Synthesis of Easily-Transferred 2D Layered BiI3 Nanoplates for

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Synthesis of Easily-Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photo-detectors Qi Wei, Jinhui Chen, Ping Ding, Bo Shen, Jiang Yin, Fei Xu, Yidong Xia, and Zhi-guo Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02582 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Synthesis of Easily-Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light PhotoDetectors Qi Wei†, Jinhui Chen‡, Ping Ding†, Bo Shen†, Jiang Yin†, Fei Xu‡, Yidong Xia†*and Zhiguo Liu† † Department of Materials Science and Engineering, College of Engineering and Applied Sciences, National Laboratory of Solid State Microstructures, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ Department of Quantum Electronics and Optical Engineering, College of Engineering and Applied Sciences, National Laboratory of Solid State Microstructures, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China KEYWORDS: Bismuth iodide, 2D layered materials, Nanoplates, Flexible, Photo-detector.

ABSTRACT

Bismuth triiodide, BiI3, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room temperature gamma/x-ray detectors, high-efficiency photovoltaic absorbers and Bi-based

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organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here we report the synthesis of vertical 2D BiI3 nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 AW-1), fast stable photoresponse (3/9 ms for raise/decay times) and remarkable specific detectivity (1.2×1012 Jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photo-detectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI3 for flexible devices.

INTRODUCTION

For years, BiI3 has been regarded as a key material for room-temperature gamma-ray detection and x-ray imaging sensors.1-4 Both theories and experiments demonstrate its band structure (indirect band gap of 1.69 eV) for high-efficiency photovoltaic absorber, room-temperature photoluminescence, solar cells, photoelectric signal converters and sensors.5-13 Recent studies have also suggested that the organic-inorganic hybrid perovskites taking BiI3 as an important precursor own considerable research and application prospects.14-17 Besides, BiI3 is a kind of layered material that possesses a repeating unit of sandwiched I-Bi-I layers with strong Bi-I ionic bonds in monolayers and weak van der Waals interaction between them (see Figure 1a ).18 Such structure offers opportunities for the preparations of monolayer and few layered BiI3 as twodimensional layered materials (2DLMS),18-20 which have been widely sought after as one of the


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most promising materials for the next-generation photonic and optoelectronic technologies.20-25 Thus it is significant to explore the synthesis as well as the properties of 2D layered BiI3 materials and the performances of related optoelectronic components are worthy of expectation. Furthermore, although 2D BiI3 materials are identified as slightly brittle ones, theoretically, thin layers of BiI3 can still afford a biaxial strain within 13%.18 Then, flexible optoelectronic devices including photo-detectors based on 2D layered BiI3 can be expected.

However, given such promising applications though, the layers of BiI3 have received less attention and there were very limited experimental reports in those fields to date. Herein, we report that BiI3 is fully competent for flexible 2D visible-light sensor, which is implemented by synthesizing BiI3 nanoplates through physical vapor deposition (PVD) method. The products in this work grow in vertical to the SiO2/Si substrate and it is very facile to transfer such BiI3 nanoplates to other substrates or maybe up to other 2D materials, bringing about expanded applications. The reproducible and sizeable photoresponses of the photo-detectors based on the transferred BiI3 nanoplates on polyethylene terephthalate (PET) substrate under red laser are demonstrated to corroborate the visible light detecting. The flexibility is authenticated by the excellent mechanical stability and reliability when the devices are tested with different bending angles and after being bent hundreds of times, respectively.

RESULTS AND DISCUSSION

Figure 1b schematically describes the core preparation process of the BiI3 nanoplates. BiI3 powder is sublimated in the high temperature zone of a quartz tube and the vapor is swept forward by the continuous argon flow then crystallizes into perpendicularly grown nanoplates and micron particles, as exhibited in Figure 1c and Figure 1d, in the low temperature zone.


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Details of the synthesis method are described in the Experimental Section and Figure S1, Supporting Information. It’s also found that such vertical products can be separated then transferred much more easily comparing with other horizontally grown 2D materials, of which the difficult transfers always involve auxiliary carriers and inevitable damages.26-28 The small contact area, which means limited chemical bonds between the vertically grown BiI3 nanoplates and the SiO2/Si substrate, offers the opportunity to separating and transferring the nanoplates with the assistance of their self-gravity via simple mechanical method. Specifically, in a typical transfer procedure in this work, one just need to invert a SiO2/Si substrate grown with BiI3 onto another PET substrate and knock its back with tweezers, then the nanoplates would fall off and lie themselves flat onto the PET substrate (Figure S2, Supporting Information). The optical microscope (OM) images and atomic force microscope (AFM) studies of the typical transferred nanoplates displayed in Figure S3, Supporting Information demonstrate that they mostly exhibit an approximate semi hexagonal morphology with thickness in the range of 15-150 nm and varied lateral dimension of 30-60 µm. In addition, some overlaps between two transferred nanostructures can be observed occasionally and a representative example is offered in Figure 1e. The overlap has a step shape with two smooth ~90o bends according to the results of AFM studies shown in Figure 1f, which indicates good mechanical flexibility of the BiI3 nanoplate.


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Figure 1. (a) Layered crystal structure of BiI3. (b) Schematic demonstration of the core growth process through PVD method. (c) Optical image of the as-grown BiI3 materials on the SiO2/Si substrate. (d) Scanning electron microscope (SEM) image of a single vertical BiI3 nanoplate. (e) Optical image and (f) atomic force microscope (AFM) studies of an overlap between two transferred BiI3 nanostructures. Inset of (f): the height data collected along the green line.

The phase structure of the as-synthesized BiI3 nanoplates was characterized by the X-ray diffraction (XRD) and transmission electron microscopy (TEM). As depicted in Figure 2a, the XRD spectrum coincides well with the main peaks of rhombohedral BiI3 (PDF#89-0307) without any impurity phases over the 2θ range of 10o-60o. The sharp diffraction peaks designate good crystallization of such nanoplates and the only (00Ɩ) diffraction signals validate the [003] direction in which the transferred, flat ones are oriented. These results are further confirmed by the outcomes of high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) characterization. The predominantly appeared inter-planar distance of 0.217 nm in Figure 2b can be identified to the (300) family planes of rhombohedral BiI3 structure and well matches the parameters from PDF#89-0307. While in Figure 2c, the diffraction spots in hexagonal symmetry further affirm the high quality single-crystalline nature as well as the [003] stacking direction of the BiI3 nanoplates. The energy disperse spectroscopy (EDS) spectra was also explored for checking the chemical composition and element distribution of the BiI3 nanoplate, and the results are shown in Figure 2d. In a randomly selected area, Bi and I atoms scatter uniformly and the atomic ration of Bi to I is 1:3, in line with the expected stoichiometric ratio.


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Figure 2. (a) XRD pattern of the BiI3 nanoplates. (b) and (c) HRTEM and SAED images of the as-grown BiI3 nanoplate. (d) EDS element mapping profiles of the BiI3 nanoplate. Insets: the respective distributions and the atom ratio of I and Bi atoms.

To study the optical absorption properties of the as-synthesized BiI3 nanoplates, the diffuse reflectance spectrum was measured and transformed to an intuitive absorbance spectrum through the Kubelka-Munk (K-M) transformation. As shown in Figure 3a, the absorption range covers almost the whole of the visible spectrum. The onset absorption occurs at ~725 nm and the intensity changes along the absorption edge going through two stages, which matches the reported indirect band gap nature of the BiI3.5,18,19 Given the diffuse reflectance, one can calculate the band value with the following formula around the fundamental absorption edge29 / ,

(1)


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where F(R) is K-M function, h is the Planck’s constant, v is light frequency, A is a constant and Eg is the optical band gap. The type of optical transition determines the exponent n. For direct transition, n = 1/2; for indirect transition, n = 2. Hence, the dependence of / on the photon energy hv is given as Figure 3b to study the indirect band gap nature of the BiI3 nanoplates. For the indirect bang gap materials, the interband transition always occurs accompanied by the absorption or emission of phonons.30 This is well presented by the two straight fitting lines with different slops, of which the interceptions on the energy axis give the value of Eg + Ep and Eg – Ep. With the high-adjusted coefficients of determination (R 2adj ) guaranteeing the suitability of the alternative modes by such fittings, the band gap can be calculated as 1.69 eV by taking the average of the two intercepts.


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Figure 3. (a) UV-vis absorbance spectrum of the BiI3 nanoplates. (b) (F(R)hν)1/2 vs photon energy to calculate the band gap.

The outstanding single-crystal quality, appropriate band gap (1.69 eV, corresponding to an absorption range overlapping most of the visible spectrum) and aforementioned mechanical flexibility make such BiI3 nanoplate an ideal candidate for flexible functional optoelectronic elements operating within the visible-light region. Herein, some flexible photo-detectors on the PET substrate, of which the structure is schematically displayed in Figure 4a, were assembled and the fabrication method is provided in the Experimental Section. For the absorption spectrum reaches a peak around 635 nm (see Figure 3a), such red laser was chosen as the incident light for the later tests of photo-sensing performances.

Figure 4. (a) The schematic diagram of the planar optoelectronic component. (b) Current-voltage characteristics in the dark and under illumination. (c) Photocurrent and responsivity (R) as functions of illumination intensity. (d) Dependences of external quantum efficiency (EQE) and


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specific detectivity (D*) on different light intensity. (e) The reproducible photoswitching characteristics under excitation of 0.7 mWcm-2. Inset: photoresponse stability with the laser on for 30 min. (f) Light current rise and decay measured under light intensity of 100 mWcm-2. All the above measurements presented in (c)-(f) were carried out under an illumination of 635 nm laser at 5 V bias.

Figures 4b-4f give the performances of the photo-detector based on an 80 nm thick BiI3 nanoplate without any deformation. The channel dimensions of the device are 10 and 12 µm for width and breadth, respectively. As shown in Figure 4b, the illumination with the laser of 635 nm in wavelength makes the irradiated current quite a contrast to the dark current. The light current would further boost up with increasing incident light intensity from 0.25 to 1100 mWcm2

because more excited photo-carriers are produced by interband transition and contribute to the

transport in the case of higher intensity. In addition, the dark current under 5V bias is only 21.3 pA, which would be of great benefit to improving the detectivity, and the ON/OFF ratio reaches as high as 1.16×104 under the illumination with an optical power of 1100 mWcm-2. It also should be noted that the I-V characteristics are non-liner, which can be attributed to the Schottky barriers at the interfaces of Au/ BiI3 nanoplate. To further study the influence of incident light intensity on photocurrent, the incident power (Pin) dependent photocurrent (Iph) is depicted as the red fitting curve in Figure 4c on a logarithmic scale. Such dependence can be described as

Iph = BPβin ,

(2)

where Iph = Iilluminated － Idark , B and β are fitting parameters. Then the exponent β can be determined by taking the slope of the fitting line of the measure data on a logarithmic scale. In


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this work, β is determined as 0.68. The sublinear dependence is common in 2DLMS based photoconductors and lies on a complex process involving the generation, separation, trapping and recombination of the photo-induced carriers (electrons and holes) within the materials as well as the electrode/materials surfaces.31 Furthermore, this dependence works stable when the power intensity changes from 0.25 to 1100 mWcm-2, revealing a wide application range (an important parameter for a photo-detector) covering over 4 orders of magnitude in power intensity. The responsivity R, another important parameter, is defined as

×

,

(3)

in which S means the active area. The blue fitting line in Figure 4c plots the responsivity at 5 V bias as a function of the incident power. On the basis of Equations (2) and (3), the relationship -0.32 between them could be deduced as R∝Pβ-1 with β=0.68, for which the measured data in or R∝Pin

can be fitted as a falling line on a logarithmic scale. Thus, a maximal responsivity of 2.8 AW-1 is figured out taking the minimum illumination intensity of 0.25 mWcm-2. In addition, the responsivity of each device built on BiI3 nanoplates with different thicknesses was also measured and given in Figure S4, Supporting Information. It’s found that the device based on thicker BiI3 nanoplate tends to have higher responsivity. The external quantum efficiency (EQE) and specific detectivity (D*) are other two important figures of merit for a photo-detector and can be gained by the relationships ·

(4)

and under short-noise-limit condition (assume the shot noise caused by dark current is the major contribution to the total noise.)


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!* · # . $

(5)

The parameter c is the velocity of light, e is the electronic charge, λ is the illumination wavelength and Id is the dark current. Obviously EQE and D* are both proportional with R and the same variation trends of EQE, D* (in Figure 4d) and R (in Figure 4c) well accord with it. Taking the maximal R as 2.8 AW-1 at 5 V bias, the highest EQE and D* to 635 nm laser are then calculated to be 540% and 1.2×1012 Jones, respectively. The good responsivity of 2.8 AW-1 assures this BiI3 nanoplate based photo-detector of quite a match of the recently reported layered materials based photo-sensors and the excellent specific detectivity of 1.2×1012 Jones even brings it an advantage in contrast to most of the 2DLMS based photo-detectors.31-36

Besides R, EQE and D*, the repeatability, stability and speed of the photoresponse are vital performances in evaluating photo-sensors as well. As one can see in Figure 4e, the current of the assembled device displays efficient and stable responses to the 635nm excitation. The reproducible switches between the ON and OFF states promise the identical photoswitching characteristics under 5 V with repetitive illumination. Additionally, the remarkable stabilities of the BiI3 nanoplate and the circuit are fully corroborated by the steady light current without any obvious deviation for up to 30 min (inset of Figure 4e). Figure 4f depicts a more detailed time response curve with much higher sampling frequency during the laser switches. At 5 V bias, the rise and decay times are 3 and 9 ms respectively, which are faster than that of many light sensors based on other 2D nanomaterials.35-39 To better evaluate the above performances as whole, related parameters of some state-of-the-art photo-sensors built on other 2DLMS are listed in Table 1 for comparison. In general, the fabricated photo-detector in this work are equipped with


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highly comparable or even better performances than those outstanding ones reported recently, indicating promising potentials of such BiI3 nanoplates for highly efficient optical sensors.

Table 1. Performance Comparison of Different 2DLMS Based Photo-detectors

Table Footnote: The samples are selected with similar structures or similar test conditions at least (for the transistor, data with Vg = 0V are selected) to ensure the fairness and scientificity. The data listed in the column for response times are given as rise/ decay times and in the column for detectivity are all calculated by the formula (5) , assuming that shot noise caused by dark current contributes to the major of the total noise.

The mechanical flexibility of the photo-detector was also studied with the results given in Figure 5. For introducing a deformation into the measurements, the PET substrate was bent with different angles (see Figure 5a). According to Figure 5b, the current possesses outstanding photoresponse with different bending angles except some slight attenuation. Fortunately, these


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attenuations are weak (within 6%, see the inset of Figure 5b) in the whole bending process and may be related to the variable band structure of the BiI3 nanoplate under different strains.18,19,34Besides the stability under different deformations, the fatigue property is also very important for a flexible element. Thus, the photoresponse was measured after the photo-detector being bent hundreds of times with an angle of 60o and the results are plotted in Figure 5c. It can be noticed that the light current shows very slight changes after bending 100 and 200 times (within 4%, see the inset of Figure 5c), indicating a good bending fatigue property. Those results exhibited in Figure 5 confirm the excellent flexibility of the fabricated photo-detector with admirable mechanical stability and reproducibility and promise such BiI3 nanoplate an idea candidate for advanced flexible optoelectronic technologies.

Figure 5. (a) Optical image of the bent photo-detector. Inset: schematic illustration of the bending device. (b) Current-voltage characteristics measured at varied bending conditions. Inset: details of the light current around 5 V bias. (c) Curves of the light current measured after the device being bent 0,100 and 200 times. Inset: details of the light current around 5 V bias. The illumination for the photocurrent measurements has an intensity of 1 mWcm-2.


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CONCLUSIONS

To summarize, we have synthesized easily-transferred vertical 2D layered BiI3 nanoplates with thickness in the range of 15-150 nm and lateral dimension of 30-60 µm via PVD method. The obtained nanoplates are identified as rhombohedral BiI3 single crystals with an indirect band gap of 1.69 eV. The response to 635 nm laser has been investigated by fabricating prototype photodetectors based on such nanoplates. Given the promising performances including high on/off ratio, good responsivity, outstanding specific detectivity and fast stable photoresponse, the vertical BiI3 nanoplates grown by PVD method are qualified optical material for visible light sensors. Moreover, the photo-detectors on PET substrate show excellent mechanical flexibility (including stability and durability) under different bending strains. These results pave the way for BiI3 material toward flexible 2D optoelectronic devices for the advanced technologies, such as wearable light sensors, flexible imaging technology, optical fiber communication and complex environment monitoring.43

EXPERIMENTAL SECTION

Materials Preparation and Characterization. The BiI3 nanoplates were synthesized via PVD method and the synthesis apparatus is schematically illustrated in Figure S1, Supporting Information. BiI3 powder used as source materials was placed in the hot zone of the quartz tube while SiO2/Si substrate was positioned downstream from the source to the cold end. The heating rate was controlled as 10 °C/min from room temperature to 150 °C then increased to 20 °C/min from 150 °C to 270 °C. This heating condition (150 °C) was preserved for 5 min before furnace cooling under ambient conditions. Constant argon flow of 6 sccm (standard-state cubic centimeter per minute) was applied as the carrier through the whole synthesizing process. The


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morphologies of the as-synthesized BiI3 samples were characterized by using optical microscope (ZEISS Imager.M1m), SEM (ZEISS ULTRA 55), and AFM (Cypher-ES). EDS (ZEISS ULTRA 55) was performed to verify the elementary composition of the products and XRD (Rigaku Ultima III, Cu Ka radiations) analysis confirmed the main diffraction peaks of BiI3. The microstructures of the products were identified by HRTEM (Tecnai G2F20 STEM) operated at 380 KV.

Device Fabrication and Analysis. The BiI3 nanoplate based photo-detection device has a planar structure on the PET substrates. Two parallel Au films with the separation of 10 µm were deposited by magnetron sputtering using polymethyl methacrylate (PMMA) film as the mask at room temperature. As described in Figure S5, Supporting Information, the BiI3 nanoplate was firstly transferred onto the PET substrate before a strip of PMMA film with a width of 10µm was taped on it with a pointed fiber. Then, 100 nm Au film was deposited onto the PET substrate by magnetron sputtering. At last, the PMMA and redundant Au film shall be removed to complete the final circuitry. The transport measurements were carried out by a Keithley 4200-SCS semiconductor characterization system and the time responses measurements were carried out by the Keithley 2400 semiconductor characterization system and an oscilloscope (Lecroy, WaveRunner 62Xi). All the aforementioned performances of the devices were tested at room temperature.

SUPPORTING INFORMATION

Schematic demonstration of the experimental installation and supplementary details of the synthetic method (Figure S1); Schematic demonstration and details of the transfer process (Figure S2); Morphology characterizations of the transferred BiI3 nanoplates on the 90 nm


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SiO2/Si substrate (Figure S3); The responsivity of the devices built on the BiI3 nanoplates with different thicknesses (Figure S4); Schematic demonstration and details of the device fabrication method (Figure S5) (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Y. D. X.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (11774159 and 11504163), the Fundamental Research Funds for the Central Universities and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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FIGURE ABSTRACT


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Synthesis of Easily-Transferred 2D Layered BiI3 Nanoplates for

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