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Sulfur Capped Germanium Nanocrystals: Facile Inorganic Ligand Exchange Andrew T. Kerr, Diogenes Placencia, Meagan E. Gay, Janice E. Boercker, Denisse Soto, Michael H. Davis, Nathan Alexander Banek, and Edward E. Foos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04045 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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The Journal of Physical Chemistry

Sulfur Capped Germanium Nanocrystals: Facile Inorganic Ligand Exchange Andrew T. Kerr,1 Diogenes Placencia,2 Meagan E. Gay,3,4 Janice E. Boercker,2 Denisse Soto,4 Michael H. Davis,1 Nathan A. Banek5 and Edward E. Foos4* 1

American Society for Engineering Education, Washington, DC

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Naval Research Laboratory, Washington, DC

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Nova Research, Inc, Alexandria, VA

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Naval Surface Warfare Center Indian Head EOD Technology Division, Indian Head, MD

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The George Washington University, Washington, DC

ACS Paragon Plus Environment

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Abstract The development of applications for germanium nanocrystals has been hindered by the limited availability of synthetic methods coupled with poorly understood ligand-exchange chemistry. Herein we describe the synthesis of germanium nanocrystals and ligand exchange experiments designed to establish facile routes toward ligand replacement and, consequently, layers that are amenable to charge-transfer. After assessing thiols, carboxylates, and dithiocarbamates, sulfur dissolved in 1-ocatadecene was determined to be the most amenable to ligand exchange, with over 95% of the initial alkylamine ligand replaced as determined by FTIR. These results were in good agreement with DFT calculations showing a strong preference for Ge-S bonding. The materials were fully characterized via powder X-ray diffraction, FTIR, TEM, XPS and UPS. This new ligand exchange procedure provides a possible route toward the fabrication of thin films that may be employed in such applications as photovoltaic devices. Introduction Inorganic nanocrystals (NCs) represent a potential route to next-generation solar cells,1-3 lightemitting devices,2,4-6 electronics,2,7-9 bioimaging10,11 and batteries.12-14 The broad interest in these materials results from quantum confinement effects that may be tuned via careful control of the size, shape, composition, and surface characteristics of the nanocrystals.1,2,4,7,12-14 Germanium NCs possess a bulk band gap of 0.67 eV that can be tuned to the near-IR, making them attractive for solar energy conversion and photodetection,15-17 bioimaging,14,18,19 and solid-state lighting.14,18-23 Recent advances in the size-selective synthesis of Ge NCs have shed additional light on the properties and stability of these materials,9,14,16,19,21-25 enabling their use in devices. The original goal in this work was to fabricate a solar cell that employs the attractive


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characteristics of Ge while circumventing the need for high temperature sintering or sensitivity to air that are caveats exhibited by the CdTe and PbS material systems, respectively.26-29 By using oleylamine capped Ge NCs followed by a novel ligand exchange process that makes use of elemental S dissolved in alkene, the preparation of thin films that could be implemented in the fabrication of such devices was successfully demonstrated.. Given the interest in Ge NCs as a photovoltaic material, there are surprisingly few examples of solar cells prepared using solution processed Ge NCs.15-17 It is important to note that ligands, and the ability to manipulate them via displacement, play a significant role in the overall properties of the NCs (e.g. charge-transfer characteristics).14,16,22,26-28As Kauzlarich et al. show, long-chain aliphatic ligands passivate the NC surface and provide stability in air for a limited time. However, these same ligands prevent efficient charge-transfer which would in turn inhibit properties such as photo-response or electroluminescence.22 The replacement of the insulating oleylamine ligands with straight-chain aliphatic thiols and thiolates provide efficient chargeseparation that was observed via surface photovoltage spectroscopy.22 Further examples of ligands that have been substituted on the surface of Ge NCs include phosphines,30 Na+, methylammonium, dodecylammonium, and hydrazinium.16 Wheeler et al. similarly show that control over the surface ligands provides better control over the resulting material properties and subsequently prepared a solar cell via treatment of oleylamine-capped Ge NCs with sodium tertbutoxide to form continuous Na+ passivated films.16 It is important to note that the starting ligand should be labile, as covalent bonds to the surface of the Ge often lead to kinetically-inert interactions that do not undergo exchange. This amine binding to the surface in the case of the oleylamine provides a labile handle for ligand replacement and is likely due to the dative bonds formed by the amine as opposed to the covalent bonds that are formed with 1-octadecene.16,22


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Beyond the traditional organic ligand exchange operations, inorganic ligands have become a recent focus of interest with an emphasis on species that are both passivating and allow for efficient charge-separation. Wheeler et al.16 and Talapin et al.31 have made strides in the formation of nanocrystals that are passivated with inorganic ions that exhibit small ionic radii. These ligands not only passivate the surface, but allow for reduced inter-nanocrystal distances. Sulfur has been employed as one such inorganic ligand which exhibits a strong passivating effect with Ge,32 and in the past has been deposited via the thermolysis of a dithiocarbamate33 or thiol.14 Alternatively, elemental S dissolved in long-chain organic alkene solvents has been employed as a S source in the formation of metal sulfide nanocrystals.34-39 Though the mechanism of exchange35,37,38 is admittedly not fully understood, organosulfur compounds are formed upon heating sulfur in the presence of 1-octadecene.40 Though some control over the identity of the organosulfur species is afforded by controlling the duration of heating, a complex mixture often persists.40 This route provides a relatively benign solution as opposed to the use of CS2 or the heating that would be required in the use of the thiols and dithiocarbamates. Thus, we hypothesized that this reagent could also be employed in ligand exchange operations as a method of S passivation. The efforts reported herein make use of the known routes to size-controlled Ge NCs,21 with an emphasis on use of the labile oleylamine ligand while verifying and expanding on the need to slowly heat the reaction to isolate the desired NC sizes.21 We have found that this asprepared ligand may be exchanged via treatment with elemental sulfur dissolved in 1-octadecene (ODE) to yield S passivated Ge NCs. This S exchange was further probed via density functional theory (DFT) calculations which provided good agreement with our observations that the elemental S was replacing the oleylamine. Our goal with this exchange chemistry was to utilize a


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small ligand that would not inhibit charge-transfer while effectively passivating the surface of the Ge and be amenable to low-temperature device fabrication. X-ray and UV photoelectron spectroscopy (XPS/UPS) were then collected on a set of ligand exchanged materials to determine the work functions and ionization potentials of the Ge NC films as well as inform the fabrication of diodes using these materials. Experimental Section General Considerations: S powder (100 mesh sublimated), oleylamine (technical grade 70%), 1-octadecene (technical grade 90%), n-butyllithium (1.6 M in hexane), toluene (≥99.5%), methanol (≥99.5%), and ethanol (≥99.5%) were all purchased from Sigma Aldrich. Germanium (IV) iodide (99.999%) and germanium (II) iodide (99.99%) were purchased from Strem Chemical, Inc. The oleylamine and 1-octadecene were degassed and stored over molecular sieves prior to use, while the n-butyllithium and germanium salts were stored under inert atmosphere in the glove box.

All other chemicals were used as received. Unless noted otherwise, all

manipulations were carried out on a vacuum/inert gas manifold using standard Schlenk technique. Ge nanocrystal Synthesis: The synthesis that follows was adapted from the procedure outlined by Neale et. al.21 ~5 nm Ge NC: In an Ar filled glove box, oleylamine (8.58 g; 32 mmol) was loaded into a 50-mL three-neck round-bottom flask with GeI2 (0.195 g; 0.6 mmol) and GeI4 (0.348 g; 0.6 mmol) and fitted with a vacuum adapter and two stoppers. The flask was removed from the glove box and the mixture sonicated for about 5 minutes then transferred to a Schlenk line. Under Ar purge the stoppers were replaced with two rubber septa, one of which was filled with a thermocouple inserted into the reaction solution. The mixture was heated under Ar to 200


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°C at about 5 °C/minute. The coloration evolved from a pale yellow to a dark brown/red, progressing through orange and red while heating. Upon reaching temperature, n-BuLi (2.5 mL of a 1.6 M solution) was quickly injected into the Ge solution. The temperature of the mixture decreased by about 10 – 15 °C upon injection. At this point, the solution was allowed to return to 200 °C and was then further heated to 300 °C at a rate of ~10 °C/minute. After 1 hour, the reaction was allowed to cool to room temperature over about 10 minutes by removing the heating mantle. The nanocrystal solution was transferred to a centrifuge tube and residual solution in the reaction flask was collected by rinsing with toluene (about 15 mL) under ambient conditions. The nanocrystals were precipitated by adding ~50 mL ethanol, and separated by centrifugation (5 minutes at 4000 RPM). The supernatant was removed by decanting and the precipitate redissolved in 20 mL of toluene. The nanocrystals were further rinsed via an additional precipitation with ~30 mL of a 1:1 (v/v) methanol/acetone mixture, and the resulting solid recollected by centrifuge. This material was dissolved in 20 mL of toluene to result in a stock solution with a concentration of about 10 mg/mL. This solution was stored in the glove box until use. It was observed that allowing this stock solution to stand for an extended time resulted in the precipitation of insoluble Ge NCs. Preparation of Ligand Exchange Solutions: S in 1-octadecene was prepared by combining elemental S and ODE in a round bottom flask to target a final concentration of 0.1 M.34 This flask was then heated under vacuum at 100 °C to degas the solvent. The mixture was then brought to 180 °C under Ar with stirring to dissolve the sulfur resulting in a pale yellow solution. Higher concentrations of S in ODE (0.80 M) required longer heating times to dissolve. Following preparation these solutions were moved into a glove box to maintain an oxygen-free environment.


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Hydrazine solutions (0.1 M or 5 M) were prepared by dissolving an appropriate amount of hydrazine in acetonitrile. The solutions were then placed into a glove box to maintain an oxygen-free environment. Ligand Exchange: Combining the desired amount of Ge NC stock with an appropriate ligand in solution results in the formation of large agglomerates of Ge NCs. For example, the stock Ge NC solution was combined with the elemental S/ODE solution in a 1:1 ratio by volume. Upon stirring, both in the presence and absence of heat, a solid would begin to precipitate out of solution within 1 – 5 minutes. This solid was collected via centrifugation (5 minutes at 4000 RPM) and washed with toluene for analysis. Similarly, upon combining the Ge NC stock with the hydrazine solution in a 1:1 ratio by volume, a precipitate would form immediately, as observed previously.22,24 This solution was stirred for an additional 30 minutes at room temperature to allow for a complete reaction. The solid was collected via centrifugation (5 minutes at 4000 RPM) under ambient conditions and washed with toluene at least two times to verify that no residual hydrazine remained on the nanocrystals prior to analysis. It is of note that if the reaction was exposed to air prior to transfer to the centrifuge tube (i.e. the Ge NC and hydrazine are mixed outside of the box) or if hydrazine persists on the products after washing, the nanocrystals would turn from a black/red solid to white. This is likely a result of oxidation of the nanocrystals; however, attempts to definitively identify the white precipitate were not successful. Film formation: Prior to film formation experiments an additional precipitation of the stock Ge NCs via a 1:1 (v/v) MeOH/acetone mixture was performed. Films of Ge NCs were prepared for analysis or device fabrication via dip coating or spin coating. In the case of dip coating, pieces of


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glass microscope slide were cleaned with EtOH, dried, then dipped into the Ge NC stock solution and allowed to evaporate. Following this stock solution exposure, the samples were dipped into the sulfur or hydrazine ligand solutions and then immediately rinsed in ethanol or toluene to clean the surface of excess ODE or acetonitrile, respectively. The spin coated samples were similarly prepared by coating Ge NC stock solution on the desired substrate then immediately spinning at 800 rpm for 30 seconds, followed by dipping into the sulfur or hydrazine ligand solutions and rinsing in ethanol or toluene. In both cases this process could be repeated multiple times to obtain a thicker coating of Ge NCs on the substrate. Glass substrates were used for profilometry or FTIR analysis, gold coated glass substrates for XPS/UPS data collection, and ZnO coated ITO glass for the devices. Device Fabrication & Testing: Commercial indium-tin oxide-coated glass (Delta Technologies Ltd.) with approximately 150 nm and a sheet resistance ~10 ohms per square was used for all experiments. Substrates of 1 in. x 1 in. size were scrubbed with 10% Triton X-100 (Sigma Aldrich), followed by sonication in 10% Triton X-100 (15 minutes), nanopure water (5 minutes), and absolute ethanol (15 minutes). After, the ITO on the substrates was patterned with a positive photoresist (Rohm and Haas – S1813), followed by the necessary exposure, etching, heating, and removal steps. Each patterned ITO substrate was etched with aqua regia (3:1 ratio) at 120 °C for ca. 75 s. Prior to use, each substrate was sonicated in absolute ethanol for 15 minutes. ZnO was synthesized and deposited according to a procedure detailed elsewhere.41 After deposition and processing of ZnO, 2,3,4,5,6-pentafluorophosphonic acid was deposited according to the procedure developed by Cowan et al.42 The Ge NCs (~5 nm) were deposited via spin coating as described above using 0.80 M S/ODE. An ~200 nm total layer of Ge NC was deposited using ~40 spin coating/ligand exchange cycles.

Substrates were then placed into an evaporator


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connected to an inert gas glovebox, and deposition of MoOx (15 nm), Au (50 nm), and Ag (100 nm) took place. Current-voltage (J/V) were obtained for at least 16 devices per substrate, with Figure 10 displaying the “champion” device. The geometry of the device design provided for an active area of 0.019 cm2, which were all tested in air. Testing under an applied field, under ambient conditions, resulted in the devices degrading rapidly (within 2 voltage sweeps), while the rest of the devices proceeded to degrade under ambient, noticeable during the systematic testing of all devices. Data acquisition was conducted with a Keithley 4200, while illumination was provided by the spectral output from a 150 W solar simulator (Newport Corporation) using an AM 1.5 G filter. The irradiance (100 mW/cm2) of the solar simulator was adjusted using a standard Si photodetector (818-SL-L, Newport Corporation) that had been crosscalibrated by a reference Si cell traceable to the National Renewable Energy Laboratory (NREL). Modeling: Ligand interactions with the Ge NCs were probed using high level atomistic quantum chemical calculations in order to acquire information about the stabilities of the exchanged products. Hexylamine was used as a surrogate for oleylamine in order to reduce the complexity of the calculations. This substitution assumes that the C-C double bond in oleylamine does not participate in the binding to the Ge NC surface. Although the identity of the sulfur species in the 1-octadecene is likely complex, single atoms were employed to provide a less complex scenario in which the focus was on the sulfur binding. Optimizations of the geometries of the ligand as well as the Ge clusters containing 8 or 10 atoms were performed separately. Once the minimum energies were found, the ligand and Ge NC structures were combined to obtain the minimum energy conformation for each combination. All geometries and harmonic vibrational frequencies calculated herein were obtained using close-shell density functional theory (DFT) methods,


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employing the M0643 functional and the aug-cc-pVDZ basis set44 implemented in the GAMESS45,46 quantum chemistry code. Characterization: Transmission electron microscopy (TEM) images were collected on a JEOL 2200FS operating at 200 kV and equipped with an EDS spectrometer. Several batches of Ge NCs were analyzed by TEM and the resulting sizes were comparable to the sizes obtained from PXRD using the Scherrer equation. Thereafter PXRD data was used to estimate the Ge NC sizes before ligand exchange and device fabrication. EDS data were also collected in the TEM to verify the presence of Ge and S in the exchanged materials. Powder X-ray diffraction (PXRD) patterns were collected by depositing solid Ge NCs while still damp with toluene on a glass substrate. Evaporation of the solvent allowed for the collection of data on a Bruker D8 Discover diffractometer using CuKα radiation (50kV, 1000µA). The data were collected between 25° - 70° 2θ with a 2-D detector at a step size of 15° and verified via the calculated pattern for Ge metal. The crystallite sizes of the NCs were estimated via the DebyeSherrer method that was applied in the Eva software. UV-Vis data were collected on a Varian CARY 100 Bio UV/Vis spectrometer. The dissolved nanocrystals were placed into a quartz cuvette and diluted with additional toluene. Data were collected between 300 to 900 nm. As the Ge NCs do not exhibit a specific absorption band, the data were plotted in eV against the log of the absorbance as described previously.30 FTIR samples were prepared by drop-casting, dipping or spin coating stock solution on a glass substrate followed by treatment with the appropriate ligand of choice. The insoluble aggregates which formed upon the reaction of Ge NCs with ligands in solution were placed directly onto glass substrates. FTIR data were collected on a Thermo Nicolet Avatar 370 FTIR using an ATR


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accessory. In order to compare the various extent of ligand exchange, the same ~5 nm Ge NCs were deposited onto three separate glass substrates. The substrates were then treated with the ligands (hydrazine, 0.1 M S/ODE) or remained untreated as a control and all three were rinsed with toluene after each step to maintain the same number of exposures to solvent. Data were then collected by placing the substrates face down on the ATR surface and a comparison between each sample could be drawn to the extent of ligand replacement. The experiment was repeated with 0.8 M S/ODE on a new batch of Ge NCs to determine the extent of ligand replacement. The solid samples and thin films were prepared for X-ray fluorescence (XRF) similar to the FTIR samples, to verify the presence of both Ge and S in the NCs that were exposed to S reagents. The solid samples were placed in a sample holder with a mylar film in front of the Xray eye. The thin film samples were placed directly over the detector eye. The XRF data were collected on a Shimadzu EDX-700. TGA data were collected on a Perkin Elmer Pyris 1. The large insoluble agglomerates formed from precipitation of the ligand exchange reaction conducted on a solution of ~5 nm Ge NC were placed into a platinum pan with a platinum stirrup and hung on a glass hook. The furnace was then raised under the pan and heated at a constant rate (5° - 10°/minute) to 600°C under a nitrogen flow. XPS/UPS characterization of the Ge NCs (~5 nm) was carried out in a photoelectron spectrometer (Kratos, model Axis-Ultra) using an Al Kα source at 1486.6 eV and He(I) excitation source (21.2 eV) for XPS and UPS, respectively. A -10.0 V bias was applied to the sample to further enhance collection of the lowest kinetic energy electrons.47-49 For XPS data, the spot size was 300 x 700 µm, while for UPS, spot size ranged between 1-3 mm. Ge NC samples


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were spin coated onto gold-coated glass at various thicknesses in order to determine the appropriate layer thickness to prevent charging during UPS measurements. Charging (or lack thereof) was verified by acquiring C 1s spectra with and without the charge neutralizer to determine if any shifts in the core level peaks were noticeable. Results and Discussion Synthesis and passivation chemistry: Oleylamine-capped Ge NCs were synthesized via an adapted reported procedure. The identity and size distributions of the Ge NCs were verified via TEM (Figure 1), EDS (supporting information Figure S1) and PXRD (Figure 2). Similar to previous studies,21 a portion of the Ge NCs synthesized were polycrystalline/amorphous as evidenced by the poorly defined grain boundaries. Though these polycrystalline products were observed, we were not able to determine the extent to which this occurs in a given sample. Furthermore, the extent to which these polycrystalline species were present may influence the properties of any photovoltaic devices prepared with these materials.


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(a)

(b)

20 nm

20 nm

(c)

(d)

10 nm

10 nm

Figure 1. TEM images showing the size and size distribution of two different samples of Ge NCs, 3.0 ± 0.4 nm (a and c), and 5.1 ± 0.7 nm (b and d). Lattice fringes observed in the magnified images index to Ge (JCPDS #9008567)

Figure 2. PXRD patterns that exhibit the sizes of Ge NCs that were synthesized. The black lines show the position of Ge (JCPDS #9008567). In addition to TEM and PXRD, the size could be monitored, to a lesser extent, via the use of UV-Vis spectroscopy. Figure 3 shows the shift in absorption that is observed upon changing the


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size of the product from ~3 to ~5 nm. The linear UV-Vis spectra can be found in the supporting information, Figure S2. It is of note that the data must be plotted on a log scale due to the lack of any clearly defined absorbance features, therefore this method provides only an estimate at which the absorption begins.30 This absorption edge may then be used to estimate the band gap of the Ge NCs. By changing the size of the Ge NCs via introduction or omission of GeI2, the absorbance onset shifted. As the GeI2 undergoes disproportionation, Ge0 seeds were produced. Upon the reduction of the remaining GeI4, additional crystal growth occurred preferentially at these preformed nucleation sites.21 Therefore the introduction of GeI2 would result in larger nanocrystals, and reducing the Ge(II):Ge(IV) ratio would further increase their average size, but in our experience also increase the size distribution, as fewer seeds would be present at these lower ratios. Once these trends were established, PXRD data was sufficient to provide estimated sizes for the exchange chemistry and device fabrication experiments.


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Figure 3. UV-Vis spectra illustrating the red shift in the absorbance of the nanocrystals as the average size increases.

In addition to the starting material ratio, it is also of note that the heating rate of this reaction strongly influenced the size of the material isolated.21 During our experiments we have observed that attempts to heat at faster rates prior to n-BuLi addition resulted in the comproportionation of the GeI4 and Ge metal to reform GeI2 as evidenced by the loss of the dark color indicative of Ge0 and the change back to a yellow color. When the n-BuLi was injected immediately upon observing this color change at 200 °C, ~3 nm Ge NCs were isolated from the reaction similar to when GeI2 was used as the only Ge reagent. When the reaction mixture was instead allowed to stir at 200°C for ~20 minutes before the injection was performed, the dark color returned, presumably as a result of reestablishing the Ge(II) disproportionation equilibrium. However, the


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subsequent injection of n-BuLi at this point would result in ~15 nm Ge NCs (as determined via PXRD) that exhibit a wide distribution of sizes in the final material. Due to solubility issues resulting from agglomeration, the 15 nm particles could not be characterized via UV-Vis and TEM. These observations were critical to the isolation of high quality material and it should be noted that the slow heating rates should be followed carefully if targeted sizes are desired. It is of note that 1-octadecene capped Ge NCs were initially employed in an effort to perform ligand replacement. While the ODE capped Ge NCs were more stable than those synthesized with oleylamine (as evidenced by longer shelf life and solubility), no evidence of ligand replacement was observed. This observation is likely due to the stability of the covalent bond formed between the ODE and the Ge.16,22 Additionally, prior to using a S/ODE solution for the ligand exchange, we attempted to replace the as-prepared oleylamine with thiols and dithiocarbamate ligands via sonication or by heating and stirring the ligand of choice with the oleylamine-capped Ge NCs in toluene. This resulted in partial ligand replacement as evidenced by the presence of S in the XRF data that were collected on these preliminary samples. However, the S:Ge ratios were always lower than expected. These results lead us to look into S/ODE and hydrazine solutions in an effort to fully replace the oleylamine. As our desired product would exhibit short inter-nanocrystal distances in a film while the Ge NCs remain passivated, we decided to attempt the ligand replacement with S in 1-ODE. This S in ODE solution has proved successful as a sulfur source for metal-sulfide nanocrystal synthesis34-39 and could potentially serve as a route to passivated metal nanocrystals. Since the size of a single S atom is much smaller than a thiol molecule, but presumably will interact with the Ge in a similar fashion, if successful the S would be expected to support shorter inter-nanocrystal distances. Upon reaction of the oleylamine-capped Ge NCs with the S/ODE solution (0.1 M), a solid began


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to precipitate that would no longer dissolve in any common solvent. The presence of sulfur in this precipitate was verified via XRF at a greater S:Ge ratio than was observed in the attempted ligand replacement reactions that were performed with thiols or the dithiocarbamate ligands. Though the ratio of S:Ge in the product was greater, complete ligand replacement was not observed by FTIR (Figure 4a), but oleylamine was displaced. Comparison of the samples before and after ligand replacement shows a small reduction in the amount of hydrocarbon chains present on the surface. The full FTIR spectra can be found in the Supporting Information, Figure S3. For comparison, nanocrystals were also treated with 5 M hydrazine in an attempt to strip the ligands completely from the surface of the Ge NCs. Upon comparing all three samples (untreated, S treated, and hydrazine treated), a trend of ligand loss is observed. As the oleylamine is displaced from the nanocrystal surface upon treatment with hydrazine, the hydrocarbon peak in the FTIR diminishes and almost entirely disappears, consistent with previous work.22 This ligand displacement was further verified via thermogravimetric analysis as shown in Figure 5. As would be expected, when the treated samples are compared to the untreated Ge NCs, a smaller mass loss corresponding to the ligand is observed (Figure 5).


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Figure 4. FTIR data exhibiting partial ligand replacement on the Ge NCs when treated with the 0.1 M S/ODE (a) and full ligand replacement when treated with the 0.80 M S/ODE (b).

Figure 5. TGA data showing mass loss of Ge NC samples treated with 0.1 M S/ODE, 0.80 M S/ODE and hydrazine in comparison to the untreated starting material. Both the 0.80 M S/ODE and hydrazine treated Ge NCs appear to have volatiles on the surface of the nanocrystals that make the ligand mass appear artificially high (see text).


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Increasing the concentration of the S/ODE solution to 0.8 M was then examined in an attempt to replace additional oleylamine. Upon the combination of the Ge NC stock solution with the concentrated S in ODE products were obtained that exhibited greatly reduced hydrocarbon peaks in the IR data (Figure 4b). The full FTIR spectra can be found in the Supporting Information, Figure S4. The ligand loss was estimated to be nearly 100% though we assume some residual oleylamine may persist on the surface and oxidation to some extent has likely occurred, and the products were no longer soluble in any common solvents. Though the use of FTIR to probe ligand loss or exchange has been described previously,16,22 it is arguably qualitative. However, there is a definitive change in the spectra upon treatment with the S/ODE or hydrazine, and while some loss of Ge NCs upon treatment is expected, it is unlikely that this would account for the entire loss of signal observed in Figure 4. This is also supported by the TGA data in Figure 5. With regard to the TGA data, it is important to point out that it appears as though both the hydrazine and the 0.80 M S/ODE treated Ge NCs have lost more mass than those that were treated with 0.1 M S/ODE. Though the overall mass loss is greater, in these two samples there was a significant evolution of volatiles (~7 – 8%) between 50 and 100 oC presumed to be weakly bound atmospheric water or residual solvent present on the inorganic surface. This would be consistent with a reduction of surface organic species. Examining the higher temperature event between ~300 and 500 °C, which can be attributed to ligand loss, suggests that much less mass is evolved in this region from the 0.1 M S (~11%), hydrazine (~9%) and 0.80 M S (~8%) treated Ge NC samples compared to untreated samples (~23%). Overall, these results support the S/ODE treatment as a low temperature and more benign alternative for the replacement of oleylamine on the Ge NC surface as opposed to the harsher


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treatments that have been employed in the past. Further, this route is amenable toward solution processing under ambient conditions (vide infra). Modeling: The experimental observation of S displacement of oleylamine on the surface of a Ge NC has not to our knowledge been described previously, thus modeling the exchange chemistry using DFT was expected to provide additional insight. Clusters containing 8 or 10 Ge atoms were used to model the Ge NC surface, while hexylamine was used as a substitute for oleylamine. Three initial structures for each (not shown) were chosen in order to sample multiple local minima and thus obtain an indication of the range of interaction energies. The energies and structures of these minimized clusters can be found in the SI, specifically Table S1 and Figures S5 and S6. The M06 optimized structures slightly overestimate all bond lengths for Ge8 with the averages of all bonds ranging from 2.55 Å to 2.61 Å. For Ge10 the average bond lengths range from 2.44 Å to 2.55 Å, which are relatively close to previous models of Ge clusters calculated with DFT/B3LYP theory50. However, the M06 family of functional calculations employed in the present study has been shown to outperform B3LYP in predictions of both structures and energetics of small clusters51 as well as systems with significant dispersion interactions.52 The binding energies of the Ge8 clusters with hexylamine are primarily governed through the intermolecular interactions of the Ge atoms with the amine nitrogen atom (Figure 6). The most stable ligand cluster, Ge8·hexylamine, with a binding energy of 2.73 eV, has the amine molecule orientated close to 4 Ge atoms, which allows for a close intermolecular approach of the terminal amine group, with N----Ge distances ranging from 2.9 Å to 4.8 Å. Similar N----Ge interactions were observed in both Ge10·hexylamine conformations (as shown in the SI Figure S4). In both cases the ligand aligns with the Ge10 cluster positioning the amine group between 5


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Ge atoms with N----Ge distances ranging from 2.7 Å to 5.1 Å. The second Ge8·hexylamine configuration, which has a binding energy of 2.48 eV, exhibits a simple configuration in which the amine group interacts with only one Ge atom in a distance of 2.4 Å. In both Ge8·hexylamine conformers the angle in the amine group reduced to 1050 while for Ge10·hexylamine the amine angle reduced to 1060. Comparison of the Gen·hexylamine (n=8 or 10) structures revealed that all the conformers were held together primarily through electrostatic interactions between the amine group and Ge atoms. Furthermore, the variation in binding energies across the complete set of dimers considered in this study was rather small, with an average of 2.64 eV. In contrast to the above results, for all examples of Gen·S (n=8 or 10) the metal cluster deformed and Ge----Ge distances elongated (Figure 6 for n=8 and in the SI Figure S7 for n=10). The binding energy for these conformers tended to depend on the number of Ge atoms available to interact with sulfur, but overall appeared to be far more stable than the structures calculated with the amine. Further, the sulfur appeared to form a covalent bond to the Ge. Together these results suggest that the sulfur would likely displace the amine for a more stable conformer. A final calculation was then performed in which both the hexylamine and a sulfur atom were included and placed at a starting distance of 4.3 Å from a Ge10. These were allowed to compete to see which species would bind more readily. This calculation again resulted in the sulfur atom forming a covalent interaction with the Ge atoms and the amine remaining at a distance more consistent with an electrostatic interaction to both the S and Ge atoms as seen in Figure 7. All of these results support the experimentally observed exchange chemistry described in the previous section.


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Figure 6. Minimized structures of Ge8·hexylamine with binding energies of 2.73 eV (a) and 2.48 eV (b), and Ge8·S with binding energies of 128.98 eV (c) and 130.56 eV (d) optimized using M06/aug-cc-PVDZ. Two examples of each interaction are shown that employed the two lowest energy conformations for comparison.


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Figure 7. Ge10 in the presence of both sulfur and hexylamine modeled at a distance of 4.3 Å before the calculation (a), and the bound sulfur and electrostatically bound hexylamine after the calculation (b).

Photoelectron spectroscopy characterization of films: Photoelectron spectroscopy scans (XPS/UPS) obtained for the various surface treatments are shown in Figure 8. XPS data (Figure 8a) shows the assignments for Ge2+, Ge3+, and Ge4+.53-58 For the Ge 3d binding energy components, assignments for all peaks have been previously attributed to oxides, while significant overlap exists between these species and GeS, GeN.53-57 In the absence of literature assignments for Ge nanocrystals with our novel surface treatments, we proceeded to treat the binding energy components for the hydrazine and hydrazine/sulfur surface treatment as a convolution of either Ge4++GeN (hydrazine), Ge4++GeS (sulfur), or Ge4++GeS+GeN (hydrazine/sulfur). Any further deconvolution is extremely difficult as many of the species that may be present fall under the observed peaks. The species associated with the varying oxidation states are most likely due to the oxides of germanium (e.g., GeO, Ge2O3, GeO2), although other compounds associated with sulfur and/or nitrogen treatment cannot be ruled out. Quantification of sulfur and nitrogen content from the ligand exchange was therefore


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unattainable, further accented by the relatively low photoionization cross-section of sulfur and nitrogen, which would require long acquisition times known to produce x-ray-induced changes at the sulfur core levels.59 When comparing the as-deposited film to the other surface treatments, a growth in the convoluted peak is seen, which is in full agreement with an increase in new Ge species being formed via the introduction of GeS and/or GeN surface species. It is of note that in this interpretation of Ge 3d there is no indication of the presence of Ge0. Upon looking closely at the Ge 2p binding energy component, which can be found in the supporting information, a peak at 1217.3 eV is observed which indicates the presence of Ge0 (Figure S8). Taking into consideration that these films were prepared under ambient laboratory conditions, a certain level of oxidation can be expected, and further bolsters the argument for the preparation of these films under fully inert conditions. UPS data (Figure 8b) shows the secondary kinetic energy edge (SKE - left panel), used to determine the work function of the system, and the high kinetic energy edge (HKE – right panel), which is used to determine the ionization potential (IP).60-63 A linear subtraction of the underlying contributions from the He (I) and He (II) photoemission signatures of the Au substrate was performed to obtain the IP, as described in detail by Munro et al.64 The work function changes between the as-deposited and hydrazine treatments are smaller in magnitude (3.2 eV and 3.0 eV, respectively) relative to the S and hydrazine/sulfur treatment (2.3 eV and 3.6 eV, respectively). Changes in the work functions to either lower or higher values when compared to the as-deposited material can be attributed to the orientation of the dipole post-treatment, either towards the nanocrystal for lower values, or opposite for the higher value).64-66


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Figure 8. a) XPS data of the Ge 3d fitted for Ge2+, Ge3+, and Ge4+; b) UPS data for all surface treatments showing the work function (left) and energy with respect to the Fermi level (right). Based upon the photoemission data, we attempted to fabricate devices from the S-treated Ge NCs due to the ability of this method to provide a less chemically-harsh treatment for thin film formation. Further, we modified the ZnO surface with a small-molecule phosphonic acid to stabilize both its work function and overall surface chemistry.42 While diode-like behavior was consistently observed in the IV curves, we were unable to obtain definitive photovoltaic responses in this system. One example of a device that exhibits a weak photovoltaic effect similar to that prepared by Wheeler et. al.16 is shown in the supporting information, Figure S9. It is of note that there is likely oxidation of the Ge NCs to some extent and thus the presence of GeO2 should not be overlooked. The GeO2 likely contributes to the active layer as the fabrication is performed under ambient conditions. This observation may lend itself to the future preparation of devices that can perform at a higher level if prepared under conditions that mitigate oxidative effects.


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Conclusions In this work, we have prepared solution processed thin films of Ge NCs diodes that no longer required harsh chemicals. This was accomplished through the experimental and theoretical development of a novel exchange chemistry based on the use of S in 1-ODE as an inorganic ligand source.

Employing this ligand exchange procedure successfully required

careful synthetic control over the Ge NC starting material and use of oleylamine as the initial ligand on the surface of the Ge. We expect that the exchange chemistry outlined here will be applicable to alternative nanocrystal material systems, where the ability of S to serve as an anionic surface passivating ligand can be further exploited. Supporting Information. Additional EDS data and DFT modeling results, photovoltaic device data, in addition to complete FTIR and UV/vis spectra. ACKNOWLEDGMENT This work was supported through an In-House Laboratory Independent Research (ILIR) project funded by the Office of Naval Research (ONR). ATK acknowledges the American Society of Engineering Education (ASEE) Postdoctoral Program and MHD acknowledges the ASEE Naval Research Enterprise Internship Program (NREIP). We also thank Michael Wagner (The George Washington University) for the use of the TGA and XRF instrumentation, and Joseph Tischler (NRL) for additional instrumentation access and support. References: 1. Debnath, R.; Bakr, O.; Sargent, E. H. Solution-Processed Colloidal Quantum Dot Photovoltaics: A Perspective. Energy Environ. Sci. 2011, 4, 4870-4881. 2. Panthani, M. G.; Korgel, B. A. Nanocrystals for Electronics. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 287-311.


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Sulfur-Capped Germanium Nanocrystals: Facile ... - ACS Publications

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