-grown MoTe2 film


9


Page 10 of 31

is first patterned into ribbons by lithography and reactive ion etching (RIE), followed by a second lithography step to deposit metal contacts by electron beam evaporation (Ti/Au, 5 nm/45 nm). Due to the apparent visual difference between 1T’ and 2H MoTe2, the two phases can be easily identified for separate device fabrication. We first study the electrical properties of 1T’ and 2H MoTe2 ribbons respectively. The transfer length method (TLM) is used to extract the contact resistance between the Ti/Au- MoTe2 interface as well as the intrinsic sheet resistance of MoTe2.74,75 TLM assumes contact resistance independent of device length and enables extraction of contact resistance (Rc in unit of Ω·µm) as well as intrinsic sheet resistance (Rs in unit of Ω/□ or ‘Ω/sq’) by combined studies on I-V curves from devices of varied channel lengths as displayed in Figure 4c and 4d. Rs and Rc can be extracted by the following equation Rtot = ⅀Rc/W + ⅀Rs·L/W

Equation 1

Where Rtot refers to the total device resistance, L, W refers to the device channel length and width respectively. Rs and Rc of MoTe2 have been extracted as follows: Rc,1T’=3.26 kΩ·µm, Rs,1T’=1.71 kΩ/□, Rc,2H=326.5 MΩ·µm, Rs,2H=35.2 MΩ/□. We can see an apparent differential sheet resistance in an order of ~20,000 between 2H and 1T’ MoTe2, and the differential contact resistance between 2H/metal and 1T’/metal is in an order of 100,000. In addition, another device has been characterized with similar electrical characteristics, as shown in Figure S7. 2H MoTe2 has been transferred and studied in a back-gated FET geometry, where Ti/Au is used as the contact metal. 2H-MoTe2 shows field-effect behavior, which is not observed for 1T’MoTe2. The typical transfer characteristics of the back-gated devices are shown in Figure 4e and


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4f. P-type behavior is observed on 2H-MoTe2 FET devices, consistent with previous theoretical and experimental results.50,58 The field-effect mobility can be calculated by using the equation µ=[dId/dVbg]×[L/(WCgVd)], where L and W are the channel length and width. Cg=ε0εr/d is the gate capacitance per unit area, where d=285 nm is the SiO2 layer thickness, ε0=8.854×10-12 F/m is the free-space permittivity and εr=3.9 is the relative permittivity of SiO2. dId/dVbg is estimated from the slope of the linear fit of the data Id-Vbg. The on/off ratio is estimated to be around 126 at Vbg=-80V, while the electron mobility of ~0.5 cm2V-1s-1, a number comparable to the highest performing CVD grown MoTe2 reported so far.50,52 The mobility could be underestimated due to the relatively high contact resistance. In addition to studying the electrical performance of phase pure 1T’ and 2H MoTe2, we examine the electrical properties of the 1T’/2H junction. Based on our measurement, CVD grown 1T’/2H/1T’ structure provides a low resistance contact for 2H-MoTe2. An optical image of a metal/1T’/2H/1T’/metal MoTe2 device is shown in Figure 4g, in which a 2H-MoTe2 channel is planarly sandwiched by two 1T’-MoTe2 strips, which are contacted by Ti/Au electrodes for measurement. A Raman intensity mapping of 2H phase E12g mode (232 cm-1) (Figure S8a), and 1T’ phase Bg mode (161 cm-1) (Figure S8b) is conducted over the device, revealing the spatial distribution of 1T’/2H/1T’ MoTe2 heterostructure. The I-V curve of the 1T’ contacted 2H MoTe2 device is shown in Figure S9. Compared with Figure 4d, the I-V curve appears to be more linear, indicating smoother increasing of drift carriers cause by increasing drain-source voltage, overcoming the problem caused by the Schottky barrier between 2D materials and common metals. To compare the electrical characteristics of the device with 1T’-phased electrodes and with Ti/Au on the 2H phase, I-V curves of electrodes 7-8 and electrodes 4-5, normalized by the dimension of the 2H channel (Inormalized=I·L/W), are co-plotted in Figure 4h. By using metal/1T’


11


Page 12 of 31

as the electrodes for 2H MoTe2 channel, the slope of the I-V curve is increased by 7-8 times compared with the device with metal/2H geometry, implying that more current can be collected through this device, and lower contact resistance is formed at the interface. This can be attributed to the different contact types between 2H/1T’ MoTe2 and metal/2H MoTe2. 2H/1T’ MoTe2 belongs to edge contact, while metal/2H MoTe2 is top contact. It has been reported that the vdW gap existing in top contact introduces an additional tunnel barrier before the Schottky barrier.28,30 Therefore the covalently bonded in-plane 2H/1T’ MoTe2 interface provides a promising solution to lower contact resistances. To reveal the mechanism of transport phenomena in MoTe2 with different phases, density functional theory (DFT) calculations are carried out to compare the Schottky barriers of 2H MoTe2/Ti vertical junction and MoTe2 2H/1T’ in-plane heterostructure. Since our experiment shows that 2H MoTe2 prepared by CVD method is a p-type semiconductors, we focus on the Schottky barrier for holes Φh, which is determined by the energy difference between the Fermi level and the valence band maximum (VBM) of the semiconductor in the junction: 76 Equation 2

Φh = EF - EVBM

where EF and EVBM are the energy of the Fermi level and VBM of the semiconductor in the junction, respectively. MoTe2 2H/1T’ in-plane heterostructure is modeled by in-plane splicing of MoTe2 with 2H and 1T’ phases as shown in Figure 5a, while 2H MoTe2/Ti vertical junction is constructed by a vertical stacking of three layers of MoTe2 with 2H phase and six layers of titanium as shown in Figure 5c, and the lattice mismatch is less than 3.5% for each junction. Structural optimization and self-consistent total energy calculations were performed adopting generalized gradient approximation

(GGA),

with

the

Perdew-Burke-Ernzerhof

(PBE)

exchange-correlation


12

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


functional, along with the projector-augmented wave (PAW) potentials, using the Vienna Ab initio Simulation Package (VASP).77 Electronic wavefunctions are expanded in a plane wave basis set with the kinetic energy cutoff of 280 eV and for the Brillouin zone integration 1 × 5 × 1 and 3 × 5 × 1 Monkhorst-Pack k-point mesh are used for MoTe2 2H/1T’ in-plane heterostructure and 2H MoTe2/Ti vertical heterostructure, respectively. The energy convergence criterion for electronic wave-function is set to be 10-5 eV. A vacuum layer of about 10 Å is chosen to guarantee no spurious interaction between layers in monolayer simulations using periodic boundary conditions. The projected electronic bands onto Mo of MoTe2 with 2H phase in MoTe2 2H/1T’ in-plane heterostructure, and Mo in 2H MoTe2/Ti vertical junction, are plotted in Figure 5b and 5d, respectively. For a comprehensive analysis, the projected electronic bands onto Mo of MoTe2 with 1T’ phase in MoTe2 2H/1T’ in-plane heterostructure, and Mo in 1T’ MoTe2/Ti vertical junction, are shown in Figure S10a and S10b, respectively. In Figure 5b the VBM of MoTe2 with 2H phase is folded to the Γ point and 0.25 eV lower than Fermi level in MoTe2 2H/1T’ in-plane heterostructure, while that in 2H MoTe2/Ti vertical junction is also folded to the Γ point but 0.5 eV lower than Fermi level as in Figure 5d. Besides, MoTe2 with 1T’ phase remains metallic in both MoTe2 2H/1T’ in-plane heterostructure and 1T’ MoTe2/Ti vertical junction, as shown in Figure S10. The electronic bands of the heterostructure can be projected into respective 2H and 1T’ phases that are the same as the isolated phases, implying the validity of the MoTe2 2H/1T’ in-plane heterostructure.78 These DFT results indicate that the contact barrier of MoTe2 2H/1T’ in-plane heterostructure is 0.25 eV lower than that of 2H MoTe2/Ti vertical junction, as sketched in Figure 5e, confirming that 1T’/2H/1T’ MoTe2 structure has much lower contact resistance than Ti/2H MoTe2/Ti structure.


13


Page 14 of 31

Conclusions In conclusion, we report the synthesis of large-area MoTe2 films through tellurizing thin Mo films by CVD method. The phase of MoTe2 can be controlled by reaction temperature and time. Systemic characterizations of MoTe2 have been conducted. The micro-scale optical transmission and absorption spectra of MoTe2 is investigated by MExS, providing useful information on its optical properties. We demonstrate an electrical device based on direct-grown semiconductormetal heterostructure 2H/1T’ MoTe2, where 1T’ phase serves as the electrode contacts for the 2H phase channel. The normalized current density through this device is higher than that measured by using metal contacts directly on 2H phase. Both experimental results and DFT calculations indicate the sharp and seamless bonded 2H/1T’ MoTe2 heterostructure provides a lower contact resistance than 2H MoTe2/Ti vertical junction. This work not only demonstrates the directsynthesis of the semiconductor-metal heterostructure, but also provides a strategy to achieve low contact resistance in 2D electronic devices. This advanced contact method can improve the performance of 2D electronic and optoelectronic devices, laying the groundwork for large-scale manufacturing.


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Synthesis and characterizations of MoTe2 films. (a) Schematic diagrams of 2H and 1T’ MoTe2 structures. Unit cells are marked with black dash lines. (b) Schematic illustration of the synthesis process. (c) An optical image of bare SiO2/Si, pure 2H MoTe2, pure 1T’ MoTe2, and mixed phased 2H/1T’ MoTe2 (from left to right). (d) Raman spectra of 2H (red) and 1T’ (blue) MoTe2.


15


Page 16 of 31

Figure 2. Raman intensity mapping, AFM measurement, and optical properties measurement of 2H/1T’ MoTe2 in-plane heterostructure. (a) An optical image of the 2H/1T’ interface. Raman intensity maps at (b) 232 cm

-1

(E12g mode of 2H phase) and (c) 161 cm

-1

(Bg mode of 1T’

phase). (d) An optical image of the 2H/1T’ interface. (e) AFM height image of a square region in (d). (f) Height profiles along the blue and red lines, showing both of 1T’ and 2H regions have a thickness of ~10 nm. (g) An optical image of 2H/1T’ MoTe2 in-plane heterostructure grown on sapphire. (h) A transmission intensity map of the same region in (g). (i) The evolution of absorbance spectra collected from 1T’ phase (blue) to 2H phase (red).


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Structure analysis of MoTe2. TEM images and corresponding FFTs for 2H phase (a and b) and 1T’ phase (c and d). (e) High defocus STEM Ronchigram image of the interface between 2H and 1T’ phases in MoTe2 heterostructure. (f) High magnification STEM image of the interface, showing that the two phases directly connect with one another. (g) FFT of the region marked by a blue outline in the STEM image in (f) demonstrating the region is single crystal and in the 2H phase. (h) FFT of the region marked by a red outline in of STEM image in (f) demonstrating the region is single crystalline and 1T’ phase.


17


Page 18 of 31

Figure 4. Electrical measurement of MoTe2 devices. (a) Schematic illustration of two types of MoTe2 devices. For the device on top, a 2H MoTe2 channel is directly contacted by metal electrodes. For the device on bottom, 1T’ MoTe2 interconnects a 2H MoTe2 channel and metal electrodes. (b) An optical image of a MoTe2 ribbon contacted by metal electrodes labeled 1 to 6. Electrodes 1 to 3 are in contact with the 1T’ phase, while electrodes 4 to 6 are in contact with the 2H phase. Ids-Vds curves of the 1T’ phase and 2H phase are shown in (c) and (d) respectively. Inset: Resistance obtained from different pairs of electrodes and channel lengths. (e) Roomtemperature field effect transfer characteristic for the 2H MoTe2 FET at 10 V drain-source voltage. Inset: an optical image of the 2H MoTe2 FET. (f) Ids-Vds curves acquired for the backgate voltage Vbg values at 0, -20, -40, and -80 V. (g) An optical image of a 1T’/2H/1T’ MoTe2 ribbon, where metal electrodes 7 and 8 are in contact with the 1T’ phase. (h) Normalized Ids-Vds curves acquired by electrodes 4-5 and 7-8.


18

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Band alignments of 2H/1T’ MoTe2 in-plane heterostructure and 2H MoTe2/Ti vertical junction. (a) Structure and (b) the projected electronic bands onto Mo of MoTe2 with 2H phase in MoTe2 2H/1T’ in-plane heterostructure. (c) Structure and (d) the projected electronic bands onto Mo in 2H MoTe2/Ti vertical junction. Fermi level is set to 0 eV. (e) Sketch of the band alignment for 2H MoTe2/Ti vertical junction and MoTe2 2H/1T’ in-plane heterostructure.

ASSOCIATED CONTENT

Supporting Information. Experimental methods, optical images of MoTe2 films grown at controlled temperature, and with controlled reaction time, XRD patterns and XPS spectra of 2H and 1T’ MoTe2, SEM images of 2H/1T’ MoTe2 in-plane heterostructure, optical image and Raman spectra of MoTe2 films on sapphire, absorbance in the visible to near-infrared of 2H and ACS Paragon Plus Environment

19


Page 20 of 31

1T’ MoTe2, electrical measurements of another MoTe2 device, optical images and Raman intensity maps of 1T’/2H/1T’ MoTe2 device, Ids-Vds curve measured by electrodes 7-8 in Figure 4g, band alignments of 2H/1T’ MoTe2 in-plane heterostructure and 1T’ MoTe2/Ti vertical junction. AUTHOR INFORMATION

Corresponding Author * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

X.Z. and Z.J. conceived the idea and designed the experiments. X.Z. performed CVD synthesis in collaboration with R.V. X.Z. also performed optical imaging, Raman spectroscopy, AFM measurement, XPS measurement, SEM imaging. Z.J. carried out device fabrication and electrical measurements. L.W. performed DFT simulations. J.A.H. and J.C.I. performed STEM imaging at Oak Ridge National Laboratory’s Center


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility. E.V. performed micro-extinction spectroscopy. Z.W. performed XRD measurement. T.H. deposited Mo films on sapphire at NG NEXT, Northrop Grumman. Y.N. performed HRTEM imaging. C.S.T. contributed to TEM imaging. J.L. contributed to sample transfer. L.D. contributed to UV-vis absorbance measurement in Naomi Halas group. J.Y. contributed to DFT simulations.

ACKNOWLEDGMENT This work is supported by BRI, Air Force Office of Scientific Research (AFOSR-Grant No. FA9550-14-1-0268), and by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. Z.J. and J.L. acknowledge the support from the Welch Foundation (grant C-1716). L.W., J.Y., and B.I.Y. acknowledge the Office of Naval Research grant N00014-15-1-2372. L.W. also acknowledges 2017/18 Oil & Gas HPC Conference Graduate Fellowship funded by the Ken Kennedy Institute for Information Technology. E.V. acknowledges the National Science Foundation Graduate Research Fellowship (Grant 0940902). E.R. and E.V. acknowledge support from the Air Force Office of Scientific Research Grant No. AFOSR-YIP FA9550-17-1-0202. This research made use of instruments in the Shared Equipment Authority at Rice University.


21


Page 22 of 31

REFERENCES (1)

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field in Atomically Thin Carbon Films.

Science 2004, 306, 666–669. (2)

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197–200.

(3)

Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722–726.

(4)

NG, Y.; Chang, T. R.; Zhou, B.; Cui, Y. T.; Yan, H.; Liu, Z.; Schmitt, F.; Lee, J.; Moore, R.; Chen, Y.; Lin, H.; Jeng, H. T.; Mo, S. K.; Hussain, Z.; Bansil, A.; Shen, Z. X. Direct Observation of the Transition from Indirect to Direct Bandgap in Atomically Thin Epitaxial MoSe2. Nat. Nanotechnol. 2014, 9, 111–115.

(5)

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.

(6)

Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P. H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2.

ACS Nano 2013, 7, 791–797. (7)

Fang, Z.; Wang, Y.; Liu, Z.; Schlather, A.; Ajayan, P. M.; Koppens, F. H. L.; Nordlander, P.; Halas, N. J. Plasmon-Induced Doping of Graphene. ACS Nano 2012, 6, 10222–10228.

(8)

Sun, J.; Lee, H. W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y. A Phosphorene-Graphene Hybrid Material as a High-Capacity Anode for SodiumIon Batteries. Nat. Nanotechnol. 2015, 10, 980–985.

(9)

Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Room-Temperature Quantum Hall Effect in Graphene. Science 2007, 315, 1379.


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Bertolazzi, S.; Brivio, J.; Kis, A. Stretching and Breaking of Ultrathin MoS2. ACS

Nano 2011, 5, 9703–9709. (11) Xia, F.; Wang, H.; Xiao, D.; Dubey, M.; Ramasubramaniam, A. Two-Dimensional Material Nanophotonics. Nat. Photonics 2014, 8, 899–907. (12) Zhu, H.; Wang, Y.; Xiao, J.; Liu, M.; Xiong, S.; Wong, Z. J.; Ye, Z.; Ye, Y.; Yin, X.; Zhang, X. Observation of Piezoelectricity in Free-Standing Monolayer MoS2. Nat.

Nanotechnol. 2015, 10, 151–155. (13) Hong, X.; Kim, J.; Shi, S.-F.; Zhang, Y.; Jin, C.; Sun, Y.; Tongay, S.; Wu, J.; Zhang, Y.; Wang, F. Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures. Nat. Nanotechnol. 2014, 9, 682–686. (14) Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and In-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135–1142. (15) Chen, K.; Wan, X.; Wen, J.; Xie, W.; Kang, Z.; Zeng, X.; Chen, H.; Xu, J. Bin. Electronic Properties of MoS2-WS2 Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy. ACS Nano 2015, 9, 9868–9876. (16) Rivera, P.; Schaibley, J. R.; Jones, A. M.; Ross, J. S.; Wu, S.; Aivazian, G.; Klement, P.; Seyler, K.; Clark, G.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Yao, W.; Xu, X. Observation of Long-Lived Interlayer Excitons in Monolayer MoSe2WSe2 Heterostructures. Nat. Commun. 2015, 6, 6242. (17) Gong, Y.; Lei, S.; Ye, G.; Li, B.; He, Y.; Keyshar, K.; Zhang, X.; Wang, Q.; Lou, J.; Liu, Z.; Vajtai, R.; Zhou, W.; Ajayan, P. M. Two-Step Growth of Two-Dimensional WSe2/MoSe2 Heterostructures. Nano Lett. 2015, 15, 6135–6141. (18) Paul, A. K.; Kuiri, M.; Saha, D.; Chakraborty, B.; Mahapatra, S.; Sood, A. K.; Das, A.

Photo-Tunable

Transfer

Characteristics

in

MoTe2–MoS2

Vertical

Heterostructure. npj 2D Mater. Appl. 2017, 1, 17. (19) Zhang, K.; Zhang, T.; Cheng, G.; Li, T.; Wang, S.; Wei, W.; Zhou, X.; Yu, W.; Sun, Y.; Wang, P.; Zhang, D.; Zeng, C.; Wang, X.; Hu, W.; Fan, H. J.; Shen, G.; Chen, X.; Duan, X.; Chang, K.; Dai, N. Interlayer Transition and Infrared Photodetection in Atomically Thin Type-II MoTe2/MoS2 van Der Waals Heterostructures. ACS


23


Page 24 of 31

Nano 2016, 10, 3852–3858. (20) Li, Q.; Liu, M.; Zhang, Y.; Liu, Z. Hexagonal Boron Nitride-Graphene Heterostructures: Synthesis and Interfacial Properties. Small 2016, 12, 32–50. (21) Zuo, Z.; Xu, Z.; Zheng, R.; Khanaki, A.; Zheng, J. G.; Liu, J. In-Situ Epitaxial Growth of Graphene/h-BN van Der Waals Heterostructures by Molecular Beam Epitaxy. Sci. Rep. 2015, 5, 14760. (22) Liu, Z.; Ma, L.; Shi, G.; Zhou, W.; Gong, Y.; Lei, S.; Yang, X.; Zhang, J.; Yu, J.; Hackenberg, K. P.; Babakhani, A.; Idrobo, J. C.; Vajtai, R.; Lou, J.; Ajayan, P. M. In-Plane Heterostructures of Graphene and Hexagonal Boron Nitride with Controlled Domain Sizes. Nat. Nanotechnol. 2013, 8, 119–124. (23) Das, S.; Chen, H. Y.; Penumatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS2 Transistors with Scandium Contacts. Nano Lett. 2013, 13, 100– 105. (24) English, C. D.; Shine, G.; Dorgan, V. E.; Saraswat, K. C.; Pop, E. Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition. Nano Lett. 2016, 16, 3824–3830. (25) Chuang, H. J.; Chamlagain, B.; Koehler, M.; Perera, M. M.; Yan, J.; Mandrus, D.; Tománek, D.; Zhou, Z. Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors. Nano Lett. 2016, 16, 1896–1902. (26) Aras, M.; Klllç, Ç.; Ciraci, S. Planar Heterostructures of Single-Layer Transition Metal Dichalcogenides: Composite Structures, Schottky Junctions, Tunneling Barriers, and Half Metals. Phys. Rev. B 2017, 95, 75434. (27) Fan, Z. Q.; Jiang, X. W.; Luo, J. W.; Jiao, L. Y.; Huang, R.; Li, S. S.; Wang, L. W. In-Plane

Schottky-Barrier

Field-Effect

Transistors

Based

on

1

T/2H

Heterojunctions of Transition-Metal Dichalcogenides. Phys. Rev. B 2017, 96, 165402. (28) Allain, A.; Kang, J.; Banerjee, K.; Kis, A. Electrical Contacts to Two-Dimensional Semiconductors. Nat. Mater. 2015, 14, 1195–1205. (29) Popov, I.; Seifert, G.; Tománek, D. Designing Electrical Contacts to MoS2 Monolayers: A Computational Study. Phys. Rev. Lett. 2012, 108, 156802.


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Kang, J.; Liu, W.; Sarkar, D.; Jena, D.; Banerjee, K. Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors. Phys.

Rev. X 2014, 4, 031005. (31) Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X. Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions. ACS Nano 2016, 10, 4895–4919. (32) Liu, W.; Kang, J.; Sarkar, D.; Khatami, Y.; Jena, D.; Banerjee, K. Role of Metal Contacts in Designing High-Performance Monolayer n-Type WSe2 field Effect Transistors. Nano Lett. 2013, 13, 1983–1990. (33) Jeong, H.; Oh, H. M.; Bang, S.; Jeong, H. J.; An, S. J.; Han, G. H.; Kim, H.; Yun, S. J.; Kim, K. K.; Park, J. C.; Lee, Y. H.; Lerondel, G.; Jeong, M. S. MetalInsulator-Semiconductor Diode Consisting of Two-Dimensional Nanomaterials.

Nano Lett. 2016, 16, 1858–1862. (34) Roy, K.; Padmanabhan, M.; Goswami, S.; Sai, T. P.; Ramalingam, G.; Raghavan, S.;

Ghosh,

A.

Graphene–MoS2

Hybrid

Structures

for

Multifunctional

Photoresponsive Memory Devices. Nat. Nanotechnol. 2013, 8, 826–830. (35) Roy, T.; Tosun, M.; Kang, J. S.; Sachid, A. B.; Desai, S. B.; Hettick, M.; Hu, C. C.; Javey, A. Field-Effect Transistors Built from All Two-Dimensional Material Components. ACS Nano 2014, 8, 6259–6264. (36) Huang, X.; Zeng, Z.; Fan, Z.; Liu, J.; Zhang, H. Graphene-Based Electrodes. Adv.

Mater. 2012, 24, 5979–6004. (37) Chuang, H. J.; Tan, X.; Ghimire, N. J.; Perera, M. M.; Chamlagain, B.; Cheng, M. M. C.; Yan, J.; Mandrus, D.; Tománek, D.; Zhou, Z. High Mobility WSe2 p- And nField-Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts. Nano Lett. 2014, 14, 3594–3601. (38) Das, S.; Gulotty, R.; Sumant, A. V; Roelofs, A. All Two-Dimensional, Flexible, Transparent, and Thinnest Thin Film Transistor. Nano Lett. 2014, 14, 2861–2866. (39) Yu, H.; Kutana, A.; Yakobson, B. I. Carrier Delocalization in Two-Dimensional Coplanar p-n Junctions of Graphene and Metal Dichalcogenides. Nano Lett. 2016, 16, 5032–5036. (40) Kim, Y.; Kim, A. R.; Yang, J. H.; Chang, K. E.; Kwon, J. D.; Choi, S. Y.; Park, J.;


25


Page 26 of 31

Lee, K. E.; Kim, D. H.; Choi, S. M.; Lee, K. H.; Lee, B. H.; Hahm, M. G.; Cho, B. Alloyed 2D Metal-Semiconductor Heterojunctions: Origin of Interface States Reduction and Schottky Barrier Lowering. Nano Lett. 2016, 16, 5928–5933. (41) Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A. HighPerformance Single Layered WSe2 p-FETs with Chemically Doped Contacts.

Nano Lett. 2012, 12, 3788–3792. (42) Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M. Phase-Engineered Low-Resistance Contacts for Ultrathin MoS2 Transistors. Nat. Mater. 2014, 13, 1128–1134. (43) Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J.; Keum, D. H.; Baik, J.; Choe, D. H.; Chang, K. J.; Suenaga, K.; Kim, S. W.; Lee, Y. H.; Yang, H. Phase Patterning for Ohmic Homojunction Contact in MoTe2. Science 2015, 349, 625–628. (44) Kim, S.; Kim, J. H.; Kim, D.; Hwang, G.; Baik, J.; Yang, H.; Cho, S. PostPatterning of an Electronic Homojunction in Atomically Thin Monoclinic MoTe2. 2D

Mater. 2017, 4, 024004. (45) Lin, Y. C.; Dumcenco, D. O.; Huang, Y. S.; Suenaga, K. Atomic Mechanism of the Semiconducting-to-Metallic Phase Transition in Single-Layered MoS2. Nat.

Nanotechnol. 2014, 9, 391–396. (46) Saha, D.; Mahapatra, S. Atomistic Modeling of the Metallic-to-Semiconducting Phase Boundaries in Monolayer MoS2. Appl. Phys. Lett. 2016, 108, 253106. (47) Saha, Di.; Mahapatra, S. Asymmetric Junctions in Metallic-SemiconductingMetallic Heterophase MoS2. IEEE Trans. Electron Devices 2017, 64, 2457–2460. (48) Ma, Y.; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A. N.; Ge, M.; Mecklenburg, M.; Zhou, C. Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices. ACS Nano 2015, 9, 7383–7391. (49) Katagiri, Y.; Nakamura, T.; Ishii, A.; Ohata, C.; Hasegawa, M.; Katsumoto, S.; Cusati, T.; Fortunelli, A.; Iannaccone, G.; Fiori, G.; Roche, S.; Haruyama, J. GateTunable Atomically Thin Lateral MoS2 Schottky Junction Patterned by Electron Beam. Nano Lett. 2016, 16, 3788–3794. (50) Zhou, L.; Xu, K.; Zubair, A.; Liao, A. D.; Fang, W.; Ouyang, F.; Lee, Y. H.; Ueno,


26

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K.; Saito, R.; Palacios, T.; Kong, J.; Dresselhaus, M. S. Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2. J. Am. Chem. Soc. 2015, 137, 11892– 11895. (51) Park, J. C.; Yun, S. J.; Kim, H.; Park, J. H.; Chae, S. H.; An, S. J.; Kim, J. G.; Kim, S. M.; Kim, K. K.; Lee, Y. H. Phase-Engineered Synthesis of Centimeter-Scale 1T′- and 2H-Molybdenum Ditelluride Thin Films. ACS Nano 2015, 9, 6548–6554. (52) Empante, T. A.; Zhou, Y.; Klee, V.; Nguyen, A. E.; Lu, I.-H.; Valentin, M. D.; Naghibi Alvillar, S. A.; Preciado, E.; Berges, A. J.; Merida, C. S.; Gomez, M.; Bobek, S.; Isarraraz, M.; Reed, E. J.; Bartels, L. Chemical Vapor Deposition Growth of Few-Layer MoTe2 in the 2H, 1T′, and 1T Phases: Tunable Properties of MoTe2 Films. ACS Nano 2017, 11, 900–905. (53) Sung, J. H.; Heo, H.; Si, S.; Kim, Y. H.; Noh, H. R.; Song, K.; Kim, J.; Lee, C. S.; Seo, S. Y.; Kim, D. H.; Kim, H. K.; Yeom, H. W.; Kim, T. H.; Choi, S. Y.; Kim, J. S.; Jo, M. H. Coplanar Semiconductor-Metal Circuitry Defined on Few-Layer MoTe2 via Polymorphic Heteroepitaxy. Nat. Nanotechnol. 2017, 12, 1064–1070. (54) Yoo, Y.; DeGregorio, Z. P.; Su, Y.; Koester, S. J.; Johns, J. E. In-Plane 2H-1T′ MoTe2 Homojunctions Synthesized by Flux-Controlled Phase Engineering. Adv.

Mater. 2017, 29. (55) Yang, L.; Zhang, W.; Li, J.; Cheng, S.; Xie, Z.; Chang, H. Tellurization VelocityDependent Metallic-Semiconducting-Metallic Phase Evolution in Chemical Vapor Deposition Growth of Large-Area, Few-Layer MoTe2. ACS Nano 2017, 11, 1964– 1972. (56) Zhou, L.; Zubair, A.; Wang, Z.; Zhang, X.; Ouyang, F.; Xu, K.; Fang, W.; Ueno, K.; Li, J.; Palacios, T.; Kong, J.; Dresselhaus, M. S. Synthesis of High-Quality LargeArea Homogenous 1T′ MoTe2 from Chemical Vapor Deposition. Adv. Mater. 2016,

28, 9526–9531. (57) Ruppert, C.; Aslan, O. B.; Heinz, T. F. Optical Properties and Band Gap of Singleand Few-Layer MoTe2 Crystals. Nano Lett. 2014, 14, 6231–6236. (58) Pradhan, N. R.; Rhodes, D.; Feng, S.; Xin, Y.; Memaran, S.; Moon, B.; Terronees, H.; Terrones, M.; Balicas, L. Field-Effect Transistors Based on Few-Layered aMoTe2. ACS Nano 2014, 8, 5911–5920.


27


Page 28 of 31

(59) Lin, Y. F.; Xu, Y.; Wang, S. T.; Li, S. L.; Yamamoto, M.; Aparecido-Ferreira, A.; Li, W.; Sun, H.; Nakaharai, S.; Jian, W. Bin; Ueno, K.; Tsukagoshi, K. Ambipolar MoTe2 Transistors and Their Applications in Logic Circuits. Adv. Mater. 2014, 26, 3263–3269. (60) Choi, K.; Lee, Y. T.; Kim, J. S.; Min, S. W.; Cho, Y.; Pezeshki, A.; Hwang, D. K.; Im, S. Non-Lithographic Fabrication of All-2D α-MoTe2 Dual Gate Transistors.

Adv. Funct. Mater. 2016, 26, 3146–3153. (61) Larentis, S.; Fallahazad, B.; Movva, H. C. P.; Kim, K.; Rai, A.; Taniguchi, T.; Watanabe, K.; Banerjee, S. K.; Tutuc, E. Reconfigurable Complementary Monolayer MoTe2 Field-Effect Transistors for Integrated Circuits. ACS Nano 2017,

11, 4832–4839. (62) Chen, F. C.; Lv, H. Y.; Luo, X.; Lu, W. J.; Pei, Q. L.; Lin, G. T.; Han, Y. Y.; Zhu, X. B.; Song, W. H.; Sun, Y. P. Extremely Large Magnetoresistance in the Type-II Weyl Semimetal MoTe2. Phys. Rev. B 2016, 94, 235154. (63) Qi, Y.; Naumov, P. G.; Ali, M. N.; Rajamathi, C. R.; Schnelle, W.; Barkalov, O.; Hanfland, M.; Wu, S. C.; Shekhar, C.; Sun, Y.; Süß, V.; Schmidt, M.; Schwarz, U.; Pippel, E.; Werner, P.; Hillebrand, R.; Förster, T.; Kampert, E.; Parkin, S.; Cava, R. J.; Felser, C.; Yan, B.; Medvedev, S. A. Superconductivity in Weyl Semimetal Candidate MoTe2. Nat. Commun. 2016, 7, 11038. (64) Duerloo, K. A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in TwoDimensional Mo-and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5, 4214. (65) Song, S.; Keum, D. H.; Cho, S.; Perello, D.; Kim, Y.; Lee, Y. H. Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain. Nano Lett. 2016, 16, 188–193. (66) Keum, D. H.; Cho, S.; Kim, J. H.; Choe, D.-H.; Sung, H.-J.; Kan, M.; Kang, H.; Hwang, J.-Y.; Kim, S. W.; Yang, H.; Chang, K. J.; Lee, Y. H. Bandgap Opening in Few-Layered Monoclinic MoTe2. Nat. Phys. 2015, 11, 482–486. (67) Deng, K.; Wan, G.; Deng, P.; Zhang, K.; Ding, S.; Wang, E.; Yan, M.; Huang, H.; Zhang, H.; Xu, Z.; Denlinger, J.; Fedorov, A.; Yang, H.; Duan, W.; Yao, H.; Wu, Y.; Fan, S.; Zhang, H.; Chen, X.; Zhou, S. Experimental Observation of


28

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Topological Fermi Arcs in Type-II Weyl Semimetal MoTe2. Nat. Phys. 2016, 12, 1105–1110. (68) Naylor, C. H.; Parkin, W. M.; Ping, J.; Gao, Z.; Zhou, Y. R.; Kim, Y.; Streller, F.; Carpick, R. W.; Rappe, A. M.; Drndić, M.; Kikkawa, J. M.; Johnson, A. T. C. Monolayer Single-Crystal 1T′-MoTe2 Grown by Chemical Vapor Deposition Exhibits Weak Antilocalization Effect. Nano Lett. 2016, 16, 4297–4304. (69) Chen, B.; Sahin, H.; Suslu, A.; Ding, L.; Bertoni, M. I.; Peeters, F. M.; Tongay, S. Environmental Changes in MoTe2 Excitonic Dynamics by Defects-Activated Molecular Interaction. ACS Nano 2015, 9, 5326–5332. (70) Kim, T.; Park, H.; Joung, D.; Kim, D.; Lee, R.; Shin, C. H.; Diware, M.; Chegal, W.; Jeong, S. H.; Park, H.; Diware, M.; Chegal, W.; Jeong, S. H.; Shin, J. C.; Park, J.; Kang, S. Wafer-Scale Epitaxial 1T′, 1T′-2H Mixed, and 2H Phases MoTe2 Thin Films Grown by Metal-Organic Chemical Vapor Deposition. Adv. Mater. Interfaces 2018, 5, 1800439. (71) Yamamoto, M.; Wang, S. T.; Ni, M.; Lin, Y. F.; Li, S. L.; Aikawa, S.; Jian, W. Bin; Ueno, K.; Wakabayashi, K.; Tsukagoshi, K. Strong Enhancement of Raman Scattering from a Bulk-Inactive Vibrational Mode in Few-Layer MoTe2. ACS Nano 2014, 8, 3895–3903. (72) Kan, M.; Nam, H. G.; Lee, Y. H.; Sun, Q. Phase Stability and Raman Vibration of the Molybdenum Ditelluride (MoTe2) Monolayer. Phys. Chem. Chem. Phys. 2015,

17, 14866–14871. (73) Kumar, A.; Villarreal, E.; Zhang, X.; Ringe, E. Micro-Extinction Spectroscopy (MExS): A Versatile Optical Characterization Technique. Adv. Struct. Chem.

Imaging 2018, 4, 8. (74) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147–150. (75) Berger, H. H. Models for Contacts to Planar Devices. Solid State Electron. 1972,

15, 145–158. (76) Liu, Y.; Stradins, P.; Wei, S. H. Van Der Waals Metal-Semiconductor Junction: Weak Fermi Level Pinning Enables Effective Tuning of Schottky Barrier. Sci. Adv. 2016, 2, e1600069.


29


Page 30 of 31

(77) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169– 11186. (78) Chiu, M.-H.; Zhang, C.; Shiu, H.-W.; Chuu, C.-P.; Chen, C.-H.; Chang, C.-Y. S.; Chen, C.-H.; Chou, M.-Y.; Shih, C.-K.; Li, L.-J. Determination of Band Alignment in the Single-Layer MoS2/WSe2 Heterojunction. Nat. Commun. 2015, 6, 7666.


30

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents (TOC)


31

1T' MoTe2 in-plane Heterostructure

Recommend Documents