P Nanocrystals Synthesized in a Nonthermal Plasma

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Morphological Control of InGa P Nanocrystals Synthesized in a Nonthermal Plasma Noah D Bronstein, Lance M. Wheeler, Nicholas C. Anderson, and Nathan R. Neale Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01358 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Chemistry of Materials

Morphological Control of InxGa1-xP Nanocrystals Synthesized in a Nonthermal Plasma Noah D. Bronstein, Lance M. Wheeler, Nicholas C. Anderson, Nathan R. Neale* Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Email: [email protected]

Abstract We explore the growth of InxGa1-xP nanocrystals (x = 1, InP; x = 0, GaP; and 1 > x > 0, alloys) in a nonthermal plasma. By tuning the reactor conditions, we gain control over the morphology of the final product, producing either 10 nm diameter hollow nanocrystals or smaller 3 nm solid nanocrystals. We observe the gas-phase chemistry in the plasma reactor using plasma emission spectroscopy to understand the growth mechanism of the hollow versus solid morphology. We also connect this plasma chemistry to the subsequent native surface chemistry of the nanocrystals, which is dominated by the presence of both dative- and lattice-bound phosphine species. The dative phosphines react readily with oleylamine in an L-type ligand exchange reaction, evolving phosphines and allowing the particles to be dispersed in nonpolar solvents. Subsequent treatment by HF causes the solid InP1.5 and In0.5Ga0.5P1.3 to become photoluminescent, whereas the hollow particles remain non-emissive.

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Introduction Semiconductor nanocrystals (NCs)1 have achieved commercial success for applications requiring efficient and spectrally pure fluorophores like bioimaging2 and electronic displays.3 Core-shell NCs based on CdSe are excellent red and green emitters with quantum yields approaching unity and emission linewidths of around 20 nm.4 The toxicity of cadmium5 is a major barrier to widespread adoption, and this has inspired an effort in both research and commercial labs to replace CdSe with InP. In addition, there are other III–V compounds like InAs, InSb, GaAs, etc. that could lower the emission energy of semiconductor NCs into the nearand mid-infrared if they could be made of a sufficiently high quality and with the appropriate passivation layers. To date, the emission efficiency and linewidth of colloidally synthesized III–V NCs has lagged behind their II–VI and IV–VI cousins. The problem is multifaceted and related to both the oxidative instability and the highly covalent nature of group III and V elements which makes precursor selection and NC growth reactions challenging.6-10 These challenges are even more severe with the more strongly covalent group IV semiconductors and were substantially solved with a nonthermal plasma synthesis technique.11-15 In addition, there are reports on the plasma synthesis of ZnO16, Cu2S17, SiC18, GaN19, and InP20 NCs. The lone report on InP by Gresback et al.20 demonstrated size control and luminescence after post-synthetic treatment with ZnS via colloidal methods. A better understanding of the mechanism of formation, plasma chemistry, and resulting particle structure and surface chemistry would help assess the benefits and limitations of nonthermal plasma synthesis for III–V semiconductor NCs. In this study, we extend the work by Gresback et al.20 to synthesize both solid and hollow InxGa1-xP NCs in a nonthermal plasma reactor (Scheme 1). The particles are made by flowing metal-organic chemical vapor deposition precursors M(CH3)3 (where M = Ga or In), PH3, Ar, and H2 into a quartz reactor tube at low pressure (4 Torr) and igniting the capacitively-coupled plasma using a 13.56 MHz radio-frequency (RF) power source. We control the particle morphology from hollow to solid by changing the gas residence time and plasma power density. We show that the chemical species observable in the plasma by emission spectroscopy are good reporters on the complex reaction mechanisms that control the NC morphology. We conclude that low plasma power densities (3 W/cm3) and short residence times (4 ms) yield solid InxGa1-xP NCs, as In0 and/or In-rich InPx nanoparticle formation does not occur under these conditions. Chemical treatment using HF renders solid InP and InGaP NCs emissive, whereas hollow NCs remain dark, presumably because fluoride ions cannot diffuse through the InxGa1-xP to reach and passivate defects at the inner surface of the NC void. We additionally investigate the surface chemistry and reactivity of the InxGa1-xP NCs. We show that the chemistry present in the plasma dictates the native surface chemistry of the solid or hollow InxGa1-xP NCs. In all cases the as-synthesized surface is composed of methyl metals, metal hydrides, and both dative and covalently-bound phosphines. Under the high power density/short residence time conditions, we also observe methylphosphine and dimethylphosphine, indicating some methyl transfer to phosphorus in the plasma. We demonstrate an L-type ligand exchange reaction on the NC surface, where oleylamine displaces


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datively-bound phosphines. This work demonstrates the utility and flexibility of nonthermal plasmas in synthesizing III–V semiconductor NCs, which affords good control over size and composition while generating a surface chemistry that is amenable to further reactivity. This work provides a key foundation for further optimization of this technique and subsequent surface chemistry manipulation (e.g., shell growth) that could ultimately enable plasma-synthesized III– V NCs to compete with colloidally grown metal chalcogenides in emission and related optoelectronic applications.

Scheme 1. Formation mechanisms for hollow and solid InxGa1-xP NCs. The plasma power densities (W/cm3) and residence times are controlled by changing the delivered power, gas flow rate, pressure, and reactor volume (reactor tube inner diameter). Low power density and long residence times result in hollow NCs with a 9–11 nm outer diameter and 2–3 nm internal void. In contast, high power density and short residence times result in solid 3–4 nm diameter NCs with no internal void. Both hollow and solid NCs react with oleylamine in toluene at room temperature to form colloidal dispersions via L-type ligand exchange reactions. Yellow spheres = P atoms. Gray spheres = In/Ga atoms.

Experimental A. Materials Benzene-d6 (Sigma Aldrich, 99.6%) and toluene (solvent grade) were dried over NaK eutectic alloy, distilled, and then vacuum transferred after three freeze-pump-thaw cycles on a Schlenk line. Oleylamine (Sigma Aldrich, tech grade) was heated to 100 °C under vacuum overnight before storing in an argon-filled glovebox with a solvent trap. The glovebox is maintained to achieve between 0.1 and 0.3 ppm O2 and H2O at all times. B. Gas-phase Synthesis of InP NCs InP NCs were prepared using a custom-built nonthermal plasma reactor using hydrogen, argon, phosphine (PH3, Matheson ULSI 5N7), trimethylindium (TMI, SAFC Hitech, Adduct



Grade, Solution TMI) and trimethylgallium (TMG, SAFC Hitech, Adduct Grade, Solution TMI) vapors. Process flow and power conditions are listed in Table 1 and are briefly described below. Argon was flowed through the TMI and/or TMG bubblers, which are temperature controlled with a thermoelectric cooler and resistive heater. The bubbler pressure was controlled to deliver a combined trimethylmetal flow rate of 0.05 mmol/min, following vapor pressure curves from literature for TMI22 and TMG.23 The molar ratio of PH3 to M(CH3)3 is above 4:1 in all cases, as a lower excess of PH3 causes In0 metal film formation on the reactor walls. Additional Ar (sometimes necessary to reach 50 sccm of total Ar flow) and H2 (100 sccm) was blended with the TMI/Ar just upstream of the plasma, which is contained in a quartz tube with either 7 mm or 19 mm inner diameter (I.D). Changing the reactor tube diameter (and consequently the tube volume) affects changes on the residence time of the reactive species in the plasma but allows other pieces of the experimental apparatus to be held constant. For example, it is easy to change the tube volume (and therefore residence time) by a factor of 10. The plasma was ignited by applying a forward power of 75–100 W at 13.56 MHz via an Advanced Energy Cesar 136 generator through an Advanced Energy VM1000 matching network (tuned to give a reflected power of 0–1 W) to a copper electrode wrapped around the quartz reactor tube. A grounded electrode was positioned downstream and separated from the hot electrode by 1 cm for the 19 mm I.D. reactor tube and 2 cm for the 7 mm I.D. reactor tube. A copper mesh cage was wrapped around the hot electrode and quartz tube to form a Faraday cage to mitigate the strong radio field. An Advanced Energy Z’Scan device connected directly at the working electrode head (corrected for the impedance of the electrode and tube configuration) was used to dynamically monitor the plasma conditions from which the delivered power could be monitored. In the case of the small solid GaP, the Z’Scan was not used, but the dilvered power was estimated assuming the same coupling efficiency (P /P ) as the other 7 mm diameter tube runs. The pressure of the reactor is stabilized at 4.00 +/- 0.01 Torr by a PID-tuned butterfly valve. The resulting NCs were collected downstream from the plasma on a 400-mesh stainless steel filter, cut and press-fit into KF40 connector, and transferred via load-lock to the inertatmosphere glove box for collection. Typical isolated NC yields were 10–30% based on the In/Ga precursor flow (phosphorus is in excess). Different combinations of plasma power density and residence time quickly brought the experiment away from the conditions that form crystalline nanoparticles. Under short residence times, reducing the power from ~4 W/cm3 to ~3 W/cm3 decreases the particle size below 3 nm. For long residence times, decreasing the power density below the values reported in Table 2 results in an amorphous crystal structure. Also for the long residence time case, increasing the power density from 1.3 W/cm3 to 2.6 W/cm3 results in no particulate material being collected at all, presumably as all of the In0(Ga0) and/or In(Ga)-rich InxGa1-xP particles in the pre-glow region deposit on the reactor tube wall. Therefore, we find that the conditions reported in this manuscript represent two local optima in the nonthermal plasma reactor condition space. We were able to achieve some size control of solid InP NCs, similar to Gresback et al.,20 by changing the reactor pressure and gas concentrations (see Fig. S5 and Table S1).


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Table 2. Reactor conditions for plasma synthesis of InxGa1-xP NCs. Tube I.D.

P Fwd

P Dlv

P Den

W

W

W/ cm3

mm

PH3

H2

Ar+ TMI

TMI T

TMI P

Ar+ TMG

TMG T

TMG P

Extra Ar

sccm

sccm

sccm

°C

Torr

sccm

°C

Torr

sccm

InP InGaP

7 7

75 75

16 14.4

4.2 3.7

5.4 5.4

100 100

50 40

20 23

49 89

0 10

--11

-500

0 0

GaP InP

7 19

100 100

-37

-1.3

5.4 5

100 100

0 50

-23

-95

25 0

-16 --

500 --

25 0

InGaP GaP

19 19

75 100

30 40

1.1 1.4

5 2.7

100 100

36 0

23 --

80 --

10 20

-1 -1

950 950

0 26

C. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFTS measurements were performed on Bruker Alpha FTIR spectrometer inside the argon-atmosphere glovebox. Spectra were collected by averaging between 24 and 128 scans at 2 cm–1 resolution. Samples were prepared by evaporating a toluene slurry of the as-prepared NC powder on a gold-coated silicon wafer. D. Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) TEM images were acquired by drop-casting dispersed particles from toluene and oleylamine onto ultrathin carbon on holey carbon TEM grids (Ted Pella). For the small particles, the samples were washed by a precipitation/centrifugation method using toluene as the solvent and methyl acetate as the antisolvent. These grids were then imaged with a Technai T30 S-Twin 300 transmission electron microscope at 300 kV and a Gatan Orius 830 CCD camera. These images were analyzed using a custom script in MATLAB. In short, the script does a rolling ball background subtraction, a threshold to turn the images into binary (with threshold values chosen by hand) Voronoi analysis to cut apart particles that are hard to distinguish by threshold, and a watershed image segmentation to identify isolated shapes. Then particles are identified from this list of shapes by filtering for size, roundness, and circularity. EDS data were collected on a Li-drifted silicon detector attached to the Tecnai ST30, operated at 300 kV. The detector is a AMETEK EDAX Model PV97-61730-ME with a 30 mm^2 active area, amplifier model 204 B+, and window type 3.3. The measurements were performed with 10 eV/channel, resulting in a FWHM of 147 eV. Electron beam current and dead time were optimized to allow sufficient signal to be collected in 10 to 30 minutes. The analysis procedure was according to the normal method for the built-in software: first we fit the background to a 3rd order polynomial locally around each group of peaks, then fit each peak and integrate the fit. The atomic composition was extracted from these peak areas according to the standard procedure, which is detailed in the Supporting Information.



E. 31P NMR Spectroscopy All NMR samples were prepared in J. Young tubes under an inert-atmosphere. Nanocrystals were dissolved in 600 µL to 1 mL of a 10 % v/v oleylamine solution in benzene-d6. NMR spectroscopy was performed on a Bruker 400 Avance III NMR instrument. 31P spectra for the NCs made at low plasma power density were acquired using standard phosphorus pulses, 0.5 s collection times, and 2.0 s delay between scans at 25° C for 15 h. 31P spectra for the NCs made at high plasma power density were acquired using a slightly different pulse sequence with a 0.2 s collection time and no delay between scans, for a total acquisition time of 15 h. F. X-Ray Diffraction (XRD) Samples were prepared by evaporating a toluene slurry of the nanocrystal powder on a silicon zero-diffraction plate (MTI corporation, B-doped polished Silicon crystal). Spectra were acquired on a Rigaku D-Max with a Cu Kα source and a monochromated point detector. CAUTION: The XRD samples were left in a chemical hood for at least 1 h before data acquisition due to the odiferous and toxic off-gassing of phosphine species upon air exposure. The instrument’s and 2 were aligned for each sample using the automatic alignment procedure. The spectra were then acquired at the rate of ~2° per min using settings that result in a total instrument broadening of around 0.17°. Total acquisition time in the Rigaku is less than 1 h to avoid a broad shoulder peak near 20–22° that grows in slowly over the course of several h of X-ray irradiation, which may be due to the growth of a surface oxide. G. Absorption and Photoluminescence (PL) Spectroscopies Absorption and PL spectra were acquired using an OceanOptics OceanFX fiber-optically coupled Silicon CCD array. The OceanFX was controlled with custom LabVIEW software that allows extremely long averaging times (from ms to h) while maintaining a correct dark signal by using a light on-off acquisition sequence with a shutter cycle time of a few hundred ms. For absorption spectroscopy, the light source was an Ocean Optics HL2000-HP halogen lamp. To achieve air-free measurement, the light is coupled into the argon-filled glovebox and back out using 1000 m diameter low OH fibers and a 1000 m diameter high OH coupler. The final fiber, coupling the light back into the OceanFX detector after passing out of the glovebox through the coupler is a 400 m diameter fiber. Typical acquisition times for absorption are an integration time of 25 s with an averaging time of a few s. For PL spectroscopy, the samples were measured in air after treatment with HF (see below). The light source is a ThorLabs M405FP1 fiber coupled 405 nm LED, controlled by a ThorLabs DC2200 high power LED Driver. Typical output power after coupling through a single 1000 m diameter fiber is around 250 mW, which is allowed to have two passes through the sample by the use of a mirror on the back side of the cuvette. Typical acquisition times for PL are an integration time of 100 ms and an averaging time of a few min. The spectral sensitivity of the detector was calibrated against the HL2000-HP tungsten halogen lamp, assuming it is a perfect blackbody with a temperature of 3000 K. For both absorption and emission spectroscopy, the Low-OH fibers have a shortwavelength cutoff around 350 nm and a long-wavelength cutoff around 2200 nm. H. Fluoride treatment of InxGa1-xP NCs Two different methods were used, both with similar results. The first method is in accordance with literature24: HF (48% w/w) was diluted into butanol to a concentration of


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approximately 1% w/w. This solution was then added dropwise to the InxGa1-xP NC/toluene/oleylamine solution and heated gently (80 °C) until the NCs emitted light upon 405 nm excitation. In the second method, NH4F crystals were added to an acetone/methyl acetate/water mixture (~10 mg NH4F/mL solvent, roughly equal parts each of acetone, methyl acetate, and water), which was then added to the InxGa1-xP NC/toluene/oleylamine solution and heated gently (80 °C) until the NCs emitted light upon 405 nm excitation. This alternative method tended to result in precipitation of the particles, but was successful in creating an emissive state and is considerably safer since it does not use concentrated HF. I. Plasma Emission Spectroscopy The OceanFX and custom LabVIEW script were used again to acquire the emission spectrum of the plasma. A UV-grade free-space to fiber coupler (OceanOptics 84-UV-25) was used to collect the plasma emission light and send it to the OceanFX. The coupler was scanned along the length of the tube using a ThorLabs DDS300-E linear translation stage and BBD201 motion controller. The light on-off pulse sequence was not used in this case. Instead, a dark spectrum was collected with the plasma off at each location along the tube, and then a light spectrum was collected with the plasma ignited for each location. The OeanFX integration time was 100 ms for the long residence time reaction condition and 200 ms for the short residence time condition. The scan speed was set to around 40 mm per second for a total experiment time of less than 10 s. The OceanFX entrance slit was 100 m in both cases, resulting in a spectral resolution of 4.6 nm FWHM. The spectral sensitivity of the detector was calibrated against the HL2000-HP tungsten halogen lamp, assuming it is a perfect blackbody with a temperature of 3000 K. The short-wavelength cutoff of this experiment is dictated by the choice of High-OH fiber (ThorLabs M93L01) at around 250 nm. The resulting spectra were fit over a narrow wavelength range (from 310 to 360 nm) using a 2-peak Gaussian model with a Gaussian background to subtract the background broad emission from H2 centered at 330 nm and extract the peak heights and areas for the In+ peak at 325 nm and the PH* peak at 340 nm.

Results and Discussion Figure 1a–f shows typical transmission electron miscroscope (TEM) images of the plasma-grown NCs. A summary of the morphology, composition, and size is found in Table 1. For all compositions, long residence time (30 ms) and low power densities (3 W/cm3) produces much smaller particles, with average diameters of 3–4 nm (Fig. 1d–f). X-ray diffraction (XRD) analysis (Fig. 1g-h) confirms the zincblende phase of all the InxGa1-xP NC samples. For the large hollow NCs, the grain size by Scherrer broadening25 is determined to be 4–5 nm (Supporting Information, Sections S4 and S5), indicating that the internal void is surrounded by a polycrystalline shell with a domain size roughly equal to the shell thickness. High-resolution TEM micrographs of this polycrystallinity can be found in the Supporting Information (Figs. S1– S3). In contrast, the solid NCs have diameters consistent with the grain size determined by XRD suggesting that these are single crystalline. A full description of the different InxGa1-xP NC samples including the synthesis conditions and sizes is shown in Table 1.



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Figure 1. Structural and Optical Characterization. TEM micrographs of (a) hollow InP, (b) hollow InGaP, and (c) hollow GaP NCs; and TEM micrographs of (d) solid InP, (e) solid InGaP, (f) solid GaP NCs. In all TEM micrographs, the scale bar is 20 nm. XRD patterns of InP, InGaP, and GaP NCs with (g) solid and (h) hollow morphology; optical absorption (solid lines) and photoluminescence (PL, dashed lines) spectra of solubilized InP, InGaP, and GaP NCs with (i) solid and (j) hollow morphology. Table 1. InxGa1-xP NC morphology, composition, and size. *Composition was measured via a Vegard’s law analysis of the XRD peak position for the large hollow particles (Figs. S6–S11 and associated discussion) and by TEM-EDS for the small solid particles due to the broad and weak XRD signal (Fig. S12, Table S2, and associated discussion). **The power density for solid GaP is estimated; see Experimental. ***The residence time is determined as the length of the plasma where the intensity of the 737 nm Ar+ emission line is ≥5% of its maximum value (Figs. S14– S15). Morphology

Power Density

Residence Time***

W/cm3

ms

Composition*

Grain size (XRD)

Diameter (TEM)

Void size (TEM)

nm

nm

nm

Solid

4.2

4

InP1.5

3.4

3.3 +/- 1

--

Solid

3.7

4

In0.5Ga0.5P1.3

2.8

3.0 +/- 0.8

--

Solid

4.5**

4

GaP0.9

3.8

3.9 +/- 0.7

--

Hollow

1.3

30

InPx

5.2

10.7 +/- 2

3.3 +/- 1

Hollow Hollow

1.1 1.4

30 30

In0.5Ga0.5Px GaPx

4.5 4.1

8.7 +/- 2 9.0 +/- 2

2.5 +/- 1 2.5 +/- 1


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The as-prepared InxGa1-xP NCs are easily solvated into 0.1 M oleylamine solutions in toluene via ligand exchange (see below). The absorption spectra for solubilized small solid InP and InGaP NCs show broad excitonic features near 625 and 550 nm, respectively (Fig. 1i). When these NCs are treated with a fluoride source in the form of HF or NH4F—a strategy known to temporarily passivate surface defects in InP NCs24,26,27—photoluminesce (PL) is observed with emission peaks at 665 and 580 nm, respectively (Fig. 1i). The GaP NCs do not an exhibit an excitonic feature and do not emit following treatment with HF, most likely because GaP has an indirect bandgap. This implies that even if the radiative rate of GaP is enhanced due to symmetry-breaking at the surface,28 it still is much slower than the nonradiative recombination rate. Similarly, the hollow NCs form solutions when treated with 0.1 M oleylamine in toluene. However, in contrast to solutions of the solid NCs, the absorption spectra of the hollow NCs do not exhibit distinct excitonic features (Fig. 1j). The bandgap of the hollow InP NC is around 1.5 eV (820 nm) from its optical absorption onset, which suggests a particle size of ~10 nm by the effective mass approximation (EMA, see Supporting Information), consistent with the outer diameter of the polycrystalline NCs determined by TEM. Given that the EMA would predict a much larger bandgap of 2.1–2.5 eV based on the XRD grain size of 4–5 nm, it appears that these grains and the internal void have minimal effect on the observed band gap. Also in contrast to the solid NCs, fluoride treatment did not result in measurable PL for the hollow NCs. This is likely due to the fact that the fluoride ions cannot diffuse through the InxGa1-xP lattice to access the internal surfaces of the NCs (and/or grain boundaries between individual crystallites making up the walls), leaving unpassivated surfaces with fast nonradiative recombination sites. In sum, these fluoride-treatment experiments provide strong evidence that the 3–4 nm NCs produced at high plasma power density and short residence time are in fact solid, since emission would not be expected if void space were present. Hollow InP micoparticles have have been formed in previous reports by the hollowing of indium micropowders during phosphidization.29-31 Since the indium diffusion is much faster than the phosphorus diffusion32 through the growing InP shell on the surface of the In0 metal particle under the growth conditions, vacancies are left at the particle core when In atoms diffuse out through the shell to react with incoming phosphorus atoms. This phenomenon is the well-known nanoscale Kirkendall Effect21 that has been invoked in the formation of hollow transition metal phosphides33,34, sulfides35, selenides, tellurides, and oxides.36,37 This implies that our synthetic process likely first generates In0, Ga0, or InxGa1-x0 metal nanoparticles that then react with phosphorus sources to make InP, GaP, and InxGa1-xP NCs. To probe the growth mechanism in detail, we performed plasma emission spectroscopy by coupling the emitted light from the plasma through a fiber and onto a linear silicon CCD array detector. A cartoon of this measurement is depicted in Figure 2. The fiber was scanned along the length of the reactor tube to allow measurement of the emission spectrum at many different locations upstream from the working electrode (the pre-glow region), near the working electrode (the most intense part of the plasma), and downstream from the working electrode (the afterglow region). The emission spectrum of the different component gases under these conditions is shown in Figures S13, as well as the emission spectra during NC growth over the entire range of the detector from 200 to 1000 nm (Figs. S16–S17). Whereas analysis of the absolute emission intensity, and therefore absolute concentration of active species in the plasma, would be desirable, it is not possible. The PH* emission peak arises from both electron impact excitation of PH and dissociative excitation of PH3 to PH* and H2. The electron density and energy



distribution would be required to know the excitation rate of both the PH* and PH3 in the measurement volume. Rapid deposition of byproducts—strongly absorbing bulk metallic and/or semiconductor material—on the walls of the reactor tube make an absolute calibration of the optical detection efficiency during the reaction impossible.

Figure 2. Plasma emission spectroscopy for InP NC growth under both reaction conditions. Red circles: short residence time (4 ms) with high power density (4.2 W/cm3). Blue triangles: long residence time (30 ms) with low power density (1.3 W/cm3). The spectra are shown (a) far above the electrodes (–65 mm), (b) nearing the electrodes (–50 mm),(c) in between the electrodes where the plasma is the most intense (0 mm), and (d) after the ground electrode (+40 mm). (e) The ratio of the two labeled peak areas, In+/PH* (at 325/340 nm), are plotted as a function of position along the reactor tube for both reaction conditions. Full plasma emission spectra for each data point are presented in Figures S16–S17. It is important to note that, due to the rapid deposition of byproduct material on the walls of the reactor tube, the entire emission profile must be collected in only a few seconds.


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Therefore, the instrument conditions were optimized for speed at the expense of spectral resolution. This results in a relatively broad resolution of 5 nm. The higher resolution spectra of the component gases without In precursor acquired with longer acquisition times make clear that there were no overlapping peaks in the region of interest (Fig. S13). Given these experimental limitations, we compare the relative peak areas to understand the relative concentration of In+ and PH*. To compare the height of any two emission peaks in a non-thermal plasma, the activation energy (Ea) for those two excited states should be similar enough that any changes in electron energy distributions over the range of conditions in the experiment have minimal impact on the relative activation of the two different species.38 Further, because a byproduct semiconductor thin film deposits rapidly on the wall of the reactor tube, the two diagnostic peaks should be as close together as possible in wavelength to minimize differences in the transmissivity of the reactor tube itself. In this experiment, both of these conditions are met: (1) the two diagnostic peaks are sufficiently close in energy for robust analysis, with a characteristic sharp PH* emission line39,40 at 340 nm and a In+ transition at 325 nm41; and (2) the activitation energies are similar, with the PH* emission line arising from either dissociative excitation of PH3 (Ea = 7.2 eV) or direct excitation of PH (Ea = 3.6 eV), whereas that for In+ is in between these two values (Ea = 5.2 eV)42,43. This similarity in emission wavelength between the PH* and In+ emission lines indicate that trends in their relative intensity will be a good reporter on trends in the relative concentration of the two species, assuming the electron energy distribution does not change too much over the length of the reactor tube. It is also crucial that there are no In+ emission lines in the 335 nm to 345 nm spectral range, and we never detect the presence of any In2+ or In3+. Further, PH* does not have any lines that could overlap with the 325 nm emission from In+. Evaluation of the ratio of the In+ to the PH* peak areas over the entire length of the plasma (Fig. 2e) reveals key differences in the reaction pathways of the two different reaction conditions. The In+/PH* peak area ratio for the two conditions track together in the pre-glow region far above the most intense part of the plasma (≥40 mm). At 40 mm upstream from the working electrode, the In+/PH* peak area ratio diverges for the two reaction conditions. For conditions that generate hollow NCs (30 ms residence time and 1.3 W/cm3 power density; blue triangles, Fig. 2e), the In+/PH* ratio drops precipitously to near zero at 20 mm above the most intense part of the plasma. In contrast, conditions that generate solid NCs (4 ms residence time and 4.2 W/cm3 power density; red circles, Fig. 2e), the In+/PH* peak area ratio remains ≥1 until 20 mm downstream of the ground electrode and does not approach zero until ≥40 mm downstream of the working electrode, well into the afterglow region. Based on these results and the wealth of literature on plasma growth processes, we propose a growth mechanism that accounts for the hollow and solid NCs observed under these two reaction conditions. Far upstream from the working electrode (≥80 mm), the precursors begin to be activated. It is well known that precursor activation in this pre-glow region generates neutral or singly negatively charged nuclei at a large concentration.11 As these nuclei and additional precursor gases pass through the pre-glow region, neutral nuclei and small clusters coagulate and coalesce into larger, negatively charged particles. Given sufficient time in this zone (i.e., long 30 ms residence time), the very reactive In(CH3)3 forms In0 metal clusters11 and/or In-rich InPx clusters that coalesce into larger In0 metal and/or In-rich InPx particles, effectively depleting the gas mixture of available In precursor. As these In0 and/or In-rich InPx particles move downstream, the higher intensity plasma near the working electrode (starting at ~40 mm upstream of the working electrode) provides sufficient energy to promote substantial



surface reaction of PH* with these particles. Given the rapidly dwindling availability of In and abundance of P precursors, the only available growth pathway for In0 and/or In-rich InPx particles is through surface reaction with PH*, where growth proceeds via the nanoscale Kirkendall Effect. Consequently, hollow InP NCs are formed. In contrast, the short 4 ms residence time is insufficient for formation of large In0 metal and/or In-rich InPx particles in the pre-glow region. Instead, nuclei and precursor gases are rapidly swept into the more intense part of the plasma, where there is an abudance of both In+ and PH* species available. The In+/PH* ratio under this short 4 ms residence time condition shows that this process only fully depletes the In+ content in the gas stream after the NCs have left the most intense part of the plasma, well into the afterglow region (20–40 mm downstream of the working electrode). The result is that cluster coagulation and nanoparticle surface growth occurs via addition of both In and P atoms, and solid InP NCs are formed. The hydrogen-rich plasma growth environment and methyl-metal starting materials are evident in the surface chemistry of the resulting InxGa1-xP NCs as confirmed by diffusereflectance infrared Fourier transform spectroscopy (DRIFTS). The DRIFTS spectra in Figure 3a clearly demonstrate that the surfaces of all NC compositions and morphologies are composed of phosphines, metal hydrides, and metal methyls. Aside from the intense P–Hx stretches at ~2300 cm–1, the presence of methyl C–H stretches is apparent from the characteristic features at ~2900 cm–1, with some or all of these methyl groups bound to In (Ga) due to the presence of the In–CH3 (Ga–CH3) umbrella mode44 at 1150 cm–1 (1190 cm–1). We cannot determine from DRIFTS whether any methyl is bound to phosphorus because the C–P stretch is very weak and would be obscured by the many other peaks in the fingerprint region below 1000 cm–1. These peaks below 1000 cm–1 are composed of a combination of second-order phonons45, P–Hx deformations,46 In(Ga)–CH3 stretches,47 and out-of-plane CH3 deformations.48 We cannot conclusively assign any of these low-energy peaks due to the significant overlap between them and because the intensity of the second-order phonons is likely modified by the broken symmetry of the NCs. In addition to P–Hx and In(Ga)–CH3 surface species, we also observe terminal In–H and Ga–H stretches49,50 at 1630 cm–1 and 1850 cm–1, in InP and GaP NC samples, respectively. In the InGaP alloy, we observe In–H but not Ga–H. Finally, the unusual feature around 2672 cm–1 in the hollow GaP NCs corresponds to C–H sitting on a P vacancy ( ) with C3V symmetry (C–H aligned along a C–Ga bond).51 It is unclear why this happens for GaP but not InGaP or InP particles but one possible explanation is that during formation of a Ga0 metal or Ga-rich GaPx particle, CHx species either stay on the surface or are dissolved into the molten metal. Then during phosphidization and crystallization, CH is trapped only for GaP and not for InGaP or InP. It is possible that differences in solubility of CHx in the molten metal or differences in diffusion of CHx out through a crystallizing metal phosphide are responsible.


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Figure 3. (a) DRIFTS characterization of solid and hollow InxGa1-xP NCs; (b) Proton-coupled P NMR spectra of hollow InxGa1-xP NCs; (c) Proton-coupled 31P NMR spectra of solid InxGa1-xP NCs. 31

Differences in surface chemical bonding for hollow and solid NCs that are not discernible via DRIFTS were analyzed with 31P NMR spectroscopy (Figs. 3b–c). Similar to the optical studies conducted above, hollow InP and solid InP NCs were solvated in oleylamine solutions (0.1 M) in benzene-d6, but in these cases the NMR tube was quickly sealed following addition of oleylamine solution to the dry powder to prevent the escape of volatiles (Scheme 2). In all cases, PH3 gas is easily detectable as a quartet at δ –242.4 (JP-H = 186.6 Hz), indicating that oleylamine displaces PH3 from the surface to form soluble oleylamine-bound InP NCs. For solid NCs



produced at high plasma power density, methyl group transfer from the organometallic precursors onto the phosphorus precursor is evident in the 1H-coupled 31P spectrum52 (Fig. 3c) based on peaks at δ –161.94 (t, JP-H = 188.21 Hz; CH3PH2) and δ –99.25 (d, JP-H = 195.92 Hz; (CH3)2PH) in addition to PH3 at δ –242.44 (q, JP-H = 186.55 Hz). For the hollow NCs, broad peaks are observed for P in the lattice of InP and GaP using 31P NMR spectroscopy. The signal for InP is shifted down field and more narrow compared to previously reported magic angle spinning 31P{1H} NMR spectra of solid InP NCs, likely due to the hollow NC containing less core-like tetrahedrally coordinated 31P nuclei.7 The spectra for the GaP NCs contains an out-ofphase component likely due to the quadrupolar 69Ga and 71Ga nuclei that cause significant broadening in addition to the homogeneous linebroadening due to slow tumbling and the inhomogeneous broadening from slight differences in the chemical environment across the 31P nuclei. For the hollow In0.5Ga0.5P (green line in Fig. 4b), the lattice P is broadened by the presence of quadrupolar 69Ga, 71Ga, 113In, and 115In which make it unmeasurable under the conditions used here. No signal from 31P could be detected for any of the solid InxGa1-xP NCs under our experimental conditions. Improving the quality of these spectra would require acquisition times much longer than 16 h or solid-state experiments for which we are unequipped.

Scheme 2. Surface chemistry of as-grown InxGa1-xP NCs, followed by an L-type ligand exchange reaction where oleylamine (R-NH2) displaces phosphine and methyl phosphines as revealed by FTIR and NMR spectroscopies. Yellow spheres = P atoms. Gray spheres = In/Ga atoms. We postulate that the most likely explanation to account for the observed methyl group transfer to phosphorus in the small solid but not the large hollow NCs is the differences in residence time. For the small solid particles made with short residence times, both In(CH3)3 and PH3 precursors are present through the most intense part of the plasma, allowing direct reaction of methyl radicals with phosphine fragments (PH or PH2). In contrast, long residence times deplete the In precursor before it can reach the high intensity plasma near the electrode (and presumabaly generates unreactive alkanes such as methane and ethane), preventing this bimolecular reaction. An alternative explanation is that the stochastic temperature of the NC lattice in the higher power density condition is much hotter than in the low power density condition due to a higher concentration of reactive hydrogens and high energy electrons. This allows the small solid NCs to melt and crystallize, and could facilitate surface reorganization such as the observed methyl group transfer. Finally, we examined the chemical reactivity of the L-type ligand exchange reaction between oleylamine and surface-bound PHx(CH3)3-x species. The DRIFTS spectra before and after this reaction using hollow GaP NCs (Fig. S18) shows that around half of P–Hx bonds remain in the sample even after exchange with a large excess of oleylamine (over 1,000-fold


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molar ratio) to surface P. Combined with the NMR data, this observation provides several siginificant conclusions about the surface chemistry of the InxGa1-xP NCs. First, the fact that PHx(CH3)3-x stays on the surface (even after pumping the reactor pressure down to

P Nanocrystals Synthesized in a Nonthermal Plasma

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