Jangyeol Yoon, Jaehyun Park, Junsung Kim, Gyu Tae Kim, * and Jeong Sook Ha * FULL PAPER. 1. Introduction

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1 Array of Single-Walled Carbon Nanotube Intrajunction Devices Fabricated via Type Conversion by Partial Coating with β -Nicotinamide Adenine Dinucleotide Jangyeol Yoon, Jaehyun Park, Junsung Kim, Gyu Tae Kim, * and Jeong Sook Ha * The fabrication of aligned single-walled, carbon nanotube (SWCNT) intratube junction devices by partially coating pristine SWCNTs with a β-nicotinamide adenine dinucleotide (NADH) solution and subsequent annealing at 15 C is reported. Gate-bias-dependent rectification behavior is observed with a rectification ratio of >1 3 at ± 1 V. A comparative study with p n-junction devices of randomly networked SWCNTs confirms the advantage of using aligned SWCNTs with substantially better rectifying characteristics due to the selective removal of metallic tubes by electrical breakdown. The gate dependence of the intratube p n-junction in the forward and backward directions is attributed to the difference in the shift of the Fermi levels (forward bias) and the enhanced direct tunneling (reverse bias), as suggested by band-diagram modeling. This work suggests a potential application of aligned SWCNT intratube p n-junction devices in the future of nanoelectronic circuits. 1. Introduction During the last two decades, carbon nanotubes (CNTs) have been verified as novel materials for nanoscale electronic devices. [ 1 5 ] Since the discovery of CNTs by Iijima, [ 6 ] various growth techniques have been developed. [ 7 11 ] Among them, the growth of aligned CNTs by chemical vapor deposition (CVD) has been given great attention due to the relative ease of selective removal of metallic CNTs from a mixture of metallic and semiconducting CNTs by electrical breakdown, which is essential for the fabrication of semiconducting devices. [ 1 ] Aligned single-walled CNTs (SWNTs) have been grown by patterning Fe catalysts on an AT-cut crystalline quartz substrate, which is a type of rotated Y-cut quartz with a cut angle of and a miscut angle of 58. [ 8 ] After the growth, transfer of the SWCNTs onto the desired device structures has been performed J. Yoon, J. Park, J. Kim, Prof. J. S. Ha Department of Chemical and Biological Engineering Korea University Seoul , Korea jeongsha@korea.ac.kr Prof. G. T. Kim School of Electrical Engineering Korea University Seoul , Korea gtkim@korea.ac.kr DOI: 1.1/adfm.1165 in a few different ways including transfer using thermal tape [ 13 ] and printing transfer of a CNT-embedded Au layer on a poly(dimethyl siloxane) (PDMS) stamp. [ 1 ] Selective removal of the metallic tubes has been performed by applying a high bias across the aligned SWCNT channel while keeping the gate bias positive, i.e., by electrical breakdown in a field-effect transistor (FET). [ 15 ] After electrical breakdown, SWCNT FETs exhibit high mobility, high on/off current ratio, and high stability under ambient conditions. [ 16 ] A p n diode is a basic building block for various devices including lightemitting devices and integrated circuits. p n-junction structures consisting of SWCNTs have been fabricated in several different ways. First, p-type SWCNTs and n-type semiconducting films have been used for the heterojunction. [ 17 ] A thus-formed heterojunction showed rectification behavior and photovoltaic effect, but the contact resistance between the SWCNT and the film was high. Second, the heterojunction between two different nanowires using pristine SWCNT and an n-type metal oxide nanowire was fabricated, but it also showed a large contact barrier for successful device performance despite more complicated fabrication processes, [ 18 ] which degrades device performance. However, an SWCNT intratube junction can be an alternative to the above-mentioned SWCNT-based p n-heterojunction devices due to its relatively easy fabrication and lower contact resistance in the junction area. Semiconducting pristine SWCNTs showed p-type transfer properties and could be converted to n-type by doping. [ 19 ] Doping of p-type SWCNTs with alkali metals produced n-type behavior, but it was not as stable in ambient air. [ ] Coating poly(ethylene-imine) (PEI), which easily donates electrons, onto the pristine p-type SWCNTs induced their conversion to n-type SWCNTs. [ 1 ] However, these were also unstable under humid conditions and returned to the original p-type nature within a few days. [ ] Recently, it was reported that β-nicotinamide adenine dinucleotide, reduced dipotassium salt (NADH), functioned as a stable typeconverting polymer for p-type SWCNTs. [ 3 ] Herein, we report on the fabrication of SWCNT intratube junction devices by partially coating pristine SWCNTs with NADH and subsequent postannealing. Aligned SWCNTs were grown on ST-cut quartz substrate by CVD. The aligned SWCNTs Adv. Funct. Mater. 11, 1, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 515

2 were transferred onto the SiO substrate using thermal tape. After the selective removal of metallic tubes by electrical breakdown, NADH solution was spin-coated onto half of the SWCNT channel, and it was postannealed at 15 C. The formed SWCNT intrajunction exhibits rectifying behavior with a high rectification ratio >1 3 at a low bias of 1 V. Under humid, air, ambient conditions, the rectifying behavior degrades noticeably within a few days, probably due to the instability of the NADH coating. However, it can be immediately recovered by thermal annealing at 15 C. The p n junction also shows strong gate dependence, which is useful in the operation of gate-modulated rectification devices. Compared to randomly networked SWCNT p n-intratube junctions, a much higher rectification ratio and lower reverse current are observed. The gate-dependent rectifying behavior is explained by the energy-band diagram.. Results and Discussion Figures 1 a,b show scanning electron microscope (SEM) images taken from SWCNTs grown on ST-cut quartz substrates using patterned Fe catalysts. As clearly seen in the images, wellaligned SWCNTs grow along the crystalline direction of the quartz substrate. The average density of SWCNTs was estimated to be 5 SWCNTs μm 1. The CVD-grown SWCNTs were transferred onto the SiO substrate using thermal-release tape (Revalpha, Nitto Denko). Figure 1 c is an optical microscope image of an array of SWCNT intratube-junction devices with back gate electrodes. It is shown that half of the pristine SWCNT channel was covered with NADH solution. After coating with NADH, the devices were thermally annealed at 15 C on a hot plate for successful release of electrons via dissociation and for formation of ohmic S p 1mm SiO G n D 3µm Figure 1. a) SEM image of the CVD-grown aligned SWCNTs on ST-cut quartz substrate. b) A magnifi ed SEM image of aligned SWCNTs with an average density of 5 CNTs μm 1. c) Optical microscope image of fully fabricated p n intratube-junction arrays. The NADH-coated region is shown in the image, and the inset shows the schematic of the p n intratube junction device with a bottom gate. d) SEM image of the SWCNT channel fabricated on the SiO substrate after transfer of the CVD-grown CNTs. The inset is the magnifi ed SEM image of the channel region. (b) S D µm 5µm 5µm contact between electrodes and the aligned SWCNTs. It is believed that thermal annealing at 15 C induces the oxidation of NADH to release two electrons via detachment of a hydrogen atom from NADH, as represented in Equation (1). [ ] NADH NAD + +H + +e (1) During the transfer and subsequent device fabrication including electrode deposition, lift-off and reactive-ion etching, it was observed that some of the SWCNTs were lost, which resulted in a reduced density of SWCNTs μm 1, as shown in Figure 1 d. Here, we define a pristine SWCNT FET as the SWCNT FET after electrical breakdown. Electrical performance of both the pristine and the NADH-coated SWCNT FETs were measured by using an HP15B. Figure a shows the I V curves of the pristine SWCNT FET, and the inset denotes its transfer characteristics. At the negative gate bias ( < ) the source-drain currents ( ) are bigger, which indicates a typical p-type nature. The curve obtained from the NADH-coated SWCNT FETs showed typical n-type transfer characteristics, where the increased as the positive gate bias was applied, as seen in the inset of Figure b. The field-effect mobility ( μ fe ) of the SWCNT FET device can be calculated using Equation () based on the cylinder-on-plane model, [ 5 ] : fe = s L NC g (L) () in the linear operation regime, where L is the channel length, N is the number of individual nanowires between the source and drain, and C g is the gate capacitance of the FET device. The field-effect mobility values of the pristine and the NADH-coated SWCNT FET were estimated to be 118 and 1696 cm V 1 s 1, respectively. The on/off current ratio exceeded 1 3 for both devices. High mobilities with good current on/off ratio indicate good performance of the SWCNT FETs. Figure c shows the statistical distribution of the on/off current ratio for 3 SWCNT FETs with and without NADH coating. In particular, it was shown that the doping of the pristine SWCNTs by NADH coating did not deteriorate the electrical performance; even the mobility and the on-current increased. Maintaining stable device performance for a long time in ambient air is essential for the practical application of the nanowire electronic devices. Under the stabilized ambient conditions of 5 C and 5% humidity, the current on/off ratio was measured. The pristine p-type SWCNT FETs kept the on/ off current ratio stable for 5 days, but the NADH-coated n-type FETs showed a dramatic degradation within a few days due to the increase of the leakage current (Figure d). The instability in the n-type behavior of NADH coated SWCNT FETs under ambient conditions is attributed to the adsorption of water molecules, which prohibits the donation of electrons to SWCNTs. Therefore, ambipolar transfer characteristics became noticeable as time increased, which greatly contrasts with the previously reported work, in which the NADH-coated SWCNTs kept an n-type nature for a long time: up to approximately one month. [ 3 ] However, thermal annealing of a degraded device at 15 C immediately recovered the electrical performance of the 516 wileyonlinelibrary.com 11 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 11, 1,

3 # of devices V Mobility : 118 cm I on /I off Intrinsic NADH coated /V s (b) R normalized R normalized Annealing 1 1V Day Mobility : 1696 cm Day /V s Intrinsic NADH coated Annealing Figure. a) Output characteristics ( ) of the pristine SWCNT FET with variation of the gate bias ( ), after the electrical breakdown. The inset shows the transfer curve of the same device. b) Output characteristics ( ) of the NADH-coated SWCNT FET after thermal annealing at 15 C. The inset shows the transfer curve of the same device. was varied between + 1 and 1 V. c) Statistical distribution of on/off current ratio for the pristine and the NADH-coated SWCNT FETs, respectively. The on (off) current was measured at = 1 ( + 1) V and = + 1 ( 1) V for the pristine and the NADH-coated SWCNT FETs, respectively. d) On/off current ratio of the pristine (fi lled circles) and the NADH-coated (open squares) SWCNT FETs as a function of time in ambient air. The fi lled square shows the full recovery of the on/off ratio from the aged device by thermal annealing. The inset shows the stable on/off current ratio of the annealed device in ambient air. newly fabricated FETs, which can be understood as a result of the removal of the water molecules. Figure 3 shows the I V characteristics of the intratube junction with the positive (negative) bias at the pristine SWCNTs for the forward (reverse) current. It shows a clear rectification behavior with a rectification ratio greater than 1 3, which indicates a high on current and low off current. The rectifying I V curve was analyzed by using an equivalent circuit model composed of a serial resistor and an ideal diode, as shown in Figure 3 b. The voltage across the diode and the resistor in a serial connection will be shared following Equation (3), [ 6 ] V = IR+ k BT q ln ( l l ) where I is a temperature-dependent saturation current, η an ideality factor, k B the Boltzmann constant, and T absolute temperature (K). With differentiation by the current, the differential resistance can be obtained from Equation (). dv di = R + k BT ql (3) () Analysis of the data given in Figure 3 a shows a linear relationship between dv /di and 1/ I in Figure 3 b, which confirms the validity of the serially connected diode model. The ideality factor, which implies the level of the influence of interface states between the p and n junctions and the contact resistance are estimated to be and 5.7 M Ω, respectively. The electrical stability of the fabricated intratube p n junction device was monitored in ambient air for 3 days. The change of on and off currents at + 1 and 1 V (left y axis) and the resultant time dependence of the normalized rectification ratio (right y axis) in the forward direction are shown in Figure 3 c. The on current (filled circle) remained nearly the same for one month while the off current (open circle) dramatically increased within a few days, which resulted in a dramatic decrease of the rectification ratio during the same period. Interestingly, this degraded one-month-old junction device recovered its original rectification ratio immediately after thermal annealing of the device at 15 C, probably due to the desorption of water molecules from the NADH-coated SWCNTs. Dramatic decrease of the rectification ratio within a few days corresponds to the drastic increase of the off current. These phenomena correspond to the fast degradation of the NADH-coated SWCNT FETs within a few Adv. Funct. Mater. 11, 1, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 517

4 I I (A).1 forward.8 reverse V 1-5 without annealing after annealing Day R normalized (b) dv/di (Ω) 1.x1 9 8.x1 8 6.x1 8.x1 8.x R : 5.7X1 6 Ω η :.. 5.x1 9 1.x x1 1 1/ I (A -1 ) I R Vg = V -V V 1V V Figure 3. a) I V curve of the intratube junction device. b) dv /di vs. 1/ I taken from the data of showing the suggested equivalent circuit model with a serial resistor component. c) The stability of the intratube junction: left y axis shows the on and off currents measured at 1 and 1 V, respectively. Right y axis is the normalized rectifi cation ratio ( R normalized ) at ± 1 V. The red data point is the R normalized value obtained after annealing the 3-day-old intratube junction device at 15 C. d) Gate dependent I V curve of the intratube junction. was swept between and + V in steps of 1 V. days, as shown in Figure d. So the increase of the off current with time under ambient air conditions breaks the rectifying properties of the intratube p n junction, which requires more improvements and understanding. Figure 3 d shows the strong gate dependence of the I V curve of the p n-junction. A back gate electrode composed of Ti (1 nm)/au (5 nm) was attached to the intratube p n junction device. As the negative bias was applied, the forward current increased in a manner similar to that of the p-type FET. Similar gate-dependent behavior of the SWCNT p n-junction devices was previously reported by Abdula and Shim. [ 7 ] Naturally grown SWCNTs are a mixture of metallic and semiconducting tubes. Depending on the density of the randomly grown SWCNTs, the metallic percolation path can be controlled. Our recent work [ 8 ] showed that the density of SWCNTs can be controlled by controlling the ferritin catalyst. Figure a,b show the curves of the randomly networked SWCNT FETs with high and low density of SWCNTs, respectively. In these figures, diluted (by and 1, respectively) ferritin solutions were used. In Figure a, the gate dependence is quite weak, which indicates that the conduction path was dominated by metallic tubes; the on/off current ratio was about 3. Correspondingly, the SEM image taken from the channel region shows a very high density of SWCNTs. The transfer curve (the left top inset) shows high on and off currents. The FETs of low-density SWCNTs demonstrate more improved gate dependence with an on/off current ratio of 1, as shown in Figure b. Accordingly, the SEM image shows a lower density of SWCNTs in the channel region. In the same way as that used in the fabrication of aligned SWCNT intratube p n-junction devices, p n-junction devices were made of randomly networked SWCNTs, as shown in Figures a and b. Without any electrical breakdown, both devices showed rectifying I V curves but a much poorer rectification ratio, as shown in Figures c and d; the rectification ratios were estimated to be 3 and 1, respectively. These results indicate an important advantage of using well-aligned SWCNTs after the selective removal of metallic tubes. The bias voltage across aligned conduction paths can break the lower resistance paths more easily than the random network owing to the parallel connection of the circuit. Inclusion of a metallic conduction path in the randomly networked film deteriorated the rectifying behavior with much lower rectification ratio, mostly due to the larger reverse current. In both high- and low-density SWCNTs, the forward current showed the strong gate dependence of p-type semiconductors. However, the reverse current 518 wileyonlinelibrary.com 11 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 11, 1,

5 V Vg = V 1µm Vg = -V V 1V V.. 1V (b). Vg = V 1µm Vg = -V V 1V V Figure. Output characteristics ( ) of the randomly networked SWCNT FETs with a) high and b) low density of SWCNTs, respectively. Gate bias ( ) was varied from 1 to + 1 V. The left top insets are the transfer curves, and the right bottom insets are the SEM images of the channel regions. Gate-bias-dependent I V curves of the randomly networked SWCNT intratube p n-junction devices made of c) high and d) low density SWCNTs. Gate bias was swept from to + V. of the p n-junction made of the low-density SWCNTs network exhibited n-type gate dependence. To explain the opposite-gate dependence of a homojunction diode, we drew an energy-band diagram in Figure 5 for both the forward bias ( + 1 V) and the reverse bias ( 1 V) applied to p-type SWCNTs. The band offset increased as the gate voltage increased from V to V as in Figure 5 a. The built-in potential ( q φ Bi ) across the p n junction is defined as the difference of the Fermi level energies ( E Fn E Fp ), and it can be changed by tuning the external gate voltage ( q Δ φ Bi ), which results in apparent gate dependence of the homojunction. By using the current-density equation, the difference of the Fermi level could be estimated with the gate voltages. [ 9 ] Here, n and p represent the electron concentration at the n side and the hole concentration at the p side for the zero gate voltage, respectively. In the n-type regime, the increase of the electrons is described by Equation (5), n + n = n e E Fn k B T (5) where Δ n is given by the change of the electron concentration by the gate voltages. Likewise, the hole concentration also is described in the p-type regime by Equation (6). Figure 5. The energy-band diagram for the homojunction diode. a) The built-in potential increased as the gate voltage increased from V to V ( q Δ φ Bi,1 q Δ φ Bi, ) under forward bias. b) Under reverse bias, the tunneling effect becomes signifi cant due to Zener breakdown in the randomly networked homojunction diode. With increase of the gate bias from V to V, the electron accumulates in the valence band of the p-type region, enhancing tunneling probability through the barrier. E Fp p + p = p e k B T (6) Using the above equations, we can express the change of the Fermi level with gate voltage in terms of the change in electrons and holes as following Equations (7) and (8), Adv. Funct. Mater. 11, 1, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 519

6 { E Fn = k B T ln 1 + n } n { E Fp = k B T ln 1 + p } (8) p so, the change of the built-in potential can be given by Equation (9). { E Fn E Fp = q n Bi = kb T ln 1 + p } n n (9) n p The unbalancing initial carrier concentration of electrons n and holes p at each side induces the apparent gate voltage dependence. As shown in Figure 5 a with a forward bias ( +1 V), the negative gate voltage ( = V) shifts the Fermi level downward in both the p-type and the n-type sides. However, the initial charge concentration of the hole at the p side is expected to be higher than the electron at the n side ( n < p ). Therefore, the built-in potential would decrease at the negative gate-bias voltage with Δ n <. Conversely, at the positive gate voltage ( = V), the built-in potential could increase due to the increase of the difference of the Fermi level. These behaviors were observed both in the aligned and in the random network cases, which indicates an apparent p-type gate dependence of the forward current in our carbon-nanotube p n-junction devices. At the reverse bias ( 1 V), the direct tunneling current possibly caused by Zener breakdown, as in Figure 5 b, was observed in a random network homojunction diode. The gate voltage could change the electron concentration in the valance band at the p-side, which affects the probability of the tunneling from the valance band of the p side to the conduction band of the n side. The positive gate voltage enhanced the tunneling effect at the depletion region. However the negative gate voltage reduces the tunneling effect oppositely, which gives the increase of the reverse current at the positive gate voltage, i.e., the apparent n-type behavior. It is worthwhile to note that this apparent n-type behavior at the reverse current appears only for the random network device case. We assumed that the metallic portion in the channel area at the random network easily induced Zener breakdown at 1 V due to a greater electron concentration. Therefore, the varying gate dependence in the homojunctions can be explained by the difference of the shift of the Fermi levels (forward bias) and the enhanced direct tunneling (reverse bias) based on our suggested model in Figure Conclusion Coating NADH solution onto p-type SWCNTs and subsequent annealing at 15 C yielded a type conversion into n-type SWCNTs. Intratube p n-junctions made of well-aligned pristine and NADH-coated SWCNTs exhibited gate-dependent rectifying characteristics with low contact resistance and an ideality factor of approximately two. A comparative study with the intratube p n-junction made of randomly networked SWCNTs confirmed the advantages of using well-aligned SWCNTs after the selective removal of metallic SWCNTs by breakdown. However, the (7) instability of NADH-coated SWCNTs in ambient air should be improved for stable operation of the intratube p n-junction devices. This work clearly shows the easy fabrication of SWCNT intratube p n-junction devices via a growth of well-aligned SWCNTs and its type conversion with NADH coating, which will be widely used as a building block of future nanoelectronic circuits.. Experimental Section Growth of Aligned SWCNTs : Aligned SWCNTs were grown on AT-cut quartz wafers (Hoffman materials), which were miscut by 58 by CVD. Prior to the growth, the quartz substrate was annealed at 9 C for 1 h, followed by slow cooling to room temperature. The thermal annealing resulted in recrystallization, inducing the guidelines for the growth of aligned SWCNTs. Fe was evaporated to make a catalyst line of 1 μ m, and a catalyst interval of 1 mm using a metal mask. Without photolithography, dense SWCNTs could be grown due to lack of residue on the quartz wafer. After Fe evaporation, it was annealed at 9 C to form 1 nm-sized Fe O 3 nanoparticles. H flow (3 sccm) was maintained while the temperature was increased to 95 C. Carbon sources of a methanol/ethanol bubbler and methane were used in the growth of SWCNTs. During the growth of well-aligned SWCNTs, the miscut direction of the quartz substrate was kept perpendicular to the gas flow. After growth, the substrate was rapidly cooled down to room temperature under Ar (8 sccm) flow. Transfer of CVD-Grown SWCNTs Onto SiO Substrate : The CVD-grown SWCNTs were transferred onto the thermally grown SiO substrate. On the grown SWCNT fi lm, a 1 nm-thick Au fi lm was deposited by e-beam evaporation to form the SWCNT-embedded Au fi lm. Thermalrelease tape (Revalpha, Nitto Denko) was attached to the Au fi lm and subsequently peeled off. The tape was then contacted onto the SiO substrate and heated at 1 C to activate adhesion. After detachment of the tape, the Au fi lm was etched away using Au etchant. Device Fabrication : After the transfer of the CVD-grown SWCNTs, source and drain electrodes of Ti (1 nm)/au (5 nm) were fabricated by photolithography. Residual SWCNTs, except for the channel area, were removed by reactive-ion etching (O, sccm, 5 torr, 3 s, 1 W). Among the transferred SWCNTs, metallic tubes were selectively removed by electrical break-down under a gate bias of + V. Afterward, half of the SWCNT channel was spin-coated ( rpm for 5 min) with 1 m M NADH solution. The device was dried at 6 C, followed by annealing at 15 C for 5 min. Electrical measurements were performed on an HP 15B. Acknowledgements This work was supported by National Research Foundation grants funded by the Ministry of Education, Science and Technology, Korea (No. ROA , No. ROA , and No. ROA-1 137). We thank Korea Basic Science Institute for SEM imaging. Received: February 1, 11 Published online: May, 11 [1 ] X. Zhou, J.-Y. Park, S. Huang, J. Liu, P. L. McEuen, Phys. Rev. Lett. 5, 95, [ ] T. Durkop, S. 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