Synthesis of Superlattice InSe Nanosheets with Enhanced

May 6, 2019 - Multilayer InSe has emerged as a promising candidate for applications in novel electronic and optoelectronic devices due to its direct b...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 18511−18516

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Synthesis of Superlattice InSe Nanosheets with Enhanced Electronic and Optoelectronic Performance Wei Feng,†,‡ Fanglu Qin,† Miaomiao Yu,† Feng Gao,§ Mingjin Dai,§ Yunxia Hu,§ Lifeng Wang,∥ Juan Hou,†,‡ Bin Li,*,†,‡ and PingAn Hu*,§ Department of Chemistry and Chemical Engineering, College of Science and ‡Post-doctoral Mobile Research Station of Forestry Engineering, Northeast Forestry University, Harbin, 150040, China § Key Lab of Microsystem and Microstructure of Ministry of Education, Harbin Institute of Technology, Harbin, 150080, China ∥ Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Geelong, Victoria 3216, Australia Downloaded via NOTTINGHAM TRENT UNIV on August 13, 2019 at 20:38:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Multilayer InSe has emerged as a promising candidate for applications in novel electronic and optoelectronic devices due to its direct bandgap, high electron mobility, and excellent photoresponse with a broad response range. Here, we report synthesis of superlattice InSe nanosheets by simple thermal annealing for the first time. The mobility is increased to 299.1 cm2 V−1 s−1 for superlattice InSe FETs and is 4 times higher than 63.5 cm2 V−1 s−1 of pristine InSe device. The superlattice InSe photodetector shows an ultrahigh responsivity of 1.7 × 104 A/W (700 nm), which is 8.5 times greater than the pristine photodetector. Superlattice InSe photodetectors hold a good photoresponse stability and rapid response time of 20 ms. The electronic and photoresponse performance improvement of superlattice InSe is attributed to higher carrier sheet density and lower contact resistance for more effective electron injection and more photogenerated carrier injection, respectively. Those results suggest that superlattice is an effective method to further improve electronic and optoelectronic properties of two-dimensional InSe devices. KEYWORDS: superlattice, InSe, field-effect transistors, photodetectors, thermal-annealing

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model22−28 and modified substrates.3,29 To adjust intrinsic electronic properties of semiconductors, ion doping is the most common method in modern microelectronics. However, this is a challenge for 2D materials because of their atomically thin structure. The conductivity30 and photoresponse31 of 2D semiconductors can be effectively adjusted by surface modifying molecules and ions but those strategies suffer from long-term stability. Bandgap, conductivity, and photoluminescence (PL) of 2D semiconductors can also be altered by external strain.32−34 Phase-engineering is another effective tool to modulate conductivity and catalytic properties of 2D semiconductors via lithium intercalation35 and thermal annealing.36 For 2D InSe, high performance FETs have been demonstrated by choosing contact metals and modified substrate,14,37 and high photoresponse photodetectors have been achieved using InSe/Graphene heterostructures.20 Theory simulation claims that monolayer InSe transforms to a new phase with inversion symmetry under 25% tensile strain.38 The optical gap of 2D InSe is validly regulated by strain with a shift rate of 90−100 meV per 1% strain,39

INTRODUCTION Two-dimensional (2D) layered semiconductors have attracted significant attention because of their great potential applications in next generation novel electronic and optoelectronic devices. A wealth of 2D layered semiconductors have been explored, such as transition metal dichalcogenides (TMDs: MoS21−4 and WSe25-67), black phosphorus,8−10 and III−VI layered semiconductors (such as GaSe11,12 and InSe13−16). Among these 2D layered semiconductors, InSe shows a high mobility of 104 cm2 V−1 s−1 and high responsivity.15,16 Different from TMDs, multilayer InSe owns a direct bandgap of 1.26 eV (≥20 nm).17 Two-dimensional InSe is more stable than black phosphorus in atmospheric environment.18 These merits clarify that 2D InSe nanosheets have a huge potential applications in high performance of rigid and flexible photodetectors,15 field effect transistors (FETs),14,16 vertical and lateral p−n heterojunctions,19,20 and image sensors.21 For designing high-performance 2D materials-based electronic and optoelectronic devices, low contact resistance electrodes, trap-free interfaces between layered semiconductors and dielectric substrates, device configurations, and intrinsic properties of channel materials are crucial parameters. Highperformance 2D electronic devices have been achieved by topgated structures,1 choosing ideal contact and contact © 2019 American Chemical Society

Received: January 27, 2019 Accepted: May 6, 2019 Published: May 6, 2019 18511

DOI: 10.1021/acsami.9b01747 ACS Appl. Mater. Interfaces 2019, 11, 18511−18516

Research Article

ACS Applied Materials & Interfaces

consistent with an early report.17 After annealing, PL is drastically quenched, indicating that the microstructure of βInSe nanosheets is modified by thermal annealing. The quenched PL of annealed InSe should arise from indirect bandgap or metallic property, and we will discuss this later. The microstructures of the pristine and annealed InSe samples were investigated via selective area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). Figure 2a exhibits the SAED pattern of pristine

indicating that 2D InSe is suitable for tunable light emitters and photodetectors. Up to date, there is no experimental reports on phase-engineering tuned electronic and optoelectronic properties of 2D InSe. In this Letter, the superstructure phase is observed in thermal-annealed multilayer β-InSe nanosheets, and the electronic and optoelectronic properties of superlattice InSe nanosheets are improved by phase-engineering. The mobility of superlattice InSe FETs is 299.1 cm2 V−1 s−1, which is 4 times for pristine device. After thermal annealing, responsivity of superlattice multilayer InSe photodetector is calculated to be 1.7 × 104 A/W, which is 8.5 times larger for pristine photodetector. Superlattice photodetectors still hold a good stability and rapid response speed of 20 ms. The electronic and photoresponse performance improvement of superlattice InSe is attributed to higher carrier sheet density and lower contact resistance for more effective electron injection and more photogenerated carrier injection, respectively. The electronic and optoelectronic performance improvement of superlattice InSe devices suggests that superlattice is an powerful way to further design high performance InSe nanodevices.

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RESULTS AND DISCUSSION In this study, we focus on multilayer InSe nanosheets, because they are direct bandgap materials and have better electronic and optoelectronic performance.14,17 In order to prevent βInSe transfers to In2Se3 or InSe1−xOx due to oxidation of InSe,40,41 the pristine multilayer InSe nanosheets were annealed at 623 K and reducing atmosphere (Ar/H2) for 30 min (more details in Supporting Information). The crystal structure of pristine and annealed multilayer InSe nanosheets were investigated by Raman spectroscopy and transmission electron microscopy (TEM). Raman is a convenient and fast characterization method to detect nanomaterials. Raman spectra of pristine and annealed multilayer InSe nanosheets are depicted in Figure 1a. Both the pristine and annealed InSe

Figure 2. Microstructure of pristine and annealed InSe nanosheets. SAED pattern of pristine (a) and annealed (b) InSe nanosheets. Insets are corresponding TEM images. HRTEM image of pristine (c) and annealed (d) InSe nanosheets.

InSe nanosheets. The SAED pattern demonstrates good crystallinity of the pristine β-InSe sample, and the hexagonal pattern indicates the crystal orientates along the ⟨001⟩ axis, which agrees well with our early report.14 The SAED pattern of annealed InSe nanosheets is shown in Figure 2b. The SAED pattern shows the same main Bragg spots, indicating the lattice type remains β-type. Interestingly, there is a superstructure appearing in annealed InSe nanosheets. The superlattice is further confirmed by HRTEM image. As shown in Figure 2c, the pristine InSe nanosheet reveals a hexagonal lattice with a lattice spacing of 0.34 nm, which is consistent with the lattice spacing of (100) planes in the simple hexagonal phase. As shown in Figure 2d, the annealed InSe nanosheet has a periodicity of 0.68 nm and one period consists of two (100) planes, which agrees well with the SAED pattern with the distance between two basic spots subdivided into two equal parts. The appearing superstructure in annealed InSe nanosheets indicates a periodic lattice distortion induced by thermal annealing, which has been observed in other layered compounds such as In2Se3,36,43,44 TaS2,45 and TaSe2.46 In order to further determine the composition of annealed InSe nanosheets, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed and the corresponding results are displayed in Figure S1. The annealed InSe nanosheets have the same XRD peaks as pristine InSe nanosheets in Figure S1a, and there are

Figure 1. Characterization of pristine and annealed InSe nanosheets. (a) Raman and (b) PL spectra of pristine and annealed InSe nanosheets.

nanosheets exhibit three of the same Raman peaks at 116, 178, and 226 cm−1, which are indexed to A11g, E12g, and A21g modes of β-InSe, respectively. There are three polytypes of layered InSe materials, which are β, γ, and ε. Different from β-InSe, there is another Raman peak at 202 cm−1 in γ- and εsamples.13,42 After annealing, there is no new Raman peak appearing at 202 cm−1, indicating annealed InSe nanosheets still are β-phase and do not transfer to γ- or ε-phase. This phenomenon is totally different with the multilayer α-In2Se3 sample, which can transfer to β-In2Se3 under thermal annealing.36 Figure 1b are PL results of pristine and annealed InSe samples. The PL peak of pristine InSe is 1.26 eV, which is 18512

DOI: 10.1021/acsami.9b01747 ACS Appl. Mater. Interfaces 2019, 11, 18511−18516

Research Article

ACS Applied Materials & Interfaces no new XRD peaks appearing. The EDS results demonstate that annealed InSe nanosheets only have two elements and the element ratio of In/Se is close to 1:1, which is similar to pristine InSe. The binding energy postions of annealed InSe nanosheets do not shift and no new XPS peak appears. All of the above results demonstrate that annealed samples only contain the InSe constituent and the annealing process does not produce new materials. Our results are totally different from early reports on InSe annealing, which is due to our annealing process being operated in reducing atmosphere (Ar/ H2) and avoiding oxidation of InSe. The thickness of the InSe nanosheet is measured by atomic force microscopy (AFM) before and after thermal annealing. The thickness of superlattice InSe is almost the same as pristine InSe as shown in Figure S2. Two-dimensional In2Se3 nanosheets with superstructure phase exhibit metallic behavior,43 demonstrating that superstructure has a profound effect on electronic properties of 2D layered semiconductors. Here, back-gated FETs made of pristine and superlattice multilayer InSe nanosheets with a thickness of 20−40 nm were fabricated (fabrication process in Supporting Information). Figure 3a is the 3D structure of InSe

Figure 4. Electronic transport properties. (a,b) The transfer curves of pristine and superlattice InSe transistor at Vds = 1 V. (c,d) The output curves of pristine and superlattice InSe FETs, respectively.

superlattice devices exhibit totally different output characteristics in Figure 4d and Figure S3; the current shows a good linear behavior in measured bias region and larger output current. The larger current of superlattice InSe is attributed to two factors: (1) larger channel conductance coming of higher carrier density and (2) lower contact resistance (Rc) of Cr− InSe for more electron injection. To elaborate the electronic properties’ improvement of the superlattice InSe sample, we calculate carrier sheet density (n2d) and Rc values of pristine and superlattice samples. The calculated n2d values are 0.996 × 1012 and 6.37 × 1012 cm−2 for pristine and superlattice InSe FETs, respectively (more calculation details in Supporting Information). After annealing, the carrier density increases 6 times due to superlattice structure. The Rc value of pristine is calculated to be 506 KΩ, which is consistent with our earlier study.47 After annealing, the contact resistance is drastically degenerated to 2.4 KΩ, which is reduced 210 times (more calculation details in Supporting Information). Those results demonstrate that the electronic transport properties’ improvement of superlattice InSe device is attributed to higher n2d and lower Rc values. To reveal the photoresponse of pristine and superlattice multilayer InSe nanosheets, various illumination lights were vertically illuminated onto InSe photodetectors. As shown in Figure 5a,b, pristine and superlattice multilayer InSe photodetectors show an obvious broadband photoresponse from 254 to 700 nm illumination light. It is clear that the photocurrent (Iph, Iph = Iillumination − Idark) of pristine and superlattice InSe photodetectors increases with applied bias voltage. Figure S4 shows the calculated Iph−λ curves of pristine and superlattice multilayer InSe photodetectors at an applied voltage of 1 V. Obviously, superlattice InSe photodetector generates larger Iph under various illumination lights. This phenomenon is due to more photogenerated carrier injection arising from lower contact resistance of superlattice InSe. For example, the Iph value is 15.1 μA for superlattice InSe photodetector under 700 nm and a bias of 1 V, which is approximately 8.4 times higher than 1.8 μA for pristine InSe. Responsivity (R), detectivity (D*), and external quantum efficiency (EQE) are three important parameters to evaluate a performance of photodetector. Superlattice InSe photodetector exhibits larger R

Figure 3. Multilayer InSe nanosheets field effect transistor. (a) Threedimensional schematic structure of InSe FETs. (b) An optical image of InSe FETs. (c) The AFM height profile of multilayer InSe channel. Inset: the AFM image of InSe channel.

device and Figure 3b is an optical image of a InSe device with a channel length of 20 μm and channel width of 15 μm. Figure 3c is the AFM image of the InSe channel and the thickness is 31 nm. Figure 4a,b is electronic transfer characteristics of pristine and superlattice InSe FETs. The pristine InSe nanosheets shows n-type transport property and a large current on/off ratio of 107. After annealing, superlattice InSe nanosheets maintain n-type conductance property but the current switching drastically degrades with a small current on/off ratio of 2.9, which is similar to electronic property of the 2D superlattice In2Se3 nanosheet,43 demonstrating its metallic property. The electrical performance suggests that the quenched PL of annealed InSe is mainly ascribed to its metallic property. The mobility value of pristine InSe FETs is 63.5 cm2 V−1 s−1 calculated from Figure 4b, which is consistent with our early report41 (more calculation details in Supporting Information). After annealing, the mobility value is increased to be 299.1 cm2 V−1 s−1, which is 4 higher times for pristine InSe FETs. This result further demonstrates that electronic performance is improved via phase engineering. Figure 4c,d are output curves of pristine and superlattice InSe FETs, respectively. The output curves of pristine InSe FETs display a linear region and a saturation region at low Vds and high Vds, respectively, which are consistent with our early reports.47 The 18513

DOI: 10.1021/acsami.9b01747 ACS Appl. Mater. Interfaces 2019, 11, 18511−18516

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CONCLUSIONS In summary, the superstructure phase is obtained in multilayer β-InSe nanosheets by simple thermal annealing for the first time. The electronic and optoelectronic properties of superlattice InSe nanosheets are significantly enhanced. After annealing, superlattice InSe nanosheets exhibit a semimetal transport behavior with a low current on/off ratio of 2.9. A high mobility of 299.1 cm2 V−1 s−1 is realized by superlattice InSe FETs, which is 4 times that for pristine InSe device. The electronic properties of the superlattice InSe nanosheet are effectively improved, which is due to a higher n2d value of 6.37 × 1012 cm−2 and a lower Rc of 2.4 kΩ induced by a superlattice. An ultrahigh responsivity of 1.7 × 104 A/W is obtained by the superlattice InSe photodetector, which is 8.5 times for the pristine photodetector. The photodetection improvement of superlattice InSe is attributted to lower contact resistance for more photogenerated carrier injection. Superlattice InSe photodetectors hold a good photoresponse stability and rapid response time of 20 ms. All of the above results demonstrate that the superlattice is a potential pathway to further improve electronic and optoelectronic properties of InSe devices.

Figure 5. Optoelectronic properties. (a,b) I−V curves for pristine and superlattice InSe photodetectors measured under various light illuminnation (254, 365, 490, 610, and 700) with an intensity of 0.29 mW/cm2, respectively. (c) The calculated responsivity values at applied voltage of 1 V. (d) Time-dependent photoresponse characteristics of pristine and superlattice InSe photodetectors illuminated by 700 nm light at V = 0.1 V with a light intensity of 0.29 mW/cm2.

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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/acsami.9b01747. values than those of pristine InSe photodetector under various illumination lights in Figure 5c (more calculated details in Supporting Information). For example, under 700 nm light illumination, R of the superlattice InSe photodetector is calculated to be 1.7 × 104 A/W at a bias of 1 V, which is almost 8.5 times larger than 2.0 × 103 A/W of pristine InSe photodetector. Superlattice InSe photodetector shows superior D* and EQE values than those of pristine InSe photodetector as shown in Figure S5. The D* and EQE values of the superlattice InSe photodetector are calculated to be 1.4 × 1013 Jones and 1.7 × 107% with 490 nm light illumination and a bias of 1 V, which are larger than 9.1 × 1012 Jones and 7.1 × 105% of pristine InSe photodetector. The superior photodetection performance in the superlattice InSe photodetector is attributed to low contact resistance between the superlattice InSe and contact metal electrodes, which leads to more photogenerated carrier injection. The photoresponse performance of the superlattice InSe photodetector is superior to most photodetectors based on other 2D semiconductors (find more details in Table S1). Those results clarify optoelectronic properties of the InSe photodetector can be effectively improved by phase-engineering. Stability and response time are another two key parameters for evaluating a photodetector. The stability of pristine and superlattice InSe photodetectors was investigated. The on/off states of pristine and superlattice InSe photodetectors keep the same level for six cycles in Figure 5d, demonstrating a good optical switching stability for both types of InSe photodetectors. Figure S6 shows the rise and decay photoresponse behaviors of pristine and superlattice multilayer InSe photodetectors. Both types of InSe photodetectors exhibit a rapid response time of 20 ms for rise and decay process independent of annealing.

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Experimental section including the calculated mobility, carrier density, contact resistance and responsivity, AFM, output characteristics of annealed InSe FETs with large Vds range, XRD, EDS, XPS, Iph−λ curves, Iph− V curves, and response time of pristine and annealed InSe photodetectors, comparison of the critical parameters for photodetectors based on 2D semiconductors (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(B.L.) E-mail: [email protected]. *(P.A.H.) E-mail: [email protected]. ORCID

Wei Feng: 0000-0001-6902-0024 Mingjin Dai: 0000-0001-6009-1715 Bin Li: 0000-0003-2696-6930 PingAn Hu: 0000-0003-3499-2733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from National Natural Science Foundation of China (NSFC, No. 51802038), China Postdoctoral Science Foundation (No. 2018M630329), Heilongjiang Postdoctoral Special Fund (LBH-TZ1801), and Fundamental Research Funds for the Central Universities (No. 2572018BC14).

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DOI: 10.1021/acsami.9b01747 ACS Appl. Mater. Interfaces 2019, 11, 18511−18516