Synthesis and Optoelectronic Applications of a Stable p-Type 2D

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Synthesis and Optoelectronic Applications of a Stable p-Type 2D Material: α‑MnS Ningning Li,†,§,¶,# Yu Zhang,†,‡,# Ruiqing Cheng,† Junjun Wang,† Jie Li,† Zhenxing Wang,† Marshet Getaye Sendeku,† Wenhao Huang,† Yuyu Yao,†,§,¶ Yao Wen,† and Jun He*,†,‡

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†

CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡ School of Physics and Technology, Wuhan University, Wuhan, 430072, P. R. China § Sino-Danish College, University of Chinese Academy of Sciences, Beijing, 100049, P. R. China ¶ Sino-Danish Center for Education and Research, Beijing, 100049, P. R. China S Supporting Information *

ABSTRACT: α-MnS, as a nonlayered p-type material with a wide band gap of 2.7 eV, has been expected to supplement the scarcity of two-dimensional (2D) p-type semiconductors, which are desperately required for constructing atomically thin p-n junctions. However, the preparation and property investigation of 2D α-MnS has scarcely been reported so far. Herein, we report the controlled synthesis of ultrathin large-scale α-MnS single crystals down to 4.78 nm via a facile chemical vapor deposition (CVD) method. Importantly, top-gating field-effect transistors based on the as-synthesized α-MnS nanosheets show p-type transport behavior with an ultrahigh on/off ratio exceeding 106, surpassing most reported p-type 2D materials. Meanwhile, α-MnS phototransistors exhibit an ultrahigh detectivity of 3.2 × 1014 Jones, as well as an excellent photoresponsivity of 139 A/W and a fast response time of 12 ms. Besides, outstanding environmental stability and admirable flexibility have also been demonstrated in the as-synthesized α-MnS nanosheets. We believe that this work broadens the scope of the CVD synthesis strategy for various p-type 2D materials and demonstrates their significant application potentials in electronics and optoelectronics. KEYWORDS: α-MnS, 2D materials, p-type transport behavior, air-stable, phototransistors

T

with a high work function and electrical gating, ambipolar materials such as WSe2, MoTe2, and BP have been proven to exhibit p-type properties,19−22 but the expected result may be weakened by Fermi level pinning originating from metal− semiconductor contact.20,23 Recently, the chemical doping process was utilized to obtain a p-type MoS2, but the carrier mobility of the p-type MoS2 may have been restrained by the dopant scattering, along with an undesirable impact on its intrinsic properties.24 Moreover, the instability in an ambient environment is a critical issue in some reported p-type materials, which always restricts their practical applications.25 Hence, exploring intrinsic p-type 2D materials with high stability and excellent performance is desperately required. Very recently, 2D intrinsic p-type Te and Se have been successfully obtained by a solution process and vapor

wo dimensional (2D) materials have attracted considerable attention in recent years due to their tunable electronic structures and attractive electrical/ optoelectronic properties.1−4 Benefiting from their atomically sharp heterointerface, tunable energy band alignment, and fascinating interlayer coupling,5−8 the atomically thin p−n junctions based on 2D materials have emerged as an essential building block for fundamental and applied studies in the field of nanoelectronics and optoelectronics, including diodes,9 photodetectors,10,11 light-emitting diodes,12 and solar cells,13 as well as recent memristors14,15 and quantum transport devices.16 So far, several 2D p−n junctions, such as WSe2/ MoS2, WSe2/WS2, and black phosphorus (BP)/MoS2, have demonstrated significant electrically tunable electronic and optoelectronic properties.10,17,18 However, owing to the scarcity in intrinsic 2D p-type materials, most of the previous investigations on the p-type conduction behavior of 2D materials were carried out via employing additional procedures, such as special metal contact, electrical gating, and chemical doping methods, which are not suitable for practical applications. For examples, with the use of a specific electrode © XXXX American Chemical Society

Received: May 30, 2019 Accepted: August 19, 2019 Published: August 19, 2019 A

DOI: 10.1021/acsnano.9b04205 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Morphologies and characterization of 2D α-MnS nanosheets. (a) Crystal structure diagrams of α-MnS (111) plane with top and side view. The Mn atom is purple, and the S atom is yellow. Inset shows the octahedral coordination. (b) A typical AFM image and the corresponding height profile of α-MnS nanosheet with thickness of 7.5 nm. Scale bar: 2 μm. (c) OM image of a typical α-MnS nanosheet with the edge length of 86 μm. Scale bar: 20 μm. (d) XRD pattern for α-MnS nanosheets transferred onto sapphire substrate. (e) XPS characterization of Mn 2p and S 2p of as-grown α-MnS. (f) HRTEM image of an ultrathin α-MnS nanosheet. Inset shows the corresponding SAED pattern. Scale bar:1 nm. EDS elemental mapping of (g) Mn and (h) S in a typical α-MnS nanosheet.

method. The as-synthesized α-MnS crystals possess rock-salttype structure, and exhibit hexagonal and triangular shapes with the domain size of over 100 μm and a thickness of 4.78 nm. Moreover, the top-gating field-effect transistors (FETs) based on α-MnS crystals show a typical p-type transport characteristic with an ultrahigh on/off ratio exceeding 106, surpassing most of reported p-type 2D materials. Meanwhile, the carrier transport mechanism in α-MnS crystals is systematically studied by extracting Schottky barrier height from temperature-dependent electrical measurements. Notably, the α-MnS phototransistor demonstrates a considerable specific detectivity of 3.2 × 1014 Jones, as well as a high responsivity of 139 A/W and a fast response time of 12 ms. Besides, excellent environmental stability and flexibility have been demonstrated in our as-synthesized α-MnS crystals. We believe that the findings in our work enrich the 2D p-type family and provide considerable application potential in flexible electronic and optoelectronic devices.

deposition method, respectively, which raises the possibility for preparing other 2D p-type materials.26,27 As an important member of the group VIIB transition metal chalcogenides, MnS possesses attractive electronic, optoelectronic, and magnetic properties.28,29 It occurs in three distinct crystalline structures: the stable α-MnS (rock-salt type structure), metastable structures β-MnS (zinc-blende type structure), and γ-MnS (wurtzite type structure).30 Both β-MnS and γ-MnS can only exist at low temperature and are easily converted to α-MnS at a high temperature. According to previous works, the self-contained manganese vacancies in αMnS act as acceptors, giving rise to p-type conduction behavior with a wide band gap of 2.7 eV.31,32 Therefore, it is presently of significant interest to explore this p-type material and investigate its applications in electronics and optoelectronics. However, to the best of our knowledge, the preparation, basic properties, and applications of 2D MnS crystals have rarely been reported. Only a few reports were communicated specifically on bulks or polycrystalline films obtained by the solvothermal process.33,34 Different from van der Waals layered materials, α-MnS owns non-van der Waals structure. Limited by strong covalent bonding in all three dimensions, the controllable growth of 2D ultrathin α-MnS single crystals is still a challenge. In this work, we successfully synthesize high-quality ultrathin nonlayered α-MnS single crystals via a halide-assisted CVD

RESULT AND DISCUSSION The α-MnS possesses a rock-salt-type structure and crystallizes into space group of Fm--3̅m (225) with the lattice parameter a = 5.224 Å. The top and side views of α-MnS (111) plane are demonstrated in Figure 1a. The top view depicts the hexagonal arrangement of the atoms with the inset showing the octahedral coordination for α-MnS (111). From the side B

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Figure 2. Electrical properties of top-gating α-MnS FETs. (a) Output characteristics of a typical top-gating α-MnS FET at different gate voltages. Inset shows the AFM image of the device with the thickness of 16.78 nm, scale bar is 5 μm. (b) Transfer characteristics of this αMnS FET at different bias voltages. (c) Thickness dependence of the field-effect on/off ratio for α-MnS nanosheets with thickness varying from 5.97 to 42.63 nm. (d) Temperature dependence of transfer characteristics of the16.78 nm-thick device. (e) Extracted effective barrier height as a function of gate voltages for α-MnS FET. Inset is the band diagram at flat band condition. (f) Environmental stability measurement for the two-terminal 14.50 nm-thick device without the encapsulation of HfO2 on channel.

substrates are well indexed to the standard α-MnS pattern (PDF no. 06-0518), which confirms the formation of pure αMnS crystals and their single-crystal nature. Further, X-ray photoelectron spectroscopy (XPS) was utilized to determine the elemental composition and chemical states of as-grown samples. As shown in Figure 1e, the peaks at binding energies 653.6 and 641.65 eV are designated as Mn 2p1/2 and Mn 2p3/2 states, respectively, while the other two peaks at 162.05 and 160.90 eV are assigned to S 2p1/2 and S 2p3/2 states of αMnS,38,39 respectively. Besides, the detailed atomic structure information and crystalline quality of the as-synthesized MnS nanosheets were investigated by high-resolution transmission electron microscope (HRTEM). As shown in Figure 1f, the HRTEM image exhibits clear lattice fringes and a perfect hexagonal arrangement of atoms, which reveal the high crystalline quality of α-MnS. The plane spacing of (220) planes is measured to be 0.18 nm, which is in good accordance with the standard XRD results (PDF no. 06-0518). The corresponding selected area electron diffraction (SAED) patterns exhibit only one set of hexagonal diffraction spots (inset of Figure 1f), indicating the perfect single-crystal nature of as-synthesized α-MnS nanosheets. Also, the (220) lattice plane spacing extracted from the SAED patterns is 0.18 nm, which is consistent with the lattice spacing observed from the HRTEM image. Additionally, a series of SAED patterns obtained from different regions in the triangular α-MnS nanosheet show a nearly identical orientation (deviation within ±0.1°), again verifying the single crystal nature of the α-MnS (Figure S4). Moreover, the uniform elemental distribution of Mn and S in the as-synthesized sample is confirmed by energydispersive spectroscopy (EDS) mapping (Figure 1g,h). Additionally, the EDX spectrum (Figure S5) shows that the atomic ratio of Mn and S in the as-grown α-MnS is approximately 1:1. Hence, all the results discussed above indicate the successful synthesis of high-quality 2D single-crystal α-MnS which

view, it can be seen that the nonlayered α-MnS (111) is constituted by an alternating Mn layer and S layer, which are bonded with strong chemical bonds; the thickness of one unit cell between the top and bottom MnS2 layer is 9.05 Å. In our work, ultrathin α-MnS single crystals were synthesized on mica substrates via a halide-assisted CVD method. MnCl2 and S powders were used as Mn and S sources, respectively. A mixture of argon (Ar) and hydrogen (H2) was used as a carrier gas throughout the growth process. More experimental details are provided in the Experimental Section. Notably, NaCl was introduced during the process and mixed with metal-based precursors, which could increase the vapor pressure of metal precursors and further promote the chemical reaction.35−37 Figure 1b demonstrates a typical atomic force microscope (AFM) image of the as-grown α-MnS crystal with a thickness of 7.5 nm. Notably, by controlling the growth parameters, the thickness of the α-MnS crystal can be as thin as 4.78 nm (Supporting Information, Figure S3), corresponding to approximately five unit cells. Moreover, the domain size, morphology and thickness of α-MnS crystals can be efficiently controlled by the growth parameters (such as growth temperature, growth time, precursors ratio, and carrier gas). The details of the growth modulation are described in Figure S1. Besides, in contrast to those small and thick flakes prepared without using NaCl, large amounts of ultrathin α-MnS nanosheets are obtained after adding NaCl, as displayed in Figure S2. Furthermore, the optical microscopy (OM) image demonstrates the achievement of a triangular α-MnS crystal with edge length of 86 μm (Figure 1c). And the maximum edge length of an as-grown α-MnS single-crystal can be up to 125 μm (Figure S1). As mentioned above, MnS possesses three possible phases with distinct structures. Thus, X-ray diffraction (XRD) was employed to clarify the crystalline structure of the obtained MnS samples. As shown in Figure 1d, the XRD results of the samples transferred onto sapphire C

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Figure 3. Optoelectronic properties of top-gating α-MnS FETs at room temperature. (a) Schematic illustration of top-gating α-MnS phototransistor. (b) Transfer characteristics of the α-MnS phototransistor under various power densities at Vds = 2 V. (c) Output characteristics of the α-MnS phototransistor under various power densities at Vgs = −6 V. (d) Responsivity and detectivity as a function of power densities at Vgs = −6 and 6 V, respectively. (e) Time-dependent photoresponse of MnS phototransistor under periodic illumination with different power densities at Vgs = −6 V and Vds= 2 V. (f) Temporal photoresponse of α-MnS phototransistor at different gate voltages (left) and the extracted response time as a function of gate voltages (right). (g) The energy band diagrams of α-MnS phototransistor taking the Schottky barrier at the contacts into account. EF is the Fermi energy level, EC is the bottom of the conduction band, EV is the top of the valence band, and ΦB is the Schottky barrier height.

excellent field-effect on/off ratio (over 106) is achieved. Interestingly, this result is much higher than previously reported p-type materials, such as GeSe (≈102), Te (≈103), and BP (≈105).40−42 Additionally, the leakage current of the device is in the order of 10−13, which is almost negligible (Figure S6). Moreover, a series of α-MnS FETs with different thickness (ranging from 5.97 to 42.63 nm) were fabricated to investigate the thickness-dependent electrical properties. The OM and AFM images of 14 representative devices with varying thicknesses are provided in Figure S7. The extracted field-effect on/off ratios of the fabricated α-MnS FETs are shown in Figure 2c. The maximum on/off ratio of 3.8 × 106 is obtained at a thickness of about 18.72 nm, and then gradually decreases with further increasing thickness. Particularly, the α-MnS FET cannot be fully turned off when the thickness of the nanosheet exceeds 40 nm, as shown in Figure S8b, which is very common in the other 2D semiconductors materials. This can be explained by the fact that only the channel region within Debye length can be depleted by the gate electric field, while the channel region out of Debye length cannot be depleted and remains at a high off-state current level.43 The field-effect hole mobility of α-MnS FETs was calculated by μ = Lgm/[W(εrε0/ d)Vds], where L is the channel length, W is the channel width,

deserves further investigation in electrical and optoelectronic applications. To investigate the electrical transport characteristics of the as-synthesized α-MnS nanosheets, top-gating FETs based on nanosheets with high-k HfO2 dielectric layer were fabricated on mica substrate. Briefly, the source and drain electrodes were first patterned by electron-beam lithography (EBL) for the subsequent thermal deposition of Cr/Au (8/60 nm). Then, 1 nm-thick aluminum (Al) was thermally deposited on the preprepared two-terminal α-MnS device as a buffer layer, and subsequently coated by a hafnium oxide, HfO2 (30 nm), layer via atomic layer deposition (ALD). Lastly, the top-gate electrode α-MnS was defined by EBL and thermally deposited with Cr/Au (5/15 nm). The output and transfer characteristics of a typical top-gate MnS transistor were studied at room temperature. Figure 2a shows the output characteristic curves at different gate voltages, where the linear relationship indicates the passable ohmic contact between metal electrodes and α-MnS crystal. The AFM image (inset of Figure 2a) shows that the thickness of the α-MnS crystal is about 16.78 nm. Further, the transfer characteristics of our device was investigated at different bias voltages. As can be seen from the Figure 2b, a typical p-type conduction behavior with an D

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dependent characteristic of α-MnS phototransistor, which suggests the photocurrent is mainly dominated by the absorption of photons. To further quantitatively analyze its photodetection performance, responsivity (R) and detectivity (D*) as two critical parameters were calculated. Responsivity can be defined as R = Iph/PS, where Iph is photocurrent, P is illumination power density, and S refers to the effective illumination area. It represents the effectiveness of conversion of incident optical signals into electrical signals. Detectivity determines the capacity to detect weak optical signals, which can be expressed as D* = RS 1/2/(2qIdark)1/2, where R, S, q, and Idark refer to responsivity, effective illumination area of a photodetector, electron charge, and dark current, respectively. On the basis of the above formulas, the responsivity and detectivity are plotted as a function of illuminated power densities (Figure 3c). It can be seen that the maximum responsivity of 139 A/W is obtained under Vgs = −6 V and Vds = 2 V with a power density of 0.283 mW/cm2. This result appears to be superior when compared with other 2D materials such as BP (≈ 4.0 × 10−4 A/W), MoS2 (≈ 4.2 × 10−4 A/W), and SnS2 (≈ 2 A/W).22,49,50 Besides, Figure 3c demonstrates that the responsivity of the device decreases with the increase of incident power densities, which can be attributed to the defect or trap states from α-MnS or the interface between αMnS and the dielectric layer. In addition, benefiting from the ultralow off-state current of α-MnS phototransistor, as high as 3.2 × 1014 Jones of detectivity is achieved at Vgs = 6 V. Notably, this result far outperforms those previously reported photodetectors based on 2D materials, such as MoS2 (≈ 107 Jones), CdTe (≈ 109 Jones), and GaS (≈ 1010 Jones).49,51,52 Actually, photoconductive gain (G) is also a critical parameter to characterize the performance of photodetectors, which can be described as τlifetime/τtransit, in which τlifetime and τtransit refer to carrier lifetime and carrier transit time, respectively. It can be quantitatively understood as the number of charge carriers extracted by one incident photon. The gain can be calculated by G = Rhc/qλ, where h is the Planck constant, c is the light velocity, and λ is the light wavelength. As shown in Figure S11, the gain reaches a maximum value of 361 at a power density of 0.283 mW/cm2, which again confirms the significant performance of α-MnS photodetector. Moreover, a repeatable and invertible photosensing behavior is also an important aspect to evaluate the capability of a photodetector. Figure 3e displays the photocurrent traces of the phototransistor at Vgs = −6 V under periodic illumination with three different power densities of 5.23, 23.77, and 105.53 mW/cm2. The highly stable and reversible photoswitching behavior between two states indicates excellent periodicity and long-term stability in a α-MnS phototransistor. It is worth noting that a high photoswitching ratio close to 104 is demonstrated at Vgs = 6 V (see Figure S10b). Also, the temporal response of phototransistor as a function of gate voltages is presented in Figure 3f. In general, the rising time and decay time can be defined as the time required for the photocurrent to reach its (1 − e−1) ≈ 63% and e−1 ≈ 37% of maximum value, respectively. It is clear that the response time is strongly dependent on the gate voltages, and the fastest response speed is obtained (τrising = 21 ms and τdecay = 13 ms) at Vgs = −6 V. The observed behavior in our α-MnS phototransistors can be interpreted by using energy band diagrams in Figure 3g. Without illumination and gate voltage, the device maintains an equilibrium state accompanied by two small Schottky barriers

gm is the transconductance extracted from transfer characteristic curves, εr is the dielectric constant for HfO2 (εr = 19), ε0 is vacuum dielectric constant of 8.85 × 10−12 F m−1, d is the thickness of HfO2 (d = 30 nm), and Vds is the bias voltage. The field-effect hole mobility increases with thickness and the maximum value (0.1 cm2 V−1 s−1) is obtained at about 34 nm, as shown in Figure S8c. The relatively low hole mobility of the synthesized α-MnS crystals may be attributed to defects scattering, metal contact, or its own low conductivity.44−46 To further take a deep insight into the carrier transport mechanism, the temperature-dependent electrical properties were studied. Figure 2d demonstrates the detailed temperature-dependent transfer characteristics. According to thermionic emission model: Isat = AA*T2 exp[−eΦB/(kBT)],43,47 where A is the contact area, A* is the Richardson constant (4πem*kB2h−3, in which m* and h refer to effective mass and Plank constant, respectively), e is the electron charge, ΦB is the Schottky barrier height, kB is the Boltzmann constant, and T is temperature. The Richardson plot of ln(Ids/T2) versus 1000/T at various gate voltages extracted from the temperaturedependent transfer curves are provided in Figure S9a, in which the slope of Richardson plot is related to the ΦB. The extracted ΦB as a function of gate voltages is depicted in Figure 2e. Accordingly, the barrier height corresponding to the flat-band condition is 70.8 meV. The carrier transport in the linear region is in accordance with thermionic emission model, which indicates that the channel current is dominated by thermionic emission at Vgs > VFB (flat-band gate voltage), whereas, due to the presence of tunneling current at Vgs < VFB, the curve deviates significantly from linearity. In this range, the channel current is dominated by thermionic emission and tunneling. This additional tunneling current contributes to a passable ohmic contact even with noticeable Schottky barrier.47,48 And this noticeable Schottky barrier may be responsible for the decreased linearity of output characteristics at low temperature (Figure S9b).43 Moreover, the stability of the 2D materials is still an important criterion. In order to investigate the environmental stability of the as-grown α-MnS nanosheets, two-terminal α-MnS transistors without the encapsulation of HfO2 on the channels were constructed. As is shown in Figure 2f, no significant degeneration of device performance was observed even after being exposed to air for 30 days. Hence, the obtained α-MnS nanosheets possess excellent stability in ambient conditions. Further, α-MnS phototransistors were fabricated to investigate their optoelectronic characteristics. Figure 3a shows the schematic diagram of the top-gate α-MnS phototransistor with 30 nm high-k HfO2 dielectric layer. The transfer characteristic curves of the fabricated phototransistor with a thickness of 18.72 nm were measured (Figure 3b) in the dark and under illumination with a wavelength of 473 nm, which exhibits distinct gate tunability of photoresponse. It can be seen that a significant photoresponse can be observed even at an extremely weak illumination intensity of 0.283 mW/cm2. The linear output characteristics of α-MnS phototransistor at different power densities were further measured, which demonstrates that the irradiated current gradually increases with the increased power densities. The photocurrent (Iph), defined by Iph = Ilight − Idark, where Ilight is the irradiated current under illumination and Idark is the dark current without illumination, was extracted from output characteristic curves, as shown in Figure S10a. The linear relationship between the photocurrent and power densities indicates a typical photonE

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Figure 4. Temperature-dependent optoelectronic properties of top-gating α-MnS FETs. (a) Transfer characteristics of the α-MnS phototransistor under various power densities at 80 K. (b) Time-dependent photoresponse of α-MnS phototransistor under periodic illumination with different power densities at 80 K (Vgs = −6 V, Vds = 2 V). (c) A separated photoresponse cycle at 80 K with rising time of 23 ms and decay time of 12 ms. (d) Photocurrent (Iph) and dark current (Ids), (e) responsivity (R) and detectivity (D*), (f) rising time (τrising) and decay time (τdecay) of an α-MnS phototransistor as a function of temperatures.

to that of 300 K, an obvious decrease in the dark current is observed in the device, which is ascribed to the suppression of thermal emission carriers. Figure 4 panels b and c exhibit stable photocurrent traces under periodic illumination and a photoresponse cycle with response times of τrising = 23 ms and τdecay = 12 ms at 80 K, which are similar to those at 300 K. Additionally, the tendency of photocurrent and dark current as a function of temperature at Vgs = −6 V is depicted in Figure 4d. Both dark current and photocurrent decrease as the temperature changes from 300 to 80 K. This can be attributed to the suppression of thermal emission and the thermalassisted tunneling phenomenon at lower temperature. Similarly, the responsivity and detectivity also show an obvious temperature dependence, and decrease with decreasing temperature, as shown in Figure 4e. Moreover, the response time slightly varies with temperature (Figure 4f), which suggests that few trap states exist in our α-MnS nanosheets.54 Afterward, the performances of our 2D α-MnS phototransistors were compared with previously reported 2D-based devices (Table 1). The ultrathin α-MnS nanosheets in our work exhibit ultrahigh responsivity and detectivity, and fast response time, which make α-MnS a very promising 2D material for optoelectronic applications. Benefiting from the excellent flexibility and insulativity of mica substrates, the in situ fabricated MnS phototransistor on a mica substrate also demonstrates significant flexible properties,

at the contacts. When applying a positive gate voltage (Vgs > Vth), the Fermi energy level of MnS can move up, and induce a higher Schottky barrier at the contacts, which corresponds to the off state. And then the dark current is strongly suppressed due to the inhibition of carriers transport caused by the higher contact barrier. Hence, the photogenerated current shows the dominating effect on the channel current instead of thermionic and tunneling currents, giving rise to an extremely high sensitivity to illumination in the off state.40 In contrast, the Fermi energy level of α-MnS moves down to approach its valence band when applying a negative gate voltage (Vgs < Vth), which brings about a lower barrier at the contact interface. The reduced barrier promotes a more efficient extraction of photocurrent and a rising photoresponse at the on state.53 In such a case, both thermionic and tunneling current provide the major contribution to the channel current because the carriers can easily overcome the lower barrier. This is also evidenced in the relationship between photoresponse ratio (Iph/Idark) and gate voltages (Figure S10c), in which the Iph/Idark decreases with decreasing Vgs because the thermionic and tunneling currents account for a larger proportion in the channel current. Further, the optoelectronic performance of the α-MnS phototransistor at different temperatures ranging from 80 to 300 K was also conducted. Figure 4a demonstrates the transfer characteristic curves of the device in the dark and under illumination with different power densities at 80 K. In contrast F

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is much higher than most other reported 2D p-type materials. Meanwhile, the α-MnS phototransistor exhibits an ultrahigh detectivity of 3.2 × 1014 Jones, as well as an excellent photoresponsivity of 139 A/W and a fast response time of 12 ms, superior to the vast majority of 2D materials. Additionally, the obtained α-MnS nanosheets possess an excellent environmental stability and an admirable flexibility. All these results indicate significant potential applications of the synthesized ultrathin α-MnS nanosheets in flexible electronics and optoelectronics.

Table 1. Comparison of the Photodetector Performance with Other Reported 2D Materials device BP MoS2 SnS2 CdTe GaS WS2 InSe MnS

responsivity [AW−1]

detectivity [Jones]

−4

4.0 × 10 4.2 × 10−4 2 6 × 10−4 7.74 × 10−4 2.2 × 10−6 12.3 139

response time [ms] −2

109 1010 1011 1014

4 × 10 50 42 18.4 30 5.3 50 12

detection wavelength 400 676 450 473 550 458 450 473

nm nm nm nm nm nm nm nm

refs 22 49 50 51 52 55 56 this work

EXPERIMENTAL SECTION Synthesis of α-MnS Nanosheets. Ultrathin α-MnS nanoflakes were synthesized by a halide-assisted CVD growth technique using a horizontal vacuum quartz tube furnace with dual temperature zone. High-purity MnCl2 powder (99.999%,Alfa) and S powder (99.98%, Alfa) were used as precursors. Typically, MnCl2 powder was mixed with a small amount of NaCl and placed in a quartzite boat. The quartzite tube was then placed at the center of the high-temperature zone, and the temperature was maintained at 660−680 °C. Then, a weighed amount of S powder was placed in a separate quartzite boat and kept at the center of low-temperature zone, and the temperature was kept at 150 °C. The fluorophlogopite mica [KMg3(AlSi3O10)F2] sheets were cleaved and positioned 3−6 cm away from the center of the high-temperature zone. Before heating, the tube was first evacuated by a mechanical pump and then high-purity Ar gas was purged to flush for 10 min to remove O2 residual left in the tube. Throughout the whole growth process, a mixture of Ar (100 sccm) and H2 (20 sccm) gases flowed through the furnace at atmospheric pressure. After 30 min reaction time, the system was cooled naturally. The obtained ultrathin MnS nanoflakes on the mica substrate were then carefully collected. Characterizations. The morphology and thickness of assynthesized MnS nanoflakes were characterized by optical microscopy (Olympus, BX51) and AFM (Dimension, 3100). The composition and crystal structure information were obtained from XPS (ESCALAB 250 Xi), X-ray diffractometer (XRD) (Bruker, D8 Focus, Cu Kα line), and a high-resolution transmission electron microscope (HR-TEM) (JEM-2100F) equipped with EDX. Device Fabrication and Measurements. The top-gate MnS device was fabricated directly on the mica substrate. The source and drain electrodes were first defined by EBL, and followed by thermal deposition of Cr/Au (8/60 nm). After completing the two-terminal MnS device, 1 nm aluminum (Al) was thermally deposited on the preprepared two-terminal MnS device as a buffer layer. Subsequently, hafnium oxide HfO2 (30 nm) was grown on it via the atomic layer deposition (ALD) technique. Lastly, the top-gate electrode is defined by EBL and thermally deposited with Cr/Au (5/15 nm). The electrical properties were measured on a probe station (Lakeshore, TTP4) equipped with a liquid-nitrogen cooling system, and the data

which shows promising application in next-generation 2D flexible electronics and optoelectronics. The OM image of the as-prepared device is shown in the inset of Figure 5a, which was then folded with a curvature radius (r) of ≈5 mm. Figure 5a shows the transfer characteristic curves of the device in the dark state before and after bending it 100 times and 500 times. The measurement was made at a fixed bias voltage of 2 V. It can be seen that the transfer curve shows a negligible decrease compared with the original one even after bending the device 500 times. A similar tendency is also obtained in the output characteristic curves at a fixed gate voltage of −6 V, as shown in Figure 5b. These results indicate the high stability of the fabricated α-MnS FETs. To further investigate the photoresponse stability of this flexible device, the time-dependent photoresponse measurement was conducted. As demonstrated in Figure 5c, our ultrathin α-MnS flexible devices maintain a reversible and long-term stable photoresponse even after bending 500 times. The on-state and off-state currents at different bending states exhibit very small differences, which are estimated to be less than 2.9% and 2.5%, respectively. The results suggest that the obtained α-MnS nanosheets have marvelous potential to be used in 2D wearable and bendable phototransistors.

CONCLUSION In summary, the controlled synthesis of an ultrathin large-scale α-MnS single crystal via a halide-assisted CVD method has been demonstrated. The as-synthesized MnS nanosheet exhibits a large lateral size of over 100 μm and a minimum thickness of 4.78 nm. Moreover, the fabricated top-gating αMnS FETs demonstrate a typical p-type semiconducting behavior with an ultrahigh on/off ratio exceeding 106, which

Figure 5. Flexible measurement of α-MnS nanosheets. (a) Transfer and (b) output characteristics of the α-MnS phototransistor in the dark before and after bending. The inset shows the OM image of the bending device with a curvature radius (r) of ≈5 mm. (c) Time-dependent photoresponse of the α-MnS phototransistor before and after bending. G

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04205. Detailed growth modulation of α-MnS nanosheets; TEM, SAED, and EDX characterization of the synthesized nanosheets; AFM characterization of a series of devices, thickness-dependent on output and transfer characteristics of the devices; extracted thickness-dependent field-effect mobility; Richardson plot of ln(Ids/T2) versus 1000/T at various gate voltages; temperature-dependent output characteristics of the device; additional photoresponse results (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Ruiqing Cheng: 0000-0002-3618-4759 Jun He: 0000-0002-2355-7579 Author Contributions #

N. Li and Y. Zhang contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.H. acknowledges the support from the Ministry of Science and Technology of China (No. 2016YFA0200700), National Natural Science Foundation of China (Nos. 61625401, 61574050, and 11674072), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09040201). The authors appreciate the support from CAS Key Laboratory of Nanosystem and Hierarchical Fabrication. Y.Z. acknowledges the support from National Natural Science Foundation of China (No. 21703047). Z.W. also acknowledges the support from the Youth Innovation Promotion Association CAS. REFERENCES (1) Yang, H.; Kim, S. W.; Chhowalla, M.; Lee, Y. H. Structural and Quantum-State Phase Transition in van der Waals Layered Materials. Nat. Phys. 2017, 13, 931−937. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Lin, Z.; Liu, Y.; Halim, U.; Ding, M.; Liu, Y.; Wang, Y.; Jia, C.; Chen, P.; Duan, X.; Wang, C.; Song, F.; Li, M.; Wan, C.; Huang, Y.; Duan, X. Solution-Processable 2D Semiconductors for High-Performance Large-Area Electronics. Nature 2018, 562, 254−258. (4) Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D Transition Metal Dichalcogides. Nat. Rev. Mater. 2017, 2, 17033. (5) Lee, C. H.; Lee, G. H.; van der Zande, A. M.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P. Atomically Thin p-n Junctions with van der Waals Heterointerfaces. Nat. Nanotechnol. 2014, 9, 676−681. H

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