PbI2 Nanosheets for Photodetectors via the Facile Cooling Thermal

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C: Physical Processes in Nanomaterials and Nanostructures

PbI2 Nanosheets for Photodetectors via the Facile Cooling Thermal Supersaturation Solution Method Rendong Wang, Shutao Li, Pengfei Wang, Junshan Xiu, Gongxiang Wei, Meiling Sun, Zhao Li, Yunyan Liu, and Mianzeng Zhong J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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The Journal of Physical Chemistry

PbI2 Nanosheets for Photodetectors via the Facile Cooling Thermal Supersaturation Solution Method Rendong Wang,† Shutao Li,† Pengfei Wang,† Junshan Xiu,† Gongxiang Wei,† Meiling Sun,† Zhao Li,† Yunyan Liu,*† Mianzeng Zhong*‡ †School

of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo, Shandong, 255049, China ‡Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, China

Abstract: As a potential candidate for modern optoelectronics, two-dimensional (2D) PbI2 nanosheets have received much attention. In this paper, we report a simple, and low-cost cooling supersaturated aqueous solution method for the controllable morphology growth of high quality PbI2 single crystals. The fluorescence kinetics of PbI2 nanosheets were investigated by the temporally and spatially resolved spectra measurements. The body center region of as-prepared PbI2 nanosheets showed higher density of defect states than that at the edges or corners. In addition, the as-prepared PbI2 nanosheets based photodetectors exhibit stable and efficient performance, including large on/off ratio (1.371×103), high light response (40 mA/W) and high detection sensitivity (3.31 × 1010 J). 1. Introduction Typical layered lead iodide (PbI2) crystal has the (I-Pb-I) hexagonal close packed structure, and each layer consists of ionic bonded I-Pb-I repeating unit and adjacent layers are weakly held by the van der Waals force.1-3 The large average atomic number makes PbI2 an excellent room temperature nuclear radiation detector and x-ray imaging device material.4-6 Due to its high resistivity and large carrier mobility, the detector made of PbI2 crystal can maintain low leakage current even under high 1

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bias voltage, good energy resolution, high detection efficiency and excellent stability.7,8 Many studies showed that the bulk PbI2 crystal is a direct band gap material (Eg = 2.2-2.55eV).9-11 Meanwhile, theoretical calculations indicate that the Eg of low-dimensional PbI2 increases with the decreasing number of layers, and it becomes an indirect band gap semiconductor with Eg = 3.72 eV when decreasing to one or two layers.12,13 Due to these thickness-dependent electronic properties, PbI2 has potential application prospects in the fields of optoelectronic devices such as sensors, lasers, photo-detectors and other advanced electronics.8,14-16 Moreover, lead-halide perovskites synthesized from the precursor of PbI2 have received great attention in the field of photodetector, solar cell, and field effect transistors.3,17-20 In general, the nucleation of PbI2 is highly anisotropic, and is easy to form low-dimensional nanostructures perpendicular to the c-axis under appropriate synthesis conditions. The low dimensional PbI2 nanostructures of nanosheets, nanowires and nanobelts, are well compatible with the traditional micro-nano fabrication processing, and have broad application prospects. Recently, many physical and chemical approaches including melt method,21,22 mechanical exfoliation,2,23 sol-gel24 and solution method,15,

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have been successfully used to synthesize the

micro- and nano-structured PbI2 crystals with outstanding performances. However, high temperature applied in the melt method results in polytype admixture and defects in crystals. The application of exfoliation method is limited due to incomplete surface morphology, uneven distribution and low productivity. Sol-gel and solution method is easy to introduce impurities into PbI2 and is time consuming. The vapor deposition is 2


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considered to be a good method for preparing high-quality nanostructured PbI2.1,26,27 For example, Yang et al.28 reported two-step vapor deposition of large-size PbI2 nanobelts which based photodetectors showed high responsivity. Wang and coworkers achieved highly uniform PbI2 flakes by a space confined physical vapor deposition.29 Nevertheless, vapor deposition demand relatively complicated processes and the instability of kinetics easily cause crystal defects. Despite the PbI2 nanostructures and photodetectors obtained great achievement, studies on the facile synthesize processes of homogenous nanostructures PbI2 and controllable morphology for facile fabrication of photodetectors still consist of great challenges. In this work, we aim to develop a simple and controllable cooling hot supersaturated aqueous solution method to controllable prepare PbI2 single crystals with uniform distribution at room temperature. Three typical different morphologies of PbI2 crystals (pyramid, hexagonal and truncated triangular shapes) were obtained by controlling the temperature of the aqueous solution. The nucleation rate, number and evolution of the crystal shape are determined by the degree of supersaturation of the solution. Furthermore, the fluorescence kinetics of as-prepared PbI2 nanosheets by spatially and temporally resolved PL measurements gives direct evidence of the distinct recombination processes and defect -state in the edge and bulk region of the nanosheet. The fabricated PbI2 nanosheet photodetectors show a small dark current of 0.14 nA and an excellent on/off current ratio of 1.371 × 103. The rise and decay response time are about 161.7 and 192.1 ms, respectively. We provide a high efficiency, low cost route for the fabrication of high quality PbI2 crystals; this method 3



may has the potential in the preparation of other 2D materials and optoelectronic devices, as well as for the research of halogen-lead perovskites and devices. 2. Experimental The substrates were ultrasonic cleaned in isopropyl alcohol (99.7%, AR), acetone (99.5%, AR), and alcohol (99.7%, AR) in sequence. Supersaturated aqueous solution was prepared by dissolving 20 mg PbI2 powder (99%, Aldrich) in 10 mL deionized water for heating. 30 μL of PbI2 supersaturated solution was taken out with a pipette and dropped on the substrate, and the PbI2 nanosheets was obtained after natural evaporation of water at room temperature. By controlling the temperature range of the supersaturated solution from 35 °C to 95 °C, various morphology of PbI2 nanosheets were obtained. The substrates were washed successively with acetone, ethanol and deionized water. The Ti / Au interdigital electrode was vapor deposited on the substrate to a thickness of 250 nm using a lift-off process and electron beam evaporation. The interdigital electrode fingers were 2 mm long, the finger width was 40 μm, and the finger pitch was 65 μm. In order to reduce the interface contact resistance and form a good ohmic contact, the sample was kept at 200 °C in Ar for 30 min. The morphology and element distribution of the samples were characterized using field emission scanning electron microscope (SEM, FEI Sirion 200) and energy dispersive spectrometer (EDS). The phase composition and crystallographic structure of the as-prepared materials were examined by X-ray diffraction (XRD, Bruker AXS D8 Advance) and high-resolution transmission electron microscopy (HRTEM; 4


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JEM-2100, JEOL). UV-Vis absorption spectra were measured by spectrophotometer testing system (UV-3600Plus, SHIMADZU). The Raman spectrum was measured from 50 to 400 cm-1 using a laser with the wavelength of 532 nm. The PL spectrum was measured by a spectrofluorometer (PL, F-4500) at the excitation wavelength of 405 nm. Steady-state and time-resolved photoluminescence (TRPL) was collected by confocal optical microscopy (Nanofiner FLEX2, Tokyo Instrument Inc.) combined with CCD (DU420A-OE, ANDOR) and time-dependent single photon counting module (SPC-150, Becker & Hickl). The 500 nm excitation light was generated by frequency doubling of a femtosecond titanium sapphire laser (Maitai HP, Spectra-Physics) at 80 MHz. The current-voltage (I-V) characteristics of the devices were recorded using a transistor test system (Agilent-B2902). The optoelectronic properties of the prepared photodetectors were obtained by using a 450 nm laser irradiation at 5 V bias. 3. Results and discussion The diagram of growth process is shown in Fig. S1. When the hot solution drops onto the cold substrates, it rapidly cools down. The temperature of the substrate is not intentionally heated at room temperature. The solution temperature is kept unchanged by putting it in a sealed container on the thermostatic heater. PbI2 nucleates on the substrate and grows into nanosheets. It takes only a few minutes to complete the growth of the nanosheets at the high temperature and supersaturation conditions. The lower the temperature of the supersaturated solution, the longer time (≥20 min) it takes to complete the growth. Fig. 1 (a-f) are the scanning electron microscope (SEM) 5



Fig. 1 SEM images of the PbI2 nanosheets at 35 ℃, 80 ℃ and 90 ℃, respectively. (a) and (d) is pyramidal triangle, (b) and (e) is hexagonal, and (c) and (f) is truncated triangular of PbI2 nanosheets.

images of three different morphologies (pyramid, hexagonal and truncated triangular shapes) of PbI2 crystals prepared at different temperatures. The optical microscopies (OM) images of PbI2 crystals prepared under different temperatures are shown in Fig. S2. It can be seen that the crystals have uniform and regular shapes and even size distributions at various temperatures. The solution temperature determines the degree of supersaturation. This is critical for the nucleation rate, nucleation number and morphology of the PbI2 crystals. At the temperature of 35 °C, the morphology of PbI2 is pyramidal as shown in Fig. 1a, 1d and Fig. S2a. A small amount of these pyramidal-like crystals also can be occasionally observed at the edge of the droplet at higher solution temperatures. Most of the PbI2 crystals are hexagonal. They formed in wide solution temperature range of 40-80 °C (Fig. S1 and Fig. S2 (b-g)). The largest hexagonal nanosheets with the lateral size about 500 μm are obtained at the solution temperature of 40 °C. In the temperature range of 40 °C-70 °C, the nucleation rate increased with the increase of 6


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the supersaturation, leading to the formation of hexagonal nanosheet with smaller size. The mean lateral size and thickness of the nanosheet grown from the solution temperature of 60 °C is about 80 μm and 100 nm, respectively (Fig. S2 (e) and Fig. S3 (a), (b)). This is consistent with the nucleation theory27-29 that the nucleation rate exponentially grows with the increase of supersaturation.30-32 The relationship between the nucleation rate and the supersaturation is derived as: 𝑉1 ∝ 𝑒𝑥𝑝[ ― 1/𝜎2 ], where V1 is the crystal nucleation rate and σ is the degree of supersaturation of the solution. However, after the temperature higher than 70 °C, the number of nucleation, the lateral size and thickness of the crystals no longer change significantly (Fig. S2 (g-i)). Fig. 1b and 1e are SEM images of PbI2 nanosheets grown at the temperature of 80 °C. It can be seen that the lateral size and thickness of nanosheet is about 20 μm and 600 nm, respectively (Fig. S3 (c), (d)). When the temperature increases to 85 - 95 °C, most of the crystals become truncated triangles (Fig. 1 (c), (f) and S2 (h, i)), but the nucleation number, lateral size and thickness of these nanosheets are similar to the ones grown at 80 °C (Fig. S3 (e), (f)). Crystal growth is a non-equilibrium process and the supersaturation degree is a dominate factor in controlling the morphology of crystal. The change in the morphology of the crystal is derived from the difference of growth rate of each crystal face. The sheet-like structure indicates the anisotropic crystal growth of PbI2. Theoretical calculations show that the low index (001) plane of PbI2 has surface energy of 0.428 J m-2, this is much lower than the other crystal surfaces.12 Low index 7



plane with low surface energy is beneficial to nucleation probability.1 The lack of solute of low supersaturation degree preferentially supplies the growth along the low index (001) plane other than nucleation for forming a new layer. Thus, the growth rate of PbI2 along low index (001) plane perpendicular to the c-axis is much higher than that along the c-axis, which leads to the formation of nanosheet with larger width-to-thickness ratios. As the increase of supersaturation, the two-dimensional nucleation rate in the low index (001) plane increases, and as a result, it speeds the forming of a new layer. The improvement of the layered growth along c-axis results in the smaller width-to-thickness ratios of the nanosheet of higher degree of supersaturation. As supersaturation increases further, the rapid growth of some high-index crystal faces along its vertical direction appear. According to Brava's law, the fast-growing crystal faces will shrink or even disappear, and this will finally lead to the formation of truncated triangles crystals. OM images in Fig. S2 indicate that most of the nanosheets are uniform in color, implying that the surface is relatively flat. Instead, a fraction of PbI2 nanosheets show a distinct color interference pattern (Fig. S4 (a)). Fig. S3 and Fig. S4 (b, c) exihibit the atomic force microscopy (AFM) images and the corresponding 1D cross-section scans of surface profile of these different surface morphologies of the PbI2 nanosheets. The radial color change of the pattern suggests a symmetry thickness variation within the nanosheets. This interference pattern has also been found in the zinc hydroxyl sulfate and SnS2 nanoplates which contain a screw-dislocation-driven spiral growth hillock on the surface.30,31,33 The screw-dislocation can affect the energy band and carrier 8


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recombination in the crystal, or to be the center of carriers scattering. In order to further study the size distribution of the three topographies, the nucleation number and size of the typical PbI2 nanosheets prepared at the temperature of 35 °C, 80 °C, and 95 °C were statistically analyzed in the bar graph as shown in Fig. S5. Hundreds of isolated PbI2 nanosheets range from 10 μm to 50 μm, which are enough for future large-scale device fabrication. The size of PbI2 nanosheets satisfy the upper limit on the spatial resolution and pixel size for x-ray imaging and thin film transistor (TFT) pixel arrays. In addition to the supersaturation, the substrate also affects the nucleation rate, size and morphology of PbI2. Since the surface of the two-dimensional material lacks of dangling bonds and the growth is no selectivity for the substrate. Thus as-grown nanosheet can be easily formed on various substrates, such as FTO, silicon wafers, flexible plastics and surface-charged adhesion slides, as shown in the optical images of Fig. S6. But the nanosheets prepared under the same conditions have different distributions and sizes on different substrates. Fig. 2a shows the X-ray diffraction (XRD) results for typical hexagonal PbI2 nanosheets grown on glass substrates. The XRD pattern of PbI2 shows four strong and sharp diffraction peaks, (001) (002) (003) and (004), which indicate good crystallization with lamellar stacking of the I–Pb–I sandwich layer perpendicular to the substrate along the c-axis. All the diffraction peaks can be well indexed to the hexagonal lattice of PbI2 (JCPDS No. 07-0235). The calculated unit cell constants a = 4.564 Å and c = 6.978 Å are in good agreement with the previous literature values,2 belonging to the space group: P3m1 (164). The Raman spectrum of the PbI2 sample 9



Fig. 2 (a) XRD pattern of the PbI2 nanosheet. (b) Raman spectrum of the as-grown PbI2. (c) UV-vis spectrum of PbI2 nanosheets. Inset: Corresponding K-M transformation of UV-Vis absorption spectra to the band gap. (d) Room temperature photoluminescence spectrum of a PbI2 sample.

excited by 532 nm wavelength laser is shown in Fig. 2b. Five Raman peaks were observed in the range of 50-400 cm-1, which were 73, 95, 111, 167, and 214 cm-1, respectively. The observed 95 and 73 cm-1 are the two main Raman peaks, assigned to the A1g vibration mode of symmetrically stretched and the Eg vibration mode of the layered shear motion, respectively.34,

35

The peak 111 cm-1 belongs to the A2u

vibration mode.12 The peak of 167 and 214 cm-1 are also observed in the mechanical exfoliation PbI2 single crystal nanosheet, and these two peaks are obvious only in thicker nanosheets.2 The sharp Raman peaks further supports the results of XRD that the prepared samples are all in a highly crystalline state. UV-visible absorption spectroscopy (Fig. 2c) shows that the absorption edge of PbI2 is at about 531 nm. 10


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According

to

Kubelka-Munk

(K-M)

conversion

of

UV-visible

absorption

spectroscopy: [F(R) hν] 2 = hv - Eg, where F(R) is a diffuse reflectance spectrum, the optical bandgap of PbI2 is calculated to be about 2.31 eV (Inset of Fig. 2 (c)), which is consistent with previous reports.11,29 Fig. 2d shows the room temperature photoluminescence (PL) spectra of PbI2 nanosheets. The strong PL peak at 476 nm and the other three weak peaks at 496 nm, 512 nm and 571 nm were observed, which correspond to the emission peak energy of 2.6 (EF peak), 2.5 (EB peak), 2.41 (D peak) and 2.17 (G peak), respectively.36 The strongest EF peak and the relatively strong EB peak may be attributed to free excitons and bound excitons, respectively.37 It is generally believed that free excitons are related to the crystallinity of the crystal. The stronger the peak intensity of the free exciton, the better the crystallinity of the material, and the bound exciton luminescence peak is opposite.21,37 Compared with the previous literature, the nanosheets fabricated by our method show high crystal quality.37 In addition, the D peak comes from the donor acceptor pair, and the G peak is related to surface roughness or surface defects.21,36-38 Energy dispersive spectrometer (EDS) test was performed at three points A, B and C at the center, edge and corner position of hexagonal PbI2 nanosheets, respectively (Fig. S7). Only the peaks of the Pb and I elements can be found in Figure S7, and there is no evidence of O element. The ratio of the atoms of the Pb and I elements in different regions is about 32% and 68%, respectively. EDS exhibits that the signals of Pb and I elements at A, B and C are almost the same, with an atomic ratio of ≈ 1:2, 11



Fig. 3 (a-c) Mapping profiles, (d) transmission electron microscopy (TEM), (e) the high resolution transmission electron microscopy (HRTEM) and (f) selected-area electron diffraction images (SAED) of the PbI2 nanosheet.

agreeing well with the stoichiometric ratio of high purity PbI2 crystals. The corresponding elemental mapping images of Pb and I (Fig. 3 (a-c)) further confirm the highly uniform distribution of Pb and I elements. The hexagonal structures of the PbI2 nanosheet crystals were supported by transmission electron microscope (TEM). Fig. 3d is the low resolution TEM image of hexagonal PbI2 nanosheets. High-resolution TEM (HRTEM) of the PbI2 nanosheet in Fig. 3e reveals that the interplanar distance along two different directions are 3.83 Å and 2.29 Å with an intersection angle of 30°, which are attributed to the {001} family of planes. Fig. 3f is the selected-area electron diffraction (SAED) recorded from the PbI2 nanosheet. The spot pattern demonstrates the high quality hexagonal single crystal structure of as-grown PbI2 nanosheet. The crystallographic orientation 12


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Fig. 4 (a) Optical micrograph of PbI2 nanosheet. (b) Fluorescence intensity of a single PbI2 nanosheet. (c) Fluorescence lifetime mapping of a fluorescence microscope. (d) Fluorescence decay profiles recovered from different region of fluorescence lifetime image.

demonstrated in the HRTEM and SAED match the results that the PbI2 crystals are stacked along the [001] zone-axis as revealed in XRD. The diversity of micro-region fluorescence decay behavior, such as the recombination process in the interfaces and bulk region, can be distinguished by the combination of temporally and spatially resolved spectra measurements. To further investigate the fluorescence kinetics of as-prepared PbI2 nanosheets, we selected a typical hexagonal PbI2 nanosheets (Fig. 4a) for 2D and time-resolved PL 13



measurements. Fig. 4b and 4c show the corresponding fluorescence intensity distribution and fluorescence lifetime mapping. Fig. 4b shows that the nanosheets have an inhomogeneous fluorescence intensity distribution. The different color of blue, green, and orange represent three typical regions of the center, edge, and top corner of the nanosheet, labeled A, B, and C, respectively. The fluorescence intensity at positions B and C is stronger than that of the center position at A. Fig. 4d shows the corresponding fluorescence decay curves of three different positions A, B and C extracted from the mapping of Fig. 4c. A double exponential function is used to fit the fluorescence decay curve and the calculated fluorescence lifetime and its corresponding proportion are listed in Table 1. The results in Table 1 exhibit that the fluorescence lifetime gradually decreases from the corner region to the center of the sheet. Besides, all fluorescence decay curves consist of two parts, a fast component process of τ1 and a slow component process of τ2. The averaging weighted lifetime is obtained by 𝜏𝑎𝑣𝑒 = 𝐴1𝜏1 + 𝐴2𝜏2 , and are listed in Table 1. Compared with the regions B and C, the ratio of the short-lifetime components in the region A is the highest, reaching 88.72 %, while the short-lifetime components in the regions B and C are only about 70 % and 60 %. This difference further illustrates the inhomogeneous distribution of fluorescence lifetime in the PbI2 nanosheet. Previous studies have shown that when the material contains a large number of defects and grain boundaries, non-radiation deactivation process caused by these trap states defect leads to the short carrier lifetime.39-41 From the weak G peak as revealed in the above photoluminescence spectrum, it can be seen that the as-grown PbI2 nanosheet may 14


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also have the same defects, resulting in uneven distribution of fluorescence lifetime. The shorter lifetime and high short-lifetime components indicate more carriers decay because of the higher defect-state density in the body center of PbI2 nanosheet. Regions B and C exhibit longer fluorescence lifetimes and a higher proportion of long-lifetime components than region A, owning to fewer defects and higher crystal quality at the edge and corner position. The long-lifetime fluorescence decay has been attributed to the band-edge exciton recombination of electrons and holes. Table 1

Fluorescence decay time, relative intensity and weighted average lifetime of three

typical regions in PbI2 nanosheet. A

B

C

τ1(ps)

60.43 (88.82%)

65.08 (70.56%)

154.99 (59.89%)

τ2(ps)

204.9 (11.18%)

195.3 (29.44%)

412.3 (40.11%)

τave(ps)

76.58

103.42

258.21

For further investigating the photoelectric properties of PbI2 nanosheets, MSM (metal-semiconductor-metal) photoconductive detector was prepared. Fig. 5a shows the current-voltage (IV) characteristics of PbI2 nanosheets at different power incident light. The linear and symmetric features under different incident light intensity show good ohmic contact between the gold electrode and the PbI2 nanosheet. The dependence of photocurrent on the incident light intensity can be represented by a power function of I = αPβ, as shown in Fig. 5b, where I is the photocurrent, α is the proportional constant, P is the optical intensity, β is an exponent and the ideal value is 15



Fig. 5 (a) I-V characteristics of the PbI2 nanosheet based photodetector at different incident light powers of a 450 nm wavelength laser. (b) Dependence of photocurrent and light power density at a bias voltage of 5V. Inset: microscope image of the as-fabricated photodetector. (c) Time-dependent photocurrent measurement under irradiation of the 450 nm laser with a light intensity of 60.75 mW cm-2 and a bias voltage of 5 V. (d) Photocurrent rise and decay time.

1, which indicating the utilization of light. A power dependence of I = 7.969 P0.79 was obtained, where β value was 0.79, relating a complex process of trap states, carriers recombination within the PbI2 nanosheet, this has also been observed in other semiconductor photodetectors.42,43 Moreover, the β is higher than the previous literature,29 showing the superior photocurrent capability and crystallinity of PbI2 nanosheets. To detect the stability of the PbI2 nanosheet, the time resolved photoresponse was measured at a bias of 5 V and 60.75 mW cm-2 power illumination. As shown in Fig. 5c, the photocurrent has been maintained a stable photocurrent for more than 4 minutes. Very low dark current of about 0.14 nA is obtained. When the 16


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light is turned on, the photocurrent of the detector rapidly increases to a saturation value of 0.192 μA with a high on/off ratio of 1.371×103. The rise and decay time are also important parameters for photodetector. The rise time is defined as the time required for dark current to reach (1- 1/e) of the maximum value of photocurrent, and decay time refers to the time needed to drop to 1/e of the maximum photocurrent. The PbI2 nanosheet photodetectors exhibit a fast response speed, and the calculated rise and decay time are of approximately 161.7 and 192.1 ms (as shown in Fig. 5d). The light responsivity (R) and detection sensitivity (D*) are critical for studying the sensitivity of photodetectors. They are calculated by R = Iph / PS and D * = R × (S / 2eId)

1/2,

where Iph is the difference between the light and dark currents, S is the

effective area, P is the irradiance of the incident light, e is the basic electronic charge, and Id is the dark current.1,15 A high photoresponse of 40 mA/W and a detection sensitivity of 3.31×1010 J were obtained under the illumination of 450 nm at a bias of 5 V. We noticed that carrier trapping and recombination due to the presence of defect states in the nanosheet inevitably affect the device properties such as sensitivity, on/off ratio and response time. Whatever, the detector exhibits excellent properties performance comparable to those recently reported literature (Table 2),10,28,45 illustrating high quality of the PbI2 nanosheet.

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Table 2 Comparison of various figures of merit of different representative PbI2 based photodetectors

Morphology

Light

bias

ON/OFF

rise time

decay time

Responsivity

Ref

ratio Nanosheet

450nm

5V

103

161.7ms

192.1ms

40 mA/W

This work

Nanosheet

405nm

1V

2000

-

-

1.3 mA/W

10

Nanosheet

450nm

1.9V

102

55μs

110μs

0.1 mA/W

44

Nanowire

450nm

3V

48

0.79s

0.91s

100 mA/W

45

single crystal

450nm

5V

103

323μs

520μs

180 mA/W

15

Nanobelt

445nm

5V

103

425ms

41ms

13mA/W

28

4. Conclusions Uniform size and controllable morphology of PbI2 nanosheets were prepared by a facile cooling supersaturated aqueous solution method. It is shown that the nucleation and morphology of the PbI2 nanosheets can be controlled by varying the degree of supersaturated solution. High quality of as-prepared PbI2 single crystals nanosheet is verified by XRD, TEM and Raman spectra. Weak G peak associated with defect state photoluminescence is demonstrated in PL test. Spatially and temporally resolved fluorescence measurements further reveal the inhomogeneous defect state distributions and recombination processes in different micro-regions of the nanosheet. The body center region has a higher density of defect states than that of the edge and corner. In addition, the as-fabricated photodetectors show excellent light responsivity 18


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and large on-off ratio of 1.371×103, with fast rise and decay time of ~161.7 and 192.1 ms, respectively. This low energy-consuming, efficient fabrication of PbI2 nanosheet provides a promising route for exploring its new applications in the field of optoelectronics, and promoting the research of lead halide perovskite and its devices. ASSOCIATED CONTENT Supporting Information: Schematic diagram of the growth process of PbI2 nanosheets (Figure S1), optical images of nanosheets grown at different temperatures (Figure S2), AFM measurements (Figure S3, S4), particle size analysis (Figure S5), optical images of PbI2 nanosheets grown on four different substrates (Figure S6), EDS of different regions of nanosheets (Figure S7) (PDF) AUTHOR INFORMATION Corresponding Author: * (Y.Y. Liu): E-mail: [email protected] * (M.Z. Zhong): E-mail: [email protected]

Notes The authors declare no conflict of interest. ACKNOWLEDGMENTS The authors acknowledge the Natural Science Foundation of China (Grant No. 11404191 and 11704228).

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References 1.

Zhong, M.; Huang, L.; Deng, H.-X.; Wang, X.; Li, B.; Wei, Z.; Li, J. Flexible Photodetectors Based on Phase Dependent PbI2 Single Crystals. J. Mater. Chem. C 2016, 4, 6492-6499.

2.

Wangyang, P.; Sun, H.; Zhu, X.; Yang, D.; Gao, X. Mechanical Exfoliation and Raman Spectra of Ultrathin PbI2 Single Crystal. Mater. Lett 2016, 168, 68-71.

3.

Ha, S. T.; Liu, X.; Zhang, Q.; Giovanni, D.; Sum, T. C.; Xiong, Q. Synthesis of Organic-Inorganic

Lead

Halide

Perovskite

Nanoplatelets:

Towards

High-Performance Perovskite Solar Cells and Optoelectronic Devices. Adv. Opt. Mater. 2014, 2, 838-844. 4.

Wang, J.; Fang, H.; Wang, X.; Chen, X.; Lu, W.; Hu, W. Recent Progress on Localized Field Enhanced Two-Dimensional Material Photodetectors from Ultraviolet-Visible to Infrared. Small 2017, 13, 1700894.

5.

Zhu, X.; Sun, H.; Yang, D.; Wangyang, P.; Gao, X. Comparison of Electrical Properties of X-Ray Detector Based on PbI2 Crystal with Different Bias Electric Field Configuration. J. Mater. Sci.: Mater. Electron. 2016, 27, 11798-11803.

6.

Kasap, S. Low-Cost X-Ray Detectors. Nat. Photonics 2015, 9, 420-421.

7.

Cong, C.; Shang, J.; Niu, L.; Wu, L.; Chen, Y.; Zou, C.; Feng, S.; Qiu, Z.-J.; Hu, L.; Tian, P., et al. Anti-Stokes Photoluminescence of Van Der Waals Layered Semiconductor PbI2. Adv. Opt. Mater. 2017, 5, 1700609.

8.

Street, R. A.; Ready, S. E.; Van Schuylenbergh, K.; Ho, J.; Boyce, J. B.; Nylen, P.; Shah, K.; Melekhov, L.; Hermon, H. Comparison of PbI2 and HgI2 for Direct 20


Page 21 of 26 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


Detection Active Matrix X-Ray Image Sensors. J. Appl. Phys 2002, 91, 3345-3355. 9.

Lan, C.; Dong, R.; Zhou, Z.; Shu, L.; Li, D.; Yip, S. P.; Ho, J. C. Large‐Scale Synthesis of Freestanding Layer ‐Structured PbI2 and MAPbI3 Nanosheets for High‐Performance Photodetection. Adv. Mater. 2017, 29, 1702759.

10. Frisenda, R.; Island, J. O.; Lado, J. L.; Giovanelli, E.; Gant, P.; Nagler, P.; Bange, S.; Lupton, J. M.; Schüller, C.; Molina-Mendoza, A. J., et al. Characterization of Highly Crystalline Lead Iodide Nanosheets Prepared by Room-Temperature Solution Processing. Nanotechnology 2017, 28, 455703. 11. Sun, H.; Zhu, X.; Yang, D.; Yang, J.; Gao, X.; Li, X. Electrical Characteristics of Lead Iodide Crystal Devices on Flexible Substrate under Mechanical Tensile Strain. Phys. Status Solidi A 2014, 211, 823-827. 12. Zhong, M.; Zhang, S.; Huang, L.; You, J.; Wei, Z.; Liu, X.; Li, J. Large-Scale 2d PbI2 Monolayers: Experimental Realization and Their Indirect Band-Gap Related Properties. Nanoscale 2017, 9, 3736-3741. 13. Toulouse, A. S.; Isaacoff, B. P.; Shi, G.; Matuchová, M.; Kioupakis, E.; Merlin, R. Frenkel-Like Wannier-Mott Excitons in Few-Layer PbI2. Phys. Rev. B 2015, 91, 165308. 14. Kong, W.; Li, G.; Liang, Q.; Ji, X.; Li, G.; Ji, T.; Che, T.; Hao, Y.; Cui, Y. Controllable Deposition of Regular Lead Iodide Nanoplatelets and Their Photoluminescence at Room Temperature. Phys. E (Amsterdam, Neth.) 2018, 97, 130-135. 21



15. Wei, Q.; Shen, B.; Chen, Y.; Xu, B.; Xia, Y.; Yin, J.; Liu, Z. Large-Sized PbI2 Single Crystal Grown by Co-Solvent Method for Visible-Light Photo-Detector Application. Mater. Lett. 2017, 193, 101-104. 16. Jeon, Y. P.; Woo, S. J.; Kim, T. W. Transparent and Flexible Photodetectors Based on CH3NH3PbI3 Perovskite Nanoparticles. Appl. Surf. Sci. 2018, 434, 375-381. 17. Dang, Y.; Ju, D.; Wang, L.; Tao, X. Recent Progress in the Synthesis of Hybrid Halide Perovskite Single Crystals. CrystEngComm 2016, 18, 4476-4484. 18. Cao, X.; Zhi, L.; Jia, Y.; Li, Y.; Zhao, K.; Cui, X.; Ci, L.; Ding, K.; Wei, J. Enhanced Efficiency of Perovskite Solar Cells by Introducing Controlled Chloride Incorporation into MAPbI3 Perovskite Films. Electrochim. Acta 2018, 275, 1-7. 19. He, Y.; Lei, Y.; Yang, X.; Lu, K.; Liu, S.; Gu, L.; Zheng, Z. Using Elemental Pb Surface as a Precursor to Fabricate Large Area CH3NH3PbI3 Perovskite Solar Cells. Appl. Surf. Sci. 2016, 389, 540-546. 20. Zhao, Y.; Liu, J.; Lu, X.; Gao, Y.; You, X.; Xu, X. Improving the Efficiency of Perovskite Solar Cells through Optimization of the CH3NH3PbI3 Film Growth in Solution Process Method. Appl. Surf. Sci. 2015, 359, 560-566. 21. Shoji, T.; Ohba, K.; Suehiro, T.; Hiratate, Y. Fabrication of Radiation Detector Using PbI2 Crystal. IEEE Trans. Nucl. Sci. 1995, 42, 659-662. 22. He, Y.; Jin, Y.; Chen, B.; He, Z.; Wang, L. Growth of PbI2 single Crystal by the Top Seed Vertical Zone Melting Method. Cryst. Res. Technol. 2015, 50, 22


Page 22 of 26

Page 23 of 26 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


823-827. 23. Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1420. 24. Liu, W.; Liu, Y.-J.; Chen, L.; Ye, T.; Chen, H.-Z.; Li, H.-Y. Gel-Incorporated Pbs and PbI2 Single-Crystals. Chin. Chem. Lett. 2015, 26, 504-508. 25. Zhang, J.; Song, T.; Zhang, Z.; Ding, K.; Huang, F.; Sun, B. Layered Ultrathin PbI2 Single Crystals for High Sensitivity Flexible Photodetectors. J. Mater. Chem. C 2015, 3, 4402-4406. 26. Koffman-Frischknecht, A.; Soldera, M.; Soldera, F.; Troviano, M.; Carlos, L.; Perez, M. D.; Taretto, K. Tuning Morphological Features of Lead Iodide by Low Pressure Vapor Phase Deposition. Thin Solid Films 2018, 653, 249-257. 27. Ioakeimidis, A.; Christodoulou, C.; Lux-Steiner, M.; Fostiropoulos, K. Effect of PbI2 Deposition Rate on Two-Step Pvd/Cvd All-Vacuum Prepared Perovskite. J. Solid State Chem. 2016, 244, 20-24. 28. Han, M.; Sun, J.; Bian, L.; Wang, Z.; Zhang, L.; Yin, Y.; Gao, Z.; Li, F.; Xin, Q.; He, L., et al. Two-Step Vapor Deposition of Self-Catalyzed Large-Size PbI2 Nanobelts for High-Performance Photodetectors. J. Mater. Chem. C 2018, 6, 5746-5753. 29. Wang, Y.; Gan, L.; Chen, J.; Yang, R.; Zhai, T. Achieving Highly Uniform Two-Dimensional PbI2 Flakes for Photodetectors Via Space Confined Physical Vapor Deposition. Sci Bull 2017, 62, 1654-1662. 30. Liu, D.; Zhou, W.; Tang, H.; Fu, P.; Ning, Z. Supersaturation Controlled Growth 23



of MAFAPbI3 Perovskite Film for High Efficiency Solar Cells. Sci. China: Chem. 2018, 61, 1278-1284. 31. Morin, S. A.; Forticaux, A.; Bierman, M. J.; Jin, S. Screw Dislocation-Driven Growth of Two-Dimensional Nanoplates. Nano Lett. 2011, 11, 4449-55. 32. Weimarn, P. P. V. The Precipitation Laws. Chem. Rev. 1925, 2, 217-242. 33. Xia, J.; Zhu, D.; Wang, L.; Huang, B.; Huang, X.; Meng, X.-M. Large-Scale Growth of Two-Dimensional SnS2 crystals Driven by Screw Dislocations and Application to Photodetectors. Adv. Funct. Mater. 2015, 25, 4255-4261. 34. Baltog, I.; Baibarac, M.; Lefrant, S. Quantum Well Effect in Bulk PbI2 Crystals Revealed by the Anisotropy of Photoluminescence and Raman Spectra. J. Phys. Condens. Matter 2009, 21, 025507. 35. Condeles, J. F.; Ando, R. A.; Mulato, M. Optical and Structural Properties of PbI2 Thin Films. J. Mater. Sci. 2007, 43, 525-529. 36. Condeles, J. F.; Ghilardi Netto, T.; Mulato, M. Lead Iodide Films as X-Ray Sensors Tested in the Mammography Energy Region. Methods Phys. Res., Sect. A 2007, 577, 724-728. 37. Shkir, M.; Abbas, H.; Siddhartha; Khan, Z. R. Effect of Thickness on the Structural, Optical and Electrical Properties of Thermally Evaporated PbI2 Thin Films. J. Phys. Chem. Solids 2012, 73, 1309-1313. 38. Kapustyanyk, V. B.; Pasternak, R. M.; Kalush, A. Z.; Panasyuk, M. R.; Tsibul’Skii, V. S.; Rudyk, V. P.; Serkiz, R. Y. Exciton Spectra of Layered PbI2 and PbI2 :Zr Crystals. J. Appl. Spectrosc. 2007, 74, 283-288. 24


Page 24 of 26

Page 25 of 26 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


39. Yang, X.; Zheng, F.; Xu, W.; Bi, P.; Feng, L.; Liu, J.; Hao, X. Improving the Compatibility of Donor Polymers in Efficient Ternary Organic Solar Cells Via Post-Additive Soaking Treatment. ACS Appl. Mater. Interfaces 2017, 9, 618-627. 40. Xu, W.-L.; Niu, M.-S.; Yang, X.-Y.; Bi, P.-Q.; Zhang, K.-N.; Xiong, C.; Yuan, H.-C.; Smith, T. A.; Ghiggino, K. P.; Hao, X.-T. Spatially Resolved Photophysical Dynamics in Perovskite Microplates Fabricated Using an Antisolvent Treatment. J. Phys. Chem. C 2017, 121, 26250-26255. 41. Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Recombination Kinetics in Organic-Inorganic Perovskites: Excitons, Free Charge, and Subgap States. Phys. Rev. Appl. 2014, 2, 034007. 42. Zeng, L.; Tao, L.; Tang, C.; Zhou, B.; Long, H.; Chai, Y.; Lau, S. P.; Tsang, Y. H. High-Responsivity Uv-Vis Photodetector Based on Transferable WS2 Film Deposited by Magnetron Sputtering. Sci. Rep. 2016, 6, 20343. 43. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501. 44. Zheng, W.; Zhang, Z.; Lin, R.; Xu, K.; He, J.; Huang, F. High-Crystalline 2D Layered PbI2 with Ultrasmooth Surface: Liquid-Phase Synthesis and Application of High-Speed Photon Detection. Adv. Electron. Mater. 2016, 2, 1600291. 45. Liu, J.; Liang, Z.; Xu, B.; Xiang, H.; Xia, Y.; Yin, J.; Liu, Z. Synthesis of PbI2 Nanowires for High Sensitivity Photodetectors. RSC Adv. 2016, 6, 59445-59449. 25



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PbI2 Nanosheets for Photodetectors via the Facile Cooling Thermal

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