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Surface Modification of CdSe Quantum-Dot Floating Gates for Advancing Light-Erasable Organic Field-Effect Transistor Memories Yong Jin Jeong, Dong-Jin Yun, Sung Hoon Noh, Chan Eon Park, and Jaeyoung Jang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01413 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Nano

Surface Modification of CdSe Quantum-Dot Floating Gates for Advancing Light-Erasable Organic Field-Effect Transistor Memories

Yong Jin Jeong,†, ‡, § Dong-Jin Yun,# Sung Hoon Noh,§ Chan Eon Park,‡ and Jaeyoung Jang,*,§

†

The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of

Korea. ‡

Polymer Research Institute, Department of Chemical Engineering, Pohang University of

Science and Technology, Pohang 37673, Republic of Korea. §

Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea.

#

Analytical Science Laboratory of Samsung Advanced Institute of Technology, SAIT, Suwon

16678, Republic of Korea.

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ABSTRACT Photo-responsive transistor memories that can be erased using light-only bias are of significant interest owing to their convenient elimination of stored data for information delivery. Herein, we suggest a strategy to improve light-erasable organic transistor memories, which enables fast “photo-induced recovery” under low-intensity light. CdSe quantum dots (QDs) whose surfaces are covered with three different organic molecules, are introduced as photoactive floating-gate interlayers in organic transistor memories. We determine that CdSe QDs capped or surface-modified with small molecular ligands lead to efficient hole diffusion from the QDs to the conducting channel during “photo-induced recovery,” resulting in faster erasing times. In particular, the memories with QDs surface-modified with fluorinated molecules function as normally-ON type transistor memories with non-destructive operation. These memories exhibit high memory ratios over 105 between OFF and ON bi-stable current states for over 10000 s and good dynamic switching behavior with voltage-driven programming processes and light-assisted erasing processes within 1 s. Our study provides a useful guideline for designing photoactive floating-gate materials to achieve desirable properties of light-erasable organic transistor memories.

Graphical Table of Contents

KEYWORDS: quantum dots, surface ligands, photo-induced recovery, optical memories, organic field-effect transistors, normally-ON type, non-destructive


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Photo-responsive flash memories based on organic field-effect transistor (OFET) architecture have attracted significant interest because photo-bias can act as the fourth terminal capable of controlling the threshold voltage and enabling large memory windows without applying high electrical stresses.1-6 For example, OFET-type light-programmable memories, which are flash memories programmed by an electrical pulse with the aid of light, are of significant interest owing to their specialized potential applications such as flexible imaging circuits, infrared-sensing memories, and multibit-storage memory cells.6-8 Meanwhile, OFET-type light-erasable memories have recently emerged as a promising element for information delivery with the advantage of convenient elimination of stored charges.9-14 The erasing process in this type of memory is usually controlled by light-only bias, following a “photo-induced recovery” mechanism, which is based on photo-induced charge transfer across the interface between the semiconductor and floating-gate layers.9-10 Early studies on the light-erasable transistor memories required high-intensity light sources ranging from 1.8 mW/cm2 to 188 mW/cm2.10-14 With the rapid progress in this field, there is an urgent issue of data erasure under low-intensity light conditions (below 1 mW/cm2). This will enable not only the reduction of operating power consumption but also integration with highly sensitive photodetectors for use in the fields of flexible imaging, integrated sensors, and biomedical applications.7, 15-17 We previously reported organic transistor memories employing a polymer/C60 composite floating-gate interlayer, which can be erased by simple light illumination.9 In this memory, low-intensity light (0.5 mW/cm2) could effectively remove trapped charge carriers and thereby enable the recovery of the initial state. However, these devices still encounter the following issues regarding process efficiency and effectiveness: 1) the erasing process required a significantly long time (30 s) to completely remove the stored information, and 2) for the reading process of p-type operation, it was necessary to apply gate voltages, which


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could possibly act as sustained gate-bias stresses to the device. In general, hole storage is preferred in p-type transistor memories owing to its higher mobility and density.9, 11-12, 14, 18-19 However, hole storage usually requires negative reading gate voltage to distinguish between ON and OFF current states, which is undesirable for non-destructive reading of memory devices.9, 11-12, 14 These limitations should be overcome for practical applications of OFETtype light-erasable memories. A possible strategy to facilitate fast erasing process under low-intensity light and non-destructive reading process is to employ tailor-made floating-gate materials for transistor memory devices. This is because floating gates perform crucial tasks including charge carrier generation with light absorption and charge transfer to the semiconductor layer. The systematic engineering of floating-gate materials may be possible by using colloidal quantum-dot (QD) nanocrystals; the use of QDs has many advantages compared with that of C60 or other widely used floating-gate materials because tunable optical and electronic properties can be achieved through the engineering of particle size and surface ligands and the choice of core materials.6, 10 In this regard, developing QD-based floating-gate interlayers is necessary and further systematic studies on the effects of properties of QDs on the performance of memory devices are strongly required. Herein, we report on light-erasable transistor memories employing QD floating-gate interlayers and demonstrate their “photo-induced recovery” under low-intensity light illumination for a significantly short erasing time of 1 s and non-destructive readout, by engineering the surface chemistry of QDs. Organic ligand-capped CdSe QDs act as polymerbinder-free photoactive charge storage materials in the floating-gate layer of transistor memories. Three types of QDs were prepared using different surface modifications—1) octadecylphosphonic acid-capped QD (ODPA-QD); 2) perfluorinated thiol-deposited QD (FQD); 3) brush-type polystyrene-capped QD (PS brush-QD)—as shown in Fig. 1a. By


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inserting these QD layers between the pentacene semiconductor and SiO2 charge-blocking dielectric layers, we fabricated OFET-type light-erasable memories (Fig. 1b). In addition, we investigated the effects of the surface of QDs on the performance of the memories and found that the use of small molecular surface ligands and the control of their dipole moments are important for the fast “photo-induced recovery” and non-destructive memory operations.

RESULTS AND DISCUSSION As-synthesized CdSe QDs were capped with ODPA and stably dispersed in toluene as shown in the photograph in Fig. 1c (See Methods for detailed procedures of the synthesis and surface modifications of CdSe QDs). The UV–visible absorption spectrum of the QDs shown in Fig. 1c reveals that they can effectively absorb green laser light (wavelength 532 nm), which was used for light illumination during this study. The QDs were observed to have an average size of ~5.4 nm, calculated from the first excitonic peak position (see Supporting Information, Fig. S1a). When spin-cast from the solution, the QDs formed uniform thin films where a pile of nanometer-sized QDs can be observed from the atomic force microscopy (AFM) topograph (Fig. 1d). The films included a crystalline morphology whose structure corresponded to the typical wurtzite structure of CdSe, identified using X-ray diffraction (XRD) analysis (Fig. S1b).20-21 After surface modifications of QDs with a perfluorinated thiol and brush-type polystyrene, we performed X-ray photoemission spectroscopy (XPS) measurements to investigate the atomic compositions at the surface of the QDs. From the XPS spectra (Fig. S2) over a wide range of energies (20 eV to 1250 eV), the atomic compositions of ODPA-QDs, F-QDs, and PS brush-QDs were calculated and the results are summarized in Fig. 1e. As shown in Figs. 1e and 1f, only F-QDs showed the F 1s peak, whereas others did not, demonstrating the existence of fluorine element in F-QD films.


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Furthermore, PS brush-QDs exhibited a higher C 1s composition and lower Se 3d and Cd 3d5 compositions compared with ODPA-QDs (Fig. 1e). We believe that these composition variations are due to a larger amount of carbon element in polystyrene than in ODPA. Figure 1e also shows that both F-QDs and PS brush-QDs still exhibited P signals although their intensities were reduced compared to that of ODPA-QDs. This implies that some ODPA ligands remained unexchanged in F-QD and PS brush-QD layers even after the surface modifications. To study the effects of surface modifications of the QDs on the growth of the overlying pentacene molecules, we investigated the morphologies and crystalline structures of the pentacene films and the surface properties of each QD film. AFM topographs in Figs. 2a-2c reveal that all the three QDs formed continuous and uniform thin films on SiO2/Si substrates without notable aggregated QD clusters. The surface of F-QD films was observed to be slightly more hydrophobic compared with that of ODPA-QD films, likely due to the Fatom-containing perfluorinated thiol treatment; the water contact angle on the F-QD film (101°, inset of Fig. 2b) was observed to be higher than that on the ODPA-QD film (92°, inset of Fig. 2a). Based on the contact angles of water and diiodomethane drops, we calculated the surface energies (γs) of the ODPA-QD, F-QD, and PS brush-QD films (32.59 mJm-2, 25.78 mJm-2, and 44.63 mJm-2, respectively), as summarized in Table S1 in the Supporting Information. The lower surface energy of F-QD films might result from the inherent characteristics of fluorine atoms in the films, which are known to induce a nonpolar and weakly interactive surface.22-23 As shown in the AFM topograph of Fig. 2c, PS brush-QD films exhibited a very smooth surface morphology with a remarkably lower root-mean-square roughness (Rq) value of 0.46 nm. As the exchanged brush-type polystyrene ligands may act as a polymer matrix, we infer that the surface of PS brush-QD films was mainly covered with the polymers; consequently, QDs did not appear on the surface.


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The AFM topographs in Figs. 2d-2f illustrate the morphologies of 50-nm-thick pentacene films grown on the three types of QD films, respectively. The pentacene films grown on the ODPA-QD and F-QD films showed mostly similar morphologies whereas that grown on the F-QD film showed a slightly smaller pentacene grain size and higher color contrast (Figs. 2d and 2e). To further investigate this difference, we fabricated 10–20-nmthick pentacene films on the ODPA-QD and F-QD films and examined their morphologies (see Supporting Information, Fig. S3). As shown in Fig. S3, the average height of pentacene grains on an F-QD film (22.5 nm) was observed to be higher than that on ODPA-QD film (17.4 nm). This might be because the lower γs value of the F-QD surface led to a stronger interaction between pentacene molecules compared with the interactions between the pentacene and the surface of the F-QD film, resulting in more distinct three-dimensional island growth of pentacene.22, 24-26 The pentacene film grown on the PS brush-QD film (Fig. 2f) exhibited the largest grains, most likely due to the highest γs value and the smoothest surface of the PS brush-QD film among the three types of QD films. This result is consistent with previous reports, which indicated that pentacene shows two-dimensional layer-by-layer growth with large grains on polystyrene gate dielectrics.27-28 Furthermore, γs of 44.63 mJm-2 is similar to that of neat polystyrene thin films, which further demonstrates the successful surface modification of PS brush-QDs.28-29 The crystalline structures of the pentacene films were investigated by performing two-dimensional grazing incidence X-ray diffraction (2DGIXD) analyses. It can be observed from Figs. 2g-2i that all the 2D-GIXD patterns of pentacene films showed (00l) and (00l)* reflections along the out-of-plane (qz) axis, suggesting the presence of both thin-film and bulk phase crystals.25, 27 However, the F-QD samples showed the clearest (00l)* reflections in both 10-nm- and 50-nm-thick pentacene films compared with the others (Figs. 2g-2i and insets). This corresponds well with the AFM results because three-dimensionally grown pentacene islands on fluorinated surfaces usually


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include a large amount of bulk phase crystals.22, 24 Collectively, based on the XPS, AFM, contact angle, and 2D-GIXD analyses, we could conclude that the surfaces of QDs were successfully modified by fluorinated molecules (F-QD) or polystyrene (PS brush-QD), and these surface modifications apparently affected the growth of the overlying pentacene molecules. Bottom-gate-top-contact OFETs employing floating-gate layers were fabricated with the three types of CdSe QD films inserted between the pentacene and SiO2 layers, as shown in Fig. 1b. Figures 3a-3c show the transfer characteristics (source–drain voltage (VD) = −20 V) of the three different OFETs. These OFETs with CdSe QD layers showed typical p-type transfer characteristics with different electrical properties during the initial sweep, as summarized in Table 1. Compared with the OFETs with ODPA-QD layers, PS brush-QDbased OFETs showed higher field-effect mobilities (µ) and saturation source–drain currents (ID), likely due to the favorable crystallinity and large grain size of pentacene on the smooth PS

brush-QD

surface.28,

30

Surprisingly,

the

OFETs

with

F-QD

layers exhibited

superior µ values with considerably more positive (+) turn-on voltages (Von) compared with those of PS brush-QD devices, despite the poorer crystallinity and morphology of pentacene films on F-QD layers. The positive Von shift may result from the permanent dipole moment at the interface between the pentacene and F-QD layers because the fluorinated molecules are known to induce increased hole accumulation in the conducting channel.31-32 We believe that the increased induction of holes effectively filled the trap states in the channel, leading to the superior µ.20, 33 After applying a source–gate voltage (VG) of −40 V for 100 ms, all three transfer curves completely shifted toward the negative VG direction (black square) and could not be turned on even at VG = −20 V (i.e., the devices became fully depleted and maintained OFF current state during a VG sweep of 20 V ~ −20 V). These shifts originated from the positive


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space charges accumulated in the QD floating-gate layers owing to the applied negative VG bias stress.9, 12, 14 When a negative electric field was applied between the source and gate electrodes, numerous holes were transferred to QDs in the floating-gate layers and stored there until being erased. These stored holes, which acted as positive space charges, hindered the formation of a hole-conducting channel even at large negative (−) biases of VG. We also investigated the effects of magnitude and time of (−) VG biases on the changes in transfer characteristics of the OFETs and found that the negative bias condition (−40 V for 100 ms) was sufficient for complete shifts of the transfer curves (see Supporting Information, Figs. S4-S6). Notably, all the negatively shifted transfer curves returned to their initial states after exposing the OFETs to green light with a wavelength of 532 nm and an intensity of 0.7 mW/cm2. In other words, all the OFETs with CdSe QD layers, regardless of the surface modification, showed such “photo-induced recovery” characteristics.9 We think this effect mainly originated from the photoresponse of CdSe QDs, based on the mechanism explained in Fig. 3d (see Supporting Information for further explanations about the mechanism, Figs. S7-S12, and the effects of the possible photoresponse of pentacene on the “photo-induced recovery”, Figs. S13-S14). It should be noted that the recovery process required different lengths of light exposure time for the three OFETs using QD layers with different surface states. As summarized in Fig. 3e, light exposure of 1 s was sufficient for the transfer curves of ODPAQD and F-QD devices to return to the initial state, whereas it took more than 20 s for the transfer curve of PS brush-QD devices to return to the initial state. Considering that PS brushQDs are mainly covered with long-chain insulating polymers, such a long time for “photoinduced recovery” might be due to the presence of polymers at the interface between the pentacene and QDs. In other words, we believe that these polymers hindered and delayed the diffusion of high-energy carriers (separated from excitons) from the QDs to the conducting


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channel. The average interparticle distance of PS brush-QDs was calculated to be 3.02 ± 0.89 nm from several TEM images (see Supporting Information, Fig. S15). In contrast, small molecules like ODPA and perfluorinated thiol could show more efficient carrier transfers between the QDs and the conducting channel. Consequently, much shorter exposure times were sufficient to exhibit “photo-induced recovery” for both ODPA-QD and F-QD devices. As a certain level of tunneling barriers is required for memory operations to prevent serious lateral charge losses,34-35 capping ligands are indispensable for QDs used as floating gates. Therefore, our study suggests that the use of small molecular surface ligands is an effective way to improve the performance of light-erasable transistor memories and further optimization may be possible by developing tailor-made surface ligands. As summarized in Fig. 3e and Table S2, the memories with ODPA-QD and F-QD layers showed a much faster recovery time (1 s) than our previously reported memories with polymer/C60 (30 s),9 and functioned with a lower light intensity for complete recovery compared with other reported devices.10-14 We also investigated the effects of the light exposure time and light power intensity on the “photo-induced recovery” of the devices and found that the devices showed intermediate states at shorter light exposure times and lower light power intensities (see Supporting Information, Figs. S16 and S17). Besides, it is noteworthy that the OFETs with F-QD layers were turned on at positive (+) VG with a large Von of 6 V. Consequently, the ID at VG = 0 V and saturation ID at VG = −20 V are larger by 2 orders and more than 1 order of magnitude, respectively, for F-QD devices compared with those of ODPA-QD devices (Table 1). The ID values of F-QD devices at VG = 0 V and −20 V are also far larger than those of the OFETs using the CdSe QDs surface modified with 1-octanethiol, which is a hydrocarbon molecule with the same thiol end group and carbon chain length with perfluorinated thiol (see Supporting Information, Figs. S18 and S19). As discussed above, the larger ID of the F-QD devices might result from the presence of


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permanent dipole moments at the interface between the pentacene and F-QD layers owing to the fluorinated molecules.31-32, 36 As permanent dipole moments can modulate the energy level at the semiconductor/dielectric interface, the relative direction and strength of the surface dipoles of F-QD and ODPA-QD films were investigated by performing ultraviolet photoelectron spectroscopy (UPS) analyses. Figure 4a shows the UPS spectra at the secondary electron cutoff region. The kinetic energy at the onset point of secondary electrons was observed to be higher for F-QDs than for ODPA-QDs, indicating the relative permanent dipole moments at the F-QD surface owing to the electron-withdrawing nature of fluorinated molecules. Based on the UPS results, we schematically illustrated the energy level diagrams at the pentacene/QD interfaces in Fig. 4b. The electron-withdrawing properties of fluorinated molecules generated a local built-in electric field and raised the vacuum level of the surface of F-QDs.31, 36 When pentacene was deposited on the F-QD film, the dipole moments of fluorinated molecules induced band bending of the HOMO and lowest unoccupied molecular orbital (LUMO) levels of pentacene toward the higher energy direction.36-37 Therefore, mobile holes could be accumulated in the conducting channel even at (+) VG (0 V ~ 6 V) and thus, the Von and Vth of F-QD devices were correspondingly shifted toward the positive direction. Consequently, F-QD devices could function as normally-ON type transistors, where ID can flow without the application of VG (i.e., ID flows at 0 V of VG).31, 38 In contrast, the accumulation of holes hardly occurred for ODPA-QD devices because ODPA induced the band bending slightly toward the lower energy direction. This UPS study explains the reason for the higher ID of the F-QD devices compared with that of ODPA-QD devices at a given VG. The light responsivity and electron-withdrawing properties of F-QD floating-gate layers facilitated the fabrication of normally-ON type, high-performance light-erasable transistor memories. For comparison, we measured the memory characteristics of both F-QDbased OFETs and ODPA-QD-based OFETs (Figs. 4c and 4d). Figure 4c shows the retention


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test results of ODPA-QD-based OFETs. The ID in OFF state (ID,OFF), acquired after voltagedriven programming by applying −40 V for 100 ms, was observed to be ~10-11 A at a reading VG of 0 V (VD = −20 V). After light-assisted erasing for 1 s, the ID in ON state (ID,ON) was ~10-10 A. The memory ratio, which is the ratio of ID in ON/OFF states, was approximately 101 (orange circles). Although the ID,ON could increase up to ~10-7 A at a reading VG of −5 V, severe current decay was observed upon continuous reading. This might be because the reading process with VG of −5 V could act as a sustained gate-bias stress of VG = −5 V to the devices. However, the retention characteristics of the normally-ON type OFETs with F-QD layers showed different results. As shown in Fig. 4d, the ID,ON was over 10-7 A and 10-6 A at the reading VG of 0 V and −5 V, respectively. Interestingly, the ID,ON gradually increased during the reading process at the VG of 0 V, presumably because the local built-in electric field discussed above induced the positive shift of transfer curves. Moreover, the reading at VG of −5 V did not show considerable ID,ON drops for over 10000 s, indicating a desirable non-destructive readout property. Accordingly, the F-QD-based OFETs showed high memory ratios over 105 with clear separations between the ON and OFF current states. Finally, we performed programing-reading-erasing-reading (P-R-E-R) cycle tests with the F-QD-based memory devices. Figure 5a shows the process conditions for the cycle test in which a cycle consists of sequential voltage-driven programing and reading, lightassisted erasing, and again voltage-driven reading processes. Note that we carried out two kinds of cycle tests depending on the reading voltage while maintaining the other conditions: 1) reading at a VG of −5 V (blue) and 2) reading at a VG of −0 V (orange). Figure 5b shows the corresponding ID switching responses of the F-QD-based memory devices during the dynamic P-R-E-R cycle tests. The memories exhibited stable ID switching behavior between the OFF and ON current states according to repetitive P-R-E-R processes for both reading conditions of VG = 0 V and VG = −5 V. It should be emphasized that the use of low-intensity


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light for 1 s was sufficient to recover the programmed memory devices and thereby to obtain the distinct ON current states during the cycle test. From these results, we could conclude that normally-ON type organic transistor memories were successfully demonstrated by using FQD layers, which can function with non-destructive reading processes and light-assisted erasing processes within 1 s.

CONCLUSION In summary, we introduced CdSe QDs whose surfaces were covered with three different organic molecules as photoactive floating-gate interlayers in light-erasable transistor memories and studied the effects of surface modification of QDs on the performances of the memories. The ODPA-, PS-brush-, and F-QD floating-gate films showed different surface properties, which affected the morphology and crystallinity of the overlying pentacene films and thereby the OFET performances. Small molecular ligands of ODPA- and F-QDs facilitated hole diffusion between the QDs and the conducting channel, enabling “photoinduced recovery” of memories with low-intensity light exposure of 1 s. UPS analyses revealed that permanent dipole moments existed on the surface of electron-withdrawing FQD films, which induced increased hole accumulation in the conducting channel. Consequently, the OFETs with F-QD layers exhibited positively shifted transfer curves compared with ODPA-QD devices and could function as normally-ON type transistor memories with non-destructive operation. The F-QD-based memories showed clearly separated OFF and ON bi-stable current states with high memory ratios over 105 for more than 10000 s after voltage-driven programming processes and light-assisted erasing processes within 1 s. These memories also showed good OFF/ON dynamic switching behavior during consecutive P-R-E-R cycle tests. Our study demonstrated that engineering the surface of QDs in floating-gate layers can be an effective way to advanced OFET-type light-erasable


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memories to satisfy the requirements of their practical applications.


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METHODS Chemicals and substrates. Cadmium oxide (CdO, ≥ 99.99 %, Aldrich), selenium (Se, powder, 99.99 %, Aldrich), ODPA (97 %, Alfa Aesar), trioctylphosphine oxide (TOPO, 99 %, Aldrich), trioctylphosphine (TOP, 98 %, Aldrich), thiol-terminated polystyrene (MW = 2700, Polyscience), perfluorinated thiol (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanethiol, Aldrich), toluene (≥ 99.8 %, anhydrous, Aldrich), ethanol (95 %, Samchun), acetone (≥ 99.5 %, Daejung), and isopropyl alcohol (≥ 99.5 %, Daejung) were used as received. Pentacene (99 %, Aldrich) were used after purification via sublimation. Highly n-doped silicon (Si) wafers (-oriented, resistivity < 0.005 Ωcm, thickness = 500 ± 50 µm) coated with 100 nm of thermal oxide (SiO2) growth were purchased from Namkang Hi-tech. Au (100 nm)-coated wafers were purchased from Aldrich. Synthesis of CdSe colloidal nanocrystals. CdSe QDs were synthesized by following literature methods20,38 with slight modifications. In detail, CdO (0.12 g), ODPA (0.56 g), and TOPO (6 g) were placed in a 50 mL three-neck round bottom flask and pumped at 180 °C for 1 h. After refilling the flask with N2 gas, the solution was heated to 320 °C. The reddish color of the solution changed to pale yellow. The resultant solution was allowed to cool down to 180 °C and degassed for 30 min under vacuum. Subsequently, the solution was heated to 390 °C under N2 condition and TOP (3.6 mL) was slowly injected (drop by drop). After the system regained a temperature of 390 °C, a solution of Se in TOP (1.7 M) was quickly injected and the reaction was allowed by stirring for 5 min. Subsequently, heating was stopped and the solution was quickly cooled to 100 °C. At that temperature, 10 mL of toluene was injected and the solution was further cooled down to room temperature. To collect the synthesized CdSe QDs, the solution was centrifuged at 3000 rpm for 5 min after adding ethanol and the precipitated QDs were re-dispersed in toluene. This process was


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repeated three times before storing QD solutions in a N2-filled glovebox. These assynthesized CdSe QDs were capped with ODPA (ODPA-QDs) and dispersed in toluene with a concentration of ~50 mg/mL. Sample preparation. Before surface modifications and film fabrications of CdSe QDs, excess organic ligands were carefully removed via additional multiple washings with ethanol and toluene, as described above. For surface modifications of CdSe QDs, thiolterminated organic compounds were chosen because thiols are known to be excellent capping agents for most QDs owing to the strong binding affinity between thiol groups and the surface of QDs.40-41 PS brush-QDs were prepared by exchanging ODPA ligands with brushtype polystyrene in solution state. At first, ODPA-capped QDs were precipitated by centrifugation with ethanol. After the removal of supernatant, the QDs were mixed with a solution of excessive thiol-terminated polystyrene (50 mg) in toluene (5 mL). This solution was stirred for several minutes to promote the ligand exchange of QDs from ODPA to brushtype polystyrene. The resulting PS-brush-QDs were washed with ethanol/toluene for multiple times to remove excessive organic ligands, re-dispersed in toluene, and filtered through a 0.2 µm polytetrafluoroethylene filter for further purification. For the fabrication of QD floatinggate films, the QD solutions were spin-coated on Si/SiO2 substrates and baked at 150 °C for 30 min to remove the residual solvents. Before the spin-coating process, Si/SiO2 substrates were cleaned with a boiled acetone solution, rinsed multiple times with acetone, isopropyl alcohol, and distilled water, and treated with UV-ozone. For the surface modification of ODPA-QD films with fluorinated molecules, we exposed the vapor of a perfluorinated thiol to ODPA-QD films. In detail, spin-coated ODPA-QD films were exposed to a perfluorinated thiol at 140 °C for 2 h, and thereafter, the films were baked at 150 °C for 20 min. These processes were performed in a N2 rich condition. Bottom-gate-top-contact OFET memories incorporating three types of QD floating-gate films were fabricated by depositing 50-nm-


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thick pentacene films on the QD films at a rate of 0.1–0.2 Ås-1 by using an organic molecular beam deposition system. Finally, 100-nm-thick gold source and drain (S/D) electrodes were deposited on these devices via thermal evaporation through a shadow mask. The channel length (L) and width (W) of the OFET memories were 50 µm and 1000 µm, respectively. Characterization. The electrical properties of memory devices were measured using a Keithley 4200 SCS in a N2-rich glove box. The capacitance (Ci) was measured using an Agilent 4284 Precision LCR meter. The µ in the saturation regime was calculated from the slope of square root drain current (ID1/2) versus VG using the equation, ID = µCiW(2L)−1(VG − Vth)2, where Ci is 30 nF/cm2. For light-assisted erasing of memory devices, a green laser light source was used, whose wavelength and light output were 532 nm and 1 mW, respectively. The intensity of light was measured using a photodetector (Thorlabs). UV–visible absorption spectrum of CdSe QDs were obtained using Cary UV–Vis spectrophotometer (Varian Co.). The thicknesses of the CdSe QD films were measured using an ellipsometer (FQTH-100, J. A. Woollam Co., Inc.). The chemical structures of the CdSe OD films were characterized by performing XPS measurements (Quantera 2, ULVAC PHI). The morphologies of the CdSe QD and pentacene films were investigated by using an atomic force microscope (Bruker Nanoscope). The X-ray diffraction analysis of the as-synthesized CdSe QD films was conducted using a synchrotron X-ray beam source at the 5A beamline in Pohang Accelerator Laboratory (PAL). The crystallinity of the pentacene films was analyzed by performing 2DGIXD experiments with a synchrotron X-ray beam source at the 3C beamlines in PAL. The energy levels of the ODPA-QD and F-QD films and the pentacene films deposited on the QD films were characterized by performing UPS measurement at the 4D beamline in PAL. Ultraviolet light of 90 eV was exposed to the sample and kinetic energy data were obtained under an applied bias of −5 V. To prepare samples for XPS and UPS measurements, Aucoated wafers were used as substrates after cleaning (the same procedure as the cleaning of


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Si/SiO2 substrates). After the three types of QD samples were mounted on a sample holder, XPS and UPS data were collected under ultrahigh vacuum condition.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Jang). ORCID Jaeyoung Jang: 000-0002-5548-8563

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(2018R1D1A1A02050420).

ASSOCIATED CONTENT Supporting Information Available: Supporting Figures S1−S19 and Supporting Tables S1−S2. The material is available free of charge via the Internet at http://pubs.acs.org.


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FIGURES

Figure 1. Schematic illustration showing (a) three types of CdSe QDs and (b) OFET incorporating the CdSe QD film used in this study. (c) UV–visible absorption spectrum of an as-synthesized CdSe QD solution. The inset photograph shows a 20 mL vial containing the as-synthesized CdSe QD solution dispersed in toluene. (d) AFM topography image of an assynthesized CdSe QD film. The inset graph displays the corresponding AFM cross-sectional height profile of the QD film. (e) Composition ratio graphs showing atomic compositions of ODPA-QD, F-QD, and PS brush-QD films, summarized from XPS results. (f) XPS profiles showing F 1s peaks for ODPA-QD, F-QD, and PS brush-QD films.


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Figure 2. AFM topography images of (a) ODPA-QD, (b) F-QD, and (c) PS brush-QD films. The inset images of (a) and (b) show seeding water droplets on ODPA-QD and F-QD films, respectively. AFM topography images of pentacene films (50 nm) deposited on (d) ODPAQD, (e) F-QD, and (f) PS brush-QD films. 2D-GIXD patterns of pentacene films (50 nm) deposited on (g) ODPA-QD, (h) F-QD, and (i) PS brush-QD films. The inset images of (g), (h), and (i) show 2D-GIXD patterns of 10-nm-thick pentacene films grown on ODPA-QD, FQD, and PS brush-QD films, respectively.


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Figure 3. Transfer characteristics and their changes under voltage or photo bias for the OFETs with (a) ODPA-QD, (b) F-QD, and (c) PS brush-QD layers. (d) Schematic diagram showing the pentacene/CdSe-QD interface for the possible mechanism of “photo-induced recovery.” (e) Recovery times of the devices in (a), (b), (c), and the reference #9 (OFETs with polymer/C60 layers). The recovery time is defined as the light exposure time required for the negatively shifted (programmed) transfer curve to return to the initial state. The recovery time for the OFETs with polymer/C60 layers was referred from our previous results.9 The intensity of green light was ~0.7 mW/cm2.


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Figure 4. (a) Comparative UPS spectra at the secondary cutoff region of an ODPA-QD film, F-QD film, pentacene (10 nm) deposited on an ODPA-QD film, and pentacene (10 nm) deposited on an F-QD film. (b) Schematic energy level diagrams at the (left) pentacene/ODPA-QD interface and (right) pentacene/F-QD interface. Retention test results of OFET memories with (c) ODPA-QD and (d) F-QD floating-gate layers (blue circles: reading at VG of −5 V, orange circles: reading at VG of 0 V).


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Figure 5. (a) Consecutive and sequential P-R-E-R process conditions for the operation of the OFET memories with F-QD layers: reading ID at VG = −5 V (blue line) or VG = 0 V (orange line) (VD = −20 V) after programming with VG = −40 V for 100 ms and erasing with light exposure for 1 s. (b) Corresponding dynamic ID switching responses during the P-R-E-R tests (blue circles: reading at VG of −5 V, orange circles: reading at VG of 0 V).


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Table 1. OFET characteristics of the transistor memories used in this study Thickness (nm)

Ci [nF/cm2]

Initial Von, ave µave [cm2/Vs] [V]

Initial Vth, ave

Saturation ID

[V]

[µA]

ODPA-QD

15.1

26.7

0.08

0.5

0.17

0.18

F-QD

16.5

26.7

0.21

6

4.63

2.9

PS brush-QD

24.2

24.8

0.14

1.5

1.02

4.1


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