Article pubs.acs.org/JPCC
Low-Temperature, Solution-Based Sulfurization and Necking of PbS CQD Films Alexandros Stavrinadis,† David So,† and Gerasimos Konstantatos*,†,‡ †
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain ICREA−Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
‡
S Supporting Information *
ABSTRACT: Lead sulfide colloidal quantum dots (CQDs) are a promising optoelectronic material. The optoelectronic functionality of PbS CQD films largely depends on the anionic ligands that passivate the Pb-rich surface of the CQDs’ inorganic cores. Herein, we report a simple solution-based method for fabricating PbS CQD films using sulfur as the ligand. In turn, passivation of the CQDs with sulfur promotes the chemisorption of oxygen. Overall, this approach results in efficient removal of the original organic ligands and in enhanced interdot electronic coupling. The CQD films present increased p-type doping, higher majority carrier mobility, and higher photoresponsivity as compared to state-of-the-art halide-passivated CQD films. The simple postsynthetic sulfurization strategy described herein can be potentially applied in a variety of metal sulfide and selenide nanomaterials whose optoelectronic functionalities in part depend on the chalcogen to metal atomic ratio.
1. INTRODUCTION PbS CQD films are being utilized in high-performing optoelectronic devices including solar cells,1,2 photodetectors,3 and LEDs.4 The optoelectronic properties of these films depend on the dimensions of the dots’ inorganic core as well as on their surface chemistry.5−7 For device-quality films, the dots’ surface is usually passivated by short organic or inorganic anions like thiolates8,9 and carboxylates,10,11 thioacyanates,12 hydroxides,13 halides,14 and oxides.9,15 These serve multiple purposes: they render the films insoluble to various solvents used during film processing,8 balance the dots’ cation-rich surface stoichiometry,16 affect the interdot electronic coupling,17 affect the majority carrier type and concentration,16 passivate or induce midgap trap states, and affect the dots’ redox potential.18,19 In short, the surface ligands affect crucially the CQD films’ optical and electronic properties. It is thus intriguing to investigate what these properties are if the use of ligands is minimized. This would require the deposition of ligand-free stoichiometric PbS CQD cores. Here, we implement this approach via the use of S2− as a postsynthetically applied CQD surface ligand. PbS CQDs are synthesized via the classic method originally reported by Hines and Scholes using hexamethyldisilathiane (TMS) as the sulfur precursor.20 Therein, the TMS quantity used is less as compared to the Pb-precursor, which is lead oleate produced by the reaction of lead oxide and oleic acid. Consequently, the final CQD product is Pb-rich with the excess Pb located on the surface of the dots and passivated by oleic acid.9 Excess TMS during synthesis has been found to yield noncolloidal aggregated suspensions of PbS nanoparticles. This © 2016 American Chemical Society
suggests that TMS can act as a sulfur source for termination of the inorganic surface of the dots. We therefore hypothesized that a TMS solution could be simply used to sulfurize asdeposited PbS CQD films, and this process would be accompanied by removal of the oleic acid ligands from the surface of the dots and a drastic decrease of the interdot spacing inside the films. To estimate the impact of that processing method on some of the most basic physicochemical and optoelectronic properties of the TMS-treated films, we compare these with iodide-passivated films made using tetrabutylammonium iodide (TBAI)14 as another example of atomic ligand passivation. Furthermore, iodide-passivated PbS CQD films are used in state-of-the art optoelectronic applications1,2 and thus make a technologically meaningful reference for our sulfur-passivated films.
2. METHODS Chemicals. Lead(II) oxide (99.999%), hexamethyldisilathane (TMS) (synthesis grade), 1-octadecene (technical grade 90%), oleic acid (technical grade 90%), toluene (anhydrous, 99.8%), and tetrabutylammonium iodide (TBAI) (98% reagent grade) were purchased from Sigma-Aldrich; acetone, acetonitrile, and methanol were from Panreac; and bis(trimethylsilyl)selenide (TMSe) was from Gelest. For the synthesis of PbS CQDs with an exciton optical absorption peak in toluene solution at approximately 980 nm, Received: June 10, 2016 Revised: August 24, 2016 Published: August 24, 2016 20315
DOI: 10.1021/acs.jpcc.6b05858 J. Phys. Chem. C 2016, 120, 20315−20322
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Figure 1. (a) TEM micrograph of PbS CQDs on an amorphous carbon film. (b) TEM micrograph of PbS CQDs forming aggregates after being treated on film with TMS. (c) FTIR spectra (C−H stretch spectral region) of PbS CQDs films before and after treatment with TBAI and TMS. (d) Optical absorption spectra of PbS CQDs films treated with TBAI, TMS, and TMSe.
0.45 g of PbO, 1.5 mL of oleic acid, and 3 mL of 1-octadecene (ODE) were mixed and heated at 90 °C under vacuum for approximately 16 h inside a three-neck flask connected to a Schlenk line. Subsequently, the atmosphere of the flask was switched to Ar, 15 mL of ODE was added, the temperature was raised to 120 °C, and 210 μL of TMS diluted in 10 mL of ODE was injected in the flask. Heating then was removed, and the flask was left to cool to room temperature. The CQD product was purified by precipitating the CQDs from solution three times by adding acetone, centrifuging at 3500 rpm for 5 min, and redispersing them in toluene. The final CQD solid precipitate was dried under a stream of N2 before finally being dispersed in toluene (30 g/L) for subsequent thin film fabrication and characterization. All measurements were performed on these CQDs, except when specified otherwise. Fabrication of Thin Films. All related processing and material/chemical handling took place inside a fume hood, because TMS and TMSe react with ambient moisture producing H2S and H2Se gases, respectively, which are both highly toxic. Therefore, great caution is advised when handling these chemicals. For the work described here, TMS and TMSe were kept in a N2 filled drybox, and only very small and diluted quantities were transferred inside sealed containers from the drybox to the fume hood for subsequent processing. CQD films of various thicknesses were fabricated via spin coating in a layerby-layer fashion. Each layer is deposited using the following steps: (1) covering the substrate with the CQD/toluene solution; (2) spinning at 2000 rpm for 30 s; (3) covering the substrate with the ligand of choice diluted in methanol (10 g/L for TBAI and 1−0.02% volume concentration for TMS) and
leaving it as is for 30 s; and (4) spinning at 2000 rpm for 30 s while washing three times with 100 μL of methanol. Steps 3 and 4 were repeated once for the TMS treated films. The TMS treatment also works if acetonitrile is used instead of methanol. Each layer is approximately 15−20 nm thick. Characterization. Optical absorption of three-layer films on soda lime glass substrates was measured using a Varian 5000 UV−vis−IR spectrophotometer. FTIR spectra of 2-layer films on double-side polished Si substrates were measured using a Cary 660 FTIR spectrophotometer. The cross section of CQD films on glass/ITO substrates was imaged using an in-lens secondary electron detector of a FIB-SEM Zeiss Auriga microscope operated at 5 kV. The cross section was prepared in situ by first depositing atop the device a layer of Pt via a gas injection system integrated in the microscope, and subsequently by using the FIB mode of the microscope to mill down a 6 um × 2 um rectangular region of the top surface. TEM was performed with a JEOL JEM 2100 transmission electron microscope. TEM samples were prepared by depositing on a lacey carbon film one drop of CQDs dissolved in toluene, and then allowing the drop to dry on the film. By adjusting the concentration of the CQD/toluene solution to approximately 0.1 g/L, after the drying process a monolayer of closely packed CQDs had been deposited on the carbon film. This approximate CQD monolayer on the carbon film was subsequently treated with TMS by immersing the CQD/lacy carbon specimen inside a 1% v/v TMS/methanol solution for 30 s, and subsequently immersing it in methanol for 1−2 s. During this process, the CQDs do not redissolve in the solution (i.e., they remain attached on the lacey carbon film), while the 20316
DOI: 10.1021/acs.jpcc.6b05858 J. Phys. Chem. C 2016, 120, 20315−20322
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Figure 2. Deconvoluted XPS spectra of (a,b) Pb 4f and (c,d) O 1s spectral regions for films treated with (a,c) TBAI and (b,d) TMS, respectively. Components assigned to PbS and PbSOx are indicated.
TMS results in the removal of the fatty ligands from the surface of the dots. As a result, the CQDs are rearranged on the lacey carbon and become more closely packed. XPS and UPS measurements were performed with a SPECS PHOIBOS 150 hemispherical analyzer (SPECS GmbH, Berlin, Germany) in ultrahigh vacuum conditions (10−10 mbar). XPS measurements were performed with a monochromatic Al Kα Xray source (1486.74 eV), and UPS measurements were done with a monochromatic HeI UV source (21.2 eV). The XPS spectra are calibrated to C 1s at a binding energy of 284.8 eV. The deconvolution of the XPS spectra was performed by fitting a sum of Lorentzian−Gaussian functions (always with 80% Gaussian weighting) to the experimental data. For consistency, the variation of the full-width- at half-maximum (fwhm) of all Gaussian−Lorentzian functions for the same spectral regions and peaks of different samples was restricted to ±0.1 eV. The TBAI- and TMS-treated samples used for XPS analysis were CQD films on 15 mm × 15 mm glass/ITO substrates. The films were fabricated via spin-casting using the layer-by-layer method described above. Five CQD layers were deposited for each sample. For the XPS measurements, all samples were grounded on the sample/stage holder via one of the ITO corners of the substrate. The CQD film on top of the ITO is semiconductive, and no charging was observed during the subsequent XPS spectral measurements. The untreated CQD film (as made CQDs) was fabricated via a single spin-coating step on top of the glass/ITO substrate. This specific film however was not particularly conductive (due to the long and insulating molecules, which prevent inter-CQD charge transfer). Hence, and to avoid charging during subsequent XPS spectral measurements, for the untreated CQD sample (and only for that) additional charge neutralization of the surface of the sample was provided by a source located close to the sample. As a further precaution, for all samples, the In 3d XPS
region (436−460 eV) was monitored to ensure that none of the C 1s and O 1s XPS components is associated with the samplés substrate; no indium was detected. For field effect transistor (FET) fabrication and characterization, PbS QD FETs were fabricated on p-doped silicon with a 285 nm-thick thermally grown oxide layer. The deposition of a 40 nm-thick PbS QD layer on the silicon substrate was asdescribed above. The source and drain contacts were fabricated by thermally evaporating 50 nm-thick Au through masks (Ossila) with 1 mm channel width and 30 μm channel length. The gate was contacted by scratching the silica layer to expose the p-doped silicon. Individual transistors were isolated from neighboring transistors by scratching away the deposited material. Annealing of FETs at 80 °C for 5 min was performed in a N2-filled glovebox. Characterizations were performed in a probe station together with Agilent B1500A in air under dark, or under pulsed laser (635 nm) illumination.
3. RESULTS AND DISCUSSION When TMS is applied on as-spun CQDs, the original oleic acid ligands are removed, leading to a drastic reduction of the interdot distance to the point that PbS QDs are necked (Figure 1a,b). This is the first indication on the effectiveness of TMS in completely removing the oleic acid. The latter is further confirmed by the FTIR spectra of CQD films treated with either TBAI (an iodide source) or TMS (the sulfur source), as shown in Figure 1c. After either treatment, the C−H stretching absorption peaks (Figure 1c), which are attributed to the oleic acid, disappear almost completely. Replacement of the long oleate ligands by shorter ones results in increased electronic coupling between adjacent dots, which reflects upon a red-shift and broadening of their exciton absorption peak.17 Yet, for many previously studied ligands,8,11,12,21 including iodide,14 the exciton peak of the QDs was still preserved (Figure 1d). On the 20317
DOI: 10.1021/acs.jpcc.6b05858 J. Phys. Chem. C 2016, 120, 20315−20322
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The Journal of Physical Chemistry C Table 1. XPS-Measured Stoichiometry and Ionic Charge Balance of PbS CQD Films Treated with Different Ligands TBAI TMS 0.02% TMS 1% TMS 1% + anneal oleic acid (as-made CQDs)
Pbtotal (PbPbSOx)
Stotal (SPbSOx)
Ototal (OPbSOx)
C
I
ionic ratio for inorganic components (Stotal+0.5*I)/Pb
1.00 1.00 (0.06) 1.00 (0.06) 1.00 (0.04) 1
0.70 0.84 (0.05) 0.87 (0.05) 0.90 (0.03) 0.56
0.12 0.24 (0.11) 0.26 (0.13) 0.18 (0.07) 0.96
0.76 0.81 0.67 0.57 9.84
0.64
1.02 0.84 0.87 0.9 0.56
other hand, TMS-treated films do not exhibit any exciton absorption peak (Figure 1d). This is an indication of strong interdot electronic coupling in the respective films to the point of necking.7,17,22,23 These effects also occur for larger dots having an exciton absorption peak at ∼1600 nm, and the respective TEM/HREM micrographs and optical absorption measurements are shown in Figures S1 and S2. In certain areas of the TEM/HREM images, it was observed that adjacent dots seem to exhibit crystallographic alignment having parallel lattice fringes (inset in Figure S1b). In addition, the exciton peak of the TMS treated large CQDs is greatly smeared and seemingly red-shifted >1700 nm. This is an indication that quantum confinement is, to some extent, preserved in TMS-treated films. This last point is further corroborated by the fact that the overall optical absorption traces of the different CQD size TMS-films follow closely the traces of the respective TBAI films as seen in both Figures 1d and S2b. The use of sulfur as a ligand is expected to promote both necking and coupling because at the points of contact between adjacent dots the chemical phase of the interface will be very similar to the phase of the dots’ cores. Moreover, the oxidation state of S provided by the TMS is −2, enabling those sulfur atoms to bind with Pb atoms of adjacent CQDs. It is thus not surprising that the complete disappearance of the exciton absorption peak for the TMS films is similar to what happens upon necking/sintering of CQDs in solids upon annealing.22 At the same time, a chalcogen-rich termination of the PbS CQD surface may promote oxidation and also promote fusion/ necking of neighboring CQDs, with both mechanisms being previously suggested as underlying causes for the smearing of the exciton peak in chalcogen-rich PbS(e) CQD films.7 Finally, it must be mentioned that the processing method presented here can be used not only for the sulfurization of the CQDs surface, but also for its selenization via treating it with the selenium equivalent of TMS, that is, bis(trimethylsilyl)selenide (TMSe). For the sole purpose of demonstrating this concept, Figure 1d contains the absorption spectrum of TMSe-treated PbS dots, and the absorption onset of those appears red-shifted as compared to the TMS-treated dots, in accordance with the narrower bandgap of PbSe as compared to PbS. The bandgap of ternary PbSxSe1−x nanoparticles decreases with increasing selenium content x.24,25 The surface chemistry of the untreated and the TMS- and TBAI-treated films was investigated using X-ray photoelectron spectroscopy (XPS). Specifically, the O 1s, S 2s, S 2p, Pb 4f, C 1s, and I 3d XPS spectra were analyzed to identify the surface elemental stoichiometry and chemical species as a function of surface treatment. Figure 2 contains examples of such analyzed (deconvoluted to component peaks) Pb 4f and O 1s spectra for the TBAI- and TMS-treated films. Our analysis of the Pb 4f spectra (Figure 2a,b) indicates that apart from a 4f 5/2 component that appears at 137.6 eV and can be assigned to PbS,26 a secondary component appears at 138.3 eV. That value is close to the reported values for PbS2O3 (138.4 eV) and
PbSO3 (138.6 eV),27 and therefore we assign the respective component observed in our spectra to Pb2+ bound to a SOx2− oxoanion of sulfur. For clarity of presentation in our respective spectra, this is symbolized as PbSOx. It is evident that this secondary chemical species is increased relative to PbS for the TMS-treated films. This suggests that the PbS CQDs become strongly susceptible to oxidation upon termination of their surface with sulfur. This hypothesis is confirmed by examination of the O 1s spectra shown in Figure 2c,d. Two peaks at 532.3 and 533.8 eV are present in all O 1s spectra and can be, respectively, assigned to oxygen in −COO− (reference value at 532.1 eV)13 and in −OH (reference value at 533.8 eV)28 of residual oleic acid. However, the TMS-treated films also exhibit a strong peak at 531.3 eV, which is assigned to PbSOx (reference values of 531 and 531.7 eV for PbSO3 and PbSO4, respectively).29 Therefore, the O 1s spectra clearly confirm the formation of PbSOx on the surface of the CQDs upon sulfurization. Finally it must be mentioned that while the S 2p and S 2s spectra of the samples were measured and respective analysis confirmed the increase of sulfur in PbS phase upon TMS treatment (compared to TBAI), the S 2s spectra cannot be used for properly monitoring the PbSOx species because the peaks from such species are usually reported to be significantly shifted (>6 eV)27,29,30 to higher energies as compared to the PbS S 2s/S 2p peaks, falling outside the energy range of our measurements. However, the S 2s spectra of the TMS- and TBAI-treated samples (Figure S3) confirm that, apart from a peak at 225.2 eV assigned to PbS,30 an additional peak at 227.2 eV is present for the TMS samples. That peak could be potentially assigned to either polysulfides Sx2− (reference value at approximately 238 eV)30 or indeed Pb−SOx (reference values ranges from 226.1 to 232.7 eV depending on x).27 We note that this peak is also not present/ significant in the S 2s spectrum of the untreated CQDs (Figure S3), which due to their passivation by the original fatty ligands are expected to be less prone to oxidation than either the TMS and the TBAI films. The Pb 4f and O 1s spectra of the untreated CQD films are also presented in Figure S4, and regarding these, we note the following: as expected, the Pb 4f spectrum presents components assigned to PbS and Pb-oleate (the Pb-oleate peaks are shifted by +0.2 eV with respect to the PbSOx components of the Pb 4f spectra of the TMS samples), and the O 1s spectrum is dominated by a new peak at 530.05 eV (shifted by −0.25 eV with respect to the PbSOx component of the O 1s spectra of the TMS samples), which we also assign to oxygen of oleate bound on Pb. We further compared the XPS spectra of TMS samples for different processing parameters: varying the TMS concentration (volume concentration in methanol) between 1% and 0.02% and also applying postfabrication mild annealing at 80 °C for 5 min (under a N2 atmosphere). No qualitative differences were observed between these samples, but quantitative analysis indicates subtle changes in the stoichiometry of the films; the respective results are summarized in Table 20318
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Figure 3. (a) Inelastically scattered electron cutoff region of UPS spectra of differently treated PbS CQD films (on ITO), which are biased at 10 V. Linear fitting (indicated by black lines close to x-axis) is used to extrapolate the values of the Fermi level relative to Evac = 0 eV. (b) Onset region of UPS spectra of films (no bias applied). Linear fitting (indicated by black lines close to x-axis) is used to extrapolate the valence band (EV) position relative to EF. (c) Optical absorption spectra close to onset. Arrows, from right to left, indicate energy values for exciton (only for TBAI-film) and bandgap (EV → EC) absorption of TBAI- and TMS-treated films, respectively. (d) Energy level values for differently treated films according to data in (a)−(c). Energy is scaled with respect to vacuum energy (0 eV).
used during XPS measurements at room temperature. Closing the discussion on the XPS characterization of the CQD films, it must also be mentioned that the Si 2p binding energy region (97−107 eV) was also monitored for all of the TMS-treated films. No feature signifying the presence of Si as a residue of the original TMS precursor or any Si-containing byproduct was detected. As CQD ligands affect the relative positions of the dots’ valence and conduction levels relative to vacuum zero energy (Evac),18,21 we measured the impact of TMS treatment on the respective energy levels using ultraviolet photoelectron spectroscopy (UPS), similar to previous reports.32 The results are summarized in Figure 3. The Fermi energy (EF) level relative to Evac for each sample was calculated using the high energy cutoff (Figure 3a) of the UPS spectra. The position of the valence band edge (EV) relatively to EF was measured as the low-energy onset of the UPS spectra (Figure 3b). Finally, the position of the conduction band (Ec) relative to EV was calculated by adding to EV the electronic bandgap (Eg) of the CQD films as measured by optical absorption spectroscopy (Figure 3c). We note that while for the TMS films Eg is taken as the apparent optical absorption onset at 1.39 eV, for the TBAI films for which an exciton absorption peak exists at 1.21 eV, Eg is taken as 1.21 eV + 0.08 eV = 1.29 eV to account for the 80 meV Coulombic stabilization energy of the confined electrons and holes calculated for the specific CQDs similarly to previous reports.18 All calculated energy levels relative to Evac = 0 eV are provided in Figure 3d, and two major conclusions can be drawn when comparing the TMS with the TBAI samples: (i) the energy levels of all TMS samples are significantly shifted, and
1. All TMS-treated samples have increased S content as compared to the TBAI-treated sample. In addition, the carbon content for each TMS sample is comparable to that of the TBAI sample, and is progressively reduced with increasing TMS concentration due to more effective removal of the original organic ligands. Finally, quantitative analysis of the O 1s peaks confirms that the content of oxygen in nonsulfate forms is very similar between all samples. Thus, the apparent increase of the total oxygen of each of the TMS samples relative to the TBAI sample can be attributed to increased oxidation of the sulfur-terminated surface of the TMS-treated dots. We calculate the hypothetical ionic ratio of the dots’ inorganic component for the various samples, taking into account the elements’ and sulfur oxoanions’ most probable ionic state: Pb2+, S2−, SOx2−, I−. In these calculations, the total sulfur amount was used for estimating the sum of S2− and SOx2− quantities. The results, shown in Table 1, indicate that all TMS- and TBAItreated samples have a significantly improved charge balance ratio of their measured inorganic components with respect to the untreated CQDs. This further indicates that sulfur and the accompanying oxygen act as an all-inorganic shell that balances, to a significant extent, the CQDs’ excess cationic charge. From Table 1 it can also be deduced that while the measured S/Pb ratio of the assumed PbSOx species is, as expected, nearly 1, the O/Pb ratio varies in the 1.7−2.1 value range, which is smaller than the 3 and 4 values expected for lead sulfite and sulfate, respectively. A plausible explanation for this discrepancy, which may also be related to the imperfect charge balance of the TMS samples when compared to the TBAI one, is the previously reported31 volatility of the oxide species of the oxidized surface of PbS, under the radiation and the high vacuum conditions 20319
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Figure 4. (a) Schematic description of CQD FET devices. (b) ISD−VG characteristics of FET devices in the dark under a source−drain bias of 0.1 V. (c) Responsivity versus optical power density for a TBAI and TMS FET devices operated at a source−drain bias of 0.1 V under illumination (λ = 635 nm, laser). (d) Transient photocurrent−time characteristics of FET devices under pulsed laser illumination (red trace for the TMS-treated film, lower blue trace for the TBAI-treated film). For clarity of presentation, the latter is further magnified by 8 times and appears as the middle blue trace as indicated by the arrow.
in Table 2. TMS-treated films exhibit up to 2.7 times higher mobility values as compared to the iodide-passivated dots and
(ii) TMS samples appear as more p-type as compared to the TBAI samples. The aforementioned relative energy shifts can be explained by considering the total dipole moment at the film/vacuum interface as the sum of the CQD core/ligand dipole moment and the ligands’ internal dipole moment.18 The core/ligand dipole moment is the dipole moment of the bond formed between the ligand and the Pb2+ cations located on the surface of the dots. This moment should be higher for the I− ligand (as compared to S2−) due to its higher electronegativity. Therefore, iodide passivation should push the CQDs’ energy levels deeper, as it does. That should also stand true when part of the TMStreated films are not passivated by S2− but by SOx2−, because the internal dipole moment of SOx2− would further contribute to the apparent difference seen between the TMS- and the TBAI-treated films. The p-type shift of the EF of the TMS-treated films as compared to those treated with TBAI further confirms that the TMS-treated films are oxidized. Oxidation of PbS CQDS, especially via the formation of surface sulfate/sulfite species, is known to induce shallow electron acceptors near the valence band that are responsible for the commonly observed p-type character of PbS CQDs in electronic devices. Further confirmation on the aforementioned electronic impact of the TMS treatment and of its accompanying oxidation effect is given by electronic characterization of field effect transistors based on CQD thin films. The FET devices are schematically described in Figure 4a. The source−drain current characteristics as a function of gate bias depend strongly on the TMS concentration used during processing, as shown in Figure 4b. These characteristics, which are all indicative of p-type conduction, are used for calculating the majority carrier (hole) mobility and concentration, and the highest values from at least three devices for each chemical process are shown
Table 2. Highest Mobility and Carrier Concentration Values Extracted from PbS CQD FETs ISD−VG Characteristics μ (cm2/V·s) TBAI TMS 0.02% TMS 0.1% TMS 1%
1.5 2.8 4.1 6.8
× × × ×
10−2 10−2 10−2 10−3
N (cm−3) 5.6 1.6 1.8 2.3
× × × ×
1017 1018 1018 1018
up to 4.6 times higher hole concentration. These results confirm that the dots’ p-type electronic character increases upon surface passivation with sulfur. Relevant to our material/ device characterization, we note that both TBAI- and TMSbased film fabrication methods yield films of almost identical morphology and thickness as studied by FIB/SEM as shown in Figure S5. The TMS-treated PbS CQD films are photoresponsive, and this is illustrated in Figure 4c,d that shows the responsivity and transient photocurrent response of FET devices to monochromatic (635 nm) pulsed laser illumination. Comparison of the TMS 0.1% and TBAI films indicates that the TMS-treated film exhibits approximately 4−100 higher responsivity as compared to the TBAI film as the illumination power density varies from 0.1 to 100 W/m2, respectively. It is known that the photosensing functionality of this type of photoconductive CQD devices relies on trapping of the photogenerated minority carriers (electrons) in oxidation-induced sensitizing trap states. In turn, this allows for the majority carriers (holes) to travel many times via the photoconductive channel under an applied bias.33 Within this context, we ascribe the increased photoresponse of the TMS devices to the midgap traps that are 20320
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The research leading to these results has received funding from Fundació Privada Cellex and the European Community’s Seventh Framework program (FP7-ENERGY.2012.10.2.1) under grant agreement 308997. We also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) and the “Fondo Europeo de Desarrollo Regional” (FEDER) through grant MAT201456210-R. This work was also supported by AGAUR under the SGR grant (2014SGR1548). G.K. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0522).
associated with surface PbSOx species. At the same time, however, due to the high density and position of these traps deep in the bandgap, the TMS films exhibit slower photoresponse than the TBAI films, as is evident in Figure 4d and Figure S6. Closing this discussion, it should be emphasized that some of the specific physicochemical effects that the TMS treatment of CQD films induces, the loss of exciton peak due to enhanced necking and electronic coupling of the CQDs, enhanced surface oxidation, and increased p-type conductivity at the cost of midgap electronic trap formation, are different not only with respect to the effects caused by iodide passivation but also to the effects caused by passivation with commonly used molecular ligands like 1,2-ethanedithiol (EDT). For example, it is well documented8,9,15 that EDT-passivation of CQD films largely preserves their exciton absorption peak, may hinder oxidation of the CQD surface, and may inhibit formation of surface-related electronic trap states. We have recently confirmed that, although optimized EDT treatment efficiently increases the S/Pb stoichiometric ratio of the CQD films to values ≥1 (as studied by XPS),32,34 it also passivates efficiently surface midgap trap states, and yields films with low oxygen content and of high electronic quality as proved by their used in efficient solar cells.34 Still, we had found that the majority carrier mobility of EDT films is smaller (∼10−3 cm2 V−1 s−1)33 as compared to the values for TMS treatment under the same post deposition annealing at 80 °C.
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4. CONCLUSIONS We have presented a simple solution-based method for the post synthetic sulfurization of PbS CQD films. This method results in the efficient removal of the organic ligands originally passivating the surface of the CQDs and their replacement by sulfur. Sulfur termination of the CQDs further renders their surface prone to oxidation, leading to the formation of sulfur oxoanions. This method can be used for controlling the pdoping of the respective films and improving their mobility as compared to reference iodide-passivated CQD films. Overall, the chemical processing strategy presented here may be applicable to a variety of other inorganic chalcogenide semiconductor nanomaterials for which controlling their stoichiometry and surface chemistry means engineering their optoelectronic and physicochemical functionality.
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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/acs.jpcc.6b05858. Additional XPS data, optical absorption data, transmission electron microscopy micrographs, and processed photoresponce measurement data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: 0034 935534182. E-mail: gerasimos.konstantatos@icfo. es. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Shuchi Gupta for providing us with large PbS CQDs having an exciton optical absorption peak at 1600 nm. 20321
DOI: 10.1021/acs.jpcc.6b05858 J. Phys. Chem. C 2016, 120, 20315−20322
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