Subscriber access provided by UNIV OF DURHAM
Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate Tunable Responsivity Yu Huang, Fuwei Zhuge, Junxian Hou, Liang Lv, Peng Luo, Nan Zhou, Lin Gan, and Tianyou Zhai ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02380 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 38 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
ACS Nano
Van der Waals Coupled Organic Molecules with Monolayer
MoS2
for
Fast
Response
Photodetectors with Gate Tunable Responsivity Yu Huang, † Fuwei Zhuge,*, † Junxian Hou, ‡ Liang Lv, † Peng Luo, † Nan Zhou, † Lin Gan, † Tianyou Zhai*,†,ξ †
State Key Laboratory of Material Processing and Die & Mould Technology, School of
Material Sciences and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China ‡
Department of Composite Materials and Engineering, College of Materials Science and
Engineering, Hebei University of Engineering, Handan, 056038, P. R. China ξ
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry,
Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, P. R. China
KEYWORDS: Transition metal dichalcogenides, Van der Waals junction, organic molecules, charge transfer interaction, photodetection, response dynamics
ACS Paragon Plus Environment
1
ACS Nano 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
Page 2 of 38
ABSTRACT: As a direct bandgap transition metal dichalcogenide, atomic thin MoS2 has attracted extensive attentions in photodetection, whereas the hitherto unsolved persistent photoconductance (PPC) from the ungoverned charge trapping in devices has severely hindered their employments. Herein, we demonstrate the realization of ultrafast photoresponse dynamics in monolayer MoS2 by exploiting a charge transfer interface based on surface assembled zinc phthalocyanine (ZnPc) molecules. The formed MoS2/ZnPc Van der Waals interface is found to favorably suppress PPC phenomenon in MoS2 by instantly separating photogenerated holes to the ZnPc molecules, away from the traps in MoS2 and the dielectric interface. The derived MoS2 detector then exhibits significantly improved photoresponse speed by more than 3 orders (from over 20 s to less than 8 ms for the decay) and a high responsivity of 430 A/W after Al2O3 passivation. It is also demonstrated that the device could be further tailored 2~10 fold sensitive while without severely sacrificing the ultrafast response dynamics using gate modulation. The strategy presented here based on surface assembled organic molecules may thus pave the way for realizing high performance TMDs based photodetection with ultrafast speed and high sensitivity.
Two dimensional transition metal dichalcogenides (2D TMDs) with tailored layer number, planar and vertical assembly are finding increasing interests1,
2
due to their tunable
optoelectronic properties for field effect transistors,3 memories,4 photodetectors,5,
6
and
neuromorphic devices.7, 8 As a prominent member in the TMDs family, MoS2 possesses layer dependent bandgaps of 1.2~1.9 eV,9 making it particularly suitable for application in low power devices with high on-off ratio.3, 10 In the last few years, rapid progresses have been made in the large scale synthesis, transfer and Van der Waals (VdW) assembly of mono-tofew layer MoS2,11-13 and also on their tunable optical properties, carrier dynamics, etc.14-16 However, in photodetectors, a notorious issue for MoS2 based devices exists due to their
ACS Paragon Plus Environment
2
Page 3 of 38 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
ACS Nano
persistent photoconductance (PPC),6 which has been debatably attributed to the minority carrier trapping by surface absorbents, inherent trap states in MoS2 and the neighboring dielectrics.17-19 Though such minority carrier trapping can improve the gain mechanism in detectors by elongating the apparent carrier lifetime,20 a dramatic sacrifice on the response speed is compromised in devices since the PPC effect often sustain minutes to hours.5, 21 Attempts to improve the response dynamics, e.g. using surface passivation and field effect methods that modulate the density of trap states and their occupation,18 on the other hand often lead to the loss of sensitivity due to the adverse suppression of trap induced gain mechanisms.19 Deliberately designed photoconductive detectors that exploit vertical carrier separation and trapping at heterostructured interfaces have been brought in for balanced detection bandwidth and responsivity, e.g. by using vertically stacked VdW materials22 or surface coupling of quantum dots.23 The essence here relies on the fast gain mechanisms from the rapid photovoltaic charge transfer at the interface under the built-in electric field and type II band alignment.24, 25 However, the prevailing VdW junctions from mechanical exfoliation confront significant challenges in device integration and scaling, while the devices with integrated quantum dots tend to suffer from surface trap states and stability issues.25 Assembly of alternative viable charge transfer interfaces on 2D surfaces are thus very attractive for the engineering of MoS2 and TMDs based detectors. Because of the ultrathin nature, 2D TMDs are known to exhibit charge transfer interactions with their neighboring substrates and surface absorbents including O2 and humidity from ambient.26,
27
Such characteristic however renders a feasible strategy of using surface
assembled charge transfer molecules for the tailorable engineering of their optoelectronic properties. Organic molecules that can be assembled on the surface of TMDs in solution or vapor phase using the dipole or VdW interactions have been intensively explored to passivate
ACS Paragon Plus Environment
3
ACS Nano 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
Page 4 of 38
the surface defects,28 tailor the doping polarity,29 and modulate the photoluminescence properties of TMDs.30 However, their impacts to the photoresponse dynamics in detectors remains to be unraveled. As the conventional organic photovoltaic materials, phthalocyanine (Pc) and its metal derivatives (MPcs) are known to exhibit excellent chemical stability and have been recently demonstrated to hold fast charge transfer interactions with TMDs (MoS2, MoSe2, WSe2, etc.).31-35 Their large π-conjugated structure renders feasible VdW molecule assembly on the surface of TMDs, therefore displays foreseeable potential in forming facile charge transfer interfaces that may enhance the photoresponse behavior of MoS2 and the like other materials. Herein, we present the achieving of ultrafast photoresponse dynamics in monolayer MoS2 detectors by the surface assembly of ZnPc molecules. It is found that the assembly of ZnPc molecules tends to compensate the intrinsic electron doping in MoS2 by withdrawing electrons from MoS2. Such charge transfer renders a spontaneous reverse separation of electron-hole pairs under illumination, driving holes to ZnPc molecules. This is found to favorably suppress the rather slow minority carrier trapping to the inherent trap states in MoS2 and the substrates, providing the accelerated response speed (0.2 V. Such characteristic confirms the presence of interfacial dipoles between n-type MoS2 and p-type ZnPc molecules due to their charge transfer interaction. As indicated by the solid line in figure, the junction current is well fitted by:47 I =
( ) (2)
using an ideal factor n of 2.7, where I0 and Rs represent respectively the saturation current and serial resistance of the junction, kB and T are the Boltzmann constant and temperature. We found the ideal factor is smaller than previous reported values for inorganic VdW junctions of BP/MoS2 (4~7),47 and the organic-inorganic junction of rubene/MoS2 (~3.9).48
ACS Paragon Plus Environment
8
Page 9 of 38 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
ACS Nano
Since the ideal factor is intimately related to the recombination of electrons and holes at the interface (n=2 for Shockley-Read-Hall recombination, and become >2 if significant trap states are present), the small value here demonstrate the high interface quality of the ZnPc/MoS2 VdW junction, which is highly desired in optoelectronic devices. To exploit the charge transfer interface between ZnPc and MoS2 in optoelectronic devices, we focus in the following to the photoresponse behavior of ZnPc treated MoS2 detectors. This is conducted by investigating MoS2 phototransistors after various ZnPc treatments from 5 to 40 min. Figure 4a shows the schematic illustration of the phototransistor based on ZnPc decorated MoS2. All the devices are prepared on Si substrates with 300 nm SiO2 layer while using thermal evaporated Cr/Au (5/50 nm) electrodes for electrical contact. The conductive Si substrate is used as the back gate for field effect modulation. The field effect transfer curves for the same MoS2 device before and after varied ZnPc treatments (10, 40 min) are first measured, as shown in Figure 4b. The measurements were conducted in both dark and illuminated (532 nm, 3.64 mW/cm2) conditions using a source-drain bias (Vds) of 1.6 V. As displayed in the figure, the measured transfer curves for all devices manifest n-type conduction with electrons as the majority carriers. Increasing the ZnPc treatment leads to the decrease of source-drain current (Ids) and positive shift of threshold voltage (Vth), which are consistent with electron compensation effect of ZnPc molecules to MoS2. After 40 min ZnPc treatment, one witnesses the emerging of bipolar characteristics (p-type at Vg5 V) in MoS2 under dark condition. However, under light illumination, the samples display again n-type conduction with considerably negatively shifted threshold voltages of
