How to Make Quantum Dot for Electronic Ink

ACS Omega. 2019 Feb 28; 4(2): 4161–4168.

Surface-Modified Substrates for Quantum Dot Inks in Printed Electronics

Lingju Meng

^†Department of Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Canada, T6G 2V4

Tao Zeng

^†Department of Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Canada, T6G 2V4

^‡School of Material Science and Engineering, Jingdezhen Ceramic Institute (Xianghu Campus), Xianghu Road, Jingdezhen 333000, Jiangxi, P. R. China

Yihan Jin

^†Department of Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Canada, T6G 2V4

^§School of Optoelectronics, Beijing Institute of Technology, No. 5 South Zhong Guan Cun Street, Beijing 100081, P. R. China

Qiwei Xu

^†Department of Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Canada, T6G 2V4

Xihua Wang

^†Department of Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Canada, T6G 2V4

Received 2019 Jan 22; Accepted 2019 Feb 13.

This article has been cited by other articles in PMC.

Abstract

Printed electronics fill the niches for low-cost, flexible devices in electronics. Developing substrates suitable for various printable electronic inks becomes an important topic in both academia and industry. Because of their extraordinary properties like solution processability, colloidal quantum dots (QDs) are gradually emerging in this field as promising candidates for electronic inks. In recent years, researchers have successfully produced high quality PbS QD inks in polar solvents. However, the incorporation of electronic inks onto a well-passivated substrate remains challenging due to the processing incompatibility between polar solvents and hydrophobic substrates. Here, we propose a surface modification strategy by using chlorine to achieve both trap-site suppression and a hydrophilic surface. The chlorine can effectively passivate the surface dangling bonds and charged hydroxyls while creating a hydrophilic surface. On this modified substrate, the contact angle between the water droplet and the SiO₂ substrate can be as small as 20° and this strategy is also feasible for other polymer and inorganic substrates. For a proof-of-concept demonstration, we fabricated a PbS QD ink-based field-effect transistor on a Cl-passivated substrate, and the device showed a mobility as high as 4.36 × 10^–3 cm²/V s, which indicates effective trap-site suppression. This device also enables the potential of the Cl-passivated substrates for QD inks with water or other polar solvents.

Introduction

Printed electronics enables electronic and photonic devices to be fabricated by printing-based technologies such as screen printing, inkjet printing, roll-to-roll printing, and so on.^1,2 As an additive manufacturing technique, the printing technique can effectively reduce costs by high volume manufacturing and less utilization of complicated semiconductor manufacturing instruments (e.g., metal organic chemical vapor deposition). Thanks to these advantages, the market of printed electronics is fastly growing and it is predicted to reach 330 billion USD by 2027.³ Consequently, researchers from both industry and academia are making efforts in developing high-performance functional devices, from simple circuits⁴ to complex transistors⁵ and light-emitting devices,⁶ by the printing technique. However, there are still challenges in this field, which need to be conquered, such as seeking of high-performance electronic inks and higher-resolution printing technologies,^7,8 while the substrate is another major consideration in printed electronics and one needs to match the properties of the electronic ink with the surface properties of the substrate. One of the most important properties of the substrate surface is its hydrophobicity or the surface energy. If the surface is more hydrophilic, the water-based ink tends to adhere better to the surface. Plasma treatment is a popular method to create a hydrophilic surface, which is brought about by dangling bonds and hydroxyl groups. Other than hydrophobicity, the interface trap is another important issue on the substrate surface in printing functional electronic devices.⁹ Dangling bonds and hydroxyls on the surface will form interface trap sites that will affect the performance of printed electronic devices. Therefore, an extra passivation strategy is needed to eliminate these trap sites.¹⁰ The use of self-assembled monolayers is a common route to surface passivation. Usually, self-assembled monolayers are long-chain, complex organic molecules, such as bis(trimethylsilyl)amine,¹¹ octadecyltrichlorosilane (OTS), divinyltetramethyldisiloxane-bis(benzocyclobutene), and polystyrene.¹² However, these molecules will create an extremely hydrophobic surface, which is a problem in printing manufacturing.

Colloidal quantum dots (QDs) are gradually attracting the attention of researchers, not only because of their extraordinary features such as tunable absorption spectrum and narrow emission spectrum that make them widely used in solar energy harvesting,^13,14 light-emitting devices,¹⁵ and light sensing¹⁶ but also due to their solution processability and amorphous nature that enable a lot of flexible electronic devices, such as flexible touch sensors,¹⁷ flexible gas sensors,¹⁸ and flexible logic circuits.¹⁹ The solution-processed QD-based devices also have the potential to be mass-produced by printing methods. To fit in this trend, solution-phased in situ ligand exchange is developed to acquire QDs with short ligands.²⁰ Among different types of QDs, PbS colloidal QDs are one of the most promising candidates because of their unique properties and multiexciton generation, which makes PbS QDs gain a lot of attention in photonic applications, such as photovoltaic devices²¹ and photodetectors.²² Previously, because of the existence of long ligands, layer-by-layer deposition was demonstrated to be an effective way to create high-quality, device-level PbS QD films.²³ However, this deposition method heavily restricts the application of printing PbS QD devices because it does not allow the QD films to be deposited in one cast. Recently, phase-transfer ligand exchange was introduced into fabricating PbS QD inks.²⁴⁻²⁶ Phase-transfer ligand exchange allows PbS QDs to obtain extremely short ligands, such as halide atoms, in suspension conditions. Thus, the ligand-exchanged QD suspension is able to form a high-quality film in one cast. It is the QD ink that has the potential to be used in printing instruments. However, the PbS QD ink is usually made using a polar carrying solvent like N,N-dimethylformamide (DMF) which does not allow the PbS QD ink to be printed on well-passivated hydrophobic substrates. However, researchers only developed ways to suppress the substrate trap sites by using self-assembled monolayers with nonpolar head groups;²⁷ the electronic-device-friendly, trap-free passivated hydrophilic surface is still an obstacle for the PbS QD ink to be fully printed.

Here, we propose a new strategy to create a trap-free substrate surface suitable for PbS QD ink by chlorine modification. Iodine atoms were previously utilized to eliminate trap states at the heterojunction interface to fabricate high-performance photonic devices.²⁸ We extend it to halide modification for various rigid and flexible substrate surfaces and apply Cl-modified SiO₂ surfaces for high-performance QD field-effect transistors (FETs). In this study, we first verified the successful passivation of chlorine on the SiO₂ surface using X-ray photoelectron spectroscopy (XPS). The contact angle measurement further showed the reduced contact angle between a droplet of DMF and the SiO₂ substrate. Then, this modification method was extended to other rigid and flexible substrates, such as Si and polyimide (Kapton films from DuPont). Finally, PbS QD ink was utilized to make an FET with a Cl-modified interface, which shows an obvious higher mobility than the unmodified control device due to suppressed surface trap sites by Cl-modification. The whole solution process shows the potential of PbS QD inks to be introduced into printing manufacturing, which will foster the development of QD materials for future printed electronics.

Results and Discussion

The surface modification process is shown in Figure ​ 1a. The substrates were treated by oxygen plasma to acquire a surface that contains a lot of silicon dangling bonds. Then, the substrate would be placed in a Petri dish that contains OTS/toluene solution or NH₄Cl/water solution for 30 min. The detailed experimental procedures are described in the Experimental Section. By this simple solution surface modification, passivated surfaces can be obtained. Normally, SiO₂ surfaces have a large number of hydroxyls and Si dangling bonds as shown in the top panel of Figure ​ 1b. These surface groups can act as trap sites that will trap charge carriers. Thus, the dangling bonds and hydroxyls will affect carrier transport as indicated as trap states. So, the passivated surface is an essential part in making a high-performance electronic device. Either the modification molecule of OTS or the chlorine as we proposed can eliminate hydroxyls and dangling bonds as shown in the bottom panel in Figure ​ 1b, which will make carrier transport more efficient.

(a) Illustration of the surface passivation process. (b) Schematic of the interface trap mechanism.

To verify that the chemical reactions in modification did occur on the surfaces, we carried out Fourier transform infrared spectroscopy (FTIR) and XPS on substrates with different modifications. From Figure ​ 2a, it can be easily seen that the bare SiO₂ and Cl-modified substrate show no abnormal absorption in the wavenumber range 2700–3200 cm^–1. However, there are two absorption peaks observed at 2849 and 2917 cm^–1, which coincide with the results of previously reported OTS absorption on the SiO₂/Si substrate.²⁹ Because the characteristic peaks of Cl cannot be observed in the infrared range, XPS was utilized to confirm Cl on the SiO₂ substrate. It is shown in Figure ​ 2b that the Cl 2p peak, which has a similar binding-energy value to that in a previous report,³⁰ can be seen in the right panel (the fitted red dashed line), which represents the Cl-modified interface, whereas the OTS-modified interface only shows noise. Energy dispersive X-ray spectroscopy (EDX) measurement (Figure S1) was also performed on a bare SiO₂ substrate, which shows no Cl characteristic peaks.

(a) FTIR results of substrates with different modifications. (b) XPS results of the OTS-passivated surface and the Cl-passivated surface. The red dashed line is fitted with the Cl 2p peak.

To investigate the processing compatibility of Cl-modified surfaces for PbS QD inks, we made contact angle measurement of water and DMF (carrying solvent of PbS QD inks) on glass substrates. The OTS-modified SiO₂ substrate, a previously reported²⁷ substrate for mobility measurement of PbS QD films, is used as the control sample here. From the left panel of Figure ​ 3, it can be seen that the contact angles of water and DMF on OTS-modified substrates are 96 and 51°, which may induce some problems in the printing process due to the large contact angles. However, the Cl-modified surface can significantly reduce these contact angles to 20 and 11°, respectively. The small contact angles are in favor of printing PbS QDs in water and DMF. We also performed experiments to investigate the stability of the Cl-passivated substrates in ambient conditions. As shown in Figure S2, it can be clearly seen that after 1, 4, and 8 days, the water contact angles remain in a relatively similar value, whereas the value of the DMF contact angle with the substrate is slightly increasing. We attributed it to a small amount of desorption of Cl atoms and this indicates that it is preferred to fabricate devices on the substrate before 4 days after Cl treatment. The Cl-modification strategy includes NH₄Cl treatment but not limited to it. Other chloride ion compounds (NaCl, 3 mg/mL; tetrabutylammonium chloride (TBAC), 15 mg/mL) are also tested by the same treatment process. As shown in Figure S3, NaCl and TBAC treatments can achieve similar results to NH₄Cl treatment. It can be concluded that other chloride ion compounds can also achieve effective substrate modification.

Contact angle measurement of water and DMF on OTS-passivated and Cl-passivated surfaces.

We further applied this surface modification strategy to polyimide (Kapton) and silicon substrates. The results of contact angle measurement are shown in Figure ​ 4. It is seen in the first row that Cl-modification will largely decrease the water contact angle on the Kapton substrate, which makes it suitable for aqueous inks. Because both Kapton and DMF have a similar structure, the contact angle of DMF on the orginal Kapton substrate is already small. Cl-modification will not make a big difference on it. The difference between the values of modified and unmodified surfaces should be a result of system error.

Contact angle measurement of solvents on chlorine-passivated Kapton and silicon substrates.

The contact angles on the silicon substrate is also shown in Figure ​ 4. Due to the native oxide (low-quality silicon oxide grown in ambient environment) on top of the silicon substrate, the contact angles between water/DMF and the silicon substrate are small. However, after Cl-modification, the contact angles become smaller, which proved that the proposed substrate modification strategy is more effective than the use of a bare silicon substrate. More importantly, the proposed strategy can effectively passivate silicon substrates using Cl-modification, which is confirmed in the demonstration below. Considering that the Cl-modification strategy is effective on the polymer substrate and other silicon-based materials, we conclude that this strategy has potential to act as a universal method to passivate various substrates in printed electronics.

To characterize the interface traps, we performed experiments to examine the carrier decay dynamics. PbS QD films were deposited between gold electrodes by the traditional layer-by-layer strategy.²³ The detailed procedure of PbS QD synthesis is in the Experimental Section. As shown in Figure S4 in the Supporting Information, the first excitonic peak is observed at 1430 nm. The size of the quantum dots is around 6 nm as shown in the transmission electron microscopy (TEM) image in Figure S5. The device we used in this testing has two terminals as shown in Figure S6. The detailed device fabrication procedure is shown in the Experimental Section. The testing set-up is shown in Figure S7 in the Supporting Information. In the measurement, the device is driven by a function generator. The current response of the quantum dot device is linearly converted to a voltage signal by an LM741 operational amplifier with a feedback loop. The output voltage signals are collected by an oscilloscope. The function generator has the output voltage of the square waveform and the device should respond in current signals of the square waveform. However, the rising and falling edges are not as sharp as the original signal from the function generator, as shown in Figure ​ 5a.

(a) Voltage response of a quantum dot device to a 100 Hz square wave. (b) Zoom-in graphs of decay on the rise edge. Inset: curve-fitting of different devices. (c) Fitting curves which show different decay times.

According to previous reports, the rising and falling edges of measured currents can be explained from the trapping and detrapping of charges in PbS quantum dot films and SiO₂ interfaces.^31,32 Therefore, the I–t curves (transient responses) of devices can be expressed as multiple exponential decays considering trapping dynamics:

1

In eq 1, V _amp is the voltage generated from the operational amplifier, which is linearly proportional to the current (I _ds) flowing through the device, whereas τ _i is related to the lifetimes of different types of carrier traps. If the decay curve is on the rising edge, the operator between I ₀ and ∑ should be minus because it is a trapping process; Otherwise, the operator should be plus because it is a detrapping process. Here, as shown in Figure ​ 5b, the rising edge is taken for analysis. We first used a 1 MΩ resistor (similar resistance to the QD devices) to extract the RC charge time (τ₁) of the measurement circuit. Then, the I–t curves of the 1 MΩ resistor, PbS QD device with the Cl-passivated interface and PbS QD device with the SiO₂ interface were fitted to eq 1, as shown in Figure ​ 5c. The various decay times (τ _i ) of different samples are shown in Table 1. In addition to τ₁, the device with the Cl-passivated interface shows another exponential component with a decay time of 5.29 ms and the device with the SiO₂ interface shows two more exponential components with decay times of 5.40 and 0.49 ms. Since 5.29 and 5.40 ms are very close, we attributed them to the trapping time inside PbS QD films and it may originate from charging the electronic states on QD surfaces. Another decay time of 0.49 ms is only shown in the device with the SiO₂ interface, and we attributed it to charging the interface traps on SiO₂ substrates. Therefore, we conclude that the Cl-passivation strategy can eliminate SiO₂ interface trap states.

Table 1

Decay Times of Different Devices

	pure resistor (ms)	device with the Cl-passivated interface (ms)	device with the SiO2 interface (ms)
τ1	3.28	3.40	3.28
τ2		5.29	5.40
τ3			0.49

To verify the potential of Cl-passivation as an effective way towards hydrophilic, trap-free substrates, we fabricated PbS QD FETs with different interfaces. The original PbS QDs are capped with long oleic acid suspending in octane. To acquire PbS QD ink, the phase-transfer ligand exchange procedure is followed. As shown in Figure ​ 6a, before ligand exchange, the PbS QDs were in nonpolar octane solvent. After agitation, the PbS QDs were covered by iodide. Thus, they were transferred to the bottom polar DMF solvent. The XPS spectrum in Figure S8 indicated the successful ligand exchange by observing the I 3d peaks in CQDs. Finally, the QD FETs were fabricated as the steps described in the Experimental Section. The device schematic is shown in Figure ​ 6b. In the FET device, bottom highly doped silicon can act as a gate, whereas the source and drain gold electrodes are made by sputtering. The channel width between the source and drain electrodes is 5 μm. The PbS QD film is deposited on the substrate by an one-time spin-coating of QD inks. The I _ds–V _ds curves are shown in Figure S9, whereas the transfer curves of these FETs are shown in Figure ​ 6c. The gate currents were also monitored to rule out the gate leakage in FET devices. As shown in Figure S10, the gate current is within noises floating around zero and is several orders of magnitude smaller than the channel current (I _ds) at any given voltage (V _gs). The log-scaled transfer curves are also plotted as shown in Figure S11. The hole mobilities of QD films are retrieved from the transfer curves. The detailed calculation method is shown in the Supporting Information. The transconductance is calculated from the dashed tangent lines. By testing multiple devices, the mobilities of plasma-treated FET and Cl-modified FET can be averaged as 2.75 ± 1.16 × 10^–3 and 4.36 ± 0.93 × 10^–3 cm²/V s, respectively. The histograms are shown in the Supporting Information as Figure S12. We attributed the low mobility of the plasma-treated FET to trap sites on the plasma-treated SiO₂ surface. The Cl-modified FET showed a large mobility, 60% larger than the plasma-treated one, because Cl can effectively passivate the surface trap sites. We also tried to fabricate a PbS QD ink FET on an OTS-modified substrate. However, we cannot retrieve any reliable data out of it, due to the poor film quality on the OTS-modified surface as shown in Figure S13. From this figure, we can see that almost nothing was left after PbS QD ink spin-coating. This agrees with the large contact angle between QD ink and the OTS-modified surface.

(a) Pictures of phase-transfer ligand exchange. (b) Schematic of a QD ink field-effect transistor. (c) Transfer curves of the QD ink FET with a plasma-treated SiO₂ surface, OTS-passivated surface, and Cl-passivated surface.

Conclusions

In this work, we showed a Cl-modification strategy to passivate rigid and flexible substrates for printed electronics. This method can create polar-solvent friendly surfaces on various polymer and inorganic substrates. FTIR and XPS were utilized to verify the passivation. Surface hydrophobicity is characterized by contact angle measurement. Then, carrier dynamic analysis was performed to verify the elimination of surface traps. Finally, we fabricated PbS QD ink-based FETs on Cl-modified substrates. They showed much higher mobilities in PbS QD films on Cl-modified substrates than the values extracted from QD films on plasma-treated substrates and OTS-modified substrates due to effective suppression of surface trap sites by Cl-modification. It can be concluded that Cl-modification can suppress the surface trap sites while creating a hydrophilic surface, showing its potential to be applied in printing techniques.

Experimental Section

Materials

Lead acetate (PbAc), n-octane, ammonium acetate (NH₄Ac), and N,N-dimethylformamide (DMF) were purchased from Fisher Scientific Inc. Ammonium chloride (NH₄Cl), 1-octadecene (ODE), oleic acid, hexamethyldisilathiane (TMS2-S), tetrabutylammonium iodide, and lead iodide (PbI₂) are all acquired from Sigma-Aldrich Inc. The polyimide (Kapton, 5 mil) films we used in contact angle testing were purchased from DuPont Inc. (Delaware). All of the silicon and oxide wafers utilized in the experiments were from University Wafer Inc. (Boston, MA).

Chlorine Modification

All substrate surfaces were activated by oxygen plasma (ICP-RIE, Alcatel) first. After surface activation, the substrates were immediately transferred into an NH₄Cl solution with a concentration of 3 mg/mL. After 30 min, the substrates were taken out and washed with water and ethanol.

Characterization

The absorption spectrum of QDs was acquired on a PerkinElmer NIR-UV spectrophotometer. A Nicolet 8700 Fourier transform infrared spectrometer was utilized to obtain the infrared absorption spectra, whereas X-ray photoelectron spectroscopy was performed on a Kratos AXIS 165 system. The contact angle measurement was performed on an FTA-200 system and the field-effect transistor measurement was done by using a Wentworth probe station and Keithley 2400 soucemeter.

Quantum Dot Synthesis

The synthesis process can be referred to in our previous publication.¹⁷ The colloidal lead sulfide quantum dots were synthesized by using a classic two-step hot-injection method. For this, 570 mg of PbAc and 1.05 mL of oleic acid were dissolved in 15 mL of ODE at 100 °C in a three-neck flask with a vacuum environment. After 8 h of heating and stirring, the lead oleate precursor would be formed. The vacuum environment was then switched to a nitrogen environment quickly by a Schlenk line. The flask was then heated up to 160 °C. After the above steps, another reactant, TMS2-S (160 μL), was prepared by mixing with 7.5 mL of ODE. The reactant solvent was injected quickly to the reaction flask by a syringe. Then, the flask was immediately removed out of the heating mantle to cool down while continuously stirring. When the temperature of the product cooled down to room temperature, it was transferred to centrifuge tubes and washed with acetone two times. Finally, the product was dried in a vacuum and redispersed in octane at a concentration of 50 mg/mL.

TEM and EDX Characterization

TEM images were taken using a JEOL JEM-ARM200CF S/TEM system. To prepare the samples, we diluted the PbS QD suspension in octane to approximately 2 mg/mL and ultrasonicated for 10 min. The diluted suspension was then added dropwise onto a copper grid. After being dried overnight at ambient conditions, the samples were ready for imaging.

The EDX data were acquired by an Oxford EDX on a Zeiss EVO M10 SEM. The samples used were surface-modified quartz glass.

Fabrication of Samples for Carrier Decay Dynamic Analysis

The two-terminal device is fabricated on a borosilicate glass. A conventional lift-off photolithography process was utilized to create electrodes on top. At first, an HMDS hydrophobic monolayer was assembled on top of the silicon oxide layer by an YES HMDS oven. A HPR-504 photoresist layer was then created by spin-coating. The spread step is performed at 500 rpm for 10 s, whereas the spin step is conducted at 4000 rpm, which will last for 40 s. After spin-coating, the wafer was then transferred onto a hotplate for soft-baking (90 s @ 115 °C) to evaporate all solvents. The exposure step was conducted on a mask aligner from ABM Inc. (San Jose, CA). The exposure time is 3 s with an intensity of 66.7 mW/cm² at 365/405 nm. Developer 354 was then applied to develop photoresist patterns for 45 s with agitation. The electrode deposition (5 nm of chromium as the adhesion layer and 65 nm of gold) was performed on a home-made sputtering system. Finally, the wafer was soaked in acetone to lift off extra gold.

Before deposition of the CQD film, the substrate was treated by oxygen plasma to remove all HDMS molecules. It was then modified by chlorine as shown in the previous step. The control device is fabricated on an OTS-modified substrate. The substrate was treated in 1% (v/v) OTS solution for 2 h.

The CQD film deposition was done by a traditional layer-by-layer strategy as described in a previous report.²³ The original CQD suspension (50 mg/mL) was diluted to 16.7 mg/mL in octane. Each layer iteration included three substeps: (1) three drops of lead sulfide quantum dot (QD) suspension was applied on the surface and spun (2500 rpm) for 10 s; (2) 0.25 mL of ammonium chloride (NH₄Cl, Sigma-Aldrich Inc.) solution in methanol was added dropwise on the CQD film and remained there for 5 s and then spun for 10 s to flush the solution away; (3) methanol was employed to wash the CQD film finally. We let the methanol remain on the film for 10 s and then spun it away. This washing step should be repeated two times.

Phase-Transfer Ligand Exchange

To enable biphasic ligand exchange, 0.235 g of PbI₂ and 0.015 g of NH₄Ac were dissolved in 5 mL of DMF. Then, 5 mL of PbS QD suspension (10 mg/mL, in octane) was added. After 1 min shaking by hand and centrifuging, the PbS QDs were transferred from the octane phase to the DMF phase. The octane should be removed by a pipette. New octane was added and mixed by a vortex mixer to wash out the residual oleic acid. After that, the product was centrifuged to separate the octane. This washing iteration should be repeated three times. After these steps, 5 mL of toluene was added to precipitate the PbS QDs. The supernatant was then poured out and the QDs were dried in a vacuum for 20 min. Finally, the QDs were redispersed in DMF.

Field-Effect Transistor Fabrication

The FET devices were based on a silicon oxide wafer with highly doped degenerated silicon, which can act as a back gate. At first, backside silicon oxide was etched off by inductively coupled plasma plasma etching. The fabrication process is exactly the same as in the fabrication of samples for carrier decay dynamic analysis.

The substrate was then treated by oxygen plasma to remove all HDMS molecules. It was then modified by chlorine as shown in the previous step. Since the CQD ink can form a highly coupled film by a single cast, the traditional layer-by-layer deposition can be abandoned. Three drops of CQD ink were casted on the top of the substrate. Then, it was spun for 1 min at 2500 rpm. Finally, the device is baked at 150 °C for 30 min to evaporate all residual solvents and accomplish a highly coupled thin film.

Acknowledgments

The authors thank the Natural Sciences and Engineering Research Council (NSERC) and Alberta Innovates-Technology Futures (AITF) for funding support. T.Z. thanks the Chinese Scholarship Council (CSC) and the Hundreds Voyage Project of Jiangxi Province for providing scholarships.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00195.

• EDX spectrum of the SiO₂ substrate; contact angles of the Cl-passivated SiO₂ substrate after a long time; contact angles between different substrates and water/DMF; colloidal quantum dot absorption; TEM image of PbS CQDs; schematic of the device for carrier decay testing; carrier decay testing set-up; XPS spectrum of CQD ink; FET characterization; calculation of hole mobility in the CQD thin film; PbS QD ink spin-coating on the OTS-modified substrate (PDF)

Notes

The authors declare no competing financial interest.

Supplementary Material

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How to Make Quantum Dot for Electronic Ink

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648829/

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