Aging of Silicon Nanocrystals on Elastomer Substrates

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Aging of Silicon Nanocrystals on Elastomer Substrates: Photoluminescence Effects Rajib Mandal, and Rebecca Joy Anthony ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10155 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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ACS Applied Materials & Interfaces

Aging of Silicon Nanocrystals on Elastomer Substrates: Photoluminescence Effects Rajib Mandal and Rebecca J. Anthony* Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48824, United States KEYWORDS: Silicon, Nanocrystals, PDMS, Photoluminescence, Plasma.

ABSTRACT: Nanocrystalline silicon is widely known as an efficient and tunable optical emitter and is attracting great interest for applications such as light-emitting devices (LEDs), electronic displays, sensors and solar-photovoltaics. To date, however, luminescent silicon nanocrystals have been used exclusively in traditional rigid devices, leaving a gap in knowledge regarding how they behave on elastomeric substrates. The present study shows how the optical and structural/morphological properties of plasma-synthesized silicon nanocrystals (SiNCs) change when they are deposited on stretchable substrates made from polydimethylsiloxane (PDMS). Our results indicate that SiNCs deposited directly from the gas-phase onto PDMS exhibit morphological changes, as well as modified aging characteristics due to enhanced oxidation. These results begin to fill the knowledge gap, and point to the potential of using luminescent SiNC layers for flexible and stretchable electronics such as LEDs, displays and sensors.

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1. INTRODUCTION Silicon (Si) is one of the most commonly-used semiconductor materials and offers advantages over some other semiconductors in that it is inexpensive, nontoxic, abundant, and has benefitted from decades of experience in purification, growth and device fabrication. Bulk Si is a major contributor in today’s microelectronics,1–3 photonics4–6 and solar-photovoltaics industries.7–10 In addition, new properties and functionalities arise for nanoscale Si, which exhibits efficient and tunable luminescence due to quantum size effects.11–14 Si nanocrystals (SiNCs) have opened the way for new and interesting applications in photovoltaics, photonics, microelectronics and nanobiotech industries.15–17 Additionally, emerging applications involving stretchable and flexible electronics for optoelectronic applications such as in-situ health monitoring, display technology, and inexpensive, versatile LEDs and solar cells are not feasible using bulk semiconductors. Hence, SiNCs offer a huge technological and scientific interest in these areas. In any NC-based application, incorporating the nanocrystals into the device architectures presents a novel challenge, particularly when deformable substrates for flexible/stretchable devices are required. Some groups have had success creating SiNC/elastomer mixed materials starting from the liquid phase, including combining SiNCs and PDMS, to make bulk hybrid NC/elastomers.18,19 When it comes to creating thin layers of NCs on top of substrates, many deposition schemes involve solvent processing (spin- or drop-casting) or high temperatures, which can dissolve or thermally decompose polymer substrates, precluding deposition directly onto elastomers like PDMS. Gas-phase inertial impaction, however, can sidestep many of these issues by allowing NCs to directly impinge upon substrates from the vapor phase, eliminating solvents and high temperatures.20–22 This inertial impaction opens the door to creating next-


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generation stretchable and flexible devices using the exciting tunable properties of NCs. Here, we present results on inertial impaction of plasma-produced SiNCs onto elastomer substrates, and the effects of aging on the photoluminescence properties of the resulting SiNC films. This research is the first investigation to our knowledge of inertial impaction of nanomaterials onto elastomeric substrates. 2. EXPERIMENTAL DETAILS Plasma reactor: SiNCs were synthesized using a nonthermal plasma reactor, as has been demonstrated for SiNCs as well as for NCs of other materials including germanium and zinc oxide.11,23–26 A schematic diagram of the plasma reactor is shown in Figure 1. The flow-through reactor consists of a Pyrex tube with an outer diameter (O.D.) of 1.27cm in the top portion and 2.54cm in the bottom portion. The tube is 30.5cm long and the expansion area is 17.8cm from the top. Argon (Ar) and silane (SiH4, 1% in Ar) were flown through the Pyrex tube, and 13.56 MHz radiofrequency (rf) power was supplied via dual ring electrodes encircling the upper portion of the tube. The gas flowrates for Ar and SiH4/Ar were 5-30 sccm and 50-80 sccm respectively. The pressure in the reactor was kept at 1.85-2.7 Torr using a slit-shaped nozzle orifice, and was 160-300 mTorr downstream of the orifice. The slit-shaped orifice at the reactor base accelerated the SiNCs and they inertially impacted onto the substrates underneath.20–22 The described conditions led to the synthesis of crystalline SiNCs of diameter 4-5 nm, as confirmed using transmission electron microscopy (TEM), x-ray diffraction (XRD), and Raman spectroscopy (Figure 1). Sample deposition and storage: SiNCs were deposited via inertial impaction onto polydimethylsiloxane (PDMS) films and bulk silicon wafers. The deposition was performed by


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rastering the substrates beneath the orifice at a standoff distance of ~3mm. Following synthesis, the samples were stored in air. PDMS preparation: PDMS was prepared in the lab using Sylgard 184 (Dow Corning Corp.). The prepolymer base (PDMS monomer) and the cross linking agent were vigorously mixed in a 10:1 weight ratio, then degassed in a vacuum desiccator for 20-25 minutes until all the air bubbles were removed from the mixture. Pre-measured amounts of the mixture were cast into plastic petri dishes, leading to film thicknesses of ~ 0.5 mm. The PDMS was then heat-cured using a hot plate at 60ºC for 2-3 hrs. The PDMS was cooled at room temperature and cut into small substrates of dimensions 28mm × 12mm. Photoluminescence Measurements: Photoluminescence (PL) measurements were performed using an Ocean Optics, Inc. USB spectrometer and optical fiber. The PL was excited using a UV/blue LED (peak at 395nm). The measurements were performed in air.


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Figure 1. (a) Schematic of the Plasma Reactor with (b) TEM image of SiNCs (circles indicate nanocrystallites), (c) XRD pattern, and (d) Raman analysis. The Raman peak is shifted to lower wavenumbers compared to the bulk silicon value (520 cm-1)27 due to the small nanocrystal size. 3. RESULTS AND DISCUSSION We deposited ~1µm layers of SiNCs onto relaxed PDMS substrates as well as silicon wafer substrates to compare the appearance and photoluminescence of the resulting films. For some cases, we also pre-stretched the PDMS substrates using a house-built stretching stage (Figure 2). For our measurements using pre-stretched PDMS, we deposited the SiNCs while the PDMS was stretched and then allowed the PDMS and SiNC films to relax before performing any measurements. Henceforth, we refer to samples produced on prestretched PDMS according to the applied strain during SiNC deposition, meaning the ratio of the length of the PDMS during


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the stretching/deposition to its length when relaxed. We produced SiNCs on silicon wafer, relaxed PDMS (0% pre-strain), and PDMS at pre-strain conditions of 10%, 20%, and 40%.

Figure 2. Flowchart showing different steps for PDMS and wafer substrates. The SiNCs as-produced have a nontrivial surface defect density and thus photoluminescence quantum yields of only several percent immediately after deposition.23 Thus, to measure their photoluminescence, we waited 2 days after deposition to allow ambient-air oxidation to begin to cap the defects and increase the samples' PL intensities: as such, oxidation of these SiNC films was our default passivation method. Despite using exactly the same recipe for synthesis, the PL from SiNCs on relaxed (0% prestrain) PDMS was significantly blueshifted in comparison to the PL from SiNCs on silicon wafer (up to 80 nm) (Figure 3a). The PL peak should, in principle, depend on the SiNCs themselves and not their substrate. We also found that the PL peak for the SiNCs on prestretched PDMS actually redshifted back towards the peak position for SiNCs on silicon wafer.


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Figure 3. Photoluminescence spectra for (a) SiNCs on relaxed (0%) PDMS and on silicon wafer, (b) SiNCs on PDMS with different pre-strain conditions, and (c) SiNC layers with different thicknesses of SiNCs on relaxed (0%) PDMS films. As the SiNC synthesis recipe was the same for all these samples, we expected the PL peak to remain constant regardless of substrate. Visual analysis via both top-down and cross-sectional


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SEM images (Figure 4) showed that the Si NC layers on relaxed (0%) PDMS were more densely packed compared to layers on silicon wafer.

Figure 4. SEM images of gas-phase-impacted SiNC films on Si wafer (a, c) and relaxed PDMS (0% strain) (b, d). As a next step, we deposited films of varying thickness on PDMS to investigate any whether the observed PL shift was dependent on layer thickness. Our initial measurements had been made on fairly thick layers (~1µm) which required 100 deposition rastering cycles. We deposited SiNCs on relaxed PDMS with 10 cycles, 50 cycles, and 100 cycles (corresponding to ~100nm, 500nm, and 1µm). PL measurements taken after 2 days showed that the thinner films exhibited even more blueshifting compared to the thicker films. (see Figure 3 c). Again, as the SiNCs are the


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same before deposition, this indicated a change that occurred for the NCs as a result of the PDMS substrate and film properties, possibly as a result of modified aging characteristics. Aging is important for SiNCs because they are susceptible to oxidation in ambient conditions. Oxidation of SiNCs causes an SiOx layer to grow at the NC surface, corresponding to a core size shrinkage and blueshifting photoluminescence.28 Increasing the fractional volume of oxidized SiNC films on PDMS compared to on silicon wafer would explain the difference in peak PL wavelength, and would also explain the thickness dependence (thinner films oxidize more rapidly). In order to test this, we needed to eliminate oxygen-exposure and compare the results for the as-produced and air-shielded samples with the results from samples that were aged in air. We made two sets of SiNC films on relaxed (0%) and 20% pre-strained PDMS as well as on Si wafers: one set was stored in a nitrogen-purged glove bag immediately after removal from the reactor, and the second set of films was stored in air. PL from all samples was measured immediately after synthesis and after 2 days of aging to observe whether the oxygen-free environment affected the peak luminescence. The results are shown in Figure 5. For samples stored in the glove bag, the PL peaks overlap on the first day and on subsequent days. For samples stored in air, the peaks began to shift from one another (blueshifting for the samples on PDMS) within a day of synthesis. This confirmed the hypothesis that oxidation is enhanced for SiNC films on PDMS. The air-permeability of PDMS has been demonstrated by previous groups, and thus the SiNC films on PDMS were oxidizing not only from the top but also through the bottom of the films.27,28 The PL measurements are ensemble measurements, collecting light from the entirety of the film thickness – thus, additional oxidation at the back of the films would


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lead to blueshifting compared to the films on silicon wafer, which were only oxidizing from the top.

Figure 5. Photoluminescence spectra from oxidation dependence experiment for samples stored in N2-purged glove bag (a and b) and stored in air (c and d). The dependence on pre-strain can be explained by examining the surface of a SiNC film on prestretched PDMS before and after relaxing the substrate. To do this, we deposited SiNCs on a PDMS substrate pre-stretched to 40% of its initial length, and left the substrate and SiNC film in place on the stretching stage while imaging in the scanning electron microscope (SEM). The SEM image shows a fairly flat layer of nanocrystals. Then, we removed the stage and relaxed the film of SiNCs to image it again. After relaxing the substrate, the SiNC film developed wrinkles across its surface. Due to the very thin nature of the SiNC film in comparison with the elastomer


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substrate, these instabilities are limited to the surface of the SiNC film as described by Bigoni.31 The surface instabilities cause the SiNC film to be thicker in some places compared to the same thickness SiNC film on relaxed PDMS or on silicon wafer. The increased thickness would result in reduced fraction of SiNCs that are oxidized, compared to the entirety of the film. This explains why increasing stretching ratio causes reduced blueshifting of the PL peak. Figure 6 shows a cartoon of this process.

Figure 6. (a) SEM image showing surface instability formation. (b) Proposed stages of oxidation for samples on PDMS vs. Si wafer.


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4. CONCLUSION In conclusion, we have demonstrated the first all-gas-phase deposition of luminescent nanocrystals onto elastomeric substrates. The films showed wrinkling in response to pre-stretch of the PDMS substrates, as well as structural changes compared to SiNCs deposited on Si wafers. The SiNCs on relaxed PDMS exhibited blueshifted luminescence compared to SiNCs on Si wafer, with decreased blueshifting observed for the wrinkled SiNC films which had been deposited on prestretched PDMS. Our experiments indicate that the blueshift effect is due to enhanced oxidation on PDMS, caused by the air-permeability of PDMS and thus oxidation of the SiNC films from all sides as opposed to the top-down-only oxidation that occurs for SiNCs on Si wafers. These results will aid ongoing and future development of stretchable and flexible luminescent NC layers for next-generation displays, LEDs, and other devices.

AUTHOR INFORMATION Corresponding Author *E‒mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by faculty startup funds provided by Michigan State University’s College of Engineering and Department of Mechanical Engineering. The work was also performed with partial support from NSF CMMI Grant #1561964.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to thank Dr. Xudong Fan from Center For Advanced Microscopy, Michigan State University, for his help with TEM analysis. We used Siemens NX 10 to create the reactor schematic (Figure 1a). The authors would also like to thank Michael Bigelow, Duncan

Kroll

and

Naomi

Carlisle

for

their

help

with

schematic

and

sample

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Aging of Silicon Nanocrystals on Elastomer Substrates

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