Synthesis and Single Crystal X-ray Diffraction Structure of an Indium Arsenide Nanocluster

The discovery of magic-sized clusters as intermediates in the synthesis of colloidal quantum dots has allowed for insight into formation pathways and provided atomically precise molecular platforms for studying the structure and surface chemistry of those materials. The synthesis of monodisperse InAs quantum dots has been developed through the use of indium carboxylate and As(SiMe3)3 as precursors and documented to proceed through the formation of magic-sized intermediates. Herein, we report the synthesis, isolation, and single-crystal X-ray diffraction structure of an InAs nanocluster that is ubiquitous across reports of InAs quantum dot synthesis. The structure, In26As18(O2CR)24(PR'3)3, differs substantially from previously reported semiconductor nanocluster structures even within the III–V family. However, it can be structurally linked to III–V and II–VI cluster structures through the anion sublattice. Further analysis using variable temperature absorbance spectroscopy and support from computation deepen our understanding of the reported structure and InAs nanomaterials as a whole.


Synthesis of In26As18(O2C(CH2)12CH3
)24(TOP) 3.In a typical synthesis, myristic acid (0.548 g, 2.4 mmol) is added to an oven-dried 15 mL 3-neck round-bottom flask fitted with a thermowell, T-adapter, and septum.The flask is placed under vacuum at 50 mtorr for 30 minutes after which the system is placed under positive nitrogen flow and anhydrous toluene (2 mL) is injected into the flask.Trimethylindium (128 mg, 0.8 mmol) is dissolved in 1 mL of toluene and added dropwise to the flask with 800 rpm stirring at 2 drops per second.The solution is left to stir for 30 minutes after which trioctylphosphine (1.0 mL, 0.831 g, 2.24 mmol) is injected followed by stirring at 1200 rpm for an additional 15 minutes.The temperature is raised to 110 °C and within 5 minutes of reaching this temperature a solution of As(SiMe3)3 (60 uL, 0.2 mmol) in anhydrous toluene (0.5 mL) is then promptly injected into the flask.After 40 minutes at 110 °C, the reaction is allowed to cool to room temperature.Once cool, the crude reaction is purified directly by size-exclusion chromatography in toluene in 0.75 mL increments.The toluene is removed under vacuum after purification to yield a dark orange, waxy solid.O2CCH2Ph)24(PEt2Ph)3. Phenylacetic acid (0.654 g, 4.8 mmol) is added to an ovendried 15 mL 3-neck round-bottom flask fitted with a thermowell, t-adapter, and septum.The flask is placed under vacuum at 50 mtorr for 30 minutes after which the system is placed under positive nitrogen flow and anhydrous toluene (2 mL) is injected into the flask.Trimethylindium (256 mg, 1.6 mmol) is dissolved in 1 mL of toluene and added dropwise to the flask with 800 rpm stirring at 2 drops per second.The solution is left to stir for 30 minutes after which diethylphenylphosphine (0.226 g, 279 uL, 1.6 mmol) is injected followed by stirring at 1200 rpm for an additional 15 minutes.The temperature is raised to 110 °C and within 5 minutes of reaching this temperature a solution of As(SiMe3)3 (120 uL, 0.4 mmol) in anhydrous toluene (0.5 mL) is then promptly injected into the flask.After 20 minutes at 110 °C, the reaction is allowed to cool to room temperature.Once cool, the crude reaction is purified directly by sizeexclusion chromatography in toluene in 0.75 mL increments.The toluene is removed under vacuum after purification to yield a dark orange, crystalline solid.ICP-OES of the purified material showed an In:As ratio of 1.42:1.Elemental analysis In26As18P3O48C222H213: calculated C 33.0%, H 2.60%, N 0.00%; actual C 40.55%, H 3.64%, N 0.00%.

Synthesis of In26As18(
For crystallization, under a nitrogen atmosphere, 4 mg of isolated cluster is dissolved in 500 μL of toluene in a 2 mL screw top scintillation vial and the opening of the vial is completely covered with aluminum foil.This vessel is then sealed in a 20 mL scintillation vial with 3 mL of pentane.After 6 days of pentane diffusion into the toluene, bright orange crystals grew at the bottom of the inner vial.Single crystals of this material could also be grown using an identical method with diethyl ether in place of toluene. Synthesis of In26As18(O2CCH2Ph)24(PBu3)3.The procedure is the same as reported above for In26As18(O2CCH2Ph)24(PEt2Ph)3 replacing diethylphenylphosphine with tri-n-butylphosphine (0.324 g, 395 uL, 1.6 mmol).
Single Crystal X-ray Diffraction Methods.An orange block from a toluene/pentane vapor diffusion, measuring 0.07 x 0.06 x 0.05 mm 3 was mounted on a loop with oil.Data was collected at -173°C on a Bruker APEX II single crystal X-ray diffractometer, Mo-radiation, equipped with a Miracol X-ray optical collimator.Crystal-to-detector distance was 40 mm and exposure time was 120 seconds per frame for all sets.The scan width was 0.7°.Data collection was 99.9% complete to 24.407° in ϴ.A total of 176486 reflections were collected covering the indices, -44<=h<=44, -24<=k<=24, -43<=l<=43.43904 reflections were symmetry independent and the elevated Rint = 0.2442 relates to the small sample size.Indexing and unit cell refinement indicated a monoclinic lattice.The space group was found to be P 21/c (No.14).The data was integrated and scaled using SAINT, SADABS within the APEX2 software package by Bruker. 12Solution by direct methods (SHELXT 13 ) produced a complete heavy atom phasing model consistent with the proposed structure.The structure was completed by difference Fourier synthesis with SHELXL. 14,15Scattering factors are from Waasmair and Kirfel. 16Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C---H distances in the range 0.95-1.00Angstrom.Isotropic thermal parameters Ueq were fixed such that they were 1.2Ueq of their parent atom Ueq for CH's and 1.5Ueq of their parent atom Ueq in case of methyl groups.All nonhydrogen atoms were refined anisotropically by full-matrix least-squares.The contribution of disordered toluene and possibly pentane solvent to the diffraction pattern was removed with SQUEEZE, 17 and some disorder of the bound ligands was modeled.The crystallographic data for the structure has been deposited in the Cambridge Crystallographic Database under deposition number 2308638.

Figure S1 .
Figure S1.As(SiMe3)3 was injected into indium carboxylate in toluene as described in the synthesis of In26As18(Myr)24(TOP)3 in the absence of phosphine.With no development of recognizable absorbance features, 1.4 equivalents of trioctylphosphine with respect to indium was injected into the reaction causing the immediate formation of InAs clusters.

Figure S2 .
Figure S2.Equivalent InAs cluster reactions using two different phosphine ligands.The overall outcome of the synthesis as well as the kinetics of conversion do not seem to vary with n-alklyphosphine chain length.

Figure S4 .
Figure S4.Final crude normalized absorbance traces from the reactions shown in FigureS3suggesting the reaction outcome does not correlate meaningfully with concentration despite the stark differences in conversion kinetics.

Figure S5 .
Figure S5.Photoluminescence of In26As18(O2CCH2Ph)24(PBu3)3 at room temperature (green) and 77 K (red) with the absorbance shown in black.The sharp feature at 530 nm in the room temperature spectrum designated by the asterisk is a Raman feature from the solvent and does not represent photoluminescence from the sample.

Figure S6 .
Figure S6.TEM micrographs of InAs clusters in a superlattice assembled by slow evaporation of toluene (left) and as discrete particles (right).

Figure S8 .
Figure S8.Direct purification of an InAs nanocluster reaction by size-exclusion chromatography.The yellow band is the In26As18 cluster and the dark red band is quantum dot impurity.

Figure S9 .
Figure S9.Photograph of the isolated single crystals grown from an ether against pentane vapor diffusion.

Figure S10 .
Figure S10. 31P-NMR of purified In26As18 cluster with phenylacetate and tributylphosphine ligands.The inset focuses on the single feature at -18.4 ppm which is indicative of bound phosphine.

Figure S11. 1 H
Figure S11. 1 H NMR spectrum of phenylacetate and tributylphosphine ligated InAs clusters.The inset shows the methylene region of the phenylacetate ligands.

Figure S12 .
Figure S12.Solution-phase FTIR of phenylacetate and tributylphosphine ligated InAs clusters in tetrachloroethylene.The isolated ring breathing modes from phenylacetate are marked by asterisks.

Figure S13 .
Figure S13.Fully ligated In26As18(O2CCH2Ph)24(PEt2Ph)3 cluster (top).Fully ligated In37P20(O2CCH2Ph)51 cluster (bottom).All carbons are presented through a space filling model and the underlying structures of the clusters are shown in wireframe.Hydrogens removed for clarity.3-coordinate arsenic atoms that are present in the InAs cluster lead to less surface protection by carboxylates compared to the more cation-rich In37P20 cluster.

Figure S17 .
Figure S17.Structural overlap between the core In26As18 and bulk wurtzite InAs.The mirrored tetrahedra in the bulk wurtzite structure maintain an eclipsed relationship with no rotation as the lattice extends.With the pseudo-wurtzite phase of the cluster, the mirrored, alternating tetrahedra rotate slightly along the c axis misaligning the cluster phase with that of the bulk.For clarity, the core In and As-based tetrahedra of the cluster are shown in ball-and-stick whereas the atoms that extend beyond the central tower of tetrahedra are shown in wireframe.Color key: As from In26As18 (purple), In from In26As18 (green), As from bulk wurtzite InAs (pink), In from bulk wurtzite InAs (brown).

Figure S18 .
Figure S18.Powder x-ray diffraction of phenylacetate and tributylphosphine ligated InAs clusters (red).The diffraction standard of bulk wurtzite InAs is shows in black.

Figure S19 .
Figure S19.In26As18 structure with the surface In atoms removed shown in wireframe except for the three In3As units that form the base of the bullet-like shape shown in ball-and-stick (top left).Cd26Se17 structure shown in wireframe except for the three Cd3Se units that form the three corners of the base of the pseudotetrahedral shape shown in ball-and-stick (top right).Superimposed Cd26Se17 and In26As18 structures showing that the underlying M17E14 structure is homologous and the placement of the three M3E units differentiate the two structures viewed from the side (bottom right) and down the C3 axis (bottom left).

Figure S20 .
Figure S20.Structural overlay of experimentally determined x-ray structure and computed x-ray structure after geometric relaxation.The high degree of overlap indicates little geometric change for the computed structure.Color key: Experimental: As (purple), In (green), P (orange), O (red), C (grey) Computational: As (blue), In (brown), P (pink), O (red), C (grey).

Figure S21 .
Figure S21.Experimental Raman spectrum of phenylacetate and tributylphosphine ligated InAs clusters using λex = 532 nm for exposure.We attribute the signal from 160 -270 cm -1 to the molecular vibrations of the InAs lattice collective.The feature at 522 cm -1 , marked by the asterisk, is from the silicon substrate.

Table S1 :
Crystallographic data for the structures provided.