Zero-Dimensional Quantum Dot

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Functional Nanostructured Materials (including low-D carbon)

Hybrid Black Phosphorus-0D Quantum Dots Phototransistors; Tunable photodoping and Enhanced photoresponsivity A-Young Lee, Hyun-Soo Ra, Dohyun Kwak, Jeong Min-Hye, JeongHyun Park, Yeon-Su Kang, Weon-Sik Chae, and Jong-Soo Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03285 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Materials & Interfaces

Hybrid Black Phosphorus-0D Quantum Dots Phototransistors; Tunable photodoping and Enhanced photoresponsivity A-Young Leea, Hyun-Soo Raa, Do-Hyun Kwaka, Min-Hye Jeonga, Jeong-Hyun Parka, Yeon-Su Kanga, Weon-Sik Chaeb, Jong-Soo Leea* a

Department of Energy Science and Engineering, DGIST, Daegu, 42988, Republic of Korea b

Analysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 41566, Republic of Korea

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT Recently, black phosphorus (BP) with direct band gap exhibited excellent potential for optoelectronic applications due to its high charge carrier mobility and low dark current as well as the variable band gap of 0.3 eV to 1.5 eV depending on the number of layers. However, few-layer BP-based phototransistors (photo-FETs) have been limited in sensitivity and wavelength selectivity. To overcome the drawback of these photo-FETs, we studied hybrid photo-FETs combined with the novel properties of the two materials between the channel and sensitizer layers. By combining a strong absorbance of quantum dots (QDs) layer and a 2D layer material with high carrier mobility, the hybrid photo-FETs are expected to produce high-performance photodetector that can effectively control the responsivity, detectivity, and response time. In this study, we demonstrate that the photogenerated carriers formed from QDs sensitizer layers migrate to BP transport layer with high charge mobility, and not only improve the photodetector performance, but also enhance the photodoping effect of BP transport layer with an ambipolar characteristic by electrons transferred from n-type CdSe QDs or holes injected from p-type PbS QDs. The responsivity and detectivity of hybrid BP/0D photo-FETs exhibits 1.16×109 A W-1 and 7.53×1016 Jones for the BP/CdSe QDs photo-FET and 5.36×108 A W-1 and 1.89×1016 Jones for the BP/PbS QDs photo-FET, respectively. The photocurrent rise (τrise) and decay (τdecay) times were τrise = 0.406 s and τdecay = 0.815 s for BP/CdSe QDs photo-FET, and τrise = 0.576 s and τdecay = 0.773 s for BP/PbS QDs photo-FET, respectively.

Keywords: Black phosphorus, Quantum dots, Phototransistor, hybrid BP-QDs, Photodoping


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Introduction The channel materials used in high-performance photo-FETs should exhibit high chargecarrier mobility, a direct and moderately wide bandgap, and an extremely low-trap state density.1-2 Graphene was first employed as a channel material owing to its high electron mobility.3 Recently, the family of two-dimensional transition metal dichalcogenides (TMDCs) has been spotlighted as an alternative for conventional optoelectronic devices due to their tunable band gap and high charge carrier mobility.4-7 However, the reported TMDC photo-FETs exhibit a low photo-responsivity on the range from 10-3 to 103 A W-1 and slow response time in the order of several miliseconds.8-9 Also, the most prominent disadvantage of TMDC photo-FETs is the limited selectivity of detection wavelength range.1, 10 On the other hand, QDs possess the ability to tune their band gap with diameter, which can control their light absorbance and emission wavelength.11-12 Therefore, QDs have been considered as potential candidates for low-cost, high-sensitivity photodetectors with selective detection wavelength.11-13 However, colloidal photodetectors exhibit a low charge carrier mobility and a surface trap states, which makes it difficult to efficiently extract the photogenerated carriers from the active layer, resulting in low photoconductive gain and response time.11, 14 Twodimensional (2D)/zero-dimensional (0D) hybrid systems are being considered as functional materials with complementary advantages, which are difficult to realize in a single component.1, 5, 10, 15-16 It allows them to be used for the transport and the sensitizer layer for the high-sensitivity photo-FETs.1-2, 5, 10, 15-16 In the previously reported hybrid photo-FETs, graphene-PbS QDs photo-FETs showed a high responsivity up to 107 A W-1 due to the photogating effect through capacitive coupling.15 However, the graphene-based photo-FETs suffered from the high dark currents, low on-off ratios, and slow response time due to the semimetal property of graphene.15, 17 Also, MoS2-PbS QDs photo-FETs showed the improved



dark current and response time by introducing n-type MoS2 channels.5 The responsivity of hybrid MoS2 photo-FETs improved several orders of magnitude higher than that of MoS2 photo-FETs and reached values of 105-106 A W-1 due to large carrier multiplication. However, the device can only operate in the OFF state under a large applied gate voltage. Without the depletion effect of the gate function, the responsivity of MoS2-PbS QDs photoFETs shows very low photoresponse.5 Although researchers on hybrid photo-FETs have been reported, it is not yet optimized for various conditions such as a combination of TMDCs and QDs, device thickness, and surface functionalization. Recently, black phosphorus (BP) with direct band gap emerged as a highly promising candidate for 2D optoelectronic applications due to its high charge carrier mobility and low dark current as well as the variable band gap of 0.3 eV to 1.5 eV depending on the number of layers.18-19 However, BP is known to be notoriously unstable in the ambient condition. It is mainly originated from the formation of phosphoric acid on the BP surface under ambient conditions.20 Recently, we have demonstrated that the phosphoric acid that causes fatal degradation of the BP device is fully recovered by 1.2-ethandithiol (EDT, C2H4(SH)2) treatment.20 The device characteristics of the degraded BP FETs were completely recovered to the level of the pristine states by the EDT treatment. At the same time, EDT molecules with a short ligand length strengthen the electronic coupling between QDs by displacing the ligand on the nanoparticle surface.20 Therefore, the EDT chemistry can play an essential role in the curing of degradation of BP surface and the improving the charge transfer characteristics between the QDs and the BP by substituting the ligands of the QDs. In this study, we demonstrate that the photogenerated carriers formed from QDs sensitizer layers transferred to BP transport layer with high charge mobility to improve the photodetector performance, as well as the enhance the photodoping effect of BP transport layer with an


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ambipolar characteristics by electrons transferred from n-type CdSe QDs or holes injected from p-type PbS QDs. Besides, we employ EDT molecules to significantly improve the device characteristics by enhancing the electronic coupling at the interfaces between BP and QDs.21 The mechanism of the photo-doping and the excellent photoelectric properties were verified by time-resolved photoluminescence (TRPL) and photoelectric characteristics studies. The schematics of the BP/CdSe QDs and BP/PbS QDs hybrid photo-FETs are illustrated in Fig. 1a and 1e, respectively. As shown in Fig. 1b and 1f, the channel length (L) and width (W) of the hybrid photo-FETs were 2.7 µm and 9.9 µm for BP/CdSe QDs, 2.7 µm and 13.0 µm for BP/PbS QDs photo-FETs, respectively. Fig. S1a shows a high-resolution TEM (HRTEM) image and Fast Fourier Transforms (FFT) patterns of selected areas of asexfoliated BP flake used in this study. It is demonstrated in the TEM image that as-exfoliated BP flake shows a high crystallinity showing lattice fringes without any defects, which is also confirmed by FFT patterns in the inset. Fig. S1b-i present the cross-section TEM images and EDX analysis of hybrid BP/CdSe QDs and BP/PbS QDs photo-FETs shown in Fig.1b and 1f, respectively. It was confirmed that CdSe and PbS QDs are deposited uniformly on the BP device with a thickness of 10 nm. We also performed the TRPL measurement to understand the charge transfer of the photogenerated carriers in the 2D/0D heterojunction region.22-26 Fig. 1c and 1d indicate the PL mapping images and decay profile of BP/CdSe QDs photo-FETs. The dark area (#1) in Fig. 1c shows the BP/CdSe QDs heterojunction; The PL of CdSe QDs layers (#2) on the substrate in Fig. 1b is strongly illuminated, whereas the PL of CdSe QDs layers (#1) deposited on the BP channel is almost completely quenched. The shortened PL lifetime and the lower PL intensity of hybrid device are attributed to the electron transfer from donor QDs to acceptor BP channel. In contrast, in the BP/PbS QDs photo-FETs, the PL



intensity of PbS QDs was lower than that of CdSe QDs except in the electrode regions (Fig. 1g). OA-capped PbS QDs have a significant effect on the PL intensity by EDT treatment compared to TOP-capped CdSe QDs. The lower PL intensity and the shortened carrier lifetime of close-packed PbS QDs could be originated from the formation of non-radiative recombination pathways by the formation of deep mid-bandgap trap centers.27-30 To evaluate photoresponse of hybrid photo-FETs, we first measured the ID-VG characteristics of the pristine BP device in a glovebox filled with nitrogen. As shown in Fig. S2b, the pristine BP photo-FET exhibits an ambipolar transport behavior in the dark with the hole mobility of 208 cm2 V-1 s-1 and the electron mobility of 70.1 cm2 V-1 s-1, respectively. The photoresponse of pristine BP photo-FET under illumination with 405 nm laser diode (425.96 µW cm-2) was hardly measured, while the hole and electron mobility almost remained unchanged. The carrier mobility in the linear regime calculated from the transconductance (gm) by plotting drain current (ID) versus gate voltage (VG) at a constant drain-source voltage (VDS). The slope of this plot is equal to gm: µlin = Lgm / (WCiVDS), where L is channel length, W is the width of the active area, gm = transconductance, Ci is the capacitance of 300 nm SiO2, and the channel length and width ratio was determined by the dimension of device. However, the hybrid BP photo-FET deposited n-type CdSe QDs shows a very different transport behavior than the pristine BP FET. As shown in Fig. S2c, once the deposition of CdSe QDs on the pristine BP device, the transport behavior of the BP/CdSe QDs photo-FET exhibits an excellent surface charge doping effect toward n-type. In particular, in the BP /QDs hybrid device hybridized with n-type CdSe QDs, the hole mobility was significantly reduced from 208 cm2 V-1 s-1 to 113 cm2 V-1 s-1 compared with the pristine BP device in Fig. S2b, while the electron mobility was increased from 70.1 cm2 V-1 s-1 to 93.0 cm2 V-1 s-1. In contrast, the BP /QDs hybrid combined with p-type PbS QDs shown in Fig.


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S2f was clearly observed the surface charge doping effect toward p-type. The measured hole mobility of BP/PbS QDs photo-FETs was 160 cm2 V-1 s-1, which was significantly increased by 15 cm2 V-1 s-1 compared with 145 cm2 V-1 s-1 of the pristine BP devices of Fig. S2e. On the other hand, the electron mobility of the hybrid device was decreased from 50.8 cm2 V-1 s-1 to 26.1 cm2 V-1 s-1. The doping phenomenon could be explained by the charge transfer in the interfaces between BP layer and QDs layer. Comparison the energy levels between BP and QDs can predicts a favorable system for the charge transfer from the 1Se quantum confined state of n-type CdSe QDs and the 1Sh quantum confined state of p-type PbS QDs to the BP transport layer. As shown in Fig. S1, the interfaces of the hybrid device improved by the EDT can facilitate such charge displacement between BP and QD QDs. The similar phenomena was recently reported as the surface doping of Cs2CO3, MoO3/BP hybrid devices.31 To understand additional photo-induced doping effect in detail, we studied systematically BP/QDs hybrid photo-EFTs according to the intensity of incident light as shown in Fig. 2. As shown in Fig. 2a and 2c, the neutral point shift (∆VNP) of the BP/CdSe QDs photo-FETs was negatively shifted from -10 VG to -18 VG as the light intensities increased from 0.24 µW/cm2 to 575.04 µW/cm2, which imply the n-type doping. The doping concentrations of photoinduced hybrid photo-FETs were calculated from the linear plots of Fig.S3a as following the equation; p (or n) = IDL/qWµlinVDS,32 where q is the electron charge, L and W are the length and width of the channel, µlin is a linear mobility, respectively. As shown in Fig. S4a, the electron (n) and hole (p) concentration of BP/CdSe QDs photo-FETs measured at 80VG and 80 VG under the dark condition were 5.08×1011 cm-2 and 42.72×1011 cm-2, respectively. The electron doping concentration of BP/CdSe QDs device clearly increased from 5.41×1011 cm-2 to 6.46×1011 cm-2, the hole concentration decreased from 33.94×1011 cm-2 to 21.51×1011 cm-2, as the light intensities increased from 0.24 µW/cm2 to 575.04 µW/cm2. The changes in the



carrier concentration are consistent with the electron mobility increasing from 45.2 cm2 V-1 s1

to 59.3 cm2 V-1 s-1 and the hole mobility decreasing from 93.4 cm2 V-1 s-1 to 53.8 cm2 V-1 s-

1

. On the other hand, the ∆VNP in the BP/PbS QDs photo-FETs, as shown in Fig. 2b and 2d,

demonstrated a p-type doping effect that positively shifted from 5 VG and 16 VG as the light intensities changed from 0.24 µW/cm2 and 24.49 µW/cm2. The hole mobility increased from 208 cm2 V-1 s-1 to 211 cm2 V-1 s-1 and the electron mobility decreasing from 89.1 cm2 V-1 s-1 to 40.1 cm2 V-1 s-1 as the light intensity changed. The variation of carrier concentrations of hybrid BP/PbS QDs photo-FETs were calculated from the linear plots of Fig.S3b. As shown in Fig. S4b, the concentration of hole and electron of BP/PbS QDs photo-FETs measured at 80 VG and at 80 VG under dark condition were 55.0×1011 cm-2 and 2.57×1011 cm-2, respectively. As the light intensities increased from 0.24 µW/cm2 to 24.49 µW/cm2, the hole concentration of BP/PbS QDs photo-FET increased from 60.2×1011 cm-2 to 64.0×1011 cm-2, on the other hand, the electron concentration decreased from 2.51×1011 cm-2 to 1.16×1011 cm2

. The photocurrent (Iph) of photo-FETs occurs at ∆VNP of a transistor as following equation,

Iph = gm ∆VNP. Where gm is the transconductance, and ∆VNP is the change in the neutral point voltage, and generally increases sublinearly with the light intensity (∆VNP =Pinα, for α

Zero-Dimensional Quantum Dot

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