Investigation of Chemical Effect of Solvent during Ligand Exchange on

Subscriber access provided by UNIV OF LOUISIANA

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Investigation of Chemical Effect of Solvent during Ligand Exchange on Nanocrystal Thin Films for Wearable Sensor Applications Sanghyun Jeon, Junhyuk Ahn, Haneun Kim, Ho Kun Woo, Junsung Bang, Woo Seok Lee, Donggyu Kim, Md Ashraf Hossain, and Soong Ju Oh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01340 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Investigation of Chemical Effect of Solvent during Ligand Exchange on Nanocrystal Thin Films for Wearable Sensor Applications

Sanghyun Jeon†, Junhyuk Ahn†, Haneun Kim†, Ho Kun Woo†, Junsung Bang†, Woo Seok Lee†, Donggyu Kim‡, Md Ashraf Hossain†, Soong Ju Oh*,†

†

Department of Materials Science and Engineering, Korea University, 145, Anam-ro Seongbuk-gu Seoul, 02841, Republic of Korea

‡

Department of Semiconductor Systems Engineering, Korea University, 145, Anam-ro Seongbuk-gu Seoul, 02841, Republic of Korea

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 55

ABSTRACT: Ligand exchange processes have been attracting tremendous interest and are necessary when fabricating nanocrystal (NC) thin films for various applications. As ligand exchange processes are based on solution treatment processes, understanding solvents’ effects on the ligand exchange process is necessary. Herein, we investigated the effects of exchanging solvents and rinsing solvents on silver (Ag) NC thin films during the ligand exchanging and rinsing steps. We studied the relationships between solvent properties, such as polarity and steric hindrance, and the structural, electronic, and electromechanical properties of NC thin films. A model system was proposed to explain the obtained relationships. We found that exchanging solvents and rinsing solvents during the ligand exchange process should be separated to regulate the ligand exchange process so that films with desired properties can be obtained. Films optimized for different purposes (highly conductive / highly electromechanically sensitive) were fabricated with the same materials and ligands using different solvents for each process. Based on these films, we fabricated a flexible strain sensor using an allsolution-process at room temperature. This device exhibits excellent performance, 2


Page 3 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


including a high gauge factor up to 400, and high reliability and stability; furthermore, it can detect minute human motion and sound.

1. INTRODUCTION Colloidal nanocrystals (NCs) are promising materials as their properties can be precisely controllable depending on its size1–3, shape1,3,4, and composition3,5. Colloidal NCs can be synthesized on a large scale using various methods such as hotinjection1,6–8, wet chemical synthesis9–12 and ligand assisted reprecipitation methods4,13. 3



Page 4 of 55

They can be easily coated onto various type of substrates via low-cost solution processes such as spin-coating, spray-coating, and inkjet printing, yielding large surface area and flexible NC thin films. Owing to their simple and large-scale fabrication as well as controllable properties, they have attracted a considerable amount of attention in various fields, such as the electronic5,14–16, optoelectronic17–20, photovoltaic21,22 device, and bio-related sensor application23–26 fields. Colloidal NCs, are surrounded by organic materials called ligands to ensure their colloidal stability when they are synthesized. These ligands are particularly important because they can control not only the size and shape of NCs4,27–29, but also their physical and chemical properties2,30–32. Depending on the characteristics of the ligand, such as length2,30,31, structure33,34 or dipole moments35, the NCs’ properties can be further modulated, rending NCs suitable for each proposed application. In assynthesized NC thin films, long ligands increase the distance between NCs and prevent efficient electron transfer between them, yielding insulating properties. Therefore, extensive studies have been conducted to replace the long and insulating ligands with 4


Page 5 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


short and functional ligands through ligand exchange steps. Short and functional ligands not only improve the electron transport of NC films, but also modify their properties through a doping effect36–38, energy-level modulation39, and so on. Most of ligand exchanging processes are done with the solid-state-ligand exchange method16,23–26,39. This method, which is proceeded on the NC deposited film, can be divided into the two steps. The first step is the ligand exchanging step. Ligand exchanging, a chemical reaction in which ligands on the NC surface are replaced by other ligands, is driven by differences in interaction forces between ligands and metal surfaces, ligand bonding type, and concentration gradient40. The second step is the rinsing step. This process is performed not only to eliminate residues such as exchanged original ligands or excess ligands generated during the ligand exchanging step, but also to modulate the defects or ligands attached to the NC surface for various purposes. In these sequential processes, the solvent is highly important. For example, to approach the surface of a NC and displace the original ligand during the ligand 5



Page 6 of 55

exchanging step, target ligand molecules should break the connection with solvent molecules. Furthermore, to eliminate or modulate the ligands on the NC surface during the rinsing step, solvents molecules should interact with ligand molecules. The entire process is, therefore, significantly affected by solvent properties such as polarity or steric hinderance. The type of solvent must be carefully considered when a ligand exchange process is conducted. Nevertheless, most of ligand exchange processes have been conducted by simply using conventional solvents such as methanol or acetonitrile and only few solvents have been utilized and studied21,23,24,39. Furthermore, they did not separate exchanging solvent and rinsing solvent in ligand exchange process. Indeed, many studies have been conducted to find better or new ligand materials30,32,39 or exchanging methods to modulate the properties41, but none of them have focused on the exchanging solvents’ and rinsing solvents’ effects on NC thin films properties yet. In this work, we investigate the effect of the polarity and steric hinderance of exchanging solvent and rinsing solvent on the degree of ligand exchange separately. To 6


Page 7 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this end, we chose same materials and ligands but different type of solvents, i.e., methanol, ethanol, and iso-propanol, which have different polarities and numbers of carbon chains. We characterize the structural, electrical, and electromechanical properties of ligand exchanged Ag NC thin films as a function of exchanging solvents and rinsing solvents, using scanning electron microscopy (SEM), energy dispersive Xray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and atomic force microscopy (AFM). We found that exchanging solvents with higher polarities and lower carbon numbers, and rinsing solvents with lower polarities and higher carbon numbers lead to the formation of NC thin films with higher resistivities and sensitivities to mechanical stress. The origin of this behavior was investigated, and a mechanism was proposed. Taking advantage of the findings that, by using different solvents, the electrical and electromechanical properties of Ag NC thin films can be controlled in a wide range, we fabricated wearable strain gauges with highly conductive Ag NC thin film electrodes and mechanically sensitive Ag NC thin film sensing layers. The developed 7



Page 8 of 55

sensor showed high gauge factors up to 400 and can successfully detected human motion and sound.

2. EXPERIMENTAL SECTION 2.1. Materials. Silver nitrate (99%, AgNO3) powder and Tetra-n-butylammonium iodide (98%) were purchased from Alfa Aesar Co., Inc. Oleic acid (90%), oleylamine (70%), Tetrabutylammonium chloride (97%), Tetrabutylammonium bromide (99.0%), (38


Page 9 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mercaptopropyl) trimethoxysilane (95%), methanol (99.8%), ethanol (99.5%), and isopropanol (99.5%) were purchased from Sigma-Aldrich. Polyethylene terephthalate (PET) films (SKC films) with a thickness of 250 m were used as flexible substrates. 2.2. Synthesis of Ag NCs. Ag NCs were synthesized using previously reported methods42. AgNO3 (1.7 g), oleic acid (45 mL), and oleylamine (5 mL) were added to a three-neck flask and were mix vigorously using magnetic stirring. To remove moisture and oxygen, the mixed solution was degassed at 70 °C for 1.5 h. After degassing, the temperature was increased up to 180 °C at a rate of 1 °C·min–1. Then the solution was cooled to room temperature, As-synthesized Ag NCs were washed a few times using toluene and ethanol for solvent and anti-solvent by centrifugation at 5000 rpm for 5 min. The precipitated Ag NCs were dispersed in octane, where the concentration was controlled at 200 mg·mL–1. 2.3. Ligand exchange. TBAB ligand exchange solutions were prepared at a concentration of 30 mM in methanol, ethanol, and iso-propanol. Ag NC thin films were

9



Page 10 of 55

immersed in these solutions for 120 s for the ligand exchange process and then rinsed with each solvent for 120 s. 2.4. Fabrication of the strain sensor. 250 m PET substrates were sequentially sonicated in acetone, iso-propanol, and deionized water for 5 min. They were then treated with UV-ozone so that hydroxyl groups could form on their surfaces. Finally, they were immersed in 5 vol% 3-mercaptopropyltrimethoxysilane solution in toluene so that the self-assembled monolayer (SAM) could form. The prepared substrates were partially covered with Kapton tape. The as-synthesized Ag NCs at 200 mg·mL–1 were spin-coated at a speed of 1000 rpm and treated with 30 mM TBAB ligand solution (isopropanol) for 2 min and then the substrate was immersed in methanol for 2 min. After formation of the electrode, the Kapton tape was peeled off, and then a coating of Ag NC was applied once again under the same conditions. The substrate was then treated with a 10 mM TBAB ligand solution (methanol) for 10 s and immersed in iso-propanol for 2 min. Finally, a 2% strain was applied to the substrate.

10


Page 11 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5. Characterization. The surface chemistry of the obtained Ag NC thin films were analyzed using an FT-IR spectroscope (LabRam ARAMIS IR2, Horiba Jobin Yvon) in the attenuated total reflection mode. The structural properties and morphologies of the films were measured by X-ray diffraction spectroscopy (MAX-250V, Rigaku), atomic force microscopy (XE100, PSIA), and scanning electron microscopy with energy dispersing X-rays (S-4300, Hitachi High technologies America, Inc.), and Transmission electron microscopy (model Tecnai G2 F30, FEI, Korea Basic Science Institute). The optical properties were investigated by ultraviolet-visible spectroscopy (Cary 5000, Agilent Technologies). To analyze the electrical performance, a probe station (MST 4000, MS TECH) and a multimeter (Fluke 289, Fluke Corporation) were used.

11



Page 12 of 55

3. RESULTS AND DISCUSSION We synthesized 4 nm silver (Ag) NCs by wet chemical methods42 (Figure S1). Ag NC thin films were formed by spin-coating an Ag NC solution on pre-treated glasses. To increase the conductivity, ligand exchange and rinsing process were conducted. Tetran-butylammonium bromide (TBAB) was chosen as ligand exchange materials as it is known to increase the conductivity of NCs24,25. Methanol (M), ethanol (E), and isopropanol (I) were used as exchanging solvents and rinsing solvents to investigate the effects of the polarity and the number of carbon chain of each solvents. For brevity, in this text, Ag NC thin film treated with TBAB using A as an exchanging solvent and B as a rinsing solvent will be referred to as EARB films. For example, Ag NC thin film treated with TBAB dissolved in methanol and rinsed with iso-propanol will be referred to as EMRI film.

12


Page 13 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) FTIR spectra of EMRM films depending on the treating time. (b) Normalized C-H peak intensity for three exchanging solvents depending on treating times. (c) FTIR spectra of TBAB treated films depending on the different exchanging and rinsing solvents after 2-min treatments. (d) XRD spectra of Ag NCs thin films treated with TBAB dissolved in methanol (blue), ethanol (yellow), and iso-propanol (green). (e) XRD 13 spectra of Ag NCs thin films treated with TBAB dissolved in methanol; the films were not ACS Paragon Environment rinsed(red) or rinsed with methanol (blue), Plus ethanol (yellow), and iso-propanol (green). (f)


Page 14 of 55

To investigate the effect of each solvent on the ligand exchange process, the surface chemistry of NCs was examined using FTIR analysis. As-synthesized Ag NCs are surrounded by oleate ligands, which have a strong C-H stretching vibration band peak around 2800–3000 cm-1. The C-H stretching peak was monitored as a function of ligand exchange time with three different solvents. In this experiment, methanol was used as the rinsing solvent for all cases to investigate only the effects of the exchanging solvents. Upon ligand exchange, the peaks gradually vanished with ligand exchanging time (Figure 1a), which indicates that the original oleate ligands were gradually replaced by an inorganic ligand of Br- ions16,24,25. When methanol was used as the exchanging solvent, some oleate ligands could be detected after 10 s, but most of them vanished after 20 s. When ethanol or iso-propanol was used, ligand exchanging progressed quickly (Figures S2a and S2b). The peak almost vanished after 10 s for two cases. The normalized peak intensity of C-H peaks depending on ligand exchange times and solvents are summarized and shown in Figure 1b. The EMRM film shows the slowest exchanging speed, and the EIRM film shows the fastest exchanging speed. The C-H 14


Page 15 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stretching peaks could not be detected after treatments lasting 2 min in all cases. When other solvents were used as rinsing solvents, the FTIR spectra did not show any peaks around 2800–3000 cm-1 after 2-min treatments (Figure 1c). The structural properties of TBAB-treated Ag NC thin films using different exchanging solvents (Figure 1d) and rinsing solvents (Figure 1e) were investigated by X-ray diffraction (XRD) analysis. All XRD data showed a peak at 31.0° attributed to the AgBr (200) crystal plane and 38.3° attributed to the (111) Ag crystal plane, which proves that during the ligand exchange process, AgBr is formed and attaches to the Ag NC surface. To investigate the effect of exchanging solvents, the XRD data of the EMRM, EERM and EIRM films were compared (Figure 1d). There were no significant differences in Ag and AgBr peak intensity of three films, representing all films contained similar amount of AgBr (AgBr/Ag ratio : EMRM : 0.19, EERM : 0.21, EIRM : 0.20). The grain size of each samples were calculated by Scherrer equation using a peak at 38.3°. The average value was 14.64 nm for EMRM, 15.97 nm for EERM, and 17.57 nm for EIRM. To investigate the effect of rinsing solvents, the XRD spectra of EMRM, EMRE, EMRI, and 15



Page 16 of 55

EMRX (RX represents the film that was not rinsed by any solvent) films were also compared (Figure 1e). In this case, there were no differences in grain size (EMRX : 14.41 nm, EMRM : 14.64 nm, EMRM : 14.41 nm, EMRM : 14.64 nm). However, there are significant differences in AgBr (200) crystal plane peak. The AgBr/Ag ratio was analyzed to measure the amount of AgBr components on the films (Figure 1f). The EMRX film shows the highest value (0.45), and the EMRI film shows the lowest value (0.19). As the polarity of the solvent increases, the value of the AgBr peak decreases (EMRX : 0.45, EMRI : 0.38, EMRE : 0.24, EMRM : 0.19). The EDX analysis also showed similar trends, i.e., there were no differences in intensity of Br depending on exchanging solvents (EMRM : 6.7%, EERM : 7.2%, and EIRM : 5.7%) but, as the polarity of rinsing solvent increases, the intensity of Br decreases (EMRX : 15.6%, EMRI : 10.0%, EMRE : 9.4%, EMRM : 6.7%) (See Supporting Information S3 and the discussion that follows). Differences in ligand exchanging rates and the amount of bromide components are closely related to the properties of each solvent. The ligand exchanging rate can be understood by both the polarity and the number of carbon chains of each solvent 16


Page 17 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Figure 1g). When TBAB was dissolved in the solvent, Tetra-n-butylammonium+ (TBA+) was surrounded by the δ- portion of solvent and Br- was surrounded by the δ+ portion. Although the detailed kinetics of the ligand exchange process can be predicted with complicated simulations and theories, such as the hard and soft acid bases (HSAB)43 and Derjaguin–Landau–Verwey–Overbeek (DLVO)44–46 theories, a simplified model can be used in our experiments as the kind of solute, and its concentration, and reaction time were the same. In this condition, it can be simplified as two steps. The first step is disconnection between Br- and solvent molecules. The second step is interaction between Br- and the Ag NCs. Assuming that the forces related to the second step are constant, the interaction between Br- and solvent molecules can play a more important role. When it comes to the binding force between Br- and solvent, two factors should be considered. The first is the polarity of the solvent and the second is the number of carbon chains, which affects the steric effect. The polarities of methanol, ethanol, and iso-propanol are 0.762, 0.675, and 0.546. The solvent that has a higher polarity can 17



Page 18 of 55

bind with Br- more tightly; therefore, methanol binds the strongest with Br-, and isopropanol binds the weakest with Br- in perspective of the one solvent molecule. When it comes to the number of carbon chains, methanol, ethanol, and iso-propanol have one, two, and three carbons in their structures, respectively. The greater the number of carbon chains, the greater the steric hinderance; therefore, fewer solvent molecules will surround the Br-, and the total bonding forces will be weakened. In summary, methanol, which has the highest polarity and the lowest number of carbons, has the strongest binding force with Br-. Iso-propanol, which has the lowest polarity and the highest number of carbons, has the lowest binding force. A weak binding force promotes interaction between Br- and Ag NCs. Therefore, iso-propanol shows the fastest rate of ligand exchange, and methanol shows the lowest rate, which agrees with our experimental results (Figure 1b). Ethanol has an intermediate polarity and carbon number; therefore, it exhibits an intermediate rate. On the other hand, the polarities of the rinsing solvents can be used to explain why the amounts of AgBr and Br- decreased in the order EMRM, EMRE, EMRI film. During the 18


Page 19 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rinsing process, it is known that a polar solvent eliminates the residues generated during the ligand exchanging process25. However, besides the residues, weakly bound Br components are also affected by the polar rinsing solvents. Therefore, a solvent that has a higher polarity attacks the Ag NC surface more aggressively and removes Br- and AgBr components from the surface. As methanol has the highest polarity and isopropanol has the lowest polarity, the amount of AgBr and Br- decreased in the order EMRM, EMRE, EMRI film, as seen in Figure 1f.

19



Page 20 of 55

Figure 2. SEM images of the surface of Ag NC thin films treated with TBAB dissolved in (a, d, g) methanol, (b, e, h) ethanol and (c, f, i) iso-propanol; the films were rinsed with (a, b ,c) methanol, (d, e ,f) ethanol and (g, h, i) iso-propanol after ligand exchange. Scale bar = 2.5 m

The morphologies of Ag NC thin films treated with different solvents were examined by SEM analysis. Depending on the type of exchanging solvents, Ag NC thin films

20


Page 21 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


showed different morphologies characterized by different uniformities, roughness, and skewness. In the case of the EMRM film (Figure 2a), there were 3~4 big holes per 100 m2 on average and many pinholes on the surface (A big hole is defined as a hole with a size exceeding 2 μm, and a pinhole represents a hole with a size smaller than 500 nm). In the case of the EERM film (Figure 2b), big holes were not observed, and the number of pinholes also reduced. In addition, small precipitations with sizes of 100–200 nm were observed. In the case of the EIRM film (Figure 2c), both big holes and pinholes were not observed. Furthermore, the size of the precipitations increased to 1m. EDX mapping analysis revealed that precipitates had higher ratios of Br:Ag compared to other parts. This indicates that the precipitates are insulating AgBr compounds (Figure S3c, S3d). The surface morphology according to the rinsing solvent was also monitored. EMRM (Figure 2a), EMRE (Figure 2d), and EMRI (Figure 2g) films did not have significant differences on their surfaces. There were also no significant changes in case of EE (Figures 2b, 2d, and 2h) or EI (Figures 2c, 2f, and 2i) films. In other words, the structural properties of the NC thin films were affected by the type of exchanging solvent, but the 21



Page 22 of 55

influence of the type of rinsing solvent was almost negligible. This tendency, which is related to the exchanging solvent, was also observed when other halide ligands were used for the ligand exchange, such as TBAC or TBAI (Figure S4). In order to analyze the surface roughness more quantitatively, AFM analysis was also conducted (Figure S5). Similar to SEM data, EMRM shows many holes and EERM shows both holes and precipitates, and EIRM shows many precipitates. The root mean square roughness value (Rq) of the EMR, EERM, and EIRM, are calculated as 98 ± 12nm, 118 ± 14nm, 108 ± 11nm. While they show similar values, EERM films show slightly higher values compared to others, as they have both holes and precipitates. We ascribe the differences in the surface morphology of the films to the differences in the ligand exchange speeds of the solvents. When the ligand exchanging rate is fast, the film forms without holes, as each Ag NC can attach to the adjacent Ag NC plane directly because of the rapid elimination of the surface ligand. On the other hand, when the rate is slow, the film has several holes on the surface. This might be because each Ag NC attaches to the empty plane of an adjacent Ag NC selectively because of 22


Page 23 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hinderance by the left ligand. A detailed explanation of the film formation mechanism can be found in the Supporting Information (Figure S6 and the discussion).

23



Page 24 of 55

Figure 3. (a) I–V characteristic curves of Ag NC thin films treated with TBAB dissolved in methanol (red), ethanol (blue) and iso-propanol(yellow); the films were rinsed with methanol after ligand exchange. (b) I–V characteristic curves of Ag NC thin films treated with TBAB dissolved in methanol; the films were rinsed with methanol (red), ethanol (blue), and iso-propanol(yellow) after the ligand exchanging step. Electrical resistivity of Ag NC thin films with various ligand-exchange(c) and rinsing times(d) using various exchanging solvents. Methanol (red), ethanol (blue), and iso-propanol(yellow). 24


Page 25 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Resistivity of TBAB-treated Ag NC thin films depending on exchanging and rinsing solvents.

Resistivit

EM

EE

EI

y [𝛀 ∙ 𝐜𝐦] RM

𝟑.𝟒 × 𝟏𝟎 ―𝟓 ± 𝟔.𝟕 × 𝟏𝟎 ―𝟔 𝟐.𝟏 × 𝟏𝟎 ―𝟓 ± 𝟗.𝟖 × 𝟏𝟎 ―𝟔 𝟕.𝟐 × 𝟏𝟎 ―𝟔 ± 𝟖.𝟑 × 𝟏𝟎 ―𝟕

RE

𝟑.𝟕 × 𝟏𝟎 ―𝟐 ± 𝟐.𝟒 × 𝟏𝟎 ―𝟑 𝟏.𝟑 × 𝟏𝟎 ―𝟐 ± 𝟖.𝟒 × 𝟏𝟎 ―𝟑 𝟐.𝟖 × 𝟏𝟎 ―𝟑 ± 𝟒.𝟓 × 𝟏𝟎 ―𝟒

RI

𝟏.𝟗 ± 𝟏.𝟐

𝟏.𝟎 ± 𝟏.𝟐

𝟐.𝟏 × 𝟏𝟎 ―𝟏 ± 𝟑.𝟕 × 𝟏𝟎 ―𝟏

To understand how the morphology and surface chemistry of an Ag NC thin film affects its electronic properties, I–V measurements were conducted. Table 1 shows the resistivities of TBAB treated Ag NC thin films that were fabricated using different solvents. When it comes to the exchanging solvents, the EMRM shows the highest resistivity of 3.4 × 10 ―5 ± 6.7 × 10 ―6 Ω ∙ cm, and the EIRM film shows the lowest resistivity of 7.2 × 10 ―6 ± 8.3 × 10 ―7 Ω ∙ cm. In the case of the rinsing solvent, the tendency is opposite that of the exchanging solvent. The EMRM film shows the lowest resistivity of 3.4 × 10 ―5 ± 6.7 × 10 ―6 Ω ∙ cm, and the EMRI film shows the highest 25



Page 26 of 55

resistivity of 1.9 ± 1.2 Ω ∙ cm. Furthermore, all films show stable I–V curves (Figure 3a, b). We also measured the change in resistivity with ligand exchanging and rinsing time (Figure 3c, d). When effect of exchanging time in resistivity was measured, EMRM shows the slowest decrease rate, and EIRM shows the fastest decrease rate (Figure 3c), which well matches previous experimental results (Figure 1b). Regarding rinsing solvents, EMRM shows the fastest decrease rate and EMRI shows the slowest decrease rate (Figure 3d) which is related to decrease of Br components.

26


Page 27 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. I–V curve of ligand-exchanged Ag NC thin films with TBAB dissolved in (a) methanol and (b) iso-propanol; the films were rinsed with methanol without (red) and with (blue) 1.0% strain. (c) Resistance change in Ag NC thin films using TBAB dissolved in methanol (red), ethanol (blue), and iso-propanol (yellow); the films were rinsed with methanol during strain and release tests with a 1.0% strain. I–V curve of ligandexchanged Ag NC thin films with TBAB dissolved in methanol; the films were rinsed with (d) methanol and (e) iso-propanol without (red) and with (blue) a 1.0% strain. (f) Resistance change in Ag NC thin films using TBAB dissolved in methanol; the films To evaluate potential application of the films in strain sensors, we investigated the were rinsed with methanol (red), ethanol (blue), and iso-propanol (yellow) during strain effects of both exchanging solvents and rinsing solvents on the electromechanical and release tests with a 1.0% strain. ACS Paragon Plus Environment

27


Page 28 of 55

properties of Ag NC thin films. We fabricated NC thin films on a 250 μm PET substrate and monitored the electrical resistance change upon applying mechanical strain by bending the substrates. One of the most important parameter of a strain sensor is the gauge factor 𝐺, which represents the sensor’s sensitivity. The gauge factor can be calculated with the following equation47 : 𝐺 = (∆𝑅 𝑅0)/𝜀 where ∆𝑅 is the resistance change, 𝑅0 is the initial resistance, and 𝜀 is the strain applied to the device. When a 1.0% strain was applied to the EMRM film, the resistance increased by 12.2%, indicating that the gauge factor was 12.2 (Figure 4a), and in case of the EIRM film, the resistance increased by 3.4%, representing a gauge factor of 3.4 (Figure 4b). We also conducted strain-release cycle tests with a 1.0% strain for Ag NC thin films using three different exchanging solvents (Figure 4c). The average gauge factor was 12.4 ± 0.4. for the EMRM film, 9.2 ± 0.8 for the EERM film, and 3.1 ± 0.4 for the EIRM film. Furthermore, the resistance increased by 12.2% for the EMRM film and by 77.2% for the EMRI film, indicating that the gauge factors were 12.2 and 77.2, 28


Page 29 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively (Figure 4d, e). When we conducted a strain release cycle test with a 1.0% strain for Ag NC thin film using three different rinsing solvents (Figure 4f), the average gauge factor was 12.4 ± 0.4 for the EMRM film, 44.1 ± 3.3 for the EMRE film, and 80.2 ± 10.7 for the EMRI film. It can be summed up that high polarity and low carbon number of exchanging solvents lead the NC thin film to have higher resistivity and gauge factor. On the other hands, high polarity of rinsing solvents leads the NC thin film to have lower resistivity and gauge factor.

29



Page 30 of 55

Figure 5. Schematic of charge transport in a ligand-exchanged Ag NC thin film when using (a) methanol and (b) iso-propanol as exchanging solvents and rinsing with (c) methanol and (d) iso-propanol before(left) and after(right) applying the strain (red arrows indicates electron flow).

To understand the origin of the different resistivities and gauge factors, we established a model system based on our quantitative structural and chemical analyses (Figure 5).

30


Page 31 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For the exchanging solvent, the higher polarity solvent created films with higher resistivity and higher gauge factors than the lower polarity solvent. This property is deeply related to the grain size and surface morphology of the thin film. In the case of the EM film, it has the smallest grain size representing the highest number of electron transport barriers (grain boundaries) (Figure 1d). Furthermore, there were several holes on the surface (Figure 2a). As electron transport paths did not form perfectly, it showed a high resistivity. When the film was bent, imperfectly formed electron transport paths near defects were easily disconnected because of stress concentration25,48,49 (Figure 5a). As a result, the number of electron paths decreased, and the film showed a relatively high gauge factor. Meanwhile, the EI film had the largest grain size and did not have holes (Figure 1d, 2c). As electron paths formed perfectly with the smallest number of barriers (grain boundaries), it exhibits a low resistivity. Furthermore, the paths were little affected by the bending because of the perfectly formed paths; thus, the film had a relatively low gauge factor when strain was applied (Figure 5b).

31



Page 32 of 55

For the rinsing solvent, the higher polarity solvent yielded films with lower resistivity and gauge factors than the lower polarity solvent. In the case of RM film where a high polarity is used as the rinsing solvent, small amount of Br components exist on the surface of the Ag NCs. The Ag NCs can make direct contact with each other by physical touching or sintering, and the effects of insulating Br components can be reduced, leading to low resistivity. As the applied strain does not affect electron paths due to sintered NCs, the film shows a low gauge factor. (Figure 5c). In the case of RI film where a solvent with a lower polarity is used, a considerable number of Br components attach to the surface, enlarging the particle distance and interrupting the electron paths between Ag NCs16,24,50–52. In this case, transport occurs by tunneling through enlarged interparticle distances. When strain is applied, the distance between NCs increases, making electron tunneling more difficult. This can explain both high resistivity and high gauge factor.

16,24,50–52

(Figure 5d). This tendency that film containing insulating

component exhibits high resistivity and high gauge factor is well matched with previous reports16,24–26,50–52. 32


Page 33 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Considering two steps were combined, the most electrically conductive and the least mechanically sensitive film is when exchanging solvent with the lowest polarity and the highest number of carbon and rinsing solvent with the highest polarity were used in ligand exchange process (EIRM). On the other hands, the least electrically conductive and the most electromechanically sensitive film is when exchanging solvent with the highest polarity and the lowest number of carbon and rinsing solvent with the lowest polarity were used in ligand exchange process (EMRI). The former can be a promising candidate for the electrode and the latter can be utilized for the strain gauge.

33



Page 34 of 55

34


Page 35 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35



Page 36 of 55

Figure 6. (a) Process for the fabrication of a strain gauge device; (b) The I–V characteristic curves of the strain sensor without strain (red) and with 0.4% strain (blue); (c) Hysteresis of the strain sensor at various periods; (d) Relative resistance change in the strain sensor as a function of applied strain; (e) Relative resistance change in the sensor when over 1000 strain-release cycles are applied. Insets show two of several cycles after 500 and 2000and s, respectively. Detection fingertreated motion Ag (f) and (g). Exploiting thes various unique properties of of TBAB NC sound thin films fabricated using different exchanging and rinsing solvents, we fabricated a strain sensor through all-solution processes that allow devices to be fabricated at low cost. Among the nine films, we chose the EMRI film as the sensing layer and the EIRM film was chosen for the electrodes because of reasons mentioned above. To further increase the gauge factor, we used a partial ligand exchanging method for the active sensing layer. It is known that when the film is fabricated using a partial ligand exchanging method, it is more insulating; thus, the gauge factor could be further increased25. Our experiment also proved that this process could increase the gauge factor (Figure S7). All solution 36


Page 37 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and room temperature-based fabrication processes of the strain sensor are presented in Figure 6a. Briefly, we covered a PET substrate with Kapton tape and deposited Ag NC onto it using the spin coating method. The ligand exchanging step was performed using TBAB dissolved in iso-propanol. The substrate was then rinsed with methanol for 2 min to create a high conductive and strain-insensitive electrode. After peeling off the Kapton tape, Ag NCs were deposited onto PET substrate again to make an active sensor. This time, the ligand exchange process was performed using TBAB dissolved in methanol for 10 s. The substrate was rinsed with iso-propanol for 2 min and then a 2% pre-strain was applied to make a highly sensitive active sensing layer. In this way, the strain gauges were fabricated without the use of a high vacuum, evaporators, a lithography system, etc. We evaluated the performance of the strain sensor. When a 0.4% strain was applied to the sensor, the resistance increased by 132.5%, indicating that the sensor was highly sensitive to strain (Figure 6b). In addition, the sensor exhibited a stable response with negligible hysteresis when strain was applied and released at various speeds (Figure 37



Page 38 of 55

6c). In addition, when the resistance change was measured according to the strain (Figure 6d), it was found that the sensor had a linear resistance change depending on applied strain and showed high gauge factor. Moreover, when the strain-release cycle test was performed over 1000 cycles, the sensor showed reliable and stable performance with excellent durability over 1000 cycles (Figure 6e). To verify the performance of the sensor, we attached it to the joint of a finger and measured the resistance change when using a computer mouse (Figure 6f). It was confirmed that the resistance increased when the finger bent when the mouse wheel was used and recovered when we returned the finger to its original state. This result showed that our strain sensor was reliable and had high sensitivity. We also tested the sensor using sound waves produced by a Bluetooth speaker and a drum application program named Real Drum (Figure 6g). When a vibrating sound (Tom) was applied, the sensor vibrated, so the resistance oscillated as well. When a sound that ended with one shock was applied (Kick), the sensor bent upwards, creating a compressive stress, so the resistance reduced and then recovered. This device showed not only high sensitivity, 38


Page 39 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


but also a quick response time. We confirmed that this vibration was detected only when high gauge factor sensor was used (Figure S8). This demonstrates the efficient sensing capability and dynamic response of our sensors.

4. CONCLUSIONS The effect of solvents on the ligand exchange process were investigated by monitoring each step using different solvents. The Ag NC thin films showed different morphological, structural, and electrical properties, depending on the solvents used. We elucidated the origin of these differences by correlating with the polarity and number of carbon chain of the solvents. This unique property was utilized to fabricate a high performing, stable, low-cost wearable strain sensor. We believe this study is of significance because it provides fundamental information of surface chemistry in nanomaterials and advanced methods for fabricating not only wearable devices but also various applications for electronics, optoelectronics, photovoltaics, etc.

39



Page 40 of 55

ASSOCIATED CONTENT

Supporting Information. Supporting information contains TEM, FTIR, UV-Vis, SEM/EDX, AFM data and an extra discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgement

40


Page 41 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2019R1C1C1003319), and Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT(NRF-2018M3D1A1059001).

REFERENCES (1)

Li, H.; Chen, D.; Li, L.; Tang, F.; Zhang, L.; Ren, J. Size- and Shape-Controlled Synthesis of PbSe and PbS Nanocrystals via a Facile Method. CrystEngComm 2010, 12 (4), 1127–1133.

(2)

Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in Pbse Nanocrystal Solids. Nano Lett. 2010, 10 (5), 1960–1969.

41



(3)

Page 42 of 55

El-Sayed, M. A. Small Is Different: Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals. Acc. Chem. Res. 2004, 37 (5), 326–333.

(4)

Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10 (3), 3648– 3657.

(5)

Oh, S. J.; Berry, N. E.; Choi, J. H.; Gaulding, E. A.; Paik, T.; Hong, S. H.; Murray, C. B.; Kagan, C. R. Stoichiometric Control of Lead Chalcogenide Nanocrystal Solids to Enhance Their Electronic and Optoelectronic Device Performance. ACS

Nano 2013, 7 (3), 2413–2421.

(6)

Hines, M. A.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable NearInfrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15 (21), 1844–1849.

42


Page 43 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7)

Zhang, L.-J.; Shen, X.-C.; Liang, H.; Yao, J.-T. Multiple Families of Magic-Sized ZnSe Quantum Dots via Noninjection One-Pot and Hot-Injection Synthesis. 2010, 21921–21927.

(8)

De Mello Donegá, C.; Liljeroth, P.; Vanmaekelbergh, D. Physicochemical Evaluation of the Hot-Injection Method, a Synthesis Route for Monodisperse Nanocrystals. Small 2005, 1 (12), 1152–1162.

(9)

Nütz, T.; Zum Felde, U.; Haase, M. Wet-Chemical Synthesis of Doped Nanoparticles: Blue-Colored Colloids of n-Doped SnO2:Sb. J. Chem. Phys. 1999,

110 (24), 12142–12150.

(10) Maaz, K.; Mumtaz, A.; Hasanain, S. K.; Ceylan, A. Synthesis and Magnetic Properties of Cobalt Ferrite (CoFe2O4) Nanoparticles Prepared by Wet Chemical Route. J. Magn. Magn. Mater. 2007, 308 (2), 289–295.

43



Page 44 of 55

(11) Nütz, T.; Haase, M. Wet-Chemical Synthesis of Doped Nanoparticles: Optical Properties of Oxygen-Deficient and Antimony-Doped Colloidal SnO 2. J. Phys.

Chem. B 2000, 104 (35), 8430–8437.

(12) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Haase, M. Wet-Chemical Synthesis of Doped Colloidal Nanomaterials: Particles and Fibers of LaPO4:Eu, LaPO4:Ce, and LaPO4:Ce,Tb. Adv. Mater. 1999, 11 (10), 840–844.

(13) Zhang, F.; Zhong, H.; Chen, C.; Wu, X. G.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9 (4), 4533–4542.

(14) Kim, D. K.; Lai, Y.; Diroll, B. T.; Murray, C. B.; Kagan, C. R. Flexible and LowVoltage Integrated Circuits Constructed from High-Performance Nanocrystal Transistors. Nat. Commun. 2012, 3, 1–6.

44


Page 45 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Kang, M. S.; Sahu, A.; Norris, D. J.; Frisbie, C. D. Size-Dependent Electrical Transport in CdSe Nanocrystal Thin Films. Nano Lett. 2010, 10 (9), 3727–3732.

(16) Kang, M. S.; Joh, H.; Kim, H.; Yun, H.-W.; Kim, D.; Woo, H. K.; Lee, W. S.; Hong, S.-H.; Oh, S. J. Synergetic Effects of Ligand Exchange and Reduction Process Enhancing Both Electrical and Optical Properties of Ag Nanocrystals for Multifunctional Transparent Electrodes. Nanoscale 2018, 18415–18422.

(17) Yu, Y.; Zhang, Y.; Song, X.; Zhang, H.; Cao, M.; Che, Y.; Dai, H.; Yang, J.; Zhang, H.; Yao, J. High Performances for Solution-Pocessed 0D–0D Heterojunction Phototransistors. Adv. Opt. Mater. 2017, 5 (24), 1–6.

(18) Shi, Z.; Li, S.; Li, Y.; Ji, H.; Li, X.; Wu, D.; Xu, T.; Chen, Y.; Tian, Y.; Zhang, Y.; et al. Strategy of Solution-Processed All-Inorganic Heterostructure for Humidity/Temperature-Stable Perovskite Quantum Dot Light-Emitting Diodes.

ACS Nano 2018, 12 (2), 1462–1472.

45



Page 46 of 55

(19) Pan, J.; Quan, L. N.; Zhao, Y.; Peng, W.; Murali, B.; Sarmah, S. P.; Yuan, M.; Sinatra, L.; Alyami, N. M.; Liu, J.; et al. Highly Efficient Perovskite-Quantum-Dot Light-Emitting Diodes by Surface Engineering. Adv. Mater. 2016, 28 (39), 8718– 8725.

(20) Mashford, B. S.; Nguyen, T. L.; Wilson, G. J.; Mulvaney, P. All-Inorganic Quantum-Dot Light-Emitting Devices Formed via Low-Cost, Wet-Chemical Processing. J. Mater. Chem. 2010, 20 (1), 167–172.

(21) Chuang, C. H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13 (8), 796–801.

(22) Crisp, R. W.; Kroupa, D. M.; Marshall, A. R.; Miller, E. M.; Zhang, J.; Beard, M. C.; Luther, J. M. Metal Halide Solid-State Surface Treatment for High Efficiency PbS and PbSe QD Solar Cells. Sci. Rep. 2015, 5, 1–6.

46


Page 47 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Kim, H.; Lee, S. W.; Joh, H.; Seong, M.; Lee, W. S.; Kang, M. S.; Pyo, J. B.; Oh, S. J. Chemically Designed Metallic/Insulating Hybrid Nanostructures with Silver Nanocrystals for Highly Sensitive Wearable Pressure Sensors. ACS Appl. Mater.

Interfaces 2018, 10 (1), 1389–1398.

(24) Joh, H.; Lee, S. W.; Seong, M.; Lee, W. S.; Oh, S. J. Engineering the Charge Transport of Ag Nanocrystals for Highly Accurate, Wearable Temperature Sensors through All-Solution Processes. Small 2017, 13 (24), 1–11.

(25) Lee, S. W.; Joh, H.; Seong, M.; Lee, W. S.; Choi, J. H.; Oh, S. J. Transition States of Nanocrystal Thin Films during Ligand-Exchange Processes for Potential Applications in Wearable Sensors. ACS Appl. Mater. Interfaces 2018, 10 (30), 25502–25510.

(26) Lee, W. S.; Lee, S. W.; Joh, H.; Seong, M.; Kim, H.; Kang, M. S.; Cho, K. H.; Sung, Y. M.; Oh, S. J. Designing Metallic and Insulating Nanocrystal

47



Page 48 of 55

Heterostructures to Fabricate Highly Sensitive and Solution Processed Strain Gauges for Wearable Sensors. Small 2017, 13 (47), 1–11.

(27) Li, Z.; Peng, X. Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects. J. Am. Chem. Soc. 2011, 133 (17), 6578– 6586.

(28) Yu, W. W.; Wang, Y. A.; Peng, X. Formation and Stability of Size-, Shape-, and Structure-Controlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals. Chem. Mater. 2003, 15 (22), 4300–4308.

(29) Mokari, T.; Zhang, M.; Yang, P. Shape, Size, and Assembly Control of PbTe Nanocrystals. J. Am. Chem. Soc. 2007, 129 (32), 9864–9865.

(30) Weidman, M. C.; Nguyen, Q.; Smilgies, D. M.; Tisdale, W. A. Impact of Size Dispersity, Ligand Coverage, and Ligand Length on the Structure of PbS Nanocrystal Superlattices. Chem. Mater. 2018, 30 (3), 807–816.

48


Page 49 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Gao, Y.; Aerts, M.; Sandeep, C. S. S.; Talgorn, E.; Savenije, T. J.; Kinge, S.; Siebbeles, L. D. A.; Houtepen, A. J. Photoconductivity of PbSe Quantum-Dot Solids: Dependence on Ligand Anchor Group and Length. ACS Nano 2012, 6 (11), 9606–9614.

(32) Kalyuzhny, G.; Murray, R. W. Ligand Effects on Optical Properties of CdSe Nanocrystals. J. Phys. Chem. B 2005, 109 (15), 7012–7021.

(33) Lee, K. Y.; Lee, Y. W.; Lee, J. H.; Han, S. W. Effect of Ligand Structure on the Catalytic Activity of Au Nanocrystals. Colloids Surfaces A Physicochem. Eng.

Asp. 2010, 372 (1–3), 146–150.

(34) He, Y. P.; Miao, Y. M.; Li, C. R.; Wang, S. Q.; Cao, L.; Xie, S. S.; Yang, G. Z.; Zou, B. S.; Burda, C. Size and Structure Effect on Optical Transitions of Iron Oxide Nanocrystals. Phys. Rev. B - Condens. Matter Mater. Phys. 2005, 71 (12), 1–9.

49



Page 50 of 55

(35) Holm, A. H.; Ceccato, M.; Donkers, R. L.; Fabris, L.; Pace, G.; Maran, F. Effect of Peptide Ligand Dipole Moments on the Redox Potentials of Au 38 and Au140 Nanoparticles. Langmuir 2006, 22 (25), 10584–10589.

(36) Ibáñez, M.; Korkosz, R. J.; Luo, Z.; Riba, P.; Cadavid, D.; Ortega, S.; Cabot, A.; Kanatzidis, M. G. Electron Doping in Bottom-up Engineered Thermoelectric Nanomaterials through HCl-Mediated Ligand Displacement. J. Am. Chem. Soc. 2015, 137 (12), 4046–4049.

(37) Liu, H.; Li, M.; Shao, G.; Zhang, W.; Wang, W.; Song, H.; Cao, H.; Ma, W.; Tang, J. Enhancement of Hydrogen Sulfide Gas Sensing of PbS Colloidal Quantum Dots by Remote Doping through Ligand Exchange. Sensors Actuators, B Chem. 2015, 212 (3), 434–439.

(38) Sayevich, V.; Guhrenz, C.; Sin, M.; Dzhagan, V. M.; Weiz, A.; Kasemann, D.; Brunner, E.; Ruck, M.; Zahn, D. R. T.; Leo, K.; et al. Chloride and IndiumChloride-Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in

50


Page 51 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solution and Doping of Nanocrystal Solids. Adv. Funct. Mater. 2016, 26 (13), 2163–2175.

(39) Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8 (6), 5863–5872.

(40) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nat. Mater. 2016, 15 (2), 141–153.

(41) Kroupa, D. M.; Vörös, M.; Brawand, N. P.; McNichols, B. W.; Miller, E. M.; Gu, J.; Nozik, A. J.; Sellinger, A.; Galli, G.; Beard, M. C. Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification.

Nat. Commun. 2017, 8 (May), 2–9.

(42) Park, J.; Kwon, S. G.; Jun, S. W.; Kim, B. H.; Hyeon, T. Large-Scale Synthesis of Ultra-Small-Sized Silver Nanoparticles. ChemPhysChem 2012, 13 (10), 2540– 2543. 51



Page 52 of 55

(43) Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85 (22), 3533–3539.

(44) Huynh, K. A.; Chen, K. L. Aggregation Kinetics of Citrate and Polyvinylpyrrolidone Coated Silver Nanoparticles in Monovalent and Divalent Electrolyte Solutions.

Environ. Sci. Technol. 2011, 45 (13), 5564–5571.

(45) El Badawy, A. M.; Scheckel, K. G.; Suidan, M.; Tolaymat, T. The Impact of Stabilization Mechanism on the Aggregation Kinetics of Silver Nanoparticles. Sci.

Total Environ. 2012, 429, 325–331.

(46) Stein, B.; Zopes, D.; Schmudde, M.; Schneider, R.; Mohsen, A.; Goroncy, C.; Mathur, S.; Graf, C. Kinetics of Aggregation and Growth Processes of PEGStabilised Mono- and Multivalent Gold Nanoparticles in Highly Concentrated Halide Solutions. Faraday Discuss. 2015, 181, 85–102.

52


Page 53 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47) Herrmann, J.; Müller, K. H.; Reda, T.; Baxter, G. R.; Raguse, B.; De Groot, G. J. J. B.; Chai, R.; Roberts, M.; Wieczorek, L. Nanoparticle Films as Sensitive Strain Gauges. Appl. Phys. Lett. 2007, 91 (18), 1–4

(48) Transactions, P.; Society, R.; Series, L.; Physical, A.-M.; Sciences, E. A Novel Strain Sensor Based on the Campaniform Sensillum of Insects A. Skordos, P. H. Chan, J. F. V. Vincent and G. Jeronimidis. Strain 2002, 360, 239–253.

(49) Rein, M. D.; Breuer, O.; Wagner, H. D. Sensors and Sensitivity: Carbon Nanotube Buckypaper Films as Strain Sensing Devices. Compos. Sci. Technol. 2011, 71 (3), 373–381.

(50) Tanner, J. L.; Mousadakos, D.; Giannakopoulos, K.; Skotadis, E.; Tsoukalas, D. High Strain Sensitivity Controlled by the Surface Density of Platinum Nanoparticles. Nanotechnology 2012, 23 (28).

53



Page 54 of 55

(51) Moreira, H.; Grisolia, J.; Sangeetha, N. M.; Decorde, N.; Farcau, C.; Viallet, B.; Chen, K.; Viau, G.; Ressier, L. Electron Transport in Gold Colloidal NanoparticleBased Strain Gauges. Nanotechnology 2013, 24 (9).

(52) Hossain, M. A.; Jeon, S.; Ahn, J.; Joh, H.; Bang, J.; Oh, S. J. Control of Tunneling Gap between Nanocrystals by Introduction of Solution Processed Interfacial Layers for Wearable Sensor Applications. J. Ind. Eng. Chem. 2019, 73, 214–220.

54


Page 55 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

55


Investigation of Chemical Effect of Solvent during Ligand Exchange on

Recommend Documents