High-Performance Quantum Dot Thin-Film Transistor with

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High-Performance Quantum Dot Thin-Film Transistor with Environmentally Benign Surface Functionalization and Robust Defect Passivation. Su Min Jung, Han Lim Kang, Jong Kook Won, Jae Hyun Kim, Cha Hwan Hwang, Kyunghan Ahn, In Chung, Byeong-Kwon Ju, Myung-Gil Kim, and Sung Kyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13997 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

High-Performance Quantum Dot Thin-Film Transistor with Environmentally Benign Surface Functionalization and Robust Defect Passivation.

Su Min Jung1†, Han Lim Kang2†, Jong Kook Won3†, JaeHyun Kim1, ChaHwan Hwang3, KyungHan Ahn4, In Chung4,5, Byeong-Kwon Ju2*, Myung-Gil Kim3* and Sung Kyu Park1*

1

School of Electrical and Electronic Engineering, Chung-Ang University, Seoul 06974, Republic of Korea 2

School of Electrical and Electronic Engineering, Korea University, Seoul, Republic of Korea 3

Department of Chemistry, Chung-Ang University, Seoul 06974, Republic of Korea

4

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea

5

School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea

†

: Equal contribution

*Corresponding Author: Prof. Sung Kyu Park, Prof. Myung-Gil Kim, and Prof. Byeong-Kwon Ju Phone: 82-2-820-5347, Fax: 82-2-820-5347 Email: [email protected], [email protected], and [email protected] ACS Paragon Plus Environment

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Abstract The recent development of high performance colloidal quantum dot (QD) thin-film transistor (TFT) has been achieved with removal of surface ligand, defect passivation, and facile electronic doping. Here, we report on high performance solution-processed CdSe QD TFTs with an optimized surface functionalization and robust defect passivation via hydrazine-free metal chalcogenide (MCC) ligands. The underlying mechanism of the ligand effects on CdSe QDs has been studied with hydrazine-free ex-situ reaction derived MCC ligands, such as Sn2S64-, Sn2Se64- and In2Se42-, allowing benign solution-process available. Furthermore, the defect passivation and remote n-type doping effects have been investigated by incorporating indium nanoparticles over the QD layer. Strong electronic coupling and solid defect passivation of QDs could be achieved by introducing the electronically active MCC capping and thermal diffusion of the indium nanoparticles, respectively. It is also noteworthy that the diffused indium nanoparticles facilitate charge injection not only interQDs but between source/drain electrodes and the QD semiconductors, significantly reducing contact resistance. With benign organic solvents, the Sn2S64-, Sn2Se64- and In2Se42- ligands based QD-TFTs exhibited field-effect mobilities exceeding 4.8, 12.0, and 44.2 cm2V-1s-1, respectively. The results reported here imply that the incorporation of MCC ligands and appropriate dopants provide a general route to high-performance, extremely stable solution-processed QD-based electronic devices with marginal toxicity, offering compatibility with standard CMOS processing and large-scale onchip device applications.

Key words: Quantum Dots, Field-effect transistor, Cadmium-selenide, High mobility, Metal Chalcogenide, doping, Thermal diffusion, Non-hydrazine.

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Introduction Recently, the thin film field-effect transistors based on solution deposition have been intensively studied to achieve the low cost, simple processing, multi-functionality, and large area uniformity for next generation large area electronics, such as flexible displays, and large area sensor arrays, which require new materials and processing strategies. Diverse semiconductor materials, such as metal oxides1, carbon nanotubes2 and organics3 are investigated for the industrial implementation on highperformance thin-film transistor (TFT) over large area, with low cost solution processes. Among diverse materials, colloidal quantum dots (QDs) have emerged as one of promising candidates as a soluble semiconductor material with the tunable band gap, novel magnetic properties, and high carrier mobility (>10 cm2V-1s-1)4–7. The colloidal QDs are typically synthesized with long organic ligands, such as oleic acid, oleylamine, trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), to allow low surface defect, easy growth control, and stable dispersion in solvent8,9. Unfortunately, the electrically insulating long organic ligands typically form large potential barrier for efficient charge transport, usually, between QDs, making the QD arrays inappropriate for electrical applications10. Although the ligand is removed with high temperature treatment, the significant volume loss or carbon residue are still problematic, indicating poor electrical performance11. To achieve efficient electronic coupling and facile charge transport between QDs, there have been significant efforts to induce a short inter-particle distance, electronically conducting bridges, and direct electrical contact with an inorganic shell. This was demonstrated with a ligand exchange to small molecules on the deposited QD arrays, along with organic semiconductor based ligand, and low-temperature decomposable short inorganic ligands. Of special note is the short inorganic ligands, such as halide/pseudohalide ions (Cl-, Br-, I-, SCN-)12–14, oxoanions (PO43-, MoO42-)15 and MCC capping ligands (Sn2S64-, In2Se42-, Cu7S4-)16–18, which demonstrated excellent charge carrier mobilities and high photovoltaic efficiencies15,19,20. Compared to monodentate ligands, such as I- and SCN-, the multidentate MCC ligands could achieve improved long term dispersion stability, which are preferable to facile and reliable device fabrication21.



The MCC ligands typically have been reported accompanying in-situ reaction between chalcogen and metal chalcogenide using hydrazine as a solvent and a reducing agent. Subsequently, the chalcogenidometallate anions replace the long organic ligand on QD surface as MCC ligands and disperse the QDs into polar hydrazine with excellent stability17,22. Although the carbon free nature of hydrazinium chalcogenidometallate allows for high purity inorganic shell formation and excellent electrical connection between semiconducting QDs23 and metallic QDs22 with electron mobility of 30 cm2/Vs and electrical conductivity of 1,250 S/cm, the significant toxicity and explosive nature of the hydrazine often deter the process from being an industry standard compatible for large-scale fabrication. Even further, the small amount oxygen or water exposure could significantly degrades the electrical performance of QD TFTs with surface oxidation/hydrolysis and subsequent defect formation, which results in significant challenges for reliable device fabrication24. Here we report on high performance QD TFTs with a robust process and a relatively environmentally benign organic solvent (dimethylformamide: DMF) with minimal toxicity (hydrazine-free). The premade chalcogenidometallate anion from ex-situ reaction is capable of direct dispersion of QDs into DMF without in-situ reductive chalcogenidometallate anion generated by hydrazine. Additionally, with the solution-processed indium nanoparticle overlayer, the thermal diffusion of indium into the QD layer achieved the surface defect passivation and subsequent electron mobility improvement in TFT devices. After the intensive investigation on the optimized QD thickness and capping ligand for efficient charge transport and defect passivation, the QD TFT devices showed saturation field-effect electron mobility as high as 44 cm2/Vs with an on-off current ratio of about 105.


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Experimental Chemical All the chemicals were purchased from Sigma-Aldrich and used without further purification.

Synthesis of nanoparticles and alkali metal chalcogenidometallates CdSe quantum dots. In a synthesis of CdSe QDs, 4.1 mmol of cadmium oxide (CdO) were mixed with 5 mL of oleic acid (OA) in 125 mL two-neck flask connected to Schlenk line. To remove water and volatile impurities, the mixture was heated at 150 oC for 1 hr under Ar atmosphere. After adding 15 mL of 1-octadecene (ODE) into the reactor, the whole mixture was further heated to 310 oC. Separately, a Se source solution was prepared by dissolving 3.2 mmol of Se in 2.0 mL of trioctylphosphine (TOP). The 2.0 mL of Se-TOP solution was swiftly injected into the above hot mixture and the reaction proceeded at 310 oC for 10 min to grow CdSe QDs. After the QD growth, the reaction mixture was naturally cooled down to room temperature. After adding 90 ml hexane to the reaction mixture, 180 ml ethanol was added into the solution and centrifuged at 10,000 rpm for 60 min. The colorless supernatant was discarded and the bottom QD precipitate was re-dispersed in hexane. The process was repeated 5 times. Indium nanoparticles (InNPs). Indium NPs were synthesized according to the reported procedure25. In a 3-neck flask with a reflux condenser, 2.5 g triotylphosphine oxide (90 %, SigmaAldrich), 5 mL oleylamine (70 %, Sigma-Aldrich) and 0.83 g indium trichloride (99.99 % SigmaAldrich) were completely dissolved in 200 mL isobutylamine (99 %, Sigma-Aldrich) to form a clear colorless solution. After pumping the reaction mixture under a vacuum for 15 min to remove air, the flask was filled with Ar gas and naturally warmed up to room temperature. Because the solvent, isobutylamine, is highly volatile with boiling point of 63 oC, almost half of solvent was left after pumping. 5.0 mL of 2 M lithium borohydride in tetrahydrofuran (Sigma-Aldrich) was added into the reaction mixture solution dropwise over 15 min. The solution was further stirred over 12 min at room temperature and subsequently heated to reflux condition (70 oC). After 1 hr reaction at the reflux condition, the reaction mixture was cooled down at room temperature. Methanol (anhydrous, ACS Paragon Plus Environment


99.8 %, Sigma-Aldrich) was added to the reaction mixture as 1:1 ratio. The mixed solution was centrifuged at 4,000 rpm for 3 min and the colorless supernatant was discarded. The indium NPs were redispersed in 15 ml toluene (anhydrous, 99.8 %, Sigma Aldrich). Na4Sn2S6·14H2O. The synthesis followed the reported literature method26. The 1 ml of 2 M SnCl4·5H2O aqueous solution was slowly added into 10 ml of 0.6 M Na2S·9H2O aqueous solution with vigorous stirring. The clear yellow solution was slowly added to 30 ml acetone and yellow oil was formed at the bottom of the flask. The supernatant was decanted away and then 10 ml of acetone was added again. The white precipitates were collected, filtered, and then dried under vacuum at room temperature. Na4Sn2Se6 and KInSe2. The stoichiometric amount of Na (114 mg), Sn (296 mg), Se (588 mg) and K (125 mg), In (368 mg), Se (506 mg) were heated in an evacuated graphite crucible at 750 oC for 24 hr, respectively. The heating rate was 30 oC/hr and cooling rate was natural. The characterization of synthesized ligands was shown in supporting information figure S.127–29.

Ligand exchange All of the ligand exchange process was performed in a nitrogen-filled glovebox with anhydrous solvents. 5.0 mg of metal chalcogenide capping ligand, such as Na4Sn2S6·14H2O, Na4Sn2Se6, and KInSe2, was dissolved in 3.0 mL of the dimethylformamide (DMF) (99.8 %, Sigma Aldrich) / ethanolamine (EA) (99.5 %, Sigma Aldrich) mixture (2:1 by volume). The solution was added into the 2.0 mL of the oleic acid capped QDs solution in hexane. After stirring for 3-4 hr, the upper colorless hexane part was carefully removed and the solution was further washed with 2.0 ml hexane several times. The ligand exchanged QD solution was mixed with 9.0 ml acetonitrile (99.8 %, Sigma Aldrich) and centrifuged at 5,000 rpm for 5 min. The upper solution was discarded and the bottom part was redispersed in DMF.

Device fabrication


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Highly p-doped Si substrate was cleaned by sonication in acetone and IPA for 10 min, respectively. The substrate was treated with oxygen plasma at 100 W for 1 min to remove the organic impurities and to form the hydrophilic surface before spin-coating. The prepared AlOx solution was spin coated on the substrate at 3,000 rpm for 20 s in air. To prepare the AlOx solution, aluminum nitrate nonahydrate (Al(NO3)3·9(H2O)) (Sigma-Aldrich) was dissolved in 2-ME (anhydrous, SigmaAldrich) in a concentration of 0.8 M. After dissolving the precursor, the solution was stirred for 12 hr at 75 oC. The spin-coated substrate was DUV-annealed for 2 hr in nitrogen atmopshere30. The electrical characterization and thickness information of Al2O3 were provided in supporting information in figure S.2. The AlOx film on the Si substrate was treated with oxygen plasma at 100 W for 1 min. The QD film was spin-coated on the substrate at 1,000 rpm for 30 s and subsequently annealed at 250 oC in a nitrogen-filled glove box. The indium NPs were spin-coated on the QD film and annealed at 50 oC. The 50 nm gold electrodes were deposited by thermal evaporation. Finally, the QD TFT device was annealed at 300 oC under inert atmosphere.

Characterization Electrical measurements. All the measurements were performed in a nitrogen- filled glove box and darkness at room temperature. The capacitance value to calculate the device mobility was obtained using an Agilent LCR 4284A. The TFT performance characterizations, such as transfer curve and output curve, were done with Agilent 4156C analyser (Agilent, U.S.A.). Mobility calculation. The field-effect mobility was calculated by using the following equation.

2 =

ξ-potential measurement. The data were collected using an Otsuka ELSZ-1000 (Otsuka Electronics, Japan). Colloidal solutions were filled into a quartz cuvette using a flow cell. The measurement was done without air bubble in the cell. Fourier transform infrared (FTIR) spectroscopy. FTIR spectra were obtained in the transmission mode using Nicolet 6700 (Thermo Sceoencetific, U.S.A.). The QD films were coated on the intrinsic Si wafer.



Optical absorption measurements. The absorption spectra of the solution and thin film were obtained by using each S-3100 (Scinco, South Korea) and V-730 (Jasco, U.S.A.). The data of the solution were acquired from the QD solution in the quartz cell. The spectra of the film were measured after the QDs or InNPs were dropped on glass and dried completely. Transmission electron microscopy (TEM). The TEM images of the device were obtained by using Titan 80-300 (FEI, U.S.A.). Energy dispersive X-ray spectroscopy (EDS). The EDS data were obtained by using Talos F200X (FEI, U.S.A.). Wide-angle powder X-ray diffraction (XRD). The XRD patterns of the QD films and InNPs powder were obtained by using Dmax 2500/PC (Rigaku, Japan) and D8-Advance (Bruker AXS, Germany), respectively. The XRD patterns were measured with Cu Kα X-ray source operating either at 40 kV and 200 mA or at 40 kV and 40 mA for QD films or InNPs powder, respectively. Thermogravimetric analysis (TGA). The thermogravimetric analysis data were acquired using N1000 thermal analyser (Scinco, South Korea) at a heating of 10 oC/min under Ar flow 40 cc/min Atomic force microscope (AFM). The thickness of QD films was measured using XE-100 (Park Systems, South Korea) after scratching the film.


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Result and Discussion Figure 1a shows the schematic diagram of ligand exchange and defect passivation for QD TFT fabrication. For the synthesis of alkali metal chalcogenidometallate, such as Na6Sn2S6·14H2O, K6Sn2Se6, and KInSe2, both high-temperature solid state and solution phase routes31–33 were utilized. The premade chalcogenidometallate anions relieved the needs of toxic hydrazine to generate chcalcometallate with chalcogen reduction. By dissolving the alkali metal chalcogenidometallate into polar organic solvents, such as DMSO, DMF, and EA, the MCC ligands are easily generated which can displace the long organic ligands on the QD surface (Figures 1a and S.3). The short inorganic ligands (≥ 0.5 nm) could reduce the interparticle distance between QDs, compared to long organic ligands (≥ 1.5 nm)34. The thermal decomposition of MCC ligands could more significantly reduce the distance between QDs. Regarding formula, the quantum mechanical coupling energy can &

∆" $

be mathematically expressed as: ≈ ℎ ≈ [− 2∗ $ % ∆] approximately, where h is ħ Plank’s constant, Γ is the tunneling rate between two nanocrystals, m* is the carrier effective mass, ħ is Planck’s constant divided by 2π, and ∆E and ∆ are the height of the tunneling barrier and the shortest distance between two nanocrystals, respectively17,35. By replacing the long hydrocarbon chain with the thin metal chalcogenide shell, QDs can easily communicate electronically with reduced ∆E and ∆. For instance, the conversion of Sn2S64- ligand into thin SnS2 shell after thermal treatment, the energy barrier and inter-particle distance between QDs could be reduced which results in facile electron transport through the QD arrays17. In general, the metal chalcogenide QD surface could be easily oxidized or hydrolyzed during the initial synthesis, ligand exchange, device fabrication, and/or electrical measurement. Although the strongly reductive hydrazine based solvent and MCC ligands might protect and recover the QD surface from undesirable degradations, the conventional organic solvent and nonreductive alkali metal chalcogenidometallate in our approach could not provide additional defect passivation routes36,37. Furthermore, when the hydrazine is used as a solvent to fabricate QD-TFT, the solvent impurity will be removed cleanly as nitrogen, hydrogen and ammonia23,38. The residue free nature of hydrazine makes it an ideal solvent to fabricate the high-performance quantum dot devices, ACS Paragon Plus Environment


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regardless of its extreme toxicity and explosiveness. Unfortunately, the typical polar solvent, such as NMF, DMF, EA, and DMSO, could not provide high enough reducing environment and residue free thermal decomposition, which could result surface defect within QD arrays and poor charge carrier transport. The indium diffusion has been reported as an effective method for the reduction of oxidized QD surface and the electrical defect passivation within QD films5,39. To achieve the solution processed indium overlayer on QD arrays, the indium nanoparticles (InNPs) dispersion is synthesized with the reported process25. In Figure S.4, the well matched XRD peaks with a theoretical pattern confirmed successful synthesis of highly crystalline indium nanoparticles. Also the absorption spectrum of InNPs showed a plasmon resonance peak at 270 nm, which is similar to previous reports40. As shown in Figure 1b, the spin-coated and thermally diffused InNPs overlayers are used to reduce the oxidized QD surface and to passivate the electrical defects of QD arrays. In Figures 1c-d, the ligand exchange of long oleic acid ligand with alkali metal chalcogenidometallates, such as Na4Sn2S6 and K6Sn2Se6 and KInSe2 has been characterized with zeta potential measurement and FTIR spectra. As reported previously, the dissolution of alkali metal chalcogenidometallates into polar solvent generate chalcogenidometallate anions, such as polymeric In2Se42-, molecular Sn2S64- and Sn2Se64-.7,17,22 During the ligand exchange process, the nucleophilic negative chalcogenidometallate anions strongly binds onto the electrophilic undercoordinated cadmium atoms at the QDs surface17. The negatively charged QD surface with the MCC ligand anion is confirmed by the zeta potential measurement, as shown in Figure 1c. The initial oleic acid bound QDs have the zeta potential value of ~1.6 mV. Despite the small zeta-potential, its long length ligand makes space between QDs, which prevents the QD aggregation caused by Van der Waals’ force41. In contrast, the MCC ligands exchanged QDs shows the negative zeta potential values charge around -50 to -70 mV, which confirmed the presence of negatively charged MCC ligands on QD surface because of ligand exchange. The large absolute zeta potential values for the MCC ligand QD dispersion in DMF induce strong electrostatic repulsion and stabilize the QD solution for a long time without significant aggregation. The further confirmation of ligand exchange was investigated with Fourier transform infrared (FTIR) spectroscopy. In Figure 1d, the absorption FTIR spectra of oleic acid capped QD shows a strong absorption around 2,800 – 3,000 ACS Paragon Plus Environment

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cm-1 from C-H bond in oleic acid. In contrast, the MCC ligands (In2Se42-, Sn2S64-, and Sn2Se64-) bound QDs shows an absence of C-H bond derived absorption peak, which confirms complete exchange of oleic acid with MCC ligands. Although the surface ligand is replaced, the inherent physical properties of QDs should be maintained to be successfully implemented into emerging electronic or optoelectronic applications. In Figure 1e, the absorption spectra of MCC ligands bound QDs show that the quantum confinement effect and excitonic absorption peaks are preserved during the ligand exchange process, without significant aggregation or chemical changes. Compared with the absorption wavelength (551 nm) of oleic acid ligand QD, the absorption spectra of the QDs-MCC ligands shows a red-shift about 1-2 nm. In the case of aliphatic ligands such as oleic acid, the ligand’s orbital energies are separated from QD’s state, so wave function is well confined. Nevertheless, if the frontier orbital energy between ligands and inorganic core are aligned, for example HOMO of CdSe (mostly Se 4p) and HOMO of ligand, the excitons are delocalized through a mixed state. While the similar HOMO levels of QDs and ligands are aligned, interfacial states are mixed and wave functions are extended across the interface42,43. As a result, the optical bandgap reduced (in here, ~10 meV) and the absorption peak is a red shift. To achieve a thin inorganic shell and solvent removal for efficient charge transport within QD arrays, the thermal decomposition of MCC ligand bound QD was investigated with thermogravimetric analysis and XPS. As shown in Figure S.5, the vacuum dried QD samples show a loss of mass between 100 to 250 oC. This small loss of mass at a relatively low temperature region around 100 to 150 oC is trapped solvent molecules, considering the boiling point of DMF (b.p. 153 o

C). Further loss of mass at a higher temperature could be attributed to the thermal decomposition of

chalcogenidometallate ligands. Although the initial alkalimetal chalcogenidometallates are thermally stable, the surface bound chalcogenidometallate anions could have thermally decomposable organic counter ions during ligand exchange process44. The complicated rearrangement and surface reaction of chalcogenidometallate requires further detailed investigations of QD surface chemistry with exsitu derived chalcogenidometallate45. Also, as shown in table S.1 and figure S.6, the relative sulfur to selenium ratio was decreased from 100 % to 70 % after annealing at the temperature of 250 oC. ACS Paragon Plus Environment


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During the ligand decomposition, it is believed that the sulfurs can be evaporated, leading to the formation of SnS2 shell. In addition to the optimized ligand decomposition, the detailed investigation on thermal diffusion of indium into QD channel layer was investigated with 300 oC annealed QDs-Sn2Se64- layers. The QDs-Sn2Se64- with a spin-coated indium overlayer were post-annealed at various temperatures from 150 oC to 350 oC. As shown in Figure S.7, thermal energy below 200 oC is not enough to diffuse indium into QD layer. The QDs-Sn2Se64 with post-annealing temperature above 250 oC shows high source-drain current flows with the maximum being at 300 oC. After further post-annealing above 350 oC, the QD aggregation might induce disconnection between QDs and accordingly current decrease. From the transfer curve in figure S.7, the carrier concentration with indium doping was roughly estimated using following equation ∆. =

/ ∙ 12. × 6786819:1;78 34

where ∆Vth is the threshold voltage shift quantity, q is the electric charge, tch is the thickness of channel and Cox is gate dielectric capacitance. This formula was induced from Q = CV 46. Using the formula, we approximately estimate the doping induced carrier concentration. ∆Vth is the value obtained by subtracting Vth value of high temperature processed device from the Vth value of 200 oC processed device. The capacitance is 90 nF/cm2 and thickness is 23 nm. As a result, the carrier concentrations of 250 and 300 oC annealed device are 3.91×1017and 4.48×1017 cm-3, respectively. For 350 oC annealed device, the data may be unreliable possibly due to QD aggregation at the high temperature. Overall, we can roughly estimate the change of carrier concentration about 1017 cm-3 with the indium doping process. With the optimized processing condition, Figure 2 showed the variation of QD-TFT performance with ligand type and QD layer thickness. As shown in Figures 2(a), 2(d) and 2(g), the indium doping resulted in remarkable increase in drain currents. The Sn2S64--QD TFT shows field effect mobility (µ) of > 4.8 cm2/V·s and ION/IOFF ≈ 105 (Figures 2a-b). Also, Sn2Se64--QD TFT shows µ > 12.0 cm2/V·s and ION/IOFF ≈ 2×105 (Figure 2d-e). When the In2Se42- ligands are used for TFTs,

exceptional electron saturation field effect mobility higher than 44.2 cm2/V·s was achieved with ACS Paragon Plus Environment

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decent ION/IOFF ≈ 7×104 (Figures 2g-h), which is comparable to hydrazine based high performance devices4. Compared to other chalcogenidometallate ligands, such as Sn2S64- and Sn2Se64-, the superior electronic coupling and defect passivation effects of the In2Se42- ligands could enhance the electron transport between QDs and fill the traps between QD and ligands43,47. Additionally, to achieve the higher performance QD-TFTs, there seems to be an optimized thickness for each ligand (Figure 2c, 2f, 2i, and S.8). For Sn2Se64--QDs, the optimized thickness is around 15 nm, while the Sn2S64--QDs and In2Se42--QDs show optimized electrical performances with thickness around 23~24 nm. It is still not clear, but we believe that when the QD layer is too thin, the channel formation is not enough to induce charge accumulation and thus to allow enough current flow. On the other hands, if the QD layer is too thick, the indium can’t penetrate deep enough to passivate the channel layer near the dielectric layers. To confirm the existence of indium in QD layers after thermal diffusion treatment, the MCC ligand QD-TFTs were investigated with transmission electron microscope (TEM) and energy dispersion xray spectroscopy (EDS), as shown in Figure 3. The Sn2S64- ligands were used for these analyses to distinguish the diffused indium atoms of the overlayer from the atomic indium of the ligand. By comparing the pristine QD sample with the thermally diffused one, the significant reduction of InNPs layer thickness is observed in Figures 3a-b. The EDS analysis in Figures 3d-e clearly showed that the indium atoms were deeply diffused into CdSe QD layers. In Figure 3d, the pristine In NPs coated on QDs without post-annealing showed the lack of In atoms in the CdSe QDs layers. In contrast, after the thermal diffusion treatment at 300 oC, the In atom percentage is increased up to 10 % in the CdSe QDs layers. The clear reduction of indium rich layer thickness is also observed from the EDS analysis. The TFT using only InNPs in Figure S.9 showed that InNPs may not allow the electron transport because its long ligands typically form potential barriers between nanoparticles. When considering the charge transport in nanoparticle thin films, it is well known that Coulomb energy between electrons is the one of important factors. If the coupling energy is smaller than Coulomb energy, the QD arrays can behave as less semiconducting materials, and more like insulators. For this reason, Coulomb repulsion restricts the electrons that are inserted into the QDs assembly, unless the charge of electrons is compensated by positive charge35. Therefore, the diffused ACS Paragon Plus Environment


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indium atoms may promote electron transport by screening the electron-electron repulsion to compensate the charge of electrons as well as the doping effects. Overall, it is considered that the insertion of In3+ into the QD assembly, the Coulomb repulsion energy can be reduced, resulting in an enhanced charge transport between the QD nanoparticles. From the TEM images in Figures 3a-c, the additional role of the diffused indium is the prevention of QD aggregation at high temperature. The thermal treatment of CdSe QD layer at 300 oC without indium diffusion typically induce significant volume shrinkage of the film from 30 nm to 10 nm, whereas with indium diffusion, the CdSe QD layer shows the reduced thickness change from 30 nm to 20 nm. Figures 4(a)-(c) and S.10 show the UV-Vis absorption spectra of the QDs as functions of MCC ligands, various annealing temperatures, and existence of indium doping. In addition to the TEM data in Figures 3(a)-(c), the absorption spectra can explain that the diffused indium prevents aggregation of CdSe QDs at a high temperature. Even after high temperature annealing above 300 o

C, the quantum confinement effects are preserved in the indium doped MCC ligands QDs, which is

clearly observed by distinctive shoulder like excitonic absorption peaks in Figures 4(a)-(c). Most especially, the solid quantum confinement effects are noted for Sn2S64- and Sn2Se64- ligands coated CdSe QDs with indium doping (Figures 4a-b). As reported, the thermally diffused indium and inorganic ligand could form inorganic chalcogenide shell, which could effectively prevent the aggregation of CdSe QDs with suppression of the Cd and Se migration at high temperature17,39. In case of the In2Se42- ligands, after 250 oC annealing of In2Se42- capped CdSe QDs, UV-Vis-NIR spectra (figure S.11) shows the absorption peak at 880 nm, which is well matched with a reported band gap value of In2Se3 (1.4 eV)48. While the ligand decomposes, it becomes the thin inorganic shell as In2Se3 and stabilizes the CdSe quantum dot47. Furthermore, the oxidation state of the diffused indium can be confirmed through XPS analysis (figure S.12). Before high temperature diffusion process, the indiums of InNPs are distinguished as In(0) (B.E 443.8 eV) from the indiums of MCC ligand (B.E 444.8 eV). In contrast, the indiums of MCC ligands are confirmed as In3+ in In2Se42- (figure S.12 (a)). After high temperature indium diffusion, there are only In3+ from the In2Se3. So, we can expect that the indiums of the InNPs react with selenium in the MCC ligands or CdSe to form the In2Se3. ACS Paragon Plus Environment

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The relatively lower excitonic absorption peak of In2Se42- ligands comparing to other ligands (Figure 4c) may indicate that the In2Se42- ligands can induce a more effective crosslink between the QDs, enhancing the electronic coupling4,17,43. Additionally, the broad XRD patterns in Figures 4d-f show that the CdSe QDs maintain discrete nano crystallinity and quantum confinement effect, even after the high temperature thermal diffusion of indium7. We believe the more effective electronic coupling between QDs with In2Se42- ligands could significantly enhance the electron transport thorough the QD layers, maintaining their quantum confinement properties. Furthermore, since Sn and In are known as dopants of the CdSe QDs49, the MCC ligands may induce electrical doping effect for the QDs47. For the electrical doping effect, the metal ions should be dissociated from the nanoclusters (inside the MCC ligands) during thermal annealing treatment. The bond dissociation energy value of Sn-S (464 kJ/mol) and Sn-Se (401.3 kJ/mol) is much higher than the value of In-Se (245.2 kJ/mol). The strong bonds between Sn and chalcogens (S and Se) prevent efficient diffusion of Sn4+ into the CdSe. In contrast, the weak In-Se bond enables efficient In3+ diffusion into CdSe, forming In2Se3 shell and showing the highest electrical performance of QD TFT with In2Se42- ligands. Moreover, for the environmental stability of the devices, it is noted that the diffused indium healed the damaged QD layers from oxygen and moisture in air ambient6. As shown in Figure S.13, the mobility of the devices was significantly decreased by air exposure, while most of the mobility could be recovered by simple annealing of the devices under inert atmosphere (200 oC for 5 min in nitrogen). While the air stable SnS2 and SnSe2 shell based TFT devices showed complete recovery of the device performance, the In2Se42--CdSe TFT showed a moderate electron mobility recovery of 46 %. As reported, the poor electrical stability of In2Se3 under ambient conditions could result in the irreversible damage to the In2Se42--CdSe TFTs50. Also, the indium nanoparticle between channel and electrode improves the contact resistance. Since the Au typically has high work-function (4.8-5.2 eV)51, the electron transport between source/drain electrodes and the n-type semiconducting layer can be hindered52,53. The indium diffusion may contribute to adjust the electrode’s work function50 and accordingly lower the contact resistance54. Figure 5 shows the contact resistance between the electrode and the channel layer. The ACS Paragon Plus Environment


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contact resistance was measured by using transmission line measurement (TLM) and calculated from the formula, 30 cd/A) hybrid EL material/devices] funded by MOTIE/KEIT, by the Human Resources Development (No. 20174030201810) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy, and by the Expansion Division on Basic Utilization of Atomic Power through the National Research Foundation of Korea (NRF-2017M2B2A9A02049820). ACS Paragon Plus Environment


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FIGURE CAPTIONS

Figure 1 (a) Schematic diagram shows how the ligands attach to QDs. (b) Schematic diagram shows fabrication process of QD-TFTs (c) Zeta potential according to ligands (d) Fourier transform infrared (FT-IR) transmission spectra of oleylamine (OA) and metal chalcogenide capping (MCC) ligands bound QD films (e) UV-Vis absorption spectra of OA and MCC ligands bound CdSe QD solutions.

Figure 2

The electrical characteristics of CdSe QD-TFTs with MCC ligands (Sn2S64-, Sn2Se64-, In2Se42-).

(a), (d), (g) Transfer curves of QD-TFTs according to ligands before and after indium doping. (W = 1,000 µm, L = 100 µm, VD = 10 V) (b), (e), (h) Output curves of QD-TFTs according to various MCC ligands. (W = 1,000 µm, L = 100 µm, VG = 10 V, step : 2 V) (c), (f), (i) Plots of electron mobility versus channel thickness according to ligand types.

Figure 3 The characterization of thermal diffusion of doped indium nanoparticles into QD films. (a), (b), (c) TEM image of cross-section of QD TFTs (scale bar = 10 nm). After the QDs were spin-coated and annealed at 250 oC, (a) shows the TEM image about that indium nanoparticles (InNPs) weren’t annealed yet and (b) shows InNPs were annealed at 300 oC. (c) shows only QD layer was annealed at 300 oC. (d), (e) Energy dispersive X-ray spectroscopy to confirm the indium diffusion.

Figure 4 The effect of indium nanoparticle doping into CdSe QD films. (a), (b), (c) UV-Vis Absorption spectra of thin film of CdSe QDs capped with MCC ligands according to temperature. The black and blue lines are only with QD film and the red one is with InNPs. (d), (e), (f) The grazing incidence angle X-ray diffraction pattern of MCC-CdSe QD thin films according to post annealing temperature.

Figure 5 The transmission line measurement (TLM) to confirm contact resistance and sheet resistance of QD films. TLM result according to (a) ligands and (b) InNP annealing temperature. (W= 1,000 µm, VG = 10 V) ACS Paragon Plus Environment

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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



TOC graphic


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High-Performance Quantum Dot Thin-Film Transistor with

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