Figure of Merit for Carbon Nanotube Photothermoelectric Detectors

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Figure of Merit for Carbon Nanotube Photothermoelectric Detectors Kristopher J Erickson, Xiaowei He, A. Alec Talin, Bernice Mills, Robert H. Hauge, Takashi Iguchi, Naoki Fujimura, Yukio Kawano, Junichiro Kono, and Francois Leonard ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06160 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on November 3, 2015

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Figure of Merit for Carbon Nanotube Photothermoelectric Detectors Kristopher Erickson1, Xiaowei He2, A. Alec Talin1, Bernice Mills1, Robert H. Hauge3, Takashi Iguchi4, Naoki Fujimura4, Yukio Kawano4, Junichiro Kono2*, François Léonard1* 1 2

Sandia National Laboratories, Livermore, California 94551, United States

Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States 3 4

Department of Chemistry, Rice University, Houston, Texas 77005, United States

Quantum Nano-electronics Research Center, Department of Physical Electronics, Tokyo Institute of Technology, Meguro-ku,Tokyo 152-8552, Japan *[email protected]; [email protected]

ABSTRACT Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our 1 ACS Paragon Plus Environment

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measurements determine the quantities that enter the figure of merit, and allow us to establish a path towards future performance improvements. Keywords: carbon nanotubes, photodetector, thermoelectric, photothermoelectric, Joule heating

(TOC) Thin films of carbon nanotubes (CNTs) possess unique properties that make them attractive for a number of applications in electronics1, sensing2, thermal management3, thermoelectrics4, and photonics5. Recently, novel photodetectors based on CNT thin films have emerged6-9 and have been shown to operate based on the photothermoelectric effect (PTE). Understanding the factors that govern the efficiency of these types of detectors is important to assess their potential for technology development, and for establishing the path towards performance improvements. This can be accomplished by deriving an appropriate figure of merit for CNT PTE detectors, such as the Noise-Equivalent-Power (NEP), and evaluating the quantities that enter this figure of merit. While initial measurements have obtained the NEP for one type of CNT PTE device7 and suggested that the thermoelectric ZT factor plays a key role in determining the NEP, a general expression for the dependence of the NEP on device and material parameters is still missing. In this manuscript, we present such an expression for the NEP which explicitly shows its functional dependence on the ZT factor and the heat transfer coefficient to the environment. We 2 ACS Paragon Plus Environment

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then evaluate the quantities that enter the figure of merit through a combination of thermoelectric, electrothermal, and photothermal measurements on thin film devices made of dense and aligned CNTs. The techniques are applied to p-type and n-type single film devices, and to intra-film and overlapping-film p-n junctions, allowing the measurement of the Seebeck coefficient, thermal conductivity, junction electrical resistivity, and heat transfer coefficient to the environment. Based on these measurements, we evaluate the figure of merit and discuss the potential for performance improvements. RESULTS AND DISCUSSION Figure 1 shows a general schematic of a thin film CNT PTE detector, which consists of a p-n junction connected by source and drain electrodes. Focused illumination causes local heating of the p-n junction, and thus a temperature difference between the p-n junction and the contacts. As a consequence, a voltage ∆V is generated across the device due to the thermoelectric effect, ∆V = S p ∆Tp − S n ∆Tn = ( S p − S n ) ∆T , where S p (S n ) is the Seebeck coefficient of the p-type (ntype) portion of the film, and ∆Tp ( n ) is the temperature difference between the p-n junction and the contact on the p-type side (n-type). The last equality assumes ∆Tp = ∆Tn = ∆T . The responsivity of the photodetector is defined as

R =

( S p − S n ) ∆T . voltage generated = incident optical power P

(1)

The maximum ∆T is generated when the p-n junction is fully illuminated along its width. In that case the maximum temperature increase is given by8

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∆Tmax =

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P W κ hG

(2)

where h is the film thickness, W its width, κ is the film thermal conductivity, and G is the heat transfer coefficient to the substrate. The maximum responsivity is thus

R =

(S

p

− Sn )

W κ hG

.

(3)

The noise-equivalent power is defined as

NEP =

noise spectrum responsivity

(4)

where the noise spectrum has units of V / Hz and the responsivity has units of V / W . The NEP represents the minimum optical power that can be detected, corresponding to a signal-to-noise ratio of 1. High performing devices correspond to low values of NEP. Alternatively, the specific detectivity D* = A / NEP is used where A is the active area of the detector. In our case, the active area is determined by the thermal length λ, i.e. the length scale over which the temperature decays away from the focused light, see experimental results further below. Because PTE devices generate photocurrent at zero bias, the relevant noise is the zero-bias dark noise spectrum10. As we will show below, our experimental measurements indicate that the zero-bias dark noise of CNT thin films approaches the Johnson noise limit, in agreement with previous measurements on a variety of CNT materials9, 11-13.


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Figure 1: Illustration of a CNT p-n junction device under focused illumination. Light absorption at the p-n junction causes local heating, and a temperature difference between the p-n junction and the contacts. This temperature difference creates a voltage through the thermoelectric effect.

For a device limited by Johnson noise, the noise spectrum is given by

4kBTR , where kB

is Boltzmann’s constant, T is the temperature, and R is the device electrical resistance. The NEP is therefore

NEP =

2 k BT 2WL G ZT

(5)

where

ZT =

S 2σ T

κ

(6)

is the usual figure of merit used to qualify thermoelectric materials. Equation (6) shows that reducing the NEP depends on two important factors: minimizing the heat transfer coefficient to the environment G and maximizing the ZT factor.


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To assess the responsivity and NEP for CNT PTE detectors and gain insight into the impact of different CNT thin film material, we studied two types of CNT thin-film devices as illustrated in Fig. 2. Both types of devices have previously been demonstrated to operate based on the PTE6-8, and utilize thin films that contain a mixture of semiconducting and metallic singlewall CNTs14. In the first type of device, the thin film (about 1 µm in thickness) consists of CNTs aligned perpendicular to the electronic transport direction; these devices are referred to as “fin” devices (see Methods for explanation). We fabricated a p-type device (Device 1), an n-type device (Device 2), and three intra-film p-n junction devices (Devices 3-5) by utilizing the asgrown p-type films, and solution-based doping with benzyl viologen (BV) to create the n-type doping. Devices 3-5 differ in the substrate on which the CNT film sits: for Device 3 the CNT film is on suspended Teflon, for Device 4 the Teflon is on AlN, and for Device 5 the CNT film is directly on AlN. The second type of device utilizes CNT thin films obtained by directionally compressing a vertical forest of CNTs, with the CNTs aligned parallel to the electronic transport direction. The p-n junctions were fabricated by physically overlapping p-type and n-type films, with overlap distances of 50 microns (Device 6) and 75 microns (Device 7), again utilizing BV for the n-type doping. Devices 6 and 7 were fabricated on Teflon tape, and then either supported on a glass slide or suspended. Optical images of devices are shown in Figs 2a-e while Fig. 2f and Fig. 2g show scanning electron microscope (SEM) images of a compressed film and a fin, respectively. Devices 3-7 were chosen in order to compare the impact of device geometry and CNT material on the performance of p-n junction devices. For example, in Devices 3-5 the CNTs are aligned in the direction perpendicular to the channel, while in Devices 6-7 they are aligned parallel to the channel. In addition, Devices 6-7 have an “overlap” p-n junction geometry, while 6 ACS Paragon Plus Environment

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Devices 3-5 has an “intra-film” p-n junction geometry. Devices 6 and 7 differ in the amount of the p-n junction geometrical overlap which has been previously found to affect device behavior6. Finally, we studied Devices 1 and 2 because they are made only of p-type or n-type CNT films which serves as a way to extract fundamental properties of the fin CNT thin film material (such as dominant carrier type) as will be discussed further below.

Figure 2: (a-e) Optical images of CNT film devices on Teflon with a schematic beneath for (a) p-type “fin” device (Device 1) (b) n-type “fin” device (Device 2) (c) p-n junction “fin” device (Devices 3-5) (d) small overlap p-n junction device (Device 6), and (e) large overlap p-n junction device (Device 7). Scale bars are 1 mm. (f) SEM image of an overlap CNT thin film after growth and compression. (g) SEM image of a CNT fin while still on the growth substrate.


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In order to assess the figure of merit of Devices 3-7, we utilize two approaches: in the first approach, we directly measure the responsivity during illumination, and obtain the NEP from Eq. (4). In the second approach, we measure the heat transfer coefficient G and each of the material properties S, σ, and κ that enter in the ZT factor. We then use Eq. (5) to obtain the NEP. In the rest of this section, we describe our approach to obtain each of the material quantities that enter the NEP, and in the Discussion section, we compare the two approaches.

Noise assessment. To assess whether Johnson noise is the main source of noise at zero bias in the dark for devices relevant to the present study, we measured the noise spectrum for Devices 45 with resistances of 56 kOhm and 280 kOhm. The noise spectrum was obtained from the intrinsic noise function of a lock-in amplifier. Figure 2 shows the noise spectrum for both devices, measured at zero bias and at a small bias giving a current of 10 nA. Although the noise values fluctuate, the zero-bias noise is found to be close to the Johnson noise limit for both devices. Furthermore, while the finite-bias noise shows 1/f behavior at low frequencies, it also approaches the Johnson noise limit at larger frequencies. These results indicate that regardless of the substrate, the fin CNT thin films at zero bias in the dark have a noise spectrum close to the Johnson limit. We did not measure the noise for the overlap devices because they were no longer functioning after the extensive set of experiments they were subjected to. However, since previous measurements on a broad range of CNT materials9, 11-13 have also reported zero-bias noise close to the Johnson noise limit, we use this limit in the rest of this paper.


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Figure 3: Measured noise spectrum for two CNT fin p-n junction devices. Panel (a) is for Device 4 (resistance 56 kΩ). Panel (b) is for Device 5 (resistance 280 kΩ). The 50Hz peak is due to the electrical power frequency.

Thermoelectric characterization to obtain S. We determined the device Seebeck coefficient, S = −∆ V / ∆ T , by measuring the voltage ∆V generated across the CNT devices while the p-n

junction is heated, as illustrated in Fig. 3a. There, a resistive heater is placed in contact with the substrate directly underneath the p-n junction by using thin copper foil, thereby creating a temperature gradient between the center of the device and the contacts. Infrared (IR) images of the temperature distributions for Device 3 (Fig. 3b), Device 6 (Fig. 3c), and Device 7 (Fig. 3d) 9 ACS Paragon Plus Environment

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all show a hot vertical segment at the location of the underlying heated copper foil, as well as a horizontal hot stripe that is the heated CNT film. The electrical contacts (dark and light blue) are discernible owing to their different emissivity compared to the CNT thin film. Indeed, their apparent lower temperature is an artifact of the measurement since the IR camera uses a constant emissivity for temperature determination for all the pixels when collecting an image. Notably, there is a temperature difference of a few degrees between the center of the device and the nearcontact CNT film. In the case of Devices 3 and 6, the location of the p-n junction made it possible to obtain ∆T p and ∆Tn within 0.3K of each other, and thus to directly use the expression

∆V = S ∆T (valid when ∆T p = ∆Tn = ∆T ) to obtain the Seebeck coefficient. From such data, shown in Fig. 3e-g, we obtain S = 138 µV/K for Device 3 and S = 140 µV/K for Device 7. To measure the Seebeck coefficients of the p and n segments in the overlap devices, we employed an approach where the heater was positioned in the middle of the p segment in Device 7 (see Supporting Information). In such a measurement, there is no temperature difference across the p segment, and the device Seebeck is equal to the Seebeck coefficient of the n segment. We obtain Sn = −86 µ V/K and S p = 54 µ V/K. Since the p-type segment of Device 6 was fabricated from the same as-grown material, we expect that it will have the same Seebeck coefficient. The situation for the analysis of Device 6 was not as favorable because the junction is closer to one electrode, and it was not possible to generate near-equal temperatures at the two contacts. In particular, the temperature difference was always larger on the n-type side. We can correct for this effect by adding the value S p ( ∆Tn − ∆Tp ) to the measured thermovoltage; the resulting data (Fig. 3f) gives S = 6 µV/K for Device 6. The low S value implies that in this case the n-type doping was not effective at converting the original p-type film in to a n-type film; in 10 ACS Paragon Plus Environment

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fact, using the above value of S p we obtain Sn = +48 µ V/K , i.e. the n-type segment is actually ptype.

Figure 4: (a) Schematic illustrating the setup for measuring the Seebeck coefficient of p-n junction devices. Infrared image of the direct heating of (b) suspended Device 3, (c) suspended Device 6, and (d) suspended Device 7. Panels (e-g) show the voltages generated across the electrodes as a function of the average temperature difference between the p-n junction and the two electrodes for (e) suspended Device 3, (f) suspended Device 6, and (g) suspended Device 7. In the case of fin devices, we were previously able to use Devices 1 and 2 to measure the p-type and n-type material Seebeck coefficient7. A value for the Seebeck coefficient of S p = 75 ± 9 µ V/K was obtained for the as-grown film (Device 1), confirming its p-type

character, while the CNT film after BV treatment (Device 2) has a negative Seebeck coefficient, S n = −71 ± 7 µ V/K , showing the effectiveness of BV at converting the as-grown p-type film to n-

type character due to the electron donating nature of BV towards CNTs15. Furthermore, if a p-n 11 ACS Paragon Plus Environment

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junction were created from these two films, the device Seebeck coefficient would be

S p − Sn = 146 µ V/K, remarkably close to the value 138 µV/K measured for Device 3. (Note that the average temperature of the CNT films increases with increasing ∆T by as much as 10K during these measurements. The linearity of the measured thermovoltage vs ∆T indicates that the Seebeck coefficient is not strongly temperature-dependent over that temperature range.) The measured values of the Seebeck coefficient can be compared with previous measurements on disordered CNT thin films: for films consisting of a mixture of semiconducting and metallic CNTs9, typical values are ~ 30 µ V/K. Solution doping of such films leads to p-type and n-type CNT films with Seebeck coefficients at room temperature that vary between +90 µV/K and -70 µV/K4. In contrast to these measurements, the CNT films measured in our study consist of nanotubes aligned perpendicular or parallel to the transport direction; despite this alignment and its variation versus the electronic transport direction, the Seebeck coefficients are still similar to disordered films, which suggests that CNT alignment may not provide a significant advantage for increasing the Seebeck coefficient. Alternatively, semiconductorenriched CNT films have recently been reported with Seebeck coefficients as large as 160 µV/K16, implying that control over CNT chirality distribution may be a promising route to improve thermoelectric properties.

Electrothermal characterization to obtain κ and G. We analyzed thermal images taken during Joule heating to extract the thermal conductivity and the heat transfer coefficient to the environment, as illustrated in Fig. 5a. As reported previously6, 7, both device types show linear current-voltage characteristics with resistances measured here to be 100 kΩ, 56 kΩ, 280 kΩ, 146 Ω, and 287 Ω for devices 3-7, respectively (the absence of rectification even in the presence of a


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p-n junction is a consequence of the presence of metallic CNTs in the samples). Figures 5(b-d) show examples of temperature profiles obtained for fin-CNT devices (Devices 1, 2, and 3). The IR images indicate a relatively uniform temperature throughout the length of the CNT film, even in the presence of the p-n junction. This indicates that solution-based doping with BV of a continuous CNT film does not create a significant additional resistance at the p-n junction.

Figure 5: (a) Schematic illustrating the experimental setup for the electrothermal experiments. Thermal images acquired during Joule heating experiments for (b) Device 1, (c) Device 2, and 13 ACS Paragon Plus Environment

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(d) Device 3. Panels (e-g) show temperature rise line profiles extracted along the channel length for the same order of devices as (b-d). Thermal images during Joule heating experiments for (h) suspended overlap Device 6 and (i) suspended overlap Device 7. (j) Temperature rise line profiles for Device 6 extracted along the channel length from images like those in (h) for different values of the applied voltage, as indicated. The data is represented as circles and the fits (see text) as lines. (k) Same as in (j) for Device 7. Because the p-n junction does not create an additional resistance, the measured thermal profiles can be analyzed by solving the one-dimensional heat conduction equation assuming uniform material properties along the thin film

κ hW

d 2T − GWT = − p( x) dx 2

(7)

where h is the film thickness, W its width, κ is the film thermal conductivity, G is the heat transfer coefficient to the substrate, and p is the power dissipated per unit length due to Joule heating. In the case of a uniform power dissipation in the device channel and assuming temperature-independent material properties (Supporting Information) the heat conduction equation gives

∆T ( x) =

RI 2 GWLtot

 cosh ( ( x − Ltot / 2 ) / λ )  1 −  cosh ( Ltot / 2λ )  

(8)

where I is the current, Ltot is the total channel length, and λ = κ h / G is the thermal length scale. With the caveat that variations in film adherence and width along its length leads to deviations from this equation, fitting the experimentally measured profiles gives the CNT film thermal


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conductivity, κ, and the heat transfer coefficient to the substrate, G. Applying this equation to the suspended fin p-n junction device (Device 3), we obtain κ = 32 ± 7 W/mK and

G = ( 2.27 ± 0.05) ×103 W/m2 K .

In contrast to the fin devices, Joule heating experiments on overlap p-n junction devices show a different profile, with a pronounced temperature increase at the p-n junction (Figs 5e,f) that clearly appears in line profiles taken along the channel length (Figs 5j,k). To extract material properties from such profiles, we consider the situation where the resistivity in the overlap region is different from that in the two arms. We thus write the power dissipation in the form  ρ p I 2 /Wh 0 < x < L p  p ( x ) =  ρ pn I 2 /Wh L p < x < L p + L pn  ρ I 2 /Wh L + L < x < L p pn tot  n

(9)

where I is the current, ρ is the resistivity, and L is length. The subscripts p, n, and pn refer to the p-type region, n-type region, and p-n overlap region. Solving the heat conduction equation with this spatially-varying power dissipation (Supporting Information) provides analytical expressions for the temperature profile which we then numerically fit to the data of Figs 5j,k. We find excellent agreement between the theory and the experimental data for Devices 6 and 7, at all applied voltages. The fitted values of ρ n = ( 2.9 ± 0.2 ) × 10 −5 Ω ⋅ m, ρ p = ( 2.1 ± 0.3 ) × 10 −5 Ω ⋅ m, and ρ pn = (1.5 ± 0.3 ) × 10 −3 Ω ⋅ m for suspended Device 6 and

ρ n = ( 7.9 ± 0.5 ) × 10 −5 Ω ⋅ m, ρ p = (1.4 ± 0.1) × 10 −4 Ω ⋅ m, and ρ pn = ( 4.0 ± 0.3 ) × 10 −3 Ω ⋅ m for suspended Device 7 show that the resistivity in the overlap region is much larger than that in the two arms, explaining the large temperature increase in the middle of the device. Furthermore, comparison of the extracted resistivities in the two arms shows that it is at least an order of 15 ACS Paragon Plus Environment

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magnitude lower than for Device 3, showing that the perpendicular nanotube orientation in Device 3 is responsible for the higher total device resistance. The extracted values for the thermal conductivity were κ = 56±12 W/mK for suspended Device 6 and κ = 24±2 W/mK for suspended Device 7. These values are comparable to those previously reported17 in the literature although large variations exist depending on the types and density of CNTs, as well as the degree of alignment. It is worth noting that the fin and overlap devices have similar thermal conductivities despite the fact that the CNTs are aligned in different directions with respect to the transport direction, most likely because the length of the CNTs in the overlap devices (60 µm) is smaller than the channel length so that the thermal transport is dominated by nanotube-nanotube interfaces as is the case for CNTs oriented perpendicular to the channel. The values of the heat transfer coefficient are significantly lower than those previously measured for CNT films in contact with inorganic solids17 but in line with those for polymer interfaces18; indeed, for suspended Device 6 we find ( 4.7 ± 0.5) ×102 W/m2 K while it is 708±8 W/m2K for suspended Device 7.

Photothermal characterization to obtain S, κ, and G. To confirm the values of the Seebeck coefficient, thermal conductivity, and the heat transfer coefficients extracted using the thermoelectric and electrothermal experiments, we further characterized the CNT material and devices by performing photothermal experiments by focusing a red laser beam through a 50x objective (NA=0.55) onto the CNT film surface, and measuring the resulting temperature landscape with the IR camera (Fig. 6a). The maximum temperature rise as a function of laser power for suspended Device 1 and Device 7 on a glass slide is shown in Figs 6b,c and was obtained by analyzing IR images such as those in Figs 6d,e. The maximum temperature is found


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to depend linearly on the laser power, with a slope of 10.4±0.2 K/mW for Device 1 and 2.3±0.1 K/mW for Device 7. These relatively large temperature increases are consistent with the high absorptivity of CNT films. The linear dependence on laser power supports the use of the linear heat conduction equation to analyze the photothermal profiles. Indeed, we can analyze the thermal images to extract material properties using the procedure described in the Supporting Information. For example, we show in Figs 6d,e thermal images for Device 1 and Device 7 during laser illumination. Temperature line profiles are extracted from the thermal images along the channel length (Figs 6f,g) and analyzed using the procedure described in the Supporting Information, assuming temperature-independent material properties; this gives

G = ( 4.5 ± 0.6) ×103 W/m2 K and κ = 42±2 W/mK for suspended Device 1, while analysis of the data for Device 7 on a glass slide gives κ = 45±3 W/mK and G = ( 4.1 ± 0.6 ) ×103 W/m2 K . The values obtained from the photothermal characterization are in reasonably good agreement with those obtained from the Joule heating experiments, with the differences most likely due to the assumptions made in deriving the models. We also measured the photothermal properties of suspended Device 7 and found the thermal conductivity to be κ = 56±12 W/mK, while the heat transfer coefficient decreased to G = ( 8 ± 1) ×102 W/m2 K , consistent with the expected decrease in heat dissipation to the environment. We also used the photothermoelectric characterization to extract the device Seebeck coefficient and compare with the values obtained from direct heating. This was done by measuring the generated voltage during the photothermal experiments, and obtaining the Seebeck coefficient from the temperature differences measured with the IR camera during illumination. For example, Device 3 gives S = 138 µV/K from direct heating and S = 136 µV/K


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from laser illumination. Device 6 gives S = 6 µV/K from direct heating and S = 6.6 µV/K from laser illumination, while for Device 7 we get S = 140 µV/K from direct heating and S = 164 µV/K from laser illumination. (The values are not expected to agree perfectly due to systematic errors when comparing the two different types of experiments. For example, the photovoltage depends on the specific location of the focused light on the p-n junction and the voltage measured during direct heating depends on the precise positioning of the heater under the p-n junction. In addition, we cannot rule out a small photovoltaic contribution in parallel with the photothermoelectric effect.)


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Figure 6: (a) Schematic illustrating the setup for the photothermal experiment. (b) Maximum temperature rise as a function of laser power for suspended Device 1. (c) Maximum temperature rise as a function of laser power for Device 7 on a glass slide. Infrared image of illuminated (d) Device 1 and (e) Device 7. (f-g) Line profiles extracted along the channel length for (f) Device 1 and (g) Device 7, for different values of laser power, as indicated. The data is represented as circles and the fits (see text) as lines.

Figure of merit. As discussed earlier, we evaluated the NEP in two ways. First, we measured directly the responsivity under illumination with visible light and used the measured device resistance to obtain the device NEP based on Eq. (4) in the Johnson noise limit. Second, we used the above measurements of each of the quantities that enter the NEP in Eq. (5), and we can thus evaluate the NEP for the three devices based on the measured material properties. (We used the quantities measured using the photothermal approach since this is closest to the direct responsivity measurements.)


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Figure 7: (a) Laser power dependence of the photovoltage for the devices when illuminated with red light focused on the p-n junction. (b) Time-dependence of the photocurrent for Device 3 when a red light source of 1 mW power is turned on and focused on the p-n junction. Figure 7 shows the photovoltage generated while a red laser beam is focused at the p-n junction. We find a linear dependence of the photovoltage on incident power with responsivities that range between 0.06 V/W and 3 V/W. The temporal dependence of the photovoltage upon turn-on of the light shows a response time of 49 ms for Device 3, with comparable or longer time scales observed for overlap p-n junction devices6. Table 1 shows the NEP obtained from Eq. (4); the NEP for Device 3, 4 and 6 are quite similar, while Device 7 has the smallest NEP and Device 20 ACS Paragon Plus Environment

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5 the largest. Similar results are obtained when evaluating the NEP from Eq. (5) using the measured material properties. In particular, the values of NEP obtained from the two approaches are comparable, and both approaches suggest that Device 7 has the lowest NEP. In addition, it is worth noting that the responsivity alone (Fig. 7a) is not enough to predict which device will have the best performance. Table 1 shows that the ZT factor varies by more than an order of magnitude in these devices. Devices 3 and 7 have similar Seebeck coefficients but Device 7 is superior because of its lower resistance. Devices 3-5 have similar values of ZT, but Device 5 has the worst performance because the heat transfer coefficient to the AlN substrate is about one order of magnitude higher than for Devices 3 and 4. Devices 6 and 7 have similar resistances but Device 7 has larger Seebeck coefficient. Still, the value ZT ~ 0.0002 for Device 7 is quite small when compared with the value ZT ~ 1 for the best thermoelectric materials, which shows that the performance of CNT photothermoelectric detectors could be significantly improved by tailoring the thermoelectric properties of the CNT thin films. For example, ZT ≈ 0.08 has been reported recently4 for CNT thin films and this would improve the NEP by almost an order of magnitude according to Eq. (5). From the perspective of reducing thermal transfer to the environment, the measured heat transfer coefficients can be compared with the heat transfer coefficient due to radiation G = 4εσ SB T 3 , where σ SB is the Stefan-Boltzmann coefficient and ε is the emissivity, equal to

0.98 for the CNT thin film19. At room temperature this gives G ≈ 6 W/m 2 K , a value that is two orders of magnitude smaller than the smallest values measured here, suggesting that the NEP


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could be improved by one order of magnitude by fully suspending the CNT thin films (as opposed to our “suspended” devices, which were still on Teflon), due to the

Device 3, suspended

ZT from Eq. (6)

NEP from Eq. (4)

NEP from Eq. (5)

1.5 × 10 −5

25 nW/ Hz

23 nW/ Hz

23 nW/ Hz

23 nW/ Hz

418 nW/ Hz

383 nW/ Hz

26 nW/ Hz

24 nW/ Hz

6 nW/ Hz

7 nW/ Hz

−5

4, Teflon on AlN

1.0 × 10

5, AlN

1.2 × 10 −5

6, suspended 7, suspended

G behavior.

1.2 × 10

−5

1.8 × 10

−4

Table 1: Figures of merit for CNT photothermoelectric detectors. Finally, the obtained values of NEP can be compared with existing thermoelectric photodetector technologies, although a complete consideration of device properties including ease of fabrication, cost, reliability, spectral bandwidth, response time, etc. should really be taken into account when comparing different photodetector technologies. With this caveat, commercial thermopile detectors20 with rise times of tens of milliseconds, comparable to the suspended devices reported here, typically have NEP around 1 nW/ Hz . Together with the above path for performance improvement, this suggests that CNT photodetectors are promising for future applications.

CONCLUSION In summary, we present a figure of merit for evaluating the performance of carbon nanotube photodetectors that operate based on the photothermoelectric mechanism. The thermal transport to the environment and the thermoelectric figure of merit emerge as key quantities, thus providing a path to evaluate and improve the performance of such photodetectors. A suite of characterization methodologies to measure coupled electrical, thermal, and optical properties of 22 ACS Paragon Plus Environment

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CNT thin-film devices allows the measurement of the quantities that enter the figure of merit. More generally, the development of a figure of merit will also allow future comparison with other photodetection mechanisms, such as the photovoltaic effects reported using split gate geometries21 or asymmetric contacts22 in individual CNTs.

Methods Fabrication of CNT devices. The CNT thin-film devices in this study were fabricated in two ways. In the first approach6, a forest of vertical CNTs (about 60 microns in thickness) was grown by chemical vapor deposition (CVD), and then pressed down to form a compact film of horizontal CNTs about 3-4 microns in thickness. The compressed-CNT films were transferred onto Teflon tape, and then positioned on top of glass slides. The p-type films were obtained directly from the as-grown material, while n-type films were obtained by depositing a drop of benzyl viologen (BV) on the p-type films6. The p-n junctions were created by physically overlapping p-type and n-type films, with overlap distances of 50 microns (Device 6) and 75 microns (Device 7). Electrical contacts were created at the two ends using silver paste. After completion of device assembly, the devices were measured while still on the glass slide or by suspending the Teflon. These devices are referred to as “overlap” devices throughout the text. In the second approach8, the CVD growth was performed on patterned catalyst lines, resulting in fins of vertically aligned CNTs. These vertical fins were then wet-transferred onto Teflon tape positioned on top of glass slides. Electrical contacts were created by depositing Au using electron-beam lithography. The as-grown p-type films were converted to n-type using BV, and intra-film p-n junctions created by treating only one half of the film with BV. These devices are referred to as “fin” devices throughout the text.


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Thermal measurements. The temperature profiles for all the measurements were obtained with an Inframeterics 760 IR thermal imaging camera, with spatial resolution of 10 µm and thermal resolution of 25 mK, using an emissivity of 0.98 for the CNT films19. For the thermoelectric measurements, the temperature difference between the two electrodes was extracted from the data points on the CNT film nearest to the contacts. Conflict of interest. The authors declare no competing financial interest. Acknowledgement. This work was supported by the US Department of Energy, Office of Science under the National Institute for Nano Engineering (NINE) at Sandia National Laboratories, and by the Lockheed-Martin Rice University LANCER Program. X.H. and J.K. were supported by DOE BES DE-FG02-06ER46308 (preparation and teraherz/infrared characterization of aligned carbon nanotubes) and the Robert A. Welch Foundation Grant No. C-1509 (detector fabrication). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000.

REFERENCES AND NOTES (1) Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.; Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible Substrates. Nano Lett. 2013, 13, 38643869. (2) Cao, Q.; Rogers, J. A. Ultrathin Films of Single-Walled Carbon Nanotubes for Electronics and Sensors: A Review of Fundamental and Applied Aspects. Adv. Mater. 2009, 21, 29-53. (3) Fu, Y.; Nabiollahi, N.; Wang, T.; Wang, S.; Hu, Z.; Carlberg, B.; Zhang, Y.; Wang, X.; Liu, J. A Complete Carbon-Nanotube-Based on-Chip Cooling Solution with Very High Heat Dissipation Capacity Nanotechnology 2012, 23, 045304. (4) Nonoguchi, Y.; Ohashi, K.; Kanazawa, R.; Ashiba, K.; Hata, K.; Nakagawa, T.; Adachi, C.; Tanase, T.; Kawai, T. Systematic Conversion of Single Walled Carbon Nanotubes into N-Type Thermoelectric Materials by Molecular Dopants. Sci. Rep. 2013, 3, 3344. 24 ACS Paragon Plus Environment

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(5) Jung, Y.; Li, X.; Rajan, N. K.; Taylor, A. D.; Reed, M. A. Record High Efficiency Single-Walled Carbon Nanotube/Silicon P–N Junction Solar Cells. Nano Lett. 2012, 13, 95-99. (6) He, X.; Wang, X.; Nanot, S.; Cong, K.; Jiang, Q.; Kane, A. A.; Goldsmith, J. E. M.; Hauge, R. H.; Léonard, F.; Kono, J. Photothermoelectric P–N Junction Photodetector with Intrinsic Broadband Polarimetry Based on Macroscopic Carbon Nanotube Films. ACS Nano 2013, 7, 7271-7277. (7) He, X., et al. Carbon Nanotube Terahertz Detector. Nano Lett. 2014, 14, 3953-3958. (8) Nanot, S.; Cummings, A. W.; Pint, C. L.; Ikeuchi, A.; Akiho, T.; Sueoka, K.; Hauge, R. H.; Léonard, F.; Kono, J. Broadband, Polarization-Sensitive Photodetector Based on Optically-Thick Films of Macroscopically Long, Dense, and Aligned Carbon Nanotubes. Sci. Rep. 2013, 3, 1335. (9) St-Antoine, B. C.; Ménard, D.; Martel, R. Single-Walled Carbon Nanotube Thermopile for Broadband Light Detection. Nano Lett. 2010, 11, 609-613. http://www.hamamatsu.com/resources/pdf/ssd/e07_handbook_Thermal_detectors.pdf (10) (8/10/2015) (11) Tarkiainen, R.; Roschier, L.; Ahlskog, M.; Paalanen, M.; Hakonen, P. Low-Frequency Current Noise and Resistance Fluctuations in Multiwalled Carbon Nanotubes. Physica E 2005, 28, 57-65. (12) Santavicca, D. F.; Chudow, J. D.; Prober, D. E.; Purewal, M. S.; Kim, P. Energy Loss of the Electron System in Individual Single-Walled Carbon Nanotubes. Nano Lett. 2010, 10, 4538-4543. (13) Sayer, R. A.; Sunkook, K.; Franklin, A. D.; Mohammadi, S.; Fisher, T. S. Shot Noise Thermometry for Thermal Characterization of Templated Carbon Nanotubes. IEEE Trans. Comp. Pack. Technol. 2010, 33, 178-183. (14) Pint, C. L., et al. Dry Contact Transfer Printing of Aligned Carbon Nanotube Patterns and Characterization of Their Optical Properties for Diameter Distribution and Alignment. ACS Nano 2010, 4, 1131-1145. (15) Kim, S. M., et al. Reduction-Controlled Viologen in Bisolvent as an Environmentally Stable N-Type Dopant for Carbon Nanotubes. J. Am. Chem. Soc. 2008, 131, 327-331. (16) Nakai, Y.; Honda, K.; Yanagi, K.; Kataura, H.; Kato, T.; Yamamoto, T.; Maniwa, Y. Giant Seebeck Coefficient in Semiconducting Single-Wall Carbon Nanotube Film. Applied Physics Express 2014, 7, 025103. (17) Marconnet, A. M.; Panzer, M. A.; Goodson, K. E. Thermal Conduction Phenomena in Carbon Nanotubes and Related Nanostructured Materials. Rev. Mod. Phys. 2013, 85, 1295-1326. (18) Dawson, A.; Rides, M.; Allen, C. R. G.; Urquhart, J. M. Polymer–Mould Interface Heat Transfer Coefficient Measurements for Polymer Processing. Polym. Test. 2008, 27, 555-565. (19) Mizuno, K.; Ishii, J.; Kishida, H.; Hayamizu, Y.; Yasuda, S.; Futaba, D. N.; Yumura, M.; Hata, K. A Black Body Absorber from Vertically Aligned Single-Walled Carbon Nanotubes. Proc. Nat. Acad. Sci. U.S.A. 2009, 106, 6044-6047. (20) www.hamamatsu.com; www.excelitas.com (9/1/2014) (21) Lee, J. U. Photovoltaic Effect in Ideal Carbon Nanotube Diodes. Appl. Phys. Lett. 2005, 87, 073101. (22) Yang, L.; Wang, S.; Zeng, Q.; Zhang, Z.; Pei, T.; Li, Y.; Peng, L.-M. Efficient Photovoltage Multiplication in Carbon Nanotubes. Nat. Photon. 2011, 5, 672-676.


Figure of Merit for Carbon Nanotube Photothermoelectric Detectors

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