Supporting information
Conformal, Graphene-based Triboelectric Nanogenerator for Self-powered
Wearable Electronics
Hyenwoo Chu*, Houk Jang*, Yongjun Lee, Youngcheol Chae, Jong-Hyun Ahn**
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seoul,
03722, Republic of Korea
*H. Chu and H. Jang contributed equally to this work
**Corresponding author: [email protected]
Modeling the surface of human skin To investigate the critical thickness to achieve conformal TENGs on human skin with randomly curved surface, we carried out the quantitative analysis based on the previous reports, assuming the microscopic morphology of skin roughness as a sinusoidal form
(S1)
with skin roughness amplitude hrough ~110 m, wavelength rough ~600 m and young’s modulus Eskin ~130 kPa.
Calculation of bending stiffness The stiffness of conformal TENGs consisting of PDMS (1.5 m), graphene (0.3 nm) and polymer layers was calculated by following Eq. S2 and S3. In this equation graphene term can be neglected because graphene layer is extremely thin.
(S2)
(S3)
where z0 is the Neutral Mechanical Plane (NMP), t is the thickness, E is the Young’s modulus (EPDMS = 1.7 MPa, EPET = 2 GPa), b is the width of device and EI was the stiffness of device.
Calculation of total energy associated with interface We carried out calculation for the total energy associated with interface with an assumption the boundary condition whose skin deformation is negligible considering that skin deformation would cause the feeling of irritation to a user. Therefore, the total energy for conformal contact could be expressed as Uinterface = Ubending + Uadhesion, where the bending energy of conformal TENG is
(S4)
and the interfacial adhesion energy is
(S5)
where g was the effective work of adhesion between the TENGs and skin (0.25 N/m).
The conformal contact of TENGs occurs when magnitude of Uinterface is larger than Ubending. In the other word, when the Uinterface < 0, the conformal TENGs preferred to be deformed and adhere to the skin.
2
x2cos1h
)x(yrough
rough
2t
t2EtEt1t
z PET
PDMS
PETPET
PDMSPDMSPET
0
2
00PET2
PETPETPET2
00PDMS2
PDMSPDMSPDMS zztt31btEzztt
31btEEI
rough
roughroughCTENGCTENGrough
bending hEIdxyEIU
0
424)")(2(1
rough
roughroughadhesion hdxyU
gg0
2222 41)'(1
Operation mechanism of the conformal TENGs The conformal TENGs was operated in single-electrode manner by using the foreign object as a counter electrification material. The working principle of the conformal TENG is schematically shown in S3 and could be understood based on the previous reports.4
1) When the foreign object was contacted to the surface of the conformal TENG, the negative charges transferred from foreign object to fluorinated PDMS surface due to the highest electronegativity of fluorine atom.
2) In the releasing state positive charges were induced on the graphene electrode because of uncompensated negative charges on the fluorinated PDMS surface, driving a flow of electron from the graphene electrode to the ground, resulting in output voltage and current.
3) When the fluorinated surface perfectly released from the foreign object, fully free-electron cannot flow due to the screening effect of induced positive charge on graphene electrode terminating output voltage and current.
4) In the contacting state, the induced positive charge decrease since negative charge on fluorinated surface was neutralized by foreign object, resulting in the reversed output signal until full contact state.
Analysis of effective contact area of four kind of clothes When cloth contact to surface of the conformal TENG, the effective contact area was important parameter for the triboelectric performance. Thus, precise measurement of the effective contact area is necessary. For the reason, the clothes were soaked with ink and then imprinted on a paper. Effective contact area can be calculated by image processing of marked paper as shown in Fig. S11. The average and deviation of contact area of each fabric were shown in Table. S1.
Reducing pressure-induced noise by controlling surface roughness When we contact the fingers to the conformal TENGs, the contact pressure was generally measured in the range from 30 to 40 kPa. However, if we intentionally contact softly or strongly, the range of contact pressure was measured from 20 kPa to 100 kPa [51]. In this regime, the pressure-induced noise exceed the contact-area dependent signal, inducing malfunction of the self-powered communication devices.
To manage the malfunction induced by different contact pressure, we investigated the output signal of two TENGs with and without O2 plasma treatment (corresponding to RMS of 13.3 nm and 0.5 nm, respectively) as summarized in Fig. S15.
In case of O2 plasma treated TENGs, the contact of one finger at strong pressure over 40 kPa can generate higher VOC of (~8 V) than VOC with contact two fingers at soft pressure of 20 kPa (~5 V), indicating pressure-induced malfunction of communication devices. In contrast, in case of TENGs without O2 plasma treatment, VOC with contact of one finger at strong pressure (~4 V) cannot overcome the VOC with contact of two fingers at soft pressure (~5 V). Thus, a malfunction induced by environmental effect such as different contact pressure could be avoided by controlling the surface roughness of TENGs.
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Figure S1. SEM image of the conformal TENGs with stacking structure of PDMS, 1 layer graphene, PET substrate. Total thickness was ~2.4 m.
Figure S2. Schematic illustration of the fabrication process of the conformal TENGs with nanostructured and functionalized surface.
Figure S3. Schematic illustration that illustrate the operating principle of the conformal TENGs based on single electrode mode whose the PDMS surface contact to the foreign object.
Figure S4. Calculated bending stiffness of the previous reports using fluorinated polymer such as PTFE and FET(black circle). The red star indicates the bending stiffness of the conformal TENGs.
Figure S5. (A) Surface profile and (B) XPS spectra of the sample with different surface including I: as-prepared, II: nanostructured, III: functionalized and IV: nanostructured and functionalized surface
Figure S6. A comparison of the open circuit voltages of the conformal TENGs formed on a glass substrate and human skin.
Figure S7. Open circuit voltage of the conformal TENGs with various contact force from 20 kPa to 100 kPa.
Figure S8. Short circuit current and open circuit voltage of the conformal TENGs as function of (A) O2 plasma treatment time and (B) SF6 plasma treatment time
Figure S9. Roughness of the PDMS surface according to O2 plasma treatment time.
Figure S10. (A) Short circuit current and (B) open circuit voltage of the conformal TENGs generated with contact of the four different clothes.
Figure S11. Optical image of stamped paper by four kind of cloth with ink for investigation of contact area. The brighter area is the non-contacted region while dark is the contacted one.
Figure S12. Short circuit current and open circuit voltage of the conformal TENGs as function of (A) contact area and (B) ∆EA.
Figure S13. Photographs of (A) the conformal TENGs transferred onto forearm and (B) contact between intending for contact between cotton cloth and the conformal TENGs
Figure S14. 50 trials of contact with sequence of alphabet ‘L’, which is dot, dash, dot, dot. The success was represented with green and the failure was represented with red.
Figure S15. Open circuit voltage (VOC) measured with contact of different number of fingers and contact pressure from 20 kPa to 100 kPa on the conformal TENGs whose roughness is (A) 13.3 nm and (B) 0.5 nm, respectively
Nylon Silk Cotton Latex
Average 20.7% 33.3% 36.6% 97.5%
deviation ±2.85% ±2.73% ±2.54% ±0.7%
∆ EA 220 nC/J 200~215 nC/J 195 nC/J 85 nC/J
Table S1. Average and deviation of contact area when each cloth contacts to the conformal TENGs and ∆EA between each cloth and fluorinated surface.(Nylon : +30 nC/J, Silk : +10~+25 nC/J, Cotton : +5 nC/J, Latex : -105 nC/J, Teflon : -190 nC/J)
