N-Doped Helical Carbon Nanotubes: Single Helix Photoconductivity

Publication Date (Web): June 19, 2012. Copyright © 2012 American .... Hideya Nakamura , Satoru Watano. Chemical Vapor Deposition 2013 19, 323-326 ...
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N-Doped Helical Carbon Nanotubes: Single Helix Photoconductivity and Photoluminescence Properties Yuan Liu,† Nujiang Tang,*,† Watson Kuo,*,‡ Chengwei Jiang,‡ Jianfeng Wen,† and Youwei Du† †

Physics Department & Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China Department of Physics, National Chung Hsing University, Taichung 402, Republic of China

‡

ABSTRACT: The single nanodevice-based ultrathin N-doped helical carbon nanotube (N-doped HCNT) was fabricated for systematical examinations of photoconductivity. A 515 nm focused beam of about 300 μm in diameter was applied for irradiation. The study showed that the N-doped HCNT has high generation of photocarriers and high bimolecular recombination rate. The photoluminescence (PL) properties of the N-doped HCNTs were examined systematically. The results indicated that the photoresponse can be improved further by selecting an appropriate wavelength excitation.

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INTRODUCTION Since the discovery of the CNTs,1 the extensive fundamental research attention has been devoted to experimental and theoretical study. Because of the special one-dimensional (1D) character, the CNTs have extraordinary performance in optical and electro-optical properties. For example, the strong 1D confinement results in large Coulombic coupling between photogenerated electrons (e) and holes (h) to create strongly bound excitons2,3 and enhances the absorption intensity because of the increased e−h overlap.4 Consequently, many groups found high photocarrier generation rate in CNT-based devices, which make CNTs as the promising active elements in novel optoelectronic devices.5−15 HCNTs, because of their special helical tubular structure, unique electrical properties, and great potential applications in nanoengineering, have aroused much attention in recent years.16−18 The specific helical geometry (such as diameter and coil pitch, etc.) is determined by the periodic incorporation of pentagon and heptagon pairs in hexagonal carbon network.17 Interestingly, depending on the helical geometry, a HCNT can behave as a semiconductor, a semimetal, a metal, or even a superconductor, which is impossible in the case of straight CNTs.18 As a semiconductor, a HCNT is a direct or indirect narrow gap semiconductor, and its band gap can be easily tuned by varying the helical geometry.17 Moreover, because doping nitrogen into carbon materials could be a promising way to improve their electronic, mechanical, and optical properties, many efforts have been devoted to study N-doped CNTs.19−29 For example, many researches confirmed that the doping of N atoms into carbon materials can favor PL emission,27,28 and the electronic properties of carbon-based materials can effectively be tuned via doping N,29 which is determined by the doping type, doping content, and so forth. Compared to undoped CNT diode, the N-doped CNT diode exhibits a large © 2012 American Chemical Society

photocurrent and a large photovoltage under illumination and shows a clear rectification effect.29 Up to now, despite the fascinating geometry-dependent electrical properties forecasted by theoretical calculation, the electrical and photoconductivity properties of single N-doped HCNT have not been investigated experimentally so far. Herein, we report its electrical and photoconductivity properties under 515 nm irradiation. It will be shown that the Ndoped HCNT has high photocarrier generation rate. Furthermore, because of the uneven photocarrier distribution in the helical structure, the N-doped HCNT has high bimolecular recombination rate. Even more interesting is the fact that a N-doped HCNT has relatively large diameter of ca. 100 nm, however, only has ultrathin wall of ca. 1 nm, and still is thermodynamically stable. With a large hollow degree, such a semiconducting N-doped HCNT is ideal for specific applications.

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EXPERIMENTAL SECTION We prepared N-doped HCNTs as reported previously.27 The process contained two steps, viz., the synthesis of pristine HCNTs and doping of nitrogen via annealing pristine HCNTs in an NH3 atmosphere. For the synthesis of N-doped HCNTs, the doping of nitrogen was performed by annealing pristine HCNTs in an NH3 atmosphere at 700 °C for 4 h. After the furnace cooled to room temperature, the N-doped HCNT sample was obtained. The single N-doped HCNT device was fabricated as described previously.30 In short, the substrate was diced from silicon wafers coated with 300 nm thick SiO2 insulating layer on which micrometer-sized Pt pads, and alignment marks were made by using standard photoReceived: March 28, 2012 Revised: June 17, 2012 Published: June 19, 2012 14584

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of the N-doped HCNTs, X-ray photoelectron spectroscopy was employed.27 The results showed that the N content defined as 100N/(C + N) atom % of the sample is 4.6 atom %, and the specific contents are for the pyrrolic N, pyridinic N, and graphitic N are 2.35538%, 1.14893%, and 1.09569%, respectively. Generally, annealing in vacuum can contribute to the healing of defects, whereas thermal treatment in air can contribute to the generation of defective sites. Additionally, we determined the Raman shift of the pristine and N-doped HCNTs (not shown). The Raman spectra were fitted and deconvoluted. It is found that the intensity ratios of the defect (D) and graphite (G) bands, ID/IG, are 0.8 for pristine HCNTs and 0.96 for N-doped HCNTs. Compared to the ID/IG of the pristine HCNTs, the values of N-doped HCNTs show a clear increasing, implying the higher density of defects in the Ndoped HCNTs. It indicates that there is a etching effect in the case of annealing HCNTs in NH3. Electrical and Photoconductivity Properties of the Single N-Doped HCNT. Shown in the inset of Figure 2a is a scanning electron microscopy (SEM) image of a N-doped HCNT nanodevice, in which multiple 700 nm wide Cr/Au electrodes with thickness of 150 nm/250 nm are arranged with an edge-to-edge spacing of 1.9 μm on a 100 nm in diameter Ndoped HCNT. We measured the electrical property and photoconductivity under ambient conditions. To minimize the effects of IR heating, and O2 desorption from CNTs under UV irradiation, a 515 nm green laser is often used as the irradiation source,9 which is also applied in our case. Shown in Figure 2a is the I−V curve of electrodes 2−3 in the device by 4-terminal measurement, revealing that the resistance is ca. 1.56 MΩ and the specific resistance of the device is ca. 0.82 MΩ/μm. Considering that the length of full tube of the Ndoped HCNT is roughly 2 times longer than the helical length, one can know that the resistance is ca. 410 kΩ/μm. On the basis of the fact that the diameter and the thickness of the Ndoped HCNT is ca. 100 and 1 nm, respectively, one can conclude that the resistivity ρ of the N-doped HCNT is ca. 1.24 × 10−2 Ω cm. It is known that σ = 1/ρ = Nμq, where N is the carrier density of N-doped HCNTs, μ is the carrier mobility, and q is electron charge. Assuming that the carrier density N of the N-doped HCNT is similar to that of multiwalled CNTs, which is from 1 × 1018 to 1019 cm−3.9 Thus, one can estimate that μ in N-doped HCNT is about 48−480 cm2/(V s). Shown in Figure 2b are the I−V curves of electrodes 1−2 with/without irradiation. One can know that the resistance without irradiation is 3.6 MΩ. As mentioned above, the resistance of N-doped HCNT without irradiation is ca. 0.82 MΩ/μm; one can calculate the resistance is ca. 0.82 MΩ/μm × 4.5 μm = 3.69 MΩ, a little higher than the measured value of 3.6 MΩ. Furthermore, the I−V curves of both with/without irradiation are well-linear and low-noise, revealing that the device is ohmically contacted device with very low contact resistance, and the contact is stable even under irradiation. Shown in Figure 2c is the current transported through electrodes 1−2 under a constant bias voltage (Vbias) of 5 mV when 7.92 kW/ cm2 irradiation is on and off. It is clear that the current increased sharply and approximately saturated within 1 s when the irradiation is on and decayed also within 1 s when the irradiation is off. Note that we have recorded the data under a much faster delay time (ca. 50 ms) and found no significant difference from the result under ca. 1 s delay time (not shown). Shown in Figure 2d is the current transported through electrodes 1−2 when various power density lasers are turned on

lithography. Cr/Au electrodes were then patterned using ebeam lithography and were placed on the top of the selected HCNT by thermal evaporation and followed by the lift-off process. The electrical property and photoconductivity of single Ndoped HCNT devices were measured under ambient conditions. For the photoconductivity measurements, a green light source with a wavelength of 515 nm for irradiation was used. A piezoelectrically driven mirror mounted before the objective allowed the beam to be positioned on the sample approximately. The green laser is reduced and focused a spot with the diameter ca. 300 μm.30 The current signal was amplified by a current preamplifier (SR 570) and detected by using the lock-in technique (Signal Recovery 7265) with a frequency of ∼1 Hz. The PL and PLE spectra were obtained at ambient conditions by a spectrofluorophotometer (Edinberge, FLS920, England) using a xenon arc lamp as the light source. For PL spectra investigation, 0.4 mg of powder samples was ultrasonically dispersed in 4 mL of distilled water for 9 h and kept in quiescence for about 2 h. After that, the supernatant was used.

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RESULTS AND DISCUSSION Microstructures of the N-Doped HCNT. We have prepared N-doped HCNTs and found that they have ultrathin walls.27 Shown in Figure 1a,b are the typical transmission

Figure 1. (a) A typical TEM image of a N-doped HCNT. (b) Magnified image of the area marked in (a). (c) HRTEM image.

electron microscope (TEM) images of a N-doped HCNT after being ultrasonically dispersed in acetone for 2 min, and the white arrow indicates the rupture part of the N-doped HCNT. The N-doped HCNT has a helical thin-walled tubular structure with the diameter of ca. 100 nm. Surprisingly, one can see that the thickness of the wall is only ca. 1 nm (shown in Figure 1b). According to high-resolution TEM (HRTEM) observation (shown in Figure 1c), the interlayer spacing of the graphite shell of the HCNT is ca. 0.348 nm, a little larger than that of conventional graphite shell. Furthermore, to detect the nitrogen content and the bonding environment of the C and N species 14585

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Figure 2. (a) I−V curve of electrodes 2−3 in the device by 4-terminal measurement. Inset is the SEM image of the N-doped HCNT nanodevice. (b) I−V curves of electrodes 1−2 in the device with/without 5.64 kW/cm2 irradiation. (c) Current transported through electrodes 1−2 at a fixed bias voltage (Vbias = 5 mV) when 7.92 kW/cm2 515 nm laser is on and off. (d) Similar time trace of the device current during the laser on−off cycles with decreasing power density from 14.2 to 0.56 kW/cm2.

Figure 3. (a) Current under amplified scale bar at four steps of Figure 2d. (b) Dependence of irradiation power density p on the increasing rate of on−state current. (c) Dependence of p on the decreasing rate of off−state current. The dotted lines are drawn to guide the eye. (d) The temperature coefficient of resistance measured from 80 to 300 K. The black curve is the fitting curve. Inset is the logarithm of R as a function of reciprocal temperature.

and off (Vbias= 5 mV). Under 14.2 kW/cm2 irradiation, the Ipc and (Ion − Ioff)/Ioff = Ipc/Ioff reach high up to ca. 0.426 nA and 30.4%, respectively, where Ion and Ioff are on−state and off− state current. Considering that the highest power density used is 14.2 kW/cm2, one can observe a higher Ipc under a higher power density irradiation. Moreover, the Ioff almost remains 1.4 nA even after these on−off cycles, implying that the influence

of irradiation process on the device resistance is weak. It is different from the case in the Schottky-contacted device based on single carbon nanocoil, where the contact resistance decreased greatly after the 515 nm irradiation.30 Shown in Figure 3a is current change within four steps illustrated in Figure 2d. One can find that (i) the current keeps stable initially without irradiation (step 1); (ii) when irradiation 14586

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is on (step 2), it gradually increases with a rate of ca. 1.2 × 10−4 /s; and when irradiation is off (step 3), it also gradually decreases with a rate of ca. 1.2 × 10−4 /s; and (iii) the current increasing rate under high power density irradiation (step 2) is obviously larger than that of ca. 2 × 10−6 /s under low power density irradiation (step 14). Detailed results on various power densities are shown in Figure 3b,c. One can see that with the increasing of the power density the change rates of both on− state and off−state currents increased approximately linear. According to Figure 3a−c, we assume that bolometric effect contributes to the slow photoresponse, similar to the case in CNT films.7 To clarify the role of bolometric effect in the photocurrent, the temperature coefficient of resistance (TCR) was measured (shown in Figure 3d). One can find that the TCR value is negative, and the curve of logarithm of R as a function of reciprocal temperature is well linear (inset of Figure 3d). Apparently, bolometric effect can attribute to the photoresponse. One can expect resistivity to follow as R(T) = R0 exp(T0/T). Therefore, R(T)/R300 K = exp(T0/T − T0/300). It fits well with Figure 3d with T0 = 17. Considering that (i) the HCNT will be destroyed at 750 K in air and (ii) the HCNT is not destroyed even under 25 kW/cm2 irradiation, one can conclude that the temperature of HCNT under 14.2 kW/cm2 is below 750 K and can roughly deduce that the photocurrent resulted from the bolometric effect is below 3%. It is much lower than the measured value of 30.4% under 14.2 kW/cm2. Additionally, the photocurrent change also may come from other parameters such as device resistance, current and thermal conductivity, etc. However, in view of the facts that (i) the photoresponse is fast and (ii) there are interband transitions for semiconductors confirmed by the vis−NIR spectrum of the Ndoped HCNT thin film (not shown), it is reasonable to infer that the photocarriers play the key role in the instantaneous photocurrent. Namely, as illustrated in Scheme 1, the N-doped

(shown in Figure 5a), implying that there is interband transition under 515 nm excitation. Figure 4a shows the photocurrent transported through electrodes 1−2 when various power density irradiation are turned on and off (Vbias = 5 mV). The observed photocurrent can be interpreted in terms of a simple kinetic model by taking into account processes of photocarrier generation and relaxation. In a uniform photocarrier distribution system, the rate equation for these processes is given as8,31 dn n = ηI − − γn2 dt τ

(1)

where n is mean concentration of photocarriers, proportional to photocurrent Ipc, η is photocarrier generation rate, I is mean intensity of irradiation, τ is nonradiative lifetime of photocarriers, and γ is rate of bimolecular recombination. The τ can be expressed as (ka + kt)−1, where ka is rate for annihilation of carriers at electrodes and kt is rate for carrier trapping by deep traps or carrier localization. As we know that I is equivalent to pAeffλ/Vhc, Aeff is effective device area which is equivalent to the helix length multiplied by helical diameter (Aeff = 4.5 μm × 250 nm), V is the volume of the HCNT which is equivalent to the cross-sectional area S of tube multiplied by its tube length L (ca. 9 μm), h is the Planck constant, c is velocity of light, and λ is irradiation wavelength. At steady-state excitation (dn/dt = 0), two limits exist: when the annihilation rate for both at the electrodes and carrier trapping is much higher than bimolecular recombination rate (viz. ka + kt ≫ γ), the dependence is n ∼ I; conversely, n ∼ I1/2. Generally, in most CNTs-based devices reported, the dependence is n ∼ I because ka + kt ≫ γ in these devices.4,10 However, as shown in Figure 4b, one can observed that there is a nonlinear increase in Ipc with increasing of p and which can be well expressed as Ipc ∼ p1/2 (shown in Figure 4c). It indicates that the photocarriers mainly disappeared by bimolecular recombination in the N-doped HCNT. Compared to the case in straight CNTs, indeed, the photocarrier distribution in the N-doped HCNT system is uneven. As shown in Scheme 1, the photocarriers are generated under the irradiation. Because the distribution of incident photons is uniform at horizontal plane, the photon concentration is approximately proportional to the effective irradiation area Seff. The Seff is equivalent to the helical surface S multiplied by cos θ, where θ is interangle of axis of tube and horizontal plane. Apparently, the top and dip horizontal planes are strong irradiation planes with θ = 0, which are the planes with high concentrations of incident photon and photocarrier. By contrast, the inclined plane between top and dip planes is weak irradiation plane with θ ≠ 0, where the concentrations of photon and photocarrier are relatively low. In other words, the photocarrier distribution within one helical cycle of N-doped HCNT is uneven. The photocarriers can be annihilated by two routes: one is annihilated at electrodes or trapped by deep traps (route ①). The other is bimolecular recombination which includes band-to-band recombination (route ②−1) and diffusion recombination (route ②−2) in the N-doped HCNT. As reported previously, in uneven photocarrier distribution system, the photocarrier recombination rate is mainly determined by the bimolecular recombination because of photocarrier diffusion recombination.32,33 Thus, the bimolecular recombination rate in the N-doped HCNT will be higher than the annihilated rate at electrodes or trapping by deep traps.

Scheme 1. Diagram for the Generation, Recombination, and Distribution of Photocarriers in a N-Doped HCNT

HCNT can effectively absorb photon energy under 515 nm irradiation, resulting in the quick generation of photocarriers. Conversely, after turning off irradiation, the carrier density will decrease immediately because the electrons and holes recombine fast. To confirm further this postulation, a PL study was carried out at RT by using 515 nm excitation. One can find that there are many clear strong peaks emission 14587

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Figure 4. (a) Photocurrent transported through electrodes 1−2 (Vbias = 5 mV) with decreasing power density. (b) Dependence of the p on photocurrent Ipc. (c) Dependence of the square root of the power density p1/2 on Ipc. The solid lines represent linear fits.

Figure 5. (a) PL spectra of the pristine and N-doped HCNTs measured at RT with the excitation wavelength of 515 nm. (b, c) PL excitation (PLE) spectra of the N-doped HCNTs with emission at (b) 578 and (c) 643 nm.

in N-doped HCNT is similar to ca. 10 ps in SWNTs,34 one can roughly estimate the 1/[2(ηIγ)1/2] in N-doped HCNT, which is about 5.81 ps under 0.566 kW/cm2 irradiation. Moreover, the nsteady can be calculated from the equation σpc = (IpcL)/(VbiasS) = nsteadyμ·2q, which is about 1.8 × 1016 cm−3 under 0.566 kW/ cm2 irradiation. As the bimolecular recombination rate plays the key role in the photocarrier recombination, eq 2 can be simplified to nsteady = (ηI/γ)1/2, and one can know that γ = (ηIγ)1/2 (1/nsteady) = 4.7 × 10−6 cm3/s. The external quantum efficiency ηQ = (Ipc/pAeff)(hc/2qλ), which is defined as the number ratio between the electrons collected and photons per unit time, can be obtain to be ca. 9.4 × 10−6. Additionally, the following equation η = 2ηQttran(ηIγ)1/2 can be also deduced. Considering that the time for photocarrier transmit ttran = L2/

However, despite the uneven photocarrier distribution within one cycle, it is reasonable to assume that the mean photocarrier distribution in different helical cycle is uniform. Approximately, eq 1 can be used to interpret these processes. In steady state that dn/dt = 0, the following equation can be derived from eq 1: nsteady = −

1 1 + 2γτ 2γ

1 + 4ηIγ τ2

(2)

Actually, the nonlinear dependence of power density p on Ipc is fitting well with Ipc ∝ nsteady = −(1/2γτ) + (1/2γ)(1/τ2 + 4ηIγ)1/2 (shown in Figure 4b). From the fitting parameters, one can know that (1/τ)[1/2(ηIγ)1/2] = 0.437P−1/2. By assuming τ 14588

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low Vbias of 5 mV are ca. 0.426 nA and 2 × 10−2, respectively, and the photocarrier generation rate η is high of ca. 27%. The N-doped HCNT has high bimolecular recombination rate, which may attribute to the uneven photocarrier distribution in the helical structure. Moreover, by selecting an appropriate wavelength excitation, a higher photoresponse can be obtained.

(2μVbias) can be calculated to be 168.7 ns, one can know that the η is high up to 27.2%. On the basis of the fact that the absorption coefficients of SWNT4 and graphene35 are ca. 0.55% and 2.3%, one can conclude that the η of N-doped HCNT are much higher than those of SWNT and graphene. Recently, the relatively high ηQ has been investigated in some carbon-based semiconductors. For example, it has been observed to be ca. 4.9 × 10−5 at 5 V in stacked graphene nanotube,10 ca. 7.9 × 10−4 at 0.6 V in SWCNT,4 and ca. 2.4 × 10−3 at 0.4 V in grapheme.36 By contrast, the ηQ of the Ndoped HCNT is relatively low, which can attribute to the low Vbias used. It is about 3 orders of magnitude lower than the 1−5 V voltage applied;4,10,36 one can reduce ttran and improve ηQ further via applying larger voltage. Considering the well-linear I−V curve, one can obtain an improving ηQ of ca. 2.0 × 10−2 under the similar electrical field of 12 000 V/cm.4 PL Properties of Pristine and N-Doped HCNTs. To explore the optical properties of the N-doped HCNTs, a detailed PL study was carried out at RT by using 515 nm excitation. Note that we have reported previously that by doping the N into the pristine HCNTs, the UV PL could be enhanced.27 As a contrast, the corresponding PL properties of the pristine sample were also investigated. Figure 5a shows the PL spectra of the pristine and the N-doped HCNTs under 515 nm excitation. The PL intensities of both samples are so high that the emission spots can be easily observed. It is interesting to see that there are three clear strong peaks centered at 567, 578, and 643 nm and several sideband peaks centered at different wavelengths. Namely, it demonstrates that both of the two samples are multicolor PL. The PL observed in HCNT is greatly different from other CNTs which exhibit infrared PL reported previously.37−39 Moreover, interestingly, one can find that (i) the N-doped HCNTs showed a great enhancement in the intensity of all peaks compared to the pristine HCNTs and (ii) there is no clear shift in the peak position in the pristine and the N-doped samples. It is similar to the result reported previously.27 Apparently, one can obtain strong PL in HCNTs doped with high-content N. In other words, by doping the HCNTs with N, one can increase the concentrations of exciton and photocarrier and obtain a high photoresponse in the Ndoped HCNT device. Figures 5b and 5c show the PLE spectra of N-doped HCNTs with emission at 578 and 643 nm, respectively. As shown in Figure 5b, with emission at 578 nm, the N-doped HCNTs exhibit strong PLE at 306, 329, and 462.5 nm, respectively, and all of them are much stronger than that at 515 nm. It indicates that compared to that under the 515 nm excitation, the concentration of exciton and photocarrier under 462.5 nm excitation is maximum and which also can be enhanced under 306 or 329 nm excitation. Similarly, the concentration of exciton and photocarrier will be much higher under excitation of 341, 366, 378.5, or 466.5 nm (shown in Figure 5c). Therefore, it is reasonable to conclude that the concentration of exciton and photocarrier can be enhanced under some other wavelength excitation, especially under UV excitation. As a result, the N-doped HCNT device can obtain a higher photoresponse.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.T.); [email protected]. edu.tw (W.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the State Key Program for Basic Research (Grants 2012CB932304 and 2010CB923402) and NSFC (Grant 51072079), P. R. China, and the National Science Council of Taiwan (NSC 99-2112-M005-007-MY3) and Center of Nanosicence and Nanotechnology, NCHU.

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CONCLUSIONS In summary, we have fabricated a single N-doped HCNT device and systematically investigated its photoconductivity. The results showed that under 515 nm excitation the maximum photocurrent Ipc and external quantum efficiency ηQ under the 14589

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dx.doi.org/10.1021/jp302962r | J. Phys. Chem. C 2012, 116, 14584−14590