Effective Synthesis of Pb5S2I6 Crystals at Low Temperature for

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Effective Synthesis of Pb5S2I6 Crystals at Low Temperature for Fabrication of High Performance Photodetector Hongrui Wang, Guihuan Chen, Jianhang Xu, Yunpeng Xu, and Qing Yang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01358 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Crystal Growth & Design

Effective Synthesis of Pb5S2I6 Crystals at Low Temperature for Fabrication of High Performance Photodetector Hongrui Wang,†,‡,§, Guihuan Chen,†,‡,§, Jianhang Xu,# Yunpeng Xu‡ and Qing ∥

Yang*,†,‡,§,

†

∥

∥

Hefei National Laboratory of Physical Sciences at the Microscale, University of

Science and Technology of China, Hefei 230026, Anhui, P. R. China. ‡

Department of Chemistry, University of Science and Technology of China, Hefei

230026, Anhui, P. R. China. §

Laboratory of Nanomaterials for Energy Conversion, University of Science and

Technology of China, Hefei 230026, Anhui, P. R. China. ∥

Synergetic Innovation Center of Quantum Information & Quantum Physics,

University of Science and Technology of China, Hefei 230026, Anhui, P. R. China #

Department of Physics, College of Science and Technology, Temple University,

Philadelphia 19122-1801, Pennsylvania, the USA. E-mail: [email protected]; Fax: +86-551-63606266; Tel: +86-551-63600243

Abstract: Sulfoiodide crystal possesses promising properties and functionalities that would be used for technical applications in many areas. In this work, rod-like lead sulfoiodide (Pb5S2I6) crystals in length of about 3 mm have been fabricated via a rapid hydrothermal process at temperature low down to 160 °C for 10 h with the assistance of acid media (hydrochloride acid). Meanwhile, structure of Pb5S2I6 is characterized

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Crystal Growth & Design 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

and optical property of Pb5S2I6 is measured and investigated based on DFT calculations. In addition, an individual Pb5S2I6 crystal based photodetector is first constructed on SiO2/Si substrate, which sensitively responses to the stimulated sunlight especially in visible region with high responsivity (0.567 mA W−1), high detectivity (2.69 × 109 Jones), and high photoswitching ratio (high to 650). And also, the device presents short rise/decay time of less than 0.2 s and low noise equivalent power (NEP) (4.08 × 10−13 W Hz−1/2).

Keywords: lead sulfoiodide, single crystal, microrod, solvothermal, crystal growth, photodetector

■ Introduction Photoconductivity of semiconducting materials is attracting more and more attentions as a very important property which shows extensive applications in many aspects, especially in imaging techniques.1 Most of the photodetectors that have been reported to date are based on elemental silicon and binary compound semiconductors.2−6 Recently, lead compounds have gained considerable interest due to their narrow band gaps, size-tunable optical properties and great performances in infrared (IR) photodetectors, light-emitting diodes (LEDs) and photovoltaics,7−10 among which lead chalcogenides have been intensively studied for technical and potential applications in thermoelectrics, optoelectronics, biological labeling, fluorescence resonance energy transfer and photovoltaic devices.11−14 Lead sulfoiodide (Pb5S2I6), as a kind of ferroelectric semiconductors, possesses ACS Paragon Plus Environment

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high dielectric, piezoelectric and photoconducting properties15,16 that would be promising in diverse areas. The one-dimensional (1D) Pb5S2I6 crystals was firstly synthesized in 1968 via hydrothermal processes from PbS and PbI2 at least over 300 °C.17 Then, needle-like Pb5S2I6 crystals were fabricated at over 250 °C and 30 to 100 atm in 1989.18 Meanwhile, we modified a synthetic process to Pb5S2I6 crystals using PbCl2, (NH2)2CS and NaI as sources performed hydrothermally at 200 °C for above 20 h.19 After that, we have not found any related reports on the fabrication of Pb5S2I6 crystals by now. To the best of our knowledge, after it has been synthesized, the ferroelectric properties of this material have been studied primarily while its photoconducting properties remain unexplored to date. Motivated by the above background in addition to our recent growth of antimony sulfoiodide (SbSI) microrods,20 we developed a mild growth process to Pb5S2I6 crystals effectively in hydrochloric acid (HCl) solutions. In detail, the 1D Pb5S2I6 single crystals were successfully synthesized from PbCl2, (NH2)2CS and NH4I in the acidic environment at 160−200°C for 10 h by modifying the previous work.19,20 Meanwhile, an individual Pb5S2I6 microrod based photodetector was constructed using two ITO glass-electrodes on a SiO2/Si substrate and the photoresponse property of the microscale Pb5S2I6 single crystal was investigated for the first time. ■ Experimental Section Growth and Characterization of Pb5S2I6 Microrods. In the current synthesis, the analytical-pure chemicals mainly including PbCl2, (NH2)2CS, NH4I, hydrochloride acid (HCl) and ethanol were all purchased from commercial sources (Sinopharm Chemical Reagent Ltd). These chemicals are used as purchased and the synthetic



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procedure to Pb5S2I6 is adopted from the one for the SbSI crystals20 with modifications. Representatively, 1.0 mmol PbCl2 and (NH2)2CS with 2.5 mmol NH4I are added into 20 mL 0.8 mol L−1 HCl solution, and then loaded to an 30-mL autoclave after 15 min stirring for the preparation of the Pb5S2I6 single crystals at 160−200 °C referring to procedure for SbSI.20 After 10 h reaction, the autoclave is naturally cooled, and the samples are collected after fully washed by a mixture of ethanol and deionized water.3,20 The structure, purity, size, morphology, chemical status and optical property of the Pb5S2I6 crystals was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman and UV-Vis-NIR absorption spectrophotometers. The detailed instruments and detecting conditions are presented in Supporting Information. Computational Methodologies. We selected first-principle density functional theory (DFT) to calculate the band structure and density of states (DOS) of the Pb5S2I6 crystals by using the Vienna ab initio simulation package (VASP).21 Perdew-Burke-Ernzerhof revised for solids (PBEsol) exchange and correlational functional was applied.22 Energy cutoff was set to 500 eV, with a 10−7 eV total energy convergence threshold for structure relaxation. The model of the Pb5S2I6 crystal was constructed by a standard unit cell of Pb5S2I6 with periodic boundary conditions. Measurements

of

Photodetective

Performances.

The

photodetective

performances of the as-grown rod-like lead sulfoiodide crystals were investigated. Typically, the device was constructed by an individual Pb5S2I6 crystal on a SiO2/Si substrate to measure the photoresponse of simulated sunlight in the light of our recent


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work for the detection of Bi2S3 nanosheets and SbSI microscale crystals.3,20 In detail, a Pb5S2I6 single crystal in length of 3 mm and diameter of 30 μm was flattened and pressed on the substrate. For detection, we pressed two pieces of indium tin oxide (ITO) glass directly against the microscale rod-like crystal as electrodes and kept an interval of 1.5 mm in the middle according to our early work.3,20 For comparison, we also made a new photodetector with silver electrodes. In detail, we pressed another Pb5S2I6 microrod on SiO2 substrate and pasted silver paints (purchased from SPI Supplies) firmly on both ends of the Pb5S2I6 microrod, and then two filamentary silvers were fixed to the silver paints to form two silver electrodes. To evaluate the optoelectronic properties of the Pb5S2I6-based photodetectors, all photoresponsed I−V curves from the devices were recorded on a Shanghai CHI660E electrochemical workstation performed at room temperature (300 K).23 The incident light source ranging from 200 to 2500 nm was generated from a PLS-SXE300/PLS-SXE300UV xenon lamp22 without any filter to simulate the sunlight. ■ Results and Discussion The structure and purity of the as-prepared samples from the acidic process was first determined by XRD. As shown in Figure 1, the XRD patterns demonstrate that the Pb5S2I6 crystals were successfully synthesized in the present route. The crystals are in high purity since there are not any impurities detected in the samples. In detail, all diffraction peaks can be labeled to the monoclinic lead sulfoiodide (JCPDS Card, No. 23-0329, bottom) whether the crystals are well ground (top profile in red) or not (middle in blue). The space group of the monoclinic Pb5S2I6 crystals is C2/m, which is different from the one of Pnma for the orthorhombic SbSI crystals (JCPDS Card, No. ACS Paragon Plus Environment


88-0988).20 And also, the XRD characterizations clearly reveal that the crystals grow anisotropically with high quality. Notably, the diffraction peaks of (311), (113), (312), (114), (313), (314), (511), (020), and (515) observed in the ground pattern (top) cannot be easily detected in the pattern for the same samples unground (middle). Moreover, (201), (400) and (402) peaks (middle, unground) are much stronger than those of the ground (top) and the standard patterns (bottom). This result reveals the Pb5S2I6 crystals tend to preferentially grow along a b-zone axial direction other than the c-direction for the SbSI crystals.20

Figure 1. The XRD patterns for samples solvothermally fabricated in 0.8 mol L−1 hydrochloric acid solution at 200 °C for 10 h: after completely ground (top in red), without ground (middle in blue) and standard data of JCPDS Card No. 23-0329 for


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Pb5S2I6 (bottom). Figure 2a is a SEM image for a typical individual Pb5S2I6 crystal. Obviously, it is found that the crystal shows rod-like shape with a length of about 3 mm. Figure 2b is a magnified SEM image for the crystal shown in Figure 2a, and its width is observed to be about 50 μm. The microcrystal has smooth surface (Figure 2b). Figure 2c is a photograph for the as-grown microcrystals at a bulk scale, and it is found that the high-quality crystals display glittering metallic luster that is darker than that of SbSI.20 Although both Pb5S2I6 and SbSI crystals have similar size grown in the acidic route while their growth behavior, the crystal color and the surface texture are different. Typically, the Pb5S2I6 crystals tend to grow individually but the SbSI crystals to grow with bundles.20 In Figure 2d, the structure of the Pb5S2I6 crystals is schematically demonstrated as viewed along [010] direction (b-axis, left) with stacking of building blocks (right), different from the SbSI crystals grown along the c-direction.20



Figure 2. The as-grown Pb5S2I6 crystals: an individual with width of about 50 μm in low- (a) and high-magnified SEM images (b), collected samples photograph (c), and corresponding schematic crystal structure along b-axis direction (d). The chemical status and composition of the as-grown Pb5S2I6 crystals is determined by XPS. In Figure 3a, elements Pb, S, I, O and C can be easily detected in the Pb5S2I6 crystals based on the survey spectra. Typically, the binding energies of Pb 5d, 4f, 4d, 4p, S 2p and I 4d, 3d, 3p, MNN Auger in addition to O 1s and C 1s can be apparently detected in the spectra. Figure 3b demonstrates the close-up spectra for Pb 4f, from which the corresponding binding energies are observed at 137.9 and 142.7 eV, for Pb 4f7/2 and Pb 4f5/2, respectively. The detected binding energies of Pb 4f suggest that the chemical state of element Pb is +2 in the crystals, matching well with the data


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reported on Pb(II) in literature.24 Figures 3c and 3d illustrated the high-resolution XPS spectra of S 2p and I 3d, and it is seen that the binding energies appeared at 160.3, 161.3, 618.8 and 630.3 eV for S 2p3/2, S2p1/2, I 3d5/2 and I 3d3/2, respectively. However, the determined binding energies of the three elements in the Pb5S2I6 crystals are all smaller than those in the early report,19 while the binding energy of I 3d in Pb5S2I6 is very close to the one in SbSI.20

Figure 3. XPS spectra of the Pb5S2I6 crystals: (a) survey, (b) close-up Pb 4f, (c) S 2p, and (d) I 3d. The Raman spectrum of the Pb5S2I6 microscale rod-like crystals is shown in Figure 4. It is found that the Raman active modes of the microrods are generally close to the ones of the nanostructures reported in our early work,19 however, the intensity of



some optical vibrations is varied. In detail, the vibrations at 77 cm−1 and 98 cm−1 are so intense although they could be assigned to the Pb-I bindings, similar to the optical vibrations in PbI2.25 The increased vibration intensity at 78 cm−1 and 98 cm−1 is thought probably due to the surface and microstructure change of the crystals obtained in the system with high concentration of acid in addition to the differently preferential growth of the crystals in the route (Figure 1). The intense peak at 134 cm−1 and the weak peak at 214 cm−1 can be considered as the vibrations related to the Pb-I bindings in the crystals by referring the early investigations of PbS in literature.26,27

Figure 4. The Raman spectrum of the Pb5S2I6 crystals obtained at room temperature using 514.5 nm as excitation lines. In the current investigation, pure Pb5S2I6 crystal can be grown in the acidic hydrothermal synthetic route at temperatures low down to 160 °C, as determined by XRD in Supporting Information (Figure S1). It is noted that this is first report on the growth of the pure crystalline Pb5S2I6 compound at such low temperature. It is the lowest temperature reported on the growth of the Pb5S2I6 crystals to the best our


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knowledge, and the reason is that acidic condition would limit the generation of binary compounds of PbS and PbI2 in the reaction system on the basis of our recent study on the SbSI crystal growth.20 Figure S2 is the SEM image for the pure Pb5S2I6 crystals grown at 160 °C via keeping other reaction conditions constant, and the results indicate that the samples are still rod-like crystals but there are some short ones mixed in the samples compared with the optimized ones synthesized at 200 °C (Figure 2a-c). Meanwhile, experimental investigations suggest that the Pb5S2I6 crystals are difficult to grow for short reaction time in this acidic route compared with the SbSI crystals since it is found that the pure Pb5S2I6 crystals can only be obtained for about 10 h while pure SbSI just fabricated for 4 h.20 As shown in Figure S3, the samples synthesized for 4 h contain some impurities besides Pb5S2I6. The difficulties of growing Pb5S2I6 crystals for a short time are probably due to the slow dissolution of PbI2, PbClI and PbS kinetically in such acid environment.



Figure 5. The room-temperature absorption spectrum of Pb5S2I6 crystals obtained at 200 °C. The illustration in insert is the curve of (αhν)2/3 versus hν. Figure 5 is a typical room-temperature absorption spectrum for the Pb5S2I6 microcrystals synthesized at 200 °C, detecting in the range of 300-1200 nm. It is found that there are two slopes of the absorbance in the spectrum in the region of about 550-750 nm. It may be caused by the defects of the cations in the crystal which are formed when the reaction is carried out in the acidic solution.20 On the other hand, this phenomenon corresponds to the exciton transitions of Pb5S2I6 which originates from energy split of valence band and spin-orbital coupling according to the investigations on the WS2 nanoparticle thin film.28 As a direct gap material of Pb5S2I6, its band gap (Eg) is determined by the formula of (αhν)n = A(hν - Eg) based on the early reports, in which α, hν and A represent absorption coefficient, photon energy and constant, respectively, and n of 2/3 is suitable for the direct semiconductor29,30 in the present work. According to this formula, the direct gap is determined to be 1.73 eV for Pb5S2I6, which fits well with the DFT simulation as shown below while it is slight smaller than that of the indirect SbSI microrods (1.80 eV).20,31,32


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Figure 6. (a) Band structure of Pb5S2I6 microrods, where the Fermi energy is set to zero, and (b) calculated density of states (DOS) of the Pb5S2I6 microrods. To prove the consequence of the band gap of the Pb5S2I6 microrods, we carried out first-principle DFT calculations to study the band gap and density of states of this material. As can be seen, Figure 6a shows the band structure and Figure 6b shows the DOS of the Pb5S2I6 crystals. The band structure indicates that this material has direct gap at 1.68 eV at Y point, which is close to the result of UV-Vis absorption spectrum in Figure 5, thus certificates its credibility. The band gap of Pb5S2I6 is higher than either bulk PbS (0.41 eV) or PbS quantum dots (about 1.2 eV).33,34 The gap opening of the Pb5S2I6 microrods is caused by the existence of iodine in the samples, and the expanded band gap leads to a promising application in photoelectric devices under visible light for this sulfoiodide material, which can be used more widely than the same application of PbS under infrared ray to some extents.35



Figure 7. (a) The schematic of singular Pb5S2I6 crystal-based photodevice, (b) current-voltage (I−V) curves under light and dark conditions at Vbias = 3 V, (c) photoswitching current-time (I-t) curves over different input power of incident light illuminations (20 to 160 mW cm−2), (d) response/recovery time under 80 mW cm−2 illumination, (e) curve of photocurrent to incident light intensity, and (f) I-t curves of


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the photodetector by switching wavelengths at 60 mW cm−2 (total light input before filtering). The present Pb5S2I6 photodevice is designed and constructed on a Si/SiO2 substrate with two ITO glass-electrodes using the way shown by the schematic illustration (Figure 7a) adopted from our early work,20 and it is observed that there is a tight contact between the semiconducting Pb5S2I6 microrod and the ITO glass electrode, as seen in the SEM images for the device (Figure S4). In photodetection, the I-V curves of the Pb5S2I6 crystal-based photodetecting device are measured by switching a simulated sunlight on and off. The wavelengths of the incident light are ranging from 200 to 2500 nm, which are generated from Xe lamp without using any filter to simulate the sunlight during all measurements at room temperature (300 K). As seen in Figure 7b, the current-voltage (I-V) behavior suggests that there are Schottky contacts between Pb5S2I6 and ITO in the photodevice (at Vbias = 3 V), which are different from the Ohmic contacts between the ITO substrate and the singular SbSI crystal.20 The Schottky contacts are thought to be resulted from the work-function variation between ITO electrodes and Pb5S2I6 microrod, consistent with the early studies.30 This behavior indicates that the Pb5S2I6 crystals are potential for the fabrication of self-powered photodetectors.36-38 Meanwhile, the I-V curves under illumination condition are not symmetrical about the origin. The asymmetric and nonlinear behavior in I-V curves is mainly resulted from the following aspects. First, the different lengths of the two contacts between the Pb5S2I6 microrod and the electrodes result in the asymmetry responsed behavior. Second, bimolecular recombination of the material at higher powers would lead to the nonlinear I-V curves



via referring the investigations on the PbS quantum dot infrared photodetectors.33 As noted, the photoresponse speed is one of the important properties for a device, and the detector with fast response would be favorable for a high-speed photodetection. Figure 7c displays the photoswitching current versus time (I-t) over varied input power of incident light illuminations from 20 to 160 mW cm−2 at 1 V gate voltage. It is easy to figure out that more photogenerated carriers are excited when the detection is performed under higher light intensity illumination, which contributes to the high photocurrent accordingly.39 Meanwhile, the observed on-off photoswitching with sharp rise and decay implies that the device responses fast that would be favorable for high-speed photodetection (Figure 7c). Figure 7d illustrates a response/recovery time for the photodevice, typically performed at 80 mW cm−2, and it is found that its response time is shorter than 0.2 s. Some more detailed performances of the device are listed in Table 1, measured under different light illuminations (20 - 160 mW cm−2). Figure 7e illustrates a corresponding dependence of photocurrent and power of illumination (photocurrent versus light intensity), which should be in accordance with a simple power law of Ip ~ Pθ. In the present condition, the fitting result is determined to be Ip ~ P0.63 with a sub-liner behavior (θ of 0.63 < 1), resulted from a complicated process of electron-hole generation, trapping, and recombination in Pb5S2I6 according to the early investigations.33,40 Besides the high sensitivity to light intensity (Figure 7c) and high-speed response (Figure 7c, 7d and Table 1), the Pb5S2I6-based photodetector also exhibits excellent wavelength sensitivity at the same time. As is shown in Figure 7f, the photodetector


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has an obvious response to the visible light ranging from 420 to 650 nm while the corresponding photocurrent is varied with the change of wavelength under a fixed light intensity of 60 mW cm−2 (the same total incident light power before filtering). In detail, it has a higher photocurrent at 420, 475, 520 and 550 nm than that at 650 nm, which is in a line with the results in Figure 5. As observed, the highest current shows at 520 nm for the Pb5S2I6-based photodetector, which is different from the SbSI device at 650 nm.20 For comparing the effect of electrodes, we have also measured the photodetective performances of the Pb5S2I6-based photodetector made by silver electrodes and it is found that the photodetector with silver electrodes has excellent sensitivity (Figure S5); however, its response performance (Figure S6) is not as fast as the one via directly pressed on the ITO glass (Figure 7). In addition to the sensitivity of the device, it is well known that the photoswitching behavior (Ilight/Idark or Ion/Ioff ratio) is also important for a device. When the light intensity illumination is higher, there will be generally more photogenerated carriers in the device, which contributes to a higher photocurrent and on-off ratio. Moreover, higher

on-off

ratio

of

the

current

means

higher

efficiency

carrier

separation/generation20 for the device under the incident light illumination. Meanwhile, the photoswitching time including response (ton) and recovery time (toff) is also investigated and summarized in Table 1. As is shown in the table, the on-off ratio of the device can reach as high as 650 at 160 mW cm−2, which is higher than those devices made of PbS quantum dots.33 And also, the fast response/recovery time (

Effective Synthesis of Pb5S2I6 Crystals at Low Temperature for

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