Nonlayered Two-Dimensional Defective Semiconductor Î³-Ga2S3

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Article

Non-Layered Two-Dimensional Defective Semiconductor #-Ga2S3 toward Broadband Photodetection Nan Zhou, Lin Gan, Rusen Yang, Fakun Wang, Liang Li, Yicong Chen, Dehui Li, and Tianyou Zhai ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00276 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 16, 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 32 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 Nano

Non-Layered Two-Dimensional Defective Semiconductor γ-Ga2S3 toward Broadband Photodetection Nan Zhou,†, ‡ Lin Gan,† Rusen Yang,‡ Fakun Wang,† Liang Li,† Yicong Chen,† Dehui Li,§ and Tianyou Zhai*,† †State

Key Laboratory of Material Processing and Die & Mould Technology, School of Materials

Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China ‡School

of Advanced Materials and Nanotechnology, Xidian University, Xi'an 710126, P. R.

China §School

of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics,

Huazhong University of Science and Technology, Wuhan 430074, P. R. China E-mail: [email protected];

KEYWORDS: 2D γ-Ga2S3, non-layered materials, defective semiconductor, space-confined chemical vapor deposition, photodetection

1 ACS Paragon Plus Environment

ACS Nano 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 32

ABSTRACT: Two-dimensional (2D) materials exhibit high sensitivity to structural defects due to the nature of interface-type materials, and the corresponding structural defects can effectively modulate their inherent properties in turn, rendering them a wide application range in highperformance and functional devices. 2D γ-Ga2S3 is a defective semiconductor with outstanding optoelectronic properties. However, its controllable preparation has not been implemented yet, which hinders exploring its potential applications. In this work, we introduce non-layered γ-Ga2S3 into 2D materimals family, which were successfully synthesized via space-confined chemical vapor deposition (CVD) method. Its intriguing defective structure are revealed by high-resolution transimission electron microscope (HRTEM) and temperature-dependent cathodoluminescence (CL) spectra, which endow the γ-Ga2S3-based device with broad photoresponse from ultraviolet to near-infrared region and excellent photoelectric conversion capability. Simultaneously, the devices also exhibit excellent ultraviolet detection ability (Rλ = 61.3 A W-1, Ion/Ioff = 851, EQE = 2.17 104 %, D* = 1.52 1010 Jones @350 nm) and relatively fast response (15 ms). This work provide a feasible way to fabricate ultrathin non-layered materials, and explore the potential applications of 2D defective semiconductor in high-performance broadband photodetection, which also suggest a promising future of defect creation in optimizing photoelectric properties.


Page 3 of 32 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 Nano

Two-dimensional (2D) semiconductors are rapidly enriched by rediscovering atomic thin properties of bulk crystals,1 which process the advantages of strong electron confinement and lightsubstance interaction,2,3 highly tunable bandgap,4,5 and superior immunity to short channel effects,6,7 thus having broad prospects in the applications of monolithic integration, nanoelectronics and optoelectronic devices.8-13 Moreover, due to the reduced dimensions14 and the nature of interface-type materials,15 2D materials exhibit high sensitivity to structural defects, and the electronic band structure, optical, electronic and photoelectric properties can be flexibly modulated though the structure defects,16-22 which is especially suitable for defective 2D semiconductors, including considerable inborn defects in crystal structure. For instance, defects often cause changes in electronic band structure, which are often accompanied by appreciable changes in excitonic transitions, such as position shift or intensity enhancement in the photoluminescence (PL) or absorption peaks.16 Besides, 2D MoS2 can either be n-type or p-type conductors dictated by the dominant sulfur (S) or molybdenum (Mo) vacancies in samples, respectively.18 Moreover, the responsivity and response time of the photoconductors can also be tuned by the defects that may be as the trapping or recombination centers, depending on the energy levels of the defect states and the Fermi level,19-21 and response spectrum range has also been broadened due to the inner energy level induced by defects.22 Here, we consider that defective structure renders 2D materials higher degree of freedom in the modulation of properties, which extend its application range beyond the capability of conventional stoichiometric structures. Therefore, it is of great significance to explore more species of 2D defective semiconductors, and to understand the relationship between interesting properties and their defective structures, which would benefit to develop high-performance and functional devices in future.


ACS Nano 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 4 of 32

Cubic Ga2S3 (γ-Ga2S3) crystal is one of the typical defective semiconductors with p-type behavior,23 and it is also a relatively stable phase at room temperature among the three existing crystal phases including monoclinic,24-27 hexagonal,28 and cubic,23,29 which process a series of excellent electronic and optoelectronic properties including direct wide bandgap (~3.4 eV), temperature-dependent optical absorption edge and excellent photoconduction behavior with high photoresponsivity.23,29 Moreover, due to the crystalline structures enriched by a variety of intrinsic defects, γ-Ga2S3 crystal can be generally regarded as a quasi-ternary compound consisting of defects, gallium (Ga) atoms and S atoms, which is expected to produce attractive features as well as the enhanced device performance. However, due to the property tending to form three dimensional structures of non-layered materials,30 the preparation of 2D γ-Ga2S3 flakes faces enormous difficulties and challenges. Until now, the related research on 2D γ-Ga2S3 is limited to thin films ( ∼500 nm) synthesized via thermal vapor sulfurization,23 and its controllable preparation has not been implemented, which obviously hinders the research progress on interesting properties in 2D γ-Ga2S3 and its potential applications. In this work, non-layered defective semiconductor γ-Ga2S3 was introduced into 2D materials family, which was synthesized by space-confined chemical vapor deposition (CVD), and their luminescence mechanism and photoelectric properties were systematically explored. The inherent Ga or S vacancies in the crystal structure have been verified by high-resolution transmission electron microscopy (HRTEM). Then the luminescence performance of the sample was investigated via low-temperature cathodoluminescence (CL), the luminescence mechanism of which was attributed to defect-induced emission, the main path of luminescence for γ-Ga2S3. Besides, γ-Ga2S3-based photodetectors demonstrated a broadband response, originating from the existence of inherent defects. Simultaneously, the device also exhibits an outstanding ultraviolet


Page 5 of 32 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 Nano

detection ability (Rλ = 61.3 A W-1, Ion/Ioff = 851, EQE = 2.17 104 %, D* = 1.52 1010 Jones @350 nm) and relatively fast photoresponse (15 ms). In addition, we also studied the performance of second harmonic generation (SHG) of γ-Ga2S3 flakes in which the intensity of signal was enhanced with the increase of thickness, rendering γ-Ga2S3 flakes high frequency conversion efficiency. Thus, the successful preparation of 2D γ-Ga2S3 flakes has expanded the family of 2D defect semiconductors, and also provided valuable reference for enhancing the performance of devices.


ACS Nano 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 6 of 32

RESULTS AND DISCUSSION Here, the γ-Ga2S3 flakes were synthesized by CVD, which can be used to realize the controllable preparation of 2D materials with various morphologies and thicknesses, further to explore the physical properties varying the thickness. The schematic diagram of the synthesis installation was shown in Figure 1a.The crystal structure model of Ga2S3 was depicted on the right side of the diagram, of which the Ga atoms layer and S atoms layer stack alternately. Specifically, each Ga atom is connected to the surrounding four S atoms, and vice versa, which implied its nonlayered nature by the absence of layered structure in each crystal direction. Moreover, as a defective semiconductor, about two-thirds of the cationic sites in the crystal structure of γ-Ga2S3 are occupied by Ga atoms, and the remaining one-thirds of cationic sites are vacancies with random distribution, inevitably constituting the structural vacancies.31 In addition, the vacancies proportion is an average for a crystal with the "ideal" Ga2S3 structure, and in fact, the value maybe greater than 1/3, caused by the synthesis process. Ultrathin Ga2S3 flakes with high-quality were obtained by the optimized CVD methods, in which Ga2Cl4 powder were employed as precursors and the S powder was heated independently for better controlling the precursor ratio during growth. Mica substrates were employed to facilitate the crystal growth due to its ultra-smooth surface in absence of suspending bonds and low energy migration barrier.32 In addition, the stacking mica-constructed confinement growth method was identified as the optimized CVD methods to improve the sample quality and crystallinity, in which a top shadow mica is introduced to partly encapsulate the surface of the bottom mica substrate to generate confined space, then furnishing a relatively constant air flow transport environment, thus reducing the mobility of adsorbed atoms and increasing the controllability of the final sample via the growth temperature and duration.33


Page 7 of 32 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 Nano

The optical microscopy (OM) of Ga2S3 flakes obtained by space-confined CVD method and the direct growth method are shown in Figure S1, in which triangle grains were obtained with the space-confined CVD, but single crystal flakes with regular shape were absent with the direct growth method. As a comparison, we found that the former is more applicable to the synthesis of non-layered 2D materials. As shown in Figure 1b and 1c, the as-synthesized triangle flakes with the length of side up to 8-15 µm are distributed on mica substrate randomly. There is slightly difference in the thickness of the flakes, and the thicknesses for the relative thin samples are approximately 9 nm. Other obtained slightly thicker samples are about 15, 25, 42, 60 nm respectively, verified by atomic force microscopy (AFM) (Figure S2). Currently, the obtained thinnest sample is about 3 nm (Figure S3), and its slightly poor crystallinity may be related to the non-layered nature. To further identify their composition, Raman spectra were employed to characterize the as-grown flakes. To avoid the influence of background peak from mica substrates, the samples were first transferred onto SiO2 (300 nm)/Si substrate by the polypropylene carbonate (PPC)/poly (methyl methacrylate) (PMMA)-mediated technology,34 and the Raman peaks varying with the thicknesses of flakes are displayed in Figure 1d. Five dominant peaks have observed in Raman spectrum locating at 114, 145, 233, 330 and 390 cm-1, which are attributed to E, E’, A1, A’ and F2 vibration modes of Ga-S bonds, respectively, confirming the flakes are Ga2S3 crystals.23,35 The Raman peaks for the 3 nm sample are too weak to be observed that only the peaks of the SiO2 substrate are identified, and the intensities of all peaks raise with the augment of the flakes thickness. However, the peak positions is almost unchanged with different thickness for all samples, strikingly different from the reported 2D MoS2 flakes which show strong thickness dependence,36 but similar to the 2D materials of III–VI group resulted from the unstrained of van der Waals bonds between layers.37-39


ACS Nano 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 8 of 32

Atomic lattice image from transmission electron microscopy (TEM) can provide visualized crystalline information, so a more precise identification can be achieved. A low-magnification TEM image of Ga2S3 single crystal flake was described in Figure 2a, which was also obtained by transferring the sample to the copper grid via the above-mentioned PMMA-mediated technology,34 and the indistinct contrast confirmed the synthesized sample with thinner thickness. The obtained selected area electron diffraction (SAED) pattern in Figure 2b and Figure S4 exhibited a standard hexagonal diffraction pattern, indicating single-crystalline structure of the obtained sample. The equal inter-planar distance confirmed the family of crystal planes in cubic phase. The corresponding high resolution TEM image was shown in Figure 2c, from which clear lattice fringe were observed, and the distance between planes were measured ~ 0.368 nm, corresponding to the (202) planes. Moreover, the location highlighted by white circles confirmed the existence of vacancy defects, as shown in the inset of Figure 2c and Figure S5, thus, the γ-Ga2S3 include various vacancies, Ga atoms and S atoms, which can be viewed as a quasi-ternary compound. The vacancies and defects existed in the crystal structure can strongly influence the electronic structure and optical properties, thus affecting electrical properties and photodetection, which will be discussed later.17 Fast Fourier Transforms (FFT) pattern based on the [-111] crystal axis by CrystalMaker showed a coincident diffraction pattern in Figure 2d, supporting the SAED result as well. Besides, elemental mapping analysis revealed that the distribution of element Ga and S was rather uniform (Figure 2e and 2f). Moreover, from the EDS spectrum, the atomic ratio of Ga and S approximately was equal to 2:3, demonstrating the obtained sample to be Ga2S3 again (Figure S6). According to the TEM data, it is safe to conclude that the as-synthesized Ga2S3 flakes is a special defect semiconductor, with cubic structure rather than most reported monoclinic phase.27


Page 9 of 32 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 Nano

Moreover, to further check the composition and accurate binding information of materials, X-ray photoelectron spectroscopy (XPS) was also employed to characterize triangular Ga2S3 flakes, as shown in Figure 2g and 2h. From the Ga 2p and S 2p detailed core spectra of Ga2S3, the signals of elements of Ga and S are clearly identified. Besides, the peaks located at 1118.4 eV and 1145.3 eV of Ga 2p spectrum can be indexed to the core level peaks of Ga 2p1/2 and Ga 2p3/2, and the other two fitted peaks around 162.4 eV and 163.5 eV of S 2p spectrum implied the binding energies of S2- in Ga2S3. The above XPS spectrum matched well with the standard spectrum, thus further identifying the materials as Ga2S3, which also revealed high chemical purity and high quality of the obtained Ga2S3 crystals. The results of the X-Ray Diffraction (XRD) pattern for Ga2S3 was consistent with the SAED results mentioned above (Figure 2i), identifying the crystal structure as cubic phase. However, it is different from the results of the thick Ga2S3 film, which clearly show the peaks at (111), (220) and (311),23 due to the thinner thickness and orientation growth. Here, the (111) plane is considered to be the most preferred growth orientation while the peak near 69 degree is the silicon substrate peak. To understand the thermal conductivity, thermal expansion, and atomic bonds of 2D Ga2S3 flakes,40,41 temperature-dependent Raman spectroscopy of the Ga2S3 triangle flakes with thickness of 15 nm were measured (Figure S7). As for the most intense peak of Raman spectra for Ga2S3, the A1 mode versus temperature are shown in Figure S7a. It can be concluded that the increasing temperature from 100 K to 300 K leads to a redshift of A1 phonon modes (233 cm-1 for 300 K) for Ga2S3 flakes under a 532-nm laser excitation (Figure S7b). The temperature-dependent peak positions can be fitted according to the following equation: 𝜔(𝛵) = 𝜔 + 𝜒𝛵

(1)


ACS Nano 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 10 of 32

where 𝜔 and 𝜒 reprent the peak position of A1 modes at zero kelvin and the first-order temperature coefficient to describe the peak positions depending on the temperature, respectively. The first-order temperature coefficient for A1 mode is − 0.00739 cm −1 K −1, which is higher than 2D Bi2Se3 flake (A1 mode, χ = − 0.00544 cm −1 K −1 ),42 but smaller than those of other 2D layered materials such as MoS2 (E12g mode, χ = − 0.0136 cm −1 K −1 ),43 and SnSe2 (A1 mode, χ = − 0.0129 cm −1 K −1 ).41 Here, the relatively small coefficients may be related to its intrinsic properties, which indicates that the phonon frequencies are relatively less sensitive to the temperature modulation. Figure S7c shows the full-width at half-maximum (FWHM) varying with temperature for the A1 mode, which increases from 15.9 cm−1 to 17.2 cm−1, as the temperature rises from 100 K to 300 K. In addition, we also found that the width of A1 mode shows relatively significant increase when the temperature exceeds 200 K, while it is almost unchanged at low temperature, which may be related to the higher order anharmonic phonon coupling.41 Electronic structure and optical properties are strongly influenced by the defects in 2D materials. Here, owing to processing higher excitation energy and spatial resolution over PL, CL spectra based on electron beam bombardment were employed to explore the luminescence mechanism, and detect excitonic states, defects and impurity levels.44 The CL spectra of Ga2S3 flakes with different thickness were measured at room temperature (Figure 3a), and the sample exhibited a broad emission ranging from 380 nm to 950 nm, in according with the result of previously reported films prepared by thermal vapor sulfurization.23 The primary emission located at 667 nm (1.86 eV) is not attributed to the bandgap emission (365 nm, ~3.40 eV), which maybe corresponding to the recombination of intrinsic S vacancies (VS) with Ga vacancies (VGa) below the band-gap. Moreover, the intensity of the defect-induced emission exhibit enhancement with the increasing thickness, stemming from the increase of defect concentration. The experimental


Page 11 of 32 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 Nano

results show that the bandgap emission was undetectable, owing to the quenching effect of the transition below the bandgap, and the comparison of intensities between the above two kinds of emissions implied that defect-induced emission is the main emission path. The interatomic distance can be strongly affected by the surrounding temperature, due to the overlap among adjacent electron orbitals.45 Therefore, temperature-dependent CL measurements were carried out to further evaluate the physical origin of the emission properties of Ga2S3. The CL spectra of Ga2S3 flake with 9 nm within the temperature range from113 K to 293 K were displayed in Figure 3b, which showed that the PL intensity decayed with the increasing temperature. Here, the above phenomenon is further analyzed, along with the increasing temperature, the recombination of defects were enhanced and maybe the VGa state was gradually occupied by electrons, and by the above nonradiative process, more photogenerated carriers are consumed and more phonons are dissipated in the form of thermal radiation. Then the decrease of photons result in the radiation transition process weakened and the recombination of VS→VGa lowered down, and the PL intensity decreased. Therefore, the emission intensity decreasing dramatically as the increasing temperature can be attributed to the enhancement of nonradiative process, corresponding to the decrease of carrier lifetime.46-48 Here, the activation energy can be obtained by fitting the following Arrhenius formula as shown in Figure 3c, I(T) = I(T = 0)/[1 + C exp(-ΔE/kB T)]

(2)

in which I(T), C, kB and Δ E represent the temperature-dependent emission intensity, a constant to describe the capture cross-section of carriers at the center, Boltzmann’s constant and activation energy relative to the defects or the traps, respectively.49,50 The obtainedΔE is 0.066 eV for the 677 nm emission peak, thus the emission may be ascribed to the shallow donor level


ACS Nano 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 12 of 32

such as VGa. It is also an indication that Ga2S3 is a naturally defect semiconductor with the existence of intrinsic defects coming from the imperfection levels, thus exhibiting defect-induced CL peaks. The schemes of possible transition mechanism for the defects at low temperature are depicted in Figure 3d, which is from VS by native chalcogen deficiency in the crystal growth and VGa inside the imperfection crystal structure, and similar phenomenon has been observed in the bulk Ga2S3.23 Simultaneously, these vacancy defects are also of significance for optoelectronic devices. Cubic Ga2S3 was a non-centrosymmetric structure and belong to the P-43m space group, therefore, nonlinear optical property like SHG should be present. Figure 4a showed the schematic diagram of SHG characterization for the Ga2S3 flakes, which could be used for laser frequency multiplier. We then explored the SHG effect on the 9 nm Ga2S3 flake under non-resonant condition. The wavelength of ~800 nm laser was employed as the excitation wavelength and the corresponding SHG signal appeared at 400 nm. The intensity of SHG signal is influenced by many factors, such as crystal structure, external laser power, thickness, sample inner quality, etc.51 The influence of crystal structure on SHG intensity were first investigated by polarization-dependent SHG in Figure 4b, and the SHG signal was collected by setting excitation field parallel to the emission polarization field. A clear six-petal pattern of SHG intensity were acquired to describe SHG intensity varying with azimuthal angles, similar to the previously reported TMDs.52,53 Then the external laser excitation power-dependent SHG intensity were investigated (Figure 4c), and the incident laser power-resolved SHG measurement was taken under a power ranging from 0.8 mW to 9.0 mW. The results has confirmed that the intensity of SHG was monotonically increased with enhancement of power, and the relation of SHG intensity and incident power can be fitted by the electric dipole theory under the first-order perturbation ISHG = |E(2ω)|2 ∝ |P(ω)|2 , where ISHG is the the SHG intensity, E(2ω) is SHG electric field vector and P(ω) is the excitation power,


Page 13 of 32 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 Nano

respectively. Here, the relation of SHG intensity and incident power can be linearly fitted with a slope of ~ 2.06 (Figure 4d), approximately equal to the theoretical value of 2, which confirmed the SHG intensity exhibiting a quadratic dependence on the excitation power. Thickness-dependent test (Figure 4e) indicated that the intensity of SHG increased with sample thickness from 3 to 15 nm, evidencing that Ga2S3 flake has a non-centrosymmetric structure regardless the layer number. Such a trait was very important to nonlinear optical applications because it guaranteed stable frequency conversion efficiency under various thicknesses. Figure 4f showed a SHG mapping image on a typical triangle Ga2S3 flake. Although a large number of vacancy defects have been confirmed by TEM and CL, the SHG mapping signal still maintains good uniformity, suggesting that defects were evenly distributed in the whole sample. Besides, SHG intensity in Ga2S3 under different wavelengths with the same power ranging from 780-1080 nm and 1080-1300 nm are shown in Figure S8, which proved the broad spectral range of 2D Ga2S3 crystal for efficient SHG. In addition, the overall trend is that the SHG intensity decreases with the increase of excitation wavelength except for several wavelengths which may be related to the interference effect and the enhanced susceptibility of materials.54,55 How defects influence on the electrical properties of Ga2S3 flakes are studied by electric transport measurement. Field effect transistor (FET) device based on as-synthesized flakes are fabricated on a 300 nm SiO2/Si wafer substrate via standard electron beam lithography (EBL), and the corresponding OM of the device is shown in Figure S9. Then the corresponding output characteristic curve and transfer characteristic curve of the Ga2S3 flake with 9.0 nm are shown in Figure 5a and 5b, respectively. The output source-drain current Ids raised with the increasing of bias Vds. Moreover, the Ids decreases as Vg increases, which confirmed Ga2S3 semiconductor is a ptype semiconductor with an on/off current ratio approximately equal to 105, better than that of


ACS Nano 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 14 of 32

other 2D materials.56-58 The p-type electrical conductive property of the obtained Ga2S3 flakes was attributed to the VGa, which are generally acting as acceptors.59 In addition, we also investigated the variation of carrier mobility with sample thickness at Vds = 1 V, as shown in Figure 5c and 5d, which show that the carrier mobility raised with the increase of the sample thicknesses ( Figure S10). Based on the equation µ = (ΔIds/ΔVg) (L/(W Cox Vds)), where L is the length of the channel, W is the width of the channel and Cox represents the gate capacitance. Here, annealing has been employed to enhance electrode-2D materials contacts and improve carrier mobility (Figure S11), and the obtained carrier mobility was about 1.06 cm2 V−1 s −1, which is comparable to the value of the devices based on ReS2 and ReSe2 flakes.60,61 The relatively general value may be related to the internal vacancy defects in crystal structure and the effect of adsorbates, and the low carrier mobility for the thinner flake may be owning to its being more sensitive to external impurities scattering at the interface, compared with the thicker samples. Further improvement of the carrier mobility might be obtained by further optimizing film quality,62,63 contact engineering,64,65 and controlled doping.66,67 The lower modulation ability of the thick samples may be owing to charge screening,68 which cause free carriers only be gated in the bottom layers, in according with the reported work.42 The uniform of film quality have been proved by the electrical properties of twenty devices, which are based on the sample from different substrates of the same synthesis process, and the obtained carrier mobility are on the same order of magnitude. (Figure S12) To further investigate the photoelectric performance of Ga2S3 with special native defective structure, the photodetectors based on 9 nm Ga2S3 flakes were fabricated by the above-mentioned method with 10/50 nm Cr/Au metal contacts, which are expected to exhibit enhanced photoelectric properties. The photoelectronic properties of Ga2S3-based device under air and room temperature condition are summarized in Figure 6. Figure 6a shows the current-voltage (I-V) of Ga2S3


Page 15 of 32 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 Nano

photodetector under illumination with the incident wavelength of 350-900 nm and in dark at a bias voltage of 1 V, from which a broad spectral response ranging from ultraviolet to near-infrared region has been achieved and a maximum photocurrent was obtained for the 350 nm incident light, in according with the CL spectrum, which arises from that the vacancy defects change the electronic structure of Ga2S3, and transition between defects below the bandgap generates ultrabandgap photoresponse under the illumination. Then the corresponding photoresponsivity (Rλ), on-off ratio (Ion/Ioff), external quantum efficiency (EQE), and detectivity (D*) under different wavelengths are obtained as shown in Figure 6b and 6c, which are important indicators to describe the performance of photodetection. Based on the formula Rλ = Iph/PS and Ion/Ioff = Iph/Idark, (where Iph is the photogenerated current; P is the incident light power; S is the effective area between channels; and Idark is the dark current) the device exhibits a maximum Rλ of 61.3 AW−1 and Ion/Ioff of 851 under the 350 nm incident light with a power density of 7.07 mW cm−2, which is higher than the devices based on 2D ReSe2,69 and other Ⅲ-Ⅵ materials (GaS, GaSe ).70,71 Based on the formula EQE = hc Rλ/eλ and D* = RλS1/2/(2eIdark )1/2, we further obtained the maximum EQE of 2.17 × 104 % and D* of 1.52 × 1010 Jones for 350 nm illumination, where h, c, Rλ, e, λ and Idark are Plank’s constant, light velocity, responsivity, elementary charge, excited wavelength and dark current, respectively. In addition, all the above four performance parameters decreases along with the red shift of incident wavelength, which reflects that photoelectric properties are closely depending on the incident wavelength. In order to explore the stable and repeatability of the photocurrent, the incident light with different wavelength was switched on and off alternatively with a time interval of 35 s under a bias of 1.0 V (Figure 6d), and the periodically repeated current suggests the stability of device and the fast response and decay rate. As the response time is close to the record limit of instrument (0.2 s), we employed modulated light to test the temporal response


ACS Nano 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 16 of 32

of the Ga2S3 devices to further measure the specific response time and decay time, which are evaluated to be 15 ms and 10 ms (Figure 6e), respectively, comparable with 2D In2S3 photodetector,22 and higher than MoS2 and other 2D materials.72,73 However, the responsivity and response time are usually opposite merit for photoconductors, and a relatively fast response time often accompanied by a relatively low responsivity. Here, we attribute the relatively high photoresponsivity to its direct bandgap property and the existence of shallow defect energy level, which lead to that minority carriers are trapped by defects, and majority carriers circulate in the channel, resulting in higher responsivity. Moreover, shallow traps play dominant roles in determining the decay time of the device, which can be several orders of magnitude faster than the deep traps.22,74 To further understand the excellent optoelectronic properties of Ga2S3 flakes, the photocurrent dependence on light intensity (350 nm) are summarized in Figure 6f, in which the photocurrent raised with the increasing laser power density. The relation between Iph and incident light power density (P) can be fitted by a power law (Iph∝Pα; where α is in the range of 0 ~ 1) in Figure 6g. The fitting parameter is 0.96 which show the approximate linear behavior, much higher than previously reported semiconductors, such as SnSe2 and HfS2,75,73 verifying that Ga2S3 flake device possesses high efficient photoelectric conversion capability. The fitting factor mainly depends on the optical intensities as well as the properties of the materials.76 We have ruled out the impact of power intensity according to the power data in the literature. For the majority of semiconductor, the photogenereted holes would be captured by the surface traps states under low power intensity; with the increase of light intensity, photogenerated holes also increased; when all the trap states are occupied completely, extra electrons and holes will generate rapid recombination. Therefore, the number of defects trap states and light intensity are crucial to the fitting factor here.


Page 17 of 32 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 Nano

For the semiconductor with recombination centers of different energies and capture cross-sections, the photocurrent usually exhibit approximate linear or linear dependence on laser intensity in ambient.76 Here, for the native defect semiconductor Ga2S3, the VS (donor) and VGa (acceptor) traps can act as coulomb repulsive centers with certain capture cross-section. For the excited electrons/holes, their surplus energy can also be absorbed by the trap states within the bandgap, then extra electron/hole will be generated and enter into the conduction/valence band. Thus, more than one carrier can be produced by one photon, generating an approximate linear/ linear behavior.77,78 In short, the existence of capture cross-section ensures high photocurrent generation efficiency. To further survey the photodetection performance of Ga2S3 flakes under near-infrared light, a 850-nm illumination was performed as the excitation source. The corresponding I-V curves with different power intensities, and the relation between Iph and P are shown in Figure 6h and 6i. The photocurrent increases as the increase of incident light power density, and the fitting factor is about 0.94, close to that under 350-nm illumination. The performance comparison of the devices based on 2D γ-Ga2S3 flakes with many previously reported devices based on other 2D materials were summarized in Table S1. It is obvious that our device performance is superior many preexisting devices. CONCLUSION In summary, we have synthesized non-layered 2D γ-Ga2S3 flakes via space-confined CVD method and explored their optoelectronic properties systematically. The existence of defects in 2D γ-Ga2S3 endow itself with a wide spectrum response range and excellent photoelectric conversion capability, and the devices also exhibit excellent ultraviolet detection ability and a relatively fast response speed. In addition, the non-centrosymmetric structure of Ga2S3 guaranteed it a high frequency conversion efficiency in SHG. Thus, this defective semiconductor may possess great


ACS Nano 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 18 of 32

potential in high-performance broadband photodetection and nonlinear optics, and the work may be beneficial for the scalable electronic application of the family of defective semiconductors.

METHODS Synthesis of Ga2S3 flakes: Schematic diagram and temperature curve for the synthesis of Ga2S3 flakes by CVD is shown in Figure 1a and S13, in which the precursor Ga2Cl4 (99.999%, Alfa) powder was located at a suitable location of quartz tube with the temperature close to 200 ℃, and S (99.9%, Alfa) power was heated up by heating belt, separately. Mica substrates were used to grow Ga2S3 triangular flakes and stacking mica-constructed confinement space were used to improve the sample quality. The growth process is as follows: firstly, the quartz tube was cleaned with high purity argon; then the stove was heated to 780°C following the program with 60 sccm Ar flow under atmospheric pressure; ultimately, the furnace was cooled down naturally after the reaction is over. Characterization and measurement: The Ga2S3 flakes are characterized by the following means: an optical microscope (BX51, OLYMPUS), a transmission electron microscope (Tecnai G2 F30, FEI) equipped with an energy-dispersive X-ray (EDX), an atomic force microscope (Dimension FastScan, Bruker), a X-ray photoelectron spectroscopy (ESCALab250, Thermo Fisher Scientific), X-Ray Diffraction (XRD-7000,Shimadzu), and a confocal Raman/PL system (Alpha 300R, WITec). The temperature-dependent CL spectra of the flakes were measured by a cathode-ray fluorescence spectrometer (MP 325, Horiba) equipped with a computer-controlled temperature stage (PP3005 Quorum Technologies Ltd).


Page 19 of 32 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 Nano

Second Harmonic Generation Measurements: An alpha 300RS+ Raman spectrometer equipped with a femtosecond laser was employed as the excitation source to measure SHG, which can generate a continuously adjustable wavelength from 340 nm to 1600 nm. The sample was excited by the output laser with a spot size of about 1.8 mm by 100× objective. Then to measure the SHG polarization, the sample was rotated by operating the sample stage manually with a step of 10°, while the polarization light was fixed. All the following experiments were performed at room temperature. Device Fabrication and Characterization: The two-probe electrodes are fabricated via electron beam lithography (FEI Quanta 650 SEM and Raith Elphy Plus) and thermal evaporation of Cr (10 nm) and Au (50 nm), respectively. To enhance the contact between the metal electrode and the sample, we applied annealing to the device at 250°C in a tube furnace under high purity Ar gas. For photodetection, a laser source (LDLS, EQ-1500, Energetiq) calibrated by a silicon photodiode was used as the incident light, then a probe station (CRX-6.5K, Lake Shore) equipped with a semiconductor test system (4200-SCS and 2400, Keithley) was employed to measure the electric properties. To record temporal response, a semiconductor characterization system (B1500A, Agilent) under modulated laser chopped at a frequency of 1 Hz was employed. All the above electrical and photoelectrical measurements were performed at room temperature and in the air. ASSOCIATED CONTENT Supporting Information The supporting information is available for free of charge on the ACS Publications website at DOI:


ACS Nano 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 20 of 32

OM images, AFM images and the corresponding height profile of the obtained Ga2S3 flakes; TEMSAED with larger area, high resolution TEM, TEM-EDX spectrum and the weight ratio and quantity ratio of the two elements, XPS and XRD characterization; The temperature dependence of Raman spectra for Ga2S3 flakes under 532 nm laser; Wavelength dependence of SHG intensity; OM image of the device; Output characteristics of Ga2S3 flakes with different thickness; The obtained carrier mobility of the twenty devices fabricated across the large area; comparison of the critical parameters for the devices based on γ-Ga2S3 flakes and other previously reported 2D materials; Temperature sequence for the synthesis of Ga2S3 flakes by CVD.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21828103 and 51727809), National Basic Research Program of China (2015CB932600), and the Fundamental Research Funds for the Central University (2015ZDTD038 and 2017KFKJXX007). Here we also want to thank the technical support from Analytical and Testing Center in Huazhong University of Science and Technology.


Page 21 of 32 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 Nano

REFERENCES 1 2 3

4 5 6 7 8 9

10 11 12 13 14 15 16

Guo, J.; Liu, Y.; Ma, Y.; Zhu, E.; Lee, S.; Lu, Z.; Zhao, Z.; Xu, C.; Lee, S. J.; Wu, H.; Kovnir, K.; Huang, Y.; Duan, X. Few-Layer GeAs Field-Effect Transistors and Infrared Photodetectors. Adv. Mater. 2018, 30, e1705934. Wang, T.; Andrews, K.; Bowman, A.; Hong, T.; Koehler, M.; Yan, J.; Mandrus, D.; Zhou, Z.; Xu, Y.-Q. High-Performance WSe2 Phototransistors with 2D/2D Ohmic Contacts. Nano Lett. 2018, 18, 2766-2771. Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Castro Neto, A. H.; Novoselov, K. S. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311-1314. Ni, Z.; Liu, Q.; Tang, K.; Zheng, J.; Zhou, J.; Qin, R.; Gao, Z.; Yu, D.; Lu, J. Tunable Bandgap in Silicene and Germanene. Nano Lett. 2012, 12, 113-118. Sun, Z.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat. Photonics 2016, 10, 227. Desai, S. B.; Madhvapathy, S. R.; Sachid, A. B.; Llinas, J. P.; Wang, Q.; Ahn, G. H.; Pitner, G.; Kim, M. J.; Bokor, J.; Hu, C.; Wong, H.-S. P.; Javey, A. MoS2 Transistors with 1Nanometer Gate Lengths. Science 2016, 354, 99-102. Xu, K.; Chen, D.; Yang, F.; Wang, Z.; Yin, L.; Wang, F.; Cheng, R.; Liu, K.; Xiong, J.; Liu, Q.; He, J. Sub-10 nm Nanopattern Architecture for 2D Material Field-Effect Transistors. Nano Lett. 2017, 17, 1065-1070. Kong, X.; Liu, Q.; Zhang, C.; Peng, Z.; Chen, Q. Elemental Two-Dimensional Nanosheets beyond Graphene. Chem. Soc. Rev. 2017, 46, 2127-2157. Qin, J.-K.; Qiu, G.; He, W.; Jian, J.; Si, M.-W.; Duan, Y.-Q.; Charnas, A.; Zemlyanov, D. Y.; Wang, H.-Y.; Shao, W.-Z.; Zhen, L.; Xu, C.-Y.; Ye, P. D. Epitaxial Growth of 1D Atomic Chain Based Se Nanoplates on Monolayer ReS2 for High-Performance Photodetectors. Adv. Funct. Mater. 2018, 28, 1806254. Zhou, X.; Hu, X.; Zhou, S.; Song, H.; Zhang, Q.; Pi, L.; Li, L.; Li, H.; Lu, J.; Zhai, T. Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity. Adv. Mater. 2018, 30, 1703286. Tan, C.; Cao, X.; Wu, X. J.; He, Q.; Yang, J.; Zhang, X.; Chen, J.; Zhao, W.; Han, S.; Nam, G. H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev. 2017, 117, 6225-6331. Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 10671075. Yu, Y.; Ran, M.; Zhou, S.; Wang, R.; Zhou, F.; Li, H.; Gan, L.; Zhu, M.; Zhai, T. PhaseEngineered Synthesis of Ultrathin Hexagonal and Monoclinic GaTe Flakes and Phase Transition Study. Adv. Funct. Mater. 2019, 29, 1901012. Huo, N.; Gupta, S.; Konstantatos, G. MoS2-HgTe Quantum Dot Hybrid Photodetectors beyond 2 µm. Adv. Mater. 2017, 29, 1606576. Hu, Z.; Wu, Z.; Han, C.; He, J.; Ni, Z.; Chen, W. Two-Dimensional Transition Metal Dichalcogenides: Interface and Defect Engineering. Chem. Soc. Rev. 2018, 47, 3100-3128. Chen, Y.; Huang, S.; Ji, X.; Adepalli, K.; Yin, K.; Ling, X.; Wang, X.; Xue, J.; Dresselhaus, M.; Kong, J.; Yildiz, B. Tuning Electronic Structure of Single Layer MoS2 through Defect


ACS Nano 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

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Page 22 of 32

and Interface Engineering. ACS Nano 2018, 12, 2569-2579. Wu, Z.; Zhao, W.; Jiang, J.; Zheng, T.; You, Y.; Lu, J.; Ni, Z. Defect Activated Photoluminescence in WSe2 Monolayer. J. Phys. Chem. C 2017, 121, 12294-12299. McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8, 2880-2888. Fang, H.; Hu, W. Photogating in Low Dimensional Photodetectors. Adv. Sci. 2017, 4, 1700323. Furchi, M. M.; Polyushkin, D. K.; Pospischil, A.; Mueller, T. Mechanisms of Photoconductivity in Atomically Thin MoS2. Nano Lett. 2014, 14, 6165-6170. Jiang, J.; Ling, C.; Xu, T.; Wang, W.; Niu, X.; Zafar, A.; Yan, Z.; Wang, X.; You, Y.; Sun, L.; Lu, J.; Wang, J.; Ni, Z. Defect Engineering for Modulating the Trap States in 2D Photoconductors. Adv. Mater. 2018, e1804332. Huang, W.; Gan, L.; Yang, H.; Zhou, N.; Wang, R.; Wu, W.; Li, H.; Ma, Y.; Zeng, H.; Zhai, T. Controlled Synthesis of Ultrathin 2D β-In2S3 with Broadband Photoresponse by Chemical Vapor Deposition. Adv. Funct. Mater. 2017, 27, 1702448. Liu, H. F.; Antwi, K. K. A.; Yakovlev, N. L.; Tan, H. R.; Ong, L. T.; Chua, S. J.; Chi, D. Z. Synthesis and Phase Evolutions in Layered Structure of Ga2S3 Semiconductor Thin Films on Epiready GaAs (111) Substrates. ACS Appl. Mater. Interfaces 2014, 6, 3501-3507. Goodyear, J.; Steigmann, G. A. The Crystal Structure of α-Ga2S3. Acta Cryst. 1963, 16, 946949. Zhang, M.-J.; Jiang, X.-M.; Zhou, L.-J.; Guo, G.-C. Two Phases of Ga2S3: Promising Infrared Second-Order Nonlinear Optical Materials with Very High Laser Induced Damage Thresholds. J. Mater. Chem. C 2013, 1, 4754-4760. Yoon, C.-S.; Medina, F. D.; Martinez, L.; Park, T.-Y.; Jin, M.-S.; Kim, W.-T. Blue Photoluminescence of α-Ga2S3 and α-Ga2S3: Fe2+ Single Crystals. Appl. Phys. Lett. 2003, 83, 1947-1949. Ho, C. H.; Chen, H. H. Optically Decomposed Near-Band-Edge Structure and Excitonic Transitions in Ga2S3. Sci. Rep. 2014, 4, 6143. Jones, C. Y.; Edwards, J. G. Observation of a Phase Transformation of Ga2S3 in a Quartz Effusion Cell above 1230 K by Means of Neutron Scattering. J. Phys. Chem. B 2001, 105, 2718-2724. Barbouth, N.; Berthier; Y., Oudar; J., Moison; J.-M. & Bensoussan, M. The First Steps of the Sulfurization of III-V Compounds. J. Electrochem. Soc.: Solid-State Sci. Technol. 1986, 133, 1663-1666. Hu, X.; Huang, P.; Jin, B.; Zhang, X.; Li, H.; Zhou, X.; Zhai, T. Halide-Induced Self-Limited Growth of Ultrathin Nonlayered Ge Flakes for High-Performance Phototransistors. J. Am. Chem. Soc. 2018, 140, 12909-12914. Guymont, M.; Tomas, A.; Pardo, M. P.; Guittard, M. Electron Microscope Study of γ-Ga2S3. Phys. Status Solidi A 1989, 113, K5-K7. Ji, J.; Song, X.; Liu, J.; Yan, Z.; Huo, C.; Zhang, S.; Su, M.; Liao, L.; Wang, W.; Ni, Z.; Hao, Y.; Zeng, H. Two-Dimensional Antimonene Single Crystals Grown by Van der Waals Epitaxy. Nat. Commun. 2016, 7, 13352. Liu, H.; Chi, D. Dispersive Growth and Laser-Induced Rippling of Large-Area Singlelayer MoS2 Nanosheets by CVD on c-Plane Sapphire Substrate. Sci. Rep. 2015, 5, 11756. Wang, J. I. J.; Yang, Y.; Chen, Y.-A.; Watanabe, K.; Taniguchi, T.; Churchill, H. O. H.; JarilloHerrero, P. Electronic Transport of Encapsulated Graphene and WSe2 Devices Fabricated by


Page 23 of 32 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 Nano

35 36 37

38 39 40 41 42 43 44 45 46 47 48 49 50 51

Pick-Up of Prepatterned hBN. Nano Lett. 2015, 15, 1898-1903. Lucazeau, G.; Leroy, J. Etude Vibrationnelle de α Ga2S3. Spectrochim. Acta, Part A 1978, 34, 29-32. Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D. From Bulk to Monolayer MoS2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22, 13851390. Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano 2014, 8, 12631272. Yang, Z.; Jie, W.; Mak, C.-H.; Lin, S.; Lin, H.; Yang, X.; Yan, F.; Lau, S. P.; Hao, J. WaferScale Synthesis of High-Quality Semiconducting Two-Dimensional Layered InSe with Broadband Photoresponse. ACS Nano 2017, 11, 4225-4236. Mudd, G. W.; Svatek, S. A.; Ren, T.; Patane, A.; Makarovsky, O.; Eaves, L.; Beton, P. H.; Kovalyuk, Z. D.; Lashkarev, G. V.; Kudrynskyi, Z. R.; Dmitriev, A. I. Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement. Adv. Mater. 2013, 25, 5714-5718. Zhang, S.; Yang, J.; Xu, R.; Wang, F.; Li, W.; Ghufran, M.; Zhang, Y.-W.; Yu, Z.; Zhang, G.; Qin, Q.; Lu, Y. Extraordinary Photoluminescence and Strong Temperature/Angle-Dependent Raman Responses in Few-Layer Phosphorene. ACS Nano 2014, 8, 9590-9596. Luo, S.; Qi, X.; Yao, H.; Ren, X.; Chen, Q.; Zhong, J. Temperature-Dependent Raman Responses of the Vapor-Deposited Tin Selenide Ultrathin Flakes. J. Phys. Chem. C 2017, 121, 4674-4679. Wang, F.; Li, L.; Huang, W.; Li, L.; Jin, B.; Li, H.; Zhai, T. Submillimeter 2D Bi2Se3 Flakes toward High-Performance Infrared Photodetection at Optical Communication Wavelength. Adv. Funct. Mater. 2018, 28, 1802707. Pawbake, A. S.; Pawar, M. S.; Jadkar, S. R.; Late, D. J. Large Area Chemical Vapor Deposition of Monolayer Transition Metal Dichalcogenides and Their Temperature Dependent Raman Spectroscopy Studies. Nanoscale 2016, 8, 3008-3018. Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S. W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T.; Ginsberg, N. S.; Wang, L.-W.; Alivisatos, A. P.; Yang, P. Atomically Thin Two-Dimensional Organic-Inorganic Hybrid Perovskites. Science 2015, 349, 1518-1521. Béguin, L.; Vernier, A.; Chicireanu, R.; Lahaye, T.; Browaeys, A. Direct Measurement of the Van der Waals Interaction between Two Rydberg Atoms. Phys. Rev. B 2013, 110, 263201. Ho, C.-H. Growth and Characterization of Near-Band-Edge Transitions in β-In2S3 Single Crystals. J. Cryst. Growth 2010, 312, 2718-2723. Li, H.; Zhu, X.; Tang, Z. K.; Zhang, X. H. Low-Temperature Photoluminescence Emission of Monolayer MoS2 on Diverse Substrates Grown by CVD. J. Lumin. 2018, 199, 210-215. Chen, Y.; Wen, W.; Zhu, Y.; Mao, N.; Feng, Q.; Zhang, M.; Hsu, H. P.; Zhang, J.; Huang, Y. S.; Xie, L. Temperature-Dependent Photoluminescence Emission and Raman Scattering from Mo1-xWxS2 Monolayers. Nanotechnology 2016, 27, 445705. Le, B. H.; Zhao, S.; Liu, X.; Woo, S. Y.; Botton, G. A.; Mi, Z. Controlled Coalescence of AlGaN Nanowire Arrays: An Architecture for Nearly Dislocation-Free Planar Ultraviolet Photonic Device Applications. Adv. Mater. 2016, 28, 8446-8454. Ishii, R.; Funato, M.; Kawakami, Y. Huge Electron-Hole Exchange Interaction in Aluminum Nitride. Phys. Rev. B 2013, 87, 161204. Autere, A.; Jussila, H.; Dai, Y.; Wang, Y.; Lipsanen, H.; Sun, Z. Nonlinear Optics with 2D


ACS Nano 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

52 53 54 55 56 57 58 59 60 61 62 63

64 65 66 67

Page 24 of 32

Layered Materials. Adv. Mater. 2018, 30, 1705963. Jiadong, Z.; Jia, S.; Qingsheng, Z.; Yu, C.; Lin, N.; Fucai, L.; Ting, Y.; Kazu, S.; Xinfeng, L.; Junhao, L.; Zheng, L. InSe Monolayer: Synthesis, Structure and Ultra-High SecondHarmonic Generation. 2D Mater. 2018, 5, 025019. Lin, X.; Liu, Y.; Wang, K.; Wei, C.; Zhang, W.; Yan, Y.; Li, Y. J.; Yao, J.; Zhao, Y. S. TwoDimensional Pyramid-Like WS2 Layered Structures for Highly Efficient Edge SecondHarmonic Generation. ACS Nano 2018, 12, 689-696. Shi, J.; Yu, P.; Liu, F.; He, P.; Wang, R.; Qin, L.; Zhou, J.; Li, X.; Zhou, J.; Sui, X.; Zhang, S.; Zhang, Y.; Zhang, Q.; Sum, T. C.; Qiu, X.; Liu, Z.; Liu, X. 3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device. Adv. Mater. 2017, 29, 1701486. Zhang, X. Q.; Lin, C. H.; Tseng, Y. W.; Huang, K. H.; Lee, Y. H. Synthesis of Lateral Heterostructures of Semiconducting Atomic Layers. Nano Lett. 2015, 15, 410-415. Zhao, Y.; Qiao, J.; Yu, P.; Hu, Z.; Lin, Z.; Lau, S. P.; Liu, Z.; Ji, W.; Chai, Y. Extraordinarily Strong Interlayer Interaction in 2D Layered PtS2. Adv. Mater. 2016, 28, 2399-2407. Zhao, Y.; Qiao, J.; Yu, Z.; Yu, P.; Xu, K.; Lau, S. P.; Zhou, W.; Liu, Z.; Wang, X.; Ji, W.; Chai, Y. High-Electron-Mobility and Air-Stable 2D Layered PtSe2 FETs. Adv. Mater. 2017, 29, 1604230. Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74-80. Aono, T.; Kase, K. Green Photoemission of α-Ga2S3 Crystals. Solid State Commun. 1992, 81, 303-305. Zhang, E.; Jin, Y. B.; Yuan, X.; Wang, W. Y.; Zhang, C.; Tang, L.; Liu, S. S.; Zhou, P.; Hu, W. D.; Xiu, F. X. ReS2-Based Field-Effect Transistors and Photodetectors. Adv. Funct. Mater. 2015, 25, 4076-4082. Wang, X.; Huang, L.; Peng, Y.; Huo, N.; Wu, K.; Xia, C.; Wei, Z.; Tongay, S.; Li, J. Enhanced Rectification, Transport Property and Photocurrent Generation of Multilayer ReSe2/MoS2 pn Heterojunctions. Nano Res. 2016, 9, 507-516. Sangwan, V. K.; Arnold, H. N.; Jariwala, D.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. LowFrequency Electronic Noise in Single-Layer MoS2 Transistors. Nano Lett. 2013, 13, 43514355. Chen, W.; Zhao, J.; Zhang, J.; Gu, L.; Yang, Z.; Li, X.; Yu, H.; Zhu, X.; Yang, R.; Shi, D.; Lin, X.; Guo, J.; Bai, X.; Zhang, G. Oxygen-Assisted Chemical Vapor Deposition Growth of Large Single-Crystal and High-Quality Monolayer MoS2. J. Am. Chem. Soc. 2015, 137, 15632-15635. Aji, A. S.; Solis-Fernandez, P.; Ji, H. G.; Fukuda, K.; Ago, H. High Mobility WS2 Transistors Realized by Multilayer Graphene Electrodes and Application to High Responsivity Flexible Photodetectors. Adv. Funct. Mater. 2017, 27, 1703448. Yoon, J.; Park, W.; Bae, G. Y.; Kim, Y.; Jang, H. S.; Hyun, Y.; Lim, S. K.; Kahng, Y. H.; Hong, W. K.; Lee, B. H.; Ko, H. C. Highly Flexible and Transparent Multilayer MoS2 Transistors with Graphene Electrodes. Small 2013, 9, 3295-3300. Yang, L.; Majumdar, K.; Liu, H.; Du, Y.; Wu, H.; Hatzistergos, M.; Hung, P. Y.; Tieckelmann, R.; Tsai, W.; Hobbs, C.; Ye, P. D. Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2. Nano Lett. 2014, 14, 6275-6280. Fang, H.; Tosun, M.; Seol, G.; Chang, T. C.; Takei, K.; Guo, J.; Javey, A. Degenerate nDoping of Few-Layer Transition Metal Dichalcogenides by Potassium. Nano Lett. 2013, 13, 1991-1995.


Page 25 of 32 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 Nano

68 Li, S.-L.; Tsukagoshi, K.; Orgiu, E.; Samorì, P. Charge Transport and Mobility Engineering in Two-Dimensional Transition Metal Chalcogenide Semiconductors. Chem. Soc. Rev. 2016, 45, 118-151. 69 Hafeez, M.; Gan, L.; Li, H.; Ma, Y.; Zhai, T. Chemical Vapor Deposition Synthesis of Ultrathin Hexagonal ReSe2 Flakes for Anisotropic Raman Property and Optoelectronic Application. Adv. Mater. 2016, 28, 8296-8301. 70 Zhou, Y.; Deng, B.; Zhou, Y.; Ren, X.; Yin, J.; Jin, C.; Liu, Z.; Peng, H. Low-Temperature Growth of Two-Dimensional Layered Chalcogenide Crystals on Liquid. Nano Lett. 2016, 16, 2103-2107. 71 Xiong, X.; Zhang, Q.; Zhou, X.; Jin, B.; Li, H.; Zhai, T. One-Step Synthesis of n-Type GaSe Nanoribbons and Their Excellent Performance in Photodetectors and Phototransistors. J. Phys. Chem. C 2016, 4, 7817-7823. 72 Huang, Y.; Zhuge, F.; Hou, J.; Lv, L.; Luo, P.; Zhou, N.; Gan, L.; Zhai, T. Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate Tunable Responsivity. ACS Nano 2018, 12, 4062-4073. 73 Yan, C.; Gan, L.; Zhou, X.; Guo, J.; Huang, W.; Huang, J.; Jin, B.; Xiong, J.; Zhai, T.; Li, Y. Space-Confined Chemical Vapor Deposition Synthesis of Ultrathin HfS2 Flakes for Optoelectronic Application. Adv. Funct. Mater. 2017, 27, 1702918. 74 Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional Van der Waals Semiconductors. Chem. Soc. Rev. 2015, 44, 3691-3718. 75 Zhou, X.; Gan, L.; Tian, W.; Zhang, Q.; Jin, S.; Li, H.; Bando, Y.; Golberg, D.; Zhai, T. Ultrathin SnSe2 Flakes Grown by Chemical Vapor Deposition for High-Performance Photodetectors. Adv. Mater. 2015, 27, 8035-8041. 76 Klee, V.; Preciado, E.; Barroso, D.; Nguyen, A. E.; Lee, C.; Erickson, K. J.; Triplett, M.; Davis, B.; Lu, I. H.; Bobek, S.; McKinley, J.; Martinez, J. P.; Mann, J.; Talin, A. A.; Bartels, L.; Léonard, F. Superlinear Composition-Dependent Photocurrent in CVD-Grown Monolayer MoS2(1–x)Se2x Alloy Devices. Nano Lett. 2015, 15, 2612-2619. 77 Yang, S.; Li, Y.; Wang, X.; Huo, N.; Xia, J.-B.; Li, S.-S.; Li, J. High Performance Few-Layer GaS Photodetector and Its Unique Photo-Response in Different Gas Environments. Nanoscale 2014, 6, 2582-2587. 78 Feng, W.; Wang, X.; Zhang, J.; Wang, L.; Zheng, W.; Hu, P.; Cao, W.; Yang, B. Synthesis of Two-Dimensional β-Ga2O3 Nanosheets for High-Performance Solar Blind Photodetectors. J. Mater. Chem. C 2014, 2, 3254-3259.


ACS Nano 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 26 of 32

Figure 1. Synthesis, crystal structure and characterization of Ga2S3 flakes by CVD. (a) Schematic diagram of preparing ultra-thin Ga2S3 flakes and crystal structure schematic diagram of the Ga2S3 defect semiconductor, in which one-third of the cation positions retain vacancies. (b) OM images of the grown Ga2S3 flakes on mica substrate and (c) the corresponding AFM image with the associated height profile. (d) Raman spectra of Ga2S3 with various thicknesses under a 532-nm excitation laser.


Page 27 of 32 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 Nano

Figure 2. TEM, XPS and XRD characterizations of the obtained Ga2S3 flakes. (a) Low resolution TEM image of the Ga2S3 flakes. (b) SAED pattern collected from the edge of the Ga2S3 flakes in (a). (c) An enlarged HRTEM image of the Ga2S3 flakes, with lattice spacing of ~0.368 nm. (d) FFT patterns obtained from the CrystalMaker, consistent with the Electron diffraction pattern in (b). (e) and (f) Ga and S elemental mapping of the as-grown the Ga2S3 flakes. (g-h) XPS characterization of the obtained Ga2S3 flakes. (i) XRD pattern of the obtained Ga2S3 flakes (blue line) and the standard diffraction pattern of Ga2S3 (pink line), which proved the as-synthesized Ga2S3 with facecentered cubic structure.


ACS Nano 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 28 of 32

Figure 3. (a) CL spectroscopy of Ga2S3 triangle flakes with different thickness at room temperature, respectively. (b) CL spectroscopy of Ga2S3 triangle flakes measured over the temperature range from 113 K to 293 K. (c) Temperature-dependent integrated CL intensity around 667 nm versus 1000/T fitted by the Arrhenius formula. (d) Energy level diagram of Ga2S3 crystal.


Page 29 of 32 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 Nano

Figure 4. SHG characterizations of the obtained Ga2S3 flakes under a 800-nm excitation laser. (a) Schematic diagram of SHG. (b) The polarization angle θ dependence of the SHG intensity, exhibiting a obviouse six-fold rotational symmetry (I = I0 sin 2 (3θ)). (c-d) The excitation powerdependent SHG intensity, with the coefficient fitted to be 2.06. (e) Thickness-dependent SHG intensity, with the laser power of 5.5 mW. (f) 2D SHG mapping of a triangle Ga2S3 flake. Note: the excitation power used in (b) and (e-f) are all 5.5 mW, and the Ga2S3 flake provided for (b-d) and (f) are the same sample with the thickness of 9 nm.


ACS Nano 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 30 of 32

Figure 5. (a) Output characteristics and (b) Transfer characteristics of FET based on the obtained 9-nm Ga2S3 with different bias at room temperature. (c) Transfer characteristics (in a logarithmic form) of FET based on Ga2S3 with various thicknesses and (d) the corresponding carrier mobility.


Page 31 of 32 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 Nano

Figure 6. Optoelectronic properties of Ga2S3 nanoflakes based photodetector. (a) The I-V of Ga2S3 photodetector under illumination with the incident wavelength of 350-900 nm and in dark at a bias voltage of 1 V, and the inset is a schematic diagram of the device. The corresponding (b) Rλ and Ion/Ioff (c) EQE and D* under different wavelengths. (d) The stable and repeatibility of the current under incident light with different wavelength. (e) Temporal response of the Ga2S3-based photodetectors measured by modulated light. (f) Comparison of I-V curves under 350-nm incident light with different intensities and in dark. (g) The photocurrent versus incident power density and the obtained fitting curve by the power law. (h) Comparison of I-V curves under 850-nm incident light with different intensities and in dark. (i) The photocurrent versus incident power density and and the obtained fitting curve by the power law.


ACS Nano 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 32 of 32

The table of contents: Non-layered defective semiconductor γ-Ga2S3 was introduced into 2D materimals family, which was synthesized by space-confined chemical vapor deposition. Impressively,

the

Ga

vacancies

or

S

vacancies

confirmed

by

low

temperature

cathodoluminescence lead to a broadband photoresponse. Simultaneously, the device exhibits excellent ultraviolet detection ability (Rλ = 61.3 A W-1, Ion/Ioff = 851 @350 nm) and relatively fast photoresponse (15 ms).

TOC Figure:


Nonlayered Two-Dimensional Defective Semiconductor Î³-Ga2S3

Recommend Documents