Charge Transport Modulation in PbSe Nanocrystal Solids by AuxAg1

Aug 27, 2018 - Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge , Tennessee ... *E-mail: [email protected]...
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Charge Transport Modulation in PbSe Nanocrystal Solids by AuxAg1−x Nanoparticle Doping ACS Nano 2018.12:9091-9100. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/26/18. For personal use only.

Haoran Yang,† Eric Wong,‡ Tianshuo Zhao,§ Jennifer D. Lee,† Huolin L. Xin,∥ Miaofang Chi,⊥ Blaise Fleury,† Han-Yu Tang,§ E. Ashley Gaulding,§ Cherie R. Kagan,†,§,# and Christopher B. Murray*,†,§ †

Department of Chemistry, ‡Department of Physics, §Department of Materials Science and Engineering, and #Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States S Supporting Information *

ABSTRACT: Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as “artificial atoms” in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables “designer” materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of AuxAg1−x alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transport. These studies point to charge carrier injection from the metal NPs into the PbSe NC matrix. The charge carrier density and charge transport dynamics in the doped NC solids are adjustable in a wide range by employing the AuxAg1−x NP with different Au:Ag ratio as dopants. This doping strategy could be of great interest for thermoelectric applications taking advantage of the energy filtering effect introduced by the metal NPs. KEYWORDS: nanocrystal solids, doping, lead selenide nanocrystals, gold silver alloy nanoparticles, charge transport, thermoelectric

C

Constructing multicomponent systems from dissimilar building blocks38,39 has also emerged as a viable route to tailor the electronic properties of NC solids. Incorporation of impurity NCs randomly distributed in a semiconductor NC solid will introduce electronic states that differ from those of the host NCs, modifying the properties of the solid as a whole. This substitutional NC doping resembles atomic doping, where atomic impurities create additional energy states that alter the electronic properties of the semiconductors. Early reports have described p-type doping of ordered or glassy PbTe NC solids by incorporation of Ag2Te NCs.40,41 More recently, Cargnello et al. showed random substitutional doping of ordered NC

olloidal semiconductor nanocrystals (NCs) are attractive materials owing to their size-dependent optical and electronic properties.1,2 In analogy with atomic systems, NCs exhibit discretized electronic energy levels and are sometimes regarded as “artificial atoms”.3−7 If multiple NCs are brought in sufficiently close proximity, their electronic wave functions will start to overlap as the interparticle distance decreases.8,9 Consequently, conductive pathways are formed to allow more efficient charge transport.6 This parallels the construction of solids from atoms. Semiconductor NC solids are being explored for a range of devices including photovoltaics,10−12 photodetectors,13,14 lightemitting diodes,15 field-effect transistors,16−21 and thermoelectric devices.22,23 Engineering the electronic properties of NC solids for different functionalities can be achieved through ligand exchange or stripping,24−31 surface doping/passivation,23,32−36 and through control of NC stoichiometry.19,20,37 © 2018 American Chemical Society

Received: April 25, 2018 Accepted: August 27, 2018 Published: August 27, 2018 9091

DOI: 10.1021/acsnano.8b03112 ACS Nano 2018, 12, 9091−9100

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metallic dopants and to construct the semiconductor matrix, respectively. The sizes of these building blocks are matched to minimize phase segregation of the metal NPs and PbSe NCs upon assembly.42,49 In addition, size matching allows us to study the doping effect as a function of only the composition of the dopant NPs, as all the dopant NPs are nearly identical in morphology. Transmission electron microscopy (TEM) images and small-angle X-ray scattering (SAXS) exemplify the size uniformity of all the NCs (Figure 2a,b). Fitting of the SAXS patterns provides a measure of the average 6 nm diameter size (5.54 to 6.48 nm) and the size distributions ranging from 6% to 12% for all the NCs (see Supporting Information Table S1). The compositions of the NCs are quantified using energy dispersive X-ray spectroscopy (EDS), and the results are summarized in Supporting Information Table S2. For the metal alloy NPs, the measured composition is relatively close to the nominal composition based on the relative concentration of precursors in the synthesis. As such, we will use the nominal composition (Au4Ag1, Au3Ag2, and Au2Ag3) to denote the AuxAg1−x NPs hereafter. To confirm the nature of alloying in the AuxAg1−x NPs, we performed detailed elemental mapping, using Au3Ag2 NPs as a representative sample (Figure 2c). We observe relatively uniform mixing of Au and Ag elements throughout each NP, although the surface is slightly Ag-rich. The reason for the Ag-rich surface is the seeded growth method used for the synthesis of AuxAg1−x NPs, in which Ag diffuses into Au NP seeds (refer to the Methods section for the details). UV−vis extinction spectra of AuxAg1−x NP dispersions (Figure 2d) exhibit extinction peaks between the 413 and 519 nm localized surface plasmon resonances characteristic of Ag and Au NPs, respectively. The extinction peak redshifts as the percentage of Au in the AuxAg1−x NPs is increased, consistent with previous reports for Au−Ag alloy NPs.50,51 The UV−vis spectrum of the PbSe NC dispersion (Figure 2d) shows the first excitation peak at 1774 nm, which leads to an estimated optical band gap of 0.59 eV, consistent with their 6 nm diameter size. Figure 2e displays cyclic voltammograms (CV) of spincoated films of the metal alloy NPs and PbSe NCs after thiocyanate (SCN) ligand exchange. CV measurements have been employed in previous literature to estimate energy of the lowest unoccupied molecular orbital (LUMO) level and the highest occupied molecular orbital (HOMO) level of semiconductor NCs.10,52,53 Likewise, the positions of the onsets of redox peaks of metal NPs provide a measure of the Fermi level relative to that of the reference electrodes. The HOMO/ LUMO energy of the semiconductor NCs and the Fermi energy of the metal NPs allow us, in principle, to estimate the energy “band” alignments between the two components. The reduction peak onset of PbSe NCs at −0.12 V relative to the ferrocenium/ferrocene couple indicates the LUMO level of PbSe NCs at 4.68 eV below the vacuum level. As the optical band gap is 0.59 eV, we calculate the HOMO level of PbSe NCs to be at 5.27 eV below the vacuum level, corresponding to the oxidation peak in the plot. The onset of the redox peak pair of Ag NPs centers around −0.15 V, while that of Au NPs occurs at a more positive voltage of 0.7 V, which leads to estimated work functions of 4.65 and 5.5 eV for Ag and Au NPs, respectively. For the alloy NPs, the CV results include the redox features of both Ag and Au, which does not allow us to estimate their work functions. However, it is reasonable to assume the work functions are bounded in between 4.65 and 5.5 eV based on previous reports for bulk Au−Ag alloys.47

superlattices with metal nanoparticles (NPs), and subsequent modification of carrier transport as percolation pathways developed with increasing concentration of metal NPs.42 The strategy of mixing different NCs to tailor the optoelectronic properties has also been applied in the design of NC-based solar cells by Rath et al.43 They reported increased carrier lifetime and enhanced device performance when a layer of mixed Bi2S3/PbS NCs is sandwiched in between a layer of neat Bi2S3 NCs and a layer of neat PbS NCs due to suppressed recombination in the NC mixture layer. While atomic doping is limited to individual elements, a greater degree of freedom is offered in doping NC solids. By independently engineering the size, shape, composition, and organization of both dopant and host NCs, a wide range of “designer” materials with tunable electronic properties can be realized. Here, we study the doping of PbSe NC solids with AuxAg1−x alloy NPs. Metal NP doped PbSe NC solids are of particular interest for thermoelectric applications, as the incorporation of metal NPs in a thermoelectric matrix could create energy barriers for electron/hole transport and enhance the Seebeck coefficient by blocking low-energy electrons/ holes, a desired effect known as energy filtering.44,45 However, energy barriers also reduce electrical conductivity, which is harmful for thermoelectric performance. Therefore, the energy barrier height must be carefully engineered to gain a net benefit in the thermoelectric figure of merit (ZT).45 Here, we choose AuxAg1−x alloy NPs as the dopants since they exhibit composition-dependent work functions between 4.2 and 5.1 eV, characteristic of the end points for Ag and Au, respectively.46,47 The tunable work function of the AuxAg1−x alloy NPs provides a feasible route to modulate the charge carrier density and the energy barrier heights within the NC solids through doping. We synthesized a series of AuxAg1−x (x = 0−1) NPs with five different compositions, but nearly identical sizes of 6 nm. The AuxAg1−x NPs are mixed with ca. 6 nm PbSe NCs and subsequently spin-cast onto substrates to form metal NP doped NC solids, as schematically shown in Figure 1. Structural characterization confirms the random

Figure 1. Schematic of metal NP doping of semiconductor NC solids.

distribution of the metal NP dopants in the PbSe NC matrix. Temperature-dependent measurements of the Seebeck coefficient and electrical conductance are performed to probe the electronic properties of the doped NC solids.40,48 We also employ room-temperature Hall effect measurements to confirm the carrier type and to extract carrier concentration. These measurements demonstrate the fine-tuning of the electronic properties of NC solids by engineering the composition of metal alloy NP dopants.

RESULTS AND DISCUSSION Figure 2 summarizes the characterization of the AuxAg1−x alloy NP and PbSe NC building blocks we employ to serve as 9092

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Figure 2. Characterization of the AuxAg1−x NPs and PbSe NCs building blocks via (a) TEM, (b) small-angle X-ray scattering, (c) STEM-EDS elemental mapping, (d) UV−vis extinction spectra, and (e) cyclic voltammetry measurements.

Figure 3. (a) A TEM image and (b) a HRTEM image of a drop cast PbSe-Au monolayer. (c) FFT of a PbSe NC in (b). (d) FFT of a Au NP in (b). (e) HAADF-STEM of a 7% Au3Ag2 NP doped PbSe NC solid, in which the brighter metal dopants are marked. (f) Low- and (g) highresolution SEM images of spin-cast NC films after SCN ligand exchange. (h) FFT of (f).

representative TEM and high-resolution TEM (HRTEM) images are shown in Figure 3a,b (TEM images on larger scales are provided in Figure S1). Due to the higher electron density of the Au NPs relative to that of the PbSe NCs, the Au NPs appear darker in the TEM images, as shown in Figure 3a. The HRTEM image shown in Figure 3b and the fast Fourier transform (FFT) of the lighter and the darker particles shown

Nevertheless, CV measurements on the AuxAg1−x NPs provide further evidence of their alloy nature since their redox peak onsets shift relative to that of pure Au or Ag as the Au:Ag ratio changes.54 To confirm the uniform mixing of the metal NPs and the PbSe NCs, a diluted dispersion of Au NPs and PbSe NCs (ca. 15 vol % of Au) was drop cast onto TEM grids, and 9093

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“undoped” refers in particular to NC doping unless otherwise stated, which is not to be confused with atomic doping. The Seebeck effect is a phenomenon in which a temperature difference ΔT applied across a conductor drives the migration of charge carriers and generates a voltage difference ΔV, and the Seebeck coefficient S is defined as −ΔV/ΔT. Positive (or negative) S indicates the majority carriers are holes (or electrons), and the temperature dependence of S contains information on the electronic band structures of the materials.57−59 Previously, temperature-dependent Seebeck coefficient measurements have been used to probe the electronic properties of semiconductor NC solids based on the theory developed by H. Fritzsche.40,48,60 For a nondegenerate semiconductor, where carrier transport at the transport energy level ET operates via either band or hopping transport, a general expression for Seebeck coefficient can be written as

in Figure 3c,d provide further evidence to discriminate the two materials. The darker particles exhibit a pentatwinned structure and a lattice constant of 0.24 nm, which is assigned to facecentered cubic (fcc) Au (111) planes. Meanwhile, the lighter particles show a cubic structure with a lattice constant of 0.31 nm, which corresponds to the rock salt (200) planes of PbSe. It is seen that the two types of particles are well mixed and there is no phase segregation observed. To further characterize the distribution of the metal NP dopants in multilayer doped PbSe NC films, we performed high-angle annular dark-field imaging in a scanning transmission electron microscope (HAADF-STEM) on one representative sample (PbSeAu3Ag2). As shown in Figure 3e, the brighter spots of 13.6 ± 6.3 × 7.0 ± 2.6 nm, attributed to Au3Ag2 NPs or their aggregates, are distributed in a matrix of PbSe NCs. The insignificant aggregation of the metal NPs is possibly due to phase segregation upon spin coating, as it has been shown that a small size mismatch between the two particles could lead to this behavior.42 Nevertheless, as the electrical measurements in the latter study are performed on millimeter-scale samples to obtain the macroscopic properties of the NC solids, the samples can still be described as homogeneously doped. In addition, the work function of metal NPs shows weak size dependence at sizes larger than 6 nm (e.g., a difference of 50 meV in work function for Ag NPs of 6 and 14 nm is estimated according to Wood).55 Therefore, the slight metal NP aggregation is not expected to significantly affect the band alignment between the metal NPs and the PbSe matrix. The AuxAg1−x NP doped PbSe NC films for electrical measurements were fabricated through spin coating the dispersions of mixed AuxAg1−x NPs and PbSe NCs in octane on cover glass or sapphire substrates under similar conditions. After spin-coating, the NC films were treated with a solution of ammonium thiocyanate in methanol to exchange the organic ligand for SCN−. The ligand exchange is very effective for both the metal NPs and the PbSe NCs, as demonstrated by the diminishing C−H stretching peaks at 2850 cm−1 to 2950 cm−1 in the FTIR spectra (Figure S2a−c) and the shrinkage of interparticle distance under TEM (Figure S2d,e). The highmagnification SEM images (Figure 3f,g) of the SCN− ligandexchanged film clearly show individual NCs and no fusing of the NCs is observed. The ligand exchange leads to the development of tiny cracks of sizes about 40 ± 15 nm × 9 ± 2 nm. However, the NC films appear continuous and uniform on the micrometer scale, as shown in the low-magnification SEM images in Figure S3. The FFT of Figure 3f shows no distinct pattern, indicating the NC films are disordered, which is expected as the fast evaporation of the solvent during spin coating prevents the formation of highly ordered superlattices, and the subsequent ligand exchange may further disturb the ordering. To study the modulation of the electronic properties of the NC solids through dopant NP engineering, we prepared a series of AuxAg1−x NPs doped PbSe NC thin films. The volume percentage of the NPs dopant is kept at ca. 7% for all doped samples, which was estimated through inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (refer to Table S3). This dopant concentration is high enough to show an observable doping effect, but is far below the percolation threshold (19.5% for hexagonal close-packed arrangement and 31% for randomly packed spheres, respectively)56 of the metal NPs. As a control, the undoped PbSe NC solids were measured as well. Note the term

S=

y k ij ES jj + C zzz e k kT {

(1)

where k, e, and ES stand for Boltzmann constant, the elementary charge, and the energy difference between the Fermi level EF and the transport energy level ET (ES = EF − ET), respectively. C is a constant independent of temperature that describes the carrier distribution within the conduction (for n-type semiconductor) or valence (for p-type semiconductor) band. Charge transport in NC solids can be tuned from carrier hopping through localized quantum-confined states to bandlike transport in delocalized and hybridized states.6 In this study, we found the NC solids exhibit relatively low Hall mobilities in the range of 0.7−2.2 cm2/(V s) and an Arrhenius type temperature dependence of the electrical conductance (refer to the sections below) and are therefore better described by hopping transport. Assuming thermally activated carriers to the transport energy level dominate the charge transport, the conductance G can be written as G=

A A σ = ·eNTe EF − ET / kT ·μ0 e Eμ / kT = G0e EF − ET + Eμ / kT l l

= G0e Eσ / kT

(2)

where A is the area of the cross section of the sample, l is the length of the sample, σ is conductivity, NT is the density of states at the transport energy, μ0 is a mobility prefactor, Eμ is an energy that describes the activation energy of the carrier mobility, which is the additional energy needed to overcome the highest energy barrier in the fluctuating energy landscape, and G0 is an aggregated conductance prefactor, which is a constant for a sample of fixed sizes. The aggregate energy Eσ thus contains the activation energy for exciting carriers from Fermi level to the transport level (EF − ET) as well as the additional activation energy for hopping (Eμ) through a fluctuating energetic landscape. These energies are represented schematically in Figure 4. This equation has also been previously applied to analyze the charge transport of PbTe and Ag2Te NC solids in similar temperature ranges.40 By combining information on Eσ from temperature-dependent conductance (G) measurements and ES from Seebeck coefficient measurements, we can get a picture of the energetic landscape of the states responsible for transport in NC solids. For the undoped PbSe NC solids, S is positive and increases as temperature decreases (Figure 5a). From the slope of the S 9094

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activation energy Eσ and the disordering parameter Eμ (Eμ = Eσ − ES). For undoped PbSe NC solids, Eσ and Eμ are 90 ± 7 meV and 65 ± 9 meV, respectively. This agrees well with the energy fluctuation of the HOMO levels estimated through the full width at half-maximum (fwhm = 70 meV) of the first excitonic absorption peak of PbSe NCs in Figure 2d. The metal NP doping is expected to affect both ES and Eμ of the PbSe NC solids, depending on the alignments between the Fermi levels of the metal NPs and the PbSe NC hosts. Based on the electrochemical analysis, the estimated band alignments between the dopant and the host are plotted in Figure 4. As the AuxAg1−x NPs are incorporated into the PbSe NC solids, there might be carrier injection from the dopant NPs into the adjacent PbSe NCs, which alters the overall carrier density of the NC solids and tunes the position of the Fermi level, as given by the parameter ES. Meanwhile, the metal NPs might also perturb the energetic landscape of the NC solids and modify Eμ. Figure 5 compares the measured Seebeck coefficient (S) and electrical conductance (G) of the doped PbSe NC solids with the undoped samples in temperature range between 200 and 300 K. Note that the absolute values of electrical conductance of are greatly affected by the differences in thickness and local defects from sample to sample (see Methods section for more details), so that they should not be compared directly across samples. We will focus on the temperature dependence of the conductance data here and will discuss the resistivity of the samples in the later section. As shown in Figure 5a, all the doped samples exhibit positive S, which suggest the majority carriers are still holes after metal NP doping. However, doping leads to a change of the absolute value of S, and a change in the slope of S with temperature, which is dependent on the type of the metal NPs. Ag NP doping leads to a significant increase in Seebeck coefficients, and ES also increases to 232 ± 29 meV, indicating that the Fermi level moves away from the HOMO levels and moves toward midgap, that is, the Ag NP compensates the holes of the native PbSe NC matrix. On the contrary, the Au NPs doping only leads to a small change in S compared with the undoped sample. ES drops to 9 ± 6 meV, which might suggest a small p-type doping effect of the Au NPs to the PbSe NC host. To verify the p-type doping effect of Au NPs, samples of higher Au volume fractions were made and their Seebeck coefficients were measured, as shown in Figure S4. The samples of 8.0% and 17.1% Au dopant volume fractions (still below percolation threshold) exhibit increasing positive Seebeck coefficient with increasing temperature, which is a characteristic of a degenerately p-type doped semiconductor, confirming the p-type doping effect of Au NPs to the PbSe NC host. Likewise, the doping-concentration-dependent study was performed for Ag doping, as shown in Figure S5. When Ag volume fractions are increased to more than 10.8%, the Seebeck coefficient became negative, indicating a p-type to ntype transition, which further proves the n-type doping effect of Ag NPs. The different doping effect of Au NPs and Ag NPs may be understood by considering the band alignments between the metal NPs and the PbSe NCs. As shown in Figure 4, the Fermi levels of the Ag NPs are higher than those of the PbSe NCs. Consequently, when the PbSe NC solids are doped with Ag NPs, electrons are injected from the Ag NPs into the adjacent PbSe NCs (n-type doping). Conversely, the Fermi levels of the Au NPs are slightly lower than that of the PbSe NCs, and in PbSe-Au NC solids, electrons are extracted from the PbSe NCs

Figure 4. Schematic of the energy landscape for PbSe NC solids and the band alignments between Ag or Au NPs and the PbSe NC matrix. The x- and y-axes represent space and energy, respectively. Note the positions of the energy levels are not drawn to scale for better visual effect.

Figure 5. Temperature-dependent Seebeck coefficient (a) and electrical conductance (b) of the doped and undoped PbSe NC solids.

versus 1/T curve, we estimate ES to be 25 ± 6 meV. Since ES = EF − ET, this indicates that the Fermi level (EF) of the undoped PbSe NC solid is very close to the HOMO levels (ET), that is, the material is heavily p-type doped (atomic doping). The origin of the p-type conduction might come from the lead deficiency induced by the SCN ligand exchange, as the Pb:Se ratio of the SCN treated PbSe NCs obtained from EDS analysis is 0.95 ± 0.02. Meanwhile, by applying a linear fit to the plot of ln(G) vs T−1 (Figure 5b), we can extract the 9095

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Table 1. ES, Eσ, and Eμ of the Doped and Undoped PbSe NC Solids Estimated from the Temperature-Dependent Seebeck Coefficient and Conductance Measurements dopant

none

Ag

Au2Ag3

Au3Ag2

Au4Ag1

Au

ES (meV) Eσ (meV) Eμ (meV)

25 ± 6 90 ± 7 65 ± 9

232 ± 29 79 ± 3 NA

98 ± 14 178 ± 8 80 ± 16

63 ± 12 177 ± 6 114 ± 13

29 ± 7 70 ± 18 41 ± 19

9±6 90 ± 4 81 ± 7

panied by a two-order-of-magnitude change in the hole density (from (1.1 ± 0.1) × 1018 cm−3 to (2.1 ± 0.6) × 1016 cm−3). Ag NP doping results in a substantial reduction of hole density, consistent with the large increase in S and ES. In contrast, Au NP doping has a much smaller effect on the hole density of the NC solids, in agreement to the smaller change in S and ES. A slightly lower hole concentration is estimated for the Au NP doped sample compared to that of the undoped sample, which is not expected based on the Seebeck coefficient measurements. We attribute this to the additional charge carrier scattering effect in the metal NP doped sample, which is not taken into account for in the Hall carrier concentration calculation. Note that the measured Hall mobility falls in the range of 0.7 to 2.2 cm2/(V s), which is generally regarded as a transition regime from hopping transport to band-like transport.6 This is consistent with the aforementioned analysis on the conductance based on the thermally activated hopping. In this regime, a suppressed or improper Hall effect has often been observed, which leads to a discrepancy of the measured carrier density and mobility compared to the true values.17,65 Although the apparent carrier density data show the expected trend for the AuxAg1−x NP dopants, the true carrier density and mobility of the NC solids remain to be elucidated.

to the Au NPs, leaving more holes in the PbSe (p-type doping). For the AuAg alloy NP dopants, we find the doping effect is within the two extremes set by Au and Ag, respectively. As the Ag percentage in the dopant NPs increases, the n-type doping effect becomes stronger and stronger, indicated by an increasing ES. It is worth noting that when comparing the Au4Ag1 doped sample with the undoped sample, we find that ES is very similar, yet the absolute Seebeck coefficient of the Au4Ag1 doped sample is ca. 20% higher than that of the undoped sample at all temperatures, which is possibly due to the energy filtering effect.60−64 The activation energies Eσ and disordering parameters Eμ of the doped samples extracted from temperature-dependent G measurements (Figure 5b) are summarized in Table 1. The smallest Eμ (41 ± 19 meV) is observed in Au4Ag1, which is consistent with the energetic band alignment shown in Figure 4. By contrast, larger disorder parameters are observed in Au2Ag3, Au3Ag2, and Au NPs doped samples (80 ± 16, 114 ± 13, and 81 ± 7 meV, respectively), suggesting the creation of energy barriers within the NC solids. Finally, we note that the Ag NP doped samples exhibit a small Eσ of 79 ± 3 meV and a large ES of 232 ± 29 meV. We believe this sample is most likely dominated by a different charge transport mechanism, such as variable range hopping through the deep defect levels, since the thermal energy (3kT = 76 meV for T = 300 K) is much smaller than ES. Therefore, the analysis based on eq 2 is no longer applicable to the Ag NP doped samples. Finally, room-temperature Hall effect measurements were employed to obtain the information on the carrier type, the effective carrier density, and mobility, and the results are summarized in Figure 6. The undoped PbSe NC solids show the lowest resistivity (1.5 ± 0.3 Ω cm), the highest hole density ((1.9 ± 0.6) × 1018 cm−3), and the highest hole mobility (2.2 ± 0.3 cm2/(V s)). Through AuxAg1−x doping, the resistivity of the NC solids is tunable by 2 orders of magnitude (from 4.0 ± 0.3 Ω cm to (3.9 ± 0.8) × 102 Ω cm) through composition engineering of the AuxAg1−x NP dopants, which is accom-

CONCLUSIONS In conclusion, we have studied the doping of PbSe NC solids with a series of AuxAg1−x NPs. Doping of NC solids is shown to be strongly analogous to atomic doping. Much like the atomic case, NC doping allows for the modification of both carrier densities and densities of states, enabling a broad range of tailoring of electronic transport properties. Unlike atomic doping, doping at the NC level offers much more complexity to allow more precise control of the electronic properties. We have demonstrated that by employing AuxAg1−x alloy NPs as the dopant and fine-tuning the Au:Ag ratio, we were able to fine-tune both the carrier density and the charge transport dynamics of the doped PbSe NC solids, which cannot be achieved through pure Au or pure Ag NPs doping. This could be valuable for developing the next-generation thermoelectric materials based on the energy filtering effect, where both carrier density and transport dynamics must be well controlled to enhance ZT. Moreover, this NC doping strategy can be utilized in parallel with the other approaches to controlling the electrical properties of the NC solids, such as the atomic doping and surface modification of the constituent NCs, which provides a wider range of tunability to the NCs based electronic materials and devices. METHODS Chemicals. Hydrogen tetrachloroaurate (III) hydrate (HAuCl4· 3H2O, ≥49% Au), oleylamine (OLAM) (C18 content: 80−90%), 1,2,3,4-tetrahydronaphthalene (tetralin, 98+%), anhydrous hexane, anhydrous toluene, anhydrous methanol, and anhydrous isopropanol were purchased from Acros Organics. tert-Butylamine-borane complex (TBAB, 97%), lead oxide (PbO, 99.999%), silver acetate (99.0%),

Figure 6. Resistivity, mobility, and hole density of the doped and undoped PbSe NC solids at room temperature determined from Hall effect measurements. 9096

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ACS Nano trioctylamine (98%), oleylamine (70%), 1-octadecene (ODE), OLAM (70%), oleic acid (OLAC, 90%), (3-mercaptopropyl)trimethoxysilane (MPTS, 95%), diphenylphosphine (DPP, 98%), tri-n-octylphosphine (TOP, 90%), selenium pellets (Se, 99.999%), ammonium thiocyanate (99+%), and anhydrous octane were purchased from Sigma-Aldrich. Tetradecylphosphonic acid (>99%) was purchased from PCI synthesis. The ammonium thiocyanate was recrystallized from methanol before use. The rest of the chemicals were used as received without further purification. Synthesis of Building Blocks. Ag NPs. Six nm Ag NPs were synthesized using a previously reported method with slight modification.66 Typically, 1 mmol of tetradecylphosphonic acid (PCI synthesis), 2 mmol of silver acetate, and 20 mL of trioctylamine were loaded into a 50 mL round-bottom flask. Under a nitrogen atmosphere and vigorous stirring, the reaction mixture was heated to 150 °C in 10 min and held at 150 °C for another 20 min. Au NPs. Four nm Au NPs were synthesized following a modified literature method.67 Typically, 0.5 mmol TBAB was dissolved in 1 mL OLAM and 1 mL tetralin. Meanwhile, 100 mg HAuCl4·3H2O was dissolved in 10 mL OLAM and 10 mL tetralin in a 50 mL flask by brief sonication. The reaction mixture was purged with nitrogen for 15 min, and the TBAB solution was swiftly injected into the flask. Upon injection, the solution changed color instantly from orange to dark red, but the reaction was left stirring in air for 1 h. AuxAg1−x NPs. Six nm AuxAg1−x alloy NPs were synthesized by a two-step method. In the first step, Au NPs with larger sizes were seeded grown from the 4 nm Au seeds. Typically, a gold precursor solution was freshly made by dissolving 200 mg HAuCl4·3H2O in 5 mL OLAM and 5 mL ODE. A calculated amount of 4 nm Au seeds and the precursor solution were mixed in a 50 mL flask. Under a nitrogen atmosphere and vigorous stirring, the reaction mixture was heated to 80 °C and held at 80 °C for 2 h. In the second step, Ag was grown onto and diffused into the Au NPs to form the AuxAg1−x alloy NPs. Typically, a silver precursor solution was freshly prepared by dissolving 100 mg of silver acetate in 2.5 mL OLAM and 2.5 mL ODE. In the meanwhile, 40 mg of Au seeds was mixed with 10 mL OLAM in a 50 mL flask. Under a nitrogen atmosphere, the reaction mixture was heated to 225 °C at a rate of ca. 15 °C/min. A calculated amount of silver precursor was dropwisely injected into the flask at a rate of 5 mL/h, starting when the temperature reached 120 °C. After completion of the injection, the reaction mixture was held at 225 °C for another 20 min. After synthesis, all subsequent steps were carried out in ambient air conditions. To purify the product, the NCs were flocculated by adding isopropanol and isolated by centrifugation. NCs were subsequently dispersed in hexane, flocculated a second time using isopropanol, and isolated by centrifugation to ensure the complete removal of excess ligands. The NCs were dispersed in octane for spincoating. PbSe NCs. Six nm PbSe NCs were synthesized following a previously reported method with slight modification.37 The synthesis and purification were both carried out under nitrogen atmosphere with a Schlenk line system using standard air-free procedures. Typically, 892 mg PbO, 3 mL OLAC, and 20 mL ODE were loaded into a 50 mL flask and heated to 120 °C for 1.5 under vacuum. Then the reaction mixture was switched to nitrogen atmosphere, and the temperature was then raised to 180 °C, at which point 8 mL of the Se precursor (1 M TOP:Se and 69 mL DPP) was rapidly injected. Upon injection, the temperature dropped to 160 °C, and the reaction continued at 160 °C for 10 min and was then quenched by ice-water cooling. The NCs were purified in a nitrogen glovebox by first precipitating the NCs with isopropanol and then centrifuging at 8000 rpm for 3 min. The NCs were redispersed in toluene and flocculated by adding isopropanol. The wash procedure was repeated three times, and the purified NCs were dispersed in octane for spin-coating. Characterization. Low-magnification TEM images were obtained using JEOL JEM-1400 transmission electron microscope. HRTEM images were obtained using JEOL JEM-2010 transmission electron microscope. SEM images and EDS composition data were obtained using JEOL JSM-7500F field emission scanning electron microscope.

Optical absorption spectroscopy was performed on samples dispersed in tetrachloroethylene using Analytical Spectral Devices QSP 350− 2500. FTIR spectroscopy was performed in transmission mode, and samples were prepared on double-side polished silicon substrates using Thermo-Fisher 6700 FTIR spectrometer. The EDS elemental mapping on AuAg alloy NPs was carried out on an aberrationcorrected FEI Titan 80/300 microscope equipped with an EDAX Optima T30 energy dispersive X-ray spectrometer at 200 kV. The images were acquired with a convergence semiangle of 30 mrad and collection semiangles of 55−200 mrad. For the case of the present work, to minimize the effect of the electron beam on the sample, images and spectra were acquired at particles without previous beam exposure using a low-beam dose. The high-angle annular dark-field STEM tomography tilt series was acquired in a field emission TEM (JEOL JEM-2100F) operated at 200 keV from −70 to +70 degree with 2° intervals. Small angle X-ray scattering (SAXS) was recorded on an in-house setup. The X-ray source was a Bruker Nonius FR591 rotating-anode coupled with Osmic Max-Flux optics and pinhole collimation to produce a bright, highly collimated beam. The scattered intensity was collected on a Bruker Hi-Star multiwire 2D detector. The whole system, including the samples, was kept under vacuum. The sample to detector distance was 54 cm. The PySAXS software was used to obtain the NP average size and size dispersion. ICP composition data were obtained using Spectro Genesis ICPOES. For the Ag NP doped PbSe NC solids, the NC mixture was digested in HNO3 (70%, Fisher) and H2O2 (30%, Fisher) with 1:1 volume ratio. The dopant volume percentage was calculated based on the Ag:Pb ratio. For all the other samples, the NC mixtures were digested in aqua regia, and the dopant volume percentage was calculated based on the Au:Pb ratio. Cyclic voltammetry measurements were performed with an electrochemistry workstation (Epsilon, C-3 cell stand) mounted inside a nitrogen-filled glovebox. In the three-electrode system, Ag/ AgNO3 in the electrolyte solution (10 mM) was used as the reference electrode, and a Pt wire worked as the auxiliary electrode, and NC or NP films spin-coated on conductive substrates served as the working electrode. Tetrabutylammonium hexafluorophosphate (TBAHP) in acetonitrile (10 mM) was the electrolyte. The conductive substrates were made by depositing Cr/Pd (5 nm/40 nm) on Si wafers following the MPTS treatment. The final potential was plotted relative to ferrocene/ferrocenium redox couple measured under the same condition. Film Deposition and Ligand Exchange. Cover glass slides were used as substrates for the Seebeck coefficient and Hall effect measurements. Sapphire substrates were used for temperaturedependent I−V measurements. All substrates were washed with a 5% detergent/DI water solution, a 1:1:1 DI water:acetone:ethanol solution, and cleaned with UV-ozone for 20 min. Then the substrates were treated with a 5 vol % mixture of MPTS in anhydrous toluene for 12 h to develop an MPTS monolayer. Then the substrates were rinsed with clean toluene and methanol to remove the excess MPTS solution. 50 mg/mL of PbSe NC dispersions in octane and 55−92 mg/mL of AuxAg1−x NC dispersions in octane were mixed at different ratios according to the desired doping concentration. The NC dispersions were spin-cast at 1000 rpm for 30 s onto desired substrates. For ligand exchange, 300 μL of a 130 mM SCN solution in methanol was dispensed onto the film surface, enough to completely cover. After 60 s, the solution was then spun off until dry. Then the film was rinsed three times with pure methanol to remove the striped ligands and the access SCN. For Seebeck coefficient, Hall effect, and I−V measurements, the above-mentioned steps were repeated for 5 times to develop ca. 200 nm thick NC films. Electrical Measurements. Seebeck Coefficient Measurements. Temperature-dependent Seebeck coefficient measurements were conducted using an MMR SB-100 Seebeck coefficient measurement system with an MMR K-20 temperature controller. The as-deposited NC films on cover glass slides were cut with a diamond scriber into ca. 1.5 mm × 4 mm pieces, which were mounted on the measurement 9097

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ACS Nano stage using silver paste. A constantan wire (an alloy of 45% nickel and 55% copper) was mounted in symmetry to the sample on the stage as a reference to monitor temperature difference ΔT. Throughout the measurements, ΔT was adjusted to around 1.5 K by adjusting the output power of the heater. All samples were evacuated under 12 mTorr for more than 3 h prior to the measurements. Electrical Conductance Measurements. Variable-temperature conductivity of doped NC solids was measured on a model Lake Shore Cryotonics (formerly Desert Cryogenics) probe-station using a model 4156C semiconductor parameter analyzer (Agilent). Samples were deposited onto sapphire substrates with Cr−Au (3−40 nm thick) bottom contact electrodes deposited to form channel lengths of 150 and 200 μm with corresponding W/L ratios of 15. Samples were sealed with a glass coverslip using epoxy (ITW Devcon) and transferred to the probe station. All measurements were then performed under high vacuum (ca. 10−5 Torr) and at various temperatures between 200 and 300 K by introducing liquid N2. Hall Effect Measurements. For the Van der Pauw geometry Hall effect measurements, four 40 nm-thick Au electrodes of 1 mm2 were deposited on to the NC films in a square geometry 2.5 mm apart via thermal evaporation (angstrom). The perimeter of the square was then carefully scratched away to isolate the film to be measured. Room-temperature Hall effect measurements were conducted using an MMR H-50 Hall effect measurement system equipped with a 0.65 T permanent magnet. The thickness of the film needed to calculate the conductivity and carrier density was measured by AFM (Asylum Research Corp. MFP-3D-BIO).

ization supported by NSF through the University of Pennsylvania Materials Research Science and Engineering Center (MRSEC) (DMR-1720530) and through the use of facilities and instrumentation maintained under this award. M.C. acknowledges the support from ORNL’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b03112. Size and composition of the NCs, composition of the NC solids, supplementary TEM and SEM images of the NC solids, the effect of ligand exchange demonstrated by FTIR and TEM, Seebeck coefficients of Au- and Agdoped NC solids where dopant fraction is varied (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Haoran Yang: 0000-0002-9709-1062 Jennifer D. Lee: 0000-0003-2644-3507 Huolin L. Xin: 0000-0002-6521-868X Miaofang Chi: 0000-0003-0764-1567 Cherie R. Kagan: 0000-0001-6540-2009 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The primary support of this project was provided by The Nature Conservancy and the generosity of Sarah W. Fuller through a Nature Net Science Fellowship awarded to H.Y. E.W., T.Z., H.Y.T., and C.R.K. were supported in their contributions to the electrical characterization by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. J.D.L.’s materials effort in characterization was supported by Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001004. B.F. and E.A.G. were supported in materials synthesis and character9098

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