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Characteristics of Reduced Graphene Oxide Quantum Dots for a Flexible Memory Thin Film Transistor Yo-Han Kim, Eun Yeol Lee, Hyun Ho Lee, and Tae Seok Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Characteristics of Reduced Graphene Oxide Quantum Dots for a Flexible Memory Thin Film Transistor

Yo-Han Kim§, Eun Yeol Lee†, Hyun Ho Lee*§, Tae Seok Seo*†

†

Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

§

Department of Chemical Engineering, Myongji University, 116 Myongji-ro, Cheoin-gu, Yonginsi, Gyeonggi-do 17058, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT Reduced graphene oxide quantum dot (rGOQD) devices in formats of capacitor and thin film transistor (TFT) were demonstrated and examined as the first trial to achieve non-ambipolar channel property. In addition, through gold nanoparticles (Au NPs) layer embedded between the rGOQD active channel and dielectric layer, memory capacitor and TFT performances were realized by capacitance-voltage (C-V) hysteresis and gate program, erase, and reprogram biases. First, capacitor structure of the rGOQD memory device was constructed to examine memory charging effect featured in hysteretic C-V behavior with a 30 nm dielectric layer of crosslinked polyvinyl alcohol (PVA). For the intervening Au NP charging layer, self-assembled monolayer (SAM) formation of the Au NP was executed to utilize electrostatic interaction by a dip-coating process under ambient environments with a conformal fabrication uniformity. Second, the rGOQD memory TFT device was also constructed in the same format of the Au NPs SAMs on a flexible substrate. Characteristics of the rGOQD TFT output showed novel saturation curves unlike typical graphene-based TFTs. However, The rGOQD TFT device reveals relatively low on/off ratio of 101, and mobility of 5.005 cm2 /V s. For the memory capacitor, the flat-band voltage shift (∆VFB) was measured as 3.74 V for ± 10 V sweep and, for the memory TFT, the threshold voltage shift (∆Vth) by the Au NP charging was detected as 7.84 V. In summary, it was concluded that the rGOQD memory device could accomplish an ideal graphene based memory performances which could have provided a wide memory window and saturated output characteristics.

KEYWORDS: Graphene oxide quantum dot, Memory device, Thin film transistor, Flexible electronics, Self-assembled monolayer, Gold nanoparticles


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INTRODUCTION Graphene has been expected to be significantly utilized in wide ranged applications at various areas, such as optoelectronics, battery or energy storage, flexible-electronics, nano-electronics, and sensor applications.1,2 Furthermore, the graphene material has many superior aspects as like as mechanical strength, electrical property, thermal conductivity, and so on.1 However, as it has been recognized as a major drawback for the electronic device application, the graphene is known to have a perfect zero band gap.1-3 Therefore, in order to effectively use semi-conducting property of the graphene for the electronic device applications, the graphene need to be modified to have a vitally opened band gap. As a consequence, many researchers have been demonstrating facile methods of controlling sp2 carbon domain, for example, including a reduction of graphene oxide (GO) to reduced graphene oxide (rGO).4 Moreover, several groups have recently reported to focus on a new graphene-based material having lateral size of graphene less than 100 nm, which is called as graphene quantum dot (GQD).4-6 The GQDs are conductive in two dimensional (2D) nano-sheets of sp2 hybridized carbons, of which electronic properties are characterized with high intrinsic mobility and excellent thermal conductivity due to π clouds.4,7,8 Ever since the GQDs have been introduced, their applications have been employed initially for optical utilizations based on their quantum confinement showing photo luminescence (PL) emission.4 Here, the GQD was different from a typical graphene 2D sheet, which consisted of light atoms, thus, it has a small dielectric constant and a weak spin-orbit coupling. These natures of the GQDs have leaded to strong carrier-carrier interactions and electronic states with the welldefined spin multiplicity.5 Meanwhile, the GQDs have been known to be zero-dimensional and they have a zero effective mass for a role of charge carrier. Therefore, we could expect some unique electronic and optical properties with the GQDs, when they were applied in active layer


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of the humidity and pressure sensor devices, where the GQDs were not adopted as a mere electrode.6 Moreover, light emitting diodes (LEDs), bio imaging device, photo-catalytic device, and metal ion detection sensor have been demonstrated with the advantageous quantum effects of the GQDs. In this end, the bandgap of the GQDs has been even controlled by size, shape, percolation, fraction of the sp2 domains in sp3 matrix, and derivatives of O, S, N atoms.4-6 Application of colloidal quantum dots (QDs) in a form of film layer has been proposed with a quantum size effect, where they were fabricated in semiconductor nanostructure of various devices. For example, quantum dot field effect transistors (QD-FETs), LEDs, photo-devices, and various sensors have been demonstrated with the semiconducting quantum effects.7,8 Particularly, for thin film transistor (TFT) applications, there have been many reports about the fabrication of the TFT based on CdSe, InAs, and PbSe QDs, which were based on II-VI and III-V semiconducting materials. Especially, CdSe-QD,9 InAs-QD,10 and PbSe-QD11 have been demonstrated to perform remarkable high electron mobility (µe) (each device showed 30 cm2 / V*s, 15 cm2 / V*s, 10 cm2 / V*s, respectively). In these cases, improvements of QD-FET’s semiconductor performances could have been determined by various methods such as annealing step and chemical sources. In addition, depending on the QDs’ chemical component, its size and surface nature of subsequent dielectric layer, the ligand chemistry for the QD should be critically controlled including an appropriate choice of solvent.7-11 However, material issue of the QDs have been a hurdle to be widely used in the TFTs fabrication with the hazardous Cd, As and Pb elements. Additionally, for the device performance of the QD-FET, there are several occurrences of inhabitant hysteresis in transfer curves without any embedding of floating gate (FG) layer.10,11 Along with the intensive progress of the QD electronics, there have been a huge interest growing in the 2D atomic sheet electronics or TFTs including memory device having


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nanoparticles.6,12 There have been many reports about the 2D semiconductors including dichalcogenides and buckled nanomaterials with non-zero bandgap.12 Their hole and electron mobilities have been reported to be upto 500~1,000 cm2 / V*s for MoS2 and phosphorene active layers.6,12 However, the 2D active semiconducting layer frequently required specific ideal dielectric layers such as hexagonal boron nitride (h-BN) and peculiar bindings at the interfaces between 2D active layer and dielectric or metal layer. Therefore, these requirements could limit corresponding fabrication process due to the lack of the dangling chemical groups.6,12 From many reports, for the preparations of GQDs, there have been several methodologies including solution-based Hummer`s method.4-6,13 In fact, the method could provide graphene oxide quantum dots (GOQDs), not exactly the ingenuine GQDs. The GOQDs would not show a sufficient conducting property as much as the GQDs associated with 5 ~ 10 % oxygenous sites. To overcome the low conducting property, hydrazine treatment was generally known to increase the conductivity.4-6 Therefore, if the GOQD is employed for an active layer of a FET, hydrazine hydrate (H2N4) will be a good resource again for the GOQD transformation into the reduced GOQD (rGOQD) and for the treatment of interparticle contact problems. In addition, the size of the 2D rGOQD has been reported as small as ~5 nm.4 As a consequence, the rGOQD can be easily handled without a high risk of fold and wrinkle formations, when they are applied to accomplish a smooth layer unlike conventional graphene, GO, and rGO sheets. In this article, we present a facile solution based fabrication process of rGOQD memory thin film transistor (rGOQD-MTFT) with high mobility on flexible substrate. To the best of our knowledges, the first report of TFT device having the rGOQD active layer is introduced. To construct NPs based FG layer, the fabrication was based on the self-assembly monolayer (SAM) formation of Au NPs as charge storage elements and the rGOQD monolayer as the active layer,


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which were sequentially stacked by dip-coating process at room temperature. Throughout many reports, the SAM of nano-materials could lead to the wide range of structural design integrating of nano-structures along with a conventional process for microelectronic fabrication.14 Here, a simple dip-coating process could be a remarkable method of nanomaterial 2D building block along with a ultrathin film formation through well-established vacuum filtration method.5,13,14 The rGOQD channel was constructed to have effective transports of charge carriers between source and drain electrodes in this study.14,15,17


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RESULT AND DISCUSSION Schematic diagrams of capacitor and TFT device structures are represented in Figure 1. Fig. 1(a) corresponds to rGOQD memory capacitor, Fig. 1(b) corresponds for rGOQD thin film transistor (rGOQD-TFT), and Fig. 1(c) corresponds for rGOQD memory thin film transistor (rGOQDMTFT). Entire process of device fabrications to prepare rGOQD-TFT and rGOQD-MTFT was described in Figure S1. The GOQD solution was prepared by the modified Hummers method.13 The shape and morphology of the GOQD were analyzed using high resolution transmission electron microscope (HR-TEM, Tecnai G2 F30, see Supporting Information, Figure S2). In the TEM image, the 2D lateral size of GOQD was measured about 5 ± 1 nm. Additionally, selected area electron diffractions (SAED) was used to characterize the crystalline structures of the GOQD. As shown in Fig. S2, the GOQD could show a high crystalline structure with a lattice parameter of ~ 0.24 nm. Also, the GOQD surface was further investigated by atomic force microscopy (AFM, Veeco D3100, USA, see Supporting Information, Figure S3 to investigate surface morphology). As shown in the line scans in Fig. S3, the surface roughness was about 1 nm having an average height of 0.5 nm, which could be originated from folds or wrinkles at the GOQD flake edges. Since it has been typically known that singly and doubly over layered rGO behave like a semiconductor, the rGOQD layer in Fig. S3 is expected to be the semiconducting layer.15,18 At the same time, X-ray photoelectron spectroscopy (XPS) and Raman spectra were characterized on the GOQD and rGOQD. The GOQD XPS spectrum as shown in Fig. S4 represents C=O bond at 288.10 eV, C-O bond at 285.80 eV, and C-C bond at 284.30 eV, respectively.16 The rGOQD C-C bond was shown at 284.19 eV.4 The GOQD Raman spectra as shown in Fig. S5 exhibited that the characteristic D band of graphene was located at 1365 cm-1, G peak is located at 1608 cm-1 of 2D peak. Also, the peak of rGOQD was identified that the D


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band was located at 1377 cm-1, and the G peak was located at 1603 cm-1 of 2D peak. (See Supporting Information, Figure S4, and Figure S5) The Raman spectra proved that GOQD and rGOQD were identical to themselves as already investigated and reported in earlier literatures.4 As the first step for the characterization of the active layer behavior, we examined semiconducting functionality of the rGOQD layer, which was constructed as charge injection layer for the rGOQD capacitor without Au NPs intervention in Figure 2(a).18 It is known that even though the reduction process is applied to the GOQD, the oxygenous regions of the rGOQD are inevitable at the level of 5 ~ 10 %.4 In the capacitor format, the rGOQD layers could be formed in a uniform coverage due to the electrostatic attraction between positively charged APTES surface and negatively charged rGOQD, which would drive a maximal advantage of the dip-coating method. Apparently, no embedding of the Au NPs layer had to result in near-zero hysteretic C-V curve as shown in Fig. 2(a). In the C-V curve, the slope of depletion area was sharply measured from accumulation area to inversion area. It indicated that the interface between the rGOQD layer and the APTES layer on PVA surface had a low content of traps and mobile ions, which would guarantee the charging phenomena effects selectively only upon existence of the Au NPs embedding.19,20 In Figure 2(b) and Figure 2(c), the Au NPs were adopted in nano-floating-gate-memory (NFGM) device structure in the rGOQD memory capacitor. The intervention layer of the 5 nm Au NPs have been continuously adopted and developed in many previous reports about memory devices, which were very successful to show electrical charging phenomena, especially on PVA dielectric layer.18-20 As shown in Fig. S6, the Au NPs were embedded efficiently and relatively densely layered by SAMs formation through the 3-aminopropyl-triethoxysilane (APTES)


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functionalized substrate by vertical immersion (dip-coating) in the solution of citrate capped Au NPs.18,19 In Fig. 2(b), the device’s C-V characteristic was shown with voltage bias ranging from ± 5 V to ± 10 V measured at 1 M Hz. Both frequency change under various voltage sweep ranges and enlargement of voltage sweeping range could produce a flat-band voltage shift (∆VFB). The ∆VFB of ± 5 V sweep range was measured as 1.1 V, where it was increased to 3.74 V for ± 10 V sweep range. Particularly, the hysteresis loops were measured in counter-clockwise directions as shown in Fig. 2 (b).15,18 These characteristics should come from a mechanism that electrons were injected and charged efficiently through the thin film of PVA layer in the depletion region.19,20 As the sweep range was increased to ± 10 V, the ∆VFB was shifted to both negative and positive directions with different magnitudes depending on the forward or backward sweep direction. These results indicate that the charge injection from PVA rather than from top electrode through the rGOQD layer was comparatively efficient with a slim discrepancy.18-20 Fig. 2(c) shows a retention characteristic of electrical charging on the rGOQD memory capacitor with Au NPs embedding. The retention results for the charge storage of the Au NPs were measured at 1 M Hz frequency as well. The measurements were performed by biasing -7 V for 10 s at first. Then, C–V measurements were executed sequentially at 0 s, 10 s, 100 s, 1,000 s, 10,000 s and 100,000 s after the bias. As a consequence, the C-V curve was shifted from the initial Au NPs charging and it was unchangeably kept even after 100,000 s. At 10 s, the ∆VFB was shown as 1.8 V. However, as the retention time has passed to 100 s, 1,000 s, 10,000 s and 100,000 s, the ∆VFB was gradually converged as 1.3 V, 1.2 V, 0.8 V, and 0.7 V, respectively. The retention characteristic implicates that the rGOQD memory capacitor with Au NPs was compatible with performance of traditional memory capacitor.


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Moreover, we examined a semiconducting property of the rGOQD active layer and a switch effect in the form of the rGOQD-TFT. Figure 3(a) shows output (Id-Vd) characteristics of rGOQD-TFT and Figure 3(b) shows transfer (Id-Vg) characteristic of the same device. The device demonstrated that output characteristics of rGOQD-TFT without Au NPs SAMs accomplished a semi-saturated linear current behavior. In Fig. 3(b), unlike conventional rGO TFT’s transfer curves, the rGOQD TFTs did not exhibit an ambipolar characteristics having both n-type and p-type current behavior at zero gate voltage, where “V” shaped curves were typically pronounced.14-17 For the phenomena of the non-ambipolar transfer curves, it can be suggested that, meanwhile the typical electronic property of the graphene or rGO have been known to have a zero bandgap, the electronic characteristics of the rGOQD could have a non-zero or open bandgap.4-6 However, more intensive investigations about the non-zero bandgap of the rGOQD on the electronic device application should be addressed in the future. At the same time, electrical hysteresis was not detected after consecutively positive and negative sweeps of Vg between - 30 V to + 10 V.14-16 From the linear region of the Id-Vg curve, mobility of the device could be derived from the equation of (dId / dVg) = (W / L) * µ * Co * Vd, which could be typically applied for the TFT device physics. The Co was the capacitance per unit area that was given by Co = ε*εo / t, where ε (the dielectric constant for PVA), εo (the electric constant at vaccum), and the t (the thickness of the PVA dielectric layer) were 3.88, 8.854 × 10-12 F m-1, and 300 nm, respectively.19 From these values, Co was obtained as 11.45 nF / cm2, Vd = -1 V. Thus, hole mobility was measured as µh = 11.575 cm2 / V*s.


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Finally, we demonstrated rGOQD-MTFT memory effect and its switch characteristics between program and erase biases. Figure 4(a) shows Id-Vd characteristics of the rGOQD-MTFT. It again showed a successful saturation output characteristic, which was significantly different from a unique result for conventional graphene based TFTs. For example, first, most of rGO TFTs have shown ambipolar characteristics, where both n-channel and p-channel were workable together.1417

Second, typical active layers of graphene TFTs seldom have shown a successful saturation

output. Therefore, by the adoption of the GOQD, the authentic output characteristic of TFT could be accomplished even with the graphene material. Figure 4(b) has shown transfer Id-Vg characteristic of the Au NPs embedded rGOQD-MTFT with + 40 V program, re-program, and 40 V erase biases, respectively. Threshold voltage (Vth) shifted approximately as much as 7.84 V when the rGOQD-MTFT devices were operated with the program/erase biases. Program and reprogram biases did not produce significant differences in output characteristics as shown in Fig. 4 (b). It proves their stable and reliable operations of memory TFTs after repetitive bias inducements without significant hysteresis, which have been occasionally observed in graphenebased memory TFTs.14-17 In addition, under a separate experiment, in the presence of 1 cm radius bending with the rGOQD-MTFT devices on the flexible PET substrate, there was no changes in threshold voltage shift (∆Vth) (data not shown). The rGOQD-MTFT was characterized with hole mobility of µh = 5.005 cm2 / V*s derived from transfer characteristic in Fig. 4(b). The µh of the Fig. 4(b) was lower than that of Fig. 3(a) without Au NPs embedding. Generally, the graphene-based TFTs have shown more apparent pchannel characteristics rather than n-channel property as like as the rGOQD-MTFT studied in this study.14-16 It indicated that the rGOQD channel was free from ambipolar transfer characteristics, which have been a common obstacle with the graphene-based active layer.


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Therefore, a critical hurdle for utilization of graphene-based semiconducting layer could be overcome using the QD formats as the active channel for the TFTs. In addition, as the memory TFTs, the relatively high mobility of µh = 5.005 cm2 / V*s of the rGOQD-MTFT implicated that carrier trapping in the Au NPs of the rGOQD channel was minimized and the charging phenomena were taken placed mainly on the Au NPs. Moreover, when the transfer curves in Fig. 3(a) and Fig. 4(a) were compared, it could be found that the saturation region was early saturated in the curve of the Fig. 4(a) with embedding of the Au NPs. The reason could be simply an improvement of electrical contact between the source/drain electrode and the active rGOQD layer through the intervention of the metallic Au NPs. In fact, there was a report that conjugations of Au NPs on semiconducting sheet like MoS2 could produce a so-called effective gate capacitance, which could increase overall current level at its corresponding TFT transfer curve, and even could provide an enhanced pseudo-mobility as well.21 Therefore, such effect could be another reason for the earlier saturation and the flatter transfer curves. For the program, erase, and re-program biases, separate measurements at the gate bias of ±40 V were tried as shown in Fig. 4(b), where the limited bias voltages (±90 V) attainable for program and erase were also reported in previous literature with polyaniline NPs.19


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CONCLUSIONS We demonstrated newly implemental fabrications and characterizations of solution processed rGOQD memory capacitor, rGOQD thin film transistor (rGOQD-TFT) and rGOQD memory thin film transistor (rGOQD-MTFT). The rGOQD channel layer showed perfect characteristic of semiconducting property pertaining to output and transfer characteristics compatible to nongraphene based TFT devices. Also, Au NPs were adopted and utilized as charging elements reliably in nonvolatile memory devices of rGOQD memory capacitor and rGOQD-MTFT structures. Memory effect of rGOQD capacitor was 3.74V in ∆VFB and its retention characteristic was shown to be efficient to show longer than 10,000 s. The rGOQD-TFT hole mobility was µh = 11.575 cm2 / V*s and rGOQD-MTFT characteristic was µh = 5.005 cm2 / V*s, respectively. In addition, a perfect non-ambipolar p-channel behavior in the TFT structure could be provided. Finally, the threshold voltage (∆Vth) shifts approximately ranging upto 7.84 V with rGOQD-MTFT devices were detected with the program/erase biases and its window memory was shown 6.83 V. These results could contribute to a further step to realize quantum dot electronics and an entirely organic or plastic electronics for a fast operable memory device.


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Corresponding Authors *E-mail: [email protected], [email protected] Present Addresses To whom correspondence should be addressed at Department of Chemical Engineering, Myongji University, 116 Myongji-ro, Cheoin-gu, Yongin 17058, Republic of Korea Acknowledgement This research was supported by the Pioneer Research Center Program through the NRF of Korea (No. 2014M3C1A3053035) funded by the Ministry of Science, ICT & Future Planning and also supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (No.2015R1D1A1A09060662). Supporting Information Fabrication of the reduced graphene oxide quantum dot (rGOQD); Capping of Au nanoparticles with citric acid; Analysis of GOQD and rGOQD; was detailed information and statistical data.


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Figure captions

Figure 1. Images of fabrication process of reduced graphene oxide quantum dot (rGOQD) based flexible devices. Final structure of (a) rGOQD memory capacitor (b) rGOQD TFT without Au NPs (c) rGOQD-MTFT with Au NPs embedding. Figure 2. Characteristics of rGOQD capacitor and rGOQD memory capacitor (a) No Au NPs intervention for the rGOQD device. C-V characteristic has shown ± 3 voltage range bias. (b) Counter-clockwise C-V characteristic of rGOQD memory capacitor with Au NPs embedding, where device voltage bias range ± 5 V to ± 10 V. (c) Retention property of rGOQD memory capacitor and measured at 10 s to 100,000 s. Figure 3. rGOQD-TFT electric property showing p-type TFT without charging of Au NPs. (a) Output (Id-Vd) curve of reduced graphene oxide thin film transistor (rGOQD-TFT). (b) Transfer (Id-Vg) curve of rGOQD-TFT. Figure 4. Au NPs embedded rGOQD-MTFT electrical characteristic. (a) Output (Id-Vd) curve of reduced graphene oxide memory thin film transistor (rGOQD-MTFT). (b) Transfer (Id-Vg) curve with +40V program or re-program bias and -40V erase bias.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Table of Contents Graphic


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Characteristics of Reduced Graphene Oxide Quantum Dots for a

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