Infrared Photoresponses from PbS Filled Multiwall Carbon

(1, 2) Chemists in turn have alluded to the similarity of CNTs with test tubes and ..... relatively low photocurrent magnitudes below 0.6 eV until app...
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J. Phys. Chem. C 2010, 114, 22703–22709

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Infrared Photoresponses from PbS Filled Multiwall Carbon Nanotubes Gustavo E. Fernandes,*,† Marian B. Tzolov,‡ Jin Ho Kim,† Zhijun Liu,† and Jimmy Xu† School of Engineering, Brown UniVersity, ProVidence, Rhode Island 02912, United States, and Department of Geology and Physics, Lock HaVen UniVersity of PennsylVania, Lock HaVen, PennsylVania 17745, United States ReceiVed: September 14, 2010; ReVised Manuscript ReceiVed: NoVember 17, 2010

The void space inside carbon nanotubes provides an interesting platform to study the interaction of nanotubes with other spatially confined materials. Here, we report on the feasibility of creating broad-band infrared and room temperature multiheterojunction diodes by filling the interior space of multiwalled carbon nanotubes with a semiconductor material via a solution based method. Lead sulfide is chosen because of its potential use in mid-infrared sensing. However, the approach described here can be extended to other materials. The device structure shows electrical rectification and photocurrent generation for a broad band of wavelengths in the near- and mid-infrared. We studied our device structure by scanning electron microscopy, energy dispersive X-ray spectroscopy, and micro-Raman spectroscopy and demonstrate compact filling of the carbon nanotubes with lead sulfide over a length of 880 nm with an aspect ratio of more than 20. These findings open the possibility of engineering devices with different functionality with multiheterojunctions at nanometer scales while leveraging on the unique properties of carbon nanotubes and their interplay with the filling materials. Introduction The void space inside carbon nanotubes (CNTs) is thought provoking. Many have contemplated the possibility of filling CNTs with various types of materials and the functional implications that might result. Cell biologists have viewed CNTs as robust and chemically inert containers that may be suitable for the delivery of drugs and other substances to cells.1,2 Chemists in turn have alluded to the similarity of CNTs with test tubes and contemplated performing chemical reactions inside CNTs.3,4 The filling of CNTs is also interesting from the point of view of materials and electronic engineering. Filled CNTs may provide a path to new functional materials and electronic structures of reduced dimensionality and extended functionality. Ajayan and Iijima5 were the first to report almost two decades ago the successful filling of CNTs with lead. Since then a number of reports have surfaced describing various different techniques for filling CNTs with a variety of materials, including metals,6-8 inorganic and organic salts,9 semiconductors,10 and polymers.11,12 Apart from such notable advances in their synthesis, less advance has been made in terms of possible functionality and applications of filled CNTs. Additional challenges are associated with electronic applications, where the need to precisely position and contact the CNTs (or to make the CNT part of a larger functional structure) is hampered by the inhomogeniety of dispersion of CNTs on substrates or by the difficulty in making electrical contacts to the selected portion of the filled CNTs. One way to circumvent such difficulties may be via the use of oriented CNTs in cases where in situ CNT growth is permissible. Anodic aluminum oxide (AAO)13-15 is one such platform that has been shown to produce arrays of homogeneously distributed and oriented CNTs that are uniform in size and diameter. In addition, CNTs grown in the AAO matrix may be opened via reactive ion etching in order * To whom correspondence should be addressed. E-mail: Gustavo_ [email protected]. † Brown University. ‡ Lock Haven University of Pennsylvania.

to facilitate the filling of CNTs with materials in the molten or solution phases or solid particulates such as quantum dots. However, little effort has been directed to the possibility of extending the reach of the AAO based nanotube array to heterojunction electronics. Here we study the electronic and photoresponse properties of CNTs that are filled with lead sulfide (PbS). The CNTs, which are vertically grown on an AAO matrix on a Si substrate, have been shown to form heterojunctions with the Si substrate.16 In the system studied in this work, we have additional electronic interfaces between (1) the quantum confined PbS filling and the bulk PbS top layer that is formed at the end of the PbS filling process, (2) the interface between the PbS filling and the CNT inner shell, and (3) the PbS filling and the Si substrate. These heterojunctions are manifested in the electrical transport characteristics of the device as well as in the photocurrent response spectra. Analyses of the resulting structure with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy confirm the presence of PbS and CNTs in the locations where they are expected to be. These results show that CNTs grown in this manner and filled with PbS effectively form electronically functional heterojunction devices. The device functionality includes, but is not limited to, broad-band infrared photodetection at room temperature, which is experimentally demonstrated here. Experimental Methods The multiwalled CNTs were grown via chemical vapor deposition (CVD) inside the pores of an anodic aluminum oxide (AAO) membrane that was formed on a 10 Ω · cm, 〈100〉 orientation, n-type silicon (Si) substrate, using the method previously described.13 CNTs thus formed are oriented perpendicularly to the Si substrate and are highly uniform in diameter, length, and spacing.13 As previously reported,16,17 through a carefully calibrated process the AAO pore bottom layers can be opened to the Si underneath, allowing CNTs to directly grow

10.1021/jp1087724  2010 American Chemical Society Published on Web 12/07/2010

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Figure 1. SEM image (a) and schematics (b) of the cross section of the device studied. The scale bar in the SEM image is 200 nm.

from Si. CNTs grown in this manner exhibit semiconductor behavior18,19 with a diameter dependent band gap and form a heterojunction with the Si substrate.16,17,20 After CNT growth, oxygen plasma cleaning was performed in order to remove a layer of amorphous carbon that was left covering the AAO and multiwalled CNTs on the sample by the CVD process. This cleaning step left the top of the CNTs open and exposed, which facilitated the infiltration of solution-phase PbS into the CNTs and subsequent deposition of metal contacts. A solution based method was used to grow a PbS film on top of the open CNTs.21 In brief, growth solution is composed of 10 mL of 0.57 M NaOH (Sigma-Aldrich), 1 mL of 0.175 M lead acetate (Sigma-Aldrich), 1 mL of 1 M thiourea (Sigma-Aldrich), and 25 mL of deionized water. The substrate is placed into the solution at room temperature, and the growth time is 30 min. During the PbS film formation, the solution-phase PbS infiltrated into the CNTs and crystallized into PbS nanowires filling the inside space of the CNTs. After PbS film growth, the samples were studied via scanning electron microscopy (SEM, LEO), energy dispersive X-ray spectroscopy (EDX, INCA-Oxford Instruments), and microRaman spectroscopy (Almega, Thermo-Nicolet). For the photocurrent measurements, a thin semitransparent gold metal contact was evaporated on the PbS side where the device was illuminated as shown in Figure 1b. Contact on the Si side of the sample was made by bonding the sample to a copper sheet with silver paste. The photocurrent spectra of the samples were measured in short-circuit conditions using a current-to-voltage amplifier connected to the external detector port of a Fourier transform infrared (FTIR) spectrometer (Equinox 55-Bruker Optics). FTIR photocurrent measurements were performed in step-scan mode with phase modulation, which ensured uniform modulation in the measured spectral range. Any additional effects due to the amplitude of the phase modulation were canceled out during the normalization procedure. Two sets of FTIR optics and light sources were used for the measurements in the near- or mid-infrared spectral ranges. The near-infrared spectra were measured using a quartz beam splitter and a 20 W tungsten lamp as illumination source, while the mid-infrared measurements were conducted with a KBr beam splitter and a Globar infrared source. Normalization was performed by measuring the spectra of the sources with a calibrated thermopile detector (Newport, model 818P-001-12) with spectrally flat responsivity up to 20 µm wavelength. The thermopile was connected to a homemade voltage preamplifier (voltage gain of 1012), and the amplified signal was fed into the external detector port of the FTIR spectrometer.

The electrical transport measurements were performed with a semiconductor parameter analyzer (HP 4145B), a source measure unit (Keithley 2602), and an impedance analyzer (HP 4192A). All electrical transport measurements were performed at room temperature and ambient conditions. Results Scanning Electron Microscope Characterization. The SEM image in Figure 1a shows the device structure. The details are labeled on the device schematics in Figure 1b. The thicknesses of the top PbS film and the AAO membrane are 200 and 880 nm, respectively. The PbS filling of the pores is complete as seen in Figure 1a and reaches the bottom of the membrane to contact the Si substrate. These PbS columns are ∼880 nm long and ∼50 nm in diameter. The grainy layer on top of the entire structure is the thin semitransparent gold contact. Energy Dispersive X-ray Spectra. Figure 2a shows the EDX spectra from the two end regions of the device. The accelerating voltage for the EDX measurements was 7 kV. Despite this low value for the accelerating voltage, the range of generated X-rays is quite large. For example, when the electron beam was directed to the Si region, about 200 nm away from the interface with the AAO, the signal from aluminum and oxygen was still detected, as shown in Figure 2a. This may depend on various parameters of the experiment and material properties. Similarly, by probing the PbS top film, oxygen and aluminum were detected, but the amounts of lead and gold are much higher. We note here that the most intense lines in the EDX spectrum from lead and sulfur are the MR1 at 2.346 keV and KR1 at 2.308 keV lines, respectively. We cannot resolve these two lines within the bandwidth of the recorded spectra. Therefore, we will further refer only to lead. In Figure 2b the electron probes are directed on the material inside of an AAO pore and on the wall of the AAO pore. Both spectra show the presence of aluminum, oxygen, and lead, which are expected considering the expected thicknesses of less than 100 nm for each of the regions. Careful comparison between the two spectra in Figure 2b shows that the relative amount of lead is higher in the material filling AAO pores. Within the resolution of our imaging system, this high lead content material appears continuous throughout the entire pores up to the Si wafer. This supports the prior finding from the SEM analysis that the CNTs are filled with PbS. Raman Scattering. EDX measurements are not suitable for the detection of CNTs, the other constituent of our device. Raman scattering was used in order to confirm the presence of CNTs, as shown in Figure 3. Spectra were collected at two

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Figure 2. (a) X-ray spectra obtained by electron beam excitation of the sample on the Si substrate site 2 (blue trace) and on the PbS film site 1 (red trace), as labeled on the SEM image. The scale bar is 200 nm. (b) X-ray spectra obtained by electron beam excitation on two different sites within the AAO region of the sample as labeled on the SEM image. The scale bar is 100 nm.

Figure 3. Raman spectra of the device structure excited by 785 nm laser light (black curve) and 532 nm laser light (red curve). The red curve was collected from a region where the top PbS layer was disrupted.

excitation wavelengths at 785 and 532 nm. Although the scattering efficiency of CNTs is larger at 532 nm, PbS deposited from solution shows photoluminescence in this spectral range, which obscures the Raman signal.22 On the other hand, excitation at 785 nm from the top surface of our device gives the spectral features expected for PbS at 456 and 968 cm-1.22-26 The band at 456 cm-1 is due to second order scattering, i.e., 2 LO.26 The band at 968 cm-1 is too energetic for the PbS and is ascribed by Zhang et al.26 to the presence of sulfates that are possibly generated during the illumination with the laser beam. The strong absorption in the top PbS layer in the visible range does not allow collection of the Raman signal from the underlying AAO layer. We were able to study this region of the device after disrupting the top PbS layer. Disruption of the top PbS layer was performed either mechanically or by using the maximum power of the excitation laser, which creates a crater in the top PbS film. The use of the laser has the advantage that

it ensures that only the top, highly absorbing layer is disrupted. In both cases, the Raman spectrum clearly shows the signature of CNT bands at 1346 and 1594 cm-1. The TO band of Si at 521 cm-1 is also present in the spectrum, which indicates that the AAO is transparent enough to transmit the green light from the excitation laser source, despite the filling of the AAO pores with low band gap PbS. It is remarkable that the PbS related band at 968 cm-1 is also present, suggesting once again that PbS infiltrated the CNTs. Optical Properties. The photocurrent is a complex phenomenon consisting of several processes that occur in sequence: optical absorption, charge carrier diffusion and separation, and collection at the electrode terminals. The very first step is absorption, which is why we have tried to estimate the optical properties of the materials as they are in the device structure. The top PbS layer is expected to give the main contribution to the optical absorption for photon energies above the PbS band gap, as this layer is illuminated first in our device structure. The geometry of our optical spectrophotometric studies was normal incidence and collection in retro-reflection. The collection is based mainly on specularly reflected light, and scattered light was not analyzed. Formally treated, this experiment is a reflection measurement. This measurement can also be regarded as double transmission through the PbS/AAO part of the device for the following reasons: i. The refractive index of Si is 3.4. The refractive index of the composite film of AAO, CNT, and PbS can be estimated using the Maxwell Garnett effective medium approximation. In this model, considering that the volume fraction of CNT is relatively much smaller, the contribution of the CNT is negligible. Using the bulk refractive index of PbS of 4 and the volume fraction of the AAO of ∼23%, the effective refractive index is ∼2. This value is significantly smaller than the refractive index of Si and is thus expected to lead to significant reflection from the Si/AAO interface. ii. The above-described reflectance measurements yielded spectra whose intensity continuously decreased toward high

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Figure 4. Absorbance spectrum of the device measured from the PbS side (line). Absorbance calculated using data from ref 27 (triangles).

Figure 5. (a) Normalized photocurrent spectrum measured at 300 K in ambient conditions. (b) Photocurrent spectra measured in vacuum in a cold-finger cryostat at 300 and 77 K.

energies. This is not expected for reflectance spectra but is typical for spectra where the dominant term is an exponent involving the absorption coefficient (as well as diffuse scattering), which is a common trait in transmission spectra. iii. The top PbS layer is only ∼200 nm thick. Using literature values for the absorption coefficient of bulk crystalline PbS,27 we calculated the absorbance spectrum of a film with effective thickness of 400 nm (twice the thickness because of the double transmission). Our data, interpreted as a double transmission measurement, are in relatively good agreement with literature values (Figure 4). There are two prominent differences between the two spectra in Figure 4. The slope of our experimental spectrum is smaller because of reflectance from the top surface. Fabry-Pe´rot interference effects take place, and their signatures are seen especially at photon energy less than 0.5 eV. As the experimental spectrum shows, the interference fringes are relatively narrow, and the difference between the maximum and minimum in the interference pattern is only ∼5%. This value is surprisingly small considering that the effective refractive index AAO with PbS filled channels is estimated to be ∼2. One possible explanation is that the refractive index of our PbS film is lower than that of the crystalline phase. Such a hypothesis is confirmed by the second difference between the two spectra in Figure 4: the sharp decrease in the absorbance of bulk crystalline PbS starts at 0.48 eV, whereas a similar feature in our experimental data occurs at ∼0.57 eV. Therefore, we expect our film to have wider band gap than bulk crystalline PbS. Typically, a higher refractive index is associated with a lower band gap. Photocurrent Spectra. The photocurrent measurements shown in Figure 5 span over 5 orders of magnitude. The measurements were performed with Fourier transform spectroscopy. Two different sets of optics and sources for the FTIR spectrometer were used resulting in different light spectra illuminating the sample and allowing us to achieve such a large dynamic range in the measurements. The first configuration used

Fernandes et al. a Globar as light source and a KBr beam splitter. The Globar spectrum was measured in the sample chamber by a calibrated, spectrally flat thermopile. The Globar spectrum peaks at 0.33 eV, where its intensity is at least 100 times larger than at 1.1 eV, the start of the band-edge absorption in Si. Such suppression of the photocurrent due to absorption in Si is beneficial for detecting the less intense absorption at smaller photon energies. The second configuration consisted of a halogen lamp as a light source and a quartz beam splitter. The lamp spectrum in the sample compartment peaks at 0.65 eV, and the intensity at 1.1 eV is about 3 times smaller. The photocurrent spectra due to the two optics/source configurations are combined at 1.05 eV to produce the curve shown in Figure 5a. The gradual decrease of the illumination intensity toward higher photon energies is another factor contributing to the high dynamic range of the final results presented in Figure 5. In order to verify the reliability of the presented results, we used long pass filters (Si and Ge double side polished wafers) in order to eliminate any possible effect due to folding of high intensity features at high photon energies to low photon energies. All features reported in Figure 5 have been reconfirmed with such controlled filter measurements. The photocurrent spectrum measured at room temperature peaks in the range of Si absorption. An additional photocurrent develops below 1.05 eV and extends down to 0.6 eV. Such photocurrent contribution was not observed in samples prepared in the same way but without the presence of PbS.28 Features in the same spectral region were observed previously for PbS films on Si.29 The room temperature spectrum contains relatively low photocurrent magnitudes below 0.6 eV until approximately 0.2 eV. This is the region where we previously observed photoresponse from a similar device structure without PbS.28 This photocurrent was attributed to photocurrent generation at the CNT/Si interface. It is not surprising, in light of our device structure, that the photoresponse due to the CNT/Si interface is small. The light absorption in the CNT competes with the absorption in the PbS, which is of larger volume. In addition, any photocurrent generated at the CNT/Si interface has to traverse an additional junction between the CNTs and PbS. The result presented in Figure 5 was measured at a modulation frequency of 11 Hz. Measurements were also performed at higher modulation frequencies (up to 732 Hz) in step-scan mode and showed similar spectra. The room temperature photocurrent spectra at frequencies higher than 45 Hz do not show measurable signal below 0.6 eV. It was previously reported28 that the photocurrent due to the CNT/Si is slow because of the large heterobarrier and decreases rapidly for frequencies above 20 Hz. Similarly, we may attribute the photocurrent below 0.6 eV reported here to the photocurrent generation in the CNT/Si part of the device. A more significant finding is perhaps from the photocurrent at liquid nitrogen temperatures, as shown in Figure 5b. The photocurrent at photon energies close to the expected band edge of PbS increased by almost 3 orders of magnitude. It also shows the unusual shift to lower energies upon cooling, a signature of the photoresponse of PbS that is infiltrated in the CNTs. The low temperature measurements were performed in a cryostat with CsI window, while the main photocurrent spectrum in Figure 5a was performed out of the cryostat in ambient conditions. The ordinates of Figure 5b are therefore not in the same scale as Figure 5a. For better comparison, the spectrum measured in the cryostat, in vacuum, and at room temperature is also shown in Figure 5b. Cooling leads to a significant increase of the photocurrent that finishes with a very steep

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Figure 6. Current-voltage (I-V) characteristics and energy band diagrams corresponding to the two possible heterojunctions in the device structure.

Figure 7. Voltage dependence of the photocurrent measured with direct illumination with an incandescent light bulb. The straight line is a guide to the eye.

decline of two and a half orders of magnitude at low energies (below ∼0.4 eV), where the absorption edge of PbS is expected to occur. Electrical Transport Measurements. The rectifying properties of the device are shown in the current-voltage (I-V) characteristics measured in the dark in Figure 6. The forward current corresponds to a positive voltage applied on the PbS side of the device. The polarity of the observed photocurrent is consistent with the I-V characteristics. The voltage dependence of the photocurrent (defined as the difference between the current under illumination and in the dark), shown in Figure 7, has an almost constant value at reverse bias and increases linearly with the bias toward positive values. The open circuit voltage is disproportionately small, 1.6 mV, considering the values of the photocurrent in the reverse direction in Figure 7. This extremely low value suggests the existence of competing junctions in the device (see Discussion section). For the present device, an incident integral optical power of 9 mW from the Globar produced a photocurrent of ∼10 nA, placing the responsivity (device area ∼2.5 mm × 2.5 mm) at ∼18 µA W-1 cm-2. Impedance spectroscopy was performed in the dark with external bias in the range of voltages used for the current-voltage characteristics. The transport resembles the one revealed by the dc current-voltage characteristics (Figure 6) up to at least 6 kHz. A significant phase shift between the current and voltage develops at higher frequencies carrying the signatures of a displacement current through capacitive structures. Typical results are shown in Figure 8 with break point frequency at 6630 Hz. These results can be modeled with an equivalent circuit of an equivalent resistance and equivalent capacitance connected in parallel. The existence of an additional equivalent resistance in series cannot be excluded, but the results in Figure 8 suggest

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Figure 8. Impedance spectroscopy results measured in the dark and at -0.5 V external bias. The red trace represents the magnitude of the admittance and the black trace the phase.

Figure 9. Capacitance vs voltage, as calculated from the impedance spectroscopy measurements in Figure 8.

that it is less than 100 Ω. Considering the device schematics in Figure 1, the series resistance corresponds to the resistances of the 300 µm Si wafer, 200 nm thick PbS film, gold electrode, and supply leads. It is expected that their combined contribution is much less than 100 Ω, in agreement with the experimental results. This allows us to calculate the equivalent capacitance and to follow its changes with the external bias, as shown in Figure 9. The capacitance does change with the external voltage, confirming the existence of a depletion region in the device. The equivalent parallel resistance also varies with the applied bias. It follows the same trend as the current-voltage characteristics in Figure 6. The forward part of the I-V curve follows a quadratic dependence for voltages higher than ∼1 V, which indicates a trap-free space charge limited current. The most probable place for such transport in our device is in the PbS, because it was grown from solution and at low temperatures. Although we cannot rule out the existence of traps in the PbS, the low band gap suggests that the trapped charges are likely to be reactivated very efficiently at room temperature. Discussion The observed filling of PbS into the CNTs may come as a surprise. As described above, an aqueous solution method was used for the PbS synthesis. The thorough filling of the CNTs with PbS suggests that our AAO template grown CNTs are not strongly hydrophobic, in agreement with prior observation by many groups. One possible factor contributing to the apparent hydrophilic character of the CNTs is the oxygen plasma treatment of the sample prior to the PbS infiltration. The oxygen plasma may help oxidize the interior CNT surface thus rendering the CNT interior more hydrophilic. Orikasa et al.30 reported that AAO grown CNTs with lengths smaller than ∼5 µm are soluble in water postgrowth without any surface treatment. Their CNT growth and processing scheme is very similar to ours.

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We note further that our solution grown PbS is probably not as dense as bulk crystalline PbS, as is evident from the apparent lower refractive index and the larger band gap, as discussed in the Results section. This is expected from solution-grown semiconductors, where the possible formation of voids between the crystallites that form the films and the incorporation of other byproducts from the growth reaction in the final structure may generate a material with an effective refractive index and band gap different from the bulk crystalline values. Films of amorphous PbS and PbS nanoparticles have been shown to exhibit larger apparent band gap than crystalline PbS.31,32 This is partly due to the relatively large exciton bohr radius of PbS (∼18 nm), which leads to quantum confinement effects in the smaller crystallites that form in such films. It is notable, however, that despite the seemingly imperfect crystalline structure of the PbS studied here, photocurrent and rectification are still observed in our device. As noted in the Results section, two paths for the photocurrent generation may be identified in our device structure, one across the PbS/Si heterojunction, which is responsible for strong photoresponse above 0.5 eV, and the other across the PbS/CNT/Si heterojunction, which gives the weak photoresponse contribution between 0.2 and 0.5 eV. The inset of Figure 6 shows the band diagrams for the PbS/ CNT/Si and PbS/Si heterojunctions drawn according to the Anderson model and the known work functions. The values for the work function and electron affinities used were estimated from published data and from our previous optical measurements of the band gap energy of PbS and CNTs.33-36 As previously noted,28 the charge separation process in the CNT/Si interface depends on three mechanisms: (1) direct tunneling, (2) phononassisted cascade tunneling through defect states, and (3) thermionic emission over the barrier. This model is supported by the energy band diagram, the observed temperature dependence of the CNT response, and the large time constant of the CNT photocurrent response band compared with that of PbS and Si. We note that the device studied here exhibited a decrease of the intensity of the CNT photoresponse band (0.2-0.5 eV) at lower temperatures and that the CNT band also disappeared at higher modulation frequencies in the room temperature measurements, consistent with the previously reported behavior of the CNT-Si heterojunction. We observe, however, that the sign of the generated photocurrent cannot be explained by the energy band diagrams in Figure 6, which predict the opposite polarity for the case of the PbS/Si or PbS/CNT interfaces. Similar contradictions in the polarity of the photocurrent in PbS heterojunctions were previously observed.37,38 One suggested explanation is that the interface between the gold (work function 5.1 eV) contact and the PbS film forms a Schottky barrier that is in series with the PbS/Si junction and with opposite polarity.38 This Schottky junction may dominate the electrical characteristics of our device and may determine the direction of rectification and the sign of the photogenerated voltage. The higher work function of gold relative to that of PbS determines its polarity to be reversed to the polarity of the PbS/Si junction. Considering the small band gap of PbS and the almost pointlike contact between Si and PbS, the reverse current for the PbS/Si junction is expected to be substantial. This allows the Au/PbS junction to dominate the electrical characteristics. As the data in Figure 6 suggest, the current limiting factor for forward external bias is not the PbS/Si junction but the bulk transport in the PbS. Therefore, the change in the conductivity of PbS upon illumination will also dominate the photocurrent in the forward direction, as

Fernandes et al. suggested by the results in Figure 7. The opposite polarity of the Au/PbS and PbS/Si junctions explains the very low open circuit voltage. At reverse external bias, however, the Au/PbS dominates the electrical transport and the typical almost constant photocurrent is observed in Figure 7. It is important to notice, however, that the Au/PbS Schottky junction alone cannot explain the photocurrent generation due to absorption in Si. Furthermore, the existence of the photocurrent characteristic for the silicon absorption in Figure 5a is evidence for the formation of a junction between silicon and PbS. The capacitance-voltage characteristics shown in Figure 9 allow us to derive the built-in potential at the junction, which is 0.8 V. As discussed above, our data suggest that the Au/PbS junction governs the electrical characteristics of the device. The value of 0.8 eV is slightly lower than that expected by considering only the work functions of Au and bulk PbS. There can be, however, additional factors that may lower this value and make the 0.8 eV value for the built-in potential within the expected range. The value of 0.8 eV is too large for the PbS/Si junction. In general, the built-in potential is expected to be very close to the open circuit voltage. Our results show that the builtin potential is much larger than the experimentally measured open circuit voltage of 1.6 mV, supporting further the hypothesis for the existence of competing junctions in our device. We believe that the charge separation in Si takes place at the interface with AAO/PbS/CNT because the photocurrent spectrum does not show the filtering effect from the absorption in the Si wafer that would result if charge separation were to take place at the Si/silver paste contact. Therefore, a possible Schottky junction between Si and the silver paste can be ruled out as the main mechanism for photocurrent response above 1.1 eV. Conclusion We have demonstrated a novel functional infrared photodetector diode device made of PbS filled multiwalled CNTs on Si. The PbS is grown in solution phase and shown to efficiently fill the CNTs as evidenced by our material characterization using SEM, EDX, and micro-Raman spectroscopy. The device structure studied here shows electrical rectification and photocurrent generation in the visible to mid-infrared with observed fast photoresponse at up to 732 Hz phase modulation frequency, which makes this device attractive for further development in infrared sensing and light energy harvesting platforms. The proof of concept device demonstrated here can be further optimized for performance by more careful removal of possible oxide barriers from the AAO pore bottoms and by perfecting the aqueous PbS film synthesis in order to improve PbS film crystallinity. Further investigation into the nature of the Au/ PbS contact may also be fruitful; other metals or alloys with work function lower than gold may be used for contact to the PbS in order to clarify the observed discrepancy between the energy band diagram and polarity of the photocurrent. The findings presented here open the possibility of engineering devices with multiple heterojunctions at small scales while leveraging on the unique properties of CNTs and their interplay with the filling materials. Acknowledgment. We thank ARO and DARPA for their support for the development of the nanotube array fabrication methods and processes, AFOSR for its support for the development of spectrally selective sensing strategies, and the WCU program of Korea at SNU for supporting the research of hybrid materials. This work was partially supported by NSF grant MRIDMR-0923047.

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