pubs.acs.org/cm Methods/Protocols Methods for the ICP-OES Analysis of Semiconductor Materials Calynn Morrison, Haochen Sun, Yuewei Yao, Richard A. Loomis,* and William E. Buhro* Cite This: Chem. Mater. 2020, 32, 1760−1768 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: The techniques employed in the compositional analysis of semiconductor materials by inductively coupled plasma optical emission spectroscopy (ICP-OES) dramatically influence the accuracy and reproducibility of the results. We describe methods for sample preparation, calibration, standard selection, fi C). and data collection. Speci c protocols are suggested for the analysis of II−VI compounds and nanocrystals containing the elements Zn, Cd, S, Se, and Te. We expect the methods provided will apply more generally to semiconductor materials from other families, such as to III−V and IV−VI nanocrystals. o legitimately share published articles. ■ INTRODUCTION lighter or volatile elements, degrade the surface, or change the oxidation state and crystal structures during the measure- Determination of the stoichiometries of semiconductor − ment.20 22 Difficulties determining exact stoichiometries have nanocrystals is a key aspect of their characterization. Most ff standard syntheses afford nanocrystals having a superstoichio- been reported in XPS, with up to 46% di erence between fi metric layer of metal cations at their surfaces, such that the initial and nal measurements of metal ions in samples during − fi 22 metal-to-nonmetal ratio M/E exceeds one.1 6 The excess depth pro ling. Some techniques, such as XRF, require that “ fi charge from the superstoichiometric cations is counterbalanced samples must be homogeneous and meet in nite thickness 23 by surface-bound anionic ligands. Such nonstoichiometric requirements” in order to produce accurate results. Ligand semiconductor nanocrystals may be interconverted with exchange or surface-chemistry experiments may only result in stoichiometric nanocrystals having charge-neutral surface differences of a single monolayer of atoms between ligation.2,4 Thus, the core stoichiometries of semiconductor nanostructures,2,4,12,24 and thus, beam-analysis techniques nanocrystals are variable and dependent on the surface ligation could yield stoichiometries that deviate from actuality. and nanocrystal size. Understanding and controlling the Errors in composition similar to those in beam techniques Downloaded via WASHINGTON UNIV on June 24, 2020 at 16:36:41 (UT surface chemistry benefits from the precise determination of often accompany digestion-requisite techniques. The forma- nanocrystal core stoichiometry. tion of volatile products of acid digestion results in losses before Compositional analyses of semiconductor nanocrystals have the measurement.25,26 However, by altering the digestion and been conducted by X-ray photoelectron spectroscopy sample-preparation method, volatile elements and surface See https://pubs.acs.org/sharingguidelines for options on how t − − (XPS),7 10 Rutherford back scattering (RBS),1,2,7,8,11 13 components may be retained for measurement, as we 14 atomic-absorption spectroscopy, inductively coupled plasma demonstrate in this paper. Analyses performed by ICP-OES, 3,15,16 mass spectrometry (ICP-MS), inductively coupled ICP-MS, and FAAS are total-composition measurements. 6,15,17,18 plasma optical-emission spectroscopy (ICP-OES), X- Because the samples are digested prior to analysis, particle fl 19 ray uorescence spectroscopy (XRF), and energy-dispersive size or sample thickness is irrelevant. ICP-OES and ICP-MS X-ray spectroscopy (EDS) using a transmission electron 6 are capable of detecting concentrations in the parts per million microscope (TEM). These techniques can be divided into (mg/L) and parts per billion range, respectively.25 The two categories: “beam techniques”, which include XPS, RBS, “ ” detection limits for S and Se, for example, may be as low as XRF, and EDS, and digestion-requisite analyses , with ICP- 0.3 mg/L and 0.2 mg/L, respectively.27 ICP techniques are OES, ICP-MS, and flame atomic absorption spectroscopy (FAAS) requiring that samples be digested prior to analysis. Whileeachanalysismethodbringsadvantagesand Received: January 20, 2020 disadvantages, accurate and precise determination of lighter Revised: February 21, 2020 elements or volatile components of semiconducting materials Published: February 21, 2020 can be challenging regardless of the analytical technique employed. In the case of beam techniques, high-energy photons, electrons, or ions strike the material and may eject © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.chemmater.0c00255 1760 Chem. Mater. 2020, 32, 1760−1768 Chemistry of Materials pubs.acs.org/cm Methods/Protocols capable of analyzing upward of 70 elements simultaneously. Ultrapure water was obtained from a Millipore Direct-Q UV- With multiple emission wavelengths per element, interferences 3 filtration system. from spectral overlaps can generally be overcome. ICP-OES Polypropylene and high-density polyethylene screw-cap also offers the convenience to the user of high sample centrifuge tubes were obtained from Corning and VWR throughput. (Figure S1). The 1000 μL solvent-resistant Nichiryo Nichipet In the majority of studies reporting the stoichiometries of EX Plus II and Eppendorf ResearchPlus Micropipettes were semiconductor nanocrystals cited above, the measured M/E employed during ICP-OES sample preparation procedures. ratios have fallen in the range of 1.0−1.8,1,3,7,9,13,15,16,19 and in Analyses were conducted using a PerkinElmer ICP-OES PE about a third of these studies the determined values were Optima 7300DV with Syngistix for ICP, Version 2.0.0.22336 larger.6,14,17,18 Interestingly, all but one of these latter studies software. Detailed instrument settings can be found in Table employed a digestion technique prior to analysis. In the S1. specific case of zinc blende (ZB) CdSe nanoplatelets, four- monolayer specimens with an expected Cd/Se ratio of 1.25 ■ PROCEDURE 15 were analyzed for 1.80 ± 0.07, and five-monolayer specimens Estimation of Concentration from Synthesis. Analyte 28 with an expected Cd/Se ratio of 1.20 were analyzed for 1.72 solutions were ultimately prepared in the concentration range 15 ± 0.11, by ICP-OES. (The expected Cd/Se ratios are taken of 0.5−20 mg/L (see Note S1 in the Supporting Information). from an analysis of ZB CdS nanoplatelets having the same This required estimation of the amount of semiconductor numbers of monolayers.28) While these measured ratios are compound contained in an analytical sample. We assumed close to expectation, they are higher, matching the outcomes of 100% yield in a given semiconductor nanocrystal synthesis, on most of our initial efforts at ICP-OES analysis. These the basis of the limiting reagent. The following example is 29,30 observations and the experiences conveyed to us by others given. In a typical synthesis of {CdSe[n-octylamine]0.53} · motivated us to conduct a thorough investigation of the ICP- quantum belts (QBs), Cd(OAc)2 2H2O (0.039 g, 1.5 mmol) OES analysis of semiconductor materials. and selenourea (0.029 g, 2.4 mmol) were allowed to react in n- · The ICP-OES method detailed in this paper was developed octylamine (3.825 g), making Cd(OAc)2 2H2O the limiting using samples with known or strong theoretical bases for reagent.31 The total mass of the crude reaction mixture was compositions, and protocols were optimized to produce 3.893 g. reproducible results that could be compared to the expected An aliquot (0.157 g, 200 μL) was withdrawn from the values. We report results for four samples of semiconductor reaction mixture, constituting 4% of the mixture and − nanocrystals, one nanocluster, and one molecular compound as containing 4% of the original cadmium (6.6 × 10 4 g, 5.8 test cases. μmol). This aliquot was ultimately digested in hydrogen We identify several pitfalls associated with conventional peroxide (500 μL) and nitric acid (500 μL) to produce a ICP-OES analysis. We offer suggestions for improving concentrated digestion solution. The estimated Cd and Se reproducibility and accuracy in methods for sample digestion, concentrations in this solution were therefore 660 mg/L. As calibration-curve construction, and emission-line and calibra- described below, aliquots of the concentrated digestion tion-standard selection. We provide specific protocols for the solution were diluted to 10.00 mL, to produce analytical analysis of compounds and nanocrystals containing cadmium, solutions having Cd-analyte concentrations of 3.3−13.1 mg/L, zinc, sulfur, selenium, and tellurium, which we expect to apply which were within the desired range for analysis. more generally to the analysis of other semiconductor General Sample Preparation and Digestion. An aliquot materials, such as InP and PbS. We demonstrate that of a nanocrystal dispersion estimated to yield an analyte seemingly small variations in sample preparation, calibration, solution with a final concentration below 20 mg/L (see above) and data collection can have a large impact on the reliability of was purified by three centrifugation−washing cycles using the results. centrifugation at 2000 rpm for 3 min, 2 mL of toluene, and a borosilicate test tube in each cycle, each time disposing of the ■ MATERIALS supernatant to remove excess organic material and unreacted precursors. The pellet obtained from the third centrifugation The following nanomaterials were prepared as described in step was then suspended in isopropanol or ethanol, which are previous reports: {CdSe[n-octylamine]0.53} quantum belts solvents compatible with polypropylene and high-density 31 4 33 (QBs), {CdSe[Cd(oleate)2]0.19}QBs, [(CdSe)13(n- polyethylene (HDPE) centrifuge tubes. The dispersion was 32 PrNH2)13] clusters, and {CdTe0.50Se0.50[oleylamine]x} and transferred to a centrifuge tube and again centrifuged (2000 26 {CdTe0.73S0.27[oleylamine]z} quantum platelets (QPs). rpm, 3 min), and the supernatant was removed. The tube was Trace-metal grade nitric acid (70%) and trace-metal grade sealed with a rubber septum and dried under vacuum via hydrogen peroxide (32%) were obtained from Millipore Sigma.