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Flexible logic circuits based on top-gate thin film transistors with printed semiconductor carbon nanotubes and top electrodes† Weiwei Xu,‡a,b Zhen Liu,‡a Jianwen Zhao,*a Wenya Xu,a Weibing Gu,a Xiang Zhang,a Long Qiana and Zheng Cui*a In this report printed thin film transistors and logic circuits on flexible substrates are reported. The topgate thin film transistors were made of the sorted semiconducting single-walled carbon nanotubes (scSWCNTs) ink as channel material and printed silver lines as top electrodes and interconnect. 5 nm HfOx thin films pre-deposited on PET substrates by atomic layer deposition (ALD) act as the adhesion layers to significantly improve the immobilization efficiency of sc-SWCNTs and environmental stability. The immobilization mechanism was investigated in detail. The flexible partially-printed top-gate SWCNT TFTs display ambipolar characteristics with slightly strong p-type when using 50 nm HfOx thin films as dielectric layer, as well as the encapsulation layer by atomic layer deposition (ALD) at 120 °C. The hole mobility, on/ off ratio and subthreshold swing (SS) are ∼46.2 cm2 V−1 s−1, 105 and 109 mV per decade, respectively. Furthermore, partially-printed TFTs show small hysteresis, low operating voltage (2 V) and high stability in air.
Received 19th September 2014, Accepted 12th October 2014
Flexible partially-printed inverters show good performance with voltage gain up to 33 with 1.25 V supply voltage, and can work at 10 kHz. The frequency of flexible partially-printed five-stage ring oscillators can
DOI: 10.1039/c4nr05471g
reach 1.7 kHz at supply voltages of 2 V with per stage delay times of 58.8 μs. This work paves a way to
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achieve printed SWCNT advanced logic circuits and systems on flexible substrates.
Introduction Printable electronics have received significant attention in recent years because of their potential applications in electronics papers,1 solar cells,2 radio frequency identification tags (RFID),3–5 logic gates and circuits,6–24 information display,25–31 and sensors.32–34 With printed electronics technology, all these devices and applications can be prepared with large area, flexible and at low cost. Printed thin film transistors (TFTs) are at the heart of printed electronics. The TFT performance is greatly dependent on its channel semiconducting materials. Compared to organic semiconductors, semiconducting singlewalled carbon nanotubes (sc-SWCNTs) show considerably higher charge mobility, better physical and chemical stability, and process temperatures compatible with flexible suba Printable Electronics Research Centre, Suzhou Institute of Nanotech and nano-bionics, Chinese Academy of Sciences, No. 398 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu Province 215123, PR China. E-mail: [email protected], [email protected]; Fax: +86-512-62603079; Tel: +86-512-62872705 b School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, PR China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr05471g ‡ These authors contributed equally to this work.
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strates.31 Complicated logic circuits composed of carbon nanotube based transistors have been demonstrated.13–18,20–24,35–38 In contrast to TFTs made of individual carbon nanotube or CVD grown carbon nanotubes, solution processed SWCNT TFTs are relatively uniform in characteristics and the fabrication process is considerably simpler.39,40 Moreover, the solution process makes it possible to fabricate TFT arrays on large size and flexible substrates. Sc-SWCNT solutions have been developed as printable inks to form conducting channels in TFTs for low-cost, large-area fabrication of printable electronics. With the advances in purification techniques and printing processes, the electrical properties of partially-printed SWCNT TFTs, such as mobility and on/off ratio, have been greatly improved in the last few years. Furthermore, all-printed carbon nanotube TFTs have previously been successfully demonstrated on flexible substrates,41–43 however, there is still a challenge for further improvement in device performance. SWCNTs usually show p-type characteristics at ambient conditions because of the adsorption of oxygen and water constructing a logic circuits with unipolar transistors is not easy. These transistors should not only be spatially well-separated, but also their threshold voltages are required to be precisely tuned.35,44,45 The reported tuning methods were chemical doping (such as p-doping by F4TCNQ (tetrafluorotetracyano-pquinodimethane)) or control of the ratios of device channel
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length to width.35,44,45 However, these methods are difficult to implement in solution process. Several groups, such as Bao’s,8 Zhou’s,18,20 Hersam’s,23 Dodabalapur’s,24 Zhang’s,37 Peng’s,38 Lee’s46,47 and Frisbie’s groups48 were trying to develop some n-type SWCNT or metal oxide transistors such that CMOS logic gates can be constructed from both p-type and n-type TFTs. However, it is not easy to obtain air-stable n-type TFTs on flexible substrates at a relatively low temperature (less than 150 °C). Consequentially, there are few reports on printed or solution processed SWCNT CMOS circuits at low temperature on flexible substrates.9,14,16,17 Recently, the ambipolar transistors have attracted significant attention, which can have both p-type and n-type transistors characteristics in a single transistor.49 Frisbie’s group reported high-performance logic circuits with ambipolar transistors.9,14,16,17 However, their circuits have to be encapsulated or in vacuum because the n-type transistors are sensitive to oxygen and water in ambient. Furthermore, to realize many foreseeable flexible printed circuit applications, flexible printed TFTs need to have several other characteristics, including high environmental stability, good uniformity, small hysteresis, high mobility, high on/off ratio, high operating frequency and low operating voltages (less than 3 V) compatible with thin film batteries.14 In this paper, we report partially-printed high-performance top-gate ambipolar SWCNT TFTs, inverters, NOR and fivestage ring oscillators with only one-step lithography. Poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1-3-thiadiazole)] (PFO-BT)-sorted sc-SWCNT solutions were used as printed semiconductor inks, and printed silver lines were used as top electrodes and interconnects. Large-area printed top-gate SWCNT TFT arrays (216 devices) and logic gate arrays, and ring oscillators were fabricated on flexible PET substrates with predeposited 5 nm HfOx. Printed devices showed low operating voltage (2 V) and high stability in air because of high-capacitance HfOx thin films as dielectric layer, as well as the encapsulation layer. Partially-printed TFTs showed good uniformity and excellent electrical properties with hole mobility and on/ off ratios. 94.2% of TFTs have their mobility in the ranges of 10–50 cm2 V−1 s−1, and the on/off ratios of 92.6% of devices are up to 104 at Vdd = 0.1 V with low subthreshold swing (SS) (100–117) and small hysteresis. Then, these printed TFTs were interconnected to form 108 inverters by printed silver lines. Printed inverters worked well at 1 kHz, and the voltage gain was more than 20 at Vdd = 1 V. The frequency of the partiallyprinted five-stage ring oscillators reached to 1.7 kHz at supply voltages of 2 V with per stage delay times of 58.8 μs. Partiallyprinted NOR logic gates were also demonstrated.
Experimental section Materials and instruments Arc discharge SWCNTs were purchased from Carbon Solution (USA). Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1-3thiadiazole)] (PFO-BT) was purchased from Shenzhen (China)
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Derthon Optoelectronic Materials Science & Technology. Conducting nanosilver inks were purchased from Beijing Institute of Graphic Communication. All the products were directly used without further purifications. Optical absorption measurements were performed in a Perkin Elmer Lambda 750 UV-Vis-NIR spectrometer. All the electrical measurements were carried out at ambient condition using a Keithley semiconductor parameter analyzer (model 4200-SCS). A NSCRIPTOR DPN system (NanoInk Inc., IL, USA) and Dimension 3100 AFM (Veeco, Santa Clara, CA) were used in AFM imaging. Sorted scSWCNT solutions and nanosilver inks were printed by an aerosol Jet 300P system (Optomec Inc., USA). X-ray photoelectron spectroscopy (XPS) analysis used Al Kα radiation in an ESCA Lab 220I-XL. Preparation of sc-SWCNT inks To obtain printable sc-SWCNT inks, a certain amounts of arc discharge SWCNTs were dispersed in 20 mL xylene with the aid of PFO-BT via probe-ultrasonication for 30 min (Sonics & Materials Inc., Model: VCX 130, 60 W). Then, the resulting SWCNT solutions were centrifuged at 21 000g for 1 h to remove metallic species and big bundles, and the supernatant was drawn out from the centrifuge tube and used to print SWCNT TFTs without any other purification. Fabrication and electrical properties of printed SWCNT TFT arrays and printed logic gates and circuits on PET substrates The preparation process of printed sc-SWCNTs inks was described in our previous paper.19 To effectively immobilize sc-SWCNTs onto the PET surfaces, 5 nm thickness HfOx adhesion layers were first deposited on the PET substrates at 120 °C by atomic layer deposition (ALD) (Cambridge NanoTech Inc.), and then pre-patterned interdigitated gold electrode arrays were fabricated on the HfOx modification of PET substrates by photolithography (the channel width, length and interfinger space are 200, 20 and 20 μm, respectively.). After that, the devices were treated by oxygen plasma (100 W) for 1 min, and then the sc-SWCNT solutions were printed or dropped onto the device channels, followed by washing 3 times with xylene. The printing procedure was repeated only 2 times, and the density of the sorted sc-SWCNTs was high enough to form a percolation path and to reach the desired current level. After that, the devices were dipped into xylene for 10 min to further remove the residual polymers. 50 nm thick HfOx thin films were deposited on top of the predeposited SWCNT thin films at 120 °C by ALD. Then, silver top-gate electrodes were printed on the top of the gold electrodes by aerosol jet printing. The holes in probing pads were created in HfOx thin films using the probe needle. Finally, printed top-gate TFTs were integrated to inverter arrays, NOR and ring oscillators with printed silver lines. All the devices were used under ambient atmosphere. The mobility of the printed top-gate SWCNT TFTs were estimated dI d L 1 19,25 by the equation μ ¼ . Here, Ci is the W Ci V ds dV g 50 nm HfOx capacitance per unit area, ∼0.196 μF cm−2
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(as shown in Fig. S3†), L and W represent the channel length and width, respectively. The input signal of inverters was generated by Agilent 33220 arbitrary-waveform generators. The dynamic responses of inverters and ring oscillators were performed by Tektronix MSO 2024 mixed signal oscilloscope.
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Results and discussion It has been demonstrated that sc-SWCNTs can be selectively separated by PFO-BT in xylene when the weight ratio of SWCNTs to PFO-BT is 2 : 9.19 However, it is found that selectivity cannot be observed when using different batches of PFO-BT from Shenzhen Derthon Optoelectronic Materials Science & Technology in the same weight ratio of SWCNTs to PFO-BT (2 : 9). This can probably be attributed to that different batches of PFO-BT, which have different molecular weight (Mw), resulting in different separation efficiency. To achieve sorted sc-SWCNTs, the relationships of selectivity and the weight ratio of SWCNTs to PFO-BT were studied. Fig. 1 is the absorption spectra of arc discharge SWCNTs with different weight ratios of SWCNTs to PFO-BT in xylene after centrifugation with 21 000g for 1 h. As shown in Fig. 1, the metallic peaks and high background absorption could be observed when the ratios of SWCNTs to PFO-BT were less than 4 : 12. However, the metallic peaks disappeared and background absorptions were reduced when the ratios of SWCNTs to PFO-BT were more than 4 : 8. The peak height was only 0.05 when the ratio was 4 : 5. Thus, the ratio of 4 : 8 (4 mg SWCNTs and 8 mg PFO-BT in 20 mL xylene) was chosen in the following experiments. The supernatant was used to directly fabricate TFTs without any other purification. High-density and homogeneous sc-SWCNT thin films exhibit higher performance.19,37,50–54 To increase the density of SWCNTs in TFT channels, the substrates are usually modified with aminopropyltriethoxysilane (APTES), or poly-L-lysine, 4-(N-hydroxycarboxamido)-1-methypyridiniu iodide (NMPI), or annealed in vacuum or treated with oxygen plasma.37,50–54 ScSWCNTs in organic solvents cannot be effectively immobilized on amine-functionalized substrates.19 However, the electrical
Fig. 1 Adsorption spectra of arc discharge SWCNTs with different weight ratios of SWCNTs to PFO-BT in xylene after centrifugation.
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Fig. 2 AFM images of SWCNT thin films in device channels on PET substrates modified (a) without and (b) with 5 nm HfOx thin films. (White scale bar is 1 µm, and the insets in Fig. 2 are the heights of SWCNTs.)
properties of transistors are significantly improved after the substrates were treated with oxygen plasma.19 Unfortunately, it cannot help to evidently increase the CNT network density on PET substrates, probably due to the quick hydrophobic recovery of PET surfaces.55 Fig. 2 shows the AFM images of SWCNT in the device channel on bare PET substrates treated with oxygen plasma. It was determined that only 3–5 SWCNTs per μm2 were observed on bare PET substrates. To obtain highdensity SWCNT thin films in the flexible device channel, a thin layer (thickness ∼5 nm) of HfOx by ALD at 120 °C was used as an adhesive layer for immobilizing sc-SWCNTs. Fig. 2b displays the typical AFM images of the SWCNT thin film on PET substrates modified with 5 nm HfOx. As shown in Fig. 2, the diameters of sorted sc-SWCNTs were ∼2 nm, suggesting that SWCNTs in the channel were individuals and few small SWCNT bundles. As evidenced, PET modified with 5 nm HfOx resulted in a higher density of SWCNTs, estimated to be about 36 SWCNTs per μm2. To better understand the immobilization mechanism, the X-ray photoelectron spectra (XPS) analysis of HfOx thin films before and after immobilizing sc-SWCNTs was performed. Fig. 3 was the O 1s, Hf4f and N 1s XPS spectra of the untreated HfOx and oxygen plasma treated HfOx with and without scSWCNTs. As shown in Fig. 3a, the O 1s peaks shifted from 532.81 to 532.39 eV after oxygen plasma treatment for 1 min at 100 W, at the same time, the height of the O 1s peaks obviously increased after oxygen plasma treatment, which was attributed to the oxygen vacancies occupied by the additional oxygen atoms in the course of oxygen plasma treatment. After immobilization of sc-SWCNTs on substrates, the O 1s peak positions had no obvious changes. It demonstrated that there are no strong interactions between oxygen and PFO-BT wrapped sc-SWCNTs, which is different from our previous report.19 Fig. 3b shows the typical Hf4f XPS spectra of HfOx with different modifications. As shown in Fig. 3b, the Hf 4f7/2 peak positions at 17.75 eV shifted to the lower binding energy levels at 17.52 eV due to the incorporation of oxygen atoms at HfOx surfaces after oxygen plasma treatment.56 The binding energy of Hf4f 7/2 shifted to the lower binding energy from 17.52 to 17.25 eV when sc-SWCNTs were immobilized on HfOx surfaces. The shift of the binding energy indicated that
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Fig. 3 The (a) O 1s, (b) Hf4f and (c) N 1s XPS spectra of HfOx-modified PET substrates with different modifications. (1) No treatment, (2) oxygen plasma treatment and (3) the immobilization of PFO-BT sorted sc-SWCNTs.
Fig. 4 Typical electrical properties of printed top-gate SWCNT TFTs on PET substrates. (a) Transfer characteristics and (b) output characteristics. The shade area of AB in Fig. 4a represents the subthreshold region.
hafnium atoms in HfOx thin films have strong interaction force with sc-SWCNTs. As shown in Fig. 3c, a strong N1s peak was observed. As shown in Fig. 3c, the N 1s peak was observed in the device channels after the deposition of PFO-BT sorted sc-SWCNTs. Hafnium with unoccupied orbits and electronrich nitrogen atom in 1,4-benzo-2,1-3-thiadiazole can form the strong Hf-N coordination bonding, resulting in the highdensity sc-SWCNTs on HfOx-modified PET substrates.57 The electrical properties of printed top-gate TFTs were measured using Keithley 4200 at ambient condition, as shown in Fig. 4a and 4b, which exhibit ambipolar characteristics, indicating that both electrons and holes inject from source and drain electrodes. The hole and electron effective mobility of the printed TFTs were 46.2 and 33.2 cm2 V−1 s−1, respectively. The on/off ratio was about 2 × 105 at Vdd = 0.1 V, and the values of SS were 117, 101, 84 and 80 mV per decade for p-type and n-type TFTs, which were less than those of printed TFTs using ion-gel gate dielectrics.17 The off currents were at the level of 10−10–10−11 A, which is 2–3 orders of magnitude lower than those of printed ion gel-gate SWCNT TFTs.17 In addition, the printed devices show low operating voltages (2 V) and gate leakage currents (Fig. S1a†). It was attributed to the HfOx thin film, which was compact and homogeneous, and provides large capacitance (about 0.196 μF cm−2. Fig. S1b†), resulting in low power dissipation, low operating voltage and small hysteresis. Such performances enable the construction of logic circuits based on the printed SWCNT TFTs. TFT arrays (216 devices), inverter arrays (108 devices) and logic circuits were fabricated by printed SWCNT as the channel material and printed silver lines as top electrodes and
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Fig. 5 The optical image of printed top-gate SWCNT TFT and inverter arrays, and logic circuits on the PET substrate. The right picture represents the enlarged images of printed top-gate TFTs, inverters and a five-stage ring oscillator. S, D and G are source, drain and gate electrodes, respectively. T1 and T2 represent transistor 1 and 2, respectively.
interconnects. Fig. 5 shows the optical images of printed topgate TFT, logic gate and circuit arrays on the PET substrate. Their measured electrical properties are shown in Fig. 6, including the histogram of hole mobility and on/off ratios. It was noted that 81% of TFTs have their mobility in the ranges of 10–40 cm2 V−1 s−1 and 191 TFTs have the on/off ratio in the ranges of 104–107. The performance variation of these TFTs was attributed to the uniformity of sc-SWCNT thin films in the device channels. Because sc-SWCNTs in outermost TFT channels were more easily washed away when rinsing with xylene, the mobility of the outermost TFTs generally was slightly lower than those of other devices. These TFTs maintained their
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Fig. 6 Transfer characteristic curves of the same printing batch of 216 devices on PET substrates with (a) linear scale and (b) log scale, and histogram of (c) hole mobilities and d) on–off ratios of 216 printed topgate TFTs.
on/off ratios, mobilities, hysteresis and threshold voltages after exposed in air for 2 months (Fig. S2†) because of the 50 nm HfOx thin film, which acted as an encapsulation layer. The performances of printed inverters, NOR, and ring oscillators on PET substrates were studied. Inverters composed of two top-gate TFTs were fabricated on PET substrates using printed silver lines. Fig. 7a is the schematic and the circuit diagram of the printed inverter, which consists of two printed top-gate TFTs in series. Fig. 7b and 7c are the voltage input– output characteristics of the inverter, which exhibited a maximum voltage gain of 9, 18, 24 and 33 at Vdd = 0.5, 0.75, 1 and 1.25 V, respectively. It can be seen from Fig. 7c, the printed inverters showed small hysteresis, which has a close relationship with the hysteresis of the printed TFTs (Fig. 6a
Fig. 7 (a) Schematic of a printed inverter on HfOx/PET substrates (top), T1 and T2 represent printed transistor 1 and transistor 2, respectively, and the symbol of the inverter (bottom), (b) the voltage input–output, and (c) gain characteristics and d) the power consumption of a printed inverter at Vdd = 0.5, 0.75, 1 and 1.25 V.
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Fig. 8 The Input and output characteristics of printed inverters on PET substrates at (a)10, (b)100, (c)1 k and (d)10 kHz. (Vdd = 2 V).
and b).14 It would be beneficial to minimize the inverter hysteresis when using small hysteresis n-type and p-type TFTs to construct CMOS inverters. Fig. 7d shows the static power consumption of the printed inverter at Vdd = 0.5, 0.75, 1 and 1.25 V. The static power consumption reaches 2.5 μW when the input voltage is low at Vdd = 1.25 V, which is higher than that of CMOS inverters.23,24 Furthermore, the electrical properties of printed inverters were measured after repeatedly bending them 100 times. As shown in the following Fig. S4,† the voltage input–output, gain characteristics and the power consumption of the printed inverter have no obvious changes. The dynamic responses of the inverters are shown in Fig. 8. It can be seen from Fig. 8 that the inverter can work properly in the kilohertz ranges. i.e., output curves were totally opposite to the input curves with little voltage loss and 60 μs delay time at 1 kHz. It was noted that the inverters can still work at 10 kHz (Fig. 8d). The switching time of the printed devices were dependent on the SWCNT density, the channel length, the parasitic capacitance of devices and the conductivity of interconnects.14 Printed NOR logic gates were also demonstrated on PET substrates. A NOR gate consists of two TFTs in parallel and two TFTs in serials. As shown in Fig. 9, the output voltages of 0 V were observed when one input voltage or both input voltages were set to 2 V. When VA and VB are 0 V, output voltages were 1.5 V at Vdd = 2 V. In summary, flexible printed NOR logic gates showed the capabilities to integrate into complicated flexible logic circuits. To demonstrate the ability of the flexible printed inverters in driving subsequent printed inverters, six printed inverters were integrated to a five-stage ring oscillator using printed silver lines. Fig. 10a and b represent the image and circuit diagram of a printed oscillator on the PET substrate. Fig. 10c and the video in ESI† show the output characteristics of a printed ring oscillator on the PET substrate. As shown in Fig. 10c and the video in ESI.† The oscillating frequency of the devices were measured to be 1.7 kHz at Vdd = 2 V. The delay time can be calculated using f = 1/(2tpN), where f, tp and N are
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high air-stability and excellent electrical properties with high on/off ratios (up to 105), high mobility (up to 46.2 cm2 V−1 s−1), low SS (109 mV per decade), low operation voltage and small hysteresis. Inverters, NOR and five-stage ring oscillators based on flexible printed top-gate TFTs have also been demonstrated. Printed inverters showed the maximum voltage gains up to 33 at Vdd = 1.25 V. The frequency of printed five-stage oscillators can be up to 1.7 kHz at Vdd = 2 V with a delay time of 58.8 μs per stage. This work provides a valid pathway to fabricate better performance printed circuits with large area and stability on flexible substrates.
Acknowledgements Fig. 9 (a) An optical image and (b) a circuit diagram of printed NOR, and (c) the output characteristics. The operation voltage is Vdd = 2 V, and two input voltages (VA and VB) are 2 V. Input voltages of 0 and 2 V are regarded as logic “0” and “1”, respectively. T1, T2, T3 and T4 represent transistor 1, 2, 3 and 4. S, D and G represent source (Au), drain (Au) and gate ( printed Ag) electrodes, respectively.
This work was supported by Natural Science Foundation of China (91123034, 61102046), Strategic Priority Research Program of the Chinese Academy of Science (XDA09020201), and Project supported by National Science and Technology Ministry (2012BAF13B05-402), the Knowledge Innovation Programme of the Chinese Academy of Sciences (KJCX2-EW-M02) and Basic Research Programme of Jiangsu Province (BK2011364).
References
Fig. 10 Five-stage printed SWCNT ring oscillator with output buffer. (a) Photograph of a printed oscillator on PET substrate, (b) circuit diagram of the printed oscillator, (c) the output characteristics of printed oscillator on PET substrates with 1.7 kHz oscillator frequency at Vdd = 2 V.
the oscillating frequency, delay time per stage and the number of stages.17 The average tp about 58.8 μs per stage can be obtained from the above equation. As the frequency of the ring oscillator greatly depends on the RC time constant of SWCNT TFTs and their connection lines,17 the frequency of flexible printed ring oscillator can be further enhanced by increasing the SWCNT density, scaling down the channel length, decreasing the parasitic capacitance and increasing the conductivity of the printed silver lines. This work is in progress.
Summary In conclusion, we have developed a reliable method to print large-area top-gate SWCNT TFT arrays on PET substrates. Predeposited 5 nm HfOx thin films on PET substrates served as good adhesive layers to greatly increase the density of SWCNT thin film. 50 nm HfOx thin films deposited on top of SWCNT thin films were used as not only dielectric layers but also as the encapsulation layer, and printed top-gate TFTs showed
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12 T. N. Ng, D. E. Schwartz, L. L. Lavery, G. L. Whiting, B. Russo, B. Krusor, J. Veres, P. Bröms, L. Herlogsson, N. Alam, O. Hagel, J. Nilsson and C. Karlsson, Sci. Rep., 2012, 2, 585. 13 C. Wang, A. K. Takei, A. T. Takahashiab and A. Javey, Chem. Soc. Rev., 2013, 42, 2592. 14 M. J. Ha, J. W. T. Seo, P. L. Prabhumirashi, W. Zhang, M. L. Geier, M. J. Renn, C. H. Kim, M. C. Hersam and C. D. Frisbie, Nano Lett., 2013, 13, 954. 15 P. H. Lau, K. Takei, C. Wang, Y. Ju, J. Kim, Z. B. Yu, T. Takahashi, G. Cho and A. Javey, Nano Lett., 2013, 13, 3864. 16 K. Hong, S. H. Kim, K. H. Lee and C. D. Frisbie, Adv. Mater., 2013, 25, 3413. 17 M. J. Ha, Y. Xia, A. A. Green, W. Zhang, M. J. Renn, C. H. Kim, M. C. Hersam and C. D. Frisbie, ACS Nano, 2010, 4(8), 4388. 18 J. Zhang, C. Wang, Y. Fu, Y. Che and C. Zhou, ACS Nano, 2011, 5(4), 3284. 19 Z. Liu, J. W. Zhao, W. Y. Xu, L. Qian, S. H. Nie and Z. Cui, ACS Appl. Mater. Interfaces, 2014, 6, 9997. 20 H. Chen, Y. Cao, J. Zhang and C. Zhou, Nat. Commun., 2014, 5, 4097. 21 J. Zhang, H. Gui, B. Liu, J. Liu and C. Zhou, Nano Res., 2013, 6(12), 906. 22 C. Wang, J. Zhang and C. Zhou, ACS Nano, 2010, 4(12), 7123. 23 M. L. Geier, P. L. Prabhumirashi, J. J. McMorrow, W. C. Xu, J. W. Seo, K. Everaerts, C. H. Kim, T. J. Marks and M. C. Hersam, Nano Lett., 2013, 13(10), 4810. 24 B. Kim, S. Jang, M. L. Geier, P. L. Prabhumirashi, M. C. Hersam and A. Dodabalapur, Nano Lett., 2014, 14(6), 3683. 25 W. Y. Xu, J. W. Zhao, L. Qian, X. Y. Han, L. Z. Wu, C. C. Wu, M. S. Song and Z. Cui, Nanoscale, 2014, 6, 1589. 26 F. N. Ishikawa, H. K. Chang, K. Ryu, P. C. Chen, A. Badmaev, L. G. D. Arco, G. Z. Shen and C. W. Zhou, ACS Nano, 2008, 3, 73. 27 J. Zhang, C. Wang and C. Zhou, ACS Nano, 2012, 6(8), 7412. 28 C. Wang, A. Badmaev, A. Jooyaie, M. Bao, K. L. Wang, K. Galatsis and C. Zhou, ACS Nano, 2011, 5(5), 4169. 29 J. Zhang, Y. Fu, C. Wang, P. Chen, Z. Liu, W. Wei, C. Wu, M. E. Thompson and C. Zhou, Nano Lett., 2011, 11(11), 4852. 30 P. Chen, Y. Fu, R. Aminirad, C. Wang, J. Zhang, K. Wang, K. Galatsis and C. Zhou, Nano Lett., 2011, 11(12), 5301. 31 T. Takahashi, Z. B. Yu, K. Chen, D. Kiriya, C. Wang, K. Takei, H. Shiraki, T. Chen, T. Ma and A. Javey, Nano Lett., 2013, 13, 5425. 32 K. Takei, T. Takahashi, J. C. Ho, H. A. G. Ko; Gillies, P. W. Leu, R. S. Fearing and A. Javey, Nat. Mater., 2010, 9, 821. 33 T. Takahashi, K. Takei, A. G. Gillies, R. S. Fearing and A. Javey, Nano Lett., 2011, 11, 5408. 34 http://www.idtechex.com/research/reports/printed-organicand-flexible-electronics-forecasts-players-and-opportunities2012-2022-000301.asp.
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Flexible logic circuits based on top-gate thin film transistors with printed semiconductor carbon nanotubes and top electrodes† Weiwei Xu,‡a,b Zhen Liu,‡a Jianwen Zhao,*a Wenya Xu,a Weibing Gu,a Xiang Zhang,a Long Qiana and Zheng Cui*a In this report printed thin film transistors and logic circuits on flexible substrates are reported. The topgate thin film transistors were made of the sorted semiconducting single-walled carbon nanotubes (scSWCNTs) ink as channel material and printed silver lines as top electrodes and interconnect. 5 nm HfOx thin films pre-deposited on PET substrates by atomic layer deposition (ALD) act as the adhesion layers to significantly improve the immobilization efficiency of sc-SWCNTs and environmental stability. The immobilization mechanism was investigated in detail. The flexible partially-printed top-gate SWCNT TFTs display ambipolar characteristics with slightly strong p-type when using 50 nm HfOx thin films as dielectric layer, as well as the encapsulation layer by atomic layer deposition (ALD) at 120 °C. The hole mobility, on/ off ratio and subthreshold swing (SS) are ∼46.2 cm2 V−1 s−1, 105 and 109 mV per decade, respectively. Furthermore, partially-printed TFTs show small hysteresis, low operating voltage (2 V) and high stability in air.
Received 19th September 2014, Accepted 12th October 2014
Flexible partially-printed inverters show good performance with voltage gain up to 33 with 1.25 V supply voltage, and can work at 10 kHz. The frequency of flexible partially-printed five-stage ring oscillators can
DOI: 10.1039/c4nr05471g
reach 1.7 kHz at supply voltages of 2 V with per stage delay times of 58.8 μs. This work paves a way to
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achieve printed SWCNT advanced logic circuits and systems on flexible substrates.
Introduction Printable electronics have received significant attention in recent years because of their potential applications in electronics papers,1 solar cells,2 radio frequency identification tags (RFID),3–5 logic gates and circuits,6–24 information display,25–31 and sensors.32–34 With printed electronics technology, all these devices and applications can be prepared with large area, flexible and at low cost. Printed thin film transistors (TFTs) are at the heart of printed electronics. The TFT performance is greatly dependent on its channel semiconducting materials. Compared to organic semiconductors, semiconducting singlewalled carbon nanotubes (sc-SWCNTs) show considerably higher charge mobility, better physical and chemical stability, and process temperatures compatible with flexible suba Printable Electronics Research Centre, Suzhou Institute of Nanotech and nano-bionics, Chinese Academy of Sciences, No. 398 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu Province 215123, PR China. E-mail: [email protected], [email protected]; Fax: +86-512-62603079; Tel: +86-512-62872705 b School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, PR China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr05471g ‡ These authors contributed equally to this work.
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strates.31 Complicated logic circuits composed of carbon nanotube based transistors have been demonstrated.13–18,20–24,35–38 In contrast to TFTs made of individual carbon nanotube or CVD grown carbon nanotubes, solution processed SWCNT TFTs are relatively uniform in characteristics and the fabrication process is considerably simpler.39,40 Moreover, the solution process makes it possible to fabricate TFT arrays on large size and flexible substrates. Sc-SWCNT solutions have been developed as printable inks to form conducting channels in TFTs for low-cost, large-area fabrication of printable electronics. With the advances in purification techniques and printing processes, the electrical properties of partially-printed SWCNT TFTs, such as mobility and on/off ratio, have been greatly improved in the last few years. Furthermore, all-printed carbon nanotube TFTs have previously been successfully demonstrated on flexible substrates,41–43 however, there is still a challenge for further improvement in device performance. SWCNTs usually show p-type characteristics at ambient conditions because of the adsorption of oxygen and water constructing a logic circuits with unipolar transistors is not easy. These transistors should not only be spatially well-separated, but also their threshold voltages are required to be precisely tuned.35,44,45 The reported tuning methods were chemical doping (such as p-doping by F4TCNQ (tetrafluorotetracyano-pquinodimethane)) or control of the ratios of device channel
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length to width.35,44,45 However, these methods are difficult to implement in solution process. Several groups, such as Bao’s,8 Zhou’s,18,20 Hersam’s,23 Dodabalapur’s,24 Zhang’s,37 Peng’s,38 Lee’s46,47 and Frisbie’s groups48 were trying to develop some n-type SWCNT or metal oxide transistors such that CMOS logic gates can be constructed from both p-type and n-type TFTs. However, it is not easy to obtain air-stable n-type TFTs on flexible substrates at a relatively low temperature (less than 150 °C). Consequentially, there are few reports on printed or solution processed SWCNT CMOS circuits at low temperature on flexible substrates.9,14,16,17 Recently, the ambipolar transistors have attracted significant attention, which can have both p-type and n-type transistors characteristics in a single transistor.49 Frisbie’s group reported high-performance logic circuits with ambipolar transistors.9,14,16,17 However, their circuits have to be encapsulated or in vacuum because the n-type transistors are sensitive to oxygen and water in ambient. Furthermore, to realize many foreseeable flexible printed circuit applications, flexible printed TFTs need to have several other characteristics, including high environmental stability, good uniformity, small hysteresis, high mobility, high on/off ratio, high operating frequency and low operating voltages (less than 3 V) compatible with thin film batteries.14 In this paper, we report partially-printed high-performance top-gate ambipolar SWCNT TFTs, inverters, NOR and fivestage ring oscillators with only one-step lithography. Poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1-3-thiadiazole)] (PFO-BT)-sorted sc-SWCNT solutions were used as printed semiconductor inks, and printed silver lines were used as top electrodes and interconnects. Large-area printed top-gate SWCNT TFT arrays (216 devices) and logic gate arrays, and ring oscillators were fabricated on flexible PET substrates with predeposited 5 nm HfOx. Printed devices showed low operating voltage (2 V) and high stability in air because of high-capacitance HfOx thin films as dielectric layer, as well as the encapsulation layer. Partially-printed TFTs showed good uniformity and excellent electrical properties with hole mobility and on/ off ratios. 94.2% of TFTs have their mobility in the ranges of 10–50 cm2 V−1 s−1, and the on/off ratios of 92.6% of devices are up to 104 at Vdd = 0.1 V with low subthreshold swing (SS) (100–117) and small hysteresis. Then, these printed TFTs were interconnected to form 108 inverters by printed silver lines. Printed inverters worked well at 1 kHz, and the voltage gain was more than 20 at Vdd = 1 V. The frequency of the partiallyprinted five-stage ring oscillators reached to 1.7 kHz at supply voltages of 2 V with per stage delay times of 58.8 μs. Partiallyprinted NOR logic gates were also demonstrated.
Experimental section Materials and instruments Arc discharge SWCNTs were purchased from Carbon Solution (USA). Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1-3thiadiazole)] (PFO-BT) was purchased from Shenzhen (China)
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Derthon Optoelectronic Materials Science & Technology. Conducting nanosilver inks were purchased from Beijing Institute of Graphic Communication. All the products were directly used without further purifications. Optical absorption measurements were performed in a Perkin Elmer Lambda 750 UV-Vis-NIR spectrometer. All the electrical measurements were carried out at ambient condition using a Keithley semiconductor parameter analyzer (model 4200-SCS). A NSCRIPTOR DPN system (NanoInk Inc., IL, USA) and Dimension 3100 AFM (Veeco, Santa Clara, CA) were used in AFM imaging. Sorted scSWCNT solutions and nanosilver inks were printed by an aerosol Jet 300P system (Optomec Inc., USA). X-ray photoelectron spectroscopy (XPS) analysis used Al Kα radiation in an ESCA Lab 220I-XL. Preparation of sc-SWCNT inks To obtain printable sc-SWCNT inks, a certain amounts of arc discharge SWCNTs were dispersed in 20 mL xylene with the aid of PFO-BT via probe-ultrasonication for 30 min (Sonics & Materials Inc., Model: VCX 130, 60 W). Then, the resulting SWCNT solutions were centrifuged at 21 000g for 1 h to remove metallic species and big bundles, and the supernatant was drawn out from the centrifuge tube and used to print SWCNT TFTs without any other purification. Fabrication and electrical properties of printed SWCNT TFT arrays and printed logic gates and circuits on PET substrates The preparation process of printed sc-SWCNTs inks was described in our previous paper.19 To effectively immobilize sc-SWCNTs onto the PET surfaces, 5 nm thickness HfOx adhesion layers were first deposited on the PET substrates at 120 °C by atomic layer deposition (ALD) (Cambridge NanoTech Inc.), and then pre-patterned interdigitated gold electrode arrays were fabricated on the HfOx modification of PET substrates by photolithography (the channel width, length and interfinger space are 200, 20 and 20 μm, respectively.). After that, the devices were treated by oxygen plasma (100 W) for 1 min, and then the sc-SWCNT solutions were printed or dropped onto the device channels, followed by washing 3 times with xylene. The printing procedure was repeated only 2 times, and the density of the sorted sc-SWCNTs was high enough to form a percolation path and to reach the desired current level. After that, the devices were dipped into xylene for 10 min to further remove the residual polymers. 50 nm thick HfOx thin films were deposited on top of the predeposited SWCNT thin films at 120 °C by ALD. Then, silver top-gate electrodes were printed on the top of the gold electrodes by aerosol jet printing. The holes in probing pads were created in HfOx thin films using the probe needle. Finally, printed top-gate TFTs were integrated to inverter arrays, NOR and ring oscillators with printed silver lines. All the devices were used under ambient atmosphere. The mobility of the printed top-gate SWCNT TFTs were estimated dI d L 1 19,25 by the equation μ ¼ . Here, Ci is the W Ci V ds dV g 50 nm HfOx capacitance per unit area, ∼0.196 μF cm−2
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(as shown in Fig. S3†), L and W represent the channel length and width, respectively. The input signal of inverters was generated by Agilent 33220 arbitrary-waveform generators. The dynamic responses of inverters and ring oscillators were performed by Tektronix MSO 2024 mixed signal oscilloscope.
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Results and discussion It has been demonstrated that sc-SWCNTs can be selectively separated by PFO-BT in xylene when the weight ratio of SWCNTs to PFO-BT is 2 : 9.19 However, it is found that selectivity cannot be observed when using different batches of PFO-BT from Shenzhen Derthon Optoelectronic Materials Science & Technology in the same weight ratio of SWCNTs to PFO-BT (2 : 9). This can probably be attributed to that different batches of PFO-BT, which have different molecular weight (Mw), resulting in different separation efficiency. To achieve sorted sc-SWCNTs, the relationships of selectivity and the weight ratio of SWCNTs to PFO-BT were studied. Fig. 1 is the absorption spectra of arc discharge SWCNTs with different weight ratios of SWCNTs to PFO-BT in xylene after centrifugation with 21 000g for 1 h. As shown in Fig. 1, the metallic peaks and high background absorption could be observed when the ratios of SWCNTs to PFO-BT were less than 4 : 12. However, the metallic peaks disappeared and background absorptions were reduced when the ratios of SWCNTs to PFO-BT were more than 4 : 8. The peak height was only 0.05 when the ratio was 4 : 5. Thus, the ratio of 4 : 8 (4 mg SWCNTs and 8 mg PFO-BT in 20 mL xylene) was chosen in the following experiments. The supernatant was used to directly fabricate TFTs without any other purification. High-density and homogeneous sc-SWCNT thin films exhibit higher performance.19,37,50–54 To increase the density of SWCNTs in TFT channels, the substrates are usually modified with aminopropyltriethoxysilane (APTES), or poly-L-lysine, 4-(N-hydroxycarboxamido)-1-methypyridiniu iodide (NMPI), or annealed in vacuum or treated with oxygen plasma.37,50–54 ScSWCNTs in organic solvents cannot be effectively immobilized on amine-functionalized substrates.19 However, the electrical
Fig. 1 Adsorption spectra of arc discharge SWCNTs with different weight ratios of SWCNTs to PFO-BT in xylene after centrifugation.
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Fig. 2 AFM images of SWCNT thin films in device channels on PET substrates modified (a) without and (b) with 5 nm HfOx thin films. (White scale bar is 1 µm, and the insets in Fig. 2 are the heights of SWCNTs.)
properties of transistors are significantly improved after the substrates were treated with oxygen plasma.19 Unfortunately, it cannot help to evidently increase the CNT network density on PET substrates, probably due to the quick hydrophobic recovery of PET surfaces.55 Fig. 2 shows the AFM images of SWCNT in the device channel on bare PET substrates treated with oxygen plasma. It was determined that only 3–5 SWCNTs per μm2 were observed on bare PET substrates. To obtain highdensity SWCNT thin films in the flexible device channel, a thin layer (thickness ∼5 nm) of HfOx by ALD at 120 °C was used as an adhesive layer for immobilizing sc-SWCNTs. Fig. 2b displays the typical AFM images of the SWCNT thin film on PET substrates modified with 5 nm HfOx. As shown in Fig. 2, the diameters of sorted sc-SWCNTs were ∼2 nm, suggesting that SWCNTs in the channel were individuals and few small SWCNT bundles. As evidenced, PET modified with 5 nm HfOx resulted in a higher density of SWCNTs, estimated to be about 36 SWCNTs per μm2. To better understand the immobilization mechanism, the X-ray photoelectron spectra (XPS) analysis of HfOx thin films before and after immobilizing sc-SWCNTs was performed. Fig. 3 was the O 1s, Hf4f and N 1s XPS spectra of the untreated HfOx and oxygen plasma treated HfOx with and without scSWCNTs. As shown in Fig. 3a, the O 1s peaks shifted from 532.81 to 532.39 eV after oxygen plasma treatment for 1 min at 100 W, at the same time, the height of the O 1s peaks obviously increased after oxygen plasma treatment, which was attributed to the oxygen vacancies occupied by the additional oxygen atoms in the course of oxygen plasma treatment. After immobilization of sc-SWCNTs on substrates, the O 1s peak positions had no obvious changes. It demonstrated that there are no strong interactions between oxygen and PFO-BT wrapped sc-SWCNTs, which is different from our previous report.19 Fig. 3b shows the typical Hf4f XPS spectra of HfOx with different modifications. As shown in Fig. 3b, the Hf 4f7/2 peak positions at 17.75 eV shifted to the lower binding energy levels at 17.52 eV due to the incorporation of oxygen atoms at HfOx surfaces after oxygen plasma treatment.56 The binding energy of Hf4f 7/2 shifted to the lower binding energy from 17.52 to 17.25 eV when sc-SWCNTs were immobilized on HfOx surfaces. The shift of the binding energy indicated that
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Fig. 3 The (a) O 1s, (b) Hf4f and (c) N 1s XPS spectra of HfOx-modified PET substrates with different modifications. (1) No treatment, (2) oxygen plasma treatment and (3) the immobilization of PFO-BT sorted sc-SWCNTs.
Fig. 4 Typical electrical properties of printed top-gate SWCNT TFTs on PET substrates. (a) Transfer characteristics and (b) output characteristics. The shade area of AB in Fig. 4a represents the subthreshold region.
hafnium atoms in HfOx thin films have strong interaction force with sc-SWCNTs. As shown in Fig. 3c, a strong N1s peak was observed. As shown in Fig. 3c, the N 1s peak was observed in the device channels after the deposition of PFO-BT sorted sc-SWCNTs. Hafnium with unoccupied orbits and electronrich nitrogen atom in 1,4-benzo-2,1-3-thiadiazole can form the strong Hf-N coordination bonding, resulting in the highdensity sc-SWCNTs on HfOx-modified PET substrates.57 The electrical properties of printed top-gate TFTs were measured using Keithley 4200 at ambient condition, as shown in Fig. 4a and 4b, which exhibit ambipolar characteristics, indicating that both electrons and holes inject from source and drain electrodes. The hole and electron effective mobility of the printed TFTs were 46.2 and 33.2 cm2 V−1 s−1, respectively. The on/off ratio was about 2 × 105 at Vdd = 0.1 V, and the values of SS were 117, 101, 84 and 80 mV per decade for p-type and n-type TFTs, which were less than those of printed TFTs using ion-gel gate dielectrics.17 The off currents were at the level of 10−10–10−11 A, which is 2–3 orders of magnitude lower than those of printed ion gel-gate SWCNT TFTs.17 In addition, the printed devices show low operating voltages (2 V) and gate leakage currents (Fig. S1a†). It was attributed to the HfOx thin film, which was compact and homogeneous, and provides large capacitance (about 0.196 μF cm−2. Fig. S1b†), resulting in low power dissipation, low operating voltage and small hysteresis. Such performances enable the construction of logic circuits based on the printed SWCNT TFTs. TFT arrays (216 devices), inverter arrays (108 devices) and logic circuits were fabricated by printed SWCNT as the channel material and printed silver lines as top electrodes and
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Fig. 5 The optical image of printed top-gate SWCNT TFT and inverter arrays, and logic circuits on the PET substrate. The right picture represents the enlarged images of printed top-gate TFTs, inverters and a five-stage ring oscillator. S, D and G are source, drain and gate electrodes, respectively. T1 and T2 represent transistor 1 and 2, respectively.
interconnects. Fig. 5 shows the optical images of printed topgate TFT, logic gate and circuit arrays on the PET substrate. Their measured electrical properties are shown in Fig. 6, including the histogram of hole mobility and on/off ratios. It was noted that 81% of TFTs have their mobility in the ranges of 10–40 cm2 V−1 s−1 and 191 TFTs have the on/off ratio in the ranges of 104–107. The performance variation of these TFTs was attributed to the uniformity of sc-SWCNT thin films in the device channels. Because sc-SWCNTs in outermost TFT channels were more easily washed away when rinsing with xylene, the mobility of the outermost TFTs generally was slightly lower than those of other devices. These TFTs maintained their
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Fig. 6 Transfer characteristic curves of the same printing batch of 216 devices on PET substrates with (a) linear scale and (b) log scale, and histogram of (c) hole mobilities and d) on–off ratios of 216 printed topgate TFTs.
on/off ratios, mobilities, hysteresis and threshold voltages after exposed in air for 2 months (Fig. S2†) because of the 50 nm HfOx thin film, which acted as an encapsulation layer. The performances of printed inverters, NOR, and ring oscillators on PET substrates were studied. Inverters composed of two top-gate TFTs were fabricated on PET substrates using printed silver lines. Fig. 7a is the schematic and the circuit diagram of the printed inverter, which consists of two printed top-gate TFTs in series. Fig. 7b and 7c are the voltage input– output characteristics of the inverter, which exhibited a maximum voltage gain of 9, 18, 24 and 33 at Vdd = 0.5, 0.75, 1 and 1.25 V, respectively. It can be seen from Fig. 7c, the printed inverters showed small hysteresis, which has a close relationship with the hysteresis of the printed TFTs (Fig. 6a
Fig. 7 (a) Schematic of a printed inverter on HfOx/PET substrates (top), T1 and T2 represent printed transistor 1 and transistor 2, respectively, and the symbol of the inverter (bottom), (b) the voltage input–output, and (c) gain characteristics and d) the power consumption of a printed inverter at Vdd = 0.5, 0.75, 1 and 1.25 V.
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Fig. 8 The Input and output characteristics of printed inverters on PET substrates at (a)10, (b)100, (c)1 k and (d)10 kHz. (Vdd = 2 V).
and b).14 It would be beneficial to minimize the inverter hysteresis when using small hysteresis n-type and p-type TFTs to construct CMOS inverters. Fig. 7d shows the static power consumption of the printed inverter at Vdd = 0.5, 0.75, 1 and 1.25 V. The static power consumption reaches 2.5 μW when the input voltage is low at Vdd = 1.25 V, which is higher than that of CMOS inverters.23,24 Furthermore, the electrical properties of printed inverters were measured after repeatedly bending them 100 times. As shown in the following Fig. S4,† the voltage input–output, gain characteristics and the power consumption of the printed inverter have no obvious changes. The dynamic responses of the inverters are shown in Fig. 8. It can be seen from Fig. 8 that the inverter can work properly in the kilohertz ranges. i.e., output curves were totally opposite to the input curves with little voltage loss and 60 μs delay time at 1 kHz. It was noted that the inverters can still work at 10 kHz (Fig. 8d). The switching time of the printed devices were dependent on the SWCNT density, the channel length, the parasitic capacitance of devices and the conductivity of interconnects.14 Printed NOR logic gates were also demonstrated on PET substrates. A NOR gate consists of two TFTs in parallel and two TFTs in serials. As shown in Fig. 9, the output voltages of 0 V were observed when one input voltage or both input voltages were set to 2 V. When VA and VB are 0 V, output voltages were 1.5 V at Vdd = 2 V. In summary, flexible printed NOR logic gates showed the capabilities to integrate into complicated flexible logic circuits. To demonstrate the ability of the flexible printed inverters in driving subsequent printed inverters, six printed inverters were integrated to a five-stage ring oscillator using printed silver lines. Fig. 10a and b represent the image and circuit diagram of a printed oscillator on the PET substrate. Fig. 10c and the video in ESI† show the output characteristics of a printed ring oscillator on the PET substrate. As shown in Fig. 10c and the video in ESI.† The oscillating frequency of the devices were measured to be 1.7 kHz at Vdd = 2 V. The delay time can be calculated using f = 1/(2tpN), where f, tp and N are
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high air-stability and excellent electrical properties with high on/off ratios (up to 105), high mobility (up to 46.2 cm2 V−1 s−1), low SS (109 mV per decade), low operation voltage and small hysteresis. Inverters, NOR and five-stage ring oscillators based on flexible printed top-gate TFTs have also been demonstrated. Printed inverters showed the maximum voltage gains up to 33 at Vdd = 1.25 V. The frequency of printed five-stage oscillators can be up to 1.7 kHz at Vdd = 2 V with a delay time of 58.8 μs per stage. This work provides a valid pathway to fabricate better performance printed circuits with large area and stability on flexible substrates.
Acknowledgements Fig. 9 (a) An optical image and (b) a circuit diagram of printed NOR, and (c) the output characteristics. The operation voltage is Vdd = 2 V, and two input voltages (VA and VB) are 2 V. Input voltages of 0 and 2 V are regarded as logic “0” and “1”, respectively. T1, T2, T3 and T4 represent transistor 1, 2, 3 and 4. S, D and G represent source (Au), drain (Au) and gate ( printed Ag) electrodes, respectively.
This work was supported by Natural Science Foundation of China (91123034, 61102046), Strategic Priority Research Program of the Chinese Academy of Science (XDA09020201), and Project supported by National Science and Technology Ministry (2012BAF13B05-402), the Knowledge Innovation Programme of the Chinese Academy of Sciences (KJCX2-EW-M02) and Basic Research Programme of Jiangsu Province (BK2011364).
References
Fig. 10 Five-stage printed SWCNT ring oscillator with output buffer. (a) Photograph of a printed oscillator on PET substrate, (b) circuit diagram of the printed oscillator, (c) the output characteristics of printed oscillator on PET substrates with 1.7 kHz oscillator frequency at Vdd = 2 V.
the oscillating frequency, delay time per stage and the number of stages.17 The average tp about 58.8 μs per stage can be obtained from the above equation. As the frequency of the ring oscillator greatly depends on the RC time constant of SWCNT TFTs and their connection lines,17 the frequency of flexible printed ring oscillator can be further enhanced by increasing the SWCNT density, scaling down the channel length, decreasing the parasitic capacitance and increasing the conductivity of the printed silver lines. This work is in progress.
Summary In conclusion, we have developed a reliable method to print large-area top-gate SWCNT TFT arrays on PET substrates. Predeposited 5 nm HfOx thin films on PET substrates served as good adhesive layers to greatly increase the density of SWCNT thin film. 50 nm HfOx thin films deposited on top of SWCNT thin films were used as not only dielectric layers but also as the encapsulation layer, and printed top-gate TFTs showed
14896 | Nanoscale, 2014, 6, 14891–14897
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