All-Inorganic Germanium Nanocrystal Films by Cationic Ligand

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Letter pubs.acs.org/NanoLett

All-Inorganic Germanium Nanocrystal Films by Cationic Ligand Exchange Lance M. Wheeler,*,† Asa W. Nichols,†,‡ Boris D. Chernomordik,† Nicholas C. Anderson,† Matthew C. Beard,† and Nathan R. Neale*,† †

Chemistry & Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States ‡ Department of Chemistry, West Virginia Wesleyan College, 59 College Avenue, Buckhannon, West Virginia 26201, United States S Supporting Information *

ABSTRACT: We introduce a new paradigm for group IV nanocrystal surface chemistry based on room temperature surface activation that enables ionic ligand exchange. Germanium nanocrystals synthesized in a gas-phase plasma reactor are functionalized with labile, cationic alkylammonium ligands rather than with traditional covalently bound groups. We employ Fourier transform infrared and 1H nuclear magnetic resonance spectroscopies to demonstrate the alkylammonium ligands are freely exchanged on the germanium nanocrystal surface with a variety of cationic ligands, including short inorganic ligands such as ammonium and alkali metal cations. This ionic ligand exchange chemistry is used to demonstrate enhanced transport in germanium nanocrystal films following ligand exchange as well as the first photovoltaic device based on an all-inorganic germanium nanocrystal absorber layer cast from solution. This new ligand chemistry should accelerate progress in utilizing germanium and other group IV nanocrystals for optoelectronic applications. KEYWORDS: Germanium nanocrystal, ligand exchange, inorganic ligand, plasma synthesis, quantum dot

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with an alkene via hydrosilylation or hydrogermylation) is an effective method to minimize the impacts of oxidation as well as enhance photoluminescence, these group IV−C bonds are kinetically inert and do not undergo exchange. Despite this, there have been some reports of ligand exchange reactions at group IV NC surfaces. Previously, we found that strongly coordinating small molecules such as ketones and nitriles interact quasi-reversibly with chlorideterminated Si NCs via a hypervalent surface Si atom.21 Korgel and co-workers recently demonstrated an irreversible, hightemperature (190 °C) ligand exchange on Si NCs by displacing dodecanthiolate (Si−SR) with dodecyl (Si−R) ligands,22 presumably driven by the stronger covalent Si−C bond compared to Si−S. For Ge NCs, the earliest published account of ligand substitution described an incomplete exchange of hexadecylamine with trioctylphosphine or octadecanethiol.23 Subsequent work on colloidally grown Ge NCs has shown that surface-bound alkylamines can be exchanged with polyethylenimine24 at room temperature or hydrazine/dodecanethiol at 80 °C.25 Finally, a recent report by Vela and co-workers described Ge/CdS or Ge/ZnS core−shell NCs accessed by high-temperature (230 °C), sulfur-based surface “priming” of

ize-tunable optical properties and the ability to process thin films using scalable, cost-efficient printing techniques have long made colloidal nanocrystals (NCs) attractive candidates for solar cells,1 light-emitting devices,2,3 transistors,4,5 photodetectors,6 and batteries.7 Conventional colloidal synthesis of NCs comprised of metal-based compound semiconductors (groups III−V, II−VI, and IV−VI) yields aliphatic ligands bound to the NC surface through labile Lewis acid−base or ionic surface bonds.8 Recent progress in NC optoelectronics has hinged on the ability to manipulate the NC surface through displacement of the labile, native insulating ligands. Key examples of this approach include: (i) Schottky solar cells based on colloidal NC films9 as well as bulk-like electronic transport in NC thin films10 achieved by solid-11 or solutionstate12 reactions to exchange insulating aliphatic ligands for short conductive species; (ii) shell and heterostructure growth to enhance multiple exciton generation13 and slow hot carrier cooling;14 and (iii) tunable band alignments by ligand exchange in NC thin film heterojunctions to control carrier extraction/ injection in optoelectronic devices.15 Whereas surface manipulation has launched metal-based NCs to the forefront of NC-based optoelectronics research, similar strategies using nontoxic and earth-abundant group IV (Si, Ge) NCs have been largely unsuccessful owing to the covalent bonds that dominate the surfaces of these nanostructures. Though functionalization of group IV NCs with covalent Si−C16−18 or Ge−C19,20 bonds (primarily through reaction © XXXX American Chemical Society

Received: December 19, 2015 Revised: January 14, 2016

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DOI: 10.1021/acs.nanolett.5b05192 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Reaction scheme for hydride-terminated Ge NCs synthesized in a RF plasma and functionalized with a OAm/(NH4)2S solution (see Supporting Information for details). The reaction yields soluble, singly isolated Ge NCs, as seen in the TEM image for a 7.8 nm sample. (b) FTIR spectra of as-synthesized 7.8 nm Ge NCs (black), Ge NCs after functionalization (green), and neat oleylamine (OAm) as a reference (blue). (c) 1H NMR spectrum of functionalized 7.8 nm Ge NCs in toluene-d8 (black) and subsequent spectra following dilution with OAm (shades of red). The blue spectrum is a reference spectrum of OAm without Ge NCs. Spectra are normalized to a Cp2Fe internal standard.

characterized by stretching (∼2000 cm−1) and deformation (400 to 900 cm−1) modesare removed completely, and vibrations consistent with OAm (blue spectrum, Figure 1b) dominate the spectrum after reaction with sulfide. Interestingly, germanium oxide peaks, indicated by Ge−O−Ge stretching modes expected to be centered at 850 cm−1,29 are not observed despite reaction in aqueous solution. This is consistent with the well-known solubility of GeOx in water and aqueous stability of planar Ge in the presence of a strong reducing agent such as the citrate anion.30 Though Robinson and co-workers used (NH4)2S as a S2− precursor28 and other reports31,32 have suggested that S2− can act as an anionic ligand for metal chalcogenide NCs, no Ge−S frequencies (expected between 550 and 670 cm−1)33 are found in our samples. Although the nature of this sulfide activation chemistry is currently unclear, these FTIR data suggest that sulfur is not incorporated into the NCs and that OAm interacts directly with the Ge NC surface. Thus, the sulfide treatment appears to reduce the as-grown *GeHx surface, possibly via production of sulfur, hydrogen, hydrogen sulfide, or related byproducts. We used 1H nuclear magnetic resonance (NMR) experiments to further probe the hypothesis that sulfide activation chemistry results in OAm-functionalized Ge NCs. Consistent with the FTIR data, the 1H NMR spectra comprised only resonances from OAm protons, with no trace of *GeHx (expected to occur from 3−6 ppm; e.g., branched oligogermanes have a shift of ∼4.6 ppm34). The 1H NMR spectra in Figure 1c show the amino proton resonance (HA) region of OAm. We found that functionalized Ge NCs purified by washing three times with methanol require small amounts of OAm (∼5 μL/mg Ge NCs) to redisperse in toluene-d8, resulting in an OAm concentration of 0.8 mM as determined by integration of the vinylic proton resonance of OAm relative to a

as-synthesized hexadecylamine- and/or dodecyl-capped Ge NCs.26 Unlike the two Si NC studies, the nature of the bonding that facilitates the Ge NC surface exchange chemistry in these examples is unclear. Here, we demonstrate functionalization of plasma-synthesized Ge NCs27 by surface reduction that yields Ge NCs capped with alkylammonium ligands that are suitable for a variety of cationic ligand exchanges. Nonthermal plasma decomposition of GeH4 gas gives hydride-terminated Ge NCs that are activated by a mixture of aqueous ammonium sulfide (20 wt %)/oleylamine (OAm)/ toluene (1:10:50 by volume).28 Typical reaction times were 12 h to achieve a dark, uniform toluene phase containing the functionalized Ge NCs. The reaction was successful for a number of different Ge NC sizes ranging from 3.4 to 16.5 nm according to Scherrer broadening analysis of the X-ray diffraction patterns for as-synthesized NCs (Figure S1). All subsequent experiments and discussion are based on the 7.8 nm sample. Full details on Ge NC synthesis via nonthermal plasma decomposition of GeH4 and post-synthesis surface activation and ligand functionalization are included in the Supporting Information. Figure 1a shows the reaction scheme as well as a transmission electron microscopy (TEM) image of 7.8 nm Ge NCs following surface activation with sulfide. The TEM image clearly shows the Ge NCs form a soluble product of singly isolated Ge NCs that is characteristic of ligand-functionalized NC surfaces. The internanocrystal spacing of ∼2.7 nm is consistent with the distance of interpenetrating OAm molecules, suggesting that the OAm is the species at the Ge NC surface following sulfide treatment. Further insight into the Ge NC surface chemistry is revealed by FTIR spectroscopy, which shows that the native surface hydrides *GeHx B

DOI: 10.1021/acs.nanolett.5b05192 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. 1H NMR spectra of Ge NCs in (a) toluene-d8 or (b) CTAB after three methanol washes (black) and dilution with (a) hexadecylamine or (b) CTAB (shades of red) at various ratios relative to OAm. The blue spectrum is of sample of (a) OAm/hexadecylamine (1:4) or (b) OAm/CTAB (1:1) without Ge NCs.

with Ge amides require acidic conditions at room temperature.37,38 An additional experiment was used to confirm this amine exchange chemistry at the Ge NC surface. Figure 2a shows the 1 H NMR spectra of the vinylic proton resonance (Hv) region of OAm, where n-hexadecylamine is used to dilute the same functionalized Ge NC sample in toluene-d8. At a 1:4 ratio of OAm/hexadecylamine (black), the vinylic resonance of OAm is buried into the baseline in comparison to the reference solution without Ge NCs (blue). When the dilution ratio is increased to 1:100, the vinylic resonance shifts upfield due to concentration effects and sharpens to resemble the free OAm molecule as the initial groups functionalizing the Ge NCs are displaced. This experiment confirms the result from exchange experiments with excess OAm that exchange with amines is rapid on the NMR time scale. Since the above data rules out an amide interaction (Ge− NHR), we considered two other bonding motifs between the amine-based ligands and the Ge NC surface. First, neutral alkylamines can bind as an L-type, neutral inner sphere complex (also called a dative or dipolar bond) to form hypervalent Ge NC surface atoms, as has been observed for many molecular Ge complexes. 39,40 Alternatively, cationic alkylammonium (RNH3+) groups can interact through a noncoordinative ionic bond with negatively charged Ge NC surface atoms to form an outer sphere complex. To differentiate between the two motifs, we dilute the same functionalized Ge NC sample with cetyltrimethylammonium bromide (CTAB; cetyl = hexadecyl) in dichloromethane-d2 (DCM) at varying ratios to the existing OAm ligand (DCM was used owing to the greater solubility of CTAB in this solvent relative to toluene). The 1H NMR spectra in Figure 2b show the aminomethyl (Hm) and methylene protons (Hα) of the cetyltrimethylammonium cation (CTA+). At a 1:1 ratio OAm/CTA+, the Hm and Hα proton resonances

ferrocene (Cp2Fe) internal standard (see Figure S2 for Ge NC concentration determination and Figure S3 for full 1H NMR spectra showing aliphatic region). There are two notable features in the 1H NMR spectrum of the functionalized and redispersed Ge NCs (black) when compared to an identical 0.8 mM solution of OAm without Ge NCs (blue): (1) All proton resonances in the spectrum of functionalized Ge NCs are broadened in comparison to the resonances from the free molecules; and (2) the signal from the amino protons in the Ge NC sample is so broad that it is buried in the baseline, which is in stark contrast to the sharper amino proton resonance of the OAm reference at the same concentration. These single, borad NMR resonances indicate that the OAm molecules are tightly bound to the slowly tumbling Ge NC or exchanging between chemically distinct environments, such as the NC surface and the toluene-d8 solution. This rapid exchange is observed even when ligands greatly outnumber the Ge NCs. A concentration of 0.8 mM OAm (black spectrum) corresponds to 1.6 × 103 ligands/NC for 7.8 nm Ge NCs. This is a factor of 5 more ligands than could possibly closely pack at the Ge NC surface (assuming a ligand cross-sectional area of 0.18 nm2 typical of dense monolayers of molecules with a long hydrocarbon chain35). This result verifies that these ligands are highly fluxional. Broadening is apparent even as the OAm concentration is increased by 3 orders of magnitude (Figure 1c, shades of red). The OAm amino proton resonance emerges from the baseline and shifts upfield due to concentration36 but is still broader than the free OAm amino peak, which indicates that ligands freely exchange with the Ge NC surface at room temperature on the NMR time scale. This result appears to rule out covalent attachment of OAm groups by a Ge−NHR interaction, where no surface exchange with free RNH2 would be expected since such transamination reactions C

DOI: 10.1021/acs.nanolett.5b05192 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters are significantly broadened in comparison to a reference solution without Ge NCs (blue spectrum). Similar to the two previous experiments, the vinylic proton resonance of the OAm molecules also emerges from the spectral baseline with increasing CTAB concentration, which is consistent with ligand exchange. The resonances shift with concentration and sharpen as the OAm/CTA+ ratio is increased to 1:20. Since there is no driving force for CTA+ to exchange with a neutral amine (i.e., proton exchange is not possible), we conclude that an ionic bonding motif exists at the Ge NC surface where CTA+ exchanges with olelyammonium cations (OAmH+) residing at the negatively charged Ge NC surface. Figure 2c depicts this new cationic ligand exchange process in the presence of excess neutral amine or quaternary ammonium salt. For exchange with a neutral primary (or secondary or tertiary) amine to occur, a proton is transferred from an alkylammonium (RNH3+) ligand at the Ge NC surface to the incoming neutral amine (R′NH2) to yield a R′NH3+ ligand and a free RNH2 molecule. We find this exchange process from surface-bound OAmH+ requires a Lewis base as least as basic as OAm. For instance, exchange proceeds with primary alkylamines such as hexadecylamine (pK a of alkylammonium is ∼10) but does not for aniline (pKa of anilinium is ∼5).41 In the quaternary ammonium salt case, charge balance necessitates the anion (A−) is transferred from the quaternary ammonium (RN(CH3)3+) salt to the surfacebound RNH3+ to yield an alkylammonium salt (RNH+A−) in solution and the RN(CH3)3+ ligand associated with the Ge NC as an outer sphere complex. This is the first example of ionic ligand functionalization and exchange on group IV NCs. The versatility of this ligand motif is illustrated in Figure 3a, which shows FTIR spectra for Ge NC films with a variety of cationic ligands. The green spectrum is of a dip-coated film of Ge NCs functionalized with the native OAmH+ ligand. These ligands are exchanged in solution by adding 5 mL of a 1 M solution of dodecylamine in toluene, and exchanged Ge NCs are isolated by the conventional antisolvent/solvent method using acetonitrile and toluene. Exchange is apparent from the disappearance of the OAm vinylic stretching and deformation modes at 3005 and 1667 cm−1, respectively, in the blue spectrum. Solid-state exchange of ligands has been described extensively for metal chalcogenide NCs11 by submersing an insoluble film of NCs in a ligand-exchange solution. We demonstrate this strategy also is effective for the OAmH+functionalized Ge NC films developed here by assembling films in a layer-by-layer fashion. Exchange for short ligands, such as methylammonium (CH3NH3+; yellow spectrum, Figure 3a) and ammonium (NH4+; data not shown), is achieved by dipping films in iodide salt solutions in dimethylformamide (DMF). In addition to NH4+, an all-inorganic Ge NC film is produced after exchange in a 0.1 M hydrazine solution in acetonitrile to produce hydrazinium (N2H5+)-functionalized Ge NCs as shown in the orange spectrum. We further demonstrate a third all-inorganic Ge NC film with an extremely small alkali metal ligand, the sodium cation (Na+, red spectrum), by performing the exchange reaction in a saturated DMF solution of sodium tert-butoxide. The tert-butoxide anion provides a convenient handle to highlight that sodium binds to the Ge NC surface, not the organic anion, since the C−Hx stretching modes of the native OAmH+ have disappeared and the film is virtually devoid of any hydrocarbon residue following ligand exchange. Successful exchange with potassium (K+; data not

Figure 3. (a) FTIR spectra of oleylammonium (OAmH+)-functionalized Ge NC films (green) exchanged with cationic ligands: dodecylammonium (CH3(CH2)11NH3+, blue), methylammonium (CH3NH3+, yellow), hydrazinium (N2H5+, orange), and sodium (Na+, red). (b) Photograph of a solution of OAmH+-functionalized Ge NCs in hexane (hex) that do not phase transfer to dimethyl sulfoxide (DMSO). (c) Photograph of N2H5+-functionalized Ge NCs in hydrazine after phase transfer from hexane.

shown) also is achieved using potassium tert-butoxide in a saturated DMF solution. FTIR spectra of the free molecules used for exchange in Figure 3a are shown in Supporting Figure S4. Cationic ligand exchange to produce Ge NCs with inorganic ligands is also successful in solution using a biphasic strategy reminiscent of previous work on metal and metal chalcogenide NCs.12,32 Figure 3b is a photograph of OAmH+-functionalized Ge NCs in hexanes (hex) that do not phase transfer into the polar solvent dimethyl sulfoxide (DMSO), as expected. If the same hexanes solution is layered over hydrazine, then the Ge NCs transfer to the hydrazine phase to yield a solution of N2H5+-functionalized Ge NCs in less than 5 min. Notably, the colloidal Ge NCs exhibit no change in their absorption properties following phase transfer, as evidenced by the absorption spectra of OAmH+ and N2H5+-functionalized Ge NCs (Figure S5). Thus, the phase-transfer process could enable deposition of all-inorganic Ge NC films without altering optical properties or need for further chemical treatment to remove ligands. As noted above, ligand exchange has been an integral tool in the development of NC-based optoelectronic devices.42 Here, we show two proof-of-principle devices that leverage cationic ligand exchange. The first device demonstrates this ligand exchange greatly enhances inter-NC electronic coupling in a Ge NC film. Ge NCs are deposited by dipping a silicon wafer with a 110 nm-thick thermal oxide and predeposited aluminum electrodes into a 20 mg mL−1 solution of OAmH+-functionalized Ge NCs. There is no measurable current through the film when a bias up to 70 V is applied (Supporting Figure S6). Ligand exchange is performed on the film by submersing it into a 0.1 M solution of hydrazine in acetonitrile and rinsing with D

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istics are comparable to previous work on Ge NC poly(3hexylthiophene) composite absorbers.44 Finally, we note there is a significant decrease in current from the control device to that with Ge NCs, which could suggest that the Ge NCs introduce significant resistance into the device stack. To probe this possibility, we modeled the device circuits of the control device with no Ge NCs, the device stack with Ge NCs assuming the Ge NCs introduce significant series resistance, and the device stack with Ge NCs exhibiting the photovoltaic effect. Figure S8 shows the modeling parameters used to simulate J−V curves. These models clearly demonstrate that a diode effect is necessary to reproduce the observed J−V responses. In this work, we demonstrate room-temperature alkylammonium functionalization of Ge NCs synthesized from germane in a nonthermal plasma. We employ FTIR and quantitative 1H NMR to show the alkylammonium ligands are readily exchanged in solution as well as the solid state. Solid-state exchange utilizing inorganic cationic ligands yields all-inorganic Ge NC films, which enabled demonstration of photodetectors and photovoltaic behavior from all-inorganic Ge NCs films cast from solution. Most importantly, the cationic ligand exchange chemistry at group IV NC surfaces established by this work finally opens the ligand exchange toolbox that has been leveraged by the metal chalcogenide NC community for years to make significant strides toward viable optoelectronic technologies based on earth-abundant, nontoxic group IV nanomaterials.

neat acetonitrile. The process is repeated 2−3 times to build up a continuous film of N2H5+-functionalized Ge NCs. The exchanged films are conductive, and current is proportional to V2 at biases greater than 10 V, which is characteristic to spacecharge-limited current (SPLC). We apply the SPLC model43 to calculate a space-charge-limited carrier mobility of 0.15 cm2 V−1 s−1 at bias between 10 and 70 V. This value is an order of magnitude larger than the field effect mobility observed in a previous report of Ge NC thin film transistors.4 We further leverage cationic ligand exchange to produce a photoactive device based on all-inorganic Ge NCs films cast from solution.44 Figure 4a shows a scanning electron

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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.nanolett.5b05192. Details on materials, Ge NC synthesis and functionalization methods, XRD patterns for a number of functionalized Ge NC sizes, the standard curve to determine Ge NC concentration, full 1H NMR spectra of data shown in Figure 1c and Figure 2a,b, FTIR spectra of molecules used for ligand exchange, UV−vis absorption spectra of OAmH+- and N2H5+-functionalized Ge NC solutions, current−voltage curve of the coplanar Ge NC devices, mobility calculation details, current−voltage curve of the control PV device, and details of circuit analysis on the PV device (PDF)

Figure 4. (a) SEM image showing Ge NC-based PV device stack. (b) Light (red) and dark (dashed black) current−voltage curves of the device fabricated with a Na+-functionalized Ge NC absorber layer. (c) Zoom-in of the same data to show PV characteristics.

microscopy (SEM) image of the device cross-section. The device features a 100 nm-thick absorber layer of Na+functionalized Ge NCs deposited using layer-by-layer assembly between hole (MoOx/Au) and electron (TiO2/ITO) contact layers that are optimized for metal chalcogenide NC photovoltaic devices.45 No optimization of the Ge NC absorber layer or contacts was performed. Figure 4b shows the current density as a function of voltage for the device in the dark (dashed black curve) and under AM 1.5 illumination (solid red curve). The device displays current rectification and excellent diode behavior under illumination with an on/off ratio of 560 at 0.8 V, which is a factor of ∼5 greater than a previous report on Ge NC-based photodetectors by Church et al.46 A control device without the Ge NC layer was fabricated and showed an on/off ratio of