A Display Module Implemented by the Fast High-Temperatue

Apr 11, 2012 - 0.79 mS, and the succeeding cool-down time is 0.36 mS. The fast .... center of the suspending yarns, and the two ends are cool ends. Fi...
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Letter pubs.acs.org/NanoLett

A Display Module Implemented by the Fast High-Temperatue Response of Carbon Nanotube Thin Yarns Yang Wei,* Peng Liu, Kaili Jiang, and Shoushan Fan* Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: Suspending superaligned multiwalled carbon nanotube (MWCNT) films were processed into CNT thin yarns, about 1 μm in diameter, by laser cutting and an ethanol atomization bath treatment. The fast high-temperature response under a vacuum was revealed by monitoring the incandescent light with a photo diode. The thin yarns can be electrically heated up to 2170 K in 0.79 mS, and the succeeding cool-down time is 0.36 mS. The fast response is attributed to the ultrasmall mass of the independent single yarn, large radiation coefficient, and improved thermal conductance through the two cool ends. The millisecond response time makes it possible to use the visible hot thin yarns as lightemitting elements of an incandescent display. A fully sealed display with 16 × 16 matrix was successfully fabricated using screenprinted thick electrodes and CNT thin yarns. It can display rolling characters with a low power consumption. More applications can be further developed based on the addressable CNT thermal arrays. KEYWORDS: Carbon nanotube, thin yarn, incandescence, display

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applicability of the new devices. The appearing applications in fast thermal response will be significantly promoted by the progress of these issues. Here we report the fast high-temperature response of suspending MWCNT thin yarns. The freestanding aligned CNT film was first tailored to CNT strips by a focused laser beam, and the strips were then shrunk into CNT thin yarns, ∼1 μm in diameter, by ethanol atomization bath. The electrical heating and natural cooling processes of the suspending CNT yarns were studied under vacuum by observing the incandescent light from the hot thin yarns with a photo diode. The thin yarns can be electrically heated up to 2170 K in about 0.79 mS, and the corresponding cool-down time is only about 0.36 mS. We attributed the fast response to the ultrasmall mass of the independent single yarn, large radiation coefficient, and thermal conductance through the two cool ends. On the basis of the millisecond response time, we proposed a new incandescent display by using the visible hot thin CNT yarns as light-emitting elements. A fully sealed display with a 16 × 16 matrix was successfully fabricated by suspending CNT thin yarns between screen-printed electrodes, and it can display rolling Chinese characters. More potential applications will greatly benefit from the improved mechanical strength, electrical conductivity, and thermal conductance in comparison with CNT films.

ncandescent light is an important source of illumination since Thomas A. Edison invented electric light in 1879.1 In the beginning the filament was carbon fiber.2 But the carbon materials were upgraded to tungsten for its longer lifetime. The finding of carbon nanotubes (CNTs) entitles new applications and opportunity to carbon materials to be used in incandescent light sources because of their unique properties.3−5 Superaligned multiwalled CNT (MWCNT) arrays are novel carbon materials, and aligned CNT film can be directly drawn from such CNT arrays due to the strong van der Waals interactions.4,6 The freestanding aligned CNT films have opened up the possibility of fabricating CNT devices with uniform properties on a large scale, such as touch panels,7 transmission electron microscopy (TEM) grids,8 aerogel muscles,9 electrodes of lithium-ion batteries,10 field emitters,11,12 etc. The applications of the aligned CNT film as incandescent light sources also have attracted research interest,13,14 and it will be a promising field for CNT applications. Recently, the ultrasmall heat capacity per unit area (HCPUA) of the aligned CNT films was reported by Xiao et al., and a thermoacoustic loudspeaker was developed by using this unique performance.15 Liu et al. found that such CNT films keep the fast thermal response even at the incandescent state.16 These studies revealed the ultrasmall HCPUA property and possible applications of the freestanding CNT films. The nanoscaled thickness and sparse network structure not only bring the ultrasmall HCPUA to the aligned CNT film but also make it weak and flimsy. This affects the process compatibility of these novel nanomaterials and reduces the practical © 2012 American Chemical Society

Received: February 24, 2012 Revised: March 29, 2012 Published: April 11, 2012 2548

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can be found in the Supporting Information (movie 1). Figure 1e is an optical micrograph of the CNT thin yarns, and scanning electron microscopy (SEM) observation shows their diameter is only about 1 μm, as presented in Figure 1f. The CNT thin yarns were further studied by integrating them into a circuit. They were set across a ∼460 μm gap between two Al coated silicon wafers attached on a glass plate. The two Al panels were used as the electrodes to feed electric currents through them. The current−voltage (I−V) characteristic of these CNT thin yarns is plotted in Figure 2a. The

Superaligned MWCNT arrays were grown on a 4 in. Si wafer by a low-pressure chemical vapor deposition (LPCVD) method. The as-synthesized CNT arrays were about 300 μm in height and 6−15 nm in diameter and had 3−10 graphite layers. The detailed messages can be referred to our previous works.4,17,18 A piece of the aligned CNT film drawn from the superaligned CNT arrays on a 4 in. wafer is shown in Figure 1a,

Figure 1. (a) CNT thin film dry spun from superaligned CNT arrays on a 4 in. wafer. (b) Sketch of the suspending CNT film and the laser cutting process. (c) An optical micrograph of the aligned CNT film across a 380 μm groove. (d) CNT strips across the groove tailored by laser. The width of the strips is ∼30 μm, and the pattern period is 120 μm. (e) CNT thin yarns were formed by ethanol atomization bath. (f) SEM image of a CNT thin yarn, ∼1 μm in diameter.

Figure 2. (a) I−V curve of the suspending CNT thin yarns. (b) Heating-voltage-dependent temperature and brightness of the CNT yarns. (c−f) Optical micrographs of the incandescent thin yarns at different temperatures.

and it can be seen that the freestanding CNT film is transparent. The CNTs in the as-spun film are arranged almost parallel to each other along the drawn direction with uniform density. A piece of silicon nitride coated Si wafer was used as a carrier for the aligned CNT film. To get a suspending section, a slot was formed in the wafer mainly by photolithography, reactive ion etching (RIE), and silicon wet etching processes. The opening on the polished surface was 10 mm × 380 μm. A sheet of the aligned CNT film was then coated on the wafer surface across the groove as schematically illustrated in Figure 1b. Figure 1c shows the aligned CNT film across the 380 μm-wide groove, illustrating the parallel and uniform assembly. A beam of YAG laser was introduced to cut the CNT film into parallel arranged CNT strips (Figure 1b). The wavelength of the laser is 1.06 μm, and the full width at half-maximum (FWHM) of the focus is 30 μm. The output power and the scanning speed of the focus were set as 3.6 W and 100 mm/S, respectively. CNTs irradiated by the focused laser were burnt quickly in air, and the film was thus cut into a designed pattern. The suspending CNT strips shown in Figure 1d are ∼30 μm in width, and the period of the pattern is 120 μm. The CNT strips were then shrunk into parallel thin yarns by ethanol atomization bath. The treatment process was monitored in an optical microscope and

change of slope is ascribed to the temperature-dependent resistance of the CNTs.14 The suspending CNT yarns were heated to incandescence by a direct current (dc) under a dynamic vacuum. The temperatures of the hot CNT thin yarns were determined by fitting the incandescent light spectrum with the blackbody radiation law.14,19 The brightness was measured by a spectroradiometer with a macro lens (Konica Minolta CS-1000S). The heating-voltage-dependent temperature and brightness are presented in Figure 1b. It can be seen that the temperature increases linearly with voltage, while the brightness exhibits an exponential increase. This is attributed to the principle of the blackbody radiation, as the radiation power is proportional to T4.14 The CNT thin yarns can be heated up to 2250 K by a 10.25 V heating voltage, as Figure 2b shows. The hot CNT yarns were observed by an optical microscope, and Figure 2c−f shows hot and incandescent CNT thin yarns at different temperatures. These images tell that the as-fabricated CNT yarns can be uniformly heated by electric currents with good consistency. It is general that the hottest spots are at the center of the suspending yarns, and the two ends are cool ends 2549

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Figure 3. (a,b) The ramp-up and the cooling-down courses of the CNT thin yarns. (c) Temperature-dependent ramp-up and cooling-down times. (d−f) Frequency-dependent responses at 200 Hz, 1 kHz, and 20 kHz, respectively.

experiments. The specific variation of the cooling-down time dependence on temperature still needs a more accurate methodology. We have also studied the frequency response of the CNT thin yarns by heating them with pulsed square wave. The duty cycle was set as 50%, and the pulsed voltage was 10 V during the experiments. The representative results are shown in Figure 3d−f. For pulses of frequency less than 200 Hz, the incandescence signal can show almost the same waveform as the heating signal. For a 1 kHz heating input, it became a triangle wave without obvious bias. For higher frequencies, the signal was biased by a positive offset, and we even observed a biased sine waveform at 20 kHz by observing the incandescent light. These results agreed well with the measurements of the ramp-up and cooling-down rates of the suspending CNT thin yarns, as the sum of them is only about 1.15 mS. These experiments sufficiently presented the fast high-temperature response of the as-fabricated CNT structures. The fast high-temperature response of the CNT thin yarns is an interesting phenomenon and can be understood by their specific properties. The fast response of CNT films is attributed to their ultrasmall HCPUA, large surface area, and large emissivity, since the CNT films are made up of sparsely disturbed CNT networks.15,16 But things have changed for the ethanol atomization bath. The CNTs are tightly compacted together in the yarns, as shown in Figure 1f. The surface area is greatly decreased by the densifying process,14 and the ultrasmall HCPUA is not fit for CNT thin yarns any more. Although the density is increased by condensation, the mass is still the same

as they are connected to Al coated wafers which are heat sinks in these measurements. A photo diode sensitive to the visible light was set to observe the heating-up and cooling-down courses through a quartz window. The photo diode displayed a response to the hot CNTs when the temperature was above ∼930 K. The heating signal and the output signal from the photo diode were recorded by an oscilloscope (Agilent 54832B). The yarns were heated up to 2170 K by a 10 V voltage, and the ramp-up and cooling-down courses are shown in Figure 3a,b, respectively. The temperature ramp-up time was defined as the time from starting heating to reaching 90% of the maximum radiation. The cooling-down time was defined as the time from stopping heating to reaching 10% of the falling edge of the photoelectricity signal. As shown, the ramp-up and cooling-down times are 0.79 and 0.36 mS, respectively. The ramp-up and cooling-down times were further studied at different temperatures, and the results are presented in Figure 3c. The experiments showed that the ramp-up time decreased with the increasing equilibrium temperature. As shown in Figure 2a,b, the temperature is decided by the heating power, and a higher temperature requires a larger power. Therefore, the principle is that a higher temperature ramp-up rate induced by the larger heating power decides the decreasing ramp-up time. We did not observe apparent change of the cooling-down time by this method, although it should increase with the increasing equilibrium temperature. This should be ascribed to the short cooling-down time and the small temperaturedependent variation, as it exceeded the capability of our 2550

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Figure 4. (a) The heating power versus σT4. (b) The in situ observation of resistance reduction of CNT film to thin yarns. (c) The stress−strain curves of CNT film without and with atomization treatment. The specimens are 5 mm wide and 10 mm long.

Figure 5. (a) The sketch of the pixel structure and the laser cutting process. (b) Screen-printed electrodes on glass with 16 × 16 matrix. (c) A fully sealed incandescent display. (d) A CNT film suspended across a ∼400 μm gap. (e) Three parallel thin CNT yarns bridging the two electrodes. (f) An optical micrograph of a lit pixel. The heating voltage is 5.5 V. The temperature and the brightness are 2036 K and 1632 cd/m2, respectively. (g) A full on state of the 16 × 16 matrix. The driving voltage and current are 5 V and 167 mA, respectively. It consumes 0.8W. (h,i) A Chinese word, Beijing.

as 0.2 ng.15 The fast ramp-up rate can thus be attributed to their small heat capacity. In the case of hot aligned CNT films in vacuum, the heat is efficiently dissipated by radiation through the large surface area and the large emissivity. For this case, thermal conductance is the other heat dissipation way besides radiation. It can be proved by the optical micrographs, as shown in Figure 2. The cool end effect tells that the thermal conductance dissipation works during cooling down. The energy loss due to photo emission can be well predicted by the Stefan−Boltzmann law, which tells that the total radiation power is proportional to σT4 and can be expressed as Φph = σT4S, where σ is the Stefan−Boltzmann constant and S is the effective surface area for photo emission. If the thermal conductance can be neglected, the heating power can be expressed as P ≈ σT4S. Our previous work has shown that the superaligned CNT films obey this principle, as the experimental curves of P vs σT4 are perfect straight lines.14 Here the introduction of the thermal conductance through the two cool

ends makes the curve deviate from a straight line, as shown in Figure 4a. This is also an important evidence. The improved thermal conductance can be further ascribed to the improved intertube contacts introduced by the shrinking process. The improved electric conductivity and mechanical strength can support this point, as both are sensitive to the intertube contacts. Figure 4b shows the observation of resistance reduction from 3.9 to 2.2 KΩ induced by the shrinking process. The stress−strain measurements shows that an ethanol atomization bath makes the CNT film stronger, as presented in Figure 4c. On the basis of these facts, the fast cooling-down rate can be attributed to the small heat capacity, the efficient radiation, and the improved thermal conductance. Both the experiments on the response time of the incandescent light to the heating signal and the frequencydependent measurements told that the incandescence response of CNT film is faster than that of liquid crystal displays (LCDs).20,21 Furthermore, the incandescent light is bright 2551

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Figure 6. Comparative studies of CNT films and thin yarns. The results show that the incandescent display having thin yarns has lower power consumption (a), can work at lower voltage (b), and has higher working temperature (c).

which comprise a word Beijing. Besides displaying characters, it can also display dynamic rolling information. A video of dynamic display is attached as movie 2 in the Supporting Information. It should be noted that the power consumption is lower under the dynamic display mode, as only a part of the pixels is on. The CNT thin yarns have advantages in the incandescent display application in comparison with the CNT films. To illustrate the performance improvements, a single pixel was comparatively studied before and after shrinking process. Figure 6 presents the results. Power consumption and the driving voltage are obviously reduced, as shown in Figure 6a,b, since both the experimental curves of the thin yarns are below that of the CNT films. Further analysis shows that at 1000 cd/m2, the power consumption is reduced from 4.4 to 3.1 mW, and the driving voltage reduction is from 6.7 to 5.3 V. The lower driving voltage can be ascribed to the reduction of the resistance, which has been exhibited in Figure 4b, and is beneficial to reducing costs of the driving circuits. Taking into account the temperature, we can understand the power consumption reduction. The temperature was plotted against the measured brightness, as shown in Figure 6c. It can be seen that the temperature of the hot yarns is higher than that of the CNT film at the same brightness. At 1000 cd/m2, the increase of the temperature is from 1890 to 1990 K. It is known that the radiation power is proportional to T4, and the efficiency of incandescent emission can be greatly improved by increasing the working temperature. Therefore, the reduction of power consumption can be attributed to the increased temperature. The CNT thin yarns can also eliminate the defects which are often encountered in incandescent CNT films. Defects are the localized resistance ununiformity, which are often at the edge of the CNT films caused by laser cutting, and sometimes in the film. The defects appear as bright hot spots in incandescent films, and only one such bright spot can even break a whole CNT film immediately as soon as its local temperature exceeds the CNTs’ endurance. These defects can be effectively concealed if the CNT films were densified into thin yarns, because the improved intertube contact can reduce the ununiformed resistance and the heat of the hot spots can be efficiently dissipated into the yarns through neighboring contacted CNTs. That is also the reason for the higher temperature endurance of the shrunk yarns, which has been revealed in Figure 6c and our previous works.14 We also note that CNTs can retain their perfection even if most of their body has expired in vacuum, and the phenomenon is different to conventional materials.22−24 The shrinking process also greatly reduces the surface area for carbon evaporation.14 These make the stable incandescent CNTs possible. It is necessary to

enough to eyes (Figure 2b). On the basis of these features, we proposed a new self-luminous type display by employing the incandescent CNT yarns as the light-emitting units, and the pixel structure is sketched in Figure 5a. The CNT thin yarns are assembled between the two thick-film electrodes on the glass plate. An incandescent display with a 16 × 16 matrix has been successfully fabricated. The fabrication was mainly based on the screen printing, aligned CNT coating, CNT patterning, and conventional vacuum sealing. All the electrodes on the glass plate were made by screen printing, a commercialized technology which can make thick film with tens of micrometers in thickness. The line and column electrodes used to connecting pixels were made by single printing; the pixel electrodes to suspend CNTs were made by multiprinting; a dielectric layer was printed between the line and column electrodes at the cross sites. An as-fabricated glass plate is shown in Figure 5b. A layer of the superaligned CNT film was then coated on the screen-printed glass plate, and a laser beam was introduced to tailor the CNT film into 16 × 16 independent units. The CNTs only remained on the pixel electrodes and were suspended across the ∼400 μm gap between the electrodes, as shown in Figure 5d. CNTs in every pixel were further cut into 3 CNT strips, 30 μm in width, and then shrunk into 3 CNT thin yarns by atomized ethanol. Figure 5e is a top view of such a pixel. The as-processed glass plate was sandwiched between a front and a back planar vacuum chamber with a conventional vacuum sealing process. The two vacuum chambers were connected with each other through two or three holes drilled on the glass plate (Figure 5b). Figure 5c is a top view of the fully sealed incandescent display, and nonevaporable getters were assembled in the back vacuum chamber to ensure a high vacuum in the sealed device. The display was then lit by applying electric currents through the fingers. Figure 5f is an optical micrograph of a hot pixel lit at 5.5 V. The temperature and the brightness are 2036 K and 1632 cd/m2, respectively. We can find that the hottest spots shift toward the left contact, and it can be ascribed to the asymmetric contacts at the two ends. This feature also suggests that heat dissipation is not just dominated by radiation. Thermal conductance through the yarn ends also plays an important part as the temperature distribution along the yarns is sensitive to the contacts on the electrodes. A full on state of the 16 × 16 matrix is shown in Figure 5g. All the pixels are uniformly lit, which reveals the reliability of the as-developed processes. The driving voltage and current are 5 V and 167 mA, respectivley, corresponding to a power consumption of 0.8 W. The incandescent display was further driven by a homemade driving circuit. Figure 5h,i presents two Chinese characters, 2552

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mention the improved mechanical properties, as Figure 4c shows. The improved strength and the strain reduction can effectively improve the device intension and decrease affects induced by the deformation of the CNT structures. These issues decide that the device should have higher reliability. We did not observe any appreciable brightness and current reduction during an over 5 h endurance measurement driven by a 5 V dc. Other than the incandescent display, some new applications can be developed with the CNT thin yarns. The addressable CNT matrix with fast thermal response can be applied as a novel new microheater array, which might be useful in microand nanoscaled chemical reactions,25 the polymerase chain reaction (PCR),26,27 and the materials genome project.28 The CNT thin yarns can also be used in thermoacoustic loudspeakers.15 Furthermore, the color can be introduced by decorating polymers on to these thin yarns.29 In summary, CNT thin yarns were efficiently fabricated by tailoring a MWCNT film with a laser and succeeding ethanol atomization bath. Their fast high-temperature response was revealed by observing the incandescent light with a photo diode. The thin yarns can be heated up to 2170 K in 0.79 mS, and the corresponding cooling-down time is 0.36 mS. The fast response is attributed to the ultrasmall mass of the independent single yarn and large radiation coefficient as well as thermal conductance through the two cool ends. A fully sealed incandescent display with a 16 × 16 matrix was successfully fabricated by employing the hot CNT thin yarns as lightemitting elements, and it can dynamically display Chinese characters. More applications can be further developed based on the CNT thin yarns and the addressable CNT thermal array, and they will profit from the improved mechanical strength, electrical conductivity, and thermal conductance in comparison with CNT films.

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ASSOCIATED CONTENT

S Supporting Information *

Movies depicting the treatment process in an optical microscope and the display of dynamic rolling information. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Basic Research Program of China (2012CB932301) and the NSFC (51102147, 51102144, and 90921012). We thank Dr. Yang Wu for proof reading.

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REFERENCES

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