Nanocrystal Inks without Ligands: Stable Colloids of Bare Germanium

LETTER pubs.acs.org/NanoLett

Nanocrystal Inks without Ligands: Stable Colloids of Bare Germanium Nanocrystals Zachary C. Holman* and Uwe R. Kortshagen Mechanical Engineering Department, University of Minnesota, 111 Church Street SE, Minneapolis, Minnesota 55455, United States

bS Supporting Information ABSTRACT: Colloidal semiconductor nanocrystals typically have ligands attached to their surfaces that afford solubility in common solvents but hinder charge transport in nanocrystal films. Here, an alternative route is explored in which bare germanium nanocrystals are solubilized by select solvents to form stable colloids without the use of ligands. A survey of candidate solvents shows that germanium nanocrystals are completely solubilized by benzonitrile, likely because of electrostatic stabilization. Films cast from these dispersions are uniform, dense, and smooth, making them suitable for device applications without postdeposition treatment. KEYWORDS: Nanocrystal, quantum dot, colloid, germanium, benzonitrile

S

emiconductor nanocrystals (NCs) have received increasing attention in the past 15 years for their potential use in new optical and electronic devices. Colloidal NCs are particularly interesting, as they promise a new era in inorganic semiconductor processing in which quantum-confined thin films are inexpensively printed. Methods have been developed to synthesize high quality II VI and IV VI NCs in solution with ligands such as trioctylphosphine/trioctylphosphine oxide or oleic acid on their surfaces.1 4 The ligands provide steric stabilization in nonpolar solvents so that aggregation of NCs due to van der Waals forces is suppressed. Unfortunately, the ligands also hinder charge carrier transport between NCs after film casting, rendering as-deposited films electrically insulating.5 7 While the ligands may be exchanged in solution for shorter molecules that reduce the interparticle spacing in films,1,8 shorter molecules provide less steric stability, encouraging flocculation. An alternative approach is to reduce the ligand length after depositing films. Excellent electronic properties have been obtained in PbSe NC films treated with hydrazine and 1,2-ethanedithiol, but the films crack after the treatments.6,9 Depositing conductive thin films from group IV colloidal NCs is more difficult. Silicon and Ge have comparatively large crystallization temperatures because of their covalent bonds, making solution synthesis challenging.10 Furthermore, once ligands have been attached either during solution synthesis or via grafting to gas-phase synthesized NCs, they resist removal. Attempts to exchange ligands in solution or chemically treat films after deposition are ineffectual, again because of the covalent bonds formed. We previously reported that Ge NC films fabricated by synthesizing Ge NCs in a plasma, functionalizing their surfaces with 1-dodecene, and casting them from solution were electrically insulating until annealed at 250 °C or higher.11 At these temperatures, the 1-dodecene molecules still did not cleave from r 2011 American Chemical Society

the NC surfaces; they decomposed instead. In this Letter, we report stable Ge NC colloids formed by dispersing “bare” NCs in select solvents. Such ligand-free colloids offer new opportunities for NC processing and device fabrication in which semiconducting films may be printed from solution without postprocessing. Germanium NCs were synthesized with a nonthermal, continuous-flow plasma process.12 Germanium tetrachloride (GeCl4) vapor and hydrogen gas (H2) precursors were dissociated in the plasma, allowing nanometer-sized Ge crystallites to nucleate via chemical clustering of the dissociation products. The size of the NCs scales with the duration of their residence in the plasma, and 6 nm diameter Ge NCs were used throughout this study. The chemistry at the surface of the NCs is a mixture of H and Cl passivation, the ratio being determined by the relative concentrations of the GeCl4 and H2 introduced into the reactor.11 Ion beam measurements in which Rutherford backscattering spectrometry (RBS) and forward recoil elastic spectrometry (FRES) signals were simultaneously collected indicate that the NCs studied here have three times as much H on their surfaces as Cl.13 Full synthesis and characterization details can be found elsewhere.11 13 Following a serendipitous observation, we recently reported that plasma-synthesized Si NCs form stably suspended agglomerates in 1,2-dichlorobenzene, yielding cloudy dispersions that do not settle, while Ge NCs form optically transparent colloids in the same solvent.13,14 Here, we investigated the solubility of Ge NCs in a host of solvents chosen for their similarity to 1,2dichlorobenzene in dielectric constant, chemical structure, or Hansen Solubility Parameters. Many of the solvents contained Received: March 8, 2011 Revised: April 21, 2011 Published: April 26, 2011 2133

dx.doi.org/10.1021/nl200774y | Nano Lett. 2011, 11, 2133–2136

Nano Letters

LETTER

Figure 1. Bare Ge NCs dispersed in benzonitrile. From left to right, the colloid concentrations are 5, 0.5, and 0.04 mg/mL. Colloids with concentrations up to 50 mg/mL were prepared.

benzene rings, and nearly all had halogens or nitrile groups. Common solvents such as toluene, hexanes, and water were also tried for completeness. Germanium NCs were collected as a powder downstream of the plasma reactor and transferred airfree to a glovebox with oxygen and water concentrations below 5 ppm, where they were weighed and added to vials. Degassed solvents were then added to the vials to form 0.25 mg/mL dispersions. The NCs remained in powder form at the bottom of their vials upon addition of all solvents except benzonitrile and acetonitrile, and 5 min of ultrasonication in the sealed vials was required to break up the powder and suspend the NCs. Remarkably, benzonitrile, and to a lesser extent acetonitrile, solubilized bare Ge NCs upon contact, and an optically transparent colloid was formed with a brief shake (Figure 1). The process is visually similar to the addition of nonpolar solvents to dried Ge NCs that have been functionalized with 1-dodecene. The stability of Ge NCs in each solvent was studied by comparing the initial Ge NC concentration to the concentration of NCs that remained suspended after filtering the dispersions through a 0.2 μm PTFE filter. The concentrations were determined from UV vis absorption measurements. Scattering from agglomerates dominated the spectra of most suspensions before filtration, and many samples looked like pristine solvent after filtration. Germanium NCs in these solvents, which included 2-chloropropane, heptyl cyanide, 1,2-dibromoethane, bromoethane, dichloromethane, pyridine, 1,3-dichlorobenzene, chloroform, as well as all very nonpolar solvents tried, were unstable and quickly formed aggregates larger than the filter pore size. The absorption spectra of Ge NCs in benzonitrile, by contrast, were identical before and after filtration, indicating small (or no) aggregates. Because scattering was insignificant in these samples, the Beer Lambert law could be used to correctly relate absorption to concentration, which in this case was the same before and after filtration (0.25 mg/mL). Consequently, the absorption of unfiltered Ge NCs in benzonitrile was used as a reference to determine the unknown concentrations of Ge NCs remaining in other solvents after filtration, as shown in Figure 2a. The ratio of absorbance, or the normalized concentration, was calculated using absorbance data at 525 nm, although the result was nearly wavelength-independent. Figure 2b displays the relative concentrations after filtration for all solvents that had nonzero postfiltration concentrations. In Figure 2c, the postfiltration concentrations are plotted against the static dielectric constant of the solvents, which was one parameter used to select

Figure 2. (a) Absorption spectra of filtered Ge NCs in three select solvents (solid colored lines). The spectrum of unfiltered Ge NCs in benzonitrile, which was used as a standard against which the spectra of all filtered samples were compared, is plotted in black, and the ratios of the absorbance of the filtered samples to this standard are plotted in dashed colored lines. Normalized concentration (ratio of absorbance at 525 nm) of filtered samples versus (b) solvent and (c) solvent static dielectric constant.

solvents for this study. Successful solvents fall within a broad range of dielectric constants that excludes both very polar and very nonpolar solvents. Dielectric constant alone, however, does not afford predictive power of solubility, as some solvents fall within the acceptable range and yet will not solubilize bare Ge NCs. Similar trends were found for Hansen Solubility Parameters; it is necessary but not sufficient for solvents to lie within a particular sphere in Hansen space to be effective at solubilizing Ge NCs (see Supporting Information). The data in Figure 2 represent a metric for colloidal stability, albeit a somewhat unusual one. While techniques such as dynamic light scattering (DLS) are commonly employed to characterize colloids, they do not easily measure the quantities desired here. From a device applications perspective, an ideal NC colloid has a minimum flocculation time on the order of several months and produces homogeneous films when cast. Dynamic light scattering gives the size distribution of colloidal particles. However, DLS data may indicate that the “best” sample is one with 99% of the material precipitated at the bottom of the cuvette, provided the 1% remaining suspended consists of isolated NCs. We would like to find solvents that have the fewest agglomerates larger than a few tens of nanometers (as these are sure to either precipitate or ruin film morphology), and filtration measures this. Films were cast from unfiltered Ge NC colloids in several solvents to verify the quality of the concentration-after-filtration metric. For solutions in which no Ge NCs remain after filtration, 2134

dx.doi.org/10.1021/nl200774y |Nano Lett. 2011, 11, 2133–2136

Nano Letters

Figure 3. Cross-sectional scanning electron micrographs of bare Ge NCs cast from solution. (a) A highly nonuniform film of Ge NCs deposited from a sonicated chloroform dispersion. (b) The edge of a field-effect transistor channel in which a 40 nm thick Ge NC film was deposited from benzonitrile after source and drain contact evaporation. (c) A 3 μm thick Ge NC film flake that detached from the substrate (the film was cast from benzonitrile). All images were taken at a 3 5° tilt from normal.

lumpy layers were observed with bare substrate exposed in some areas, and NC aggregates micrometers in size in others (Figure 3a). For NCs in benzonitrile, however, uniform films a few nanometers to a few micrometers in thickness were produced on glass and Si wafer chips depending on the colloid concentration and spin speed (Figures 3b,c). No large agglomerates are visible, confirming their absence in solution. Note that although homogeneous films were obtained here with little attention to the casting process this is not ensured by a favorable score on the concentration-after-filtration metric. Well-dispersed colloidal NCs are an important prerequisite to nice films, but substrate surface chemistry and solution drying kinetics, among other factors, may cause even the most stable colloids to yield nonuniform films.15,16 We turn now to the mechanism by which the colloids are stabilized. It is not evident that the dominant stabilizing forces are the same for each solvent, and we focus on only benzonitrile here. Nonetheless, with the exception of pyridine and water (GeO2 dissolves in water), NCs were not observed to undergo structural changes in any solvents. X-ray diffraction spectra show that assynthesized NC powder and NC films cast from various solvents contain indistinguishable 6 nm diamond cubic Ge NCs (see Supporting Information). We also did not observe evidence of

LETTER

Figure 4. (a) FTIR and (b) RBS spectra of Ge NC powder assynthesized, annealed for 1 h in a N2-purged glovebox at 300 °C, dipped in diluted HF for 30 s, and oxidized in ambient air for 75 min. Spectra are offset for clarity. Air exposure was minimized for all unoxidized samples, and the FTIR spectra were recorded in a glovebox. The FTIR spectra were measured in diffuse reflectance mode on Ge NCs deposited from the gas phase onto Au-coated Si substrates. The RBS spectra were measured at 1.00 MeV (2 μC charge collected) on Ge NCs deposited from the gas phase onto Si substrates. The approximate energy of the leading edge of each element’s signal is identified; the variation in peak width and onset is due to film thickness variation between samples.

reaction between the NCs and most solvents. RBS and Fourier transform infrared (FTIR) spectroscopy revealed similar NC chemistry before and after colloid formation in benzonitrile, and thermogravimetric analysis (TGA) showed that residual solvent in the film evaporates at or below the boiling point of benzonitrile, as expected for unreacted solvent (see Supporting Information). However, solubility depends entirely on the state of the NC surfaces, at least when benzonitrile is used as a solvent. Colloids that were kept in sealed vials were stable for months without visible change, while those that were allowed to oxidize flocculated over the course of days or weeks, depending on their concentration (see Supporting Information). In an experiment in which Ge NC surface chemistry was controllably altered, benzonitrile was added to as-synthesized Ge NCs, NCs that had been heated to 300 °C for one hour in a glovebox, NCs that had been dipped in diluted hydrofluoric (HF) acid for 30 s, and NCs that were oxidized in ambient air for 75 min. The FTIR and RBS spectra in Figure 4 show the surface chemistry of these NCs prior to the addition of benzonitrile. The as-synthesized Ge NCs had a combination of H, H2, and H3 termination in addition to the Cl observable in RBS but not in FTIR. It has been shown that both H and Cl surface species are removed by annealing at 300 °C in an inert atmosphere,13 leaving an unknown, possibly reconstructed surface. The sample dipped in HF had a purely monohydride surface, and virtually no Cl. Finally, the air-exposed sample showed residual Hx termination and trace amounts of Cl, but a significant oxide shell had already formed. 2135

dx.doi.org/10.1021/nl200774y |Nano Lett. 2011, 11, 2133–2136

Nano Letters Of these samples, only the as-synthesized NCs were soluble in benzonitrile. The others yielded NC concentrations of zero after filtering. This indicates that the Cl on the as-synthesized NCs plays an important role in their solubilization in benzonitrile, and that H- and Cl-free surfaces, purely H-terminated surfaces, and oxidized surfaces do not provide the colloidal force(s) necessary for stable suspension. Steric forces are absent in these ligand- and surfactant-free colloids and there are no species present to participate in hydrogen bonding, suggesting that electronegative Cl surface species induce electrostatic stabilization. Indeed, we measured an average zeta potential of 25 mV over three different batches of Ge NCs in benzonitrile, on the cusp of the (30 mV often regarded as sufficient for long-term stability. While not common, others have reported electrical double layer formation for colloids in similar polar organic solvents.17 We hypothesize that the most likely routes to NC surface charging in this case are Lewis acid base interactions with the Cl surface groups acting as the acid, or dissociation of Hx groups. Others have studied GeCl4 and trichlorogermane (HGeCl3)—molecules with similar chemistry to our NCs— as Lewis acids and found that complexes are readily formed with N-donor neutral Lewis bases.18,19 The degree to which a solvent can support electrostatic forces depends on its polarity, suggesting that the trend in colloid stability with dielectric constant in Figure 2c may in fact be attributable to a single stabilization mechanism. That is, among solvents that “work” (those that do not must be investigated individually for signs of, e.g., reaction), Ge NC solubility may be due exclusively to electrical double-layer formation. Colloid stability then depends on a solvent’s ability to exchange charge with the NCs and support the resulting electric fields. The solvents 1,2-dichlorobenzene and 1,3-dichlorobenzene provide an illustrative example. These isomers are similar in structure, boiling point, and reactivity, yet the static dipole moment of 1,3dichlorobenzene is half that of 1,2-dichlorobenzene (5 vs 10.1) because of the more symmetric positioning of the electronegative Cl atoms. We observed that nearly 60% of Ge NCs remain in 1,2dichlorobenzene after filtration while none remained in 1,3dichlorobenzene. We have demonstrated that it is possible to form stable colloids of bare Ge NCs with H- and Cl-terminated surfaces using select solvents. This pick-the-solvent-to-match-the-NCs approach is limiting in that only a few solvents may exist that solubilize the NCs, and these solvents may be unsatisfactory in other regards. However, if a solvent can be found for each NC material that is as successful as benzonitrile is for Ge NCs, this will be a great boon for NC processing and device fabrication. By moving from steric stabilization to electrostatic stabilization, NC colloids that are stable for months may be formed without the use of ligands. Nanocrystals in films cast from these colloidal solutions are not separated by insulating molecules that prohibit charge carrier transport, allowing semiconductor NC devices to be constructed from as-cast films.

LETTER

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSF under Grant CBET0756326, IGERT Grant DGE-0114372, and MRSEC Grant DMR-0819885. Partial support was also provided by the DOE Energy Frontier Research Center for Advanced Solar Photophysics and by the UMN Center for Nanostructure Applications. We thank Dr. Greg Haugstad and Moon Sung Kang for their assistance with RBS and TGA measurements, and Lance Wheeler for helpful discussions. ’ REFERENCES (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (2) Micic, O. I.; Sprague, J. R.; Curtis, C. J.; Jones, K. M.; Machol, J. L.; Nozik, A. J.; Giessen, H.; Fluegel, B.; Mohs, G.; Peyghambarian, N. J. Phys. Chem. 1995, 99, 7754–7759. (3) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655–3657. (4) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P. R.; Micic, O. I.; Ellingson, R. J.; Nozik, A. J. J. Am. Chem. Soc. 2006, 128, 3241–3247. (5) Law, M.; Luther, J. M.; Song, O.; Hughes, B. K.; Perkins, C. L.; Nozik, A. J. J. Am. Chem. Soc. 2008, 130, 5974–5985. (6) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86–89. (7) Drndic, M.; Jarosz, M. V.; Morgan, N. Y.; Kastner, M. A.; Bawendi, M. G. J. Appl. Phys. 2002, 92, 7498–7503. (8) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (9) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. ACS Nano 2008, 2, 271–280. (10) Heath, J. R.; Shiang, J. J. Chem. Soc. Rev. 1998, 27, 65–71. (11) Holman, Z. C.; Kortshagen, U. R. Langmuir 2009, 25, 11883–11889. (12) Gresback, R.; Holman, Z.; Kortshagen, U. Appl. Phys. Lett. 2007, 91, 093119. (13) Holman, Z. C.; Liu, C. Y.; Kortshagen, U. R. Nano Lett. 2010, 10, 2661–2666. (14) Liu, C. Y.; Holman, Z. C.; Kortshagen, U. R. Nano Lett. 2009, 9, 449–452. (15) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003, 426, 271–274. (16) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827–829. (17) Christenson, H. K.; Horn, R. G. Chem. Phys. Lett. 1983, 98, 45–48. (18) Levason, W.; Reid, G.; Zhang, W. Coord. Chem. Rev. 2011, 255, 1319–1341. (19) Nogai, S.; Schriewer, A.; Schmidbaur, H. Dalton Trans. 2003, 16, 3165–3171.

’ ASSOCIATED CONTENT

bS

Supporting Information. Normalized concentration of filtered, dispersed Ge NCs versus Hansen Solubility Parameters; XRD spectra of Ge NCs before and after dispersal in select solvents; RBS, FTIR, and TGA spectra of Ge NCs before and after dispersal in benzonitrile; and a photograph of fresh and oxidized NCs in benzonitrile. This material is available free of charge via the Internet at http://pubs.acs.org. 2136

dx.doi.org/10.1021/nl200774y |Nano Lett. 2011, 11, 2133–2136