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Seung Jin Heo,a Seokhyun Yoon, a Sang Hoon Oh, a Doo Hyun Yoon,a and Hyun Jae Kim*a 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We investigated the effects of high-pressure treatment on charge carrier transport in PbS colloidal quantum dot (CQD) solids. We applied high pressure to PbS CQD solids using nitrogen gas to reduce the inter-dot distance. Using this simple process, we obtained conductive PbS CQD solids. Terahertz timedomain spectroscopy was used to study charge carrier transport as a function of pressure. We found that the minimum pressure needed to increase the dielectric constant, conductivity, and carrier mobility was 4 MPa. All properties dramatically improved at 5 MPa; for example, mobility increased from 0.13 cm2 V-1 s-1 at 0.1 MPa to 0.91 cm2 V-1 s-1 at 5 MPa. We propose this simple process as a nondestructive approach for making conductive PbS CQD solids that are free of chemical and physical defects.

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Colloidal quantum dots (CQDs) have been suggested as an alternative next-generation electronic and optoelectronic material because of their numerous advantages. These include multiple exciton generation1, bandgap engineering by tuning the particle size2, and low-cost solution-based fabrication3. CQDs, which are zero-dimensional semiconductor systems, are synthesized and dispersed in solution. The dispersion must be converted into a monolayer or multilayer film, a so-called CQD solid, for industrial applications such as light-emitting diodes4, photovoltaics (PV)5, and field-effect transistors6. However, there are several issues: the electrical or optical properties of CQDs in a zero-dimensional system cannot be maintained in CQD solids. Many researchers have attempted to improve the optical and electrical properties of CQD solids.7-9 Most studies have focused on conductive CQD solids because of their potential utility in various devices. Two simple rules have developed concerning conductivity enhancement: stable and partial occupation of the sparse and identifiable quantum dot states is required, and the carriers must be able to hop between these states on separate quantum dots within a reasonable time scale.7 One way to improve the conductivity of CQD solids is to decrease the interdot distance. Previous studies attempted to do this by removal of long ligands, or exchange with short ligands, or by thermal annealing of the CQD solids.10-12 Houtepen et al. reported the effects of inter-dot distance on charge carrier mobility in PbSe CQD solids.13 They used ligands of different lengths to modify inter-dot distance. They found that the PbSe CQD solids with the shortest ligands had the highest carrier mobility because the short inter-dot distance enabled the charge carrier to transport within the carrier lifetime. Chemical and thermal treatments of CQD solids affect the inter-dot distance, which can improve carrier mobility. However, such treatments are a major cause of This journal is © The Royal Society of Chemistry [2013]

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aggregation, voids14, and sintering.11 In this paper, for the first time, we present a systematic study of the influence of high-pressure treatment on charge carrier transport in PbS CQD solids. To minimize the inter-dot distance, we applied a treatment post film formation. This approach is a nondestructive method to decrease the inter-dot distance and has the possibility of controlling inter-dot distance by adjusting the applied pressure. We used terahertz time-domain spectroscopy (THz–TDS) to analyze differences with respect to the applied pressure. These experiments provided optical parameters, such as the complex refractive index and optical conductivity, and the dielectric function of high-pressure-treated PbS CQD solids.

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All chemicals were purchased from Sigma-Aldrich. To synthesize the PbS CQD solution, a slurry of lead chloride in oleylamine (OLA) (1:2 molar ratio) was mixed for several hours in a threenecked flask at 100°C under a flow of N2. Elemental sulfur was dissolved in OLA (0.1:0.2 molar ratio) at 80°C over 30 min. The temperature of the lead chloride-OLA slurry was increased to 120°C over 30 min and then the sulfur–OLA solution was added. The temperature was reduced to the growth temperature of 100°C and held there for several hours until the system became homogeneous. The mixture was removed and quenched by pouring into cold toluene. Oleic acid (OA) was added to the PbS CQD suspension (20:3 volume ratio) at room temperature to exchange the OLA ligands for OA. The suspension was ultrasonicated and then centrifuged to remove excess lead chloride and free ligands. Ethanol was added to the retained supernatant. The suspension was centrifuged, the supernatant was discarded, and the precipitate was redispersed in toluene. The suspension was washed twice, and finally dispersed in toluene (~50 mg mL-1). The PbS CQD solids were treated with a methanolic solution of cetyltrimethylammonium bromide [Nanoscale], [2013], [vol], 00–00 | 1

Nanoscale Accepted Manuscript

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Influence of high-pressure treatment on charge carrier transport in PbS colloidal quantum dot solids

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(CTAB) to exchange OA with Br- ions. This treatment consisted of a three-step spin-coating cycle: 1.) 50 mg mL-1 of the PbS CQD solution was spin coated; 2.) 0.5 mL of the methanolic CTAB solution was coated onto the PbS CQD film; and 3.) the film was washed with methanol. To fabricate PV devices, solution-processed ZnO thin films were spin-coated onto an ITO substrate and annealed at 500°C for 4 h. The PbS CQD solids were deposited as explained above. Layers of MoO3 (3 nm) and Ag (100 nm) were deposited onto the PbS CQD solids by thermal evaporation. For high-resolution transmission electron microscopy (HR-TEM) study of the devices, samples were prepared by field ion beam milling (Helios Nanolab, FEI) using a Ga+ beam followed by argon-ion nano-milling. The HR-TEM (JEM-4010 TEM, JEOL) operated at 200 kV. The J–V curves were obtained with an Air Mass 1.5 solar simulator (ABET Technologies, Sun 2000) providing illumination at an intensity of 100 mW cm-2. Atomic force microscopy (AFM) was performed in non-contact mode (XE-100, Park System). All samples were prepared onto sapphire substrate. We scraped surface of samples by cutter with extremely sharp blade for artificial steps. Absorption spectra were measured with a spectrophotometer (Agilent Technologies, Agilent Cary 5000). The THz–TDS (TPS Spectra 1000, Teraview) study used a Ti:sapphire ultra-short pulsed laser. The resolution was 0.2 cm-1. The sample chamber was maintained under a vacuum to remove atmospheric water vapor.

3 Results and discussion

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We fabricated as-prepared (0.1 MPa condition) and highpressure-treated PbS CQD solids to analyze the fundamental effects of the pressure treatment. As-prepared samples were dried in ambient air without thermal annealing, because thermal treatment can affect the inter-dot distance.15 For the highpressure-treated samples, nitrogen gas was introduced into a pressure chamber until the pressure reached 5 MPa. This static pressure was then held for 2 h. During this time, the PbS CQDs on the surface of the solids underwent collisions with the gaseous nitrogen atoms. These collisions occurred randomly and continuously. These collisions caused the dots within the PbS CQD solids to become rearranged, decreasing the inter-dot distance. This phenomenon was observed by cross-sectional HRTEM, as shown in Fig. 1 and Fig. S1. To confirm that the changes would occur within a PV device and not only within a thin film, we imaged a Ag/MoO3/PbS CQD solids/ZnO/indium tin oxide (ITO) device. The HR-TEM images revealed changes inside the PbS CQD solids and at the interface between the PbS CQD solids and the ZnO. The inter-dot distance was laterally and vertically reduced in the PbS CQD solids after the high-pressure treatment: the thickness of the PbS CQD solids decreased from 43.9 to 36.7 nm (Fig. S1). In order to estimate the overall change in thickness by high-pressure treatment, we used AFM analysis. Fig. S2 and Fig. S3 show the thickness of as-prepared PbS CQD solids and high-pressure-treated PbS CQD solids. There was some debris on both sides of step coming from scraping PbS CQD solids. The thickness was obtained as the difference between the heights of the two regions defined by average cursors at each side of the step. The average thickness of as-prepared PbS CQD solids and high-pressure-treated PbS CQD solids is 44.3 nm 2 | Nanoscale, [2013], [vol], 00–00

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and 35.3 nm, respectively. From HR-TEM and AFM analysis, we clearly observed decreasing thickness of PbS CQD solids by high-pressure treatment. It indicates that the inter-dot distance was decreased in high-pressure-treated PbS CQD solids, because there is no loss of PbS CQD and no reduction of area in PbS CQD solids during high-pressure treatment, just decreasing height of PbS CQD solids. In order to clarify the effects of highpressure treatment mentioned above, we measured UV-Vis-NIR spectroscopy. Absorption spectra of as-prepared and highpressure-treated PbS CQD solids are shown in Fig. 2. We fitted first exciton peak using Lorentzian function.16 The first exciton peak is 1.967 eV and 2.042 eV in as-prepared PbS CQD solids and high-pressure-treated PbS CQD solids, respectively. There is a 75 meV red-shift in high-pressure-treated PbS CQD solids. It provides additional evidence for the relatively strong electronic coupling between PbS CQD due to decreasing inter-dot distance.17 Fig. 1 shows that the high-pressure treatment caused the PbS CQD solids to become homogeneous and crack-free; voids disappeared, which increased the density. High-quality PbS CQD solids can thus be obtained using this simple treatment with no loss of ligands or destruction of the geometry within the PbS CQD solids. Also, a reduced inter-dot distance leads to a conductive PbS CQD solid.18 We also observed an enhanced interface between the PbS CQD solids and ZnO after the highpressure treatment. In general, a poor interface will lead to inefficient carrier transport; it is a severe interface trap in electronic devices.19 Our high-pressure treatment can enhance CQD-based electronic and optoelectronic device performance in two ways: a reduction in the inter-dot distance and an improvement in the interface quality. To confirm the improvements, we fabricated a PbS CQD-based PV and measured the current density-voltage (J–V) characteristics, as shown in Fig. S4. We compared the as-prepared cell with the high-pressure-treated cell. The increased open-circuit voltage (VOC) and short-circuit current (JSC) in the high-pressure-treated cell were attributed to an improved interface quality and a reduced inter-dot distance. These experimental results are in good agreement with the conclusions drawn from the HR-TEM images. As noted above, the inter-dot distance was decreased by the high-pressure treatment. We used THz–TDS to probe the changes in conduction and carrier dynamics with this treatment; this technique provided information on the influence of nm-scale disorder on carrier motion by examining the response of an electromagnetic field in the THz frequency range.20 Cracks and voids in the long range and the inter-dot distance in the short range both affect THz–TDS results. The technique provides a direct determination of both the amplitude and the phase at each frequency, and thus both the theoretical and actual parts of the refractive index can be calculated without using the Kramers– Kronig relations.21 Complex dielectric and complex conductivity functions can be obtained from the refractive index. We prepared PbS CQD solids on a fused silica substrate, which is almost transparent in the THz range (Fig. S5). Pressure on the samples was raised over 2 h from 0.1 to 5 MPa at intervals of 1 MPa using nitrogen gas. The thickness of each sample was measured by an alpha-step thickness profiler. Fig. 3 shows the dielectric constant for each sample. There was a dramatic increase in this property at

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Nanoscale Accepted Manuscript

DOI: 10.1039/C3NR03641C

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4 MPa. The average dielectric constant at 0.5 to 1 THz as a function of pressure is summarized in Table 1. The dielectric constant for 5 MPa was double that for 0.1 MPa as a result of the decrease in the inter-dot distance, as evident in HR-TEM images.11 There was a threshold pressure for the change in the dielectric constant, with the smallest value found for 4 MPa. Therefore, we concluded that the effective minimum pressure for the technique is 4 MPa. The increase in the dielectric constant affects charge transport. The charging energy (EC) is the energy required to add or remove additional charge to a particle. For a spherical CQD, this can be expressed by:  

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1  ∑!  "

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where ω is the radial frequency, τ is the carrier collision time, n is the carrier density, and m* is the carrier effective mass. In practice, this formula can be rewritten by assuming that the duration of a collision is negligible in comparison with the interval between collisions. Therefore, we can sum the series to infinity and then obtain

 

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(1)

where e is the fundamental unit of charge, ε is the dielectric constant of the material, and r is the radius of the CQD; our PbS CQDs were almost spherical, as shown in Fig. S6.22 This equation indicates that the charging energy decreases with increasing dielectric constant. Therefore, it is easier to transport carriers with a higher dielectric constant because the charging energy acts against the facile migration of carriers from one dot to another. At 5 MPa, the dielectric constant at 33.3 cm-1 (1 THz) was twice that at 0.1 MPa. This means that the energy for carrier transport dwindles to half of its previous value in terms of energy. The complex conductivity function  was obtained simultaneously with the complex dielectric function ̃ . We used the Drude-Smith model to fit the THz conductivity data.23 This model adds an additional term, cj, which is the fraction of the carrier’s initial velocity that is retained after the jth collision. The Drude-Smith model describes the complex conductivity as

 

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When c = 0, this formula represents the standard Drude model. The conductivity calculated from our experimental data was welldescribed by the Drude-Smith model for c = - 0.99 (Fig. S7a and S7b). Fig. 4 shows our calculated THz conductivities. The average conductivity from 0.5 to 2 THz dramatically increased from 5.1 to 17 Ω-1cm-1 for the 0.1MPa and 5MPa conditions, respectively. The threshold pressure for the change of conductivity was also 4 MPa, as for the dielectric constant. A pressure over 4 MPa can thus rearrange the dots within the CQD solids in the solid state, reduce inter-dot distance, and form a conductive PbS CQD solid. The enhanced conductivity can also be explained in terms of carrier collision time and carrier density, which are Drude-Smith model fitting parameters. The carrier collision time and carrier density increased at 5 MPa, as shown in Fig. 5. The decrease in the inter-dot distance leads to denser packing of the carrier in PbS CQD solids. Therefore, the carrier This journal is © The Royal Society of Chemistry [2013]

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density is approximately doubled at 5 MPa. Moreover, a carrier can spend more time being transported in high-pressure-treated PbS CQD solids at 5 MPa. The carrier collision time increased from 48 fs at 0.1 MPa to 59 fs at 5 MPa. This indicates that PbS CQDs are crowded and that the carriers in strongly-coupled PbS CQD solids can transport efficiently because of the enhanced CQD interface after the high-pressure treatment. The real part of the refractive index and conductivity from 0.5 to 2 THz is shown in Fig. 6. The refractive index also rapidly increased at the highest pressure. This behavior is again consistent with a reduced inter-dot distance; a high refractive index indicates a dense solid. From these results, we concluded that the quality of the PbS CQD solids was improved by the high-pressure treatment; the higher density and conductivity should lead to better carrier mobility. The carrier mobility under each pressure condition was estimated using the fitting parameters given in Table 2. The carrier mobility increased nine-fold under 5 MPa of applied pressure, which is consistent with the origin of the improvement being decreased inter-dot distance. The hopping rate (Γ) between two orbitals of CQD neighbors and the electronic exchange coupling energy (β) between two neighboring quantum dots (a, b) can be expressed by: - +Ψ. / β  (Ψ* +Η

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where h is Planck’s constant, Γ is the hopping rate between two orbitals of CQD neighbors, m* is the carrier effective mass, Ebar is the barrier height, and x, which is the inter-dot distance in this study, is the width of the barrier. From eqn (5), we expect the electronic exchanging coupling energy to increase with decreasing inter-dot distance because of greater spatial overlap of the wave functions. This means that as the inter-dot distance decreases, exchange interactions become significant, and the electronic wave functions of the individual CQD spread out into the longer range.24 Therefore, the high-pressure treatment can provide strongly-coupled domains in PbS CQD solids, and bandlike transport. Specifically, these strongly-coupled domains are the cause of efficient carrier transport and decreased exciton binding energy, which result in an increased possibility of exciton dissociation.11,25 This is consistent with improved power conversion efficiency, as mentioned above. Moreover, the hopping rate and the electronic exchange coupling energy increase exponentially with decreasing inter-dot distance. Barrier height and carrier effective mass are weakly dependent on the hopping rate, whereas inter-dot distance has a strong dependence. From these results, we concluded that the inter-dot distance can be modified by high-pressure treatment. This treatment results in dramatic changes in properties such as the refractive index, dielectric constant, conductivity, and carrier mobility.

Conclusions

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Summarizing, we demonstrated that PbS CQD solids can be modified into dense and conductive solids by high-pressure treatment. We investigated changes in their properties through HR-TEM and THz–TDS analyses. We observed a reduced interdot distance at 4 MPa and over, which resulted in increases in the Nanoscale, [2013], [vol], 00–00 | 3

Nanoscale Accepted Manuscript

DOI: 10.1039/C3NR03641C

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This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Education, Science, and Technology (MEST) (no. 2011-0028819). The authors acknowledge Taeyoon Hong, Kyujin Choi, and Professor Jae Hoon Kim at Yonsei University for access to the terahertz time-domain spectrometer.

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O.E. Semonin, J.M. Luther, S. Choi, H.-Y. Chen, J. Gao, A.J. Nozik, and M.C. Beard, Science, 2011, 334, 1530-1533. T. Hirai, Y. Tsubaki, H. Sato, and I. Komasawa, J. Chem. Eng. Jpn., 1995, 28, 468-473. C. Ippen, T. Geco, and A. Wedel, J. Inf. Disp., 2012, 13, 91-95. L. Sun, J. J. Choi, D. Stachnik, A.C. Bartnik, B.-R. Hyun, G.G. Malliaras, T. Hanrath, and F.W. Wise, Nat. Nanotechnol., 2012, 7, 369-373. S.A. McDonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, and E.H. Sargent, Nat. Mater., 2005, 4, 138-142. S. Yang, N. Zhao, L. Zhang, H. Zhong, R. Liu, and B. Zou, Nanotechnology, 2012, 23, 255203. P. Guyot-Sionnest, J. Phys. Chem. Lett., 2012, 3, 1169-1175. D. Yu, C. J. Wang, and P. Guyot-Sionnest, Science, 2003, 300, 12771280. D. Vanmaekelbergh, and P. Liljeroth, Chem. Soc. Rev., 2005, 34, 299-312. J.H. Warner, Adv. Mat., 2008, 20, 784-787. K.J. Williams, W.A. Tisdale, K.S. Leschkies, G. Haugstad, D.J. Norris, E.S. Aydil, and X.-Y. Zhu, ACS Nano, 2009, 3, 1532-1538. W. Lu, F. Yamada, and I. Kamiya, J. Vac. Sci. Technol. B, 2010, 28, C5E8-C5E11. Y. Gao, M. Aerts, C.S.S. Sandeep, E. Talgorn, T.J. Savenije, S. Kinge, L.D.A. Siebbeles, and A. J. Houtepen, ACS Nano, 2012, 6, 9606-9614. E. J. D. Klem, H. Shukla, S. Hinds, D.D. MacNeil, L. Levina, and E.H. Sargent, Appl. Phys. Lett., 2008, 92, 212105. S. C. Chiu, J. S. Jhang, J. F. Chen, J. Fang, and W. B. Jian, Phys. Chem. Chem. Phys., 2013, 15, 16127-16131. D. S. Chemla, D. A. B. Miller, P. W. Smith, A. C. Gossard, and W. Wiegmann, IEEE J. Quantum Electron., 1984, QE-20, 265-275. X. Ma, F. Xu, J. Benavides, S. G. Cloutier, Org. Electron., 2012, 13, 525-531. Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H.W. Hillhouse, and M. Law, Nano Lett., 2010, 10, 1960-1969. M.J. Panzer, K.E. Aidala, P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, and V. Bulovic, Nano Lett., 2010, 10, 2421-2426. D.G. Cooke, A.N. MacDonald, A. Hryciw, J. Wang, Q. Li, A. Meldrum, and F.A. Hegmann, Phys. Rev. B., 2006, 73, 193311. S.L. Dexheimer, Terahertz Spectroscopy Principles and Applications; Taylor & Francis Group, London, U.K., 2007, ch. 4, pp. 119-160. D.V. Talapin, J. Lee, M.V. Kovalenko, and E.V. Schevchenko, Chem. Rev., 2010, 110, 389-458.

4 | Nanoscale, [2013], [vol], 00–00

a

School of Electrical and Electronic Engineering, Yonsei University, Seoul, Republic of Korea. E-mail: [email protected]

Table 1 The average value of dielectric constant from 0.5 THz to 2 THz. Pressure condition (MPa)

Dielectric constant

0.1 1 2 3 4 5

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Table 2 Carrier mobility calculated from THz–TDS data.

Notes and references 1

23 N.V. Smith, Phys. Rev. B., 2001, 64, 155106. 24 M.C. Beard, G.M. Turner, J.E. Murphy, O.I. Micic, M.C. Hanna, A.J. Nozik, and C.A. Schmuttenmaer, Nano Lett., 2003, 3, 1695-1699. 25 A. Franceschetti, Zunger, A. Phys. Rev. B., 2001, 63, 153304.

Pressure condition (MPa)

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0.1 1 2 3 4 5

0.13 0.13 0.13 0.13 0.33 0.91

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Fig. 1 Cross-sectional HR-TEM images of the Ag/MoO3/PbS CQD solids/ZnO stack on an ITO substrate. (a) The sample was prepared only by evaporating toluene, without thermal annealing. Yellow arrows indicate voids within the PbS CQD solids or large inter-dot distances, and blue arrows indicate poor-quality interfaces between the PbS CQD solids and ZnO. (b) The high-pressure-treated sample was prepared using N2 at 5 MPa gas for 2 h.

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refractive index, dielectric constant, and conductivity. Also, the carrier mobility of the PbS CQD solids treated at 5 MPa increased nine-fold to almost 1 cm2 V-1 s-1, compared with 0.1 MPa for the untreated material. The enhanced carrier transport is for two reasons: a high-quality film formation and an increase in the exchange coupling energy. Efficient carrier transport is achieved by decreasing the energy required for transport to neighboring dots. This is a remarkable finding for conducting CQD solids. The nondestructive high-pressure treatment strongly affects the inter-dot distance. Therefore, it is a very attractive approach to improve the performance of CQD-based electronics and optoelectronics.

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Fig. 2 Absorption spectra of as-prepared and high-pressure-treated PbS CQD solids. The dashed line is from Lorentzian fitting for first exciton peak and their peak position is represented by triangle.

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Fig. 5 (a) Carrier collision time (τ) and (β) carrier concentration (N) extracted from the Drude-Smith model as a function of pressure.

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Fig. 3 The real part of the complex dielectric function for as-prepared and high-pressure-treated PbS CQD solids.

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Fig. 6 The real part of the refractive index and conductivity of the PbS CQD solids after various high-pressure-treatments.

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Fig. 4 The real part of the complex conductivity function for as-prepared and high-pressure-treated PbS CQD solids.

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DOI: 10.1039/C3NR03641C

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We propose high-pressure treatment as a nondestructive approach for making conductive CQD solids that are free of chemical and physical defects. 33x14mm (300 x 300 DPI)

Nanoscale Accepted Manuscript

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DOI: 10.1039/C3NR03641C

Influence of high-pressure treatment on charge carrier transport in PbS colloidal quantum dot solids.

We investigated the effects of high-pressure treatment on charge carrier transport in PbS colloidal quantum dot (CQD) solids. We applied high pressure...
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