Transition States of Nanocrystal Thin Films during Ligand-Exchange

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Functional Inorganic Materials and Devices

Transition States of Nanocrystal Thin Films During Ligand Exchange Processes for Potential Application in Wearable Sensors Seung-Wook Lee, Hyungmok Joh, Mingi Seong, Woo Seok Lee, Ji-Hyuk Choi, and Soong Ju Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06754 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Transition States of Nanocrystal Thin Films During Ligand Exchange Processes for Potential Application in Wearable Sensors

Seung-Wook Lee†, Hyungmok Joh‡, Mingi Seong‡, Woo Seok Lee‡, Ji-Hyuk Choi*§ and Soong Ju Oh*‡ †

Department of Semiconductor Systems Engineering & ‡Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea §Rare

Metals Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

KEYWORDS: nanocrystals, ligand exchange, structural transformation, metal-insulator transition, intermediate state, strain gauge, wearable sensor

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Abstract Ligand exchange is an advanced technique for tuning the various properties of nanocrystal (NC) thin films, widely used in NC thin film device applications. Understanding how NC thin films transform into functional thin film devices upon ligand exchange is essential. Here, we investigated the process of structural transformation and accompanying property changes in NC thin films, by monitoring the various characteristics of silver (Ag) NC thin films at each stage of ligand exchange process. A transition state was identified in which the ligands are partially exchanged, where the NC thin films showed unexpected electromechanical features with high gauge factors up to 300. A model system was established to explain the origin of the high gauge factors, supported by observation of spontaneously formed nanocracks and metal-insulator transition from structural analysis and charge transport study, respectively. Taking advantages of the unique electromechanical properties of the NC thin films, we fabricated flexible strain gauge sensor devices with high sensitivity, reliability, and stability. We introduce a one-step fabrication process, namely “time- and spatial-selective ligand exchange process” for the design of low-cost and highperformance wearable sensors that effectively detect human motion, such as finger or neck muscle movement. This study provides fundamental understanding of the ligand exchange process in NCs, as well as insight into the functionalities of NC thin films for technological applications.


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Introduction

Colloidal nanocrystal (NC) thin films have attracted huge attention in various applications, such as electronics and sensors, owing to their ease of fabrication and tunable properties.1–5 Colloidal NCs can be synthesized on a large scale using wet chemical methods and are easily converted into large area, flexible NC thin films via low-cost solution processes such as roll-to-roll processing, ink-jet printing, spin-coating, and spray-coating.2–4,6–8 NC thin films have a broad range of properties that can be precisely controlled in two ways. First, analogous with bulk solids for which the properties are determined by the elemental composition, the properties of NC thin films are fundamentally determined by the NC elements called artificial atoms. The properties of these individual NC elements can be tailored by tuning their size, shape, composition, and surface.1–3,9– 14

Second, the interactions between individual NCs can further affect the overall properties of the

NC thin films, as NCs act as building blocks. This is analogous to solids, where the interactions between each atom or the interatomic spacing affect the structural, electronic, optical, mechanical, magnetic, and thermal properties. For NC thin films, the ligand is a key parameter used to control the properties of the individual NCs and the interactions between the NCs, as it influences the surface and interface of the NCs.2,3,9– 11,14–18

Depending on the type of ligand used, the properties of the NCs can be modified and the

interactions between the NCs can be enhanced or reduced. However, as-synthesized NC thin films generally do not retain applicable electronic properties or functionalities. Long interparticle distance between adjacent NCs coming from initial long ligands prohibit efficient electron exchange in NC thin films, leading to electronically insulating and functionally inactive thin films. When ligands are exchanged for shorter counterparts via ligand exchange processes, the distance



between the NCs is reduced and interactions occur, thereby enhancing NC coupling and facilitating charge carrier (electron or hole) transport. Therefore, the ligand exchange process activates the electronic, magnetic, optoelectronic, thermoelectronic, and electromechanical functionalities of the NCs, and can be utilized to create functional NC thin film devices. By carefully designing the length and type of new ligands, it is possible to further tailor the charge carrier statistics or transport mechanism of semiconductor and metal NC thin films.9,11,19,20 This may have a huge impact on the device performance, leading to higher on/off ratios or swing in transistors, light emission efficiency in light emitting diodes, power conversion efficiency in solar cells, responsivity in photodetectors, gauge factor in strain gauges, temperature coefficient of resistance in temperature sensors, and so on.2,9,11,14,15,21–25 Given that the ligand exchange process transforms insulating and nonfunctional NC thin films into functional devices, the details of this process have been extensively investigated. For example, the effects of the mother solvent of the ligand exchange solution, the binding group and counter ion of the ligands on the properties of NC thin films have been studied. 26–28 In-situ observation of the ligand exchange process using transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetry (TGA) has also been carried out to monitor the changes in the surface chemistry.29–32 However, the details of the changes in the structures and properties of the NC thin films during ligand exchange process are still not fully understood. In particular, the mechanisms and phenomena underlying the microand macroscopic structural transformations and how the properties are affected remain unknown. To fully exploit the ligand exchange process, it is crucial to understand the relationship between the structures and properties of NC thin films during ligand exchange process. It can provide insight into controlling the functionalities of the NCs for various applications.


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Herein, we investigate the transformation and accompanying property changes of NC thin films during the ligand exchange process by monitoring the structural states and various properties of the NC thin films at each stage of the ligand exchange process (Figure 1). Specifically, silver (Ag) NCs are selected, and we examine how insulating NC thin films are transformed into the metallic counterparts. Interestingly, we identify a transition state involving partial ligand exchange, where the film is neither completely electrically insulating nor conductive. Evaluation of the charge transport mechanism reveals that this state belongs to the metal-insulating transition regime. The transition state exhibits unexpected electromechanical properties of highly sensitive resistance behavior in response to external mechanical strain. We attribute this response to the nanocracks that are spontaneously formed during transformation in both nano- and microscale structures. These unique electromechanical properties render the NC thin films suitable for flexible strain gauges, which is the core of wearable sensor technology. Taking advantage of this feature, we fabricate NC thin film-based strain gauges that demonstrate high sensitivity, reliability, and stability. We develop an extremely simple, one step fabrication method of time- and spatialselective ligand exchange process to manufacture the strain gauges by controlling the degree of the ligand exchange, which can significantly lower the device fabrication steps and cost. The ability of this sensor to sensitively detect human motions, such as finger or neck muscle movement, is demonstrated. This work provides fundamental insight into the ligand exchange process for NC thin films and highlights a promising avenue for the development of low-cost and highperformance NC-based flexible and wearable electronic devices and sensors.



Figure 1. Schematic of ligand exchange process of NC thin films.


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Results and discussion Ag NCs and tetrabutylammonium bromide (TBAB) were used as the representative NC material and ligand exchange material, respectively. Ag NCs are one of the most extensively studied and widely used NCs for various applications such as optics, electronics, and bio-sensors.13,21–23,33–36 TBAB treatment is known to greatly reduce the interparticle distance of Ag NCs from ~3 nm to 0 nm by replacing the oleate ligands with inorganic halide ligands of Br−, dramatically changing their plasmonic, electronic, and electromechanical properties in thin films. Particularly, TBAB treatment enhances the electrical conductivity by more than twelve orders, allowing us to readily monitor the transition between insulating and metallic properties.22,23 3.5 nm Ag NCs were synthesized through wet chemical methods.37 Ag NC thin films were formed by spin-coating the Ag NC solutions onto pre-treated substrates. TBAB treatment was conducted for different durations of 0, 2, 5, 15, 30, and 60 secs to investigate each state of ligand exchange process.



Figure 2. a) FT-IR, b) UV-vis absorption, c) XRD, and d) EDX spectra for spin-coated Ag NC thin films subjected to Br− ligand exchange for various times. (red : 0 sec, orange : 2 sec, yellow : 5sec, green : 15 sec, blue : 30 sec, violet : 60 sec)

To investigate the changes in the surface chemistry of the Ag NCs, Fourier transform infrared (FT-IR) analysis was conducted on the Ag NC thin films treated with TBAB for various times


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(Figure 2a). The CH stretching vibration band, which arises from the oleate ligands, was observed around 2850−3000 cm-1 in the FT-IR profile of the Ag NC thin.9,38 The intensity of the CH band decreased with increasing TBAB treatment time. At treatment time of 15 sec, the peak intensity was greatly reduced, indicating only small amounts of ligand remained. At longer treatment times such as 30 and 60 sec, this band was no longer observed, indicating complete removal of the original ligands. Changes in the optical properties were observed by UV-vis analysis (Figure 2b). Initially, the Ag NC thin film showed a localized surface plasmonic resonance peak (LSPR) around 450 nm.22,37,38 This peak is known to disappear when the individual NCs are coupled together through ligand exchange.22,23,33 For short treatment times such as 2 and 5 sec, the LSPR peak still remained, indicating that a large part of the original ligands still existed. After treatment for 15 sec, the intensity of the LSPR signal declined greatly, although not completely. With treatment times over 30 sec, this peak completely disappeared, implying that the characteristics of the isolated NCs were lost in the NC thin film. The structural properties of the Br− treated Ag NC thin films were examined by X-ray diffraction (XRD) analysis (Figure 2c and Table S1). The profile of the as-synthesized Ag NC thin films showed a peak at 38.3o, attributed to the (111) Ag crystal plane.39,40 With treatment times longer than 15 sec, a peak at 44.2o attributed to the Ag (200) crystal plane is formed. With increasing TBAB treatment times, these peaks gradually became stronger and sharper, indicating growth in the size of the NCs. According to Debye Scherrer equation, the size of the as-synthesized and ligand exchanged Ag NCs with TBAB treatment time for 2, 5, 15, 30, and 60 sec was 2.94, 3.28, 5.18, 6.56, 16.2, and 21 nm, respectively. After 15 sec of TBAB treatment, a new peak appeared at 30.7o, corresponding to AgBr (200).39,40 When the inorganic Br− ligands replace the original ligands, the Br− ions bond to Ag, forming AgBr. Thus, the corresponding peak gradually increased



with increasing TBAB treatment time. The changes in the elemental composition of the Ag NC thin film were also examined using energy-dispersive X-ray (EDX) analysis (Figure 2d). A peak of Ag was observed at 2.99 keV in the EDX profile of the as-synthesized Ag NC thin films. No noticeable change was observed until 15 sec of treatment, where a small peak appeared at 1.5 keV, corresponding to Br.40 With increasing treatment time, the peak of Br became distinct and sharp, indicating successful ligand exchange from oleate to Br−.


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Figure 3. TEM images of a) as-synthesized Ag NCs subjected TBAB ligand exchange for b) 2, c) 5, d) 15, e) 30, and f) 60 sec. (scale bar: 20 nm) SEM images of g) as-synthesized Ag NC thin films subjected ligand exchange for h) 15 sec and i) 60 sec. (scale bar: 2μm) k) Schematic of structural transformation of NC thin films, creating and filling nanocracks upon ligand exchange process at different time intervals. (Red arrow indicates the migration of Ag atom)



TEM analysis was conducted to directly monitor the structural transformation of the Ag NCs as a function of the ligand exchange time. Figure 3(a−f) show the transformation of the Ag NCs upon TBAB treatment. Initially, the Ag NCs had a spherical shape with a diameter of 3.52±1.09 nm and interparticle distance of 2.72±0.36 nm. After treatment for 2 sec, the size remained the same, with reduced interparticle distances; overlaps of NCs were frequently observed. After 5 sec of treatment, two types of Ag NCs co-existed with different diameters: 3.66±0.67 nm and 19.03±2.27 nm. The former corresponds to the unchanged, as-synthesized NCs, whereas the latter indicates a growth of about 100 adjacent small NCs through fusion or sintering. Coalescence and growth of the NCs occur by migration of mobile Ag atoms to reduce the surface energy. Some small NCs that did not participate in the growth reaction remained, due to partially remaining organic ligands. For the sample subjected to 15 sec treatment, only large sized NCs were observed with an average size of 31.4±8.7 nm, where the large particles were adjacent to each other. Small sized Ag NCs seem to grow to form large particles by oriented attachment, displaying mostly five-fold symmetry with (111) facets and multiple twinning, as shown in a high magnification TEM image (See discussions in Supporting Information and Figure S1). When the treatment time was increased to 30 or 60 sec, further coalescence and growth between the larger NCs occurred simultaneously in non-specific directions. Upon the complete removal of initial organic ligands which serve as surfactants to make uniform spherical shape, enlarged NCs become irregular shapes with an average size of 48.58± 19.83 nm. Scanning electron microscopy (SEM) was used to monitor the macroscopic morphological changes of the Ag NC thin films upon TBAB treatment. The as-synthesized Ag NC thin films were


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characterized by a smooth and continuous surface without any holes or cracks (Figure 3g). When the NC thin films were treated with TBAB for short time of 15 sec, the smooth NC thin films changed to segmented films of multiple micro-sized clusters with boundaries or cracks (Figure 3h). The formed Ag microclusters had a size of 1.28±0.41 µm2 and the density of the cracks was calculated as 1.03±0.11 µm−2. After 60 s of TBAB treatment, relatively less rough surfaces were observed, and the cracks disappeared (Figure 3i). Based on TEM and SEM analysis, we developed a model system to interpret the structural transformation on the nanoscale and microscale (Figure 3k). At the early stage of short-ligand treatment, multiple Ag NCs are fused together (stage I in figure 3k). The enlarged NCs are physically connected to each other, forming microclusters. This in turn forms empty spaces or larger gaps through the entire film, like nanocracks. The overall structure of the microclusters and spaces resemble a system of a metallic granular system.41 We believe that small amount of residual ligands block a specific side of the NCs located at the outside of microcluster and mitigate further migration of Ag atoms. This effectively hinders the complete growth of microclusters and forms nanocracks. Although the overall surface area decreased during partial ligand exchange, this state was still metastable, favoring further reduction of the surface energy. As the ligand exchange process progressed, continuous fusion of the enlarged NCs allowed microclusters to collapse into thinner and smoother NC thin films due to complete removal of initial organic ligands. In this way, the voids such as nanocracks were filled. (stage II in figure 3k). Although the process occurred in the solid state, the mobile nature of the Ag atoms enabled this transformation, reducing total surface area and energy.



Figure 4. a) Electrical resistivity of Ag NC thin films with various ligand exchange times. b) I-V curve of partially ligand exchanged Ag NC thin films with 15 sec treatment (upper) and fully ligand exchanged Ag NC thin films with 60 sec treatment (lower) with (red) and without (black) strain. c) Gauge factor of Ag NC thin films subjected to ligand exchange for various times. Electrical resistivity vs. temperature graphs of thin films with d) partially and e) fully ligand treated Ag NC thin film. f) TCR and gauge factors of Ag NC thin films with various ligand exchange times. Schematic of charge transport in partially ligand exchanged Ag NC thin films (intermediate state) g) before and h) after applied strain. (red arrow indicates electron flow)


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The effects of the structural transformation during ligand exchange on the electrical properties of the Ag NC thin films were investigated. The changes in the electrical properties were monitored via 2-point probe measurement. The sample was prepared as a simple resistor type consisting of two electrodes and one active area. The size of the active area was 0.5 × 1.5 cm. The electrical resistivity of the Ag NC thin films is plotted in Figure 4a. Due to the existing oleate ligands, the Ag NC thin film without ligand exchange was too electrically insulating for measurement of its electrical resistivity. The NC thin films treated for 2 and 5 sec also showed insulating properties. With longer TBAB treatment times, the Ag NC thin films became electrically conductive. With 15 sec of treatment, the Ag NC thin film showed a measurable resistivity of 5.31±6.88 × 102 ohm·cm. The high electrical resistivity is attributed to the partially remaining oleate ligands and multiple cracks, as seen in the FTIR and SEM image, respectively. The resistivity decreased exponentially with increasing treatment time, and saturated around 8.98±1.92 × 10-4 ohm·cm after 60 sec treatment. Sufficient charge transport paths were formed due to a reduction of the number of cracks and further fusion between the Ag microclusters in the final stage. Combinational study of chemical, optical, structural, and electrical analyses imply that 2 or 5 sec treatment yields ineffectively ligand exchanged Ag NC thin films with very high resistivity, 15 sec treatment forms partially ligand exchanged Ag NC thin films with intermediate resistivity, and 30 or 60 sec treatment creates fully ligand exchanged Ag NC thin films with very low resistivity. The electromechanical properties of the Ag NC thin films were explored. The current-voltage (I-V) curve was recorded with or without bending the NC thin films. As representative samples, partially ligand exchanged NC thin film (treated with TBAB for 15 sec) and fully ligand exchanged



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NC thin film (treated with TBAB for 60 sec) were prepared (Figure 4b). We analyzed the gauge factor of these Ag NC thin films, which is an important parameter for evaluating the electromechanical properties. The gauge factor is expressed by Eq. (1): G = (△R/R0)/ε

Eq. (1)

where △R is the change in the resistance, R0 is the initial resistance, and ε is the strain. The strain was calculated from t/2r, where t is the thickness of the sample substrate and r is the bending radius.42–44 When 0.4 % strain was applied to the partially ligand exchanged thin films, the resistance increased by 134 %, and the gauge factor was calculated to be 335 (Figure 4b, upper graph). On the other hand, the fully ligand exchanged thin film showed little current change when a strain of 0.4 % was applied (Figure 4b, lower graph). The gauge factor was calculated as a very small value of 3.6, in good agreement with a previous report.22 The resistivity and gauge factor of the Ag NC thin films subjected to ligand exchange for various times are summarized in Table 1. The average gauge factor was 144.0±98.4 and 3.4±0.8 for treatment times of 15 and 60 sec, respectively (Table 1). The gauge factor was plotted as a function of the ligand exchange time (Figure 4c), exhibiting that the resistivity and gauge factor data followed similar trends. In metalinsulator composites such as NC thin films with ligands or granular system, high gauge factor is expected in highly resistive thin films, as gauge factor is proportional to the slope of resistanceconcentration of insulator plot.21,42,43 In our partially ligand exchanged Ag NC thin films, remaining ligands and nanocracks serve as insulating components that increase both resistivity and gauge factor. This leads to abnormally high gauge factor that is even higher than theoretical values of 20 to 50 determined by the variety ligand lengths in particle-based strain sensors.42 It should be


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noted that the partially ligand exchanged Ag NC thin films shows relatively large deviation in resistance and gauge factor, which is attributed to the nature of randomness and variation in posttreatment time, etc.21 (See discussions in Supporting Information). We believe that optimization process such as synthesis and purification of NCs and the control of the type and concentration of ligands can decrease the variations and improve the device performance.

Table 1. Resistivity, gauge factors, and TCR values of Ag thin films with various TBAB treatment times TBAB Treatment

2

5

15

30

60

Time (sec) MetalTransport Insulating

Insulator

Metallic

behavior transition Resistivity

5.31±6.88 -

6.34±5.74

8.98±1.92

-

(ohm·cm)

× 102

× 10-3

× 10-4

Gauge -

-

144.0±98.4

factor


25.3±14.6

3.4±0.8


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Temperature -2.47±0.69

Coefficient of -

0.23±0.14

1.11±0.21

-

Resistance

× 10−3

× 10−3

× 10−3

(Κ−1)

To further understand the differences in the electrical and electromechanical properties of the partially and fully ligand exchanged Ag NC thin films, we investigated the charge transport mechanism using variable-temperature electrical measurements. The changes in the electrical resistance of each thin film sample with variation of temperature from 85 to 285 K are displayed in Figures 4d and e, respectively. Generally, the electrical resistance of a pure metal is proportional to the temperature45, and is expressed by Eq. 2: (dR/R0) = αdT

Eq. (2)

where α is the temperature coefficient of resistance (TCR), dT is the temperature change in the sample. From the graph 4d and e, the TCR values of the partially and fully Ag NC thin films were calculated to be -2.47±0.69 × 10−3 and 1.11±0.21 × 10−3 Κ−1, respectively. The positive slope of the resistance versus temperature plot for fully ligand exchanged Ag NC thin film indicates that the charge transport mechanism involves metallic transport, commonly observed for metallic films (Figure 4e). In contrast, for the partially ligand exchanged Ag NC thin films, the slope was negative, which is opposed to the characteristics of metals. The negative TCR value indicates a hopping mechanism for charge transport in the thin film (Figure 4d). Hopping transport is generally described by the Arrhenius activation energy equation46:


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R = R0 exp (Ea/kBT)

Eq. (3)

Where kB, Ea, and T are the Boltzmann constant, the activation energy, and temperature, respectively. The activation energy was calculated as 1.96 × 10−2 eV in the high temperature range and 3.60 × 10−3 eV in the low temperature range (Figure S2). It should be noted that a positive slope was sometimes observed for these samples, although not frequently (see discussions in Supporting Information and Figure S3). Based on these results, the Ag NC thin films were classified into three regimes according to the TBAB treatment time: insulating (ineffective ligand exchange with 2 or 5 sec treatment), metalinsulator transition (partial ligand exchange with 15 sec treatment), and metallic (fully ligand exchange with 30 or 60 sec treatment) regimes (Figure 4f). First, the Ag NC thin film treated with TBAB for 2 or 5 sec is too insulating to measure resistivity, gauge factor, and TCR. Second, partially ligand exchanged Ag NC thin film belongs to the metal-insulator transition regime, and referred as intermediate state. In this regime, the resistivity spanned a wide range from 101 ohm·cm to 103 ohm·cm, and the TCR was mostly negative but was also positive in a few instances. The high resistivity and negative TCR values arise from the incompletely fused NCs and many existing cracks. The numerous cracks interrupt charge transport from one microcluster to another in the thin film. These samples also had a high gauge factor. The width of the cracks between the Ag microclusters increased when strain was applied. Thus, the electron pathway was cut off, restricting charge transport and increasing the resistance (Figures 4g and h). Last, fully ligand exchanged Ag NC thin films belong to the metallic regime. In this regime, the sample showed very low resistivity, a low gauge factor, and a positive TCR value. This is because the interparticle distance was almost zero, and more importantly, the NCs were completely fused due to the



relatively long TBAB treatment time; thus, the electron pathways were completely secured. The formed electron pathways were not broken by extreme strains such as 2%, resulting in a low gauge factor. It should be noted that the sample prepared with 30 sec treatment had a lower resistivity, positive TCR and higher gauge factor than the sample treated 60 sec, indicating that the degree of metallic property is relatively low (Figure S4). Our study clearly shows the strong features and potentials of nanomaterials; controlling the surface and interface of NCs allows to obtain all of metallic, intermediate, and insulating thin films, while it is very difficult for bulk materials.

Figure 5. a) Resistance changes in ligand-exchanged Ag NC-based thin film treated for 15 sec during strain and release tests with 0.4% strain. b) Fast and c) slow response profile for Ag NCbased thin film subjected to 15 sec ligand exchange. d) Strain response profile Ag NC thin films where 0.4% strain was applied and released.


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The electronic and electromechanical properties of the Ag NC thin films could be precisely tuned over a wide range by controlling the duration of the ligand exchange process. These tunable properties may enable the construction of many functional devices. Herein, we attempted to design high performance strain sensors by exploiting the tunable and high gauge factors of the thin films. Indeed, the strain sensor is one of the most important key components in wearable electronics.5 Wearable strain sensors are currently widely used in various fields such as bio-integrated electronics, robotics, and human healthcare systems.5,21,22,47–49 For these applications, the strain sensor must have high sensitivity in order to detect the delicate signals such as heart pulses or muscle movement.47 The gauge factor is a measure of the sensitivity in strain sensors, as defined. One effective way to improve the gauge factor is to form cracks on the thin film surface by applying high strain.21,22,49–51 However, this mechanical process can cause damage to the sample or substrate when high strain exceeding the plastic deformation range is applied. In our method, nanocracks were chemically and spontaneously generated by engineering the ligand exchange process without any sample or substrate damage. The performance of the fabricated strain sensor devices with naturally formed nanocracks was evaluated through various stability, reliability, and time responsibility tests. The sensor devices showed a linear change in the resistance as the strain increased in the range of 0.08 to ~ 3 % of strain, indicating that they can be used as strain gauges (Figure S5). The resistance of the strain sensor device changed by more than 40% when 0.4% strain was applied. The resistance returned to its original value when the strain was removed, as proved in several strain cycling tests (Figure 5a). We investigated the response of the sensor device under different frequencies. When strain was applied to the device quickly, the resistance changed immediately (Figure 5b). The changes in the resistance of these samples during slow cycling at 0.4 Hz also showed a stable change and recovery without any hysteresis (Figure 5c). Strain cycle tests



were conducted more than 1000 times at a frequency of 1 Hz. The device showed a stable change in the resistance without degradation, indicating that NC thin films show robust mechanical property (Figure 5d). These experimental data demonstrate that the fabricated device is a promising candidate for strain sensors with high sensitivity, reliability, and stability using simple and low-cost processes.


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Figure 6. a) Time- and spatial-selective ligand exchange process for fabrication of a strain gauge device: b) images of a strain sensor attached to a finger with strain and release motion, and c) resistance change during several cycles of finger bending. d) Images of strain device attached to neck showing response to very small muscle motion of` swallowing, and e) resistance changes in response to this motion.

To realize practical wearable sensors, high sensitivity and reliability are key factors, and it is also crucial to utilize low-cost materials and fabrication processes. Typically, conventional sensor devices are fabricated via expensive and complicated methods because each sensor device consists of various parts, such as electrodes and sensing components.42,43,52–55 Each part typically requires different materials and many expensive vacuum-based processes, which greatly increases the fabrication cost. To address these limitations, we developed simplified and cost-effective strategies, namely time- and spatial-selective ligand exchange processes for fabricating strain sensors (Figure 6a). In these methods, only Ag NCs and TBAB ligands are used, but the construction of both electrodes and sensing components were achieved by controlling the degree of ligand exchange. The Ag NC thin films with long ligand exchange time were used as a building block to construct the electrodes as they have a very low resistivity and gauge factor. The Ag NC thin films with short ligand exchange time were used to fabricate strain gauges windows due to their high sensitivity. To construct electrodes and sensing components simultaneously, a PDMS layer was covered and peeled off at a desire time and place. In detail, the devices were immersed in TBAB solution for 45 sec with the sensing segments covered with PDMS. The PDMS layer was then peeled off in the liquid phase, and the device was further exposed to TBAB solution for another 15 sec. This layer was washed with methanol, completing the device fabrication. Each process was



performed via solution processing at room temperature and pressure. The fabricated strain sensor device was attached to the finger, and the sensor performance was determined by measuring the resistance change when the finger was bent and extended (Figure 6b and c). The resistance of the sample attached to the finger successfully changed and recovered with a resistance change of about 125% and remained stable over repeated cycles. To further demonstrate the functionality of the developed sensor devices, we also placed the sensor on the skin of the neck and examined the very small resistance change of 1% as a means of detecting very small muscle motions such as swallowing (Figure 6d). The resistance changes were successfully measured in real time, detecting even small muscle movement (Figure 6e). While the experiment demonstrates high performance of our sensor, more elaborate study such as encapsulation, mechanical stability should be also conducted to realize practical applications.

Conclusion The transformation of NC thin films during ligand exchange was investigated by monitoring various properties at each stage. An intermediate state that exhibits unexpected electromechanical properties was found during the partial ligand exchange process. The unique properties were explained based on combined evaluation of the charge transport mechanism and structural analysis. These novel properties were exploited for the design of low-cost, high performance, wearable strain sensors that can efficiently detect delicate human motion. This work presents scientifically important fundamental information for nanoscience and provides a technologically advanced strategy for achieving wearable electronics.


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Methods Materials Silver nitrate (99%, AgNO3) powder was purchased from Alfa Aesar Co., Inc. Oleic acid (90%), oleylamine (70%), tetra-n-butyl ammonium bromide (99.0%, TBAB), (3-mercaptopropyl) trimethoxysilane (95%, MPTS), and methanol (99.8%) were purchased from Sigma Aldrich. Polyethylene terephthalate (PET) films (SKC films) with a thickness of 100 μm were used as flexible substrates.

Synthesis of Ag NCs Ag NCs were synthesized by a previously reported process with slight modification.37 AgNO3 (1.7 g), oleic acid (45 mL), and oleylamine (5 mL) were prepared in a three-neck flask and mixed with magnetic stirring. The mixture was degassed at 75°C for 1.5 h. The solution temperature was increased to 180°C at a heating rate of 1°C·min−1, followed by cooling to room temperature. The synthesized Ag NCs were washed by centrifugation at 5,000 rpm for 5 min with toluene and ethanol. The precipitate was redispersed in toluene. The washing process was repeated three times with the same solvent. The washed precipitated Ag NCs were dispersed in octane, where the concentration was controlled to 200 mg·mL−1. Ag NC thin film deposition The Ag NCs dispersed in octane were spin-coated onto MPTS-treated PET (thickness 100 μm) at 1,000 rpm to form Ag NC thin films for the strain sensor device.22 The ligand exchange process was performed using TBAB (10 mM) in methanol. The spin-coated Ag NC thin films were immersed in the TBAB ligand exchange solution and removed after various exchange times. These samples were washed with methanol three times.



Characterization The surface chemistry of the Ag NC thin films subjected to various ligand exchange times was analyzed using Fourier transform infrared spectroscopy in the attenuated total reflection (ATR) mode. The optical properties of the Ag NC thin films coated on transparent glass were measured by UV-vis spectroscopy using a Cary 5000 apparatus (Agilent Technologies). To observe the changes in the crystal structure and surface morphology of the Ag NC thin film as a function of the ligand exchange time, X-ray diffraction (MAX-250V, Rigaku), TEM (Model Tecnai G2 F30, FEI, Korea Basic Science Institute), and SEM (S-4300, Hitachi) analysis were conducted. To analyze the electrical performance, electrical measurements of the Ag NC thin films were performed using a probe station (MST 4000, MS Tech.) and a semiconductor analyzer (HP 4155B). The electromechanical performance of the Ag NC thin film devices was characterized based on a strain test using a multimeter or a probe station and a semiconductor analyzer with a home-made linear moving stage that is moved by a stepwise motor.

Fabrication of strain sensor device The strain sensor device was fabricated by a one-step process. The electrodes and sensor segments were constructed using a Ag NC solution and TBAB ligand solution. First, The synthesized Ag NCs were spin-coated onto MPTS-treated PET (100 µm thickness) at 1000 rpm for 1 min. The Ag NC coated thin film was partially covered (on the sensing segment) with a PDMS film. The Ag NC thin film partially covered with PDMS was dipped in a TBAB/methanol solution (= 30 mM) for the first ligand exchange step. After 45 sec, the PDMS film was peel off in liquid phase, and additional ligand treatment was conducted for 15 sec. The sample was removed


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and washed with methanol three times. Finally, the resistor type strain sensor was successfully obtained.



ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website at DOI: Discussions of growth of Ag NCs upon ligand exchange and transport behavior in Ag NC thin films, TEM images of Ag NCs, resistivity-temperature plot and strain sensor performance data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [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.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT and Future Planning (2016R1C1B2006534). This was also supported by Korea Electric Power Corporation (Grant number: 18A-002).


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Transition States of Nanocrystal Thin Films during Ligand-Exchange

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