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Abstract
A terahertz (THz) nonvolatile in situ electrically erasable-rewritable photo-memory based on indium oxide (In2O3) nanoparticles is reported. The In2O3/PEDOT:PSS/quartz sample increases its conductivity and attenuates its THz transmission under optical excitation. When this optical excitation is terminated, the modulated THz transmission recovers to its original value in an air environment slightly. The modulated THz transmission recovered more rapidly with increasing bias voltage. Nonvolatile digital information storage is enabled when the In2O3/PEDOT:PSS/quartz structure is encapsulated in nitrogen. The photo-memory can be rewritten after in situ electrical erasure. The results show that in situ electrically erasable terahertz nonvolatile rewritable photo-memories are feasible.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) waves have low photon energy, high transparency, and broadband characteristics that make these waves attractive for use in communications, imaging, biological, medical, and security applications [1–5]. With the rapid continuing development of THz wave technology, considerable efforts have been devoted to the development of functional terahertz devices, including modulators [6–11], filters [12], absorbers [13–24], polarizers [22,23], and metamaterials [24–28]. THz read-only multi-order nonvolatile rewritable information storage based on use of indium oxide nanoparticles was investigated in our previous work [29]. Light-based excitation of In2O3/quartz structures increases their conductivity while simultaneously attenuating their THz transmission properties. Modulated THz waves show no obvious changes over long periods in nitrogen gas after the optical excitation is terminated. Multi-order nonvolatile digital information storage based on use of various light intensities and the storage characteristics of indium oxide/quartz can thus be realized using the In2O3/quartz structure. However, if the THz storage must be rapidly restored to its original state, the sample must be placed on a hot table for 15 min for thermal annealing at 100°C [29]. This process is both complex and time-consuming. Therefore, use of electric modulation may provide a more effective way to erase such a rewritable photo-memory.
The polymer mixture poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) offers properties that include high transparency, excellent thermal stability, high carrier mobility and high conductivity [30–34]. In particular, the conductive properties of PEDOT:PSS can be tuned via chemical doping using ethylene glycol (EG) and dimethylsulfoxide (DMSO) [32]. Kim et al. reported the fabrication and characterization of an organic light-emitting device that was used a highly conductive form of PEDOT:PSS as a hole-conducting layer [33]. Consequently, PEDOT:PSS has been widely used as a transparent conductive layer in a manner similar to the use of graphene in organic electronics and in THz functional devices such as capacitors and transparent electrodes. Du et al. reported the use of DMSO-doped-PEDOT:PSS films as transparent electrodes in an electrically tunable THz liquid crystal phase shifter [34]. Mixing PEDOT:PSS with various solvents, e.g., DMSO, allows the polymer mixture’s conductivity to be improved by up to two or even three orders of magnitude [30,32]. A high-efficiency active bidirectional electrically-controlled THz device based on 10 vol.% DMSO-doped PEDOT:PSS with low-power photo-excitation was investigated in our previous work [30]. Therefore, DMSO-doped-PEDOT:PSS films with their easy soluble processing, high transparency and high conductivity are suitable materials for replacement of graphene films in THz functional devices [30].
In this work, a THz in situ electrically-erasable photo-memory based on indium oxide nanoparticles is investigated. Light excitation of the In2O3/DMSO-doped-PEDOT:PSS/quartz structure increases its conductivity while simultaneously attenuating its THz transmission. After the optical excitation is terminated and a bias voltage is applied to both ends of the sample, the change in the modulated THz wave can be measured over a long period in air or nitrogen atmospheres.
2. Experimental details
A schematic of the measurement setup used and a detailed image of the electrically-erasable rewritable THz device structure are shown in Fig. 1(a). A 30-nm-thick 10 vol.% DMSO-doped PEDOT:PSS solution was spin-coated onto a clean 1-mm-thick quartz substrate. The resulting DMSO-doped PEDOT:PSS layer was dried at 100°C for 15 min. Parallel silver line electrodes were then deposited by thermal evaporation on top of the DMSO-doped PEDOT:PSS layer. The gap between the line electrodes is 1 mm. In the next step, In2O3 nanoparticles (particle diameter: 10 nm) were dissolved in ethanol and spin-coated on the prefabricated sample. The spin-coated In2O3 film was then annealed at 340°C for 1 h in air environment.
Fig. 1. (a) Schematic of the terahertz time-domain spectroscopy (THz-TDS) system and structure of the electrically-erasable rewritable THz device. (b) Scanning electron microscopy (SEM) images of the indium oxide sample.
A terahertz time-domain spectroscopy (THz-TDS) system was used to measure the transmission spectra of the fabricated In2O3/PEDOT:PSS/quartz hybrid device under the application of various bias voltages (Vg) and continuous wave (CW) optical pump illumination at 450 nm with power P at normal incidence. All measurements were performed at room temperature under ambient conditions with humidity of less than 5%. The In2O3 film was also observed by scanning electron microscopy (SEM; Hitachi S-4800) at an accelerating voltage of 15 kV. The surface and fracture surface images of the In2O3 sample were observed, as shown in Fig. 1(b).
3. Results and discussion
The THz time-domain spectra through the In2O3/PEDOT:PSS/quartz structure were measured using a semiconductor continuous wave (CW) laser at various intensities; the laser had an operating wavelength of 450 nm, as shown in Fig. 2(a). When the semiconductor laser’s optical fluence was 62 mW/cm2, the THz transmission intensity decreased to 50%. Figure 2(b) shows the THz transmission spectra obtained through an In2O3/quartz structure under semiconductor laser irradiation at various intensities. Upon laser irradiation, the THz light transmittance of the structure decreased significantly. When the laser fluence was 119 mW/cm2, the structure’s THz transmission intensity decreased to almost 20%. To evaluate the modulation properties of the In2O3/PEDOT:PSS/quartz sample, we introduced a modulation factor (MF) that represented the variation of the total transmitted THz power caused by light excitation. The formula for the MF is as follows [35]:
Fig. 2. (a) TDS through an In2O3/quartz structure irradiated using 450 nm light. (b) THz transmission spectra through an In2O3/quartz structure irradiated using 450 nm light. (c) Modulation factors of the In2O3/quartz sample under external excitation at various wavelengths and laser fluences. (d) Carrier density and photoconductivity of the In2O3/quartz sample under external excitation at various wavelengths and fluences.
To determine the modulation mechanism in the In2O3/PEDOT:PSS/quartz structure, the conductivity was calculated by measuring the resistance per unit volume of the sample. Figure 2(d) shows the relationship between the conductivity of the In2O3/PEDOT:PSS/quartz structure and the various external excitation light intensities. The conductivity of the In2O3/PEDOT:PSS film was enhanced as the excitation light intensity increased. When the intensity of the 450 nm laser was 119 mW/cm2, the conductivity of the In2O3/PEDOT:PSS/quartz structure reached a maximum. The conductivity was enhanced upon 450 nm laser excitation and showed an increase from 12.1 S/cm to 12.7 S/cm. To provide further confirmation of the explanation of the modulation mechanism, we calculated the carrier density for the In2O3/PEDOT:PSS/quartz structure. To perform this calculation, we extracted the complex dielectric constant of the sample from the transmission spectra measured by THz-TDS under laser irradiation at various power levels. The carrier density N can then be calculated based on the Drude model using the equation [35]
where m is the effective electron mass, ${\varepsilon _0}$ is the permittivity of a vacuum, e is the electronic charge, and ${\omega _p}$ is the plasma frequency, which can be expressed approximately as follows [35]: where $\varepsilon {}_r$ and ${\varepsilon _i}$ are the real and imaginary parts of the dielectric constant, respectively. $\omega$ is the THz frequency and the value of $\omega$ was chosen to be 1 THz, at which the THz power is at a maximum in the transmission spectra shown in Fig. 2(a). The calculated carrier density of the In2O3/PEDOT:PSS/quartz structure is shown in Fig. 2(d). Clearly, the trend for the change in carrier density showed good agreement with that for the MF and conductivity values, as shown in Figs. 2(c)–(d).Figure 3(a) shows the power spectra obtained for THz transmission through the DMSO-doped PEDOT:PSS/quartz structure and the quartz substrate. The spectra indicate that the insertion loss of the DMSO-doped PEDOT:PSS layer was approximately 10%. Figure 3(a) shows the measured normalized time-domain signals from the PEDOT:PSS/quartz sample under various bias voltages ranging from 0 V to 5 V. The results showed that increases in the bias voltage also caused a slight rise (almost 8%) in the THz transmission intensity, because the carrier concentration of DMSO-doped PEDOT:PSS layer decreased under the action of bias voltage. In particular, the distinctions between the THz transmission spectra measured under application of a bias voltage and under no bias voltage were unconspicuous, although the current increased to 180 mA. Figure 3(b) shows the bias voltage dependence of the THz intensities of the transmission and the current from the PEDOT:PSS/quartz structure. The results show that the PEDOT:PSS layer has little effect on THz transmission under application of an electric current and it thus has great potential for use as a THz transparent electrode.
Fig. 3. (a) Measured normalized frequency-domain signals from PEDOT:PSS/quartz sample under various bias voltages ranging from 0 V to 5 V. The inset shows the THz transmission frequency-domain signals through the PEDOT:PSS/quartz structure and the quartz substrate. (b) Bias voltage dependences of THz intensities for transmission (black symbol) and current (red symbol) of the PEDOT:PSS/quartz structure.
When the optical excitation was terminated, the modulated THz transmission recovered to its original value in an air environment in slightly more than 2 h, as shown by the black line in Fig. 4(a). This phenomenon has been discussed in our previous work. The process of hole absorption by oxygen defects is slow, which causes the photo-excitons to dissociate slowly [29]. For the THz storage structure to be restored quickly to its original state, the sample must be placed on a hot table for 15 min for thermal annealing at 100°C. Additionally, this method involves the samples being removed from the measurement system, which could take a long time, and the process is both complex and time-consuming. In the experiments in this work, an in situ electrically-erasable rewritable photo-memory was constructed, where the DMSO-doped PEDOT:PSS layer acted as the THz transparent electrode and the In2O3 layer acted as the material with the photoconductivity change. Figure 4(a) shows the attenuation of the THz transmittance over time of an In2O3/PEDOT:PSS/quartz structure under various bias voltages (0 V, 2.5 V, 5 V and 7.5 V). It was observed that the modulated THz transmission recovered more rapidly as the bias voltage increased. In particular, the modulated THz transmission recovered in 40 s at 7.5 V, as shown by the red line in Fig. 4(a). Figure 4(b) shows the attenuation over time of the THz transmittance of an In2O3/graphene/quartz structure under various bias voltages. The modulated THz transmission also recovered rapidly with increasing bias voltage in this structure. When compared with the graphene-based device, the In2O3/PEDOT:PSS/quartz sample achieved quicker recovery at a lower bias voltage than that applied to the In2O3/graphene/quartz sample, particularly at application of a bias voltage (5 V). Figure 4(c) shows the relationship between the THz transmission and the resistance during the recovery process. With increasing THz transmission, the resistance of the In2O3/PEDOT:PSS/quartz sample also increased. These results indicate the mechanism of the electrically-erasable modulation process. The bias voltage applied to the transparent electrode changes the energy level of the PEDOT:PSS layer to promote transfer of photo-generated carriers in In2O3 to the PEDOT:PSS layer. There is a bend in the energy band in PEDOT:PSS under application of the bias voltage that could also cause the photo-generated carriers of In2O3 to be transferred to the PEDOT:PSS layer. This process induces a reduction in the photo-carrier density in In2O3, which then enhances the THz transmission. In general, higher bias voltages led to higher modulation recovery speeds.
Fig. 4. (a) Attenuation of THz transmittance over time of an In2O3/PEDOT:PSS/quartz structure under various bias voltages. (b) Attenuation of THz transmittance over time of an In2O3/graphene/quartz structure under various bias voltages. (c) Variation of resistance (red symbol) and THz transmittance (black symbol) over time.
When the In2O3/PEDOT:PSS/quartz structure was encapsulated in an inert gas (nitrogen), the THz transmission showed no obvious change after optical excitation, as indicated by the black line in Fig. 5(a). The high density of the carriers and the written information were retained for more than 60 min because the holes could effectively be captured by oxygen vacancies and the photo-electrons could then persist for longer periods of time in an oxygen-free atmosphere that contained no excess holes with which they could recombine [29,36–38]. An in situ electrically-erasable rewritable photo-memory was thus demonstrated in a nitrogen gas atmosphere. Figure 5(a) shows the attenuation of the THz transmittance over time of an In2O3/PEDOT:PSS/quartz structure under various bias voltages (0 V, 4 V, 5 V) in nitrogen. Similarly, it was also observed that the modulated THz transmission recovered rapidly with increasing bias voltage. This modulated THz transmission recovered in 1 min at a bias voltage of 5 V, as shown by the red line in Fig. 4(a). In this case, the high density of the carriers and the written information could again be retained for more than 60 min. To evaluate the storage characteristics of the In2O3/PEDOT:PSS/quartz sample, we took measurements while repeating the optical excitation and recovery process under application of a bias voltage, as shown in Fig. 5(b). These results show that an in situ electrically-erasable THz nonvolatile rewritable photo-memory can be realized based on In2O3 nanoparticles.
Fig. 5. (a) Attenuation of THz transmittance over time of In2O3/PEDOT:PSS/quartz structure under various bias voltages in a nitrogen atmosphere. (b) Storage and repetition characteristics of the In2O3/PEDOT:PSS/quartz sample under bias voltage of 5 V.
4. Conclusions
In summary, a THz nonvolatile in situ electrically-erasable rewritable photo-memory based on indium oxide (In2O3) nanoparticles is investigated. The In2O3/PEDOT:PSS/quartz sample caused its conductivity to increase while attenuating its THz transmission under optical excitation. When the optical excitation was terminated, the modulated THz transmission recovered to its original value in an air environment in a little over 2 h. The modulated THz transmission also recovered more rapidly with an increase in the applied bias voltage. Nonvolatile digital information storage was obtained when the In2O3/PEDOT:PSS/quartz structure was encapsulated in nitrogen. This photo-memory could then be rewritten after in situ electrical erasure. These results show that an in situ electrically-erasable THz nonvolatile rewritable photo-memory can be realized in practice for digital information storage.
Funding
National Natural Science Foundation of China (61505125); Capital Normal University (Youth Innovative Research Team); Project of High-level Teachers in Beijing Municipal Universities in the Period of 13th Five-year Plan.
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