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Effects of graphene quantum dots interlayer on performance of ZnO-based photodetectors
Accepted Manuscript Effects of graphene quantum dots interlayer on performance of ZnO-based photodetectors
S. Soraya Mousavi, Ali Kazempour, Babak Efafi, Mohammad Hossein Majles Ara, Batool Sajad PII: DOI: Reference:
S0169-4332(19)32181-6 https://doi.org/10.1016/j.apsusc.2019.07.145 APSUSC 43403
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
7 May 2019 29 June 2019 16 July 2019
Please cite this article as: S.S. Mousavi, A. Kazempour, B. Efafi, et al., Effects of graphene quantum dots interlayer on performance of ZnO-based photodetectors, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.07.145
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ACCEPTED MANUSCRIPT
Effects of Graphene Quantum Dots interlayer on Performance of ZnO-Based Photodetectors S. Soraya Mousavia, b, *, Ali Kazempourb, Babak Efafi b,c, Mohammad Hossein Majles Arab, c,*,
c
Photonics Laboratory, Department of Physics, Kharazmi University, Alborz, Iran
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b
Photonics Laboratory, Department of Physics, Alzahra University, Tehran, Iran
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a
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Batool Sajada
Nanophotonics Laboratory, Research Institute of Applied Science Center, Kharazmi University,
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Alborz, Iran
KEYWORDS: Graphene Quantum dots; ZnO-based photodetector; Ultraviolet; Sandwich structure; Fast response; High signal to noise ratio;
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ABSTRACT
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In this paper, the effects of inserting graphene quantum dots (GQDs) as an interlayer
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between two zinc oxide (ZnO) thin films is studied comprehensively. This kind of sandwich structure offers various advantages over similar structures modified using GQDs. It is obvious
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that exploiting GQDs properties can thoroughly modify the optical and electrical properties of ZnO thin films. Due to the two Schottky barriers created at the interfaces, low amounts of dark
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current (about 127 nA) and a high signal to noise ratio (SNR) of 3.52×102 is measured for a
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sandwich structure comprised of ZnO/GQDs/ZnO (ZGZ) layers. Furthermore, the high-speed photodetector, referred in this paper, responded at least 23 times faster compared to pure ZnO (ZZ), resulting in 370 µs and 8.8 ms of response time for ZGZ and ZZ, respectively. The high amounts of responsivity and detectivity achieved by this visible-blind photodetector are other unique features this form of sandwich hybrid structures benefit from. Four orders of magnitudes enhancement in detectivity accomplished by inserting a thin film of GQDs.
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ACCEPTED MANUSCRIPT 1. Introduction: The discovery of Graphene as a two-dimensional honeycomb lattice by Novoselov et al. in 2004 has caused significant progress in the semiconductor industry.1 Due to superior electronic, thermal, and mechanical properties as well as higher chemical stability, Graphene has
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drawn the attention of scientists as a promising building block for the next generation of
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semiconductor devices. Other properties that have made this novel form of carbon the best
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candidate for optoelectronic devices include high carrier mobility at room temperature (above
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15000 cm2V−1s−1), the optical transmittance of about 97.9% to the white light, flexibility, and light -weight.2 This kind of carbon allotrope can be seen as the basic unit of all other dimensional
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carbon materials. For example, it can be grown as zero-dimensional fullerene and rolled into 1D carbon nanotubes or stacked into 3D graphite.3-5 So far, several techniques have been developed
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to produce graphene, including micromechanical exfoliation of highly oriented pyrolytic graphite
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(HOPG), ultrasonication exfoliation of graphite, chemical reduction of graphite oxide (GO),
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carbon nanotube (CNT) unzipping, and epitaxial and chemical vapor deposition (CVD) growth. 6 These different morphologies exhibit different physical and chemical properties compared with
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pristine graphene. With a graphene-like structural framework of sp2 hybridized carbon, graphene quantum dots (GQDs) are highly heterogeneous, nanoscale, 0D particles with a typical diameter
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of 1–20 nm. 7 The quantum confinement and edge effects in GQDs provide them with robustness towards limitations faced by Graphene (zero band-gap nature and semi-metal conductivity), offering specific characters for direct use in optoelectronics.8 In recent studies, additional excellent properties of GQDs, such as high transparency and surface area, have been proposed for energy and display applications.9 In this way, different synthesis methods have been
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ACCEPTED MANUSCRIPT investigated to get graphene quantum dots such as a novel hydrothermal method, chemical oxidation and exfoliation method, and citric acid splitting method.10 On the other hand, unique optical and electrical properties of ZnO (as one of the most applied semiconductors) can be modified using graphene quantum dots. The ZnO and GQDs
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hybrids (e.g. core-shell structures) are of particular interest because they combine the superior
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wavelength selectivity of ZnO and the charge mobility of graphene, both of which are critical to
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applications of ultraviolet (UV) photodetectors, electron emitters, and many other optoelectronic
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devices. 11-16 Tuning the band gap of GQDs by controlling the size makes it suitable for use in a wide range of optical applications, especially in the ultraviolet region of light.
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Until now, various devices based on ZnO/GQDs hybrids have been reported.11These
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hybrids are ideal candidates for both light emitting diodes (LEDs) and visible-blind UV detectors needed in various applications, such as flame detection, environmental monitoring, and UV
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astronomy.17 In this way, a novel-designed UV photodetector based on a sandwich structure
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composed of ZnO and GQDs thin films (ZnO/GQDs/ZnO) was fabricated and characterized. This applicable structure due to the possessing two interfaces offers many advantages over
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similar structures with one interface, including a higher degree of control over its properties. Due
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to differences between the work functions of each layer as well as the arrangement of layers, optical and electrical characters of the multilayer structure are accurately modified. Moreover, studying the photoluminescence (PL) spectra, the absorption spectra, and the photoresponsivity, indicates that the carriers’ separation and transfer between graphene quantum dots and ZnO should play an important role in the functionality of ZnO/GQDs/ZnO UV photodetectors. Our investigations provide a promising route to design high-performance photodetectors by controlling the interfaces between GQDs films and semiconductors. According to our obtained
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ACCEPTED MANUSCRIPT results, introducing a thin film consisting of GQDs between two ZnO layers can considerably improve all electro-optical features of the ZnO-based photodetectors, especially enhances its response speed. This visible-blind photodetector presents unique properties; all of which are properly enhanced, unlike most reported devices that offer a maximum of only one or two
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excellent characteristics. 2. Experimental Section
2.1.
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(ZnO/GQDs/ZnO) are prepared via the following steps:
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A number of the photodiodes, based on a sandwich structure consisting of GQDs
ZnO thin films
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High-quality ZnO thin films, deposited on p-type silicon substrates using a ZnO
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precursor, were prepared through the sol-gel method followed by spin coating (Figure 4). At first, a mixture of absolute ethanol (used as solvent) and Triethylamine (TEA) is preheated up to
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60 ̊ C. After that, Zinc acetate dehydrates (99.5% Merck) is added to obtain 1M of ZnO sol. To
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do so, the molar ratio of TEA to zinc acetate is kept constant at 2:5. 18 A clear and homogeneous solution yields after 60 minutes of stirring. Before the coating step, the substrates are first
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ultrasonically cleaned. They are then cleaned according to the RCA-1 standard (standard sets of
hour. 19 2.2.
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wafer cleaning steps). After the coating process, the samples are dried in an oven at 75°C for one
Graphene Quantum Dots thin films Up until now, many synthesis methods for producing GQDs have been suggested by
different groups. Depending on the synthesis routes, the optical and electrical properties of quantum dots vary widely. In this paper, GQDs was synthesized through the method suggested
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ACCEPTED MANUSCRIPT Ag ZnO GQDs
Ag
ZnO
SiO2 Si 20
The GQDs was prepared by pyrolyzing citric acid. According to the
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by Dong and et al.
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Scheme 1. Schematic structure of ZGZ photodetector with sandwich structure
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procedure, 2g of citric acid is added to a 5 ml beaker and heated to 200̊ C. After 5 minutes, the citric acid powder is liquidated. Subsequently, the liquid discolors from colorless to pale yellow,
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and then orange in 30 min, implying the formation of GQDs as illustrated in Figure. The
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obtained orange liquid is neutralized to pH 7.0 by adding NaOH solution under vigorous stirring. After preparing the GQDs precursor, thin films are deposited using a spin coater. Thereafter, the
Fabrication of Photodiodes
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2.3.
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thin films dry in an oven at 75°C for one hour.
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A sandwich-based photodiode is fabricated after three steps of depositing ZnO, GQDS, ZnO thin films respectively as previously described. The prepared structures are then annealed at
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400̊ C. Before creating silver electrical contacts during the last step, the conventional
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photolithography performs to remove part of the ZnO layer from the Silicon substrates. The thickness of ZnO and GQDs layers measured about 400 nm, 20 nm, respectively A schematic structure of the prepared device with the mentioned sandwich structure is depicted in Scheme 1. Different characterizations were performed to investigate the effects of introducing the GQDs layer on the functionality and specifications of the prepared photodiodes. Photoluminescence, UV-vis spectroscopy, cyclic voltammetry, scanning electron microscopy,
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ACCEPTED MANUSCRIPT transmission electron microscopy, Ellipsometry and optoelectrical measurements were used to analyze the characteristics of the prepared samples. 3. Results and Discussions 3.1.
Morphology and band gap estimation of materials
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Figure 1 shows TEM images of GQDs and Figure 2 indicates the schematic of its structure and formation process. According to TEM images, the size of GQDs is approximately 1-5 nm,
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with an average diameter of 2.24 nm.
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Based on the effective mass approximation model (EMA), the shift of band gap energy (∆Eg), due to the confinement of an exciton in a quantum dot with size R can be expressed as
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h2 1 1 1.78e 2 * E g ( ) 0.248E Ry 2 8R me m h R
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(1)
Where me and mh are the effective masses of electrons and holes and Ɛ, h, e, and R are the
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dielectric constant, the Planck constant, the charge of an electron, and the Rydberg energy,
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respectively. 21 Therefore, the bandgap of the prepared GQDs is estimated according to the above (b)
(e)
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(a)
(c)
(d)
Figure 1. (a, b, c) TEM images with different magnification and (d) size distribution of the prepared GQDs and (e) A prepared diluted solution of graphene quantum dots
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ACCEPTED MANUSCRIPT equation. Due to the extreme value of the graphene dielectric constant, the second and third terms of the above equation are negligible. 22, 23 Given that TEM images (Figure 1) illustrate an average size of about 2.24 nm for GQDs, its
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energy band gap is calculated approximately at 2.71 eV, demonstrating a close correlation with
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Pyrolyzing
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Citric acid
COOH
OH
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Epoxy
GQDs
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Figure 2. Formation of GQDS from Citric Acid through pyrolyzing process and its components
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the results from the cyclic voltammetry (2.70 eV) and the PL analysis (2.72 eV).
(a)
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The X-Ray diffraction (XRD) analysis was employed to determine the crystal structure of the
(b)
Figure 3. (a) SEM micrograph of ZnO thin film, (b) XRD patterns of ZnO thin
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ACCEPTED MANUSCRIPT ZnO thin film. The recorded patterns presented in Figure 3(b) reveal the hexagonal wurtzite phase of the ZnO thin film. Figure 3(a) illustrates the SEM micrograph of the ZnO thin film. A highly
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compact and uniform thin film is observable.
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3.2.UV-vis Spectroscopy
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Figure 4 shows the UV-vis spectra of the samples with the same film thickness. A comparison between the spectra of the sample with (ZGZ) and without (ZZ) the GQDs layer 24, 25
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demonstrates that the absorption peak of both samples is located in the UV region.
It is
worth mentioning that due to the effects of quantum confinement, GQDs absorbs light mainly in
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the UV region and remain transparent in the visible region. 26, 27, 28 Based on other investigations,
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a prominent absorption peak is observed between 230 and 270 nm, attributing to the π → π*
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transition of sp2 C–C bonds. The shoulder peaks at around 300-320 nm are related to the n → π* transition of C═O bonds and other functional groups attached to the surface of GQDs.
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2
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1.5 1.25
ZGZ
1 0.75 0.5 0.25 0 300
350
400
450
500
Wavelength (nm)
550
600
240 nm
3
ZZ
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Absorbance (arb. un.)
1.75
(b)
Absorbance (arb. un.)
(a)
29, 30
305 nm
2.5
324 nm
2 1.5 1 0.5 0 200
300
400
500
600
700
800
Wavelength (nm)
Figure 4. (a) UV-Vis spectra of ZZ and ZGZ thin films (b) Absorbance of GQDs
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ACCEPTED MANUSCRIPT Following a similar trend, the absorption band of GQDs recorded in our work is roughly 240 nm, which corresponds to the π → π* transition of the aromatic sp2 C domains. The other recorded transitions occurred during the n → π* transition at 305 nm and 324 nm. Embedding a GQDs layer between two layers of ZnO offers a thoroughly different optical
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structure that merely responds to the UV light (visible-blind photodetector). Since part of the
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absorbance of GQDs is related to its chemical groups, after the deposition of the last ZnO layer,
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the functional groups attached to GQDs are bonded with the ZnO surface dangling bonds at the interfaces. Consequently, its UV absorption peak is increased significantly. It is while the
Photoluminescence measurements
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3.3.
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thickness of both samples is approximately the same.
Figure 5(a) indicates the photoluminescence behavior of the samples excited with the
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wavelength of 350 nm. For better understanding, ZG sample composed of ZnO layer coated with 800
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600
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500 400 300 200 100 0
410
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360
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Intensity (arb. un.)
700
(c)
Conduction band
e
e
460
50
(b)
Photoluminescence
Eg=2.701 eV 0
GQDs ZZ ZG ZGZ
Potential(v)
(a)
-50
V = 1.253
-100 -150 V = -1.448
-200 510
-2
560
Zni
e Zni+
0
1
2
Current(μA)
Wavelength (nm)
e
-1
e
e
e
(d) LUMO
𝜋 − 𝜋∗
𝑛 − 𝜋∗
VZn2-
Vo OZn
Vo+
HOMO
Valence band 360-410 nm
430-450 nm
460-480 nm
480-520 nm
Figure 5. (a) Photoluminescence spectra of the prepared sample in each step, (b) Band gap energy of GQD measured by cyclic voltammetry, (c) energy level of ZnO defects, 9 (d) allowed transition of electrons through energy levels of GQDs
ACCEPTED MANUSCRIPT GQDs thin film and the same thickness similar to the examined samples prepared and compared in this section. GQDs possess a graphene core and some attached chemical groups, both of which can control PL. Specifically, its core determines the intrinsic emission, while the attached chemical
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groups control the surface state. 29 It is also highly fluorescent due to the extended π-conjugation
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structure of carbon atoms. In Figure 5(a), GQDs shows emission at a wavelength of 455 nm, this
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corresponds to a band gap of 2.725 eV. Furthermore, according to cyclic voltammetry results, the band gap energy of the prepared GQDs is measured at about 2.701 eV (Figure 5(b)). The
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excellent fluorescence emission of GQDs is due to both its quantum confinement and its
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boundary effect. Compared to other findings, the prepared GQDs only showed a blue emission, implying there are no other peak emissions related to the defect states. 31, 32, 33
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Two main peaks, identified in the photoluminescence of the samples, contain a ZnO thin 30, 34, 35
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film: one in the range of 380 nm to 390 nm, and the other at the wavelength of 436.06.
The former is attributed to the electron transition between the valence and the conduction bands
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(ZnO band gap). In this case, two valence atomic orbitals, i.e. Zn-4s and O-2p, contribute to the
orbitals (LUMO).
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formation of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular
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The positions of HOMO and LUMO are estimated from the onset potential of oxidation and reduction peaks according to the relations: EHOMO = −e (Eox + 4.14) and ELUMO = −e (Ered + 4.14). Accordingly, the measured absorption peak at 381 nm is attributed to the transition of an electron from HOMO to LUMO (ZnO band gap). As shown in Figure 5 (a), the blue emission peaks in the range of 430 nm to 445 nm are observed for ZGZ, ZZ, and ZG samples, and are related to the defects such as O and Zn interstitial defects or vacancies (Figure 5(c)). Moreover,
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ACCEPTED MANUSCRIPT the emission peak of ZGZ at 390 nm experienced a red shift compared to that of ZZ at 381 nm. Besides, ZG behaved completely different from ZZ and ZGZ. The intensity of its first peak position at 381 nm is lower than that of the others. But the intensity of its other peaks in the range of 430 nm to 460 nm was increased significantly. As Son mentioned, quenching in the UV
32
The three functional groups (Figure 2), carboxyl (–
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quenching and charge transfer reactions.
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emission of ZnO in the presence of graphene quantum dots is primarily caused by static
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COOH), hydroxy (–OH), and epoxy, which are formed on the GQDs surface, can easily bond with the ZnO surface, resulting in PL quenching. Furthermore, the coupling between graphene
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quantum dots and ZnO induced splitting of the pristine LUMO level to several levels in the
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graphene layer, which transfer the electrons to the ZnO ground state. These transitions create new features in the PL spectrum and are only allowed from specific levels (Figure 5 (d)). Pan et
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al. suggested that the blue PL of the GQDs is attributed to free zigzag sites with a 36
Nevertheless, after the deposition of another ZnO
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carbene/carbyne-like triplet ground state.
layer, quenching effects of using a GQDs thin film compensates to some extent near the UV
Electro-optical measurements
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3.4.
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peak emission in ZGZ samples.
Charge carrier transmission in a multilayer structure can mainly be accomplished through
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one of the following three methods: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. Direct tunneling through the barrier is the predominant mechanism for ZnO/ GQDs Schottky barriers at room temperature as well as cases with a low bias voltage. The presence of a large number of the majority carriers (high GQDs electron density) near the interfaces and the small width of the barrier allow electrons to overcome the interface barrier.
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ACCEPTED MANUSCRIPT (a)
(b) Vacuum Level ΔEc=0.9eV LUMO
ZnO
GQDs
ZnO
ΔEv=1.45 eV
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HOMO
(d)
(c)
+
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-
Tunneling
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Photoexcitation
Scheme 2. (a) Band gap and HOMO and LUMO energies of ZnO and GQDs and their positions,
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(b) energy band diagram of ZnO/GQDs/ZnO heterostructure at equilibrium, (c) Tunneling
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electrons through energy band levels of ZGZ device in reverse bias (dark), (d) energy band
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diagram of ZGZ device in forward bias (under illumination) The work function of GQDs has recently been investigated both theoretically and
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experimentally. The calculated work function of GQDs is approximately found to be 3.5–3.8 eV using different methods.37 The estimated height of the Schottky barrier is about 1 eV for the ZnO/GQDs interface. Thus, at room temperature with 0.026 eV of energy, electrons are disabled to overcome the barriers. Furthermore, according to the cyclic voltammetry results, LUMO and HOMO energy levels of the prepared GQDs and their position with respect to those of the ZnO layers are measured and depicted schematically before and after formation in Scheme 2 (a, b).
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ACCEPTED MANUSCRIPT According to Scheme 2 (c), a few of the electrons with higher energy can pass through the barriers and participate in dark current at room temperature in reverse bias. Nevertheless, the existence of two interface barriers at this sandwich structure can decrease the number of charge carriers contributing to the dark current. In contrast, lower values of dark
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current are recorded for this type of structure, indicating at least one order of magnitude smaller
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values compared to that of the pristine ZnO photodetectors (1.4 μA). The recorded numbers
(a) Id-ZGZ
1.6 1.4
50 45 40 35 30 25 20 15 10 5 0 -5
Iphoto-ZGZ
Iphoto-ZZ
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1.2
PhotoCurrent (μA)
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(b) Id-ZZ
1 0.8
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0.6 0.4 0.2 0 -0.2 -5
-4
-3
-2
-1
0
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Dark current (µA)
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reached as low as 127 nA for the ZGZ photodetectors at 5V bias voltage (Figure 6 (a)).
1
2
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Voltage (V)
3
4
5
-5
-4
-3
-2
-1
0
1
2
3
4
Voltage (V)
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Figure 6. A comparison between (a) dark and (b) photo currents of ZZ and ZGZ photodetectors Under light illumination, the charge transfer mechanism is interpreted as the following:
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electron-hole pairs are formed during the excitation of both the ZnO and the GQDs layers under UV light (Scheme 2 (d)). Due to a strong chemical bonding created at the interface between the ZnO and the GQDs layers, the photo excited electrons are transferred and flowed through the circuit by voltage bias. Based on Figure 6 (b), the magnitude of the ZGZ photocurrent seems at least one order of magnitude higher compared to that of the pure ZnO photodetectors. This enhancement in ZGZ photocurrent is attributed to the higher amount of UV light absorption (based on UV-vis spectra) as well sandwich structure. The recorded values for the
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ACCEPTED MANUSCRIPT photocurrent of the ZGZ and ZZ photodetectors were 4.22×10-5 and 6.5 ×10-6 as a higher density of charge carriers flow in ZGZ at 5V, respectively. Based on He and et al. calculations, the resistivity of the thin film depends on electron density and mobility. The arguments presented in this paper are similar to the work in a previous 38
with the exception of
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theoretical study regarding a sandwich structure of ZnO/metal/ZnO
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GQDs instead of metal.
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It is clear that inserting a layer of GQDs significantly increases the electron density. In this manner, the ZnO thin films are regarded as two parallel resistors, which may reduce the total
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resistance of the whole multilayer. Therefore, the total resistivity of the ZnO/GQDs structure is
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lower than that of the single ZnO film39. Accordingly, the resistivity of ZnO thin films can directly be obtained through the following relation: 1 ne 1 d n 0 d V n .e 0 T
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1
(2)
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R
Where n is the number of electrons flowing from the GQDs layer into the ZnO thin films
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via the tunneling effect. µ, d and d0, n0 and vT correspond to electron mobility, GQDs and ZnO
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film thickness, the intrinsic electron density of the ZnO layer and the average speed of electrons at temperature T, respectively.39 On the other hand, due to the smaller contribution of thermionic emission mechanism to the current, its portion is negligible. Moreover, under illumination, the number of electrons which can get higher energy, pass through the interface barrier, and contribute to the current is increased. This leads to higher amounts of photocurrent and lower device resistivity.
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ACCEPTED MANUSCRIPT Accordingly, high photocurrent and low dark current increases SNR. A high SNR value of 3.52×102 was measured for ZGZ at 5V, while the SNR value was 2.9 for ZZ sample. Based on the UV-Vis spectra, both samples have a maximum responsivity for wavelengths in the range
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380-390 nm, so the measurements performed in this range. The extent of photogeneration of
Figure 7. A comparison between the time-dependent photoresponse of ZZ and ZGZ photodetectors
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at 5V under illumination of UV light at 380 nm wavelength
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electron-hole pairs is directly proportional to the intensity of incident UV light. A large number of high energy UV photons create more electron-hole pairs, which in turn increases the current
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density. Additionally, the ZGZ sample shows a higher response speed compared to that of the ultraviolet light. It has a short rise time of about 370 µs, scoring at least 23 times faster than that of the pure ZnO (8.8 ms), which is deemed relatively fast compared to similar photodetectors (Figure 7). 33, 35, 40 This short response time corresponds to higher electron mobility of GQDs electrons which flow into the ZnO layers. As it is well-understood, an increase in carrier density will reduce the
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ACCEPTED MANUSCRIPT width of the depletion layer (W 2 (V a V bi ) ).
35
Furthermore, Cj (junction capacitance) is
inversely proportional to W, the width of the depletion layer ( C j
A W
). The junction
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CR
IP
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capacitance is in parallel with other components of photodiodes.
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Scheme 3. Formation and arrangements of junction capacitance in ZGZ multilayer
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Accordingly, ZnO/GQDs/ZnO heterojunctions can act as two parallel capacitors (C2),
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resulting in total capacitance of 2C2. Moreover, this capacitance is in series with the junction capacitance pertaining to p-Si/ZnO (C1), leading to a much lower overall capacitance Cmulti
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(Scheme 3), which is much smaller in value compared to that of ZZ. Therefore, given the equation for the time constant (τ=RC), a reduction in structure resistivity and capacitance is the reason for considerably shorter response time. Specifications of both kinds of photodetectors are measured at 5V and listed in Table.1.
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ACCEPTED MANUSCRIPT Table 1. Specifications of ZZ and ZGZ photodetectors at 5-volt bias under 385 nm illumination Measuring Name
wavelength (nm)
IPhoto
R
(µA)
(KΩ)
SNR
Rise time
Responsivity
Detectivity
(s)
(A/W)
(Jones)
385
6.5
769.2
2.9
8.8×10-3
12.56
1.22×1010
ZGZ
385
42.2
116. 2
3.52×102
370×10-6
89.3
1.54×1014
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T
ZZ
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According to Table 7, high amounts of photodetectivity (1.54×1014 Jones), as well as responsivity about 89 A/W, calculated for ZGZ device. By contrast, the high value of detectivity
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about four orders of magnitudes higher than of ZZ sample, obtained as a result of the lower density of dark current and higher amount of photocurrent recorded for ZGZ.
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These result can also be modified by optimization of the layer's thickness according to the
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equation (2). In this manner, the thickness of both thin films (ZnO and GQDs) can considerably
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affect the resistivity and consequently response time of the device. Conclusion
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In summary, inserting a thin layer of GQDs between two ZnO thin films improves all specifications of ZnO-based photodetectors. The sandwich structure not only reduces dark
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currents (due to the formation of Schottky barriers near the interfaces) but also increases the
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photocurrents because of injection of higher amounts of photoexcited charge carriers from GQDs layer. As a result, a higher amount of detectivity about four orders of magnitudes was recorded for the sample composed of GQDs layer. For ZGZ sample, the photoresponsivity and detectivity observed are ∼89 A/W and ∼1.5 × 1014 Jones at 385 nm wavelength and 5V bias. It is experimentally and theoretically proven that higher response speed can be achieved due to the device configuration which offers lower total capacitance and resistance. This modified structure offers a simple and cost effective route for improving the ZnO-based photodetectors.
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ACCEPTED MANUSCRIPT AUTHOR INFORMATION Corresponding Author *Email: [email protected]
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*Email: [email protected]
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ORCID
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Seyedeh Soraya Mousavi: 0000-0003-4895-2425
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Author Contributions
The manuscript was written through the contributions of all authors. All authors have approved
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the final version of the manuscript. Seyedeh Soraya Mousavi gave the idea of the manuscript.
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She, Ali Kazempour and Babak Efafi contributed to preparing and analyzing the samples. Mohammad Hossein Majlesara and Batool Sajad are both supervisors and contributed in whole
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ABBREVIATIONS
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the process.
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GQDs, Graphene Quantum Dots; ZZ, ZnO/ZnO structure; ZGZ, ZnO/GQDs/ZnO structure; RCA-1, the Radio Corporation of America. TEA, Triethylamine;
(1)
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
Obtaining a lower amount of dark current for sandwich structured device
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Enhancing SNR at least about two orders of magnitudes.
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Recording higher response speed for the device with sandwich structure
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Increasing in detectivity of the ZnO/GQD/ZnO device about four orders of magnitudes compared to that of ZnO/ZnO design.
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interlayer in sandwich structure
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Comprehensive discussion about the effects of using GQDs layer as an
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S. Soraya Mousavi, Ali Kazempour, Babak Efafi, Mohammad Hossein Majles Ara, Batool Sajad PII: DOI: Reference:
S0169-4332(19)32181-6 https://doi.org/10.1016/j.apsusc.2019.07.145 APSUSC 43403
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
7 May 2019 29 June 2019 16 July 2019
Please cite this article as: S.S. Mousavi, A. Kazempour, B. Efafi, et al., Effects of graphene quantum dots interlayer on performance of ZnO-based photodetectors, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.07.145
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ACCEPTED MANUSCRIPT
Effects of Graphene Quantum Dots interlayer on Performance of ZnO-Based Photodetectors S. Soraya Mousavia, b, *, Ali Kazempourb, Babak Efafi b,c, Mohammad Hossein Majles Arab, c,*,
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Photonics Laboratory, Department of Physics, Kharazmi University, Alborz, Iran
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b
Photonics Laboratory, Department of Physics, Alzahra University, Tehran, Iran
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a
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Batool Sajada
Nanophotonics Laboratory, Research Institute of Applied Science Center, Kharazmi University,
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Alborz, Iran
KEYWORDS: Graphene Quantum dots; ZnO-based photodetector; Ultraviolet; Sandwich structure; Fast response; High signal to noise ratio;
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ABSTRACT
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In this paper, the effects of inserting graphene quantum dots (GQDs) as an interlayer
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between two zinc oxide (ZnO) thin films is studied comprehensively. This kind of sandwich structure offers various advantages over similar structures modified using GQDs. It is obvious
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that exploiting GQDs properties can thoroughly modify the optical and electrical properties of ZnO thin films. Due to the two Schottky barriers created at the interfaces, low amounts of dark
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current (about 127 nA) and a high signal to noise ratio (SNR) of 3.52×102 is measured for a
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sandwich structure comprised of ZnO/GQDs/ZnO (ZGZ) layers. Furthermore, the high-speed photodetector, referred in this paper, responded at least 23 times faster compared to pure ZnO (ZZ), resulting in 370 µs and 8.8 ms of response time for ZGZ and ZZ, respectively. The high amounts of responsivity and detectivity achieved by this visible-blind photodetector are other unique features this form of sandwich hybrid structures benefit from. Four orders of magnitudes enhancement in detectivity accomplished by inserting a thin film of GQDs.
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ACCEPTED MANUSCRIPT 1. Introduction: The discovery of Graphene as a two-dimensional honeycomb lattice by Novoselov et al. in 2004 has caused significant progress in the semiconductor industry.1 Due to superior electronic, thermal, and mechanical properties as well as higher chemical stability, Graphene has
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drawn the attention of scientists as a promising building block for the next generation of
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semiconductor devices. Other properties that have made this novel form of carbon the best
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candidate for optoelectronic devices include high carrier mobility at room temperature (above
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15000 cm2V−1s−1), the optical transmittance of about 97.9% to the white light, flexibility, and light -weight.2 This kind of carbon allotrope can be seen as the basic unit of all other dimensional
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carbon materials. For example, it can be grown as zero-dimensional fullerene and rolled into 1D carbon nanotubes or stacked into 3D graphite.3-5 So far, several techniques have been developed
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to produce graphene, including micromechanical exfoliation of highly oriented pyrolytic graphite
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(HOPG), ultrasonication exfoliation of graphite, chemical reduction of graphite oxide (GO),
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carbon nanotube (CNT) unzipping, and epitaxial and chemical vapor deposition (CVD) growth. 6 These different morphologies exhibit different physical and chemical properties compared with
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pristine graphene. With a graphene-like structural framework of sp2 hybridized carbon, graphene quantum dots (GQDs) are highly heterogeneous, nanoscale, 0D particles with a typical diameter
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of 1–20 nm. 7 The quantum confinement and edge effects in GQDs provide them with robustness towards limitations faced by Graphene (zero band-gap nature and semi-metal conductivity), offering specific characters for direct use in optoelectronics.8 In recent studies, additional excellent properties of GQDs, such as high transparency and surface area, have been proposed for energy and display applications.9 In this way, different synthesis methods have been
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ACCEPTED MANUSCRIPT investigated to get graphene quantum dots such as a novel hydrothermal method, chemical oxidation and exfoliation method, and citric acid splitting method.10 On the other hand, unique optical and electrical properties of ZnO (as one of the most applied semiconductors) can be modified using graphene quantum dots. The ZnO and GQDs
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hybrids (e.g. core-shell structures) are of particular interest because they combine the superior
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wavelength selectivity of ZnO and the charge mobility of graphene, both of which are critical to
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applications of ultraviolet (UV) photodetectors, electron emitters, and many other optoelectronic
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devices. 11-16 Tuning the band gap of GQDs by controlling the size makes it suitable for use in a wide range of optical applications, especially in the ultraviolet region of light.
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Until now, various devices based on ZnO/GQDs hybrids have been reported.11These
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hybrids are ideal candidates for both light emitting diodes (LEDs) and visible-blind UV detectors needed in various applications, such as flame detection, environmental monitoring, and UV
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astronomy.17 In this way, a novel-designed UV photodetector based on a sandwich structure
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composed of ZnO and GQDs thin films (ZnO/GQDs/ZnO) was fabricated and characterized. This applicable structure due to the possessing two interfaces offers many advantages over
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similar structures with one interface, including a higher degree of control over its properties. Due
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to differences between the work functions of each layer as well as the arrangement of layers, optical and electrical characters of the multilayer structure are accurately modified. Moreover, studying the photoluminescence (PL) spectra, the absorption spectra, and the photoresponsivity, indicates that the carriers’ separation and transfer between graphene quantum dots and ZnO should play an important role in the functionality of ZnO/GQDs/ZnO UV photodetectors. Our investigations provide a promising route to design high-performance photodetectors by controlling the interfaces between GQDs films and semiconductors. According to our obtained
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ACCEPTED MANUSCRIPT results, introducing a thin film consisting of GQDs between two ZnO layers can considerably improve all electro-optical features of the ZnO-based photodetectors, especially enhances its response speed. This visible-blind photodetector presents unique properties; all of which are properly enhanced, unlike most reported devices that offer a maximum of only one or two
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excellent characteristics. 2. Experimental Section
2.1.
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(ZnO/GQDs/ZnO) are prepared via the following steps:
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A number of the photodiodes, based on a sandwich structure consisting of GQDs
ZnO thin films
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High-quality ZnO thin films, deposited on p-type silicon substrates using a ZnO
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precursor, were prepared through the sol-gel method followed by spin coating (Figure 4). At first, a mixture of absolute ethanol (used as solvent) and Triethylamine (TEA) is preheated up to
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60 ̊ C. After that, Zinc acetate dehydrates (99.5% Merck) is added to obtain 1M of ZnO sol. To
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do so, the molar ratio of TEA to zinc acetate is kept constant at 2:5. 18 A clear and homogeneous solution yields after 60 minutes of stirring. Before the coating step, the substrates are first
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ultrasonically cleaned. They are then cleaned according to the RCA-1 standard (standard sets of
hour. 19 2.2.
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wafer cleaning steps). After the coating process, the samples are dried in an oven at 75°C for one
Graphene Quantum Dots thin films Up until now, many synthesis methods for producing GQDs have been suggested by
different groups. Depending on the synthesis routes, the optical and electrical properties of quantum dots vary widely. In this paper, GQDs was synthesized through the method suggested
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ACCEPTED MANUSCRIPT Ag ZnO GQDs
Ag
ZnO
SiO2 Si 20
The GQDs was prepared by pyrolyzing citric acid. According to the
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by Dong and et al.
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Scheme 1. Schematic structure of ZGZ photodetector with sandwich structure
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procedure, 2g of citric acid is added to a 5 ml beaker and heated to 200̊ C. After 5 minutes, the citric acid powder is liquidated. Subsequently, the liquid discolors from colorless to pale yellow,
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and then orange in 30 min, implying the formation of GQDs as illustrated in Figure. The
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obtained orange liquid is neutralized to pH 7.0 by adding NaOH solution under vigorous stirring. After preparing the GQDs precursor, thin films are deposited using a spin coater. Thereafter, the
Fabrication of Photodiodes
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thin films dry in an oven at 75°C for one hour.
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A sandwich-based photodiode is fabricated after three steps of depositing ZnO, GQDS, ZnO thin films respectively as previously described. The prepared structures are then annealed at
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400̊ C. Before creating silver electrical contacts during the last step, the conventional
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photolithography performs to remove part of the ZnO layer from the Silicon substrates. The thickness of ZnO and GQDs layers measured about 400 nm, 20 nm, respectively A schematic structure of the prepared device with the mentioned sandwich structure is depicted in Scheme 1. Different characterizations were performed to investigate the effects of introducing the GQDs layer on the functionality and specifications of the prepared photodiodes. Photoluminescence, UV-vis spectroscopy, cyclic voltammetry, scanning electron microscopy,
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ACCEPTED MANUSCRIPT transmission electron microscopy, Ellipsometry and optoelectrical measurements were used to analyze the characteristics of the prepared samples. 3. Results and Discussions 3.1.
Morphology and band gap estimation of materials
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Figure 1 shows TEM images of GQDs and Figure 2 indicates the schematic of its structure and formation process. According to TEM images, the size of GQDs is approximately 1-5 nm,
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with an average diameter of 2.24 nm.
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Based on the effective mass approximation model (EMA), the shift of band gap energy (∆Eg), due to the confinement of an exciton in a quantum dot with size R can be expressed as
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h2 1 1 1.78e 2 * E g ( ) 0.248E Ry 2 8R me m h R
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(1)
Where me and mh are the effective masses of electrons and holes and Ɛ, h, e, and R are the
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dielectric constant, the Planck constant, the charge of an electron, and the Rydberg energy,
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respectively. 21 Therefore, the bandgap of the prepared GQDs is estimated according to the above (b)
(e)
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(a)
(c)
(d)
Figure 1. (a, b, c) TEM images with different magnification and (d) size distribution of the prepared GQDs and (e) A prepared diluted solution of graphene quantum dots
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ACCEPTED MANUSCRIPT equation. Due to the extreme value of the graphene dielectric constant, the second and third terms of the above equation are negligible. 22, 23 Given that TEM images (Figure 1) illustrate an average size of about 2.24 nm for GQDs, its
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energy band gap is calculated approximately at 2.71 eV, demonstrating a close correlation with
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Pyrolyzing
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Citric acid
COOH
OH
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Epoxy
GQDs
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Figure 2. Formation of GQDS from Citric Acid through pyrolyzing process and its components
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the results from the cyclic voltammetry (2.70 eV) and the PL analysis (2.72 eV).
(a)
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The X-Ray diffraction (XRD) analysis was employed to determine the crystal structure of the
(b)
Figure 3. (a) SEM micrograph of ZnO thin film, (b) XRD patterns of ZnO thin
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ACCEPTED MANUSCRIPT ZnO thin film. The recorded patterns presented in Figure 3(b) reveal the hexagonal wurtzite phase of the ZnO thin film. Figure 3(a) illustrates the SEM micrograph of the ZnO thin film. A highly
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compact and uniform thin film is observable.
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3.2.UV-vis Spectroscopy
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Figure 4 shows the UV-vis spectra of the samples with the same film thickness. A comparison between the spectra of the sample with (ZGZ) and without (ZZ) the GQDs layer 24, 25
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demonstrates that the absorption peak of both samples is located in the UV region.
It is
worth mentioning that due to the effects of quantum confinement, GQDs absorbs light mainly in
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the UV region and remain transparent in the visible region. 26, 27, 28 Based on other investigations,
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a prominent absorption peak is observed between 230 and 270 nm, attributing to the π → π*
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transition of sp2 C–C bonds. The shoulder peaks at around 300-320 nm are related to the n → π* transition of C═O bonds and other functional groups attached to the surface of GQDs.
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2
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1.5 1.25
ZGZ
1 0.75 0.5 0.25 0 300
350
400
450
500
Wavelength (nm)
550
600
240 nm
3
ZZ
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Absorbance (arb. un.)
1.75
(b)
Absorbance (arb. un.)
(a)
29, 30
305 nm
2.5
324 nm
2 1.5 1 0.5 0 200
300
400
500
600
700
800
Wavelength (nm)
Figure 4. (a) UV-Vis spectra of ZZ and ZGZ thin films (b) Absorbance of GQDs
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ACCEPTED MANUSCRIPT Following a similar trend, the absorption band of GQDs recorded in our work is roughly 240 nm, which corresponds to the π → π* transition of the aromatic sp2 C domains. The other recorded transitions occurred during the n → π* transition at 305 nm and 324 nm. Embedding a GQDs layer between two layers of ZnO offers a thoroughly different optical
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structure that merely responds to the UV light (visible-blind photodetector). Since part of the
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absorbance of GQDs is related to its chemical groups, after the deposition of the last ZnO layer,
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the functional groups attached to GQDs are bonded with the ZnO surface dangling bonds at the interfaces. Consequently, its UV absorption peak is increased significantly. It is while the
Photoluminescence measurements
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3.3.
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thickness of both samples is approximately the same.
Figure 5(a) indicates the photoluminescence behavior of the samples excited with the
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wavelength of 350 nm. For better understanding, ZG sample composed of ZnO layer coated with 800
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600
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500 400 300 200 100 0
410
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360
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Intensity (arb. un.)
700
(c)
Conduction band
e
e
460
50
(b)
Photoluminescence
Eg=2.701 eV 0
GQDs ZZ ZG ZGZ
Potential(v)
(a)
-50
V = 1.253
-100 -150 V = -1.448
-200 510
-2
560
Zni
e Zni+
0
1
2
Current(μA)
Wavelength (nm)
e
-1
e
e
e
(d) LUMO
𝜋 − 𝜋∗
𝑛 − 𝜋∗
VZn2-
Vo OZn
Vo+
HOMO
Valence band 360-410 nm
430-450 nm
460-480 nm
480-520 nm
Figure 5. (a) Photoluminescence spectra of the prepared sample in each step, (b) Band gap energy of GQD measured by cyclic voltammetry, (c) energy level of ZnO defects, 9 (d) allowed transition of electrons through energy levels of GQDs
ACCEPTED MANUSCRIPT GQDs thin film and the same thickness similar to the examined samples prepared and compared in this section. GQDs possess a graphene core and some attached chemical groups, both of which can control PL. Specifically, its core determines the intrinsic emission, while the attached chemical
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groups control the surface state. 29 It is also highly fluorescent due to the extended π-conjugation
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structure of carbon atoms. In Figure 5(a), GQDs shows emission at a wavelength of 455 nm, this
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corresponds to a band gap of 2.725 eV. Furthermore, according to cyclic voltammetry results, the band gap energy of the prepared GQDs is measured at about 2.701 eV (Figure 5(b)). The
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excellent fluorescence emission of GQDs is due to both its quantum confinement and its
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boundary effect. Compared to other findings, the prepared GQDs only showed a blue emission, implying there are no other peak emissions related to the defect states. 31, 32, 33
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Two main peaks, identified in the photoluminescence of the samples, contain a ZnO thin 30, 34, 35
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film: one in the range of 380 nm to 390 nm, and the other at the wavelength of 436.06.
The former is attributed to the electron transition between the valence and the conduction bands
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(ZnO band gap). In this case, two valence atomic orbitals, i.e. Zn-4s and O-2p, contribute to the
orbitals (LUMO).
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formation of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular
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The positions of HOMO and LUMO are estimated from the onset potential of oxidation and reduction peaks according to the relations: EHOMO = −e (Eox + 4.14) and ELUMO = −e (Ered + 4.14). Accordingly, the measured absorption peak at 381 nm is attributed to the transition of an electron from HOMO to LUMO (ZnO band gap). As shown in Figure 5 (a), the blue emission peaks in the range of 430 nm to 445 nm are observed for ZGZ, ZZ, and ZG samples, and are related to the defects such as O and Zn interstitial defects or vacancies (Figure 5(c)). Moreover,
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ACCEPTED MANUSCRIPT the emission peak of ZGZ at 390 nm experienced a red shift compared to that of ZZ at 381 nm. Besides, ZG behaved completely different from ZZ and ZGZ. The intensity of its first peak position at 381 nm is lower than that of the others. But the intensity of its other peaks in the range of 430 nm to 460 nm was increased significantly. As Son mentioned, quenching in the UV
32
The three functional groups (Figure 2), carboxyl (–
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quenching and charge transfer reactions.
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emission of ZnO in the presence of graphene quantum dots is primarily caused by static
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COOH), hydroxy (–OH), and epoxy, which are formed on the GQDs surface, can easily bond with the ZnO surface, resulting in PL quenching. Furthermore, the coupling between graphene
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quantum dots and ZnO induced splitting of the pristine LUMO level to several levels in the
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graphene layer, which transfer the electrons to the ZnO ground state. These transitions create new features in the PL spectrum and are only allowed from specific levels (Figure 5 (d)). Pan et
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al. suggested that the blue PL of the GQDs is attributed to free zigzag sites with a 36
Nevertheless, after the deposition of another ZnO
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carbene/carbyne-like triplet ground state.
layer, quenching effects of using a GQDs thin film compensates to some extent near the UV
Electro-optical measurements
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3.4.
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peak emission in ZGZ samples.
Charge carrier transmission in a multilayer structure can mainly be accomplished through
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one of the following three methods: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. Direct tunneling through the barrier is the predominant mechanism for ZnO/ GQDs Schottky barriers at room temperature as well as cases with a low bias voltage. The presence of a large number of the majority carriers (high GQDs electron density) near the interfaces and the small width of the barrier allow electrons to overcome the interface barrier.
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ACCEPTED MANUSCRIPT (a)
(b) Vacuum Level ΔEc=0.9eV LUMO
ZnO
GQDs
ZnO
ΔEv=1.45 eV
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HOMO
(d)
(c)
+
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-
Tunneling
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Photoexcitation
Scheme 2. (a) Band gap and HOMO and LUMO energies of ZnO and GQDs and their positions,
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(b) energy band diagram of ZnO/GQDs/ZnO heterostructure at equilibrium, (c) Tunneling
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electrons through energy band levels of ZGZ device in reverse bias (dark), (d) energy band
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diagram of ZGZ device in forward bias (under illumination) The work function of GQDs has recently been investigated both theoretically and
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experimentally. The calculated work function of GQDs is approximately found to be 3.5–3.8 eV using different methods.37 The estimated height of the Schottky barrier is about 1 eV for the ZnO/GQDs interface. Thus, at room temperature with 0.026 eV of energy, electrons are disabled to overcome the barriers. Furthermore, according to the cyclic voltammetry results, LUMO and HOMO energy levels of the prepared GQDs and their position with respect to those of the ZnO layers are measured and depicted schematically before and after formation in Scheme 2 (a, b).
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ACCEPTED MANUSCRIPT According to Scheme 2 (c), a few of the electrons with higher energy can pass through the barriers and participate in dark current at room temperature in reverse bias. Nevertheless, the existence of two interface barriers at this sandwich structure can decrease the number of charge carriers contributing to the dark current. In contrast, lower values of dark
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current are recorded for this type of structure, indicating at least one order of magnitude smaller
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values compared to that of the pristine ZnO photodetectors (1.4 μA). The recorded numbers
(a) Id-ZGZ
1.6 1.4
50 45 40 35 30 25 20 15 10 5 0 -5
Iphoto-ZGZ
Iphoto-ZZ
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1.2
PhotoCurrent (μA)
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(b) Id-ZZ
1 0.8
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0.6 0.4 0.2 0 -0.2 -5
-4
-3
-2
-1
0
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Dark current (µA)
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reached as low as 127 nA for the ZGZ photodetectors at 5V bias voltage (Figure 6 (a)).
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2
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Voltage (V)
3
4
5
-5
-4
-3
-2
-1
0
1
2
3
4
Voltage (V)
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Figure 6. A comparison between (a) dark and (b) photo currents of ZZ and ZGZ photodetectors Under light illumination, the charge transfer mechanism is interpreted as the following:
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electron-hole pairs are formed during the excitation of both the ZnO and the GQDs layers under UV light (Scheme 2 (d)). Due to a strong chemical bonding created at the interface between the ZnO and the GQDs layers, the photo excited electrons are transferred and flowed through the circuit by voltage bias. Based on Figure 6 (b), the magnitude of the ZGZ photocurrent seems at least one order of magnitude higher compared to that of the pure ZnO photodetectors. This enhancement in ZGZ photocurrent is attributed to the higher amount of UV light absorption (based on UV-vis spectra) as well sandwich structure. The recorded values for the
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ACCEPTED MANUSCRIPT photocurrent of the ZGZ and ZZ photodetectors were 4.22×10-5 and 6.5 ×10-6 as a higher density of charge carriers flow in ZGZ at 5V, respectively. Based on He and et al. calculations, the resistivity of the thin film depends on electron density and mobility. The arguments presented in this paper are similar to the work in a previous 38
with the exception of
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theoretical study regarding a sandwich structure of ZnO/metal/ZnO
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GQDs instead of metal.
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It is clear that inserting a layer of GQDs significantly increases the electron density. In this manner, the ZnO thin films are regarded as two parallel resistors, which may reduce the total
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resistance of the whole multilayer. Therefore, the total resistivity of the ZnO/GQDs structure is
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lower than that of the single ZnO film39. Accordingly, the resistivity of ZnO thin films can directly be obtained through the following relation: 1 ne 1 d n 0 d V n .e 0 T
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1
(2)
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R
Where n is the number of electrons flowing from the GQDs layer into the ZnO thin films
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via the tunneling effect. µ, d and d0, n0 and vT correspond to electron mobility, GQDs and ZnO
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film thickness, the intrinsic electron density of the ZnO layer and the average speed of electrons at temperature T, respectively.39 On the other hand, due to the smaller contribution of thermionic emission mechanism to the current, its portion is negligible. Moreover, under illumination, the number of electrons which can get higher energy, pass through the interface barrier, and contribute to the current is increased. This leads to higher amounts of photocurrent and lower device resistivity.
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ACCEPTED MANUSCRIPT Accordingly, high photocurrent and low dark current increases SNR. A high SNR value of 3.52×102 was measured for ZGZ at 5V, while the SNR value was 2.9 for ZZ sample. Based on the UV-Vis spectra, both samples have a maximum responsivity for wavelengths in the range
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380-390 nm, so the measurements performed in this range. The extent of photogeneration of
Figure 7. A comparison between the time-dependent photoresponse of ZZ and ZGZ photodetectors
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at 5V under illumination of UV light at 380 nm wavelength
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electron-hole pairs is directly proportional to the intensity of incident UV light. A large number of high energy UV photons create more electron-hole pairs, which in turn increases the current
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density. Additionally, the ZGZ sample shows a higher response speed compared to that of the ultraviolet light. It has a short rise time of about 370 µs, scoring at least 23 times faster than that of the pure ZnO (8.8 ms), which is deemed relatively fast compared to similar photodetectors (Figure 7). 33, 35, 40 This short response time corresponds to higher electron mobility of GQDs electrons which flow into the ZnO layers. As it is well-understood, an increase in carrier density will reduce the
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ACCEPTED MANUSCRIPT width of the depletion layer (W 2 (V a V bi ) ).
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Furthermore, Cj (junction capacitance) is
inversely proportional to W, the width of the depletion layer ( C j
A W
). The junction
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capacitance is in parallel with other components of photodiodes.
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Scheme 3. Formation and arrangements of junction capacitance in ZGZ multilayer
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Accordingly, ZnO/GQDs/ZnO heterojunctions can act as two parallel capacitors (C2),
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resulting in total capacitance of 2C2. Moreover, this capacitance is in series with the junction capacitance pertaining to p-Si/ZnO (C1), leading to a much lower overall capacitance Cmulti
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(Scheme 3), which is much smaller in value compared to that of ZZ. Therefore, given the equation for the time constant (τ=RC), a reduction in structure resistivity and capacitance is the reason for considerably shorter response time. Specifications of both kinds of photodetectors are measured at 5V and listed in Table.1.
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ACCEPTED MANUSCRIPT Table 1. Specifications of ZZ and ZGZ photodetectors at 5-volt bias under 385 nm illumination Measuring Name
wavelength (nm)
IPhoto
R
(µA)
(KΩ)
SNR
Rise time
Responsivity
Detectivity
(s)
(A/W)
(Jones)
385
6.5
769.2
2.9
8.8×10-3
12.56
1.22×1010
ZGZ
385
42.2
116. 2
3.52×102
370×10-6
89.3
1.54×1014
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ZZ
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According to Table 7, high amounts of photodetectivity (1.54×1014 Jones), as well as responsivity about 89 A/W, calculated for ZGZ device. By contrast, the high value of detectivity
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about four orders of magnitudes higher than of ZZ sample, obtained as a result of the lower density of dark current and higher amount of photocurrent recorded for ZGZ.
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These result can also be modified by optimization of the layer's thickness according to the
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equation (2). In this manner, the thickness of both thin films (ZnO and GQDs) can considerably
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affect the resistivity and consequently response time of the device. Conclusion
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In summary, inserting a thin layer of GQDs between two ZnO thin films improves all specifications of ZnO-based photodetectors. The sandwich structure not only reduces dark
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currents (due to the formation of Schottky barriers near the interfaces) but also increases the
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photocurrents because of injection of higher amounts of photoexcited charge carriers from GQDs layer. As a result, a higher amount of detectivity about four orders of magnitudes was recorded for the sample composed of GQDs layer. For ZGZ sample, the photoresponsivity and detectivity observed are ∼89 A/W and ∼1.5 × 1014 Jones at 385 nm wavelength and 5V bias. It is experimentally and theoretically proven that higher response speed can be achieved due to the device configuration which offers lower total capacitance and resistance. This modified structure offers a simple and cost effective route for improving the ZnO-based photodetectors.
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ACCEPTED MANUSCRIPT AUTHOR INFORMATION Corresponding Author *Email: [email protected]
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*Email: [email protected]
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ORCID
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Seyedeh Soraya Mousavi: 0000-0003-4895-2425
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Author Contributions
The manuscript was written through the contributions of all authors. All authors have approved
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the final version of the manuscript. Seyedeh Soraya Mousavi gave the idea of the manuscript.
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She, Ali Kazempour and Babak Efafi contributed to preparing and analyzing the samples. Mohammad Hossein Majlesara and Batool Sajad are both supervisors and contributed in whole
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ABBREVIATIONS
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the process.
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GQDs, Graphene Quantum Dots; ZZ, ZnO/ZnO structure; ZGZ, ZnO/GQDs/ZnO structure; RCA-1, the Radio Corporation of America. TEA, Triethylamine;
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
Obtaining a lower amount of dark current for sandwich structured device
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Enhancing SNR at least about two orders of magnitudes.
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Recording higher response speed for the device with sandwich structure
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Increasing in detectivity of the ZnO/GQD/ZnO device about four orders of magnitudes compared to that of ZnO/ZnO design.
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interlayer in sandwich structure
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Comprehensive discussion about the effects of using GQDs layer as an
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