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Fermi-level-controlled semiconducting-separated carbon nanotube films for flexible terahertz imagers Daichi Suzuki, Yuki Ochiai, Yota Nakagawa, Yuki Kuwahara, Takeshi Saito, and Yukio Kawano ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00421 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Nano Materials

Fermi-level-controlled semiconducting-separated carbon nanotube films for flexible terahertz imagers Daichi Suzuki1, Yuki Ochiai1, Yota Nakagawa1, Yuki Kuwahara2, Takeshi Saito2, and Yukio Kawano1,* 1

Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of

Technology, Tokyo 152-8552, Japan 2

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), Ibaraki 305-8565, Japan KEYWORDS Terahertz, Flexible imager, Carbon nanotube, Photothermoelectric effect, Chemical doping

ABSTRACT

Carbon-nanotube-related (CNT-related) materials and structures are highly anticipated as potential building blocks for future flexible electronics and photonics. Despite the various promising applications of CNT-related materials, one obstacle is the lack of ability to globally control and tune the Fermi level of microscale-thick CNT films because these films require a certain thickness to maintain their free-standing shape and freely bendable flexibility. In this work, we report on Fermi-level-controlled flexible and bendable terahertz (THz) imagers with chemically adjustable Fermi-level-tuning methods for CNT films. By utilizing the electronicdouble-layer technique with ionic liquids, we obtained an on/off resistance ratio (2758) for a

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semiconducting-separated CNT film with a thickness of 30 µm and tuned the Fermi level at an optimal gate voltage to maximize the THz detector performance. In addition, development of a gate-free tunable doping technology based on a variable-concentration dopant solution enabled the fabrication of a Fermi-level-tuned p-n junction CNT THz imager. The demonstrated chemically tunable doping capability will facilitate the realization of flexible THz imaging applications, and, when combined with a low-cost fabrication method such as an ink-jet coating process, will lead to large-area THz photonic devices.

INTRODUCTION Carbon-nanotube-based (CNT-based) electronics and photonics are attracting considerable attention because of their unique electronic, optical, thermal, and mechanical properties1-5. CNTrelated materials and structures can be utilized in various applications, such as transparent conducive films for touch panels and electronic paper6,7, electromagnetic absorbers8,9, interconnection for electronic devices10,11, and nano/microelectromechanical systems12-14. Among the many potential applications of CNTs, their use in THz devices is highly anticipated and one of the more promising application fields. Previous studies have reported CNT-based THz detection involving several mechanisms: the bolometric effect15, high-frequency mixing16, photon-assisted tunneling17,18, and the photothermoelectric effect19-21. Though the detector operation of the former three mechanisms requires low-temperature environments, the utilization of the photothermoelectric effect has been demonstrated to enable room-temperature THz detection.


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In current THz technologies, one of the most critical issues is the development of THz cameras that are inevitably required for in-line industrial inspections, real-time daily monitoring, etc.22,23. Recently, we developed uncooled, bendable, and wearable THz cameras based on multi-arrayed CNT devices and demonstrated 360-degree omnidirectional THz imaging of bent samples24. This technology has enabled easy mapping of THz images, even for samples with three-dimensional surface curvatures, without the need for bulky systems, thus making flexible THz photonics possible. Despite strong advantages, our early type of CNT THz camera has plenty of room for performance enhancement with respect to the following points: The photothermoelectric effect originates from carrier drift generated by variations in the density of states (DOS)25,26. The CNT exhibits sharp variations in the DOS because of the quantum confinement effect and the onedimensional nature of the CNT, leading to excellent photothermoelectric performance27,28. Therefore, distinguishing semiconducting CNTs and metallic CNTs and tuning their Fermi level are important. Recent advances in CNT semiconductor-metal-separation technology29-31 provide strong possibilities for enhancing THz-sensing performance. However, no systematic studies that examine the physical factors governing the performance of CNT THz imagers, such as the Seebeck coefficient, THz coupling efficiency, and detector noise in terms of the Fermi-level position, have been reported. In this work, we investigated the THz detection performance for various types of CNT films, including a metallic-rich type and a semiconducting-rich type, and realized chemically tunable control of the Fermi-level position. From the viewpoint of an application to flexible THz cameras, although relatively high-thickness non-aligned CNT films (typically >10 µm) are needed to maintain mechanical strength and show freely bendable flexibility, globally and finely


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controlling the Fermi level of such thick CNT films is difficult because of their huge surface area32. Here, we successfully tuned the carrier density and resulting Fermi level of thick (~30 µm) CNT films using two chemical methods: gate-voltage tuning using ionic liquids as a gate dielectric and gate-free tunable doping using dopant solutions with various concentrations. We found that semiconducting-separated CNT films showed higher performance for THz detection and demonstrated an on/off resistance ratio (2758) through gate control, even for such a thick CNT film. This capability enabled us to evaluate an optimal gate voltage in terms of the Seebeck coefficient, THz absorption (THz coupling efficiency with the CNT film), and noise voltage. Furthermore, the tunable doping technology based on the concentration of dopant solution enabled us to control the Fermi level without using the gate electrode. Using this method, we fabricated a Fermi-level-tuned p-n junction CNT THz imager with an optimally positioned Fermi level. The technologies presented for Fermi-level tuning and enhancement of the THz detection properties for the CNT film are a general feature, and, along with THz detection, can be used in a broad range of CNT-based applications.

RESULTS AND DISCUSSION

The THz detection mechanism used here is based on the THz-induced photothermoelectric effect, which is described according to the simple model shown in Fig. S1 of the Supporting Information24. The generated voltage VTotal (the THz response) is expressed as ∝ − ×

(1)


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where SA and SB are the Seebeck coefficients of the materials [µV/K], and α is the absorption of incident THz waves [%]. One of the figures of merit of the detector performance is the noise equivalent power (NEP), which is defined as

NEP =

Noise voltage [V/√Hz] [W/√Hz] Sensitivity [V/W]

(2)

In this work, we studied three major factors closely linked to detector performance: the Seebeck coefficient, THz absorption rate, and noise voltage. We prepared ten types of flexible CNT films according to differences in structural and electrical properties: single-wall CNTs (SWCNTs) and multiwall CNTs (MWCNTs); metal-rich CNTs, semiconductor-rich CNTs, and metal/semiconductor-mixed CNTs; chemically doped (n-doped) CNTs33 and nondoped (naturally p-doped) CNTs; and aligned CNTs and randomly oriented CNTs. For the film preparation of distinct metal-semiconductor-separated SWCNTs, we used SWCNTs with an average diameter of 1.3 nm synthesized by an enhanced direct-injection pyrolytic synthesis (eDIPS) method. The SWCNTs were separated into metallic and semiconducting SWCNTs using the electric-field-induced layer formation (ELF) method, which is a carrier-free electrophoresis process conducted as follows: The SWCNTs were dispersed into an aqueous solution of 1 wt% polyoxyethylene (100) stearyl ether as a nonionic surfactant and then sonicated by a horn-type ultrasonic homogenizer. After precipitation of bundles and impurities through an ultracentrifugation process, an electric field was vertically applied to a monodispersed SWCNT solution, resulting in the preparation of electrophoretically separated metallic and semiconducting SWCNT solutions29,30. Figure 1 shows photographic images of the CNT solutions. Flexible CNT films were deposited via a filtration process (see Fig. S2 of the Supporting Information)34. To obtain a free-standing


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shape and flexibility, we fabricated CNT films with a typical thickness of 30 µm. These CNT films are naturally p-type in air. To prepare stable n-type CNT films, we used chemical doping techniques involving sodium hydroxide (NaOH) solutions containing crown ethers. We measured and evaluated the Seebeck coefficients of all the CNT films to obtain a higher photothermoelectric THz response. (For the measurement setup, see Fig. S3 of the Supporting Information.) Table 1 compares the Seebeck coefficients of all the CNT films prepared here. The results show that the Seebeck coefficients of the semiconducting SWCNT films (a few hundred µV/K) are much higher than those of other CNT films. This large difference in the Seebeck coefficient is basically attributed to their electronic structures. It is generally known that the Seebeck coefficient strongly depends on the derivative of the DOS at the Fermi level. In semiconducting SWCNT films, because the Fermi level can be shifted to the Van Hove singularity point of the DOS with a slight change in carrier concentration, the derivative of the DOS at the Fermi level is more prominent than other CNT films. This feature results in the large Seebeck coefficients of the semiconducting SWCNT films, indicating that the semiconducting SWCNT film is potentially more effective for a flexible THz imager. We also observed that the electrical conductivity in parallel with respect to the CNT alignment was 10 times higher than that in perpendicular. Therefore, the device noise can be reduced by a factor of √10 by aligning the SWCNTs in parallel. In addition, aligned SWCNTs exhibit strong polarization sensitivity in a broad frequency region. Despite the advantages of its low noise operation and polarization sensitivity, such aligned films are easily torn along the alignment direction of the tubes and its strong polarization dependence limits area of the THz detection only to the parallel direction


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with the alignment of SWCNTs. These features hamper the practical use of flexible and bendable applications. For this reason, we here used non-aligned SWCNT films. We next examined the noise voltage, which is one of the important factors for detector performance35. One of the advantages of using the CNT THz detector is that it can suppress the effect of noise arising from the dark current such as the shot noise and the flicker noise because it works under zero bias voltage (see Fig. S1 for the experimental result showing the non-bias operation). Accordingly, as shown in Fig. S4, the minimum limit for the noise voltage of this detector is primarily determined by the thermal noise given by+4-. ∆012, where kB is the Boltzmann constant ∆f is the frequency bandwidth, T is the temperature, and R is the resistance. The flicker noise appeared below 10 Hz, but we can avoid it by choosing a proper operation frequency of the CNT THz detector. However, compared with the metallic and mixed CNT films, the semiconducting CNT films generally tend to exhibit a higher resistivity, leading to a larger thermal noise voltage, i.e., a higher NEP. The reduction in the resistivity is therefore an effective approach to suppressing the thermal noise voltage. Consequently, we performed Fermi-level tuning of the semiconducting CNT film to increase its carrier density and reduce its resistivity. The gate control of the electrical properties of individual CNTs can be easily realized through well-known field effects using metal-insulator-CNT layered structures36-38; however, such a gating approach cannot work effectively deep inside microscale-thick CNT films, as used in this work, because of their randomly tangled structure and the resulting huge surface area. This feature leads to a low resistance on/off ratio relative to the gate voltage (low resistance controllability). There are several kinds of methods to adjust the Fermi level of the synthesized CNT film: aqueous/organic doping solvents, gaseous environment, and so on39. However, in order to study THz absorption


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phenomena with the variation in the Fermi level, it is necessary to continuously change the Fermi level of the thick CNT film after the fabrication of CNT THz detectors. For this reason, we utilized the electronic-double-layer (EDL) technique to continuously tune the Fermi level of the thick, flexible semiconducting CNT film after the filtration process. Figure 2(a) shows schematic and photographic illustrations of the EDL CNT transistor. The semiconducting CNT film (channel) and the mixed CNT film (gate) with Au electrodes were placed onto a glass substrate before lead wires were attached to the source and drain electrodes using Ag paste. All the metals were covered with a silicone sealant to prevent electrochemical reactions with the ionic liquid. The inside of the silicone sealant receptacle was filled with the ionic liquid (DEMETFSI, Kanto Chemical Co., Inc.). When a gate voltage was applied, electrons or holes were globally generated in the whole CNT network channel through the potential shift via local ionization. Figure 2(b) shows the experimental data obtained for the gate-controlled resistance variation in the CNT film. Figure 2(b) shows the resistance change in the semiconducting CNT film with a much higher on/off ratio of 2758 compared with those obtained for the metallic and mixed CNT films (Fig. S5). Additionally, the reduced resistance reached a value as low as that of the metallic CNT film. These results demonstrate that the EDL technique is useful for continuous tuning the Fermi level even for a microscale-thick CNT network and contributes to resistance reduction in a semiconducting CNT film. Based on the aforementioned technology and measurements, we compared the noise voltage spectra with and without Fermi-level optimization. Figure 2(c) shows that the minimum noise voltage reached the theoretical thermal noise limit of 0.6 nV Hz-1/2 at VG = -1.5 V and 57 nV Hz-


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1/2

at VG = 0.7 V and that the measured noise (~0.6 nV Hz-1/2) at VG = -1.5 V is approximately 87

times lower than that (~52 nV Hz-1/2) at VG = 0.7 V, leading to lower noise operation. We further studied the THz absorption spectra of the semiconducting CNT film as a function of the Fermi level. (See Fig. S6 of the Supporting Information for details of the measurement setup.) Figure 3(a) shows a relatively flat THz absorption over a broad THz frequency region that strongly depends on the gate voltage. As shown in Fig. 3(b), the data for the 2-THz absorption versus the gate voltage reveal that the THz coupling efficiency was varied by a factor of 16 (6.1% at the minimum and 98.2% at the maximum), indicating that the efficiency of the THz wave coupling with the CNT film could be controlled via the external gate voltage. As schematically shown in the energy band diagram of the semiconducting CNT film (Fig. 3(c)), the THz coupling efficiency mainly originates from the intraband transition due to the lower photon energy of THz waves (~4 meV at 1 THz) compared to the bandgap energy of semiconducting materials. When the Fermi level lies in the band gap, most of the irradiated THz waves are transmitted through the CNT film. By contrast, when the Fermi level enters the higher or lower band, the THz waves are well absorbed by the finite carriers in the CNT films. The characteristics observed for the gate-voltage dependence of the THz absorption spectra are consistent with this explanation. The results therefore indicate that the carrier doping via the gate voltage strongly contributes to THz wave coupling with the CNT film, indicating the importance of Fermi-level control of the performance of the optimizing detector. Based on all the findings presented, we fabricated a flexible THz imager utilizing a Fermi-levelcontrolled semiconducting CNT film. Although the EDL technique is a powerful method to tune the physical properties of CNT networks, the presence of the ionic liquid inevitably increases the


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total heat capacity of the device and hampers the temperature rise induced by THz irradiation, which in turn deteriorates and slows down the THz response generated by the photothermoelectric effect (See Fig. S7 of the Supporting Information for the transient response time of CNT THz imagers).. For this reason, as an alternative to the ionic liquid, we used crown ether as an n-type doping solution for CNT films. Figure 4 illustrates the n-type doping mechanism of the crown ether solution. When a simple salt of NaOH and 15-crown-5-ether are mixed in pure water, the cation of Na+ is tightly bound inside the ring structure of the crown ether. The counter anion of OH- chemically reacts with the CNT and transfers electrons into the CNT film, which tunes the Fermi level of the CNT film. The results of the Seebeck coefficient as a function of the concentration of NaOH and crown ether solution (Fig. 4(b)) show that the pristine p-type CNT films (Seebeck coefficient of 44.4 µV/K) were gradually shifted to n-type (Seebeck coefficient of -41.1 µV/K) with an increasing concentration of the NaOH doping solution. We confirmed the stability of the n-doping. Figure 4(c) displays the THz response mapping of the CNT THz detector. In general, the photothermoelectric effect were generated at the interfaces of both the p-n junction and the metal-CNT interfaces. However, Equation (1) indicates that the relative Seebeck coefficient of the two composite materials, SB-SA is enhanced when SA and SB has an opposite sign, respectively. As shown in Figure 4 (b), the n-doped CNT films have the Seebeck coefficient of -41.1 µV/K, whose polarity is opposite to that of the p-type CNT films (44.4 µV/K), resulting in a large relative Seebeck coefficient of the p-n junction (44.4 µV/K-(-41.1 µV/K)). Note here that since the Seebeck coefficient of electrode metals such as Au, Ag, and Cu are low enough, positive values (1.5 µV/K), there is no such enhancement in the relative Seebeck coefficient. As a result of the larger relative Seebeck coefficient of the p-n junction than that of the metal-CNT interface, the photothermoelectric effect at the p-n junction


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predominated as shown in the response profile of Fig. 4 (c), where the strong positive peak (X = 0 mm) was generated at the p-n junction and the weak negative peaks (X = ±5 mm) were generated at the metal-CNT interface, respectively. The results indicate that the electrical properties, the position of the p-n junction and the THz response of the CNT THz sensor, remain almost unchanged over 25 days, leading to stable operation for THz sensing. We thus successfully tuned the Fermi level of the CNT film without using a gate voltage, by adjusting the concentration of NaOH. In addition, as a result of the reduction in the total heat capacity, we obtained the detection speed faster than that observed for the EDL device (See Fig. S7). Figure 5(a) shows a photograph and the THz response of the flexible THz imager. A part of the flexible CNT film was chemically doped using the aforementioned method; consequently, a p-n junction was formed. Lead wires were connected to the sample using Ag paste. We measured the relative Seebeck coefficient at the p-n junction to be as high as 428 µV/K, corresponding to a subtraction of 262 µV/K for the p-type CNT film and –166 µV/K for the n-type CNT film. After THz irradiation of the p-n junction, we observed a THz response voltage induced via the photothermoelectric effect (the right-hand side of Fig. 5(a)). When the Fermi level is set to an optimal operation position, our flexible CNT THz sensor shows an NEP value of 31 pW Hz-1/2, which is a somewhat better value compared to room-temperature thermal-type solid-state detectors40. With this CNT THz imager, we carried out a noncontact and nondestructive inspection measurement. Figure 5(b) shows a schematic of the measurement system used for the THz imaging. A 1-THz wave radiated from a multiplier-based source (Virginia Diodes, Inc.) with an amplitude modulation frequency of 20 Hz was guided through a horn antenna onto an envelope, and the CNT THz imager detected the THz wave transmitted


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through the envelope. By utilizing high THz penetration into the paper, the THz image observed clearly visualized the metal wires concealed inside the envelope in a nondestructive manner. Additionally, we performed an omnidirectional nondestructive inspection with our CNT THz detector. Figure 6 shows the photographic image of wearable CNT THz scanners and the experimental result of multi-view imaging. Owing to the bendability of our flexible THz imager, the imagers can be easily wrapped around curved surfaces, even such as fingertips, as presented in Fig. 6(a). As one example of omnidirectional scanning, we detected a breakage of a pipe just by inserting and rotating the flexible THz imager attached to the fingertip (Fig. 6(b)). This result demonstrates that our device relaxes restrictions of imaging conditions such as shapes and locations of objects to be measured, thus widely enlarging the adaptable range of THz imagers. Finally we would like to make comments about comparison between CNT film devices and graphene devices41-43. Advantages of using CNT films are high THz absorption rate of the Fermi-level-tuned CNT films (above 90%) and easy handling without substrate. In contrast, one of disadvantages is that lithography process is not applicable to thick CNT films, leading to some difficulties in designing a desired device pattern. Besides mono-layer CNT films currently can not be produced, preventing use in ultra-thin CNT film devices.

CONCLUSION

In conclusion, we presented the fabrication and evaluation of a bendable THz imager based on chemically Fermi-level-tuned semiconducting CNT films. By implementing ionic-liquid gating technology into thick and flexible CNT films and tuning their Fermi level, we systematically investigated and evaluated the characteristics of the Seebeck coefficient, resistance (noise voltage), and THz coupling efficiency, which all govern the THz detection performance. We


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obtained an on/off resistance ratio (2758) for a semiconducting-separated CNT device with a film thickness of 30 µm, providing a high degree of controllability of physical parameters strongly relevant to CNT THz detector performance. Additionally, we developed gate-free Fermi-level tuning based on variable-concentration dopant solutions and fabricated a Fermilevel-tuned p-n junction CNT THz imager. The use of this THz imager led to a noncontact and nondestructive inspection of a metal wire concealed inside an opaque object. The series of experimental results demonstrated adjustable Fermi-level tuning even for such thick CNT films while retaining the advantageous features of mechanical strength and bendability. We expect that further enhancement is possible by optimizing the n-type inducers and the valence and electronegativity of the chemical doping solution. The technology presented is thus anticipated to expand the adaptable range of flexible THz imaging and to also facilitate the realization of CNTbased flexible devices and their applications such as touch panels and electromagnetic filters.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available. Schematic image of the photothermoelectric effect of the THz detection mechanism, schematic image of the filtration process, homemade Seebeck coefficient measurement system, noise voltage spectrum of the pristine semiconducting CNT THz detector, variation in the resistance of the metallic CNT film with the gate voltage,


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schematic image of the THz-time domain spectroscopy, and transient response time of CNT THz detectors that were doped by ionic liquids or crown ethers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.K.) ORCID Daichi Suzuki: 0000-0001-8803-080X Yuki Kuwahara: 0000-0002-3441-0476 Takeshi Saito: 0000-0001-5664-6407 Yukio Kawano: 0000-0002-8456-2937 Notes The authors declare the following competing financial interest: Y.K., D.S., and Y.O. have filed Japanese and PCT patent applications related to this work. ACKNOWLEDGMENT We thank ZEON Corporation for providing the CNT film. This work was supported in part by the JST-Mirai Program, the Matching Planner Program, and the Center of Innovation Program from the Japan Science and Technology Agency, JSPS KAKENHI Grant Numbers JP16J09937, JP17K19026, JP17H02730, JP16H00798, JP16H00906 from the Japan Society for the Promotion


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of Science, the Murata Science Foundation, and Support for Tokyo Tech Advanced Researchers (STAR)

ABBREVIATIONS THz, Terahertz; CNT, Carbon nanotube; DOS, Density of states; eDIPS, enhanced directinjection pyrolytic synthesis; EDL, Electronic-double-layer;

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Table 1. Measurement results for the Seebeck coefficients of various types of CNT films.

Structure

Electrical

Semicon ductor

Figure 1. Photographs of metallic, semiconducting, and metal/semiconductor-mixed CNT solutions and a flexible CNT film formed via the filtration process. The CNT solutions were filtered through a membrane filter using a vacuum pump. The typical thickness of the CNT film is 30 µm; such a film holds its free-standing shape and shows flexibility.

SWCNT

Type

262

N

-166

N+

-152

N

Random

N+ Metal

Metal

70 -48 -46

P

2

N

-11 Aligned

MWCNT

Seebeck coefficient [µV/K]

P

P Mixture

Orientation

None

(Parallel)

Aligned (Perpendicular)

30 30

Figure 2. (a) Schematic and photographic images of the EDL transistor with an ionic liquid (DEMETFSI). According to the gate voltage application, ions move inside the liquid and reach the CNT film and gate electrode. As a result, the Fermi level can be tuned for CNT films with thicknesses of several tens of micrometers. (b) Resistance of the CNT film versus the gate voltage. An on/off ratio of 2758 was obtained. (c) Noise voltage spectra at VG = 0.7 V and VG = -1.5 V, where the dashed lines indicate the theoretical values for the thermal noise. The peaks at 50 Hz are due to the electrical line noise, not from the CNT device itself.


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Figure 3. (a) THz absorption spectra of the semiconducting CNT film. (b) THz absorption of an irradiation frequency of 2 THz as a function of the gate voltage. (c) Schematic of the energy band diagram of the semiconducting CNT film. Because the THz absorption is induced by an intraband transition, the carrier doping via the gate voltage contributes to highly efficient THz absorption.

Figure 4. (a) Fermi-level control of the CNT film via a chemical doping solution of NaOH and 15-crown-5-ether. The left figure shows the mechanism of chemical doping. In the doping solution, crown ethers strongly bind cations (Na+) and form complexes. The counter anions (OH-) inject electrons into the CNT film, which changes the Fermi level of the CNT film. The right figure illustrates the device structure and measurement system. (b) Measurement results for the Seebeck coefficient of a CNT film as a function of the concentration of the dopant (NaOH). (c) Stability check for the n-doping. The position of the p-n junction and the THz response of the CNT device remained almost unchanged over 25 days, leading to stable operation of THz sensing.


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Figure 5. (a) Flexible CNT THz imager based on a Fermi-leveltuned p-n junction CNT film. The voltage/current signal was generated by irradiation at 29 THz via the THz-induced photothermoelectric effect. (b) Sketch of the THz imaging measurement setup. (c) Photographic image of the envelope and THz image acquired by the CNT THz imager. The metal wire concealed inside the envelope was clearly visualized in a noncontact and nondestructive manner.

Figure 6. (a) Photographic image of wearable THz scanners. Owing to the bendability of our flexible THz imager, the imagers can be attached even at fingertips, which relaxes restrictions of imaging conditions such as shapes and locations of objects to be measured. (b) Omnidirectional THz scanning of a pipe. A breakage of a pipe was clearly detected just by inserting and rotating the flexible THz imager.


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Table of Contents.


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Fermi-Level-Controlled Semiconducting ... - ACS Publications

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