Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe2

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Applications of Polymer, Composite, and Coating Materials

Flexible Vertical p-n Diode Photodetectors with Thin Ntype MoSe Films Solution-processed on Water Surfaces 2

Ihn Hwang, Jong Sung Kim, Beomjin Jeong, Sung Hwan Cho, and Cheolmin Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Applied Materials & Interfaces

Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe2 Films Solution-processed on Water Surfaces

Ihn Hwang,∇ Jong Sung Kim,∇ Sung Hwan Cho, Beomjin Jeong, and Cheolmin Park *

Department of Materials Science and Engineering, Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea E-mail: [email protected]

KEYWORDS: Transition metal dichalcogenides, Vertical p-n diode, Photodetector, MoSe2 nanosheets, Liquid exfoliation, Amine-terminated polymers, Water-air interface

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ABSTRACT Two-dimensional (2D) nanosheets of transition metal dichalcogenides (TMDs) are of significant interest for potential photo-electronic applications. However, the fabrication of solution-processed arrays of mechanically flexible thin TMD films-based vertical type pn junction photodetectors over a large area is a great challenge. Our method is based on controlled solvent evaporation of MoSe2 suspension spread on water surface. Single or few-layered MoSe2 nanosheets modified with the dispersant amine-terminated poly(styrene) (PS-NH2) were homogeneously deposited and stacked on water upon solvent evaporation, giving rise to uniform MoSe2/PS-NH2 composite films that can be readily transferred onto other substrates. A pn junction vertical diode of Al/p-type Si/p-type poly(9,9-di-n-octylfluorenyl-2,7-diyl)/ntype MoSe2 composite/Au stacked from bottom to top exhibited characteristic rectifying current behavior upon voltage sweep with a rectification ratio of 103. Subsequent illumination of near infrared light on the device resulted in a substantially enhanced dark current of approximately 103 times greater than that of the non-exposed device. The photodetection performance i.e., switching time, responsivity, and detectivity, were 100.0ms, 2.5AW−1 and 2.34×1014, respectively. Furthermore, the performance of mechanically flexible photodetectors devices was comparable with that of the devices fabricated on the hard Si substrate even after 1000 bending cycles at a bending diameter of 7.2 mm.


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INTRODUCTION The extraordinary materials properties of two-dimensional (2D) nanosheets of transition metal dichalcogenides (TMDs), such as MoS2, WS2, and MoSe2, ranging from insulators, direct band gap semiconductors to metals, make these materials suitable for a variety of emerging applications in electronics, optics, energy conversion and storage.1-6 In particular, the photocurrent conversion of TMD nanosheets with their characteristic optical absorption energies is unprecedentedly impressive, offering efficient wavelength selective photo-detection. Recent studies have evidenced that single or few-layered TMDs, including MoS2 and WS2, elaborately fabricated by mechanical exfoliation with a Scotch tape are extremely photoconductive in the visible range. 7-9 Photodetectors (PDs) based on TMD nanosheets are categorized mainly into three types in terms of device architecture: two-terminal parallel and vertical PDs10-12 and three-terminal field effect transistor (FET) type PDs (Table S1).13,14 Two-terminal parallel devices, in which two metal electrodes are precisely defined across a mechanically exfoliated nanosheet based on electron beam or photolithography, have been studied extensively owing to the ease of device fabrication.15,16 A FET type PD contains a gate electrode between a gate insulator and a twoterminal parallel device, and its photo-conversion properties can be further controlled by the gate field.17-19 Apart from the advantage of ease of fabrication of two electrical contacts on nanosheets largely varied in size and position, the parallel type devices enable direct exposure of light on the nanosheets. However, they have limitations of high operating voltage and large device area compared with vertical devices, making it difficult to fabricate high-density PDs. Considering that common PDs based on Si and organic semiconductors adopt vertical diode structures, 20-23 there is an increasing demand for the development of arrays of vertical diode PDs of TMD nanosheets for their technological implementation. Even though mechanical exfoliation using a Scotch type is convenient for the fabrication of PDs in most cases, it is not applicable for fabrication of arrays of devices. In addition, single or 3 ACS Paragon Plus Environment


few-layer nanosheets are rarely suitable for vertical diode PDs, owing to a possible electrical leakage upon application of voltage for separation of photo-excited carriers. A TMD film comprising multi-stacked nanosheets is beneficial to avoid the leakage problem. In addition, the nanosheets should be evenly distributed in the film, as the formation of aggregates may hinder photoconduction between the top and the bottom electrodes. Therefore, it is important to develop an efficient method for fabricating thin TMD films with a uniform distribution of nanosheets that exhibits not only numerous photoconduction pathways but also excellent mechanical stability. Furthermore, considering the mechanical flexibility of 2D TMDs, arrays of photodetectors on either plastic or paper would be beneficial for wearable and patchable applications. The fabrication of a thin TMD film involves efficient exfoliation of the sheets of 2D TMDs from stacked bulk samples as well as prevention of re-aggregation of the sheets upon film formation. Many studies have been devoted to liquid phase exfoliation and stabilization of TMDs, not only because of the extra driving force offered by the solvent medium for the separation of the sheets, but also because of the suitability of 2D TMDs dispersed in a solvent for various solution-based film processes such as spin-coating, dip-coating, and layer-by-layer assembly.24-29 To further promote the separation of the sheets, additional interactions with TMDs are required, including ion intercalation, surfactant driven interaction, and a highly boiled medium.30-36 Although these approaches are promising and scalable, the photoconduction properties of solution-processed TMD films have not been extensively investigated (Table S2). We have recently demonstrated non-destructive modification of TMD sheets with amine-terminated polymers, in which the amine moiety adhered to the surface of the TMDs while the long and flexible polymer chain provided sufficient physical gaps between two sheets to mitigate the strong van der Waal interactions of the sheets. The two-terminal parallel PD fabricated using the thin TMD/polymer film was mechanically flexible when prepared on a conventional filter paper and exhibited excellent photoconduction upon layer 4 ACS Paragon Plus Environment

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illumination. In spite of the successful development of the parallel PDs with the TMD films, vertical PDs could not be realized using common solution casting processes such as spincoating, dip-coating, and bar-coating, owing to the difficulty in preparing a TMD film thick enough to produce substantially large amount of photo-carriers on either electrode or p-type semiconducting layer, using a suspension with a relatively low concentration of highlydispersed TMD nanosheets. It is, therefore, on demand to develop a novel solution-based, scalable process for fabricating thickness-controlled and uniform TMD composite films suitable for arrays of vertical PDs. In this work, we fabricated high-performance vertical-type diode TMD PDs based on thickness-controlled MoSe2 films solution-processed with amine-terminated polymers on a water surface. The thin composite film, quiescently developed upon solvent evaporation on water surface, was uniform in thickness without significant aggregates and could be easily transferred on a variety of substrates. Arrays of pn junction type vertical PDs were fabricated, consisting of a thin TMD film deposited on a p-type poly(9,9-di-noctylfluorenyl-2,7-diyl) (PFO) polymer film spin-coated on a boron doped Si/Al electrode, followed by deposition of a top Au electrode. The devices were found to be reliable with excellent photodetection performances of high ON/OFF photocurrent ratio, fast switching time, high responsivity and detectivity. Vertical PDs were also fabricated on a mechanically flexible polymer substrate, forming arrays of flexible PDs, and their performance was found to be comparable to that of PDs fabricated on a hard substrate.

EXPEIMENTAL SECTION Exfoliation of MoSe2 with amine-end-functionalized polymers: MoSe2 powders were purchased from Alfa Aesar and Sigma Aldrich. A PS-NH2 with the molecular weight and polydispersity index of 9500 g mol-1 and PDI ~ 1.16, respectively was produced from Polymer Source Inc., Doval, Canada. A PFO (MW: ∼ 58200, PDI: ∼ 3.7) was purchased from Sigma 5 ACS Paragon Plus Environment


Aldrich. All of the solvents used in the present work were also purchased from Sigma-Aldrich. All the materials were used as received unless otherwise stated. In a typical procedure, 250.0 mg of bulk MoSe2 powder and 25.0 mg of PS-NH2 were added into a 30.0-mL glass vial containing 25.0 mL of toluene. The solution was sonicated for 45.0 min using a tip sonicator with a 10.0-s ON pulse and a 5.0-s OFF pulse at amplitude of 50% in an ice bath. The dispersions were allowed to settle for 24.0 h, and the top dispersion was subsequently decanted and centrifuged for 30.0 min at 1500.0 rpm to remove unexfoliated and large particles. After centrifugation, the top half of the dispersion was collected, and the concentration of MoSe2 nanosheets was determined by standard gravimetric analysis. Device fabrication process: A back-side Al electrode was deposited on a p-type (boron doped) Si wafer (purchased from Buysemi, Inc., South Korea) after spin-coating of the PFO layer (0.7 wt.%) at 1000.0 rpm for 60.0 s. A thin MoSe2/PS-NH2 film transferred on the PFO spin-coated p-type silicon wafer was dried at 80 C for 1.0 h. Using a stainless-steel shadow mask (SUS-Mask) of size 400.0 µm, 20.0~30.0 nm-thick Au film (top electrode) was deposited on the transferred MoSe2 film by thermal evaporation (MEP 5000, SNTEK Co., Ltd.) under a pressure of 10−6 mbar at a growth rate of 1.0 nm s−1. For mechanically flexible vertical p-n junction PDs, ITO bottom electrodes deposited on PET substrates (Freemteck, Inc., South Korea) were employed. Thin PFO films were deposited on an ITO surface, followed by the transfer of thin MoSe2/PS-NH2 films. Ag nanowire network top electrodes were spray-coated on the thin MoSe2/PS-NH2 film through the shadow mask, leading to the arrays of mechanically flexible vertical p-n junction diode PDs. To avoid overestimation of photo-current arising from peripheral current, we completely isolated the individual devices with appropriate solvent (Supporting information, Figure S1). The thickness of all the films employed in our photodetectors was characterized with both Alpha Step (500 KLA Tencor) and tapping modeAFM in height mode (Digital Instruments NanoScope 3100). Characterization Methods: The nanostructures of a thin MoSe2/PS-NH2 film were 6 ACS Paragon Plus Environment

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observed by tapping mode AFM in height and phase contrast, spherical aberration correction scanning transmission electron microscope (STEM; JEOL JEM-ARM 200F) and HRTEM (JEOL 2100F) at 200.0 kV in bright field mode. The nanostructures of a thin MoSe2/PS-NH2 film were characterized using field-emission scanning electron microscopy (FE-SEM) (JEOLSM 7601f-plus) with an acceleration voltage of 10 kV. The vertical layer structures of the photodetector were analyzed using a focused-ion-beam transmission electron microscope (FIBTEM; JIB-4601F, JEOL) and STEM (JEOL JEM-ARM 200F). The absorbance and the transmittance spectra of both the composite solutions and the films were recorded using a UVvis-near-infrared spectrophotometer (Varian Cary 5000) and a UV visible spectrophotometer (JASCO V-530). X-ray photoelectron spectroscopy (XPS, K-alpha Thermo VG, K.) measurements were acquired using a monochromated Al X-ray source (Al Kα line: 1486.6 eV). The electrical properties of all the devices were measured under illumination with an NIR laser source (power ranging from 5.0 to 170.0 mW, wavelength of 835.0 nm) at low power (40.0 µW~5.0 mW) using a film-type polarizer (Qbic Laser system, Model number: QBFDL-835150-1000). The electrical properties were characterized using a Hewlett-Packard 4145B semiconductor parameter analyzer. All the measurements were carried out in a metallic dark box at room temperature in air. The electrical properties of vertical p-n junction diode PD at low temperature were characterized using an MSTECH cryogenic probe station in a vacuum chamber, and the cryogenic temperature was maintained using liquid nitrogen.

RESULTS AND DISCUSSION Thin polymer composites with highly dispersed MoSe2 nanosheets was prepared by spreading the suspension of MoSe2 nanosheets mixed with amine terminated poly(styrene) (PSNH2)37 on water surface, as shown schematically in Figure 1a. Efficient exfoliation and dispersion of MoSe2 nanosheets in toluene was obtained with the use of PS-NH2 combined with sonication and ultracentrifugation, as described in our previous work.26 Few-layered MoSe2 7 ACS Paragon Plus Environment


nanosheets with PS-NH2 were successfully stabilized in toluene with a maximum concentration of approximately 0.2 mg mL−1. When several droplets of the suspension were deposited on water surface, they immediately spread on the surface because of the Marangoni effect (surface tension of water and toluene being 73.0 and 28.0 dynes/cm, respectively), giving rise to a thin composite film after evaporation of toluene (Supporting Information, Figure S2). 37 Therefore, a solvent with a surface energy lower than that of water should be selected to obtain a good spread. Dispersed TMD solutions were prepared using various solvents in our previous work, and we selected a solvent with good dispersion and low surface tension. The thickness of the nanocomposite films was controlled, from approximately 50.0 to 350.0 nm, by controlling the concentration of suspension. The nanocomposite film with n-type MoSe2 nanosheets on the water surface was mechanically transferred onto a thin p-type PFO film (approximately 100.0 nm thickness), spincoated on a boron doped p-type silicon substrate, as shown schematically in Figure 1b. The polymeric p-type layer is advantageous because of its mechanical flexibility as well as firm interface with the n-type MoSe2/polymer nanocomposite. The MoSe2/polymer nanocomposite and the PFO layers vertically stacked on the p-type Si substrate are evident in the cross-sectional high resolution TEM images, recorded simultaneously with two-dimensional energy-dispersive X-ray spectrometry (2D EDX) measurements, as shown in Figure 1c. The bright-field TEM results in Figure 1c show the bilayer of the MoSe2/polymer nanocomposite and the PFO layer of approximately 150.0 and 100.0 nm thickness, respectively. The 2D EDX maps of specific atomic elements of the constituent layers confirm the formation of the discrete layers. The subsequent thermal evaporation of the thin Au top electrodes through a shadow mask on the nanocomposite completed the fabrication of the arrays of vertical PDs. For photo-excitation of the MoSe2 nanosheets, laser with a wavelength of 835.0 nm was directly irradiated on the Au electrode, which was thin enough for the laser to penetrate and reach the nanocomposite, as illustrated schematically in Figure 1b. 8 ACS Paragon Plus Environment

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Figure 1. (a) Schematics and photographs illustrating fabrication of a thin MoSe2/PS-NH2 film at the liquid/air interface. Step I: Preparation of a suspension of MoSe2 nanosheets exfoliated and dispersed with PS-NH2 in toluene. The schematic shows the nanosheets modified with PSNH2 chains via Lewis acid-base interaction between primary amines of PS-NH2 and Mo atoms. In the step II, droplets of the suspension deposited on water surface uniformly spread with time by Marangoni effect. A thin MoSe2/PS-NH2 film was developed after complete evaporation of toluene as shown in the step III. The nanocomposite film was readily transferred on a substrate (Step IV). Dotted lines are added for clarification. (b) Schematic illustration of arrays of p-n vertical diode PDs. The zoomed scheme shows an individual PD with PS-NH2 modified MoSe2 nanosheets evenly distributed in the composite. (c) High-resolution cross-sectional TEM image of a p-n vertical diode PD. Scale bar is 100 nm. TEM-EDX results are also shown with multivariate statistical analysis of characteristic atomic elements of the constituent layers. Scale bar is 100 nm. 9 ACS Paragon Plus Environment


The 835.0 nm wavelength was chosen based on the UV-vis spectroscopy results of the MoSe2 suspension with PS-NH2, in which the characteristic A-exciton absorption was observed near 800.0 nm (Supporting Information, Figure S3a). Prior to the evaluation of the photodetection performance of the vertical PDs, the photoelectric properties of the thin nanocomposite film containing MoSe2 nanosheets were examined, and the results are shown in Figure 2. The few-layered MoSe2 nanosheet flakes modified with PSNH2 are closely packed, as shown in the scanning electron microscopy (SEM) image in Figure 2a (Supporting information, Figure S4). The transmission electron microscopy (TEM) images obtained from the diluted MoSe2 suspension in Figure 2b, c and d confirm a few layer exfoliations of the nanosheets and shows that individual flakes are approximately 5.0 nm in thickness. The distribution of the number of layers of the exfoliated nanosheets was evaluated, based on a set of AFM images in height contrast (Supporting information, Figure S5). The content of MoSe2 nanosheets in the nanocomposite was approximately 70.0 wt.% according to the Thermogravimetric analysis results (Supporting Information, Figure S3b). From the above results, it is believed that the highly dispersed MoSe2 flakes can form numerous photoconductive pathways from the top to the bottom of the 300 nm-thick composite film. Since the performance of a TMD containing photodetector increases with the number of continuous vertical pathways of TMDs, the amount of TMDs in a composite should be maximized as long as uniform film is developed with good processability. As expected, the device performance would be degraded when the TMD content were decreased.


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Figure 2. (a) A SEM image of the top view of a thin MoSe2 /PS-NH2 film. (b) and (c) Bright field TEM images of a thin MoSe2/PS-NH2 film prepared from a dilute suspension. (d) A HRTEM image of a single-layer MoSe2 nanosheet. The inset shows the Fast Fourier transform (FFT) of the HR-TEM image. (e) A Raman spectrum of a MoSe2 /PS-NH2 film. A spectrum of bare MoSe2 film is also shown for comparison (black). The inset shows the Raman spectra zoomed at the wavelength regime from 250 to 320 cm-1. (f) Photoluminescence (PL) spectrum of a MoSe2/PS-NH2 film on a glass substrate obtained with an excitation laser of 532 nm in wavelength.



The Raman spectroscopy results in Figure 2e confirm the formation of few-layered MoSe2 nanosheets.38 The Raman spectrum of the MoSe2/PS-NH2 composite on the SiO2 substrate is compared with that of bulk MoSe2 in Figure 2e. The A1g vibration mode of MoSe2 softens in the Raman spectrum of the MoSe2 film, because of weakening of interlayer coupling upon decrease in the number of layers. On the other hand, the E12g vibration mode stiffens because of strong dielectric screening of the long-range Coulombic interaction with decrease in the number of layers. Based on the degrees of blue-shift of E12g mode and red-shift of A1g mode of the exfoliated MoSe2, we expect that few-layered nanosheets are predominant in the film, which is consistent with the previously reported distribution of number of layers of exfoliated TMD nanosheets.37 The intensity of the E12g peak increased because of the physical repulsion of the MoSe2 flakes arising from the incorporation of PS-NH2. Thus, the Raman spectroscopy results reveal that few-layered MoSe2 nanosheets are dominant in the film, in which the E12g and the A1g peaks, corresponding to out-of-plane and in-plane modes, are blue- and red-shifted, respectively, with a frequency difference of approximately 0.4 cm−1. 39 The photoluminescence (PL) spectrum of the MoSe2/PS-NH2 composite on the glass substrate was recorded at an excitation wavelength of 532 nm, and the results are shown in Figure 2f. The PL peak centered at approximately 823.0 nm (1.53±0.01 eV), arising from the A excitons of MoSe2, is attributed to an indirect-to-direct bandgap transition which occurs at the K high symmetry point of the Brillouin zone associated with quantum confinement in the perpendicular direction.50 This implies the presence of substantial amounts of single layered MoSe2 nanosheets in the film prepared on water surface. The single and the few-layered MoSe2 nanosheets exfoliated by our method were characterized by atomic force microscopy (Supporting Information, Figure S5). A possible oxidation during film fabrication on water was also examined with XPS and FT-IR and the results show that no oxidation occurred during the fabrication process (Supporting information, Figure S6). The photo-response of the vertical PD with the solution-processed MoSe2/PS-NH2 film was 12 ACS Paragon Plus Environment

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examined as a function of wavelength to select an appropriate input photon source for detailed photo-detection characterization, and the results are shown in Figure 3a. Substantial photocurrent was observed in the NIR regime including a characteristic absorption peak at 800 nm arising from the A exciton of MoSe2, as also observed in the UV-vis spectroscopy results (Supporting Information, Figure S3a). Based on the wavelength dependent photocurrent results, we chose a laser wavelength of 835.0 nm for further investigation of the PDs.

Figure 3. (a) Photo-current as a function of the wavelength of input light source of a p-n junction vertical diode PD with a MoSe2/PS-NH2 film. (b) Log scale current-voltage characteristics of a p-n junction vertical diode PD with a MoSe2/PS-NH2 film before (black) and upon NIR exposure (red). (c) UPS results of a thin MoSe2/PS-NH2 film. Schematic of energy band diagram of the constituent layers of a p-n junction vertical diode PD is shown in the inset. (d) Statistical device population of log scale ON/OFF photocurrent ratios of 90 PDs (e) Photo switching behavior of a p-n junction vertical diode PD with a MoSe2/PS-NH2 film characterized by variation of photo current with periodic illumination of NIR laser. The pulse periodicity was 15 seconds. The inset shows the photocurrent switching occurs within approximately 140 msec. (f) The corresponding responsivity and photo current values of the photodetector as a function of the NIR intensity at a Vbias=-5V.

The vertical PD exhibits a characteristic pn junction diode behavior at a relatively small voltage sweep of 5.0 V with a rectification ratio of approximately 104 as shown in Figure 3b. When the laser was exposed onto the thin Au electrode of the vertical PD, the dark current in the negative voltage region was significantly increased, because of photocurrent arising from 13 ACS Paragon Plus Environment


efficient photo-carrier generation and dissociation upon the application of a biased electric field. The substantially enhanced dark current upon exposure is explained with the energy band diagram of the device shown in Figure 3c. The highest occupied molecular orbital (HOMO) level of the MoSe2/PS-NH2 nanocomposite film obtained using ultraviolet photoemission spectroscopy (UPS) was approximately 5.3 eV, which is very close to the theoretical HOMO level of MoSe2.40 Photo-carriers produced in the MoSe2/PS-NH2 nanocomposite upon laser excitation are drifted to the corresponding electrodes upon the application of a negative bias. In other words, electrons in the lowest unoccupied molecular orbital (LUMO) of MoSe2 are moved to the top Au electrode, while holes in the HOMO drift through the PFO and the boron doped Si electrode, resulting in an increase in the dark current. In our system, 30 arrays of vertical PDs were fabricated in a single batch using the thin MoSe2/PS-NH2 nanocomposite film prepared by the water-floating and transfer process. To evaluate the device-to-device reliability of the PDs, we statistically examined 90 devices. The ON/OFF ratios of the devices upon laser exposure were varied from one device to the other, but the majority of the devices, approximately 70.0 %, showed ON/OFF ratios greater than 103 with a highest population of 104, as shown in Figure 3d. Furthermore, the vertical PD quickly responded to consecutive pulsed NIR input with a switching speed of approximately 140.0 ms, as shown in Figure 3e. The multiple photo-switching performance of our device was also examined over 3600 and the results show the excellent reliability of our photodetector (Supporting information, Figure S7). It should be noted that our efforts to develop a vertical photodetector based on spin coating were unsuccessful because of the difficulty in fabricating sufficiently thick nanocomposite films with sufficient photocarriers. The results shown in Figure S8 are mostly optimized after lots of trial and error in our system. Most of spin-coated devices were electrically failed with very low yield. The photodetection performance of a PD based on spin-coating was even much worse than that on water floating method (Supporting Information, Figure S8). The photo-detection performance of the vertical PD with the 14 ACS Paragon Plus Environment

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MoSe2/PS-NH2 nanocomposite film was further examined as a function of the laser power, and the results are shown in Figure 3f. The photocurrent values at a bias voltage of −5.0 V increases with increase in the laser power from 40.0 W to 5.0 mW. The responsivity of the vertical PD was measured as a function of the laser power based on the photocurrent results, and the plot in Figure 3f shows that the responsivity of the device decreases from approximately 2.5 to 0.85 A W−1 with increase in the laser power, consistent with the results previously reported for TMD nanosheets.41,42 A maximum responsivity of 2.5 A W−1 was obtained at a laser power of 40.0 W based on the equation. The specific detectivity (D*) of the device was approximately 2.34 × 1014 Jones at a bias voltage of −5.0 V and a laser power of 40.0 W, assuming that the device noise is primarily thermal.

Figure 4. (a) Log scale ON and OFF photocurrent ratio values (black) and dark current values (red) of p-n junction vertical diode PDs as a function of thickness of MoSe2/PS-NH2 films. (b) Log scale ON and OFF photocurrent ratio values of p-n junction vertical diode PDs with MoSe2 /PS-NH2 films as a function of PFO film thickness. (c) Log scale ON and OFF photocurrent ratio values of a p-n junction vertical diode PD with a MoSe2 /PS-NH2 film as a function of temperature. The variation of dark current of the device as a function of temperature is also shown (red). Based on the dark current variation results with temperature, the contribution of photocurrent arising from exciton-dissociation was calculated.

A more systematic investigation of the performance of the vertical PDs was carried out as a function of the film thickness of the nanocomposites. Figure 4a shows that the film with a thickness of approximately 45.0 nm prepared from a 0.05 wt.% MoSe2 suspension exhibited excellent photocurrent with the maximum ON/OFF ratio greater than 103. The highest ON/OFF ratio of approximately 104 was obtained for the 130.0 nm-thick nanocomposite film prepared 15 ACS Paragon Plus Environment


from a 0.2 wt.% MoSe2 suspension (see Figure 3b and Supporting Information, Figure S9). The maximum ON/OFF ratio decreased at film thicknesses greater than 130.0 nm showing a value of approximately 102 for a nanocomposite thickness of approximately 350.0 nm. In the process of increasing the concentration of the MoSe2 solution to 0.8 wt.%, the MoSe2 flakes tented to reaggregation. The reaggregated MoSe2 shows a tendency to increase conductivity, thus dark current of device is increase. The ON/OFF ratio of the vertical PD was also examined as a function of the thickness of the PFO layers, and the results are shown in Figure 4b. No significant variation in the ON/OFF ratio was observed in the vertical PDs for PFO layer thicknesses ranging from approximately 55.0 to 273.0 nm (Supporting Information, Figure S10). There exist three main sources of photocurrent arising from TMDs on visible and NIR exposure: (1) photoconduction by photo-induced band excited carriers that can be dissociated into free electrons and holes thermally or by a large electric field, (2) photoconduction by photoexcited carriers decaying into heat that makes TMDs warm, resulting in the reduction of electrical resistance, i.e., bolometric photocurrent, and (3) photothermoelectric generation of current by light illumination across the metal–TMD interface. Considering that photodetectors with polymer composites frequently show bolometric photocurrent upon NIR exposure, 43,44 the bolometric photocurrent of the vertical PD with the MoSe2/PS-NH2 nanocomposite was also investigated in Figure 4c. For this, we initially monitored the variation of dark current of the device as a function of temperature upon heating at a bias voltage of -5.0 V. After the photocurrent measurement of the device upon NIR exposure as a function of the device temperature, the temperature dependent dark current was compared with the total photocurrent values as a function of temperature to determine the contributions of both photo-exciton dissociation and bolometric photocurrent.26, 45 The results in Figure 4c indicate that the thermal contribution is approximately less than 18% even at a temperature of 105.0 C which is considerably higher than the temperature expected to be attained by the device upon NIR exposure. Thus, the photo-induced band excitation is expected to be dominant in the vertical 16 ACS Paragon Plus Environment

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PD with the MoSe2/PS-NH2 nanocomposite, while both the bolometric and the photothermoelectric effects are negligible. 45 The pn type vertical PD with the MoSe2/PS-NH2 nanocomposite film was fabricated on a mechanically flexible plastic substrate, since the nanocomposite film prepared on the water surface could be easily transferred onto any water-insoluble substrates. The nanocomposite film was transferred onto a PFO layer spin-coated on an indium tin oxide (ITO) electrode deposited on a poly (ethylene terephthalate) (PET) substrate.

Figure 5. (a) Schematic illustration of a mechanically flexible p-n junction vertical diode PD with a MoSe2 /PS-NH2 film. A device consists of a spray-coated networked Ag nanowire electrode, a thin MoSe2/PS-NH2 film, a PFO and an ITO bottom electrode vertically stacked on a PET substrate. A photograph of arrays of the flexible PDs is shown in the inset. (b) Log scale current-voltage characteristics of a flexible p-n junction vertical diode PD before (black) and upon NIR exposure (red). (c) The corresponding responsivity (red) and photo current (black) values of a flexible p-n junction vertical diode PD as a function of the NIR power at a Vbias=5V. (d) Photo switching behavior of a flexible p-n junction vertical diode PD characterized by variation of photo current with periodic illumination of NIR laser (the intensity of 123 mWcm2 ). The pulse periodicity was 15 seconds. (e) Log scale ON/OFF photo current values of a flexible p-n junction vertical diode PD measured at Vbias=-5V as a function of (e) the bending diameter as well as (f) the bending cycles up to 1000 times. The inset photograph shows in-situ bending measurement set-up of the flexible photodetectors at different bending radii.

Subsequently, patterned Ag nanowire network electrodes were fabricated on the nanocomposite by spray coating with a pattern mask, forming arrays of mechanically flexible 17 ACS Paragon Plus Environment


vertical PDs as shown in Figure 5a (also see the inset). The characteristic pn type diode behavior was observed with a rectification ratio of approximately 103, comparable with that observed in the device prepared on the p-type Si substrate, as shown in Figure 5b. Upon direct illumination of the top Ag network electrode of the vertical PD with the 835.0 nm laser, substantial photocurrent was detected for a voltage sweep of ±5.0 V. Similar to the case of the vertical PD on the hard substrate, the photocurrent increases with increase in the laser power from 7.0 to 120.0 mW, owing to the presence of photo-excited carriers in the MoSe2 nanosheets, followed by their dissociation at the pn interface, as shown in Figure 5c. The lower photocurrent with a flexible device than one on a hard substrate mainly arises from ITO electrode, in particular, deposited on a PET substrate in the flexible device. We believe that the higher resistance of the ITO (15Ω) than p-doped electrode (0.001Ω) gave rise to the relatively lower device performance. The responsivity of the device was obtained as a function of the laser power based on the photocurrent results, and the resulting plot in Figure 5c shows that the responsivity of the device decreases with the laser power, consistent with the results reported previously for TMD nanosheets. A maximum responsivity of approximately 1.5 A W−1 was obtained at a laser power of 120.0 mW, slightly lower than that for the PD on the hard substrate owing to the lower ON current in the flexible device without the p-type Si layer. The photo-response switching behavior of the flexible device is also comparable with that of the PD on the hard Si substrate with a switching speed of approximately 100.0 ms, as shown in Figure 5d. The photodetection performance of the flexible PD with the MoSe2/PS-NH2 nanocomposite film was investigated as a function of the bending diameter as well as the number of bending events at a fixed bending diameter. There was no significant change in the ON/OFF ratio even at a bending diameter of 3.0 mm as shown in Figure 5e. The mechanically flexible PDs were also reliable after multiple bending events, more than 1000 cycles, at a 7.2 mm bending diameter as shown in Figure 5f and Supporting Information, Figure S11. The ON/OFF ratio rapidly decreases with the number 18 ACS Paragon Plus Environment

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of bending events when the bending diameter is less than 7.2 mm. Main reason for the degradation upon repetitive bending was due to the mechanical failure of transparent ITO electrode rather than the failure of the TMD layer (Supporting information, Figure S12 and S13). It would be eventually interesting to develop a photodetector with both p- and n-type TMD layers which is our on-going effort.

CONCLUSIONS We successfully fabricated thickness-controlled MoSe2 films consisting of single and fewlayered nanosheets by solution exfoliation of bulk MoSe2 with amine-terminated polymers, followed by solvent evaporation on water surface based on the Marangoni effect. The facile transfer of the nanocomposite prepared on water onto another substrate was useful for the fabrication of high-yield arrays of vertical type diode MoSe2 PDs with device architecture of Au/MoSe2 nanocomposite/PFO/p type-Si/Al from top to bottom. Upon exposure to a NIR laser of 835.0 nm wavelength, the devices exhibited excellent photodetection performance with an ON/OFF current ratio, operation voltage, switching time, and maximum responsivity of 10 4, 5.0 V, 100.0 ms, and 2.5 AW−1, respectively. Furthermore, by transferring the thin MoSe2 nanocomposite film on a bilayer of PFO/ITO on a PET substrate, followed by spray printing of an Ag nanowire network electrode, we were able to fabricate arrays of mechanically flexible PDs with photodetection performance comparable with that of PDs fabricated on a hard substrate even after 1000 bending cycles at a bending diameter of 7.2 mm.



ASSOCIATED CONTENT

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Office phone: +82-2-2123-2833 Fax:+82-2-312-5375 ORCID Cheolmin Park: 0000-0002-6832-0284 Author Contributions I.H. and J.S.K. are equally contributed. Notes The authors declare no competing financial interest.

Acknowledgements ∇

These authors contributed equally to this work. This research was supported by a grant from

the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (Nos. 2017R1A2A05001160) and (NRF-2016M3A7B4910530). This material is based on work supported by the Air Force Office of Scientific Research under award number FA2386-16-14058.

Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))


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High performance vertical p-n diode Near Infrared photodetectors are successfully fabricated with solution-processed, thin n-type MoSe2 nanosheet composite films. The thickness-controlled MoSe2 films consisting of single and few-layered nanosheets are developed by liquid-exfoliation of bulk MoSe2 with amine-terminated polymers, followed by solvent evaporation on water surface based on the Marangoni effect. Furthermore, mechanically flexible photodetectors fabricated on polymer substrates exhibit excellent performance after 1000 bending cycles.

KEYWORDS: Transition metal dichalcogenides, Vertical p-n diode, Photodetector, MoSe2 nanosheets, Liquid exfoliation, Amine-terminated polymers, Water-air interface


Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe2

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