Ultrasensitive Flexible Proximity Sensor Based on Organic Crystal for

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Ultra-sensitive flexible proximity sensor based on organic crystal for location detection Haiting Wang, Qingxin Tang, Xiaoli Zhao, Yanhong Tong, and Yichun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15352 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on January 2, 2018

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ACS Applied Materials & Interfaces

Ultra-sensitive flexible proximity sensor based on organic crystal for location detection Haiting Wang, Qingxin Tang,* Xiaoli Zhao, Yanhong Tong, and Yichun Liu* Center for Advanced Optoelectronic Functional Materials Research, and Key Lab of UVEmitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China E-mail address: [email protected]; [email protected] Tel./fax: +86-431-85099873.

ABSTRACT: A new type of flexible proximity sensor that uses the micro-sized organic crystal as the sensing element is demonstrated. The two-terminal organic sensor can accurately perceive the external objects, such as the human finger, fibre and even AFM tip. The proximity sensor shows an unprecedented distance resolution of 0.05 mm, which is two order of magnitude higher than that of previously reported conventional capacitor proximity sensors. A novel method has been proposed to realize the location detection of the approaching unknown-charge object by changing the distance between stimuli and sensor. Our results open a new route to realize ultra-sensitive perception of objects, making it promising candidate for applications in artificial intelligence, healthcare systems and high precision robots.

KEYWORDS: organic crystals, ultra-sensitive perception, proximity sensors, distance resolution, location detection, wearable electronics. 1 ACS Paragon Plus Environment

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1. INTRODUCTION With the recent progress of robotic systems, prosthetics and wearable medical devices, efforts have been devoted towards realization of highly sensitive and skin-mountable sensors.1-9 Among the various sensing capabilities, proximity sensation is of great importance in safety precaution, and noninvasive medical diagnostics and therapy.10-14 They could help the robotics and prosthetic devices “feel” the real world during interactions, and give timely prediction aiming to deliver target feedback.15-19 Current, most of the reported proximity sensors applied a mechanism similar to that of the extensively used capacitive touch screens,11,20 where the device configuration of the parallel plate capacitor is used by inserting a sensing dielectric layer between one pair of metal plates, 10-12,19 and the sensing is based on the interference of electric field.10-14,21,22 The proximity of the objects like human finger and human hand enables the change of capacitance.10-14,16,18,19 For example, Ahn et al. fabricated a wearable capacitor proximity sensor by sandwiching the insulated acrylic polymer between graphene electrodes, to perceive a human finger with a distance resolution of 5 mm. When the finger comes close, the reduced distance between the human finger and the sensor makes part of the fringing electric field absorbed, resulting in the decreased capacitance of the capacitor.11 Such a simple device configuration shows the outstanding advantages like facile fabrication, low power consumption, and low cost. Until now, the reported target objects for capacitor-type proximity sensors are limited to human finger and metal conductors, with the highest distance resolution at 5 mm.10-12 Highersensitivity and higher-resolution proximity sensors for detection of micro-objects are desirable for electronic skin, humanoid robotics, biomedicine, and even security defense.15-19 More importantly, the previously reported location detection depends on the pre-measured criterion curve of electrical signal versus distance, which is used to compared with the 2 ACS Paragon Plus Environment

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measured capacitance value to determine the distance between sensor and stimuli.10-14 In this case, the location detection only can be achieved for fixed objects, since the capacitance change induced by different external objects is different. Therefore, it is requisite to find a method to realize the accurate location detection of the unknown-charge objects, which is important for practical application such as behavior control of humanoid robots and security defense. Herein, we present a new type of ultra-sensitive flexible proximity sensors, in which one key is that the flexible micro-sized organic single crystal serves as the sensing element for the first time. Organic single crystal is free from defects and disorders, and hence is very appropriate to study the sensing mechanism. Different from the conventional capacitor-type proximity sensor, here the electrodes are located on the two ends of the organic single crystal to form a two-terminal planar device configuration. Such a new-type sensor can accurately perceive the external objects such as 2 nm-AFM tip, fibre and human finger. The highly sensitive detection towards AFM tip can reach an unprecedented distance resolution of 0.05 mm, which is two orders of magnitude higher than that of previously reports for conventional capacitor-type proximity sensors. Moreover, our sensors realize the location detection of the stimuli for the first time by changing the distance between stimuli and sensor. Our novel results provide a new way for the high-sensitivity detection of the external objects, showing a big application potential in high precision robots for automated handing of external object. 2. EXPERIMENT SECTION Device fabrication: Flexible polyethylene terephthalate (PET) substrates with a thickness of 100 µm were cleaned with deionized water, ethanol and acetone, respectively, and then were blown using a nitrogen gun. The insulated polymethyl methacrylate (PMMA) was deposited on PET to provide the good electrical stability for the two-terminal devices, which is a 3 ACS Paragon Plus Environment


necessary premise for further sensing measurements. The PMMA in anisole was spin-coated on the PET substrates at 4000 rpm for 40 s, followed by thermal treatment at 100 ℃ to remove the solvent, Semiconductor single crystalline nanobelts of rubrene (Aldrich) were synthesized by physical vapor transport method at atmospheric pressure in high-purity argon at approximately 100 sccm. Crystals varied in thickness from tens to hundreds of nanometers. Only very thin, flexible crystals were chosen, then were transferred to the substrates by a mechanical probe. Gold electrodes were deposited on the substrates via vacuum evaporation through a shadow mask at a pressure of 10-4 Pa and a rate of 0.1 Ås-1. Measurements: Optical microscope images were measured with an Olympus BX51. Scanning electron microscope (SEM) images were obtained on a Philip XL30 instrument. Atomic force microscope (AFM) images were obtained via a Dimension Icon atomic force microscope produced by Bruker. AFM tip proximity measurements were carried out in scanasyst-air mode. The electrical characteristics were real-time recorded with Keithley 2450. The current value remains stable at the fixed distance between stimuli and sensor, which is used as the electrical signal to perceive the stimuli (S1 in Support Information). 3. RESULTS AND DISCUSSION Figure 1a presents the schematic illustration of a flexible proximity sensor based on a twoterminal planar device configuration. Micro-sized rubrene single crystal is selected as the sensing element of the sensor, owing to the fact that rubrene is one of the most promising organic materials with stable electrical performance, and its single crystal can be easily obtained by a simple vapor transport method.23-26 Rubrene single crystals were mechanically transferred onto the insulated PMMA/PET supporting substrate with a mechanical probe, and then the Au was used as electrodes (Supporting Information). The optical image of a real twoterminal device is showed in the inset of Figure 1a. Figure 1b schematically shows the sensing 4 ACS Paragon Plus Environment

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mechanism. And Figure 1d and 1e are SEM and AFM images of the rubrene crystals. The crystals present the regular belt-like structure with flat surface and the length ranging from tens to hundreds of micrometers. The bendable and flexible morphology of rubrene nanobelts shows that they can integrate with the flexible substrates to meet the requirements of flexible and conformal electronics. The electrical characteristics of the flexible device were measured after the device was repeatedly bent with the strain of 0.23% and then was recovered for 200 times (Figure S3). As shown in Figure 1b, the perception of sensors is based on charges of objects. Many of the external objects are unintentionally electrically charged with tiny charge quantity, for example, human finger, hair, fiber, and paper.27-30 When the charged objects approach the organic crystal, the charges induced electric field functions as a gate voltage of field-effect transistor that can effectively modulate the carriers in organic semiconductor,31,32 resulting in the current change of organic device. In this case, air can serve as the dielectric of field-effect transistor that has been reported in our previous studies.33 That is to say, the approaching charged object introduces a “gate” electrode into the two-terminal organic semiconductor device, causing the dramatic current change. A similar “gate” effect has been demonstrated in InSb and graphene transistors, where the pre-contact between sensor and stimuli was processed prior to sensing detection to artificially produce triboelectric charges, and the charged stimuli behaves as the second “gate” electrode31,34 In contrast, only a few carriers in most of organic semiconductors, and the gate voltage play a decisive role in carrier transport of organic field-effect transistors. The gate voltage can induce a large number of charges at the interface between semiconductor and insulator, which contributes to carrier transport. Here, one key is the flexible organic crystal as sensing material for the first time in proximity sensors. The ultra-low carrier concentration in organic crystal amplifies the weak output signal changes. Eventhough the stimuli with tiny charges only can induce very few changes in organic semiconductor crystal, the number of the induced carriers is high compared with the initial carriers in organic semiconductor crystal. Such a dramatic relative 5 ACS Paragon Plus Environment


carrier change therefore results in ultrahigh-sensitivity detection towards external charged objects. To demonstrate the proximity-sensing capability of our fabricated flexible rubrene singlecrystal device, different stimuli were applied to simulate the real external environment by approaching and moving away from the device. It is found that the sensors do not response towards grounded metallic objects like stainless steel ruler, AFM conductive tip and Au film, but presents the dramatic current change towards charged objects like human finger, fiber, hair, paper et al., which further confirms our proposed sensing mechanism mentioned above. As a typical example, Figure 2 shows the sensing performance of sensor by using a human finger as stimuli. Figure 2a is the schematic image of experimental process. In order to exclude the effect of illumination in measurement process, the electrical signal of the sensor was measured under white light irradiation with different light power. As shown in Figure 2b, the rubrene sensors show the high light stability, which further confirms that the following current signal change originates from the charge induced gate effect rather than photocurrent. Figure 2c is the dynamic switch of the sensor by the approaching and moving away the finger. When the finger came close to the device, the distance between the finger and device was controlled at ~1 cm. The current of the rubrene single-crystal device shows the obvious increase with the proximity of the finger towards the device. On the contrary, the current rapidly decreases after moving the finger away from the device. During 70 continuous cycles, the device shows the outstanding reversibility and stability with fast response and recovery (Video S1). Here, the switch of current (ΔI = I - I0, denoting the absolute current change) depends on the switch of the distance between the finger and device, where I and I0 denote the current of the crystal with and without proximity stimuli, respectively. When the finger moves away from the device, the device restores its original current value. The same interesting phenomenon also can be observed in other objects. For example, when the fiber as external 6 ACS Paragon Plus Environment

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stimuli approaches the device, our device can perceive it and shows a significant current change (Figure S4). These results indicate that our fabricated flexible single-crystal device can be used as proximity sensor to perceive the objects to move away and closer, suggesting the strong potential for human-machine interaction and electronic skin. Further, an AFM probe tip with normal radius of 2 nm was used as external stimuli (Figure S5). The schematic image and the corresponding optical microscopy image of the measured device are illustrated in Figure 3a and the inset of Figure 3a, respectively. The measurements were carried out in scanasyst-air mode, and the AFM tip was controlled between “Engage” and “Withdraw” commands. The “Engage” command brings the tip to almost contact with the crystal surface, and the “Withdraw” command makes the tip move away from the crystal surface. In order to exclude the effect of red light illuminated on AFM cantilever, the light stability of the sensor was shown in Figure S6. Figure 3b is the single switch from “Withdraw” to “Engage”, which shows that the current remains stable at a fixed distance d. This result is different from the previously reported approaching-leaving switch of the capacitor-type triboelectric nanogenerators where the transient current was produced in contact-separation process due to charge transfer.35 The real-time current response to the probe tip engaging and withdrawing (ΔI = I - I0) for 10 cycles is shown in Figure 3c, where the distance of the tip between “Engage” and “Withdraw” states was controlled at 1 mm (by setting the SPM parameter “Sample clearance” at 1 mm). Here, I and I0 denote the current of the crystal in “Withdraw” and “Engage” state, respectively. The current increases fast and accurately when the probe tip approaches to the crystal, and fast decreases when the probe tip is removed (Video S2 and Video S3). By cyclically controlling the tip between “Engage” and “Withdraw” states, the device current demonstrates the impressive repeatability. No degradation is observed after multiple cycles and continuous work. Further, to better understand the proximity-sensing performance of our sensor, Figure 3d gives the real-time 7 ACS Paragon Plus Environment


current response to the dynamic switches with the increased tip distance between “Engage” and “Withdraw” d from 0.2 to 1 mm with a step of 0.05 mm. The current changes obviously with the changed d, with excellent response and quick relaxation times. With the increase of d, the distance between tip and crystal surface in “Withdraw” state increases, resulting in the increased current change. Our sensors have the capability to detect the approached microobjects with a distance change of 0.05 mm, which is the control limit of AFM equipment, and such a height resolution value is two orders of magnitude higher than the previous report results in conventional capacitor-type proximity-type sensors,10-12 showing the ultra-high sensitivity and resolution of our rubrene single-crystal sensor. For proximity sensors, keeping accurate location and distance detection is crucial in safety precaution, and noninvasive medical diagnostics and therapy.36,37 Previous reports on capacitor proximity sensors only gave the experimentally measured plot of capacitance versus distance,10-12,16,18 which is used as the pre-measured criterion curve to realize location detection of the same object by comparing the capacitance values. Obviously, the capacitance response is dependent on the stimuli. Therefore, based on this method, the location detection requires the pre-measurement process of each stimuli, which makes the measurement process complex. Therefore, it is requisite to find a simpler method to realize the accurate location detection of the object. Here, we propose an effective method to realize the location detection of unknown-charge objects in proximity sensors, by changing the distance between object and sensor. In contrast, in our experiments, no pre-measurement process for each stimuli is required because the distance can be calculated by an equation unrelated with stimuli charges. As shown in Figure 1b, according to the sensing mechanism, it can be easily observed that the current of device is determined by the charges of the stimuli Q and distance between device and stimuli d. We define the current of the device I  y(Q)  y(d) . According to this equation,

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for the stimuli with the fixed charges, when the stimuli moves from d0 to d (as shown in the schematic measurement process of Figure 4a), we obtain I0 y(Q)  y(d0 ) y(d0 ) y(d  Δd) = = = , I y(Q)  y(d) y(d) y(d)

(1)

where I0 and I is the current values of the organic device before and after movement, Δd is the movement distance. Eq. (1) shows that the ratio of I0 to I is not related to charges. Further, our experimental results well confirm this point. As shown in Figure 4b1,b2,c1,c2, we applied the two objects with different charges to respectively obtain the relation of I0, I, d and Δd. Here, polydimethylsiloxane (PDMS) slices were used as stimuli. The unintentional charges in PDMS slice are confirmed by the scanning kelvin probe microscopy (SKPM), which presents the uniform surface potential of PDMS slice at –1.4 V (Figure S7). The changed charges in PDMS slice are controlled by the size of PDMS slice, which is similar to previous report.38 For the two PDMS slices with different charges, the real-time current response to the dynamic switches with the increased distance between sensor and stimuli was respectively shown in Figure 4b1 and 4c1, and the experiment results of I as a function of d were given as scatter data in Figure 4b2 and 4c2. By fitting the experiment data, the obtained empirical functions confirm I∝e-0.9d+15, as shown in Figure 4b2 and 4c2. In this case, -0.9(d  Δd) I0 e  15 = . - 0.9d I e  15

(2)

Eq.(2) suggests that the distance between sensor and stimuli d is independent on stimuli charges. When a stimuli with unknown charges approaches the sensor, by moving the stimuli with established Δd, and by recording the output current values before and after moving I0 and I, the distance between sensor and stimuli d can be calculated according to Eq.(2).



To confirm the proposed method, a new stimuli (plastic ruler) with unknown charges is used for the detection of the distance between sensor and stimuli d, as shown in Figure 5a. When moving the plastic ruler at different position, the current values before and after moving (I0, I) were respectively recorded, and the moved distance Δd was also recorded, as shown in Figure 5b and Table S1. According to Eq.(2), Figure 5c gives the theoretically calculated d based on Δd, I0 and I (details see Table S1). For comparison, the experimentally measured d is also shown. It can be observed that the theoretically calculated d values are in good agreement with our measured values. For example, the initial current I0 = 0.43934 µA. When Δd = 2 mm, I = 0.4397 µA and the calculated d = 4.7 mm, which is similar to the measured result (5 mm). These results powerfully confirm our sensors can directly realize the detection of the distance between stimuli and sensor eventhough the charges of the stimuli is unknown. For example, here, the charges on plastic ruler is unknown, and the pre-measured criterion curve of the sensor towards plastic ruler need not be proceeded. Today, the development of modern smart products requires more and more accuracy and the tendency moves towards smaller components. However, there is still a lack of tools for detection of extremely tiny object which requires high accuracy to be manipulated or assembled.39,40 In our experiments, it is found our device is very favorable for the detection of tiny object and small distance, for example, the unintentionally charged AFM tip with the distance resolution of 0.05 mm, and the unintentionally charged PDMS slice with 1.6 mm×1.6 mm (as shown in Figure 4b1) in the maximum distance of 9 mm with the distance resolution of 0.5 mm. The outstanding advantages of our two-terminal organic sensors in high precision and high distance resolution show a big application potential in high precision robots for automated handing of microobject. It should be mentioned that the distance detection is independent on charges while the detection distance range is dependent on the charge quantity of object, since the current response of the device originates from the charged object induced electric field as gate


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electrode. Therefore, the detection distance limit can be broaden with the increased charge quantity of objects. 4. CONCLUSION In conclusion, for the first time, a flexible rubrene crystal was employed as the sensing element, to realize the sensor with proximity sensing capabilities. This new-type facile twoterminal sensor utilizes the ultra-low carrier concentration of organic semiconductor to amplify the weak output signal changes, and therefore can perceive extremely tiny charge quantity, so that those unintentionally electrified objects, such as hair, fiber, human finger, and even AFM tip can be easily detected. The AFM tip can be detected with the height resolution of 0.05 mm, which is two orders of magnitude higher than that of previously extensively reported capacitor proximity sensors. And we give a resolution to realize the accurate location of the unknown charged object. Our discovery opens a new route to realize ultra-sensitive perception of objects, making our sensors promising candidates for applications in artificial intelligence and healthcare systems. ASSOCIATED CONTENT Supporting Information. Materials and Methods, electrical signal measurement (Figure S1 and Figure S2), electrical properties of the flexible organic sensor before and after repeated bending (Figure S3), dynamic current response of the proximity sensor to the fiber cloth approaching and moving away from the device (Figure S4),basic parameters of the applied AFM probe in our experiments (Figure S5), current response of the flexible rubrene proximity sensor to red light (Figure S6), surface potential of PDMS slice measured by the scanning kelvin probe microscopy (Figure S7), dynamic current response of the proximity sensor to human finger approaching and moving away (Video S1), dynamic current response of the sensor to AFM probe tip under “Engage” and “Withdraw” states (Video S2 and Video S3). 11 ACS Paragon Plus Environment


Corresponding Author *E-mail address: [email protected]; [email protected] ACKNOWLEDGEMENT This work is supported by NSFC (51322305, 61574032, 61376074, 91233204), 111 Project (B13013). REFERENCES (1)

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Codourey A.; Zesch W.; Büchi R.; Siegwart R. High Precision Robots for Automated Handling of Micro Objects. Seminar on Handling and Assembly of Microparts, 1994.

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Fatikow S., Wich T.; Krohs F.; Dahmen C. Towards automated handling on the nanoscale. International Conference on Intelligent Robots and Systems, 2008, 22-26.


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Page 17 of 22

Object

a

b Au

Au

Rubrene

Rubrene crystal

proximity

Electrode Supporting layer

Supporting layer Voltage

c

Voltage

e

d

0.8 nm 120

50 μm

0.7

60 80 μm

60

60

40

40 20

50 μm

50 μm

20

5 μm

80 μm

I (A)

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


0.6 0.5 0.4 0

50

100 150 Bend times

200

Figure 1. (a) Schematic illustration image and optical microscopy image of the flexible twoterminal proximity sensor based on a micro-sized rubrene single crystal. Scale bar: 50 µm. (b) Sensing mechanism of the organic semiconductor-based sensor: Initial state, proximity state. (c,d) SEM and AFM images of a typical rubrene single crystal. SEM images show the good flexibility of the rubrene crystals. AFM result indicates that the crystal surface is flat and the thickness of the crystal is only ~100 nm. (e) Current versus bending times curves at V = 10 V.



a

b

Dark 0.6 mW 5 mW 10 mW 25 mW 40 mW

I (A)

1.0

0.5

White light

0.0 0

c

2

4

V (V)

6

8

10

0.02

0.00 I (A)

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 18 of 22

0.02

Finger remove

Hold

-0.02

0.00

-0.04

Finger approach 190

0

100

195

200 Time (s)

200

300

Figure 2. Proximity-sensing performance of the flexible two-terminal sensor to a human finger. (a) Photograph of a finger approaching the device. (b) I-V curves of the sensor measured in the dark and under white light irradiation (light power from 0.6 mW to 40 mW) to exclude the light effect. (c) Dynamic current response (ΔI = I - I0) of the proximity sensor to human finger approaching and moving away for 70 cycles at a constant voltage of 10 V (see also Video S1). I and I0 denote the current of the crystal with and without proximity stimuli, respectively.


Page 19 of 22

AFM probe tip

a

b

AFM tip

I (A)

10.0

d Au

Au

Rubrene

Au

Au

Tip engage

Supporting layer

9.8 0

c -0.2

100 Time (s)

200

d = 1 mm Hold Tip engage

I (A)

-0.1 0.0

Tip withdraw 0.1 0

200

Time (s)

I (A)

d

400

600

-0.06 Hold Tip engage -0.03 0.00 d (mm)

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


Δd = 0.05 mm

Tip withdraw

1 0 0

200

Time (s)

400

Figure 3. Proximity-sensing performance of the two-terminal sensor to an AFM probe tip. (a) Schematic illustration and optical microscopy of the device under the AFM probe tip. (b) Current change from “withdraw” to “Engage” which shows the stable current at fixed distance. Insets are the schematic images of measure device. (c) Real-time current response (ΔI = I - I0) to the probe tip engaging and withdrawing for 10 cycles at the fixed distance d of 1 mm. d is the distance of the tip between “Engage” and “Withdraw” states. I and I0 denote the current of the crystal in “Withdraw” and “Engage” states, respectively. (d) Real-time current response to the dynamic switches with the increased d from 0.2 to 1 mm with a step of 0.05 mm (down yaxis). 19 ACS Paragon Plus Environment


a

b1

Stimuli

b2

0.462

2 mm

I0

I (A)

I (A)

0.459 3

0.456

4

6

Δd

0

20

40 60 Time (s)

2

80

c2

4

6 d (mm) -7

2 mm 2.5 3

0.456

Au 7

0.454

5

-0.9d

+15)

0.456

4

0.454

Fitting Experiment value

Supporting layer

0

8

0.458

I (A)

0.458

Rubrene

Fitting Experiment value

I = 0.29*10 (e

PDMS

Au

+15)

0.456

0.460

d

-0.9d

0.459

0.453

c1

I

I = 0.31*10 (e

8

0.453

d0

-7

0.462

PDMS

I (A)

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 20 of 22

50 Time (s)

100

2

4 d (mm)

6

Figure 4. Location detection of the object with unknown charges. (a) Schematic image showing the charged stimuli approaching the device from d0 to d. (b1, b2, c1, c2) Real-time current response to dynamic switches, and the current I versus d with the charged PDMS slices as stimuli with size of 1.6 mm×1.6 mm and 1.1 mm×1.1 mm, respectively. The red curves are the empirical fitting results of the experimental data. Insets are the photographs of PDMS slices.


Page 21 of 22

c

b

a

-0.9(d  Δd)

I0 e  15 = - 0.9d I e  15

0.450 I (A)

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


Plastic ruler

0.445

Theoretical value d (mm)

Measured value d (mm)

4.7

5

2.9

3

1.8

2

0.440

0

50 Time (s)

Figure 5. Experimental verification of location detection with a plastic ruler as stimuli. (a) Photograph of the measurement process with the plastic ruler. (b) Real-time current response to the changed distance. The inset shows the empirical equation obtained from Figure 4b2 and c2. (c) Theoretical and measured value d. The theoretical d values were calculated from I0, I, and Δd according to the empirical equation shown in the inset of Figure 5b.



Table of Contents Graphic AFM tip

Proximity sensor -60

Au

Rubrene

Au

Supporting layer

I (nA)

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

-30

0

0

200 400 Time (s)


Page 22 of 22

Ultrasensitive Flexible Proximity Sensor Based on Organic Crystal for

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