In this paper, we present a large-volume (non-micro) atmospheric pressure glow plasma capable of rapid, large-scale zinc oxide nanocrystal synthesis and deposition (up to 400 μg/min), whereas in the majority of the literature, nanoparticles are synthesized using micro-scale or filamentary plasmas. The reactor is an RF dielectric barrier discharge with a non-uniform gap spacing. This design encourages pre-ionization during the plasma breakdown, making the discharge uniform over a large volume. The produced zinc oxide nanocrystals typically have diameters ranging from 4 to 15 nm and exhibit photoluminescence at ≈550 nm and localized surface plasmon resonance at ≈1900 cm−1 due to oxygen vacancies. The particle size can be tuned to a degree by varying the gas temperature and the precursor mixing ratio.

Plasma-synthesized nanocrystals exhibit highly tunable optical and electrical properties and can serve as low-cost building blocks for optoelectronic devices.1–5 While the synthesis process usually uses low pressure (<10 Torr) plasmas, atmospheric-pressure plasma synthesis is an increasingly popular field of research due to the potential benefit of lower manufacturing cost.6–15 Today, most nanocrystal-synthesizing atmospheric-pressure glow plasmas are microplasmas—miniature reactors with characteristic lengths less than 1 mm. While the small size ensures the nonthermal plasma condition (gas temperature Tg ≪ electron temperature Te), it limits the nanoparticle production rate and yield. The main challenges in the development of a large-volume atmospheric-pressure glow discharge (APGD) include maintaining the nonthermal condition and ensuring discharge uniformity over a large volume. Since Kanazawa et al. developed the first stable APGD in the 1970s,16 various reactor designs and ignition techniques have been developed to ensure the uniformity of APGDs with characteristic lengths ranging from a few mm up to a few cm.17–29 For the purpose of nanocrystal synthesis, the APGD's large volume and uniformity should enable rapid and controllable production. However, there are no published reports on nanocrystal synthesis using the APGD. To fill this knowledge gap, here we present an APGD reactor for zinc oxide (ZnO) nanocrystal synthesis. The reactor is an RF dielectric barrier glow discharge with a characteristic length of 2.4 mm and a reactor volume approximately 10 times larger than that of a microplasma. The applied power typically ranges from 25 W to 45 W; this is comparable to the applied power (35 W) used in the microplasma for photoluminescent silicon nanocrystal synthesis.9 

Zinc oxide nanocrystals have shown promise as essential components of a variety of optoelectronic devices, including white LEDs30 and high-efficiency perovskite solar cells.31 Additionally, their mid-IR localized surface plasmon resonance (LSPR), highly tunable by Al doping32 and/or quantum confinement,33 may be suitable for plasmonic technologies such as surface-enhanced infrared spectroscopy.34 The ZnO nanocrystals produced in our APGD reactor exhibit green (≈550 nm) photoluminescence (PL) and a mid-IR (≈1900 cm−1) LSPR.

Figure 1(a) shows the APGD reactor schematic. The reactor has a concentric cylinder geometry. The outer copper electrode is a coil wrapped around a quartz tube functioning as the dielectric barrier, and the inner electrode is a grounded tungsten wire. While the helical electrode design resembles an induction coil, the discharge is capacitively coupled, as one end of the coil is floating. The capacitive nature is confirmed by the reactor's current-voltage waveform, in which the current leads the voltage by 60°–75°. The quartz tube has an outer diameter of 7 mm and a thickness of 1 mm; the coil is 55 mm in length. The discharge is generated between the electrodes using a 13.56 MHz RF source and a 3–23 standard liters per minute (slm) helium or argon flow. The discharge appears diffuse when viewed by the eye, though at large power densities and low gas flow rates, localized filament formation is visible in argon-oxygen discharges.

FIG. 1.

Schematic of the atmospheric-pressure glow discharge reactor for nanocrystal synthesis. (a) The electrical components and characterization tools. (b) The gas injection system. The outlet is not capped; the gas injects directly into the atmosphere.

FIG. 1.

Schematic of the atmospheric-pressure glow discharge reactor for nanocrystal synthesis. (a) The electrical components and characterization tools. (b) The gas injection system. The outlet is not capped; the gas injects directly into the atmosphere.

Close modal

As shown in Figure 1(a), the center tungsten electrode is slanted so that the reactor gap spacing is non-uniform with an average gap spacing of 2.4 mm (the slanted center electrode is mounted onto the oxygen injection tube shown in Figure 1(b) for physical support). This non-uniform gap spacing ensures discharge uniformity by encouraging the production of a pre-ionization electron population, similar in nature to that of the double-discharge excitation gas laser developed by Laflamme.27 The discharge is first initiated in a local region where the gap spacing is minimal, then the discharge expands to fill the entire volume as the electrode voltage is increased. This way, the discharge remains uniform over a large volume. This could be due to the free electrons (and helium metastables in the case of a helium discharge) created by the initial discharge facilitating the formation of a uniform plasma downstream of the excited species, suppressing streamer formation and the glow-to-arc transition. The physics of this process was discussed by Palmer.35 

To inject the precursors (oxygen and diethylzinc (DEZ)), the reactor tube is attached to a three-way Swagelok® connector as shown in Figure 1(b). A stream of fast-flow carrier gas (on the order of slm), either argon or helium, is injected into the reactor through the bottom branch. A separate stream of slow-flow helium (on the order of standard cubic centimeters per second (sccm)) is injected into a stainless steel bubbler containing the DEZ. The slow-flow helium and the DEZ vapor then mix with the carrier gas as they enter the reactor. Meanwhile through separate gas lines, a stream of slow-flow oxygen (on the order of sccm) is injected into a small ceramic tube which is fed through the Swagelok connector into the plasma; this design prevents the oxygen and DEZ from mixing prior to entering the plasma.

The diagnostics tools include a Tektronix® P6015A voltage probe, a Pearson® current monitor, a Picoscope®, a substrate holder, and a 1 mm outer-diameter thermocouple. The thermocouple and the substrate holder cannot be used simultaneously since they occupy the same position; Figure 1(b) shows the schematic when the substrate holder in place. The voltage probe measures the electrode voltage at the driven electrode and the current monitor measures the current at the RF input. For particle collection, a transmission electron microscope (TEM) grid or a silicon substrate is mounted onto the substrate holder, which is positioned approximately 2 mm away from the tube outlet to collect the synthesized nanoparticles carried by the carrier gas flow. The thermocouple is inserted directly into the plasma through the tube opening for ex situ gas temperature measurement. The dissipated power is measured by subtracting the power dissipated in the matching network from the total applied power.

The plasma electron density n and the electron temperature Te can be estimated using zero-dimensional power balance models, as discussed in Refs. 37 and 38. Assuming the displacement current is small in the bulk plasma, the current density Jrf can be expressed as

Jrf=neμEbulk,
(1)

where μ is the electron mobility and Ebulk, the electric field strength in the bulk region, can be estimated by subtracting the sheath voltage from the total voltage and the sheath thickness from the gap spacing. The sheath properties are calculated using collisional sheath models.37,38 Similarly, Te can be estimated using a zero-dimensional power balance that equates the electron heating by the RF fields to the electron energy lost due to electron-neutral collisions, as discussed by Moravej.38 

To investigate the effect of Tg on the particle morphology, it is necessary to control Tg independently of the power density and the residence time. This is accomplished by placing the reactor in a container filled with dry ice or insulating the reactor using styrofoam. At a 23 slm carrier gas flow rate and a 41 W cm−3 power density, Tg is approximately 160 °C; the dry ice lowers Tg by approximately 32 °C and the styrofoam insulator increases the temperature by about 46 °C. Tg is much higher at lower carrier gas flow rates (at a flow rate of 3 slm and a power density of 30 W cm−3, Tg is 314 °C). Overall, Tg is much lower than the microplasma gas temperature (1500 K) reported in Ref. 9 and 10.

The experiment was conducted in the following sequence. First, the system was purged using argon to clear any residual ZnO deposits in the gas lines. Then, the precursors were injected into an argon or helium plasma to form the ZnO nanoparticles. After collecting the nanoparticles, the substrate was removed from the particle stream, the DEZ was turned off, and then the plasma was turned off. Without the plasma, the electrode voltage was adjusted to match the voltage during the collection; this information was used to calculate the power loss. Finally, to measure the gas temperature, a plasma was initiated under the same conditions as the nanoparticle synthesis plasma and the thermocouple was inserted into the plasma.

Figure 2(a) shows the voltage-current characteristic of the APGD reactor under various carrier gas compositions. For all cases, the voltage-current characteristic shows a positive slope, indicating that the discharge operates in the alpha mode.28,36 Figure 2(b) shows the electron density as a function of the power density with and without the precursor flow. The electron densities are slightly higher for the case without the precursor flow, showing that a portion of the applied power is dissipated in precursor dissociation and excitation. Compared to the microplasmas reported in Refs. 9 and 10 where the electron densities are on the order of 1015 cm−3, the electron densities in the APGD reactor is much lower (on the order of 1011 cm−3). Figure 2(c) shows the gas temperature as a function of the power density, and Figure 2(d) shows the gas temperature as a function of the carrier gas flow rate. Figure 2(d) illustrates that adding the oxygen impurity results in little difference in the gas temperature. Figure 2(e) shows the current-voltage waveform of the APGD reactor. The electron temperature is typically around 2 eV.

FIG. 2.

Electrical and thermal properties of the APGD reactor. (a) Voltage-current characteristics. (b) Electron density vs. power density in a 23 slm argon plasma. (c) Gas temperature vs. power density in a 3 slm argon plasma with 20 sccm oxygen impurity. (d) Gas temperature vs. carrier gas flow rate in a 3 slm argon plasma with and without the 20 sccm oxygen impurity; the power density was held between 20 and 23 Wcm−3. (e) Current-voltage waveform of the APGD reactor in an argon plasma with oxygen impurity; the current (total current) leads the voltage by approximately 74°.

FIG. 2.

Electrical and thermal properties of the APGD reactor. (a) Voltage-current characteristics. (b) Electron density vs. power density in a 23 slm argon plasma. (c) Gas temperature vs. power density in a 3 slm argon plasma with 20 sccm oxygen impurity. (d) Gas temperature vs. carrier gas flow rate in a 3 slm argon plasma with and without the 20 sccm oxygen impurity; the power density was held between 20 and 23 Wcm−3. (e) Current-voltage waveform of the APGD reactor in an argon plasma with oxygen impurity; the current (total current) leads the voltage by approximately 74°.

Close modal

Figure 3 shows the TEM images of typical ZnO particles synthesized in the APGD tube reactor. Depending on the reactor parameters, four types of particles were produced: (1) relatively mono-disperse spherical particles with diameters of 20 nm or less (Figure 3(a)); (2) a mixture of spherical and nonspherical particles with diameters of 20 nm or less (Figure 3(b)); (3) mainly nonspherical particles with diameters of 20 nm or less (Figure 3(c)); and (4) mainly high-aspect-ratio nonspherical platelets with diameters larger than 20 nm (Figure 3(d)). Large particles tend to be nonspherical rather than spherical, indicating that the growth along the a axis (the crystallographic axis) is energetically favorable.

FIG. 3.

Four kinds of ZnO nanoparticles made using the APGD tube reactor. (a) Relatively mono-disperse spheres. (b) A mixture of spheres and nonspherical particles. (c) Mainly nonspherical particles. (d) Mainly high-aspect-ratio nonspherical platelets formed occasionally in large-flow-rate plasmas; the rod-shaped objects in the image are platelets viewed from the side.

FIG. 3.

Four kinds of ZnO nanoparticles made using the APGD tube reactor. (a) Relatively mono-disperse spheres. (b) A mixture of spheres and nonspherical particles. (c) Mainly nonspherical particles. (d) Mainly high-aspect-ratio nonspherical platelets formed occasionally in large-flow-rate plasmas; the rod-shaped objects in the image are platelets viewed from the side.

Close modal

The presence of the plasma greatly improves the particle size distribution and the production rate. Without the plasma, the oxygen-DEZ reaction produces large amounts of ZnO agglomerates ranging from a few hundred nanometers up to a micrometer in size. When the plasma is present, the synthesized particles are negatively charged so that agglomeration is suppressed. The presence of the plasma also increases the production rate from 20 μg/min to 400 μg/min, with higher power density leading to higher production rates. This demonstrates that the plasma electrons significantly enhance the precursor dissociation rate. This production rate is much higher than the production rate (3 μg/min) reported in the microplasma used for photoluminescent silicon nanocrystal synthesis.9 Due to the particle bounce effect at large Stokes number at atmospheric-pressure,39,40 the production rate greatly drops at large carrier gas flow rate (dropping from 400 μg/min at a 3 slm flow rate to 4 μg/min at a 23 slm flow rate).

A certain degree of control over the particle size has been achieved by varying the oxygen:DEZ ratio and the gas temperature, as shown in Figures 4(a) and 4(b). The average particle size is calculated by measuring 30 randomly selected particles collected on the TEM grid. Figure 4(a) shows that the particle size increases for ratios less than 20:1 and decreases for ratios beyond 22:1. This shows that while an oxygen-poor plasma leads to small particles due to insufficient oxygen for complete reaction, an oxygen-rich plasma also suppresses the particle growth due to the diminished plasma density. Figure 4(b) shows that smaller particles are produced at higher gas temperatures. For all cases, larger particle size is often accompanied by a larger standard deviation in particle size, since small particles less than 10 nm are always present, even when the majority of the particles are larger than 10 nm.

FIG. 4.

Particle size as a function of reactor parameters. (a) Particle size vs. oxygen:DEZ ratio. (b) Particle size vs. gas temperature. Error bars correspond to the standard deviations of the particle size. Other factors are kept approximately constant.

FIG. 4.

Particle size as a function of reactor parameters. (a) Particle size vs. oxygen:DEZ ratio. (b) Particle size vs. gas temperature. Error bars correspond to the standard deviations of the particle size. Other factors are kept approximately constant.

Close modal

Figure 5 shows the optical properties of the ZnO nanocrystals. 4–10 nm ZnO nanocrystals dispersed in ethanol and illuminated by a 350 nm excitation source exhibit a luminescent peak at ≈550 nm (Fig. 5(c)), possibly due to the hydroxyl (OH) groups on the nanocrystal surfaces (Fig. 5(b)). In ZnO nanocrystals, surface OH groups are known to provide intermediary states through which holes migrate to singly ionized oxygen vacancies, creating doubly ionized recombination centers which enable green emission;41 emission at ≈550 nm from low-pressure-plasma-synthesized ZnO nanocrystals was attributed to this mechanism.42 These OH groups, however, suppress the LSPR43 (Fig. 5(b)). Following a procedure previously applied to low-pressure-plasma-synthesized ZnO nanocrystals,43 we use atomic layer deposition (ALD) of 7.7 nm of Al2O3 to remove the OH groups and seal the ZnO nanocrystals, resulting in an air-stable LSPR at ≈1900 cm−1 in 11–13 nm nanocrystals (Fig. 5(b)). Fitting the LSPR feature using Mie theory with the classical Drude dielectric function (for details, see Ref. 43), we find the free electron density (ne) and the intraparticle electron mobility (μe) to be 7 × 1019 cm−3 and 20 cm2 V−1 s−1, respectively. These values are similar to those of the ZnO nanocrystals synthesized in a low-pressure plasma and are suitable for devices requiring high n-type electrical conductivity; the high ne is likely due to electron donation by the oxygen vacancies evidenced by the green PL.

FIG. 5.

(a) X-ray diffraction pattern of 11–13 nm nanocrystals with zincite powder diffraction pattern (ICDD-PDF 36-1451). (b) Diffuse reflectance Fourier transform infrared spectrum of 11–13 nm nanocrystals deposited on a gold-coated silicon substrate before (blue curve) and after (red curve) ALD treatment. In the before-ALD spectrum, the absorption features at 1600 and 3400 cm−1 are due to OH groups on the nanocrystal surfaces, and the absorption feature at 500 cm−1 is due to Zn-O bonds. In the after-ALD spectrum, the absorption feature at 800 cm−1 is due to Al-O bonds. (c) Photoluminescence spectrum of 4–10 nm nanocrystals dispersed in ethanol and excited by a 350 nm light source. The near-band-gap emission is not visible due to overlap with the source.

FIG. 5.

(a) X-ray diffraction pattern of 11–13 nm nanocrystals with zincite powder diffraction pattern (ICDD-PDF 36-1451). (b) Diffuse reflectance Fourier transform infrared spectrum of 11–13 nm nanocrystals deposited on a gold-coated silicon substrate before (blue curve) and after (red curve) ALD treatment. In the before-ALD spectrum, the absorption features at 1600 and 3400 cm−1 are due to OH groups on the nanocrystal surfaces, and the absorption feature at 500 cm−1 is due to Zn-O bonds. In the after-ALD spectrum, the absorption feature at 800 cm−1 is due to Al-O bonds. (c) Photoluminescence spectrum of 4–10 nm nanocrystals dispersed in ethanol and excited by a 350 nm light source. The near-band-gap emission is not visible due to overlap with the source.

Close modal

Here, we presented a large-volume (non-micro) APGD reactor for rapid, large-scale ZnO nanocrystal synthesis. The reactor uniformity over large volume was ensured by a pre-ionization technique utilizing a varied gap spacing design. We found that the presence of the plasma greatly improved the particle size distribution and the production rate. The produced nanocrystals typically had diameters ranging from 4 to 15 nm and exhibited PL at ≈550 nm and LSPR at ≈1900 cm−1 due to oxygen vacancies. The particle size was shown to be somewhat tunable by varying the temperature and the oxygen-to-DEZ ratio. A larger size was observed at low gas temperatures and oxygen to DEZ ratio of about 20:1.

N.B. was supported in part by the DOE Plasma Science Center and the Doctoral Dissertation Fellowship from the University of Minnesota. B.L.G. was supported by NSF through MRSEC Grant No. DMR-1420013. J.Y. acknowledges the support by the Army Office of Research under MURI Grant No. W911NF-12-1-0407. Part of this work was carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. The authors would like to thank Juyong Jang for his contribution to the reactor design and construction, and Yunxiang Qin for performing scanning electron microscope measurements on the nanocrystal deposits.

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