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Paper Thermoelectrics: Merging Nanotechnology with Naturally Abundant Fibrous Material Chengjun Sun, Amir Hossein Goharpey, Ayush Rai, Teng Zhang, and Dong-Kyun Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05843 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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ACS Applied Materials & Interfaces

Paper Thermoelectrics: Merging Nanotechnology with Naturally Abundant Fibrous Material Chengjun Sun†, Amir Hossein Goharpey†, Ayush Rai†, Teng Zhang†, Dong-Kyun Ko†,*. †

Department of Electrical and Computer Engineering, New Jersey Institute of Technology,

University Heights, Newark, NJ, 07102 KEYWORDS: colloidal quantum dots, paper electronics, thermoelectrics, energy harvesting, nanocomposites

ABSTRACT: The development of paper-based sensors, antennas, and energy harvesting devices can transform the way how electronic devices are manufactured and used. Herein we describe an approach to fabricate paper thermoelectric generators for the first time by directly impregnating naturally abundant cellulose materials with p- or n-type colloidal semiconductor quantum dots. We investigate Seebeck coefficients and electrical conductivities as a function of temperature between 300 and 400 K as well as in-plane thermal conductivities using Angstrom’s method. We further demonstrate equipment-free fabrication of flexible thermoelectric modules using p- and n-type paper strips. Leveraged by paper’s inherently low thermal conductivity and high flexibility, these paper modules have the potential to efficiently utilize heat available in natural

1 ACS Paragon Plus Environment


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and man-made environments by maximizing the thermal contact to heat sources of arbitrary geometry.

INTRODUCTION Breakthroughs in energy harvesting hold promise for "deploy-and-forget" wireless sensor networks1 and other distributed or portable power applications. In this regard, efficient energy harvesting from ubiquitous temperature gradients by means of silent, maintenance-free thermoelectric generators has been a long-standing goal. The efficiency of a thermoelectric device is governed by three interrelated materials parameters: Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (k). These parameters grouped together define the thermoelectric figure of merit ZT (= S2σT/k) which is used as a main criterion for determining the heat-to-electricity conversion efficiency. During the last two decades, large improvements in ZT have been demonstrated in nanostructured thermoelectrics by suppressing the thermal conductivity with great success.2-3 Recent efforts are devoted to enhancing the thermoelectric power factor (S2σ)4-5 and finding alternative materials with elemental abundance.6 Efficient heat recovery requires thermoelectric generators to have maximum thermal contact with heat sources that often have arbitrary geometries, such as pipes or human bodies. Thus, it is highly desirable to produce thermoelectric materials that are physically flexible. However, conventional bulk thermoelectrics are rigid, brittle, and are not designed to be installed conformally on curved surfaces presenting a major roadblock toward developing high performance energy harvesting modules. Furthermore, the preparation of bulk thermoelectric materials often involves energy-intensive processing which in turn leads to high cost of


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production. Herein, we demonstrate a facile approach to fabricate low-cost, flexible thermoelectric materials and modules based on cellulose papers and colloidal semiconductor quantum dots (QDs). Solution-synthesized QDs are a versatile class of nanomaterials that offer lowtemperature, large-area integration with the ease of materials processing. With diameters typically less than about 10 nm, QDs can be prepared with uniform size and shape which is hardly achievable using traditional ball-milling technique7 and the strong quantum confinement modifies the electronic density of states in QD solids that can potentially enhance the Seebeck coefficient.8-9 Bottom-up preparation of thermoelectrics through compaction of QDs have proven to be effective in scattering phonons at crystal interfaces.10-13 Solids made from two different materials of QDs14 or core-shell QDs,15 and polymer-nanocrystal hybrid composites16 are also particularly appealing approaches. Cellulose papers, on the other hand, are traditionally used for thermal insulation materials (k = 0.05 Wm-1K-1)3 and are flexible and abundant. Various paper-based devices including transistors,17 sensors,18 antennas,19 and batteries20 have routinely demonstrated high performance under mechanically strained conditions. Furthermore, the price is substantially lower than commonly used polyethylene terephthalate and polyimide flexible substrates.21 Paper being the Earth’s major biopolymer, the yearly production is about 100 million tons and its roll-to-roll processing technology exceed 100 km·h-1,22 making it one of the most widely available material. Motivated by these advantageous attributes, the present work investigates the fabrication and characterization of cellulose-QD nanocomposites for thermoelectric energy harvesting. We utilize solutions of p- or n-type QDs as “impregnating solutions” to incorporate QDs throughout



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the entire thickness of the paper via dip-casting. In contrast to previous studies that uses QD solutions23-24 or other inks25-26 to deposit uniform thin-films or printed patterns on smooth substrate surfaces, limitations arising from paper’s porosity and surface roughness do not apply to our approach, thus making the fabrication potentially amenable to high-yield, high-throughput processing. A complete suite of thermoelectric transport measurement was carried out and the fabrication of prototype paper thermoelectric modules is further demonstrated in this study.

EXPERIMENTAL SECTION Chemicals. Lead oxide (PbO, 99.999%), lead acetate trihydrate (Pb(Ac)2·3H2O, 99.999%), oleic

acid

(OA,

tech.

grade,

90%),

bismuth(III)

acetate

(Bi(Ac)3,

99.999%),

trioctylphosphine(TOP, tech. grade, >90%), 1-octadecene (ODE, 90%), squalane (99%), tellurium shot (99.999%), hexamethyldisilathiane (TMS, synthesis grade), potassium hydroxide (KOH pellets, >85%), ammonium iodide (NH4I, >99%), hexane (anhydrous, 95%), octane (anhydrous,

>99%), tetrachloroethylene (anhydrous, >99%), methanol (anhydrous, 99.8%),

methanol (ACS reagent, >99.8%) were purchased from Sigma Aldrich. Acetone (99.8%, extra dry) was purchased from Acros Organics. Synthesis of 8.1 nm PbS QDs. PbS QDs with the first absorption peak around 1850 nm were synthesized using a standard air-free Schlenk line technique.27 Briefly, 0.446 g PbO and 20 ml OA were added in a 50 mL three-neck flask. The mixture was stirred and dried at 110 °C under vacuum for 2 ~ 3 hours to form a clear solution. The flask was filled with N2, and the temperature of the solution was increased to 150 °C. Then 0.2 ml TMS (1 mmol) in 10 ml ODE was swiftly injected. The temperature of the solution dropped to ~ 120 °C immediately. The reaction was remained at 120 ± 6°C for 16 min and was quenched using a water bath. The QDs


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were dispersed in hexane, precipitated using acetone, and centrifuged inside N2 filled glovebox. The purification step was repeated twice and the QDs were re-dispersed and stored in hexane. Synthesis of 8.5 nm Bi-doped PbTe QDs. Prior to the synthesis, 0.75 M TOPTe stock solution was prepared inside the glovebox by dissolving 5 g Te shot in 50 ml TOP at 55 °C overnight. 2.856 g Pb(Ac)2·3H2O (7.5 mmol), 0.051 g Bi(Ac)3 (0.13 mmol), 50 ml ODE and 10 ml OA were added to a 250 mL three-neck flask. The mixture was stirred and dried at 90 °C under vacuum for 2 ~ 3 hours. The flask was filled with N2, and the temperature was increased to 140 °C. Then 10 ml of 0.75 M TOPTe solution was rapidly injected. The temperature of the solution dropped to ~ 130 °C immediately. The reaction was remained at 130 ± 6°C for 2 min and was quenched using a water bath. For the size-selective precipitation, the reaction mixture was transferred to a glovebox, dispersed in hexane, precipitated in acetone and centrifuged. The precipitate was dispersed in hexane, centrifuged, and the remaining precipitate was discarded. Acetone was added to the supernatant drop wise until the solution became translucent. The precipitate was collected by centrifugation. A small amount of acetone was next added to the supernatant and a new centrifugation was performed, yielding another fraction of supernatant and precipitate. The separation was repeated 8 times resulting in fractions of QDs with different sizes. Typically monodispersed QD samples were found in 4 or 5th fraction. The synthesis of 8.5 nm undoped PbTe QDs was carried out following the previous reported procedure.5 QD paper strip preparation. A filter paper (Whatman 40) was cut to size (2.5 × 1.0 cm for Seebeck coefficient and module testing, 1.5 x 1.5 cm for van der Pauw measurement, and 1 x 7 cm for thermal diffusivity determination) was dipped into a 50 mg ml-1 QD solution (hexane/octane 20:1 mixture). Then the ligand exchange procedure was carried out by dipping into 0.1M KOH in methanol solution for PbS QDs and 0.1M NH4I in methanol for Bi-doped



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PbTe QDs for 1 min. The sample strip was subsequently rinsed with copious amount of methanol to remove unbound residual ligands. The dip casting/ligand exchange process was repeated 15 times through layer-by-layer fashion. Characterization. Transmission electron microscope (TEM) images were obtained using JEOL 2100F operating at 200kV. Scanning electron microscopy (SEM) images were acquired using LEO 1530VP FESEM. Optical absorption spectra of QD solutions were obtained using StellarNet UV-Vis-NIR spectrometer (RW-NIRX-SR and BLK-CXR). Seebeck coefficients of QD paper strips were measured using MMR SB1000 Seebeck measurement system and K2000 digital temperature controller. Van der Pauw sheet resistivity was measured using MMR fourprobe chamber coupled with Agilent 4155A semiconductor parameter analyzer. In-plane thermal diffusivity was measured using a home-made system based on Angstrom’s method. All thermoelectric parameter characterizations were carried out under 1018 /cm3 of carriers. To fabricate p- and n-type paper strips, a filter paper was used as it provides the open matrix structure on submicron scale (Whatman 40) and does not contain any filler materials or surface coating. A 2.5 x 1 cm paper strip was simply dip-casted into a 50 mg mL-1 of QD solution prepared in hexane/octane (20:1). Similar to previous report on QD thin-films44-45 the as-casted QD-paper was electrically insulating due to the long ligands (oleic acid) that inhibit charge transport between dots. In order to facilitate electrical conductivity, a ligand exchange procedure that replaces the long native ligand to a shorter one was performed. For the preparation of p-and n-type strips, hydroxide (OH-) and iodide (I-) ligands, respectively, were employed in this study. They are known to provide stable protection from atmospheric oxygen which will p-dope lead chalcogenide QDs over time45-46 and induce long-term degradation.47 Furthermore, iodide is known to maintain stable n-type under air-exposure48 which is highly preferable for n-type paper thermoelectric studies. Notably, these atomic halide ligands are also non-toxic compared to the commonly used hydrazine44 or ethandithiol.45 Briefly, a p-type PbS QD paper strip was dipped into 0.1M of KOH in methanol (0.1M of NH4I in methanol for n-type Bi-doped PbTe) solution for 1 minute, followed by cleaning with copious amount of methanol to wash away unbound oleic acid ligands (Supporting Information S4). The QD dip-casting/ligand exchange steps were repeated 15 times. Plain filter papers contain randomly interconnected network of cellulose fibers and thus, are highly porous (60 ±


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2%, calculated from known cellulose density and geometrical volume) as confirmed by the surface and cross-sectional images obtained from scanning electron microscope (SEM), as shown in Figure 3a,c. Upon dip-casting, QDs start to permeate inside the pores and after 15 iterations, a smooth continuous surface (Figure 3b) with QD impregnated internal structure (Figure 3d) is formed. Notably, individual cellulose fibers that were distinctly resolved in Figure 3c are no longer observed in Figure 3d. The infiltration of QDs throughout the entire thickness of the paper is further confirmed by cross-sectional energy dispersive X-ray (EDX) line scan analysis (Figure 3c,d). After full QD impregnation, the density increases from 0.60 ± 0.03 g cm-3 (plain filter paper) to 2.06 ± 0.05 and 2.10 ± 0.05 g cm-3 for PbS and Bi-doped PbTe QD paper strips, respectively (see Supporting Information S5). If the pores inside the paper are completely filled with QDs, the densities would be 2.48 ± 0.15 for PbS QD paper strips and 2.66 ± 0.15 g/cm3 and Bi-doped PbTe QD paper strips. By comparing these values, the percentages of pore filling are calculated as 78 ± 12% and 73 ± 10% for PbS and Bi-doped PbTe QD paper strips, respectively (see Supporting Information S5). Open-circuit voltage (VOC) as a function of temperature difference (∆T) was studied to determine the majority carrier type. The paper strip fabricated from PbS QDs and treated with KOH showed positive VOC (Figure 4a) indicative of a p-type semiconductor. The slope of +438 µV K-1 represents the Seebeck coefficient of this sample. Figure 4b shows the plot obtained from the Bi-doped PbTe QD paper strip treated with NH4I. The negative open-circuit voltage and Seebeck coefficient of -136 µV K-1 indicate that the sample is n-type. We have also investigated paper strips prepared from undoped PbTe QDs treated with the same NH4I. The positive VOC (see Supporting Information S6) suggests that n-type behavior observed in Bi-doped PbTe samples primarily arises from bismuth doping and not from I- ligands.



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In practical applications, the figure of merit (ZT) of thermoelectric device varies with temperature. Thus, understanding the temperature dependence of thermoelectric parameters is important for obtaining the optimal temperature range of operation. Figure 4c shows the temperature-dependent Seebeck coefficient (S) of PbS and Bi-doped PbTe paper strips obtained from 300 to 400 K. Although the Bi-doped PbTe paper strip shows a weaker temperature dependence (increasing from |-152| µV K-1 at 310K to |-181| µV K-1 at 400 K), both PbS and Bidoped PbTe paper strips show increase in the magnitude of Seebeck coefficient with increasing temperature. Since typical heavily doped semiconductors show S ~T relation49 while nondegenerately doped semiconductors exhibit S ~1/T trend,50 these observation suggest that both KOH-treated PbS and NH4I-treated Bi-doped PbTe paper strips prepared in the present study have high level of carrier concentrations. Electrical conductivity (σ) versus temperature plot is shown in Figure 4d, which was obtained using 4-probe van der Pauw method. A 1.5 x 1.5 cm square paper sample was cut to size and Ohmic contacts were made using a silver paste (see Supporting Information S7). A small current was used to minimize the heat dissipation (< 5 mW) and the contact quality, sample uniformity, and reciprocity assessments were less than 5% in probe-to-probe variations. The electrical conductivities of both samples in the temperature range between 300 - 400 K were orders of magnitude lower than bulk PbS51 and PbTe.52 This is mainly due to the fact that about 40% of the volume in the paper sample is composed of non-conducting cellulose and the carriers hopping between semiconductor dots typically exhibit low mobility.53 Finally, combining electrical conductivity and Seebeck coefficient results, temperaturedependent power factors (S2σ) are obtained and are shown in Figure 5a. Both samples show a maximum at 400 K with PbS and Bi-doped PbTe paper strips reaching 0.1 and 0.05 µWcm-1 K-2 (10-4 Wm-1K-2), respectively. These results are comparable to the previously reported QD-based


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thermoelectrics including I-doped PbTe QDs31 and Ag2Te QDs treated with various ligands,33 but are 1 ~ 2 orders of magnitude lower than the bulk.51-52 The electrical conductivity and thus the power factor can potentially be improved by controlled doping54 of QDs and enhancing the carrier mobility.55-57 Cellulose has been traditionally used as a building thermal insulation material and is known to have thermal conductivity as low as 0.05 Wm-1 K-1. Moreover, films composed of colloidal QDs also exhibit low thermal conductivity, varying from 0.1 - 0.4 Wm-1 K-1, that depends largely on QD core/surface ligand volume fraction and type of ligands.58-59 In these respects, a QD paper strip, a composite of colloidal semiconductor QDs and cellulose fibers, is expected to have heat transport property beneficial for thermoelectric applications. The in-plane thermal diffusivity of paper strip samples was measured under vacuum using Angstrom’s method60 at room temperature (see Supporting Information S8). The measurement setup was calibrated using a glass slide as a reference sample. In a typical measurement, a 1 x 7 cm long paper strip was prepared following the identical dip-casting/ligand exchange procedure described previously. A sinusoidal heat wave is applied to on one end of the sample strip using a Peltier device. Two 10 KΩ thermistor beads (T1 and T2), which are capable of detecting small temperature variations due to their high sensitivities, were made in thermal contacts with the paper strip using a thermal paste separated by a known distance (∆x). The temperature as a function of time is then recorded. A typical plot obtained from PbS paper strip is shown in Figure 5b. From the time delay (∆t) between two waves and ∆x, the speed of the thermal wave propagating through the length direction of the paper strip was estimated to be 3.3 ± 0.3 × 10-4 m/s (2.8 ± 0.2 × 10-4 m/s for Bidoped PbTe paper strip), which hints to the fact that the heat transports in these samples are slow. Compared, high thermal conductivity brass (60% Cu) showed thermal wave propagating at



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the speed of 2.7 × 10-3 m/s, an order of magnitude faster than paper strip samples. Analysis of amplitude ratio, ∆t, and ∆x directly give the thermal diffusivity value without any parameter fittings. The thermal diffusivities of PbS and Bi-doped PbTe paper strips were 2.3 ± 0.3 × 10-7 and 1.7 ± 0.3 × 10-7 m2/s, respectively. Using specific heat capacities measured by differential scanning calorimetry (see Supporting Information S9: 0.49 ± 0.04 and 0.83 ± 0.01 J g-1 K-1 for PbS and Bi-doped PbTe paper strips, respectively) and density values obtained previously, the thermal conductivities (k) were calculated as 0.23 ± 0.06 Wm-1 K-1 for PbS and 0.30 ± 0.06 Wm-1 K-1 for Bi-doped PbTe paper strips. These k values are approximately an order of magnitude lower than the bulk counterparts. Ultimately, room temperature ZT300K is calculated to be 7 × 10-3 and 3 × 10-3 for PbS and Bi-doped PbTe paper strips, respectively. Theses thermoelectric transport investigations points out to a fact that about two orders or more improvements in ZT, especially from the electrical conductivity, will make paper thermoelectrics a viable option for power generation as an alternative to bulk thermoelectric materials. The availability of both p- and n-type paper strips enables us to further extend our study to construct a paper thermoelectric module. By simply cutting multiple p- and n-type papers to size and bonding them electrically together with a silver paste coated paper, flexible thermoelectric modules are fabricated without the need of complex patterning or fabrication process. Two separate p-type (PbS) and n-type (Bi-doped PbTe) strips showed positive and negative Seebeck voltage of 5.3 mV and -1.6 mV, respectively, under ∆T of 33 °C formed across the heat sink (metal plate) and hot plate (Figure 6a). A thermocouple was formed by connecting two strips electrically in series but thermally in parallel, as shown in Figure 6b. 6.7 mV of Seebeck voltage was generated which is in close agreement with the magnitude obtained by summing up two voltage components (5.3 + 1.6 mV). Total of 6 paper thermoelectric strips were used to form


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three thermocouples electrically connected in series (Figure 6c) generating three times the voltage (21.1 mV) produced by a single thermocouple. Alternatively, we have also investigated a vertically-stacked design for applications where using large lateral area may not be available. In this design, a thermocouple was composed of a p- and n-type strips sandwiching a blank insulating paper, as shown in Figure 6d inset. Although two ends were electrically connected through a silver paste, the rest of the portion was electrically isolated. Total of three thermocouples were stacked together. Under the same ∆T (33 °C) condition, the measured Seebeck voltage was 14.2 mV which is slightly less than the lateral design module (Figure 6c) since the thermoelectric strips in the upper stack were not in direct contact with the heat source. It is envisioned that a thicker stack can be used as a single thermoelectric leg in a traditional module design. A single leg that is composed of numerous thermocouples can generate a high voltage and all the electrical connections to each thermoelectric leg can be made at the bottom of the module. Furthermore, modules made from paper thermoelectric leg can potentially be more resilient to mechanical stresses. Bending tests conducted on our paper thermoelectric strips indicate minimal changes in the electrical resistance (< 9%) up to the bending radius r = 3 mm (Supporting Information S10), suggesting high operational stability of our cellulose-QD nanocomposites under mechanically bent conditions.

CONCLUSION In summary, we have fabricated both p- and n-type thermoelectric paper strips by directly impregnating colloidal semiconductor QDs into naturally abundant cellulose paper. We have further demonstrated equipment-free fabrication of flexible paper modules. Prototype paper modules introduced in this study can potentially enable a seamless integration of thermoelectric



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energy harvesting into everyday environment as they can be installed onto walls, window shades, pipes, and vents conformally with minimal invasiveness. Although ZT of both p-and n-type paper strips estimated from S, σ, and k measurements are low, this study points to a direction that further increase in the ZT can be achieved with efforts focused on electrical conductivity enhancement. Recent high carrier mobility reports in QD solids55-57 may serve as a guide to this improvement.


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Figure 1. Schematic illustration of monodisperse and polydisperse QD solids affecting (a) Seebeck coefficient and (b) carrier transport. Eavg, EF, D(E), f(E), and S denote average energy of carriers, Fermi energy level, density of state of lowest unoccupied conduction levels, FermiDirac distribution function, and Seebeck coefficient, respectively. Note that the centers of the D(E) distribution are the same for monodisperse and polydisperse (same average dot size). Green shaded area indicates the distribution of carriers estimated by multiplying D(E) and f(E).



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Figure 2. Transmission electron microscope (TEM) images of (a) 8.1 ± 0.4 nm p-type PbS and (b) 8.5 ± 0.3 nm n-type Bi-doped PbTe QDs. (c) and (d) show optical absorption spectra of PbS and Bi-doped PbTe QDs, respectively. An absorption spectrum obtained from undoped PbTe with the same average dot size is overlaid on (d).


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Figure 3. SEM images of a filter paper before ((a) and (c)), and after ((b) and (d)) QD dipcasting. (a) and (b) are surface images, (c) and (d) are cross-sectional images with EDX line scan analysis. After QD dip-casting, smooth surface (b) and QD impregnated internal structure (d) are formed.



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Figure 4. Open-circuit voltage versus temperature difference (∆T) obtained from (a) p-type PbS and (b) n-type Bi-doped PbTe QD paper strip at 300K. (c) and (d) show temperature-dependent Seebeck coefficient and electrical conductivity, respectively, of PbS (blue dot) and Bi-doped PbTe (red diamond) samples. Small temperature difference was applied (∆T = 2K) in order not to perturb the overall sample temperature.


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Figure 5. (a) Thermoelectric power factor plotted as a function of temperature for PbS (blue dot) and Bi-doped PbTe (red diamond). (b) shows a typical temperature versus time plot obtained from two thermistors (T1 and T2) placed on a PbS QD paper strip. Dotted lines represent sinusoidal fitting.



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Figure 6. Thermoelectric module constructed from n- and p-type paper thermoelectric. (a) shows the Seebeck voltage of 5.3 mV and -1.6 mV generated from separate p-type and n-type strip, respectively, using 33 °C temperature difference. (b) shows the Seebeck voltage of 6.7 mV (≈ 5.3 + 1.6 mV) measured from the single thermocouple. (c) shows a module constructed from 3 thermocouple pairs generating 21.1 mV and (d) shows vertically-stacked 3-thermocouple module generating 14.2 mV.


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ASSOCIATED CONTENT Supporting Information Size distribution analysis, elemental composition analysis, XRD characterization, FTIR characterization, calculation of QD-cellulose nanocomposite density, open-circuit measurement of undoped PbTe QDs treated with NH4I, van der Pauw resistivity measurements, thermal diffusivity measurement using Angstrom’s method, measurement of specific heat capacity, and resistance change under bending configuration. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Dong-Kyun Ko. Address: Department of Electrical and Computer Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ, 07102. Email: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources

ACKNOWLEDGMENT The authors wish to thank Dr. D. Su for assistance with TEM. This work was supported by NJIT start-up grant, NJIT faculty seed grant, and Otto York center FIUSG grant. This research



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used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

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