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Hysteretic behavior in ion gel-graphene hybrid terahertz modulator
Carbon 155 (2019) 514e520
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Hysteretic behavior in ion gel-graphene hybrid terahertz modulator Xieyu Chen a, Zhen Tian a, *, Jin Wang c, Yinghui Yuan a, Xueqian Zhang a, Chunmei Ouyang a, Jianqiang Gu a, Jiaguang Han a, Weili Zhang a, b, ** a Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Opto-electronics Information and Technology (Ministry of Education), Tianjin University, Tianjin, 300072, China b School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma, 74078, USA c School of Electronic Engineering, Tianjin University of Technology and Education, Tianjin, 300222, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2019 Received in revised form 23 August 2019 Accepted 2 September 2019 Available online 3 September 2019
Hysteretic behavior in ion gel-graphene hybrid terahertz (THz) modulator is presented. The ion gel gated graphene modulator was designed and fabricated by conventional wet-based graphene transfer method. The modulation performance and hysteretic behavior of the device was characterized by THz timedomain spectroscopy. The dependence of hysteresis on the sweeping voltage rate and sweeping range was explored in a continuous wave terahertz system. The temporal response of the sample was also measured and fitted by a double-exponential expression, which indicated that there were two mechanisms that might cause the hysteresis in the THz modulator. This hysteretic behavior is promising in developing nonvolatile memory devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Terahertz (THz) frequencies are located between the microwave and far infrared spectra of the electromagnetic radiation. In the past twenty years, many fascinating properties of THz waves were observed, suggesting promising applications in biology, medical science, communications, homeland security and non-destructive evaluations [1]. However, due to shortage of high performance optical components, such as power modulators, phase modulators, polarization devices and absorbers, THz applications have been very limited. Among those devices, intensity modulators [2e5] are essential elements in THz communication and imaging systems. Thus, investigating high performance modulators is very meaningful to practical THz applications. Graphene and its derivatives have attracted much attention for the past decade because of their extraordinary electronic and optoelectronic properties [6e8]. As a two-dimensional semiconductor, the carrier concentration of graphene can be extensively
* Corresponding author. ** Corresponding author. Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin University, Tianjin, 300072, China. E-mail addresses: [email protected] (Z. Tian), [email protected] (W. Zhang). https://doi.org/10.1016/j.carbon.2019.09.007 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
changed by optical pump [9e11], external gate voltage [12] or chemical doping [13] and thus its properties. Moreover, compared with traditional bulk semiconductors and other 2D materials, graphene has many benefits: (1) Graphene exhibits mechanical flexibility, broad spectral response, compactness and isotropy. (2) Graphene has outstanding carrier mobility compared with other 2D materials owing to its symmetric conical band structure. (3) The uniform wafer-scale growth of graphene can be achieved by chemical vapor deposition technique, which makes graphene costeffective. (4) Graphene is stable in the air environment without any protection sealing. Therefore, graphene has widely been used in optical modulators from the microwave [14,15], terahertz [16,17], infrared [18,19] to visible [20] regimes. At THz frequencies, the transmission of graphene is dominated by the intraband transitions and can be significant modulated up to nearly 100% in theory. A conventional graphene-SiO2-semiconductor structure was initially designed to modulate the transmission of THz wave [17,21e30]. However, due to relatively small capacitance of such devices, the bias voltage applied to the graphene was always too high. In recent years, by replacing the insulator layer, such as SiO2, with a solid state ion gel electrolyte, the THz transmission of graphene can be modulated efficiently by a small gate voltage [31e37]. Although a large number of works on ion gel-graphene hybrid modulators were reported [19,20,31e37], the modulation speed and hysteretic behavior in such devices remain unexplored in the THz regime. In this article, we present a systematic investigation of the
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hysteretic behavior in an ion gel gated graphene THz modulator. The device was designed and fabricated using conventional wetbased graphene transfer method and we experimentally demonstrated that the device was able to deliver 43% modulation depth by switching the gate voltage from 0.4 V to 2 V. The transmission of THz peak was measured 60 s after the voltage was changed during the process of sweeping the gate voltage forward and backward from 1 V to 1 V. A hysteretic behavior was experimentally observed and characterized. The dependence of the hysteresis on the sweeping rate and sweeping range of the gate voltage was also studied in a continuous wave THz system. The temporal response of the modulator was consequently investigated and the results indicated that there were two mechanisms that could lead to the hysteretic behavior. The hysteresis of the ion gel-graphene hybrid THz modulator is meaningful to improve the performance of the modulator and promising in developing photonic memory devices. 2. Device fabrication As shown schematically in Fig. 1a, the proposed device is composed of three main components: ion gel, monolayer graphene, and quartz substrate. The device was fabricated by conventional wet-based transfer method. First, a commercial CVD-grown monolayer graphene (1 cm 1.2 cm) covered with PMMA film was transferred onto the quartz wafer. Then, after removing the PMMA by acetone, a conductive silver adhesive electrode was fabricated on the graphene. Second, the ion gel solution was prepared by dissolving poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) (purchased from SigmaAlorich) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSA]) (purchased from Aladdin) in acetone. The weight ratio between the polymer, ionic liquid and the solvent was 1:4:7. The solution was then stirred at 50 C for 2 h to form a homogenous solution. Next, the solution was spin-coated on the graphene at a rate of 3000 r.p.m. Then the device was baked at 70 C for 24 h to remove the acetone solvent [36]. Finally, another electrode was fabricated on the ion gel. An optical image of the free standing ion-gel used in our sample is shown in Fig. S1. The modulation mechanism of the device is illustrated in Fig. 1b. When there is no bias voltage, free cations and anions will be distributed in the ion gel randomly because of thermal motion. The graphene is p-doped due to the adsorbed oxygen. When an external
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voltage is applied to the device, the electrodes will create an electric field and attract oppositely charged ions from the solution, forming what is known as an electrical double layer (EDL) near the graphene and the upper electrode. Since the thickness of EDL is very thin, this configuration yields very large electric field next to the graphene and thus large carrier concentration on it [34]. Therefore, the Fermi energy of the graphene can be efficiently modulated by a small bias voltage, which is highly desired in THz modulators. 3. Results and discussion To demonstrate the modulation ability of the device, we experimentally measured the sample shown in the insert of Fig. 2c using a typical THz time-domain spectroscopy (THz-TDS) system (Fig. S4). The electric field transmissions of the air reference and the device under different voltages are shown in Fig. 2a. When the bias voltage is increased from 0 V to 2 V, the THz pulse gradually decreases in magnitude. However, because the graphene is p-doped in air, the transmission pulse is not the highest under 0 V voltage. When we applied 0.4 V gate voltage to the device, the amplitude of THz pulse reaches the maximum, which means the Fermi level of the graphene is close to the charge neutral point (CNP) under 0.4 V voltage and the graphene keeps hole-doped with the gate voltage swept from 0.4 V to 2.0 V. It should be noted that owing to the symmetric conical band structure of graphene, the terahertz modulation property of the graphene under electrondoped is similar to the hole-doped graphene. Therefore, we only show the terahertz amplitude modulated by hole-doped graphene (0.4 Ve2.0 V) here, which can clearly show the modulation ability of our device. However, to further understand the physical mechanism of the device, we also investigate the modulation property of the electron-doped graphene, as illustrated in Fig. S2. In Fig. 2b, the ~R ðuÞ normalized transmission spectra defined as j~tðuÞj ¼ ~ ES ðuÞ=E ~S ðuÞ and E ~R ðuÞ are Fourier transforms of the are also shown, where E measured terahertz pulses of the sample and air reference, respectively. The THz transmission of the sample gradually decreases as the gate voltage is increased from 0.4 V to 2 V. It should be noted that the transmission is lower at high frequency due to absorption of the quartz substrate and ion gel layer, which is further explained in supplementary materials section 3. We also show the normalized transmission of the peak THz field as a function of gate voltages applied to the graphene layer in Fig. 2c. The device achieves a modulation depth up to 43% by switching the gate voltage
Fig. 1. (a) Schematic diagram of the ion gel-graphene hybrid THz modulator. (b) Manipulation of the Fermi level of graphene in contact with an ion gel electrolyte under a gate voltage Vg. (A colour version of this figure can be viewed online.)
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Fig. 2. (a) Measured time-domain transmission signals of the air reference and the sample under different gate voltages. (b) Normalized experimental transmission spectra of the sample with the gate voltage varying from 0.4 V to 2.0 V. (c) Normalized transmission of the peak THz field as a function of gate voltages. Inset: image of the fabricated sample. (A colour version of this figure can be viewed online.)
from 0.4 V to 2 V. The modulation depth is defined as M ¼ ðTVg T0:4V Þ=T0:4V 100%, where TVg and T0:4V are the transmissions of the sample at peak THz field under Vg and 0.4 V voltages. As described above, the device can efficiently modulate the THz wave by only applying a small bias voltage. It should be noted that by further increasing the gate voltage above 2.0 V, a larger modulation depth can be achieved. However, this may introduce irreversible damage to the modulator due to the relatively small electrochemical window of the electrolyte [32]. Therefore, the highest gate voltage used in our manuscript is 2.0 V which is quite safe. To explore the hysteretic behavior of the ion gel-graphene integrated THz modulator, we measured the transmission at the peak THz field 60 s after the gate voltage was changed. As shown in Fig. 3a, the voltage was swept from 0 to 1 V and back to 0 V, then to 1 V and back to 0 V with a voltage interval of 0.2 V. A considerable amount of gate controlled optical hysteresis can be observed. As the gate voltage is increased from 0 V to 1 V, the transmission decreases gradually as mentioned earlier. When the voltage is swept from 1 V back to 0 V, the transmission increases continuously. However, under the same voltages of two sweeping processes, the transmission of the sample is lower when the gate voltage is decreased back to 0 V. A similar hysteretic behavior can be observed when we sweep the voltage from 0 to 1 V and back to 0 V. Moreover, the voltage scan leads to a positive shift of the neutral point in the graphene. The hysteresis in the modulator may origin from the slow polarization response time of ions in the ion gel and the charge trapping phenomena at the defect sites or grain boundaries in a large-area CVD monolayer graphene [17,19,38].
Next, we schematically describe the Fermi level of the graphene as a function of sweeping voltages, as shown in Fig. 3b. Without any gate voltage (point A), the Fermi level is slightly lower than the Dirac point due to unintentional p-dopants. When the gate voltage is increased from 0 V to 1 V (point B), the Fermi level decreases, leading to the enhancement of THz absorption and reflection due to greater intraband transition. Then the voltage falls back to 0 V, and the Fermi level increases gradually. However, due to the hysteretic behavior, the Fermi level under 0 V is lower than initial state. As the voltage is swept from 0 V to 1 V (point D), the Fermi level continuously rises. During the process, the Fermi level will reach the Dirac point (point C) at VDirac , corresponding to the maximum transmission. When the gate voltage is swept back to 0 V, a positive shift of VDirac is observed due to the hysteresis in the modulator. To explore the relationship between the magnitude of the hysteresis and the sweeping rate of the gate voltage, the sample was measured in a continuous wave THz system (Fig. S5) under different sweeping rates. The system comprises a 0.1 THz source and a fast THz detector, both purchased from Terasense Inc. The output power of the source is 88 mW and the response time of the detector is 1 ms. Thus, the time resolution of our experimental system can reach microsecond which is much smaller than the time scale of hysteretic behavior in the modulator. And a multi-function generator (WF 1974 purchased from NF Corporation) was used to apply the gate voltage to the modulator. The transmission power of our sample with the gate voltage swept from 0 V to 1.5 V and back to 0 V, then to 1.5 V and back to 0 V under different sweeping rates is shown in Fig. 4. A hysteretic behavior can be observed at each of the four sweeping rates. However, the magnitude of the hysteresis
Fig. 3. (a) Measured transmission of the peak THz field 60s after the voltage was changed. And the voltage was swept forward and backward from 1 V to 1 V. The arrows denote the sweeping direction of bias voltage (b) Schematic of the Fermi level as a function of the swept gate voltage. (A colour version of this figure can be viewed online.)
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Fig. 4. Transmission power of the sample recorded under four different gate voltage sweeping rates (1.2, 0.6, 0.3, 0.06 V/s). The arrows denote the sweeping direction of the bias voltage. (A colour version of this figure can be viewed online.)
exhibits a significant dependence on sweeping rate of the gate voltage. When the voltage is swept under the sweeping rate of 1.2 V/S, as illustrated in Fig. 4a, the hysteresis approaches the maximum and the shift of VDirac due to the hysteretic behavior is ~0.68 V. As the sweeping rate is decreased, the shift of VDirac decreases from 0.68 V (1.2 V/S) to 0.35 V (0.6 V/S) then to 0.1 V (0.3 V/ S), as shown in Fig. 4a, b, c, respectively. When the sweeping rates is as slow as 0.06 V/S, the shift of VDirac almost disappears and the hysteretic behavior is minimum. It indicates that the hysteresis in our device is on a time scale of a few seconds, which is consistent with the previous report [38]. It should be noted that the hysteretic behavior resulting from the slow polarization response time of ions is very common in ion gel based devices [19,39]. As shown above, when an external voltage is applied to the device, the electrodes will create electric field and attract oppositely charged ions from the solution, forming what is known as an electrical double layer near the graphene layer. This configuration yields the efficient modulation of the carrier concentration on graphene. It indicates that the Fermi level of graphene under certain gate voltage is determined by ions concentration next to the graphene. When the gate voltage is swept the ions distribution will changed gradually, and thus the Fermi level of graphene. However, the polarization response time of ions is relatively slow. Therefore, if the sweeping rate of gate voltage is very high, the redistribution of ions will not be completed. In this case, compared with the state of equilibrium, the transmission of our device will be different under certain gate voltage. So the hysteretic behavior caused by the ion gel will be observed with the gate voltage sweeping forward and backward. Next, the hysteretic behavior of the modulator under different gate voltage sweeping ranges is investigated, as shown in Fig. 5. The voltage sweeping rate is 0.8 V/s in measurement. The hysteretic can be observed under all sweeping ranges, however the shift of VDirac (dVDirac ), namely the magnitude of the hysteresis, varies with sweeping range. When the gate voltage is swept between 2 V and 2 V, dVDirac is ~0.66 V. As the voltage sweeping range is decreased, dVDirac gradually decreases from 0.62 V (in the range of 1.6 V) to 0.52 V (in the range of 1 V) then to 0.49 V (in the range of 0.8 V), as illustrated in Fig. 5f. When the range of voltage decreases to 0.2 V, we cannot observe the shift of VDirac due to lower voltage of VDirac , as shown in Fig. 5e. The decrease of dVDirac indicates that the hysteretic behavior in the sample will reduce by decreasing the
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sweeping range of the gate voltage. However, as demonstrated previously, the hysteresis attributed to the slow polarization response time of ions in the ion gel is not sensitive to the sweeping range due to the similar concentration of inversed charge at a certain gate voltage under a similar sweeping rate [38]. Thus, there may exist another mechanism causing the hysteretic behavior which coexists with the slow ion mobility in the ion gel. The resulted hysteresis exhibits a significant dependence on sweeping range of the gate voltage. To further clarify the underlying mechanism of the hysteretic behavior in the modulator, we measured the temporal behavior of the output THz power under different gate voltages in the continuous wave THz system. The top panels of Fig. 6a and b shows the square-wave voltage applied to the modulator and the lower panels illustrate the transmission of the sample normalized to the output power under 0 V voltage. The transmission power decreased when a positive gate voltage of Vg was switched and then returned to the initial value as the gate voltage was switched back to 0 V. However, the rise time under 1 V voltage was much slower than that under 0.5 V, which indicates that the hysteretic behavior in our sample will increase with increasing voltage sweeping range. In order to fit the time response, a double-exponential expression with the fast and slow relaxation processes was used under different gate voltages. The fitting results are shown as the red lines in the lower panel of Fig. 6a. It can be seen that excellent agreement is obtained between the fitting lines and the experimental measurements. The double-exponential used in the fitting is
TðtÞ ¼ A0 þ A1 eðtt0 Þ=t1 þ A2 eðtt0 Þ=t2 where t1 and t2 are the time constants of two relaxation processes, respectively. The retrieved fitting parameters are summarized in Table 1. When the gate voltage was changed between 0 V and 1 V, the time constants were 0.94 s and 14.00 s for the rise. When the positive voltage is decreased to 0.5 V, the time constant corresponding to the fast decay process is varied a little to 1.04 s while the other time constant is decreased a lot to 7.24 s. It indicates that the hysteretic behavior in the sample can be attributed to two different mechanisms. The fast decay process originated from the slow ion mobility in the ion gel is not sensitive to the gate sweeping range, as demonstrated in the previous work [38]. While the slow process may be resulted from the charge trapping phenomena at the defect sites and grain boundaries due to non-idealities in a CVD monolayer graphene or the sample fabrication [17]. Therefore, when the gate voltage is increased, the charge trapping will be enhanced and thus the hysteretic behavior. We also investigated the temporal response of the modulator under negative gate voltages, as illustrated in Fig. 6b. When the gate voltage is switched from 0 to 1 V, the transmission power is increased first and then decreased. It indicates that the Fermi level of the graphene rises from the initial location lower than Dirac point to Dirac point and finally to the location above it. As the gate voltage is turned off, the transmission power goes through a similar process which means that the Fermi level passes through the Dirac point again. The phenomenon can be observed only when the negative gate voltage Vg is lower than VDirac . As shown in the inset of Fig. 6b, when the gate voltage is only 0.2 V, the Fermi level would not pass through the Dirac point. As illustrated in Table 1, the decay time of the THz modulator can be on a time scale of 20 s, which will be a hurdle to the fast response to gate voltage and severely constrain the modulation speed of the modulator. The temporal response can be improved by using high quality graphene and ionic liquids with higher ionic conductivity. Although the slow response time of our sample is not ideal for high-speed THz modulation, it could be applicable in other fascinating fields, such as
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Fig. 5. Measured power transmission under five different gate voltage sweeping ranges (2, 1.6, 1, 0.8, 0.2 V) without changing the sweeping rate (0.8 V/s) are shown in (a), (b), (c), (d) and (e), respectively. The arrows denote the sweeping direction of the gate voltage. (f) Shift of VDirac as a function of gate voltage sweeping range. (A colour version of this figure can be viewed online.)
Fig. 6. Temporal behavior of the ion gel-graphene hybrid THz modulator. (a) The positive square-wave voltage applied to the graphene (top panel) and the time response of the modulator (low panel) under different gate voltages (1 V and 0.5 V). The fitting is shown as red lines. (b) Negative square-wave voltage applied to the graphene (top panel) and the time response of the modulator (low panel) under three different gate voltages (1, 0.5, 0.2 V). (A colour version of this figure can be viewed online.)
Table 1 Retrieved fitting time constants under different gate voltages. Time Constant
t1 t2
0 V~1 V
0 Ve0.5 V
Fall
Rise
Fall
Rise
0.48s 7.06s
0.94s 14.00s
0.63s 7.34s
1.04s 7.24s
electrically controllable photonic memory devices. It should be mentioned that the modulation performance of our device is closely related to the electrical properties of ion gel such as capacitance and resistance. First, the modulation depth of our device is determined by the graphene conductivity under different gate voltages, and thus the Fermi level of graphene. As demonstrated in previous work [40], in such modulation configuration the Fermi level is related to the gate voltage by jEF j ¼ ZvF ðpNÞ1=2. Here,
- and vF are the reduced Planck's constant and the Fermi velocity, respectively. N is the total carrier density expressed by N ¼ 1=2 ðn20 þ a2 jDVj2 Þ . Where n0 , DV and a are the carrier density at the conductivity minimum, the gate voltage relative to the charge neutral point and the specific capacitance of the ion gel dielectric, respectively. According to the formula above, the modulation depth of our device will increase with the capacitance increasing. Second, in such field effect transistor, the modulation speed is mainly dominated by the RC time constant of ion gel given by RC ¼ C 0 L=s [41]. Where C 0 , L, s are the specific capacitance, the distance between the graphene and the electrode on the ion gel layer and the conductivity of ion gel, respectively. Thus, the response time of our device is also related to the electrical properties of ion gel. The thickness of the ion gel used in our sample is a constant. However, the physical properties of ion gel are related to its thickness, and thus the modulation performance of our device. In previous work,
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the relationship between the electrical properties and the thickness of the ion gel has been systematically investigated [41]. It can be seen that the capacitance of the ion gel mainly depends on the thickness of the double layer rather than the ion gel film. Therefore, the capacitance is insensitive to the film thickness and so is the modulation depth of our device. Moreover, the conductivity of the ion gel is also independent of the film geometry. However, the distance between the graphene and the second electrode increases with the film thickness increasing, and thus the RC time constant of ion gel. So when the ion gel film is thicker the modulation speed of our device will decrease. Moreover, due to the terahertz absorption of ion gel, the insert loss of the modulator will monotonically increase with the thickness of film. It seems that the thinner the ion gel film is, the better the modulator performs. However, the mechanical strength of the ion gel will reduce with the thickness decreasing. Therefore, a proper thickness of ion gel film is important to balance the modulation properties of such devices. It should be noted that the hysteresis effect resulting from the slow polarization response time of ions in the ion gel won't vanish in such devices, yet we can reduce it to improve the modulation speed of our device. We can choose ionic liquid with faster polarization response to increase the conductivity of ion gel. Geometry properties of our device can be further optimized to decrease the distance between graphene and the second electrode, and thus the RC time constant of ion gel. Moreover, small-molecule solvents can be introduced into the ion gel to improve the electrical properties of it [42], and thus the modulation performance of our devices. 4. Conclusions We demonstrated an ion gel-graphene hybrid THz modulator, which can be efficiently modulated by an external gate voltage. The device exhibits 43% modulation depth by changing the bias voltage from 0.4 V to 2 V. By sweeping the gate voltage forward and backward from 1 V to 1 V and measuring the transmission of the peak THz field 60 s after the voltage is changed, a hysteretic behavior is observed in the device. Moreover, the sample was measured in a continuous wave THz system and the results show that the hysteresis gradually decreases with the sweeping rate being decreased. It is also found that by increasing the voltage sweeping range, the hysteretic behavior can be enhanced. We also measured the temporal response of the device and fitted the experimental results by a double-exponential expression, which indicates there are two mechanisms that may cause the hysteresis. One is the slow polarization response time of ions in the ion gel, and the other is the charge trapping phenomenon due to the nonidealities in a CVD monolayer graphene. The slow response time in such structures could be promising in developing nest-generation nonvolatile memory devices. Acknowledgments This work was supported by the National Key Research and Development Program of China (with grant NO. 2017YFA0701004), the National Natural Science Foundation of China (Grant Nos. 61675145, 61722509, 61735012, 61420106006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.007. References [1] M. Tonouchi, Cutting-edge terahertz technology, Nat. Photonics 1 (2007)
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Carbon journal homepage: www.elsevier.com/locate/carbon
Hysteretic behavior in ion gel-graphene hybrid terahertz modulator Xieyu Chen a, Zhen Tian a, *, Jin Wang c, Yinghui Yuan a, Xueqian Zhang a, Chunmei Ouyang a, Jianqiang Gu a, Jiaguang Han a, Weili Zhang a, b, ** a Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Opto-electronics Information and Technology (Ministry of Education), Tianjin University, Tianjin, 300072, China b School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma, 74078, USA c School of Electronic Engineering, Tianjin University of Technology and Education, Tianjin, 300222, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2019 Received in revised form 23 August 2019 Accepted 2 September 2019 Available online 3 September 2019
Hysteretic behavior in ion gel-graphene hybrid terahertz (THz) modulator is presented. The ion gel gated graphene modulator was designed and fabricated by conventional wet-based graphene transfer method. The modulation performance and hysteretic behavior of the device was characterized by THz timedomain spectroscopy. The dependence of hysteresis on the sweeping voltage rate and sweeping range was explored in a continuous wave terahertz system. The temporal response of the sample was also measured and fitted by a double-exponential expression, which indicated that there were two mechanisms that might cause the hysteresis in the THz modulator. This hysteretic behavior is promising in developing nonvolatile memory devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Terahertz (THz) frequencies are located between the microwave and far infrared spectra of the electromagnetic radiation. In the past twenty years, many fascinating properties of THz waves were observed, suggesting promising applications in biology, medical science, communications, homeland security and non-destructive evaluations [1]. However, due to shortage of high performance optical components, such as power modulators, phase modulators, polarization devices and absorbers, THz applications have been very limited. Among those devices, intensity modulators [2e5] are essential elements in THz communication and imaging systems. Thus, investigating high performance modulators is very meaningful to practical THz applications. Graphene and its derivatives have attracted much attention for the past decade because of their extraordinary electronic and optoelectronic properties [6e8]. As a two-dimensional semiconductor, the carrier concentration of graphene can be extensively
* Corresponding author. ** Corresponding author. Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin University, Tianjin, 300072, China. E-mail addresses: [email protected] (Z. Tian), [email protected] (W. Zhang). https://doi.org/10.1016/j.carbon.2019.09.007 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
changed by optical pump [9e11], external gate voltage [12] or chemical doping [13] and thus its properties. Moreover, compared with traditional bulk semiconductors and other 2D materials, graphene has many benefits: (1) Graphene exhibits mechanical flexibility, broad spectral response, compactness and isotropy. (2) Graphene has outstanding carrier mobility compared with other 2D materials owing to its symmetric conical band structure. (3) The uniform wafer-scale growth of graphene can be achieved by chemical vapor deposition technique, which makes graphene costeffective. (4) Graphene is stable in the air environment without any protection sealing. Therefore, graphene has widely been used in optical modulators from the microwave [14,15], terahertz [16,17], infrared [18,19] to visible [20] regimes. At THz frequencies, the transmission of graphene is dominated by the intraband transitions and can be significant modulated up to nearly 100% in theory. A conventional graphene-SiO2-semiconductor structure was initially designed to modulate the transmission of THz wave [17,21e30]. However, due to relatively small capacitance of such devices, the bias voltage applied to the graphene was always too high. In recent years, by replacing the insulator layer, such as SiO2, with a solid state ion gel electrolyte, the THz transmission of graphene can be modulated efficiently by a small gate voltage [31e37]. Although a large number of works on ion gel-graphene hybrid modulators were reported [19,20,31e37], the modulation speed and hysteretic behavior in such devices remain unexplored in the THz regime. In this article, we present a systematic investigation of the
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hysteretic behavior in an ion gel gated graphene THz modulator. The device was designed and fabricated using conventional wetbased graphene transfer method and we experimentally demonstrated that the device was able to deliver 43% modulation depth by switching the gate voltage from 0.4 V to 2 V. The transmission of THz peak was measured 60 s after the voltage was changed during the process of sweeping the gate voltage forward and backward from 1 V to 1 V. A hysteretic behavior was experimentally observed and characterized. The dependence of the hysteresis on the sweeping rate and sweeping range of the gate voltage was also studied in a continuous wave THz system. The temporal response of the modulator was consequently investigated and the results indicated that there were two mechanisms that could lead to the hysteretic behavior. The hysteresis of the ion gel-graphene hybrid THz modulator is meaningful to improve the performance of the modulator and promising in developing photonic memory devices. 2. Device fabrication As shown schematically in Fig. 1a, the proposed device is composed of three main components: ion gel, monolayer graphene, and quartz substrate. The device was fabricated by conventional wet-based transfer method. First, a commercial CVD-grown monolayer graphene (1 cm 1.2 cm) covered with PMMA film was transferred onto the quartz wafer. Then, after removing the PMMA by acetone, a conductive silver adhesive electrode was fabricated on the graphene. Second, the ion gel solution was prepared by dissolving poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) (purchased from SigmaAlorich) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSA]) (purchased from Aladdin) in acetone. The weight ratio between the polymer, ionic liquid and the solvent was 1:4:7. The solution was then stirred at 50 C for 2 h to form a homogenous solution. Next, the solution was spin-coated on the graphene at a rate of 3000 r.p.m. Then the device was baked at 70 C for 24 h to remove the acetone solvent [36]. Finally, another electrode was fabricated on the ion gel. An optical image of the free standing ion-gel used in our sample is shown in Fig. S1. The modulation mechanism of the device is illustrated in Fig. 1b. When there is no bias voltage, free cations and anions will be distributed in the ion gel randomly because of thermal motion. The graphene is p-doped due to the adsorbed oxygen. When an external
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voltage is applied to the device, the electrodes will create an electric field and attract oppositely charged ions from the solution, forming what is known as an electrical double layer (EDL) near the graphene and the upper electrode. Since the thickness of EDL is very thin, this configuration yields very large electric field next to the graphene and thus large carrier concentration on it [34]. Therefore, the Fermi energy of the graphene can be efficiently modulated by a small bias voltage, which is highly desired in THz modulators. 3. Results and discussion To demonstrate the modulation ability of the device, we experimentally measured the sample shown in the insert of Fig. 2c using a typical THz time-domain spectroscopy (THz-TDS) system (Fig. S4). The electric field transmissions of the air reference and the device under different voltages are shown in Fig. 2a. When the bias voltage is increased from 0 V to 2 V, the THz pulse gradually decreases in magnitude. However, because the graphene is p-doped in air, the transmission pulse is not the highest under 0 V voltage. When we applied 0.4 V gate voltage to the device, the amplitude of THz pulse reaches the maximum, which means the Fermi level of the graphene is close to the charge neutral point (CNP) under 0.4 V voltage and the graphene keeps hole-doped with the gate voltage swept from 0.4 V to 2.0 V. It should be noted that owing to the symmetric conical band structure of graphene, the terahertz modulation property of the graphene under electrondoped is similar to the hole-doped graphene. Therefore, we only show the terahertz amplitude modulated by hole-doped graphene (0.4 Ve2.0 V) here, which can clearly show the modulation ability of our device. However, to further understand the physical mechanism of the device, we also investigate the modulation property of the electron-doped graphene, as illustrated in Fig. S2. In Fig. 2b, the ~R ðuÞ normalized transmission spectra defined as j~tðuÞj ¼ ~ ES ðuÞ=E ~S ðuÞ and E ~R ðuÞ are Fourier transforms of the are also shown, where E measured terahertz pulses of the sample and air reference, respectively. The THz transmission of the sample gradually decreases as the gate voltage is increased from 0.4 V to 2 V. It should be noted that the transmission is lower at high frequency due to absorption of the quartz substrate and ion gel layer, which is further explained in supplementary materials section 3. We also show the normalized transmission of the peak THz field as a function of gate voltages applied to the graphene layer in Fig. 2c. The device achieves a modulation depth up to 43% by switching the gate voltage
Fig. 1. (a) Schematic diagram of the ion gel-graphene hybrid THz modulator. (b) Manipulation of the Fermi level of graphene in contact with an ion gel electrolyte under a gate voltage Vg. (A colour version of this figure can be viewed online.)
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Fig. 2. (a) Measured time-domain transmission signals of the air reference and the sample under different gate voltages. (b) Normalized experimental transmission spectra of the sample with the gate voltage varying from 0.4 V to 2.0 V. (c) Normalized transmission of the peak THz field as a function of gate voltages. Inset: image of the fabricated sample. (A colour version of this figure can be viewed online.)
from 0.4 V to 2 V. The modulation depth is defined as M ¼ ðTVg T0:4V Þ=T0:4V 100%, where TVg and T0:4V are the transmissions of the sample at peak THz field under Vg and 0.4 V voltages. As described above, the device can efficiently modulate the THz wave by only applying a small bias voltage. It should be noted that by further increasing the gate voltage above 2.0 V, a larger modulation depth can be achieved. However, this may introduce irreversible damage to the modulator due to the relatively small electrochemical window of the electrolyte [32]. Therefore, the highest gate voltage used in our manuscript is 2.0 V which is quite safe. To explore the hysteretic behavior of the ion gel-graphene integrated THz modulator, we measured the transmission at the peak THz field 60 s after the gate voltage was changed. As shown in Fig. 3a, the voltage was swept from 0 to 1 V and back to 0 V, then to 1 V and back to 0 V with a voltage interval of 0.2 V. A considerable amount of gate controlled optical hysteresis can be observed. As the gate voltage is increased from 0 V to 1 V, the transmission decreases gradually as mentioned earlier. When the voltage is swept from 1 V back to 0 V, the transmission increases continuously. However, under the same voltages of two sweeping processes, the transmission of the sample is lower when the gate voltage is decreased back to 0 V. A similar hysteretic behavior can be observed when we sweep the voltage from 0 to 1 V and back to 0 V. Moreover, the voltage scan leads to a positive shift of the neutral point in the graphene. The hysteresis in the modulator may origin from the slow polarization response time of ions in the ion gel and the charge trapping phenomena at the defect sites or grain boundaries in a large-area CVD monolayer graphene [17,19,38].
Next, we schematically describe the Fermi level of the graphene as a function of sweeping voltages, as shown in Fig. 3b. Without any gate voltage (point A), the Fermi level is slightly lower than the Dirac point due to unintentional p-dopants. When the gate voltage is increased from 0 V to 1 V (point B), the Fermi level decreases, leading to the enhancement of THz absorption and reflection due to greater intraband transition. Then the voltage falls back to 0 V, and the Fermi level increases gradually. However, due to the hysteretic behavior, the Fermi level under 0 V is lower than initial state. As the voltage is swept from 0 V to 1 V (point D), the Fermi level continuously rises. During the process, the Fermi level will reach the Dirac point (point C) at VDirac , corresponding to the maximum transmission. When the gate voltage is swept back to 0 V, a positive shift of VDirac is observed due to the hysteresis in the modulator. To explore the relationship between the magnitude of the hysteresis and the sweeping rate of the gate voltage, the sample was measured in a continuous wave THz system (Fig. S5) under different sweeping rates. The system comprises a 0.1 THz source and a fast THz detector, both purchased from Terasense Inc. The output power of the source is 88 mW and the response time of the detector is 1 ms. Thus, the time resolution of our experimental system can reach microsecond which is much smaller than the time scale of hysteretic behavior in the modulator. And a multi-function generator (WF 1974 purchased from NF Corporation) was used to apply the gate voltage to the modulator. The transmission power of our sample with the gate voltage swept from 0 V to 1.5 V and back to 0 V, then to 1.5 V and back to 0 V under different sweeping rates is shown in Fig. 4. A hysteretic behavior can be observed at each of the four sweeping rates. However, the magnitude of the hysteresis
Fig. 3. (a) Measured transmission of the peak THz field 60s after the voltage was changed. And the voltage was swept forward and backward from 1 V to 1 V. The arrows denote the sweeping direction of bias voltage (b) Schematic of the Fermi level as a function of the swept gate voltage. (A colour version of this figure can be viewed online.)
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Fig. 4. Transmission power of the sample recorded under four different gate voltage sweeping rates (1.2, 0.6, 0.3, 0.06 V/s). The arrows denote the sweeping direction of the bias voltage. (A colour version of this figure can be viewed online.)
exhibits a significant dependence on sweeping rate of the gate voltage. When the voltage is swept under the sweeping rate of 1.2 V/S, as illustrated in Fig. 4a, the hysteresis approaches the maximum and the shift of VDirac due to the hysteretic behavior is ~0.68 V. As the sweeping rate is decreased, the shift of VDirac decreases from 0.68 V (1.2 V/S) to 0.35 V (0.6 V/S) then to 0.1 V (0.3 V/ S), as shown in Fig. 4a, b, c, respectively. When the sweeping rates is as slow as 0.06 V/S, the shift of VDirac almost disappears and the hysteretic behavior is minimum. It indicates that the hysteresis in our device is on a time scale of a few seconds, which is consistent with the previous report [38]. It should be noted that the hysteretic behavior resulting from the slow polarization response time of ions is very common in ion gel based devices [19,39]. As shown above, when an external voltage is applied to the device, the electrodes will create electric field and attract oppositely charged ions from the solution, forming what is known as an electrical double layer near the graphene layer. This configuration yields the efficient modulation of the carrier concentration on graphene. It indicates that the Fermi level of graphene under certain gate voltage is determined by ions concentration next to the graphene. When the gate voltage is swept the ions distribution will changed gradually, and thus the Fermi level of graphene. However, the polarization response time of ions is relatively slow. Therefore, if the sweeping rate of gate voltage is very high, the redistribution of ions will not be completed. In this case, compared with the state of equilibrium, the transmission of our device will be different under certain gate voltage. So the hysteretic behavior caused by the ion gel will be observed with the gate voltage sweeping forward and backward. Next, the hysteretic behavior of the modulator under different gate voltage sweeping ranges is investigated, as shown in Fig. 5. The voltage sweeping rate is 0.8 V/s in measurement. The hysteretic can be observed under all sweeping ranges, however the shift of VDirac (dVDirac ), namely the magnitude of the hysteresis, varies with sweeping range. When the gate voltage is swept between 2 V and 2 V, dVDirac is ~0.66 V. As the voltage sweeping range is decreased, dVDirac gradually decreases from 0.62 V (in the range of 1.6 V) to 0.52 V (in the range of 1 V) then to 0.49 V (in the range of 0.8 V), as illustrated in Fig. 5f. When the range of voltage decreases to 0.2 V, we cannot observe the shift of VDirac due to lower voltage of VDirac , as shown in Fig. 5e. The decrease of dVDirac indicates that the hysteretic behavior in the sample will reduce by decreasing the
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sweeping range of the gate voltage. However, as demonstrated previously, the hysteresis attributed to the slow polarization response time of ions in the ion gel is not sensitive to the sweeping range due to the similar concentration of inversed charge at a certain gate voltage under a similar sweeping rate [38]. Thus, there may exist another mechanism causing the hysteretic behavior which coexists with the slow ion mobility in the ion gel. The resulted hysteresis exhibits a significant dependence on sweeping range of the gate voltage. To further clarify the underlying mechanism of the hysteretic behavior in the modulator, we measured the temporal behavior of the output THz power under different gate voltages in the continuous wave THz system. The top panels of Fig. 6a and b shows the square-wave voltage applied to the modulator and the lower panels illustrate the transmission of the sample normalized to the output power under 0 V voltage. The transmission power decreased when a positive gate voltage of Vg was switched and then returned to the initial value as the gate voltage was switched back to 0 V. However, the rise time under 1 V voltage was much slower than that under 0.5 V, which indicates that the hysteretic behavior in our sample will increase with increasing voltage sweeping range. In order to fit the time response, a double-exponential expression with the fast and slow relaxation processes was used under different gate voltages. The fitting results are shown as the red lines in the lower panel of Fig. 6a. It can be seen that excellent agreement is obtained between the fitting lines and the experimental measurements. The double-exponential used in the fitting is
TðtÞ ¼ A0 þ A1 eðtt0 Þ=t1 þ A2 eðtt0 Þ=t2 where t1 and t2 are the time constants of two relaxation processes, respectively. The retrieved fitting parameters are summarized in Table 1. When the gate voltage was changed between 0 V and 1 V, the time constants were 0.94 s and 14.00 s for the rise. When the positive voltage is decreased to 0.5 V, the time constant corresponding to the fast decay process is varied a little to 1.04 s while the other time constant is decreased a lot to 7.24 s. It indicates that the hysteretic behavior in the sample can be attributed to two different mechanisms. The fast decay process originated from the slow ion mobility in the ion gel is not sensitive to the gate sweeping range, as demonstrated in the previous work [38]. While the slow process may be resulted from the charge trapping phenomena at the defect sites and grain boundaries due to non-idealities in a CVD monolayer graphene or the sample fabrication [17]. Therefore, when the gate voltage is increased, the charge trapping will be enhanced and thus the hysteretic behavior. We also investigated the temporal response of the modulator under negative gate voltages, as illustrated in Fig. 6b. When the gate voltage is switched from 0 to 1 V, the transmission power is increased first and then decreased. It indicates that the Fermi level of the graphene rises from the initial location lower than Dirac point to Dirac point and finally to the location above it. As the gate voltage is turned off, the transmission power goes through a similar process which means that the Fermi level passes through the Dirac point again. The phenomenon can be observed only when the negative gate voltage Vg is lower than VDirac . As shown in the inset of Fig. 6b, when the gate voltage is only 0.2 V, the Fermi level would not pass through the Dirac point. As illustrated in Table 1, the decay time of the THz modulator can be on a time scale of 20 s, which will be a hurdle to the fast response to gate voltage and severely constrain the modulation speed of the modulator. The temporal response can be improved by using high quality graphene and ionic liquids with higher ionic conductivity. Although the slow response time of our sample is not ideal for high-speed THz modulation, it could be applicable in other fascinating fields, such as
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Fig. 5. Measured power transmission under five different gate voltage sweeping ranges (2, 1.6, 1, 0.8, 0.2 V) without changing the sweeping rate (0.8 V/s) are shown in (a), (b), (c), (d) and (e), respectively. The arrows denote the sweeping direction of the gate voltage. (f) Shift of VDirac as a function of gate voltage sweeping range. (A colour version of this figure can be viewed online.)
Fig. 6. Temporal behavior of the ion gel-graphene hybrid THz modulator. (a) The positive square-wave voltage applied to the graphene (top panel) and the time response of the modulator (low panel) under different gate voltages (1 V and 0.5 V). The fitting is shown as red lines. (b) Negative square-wave voltage applied to the graphene (top panel) and the time response of the modulator (low panel) under three different gate voltages (1, 0.5, 0.2 V). (A colour version of this figure can be viewed online.)
Table 1 Retrieved fitting time constants under different gate voltages. Time Constant
t1 t2
0 V~1 V
0 Ve0.5 V
Fall
Rise
Fall
Rise
0.48s 7.06s
0.94s 14.00s
0.63s 7.34s
1.04s 7.24s
electrically controllable photonic memory devices. It should be mentioned that the modulation performance of our device is closely related to the electrical properties of ion gel such as capacitance and resistance. First, the modulation depth of our device is determined by the graphene conductivity under different gate voltages, and thus the Fermi level of graphene. As demonstrated in previous work [40], in such modulation configuration the Fermi level is related to the gate voltage by jEF j ¼ ZvF ðpNÞ1=2. Here,
- and vF are the reduced Planck's constant and the Fermi velocity, respectively. N is the total carrier density expressed by N ¼ 1=2 ðn20 þ a2 jDVj2 Þ . Where n0 , DV and a are the carrier density at the conductivity minimum, the gate voltage relative to the charge neutral point and the specific capacitance of the ion gel dielectric, respectively. According to the formula above, the modulation depth of our device will increase with the capacitance increasing. Second, in such field effect transistor, the modulation speed is mainly dominated by the RC time constant of ion gel given by RC ¼ C 0 L=s [41]. Where C 0 , L, s are the specific capacitance, the distance between the graphene and the electrode on the ion gel layer and the conductivity of ion gel, respectively. Thus, the response time of our device is also related to the electrical properties of ion gel. The thickness of the ion gel used in our sample is a constant. However, the physical properties of ion gel are related to its thickness, and thus the modulation performance of our device. In previous work,
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the relationship between the electrical properties and the thickness of the ion gel has been systematically investigated [41]. It can be seen that the capacitance of the ion gel mainly depends on the thickness of the double layer rather than the ion gel film. Therefore, the capacitance is insensitive to the film thickness and so is the modulation depth of our device. Moreover, the conductivity of the ion gel is also independent of the film geometry. However, the distance between the graphene and the second electrode increases with the film thickness increasing, and thus the RC time constant of ion gel. So when the ion gel film is thicker the modulation speed of our device will decrease. Moreover, due to the terahertz absorption of ion gel, the insert loss of the modulator will monotonically increase with the thickness of film. It seems that the thinner the ion gel film is, the better the modulator performs. However, the mechanical strength of the ion gel will reduce with the thickness decreasing. Therefore, a proper thickness of ion gel film is important to balance the modulation properties of such devices. It should be noted that the hysteresis effect resulting from the slow polarization response time of ions in the ion gel won't vanish in such devices, yet we can reduce it to improve the modulation speed of our device. We can choose ionic liquid with faster polarization response to increase the conductivity of ion gel. Geometry properties of our device can be further optimized to decrease the distance between graphene and the second electrode, and thus the RC time constant of ion gel. Moreover, small-molecule solvents can be introduced into the ion gel to improve the electrical properties of it [42], and thus the modulation performance of our devices. 4. Conclusions We demonstrated an ion gel-graphene hybrid THz modulator, which can be efficiently modulated by an external gate voltage. The device exhibits 43% modulation depth by changing the bias voltage from 0.4 V to 2 V. By sweeping the gate voltage forward and backward from 1 V to 1 V and measuring the transmission of the peak THz field 60 s after the voltage is changed, a hysteretic behavior is observed in the device. Moreover, the sample was measured in a continuous wave THz system and the results show that the hysteresis gradually decreases with the sweeping rate being decreased. It is also found that by increasing the voltage sweeping range, the hysteretic behavior can be enhanced. We also measured the temporal response of the device and fitted the experimental results by a double-exponential expression, which indicates there are two mechanisms that may cause the hysteresis. One is the slow polarization response time of ions in the ion gel, and the other is the charge trapping phenomenon due to the nonidealities in a CVD monolayer graphene. The slow response time in such structures could be promising in developing nest-generation nonvolatile memory devices. Acknowledgments This work was supported by the National Key Research and Development Program of China (with grant NO. 2017YFA0701004), the National Natural Science Foundation of China (Grant Nos. 61675145, 61722509, 61735012, 61420106006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.007. References [1] M. Tonouchi, Cutting-edge terahertz technology, Nat. Photonics 1 (2007)
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