Solution-Processed 2D PbS Nanoplates with Residual Cu2S

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Solution-Processed 2D PbS Nanoplates with Residual Cu2S Exhibiting Low Resistivity and High Infrared Responsivity Wen-Ya Wu, Sabyasachi Chakrabortty, Asim Guchhait, Gloria Yan Zhen Wong, Goutam Kumar Dalapati, Ming Lin, and Yinthai Chan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04330 • Publication Date (Web): 26 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Chemistry of Materials

Solution-Processed 2D PbS Nanoplates with Residual Cu2S Exhibiting Low Resistivity and High Infrared Responsivity Wen-Ya Wu†‡∆, Sabyasachi Chakrabortty†∆, Asim Guchhait†∆, Gloria Yan Zhen Wong†, Goutam Kumar Dalapati‡, Ming Lin‡ and Yinthai Chan†‡* †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore

117543, Singapore. E-mail: [email protected] ‡

Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way,

Innovis, Singapore, 138634, Singapore.

ABSTRACT: We report the synthesis of colloidal 2D PbS nanoplates with residual Cu2S domains via a partial cation-exchange process involving Pb2+ and pre-synthesized hexagonal Cu2S nanoplates with an average thickness of ~3 nm and edge lengths of ~150 nm. Different from previously reported PbS nanosheets whose basal planes are ±{100}PbS, our approach yields nanoplates whose basal planes are ±{111}PbS, which was previously theoretically predicted to have better surface ligand passivation. Subsequently, we found that the PbS nanoplates showed improved colloidal stability and did not suffer from severe aggregation despite numerous solvent wash steps. We further incorporated a film of nanoplates into a planar photodetector device with lateral Au electrodes. The amount of residual Cu2S in the PbS nanoplates, which can be tuned by adjusting the reaction time of the cation-exchange process, was found to play a crucial role in determining the in-plane conductivity of the film

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and therefore its photodetection efficiency. For PbS nanoplates with 7.8% residual Cu+, the responsivity and specific detectivity at 808 nm was ~1739 A/W and ~2.55 x 1011 Jones respectively. The high responsivity was attributed to the very low PbS nanoplate film resistivity of 8.04 ohm•cm, which is comparable to commercial doped semiconductors.

1. INTRODUCTION Colloidal semiconductor nanocrystals are promising materials for optoelectronic devices because of their ease of fabrication, tunable band-gap based on size and solution processability that allows them to be easily incorporated into various device architectures.1-3 Unfortunately, because the surface ligands that facilitate particle dispersion in solvents are highly insulating, charge transport across a film of nanocrystals is generally poor.4-6 Twodimensional (2D) semiconductor nanoparticles with large lateral dimensions potentially ameliorate this issue by reducing the number of charge hopping and tunneling events across a film of highly crystalline nanosheets. The spectral positions of the band-edge transitions may be varied by the thickness of the 2D particles, and synthesis is straightforward to scale-up in a manner suitable for low cost, large-scale production. Of the different colloidal semiconductor nanocrystals, PbS has been amongst the most intensively investigated as a candidate material for solution-processed optoelectronics.7 This is primarily due to its small bandgap and large Bohr exciton radius, which collectively give it wide spectral tunability in the infrared. Recent efforts employing micron-sized ultrathin 2D PbS nanoplates as the active material in photodetector devices showed efficient detection in the infrared,8 however these nanoplates were fabricated via a chemical vapor deposition process which can be expensive and challenging to scale-up in terms of production volume. In 2010, Schliehe et. al. introduced the wet-chemical synthesis of 2D PbS nanosheets based on the 2D oriented attachment of sphere-like PbS particles into ultrathin planar structures.9 2 ACS Paragon Plus Environment

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Their approach was subsequently adopted by other researchers who explored different synthesis parameters that allowed for shape10 and thickness control.11 Aside from oriented attachment, nucleation and growth within a lamellar n-octylamine bilayer template has also been shown to produce pseudohexagonal PbS quantum plates.12 Despite theoretical predictions of improved carrier mobility and stronger photon absorption,13 a thin film of nanosheets produced via the method of Schliehe et. al. yielded relatively low responsivity when utilized within a photodetector configuration.9 Additionally, we found that these nanosheets are prone to severe aggregation upon processing, and cannot easily be separated into isolated sheets even after prolonged sonication. These limitations prompted us to develop an alternative approach to the synthesis of 2D PbS nanocrystals. Herein, we describe the wet-chemical preparation of hexagonally-shaped, thin PbS nanoplates (NPLs) with small Cu2S domains that were obtained via Pb2+ exchange of presynthesized Cu2S NPLs. Different from previously reported PbS nanosheets whose basal planes are ±{100}PbS,9 the judicious combination of nanoplate synthesis with cationexchange14 yields NPLs with ±{111} basal planes (Scheme 1) that have been suggested to exhibit comparatively better surface passivation with native oleic acid ligands.15 A thin film of PbS NPLs sandwiched between two lateral Au electrodes atop a SiO2/Si substrate showed a high responsivity of ~1739 A/W and specific detectivity of 2.55 x 1011 Jones at an excitation wavelength of 808 nm. This was attributed to the sheet-like shape of the PbS nanocrystals and a remarkably low resistivity of 8.04 Ω·cm that is most likely due to the presence of residual Cu2S.



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Scheme 1. Schematic illustrating that the direct synthesis of 2D PbS nanostructures using oleic acid as the main capping group typically results in ±{100} basal facets that are not well passivated, strongly affect surface ligand passivation that further influence colloidal stability of 2D PbS NPLs. 2. EXPERIMENTAL SECTION Chemicals and materials. Copper (I) chloride (CuCl, 99.99%), trioctylphosphine oxide (TOPO, 99%), 1-octadecene (ODE, 90%), 1-dodecanethiol (1-DDT, 97%), lead (II) acetate (Pb(Ac)2, 3H2O), oleic acid (90%, technical grade), trioctylphosphine (TOP, 97%), 3-mercaptopropionic acid (MPA, 99%) were purchased from Sigma Aldrich. All the chemicals were used as received without further purification. Unless stated otherwise, all reactions were conducted in oven-dried glassware under nitrogen atmosphere using standard Schlenk techniques.


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Synthesis of Cu2S nanoplatelets (NPLs). A typical synthesis of Cu2S NPLs 16 is as follows: CuCl (0.4 mmol), TOPO (2.5 mmol), ODE (20 mL) were degassed under vacuum at 90 ℃ in a round bottom 3-neck flask for 1 hour. 1DDT (4 mmol) was then swiftly injected into the 3-neck reaction flask at 160℃, upon which the mixture was continuously heated until a growth temperature of 210 ℃ was reached. The reaction was then kept at this temperature for another 2~3 hours. This yielded Cu2S NPLs with thickness of ~3 nm and edge lengths of ~120-150 nm. To process the NPLs from their growth solution, the entire reaction mixture was cooled to room temperature, whereupon 1 equivalent volume of toluene and 8 equivalent volumes of ethanol were added to 1 equivalent volume of growth solution. This was followed by centrifugation at 3900 rpm for 5 min, and the supernatant was discarded. The remaining NPLs precipitate was then re-dispersed in 1 equivalent volume of toluene, precipitated by 8 equivalent volumes of ethanol and centrifuged. This process was continued a second time before the processed NPLs were dispersed in toluene and sonicated for 1 hour (to ensure they are fully dispersed) for subsequent use. Cation exchange of Cu2S NPLs to PbS NPLs. The Pb2+ exchange process was carried out by adopting a previously reported procedure with modifications.17 First, a solution of Pb (II) oleate was prepared by mixing Pb(Ac) 2•3H2O (3.3 mmol) with oleic acid (7.9 mmol) and ODE (6 mL) in a three-neck RBF flask. The mixture was then degassed at 80°C for 30 min followed by heating under N2 at 250°C for 15-20 min. Second, in a separate reaction, Pb(Ac) 2•3H2O (0.7 mmol) was dissolved in 3 mL of methanol and added swiftly to a stirring solution of Cu2S NPLs that were dispersed in 3 mL of toluene. Third, 3 mL of the lead (II) oleate solution was added to the NPL mixture to enhance their dispersibility in the solvent mixture. It should be noted that a large excess of Pb2+ ions was 5 ACS Paragon Plus Environment


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used for this exchange reaction. Finally, 0.6 mL of TOP was added to the NPL mixture to drive the exchange reaction forward via the formation of an energetically favorable complex between Cu+ and TOP. A progressive color change from brown to black was observed during the entire course of the reaction. The exchange reaction was allowed to proceed over a course of around two weeks at room temperature before being quenched with excess methanol (which causes precipitation of the NPLs), centrifuged and then re-dispersed in chloroform. Longer reaction times did not appear to appreciably increase the extent of cation-exchange while carrying out the reaction at temperatures larger than 60oC resulted in noticeable structural degradation and subsequent aggregation of the samples. This may be due to the stresses and strains induced by a faster rate of cation-exchange at higher temperature. Device fabrication. The photodetector device was fabricated over a patterned pair of gold electrodes (each pair having a length of 1.75 mm and thickness of ~50 nm) atop a ~300 nm thick SiO2 on a Si substrate. The gap between the electrodes was 1 µm and spanned a width of 5 µm. Prior to device fabrication, the substrate was cleaned by sonication in acetone and then isopropyl alcohol (IPA) for 15 minutes each. The cleaned wafer was then dried using a nitrogen gun. Processed PbS NPLs were then deposited onto the wafer via a layer-by-layer spin coating technique. Ligand exchange with mercaptopropionic acid (MPA) was performed on each deposited layer. The details of the procedure are as follows: first, a 50 µL chloroform solution of PbS NPLs (5 mg/mL) was spin coated onto the SiO2/Si substrate at a speed of 3000 rpm for 10 seconds. Subsequently, a few drops (~ 30 µL) of MPA solution in methanol (10% in volume) was added to the film on the wafer and allowed to stand for 20 seconds spinning at 3000 rpm for 10 seconds. The deposited layer(s) were then washed by methanol and chloroform by spinning at same speed and time. The entire process was repeated another 7


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times, yielding an 8-layer film whose thickness was around 57 nm. The entire device was then annealed inside a tube furnace at 250 oC for 1 hour in a N2 environment. Current-voltage characterization of the photodetector device was carried out using a Keithley 2612B electrometer and under the illumination of a 808 nm continuous wave laser. 3. RESULTS AND DISCUSSION The fabrication of PbS NPLs begins with the synthesis of hexagonally shaped Cu2S sheets via a slight modification of a previous procedure reported by our group (As described in EXPERIMENTAL SECTION).16 Given the similarity between the lattice spacing of the ±{002}Cu2S and ±{111}PbS facets, we hypothesized that the cation exchange of Cu2S NPLs with Pb2+ at room temperature should yield PbS NPLs with ±{111}PbS basal plane facets that would yield better surface ligand passivation and therefore better colloidal stability. The synthesized Cu2S NPLs possessed a thickness of ~3.5 nm and edge lengths of ~120 – 150 nm. Upon exposure to Pb2+, cation-exchange primarily takes place at the edges and slowly migrates towards the center of the NPL over the course of ~2 weeks, as revealed in Figure 1a which was obtained by combining electron energy loss spectroscopy (EELS) with aberration corrected scanning transmission electron microscopy (STEM). During the course of the relatively long reaction time, small amounts of residual Cu in the NPLs were observed in the elemental map and may be attributed to the slow diffusion rate of Cu+ within PbS (Table S1).18,19 These changes in the elemental map corresponded well with the evolution of crystal structure as monitored by XRD characterization of the samples at different reaction times (shown in Figure S1). While occurring predominantly at the more reactive edge facets, some cation-exchange between Cu+ and Pb2+ is also likely to take place at the basal planes, resulting in spatially inhomogeneous cation exchange rates. Subsequently, the synthesized PbS NPLs possess a roughened surface, as illustrated in Figure 1b. Accordingly, the use of



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very thin Cu2S samples (< 2 nm) resulted in randomly located holes upon Pb2+ exchange (Figure S2), which motivated our use of thicker NPLs. Selected area electron diffraction (SAED) measurements projected from zone axis, as seen in the inset in Figure 1b, show that the exchanged NPLs are single crystalline with basal plane ±{111}PbS.

Figure 1. (a) Electron energy loss spectroscopy (EELS) data providing an elemental mapping of Cu2S NPLs undergoing progressive Pb2+ exchange. The red color represents Cu while cyan indicates Pb. (b) Typical low resolution TEM image of PbS NPLs produced via Pb2+ exchange with Cu2S NPLs, with SAED on a single NPL in the insets. (c) and (d) are HRTEM images of PbS NPLs projected from top-down and side-view respectively, with their corresponding FFT images in the insets. Lattice d-spacing values of 0.21 nm and 0.34 nm that correspond to the ±{220}PbS and ±{111}PbS planes were derived from the pair of spots circled in yellow in the insets of (c) and (d) respectively.

High resolution TEM (HRTEM) was used to characterize the facet distribution on the Pb2+ exchanged NPLs, which is expected to be similar to that of Cu2S due to the invariance of the sulfur positions during the exchange process.20 The HRTEM images illustrated in Figure 1c and 1d indicate good crystallinity and clear lattice fringes from ±{220}PbS and ±{111}PbS


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with lattice spacings of 0.21 nm and 0.34 nm respectively. These values are close to those of ±{110}Cu2S and ±{002}Cu2S with corresponding lattice spacings of 0.198 nm and 0.34 nm. Further support for the transformation of the above-mentioned facets of Cu2S to PbS is given by the close similarity between their theoretical reciprocal lattices considering a zone axis of and respectively (Figure S3). Commensurate with our hypothesis stated earlier, the PbS NPLs possess a zone axis with exposed ±{111}PbS on the basal plane which, to the best of our knowledge, has not been reported. Indeed, 2D PbS nanosheets obtained via direct synthesis (oriented attachment) result in basal planes comprised of ±{100}PbS, which is energetically favored over ±{111}PbS.9 In the case of PbSe NCs (structurally similar to PbS) directly synthesized via hot injection of Pb and Se precursors, the terminal ±{111}PbSe are eventually replaced by the lower energy ±{100}PbSe upon growing into larger nanoparticles.15 To better understand the transformation of ±{002}Cu2S to ±{111}PbS, we examined the interface between the Cu2S and PbS domains of a partially exchange NPL via inverted HRTEM as depicted in Figure 2a. The two distinct regions were identified and color coded in red (Cu2S) and green (PbS). A zigzag-shaped line, oriented at an angle of 120o between turns, may be observed at the interface between the two domains. This suggests the presence of two distinct planes, at 120o of each other, in which there is near epitaxial matching between the basal facets of Cu2S and PbS. Fourier transform analysis of the interface region of the image revealed a pair of very closely spaced spots that were ascribed to ±{110}Cu2S and ±{220}PbS, as illustrated in Figure 2b. Accordingly, the d-spacings of these planes are very close, which is consistent with the notion that the two domains are lattice-matched along the zig-zag-shaped interface. It is also seen that the spots corresponding to ±{110}Cu2S and ±{220}PbS lie on a common line, which indicates that the growth direction of ±{002}Cu2S and ±{111}PbS are the same. Furthermore, atomic models of the Cu2S (002) plane and PbS (111) plane, as illustrated in Figure 2c and 2d, suggest that the sulfur sub-lattice undergoes only a 9 ACS Paragon Plus Environment


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slight expansion of 3.5% during the transformation of Cu2S to PbS. Taken together, these considerations rationalize why room temperature cation-exchange between Cu+ and Pb2+ results in conversion of ±{002}Cu2S to ±{111}PbS. One important consequence of this is the improved colloidal stability of the synthesized NPLs, where we did not observe significant aggregation in spite of numerous repeated processing steps. This is in contrast to directly synthesized PbS nanosheets with ±{100} basal facets, where we observed severe aggregation occurring after just 1-2 processing steps.

Figure 2. (a) Reconstructed HRTEM image of a partially exchanged Cu2S-PbS NPL, where the Cu2S and PbS regions are identified according to their characteristic lattice d-spacings and color coded in red and green respectively. The interface between the two regions shows a zigzag border with 120o turns. (b) FFT of the interface region showing two pairs of reflected spots belonging to the ±{110}Cu2S and ±{220}PbS planes. Illustration of the structure and atomic composition of the (c) Cu2S (002) and (d) PbS (111) facets.

Depending on the extent of cation-exchange reaction between Cu+ and Pb2+, the PbS NPL products can retain appreciable domains of Cu2S. To characterize the composition and elemental oxidation states of the NPLs within a thin film configuration that is relevant to 10 ACS Paragon Plus Environment

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device applications, X-ray photoelectron spectroscopy (XPS) was employed. The NPL film was fabricated via the successive deposition of NPL layers, with 3-mercaptopropionic acid (MPA) treatment between deposition steps to passivate the surface of the NPLs.21 Figure 3a depicts a high resolution XPS spectrum of the NPL film, whereby the 4f peak of Pb is prominently featured. The fitted peak position closely matches with Pb2+ corresponding to PbS at 137.95 eV (4f7/2) and 142.8 eV (4f5/2).22 Two other small peaks in the Pb 4f spectrum could also be identified as that of PbSO4 (at 138.9 eV for Pb 4f7/2) and Pb-COO- (at 139.95 eV for Pb 4f7/2). The former species stems from a partial oxidation of PbS while the latter species may be attributed to the treatment of the film with MPA.23 Analysis of the S 2p peaks, as shown in Figure 3b, revealed that the peak positions for S2p3/2 at 161.15 eV and S 2p1/2 at 162.20 eV are well-matched with the S2- peak positions corresponding to PbS. Small peaks that may be attributed to Cu2S (at 162.1 eV for S 2p3/2) and unbound thiol (163.4 eV for S 2p3/2) are also observed.22 The presence of Cu+ due to the



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Figure 3. High-resolution XPS spectra of a film of PbS NPLs produced via Pb2+ exchange with Cu2S NPLs. (a) Analysis of the Pb 4f peaks indicate a dominant presence of Pb-S, a lesser amount of Pb-SO4 and trace amounts of Pb-COO-. (b) Analysis of the S 2p peaks reveal a strong presence of Pb-S, a much weaker presence of Cu-S and trace amounts of RS-. (c) Analysis of the Cu 2p peaks show a strong presence of Cu+ and a much weaker presence of Cu2+. (d) EDX spectrum of a PbS NPL showing an elemental composition comprising mainly PbS and a small amount of residual Cu2S.

residual amount of Cu2S present within the partially cation-exchanged NPLs may be confirmed by analyzing the high resolution spectrum for Cu 2p as illustrated in Figure 3c. The fitted peaks at 932.5 eV (Cu 2p3/2) and at 952.4 eV (Cu 2p1/2) match well with that of Cu+, and the strong presence of Cu2+ may be ruled out due to the very weak signals from its characteristic peaks.22 The intensity of the satellite peaks corresponding to Cu2+ do not increase appreciably even with larger amounts of residual copper, which strongly indicates that the Pb2+ exchange process does not result in the oxidation of Cu+. The percentage of residual copper in each NPL is determined by comparing the relative peak areas in the XPS spectrum, which for the particular sample analyzed in Figure 3 yields a value of ~7.8%. Analysis via EDX, as shown in Figure 3d, suggested an NPL elemental composition similar to that indicated by the XPS measurements. To evaluate the potential of the PbS NPLs as the active material in a photodetector, we incorporated the NPLs into a planar device comprised of a pair of pre-patterned lateral Au electrodes atop a ~300 nm thick SiO2 layer on a Si substrate (Figure 4a). The height of the Au electrodes was ~ 50 nm while the gap between the electrodes was ~ 1.0 µm. The width of the electrodes was ~ 5.0 µm, which yields a theoretical active area of 5.0 µm2. The active NPL film in between the Au electrodes was deposited layer-by-layer via the same process used in the XPS measurements. The films were subsequently allowed to undergo thermal annealing at a mild temperature of ~250 oC for 1 hour in N2 to reduce defect states that may be introduced during the cation-exchange process.24,25 Analysis via Atomic Force 12 ACS Paragon Plus Environment

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Microscopy (AFM) of a thermally annealed 8-layer thin film over a 5 µm x 5 µm area in the device, as illustrated in Figure 4b, revealed a film thickness of ~57 nm (via a scratch across the film, see figure S4) and a surface roughness (RMS) of ~14 nm. The rough morphology relative to spherical particle based films

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may be due to the tilting and stacking of large

NPLs within the film. It may also be seen from the AFM image that the NPLs dispersed across the film are particle-like and relatively uniform in size, which suggests that the thermal annealing did not cause the NPLs to fuse into a bulk solid. This is further corroborated by UV-Vis absorption spectroscopy of the NPL films, as exemplified in Figure 4c, which shows that the spectral feature at ~650 nm persists even after the thermal annealing process. The presence of the spectral feature, which may be ascribed to a quantum-confined higher order transition, provides additional evidence that the overall morphology of the NPLs were preserved. Finally, XPS measurements on the annealed films showed that the Cu+ peaks remained unchanged, suggesting that the oxidation state of copper did not change during the thermal treatment (Figure S5).



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Figure 4. (a) Schematic of the PbS NPL photodetector device architecture. (b) AFM image of a typical 5 x 5 µm2 area of a thermally annealed PbS NPL film. (c) UV-vis spectrum of a typical PbS NPL film. (d) Current-voltage characteristics of the photodetector device in the dark and under illumination from a 808 nm laser. (e) Table summarizing the different metrics obtained from a photodetector device based on PbS NPLs and PbS QDs respectively.

To evaluate the near-infrared (NIR) photoresponse of the NPL based device, current-voltage (I-V) measurements were conducted using 0.5 mW illumination from an 808 nm laser under ambient conditions. Figure 4d shows the I-V response of the device containing an 8-layer film of PbS NPLs. The external applied bias was varied from -4.0 V to +4.0 V, with a corresponding increase in photocurrent in the range of microamps. By taking into account the area of illumination, we determined the responsivity of the device to be ~1739 A/W. This value is comparable to devices employing epitaxially grown 2D PbS nanoplates

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amongst the highest for non-hybrid colloidal PbS nanocrystal based devices.27 Unfortunately, 14 ACS Paragon Plus Environment

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the dark current obtained was also high, giving a more modest ratio of photo- to dark current and subsequently a specific detectivity of ~ 2.55 x 1011 Jones. We attribute the large dark current measured to the residual small Cu2S domains that can increase the overall conductivity of the PbS NPLs. Indeed, a control experiment using a pristine (unannealed) film of Cu2S NPLs within the same device architecture showed a much larger dark current than a pristine film of PbS NPLs but no measurable net photocurrent despite the excitation wavelength being well within the absorption profile of the NPLs (Figure S6). Subsequently, the relative amounts of PbS and Cu2S within each NPL can have a profound effect on its photodetection efficiency. For example, a PbS NPL sample with 30% residual copper yielded a dark current nearly 6 times larger than PbS NPLs with 7.8% residual copper at an applied bias of 4.0 V. However, only a small photocurrent was detected under 10 mW illumination, making the device unsuitable as a photodetector (Figure S7). A systematic tuning of the relative PbS and Cu2S amounts, which can be carried out by simply varying the reaction time of the cation exchange process, should afford the most optimal values for responsivity and specific detectivity. A log-log plot of the responsivity versus the square root of the excitation light intensity revealed a linear relationship, suggesting that carrier recombination in these NPLs is primarily bimolecular (Figure S8). To verify that the large responsivity measured for the PbS NPLs with 7.8% residual copper was due to high in-plane conductivity, we determined the resistivity of the NPL film by measuring its sheet resistance via a standard four-point probe system. In this probe system, current is passed through two outer probes while voltage is measured across two inner probes. The distance between the probes was fixed at 1 mm while the film thickness was ~57 nm, allowing us to estimate the bulk resistivity of the film from its sheet resistance. We measured the sheet resistance at five distinct regions of the NPL film and found a relatively small variation (Table S2). Subsequently, the resistivity was calculated by multiplying the 15 ACS Paragon Plus Environment


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average sheet resistance by the thickness of the film. This gave a remarkably low resistivity of 8.04 ohm·cm, which is within the range of resistivities exhibited by commercial doped semiconductors.28,29 Even lower resistivities may be expected for the PbS NPLs if structural defects and vacancies introduced during the cation-exchange reaction, which can act as charge scattering centers,30 can be reduced. We benchmarked the performance of our PbS NPL based devices by carrying out a control experiment using wet-chemically synthesized colloidal PbS quantum dots (QDs) as the active material in the same photodetector device architecture. Key parameters and processes such as the optical density of the film, its thickness, deposition procedure and thermal annealing treatment were kept as identical as possible to that of the PbS NPL based devices. The average resistivity of a thin film of spherical PbS QDs was determined to be 30.47 ohm·cm, which is more than 3 times higher than that of a PbS NPL film. Additionally, the photo- and dark current for a given applied bias is significantly less in PbS QD devices than PbS NPL devices (Figure S9). Compared with the NPL films, the responsivity and specific detectivity of the PbS QD films are about 69% and 56% lower respectively, as tabulated in Figure 4e. This is consistent with the larger resistivity exhibited by the film of PbS QDs. 4. CONCLUSION In summary, we fabricated solution processed, colloidal PbS NPLs with residual Cu2S domains via partial cation exchange between Pb2+ and pre-synthesized Cu2S NPLs. The synthesized PbS NPLs possess a unique facet distribution, namely that its basal planes comprise {111} facets, which is distinctively different from previously reported 2D PbS nanostructures. Characterization by XPS and EDX showed that the partial exchange with Pb2+ at room temperature over the course of ~2 weeks produced PbS NPLs with ~7.8 % residual copper. A thin film of these PbS NPLs were incorporated into a planar photodetector 16 ACS Paragon Plus Environment

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device that exhibited high photodetection efficiency at a wavelength of 808 nm, with a responsivity and specific detectivity value of ~1739 A/W and ~2.55 x 1011 Jones respectively. This was attributed to a very low film resistivity of 8.04 ohm·cm, which is comparable to commercial doped semiconductors. This work describes the use of partial cation exchange and plate-like morphology in colloidal PbS nanocrystal films to improve their in-plane conductivity while retaining good photoresponse. Better photodetection efficiencies may be expected with higher nanocrystal packing densities and a more judicious choice of ligands, making these heterostructured PbS NPL films possible candidates for low cost, high performance solution processed NIR photodetectors.

ASSOCIATED CONTENT Supporting Information. Structural and optical characterization, additional experiments including TEM, XRD, XPS, AFM data analysis, control experiments on device performance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email (Yinthai Chan): [email protected] Author Contributions ∆These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



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The authors acknowledge funding support from a NRF CRP (NRF-CRP8-2011-07) grant, a JSPS-NUS Joint Research Projects grant (WBS R143-000-611-133) and a A*STAR-JCO (No. 1437c00135) grant. REFERENCES (1) Kramer, I. J.; Sargent, E. H. The Architecture of Colloidal Quantum Dot Solar Cells: Materials to Devices. Chem. Rev. 2014, 114, 863-882. (2) Konstantatos, G.; Sargent, E. H. Nanostructured Materials for Photon Detection. Nat.

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Solution-Processed 2D PbS Nanoplates with Residual Cu2S

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